Performance of SO42-/AC catalysts prepared with different precursors in the synthesis of polyoxymethylene dimethyl ethers from dimethyl ether direct oxidation
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摘要: 分别用硫酸、硫酸铵作为前驱体，活性炭（AC）作为载体，采用浸渍法制备了SO42-/AC双功能催化剂（SO42-离子和AC载体分别提供催化剂的酸性和氧化还原性），考察了其在二甲醚（DME）直接氧化合成聚甲氧基二甲醚（DMMx）反应中的催化性能。结果表明，不同前驱体制备的SO42-/AC催化剂表现出显著的催化活性差异。40% H2SO4/AC催化剂具有较好的反应活性，DME转化率为8.4%，DMM1-2的选择性达到59.7%，并且没有COx的生成；而在40%（NH4）2SO4/AC催化剂上，反应主要生成了COx，DMM选择性仅2.7%，且无DMM2生成。XRD、BET、NH3-TPD及O2-TPD-MS等表征结果显示，H2SO4/AC催化剂中适量的弱酸性位和氧化还原性位有利于DME直接氧化合成DMMx。经过SO42-修饰的催化剂促进了O2在活性炭表面的活化；前驱体H2SO4的加入提高了催化剂表面的弱酸性位数量，而（NH4）2SO4的引入却促使催化剂表面产生中强酸性位。Abstract: A series of SO42-/AC bifunctional catalysts, in which SO42- and AC acted as the acid sites and redox sites, respectively, are prepared by impregnation with H2SO4 and (NH4)2SO4 as precursors and active carbon as support; their catalytic performance in the direct oxidation of dimethyl ether (DME) to polyoxymethylene dimethyl ethers (DMMx) was investigated. The results show that the catalytic performance of SO42-/AC catalysts is significantly related to the precursor used. Over the 40%H2SO4/AC catalyst, the selectivity to DMM1-2 reaches 59.7%, with a DME conversion of 8.4%; in addition, there is no COx observed in the products. However, a large amount of COx by-product is formed over the 40%(NH4)2SO4/AC catalyst; meanwhile, the selectivity to DMM is only 2.7% and no DMM2 is formed. The XRD, N2 sorption, NH3-TPD and O2-TPD-MS characterization results illustrate that the suitable amount of weak acid sites and redox sites of the H2SO4/AC catalyst is beneficial to the formation of long chain DMMx from DME oxidation. The modification of AC with SO42- promotes the activation of O2 over the surface of AC support, whereas the introduction of H2SO4 improves the weak acid sites of the catalyst. On the contrary, the number of medium-strong acid sites of the catalyst is increased by modifying AC with (NH4)2SO4.
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Key words:
- dimethyl ether /
- polyoxymethylene dimethyl ethers /
- direct oxidation /
- precursor /
- SO42-/AC
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图 1 不同前驱体制备SO42-/AC催化剂的XRD谱图
Figure 1 XRD patterns of SO42-/AC catalysts with different precursors
图 2 30%H2SO4/AC以及30%(NH4)2SO4/AC催化剂的N2吸附-脱附曲线(a)，30%H2SO4/AC以及30%(NH4)2SO4/AC催化剂的BJH孔分布(b)
Figure 2 N2 adsorption-desorption isotherms of 30%H2SO4/AC and 30%(NH4)2SO4/AC catalysts (a), BJH pore size distribution of 30%H2SO4/AC and 30%(NH4)2SO4/AC catalysts (b)
图 3 不同前驱体制备SO42-/AC催化剂的NH3-TPD谱图
Figure 3 NH3-TPD profiles of SO42-/AC catalyst with different precursors
图 5 由DME直接氧化合成DMMx的可能反应路径示意图
Figure 5 Possible reaction pathway of DME direct oxidation to DMMx
表 1 不同前驱体制备SO42-/AC催化剂的反应性能
Table 1 Effects of different precursors on the performance of SO42-/AC catalysts
Catalyst DME Conversion x/% Selectivity sC-mol /% DMM DMM2 DMM1-2 CH3OH HCHO MF CH4 COx AC 6.8 0 0 0 54.2 45.8 0 0 0 20%H2SO4/AC 8.3 8.6 6.8 15.4 65.2 19.4 0 0 0 30%H2SO4/AC 8.3 24.9 6.0 30.9 47.4 17.9 3.8 0 0 40%H2SO4/AC 8.4 55.3 4.4 59.7 22.7 13.1 4.5 0 0 20%(NH4)2SO4/AC 10.0 3.4 0 3.4 12.3 3.1 5.2 0 76.0 30%(NH4)2SO4/AC 9.6 5.2 0 5.2 8.1 2.9 3.9 0 79.9 40%(NH4)2SO4/AC 9.5 2.7 0 2.7 13.1 1.3 0.8 0 82.1 reaction conditions: atmospheric pressure, 240 ℃, catalyst: 1 mL, 3 600 h-1, O2/DME(molar ratio)= 1.3:1.2 
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表 2 载体及催化剂的孔结构参数
Table 2 Textural properties of support and various catalysts
Catalyst BET surface area A/(m2·g-1) Total pore volume v/(cm3·g-1) Average pore diameter d/nm AC 948.5 0.452 1.910 30%H2SO4/AC 636.9 0.383 2.006 30%(NH4)2SO4/AC 146.2 0.108 1.945
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表 3 NH3-TPD谱图中催化剂的酸量
Table 3 Integration results of NH3-TPD profiles
Catalyst Weak acid sites /(μmol·g-1) Medium-strong acid sites /(μmol·g-1) AC 1.54 - 20%H2SO4/AC 9.57 - 30%H2SO4/AC 11.24 - 40%H2SO4/AC 12.30 - 20%(NH4)2SO4/AC - 17.26 30%(NH4)2SO4/AC - 21.45 40%(NH4)2SO4/AC - 24.04
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