Applied Catalysis A, General (v.358, #1)

Contents (iii-vi).

Modeling of a heterogeneous industrial fixed bed reactor for selective dehydrogenation of heavy paraffin with Pt–Sn–Al2O3 catalyst has been the subject of current study. A detailed model of the reactor has been developed. In order to estimate reactor production by the passage of time, the reactor model which is a set of partial differential equations (PDEs), ordinary differential equations and algebraic equations has been solved numerically.Modeling of a heterogeneous industrial fixed bed reactor for selective dehydrogenation of heavy paraffin with Pt–Sn–Al2O3 catalyst has been the subject of current study. Using mass balance, momentum balance for appropriate element of reactor and applying pressure drop, rate and deactivation equations, a detailed model of the reactor has been obtained. Mass balance equations have been used for five different components. In order to investigate reactor performance in time, the reactor model which is a set of partial differential equations (PDEs), ordinary differential equations and algebraic equations has been solved numerically.Variation of paraffins, olefins, dienes, aromatics and hydrogen mole percent as a function of time and reactor radius have been found by numerical solution of the model. Modeling results have been compared with an industrial reactor data at different operating times. The comparison successfully confirms validity of the proposed model.It was found that at radius of 0.75–0.68 m of catalytic bed, there is not sensible conversion of paraffin as surface reaction is rate determining. At entrance of the bed variation of diene was near zero. Hydrogen production was high at entrance of the catalytic bed. Hydrogen production decreases along the reactor bed because of olefin cracking.
Keywords: Linear alkyl benzene; Paraffin; Dehydrogenation; Fixed bed reactor; Modeling;

Steam reforming of methanol over CuO/ZnO/CeO2/ZrO2/Al2O3 catalysts by Gang Huang; Biing-Jye Liaw; Cheng-Jyun Jhang; Yin-Zu Chen (7-12).
During the design of CuO/ZnO/CeO2/ZrO2/Al2O3 catalysts for steam reforming of methanol (SRM), the effects of CeO2, ZrO2 and Al2O3 were clearly identified. ZrO2 promoted the SRM reaction, but CeO2 and Al2O3 weakened it. An appropriate amount of Al2O3 was still needed for the mechanical strength of catalysts. The CuO/ZnO/ZrO2/Al2O3 (30/40/20/10 and 40/30/20/10) catalysts are good candidates for SRM as compared with G66B (CuO/ZnO/Al2O3  = 30/60/10).The composition (CuO/ZnO/Al2O3  = 30/60/10) of a commercial catalyst G66B was used as a reference for designing CuO/ZnO/CeO2/ZrO2/Al2O3 catalysts for the steam reforming of methanol (SRM). The effects of ZnO, CeO2, ZrO2 and Al2O3 on the SRM reaction were clearly identified. CeO2, ZrO2 and Al2O3 all improved the dispersions of CuO and ZnO in CuO/ZnO/CeO2/ZrO2/Al2O3 catalysts. Zirconium oxide promoted the SRM reaction and slightly reduced the concentration of CO, but CeO2 and Al2O3 weakened the SRM reaction. The introduction of ZrO2 into CuO/ZnO/Al2O3 (30/60/10) improved the reducibility and stability of the catalyst. The addition of CeO2 or Al2O3 hindered the reducibility of the catalyst and weakened the interaction between CuO and ZnO. Nevertheless, an appropriate amount of Al2O3 was needed for the stability and the mechanical strength of the catalysts. The CuO/ZnO/ZrO2/Al2O3 (30/40/20/10) and CuO/ZnO/ZrO2/Al2O3 (40/30/20/10) catalysts are good candidates for the SRM, as determined by comparison with the commercial catalyst G66B.
Keywords: Steam reforming of methanol (SRM); Copper catalyst; Hydrogen production; CuO/ZnO/ZrO2/Al2O3; G66B;

Elaboration and study of poly(vinylidene fluoride)–anatase TiO2 composite membranes in photocatalytic degradation of dyes by Ouafa Tahiri Alaoui; Quang Trong Nguyen; Chamekh Mbareck; Touria Rhlalou (13-20).
The adsorption capacity of the TiO2/PVdF membrane is enhanced by a prior conditioning in ethanol. The photocatalytic efficiency of TiO2 immobilized in PVdF was compared with that of the conventional TiO2 Degussa P25 catalyst. The rate of TiO2/PVdF membrane degradation photocatalytic was 6.1 and 2.4 μmol L−1  min−1 for IC and BG respectively, against 8.7 and 5.9 μmol L−1  min−1 for the P25 TiO2.Porous PVdF-based membranes filled with anatase (TiO2) nanocrystalline particles were prepared by using a new phase inversion technique for entrapment of TiO2. The prepared samples were characterized by scanning electron microscopy, thermal gravimetry analysis, and contact angle measurements. The SEM investigation showed a fair distribution in the membranes of the TiO2 particles, mainly in aggregate form. The best photocatalytic membrane had a pore size of 0.96 μm, with a maximum porosity of 86% at 0.5 TiO2/PVdF weight ratio. The original porous TiO2/PVdF membranes showed a very weak degradation ability of dyes in an aqueous medium due to their poor wetting. We showed that a prior membrane conditioning in ethanol increased the membrane wetting, thus the dye degradation rate by making the TiO2 particles in the internal structure accessible to the external medium. The adsorption capacity of the TiO2/PVdF membrane was studied with the aqueous solutions of Brilliant Green (BG) and Indigo Carmin (IC) dyes, and the photocatalytic efficiency of TiO2 immobilized in PVdF was compared with that of the conventional TiO2 Degussa P25 catalyst. The rate of TiO2/PVdF membrane degradation photocatalytic was 6.1 and 2.4 μmol L−1  min−1 for IC and BG respectively, against 8.7 and 5.9 μmol L−1  min−1 for the P25 TiO2. The Langmuir-Hinshelwood model was adequate to describe the rate of dye degradation only when the amount of dyes adsorbed on photocatalytic material is significant.
Keywords: PVdF-TiO2 catalyst; Photodegradation; Dyes; Microporous membranes; Phase inversion;

Evaluation of the catalytic activity of the water-soluble organometallic complex [Rh(μ-Pz)(CO)(TPPTS)]2 in the hydroformylation of short-chain olefins in a refinery's naphtha cut by Victor J. Guanipa Q.; Luis G. Melean; Maria Modroño Alonzo; Angel Gonzalez; Merlin Rosales; Francisco Lopez-Linares; Pablo J. Baricelli (21-25).
A water-soluble organometallic complex of rhodium, namely [Rh(μ-Pz)(CO)(TPPTS)]2 [Pz = pyrazolate; TPPTS = P(C6H4SO3Na)3, tris(sodium-m-trisulphonated-triphenyl-phosphine)] was evaluated in the catalytic hydroformylation of the short-chain olefins contained in a naphtha cut from “El Palito” refinery in Venezuela. The reactions were carried under moderate conditions in an aqueous/organic biphasic medium. The naphtha current was characterized. Under the best reaction conditions (825 psi of syn-gas, T  = 70 °C, S/C = 200:1) a very good conversion to aldehydes (96%) was obtained after 72 h. The mercury test was performed to the system proving that the reaction proceeds through molecular species. The recycling of the aqueous solution of the catalytic precursor was performed four times resulting in a decrease of activity from 96% to 60%.A water-soluble organometallic complex of rhodium, namely [Rh(μ-Pz)(CO)(TPPTS)]2, was evaluated in the catalytic hydroformylation of the short-chain olefins contained in a naphtha cut from “El Palito” refinery in Venezuela. The reactions were carried under moderate conditions in an aqueous/organic biphasic medium. The naphtha current was characterized. Under the best reaction conditions (825 psi of syn-gas, T  = 70 °C, S/C = 200:1) a very good conversion to aldehydes (96%) was obtained after 72 h. The mercury test was performed to the system proving that the reaction proceeds through molecular species. The recycling of the aqueous solution of the catalytic precursor was performed four times resulting in a decrease of activity from 96% to 60%.
Keywords: Hydroformylation; Rhodium; Phosphines; Naphtha; Aqueous biphasic catalysis;

Characterisation and reactivity of Re/carbon catalysts in hydrodesulphurisation of dibenzothiophene: Effect of textural and chemical properties of support by G. Lagos; R. García; A. López Agudo; M. Yates; J.L.G. Fierro; F.J. Gil-Llambías; N. Escalona (26-31).
This work examines the effect of the textural and chemical properties of two activated carbons on the activity of carbon-supported Re catalysts for the hydrodesulphurisation (HDS) of dibenzothiophene (DBT). For this purpose, two series of Re-containing catalysts with Re loadings between 0.06 and 0.33 atoms Re nm−2 were prepared by impregnation of microporous (CMI) and micro–mesoporous (CME) activated carbons with solutions of NH4ReO4. Characterisation of the carbon supports by textural analysis showed that the CMI carbon was exclusively microporous while the CME was micro- and mesoporous. Temperature-programmed decomposition (TPD), FTIR and electrophoretic migration and acidity measurements showed that the CMI presented a greater amount of highly stable oxygen surface groups (carbonyl and quinone type) and less strong acidic carboxylic groups than CME. The textural properties of the catalysts indicated that the oxidic Re species of the catalyst precursors were mostly deposited over the external surface of the carbons. Upon sulphidation, XPS results showed that Re was fully sulphided for both catalyst series, and that the deposited oxidic Re species were redistributed during sulphidation due to the interaction with the stable oxygen surface groups of the carbons. The greater concentration of these oxygen surface groups on the CMI carbon led to better Re dispersion in the Re(x)/CMI catalysts than in the Re(x)/CME catalysts, especially at low Re loadings. Thus, the Re(x)/CMI catalysts showed higher HDS activity and greater increases in activity with increasing Re loadings up to about 0.15 Re atoms nm−2 than the Re(x)/CME catalysts. Above this Re content, the texture of the carbons controlled the HDS activity. The activity of the Re(x)/CMI catalysts decreased markedly due to a decrease in ReS2 dispersion caused by formation of Re multilayers and plugging of micropores, while the activity of the Re(x)/CME catalysts continued increasing because the presence of larger pores on the CME carbon delayed the effects of pore blocking and ReS2 sintering to higher Re loadings.Textural, chemical properties and activities of carbon-supported Re catalysts for the hydrodesulphurisation of dibenzothiophene was studied. Two series of Re-containing catalysts were prepared by impregnation of microporous (CMI) and micro–mesoporous (CME) activated carbons. The activity was explained by difference in the textural properties and dispersion of the active phases in both catalysts systems.
Keywords: Re catalysts; HDS of DBT; Micro- and micro/mesoporous carbons; Re sulphide species;

Selective liquid-phase hydrogenation of citral over supported bimetallic Pt–Co catalysts by Nicolás M. Bertero; Andrés F. Trasarti; Bernard Moraweck; Armando Borgna; Alberto J. Marchi (32-41).
Bimetallic Pt–Co/C catalysts proved more active and selective than monometallic Pt/C and Co/C samples in the liquid-phase hydrogenation of citral. The improved catalytic performance of Pt–Co/C samples was attributed to (1) the formation of a bimetallic Pt–Co phase; (2) electron transfer from Co to Pt; (3) a highly activated citral adsorption in the on-top, di-σCO and πCO modes over Pt–Co phase.The liquid-phase hydrogenation of citral was studied at 393 K and 10 bar on Pt–Co/C catalysts, having different Pt/(Pt + Co) ratios and containing a total metal load of about 2%. The monometallic and bimetallic Pt–Co/C catalysts were prepared by impregnation and co-impregnation, respectively, with cobalt tris(acetylacetonate) and platinum bis(acetylacetonate). Monometallic Pt/C and Co/C catalysts showed very low activity and selectivity to the desired products. Undesirable reactions, such as citral decarbonylation and hydrogenolysis, were observed with these monometallic catalysts. Instead, bimetallic Pt–Co/C proved very active and selective to geraniol/nerol and the main products detected were geraniol/nerol, citronellal and citronellol. Hydrogenation kinetic constants were determined by modeling catalytic data and using a pseudo-homogeneous kinetics. From the analysis of the kinetic parameters, an optimum Pt/(Pt + Co) ratio was found for both the catalytic activity and selectivity towards geraniol/nerol. Furthermore, it was determined that this optimum ratio depends on the activation conditions. Temperature-programmed reduction (TPR) experiments and X-ray absorption spectroscopy (XAS) demonstrated the existence of Pt–Co bimetallic compounds on the carbon support. On the basis of these results, it was proposed that cobalt improves the catalytic performance of platinum by electron transfer. This electron transfer is favored by the high interaction of both metals existing in these types of bimetallic compounds.
Keywords: Citral hydrogenation; Aldehydes (α,β-unsaturated); Platinum-based catalysts; Cobalt-based catalysts; Bimetallic compounds;

Insight to sulfur species in the hydrodeoxygenation of aliphatic esters over sulfided NiMo/γ-Al2O3 catalyst by Eeva-Maija Ryymin; Maija L. Honkela; Tuula-Riitta Viljava; A. Outi I. Krause (42-48).
The hydrodeoxygenation (HDO) network of aliphatic esters was investigated over reduced and sulfided NiMo/γ-Al2O3 catalysts. The participation of surface OH and SH groups in the various reaction steps and the stability of the surface sulfur are discussed. Stronger nucleophilic strength of the SH groups compared to the OH groups and formation of sulfur-containing intermediates were of special importance.The hydrodeoxygenation (HDO) network of aliphatic esters was investigated over reduced and sulfided NiMo/γ-Al2O3 catalysts in a batch reactor at 250 °C and 7.5 MPa. Methyl and ethyl heptanoate and their main HDO reaction intermediates were used as reactants. The participation of surface OH and SH groups in the various reaction steps and the stability of the surface sulfur are discussed. Although the surfaces of the reduced and sulfided catalysts under H2 have identical properties, the reactivity of the oxygen-containing compounds is greater over the sulfided catalyst. This is due to the stronger nucleophilic strength of the SH groups compared to the OH groups. Methyl and ethyl heptanoate react via various reaction pathways, of which alkaline hydrolysis is demonstrated to be of special importance. Sulfur-containing intermediates are formed in the HDO reactions with surface SH groups. Accordingly, sulfur level on the catalyst surface is diminished.
Keywords: Hydrodeoxygenation (HDO); Methyl heptanoate; Sulfided NiMo; Reduced NiMo; Surface sulfur;

Catalytic activity of silica gel in the synthesis of sulfonamides under mild and solvent-free conditions by Maasoumeh Jafarpour; Abdolreza Rezaeifard; Marzieh Aliabadi (49-53).
Efficient synthesis of various sulfonamides via the nucleophilic attack by aliphatic and aromatic amines with sulfonyl chlorides using silica gel as a cheap, available, safe and heterogeneous catalyst under mild and solvent-free conditions is described. The reusability of the catalyst has been successfully examined without noticeable loss of its catalytic activity.Silica gel as a cheap, heterogeneous and reusable catalyst is effectively utilized for the preparation of sulfonamides via condensation of aliphatic and aromatic amines with sulfonyl chlorides under solvent-free conditions at room temperature.
Keywords: Sulfonamide; Silica gel; Solvent-free; Amines; Sulfonyl chloride;

In co-oxidation of p-xylene (PX) and p-toluic acid (p-TA) to terephthalic acid (TA) in water with Mn acetate catalyst, PX was transformed to p-TA while p-TA was converted to TA. The radicals generated from facile PX-to-p-TA conversion would assist more difficult p-TA to TA conversion. The promotional effect of CO2 and co-catalysts was demonstrated.Co-oxidation of p-xylene (PX) and p-toluic acid (p-TA) to terephthalic acid (TA) in water with Mn acetate catalyst has been studied. Mn acetate showed better catalytic activity than Co acetate and no synergism was observed between two catalysts. During the reaction, PX was transformed to p-TA while p-TA was converted to TA by a radical reaction. In the co-oxidation, radicals generated from facile PX-to-p-TA conversion would assist more difficult p-TA to TA conversion. Addition of CO2 in the oxygen promoted the TA formation and suppressed the CO2 formation from the combustion of reactants. Additional transition metals such as Ni and Ti also promoted the catalytic activity of Mn catalyst. Furthermore, a synergism was observed between added CO2 and the added transition metal. Finally, the yield of terephthalic acid reached about 95% with Mn + (Ti, Ni or Cr) catalyst system and CO2 addition.
Keywords: Terephthalic acid; Co-oxidation; Water solvent; Manganese acetate; CO2 effect;

Cu/K2Ti2O5, synthesized as novel NO x storage-reduction catalyst, adsorbs NO x at both low (100–400 °C) and high temperature (500–600 °C). The experimental data reveal that the oxygen vacancies are the sites for NO x adsorption at low temperature, whereas the high temperature NO x adsorption is accompanied by a structure change between K2Ti2O5 and K2Ti6O13.A novel NO x storage-reduction catalyst Cu/K2Ti2O5, which uses K2Ti2O5 rather than Ba and Al2O3 as NO x storage compound as well as support, has been synthesized and investigated. Activity tests including temperature-programmed ramping, isothermal storage, and lean-rich cycling reveal that Cu/K2Ti2O5 adsorbs NO x over a very wide temperature range (200–600 °C), with two peak temperatures at 280 and 550 °C, respectively. The NO x storage and reduction mechanism over Cu/K2Ti2O5 is studied in detail. We demonstrate that, at low temperature (200–400 °C), NO x is adsorbed on the oxygen vacancy sites which are formed on K2Ti2O5 support during the lean-period; whereas at high temperature (500–600 °C), the NO x storage and reduction is explained by a structure switching between K2Ti2O5 and K2Ti6O13 caused by NO2 adsorption and de-sorption. Our results also provide another option for synthesizing NSR catalysts with K2Ti2O5 or even with other potassium containing compounds.
Keywords: Potassium titanate; NO x storage; Oxygen vacancy; Structure change; K2Ti6O13; Vehicle emission;

A batch microreactor has been utilized to study the near-critical water gasification of glucose in the presence of metal catalysts. The reaction temperature and pressure were 340–380 °C and 150–250 bar, respectively. The following catalytic activities have been observed: Raney-nickel 4200 > Raney-nickel 3202 > ruthenium on alumina and ruthenium on carbon > Raney-copper > Raney-cobalt.Catalytic gasification of biomass in sub- and supercritical water is a promising process for the production of fuel gaseous. In this paper, a batch microreactor has been utilized to study the near-critical water gasification of glucose in the presence of supported and unsupported metal catalysts consisting of Raney-nickel, Raney-cobalt, Raney-copper, carbon-supported ruthenium, and alumina-supported ruthenium. The reaction temperature and pressure were 340–380 °C and 150–250 bar, respectively. Effects of reaction temperature, reaction time, and catalyst loading on the amount of the generated gas as well as its composition were investigated. Results indicated that within our operating conditions, the conversion of glucose was sensitive to temperature but varying the catalyst loading in the range of 30–100 wt% did not significantly affect the conversion, implying that the experiments were mainly conducted at saturated amounts of catalyst. The following catalytic activities for the decomposition of glucose have been observed: Raney-nickel 4200 > Raney-nickel 3202 > ruthenium on alumina and ruthenium on carbon > Raney-copper > Raney-cobalt. However, the relatively small difference in the gas yields obtained by Raney-copper catalyzed reaction and those of Raney-nickel and ruthenium suggested this relatively inexpensive spongy structure of copper metal could be very useful for gasification of biomass in subcritical water environments.
Keywords: Gasification; Subcritical water; Supercritical water; Biomass; Glucose; Catalyst;

The activity of mesoporous titania, obtained by sol–gel method using urea as pore-forming agent, directly modified with different concentrations of tungstophosphoric acid, in the photodegradation of methyl orange was higher when the TPA content and the calcination temperature of the samples were raised. A lower recombination of electron–hole pairs, lower band gap, and higher crystallinity of the samples are the main responsible effects.The materials were synthesized by the sol–gel method, using titanium isopropoxide as precursor and urea as pore-forming agent. During the gelation, an alcoholic solution of tungstophosphoric acid (TPA) was added. Its concentration was varied in order to obtain TPA contents of 0, 10, 20 and 30% (w/w) in the solid. The urea was extracted with water and the solids were thermally treated between 100 and 600 °C. Mesoporous solids were obtained, with a mean pore diameter (D p) larger than 3.1 nm. The specific surface area (S BET) decreased and D p slightly increased when the TPA content was raised. S BET also diminished when the calcination temperature was increased, the decrease being lower for higher TPA contents. The X-ray diffraction patterns of the solids modified with TPA only presented the characteristic peaks of the anatase phase. The crystalline degree and the crystal size increased for higher calcination temperatures. According to the Fourier transform infrared and 31P magic angle spinning-nuclear magnetic resonance spectra, the [PW12O40]3− species is the one mainly present in the solids. The band gap values obtained from the UV–vis diffuse reflectance spectra are slightly lower than those reported for the anatase phase. The catalytic activity of the materials in the photodegradation of methyl orange was higher when the TPA content and the calcination temperature of the samples were raised. The lower recombination degree of the photogenerated electrons and holes caused by the increase in the crystalline degree, implying a lower number of defects which act as recombination centers, and in the modified samples by TPA, due to its effect as charge trapping center, added to lower band gap values, are the main effects responsible for the activity increase.
Keywords: Mesoporous titania; Tungstophosphoric acid; Urea; Photocatalyst;

Synthesis and characterization of supported bimetallic overlayer catalysts by Michael P. Latusek; Rebecca M. Heimerl; Brett P. Spigarelli; Joseph H. Holles (79-87).
Directed deposition was used to synthesize alumina-supported bimetallic overlayer Pd on Re (Re@Pd) and Pt on Ni (Ni@Pt) catalysts. The directed deposition technique combines the use of inhibitors and a surface reaction to produce the desired overlayer structure. Re@Pd catalysts showed decreased hydrogen heat of adsorption compared to Pd or Re monometallic catalysts as predicted by computational studies.The directed deposition technique was used to synthesize alumina-supported bimetallic overlayer Pd on Re (Re@Pd) and Pt on Ni (Ni@Pt) catalysts. Computational and single crystal studies have predicted unique adsorption properties for similar overlayer type catalysts compared to the monometallic catalysts. The directed deposition technique combines the use of inhibitors and a surface reaction to produce the desired overlayer structure. The Re@Pd and Ni@Pt catalysts were characterized using hydrogen chemisorption, transmission election microscopy (TEM), and energy dispersive spectroscopy (EDS) to correlate properties with synthesis conditions. H2 chemisorption indicated the proper synthesis conditions to deposit the overlayer metal on the base metal catalyst particle as desired. When a surface deposition inhibitor was not used, evidence for the formation of isolated particles of the overlayer metal was detected. Re@Pd catalysts showed decreased hydrogen heat of adsorption compared to Pd or Re monometallic catalysts as predicted by the literature. Ni@Pt catalysts demonstrated intermediate H2 heat of adsorption compared to Pt and Ni. TEM/EDS were used to demonstrate that the overlayer metal was associated with the base metal particles as desired.
Keywords: Pd/Re; Pt/Ni; Hydrogen chemisorption; Hydrogen heat of adsorption;

Synthesis variables such as synthesis time, molar SiO2/SnO2 ratios in the starting gel composition, silica source and tin sources affect the physico-chemical properties of mesoporous stannosilicate molecular sieves with MCM-48 structure (Sn-MCM-48). For the first time, Sn-MCM-48 has been used as a new heterogeneous catalyst in solvent-free Mukaiyama-type aldol reaction.The influence of various synthesis variables on the physico-chemical properties of mesoporous stannosilicate molecular sieves with MCM-48 structure (Sn-MCM-48) has been investigated. Sn-MCM-48 has been used as a new heterogeneous catalyst in solvent-free Mukaiyama-type aldol reactions. Invariably, the β-hydroxy ester selectivity was found to be 100% irrespective of the conversion level. The product yield was found to increase, to reach a maximum (70%), and then to decrease with the increase in tin content.
Keywords: Sn-MCM-48; Mesoporous molecular sieves; Hydrothermal synthesis; Characterization; Heterogeneous catalysis; Mukaiyama–aldol condensation;

Ionone synthesis by cyclization of pseudoionone on silica-supported heteropolyacid catalysts by V.K. Díez; B.J. Marcos; C.R. Apesteguía; J.I. Di Cosimo (95-102).
The liquid-phase cyclization of pseudoionone to ionones (α, β and γ isomers) is studied on silica-supported heteropolyacid (HPA) catalysts. The reaction is promoted on strong Brønsted acid sites. The pseudoionone conversion to ionones increases with HPA loading and reaction temperature. The latter also affects the isomer distribution. The α isomer is mainly obtained.The liquid-phase cyclization of pseudoionone to ionones (α, β and γ isomers) was studied on silica-supported heteropolyacid (HPA) catalysts containing between 18.8 and 58.5% HPA. The HPA used was tungstophosphoric acid (H3PW12O40). The catalyst surface and structural properties were thoroughly characterized by several techniques. The density, chemical nature and strength distribution of the surface acid sites were determined by adsorbing and monitoring by temperature-programmed desorption and infrared spectroscopy probe molecules such as NH3 and pyridine. The pseudoionone conversion to ionones increased linearly with the Brønsted acid site density until the HPA loading approached to the monolayer saturation coverage. For higher HPA contents, the pseudoionone adsorption and conversion were hampered because of spatial constraints that diminished the reactant accessibility to the proton active sites. The highest ionone yield, 79%, was obtained on a 58.5 wt% HPA/SiO2 catalyst at 383 K and is comparable to the best values reported in literature for the homogeneously catalyzed reaction using sulfuric acid. The ionone isomer distribution was modified by varying both the temperature and the reaction time. A reaction mechanism was postulated in which ionone isomers (α, β and γ) are primary products, but γ-ionone is isomerized to α-ionone while β-ionone is not converted in the other isomers.
Keywords: Ionone; Heteropolyacid; Acid catalysis; Tungstophosphoric acid; HPA;