Dual Effects of Zinc Species on Active Sites in Bifunctional Composite Catalysts Zr/H[Zn]ZSM-5 for Alkylation of Benzene with Syngas

Bifunctional composite catalysts Zr/H[Zn](x)Z with different Zn contents were prepared by combining the impregnation and physical mixing method. The effect of zinc species on two active sites in bifunctional composite catalysts Zr/ H[Zn]ZSM-S for alkylation of benzene with syngas has been investigated. The results indicated that the introduction of Zn greatly improves the catalytic performance and the selectivity of target products and changed with the change of Zn content. The sample Zr/H[Zn](3)Z exhibited the highest total selectivity of toluene and xylene of 91% and the benzene conversion of 18% at 400 degrees C under 3.3 MPa; nevertheless, the sample Zr/H[Zn](5)Z showed the highest benzene conversion and xylene selectivity, which reached 31 and 27%, respectively. The acidic properties and state of Zn species of H[Zn](x)Z zeolites have been explored. All evidence suggested that most of Zn introduced into HZ zeolite existed in the catalyst as Zn(OH)(+), forming new Zn-Lewis acid at the expense of Bronsted acid. The Zn species plays a role in three aspects in the alkylation reaction between benzene and syngas. First, the Zn species can provide H species for ZrO2 activation of CO by promoting H-2 activation, thus generating more methanol; second, Zn(OH)(+) species formed in zeolite can inhibit the side reactions of methanol so as to promote the alkylation of benzene and methanol; finally, the Zn-Lewis acidic sites can inhibit the cracking reaction of C-6(+) olefins and further promote the dehydrogenation aromatization reaction.