Kinetics and mechanism of acid catalyzed alkylation of phenol with cyclohexene in the presence of styrene divinylbenzene sulfonic resins
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► No relevant influence of the type of the sulfonic resin on the cyclohexylation kinetics. ► Zero reaction order for cyclohexene and higher than 1 for phenol concentration. ► A consecutive adsorption equilibrium of cyclohexene on pre-adsorbed phenol occurs. ► The kinetics of cyclohexene dimerization, di-cyclohexylation, etherification and adsorption equilibria are evaluated. ► The experimental data fits a Eley–Rideal type model, which takes into account equilibria and side reactions kinetics.
Introduction
Alkylphenols and alkyl phenyl ethers are molecules of considerable interest because of their industrial relevance [1], [2], [3]. As a matter of fact, the world production of alkylphenols exceeds 460,000 t/yr. The vast majority of alkylphenols is used to synthesize derivatives which have applications ranging from surfactants to pharmaceuticals. In additions, the use of alkylphenols in the production of both polymer additives and monomers for engineering plastics is expected to show a constant growth during the next years. Alkyl phenyl ethers are compounds of great interest in the field of fine chemicals and their production is in continuous expansion especially in the new markets [1], [2], [3].
The cation-exchange resins are used as catalysts in a large number of industrial processes, such as the manufacture of alkyl phenols, the esterification of carboxylic acids, the synthesis of ethers, the hydration of alkenes, etc. [4], [5], [6], [7], [8], [9]. The use of these heterogeneous systems is a suitable alternative to the usual procedures in a homogeneous system in the presence of mineral acids and ion-exchange resins appear to be ideal catalysts to convert polluting processes into greener ones [4], [5], [6], [7], [8], [9], [10], [11], [12].
Phenol is an activated molecule to the electrophilic aromatic substitutions and its mechanism of alkylation has been known for a long time [1], [3], [13], [14], [15]. Some aspects, connected to kinetics and selectivity of phenol cyclohexylation, however, are not completely elucidated. In particular, the studies of Sharma and coworkers carried out in the early nineties, relative to the reactivity of several olefins toward phenol in the presence of sulfonated resins, pointed out that the ortho–para selectivity in the ring alkylation of phenol is strictly related to the nature of the olefin. In particular, propene and 1-butene give an ortho–para ratio close to 2, while isobutene, α-methyl styrene and diisobutene give almost exclusively para alkylation [16]. More recently, Hölderich and coworkers showed that high para selectivity is obtained in the alkylation of phenol with isobutene and the nature of the acid catalysts does not influence the selectivity. On the contrary, catalyst activity is influenced by the amount and the strength of the acid sites [17]. In these studies there are no evidences of cyclohexylphenyl ether formation; in contrast, by using cyclohexene as alkylating agent, the formation of the ether occurs with high yield by using solid acid catalysts with different nature [18], [19], [20]. For instance, Yadav and Kumar have recently studied the kinetics of phenol cyclohexylation catalyzed by many solid acids in solvent-less conditions at 333 K [18]. The time concentrations profile shows the formation of cyclohexyl phenyl ether, as the most abundant product (70%) after 4 h of reaction. Ring alkylation occurs in lesser extent and the ortho–para ratio is in the range 2–7 depending on the type of the catalyst used [18]. In that work the best fit of the data are obtained by an Eley–Rideal kinetic model, but several aspects of the kinetics were not studied. More recently, Yadav and Pathre studied the cyclohexylation of guaiacol catalyzed by several solid catalysts. Also in this work, the formation of the ether is observed but its concentration passes through a maximum and finally decreases to complete consumption [21], [22]. Also in this case the best fit of the data was obtained by an Eley–Rideal kinetic model, but a consecutive rearrangement of the ether was implemented in the model thus explaining its disappearance [21]. Quite surprisingly, there is no mention of the etherification equilibrium, which is well known for aliphatic ether, and it is evident also for the cyclohexylphenyl ether [23], [24].
The mechanistic aspect of the electrophilic attack to the phenol is investigated from a theoretical point of view with DFT studies by Tang and coworkers. These authors suggested that an olefin reacts with a sulfonic acid leading to the formation of a sulfonic ester intermediate, which, in turns, reacts with phenol to form the products of alkylation [25].
In this paper we study the kinetics of phenols cyclohexylation catalyzed by some sulfonic resins in order to highlight some unclear aspects of this reaction. In particular we try to develop a kinetic model taking into account the different equilibria, since both the cyclohexylphenyl ether and heterogeneous equilibria affect the overall kinetics. In addition, the reactivity of the cyclohexyl phenyl ether, recognized as a transient intermediate, is implemented in the model [24].
Section snippets
Materials
Reagents and solvents were used after purification of the commercially available samples and their purity was checked by melting point, thin layer chromatography (TLC), high performance liquid chromatography (HPLC), gas chromatography (GC) and gas chromatography coupled to a mass spectroscopy (GC–MS). The solvents were treated in a double bed column, filled with H2SO4/SiO2 and SiO2 to adsorb water and impurities. The residual water content was checked by HPLC analysis and its concentration is
Synthesis and characterization of catalysts
Sulfonated styrene divinyl benzene resins (Amberlyst™ 15 and 36) have been treated with nitric acid and sulfuric acid in order to obtain new acid catalysts to be tested in the alkylation of phenol with cyclohexene. Average pore diameter, pore volume, BET surface area and TIEC of the resins are summarized in Table 1. Average pore diameter (AVP) does not significantly change upon treatments. Only a moderate increase of the AVP is observed for the sulfonated Amberlyst 15 and for the nitrated
Conclusions
We tested commercial and modified macroreticular sulfonic styrene divinylbenzene resins (Amberlyst™ 15 and Amberlyst™ 36) in the cyclohexylation of phenol. The modification of these materials by nitration and sulfonation allowed obtaining new catalysts with activities higher than the commercial materials. The reaction showed a complex kinetic path, which is characterized by the formation of the cyclohexyl phenyl ether as an equilibrium intermediate. The influence of the reagents on the initial
Acknowledgements
Financial support by Ca’ Foscari University of Venice is gratefully acknowledged (Ateneo fund 2009). A thank to Dr. Davide Montin for some preliminary experiments carried out during his degree in Industrial Chemistry. Finally, a special thank to Mr. Claudio Tortato for the helpful discussions.
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