Sulfated Zirconia Catalysts Research Articles

Rapid genesis of active phase during calcination of promoted sulfated zirconia catalysts

Amorphous sulfated zirconium hydroxide was promoted with iron or manganese via the incipient wetness technique to give 0.5–3.5 wt% promoter in the final catalyst. An exothermic reaction during the calcination temperature ramp (to 823 or 923 K) leads to a rapid overheating (“glow”) of the sample. X-Ray diffraction shows that crystallization starts before and progresses during the overheating. The surface area shrinks during the glow, and its final size (85–120 m 2 g −1) and the porosity appear to be largely determined by the glow event. Manganese and iron ions prevent the coalescence of particles and lead to high surface areas. Variation of the batch volume (2.2, 8.4, or 17.1 ml) for calcination produced different catalysts from the same precursor. For both promoters, samples calcined in large batches exhibited the highest surface areas, interconnected mesopores (2.2–3.8 nm), and the highest maximum rate in n-butane isomerization (338 K, 1 kPa n-butane). Long-term performance was independent of surface area and morphology. The concentration of the active sites depended on the calcination batch size, indicating that the active phase is formed in kinetically controlled reactions during the rapid overheating. The catalytic and textural properties of sulfated zirconia catalysts can be reproduced by controlling the batch size used for calcination.

Solvent free isomerisation of longifolene with nano-crystalline sulphated zirconia

Nano-crystalline sulphated zirconia catalyst was synthesised using sol–gel technique in acidic, basic and neutral medium. The samples were characterised by X-ray diffraction, DRIFT spectroscopy, nitrogen adsorption–desorption and TG analysis. Bronsted acidity was measured by cyclohexanol dehydration model reaction. The samples were studied for single step, solvent free isomerisation of longifolene to iso-longifolene in a liquid phase batch reactor. The catalyst yielded >90% conversion and ∼100% selectivity for iso-longifolene.

Surface characterization of zirconia, holmium oxide/zirconia and sulfated zirconia catalysts

Zirconia-supported holmium oxide, as well as their sulfated catalysts using (NH 4) 2SO 4 were prepared by wet impregnation and calcined at different temperatures. The samples thus obtained were subjected to bulk and surface texture analysis using X-ray powder diffractometry (XRD) and N 2-sorptiometry, respectively. The surface acidic properties were characterized by in situ FT-IR spectroscopy of adsorbed pyridine molecules. The decomposition of gas phase 2-propanol molecules on the catalysts was examined at different temperatures, also using in situ FT-IR spectroscopy. The results obtained revealed that the presence of Ho 2O 3 on ZrO 2 induces the formation of cubic and/or tetragonal ZrO 2 phase at low temperatures (≤800 °C) without significant effect on the surface area. Whereas, the presence of Ho 2O 3 and sulfate ions on ZrO 2 results in the highest formation of cubic and/or tetragonal ZrO 2 phase at low temperatures and induces stabilization of the surface area. The surface of Ho 2O 3/ZrO 2 samples exposes Lewis acid and reactive basic sites, whereas the sulfated samples show at least two types of Lewis and Brönsted acid sites with enhanced acid strength. The decomposition of 2-propanol results show that the sulfate-free samples are dual functioning catalysts at 300 °C; 2-propanol molecules decompose thereon via dehydration into propene and dehydrogenation into acetone. However, the sulfated samples are found to only catalyze the dehydration into propene at all reaction temperatures applied.

On the catalytic nature of Mn/sulfated zirconia for selective reduction of NO with methane

In this work, the catalytic nature of Mn loaded sulfated zirconia (SZ) catalysts for the selective catalytic reduction (SCR) of NO with methane was investigated by a combination of reactions and characterizations such as FT-IR spectroscopy, H2-TPR, UV–vis diffuse reflectance spectroscopy (DRS) and NO-TPD. It was found from the results of reactions and FT-IR spectra that the strong Brønsted and Lewis acid sites in the Mn/SZ catalysts were essential for the SCR of NO with methane. The loading of Mn increased the number of strong Lewis acid sites on the surface of SZ catalyst, which is one reason for its promoting effect. On the other hand, FT-IR spectra, H2-TPR and UV–vis DRS of the catalysts demonstrated that the presence of the SO42− species occupied the terminal OH sites on the surface of ZrO2 support and thereby restrained the formation of more oxidative and nonstoichiometrically dispersed MnOx (1.5<x<2) phase. Such an effect of SO42− suppressed the combustion reaction of CH4 by O2 and increased the selectivity towards NO reduction. The NO-TPD showed that the loading of Mn increased the adsorption of NO over SZ catalyst, which is another reason for the promoting effect of Mn.

Isomerization of light alkanes: preparation and characterization of platinum promoted sulfated zirconia catalysts

Platinum-promoted sulfated zirconia catalysts active in n-hexane isomerization were prepared in a one-step synthesis and characterized. The crystallinity and sulfur contents depend mainly on the calcination temperature. The stability of the sulphate in the presence of hydrogen is reduced by the presence of Pt.

Study of alumina-promoted SO2-4/NiO/ZrO2 catalyst performance

Improvement of the sulfated zirconia catalysts (SZN) was studied by adding different amounts of alumina as follows: 17.5, 29.8, 45.9, and 56wt %. Even though, alumina, is expect to promote higher specific area and increase the amount of acid sites, stability of the zirconia tetragonal phase, the active zirconia phase of the SZN catalyst isisomerization reaction was obtained for catalysts containing between 17.5, and 29.8wt % of alumina, which reflects the promotional effect of alumina. On the contrary, a strong reduction of this reaction rate is observed when higher amounts of alumina are added to the catalyst, which indicates the dilution effect of alumina.

Sulfated zirconia as an efficient catalyst for organic synthesis and transformation reactions

The efficacy of sulfated zirconia catalyst was investigated towards various acid-catalyzed organic syntheses and transformation reactions in the liquid phase. The SO 4 2−/ZrO 2 efficiently catalyzes synthesis of 1,5-benzodiazepine derivatives, electrophilic substitution of indoles with aldehydes to afford the corresponding bis(indolyl)methanes, synthesis of 3,4-dihydropyrimidinones, synthesis of diaryl sulfoxides, and tetrahydropyranylation of alcohols and phenols. Various advantages associated with these protocols include, simple work-up procedure, solvent-free conditions, short reaction times, high product yields and easy recovery and reusability of the catalyst. The SO 4 2−/ZrO 2 catalyst was obtained by immersing a finely powdered hydrous Zr(OH) 4 into 1 M H 2SO 4 solution and subsequent drying and calcination at 923 K. The Zr(OH) 4 was prepared from aqueous ZrOCl 2·8H 2O solution by hydrolysis with dilute ammonium hydroxide. The bulk and surface properties of the prepared catalysts were examined by X-ray powder diffraction, BET surface area, ammonia-TPD and Raman spectroscopy techniques. All characterization results revealed that the incorporated sulfate ions show a significant influence on the surface and bulk properties of the ZrO 2. In particular, XRD and Raman results suggest that impregnated sulfate ions stabilize the metastable tetragonal phase of ZrO 2 at ambient conditions. Ammonia-TPD and BET surface area results indicate that sulfated catalyst exhibits enhanced acid strength and specific surface area than that of unprompted ZrO 2.

In situ 13C MAS NMR Study on the Mechanism of Butane Isomerization Over Catalysts with Different Acid Strength

Reaction mechanism of skeletal isomerization of n-butane over sulfated zirconia (SZ), Cs2.5H0.5PW12O40 (Cs2.5) and H-form mordenite (H-MOR) catalysts was studied using 13C MAS NMR with 13C-labeled n-butane. The isomerization of n-butane over SZ type catalysts proceeds predominantly via a monomolecular mechanism below 333 K and gradually changes to a bimolecular alkylation-β-scission mechanism as the reaction temperature is increased to 423 K. Iron promoter in SZ catalyst facilitates the bimolecular process. The n-butane isomerization over Cs2.5 also proceeds mainly via a monomolecular mechanism below 373 K. The bimolecular mechanism becomes significant as the reaction temperature is increased to 423 K. On both SZ and Cs2.5 catalysts hydrogen inhibits the isomerization reaction, in particular the bimolecular process. In contrast, the n-butane isomerization over H-MOR with relatively moderate acid strength proceeds mainly via a bimolecular mechanism at 473 K. The kinetics of n-butane isomerization on SZ below 333 K and Cs2.5 below 373 K are well represented by the Langmuir–Hinshelwood equation for a reversible first order surface reaction, further supporting that a monomolecular mechanism proceeds primarily on SZ and Cs2.5 catalysts at early reaction stage. All results suggest that the stronger the acidity of the catalyst the lower the reaction temperature of n-butane isomerization and the more contribution of the monomolecular mechanism. The overall mechanism of 1−13C-n-butane reaction on SZ, Cs2.5 and H-MOR catalysts including 13C scrambling and butane isomerization is proposed.

In situ diffuse-reflectance infrared spectroscopic investigation of promoted sulfated zirconia catalysts during n-butane isomerization

Sulfated zirconia, unpromoted or promoted with either 2 wt% iron or manganese, was investigated by in situ diffuse reflectance IR spectroscopy during activation, n-butane isomerization (323–378 K, 1–5 kPa n-butane), and regeneration. During activation at 773 K in N 2 or O 2, more than 95% of the adsorbed water was removed, and comparison of the resulting pattern of sulfur–oxygen vibrations with DFT calculations (A. Hofmann and J. Sauer, J. Phys. Chem. B 108 (2004) 14652) suggests S 2 O 7 2 − as a major surface species. All catalysts underwent an induction period before maximum rates were measured, which at 1 kPa n-butane were 350 and 180 μmol g −1 h −1 for the Fe- or Mn-promoted catalyst (323 K) and 5 μmol g −1 h −1 for sulfated zirconia (358 K). During isomerization several bands increased in the range of 1700–1600 cm −1; the growth of a band at 1600 cm −1 was restricted to the period of increasing catalytic activity. The rate of isomerization was linearly correlated with the total area increase of the bands in this region, indicating that the absorbing species are either active surface intermediates or side products (possibly water) formed in an amount proportional to the active surface species. The rate per such species was much higher in the presence of Mn; that is, promoters facilitate reaction steps after the initial alkane activation. The promoted materials deactivated within hours under the applied conditions, but complete recovery of the spectral and catalytic properties of Mn-promoted sulfated zirconia was possible through regeneration in O 2 at 773 K. Attempts to regenerate in N 2 produced unsaturated surface species absorbing at 1532 and 1465 cm −1.

Optimization of synthesis variables in the preparation of active sulfated zirconia catalysts

The synthesis of a series of sulfated zirconia catalysts was optimized using the isomerization of n-butane as a reaction probe. The normality of the H2SO4 solution used in the sulfation step was found to be the most important variable. A systematic change in the concentration of the H2SO4 solution showed that the optimum acid concentration was 0.25 N. When a catalyst prepared with this acid concentration was used, the conversion of n-butane at 200 °C was 35% at 5 min t-o-s. This was close to the thermodynamic equilibrium value of 56% conversion. This maximum was coincident with a catalyst with the highest specific surface area. An increase in the concentration of the H2SO4 solution above 0.25 N resulted in a decrease in both surface area and zirconia crystallinity. XPS studies showed a linear relationship between the H2SO4 solution concentration and the surface sulfur concentration. Bulk concentrations were determined by elemental analysis. The surface area increased to a maximum for a H2SO4 concentration of 0.25 N, while the concentration of bulk sulfur continued to increase when the acid concentration was progressively increased to 2.00 N. The use of a mordenite trap in the reactant stream resulted in an increase in n-butane conversion and a decrease in the rate of catalyst deactivation. XPS studies showed that the sulfur was present as sulfate species and that the oxidation state was not affected by the reaction.

Isomerization of n-butane over sulfated zirconia catalyst under supercritical conditions

Heterogeneous catalytic processes in supercritical fluids offer environmentally benign alternatives to current catalyst technologies. In the case of isomerization of light alkanes over solid acid catalysts at atmospheric pressure, the catalysts deactivate rapidly due to coke formation on the catalyst surface. To solve this problem, we studied the isomerization of n-butane over sulfated zirconia in a n-butane supercritical condition of reactant and products. Under the supercritical condition, no significant deactivation was observed, and the steady state activity was maintained. Furthermore, the active sites of sulfated zirconia were characterized by FT-IR spectroscopy: it was found that Lewis acid sites on sulfated zirconia play an important role in the isomerization of n-butane.

Comparison between Ni and Pt promoted SO42??ZrO2 catalysts for n-octane hydroisomerization-cracking

The production of C4–C6 branched isoalkanes by isomerization-cracking of n-octane (300 °C, 0.1 MPa) using sulfated zirconia catalysts was studied. The effect of Ni addition on the activity and selectivity was compared to the effect of Pt. Pt enhanced the activity and stability of the catalysts. Ni mainly improved the selectivity to isoparaffins (maximum yield at 1% Ni). n-Octane conversion fully correlated with total acidity. Average and final stable conversion values were in comparison lower for Ni catalysts. This result was addressed to the lower metal activity of Ni and to the detrimental effect of sulfated zirconia on the reducibility of the metal.

Approaching Equilibrium Conversion in the Isomerization of Supercritical n-Butane by Using Sulfated Zirconia Catalysts Promoted with Iron

The application of supercritical temperatures and pressures in the isomerization of n-butane is a means to avoid deactivation of the catalyst by coke and to enhance the throughputs. However, catalysts consisting of unpromoted sulfated zirconia are not sufficiently active to attain equilibrium conversions of n-butane. Through impregnation of iron salts on sulfated zirconium hydroxide, we produced catalysts containing 1 wt % iron with strongly enhanced activity. In pelletized form, these catalysts enabled the processing of n-butane at conversions close to thermodynamic equilibrium. At the preferred reaction temperature of 488 K, the yield of isobutane amounted to 43%. Catalysts manufactured by extrusion were considerably less active.

Iron and manganese promoted sulfated zirconia: acidic properties and n-butane isomerization activity

Sulfated zirconia catalysts promoted with Fe and Mn have been synthesized and the acidity characterized by IR spectroscopy of adsorbed pyridine. The catalytic isomerization of n-butane was investigated in a fixed bed reactor operated at low conversion at atmospheric pressure and 250 °C. The main effect of the promoters was to change the ratio Brönsted:Lewis acidity of the samples. The catalytic activity was found to be correlated with the number of strong Brønsted acid sites.

Interaction between sulfated zirconia and alkanes: prerequisites for active sites—formation and stability of reaction intermediates

Two sulfated zirconia catalysts were prepared via sulfation and calcination of zirconium hydroxide at 873 K; the zirconium hydroxide had been aged at room temperature for 1 h (SZ-1) or aged at 373 K for 24 h (SZ-2). SZ-1 was active for n-butane isomerization at 373 K; SZ-2 reached a similar performance level only at 473 K. Both materials contained about 9 wt% sulfate and were tetragonal. Because of a lower BET surface area (105 vs. 148 m 2/g) SZ-1 featured a higher sulfate density, and XRD and EXAFS analysis showed larger (ca. 10 nm) and more well ordered crystals than for SZ-2. n-Butane TPD on SZ-1 showed a butene desorption peak at low temperature, whereas no obvious butene desorption was observed with SZ-2, suggesting that SZ-1 has a higher oxidizing power at low temperature than SZ-2. The number of sites capable of dehydrogenation are less than 5 μmol/g, because the differential heats of n-butane adsorption as measured by microcalorimetry were 45–60 kJ/mol for higher coverages, indicating weak and reversible interaction. TAP experiments describe the adsorption and desorption behavior of n-butane at different activity states and are the basis for a simple adsorption model. Reactant pulses and purge experiments show that the active species, presumably formed in an oxidative dehydrogenation step, are stable at the surface under reaction conditions.

In situ XANES study of Mn in promoted sulfated zirconia catalysts

Mn-promoted sulfated zirconia catalysts (2 wt% Mn) were investigated in situ, during the catalyst activation, isomerization of n-butane, and subsequent re-activation, using X-ray absorption spectroscopy of the Mn K-edge. The average valence of Mn in the catalysts, as determined from the edge position, was found to change from either 2.65 or 2.77 in the calcined samples to about 2.5 during activation in He (703 K for 30 min). During the isomerization of n-butane (1% in He, 80 ml min-1, 0.5 g catalyst at 333 K), the average Mn valence did not change further. When the catalyst was activated in 50% O2 the average valence only decreased from about 2.78 to 2.72. In this case, the average valence during the isomerization reaction decreased at a nearly constant rate both during the induction of activity and deactivation of the catalyst. The data do not support a stoichiometric redox reaction involving the promoter as initiator of the isomerization. However, a higher Mn valence after activation was indicative of a higher maximum conversion. It is concluded that the promoter cations function through modification of the structure of the zirconia.

Effect of metals on the catalytic activity of sulfated zirconia for light naphtha isomerization

We studied on the function of the metal in the sulfated zirconia(SO 4 2− /ZrO2) catalyst for the isomerization reaction of light paraffins. The addition of Pt to the SO 4 2− /ZrO2 carrier could keep the high catalytic activity. The improvement in this isomerization activity is because Pt promotes removal of the coke precursor deposited on the catalyst surface. Though this catalytic function was observed in other transition metals, such as Pd, Ru, Ni, Rh and W, Pt exhibited the highest effect among them. It was further found that the Pd/SO 4 2− /ZrO2–Al2O3 catalyst possessed a catalytic function for desulfurization of sulfur-containing light naphtha in addition to the skeletal isomerization. The sulfur tolerance of catalyst depended on the method of adding Pd, and the catalyst prepared by impregnation of the SO 4 2− /ZrO2–Al2O3 with an aqueous solution of Pd exhibited the highest sulfur tolerance. Further, we investigated the improvement in sulfur tolerance of the Pt/SO 4 2− /ZrO2–Al2O3 catalyst by impregnation of Pd. The results of EPMA analysis indicated that this catalyst was a hybrid-type one (Pt/SO 4 2− /ZrO2–Pd/Al2O3) in which Pt/SO 4 2− /ZrO2 particles and Pd/Al2O3 particles adjoined closely. This hybrid catalyst possessed a very high sulfur tolerance to the raw light naphtha that was obtained from the atmospheric distillation apparatus, although this light naphtha contained much sulfur. We assume that such a high sulfur tolerance in the hybrid catalyst is brought about by the isomerization function of Pt/SO 4 2− /ZrO2 particles and the hydrodesulfurization function of Pd/Al2O3 particles. Besides, since the hybrid catalyst also provides high catalytic activity in the isomerization of HDS light naphtha, we suggest that the Pd/Al2O3 particles supply atomic hydrogen to the Pt/SO 4 2− /ZrO2 particles by homolytic dissociation of gaseous hydrogen and also enhance the sulfur tolerance of Pt/SO 4 2− /ZrO2 particles. Finally, we also propose the most suitable location of Pd and Pt in the metal-supported SO 4 2− /ZrO2–Al2O3 catalyst.

XAS in situ cell for measurements of Mn and Fe promoted sulfated zirconia catalysts

Promotion of sulfated zirconia (SZ) with Mn or Fe results in an orders of magnitude more active catalyst for the skeletal isomerization of n-butane. One explanation for this promotion effect, consistent with rapid deactivation observed for these catalysts, suggests that the promoter participates in non-catalytic redox reactions. In order to test this hypothesis it was necessary to measure the Mn valence under reaction conditions (in situ). However, it was found that the catalysts have a reduced activity when pressed into pellets for measurement. Therefore, we designed an in situ cell that could be used to measure spectra of a powdered catalyst sample. The cell consists of an internal powder bed reactor (heatable to 773 K), which is enclosed in a He-filled, temperature controlled outer cell to ensure that the window to the inner cell, and the catalyst it contacts, are at a well-defined temperature. The reactor characteristics of the new cell are shown to be comparable to those of a quartz tubular reactor. Spectra are measured in fluorescence mode with simultaneous measurement of a reference foil in transmission using a section of the beam that bypasses the catalyst bed. Contrary to previous reports, data obtained in situ, show that the Mn valence remains constant during reaction with n-butane (333 K) after the catalyst has been activated in He. Additionally, EXAFS measurements indicate that the local environment of the Mn changes significantly during activation in He. Neither the XANES, nor the EXAFS for the Fe K-edge of FeSZ change during activation or reaction. These results suggest that the promotion effect is either catalytic, or is indirect, perhaps through modification of the zirconia bulk.

Catalytic Oxidation of Trichloroethylene over Pd-Loaded Sulfated Zirconia

The oxidative decomposition of trichloroethylene (TCE) was investigated using palladium catalysts supported on pure and sulfated zirconia. The reactions were performed under dry and wet conditions in the temperature between 200 and <TEX>$550^{\circ}C$</TEX> keeping GHSV of 14,000 <TEX>$h^{-1}.$</TEX> The products such as <TEX>$C_2Cl_4,\;C_2HCl_5,\;CO\;and\;CO_2$</TEX> were observed in the reaction. The addition of water in the feed affected the distribution of reaction product with dramatically improved catalytic activity. The spectroscopic investigations gave an evidence that the strong acid sites play an important role on controlling the catalytic activity. Among the catalysts investigated, the Pd-loaded sulfated zirconia catalyst with 1 wt% Pd was found to exhibit the highest catalytic activity in the presence of water vapor having the stability for 30 h of the reaction at <TEX>$500^{\circ}C$</TEX>. The successful performance of the catalyst might be attributed to promotional effect of Pd active sites and strong acid sites induced from surface sulfate species on zirconia.

Supported sulfated zirconia catalysts and their properties

Abstract Sulfated zirconia (SZ), SO 4 /ZrO 2 , was supported by SiO 2 , γ-Al 2 O 3 , and K-10. Aspects of preparation, such as ZrO 2 precursor, sulfur contents, sulfating agent, and calcination conditions, were studied. The pure supports and catalysts were characterized with XRD, BET, FTIR photoacoustic spectroscopy (pyridine adsorption), NH 3 -TPD, and tested for the benzoylation of anisole with benzoyl chloride. The support strongly affects the catalytic activity; SZ supported by K-10 achieved catalytic yields higher than that of bulk SZ.