The effect of ZSM-5 zeolite crystal size on p-xylene selectivity in toluene disproportionation

The effect of crystal size was explored in this work aiming at enhancing p-xylene selectivity through toluene disproportionation over ZSM-5 zeolite. The different physicochemical properties of ZSM-5 were investigated using various characterisation techniques including X-ray diffraction (XRD), pyridine adsorption, Fourier transform infra-red (FTIR), BET surface area by N2 adsorption, inductively coupled plasma (ICP) and scanning electron microscopy (SEM). Each catalyst was tested in a fixed bed reactor at a temperature 475 deg. C, weight hourly space velocity (WHSV) 3-83 h-1 and two different pressures (1 and 10 bar). ZSM-5 zeolites with crystal sizes 5, 50 and 100 μm were synthesised in house and compared with the commercially obtained ZSM-5 having a crystal size of 0.5 μm. As a result of increasing the crystal size the p-xylene selectivity was improved. This was attributed to the longer diffusion path lengths of the large crystals which imposed more diffusion constraints on the other xylene isomers. ZSM-5 zeolite with the largest crystal size 100 μm achieved the highest p-xylene selectivity (58%) at the highest WHSV 83 h-1. However, this was accompanied by a low conversion (2 wt %). The best combination of p-xylene selectivity (40%) alongside 15 wt % toluene conversion was achieved over ZSM-5 having 5 μm crystal size.