news & views lactone monomer. It was found that by using emulsion conditions with a Lewis-acid additive, polymers with molar masses of up to 85 kg mol–1 could be produced. Analysis of the resulting polymer with NMR spectroscopy revealed that, in the absence of Lewis-acid additives, a camphor-like bicyclic structure was formed by the alternating insertion of tiglate and allylic moieties into the growing polymer chain — a process that overcomes the steric constraints that would be associated with the sequential addition of tiglates. Under the optimized conditions that used Lewis acids, however, other monocyclic rings were observed in the backbone of the polymeric product (Fig. 1b). Most notably, perhaps, is that unlike many polymers made from CO2, these materials were found to exhibit glass transitions up to 192 °C, which highlights their potential as engineering plastics. The team went on to show that the copolymers could also be made in a one-pot, two-step process in which — following lactone synthesis and venting of excess butadiene — the lactone intermediate could be polymerized without needing to be purified first. Under these conditions, the final CO2 content of the copolymers was slightly lower than for the stepwise procedure that involved purification (27 compared with 33 mol%). Moreover, the resulting polymers had reduced molar masses and glass-transition temperatures, most likely as a consequence of the
incorporation of other alkene-containing by-products into the polymer chains. Nonetheless, this method was applied to the terpolymerization of CO2 and butadiene with isoprene and 1,3-pentanediene in an attempt to broaden the applicability of this approach to materials synthesis. Incorporating up to 33 mol% (29 wt%) of CO2 into a new polymeric material is an impressive feat, and the copolymerization with butadiene — which can itself be generated from renewable sources — makes this a particularly exciting advance in the area of ‘green’ materials. Although this methodology certainly overcomes the problem of copolymerizing CO2 with an alkene and presents a new method for the production of polymer materials that are derived from CO2, the general applicability of the methodology is unclear at this stage. In addition, more needs to be understood about the mechanical and degradative properties of these materials to discover if they can be applied as replacements for existing ones — and a question remains concerning how stable these materials are in the environment. Does this perhaps offer an opportunity to generate a commodity polymer that can overcome not only the problem of CO2 utilization, but also of persistence of materials in our environment? Time will tell. Some potential versatility in the feedstock has been shown, yet to access a wider range of materials the extension of this methodology to direct copolymerization of
CO2 with other dienes would be desirable. Post-polymerization functionalization of the lactone rings and residual alkenes also offers opportunities to expand the range of properties of these polymers. Finally, despite ATRP (atom-transfer radical polymerization) and nitroxidemediated polymerization methodologies being shown to be unsuccessful for the controlled polymerization of this system, the application of chain-transfer agents — including those capable of mediating RAFT (reversible activation-fragmentation chain transfer) polymerization — may provide opportunities to specifically tailor these polymers and firmly establish them at the forefront of a new generation of ‘green’ materials. ❐ Andrew Dove is in the Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. e-mail: [email protected] References 1. Aresta, M. (ed.) Carbon Dioxide as Chemical Feedstock (Wiley-VCH, 2010). 2. Darensbourg, D. J. & Wilson, S. J. Green Chem. 14, 2665–2671 (2012). 3. Coates, G. W. & Moore, D. R. Angew. Chem. Int. Ed. 43, 6618–6639 (2004). 4. Nakano, R., Ito, S. & Nozaki, K. Nature Chem. 6, 325–331 (2014). 5. Sasaki, Y., Inoue, Y. & Hashimoto, H. J. Chem. Soc. Chem. Commum. 605–606 (1976). 6. Behr, A. & Juszak, K.D. J. Organomet. Chem. 255, 263–268 (1983). 7. Braunstein, P., Matt, D. & Nobel, D. J. Am. Chem. Soc. 110, 3207–3212 (1988). 8. Dinjus, E. & Leitner, W. Appl. Organomet. Chem. 9, 43–50 (1995). 9. Behr, A. & Becker, M. Dalton Trans. 4607–4613 (2006). 10. Haack, V., Dinjus, E. & Pitter, S. Die Angew. Makromol. Chem. 257, 19–22 (1998).
METAL–ORGANIC FRAMEWORKS
Recognizing the unrecognizable
Separating carbon monoxide from chemically similar nitrogen gas is particularly challenging. Now, a flexible porous coordination polymer has been developed that recognizes carbon monoxide over nitrogen, with structural changes in the material leading to its accelerated adsorption.
Krista S. Walton
C
arbon monoxide is an important raw material in the synthesis of a variety of basic chemicals, including methanol and aldehydes, and can also be used in Fischer–Tropsch reactions for the production of liquid fuels. One of the largest sources of CO is the incomplete combustion of hydrocarbons, with more than 100 million tons of carbon monoxide produced annually in the United States alone1. However, most of these sources of CO are gas mixtures containing various amounts of nitrogen, carbon dioxide,
hydrogen, methane or water, complicating their use as a feedstock. The separation of CO from N2 in particular is a notoriously difficult problem due to the similarity of the molecules in terms of molecular weight and physical properties, with few CO-selective adsorbents available2. Materials that do provide high selectivity for CO over N2 typically contain transition metal sites to chemisorb the CO. However, because of the strong CO–metal interactions, the high temperatures needed to desorb the CO and reuse the material offsets
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
the advantage of this high selectivity. Therefore, designing a porous material that simultaneously recognizes CO with reasonable adsorption energies and rejects N2 would be a major advance. Writing in Science, Kitagawa and co-workers describe a novel porous coordination polymer (PCP) with open metal sites, that exhibits unique CO adsorption capabilities3. PCPs (or MOFs) are crystalline materials made from organic ligands and metal salts and exhibit an array of characteristics from high surface 277
news & views
S S L PCP 1
Dehydration
PCP 1 S
Channel constriction
Channel opening CO adsorption
PCP 2
PCP 1• CO
Figure 1 | Crystal structure of PCP 1 shown in the c direction, with large (L) and small (S) channels going into the page. On removal of water from the solvated material, the structure transforms to PCP 2, where openings to the small channels become restricted along the c direction. Adsorption of CO results in breaking of the O-Cu(ii) bonds, and CO coordination at the metal sites leads to opening of the small channels and transformation of the material back to the solvated PCP 1 form.
areas to shape-changing pores4. Examples such as MIL-53 are well known to flex or breathe on exposure to carbon dioxide, but a breathing response to CO is less common5. Recently, Gumma and co-workers6 found that MIL-53(Al) can be tuned to remain in its narrow-pore form, which was found to breathe during adsorption of CO, but the adsorption behaviour for N2 was essentially identical. Rigid materials with open metal sites have also been shown to adsorb appreciable amounts of CO, but again with limited selectivity for CO over N2 (ref. 7). The material PCP 1 (Fig. 1) described by Kitagawa and co-workers3 is unique in this regard. It is formed of 2D sheets of copper paddle-wheel building units that stack, resulting in a material with two types of channel — large (L), with a diameter of 9 Å, and small (S), with a diameter of 4 Å. When axial water is removed from the copper sites in PCP 1, the dried material PCP 2 undergoes a structural change in which the S channel partitions into distinct porous volumes connected by highly constricted windows. On exposure to CO, the adsorbed molecules disrupt this partitioning and open the smaller channel back to its solvated form, PCP 1, allowing even greater adsorption of CO, an effect that the authors term ‘self-accelerating’. Importantly, exposure to N2 under the same conditions does not induce this 278
effect, and the crystal structure remains in the PCP 2 form. Binary adsorption experiments show selectivities for CO over N2 that exceed the capabilities of previously reported materials. From synchrotron powder X-ray diffraction experiments under an atmosphere of CO, a crystal structure for PCP 1•CO was obtained, which was found to contain a Cu(ii)−CO complex. To form this complex, the adsorbed CO disrupts the interaction between the carboxylate oxygen atoms and copper sites in the paddlewheel building units, leading to a global restructuring of PCP 2 (depicted in Fig. 1). This self-accelerating adsorption leads to CO loadings of approximately 1.333 CO per Cu(ii) site. Complexes between Cu(i) and CO are relatively common, but the Cu(ii)−CO complex found in this work has not been previously reported in the Cambridge Structural Database. Density functional theory calculations show that the binding energy for CO on a discrete paddlewheel complex is only 20.4 kJ mol–1, and the estimated isosteric heat of adsorption for CO in PCP 2 was found to be very similar, with a value of 19 kJ mol–1. These CO adsorption energies are surprisingly low for a material with open metal sites; for comparison, an isosteric heat of adsorption of 45 kJ mol–1 has been observed7 for CO binding to MIL-101, a rigid MOF.
Experiments combined with firstprinciples calculations indicate a threestep adsorption mechanism, with PCP 2 first adsorbing CO into the large L channels because the small S channels are inaccessible. When these channels have filled, Cu(ii)−CO bonds form, setting off the structural change that opens the constrictions in S. In the final step, the expansion of those squeezed paths facilitates additional CO adsorption into channel S. When exposed to a gas mixture, PCP 2 adsorbs both CO and N2 into channel L, however, only CO can open channel S. Even after this expansion, the coordinated CO molecules prevent N2 from diffusing into the pore, and this gives PCP 2 a clear preference for CO over N2. A remarkable characteristic of the responsive behaviour exhibited by this material is that such low CO adsorption energies lead to a breaking of Cu(ii)−O coordination bonds and reordering of the structure on formation of the Cu(ii)−CO complex. The high selectivity over N2 is also surprising and counterintuitive, as generally a material that displays modest heats of adsorption for a particular sorbate is unlikely to also exhibit good selectivities for that molecule. On the contrary, PCP 2 is not only selective for CO, but it actually responds to the adsorption with a crystal transformation that further accelerates adsorption of CO. In addition to using PCP 1 for selective adsorption of CO over N2, it would be interesting to evaluate this material for the removal of trace contaminants from air. Given the responsive behaviour of PCP 1 to CO, other strongly associating species such as ammonia may show a similar effect. PCPs for molecular recognition applications have long been a target of coordination chemistry design, and this work provides an important step in this direction. Extending the results to additional architectures and sorbate pairs would represent a significant advance in the field. ❐ Krista S. Walton is at the School of Chemical & Biomolecular Engineering at the Georgia Institute of Technology, 311 Ferst Dr. NW, Atlanta, Georgia, 30332, USA. e-mail: [email protected] References 1. Integrated Science Assessment for Carbon Monoxide Publication No. EPA/600/R-09/019F (Environmental Protection Agency, 2010). 2. Dutta, N. N. & Patil, G. S. Gas Sep. Purif. 9, 277–283 (1995). 3. Sato, H. et al. Science 343, 167–170 (2014). 4. Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. Science 341, 974 (2013). 5. Serre, C. et. al. J. Am. Chem. Soc. 124, 13519–13526 (2002). 6. Mishra, P., Edubilli, S., Prasad Uppara, H., Mandal, B. & Gumma, S. Langmuir 29, 12162−12167 (2013). 7. Pirngruber, G. D. et al. ChemSusChem 5, 762–776 (2012).
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
incorporation of other alkene-containing by-products into the polymer chains. Nonetheless, this method was applied to the terpolymerization of CO2 and butadiene with isoprene and 1,3-pentanediene in an attempt to broaden the applicability of this approach to materials synthesis. Incorporating up to 33 mol% (29 wt%) of CO2 into a new polymeric material is an impressive feat, and the copolymerization with butadiene — which can itself be generated from renewable sources — makes this a particularly exciting advance in the area of ‘green’ materials. Although this methodology certainly overcomes the problem of copolymerizing CO2 with an alkene and presents a new method for the production of polymer materials that are derived from CO2, the general applicability of the methodology is unclear at this stage. In addition, more needs to be understood about the mechanical and degradative properties of these materials to discover if they can be applied as replacements for existing ones — and a question remains concerning how stable these materials are in the environment. Does this perhaps offer an opportunity to generate a commodity polymer that can overcome not only the problem of CO2 utilization, but also of persistence of materials in our environment? Time will tell. Some potential versatility in the feedstock has been shown, yet to access a wider range of materials the extension of this methodology to direct copolymerization of
CO2 with other dienes would be desirable. Post-polymerization functionalization of the lactone rings and residual alkenes also offers opportunities to expand the range of properties of these polymers. Finally, despite ATRP (atom-transfer radical polymerization) and nitroxidemediated polymerization methodologies being shown to be unsuccessful for the controlled polymerization of this system, the application of chain-transfer agents — including those capable of mediating RAFT (reversible activation-fragmentation chain transfer) polymerization — may provide opportunities to specifically tailor these polymers and firmly establish them at the forefront of a new generation of ‘green’ materials. ❐ Andrew Dove is in the Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. e-mail: [email protected] References 1. Aresta, M. (ed.) Carbon Dioxide as Chemical Feedstock (Wiley-VCH, 2010). 2. Darensbourg, D. J. & Wilson, S. J. Green Chem. 14, 2665–2671 (2012). 3. Coates, G. W. & Moore, D. R. Angew. Chem. Int. Ed. 43, 6618–6639 (2004). 4. Nakano, R., Ito, S. & Nozaki, K. Nature Chem. 6, 325–331 (2014). 5. Sasaki, Y., Inoue, Y. & Hashimoto, H. J. Chem. Soc. Chem. Commum. 605–606 (1976). 6. Behr, A. & Juszak, K.D. J. Organomet. Chem. 255, 263–268 (1983). 7. Braunstein, P., Matt, D. & Nobel, D. J. Am. Chem. Soc. 110, 3207–3212 (1988). 8. Dinjus, E. & Leitner, W. Appl. Organomet. Chem. 9, 43–50 (1995). 9. Behr, A. & Becker, M. Dalton Trans. 4607–4613 (2006). 10. Haack, V., Dinjus, E. & Pitter, S. Die Angew. Makromol. Chem. 257, 19–22 (1998).
METAL–ORGANIC FRAMEWORKS
Recognizing the unrecognizable
Separating carbon monoxide from chemically similar nitrogen gas is particularly challenging. Now, a flexible porous coordination polymer has been developed that recognizes carbon monoxide over nitrogen, with structural changes in the material leading to its accelerated adsorption.
Krista S. Walton
C
arbon monoxide is an important raw material in the synthesis of a variety of basic chemicals, including methanol and aldehydes, and can also be used in Fischer–Tropsch reactions for the production of liquid fuels. One of the largest sources of CO is the incomplete combustion of hydrocarbons, with more than 100 million tons of carbon monoxide produced annually in the United States alone1. However, most of these sources of CO are gas mixtures containing various amounts of nitrogen, carbon dioxide,
hydrogen, methane or water, complicating their use as a feedstock. The separation of CO from N2 in particular is a notoriously difficult problem due to the similarity of the molecules in terms of molecular weight and physical properties, with few CO-selective adsorbents available2. Materials that do provide high selectivity for CO over N2 typically contain transition metal sites to chemisorb the CO. However, because of the strong CO–metal interactions, the high temperatures needed to desorb the CO and reuse the material offsets
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
the advantage of this high selectivity. Therefore, designing a porous material that simultaneously recognizes CO with reasonable adsorption energies and rejects N2 would be a major advance. Writing in Science, Kitagawa and co-workers describe a novel porous coordination polymer (PCP) with open metal sites, that exhibits unique CO adsorption capabilities3. PCPs (or MOFs) are crystalline materials made from organic ligands and metal salts and exhibit an array of characteristics from high surface 277
news & views
S S L PCP 1
Dehydration
PCP 1 S
Channel constriction
Channel opening CO adsorption
PCP 2
PCP 1• CO
Figure 1 | Crystal structure of PCP 1 shown in the c direction, with large (L) and small (S) channels going into the page. On removal of water from the solvated material, the structure transforms to PCP 2, where openings to the small channels become restricted along the c direction. Adsorption of CO results in breaking of the O-Cu(ii) bonds, and CO coordination at the metal sites leads to opening of the small channels and transformation of the material back to the solvated PCP 1 form.
areas to shape-changing pores4. Examples such as MIL-53 are well known to flex or breathe on exposure to carbon dioxide, but a breathing response to CO is less common5. Recently, Gumma and co-workers6 found that MIL-53(Al) can be tuned to remain in its narrow-pore form, which was found to breathe during adsorption of CO, but the adsorption behaviour for N2 was essentially identical. Rigid materials with open metal sites have also been shown to adsorb appreciable amounts of CO, but again with limited selectivity for CO over N2 (ref. 7). The material PCP 1 (Fig. 1) described by Kitagawa and co-workers3 is unique in this regard. It is formed of 2D sheets of copper paddle-wheel building units that stack, resulting in a material with two types of channel — large (L), with a diameter of 9 Å, and small (S), with a diameter of 4 Å. When axial water is removed from the copper sites in PCP 1, the dried material PCP 2 undergoes a structural change in which the S channel partitions into distinct porous volumes connected by highly constricted windows. On exposure to CO, the adsorbed molecules disrupt this partitioning and open the smaller channel back to its solvated form, PCP 1, allowing even greater adsorption of CO, an effect that the authors term ‘self-accelerating’. Importantly, exposure to N2 under the same conditions does not induce this 278
effect, and the crystal structure remains in the PCP 2 form. Binary adsorption experiments show selectivities for CO over N2 that exceed the capabilities of previously reported materials. From synchrotron powder X-ray diffraction experiments under an atmosphere of CO, a crystal structure for PCP 1•CO was obtained, which was found to contain a Cu(ii)−CO complex. To form this complex, the adsorbed CO disrupts the interaction between the carboxylate oxygen atoms and copper sites in the paddlewheel building units, leading to a global restructuring of PCP 2 (depicted in Fig. 1). This self-accelerating adsorption leads to CO loadings of approximately 1.333 CO per Cu(ii) site. Complexes between Cu(i) and CO are relatively common, but the Cu(ii)−CO complex found in this work has not been previously reported in the Cambridge Structural Database. Density functional theory calculations show that the binding energy for CO on a discrete paddlewheel complex is only 20.4 kJ mol–1, and the estimated isosteric heat of adsorption for CO in PCP 2 was found to be very similar, with a value of 19 kJ mol–1. These CO adsorption energies are surprisingly low for a material with open metal sites; for comparison, an isosteric heat of adsorption of 45 kJ mol–1 has been observed7 for CO binding to MIL-101, a rigid MOF.
Experiments combined with firstprinciples calculations indicate a threestep adsorption mechanism, with PCP 2 first adsorbing CO into the large L channels because the small S channels are inaccessible. When these channels have filled, Cu(ii)−CO bonds form, setting off the structural change that opens the constrictions in S. In the final step, the expansion of those squeezed paths facilitates additional CO adsorption into channel S. When exposed to a gas mixture, PCP 2 adsorbs both CO and N2 into channel L, however, only CO can open channel S. Even after this expansion, the coordinated CO molecules prevent N2 from diffusing into the pore, and this gives PCP 2 a clear preference for CO over N2. A remarkable characteristic of the responsive behaviour exhibited by this material is that such low CO adsorption energies lead to a breaking of Cu(ii)−O coordination bonds and reordering of the structure on formation of the Cu(ii)−CO complex. The high selectivity over N2 is also surprising and counterintuitive, as generally a material that displays modest heats of adsorption for a particular sorbate is unlikely to also exhibit good selectivities for that molecule. On the contrary, PCP 2 is not only selective for CO, but it actually responds to the adsorption with a crystal transformation that further accelerates adsorption of CO. In addition to using PCP 1 for selective adsorption of CO over N2, it would be interesting to evaluate this material for the removal of trace contaminants from air. Given the responsive behaviour of PCP 1 to CO, other strongly associating species such as ammonia may show a similar effect. PCPs for molecular recognition applications have long been a target of coordination chemistry design, and this work provides an important step in this direction. Extending the results to additional architectures and sorbate pairs would represent a significant advance in the field. ❐ Krista S. Walton is at the School of Chemical & Biomolecular Engineering at the Georgia Institute of Technology, 311 Ferst Dr. NW, Atlanta, Georgia, 30332, USA. e-mail: [email protected] References 1. Integrated Science Assessment for Carbon Monoxide Publication No. EPA/600/R-09/019F (Environmental Protection Agency, 2010). 2. Dutta, N. N. & Patil, G. S. Gas Sep. Purif. 9, 277–283 (1995). 3. Sato, H. et al. Science 343, 167–170 (2014). 4. Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. Science 341, 974 (2013). 5. Serre, C. et. al. J. Am. Chem. Soc. 124, 13519–13526 (2002). 6. Mishra, P., Edubilli, S., Prasad Uppara, H., Mandal, B. & Gumma, S. Langmuir 29, 12162−12167 (2013). 7. Pirngruber, G. D. et al. ChemSusChem 5, 762–776 (2012).
NATURE CHEMISTRY | VOL 6 | APRIL 2014 | www.nature.com/naturechemistry
© 2014 Macmillan Publishers Limited. All rights reserved
