Abstract
The conversion of styrene in the presence of cumene has been studied on a family of aluminosilicate acid catalysts. This study was part of a project aimed at the removal of styrene monomer traces in polystyrene (PS). The catalytic experiments were performed at 200 ◦C in a closed stirred tank reactor with 1.92mmol styrene, 49.8mmol cumene and 60mL n-dodecane as solvent. The evaluated catalytic materials were faujasite and beta zeolites, and amorphous silica-aluminas with disordered or ordered mesoporosity. Four reaction paths are involved in the styrene conversion. A free-radical chain polymerization in the homogeneous phase is mostly sensitive in the presence of catalysts of low activity. The three other transformation routes are catalyzed by the acid sites of the materials: (1) hydrogen transfer between styrene and cumene to ethylbenzene and -methylstyrene; (2) ring oligomerization of styrene to dimers and trimers; (3) alkylation of cumene by styrene into primary and secondary alkylates. The presence of acid sites is a prerequisite for the styrene transformation to occur on the materials. However, the key factor for the catalytic activity is the presence of a large mesoporous volume favoring the molecular traffic within the solid. Mesoporous silica-aluminas behave most favorably in this sense. As regards the distribution of products, a strong acid strength as well as the increase of the cumene/styrene molar ratio favor the alkylation pathway. There is a dark coloration of the reaction medium when carbocations are trapped within the micropores of zeolites. Among the catalysts investigated, the most attractive materials as regards activity, selectivity and a coloration of the solution are mesoporous silica-aluminas and, more preferably, those featuring an ordered porosity of the MCM-41 type.