甲醇无氧脱氢制甲酸甲酯Cu/SiO2催化剂的粒径效应

(厦门大学化学化工学院,醇醚酯化工清洁生产国家工程实验室,福建 厦门 361005)

甲酸甲酯; 无氧脱氢; Cu/SiO2催化剂; 粒径效应

Particle size effect on the Cu/SiO2 catalyst in the nonoxidative dehydrogenation of methanol to methyl formate
HUANG Cong,XIE Liqiang,YE Linmin*,YUAN Youzhu

(National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,China)

methyl formate; nonoxidative dehydrogenation; Cu/SiO2 catalyst; particle size effect

DOI: 10.6043/j.issn.0438-0479.202104008

备注

以甲醇无氧脱氢制甲酸甲酯为目标反应,考察了3种方法制备的Cu/SiO2催化剂的性能差异,优选出蒸氨法并通过还原温度调控Cu纳米颗粒尺寸,结合催化剂的物理化学表征进行构效关联.研究表明:Cu/SiO2催化剂上存在两种活性位,脱氢过程主要在Cu0活性位上进行,生成甲醛中间体,再与甲醇偶联脱氢生成甲酸甲酯; Cu+活性位的存在促使Cu0活性位生成的甲醛中间体分解成CO和H2,降低了甲酸甲酯的选择性.随着还原温度升高,Cu/SiO2催化剂表面Cu0含量显著提升,且Cu粒径从3.1 nm增大到13.1 nm.甲酸甲酯的选择性随Cu0含量增大而提高,转换频率(TOF)随Cu粒径呈现火山型曲线变化,当Cu粒径为9.5 nm时,TOF最高可达22.9 h-1.在优化条件下,甲醇转化率和甲酸甲酯选择性分别为38.5%和90.2%,催化剂寿命超过100 h.

Objective: Methyl formate is an important C1 chemical that has both an aldehyde group and ester group in the molecule, and it can be used to synthesize methyl acrylate, methyl glycolate and dimethylformamide. Methanol anaerobic dehydrogenation method is the simplest, cheapest, and cleanest production process for methyl formate. In general, supported copper-based catalysts is widely used in the anaerobic dehydrogenation of methanol. However, the Cu/SiO2 catalyst prepared by the traditional method usually has complex Cu active sites, resulting in unsatisfactory methyl formate selectivity. In addition, the Cu nanoparticles are easily sintered during the reaction process, leading to the deactivation of catalyst.
Methods: Taking the anaerobic dehydrogenation of methanol to methyl formate as the model reaction, we carefully investigated the catalytic performance of Cu/SiO2 catalysts prepared by ammonia evaporation (AE), hydrothermal (HT) and ammonia evaporation hydrothermal (AEH) methods. The catalytic evaluation was performed in a fix-bed reactor equipped with on-line gas chromatography (GC) with flame ion detector (FID) and thermal conductivity detector (TCD). Various physicochemical characterizations based on X-ray diffraction (XRD), transmission electron microscope (TEM), CO temperature program desorption (CO-TPD), and CO diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) were carried out to understand the structure-performance correlation.
Results: Based on the catalytic performance, AE was selected as an ideal method for preparing Cu/SiO2 catalysts. Among three catalysts (Cu/SiO2-AE, Cu/SiO2-HT and Cu/SiO2-AEH), H2-TPR showed that the Cu/SiO2-AE catalyst exhibited the lowest reduction temperature, implying the weakest interaction between Cu and SiO2. Then, Cu nanoparticle of Cu/SiO2-AE catalyst is easier to be reduced to Cu0 than that of the comparison catalysts. On the other hand, the Cu nanoparticle size of Cu/SiO2-AE catalyst could be regulated by the reduction temperature from 300 to 800 °C. The XRD results showed that the Cu nanoparticle size was approximately 3.1 nm when the reduction temperature was lower than 550 °C. Further increasing the reduction temperature led to a significant aggregation of Cu nanoparticles size over 13 nm. Meanwhile, the results of XPS, CO-TPD and CO-DRIFTS characterization showed that Cu+ content of Cu/SiO2-AE exhibited a downward trend with increasing reduction temperature. Combined with the characterization and catalytic performance, it is speculated that both the valence state and nanoparticle size of Cu govern the catalytic performance of Cu-based catalyst. The turnover frequency (TOF) appeared as a volcano-type curve with the variation of Cu particle size, and peaked at 22.9 h-1 with the Cu particle size of 9.5 nm. The methanol conversion and methyl formate selectivity remained up to 38% and 90% during 100 h reaction time under the optimal reaction conditions: 200 °C, 0.1 MPa, n (N2) / n (CH3OH) = 2.6, and weight hourly space velocity (WHSV) = 4 h1 over Cu/SiO2-AE-750 catalyst.
Conclusion: The structure-performance correlation revealed that there were two kinds of active sites on Cu/SiO2 catalyst. The dehydrogenation process mainly carried out on Cu0 site and produced HCHO as an intermediate, followed by coupling with methanol to yield methyl formate as a target product. However, the existence of Cu+ site promoted the decomposition of the adsorbed HCHO intermediate into CO and H2, which reduced the methyl formate selectivity. Therefore, the regulation of surface Cu species to form more metallic Cu is of great importance to achieve high methyl formate selectivity.

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