Studies On The Modulation Of Structure And The Regulation Of Catalytic Performance For Cu-Based Catalysts In One-Carbon Reaction

One-carbon reaction has become one of the most important research contents for the new generation of coal chemical and natural gas chemical industry, the corresponding catalyst is the key to grasp the one-carbon reaction, how to develop the catalyst with the excellent catalytic performances has become the core content of one-carbon reaction. Nowadays, the cheap, easily available Cu-based catalysts have been widely used in one-carbon reaction, wherein, the catalytic performances of Cu-based catalysts are closely related to their corresponding microstructure, such as structure, chemical composition, activity center, supporter, the structure size of active components, and so on. The microstructure of catalyst itself can best reflect the essential attribute of their catalytic performances. The microstructure of Cu-based catalysts and the reaction mechanism are two important factors to clearly illustrate the catalytic performances of Cu-based catalysts. Based on the basic researches about the microstructure of Cu-based catalysts and the catalytic reaction mechanism, the effects of structure modulation for Cu-based catalysts on the catalytic performances can be clearly understood, then, a new catalytic process and the design of novel Cu-based catalysts can be proposed, which have become a hot topic in the research field of one-carbon reaction.In this paper, based on the quantum chemical density functional theory (DFT) method, four kinds of important one-carbon reactions on Cu-based catalysts are chosen as the research objects, including synthesis of methanol by CO2hydrogenation, synthesis of ethanol by syngas, synthesis of dimethyl carbonate by methanol oxidative carbonylation and synthesis of acetic acid by CH4-CO2two-step reaction. Then, the catalytic reaction mechanism is employed as the foundation to clearly illustrate the relationship between the modulation of structure and the regulation of catalytic performances for Cu-based catalysts in one-carbon reaction. Four different modulation ways of the microstructure for Cu-based catalysts, the doping of the second metal, the selection of supporter, the replacement of coordination atom for activity center and the structure size of active components, are used to probe into the regulation of catalytic performances, such as the catalytic activity, selectivity and stability of Cu-based catalysts. As a result, the corresponding relationship between the modulation way of structure and the regulation of catalytic performance can be defined, and the essential relationship between the modulation of structure and the regulation of catalytic performance can be revealed. The fundamental structural features of Cu-based catalysts with high catalytic performances in one-carbon reaction can be obtained. The reliable and systematic theory and method can be established to describe the modulation of structure and the regulation of catalytic performances for the Cu-based catalysts. The experimental clue can be provided for the screening, modification and design of novel Cu-based catalysts. The catalytic basic theory of one-carbon reaction can be enriched. The main conclusions are obtained as follows:1. For methanol synthesis from CO2hydrogenation on Cu-based catalyst, the doping of the second metal, the selection of supporter and the structure size of active components are chosen to modulate the structure of Cu-based catalysts; as a result, the catalytic activity, selectivity and stability of Cu-based catalysts can be well regulated. Methanol synthesis from CO2hydrogenation mainly goes through three reaction pathways to form methanol via two kinds of intermediates, CO and HCOO. Our results indicate that the hydroxylation supported PdCu/y-Al2O3bimetallic catalyst can exhibit good catalytic performances for methanol synthesis via CO2hydrogenation, which includes a high activity and selectivity to methanol, as well as the higher stability of this catalyst.2. For ethanol synthesis from syngas on Cu-based catalysts, methane needs to be introduced into syngas; then, a completely new reaction mechanism for ethanol synthesis via methane-syngas reaction routes can be obtained, which can realize the change of key rate-controlling step for the catalytic reaction of ethanol synthesis; as a result, the modulation of structure for Cu-based catalysts can effectively realize the regulation of catalytic performances. Our results suggest that the second metal Rh-doped RhCu bimetallic catalyst exhibits a high activity and selectivity to ethanol synthesis from methane-syngas method. Meanwhile, the strong adsorption ability of CO and the high catalytic activity of CH4dissociation into CH3are two critical factors, which can determine that whether the bimetallic Cu-based catalysts modulated by the doping of the second metal can exhibit the high catalytic activity for the reaction of methane-synthesis to ethanol.3. For DMC synthesis by oxidative carbonylation of methanol over Cu-based catalysts, the supported chloride-free copper catalysts should be firstly considered in order to solve the problems of catalytic performances. Meanwhile, in order to improve the catalytic activity and stability of supported chloride-free copper catalysts, the selection of supporter and the replacement of coordination atom for the active center Cu species should be used to modulate the structure of Cu-based catalysts. Our results show that adjusting the Si/Al ratio of supporter Y zeolites can improve the catalytic performances of Cu/Y catalyst, and CuY catalyst with Si/Al=6.5have shown the higher catalytic performances for the oxidative carbonylation of methanol to DMC, this result has been validated by the reported experiments.4. For the synthesis of acetic acid by CH4-CO2two-step reaction on Cu-based catalysts, the second metal Co-doped CoCu bimetallic catalyst shows the excellent synergetic effect for the two-step reaction, which can effectively regulate the catalytic performances of Cu-based catalysts for the synthesis of acetic acid. CoCu bimetallic catalyst is suitable to the synthesis of acetic acid by CH4-CO2two-step reaction with the high catalytic performances. Hereinto, the second doping metal Co shows the high catalytic activity for CH4dissociation, Cu metal shows the excellent catalytic activity and the high selectivity for the synthesis of acetic acid. On the other hand, the studies on the reaction mechanism of CH4-CO2two-step reaction over CoCu bimetallic catalyst not only find the overfall process of CHx between metal Co and Cu at the electronic-molecular level, but also clearly illustrate that CoCu bimetallic catalyst can solve the thermodynamic equilibrium yield of two-step reaction; the corresponding reason is that bimetallic catalyst can realize the reaction coupling with hydrogen-assistance for two-step reaction.5. The studies about the modulation of structure and the regulation of catalytic performances for Cu-based catalysts in four kinds of typical one-carbon reactions show that the different one-carbon reactions need to adopt the different modulation methods of structure in order to achieve the corresponding regulation of catalytic performances for Cu-based catalysts.6. The catalytic reaction mechanism is the foundation to elucidate the essential relationship between the structure modulation of catalyst and the regulation of catalytic performances. Only when the reaction mechanism of the catalytic reaction is clearly illustrated at the electronic-molecular level, the key rate-controlling step for catalytic reaction can be obtained. Then, it can be determined that which step is the key step of controlling reaction to achieve the regulation of catalytic performances, as a result, the accurate information of structure modulation for the screening and design of highly efficient catalysts can be provided. On the other hand, the studies on catalytic reaction mechanism using quantum chemistry calculation method can explicitly clarify and determine the intermediates and transition states involving the reaction processes, and exactly describe the details of reaction mechanism for each elementary reaction, in which the studies only relying on experimental method, especially the transitions states, can not be well completed; moreover, the results obtained by theoretical calculation can reveal the physical and chemical essence of catalysts with the high catalytic performances at the electronic-molecular level. Finally, our calculated results in this study show that the theoretical calculation can be used as a powerful tool to provide the clear experiment clues for the screening, modification and design of novel Cu-based catalysts in one-carbon reaction.