Construction And Application Of Palladium Catalysts Supported On Chitin Microspheres

In recent years,the international academic community believes that one of the most important topics in current chemical development is "natural resources chemistry." With the depletion of fossil fuels,such as the oil,coal,etc.,the energy crisis is gradually approaching and the issue of acquiring new energy needs to be solved.Chitin,as the second largest natural macromolecule in nature after cellulose,is widely found in the arthropod shell.It has abundant functional groups,as well as excellent biocompatibility and biodegradability,which is an ideal raw material for building new materials.Meanwhile,the hydroxyl and acetylamino groups on the chitin molecular chain can adsorb and disperse organic molecules or inorganic metal ions,which have been broad applied in the fields of water treatment,gas adsorption,catalysis,etc..Therefore,the development and utilization of chitin has become increasingly attractive.Transition metal palladium catalyst has become one of the research hotspots as its unique catalytic properties.Although the traditional homogeneous palladium catalyst has good dispersion and high catalytic efficiency,it is easy to agglomerate in the reaction process and cannot be recycled,which may cause serious environmental pollution and economic problems.Heterogeneous palladium catalysts can be recycled,and not contaminate the product,which can solve the above problems well.Faced with the increasing shortage of energy and the urgent need for sustainable development,the use of natural biomass chitin as a carrier material for metal catalysts has become a very promising approach combined with the characteristics of chitin.This work utilized the NaOH/urea aqueous solution developed by our laboratory to dissolve chitin and construct it into nanofiber woven microspheres through heat induction.The chitin microspheres are used as a carrier for metal palladium catalysts.The abundant hydroxyl and acetamide groups in the chain can effectively rivet metal palladium,and the large specific surface area of the microspheres can well disperse the palladium nanoparticles.A series of characterization methods were used to study the structures and properties of these supported catalysts such as scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray absorption spectroscopy(XAS),X-ray photoelectron spectroscopy(XPS),infrared spectroscopy(FT-MR),solid-state nuclear magnetic resonance spectroscopy(13C-NMR),nitrogen adsorption instrument and so on.We also used some chemical reactions to evaluate their application in the field of catalysis.The main innovations of this work include the following.(1)Chitin microspheres with multi-hole structure were successfully constructed,and a highly dispersed nano-palladium catalyst was designed and prepared using the tiny holes of chitin,and the interaction between chitin and Pd was also elucidated.The supported catalyst showed excellent activity in hydrogenation of a nitro compounds.(2)Based on the large surface area and abundant functional groups of chitin microspheres,a highly stable Pd/chitin catalyst was prepared.It exhibited good catalytic activity both in the oxidation of carbon monoxide and in the Heck carbon-carbon coupling reaction.(3)Through the calcination activation method,size controllable palladium catalysts were successfully constructed.The results proved that the calcination process could induce the formation Pd(0)and make the catalyst more stable.The catalyst had excellent catalytic activity and stability in the hydrogenation reaction of styrene and benzaldehyde.The main contents and conclusions of this paper are summarized as follows.The NaOH/urea aqueous solution was used to dissolve chitin at low temperature,and the chitin microspheres with nanofiber structure were constructed by heat induction.Based on the large surface area and abundant pore structure of the chitin microspheres,highly dispersed palladium clusters were successfully supported on the microspheres.The micro-pores(<1.4 nm)and acetamido groups on chitin microspheres are critical for the formation of the tiny Pd clusters.X-ray absorption spectroscopy(XAS)and scanning transmission electron microscopy(HAADF)showed that the cluster had an average size of 0.6 nm.At the same time,X-ray absorption spectroscopy shows that the resulting catalyst can maintain good dispersibility and a narrow size distribution both in the oxidized state and the reduced state.In the hydrogenation reaction of 4-nitroanisole,the catalyst exhibits extremely excellent catalytic activity,and its initial reaction rate is 14 times that of commercial palladium carbon catalyst,and has a high TOF~52000 h-1.Moreover,although the nano-palladium has accumulated after 10 runs,the activity loss is very small,and the yield of the target product can reach 96%after the 10th reaction cycle.Therefore,it has a good industrial application prospect.In order to improve the shortcomings of the stability of the above-mentioned supported catalyst(Red-Pd@chitin).We obtained a stable high-dispersion Pd/chitin catalyst by calcination at high-temperature.X-ray absorption spectroscopy(XAS)and scanning transmission electron microscopy(HAADF)demonstrated the presence of highly dispersed nano-palladium.At the same time,the resulting supported catalyst was successfully used to study the Heck coupling reaction and CO oxidation reaction.In the Heck coupling reaction,the activity of Pd/chitin catalyst was significantly higher than that of commercial Pd/C,and nano-palladium had almost no aggregation after multiple cycles of reaction.However,the Red-Pd@chitin catalysts without calcination treatment have obvious accumulation after several runs.The Pd/chitin catalyst also shows excellent activity and stability in CO oxidation reaction.Further,a series of well-dispersed and size-controllable PdNPs immobilized on chitin microspheres were designed and synthesized by regulating the loading of palladium.The obtained catalysts have good dispersity and narrow size distribution,and the size of nano-palladium varies from 1 nm to 3 nm with the increase of palladium loading.As the support of these catalysts,chitin microspheres supplied abundant functional groups to capture the Pd catalysts,as well as a high surface area to spread the Pd particles evenly.Moreover,the chitin could induce the formation of PdNPs during calcination,and the partially carbonized chitin made the Pd particles more stable.Importantly,these supported Pd catalysts exhibited excellent catalytic performances and recyclability for the hydrogenation of styrene and benzaldehyde.The most active palladium catalyst has a TOF value of up to 50000 h-1 in the hydrogenation of styrene.Furthermore,the calcined catalyst had almost no loss of reactivity during the 10 reaction cycles and no palladium aggregation.This dissertation builds a series of chitin/palladium-supported catalysts and studies the relationship between the structure and properties of the materials.The experimental data also show that the catalysts have great application in the field of catalysis.The research work involves the areas of polymer materials,organic chemistry,catalysis,etc.It has the characteristics of cross-disciplinary research.Here,it is effectively solved the problem that it is difficult for palladium nanoparticles to maintain small particles while being highly dispersed,and the supported catalyst can efficiently catalyze various types of chemical reactions.It is important to use biomass resources as raw materials to make materials,which meets the needs of sustainable development in today's society.Therefore,the dissertation have academic value and application prospects.