HPW改性Hβ分子筛催化甲苯和叔丁醇烷基化反应的性能

doi:10.1016/S1872-2067(16)62587-1

摘要/Abstract

摘要：

PTBT是一种十分重要的有机材料，但传统制备工艺存在能耗较高，工艺复杂，环境污染严重等诸多问题.为解决上述问题，人们提出甲苯和叔丁醇直接一步合成PTBT来代替传统的合成工艺.甲苯和叔丁醇原料来源丰富，用酸性分子筛等催化剂催化甲苯和叔丁醇烷基化反应合成PTBT不但能节约成本，简化分离和提纯工艺，还能防止环境污染和设备腐蚀.但催化剂的活性低、稳定性差制约了该反应的工业化进程.甲苯和叔丁醇侧链烷基化反应历程复杂，需要催化剂的酸性和孔道结构的协同作用，因此设计催化活性高、选择性好、稳定性强的催化剂是一项十分具有挑战的研究课题.我们采用浸渍法成功制备了H₃PW₁₂O₄₀改性Hβ分子筛催化剂（HPW/Hβ），并采用XRD，SEM，TEM，ICP，FT-IR，BET，NH₃-TPD和Py-IR等手段对分子筛催化剂样品进行了表征，并以甲苯和叔丁醇烷基化反应为探针反应，研究了HPW/Hβ分子筛催化剂的催化性能.
由SEM分析可知，HPW/Hβ分子筛催化剂的形貌与Hβ并无明显差异，形状规整，粒度均匀，晶体形貌较好，表明HPW的引入对Hβ颗粒结构无明显影响.由XRD分析可知，与未改性Hβ分子筛相比，HPW/Hβ样品的出峰位置和峰形基本保持一致，表明HPW在Hβ表面呈均匀分散状态，但负载HPW后Hβ结晶度略有下降.由TEM分析可知，负载HPW后的Hβ分子筛依然保持规整的三维立方孔道结构，且孔径均一，表明负载HPW后的Hβ分子筛的骨架结构没有被破坏，黑色阴影部分或者斑点即为夹心型杂多酸阴离子在分子筛Hβ上的固载位.由FT-IR分析可知，HPW和Hβ之间存在键合作用，部分HPW已成功分散到Hβ骨架表面上.由BET分析可知，和Hβ原粉相比较，HPW/Hβ的比表面积、孔容、孔径均有所下降，BET比表面积从492.5下降到379.6 m²/g，而孔径从3.90下降至3.17 nm.这是因为HPW对分子筛孔道具有修饰作用，使分子筛的孔径有所降低.由NH₃-TPD和Py-IR酸性表征可知，负载HPW能有效增加Hβ沸石分子筛的酸量，尤其是B酸量.未改性Hβ的B酸含量为84.23 μmol/g，而HPW/Hβ的B酸含量为142.97 μmol/g，增加了69.74%.
由酸性表征可知，Hβ的总酸量小，B酸含量低，因而催化活性弱，甲苯转化率仅为54.0%.另外，Hβ分子筛的12元环直通道的孔道开口尺寸为0.66 nm×0.67 nm，PTBT（动力学直径0.58 nm）和MTBT（动力学直径0.65 nm）都能够从其孔道中扩散出来，因而分子筛孔道的择形作用对产物的选择性作用较小，PTBT的选择性（69.6%）较低.负载HPW能有效增加Hβ分子筛的总酸量，尤其是B酸量，而B酸量增加，有利于反应中正碳离子生成，因而增加催化活性.另外，HPW改性还能提高PTBT的选择性，这是因为HPW对分子筛孔道具有修饰作用，使分子筛的孔径有所降低.而适量减小的孔径使得分子筛的择形作用大大增加，体积较小的PTBT能从孔道中扩散出来，而体积较大的MTBT，由于空间位阻的作用，很难从其中扩散出来，从而增加了对位选择性.通过对HPW/Hβ催化甲苯和叔丁醇烷基化反应工艺条件进行考察，确定了适宜的反应条件：环己烷60 mL，催化剂1.0g，n（叔丁醇）/n（甲苯）=3/1，反应温度180℃，反应时间4 h.此条件下甲苯转化率为73.1%，PTBT的选择性为80.8%.

关键词: 烷基化, 甲苯, 叔丁醇, H₃PW₁₂O₄₀, B酸性, Hβ分子筛

Abstract:

An Hβ-supported heteropoly acid (H₃PW₁₂O₄₀ (HPW)/Hβ) catalyst was successfully prepared by wetness impregnation, and investigated in the alkylation of toluene with tert-butyl alcohol for the synthesis of 4-tert-butyltoluene (PTBT). X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fourier-transform infrared spectroscopy, inductively coupled plasma-optical emission spectrometry, the brunauer emmett teller (BET) method, temperature-programmed NH₃ desorption, and pyridine adsorption infrared spectroscopy were used to characterize the catalyst. The results showed that loading HPW on Hβ effectively increased the B acidity and decreased the pore size of Hβ. The B acidity of HPW/Hβ was 142.97 μmol/g, which is 69.74% higher than that of Hβ (84.23 μmol/g). The catalytic activity of the HPW/Hβ catalyst was much better than that of the parent Hβ zeolite because of its high B acidity. The toluene conversion over HPW/Hβ reached 73.1%, which is much higher than that achieved with Hβ (54.0%). When HPW was loaded on Hβ, the BET surface area of Hβ decreased from 492.5 to 379.6 m²/g, accompanied by a significant decrease in the pore size from 3.90 to 3.17 nm. Shape selectivity can therefore play an important role and increase the product selectivity of the HPW/Hβ catalyst compared with that of the parent Hβ. PTBT (kinetic diameter 0.58 nm) can easily diffuse through the narrowed pores of HPW/Hβ, but 3-tert-butyltoluene (kinetic diameter 0.65 nm) diffusion is restricted because of steric hindrance in these narrow pores. This results in high PTBT selectivity over HPW/Hβ (around 81%). The HPW/Hβ catalyst gave a stable catalytic performance in reusability tests.

Key words: Alkylation, Toluene, tert-Butyl alcohol, H₃PW₁₂O₄₀, B acidity, H&beta, zeolite

王园园, 宋华, 孙兴龙. HPW改性Hβ分子筛催化甲苯和叔丁醇烷基化反应的性能[J]. 催化学报, 2016, 37(12): 2134-2141.

Yuanyuan Wang, Hua Song, Xinglong Sun. Alkylation of toluene with tert-butyl alcohol over HPW-modified Hβ zeolite[J]. Chinese Journal of Catalysis, 2016, 37(12): 2134-2141.

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