DOI: 10.1002/cssc.201403099
Communications
Iridium-Catalyzed Hydrogen Production from Monosaccharides, Disaccharide, Cellulose, and Lignocellulose Yang Li, Peter Sponholz, Martin Nielsen, Henrik Junge, and Matthias Beller*[a] Hydrogen constitutes an important feedstock for clean-energy technologies as well as for production of bulk and fine chemicals. Hence, the development of novel processes to convert easily available biomass into H2 is of general interest. Herein, we demonstrate a one-pot protocol hydrogen generation from monosaccharides, disaccharide, and extremely demanding cellulose and lignocellulose substrates by using a pincer-type iridium catalyst. Applying ppm amounts of this catalyst, hydrogen is produced at temperatures lower than 120 8C. More specifically, catalyst turnover numbers (TONs) for lignocellulose from bamboo reached up to about 3000. Interestingly, even (used) cigarette filters, which are composed of cellulose acetate, produce hydrogen under optimized conditions.
accumulated catalyst turnovers, after 5 successive additions of glucose, were reached in ionic liquid. Moreover, formic acid was identified as the key intermediate for producing H2 and CO2. Thus, only one mole H2 can be produced per mole of glucose, as the authors mentioned. Therefore, it is of general interest to develop new catalyst systems that work efficiently under mild conditions and allow for higher catalyst turnover numbers (TONs). Moreover, these systems should allow to produce more than one equivalent of H2 per monosaccharide unit. Inspired by the work of Wasserscheid and co-workers and by examples of hydrogen generation from simple[12–14] and polyol alcohols,[15] we describe herein hydrogen generation from biomass in the presence of iridium catalyst[16, 17] at comparatively low temperature (< 120 8C). In fact, different monosaccharides, cellobiose, and more demanding substrates (i.e, cellulose, lignocelluloses) produce hydrogen under the optimized conditions. Furthermore, comparison of the resulting spectra of monosaccharides in the presence or absence of catalyst showed that sodium formate and other carboxylic acids salts are formed as intermediates in the reaction mixture of fructose and glucose. These results suggest the possibility of more hydrogen production as mentioned above. For our initial experiments, the monosaccharide l-( )-fructose was selected as a benchmark substrate. In order to achieve successful catalysis a suitable solvent for both the hydrophilic starting material and the sensitive transition metal catalyst had to be found. After many experiments, we found that l-( )-fructose is exothermically dissolved in diglyme in the presence of 1.3 equiv of KOH at 95 8C. After an initial rise of the temperature to 127 8C for a few seconds, a clear solution is formed. To our delight, addition of the commercially available non-innocent pincer-type catalyst Ru-MACHO [catalyst 1, (RuHCl(PNPPh)CO][18, 19] to this solution already resulted in the generation of significant amounts of hydrogen. The catalyst activity reached turnover frequencies (TOFs) of 771 h 1 (1 h) and 567 h 1 (2 h) (Table 1, entry 1). CO and CH4 contents of about 120 ppm and 40 ppm, respectively, were detected. The concomitant formation of CO2 proved C C bond cleavage of the monosaccharide, too. Acid hydrolysis of the reaction mixture proved that most carbon dioxide was trapped in solution as (bi)carbonate. Noteworthy, a blank reaction without any catalyst in the presence of base produced some carbon dioxide but no hydrogen (see Supporting Information).[20] Exchange of the Cl anion in catalyst 1 by BH4 (catalyst 2, Table 1, entry 2) resulted in similar catalyst activity. On the other hand, variation of the phosphorus substituents in cata-
Hydrogen plays a key role for future clean energy as well as for the industrial production of bulk and fine chemicals.[1] In 2011, the worldwide production of hydrogen exceeded 31 million metric tons.[2] Today, 96 % of this hydrogen comes from fossil resources through coal gasification and gas reforming processes.[2] Thus, “green” hydrogen from the combination of renewable energy and sustainable resources, such as water and biomass, is highly desirable. However, until now largescale hydrogen production from water remains challenging.[3, 4] Consequently, the development of efficient technologies to produce H2 from biomass is an attractive prospect. Although important contributions on hydrogen generation from biomass have been made by means of enzyme fermentation,[5, 6] gasification (about 800 8C to 1300 8C),[7] reforming in supercritical water (about 380 8C, 230 bar), and aqueous-phase reforming (about 220 8C, 20 bar) by heterogeneous catalysts,[8, 9] the utilization of lignocellulosic biomass is still called “a chewy problem”.[10] While the former technology is limited by the cost of enzyme, downstream processing cannot be avoided applying heterogeneous catalysts under harsh conditions, as the direct utilization of the produced hydrogen is limited by the large amount of methane present in the final gas mixture. Notably, Wasserscheid and co-workers reported seminal work on hydrogen generation accompanied by CO2 production from glucose and cellulose in ionic liquids at 150–180 8C, catalyzed by [(p-cymene)RuCl2]2/TMEDA.[11] However, less than 200 [a] Prof. Dr. Y. Li, P. Sponholz, Dr. M. Nielsen, Dr. H. Junge, Prof. Dr. M. Beller Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert Einstein Str. 29a, 18059 Rostock (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403099.
ChemSusChem 0000, 00, 0 – 0
These are not the final page numbers! ÞÞ
1
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
Communications Table 1. Hydrogen production from fructose, glucose, and cellobiose by using a variety of molecularly defined catalysts.
Entry[a] Catalyst Loading Base [mmol, ppm] (equiv)
V(H2) (1 h) V(CO2)[b] (1 h) TOF (1 h) V(H2) (2 h) V(CO2)[b] (2 h) TOF (2 h) [mL] [mL] [h 1] [mL] [mL] [h 1])
1 2 3[c] 4 5 6[c] 7 8 9[c] 10[c] 11[c,d] 12[c,e]
90.3 95.6 164.5 124.0 42.1 201.7 135.2 183.4 293.8 165.5 98.4 125.1
1 2 3 4 5 6 3 6 6 6 6 6
4.76, 50.6 4.84, 51.5 4.67, 49.7 4.75, 50.5 4.79, 51.0 4.51, 48.0 4.35, 46.3 4.73, 50.3 4.64, 49.4 0.97, 10.3 0.97, 10.3 0.50, 10.5
KOH (1.3) KOH (1.3) KOH (1.3) KOH (1.3) KOH (1.3) KOH (1.3) NaOH (1.3) NaOH (1.3) NaOH (1.5) NaOH (1.5) NaOH (1.5) NaOH (1.5)
44.7 33.4 87.5 54.0 13.9 112.8 29.3 61.1 13.3 < 0.1 0.35 86.4
771 802 1432 1060 356 1817 1263 1577 2572 6923 4116 10 269
132.9 134.9 248.2 202.3 64.4 211.5 208.2 243.1 341.3 265.8 207.0 133.0
103.1 88.1 157.3 119.7 42.6 182.5 78.8 94.9 54.2 4.5 4.3 92.0
567 566 1053 865 273 952 972 1045 1494 5559 4329 5459
[a] If not otherwise mentioned, reactions were performed with l-( )-fructose (94.03 mmol) in diglyme (10 mL) at 95 8C. Volumes were measured by gas burette after removal of blank volumes. Volumes of H2 and CO2 were based on the detected ratios of H2 and CO2, TOFs were calculated with respect to volumes (mL) of H2. [b] V(CO2) is the CO2 produced in the gas phase. [c] Reported as an average of 2 reactions and with an error margin of 8 %. [d] d-(+)-Glucose was used as substrate. [e] d-(+)-Cellobiose was used as substrate and the reaction was performed on 47.02 mmol scale (based on the monosaccharide) at 120 8C.
lyst 1 from phenyl to iso-propyl (catalyst 3, entry 3) and cyclohexyl (catalyst 4, entry 4) increased the hydrogen generation. Especially using catalyst 3 the TOF reached 1053 h 1 after 2 h. A lower activity was observed when using Milstein’s catalyst precursor (5) (entry 5).[21, 22] Surprisingly, an iridium-based catalyst (6) proved to be most active (entry 6), although it previously showed a low efficiency in hydrogen generation from iPrOH.[14] The addition of base is critical for hydrogen generation from alcohols under milder conditions.[12–15] Hence, reactions were performed in the presence of NaOH and LiOH. However, in the latter case the base was not completely dissolved. When comparing catalysts 3 and 6 in the presence of KOH and NaOH, 3 showed a slightly better reactivity using KOH (Table 1, entries 3 and 7), whereas catalyst 6 displayed a better reactivity using NaOH (entries 6 and 8). Furthermore, the reaction efficiency was investigated using both catalysts with an increased amount of base (Supporting Information, Table S1, entries 1–3; Table 1, entry 9). Using 1.5 equiv of NaOH, catalyst 6 showed distinctly higher reactivity (entry 9). However, a higher concentration of NaOH (1.7 equiv) sharply decreased the reactivity, which might be caused by the suspension produced in the reaction mixture (Table S1, entry 4). In addition to diglyme, fructose can be dissolved under basic conditions in other dipolar aprotic solvents
&
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
such as 1-methyl-2-pyrrolidinone (NMP) and dimethyl sulfoxide (DMSO) (Table S1, entries 5 and 6). Although the reaction showed improved activity at the initial stage when using NMP, it almost stopped after 1h (Table S1, entry 5). When the catalyst loading was decreased from 49.9 ppm to 9.8 ppm, an excellent TON of 7422 was obtained after 1 h, however, after that time the catalyst system was deactivated (Table S1, entry 7). On the other hand, using diglyme as solvent, the catalyst system was more stable and a TOF of 5559 h 1 was reached after 2 h (the TON reached 11106 after 3 h and 11 371 after 6 h). Here, a 2.0 % conversion (based on 6 mol of H2 produced per mol of monosaccharide) was obtained (Figure 1, Table 2). With d-(+)-glucose as substrate, a similar activity was observed (Table 1, entry 11, TOF after 1 h: 4116 h 1; after 2 h: 4329 h 1) and the total TON reached 10 863 after 3 h and
Figure 1. Hydrogen production from fructose using Ir-catalyst 6.
12 177 after 6 h with 2.1 % conversion (see Supporting Information, Figure S1). Here, the content of CO was about 170 ppm and no signal of CH4 was observed. The ratios of produced H2 to CO2 were 1:1.37 for l-( )-fructose (6 h reaction) and 1:1.29 for d-(+)-glucose (6 h reaction). The calculated amounts of carbon dioxide resulted from measurements both in gas phase and solution. They were corrected by the blank volume obtained under basic conditions without any transition-metal catalyst. The increased formation of carbon dioxide compared to the blank value demonstrates reforming of the monosaccharides to some extent. 2
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communications Inspired by reactions of the monosaccharides, subsequently d-(+)-cellobiose was investigated as an example of a disaccharide. It showed good efficiency in diglyme (Table 1, entry 12) and the ratio of produced H2/CO2 was 1:1.89. Similar to the glucose reaction, a CO content of about 220 ppm was detected and no signal for CH4 was observed. After that, we switched our attention to cellulose as the ultimate target of biomass conversion. Unfortunately, using cellulose in NMP even at 150 8C, a very low TON was observed due to the low solubility of the substrate (Table 3, entry 1). Apparently, hydrolysis of cellulose to glucose and/or cellobiose is required for more efficient hydrogen generation. Most of the known hydrolyses of cellulose are performed under acid conditions in diluted ionic liquids (about 12.5 g ionic liquid per 1 g of cellulose)[23, 24] or at
Table 2. H2 generation from l-( )-fructose (94.03 mmol) in diglyme (10 mL) with 10.7 ppm of catalyst 6: TON for 6 h was shown. Reaction conditions: Volumes were measured by gas burette and the blank volume was removed. t [h]
V [mL]
V(H2) [mL]
V(CO2)[a] [mL]
TON
TOF [h 1]
1 2 3 4 5 6
136.0 262.0 294.0 308.0 318.0 325.0
163.0 254.8 257.7 281.7 281.9 282.3
< 0.1 7.2 18.3 26.3 36.1 42.7
5666 10264 11106 11347 11355 11371
6566 5132 3702 2837 2271 1895
[a] V(CO2) is the produced CO2 in the gas phase.
In order to imTable 3. Hydrogen production from cellulose. prove the hydrogen productivity, further experiments were performed using the benchmark system l-( )-frucEntry[a] Loading T(1, 2, 3) V(H2) (1 h) V(CO2)[b] (1 h) TON (1 h) V(H2) (2 h) V(CO2)[b] (2 h) TON Yield [%][h] Yield [%][h] [mmol, ppm] [8C] [mL] [mL] [mL] [mL] (2 h) (glucose) (TRS) tose. Unfortunately, variation of the 1[c] 11.90, 506.4 –, –, 150 15.0 – 51.2 27.0 – 92.2 – – 0.62, 20.6 140, 140, 140 76.6 0.4 5049 82.6 0.4 5446 < 2.8 5.0 2[d] ligand concentra0.59, 19.6 140, 140, 140 88.7 9.1 6125 95.4 9.9 6588 < 2.8 5.6 3[e] tion or adding extra 0.64, 21.2 140, 140, 140 86.7 5.6 5547 96.9 5.2 6199 < 2.8 6.0 4[e,f] amounts of catalyst 0.71, 23.7 140, 95, 95 16.5 0.3 944 27.0 0.3 1546 < 2.8 5.6 5[e] and base did not 6 0.58, 19.3 140, 140, 95 37.8 0.7 2652 59.2 0.3 4154 < 2.8 5.6 7 0.60, 20.0 140, 140, 120 87.2 0.8 5927 100.0 1.0 6798 < 2.8 5.6 induce obvious gas 8 0.64, 21.2 140, 120, 120 86.0 3.0 5502 94.7 4.3 6059 < 2.8 5.6 generation. When 9[g] 0.62, 20.6 95, 120, 120 64.2 0.8 4232 69.4 1.1 4575 < 2.8 6.2 increasing the cata0.61, 20.3 120, 120, 120 82.2 0.3 5502 93.9 0.4 6285 < 2.8 3.5 10[e] lyst loading to [a] After heating the reaction mixture of 30.00 mmol of cellulose in NMP (24 mL), H2O (2 mL), and HCl (37 %, 4 mL) for 2.5 h at about 500 ppm, the temperature 1 (8C), the reaction temperature was adjusted to 95 8C and NaOH solution (1.5 equiv, except for NaOH solution H2 conversion was used for the neutralization) was added. After that the temperature was adjusted to temperature 2 (8C) and the reaction mixture increased to 2.6 % was stirred for 1 h, and the temperature was adjusted to temperature 3 (8C) and catalyst was added. Volumes (mL) were measured by gas burette and blank volumes were removed, volumes of H2 and CO2 were based on the detected ratios of H2 and (Supporting InforCO2, TONs were based on volumes of H2. [b] V(CO2) is the produced CO2 in gas phase. [c] The reaction was performed on mation, Table S2, 23.50 mmol scale in NMP (20 mL) in the presence of 1.5 equiv of NaOH. [d] Hydrolysis was performed in 1 h. [e] Reported on entry 1). In order to the average of 2 reactions with an error margin of 6 %. [f] Hydrolysis was performed in 4 h. [g] Hydrolysis was performed in suppress catalyst 24 h. [h] The reactions were performed on 7.5 mmol scale with the same hydrolysis procedure for detection of hydrolysis yields of glucose and total reducing sugar (TRS) (see Supporting Information). deactivation, experiments under diluted conditions much higher temperatures (> 260 8C, 5.7 MPa).[25] Based on the were also performed (0.565 g mL 1 versus 1.694 g mL 1) (entry 2), but no major influence was observed. As expected, original screening of reaction conditions, it was clear that increasing the reaction temperature to 140 8C improved the H2 a too-diluted concentration of sugars results in low reactivity. In addition, the stability of the sensitive organometallic comconversion slightly to 3.4 % (entry 4). On the other hand, variaplex is limited. With these considerations in mind, a systematic tion of the amount of fructose did not show any pronounced study to combine hydrolysis of cellulose and the subsequent effects (entries 4 and 5). hydrogen production was performed. In the initial experiTo gain some insight into the hydrogen production from ments, the hydrolysis step was performed in 1-butyl-3-methylimonosaccharides, reaction mixtures of fructose and glucose midazolium chloride (0.67 mmol mL 1) and water in the preswith and without catalyst were studied by NMR spectroscopy after 6 h of stirring at 95 8C, respectively. Comparison of the reence of hydrochloric acid (HCl). After neutralization, TONs of sulting spectra showed that sodium formate and other carbox533 (1 h) and 593 (2 h) at 140 8C were achieved in the presence ylic acids salts are formed as intermediates before addition of of catalyst 6. Unfortunately, increasing the cellulose concentrathe catalyst. Furthermore, these reaction mixtures showed tion resulted in reproducibility problems because a gelatinous almost the same intermediates. However, it was very difficult mixture formed. Comparatively, significantly improved results to identify each of the individual intermediates at this stage. could be obtained by using an aqueous mixture of NMP. Thus, ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
These are not the final page numbers! ÞÞ
3
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
Communications hydrolysis of 30 mmol of cellulose at 140 8C by hydrochloric acid (1 h), followed by neutralization with NaOH solution and addition of catalyst 6 led to 82.6 mL of hydrogen and TONs of 5049 after 1 h and 5446 after 2 h, respectively (Table 3, entry 2). Prolonging the hydrolysis time to 2.5 h resulted in even higher hydrogen volume (95.4 mL) and TONs [6125 (1 h), 6588 (2 h), Table 3, entry 3]. The effect of the water concentration on the hydrolysis step was investigated as well. Both, increasing and decreasing the amount of water resulted in lower reactivity (Supporting Information, Table S3, entries 1 and 2). This is probably explained by the lower catalyst stability and the reduced solubility of base in the presence of decreasing amount of water. Notably, the iridium-based pincer complex showed distinctly higher efficiency compared to the more common Ru-MACHO (catalyst 1) and RuHCl(PNPiPr)CO (catalyst 3) (Table S3, entries 3 and 4). Temperature variation of the different reaction steps showed that fast hydrogen generation from cellulose is possible even at 120 8C. Experiments revealed that hydrolysis (Table 3, entries 8, 9 vs. 10) and the hydrogen generation step (Table 4, 6 vs. 7) performed best at this temperature. When the reaction temperature of the hydrolysis or temperature 2 was reduced to 95 8C, a longer time was needed (Table 3, entry 9) or much lower catalyst TON is observed (Table 3, entry 5). Furthermore, the influence of the amount of base was studied. Similar to the result of fructose, an increased amount of NaOH (1.8 equiv) induced lower activity (Table S3, entry 5). Reducing the base concentration, the TON (2 h) remained constant, though an increased volume of CO2 was released from the reaction mixture (Table S3, entry 6). Due to the improved hydrogen generation in diglyme for fructose, a solvent switch to diglyme was investigated. However, the reaction activity decreased to a major extent (Table S3, entry 7). This can be explained by the favorable effect of NMP on the hydrolysis step.
To get a better understanding of the hydrogen production from cellulose, the yields of glucose and reducing sugars after hydrolysis were detected. To our surprise, only small amounts were present (Table 3, see yields of glucose and total reducing sugar content after hydrolysis). Furthermore, we compared the reactivities of cellulose, glucose, and cellobiose under similar conditions. Using glucose and cellobiose as substrates, the reactions were performed without the hydrolysis procedure but in the presence/absence of NaCl (Supporting Information, Table S4). While glucose showed the same level of reactivity as cellulose, cellobiose showed a slightly higher reaction efficiency. Moreover, the produced NaCl during the neutralization did not considerably decrease the reaction activity (Table S4, entries 2 and 4). These results suggest that considerable amounts of oligomers are formed in the initial hydrolysis step of the polysaccharide. These more soluble oligomers displayed similar reactivity compared to glucose and/or cellobiose. Finally, we applied this one-pot protocol for hydrogen generation from cellulose to lignocellulose, which constitutes one of the most abundant renewable feedstocks on earth. Two kinds of lignocellulose, from pulp and second-year bamboo, were investigated. To our delight, both feedstocks showed good activity (Table 4, entries 2 and 3). More specifically, the catalyst TON of lignocellulose from pulp reached 2188 after 4 h and 2441 after 7 h. Similarly, the TON of lignocellulose from bamboo reached 2266 (4 h) and 2844 (8 h). These experiments show that molecular-defined catalysts allow for hydrogen production even from more challenging and relevant carbohydrates. Interestingly, a comparable activity was observed when using cellulose acetate fibers. This polymer is produced on > 800 000 ton-scale in 2008 and mainly used in the production of cigarette filter tow.[26] As shown in Table 4 using plain cellulose acetate, commercial cigarette filters and even used cigarette filters allowed for hydrogen generation under optimized
Table 4. Hydrogen production from cellulose, lignocelluloses, cellulose acetate, and cigarette filters.[a]
Entry
Substrate
Amount [mmol], [ppm]
V(H2) (1 h) [ mL]
V(CO2)[b] (1 h) [mL]
TON (1 h)
V(H2) (2 h) [mL]
V(CO2)[b] (2 h) [mL]
TON (2 h)
V(H2)/V(CO2)[c]
1 2[d] 3[e] 4 5[f] 6[f]
cellulose lignocellulose (from pulp) lignocellulose (from bamboo) cellulose acetate commercial cigarette filters used cigarette filters
0.61, 20.3 0.63, 20.9 0.63, 20.9 0.61, 20.3 0.61, 40.5 1.25, 83.5
82.2 25.6 26.1 44.5 33.9 14.9
0.3 0.6 1.0 < 0.1 0.1 < 0.1
5502 1662 1695 2978 2269 484
93.9 33.7 34.9 52.3 38.9 19.4
0.4 0.5 0.5 < 0.1 0.1 < 0.1
6285 2188 2266 3500 2604 630
1.00:1.09 1.00:1.23 1.00:1.29 1.00:0.81 1.00:0.77 1.00:1.35
[a] All the reactions were performed on the optimized reaction conditions on 30.00 mmol scale (based on the monomer). Calculated concerning to volumes (mL) were measured by gas burette and blank volumes were removed, volumes of H2 and CO2 were based on the detected ratios of H2 and CO2, TONs were based on volumes of H2, all the experiments were reported on the average of 2 reactions with an error margin of 8 %. The pictures of special feedstocks were shown. [b] Produced CO2 in gas phase. [c] Volumes of CO2 are the produced CO2 in whole reactions after remove of produced CO2 in blank reactions. [d] The contents of lignocellulose from the pulp are identified as: H2O, 7.6 %, cellulose, 64.8 %, hemicellulose, 1.9 %, lignin, 0.9 %, ash, 24.0 %. 30.00 mmol scale is based on the content of cellulose and hemicellulose, 32 mL of NMP was used. Reported TONs are after 2 h and 4 h, TON can reach 2441 after 7 h. [e] The contents of lignocellulose from the second year bamboo are identified as: H2O, 10.5 %, cellulose, 45.0 %, hemicellulose, 20.2 %, lignin, 19.8 %, ash, 0.8 %. 30.00 mmol scale is based on the content of cellulose and hemicellulose, 32 mL of NMP was used. Reported TONs are after 2 h and 4 h, and TON can reach 2844 after 8 h. [f] The reaction was performed on 15 mmol scale.
&
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
4
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communications Acknowledgements
conditions (TONs of 3500 (2 h), 2604 (2 h), and 630 (2 h), respectively). The content of CO for reactions of cellulose was detected as about 410 ppm. However, using lignocellulose from pulp and bamboo, or cigarette filters, a higher content of CO was detected (0.1 %, 0.4 % and 0.3 %, respectively). It should be noted that in the gas phase of these reactions, no CH4 was detected. In summary, we describe a procedure for hydrogen generation from biomass using molecular-defined pincer-type catalysts. Monosaccharides, cellobiose, as well as demanding cellulose and lignocelluloses gave hydrogen using only ppm amounts of the non-innocent IrH2Cl(PNPiPr) complex. Stoichiometric amounts of NaOH used in our protocol promote high catalyst turnover numbers and trap the produced CO2 as (bi)carbonate. This novel process provides another opportunity for potential application of lignocellulosic biomass besides wellknown enzymatic fermentation processes and heterogeneous catalysis procedures. Notably, hydrogen generation takes place under comparatively mild reaction conditions and the reaction system is stable with regard to water, NaCl, and ash from pulp. The detection of small amounts of glucose and reducing sugars after hydrolysis of polysaccharides suggest that hydrogen generation proceeds mainly from the corresponding oligomers.
Y. L. and M. N. thank the Alexander von Humboldt Foundation for financial support. We thank Dr. Elisabetta Alberico for affording catalysts 3 and 4, Dr. Lei Wang, Institute of Pulp and Paper Technology, Hubei University of Technology, China for affording lignocellulose from the second-year bamboo and Shanghai Boning Engineering Fibers Co. Ltd. China for affording lignocellulose from pulp, Dr. Christine Fischer for analysis of glucose from cellulose hydrolysis, and Dr. Wenhua Wang and Dr. Decai Xiong for helpful discussion. Keywords: biomass · homogeneous catalysis · hydrogen · iridium · renewable energy [1] N. Armaroli, V. Balzani, ChemSusChem 2011, 4, 21 – 36. [2] Hydrogen Production Worldwide and US total Hydrogen Production, US Department of Energy Hydrogen Analysis Resource Center, http:// hydrogen.pnl.gov/cocoon/morf/hydrogen/article/706 2012. [3] F. E. Osterloh, Chem. Soc. Rev. 2013, 42, 2294 – 2320. [4] J. R. Swierk, T. E. Mallouk, Chem. Soc. Rev. 2013, 42, 2357 – 2387. [5] J. Woodward, S. M. Mattingly, M. Danson, D. Hough, N. Ward, M. Adams, Nat. Biotechnol. 1996, 14, 872 – 874. [6] R. R. O. Barros, R. S. Paredes, T. Endo, E. P. S. Bon, S.-H. Lee, Bioresour. Technol. 2013, 136, 288 – 294. [7] R. Toonssen, N. Woudstra, A. H. M. Verkooijen, Int. J. Hydrogen Energy 2008, 33, 4074 – 4082. [8] R. D. Cortright, R. R. Davda, J. A. Dumesic, Nature 2002, 418, 964 – 967. [9] P. Azadi, R. Farnood, Int. J. Hydrogen Energy 2011, 36, 9529 – 9541. [10] K. Sanderson, Nature 2011, 474, S12 – S14. [11] N. Taccardi, D. Assenbaum, M. E. M. Berger, A. Bçsmann, F. Enzenberger, R. Wçlfel, S. Neuendorf, V. Goeke, N. Schçdel, H.-J. Maass, H. Kistenmacher, P. Wasserscheid, Green Chem. 2010, 12, 1150 – 1156. [12] M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85 – 89. [13] R. E. Rodrguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-Quinones, H. Grtzmacher, Nat. Chem. 2013, 5, 342 – 347. [14] M. Nielsen, A. Kammer, D. Cozzula, H. Junge, S. Gladiali, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 9593 – 9597; Angew. Chem. 2011, 123, 9767 – 9771. [15] a) D. Morton, D. J. Cole-Hamilton, J. Chem. Soc. Chem. Commun. 1988, 1154 – 1156; b) Y. Li, M. Nielsen, B. Li, P. H. Dixneuf, H. Junge, M. Beller, Green. Chem. 2015, 17, 193. [16] Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J. Lough, K. Abdur-Rashid, Organometallics 2006, 25, 4113. [17] X. Chen, W. Jia, R. Guo, T. W. Graham, M. A. Gullons, K. Abdur-Rashid, Dalton Trans. 2009, 1407 – 1410. [18] W. Kuriyama, T. Matsumoto, Y. Ino, O. Ogata, PCT Int. Appl. WO/2011/ 048727A1, 2011. [19] M. Bertoli, A. Choualeb, A. J. Lough, B. Moore, D. Spasyuk, D. G. Gusev, Organometallics 2011, 30, 3479 – 3482. [20] A. V. Ellis, M. A. Wilson, J. Org. Chem. 2002, 67, 8469 – 8474. [21] J. Zhang, M. Gandelman, L. J. W. Shimon, H. Rozenberg, D. Milstein, Organometallics 2004, 23, 4026 – 4033. [22] C. Gunanathan, D. Milstein, Science 2013, 341, 1229712. [23] C. Li, Z. K. Zhao, Adv. Synth. Catal. 2007, 349, 1847 – 1850. [24] C.-Z. Liu, F. Wang, A. R. Stiles, C. Guo, Appl. Energy 2012, 92, 406 – 414. [25] C. Wang, F. Zhou, Z. Yang, W. Wang, F. Yu, Y. Wu, R. Chi, Biomass Bioenergy 2012, 42, 143 – 150. [26] J. Puls, S. A. Wilson, D. Hçlter, J. Polym. Environ. 2011, 19, 152 – 165.
Experimental Section Standard procedure for hydrogen production from monosaccharides and cellobiose: Starting material was added to a doublewalled thermostated reaction vessel and treated with vacuo and argon several times to remove air. After that, solvent and base were added in argon atmosphere. The catalyst was added with a small Teflon tube after the reaction mixture became stationary at the specified temperature. The amount of gas generated over time was measured by a manual gas burette (100 mL, 250 mL, and 1000 mL burettes were used). Subsequently, the gas purity was detected by GC analysis. Standard reaction procedure for hydrogen production from cellulose, lignocellulose, cellulose acetate, and cigarette filters: Substrate (30.00 mmol) and NMP (24 mL) were added to a doublewalled thermostated reaction vessel at room temperature. The mixture was treated with vacuo and argon several times to remove air. H2O (2 mL) and HCl (37 %, 4.00 mL) were added. The reaction temperature was increased to temperature 1 and stirred for 2.5 h. Then the reaction temperature was adjusted to 95 8C and NaOH solution (prepared by 61.87 g NaOH and 50 mL distilled H2O, heated by a 100 8C oil bath) (1.5 equiv, except for NaOH solution used for neutralization) was added dropwise. After that, the reaction temperature was adjusted to temperature 2 and stirred for another 1 h. Then the reaction temperature was adjusted to temperature 3, the catalyst was added with a small Teflon tube after the reaction mixture became stationary. The amount of gas generated over time was measured by manual gas burette (100 mL and 250 mL burettes were used). Subsequently, the gas purity was detected by GC analysis.
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
These are not the final page numbers! ÞÞ
Received: October 8, 2014 Revised: December 9, 2014 Published online on && &&, 0000
5
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
COMMUNICATIONS Y. Li, P. Sponholz, M. Nielsen, H. Junge, M. Beller* && – && Iridium-Catalyzed Hydrogen Production from Monosaccharides, Disaccharide, Cellulose, and Lignocellulose
&
ChemSusChem 0000, 00, 0 – 0
Doing the splits: A one-pot procedure is developed to generate hydrogen from biomass using molecular-defined pincer-type catalysts. Monosaccharides, cellobiose, as well as demanding cellulose and lignocelluloses give hydrogen
www.chemsuschem.org
6
using only ppm amounts of the non-innocent iridium-based complex (IrH2Cl(PNPiPr). Hydrogen generation takes place under 120 8C. The reaction system is stable with respect to water, NaCl, and ash from pulp.
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communications
Iridium-Catalyzed Hydrogen Production from Monosaccharides, Disaccharide, Cellulose, and Lignocellulose Yang Li, Peter Sponholz, Martin Nielsen, Henrik Junge, and Matthias Beller*[a] Hydrogen constitutes an important feedstock for clean-energy technologies as well as for production of bulk and fine chemicals. Hence, the development of novel processes to convert easily available biomass into H2 is of general interest. Herein, we demonstrate a one-pot protocol hydrogen generation from monosaccharides, disaccharide, and extremely demanding cellulose and lignocellulose substrates by using a pincer-type iridium catalyst. Applying ppm amounts of this catalyst, hydrogen is produced at temperatures lower than 120 8C. More specifically, catalyst turnover numbers (TONs) for lignocellulose from bamboo reached up to about 3000. Interestingly, even (used) cigarette filters, which are composed of cellulose acetate, produce hydrogen under optimized conditions.
accumulated catalyst turnovers, after 5 successive additions of glucose, were reached in ionic liquid. Moreover, formic acid was identified as the key intermediate for producing H2 and CO2. Thus, only one mole H2 can be produced per mole of glucose, as the authors mentioned. Therefore, it is of general interest to develop new catalyst systems that work efficiently under mild conditions and allow for higher catalyst turnover numbers (TONs). Moreover, these systems should allow to produce more than one equivalent of H2 per monosaccharide unit. Inspired by the work of Wasserscheid and co-workers and by examples of hydrogen generation from simple[12–14] and polyol alcohols,[15] we describe herein hydrogen generation from biomass in the presence of iridium catalyst[16, 17] at comparatively low temperature (< 120 8C). In fact, different monosaccharides, cellobiose, and more demanding substrates (i.e, cellulose, lignocelluloses) produce hydrogen under the optimized conditions. Furthermore, comparison of the resulting spectra of monosaccharides in the presence or absence of catalyst showed that sodium formate and other carboxylic acids salts are formed as intermediates in the reaction mixture of fructose and glucose. These results suggest the possibility of more hydrogen production as mentioned above. For our initial experiments, the monosaccharide l-( )-fructose was selected as a benchmark substrate. In order to achieve successful catalysis a suitable solvent for both the hydrophilic starting material and the sensitive transition metal catalyst had to be found. After many experiments, we found that l-( )-fructose is exothermically dissolved in diglyme in the presence of 1.3 equiv of KOH at 95 8C. After an initial rise of the temperature to 127 8C for a few seconds, a clear solution is formed. To our delight, addition of the commercially available non-innocent pincer-type catalyst Ru-MACHO [catalyst 1, (RuHCl(PNPPh)CO][18, 19] to this solution already resulted in the generation of significant amounts of hydrogen. The catalyst activity reached turnover frequencies (TOFs) of 771 h 1 (1 h) and 567 h 1 (2 h) (Table 1, entry 1). CO and CH4 contents of about 120 ppm and 40 ppm, respectively, were detected. The concomitant formation of CO2 proved C C bond cleavage of the monosaccharide, too. Acid hydrolysis of the reaction mixture proved that most carbon dioxide was trapped in solution as (bi)carbonate. Noteworthy, a blank reaction without any catalyst in the presence of base produced some carbon dioxide but no hydrogen (see Supporting Information).[20] Exchange of the Cl anion in catalyst 1 by BH4 (catalyst 2, Table 1, entry 2) resulted in similar catalyst activity. On the other hand, variation of the phosphorus substituents in cata-
Hydrogen plays a key role for future clean energy as well as for the industrial production of bulk and fine chemicals.[1] In 2011, the worldwide production of hydrogen exceeded 31 million metric tons.[2] Today, 96 % of this hydrogen comes from fossil resources through coal gasification and gas reforming processes.[2] Thus, “green” hydrogen from the combination of renewable energy and sustainable resources, such as water and biomass, is highly desirable. However, until now largescale hydrogen production from water remains challenging.[3, 4] Consequently, the development of efficient technologies to produce H2 from biomass is an attractive prospect. Although important contributions on hydrogen generation from biomass have been made by means of enzyme fermentation,[5, 6] gasification (about 800 8C to 1300 8C),[7] reforming in supercritical water (about 380 8C, 230 bar), and aqueous-phase reforming (about 220 8C, 20 bar) by heterogeneous catalysts,[8, 9] the utilization of lignocellulosic biomass is still called “a chewy problem”.[10] While the former technology is limited by the cost of enzyme, downstream processing cannot be avoided applying heterogeneous catalysts under harsh conditions, as the direct utilization of the produced hydrogen is limited by the large amount of methane present in the final gas mixture. Notably, Wasserscheid and co-workers reported seminal work on hydrogen generation accompanied by CO2 production from glucose and cellulose in ionic liquids at 150–180 8C, catalyzed by [(p-cymene)RuCl2]2/TMEDA.[11] However, less than 200 [a] Prof. Dr. Y. Li, P. Sponholz, Dr. M. Nielsen, Dr. H. Junge, Prof. Dr. M. Beller Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert Einstein Str. 29a, 18059 Rostock (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403099.
ChemSusChem 0000, 00, 0 – 0
These are not the final page numbers! ÞÞ
1
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
Communications Table 1. Hydrogen production from fructose, glucose, and cellobiose by using a variety of molecularly defined catalysts.
Entry[a] Catalyst Loading Base [mmol, ppm] (equiv)
V(H2) (1 h) V(CO2)[b] (1 h) TOF (1 h) V(H2) (2 h) V(CO2)[b] (2 h) TOF (2 h) [mL] [mL] [h 1] [mL] [mL] [h 1])
1 2 3[c] 4 5 6[c] 7 8 9[c] 10[c] 11[c,d] 12[c,e]
90.3 95.6 164.5 124.0 42.1 201.7 135.2 183.4 293.8 165.5 98.4 125.1
1 2 3 4 5 6 3 6 6 6 6 6
4.76, 50.6 4.84, 51.5 4.67, 49.7 4.75, 50.5 4.79, 51.0 4.51, 48.0 4.35, 46.3 4.73, 50.3 4.64, 49.4 0.97, 10.3 0.97, 10.3 0.50, 10.5
KOH (1.3) KOH (1.3) KOH (1.3) KOH (1.3) KOH (1.3) KOH (1.3) NaOH (1.3) NaOH (1.3) NaOH (1.5) NaOH (1.5) NaOH (1.5) NaOH (1.5)
44.7 33.4 87.5 54.0 13.9 112.8 29.3 61.1 13.3 < 0.1 0.35 86.4
771 802 1432 1060 356 1817 1263 1577 2572 6923 4116 10 269
132.9 134.9 248.2 202.3 64.4 211.5 208.2 243.1 341.3 265.8 207.0 133.0
103.1 88.1 157.3 119.7 42.6 182.5 78.8 94.9 54.2 4.5 4.3 92.0
567 566 1053 865 273 952 972 1045 1494 5559 4329 5459
[a] If not otherwise mentioned, reactions were performed with l-( )-fructose (94.03 mmol) in diglyme (10 mL) at 95 8C. Volumes were measured by gas burette after removal of blank volumes. Volumes of H2 and CO2 were based on the detected ratios of H2 and CO2, TOFs were calculated with respect to volumes (mL) of H2. [b] V(CO2) is the CO2 produced in the gas phase. [c] Reported as an average of 2 reactions and with an error margin of 8 %. [d] d-(+)-Glucose was used as substrate. [e] d-(+)-Cellobiose was used as substrate and the reaction was performed on 47.02 mmol scale (based on the monosaccharide) at 120 8C.
lyst 1 from phenyl to iso-propyl (catalyst 3, entry 3) and cyclohexyl (catalyst 4, entry 4) increased the hydrogen generation. Especially using catalyst 3 the TOF reached 1053 h 1 after 2 h. A lower activity was observed when using Milstein’s catalyst precursor (5) (entry 5).[21, 22] Surprisingly, an iridium-based catalyst (6) proved to be most active (entry 6), although it previously showed a low efficiency in hydrogen generation from iPrOH.[14] The addition of base is critical for hydrogen generation from alcohols under milder conditions.[12–15] Hence, reactions were performed in the presence of NaOH and LiOH. However, in the latter case the base was not completely dissolved. When comparing catalysts 3 and 6 in the presence of KOH and NaOH, 3 showed a slightly better reactivity using KOH (Table 1, entries 3 and 7), whereas catalyst 6 displayed a better reactivity using NaOH (entries 6 and 8). Furthermore, the reaction efficiency was investigated using both catalysts with an increased amount of base (Supporting Information, Table S1, entries 1–3; Table 1, entry 9). Using 1.5 equiv of NaOH, catalyst 6 showed distinctly higher reactivity (entry 9). However, a higher concentration of NaOH (1.7 equiv) sharply decreased the reactivity, which might be caused by the suspension produced in the reaction mixture (Table S1, entry 4). In addition to diglyme, fructose can be dissolved under basic conditions in other dipolar aprotic solvents
&
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
such as 1-methyl-2-pyrrolidinone (NMP) and dimethyl sulfoxide (DMSO) (Table S1, entries 5 and 6). Although the reaction showed improved activity at the initial stage when using NMP, it almost stopped after 1h (Table S1, entry 5). When the catalyst loading was decreased from 49.9 ppm to 9.8 ppm, an excellent TON of 7422 was obtained after 1 h, however, after that time the catalyst system was deactivated (Table S1, entry 7). On the other hand, using diglyme as solvent, the catalyst system was more stable and a TOF of 5559 h 1 was reached after 2 h (the TON reached 11106 after 3 h and 11 371 after 6 h). Here, a 2.0 % conversion (based on 6 mol of H2 produced per mol of monosaccharide) was obtained (Figure 1, Table 2). With d-(+)-glucose as substrate, a similar activity was observed (Table 1, entry 11, TOF after 1 h: 4116 h 1; after 2 h: 4329 h 1) and the total TON reached 10 863 after 3 h and
Figure 1. Hydrogen production from fructose using Ir-catalyst 6.
12 177 after 6 h with 2.1 % conversion (see Supporting Information, Figure S1). Here, the content of CO was about 170 ppm and no signal of CH4 was observed. The ratios of produced H2 to CO2 were 1:1.37 for l-( )-fructose (6 h reaction) and 1:1.29 for d-(+)-glucose (6 h reaction). The calculated amounts of carbon dioxide resulted from measurements both in gas phase and solution. They were corrected by the blank volume obtained under basic conditions without any transition-metal catalyst. The increased formation of carbon dioxide compared to the blank value demonstrates reforming of the monosaccharides to some extent. 2
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communications Inspired by reactions of the monosaccharides, subsequently d-(+)-cellobiose was investigated as an example of a disaccharide. It showed good efficiency in diglyme (Table 1, entry 12) and the ratio of produced H2/CO2 was 1:1.89. Similar to the glucose reaction, a CO content of about 220 ppm was detected and no signal for CH4 was observed. After that, we switched our attention to cellulose as the ultimate target of biomass conversion. Unfortunately, using cellulose in NMP even at 150 8C, a very low TON was observed due to the low solubility of the substrate (Table 3, entry 1). Apparently, hydrolysis of cellulose to glucose and/or cellobiose is required for more efficient hydrogen generation. Most of the known hydrolyses of cellulose are performed under acid conditions in diluted ionic liquids (about 12.5 g ionic liquid per 1 g of cellulose)[23, 24] or at
Table 2. H2 generation from l-( )-fructose (94.03 mmol) in diglyme (10 mL) with 10.7 ppm of catalyst 6: TON for 6 h was shown. Reaction conditions: Volumes were measured by gas burette and the blank volume was removed. t [h]
V [mL]
V(H2) [mL]
V(CO2)[a] [mL]
TON
TOF [h 1]
1 2 3 4 5 6
136.0 262.0 294.0 308.0 318.0 325.0
163.0 254.8 257.7 281.7 281.9 282.3
< 0.1 7.2 18.3 26.3 36.1 42.7
5666 10264 11106 11347 11355 11371
6566 5132 3702 2837 2271 1895
[a] V(CO2) is the produced CO2 in the gas phase.
In order to imTable 3. Hydrogen production from cellulose. prove the hydrogen productivity, further experiments were performed using the benchmark system l-( )-frucEntry[a] Loading T(1, 2, 3) V(H2) (1 h) V(CO2)[b] (1 h) TON (1 h) V(H2) (2 h) V(CO2)[b] (2 h) TON Yield [%][h] Yield [%][h] [mmol, ppm] [8C] [mL] [mL] [mL] [mL] (2 h) (glucose) (TRS) tose. Unfortunately, variation of the 1[c] 11.90, 506.4 –, –, 150 15.0 – 51.2 27.0 – 92.2 – – 0.62, 20.6 140, 140, 140 76.6 0.4 5049 82.6 0.4 5446 < 2.8 5.0 2[d] ligand concentra0.59, 19.6 140, 140, 140 88.7 9.1 6125 95.4 9.9 6588 < 2.8 5.6 3[e] tion or adding extra 0.64, 21.2 140, 140, 140 86.7 5.6 5547 96.9 5.2 6199 < 2.8 6.0 4[e,f] amounts of catalyst 0.71, 23.7 140, 95, 95 16.5 0.3 944 27.0 0.3 1546 < 2.8 5.6 5[e] and base did not 6 0.58, 19.3 140, 140, 95 37.8 0.7 2652 59.2 0.3 4154 < 2.8 5.6 7 0.60, 20.0 140, 140, 120 87.2 0.8 5927 100.0 1.0 6798 < 2.8 5.6 induce obvious gas 8 0.64, 21.2 140, 120, 120 86.0 3.0 5502 94.7 4.3 6059 < 2.8 5.6 generation. When 9[g] 0.62, 20.6 95, 120, 120 64.2 0.8 4232 69.4 1.1 4575 < 2.8 6.2 increasing the cata0.61, 20.3 120, 120, 120 82.2 0.3 5502 93.9 0.4 6285 < 2.8 3.5 10[e] lyst loading to [a] After heating the reaction mixture of 30.00 mmol of cellulose in NMP (24 mL), H2O (2 mL), and HCl (37 %, 4 mL) for 2.5 h at about 500 ppm, the temperature 1 (8C), the reaction temperature was adjusted to 95 8C and NaOH solution (1.5 equiv, except for NaOH solution H2 conversion was used for the neutralization) was added. After that the temperature was adjusted to temperature 2 (8C) and the reaction mixture increased to 2.6 % was stirred for 1 h, and the temperature was adjusted to temperature 3 (8C) and catalyst was added. Volumes (mL) were measured by gas burette and blank volumes were removed, volumes of H2 and CO2 were based on the detected ratios of H2 and (Supporting InforCO2, TONs were based on volumes of H2. [b] V(CO2) is the produced CO2 in gas phase. [c] The reaction was performed on mation, Table S2, 23.50 mmol scale in NMP (20 mL) in the presence of 1.5 equiv of NaOH. [d] Hydrolysis was performed in 1 h. [e] Reported on entry 1). In order to the average of 2 reactions with an error margin of 6 %. [f] Hydrolysis was performed in 4 h. [g] Hydrolysis was performed in suppress catalyst 24 h. [h] The reactions were performed on 7.5 mmol scale with the same hydrolysis procedure for detection of hydrolysis yields of glucose and total reducing sugar (TRS) (see Supporting Information). deactivation, experiments under diluted conditions much higher temperatures (> 260 8C, 5.7 MPa).[25] Based on the were also performed (0.565 g mL 1 versus 1.694 g mL 1) (entry 2), but no major influence was observed. As expected, original screening of reaction conditions, it was clear that increasing the reaction temperature to 140 8C improved the H2 a too-diluted concentration of sugars results in low reactivity. In addition, the stability of the sensitive organometallic comconversion slightly to 3.4 % (entry 4). On the other hand, variaplex is limited. With these considerations in mind, a systematic tion of the amount of fructose did not show any pronounced study to combine hydrolysis of cellulose and the subsequent effects (entries 4 and 5). hydrogen production was performed. In the initial experiTo gain some insight into the hydrogen production from ments, the hydrolysis step was performed in 1-butyl-3-methylimonosaccharides, reaction mixtures of fructose and glucose midazolium chloride (0.67 mmol mL 1) and water in the preswith and without catalyst were studied by NMR spectroscopy after 6 h of stirring at 95 8C, respectively. Comparison of the reence of hydrochloric acid (HCl). After neutralization, TONs of sulting spectra showed that sodium formate and other carbox533 (1 h) and 593 (2 h) at 140 8C were achieved in the presence ylic acids salts are formed as intermediates before addition of of catalyst 6. Unfortunately, increasing the cellulose concentrathe catalyst. Furthermore, these reaction mixtures showed tion resulted in reproducibility problems because a gelatinous almost the same intermediates. However, it was very difficult mixture formed. Comparatively, significantly improved results to identify each of the individual intermediates at this stage. could be obtained by using an aqueous mixture of NMP. Thus, ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
These are not the final page numbers! ÞÞ
3
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
Communications hydrolysis of 30 mmol of cellulose at 140 8C by hydrochloric acid (1 h), followed by neutralization with NaOH solution and addition of catalyst 6 led to 82.6 mL of hydrogen and TONs of 5049 after 1 h and 5446 after 2 h, respectively (Table 3, entry 2). Prolonging the hydrolysis time to 2.5 h resulted in even higher hydrogen volume (95.4 mL) and TONs [6125 (1 h), 6588 (2 h), Table 3, entry 3]. The effect of the water concentration on the hydrolysis step was investigated as well. Both, increasing and decreasing the amount of water resulted in lower reactivity (Supporting Information, Table S3, entries 1 and 2). This is probably explained by the lower catalyst stability and the reduced solubility of base in the presence of decreasing amount of water. Notably, the iridium-based pincer complex showed distinctly higher efficiency compared to the more common Ru-MACHO (catalyst 1) and RuHCl(PNPiPr)CO (catalyst 3) (Table S3, entries 3 and 4). Temperature variation of the different reaction steps showed that fast hydrogen generation from cellulose is possible even at 120 8C. Experiments revealed that hydrolysis (Table 3, entries 8, 9 vs. 10) and the hydrogen generation step (Table 4, 6 vs. 7) performed best at this temperature. When the reaction temperature of the hydrolysis or temperature 2 was reduced to 95 8C, a longer time was needed (Table 3, entry 9) or much lower catalyst TON is observed (Table 3, entry 5). Furthermore, the influence of the amount of base was studied. Similar to the result of fructose, an increased amount of NaOH (1.8 equiv) induced lower activity (Table S3, entry 5). Reducing the base concentration, the TON (2 h) remained constant, though an increased volume of CO2 was released from the reaction mixture (Table S3, entry 6). Due to the improved hydrogen generation in diglyme for fructose, a solvent switch to diglyme was investigated. However, the reaction activity decreased to a major extent (Table S3, entry 7). This can be explained by the favorable effect of NMP on the hydrolysis step.
To get a better understanding of the hydrogen production from cellulose, the yields of glucose and reducing sugars after hydrolysis were detected. To our surprise, only small amounts were present (Table 3, see yields of glucose and total reducing sugar content after hydrolysis). Furthermore, we compared the reactivities of cellulose, glucose, and cellobiose under similar conditions. Using glucose and cellobiose as substrates, the reactions were performed without the hydrolysis procedure but in the presence/absence of NaCl (Supporting Information, Table S4). While glucose showed the same level of reactivity as cellulose, cellobiose showed a slightly higher reaction efficiency. Moreover, the produced NaCl during the neutralization did not considerably decrease the reaction activity (Table S4, entries 2 and 4). These results suggest that considerable amounts of oligomers are formed in the initial hydrolysis step of the polysaccharide. These more soluble oligomers displayed similar reactivity compared to glucose and/or cellobiose. Finally, we applied this one-pot protocol for hydrogen generation from cellulose to lignocellulose, which constitutes one of the most abundant renewable feedstocks on earth. Two kinds of lignocellulose, from pulp and second-year bamboo, were investigated. To our delight, both feedstocks showed good activity (Table 4, entries 2 and 3). More specifically, the catalyst TON of lignocellulose from pulp reached 2188 after 4 h and 2441 after 7 h. Similarly, the TON of lignocellulose from bamboo reached 2266 (4 h) and 2844 (8 h). These experiments show that molecular-defined catalysts allow for hydrogen production even from more challenging and relevant carbohydrates. Interestingly, a comparable activity was observed when using cellulose acetate fibers. This polymer is produced on > 800 000 ton-scale in 2008 and mainly used in the production of cigarette filter tow.[26] As shown in Table 4 using plain cellulose acetate, commercial cigarette filters and even used cigarette filters allowed for hydrogen generation under optimized
Table 4. Hydrogen production from cellulose, lignocelluloses, cellulose acetate, and cigarette filters.[a]
Entry
Substrate
Amount [mmol], [ppm]
V(H2) (1 h) [ mL]
V(CO2)[b] (1 h) [mL]
TON (1 h)
V(H2) (2 h) [mL]
V(CO2)[b] (2 h) [mL]
TON (2 h)
V(H2)/V(CO2)[c]
1 2[d] 3[e] 4 5[f] 6[f]
cellulose lignocellulose (from pulp) lignocellulose (from bamboo) cellulose acetate commercial cigarette filters used cigarette filters
0.61, 20.3 0.63, 20.9 0.63, 20.9 0.61, 20.3 0.61, 40.5 1.25, 83.5
82.2 25.6 26.1 44.5 33.9 14.9
0.3 0.6 1.0 < 0.1 0.1 < 0.1
5502 1662 1695 2978 2269 484
93.9 33.7 34.9 52.3 38.9 19.4
0.4 0.5 0.5 < 0.1 0.1 < 0.1
6285 2188 2266 3500 2604 630
1.00:1.09 1.00:1.23 1.00:1.29 1.00:0.81 1.00:0.77 1.00:1.35
[a] All the reactions were performed on the optimized reaction conditions on 30.00 mmol scale (based on the monomer). Calculated concerning to volumes (mL) were measured by gas burette and blank volumes were removed, volumes of H2 and CO2 were based on the detected ratios of H2 and CO2, TONs were based on volumes of H2, all the experiments were reported on the average of 2 reactions with an error margin of 8 %. The pictures of special feedstocks were shown. [b] Produced CO2 in gas phase. [c] Volumes of CO2 are the produced CO2 in whole reactions after remove of produced CO2 in blank reactions. [d] The contents of lignocellulose from the pulp are identified as: H2O, 7.6 %, cellulose, 64.8 %, hemicellulose, 1.9 %, lignin, 0.9 %, ash, 24.0 %. 30.00 mmol scale is based on the content of cellulose and hemicellulose, 32 mL of NMP was used. Reported TONs are after 2 h and 4 h, TON can reach 2441 after 7 h. [e] The contents of lignocellulose from the second year bamboo are identified as: H2O, 10.5 %, cellulose, 45.0 %, hemicellulose, 20.2 %, lignin, 19.8 %, ash, 0.8 %. 30.00 mmol scale is based on the content of cellulose and hemicellulose, 32 mL of NMP was used. Reported TONs are after 2 h and 4 h, and TON can reach 2844 after 8 h. [f] The reaction was performed on 15 mmol scale.
&
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
4
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communications Acknowledgements
conditions (TONs of 3500 (2 h), 2604 (2 h), and 630 (2 h), respectively). The content of CO for reactions of cellulose was detected as about 410 ppm. However, using lignocellulose from pulp and bamboo, or cigarette filters, a higher content of CO was detected (0.1 %, 0.4 % and 0.3 %, respectively). It should be noted that in the gas phase of these reactions, no CH4 was detected. In summary, we describe a procedure for hydrogen generation from biomass using molecular-defined pincer-type catalysts. Monosaccharides, cellobiose, as well as demanding cellulose and lignocelluloses gave hydrogen using only ppm amounts of the non-innocent IrH2Cl(PNPiPr) complex. Stoichiometric amounts of NaOH used in our protocol promote high catalyst turnover numbers and trap the produced CO2 as (bi)carbonate. This novel process provides another opportunity for potential application of lignocellulosic biomass besides wellknown enzymatic fermentation processes and heterogeneous catalysis procedures. Notably, hydrogen generation takes place under comparatively mild reaction conditions and the reaction system is stable with regard to water, NaCl, and ash from pulp. The detection of small amounts of glucose and reducing sugars after hydrolysis of polysaccharides suggest that hydrogen generation proceeds mainly from the corresponding oligomers.
Y. L. and M. N. thank the Alexander von Humboldt Foundation for financial support. We thank Dr. Elisabetta Alberico for affording catalysts 3 and 4, Dr. Lei Wang, Institute of Pulp and Paper Technology, Hubei University of Technology, China for affording lignocellulose from the second-year bamboo and Shanghai Boning Engineering Fibers Co. Ltd. China for affording lignocellulose from pulp, Dr. Christine Fischer for analysis of glucose from cellulose hydrolysis, and Dr. Wenhua Wang and Dr. Decai Xiong for helpful discussion. Keywords: biomass · homogeneous catalysis · hydrogen · iridium · renewable energy [1] N. Armaroli, V. Balzani, ChemSusChem 2011, 4, 21 – 36. [2] Hydrogen Production Worldwide and US total Hydrogen Production, US Department of Energy Hydrogen Analysis Resource Center, http:// hydrogen.pnl.gov/cocoon/morf/hydrogen/article/706 2012. [3] F. E. Osterloh, Chem. Soc. Rev. 2013, 42, 2294 – 2320. [4] J. R. Swierk, T. E. Mallouk, Chem. Soc. Rev. 2013, 42, 2357 – 2387. [5] J. Woodward, S. M. Mattingly, M. Danson, D. Hough, N. Ward, M. Adams, Nat. Biotechnol. 1996, 14, 872 – 874. [6] R. R. O. Barros, R. S. Paredes, T. Endo, E. P. S. Bon, S.-H. Lee, Bioresour. Technol. 2013, 136, 288 – 294. [7] R. Toonssen, N. Woudstra, A. H. M. Verkooijen, Int. J. Hydrogen Energy 2008, 33, 4074 – 4082. [8] R. D. Cortright, R. R. Davda, J. A. Dumesic, Nature 2002, 418, 964 – 967. [9] P. Azadi, R. Farnood, Int. J. Hydrogen Energy 2011, 36, 9529 – 9541. [10] K. Sanderson, Nature 2011, 474, S12 – S14. [11] N. Taccardi, D. Assenbaum, M. E. M. Berger, A. Bçsmann, F. Enzenberger, R. Wçlfel, S. Neuendorf, V. Goeke, N. Schçdel, H.-J. Maass, H. Kistenmacher, P. Wasserscheid, Green Chem. 2010, 12, 1150 – 1156. [12] M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85 – 89. [13] R. E. Rodrguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-Quinones, H. Grtzmacher, Nat. Chem. 2013, 5, 342 – 347. [14] M. Nielsen, A. Kammer, D. Cozzula, H. Junge, S. Gladiali, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 9593 – 9597; Angew. Chem. 2011, 123, 9767 – 9771. [15] a) D. Morton, D. J. Cole-Hamilton, J. Chem. Soc. Chem. Commun. 1988, 1154 – 1156; b) Y. Li, M. Nielsen, B. Li, P. H. Dixneuf, H. Junge, M. Beller, Green. Chem. 2015, 17, 193. [16] Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J. Lough, K. Abdur-Rashid, Organometallics 2006, 25, 4113. [17] X. Chen, W. Jia, R. Guo, T. W. Graham, M. A. Gullons, K. Abdur-Rashid, Dalton Trans. 2009, 1407 – 1410. [18] W. Kuriyama, T. Matsumoto, Y. Ino, O. Ogata, PCT Int. Appl. WO/2011/ 048727A1, 2011. [19] M. Bertoli, A. Choualeb, A. J. Lough, B. Moore, D. Spasyuk, D. G. Gusev, Organometallics 2011, 30, 3479 – 3482. [20] A. V. Ellis, M. A. Wilson, J. Org. Chem. 2002, 67, 8469 – 8474. [21] J. Zhang, M. Gandelman, L. J. W. Shimon, H. Rozenberg, D. Milstein, Organometallics 2004, 23, 4026 – 4033. [22] C. Gunanathan, D. Milstein, Science 2013, 341, 1229712. [23] C. Li, Z. K. Zhao, Adv. Synth. Catal. 2007, 349, 1847 – 1850. [24] C.-Z. Liu, F. Wang, A. R. Stiles, C. Guo, Appl. Energy 2012, 92, 406 – 414. [25] C. Wang, F. Zhou, Z. Yang, W. Wang, F. Yu, Y. Wu, R. Chi, Biomass Bioenergy 2012, 42, 143 – 150. [26] J. Puls, S. A. Wilson, D. Hçlter, J. Polym. Environ. 2011, 19, 152 – 165.
Experimental Section Standard procedure for hydrogen production from monosaccharides and cellobiose: Starting material was added to a doublewalled thermostated reaction vessel and treated with vacuo and argon several times to remove air. After that, solvent and base were added in argon atmosphere. The catalyst was added with a small Teflon tube after the reaction mixture became stationary at the specified temperature. The amount of gas generated over time was measured by a manual gas burette (100 mL, 250 mL, and 1000 mL burettes were used). Subsequently, the gas purity was detected by GC analysis. Standard reaction procedure for hydrogen production from cellulose, lignocellulose, cellulose acetate, and cigarette filters: Substrate (30.00 mmol) and NMP (24 mL) were added to a doublewalled thermostated reaction vessel at room temperature. The mixture was treated with vacuo and argon several times to remove air. H2O (2 mL) and HCl (37 %, 4.00 mL) were added. The reaction temperature was increased to temperature 1 and stirred for 2.5 h. Then the reaction temperature was adjusted to 95 8C and NaOH solution (prepared by 61.87 g NaOH and 50 mL distilled H2O, heated by a 100 8C oil bath) (1.5 equiv, except for NaOH solution used for neutralization) was added dropwise. After that, the reaction temperature was adjusted to temperature 2 and stirred for another 1 h. Then the reaction temperature was adjusted to temperature 3, the catalyst was added with a small Teflon tube after the reaction mixture became stationary. The amount of gas generated over time was measured by manual gas burette (100 mL and 250 mL burettes were used). Subsequently, the gas purity was detected by GC analysis.
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
These are not the final page numbers! ÞÞ
Received: October 8, 2014 Revised: December 9, 2014 Published online on && &&, 0000
5
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
COMMUNICATIONS Y. Li, P. Sponholz, M. Nielsen, H. Junge, M. Beller* && – && Iridium-Catalyzed Hydrogen Production from Monosaccharides, Disaccharide, Cellulose, and Lignocellulose
&
ChemSusChem 0000, 00, 0 – 0
Doing the splits: A one-pot procedure is developed to generate hydrogen from biomass using molecular-defined pincer-type catalysts. Monosaccharides, cellobiose, as well as demanding cellulose and lignocelluloses give hydrogen
www.chemsuschem.org
6
using only ppm amounts of the non-innocent iridium-based complex (IrH2Cl(PNPiPr). Hydrogen generation takes place under 120 8C. The reaction system is stable with respect to water, NaCl, and ash from pulp.
0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
