Framework Al proximity plays an important role in the catalytic properties of zeolites. In Cu-SSZ-13 zeolites, Cu speciation and reaction kinetics are closely linked to Al proximity for selective catalytic reduction of NOx with NH3 and selective hydrocarbon oxidation. Here, we report a workflow to predict Al distributions and generate zeolite composition phase diagrams. We use Cu-SSZ-13 to show how we bridge microscopic details of exchange Cu cations (monomeric and dimeric) to the macroscopic composition of the zeolites (Si:Al and Cu:Al ratios), and to zeolite synthesis conditions (the use of organic and inorganic structure-directing agents). SSZ-13 has chabazite topology with only one symmetry-unique T-site but various symmetry-unique T-site pairs. We generate Al distributions in SSZ-13 by assigning lattice energies to Al-Al pairs based on Löwenstein’s rule (no Al-O-Al linkage) or lattice energies derived from density functional theory (DFT) calculations related to zeolite synthesis conditions, and then distributing Al using a Monte Carlo procedure. We subsequently predict Cu-SSZ-13 composition phase diagrams based on Al distributions. Previous study has predicted [CuOH]+ and Cu2+ quantities as a function of Si:Al and Cu:Al ratios. Here, we consider the formation of Cu dimers at eight-membered ring (8MR) Al pair sites based on DFT calculations. We predict quantities of [CuOH]+, Cu2+ and Cu dimers and find a significant amount of Cu dimer sites that exist under Löwenstein’s rule over tested Si/Al ratio from 3 to 40. We also find the number of Cu dimer sites maximizes at Si:Al around 10, which qualitatively agrees with reported methanol productivity in Cu-SSZ-13 in partial methane oxidation to methanol reactions, where Cu dimers are proposed to be responsible for the conversion. This workflow can be applied broadly to other zeolites to predict Al distributions and speciation of exchange cations as consequences of Al distributions.

Computational investigation of Al distribution in SSZ-13 zeolites and its influence on Cu speciation

ACS Spring Meeting 2021

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