| Hydrogen peroxide?H2O2?is mainly used as bleach and disinfectant,due to its strong oxidizing,and also chemical raw material.At present,anthraquinone process is the mainstream for the production of H2O2,which includes the 2-ethyl-9,10-anthraquinone?eAQ?hydrogenation process,autoxidation process of 2-ethyl-9,10-anthrahydroquinone?eAQH2?and other side reactions.High efficiency catalyst is the key factor for improving the yield of H2O2,and at present,the most widely used catalysts are the supported Pd pellet catalysts,which show better activities than other metal-based or metal-free pellet catalysts in the hydrogenation of eAQ.However,there exists poor mass transfer performance,low effective utilization rate of catalysts,and low production yield of unit mass catalyst in hydrogenation process.Thus,the most important work for hydrogenation process is how to solve these problems.For monolithic catalysts,the mass and heat transfer efficiency are improved in orders of magnitude,and effective utilization rate of catalyst active component is high?near 100%?,thus they are widely used in gas-solid two phases and gas-liquid-solid three phases catalytic reactions,which are controlled by transfer or the combination of transfer and chemical reaction.In this work,considering the excellent transmission performance of monolithic catalyst and characteristics of the anthraquinone hydrogenation over Pd catalysts,the monolithic catalysts are proposed to be used in the anthraquinone,and in terms of coating and active sites of monolithic catalysts,the Pd-based composite coating and Pd-based bimetallic monolithic catalysts are prepared to further strengthen the hydrogenation process,increasing the efficiency of hydrogen peroxide.In terms of reaction mechanism,catalysts preparation,structure characterization and catalytic activity test,the mass transfer and reaction of the production of H2O2 over monolithic catalysts are systematically studied to provide scientific basis and theoretical guidance for the industrial amplification of structured catalysts.The main research contents and conclusions are as follows:First,Pd/SiO2/cordierite monolithic catalysts with different Pd loadings are investigated and the appropriate experimental conditions are tested in the anthraquinone process.It was found that the STY can achieve 793.2 g H2O2·g-1 Pd h-1 over the optimum catalyst 1.5%Pd/SiO2/COR monolith catalyst on the appropriate conditions for the synthesis of H2O2:temperature 80?,atmospheric pressure,the inlet H2 and eAQ flow rates of 10 and 0.7 mL·min-1,respectively,eAQ concentration 60 g·L-1.Second,in order to further improve the catalytic activity,the Pd/washcoat/cordierite?W= MCM-41,SBA-15,MCM-22,Beta,ZIF-8?monolithic catalysts are used in the hydrogenation of eAQ.The reduction condition,metal loading,and the thickness of washcoat are investigated,and the monolithic catalysts and pellet catalysts are compared in the hydrogenation of eAQ.The optimum washcoat thickness of Pd/W/COR is 40-50 ?m;The optimum Pd loading is 2%for Pd/ZIF-8/COR and 0.8%for the other six types of monolithic catalysts.The N2H4 and H2 co-reduction is the best reduction condition.The 0.8%Pd/MCM-41/COR has the highest H202 yield(7.54 g·L-1)and selectivity?85.3%?for active quinones among the monolithic catalysts investigated.Under the same experimental conditions,the H2O2 yield over the 0.8%Pd/MCM-41/COR monolithic catalyst(7.54 g L-1),of which the Pd loading is 0.2%based on the total weight of the monolithic cordierite support and the washcoat,is a little lower than that over the commercial 0.2%Pd/Al2O3 pellet catalyst?diameter 3 mm,porosity 0.37?(10 g·L-1).However,the selectivity for active quinones?85.3%?and Pd efficiency(1573 g H2O2·g-1 Pd h-1)for 0.8%Pd/MCM-41/COR are significantly higher than those of the commercial Pd/Al2O3 pellet catalyst(70.0%and 500 g H2O2· g-1 Pd h-1).Third,CFD fluid dynamics simulation software are used to investigate the mass transfer performance in anthraquinone hydrogenation to evaluate the activity difference between monolithic catalysts and pellet catalysts from the point of mass transfer.The intrinsic kinetic equation:Reaction activation energy is 27.54kJ·mol-1.Then,the main influence factor of the hydrogenation of 2-ethylanthraquinone was investigated.It is demonstrated that gas-liquid mass transfer is the main factor.The CFD simulation results show that the gas-liquid mass transfer coefficients of monolithic catalysts can be improved 5-20 times when compared to the pellet catalysts.Moreover,in the presence of anthraquinone hydrogenation,the mass transfer process is further strengthened to explain the Pd efficiency of monolithic catalysts being higher than that of pellet catalysts.Fourth,Pd-based bimetallic monolithic catalysts are proposed to further strength the anthraquinone hydrogenation process,and the Pd-M/SiO2/COR?M = Ni,Fe,Mn,and Cu?bimetallic monolithic catalysts are prepared.Moreover,the metal type and mass ratio of Pd and the second metal are investigated in the hydrogenation of eAQ.The characterization results show that the addition of the second metal can reduce the particle size of active metal and improve distribution uniformity of active component.It is demonstrated that the catalytic activity follows the order of Pd-Ni/SiO2/COR>Pd-Fe/SiO2/COR>Pd-Mn/SiO2/COR>Pd/SiO2/COR>Pd-Cu/SiO2/COR.The optimum mass ratio of Pd and the second metal is Pd/M = 2.Finally,DFT calculations were conducted to investigate the adsorption energetics for eAQ on both Pd and PdM?M = Ni,Fe,Mn and Cu?surfaces and the hydrogenation reaction mechanism of eAQ and H4eAQ,indicating the intensification mechanism of bimetallic catalyst.The results show that the adsorption energies of eAQ on Pd3Ni1?111?,Pd3Fe,?1 1 1?,Pd3Mn1?1 1 1?,and Pd3Cu1?1 1 1?decrease when compared to that on on Pd?1 1 1?,which is due to the aromatic rings having a strong affinity for Pd,not for the second metals.However,the adsorption energies of eAQ on Pd3M1?M = Ni,Fe and Mn?does not change so much as opposed to that on Pd?1 1 1?,due to the strong bonding of the carbonyl group with the second metal atoms.This strong bonding results in the higher catalytic activity.However,the adsorption energy of eAQ on Pd3Cu1?1 1 1?changes a little more as opposed to that on Pd?111?,which is attributed to the inhibition of adsorption of eAQ on Pd3Cu1?1 1 1?.In addition,the reaction mechanism is investigated to explain the difference for catalytic activity.It is demonstrated that the reaction activity energy decrease after the addition of the second metal?Ni,Fe,Mn?,and the reaction activity energy increase after the addition of Cu,which lead to the different catalytic activity.Moreover,the reaction route is obtained,and the reaction route of different catalysts is similar.They both have four reaction transition states. |
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