Self-separation Of Pickering Emulsion Catalyst For Benzyl Alcohol Oxidation And The Catalytic Performance Of Pickering Emulsion Prepared With Nitrogen-doped GO

For the oil-water two-phase reaction system,the Pickering emulsion catalytic system increases the interfaces between the catalyst and the reactants and reduces the mass transfer resistance during the reaction.Like other nanocatalysts,the Pickering emulsion catalytic system also suffers from the problem of difficult catalyst separation.In this work,a surface-property-responsive Pickering emulsion catalytic system was prepared by using modified graphene oxide（GO）supported on phosphotungstic acid（PW）for the study of self-separation in the oxidation of benzyl alcohol.By changing the oxidation time in the low temperature（x）,middle temperature（y）and high temperature（z）stages of preparing GO,GO_x-y-z with different oxidation degrees were obtained.Using 3-aminopropyltrimethoxysilane（APTMS）to modify GO_x-y-z material,and then immobilizing PW to synthesize PW/GO_x-y-z-NH₂ catalysts,and a series of Pickering emulsion catalytic systems were prepared with PW/GO_x-y-z-NH₂.The increase of the oxidation degree of GO was used to increase the hydrophilicity of the catalyst,so that the prepared Pickering emulsion could self-separate after the reaction.In addition,the effect of the catalysts prepared under different oxidation times on the self-separation of the emulsion after the reaction was explored.At the same time,the influence of the amount of H₂O₂ on the self-separation of the Pickering emulsion system was also analyzed.The results show that the Pickering emulsion catalyst system prepared by PW/GO_1-6-1-NH₂ has the best catalytic activity and the least amount of H₂O₂ and can realize the self-separation of the emulsion after the reaction.The Pickering emulsion prepared by this catalyst still has good self-separation ability after in-situ cycling for five times.A Pickering emulsion system for catalyzing the nucleophilic substitution reaction of bromooctane with potassium iodide was also prepared.Using urea as nitrogen source,nitrogen-doped graphene oxide（NGO）was prepared,and NGO was modified by APTMS and ethylenediamine（ED）to prepare Pickering emulsion for the nucleophilic substitution reaction of bromooctane and potassium iodide.The XPS analysis showed that nitrogen atoms were successfully doped and simultaneously existed in the form of pyridinic nitrogen,pyrrolic nitrogen and graphitic nitrogen.Compared with GO and GO-NH₂（2.0）-ED（1.0）catalysts,the catalytic activities of both NGO and NGO-NH₂（2.0）-ED（1.0）after nitrogen doping were significantly increased.This is because the doping of nitrogen atoms introduces pyridine nitrogen active sites and increases the lipophilicity of GO.Next,the number of active sites was increased by adjusting the mass ratio of APTMS to ED,and the properties of Pickering emulsions were changed.When the mass ratio of APTMS to ED is 2:1,the Pickering emulsion catalyst system prepared by NGO-NH₂（2.0）-ED（1.0）has the smallest droplet size,largest density and narrowest distribution,so the catalytic activity is also the highest.After five cycle experiments,the Pickering emulsion system prepared by NGO-NH₂（2.0）-ED（1.0）catalyst still has high catalytic activity and shows good recyclability.