Studies On The Formation Mechanism Of Carbon Chain Involving In F-T Synthesis On Co Catalyst

As an important supplement way for oil-base carbon resources, Co-based Fischer-Tropsch synthesis can convert the relatively rich reserves of coal and natural gas in our country into the clean fuel or high value-added chemicals of alkane and olefin hydrocarbon. The reactant is simple, however, the reaction process is complex. Recently, many researchers have put forward the important four different reaction mechanisms and attempted to explain the reaction process based on these four mechanisms. Quantum chemical calculation is one of the important methods to investagate F-T synthesis reaction mechanism, while recent studies are lack of comparisons and analysis of the different mechanism. Therefore, it is of great s ignificance to systematically investigate the mechanism of F-T synthesis.Using density functional theory calcualtions in this study, the periodic slab model has been employed to investigate the F-T synthesis process of hydrocarbon chain initiation, growth and termination on two different surfaces of HCP Co catalyst(?Co?10-10?-A and Co?10-11? surfaces with 28% and 35% of the total surface area exposed, respectively?. First, the formations of C1 species CH_x, CHOH and CH₃ OH from syngas have been investigated, then, the hydrocarbon chain growth from C1 to C2 H_x, and C2 H_x to C3 H_x have been further investigated in order to illustrate the mechanism of F-T synthesis on Co catalyst. The main conclusions are listed as follows:?1? C1 species CHOH formation that related to the enol mechanism is difficult, indicating that enol mechanism is out of favor, while CH_x formation that related to other three kinds of mechanisms for the carbide, CO insertion and alkenyl mechanisms is easy.On Co?10-10?-A surface, CH_x is mainly formed through H-assisted CO dissociation pathway, in which CH is formed via CHO dissociation pathway of CO+H?CHO?CH+O, CH₂ and CH₃ are formed through CH₂ O with the direct and H-assisted dissociation, respectively, meanwhile, CH₂ hydrogenation also contributes to CH₃ formation, the corresponding pathways are CO+H? CHO+H?CH₂O?CH₂+O and CO+H?CHO+H?CH₂O+H?CH₂+OH are mainly responsible for CH₂ formation, CO+H?CHO+H?CH₂O+H?CH₃+O and CO+H?CHO+H?CH₂O+H?CH₂+OH+H?CH₃+OH pathways mainly contribute to CH₃ formation, CH₂ and CH₃ are the dominate existence form of CH_x species. On Co?10-11? surface, CH, CH₂ and CH₃ are formed through the hydrogenation of C species from CO direct dissociation, C and CH are the most abundant species. Thus, surface structure not only affects the pathway of CH_x formation, but also affects the main existence form of CH_x.CH₃OH is formed by the pathway of CO+H?CHO+H?CH₂O+H? CH₂OH+H?CH₃OH on Co?10-10?-A surface; two competitive pathways of CO+H?CHO+H?CHOH+H?CH₂OH+H?CH₃OH and CO+H?CHO+H? CH₂O+H?CH₂OH+H?CH₃OH are mainly responsible for CH₃ OH formation on Co?10-11? surface. CH₃ OH formation can not compete with CH_x formation on these two surfaces; as the Co?10-10?-A and Co?10-11? surfaces with the relatively high proportion of 28% and 35% of the total surface area exposed, respectively, it can be considered that the HCP Co catalyst exhibits a good catalytic performance towards CH_x formation rather than CH₃ OH, CH_x will particapate the further chain growth.?2? The hydrocarbon chain growth mechanism in F-T synthesis on two Co surface have been obtained. When the initial chain grows from CH_x to C2 H_x, the initial chain of CH₂ CH formation is not favorable on these two surfaces, respectively, indicating that the chain initiation based on the alkenyl mechanism is difficult.On Co?10-10?-A surface,CH₃CH₂ is the main C2 H_x species formed through carbide mechanism, the detailed pathways are CH₂+CH₂?C2H4+H?CH₃CH₂ and CH₃+CH₂?CH₃CH₂. For the C2 H_x species of CH₃CH₂ chain growth to C3 H_x, CH₃CH₂CH₂ is the main C3 H_x species formed through carbide mechanism with the pathway of CH₃CH₂+CH₂?CH₃CH₂CH₂. Therefore, the hydrocarbon chain growth on Co?10-10?-A surface is based on carbide mechanism of RCH₂+CH₂? R'CH₂, where R represents alkyl or H, and R' equals to RCH₂.For the chain termination, R'CH₂ can be terminated with being hydrogenated to alkanes. Since the favorable oxgeanate species RCH₂ CHO prefers to be hydrogenated to RCH₂CH₂ O rather than its desorption, resulting in the rare existence of aldehydes; the higher activation barrier of RCH₂CH₂ O hydrogenation make the alcohols formation also unlikely.On Co?10-11? surface, CH₃ CH and CH₃CH₂ are the main C2 H_x species, where CH₃ CH is formed through CHO insertion mechanism with the pathway of CH₂+CHO?CH₂CHO+H?CH₃CHO?CH₃CH+O; CH₃CH₂ is formed through carbide mechanism with the pathways of CH₂+CH₂?C2H4+H? CH₃CH₂ and CH₃+CH₂?CH₃CH₂, and through CHO insertion mechanism with the pathway of CH₂+CHO?CH₂CHO+H?CH₃CHO?CH₃CH+O+H?CH₃CH₂+O. From CH₃ CH and CH₃CH₂ chain growth to C3 H_x, CH₃CH₂ CH and CH₃CH₂CH₂ are the main C3 H_x species, where CH₃CH₂ CH formed via CHO insertion mechanism with the pathway of CH₃CH+CHO?CH₃CHCHO+H?CH₃CH₂CHO? CH₃CH₂ CH is similar to that of CH₃ CH formation; CH₃CH₂CH₂ is formed through two pathways of CH₃CH+CH₂?CH₃CHCH₂+H?CH₃CH₂CH₂ and CH₃CH₂+CH₂?CH₃CH₂CH₂ that based on carbide mechanism, and through the pathway of CH₃CH+CHO?CH₃CHCHO+H?CH₃CH₂CHO?CH₃CH₂CH+ O+H?CH₃CH₂CH₂+O that based on CHO insertion mechanism. As a result, on Co?10-11? surface, RCH and RCH₂ are the main chains, RCH chain growth are carried out by CHO insertion mechanism with the pathway of RCH+CHO?RCHCHO+H?RCH₂CHO?R'CH; meanwhile, RCH₂ chain growth are carried out by the carbide machanim with the pathways of RCH+CH₂?RCHCH₂+H?R'CH₂ and RCH₂+CH₂?R'CH₂ together with CHO insertion mechanism with the pathway of RCH+CHO?RCHCHO+H? RCH₂CHO?R'CH+H?R'CH₂.For the chain termination, R'CH can be terminated by coupling with CH₂ to form alkenes or hydrogenation to alkanes; R'CH₂ can be terminated by hydrogenation to alkanes. As RCH₂ CHO prefers to be dissociated into RCH₂ CH rather than disorption or hydrogenation to alcohols, therefore, the oxygenates of aldehydes and alcohols formation are also difficult.?3? On Co?10-10?-A surface, the main C2 H_x species of CH₃CH₂ that mainly formed through hydrogenation of ethene will participate in the further chain growth of coupling with CH₂, as a result, the hydrogeantion of ethene is crucial for chain growth; On Co?10-11? surface, the alkene of RCHCH₂ prefers to be hydrogeanted to RCH₂CH₂, and RCH₂CH₂ coupling with CH₂ can realize the chain growth, thus, the hydrogenation of alkene is responsible for RCH₂CH₂ chain growth that based on carbide mechanism. Therefore, the relatively strong adsorption capacity of alkene on Co surface will promote the chain growth of F-T synthesis.