Acidity-activity relationships in the solvent-free tert-butylation of phenol over sulfated metal oxides

Abstract

This is from web nothing original Sulfated metal oxides have long been reported to exhibit enhanced acidity properties, which in turn affect reactivity. In this study, sulfated SnO2, TiO2 and ZrO2 of varying acidic properties were synthesized and utilized for the solvent-free alkylation of phenol with tert-butyl alcohol. Herein, it was observed that tert-butylation of phenol could be carried out at 120 ◦C with significant yield towards alkylated products; the majority was accounted for by the mono-alkylated products. Among the catalysts studied, SnO2 with high content of sulfation was found to be the more active. In-situ DRIFTS in the range of 25–500 ◦ C was used to investigate the tem- perature evolution of sulfate species and differentiate between bidentate and tridentate configurations. By combining temperature-programmed desorption, in-situ DRIFTS and catalytic performance measurements a correlation between the ratio of Lewis to Brønsted acids and overall reactivity was observed. 1. Introduction Modern catalysis research has seen tremendous interest from academia and industry for the transition from classical homogeneous catalysis to environmentally-friendly, heterogeneous alternatives. Ho- mogeneous catalysis is often unfavorable, both economically and envi- ronmentally, due to difficulty in downstream separation, high capital costs, containment and management of hazardous acid waste, instru- ment corrosion, and the danger present to plant operators [1]. Amongst variegated alkylation reactions, tert-butylation of phenol is one of particular importance to industry [2–5] and is classically carried out in the presence of a homogeneous acid catalyst such as sulfuric acid, hy- drofluoric acid, phosphoric acid, aluminum chloride, or boron tri- fluoride [1–4,6–8]. Current industrial processes for the alkylation of phenol with tert-butyl alcohol (TBA) require high temperature and pressure (e.g. up to 325 ◦C and 15 atm [9]), incurring significant eco- nomic burden [9–14]. It has been reported that 450,000 tons per year of tert-butyl phenols (TBPs) are manufactured industrially for production of innumerable chemical commodities [2,3,6,15]. The reaction of tert-butyl alcohol with phenol ideally results in the following carbon-alkylated products: 2-tert-butylphenol (2-TBP), 4-tert-butylphenol (4-TBP), 2,4-tert-butyl- phenol (2,4-TBP), 2,6-tert-butylphenol (2,6-TBP), and 2,4,6-tert-butyl- phenol (2,4,6-TBP). 2-TBP is utilized in production of pesticides and fragrances; 4-TBP functions as a flavoring agent and petroleum additive and is further used to manufacture oil field chemicals, plastic and rubber products, paint and coating additives, adhesives and sealants, fra- grances, and phosphate esters. Amongst the di-alkylated products, ul- traviolet absorbers in polyolefins and PVC stabilizers are synthesized from 2,4-TBP, while 2,6-TBP finds uses in antioxidants and pharma- ceutical industry [1–7,15–22]. Traditionally catalyzed by Brønsted acidity, alkylation of phenol with tert-butyl alcohol can occur at either the oxygen of the phenolic hydroxyl group or directly to the aromatic carbon ring. Unpaired elec- trons from the oxygen on phenol stimulate an electron-releasing reso- nance effect, thereby activating the ring for alkylation. Delocalization of these unpaired electrons grants the ring stability such that preference is given to ortho and para substitution; selectivity towards meta substitu- tion is limited at best [8,23]. The presence of the hydroxyl group in phenol kinetically favors the formation of the ortho-substituted product, in spite of steric hindrance effects; the para-substituted product is however thermodynamically favored in the presence of moderately acidic media. Stronger acids and higher temperatures tend to favor the production of di- and tri-alkylated products; weak acids lead to forma- tion of oxygen-alkylated products, i.e. tert-butylphenol ether (TBPE) [16–18,24–26]. Although carbon-alkylation is thermodynamically favored due to the relative stability of resulting ortho- and para-alkylated products, oxygen-alkylation may be given