Methylation of Naphthalene over Zeolites

The selectivity of an isomer is shown by the percentage in all isomers for each alkylate unless otherwise stated. 

Weitkamp and Neuber59 found that high selectivities for the narrow methylation products, i.e., 2,6-, 2,7-, and 2,3-DMN, were obtained over HZSM-5 and HZSM-11 in the methylation of 2-MN. Over these catalysts, the isomerization to 

1-MN always decreased with time on stream, whereas the methylation of the DMN isomers either remained nearly constant or only slightly increased. After 10 h, 50% of the DMN isomers were composed of 2,6-DMN, with all three of these narrow DMN products accounting for more than 90% of the product. These results clearly indicate that shape-selectivity becomes more pronounced with time on stream. Over an HZSM-5 catalyst precoked at 400 °C by converting methanol alone (into hydrocarbons), the initial yield of 1-MN was significantly reduced (to 1.5% vs. 4% for a fresh catalyst), with the initial content of 2,6- and 

7- DMN in the DMN isomers being greater (84% ) on the precoked catalyst than on the fresh catalyst (70%). This is a well known effect in zeolite catalysis due to the partial deactivation of the acid sites by carbonaceous deposits. The authors explain these results by a product selectivity mechanism inside the pore system of ZSM-5 or ZSM-11. Even though the diffusion pathways for the product molecules decreased by the formation of carbonaceous deposits, the narrow molecules such as 2-MN, 2,6-DMN, and 2,7-DMN were less affected than bulkier ones such as 1-MN and 1,6-DMN. 

Yashima and his co-workers examined the methylation of 2-MN using MFI-metallosilicates.60 The methylation over HZSM-5 yielded predominantly 2,6-, 

3-DMN with the formation of bulky 1,2-, 1,3-, 1,6-, and 1,7-DMN (Table 3). The isomers produced initially were three P,P-DMN isomers, i.e., 2,6-(50 %), 2,7- (38 %), and 2,3- (7%) DMN at the short contact time. However, the selectivities for 2,6- and 2,7-DMN decreased with an increase in a,p-DMN isomers, i.e., 1,3-, 1,6-, and 1,7- DMN as the contact time increased. These results suggest that the increase in the selectivity for a,p-DMN is due to the isomerization of initially produced 2,6- and 2,7-DMN inside the pores. The isomerization occurred at the external acid sites because the co-feed of 2,4-dimethylquinoline (2,4-DMQ) enhanced the selectivity for 2,6- and 2,7-DMN. The isomerization decreased by the substitution of A1 with Ga and Fe, and was deactivated completely by the co-feed of 2,4-DMQ for Ga- and Fe-MFI. The substitution also enhanced the formation of 2,6-DMN the 2,6-/2,7- ratio reached 1.78 for Fe-MFI. These differences of P,P-selectivity and 2,6-12J- ratio are not due to the changes of pore dimension but to acid strength by the substitution. Weaker acid sites produce 2,6-DMN preferentially. One reason is that the electron density of HOMO of 2-MN at C-6 is significantly higher than that of C-7. 

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