MECHANISTIC STUDIES OF C-C, C-H, AND C-O BONDS TRANSFORMATION FOR SELECTIVE ACID-CATALYZED REACTIONS

Abstract

Selective transformation of C-C, C-H, and C-O bonds catalyzed by solid acids plays an essential role in chemical manufacturing, from upcycling of plastic with oxygenated functional groups to industrial processes of naphtha reforming. Establishing of structure-activity relationships through mechanistic studies, especially on reactions with massively interconnected mechanisms, becomes critical not only for controlling the selectivity, but also for rationally designing catalysts. Despite emerging interest and valuable practical applications, mechanistic studies of such systems have much inadequately been reported in literature.In this Ph.D. dissertation, I report my computational works supplemented by experimental collaborations on investigating the two following systems: i) selective conversion of polyalcohols in multi-layered plastic film on various solid acids; and ii) naphtha reforming on platinum/gamma-alumina modified by surface chlorination, mainly focused on the involvement of acid catalysis at solely acid domain and the acid-metal interface. The former demands elaborate catalyst designs and reaction engineering to selectively converting C-O bonds within a multi-component feedstock, while the later has possessed challenges for decades in determining the underlying fundamentals of the competition among multiple pathways for the C-C and C-H bonds transformation. Chapter 3 summarizes our works on selective conversion of poly(ethylenevinylalcohol) (EVOH) on Brønsted acid sites (BAS) using C5- and C6-polyalcohols as the model compounds. Combining density functional theory (DFT) calculations with supporting experiments, we detailed the complex reaction mechanisms and highlighted the effects of solvents on the catalyst activity and reaction kinetics. These findings guide catalyst designs and reaction engineering to effectively eliminate hydroxyl groups from EVOH, as well as to boost the selectivity of ketone-functionalized products for upcycling purposes. Chapters 4 presents our efforts in addressing a big question in another project: how the chlorine (Cl) promotes acid-related functions of platinum/gamma-alumina for naphtha reforming – an important industrial production of aromatics. We show that the Cl promotes the protonation and β-scission of alkenes on gamma-alumina by dynamically responding to the positively charged transition state (TS), induced by a larger surface dipole at the corresponding TS. Specifically, we proved this alternate mechanism based on surface polarizability, manifested by a transient surface-induced dipoles at the TS and supported by a combined experimental and computational study. We illustrate this concept using acid-catalyzed carbon-carbon bond cleavage of a surface-bound alkoxide on chlorinated gamma-alumina, which is of importance in many industrial processes, such as hydrocarbon cracking and reforming, as well as emerging processes such as biomass and waste polymer conversion. The DFT-calculated deprotonation energy, a typical descriptor for acid strength, and the adsorption energy of pyridine remain largely unchanged upon chlorination; however, the measured reaction rates increase significantly, agreeing with the trend in calculated activation barriers. This result can be explained by stabilization of the positively charged transition states, induced by a transient surface dipole. A linear correlation is observed between the strength of such dipole moments and the reduction of corresponding activation barriers. This linear correlation holds true for the more facile protonation of olefins on the same catalyst. We anticipate that the intrinsic role of surface dipoles can be applied to describe other acid-catalyzed reactions that involve transient charge separation at the transition states. Surface engineering of the distribution of acid sites to modify the surface dipole is likely a valuable approach to alter activity and selectivity in acid catalysis. In summary, the fundamental insights from Chapters 4 shed the light related to active site formulating and surface engineering for developing better catalysts for such important acid-catalyzed reactions. Altogether, my Ph.D. works contribute to comprehending the scientific knowledge of acid-catalyzed reactions with highly complex mechanism. These findings open different avenues to more precisely control the selectivity and rationally design the catalyst for such systems, from modifying the nature of active sites by solvents to tune the species adsorption, to manipulating the transient surface dipoles by surface dopants to alter the kinetics at selectivity-determining steps. On another hand, the fundamental insights also provide valuable implications to improve relevant industrial processes in chemical manufacturing.