(111d) 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 reactant, product or solvent are of increased importance in biomass conversion into fuels and chemicals. In this context water tolerant solid acids are highly valued. Polymer-oxide hybrid materials based on non-porous silica-supported sulfonic acid-containing polymer brush materials are proposed here as a new class of potentially water-tolerant solid acid catalysis. Atom transfer radical polymerization (ATRP) using both an established and a new ATRP initiator that is designed to improve the hydrolytic stability of the catalyst, is used to create 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-toluene sulfonic acid, and being substantially more active than an acidic polymer resin (Amberlyst 15). The heterogeneous nature of the catalyst allows for easy 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. The activity of the recycled polymer brush catalysts decreased slightly in each cycle due to 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.