Continuous Acetone Ammoximation over TS-1 in a Tubular Membrane

Jun 18, 2010 - titanium silicalites-1 (TS-1) in the tubular membrane reactor was investigated. It has demonstrated that the tubular membrane reactor s...
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Ind. Eng. Chem. Res. 2010, 49, 6309–6316

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Continuous Acetone Ammoximation over TS-1 in a Tubular Membrane Reactor Zhaohui Li,†,‡ Rizhi Chen,† Weihong Xing,*,† Wanqin Jin,† and Nanping Xu† State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing, 210009, People’s Republic of China, and College of Chemistry and Biological Engineering, Changsha UniVersity of Science & Technology, Changsha, 410076, People’s Republic of China

A new tubular membrane reactor based on tubular metallic membrane is developed, which can solve the problem concerning in situ separation of catalyst from the reaction mixture and make the production process continuous. In this article, the feasibility of continuous ammoximation of acetone to acetone oxime over titanium silicalites-1 (TS-1) in the tubular membrane reactor was investigated. It has demonstrated that the tubular membrane reactor system can maintain a more long-term steady production of acetone oxime than that of a side-stream ceramic membrane reactor and has a higher productivity than the batch reactor. The effects of operation conditions (stirring rate, residence time, temperature, catalyst concentration, molar ratio of NH3/acetone, H2O2/acetone, and t-butanol/acetone) on the performances of the reaction system were examined via single factor experiments. Results show that the operation conditions greatly affect the conversion, selectivity of acetone ammoximation, and the filtration resistance. The acetone conversion is >94.5% and the acetone oxime selectivity remains stable at ∼98% in a 30-h continuous run. 1. Introduction Recently, titanium silicalites-1 (TS-1) compounds have been extensively investigated for their high catalytic activity and selectivity in the selective oxidation of organic compounds (e.g., aromatic hydroxylation, alkene and allylic compounds epoxidation, and ketone ammoximation) under mild conditions using aqueous H2O2 as an oxidizing agent.1-6 However, there is a problem with separating TS-1 catalysts from the reaction products in the practical process,7,8 because their particle size is too fine to be removed by gravity settling or porous tube filtration.9 A simple approach to overcome this problem is to attach catalysts to a suitable substrate; however, in that case, the drawbacks are mass-transfer limitations of reactants to the catalyst surface and the decrease in the effective surface area of the catalyst particles. It was reported that catalysts in suspension have a better catalytic activity than immobilized ones.10,11 A very promising method for solving this problem is to perform the heterogeneous catalytic reaction in a membrane reactor, in which the membrane is not selective and its major role is to filter the suspended catalysts.12,13 It is the most representative feature of the reactor that the membrane reactor system can retain the catalysts in situ and make the reaction process continuous. Conventional membrane reactors are characterized by two configurations: side-stream (recirculated or external) type and submerged (immersed or integrated) type.14 The side-stream membrane reactor, in which the reaction happens in a stirred reaction vessel and the separation of the product is performed in a separate cross-flow membrane filtration unit,15 is more flexible and easier to scale up, because of the segregation of the reaction zone and the separation zone; however, the potential loss of fine catalysts could be greater in the pipe and pumping * To whom correspondence should be addressed. E-mail: [email protected]. † State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology. ‡ College of Chemistry and Biological Engineering, Changsha University of Science & Technology.

systems15 and the use of recirculation loop leads to increased energy costs.14,16 While the submerged membrane reactor, coupling the reaction zone with separation zone in a single unit of equipment,8 solves the problem concerning in situ separation of catalyst from the reaction products and consumes much less energy than an external membrane reactor, because of the absence of the high-flow recirculation pump.14 In recent years, some studies were focused on the submerged membrane reactors,13-17 and most of the studies, basically using polymeric membranes, were mainly focused on the biochemical process and wastewater treatment.15,16 Only a few investigations were based on the chemical or petrochemical production processes,8 using inorganic membranes as the separation unit. In the present work, a new configuration tubular inorganic membrane reactor system using the tubular metallic membrane as the reactor was developed and the model reaction was performed inside the tubular membrane; therefore, the system consumes much less energy than an external membrane reactor, because there is no need for the high-flow recirculation pump. Because of its excellent thermal conductivity, mechanical stability, and machinability, the tubular metallic membrane as a reactor has potential applications in some important heterogeneous catalytic reactions, such as cyclohexanone ammoximation over TS-1 and benzene hydrogenation over nickel. Acetone oxime, which has features of low toxicity, low environmental risk, and high deoxidization effect, can be widely used as a corrosion inhibitor and passivator in boilers instead of N2H4.18 The conventional routes for the production of acetone oxime involve multiple steps and hazardous chemicals such as oleum, halides, or oxides of nitrogen. In addition, large quantities of low-value byproduct such as ammonium sulfate are produced.18 The ketone ammoximation to oxime over TS-1, employing 30% aqueous H2O2 as the oxidant, is a novel and environment-friendly technology,18-22 but the problem of separating TS-1 catalyst from the reaction mixture also limits the process industrialization. In this work, the ammoximation of acetone to acetone oxime catalyzed by TS-1 catalysts was taken as a model reaction to investigate the feasibility of the continuous operation in the

10.1021/ie901912e  2010 American Chemical Society Published on Web 06/18/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010 Scheme 1. Mechanism of Acetone Oxime Formation via a Hydroxylamine Route

Figure 1. Diagram of new inorganic membrane reactor system for acetone ammoximation to acetone oxime.

tubular membrane reactor system. A porous stainless steel membrane with a nominal pore size of 100 nm was applied to construct the tubular membrane reactor, because of its excellent thermal conductivity, mechanical stability, and machinability. The characteristics of the tubular membrane reactor were evaluated by the experimental results (conversion, selectivity, and filtration resistance) of the model reaction. 2. Experimental Section 2.1. Materials. TS-1 catalyst (average particle size, 200 nm; BET surface area, 408 m2 g-1; the Si/Ti mass ratio ) 9) was provided by Baling Petrochemical Company, SINOPEC. Acetone, 30% hydrogen peroxide, 25% ammonia, t-butanol, and acetone oxime (analytical grade) were all obtained commercially and used as received. Methanol (>99.9% chromatography grade) was obtained from the Yuwang group and pure water was provided by the Hangzhou Wahaha Group (China). Deionized water (electrical conductivity of