Surface Modification Of Halloysite Nanotubes For Supporting Pt Nanocatalysts

Cinnamaldehyde(CAL) is one of representative α, β- unsaturated aldehydes and its C=O hydrogenation product cinnamyl alcohol(COL) acts as an impotant spices composition or organic synthesis intermediate. Researches show that Pt nano-catalysts exhibit high activity and selectivity for C=O hydrogenation of cinnamaldehyde, but its applications have been hindered by problems such as high cost and difficult separation. The limitation can be solved by using natural nano-materials to loading Pt catalysts, thus economically and efficiently hydrogenation can be achieved.Natural nano-mineral halloysite possesses special tubular structure and stable physicochemical properties, with cost advantages, showing potentials as ideal support of various synthetic nano-catalysts. However, the performance of halloysite nanotubes(HNTs) as catalyst support is impaired by their disadvantages of surface smoothness, adhesion weakness and so on. To improve the surface properties of HNTs and the performance for nano-catalysts depositing, HNTs were modified by simple efficient polydopamine modification and molten-salt method in the present study. Then, modified HNTs were used to support Pt nanocatalyst and typical selective hydrogenation process of cinnamaldehyde was used to evaluate the catalytic properties. The main contents and results of the present study are as follows:(1) Study on the polydopamine modification of HNTsPolydopamine modified HNTs(PDHNTs) were produced based on the self-polymerization property of dopamine. The morphology, structure and surface properties of PDHNTs were characterized with scanning electron microscope(SEM),transmission electron microscope(TEM), Fourier transform infrared spectrometer(FT-IR), specific surface area and pore size analyzer and thermogravimetry analyzer(TGA). We found that the tube wall thickness were enlarged, and polydopamine was grafted on the surface of HNTs with the active group –NH2 occuring on the surface.The content ratio of polydopamine in PDHNTs was calculated to be 4.74%. Thespecific surface area of HNTs increased significantly as well as pore size and pore volume. Polydopamine modification of HNTs would be beneficial to the enrichment for metal ions on catalytic supports.(2) Study on the loading performance of Pt catalysts on PDHNTs support and catalytic propertiesPt@PDHNTs was prepared using solvothermal method with polydopamine as reductant and ethylene glycol as solvent and auxiliary reductant. Then we obtained Pt@CHNTs through the carbonization of Pt@PDHNTs. Pt@PDHNTs and Pt@CHNTs were characterized using TEM, inductively coupled plasma atomic emission spectrometer(ICP-AES), FT-IR, specific surface area and pore size analyzer.The Pt catalyst on the PDHNTs support showed as uniform spherical nanoparticles with average diameter of 8.67 nm. Carbonization of Pt@PDHNTs resulted in the increase of specific surface area and pore volume, the disappearance of –NH2 and the remaining of Pt nanoparticle morphology. The selective hydrogenation of cinnamyl aldehyde test showed that carbonization improved the activity and selectivity and kept well reusability and leaching resistant properties of catalyst.(3) Study on the molten-salt modification of HNTsSurface modification of HNTs was conducted through molten-salt method.Roughed halloysite nanotubes(RHNTs) with defects were prepared by using weak base Na2CO3 to etch the surface of HNTs in Na NO3(molten point 308 °C) molten-salt system. The morphology, structure and surface properties of RHNTs were characterized with SEM, TEM, X-ray diffractometer(XRD), FT-IR, atomic force microscope(AFM), specific surface area and pore size analyzer and TGA. Results showed that after modification RHNTs retained the fundamental lumen structure of HNTs, and the roughness of RHNTs with defects formed on the tube wall surface has been significantly increased, as well as specific surface area and pore volume. The surface modification of HNTs might render enhanced loading performance for Pt catalysts and improved catalytic properties.(4) Study on the loading performance of Pt catalysts on RHNTs support and catalytic propertiesPt@RHNTs was prepared by using simple solvothermal method with RHNTs assupport and ethylene glycol as solvent as well as reductant. The obtained catalyst was characterized with TEM, ICP-AES and specific surface area and pore size analyzer.Results showed that few Pt nanoparticles were loaded on the surface of natural HNTs which also formed large aggregates. However, the loading amount of Pt on RHNTs(2.49%) was much higher than that of HNTs(1.45%). Pt nanoparticles was better dispersed on the surface of RHNTs with an average diameter of 3.8 nm. Furthermore,the performance of Pt@RHNTs for the selective hydrogenation of cinnamaldehyde was much better than that of Pt@HNTs and other supported Pt catalysts. In addition,Pt@RHNTs shows excellent reusability and leaching resistant.