CrO x /Ti-Al2O3催化剂结构及其催化丙烷脱氢性能

doi:10.16085/j.issn.1000-6613.2022-0169

摘要/Abstract

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

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-Cr₂O₃and 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-Al₂O₃were 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

中图分类号:

TQ031.4

张永祥, 王德龙, 郭晓燕, 邵怀启. CrO _x /Ti-Al₂O₃催化剂结构及其催化丙烷脱氢性能[J]. 化工进展, 2022, 41(11): 5879-5886.

ZHANG Yongxiang, WANG Delong, GUO Xiaoyan, SHAO Huaiqi. Structure and performance of CrO _x /Ti-Al₂O₃ catalysts for propane dehydrogenation[J]. Chemical Industry and Engineering Progress, 2022, 41(11): 5879-5886.

图/表 11

图1 CrO x /nTi-Al2O3催化剂的XRD图1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

图2 催化剂CrO x /Al2O3(a)、CrO x /0.5Ti-Al2O3(b，c，d)、CrO x /2Ti-Al2O3(e)、CrO x /3Ti-Al2O3(f)的TEM图

表1 CrO x /nTi-Al2O3催化剂的比表面积和孔径

催化剂	S_BET^①/m²·g^-1	V_P/cm³·g^-1	D_P/nm
CrO _x /Al₂O₃	199.7（0）	0.649	8.1
CrO _x /0.5Ti-Al₂O₃	194.5（16.3）	0.650	9.6
CrO _x /1Ti-Al₂O₃	189.8（31.3）	0.631	9.6
CrO _x /2Ti-Al₂O₃	187.1（28.6）	0.631	9.6
CrO _x /3Ti-Al₂O₃	180.3（19.1）	0.539	7.8

图3 CrO x /nTi-Al2O3催化剂的孔径分布1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

图4 CrO x /nTi-Al2O3催化剂的FTIR光谱图1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

图5 CrO x /nTi-Al2O3催化剂的拉曼光谱1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

表2 CrO x /nTi-Al2O3催化剂的XPS结果

催化剂	Cr 2p_3/2结合能/eV		Cr⁶⁺/Cr³⁺	Cr/Al原子比			Cr_2p/Al_2p强度比	Ti/Al原子比	晶格氧/吸附氧
催化剂	Cr⁶⁺	Cr³⁺	Cr⁶⁺/Cr³⁺	Cr^①	Cr⁶⁺	Cr³⁺	Cr_2p/Al_2p强度比	Ti/Al原子比	晶格氧/吸附氧
CrO _x /Al₂O₃	579.1	576.7	0.34	0.116	0.029	0.087	1.92	—	0.59
CrO _x /0.5Ti-Al₂O₃	579.7	577.0	0.23	0.106	0.020	0.086	1.97	0.0105^②	1.17
CrO _x /1Ti-Al₂O₃	579.6	576.9	0.19	0.106	0.017	0.089	1.98	0.0123	1.70
CrO _x /2Ti-Al₂O₃	579.8	577.1	0.19	0.112	0.018	0.094	2.07	0.0168	1.86
CrO _x /3Ti-Al₂O₃	579.6	577.0	0.31	0.111	0.026	0.085	2.06	0.0262	1.78

图6 CrO x /nTi-Al2O3催化剂的NH3-TPD图1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

表3 CrO x /nTi-Al2O3催化剂的吡啶-红外结果

催化剂	Brönsted酸量 /μmol·g^-1	Lewis酸量 /μmol·g^-1	总酸量 /μmol·g^-1
CrO _x /Al₂O₃	13.44	312.71	326.15
CrO _x /0.5Ti-Al₂O₃	0.99	47.84	48.84
CrO _x /1Ti-Al₂O₃	1.18	30.64	31.83
CrO _x /2Ti-Al₂O₃	1.23	50.14	51.37
CrO _x /3Ti-Al₂O₃	1.53	87.54	89.07

图7 CrO x /nTi-Al2O3催化剂上的丙烷脱氢反应性能1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

图8 反应后的CrO x /nTi-Al2O3催化剂的热失重结果1—CrO x /Al2O3；2—CrO x /0.5Ti-Al2O3；3—CrO x /1Ti-Al2O3；4—CrO x /2Ti-Al2O3；5—CrO x /3Ti-Al2O3

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CrO _x /Ti-Al₂O₃催化剂结构及其催化丙烷脱氢性能

Structure and performance of CrO _x /Ti-Al₂O₃ catalysts for propane dehydrogenation

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