A DFT study of the acid-catalyzed conversion of 2,5-dimethylfuran and ethylene to p-xylene

Meeting Program — November 2012

Nima Nikbin
Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing
Uni­ver­si­ty of Delaware
Newark, DE
Stu­dent Pre­sen­ta­tion

Abstract — In this paper we present the detailed mech­a­nism for the con­ver­sion of DMF and eth­yl­ene to p-xylene. The mech­a­nism was cal­cu­lat­ed by gas-phase DFT (Den­si­ty-Func­tion­al The­o­ry) for the uncat­alyzed, the Brøn­st­ed acid-cat­alyzed and the Lewis acid-cat­alyzed reac­tion. The con­ver­sion con­sists of Diels-Alder cycload­di­tion and sub­se­quent dehy­dra­tion of the cycloadduct, an oxa-nor­bornene deriv­a­tive. Even though the DMF-eth­yl­ene cycload­di­tion is ther­mal­ly fea­si­ble, we find that Lewis acids can fur­ther low­er the acti­va­tion bar­ri­ers 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 find that Brøn­st­ed 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 find that Brøn­st­ed 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 Brøn­st­ed acids. We argue that the spe­cif­ic syn­thet­ic route to p-xylene is kinet­i­cal­ly lim­it­ed by the Diels-Alder reac­tion when Brøn­st­ed 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 Brøn­st­ed acidic zeo­lite HY than in the Lewis acidic zeo­lite NaY.