Theoretical study of pinacol rearrangement using Acid-Catalyzed System and ZSM-5

Abstract

Pinacol rearrangements using acid-catalyzed system, ZSM-5 and metal substituted molecular sieve catalyst have been investigated using density functional theory method at B3LYP/6-31G (d) level of theory. The ZSM-5 catalyst was modeled as 3T and 5T cluster models. The metal substituted molecular sieves represented by Fe (II) and Cu (ll) substituted in zeolite framework were modeled using Fe-3T and Cu-3T cluster models. The results show that activation energies of pinacol rearrangement over Fe-3T and Cu-3T are 29.63 and 37.11 kcal/mol, respectively. As a result, Fe substituted in framework of zeolite presents higher catalytic activity for pinacol rearrangement than Cu substituted in zeolite framework. Moreover, the pinacol rearrangements of propylene glycol conversion to propanal and propanone via transition states TS1, TS2 and TS3 have been investigated using acid-catalyzed system and ZSM-5 in order to compare the product distribution ratio. In the acid-catalyzed system, the four water-addition models I to IV, were obtained. The number of hydration water interacting with the transition states of three concerted pathways has obviously affected the product ratio. Estimated percentages of product composition computed from activation free energies of each reaction models were carried out. The percent ratios of product propanal to propanone based on models I, II, III and IV are [99.88]:[0.12], [99.81]:[0.19], [89.01]:[10.99] and [46.59]:[53.41], respectively while the percent ratios of product propanal to propanone based on 3T and 5T cluster models are [2.36]:[97.64] and [2.56]:[97.43], respectively.