Abstract:

The Ti-modified alumina was obtained by template method and used as support to prepare CrO _x /nTi-Al₂O₃ catalysts, and the influences of Ti loading on the catalyst structure and catalytic performance for propane dehydrogenation were investigated. X-ray diffraction(XRD), N₂ adsorption-desorption, transmission electron microscopy(TEM), Fourier transform infrared spectroscopy(FTIR), UV-Raman spectroscopy, X-ray photoelectron spectroscopy(XPS), NH₃-temperature-programmed desorption(NH₃-TPD) and pyridine-infrared spectroscopy(Py-IR) characterizations were carried out to elucidate the catalyst properties. The results showed that CrO _x /nTi-Al₂O₃ catalysts possessed uniform foam-like mesoporous structure with micropore and high specific surface area of 180—195m²/g. The main chromium species on the catalysts were Cr⁶⁺ and Cr³⁺. Among them, the Cr⁶⁺ species existed in the form of monochromate and bichromate, and Cr³⁺ species existed in the form of crystalline α-Cr₂O₃ and high-dispersive non-redox Cr₂O₃. The Ti loading decreased Cr⁶⁺ content on the catalyst surface and increased high-dispersive Cr³⁺ content in the pore channel. The Ti loading decreased weak acid intensity and promoted the formation of medium acid sites, meanwhile the amounts of Brönsted and Lewis acid sites were obviously decreased. The propane conversion and propylene yield on CrO _x /nTi-Al₂O₃ were obviously improved by adding 0.5%—1.0% TiO₂(mass), but excess Ti (>2%TiO₂) could decrease the propylene selectivity and yield. The oxidation dehydrogenation of propane took places on Cr⁶⁺ species, which were reduced to redox Cr³⁺ species, and then the redox Cr³⁺ species continuously catalyzed the propane dehydrogenation. Meanwhile, the high-dispersive non-redox Cr³⁺ species in the pore channel directly catalyzed propane dehydrogenation to produce propylene. The aboved-mentioned Cr active sites provided both high catalytic activity and good stability.

Key words: propane dehydrogenation, alumina, chromia, catalyst support, titanium-modification, stability

摘要：

以模板法制备的Ti改性Al₂O₃为载体制备了CrO _x /nTi-Al₂O₃催化剂，考察了Ti含量对催化剂的结构及其催化丙烷脱氢性能的影响。采用X射线衍射（XRD）、N₂吸附-脱附、透射电子显微镜（TEM）、傅里叶变换红外光谱（FTIR）、拉曼光谱、X射线光电子能谱（XPS）、氨气程序升温脱附（NH₃-TPD）、吡啶红外吸附（Py-IR）等方法对催化剂的结构进行了表征。结果表明，CrO _x /nTi-Al₂O₃催化剂具有均匀的泡沫状介孔结构并含有少量微孔，表面积在180~195m²/g；铬主要以Cr⁶⁺和Cr³⁺形式存在，其中Cr⁶⁺主要以单铬酸盐和双铬酸盐形式存在，Cr³⁺以α-Cr₂O₃晶体和高分散Cr₂O₃形式存在，Ti的加入降低了催化剂表面Cr⁶⁺含量，增加了孔道内高分散Cr³⁺含量；Ti的加入降低了弱酸的强度，生成了少量中强酸，并使催化剂中B酸和L酸中心数量明显减少。少量的Ti（0.5%~1.0%TiO₂，质量分数）可明显提高丙烷转化率和丙烯收率，但过多的Ti（>2%TiO₂）则明显降低丙烯选择性而使丙烯收率降低。CrO _x /nTi-Al₂O₃催化剂表面Cr⁶⁺物种可催化丙烷氧化脱氢，本身还原成Cr³⁺后继续催化丙烷直接脱氢，孔道内部的高分散Cr³⁺可催化丙烷直接脱氢反应，二者结合使催化剂保持了较高的催化活性和较好的稳定性。

关键词: 丙烷脱氢, 氧化铝, 氧化铬, 催化剂载体, 钛改性, 稳定性