Preparation Of Carbon-Based Pickering Emulsifiers For Selective Oxidation Of Benzyl Alcohol

The Pickering interfacial catalysis(PIC)by making use.of nanoparticles-stabilized Pickering emulsion has received much attention across the world,which features high reaction interface and low mass transfer resistance for the catalytic reactions.The nanoparticles in the PIC system play a key role,and have two functions,i.e.both as emulsifier and support.The surface properties of the nanoparticles are one of the main factors that have great impact on the catalytic activtity of the nanoparticles-stabilized PIC system.Up to now,the design and fabrication of the nanoparticles-based emulsifier with high emulsion capability and stability still remains a big challenge.Carbon materials with large specific surface area,diversified morphology and high corrosion resistance have been regarded as one of the potential nanoparticles for making high performance emulsifiers with tuned properties.In this dissertation,we have made efforts to design and prepare carbon-based Pickering emulsifiers with a focus on the emulsion stability and the catalytic property as emulsifier and support in the PIC system,where the selective oxidation of benzyl alcohol was adoped as a probe reaction.The carbon-based emulsifiers were prepared from carbon nanotubes(CNTs)and graphene oxide(GO)by different chemical modification methods,depending on the morphology and surface properties of the nanocarbons adopted.The catalytic activities of the carbon-based Pickering emulsifiers with different noble metals were evaluated by the selective oxidation of benzyl alcohol.Below are the main summarized results.1.The amphiphilic nanohybrids composed of CNT-bridged nanosheet-shaped LDHs(termed as LDH-CNTs)were synthesized by the hydrothermal method.The emulsion stabilized by LDH-CNTs nanohybrids shows an improved emulsified capability and thermodynamic stability at the toluene-water interface due to the sheet-like structure of LDHs,which is better than the bare LDHs and CNTs.Benefiting from such a unique structure and emulsion characteristics,the Ru/LDH-CNTs catalyst made by impregnation method with LDH-CNTs as support for the Ru precursor is active for the selective oxidation of benzyl alcohol to benzaldehyde,evidenced by the high benzyl alcohol conversion of 92.3%at 85 ℃ in 5 h with the Ru/reactant molar ratio of 1/130 and the emulsifier loading of 200 mg in the emulsion system.The benzyl alcohol converstion in the case of Ru/LDH-CNTs is higher than 56.0%of Ru/CNTs and 2.0%of Ru/LDHs under similar conditions.This has demonstrated a positive correlation between the emulsified capability and the catalytic performance in terms of the benzyl alcohol conversion.2.The sheet-like GOND emulsifier was preparaed by grafting N,N-diethylethylenediamine(ND)onto the surface.of GO prepared by Hummers method.The wettability of the GO is remarkably changed,and the GOND exhibits a remarkable stability in a wide pH range of 2-10.In the case of GOND emulsion,only 20 mg of GOND is required to stabilize the emulsion,which is 90%less than 200 mg of LDH-CNTs under similar conditions.Benefiting from the excellent emulsion properties of the GOND,the Pd/GOND shows a benzyl alcohol conversion of 76.0%at 85℃ in 5 h with the Pd/reactant molar ratio of 1/160 and the Pd/GOND loading of 20 mg,which is higher than Pd/GO with a benzyl alcohol conversion below 1.0%under similar conditions.3.The amphiphilic emulsifier with a structure similar to amino acids was fabricated by selectively opening the epoxy groups on the GO surface via the ring-opening reaction scheme with the DETA as the agent.In comparison with GOND made in organic solvent in 24 h,the process to make GO-DETA is green,fast and cheap.Moreover,the GO-DETA emulsion shows a high stabilitity in a wide temperature range of 70-90℃ and pH value of 2-10,resulting in the uniform size distribution of droplets of 133-178 μm.Benefiting from the excellent emulsion properties of the GO-DETA,the Pd/GO-DETA shows a conversion of 60.0%for the selective oxidation of benzyl alcohol in 4 h with the Pd/reactant molar ratio of 1/190 and a Pd/GO-DETA loading of 14 mg.It is much higher than the conversion of benzyl alcohol in the case of Pd/GO emulsion,which is below 1.0%in 4 h under similar conditions.4.The GONR-IL Pickering emulsifier that is highly stable in a wide pH range was prepared by passivating the-COOH groups on the graphene oxide nanoribbon(GONR)by ionic liquid(IL)via the amidation reaction.The as-made GONR-IL shows an excellent emulsion capacity,and the GONR-IL supported Pd catalyst stabilized at toluene-water interface shows a benzyl alcohol conversion of 92.0%in 3 h with the Pd/reactant molar ratio of 1/288 and a Pd/GONR-IL loading of 20 mg,which is much higher than other Pickering emulsifiers,1.0%for Pd/GONR,8.0%for Pd/GO-IL and 51.0%for Pd/CNT-IL,respectively,under similar conditions.