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Magnetic nanoparticles based on magnetite (MNP) are important materials for different research applications, particularly when they are coated with inert phases. For instance, coating them with silica forms MNP@SiO2, which has various uses, including as a support for catalysts because it has surface functional groups that form bonds to a coating layer; the material presents itself with high magnetic susceptibility, turning it easily separable from the reaction in a heterogeneous medium. Herein, magnetite was synthesized and coated with tetraethylorthosilicate (TEOS) and then used to support phosphotungstic acid (H3PW12O40, HPW). The X%HPW/MNP@SiO2 materials were characterized by several techniques, such as EDXRF, XRD, SEM, FT-IR, low temperature N2 physisorption, and pyridine gas adsorption/desorption (Py-TPD) and then applied to the esterification reaction of oleic acid with ethanol. The results successfully indicated the synthesis of the catalysts, keeping the structural and magnetic characteristics of the MNP@SiO2 and providing a good dispersion and integrity of HPW on its surface. In the model reaction, high activity, selectivity (100%), and stability were observed, with yields of 98%, 66% and 54% of ethyl oleate when using 30%, 20% and 10%HPW/MPN@SiO2, respectively, for a reaction time of one hour and an acid to ethanol molar ratio of 1:6. No leaching of the active phase was detected up to four recycles of the best catalyst (30%HPW/MPN@SiO2).

Ordered mesoporous silica (OMS) is an important and useful material for a variety of applications, including catalysis, adsorption, sensing and controlled drug delivery. The surface chemistry and the silanol groups on OMS pores are key properties for the potential modification and application of this material. This research aimed to synthesize (using standard protocols) and differentiate the accessibility and strength of the H-acceptor Si–OH from FDU-12, SBA-16, MCM-41 and SBA-15 by pyridine (Py) donor, where the first two have cubic pore structures and the last two have hexagonal pore structures. Donor–acceptor properties were assessed by calculation of the surface Si–OH densities by thermogravimetry (TG), H2O-TPD/MS, and 29Si MAS and CP/MAS NMR. The nature of the Si–OH groups on these materials was determined to be hydrogen-bonding sites using FT-IR spectroscopy of Py adsorption. The reactivity of these silanol groups was probed by Py-TG and slurry microcalorimetry of Py adsorption in cyclohexane. Differences in accessibility and reactivity were discussed considering the total potential sites on the surface (nOH) versus the actual sites that can react with the Py molecule (nPy). By using microcalorimetry, it was possible to quantitatively distinguish the strength of the sites: The acidity order was approximately the same as the relative amount of silanol groups (Si–OH) and Py on the surface of the OMS materials (αPy): FDU-12 > MCM-41 ≥ SBA-16 > SBA-15.

An alternative method for the preparation of PEMFC electrodes is presented in this work based on the direct deposition of Pt particles onto the gas diffusion layer (Pt@GDL) by hydrothermal reduction of the H2PtCl6 precursor from formic acid, ethylene glycol, and ethanol reductive solutions. There is a successful anchorage of Pt particles via the formation of Pt crystal aggregates. The influence of the reducing agent concentration and temperature was studied to analyze their influence on the size, morphology, and distribution of the Pt particles on the gas GDL. The prepared Pt@GDL was tested for formic acid and ethanol high-temperature H3PO4-doped PEMFC. The Pt@GDL prepared in the formic acid reductive atmosphere presented the best performance associated with the formation of smaller Pt crystals and a more homogeneous dispersion of the Pt particles. For formic acid and ethanol-fed high-temperature PEMFC using a H3PO4-doped polybenzimidazole membrane as the solid electrolyte, maximum power densities of 0.025 and 0.007 W cm−2 were drawn at 200 °C, respectively.

The use of renewable raw materials as feedstocks for important industrial chemicals presents a challenge to the scientific community in its pursuit of sustainability. The necessary tasks include the revision of significant processes such as the ethanol-to-olefin process (ETO), and our goal was the production of ethylene. We chose 12-tungstophosphoric acid (HPW) supported on ceria-zirconia mixed oxide (Ce0.8Zr0.2O2, CZ) as a promising catalyst. The structural properties of different loadings of HPW (5–45 wt%) and different support preparation methods were examined, in addition to their activities in the gas-phase dehydration of ethanol. The maintenance of the Keggin structure was proved by infrared spectroscopy (FT-IR), 31P solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray diffraction (XRD). Scanning electron microscopy (SEM) images showed a gradual increase in the formation of agglomerates by the HPW coating on the support surface. The reaction products were followed by mass spectra in the interval of 200–400 °C. Multiple catalysts reached 100% ethanol conversion and selectivity for ethylene at low temperatures (200, 250 and 300 °C). A double effect of the support was observed: to disperse the HPW, improving its original low surface area, and to probably facilitate the coke burning process. Tests to determine the lifetime of catalysts (e.g., 26.4% HPW/CZ) performed in a pulsed microreactor coupled to a GC system showed at least 70 h of activity at 300 °C temperature, which demonstrated the potential of these catalysts.

Research on magnetic nanoparticles of magnetite (MNPs) has intensified because it can be coated with different materials leading to diverse properties. In this work, MNPs and silica coated MNPs were synthesized by co-precipitation of Fe(III) and Fe(II) precursors, different proportions of tetraethylorthosilicate (TEOS), and scales showing high reproducibility and proximity of crystalline domains. Spectra of FT-IR exhibited the formation of Si OH Fe bonds. TEM images revealed that most of the MNPs were spherical with a fair distribution of encapsulated nanoparticles. Magnetization increased as the amount of silica coating increases. Silica coating did not modify the shape and structure of the MNPs cylindrical mesoporous open in the extremities. Adsorption of pyridine on the MNPs demonstrated the presence of hydrogen bonding, Lewis and Bronsted sites. For MNPs coated with silica, there was a neutralization of the Bronsted leading to weaker Lewis (coordinatively unsaturated iron) and hydrogen bonding sites (silanol groups).

Nanostructured materials have been used in several branches of science and technology. Particulate matter is one of the major air pollution concerns. In this work, nanorods and nanoparticles of Ce0.8Zr0.2O2 (CZ) mixed oxides were prepared by different routes, and the use of an organic template was evaluated in diesel soot oxidation. The catalysts were characterized by several techniques including structural analysis (XRD, TEM, N2 adsorption–desorption) and activity (TPR/MS, TPO/MS). A fast TPR/MS method is proposed to calculate hydrogen consumption that can be correlated to the oxygen storage capacity (OSC). It was demonstrated that CZ-nanorods with twice the amount of template in the syntheses (CZ-NRs-2X) were very active for soot oxidation with T50% at 351 °C, and CO2 and H2O were the only oxidation products from Printex®-U (Evonik). This catalyst, reported for the first time, was subjected to up to three cycles and it showed fair activity, proving that this morphology is one of the best mixed oxides of CZ for oxidation.

In this study, cerium-zirconium mixed oxides were prepared using the sol-gel methodology with cetyltrimethylammonium bromide (CTAB) for tuning the specific surface area and pore volume. The composition of Ce0.8Zr0.2O2 (CZ) was selected as being one of the most active for soot oxidation, which is a concern as an environmental pollutant. The structural and morphological properties were investigated using EDXRF, XRD, SEM/EDS, Raman and N2 physisorption at low temperature. The catalytic test was evaluated by temperature-programmed oxidation coupled with mass spectrometry (TPO/MS). Any variation in the CTAB concentration during synthesis changed the particle size and surface area, indicating that the particle formation follows a mechanism in which the CTAB acts as capping agent. Additionally, the material synthesized with CTAB was found to have higher oxygen vacancy compared to the materials from the conventional sol-gel process. The catalytic test suggested that the material with the smallest surface area had the lowest temperature of oxidation (T50% = 400 °C), which demonstrates that the oxygen vacancy parameter is essentially more important than the specific surface area itself.

Catalytic materials supported on solid matrices have driven progress in classical processes in the search for high catalytic activity. This report provides a comprehensive comparison of twelve composites prepared by impregnation of two heteropolyacids (H3PW12O40 and H4SiW12O40) on zeolites BEA and Y (X%HPA/Zeolite). The materials were characterized by XRD, FT-IR, multinuclear MAS NMR, N2 and pyridine adsorption and calorimetry. The Keggin structure of the HPA and zeolite were confirmed by 31P/29Si/27Al MAS NMR, FT-IR and XRD. Generally, for X%HPA/zeolite, dealumination occurred after impregnation. Secondary intrareticular mesopores were more evident on zeolite BEA than on zeolite Y and the decreased specific surface area and pore volume with increased HPA loading was indicative of HPA deposition on their outer surface. There was a relatively strong interaction between the HPA and the zeolite surface, which involved extraframework aluminum (EFAL) sites hydrogen bonded to protons of HPA. 39%HPW/BEA and 40%HPW/Y showed Bronsted sites with high strength (average −145 kJ mol−1), which were strong enough to catalyze the esterification of n-butanol with acetic acid, yielding 83 and 78% conversion, respectively. The catalysts could be reutilized three times without any appreciable loss in the conversion.

Owing to the high thermal resistance and greater selectivity for products of interest, zeolites are widely used both as catalysts and as catalytic supports for reactions of industrial importance. During modification of the zeolite structure, several parameters should be carefully controlled to avoid structural destruction of the zeolite. A fractional factorial design (24−1) was performed to evaluate the contrasts of the dealumination variables (temperature, humidity, dealumination degree and washing) in each response (Si/Al ratio, number of acid sites, catalytic conversion). Structural characterizations (XRD, textural analysis, FT-IR, XRF, 27Al MAS NMR) and acidity (gas adsorption of pyridine) were performed. Catalytic tests were carried out with the ethanol dehydration reaction in a fixed-bed microreactor and pulsed flow. The dealumination degree and humidity have a statistically significant influence on the dealumination, producing higher changes to the Si/Al ratio and better ethanol conversion than the parent zeolite. The intrinsic acidity (number of acid sites) seemed to depend to a greater extent on the washing procedure and on the reaction temperature.

Zeolites and polyoxometalates are catalysts used in important environmental processes and their combination produces a new solid tailoring the advantages of both catalysts. In this work, supported H3PW12O40 or H4SiW12O40 on zeolite Y was prepared by incipient and aqueous impregnation, which maintained the Keggin structure. The dispersion of H3PW12O40 or H4SiW12O40 on zeolite Y surface lead to nanocrystals of 5 nm and 23 nm, respectively. Although the crystallinity decreased with HPA loading, the SEM images showed that the original morphology of zeolite Y was conserved and a strong interaction between the solids could be inferred from the MAS NMR spectra. Enhanced formation of secondary mesopores on the zeolite Y structure was related to the partial dealumination process that took place during the impregnation. Heteropolyacid impregnation on zeolite Y resulted on the disappearance of the Lewis sites, because of the interaction with the Keggin anion, thus causing the solid to have predominantly Bronsted type sites. The new sites kept the strengths higher than −100 kJ mol−1 with a generally increased number because the individual solids had about the same strength of sites (−147 kJ mol−1). Higher conversion (77%) for esterification reaction was observed for the samples prepared by incipient impregnation.

Herein, a combination of heteropolyacids and ionic liquids as a catalytic system was studied for the Biginelli multicomponent reaction; the positive ionic liquid effect associated with the acidic strength of zeolite-supported heteropolyacids made this combination an efficient catalytic system for the multicomponent synthesis of 3,4-dihydropyrimidin-2(1H)-one/thione derivatives. The acidic strength effect was evaluated, and a range was determined in which the reaction provided better results. The mechanism of the reaction was also investigated in the presence and absence of ionic liquids, and two features of paramount importance were revealed: the mechanism could be tuned to proceed through only one reaction path among three possibilities and the kinetics of the reaction was significantly faster in the presence of an ionic liquid.

In this work, we investigated the role of solid-state dealumination by (NH4)2SiF6 (25% Al removal and 13% Si insertion), the impregnation of niobium (10, 18, and 25 wt. %) on dealuminated *BEA (DB) zeolite and their catalytic properties in ethanol and xylose transformations. Among all the studied catalysts, 18%Nb-DB showed increased mesoporosity and external areas. A leveling effect in the number and strength of the proposed two sites (Br&oslash, nsted and Lewis) present in the catalyst (n1 = 0.24 mmol g&minus, 1, &minus, &Delta, H1 = 49 kJ mol&minus, 1, and n2 = 0.20 mmol g&minus, 1, &ndash, H2 = 42 kJ mol&minus, 1) in the catalyst 18%Nb-DB, might be responsible for its good activity. This catalyst presented the highest selectivity for diethyl ether, DEE (97%) with 61% conversion after 50 ethanol pulses at 230 &deg, C (turnover number, TON DEE = 1.15). These features allowed catalytically fruitful bonding of the ethanol molecules to the neighboring sites on the channels, facilitating bimolecular ether formation through a possible SN2 mechanism. The same catalyst was active and selective for transformation of xylose at 180 &deg, C, showing 64% conversion and 51% selectivity for furfural (TON Furfural = 24.7) using water as a green solvent.

Catalysis is an important tool in obtaining a range of derivative products from renewable sources, such as ethanol. Solid-state dealumination using ammonium hexafluorosilicate on the BEA zeolite under the treatment at temperature of 190 °C, 70 mol% dealumination in the presence of humidity and washing with buffer solution at 25 °C generated larger pores. The absence of washing procedure for the same sample resulted in pore blockage. Ethanol dehydration reactions showed conversion of 72% (64% ethylene and 8% diethyl ether) for this sample at 300 °C and high weight hourly space velocity (WHSV, 3247 h-1), whereas, at the same temperature but at a low WHSV (57 h-1), the conversion was 88% (49% ethylene and 39% diethyl ether). Dealuminated BEA zeolite presented better diffusion conditions, higher hydrophobicity and generated larger pores. Impregnation with H3PW12O40, H4SiW12O40 and Nb2O5 explained the selectivity, showing that Lewis acid sites from Nb2O5 favored the diethyl ether production.

Poly(lactic acid) (PLA) is a significant polymer that is based on renewable biomass resources. The production of PLA by polycondensation using heterogeneous catalysis is a focus for sustainable and economical processes. A series of samples comprising 12-tungstophosphoric acid (H3PW) supported on activated carbon, silica, and alumina induced the catalytic polymerization of D,L-lactic acid to form blends of PLA. The catalysts were characterized by multiple techniques to confirm the integrity of the Keggin anion as well as the acidity, which is the key property for relating structure to activity. The best reaction conditions were established for H3PW/C and tested for the other supported catalysts. The obtained polymer was a blend that was characterized as an enantiomeric excess (ee) of as much as 95% L-PLA (PLLA) with a mass average molar mass (Mw) of approximately 14,900 daltons. The role of H3PW in these polymerizations was demonstrated, i.e., without the Keggin acid, only oligomeric units (Mw < 10,000 daltons) could be obtained. Additionally, inverse relationships between the Mw of PLA and the enthalpy (–ΔH) of the strongest sites of the catalysts were distinguished, i.e., PLAMw-H3PW/C > PLAMw-H3PW/Al2O3 > PLAMw-H3PW/SiO2, whereas the acidity (–ΔH) order was as follows: H3PW/SiO2 > H3PW/Al2O3 > H3PW/C. These findings could be attributed to the correct tuning of strength and the accessibility of the sites to produce longer polymeric chains.

The dealumination of zeolites has been studied to tune acidity, resulting in enhanced catalytic efficiency. MOR, FER, and ZSM-5 were dealuminated to remove 5, 10, 15, or 20 mol% of Al with (NH4)2SiF6 (solid-state reaction at 80 °C). XRD, SEM, and FT-IR showed structural maintenance. Aluminum removal was confirmed by the total Si/Al ratio (XRF and AAS) and 27Al MAS NMR. The dealuminated samples were tested in the dehydration of different alcohols and compared to the protonic parent zeolites. Dealuminated MOR samples exhibited a decrease in coke formation (maximum decrease of 18.3%, 18.2%, and 6.1% for methanol, ethanol, and 1-propanol dehydration, respectively), and an increase in conversion values (maximum increase of 29.0%, 13.9%, and 6.2% for methanol, ethanol, and 1-propanol dehydration). H-FER/20% showed an improvement in methanol (19.9%) and ethanol (3.8%) conversions and a reduction of coke (decrease of 8.6% and 5.6%, respectively), corroborating the highest value of acid sites (0.73 mmol g−1). All catalysts showed 100% selectivity to each desired product. The mechanisms for all dehydrations were related to the presence of Bronsted acid sites. Methanol dehydration might involve the generation of methoxonion ion, and ethoxy groups might be formed for ethanol conversion. For ZSM-5, Lewis acid sites may also participate in the elimination mechanism. High reaction temperature (300 °C) favored intramolecular dehydration to ethylene, avoiding formation of diethyl ether and oligomers. Structural defects on H-ZSM-5/15% could facilitate the access of 1-propanol to active sites, leading to the best combination of TON (0.67), conversion (100%), and coke formation (0.35%).

A unique method for screening fifteen US Environmental Protection Agency polycyclic aromatic hydrocarbons (EPA-PAHs) in drinking and lake water samples is reported. One milliliter volume of water sample is mixed and centrifuged with 2 mg of BEA zeolite. The precipitate is subsequently treated with equal volumes (100 μL) of a 70/30 methanol–water mixture and n-octane. Fifteen EPA-PAHs are directly determined (no chromatographic separation) in the layer of n-octane via 4.2 K laser-excited time-resolved Shpol'skii spectroscopy. A mathematical equation is derived to correlate the PAH concentration in the water sample to its concentration in the layer of Shpol'skii solvent (n-octane). Qualitative and quantitative analyses are based on the collection of wavelength–time matrices, i.e. data formats that carry with them spectral and lifetime information. With 1 mL of water, the limits of detection varied from 1.1 ng L− 1 (benzo[a]pyrene) to 194 ng L− 1 (naphthalene). The analytical recoveries of the new method are in good agreement with those obtained via high-performance liquid chromatography. The simplicity of the experimental procedure and the use of microliters of organic solvent make the new method a valuable and environmentally friendly alternative for the routine monitoring of EPA-PAHs in water samples.

This work deals with preparation and characterization of H3PW12O40 (H3PW) supported on silica-alumina. Impregnation of H3PW (15, 20, 30 and 40 wt.%) on commercial silica-alumina support in acidic aqueous solution is effective for preparing this catalyst keeping its Keggin structure, according to different methods of characterization. The catalysts were tested in a model reaction of acetic acid with ethanol and 30 wt.% H3PW/SiO2-Al2O3 had the highest activity under the conditions: catalyst calcination at 300 ºC, temperature of 100 ºC, acetic acid:ethanol molar ratio of 2:1 and catalyst:acetic acid mass ratio of 10 wt.%. The reaction yield was 79 and 100% selectivity for ethyl acetate over three reutilizations, for reaction time of 2 h. The calculated total acid site distribution was 0.299 mmol g-1 (97% of the theoretical probed by pyridine), and most of these (0.236 mmol g-1) were Brønsted weak-medium strength (pyridine desorption between 300 and 500 ºC).

In this work, activities of HUSY and Ce/HUSY zeolites were studied in transesterification cycles of soybean oil and ethanol to produce biodiesel. The characterization of the materials was performed by FT-IR, XRD, BET method and pyridine adsorption followed by thermal analyses. TG/DTG results indicated a decrease of acid sites for both samples after each reaction cycle. However, Ce/HUSY zeolite showed a superior stabilization of acidic sites after three catalytic cycles and intermediary activation procedures. Biodiesel production exhibited high conversion levels (>96%) for both zeolites in all transesterification cycles. Surface area and pore volume measurements evidenced that cerium incorporation reduced the number of acid sites by interacting with OH groups in the micropore and external area of the zeolitic surface. This interaction resulted in an acid and structural stability, which provided a better activity (99%) than HUSY (96%). The higher conversion values obtained by zeolites showed a final product with a different distribution when compared with the traditional transesterification process. The identification of free fatty acids, diethyl and glycerol ethers in the final products and the reduction of unsaturated compounds indicated that parallel reactions also occurred in the studied systems. Nonetheless, the biofuel produced showed high ester content and did not present changes in its calorific power.

Poly(lactic acid) (PLA) is an important polymer because of its significant biocompatibility and biodegradability. Supported H3PW12O40 (H3PW) on activated carbon was utilized for the catalytic polymerization of D,L-lactic acid, resulting in blends of PLA. The stability of the polymer was monitored by thermogravimetry (TGA), and the decomposition temperature (Td) was used to determine the optimal production conditions (i.e., temperature of 180 °C for 15 h; 0.1 wt. % catalyst; 20 wt. % H3PW/carbon calcined at 400 °C). The best catalyst was reused three times with good activity and recovery (95 %) and was analyzed to confirm the consistency of its Keggin structure, dispersion, and acidity, which are important parameters that affect the catalyst’s activity. The obtained polymer was characterized by gel permeation chromatography (GPC), Fourier-transform infrared spectroscopy (FT-IR), 1H/13C nuclear magnetic resonance (NMR) spectroscopy, specific optical rotation ([α]D 25), powder X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The average molar mass of the polymer was 17,400 g mol−1. Blends of poly(lactic acid) with 85 % poly(L-lactic acid) stereospecific isomer were obtained.

Ammonium and cesium derivatives from H3PW12O40 (H3PW), namely (NH4)xCs2.5−xH0.5PW12O40 (x = 0.5, 1, 1.5, 2), were synthesized and structurally characterized by FT-Raman spectroscopy, and their thermal stability was evaluated by FTIR and TGA/DTA. The acidity was characterized by the adsorption/desorption of gaseous pyridine, by FTIR and TGA/DTA as well as by the reaction of oleic acid and ethanol. The stability of the mixed salts regarding the Keggin structure was much higher than the parent acid, with the onset decomposition around 520 °C. Nonetheless, calcination up to 300 °C is recommended for the integrity of the mixed salt. The FTIR of adsorbed pyridine displayed only Bronsted acidic sites, which was confirmed by TGA measurements of the formation of Py–H+⋯Py adducts. The best esterification result was for (NH4)2Cs0.5H0.5PW12O40 with TOF = 1.314 molEO mol−1 proton s−1 with a 1:6 (oleic acid:ethanol) molar ratio, at 80 °C and 10 wt% catalyst in relation to the acid.

The analytical performance of BEA – a commercial zeolite – is evaluated for the pre-concentration of fifteen Environmental Protection Agency – polycyclic aromatic hydrocarbons and their subsequent HPLC analysis in tap and lake water samples. The pre-concentration factors obtained with BEA have led to a method with excellent analytical figures of merit. One milliliter aliquots were sufficient to obtain excellent precision of measurements at the parts-per-trillion concentration level with relative standard deviations varying from 4.1% (dibenzo[a,h]anthracene) to 13.4% (pyrene). The limits of detection were excellent as well and varied between 1.1 (anthracene) and 49.9 ng L−1 (indeno[1,2,3-cd]pyrene). The recovery values of all the studied compounds meet the criterion for regulated polycyclic aromatic hydrocarbons, which mandates relative standard deviations equal or lower than 25%. The small volume of organic solvents (100 μL per sample) and amount of BEA (2 mg per sample) makes sample pre-concentration environmentally friendly and cost effective. The extraction procedure is well suited for numerous samples as the small working volume (1 mL) facilitates the implementation of simultaneous sample extraction. These are attractive features when routine monitoring of numerous samples is contemplated.

Supported heteropolyacids (HPAs) have been used as catalysts by fine-tuning their heterogenous and Bronsted acidic properties. In this work, supported H 3 PW 12 O 40 (HPW) on MCM-41 was prepared (2–40 wt%) and applied in the intramolecular cyclization of (+)-citronellal. The synthesized materials were characterized by 31 P MAS NMR, XRD, FTIR, pyridine gaseous adsorption, low temperature nitrogen adsorption and thermal analysis. The impregnation of 20% HPW on the surface of MCM-41 decreased the characteristic crystallographic reflections of the support, suggesting that HPW modifies the long-range order. The results show that the HPW was deposited preferentially inside the mesopores of the support and affected the acid strength of the supported catalyst. FTIR studies of pyridine adsorption confirmed the presence of Bronsted and hydrogen bonded acid sites. All synthesized materials were active in the cyclization reaction, but the 20%HPW/MCM-41 sample was the most active. This catalyst showed about 96% conversion and 65% selectivity to the most important stereoisomer (−)-isopulegol, under 1 h reaction. This catalyst was reused four times with a fair degree of deactivation. The decreased activity was attributed to the formation of small agglomerates in the channels of MCM-41.

This work reports the synthesis, characterization and application of rare earth (La, Ce, Sm and Gd) trisdodecylsulfate materials as Lewis acid-surfactant-combined catalysts for the production of biodiesel from waste cooking soybean oil. All the solid materials showed a bilayer lamellar arrangement with the dodecylsulfate molecules in a head to head, tail to tail packing. The reaction between commercial feedstocks (soybean oil and oleic acid) with ethanol was used separately to study La, Ce, Sm and Gd catalyst activities for transesterification (conversions of 73, 92, 53 and 73%) and esterification (conversions of 87, 80, 75 and 85%) after 1 h, respectively. Among them, cerium(III) trisdodecylsulfate showed the highest catalytic activity (TOF = 29.5 mol h −1 mol cat −1 ) for transesterification, while lanthanum(III) and gadolinium(III) trisdodecylsulfates were the most active for esterification (TOF = 28.7 mol h −1 mol cat −1 ). The application of the studied catalysts for the simultaneous transesterification and esterification reaction of waste cooking oil, containing 8.8 wt.% of free fatty acids (FFAs), with ethanol showed conversions of 76 (3.3 wt.% of FFAs), 79 (5.3 wt.% of FFAs), 81 (2.5 wt.% of FFAs) and 86% (0.0 wt.% of FFAs) after 1 h for La, Ce, Sm and Gd catalysts, respectively. Thus, gadolinium(III) trisdodecylsulfate was the best material for the conversion of waste feedstock to produce biodiesel. However, cerium(III) trisdodecylsulfate was less susceptible to deactivation, exhibiting conversion values of 92, 81 and 77% for the 1st, 2nd and 3rd reaction cycles. The catalysts were characterized by elemental analysis, X-ray powder diffraction (XRD), 1 H NMR, thermal analysis (TG/DTG/DTA) and FTIR measurements.

The physicochemical properties of three commercial zeolites – namely BEA, USY and ZSM-5 – are compared for the water removal of 15 PAHs included in the US EPA priority pollutants list. The best percentages of extraction were obtained with BEA and varied from 85.2 ± 1.7% (benzo[ g , h , i ]perylene) to 99.6 ± 1.1% (pyrene). The correlation of physicochemical properties to the percentages of extraction suggests that the predominant factor for a successful extracting performance is the adsorption of PAHs to the external surface of the extracting material. This process is facilitated by relatively large total pore volumes, large average pore diameters and a homogeneous distribution of Lewis acid sites. The presence of the latter enhances extraction performance via π-electron interactions with the aromatic rings of the PAH.

Mixed oxides of CexZr1−xO2 (0.1 ≤ x ≤ 0.9) were prepared by sol–gel method, in aqueous ammonia solution with CeCl3·7H2O and ZrOCl2·8H2O as precursors, and employed in diesel soot combustion. The catalysts were characterized by XRF/EDX, nitrogen adsorption, TGA/DTG, powder XRD, FTIR/DRIFTS and Raman. In addition, the acidity was evaluated by adsorption and desorption of pyridine. XRD indicated the formation of solid solutions that progressively distorted from cubic into tetragonal lattices. Raman studies confirmed that the Ce–O bonding was stronger in the mixed oxide series because of the cell contraction, as a result of the zirconium insertion. Ce0.8Zr0.2O2 was the most acidic and active material, shifting the combustion temperature (Tm) from 622 to 547 °C (loose contact) or 404 °C (tight contact). The calculated activation energy for the catalytic combustion of this optimized oxide attested that the combustion temperature was lower under all conditions, compared to the thermal process. The catalyst was utilized five times without any appreciable loss of activity and maintained its structural properties.

Zeolites have occupied a distinguished position due to their unique properties as solid acids and catalytic results achieved in several industrial reactions. This work studied the influence of supported WO 3 on USY zeolite structure, acidity and activity towards an esterification reaction. High dispersion of WO 3 species on USY was achieved, but at higher loading (⩾11.4%), microcrystalites of WO 3 were detected below the theoretical monolayer coverage (∼32%). Tungsten species were deposited preferentially inside the zeolite structure and interacted with the Bronsted sites of USY as well as on silanol surface groups with the formation of small aggregates. In addition, dealumination took place, especially in the samples with high WO 3 loading. USY had the most and the strongest acidic sites (Bronsted type), but the incorporation of WO 3 decreased the amount and the strength of the new sites. However, all WO 3 /USY catalysts were more active than USY in the esterification of oleic acid with ethanol (conversion above 74%, 2 h at 200 °C). The calculation of the TOF for a 1 h reaction demonstrated that 11.4% WO 3 /USY was the most active catalyst. Furthermore, it had the lowest rate of deactivation of acid sites after the reaction (∼13% after four cycles). The better performance of the 11.4% WO 3 /USY sample was also attributed to a better distribution of strength of the acidic sites and a more hydrophobic character of the synthesized material.

Este trabalho descreve detalhadamente o efeito da quantidade de cério na estrutura e morfologia da zeólita NH4USY. Ce-USY (2-25% m/m de CeO2) foi obtido por impregnação úmida de CeCl3 seguida de calcinação a 550ºC por 8 h. Em quantidades baixas (2-10%), foi observado que as espécies de cério encontram-se nas posições de troca iônica na rede, enquanto em maiores teores (15-25%) pequenos agregados formaram-se na superfície da HUSY. Difratometria de raios X (XRD) mostrou apenas reflexões relacionadas à HUSY, confirmando a alta dispersão das espécies de cério, porém as análises por espectroscopia Raman com transformada de Fourier (FT-Raman) detectaram CeOx para os materiais acima de 10%. A reação do CeCl3 com NH4USY produziu NH4Cl, o qual se decompõe em HCl, ocasionando a desaluminização da rede. Os materiais apresentaram um aumento da razão Lewis/Brønsted com o aumento da quantidade de cério, devido a interação do excesso de cério com os grupos OH da USY e consequente formação de espécies CeOx. This work describes comprehensibly the effect of cerium loading on the structure and morphology of NH4USY zeolite. The Ce-USY (2-25 wt.% of CeO2) was obtained by wet impregnation of CeCl3 followed by calcination at 550ºC for 8 h. At low loadings (2-10%), cerium species are mainly located at ion exchange positions in the framework, whereas at higher loadings (15-25%), small aggregates were formed on the HUSY surface. X-ray diffractograms (XRD) exhibited only the reflections related to HUSY, demonstrating the high dispersion of cerium species, but Fourier transform Raman spectroscopy (FT-Raman) detected CeOx for the materials above 10%. Reaction of CeCl3 with NH4USY produced NH4Cl, which decomposed to form HCl, leading to framework dealumination. The materials showed an increased Lewis/Brønsted ratio with increasing cerium loadings due to the interaction between the excess cerium and the OH groups of USY, and the consequent formation of CeOx species.

The surface acidity of the molecular sieve Si-MCM-41 was investigated by liquid phase calorimetric-adsorption analysis (Cal-Ad), gas adsorption measurements (TG-TPD) and infrared spectroscopy using pyridine as the probe molecule. The results showed that Si-MCM-41 evacuated at 200 °C had one type of hydrogen bonding site (n1 = 2.27 mmol g−1 with ΔH1 = −95.3 kJ mol−1) that was much stronger than those in silica gel. These sites corresponded to the total OH groups of the material surface and were able to retain pyridine at higher temperatures than silica gel (400–600 °C). In addition, the material had a higher entropy value (ΔS1 = −250 J K−1 mol−1) than silica gel. This indicates that pyridine is more strongly adsorbed on the surface of Si-MCM-41 than silica-gel, which leads to better catalyst properties. Solid-state 29Si MAS-NMR measurements showed a high amount of silanol groups (52%) in the Si-MCM-41 structure (for the Q2 and Q3 species). These results suggest a highly polar environment caused by the hydroxyl groups that line the walls of the hexagonal long-range ordered pores.

Mixed salts of cesium and ammonium derivatives of H3PW12O40 ((NH4)xCsyH0.5PW) were synthesized by a precipitation/ion exchange method with a different addition order for each cation. The produced salts were characterized by FTIR, XRD, 31P MAS NMR, SEM, pore size and the BET method. FTIR demonstrated that the materials maintained the Keggin structure under all tested conditions. BET results showed that salts formed by proton substitution from ammonium to cesium had much higher surface areas. Characterization by 31P MAS NMR and XRD, demonstrated that the salt structures varied sharply with the degree of hydration. After calcination, the materials were confirmed to have characteristics of solid solutions with random distribution of the cations with similar crystallographic patterns. The primary nanocrystals aggregated to form solids with mesoporous and microporous structures, created by partial unidirectional growth of the nanocrystals during formation of the bigger, round microcrystallites about 0.3–1 μm in size.

Catalysts of H3PW12O40 (H3PW) supported on Nb2O5 were prepared with a wide range of loadings (20–60 wt%) and characterized by FTIR, FT-Raman, XRD, 31P MAS NMR, the BET method and pyridine adsorption in the gas phase. In addition, the acidity was measured by the Cal-Ad method, which combines microcalorimetry and adsorption of pyridine on the solid samples in cyclohexane slurry. Preparation of the catalysts involved two procedures: impregnation in aqueous acid or acetonitrile solutions with further calcination at 200 °C for 8 h. No decomposition of H3PW was observed under these conditions. The results indicated the formation of well-dispersed H3PW species on materials with loadings up to 30 wt%, prepared preferentially in aqueous acid solution. A strong interaction between the heteropolyacid and niobia surface was observed forming species such as [ NbOH 2 ] n + [ H 3 - n PW 12 O 40 ] n - 3 . The H3PW/Nb2O5 materials containing 20–40 wt% presented type II isotherms and showed that surface area, pore volume and pore size decreased as the amount of H3PW increased, whereas the micropore area increased with H3PW loading. According to gaseous pyridine adsorption results, the catalysts presented only Bronsted sites. Microcalorimetric titration demonstrated that the 25 wt% material was the most acidic. The Cal-Ad method provided the enthalpy and number of sites for the strongest interaction with pyridine as: ΔH1 = −119.7 kJ mol−1 and n1 = 0.086 mmol g−1. About 33% of the protons were available for 25% H3PW/Nb2O5, in comparison to only 8% for pure H3PW. A considerable increase in the surface protons was achieved by H3PW on niobia surface.

Esterification of organic acids with alcohols produces an industrially important class of substances with a variety of applications. This work presents an impregnation route to support H3PW12O40 (H3PW) on zirconia (ZrO2) in acidic aqueous solution (HCl 0.1 mol L−1) at different ratios (5, 10, 15, 20, 25, 40 and 60 wt%), which were further applied in the esterification of oleic acid with ethanol. Impregnated samples calcined at 200 °C for 4 h were characterized by FTIR, FT-Raman, XRD, 31P MAS NMR and BET surface area. No decomposition of the Keggin anion structure was observed under these conditions. The XRD results, surface area determination and catalytic tests pointed out that H3PW was well dispersed over the support and only a monoclinic phase of the commercial ZrO2 was detected. An optimum reaction performance (88% of oleic acid conversion) was achieved at 20 wt% loading, 100 °C, 4 h reaction and 1:6 (acid:ethanol) molar ratio. A small leaching of 8 wt% of the initial mass of this catalyst (i.e., the actual loading was 18.4 wt%) was also observed at the end of reaction, which affects the reaction kinetics. Thermal stability studies of 20%H3PW/ZrO2 catalyst, determined by 31P MAS NMR, XRD and FT-Raman revealed that Keggin anion decomposition begins at ca. 500 °C, which was confirmed by the respective decrease of the catalytic activity. A preliminary study of recyclability indicated that a treatment of the spent catalyst involving a sequence of washing with n-hexane, drying at 100 °C and calcining at 300 °C for 4 h, recovered conversion values as high as 70%.

This article reports the synthesis, characterization and application of cerium(III) trisdodecylsulfate trihydrate (Ce[OSO3C12H25]3·3H2O) as a Lewis acid-surfactant-combined catalyst for the production of alkyl esters by solvent-free transesterification and esterification reactions. The new material presents a bilayer lamellar arrangement with an interplanar spacing of d001 = 3.76 nm, and the dodecylsulfate headgroup in a C2v symmetry. The reaction of soybean oil and oleic acid with light alcohols (methanol and ethanol) was used as model reactions and high catalytic activity (TOF = 57.4 mol h−1 molcat−1) was observed. The catalyst converted the used feedstocks to esters in less than 4 h with yields greater than 95%. In addition, it can be used to produce biofuels from unrefined and waste oils in which both triacylglyceride and free fatty acid molecules are present. After reaction, the recovered catalyst maintained its activity (≥95%) for three cycles. The catalyst was characterized by elemental analysis, X-ray powder diffraction (XRD), thermal analysis (thermogravimetry (TG)/derivative thermogravimetry (DTG)/differential thermal analysis (DTA)), pyridine adsorption, and spectroscopic analysis (FTIR and NMR).

This work deals with the preparation and application of new catalysts based on CuO, Nb 2 O 5 , and Si-MCM-41 for the abatement of diesel soot particulates. The properties of Si-MCM-41 mesoporous material were enhanced by studying the effect of the template removal method on specific surface area, terminal silanol groups, and long-range ordering. Template removal experiments indicated that thermal decomposition by double-step calcination at 300 °C for 3 h and 550 °C for 3 h produced a material with the best morphological characteristics. This material was used to prepare CuO/Nb 2 O 5 /MCM-41 catalysts with a 1:1 mass ratio (CuO:Nb 2 O 5 ) and 2, 5, 10, 15, and 25 wt.% loadings. The incorporation of CuO and Nb 2 O 5 leads to the formation of well-dispersed small crystallites and an amorphous phase, respectively, for the samples with lower oxide loadings. For the samples with higher oxide loadings, the crystallites increase in size, leading to the formation of copper–niobium mixed oxide phases and agglomerates. All the prepared materials presented considerable activity towards the oxidation of diesel soot particulates, shifting the maximum oxidation temperature ( T ox ) to lower values. The most promising catalyst was CuO/Nb 2 O 5 /MCM-41 with 25 wt.% of each oxide, which presented an onset temperature of 388 °C and appreciable activity at temperatures as low as 450 °C without the addition of any chemical promoter.

Supported niobium pentoxide catalysts are active for a variety of acid and redox catalyzed reactions such as dehydration, hydrogenation and esterification. In this work, niobium pentoxide has been supported on silica–alumina using two methods: grafting in CH2Cl2 or ethanol solution with NbCl5 precursor, and impregnating in aqueous solution with ammonium niobium oxalate. XRD, TG/DTG/DTA, FTIR, DRIFTS, FT-Raman, and calorimetric titration with pyridine were used to characterize the materials. Calcination of the materials at 800 °C produced crystalline Nb2O5 (T-phase) for the samples prepared by grafting, while a mixture of T- and H-phases are formed by aqueous impregnation, indicating the higher stability of the former materials. Thermal analysis confirmed a stronger interaction of niobium oxide species on the silica–alumina surface for the grafting procedure. On the other hand, partial neutralization of hydroxyl groups occurred on the support surface by grafting process, and the neutralization reaction is practically complete at 25 wt.% of Nb2O5 by impregnation. The catalysts with about 10 wt.% of Nb2O5 prepared by aqueous impregnation reached the monolayer, while by grafting it predominates bulk Nb2O5, as demonstrated by FT-Raman. These catalysts also showed the highest acidity, when calcined at 300 °C. Based on these results, liquid phase esterification reactions of acetic acid were carried out with different alcohols (ethanol, n-butanol, and iso-pentanol). The yields were about the same, independent of the preparation method for 10 wt.% Nb2O5 catalysts. The conversions using molar ratio 2:1 (acid:alcohol) after 8 h showed good results (83, 87, and 91%, respectively) and 100% selectivity for all esters.

Metalloporphyrin catalysts are able to carry out selective oxidation of organic substrates with several oxidizing agents. Recently, mesoporous materials have been studied as supports because they present high specific surface area, better dispersion and regeneration properties. This work presents the results of synthesis, characterization and application of three metalloporphyrin catalysts (FeTPPCl, MnTPPCl and CoTPP, where TPP = tetraphenylporphyrin) anchored on MCM-41, in the reaction of cyclohexene oxidation with hydrogen peroxide. A modified sol–gel preparation was chosen for the synthesis of the MCM-41 mesoporous material, as well as the anchoring was followed by Soxhlet extraction to ensure strong adsorption of the complex. The supported materials were much more stable than pure metalloporphyrins. The synthesized catalysts were characterized by UV–vis, FTIR, XRD, ICP-AES, 29 Si MAS-NMR and thermal analysis, before and after incorporation. Evidence of the metalloporphyrin immobilization was confirmed by elemental analysis and their activity in the oxidation reaction. FeTPPCl/MCM-41 showed higher conversion than CoTPP/MCM-41 and MnTPPCl/MCM-41. However, MnTPPCl/MCM-41 even in low concentration on the support showed a good conversion for the direct oxidation of cyclohexene with the highest turnover number (1.54 × 10 5 ). All catalysts showed similar selectivity that favors allylic oxidation products over epoxidation. No leaching of the metalloporphyrins was observed after the reaction.

Incorporation of niobium into microporous molecular sieves brings up new properties for these materials. ZSM-5 supported catalysts containing 2, 5, 13 and 19 wt.% of niobium pentoxide were prepared by aqueous impregnation and characterized by the combination of thermal (TG/DTA) and spectroscopic methods such as FTIR, FT-Raman, DRIFTS, and 29 Si and 27 Al MAS and 29 Si CP-MAS-NMR. In addition, X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and measurements of specific surface area by nitrogen adsorption (BET method) were carried out in order to obtain structural, textural and stability information about the solids. The samples were studied at different temperatures and heating time to optimize these parameters and to avoid a possible reduction of acidic properties by dehydroxylation. MAS-NMR and XRD results showed that the zeolite structure containing Nb did not undergo dealumination after thermal treatment, according to the presence of Si O Nb units on the zeolites structure in FTIR and Raman studies. Thermal analysis confirmed that Nb 2 O 5 impregnation reduced zeolite ZSM-5 dehydroxylation and thus promoted thermal stabilization. Nitrogen adsorption isotherms by the BET method indicated that surface area and pore volume decreased with the increase of Nb loading on the calcined samples at 450 °C. In addition, the nature of the zeolite active sites was investigated by spectroscopic and thermal analysis after pyridine adsorption. It was proposed that there are two different acidic sites (Bronsted and hydrogen bond site types) on the studied systems.

Silica-supported H 3 PW 12 O 40 (H 3 PW) catalysts (8, 15, 20 and 25 wt.% H 3 PW) were prepared and their catalytic activities were evaluated in benzene transalkylation with C9+ aromatics, using an industrial stream. This reaction is a convenient way to upgrade the low value of the stream produced in naphtha reforming and gasoline pyrolysis, by improving the production of xylenes. It was found that the activity and selectivity to xylenes changed with the acid loading and this was related to optimum acid strength and dispersion as well as specific surface area of the solids. The most active catalyst was the sample with 15 wt.% H 3 PW, which showed the adequate combination of the three parameters cited above. This solid was also the most stable, showing the lowest rate of deactivation under the reaction conditions, producing the least amount of coke. In addition, it presented the highest activity and selectivity to xylenes, even when compared to a mordenite commercial catalyst. Consequently, this catalyst represents a promising catalyst for industrial applications.

Copper oxide and niobium pentoxide were supported on silica–alumina with 2, 5, 10, 15 and 25 mass% loadings and 1:1 mass ratio of CuO:Nb2O5. XRD and BET measurements confirmed that monolayer coverage is reached with loadings between 5–10 mass% (∼308 m2 g−1). The DRIFTS spectra of pyridine adsorbed catalysts showed bands associated with Bronsted, Lewis and a combination of both acidic sites. Thermal analysis and liquid phase microcalorimetry provided the parameters for the best catalyst (10 mass%), which has the highest number of acidic sites (0.38 mmol g−1) and enthalpies of interaction with pyridine for Bronsted and Lewis sites (ΔH1 = −107.5 and ΔH2 = −64.4 kJ mol−1, respectively).

Biodiesel fuel (fatty acid esters) has become more and more attractive due to its environmental benefits. Transesterification is the most common and important method for making biodiesel from vegetable oils or animal fats. Several studies have focused on the development and improvement of analytical methods for monitoring biodiesel production and determining the fuel quality. Analytical procedures reported in the literature include chromatographic methods (e.g., gas chromatography, high-performance liquid chromatography, gel permeation chromatography, etc.) and spectroscopic methods [e.g., 1H and 13C NMR, near infrared, Fourier transform infrared spectroscopy, and recently, Fourier transform (FT)-Raman]. The study presented in this paper expands our previous research, in which FT-Raman spectroscopy combined with partial least squares (PLS) multivariate analysis was successfully applied to the quantification of soybean oil/ethyl ester mixtures. The FT-Raman/PLS methods developed by our group were used to m...

Copper oxide and niobium pentoxide were supported on silica-alumina by two methods: co-impregnation and sequential impregnation, with CuO:Nb 2 O 5 ratios of 1:10 and 1:1. These new materials showed different characteristics according to the preparation method. The copper and niobium precursors totally decomposed after treatment at 300 °C for 6 h. The materials prepared by sequential impregnation exhibited lower thermal stability than the analogous ones prepared by co-impregnation based on the Nb 2 O 5 transition phase. Whereas that transition occurs at about 1356 °C in the absence of CuO, it occurs at 1340 and 1290 °C using the co-impregnation and sequential processes, respectively. In addition, the materials with a CuO:Nb 2 O 5 ratio of 1:1 and copper oxide content >2 wt% did not present that transition phase. This indicates a strong influence of CuO over Nb 2 O 5 crystallites. XRD studies confirmed this interaction, showing the patterns of Cu and Nb oxides (for most of the samples prepared by both methods) and the presence of a third component (copper niobium oxide) on the samples with CuO content ⩾10 wt% when calcined at 800 °C for 6 h. FTIR, DRIFTS, and FT-Raman studies provided additional evidence of that interaction, which occurs not only on the surface hydroxyl groups of silica-alumina, but also on the surface hydroxyl groups of niobium pentoxide. The application of CuO/Nb 2 O 5 /SiO 2 -Al 2 O 3 materials in the oxidation of diesel soot particulates is promising; mixtures of the catalyst with a model particulate (Printex ® U) lowered the oxidation temperature from 622 to 518 °C.

USY (ultrastabilized Y) and Ce/USY (5 wt.% supported) zeolite acidities were characterized by microcalorimetric and adsorption studies of pyridine using liquid phase (Cal-Ad), thermogravimetry, and infrared analysis. The average adsorption enthalpies determined by microcalorimetry were −125.0 kJ mol−1 for USY and −97.2 kJ mol−1 for Ce/USY. A heterogeneous distribution of acid sites with heats of adsorption ranging from −134.0 (maximum heat value for USY) to −73.5 (minimum heat value for Ce/USY) kJ mol−1 was found for both zeolites. A two-site model was best fitted by the Cal-Ad method for HUSY (n1 = 0.1385 mmol g−1 with ΔH1 = −134.0 kJ mol−1, and n2 = 0.7365 mmol g−1 with ΔH2 = −101.5 kJ mol−1) and Ce/HUSY (n1 = 0.0615 mmol g−1 with ΔH1 = −117.6 kJ mol−1, and n2 = 0.7908 mmol g−1 with ΔH2 = −83.6 kJ mol−1). DRIFTS measurements after pyridine adsorption showed that USY zeolite possesses only Bronsted acidity and that cerium impregnation leads to the appearance of Lewis sites. Based on these results, three families of acid strength were distinguished: (i) strong Bronsted sites (ΔH > −130 kJ mol−1); (ii) Bronsted sites with intermediate strength (−100

Biodiesel (FA esters) has become very attractive as an alternative diesel fuel owing to its environmental benefits. Transesterification is the most usual and important method to make biodiesel from vegetable oils. This article investigates the potential for using Raman spectroscopy to monitor and quantify the transesterification of soybean oil to yield ethyl esters. The differences observed in the Raman spectra of soybean oil after transesterification were a peak at 2932 cm−1 ( $$v_{CH_2 }$$ ), the displacement of the v C=O band from 1748 to 1739 cm−1, and the bands at 861 (v R-C=O and v C-C) and 372 cm−1 (δ CO-O-C). Uni- and multivariate analysis methods were used to build several analytical curves and then applied in known samples, treated as unknowns, to test their ability to predict concentrations. The best results were achieved by Raman/PLS calibration models (where PLS=partial least squares regression) using an internal normalization standard (v =C-H band). The correlation coefficient (R 2) values so obtained were 0.9985 for calibration and 0.9977 for validation. Univariate regression analysis between biodiesel concentration and the increasing intensity of $$v_{CH_2 }$$ band or v C=O displacement showed R 2 values of 0.9983 and 0.9742, respectively. Although spectroscopic methods are less sensitive than chromatographic ones, the data obtained by spectroscopy can be correlated with other techniques, allowing biodiesel yield and quality to be quickly assessed.

Preparation, structural characterization and solid acidity studies of H3PW12O40 (H3PW) supported (20, 40, 50 and 80 wt.%) on γ-alumina have been performed. Impregnation of H3PW on γ-alumina was performed by the evaporation technique using water (acidified with HCl), ethanol and acetonitrile as solvents. The presence of the Keggin anion on alumina surface is very dependent upon the preparation conditions and pH, as indicated by partial decomposition in water solution. High acid concentration in aqueous solutions is required to obtain intact the Keggin anion. Ethanol showed the formation of distinct species on the alumina surface. In contrast, acetonitrile has not displayed any significant decomposition of the Keggin anion. All these conclusions are based on FTIR, Raman, 31P MAS–NMR, XRD and SEM–EDX measurements. The presence of the Keggin structure on the alumina surface can be followed by those techniques, eliminating any doubt about collapse of the supported anion. Calorimetric and adsorption of pyridine interactions in cyclohexane slurry (Cal-ad method) showed that the catalyst H3PW/Al2O3 is a weaker acid than pure H3PW or supported on silica, but stronger than alumina. The enthalpies indicated partial neutralization of the most basic sites of alumina at the expense of the strongest protons of H3PW.

Supported niobium pentoxide materials have been effective catalysts for a variety of acid and redox reactions (e.g., dehydration of alcohols, esterifications, etc.). To the best of our knowledge, there are no reports about Nb2O5 supported on silica−alumina. Catalysts of Nb2O5/SiO2−Al2O3 were prepared with 2, 5, 10, 15, 20, and 25 wt % of Nb2O5 by aqueous solution impregnation using ammonium niobium oxalate on silica−alumina. The materials, after being dried at 100 °C, were calcined at 800 °C and characterized by several methods. Investigation through X-ray diffraction showed the typical patterns of crystalline Nb2O5, which were composed of mixtures of orthorhombic and monoclinic phases (T, M, and H, respectively) present in the materials with content higher than 10 wt %. DTA curves displayed an exothermic peak at 1356 °C (average) without mass loss (confirmed by TG), which may be ascribed to a phase transition (H phase formation) of Nb2O5 supported on silica−alumina. Pure Nb2O5·nH2O showed a transition fr...