Selective Hydrogenation Of Citral Research Articles

Selective Hydrogenation of Citral on Pt-Containing Catalysts at Room Temperature and Atmospheric Pressure

It is shown that the 1% Pt/CeO2–ZrO2 (1% Pt/CZ) catalytic system allows selective hydrogenation of citral with a 94% conversion and a selectivity towards unsaturated alcohols of 59% at room temperature and atmospheric pressure. The effect of addition of alkali to the reaction mixture on the yield of the target products is studied, and the optimum conditions of the reaction are determined.

Influence of the Pt-particle size on the performance of carbon supported catalysts used in the hydrogenation of citral

The influence of the Pt-particle size in the selective citral hydrogenation is analysed. For that, a Pt/Carbon catalyst has been prepared by impregnation of a carbon xerogel and pre-treated in different conditions to obtain particle sizes varying from 3 to 10nm. The Pt particle size has been deeply characterized by HRTEM, XRD, XPS and H2 chemisorption and related with the catalytic performance in each case. Both activity and selectivity to the valuable unsaturated alcohols are favoured for catalysts with increasing Pt-particle size up to around 8nm, decreasing for larger particles. In this sense, the interaction and transformation of geranial (E-citral) is also favoured regarding neral (Z-citral).

Selective hydrogenation of citral by noble metals supported on carbon xerogels: Catalytic performance and stability

A series of monometallic Pt, Ir and Ru-catalysts deposited on carbon xerogel microspheres was prepared, exhaustively characterized and used in the selective hydrogenation of citral. A similar metal particle size is obtained in all cases after He-pretreatment, allowing the comparison between metals; the catalytic activity increases in the sense Ir<Ru<Pt. Sintering is favoured when catalysts are pretreated in H2-flow leading to an important loss of activity, especially for Ru-catalysts. Pt and Ir-catalysts are more selective than Ru-catalysts, reaching selectivity values to unsaturated alcohols of around 80%. Thus, in terms of yields to these valuable products Pt-catalysts seem to be the most appropriate active phase. Nevertheless, reutilization experiments showed that Ir-catalyst maintained a good performance while a severe deactivation is observed for Pt-catalysts. This fact is discussed on the basis of the different nature of the deposits formed during reaction.

Influence of the pretreatment conditions on the development and performance of active sites of Pt/TiO2 catalysts used for the selective citral hydrogenation

The influence of pretreatment conditions of Pt/TiO2 catalysts was analyzed by different techniques and changes in the textural and chemical properties correlated with the catalytic performance in citral hydrogenation. After H2 pretreatment, the formation of two-dimensional Pt nanoparticles and oxygen vacancies favors the diffusion of Pt species into the TiO2 structure, while three-dimensional Pt crystallites are formed after He pretreatment. The coupled effect of Pt 2-D nanoparticles and oxygen vacancies strongly enhances the catalytic activity and the selective hydrogenation of citral to unsaturated alcohols by favoring the citral adsorption on the reduced surface through the terminal CO groups in an environment rich in hydrogen provided by the highly dispersed Pt nanoparticles. These coupled metal-vacancy active sites are formed also after a combined He/H2 pretreatment; nevertheless, the formation of three-dimensional Pt particles during the previous He treatment strongly reduce the extent of TiO2–Pt interface and consequently the surface site concentration and the catalytic activity.

Effect of Fe(III) on hydrogenation of citral over Pt supported multiwalled carbon nanotube

Nano-platinum granules were affixed to MWCNTs via a liquid-reduction method. Different methods were utilized to introduce Fe(III) into the Pt/MWCNT catalyst. The influence of Fe(III) on the performance of a selective citral hydrogenation over the Pt/MWCNTs in isopropanol was investigated. Characterization techniques such as XRD, TEM, TPR, XPS and N2 physisorptions were used. Changes in the absorbance and wavenumber for the CO bond in the citral were monitored with a high-pressure in situ infrared spectrometer. When 0.25wt.% Fe(III) was added, the citral conversion remained unchanged, but the selectivity toward the unsaturated alcohols increased from 33.7 to 90.7%.

Development of carbon xerogels as alternative Pt-supports for the selective hydrogenation of citral

The role of activated carbons and carbon xerogels as Pt-supports for the selective citral hydrogenation is compared. The influence of porosity and inorganic matter impurities in activated carbons determines the performance of their derivative catalysts. The presence of Pt-particles inside mesopores and acid mineral matter favors secondary reactions decreasing the unsaturated alcohols (UA) yield. Highly active and selective monometallic Pt-catalysts were obtained using pure carbon xerogels structured in microspheres as support.

Catalysts Supported on Carbon Materials for the Selective Hydrogenation of Citral

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.

TiO2 Supported Pt Based Bimetallic Nanocatalysts for Selective Hydrogenation of Citral

TiO 2 supported Pt based bimetallic nanocatalysts were prepared by impregnation method and reduced at two different temperatures, 375 °C and 575 °C for the selective hydrogenation of citral to the corresponding unsaturated alcohols namely; geraniol (GOL) and nerol (NOL). The catalysts were characterized by BET surface area measurement, FT-IR, SEM, EDAX, TEM, XRD and XPS. The prepared nanocatalysts are uniformly dispersed with an average particles size 50-100 nm and zero valence metallic state. The SMSI (Strong Meal-Support Interaction ) effect shows that the catalysts reduced at higher temperature leads to an increase in selectivity toward unsaturated alcohols (GOL & NOL) for Pt-Ru/TiO 2 compared to Pt-Pd/TiO 2 and Pt-Au/TiO 2 nanocatalysts. In addition a second metal (Ru) also leads to an increase of the GOL & NOL selectivity during the citral hydrogenation. The generated partially oxidized second metal species due to the difference in electronegativity, strongly binds the C=O group and also paves the way for the selective activation of the C=O bond. Metal support interaction and promoter active metal interaction increased the unsaturated alcohol formation.

The relationship between the structural properties of bimetallic Pd–Sn/SiO 2 catalysts and their performance for selective citral hydrogenation

The effect of Sn addition to Pd on the selective liquid-phase hydrogenation of citral to α,β-unsaturated alcohols (UA: nerol and geraniol) was examined. Pd–Sn/SiO 2 bimetallic catalysts were prepared by successive impregnation method and were characterized by transmission electronic microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDX), temperature-programmed reduction (TPR), Fourier transform infrared (FTIR) spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS). Sn addition to Pd/SiO 2 catalysts significantly modifies their properties for citral hydrogenation performed at 130 °C, under 7 MPa and in isopropanol solvent, inducing a promoting effect on the UA selectivity. This promoting effect is related to the existence of a Pd–Sn interaction highlighted by EDX analysis, TPR under hydrogen and FTIR of adsorbed CO. The latter technique suggested the presence of a geometric effect on catalytic activity. Maximum UA selectivities (>75% at 30% citral conversion) were obtained when an alloy of the Pd 3Sn type is formed in the bimetallic particles, as confirmed by TPR and XPS. Moreover, FTIR measurements of the adsorbed CO singleton frequency as well as XPS binding energy shifts strongly imply an electron transfer from Sn to Pd, which is proposed to be responsible for enhanced adsorption of citral C O bond on the surface of Pd–Sn/SiO 2 bimetallic catalysts. To our knowledge, it is the first time that modified Pd catalysts lead to such important UA selectivity values during α,β-unsaturated aldehyde hydrogenation.

Catalytic performance in selective hydrogenation of citral of bimetallic Pt–Sn catalysts supported on MgAl2O4 and γ-Al2O3

The liquid phase citral hydrogenation, using Pt and PtSn catalysts supported on MgAl2O4 and γ-Al2O3, was studied in a stirred reactor at 70°C and atmospheric pressure. It was found that the addition of Sn to the Pt catalysts increases the selectivity to double unsaturated alcohols for both catalyst series. Besides, monometallic catalysts hydrogenate the α,β-unsaturation with a high selectivity in absence of cyclization secondary products. The performance of these catalysts in the citral hydrogenation was related with the characteristics of the metallic phase. Results from test reactions – cyclohexane dehydrogenation (CHD) and cyclopentane hydrogenolysis (CPH) – H2 chemisorption, 2-propanol dehydration, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), were used to explain the influence of the support and the Sn loading and postulate the models of the catalytic metallic surface. Results indicated that, a fraction of ionic Sn would be deposited near Pt, thus increasing the polarization of the carbonyl group, and a fraction of metallic Sn could form Pt–Sn alloy phases that would hinder the hydrogenation of the olefinic bonds and would be active to the hydrogenation of the carbonyl group. Both effects contribute to a higher selectivity to unsaturated alcohols in bimetallic PtSn/Al2O3 catalysts than PtSn/MgAl2O4 ones, which display a low alloy formation.

Selective hydrogenation of citral over Au-based bimetallic catalysts in supercritical carbon dioxide

Selective hydrogenation of citral was investigated over Au-based bimetallic catalysts in the environmentally benign supercritical carbon dioxide (scCO2) medium. The catalytic performances were different in citral hydrogenation when Pd or Ru was mixed (physically and chemically) with Au. Compared with the corresponding monometallic catalyst, the total conversion and the selectivity to citronellal (CAL) were significantly enhanced over TiO2 supported Pd and Au bimetallic catalysts (physically and chemically mixed); however, the conversion and selectivity did not change when Ru was physically mixed with Au catalyst compared to the monometallic Ru/TiO2, and the chemically mixed Ru-Au/TiO2 catalyst did not show any activity. The effect of CO2 pressure on the conversion of citral and product selectivity was significantly different over the Au/TiO2, Pd-Au/TiO2, and Pd/TiO2 catalysts. It was assumed to be ascribed to the difference in the interactions between Au, Pd nanoparticles and CO2 under different CO2 pressures.

Physically and chemically mixed TiO 2-supported Pd and Au catalysts: unexpected synergistic effects on selective hydrogenation of citral in supercritical CO 2

The selective hydrogenation of citral was studied with various TiO 2-supported monometallic and bimetallic Pd and Au catalysts and their physical mixtures in supercritical CO 2 (scCO 2). Significant synergistic effects appeared when active Pd species was chemically or physically mixed with less active Au species. The total rate of conversion was greatly enhanced and the selectivity to citronellal (CAL) was improved. The physical properties of those catalysts were characterized by TEM, HRTEM-EDS, XPS, and UV/Vis and their features of H 2 desorption were examined by TPD. The physical and chemical characterization results were used to discuss the reasons for the unexpected synergistic effects observed. The same selective hydrogenation was also conducted in a conventional non-polar organic solvent of n-hexane to examine the roles of scCO 2. The use of scCO 2 was effective for accelerating the hydrogenation of citral and improving the selectivity to CAL.

ChemInform Abstract: The Promoting Effect of a Dicyanamide Based Ionic Liquid in the Selective Hydrogenation of Citral.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

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

AbstractSelective hydrogenation of citral to unsaturated alcohol [geraniol (trans) + nerol (cis)] was carried out in supercritical carbon dioxide (scCO2) 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 CO2. 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 scCO2 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 scCO2 medium.

Effects of support and metal loading on the characteristics of Co based catalysts for selective hydrogenation of citral

Silica-, alumina- and titania-supported Co catalysts with different metal loadings were tested for selective citral hydrogenation in liquid phase. Temperature programmed desorption of hydrogen (H 2-TPD) indicated that the catalysts with the highest unsaturated alcohols selectivities present particular H-Co species. Electron diffraction and XRD measurements revealed that these species would be linked to the presence of Co 0 hexagonal phase. According to the results obtained by TEM and cyclohexane dehydrogenation, these two properties would be present on large Co particles.

Changes in the selective hydrogenation of citral induced by copper addition to Ru/KL catalysts

Bimetallic Ru–Cu catalysts supported on KL zeolite have been prepared by coimpregnation with ionic precursors and characterized by several methods, such as temperature-programmed reduction, CO and hydrogen chemisorption, nitrogen adsorption, infrared spectroscopy of chemisorbed CO and microcalorimetry of CO adsorption. The catalytic behavior of the samples was analyzed in the selective hydrogenation of citral in the liquid phase, at 323 K and 5 MPa. The presence of bimetallic entities as well as of segregated copper species was recognized by the TPR measurements. The CO-FTIR and microcalorimetry results evidence that the higher the Cu/Ru atomic ratio the larger the surface heterogeneity, with formation of Cu δ+ species. Bimetallic catalysts are more active than the monometallic ruthenium catalyst in the hydrogenation of citral, but this activity decreases with the increasing of copper concentration. In addition, selectivity towards citronellal decreases as the copper content increases, in opposite trend to the selectivity toward geraniol and nerol. For low copper loading (Cu/Ru ≈ 0.4) formation of a surface alloy is discussed. This surface structure is particularly active for hydrogenation of the conjugated C C double bond of citral. For Cu/Ru = 0.8 and Cu/Ru = 1.2 samples, three-dimensional islands of segregated copper seem to cover, in part, the surface alloy. This latter surface structure is less active than the preceding one for hydrogenation of the C C double bond, but more selective for hydrogenation of the C O group of citral.

Selective Hydrogenation of Citral in an Organic Solvent, in a Ionic Liquid, and in Substance

AbstractIt is investigated whether the catalyst system Ag‐In/SiO2 can be applied in the selective hydrogenation of citral, with the aim of synthesizing the acyclic, allylic terpene alcohols geraniol/nerol with high selectivity and space‐time yield. In addition, as liquid‐phase hydrogenations are often also influenced by the choice of solvent, it is of interest to know how the solvent, in particular in terms of its polarity and hydrogen solubility, affects the activity and the selectivity of catalysts during hydrogenation of the α,β‐unsaturated aldehyde citral.

Effect of the reduction–preparation method on the surface states and catalytic properties of supported-nickel particles

Several high surface area graphite-supported Ni catalysts were prepared and reduced by different methods. Samples were prepared by impregnation of the support and then submitted to traditional H 2 flow reduction at high temperature or to reduction with aqueous hydrazine at low temperature, and compared with a sample prepared by the deposition–precipitation method by means of the reduction of the dissolved Ni precursor by hydrazine at low temperature. The surface of these catalysts was characterised by means of CO adsorption microcalorimetry and by X-ray photoelectron spectroscopy (XPS), and a correlation of the results obtained with those on the selective hydrogenation of citral can be done. In this sense, only Ni 0 sites were detected at the surface of the Ni catalysts prepared by reduction with hydrogen leading to high catalytic activities and selectivity towards citronellal formation, while Ni oxidised species are also present (and are major) for catalysts prepared by reduction with hydrazine, what leads to an enhanced selectivity towards the formation of the unsaturated alcohols, geraniol and nerol. The oxidation of these latter catalysts seems to be only superficial, as suggested by the absence of H 2 consumption peaks in the TPR experiments. The presence of Ni 0 sites over these latter catalysts was not detected by XPS, thus our study proves that CO adsorption microcalorimetry is a very powerful tool for surface characterisation of heterogeneous catalysts.

Modelling of catalyst deactivation in liquid phase reactions: Citral hydrogenation on Ru/Al2O3

Selective hydrogenation of citral was investigated at 70 °C over Ru/Al2O3. Activity changes in a series of consecutive experiments were described by a mathematical model, which included reaction kinetics and deactivation.

Liquid phase hydrogenation of citral: suppression of side reactions

Selective hydrogenation of citral was investigated at 70 °C in four different alcohols over Ru, Rh and Ni supported catalysts. The main aim was to suppress the side reaction, like acetalization and cyclization of citronellal. Acetalization could be suppressed either by using chloride-free catalyst precursors and other supports than active carbon or by using longer chained alcohols, like 2-pentanol and 2-methyl-2-propanol as solvents. The highest maximum selectivity to citronellol (92%) was obtained in citral hydrogenation in ethanol over Ni/Al 2O 3 catalyst, which was pre-reduced ex situ at 350 °C and in situ at 350 °C for 2 h in ethanol. The acidic alumina support was covered by spillover hydrogen, suppressing the acetalization reaction. Citronellal cyclization increased with the increasing solvent hydrophobicity and was also larger over active carbon supported Ru catalyst.