This study was performed in context of a series of studies conducted investigating the use of acid zeolites for the synthesis of thymol by selective isopropylation of m‐cresol on the 6‐position. The aim of this study was to investigate the effect of the SiO2/Al2O3 ratio, i.e. the aluminium content of H‐MFI (H‐ZSM‐5) zeolites. The results are interpreted based on a deep analysis of the reaction network. Three commercial samples of the H‐MFI zeolite were employed with high, medium and low molar SiO2/Al2O3 ratios of 400, 90 and 20 respectively, whereby the first two samples consisted of agglomerates of very small crystallites, < 10 nm, while the last sample consisted of comparatively large crystallites with diameters of several 10 nm. m‐Cresol as the substrate was alkylated with isopropanol in a molar feed ratio of 1:1. Reactions were carried out in a tubular fixed‐bed reactor in the gas‐phase at reaction conditions of 200 ‐ 300°C, WHSV of 0.016 – 1.03 gm‐cresol/gcat.hr and 3 bar (abs) as the standard pressure. A pressure series at 275°C and WHSV of 0.25 – 1.03 gm‐cresol/gcat.hr was also carried out. The following was observed: Isopropanol reacts in two parallel ways namely, by O‐alkylation of the m‐cresol to form isopropyl‐3‐tolyl ether which occurs predominantly by its dehydration to propene and secondly, by alkylation of m‐cresol with propene with high preference on the 6‐position, forming thymol with high selectivity. The ether rearranges internally or transalkylates with m‐cresol to form thymol, 2‐isopropyl‐3‐methyl phenol and 4‐isopropyl‐3‐methyl phenol with a thymol selectivity < 50%. Higher reaction temperatures were found to favour the propene route. It appears that the major effect of the SiO2/Al2O3 ratio is, as expected, a direct effect on catalyst activity and an indirect effect on thymol selectivity via mass transfer control. Thymol is an intermediate product in the reaction network, that is, it can react further (by isomerisation or further alkylation to higher alkylated products). Mass transfer control on such reaction sequences reduce intermediate selectivity and hence thymol selectivity. Exactly this was observed over the low SiO2/Al2O3 ratio (i.e. high aluminium content), ‘large’ crystallite H‐MFI‐20 sample. The major conclusions and recommendations resulting from this study are the following:  Use propene as the alkylating agent rather than isopropanol in order to achieve high thymol selectivity  Avoid mass transfer control.

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