UC Berkeley

Light alkane dehydroaromatization on MFI-type zeolites, is an industrially significant process for creating value-added products. In particular, gallium modified H-MFI (Ga/H-MFI) is known to be a highly active and stable catalyst for this reaction, owing to its high dehydrogenation activity, which in turn increases selectivity for aromatization. However, the exact structure of the active gallium species in Ga/H-MFI and the mechanistic details of light alkane dehydrogenation remain a subject of debate. In this work, insights into the nature of the active gallium site in H-MFI are developed and a detailed comparison between alkane cracking and dehydrogenation on Ga/H-MFI and H-MFI is conducted, based on theoretical predictions of C2-C4 conversions and comparison with experimental kinetics. We further extend the application of our strategy towards understanding alkene oligomerization on protons, which is the step following alkane dehydrogenation in Ga/H-MFI.

We begin by highlighting the importance of using theoretical predictions which account for long-range zeolite framework interactions in order to predict accurate adsorption energies and reaction barriers, and for correctly assessing the potential of specific active centers to promote catalytic processes. The inclusion of long-range electrostatic and dispersion interactions is critical for the successful reproduction of experimentally measured adsorption energies and activation barriers. We recall that large clusters, containing at least 150 T atoms, are necessary to adequately capture long-range effects on zeolite-confined adsorption and catalysis and to achieve convergence with respect to cluster size. Long-range electrostatic interactions are observed to have the most significant impact on the geometries and stabilization of ion-pair transition states and adsorbed species in the zeolite. Consequently, mechanical embedding schemes that do not account for the polarization of the QM region by the Madelung potential of the zeolite lattice, are unable to predict intrinsic activation energies, supporting the necessity for an electrostatically embedded approach. The stabilization of transition states relative to reactant complexes by long-range electrostatic interactions is observed to increase with the increasing ionic character of the complex. Such TS stabilization can be especially significant in the case that the reactant complexes preceding the rate-limiting TS cover all bare active sites within the zeolite. On the other hand, dispersive interactions more significantly affect the stabilization of reactants relative to the gas phase and are thus more important for predicting accurate apparent activation energies. Their importance increases for substrates that provide an ideal fit to micropore environment, i.e., when the critical diameter of the substrate and pore have similar dimensions. Dispersion effects must also be included to capture TS shape selectivity trends with respect to different micropore environments or zeolite topologies. We further highlight the importance of long-range framework effects through a case study of the dehydrogenation of light alkanes on Ga/H-MFI, where we report preliminary results demonstrating that the use of a model accounting for both long-range electrostatic and dispersion effects has a very significant impact on predicted reaction barriers of up to about 17.6 kcal/mol. Our results suggest [GaH2]+ sites have been overlooked as active site candidates for light alkane dehydrogenation catalysis in previous studies, whereas, as discussed above, these sites can compete with [GaH]2+ cations for ethane dehydrogenation.

We then use our hybrid QM/MM approach to investigate the relative stability of several reduced Ga cationic species and their activity for the dehydrogenation of light alkanes. This effort has involved an examination of the free energy landscape for all processes and has used energetic span analyses to identify the rate-determining TS for each reaction process. Electronic structure calculations were carried out with a quantum mechanics/molecular mechanics model that uses a range-separated hybrid density functional and accounts for dispersive interactions, together with a high-order basis set. The molecular mechanics portion of these calculations involves parameters chosen to capture experimentally measured heats of adsorption for a wide variety of adsorbates (polar and non-polar) in different zeolites. Our analysis has revealed that univalent and divalent gallium hydrides: [GaH2]+ and [GaH]2+ respectively, are more active for dehydrogenation compared to H+ sites and thermodynamically stable Ga+ sites. Overall, [GaH]2+ is found to be the most active site for light alkane dehydrogenation. The TS for this site provides enthalpic stabilization for C-H cleavage via alkyl and carbenium dehydrogenation pathways. By contrast, the TS for carbenium-like C-H cleavage on Brønsted acid sites is found to be less enthalpically favored due to the limited electronic interactions of this TS with the atoms in the framework and the proton. Activation barriers predicted under reaction conditions for light alkane dehydrogenation are in good agreement with available experimental data which have been measured for alkane to alkene dehydrogenation in H-MFI and Ga/H-MFI. With increasing length of the alkane, we find that constrained TSs can become less favorable due to increasing entropic penalties relative to the gas phase alkanes. We also observe that the alkyl mechanism becomes more favorable with increasing chain length because of the enthalpic favorability of the TS responsible for the second C-H cleavage step leading to light alkene formation.

Next, the mechanism and kinetics of light alkane conversions over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl3 are studied using theory and experiments. C2H6 dehydrogenation is found to be catalyzed by both [GaH]2+ and [GaH2]+ cations at similar turnover frequencies. Rate measurements over Ga/H-MFI containing predominantly [GaH2]+ cations reveal that C2H6 dehydrogenation rates exhibit a Langmuir-Hinshelwood dependence on C2H6 partial pressure at elevated temperatures (> 730 K), consistent with the involvement of chemisorbed [C2H5-GaH]+ species. The reaction kinetics suggest that C2H6 dehydrogenation proceeds via heterolytic C-H cleavage of adsorbed C2H6 by [GaH2]+ cations to form H2 and [C¬2H5-GaH]+ species, which further decompose via β-hydride elimination to form C2H4. C3H8 and C4H10 dehydrogenation and cracking are catalyzed exclusively by [GaH]2+ cations. The observed reaction kinetics are consistent with an alkyl mediated mechanism occurring over [GaH]2+ for C4H10 and C3H8. The mechanism proceeds via facile, heterolytic dissociation of adsorbed C3H8 and C4H10 to form [C3H7-GaH]+-H+ and [C4H9-GaH]+-H+ cation pairs via methyl C-H activated pathways. Dehydrogenation then proceeds via β-hydride elimination respectively forming propene and butene, while cracking proceeds via C-H activated H+ attack. Methylene activation was also considered for butane dehydrogenation and cracking but was found to occur at a significantly lower rate. The apparent and intrinsic activation enthalpies extracted from the measured kinetics are in good agreement with those determined from theoretical analyses of the mechanisms, capturing the experimentally observed dehydrogenation versus cracking behavior.

For the final chapter of our work, we investigate the details of the mechanism by which ethene oligomerization occurs over H-MFI. Theoretical calculations using a hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) Model of the Gibbs free energy landscape, are performed and used together with the Energetic Span Model proposed by Kozuch and Shaikh to identify the Rate-Determining Transition States (RDTS) involved in the formation of propene, butene and pentene, primary products of ethene oligomerization on isolated Brønsted acid sites in H-MFI at 523 K. Nonclassical carbenium ion intermediates with a tertiary carbenium character are found to be relevant for describing C-C coupling and C-C cleavage steps responsible for the formation of these primary product oligomers and their respective branching pathways. For the formation of all alkene oligomers, C-C coupling and C-C cleavage steps consistently emerge as the RDTSs. These RDTSs are followed by the formation of cyclic intermediates that are cleaved by protons to form alkoxides, the desorption of which leads to the formation of the product alkene. We predict the competitive formation of butene and propene in favor of the formation of pentene, consistent with experimentally reported data for ethene oligomerization products at 523 K. Our results are found to be in good agreement with experimentally observed results for ethene oligomerization measured experimentally under conditions of differential conversion.