(327c) Catalytic Evaluation of Sulfated Metal Oxide Catalysts for the Solvent-Free Tert-Butylation of Phenol

The transition from classical homogeneous catalysis to environmentally friendly, heterogeneous alternatives presents a unique challenge. Tert-butylation of phenol is a characteristic example due to its importance in industrial applications. An estimated 450,000 tonnes per year of tert-butyl phenols (TBP) are manufactured industrially for production of innumerable chemical commodities [1]. 2-TBP is utilized in production of pesticides and fragrances; 4-TBP functions as a flavoring agent and petroleum additive, among other compounds. However, the reaction is classically carried out in the presence of liquid acid catalysts [2], inducing economic and environmental concerns [3].

Numerous research groups have carried out this reaction using heterogeneous catalysts in both vapor and liquid phases and in batch and continuous flow systems, though few have managed to perform tert-butylation of phenol under conditions relevant to industrial applications. As this reaction is catalyzed by Bronsted acidity, it is believed that solid superacid metal oxide catalysts with surface-bound sulfate species may be appealing.

At low pressure and temperature, sulfated metal oxide catalysts as well as exhibited relatively high activity for tert-butylation of phenol. The observed activity for these catalysts follows the trend Amberlyst ® 15 > SnO₂/SO₄^2- > TiO₂/SO₄^2- > ZrO₂/SO₄^2-. At complete conversion, selectivity was observed to follow the same trend: ~60-65% 2-TBP, ~30-35% 4-TBP, 3-5% 2,4-TBP, and minor amounts of 2,6-TBP and 2,4,6-TBP. Production of TBPE, the O-alkylated product, was observed at very short reaction times. Catalytic regeneration and reuse were also evaluated. Remarkably, it was found that Amberlyst ® 15 could be regenerated simply by washing with ethanol and drying at 110°C; reuse was achieved 3 times without 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 catalysts presented here offer an environmentally-friendly alternative to traditional homogeneous catalysts.