Abstract:
p-Xylene is the most important intermediate in xylene isomers. It is mainly used for producing terephthalic acid. Currently, p-xylene is produced by disproportionation of toluene and p-xylene-oriented isomerization of mixed xylene isomers. However, these processes will produce a large quantity of benzene, hence, the cost of isolation and purification is relatively high. The catalytic methylation of toluene is, therefore, a very promising alternative method of producing p-xylene without any by-product. A variety of zeolites have been used for catalyzing this reaction such as acidic zeolite, HZSM-5 because of its suitable pore size for p-xylene diffusion. On the other hand, the acid sites on the external catalyst surface would cause an isomerization of p-xylene to o- and m-xylene, resulting in decreasing the selectivity to p-xylene. The purpose of this study was to deactivation of its external acid sites. The HZSM-5 catalysts used were modified via i) chemical liquid deposition (CLD), and ii) dealumination via acid treatment. The varied amounts and cycles of tetraethyl orthosilicate (TEOS) loading in the ranges of, 0.5-2.0 ml.g-1 and 1-3 times respectively, were employed for the former method whereas the use of 0.5 M oxalic acid was for the latter. Catalytic activity testing was carried out at atmospheric pressure, 400 ℃, WHSV of 24 h-1, and toluene to methanol ratio of 4. The results showed that the catalyst modified via the CLD with a TEOS concentration of 1 ml/g.cat. with 2-cycle treatment exhibited the highest p-xylene selectivity in product from 67.29 % to 84.39 % or p-xylene selectivity in xylene from 75.17 % to 92.05 % as compared to the parent catalyst.
