Density‐Dependent Formation of the Pure trans‐Isomer of the Unsaturated Alcohol by Selective Hydrogenation of Citral in Supercritical Carbon Dioxide

Abstract

Selective hydrogenation of citral to unsaturated alcohol [geraniol (trans) + nerol (cis)] was carried out in supercritical carbon dioxide (scCO 2 ) using an MCM-41 supported plantinum catalyst (1 wt% Pt). A remarkable rate of isomerization of the unsaturated alcohol [nerol (cis) to geraniol (trans)] during the hydrogenation of citral was achieved simply by tuning the density of CO 2 . Optimum reaction conditions were developed to obtain only geraniol (trans) with a selectivity of 98.8% and citral conversion of 99.8%. A significant change in the cis:trans ratio of the product (1:82.3) from the substrate (1:1.3) was observed depending on the various reaction parameters like carbon dioxide and hydrogen pressure, reactant concentration, reaction time and, particularly, the total selectivity for unsaturated alcohol [geraniol (trans) + nerol (cis)]. It has been observed that the presence of hydrogen is necessary for isomerization. Our results were explained in terms of a density-dependent, two-step model. The kinetic behaviour shows that the rate of isomerization was higher in scCO 2 compared to other organic solvents and the pure form of geraniol (trans) was obtained exclusively. A probable reaction pathway was proposed in order to explain the isomerization during hydrogenation of citral in scCO 2 medium.

Study on selectivity of β-myrcene hydrogenation in high-pressure carbon dioxide catalysed by noble metal catalysts

“…In scCO 2 the reaction kinetics exhibited a first-order reaction rate i.e. for limonene 17a and citral 31 hydrogenations. The obtained results indicate that the hydrogenation of the first double bond is always faster in one phase than in two phases as presented in Table 1.…”

Advantageous heterogeneously catalysed hydrogenation of carvone with supercritical carbon dioxide

“…In scCO 2 the reaction kinetics exhibited a firstorder reaction rate i.e. for limonene, 10 b-myrcene 27 and citral 35 hydrogenation. Summarising, it can be stated that the pressure of CO 2 strongly affects the reaction conditions, and at lower total pressure (12.5 MPa) reactions were carried out in biphasic (liquid + gas) conditions while a total pressure of 16 MPa shifted the reaction mixture towards a one-phase region.…”

Environmentally Benign Transformations of Monoterpenes and Monoterpenoids in Supercritical Fluids

“…scCO 2 was shown to be a suitable reaction medium for the highly efficient hydrogenation of citral using monometallic Pt and bimetallic PteRu supported on MCM-48 as catalysts at 323 K (Scheme 24) [80]. The Pt catalyst was found to predominantly form unsaturated alcohols geraniol 74 and nerol 75, whereas the addition of Ru to the Pt considerably changed the selectivity for citronellal 70.…”

“…Selective hydrogenation of citral to unsaturated alcohols 74 and 75 in scCO 2 was carried out using Pt catalyst supported on another mesoporous material, MCM-41 [83]. It was found that nerol 75 can isomerize into β-Myrcene 7 SCHEME 23 Hydrogenation of myrcene.…”