CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402346
Design of a Metal-Promoted Oxide Catalyst for the Selective Synthesis of Butadiene from Ethanol Vitaly L. Sushkevich,[a, b] Irina I. Ivanova,*[a, b] Vitaly V. Ordomsky,[a, b] and Esben Taarning[c] The synthesis of buta-1,3-diene from ethanol has been studied over metal-containing (M = Ag, Cu, Ni) oxide catalysts (MOx = MgO, ZrO2, Nb2O5, TiO2, Al2O3) supported on silica. Kinetic study of a wide range of ethanol conversions (2–90 %) allowed the main reaction pathways leading to butadiene and byproducts to be determined. The key reaction steps of butadiene synthesis were found to involve ethanol dehydrogenation, acetaldehyde condensation, and the reduction of crotonalde-
hyde with ethanol into crotyl alcohol. Catalyst design included the selection of active components for each key reaction step and merging of these components into multifunctional catalysts and adjusting the catalyst functions to achieve the highest selectivity. The best catalytic performance was achieved over the Ag/ZrO2/SiO2 catalyst, which showed the highest selectivity towards butadiene (74 mol %).
Introduction Buta-1,3-diene is an important monomer in the production of polymers such as polybutadiene, styrene–butadiene rubber, styrene–butadiene latex, acrylonitrile–butadiene–styrene polymer, nitrile rubber, and some others. Currently, approximately 95 % of butadiene is produced by isolation from naphtha steam cracker fractions generated during ethylene production. However, trends in the development of the ethylene industry are such that ethylene producers are switching to more efficient ethylene production technologies. This leads to the lack of butadiene on the market and, therefore, requires the development of alternative technologies leading to butadiene. One of the most perspective sources for butadiene production is ethanol, which can be produced from carbohydrate biomass. The recent assessment of the economic, environmental, health, safety, and operation aspects of naphtha-based and bioethanol-based routes[1–3] revealed that the bioethanol to butadiene process could be a promising substitute for the dominant naphtha-based method and can contribute to a decrease in the use of fossil-fuel reserves. The one-step catalytic process to produce butadiene from ethanol was developed by Lebedev[4] and was commercialized in Russia in the beginning of the 1920s. Later on, the Carbide [a] V. L. Sushkevich, Prof. I. I. Ivanova, V. V. Ordomsky Department of Chemistry Lomonosov Moscow State University Leninskye Gory 1, bld. 3, 119991 Moscow (Russia) Fax: (+ 7) 4959393570 E-mail: [email protected] [b] V. L. Sushkevich, Prof. I. I. Ivanova, V. V. Ordomsky “UNISIT” LLC Leninskye Gory 1, bld. 75 V, 119991 Moscow (Russia) [c] Dr. E. Taarning Haldor Topsøe A/S Nymoellevej 55 2800 Kgs. Lyngby (Denmark) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402346.
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and Carbon Chemicals Corporation in the USA commercialized a two-step process[5–7] based on the Ostromislenskiy method.[8] The latter technology included ethanol dehydrogenation into acetaldehyde and conversion of the mixture of ethanol and acetaldehyde into butadiene. However, after the 1960s both processes became economically unfavorable and were stopped owing to the development of petrochemical routes of butadiene production. Nowadays, interest in Lebedev’s process has been renewed. However, the existing technologies require substantial improvement; in particular, the development of novel catalysts with high activity and selectivity is of prime importance.[1] These circumstances have generated a new wave of research activity in this direction in recent years.[1–3, 9–13] Analysis of the literature on the catalytic synthesis of butadiene from ethanol suggests that all the catalytic systems proposed so far have been based on basic or acidic oxides or their mixtures. Basic oxide catalysts were first proposed by Lebedev. According to different literature sources, Lebedev’s catalyst involved magnesia[14] or zinc oxide[13, 15] as the main active components. Magnesia–silica catalysts were thoroughly studied later on by many research groups.[9, 12, 16–20] Niiyama et al.[16] investigated the effect of the MgO content and revealed that an increase in the catalyst activity was observed with an increase in the magnesia content up to 85 %. They also pointed to the importance of the preparation method. The effect of the method of preparation was further studied by Kvisle et al.,[19] who demonstrated that wet kneading of the two oxides resulted in a much more active catalyst than that obtained by mechanical mixing of the components. Kitayama et al.[17] were the first to use sepiolite (a fibrous magnesia–silicate clay mineral) for the conversion of ethanol into butadiene. Many efforts were devoted to dope magnesia–silica catalysts with different transition-metal oxides, such as cromia,[14] manganese,[17] nickel oxide,[20] iron, cobalt, copper, and zinc ChemSusChem 2014, 7, 2527 – 2536
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CHEMSUSCHEM FULL PAPERS oxides.[12] These promoters were reported to improve the yield and the stability of the catalytic system. The best result obtained so far for magnesia–silica-based systems was reported by Ohnishi et al.[18] over a MgO/SiO2 catalyst (molar ratio = 1:1) promoted with 0.1 % Na2O. This catalyst was reported to show 100 % conversion of ethanol with ultimate selectivity to butadiene of 87 % at 623 K; however, the lack of experimental details did not allow for reliable comparison with the data of other researchers. Acidic catalytic systems were first proposed by Toussaint and co-workers[5, 6, 21] for the second step of the two-step process developed by the Carbide and Carbon Chemicals Corporation. These catalysts involved Lewis acid oxides supported on silica. In particular, supported tantalum, zirconium, niobium, thorium, uranium, and titanium oxides were shown to be active catalysts for the conversion of ethanol and acetaldehyde into butadiene. Among these catalytic systems, 2 %Ta2O5/SiO2 proved to be the best and yielded butadiene with 60 % selectivity and 30–35 % yield per pass. The one-step process was also studied over acidic catalysts. Doping of tantala–silica catalysts with metal oxides capable of effecting the dehydrogenation of ethanol into acetaldehyde proved to be efficient for this process. In particular, doping with copper and cadmium oxides as well as chromium and magnesium oxides gave active catalysts.[15] A recent investigation by Jones et al.[10] revealed that application of various binary and ternary mixed oxide systems involving zinc, zirconium, cerium, manganese, copper, and cobalt oxides supported on silica may lead to a further improvement in the selectivity and yield of butadiene. The most promising catalyst reported by these authors was a Cu/Zr/Zn oxide system supported on silica. The highest selectivity observed was 67 %. The third type of catalytic system that was used for the synthesis of butadiene from ethanol involved amphoteric oxides or mixed acid and base oxide systems. According to some literature sources,[13, 15] such systems were first proposed by Lebedev and involved zinc oxide supported on alumina. Afterwards, they were extensively studied by Bhattacharyya et al.[27] Al2O3/ MgO (60:20), Al2O3/Cr2O3 (60:40), Al2O3/CaO (60:20), and Al2O3/ ZnO (60:40) were shown to be good catalysts; however, the best result (55.8 % yield) was reached over Al2O3/ZnO (60:40). Furthermore, a considerable increase in the yield up to 72.8 % was reported over this system under fluidized bed conditions. Fripiat et al.[22, 23] used aluminated sepiolite in the synthesis of butadiene. The insertion of aluminum led to an increase in the Lewis acidity of basic sepiolite and resulted in an improvement in its activity and selectivity, although the reported yield was lower than that on mixed oxides. In our recent patent,[11] we demonstrated that the activity and selectivity of various oxide catalysts including MgO, TiO2 MgO, ZrO2, Nb2O5, and Ta2O5 could be improved significantly by doping the oxides with small amounts of metals such as Cu, Ag, Au, and some others. The preparation of such catalysts involved impregnation of oxide systems with metal precursor salts followed by their calcination in air and reduction under a hydrogen atmosphere. This discovery allowed the conversion of the oxide catalysts to be increased by a factor of 5 and also 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org allowed the selectivity to butadiene to be higher than 80 mol %. The effect was confirmed in a further study by Makshina et al.[12] over MgO/SiO2 catalysts doped with silver. Although significant achievements have been made towards catalyst improvement over the last few years, fundamental understanding of the process, which is necessary for rational catalyst design, is still lacking. As stressed in the recent review by Weckhuysen et al.,[1] the following key questions regarding optimal catalyst design remain to be answered: 1) Reaction mechanism over different catalysts and the ratedetermining step 2) Reaction pathways leading to byproducts 3) Structure–activity relationship, optimal catalytic functions (acid/base/redox), and the balance between them required to achieve the highest selectivity to butadiene 4) Catalyst stability and deactivation pathways In this contribution, we aim to answer at least some of these questions for the metal-containing oxide catalysts (M/MOx/ SiO2) discovered in our previous study,[11] in particular: 1) To clarify the main reaction pathways leading to butadiene and byproducts over metal-containing oxide catalysts 2) To select active and selective catalyst components for each reaction step 3) To merge these components into one multifunctional catalyst and to balance the functions to achieve the highest selectivity 4) To adjust the reaction conditions
Results and Discussion Reaction pathways leading to target and side products At present, two main reaction mechanisms for the conversion of ethanol into butadiene have been discussed in the literature.[1, 9–12, 16, 21, 24–27] The first one is based on aldol condensation of acetaldehyde and involves ethanol dehydrogenation into acetaldehyde, its condensation, dehydration of 3-hydroxybutanal, reduction of crotonaldehyde with ethanol, and dehydration of crotyl alcohol into butadiene (Scheme 1). This mechanistic pathway was first proposed by Toussaint and co-workers[21] and was further adopted by many research groups.[9–12, 16, 21, 24–27] However, different suggestions have been made on the rate-limiting step of this pathway over different catalysts. Thus, ethanol dehydrogenation into acetaldehyde
Scheme 1. Butadiene formation through the aldol condensation pathway.[1]
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was reported to be rate limiting over a silica–magnesia catalyst.[16] On the contrary, aldol condensation was found to be rate limiting over supported tantalum, zirconium, aluminum, and titanium oxide catalysts and over Al2O3/ZnO mixed oxides.[10, 21, 27] Finally, reduction of crotonaldehyde was suggested to be rate limiting over Lebedev’s catalyst.[24–26] An alternative mechanistic pathway is based on Prins-like reaction between acetaldehyde and ethylene (both derived from ethanol), followed by dehydration of but-3-en-2-ol to yield butadiene (Scheme 2). This reaction pathway was proposed by Natta and Rigamonti[14] and was further supported by the investigations of Fripiat and co-workers[22] over aluminated sepiolite catalysts. Figure 1. Yields of the main reaction products and intermediates versus ethanol conversion (T = 593 K).
Scheme 2. Butadiene formation through the Prins condensation pathway.[1]
Recent thermodynamic calculations[1] indicated that both pathways can operate at reaction temperature, which is considered optimal for butadiene synthesis (673–698 K). However, the acetaldehyde condensation route is slightly more favorable. Notably, the main reaction pathway leading to butadiene is usually accompanied by a number of side reactions that are poorly understood. The nature and the amount of byproducts depend on the type of catalyst and are generally not discussed. To obtain deeper insight into the reaction pathways leading to butadiene and byproducts over M/MOx/SiO2 catalysts, a kinetic study was performed over a wide range of weight hourly space velocity (WHSV) values with the model catalyst containing 1 wt % Ag and 4 wt % zirconia supported on silica (1 Ag/ 4 ZrO2/SiO2). Ethanol conversion varied from 2 to 90 %. The yields of the various products are plotted versus conversion of ethanol in Figures 1–4. The kinetic analysis of these curves is made on the basis of the models calculated in Ref. [28] for primary and secondary and stable and unstable reaction products. The kinetic curves for the main group of products, including acetaldehyde, crotonaldehyde, butadiene, and heavier products (mostly hexatrienes), are shown in Figure 1. Acetaldehyde appears at very low conversions and corresponds to primary reaction products. The kinetic curve for acetaldehyde shows a maximum; this suggests that it is involved in secondary reactions that most probably include self- and cross-condensation reactions leading to 3-hydroxybutanal and heavier C6 + products. 3-Hydroxybutanal was not detected in the reaction products, probably as a result of its fast dehydration into crotonaldehyde, observed as a secondary unstable product (Figure 1). 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Notably, broadening and the asymmetry of the maximum on the kinetic curve of acetaldehyde at high conversions can be indicative of the second reaction pathway leading to acetaldehyde. Such a secondary process may involve oxidation of ethanol with crotonaldehyde through a Meerwein–Ponndorf– Verley–Oppenauer (MPVO) reaction, as shown in Scheme 1. This observation is in line with other findings[1, 10, 21, 27] that imply that crotonaldehyde reduction with ethanol is a more favorable route than reduction with hydrogen. Observation of acetaldehyde and crotonaldehyde as intermediate products and indication of the MPVO reduction of crotonaldehyde suggests that butadiene is most probably formed over M/MOx/ SiO2 catalysts through the aldol condensation pathway shown in Scheme 1. The major side products of this pathway are hexatrienes (C6 +), which show the kinetic curve of stable secondary reaction products (Figure 1). The formation of these products can be accounted for by the self- and cross-aldol condensation of crotonaldehyde and acetaldehyde, followed by the reduction and dehydration of the aldehydes formed (Scheme 3).
Scheme 3. Formation of heavy byproducts.
Another pathway of ethanol conversion involves ethanol dehydration into diethyl ether and ethylene. Diethyl ether shows the kinetic curve corresponding to a primary unstable product (Figure 2), whereas the curve of ethylene is typical for stable secondary products. These observations suggest that ethylene is formed mostly via diethyl ether; however, the direct dehydration of ethanol into ethylene cannot be completely excluded (Scheme 4). The kinetic curves point that ethylene is not involved in further transformations and suggest that it could hardly be an intermediate in the synthesis of butadiene ChemSusChem 2014, 7, 2527 – 2536
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Scheme 5. Reaction pathways leading to butan-1-ol.
Scheme 6. Reaction pathways leading to butenes. Figure 2. Yields of the ethanol dehydration products versus ethanol conversion (T = 593 K).
dehydration of butan-1-ol and subsequent isomerization (Scheme 6): Finally, the last group of byproducts involves ethyl acetate, acetic acid, acetone, and propene. Ethyl acetate shows the curve of the unstable secondary product, which passes through the maximum at 60 % ethanol conversion (Figure 4). It can be formed from acetaldehyde by Tischenko reaction (Scheme 7).[29]
Scheme 4. Formation of ethanol dehydration products.
through Prins condensation. This result confirms the conclusion that aldol condensation is the main reaction pathway leading to butadiene over M/MOx/SiO2 catalysts. C4 reaction byproducts, including butenes and butan-1-ol, represent a significant group of byproducts over M/MOx/SiO2 catalysts. The kinetic curve of butan-1-ol shows a maximum at 30 % ethanol conversion (Figure 3) and corresponds to the unstable secondary product that can be formed by the reduction of the C=C bond in crotyl alcohol and/or crotonaldehyde, as shown in Scheme 5. But-1-ene and but-2-ene can be attributed to stable secondary products; the yield of these products sharply increases with conversion (Figure 3). Butenes can hardly be formed by butadiene hydrogenation and are most probably formed by
Figure 4. Yields of ethyl acetate and propylene versus ethanol conversion (T = 593 K).
Scheme 7. Reaction pathway leading to ethyl acetate.
Further transformations of ethyl acetate may include hydrolysis into ethanol and acetic acid, decarboxylation of acetic acid into acetone,[30] and reduction and dehydration of the latter into propylene,[31] which is observed as a stable secondary product of the reaction (Scheme 8). Reaction network Figure 3. Yields of butenes and butan-1-ol versus conversion of ethanol (T = 593 K).
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The reaction network presented in Figure 5 further rationalizes all of the above observations. The main reaction pathway leadChemSusChem 2014, 7, 2527 – 2536
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www.chemsuschem.org are also found to be stable products over M/MOx/SiO2 catalysts. Selection of catalyst active components
Scheme 8. Reaction pathway leading to propylene.
Figure 5. Reaction network.
ing to butadiene involves five reaction steps: first, ethanol dehydrogenation into acetaldehyde; second, aldol condensation of acetaldehyde; third, dehydration of 3-hydroxybutanal; fourth, MPVO reduction of crotonaldehyde with ethanol; and fifth, dehydration of crotyl alcohol into butadiene. The rate-limiting step over M/MOx/SiO2 catalysts is most probably aldol condensation, as acetaldehyde is the major intermediate product accumulating over the catalysts during the reaction (Figure 1). The most rapid reaction steps involve dehydration of 3-hydroxybutanal and crotyl alcohol. Only trace amounts of these products were detected during the reaction. Besides dehydrogenation, ethanol undergoes dehydration to give diethyl ether and ethylene. Ethylene is found to be a stable product over M/MOx/SiO2 catalysts. It is not involved in further reactions, such as Prins condensation with acetaldehyde. On the contrary, acetaldehyde is very reactive and initiates two side reactions along with the target reaction pathway leading to butadiene. A Tischenko reaction leads to ethyl acetate, which is further converted into acetone and propylene. Cross-condensation of acetaldehyde with crotonaldehyde and other aldehydes results in heavier products, such as hexatrienes. Finally, the last side reaction pathway involves hydrogenation of crotyl alcohol to give butan-1-ol and its further dehydration and isomerization into but-1-ene and but-2-ene, which 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The results of the kinetic study over the M/MOx/SiO2 catalysts suggest that among the five reaction steps that constitute the target reaction pathway, three steps involving ethanol dehydrogenation, acetaldehyde condensation, and MPVO reduction of crotonaldehyde can be considered as key reaction steps leading to butadiene. Dehydration of 3-hydroxybutanal and crotyl alcohol is shown to proceed very easily, even on the weak acid sites of silica support. This conclusion is supported by previous studies.[10] Therefore, the selection of the active components of the catalyst was focused on the three abovementioned steps; silica was chosen as a support owing to its high surface area and weak acidity, which enable it catalyze the dehydration steps. Analysis of the literature data on the dehydrogenation of alcohols revealed two main groups of catalytic systems used in these reactions. The first one involves metal-containing catalysts, including noble metals and metals from group Ib.[32] The second one is composed of basic or transition-metal oxides, such as MgO, CeO2, and Cr2O3, or their mixtures.[33] Oxidebased catalysts usually require higher temperatures than metal-containing catalysts for the dehydrogenation of alcohols and give lower yields of acetaldehyde under the conditions typical for butadiene synthesis. Therefore, supported metal catalysts were found to be a better choice of multifunctional catalyst for butadiene synthesis. Among the metal catalysts studied in the dehydrogenation of ethanol, noble metals were found to be very active but not selective; along with dehydrogenation, ethanol underwent steam reforming into CO and CH4.[32d] On the contrary, copper and nickel catalysts were shown to be active and selective in the dehydrogenation of ethanol.[32b] Even higher activity (turnover frequency = 3.4 s1) and selectivity to acetaldehyde (up to 94 %) were obtained in our recent study over silver supported on silica.[32e] It has been demonstrated that an efficient metal catalyst for the dehydrogenation of ethanol should have a high density of SiOH groups on the silica support, a high metal dispersion, and should be characterized by close proximity of SiOH and the metal sites. On the basis of these data, silver, copper, and nickel were selected as metal components for the multifunctional catalyst. Aldolization of acetaldehyde has been reported over different solid catalysts including oxide and mixed oxide systems based on ZrO2, Ta2O5, Nb2O5, TiO2, CeO2, Al2O3, MgO, zeolites, aluminophosphates, hydrotalcite-like materials, and so on.[34] Both acidic and basic systems were shown to be active catalysts for aldol condensation. Our recent studies[34c] performed by using ZrO2/SiO2 and MgO/SiO2 catalysts suggested that strong basic sites existing over MgO/SiO2 are more reactive in condensation reactions than the acidic sites of both ZrO2/SiO2 and MgO/SiO2. However, strong basic sites are quickly deactivated. Therefore, the reaction pathway involving Lewis acid sites most probably governs the conversion of acetaldehyde over both MgO/SiO2 and ZrO2/SiO2 under steady-state condiChemSusChem 2014, 7, 2527 – 2536
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tions. These results determined the choice of oxide catalysts for this study. In particular, MgO, ZrO2, Ta2O5, Nb2O5, TiO2, and Al2O3 supported on silica were selected. Traditional homogeneous catalysts used in MPVO reactions are aluminum and titanium alkoxides. The solid catalysts studied so far in the MPVO reaction include metal oxides,[35a] hydrotalcites,[35b,c] zeolites,[35d] and immobilized metal alkoxides.[35e] Zirconia- and titania-based catalytic systems such as hydrous zirconia, ZrO2 or TiO2 supported on different oxides, and ZrBEA or TiBEA zeolites were found to be the most active and selective for the MPVO reduction of different carbonyl compounds. In our previous work,[35f] we demonstrated that crotonaldehyde can be reduced with ethanol into crotyl alcohol over ZrO2/SiO2 and ZrBEA catalysts with selectivity up to 80 %. The similarity of the active catalysts for the MPVO reduction and aldol condensation reactions simplifies the selection of the active components for the multifunctional catalyst. On the basis of all these considerations, the following choices were made: metal components: Ag, Cu, and Ni; oxide components: MgO, ZrO2, Nb2O5, TiO2, and Al2O3, support: SiO2.
The nature of the acid sites was investigated by FTIR spectroscopy of adsorbed CO, a well-established probe molecule for the characterization of the acidity of various oxide catalysts.[36] The adsorption of CO at low temperature ( 100 K) leads to the formation of H-bonds with OH groups and to the coordination of CO to Lewis acid sites. Owing to this interaction, the n(CO) vibration band of adsorbed CO shifts to higher wavenumbers with respect to the band of pseudoliquid CO (n˜ = 2138 cm1). This shift is characteristic of the nature (Lewis or OH) and strength of the site. In a typical experiment, CO was gradually adsorbed dose per dose until complete saturation of the active sites and the appearance of the band corresponding to pseudoliquid CO. The results obtained are shown in Figure 6.
Preparation and characterization of the multifunctional catalysts Catalysts were prepared by incipient wetness impregnation of silica with oxide and metal precursors. Afterwards, the catalysts were calcined and reduced to obtain metal-containing supported oxide systems. The chemical and textural characteristics of the catalysts prepared are shown in Table 1. The content of the metals was within 0.29–0.33 wt %, whereas the content of oxide components was in the 4.1–4.3 wt % range. Nitrogen adsorption–desorption data point to the high surface area of all the silica-supported catalysts, which is close to parent silica. The average pore sizes determined by BJH analysis is within 90–100 . The TEM images shown in the Supporting Information for the example of the 2 Ag/4 ZrO2/SiO2 catalyst show the uniform distribution of metal particles that are 2–20 mm in size over the oxide surface The acidic properties of the catalysts were studied by temperature-programmed desorption (TPD)-NH3 and FTIR spectroscopy. The TPD-NH3 profiles observed over the samples studied reveal only one peak in the temperature range from 340 to 573 K, which corresponds to weak acid sites. The highest amount of acid sites was observed over the 0.3 Ag/4 Al2O3/SiO2 catalyst (Table 1). Table 1. Catalyst characteristics. Catalyst
Chemical composition metal [wt %] oxide [wt %]
Surface area [m2 g1]
0.3 Ag/4 ZrO2/SiO2 0.3 Cu/4 ZrO2/SiO2 0.3 Ni/4 ZrO2/SiO2 0.3 Ag/4 Nb2O5/SiO2 0.3 Ag/4 TiO2/SiO2 0.3 Ag/4 MgO/SiO2 0.3 Ag/4 Al2O3/SiO2
0.32 0.33 0.30 0.29 0.30 0.31 0.31
255 275 260 270 275 265 270
4.1 4.1 4.2 4.3 4.1 4.2 4.2
[a] Measured by TPD-NH3.
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Figure 6. FTIR spectra of adsorbed CO over silver-containing catalysts.
The adsorption of CO on a pure silica support leads to the appearance of bands at approximately n˜ = 2156 and 2138 cm1 (Figure 6). Whereas the band at n˜ = 2138 cm1 is due to physically adsorbed CO, the band at n˜ = 2156 cm1 can be attributed to vibrations of CO adsorbed on the surfaces of the OH groups.[37] Deposition of oxides on the silica surface results in the appearance of bands in the n˜ = 2170–2190 cm1 range, which correspond to Lewis acid sites. Magnesia-, titania-, and zirconiacontaining catalysts show only Average pore Total amount of one band at approximately n˜ = diameter [] acid sites[a] [mmol g1] 2170 cm1. On the contrary, 95 271 niobia- and alumina-based cata100 274 lysts are characterized by the 90 280 90 256 presence of two types of Lewis 95 198 sites. The observation of two 95 246 types of sites in the case of 90 343 niobia can be accounted for by the different oxidation states of ChemSusChem 2014, 7, 2527 – 2536
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Nb in the sample (i.e., Nb5 + and Nb4 + ). In the case of alumina, the two bands may be due to differently coordinated Al atoms.[36]
Evaluation and optimization of multifunctional catalysts Effect of the type of metal In the first step of the study, the effect of the type of metal responsible for the ethanol dehydrogenation step was investigated. The model catalysts included Cu-, Ag- and Ni-containing zirconia supported on silica. The results on ethanol conversion over these catalysts are summarized in Table 2. Copper- and silver-containing catalysts showed similar conversions and selectivities towards butadiene, whereas the nickel-containing catalyst demonstrated lower selectivity owing to the formation of butenes. To study catalyst deactivation, the yield of butadiene was plotted versus time on stream (TOS) as shown in Figure 7. The Ni-promoted catalyst showed the highest stability with TOS. This behavior of the Ni catalyst could be accounted for by the cracking of coke precursors in the presence Ni. This hypothesis is supported by the lower selectivity to heavy products over this catalyst and higher selectivity to C2C4 olefins (Table 2). Although 0.3 Ni/4 ZrO2/SiO2 demonstrated the highest stability with TOS, ethanol conversion and butadiene yield over this catalyst were the lowest. The Ag- and Cu-containing catalysts showed almost the same conversion of ethanol and selectivity to butadiene. However, 0.3 Ag/4 ZrO2/SiO2 demonstrated higher stability with TOS (Figure 7). Therefore, this catalyst was selected for further studies.
Figure 7. Effect of the type of promoter on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
The catalysts containing titania and magnesia showed rather high selectivities to heavy byproducts (up to 27 mol %). This observation is in line with previous studies[34] that showed that MgO and TiO2 are very active in aldol condensation. We suppose that the crotonaldehyde formed in the first step of the aldol addition undergoes further reaction with acetaldehyde to give C6 aldehydes. The latter undergo reduction and dehydration to give hexatrienes and their derivatives (Scheme 3). The highest selectivity towards butadiene was observed over 0.3 Ag/4 ZrO2/SiO2, which was selected for further optimization. Deactivation of the catalysts was also studied; curves for the yield of butadiene versus time on stream are presented in Figure 8. Although 0.3 Ag/4 ZrO2/SiO2 is not the most stable among all of the catalysts, it showed the highest yield at steady state.
Effect of the type of oxide Effect of Ag and ZrO2 content on ethanol conversion into butadiene over the Ag/ZrO2/SiO2 catalyst
The results obtained over Ag-promoted catalysts containing different oxides are presented in Table 2. The highest conversion of ethanol was observed over alumina-based catalyst. However, the main reaction products over this catalyst were ethylene and ethyl ether; the selectivity to butadiene was only 17 mol %. Such behavior of the alumina-containing catalyst is most probably due to the highest amount of acid sites over this catalyst (Table 1) and the presence of Brønsted acid sites, which are responsible for the ethanol dehydration step. High selectivity to ethanol dehydration products was also observed over niobia-based catalysts, which also exhibits Brønsted acidity.
The results obtained over Ag/ZrO2/SiO2 catalysts with different compositions are presented in Table 3 and Figures 9 and 10. The increase in the Ag content from 0.3 to 1.0 wt % led to a significant increase in the conversion of ethanol from 30 to 48 %; a further increase in the content of Ag to 2 wt % resulted in only a marginal increase in the conversion to 55 %. The selectivity to the products of acetaldehyde conversion increased at the expense of the products of ethanol dehydration. In particular, the contribution of heavy byproducts (mostly hexatrienes) increased as a result of the enhancement in the secondary
Table 2. Ethanol conversion and products distribution over M/MOx/SiO2 catalysts (T = 593 K, WHSV = 0.31 h1, TOS = 5 h). Sample 0.3 Ag/4 ZrO2/SiO2 0.3 Cu/4 ZrO2/SiO2 0.3 Ni/4 ZrO2/SiO2 0.3 Ag/4 Nb2O5/SiO2 0.3 Ag/4 TiO2/SiO2 0.3 Ag/4 MgO/SiO2 0.3 Ag/4 Al2O3/SiO2
Ethanol conversion [%]
butadiene
ethylene
propylene
Selectivity [mol %] butenes ethyl ether
ethyl acetate
butan-1-ol
C6 +
30.0 26.8 10.3 16.2 15.0 19.0 62.5
73.8 73.5 67.9 44.9 39.4 60.6 17.6
2.7 2.6 5.8 16.8 8.4 3.8 27.2
3.0 3.7 3.7 2.0 2.2 1.9 5.0
2.9 3.6 5.7 1.5 2.5 1.9 4.7
1.6 1.6 2.7 1.7 4.2 4.9 0.7
3.0 3.4 2.6 1.1 3.4 3.5 0.7
9.3 8.4 5.3 4.4 26.6 17.9 3.5
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3.7 3.3 6.4 27.8 13.3 5.5 40.6
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Table 3. Ethanol conversion and products distribution over Ag/ZrO2/SiO2 catalysts (T = 593 K, WHSV = 0.31 h1, TOS = 5 h). Sample 0.3 Ag/4 ZrO2/SiO2 1 Ag/4 ZrO2/SiO2 2 Ag/4 ZrO2/SiO2 1 Ag/1 ZrO2/SiO2 1 Ag/10 ZrO2/SiO2 1 Ag/18 ZrO2/SiO2
Ethanol conversion [%]
butadiene
ethylene
propylene
Selectivity [mol %] butenes ethyl ether
ethyl acetate
butan-1-ol
C6 +
30.0 48.0 55.2 18.4 45.6 45.8
73.8 72.1 71.3 67.8 73.0 72.8
2.7 1.5 2.0 2.4 2.1 2.3
3.0 3.0 3.0 2.5 2.9 2.9
2.9 2.9 3.0 2.9 2.6 2.5
1.6 3.0 1.6 3.8 2.9 3.0
3.0 1.7 1.0 2.4 2.5 2.8
9.3 12.7 15.7 14.9 11.7 10.8
Figure 8. Effect of the type of oxide on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
condensation reactions. At the same time, the selectivity towards butadiene slightly decreased from 73 to 71 mol %. Study of catalyst deactivation (Figure 9) suggests that the increase in the Ag content to values higher than 1 wt % leads to an increase in the deactivation rate. Therefore, promoting the reaction with 1 wt % Ag seems to be the most appropriate. The effect of ZrO2 content was studied over the catalyst containing 1 wt % silver (Table 3, Figure 10). The increase in the ZrO2 content from 1 to 4 wt % resulted in a sharp increase in ethanol conversion and selectivity towards butadiene, whereas a further increase in the zirconia content from 4 to 18 wt % did
Figure 9. Effect of the Ag content on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3.7 3.1 2.4 3.3 2.4 3.0
Figure 10. Effect of the ZrO2 content on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
not affect significantly the performance of the catalyst. Investigation of the stability of the activity of the catalysts with TOS revealed that an increase in the ZrO2 content up to 10 wt % led to a decrease in the catalyst deactivation rate (Figure 10). Therefore, the 1 Ag/10 ZrO2/SiO2 catalyst was selected for further studies of the effect of the reaction conditions. Effect of temperature and WHSV on ethanol conversion into butadiene over the 1 Ag/10 ZrO2/SiO2 catalyst The reaction temperature had a significant influence on the activity and selectivity of the catalyst. An increase in the temperature from 553 to 593 K boosted the selectivity to butadiene from 58 to 70 mol % (Figure 11). A further increase in the temperature resulted in a decrease in the selectivity to butadiene, which was associated with the increased contribution of the products of ethanol dehydration. Therefore, for the selective synthesis of butadiene over 1 Ag/10 ZrO2/SiO2 the 590–610 K temperature range is preferred. The effect of WHSV on the process parameters was studied in the 0.04–10 g g1 h1 range (Table 4). An increase in WHSV led to a dramatic decrease in both ethanol conversion and selectivity to butadiene. The highest selectivity of 74 mol % was reached at the lowest WHSV of 0.04 g g1 h1. However, a further decrease in WHSV was useless, as it is accompanied by a decrease in the output of butadiene. A comparison of the yields obtained over 1 Ag/10 ZrO2/SiO2 with the literature data (Table 7 in Ref. [1]) points that this catalyst is among the most efficient. The higher butadiene yields of 87 % reported by OhChemSusChem 2014, 7, 2527 – 2536
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Table 4. Effect of WHSV (T = 593 K, TOS = 5 h). WHSV [g g1 h1]
Ethanol conversion [%]
butadiene
ethylene
propylene
butenes
Selectivity [mol %] diethyl ether ethyl acetate
butan-1-ol
crotonaldehyde
C6 +
0.04 0.08 0.30 1.23 3.5 10.3
88.0 79.1 48.0 29.1 11.3 7.8
73.9 73.6 73.0 66.1 60.4 49.3
2.9 1.2 2.1 1.2 1.6 2.0
3.4 2.9 2.9 1.2 1.0 0.7
6.4 6.1 2.6 4.5 2.8 4.2
1.9 2.0 2.4 2.7 5.2 8.9
0.1 0.3 0.8 4.5 6.6 7.3
0.0 0.1 2.5 1.8 3.7 3.2
10.1 11.7 10.9 12.1 13.0 16.0
Figure 11. Effect of temperature on the conversion of ethanol into butadiene over 1 Ag/10 ZrO2/SiO2 (WHSV = 0.3 h1).
nishi[18] were obtained after 10 min of the reaction and cannot be adequately compared with the other results. Besides ethanol conversion and butadiene yield, the catalyst productivity should be considered, as it is among the most important parameters for industrial applications. The highest catalyst productivity of 0.23 g g1 h1 was achieved at the highest WHSV = 10.3 g g1 h1. However, this value was reached at a rather low selectivity to butadiene. At more acceptable selectivity to butadiene of 66 mol %, the catalyst productivity reached 0.14 g g1 h1, which is close to the highest value reported.[12]
1.3 2.1 2.9 5.9 5.7 8.3
Catalyst design included the selection of active components for the key reaction steps leading to butadiene; these components were merged into multifunctional catalysts and the catalyst functions were adjusted to achieve the highest selectivity. Among the metal-containing components (M = Ag, Cu, Ni) selected for the dehydrogenation of ethanol, a Ag promoter was found to be the most active, selective, and stable with time on stream. Among the metal oxide components (MOx = MgO, ZrO2, Nb2O5, TiO2, Al2O3) chosen for the condensation of acetaldehyde and the MPVO reduction of crotonaldehyde, ZrO2/SiO2 was shown to be the most promising. The optimized catalyst contained 1 wt % silver and 10 wt % zirconia on silica and provided selectivity to butadiene of 74 mol % at an ethanol conversion of 88 % at 593 K.
Experimental Section Catalyst preparation Catalysts were prepared by insipient wetness impregnation of silica gel (supplied by Karpov Chemical Plant) with aqueous solutions of the corresponding nitrates. After impregnation, the samples were dried overnight at ambient temperature and then at 383 K for 3 h. The samples were then heated in a flow of dry air to 873 K with a temperature ramp of 2 K min1 and then calcined at this temperature for 4 h. Reduction of the metal promoters was performed in a catalytic reactor in situ prior to the catalytic run. Catalysts are denoted as yM/zMOx/SiO2, in which y is the content of the metal loaded and z is the content of the oxide.
Conclusions Catalyst characterization The main reaction pathways of ethanol conversion over metalpromoted oxide catalysts were determined on the basis of a kinetic study of a wide range of ethanol conversions. The target reaction route leading to butadiene was demonstrated to include five consecutive reaction steps: first, ethanol dehydrogenation into acetaldehyde; second, aldol condensation of acetaldehyde; third, dehydration of 3-hydroxybutanal; fourth, MPVO reduction of crotonaldehyde with ethanol; and fifth, dehydration of crotyl alcohol into butadiene. The main side reactions were found to involve: first, ethanol dehydration into diethyl ether and ethylene; second, conversion of acetaldehyde into ethyl acetate and further into acetone and propylene; third, cross-condensation of acetaldehyde with crotonaldehyde and other aldehydes into heavy byproducts; and fourth, hydrogenation of crotyl alcohol into butan-1-ol followed by its dehydration and isomerization into but-1-ene and but-2-ene. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). Sorption–desorption isotherms of nitrogen were measured at 77 K by using an automated porosimeter (Micrometrics ASAP 2000). The acidic properties were studied by temperature-programmed desorption of ammonia (TPD-NH3) and low-temperature FTIR spectroscopy of adsorbed CO. TPD experiments were performed with a USGA-101 (UNISIT) in the temperature range from 293 to 1053 K in a flow of dry He (30 mL min1). The heating rate was 8 K min1. In a typical experiment, the samples were calcined at 673 K in a flow of dry He, saturated with ammonia at 323 K, and then the physisorbed NH3 was removed with a flow of He at 373 K. IR spectra were recorded with a Nicolet Protg 380 FTIR spectrometer at an optical resolution of 4 cm1. Prior to the measurements, the catalysts were pressed in self-supporting discs and activated in the IR cell attached to a vacuum line at 723 K for 4 h. AdChemSusChem 2014, 7, 2527 – 2536
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sorption of CO was performed in a low-temperature cell at 100 K. The pressure was measured by using a Barocell gauge.
Catalyst evaluation Catalytic experiments were performed in a flow-type fixed-bed reactor under atmospheric pressure. In a typical experiment, the catalyst (4 g, fraction 0.5–1 mm) was packed into a quartz tubular reactor and purged with nitrogen at 873 K for 0.5 h. Ethanol (95 wt %) was used as the feed. The reaction mixture was fed by using a Razel syringe pump. Helium was used as the carrier gas (molar ratio EtOH/He = 1). The WHSV was varied from 0.04 to 10.3 h1, and the reaction temperature was within 550–650 K. Gaseous products were analyzed on line with a Crystal 2000M gas chromatograph by using a 50 m SE-30 column. Liquid products were separated and analyzed by using 50 m SE-30 and 2 m Porapak Q columns. Methane was used as an external standard for the gaseous products. The conversion of ethanol [Eq. (1)], selectivity [Eq. (2)], and yields [Eq. (3)] were calculated as follows: Ethanol conversion ¼
mreactedðEtOHÞ 100 % mfedðEtOHÞ
ð1Þ
m
Selectivity ½mol % ¼
k Fj Mrjj 100 %
ð2Þ
mreactedðEtOHÞ 46
Yield ½mol % ¼ ethanol conversion
selectivity 100
ð3Þ
in which mreacted(EtOH) is mass of reacted ethanol, mfed(EtOH) is mass of fed ethanol, mj is the mass of the j product in the reaction mixture, Mrj is the molecular weight of product j, and k Fj is the number of ethanol molecules required for product formation. Catalyst productivity was determined as the grams of butadiene obtained per gram of catalyst per hour and calculated as follows [Eq. (4)]: Productivity ¼
WHSVEtOH yieldbutadiene 54 46 2 100
ð4Þ
Acknowledgements V.L.S. gratefully acknowledges Haldor Tøpsoe A/S for a PhD fellowship. Keywords: domino reactions · kinetics · metal promoters · oxide catalysts · reaction mechanisms [1] C. Angelici, B. M. Weckhuysen, P. Bruijnincx, ChemSusChem 2013, 6, 1595 – 1614. [2] J. A. Posada, A. D. Patel, A. Roes, K. Blok, A. C. Faaij, M. K. Patel, Bioresour. Technol. 2013, 135, 490 – 499. [3] A. D. Patel, K. Meesters, H. den Uil, E. de Jong, K. Bloka, M. K. Patel, Energy Environ. Sci. 2012, 5, 8430 – 8444. [4] a) S. V. Lebedev, British Patent 331402, 1929; b) S. V. Lebedev, British Patent 331482, 1930; c) S. V. Lebedev, French Patent 665917, 1929; d) S. V. Lebedev, Russ. J. Gen. Chem. 1933, 3, 698 – 708; e) S. V. Lebedev, Chem. Ztg. 1936, 60, 313 – 316. [5] W. J. Toussaint, J. T. Dunn, U. S. Patent 2,357,855, 1944. [6] W. Toussaint, J. Dunn, D. Jackson, Ind. Eng. Chem. 1947, 39, 120 – 125. [7] B. B. Corson, H. E. Jones, C. E. Welling, J. A. Hinckley, E. E. Stahly, Ind. Eng. Chem. 1950, 42, 359 – 373.
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[8] J. Ostromislenskiy, J. Russ. Phys. Chem. Soc. 1915, 47, 1472 – 1506. [9] M. Len, E. Diaz, S. Ordonez, Catal. Today 2011, 164, 436 – 442. [10] M. Jones, C. Keir, C. Di Iulio, R. Robertson, C. Williams, D. Apperley, Catal. Sci. Technol. 2011, 1, 267 – 272. [11] a) V. Ordomsky, V. Sushkevich, I. Ivanova, WO2012/156340, 2012; b) V. Ordomsky, V. Sushkevich, I. Ivanova, RU2440962, 2010. [12] E. V. Makshina, W. Janssens, B. F. Sels, P. A. Jacobs, Catal. Today 2012, 198, 338 – 344. [13] G. O. Ezinkwo, V. F. Tretjakov, R. M. Talyshinky, A. M. Ilolov, T. A. Mutombo, Catal. Commun. 2014, 43, 207 – 212. [14] G. Natta, R. Rigamonti, Chim. Ind. 1947, 29, 195 – 201. [15] B. B. Corson, E. E. Stahly, H. E. Jones, H. D. Bishop, Ind. Eng. Chem. 1949, 41, 1012 – 1017. [16] H. Niiyama, S. Morii, E. Echigoya, Bull. Chem. Soc. Jpn. 1972, 45, 655 – 659. [17] Y. Kitayama, A. Micheshita, J. Chem. Soc. Chem. Commun. 1981, 401 – 402. [18] R. Ohnishi, T. Akimoto, K. Tanabe, J. Chem. Soc. Chem. Commun. 1985, 1613 – 1614. [19] S. Kvisle, A. Aguero, R. P. A. Sneeden, Appl. Catal. 1988, 43, 117 – 131. [20] Y. Kitayama, M. Satoh, T. Kodama, Catal. Lett. 1996, 36, 95 – 97. [21] W. M. Quattlebaum, W. J. Toussaint, J. T. Dunn, J. Am. Chem. Soc. 1947, 69, 593 – 594. [22] V. Gruver, A. Sun, J. J. Fripiat, Catal. Lett. 1995, 34, 359 – 364. [23] J. B. D. Delacaillerie, V. Gruver, J. J. Fripiat, J. Catal. 1995, 151, 420 – 430. [24] Y. A. Gorin, F. N. Kogan, J. Syn. Rubber 1936, 11 – 12, 6 – 12. [25] Y. A. Gorin , Russ. J. Gen. Chem. 1946, 16, 1089 – 1092. [26] Y. A. Gorin, M. I. Danilina, Russ. J. Gen. Chem. 1948, 18, 1069 – 1075. [27] a) S. K. Bhattacharyya, S. K. Sanyal, J. Catal. 1967, 7, 152 – 158; b) S. K. Bhattacharyya, N. D. Ganguly, J. Appl. Chem. 1962, 12, 97 – 104; c) S. K. Bhattacharyya, N. D. Ganguly, J. Appl. Chem. 1962, 12, 105 – 110; d) S. K. Bhattacharyya, B. N. Avasthi, Ind. Eng. Chem. Process Des. Dev. 1963, 2, 45 – 51. [28] A. N. Ko, B. W. Wojciechowski, Prog. React. Kinet. 1983, 12, 201 – 262. [29] K. Inui, T. Kurabayashi, S. Sato, Appl. Catal. A 2002, 237, 53 – 61. [30] G. A. H. Mekhemer, S. A. Halawy, M. A. Mohamed, M. I. Zaki, J. Catal. 2005, 230, 109 – 122. [31] R. S. Rao, A. B. Walters, M. A. Vannice, J. Phys. Chem. B 2005, 109, 2086 – 2092. [32] a) J. Keuler, L. Lorencen, S. Miachon, Appl. Catal. A 2001, 218, 171 – 180; b) N. Doca, E. Segal, React. Kinet. Catal. Lett. 1985, 28, 123 – 129; c) M. Domok, M. Toth, J. Rasko, A. Erdohelyi, Appl. Catal. B 2007, 69, 262 – 272; d) H. Idriss, Platinum Met. Rev. 2004, 48, 105 – 115; e) V. Sushkevich, I. Ivanova, E. Taarning, ChemCatChem 2013, 5, 2367 – 2373. [33] a) H. Hattori, Appl. Catal. 2001, 222, 247 – 259; b) V. Dez, C. Apestegua, J. Di Cosimo, J. Catal. 2003, 215, 220 – 233; c) G. A. M. Hussein, N. Sheppard, M. I. Zaki, R. B. Fahim, J. Chem. Soc. Faraday Trans. 1991, 87, 2661 – 2668. [34] a) W. Ji, Y. Chen, H. Kung, Appl. Catal. A 1997, 161, 93 – 104; b) K. Tanabe, G. Zhang, H. Hattori, Appl. Catal. 1989, 48, 63 – 69; c) V. Ordomsky, V. Sushkevich, I. Ivanova, J. Mol. Catal. A 2010, 333, 85 – 93. [35] a) M. Glin´ski, Appl. Catal. A 2008, 349, 133 – 139; b) P. S. Kumbhar, J. Sanchez-Valente, F. Figueras, Chem. Commun. 1998, 1091 – 1092; c) Y. Z. Zhu, S. H. Liu, S. Jaenicke, G. Chuah, Catal. Today 2004, 97, 249 – 255; d) J. C. van der Waal, K. Tan, H. van Bekkum, Catal. Lett. 1996, 41, 63 – 67; e) Y. Z. Zhu, S. Jaenicke, G. K. Chuah, J. Catal. 2003, 218, 396 – 404; f) V. Sushkevich, I. Ivanova, S. Tolborg, E. Taarning, J. Catal. 2014, 316, 121 – 129. [36] a) H. Kawakami, S. Yoshida, J. Chem. Soc. Faraday Trans. 1985, 81, 1117 – 1127; b) H. Kawakami, S. Yoshida, J. Chem. Soc. Faraday Trans. 1985, 81, 1129 – 1137. [37] a) M. Bensitel, O. Saur, J. C. Lavalley, Mater. Chem. Phys. 1987, 17, 249 – 258; b) M. Maache, A. Janin, J. C. Lavalley, J. F. Joly, E. Benazzi, Zeolites 1993, 13, 419 – 426; c) A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M. Padovan, C. Otero Arean, J. Chem. Soc. Faraday Trans. 1992, 88, 2959 – 2969.
Received: April 23, 2014 Revised: June 9, 2014 Published online on August 14, 2014
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Design of a Metal-Promoted Oxide Catalyst for the Selective Synthesis of Butadiene from Ethanol Vitaly L. Sushkevich,[a, b] Irina I. Ivanova,*[a, b] Vitaly V. Ordomsky,[a, b] and Esben Taarning[c] The synthesis of buta-1,3-diene from ethanol has been studied over metal-containing (M = Ag, Cu, Ni) oxide catalysts (MOx = MgO, ZrO2, Nb2O5, TiO2, Al2O3) supported on silica. Kinetic study of a wide range of ethanol conversions (2–90 %) allowed the main reaction pathways leading to butadiene and byproducts to be determined. The key reaction steps of butadiene synthesis were found to involve ethanol dehydrogenation, acetaldehyde condensation, and the reduction of crotonalde-
hyde with ethanol into crotyl alcohol. Catalyst design included the selection of active components for each key reaction step and merging of these components into multifunctional catalysts and adjusting the catalyst functions to achieve the highest selectivity. The best catalytic performance was achieved over the Ag/ZrO2/SiO2 catalyst, which showed the highest selectivity towards butadiene (74 mol %).
Introduction Buta-1,3-diene is an important monomer in the production of polymers such as polybutadiene, styrene–butadiene rubber, styrene–butadiene latex, acrylonitrile–butadiene–styrene polymer, nitrile rubber, and some others. Currently, approximately 95 % of butadiene is produced by isolation from naphtha steam cracker fractions generated during ethylene production. However, trends in the development of the ethylene industry are such that ethylene producers are switching to more efficient ethylene production technologies. This leads to the lack of butadiene on the market and, therefore, requires the development of alternative technologies leading to butadiene. One of the most perspective sources for butadiene production is ethanol, which can be produced from carbohydrate biomass. The recent assessment of the economic, environmental, health, safety, and operation aspects of naphtha-based and bioethanol-based routes[1–3] revealed that the bioethanol to butadiene process could be a promising substitute for the dominant naphtha-based method and can contribute to a decrease in the use of fossil-fuel reserves. The one-step catalytic process to produce butadiene from ethanol was developed by Lebedev[4] and was commercialized in Russia in the beginning of the 1920s. Later on, the Carbide [a] V. L. Sushkevich, Prof. I. I. Ivanova, V. V. Ordomsky Department of Chemistry Lomonosov Moscow State University Leninskye Gory 1, bld. 3, 119991 Moscow (Russia) Fax: (+ 7) 4959393570 E-mail: [email protected] [b] V. L. Sushkevich, Prof. I. I. Ivanova, V. V. Ordomsky “UNISIT” LLC Leninskye Gory 1, bld. 75 V, 119991 Moscow (Russia) [c] Dr. E. Taarning Haldor Topsøe A/S Nymoellevej 55 2800 Kgs. Lyngby (Denmark) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402346.
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and Carbon Chemicals Corporation in the USA commercialized a two-step process[5–7] based on the Ostromislenskiy method.[8] The latter technology included ethanol dehydrogenation into acetaldehyde and conversion of the mixture of ethanol and acetaldehyde into butadiene. However, after the 1960s both processes became economically unfavorable and were stopped owing to the development of petrochemical routes of butadiene production. Nowadays, interest in Lebedev’s process has been renewed. However, the existing technologies require substantial improvement; in particular, the development of novel catalysts with high activity and selectivity is of prime importance.[1] These circumstances have generated a new wave of research activity in this direction in recent years.[1–3, 9–13] Analysis of the literature on the catalytic synthesis of butadiene from ethanol suggests that all the catalytic systems proposed so far have been based on basic or acidic oxides or their mixtures. Basic oxide catalysts were first proposed by Lebedev. According to different literature sources, Lebedev’s catalyst involved magnesia[14] or zinc oxide[13, 15] as the main active components. Magnesia–silica catalysts were thoroughly studied later on by many research groups.[9, 12, 16–20] Niiyama et al.[16] investigated the effect of the MgO content and revealed that an increase in the catalyst activity was observed with an increase in the magnesia content up to 85 %. They also pointed to the importance of the preparation method. The effect of the method of preparation was further studied by Kvisle et al.,[19] who demonstrated that wet kneading of the two oxides resulted in a much more active catalyst than that obtained by mechanical mixing of the components. Kitayama et al.[17] were the first to use sepiolite (a fibrous magnesia–silicate clay mineral) for the conversion of ethanol into butadiene. Many efforts were devoted to dope magnesia–silica catalysts with different transition-metal oxides, such as cromia,[14] manganese,[17] nickel oxide,[20] iron, cobalt, copper, and zinc ChemSusChem 2014, 7, 2527 – 2536
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CHEMSUSCHEM FULL PAPERS oxides.[12] These promoters were reported to improve the yield and the stability of the catalytic system. The best result obtained so far for magnesia–silica-based systems was reported by Ohnishi et al.[18] over a MgO/SiO2 catalyst (molar ratio = 1:1) promoted with 0.1 % Na2O. This catalyst was reported to show 100 % conversion of ethanol with ultimate selectivity to butadiene of 87 % at 623 K; however, the lack of experimental details did not allow for reliable comparison with the data of other researchers. Acidic catalytic systems were first proposed by Toussaint and co-workers[5, 6, 21] for the second step of the two-step process developed by the Carbide and Carbon Chemicals Corporation. These catalysts involved Lewis acid oxides supported on silica. In particular, supported tantalum, zirconium, niobium, thorium, uranium, and titanium oxides were shown to be active catalysts for the conversion of ethanol and acetaldehyde into butadiene. Among these catalytic systems, 2 %Ta2O5/SiO2 proved to be the best and yielded butadiene with 60 % selectivity and 30–35 % yield per pass. The one-step process was also studied over acidic catalysts. Doping of tantala–silica catalysts with metal oxides capable of effecting the dehydrogenation of ethanol into acetaldehyde proved to be efficient for this process. In particular, doping with copper and cadmium oxides as well as chromium and magnesium oxides gave active catalysts.[15] A recent investigation by Jones et al.[10] revealed that application of various binary and ternary mixed oxide systems involving zinc, zirconium, cerium, manganese, copper, and cobalt oxides supported on silica may lead to a further improvement in the selectivity and yield of butadiene. The most promising catalyst reported by these authors was a Cu/Zr/Zn oxide system supported on silica. The highest selectivity observed was 67 %. The third type of catalytic system that was used for the synthesis of butadiene from ethanol involved amphoteric oxides or mixed acid and base oxide systems. According to some literature sources,[13, 15] such systems were first proposed by Lebedev and involved zinc oxide supported on alumina. Afterwards, they were extensively studied by Bhattacharyya et al.[27] Al2O3/ MgO (60:20), Al2O3/Cr2O3 (60:40), Al2O3/CaO (60:20), and Al2O3/ ZnO (60:40) were shown to be good catalysts; however, the best result (55.8 % yield) was reached over Al2O3/ZnO (60:40). Furthermore, a considerable increase in the yield up to 72.8 % was reported over this system under fluidized bed conditions. Fripiat et al.[22, 23] used aluminated sepiolite in the synthesis of butadiene. The insertion of aluminum led to an increase in the Lewis acidity of basic sepiolite and resulted in an improvement in its activity and selectivity, although the reported yield was lower than that on mixed oxides. In our recent patent,[11] we demonstrated that the activity and selectivity of various oxide catalysts including MgO, TiO2 MgO, ZrO2, Nb2O5, and Ta2O5 could be improved significantly by doping the oxides with small amounts of metals such as Cu, Ag, Au, and some others. The preparation of such catalysts involved impregnation of oxide systems with metal precursor salts followed by their calcination in air and reduction under a hydrogen atmosphere. This discovery allowed the conversion of the oxide catalysts to be increased by a factor of 5 and also 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org allowed the selectivity to butadiene to be higher than 80 mol %. The effect was confirmed in a further study by Makshina et al.[12] over MgO/SiO2 catalysts doped with silver. Although significant achievements have been made towards catalyst improvement over the last few years, fundamental understanding of the process, which is necessary for rational catalyst design, is still lacking. As stressed in the recent review by Weckhuysen et al.,[1] the following key questions regarding optimal catalyst design remain to be answered: 1) Reaction mechanism over different catalysts and the ratedetermining step 2) Reaction pathways leading to byproducts 3) Structure–activity relationship, optimal catalytic functions (acid/base/redox), and the balance between them required to achieve the highest selectivity to butadiene 4) Catalyst stability and deactivation pathways In this contribution, we aim to answer at least some of these questions for the metal-containing oxide catalysts (M/MOx/ SiO2) discovered in our previous study,[11] in particular: 1) To clarify the main reaction pathways leading to butadiene and byproducts over metal-containing oxide catalysts 2) To select active and selective catalyst components for each reaction step 3) To merge these components into one multifunctional catalyst and to balance the functions to achieve the highest selectivity 4) To adjust the reaction conditions
Results and Discussion Reaction pathways leading to target and side products At present, two main reaction mechanisms for the conversion of ethanol into butadiene have been discussed in the literature.[1, 9–12, 16, 21, 24–27] The first one is based on aldol condensation of acetaldehyde and involves ethanol dehydrogenation into acetaldehyde, its condensation, dehydration of 3-hydroxybutanal, reduction of crotonaldehyde with ethanol, and dehydration of crotyl alcohol into butadiene (Scheme 1). This mechanistic pathway was first proposed by Toussaint and co-workers[21] and was further adopted by many research groups.[9–12, 16, 21, 24–27] However, different suggestions have been made on the rate-limiting step of this pathway over different catalysts. Thus, ethanol dehydrogenation into acetaldehyde
Scheme 1. Butadiene formation through the aldol condensation pathway.[1]
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was reported to be rate limiting over a silica–magnesia catalyst.[16] On the contrary, aldol condensation was found to be rate limiting over supported tantalum, zirconium, aluminum, and titanium oxide catalysts and over Al2O3/ZnO mixed oxides.[10, 21, 27] Finally, reduction of crotonaldehyde was suggested to be rate limiting over Lebedev’s catalyst.[24–26] An alternative mechanistic pathway is based on Prins-like reaction between acetaldehyde and ethylene (both derived from ethanol), followed by dehydration of but-3-en-2-ol to yield butadiene (Scheme 2). This reaction pathway was proposed by Natta and Rigamonti[14] and was further supported by the investigations of Fripiat and co-workers[22] over aluminated sepiolite catalysts. Figure 1. Yields of the main reaction products and intermediates versus ethanol conversion (T = 593 K).
Scheme 2. Butadiene formation through the Prins condensation pathway.[1]
Recent thermodynamic calculations[1] indicated that both pathways can operate at reaction temperature, which is considered optimal for butadiene synthesis (673–698 K). However, the acetaldehyde condensation route is slightly more favorable. Notably, the main reaction pathway leading to butadiene is usually accompanied by a number of side reactions that are poorly understood. The nature and the amount of byproducts depend on the type of catalyst and are generally not discussed. To obtain deeper insight into the reaction pathways leading to butadiene and byproducts over M/MOx/SiO2 catalysts, a kinetic study was performed over a wide range of weight hourly space velocity (WHSV) values with the model catalyst containing 1 wt % Ag and 4 wt % zirconia supported on silica (1 Ag/ 4 ZrO2/SiO2). Ethanol conversion varied from 2 to 90 %. The yields of the various products are plotted versus conversion of ethanol in Figures 1–4. The kinetic analysis of these curves is made on the basis of the models calculated in Ref. [28] for primary and secondary and stable and unstable reaction products. The kinetic curves for the main group of products, including acetaldehyde, crotonaldehyde, butadiene, and heavier products (mostly hexatrienes), are shown in Figure 1. Acetaldehyde appears at very low conversions and corresponds to primary reaction products. The kinetic curve for acetaldehyde shows a maximum; this suggests that it is involved in secondary reactions that most probably include self- and cross-condensation reactions leading to 3-hydroxybutanal and heavier C6 + products. 3-Hydroxybutanal was not detected in the reaction products, probably as a result of its fast dehydration into crotonaldehyde, observed as a secondary unstable product (Figure 1). 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Notably, broadening and the asymmetry of the maximum on the kinetic curve of acetaldehyde at high conversions can be indicative of the second reaction pathway leading to acetaldehyde. Such a secondary process may involve oxidation of ethanol with crotonaldehyde through a Meerwein–Ponndorf– Verley–Oppenauer (MPVO) reaction, as shown in Scheme 1. This observation is in line with other findings[1, 10, 21, 27] that imply that crotonaldehyde reduction with ethanol is a more favorable route than reduction with hydrogen. Observation of acetaldehyde and crotonaldehyde as intermediate products and indication of the MPVO reduction of crotonaldehyde suggests that butadiene is most probably formed over M/MOx/ SiO2 catalysts through the aldol condensation pathway shown in Scheme 1. The major side products of this pathway are hexatrienes (C6 +), which show the kinetic curve of stable secondary reaction products (Figure 1). The formation of these products can be accounted for by the self- and cross-aldol condensation of crotonaldehyde and acetaldehyde, followed by the reduction and dehydration of the aldehydes formed (Scheme 3).
Scheme 3. Formation of heavy byproducts.
Another pathway of ethanol conversion involves ethanol dehydration into diethyl ether and ethylene. Diethyl ether shows the kinetic curve corresponding to a primary unstable product (Figure 2), whereas the curve of ethylene is typical for stable secondary products. These observations suggest that ethylene is formed mostly via diethyl ether; however, the direct dehydration of ethanol into ethylene cannot be completely excluded (Scheme 4). The kinetic curves point that ethylene is not involved in further transformations and suggest that it could hardly be an intermediate in the synthesis of butadiene ChemSusChem 2014, 7, 2527 – 2536
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Scheme 5. Reaction pathways leading to butan-1-ol.
Scheme 6. Reaction pathways leading to butenes. Figure 2. Yields of the ethanol dehydration products versus ethanol conversion (T = 593 K).
dehydration of butan-1-ol and subsequent isomerization (Scheme 6): Finally, the last group of byproducts involves ethyl acetate, acetic acid, acetone, and propene. Ethyl acetate shows the curve of the unstable secondary product, which passes through the maximum at 60 % ethanol conversion (Figure 4). It can be formed from acetaldehyde by Tischenko reaction (Scheme 7).[29]
Scheme 4. Formation of ethanol dehydration products.
through Prins condensation. This result confirms the conclusion that aldol condensation is the main reaction pathway leading to butadiene over M/MOx/SiO2 catalysts. C4 reaction byproducts, including butenes and butan-1-ol, represent a significant group of byproducts over M/MOx/SiO2 catalysts. The kinetic curve of butan-1-ol shows a maximum at 30 % ethanol conversion (Figure 3) and corresponds to the unstable secondary product that can be formed by the reduction of the C=C bond in crotyl alcohol and/or crotonaldehyde, as shown in Scheme 5. But-1-ene and but-2-ene can be attributed to stable secondary products; the yield of these products sharply increases with conversion (Figure 3). Butenes can hardly be formed by butadiene hydrogenation and are most probably formed by
Figure 4. Yields of ethyl acetate and propylene versus ethanol conversion (T = 593 K).
Scheme 7. Reaction pathway leading to ethyl acetate.
Further transformations of ethyl acetate may include hydrolysis into ethanol and acetic acid, decarboxylation of acetic acid into acetone,[30] and reduction and dehydration of the latter into propylene,[31] which is observed as a stable secondary product of the reaction (Scheme 8). Reaction network Figure 3. Yields of butenes and butan-1-ol versus conversion of ethanol (T = 593 K).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The reaction network presented in Figure 5 further rationalizes all of the above observations. The main reaction pathway leadChemSusChem 2014, 7, 2527 – 2536
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www.chemsuschem.org are also found to be stable products over M/MOx/SiO2 catalysts. Selection of catalyst active components
Scheme 8. Reaction pathway leading to propylene.
Figure 5. Reaction network.
ing to butadiene involves five reaction steps: first, ethanol dehydrogenation into acetaldehyde; second, aldol condensation of acetaldehyde; third, dehydration of 3-hydroxybutanal; fourth, MPVO reduction of crotonaldehyde with ethanol; and fifth, dehydration of crotyl alcohol into butadiene. The rate-limiting step over M/MOx/SiO2 catalysts is most probably aldol condensation, as acetaldehyde is the major intermediate product accumulating over the catalysts during the reaction (Figure 1). The most rapid reaction steps involve dehydration of 3-hydroxybutanal and crotyl alcohol. Only trace amounts of these products were detected during the reaction. Besides dehydrogenation, ethanol undergoes dehydration to give diethyl ether and ethylene. Ethylene is found to be a stable product over M/MOx/SiO2 catalysts. It is not involved in further reactions, such as Prins condensation with acetaldehyde. On the contrary, acetaldehyde is very reactive and initiates two side reactions along with the target reaction pathway leading to butadiene. A Tischenko reaction leads to ethyl acetate, which is further converted into acetone and propylene. Cross-condensation of acetaldehyde with crotonaldehyde and other aldehydes results in heavier products, such as hexatrienes. Finally, the last side reaction pathway involves hydrogenation of crotyl alcohol to give butan-1-ol and its further dehydration and isomerization into but-1-ene and but-2-ene, which 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The results of the kinetic study over the M/MOx/SiO2 catalysts suggest that among the five reaction steps that constitute the target reaction pathway, three steps involving ethanol dehydrogenation, acetaldehyde condensation, and MPVO reduction of crotonaldehyde can be considered as key reaction steps leading to butadiene. Dehydration of 3-hydroxybutanal and crotyl alcohol is shown to proceed very easily, even on the weak acid sites of silica support. This conclusion is supported by previous studies.[10] Therefore, the selection of the active components of the catalyst was focused on the three abovementioned steps; silica was chosen as a support owing to its high surface area and weak acidity, which enable it catalyze the dehydration steps. Analysis of the literature data on the dehydrogenation of alcohols revealed two main groups of catalytic systems used in these reactions. The first one involves metal-containing catalysts, including noble metals and metals from group Ib.[32] The second one is composed of basic or transition-metal oxides, such as MgO, CeO2, and Cr2O3, or their mixtures.[33] Oxidebased catalysts usually require higher temperatures than metal-containing catalysts for the dehydrogenation of alcohols and give lower yields of acetaldehyde under the conditions typical for butadiene synthesis. Therefore, supported metal catalysts were found to be a better choice of multifunctional catalyst for butadiene synthesis. Among the metal catalysts studied in the dehydrogenation of ethanol, noble metals were found to be very active but not selective; along with dehydrogenation, ethanol underwent steam reforming into CO and CH4.[32d] On the contrary, copper and nickel catalysts were shown to be active and selective in the dehydrogenation of ethanol.[32b] Even higher activity (turnover frequency = 3.4 s1) and selectivity to acetaldehyde (up to 94 %) were obtained in our recent study over silver supported on silica.[32e] It has been demonstrated that an efficient metal catalyst for the dehydrogenation of ethanol should have a high density of SiOH groups on the silica support, a high metal dispersion, and should be characterized by close proximity of SiOH and the metal sites. On the basis of these data, silver, copper, and nickel were selected as metal components for the multifunctional catalyst. Aldolization of acetaldehyde has been reported over different solid catalysts including oxide and mixed oxide systems based on ZrO2, Ta2O5, Nb2O5, TiO2, CeO2, Al2O3, MgO, zeolites, aluminophosphates, hydrotalcite-like materials, and so on.[34] Both acidic and basic systems were shown to be active catalysts for aldol condensation. Our recent studies[34c] performed by using ZrO2/SiO2 and MgO/SiO2 catalysts suggested that strong basic sites existing over MgO/SiO2 are more reactive in condensation reactions than the acidic sites of both ZrO2/SiO2 and MgO/SiO2. However, strong basic sites are quickly deactivated. Therefore, the reaction pathway involving Lewis acid sites most probably governs the conversion of acetaldehyde over both MgO/SiO2 and ZrO2/SiO2 under steady-state condiChemSusChem 2014, 7, 2527 – 2536
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tions. These results determined the choice of oxide catalysts for this study. In particular, MgO, ZrO2, Ta2O5, Nb2O5, TiO2, and Al2O3 supported on silica were selected. Traditional homogeneous catalysts used in MPVO reactions are aluminum and titanium alkoxides. The solid catalysts studied so far in the MPVO reaction include metal oxides,[35a] hydrotalcites,[35b,c] zeolites,[35d] and immobilized metal alkoxides.[35e] Zirconia- and titania-based catalytic systems such as hydrous zirconia, ZrO2 or TiO2 supported on different oxides, and ZrBEA or TiBEA zeolites were found to be the most active and selective for the MPVO reduction of different carbonyl compounds. In our previous work,[35f] we demonstrated that crotonaldehyde can be reduced with ethanol into crotyl alcohol over ZrO2/SiO2 and ZrBEA catalysts with selectivity up to 80 %. The similarity of the active catalysts for the MPVO reduction and aldol condensation reactions simplifies the selection of the active components for the multifunctional catalyst. On the basis of all these considerations, the following choices were made: metal components: Ag, Cu, and Ni; oxide components: MgO, ZrO2, Nb2O5, TiO2, and Al2O3, support: SiO2.
The nature of the acid sites was investigated by FTIR spectroscopy of adsorbed CO, a well-established probe molecule for the characterization of the acidity of various oxide catalysts.[36] The adsorption of CO at low temperature ( 100 K) leads to the formation of H-bonds with OH groups and to the coordination of CO to Lewis acid sites. Owing to this interaction, the n(CO) vibration band of adsorbed CO shifts to higher wavenumbers with respect to the band of pseudoliquid CO (n˜ = 2138 cm1). This shift is characteristic of the nature (Lewis or OH) and strength of the site. In a typical experiment, CO was gradually adsorbed dose per dose until complete saturation of the active sites and the appearance of the band corresponding to pseudoliquid CO. The results obtained are shown in Figure 6.
Preparation and characterization of the multifunctional catalysts Catalysts were prepared by incipient wetness impregnation of silica with oxide and metal precursors. Afterwards, the catalysts were calcined and reduced to obtain metal-containing supported oxide systems. The chemical and textural characteristics of the catalysts prepared are shown in Table 1. The content of the metals was within 0.29–0.33 wt %, whereas the content of oxide components was in the 4.1–4.3 wt % range. Nitrogen adsorption–desorption data point to the high surface area of all the silica-supported catalysts, which is close to parent silica. The average pore sizes determined by BJH analysis is within 90–100 . The TEM images shown in the Supporting Information for the example of the 2 Ag/4 ZrO2/SiO2 catalyst show the uniform distribution of metal particles that are 2–20 mm in size over the oxide surface The acidic properties of the catalysts were studied by temperature-programmed desorption (TPD)-NH3 and FTIR spectroscopy. The TPD-NH3 profiles observed over the samples studied reveal only one peak in the temperature range from 340 to 573 K, which corresponds to weak acid sites. The highest amount of acid sites was observed over the 0.3 Ag/4 Al2O3/SiO2 catalyst (Table 1). Table 1. Catalyst characteristics. Catalyst
Chemical composition metal [wt %] oxide [wt %]
Surface area [m2 g1]
0.3 Ag/4 ZrO2/SiO2 0.3 Cu/4 ZrO2/SiO2 0.3 Ni/4 ZrO2/SiO2 0.3 Ag/4 Nb2O5/SiO2 0.3 Ag/4 TiO2/SiO2 0.3 Ag/4 MgO/SiO2 0.3 Ag/4 Al2O3/SiO2
0.32 0.33 0.30 0.29 0.30 0.31 0.31
255 275 260 270 275 265 270
4.1 4.1 4.2 4.3 4.1 4.2 4.2
[a] Measured by TPD-NH3.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 6. FTIR spectra of adsorbed CO over silver-containing catalysts.
The adsorption of CO on a pure silica support leads to the appearance of bands at approximately n˜ = 2156 and 2138 cm1 (Figure 6). Whereas the band at n˜ = 2138 cm1 is due to physically adsorbed CO, the band at n˜ = 2156 cm1 can be attributed to vibrations of CO adsorbed on the surfaces of the OH groups.[37] Deposition of oxides on the silica surface results in the appearance of bands in the n˜ = 2170–2190 cm1 range, which correspond to Lewis acid sites. Magnesia-, titania-, and zirconiacontaining catalysts show only Average pore Total amount of one band at approximately n˜ = diameter [] acid sites[a] [mmol g1] 2170 cm1. On the contrary, 95 271 niobia- and alumina-based cata100 274 lysts are characterized by the 90 280 90 256 presence of two types of Lewis 95 198 sites. The observation of two 95 246 types of sites in the case of 90 343 niobia can be accounted for by the different oxidation states of ChemSusChem 2014, 7, 2527 – 2536
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Nb in the sample (i.e., Nb5 + and Nb4 + ). In the case of alumina, the two bands may be due to differently coordinated Al atoms.[36]
Evaluation and optimization of multifunctional catalysts Effect of the type of metal In the first step of the study, the effect of the type of metal responsible for the ethanol dehydrogenation step was investigated. The model catalysts included Cu-, Ag- and Ni-containing zirconia supported on silica. The results on ethanol conversion over these catalysts are summarized in Table 2. Copper- and silver-containing catalysts showed similar conversions and selectivities towards butadiene, whereas the nickel-containing catalyst demonstrated lower selectivity owing to the formation of butenes. To study catalyst deactivation, the yield of butadiene was plotted versus time on stream (TOS) as shown in Figure 7. The Ni-promoted catalyst showed the highest stability with TOS. This behavior of the Ni catalyst could be accounted for by the cracking of coke precursors in the presence Ni. This hypothesis is supported by the lower selectivity to heavy products over this catalyst and higher selectivity to C2C4 olefins (Table 2). Although 0.3 Ni/4 ZrO2/SiO2 demonstrated the highest stability with TOS, ethanol conversion and butadiene yield over this catalyst were the lowest. The Ag- and Cu-containing catalysts showed almost the same conversion of ethanol and selectivity to butadiene. However, 0.3 Ag/4 ZrO2/SiO2 demonstrated higher stability with TOS (Figure 7). Therefore, this catalyst was selected for further studies.
Figure 7. Effect of the type of promoter on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
The catalysts containing titania and magnesia showed rather high selectivities to heavy byproducts (up to 27 mol %). This observation is in line with previous studies[34] that showed that MgO and TiO2 are very active in aldol condensation. We suppose that the crotonaldehyde formed in the first step of the aldol addition undergoes further reaction with acetaldehyde to give C6 aldehydes. The latter undergo reduction and dehydration to give hexatrienes and their derivatives (Scheme 3). The highest selectivity towards butadiene was observed over 0.3 Ag/4 ZrO2/SiO2, which was selected for further optimization. Deactivation of the catalysts was also studied; curves for the yield of butadiene versus time on stream are presented in Figure 8. Although 0.3 Ag/4 ZrO2/SiO2 is not the most stable among all of the catalysts, it showed the highest yield at steady state.
Effect of the type of oxide Effect of Ag and ZrO2 content on ethanol conversion into butadiene over the Ag/ZrO2/SiO2 catalyst
The results obtained over Ag-promoted catalysts containing different oxides are presented in Table 2. The highest conversion of ethanol was observed over alumina-based catalyst. However, the main reaction products over this catalyst were ethylene and ethyl ether; the selectivity to butadiene was only 17 mol %. Such behavior of the alumina-containing catalyst is most probably due to the highest amount of acid sites over this catalyst (Table 1) and the presence of Brønsted acid sites, which are responsible for the ethanol dehydration step. High selectivity to ethanol dehydration products was also observed over niobia-based catalysts, which also exhibits Brønsted acidity.
The results obtained over Ag/ZrO2/SiO2 catalysts with different compositions are presented in Table 3 and Figures 9 and 10. The increase in the Ag content from 0.3 to 1.0 wt % led to a significant increase in the conversion of ethanol from 30 to 48 %; a further increase in the content of Ag to 2 wt % resulted in only a marginal increase in the conversion to 55 %. The selectivity to the products of acetaldehyde conversion increased at the expense of the products of ethanol dehydration. In particular, the contribution of heavy byproducts (mostly hexatrienes) increased as a result of the enhancement in the secondary
Table 2. Ethanol conversion and products distribution over M/MOx/SiO2 catalysts (T = 593 K, WHSV = 0.31 h1, TOS = 5 h). Sample 0.3 Ag/4 ZrO2/SiO2 0.3 Cu/4 ZrO2/SiO2 0.3 Ni/4 ZrO2/SiO2 0.3 Ag/4 Nb2O5/SiO2 0.3 Ag/4 TiO2/SiO2 0.3 Ag/4 MgO/SiO2 0.3 Ag/4 Al2O3/SiO2
Ethanol conversion [%]
butadiene
ethylene
propylene
Selectivity [mol %] butenes ethyl ether
ethyl acetate
butan-1-ol
C6 +
30.0 26.8 10.3 16.2 15.0 19.0 62.5
73.8 73.5 67.9 44.9 39.4 60.6 17.6
2.7 2.6 5.8 16.8 8.4 3.8 27.2
3.0 3.7 3.7 2.0 2.2 1.9 5.0
2.9 3.6 5.7 1.5 2.5 1.9 4.7
1.6 1.6 2.7 1.7 4.2 4.9 0.7
3.0 3.4 2.6 1.1 3.4 3.5 0.7
9.3 8.4 5.3 4.4 26.6 17.9 3.5
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3.7 3.3 6.4 27.8 13.3 5.5 40.6
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Table 3. Ethanol conversion and products distribution over Ag/ZrO2/SiO2 catalysts (T = 593 K, WHSV = 0.31 h1, TOS = 5 h). Sample 0.3 Ag/4 ZrO2/SiO2 1 Ag/4 ZrO2/SiO2 2 Ag/4 ZrO2/SiO2 1 Ag/1 ZrO2/SiO2 1 Ag/10 ZrO2/SiO2 1 Ag/18 ZrO2/SiO2
Ethanol conversion [%]
butadiene
ethylene
propylene
Selectivity [mol %] butenes ethyl ether
ethyl acetate
butan-1-ol
C6 +
30.0 48.0 55.2 18.4 45.6 45.8
73.8 72.1 71.3 67.8 73.0 72.8
2.7 1.5 2.0 2.4 2.1 2.3
3.0 3.0 3.0 2.5 2.9 2.9
2.9 2.9 3.0 2.9 2.6 2.5
1.6 3.0 1.6 3.8 2.9 3.0
3.0 1.7 1.0 2.4 2.5 2.8
9.3 12.7 15.7 14.9 11.7 10.8
Figure 8. Effect of the type of oxide on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
condensation reactions. At the same time, the selectivity towards butadiene slightly decreased from 73 to 71 mol %. Study of catalyst deactivation (Figure 9) suggests that the increase in the Ag content to values higher than 1 wt % leads to an increase in the deactivation rate. Therefore, promoting the reaction with 1 wt % Ag seems to be the most appropriate. The effect of ZrO2 content was studied over the catalyst containing 1 wt % silver (Table 3, Figure 10). The increase in the ZrO2 content from 1 to 4 wt % resulted in a sharp increase in ethanol conversion and selectivity towards butadiene, whereas a further increase in the zirconia content from 4 to 18 wt % did
Figure 9. Effect of the Ag content on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3.7 3.1 2.4 3.3 2.4 3.0
Figure 10. Effect of the ZrO2 content on the yield of butadiene versus time on stream (T = 593 K, WHSV = 0.3 h1).
not affect significantly the performance of the catalyst. Investigation of the stability of the activity of the catalysts with TOS revealed that an increase in the ZrO2 content up to 10 wt % led to a decrease in the catalyst deactivation rate (Figure 10). Therefore, the 1 Ag/10 ZrO2/SiO2 catalyst was selected for further studies of the effect of the reaction conditions. Effect of temperature and WHSV on ethanol conversion into butadiene over the 1 Ag/10 ZrO2/SiO2 catalyst The reaction temperature had a significant influence on the activity and selectivity of the catalyst. An increase in the temperature from 553 to 593 K boosted the selectivity to butadiene from 58 to 70 mol % (Figure 11). A further increase in the temperature resulted in a decrease in the selectivity to butadiene, which was associated with the increased contribution of the products of ethanol dehydration. Therefore, for the selective synthesis of butadiene over 1 Ag/10 ZrO2/SiO2 the 590–610 K temperature range is preferred. The effect of WHSV on the process parameters was studied in the 0.04–10 g g1 h1 range (Table 4). An increase in WHSV led to a dramatic decrease in both ethanol conversion and selectivity to butadiene. The highest selectivity of 74 mol % was reached at the lowest WHSV of 0.04 g g1 h1. However, a further decrease in WHSV was useless, as it is accompanied by a decrease in the output of butadiene. A comparison of the yields obtained over 1 Ag/10 ZrO2/SiO2 with the literature data (Table 7 in Ref. [1]) points that this catalyst is among the most efficient. The higher butadiene yields of 87 % reported by OhChemSusChem 2014, 7, 2527 – 2536
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Table 4. Effect of WHSV (T = 593 K, TOS = 5 h). WHSV [g g1 h1]
Ethanol conversion [%]
butadiene
ethylene
propylene
butenes
Selectivity [mol %] diethyl ether ethyl acetate
butan-1-ol
crotonaldehyde
C6 +
0.04 0.08 0.30 1.23 3.5 10.3
88.0 79.1 48.0 29.1 11.3 7.8
73.9 73.6 73.0 66.1 60.4 49.3
2.9 1.2 2.1 1.2 1.6 2.0
3.4 2.9 2.9 1.2 1.0 0.7
6.4 6.1 2.6 4.5 2.8 4.2
1.9 2.0 2.4 2.7 5.2 8.9
0.1 0.3 0.8 4.5 6.6 7.3
0.0 0.1 2.5 1.8 3.7 3.2
10.1 11.7 10.9 12.1 13.0 16.0
Figure 11. Effect of temperature on the conversion of ethanol into butadiene over 1 Ag/10 ZrO2/SiO2 (WHSV = 0.3 h1).
nishi[18] were obtained after 10 min of the reaction and cannot be adequately compared with the other results. Besides ethanol conversion and butadiene yield, the catalyst productivity should be considered, as it is among the most important parameters for industrial applications. The highest catalyst productivity of 0.23 g g1 h1 was achieved at the highest WHSV = 10.3 g g1 h1. However, this value was reached at a rather low selectivity to butadiene. At more acceptable selectivity to butadiene of 66 mol %, the catalyst productivity reached 0.14 g g1 h1, which is close to the highest value reported.[12]
1.3 2.1 2.9 5.9 5.7 8.3
Catalyst design included the selection of active components for the key reaction steps leading to butadiene; these components were merged into multifunctional catalysts and the catalyst functions were adjusted to achieve the highest selectivity. Among the metal-containing components (M = Ag, Cu, Ni) selected for the dehydrogenation of ethanol, a Ag promoter was found to be the most active, selective, and stable with time on stream. Among the metal oxide components (MOx = MgO, ZrO2, Nb2O5, TiO2, Al2O3) chosen for the condensation of acetaldehyde and the MPVO reduction of crotonaldehyde, ZrO2/SiO2 was shown to be the most promising. The optimized catalyst contained 1 wt % silver and 10 wt % zirconia on silica and provided selectivity to butadiene of 74 mol % at an ethanol conversion of 88 % at 593 K.
Experimental Section Catalyst preparation Catalysts were prepared by insipient wetness impregnation of silica gel (supplied by Karpov Chemical Plant) with aqueous solutions of the corresponding nitrates. After impregnation, the samples were dried overnight at ambient temperature and then at 383 K for 3 h. The samples were then heated in a flow of dry air to 873 K with a temperature ramp of 2 K min1 and then calcined at this temperature for 4 h. Reduction of the metal promoters was performed in a catalytic reactor in situ prior to the catalytic run. Catalysts are denoted as yM/zMOx/SiO2, in which y is the content of the metal loaded and z is the content of the oxide.
Conclusions Catalyst characterization The main reaction pathways of ethanol conversion over metalpromoted oxide catalysts were determined on the basis of a kinetic study of a wide range of ethanol conversions. The target reaction route leading to butadiene was demonstrated to include five consecutive reaction steps: first, ethanol dehydrogenation into acetaldehyde; second, aldol condensation of acetaldehyde; third, dehydration of 3-hydroxybutanal; fourth, MPVO reduction of crotonaldehyde with ethanol; and fifth, dehydration of crotyl alcohol into butadiene. The main side reactions were found to involve: first, ethanol dehydration into diethyl ether and ethylene; second, conversion of acetaldehyde into ethyl acetate and further into acetone and propylene; third, cross-condensation of acetaldehyde with crotonaldehyde and other aldehydes into heavy byproducts; and fourth, hydrogenation of crotyl alcohol into butan-1-ol followed by its dehydration and isomerization into but-1-ene and but-2-ene. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). Sorption–desorption isotherms of nitrogen were measured at 77 K by using an automated porosimeter (Micrometrics ASAP 2000). The acidic properties were studied by temperature-programmed desorption of ammonia (TPD-NH3) and low-temperature FTIR spectroscopy of adsorbed CO. TPD experiments were performed with a USGA-101 (UNISIT) in the temperature range from 293 to 1053 K in a flow of dry He (30 mL min1). The heating rate was 8 K min1. In a typical experiment, the samples were calcined at 673 K in a flow of dry He, saturated with ammonia at 323 K, and then the physisorbed NH3 was removed with a flow of He at 373 K. IR spectra were recorded with a Nicolet Protg 380 FTIR spectrometer at an optical resolution of 4 cm1. Prior to the measurements, the catalysts were pressed in self-supporting discs and activated in the IR cell attached to a vacuum line at 723 K for 4 h. AdChemSusChem 2014, 7, 2527 – 2536
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sorption of CO was performed in a low-temperature cell at 100 K. The pressure was measured by using a Barocell gauge.
Catalyst evaluation Catalytic experiments were performed in a flow-type fixed-bed reactor under atmospheric pressure. In a typical experiment, the catalyst (4 g, fraction 0.5–1 mm) was packed into a quartz tubular reactor and purged with nitrogen at 873 K for 0.5 h. Ethanol (95 wt %) was used as the feed. The reaction mixture was fed by using a Razel syringe pump. Helium was used as the carrier gas (molar ratio EtOH/He = 1). The WHSV was varied from 0.04 to 10.3 h1, and the reaction temperature was within 550–650 K. Gaseous products were analyzed on line with a Crystal 2000M gas chromatograph by using a 50 m SE-30 column. Liquid products were separated and analyzed by using 50 m SE-30 and 2 m Porapak Q columns. Methane was used as an external standard for the gaseous products. The conversion of ethanol [Eq. (1)], selectivity [Eq. (2)], and yields [Eq. (3)] were calculated as follows: Ethanol conversion ¼
mreactedðEtOHÞ 100 % mfedðEtOHÞ
ð1Þ
m
Selectivity ½mol % ¼
k Fj Mrjj 100 %
ð2Þ
mreactedðEtOHÞ 46
Yield ½mol % ¼ ethanol conversion
selectivity 100
ð3Þ
in which mreacted(EtOH) is mass of reacted ethanol, mfed(EtOH) is mass of fed ethanol, mj is the mass of the j product in the reaction mixture, Mrj is the molecular weight of product j, and k Fj is the number of ethanol molecules required for product formation. Catalyst productivity was determined as the grams of butadiene obtained per gram of catalyst per hour and calculated as follows [Eq. (4)]: Productivity ¼
WHSVEtOH yieldbutadiene 54 46 2 100
ð4Þ
Acknowledgements V.L.S. gratefully acknowledges Haldor Tøpsoe A/S for a PhD fellowship. Keywords: domino reactions · kinetics · metal promoters · oxide catalysts · reaction mechanisms [1] C. Angelici, B. M. Weckhuysen, P. Bruijnincx, ChemSusChem 2013, 6, 1595 – 1614. [2] J. A. Posada, A. D. Patel, A. Roes, K. Blok, A. C. Faaij, M. K. Patel, Bioresour. Technol. 2013, 135, 490 – 499. [3] A. D. Patel, K. Meesters, H. den Uil, E. de Jong, K. Bloka, M. K. Patel, Energy Environ. Sci. 2012, 5, 8430 – 8444. [4] a) S. V. Lebedev, British Patent 331402, 1929; b) S. V. Lebedev, British Patent 331482, 1930; c) S. V. Lebedev, French Patent 665917, 1929; d) S. V. Lebedev, Russ. J. Gen. Chem. 1933, 3, 698 – 708; e) S. V. Lebedev, Chem. Ztg. 1936, 60, 313 – 316. [5] W. J. Toussaint, J. T. Dunn, U. S. Patent 2,357,855, 1944. [6] W. Toussaint, J. Dunn, D. Jackson, Ind. Eng. Chem. 1947, 39, 120 – 125. [7] B. B. Corson, H. E. Jones, C. E. Welling, J. A. Hinckley, E. E. Stahly, Ind. Eng. Chem. 1950, 42, 359 – 373.
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[8] J. Ostromislenskiy, J. Russ. Phys. Chem. Soc. 1915, 47, 1472 – 1506. [9] M. Len, E. Diaz, S. Ordonez, Catal. Today 2011, 164, 436 – 442. [10] M. Jones, C. Keir, C. Di Iulio, R. Robertson, C. Williams, D. Apperley, Catal. Sci. Technol. 2011, 1, 267 – 272. [11] a) V. Ordomsky, V. Sushkevich, I. Ivanova, WO2012/156340, 2012; b) V. Ordomsky, V. Sushkevich, I. Ivanova, RU2440962, 2010. [12] E. V. Makshina, W. Janssens, B. F. Sels, P. A. Jacobs, Catal. Today 2012, 198, 338 – 344. [13] G. O. Ezinkwo, V. F. Tretjakov, R. M. Talyshinky, A. M. Ilolov, T. A. Mutombo, Catal. Commun. 2014, 43, 207 – 212. [14] G. Natta, R. Rigamonti, Chim. Ind. 1947, 29, 195 – 201. [15] B. B. Corson, E. E. Stahly, H. E. Jones, H. D. Bishop, Ind. Eng. Chem. 1949, 41, 1012 – 1017. [16] H. Niiyama, S. Morii, E. Echigoya, Bull. Chem. Soc. Jpn. 1972, 45, 655 – 659. [17] Y. Kitayama, A. Micheshita, J. Chem. Soc. Chem. Commun. 1981, 401 – 402. [18] R. Ohnishi, T. Akimoto, K. Tanabe, J. Chem. Soc. Chem. Commun. 1985, 1613 – 1614. [19] S. Kvisle, A. Aguero, R. P. A. Sneeden, Appl. Catal. 1988, 43, 117 – 131. [20] Y. Kitayama, M. Satoh, T. Kodama, Catal. Lett. 1996, 36, 95 – 97. [21] W. M. Quattlebaum, W. J. Toussaint, J. T. Dunn, J. Am. Chem. Soc. 1947, 69, 593 – 594. [22] V. Gruver, A. Sun, J. J. Fripiat, Catal. Lett. 1995, 34, 359 – 364. [23] J. B. D. Delacaillerie, V. Gruver, J. J. Fripiat, J. Catal. 1995, 151, 420 – 430. [24] Y. A. Gorin, F. N. Kogan, J. Syn. Rubber 1936, 11 – 12, 6 – 12. [25] Y. A. Gorin , Russ. J. Gen. Chem. 1946, 16, 1089 – 1092. [26] Y. A. Gorin, M. I. Danilina, Russ. J. Gen. Chem. 1948, 18, 1069 – 1075. [27] a) S. K. Bhattacharyya, S. K. Sanyal, J. Catal. 1967, 7, 152 – 158; b) S. K. Bhattacharyya, N. D. Ganguly, J. Appl. Chem. 1962, 12, 97 – 104; c) S. K. Bhattacharyya, N. D. Ganguly, J. Appl. Chem. 1962, 12, 105 – 110; d) S. K. Bhattacharyya, B. N. Avasthi, Ind. Eng. Chem. Process Des. Dev. 1963, 2, 45 – 51. [28] A. N. Ko, B. W. Wojciechowski, Prog. React. Kinet. 1983, 12, 201 – 262. [29] K. Inui, T. Kurabayashi, S. Sato, Appl. Catal. A 2002, 237, 53 – 61. [30] G. A. H. Mekhemer, S. A. Halawy, M. A. Mohamed, M. I. Zaki, J. Catal. 2005, 230, 109 – 122. [31] R. S. Rao, A. B. Walters, M. A. Vannice, J. Phys. Chem. B 2005, 109, 2086 – 2092. [32] a) J. Keuler, L. Lorencen, S. Miachon, Appl. Catal. A 2001, 218, 171 – 180; b) N. Doca, E. Segal, React. Kinet. Catal. Lett. 1985, 28, 123 – 129; c) M. Domok, M. Toth, J. Rasko, A. Erdohelyi, Appl. Catal. B 2007, 69, 262 – 272; d) H. Idriss, Platinum Met. Rev. 2004, 48, 105 – 115; e) V. Sushkevich, I. Ivanova, E. Taarning, ChemCatChem 2013, 5, 2367 – 2373. [33] a) H. Hattori, Appl. Catal. 2001, 222, 247 – 259; b) V. Dez, C. Apestegua, J. Di Cosimo, J. Catal. 2003, 215, 220 – 233; c) G. A. M. Hussein, N. Sheppard, M. I. Zaki, R. B. Fahim, J. Chem. Soc. Faraday Trans. 1991, 87, 2661 – 2668. [34] a) W. Ji, Y. Chen, H. Kung, Appl. Catal. A 1997, 161, 93 – 104; b) K. Tanabe, G. Zhang, H. Hattori, Appl. Catal. 1989, 48, 63 – 69; c) V. Ordomsky, V. Sushkevich, I. Ivanova, J. Mol. Catal. A 2010, 333, 85 – 93. [35] a) M. Glin´ski, Appl. Catal. A 2008, 349, 133 – 139; b) P. S. Kumbhar, J. Sanchez-Valente, F. Figueras, Chem. Commun. 1998, 1091 – 1092; c) Y. Z. Zhu, S. H. Liu, S. Jaenicke, G. Chuah, Catal. Today 2004, 97, 249 – 255; d) J. C. van der Waal, K. Tan, H. van Bekkum, Catal. Lett. 1996, 41, 63 – 67; e) Y. Z. Zhu, S. Jaenicke, G. K. Chuah, J. Catal. 2003, 218, 396 – 404; f) V. Sushkevich, I. Ivanova, S. Tolborg, E. Taarning, J. Catal. 2014, 316, 121 – 129. [36] a) H. Kawakami, S. Yoshida, J. Chem. Soc. Faraday Trans. 1985, 81, 1117 – 1127; b) H. Kawakami, S. Yoshida, J. Chem. Soc. Faraday Trans. 1985, 81, 1129 – 1137. [37] a) M. Bensitel, O. Saur, J. C. Lavalley, Mater. Chem. Phys. 1987, 17, 249 – 258; b) M. Maache, A. Janin, J. C. Lavalley, J. F. Joly, E. Benazzi, Zeolites 1993, 13, 419 – 426; c) A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M. Padovan, C. Otero Arean, J. Chem. Soc. Faraday Trans. 1992, 88, 2959 – 2969.
Received: April 23, 2014 Revised: June 9, 2014 Published online on August 14, 2014
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