Cu-ZnO-ZrO2催化剂孔结构调控CO2加氢制甲醇性能研究

Pore structure modulation of Cu-ZnO-ZrO2 catalysts for methanol production from CO2 hydrogenation

  • 摘要: 采用胶质晶体模板法制备了不同孔径Cu-ZnO-ZrO2(CZZ)催化剂,并对其CO2加氢制甲醇性能进行了研究。结果表明,通过改变催化剂孔径可以实现ZnO粒径大小的调控,较小的粒径表现出更卓越的催化性能。其中,在孔径为55 nm的(CZZ-55)样品上,ZnO粒径为14.5 nm,CO2转化率为14.83%,甲醇选择性为78.8%,甲醇产率可达345.8 g/(kg·h)。原位漫反射傅里叶变换红外光谱结果表明,在CZZ催化剂上CO2加氢制甲醇遵循甲酸盐路径,ZnO-ZrO2界面是CO2吸附和活化的活性位点,而三维有序大孔结构有助于形成更分散的ZnO-ZrO2活性位,提高了CO2转化率。并且孔径大小对中间体的转化具有一定影响,孔径越小,甲酸盐更容易转化为甲醇。此外,三维有序的大孔结构为产物(水汽和甲醇)快速扩散提供了“高速通道”,有效抑制CO2加氢的副产物水汽对活性位的毒化作用,较大程度提高了催化剂的稳定性,在600 h内无明显失活。

     

    Abstract: Copper-based catalysts have attracted much attention for the hydrogenation of carbon dioxide (CO2) to synthesize methanol, however, problems including low methanol selectivity, easy sintering of the active components of the catalysts, and poor stability are commonly encountered. In this study, Cu-ZnO-ZrO2 (CZZ) catalysts with macroporous and nonporous morphology were prepared by the colloidal crystal template method and the conventional co-precipitation method, respectively, and their CO2 hydrotreating to methanol performance was investigated. In the colloidal crystal template method, polymethyl methacrylate (PMMA) was chosen as the template structure, and the diameter size of the macropores was regulated by controlling the PMMA particle size, so that samples with different pore sizes were prepared. The results show that compared with the bulk samples prepared by the co-precipitation method, the samples prepared by the template method have a permeable macroporous structure, and due to the special three-dimensional ordered structure of the macroporous holes, the ZnO can be uniformly dispersed around the pore wall formed by Cu, which effectively prevents the growth of ZnO particles. Moreover, by changing the pore size of the macropores, the regulation of ZnO particle size can be realized, and smaller ZnO particle size shows more excellent catalytic performance. Among them, excellent catalytic performance and application potential were demonstrated on a (CZZ-55) sample with a pore size of 55 nm, a ZnO particle size of 14.5 nm, a CO2 conversion of 14.83%, a methanol selectivity of 78.8%, and a methanol yield up to 345.8 g/(kg·h) which is 1.52 times higher than the performance of the nonporous catalyst. The results of in situ diffuse reflectance infrared Fourier transform spectroscopy showed that the methanol synthesis from CO2 hydrogenation over the CZZ catalyst followed the formate pathway, and the ZnO-ZrO2 interface was the active site for CO2 adsorption and activation. Moreover, the three-dimensional ordered macroporous structure provides an ideal “pedestal” for the creation of abundant interfaces and active sites, which contributes to the formation of more dispersed ZnO-ZrO2 active sites, thus significantly increasing the CO2 conversion rate. At the ZnO-ZrO2 interface, CO2 can be stably adsorbed and activated to form carbonates and bicarbonates and to adsorb formates generated by subsequent hydrogenation reactions. It is worth noting that the pore size of the catalyst has a significant effect on the conversion of the reaction intermediates. Specifically, smaller pore sizes were more favorable for the formation of the key intermediates formate and methoxide, which provided the necessary conditions for effective conversion to methanol. In addition, samples with three-dimensionally ordered macroporous structures play a unique role in the CO2 hydrogenation to methanol process compared with the non-porous bulk samples. The three-dimensional ordered macroporous structure of the pores provides a “high-speed channel” for the rapid diffusion of the reaction products (water vapor and methanol), which effectively inhibits the toxicity of the by-products of CO2 hydrogenation, water vapor, to the active components of Cu and ZnO, and improves the stability of the catalyst to a large extent. Under the actual reaction conditions of 4 MPa and 220 ℃, no obvious deactivation phenomenon was observed within 600 h, and the catalyst has a promising application. This work emphasizes the importance of catalyst morphology for the design of catalysts for methanol synthesis from CO2 hydrogenation and provides new ideas for the controlled synthesis of efficient methanol synthesis catalysts.

     

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