Liquid-phase alkylation of toluene with long-chain alkenes over HFAU and HBEA zeolites Original Research Article

The alkylation of toluene with 1-heptene and 1-dodecene were used as model reactions for the synthesis of long-chain linear alkylbenzenes, the precursors of biodegradable surfactants. The effect of the pore structure of large pore zeolites: HFAU (framework Si/Al=15) and HBEA (framework Si/Al=12.5) on their catalytic properties was determined in liquid phase at 90°C with a toluene/alkene molar ratio of 3. With these large pore zeolites, the main reactions are alkene double bond shift and toluene alkylation. Monoalkyltoluenes are the main reaction products, whereas bimonoalkyltoluenes and alkene dimers appear in low amount from, respectively, HFAU and HBEA. HBEA, despite the small size of its crystallite is practically inactive for the toluene alkylation by 1-dodecene. This low activity can be attributed to the slow desorption of the reaction product from the narrow pore of this zeolite and more particularly the non-desorption of the 3- to 6-monododecyltoluenes. These steric constraints lead to the formation of bulky aromatic compounds in the HBEA micropores. These components formed in larger amount than over HFAU, are responsible to the blockage of the alkenes (dodecene mixture and dimers) in the zeolite pores. However, the limitation effect of diffusion steps observed in the pore of HBEA leads to increase the selectivity to 2-phenylalkanes which are the most biodegradable isomers.