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Applied Catalysis A, General (v.316, #1)

Editorial Board (pp. co2).

Contents (pp. iii-vi).

Phosgene-free approaches to catalytic synthesis of diphenyl carbonate and its intermediates by Jinlong Gong; Xinbin Ma; Shengping Wang (pp. 1-21).

This review attempts to describe and summarize recent literatures of process design and catalytic chemistry of phosgene-free approaches to catalytic synthesis of diphenyl carbonate and its intermediates. The advantages and disadvantages of these processes are discussed. Strategies to overcome potential problems are provided. The perspectives to improve catalytic efficiency of phosgene-free processes are proposed. ▪Diphenyl carbonate (DPC) is considered as a substitution for phosgene to synthesize polycarbonate resins. Conventional production of DPC involves reactions of phosgene and phenol. However, the phosgene process has drawbacks such as environmental and safety problems associated with using highly toxic phosgene as the reagent, which results in the formation of chlorine salts, and copious amounts of methylene chloride as the solvent. For these reasons, environmentally friendly processes for DPC production without using phosgene have been proposed and developed in the past decades. So far, the most promising alternatives appear to be the transesterification of dimethyl carbonate (DMC) and phenol, the direct oxidative carbonylation of phenol, and transesterification of dialkyl oxalates and phenol. This paper attempts to review recent literature concerning process design and catalytic chemistry for these phosgene-free approaches. The advantages and disadvantages are discussed for each reaction. Strategies to overcome potential problems are provided. The perspectives to improve catalytic efficiency of phosgene-free process are proposed.

Keywords: Review; Catalysis; Polycarbonates; Phosgene-free; Environmental; Diphenyl carbonate; Dimethyl carbonate; Methyl phenyl carbonate; Carbonylation; Transesterification; Dimethyl oxalate; Diethyl oxalate; Diphenyl oxalate; Methyl phenyl oxalate; Decarbonylation; Heterogeneous; Homogeneous; Supported catalyst; Palladium; MoO; ₃; TiO; ₂

Preparation, characterization and testing of Cr/AlSBA-15 ethylene polymerization catalysts by G. Calleja; J. Aguado; A. Carrero; J. Moreno (pp. 22-31).

Ethylene polymerization catalysts have been prepared by grafting chromium(III) acetylacetonate onto AlSBA-15 mesoporous materials. Attachment of Cr species onto AlSBA-15 surface results from the interaction with of hydroxyl groups, while a ligand exchange reaction may occur over siliceous SBA-15. Cr–AlSBA-15 catalyst with a Si/Al=30 is almost four times more active than a conventional Cr/SiO₂ Phillips catalyst. ▪Ethylene polymerization catalysts have been prepared by grafting chromium(III) acetylacetonate onto AlSBA-15 (Si/Al=∞, 156, 86 and 30) mesoporous materials. A combination of XRD, nitrogen adsorption, TEM, ICP-atomic emission spectroscopy, H₂-TPR, TGA, UV–vis and FT-IR spectroscopy, were used to characterize the prepared Cr–AlSBA-15 catalysts. By reducing the Si/Al ratio of the AlSBA-15 supports increases the amount of chromium anchored, promotes the stabilization of chromium species as chromate and decreases the reduction temperature of Cr⁶⁺ ions determined by H₂-TPR. Attachment of Cr species onto AlSBA-15 surface results from the interaction of hydroxyl groups with the acetylacetonate ligands through H-bonds. On the contrary, a ligand exchange reaction may occur over siliceous SBA-15.The polymerization activity of Cr–AlSBA-15 catalysts is significantly improved by increasing aluminium content of the AlSBA-15 supports. Particularly, the chromium catalyst prepared with AlSBA-15 (Si/Al=30) support is almost four times more active than a conventional Cr/SiO₂ Phillips catalyst. Polymers obtained with all the catalysts showed melting temperatures, bulk densities and high load melt indexes indicating the formation of linear high-density polyethylene.

Keywords: Mesoporous; SBA-15; AlSBA-15; Chromium; Ethylene polymerization

Internal versus external surface active sites in ZSM-5 zeolite by Mehran Ghiaci; Alireza Abbaspur; Mohammad Arshadi; Behzad Aghabarari (pp. 32-46).

Vapor-phase methylation of toluene with methanol and isopropylation of toluene with 2-propanol has been investigated in a down flow reactor under atmospheric conditions using N₂ gas carrier over a series of surface modified and unmodified ZSM-5 (Si/Al=60–170) loaded with H₃PO₄, differing in the external surface treatment of the zeolites. The feed molar ratios of toluene/methanol and toluene/2-propanol were varied over a wide range (8–0.125), and the optimum feed ratio of toluene/alcohol was less than 0.5 in both cases. Space velocity employed in toluene methylation reported as WHSV (toluene)=1.2h⁻¹, and the space velocity employed in toluene isopropylation reported as WHSV (toluene)=0.8h⁻¹. The methylation reactions were carried out in the temperature range of 623–773K, and the isopropylation reactions were carried out in the temperature range of 483–583K. Atmospheric pressures was maintained in all runs. Catalysts containing 0–4.9wt.% P were prepared using modified and unmodified ZSM-5 zeolites, and their catalytic performance for vapor-phase alkylation of toluene with methanol and 2-propanol were investigated. The optimum phosphorous content for methylation was 2.1wt.% P which was greater than the optimum phosphorous loading for isopropylation (0.7wt.% P).Vapor-phase methylation of toluene with methanol and isopropylation of toluene with 2-propanol has been investigated in a down flow reactor under atmospheric conditions using N₂ gas carrier over a series of surface modified and unmodified ZSM-5(Si/Al=60–170) loaded with H₃PO₄. ▪

Keywords: Toluene alkylation; 2-Propanol; H; ₃; PO; ₄; Modified ZSM-5; Cymene; Xylene; Catalyst

CO₂ reforming of methane to syngas over highly active and stable supported CoO_x (accompanied with MgO, ZrO₂ or CeO₂) catalysts by Kartick C. Mondal; Vasant R. Choudhary; Upendra A. Joshi (pp. 47-52).

Catalytically inactive CoO_x–MgO (calcined at 900°C) becomes highly active and selective in the CO₂ reforming of methane to syngas when it is deposited on a commercial low surface-area macro-porous silica–alumina support (SA-5205) or when CoO_x is deposited on the support precoated with MgO. Both the supported CoO_x and MgO containing catalysts (CoO_x–MgO/SA-5205 and CoO_x/MgO/SA-5205) show high methane conversion (>98%) activity and H₂ selectivity (>94%) and also high stability against carbon deposition [rate of carbon deposition≤0.26mg (C) g (catalyst)⁻¹h⁻¹] in the CO₂ reforming (at 850°C, GHSV=20,000cm³g⁻¹h⁻¹). The CoO_x/MgO/SA-5205 is, however, more active and stable than the CoO_x–MgO/SA-5205. The supported catalyst prepared by depositing CoO_x on SA-5205 precoated with ZrO₂, CeO₂, Y₂O₃ or ThO₂ also shows very high stability against the carbon deposition [rate of carbon deposition<0.1mg(C)g(catalyst)−1 h⁻¹]. The choice of the metal oxide used for precoating the support for achieving better catalyst performance is in the following order: MgO>ZrO₂>CeO₂>Y₂O₃>ThO₂. The type of support (silica–alumina or zirconia–haffnia) also plays important role in deciding the catalyst performance; the catalysts with the commercial zirconia-haffnia support (SZ-5564) show, in general, poor activity with higher rates carbon deposition. In the absence of MgO or the other metal oxide, the catalyst (CoO_x/SA-5205 or SZ-5564) shows very poor activity with higher carbon deposition rate.

Keywords: Supported CoO; _x; catalysts; CO; ₂; reforming of methane; Carbon formation in CO; ₂; reforming of methane; Syngas

Improved selectivity in the toluene alkylation reaction through understanding and optimising the process variables by J.P. Breen; R. Burch; M. Kulkarni; D. McLaughlin; P.J. Collier; S.E. Golunski (pp. 53-60).

It has been demonstrated that it is possible to produce p-xylene with >99.9% selectivity by optimising the process variables of the toluene alkylation reaction over a B/ZSM-5 catalyst. This has been achieved specifically by minimising the undesirable isomerisation of p-xylene on the external acid sites of the catalyst by controlling the contact time. ▪It has been demonstrated that it is possible to produce p-xylene with near-perfect selectivity by optimising the process variables of the toluene alkylation reaction over a B/ZSM-5 catalyst. This has been achieved specifically by minimising the undesirable isomerisation of p-xylene on the external acid sites of the catalyst by controlling the contact time. This offers a superior route to high selectivity compared to multiple pre-treatments of the catalyst to eliminate external acid sites (for example, by silanisation). Additionally, a high toluene:methanol feed ratio is beneficial because it minimises the methanol dehydration reaction. A further benefit is that the maximum theoretical conversion of toluene is limited, which diminishes p-xylene product inhibition. These findings confirm that toluene alkylation is best operated in a multistage reactor, but also highlight the need for removal of p-xylene as well as addition of methanol between stages. The method of catalyst preparation is not critical, provided that there is sufficient boron present (≥6.5wt%) and any boron lost through sublimation during the reaction is replenished. It has been found that an active catalyst can be produced in situ by either loading a physical mixture of hydroboric acid and HZSM-5 into the reactor prior to reaction or even placing a bed of hydroboric acid upstream from a bed of HZSM-5. The in situ-formed catalyst readily achieves >99.9% selectivity to the desired p-xylene isomer, under optimised conditions.

Keywords: Toluene alkylation; ZSM-5 zeolite catalyst; Boron; Magnesium; Methanol; p; -Xylene

Aromatization activity of gallium containing MFI and TON zeolite catalysts in n-butane conversion: Effects of gallium and reaction conditions by Dmitry B. Lukyanov; Tanya Vazhnova (pp. 61-67).

A kinetic study of n-butane conversion over acidic and gallium (Ga) containing MFI and TON zeolites has revealed that Ga active sites create a new pathway for aromatics formation via dehydrogenation reaction steps. This pathway does not involve bulky bimolecular hydrogen transfer steps of the aromatization process over acidic zeolites, and, as a consequence, leads to considerably higher enhancement of the aromatization activity of the one-dimensional TON catalyst when compared to the three-dimensional MFI catalyst. This finding highlights fundamentally different spatial requirements for alkane aromatization over acidic and Ga containing zeolites and indicates that the zeolites with severe spatial constraints could become very selective catalysts for alkane aromatization after their modification with Ga. It is anticipated that these results will initiate the search for new, highly selective aromatization catalysts based on zeolites with different structures. The second important finding of this work is the evolution of the aromatization activity of GaH-TON and GaH-MFI catalysts during n-butane reaction that is likely associated with formation of catalytically active Ga⁺ ions. In our study, this process was completed in about 15min, i.e. much faster then similar processes that were reported earlier in the literature for the GaH-MFI catalysts. To the best of our knowledge, no data on the evolution of the aromatization activity was reported up to day for the one-dimensional GaH-TON catalysts.This paper highlights fundamentally different spatial requirements for alkane aromatization over acidic and gallium containing zeolites and indicates that the zeolites with severe spatial constraints could become very selective catalysts for alkane aromatization after their modification with gallium. It is anticipated that our results will initiate the search for new, highly selective aromatization catalysts based on zeolites with different structures. ▪

Keywords: Acid sites; Gallium sites; Dehydrogenation; Alkane aromatization; Bifunctional catalysts

On the mechanisms and the selectivity determining steps in syngas conversion over supported metal catalysts: An IR study by V. Sanchez-Escribano; M.A. Larrubia Vargas; E. Finocchio; G. Busca (pp. 68-74).

An IR study of syngas and methanol conversion has been performed over Cu–ZnO–Al₂O₃ methanol synthesis catalyst, Ni–Al₂O₃ methanation catalyst and Co–Al₂O₃ Fischer Tropsch catalyst. The data, obtained at low pressure, provide unequivocal evidence of the existence of a way via oxygenated intermediates (formates, possibly dioxymethylene, methoxy groups) in the three cases. In the selectivity determining step, methoxy groups desorb associatively as methanol on Cu–ZnO–Al₂O₃. Methoxy groups are selectively hydrogenolyzed to methane over Ni–Al₂O₃. Over Co–Al₂O₃ oxygenated surface species may be involved in the chain growth to give C₂₊ compounds. It is possible that this mechanism coexists with the via-carbide all-metallic catalysis reported for methanation and FT synthesis, on the basis of studies performed on pure metals.

Keywords: Methanol synthesis; Methanation; Fischer Tropsch synthesis; IR spectroscopy; Oxygenated intermediates; Syngas conversions; Selectivity; Cu–ZnO–Al; ₂; O; ₃; catalyst; Ni–Al; ₂; O; ₃; catalyst; Co–Al; ₂; O; ₃; catalyst

Influence of the Si/Al ratio and crystal size on the acidity and activity of HBEA zeolites by A. Simon-Masseron; J.P. Marques; J.M. Lopes; F. Ramôa Ribeiro; I. Gener; M. Guisnet (pp. 75-82).

Three HBEA samples were prepared through ion exchange followed by calcination under dry air flow at 550°C from parent samples synthesized with different total Si/Al ratios or crystal sizes and characterized by various techniques. The denomination, Si/Al ratio and crystal size were the following—Cal: 12.5, ∼20nm; LC: 14.5, 3000–10,000nm; RAl: 7.0, ∼50nm. Pyridine adsorption followed by FTIR shows very different values of the Lewis/Brønsted site concentration ratio: 1.2 with Cal, 0.2 with LC and 0.6 with RAl, which indicates a predominant positive effect of the crystal size on the framework stability. Methylcyclohexane transformation at 450°C was found to occur through both protolysis and the carbenium ion chain mechanism. Protolysis plays a significant role on Cal only. This very difficult reaction which requires very strong protonic sites, most likely sites resulting from interaction of bridging OH groups with neighbouring Lewis sites was 3 and 12 times faster on Cal than on RAl and LC samples, respectively. Furthermore, from the low turnover frequency value of the protonic sites of LC, severe diffusion limitations were demonstrated to occur on this large crystal size sample.Three HBEA samples were prepared with different values of Si/Al ratio and crystal size (from 7 to 14.5 and 20 to 10,000nm, respectively). Textural, structural and acid properties were examined. Catalytic characterization was performed by methylcyclohexane transformation; product distribution was investigated and the reaction rates of both protolytic and carbenium ion chain mechanism were estimated. ▪

Keywords: BEA zeolite; Crystal size; Si/Al ratio; Acidity; Methylcyclohexane cracking

Method of catalyst coating in micro-reactors for methanol steam reforming by Sun-Mi Hwang; Oh Joong Kwon; Jae Jeong Kim (pp. 83-89).

Micro-channels of silicon-based micro-reactors were successfully coated with deionized (DI) water-based Cu–ZnO–Al₂O₃ catalyst slurry by a fill-and-dry coating method, applicable to pre-assembled micro-reactors, for steam reforming of methanol. The 10–20μm thick catalyst layers could be formed on the inner walls of the micro-channels after the micro-channels were fully filled with catalyst slurry, because the catalyst particles in the slurry cohered to the walls of micro-channels by surface tension during drying and calcinations. The adhesion between the catalyst layer and silicon surface was improved by pre-coating the micro-channels with an alumina adhesion layer. The addition of polyvinyl alcohol (PVA) in the alumina sol resulted in better adhesion of the alumina layer at the corners of the channels. The critical minimum thickness of the alumina layer for catalyst coating was 0.15μm. The highest catalytic activity without loss of intrinsic catalytic activity was obtained using 1:5 (catalyst to solvent) DI water-based catalyst layers coated by fill-and-dry coating. The maximum H₂ production rate was 85ccm with 1650ppm of CO measured at 300°C using a methanol feed rate of 9ml/h.This paper describes a fill-and-dry coating method using slurry-type catalyst in the micro-channels of pre-assembled micro-reactors for methanol stream reforming. We found that the catalyst activity was affected by the kinds of solvents as well as the ratio of catalyst to solvent of the slurry. In addition, a thin adhesive layer was required to improve the adhesion between the substrate surface and the catalyst layer. ▪

Keywords: Micro-reactor; Micro-channel; Catalyst coating; Reformer

Coke formation and reaction pathways of catalyst-surface-generated radicals during the pyrolysis of ethane using Ni mesh catalyst by Woo Jin Lee; Chun-Zhu Li (pp. 90-99).

During the catalytic pyrolysis of C₂H₆, the radicals that formed on the nickel mesh catalyst surface easily desorb at high gas flow rates. The desorption of radicals greatly limits the formation of coke on the catalyst surface. The mesh catalyst could have dual roles as a terminator and a re-initiator for the radical chain reactions in the gas phase. ▪The use of a conventional porous catalyst in the study of the pyrolysis of ethane (or any hydrocarbon) is complicated because reactions take place both on the catalyst surface and in the gas phase (including pores). A non-porous nickel mesh catalyst was used to study the catalytic pyrolysis of C₂H₆ in a quartz tubular reactor at 750°C. Our experimental results strongly indicate that the radicals formed on the catalyst surface could easily desorb from the catalyst surface when the gas film around the mesh wires was thinned by increasing the gas flow rate passing through the mesh. The desorption of radicals from the catalyst surface greatly limited the formation of coke on the catalyst surface, leading to sustained high catalytic activities even in pure C₂H₆ at 750°C. The catalyst could have dual roles. It could act as a chain reaction terminator by providing a surface for the chain termination reactions to take place. The mesh catalyst could also act as a chain reaction re-initiator by providing additional radicals into the gas phase in which the desorbed radicals re-initiate and participate in the chain reactions. The dual roles of the mesh catalyst were largely responsible for the changes in net product formation rates and product distribution (e.g. the C₂H₄/CH₄ ratio).

Keywords: Pyrolysis; Radical; Mesh catalyst; Nickel; Ethane; Coke formation

Kinetic characterization of a bioinspired, heterogeneously active macromolecular catalyst for phenol oxidation and coupling reactions by Chandra Shekhar Purohit; Masood Parvez; Sandeep Verma (pp. 100-106).

This report describes a novel cytosine containing copper metalated polymeric matrix possessing phenol oxidative and coupling activities. ▪We have synthesized and characterized a novel copper metalated polymeric matrix of modified nucleobase cytosine that catalyses oxidation of phenolic substrates. This metalated polymer is insoluble in common organic solvents, thus catalysis is heterogeneous in nature so that the material can be reused. The molecular structure of the monomer and the metalated monomer, a Michaelis–Menten kinetic analysis of oxidative transformations using different phenolic substrates and mechanistic investigations are presented in this paper.

Keywords: Cytosine; Copper; Heterogeneous catalysis; Phenol oxidation

The effect of ceria content on the properties of Pd/CeO₂/Al₂O₃ catalysts for steam reforming of methane by L.S.F. Feio; C.E. Hori; S. Damyanova; F.B. Noronha; W.H. Cassinelli; C.M.P. Marques; J.M.C. Bueno (pp. 107-116).

One series of Pd/CeO₂–Al₂O₃ catalysts with various Ce loading was studied in the steam reforming of methane (1). The CH₄ turnover rate of catalysts with ceria loading ≥12wt.% was around four orders of magnitude higher compared to that of Pd/Al₂O₃ catalyst. The increase of the activity was attributed to increase of the accessibility of CH₄ to metal active sites. ▪The effect of CeO₂ loading on the surface properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pd catalysts was studied in the process of steam reforming of methane. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), UV–vis diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). The XRD measurements indicated that palladium particles on the surface of fresh and reduced catalysts are well dispersed. TPR experiments revealed a heterogeneous distribution of PdO species over CeO₂–Al₂O₃ supports; one fraction of large particles, reducible at room temperature, another fraction interacting with CeO₂ and Al₂O₃, reducible at higher temperatures of 347 and 423K, respectively. The PdO species reducible at room temperature showed lower CO adsorption relative to the PdO species reducible at high temperature. In contrast to Pd/Al₂O₃, the FTIR results revealed that CeO₂-containing catalyst with CeO₂ loading ≥12wt.% show lower ratio (LF/HF) between the intensity of the CO bands in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). This ratio was constant with increasing the temperature of reduction. The FTIR spectra and the measurement of Pd dispersion suggested that Pd surface becomes partially covered with ceria at all temperature of reduction and with increasing ceria loading in Pd/CeO₂–Al₂O₃ catalysts. Although the PdO/Al₂O₃ showed higher Pd dispersion compared to that of CeO₂-containing catalysts, the addition of ceria resulted in an increase of the turnover rate and specific rate to steam reforming of methane. The CH₄ turnover rate of Pd/CeO₂–Al₂O₃ catalysts with ceria loading ≥12wt.% was around four orders of magnitude higher compared to that of Pd/Al₂O₃ catalyst. The increase of the activity of the catalysts was attributed to various effects of CeO₂ such as: (i) change of superficial Pd structure with blocking of Pd sites; (ii) the jumping of oxygen (O^*) from ceria to Pd surface, which can decrease the carbon formation on Pd surface. Considering that these effects of CeO₂ are opposite to changes of the reaction rate, the increase of specific reaction rate with enhancing the ceria loading suggests that net effect results in the increase of the accessibility of CH₄ to metal active sites.

Keywords: Pd catalysts; CeO; ₂; –Al; ₂; O; ₃; carriers; Steam reforming of methane; XRD; TPR; FTIR

A new catalyst material based on niobia/iron oxide composite on the oxidation of organic contaminants in water via heterogeneous Fenton mechanisms by L.C.A. Oliveira; M. Gonçalves; M.C. Guerreiro; T.C. Ramalho; J.D. Fabris; M.C. Pereira; K. Sapag (pp. 117-124).

The present work describes the preparation of novel materials based on niobia (Nb₂O₅)/iron oxides and their use as catalyst on oxidizing reactions of organic compounds. The decomposition study was realized with a basic dye as a model molecule: the methylene blue. The analysis of the products, with electrospray ionization mass spectrometry (ESI-MS), showed that the methylene blue was successively oxidized (hydrolyzed) through different intermediate species. ESI mass spectra in the positive ion mode for monitoring the oxidation of methylene blue dye in water by the niobia:iron oxide 1:1 and H₂O₂ system at different reactions times. ▪The present work describes the preparation of novel materials based on niobia (Nb₂O₅)/iron oxides and their use as catalyst on oxidizing reactions of organic compounds in aqueous medium with hydrogen peroxide. These new composites were prepared by mixing natural niobia and iron oxides and were characterized with powder X-ray diffraction (XRD), chemical analyses, scanning electron microscopy (SEM), and⁵⁷Fe Mössbauer spectroscopy. Results showed that the main iron oxides so formed were goethite (αFeOOH) and maghemite (γFe₂O₃) with small particle sizes. The decomposition study was realized with a basic dye as a model molecule: the methylene blue. The analysis of the products, with electrospray ionization mass spectrometry (ESI-MS), showed that the methylene blue was successively oxidized (hydrolyzed) through different intermediate species. These results strongly suggest that highly reactive hydroxyl radicals generated from the H₂O₂ on the surface of the 1:1 niobia:iron oxide composite act as an efficient heterogeneous Fenton catalyst

Keywords: Oxidation; Heterogeneous Fenton; Iron oxides; Hydrogen peroxide

Corrigendum to “Hydroconversion of n-hexadecane over Pt/WO_x–ZrO₂ catalysts prepared by a PVA-template coprecipitation route The effect of tungsten surface coverage on activity and selectivity�? [Appl. Catal. A: Gen. 309 (2006) 224–236] by A. Martínez; G. Prieto; M.A. Arribas; P. Concepión (pp. 125-125).

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Applied Catalysis A, General (v.316, #1)