Promoting effects of desilication and dealumination on the catalytic performance of Al-rich HMOR for catalysing naphthalene tert-butylation with tertiary butanol

tert-Butylation of naphthalene is a potential technology to produce 2,6-di-tert-butylnaphthalene (2,6-DTBN), which is a valuable raw material for the synthesis of polyethylene naphthalate (PEN). Alkaline, acid and combined alkaline-acid treatments were adopted to regulate the porosity and acidity of Al-rich HMOR zeolite. The structure, porosity and acidity of the catalysts were well characterized and the catalytic performance in the tertbutylation of naphthalene was extensively investigated. The strong acid sites were verified as the intrinsic active sites for naphthalene alkylation for the first time. Characterization results indicated that alkaline treatment could extract silicon from the framework and intra-crystalline mesopores with diameters of 3-30 nm were successfully generated in Al-rich HMOR by one-step alkaline treatment. The introduced mesopores could enhance the mass diffusion of molecules as well as the coke-resistance, despite that the high acid density in the internal surface would lead to fast coke deposition. On the other hand, the acid treatment could remove the aluminium efficiently, leading to significantly decreased strong acid sites located at the micropores and pore entrances, which reduced coke deposition in the channels and pore entrances and brought about sharp increase in the conversion rate of naphthalene per strong acid site. Thus, higher naphthalene conversion and di-tert-butyl naphthalene (DTBN) selectivity were obtained over all the post-treated samples. Besides, the high selectivity for 2,6-DTBN (2,6-/2,7-DTBN > 28.0) remained. More importantly, combined alkaline and acid treatment had synergistic effects on the improvement of the initial activity as well as the catalytic stability due to the optimized acidity and porosity.