(305d) Kinetic Modeling of the Isomerization and Alkylation Reaction Paths Involved in the Synthesis of Linear Alkylbenzenes over Y Zeolites

The alkylation of benzene is involved in the industrial production of important chemicals and chemical intermediates such as ethylbenzene, cumene and linear alkylbenzenes. In the last years, the production of ethylbenzene and cumene shifted from homogeneous catalysts to solid acids, while most industrial plants for the production of linear alkylbenzenes still use the technology based on HF catalyst. In view of its corrosive nature, the HF catalyst requires special materials and special design of the industrial installations in order to assure safe operation. After utilization, the spent catalyst requires neutralization thereby producing large amounts of residues that cause environment problems. Due to the economical and ecological issues posed by the use of homogeneous acid catalysts, processes based on solid acid catalysts are bound to get more and more importance in the field of LAB production. The detailed knowledge of the kinetics of such processes is of great importance in the development of new processes and the improvement of existing ones. The liquid phase alkylation of benzene with 1-octene over a series of Y zeolites has been chosen as model reaction to study the kinetics of the alkylation processes occurring on solid acid catalysts, in general, and of linear alkylbenzenes production, in particular. Linear alkylbenzenes are used in the industry of biodegradable detergents and Y zeolite seems to be one of the most suitable alternative catalysts for their production by benzene alkylation with long chain olefins. Due to the particular dimensions of its channels and cavities, Y zeolite offers a good compromise between easy access to its catalytic active sites and restricted formation of bulky branched molecules. In this way, Y zeolite shows a good alkylation activity with high selectivity to desired products - linear alkylbenzenes. As the catalyst Si/Al ratio increases, the concentration of the acid sites diminishes while the acid strength of the remaining sites is increasing. Therefore, by dealumination one could fine tune the acid properties of the zeolite in order to achieve a catalyst with optimal acid properties for the desired application. Three Y zeolites with Si/Al ratios 6, 13 and 30 were selected for this study. The experimental tests were performed in liquid phase, in a Robinson-Mahoney reactor operating in continuous flow mode, in a temperature domain ranging from 343K to 373K, for molar ratios benzene to 1-octene from 1 to 10. The conversion of the olefin reactant (values between 10 and 99%) and conversions to reaction products (molar yields) were obtained for space times (quantity of catalyst divided by feed molar flow of olefin) ranging from 5 to 120 kg s mol-1. During experiments, it was observed that the catalyst deactivates with time on stream. However, at high conversion of the olefins, the deactivation of the catalyst is less important suggesting that the olefins are the components responsible for this phenomenon. Initial conversions and selectivities for the fresh catalyst are obtained by extrapolation to zero time on stream. Octene isomers (2-, 3- and 4-octene) and phenyloctanes (2-, 3- and 4-phenyloctane) have been obtained as reaction products at the investigated conditions. Dialkylated products have been observed in trace amounts at 373K for low benzene to 1-octene feed molar ratios. No octene oligomers have been observed in the reactor effluent. At low values of conversions, the selectivity to alkylated products is low and the octene isomers are the main reaction products since the isomerization occurs faster than the alkylation. As the conversion of 1-octene increases, the selectivity to alkylated products increases at the expense of the olefins. At the experimental conditions investigated, the equilibrium between the octene isomers is not reached indicating that, although with different rates, the isomerization and the alkylation processes occur on similar time scales. Among the alkylated products, 2-phenyloctane ? the most desirable isomer ? is obtained with higher selectivity, especially at low conversions of 1-octene. A slight increase in the proportion of 2-phenyl isomer among the alkylated products is observed when the temperature increases while the total selectivity to alkylated products remains the same and depends only on the 1-octene conversion. At similar space times and temperatures, the conversion of 1-octene decreases as the benzene to olefin feed molar ratio increases. Simultaneously, the conversion to phenyloctanes and, especially, to 2-phenyloctane, is favored by the high benzene to olefin ratio in the feed. The higher selectivity to 2-phenyloctane can be explained by the fact that, at the experimental conditions investigated, only 2-octene and 2-phenyloctane were observed as primary products. Although the double bond shift reaction is fast, the isomerization of the olefins does not reach equilibrium and, therefore, the distribution of the octene isomers in the reaction mixture influences the distribution of the phenyloctane isomers. The activity of the catalyst increases with increasing Si/Al ratio of the zeolite. However, the selectivity to a particular reaction product does not depend on the Si/Al ratio of the catalyst but is a function of the conversion of the fed olefin only. A reaction network was developed based on the chemistry of the process and the observed product distribution. In this reaction network the olefins undergo consecutive isomerization: 1-octene produces 2-octene while 2-octene produces directly 3-octene or transforms back to 1-octene. On the other hand, all olefins can react with benzene and produce phenyloctanes. Several mechanisms have been suggested in the literature for this type of reactions. The most successful ones involve the formation of carbenium and/or carbonium ions as intermediates. Although the detailed nature of the intermediates is the subject of ongoing investigations, in this study kinetic models have been developed based on carbenium ion chemistry. Rate equations have been developed based on the steady state approximation and the concept of multiple non-quasiquilibrated steps. To reduce the number of kinetic parameters in the model, the elementary steps involved in this mechanism were grouped in three families: (i) olefin protonation/deprotonation steps, (ii) surface alkylation of benzene steps, and (iii) desorption (deprotonation) of the alkylated products steps. The model assumes fast desorption of the alkylated products and the rate coefficients associated with a reaction family are considered equal. Two olefin protonation coefficients appear as parameters in the model in order to describe the reversibility of the all elementary steps involved in the reaction network. Preexponential factors are calculated based on transition state theory. The resulting model contains four parameters that are determined by regression to the experimental data: one olefin protonation activation energy, one alkylation activation energy and two protonation enthalpies. The concentration of acid sites of the catalysts is included in the rate equations, while the difference in the acid strength is accounted for via the protonation enthalpy. Estimates of the parameters were obtained by minimizing the residual sum of squares based on conversions using six responses: 1-, 2- and 3-octenes and 2-, 3- and 4-phenyloctanes. Student's t test was used for verifying the significance of the estimated values of individual parameters and F test for significance of the global regression. Activation energies of 40kJ/mol for the olefin protonation step and 100kJ/mol for the benzene alkylation step were obtained by non-isothermal parameter estimation over the experimental data on the three catalysts. The estimated protonation enthalpies over the three catalysts were ?56kJ/mol (Si/Al = 6), ?60kJ/mol (Si/Al = 13) and ?70kJ/mol (Si/Al = 30). The good agreement between the experimental values and the values predicted by the model shows the ability of this model to describe the experimental observations taking into account the simultaneous occurrence of both olefin isomerization and benzene alkylation processes on catalysts with different acid properties.