Heterogeneous Enantioselective Hydrogenation of Activated Ketones Catalyzed by Modified Pt‐Catalysts: A Systematic Structure‐Selectivity Study

A systematic structure‐selectivity study was carried out for the enantioselective hydrogenation of activated ketones with chirally modified Pt/Al₂O₃ catalysts. For this, 18 modifiers containing an extended aromatic system able to form a strong adsorption complex with the Pt surface, and a suitable chiral group with an amino function capable to interact with the keto group of the substrate ( HCd, Qd, HCn, Qn , and semi‐synthetic derivatives, as well as synthetic analogues) were prepared and tested on 8 different activated ketones in AcOH and toluene under standard conditions. It was found that relatively small structural changes of the substrate and/or modifier structures strongly affected the enantioselectivity, and that no “best” modifier exists for all substrates. The highest ees for all substrates were obtained with quinuclidine‐derived modifiers in combination with naphthalene or quinoline rings, either in AcOH (substrates 1 – 5 and 8 , all carrying an sp³ carbon next to the keto group) or toluene ( 6 and 7 , with an sp² carbon next to the ketone). The presence and nature of the substituent R' at the quinuclidine significantly affected the ee (positive and negative effects). Certain combinations of an aromatic system and an amino function were preferred: For the quinuclidine moiety, quinoline and to a somewhat lesser extent naphthalene were a better match, while for the pyrrolidinylmethyl group anthracene was better suited. Methylation of the OH group often had a positive effect for hydrogenations in AcOH but not in toluene. With the exception of 8 , higher ees were obtained for the Cd / Qn series [leading to (R)‐products] than for the Cn / Qd series [leading to (S)‐products]. In several cases, opposite structure‐selectivity trends were detected when comparing reactions in toluene and AcOH, indicating a significant influence of the solvent.     

Asymmetric Hydrogenation of Activated Ketones on Platinum: Relevant and Spectator Species

The heterogeneous enantioselective hydrogenation of activated ketones over chirally modified platinum is reviewed with emphasis on identifying the role of the various species observed in this catalytic system. The past years have witnessed a continuous broadening of the scope of this catalytic system including new reactants and modifiers affording over 97% ee. New reaction pathways have been uncovered and the kinetic and mechanistic studies have been faced with a number of complicating factors caused by spectator species and interactions in solution and on the Pt surface. The previously proposed mechanistic models are critically assessed in the light of these new findings.     

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.     

Hydrogenation of 2-Ethylanthraquinone over Pd/SiO2 and Pd/Al2O3 in the Fixed-Bed Reactor. The Effect of the Type of Support

The effects of the type of support and Pd concentration profile in alumina and silica supported egg-shell catalysts and their performance in the hydrogenation of 2-ethylanthraquinone (eAQ) were studied in 'Anthra' (AQ) and 'All-Tetra' systems. The activity and deactivation of catalysts were determined in the fixed-bed reactor. Solution saturated with hydrogen, (concentration of active quinones 60g/dm³, eAQ in the AQ system, 30% of eAQ and 70% of H₄eAQ–2-ethlytetrahydroanthraquinone, in the All-Tetra system) was circulated through the catalyst bed at temperature 50°C and pressure 5bar. The contents of eAQ, active quinones, H₄eAQ and degradation products were determined in the course of hydrogenation by GC method. The egg-shell palladium catalysts (1–2wt% Pd) prepared by the precipitation of palladium hydroxide onto alumina and silica supports pre-impregnated with various alkaline (NaHCO₃, NaH₂PO₄, Na₂SiO₃) solutions were used in the hydrogenation experiments. Pd concentration profile inside the grains of catalysts was characterized by scanning electron microscopy. A difference between alumina and silica carriers with respect to the course of side reactions producing degradation products was found. Degradation of quinones in the hydrogenolytic reactions predominated on alumina supported catalysts, while the catalysts with silica favoured the hydrogenation of aromatic rings resulting in H₄eAQ-active quinone. As a crucial factor for the decrease in the activity during the hydrogenation run, the reactivity of catalyst in the hydrogenolytic reactions was established. Alumina supported catalysts exhibited much higher deactivation than those of silica supported ones. Silica carrier as well as silica species introduced onto alumina under pre-impregnation with Na₂SiO₃ exhibited an advantageous role in the catalyst performance, in terms of activity and deactivation.     

Dehydrogenation of Cyclohexane Over Ni Based Catalysts Supported on Activated Carbon using Spray-pulsed Reactor and Enhancement in Activity by Addition of a Small Amount of Pt

The polycrystalline silver after pre-treated with oxygen at high temperature was found to show excellent activity and selectivity in the process of direct dehydrogenation of methanol to anhydrous formaldehyde and the active oxygen species for this reaction was observed and proposed as the active centeres with in situ Raman scattering.     

The Reductive Amination of Aldehydes and Ketones and the Hydrogenation of Nitriles: Mechanistic Aspects and Selectivity Control

This review deals with two of the most commonly used methods for the preparation of amines: the reductive amination of aldehydes and ketones and the hydrogenation of nitriles. There is a great similarity between these two methods, since both have the imine as intermediate. However, due to the high reactivity of this intermediate, primary, secondary and/or tertiary amines are obtained (often simultaneously). The relation of the selectivity to different substrate structures and reaction conditions is briefly summarised, the main focus being on the catalyst as it is the most significant factor that governs the selectivity. Different mechanisms are discussed with the view to correlate the structure of the catalyst and, more particularly, the nature of the metal and the support with selectivity. The crucial point is the presumed location of the condensation and hydrogenation steps.     

两种铵盐催化剂和不同活化工艺对粘胶基活性性碳纤维的影响

研究了磷酸二氢铵和硫酸铵两种催化剂,以及逐步碳化活化和直接活化两种工艺对粘胶活性碳纤维得率、比表面积和孔结构等的影响;新指标有效得率能满意地克服单一指标的不足.粘胶纤维的氧化、碳化及活化得率随磷酸二氢铵的用量线性增加,随硫酸铵的用量增幅趋缓.磷酸二氢铵优于硫酸铵,用前者时纤维容易活化、比表面积大、有效得率高.纤维的有效得率与催化剂种类有关,随磷酸二氢铵的增加而上升;但与活化工艺的关系不大.经N2等温吸附和WAXD测试表明,两种工艺制得的产品有相同的孔结构,但微晶结构不同.     

两种铵盐催化剂和不同活化工艺对粘胶基活性碳纤维的影响

研究了磷酸二氢铵和硫酸铵两种催化剂,以及逐步碳化活化和直接活化两种工艺对粘胶活性碳纤维得率、比表面积和孔结构等的影响;新指标有效得率能满意地克服单一指标的不足。粘胶纤维的氧化、碳化及活化得率随磷酸二氢铵的用量线性增加,随硫酸铵的用量增幅趋缓。磷酸二氢铵优于硫酸铵,用前者时纤维容易活化、比表面积大、有效得率高。纤维的有效得率与催化剂种类有关,随磷酸二氢铵的增加而上升;但与活化工艺的关系不大。经N2等温吸附和WAXD测试表明,两种工艺制得的产品有相同的孔结构,但微晶结构不同。     

Hydrogenation of But-1-yne and But-1-ene on Palladium Catalysts: Particle Size Effect

The hydrogenation of but-1-yne and but-1-ene was studied on highly (D = 97%) and lowly (D = 26%) dispersed palladium on alumina catalysts. The activity calculated in terms of accessible palladium atoms was found to depend on the particle size. For the hydrogenation of but-1-ene, the turnover number was higher on highly dispersed catalysts than on the lowly dispersed one. The reverse was true for the hydrogenation of but-1-yne. During the hydrogenation of but-1-yne the quantity of but-1-ene produced without further hydrogenation was higher on small particles. A study by X-ray photoemission spectroscopy showed an increase of the binding energy by 1 eV for the 3d core level of palladium in 0.2% Pd/Al₂O₃ (D > 80%) , either before or after a catalytic run (no coalescence occurred). The catalytic results were interpreted as being due to the variation of the adsorption strength of the hydrocarbons. The unsaturated hydrocarbons could be more strongly adsorbed on very small palladium particles which may be considered as “electron deficient”. A calculation showed that the sticking coefficient of but-1-yne was at least 2.5 times as high on small particles as on larger ones.     

Hydrogenation of Aromatic Nitrogroups with Precious Metal Powder Catalysts: Influence of Modifier on Selectivity and Activity

The development of new improved catalysts for the selective hydrogenation of aromatic nitrogroups is still a field of high interest for the chemical industry. Hydrogenation of aromatic nitrogroups to the corresponding aromatic amines is one of the most important applications for precious metal powder catalysts (PMPC). In this paper various commercial PMPC technologies such as palladium, platinum, platinum modified with copper and platinum modified with vanadium on activated carbon powder supports were compared to each other in regard to activity and selectivity for hydrogenation of 1-chloro-2-nitrobenzene. In this reaction, catalysts comprised of vanadium modified platinum on activated carbon showed excellent activities and selectivities towards 2-chloroaniline. Catalyst performance was affected by the ratio of platinum to vanadium and the properties of the carbon. The role of vanadium as an effective modifier to avoid accumulation of hazardous aromatic hydroxylamines was explained.     

涓流床反应器液体空速和床层高度对加氢效果的影响

本实验采用涓流床反应器在2-乙基己烯-2-醛催化加氢制2-乙基己醇小试和模试装置中,取得了催化活性与液体空速和催化剂床层高度的关系,并运用最小二乘法分别得到在操作条件下的经验公式: LHSV~（-2/3）=-0.206-16.853log（A_i）/（A_o） log（A_i/A_o）=-0.027-0.0027L~（-1/3） 由上式可得出在操作条件下反应物转化为产品时的最大液体空速及最小催化剂层高度。所得到的催化活性对液体空速的关系与文献相符。实验认为由床层高度的变化所取得的加氢效果的变化是由于液体质量流速的变化而引起的。 本实验所得的数据对于涓流床反应器的开发、设计、操作是有益的。     

Oxidation of Butane to Maleic Anhydride using Vanadium Phosphate Catalysts: Comparison of Operation in Aerobic and Anaerobic Conditions using a Gas-gas Periodic Flow Reactor

The use of a periodic flow reactor is described for the oxidation of butane to maleic anhydride to compare the catalytic performance of vanadium phosphate catalysts operating under aerobic and anaerobic conditions. It is found that for the catalyst prepared via a standard VPO method, operation in the absence of oxygen leads to a very small enhancement in selectivity when butane concentrations in the range 0.9–2.9% are used. Operation in the absence of oxygen leads to very small differences in conversion such that the overall yield is enhanced and this effect is maximised for reactor feeds containing 1.5% butane. However, the enhancement is negligible when the catalyst is operated at high conversion required for commercial operation, indicating that reactors operating with continuous flow with aerobic conditions are preferred. Similar experiments are conducted for a catalyst prepared by the VPD method and, in contrast, this catalyst gives lower butane conversion and maleic anhydride selectivity when operated in the absence of oxygen.     

Hydrogenation of Citral using Monometallic Pt and Bimetallic Pt‐Ru Catalysts on a Mesoporous Support in Supercritical Carbon Dioxide Medium

Supercritical carbon dioxide was shown to be a suitable reaction medium for the highly efficient hydrogenation of citral using monometallic Pt and bimetallic Pt‐Ru supported on a mesoporous material, MCM‐48, as catalyst. A remarkable change in the product distribution was observed after the addition of Ru to the monometallic Pt catalyst in supercritical carbon dioxide. The monometallic Pt catalyst was found to be highly selective to the unsaturated alcohol (geraniol+nerol) at a temperature of 323 K within 2 h whereas the bimetallic catalyst becomes selective to the partially saturated aldehyde (citronellal) under the same reaction conditions. Phase behavior plays an important role in the product distribution. Highest conversion and high selectivity to citronellal were achieved in the homogeneous phase for the Pt‐Ru catalyst while on the other hand the unsaturated alcohol (geraniol+nerol) was produced in the heterogeneous phase for the monometallic Pt catalyst. An XPS study offers strong evidence of the electronic modification of Pt after the addition of Ru in the bimetallic catalyst. The change in product distribution on the Pt‐Ru bimetallic catalyst may be explained by the appreciable interaction between the medium and the metal particles promoted by the presence of metallic Ru.     

Hydrogenation of acetophenone using a 10% Ni supported on zeolite Y catalyst: kinetics and reaction mechanism

The kinetics of hydrogenation of acetophenone was studied using a 10% Ni supported on zeolite Y catalyst in a temperature range 353–393 K. The effect of H₂ pressure, initial concentration of acetophenone and catalyst loading on the concentration-time profiles was studied. Water formed during the course of hydrogenation showed a strong inhibiting influence on the rate of reaction. A rate equation has been proposed based on L–H type rate mechanism assuming that the reaction between the non-competitively adsorbed hydrogen and the adsorbed organic substrate as rate limiting step. A semi-batch reactor model was developed to predict the concentration-time, H₂ consumption-time profiles at different sets of initial conditions. The model predictions were found to agree with experimental data very well at all temperatures. This model incorporates the inhibition of hydrogenation rate due to water. The inhibiting effect of water is also explained based on quantum chemical calculations.     

Perfectly Wetting Mixtures of Surfactants from Renewable Resources: The Interaction and Synergistic Effects on Adsorption and Micellization

This paper presents a study of the surface properties of mixtures of surfactants originating from renewable sources, i.e., alkylpolyglucoside (APG), ethoxylated fatty alcohol (AE), and sodium soap (Na soap). The main objective was to optimize the surfactant ratio which produces the highest wetting properties during the analysis of the solution of the individual surfactants, two- and three-component mixtures, and at different pH values. The results showed the existence of a synergistic effect in lowering the interfacial tension, critical micelle concentration and the formation of mixed micelles in selected solutions. We found that best wetting properties were measured for the binary AE:APG mixtures. It has been demonstrated that slightly lower contact angles values were observed on Teflon and glass surfaces for the AE:APG:soap mixtures but the results were obtained for higher concentration of the components. In addition, all studied solutions have very good surface properties in acidic, basic and neural media. However, the AE:soap (molar ratio of 1:2), AE:APG (2:1) and AE:APG:soap (1:1:1) compositions improved their wetting power at pH 7 on the aluminium and glass surfaces, as compared to solutions at other pH values tested (selected Θ values close to zero—perfectly wetting liquids). All described effects detected would allow less surfactant to be used to achieve the maximum capacity of washing, wetting or solubilizing while minimizing costs and demonstrating environmental care.