钛硅分子筛TS-1催化环氧丙烷异构反应的机理探究

doi:10.11949/0438-1157.20210519

摘要/Abstract

摘要：

丙烯氢氧环氧化一步法制备环氧丙烷（PO）相比于传统的PO工业生产方法在经济和环保方面具有不可比拟的优势。Au/TS-1双功能催化剂在该反应中展现出较优的PO性能，针对其中TS-1催化PO开环异构生成副产物进行了研究，结合PO在堵孔TS-1分子筛（TS-1-B）和Au/TS-1-B催化剂上的反应性能和红外表征结果，采用理论计算探究了Ti-Defect位点上丙醛和丙酮的生成路径以及涉及的能量变化。结果显示PO在TS-1上的异构化主要经历碳氧键断裂和氢原子转移重排两个过渡态，以及具有五元环结构的双配位丙氧基物种中间体。相比于丙醛，丙酮由于生成过程中氢原子重排的过渡态能垒较高而具有更低的异构化选择性。所揭示的TS-1上PO吸附及异构化反应机制将为钛基丙烯环氧化催化剂的结构改性以增强PO脱附从而提高PO选择性提供理论依据。

关键词: 环氧丙烷异构化, 反应机理, 钛硅分子筛, 理论计算, 丙烯环氧化

Abstract:

One-step catalytic epoxidation of propylene with H₂ and O₂ to produce propylene oxide (PO) has incomparable advantages over traditional industrial processes for PO production because of economic and environmental considerations. The formation of byproducts from PO isomerization catalyzed by TS-1 was studied considering that the bifunctional Au/TS-1 catalysts have exhibited superior PO performance in this reaction. By combining with the catalytic performance of PO isomerization and FT-IR results over the uncalcined TS-1 (TS-1-B) and Au/TS-1-B catalyst with blocked micropores, the formation pathways and corresponding energy changes of propanal and acetone over Ti-Defect site were explored with theoretical calculations. The results show that the isomerization of PO on TS-1 mainly undergoes two transition states of carbon-oxygen bond cleavage and hydrogen atom rearrangement, and the bidentatepropoxy species intermediate with a five-member ring. Compared to propanal, acetone has a lower selectivity from PO isomerization because of the higher transition state energy barrier of hydrogen atom rearrangement during its formation process. The PO adsorption and isomerization mechanism on TS-1 disclosed here will provide a theoretical basis for the structural modification of the titanium-based propylene epoxidation catalyst to enhance PO desorption, thereby improving PO selectivity.

Key words: propylene oxide isomerization, reaction mechanism, titanium silicate zeolite, theoretical calculation, propylene epoxidation

中图分类号:

TQ 426.1

王刚,段学志,袁渭康,周兴贵. 钛硅分子筛TS-1催化环氧丙烷异构反应的机理探究[J]. 化工学报, 2021, 72(10): 5150-5158.

Gang WANG,Xuezhi DUAN,Weikang YUAN,Xinggui ZHOU. Mechanistic insights into catalytic isomerization of propylene oxide over TS-1[J]. CIESC Journal, 2021, 72(10): 5150-5158.

图/表 11

图1 Au/TS-1催化丙烯氢氧环氧化生成PO以及PO异构化示意图

Fig.1 Schematic diagram of propylene epoxidation with H2 and O2 to PO and its isomerization over Au/TS-1 catalyst

图2 计算中所采用的TS-1分子筛9T模型

Fig.2 The optimized 9T clusters model of TS-1 employed for the calculation

表1 TS-1模型中Ti—O键键长与实验值比较

Table 1 Comparison of Ti—O bond lengths between calculated and experimental values

键类型	Ti—O键键长 /nm
键类型	计算值	实验值
Ti—O(1)	0.181
Ti—O(2)	0.176
Ti—O(3)	0.182
Ti—O(4)	0.175
平均值	0.179	0.1793±0.0007^[27]

图3 所合成的钛硅分子筛的结构表征

Fig.3 Structural characterization of synthesized titanosilicate

表2 TS-1-B和TS-1-O的BET比表面积和孔容参数

Table 2 BET surface area and pore parameters of TS-1-B and TS-1-O

样品

处理温度/℃

S_BET/

(m²?g^-1)

V_micro/

(cm³?g^-1)

V_total/

(cm³?g^-1)

图4 Au/TS-1-B催化剂上Au颗粒的HAADF-STEM和相应的粒径分布

Fig.4 Typical HAADF-STEM image and corresponding particle size distribution of Au/TS-1-B catalyst

图5 TS-1-B和Au/TS-1-B上PO异构化和深度氧化反应性能

Fig. 5 Catalytic performance of PO isomerization and over-oxidation over TS-1-B and Au/TS-1-B

图6 TS-1催化PO异构化生成丙醛中的关键结构和键长（单位：nm）

Fig.6 The key structures and bond lengths during PO isomerization to produce propanal over TS-1 (Unit: nm)

图7 TS-1催化PO异构化生成丙醛的能线图

Fig.7 Energy profile for PO isomerization to produce propanal over TS-1

图8 TS-1催化PO异构化生成丙酮中的关键结构和键长（单位：nm）

Fig.8 The key structures and bond lengths during PO isomerization to produce acetone over TS-1 (Unit: nm)

图9 TS-1催化PO异构化生成丙酮的能线图

Fig.9 Energy profile for PO isomerization to produce acetone over TS-1

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