Carbon Materials As Efficient Catalyst For The Direct Dehydrogenation Of Isobutane

Isobutene(i-C4H8)is a useful platform chemical for production of various important chemicals and polymers,such as tert-butyl alcohol,butyl rubber and polyisobutene.Currently,industrial production of isobutene is mainly based on steam cracking of naphtha and fluid catalytic cracking(FCC).However,it is hard to meet the growing demand because of the low output of isobutene in these processes.Instead,isobutane(i-C4H10)dehydrogenation is a good alternative for this issue because isobutane is an important and readily available raw material mainly from oil refining and natural gas exploitation process.There is a vast perspective in utilizing isobutane to produce valuable isobutene,on the other hand,the target product of isobutene can be obtained with high yield and excellent purity.In this process,Pt and Cr Ox-based catalysts are successfully applied in industry.Despite the fact that several isobutane dehydrogenation technologies exhibited relatively high performance,the high cost of Pt-based catalysts and harmfulness of Cr Ox-based catalysts severely limit their large-scale application.Therefore,high efficient and cost-effective dehydrogenation catalyst is urgently needed.Carbon materials exhibit peculiar hollow geometry,coupled with a conjugated all-carbon structure,thermal properties and anchoring sites compared with other materials,which makes them promising for various applications,including oxidative dehydrogenation of ethane,propane and ethylbenzene.Compared to direct dehydrogenation reaction,oxidative dehydrogenation has significant advantages such as exothermic reaction,moderate temperature,but the undesirable,parallel combustion reaction and the yield of alkene are not sufficient to satisfy economic feasibility.Therefore,in this work,we use commercial carbon nanotubes which was systematically modified with different amount of nitrogen and as-synthesized molybdenum/molybdenum-gallium based organic-inorganic hybrid nanomaterials and probed as a catalyst for direct dehydrogenation reaction of isobutene.In the reaction,we focused on the revealing the nature of the catalysts on the catalysis process,providing insights into the relationship between the species and catalytic activity and probable active sites were also discussed.The main contents are summarized as follow:(1)Oxidized multi-walled carbon nanotubes were modified with melamine,it exhibits isobutane conversion of 51.8 %,isobutene selectivity of 45.9 % and yield of 23.8 % when the nitrogen content is 1.93%.Oxygen functionalities are partly removed and the structural defects of carbon improved by nitrogen-doping.The increased catalytic performance is well-correlated with promoted adsorption of isobutane and decreased adsorption of isobutene,because of the increased electron density of carbon atoms.Minimal deactivation was also observed which may be ascribed to the loss of nitrogen heteroatoms and the increased graphitization.(2)A series of Mo-CT hybrid nanowire catalysts have been prepared with varieties of Mo species,which exhibit special catalytic characteristics.In particular,the optimal Mo-C700 catalyst displays well isobutane conversion of 15.5%,isobutene selectivity and yield of 73% and 11.1%,respectively.Extensive characterizations reveal that such prominent performance is ascribe to the enhanced adsorption of isobutane and decreased adsorption of isobutene for η-Mo C species.Moreover,the generation of η-Mo C species can partially suppress the cracking reaction.(3)The addition of Ga in Mo nanowire catalysts can improve the catalytic performance remarkably.In particular,the Ga-Mo-C650-0.033 catalyst displays isobutane conversion of 21.1%,isobutene selectivity and yield of 66.9% and 14.1%,respectively.Stability test showed that the catalyst remains 85% activity after reaction for 30 h.The results demonstrated that appropriate content of Ga in nanowires is beneficial for the structure characteristic and surface area,which also has effect on the phase’s evolution and there is strong interaction between Mo and Ga.When Ga/Mo=0.009,the ratio of Ga3+ to Gaδ+ and the percentage of Ga3+ reach the maximum,giving the highest catalytic activity.In η-Mo C catalyst,the Ga 2p peaks are visibly red-shifted,indicating the enriched electrons,which enhance the desorption of electron-rich isobutene and further leads to the exceptional improved catalytic performance for catalyst.