Citral-to-Menthol Transformations in a Continuous Reactor over Ni/Mesoporous Aluminosilicate Extrudates Containing a Sepiolite Clay Binder

Abstract

One-pot continuous synthesis of menthols from citral was performed over 5 wt % Ni supported on a mesoporous aluminosilicate catalyst with sepiolite as a binder at 70 °C with a selectivity of 75% to menthols. Catalyst deactivation with time-on-stream resulted in a decrease of the conversion and selectivity to menthols at the expense of higher selectivity to isopulegols. Stereoselectivity to isopulegols and menthols only slightly changed with conversion and TOS. A kinetic model capable of describing experimental data for transformations of citral to menthol in a continuous mode was developed. It was based on a detailed reaction network and also comprised deactivation on both metal and acid sites. Numerical data fitting confirmed a good correspondence between the experimental data and calculations.

One-Pot Synthesis of Menthol Starting from Citral over Ni Supported on Attapulgite-H-Beta-38 Extrudates in a Continuous Flow: Effect of Metal Location

“…This catalyst contained only weak acid sites according to ammonia TPD. In addition, 75% selectivity of menthols at 98% citral conversion over 5 wt % Ni/mesoporous aluminosilicate-sepiolite extrudates [Ni/(MAS+sepiolite)] was reported in ref . When comparing the current results with [Ni/(Beta-38+Att.)]…”

“…When comparing the current results with [Ni/(Beta-38+Att.)] with 44–45% menthol yields, it can be observed that this catalyst contains a 2.23-fold higher amount of Brønsted acid sites than (Ni/(MAS+sepiolite)). Furthermore, comparing the performance of [Ni/(H-Beta-38+Att.)]…”

“…Note that Ni particle size for [(Ni/H-Beta-38)+Att.] is 9.8 nm giving menthol selectivity of 44%, while it was 8 nm for Ni supported on a mesoporous alumino-silicate-sepiolite composite catalyst with the corresponding maximum menthol selectivity of 75%. This result shows that although Ni particle sizes were quite close to each other, the maximum menthol selectivity was much higher in the latter case.…”

“…The authors have recently demonstrated the suitability of Ni-H-Beta-38 sepiolite extrudates (the number denotes the Si/Al ratio) as catalysts in the one-pot synthesis of menthol starting from citronellal and citral. 20 This Ni-H-Beta-38-sepiolite catalyst prepared by impregnation of extrudates and subsequently having Ni located both on H-Beta-zeolite and sepiolite gave a 49% yield of menthols and 72−74% stereoselectivity. Furthermore, continuous transformation of citral over Ni supported on a mesoporous aluminosilicate using sepiolite as a binder at 70 °C was reported giving 75% selectivity to menthols.…”

“…This Ni-H-Beta-38-sepiolite catalyst prepared by impregnation of extrudates and subsequently having Ni located both on H-Beta-zeolite and sepiolite gave a 49% yield of menthols and 72–74% stereoselectivity. Furthermore, continuous transformation of citral over Ni supported on a mesoporous aluminosilicate using sepiolite as a binder at 70 °C was reported giving 75% selectivity to menthols …”