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

Abstract

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 characterization 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 °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 synthesized 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 were achieved over ZSM-5 having 5 µm crystal sized.

High‐Pressure Operando UV‐Vis Micro‐Spectroscopy of Coke Formation in Zeolite‐based Catalyst Extrudates during the Transalkylation of Aromatics

“…By comparing these UV‐vis DR spectra it becomes clear that the absorption bands are formed at a greater rate at 5 bar, indicating a higher conversion level at 5 bar. The higher pressure forces more reactants onto the catalytic active sites, therefore increasing the activity of the catalyst [37,38] . Furthermore, the intensity of the absorption bands is higher for the 20 : 80 SiO 2 5 bar reaction than for the 1 bar reaction as more polyaromatic species are formed due to a higher conversion.…”

“…Previous studies showed that the catalytic conversion increased with increasing pressure, as the higher pressure enhanced the adsorption of the reactants on the active sites of the catalyst. Furthermore, the xylene selectivity decreased with increasing pressure, as more side reactions can occur when the pressure is increased [37,38] . These side reactions cause the formation of poly‐aromatic molecules, also known as coke molecules, that block the pores of the zeolite and cause deactivation [27,39,40] …”

Improvement of adsorption and catalytic properties of zeolites by precisely controlling their particle morphology

“…Attempts are also made to selectively block the (010) surface or extension of sinusoidal channels along the a axis for enhancing shape selectivity. 93,94 Highly intergrown and twinned ZSM-5 crystals synthesized with NBA and tetrapropylammonium bromide (TPABr) as co-templates (Fig. 9F and G) exhibit a very high p-xylene selectivity (499%) (Fig.…”

Studies of propene conversion over H-ZSM-5 demonstrate the importance of propene as an intermediate in methanol-to-hydrocarbons chemistry

“…44 Alternatively, xylene products may be formed by the disproportionation of toluene produced by the Scheme 2(iii) rearrangement mechanism (Scheme 2(iv)). 45,46 Cyclopentadienyl cations with ethyl or longer side chains may release light olefins through an internal rearrangement followed by elimination, as demonstrated for ethene in Scheme 2(v). 44…”