Zeolites alkylation

The lifetime of a zeolitic alkylation catalyst depends on the concentration of Brpnsted acid sites. This has been shown by Nivarthy et al. (78), who used a series of zeolites H-BEA with varied concentrations of back-exchanged sodium ions. The sodium decreased the concentration of Brpnsted acid centers, which led to a concomitant decrease in the measured catalyst lifetime during alkylation. 

Fig. 10. Typical time on stream behavior of a CeY zeolite alkylated in a fixed-bed reactor (128).

In principle, the same rules hold true when zeolitic alkylation catalysts are used. A detailed study of the influence of PO and OSV on the performance of zeolite H-BEA in a backmix reactor was reported by de Jong et al. (80). The authors developed a simple model of the kinetics, which predicted catalyst lifetimes as a function of P/O and OSV. Catalyst lifetime (which is equivalent to the catalyst productivity, the reciprocal of acid consumption) increased with increasing P/O ratio and decreasing OSV. Furthermore, the authors persuasively demonstrated the superiority of a backmix reactor over a plug flow reactor. Qualitatively similar results were obtained by Taylor and Sherwood (222) employing a USY zeolite catalyst in a backmix reactor. The authors stressed the detrimental effect of unreacted alkene on the catalyst lifetime and product quality. Feller et al. (89) tested LaX zeolites in a backmix reactor and found the catalyst productivity to be nearly independent of the OSV within the examined OSV range. At higher values of OSV, the catalyst life was shorter, but in this shorter time the same total amount of product was produced. The P/O ratio had only a moderate influence on the catalyst performance. 

Application Advanced technology to produce high-purity cumene from propylene and benzene using patented catalytic distillation (CD) technology. The CDCumene process uses a specially formulated zeolite alkylation catalyst packaged in a proprietary CD structure and another specially formulated zeolite transalkylation catalyst in loose form. 

It follows from the above results that the presence of metal cations mostly in the zeolite intersections (MeH-ZSM-5) increases the zeolite para-selectivity, while their contribution to the zeolite alkylation activity is not significant. On the other hand, the metal cation presence in the "surface" sites (FeSiH-ZSM-5, Mn O H-ZSM-S) substantially enhances the zeolite isomerization activity. As the para-selectivity of the MeH-ZSM-5 zeolites is higher than that of the pure H-ZSM-5, the metal cation location in the channel intersections should cause steric hindrances owing to the diameter of the metal cations (Fe 0.64 S, Mn 0.80 2 and Al 0.50 2) this effect (which is also necessarily affected by the number of cations present in the zeolite) was not found with the relatively small A1 cation. Then the contribution of the metal cation to the transport limitation of the bulkier isomers and/or to the selectivity of the initial alkylation step should exceed the contribution of the cations to the isomerization activity. 

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