Heterogeneous catalysis for the liquid-phase alkylation of phenol with tert-butyl alcohol in solvent-free conditions
Creators: name: Zuber, Adam
Publisher: name: No Publisher Supplied
Year: 2023

description: The transition from classical homogeneous catalysis to environmentally friendly, heterogeneous alternatives presents a unique challenge. Among the broad banner of Friedel-Crafts reactions, the tert-butylation of phenol is one of specific interest for industrial applications and has thus received some attention from the academic research community. However, the reaction is classically carried out in the presence of liquid acid catalysts at high temperature and pressure, raising economic and environmental concerns. As this reaction is catalyzed by Brønsted acidity, it is believed that solid superacid metal oxide catalysts with surface-bound sulfate species may be appealing. Sulfated metal oxides have long been reported to exhibit enhanced acidity properties, which in turn affect chemical reactivity. In this study, select sulfated metal oxides – SnO2, TiO2 and ZrO2 – of varying acidic properties were synthesized and utilized for the solvent-free alkylation of phenol with tert-butyl alcohol (TBA). Herein, it was observed that tert-butylation of phenol could be carried out at 120°C with significant yield towards alkylated products (up to approximately 60% of the total carbon balance); the majority of the alkylated product selectivity was accounted for by the mono-alkylated products (2- and 4-tert-butylphenols), which were generated in a 2:1 ratio, respectively. Temperature-programmed desorption (TPD) of the sulfated metal oxides with pyridine and 2,6-dimethylpyridine was used to investigate Brønsted and Lewis acidity, which, in combination with the kinetic data, allowed for correlation of acidity to activity. While the tert-butylation of phenol is oft-reported to be catalyzed by Brønsted acidity, and a small minority have claimed that Lewis acidity may contribute to reactivity, this work demonstrates that the relative ratio of Lewis and Brønsted acidity is the determining factor for the activity of sulfated metal oxides towards alkylated products. Likewise, it was found that Amberlyst ® 15 could be utilized for the tert-butylation of phenol derived from biomass sources, namely glycerol, by way of microbial biosynthesis, thereby highlighting the significance and applicability of this study. It was found that Amberlyst ® 15 could be regenerated simply by washing with ethanol and drying at 110°C; reuse was achieved three times without significant loss of conversion, quite contrary to the sulfated metal oxides. Given the high conversion of the alkylating agent and selectivity towards mono-alkylated products, the materials presented here offer an environmentally-friendly alternative to traditional homogeneous catalysts. Thus, complemented by the fundamental molecular-level insights derived from in situ spectroscopic studies, this work presents a vital contribution to the understanding and advancement of heterogeneous catalysis for applications of industrial value. Furthermore, herein I aim to reconcile and summarize some of the prominent issues surrounding tert-butylation of phenol as a whole (particularly with regards to heterogeneous catalysis), addressing and critiquing some significant gaps and inconsistencies, while presenting a potential perspective for future development of the field.