Surface acidity study of Mn+-montmorillonite clay catalysts by FT-IR spectroscopy: Correlation with esterification activity

A simple method for evaluating the surface acidity of different cation-exchanged montmorillonite (mont) clay catalysts, Mⁿ⁺-mont (Mⁿ⁺=Al³⁺, Fe³⁺, Cr³⁺, Zn²⁺, Ni²⁺, Cu²⁺, and H⁺), involving treatment with pyridine is described. After treating with pyridine, the samples were heated at 120 °C and the FT-IR spectra were directly recorded in the region 1650 and 1350 cm⁻¹. The data obtained show the presence of both Lewis and Brønsted acid sites. The activities of the catalysts to bring about Brønsted acid catalysed esterification of succinic acid with iso-butanol to yield di-(iso-butyl) succinate have been studied. The Brønsted acidity data obtained for Mⁿ⁺-mont correlated well with activity in the esterification reaction. The activities of the catalysts were found to decrease in the order of exchange ions Al³⁺ > Fe³⁺ > Cr³⁺ > Zn²⁺ > Ni²⁺ > Cu²⁺ > Na⁺-mont. They also correlated well with the charge to radius ratio of the cations. The catalysts exchanged with trivalent cations showed stronger absorption bands attributed to Brønsted acidity (1540 cm⁻¹) whereas those exchanged with divalent cations showed an increased Lewis acidity (1450 cm⁻¹) and reduced Brønsted acidity along with charge to radius ratio. Zn²⁺-, Cu²⁺- and Ni²⁺-exchanged clays showed an additional peak around 1605 cm⁻¹ which is attributed to the pyridine adsorption on surface sites through its π electrons. The method suggested here to evaluate the acidity is suitable for active sites which are thermally unstable such as water molecules in the hydration shell of a cation in exchanged clay.     

Effect of ZnSO4, MnSO4 and FeSO4 on the Partial Hydrogenation of Benzene over Nano Ru-Based Catalysts

Nano Ru-based catalysts, including monometallic Ru and Ru-Zn nanoparticles, were synthesized via a precipitation method. The prepared catalysts were evaluated on partial hydrogenation of benzene towards cyclohexene generation, during which the effect of reaction modifiers, i.e., ZnSO₄, MnSO₄, and FeSO₄, was investigated. The fresh and the spent catalysts were thoroughly characterized by XRD, TEM, SEM, XPS, XRF, and DFT studies. It was found that Zn²⁺ or Fe²⁺ could be adsorbed on the surface of a monometallic Ru catalyst, where a stabilized complex could be formed between the cations and the cyclohexene. This led to an enhancement of catalytic selectivity towards cyclohexene. Furthermore, electron transfer was observed from Zn²⁺ or Fe²⁺ to Ru, hindering the catalytic activity towards benzene hydrogenation. In comparison, very few Mn²⁺ cations were adsorbed on the Ru surface, for which no cyclohexene could be detected. On the other hand, for Ru-Zn catalyst, Zn existed as rodlike ZnO. The added ZnSO4 and FeSO₄ could react with ZnO to generate (Zn(OH)₂)₅(ZnSO₄)(H₂O) and basic Fe sulfate, respectively. This further benefited the adsorption of Zn²⁺ or Fe²⁺, leading to the decrease of catalytic activity towards benzene conversion and the increase of selectivity towards cyclohexene synthesis. When 0.57 mol·L⁻¹ of ZnSO₄ was applied, the highest cyclohexene yield of 62.6% was achieved. When MnSO₄ was used as a reaction modifier, H₂SO₄ could be generated in the slurry via its hydrolysis, which reacted with ZnO to form ZnSO₄. The selectivity towards cyclohexene formation was then improved by the adsorbed Zn²⁺.     

Esterification of dicarboxylic acids to diesters over Mn+-montmorillonite clay catalysts

Esterification of dicarboxylic acids with various alcohols and phenols in presence of metal exchanged montmorillonite clay catalyst (Mⁿ⁺-mont; Mⁿ⁺ = Al³⁺, Fe³⁺, Cr³⁺, Zn²⁺, Mn²⁺, and Ni²⁺) is studied. Among the catalysts used, Al³⁺-mont was found to be the most effective, as it gave good to excellent yields of esters under mild reaction conditions. The heterogeneous catalyst presented here can be regenerated and reused. All these features indicate the high potential of the reaction as green chemistry process.     

Effect of acid treatment on the composition and structure of a natural aluminosilicate and on its catalytic properties in α-pinene isomerization

The article deals with the effect of the conditions of modification of a natural aluminosilicate catalyst with a 10% HCl solution on the chemical and phase compositions, porous and crystal structures, and acidity of the material and on its catalytic properties in α-pinene isomerization. Treatment of the aluminosilicate with 10% HCl (25–250 mL per gram of solid) causes cation exchange, increases the concentration of protonic sites on the aluminosilicate surface, and removes impurity calcite and dolomite. The specific surface area of the aluminosilicate increases from 52 to 68–82 m²/g. Treatment of the aluminosilicate with 175.0 or 250.0 mL/g of HCl removes a considerable amount framework Al³⁺, Fe^2+/3+, and Mg²⁺ cations, leading to a partial disruption of its structure and to a decrease in its acidity, specific surface area, and, as a consequence, catalytic activity relative to the same parameters of the samples treated with 50 or 100 mL/g of HCl. The highest catalytic activity is displayed by the aluminosilicate treated with 50 mL/g of HCl. The camphene and dipentene selectivity of the reaction (at 85% α-pinene conversion) with the original catalyst is 55 and 30%, respectively; modifying the aluminosilicate with HCl raises the camphene selectivity and reduces the dipentene selectivity by 5–6%. The catalyst considered here is more active than the commercial titanium catalyst.     

Interactions of Carbaryl with homoionic montmorillonite

Adsorption and catalytic degradation of the insecticide Carbaryl on montmorillonite self-supporting films (Upton, Wyoming) saturated with different cations (Al³⁺, Ca²⁺, Cu²⁺, Na⁺) was studied using thin-layer chromatography, X-ray diffraction, infrared and mass spectroscopy techniques. Carbaryl is adsorbed at room temperature by a coordination bond, through a water bridge, between the C=O group and the exchangeable cation. X-ray diffraction patterns showed that the molecule is always intercalated in the smectite layers except for Na-montmorillonite. On moderate heating (90°C for 30 h) the Carbaryl adsorbed on Cu- and Al-montmorillonite decomposes to CO₂, N-methylammonium cation, and 1-naphthol and polymerization products, di- and tri-condensate of 1-naphthol, are formed. By contrast for Ca- and Na-montmorillonite systems no degradation of Carbaryl was observed on heating at 90°C for 30 h. The Carbaryl decomposition can be ascribed to the concurrent effect, enhanced by dehydration, of the stronger polarizing power of Cu²⁺ and Al³⁺ ions and the acidity of the residual solvation water.     

Brønsted and Lewis acid catalysis with ion- exchanged clays

An acid-treated montmorillonite clay has been ion-exchanged with Al³⁺, Fe³⁺, Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺ and Na⁺. The catalytic activities of these materials have been measured in the Brønsted acid catalysed rearrangement of α-pinene to camphene, and the Lewis acid catalysed rearrangement of camphene hydrochloride to isobornyl chloride, following thermal activation at temperatures from 75 to 350°C. The surface acidities of the ion-exchanged clays have been measured using a microcalorimetric method involving ammonia adsorption, and through the infrared spectra of adsorbed pyridine. The results show that maximum Brønsted acidity is generated on thermal activation at approximately 150°C and maximum Lewis acidity at 250- -300°C. A good correlation has been found between the surface acidities and the catalytic activities of the ion- exchanged clays in both reactions. A significant result is the relatively low surface Lewis acid strength of Al³⁺- exchanged clays, for which a possible explanation is proposed.     

The catalytic conversion of CO2 to hydrocarbons over Fe–K supported on Al2O3–MgO mixed oxides

Al₂O₃–MgO mixed oxides prepared by a co-precipitation method have been used as supports for potassium-promoted iron catalysts for CO₂ hydrogenation to hydrocarbons. The catalysts have been characterized by XRD, BET surface area, CO₂ chemisorption, TPR and TPDC techniques. The CO₂ conversion, the total hydrocarbon selectivity, the selectivities of C₂–C₄ olefins and C₅₊ hydrocarbons are found to increase with increase in MgO content upto 20 wt% in Fe–K/Al₂O₃–MgO catalysts and to decrease above this MgO content. The TPR profiles of the catalysts containing pure Al₂O₃ and higher (above 20 wt%) MgO content are observed to contain only two peaks, corresponding to the reduction of Fe₂O₃ to Fe⁰ through Fe₃O₄. However, the TPR profile of 20 wt% MgO catalyst exhibits three peaks, which indicate the formation of iron phase through FeO phase. The TPDC profiles show the formation of three types of carbide species on the catalysts during the reaction. These profiles are shifted towards high temperatures with increasing MgO content in the catalyst. The activities of the catalysts are correlated with physico-chemical characteristics of the catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydroxylation of phenol with H2O2 over transition metal containing nano-sized hollow core mesoporous shell carbon catalyst

The catalytic performance of transition metal (Fe²⁺ or Cu²⁺) containing nano-sized hol low core mesoporous shell carbon (HCMSC) heterogeneous catalysts for the hydroxylation of phenol with hydrogen peroxide (H₂O₂) in water was investigated in a batch reactor. The metal-containing HCMSC catalyst showed higher activity than the same metal ion-exchanged zeolites. The nature of the metal and its content in the HCMSC had remarkable influence on the reaction results under the typical reaction conditions (PhOH/H₂O₂=3, reaction temperature=60 ‡C). Fe²⁺ containing HCMSC catalyst showed high catalytic activity with phenol conversion of 29%, selectivity to catechol (CAT) and hydroquinone (HQ) about 85%, H₂O₂ effective conversion about 70% and selectivity to benzoquinone (BQ) below 1% in the batch system.     

Application of modified clays in diazotization and azo coupling reactions in water

Diazotization and diazo coupling reaction of a series of aromatic amines and activated aromatic compounds over eco-friendly modified clay catalysts are described. The reaction was catalyzed by modified montmorillonite K-10 including Cu²⁺, Fe³⁺, and Zn²⁺ metal ions. The versatility of this method was checked by using various amines and phenols, which showed reasonable yields of products. These inexpensive, noncorrosive and reusable catalysts were found to exhibit bifunctional catalytic properties for diazotization and diazo coupling reactions. The catalysts could be reused several times without significant loss of their catalytic reactivity.     

Chemoselective C=O Hydrogenation of α,β-unsaturated Carbonyl Compounds over Quasihomogeneous and Heterogeneous Nano-Au0 Catalysts Promoted by Lewis Acidity

Lewis-acid promoted Au⁰ clusters were used as quasihomogeneous or as heterogeneous metal catalysts for the chemoselective hydrogenation of α,β-unsaturated carbonyl compounds. Addition of Lewis-acid cations like Zn²⁺ to amidic dispersions of size-optimized Au⁰ nanocolloids led to an enhanced hydrogenation rate and allylic alcohol chemoselectivity in the hydrogenation of e.g., crotonaldehyde (83% selectivity at 92% conversion). Heterogeneous Au⁰ catalysts were generated by the quantitative immobilization of the Au⁰ nanocolloids on a nano-sized ZnO support, followed by partial removal of the polyvinylpyrrolidone stabilizer. This deposition preserved the nanodispersed state of Au⁰, and resulted in a heterogeneous metal catalyst with stable activity and high C=O chemoselectivity in e.g., the hydrogenation of mesityl oxide (66% selectivity at 91% conversion).     

Basicity–FAME yield correlations in metal cation modified MgAl mixed oxides for biodiesel synthesis

In this paper Zn^2 +, Co^2 +, Fe^3 + and La^3 + were respectively introduced into Mg₃Al₁ mixed oxides to prepare Mg₁Zn₂Al₁, Mg₁Co₂Al₁, Mg₃Al_0.6Fe_0.4 and Mg₃Al_0.6La_0.4 oxides by calcinations at 773 K. The XRD, XPS, BET, ICP-AES and CO₂-TPD characterizations of samples exhibited that the effects of the substitution of trivalent cations (Fe^3 +, La^3 +) for Al^3 + on the basicity of samples were stronger than the substitution of divalent cations (Zn^2 +, Co^2 +) for Mg^2 + because of high O^2 − concentration which were coincident with the FAME yield for these catalysts. La^3 + modified catalyst exhibited highest activity in transesterification.     

Development of New Cu0–ZnII/Al2O3 Catalyst Supported on Copper Metallic Foam for the Production of Hydrogen by Methanol Steam Reforming

Copper metallic foam with thermal conductive properties, manufactured by S.C.P.S., has been investigated as a support for catalysts to improve thermal exchange inside the reactor for the endothermic steam reforming of methanol. Thus, we have developed a procedure for the in situ preparation of a Cu⁰–Zn^II/Al₂O₃ catalyst onto the copper metallic foam. The foam-based Cu⁰–Zn^II/Al₂O₃ catalyst shows an activity three times as high as commercial catalysts for a conversion of 74% of methanol into hydrogen.     

Selective oxidation of propane on MgO/γ‐Al2O3‐supported molybdenum catalyst: influence of promoters

The influence of promoters, potassium and samarium, on molybdenum supported over MgO–γ‐Al₂O₃ catalyst has been investigated in the oxidative dehydrogenation of propane. The acidities of catalysts were determined by temperature‐programmed desorption of NH₃ and by decomposition of 2‐propanol. The K‐promoted catalyst showed the lower acidity followed by the Sm, whereas the unpromoted sample showed the highest acidity. The higher the acid character of the catalyst, the lower the selectivity to propene. Redox properties determined from EPR spectra change with the addition of the promoter. A parallelism between Mo⁶⁺ reducibility and catalytic activity was found. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. II. Action of Promoters Zn and Li

By correlating the results of the NH₃-TPD characteristic study and the catalyst activity assay of the W/HZSM-5-based catalysts, we confirmed that the intensity and concentration of the surface B-acid sites have pronounced effects on the catalyst performance for dehydro-aromatization of methane (DHAM). It was found experimentally that, by addition of a proper amount of Mg²⁺, the strong B-acid sites at the catalyst surface could be effectively eliminated, whereas the addition of a proper amount of Zn²⁺ or Li⁺ resulted not only in eliminating most of the strong surface B-acid sites but also in generating a kind of new medium-strong acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for dehydro-aromatization of methane; on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. By simultaneous addition of Mg²⁺ and Zn²⁺, optimized adjustment in surface acidity of the catalyst could be realized. On the other hand, the doping of the Zn²⁺ or Li⁺ component to the tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W component at the surface of the catalyst, thus leading to a pronounced decrease in the reduction temperature for the hard-to-be-reduced W⁶⁺ species and an increase in quantity of the reducible W⁶⁺ species at the reaction temperature for DHAM, as has been evidenced by the results of a H₂-TPR study on the reducibility of the Zn²⁺ (or La³⁺, Li⁺, Mn²⁺)-promoted W/HZSM-5 system. The above two roles that Zn²⁺ and Li⁺ as promoters played both contributed to the persistence of high methane conversion and benzene selectivity, and the alleviation of coke deposition, as well as the prolongation of the catalyst lifetime.     

Vapor-phase methylation of pyridine with methanol to 3-picoline over Zn1−xCoxFe2O4 (x=0, 0.2, 0.5, 0.8 and 1.0)-type ternary spinels prepared via a low temperature method

The reaction of pyridine with methanol was carried out over Zn_1−xCo_xFe₂O₄ (x=0, 0.2, 0.5, 0.8 and 1.0)-type systems in a fixed-bed down-flow reactor. The influences of surface acidity, cation distribution in the spinel lattice and various reaction parameters are discussed. The activity and selectivity were shown to be strongly dependent on the surface acidity of the systems. Over all compositions of the systems, 3-picoline was formed as the major product, even though the activity and selectivity show a strong dependence on composition and reaction conditions. Generally, the systems possessing more acidic sites (x≥0.5) favor the production of 3-picolines and 3,5-lutidine. Pyridine conversion increased with the progressive substitution of Zn²⁺ ions by Co²⁺ ions. Cation distribution in the spinel lattice influences their acidic properties. These factors have been adequately considered as helpful to evaluate the activity of the systems.     

A Pd–Fe/α-Al2O3/cordierite monolithic catalyst for CO coupling to oxalate

A novel Pd–Fe/α-Al₂O₃/cordierite monolithic catalyst was prepared for the synthesis of diethyl oxalate from CO and ethyl nitrite. The palladium-based monolithic catalyst with an optimal thickness (15 μm) of Al₂O₃ washcoat showed excellent catalytic activity and selectivity in a continuous flow, fixed-bed microreactor. The physicochemical properties of catalyst were studied by a variety of characterization techniques. Catalytic performances of Pd–Fe/α-Al₂O₃/cordierite monolithic catalysts were dependent on particle size of alumina-sol, thickness of Al₂O₃ washcoat, pore structure, surface acidity of carrier, and distribution of active metal component on the Al₂O₃ washcoat. Under the mild reaction conditions, CO conversion was 32% and the space–time yield of diethyl oxalate was 429 g/(L h). Pd efficiency (DEO(g)/Pd(g)/h) of the monolithic catalyst (274 h⁻¹) was much higher than that of a reference pellet catalyst (46 h⁻¹), probably due to high dispersion of the Pd nanoparticles on the surface of the monolithic catalyst.     

Study on an iron–manganese Fischer–Tropsch synthesis catalyst prepared from ferrous sulfate

A systematic study was undertaken to investigate the effects of the initial oxidation degree of iron on the bulk phase composition and reduction/carburization behaviors of a Fe–Mn–K/SiO₂ catalyst prepared from ferrous sulfate. The catalyst samples were characterized by powder X-ray diffraction (XRD), Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and H₂ (or CO) temperature-programmed reduction (TPR). The Fischer–Tropsch synthesis (FTS) performance of the catalysts was studied in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that the fresh catalysts are mainly composed of α-Fe₂O₃ and Fe₃O₄, and the crystallite size of iron oxides is decreased with the increase of the initial oxidation degree of iron. The catalyst with high content of α-Fe₂O₃ in its as-prepared state has high content of iron carbides after being reduced in syngas. However, the catalyst with high content of Fe₃O₄ in its as-prepared state cannot be easily carburized in CO and syngas. FTS reaction study indicates that Fe-05 (Fe³⁺/Fe_total = 1.0) has the highest CO conversion, whereas Fe-03 (Fe³⁺/Fe_total = 0.55) has the lowest activity. The catalyst with high CO conversion has a high selectivity to gaseous hydrocarbons (C₁–C₄) and low selectivity to heavy hydrocarbons (C₅₊).     

Benzylation of benzene to diphenylmethane using zeolite catalysts

The catalytic liquid phase benzylation of benzene to diphenylmethane (DPM) with benzyl chloride (BC) is investigated over a number of zeolite catalysts at 358 K and under atmospheric pressure. Conventional homogeneous Lewis acid catalyst, AlCl₃, is also included for comparison. Zeolite H-β is found to be more selective but less active compared to HY and H-ZSM-5 zeolites in the benzylation of benzene. The conversion of BC, rate of BC conversion and selectivity to DPM over H-β after 6 h of reaction time are 33.3 wt%, 4.7 × 10⁻³ mmol g⁻¹ h⁻¹ and 89.1 wt%, respectively. For comparison, the conversion of BC, rate of BC conversion and selectivity to DPM over AlCl₃, under identical reaction condition, are found to be 100 wt%, 170 × 10⁻³ mmol g⁻¹ h⁻¹ and 58 wt%, respectively. Higher amounts of consecutive products are obtained over AlCl₃ due to its non shape selectivity. The acidity of the zeolite catalysts is measured by temperature programmed desorption method. The effect of the duration of the run, SiO₂/Al₂O₃ ratio of H-β, catalyst concentration, reaction temperature and benzene to BC molar ratio on the catalyst performance is also investigated in order to optimize the conversion of BC and selectivity for DPM. The conversion of BC using H-β is increased with the increase in the reaction time, catalyst concentration, reaction temperature and molar ratios whereas it decreases with the increase in SiO₂/Al₂O₃ molar ratio of H-β. H-β is recycled two times and a slight decrease in BC conversion is observed after each cycle, which is related to the minor dealumination of the zeolite catalyst by HCl, which is produced during the reaction as by product. The formation of DPM is explained by an electrophilic attack of the benzyl cation (C₆H₅CH₂⁺) on the benzene ring, which is produced by the polarization of BC over acidic sites of the zeolite catalysts.     

Production of middle distillate through hydrocracking of paraffin wax over Pd/SiO2–Al2O3 catalysts

Pd/SiO₂–Al₂O₃ catalysts (Pd/SA-X) with different SiO₂ contents (X, wt%) were prepared for use in the production of middle distillate (C₁₀–C₂₀) through hydrocracking of paraffin wax. The effect of SiO₂ content of Pd/SA-X catalysts on their physicochemical properties and catalytic performance in the hydrocracking of paraffin wax was investigated. High surface area and well-developed mesopores of Pd/SA-X catalysts improved the dispersion of Pd species on the SiO₂–Al₂O₃ support. Acidity of Pd/SA-X catalysts determined by NH₃-TPD experiments showed a volcano-shaped trend with respect to SiO₂ content. Conversion of paraffin wax increased with increasing acidity of the catalyst, while selectivity for middle distillate decreased with increasing acidity of the catalyst. Yield for middle distillate showed a volcano-shaped curve with respect to acidity of the catalyst. This indicates that acidity of Pd/SA-X catalysts played an important role in determining the catalytic performance in the hydrocraking of paraffin wax. Among the catalyst tested, Pd/SA-69 with moderate acidity showed the highest yield for middle distillate.     

Selective reduction of nitrogen oxides by hydrocarbons on hydrotalcite Co and Ni catalysts

Hydrotalcitelike Co-Al and Ni-Al catalysts of different compositions (with the atomic ratio M ²⁺/Al³⁺ = 0.5–3.0) were studied in the reaction of selective reduction of NO by propane, propylene, and n-decane in the presence of O₂. The higher activity of the catalysts with M ²⁺/Al³⁺ = 0.5 is connected with high dispersity of Ni or Co cations stabilized by a significant amount of Al³⁺ ions. Propylene was shown to be the most efficient reducing agent for nitrogen oxide. The highest degree of conversion to the extent of 90–99% was attained at 400 and 420–440°C for Ni-Al and Co-Al samples, respectively. When propane and decane were used as reducing agents, the conversion of both catalysts was characterized by the volcano-shaped dependence on temperature due to the fact that the catalyst took part in the concurrent reaction of hydrocarbon (reducing agent) oxidation. Hydrotalcitelike materials are promising representatives of inexpensive bi- and multicomponent systems. The design strategy for new active catalysts for processes of purification of gas exhausts from NO _x , that are stable in the presence of water and sulfur oxides, may be based on usage of hydrotalcites with modified ions introduced into them.