Development of highly effective catalyst for Beckmann rearrangement

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Vapor phase Beckmann rearrangement of cyclohexanone oxime was carried out with continuous flow system. Various zeolites were examined to clarify the effect of zeolite pore on the selectivity. High selectivity to epsilon-caprolactam was obtained on Ca-A and borosilicate with template molecules remaining in the pore. The pore sizes of these zeolites are too small for the molecules of cyclohexanone oxime to enter. Therefore, it was revealed that the reaction to produce bulky epsilon-caprolactam proceeds selectively on the external surface of these zeolite and that the side reaction to produce smaller molecules dominates inside the pore. Catalytic performances of various oxides were examined to clarify the nature of the active site. Strong acid zeolites, H-ZSM-5, H-mordenite, H-Y,etc., gave low selectivity to epsilon-caprolactam (30-60 mol%) mainly because of the formation of by-products having high boiling point. Basic zeolites, K-ZSM-5, K-ferrierite, etc., also exhibited the low selectiv … More ity (30-60 mol%). Amorphous oxides, gamma-alumina, titania, zirconia, etc., gave the selectivity lower than 40 mol%. Higher selectivity (>80 mol%) was obtained when crystalline oxides, such as H-ferrierite, Ca-A zeolite, H-SAPO-5 and silicalite-1, were used. These catalysts do not have strong acid and base sites, which also results in the higher stability due to much less accumulation of the by-products. These results indicate that neutral and weakly acidic hydroxyl groups are also active for the formation of epsilon-caprolactam and that these hydroxyls are more effective for the selective and stable formation of epsilon-caprolactam than strong acid or base sites. However, silica gel, which has also neutral hydroxyl groups, exhibited low selectivity. Treatment with aluminum chloride on silica gel, which eliminated part of surface hydroxyls, enhanced the selectivity. The surface concentration of neutral hydroxyl group may have an important role on the selective formation of epsilon-caprolactam. Less