Research On Catalytic Stability Of In-situ Liquid Catalytic Hydrogenation

In situ liquid catalytic hydrogenation is a novel liquid system of cat-alytic hydrogenation proposed by our group, of which active hydrogenproduced from the reforming of alcohol solution was used directly for thehydrogenation reactions, obviating using molecular hydrogen gas. In thenovel reaction system, the utilization of hydrogen atoms from alcohols isimproved greatly because the hydrogen atoms come from both alcoholsand water. Moreover, the novel reaction shows wide application in indus-try thanks to its advantages of high atomic utilization and environmentfriendly.N-alkylation is among the most important reactions in organic che-mistry. Using proper catalyst, the reforming of alcohol solution, the hy-drogenation of nitroaromatics to anilines and N-alkylation of anilines to N-alkyl anilines would be coupled efficiently. Therefore, one step synthe-sis of N-alkyl anilines from nitroaromatics is a desirable process owing tothe advantages such as simplifying process and separation steps, highproductivity, and low cost. In the reaction system, alcohols were used assolvent, hydrogen donor, as well as alkylating reagent. The catalyst wasacted as hydrogen producing, hydrogenation, as well as N-alkylation.However, the most problem of the in situ liquid catalytic hydrogena-tion is the stability of the catalyst. In this dissertation, assembled bimetal-lic catalyst prepared by the method of adsorption was used. The catalyticstability of in situ liquid catalytic hydrogenation was highlight. Besides,the main factors of catalytic deactivation and the methods of catalystsreactivation were discussed. Main points of this dissertation were listed asfollowing:1. The assembled Pt-based catalysts were prepared by adsorptionand applied in the in situ liquid catalytic hydrogenation of nitrobenzene tocorresponding N-alkyl anilines. The effect of the content of Sn, reactiontemperature, the concentration of nitrobenzene and the content of waterwere discussed. The results show that, with the addition of Sn, the cata-lytic activity and selectivity were increased significantly. In the optimumreaction conditions （503K,5%H2O,7.5h-1）,1wt%Pt3Sn/Al₂O₃wasproved the highest catalytic activity, with the conversion of nitrobenzene100%, and the total selectivity of N-ethyl aniline and N, N-diethyl aniline 98.2%. This is because that Sn would modify the geometric effect andelectron structure of Pt. Ionic tin act as Lewis acid make it is able to acti-vate the N=O group hydrogenation.2. The nitroaromatics derivations catalyzed by assembled Pt-basedcatalyst were studied. The results show good activity of most nitroaro-matic especially to the containing electron-donating group, with the con-version of100%, and the total selectivity more than90%. This could beexplained by the electronic effect and conjugative effect, which woulddecrease the electronic density of N atom. The electron abundant Pt atomwould absorb the electron deficiency N atom, strengthening the adsorp-tion of reactant, which is benefit for the hydrogenation reaction.3. In the study of the catalytic stability of in situ liquid catalytic hy-drogenation,1wt%Pt3Sn/Al₂O₃shows super stability with more than550h. This could be explained that, Sn existed in the formation of ionic spe-cies and metallic state. Ionic species of Sn are likely associated with thesupport, slowing down the loss of active ingredient. While metallic stateSn is inclined to form Pt3Sn alloys, which would inhibits cracking reac-tions involving cleavage of C-C bonds and resistant to coking comparedto Pt catalyst.4. The results of IR characterization indicated that, there were COcharacteristic absorption peaks on the surface of deactivated catalyst. Thiscould be illustrated that with the changes of Pt3Sn alloy, the concertration of CO increase, which poison the catalyst. Besides the catalyst stabilitywould be improved by decreasing the concentration of CO through wa-ter-gas shift （WGS） and Fischer-Tropsch （FTS）.4. The relationship between surface structure, valence state, the spe-cific surface areas, the average surface adsorption of the catalyst and cat-alytic stability was characterization by TEM、XRD、XPS、BET and IR,respectively. The results show that the catalyst deactivation is attributedto coupling of the changes of crystalline phase of active ingredient, ag-gregation and losing of active ingredient, the changes of structure of hole,and the poisoning of CO. After catalyst regenerating, it was proved thatCO adsorb on the surface of catalyst poison the catalyst was the mainfactor leading to catalyst deactivation.In conclusion, in situ liquid hydrogenation was adapted synthesis ofa series of N-alkyl anilines from nitroaromtics derivation, obtaining goodconversion and selectivity. Moreover, the catalyst stability could enhancedrasticly after tin modification. The main reason of the catalytic deactiva-tion was attributed to the changes of Pt3Sn alloy, leading the formation ofCO, poisoning the catalyst. The novel liquid hydrogenation contains theadvantages such as high atomic utilization, simplification procedure andenvironment-friendly, high catalytic stability. Therefore, the developmentof in situ liquid hydrogenation is promising both in laboratory and indus-try.