An important and challenging topic in energy research is the conversion of light hydrocarbons from natural gas to more sought after energy carriers.1 Light alkanes, such as ethane, propane, and butane are found in abundance in natural gas and have been largely exploited for their high energy content. Light alkanes can also play a significant role as chemical feedstocks for the production of more complex and useful chemicals; however, due to the high temperatures and pressures needed for their conversion, this route is difficult to exploit.2Catalysts offer an efficient way of reducing the synthetic requirements for these conversions; however, finding an efficient catalyst for selective oxidation under mild conditions remains a significant challenge.
Materials that have shown some promise in this field are comprised of just a few isolated metal centers (e.g., single site catalysts, ligated metal complexes, and the active site in the enzyme methane monooxygenase, which is comprised of a few oxidized Cu or Fe moieties).3,4 The benefit of these catalysts is that they are structurally and electronically isolated from each other, which eliminates site-to-site interactions and thereby allows the system to be directly controlled for the desired reaction.5 Accomplishing such catalysts requires a chemical environment that avoids agglomeration of the metal centers. In this work, we accomplish this by supporting small metal cluster catalysts on metal-organic frameworks (MOFs). MOFs are porous crystalline solids comprised of metal-based nodes and organic linkers. Our experimental collaborators have supported small metal cluster catalysts onto the MOF nodes using atomic layer deposition.6Our objective is to optimize such catalysts for activity and selectivity for ethane oxidation to ethanol.
One metal that has shown promise for use in the selective oxidation of alkanes is copper, whose chemistry is varied due to its ability to access multiple oxidations.7-9 Copper has been shown to selectively oxidize n-hexanes to hexanol and cyclohexane to cyclohexanol at moderate conditions,10 and thus we have used it as a starting point. Specifically, we have mapped out the catalytic pathway for ethane oxidation to ethanol on two Cu clusters, i.e., CuX+(OH)X(X= 1 or 2). We have analyzed both a direct C-H bond activation pathway, where ethane dissociates on the metal site, as well as an oxygen-assisted pathway, where the oxygen site is utilized in the pathway. Our results indicate that the two pathways are competitive.
Our ultimate goal is to design a catalyst that is active and selective for ethane oxidation to ethanol. This is a large endeavor that will likely require a precisely-tuned catalyst composition. To begin to learn the properties that will be needed, we screen various ethane oxidation pathways on numerous gas phase MX+(OH)X(M = Metals from Li-Bi; X= 1 or 2) complexes. We select the most promising compositions and study the MOF-supported analogs with the aim of identifying descriptors of activity and selectivity for ethane oxidation to ethanol. We use the results to make hypotheses about design rules for metal-based cluster catalysts that can be synthesized via ALD in MOFs.
1 Hammond, C.; Conrad, S.; Hermans, I. Oxidative Methane Upgrading. ChemSusChem, 2012, 5, 1668 – 1686.
2 Sun, M.; Zhang, J.; Putaj, P.; Caps, V.; Lefebvre, F.; Pelletier, J.; Basset, J., Catalytic Oxidation of Light Alkanes (C 1−C4) by Heteropoly Compounds. Chem. Rev., 2014, 114, 981−1019.
3 Ma, L.; Pan, Y.; Man, W.; Kwong, H., Lam, W. W. Y., Chen, G., Lau, K., Lau, T. Highly Efficient Alkane Oxidation Catalyzed by [MnV(N)(CN)4]2− Evidence for [MnVII(N)(O)(CN)4]2− as an Active Intermediate. J. Am. Chem. Soc., 2014, 136, 7680−7687.
4 Mishra, G. S.; Kumar, A.; Tavares, T. B. Single site anchored novel Cu(II) catalysts for selective liquid–gas phase O2 oxidation of n-alkanes. 357 (2012) 125–132 . J. Mol. Catal. A: Chem., 2012, 357, 125-132.
5 Thomas, J.M. and Raja, R. The advantages and future potential of single-site heterogeneous Catalysts. Top. Catal., 2006. 40, 3-17.
6 Mondloch, J. E.; Bury, W.; Fairen-Jimenex, D.; Kwon, S.; DeMarco, E. J.; Weston, M. H.; Sarjeant, A. A.; Nguyen, S. T.; Stair, P. C.; Snurr, R. Q.; Farha, O. K.; Hupp, J. T., Vapor-Phase Metalation by Atomic Layer Deposition in a Metal–Organic Framework. J. Am. Chem. Soc., 2013, 135, 10294-10297.
7 Punniyamurthy, T.; Rout, L. Recent advances in copper-catalyzed oxidation of organic compounds. Coordin. Chem. Rev., 2008, 252, 134-154.
8 Allen, S. E.; Walvoord, R. R.; Padilla-Salinas, R.; Kozlowski, M. C. Aerobic Copper-Catalyzed Organic Reactions. Chem. Rev.,2013, 113, 6234-6458.
9 Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B; Cundari, T. R.; Petersen, J. L., Single-electron Oxidation of Monomeric Copper(I) Alkyl Complexes: Evidence for Reductive elimination through Bimolecular Formation of Alkanes. Organometallics, 2006, 25, 4097-4104.
10 Conde, A.;Vilella, L.; Balcells, D.; Díaz-Requejo, M. M.; Lledós, A.;Pérez, P. J., Introducing Copper as Catalyst for Oxidative Alkane Dehydrogenation. J. Am. Chem. Soc. 2013, 135, 3887-3896.