Article pubs.acs.org/IECR
Kinetic Model of Catalytic Self-Condensation of Cyclohexanone over Amberlyst 15 David Lorenzo, Ernesto Simón, Aurora Santos, and Arturo Romero* Departamento de Ingeniería Química, Facultad de Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain S Supporting Information *
ABSTRACT: The kinetics of heterogeneously catalyzed self-condensation of cyclohexanone using Amberlyst 15 as a catalyst has been determined from data obtained in a batch reactor. Temperature and catalyst concentration ranges used were 70−110 °C and 6−66 gcat dry·kg−1, respectively. Runs were carried at both 5 bar of pressure, in order to avoid the evaporation of the water produced, and under vacuum conditions (0.4 bar) in order to remove the water produced. A negative impact of water on the reaction rate was found due to adsorption onto the catalyst surface and promotion of the reverse reaction. The main products of cyclohexanone self-condensation were dimers 2-(1-cyclohexenyl)cyclohexanone (D2) and 2-cyclohexylidenecyclohexanone (D3), this reaction being the first step in obtaining 2-phenylphenol. Undesired trimers were obtained significantly at the highest temperature used. Kinetic modeling was carried out, taking into account reactant and product concentrations (cyclohexanone, dimers, trimers, and water), catalyst concentration, and temperature. Both potential rate law and models based on Langmuir− Hinshelwood theory were tested. The Langmuir isotherm for water adsorption was also determined from experimental data. Strong water adsorption was found if water was not distillated while cyclohexanone adsorption was negligible and condensation products were weakly adsorbed onto the catalyst surface. Adsorption constants calculated for water and dimers were 0.017 and 3.63 × 10−4 kg·mmol−1, respectively. The activation energy obtained for trimer production (220.52 kJ·mol−1) was higher than that corresponding to the dimer formation (68.46 kJ·mol−1) explaining the decrease in dimers at the higher temperature. The stability of the catalyst was analyzed in a continuous fixed bed reactor (FBR), finding that no deactivation took place in the 300 h used as time on stream. The kinetic model obtained in batch mode was also able to explain the data obtained in the FBR satisfactorily. hydroxy-[1,1′-bicyclohexyl]-2-one (D1) is first formed and then dehydrated in situ, yielding an isomeric mixture of reaction products, namely, 2-(1-cyclohexen-1-yl)cyclohexanone (D2) and 2-cyclohexylidencyclohexanone (D3), as shown in Scheme 1.5 The reaction illustrated in Scheme 1 is the first stage in industrial synthesis of 2-phenylphenol.4,11,12 The second reaction step is the production of 2-phenylphenol showed in Scheme 2. The dimers, which are formed by this mechanism, can also be condensed with excess cyclohexanone in the reaction medium, consequently the selectivity for dimers is reduced because of the formation of undesired side products.4,12 The consecutive reactions are shown in Scheme 3. Among the catalyst used for cyclohexanone self-condensation (summarized in Table 1), Amberlyst 15 has been employed. Amberlyst polymer based catalysts and ion exchange resins mostly involve the use of functionalized styrene divinylbenzene copolymers with different surface properties and porosities. The functional group is generally of the sulfuric acid type. These resins are supplied as gellular or macro-reticular spherical beads. Amberlyst polymeric resins have been used for 40 years in a
1. INTRODUCTION Cyclohexanone is an important intermediate in the chemical industry. It is mainly used to produce caprolactam and adipic acid, which are used as precursors when manufacturing nylon fibers.1,2 Furthermore, cyclohexanone is used to produce 2-(1cyclohexen-1-yl)cyclohexanone (D2) and 2-cyclohexylidencyclohexanone (D3), important intermediates in the synthesis of 2-phenylphenol (OPP). This compound has been used as biocide in different fields such as agriculture, fine chemistry, and hospital and household industries. OPP is used to preserve fruits and vegetables and to control various pests. It is also applied to disinfect surface materials. It is added to products in the cosmetic, plastic, textile, and paper industries as a preservative.3 Cyclohexanone is able to self-condense to produce the dimers. This reaction is a reversible aldol condensation that can be catalyzed by acidic4 or basic5 catalysts. This reaction takes place as a side reaction5 in several stages of caprolactam production, such as in cyclohexyl hydroperoxide decomposition with aqueous caustic soda,6 in the catalytic dehydrogenation of cyclohexanol to cyclohexanone,7 or during cyclohexanone purification using an alkali metal hydroxide8,9 or a salt of sulfuric acid10 as a catalyst. Cyclohexanone is condensed by a basic or acid catalyst in liquid phase to form dimers: 2-(1-cyclohexenyl)cyclohexanone (D2) and 2-cyclohexylidenecyclohexanone (D3). In the cyclohexanone self-condensation reaction, the adduct 1′© XXXX American Chemical Society
Received: August 14, 2014 Revised: November 11, 2014 Accepted: November 17, 2014
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dx.doi.org/10.1021/ie5032265 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Scheme 1. Dimer Formation in the Cyclohexanone Self-Condensation Reaction
Owing to the water content having a negative effect on the reaction, due both to adsorption14 and reverse reaction5 and the fact that water and organic compounds have quite different boiling points, a reactive distillation (RD) could be used to avoid the negative effect of water on the reaction rate. In general, RD is a process intensification strategy that combines reaction and separation in the same vessel.15 In the last two decades the use, design, and modeling of RD reactions have risen significantly.16 In this study, we focus on obtaining a kinetic model for the self-condensation of cyclohexanone using Amberlyst 15 as catalyst in presence and absence of water. The kinetic model should quantify the effect of all variables: temperature, catalyst concentration, and reactant and product concentrations (water included) in cyclohexanone conversion and selectivity for dimer formation. For a reliable quantification and distribution of the water in vapor, liquid, and solid phases, two set-ups were used. RD assured that all water formed was in the vapor phase. A pressurized system guaranteed that all the water formed was in the liquid−solid phases. Finally, continuous operation was carried out to check both catalyst stability and validation of the kinetic model obtained in batch conditions.
Scheme 2. 2-Phenylphenol Formation
wide variety of reactions for the manufacture of bulk and fine chemicals and purification processes. These resin catalysts can advantageously replace mineral and organic acids and bases in various syntheses such as phenol alkylation with olefins, condensations of aldehydes and ketones, esterification of organic acids, or etherification of olefins with alcohols.13 Amberlyst catalysts are a significant step toward accomplishing green chemistry goals by helping to reduce byproducts and effluents in organic synthesis. The most important parameters to take into account are maximum operating temperature, moisture content, surface area, porosity, and ion exchange capacity. Water is a key specie as it can solvate the protons present in the active catalytic sites. Under high concentrations of polar compounds, the sulfonic groups can be ionized, solvated, and even dissociated. Another factor to take into account with Amberlyst 15 as catalyst is the operating temperature range because the sulfonic group hydrolysis is very important above 127 °C, and the loss of activity can be dramatic.14 As shown in Table 1, the two previous studies of cyclohexanone self-condensation using Amberlyst A-15 were carried out in batch operation at atmospheric pressure and a temperature range close to the water boiling point. Both studies12,14 describe the strong effect of water in the reaction medium. However, the water contents in both the liquid phase and on the catalyst surface were not measured. Reverse reaction were not explicitly taken into account and no kinetic studies of trimer formation were performed. Moreover, no data on catalyst stability are available.
2. EXPERIMENTAL SECTION 2.1. Reactants and Materials. Cyclohexanone (Fluka, catalogue no. 29135); 2-(1-cyclohexenyl)cyclohexanone, containing about 10% 2-cyclohexylidencyclohexanone (Alfa Aesar, catalogue no. L09798); dodecahydrotriphenylene, 99% (Aldrich, catalogue no. 106518); N-ethylbenzamide (Aldrich, catalogue no. 426385); triphenylene (Aldrich, catalogue no. T82600) were used as reactants or standards. 2.2. Catalyst. In the present work, the strongly acidic ionexchange resin Amberlyst 15 hydrogen form (Sigma-Aldrich, catalogue no. 06423) was used for catalysis. The main catalyst properties are13 maximum operating temperature 120 °C,
Scheme 3. Three (T) and Four (F) Rings Condensed Products in the Cyclohexanone Self-Condensation Process
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dx.doi.org/10.1021/ie5032265 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Table 1. Experimental Conditions for Self-Condensation of Cyclohexanone Found in the Literature year
T, °C
P, bar
catalyst
Ccatdry, g L−1
reactor
ref
1993 1993 2003 2008 2010 2013
90−130 70−110 119−137 80−100 110−130 127−149
1
