Benedict T. W. Lo

From Biomass‐Derived Furans to Aromatics with Ethanol over Zeolite

We report a novel catalytic conversion of biomass-derived furans and alcohols to aromatics over zeolite catalysts. Aromatics are formed via Diels-Alder cycloaddition with ethylene, which is produced in situ from ethanol dehydration. The use of liquid ethanol instead of gaseous ethylene, as the source of dienophile in this one-pot synthesis, makes the aromatics production much simpler and renewable, circumventing the use of ethylene at high pressure. More importantly, both our experiments and theoretical studies demonstrate that the use of ethanol instead of ethylene, results in significantly higher rates and higher selectivity to aromatics, due to lower activation barriers over the solid acid sites. Synchrotron-diffraction experiments and proton-affinity calculations clearly suggest that a preferred protonation of ethanol over the furan is a key step facilitating the Diels-Alder and dehydration reactions in the acid sites of the zeolite.

Elucidation of Adsorbate Structures and Interactions on Brønsted Acid Sites in H-ZSM-5 by Synchrotron X-ray Powder Diffraction.

Microporous H-ZSM-5 containing one Brønsted acid site per asymmetric unit is deliberately chosen to host pyridine, methanol, and ammonia as guest molecules. By using new-generation in situ synchrotron X-ray powder diffraction combined with Rietveld refinement, the slight but significant alteration in scattering parameters of framework atoms modified by the guest molecules enables the user to elucidate their adsorption geometries and interactions with the Brønsted acid sites in H-ZSM-5 in terms of atomic distances and angles within experimental errors. The conclusion, although demonstrated in the H-ZSM-5, is expected to be transferable to other zeolites. This approach provides a stepping stone towards the rational engineering of molecular interaction(s) with acid sites in zeolitic catalysis.

Decarboxylation of Lactones over Zn/ZSM‐5: Elucidation of the Structure of the Active Site and Molecular Interactions

Herein, we report the catalytic decarboxylation of γ-valerolactone (GVL) over Zn/ZSM-5 to butene, followed by aromatization at high yield with co-feeding of water. An evaluation of the catalytic performance after prolonged periods of time showed that a water molecule is essential to maintain the decarboxylation and aromatization activities and avoid rapid catalyst deactivation. Synchrotron X-ray powder diffraction and Rietveld refinement were then used to elucidate the structures of adsorbed GVL and immobilized Zn species in combination with EXAFS and NMR spectroscopy. A new route for the cooperative hydrolysis of GVL by framework Zn-OH and Brønsted acidic sites to butene and then to aromatic compounds has thus been demonstrated. The structures and fundamental pathways for the nucleophilic attack of terminal Zn-OH sites are comparable to those of Zn-containing enzymes in biological systems.

Entrapped Single Tungstate Site in Zeolite for Cooperative Catalysis of Olefin Metathesis with Brønsted Acid Site

Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Brønsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS-MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and trans-2-butene to propene, under mild reaction conditions, the propene production rate over WO x/USY is ca. 7300 times that over the industrial WO3/SiO2-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO x/USY for a wide range of reaction conditions.