Alumina-promoted Sulfated Zirconia Research Articles

Conversion of isobutane to n-butane over bifunctional PtAu-solid superacids catalyst

A platinum-substituted alumina-promoted sulfated zirconia (PtAu/SZA) was prepared and used for the conversion of isobutane to n-butane, aiming to optimize the ethylene feedstock in the steam thermal cracking process and increase the ethylene yield. PtAu bimetallic catalysts showed higher catalytic activity than Pt-alone catalyst, and the enhancement is attributed to PtAu alloying. By diluting and separating Pt with PtAu alloy agglomerates, continuous Pt agglomerates disappear and hydrogen dissociation on Pt is enhanced. In addition, the reduction temperature of Pt is decreased with the increase in Au content, while the reduction temperature of sulfate was increased which will delay the loss of sulfate according to the results of H2-TPR and NH3-TPD. However, the acidity of the catalyst decreases with the increase in Au, which is due to that PtAu alloy covers the acid sites on the surface. Overall, we found that the Pt0.5Au0.3/SZA showed the highest yield to n-butane (34.69%).

In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy Investigations on the Evolution of Surface and Catalysis Properties of Alumina-Promoted Sulfated Zirconia during n-Butane Isomerization

The changes in surface structure properties of aluminum-promoted sulfated zirconia (SZA) during NH3 ad/desorption and n-butane isomerization were investigated by in situ diffuse reflectance infrare...

Comparative study of n-butane isomerization over SO42−/Al2O3-ZrO2 and HZSM-5 zeolites at low reaction temperatures

The effects of acidity and pore structures of alumina-promoted sulfated zirconia (SZA) and two HZSM-5 zeolites with different SiO2/Al2O3 ratios on reaction properties of n-butane conversion were comparatively investigated. The results demonstrated that monomolecular isomerization reaction predominated at 433–533K for SZA catalysts and bimolecular pathway predominated at 573–673K for HZSM-5 zeolites. For both samples, the amount of coke deposits was considerably low. The differences in catalytic behavior were caused by different acid properties and pore structures. The strength and stability of catalytic Brønsted acid sites determine the conversion rate of n-butane and the deactivation rate of catalysts, respectively. While the contributions of monomolecular and bimolecular pathway, as well as the products distributions are depended on the pore structures and influenced by the density of catalytic Brønsted acid sites.

Isomerization of n-Butane over SO42–/Al2O3–ZrO2 in a Circulated Fluidized Bed Reactor: Prospects for Commercial Application

The stability of alumina-promoted sulfated zirconia (SZA) was investigated to achieve the isomerization of n-butane in a circulating fluidized bed (CFB) unit. The pilot-scale evaluation in a CFB unit showed high stability of the SZA catalyst and that the catalytic activity was dominated by the residence time of n-butane rather than its linear velocity. Increases in the reaction and regeneration temperature both led to an increase in the conversion of n-butane and a decrease in the selectivity to isobutane, caused by increasing side reactions. Although the regeneration was conducted in air, a trace of SO2 evolved during the regeneration, which could be minimized at the appropriate gas stripping temperature, low regeneration temperature, and high space time of the feed. Compared with conventional fixed-bed technologies, the CFB process shows lower selectivity to isobutane due to the inevitable axial back-mixing and severe “dimerization-cracking” reaction.

Consequence of heterogeneity of active sites for reactivity mechanism of n-butane isomerization over SO42−/ZrO2-Al2O3 catalyst

The relationship between the heterogeneity of active sites and isomerization mechanism of n-butane over alumina-promoted sulfated zirconia (SZA) was elucidated by two series of catalysts with different surface properties. The incomplete removal of coke deposition leads to a decrease in overall activity without the reduction of the rate of isobutane formation. Subjecting the fresh SZA catalyst to the thermal treatment at 650°C causes a striking decrease in activity, resulting from the decrease in the superacidity and oxidizability of the catalyst. While the thermal treatment at 600°C only selectively suppresses the side reactions. It is found that the peaks in NH3-TPD profiles at 500–700°C are caused by a redox reaction between strongly adsorbed ammonia and superacid sites by in situ NH3-TPD-MS measurements. Thus, this peak reflects the strength of superacidity and oxidizability. At the beginning of the reaction, the effective oxidative and protonative activation routes result in the higher concentration of intermediates, which facilitates the “dimerization-isomerization-cracking” reactions. The incomplete removal of coke and the appropriate decrease in the density of sulfate species on the surface selectively promoted the monomolecular pathway.

Effect of modification methods on the surface properties and n-butane isomerization performance of La/Ni-promoted SO42−/ZrO2-Al2O3

The La and/or Ni was introduced into alumina-promoted sulfated zirconia by impregnation and co-precipitation to improve the catalytic property of n-butane isomerization. Catalysts characterization shows that the addition of La/Ni has a remarkable influence on the surface and textual properties depending on the modification method. The impregnation of La/Ni facilitates the transformation of a small amount of tetragonal zirconia into monoclinic phase, while the co-precipitation improves the stability of tetragonal ZrO2. H2-TPR indicates that the addition of La/Ni changes the interaction between SO42− and supports, which affects the acidity on the surface. Specifically, the Lewis acidity is significantly enhanced by either modification method. The co-precipitation reserves almost all of the Brønsted acid sites, while the impregnation causes a remarkable decrease of Brønsted acid sites. Reaction results demonstrate that the co-precipitation exhibits a significant advantage over impregnation that the higher conversion of n-butane and selectivity to isobutane are obtained on the catalyst prepared by co-precipitation. The increase of catalytic activity is ascribed to the accelerated activation rate of n-butane molecules by hydride subtraction on the Lewis acid sites at higher reaction temperature. Furthermore, the addition of La/Ni improves the selectivity to isobutane by inhibiting the bimolecular reaction.

Nature of active sites and deactivation mechanism for n-butane isomerization over alumina-promoted sulfated zirconia

The deactivation of n-butane isomerization on alumina-promoted sulfated zirconia (SZA) was studied at 473K under atmospheric pressure. Under N2, the regeneration temperature influences the isomerization activity greatly. The change of the sulfate species during the deactivation process was determined quantitatively by a temperature-programmed evolution mass spectrometer. Two active sites, reducible sites and acid sites, exist on the catalyst surface and show a synergistic effect on the n-butane isomerization reaction. The reducible sulfate species play an important role in butene formation via oxidative dehydrogenation of n-butane, while the acid sites are responsible for the isomerization steps. The reduction of sulfate species and the accumulation of sulfite species on the surface, which cause the decrease of butene concentration in the catalyst bed, are the main reasons for catalyst deactivation. Based on these findings, a new reaction model and two possible ways to improve the catalytic stability of SZA catalyst are proposed.

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.

Alumina-Promoted Sulfated Zirconia System: Structure and Microstructure Characterization

The Al2O3 promotion of ZrO2/SO4 has been shown to influence n-butane isomerization activity. In this paper we report a study of the influence of a small addition of Al2O3 (0.6−10.3 wt %) onto the ZrO2 structure and microstructure. The addition of alumina is related to an increase of the surface area. A detailed X-ray diffraction analysis by the use of the modified Rietveld method allowed us to follow the changes in the crystalline phases and to quantify the amount of amorphous phase. When the Al2O3 content is increased, a decrease of the ZrO2 particle sizes is observed, paralleled by an increase of the amorphous phase. When the Al2O3 content is larger than 3.7 wt %, only amorphous phase can be detected. No crystalline peaks of Al2O3 were ever observed. These data were supported also by TEM and Raman measurements.

New Methods for Quantitative Determination of Brønsted Acid Sites on Solid Acids: Applicability and Limits for Al2O3-Promoted SO42−/ZrO2 Catalysts

Two quantitative methods, both based on H/D exchange, have been developed for the Brønsted acid sites titration on solid acids. The agreement between the results obtained with the two methods on unpromoted sulfated zirconia (SZ) is remarkable: 0.061 and 0.064 mmol of H/g of catalyst were respectively found with the first (anhydride method) and the second (isobutane method) method. Furthermore, the use of a small alkane as the probe molecule with the isobutane method gives us information about the capacity of the solid to activate an alkane. Nevertheless, this last method is hard to apply to very active catalysts such as 3% alumina-promoted sulfated zirconia (SZA3). Indeed, it was not possible in this case to find optimal temperature conditions, low enough to minimize secondary reactions such as isomerization or cracking and, at the same time, sufficiently high for an accurate quantitative H/D exchange measure. On the other hand, the anhydride method was always applicable to SZAn catalysts and the obtained results were very reproducible.