高分散型VOx/SBA-15催化剂的制备及其氧化活化烷烃反应研究

Compared with the conventional steam cracking process, catalytic cracking of naphtha has the advantages of low reaction temperature and energy consumption, high propylene-to-ethylene ratio, small yield of methane, and etc. However, paraffin constituent of straight-run naphtha (centered in the range of C6-C9) has a relatively low reactivity in catalytic cracking reactions. To solve this problem, activation of paraffin molecule by lattice oxygen was introduced into the catalytic cracking system in our previous study. VOx/Al2O3 was prepared and applied for providing lattice oxygen, and it promoted conversion of n-heptane over ZSM-5 catalyst. Meanwhile, the propylene selectivity was improved. However, it was also found that the dispersion of vanadia species over the VOx/Al2O3 was low, which resulted in a less amount of surface lattice oxygen and poor activity. Therefore, highly-dispersed VOx/SBA-15 is designed and prepared in this proposal. By altering the pore length and surface acidity of SBA-15, the relationship between properties of support of the vanadia catalyst and the catalytic performances is explored. Furthermore, the temperature-programmed-surface-reaction and In-situ-FT-IR reaction is employed, for clarification of the structure of surface active intermediate, and the detailed mechanism of the reaction process. This study will supplement the theory system for selective oxidation of hydrocarbons, and also provide scientific basis for solving the difficulty of paraffin cracking in naphtha catalytic cracking process.

针对直馏石脑油中富含的烷烃组分(主要集中在C6-C9)催化裂解反应活性较低的问题，前期研究中提出了烷烃的晶格氧氧化-催化裂解反应路线，可有效地提高烷烃催化裂解反应活性和丙烯选择性。然而，所制备的VOx/Al2O3作为晶格氧催化剂具有表面钒氧活性单元分散度差和活性不足的缺点。因此，本项目提出高分散型VOx/SBA-15晶格氧催化剂的设计制备，以提高表面晶格氧数量和活性。通过改变SBA-15载体的孔道长度和表面酸性质，探究钒基催化剂载体性质对其氧化活化烷烃反应性能的影响机制；并进一步通过程序升温表面反应(TPSR)和原位红外反应(In-situ-FT-IR)实验，揭示烷烃晶格氧氧化-催化裂解反应活性中间物种的结构形态，以及具体的反应机理。本项目研究结果可进一步丰富完善烃类选择性氧化理论体系，同时也为实现新型石脑油催化裂解路线提供了一定的理论基础。

相比于蒸汽裂解工艺，石脑油催化裂解在能耗、CO2排放量和产物分布上具有明显的优势，但直馏石脑油中的烷烃反应活性较差，在催化裂解反应条件下在平衡分子筛催化剂上的转化率低，前期研究提出的烷烃晶格氧氧化-催化裂解反应路线可有效地提高烷烃催化裂解反应活性和丙烯选择性。本项目制备了高分散型VOx/SBA-15晶格氧催化剂，并进一步理解正庚烷的氧化活化催化裂解反应。研究结果表明，VOx基催化剂对正庚烷的活化作用受到反应条件，载体性质，VOx分散度和结构形态等多方面的影响。通过接枝/阴离子交换法成功制备了VOx/SBA-15，VOx单元在高比表面积SBA-15上获得了很好的分散，且主要以孤立态的O=V-(-O-)-3形式存在，其表面晶格氧物种的活性低于VOx/Al2O3，致其对正庚烷转化的促进作用较低。VOx基催化剂表面的酸碱性调变后，主要对氢气和芳烃产物的选择性产生了显著影响，酸性越强，氢气和芳烃产物的选择性越高。