Studies In The Preparation And Catalytic Performance Of Noval Nanodiamond-Based Catalysts For Direct Dehydrogenation Of Ethylbenzene

Styrene is a very important organic chemical raw material,widely used in the production of chemical products like plastics,rubber and resin.It is also extensively employed as fine chemical raw material and intermediate in the field of fine chemical.Styrene is primarily produced from dehydrogenation of ethylbenzene over commercial potassium-promoted iron oxide at the present.Although high styrene selectivity and yield are achieved,the catalytic system is typically carried out with excess stream provided simultaneously to alleviate carbon deposit during the dehydrogenation process.Compared with the commercial potassiumpromoted iron oxide catalyst,the sustainable nanodiamond-based catalysts demonstrated superior stability in the dehydrogenation of ethylbenzene to styrene and outstanding anti-carbon deposition capacity without the introduction of steam,meanwhile,metal was excluded in the catalysts prepareation.This paper focus on the current problems of nanodiamond-based nanocarbon catalysts in catalyzing direct dehydrogenation of ethylbenzene to styrene,especially on the dispersion of nanodiamonds and the construction of monolithic catalysts and their catalytic performance in direct dehydrogenation of ethylbenzene.N anodiamond/nitrogen doped carbon hybrid was synthesized using hexamethylenetetramine and ammonium chloride,in which hexamethylenetetramine served as both nitrogen doped carbon precursor and dispersing agent and the ammonium chloride acted as gas template.The hybrids were investigated by various characterization techniques.To investigate the structure-activity relationship and regulation rules for the as-synthesized nanodiamond-based hybrid nanocarbon catalysts,the characterization results were correlated with the catalytic performance of direct dehydrogenation of ethylbenzene to styrene.The obtained results illustrated that the porous structure in the prepared nanodiamond/nitrogendoped carbon hybrids increased the number of exposed active sites(carbonyl and structural defects)for the dehydrogenation reaction.Meanwhile the improved nucleophilicity of active sites due to nitrogen doping is beneficial for C-H activation for the DDH reaction.The porous nanodiamond/nitrogen-doped carbon hybrids nanocarbon catalysts demonstrated a steady-state styrene rate of 5.61 mmol g-1 h-1,a styrene selectivity of 99.3%.In order to further promote the dispersion of nanodiamonds,nanodiamond/porous nitrogen doped carbon nanosheet hybrid was prepared by using molten salt to deaggregate nanodiamonds and adding nitrogen doped carbon nanosheets during the preparation process.Extensive characterizations revealed that the deaggregated nanodiamonds were smoothly inserted into the porous nitrogen doped carbon nanosheets in molten salt and nanodiamonds were highly dispersed,increasing the number of exposed active sites on the surface of nanodiamonds.With subsequent oxidation treatment in air for the as-prepared ND/NC-ms hybrid,ND/NC-ms-o exhibited more active sites and demonstrated a steady-state styrene rate of 7.06 mmol g-1 h-1,a styrene selectivity of 99.8%.To solve the intrinsic problem of powder catalysts that are difficult to be pelleted into large objects,two types of nanodiamond-based monolithic nanocarbon catalysts were prepared.First,a new nanodiamond-based monolith catalyst was prepared using carbon nanotube-modified silicon carbide foam(CNT-SiC)as the matrix.Based on the disaggregation effect of molten salts in dispersing nanodiamonds discovered in the above work,a hexamethyltetramine nitrateassisted molten salt impregnation method was used to load nanodiamonds onto CNT-SiC,realizing the disaggregation of nanodiamonds and the loading of hexamethyltetramine nitrate simultaneously,and the monolith demonstrates a steady-state styrene rate of 5.49 mmol g-1 h-1,a styrene selectivity of 99.8%and a good catalytic stability.Second,a sulphur-doped nanodiamond-based monolithic catalyst was prepared by assembling nanodiamonds and carbon nanotube in the pores of commercial silicon carbide foam with sodium dodecyl sulfate assisted freeze-drying technology.And it demonstrates a steady-state styrene rate of 6.25 mmol g-1 h-1,a styrene selectivity of 99.0%.The prepared nanodiamond-based monolithic catalyst was characterized by a series of characterization methods,and the conformational relationships of the catalysts were investigated in conjunction with the results of the direct dehydrogenation of ethylbenzene to styrene.