(583bh) Mechanism and Kinetics of Cyclohexanone Ammoximation Over Titanium Silicate Molecular Sieves

Mechanism and Kinetics of Cyclohexanone Ammoximation over Titanium Silicate Molecular Sieves

Yongxiang Li, Wei Wu and Enze Min

 State Key Laboratory of Catalytic Material and Reaction Engineering, Research Institute of

Petroleum Processing, SINOPE, Beijing China

Abstract

Cyclohexanone oxime is the key intermediate in the manufacture of e-caprolactam. Conventional production routes involve numerous steps with the use of hazardous chemicals like oleum, halides, and oxides of nitrogen. In addition, large quantities of the low value by-product ammonium sulfate and considerable amount of waste are also produced. An alternative method of cyclohexanone oxime synthesis is the reaction of cyclohexanone with ammonia and hydrogen peroxide in the liquid phase over titanium silicate molecular sieves[1]. Since the main byproduct of the reaction is water and the reaction takes place under mild conditions, it is an environmental friendly, clean and efficient process technology. Since the discovery of the catalytic effect of titanium silicate on the process[2], the literature has mostly dealt with either the identification and characterization of active sites or the experimental determination of conversion and selectivity data under specific conditions[3-6]. However, only a limited number of studies have focused so far on the mechanism and the kinetics of this reaction over titanium silicate sieves with diluted hydrogen peroxide[7,8]. The objectives of the present study are to examine the reaction framework of cyclohexanone ammoximation and develop more complete kinetic models including hydrogen peroxide decomposition by a more detailed kinetic analysis.

The titanium silicate catalysts used in this study are synthesized hydrothermally. The gel is prepared from an appropriate mixture of tetraethylorthosilicate, tetrabutylorthotitanate, and sodium-free tetrapropyl ammonium hydroxide as organic template. The hydrothermal crystallization is performed in a stainless steel reactor at higher temperature. The solid is then filtered, washed, dried and calcined at 823K. SEM observations reveal that these catalyst samples are spherical particle in the range of 0.1~0.3μm. The specific interfacial area of the catalyst and its porous volume are 429m²/g and 0.497ml·g^-1, respectively. The materials used in this study are cyclohexanone, hydrogen peroxide, ammonia, t-butyl alcohol, and they are obtained from commercial suppliers.

The kinetics of the cyclohexanone ammoximation in the liquid phase over titanium silicate molecular sieves has been studied in a slurry reactor. The reactions are carried out at different temperatures and molar ratios of NH₃/C₆H₁₀O and H₂O₂/ C₆H₁₀O under atmospheric pressure. The concentration of titanium silicate sieves in liquid is kept constant. The reaction products of cyclohexanone ammoximation are analyzed using a gas chromatograph. The H₂O₂ concentration in reaction mixtures is determined by iodometric titration. In this study the initial rates method is used for kinetic analysis. They are obtained by fitting the concentration/time data by Marquardt technique. The Gauss-Newton method is used to estimate model parameters by fitting the kinetic data.

The ammoximation of cyclohexanone is found to proceed with good cyclohexanone conversion and selectivity to form oxime over the titanium silicate sieves. The experimental results show this reaction is actually conducted in a multiphase reaction medium with a complex parallel-consecutive reaction scheme. The major product of the reactions is cyclohexanone oxime, while some by-products are also produced. We have found that more than ten trace amount of by-products appeared in the reaction product. In spite of very small amounts of organic by-products, the catalyst can irreversibly absorb them resulting in a blockage of active sites and a loss of catalytic activity. Most organic side reactions are non-catalytic homogeneous reactions. The possible reaction mechanism obtained from the detailed kinetic analysis suggests H₂O₂ is adsorbed in the active site of catalyst, the synthesis of hydroxylamine by NH₃ and H₂O₂is irreversible and this reaction rate is quicker than the one of cyclohexanone and hydroxylamine. The analysis of the reaction framework is useful to understanding the deactivation mechanism of the catalyst and increasing the catalyst stability.

On the basis of these experimental data under the reaction conditions investigated, the power law rate equations have been developed and the kinetic parameters are evaluated by simulation the initial rate data. The reactions are between zero and unity order with respect to reactant concentrations. The rate equations show that cyclohexanone ammoximation is no more sensitive than H₂O₂ decomposition with respect to H₂O₂. The activation energy is found to be 95kJ/mol and 82kJ/mol for the ammoximation of cyclohexanone and H₂O₂decomposition, respectively. These data clearly show a good agreement between the model prediction and experimental data under the present operating conditions. The kinetic models are useful to the design and operation of reactor.

Keywords: kinetics, titanium silicalite, cyclohexanone, ammoximation, cyclohexanone oxime

REFERANCES

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