Catalytic Production Of Light Olefins Via Single-atom Catalysis

MTO and ethane dehydrogenation are two kinds of industrial pathways for light olefin production.MTO technology is a bridge between coal/natural gas and petrochemicals,which is important for the production of light olefins from non-petroleum resources such as coal and natural gas in China.The pore structure and acid properties of zeolites are two key factors for acid-catalyzed reactions.The unique pore or cavity structure of zeolites presents a confinement effect not only on the structure of reaction intermediates but also on the formation of olefins.Therefore,designing a suitable topology and precise adjustment of the acid properties(acid strength and acid density)of zeolites are of great significance to control the olefins selectivity.The important criteria and core competence of MTO technology development are the selectivity of target products and the stability of the catalysts.Besides,there is still a gap between the supply and demand of light olefins(especially ethylene)from MTO process.Ethane dehydrogenation technology could produce ethylene in a targeted manner with few by-products and is expected to replace steam cracking technology based on petroleum.This dissertation explores new methods to improve the selectivity of target products and prolong catalysts lifespan for MTO/MTP and ethane dehydrogenation reaction,starting from the preparation and modification of catalysts.The main content is summarized as follows:ZSM-5 with 10-membered ring(MR)channels is the most widely studied industrial MTP catalyst.Considering the highly hydrothermal stability of all-silica zeolite and the similar chemical properties of Ga and Al from the same main group,a method has been developed to modify all-silica MFI-type zeolite by Ga by in situ hydrothermal method.It is found that Ga atoms in the GaS-1 framework possess a tetrahedral coordination,connected by SiOx-O-Gaδ+bonds and no ex-GaO+ species.The acidity of GaS-1 is adjusted by controlling the Ga/Si ratios,which in turn leads to a high propylene selectivity,high P/E ratio and long catalysts lifespan.Based on the MTP performance and acidity results,it is concluded that the L/B ratios are positively correlated with the catalyst lifetime,and the W/S ratios determine the P/E ratios of the MFI-type zeolites.Combining in situ DRIFTS and MS,as well as 12C/13C-methanol and CH3OH/CD3OD isotope switch experiments and coke species analysis,it is predicted that methanol conversion over GaS-1 follows a dual-cycle mechanism,with propylene and C3+olefins mainly generated through olefins methylation-cracking mechanism;HCP species based on the aromatic cycle preferentially form diMCP+,and trimethylbenzene and tetramethylbenzene are the main active aromatic species.DDR zeolites with 8-MR channels exhibit a high tendency to produce light olefins during methanol conversion due to their highly constrained small pore size.The synthesis of Ga-modified all-silica DDR-type zeolite and their MTO studies have not yet been reported in the literatures.Ga-DDR catalyst with atomically dispersed Ga embedded on all-silica DDR-type zeolite framework was synthesized through hydrothermal method with ethylenediamine as complexant.Combined with HAADF-STEM,MAS NMR and XAFS,it is determined that Ga atoms in Ga-DDR zeolite framwork possess tetrahedra-coordination structure and no Ga2O3 is detected.After the activity test of MTO,the selectivity of light olefins and propylene over Ga-DDR was as high as 95%and 55%,respectively.The DRIFTs,MS and the analysis of coking species reveal that ethylene was the initial olefin and the olefin cycle played an important role on Ga-DDR during methanol conversion.Ga-DDR catalysts possess ultrasmall micropores that limit the diffusion of polymethylbenzene and long-chain olefins,so that the products from the olefin-based cycle mainly include propylene.The deactivation of the Ga-DDR catalyst is associated with the formation of a large amount of monoalkylated adamantine in the cage.In addition,a series of DDR-type zeolites with different Ga/Si ratios were synthesized greenly and rapidly for the first time through adding DD3R seed into the synthesis gel,without the addition of inorganic bases,ethylenediamine and fluoride.The morphology of GaDDR is related to the Ga content and is dominated by the common rhombic crystals,which presents a polygonal crystal with small size and has not yet been reported in the literatures.AC exhibits a high specific surface area,porous structure and abundant surface functional groups,which is rarely employed as catalyst support for ethane dehydrogenation reactions.The PdIr/AC catalysts synthesized by the impregnation method reveal comparable or even higher catalytic activity than the traditional Pt-based catalysts.Firstly,the study investigates the influence of NaBH4 treatment on the AC support,the dispersion of Pd and Ir,the selectivity of methane and CO2 generation as by-products,which proves the necessity and advantages of deoxygenation treatment of the AC carrier by NaBH4.The Pd and Ir species loaded on AC include atomically dispersed Pd2+/Ir4+and Pd/Ir nanoparticles.To examine the active species for ethane dehydrogenation reaction,the content of metallic Pd0 and Ir0 and oxidized Pd2+ and Ir4+ on catalysts is manipulated by changing the pretreatment atmosphere.The activity test results confirm that Pd2+ and Ir4+ are the main active species for ethane dehydrogenation.The ethane dehydrogenation process over PdIr/AC catalysts mainly experiences two steps:OEDH by the oxygen species on the AC as the oxidant that is not limited by thermodynamics and EDH.The DFT calculation illustrates that the rate-determining step is ethane dehydrogenation on Pd/AC and PdIr/AC and*C2H5 dehydrogenation on Ir/AC.The energy barriers of rate-determining step over Pd/AC,Ir/AC and PdIr/AC for OEDH are 0.75,1.48 and 1.02 eV,respectively,while they are 2.89,2.42 and 1.81 eV for EDH,respectively,indicating that the Pd-Ir atomic pairs are more favorable for EDH.The deactivation of PdIr/AC catalysts is attributed to the coking and CNT formed during ethane dehydrogenation,which cover the catalyst surface or block the pore structure.In addition,the metal sintering caused by high temperature also aggravates the catalyst deactivation.