One-step Pyrolysis Can Control The Preparation Of Porous Carbon-limited Co Nanoparticles And Their Fischer-Tropsch Reaction

Fischer-Tropsch synthesis(FTS)is a gas-to-liquid process toward production of clean liquid hydrocarbon fuels with high quality from synthesis gas originating from coal,natural gas,or biomass feed stock.In general,the composition of active phase and support type present in catalyst play a pivotal role in FTS performance.Thereinto,cobalt-based catalysts have been extensively studied and applied to synthesize long-chain hydrocarbons due to their high intrinsic activity at low temperature reaction process,favorable ability in chain growth and low activity for the water-gas shift reaction.However,metal dispersion and particle size of catalyst surface are strongly dependent on the interaction between active phase and support.In addition,these supported-cobalt catalysts are prepared through the traditional multistep processes,including pre-treatment of support,impregnation,calcination and post-reduction.The whole process is very tedious and non-continuous,resulting in uncontrollable size distribution and inferior dispersion.Therefore,the quest for straightforward and one-step preparation method,tuning interaction between active phase and support,as well as realizing the controllable synthesis of cobalt nanoparticles(Co-NPs)are still challenge for FTS.In this regard,Co-based zeolitic imidazolate framework(ZIF-67)was employed as precursor for preparation of metallic Co-NP sconfined in porous carbon,namely Co/C catalyst.Upon the pyrolysis temperature and time,the tunable size and nanoscale distribution of Co-NPs can mediate FTS performance.On the basis of ex-situ and in-situ characterization,the mechanism toward the formation and growth of Co-NPs are systematically investigated.Furthermore,the nanosize effect and relationship of structure-function between Co-NPs and FTS performance are elucidated.Finally,the comparison of techno-economic analysis toward catalyst preparation technology show that pyrolysis in-situ reduction(PISR)technology is feasible and promising to make a profit for future industrial application.The detailed results are as follow:(1)The as-prepared ZIF-67 material was pyrolyzed under Ar atmosphere.Upon controlling pyrolysis temperature(450?900 ?)and time(4?12 h),a series of Co/C catalysts with tunable size of Co-NPs(7.5?118 nm)were prepared.The ex-situ and in-situ characterization techniques were applied to probe the formation mechanism and physico-chemical property of active species present in catalyst surface.The results indicate that ZIF-67 crystals suffer from collapse and thermal decomposition,releasing the autogenetic reducing atmosphere containing H2,CO,CH4 and NH3 at the above 490 ?.The resulting the isolated Co(?)ions within ZIF-67 framework are reduced to metallic Co-NPs.In the meantime,the rigid cagelike network of ZIF-67 is subjected to carbonization to form the graphitic porous carbon matrix,in which the metallic Co-NPs are confined.(2)All Co/C catalysts with different size of Co-NPs were evaluated in the FTS reaction at 235 ?,1.0 MPa,H2/CO = 2 and gas hourly spacevelocity(GHSV)of 5550·h-1.Evidently,the nanosize effect of Co-NPs on FTS performace can be observed and concluded to 8?16 nm giving preferable catalytic performance.Specifically,Co/C500-8 catalyst prepared by pyrolysis at 500 ? and 8 h shows a particle size distribution of 8.9 nm,and demonstrates the highest catalytic activity with CTY value of 0.9×10-5 molcogco·s-1,C5+ selectivity of 57.6%,olefin to paraffin(O/P)ratio of 0.68,and the lowest methane selectivity(27.8%).When the size of Co-NPs is beyond to 16 nm,the catalytic activity decreased significantly,while the products distributions almost keep steady-state.In addition,the test in catalytic stability with time on stream(TOS)of 130 h shows that Co/C500-8 catalyst still maintains a uniform distribution and optimal range of particles size(8-16 nm),but there is a certain extent carbon deposit.(3)The techno-economic analysis(TEA)for Co/C500-8 catalyst prepared from the pyrolysis in-situ reduction of ZIFs(PISR)technology was conducted underlab-scale.Furthermore,the benchmark Co/AC catalyst prepared from traditional impregnation and post-reduction(TIPR)technology was used to compare.It is found that total capital investment(TCI)of PISR technology in equipments,consumable and utilities is more economic than that of TIPR techonology,but the TCI of chemicals is higher than TIPR techonology because of expensive organic ligand(2-methylimidazole).As a result,the cost of Co/C500-8 catalyst(76.5 ?/g)is much higher than that from TIPR(29.2 ?/g).Nevertheless,some advantages toward PISR technology must be recoganized,such as time-saving and the tunable size of Co-NPs,as well as high spece-time yield(STY).Taking a further envisagement,therefore,PISR technology will be more overwhelming than TIPR when the catalyst synthesis in large scale and slurry reactor FTS in industrial application are targeted.