Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

Methanol synthesis by CO₂ hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In₂O₃ supported on ZrO₂ under industrially relevant conditions. This strongly contrasts to the benchmark Cu‐ZnO‐Al₂O₃ catalyst, which is unselective and experiences rapid deactivation. In‐depth characterization of the In₂O₃‐based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co‐feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.     

Development of high performance Cu/ZnO-based catalysts for methanol synthesis and the water-gas shift reaction

Our groups studies on Cu/ZnO-based catalysts for methanol synthesis via hydrogenation of CO₂ and for the water-gas shift reaction are reviewed. Effects of ZnO contained in supported Cu-based catalysts on their activities for several reactions were investigated. The addition of ZnO to Cu-based catalyst supported on Al₂O₃, ZrO₂ or SiO₂ improved its specific activity for methanol synthesis and the reverse water-gas shift reaction, but did not improve its specific activity for methanol steam reforming and the water-gas shift reaction. Methanol synthesis from CO₂ and H₂ over Cu/ZnO-based catalysts was extensively studied under a joint research project between National Institute for Resources and Environment (NIRE; one of the former research institutes reorganized to AIST) and Research Institute of Innovative Technology for the Earth (RITE). It was suggested that methanol should be produced via the hydrogenation of CO₂, but not via the hydrogenation of CO, and that H₂O produced along with methanol should greatly suppress methanol synthesis. The Cu/ZnO-based multicomponent catalysts such as Cu/ZnO/ZrO₂/Al₂O₃ and Cu/ZnO/ZrO₂/Al₂O₃/Ga₂O₃ were highly active for methanol synthesis from CO₂ and H₂. The addition of a small amount of colloidal silica to the multicomponent catalysts greatly improved their long-term stability during methanol synthesis from CO₂ and H₂. The purity of the crude methanol produced in a bench plant was 99.9 wt% and higher than that of the crude methanol from a commercial methanol synthesis from syngas. The water-gas shift reaction over Cu/ZnO-based catalysts was also studied. The activity of Cu/ZnO/ZrO₂/Al₂O₃ catalyst for the water-gas shift reaction at 523 K was less affected by the pre-treatments such as calcination and treatment in H₂ at high temperatures than that of the Cu/ZnO/Al₂O₃ catalyst. Accordingly, the Cu/ZnO/ZrO₂/Al₂O₃ catalyst was considered to be more suitable for practical use for the water-gas shift reaction. The Cu/ZnO/ZrO₂/Al₂O₃ catalyst was also highly active for the water-gas shift reaction at 673 K. Furthermore, a two-stage reaction system composed of the first reaction zone for the water-gas shift reaction at 673 K and the second reaction zone for the reaction at 523 K was found to be more efficient than a one-stage reaction system. The addition of a small amount of colloidal silica to a Cu/ZnO-based catalyst greatly improved its long-term stability in the water-gas shift reaction in a similar manner as in methanol synthesis from CO₂ and H₂.     

溶剂极性对微波辐射老化制备前驱体及CuO/ZnO/Al2O3催化剂结构的影响

在制备CuO/ZnO/Al2O3催化剂的老化过程中,采用微波辐射老化技术,着重研究了溶剂极性对前躯体物相组成,烧后CuO/ZnO/Al2O3催化剂结构及其在浆态床合成甲醇工艺中催化性能的影响。通过XRD、DTG、H2-TPR,FTIR、HR-TEM和XPS对前驱体及催化剂表征表明,沉淀母液在微波辐射条件下进行老化,溶剂的极性对前躯体物相组成及催化剂结构影响显著。随着溶剂极性的增大,Zn2+/Cu2+取代Cu2(CO3)(OH)2/Zn5(CO3)2(OH)6中Cu2+/Zn2+的取代反应增强,使得前躯体中(Cu,Zn)5(CO3)2(OH)6和(Cu,Zn)2(CO3)(OH)2物相的含量增多,结晶度提高,导致烧后CuO/ZnO/Al2O3催化剂中CuO-ZnO协同作用增强,且CuO晶粒减小,表面Cu含量增加,催化剂活性和稳定性提高。水溶剂的极性最大,制备的催化剂活性和稳定性最好,甲醇的时空收率(STY)和平均失活率分别为320 mg.g-1.h-1和0.11%.d-1。     

Visualizing Formation of Intermetallic PdZn in a Palladium/Zinc Oxide Catalyst: Interfacial Fertilization by PdHx

Controllable synthesis of well‐defined supported intermetallic catalysts is desirable because of their unique properties in physical chemistry. To accurately pinpoint the evolution of such materials at an atomic‐scale, especially clarification of the initial state under a particular chemical environment, will facilitate rational design and optimal synthesis of such catalysts. The dynamic formation of a ZnO‐supported PdZn catalyst is presented, whereby detailed analyses of in situ transmission electron microscopy, electron energy‐loss spectroscopy, and in situ X‐ray diffraction are combined to form a nanoscale understanding of PdZn phase transitions under realistic catalytic conditions. Remarkably, introduction of atoms (H and Zn in sequence) into the Pd matrix was initially observed. The resultant PdH_x is an intermediate phase in the intermetallic formation process. The evolution of PdH_x in the PdZn catalyst initializes at the PdH_x/ZnO interfaces, and proceeds along the PdH_x ?111? direction.     

Methanol‐Assisted Autocatalysis in Catalytic Methanol Synthesis

Catalytic methanol synthesis is one of the major processes in the chemical industry and may grow in importance, as methanol produced from CO₂ and sustainably derived H₂ are envisioned to play an important role as energy carriers in a future low‐CO₂‐emission society. However, despite the widespread use, the reaction mechanism and the nature of the active sites are not fully understood. Here we report that methanol synthesis at commercially applied conditions using the industrial Cu/ZnO/Al₂O₃ catalyst is dominated by a methanol‐assisted autocatalytic reaction mechanism. We propose that the presence of methanol enables the hydrogenation of surface formate via methyl formate. Autocatalytic acceleration of the reaction is also observed for Cu supported on SiO₂ although with low absolute activity, but not for Cu/Al₂O₃ catalysts. The results illustrate an important example of autocatalysis in heterogeneous catalysis and pave the way for further understanding, improvements, and process optimization of industrial methanol synthesis.     

New process of low-temperature methanol synthesis from CO/CO2/H2 based on dual-catalysis

A new process of low-temperature methanol synthesis from CO/CO₂/H₂ based on dual-catalysis has been developed. Some alcohols, especially 2-alcohol, were found to have high catalytic promoting effect on the synthesis of methanol from CO hydrogenation. At 443 K and 5 MPa, the synthesis of methanol could process high effectively, resulting from the synergic catalysis of Cu/ZnO solid catalyst and 2-alcohol solvent catalyst. The primary results showed that when 2-butanol was used as reaction solvent, the one-pass average yield and the selectivity of methanol, in 40 h continuous reaction at temperature as low as 443 K and 5 MPa, were high up to 46.51% and 98.94% respectively. The catalytic activity was stable and the reaction temperature was 80 K or so lower than that in current industry synthesis process. This new process hopefully will become a practical method for methanol synthesis at low temperature.     

In situ infrared study of formate reactivity on water–gas shift and methanol synthesis catalysts

In order to investigate the methanol synthesis reaction from CO₂/H₂, a comparative study of the reactivity of formate species on different types of catalysts and catalyst supports has been carried out. Formic acid was adsorbed on water–gas shift catalysts, Cu/ZnO/Al₂O₃ methanol synthesis catalyst and ZnO/Al₂O₃ support, Cu/ZnO/ZrO₂ and Cu/ZnO/CeO₂ methanol synthesis catalysts as well as their corresponding supports ZnO/ZrO₂ and ZnO/CeO₂. Superior reactivity and selectivity of dedicated methanol synthesis catalysts was evidenced by their behavior during the subsequent heating ramp, when these samples showed the simultaneous presence of formates and methoxy species and a higher stability of these reaction intermediates in the usual temperature range for the methanol synthesis reaction.     

Direct Catalytic Conversion of Ethanol to C5+ Ketones: Role of Pd–Zn Alloy on Catalytic Activity and Stability

Ethanol can be used as a platform molecule for synthesizing valuable chemicals and fuel precursors. Direct synthesis of C₅₊ ketones, building blocks for lubricants and hydrocarbon fuels, from ethanol was achieved over a stable Pd‐promoted ZnO‐ZrO₂ catalyst. The sequence of reaction steps involved in the C₅₊ ketone formation from ethanol was determined. The key reaction steps were found to be the in situ generation of the acetone intermediate and the cross‐aldol condensation between the reaction intermediates acetaldehyde and acetone. The formation of a Pd–Zn alloy in situ was identified to be the critical factor in maintaining high yield to the C₅₊ ketones and the stability of the catalyst. A yield of >70 % to C₅₊ ketones was achieved over a 0.1 % Pd‐ZnO‐ZrO₂ mixed oxide catalyst, and the catalyst was demonstrated to be stable beyond 2000 hours on stream without any catalyst deactivation.     

表面助剂改性对Cu/ZnO/Al2O3甲醇合成催化剂性能的影响

采用共沉淀-后浸渍方法制备了表面助剂改性的Cu/ZnO/Al₂O₃ (CZA)甲醇合成催化剂, 在固定床反应器上以合成气为原料分别考察了三种助剂(Zr、Ba和Mn)对CZA催化剂性能的影响; 以Zr为助剂时反应温度的影响; 并进行了催化稳定性试验. 利用粉末X射线衍射(XRD)、低温氮气吸脱附(N₂-sorption)、氧化亚氮(N₂O)反应吸附技术、X射线光电子能谱(XPS)、氢气程序升温吸脱附(H₂-TPD)、扫描电子显微镜(SEM)和高分辨透射电子显微镜(HR-TEM)技术对催化剂进行了表征.结果显示: 以Zr或Ba作为助剂能够明显提高CZA催化剂耐热前后的甲醇时空收率(STY); Mn的引入降低了CZA催化剂的耐热前活性; Zr的引入降低了CZA催化剂最高活性温度点, 增强了CZA催化剂的催化稳定性; 还原态CZA催化剂表面Cu⁰和ZnO都能吸附活化氢气, Cu⁰与ZnO的强相互作用有利于提高催化剂的性能, 耐热后催化剂性能的降低归因于Cu晶粒的长大. 在实验和表征结果基础上,提出了CZA催化剂上合成气制甲醇的“双向同步催化反应历程”.     

Quantification of Zinc Atoms in a Surface Alloy on Copper in an Industrial‐Type Methanol Synthesis Catalyst

Methanol has recently attracted renewed interest because of its potential importance as a solar fuel. 1 Methanol is also an important bulk chemical that is most efficiently formed over the industrial Cu/ZnO/Al₂O₃ catalyst. The identity of the active site and, in particular, the role of ZnO as a promoter for this type of catalyst is still under intense debate. 2 Structural changes that are strongly dependent on the pretreatment method have now been observed for an industrial‐type methanol synthesis catalyst. A combination of chemisorption, reaction, and spectroscopic techniques provides a consistent picture of surface alloying between copper and zinc. This analysis enables a reinterpretation of the methods that have been used for the determination of the Cu surface area and provides an opportunity to independently quantify the specific Cu and Zn areas. This method may also be applied to other systems where metal–support interactions are important, and this work generally addresses the role of the carrier and the nature of the interactions between carrier and metal in heterogeneous catalysts.     

Supported Intermetallic PdZn Nanoparticles as Bifunctional Catalysts for the Direct Synthesis of Dimethyl Ether from CO‐Rich Synthesis Gas

The single‐step syngas‐to‐dimethyl ether (STD) process entails economic and technical advantages over the current industrial two‐step process. Pd/ZnO‐based catalysts have recently emerged as interesting alternatives to currently used Cu/ZnO/Al₂O₃ catalysts, but the nature of the active site(s), the reaction mechanism, and the role of Pd and ZnO in the solid catalyst are not well established. Now, Zn‐stabilized Pd colloids with a size of 2 nm served as the key building blocks for the methanol active component in bifunctional Pd/ZnO‐γ‐Al₂O₃ catalysts. The catalysts were characterized by combining high‐pressure operando X‐ray absorption spectroscopy and DFT calculations. The enhanced stability, longevity, and high dimethyl ether selectivity observed makes Pd/ZnO‐γ‐Al₂O₃ an effective alternative system for the STD process compared to Cu/ZnO/γ‐Al₂O₃.     

Alcohol-Induced Strong Metal-Support Interactions in a Supported Copper/ZnO Catalyst

Tuning the strong metal-support interaction (SMSI) in metal catalysts is a promising strategy to improve their catalytic performance. In this article, we systematically investigated the influences of different alcohol/water mixtures on the evolution of the interfacial structure of Cu/ZnO catalysts in the reduction stage. A series of in situ characterization and theoretical simulation studies were performed to elucidate the various mechanisms of alcohol induced SMSI. It was found that when methanol/water is added to H₂ during the reduction pretreatment, more oxygen vacancies are formed on the ZnO support, which facilitates the dissociation of H₂O and the hydroxylation of ZnO species. Such promotion eventually favors the SMSI between Cu and ZnO and increases the catalytic activity for the methanol steam reforming reaction. In contrast, the addition of ethanol/water and 1-propanol/water during reduction leads to a physical blockage of the catalyst by alcohol molecules, poisoning the active Cu sites and limiting the migration of ZnO species.     

C207联醇催化剂的羰基硫中毒研究

以常压甲醇分解反应为基础实验测定了不同羰基硫含量和不同反应温度下甲醇合成铜基催化剂活性随时间的变化。结果表明，羰基硫含量增加时；失活速率增快；反应温度升高时，失活速率也加快。羰基硫中毒前后催化剂样品的ＸＲＤ谱图证实了起催化作用的活性组分为Ｃｕ＾＋／ＺｎＯ。应用改进的高斯－牛顿法对失活实验数据进行参数估值，获得了Ｃ２０７铜基催化剂羰基硫中毒的失活速率方程。     

Methanol-Assisted Autocatalysis in Catalytic Methanol Synthesis

Catalytic methanol synthesis is one of the major processes in the chemical industry and may grow in importance, as methanol produced from CO₂ and sustainably derived H₂ are envisioned to play an important role as energy carriers in a future low-CO₂-emission society. However, despite the widespread use, the reaction mechanism and the nature of the active sites are not fully understood. Here we report that methanol synthesis at commercially applied conditions using the industrial Cu/ZnO/Al₂O₃ catalyst is dominated by a methanol-assisted autocatalytic reaction mechanism. We propose that the presence of methanol enables the hydrogenation of surface formate via methyl formate. Autocatalytic acceleration of the reaction is also observed for Cu supported on SiO₂ although with low absolute activity, but not for Cu/Al₂O₃ catalysts. The results illustrate an important example of autocatalysis in heterogeneous catalysis and pave the way for further understanding, improvements, and process optimization of industrial methanol synthesis.     

Syngas conversion beyond chemical equilibrium by in situ bimolecular reaction

An in situ bimolecular reaction, in which syngas is fed with toluene as a secondary reactant (hereafter Tol in situ methylation), was studied over bifunctional catalysts comprised of methanol synthesis catalyst and H-ZSM-5 in a fixed-bed down-flow reactor at 460 psig. When physically mixed with H-ZSM-5 to form bifunctional catalysts, CrZ_HZ (Cr₂O₃/ZnO + HZSM-5) catalyst showed much higher activity than CZA_HZ (CuO/ZnO/Al₂O₃ + H-ZSM-5) in the Tol in situ methylation, while CrZ catalyst exhibited substantially lower activity than CZA in methanol synthesis. CO conversion to methanol in the Tol in situ methylation was estimated by Bz in situ methylation. The CO conversion to methanol was calculated to be in the range of 11–27 %, while that in methanol synthesis over CrZ was about 5 % at most due to chemical equilibrium limitation. By employing a silicalite-coated H-ZSM-5 (Sil/HZ) in bifunctional catalyst, xylene selectivity and para-xylene yield were much improved in the Tol in situ methylation.     

Promoting effect of polyoxyethylene octylphenol ether on Cu/ZnO catalysts for low-temperature methanol synthesis

Cu/ZnO catalysts were prepared by the co-precipitation method with the addition of OP-10 (polyoxyethylene octylphenol ether) and were chemically and structurally characterized by means of XRD, BET, H₂-TPR, CO-TPD and N₂O-titration. The effect of OP-10 addition on the activity of Cu/ZnO for the slurry phase methanol synthesis at 150 °C was evaluated. The results showed that Cu/ZnO prepared with addition of 8% OP-10 (denoted as C8) exhibited the promoted activity for the methanol synthesis. The conversion of CO and the STY (space time yield) of methanol were 42.5% and 74.6% higher than those of Cu/ZnO prepared without addition of OP-10 (denoted as C0), respectively. The precursor of C8 contained more aurichalcite and rosasite, and the concerted effect of Cu-Zn in C8 was found to be stronger than that in C0. Compared with C0, C8 showed smaller particle size, lower reduction temperature and larger BET and Cu surface areas.     

Counting of Oxygen Defects versus Metal Surface Sites in Methanol Synthesis Catalysts by Different Probe Molecules

Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N₂O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N₂O, temperature programmed desorption of H₂, and kinetic experiments. The presence of ZnO dramatically influences the N₂O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu‐ZnO interface and can be used to easily quantify the intensity of Cu‐Zn interaction. N₂O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H₂.     

Bifunctionality of Cu/ZnO catalysts for alcohol-assisted low-temperature methanol synthesis from syngas:Effect of copper content

Alcohol-assisted low-temperature methanol synthesis was conducted over Cu/ZnO_X catalysts while varying the copper content(X). Unlike conventional methanol synthesis, ethanol acted as both solvent and reaction intermediate in this reaction, creating a different reaction pathway. The formation of crystalline phases and characteristic morphology of the co-precipitated precursors during the co-precipitation step were important factors in obtaining an efficient Cu/ZnO catalyst with a high dispersion of metallic copper,which is one of the main active sites for methanol synthesis. The acidic properties of the Cu/ZnO catalyst were also revealed as important factors, since alcohol esterification is considered the rate-limiting step in alcohol-assisted low-temperature methanol synthesis. As a consequence, bifunctionality of the Cu/ZnO catalyst such as metallic copper and acidic properties was required for this reaction. In this respect, the copper content(X) strongly affected the catalytic activity of the Cu/ZnO_X catalysts, and accordingly, the Cu/ZnO_0.5 catalyst with a high copper dispersion and sufficient acid sites exhibited the best catalytic performance in this reaction.     

Oxidative Fluorination of Cu/ZnO Methanol Catalysts

The influence of a mild difluorine treatment on Cu/ZnO precatalysts for methanol synthesis was investigated. It led to the incorporation of 1.2…1.3±0.1 wt % fluoride into the material. Fluorination considerably increased the amount of ZnO_x related defect sites on the catalysts and significantly increased the space‐time yields. Although the apparent activation energy E_A,app for methanol formation from CO₂ and H₂ was almost unchanged, the E_A,app for the reverse water‐gas shift (rWGS) reaction increased considerably. Overall, fluorination led to a significant gain in methanol selectivity and productivity. Apparently, also the quantity of active sites increased.