Improvements in the Beckmann rearrangement process by using highly selective mesoporous catalysts

• The Beckmann rearrangement of CHO to e-C was studied by using Al-M(20).
• The effect of the reaction conditions on the CHO conversion was studied.
• The stability of the catalyst and the possibility of the recycling were investigated.
• Conversion values of 95% and selectivity to e-C 100% were obtained.
• Optimal reaction conditions: 350 °C, W/F = 40 gh/mol and 1-hexanol as solvent.

The Beckmann rearrangement of cyclohexanone oxime (CHO) into e-Caprolactam (e-C) at 300–380 °C and W/F = 1–60 gh/mol over a Al-MCM-41 catalyst was studied in a flow reactor at atmospheric pressure. Different reaction conditions as well as the nature of the solvents employed in the feed were evaluated. The e-C was the main product on the whole reaction conditions, but temperatures above 360 °C favored its decomposition. A reaction pathway was proposed in order to explain the results obtained at 360 °C. The stability and the possibility of recycling of the catalyst were checked by XRD, N2 adsorption, FTIR, pyridine adsorption coupled with FTIR and catalytic activity tests. Thus, the catalyst could be used during 3600 min and then recovered and reused three times without significant changes in the active species, catalytic activity and e-C yield. In addition, a modification in the operating conditions, which consisted in pretreating the catalyst with 1-hexanol, allowed to improve the yield to caprolactam by blocking the terminal silanols responsible of the byproduct formation. Thus, the better catalytic performance was observed at 350 °C and W/F = 40 gh/mol, immediately after a catalyst pretreatment with 1-hexanol during 3 h with 1-hexanol, which was also used as reaction solvent. Such conditions allowed us to achieve a high CHO conversion (95%) with a e-C selectivity of around 100%, improving thus results previously reported.

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