Metal-organic frameworks (MOFs) are nanoporous materials composed of metal centers connected by organic linkers in a three-dimensional, crystalline network. MOFs are an ideal platform for rational design of selective catalysts because they have a high accessible surface area, high density of metal sites, and a virtually limitless combinatorial design space. The synthetic tunability of MOFs gives them the potential to surpass existing zeolite-based catalysts in industrially important reactions. An example of one such reaction is the propene dimerization reaction, which is catalyzed by nickel-containing porous catalysts and yields a mixture of C6 olefin isomers. The MOF-74 class of materials has been shown experimentally to catalyze the dimerization of propene with higher selectivity for industrially valuable linear olefin products than available zeolite-based catalysts.1
In this work we develop a theoretical framework to explain and predict changes in the product distribution of the propene dimerization reaction that results from the use of different porous materials as catalysts. This product distribution is the result of phenomena across many length and time scales, including thermodynamic, kinetic, and diffusive regimes. We show that experimentally observed changes in the product distribution from the literature can be explained in terms of the contribution of the pores to the free energy of formation of the product isomers (Figure 1). This free energy is directly computed using Monte Carlo simulation, and then related to the probability of formation of each product. The molecular origins of these different formation probabilities are further interpreted using information about favorable adsorption sites, probability distribution of adsorbate configurations, and enthalpic and entropic contributions to free energy.
We use our model to screen a library of 118 existing and hypothetical MOF and zeolite structures to study how product distribution can be tuned by changing pore size, shape, and composition of porous materials (Figure 2). Using these molecular descriptors, we identify catalyst properties that increase the selective reaction of the desired linear olefin isomers. Among these, we note that a pore size which is commensurate with the size of the desired linear products enhances linear conversion by sterically hindering the branched isomers. Another promising feature is the presence of open metal sites, which interact with the olefin π bond to provide favorable binding sites for the linear isomers. We also identify a few not-yet-synthesized materials which we predict to have a higher fraction of linear isomers than the best-performing experimentally studied structures.
Our method builds on the ideas of shape selectivity2 to compute not only the relative free energy of reaction products, but also predict product distribution to good quantitative agreement with results from the literature. This work shows that molecular simulation can provide an atomistic perspective on material features such as pore shape and composition in order to better understand how porous catalysts influence product distribution.
References
[1] Mlinar, A. N.; Keitz, B. K.; Gygi, D.; Bloch, E. D.; Long, J. R.; Bell, A. T. ACS Catal. 2014, 4, 717–721.
[2] Smit, B.; Maesen, T. L. M. Nature 2008, 451, 671–678.
Advances in molecular simulation , Interfacial and confined phenomena , Carbon capture and other industrial applications
