Characterizations of aluminum-promoted sulfated zirconia on mesoporous MCM-41 silica: Butane isomerization

Sulfated zirconia (SZ) was supported on mesoporous molecular sieves MCM-41 by impregnation of zirconium sulfate followed by calcination. The nanochannels of MCM-41 provide a large surface area for the solid state dispersion of zirconium sulfate and a steric restriction on formation of zirconia nanoparticles. The catalysts were tested in n-butane isomerization. With the addition of a proper amount of alumina as a promoter, denoted as ASZ/MCM-41, the catalytic activity was dramatically improved in comparison to the activities of SZ/MCM-41. The increase of activity was determined primarily by the amount of aluminum added and the temperature of calcination. The SZ/MCM-41 catalysts were characterized by X-ray diffraction (XRD), high resolution TEM (HR-TEM), NH₃ adsorption (NH₃-TPD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption (EXAFS). In particular, the Zr K-edge EXAFS data give one a measure of the degree of dispersion of zirconia on the surface of MCM-41. The trend of the promotion effects of alumina on SZ in butane isomerization is not monotonic; there is an optimum level of Al-loading for high activity. It is explained based on three quantitative factors: increased sulfur loadings, balanced distribution of Lewis and Brønsted acid sites, and higher dispersion of zirconia.     

Catalytic behavior of nanostructured sulfated zirconia promoted by alumina: Butane isomerization

Promotion of sulfated zirconia with alumina (ASZ) improves its catalytic activities in n-butane isomerization. The activity and stability of the sulfated zirconia catalysts are investigated in three different nanostructures: ASZ supported on MCM-41, ASZ nanoparticles, and Al-promoted mesoporous sulfated zirconia. The increase of activity was determined primarily by the amount of aluminum addition and the temperature of calcination. The remarkable activity and stability of the Al-promoted catalysts are due to an improved distribution of acid sites strength. The Al loadings in all three catalysts can be adjusted so that optimum catalytic activities for butane isomerization could be found. The increase of butane conversion can be as high as 6 times of that in un-promoted SZ catalysts. This is due to an enhanced amount of weak Brønsted acid sites with intermediate strength on the optimal catalysts. For nanoparticle form of sulfated zirconia, the activity is most steady which is related to the optimum distribution of weak Brønsted acid. On the other hand, too much strong Brønsted acid leads to rapid decay of activity because of coking and cracking. The overall reaction mechanism of the isomerization of n-butane over sulfated zirconia was discussed to understand the details in product distribution.     

Hydroisomerization of n-hexane over gallium-promoted sulfated zirconia

Gallium-promoted sulfated zirconia (GSZ) catalysts were prepared by impregnation of zirconium hydroxide with aqueous Ga₂(SO₄)₂ followed by calcination. Isomerization of n-hexane was studied over GSZ at 150 °C, 2.0 MP, WHSV 2 and H₂/hexane (molar) ratio of 5. In comparison to sulfated zirconia (SZ), the conversion of n-hexane over Gallium-promoted sulfated zirconia (GSZ) was greatly improved and it remained stable at 85%. In particular, almost all the products were isomers of hexane and the selectivity of 2,2-DMB reached 20%. The results of characterization indicated that the addition of gallium onto SZ catalyst showed little difference in acid strength between SZ and GSZ catalysts while the redox properties of the SZ catalyst changed with addition of gallium. The transformation of SZ crystalline from metastable tetragonal phase, the more active phase, to monoclinic phase was retarded with the addition of gallium. Furthermore, the simultaneous promotion of Pt and Ga brings the production distribution very close to the equilibrium one.     

Well-Dispersed Gallium-Promoted Sulfated Zirconia on Mesoporous MCM-41 Silica

Gallium-promoted sulfated zirconia (SZ) was confined inside pure-silica MCM-41 (abbreviated as SZGa/MCM-41), where the latter served as a host material. It was prepared by direct dispersion of metal sulfate in the as-synthesized MCM-41 materials, followed by thermal decomposition. The SZGa/MCM-41 catalysts were characterized by XRD, N₂ adsorption, HRTEM, DRIFT, NH₃-TPD, and TPR. The experimental results showed that the ordered porous host structure was still maintained in the catalyst. SZ was in meta-stable tetragonal phase and highly dispersed on the interior surface of MCM-41 even at a high loading of 50 wt%. Additionally, a small fraction of SZ nanoparticles on the external surface of MCM-41 was obtained. The catalytic activity of SZGa/MCM-41 was examined in n-butane isomerization. In comparison to SZ/MCM-41 without promoter, the catalytic activities of the Ga-promoted catalysts were greatly improved. The reason proposed for the higher activity of the Ga-promoted catalysts was that Ga enhances the oxidizing ability of the catalysts.     

铝对固体超强酸/介孔分子筛催化性能的促进作用

采用浆态直接浸渍-焙烧方法制备了铝促进的固体超强酸／介孔分子筛材料，并采用XRD、N2吸附、吡啶吸附IR、反应表征等方法对其结构及性能进行了表征。铝的引入有利于形成和稳定样品表面的B酸，有助于稳定表面硫物种，而样品的比表面和孔容随着铝的加入变化不大。添加适量的铝能够大大提高正丁烷异构化的催化活性。     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

Well-dispersed Ceria-promoted Sulfated Zirconia Supported on Mesoporous Silica

Ceria-promoted sulfated zirconia (CeSZ) was supported on mesoporous molecular sieve of pure-silica MCM-41 (abbreviated as CeSZ/MCM-41). It was prepared by direct impregnation of metal sulfate onto the as-synthesized MCM-41, followed by solid state dispersion and thermal decomposition. The resultant catalysts were characterized by TG, XRD, nitrogen physisorption and TEM. It was showed that the hollow tubular structure of MCM-41 was retained, even with ZrO₂ loading as high as 60 wt.%. Most of CeSZ was well dispersed on the interior surface of the ordered mesopores, following a slight twist of the channels. The catalytic activity of CeSZ/MCM-41 was studied in the octadecanol oxidation. The improved performance of CeO₂-promoted catalysts was attributed to the high dispersion of sulfated zirconia (SZ) and the introduction of CeO₂ enhancing the oxidation ability of catalysts by retarding the transformation of zirconia from highly catalytic active metastable tetragonal phase to monoclinic phase.     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

Catalytic Behavior of Alumina-Promoted Sulfated Zirconia Supported on Mesoporous Silica in Butane Isomerization

Alumina-promoted sulfated zirconia was supported on mesoporous molecular sieves of pure-silica MCM-41 and SBA-15. The catalysts were prepared by direct impregnation of metal sulfate onto the as-synthesized MCM-41 and SBA-15 materials, followed by solid state dispersion and thermal decomposition. Measurements of XRD and nitrogen adsorption isotherms showed that the structures of resultant materials retain well-ordered pores, even with ZrO₂ loading as high as 50 wt%. The characterization results indicated that most of the promoted sulfated zirconia were well dispersed on the internal surface of the ordered mesopores. The catalytic behavior of the alumina-promoted sulfated zirconia supported on mesoporous silica was studied in n-butane isomerization. The supports of mesoporous structures led to high dispersion of sulfated zirconia in the meta-stable tetragonal phase, which was the catalytic active phase. The high performance of alumina-promoted catalysts was ascribed to the sulfur retention by alumina.     

Direct impregnation method for preparing sulfated zirconia supported on mesoporous silica

A new method has been developed to prepare sulfated zirconia (S–ZrO₂) supported on mesoporous silica. With direct exchange of metal containing precursors for the surfactants in the as-synthesized MCM-41 materials, the problem of fill-up of the mesoporous structure was avoided and high sulfur content was achieved. By using this method, the composite of S–ZrO₂/MCM-41 with ZrO₂ content higher than 60 wt.% can be easily obtained without serious blockage of the pore structure of MCM-41. Nevertheless, the pore size and pore volume of the resultant S–ZrO₂/MCM-41 composites were found to vary markedly with the loading of ZrO₂. The strong acidic character of the obtained composites was examined by using them as catalysts in n-butane isomerization. Introduction of other metals such as aluminum as promoter into S–ZrO₂/MCM-41 can be easily conducted by the direct impregnation method.     

Sulfated zirconia grafted on a mesoporous silica aerogel: Influence of the preparation parameters on textural, structural and catalytic properties

Silica supported sulfated zirconia catalysts were synthesized via a new method by grafting sulfated zirconia on the surface of a silica aerogel previously prepared. The main parameters studied in this work were the S/Zr, Zr/Si molar ratios and the support nature. The synthesized solids were characterized using XRD, N₂ physisorption at 77 K, TG-DTA/SM, sulfur chemical analysis and adsorption–desorption of pyridine followed by infrared spectroscopy. These solids were tested in the n-hexane isomerization reaction. Two types of mesopores were observed on the silica aerogel. This mesoporosity was affected depending on the preparation parameters.

The increase of the Zr/Si molar ratio induces the decrease of the size of zirconia particles deposed on the support. In this case, appreciable amounts of sulfur are retained with the presence of a relatively strong Brönsted and Lewis acid sites on the catalyst surface. A high density of Brönsted sites seems to be interesting in the n-hexane isomerization reaction.     

Selectivity and mechanism for skeletal isomerization of alkanes over typical solid acids and their Pt-promoted catalysts

Selectivities for skeletal isomerizations of n-butane and n-pentane catalyzed by typical solid acids such as Cs_2.5H_0.5PW₁₂O₄₀ (Cs2.5), SO₄²⁻/ZrO₂, WO₃/ZrO₂, and H-ZSM-5 and their Pt-promoted catalysts were compared. High selectivities for n-butane and low selectivity for n-pentane were observed over Cs2.5 and SO₄²⁻/ZrO₂, while H-ZSM-5 was much less selective, and WO₃/ZrO₂ was highly selective for both reactions. The Pt-promoted solid acids were usually selective for these reactions in the presence of H₂ except for Pt-H-ZSM-5 for n-butane isomerization. Both the acid strength and pore structure would be factors influencing the selectivity. Mechanism of skeletal isomerization of n-butane was investigated by using 1,4-¹³C₂-n-butane over Cs2.5 and Pt–Cs2.5. It was concluded that n-butane isomerization proceeded mainly via monomolecular pathway with intramolecular rearrangement on Pt–Cs2.5, while it occurred through bimolecular pathway with intermolecular rearrangement on Cs2.5. The higher selectivity on Pt–Cs2.5 would be brought about by the monomolecular mechanism. In the skeletal isomerization of cyclohexane, Pt–Cs2.5/SiO₂ was highly active and selective, while Pt–Cs2.5 was less selective. Control in the acid strength of Cs2.5 by the supporting would be responsible for the high selectivity.     

Characterization of palladium supported on sulfated zirconia catalysts by DRIFTS, XAS and n-butane isomerization reaction in the presence of hydrogen

Palladium supported on sulfated zirconia (PdSZ) has been characterized by the n-butane isomerization reaction in the presence of hydrogen, X-ray absorption spectroscopy (XAS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) of adsorbed carbon monoxide. Catalyst calcination at 873 K followed by hydrogen reduction at 513 K results in the formation of 30–40 Å Pd metal clusters, but the surface can only weakly adsorb CO, though stronger than Pd-free, sulfated zirconia catalysts. In the presence of hydrogen, PdSZ has a lower n-butane isomerization activity than SZ, and the Pd function cannot stabilize the reaction at low H₂/n-butane ratios.     

NO reduction by CH4 in the presence of excess O2 over Mn/sulfated zirconia catalysts

Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3 wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH₃–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N₂ adsorption and the H₂–TPR of the catalysts demonstrated that the presence of the SO₄²⁻ species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH₄ by O₂ and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO₄²⁻. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO₂ poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO₂.     

n-Butane isomerization over transition metal-promoted sulfated zirconia catalysts: effect of metal and sulfate content

n-Butane isomerization has been investigated over transition metal-promoted sulfated zirconia catalysts. Fe alone was a more efficient promoter than Mn and Cr, and mixtures of Fe–Mn, Fe–Cr, and Fe–V. Promotion of a commercial sulfated zirconia (4.8% sulfate) with iron amounts in excess of 2% depressed the conversion and increased the deactivation rate. Increasing reaction temperatures improved the conversion and decreased the induction period. At 70°C and above, the induction period was suppressed. The influence of activation temperature was studied over a Fe-sulfated zirconia catalyst (with 2% Fe and 4.8% SO₄²⁻ pre-calcined at 600°C). The best conversion was achieved when the catalyst was activated at 350°C in air. This temperature was apparently sufficient to generate the redox sites active at low reaction temperature. Activation in flowing helium depressed the catalytic activity and increased the induction period. The sulfate content had a significant effect on the catalyst performance. For 2% Fe-promoted zirconia, a maximum conversion was found for 7.5% SO₄²⁻, probably related with a better balance (or synergism) of the redox and acid sites involved in a bimolecular mechanism. The time required to reach a maximum of conversion (induction period) decreased with increasing total acidity, i.e. with sulfate content. The series of catalysts with different amounts of sulfate has been characterized by X-ray diffraction, nitrogen adsorption–desorption isotherms, TGA, ammonia-TPD, DRIFT, Raman, and X-ray photoelectron spectroscopy.     

Tetragonal structure, anionic vacancies and catalytic activity of SO4-ZrO2 catalysts for n-butane isomerization

An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO₄²⁻-ZrO₂ with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO₄²⁻-ZrO₂ catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H₂ was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work.

A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO₄²⁻-ZrO₂ is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.     

NO reduction by CH4 in the presence of excess O2 over Pd/sulfated zirconia catalysts

The selective catalytic reduction (SCR) of NO by methane in the presence of excess oxygen has been studied on a series of Pd catalysts supported on sulfated zirconia (SZ). This support is not as sensitive to structural damage by steaming as the acidic zeolites, such as H-ZSM-5 and H-Mor. In previous studies, it was shown that this type of acidic zeolites are able to stabilize Pd²⁺ ions and promote high SCR activity and selectivity, which are typically not seen in Pd catalysts. In this contribution, it has been demonstrated that SZ is able to promote the NO reduction activity in a similar way to the acidic zeolites, by stabilizing Pd²⁺ ions that is selective for NO reduction. As in the case of acidic zeolites, the stabilization of Pd²⁺ ions can occur through a transfer of Pd species from particle to particle. One of the attractive features of Pd/SZ catalysts is that they are less sensitive to water and SO₂ poisoning than Pd/H-ZSM-5 catalyst and exhibit higher reversibility after removal of water or SO₂.     

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 MnO_x (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.     

Polarisation enhanced C magnetic resonance studies of the hydrogenation of pentene over Pd/Al2O3 catalysts

This paper reports the first demonstration of ¹³C distortionless enhancement by polarisation transfer (DEPT) NMR spectroscopy at natural abundance to study the hydrogenation and isomerisation of pentenes over a 1 wt% Pd/Al₂O₃ catalyst. Single component C5 hydrocarbons and binary mixtures of hydrocarbon and hydrogen have been adsorbed on both a pure alumina support and the Pd/Al₂O₃ catalyst derived from it. The pentene species studied were 1-, cis-2- and trans-2-pentene. No isomerisation or hydrogenation was observed when single component pentene isomers or binary mixtures of 1-pentene and hydrogen, and cis-2-pentene and hydrogen were adsorbed onto the pure alumina support. However, when trans-2-pentene and hydrogen were both adsorbed onto the support, partial hydrogenation to n-pentane was observed in addition to the presence of both cis-2- and trans-2-pentenes. All pentene isomers hydrogenate over the Pd/Al₂O₃ catalyst to give predominantly n-pentane and a small amount of the trans-2-pentene isomer. For the parameters chosen here these studies show that trans-2-pentene appears to be the active isomer for hydrogenation over the pure support alone.     

Sulfated zirconia catalysts promoted with Fe and Mn: Mn effect in the Fe dispersion

n-Butane conversion was studied over Fe- and Mn-promoted sulfated zirconia by pulse reaction over the temperature range of 423–503 K to explore the role of Mn in the catalytic activity of Fe-promoted sulfated zirconia. On a sample containing 2 wt.% Fe, the n-butane isomerization activity was found to decrease with decreasing Fe/Mn molar ratio beyond 0.65 wt.% Mn. Concomitantly, XPS showed that the surface Fe/Zr intensity ratio increases with the Mn content until it reaches a maximum at 0.65 wt.% Mn, and then decreases. This behavior reflects a higher Fe dispersion at relatively low Mn content. Thus, low concentrations of Mn disperse the iron on the surface.

Temperature programmed reduction data indicate that more sulfate species are accessible to reduction when metal promoters are present over sulfated zirconia. However, increasing Mn amount decreases the fraction of sulfate being reducible.