LEWIS/BRONSTED ACID SYNERGY FOR THE CONVERSION OF FURANS TO AROMATICS IN ZEOLITES

摘要

Diels-Alder cycloaddition between a diene and a dienophile- one of the most celebrated reactions in organic synthesis-and subsequent dehydration of the resulting cycloadduct have recently been proposed in the highly selective conversion of 2,5- dimethylfuran(DMF)and ethylene to p-xylene in the Br?nsted acidic zeolite HY and HBEA(cf.Figure 1). Interest in this synthetic route stems from the fact that p-xylene is a platform chemical in the production of polyethylene terephthalate,an important polymer in consumer products. The proposed process is not only more selective to p-xylene than petroleum-based catalytic reforming ones,but also sustainable.Kinetics studies have revealed that the rate of p-xylene production is independent of the Si:Al ratio,i.e.,of the density of Br?nsted active sites,which raises intriguing questions about the catalytic pathway and rate-limiting step,and the role of Br?nsted acidic zeolites. Using Density-Functional Theory calculations,we show that HY is only beneficial to the dehydration of the cycloadduct,an oxanorbornene derivative,and investigate the alternative of Lewis acid catalysis by considering five alkali- exchanged zeolites Y.Lewis acids are known to accelerate the Diels- Alder by closing the energy gap between the frontier molecular orbitals of the reacting partners,stabilizing the transition state. Although Diels-Alder cycloaddition has received considerable attention by theoretical chemists over the years, it has not been studied in the presence of zeolite catalysts,where confinement and charge transfer from the framework to the Lewis acid centre can play a significant role.Given the impact of zeolites on commercial applications,we present here detailed mechanistic studies of the conversion of DMF and ethylene to p-xylene,analyze how Lewis acid catalysis of the Diels-Alder reaction is affected by charge screening and inductive effects and explore fruitful correlations that can guide the design of catalysts by means of in silico screening.