Vapour phase hydrogenation of phenol over Pd/C catalysts: A relationship between dispersion,metal area and hydrogenation activity

A series of palladium supported on activated carbon catalysts, with Pd varying from 0.5 to 6.0 wt%, were prepared via wet impregnation method using PdCl₂ · xH₂O as a precursor salt. The dried samples were further reduced at 573 K in hydrogen and characterized by CO adsorption at room temperature in order to determine the dispersion, metal area and particle size. The catalysts were tested for vapour phase phenol hydrogenation in a fixed-bed all glass micro-reactor at a reaction temperature of 453 K under normal atmospheric pressure. The decrease in metal surface area as well as dispersion with corresponding increase in turn-over frequency (TOF) against palladium loadings suggest the unusual inverse relationship that exist between Pd dispersion and phenol hydrogenation activity over Pd/carbon catalysts. The stability of TOF at larger crystallite size indicates that phenol hydrogenation is less sensitive reaction especially beyond 3 wt% of Pd content. It is evident from the results that structural properties of the catalysts strongly influence the availability of Pd atoms on the surface for CO chemisorption and hence for phenol hydrogenation. A comparison between selectivity and product yield of the reaction against overall phenol conversion indicates that changes in reaction selectivity for cyclohexanone or cyclohexanol is independent of phenol conversion level and either of the product is not formed at the cost of another. The stability of the catalysts with reaction time suggests that coke formation on the surface of the catalyst is less significant and the formation of cyclohexanone remains almost total even at higher reaction temperatures.     

Studies on the hydrogenation of cinnamaldehyde over Pd/C catalysts

The hydrogenation of cinnamaldehyde was investigated using a 5% Pd/C catalyst in a 250 cm³ stirred tank reactor and 500 cm³ autoclave. The experiments were carried out at 273–343 K and 0·1–1·1 MPa. Non-polar solvents, e.g. toluene, decane, methylcyclohexane, decalin, ether and heptane, and polar solvents such as methanol, ethanol, propan-1-o1, propan-2-ol, butan-1-ol and butan-2-ol were used to study the selectivity with respect to hydrocinnamaldehyde formation, the reaction kinetics and mass transfer. The additives, such as potassium acetate, ferrous chloride, ferrous sulphate and quinoline were incorporated into the catalyst in order to improve the catalyst selectivity, which was observed especially in the case of potassium acetate. © 1998 SCI     

Controlled preparation of Pd/AC catalysts for hydrogenation reactions

A commercial activated carbon was submitted to different liquid oxidizing treatments in order to create a high density of carboxylic groups on the carbonaceous surface. Characterization of the modified activated carbon shows that it is possible to generate a large range in acidic group concentration without significant change of the textural properties. This ability allows us to introduce, by ion exchange, palladium cations up to 7.3 wt.%. This way of preparation, coupled to soft formaldehyde reduction, leads to highly dispersed metallic palladium catalysts. The resulting catalysts were tested in hydrogenation of cinnamaldehyde and exhibited a high hydrogenation efficiency compared to activities reported in the literature.     

溶剂对Pd/C催化四氯化碳液相加氢反应的影响

考察了活性组分、载体、Pd含量及溶剂对四氯化碳液相催化加氢转化为氯仿反应的影响。实验结果表明,质量分数0.5%的Pd/C是四氯化碳液相催化转化的较优催化剂。溶剂对该反应的速率和产品分布有显著影响,在正庚烷、甲苯和甲醇3种溶剂中,甲醇最利于提高Pd/C催化剂的活性,且四氯化碳转化的平均反应速率随着甲醇含量的增加呈现先增大后减小的趋势,氯仿的选择性随着甲醇含量的增加有所降低。当甲醇和四氯化碳的体积比为2∶1时,四氯化碳转化速率最高。甲醇和四氯化碳的体积比为1∶10时,氯仿的选择性达90%。     

Effect of metal dispersion on CO hydrogenation over Pd/HZSM-5 catalysts

Pd/HZSM-5 catalysts prepared by ion-exchange method using Pd(NH₃) ₄ ²⁺ were calcined and reduced at different temperatures to provide different metal dispersions. The effect of Pd dispersion on CO adsorption characteristics and acidity were observed through FT-IR study. Methanol and dimethyl ether were the main products in CO hydrogénation over Pd/HZSM-5 catalyst with small Pd particles on which CO was weakly adsorbed, while the selectivity to methane increased with metal sizes.     

Deactivation of metal catalysts in liquid phase organic reactions

The paper gives a general survey of the factors contributing to the deactivation of metal catalysts employed in liquid phase reactions for the synthesis of fine or intermediate chemicals. The main causes of catalyst deactivation are particle sintering, metal and support leaching, deposition of inactive metal layers or polymeric species, and poisoning by strongly adsorbed species. Weakly adsorbed species, poisons at low surface coverage and solvents, may act as selectivity promoters or modifiers. Three examples of long term stability studies carried out in trickle-bed reactor (glucose to sorbitol hydrogenation on Ru/C catalysts, hydroxypropanal to 1,3-propanediol hydrogenation on Ru/TiO₂ catalysts, and wet air oxidation of paper pulp effluents on Ru/TiO₂) are discussed.     

Vapour phase hydrogenation of olefins by formic acid over a Pd/C catalyst

It has been found that ethylene and propylene could be effectively hydrogenated by formic acid vapour over a Pd/carbon catalyst at low temperatures (<440 K). Surface hydrogen formation from formic acid is the rate-determining step for this hydrogenation reaction. Interaction of this hydrogen with the olefins is then fast. The conversion of formic acid in the presence of either of the olefins at any temperature is higher than in their absence. This has been explained by a much lower surface hydrogen concentration in the presence of the olefins. Direct experiments have confirmed that hydrogen inhibits the formic acid decomposition. Water vapour addition has a small positive effect on the decomposition of formic acid as well as on the hydrogenation of the olefins with formic acid. Catalysts consisting of gold supported on carbon or titania are both active in the production of hydrogen from formic acid. However, in contrast to the Pd/C catalyst, neither gives hydrogenation of the olefins with this acid.     

Effects of preparation variables on the activity of alumina-supported platinum catalysts for liquid phase cinnamaldehyde hydrogenation

Alumina-supported platinum catalysts were prepared through different metal loading and reduction procedures. The platinum precursors were loaded by impregnation and ion-exchange and then reduced by hydrogen at high temperatures and by sodium tetrahydroborate solution at a low temperature of 35°C. The low-temperature reduced (LR) catalysts were found to produce cinnamyl alcohol in high selectivities in the liquid phase hydrogenation of cinnamaldehyde, irrespective of the method of metal loading. The high-temperature reduced (HR) catalysts prepared with ion-exchange produced hydrocinnamaldehyde and cinnamyl alcohol; the selectivity of the former was larger than that of the latter. In contrast, the HR catalysts prepared with impregnation produced large amounts of by-products and small amounts of hydrocinnamaldehyde and cinnamyl alcohol. When these HR catalysts were additionally treated with sodium tetrahydroborate solution, they indicated product distributions similar to the LR catalysts.     

The activity and stability of Pd/C catalysts in benzene hydrogenation

Carbon black supported Pd catalysts were prepared by an incipient wetness impregnation method and tested for benzene hydrogenation. The catalytic activity is subjected to serious deactivation. However, the deactivation is found to depend on the test procedure. When the reaction is performed under constant flow of benzene and hydrogen during a stepwise temperature-ascending-descending test sequence, no deactivation is observed. Deactivation occurs when the reaction gas is replaced by a He purge during temperature changes in the test sequence. The Pd/C catalysts prepared from different precursors show similar activation energy and similar TOF as oxide-supported Pd catalysts when deactivation is confined by applying suitable experimental procedures. EXAFS, TEM, and TGA results suggest that both Pd sintering and carbonaceous residue are responsible for the deactivation.     

Support effects in hydrogenation of cinnamaldehyde over carbon nanofiber-supported platinum catalysts: Kinetic modeling

Carbon nanofiber-supported platinum catalysts with a narrow and stable platinum particle size distribution (1-2 nm) were prepared, one with a considerable amount of oxygen support surface groups (PtCNF, 2.8 acidic O atoms/nm²) and one with a much smaller amount (PtCNF973, 0.06 acidic O atoms/nm²). Their catalytic performance was compared in a series of liquid-phase cinnamaldehyde hydrogenation experiments at 383 K using hydrogen pressures of 2.8-6.8 MPa and a cinnamaldehyde concentration in the range 14-. These results showed a different hydrogen dependency of the reaction rate for the two catalysts, demonstrating a change in the adsorption of hydrogen on platinum. Using a single-site model including both Langmuir-Hinshelwood kinetics and mass transfer effects we found that the intrinsic reaction rate increased up to a factor of 120 with the removal of the oxygen-containing surface groups. Also the adsorption constants increased significantly, indicating that adsorption on PtCNF973 is stronger than on PtCNF. These results suggest that hydrogenation is assisted by adsorption of the benzene ring of cinnamaldehyde on the non-polar CNF support surface after removal of the oxygen-containing groups.     

Synthesis of metal intercalated clay catalysts for selective hydrogenation reactions

Interlamellar space of the smectite clay minerals have been found as an excellent nano phase reactor due to their easy swelling property and the adsorption capacity. In this work size quantized nano particles of catalytically active noble metals have been generated within the interlamellar space of the smectite type clay minerals. Platinum and ruthenium intercalated/impregnated montmorillonite and hectorite catalysts were synthesised and characterized by various instrumental techniques. Catalytic activity of the synthesised catalysts was evaluated towards hydrogenation of cinnamaldehyde in liquid phase. The effect of various reaction parameters such as temperature, hydrogen pressure, solvents, and amount of catalyst and time on stream on conversion and selectivity were studied and the results are discussed. The studies revealed that noble metal intercalated/impregnated clay catalysts were very effective in hydrogenation reactions as they showed significant conversion and selectivity. Comparison of catalytic activity made with the impregnated catalysts showed that the intercalated catalysts were better than impregnated catalysts in terms of conversion and selectivity.     

钯碳催化剂的制备条件对苯酚气相加氢制环己酮的影响

采用浸渍法制备了Pd/C催化剂,考察了活性炭预处理方法、沉淀温度和还原方法对催化剂催化苯酚气相加氢制环己酮的影响。通过XRD和XPS对催化剂进行表征,考察了催化剂制备条件对Pd/C催化剂结构和催化活性的影响。结果表明:双氧水预处理可以向载体表面引入氧化性基团,促进钯在载体中的分散;在活性炭酸洗过程中进行搅拌,可以减少落粉的产生,提高催化剂的活性;沉淀温度升高会导致催化剂中Pd晶粒度变大;Pd晶粒度过小或过大都会导致催化剂活性下降;采用水合肼还原有利于提高催化剂的活性。     

Pd/HZSM-5催化剂催化加氢丙酮合成甲基异丁基酮

以HZSM-5为载体,采用浸渍法制备系列Pd/HZSM-5催化剂,在高压连续流动固定床反应器中考察Pd/HZSM-5催化剂催化加氢丙酮一步法合成甲基异丁基酮性能,并对工艺条件进行优化。结果表明,当HZSM-5载体上Pd负载质量分数为0.5%时,在反应温度140 ℃、氢压1 MPa、空速0.48 h^-1和氢酮物质的量比为1条件下,Pd/HZSM-5催化剂催化活性较高,丙酮转化率为45.91%,甲基异丁基酮选择性为94.33%。采用XRD、H₂-TPD、SEM、EDS和TGA等对催化剂进行表征,结果表明,负载质量分数0.5%的Pd在HZSM-5分子筛表面分散均匀,且0.5%Pd/HZSM-5催化剂具有较高氢吸附能力,失活的主要原因为催化剂表面积炭,采用流化床反应器取代传统的固定床反应器可以很好的解决催化剂积炭问题。     

Enhanced liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol over mesoporous carbon supported Pd catalysts

Supported Pd catalysts on ordered mesoporous carbon (OMC) and activated carbon (AC) were prepared and their catalytic behavior for the liquid phase catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol was investigated. In comparison with Pd/AC, Pd particles of Pd/OMC were effectively confined in the mesopores of OMC, resulting in high Pd dispersion and Pd²⁺ content. Accordingly, Pd/OMC exhibited higher catalytic activity than Pd/AC. Moreover, increasing catalyst reduction temperature lowered the catalytic activities and favored stepwise HDC of 2,4-dichlorophenol.     

The catalytic hydrogenation of benzene over supported metal catalysts.: II. Gas-phase hydrogenation of benzene over copper-on-silica

A study has been made of the kinetics and mechanism of the gas-phase hydrogenation of benzene over a copper-on-silica catalyst in the temperature range from 300 to 450 K and at 130 to 395 kPa total pressure. Attention has been given also to the kinetics of the hydrogenation of the reaction intermediates 1,4-cyclohexadiene and cyclohexene.Two different catalysts were prepared on Shell silica spheres as a support, one according to the Dutch Patent No. 143,139 and the other according to European Patent No. 0006313. Their texture and analytical composition were studied by means of TEM, nitrogen physisorption and capillary condensation, adsorptive decomposition of nitrous oxide, atomic absorption spectrometry and XPS/AES analysis.The catalyst prepared according to European Patent No. 0006313 had a high mechanical strength, a perfect chemical stability and a high sintering resistance. The mean copper particle diameter was 5.5 nm. The other catalyst displayed poor mechanical strength and fragmented during testing.A reaction mechanism is formulated on the analogy of the mechanism proposed for the hydrogenation of benzene over ruthenium [11].The catalytic activity of copper is compared with that of several other transition metals and presented in the form of modified Tanaka-Tamaru plots.     

选择性催化加氢合成3,4-二氯苯胺的Pd/C催化剂研究

研究了Pd/C催化剂表面金属钯平均粒径对3,4-二氯硝基苯催化加氢反应选择性的影响，发现金属钯平均粒径越大，脱氯副反应越不明显，加氢选择性越高。采用浓硝酸预处理的活性炭负载钯催化剂虽然能够增大表面金属钯平均粒径，粒径分布却较宽，不利于加氢选择性的提高。使用饱和EDTA·2Na溶液预处理活性炭后，在钯负载量降低的情况下，可以达到活性炭载体表面金属钯大粒径、狭窄的分布，实现了在不添加脱氯抑制剂的情况下高选择性合成3,4-二氯苯胺。     

Porous membrane catalysts with Pd-M-clusters for liquid phase hydrogenation of dehydrolinalool

New membrane catalysts with less than 1.5% of Pd in porous stainless steel have been prepared for liquid phase hydrogenation of unsaturated organic compounds. Ultradispersed Pd modified with catalytically active Pb, Mn, or Bi was prepared using Metal Vapour Synthesis techniques. Rates of over 12 mol/m²h with selectivities of up to 96% in hydrogenation of dehydrolinalool into linalool under atmospheric pressure of H₂ were observed. The ease of separation of this catalyst from reaction products, its regenerability, along with its corrosion, thermal stability, and mechanical strength give good prospects for these materials.