Recent advances in metal-free carbon-based nanocatalysts

Abstract

The catalytic reaction occupies an important position in the petroleum and chemical industry, and the traditional metal-based catalytic materials are hindered because of the limited reserves, poor stability, environmental pollution and so on. In recent years, with the promotion of green chemistry and sustainable chemical, metal-free carbon based nanomaterials and their applications in catalysts have attracted wide attention, and it has become a hot topic in the fields of energy, materials and chemical industry. In this paper, the relationship between the structure and catalysis of carbon-based nanomaterials is briefly introduced. When metal free carbon based materials directly used as catalysts, their catalytic active sites are mainly contain three types. Firstly, edge atoms and defect sites, whose carbon atoms are unsaturated and have high catalytic activity, can activate oxygen molecules and catalyze methane cracking. Secondly, functional groups on the surface of carbon materials. Different functional groups can catalyze different reaction systems. The oxygen-containing functional groups, for example, have certain weak acidity and they generally possess excellent catalytic performance in the acid-base reaction and multiple oxidation reactions. Thirdly, heteroatoms that replace carbon atoms into sp2 lattices will results in the changing of electron density distribution in carbon structure, which can promotion of chemical reactions by reducing the reaction energy barriers. The progress in the basic research on the application of metal-free carbon-based nanomaterials for the reaction of liquid phase, gas phase and photoelectricity in the last five years is reviewed. By in situ infrared characterization technique ( in situ FTIR) and density functional theory (DFT) calculations, the catalytic mechanism catalyzed by carbon materials was studied and the result indicates that most of the catalytic reactions are surface reactions, whose reaction rate are greatly affected by the specific surface area. The merit and demerit including, but not limited to catalytic efficient and stability of powder catalyst and monolith catalyst are list. Specially, our study shows that the construction of three-dimensional structured monolithic carbon-based catalytic materials can effectively avoid the separation problem of powder metal free carbon-based catalytic materials, and provides a reference for solving the practical bottleneck of metal free carbon-based catalytic materials in liquid phase catalytic reaction. In addition, based on the research basis of our group, the problems and future prospect of carbon-based nanomaterials are discussed from the aspect of the reaction engineering. Actually, there are few studies on the reaction engineering of metal free carbon based catalysts. At present, metal free carbon-based catalysts are mainly used in the form of ultra-fine powders. From the perspective of chemical magnification, carbon nanomaterials participate in catalytic reactions mostly in heterogeneous catalytic process, and their catalytic performance is obviously affected by catalyst dispersion, fluidity, mixed mass transfer and so on. Conventional reactors have some problems, such as weak mixing effect, blind zone of mixing, and poor inter-phase transfer effect, which lead to the contradiction between selectivity and reaction rate in the development of industrial catalytic technology of carbon nanomaterials. At the same time, there are still some problems in the traditional catalytic process, such as poor catalyst stability, easy agglomeration, and low recycling efficiency. Therefore, in order to realize the industrial scale application of metal free carbon nanomaterials catalytic reaction system, the research and development of industrial reactor suitable for the catalytic reaction process of carbon nanomaterials and its matching process is the key to form practical technology. Three-dimensional monolithic catalyst can effectively improve the efficiency of heat and mass transfer. It is expected to solve the problems of low efficiency of separation and process transfer of metal free carbon-based catalytic materials, simplify the process flow and avoid the loss of catalyst. Therefore, there is still a long way to go for the research and application of three-dimensional metal free carbon based catalysts in the future.