Performance of HZSM-5 as Catalyst for Alkylation of Methyl-naphthalene with Methanol in Supercritical Phase

The alkylation of methylnaphthalene(MN) with methanol in the presence of HZSM-5 is a promising route for producing 2,6-dimetylnaphthalene(2,6-DMN) with a high selectivity. However, the conversion of MN is very low and the catalyst will be deactivated rapidly with increasing time on stream. In this study, the effects of the reaction pressure on the reactivity, selectivity and life of the catalyst of alkylation of MN over HZSM-5 modified by BaO were investigated. It was observed that with the enhancement of pressure, the conversion of MN increased, but the selectivity of 2,6-DMN kept unchanged, which was about 40%—42%. When the alkylation was carried out under a supercritical condition, the conversion of MN was 3—6 times higher and the life of catalyst was 25—30 times longer than those at an ambient pressure. The thermogravimetric analyses of the deactivated catalysts at different reaction pressures indicate that the amount of coke deposited on the catalysts was about 10% to 12 %, and the coke-burning reactions mainly took place in a temperature range from 720 to 860 K, and the apparent activation energies of the coke-burning catalysts at 0.1 MPa(10 h) and 7.6 MPa(108 h) were, respectively, 65.90 and 84.72 kJ/mol. It is concluded from the results that the supercritical condition is advantageous to enhancing the conversion of alkylation and extraction in situ, and to transporting those high molecular-weight poly-aromatic compounds so as to extend the catalyst life successively.