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Fabrication of Pt Nano-cluster/Chitosan and Its Derivatives Hybrid Membrane and Its Catalytic Activity for Partial Hydrogenation of Benzene

Author: DiHao
Tutor: ZhangAiQing
School: Central South University for Nationalities
Course: Polymer Chemistry and Physics
Keywords: Hybrid membrane Pt nano-cluster Chitosan Cyclohexene Hydrogenation
CLC: TQ231.2
Type: Master's thesis
Year: 2011
Downloads: 39
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Abstract

Cyclohexene is an important chemical raw material with active double bond, which has widespread use in the pharmaceutical and petroleum industry. Especially in polymer field, the production route of Nylon 6 or 66 with cyclohexene as the raw material is simple and low-cost. Although cyclohexene is an intermediate product of benzene hydrogenation, cyclohexane is always produced because of the kinetically favored hydrogenation of cyclohexene to cyclohexane. Thus, investigation of catalyst for the partial hydrogenation of benzene to cyclohexene is the key.Several metals can be used as hydrogenation catalysts, such as Pt, Pd, Ru, and Ni, in which Pt has the highest catalytic activity for benzene hydrogenation. Dini et al found that cyclohexene was a reaction product of benzene hydrogenation when the Pt/polyamide was used as catalysts. In their studies, the highest selectivity to cyclohexene was 48.0%, but the conversion of benzene was 0.4%, while the conversion of benzene got to 25.9%, but then the selectivity to cyclohexene was only 0.1%. In addition, Domínguez et al reported that when catalysts of 0.5% Pt supported on gallia/alumina were used in benzene hydrogenation, the only product was cyclohexane. Nagahara reported a particularly efficient catalyst composed of Ru-black promoted with ZnO. In their studies, the yield of 56% and the selectivity of 80% were believed to be high enough to develop an industrial process. Recently, William et al have studied the activities for low-temperature hydrogenation of benzene by Pt/Ni bimetallic catalysts supported onγ-Al2O3. It was observed that the bimetallic catalysts were more active than either parent metal catalyst, but in this paper cyclohexane was the only observed reaction product.In our previous studies, we have investigated the fabrication of Pt nano-cluster/polyimide hybrid membrane and its catalytic activity to the partial hydrogenation of benzene, in which the selectivity to cyclohexene reached to 72.4%. Thus it can be supposed that benzene may contact the active center of catalyst in the hybrid membrane through swelling method, and the controlling of the hydrogenation to obtain higher selectivity of cyclohexene can be realized by controlling the swelling degree of membrane. However, the preparation process of polyimide is complex and costly, so in this research the cheap and abundant natural polymer chitosan (CS) was chosen. Chitosan is the N-deacetylated derivative of chitin, which is suitable functional material because of its biocompatibility, biodegradability, non-toxicity, adsorption properties, etc. In addition, the large number of -NH2 and -OH in CS has good chelating ability to metal. There are three parts to be included in this paper: 1. Monodispersed Pt nano-clusters were prepared by the reduction of H2PtCl6 with eth ylene glycol under microwave conditions. IR、TEM、XRD、XPS and TG were used to characterize the structure of platinum nano-particles and Pt/CS. TEM and XRD sho wed that Pt nanoclusters can be dispersed in the chitosan membrane, while keep intact crystal; XPS and TG data indicated that Pt was coordinated with N and O in CS to some extend, while hybrid membrane has good thermal properties. Experimental results showed that Pt/CS catalyst gave a high selectivity for cyclohexene of 68% in the liquid phase hydrogenation of benzene, while there was no cyclohexene in the product when the catalyst was only Pt nano-particles without chitosan hybrid membrane.2. N-arylation of chitosan was performed via a Schiff bases formed by the reaction between the 2-amino group of glucosamine residue of chitosan with different quality of propionaldehyde, hexanal, twelve carbon aldehyde under acidic condition followed by reduction of the Schiff base intermediate with sodium borohydride.IR and NMR di fferent degrees of substitution and different length of alkyl chain has been grafted on to chitosan.Pt/ N-aryl chitosans were characterized by XRD,XPS and TG. With the in creasing of extent of substitution of N-n-propyl, the swelling degrees of PCS membr anes in benzene, cyclohexene, and cyclohexane were all increased. Experimental of hydrogenation of benzene catalyzed by Pt/C3H5-CS showed that with the increasing of the extent of substitution of N-n-propyl, the conversion of benzene was gradually increased, while the selectivity of cyclohexene increased at first and then decreased. With 28.9 % of ES, the selectivity of cyclohexene was as high as 85.2 %. Experimental of hydrogenation of benzene catalyzed by Pt/C6H13-CS showed that with the increasing of the extent of substitution of N-n- C6H13, the conversion of benzene was gradually increased, while the selectivity of cyclohexene increased at first and then decreased. With 32 % of ES, the selectivity of cyclohexene was as high as 60.3 %. Experimental of hydrogenation of benzene catalyzed by Pt/C12H25-CS showed that with the increasing of the extent of substitution of N-n- C12H25, the conversion of benzene was gradually increased, while the selectivity of cyclohexene decreased.3. Chitosan grafted hydrophobic groups GMA were characterized by IR, NMR, XRD and XPS. The introduction of hydrophobic groups GMA changed the degree of crystallinity of CS-g-GMA , also made the swelling ratio of non-polar solvents such as benzene, cyclohexene and cyclohexane, in CS-g-GMA increase significantly. With the increasing of flow velocity of benzene molecules in the Pt / CS-g-GMA hybrid membrane made conversion of benzene hydrogenation reaction significantly increase, while the selectivity of cyclohexene is also essential to maintain the 55%.

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