Sulfated Zirconia Samples Research Articles

Effect of the Sulfated Zirconia Nanostructure Characteristics on Physicochemical and Electrochemical Properties of SPEEK Nanocomposite Membranes for PEM Fuel Cell Applications

AbstractThis study expresses that the characteristics of sulfated zirconia (SZ) nanostructure in sulfonated poly(ether ether ketone) (SPEEK)‐based membranes is the key to optimize their properties for fuel cell applications. Two types of SZ treating in different thermal conditions are produced by precipitation and analyzed by X‐ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, energy‐dispersive X‐ray analysis, and transmission electron microscopy. Sulfate concentration on both types of SZ is changing from 0.54 to 1.45 wt% and thus the SZ samples differ in particle size and sulfate content. The nanocomposite membranes are prepared by incorporating 6 wt% of SZ samples into SPEEK matrix in casting procedure followed by performing electrochemical characterizations and impedance spectroscopy. Moreover, morphology, mechanical, and chemical stability of the membranes are investigated by field emission scanning electron microscopy, stress–strain, and ex situ Fenton's tests. The incorporation of SZ sample having more surface sulfate groups in the SPEEK matrix results in not only excessive oxidative stability and tensile strength but also more acidic sites for ion transport, promoting conductivity. Furthermore, both types of nanocomposite membranes show improved ionic conductivity and water affinity with a lower tendency to swell rather than the plain SPEEK membrane. It is proved that desired consequences of doping SZ into SPEEK matrix can be intensified by changing the physicochemical properties of sulfated zirconia nanoparticles.

Quantitative Analysis of IR Intensities of Alkanes Adsorbed on Solid Acid Catalysts

The goal of this work was to determine the integrated molar absorption coefficients of adsorbate vibrations and to use them as a measure of the activation of chemical bonds by the catalyst surface. For this purpose, an apparatus was built for the simultaneous measurement of transmission IR spectra of the adsorbate and the corresponding adsorption isotherm; the adsorbed amount was determined by a volumetric–barometric method. The adsorption of n-butane and 2,2-dimethylpropane on self-supporting wafers of mordenite and sulfated zirconia samples was investigated at temperatures of 298–308 K and alkane pressures of 0–100 hPa. The linear range of plots of the IR band intensities vs coverage was used to determine absorption coefficients of C–H stretching vibrations. The coefficients were found to depend on particle size and morphology, and it is shown that they are overestimated because the path length is enhanced through strong scattering in the wafers. The results reveal the limitations of quantitative analysis by transmission IR spectra when fine powders (such as catalysts) are investigated.

Glycerol acetylation on sulphated zirconia in mild conditions

This work focuses on the role of the hydrolysis ratio over the physicochemical properties of sulphated zirconia (SZ) obtained by the sol–gel method. The solids obtained were tested in the esterification of acetic acid diluted in glycerol, the latter being used both as reactant and solvent. The acidic properties of the SZ samples and their performances in the acetylation reaction have been successfully correlated. The recyclability has also been studied: sample SZ-1 (sulphated zirconia prepared with a hydrolysis molar ratio=1) shows a progressive deactivation in successive tests due to sulphur leaching which induces an activity in the homogeneous phase.

In situ FTIR study of the adsorption and reaction of ortho-dichlorobenzene on Pd–Co sulfated zirconia catalysts

The adsorption and oxidation of ortho-dichlorobenzene ( o-DCB) over sulfated zirconia (SZ), Co–SZ, and Pd/Co–SZ were studied by means of in situ FTIR spectroscopy to determine the nature of surface intermediates formed on these catalysts. The results indicate that o-DCB adsorbs on Lewis acid sites, forming surface intermediates that are strongly held on the surface. These intermediates can be subsequently oxidized to yield various partial oxidation products in either the presence or absence of gas-phase oxygen, confirming that surface oxygen is involved in the oxidation process. On Pd/Co–SZ samples, the oxidation is faster in the presence of gas-phase oxygen, yielding higher concentrations of partial oxidation products on the catalyst surface.

Study on reuse of metal oxide-promoted sulphated zirconia in acylation reactions

A series of sulphated zirconia samples (SZ) promoted with Al2O3, Ga2O3, and Fe2O3 were synthesized by co-precipitation at constant pH and aged under reflux conditions. Structural, surface and catalytic properties of the samples were investigated using N2 adsorption/desorption, thermal analysis, in-situ FTIR spectroscopy, TPR–MS and EGA–MS measurements. The catalytic performance of promoted SZ in anisole acylation has been investigated. Promotion by either Fe2O3 or Ga2O3 was found to increase the catalytic activity (yield) after recycle of catalysts with respect un-promoted sample, whereas promotion by Al2O3 was observed to increase the conversion but not the yield. It is worth noting that all systems present high selectivity to the p-metoxyacetophenone product.

Precursor type affecting surface properties and catalytic activity of sulfated zirconia

Zirconium-hydroxide precursor samples are synthesized from Zr-hydroxide, Zr-nitrate, and Zr-alkoxide, by precipitation/impregnation, as well as by a modified sol-gel method. Precursor samples are further sulphated for the intended SO4 2- content of 4 wt.%, and calcined at 500-700oC. Differences in precursors? origin and calcination temperature induce the incorporation of SO4 2- groups into ZrO2 matrices by various mechanisms. As a result, different amounts of residual sulphates are coupled with other structural, as well as surface properties, resulting in various catalytic activities of sulphated zirconia samples. Catalyst activity and selectivity are a complex synergistic function of tetragonal phase fraction, sulphates contents, textural and surface characteristics. Superior activity of SZ of alkoxide origin can be explained by a beneficial effect of meso-pores owing to a better accommodation of coke deposits.

Ga-promoted sulfated zirconia systems. II. Surface features and catalytic activity

Sulfated zirconia samples having a variable Ga 2O 3 content (in the 1–15% molar range) were synthesized. The promoting effect of gallium was studied in the catalytic isomerization of n-butane at 523 K, by feeding n-butane and H 2 (with a 1:4 ratio). Catalytic activity was found to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga 2O 3 doubled the activity of sulfated zirconia, whereas for a sample with 15 mol% Ga 2O 3 the catalytic activity was completely lost. Surface chemistry of these materials was studied by means of FTIR spectroscopy and adsorption microcalorimetry, using selected probe molecules (CO and 2,6-dimethylpyridine). IR spectroscopy showed that gallium-containing sulfated zirconia samples exhibit both Lewis and Brønsted acidity. Lewis acidity is attributed to coordinatively unsaturated Zr 4+ ions located in defective surface sites, whereas Brønsted acidity is associated to surface sulfate groups. Samples with a Ga 2O 3 content between 1 and 9 mol% show a combination of Lewis and Brønsted acidity that quantitatively decreases with increasing gallium oxide content. Samples having a Ga 2O 3 content equal to 9 mol% or greater show the following specific features: (i) these samples are much more difficult to dehydrate than those with smaller gallium content, and the hydroxy groups interact by hydrogen bonding, (ii) the sulfate groups progressively lose their covalent character, and (iii) both Lewis and Brønsted acidity of the samples decreases drastically. Couples of Lewis and Brønsted acid sites appear to be needed for catalytic activity in n-butane isomerization, and they present an optimum ratio when the catalyst is brought to a medium-high dehydration degree and when its Ga 2O 3 content is of about 3–5 mol%.

Synthesis and characterization of thermally stable mesostructured sulfated zirconia by a novel sulfate-assisted alcohothermal route

Thermally stable mesostructured sulfated zirconia (MSZ) with a high surface area has been successfully prepared by a novel one-step synthetic method involving the alcohothermal processing of zirconium nitrate in the presence of ammonium sulfate. The MSZ samples were characterized using X-ray diffraction (XRD), N2 adsorption measurements, thermal analysis (TG/DTA) and transmission electron microscopies (TEM). It is shown that the resulting mesostructured sulfated zirconia samples are tetragonal crystalline, which have a high specific surface area and a well-developed textural mesoporosity with narrow pore size distribution. Catalytic testing showed that the present mesostructured sulfated zirconia was much more active than conventional sulfated zirconia for n-butane isomerization.

Synthesis and characterization of nano-crystalline sulfated zirconia by sol–gel method

Synthesis of nano-crystalline sulfated zirconia with crystallite size less than 20 nm using the one-step as well as the two-step sol–gel technique was carried out. Tetragonal phase of sulfated zirconia was obtained on calcination at 600 °C. All the sulfated zirconia samples were found to be active for benzylation of toluene showing 26–41% conversion of toluene, however, samples prepared by the one-step method showed higher conversion (33–41%) as compared to the samples prepared by the two-step method (26–28%). Furthermore, in the one-step method, the introduction of sulfuric acid prior to hydrolysis, results into improved structural and textural properties in terms of higher sulfur loading, smaller crystallite size, higher surface area, average pore diameter and higher catalytic conversion for benzylation of toluene.

Quantitative Lewis/Brönsted Ratios Using DRIFTS

The extinction coefficient ratio (ECR) of coordinatively bonded pyridine (Lpy) and protonated pyridine (Bpy) was determined in situ by diffuse-reflectance FTIR spectroscopy (DRIFTS), by exposing sulfated zirconia (SZ) containing chemisorbed pyridine to water vapor. Both pure and KBr-diluted SZ samples were analyzed. The linearity with concentration of the Kubelka−Munk transformation was verified for pyridine interacting with the undiluted SZ samples. The portability of published ECR values between transmission and diffuse-reflectance IR spectroscopy, as well as between different types of materials, was found questionable. For the SZ samples analyzed, following a statistical analysis of the data, an Lpy/Bpy ECR value of 2.05 ± 6.3% was determined, resulting in the calculation of an initial Lewis/Brönsted acidity ratio value of 1.1 in the freshly activated material. This ECR determination method should be applicable to other similar solid acids that have a window of relative transparency in the 1600−1400 cm-1 range and are capable of retaining chemisorbed pyridine upon exposure to water vapor.

Effect of Synthetic Parameters on Structural, Textural, and Catalytic Properties of Nanocrystalline Sulfated Zirconia Prepared by Sol−Gel Technique

A series of sulfated zirconia samples were synthesized by a two-step sol−gel technique using zirconium propoxide as a precursor having two different concentrations, hydrolyzed with aqueous ammonia at pH 9−10 and sulfated with sulfuric acid. Effects of various synthetic parameters were studied on the structural, textural, and catalytic properties, which were determined using XRD, FT-IR, DRIFT, N2 adsorption−desorption isotherms, and benzylation of toluene. The sol−gel technique resulted in the formation of nanocrystalline sulfated zirconia having a low crystallite size (<20 nm) predominantly in the tetragonal phase with a surface area and pore size ranging from 64 to 116 m2/g and 47−67 A, respectively. The amount of sulfur and the nature of the sulfur species bound with the zirconia surface were observed to play a significant role in determining the catalytic activity of a sample. Ultrasonication assisted sulfated zirconia samples prepared with a higher concentration of the metal precursor were found to be...

In situ infrared study of pyridine adsorption/desorption dynamics over sulfated zirconia and Pt-promoted sulfated zirconia

The dynamic behavior of adsorbed pyridine and the sulfate group has been studied over sulfated zirconia (SZ) and Pt-promoted sulfated zirconia (Pt/SZ) by in situ infrared (IR) spectroscopy coupled with mass spectrometry (MS). IR analysis confirmed that pretreating SZ in flowing He at 500 °C led to the formation of SO species. Pyridine adsorption caused desorption of sulfur in the form of SO 3. IR and MS analyses coupled with a temperature-programmed desorption (TPD) study confirmed that pyridine adsorbed onto the Lewis acid sites was oxidized to CO 2 while the pyridine–Brønsted acid site complexes showed little desorption or oxidation. Addition of Pt onto sulfated zirconia led to enhanced Brønsted acidity when treated with H 2; higher loading of Pt led to decreased thermal stability of the sulfate group, promoting desorption of SO 2 during the TPD. This is the first report that presents dynamic analyses of pyridine adsorption/desorption phenomena over sulfated zirconia samples.

Studies on sulphated zirconia: synthesis, physico-chemical characterisation and n-butane isomerisation activity

A series of sulphated zirconia samples having different weight percentages of sulphate were synthesised by wetness impregnation of zirconium hydroxide with an aqueous solution of (NH4)2S2O8. The structural, surface and catalytic properties of these samples, calcined at different temperatures were investigated using XRD, BET, FTIR, UV–VIS diffused reflectance studies (DRS), photoacoustic measurements, adsorption of pyridine from non-aqueous solvent, surface hydrophilicity and n-butane isomerisation reaction. XRD and UV–VIS DRS revealed that sulphate stabilises the tetragonal phase of zirconia by controlling embryo growth. The FTIR spectral patterns supported the presence of two different types of sulphate, which were bidentately co-ordinated to zirconium matrix. Both Brönsted and Lewis acidity were increased with increase in nominal sulphate loading. Drastic changes were observed in 15wt.% sulphated sample, which exhibited quite high values of Brönsted and Lewis acidity but very low butane isomerisation activity. The effect of pre-treatment temperature, and nominal sulphate loading on n-butane conversion suggested involvement of a Lewis–Brönsted synergy in butane isomerisation reaction.

Monoclinic and Tetragonal High Surface Area Sulfated Zirconias in Butane Isomerization: CO Adsorption and Catalytic Results

In the present study high surface area monoclinic and tetragonal sulfated zirconia samples both active in n-butane isomerization were prepared from the same hydroxide precursor in a novel one-step hydrothermal process. The process is studied via analysis of intermediate stages of the product formation. The monoclinic samples show a catalytic activity in n-butane isomerization lower by about a factor of 4 compared to the tetragonal samples, although the acidity as analyzed by CO adsorption was found to be similar. This suggests, that it is not only the acidity of sulfated zirconia that determines the catalytic performance. The results support the bimolecular mechanism, for which a favorable arrangement of surface groups is necessary. The surface structure of tetragonal zirconia seems to be better suited than that of the monoclinic zirconia, although the presented monoclinic zirconia performs substantially better than claimed in many publications, where the monoclinic phase is typically assumed to be inactive.

Manganese, iron and sulfur K edge XAFS of promoted sulfated zirconia catalysts

Promoted sulfated zirconia samples were prepared by the incipient wetness technique to produce isomerization catalysts which were active for the conversion of n-butane to isobutane at 338K (up to 10% conversion of 1% n-butane, 1atm., 0.25 h(-1) WHSV). The local structure of Fe and Mn in promoted sulfated zirconia was investigated using fluorescence yield XAS. Spectra were taken of calcined samples, activated samples, and samples after reaction with n-butane (maximum activity and deactivated). Factor analysis reveals that the Mn K edge XANES can be described by a linear combination of the spectra of two separate components, and that the ratio of these components changes with activation of the catalyst, and during use in the n-butane isomerization reaction. The change in ratio of the Mn species during activation and reaction results in a reduction of the average Mn valence from 2.4 to 2.2. The Fe K edge XANES was not similarly affected by activation and reaction with n-butane.

Catalytic acylation of aromatics with carboxylic anhydrides over sulfated zirconia

Two sulfated zirconia samples have been synthesized using different preparation procedures. Textural and acid properties of these catalysts have been characterized and compared with commercial zeolite Hβ by means of nitrogen low-temperature adsorption, temperature-programmed desorption of ammonia and microcalorimetry. The two sulfated zirconias proved to be appropriate catalysts for acylations of aromatic compounds containing electron donating substituents, e.g., for syntheses of several aromatic ketones.

Influence of the calcination temperature on the acidic and catalytic properties of sulphated zirconia

The effect of the calcination temperature ( T c from 500 to 700°C) of sulphated zirconia on acidic and catalytic properties was investigated. The sulphur content decreases from 2.5 to 1 wt% when T c increases from 500 to 700°C; the BET surface area is between 90 and 100 m 2 g −1 for all the samples with the exception of that calcined at 700°C (80 m 2 g −1). IR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Brönsted- and Lewis-acid sites from pyridine adsorption. The activities, stabilities and selectivities of the sulphated zirconia samples were compared for both n-butane and propane transformations at 250°C. For T c≤600°C, the concentrations of Lewis- and Brönsted-acid sites are practically identical. For higher calcination temperatures, the Lewis acidity increases at the expense of the Brönsted acidity. The increase with T c in the wave number of the hydroxyl band, the decrease of the ratio between the number of protonic sites retaining pyridine adsorbed at 450 and 150°C as well as the increase in the ratio of n-butane and propane reactivities suggest that the acid strength decreases when T c increases. In agreement with this proposal and with the decrease in the number of protonic sites, there is a decrease in activity when T c increases from 600 to 700°C. However, the reverse is found for lower calcination temperatures. The unexpected low activity of the samples calcined at lower temperatures may be due to their very fast initial deactivation (before the first activity measurement). Deactivation is mainly due to poisoning of the acid sites by carbonaceous deposits. However, sulphur elimination with protonic sites consumption also plays a limited role.

The role of the oxide precursor on the features of sulphated zirconia

Zirconia samples have been obtained by calcining, at 470°C, hydrous precursors prepared by following two different procedures: (a) room temperature precipitation by NH 3 at acid pH (≤2.5) followed by 96 h refluxing; (b) neutralisation (pH = 7.0–7.5) of ZrCl 4 solutions at the boiling point by KOH. The oxides, calcined starting from the two precursors, appear to be composed either of monoclinic or tetragonal (metastable) ZrO 2. Sulphated zirconia samples have been obtained by treating the precursor powders with sulphuric acid at different concentrations and subsequently calcining them at 470°C. The sulphate-doped oxides have been characterised for phase composition-crystallimity (XRD), water-sulphur content (TGA, elemental analysis), surface area (BET) and surface chemical state (XPS also in combination with ion sputtering). The different oxide precursors produce distinctive features, on the treated oxides, concerning specifically the degree of hydration/hydrophilicity of the samples and the nature of the zirconium surface sites.

Manganese, iron, cobalt, nickel, and zinc as promoters of sulfated zirconia forn-butane isomerization

First-row transition metals were tested in a flow reactor as promoters of sulfated zirconia for the isomerization ofn-butane at 373 K and 0.005 barn-butane partial pressure. The activity of each of the sulfated zirconia samples increased through a maximum and then decreased with time on stream. In order of increasing activity measured at the maximum, the effect of the promoters increased from right to left in the periodic table: zinc, nickel, cobalt, iron, and manganese. Iron and manganese increased the activity by. 2–3 orders of magnitude. The causes of the promoter action are not elucidated; the promoters may play noncatalytic roles as initiators.

Characterization and selective poisoning of acid sites on sulfated zirconia

Microcalorimetric measurements and infrared spectroscopy of ammonia adsorption were used to characterize the acidic properties of sulfated zirconia catalysts. Reaction kinetic measurements forn-butane isomerization were conducted over catalysts that were selectively poisoned with controlled amounts of ammonia. Initial heats of ammonia adsorption on the strong acid sites of sulfated zirconia were 150–165 kJ/mol, and these sites contain Bronsted acid and possibly Lewis acid centers. Sulfated zirconia samples that show high activity for the isomerization ofn-butane possess Bransted acid sites of intermediate strength, with differential heats of ammonia adsorption between 125 and 140 kJ/mol. The results of selective poisoning of sulfated zirconia with ammonia confirm that Bransted acid sites of intermediate strength are active forn-butane isomerization at 423 K while not discounting a possible role of the stronger acid sites.