THE KINETICS OF THIXOTROPIC BEHAVIOUR IN CLAY/KAOLIN AQUEOUS SUSPENSIONS

Two possible processes for deactivation in the catalytic hydrolysis of acrytonitriie over Raney copper have been examined at temperatures in the range 40 to 100°C. The first of these is the diffusion limited oxidation of the catalyst surface which produces oxides of copper. The other, more dominant, effect is thought to result from fouling of the catalyst pores by thermally polymerized acrylamide. This mechanism of catalyst poisoning which is independent of both reactant and product concentrations is evidenced by changes in specific surface areas and pore sizes of the catalyst used.     

Probing the electronic structure of an active catalyst surface under high-pressure reaction conditions: the oxidation of methanol over copper

The active phase of a bulk metallic copper catalyst is investigated by surface sensitive X-ray absorption spectroscopy at the oxygen K-edge and the Cu L-edges in the total electron yield mode under practical steady state flow-through conditions. The active catalyst surface contains oxygen atoms revealing significant spectral differences compared to those of known copper oxides. The partial oxidation of methanol to formaldehyde is correlated to the abundance of this copper suboxide. These oxygen atoms probe defects of the copper lattice, which represent catalytically active sites. The suboxide is undetectable under UHV conditions. The total oxidation of methanol is catalysed by a conventional copper(I) oxide species and the abundance of carbon dioxide in the gas phase is increasing with decreasing integrated intensity of the oxide species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics and Mechanism in the Liquid-Phase Hydrogenation of Maleic Anhydride and Intermediates

The influence of zinc oxide on the kinetics and mechanism of the liquid-phase hydrogenation of maleic anhydride (MA) and intermediates was investigated on copper-based catalysts. No influence of zinc oxide on the hydrogenation of maleic anhydride was observed in previous experiments. The discontinuous hydrogenation of succinic anhydride (SA) resulted in the formation of γ-butyrolactone (γ-BL) and 1,4-butanediol (1,4-BD) on a copper/zinc catalyst. On a zinc-free copper catalyst only γ-butyrolactone was formed while the hydrogenation of γ-butyrolactone to 1,4-butanediol was inhibited. It was observed that succinic anhydride which is adsorbed on the copper surface of the catalyst prevents the adsorption of γ-butyrolactone. On copper/zinc catalysts the reversible adsorption of succinic anhydride on the inactive zinc oxide crystallites, which led to a reversible decrease of the carbon balance, is responsible for a decrease of the succinic anhydride coverage of the copper sites. It appears that the decrease of the succinic anhydride coverage of the copper surface is proceeding by surface diffusion of succinic anhydride to the adjacent zinc oxide crystallites. On this basis two different reaction pathways via succinic anhydride adsorbed on the copper surface and via succinic anhydride adsorbed on the zinc oxide crystallites were proposed for the hydrogenation of maleic anhydride and intermediates. Kinetic modeling of the reaction pathway taking into account both reaction pathways led to good agreement of calculated and experimental results.     

Steam reforming of methanol using Cu-ZnO catalysts supported on nanoparticle alumina

Methanol steam reforming was studied over several catalysts made by deposition of copper and zinc precursors onto nanoparticle alumina. The results were compared to those of a commercially available copper, zinc oxide and alumina catalyst. Temperature programmed reduction, BET surface area measurements, and N₂O decomposition were used to characterize the catalyst surfaces. XRD was used to study the bulk structure of the catalysts, and XPS was used to determine the chemical states of the surface species. The nanoparticle-supported catalysts achieved similar conversions as the commercial reference catalyst but at slightly higher temperatures. However, the nanoparticle-supported catalysts also exhibited a significantly lower CO selectivity at a given temperature and space time than the reference catalyst. Furthermore, the turnover frequencies of the nanoparticle-supported catalysts were higher than that of the commercial catalyst, which means that the activity of the surface copper is higher. It was determined that high alumina concentrations ultimately decrease catalytic activity as well as promote undesirable CH₂O formation. The lower catalytic activity may be due to strong Cu-Al₂O₃ interactions, which result in Cu species which are not easily reduced. Furthermore, the acidity of the alumina support appears to promote CH₂O formation, which at low Cu concentrations is not reformed to CO₂ and H₂. The CO levels present in this study are above what can be explained by the reverse water-gas-shift (WGS) reaction. While coking is not a significant deactivation pathway, migration of ZnO to the surface of the catalyst (or of Cu to the bulk of the catalyst) does explain the permanent loss of catalytic activity. Cu₂O is present on the spent nanoparticle catalysts and it is likely that the Cu⁺/Cu⁰ ratio is of importance both for the catalytic activity and the CO selectivity.     

Effect of preparation method on structure and catalytic activity of Cr-promoted Cu catalyst in glycerol hydrogenolysis

Relationships between surface structure and catalytic properties were investigated for a series of copper chromium catalysts. The catalysts were prepared using methods involving impregnation and precipitation, and their catalytic activities were evaluated for the hydrogenolysis of glycerol. Catalyst (10I and 50I) prepared by the impregnation method contained a mixed phase of both individual copper and chromium oxide structures, while the catalyst (50P) prepared by precipitation showed a single phase, with a copper chromite spinel structure (CuCr₂O₄). XPS data indicated that, after the reduction step, the copper species in the impregnated catalyst was reduced to Cu⁰, but the catalyst prepared by the precipitation method retained a spinel structure evidenced by the large amount of Cu²⁺ species. In hydrogenolysis reactions, the precipitated catalyst showed a higher catalytic activity than the impregnated catalyst. Thus, the reduced copper chromite spinel structure, which constitutes a single phase, appears to be responsible for the high catalytic activity in the hydrogenolysis of glycerol to propylene glycol.     

Characterization of Copper Foam as Catalytic Material in Ethanol Dehydrogenation

Copper foam was used as a catalyst in ethanol dehydrogenation to acetaldehyde. Catalyst pretreatment, reaction temperature, liquid feed composition, and catalyst loading all affect ethanol conversion. Copper foam pretreated by oxidation yielded the highest ethanol conversion but deactivated due to copper surface reconstruction. The foam catalyst can be repeatedly reactivated by a short time exposure to air under reaction conditions. Yet, copper foam performance for ethanol dehydrogenation has been inferior in terms of activity and stability to that of supported copper catalysts.     

Steam reforming of methanol using Cu-ZnO catalysts supported on nanoparticle alumina

Methanol steam reforming was studied over several catalysts made by deposition of copper and zinc precursors onto nanoparticle alumina. The results were compared to those of a commercially available copper, zinc oxide and alumina catalyst. Temperature programmed reduction, BET surface area measurements, and N₂O decomposition were used to characterize the catalyst surfaces. XRD was used to study the bulk structure of the catalysts, and XPS was used to determine the chemical states of the surface species. The nanoparticle-supported catalysts achieved similar conversions as the commercial reference catalyst but at slightly higher temperatures. However, the nanoparticle-supported catalysts also exhibited a significantly lower CO selectivity at a given temperature and space time than the reference catalyst. Furthermore, the turnover frequencies of the nanoparticle-supported catalysts were higher than that of the commercial catalyst, which means that the activity of the surface copper is higher. It was determined that high alumina concentrations ultimately decrease catalytic activity as well as promote undesirable CH₂O formation. The lower catalytic activity may be due to strong Cu-Al₂O₃ interactions, which result in Cu species which are not easily reduced. Furthermore, the acidity of the alumina support appears to promote CH₂O formation, which at low Cu concentrations is not reformed to CO₂ and H₂. The CO levels present in this study are above what can be explained by the reverse water-gas-shift (WGS) reaction. While coking is not a significant deactivation pathway, migration of ZnO to the surface of the catalyst (or of Cu to the bulk of the catalyst) does explain the permanent loss of catalytic activity. Cu₂O is present on the spent nanoparticle catalysts and it is likely that the Cu⁺/Cu⁰ ratio is of importance both for the catalytic activity and the CO selectivity.     

Ce-Cu复合催化剂用于对硝基苯酚废水处理研究

通过络合法制得Ce-Cu复合催化剂,采用X射线衍射、扫描电子显微镜和程序升温还原对催化剂性能进行了研究,考察了该催化剂用于废水中对硝基苯酚去除性能。结果表明,催化剂中Cu氧化物能很好地溶入CeO2晶格形成表面具有多孔结构的Ce0.86Cu0.14O2固溶体催化剂,催化剂中Cu-O键得到一定程度的削弱,使其可还原性能得到提高而提高了催化剂中氧化物的活性。Ce0.86Cu0.14O2催化剂可将废水中对硝基苯酚的质量浓度由150 mg.L-1降到1.8 mg.L-1左右,对硝基苯酚去除率可达98.8%。     

Methanol Decomposition Over Copper/Silica: Effect of Temperature and Co-reactant on Carbon Deposition

The interaction of methanol with a copper/silica catalyst at 373 and 523K under reducing, oxidising and inert carrier gas flows has been studied. Under all conditions there is retained material associated solely with the copper. In general the retained species is adsorbed methanol/methoxy; only over an oxidised catalyst after treatment at 523K is there no evidence for adsorbed methanol/methoxy. Desorption of carbon dioxide is associated with an up-take in dioxygen indicating oxidation of a surface species, probably formate. After laydown under reducing or inert gas flow, the copper does not re-oxidise under the TPO gas flow, even at temperatures >673K indicating that material is still retained by the copper. Bulk re-oxidation of the reduced catalyst in the absence of retained species is rapid at 293K. Under oxidising conditions at 523K there is no evidence for adsorbed methanol/methoxy on the surface of the copper; in this case the retained species may be more akin to a carbonate.     

Selective oxidation of carbon monoxide over CuO–CeO2 catalyst: Effect of hydrothermal treatment

Copper oxide–ceria (CuO–CeO₂) catalyst for selective oxidation of carbon monoxide (CO) was prepared by co-precipitation and hydrothermal treatment methods and evaluated for catalytic activity in a reformate gas composition which simulated the produced gas from methanol steam reforming. By applying the condition of hydrothermal treatment, the catalytic activity of CuO–CeO₂ catalyst was increased and the operating temperature window, in which the concentration of carbon monoxide was lower than 10 ppm, was widened. From the thermogravimetric (TG) results of hydrothermally treated catalyst precursor, CuO–CeO₂ catalyst did not show any improvement in physical properties such as Brunauer Emmett Teller (BET) surface area, pore volume and average pore diameter, but the chemical stability might be enhanced by hydrothermal treatment. By hydrothermal treatment, cuprous ion in the CuO–CeO₂ catalyst migrated to the surface of catalyst resulting in increased surface concentration of copper and formation of cupric oxide on the surface of catalyst during calcination. While increasing the calcination temperature (i.e. above 800 °C), the phase separation occurred with a part of copper and cupric oxide was formed on the surface of catalyst which was observed in X-ray diffraction (XRD) analysis.     

Tailoring of surface melting of oxide based catalyst particles by doping to influence the growth of multi-walled carbon nano-structures

In catalytic chemical vapor deposition method, the growth of multi-walled carbon nano-tubes (MWCNT), nano-rods/fibers or nano-tapes takes place over cobalt oxide based catalyst particles at a growth temperature higher than the temperature for surface melting of the catalyst particles. Doping of cobalt oxide by copper oxide has been used to decrease the surface melting temperature of the oxide based catalyst particles in the size range of about 200–500 nm, which is important in determining the type of carbon nano structures growing over the catalyst. It has been observed that MWCNT forms at a small difference between the growth temperature and the surface melting temperature while nano-rods or nano-tapes form at relatively larger difference between these temperatures. The present study indicates the possibility of controlling surface melting temperature by the level of doping in order to get the desired nano-structure at a given growth temperature.     

Synthesis of DME from syngas on the bifunctional Cu–ZnO–Al2O3/Zr-modified ferrierite: Effect of Zr content

The catalytic activity on the coprecipitated Cu–ZnO–Al₂O₃/Zr-ferrierite (CZA–ZrFER) with different Zr content from 0 to 5 wt.% was investigated for the direct synthesis of dimethylether (DME) from H₂-deficient and biomass-derived model syngas (H₂/CO molar ratio = 0.93). The catalytic functionalities, such as CO conversion and DME selectivity, showed their maxima on the bifunctional catalyst with 3 wt.% Zr-modified ferrierite. Detailed characterization studies were conducted on the catalysts to measure their properties such as surface area, acidity by temperature-programmed desorption of ammonia (NH₃-TPD), reducibility of Cu oxide by temperature-programmed reduction (TPR), copper surface area measurements by N₂O titration method, electronic states of copper by IR analysis and particle size measurement by XRD and TEM analysis. The number of acid sites measured by NH₃-TPD on the bifunctional catalysts decreased monotonously with the increase of Zr content, meanwhile, the acidic strength is found to be minimal on the catalyst showing best performance. The reducibility of copper oxide and the surface area of metallic copper also exhibited their maximum values at the same Zr composition indicating that these are responsible for the optimum functionality of the bifunctional CZA–ZrFER catalyst. The role of easily reducible copper species with small particle size and the suppressed strong acidic sites is also emphasized in the consecutive reaction from syngas to DME on the bifunctional catalyst. The different behavior of intrinsic rate of the bifunctional catalysts is also well correlated with the metallic surface area of copper and the amount of acidic sites with their acidic strength.     

Effects of catalyst composition on methanol synthesis from CO2/H2

Effects of catalyst composition have been studied for Cu/support and Cu/ZnO/supports in methanol synthesis from CO₂/H₂. A strong effect of support has been observed. Different supports brought about different behavior in temperature-programmed reduction of copper, different copper surface areas, and different catalytic activity and selectivity. It seemed possible to find catalyst supports that might perform better than commercial Cu/ZnO/Al₂O₃ catalysts. A correlation was observed between catalytic activity and the copper surface area which was varied by using different supports. However, the sup]>orts appeared to influence other catalytic properties as well, for example, the surface oxygen coverage.     

The role of zinc oxide in Cu/ZnO catalysts for methanol synthesis and the water–gas shift reaction

All commercial catalysts for methanol synthesis and for the water–gas shift reaction in the low temperature region contain zinc oxide in addition to the main active component, copper. The varied benefits of zinc oxide are analysed here. The formation of zincian malachite and other copper/zinc hydroxy carbonates is essential in the production of small, stable copper crystallites in the final catalyst. Further, the regular distribution of copper crystallites on the zinc oxide phase ensures long catalyst life. Zinc oxide also increases catalyst life in the water–gas shift process by absorbing sulphur poisons but it is not effective against chloride poisons. In methanol synthesis, zinc oxide (as a base) removes acidic sites on the alumina phase which would otherwise convert methanol to dimethyl ether. Although bulk reduction of zinc oxide to metallic zinc does not take place, reduction to copper–zinc alloy (brass) can occur, sometimes as a surface phase only. A new interpretation of conflicting measurements of adsorbed oxygen on the copper surfaces of methanol synthesis catalysts is based on the formation of Cu–O–Zn sites, in addition to oxygen adsorbed on copper alone. The possible role of zinc oxide as well as copper in the mechanisms of methanol synthesis is still the subject of controversy. It is proposed that, only under conditions of deficiency of adsorbed hydrogen on the copper phase, hydrogen dissociation on zinc oxide, followed by hydrogen spillover to copper, is significant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions

Catalysts based on CuO/γ-alumina, CuAl₂O₄/γ-alumina, NiO/γ-alumina, NiAl₂O₄/γ-alumina and bulk CuAl₂O₄ have been structurally characterized by BET, porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their catalytic behaviors have also been tested for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature of 413 K. The copper and nickel catalysts supported on γ-alumina have surface areas of the same order as the support γ-alumina of ca. 190 m²/g and high active phase dispersions which were also confirmed by SEM, whereas the bulk copper aluminate spinel has a surface area of ca. 30 m²/g. XRD detects the phases present and shows a continuous loss of CuO by elution and the formation of a copper oxalate phase on the surface of the copper catalysts which also elutes with time. The NiO was also eluted but less than the copper catalysts. Only the copper and nickel spinel catalysts were stable throughout the reaction. Phenol conversion vs. time shows a continuous overall decrease in activity for the CuO/γ-alumina and NiO/γ-alumina catalysts. In turn, the copper and nickel spinel catalysts reach steady activity plateaus of 40 and 10%, respectively, of phenol conversion. The bulk copper aluminate spinel shows an activity plateau of 20% of the conversion which is lower than that from the copper aluminate/γ-alumina catalyst due to its lower surface area. Nickel catalysts always have lower activities than the copper catalysts for the phenol oxidation reaction. The copper catalysts drive a mechanism of partial phenol oxidation to carboxylic acids and quinone-related products with very high specific rates, and the nickel catalysts mainly drive a mechanism of CO₂ formation with lower conversion but with a potential higher catalyst life. The triphasic tubular reactor using trickle-bed regime largely avoids the mechanism of polymer formation as a catalyst deactivation process.     

Fatty methyl ester hydrogenation to fatty alcohol part II: Process issues

Fatty alcohols are produced by hydrogenating fatty methyl esters in slurry phase in the presence of copper chromite catalyst at temperatures of 250–300°C and hydrogen pressures of 2000–3000 psi. The fatty methyl ester, catalyst, and hydrogen are fed to the reactor cocurrently. The product slurry is passed through gas-liquid separators and then through a continuous filtration system for removal of the catalyst. A portion of the used catalyst in crude alcohol is recycled to the hydrogenator. The overall efficiency of the process depends upon the intrinsic activity, life, and filterability of the catalyst. The fatty alcohol producer therefore requires a catalyst with high activity, long life, and good separation properties. The main goal of the present laboratory investigation was to develop a superior copper chromite catalyst for the slurry-phase process. Two copper chromite catalysts, prepared by different procedures, were tested for methyl ester hydrogenolysis activity, reusability, and filtration characteristics. The reaction was carried out in a batch autoclave at 280°C and 2000–3000 psi hydrogen pressure. The reaction rates were calculated by assuming a kinetic mechanism that was first-order in methyl ester concentration. The catalyst with the narrower particle size distribution was 30% more active, filtered faster, and maintained activity for several more uses than the catalyst with the broader particle size distribution. X-ray photoelectron spectroscopy data showed higher surface copper concentrations for the former catalyst.     

Copper catalysts in the selective hydrogenation of soybean and rapeseed oils: IV. Copper on silica gel,phase composition and preparation

Phase composition of a copper on silica gel catalyst was studied with X-ray diffraction analysis. Activity measurements showed three periods of activity, the first two of which were ascribed to a copper surface subjected to reduction and the third one to the reduced form of the catalyst. Hydrogenation reaction over Cu/SiO₂ catalyst has a complex pressure dependence with a rate maximum at 6 atm in the low pressure range. Preparation of the catalyst was studied. On the basis of a proposed reaction model, a catalyst mixture was prepared and tested with good results. In rapeseed oil hydrogenation, Cu/SiO₂ catalysts were shown to be superior to copper chromite catalysts. In soybean oil the two types of catalyst were rather equivalent.     

Hydrogenation of dimethyl oxalate to ethylene glycol over mesoporous Cu‐MCM‐41 catalysts

The synthesis and utilization of mesoporous Cu‐MCM‐41 catalysts for hydrogenation of dimethyl oxalate to ethylene glycol is described in this article. Physicochemical properties of these Cu‐MCM‐41 catalysts have been investigated by N₂‐physisorption, X‐ray diffraction, inductively coupled plasma, N₂O titration, transmission electron microscopy, temperature programmed reduction, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. It was found that the copper loading significantly influenced the pore structure and copper surface area of the catalyst. High catalytic performance is obtained over a 20Cu‐MCM‐41 catalyst with a full DMO conversion and EG yield of 92% at a LHSV of 3.0 h^?1. The catalytic performance of optimized 20Cu‐MCM‐41 catalyst could be attributed to the fine copper dispersion and large copper surface areas. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2530–2539, 2013     

The striking behaviour of copper catalysing the gasification reaction of coal chars in dry air

When copper acts as catalyst in the reaction of oxygen with coal chars, at a certain temperature there is a very high increase in the reactivity value, reaching a region of very low apparent activation energy. This temperature has been identified with that in which the particles of the catalyst wet the surface of the carbon and become mobile. The influences of copper salt and method of preparation of the catalysts, rank of parent coals, residence temperature, time and percentage of Cu and the temperature at which the reactivity is increased have been studied. In some cases this temperature is up to 190 K below the Tammann temperature of CuO (831 K).     

The Effect of ZnAl2O4 on the Performance of Cu/ZnxAlyOx+1.5y Supported Catalysts in Steam Reforming of Methanol

This paper demonstrates the benefit of using spinel (ZnAl₂O₄) as a support for copper catalysts in hydrogen generation. We have investigated the influence of catalyst pre-treatment, support composition and copper content on the physicochemical and catalytic properties of copper catalysts supported on Zn_xAl_yO_x+1.5y in the methanol steam reforming. The physicochemical properties of the catalysts were examined by X-ray diffraction, temperature-programmed reduction, specific surface area and porosity, X-ray photoelectron spectroscopy, FTIR and chemisorption methods. The reduced copper catalysts showed higher conversion of methanol and higher hydrogen production. We also found that the presence of Cu⁺ and Cu⁰ species on the catalyst surface strongly influences the reaction yield and hydrogen production. FTIR measurements performed for copper catalysts confirmed that increasing of aluminium content in the case of catalytic systems caused the growth of adsorbed species on the catalyst surface.