Selective Toluene Disproportionation over ZSM-5 Zeolite

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Abstract

This research aimed at improving p-xylene selectivity in toluene disproportionation over ZSM-5 zeolite by exploring the effect of crystal size and various post synthetic modification methods. A comprehensive study of the effect of different modifications on the physicochemical properties of ZSM-5 was investigated using X-ray diffraction (XRD), pyridine adsorption, Fourier transform infra-red (FTIR), 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR), BET surface area by N2 adsorption, inductively coupled plasma (ICP) and scanning electron microscopy (SEM). The catalytic performance of each catalyst was studied in a fixed bed reactor at a temperature 475 °C, WHSV 3-83 h-1 and two different pressures (1 and 10 bar). ZSM-5 zeolites with different 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. The increase in crystal size improved p-xylene selectivity which was attributed to the diffusion constraints imposed by the longer diffusion path lengths of large crystals. The highest p-xylene selectivity (58 %) was achieved over ZSM-5 with the largest crystal size 100 µm at the highest WHSV 83 h-1. However, it was accompanied by a low conversion (2 wt. %). ZSM-5 with crystal size of 5 µm delivered the best results in terms of the combination of para-selectivity (40 %) and toluene conversion (15 wt. %). The p-xylene produced in the channels of ZSM-5 can quickly isomerise to o-xylene and m-xylene on the external unselective acid sites. Different post modification methods were applied in this study in attempt to suppress the fast isomerization reaction by deactivating the external acid sites. This was achieved to some extent by depositing an inert silica layer using different silica agents, amounts and number of modification cycles and as a result p-xylene selectivity was significantly improved (84 %), especially over large crystals 5 µm. The decrease in Brønsted acidity (FTIR) suggested the success of the silylation method. Furthermore, impregnation of lanthanum and phosphorus on ZSM-5 improved p-xylene selectivity (40 %). FTIR measurements showed a drastic drop in the number of Brønsted and Lewis acid sites after loading phosphorus which led to a large reduction in toluene conversion. Lanthanum impregnation had less effect on conversion and increased selectivity with decreased Brønsted sites and pore volume reduction showed by N2 adsorption suggesting some pore narrowing. There are several approaches that can be considered in future to further improve p-xylene selectivity. Improving the synthesis of large crystals to balance acidity and crystal size can lead to the enhancement of p-xylene selectivity. Also, performing toluene disproportionation on optimised pre-coked ZSM-5 large crystals at high pressure can help to maintain the conversion while increasing p-xylene selectivity. Another approach would be to apply silylation modification to extruded large crystals ZSM-5.

Keyword(s)

Disproportionation; Lanthanum; Large crystals; Phosphorus; Silylation; Toluene; ZSM-5; p-xylene

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