Bio-based nylon intermediates from γ-valerolactone: Catalyst modifications for an enhanced selectivity to methyl 4-pentenoate

Ring-opening transesterification of bio-based γ-valerolactone (GVL) to produce a mixture of methyl pentenoate isomers (MPs) such as methyl 4-pentenoate (M4P), methyl 3-pentenoate (M3P) and methyl 2-pentenoate (M2P) has been carried out using modified Zr-containing catalysts in a gas phase continuous process. As the three MP isomers are potential nylon intermediates, this approach is sustainable from a green chemistry point of view to synthesize bio-based polymers. The reaction was carried out in a fixed bed catalytic reactor at ambient pressure and in the temperature range from 255 to 335°C. In the present study, SiO₂ supported zirconia catalysts were prepared by wet impregnation with varying Zr loadings in the range from 5 to 30 wt%. The catalysts were characterized by various techniques. The surface areas were determined by N₂ adsorption, acid-base properties by NH₃-TPD and CO₂-TPD techniques. In addition, the distinction of Lewis and Brønsted sites were also estimated by Py-FTIR. Among all catalysts, 25 % Zr/SiO₂ exhibited the best performance. The influence of various key reaction parameters on the activity/selectivity was explored and suitable reaction conditions were optimized. The conversion of GVL and product distribution was found to be strongly dependent on the reaction temperature and Zr content of the catalysts. The sum selectivity of all the three MP isomers varied in the range from 97 % to 99 %, of which M4P was always the dominant product. At low reaction temperature (255°C), M4P was the major product while M2P was a minor product. However, with increase in temperature, the selectivity of M2P and M3P increased at the expense of M4P due to isomerisation of the CC double bond. In addition, the effect of support on the best Zr loading was also explored and it was found that the SiO₂ support exhibited superior performance compared to others. In the direction of enhancing the selectivity of M4P, Cr-doping was also done onto 25Zr/SiO₂, the best catalyst of this study. Such incorporation of chromium significantly enhanced the selectivity of M4P above 80 % at acceptably high conversion of GVL.