| Microchemical technology is a frontier engineering technology that integrates fluid mechanics,heat and mass transfer,chemistry,materials and other disciplines.As the core equipment of this technology,microreactor usually has a characteristic size of several hundred microns and a large specific surface area,which can significantly improve the heat and mass transfer efficiency.Since being proposed in the 1990 s,microreactors have been widely investigated and applied in the fields of energy,chemical industry,biology and medicine.Due to the high efficiency of heat and mass transfer capacity,microreactors are inherently suitable for fine chemical industry which is typically limited by mass transfer.Most of these processes need catalysts to promote the reactions.The catalyst is deposited in the microreactor in three ways: packed type,suspension type and wall-supported type.The packed catalyst will increase the diffusion resistance and the flow pressure drop.The suspended catalyst is trending to cause blockage,leading to microreactor failure.Compared with these two types,the wall-supported catalysts have been widely investigated in recent years because of the high safety and low tendency of blockage.However,due to the limitation of microscaled channels,the deposition of catalyst layer on the channel wall is still a great challenge.In this thesis,the preparation and properties of high-efficiency catalyst layer in microreactors were investgated.Firstly,as inspired by the mussel's super adhesive capability,the novel catalyst layer(palladium nanoparticles supported on the polyelectrolyte multilayer film)was prepared by using the layer-by-layer self-assembly and ion-exchange in-situ reduction on polydopamine-modified microchannel wall.The effect of preparation parameters on the load of catalyst and the dispersion of active sites were studied.Then in order to improve the catalyst stability,gold-palladium alloy nanocatalyst was synthesized and supported on polyelectrolyte multilayers film using the co-reduction method based on the synergistic effect from the double-metal theory.In addition,gold-palladium core-shell structured nanocatalyst was preaprared and supported on polyelectrolyte multilayers film using the progressive reduction method.Significant improvement in catalyst stability was obsereved.Moreover,in order to control the catalyst load,a hybrid multilayer catalyst layer(layer-by-layer polyelectrolyte thin film and palladium nanocatalyst)was prepared as well.Lastly,a weak-reduction method which enables both reduction and protection was proposed to impede the growth of catalyst nanocrystal and enhance the dispersion.The performance of the prepared catalyst layer was evaluated by the hydrogenation of nitrobenzene to aniline which is a typical chemical reaction in industry.The key findings are as follows:1)Using the layer-by-layer self-assembly technology,the microchannel wall was modified by polydopamine(PDA),and strong-strong polyelectrolyte solutions((Poly(diallyl dimethylammonium)chloride)(PDDA)and poly(styrene sulfonic acid)sodium(PSS))were injected into the microchannel to allow self-assembly of polyelectrolyte multilayer film.Then the palladium precursor ions were exchanged on the film and in situ were reduced to form palladium nanoparticles catalyst layer.The results showed that the palladium load was very low under the single-layer PSS-PDDA structure,and the palladium load could be slightly increased by adding PSS-PDDA layers,but the transfer resistance of nitrobenzene in the catalyst layer would be increased,leading to the degradation of the performance of the microreactor.In addition,due to the charge site occupancy and release in the ion-exchange and in-situ reduction process,multiple ion-exchange and in-situ reduction can effectively increase the palladium load,provide more active sites and improve the performance of the microreactor.However,when the number of layers of PSS-PDDA is fixed,increasing the load will increase the particle size and reduce the specific surface area,thus reducing the utilization efficiency of catalyst.2)Using layer-by-layer self-assembly technology,the weak-strong polyelectrolyte pairs(Poly(allylamine hydrochloride)(PAH)and PSS)were used to conduct self-assembly of the polyelectrolyte film in the PDA-modified microchannels,and then the palladium precursor ions exchange and in situ reduction were carried out on the surface to prepare the catalyst layer of palladium nanoparticles supported by PSS-PAH.Experimental studies showed that the palladium load of polyelectrolyte multilayer membrane increases with the increase of pH value of PAH solution.Increasing the concentration of the precursor or prolonging the ion exchange time at the same concentration of the precursor could increase the palladium catalyst load.Multiple ion-exchange and in-situ reduction can significantly increase the palladium load,however,the increase in palladium catalyst load will lead to the increase in particle size and reduce the specific surface area and catalyst utilization efficiency.3)Increasing palladium load in the microreactor can provide more active sites and improve the performance of the microreactor.However,the increase of palladium nanoparticles can also increase the particle size and decrease the specific surface area of palladium nanoparticles.In order to improve the stability of the microreactor,the catalyst layer of gold-palladium bimetallic alloy nanoparticles and the catalyst layer of gold-palladium bimetallic core shell nanoparticles were prepared by co-reduction and progressive reduction.Experimental results showed that the catalyst layer loaded on the nanoparticles of gold-palladium alloy could significantly improve the performance of the microreactor.Palladium as a shell metal core-shell bimetallic nanoparticles supported catalyst layer can also significantly improve the performance of microreactors.4)The catalyst layer loaded with gold-palladium bimetal can significantly improve the performance of the microreactor while the addition of gold will increase the cost.In order to reduce the cost,a multilayer polyelectrolyte film and palladium nanocatalyst composite structure were prepared.Experimental studies showed that increase the number of layers in the composite structure of polyelectrolyte film and palladium nanocatalysts could significantly improve the performance of the microreactor,but when the number of layers in the composite structure increase to a certain number,the reactant in the catalyst layer were subjected to increased transfer resistance,leading to the stability of the performance of the microreactor.5)Aiming at the agglomeration growth of palladium particles in the reduction process,an ascorbic acid reduction system with both reduction and protection effects was constructed to prepare the catalyst layer of PSS-PAH supported palladium nanoparticles.A multilayer polyelectrolyte film supported silver-palladium bimetallic catalyst layer was prepared by inhibiting the growth of gold particles.Experimental results showed that the reduction of polyelectrolyte film adsorbed palladium ion with ascorbic acid under the same conditions could significantly improve the performance of the microreactor. |
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