We present a detailed mechanism for the conversion of DMF (2,5-dimethylfuran) and ethylene to p-xylene, obtained by gas-phase DFT electronic structure calculations. The conversion consists of Diels-Alder cycloaddition and subsequent dehydration of the cycloadduct, an oxa-norbornene derivative. We present the energetics of both the uncatalyzed and acid-catalyzed (Bronsted and Lewis) reactions. Even though the DMF-ethylene cycloaddition is thermally feasible, we show that Lewis acids can further lower the activation requirements by decreasing the HOMO-LUMO gap of the addends. The catalytic effect may be significant or negligible depending on whether the Diels-Alder reaction proceeds in the normal or the inverse electron-demand direction. We also show that Bronsted acids are extremely effective at catalyzing the dehydration of the oxa-norbornene derivative, which, according to our calculations, cannot proceed uncatalyzed. On the other hand, we conclude that Bronsted acids do not catalyze the cycloaddition. Although strong Lewis acids like Li+ can catalyze the dehydration, our calculations indicate that relatively elevated temperatures would be required as they are not as effective as Bronsted acids. We argue that the specific synthetic route to p-xylene is kinetically limited by the Diels-Alder reaction when Bronsted acids are used and by the dehydration when a Lewis acid is used, with the latter being slower than the former. Finally, we adduce experimental data that corroborate the theoretical predictions: we observe no activity in the absence of a catalyst and a higher turnover frequency to p-xylene in the Bronsted acidic zeolite HY than in the Lewis acidic zeolite NaY.
Bibliographical noteFunding Information:
The work of N.N. and D.G.V. was supported partially by the Department of Energy under Grant Number DE-FG02-05ER25702 . The work of P.T.D., S.C., P.J.D., and R.F.L. was based on work financially supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001004. Calculations were performed with use of the TeraGrid resources provided by the University of Illinois’ National Center for Supercomputing Applications and at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
- Diels-Alder reaction