Hybrid Sulfonic Acid Catalysts Based on Silica-Supported Poly(Styrene Sulfonic Acid) Brush Materials and Their Application in Ester Hydrolysis

Catalytic conversions involving water as a reactant, product, or solvent are of high importance in biomass conversion into fuels and chemicals. In this context, water-tolerant solid acids are highly valued. Polymer-oxide hybrid materials based on nonporous silica-supported sulfonic acid-containing polymer brush materials are proposed here as a new class of potentially water-tolerant solid acid catalyst. Atom transfer radical polymerization (ATRP), using both (i) an established and (ii) a new ATRP initiator that is designed to improve the hydrolytic stability of the catalyst, leads to creation of poly(styrene) brushes on the surface of fumed silica. These brushes are sulfonated to produce an acid catalyst akin to an acidic Merrifield resin, but with enhanced accessibility of the active sites. The catalysts are evaluated in the hydrolysis of ethyl lactate, with the polymer brush materials having the same activity as a homogeneous catalyst, p-toluenesulfonic acid, and being substantially more active than an acidic polymer resin (Amberlyst 15). The heterogeneous nature of the catalyst allows for straightforward catalyst recovery and recycle. The stability of the polymer brush catalysts depends on the nature of the initiator used, with the new alkyl-based initiator, introduced here, giving enhanced stability relative to the standard, ester-containing initiator that is most commonly used in surface-initiated ATRP. The activity of the recycled polymer brush catalysts decreased slightly in each cycle because of both desulfonation and the gradual detachment of the polymer chains from the oxide support. Oxide-supported polymer brush materials are suggested to be a promising new architecture for hybrid catalyst materials.