Influence Factors Of Halloysite-supported Gold Catalyst On The Catalytic Performance Of Liquid-phase Selective Oxidation Of Cyclohexane

Halloysite(HNTs)is a natural clay mineral with excellent physical and chemical properties.Its unique nanotube structure and different inner and outer surface compositions are conducive to the physical or chemical modification of the inner and outer surface of the tube,which can be used as an ideal carrier for further preparation of supported catalysts by noble metal particles such as gold nanoparticles.In this paper,organosilane was used as a chemical modifier to modify the inner surface of halloysite.The precursor of chloroauric acid was successfully loaded on the organic modified halloysite by using the characteristic that-NH₂ on organosilane could coordinate and capture gold ions.The halloysite supported nano gold catalyst samples were prepared by liquid-phase reduction.The chemical modification effect of halloysite were characterized by IR and NMR.The size distribution,average particle size and valence of gold were characterized by transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS).The actual loading amount of gold in catalyst samples was measured by ICP.The liquid phase selective oxidation of cyclohexane was used as a model reaction,the activity,selectivity and stability of the prepared gold catalyst samples were evaluated,and the factors influencing the catalytic performance of the gold catalyst supported on organic modified halloysite were discussed.The reaction conditions of selective oxidation of cyclohexane in liquid phase were optimized by selecting representative gold catalyst samples.The main results showed that:(1)Gold catalyst sample 1.5wt%Au/AE-HNTs was prepared from halloysite grafted by N-(2-aminoethyl)-3-aminopropyltrimethoxysilane(AEAPTMS)with sodium borohydride as reducing agent and chloroauric acid precursor concentration of 10mmol/L.the catalyst showed the best catalytic activity and selectivity in the model reaction of cyclohexane liquid phase selective oxidation.IR and NMR characterization results showed that the chemical modification of halloysite was successful.TEM and XPS characterization results show that the gold nanoparticles on the surface of the support were highly dispersed with an average particle size of 2.6 nm,and the gold existed on the surface of the support as metallic Au⁰,meanwhile the actual load of gold was 1.43wt%.The size of reducing agent and the concentration of chloroauric acid are the main reasons for the differences in the size,distribution and valence of gold nanoparticles on the surface of the support,and the size,distribution and valence of gold nanoparticles affect the catalytic activity of gold The catalytic activity and selectivity of the catalyst are the key factors.(2)The type and amount of modifier used in the chemical modification of halloysite support,the modification solvent and the calcination pretreatment of halloysite all had certain effects on the activity and selectivity of the supported gold catalyst in the liquid phase selective oxidation of cyclohexane.The gold catalyst 1.5wt%Au/AE-HNTs exhibited best catalytic performance for the liquid phase selective oxidation of cyclohexane at 170?and 2MPa for 2h and catalyst dosage 0.15 g,the conversion of cyclohexane was 9.6%,the selectivity of Ka oil was 83.06%,and the yield of Ka oil could reach 7.97%.Moreover,the catalyst could be reused for 4 times,and the conversion of cyclohexane could still be maintained as high as 5.82%,the selectivity of Ka oil was 81.39%,which reached the requirement of industrial catalyst.(3)The catalytic performance of chemically modified halloysite and physically modified halloysite supported gold catalysts with the same gold content was compared,although physically modified halloysite supported gold catalysts showed better catalytic activity and selectivity in the liquid phase selective oxidation of cyclohexane(The conversion of cyclohexane was 10.29%and the selectivity of Ka oil was 85.75%).However,its stability was significantly lower than that of chemically modified halloysite supported gold catalyst.The former was recycled for three times,the catalytic activity and selectivity of the catalyst decreased significantly.On the contrary,the gold catalyst supported on chemically modified halloysite had better stability,and its activity and selectivity had not been significantly reduced after four cycles.The main reason was that the interaction force between the chemically modified halloysite and gold nanoparticles was strong,and the gold nanoparticles were more stable on the surface of the carrier,so it was not easy to be inactivated due to falling off or agglomeration in the reaction process.(4)The catalytic activity and selectivity of gold catalyst supported on layered kaolin support with the same chemical composition but different morphology as nano tubular structure of halloysite were investigated.Under the same preparation conditions,the tubular halloysite supported gold catalyst had better catalytic activity and selectivity than the layered kaolin supported gold catalyst.XPS characterization results showed that Gold existed in the form of Au⁰ and Au⁺simultaneously on the layered kaolinite surface,but only in the form of Au⁰ on the tubular halloysite surface,indicated that Au⁰ may be the main active site for the liquid phase selective oxidation of cyclohexane.(5)This paper took the catalyst sample 1.5wt%Au/AE-HNTs as the research object,the reaction conditions of cyclohexane liquid phase selective reaction were optimized by single factor experiment and response surface methodology.The results showed that the model reaction conditions had a great influence on the catalytic activity and selectivity of gold catalyst in the liquid phase selective oxidation of cyclohexane,and the order of influence was reaction temperature>reaction pressure>reaction time.The optimal reaction conditions were reaction temperature 167.2?,reaction pressure 2.31MPa,reaction time 3.1h and the amount of catalyst was 0.15g.Under the optimal reaction conditions,the conversion of hexane was 85.42%,the selectivity of Ka oil was 9.51%,and the yield was 8.12%.