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.
Cofeeding acetaldehyde (1−4 C%) with dimethyl ether (DME) and methanol (DME:methanol ∼9:1, on a carbon basis) on two MFI-type zeolites: a conventional (Conv) MFI zeolite (SiO 2 /Al 2 O 3 ∼80, diffusion length ∼250 nm) and a self-pillared pentasil (SPP) MFI zeolite (SiO 2 / Al 2 O 3 ∼150, diffusion length ∼1.5 nm) at 673 K resulted in a monotonic increase in selectivity toward ethene (from 9.3 to 15 C% on Conv MFI and from 1.4 to 6.4 C% on SPP MFI) and methylbenzenes (from 4.9 to 7.8 C% on Conv MFI and 2.6 to 5.3 C% on SPP MFI). The mechanistic basis for this increase in ethene and methylbenzene (MB) selectivity is acetaldehyde undergoing multiple aldol-condensation reactions to form higher homologues (e.g., sorbaldehyde) that subsequently undergo ring-closure followed by dehydration to form aromatics (e.g., benzene). Cofeeding acetaldehyde, therefore, increases the concentration of aromatics inside the zeolite pores, which in turn enhances the propagation of the aromatics-based methylation/dealkylation cycle and consequentially results in higher ethene production. In an isotopic experiment where 13 C 2 -acetaldehyde (∼4 C%) was coreacted with unlabeled DME and methanol (DME:methanol ∼9:1, on carbon basis) on Conv MFI and SPP MFI at 673 K, ethene present in the effluent was enriched with two 13 C labels and the net 13 C-content in ethene (11−12% on Conv MFI and 45−52% on SPP MFI) was higher than the 13 C-content in MBs (5−6% on Conv MFI and 9−17% on SPP MFI). Ethene, therefore, besides being formed via aromatic-dealkylation, is also being produced from acetaldehyde or its aldol-condensation products via a direct synthesis route.
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