Catalytic performance of boron-containing Na-B-ZSM-5 molecular sieves in methanol dehydrogenation to formaldehyde

Na-B-ZSM-5 molecular sieves were synthesized by hydrothermal method with silica white as silicon source, boric acid as boron source, sodium hydroxide as alkali, and tetrapropylammonium bromide (TPABr) and 1,6-hexamethylenediamine (HMDA) as the hybrid template. The synthesized Na-B-ZSM-5 samples were characterized by XRD, SEM, FT-IR, UV-vis, ¹¹B MAS NMR and NH₃-TPD. Their catalytic performance in methanol dehydrogenation to formaldehyde was evaluated in a fixed-bed reactor under atmospheric pressure and the effects of the catalyst preparation parameters like Si/B molar ratio, basicity, crystallization temperature and crystallization time as well as the reaction conditions like temperature and space velocity (WHSV) on the methanol dehydrogenation behavior were investigated. The results indicated that boron atoms are introduced into the framework of Na-B-ZSM-5 molecular sieves, present as the Bronsted acid sites from four-coordinated boron atoms and Lewis acid sites from tricoordinated boron atoms. Na-B-ZSM-5 molecular sieves contain more weak acid sites and less middle-strong acid sites. The optimized preparation parameters of the Na-B-ZSM-5 molecular sieve are as follows: 7.5 of Si/B molar ratio, 0.14 of Na₂O/SiO₂ molar ratio, 170 ℃ of crystallization temperature and 48 h of crystallization time. The conversion of methanol and the selectivity to formaldehyde reach 62.97% and 68.86%, respectively, under the reaction conditions of 550 ℃ and a WHSV of 1.85 h^-1 over the optimized Na-B-ZSM-5 molecular sieve.