Formation of ethyl benzene and styrene by side chain methylation of toluene over calcined LDHs

Abstract

Mg/Al layered double hydroxide (LDH) with various Mg/Al molar ratios 3, 4, 5, 7 and 10 were prepared, calcined and characterized by XRD, FT-IR, ICPES and BET surface area analysis. These LDHs were used for side chain methylation of toluene with methanol between 300 and 450 °C. For comparison, Cu/Al, Ni/Al, Co/Al and Zn/Al LDHs with molar ratio 3 were also prepared. The selectivity of styrene was higher over Mg/Al with low Al content. The selectivity of ethyl benzene was higher at 400 °C. At higher temperature, the side chain alkylation was favored and the selectivity of nuclear alkylation products such as xylene and mesitylene decreased as Mg/Al ratio changes from 3 to 10. The enhanced activity of calcined LDHs is due to the surface sites of low (OH − groups), medium (Mg–O pairs), and strong (O 2− anions) basicity. The surface concentration of low and medium strength basic sites increased with decreasing Al content. On Mg/Al LDHs, nuclear alkylation increased with increasing Al content. Substitution of Co 2+, Ni 2+, Zn 2+ and Cu 2+ for Mg 2+ favored nuclear alkylation. The selectivity of xylene and mesitylene increased with increase in temperature at a maximum 400 °C. Mg/Al LDHs were the most LDHs active than other LDHs for side chain methylation of toluene.