Hydrogenation of Cinnamaldehyde over Pd/Al2O3 Catalysts Modified with Thiol Monolayers

Modification of supported Pt catalysts with thiols has recently been shown to improve the hydrogenation selectivity of α,β-unsaturated aldehydes to unsaturated alcohols. Here, we apply a variety of organic thiol coatings to Pd/Al₂O₃ catalysts that typically have a much lower intrinsic selectivity for desired product formation. Thiol monolayers were found to increase hydrogenation selectivity to cinnamyl alcohol; however, unlike with Pt catalysts, the increase was independent of the identity of the organic tail.

     

Hydrogenation of Nitrobenzene with Supported Transition Metal Catalysts in Supercritical Carbon Dioxide

Transition metal catalysts such as Pd, Pt, Ru, and Rh supported on carbon, silica and alumina have been examined for the hydrogenation of nitrobenzene (NB) in supercritical carbon dioxide (scCO₂) and in ethanol. The order of hydrogenation activity is Pt>Pd>Ru, Rh in scCO₂ and in ethanol. The effectiveness of the support is C>Al₂O₃, SiO₂ for either Pt or Pd in scCO₂. For all the catalysts, higher selectivity to aniline has been obtained in scCO₂ compared with ethanol. Hydrogenation of nitrobenzene catalyzed with Pd/C and Pt/C catalysts was successfully conducted in scCO₂ with a 100% yield to aniline at a lower reaction temperature of 35 °C. The product aniline (organic phase) can be easily separated from the side‐product water (aqueous phase), solvent (scCO₂), and catalyst (solid) by a simple phase separation process. The hydrogenation of NB is a structure‐sensitive reaction in ethanol as well as in scCO₂ except for a few Pt/C catalysts in which the degree of metal dispersion is small (<0.08).     

The selective formation of unsaturated alcohols by hydrogenation of α,β-unsaturated aldehydes in supercritical carbon dioxide using unpromoted Pt/Al2O3 catalyst

When α,β-unsaturated aldehydes are reduced by hydrogen in supercritical carbon dioxide using an unmodified Pt/Al₂O₃ catalyst, unsaturated alcohols are highly selectively produced, in contrast to previous results with organic solvents in which such a selective hydrogenation is difficult to achieve with monometallic Pt catalysts. The selectivity of unsaturated alcohols depends mainly on pressure of carbon dioxide while the conversion depends on both carbon dioxide and hydrogen pressures. The selectivity increases with increasing pressure and then does not change so much at higher pressures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tuning the Behavior of Au and Pt Catalysts for the Chemoselective Hydrogenation of Nitroaromatic Compounds

Efficient catalysts for the synthesis of functionalized anilines from the corresponding substituted nitroaromatics should combine an excellent chemoselectivity with a high hydrogenation activity. Attempts to increase the activity of the Au/TiO₂ catalyst and to improve the selectivity of traditional Pt catalysts, based on a previous understanding of the mode of action of supported gold and platinum nanoparticles, are reviewed and discussed.     

Hydrogenation of vegetable oil using highly dispersed Pt/γ-Al2O3 catalyst: Effects of key operating parameters and deactivation study

In the present work, Pt/γ-Al₂O₃ catalysts with high metal dispersion were prepared and characterized using chloroplatinic acid and platinum acetylacetonate as metal precursors. The activity and selectivity of the catalysts were evaluated in the hydrogenation of sunflower oil. A comprehensive analysis of the effects of key operational parameters on catalytic performance was carried out. The experimental variables were hydrogen pressure (275.8–551.6 kPa), temperature (160–200°C), and catalyst loading (0.005–0.015 kg Pt_exp/m³_oil). Platinum catalysts were active, with a double bond conversion of 28% at 2 h. The metal precursor affected catalyst selectivity. The catalyst prepared with chloroplatinic acid exhibited a lower formation of trans-isomers compared with Pt acetylacetonate. The γ-Al₂O₃ supported platinum catalyst with a metal loading of 0.51 wt.% and a metal dispersion of 98% maintained its initial catalyst activity and selectivity after 10 consecutive uses (1200 min accumulate operation time), without changes in its catalytic properties. The obtained results suggested that Pt catalysts are an attractive alternative to conventional nickel catalysts for the hydrogenation of vegetable oil.     

Influence of Preparation Modes on Pt–Ni/CNTs Catalysts Used in the Selective Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde

Pt–Ni/CNTs catalysts are prepared by different impregnation techniques and different reduction methods (H₂, HCHO, and KBH₄) for the selective hydrogenation of cinnamaldehyde (CMA) to hydrocinnamaldehyde (HCMA) and investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (H₂-TPR) techniques. The results show that the catalytic selectivity and activity of the Pt–Ni/CNTs catalysts would significantly be improved by using KBH₄ as a reducing agent, due to the electronic synergetic effect of Pt–Ni–B, and 96% for conversion of CMA and 88% for selectivity of HCMA are obtained over Pt–Ni/CNTs catalyst reduced by KBH₄. Furthermore, the hydrogenation rate of CMA and selectivity of CMA to HCMA over Pt–Ni/CNTs catalyst are significantly improved in the presence of trace base or acid promoters again. The best result (92% for conversion of CMA and 96% for selectivity of HCMA) is obtained when NaOAc is used as base promoter.     

Role of Pt Oxidation State on the Activity and Selectivity for Crotonaldehyde Hydrogenation Over Pt–Sn/Al2O3–La and Pt–Pb/Al2O3–La Catalysts

Pt–Sn/ALa10 and Pt–Pb/ALa10 catalysts (10 wt% La₂O₃) were studied in the selective hydrogenation of crotonaldehyde. Oxidized Pt²⁺ and reduced Pt⁰ species were identified by XPS on the bimetallic catalysts. High selectivity to crotylalcohol was obtained on the Pt–Pb/ALa10 catalyst where an electron transfer effect from Pb to Pt was proposed. For the Pt–Sn/ALa10 catalyst the formation of Pt–SnO _x –La₂O₃ complexes showing low activity and low selectivity was inferred.     

Carbon xerogel supported noble metal catalysts for fine chemical applications

Carbon xerogels are mesoporous materials obtained upon pyrolysis of the dried gels resulting from polycondensation of resorcinol and formaldehyde. Treatment with nitric acid under severe conditions introduces high amounts of oxygen containing functional groups onto the surface of the material, leading however to the collapse of its porous structure. The resulting material is then used to support 1 wt.% Pt, Ir and Ru monometallic catalysts by wet impregnation using organometallic precursors. The catalysts are characterized by N₂ adsorption–desorption isotherms at 77 K, temperature programmed desorption coupled with mass spectrometry, scanning and transmission electron microscopy, and H₂ chemisorption. The liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol is used in order to assess the catalytic performance of the prepared materials. Pt and Ru catalysts are initially very selective towards the hydrogenation of the olefinic double bond, while Ir is mostly selective towards the carbonyl group. After a thermal post-reduction treatment at 973 K, selectivity towards cinnamyl alcohol is significantly improved regardless of the metal nature. The Pt catalyst exhibits the best behavior, a complete shift in C=C to C=O hydrogenation being detected. The improvement in selectivity is rationalized in terms of both an increase in metal particle size and a modification in the surface chemistry of the catalyst after the post-reduction treatment.     

Hydrogenation of Sunflower Oil over M/SiO2 and M/Al2O3 (M = Ni,Pd, Pt,Co, Cu) Catalysts

This work is aimed at evaluating the performance of several catalysts in the partial hydrogenation of sunflower oil. The catalysts are composed of noble (Pd and Pt) and base metals (Ni, Co and Cu), supported on both silica and alumina. The following order can be proposed for the effect of the metal on the hydrogenation activity: Pd > Pt > Ni > Co > Cu. At a target iodine value of 70 (a typical value for oleomargarine), the production of trans isomers is minimum for supported nickel catalysts (25.7–32.4 %, depending on the operating conditions). Regarding the effect of the support, Al₂O₃ allows for more active catalysts based on noble metals (Pd and Pt) and Co, the effect being much more pronounced for Pt. Binary mixtures of catalysts have been studied, in order to strike a balance between catalyst activity and product distribution. The results evidence that Pd/Al₂O₃–Co/SiO₂ mixture has a good balance between activity and selectivity, and leads to a very low production of trans isomers (11.8 %) and a moderate amount of saturated stearic acid (13.5 %). Consequently, the utilization of cobalt‐based catalysts (or the addition of cobalt to other metallic catalysts) could be considered a promising alternative for the hydrogenation of edible oil.     

Selective hydrogenation of (E)-2-hexenal to (E)-2-hexen-1-ol over Co-based bimetallic catalysts

Hydrogenation of (E)-2-hexenal was carried out in a liquid phase using Co-based bimetallic catalysts (M–Co/Al₂O₃, M=Pd, Pt, Ru, Rh, Sn, Fe, or Cu). Pd–Co/Al₂O₃ showed the highest activity among the catalysts tested and catalyzed the hydrogenation of CC bond predominantly to produce hexanal and 1-hexanol. Pt–Co/Al₂O₃ was more active than monometallic Co/Al₂O₃ for the hydrogenation of CO bond. The excellent result, 92% selectivity to (E)-2-hexen-1-ol formation at 90% conversion, was obtained by the hydrogenation over Pt–Co/Al₂O₃ bimetallic catalyst. No improved activities were observed for the other bimetallic catalysts.     

Tuning product selectivity of CO2 hydrogenation by OH groups on Pt/γ-AlOOH and Pt/γ-Al2O3 catalysts

Herein, we explore how OH groups on Pt/γ-AlOOH and Pt/γ-Al₂O₃ catalysts affect CO₂ hydrogenation with H₂ at temperatures from 250°C to 400°C. OH groups are abundant on γ-AlOOH, but rare at Pt-(γ-AlOOH) interface which is the most favorable site for CO₂ conversion on Pt/γ-AlOOH. This makes CO₂ hydrogenation on Pt/γ-AlOOH form CO weakly bonding to γ-AlOOH, which prefers to desorption from Pt/γ-AlOOH rather than further conversion, thus enhancing CO production on Pt/γ-AlOOH. Different from Pt/γ-AlOOH, OH groups are abundant at Pt-(γ-Al₂O₃) interface which is the most favorable site for CO₂ conversion on Pt/γ-Al₂O₃. This promotes CO₂ hydrogenation on Pt/γ-Al₂O₃ to form CO strongly bonding to Pt, which prefers to further hydrogenation to CH₄, and thereby increases CH₄ selectivity on Pt/γ-Al₂O₃. Therefore, the OH groups at metal-support interface are crucial factor influencing product distribution, and must be considered seriously when fabricating catalysts.     

Pt/CeO2 catalysts in selective hydrogenation of crotonaldehyde: high performance of chlorine‐free catalysts

The hydrogenation of crotonaldehyde was conducted in gaseous phase, at atmospheric pressure, on Pt/CeO₂ catalysts prepared from metal precursors containing or not chlorine. The activities and selectivities were studied, at 253 K, as a function of the reduction temperature of the catalyst (473–993 K). The Pt/CeO₂ catalyst, prepared from tetraammineplatinum nitrate, led to 5–20% crotyl alcohol selectivity when the catalyst was reduced at low temperature (473–673 K), while increasing the reduction temperature up to 973 K, the crotyl alcohol selectivity reached more than 80%. Repeating a series of experiments after a re‐calcination treatment at 673 K, the selectivity decreased to only 40% after 473 K reduction to reach again more than 80% after 673 K reduction temperature. A phase transformation of Pt to CePt₅ was observed by XRD analysis after 973 K reduction treatment. Differently on Pt/CeO₂ catalysts containing chlorine, prepared from either chloroplatinic acid or tetraammineplatinum chloride, the crotyl alcohol selectivity never exceeded 30% and did not form alloy up to 973 K reduction temperature. The main results are interpreted considering that the activity of CePt₅ for C=C hydrogenation is low compared to unmodified platinum catalyst and the activation of the carbonyl bond is induced by the presence of oxygen vacancies sites located at the interface between ceria and the metallic particles. The results are in good accordance with the information known at the present time on the metal–support interactions in Pt deposited on CeO₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of Metal Type on Partial Hydrogenation of Rapeseed Oil-Derived FAME

Supported SiO₂ catalysts were studied for the partial hydrogenation of rapeseed oil-derived fatty acid methyl esters (FAME) for improving its oxidative stability. The effect of metal type: Pt, Pd, and Ni, on catalytic activity and cis–trans selectivity was investigated. Hydrogenation activity was studied in terms of turn over frequency (TOF) of C18:3, C18:2, C18:1, and C18:0 FAME. The highest TOF of C18:3, C18:2, and C18:1 was found for Pd catalyst. However, C18:0 TOF of Pt is higher than that of the Pd catalyst. The higher in C18:0 TOF can explain the low selectivity towards trans-monounsaturated FAME of the Pt catalyst, which is due to the subsequent hydrogenation of the intermediate trans-monounsaturated to saturated FAME. On the other hand, Ni showed the lowest TOFs when compared with the Pt and Pd catalysts.     

Pt catalysts supported on zeolitized-pumice for the selective hydrogenation of campholenic aldehyde: A characterization and kinetic study

A detailed characterization study of Pt catalysts supported on a novel zeolitized-pumice (Z-PM) support is reported. Two catalysts, named Pt(Cl)/Z-PM and Pt(Ac)/Z-PM, prepared by impregnation from H₂PtCl₆ and Pt(acac)₂ as precursors and subsequent reduction under H₂ at 623 K, were investigated in terms of microstructure, chemical and surface properties by using X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS).Characterization data showed that the microstructural stability of the support was preserved by using Pt(acac)₂ as precursor. It was verified instead that, by using H₂PtCl₆, the microstructure of the support changed during the thermal activation step under hydrogen, favouring a strong interaction between Pt and support as well as the formation of bimetallic Pt–Fe alloys. Moreover, a moderate increase of Pt d-band vacancies with respect to metallic Pt was observed in this catalyst by XPS.The above catalysts were tested in the selective hydrogenation of campholenic aldehydes to the corresponding unsaturated alcohol, naturanol. In comparison to the Pt(Ac)/Z-PM sample, the ex-chloride Pt(Cl)/Z-PM catalyst showed a higher selectivity to naturanol. This behaviour was interpreted on the basis of the different microstructural and electronic properties as evinced by the characterization data.     

Catalysts with noble metals based on super-cross-linked polystyrene for the hydrogenation of aromatic hydrocarbons

A series of catalysts containing noble metals on a super-cross-linked polystyrene (SCP) support with a developed specific surface area (>1000 m²/g) and high thermal stability are prepared and studied to develop an effective catalyst for the low-temperature hydrogenation of aromatic hydrocarbons. A study of Pt- and Pd-containing catalysts based on SCP, carbon supports, and alumina in the hydrogenation of simple (benzene, toluene), branched (n-butylbenzene) and polycyclic (terphenyl) aromatic compounds is conducted. In the hydrogenation of aromatic hydrocarbons, the activity of the catalysts on SCP is comparable to or surpasses analogous catalysts based on Al₂O₃ and Sibunit in the content of noble metals; it is established that catalysts on SCP have greater selectivity in the hydrogenation of benzene in a benzene-toluene mixture. The electronic state of metals in the Pt(Pd)/SCP catalysts is studied by the IR spectroscopy of adsorbed CO. In testing the catalysts in the hydrogenation of terphenyl, it is found that Pt-containing catalyst on the SCP can operate in reversible hydrogenation-dehydrogenation cycles (terphenyl-tercyclohexane); this is promising for the use of such catalyst systems in creating composite materials for hydrogen storage.     

Catalytic hydrogenation of linoleic acid over platinum-group metals supported on alumina

Catalytic activity and selectivity for hydrogenation of linoleic acid (cis-9,cis-12 18:2) were studied on Pt, Pd, Ru, and Ir supported on Al₂O₃. Stearic acid (18:0) and 10 different octadecenoic isomers (18:1) in the products could be separated completely by using a new capillary column coated by isocyanopropyl trisilphenylene siloxane for gas-liquid chromatography. The monoenoic acid isomers and dienoic acid isomers in the products on the various catalysts showed different distributions. The catalysts exhibited nearly equal selectivity for stearic acid formation. The 12-position double bond in linoleic acid has higher reactivity than the 9-position double bond in catalytic hydrogenation on platinum-group metal catalysts. In addition to hydrogenation products of linoleic acid, geometrical and positional dienoic acid isomers (trans-9,trans-12; trans-8,cis-12; cis-9,trans-13; trans-9,cis-13; cis-9,trans-12 18:2), due to isomerization of linoleic acid during hydrogenation, were contained in the reaction products. Ru/Al₂O₃ exhibited the highest activity for isomerization of linoleic acid with the noble metal catalysts. Conjugated octadecadienoic acid isomers have been observed in products of the reaction on Pt/Al₂O₃, Ru/Al₂O₃, and Ir/Al₂O₃. Catalytic activities of noble metals for positional and geometric isomerization of linoleic acid during hydrogenation decreased in the sequence of Ru ≥ Pt > Ir » Pd.     

Structure and activity of platinum–silica catalysts prepared by a combustion process

By combustion of a mixture of silicone oil and chloroplatinic acid and organic solvents, Pt–SiO₂ catalysts with Pt concentrations between 37 and 90% have been prepared and their activity in benzene hydrogenation has been tested in a static system. The special structural characteristics conferred on the catalysts by the combustion method, i.e. small Pt particles with smooth round surfaces well separated by silica, ensure good resistance to de-activation by exposure to elevated temperatures as well as to atmospheres with high humidity at low temperatures.     

Intramolecular selective hydrogenation of cinnamaldehyde over CeO2–ZrO2-supported Pt catalysts

Selective liquid phase hydrogenation of cinnamaldehyde is reported, for the first time, over CeO₂, ZrO₂, and CeO₂–ZrO₂-supported Pt catalysts. Cinnamyl alcohol is the selective product. These catalysts are highly active and selective even at 25 °C and found to be superior to most of the hitherto known supported Pt catalysts. Alkali addition (NaOH) has enhanced the performance of these catalysts. At an optimized reaction condition, 95.8% conversion of cinnamaldehyde and 93.4% selectivity of cinnamyl alcohol have been obtained. Acidity of the support (due to the presence of ZrO₂ component) and higher electron density at Pt (due to CeO₂ component) are attributed to be responsible for the superior catalytic activity of Pt supported on CeO₂–ZrO₂ composite material.     

Synthesis of p‐aminophenol from the hydrogenation of nitrobenzene over metal–solid acid bifunctional catalyst

BACKGROUND: A single‐step conversion of nitrobenzene (NB) to p‐aminophenol (PAP) through catalytic hydrogenation is a widely used synthesis route for PAP. The main shortcoming of this route is the use of sulfuric acid for rearrangement of the phenylhydroxylamine (PHA) intermediate. In this paper, S₂O₈^2?/ZrO₂ (PSZ) solid acid and Pt‐S₂O₈^2?/ZrO₂ (Pt‐PSZ) bifunctional catalysts were prepared for the synthesis of PAP in non‐acid medium. RESULTS: Calcination temperature has a substantial effect on the acidity, structure and activity for PHA rearrangement of PSZ. The highest PAP yield was 33.8% over PSZ calcined at 823 K when the reaction was carried out in water at 423 K. A high PAP yield of 23.9% was achieved by a single‐step reaction of nitrobenzene over Pt‐PSZ bifunctional catalysts. CONCLUSION: PSZ solid acid exhibits high activity for PHA rearrangement. Perfect tetragonal ZrO₂ and much stronger acid sites play important roles in catalytic activity. Inhibiting the hydrogenation activity by reducing the amount of Pt loading on Pt‐PSZ can improve the competition of PHA rearrangement on acid sites with hydrogenation of PHA on metal active sites, resulting in better selectivity to PAP. Copyright © 2008 Society of Chemical Industry