含硼杂原子Na-B-ZSM-5分子筛对甲醇脱氢制甲醛反应的催化性能

以白炭黑为硅源、硼酸为硼源、NaOH为碱源、四丙基溴化铵(TPABr)和1,6-己二胺(HMDA)混合模板剂,采用水热合成法制备含硼杂原子Na-B-ZSM-5分子筛,用XRD、SEM、FT-IR、UV-vis、11B MAS NMR、NH3-TPD等方法对其进行表征。在连续流动常压固定床反应器上评价Na-B-ZSM-5分子筛对甲醇脱氢制甲醛反应的催化性能,考察n(Si)/n(B)、n(Na2O)/n(SiO2)、晶化温度和晶化时间等制备参数以及反应温度和质量空速等工艺参数对催化性能的影响。结果表明,硼进入了分子筛的骨架结构中,存在与B酸中心有关的骨架四配位硼和与L酸中心有关的骨架三配位硼,Na-B-ZSM-5分子筛含有较多的弱酸位和少量的中强酸位。催化剂的最佳制备参数为n(Si)/n(B)比值为7.5、n(Na2O)/n(SiO2)比值为0.14、晶化温度170℃、晶化时间48 h。Na-B-ZSM-5(7.5)分子筛在反应温度550℃、质量空速1.85 h-1的反应条件下对甲醇的转化率为62.97%,甲醛的选择性为68.86%。

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.