General Acid-Type Catalysis in the Dehydrative Aromatization of Furans to Aromatics in H‑[Al]-BEA, H‑[Fe]-BEA, H‑[Ga]-BEA, and H-[B]-BEA Zeolites

Al, Ga, Fe, and B
metal substituents have been examined for their
ability to change the Brønsted acid strength of BEA zeolite and
inhibit undesired hydrolysis in the production of aromatics from furan,
2-methylfuran, and 2,5-dimethylfuran. We employed electronic structure
calculations to examine this series of furans in H-[Al]-, H-[Fe]-,
H-[Ga]-, and H-[B]-BEA zeolites. These calculations were used to parametrize
a microkinetic model to make direct comparisons to experiments run
with furan and DMF in the weakest and strongest acid zeolites, H-[B]-BEA
and H-[Al]-BEA, respectively. Electronic structure calculations revealed
that the Diels–Alder reaction remains unaffected by changes
to the Brønsted acid strength of the zeolite, whereas the dehydration
and hydrolysis reactions are affected in a fashion reminiscent of
general acid catalysis. Interestingly, despite its significantly lower
acid strength, H-[B]-BEA was experimentally shown to have an activity
similar to that of H-[Al]-BEA for the production of both benzene and p-xylene from furan and 2,5-dimethylfuran, respectively.
Analysis with the microkinetic model revealed that, even with this
weaker heterogeneous acid site, the dehydration reaction is sufficiently
catalyzed, activating the aromatic production pathway. The use of
a weaker, heterogeneous Brønsted-acidic zeolite did not have
a significant effect on the product selectivity, however, indicating
that the same reaction pathways are active with both catalysts.