Various
parameters in the catalytic hydroconversion of triglycerides
(palm oil) were carefully investigated for maximizing the production
of biojet fuel. The results showed that the deoxygenation of triglyceride
via hydrotreatment should be carried out in a separate reactor prior
to the hydrocracking step (i.e., two-step reaction process). Otherwise,
the CO generated during deoxygenation can poison the metal components
in the metal/acid bifunctional catalysts (Pt/zeolites), which can
cause significant imbalance between the metal and acid functions in
hydrocracking. This leads to fast catalyst deactivation via coke formation,
heavy formation of aromatics, and overcracking of hydrocarbons, resulting
in the reduction of final biojet fuel yield. In the two-step process,
the second hydrocracking step mainly determines the final biojet fuel
yield, and thus, a rational design of the hydrocracking catalysts
that can suppress overcracking is essential. The diffusion characteristics
of the multibranched hydrocarbon (e.g., 2,2,4-trimethylpentane) in
the hydrocracking catalysts could be correlated with the yields of
the jet fuel-range C8–C16 hydrocarbons and the iso/n-paraffin ratios. The result indicates that the
facile diffusion of multibranched isomers out of catalysts before
excessive cracking is important for the suppression of the formation
of light hydrocarbons (≤C7). Consequently, Pt supported on
nanocrystalline large-pore BEA zeolite showed the largest biojet fuel
yield with the highest iso-paraffin content. Under
the optimized conditions, 55 wt % of biojet fuel with respect to palm
oil was achieved after final distillation, which satisfied all the
required fuel specifications.
Metal catalysts are generally supported on hard inorganic materials because of their high thermochemical stabilities. Here, we support Pd catalysts on a thermochemically stable but “soft” engineering plastic, polyphenylene sulfide (PPS), for acetylene partial hydrogenation. Near the glass transition temperature (~353 K), the mobile PPS chains cover the entire surface of Pd particles via strong metal-polymer interactions. The Pd-PPS interface enables H2 activation only in the presence of acetylene that has a strong binding affinity to Pd and thus can disturb the Pd-PPS interface. Once acetylene is hydrogenated to weakly binding ethylene, re-adsorption of PPS on the Pd surface repels ethylene before it is further hydrogenated to ethane. The Pd-PPS interaction enables selective partial hydrogenation of acetylene to ethylene even in an ethylene-rich stream and suppresses catalyst deactivation due to coke formation. The results manifest the unique possibility of harnessing dynamic metal-polymer interaction for designing chemoselective and long-lived catalysts.
Designed catalyst poisons can be deliberately added in various reactions for tuning chemoselectivity. In general, the poisons are "transient" selectivity modifiers that are readily leached out during reactions and thus should be continuously fed to maintain the selectivity. In this work, we supported Pd catalysts on a thermochemically stable crosslinked polymer containing diphenyl sulfide linkages, which can simultaneously act as a catalyst support and a "permanent" selectivity modifier. The entire surfaces of the Pd clusters were ligated (or poisoned) by sulfide groups of the polymer support. The sulfide groups capping the Pd surface behaved like a "molecular gate" that enabled exceptionally discriminative adsorption of alkynes over alkenes. H 2 /D 2 isotope exchange revealed that the capped Pd surface alone is inactive for H 2 (or D 2 ) dissociation, but in the presence of coflowing acetylene (alkyne), it becomes active for H 2 dissociation as well as acetylene hydrogenation. The results indicated that acetylene adsorbs on the Pd surface and enables cooperative adsorption of H 2 . In contrast, ethylene (alkene) did not facilitate H 2 −D 2 exchange, and hydrogenation of ethylene was not observed. The results indicated that alkynes can induce decapping of the sulfide groups from the Pd surface, while alkenes with weaker adsorption strength cannot. The discriminative adsorption of alkynes over alkenes led to highly chemoselective hydrogenation of various alkynes to alkenes with minimal overhydrogenation and the conversion of side functional groups. The catalytic functions can be retained over a long reaction period due to the high thermochemical stability of the polymer.
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