Preparation Of Activated Carbon From Petroleum Coke By KOH Activation And The Chemical Activation Mechanism

Activated carbon (AC) with high surface area is widely used in fuel gas storage, gas separation, catalysis and various chemical processes. Controlling pore size and pore size distribution are necessary for the application of AC in a specific end use. It is well known that petroleum coke (PC) is a good precursor for preparing high surface area AC because of its' high carbon content, low volatile and ash content. However, PC is difficult to be activated due to the stable micrographitic structure and lack of the initial pores in it, so that a large amount of KOH is needed to prepare AC from PC, which brings many problems, such as high product cost, environment pollution etc. Therefore, how to optimize the process conditions, such as mild reaction temperature with lower KOH/coke ratio, and produce AC with desired porous texture based on a deeper understanding of the chemical activation mechanism is the emphasis for further research. In this dissertation, investigations were conducted as follows:First, AC is prepared from a green PC from SINOPEC Liaoyang Petrochemical Company as precursor and KOH as activating agent, the effect of different preparation conditions, such as the method for mixing KOH and coke, carrier gas flow and particle size of coke on porosity development of AC was investigated. It has been found that the dipping of activated mixtures in different solutions before washing affects the pore formation of AC greatly; indicating the elimination of the produced potassium compounds is an important step for the porosity development of AC.To investigate the effect of the changes of surface groups and micrographitic structure of the precursor on the porosity development of AC, the PC was pre-carbonized before activation. With the increase of the heat treatment temperature, the hydrogen and oxygen content, and the surface functional groups of the PC were decreased, while the micrographitic structure of it became more stable and difficult to be activated. So the BET surface area of the corresponding AC decreased. The BET surface area of the AC made from green coke at a 3:1 KOH/Coke ratio is 1997m²g^-1, while that from 1200℃treated coke is 12m²g^-1. The pre-carbonization at relative low temperature has little effect on the BET surface area of the final carbon, but it slightly widens the micropore size of the AC.The influence of pre-oxidation of the PC with oxidizing agent H₂O₂, HNO₃ or air on the development of porosity of the produced AC was analyzed in the view of the changes of the surface chemistry and the texture of the PC. The results showed that introduction of the surface oxygen groups by the pre-oxidation leads to an increase m me reactivity of the PC in chemical activation, which results in an increase in the BET surface area and the N₂ adsorption capacity of the AC. And the addition of oxygen to the graphitic microcrystallines of the precursors slightly widens the interlayer spacing. Therefore, an AC with a surface area as high as 2744 m²g^-1 and narrow pore size distribution was prepared from the coke oxidized by 20%H₂O₂ solution with a 3:1 KOH/Coke ratio. Based on the above results, it can be indicated that the surface oxygen groups of the precursor can be acted as 'active site', which play an important role in activation.Based on above investigations, a systemic chemical activation mechanism of the PC with KOH was propposed. During the chemical activation with KOH, pores were formed by reaction of KOH with PC, and then the formed K₂CO₃ can further react with carbon, which contributes to the porosity development. However, when the carbon precursors were activated with directly added K₂CO₃, the AC yields were all very low; while the BET surface area of them were not as large as expected. It indicates that the distribution of K₂CO₃ in the formed pores of the carbon contributes to the porosity development by the reactions of K₂CO₃ with -CH-, -CH₂- in PC. XRD analysis indicated that the produced K intercalates into the lamellae of the graphitic microcrystallines of the carbon, and broadens the interlayer space, consequently further develops the porous structure. An activated coke with a BET surface area of 98 m²g^-1 was obtained from PC by activation with the produced K.Based on the proposed chemical activation mechanism, a new method to produce high surface area AC has been developed, in which additive N was used to improve the diffusion of KOH into PC and act as activation agent at the same time during the chemical activation. The AC with high solid yield of 68%, BET surface area of 2532 m²g^-1 and narrow pore size distribution can be prepared with a 3:1 KOH/Coke ratio from activation at 730℃for 30 minutes by this method.A HDS catalyst is derived from the prepared AC supported with Ni and Mo, which shows a HDS activity comparable to that of the commercial catalyst of Mo-Ni/γ-Al₂O₃. The dibenzothiophene conversion reached over 98% at 290℃. The AC with high surface area and pore size distribution between 1.0～2.0nm was used as adsorbent for methane storage, which shows an excellent methane-storing performance. Under pressure of 1.4 MPa and room temperature, the methane-storing ability of the AC reaches 0.112 g·g^-1.