Applied Catalysis A, General (v.203, #1)
The advantages of ozone treatment in the preparation of tubular silica structures by Imre Kiricsi; Ágnes Fudala; Zoltán Kónya; Klára Hernádi; Patrick Lentz; János B Nagy (L1-L4).
We applied ozone for removal of templates from meso- and nanoporous silicate of tubular structure. This treatment resulted in MCM-41 and silicon nanotube products of high quality.
Keywords: Template removal; Ozone treatment; Meso- and nanoporous silicates;
Skeletal isomerisation of n-butenes present in C4 pyrolysis residue fraction by Vendelı́n Macho; Milan Králik; Ludovit Jurecek; Emilia Jurecekova; Jarmila Balazova (5-14).
Butenes present in butadiene and isobutene free C4 fraction (a stream from the production of methyl-tert-butyl ether — so-called raffinate II) were isomerised over the hydrogen-chlorided and sulphated catalysts at 350–515°C. A natural clay (bentonite) and commercial γ-alumina were used as catalyst precursors. Hydrogen-chlorided catalysts were deactivated in about 6 h by 10–15% with respect to butene conversion, and a regeneration with air doped hydrogen chloride did not allow to renew the catalytic activity to full extent. This behaviour should be ascribed to high volatility of chlorides which, moreover, pollute the product as itself and in the form of chlorinated organic compounds. On the other hand, catalysts prepared by treatment of calcinated and evacuated γ-alumina with diluted sulphuric acid, proved to be sufficiently active in 2500 h lasting experiments, whereas regeneration with air was necessary every 4–6 h. This stability is probably caused by high melting point of aluminium sulphate which is formed in the course of the catalytic preparation and treatment. The selectivity, 50% of the isobutene formation, was achieved at approximately 50% conversion of butenes, 450°C, GHSV=0.6 g cm−3 h−1. Experiments at higher pressure allowed to increase the conversion, but larger amount of components with higher molar weight was formed. Methanol and methyl-tert-butyl ether did not effect the process of skeletal isomerisation significantly, but a presence of butadiene strongly enhanced deactivation of the catalyst.
Keywords: Butenes; Isobutene; Skeletal isomerisation; Alumina; Hydrogen chloride; Sulphuric acid; Tetraethoxysilane; Deactivation; Methanol; Methyl-tert-butyl ether; Butadiene;
Characterization of rice husk ash-supported nickel catalysts prepared by ion exchange by Ming-Tseh Tsay; Feg-Wen Chang (15-22).
The rice husk ash-supported nickel catalysts prepared by ion exchange technique and the support itself have been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). In addition to BET surface area, total pore volume and average pore diameter were also measured. The metal surface area and the crystal size of nickel were measured by the selective chemisorption of hydrogen. The effect of calcination temperature on the catalyst reduction was investigated by the temperature-programmed reduction (TPR) technique. The results show that nickel silicate with a layer structure formed after drying step. The BET surface area increases with the nickel loading of the catalyst. The nickel surface area increases with nickel loading up to 16.7 wt.% Ni and then decreases with further increase in nickel loading. The dispersion of nickel gradually decreases with nickel loading. Furthermore, the mean size of nickel crystallites increases with nickel loading. The reduction temperature increases with the increase of calcination temperature. Reduction of NiO from the thermal decomposition of the layered nickel silicates is found to be unusually difficult.
Keywords: Rice husk ash; Silica support; Nickel catalyst; Characterization; Ion exchange;
Latest developments in microactivity testing: influence of operational parameters on the performance of FCC catalysts by D Wallenstein; A Haas; R.H Harding (23-36).
An accurate assessment of catalyst performance is the most important goal in the testing of fluid catalytic cracking (FCC) catalysts since the catalyst is a key contributor to the overall unit performance. For this purpose small scale tests in the laboratory have been the workhorse in the industry because they are less expensive and time-consuming to operate than circulating riser pilot units. Two reactor types are common in microactivity testing, simple fixed bed (plug-flow) reactors and fixed fluidised bed reactors. Pitfalls have been identified for both experimental modes and this paper discusses the strengths and weaknesses of these two techniques. This work demonstrates that catalyst testing in small-scale fixed fluidised bed reactors can result in erroneous catalyst ranking, while the use of improved fixed bed reactors has a better agreement with riser pilot units. Furthermore, reactors of either type that employ in-situ regeneration result in unrealistically high coke and hydrogen yields due to the oxidation of contaminant metals, and therefore advanced deactivation procedures for metallated FCC catalysts cannot be utilised in small-scale tests with this technique.
Keywords: Fluid catalytic cracking; Microactivity tests; Fixed bed reactors; Fixed fluidised bed reactors;
Catalytic behaviour of Cl-free and Cl-containing Pd/Al2O3 catalysts in the total oxidation of methane at low temperature by Delphine Roth; Patrick Gélin; Michel Primet; Emmanuel Tena (37-45).
Pd/Al2O3 catalysts prepared by the impregnation of alumina free of chlorine with H2PdCl4 and Pd(NO3)2 precursors were characterised and their catalytic activity in methane oxidation was measured under oxygen-rich conditions. The presence of residual chlorine ions either originating from the precursor or from impregnation with HCl was shown to strongly inhibit the conversion of methane. Residual chlorine ions could be slowly removed under reaction conditions in the form of HCl. This was observed by in situ measurement of HCl departure and catalytic activity. Experimental data were explained by a simple model in which residual chlorine would block PdO surface active sites during the reaction. The resulting catalyst, free of chlorine ions, was similar to the one prepared from the nitrate precursor in terms of dispersion and activity. Chlorine-free catalysts exhibited slow deactivation with time on stream, which was tentatively attributed to the slow conversion of the active PdO phase into a less active Pd(OH)2 phase. This interpretation was supported by the poisoning effect of water vapour observed upon addition of water to the feed while CO2 had no influence. Regeneration of the catalyst could be achieved by purging in dry carrier above 500°C, which could be ascribed to the decomposition of Pd(OH)2 into PdO.
Keywords: Pd/Al2O3 catalysts; Methane; Chemisorption; Catalytic combustion; Chlorine poisoning;
Selective alkylation of biphenyl with t-butanol over large pore zeolites by Jana Horniaková; Dušan Mravec; Soňa Faboková; Milan Hronec; Patrice Moreau (47-53).
Alkylation of biphenyl with t-butanol under liquid phase conditions has been studied over HY and Hβ zeolites with different silicon to aluminum molar ratios as catalysts. HY zeolites have been shown to be more active than Hβ zeolites. A maximum of 58% for the biphenyl conversion has been obtained over HY (15) zeolite at 160°C. The initial catalytic activity of studied zeolites for t-butylation of biphenyl at 160°C can be set in the next order: HY (15)>HY (30)>Hβ (12.5)≅Hβ (25). Both types of zeolites showed a significant selectivity of the monoalkylation step versus di- and polyalkylation with a very high selectivity leading to the 4-t-butylbiphenyl derivative (4-TBB) (near 95% for HY and 87% for Hβ). Moreover, a high initial selectivity in 4,4′-di(t-butyl)biphenyl (4,4′-DTBB) was obtained (near 82% for HY and 90% for Hβ at 160°C). Such a selectivity remained constant with time over HY, while it dramatically decreased over Hβ. Higher temperatures and higher catalyst concentration led to an increase of secondary reactions, which have been evidenced as isomerization of 4-TBB to 3-TBB, 4,4′-DTBB to 3,4′- and 3,3′-DTBB, oligomerization of isobutylene, alkylation of biphenyl with oligomers to higher alkyl- and polyalkylbiphenyls together with disproportionation and transalkylation of alkylbiphenyls.
Keywords: Alkylation; Biphenyl; t-Butanol; 4,4′-Dialkylbiphenyls; Zeolites; Shape-selectivity; Heterogeneous catalysis;
Obtaining tungsten carbides from tungsten bipyridine complexes via low temperature thermal treatment by S Wanner; L Hilaire; P Wehrer; J.P Hindermann; G Maire (55-70).
Tungsten carbides were prepared by thermal treatment under nitrogen of a tungsten bipyridine complex. The catalysts obtained have been characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and elemental analysis. The reaction with 2-methylpentane was used as a chemical probe for the formation of tungsten carbide.
Keywords: Tungsten carbides: preparation; Characterization;
Heterogeneous asymmetric reactions by István Kun; Béla Török; Károly Felföldi; Mihály Bartók (71-79).
The enantioselective hydrogenation of 2-methyl-2-pentenoic acid (MPA) over a cinchona modified 5% Pd/Al2O3 catalysts is described. The main experimental variables studied were the MPA/catalyst and the cinchonidine (CD)/MPA ratios, the effect of ultrasonic irradiation and the modifier structure. Low MPA/catalyst ratio was found to be advantageous for obtaining enhanced enantioselectivity, while higher CD/MPA ratios resulted in increasing enantioselectivities according to a saturation type curve. The ultrasonic pretreatment (similarly to α-ketoesters) also slightly increased the enantiomeric excesses. These modifications of the CD–Pd/Al2O3 system, resulted in enantioselectivities up to 66% ee (0°C, 10 min sonication, CD/MPA=0.5, 50 bar H2 pressure) for (S)-2-methyl-pentanoic acid. The variation of the modifier structure provided unique information about the nature of the cinchona–substrate interaction. It has been revealed that the OH group of the alkaloid should be involved in the substrate–modifier interaction which more likely occurs in the liquid phase. An intermediate with refined structure is proposed and the mechanism is suggested to be of stoichiometric nature.
Keywords: Enantioselective hydrogenation; Cinchona derivatives; Ultrasound; Pd/Al2O3;
Partial oxidation of methane to formaldehyde over Mo/HZSM-5 catalysts by A de Lucas; J.L Valverde; L Rodriguez; P Sanchez; M.T Garcia (81-90).
Two Mo/HZSM-5 catalyst series have been tested in the partial oxidation of methane to formaldehyde at atmospheric pressure. Conventional aqueous impregnation with ammonium heptamolybdate was used for preparing catalysts with Mo loading ranging from 0.3 to 3.3 wt.%. Two different molybdenum species were detected in these catalysts: tetrahedral monomeric Mo(VI) species and octahedral coordinated polymolybdate species. For molybdenum loading higher than 1.3 wt.%, polymolybdate species are predominant causing a decrease in the dispersion and a partial blocking up of the HZSM-5 zeolite channels. A second catalyst series were prepared from HZSM-5 zeolite by aqueous impregnation and subsequent washing with an aqueous ammonia solution, yielding Mo/HZSM-5 catalysts with Mo contents ranging from 0.08 to 0.45 wt.%. A well-dispersed molybdenum oxide phase mainly constituted of tetrahedral monomeric species joined to the zeolite surface was characterized. Both catalyst series show different activity in methane conversion. Polymolybdate species of impregnated Mo/HZSM-5 catalysts are more reactive in methane activation than acid sites of HZSM-5 but lead to low HCOH selectivity values. In contrast, treated catalysts show lower methane conversion but higher HCOH selectivity. These results demonstrate the specificity of the MoO sites of monomeric species for HCOH production.
Keywords: Oxidation of methane (partial); Formaldehyde; Mo/HZSM-5 catalyst; Supported molybdenum species; MoO sites;
Synthesis of Bisphenol-A over heteropoly compounds encapsulated into mesoporous molecular sieves by Krystyna Nowiǹska; Waldemar Kaleta (91-100).
The 12-tungstophosphoric acid (HPW) and its insoluble salts containing either caesium or ammonium cations were encapsulated into channels of a MCM-41 molecular sieve and they were used as catalyst for the liquid phase synthesis of Bisphenol-A from phenol and acetone. For comparison, zeolites ZSM-5, H-Y and dealuminated H-DY have also been tested in this reaction. The CsHPW/MCM system appeared to be the most selective towards p,p′ isomer formation.
Keywords: Bisphenol-A; Phenol; Acetone; Heteropoly acids; Mesoporous material;
Comparison of nitric and oxalic acid in the dealumination of mordenite by R Giudici; H.W Kouwenhoven; R Prins (101-110).
The dealumination of mordenite was carried out at 100°C with nitric and oxalic acid; after calcination, the samples were characterised using various techniques. The bulk composition showed that both acids were very effective in removing aluminium, but that aluminium was extracted at a higher rate by oxalic acid even at low concentrations. Nitrogen adsorption isotherms and scanning electron microscopy (SEM) images showed the formation of a secondary pore system in the zeolite after treatment with nitric acid but not with oxalic acid. NMR analysis revealed that, at the same Si/Al ratio, the mordenite was almost free of extra-framework aluminium (EFAl) after nitric acid extraction, whereas after extraction with oxalic acid half of the aluminium was octahedrally co-ordinated. The Si(0Al) peak in the 29Si NMR was found to be split when dealumination with nitric acid was carried out due to the presence of less EFAl. Treatment with oxalic acid led to the formation of a larger number of lattice defects.
Keywords: Oxalic acid; Secondary pore system; NMR spectroscopy; Extra-framework aluminium; Lattice defects;
Coupling of chemical, electrochemical and ultrasonic energies for controlled generation of hydroxyl radicals by David H. Bremner; Arthur E. Burgess; Feng-Bin Li (111-120).
Hydroxyl radicals HO• are generated under Fenton-like (Fe2++H2O2→HO•+OH−+Fe3+) catalytic conditions using chemical, electrochemical and ultrasonic energies in combination. The resulting hydroxyl radicals are utilised in a model reaction in which benzene is hydroxylated directly to produce phenol. Chemical energy alone using a FeCl2 catalyst supported on silica gel gives synthetic yields of ca. 2% (calculated on benzene initially present), which are increased to 13–15% when irradiated with ultrasound. Coupled chemical and electrochemical synthesis in which the Fe2+ catalyst is generated in situ through controlled dissolution of an iron electrode produces yields of 10–13%. Higher yields (15–20%) are obtained when ultrasonic energy is coupled with the chemical–electrochemical method. The merits and limitations of these different methods and the significance of the yields of the model reaction in terms of the efficiency of radical generation are discussed. It is concluded that the combined chemical–electrochemical–ultrasonic approach offers more effective control of energy input for hydroxyl radical generation appropriate to various synthetic reactions. The direct synthesis of phenol approaches a limiting yield of ca. 20% when competing side reactions, such as further oxidation, occur.
Keywords: Hydroxyl radicals; Fenton catalytic conditions; Electrochemistry; Ultrasound; Phenol synthesis; Iron catalysis; Coupled energies;
Oxidative conversion of n-heptane over molecular sieves by T.Yu Stoylkova; Chr.D Chanev; H.T Lechert; C.P Bezouhanova (121-126).
n-Heptane was partially oxidized at 523 K to 2-, 3- and 4-heptanones and heterocyclic compounds in the presence of SAPO-5, SAPO-11, BEA and MCM-41 under air flow at different molar ratios of n-heptane/oxygen. The catalysts were characterized by X-ray diffraction (XRD), raster electron microscopy and energy dispersion analysis of X-ray (EDAX). The presence of acid and basic sites was proven by interaction with alcohols — cyclohexanol and benzyl alcohol. 2-Methyl-5-ethyl-furan and 2,6-dimethyl-pyran were the main heterocyclic products. The oxidation of n-heptane to the corresponding hydroperoxides is suggested. The cleavage of the hydroperoxides is activated on the acid active sites of the molecular sieves used as catalysts. 1-Hepten-4-ol, 4-methyl-1,4-hexadiene and 2,4-dimethyl-1,3-pentadiene were also obtained as products.
Keywords: SAPO-5; SAPO-11; BEA; MCM-41; n-Heptane; Air oxidation; 2-Heptanone; 2-Methyl-5-ethyl-furan;
Synthesis of adamantane on commercially available zeolitic catalysts by Markéta Navrátilová; Karel Sporka (127-132).
The isomerization of endo-tricyclodecane to adamantane on commercially available zeolites was studied. Zeolites beta, mordenite, Y and ZSM-5 were tested and zeolite Y was the best. The sufficient acidity for the above rearrangement was exhibited by the zeolite in the hydrogen form. The Si/Al ratio in zeolitic framework influenced also the activity and selectivity of adamantane formation. The optimum reaction temperature was found to be 270°C.
Keywords: Adamantane; Isomerization; Zeolite;
TiO2 supported vanadyl phosphate as catalyst for oxidative dehydrogenation of ethane to ethylene by P. Ciambelli; P. Galli; L. Lisi; M.A. Massucci; P. Patrono; R. Pirone; G. Ruoppolo; G. Russo (133-142).
Bulk and TiO2 supported VOPO4 has been investigated for the oxidative dehydrogenation of ethane. XRD, SEM, TG analyses and BET surface area measurements indicated that vanadyl phosphate is highly dispersed on the support up to mono-layer coverage. A fraction of vanadium is present as V(IV) in the calcined samples as evaluated by EPR and TPR techniques. Both reducibility and acidity of vanadium phosphate is strongly enhanced by deposition on TiO2 with respect to the bulk phase, as shown by TPR and NH3 TPD technique, respectively. The supported catalysts are active and selective in the oxidative dehydrogenation of ethane to ethylene in the temperature range 450–550°C, the mono-layer catalyst giving the best performances. Ethylene selectivity decreases with the contact time but increases with the temperature. The former effect indicates that ethylene is further oxidized to CO x at high contact times. The effect of the temperature was attributed to the formation of V(IV), favoured at increasing temperature. This hypothesis was supported by TPR experiments carried out after catalytic tests at 550°C that indicated a significant increase of the fraction of V(IV) after the reaction.
Keywords: Ethane oxidative dehydrogenation; Vanadium phosphate; Redox properties; Acid sites;
Study of ammonium, mixed ammonium–cesium and cesium salts derived from (NH4)5[PMo11VIVO40] as isobutyric acid oxidation catalysts by Catherine Marchal-Roch; Nathalie Laronze; Nathalie Guillou; André Tézé; Gilbert Hervé (143-150).
Thermal exchange of cesium for ammonium cations in (NH4)5[PMo11VIVO40] impregnated at incipient wetness by CsNO3 was performed under nitrogen. The structural modification of this salt, previously reported in the Part I of this work, limits the solid state exchange to three cesium per heteropolyanion. Mixed NH4 +, Cs+ and VO2+ salts, isotype of the cubic ammonium and cesium 12-molybdophosphates, are obtained. The characterizations of these low surface area compounds show that their formula is Cs2x (NH4)6−2x (VO)[PMo11VIVO40][PMo12O40], with 0≤x≤3. These compounds are active for oxidative dehydrogenation of the isobutyric acid and selective to methacrylic acid. Characterizations of the catalysts corresponding to x<3 after reaction reveal a loss of ammonia and a quantitative release of vanadium from the heteropolyanion. Above x=3, the samples which are inactive, are composed of the oxidized mixed cesium and vanadium salt Cs6(VO2)[PMo11VVO40][PMo12O40] and an excess of cesium nitrate. Like for the mixed ammonium–cesium 12-molybdophosphates, the best catalyst is the nearly stoichiometric cesium salt (x=3) which is stable in the conditions of the reaction. The presence of vanadium improves significantly both the catalytic activity and the selectivity to methacrylic acid.
Keywords: Solid state thermal exchange; Methacrylic acid; Isobutyric acid;
Synthesis, pore texture and surface acid–base character of TiO2/carbon composite xerogels and aerogels and their carbonized derivatives by Francisco José Maldonado-Hódar; Carlos Moreno-Castilla; José Rivera-Utrilla (151-159).
Synthesis of TiO2/C composite xerogels and aerogels were obtained by polymerization of a mixture of resorcinol, formaldehyde and tetrabutyl orthotitanate. Samples so obtained were dried with supercritical CO2 (aerogel) or at 110°C in an oven (xerogel). The aerogel was prepared with two different initial TiO2 contents of 30 and 49%. Both aerogels and the xerogel were subsequently carbonized in an inert atmosphere at 500 and 900°C. Surface area and porosity of the samples were measured by CO2 adsorption at 273 K and mercury porosimetry; X-ray diffraction (XRD) technique was used to determine the crystallographic structure of the inorganic constituent of the composite (anatase or rutile); and acid–base surface characteristics of these materials were studied by testing them as catalysts in the decomposition reaction of isopropanol. Finally, some of these samples were used as supports for tungsten oxide catalysts and also tested in the above reaction. The composite aerogel with the lower TiO2 content and its carbonized derivatives had the more developed meso- and macroporosity, indicative of the importance of both the supercritical drying process of these materials and of the TiO2 content. In this case, the composite aerogel carbonized at 500°C had the highest activity of propene production and therefore, the highest surface acidity, because of the good accessibility of the surface acid sites. The tungsten oxide catalyst supported on it also presented the highest conversion in the above reaction, although in this case acetone and diisopropyl ether were also obtained.
Keywords: TiO2/C composite aerogels; TiO2/C composite xerogels; Acid–base surface characteristics; Tungsten supported on TiO2/C composite aerogel;
Patent report (161-167).