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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