<p>Co-feeding H<sub>2</sub> at high
pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO)
reactions while maintaining high alkene-to-alkane ratios; however, the
mechanisms and species hydrogenated by H<sub>2</sub> co-feeds to
prevent catalyst deactivation remain unknown. This study uses periodic density
functional theory (DFT) to examine hydrogenation mechanisms of MTO product C<sub>2</sub>–C<sub>4</sub> alkenes, as
well as species related to the deactivation of MTO catalysts such as C<sub>4</sub> and C<sub>6</sub> dienes,
benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show
that dienes and formaldehyde are selectively hydrogenated in both frameworks at
MTO conditions because their hydrogenation transition states proceed via
allylic and oxocarbenium cations which are more stable than alkylcarbenium ions
which mediate alkene hydrogenation. Diene hydrogenation is further stabilized
by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from
butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation
directly leads to selective hydrogenation of dienes; pathways which hydrogenate
dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with
barriers 20 kJ mol<sup>-1</sup> higher than α,δ-hydrogenation and with
barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation
of butadiene also occurring through allylic carbocations. Hydrogenation of
formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol<sup>-1</sup> of diene hydrogenation barriers, indicating
that it may also contribute to increasing catalyst lifetimes by preventing
diene formation. Benzene, in contrast to dienes and formaldehyde, is
hydrogenated with higher barriers than C<sub>2</sub>–C<sub>4</sub> alkenes
despite proceeding via stable benzenium cations because of the thermodynamic
instability of the product which has lost aromaticity. Carbocation stabilities
predict the relative rates of alkene hydrogenation and in some cases shed
insights into the hydrogenation of benzene, dienes, and formaldehyde, but
cation stabilities alone cannot account for the poor hydrogenation of benzene
or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation.
This work suggests that the main mechanisms of catalyst lifetime improvement
with high H<sub>2</sub> co-feeds is
reduction of diene concentrations through both their selective hydrogenation
and hydrogenation of their precursors to prevent formation of deactivating
polyaromatic species.</p>
Methanol-to-Olefin Reaction over MWW and MFI Zeolites: Effect of Pore Structure on Product Distribution and Catalyst Deactivation