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Cofeeding high-pressure (16 bar) H2 with methanol (0.005 bar) during methanol-to-hydrocarbons conversion over acidic zeolites with varying topologies (CHA, AEI, FER, and BEA) results in a ∼2× to >15× enhancement in catalyst lifetime compared to He cofeeds, as determined by the cumulative turnovers attained per proton before the final methanol conversion level drops below 15%C. These beneficial effects of prolonged catalyst lifetime are observed without any impact on the carbon backbone of effluent hydrocarbon products characteristic of the particular zeolite topology. The olefins-to-paraffins ratio of C2+ hydrocarbons, however, decreases due to enhanced paraffins production, and the magnitude of this decrement depends on the specific zeolite topology. The observations of marked lifetime improvements and topology-dictated variations in the paraffin make of MTH effluent with H2 cofeeds can be interpreted based on the different proclivities of zeolitic protons confined in varying topological environments for catalyzing hydrogenation of hydrocarbons that are predominantly formed via formaldehyde-based alkylation routes (e.g., 1,3-butadiene) or methanol-based alkylation routes (e.g., ethene and propene). Independent kinetic studies reveal that measured hydrogenation rates per H+ of 1,3-butadiene are at least 1 order of magnitude (∼7× to ∼320×) higher than that of ethene or propene, which provides an explanation for the observed lifetime improvements in MTH with H2 cofeeds. Further, trends in the reactivities of ethene and propene with H2 over the different zeolites help explicate the topology-dependent variations in the paraffin content of the effluent hydrocarbons during MTH with H2 cofeeds.

Catalyst lifetime and product selectivity of methanol-toolefins (MTO) catalysis on isostructural HSSZ-13 and HSAPO-34, possessing Brønsted acid sites of different acid strengths, are examined and interpreted to elucidate the role of acid strength in MTO catalysis with and without high-pressure H 2 co-feeds. MTO catalysis without H 2 cofeeds on HSSZ-13 results in a shorter catalyst lifetime and higher paraffins-toolefins ratio than on HSAPO-34, plausibly due to faster rates of formaldehyde formation and involvement of formaldehyde in subsequent alkylation reactions that transform active chain carriers to inactive polycyclics on Brønsted acid sites of higher acid strength. Higher reactivities of protons of greater acid strength in catalyzing hydrogenation of hydrocarbons and oxygenates result in a higher increment in lifetime, paraffins selectivity, and methane selectivity on HSSZ-13 than on HSAPO-34 when coprocessing high-pressure H 2 during MTO catalysis as affirmed in independent kinetic studies and density functional theory (DFT) calculations. These results provide mechanistic insights into the critical role of acid strength in influencing catalyst lifetime and product selectivity during MTO catalysis and rationalize, from a mechanistic and kinetic vantage point, why it may be advantageous to use aluminosilicates such as HSSZ-13 in bifunctional catalytic formulations to upgrade syngas mixtures or during MTO catalysis with cofeeds of molecular hydrogen.

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Low-silica faujasite (FAU) zeolites (with Si/Al ratio of ca. 1.2–1.8) sustain framework integrity and porosity upon moderate ion exchange (0.01 M NH4NO3 solution for 1 h at ambient temperature), which introduces two kinds of protons, distinctive in reactivity and coordination to the zeolite framework, within supercages (HSUP). Moderate ion exchange limited within supercages transpires while maintaining full occupancy of Na+ cations within associated sodalite cages; this in turn helps stabilize the framework of low-silica H-FAU zeolites. Protons located on site II (H3630) and site III (H3650) within supercages on low-silica FAU zeolites can be classified and enumerated by virtue of infrared spectroscopy, providing an opportunity to compare reactivities of these distinct protons for monomolecular protolytic reactions of propane. Protons on site II exhibit prominently higher reactivity for monomolecular propane dehydrogenation and cracking than protons on site III. Low-silica proton-form FAU zeolites (zeolite X) upon moderate ion exchange possess protons on site III that are unavailable on high-silica FAU zeolites (zeolite Y) and limit ion exchange within supercages, providing unprecedented high-temperature structural and chemical stability (>773 K) and enabling their application as solid-acid catalysts.