The synthesis and characterization of isolated Ga3+ cations in Ga/H-MFI and the kinetics of light alkane dehydrogenation and cracking over [GaH]2+ and [GaH2]+ cations
Ga-exchanged H-MFI (Ga/H-MFI) is known to be a highly reactive and selective heterogeneous catalyst for the conversion of light alkanes (ethane, propane and n-butane) into alkenes and aromatics and H2 via dehydrogenation and dehydrocylization reactions. However, the chemical structure of ion-exchanged Ga complexes, their state under reaction conditions and their catalytic role during light alkane conversion, are not well understood. The aim of this dissertation is to study the synthesis of isolated and well-defined Ga species that are ion-exchanged in Ga/H-MFI, to characterize Ga structures formed as a function of the Ga/Al ratio and to examine the site requirements, kinetics and mechanisms of light alkane dehydrogenation and cracking over these catalysts. These findings enable the rational design of zeolite-based dehydrogenation catalysts that contain Ga species that are specifically tailored to be catalytically active for a given alkane reactant.
The synthesis of isolated and well-defined Ga species ion-exchanged in Ga/H-MFI remains a formidable challenge. In addition, there has been considerable debate with regard to the structure of the Ga species that is kinetically relevant during alkane dehydrogenation and dehydrocyclization. In Chapter 2, we examine synthetic protocols for the preparation of Ga/H-MFI via the vapor phase exchange of dehydrated H-MFI with GaCl3. Catalysts with a range of Ga/Al ratios (0.1-0.7) are prepared using this protocol and Ga species in these materials are characterized under oxidizing and reducing conditions using a number of chemical and spectroscopic probes. We find that gas-phase GaCl3 monomers or dimers react with Brønsted acid O-H groups in H-MFI to form monovalent [GaCl2]+ cations and HCl. The treatment of GaCl2-MFI materials with 105 Pa H2 at 823 K leads to the stoichiometric removal of Ga-bound Cl ligands as HCl and the formation of [GaH2]+ cations. These structures upon oxidation in O2 form isolated [Ga(OH)]2+ cations at low Ga/Al ratios (~ 0.1) and isolated [Ga(OH)2]+-H+ cation pairs at Ga/Al ratios higher than 0.1 but less than 0.3, as evidenced by infrared spectroscopy, NH3-TPD, Ga K-edge XANES and EXAFS. Both structures require the presence of proximate cation-exchange sites (either present as next-nearest neighbors (NNN) or next, next-nearest neighbors (NNNN)). At a Ga/Al ratio of 0.3, the available proximate cation-exchanges sites become saturated by Ga3+ species and further addition of Ga to these materials leads to the formation of neutral, condensed GaOx oligomers. Upon exposure of isolated, oxidized Ga3+ structures to H2 at elevated temperatures, H2-TPR, NH3-TPD, infrared spectroscopy, Ga K-edge XANES and EXAFS suggest that [GaH]2+ cations, [Ga(OH)H]+-H+ cation pairs and [GaH2]+ cations form. Theoretically generated thermodynamic phase diagrams suggest that the nature of Ga3+ species formed under oxidizing or reducing conditions is highly sensitive to the framework Al-Al distance between cation-exchange sites, H2 and H2O partial pressures. Under sufficiently anhydrous reducing conditions ( < 10-1 Pa H2O), these calculations together with experimental data suggest that [GaH]2+ cations and [GaH2]+-H+ cation pairs are the only structures that form in Ga/H-MFI with Ga/Al ratios ≤ 0.3.
The synthesis of isolated Ga3+ species that are well-characterized under oxidizing and reducing conditions allows the identification of catalytically relevant Ga species and the mechanisms by which these species catalyze light alkane dehydrogenation. In Chapter 3, we examine the site requirements, the kinetics and the mechanisms for propane (C3H8) dehydrogenation and cracking over Ga/H-MFI catalysts prepared in the manner described in Chapter 2. It is observed that dehydrogenation and cracking rates over Ga/H-MFI are 2 orders and 1 order of magnitude higher than the corresponding reaction rates over H-MFI. Both reactions are catalyzed by [GaH]2+ cations; [GaH2]+ cations are inactive for these reactions. It is also observed that both reactions exhibit a Langmuir-Hinshelwood dependence on C3H8 partial pressure and are inhibited H2 in a manner such that ratios of dehydrogenation to cracking are independent of C3H8 and H2 partial pressures. Experimentally measured activation enthalpies together with theoretical analysis of reaction pathways suggest that both reactions proceed over [GaH]2+ via the heterolytic dissociation of C3H8 by [GaH]2+ to form [C3H7-GaH]+-H+ cation pairs. Dehydrogenation rates are limited by β-hydride elimination within the alkyl fragment to form C3H6 and H2. Cracking rates are limited by the H+ attack of the C-C bond in the alkyl fragment. Both reactions are inhibited in the presence of H2 due to the formation of [GaH2]+-H+ cation pairs.
In Chapter 4, we extend our study of light alkane dehydrogenation over Ga/H-MFI to ethane (C2H6 ) and n-butane (n-C4H10) reactants. We find that C2H6 dehydrogenation rates are catalyzed by both [GaH]2+ and [GaH2]+ cations, consistent with the similar free energy barriers for this reaction over both structures predicted by theoretical calculations. Over [GaH2]+ cations, C2H6 dehydrogenation rates are weakly inhibited by H2 and bear a Langmuirian dependence on C2H6 partial pressure. Measured activation enthalpies together with theoretical calculations are consistent with a mechanism involving the heterolytic C-H activation of C2H6 by [GaH2]+ cations to form [C2H5-GaH]+ cations and H2. A subsequent β-hydride elimination from the C2H5 fragment results in the formation of C2H4. At low C2H6 pressures, dehydrogenation rates are first-order in C2H6 and are limited by initial C-H cleavage. At high C¬2H6 partial pressures, dehydrogenation rates are zero-order in C2H6 partial pressure and are limited by the β-hydride elimination step. On the other hand, C4H10 dehydrogenation, terminal and central cracking reactions are catalyzed only by [GaH]2+ rates (and not [GaH2]+ cations) with turnover frequencies that are 3 orders, 2 orders and 1 order of magnitude higher than the turnover frequencies of the corresponding reactions over H-MFI. Kinetic rate measurements together with theoretical results suggest the mechanism for C4H10 dehydrogenation and cracking over Ga/H-MFI is analogous to the mechanism for C3H8 dehydrogenation. In this case, [GaH]2+ cations activate secondary C-H bonds in n-C4H10 to form [sec-C4H9-GaH]+-H+ cation pairs. These intermediates then either undergo dehydrogenation via β-hydride elimination to form 1-butene, 2-butene and H2 or undergo terminal or central cracking via C-C bond attack by the proximal Brønsted acid O-H group. The transition state for central cracking is more constrained than that for terminal cracking resulting in terminal cracking rates over Ga/H-MFI that are a factor of 3 higher than central cracking rates. [GaH2]+ cations are inactive for the conversion of alkane reactants with carbon chain length greater than 2 due to entropic losses associated with the rate-limiting transition state over these species.