Liquid-phase Hydrogenation Of Citral Research Articles

Design of γ-Alumina-Supported Phosphotungstic Acid-Palladium Bifunctional Catalyst for Catalytic Liquid-Phase Citral Hydrogenation

This study primarily addresses the development of dynamic, selective and economical metal–acid (bifunctional) catalysts for one-pot menthol production by citral hydrogenation. Specifically, various metals such as Pd, Pt, Ni, Cs and Sn were doped over alumina support. Additionally, bifunctional composite catalysts were also prepared with the impregnation of heteropoly acids and Pd precursors over alumina support. Analytical techniques (e.g., BET, PXRD, FT-IR, pyridine adsorption and amine titration methods) were applied for characterization of the most efficient and selective catalysts (e.g., Al2O3 and PTA-Cat-I). Similarly, most of the essential operational variables (e.g., loading rate of metal precursor, type of heteropoly acid, temperature, gas pressure and reaction time) were examined during this study. The experimental data shows that the bifunctional catalyst (PTA-Cat-I) produced 45% menthol at full citral substrate conversion (r = 0.038 mmoles.min−1) in liquid-phase citral hydrogenation (at optimized operating conditions: 70 °C, 0.5 MPa and 8 h). However, the heteropoly acid-supported bifunctional catalysts (e.g., PTA-Cat-I, PMA-Cat-I, SMA-Cat-I and STA-Cat-I) resulted in cracking and the dehydration of isopulegol/menthol by the generation of side products (e.g., 4-isopropyl-1-methyl, cyclohex-1-ane/ene); therefore, menthol yield was extensively diminished. On the other hand, non-acidic catalysts (e.g., Cat-I, Cat-II, Cat-III, Cat-IV and Cat-V) readily promoted hydrogenation reactions. The optimum menthol yield occurred due to the presence of strong Lewis and weak Bronsted acid sites. Mass transfer and reaction rate were substantially diminished due to acidity strength, heteropoly acid type and blockage of pores by the applied bifunctional catalysts.

Design of Nickel Supported Hierarchical ZSM-5/USY Zeolites Bifunctional Catalysts for One Pot Menthol Synthesis Via Liquid Phase Citral Hydrogenation: Synthesis, Reaction Kinetics and Characterization

Design of Nickel Supported Hierarchical ZSM-5/USY Zeolites Bifunctional Catalysts for One Pot Menthol Synthesis Via Liquid Phase Citral Hydrogenation: Synthesis, Reaction Kinetics and Characterization

Hydrogenation of Citral Over IrAu/TiO2 Catalysts. Effect of the Preparation Method

Hydrogenation of citral over Ir/TiO2 and IrAu/TiO2 catalysts was studied. Catalysts were prepared by deposition–precipitation with urea (Ir/TiO2) and sequential incorporation of the metals using a combined DPU-incipient wetness impregnation approach (IrAu/TiO2). Samples were reduced in H2 at 573 K (LT) and 773 K (HT). These materials were characterized by transmission electron microscopy (HAADF), TPR and CO adsorption followed by DRIFT. The liquid phase hydrogenation of citral was studied in a Parr type batch reactor at 363 K and a H2 pressure of 0.62 MPa. An Ir/TiO2 catalyst prepared by wet impregnation was used as reference catalyst. High selectivity to unsaturated alcohols was found, the only reaction products were geraniol and nerol for Ir/TiO2 DPU and IrAu/TiO2 catalysts. The IrAu/TiO2 (HT) catalyst showed the higher conversion levels.

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.

The Influence of the Preparation Method on the Behaviour of PtGe Catalysts Supported on Activated Carbon in Citral Hydrogenation

The liquid-phase citral hydrogenation carried out over PtGe catalysts prepared by different techniques and supported on activated carbon powder purified and functionalized with nitric acid (C-PN) was investigated. In catalysts prepared by conventional successive impregnation (CI), Ge does not improve in an important way either the activity or the selectivity to unsaturated alcohols. Besides, PtGe catalysts prepared by controlled surface reaction (CSR) show much better activities and selectivities to unsaturated alcohols (UA). It is needed a large concentration of Ge to polarize the carbonyl group in the case of CI catalysts because Ge is located mainly over the support and small quantities would be deposited over the Pt particles. In PtGe catalysts prepared by CSR, Ge moieties would polarize the carbonyl group since they would be placed near PtGe nanoclusters, thus giving high selectivity to UA.

Inverse temperature dependence due to catalyst deactivation in liquid phase citral hydrogenation over Pt/Al2O3

Citral hydrogenation was studied over a Pt/Al2O3 catalysts in cyclohexane and in 2-pentanol. Several kinetic trends, which were correlated to catalyst deactivation, were achieved. Accumulation of trans isomers in both citral and in unsaturated alcohols was visible under deactivating conditions. The further hydrogenation of citronellal was nearly totally inhibited due to catalyst deactivation, whereas nerol, geraniol and citronellol formation were enhanced at higher temperatures and pressures. The main unusual kinetic phenomena in citral hydrogenation in 2-pentanol were, first, a maximum observed in the initial hydrogenation rates as a function of temperature caused by catalyst deactivation and, second, a minimum in citral conversion after prolonged reaction times. The reason for these unusual kinetic phenomena is decarbonylation reaction occurring during hydrogenation, i.e. formation of CO, which was confirmed by temperature programmed desorption of geraniol from a reduced Pt/Al2O3 catalyst.

From molecule to material: Mg 2Sn as hydrogenation catalyst

The liquid-phase hydrogenation of citral has been studied for the first time over an ionogenic Zintl phase, Mg 2Sn. By using this unsupported alloy, citral conversion was 20% at 448 K and the product mixture was mainly comprised of geraniol, nerol, citronellal and citronellol. The product ratio of the formation of unsaturated allyl-type alcohols to the formation of citronellal + citronellol is similar to that obtained with the unsupported alloy Ru 3Sn 7 and a silica supported Ru–Sn catalyst. The results clearly indicate that the polar character of active sites is a prerequisite for the chemisorption of the polar C=O group and, thus, must be a key factor in controlling the selectivity of the hydrogenation of α,β-unsaturated aldehydes.

The kinetics of citral hydrogenation over Pd supported on clinoptilolite rich natural zeolite

The liquid phase hydrogenation of citral has been investigated over Pd (2.42%) supported on clinoptilolite rich natural zeolite catalyst for different reaction conditions. The zeolite support did not affect the active metal properties. This was attributed to the large size of Pd particles on the support. High selectivity (90%) to citronellal was obtained at complete conversion of citral. The catalytic activity increased with reaction temperature (80, 100, 120 °C) following an Arrhenius behaviour, while selectivity remained constant for a given conversion. Selectivity to citronellal increased from 78 to 90% with increase in the amount of catalyst in the reaction solution (Citral/Pd mole ratio 293, 176 and 105). The spent catalyst regained its fresh activity and selectivity upon regeneration.

Liquid-Phase Citral Hydrogenation over SiO2-Supported Group VIII Metals

Liquid-phase citral hydrogenation over SiO2-supported Group VIII metals at 300 K and 1 atm was studied in the absence of all transport limitations as verified by the Madon–Boudart test and the Weisz–Prater criterion. The initial TOF (turnover frequency) for citral hydrogenation varied by three orders of magnitude and exhibited the following trend: Pd>Pt>Ir>Os>Ru>Rh>Ni>Co⪢Fe (no activity was detected over Fe/SiO2). When TOF is correlated with percentage d-character, a volcano plot results. There are significant differences in product distribution among the different metals, with Os, Ru, and Co exhibiting high selectivity toward the unsaturated alcohol isomers (geraniol and nerol), while Rh, Ni, and Pd were more selective for citronellal and isopulegol. The variation in product distribution among the various metals is rationalized on the basis of the d-band widths that affect the relative importance of the stabilizing two-electron interactions and destabilizing four-electron interactions. With the exception of Ni/SiO2, all catalysts exhibited substantial deactivation that was attributed to inhibition by adsorbed CO produced by the decomposition of either citral or the unsaturated alcohol. In contrast, Ni/SiO2 exhibited an initially low, stable TOF during the first 7 h that then increased fourfold and again remained constant to 84% citral conversion, at which point the run was terminated.