Approach Of Oriented Conversion Of Deoiled Asphalt To Advanced Carbon Functional Materials

Carbon materials, which have a close relationship with advancement of human civilization, have been one of the most important materials. The various carbon and their compounds have done a great contribution during human progress. Advanced carbon functional materials, such as vapor grown carbon fibers (VGCFs), carbon microbeads (CMBs), carbon nanotubes (CNTs), carbon nano onion-like fullerenes (NOLFs) and metal-encapsulating carbon particles, have achieved extensive application in many areas because of their excellent properties.Deoiled asphalt (DOA) is a carbon-rich by-product of petroleum industry. Vapor grown carbon materials can be synthesized from synergic effects of gaseous species, including CH₄, CO, H₂, N₂, H₂S and low molecular hydrocarbons, which are released during by the pyrolysis of DOA. Furthermore, the residue coke from DOA can be converted into graphitie-like products by heat-treatment or arc-discharge method. Therefore, it is suggested that DOA is a favorable option as a carbon source for large-scale synthetic process of advanced carbon materials. Utilization of DOA of such kind can provide a new approach for expansion of product chains of petroleum processing and comprehensive utilization of petroleum resource.In this paper, the preparation of carbon materials with high added value was achieved from DOA by chemical vapor deposition (CVD), co-carbonization and microwave plasma methods, including VGCFs, CMBs, CNTs, Fe-encapsulating NOLFs, Fe₃C-containing carbon microparticles and aligned carbon film. The structures and growth mechanisms of the products were investigated systematically with special emphasis on growth model of VGCFs. The thermal stability and H₂ storage performance of VGCFs were also explored. Based on similarities and differences between DOA and coal tar pitch (CTP), carbon materials, such as VGCFs, CMBs, CNTs and metal-encapsulating carbon nanoparticles, were also prepared from CTP by CVD.The main contents and conclusions are as follows:1. VGCFs were synthesized by CVD in argon atmosphere, using deoiled asphalt as carbon source and ferrocene as catalyst precursor. The influences of different experimental parameters, such as ferrocene content, reaction temperature, reaction time and argon flow rate, were investigated, with respect to the morphology and product yield of VGCFs. The morphologies and structures of products were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Raman spectroscopy. The formation mechanism of VGCFs was discussed in detail. The thermal stability and H₂ storage performance of VGCFs were also explored. The results revealed as follows:The synthesis of VGCFs was found to be dependent on the synergic effects of gaseous species from thermal cracking of DOA and following a growth mechanism of particle-tube-fiber: catalyst nanoparticles first agglomerate and assemble into larger nanoparticles, onto which gasous carbon species are absorbed and converted into CNTs through diffusion and precipitation of gaseous carbon species, and CNTs finally develop into VGCFs by a self-catalysis behavior. An effective mass production of controllable and pure VGCFs, with diameters ranging from 100nm to 1.2μm, was achieved under optimized preparation parameters. The VGCFs exhibited a tree annual ring type of structure and had a hollow core. Their walls can be divided into two different kinds of structures, that is, the high-graphitized inner layer shells and low-graphitized outer layer shells. CMBs were produced in the absence of catalyst. Oriented VGCFs, carbon trees and secondary VGCFs were obtained under appropriate conditions. The VGCFs with special structures (branched, bamboo-shaped and core-shell structure) were also discussed.VGCFs from direct pyrolysis of DOA are distinctively different from traditional pitch-derived carbon fibers in their simple and low-cost fabrication, unique structure and excellent properties. This new approach to VGCFs should be of potential in the production of carbon fibers in large scale.2. CNTs were prepared by CVD using DOA as carbon source and ferrocene as catalyst precursor in an atmosphere of hydrogen and argon. The influences of reaction temperature and hydrogen flow rate on CNTs growth were investigated. The optimum technological parameters for the preparation of CNTs were determined through CNTs characterization by FESEM, HRTEM, EDS, XRD and Raman spectroscopy. The result revealed that high purity and uniform CNTs, with diameter of about 35nm, were grown from DOA by CVD using ferrocene content of 10wt.% at 1000℃for 30min in an atmosphere of hydrogen (150ml/min) and argon (150ml/min). The as-synthesized CNTs were randomly oriented and tangled with each other and had good crystallinity with some defects on their surface layers. The formation of CNTs depended on the joining effects of pyrolysis gases from DOA and followed vapor-liquid-solid growth model. The metal particles observed on tube tips suggested that the main growth mode followed a tip-growth mechanism.3. A series of size-controllable CMBs were grown from DOA by CVD, with the emphasis on the influences of experimental parameters, including reaction temperature, argon flow rate and reaction zone. The products were characterized by FESEM, HRTEM, EDS, XRD and Raman spectroscopy. The formation mechanism of CMBs was discussed based on experimental results. The results showed an effective mass production of size-controllable CMBs, with diameters ranging from 100nm to 1μm, was achieved. The obtained high purity CMBs were spherical with uniform size and amorphous structure.4. Fe-encapsulating NOLFs were obtained by CVD using DOA as carbon source and ferrocene as catalyst precursor in an argon flow of 150ml/min at 900℃for 30min. FESEM, HRTEM, EDS, XRD and Raman spectroscopy were used to characterize morphologies and microstructures of the products. The results showed that Fe-encapsulated NOLFs in the outlet of quartz tube had core/shell structures with sizes ranging from 3 to 5nm and their outer shells were composed of poorly crystallized graphitic layers. Their growth mode followed vapor-liquid-solid growth mechanism and all atoms in the graphite sheets arose from carbon atoms in the Fe-carbide particles.5. DOA as carbon source and ferrocene as catalyst precursor were chosen to synthesize an Fe/C composite, in which iron nanoparticles were dispersed uniformly in carbon via co-carbonization. The resulting samples were heat treated to synthesize Fe₃C-containing carbon microparticles by high-temperature heat treatment in vacuum. All products were examined by HRTEM and XRD. The results showed that large numbers of metal particles with～3nm in size were dispersed in carbon by co-carbonization products at the temperature of about 450℃for 3h and the degree of graphitization of the products was greatly improved after high-temperature heat treatment at 2000℃for 2h. An Fe₃C microparticle with～260nm in diameter was completely coated with 20 perfect graphitic carbon layers and the spacing of the lattice fringers was about 0.34nm. Besides, partially-filled and hollow carbon microparticles and hollow carbon fibers were obtained by this method.6. Aligned carbon film with novel structure was synthesised from DOA by microwave plasma method, the high pure products were formed orientedly in the shape of strip-like wheat head. Their out-layers possessed a higher degree of graphization, the maximum width was about 65nm and the length was about 900nm, but their center parts were amorphous. It was suggested that the products experienced an outer to inner growth. In addition, some metals in DOA, such as Ni and Fe, which might play the roles of catalyst for the growth of the aligned carbon film, were in favor of the formation of carbon nanomaterials. 7. The direct synthesis of carbon materials, including VGCFs, CMBs, CNTs and metal-encapsulating carbon nanoparticles, was achieved by the pyrolysis of CTP by CVD. The influences of the process parameters, including catalyst content and species, reaction temperature and time, species and flow rate of carrier gas, the growth zone of the products, were studied in detail. The morphologies and structures of the products were characterized and analyzed by FESEM, HRTEM, XRD and Raman spectroscopy. The experimental results demonstrated that pure CMBs with about 560nm in diameter were obtained in the absence of catalyst, and VGCFs, with the diameters of about 100nm, 115nm, 320nm and 890nm, respectively, were obtained, the curl and tangled CNTs with diameter of about 40nm were produced in mixed atmosphere of argon and hydrogen, metal-encapsulating carbon nanoparticles were obtained in the outlet of quartz tube, similar to DOA-based carbon materials. However, the different elemental composition and quality between DOA and CTP resulted in the different rules of influences on vapor grown carbon materials.In brief, DOA and CTP are good precursors for preparing advanced carbon functional materials with great prospect for wide application. Appropriate selection of processing conditions can realize the controllable conversion of DOA and CTP to various advanced carbon functional materials. This research can promote the combination of chemical industry of petroleum and coal with advanced carbon functional material so as to expand product chains in process of petroleum, realize comprehensive utilization of resource, provide a new route for research of carbon nanomaterials, and contribute to the science of carbon materials.