Citral, hydrogenation, selective

Synthesis from Citral. Selective hydrogenation of citral to citronellal can be accomplished in the presence of a palladium catalyst in an alkaline alcoholic reaction medium [65]. 

Hydrogenation reactions, particularly for the manufacture of fine chemicals, prevail in the research of three-phase processes. Examples are hydrogenation of citral (selectivity > 80% [86-88]) and 2-butyne-l,4-diol (conversion > 80% and selectivity > 97% [89]). Eor Pt/ACE the yield to n-sorbitol in hydrogenation of D-glucose exceeded 99.5% [90]. Water denitrification via hydrogenation of nitrites and nitrates was extensively studied using fiber-based catalysts [91-95]. An attempt to use fiber-structured catalysts for wet air oxidation of organics (4-nitrophenol as a model compound) in water was successful. TOC removal up to 90% was achieved [96]. 

Selective hydrogenation of citral Rh/SiO, Geraniol, nerol Fragrances... 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

Citronellal, an aldehyde with a trisubstituted double bond, was hydrogenated to citronellol over a ruthenium catalyst poisoned with lead acetate in 90-100% yields (eq. 5.22)46 or over chromium-promoted Raney Ni in 94% yield in methanol at 75°C and about 0.31 MPa H2.47 Court et al. studied the selective hydrogenation of citral (1, eq. 5.24) to citronellol over unsupported Nij. o catalysts, prepared by reduction of mixtures of metal iodides with naphthalene-sodium as reducing agent, in cyclohexane and in 2-propanol at 80°C and 1.0 MPa H2.48 Higher yields of citronellol were obtained in 2-propanol than in cyclohexane, primarily via citronellal as the predominant intermediate. The yields of citronellol for the overall hydrogenation in 2-propanol over Mo-promoted catalysts were Mo0 03 96%, Mo0 06 98%, and Mo012 96%. 

The lipidic vitamins (ref.84) include vitamin A (32), a substance intrinsic to the physiology of vision, vitamin E (83), a natural protective antioxidant, and vitamins K, (84) with Kj (85), antihemorrhagic compounds, each of which is derivable from an initial natural product intermediate. Although traditionally a -ionone obtained from citral (a major constituent of lemon grass oil) was used for the synthesis of vitamin A, a synthetic source has now replaced this in a process which also gives /g-carotene. In one method the Cl4 aldehyde in that process is reacted with a C6 eneyne component and selective hydrogenation followed by dehydration and isomerisation affords the final product (ref.85). 

Neri G, Milone C, Donato A (1994) Selective hydrogenation of citral over Pt-Sn supported on activated carbon. J Chem Tech Biotech 60 83... 

The majority of published research has concentrated on the preparation of the catalyst - the effect of different supports and different metals, the addition of second metals and the effect of different preparation methods on the selectivity of the catalysts for selective hydrogenation [2,3,5,6-10]. The effects of reaction conditions on selectivity have received considerably less attention. Gallezot and Richard [4] commented on the scarcity of systematic studies on the influence of reaction parameters such as pre-reduction of the catalyst, temperature, pressure, concentration of reactant and nature of the solvent for a given catalyst and reaction. Since then Singh et al. [11] have obtained quantitative kinetic data on the liquid phase hydrogenation of citral over Pt/SiOa catalysts and have used this information to present a kinetic model which fits their data. 

In this paper we will report on the selective hydrogenation of o /3 unsaturated aldehydes using a supported palladium catalyst in a range of ionic liquids. We will also compare their reactivity and selectivity to conventionally used organic solvents. Two systems have been investigated, cinnamaldehyde to form hydrocinnamaldehyde and citral... 

Table 1 compares the results from the selective hydrogenation of citral, at a variety of pressures and temperatures in both ionic liquids and organic solvents using palladium supported on carbon (Pd/C) and platinum supported on graphite (Pt/G). It is clear that the ionic liquid systems are highly selective solvents for the reduction of C=C without any reduction of the carbonyl group over palladium and similar selectivities can be achieved for the carbonyl group over platinum. 

Surface Organometallic Chemistry on Metals Selective hydrogenation of citral on silica supported Rhodium modified by tetra-n-butyl Germanium, Tin and Lead. 

Most of the work on the selective hydrogenation of unsaturated aldehydes has been carried out on cinnamaldehyde and few other studies are available on aliphatic aldehydes which can be of potential industrial interest. Within the class of aliphatic imsaturated aldehydes, hydrogenation of citral is important for its interest in the perfumery industry and from a scientific point of view since it offers three unsaturations an aldehyde group, a coiyugated double bond and an isolated double bond. 

Court, J Janati-Idrissi, F Vidal, S. Selective hydrogenation of citral in the liquid phase over un- supported nickel-molibdenum catalysts Nii-xMox- Studies in Surface Science and Catalysis, 1991 59, 193-200. 

Similar improvement in citral selectivity as achieved with an alkaline IL containing SILCA (see above), was observed when alkaline modifiers (KOH or NajCOj) were added to the IL layer (Figure 12.6) [15]. We assume that an increase in the alkaline modifier concentration increases the hydrogen solubility and citral concentration on the catalyst surface, at elevated pressures (lObar), thus inducing an activity boost and higher selectivity toward citronellal. A similar phenomenon has been reported by Pak et al. [16], as they observed that addition of Na2C03 to a Ni-CrjO catalyst increased the hydrogen solubility and, consequently, the reaction rate. It is assumed that in the presence of NajCOj, the citral concentration on the catalyst surface increases. Sodium adsorbs on the surface of the metallic catalyst and has an influence on the adsorption of citral and its selectivity [16]. 

Worz, N., Arras, J., and Claus, P. (2011) Continuous selective hydrogenation of citral in a trickle-bed reactor using ionic liquid modified catalysts. Appl. Catal, A Gen., 391, 319-324. 

K., and Mikkola, J.-P. (2012) Alkaline ionic liquids applied in supported ionic liquid catalyst for selective hydrogenation of citral to citronellal. Frontiers in Chemistry, submitted. 

Arras, )., Steflan, M., Shayeghi, Y., and Qaus, P. (2008) The promoting eflect of a dicyanamide based ionic liquid in the selective hydrogenation of citral. Chem. Commun., 4058-4060. 

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