Preparation and Characterization of Sulfated Zirconia from a Commercial Zirconia Nanopowder

Sulfated zirconia (SZ) catalysts were prepared by impregnation from commercial zirconia nanopowder with different concentrations of sulfuric acid (0.2, 0.5, and 0.8 M) and calcination temperatures (600, 700, 800, and 900 OC). SZ catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Transmission electron microscopy (TEM), Surface area analyzer (SAA), and acidity test by ammonia adsorption. FTIR showed that the modification of zirconia with sulfuric acid enhanced the catalysts acid sites which confirmed by acidity test and it also showed the 0.8 M SZ at 600 OC was the optimal condition with the highest acidity value of 3.81 mmol/g. XRD showed that SZ crystalline structure was in a monoclinic phase and the intensity decreased with increasing concentration and decreasing calcination temperature.


INTROdUCTION
The recent widely developed heterogeneous catalyst is catalysts which had a group of metal oxides such as zirconia (ZrO 2 ).It has corrosionresistant properties, strong chemical resistance, low thermal conductivity, not conduct electricity, and better thermal power than alumina 1 .This material also has so potential in many applications such as hydrolysis, biodiesel production, pillared material, and often used for hydrocracking process [2][3][4] .It was developed and used as a catalyst because it had double characteristics as a weak acid and base catalyst depend on the treatment process, so that is possible to be used in many catalytic reaction 4 .ZrO 2 catalyst could be used as an acid catalyst by modification process using acidic compounds such as sulfuric acid 5 .The sulfated zirconia (SZ) material which obtained by modification process would produce a high acidic catalyst because of the formation many active sites of Brønsted and Lewis acid sites in the structure 6 .Among many super acid solids, SZ is the most studied material because of its ability to catalyze with its higher acidity than other inorganic acids.
The catalytic properties of SZ catalyst depend on treatment methods, ZrO 2 precursor, calcination temperature, sulfating agent, etc 7 .Preparation of SZ catalyst could be done by various methods such as hydrothermal 8 and sol gel methods 9 .However, it has advantages and disadvantages to produce a good catalyst.In this study, ZrO 2 was chosen as catalyst because of its good material properties and characteristics.Preparation of SZ catalyst was done by wet impregnation method from commercial ZrO 2 nanopowder and using sulfuric acid as sulfating agent.The different concentrations and calcination temperatures were investigated in this study in order to know the effect of its on SZ catalyst.

Sample preparation
Sulfated Zirconia (SZ) catalysts were prepared from 10 g of ZrO 2 nanopowder added into 150 mL H 2 SO 4 with different concentrations (0.2, 0.5, and 0.8 M) respectively, then stirred for 24 hours.The different concentrations were performed to determine the effect of sulfuric acid concentration on zirconia.SZ catalysts were then dried in an oven at 100 O C for overnight, calcined with different temperatures (600, 700, 800, and 900 O C) for 4 hour.After that SZ catalysts were crushed and sieved in a 250 mesh sieve.The different temperatures were conducted to learn the effect on SZ catalyst.
The acidity test of SZ was determined with ammonia adsorption by gravimetric method.The acidity value of ZrO 2 nanopowder and sulfated ziconia were calculated by using the equation:

Sample characterization
The SZ catalysts were characterized by Fourier transform infrared spectroscopy (FTIR, Shimadzu Prestige-21) in range 4000-500 cm -1 with a KBr disc technique.The presence of sulfate groups on ZrO 2 was confirmed by FTIR.X-ray diffraction (XRD, Shimadzu XRD 6000) with Cu K radiation at 40 kV and 40 mA under space conditions.Crystallite size was determined using Scherrer formula with a shape factor (K) of 0.89: D = (0.9λ)/b cos θ, where D is the crystallite size (nm), λ is the radiation wavelength (0.154056 nm), θ is the diffraction peak angle and b is the corrected half-width at halfmaximum intensity (FWHM).Transmission electron microscopy (TEM, Hitachi H8100A) at 100 kV for confirming catalyst morphology.Surface area analyzer (SAA, Quantachrome NovaWin Series version 11.0).Sample was thoroughly outgassed for 4 h, at 200 °C, bath temp. of 77.3 K, and equil time of 60/60 sec (ads/des) then the weight of the outgassed sample was that used in calculations.The specific surface area was calculated by applying the Brunauer-Emmett-Teller (BET) method.

FTIR characterization
FTIR analysis showed the absorption band at 490-745 cm -1 were corresponded by the presence of Zr-O-Zr bond 10 .The absorption band at 3400-3800 cm -1 showed the streching vibration of the O-H bond and the absorption band around at 1636 cm -1 corresponded to bending vibration of O-H bond 11 .The spesific absorption band of SZ catalyst at 1120-1224 cm -1 characterized to symmetric streching vibration of SO 4 2-bidentat species 12 .This observation also indicated the presence of covalent S=O double bonds on surface sulfate groups at SZ which could effect the acidity of catalyst 13 .In Fig. 1 showed SZ of 0.2 M and calcined at different temperatures did not clearly show the specific peak of sulfate groups.It was different from SZ of 0.5 M at 600 o C (Fig. 2), it had showed the spesific peak of sulfate groups.SZ of 0.8 M at 600 o C (Fig. 3) had the stronger sulfate groups specific peak than others.It showed that sulfate groups had impregnated on ZrO 2 well.However overall in this concentration variations, the peak of SO 4 2-would appear at the higher concentration and disappear at higher calcination temperature.Sulfate groups of SZ had the important role in catalytic activity because it need the high acidity of catalyst reaction 14 .The higher acidity would have an effect on the good catalytic reaction.

Acidity test
FTIR analysis of ammonia adsorption on solid surface could distinguish both the Brønsted acid sites and Lewis acid sites.Adsorption band at 1404 cm -1 was associated with NH 3 adsorption on the Brønsted acid sites 15,16 .Adsorption band around at 1119 cm -1 was associated with NH 3 adsorption on the Lewis acid sites 17 .In this study, acidity value for ZrO 2 of 0.059 mmol/g and SZ of 0.8 M at 600 o C had the highest acidity as shown the acidity value in Table 1 as confirmed in Fig. 6 from FTIR observation which had the stronger peak of NH 3 adsorption than others (Fig. 4 and Fig. 5).The catalytic acidity would decrease at the higher calcination temperature and increase at the higher concentration.It was caused by sulfate groups loading on SZ catalysts which lost because of the higher calcination temperature.Modification of ZrO 2 with sulfuric acid could enhance the acid strengh because of two factors i.e. the Brønsted acid sites from bridging Zr-OH groups and the Lewis acid sites from low cordinate Zr ions 18 .had reported the synthesis of SZ nanopowders with sol gel method and it found that tetragonal phase at 500 o C and was stabled at the higher temperature.However, all of the SZ synthesis process depend on the ZrO 2 precusor to obtain the stable and active catalyst.The crystal sizes of SZ catalysts were calculated  2. The results showed overall the crystallite size decreased with the addition of sulfate.However, the calculation depend on halfmaximum intensity of crystal peak.

Table 1 : Acidity value of sulfated zirconia (SZ)
Scherrer equation and crystal size for ZrO 2 and SZ were presented in Table