Decomposition of styrene dimers on Al2O3–Cr and Al2O3–SiO2–Cr catalysts of acid and redox nature

Abstract

The transformations of styrene dimers at 753 K over catalysts possessing Lewis acid centers as well as weak Bronsted sites of HO > −2.2 (γ-Al2O3); Lewis acid, weak Bronsted sites of −3.5 < HO ≤ −2.2 and redox centers (γ-Al2O3–Cr); strong Bronsted sites of −14 < HO ≤ −3.5 (SiO2–Al2O3(13 %)), strong Bronsted sites of 14 < HO ≤ −3.5 and redox centers (SiO2–Al2O3(13 %)–Cr) have been studied. It was established that the following surface groups are responsible for the catalytic activity in the transformation of styrene dimers. γ-Al2O3: OH group adjacent to Al3+ cation (alkenyl chain protonation) and Al3+ (allylic anion generation); γ-Al2O3–Cr: Bronsted acid centers originating from chromium ions at high oxidation states (alkenyl chain protonation) and Cr3+ redox sites (allylic cation generation); SiO2–Al2O3(13 %)–Cr: Bronsted acid centers of silica–alumina (alkenyl chain and aromatic ring protonation) and Cr3+ redox sites (allylic cation generation). The protonation of styrene dimer at the alkenyl chain leads to β-cleavage to toluene and α-methylstyrene as well as to styrene and ethylbenzene. The allylic species formed react to the different styrene dimer isomers and through diphenylbutadiene intermediate to coke. The use of methylcyclohexane as solvent for the styrene dimers in the presence of chromium containing catalysts results in the aromatization reaction leading to toluene and hydrogen. The presence of the later reactant in the system causes styrene and α-methylstyrene hydrogenation to ethylbenzene and cumene.