Tungstated Zirconia Catalysts Research Articles

Biodiesel synthesis from microalgal lipids using tungstated zirconia as a heterogeneous acid catalyst and its comparison with homogeneous acid and enzyme catalysts

Downstream step of catalytic conversion is scarcely investigated area in microalgal biodiesel production process. In this study a heterogeneous acid catalyst, tungstated zirconia (WO3/ZrO2) is evaluated for conversion of S. obliquus lipids. Catalytic efficiency of tungstated zirconia catalyst was compared to the homogeneous acid catalyst and enzyme catalyst in terms of conversion efficiency, reaction parameters, energy consumption and reusability. Tungstated zirconia catalyst showed maximum biodiesel conversion of 94.58% at 100°C temperature, 12:1 methanol to oil molar ratio and 15% of catalyst amount based on oil weight in 3h. Tungstated zirconia showed comparable biodiesel conversion to homogeneous catalyst and higher conversion than the enzyme catalyst. The time requirement for heterogeneous catalyst was lowest, while, the energy consumption was highest among the selected catalysts. Most of the fuel properties of biodiesel synthesized by tungstated zirconia catalyzed conversion of S. obliquus lipids comply with the specifications set by ASTM and EN standards.

From laboratory catalysts to a new prototype: a novel real candidate for the isomerization of C5–C6 paraffins

An active tungstated zirconia catalyst was developed with the aim of isomerising industrial feedstocks, and presented stable activity with an increase of the R.O.N. of 14 at 225 °C.

Acid catalytic properties of reduced tungsten and niobium-tungsten oxides

Tungsten oxide and mixed niobium–tungsten oxides were prepared by calcination of re-precipitated tungsten and niobium precursors. These materials were characterized by N2 physisorption, XRD, XPS, UV–vis and UV Raman spectroscopy and H2-TPR. With increasing calcination temperature the oxides were seen to transform from a mixture of h-WO3 and HATB (T<400°C) to predominantly h-WO3 (400°C<T<470°C) and m-WO3 (T>470°C). Ammonium ion and ammonia are essential to retain the hexagonal structure of the intermediate phase. The reducibility of the various samples was linked to their structure. In-situ XRD point to formation of β-W in the presence of Pd when the precursor was deeply reduced. When the precursor predominantly contained h-WO3, (NH4)0.33−x(H3O)yWO3−z was obtained upon reduction of tungsten and mixed niobium–tungsten oxide materials. These exhibited good performance in the bifunctional hydroisomerization of n-alkanes, outperforming amorphous silica–alumina and tungstated zirconia catalysts.

Vapour phase dehydration of glycerol to acrolein over tungstated zirconia catalysts

Abstract Tetragonal (TZ) and monoclinic (MZ) polymorphs of zirconia supports were synthesised by sol–gel method followed by variation of the calcination temperature. Tungstated (10 wt% WO 3 ) supported on the zirconia polymorphs were prepared by impregnation method by using ammonium metatungstate precursor. The physico-chemical properties of the calcined catalysts were characterised by X-ray diffraction, UV–vis diffused reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), surface area and pore size distribution measurements to gain insight into the effect of morphology of the catalyst textural properties, and structure. The surface acidic properties have been determined by NH 3 TPD method and also with FT-IR spectra of pyridine adsorption. Vapour phase dehydration of glycerol to acrolein was employed to investigate the catalytic functionalities. Glycerol conversion and acrolein selectivity was mainly dependent on the fraction of moderate acid sites with majority of them are due to Bronsted acidic sites. Monoclinic zirconia based catalysts have shown the highest activity and acrolein selectivity compared to the corresponding tetragonal zirconia catalysts.

Aberration-corrected HRTEM study of Mn-doped tungstated zirconia catalysts

Solid acids as tungstated zirconia have received significant attention for isomerization reactions. The identification of catalytically active WOx nanostructures supported on ZrO2 presents significant challenges, since different surface WOx species have been proposed. In this work the results of the structure and domain size of the very small tungsten oxides species (WOx) supported on zirconia are presented. Electron micrographs of a Mn–WOx–ZrO2 active catalyst obtained in an aberration-corrected high resolution transmission electron microscope (HRTEM) allowed direct imaging of subnanometer tungsten oxide, from mono- to poly-tungstate WOx species as well as the presence of tungsten oxides crystallites (WO3) over ZrO2 in a homogeneous distribution. It is shown that it is feasible to identify and measure these WOx nanospecies supported on zirconia by using an aberration-corrected HRTEM.

Relation between surface acidity and reactivity in fructose conversion into 5-HMF using tungstated zirconia catalysts

Catalytic dehydration of fructose and its conversion to 5-hydroxymethylfurfural was studied using tungstated zirconia oxides, with various tungsten oxide loadings (1–20 wt.%). The samples were prepared by incipient wetness impregnation and thoroughly characterized using a combination of different techniques: structural, thermal and calorimetric analyses. Zirconia was predominantly present in the investigated samples in the tetragonal phase when the WO3 loading was above 10 wt.%. The samples exhibited amphoteric characteristics, as they adsorbed both ammonia and sulfur dioxide on their surface. The number of surface acid sites increased with increasing WO3 content. Fructose dehydration tests evidenced the formation of 5-hydroxymethylfurfural and by-products (formic and levulinic acids). The results show that the ratio of basic to acidic sites of the solid catalysts is the key parameter for the selectivity in 5-HMF, while the global fructose conversion was mainly related to the presence of acid sites of a given strength with 150 > Qdiff > 100 kJ·molNH3− 1.

Hydrodechlorination of dichloromethane with mono- and bimetallic Pd–Pt on sulfated and tungstated zirconia catalysts

Monometallic (Pt or Pd) and bimetallic (Pt–Pd) catalysts supported on zirconia promoted with sulfate (SZ) or tungsten oxide (WZ) were prepared and tested in the gas-phase hydrodechlorination of dichloromethane. The catalysts showed a high selectivity to non-chlorinated products (between 80% and 90% at 250°C) being methane the main reaction product. As a general trend, the WZ catalysts yielded significantly higher dichloromethane conversion than the SZ ones, yielding all the catalysts initial conversions higher than 80% at a reaction temperature of 250°C. However, the former showed a very poor stability regardless of the metallic active phase. On the other hand, the presence of palladium in the sulfated zirconia catalyst avoids deactivation as proved in long-term experiments (80-h time on stream). XPS and elemental analyses of the used catalysts suggest that adsorption of organochlorinated species is a cause of deactivation by blocking the active sites. In the monometallic SZ Pt catalyst, deactivation occurs also by poisoning of the Pt sites by the H2S resulting from sulfate reduction under the hydrogen-rich gas atmosphere. The metal particle size appears to be a critical point with regard to stability of the catalysts. The one with the highest dispersion showed the highest stability with no signs of deactivation after more than 80h on stream.

High-throughput study of the iron promotional effect over Pt/WOx–ZrO2 catalysts on the skeletal isomerization of n-hexane

Pt supported on tungstated zirconia catalysts (Pt/WZ) doped with different Fe contents were evaluated in the n-hexane hydroisomerization reaction. High-throughput experimentation (HTE) approaches were used to investigate the effect of the iron incorporation over the catalytic activity. It was found that the presence of iron at low concentrations (0.5–1wt%) increases the conversion and the yield to the high octane 2,2-dimethyl-butane (2,2-DMB) isomer. The characterization results suggest that the presence of iron modifies the nanostructure of the WOx species on the zirconia surface which in turn produces the generation of active Brønsted acid sites thus optimizing the tungsten content toward the highest catalytic activity. It was also found that the surfactant incorporation further improves the performance of Pt/WZ catalysts, although, the activity was improved with the Fe addition at lower concentration.

Acid and redox properties of tungstated zirconia catalysts

Various tungstated zirconia catalysts with a WO3 loading of about 16 wt% were characterized both in their acid and oxidation properties. The samples have been characterized in their micro-structural and surface properties by BET, X-ray diffraction, Raman spectroscopy, temperature programmed reduction, elemental chemical analysis. The surface acidity was determined by the techniques of NH3 adsorption microcalorimetry and pyridine infrared spectroscopy (FT-IR). Improved acidity has been detected upon addition of WO3 to zirconia by both techniques. The global acid strength and the total number of acid sites increased greatly with the formation of WOx clusters on the zirconia support. This acidity increase can be attributed to the creation of Bronsted acid sites generated by the well dispersed WOx domains, as observed by FT-IR pyridine desorption.

Investigation of different modifiers for nanocrystal zirconia on W/ZrO 2 catalysts via esterification

The present work focused on investigation of different modifiers, such as Si, Al, Bi, and Pb on the tungstated zirconia catalysts. First, the different modifier precursors were introduced onto the nanocrystal zirconia having molar ratio (metal: Zr) at 0.01:1. Then, the 15 wt% of tungsten (W) was impregnated onto the modified zirconia. The physicochemical properties influenced by the modifiers were determined using the N 2 physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and electron spin resonance (ESR). In order to measure the catalytic activity, esterification of acetic acid and 1-heptanol was performed. It was found that the modifiers apparently resulted in dramatically decreased activity due to decreased acidity of the catalysts. It can be proposed that the modifiers had three major roles; (i) inhibit the phase transformation, (ii) enhance the presence of F-center (electrons stabilized in oxygen vacancy), and (iii) increase strong interaction of adsorbed species on zirconia. The third effect was the key factor leading to pronounced decrease in acidity of the catalysts.

Study on Solvent/Alkoxide Molar Ratios on Synthesis Zirconia Nanoparticles for Tungstated Zirconia Catalysts Over Esterification

AbstractA series of nanocrystal zirconia samples were synthesized, characterized and studied for the esterification reaction. The results showed that the solvothermal method under different solvent/alkoxide molar ratios led to remarkable effect on the amount of carbon residue remained on the surface of catalysts resulting in the difference in proton species-support interaction behavior. Moreover, the esterification activity of W/ZrO 2 varied distinctly with the solvothermal synthesis condition of nanocrystal zirconia.Graphical AbstractThe correlation between the amount of carbon residue and the esterification activity of the catalyst was primarily established. It was proposed that the esterification activity was strongly dependent on the amount of carbon residue on the surface of catalyst and the degree of interaction between the zirconia and adsorbed species leading to the difference in acidity of catalyst.[IMAGE]

Reaction Kinetics and Mechanisms for Hydrolysis and Transesterification of Triglycerides on Tungstated Zirconia

Hydrolysis and transesterification are two reactions which can occur during the synthesis of biodiesel. An Investigation of the mechanistic pathways in hydrolysis and transesterification were carried out at a relatively high temperature (100–130 °C) and moderate pressures (120–180 psi) with tricaprylin and water for hydrolysis or methanol for transesterification using a tungstated zirconia catalyst in a batch reactor. It was found that upon increasing the concentration of TCp, the reaction rates for both hydrolysis and transesterification increased at all conditions. In contrast, water inhibited the reaction rate of hydrolysis by poisoning the active sites. For transesterification, the apparent reaction order of methanol evolved from positive to negative as the concentration of methanol relative to TCp increased. Using a reaction model discrimination procedure, it was found that hydrolysis on WZ could be successfully described by an Eley–Rideal single site mechanism with adsorbed TCp reacting with bulk phase water. The mechanistic pathway for transesterification also seems to follow a similar mechanism, identical to the one previously proposed for transesterification on a solid acid catalyst (SiO2-supported Nafion-SAC-13) at lower temperature (60 °C).

Solvent effect on synthesis of zirconia support for tungstated zirconia catalysts

Solvothermal reaction of zirconium n-butoxide (ZNB) in different solvent media, such as 1,3-pentanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol resulted in the formation of zirconium dioxide (ZrO 2) nanostructure. Then, the 15%W/ZrO 2 (WZ) catalysts using different zirconia supports were prepared by impregnation method. The effects of solvent on preparation of zirconia on the catalytic performance of WZ catalysts in esterification of acetic acid and methanol at 60 °C were investigated. The experimental results showed that ZrO 2 particles prepared in 1,4-butanediol (ZrO 2-BG) have a spherical shape, while in other glycols the samples were irregularly-shaped particles. The reaction results of esterification illustrated that the W/ZrO 2-BG catalysts had high surface acidity and showed high acetic acid conversion. The W/ZrO 2-PeG catalysts (ZrO 2 particles prepared in 1,5-pentanediol, PeG) exhibited the lowest surface acidity among other samples due to strong interaction of proton species and the zirconia supports as proven by TGA. One of the possible reasons can be attributed to different amounts of carbon residue on the surface of catalysts.

Study of the Formation of WOx Nanostructures Supported on Zirconia Prepared by Different Synthesis Routes

AbstractDifferent synthesis procedures of Pt supported on tungstated zirconia catalysts (Pt/WOx-ZrO2) were investigated with the aim to elucidate the different WOx nanostructures developed on the zirconia surface depending on the preparation route. Pt/WOx-ZrO2 catalysts were synthesized by the coprecipitation and impregnation methods and pretreated by various procedures such as different calcinations temperatures or the use of reflux. The catalysts characterization was carried out by X-ray diffraction (XRD), Raman spectroscopy, transmission and scanning electron microscopy (TEM, SEM) and nitrogen physisorption, and the catalytic activity was evaluated in the n-hexane isomerization reaction. The results indicate that the development of active sites for isomerization of n-hexane is enhanced by the stabilization of the WOx nanostructures on the surface of zirconia, before the formation of the WO3 crystallites, and it largely depends on the synthesis method.

The Role of Zirconia Surface on Catalytic Activity of Tungstated Zirconia via Two-Phase Esterification of Acetic Acid and 1-Heptanol

It is proven that surface nature of zirconia can play crucial roles on determining the catalytic activity of tungstated zirconia catalysts. The solvothermal method for preparation of crystalline zirconia support can result in the formation of Zr–OH bond on its surface. This surface species related to the Zr-heteropolyacid, acting as strong Bronsted acid sites. It consequently affected on the catalytic activity.

Study of acidic commercial WO x /ZrO2 catalysts by adsorption microcalorimetry and thermal analysis techniques

In this work we report about the characterization of the acidic and redox properties of four different commercial tungstated zirconia catalysts with W loadings of about 12–13 mass%. The samples have been characterized in terms of their micro-structural and surface properties by BET, X-ray diffraction, temperature programmed reduction, elemental chemical analysis and adsorption microcalorimetry of NH3. Improved acidity has been detected upon addition of WO3 to zirconia and differences between the samples were pointed out thanks to the results obtained by the complementary physico-chemical techniques used in this study.

Study by high-throughput experimentation of the effect of the pretreatment and precursors on the catalytic activity of tungstated zirconia catalysts

Abstract High-throughput experimentation (HTE) was used for the synthesis, characterization and catalytic evaluation of tungstated zirconia (WZ) materials doped with manganese. A library with three series of MnOy–WOx–ZrO2 catalysts was prepared by different methods (surfactant-assisted coprecipitation and impregnation), using different precursors and pretreatments (washing and/or aging). These materials were characterized by XRD, Raman and MET (HRTEM and EDS). Different nanostructures were observed depending on the synthesis method. Highly dispersed WO3 catalysts calcined at 800 °C were obtained by coprecipitation method, while the impregnation method brings the segregation of both, the monoclinic WO3 and ZrO2 phases. The catalytic activity of the Pt-promoted MnOy–WOx–ZrO2 catalysts (0.3 wt.% Pt) was evaluated in the n-hexane hydroisomerization reaction. Although, in the coprecipitated catalysts similar WOx nanostructures were observed, the catalytic activity mainly depends on the zirconia precursor, as well as aging and washing pretreatments, following the order: ZrOCl2 (coprecipitation) > ZrO(NO3)2 (coprecipitation) ≫ Zr(OH)4 (impregnation). The catalysts with highest activity were pretreated by washing and aging.

Tungstated Zirconia Catalysts for Liquid-Phase Beckmann Rearrangement of Cyclohexanone Oxime: Structure-Activity Relationship

The performance of tungstated zirconia catalysts with different tungsten loadings and calcination temperatures for liquid-phase Beckmann rearrangement of cyclohexanone oxime to e-caprolactam was studied and the relation of their activity with the structure of tungsten species on zirconia surface was investigated. Cyclohexanone was the major product when zirconia was used as the catalyst while e-caprolactam was formed in major amounts with tungstated zirconia. The relative amounts of these products depended on the W surface density, and the maximum selectivity to e-caprolactam was observed at tungsten loadings near that required for the formation of a monolayer. UV−visible absorption edge energies suggested that the surface contains polytungstate species at these loadings. These polytungstate domains create Bronsted acid centers on the surface, as suggested by acidity measurements using NH3 adsorption microcalorimetry and FTIR of adsorbed pyridine, thereby making the tungstated zirconia catalysts active fo...

Influence of calcination treatment on the activity of tungstated zirconia catalysts towards esterification

Abstract In the present study, zirconia prepared by the solvothermal method was calcined under reductive (H2) and oxidative (O2) treatment to obtain ZrO2-H2 and ZrO2-O2, respectively. Then, they were impregnated with 15 wt% of tungsten (W) to produce W/ZrO2-H2 and W/ZrO2-O2 catalysts for esterification. It revealed that the presence of F-center was more dominant under the reductive treatment during calcinations of zirconia. Consequently, this can result in increased dispersion of tungsten indicating the existence of the surface monotungstates on the W/ZrO2-H2 catalyst. Therefore, the enhanced activity obtained from the W/ZrO2-H2 catalyst can be attributed to increased dispersion of tungsten and acidity.

EXAFS spectroscopy as a tool to probe metal–support interaction and surface molecular structures in oxide-supported catalysts: application to Al2O3-supported Ni(ii) complexes and ZrO2-supported tungstates

EXAFS spectroscopy is shown as a tool of prime importance to probe the formation of metal-oxygen-support bonds and unravel the surface molecular structure in oxide-supported systems through two examples: (i) a molecular metal complex (Ni(II) bisglycinate) characterized after impregnation and drying on Al2O3, and (ii) a tungsten oxide nanophase characterized after deposition on zirconia and high temperature thermal treatment (tungstated zirconia catalysts, i.e. WOx/ZrO2). Unlike other spectroscopic techniques, EXAFS at the Ni K-edge proves that a modest thermal activation during the impregnation step triggers the grafting of nickel(II) bisglycinate onto the support: Al next-nearest neighbours are detected when the impregnation is carried out at 60 degrees C instead of room temperature. Characterization of WOx/ZrO2 catalysts shows the presence of W next-nearest neighbours around tungsten, with W-W distances distinctive of edge-shared WO6 octahedra only. The WOx overlayer can thus be described as bidimensional, nanometric slabs of 4 to 5 WO6 units on each side. In these slabs, W octahedra are interconnected to form a more condensed structure than the one present in bulk WO3 (in which linkage through corners exists). Moreover, EXAFS results conclusively demonstrate that the WOx overlayer is directly anchored to the ZrO2 surface by means of W-O-Zr bonds with a W-Zr distance of 3.14 A.