(584b) Elucidating the Acidity-Activity Relation in Sulfated Metal Oxides for the Solvent-Free Tert-Butylation of Phenol

Tert-butylation of phenol is classically conducted via liquid acid catalysts [1,3], inducing economic and environmental concerns [4]. An estimated 450,000 tonnes per year of tert-butyl phenols are manufactured for production of innumerable chemical commodities [1]. Numerous research groups have carried out this reaction using heterogeneous catalysts, yet few have managed to do so under conditions relevant to industrial applications [1-6].

As tert-butylation of phenol is an acid-catalyzed reaction, sulfated metal oxides may be appealing. The preeminent objective of this work was thus to, firstly, characterize and, secondly, determine their efficacy at low temperature and pressure via traditional kinetic study and, uniquely, spectrokinetic analysis. Furthermore, given that the effect of Bronsted (B) and/or Lewis (L) acidities on kinetic behavior remains ambiguous in the literature [1,2], it was deemed necessary to clarify the acidity-activity relation for this system.

At the chosen reaction conditions, the sulfated metal oxide catalysts – tin oxide, titania, and zirconia – exhibited relatively high activity. At full conversion, selectivity was observed to follow similar trends, with a 2:1 ratio of mono-alkylated products, minor amounts of multi-alkylated products, and production of tert-butyl phenol ether at lower reaction time. The congruence of selectivity trends implies that the underlying mechanism is the same for these materials, though SnO₂/SO₄^2- (1.0 M) achieved performance near to that of Amberlyst ® 15 [5].

Pyridine adsorption via DRIFTS revealed significant latent B and L acidity in these catalysts; this was further confirmed via pyridine and 2,6-dimethylpyridine TPD measurements. Analysis of kinetic data alongside TPD measurements revealed that B acidity alone does not account for activity trends. Rather, the relative ratio of L to B acid sites correlates well with reactivity; the underlying reason however remains elusive. In general, this study has demonstrated that controlled modulation of acidity in these materials is necessary to optimize activity.