Sulfated Zirconia Catalysts Research Articles

Structural Transformation of (110) Ultrathin Films of Tetragonal Zirconia Induced by Polarity

The (110) surface of tetragonal zirconia is polar and hence intrinsically unstable. It is however proposed to be present in samples of sulfated zirconia catalysts. The processes enabling the stabilization of this termination of tetragonal zirconia, by relaxation or reconstruction, have been studied with periodic density functional theory. The bulk terminated surface shows huge relaxation by dimerization of layers, however keeping a high unstable surface energy. The most stable surface is oxygen terminated but with an oxygen content reduced by a factor of 2 compared to the bulk terminated case. For ultrathin films, this surface reconstruction is accompanied by a structural transformation of the bulk, which completely cancels the polarity and creates a distorted (001) surface. Other surface contents have been studied with non stoichiometric slabs, without showing more stable solutions.

Synthesis and characterization of mesoporous silica-supported nano-crystalline sulfated zirconia catalysts prepared by a sol–gel process: Effect of the S/Zr molar ratio

Abstract Mesoporous silica-supported nano-crystalline sulfated zirconia catalysts were prepared via the sol–gel process using an in situ sulfation. The parameter studied in this work was the S/Zr molar ratio. The synthesized solids were characterized using XRD, N 2 physisorption, TG-DTA/SM, sulfur chemical analysis and adsorption–desorption of pyridine, and tested in the gas-phase acid-catalysed isomerization of n -hexane. The gelation process is highly affected by the sulfate loadings. Two gelation mechanisms were evidenced depending on the S/Zr molar ratio. The first one observed when 0.15 ≤ S/Zr ≤ 0.5, is characterized by a relatively high gelation rate. This mechanism favours the formation of two types of mesopores and a low percentage of retained sulfur. The second gelation mechanism occurs for higher S/Zr ratios: 0.5 ≤ S/Zr ≤ 1.2. In that case, slower gelation rates are observed. This slower gelation process leads to materials with reduced BET surface area but the amount of retained sulfur is increased. Appreciable catalytic properties were observed for the sample prepared with the highest S/Zr ratio, which presents the smallest size of sulfated zirconia crystallites and shows both Bronsted and Lewis acid sites on its surface.

N-Hexane skeletal isomerization over sulfated zirconia catalysts with different Lewis acidity

Abstract The n -hexane skeletal isomerization over Pt/SO 4 2− /Al 2 O 3 /ZrO 2 catalysts with different density of Lewis acid sites has been studied in a fixed-bed reactor. The acidity of the catalysts was characterized by FTIR spectroscopy of low temperature CO adsorption. The calculation of the rate constants, characterizing the catalyst activity and selectivity was performed on the base of mathematical model of plug flow reactor. It was shown that the rate of hexane isomerization is mainly determined by the density of Lewis acid sites on the surface of sulfated zirconia catalysts: rate constant of n -hexane isomerization for catalyst with Lewis acidity 250 μmol g −1 is four-fold as that for catalyst with concentration of Lewis acid sites 150 μmol g −1 and the rate constant of desired product (2,2-dimethylbutane) formation is also higher by two times.

Benefits of mesopores in sulfated zirconia catalyst

Activities of two catalysts obtained from hydroxide and alkoxide precursors were correlated to their different structural and textural properties. Activity is mainly determined by the presence of active tetragonal phase, which is more stable at high temperatures in the catalyst of alkoxide origin than in one from hydroxide. An appropriate structure must be accompanied with beneficial pore structure, as is the case of mesopores of the catalyst from alkoxide. In the case of very small pores of the catalyst from hydroxide precursor, activity is negligible due to pores plugged by coke.

Pd-based sulfated zirconia prepared by a single step sol–gel procedure for lean NOx reduction

Pd-based sulfated zirconia catalysts have been prepared through a single step (one-pot) sol–gel preparation technique, in which both sulfate and Pd precursors were dissolved in an organic solution before the gelation step. Observation of the calcination procedure through TGA/DSC and mass spectrometry revealed that the addition of increasing amounts of Pd resulted in the evolution of organic precursor species at lower temperatures. In situ XRD experiments showed that tetragonal zirconia is formed at lower temperatures and larger zirconia crystallites are formed when Pd is added to the gel. Although tetragonal zirconia was the only phase observed through XRD, Raman spectra of samples calcined at 700 °C showed the presence of both the tetragonal and the monoclinic phase, indicating a surface phase transition. DRIFTS experiments showed NO species adsorbed on Pd 2+ cations. Pd/SZ catalysts prepared through this single step method were active for the reduction of NO 2 with CH 4 under lean conditions. Calcination temperature had a significant effect on this activity, with samples calcined at 700 °C being much more active than those calcined at 600 °C, despite the observed transition to the monoclinic phase. This activity may be linked to observed changes in the surface sulfate species at higher calcination temperatures.

Content of sulfates and their stability: Key factors determining the catalytic activity of sulfated zirconia catalysts

Two series of sulfated zirconia catalysts were synthesized from various precursors using mono- or multi-step sequence preparations under laboratory conditions. Their activities/selectivities in the isomerization reaction of n-hexane were correlated to their textural, structural and morphological properties. The slightly higher activity of a commercially sulfated Zr(OH)4-based catalyst is in agreement with the differences in the content of SO24 - ions and their thermal stability, textural and structural properties, i.e., crystallite size and possible imperfection of the incorporation of sulfate groups in the multi-step synthesis of the catalyst having a nitrate origin. The employment of H2 as the carrier gas resulted in no catalytic activity, regardless of the catalyst precursor, preparation method and calcination temperature. When the isomerization reaction was performed under He, the relatively short life-times of all catalyst samples were caused by fast deactivation due to coking in the absence of H2. .

Sulfated Zirconia-Catalyzed Synthesis of 3,4-Dihydropyrimidin-2(1H)-ones (DHPMs) Under Solventless Conditions: Competitive Multicomponent Biginelli vs. Hantzsch Reactions

The catalytic ability of ZrO(2)/SO(4)(2-) to promote solventless three-component condensation reactions of a diversity of aromatic aldehydes, urea or thoiurea and ethyl acetoacetate was studied. Products resulting from Hantzsch and/or Biginelli multi-component reactions are obtained in the presence of solid acid catalysts using the same reactants but different temperature conditions. The sulfated zirconia catalyst can be recovered and recycled in subsequent reactions with a gradual decrease of activity.

New process to control hydrolysis step during sol-gel preparation of sulfated zirconia catalysts

Sulfated zirconia catalysts were prepared by sol-gel process using a new method allowing the control of the hydrolysis step of zirconium alkoxides. Prepared samples were characterized by N2 adsorption (desorption) at 77 K, XRD, chemical analysis of total sulfur and XPS. Catalytic properties have been evaluated in n-butane and n-hexane isomerization reactions. Obtained results show that time necessary for gelation is significant and depends considerably on the nature of the zirconium precursor. Concerning the properties of the solids obtained by this method, it has been noted that the use of the zirconium propoxide led to a better retention of the sulfate species, which improves activity during isomerization reactions. It has been found also that an initial S/Zr molar ratio equal to 0.5 allowed to obtain an optimal sulfur content and a high specific surface area.

In situ spectroscopic investigation of activation, start-up and deactivation of promoted sulfated zirconia catalysts

Sulfated zirconia (SZ), unpromoted and promoted with 0.5–2.0 wt.% Mn or Fe, was investigated by in situ IR and UV–vis spectroscopy during activation at 723–773 K and interaction with n-butane at 298–323 K. During start-up of the catalysts, the isomerization rate increases with the amount of water that is being formed, consistent with an activation of n-butane via oxidative dehydrogenation. Sulfate is the most likely oxidizing agent because activity is also observed for unpromoted SZ and without reduction of Mn. Isomerized species were also found on the surface of Fe-promoted SZ after exposure to n-butane without water formation indicating other yet unknown initiation pathways. During isomerization, Lewis acid sites on Fe-promoted SZ are blocked by water; the valence of Mn decreases only if the catalyst was previously activated in O 2. Stable species absorbing at 2820–2840 cm −1 detected on the surface of SZ and Fe-promoted SZ after contact with n-butane at 298 K are tentatively ascribed to alkoxides, which may be side-products or intermediates. These species differ slightly for the two catalysts, reflecting the different structural and electronic properties of the materials.

A skeletal rearrangement study of a carbon-13 labelled 3-methylpentane on doped sulphated zirconia catalysts

The isomerization on sulphated zirconias of the 3-methyl(3- 13C)pentane, molecule chosen to distinguish between the methyl and ethyl migrations, is studied. The products distribution shows 0% for the 3-methylpentane isotopomers (not detected? or not formed?), what makes the interpretations difficult. Supposing these molecules formed in low proportions but not detected, a reaction pathway involving protonated cyclopropanes is proposed: the isotopomers are obtained from one- two- or three-step reactions. However, it is envisaged the possible non-formation of these molecules and a reaction pathway involving protonated cyclopropanes, cyclobutanes and bicyclopropanes is suggested: the 13C-label scrambling would be a two-step isomerization and the other isotopomers would be formed via a one-step reaction. The relative proportions of methyl migration (MM) and ethyl migration (EM) are assessed. The MM/EM ratio could be appreciated as an intrinsic characteristic of the catalyst because of correlations with, e.g. the crystalline structure, the sulphate content and the activity. Such correlations knowledge could help preparing catalysts with high activity and selective reactivity.

A solid-state NMR, FT-IR and TPD study on acid properties of sulfated and metal-promoted zirconia: Influence of promoter and sulfation treatment

The acid properties of various sulfated and metal-promoted zirconium oxide (ZrO 2) catalysts have been studied by solid-state 31P MAS NMR, FT-IR and TPD using the adsorbed trimethylphosphine oxide (TMPO), pyridine and ammonia as the probe molecule, respectively. Sulfated zirconia (SZ) catalysts having varied sulfur contents and metal promoter (M = Al, Ga and Fe) were prepared by sol–gel method. Effects of sulfation and promoter on the detailed qualitative and quantitative information of acid sites, viz. types (Brønsted versus Lewis), strengths and distributions, on various synthesized SZ and M/SZ were examined by 31P MAS NMR of adsorbed TMPO in conjunction with elemental analyses by ICP-MS. By comparison, pyridine-IR and NH 3-TPD methods are capable of providing only qualitative information of the overall acidity. It was found that while the parent ZrO 2 possesses only weak Lewis acidities, elaborated sulfation treatment leads to formation and coexistence of strong Brønsted (B) and Lewis (L) acid sites whose variations can readily be followed. On the other hand, incorporation of different metal promoters onto SZ, led to simultaneous formation/elimination and variations of B- and L-sites with varied strengths and distributions. As a result, Ga/SZ was found to possess more B- than L-sites, whereas an opposite trend was observed for Al/SZ. As for Fe/SZ, a pronounced increase in both concentration and strength of acid sites were found. With an exception of Ga/SZ, the ‘very strong’ acid sites observed for various SZ and M/SZ catalysts were found to be associated more to L- than B-sites.

Gas and liquid phase reactions on MCM-41/SZ catalysts

A series of sulphated zirconia (SZ) catalysts supported on MCM-41 were prepared by the liquid-crystal template method (LCT), using cetyltrimethylammonium bromide (CTABr) as template. The effect of metal oxides promotion (Al2O3, Fe2O3 and Ga2O3) on structural and textural property was studied. These materials were characterized by ion chromatography (for the determination of sulphates content), nitrogen-adsorption measurements, X-ray diffraction, FT-IR spectroscopy, TG–DSC analysis and TPR–MS measurements. The catalytic performance of the catalysts in both gas and liquid phase reactions was investigated. A correlation between morphological and chemical physical features of the systems and their catalytic activity is here proposed.

Silica supported sulfated zirconia prepared by a sol-gel process: Effect of solvent evacuation procedure on the structural, textural and catalytic properties

Sol-gel process was used to prepare silica supported sulfated zirconia catalysts. The main parameter studied in this work was the gel drying method through four different ways of solvent evacuation. Textural, structural as well as the acidic properties of the four samples were studied using N2 physisorption, X-ray diffraction, mass spectrometry, sulfur chemical analysis and adsorption-desorption of pyridine. The isomerization of n-hexane was used as a catalytic test. The surface areas and the pore distributions are highly affected by the drying mode. One of the four drying methods leads to a solid having improved textural properties and presenting both the crystalline ZrO2 tetragonal phase and a particular type of sulfate mode bond. It seems that these three conditions are necessary for achieving high catalytic activity.

α-Pinene isomerization to campheneEffect of thermal treatment on sulfated zirconia

Abstract The isomerization reaction of α-pinene produces bicyclic and monocyclic compounds and other products. The purpose of this study is to investigate the influence of the pre-treatment temperature of the catalyst on the concentration of Lewis and Bronsted acid sites and the activity and selectivity of the catalyst. Catalyzed reactions in heterogeneous and homogeneous phases were performed. In heterogeneous reaction, sulfated zirconia (SZ) and pre-treated SZ catalyst at several temperatures were used. The thermal treatment affects the ratio between Bronsted and Lewis sites. Results were contrasted with those obtained with SZ catalyst without thermal treatment. In homogeneous reaction the catalyst is sulfuric acid, and in the reaction medium Bronsted sites are only present coming from the H+ of the acid. The ratio between bi/monocyclic compounds for H2SO4, SZ catalyst and pre-treated catalyst at several temperatures, SZ250, SZ350 and SZ500, varies between 0.4 and 4.3. It can be observed that there exists an optimal thermal treatment where the ratio is higher. FT-IR spectra of adsorbed pyridine on catalysts pre-treated thermally show the presence of Lewis and Bronsted acid sites. The amount of acid sites was determined by TPD of adsorbed pyridine.

N-Butane Isomerization Catalyzed by Sulfated Zirconia Nanocrystals Supported on Silica or γ-Alumina

Supported sulfated zirconia catalysts with zirconia contents of 10, 20 and 50 wt% were prepared by impregnation of SiO2 and γ-Al2O3 supports with H2SO4/ Zr(SO4)2 solutions followed by calcination at 923 K. The catalysts were characterized by X-ray diffraction, extended X-ray absorption fine structure measurements, thermal analysis, UV–vis spectroscopy, and electron microscopy. Tetragonal zirconia was detected in all silica-supported samples but only in the 50 wt% zirconia-containing alumina-supported sample, indicating high dispersion of zirconia on alumina. Alumina-supported samples retained additional sulfate, at least in part as Al2(SO4)3. All samples were active in n-butane isomerization (1 kPa n-butane, 378 K). There was no relation between the presence of tetragonal zirconia in these samples and the catalytic performance.

Selective poisoning of active centers of sulfated zirconia monitored by TAP, XPS, and DRIFTS

Temporal analysis of products (TAP), XPS, and DRIFTS were used to study the activity and deactivation of acid sites on a sulfated zirconia catalyst for the n-butane isomerization. The activity of the catalyst was altered by selective poisoning of active centres by sulfation with sodium sulfate under the condition of a constant surface sulfate content. Depending on the surface sodium concentration the Brønsted acid sites were selectively poisoned, which was reflected by a stepwise decrease in the isomerization activity and an altered Brønsted-to-Lewis acid ratio. It could be shown, that only a very small amount of Brønsted acid sites were responsible for the initiation of the catalytic reaction. TAP measurements revealed that with a decreasing number of available strong Brønsted sites, the adsorption of the reactant on the surface dramatically decreased. The altered adsorption surface places prevented the formation of reactive intermediates which seemed to be necessary to start the isomerization reaction cycle. The samples were additionally characterized by N 2-physisorption, X-ray diffraction, and temperature programmed desorption of ammonia. Moreover, the number of Brønsted sites was estimated by a solid-phase exchange of sulfated zirconia with NH 4Y zeolite which could be directly correlated to the DRIFTS results.

N-ペンタンの超臨界骨格異性化反応に用いる硫酸化SO42-/ZrO2触媒へのAlの添加効果

n-ペンタンの超臨界骨格異性化反応に用いる硫酸化SO42-/ZrO2触媒へのAlの添加効果

Mesoporous sulphated zirconia by liquid-crystal templating method

A series of sulphated zirconia (SZ) samples were prepared by the liquid-crystal templating (LCT) method, using linear alcohols with different alkyl chain length as co-solvents and neutral ammine as template. The type and duration of aging was also varied in order to study their effects on the catalytic activity and texture of the synthesized catalysts. These materials were characterized by ion chromatography for determination of sulphate content, nitrogen-adsorption measurements, X-ray diffraction, and transmission electron microscopy. The results showed that the sulphated zirconia catalysts were mesoporous and that their surfaces area and pore size distribution varied as a function of the synthesis conditions. In particular, we found a direct correlation between alkyl chain length of co-solvent and the pore size. The catalytic activity was tested in the toluene and anisole acylation and it was found to increase with the pore size.

Determination of relative acid strength and acid amount of solid acids by Ar adsorption

Relative acid strength and acid amount of solid acids (alumina, silica-alumina, sulfated zirconia, mordenite, ZSM-5, beta, Y, and reduced MoO 3) are determined by argon adsorption technique. To obtain the heat of Ar adsorption and saturated adsorption amount, the adsorption isotherm is analyzed using the theory reported by Cremer and Flügge. The obtained heats of Ar adsorption and saturated adsorption amounts of sulfated zirconia catalysts and proton-type zeolites correspond well with the activities of acid-catalyzed pentane isomerization of these catalysts. The heats of adsorption were −22 kJ mol −1 for sulfated zirconia, and ca. −19 kJ mol −1 for mordenite, ZSM-5, and beta. Molybdenum oxides reduced at 623 and 773 K show large heat of adsorption (−19.3 and −19.7 kJ mol −1, respectively), and these are classified into the superacid.

Active and recyclable sulphated zirconia catalysts for the acylation of aromatic compounds

Mesoporous sulphated zirconia catalysts have been prepared by precipitation of Zr(OH) 4 at constant basic pH and tested for the acylation of anisole with benzoic anhydride. All samples have been characterized by N 2 physisorption, ion chromatography and thermal analysis coupled with MS analysis of evolved gases (EGA). The samples possessed high specific surface areas and mean pore size in the mesopore range. TG/DSC/EGA analysis allowed us to observe all phenomena related to the calcination process and to better understand the catalyst behavior. Observed phenomena are physisorbed water elimination, dehydroxylation, sulphating agent decomposition, phase transitions and sulphate species removal. Catalytic tests were carried out at different temperatures (30–50 °C) after a thermal treatment. The effects of the precipitation pH, calcination temperature and activation temperature on the catalytic performances were studied. Reuse of the catalyst was also appraised.