Free Radical Formation in ZrO 2–SiO 2Sol–Gel Derived Catalysts (original) (raw)
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Effect of Zirconia Precursor on the Properties of ZrO 2 SiO 2 Sol-Gel Oxides
Journal of Sol-gel Science and Technology, 2002
Sol-gel zirconia-silica oxides were synthesized with two zirconium precursors, zirconium n-butoxide and zirconium acetylacetonate, and two different hydrolysis catalysts, HCl and H2SO4. The samples prepared with HCl were additionally sulfated with a 1 M solution of H2SO4. Characterization was performed with FTIR and 29Si-MAS-NMR spectroscopy, as well as with nitrogen adsorption. Because zirconium and silicon alkoxides have different hydrolysis rates,
The Journal of Physical Chemistry B, 2003
Sulfated zirconia has been prepared according to three different procedures, viz., (i) conventional impregnation with sulfuric acid and calcination (3 h at 773 K) of two zirconia's (50 and 217 m 2 /g), (ii) reaction of zirconium tetrachloride with sulfuric acid giving bulk anhydrous zirconium sulfate, and (iii) deposition-precipitation of highly dispersed zirconia on silica and subsequent reaction with either H 2 S and O 2 , or SO 2 and O 2 , or SO 3. The latter two procedures lead to essentially water-free catalysts. Thermogravimetry showed that the impregnated and calcined zirconia's loose sulfate above 830 K (50 m 2 /g) and 910 K (217 m 2 /g). In a gas flow containing water, the sulfated silica-supported zirconia loses sulfate already at 673 K because of the reaction to more volatile sulfuric acid. The catalysts were employed in the gas-phase trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3) at 473 and 673 K and in the solvent-free, liquid-phase hydroacyloxy-addition of acetic acid to camphene (4) to camphene (5) to isobornyl acetate (6) at 338 K. The water-free catalysts were not active; only after addition of water was catalytic activity exhibited. The catalytic activity of the differently prepared sulfated zirconia's is governed by the equilibrium: Zr(SO 4) 2 + 4H 2 O h Zr(SO 4) 2 ‚4H 2 O + nH 2 O h ZrO 2 + 2H 2 SO 4 .aq. Addition of water vapor to the bulk sulfate at 473 K led to the reaction to the tetrahydrate, which was not active, whereas the highly dispersed silica-supported zirconium sulfate reacted to form sulfuric acid. The supported catalyst rapidly released the water at 473 K, which resulted in a rapid drop in catalytic activity. Transport of water through the porous system dominates the activity of the impregnated zirconia's. Accordingly, the slight activity of the zirconia of 50 m 2 /g rapidly dropped at 473 K, whereas the zirconia of 217 m 2 /g displayed a high and stable activity. At 673 K, the transport is much more rapid. The activity of the highly porous zirconia was therefore at 673 K much lower than at 473 K. Whereas the gas-phase reaction is governed by transport of water vapor, the liquid-phase reaction is dominated by transport of the reactants to the active sites. Consequently, the sulfated zirconia of 50 m 2 /g showed a considerably higher activity than that of 217 m 2 /g. Also the silica-supported catalyst exhibited a higher activity. The consistent results demonstrate that sulfated zirconia needs water to display activity in the gas-phase and liquid-phase reaction studied.
Studies in Surface Science and Catalysis - STUD SURF SCI CATAL, 2000
Two different processes were used in this study to prepare sulfated zirconia. The effect of both the hydrolysis ratio H2O/Zr and S/Zr molar ratio were examined. Sample characterization was carried out using XRD, NH3-TPD, N2 adsorption-desorption and FT-IR spectroscopy using adsorbed probe-molecules ( C6H6 and pyridine). Sulfur content before and after n-hexane isomerization reaction were measured. We found that samples obtained by solgel process retain high amount of sulfur and exhibit important BET surface areas. The decrease of the hydrolysis ratio improves considerably acidic and catalytic properties. Lewis and BrOnsted acid sites coexist but the Lewis ones are predominant. It was also found that the increase of the Pt lowding in mechanical mixture of Pt/Al2O3 and sulfated zirconia enhances the catalytic activity and selectivity at moderate activation temperature.
Preparation of sol-gel sulfated ZrO 2 SiO 2 and characterization of its surface acidity
Applied Catalysis A-general, 1995
Zr-Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, using tetraethoxysilane and zirconium acetate as precursors. Zirconium is incorporated into the lattice of silica, but can easily be extracted either by self steaming or by acid treatment. The solids obtained in basic medium show a low surface area and low acidity. The samples prepared in acidic medium show large surface areas between 410 and 740 m2/g and can be converted to strong acids by contact with a solution of sulfuric acid. They retain pyridine up to 773 K and contain a total number of acid sites comparable to that of HY zeolites i.e. 1.7 meq/g. The catalytic activity for the bifunctional isomerization of n-hexane expressed per gram of zirconium is comparable for the sample containing 4wt.-% Zr and sulfated zirconia.
Chinese Journal of Catalysis, 2007
Doping sulfated zirconia with silica increases the acidity of the sulfated zirconia. This modified catalyst was used for liquid-phase dehydration of methanol at 413-453 K as a model reaction. Methanol is sequentially dehydrated to dimethyl ether and ethylene over both sulfated zirconia and silica-doped sulfated zirconia in a single step. Also, significant amounts of propylene are formed by the silica-doped catalyst but not by the undoped catalyst. The data were discussed in relation to increased acid active sites after doping with silica, which electronically modified the catalyst surface.
Characterization and catalytic activity of zirconium dioxide prepared by sol-gel
Latin American Applied Research, 2005
Zirconia samples prepared from Zr(n- OC3H7)4 and ZrOCl2 were characterized by XRD, BET surface area, TG-DTA, TEM/SEM, FTIR, and D2O exchange. Oxides obtained from the alkoxide contain two types of hydroxyl groups at 3704 and 3597 cm -1 , which do not exchange with D2O. Those obtained from ZrOCl2 produced three types of hy- droxyl groups at 3774, 3733 and 3671 cm -1 , which do exchange with D2O. Surface acidity was determined by FTIR of adsorbed lutidine. Only Lewis acid sites were found on samples from ZrOCl2 hydrolysis. FTIR spectra of calcined samples exhibited strong absorption bands in the 1800 to 1000 cm -1 region, which are ascribed to various types of carbonate and bicarbonate species. These ones are strongly ad- sorbed on the surface oxide and they are removed only after 3 hours at 600 oC in stream of H2. Samples derive from the alkoxide precursor restore the origi- nal carbonate species after exposure to O2 at 600oC. Catalytic activity for i-propanol dehydra- tion/dehydroge...
Journal of Catalysis, 2004
ZrO2–SO4 powders were prepared through a single step sol–gel reaction by modulating the conditions of the alkoxide hydrolysis and polycondensation steps. Sulfuric acid or (NH4)2SO4 was employed as the sulfating agent. The samples were calcined at 890 K and characterized as to phase composition crystallinity (XRD) and surface area porosity (BET method). Surface functionalities were investigated by TGA/FTIR, FTIR, DRIFTS,
Solid acid catalyst material has many advantages over homogeneous acid catalysts due to easy separation, reusability and environmental friendliness. The objective of this work is to synthesize non-ionic surfactant-templated, ordered mesoporous silica sulphated zirconia binary oxide with improved acidic and textural properties [1,2]. Attempts have been made to make synthesis methodology more economical by using cheaper raw materials like sodium silicate and zirconium oxy-chloride as source of silica and zirconia respectively [29]. A series of mesoporous SSZ materials were prepared using tri-block co-polymers as template by changing various parameters like the precursors used, the Si:Zr ratio, pH of the gel and the effect of the ageing time. The products were characterized by XRD, TEM, SEM, TPD, FTIR, nitrogen adsorption at liquid nitrogen temperature and by chemical analysis. Acidic properties of selected samples were determined by ammonia TPD technique. The catalytic activity of the...
Synthesis of silica sulphated zirconia binary oxide with improved acidic and textural properties
2013
Solid acid catalyst material has many advantages over homogeneous acid catalysts due to easy separation, reusability and environmental friendliness. The objective of this work is to synthesize non-ionic surfactant-templated, ordered mesoporous silica sulphated zirconia binary oxide with improved acidic and textural properties[1,2].Attempts have beenmade tomake synthesis methodologymore economical by using cheaper rawmaterials like sodium silicate and zirconium oxy-chloride as source of silica and zirconia respectively[29].Aseries of mesoporous SSZ materials were prepared using tri-block co-polymers as template by changing various parameters like the precursors used, the Si:Zr ratio, pH of the gel and the effect of the ageing time. The products were characterized by XRD, TEM, SEM, TPD, FTIR, nitrogen adsorption at liquid nitrogen temperature and by chemical analysis. Acidic properties of selected samples were determined by ammonia TPD technique. The catalytic activity of the prepared...
Preparation and characterization of sulfated zirconia catalysts obtained via various procedures
Applied Catalysis A: General, 1999
Mesoporous sulfated titania xerogels with structural and catalytic properties were prepared by the modified sol-gel method using surfactant as templates. The sulfate loading of these catalyst materials were varied in the range of 0 to 50 wt% SO 4 2-. Characterization of the sulfated titania were evaluated by transmission electron microscopy, N 2 adsorption and X-ray diffraction. The addition of sulfate has decreased the crystalline size and induces the structural transformation from rutile to anatase. The best sample catalyst was attained at TS5 containing 5 wt% of sulfate and having equal strength of both Brønsted and Lewis acid site. Sulfated titania show good activity towards citronellal isomerization at 95°C using toluene as solvent. Conversion of citronellal correlates with the the surface acidity of the catalyst and the use of toluene as solvent in the reaction, while the selectivity towards (-)-isopulegol depends on the corresponding acid site strength.