Catalysts Supported on Carbon Materials for the Selective Hydrogenation of Citral

Abstract

The heterogeneously catalyzed selective-hydrogenation of citral is one of the more feasible ways for obtaining its appreciated unsaturated-alcohols, nerol and geraniol, which are present in over 250 essential oils. Thus, citral has very recently come to be produced petro-chemically in very large quantities, and so partial hydrogenation of citral has become a very economical route for the production of these compounds. However, the selective hydrogenation of citral is not easy, because citral is an α,β-unsaturated aldehyde which possesses three double bonds that can be hydrogenated: an isolated C=C bond and the conjugated C=O and C=C bonds. For this reason, in catalyst selection there are several important issues which affect the product selectivity, for example, the active metal and metal particle size which are factors related to the catalyst preparation method, catalyst precursor, or support surface area, as well as other factors such as porosity, the addition of a second catalytic metal, and, of course, the type of catalyst support. About this last one, carbon materials are very interesting supports for this type of hydrogenation reaction due to their unique chemical and textural properties. This review collects and analyzes the results obtained in the selective hydrogenation of citral catalyzed by carbon material supported metals

Effect of structural ordering of the carbon support on the behavior of Pd catalysts in aqueous-phase hydrodechlorination

“…In spite of its availability, graphitic carbons have been much less used than other carbon materials in catalysis due to their low specific surface area (10 -50 m 2 ·g -1 ) and poorly developed pore structure [4]. Nevertheless, graphites have shown fairly good performance as catalysts supports in different reactions, such as the hydrogenation of citral, where higher selectivity towards the unsaturated alcohol was obtained compared to other conventional supports such as alumina, silica or activated carbon [5]. They have been also used in CO electro-oxidation [6], oxidation of alkenes [7], catalytic decomposition of ammonia [8] and even as bare catalyst in the Baeyer-Villiger oxidation of cyclohexanone, improving the activity and selectivity towards lactones [9].…”

Catalysts based on large size-controlled Pd nanoparticles for aqueous-phase hydrodechlorination

“…Regarding to the supports, among the different materials used in catalysis such as aluminium oxides, magnesia, titania, zirconia, silica, zeolites, etc., the carbon materials can be highlighted due to their relativity inertness, facility to recover active phase or their tunable properties, among others [43]. Particularly, carbon blacks (CB) have been widely used as catalysts supports in several reactions such as hydrogenation [44,45], oxygen reduction [46] and electro-oxidation [47], being their good electrical conductivity one of the most remarkable properties that seems to play a significant role in catalysis [44,47,48].…”

Effect of Alkali Carbonate/Bicarbonate on Citral Hydrogenation over Pd/Carbon Molecular Sieves Catalysts in Aqueous Media

“…The citral was first getting hydrogenated to citronellal, and then it can be easily desorbed from Pd active sites to solution. The larger palladium particles are facilitating the desorption of citronellal from the catalyst surface [17].…”

“…However, non-traditional carbon materials such as carbon nanotubes, carbon nano-fibers, carbon xerogel and aerogels were widely studied for hydrogenation application, and few reports were also available on use of composite materials like carbon-TiO 2 in the literature for this application [17]. The CMS were very superior carbon materials, especially as catalyst supports due to their thermal stability, resistance to strong acidic or alkali media, hydrophobic nature, and their morphology properties such as micro-structure, amorphous nature, slit pore geometry, compared to zeolites.…”