Theoretical Study Of The Oxidative Dehydrogenation Of Propane Over Supported Vanadia Surface And Oxygenated Carbon Nanotubes

In the 21st century, the energy shortage issue has attracted wide attention. With the growing shortage of fossil fuel, how to convert light alkanes to value-added and useful light alkenes is a popular research issue in the petrochemical industry. The traditional industrial source of propene comes from the by-product of fossil oil cracking process. However, it is not conducive to the sustainable development. Another method of obtaining propene is the catalytic dehydrogenation of propane, which can be divided into the direct dehydrogenation (DDP) and oxidative dehydrogenation (ODP) of propane. The former one is an endothermic reversible reaction and should be carried out under the condition of high temperature and low pressure in order to improve the conversion of propane. However, this will lead to the reduction of propene selectivity and even the catalysts will be suffered from coke formation and deactivation. On the other hand, the latter one is a strong exothermic and irreversible reaction. However, due to the introduction of oxygen in the reaction, it will be easily suffered with deep oxidation of propane or propene to carbon oxides. Therefore, the choice of a high-performance catalyst is considered as the key to solve these problems. At present, supported vanadia catalysts are commonly employed as one of the efficient transition metal oxide catalysts in the ODP reaction. Meanwhile, in the recently years, modified carbon nanotubes (CNTs), which is the representative of carbon nanomaterial catalyst, has revealed a superior reactivity and propene selectivity in the ODP reaction. Furthermore, in comparison with conventional metal oxide catalysts, CNTs have advantages of corrosion resistance, unique surface properties, wide availability and environmental acceptability. It has greatly developed the prospect of metal-free materials. Although previous studies have already provided useful information about the ODP reaction on various catalysts, a systematic investigation on the origin of activity of catalysts and the physical and chemical properties that influenced the catalytic performance is still necessary. Therefore, in the present work, we use first principle density functional theory (DFT) methods to investigate the ODP reaction on the VOx/TiO2 and o-CNT catalysts. On this basis, we put emphasis on the following sections:DFT studies on structure and stability of monomeric VOx/TiO2 (anatase) catalysts:Periodic DFT method has been utilized to investigate the structure and stability of monomeric HVOx species on anatase support. Three most stable surfaces of anatase were investigated, namely the (001), (100) and (101) surfaces. Unlike previous theoretical studies it was found that on the (001) surface vanadia species with five-coordinated vanadium atom are more stable than those with tetrahedrally coordinated vanadium atom. On the other hand, on the (100) and (101) surfaces, the vanadium atom in the vanadia species is still tetrahedrally coordinated. The stability of different VOx/TiO2 structures which are not fully dehydrated has been systematically studied and the results show that the vanadia species on the three surfaces follow an order of TiO2 (001)> TiO2 (100)> TiO2 (101). This can be understood from the acidity and basicity of the three anatase surfaces. The results suggest that monomeric VOXxspecies may be better stabilized if the support exposes more (001) surfaces. Our analyses on electronic structure of the most stable VOx/TiO2 structure (D001) suggest that its bridging V-O-Ti oxygen atoms may have higher reactivity than the terminal vanadyl oxygen atoms.The influence of anatase support and vanadia dispersion of VOx/TiO2 catalysts on the ODP reaction:The ODP reaction on the terminal vanadyl (Ov=), interface (Ov-Ti) and bridging (Ov-v) oxygen sites of the monomer and dimer dispersed vanadia catalysts (VOx) supported on the anatase (001) and (100) surfaces have been investigated using DFT calculations. Gibbs free energy profile analyses indicate that on all the four catalysts the first C-H bond activation step of propane is the rate-determining step of the ODP reaction and the transition state of the propene formation step is the rate-determining transition state. It was found that the ODP activity of the VOx catalyst can be tuned by its dispersion and the support surface. The results indicate that catalysts supported on the (100) surface show higher activity than those on the (001) surface, and on the (100) surface, dimer dispersed VOX catalyst behaves better than the monomer one while vice versa on the (001) surface. Theoretical analyses indicate that the tuning of the catalytic activity is through the modification of the electronic structure of the lattice oxygen sites. On the monomer VOx/TiO2 (001) surface the Ov-Ti sites are better nucleophilic centers with higher bonding ability to hydrogen and thus have higher activity in activating the first C-H bond of propane. On the monomer VOx/TiO2 (100) surface the Ov= and Ov-Ti sites exhibit similar bonding ability and thus have similar C-H bond activating ability. On the other hand, on both dimer VOx/TiO2 surfaces the Ov= sites are better nucleophilic centers with higher C-H bond activating ability. It was also found that with increasing VOx loading, the formation of propene changes from direct hydrogen abstracting of propyl radical to a concerted propoxide mechanism. The present computational results suggest that for the ODP reaction the anatase (100) surface is a better support surface. To achieve higher catalytic performance it needs the cooperation of both the Ov= and Ov-Ti sites and they are the active centers for the first and second C-H bond activation, respectively.The origin of activity and catalytic performance of CNT on the ODP reaction:In the previous work, we have systematically investigated the ODP reaction on four VOx/TiO2 catalysts. In this work, we utilize CNTs as one of the typical carbon nanomaterial to explore the ODP reaction. We have systematically investigated the origin of the activity of CNTs and proposed how to improve the catalytic performance of CNTs on the ODP reaction. This study offers a general investigation of the microstructures of multiple active oxygen-containing sites and the detailed characterization of electronic structures. We have revealed that the essence to tune the catalytic performance is to tune the conjugation of CNT π orbital and the orbital of active CO groups. In these models, the bulk CNT serves as the electron reservoir which provides or draws electrons from the functional groups and subsequently alters their C-H activation activity. Groups that draw electrons from CNT have high C-H activation activity but the formed i-propoxide may be so stable as to poison the catalysts. On the other hand groups that transfer electrons to CNT have lower C-H activation activity but the formed i-propoxide is less stable. This conjugation can control the de localization level of the active sites and play a key role in the balance of the reaction activity and the stability of the intermediate while catalyzing the ODH of propane. This study can greatly promote our understanding on the origin of the activity of CNT catalysts from the microscopic scale and give an instructive guidance to the experimental preparation of high-performance CNT catalysts.