| Polyoxometallates(heteropoly acids)have been widely employed as oxidation catalysts in various homogeneous and heterogeneous reactions.One of the great advantages of polyoxometallates catalysts is that their oxidation catalytic properties can be controlled in a systematic way by changing the identity of charge compensating counter-cations,heteroatoms, and framework polyatoms.Another advantage that makes polyoxometallates promising catalysts is their great thermal stability,so they can be used under the middle high temperature catalytic conditions(300～350℃).However,there are some problems for polyoxometallates in the application:as homogeneous catalyst,it is difficult to recover the catalyst from the system;and as heterogeneous catalyst,their surface area is very low.So it is very important to resolve above mentioned problems for the application of polyoxometallates.Firstly,a reaction-controlled phase-transfer catalyst Q3PMo4O16(Q=[C7H7N(CH3)3]+)is synthesized by using quaternary ammonium salt to modify the polyoxometallates.Under the action of H2O2,the insoluble Q3PMo4O16formed soluble catalytic activity species Q3PMo4O24 to catalyze the oxidation reaction;after the H2O2 are consumed up,the catalyst can deposit from the system as Q3PMo4O16.Therefore,in the reaction,the catalyst not only shows the feature of homogeneous catalyst,but also can be recovered easily as heterogeneous catalyst. This catalyst is employed in the liquid phase oxidation of alcohols.It is found that the aromatic alcohols and aliphatic sec alcohol can be catalyzed oxidation more efficiently than aliphatic primary alcohol.When the molar ratio of H2O2 to benzyl alcohol is 0.75,the conversion based on H2O2 reaches 92.8%,the selectivity of main product benzaldehyde is not less than 99%,the recovery ratio is 87.6%(wt%),moreover,the catalyst can be recycled three times without decreasing the activity obviously.Then,another reaction-controlled phase-transfer polyoxotungstate catalyst Q9PW9O34 (Q=[C7H7N(CH3)3]+)is prepared.The results of characterizing with FT-IR and 31P NMR indicating that after it reacts with H2O2,the catalyst depolymerizes into one smaller active species[PW4O24]3-which is soluble in the reaction medium,when the oxidant H2O2 is consumes up,the soluble species polymerizes into insoluble larger compounds with Keggin-structure framework.The catalyst is utilized in the liquid phase oxidation of benzyl alcohol,when the molar ratio of H2O2 to benzyl alcohol is 0.9,after 0.5h,the conversion based on H2O2 is not less than 95.0%,the selectivity of benzaldehyde is over 99%,the catalyst is recycled three times and the activity is maintained well. Through kinetic experiments,the reaction rate equations of liquid phase catalytic oxidation of benzyl alcohol with Q3PMo4O16and Q9PW9O34are obtained,respectively.The equations as follows:dc/dt=k[BzOH][Q3PMo4][H2O2]and dc/dt=k[BzOH]0.5[Q9PW9]0.5 [H2O2]0.5.The reaction activation energy of these two catalytic systems are 48.6 kJ/mol and 27.0 kJ/mol,respectively.And the kinetic mathematical model of catalytic system with O3PMo4O16is built.Some supported polyoxometallates catalysts are prepared by supporting a series of polyoxometallates on the quaternary ammonium poly(phthalazinone ether sulfone ketone.In the liquid phase oxidation of benzyl alcohol,when the molar ratio of substrates to H2O2 is 1, the catalytic activity of QAPPESK-PW9 is the highest,the conversion of benzyl alcohol reaches 92.1%,and the selectivity of benzaldehyde is 98.2%within 6h.And in the catalytic oxidation of cyclohexene,QAPPESK-PMo4 is confirmed to be the best catalyst,the conversion and selectivity of cyclohexene oxidation reaction are 95.7%and 96.0%, respectively within 4 h.The studies of characterization of QAPPESK-PW9 and QAPPESK-PMo4 with FT-IR,XRD and TG demonstrate that:the heteropoly anions are finely dispersed on the polymer support,they do not form crystal phase on the support surface; the structure of polymer support and heteropoly anions are maintained well during the reaction.The QAPPESK-PW9 and QAPPESK-PMo4 can be recycled five times without decreasing the catalytic activity,indicating that no leaching of the active species occurred during the reaction.The crosslinked polystyrene D201 is employed as the support of[PW9O34]9-and [PMo4O24]3-.The catalysts are characterized with FT-IR,UV,XRD,TG and EPMA,and it is found that the heteropoly anions do not form crystal phase on the support surface,during the reaction,the polymer support and heteropoly anions show high stability,their structure are both maintained well.The D201-PMo4 shows higher catalytic activity than D201-PW9,within 6h,the conversion of cyclohexene reaches 89.5%,and the selectivity of main product is 90.5%.The catalyst can be recycled five times without decreasing the activity obviously.The catalyst with QAPPESK as support shows higher catalytic activity than that with D201 as support. |
PDF Full Text Request