Stability and Surface Acidity of Aluminum Oxide Grafted on Silica Gel Surface
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207 Mat. Res. Soc. Symp. Proc. Vol. 497 01998 Materials Research Society
EXPERIMENT Aluminum oxide was grafted on silica gel surface by immersing 70 g of silica gel (Aldrich, SBET= 500 m2 .g', average diameter pore of 6 nm, and particle sizes between 0.2 an 0.05 mm),
previously heated at 423 K under vacuum (10-3 torr), in 250 ml of 0.14 M toluene solution of aluminum isopropoxide (Aldrich). The suspension was refluxed under nitrogen atmosphere for 24 h. The mixture was filtered under nitrogen atmosphere, washed with dry ethanol and heated under vacuum in order to eliminated all solvent. Furthermore, the solid was immersed in bidistilled water to promoted the hydrolysis of the unreacted AI-O-R bond, filtered, washed with water, and dried at 393 K under vacuum for 8 h. The following chemical equations describe the preparation reactions: n=-SiOH + [(CH 3) 2 CHO] 3 A1 --> (=SiO)nA1[OCH(CH 3)2] 3.n+ n(CH 3)2CHOH (1) (-SiO)nAI[OCH(CH 3)2]3 . + (3-n)H 2 0
-+
(-SiO)nAl(OH) 3., + (3-n)(CH 3)2CHOH
(2)
where -SiOH stands for silanol group on silica gel surface. The quantity of Al incorporated in (-SiO),AI(OH) 3,, hereafter designated as SiO2 /A12 0 3, was determined by X-ray fluorescence spectroscopy. The quantity of aluminum grafted on the silica gel surface was 1.92 wt%, corresponding to 0.73 mmol.g'l. Fine-grained samples of SIO 2/A120 3 were heat treated for 8 h at the following temperatures: 423, 573, 873, 1173, 1323 and 1473 K. The specific surface areas of the samples were obtained by the BET multipoint method on a ASAP 2010 equipment. The XPS analyses were carried out on a Kratos XSAM HS spectrometer, under 10-9 torr, using Mg Kx radiation as X-ray source. The spectra were referenced to the binding energy of 284.8 eV for the C Is line of adventitious carbon. The 27Al MAS NMR spectra were obtained using a Bruker AC 300/P spectrometer. The chemical shifts of the samples were calibrated against a external AIC13 solution. The infrared spectra were obtained on a Bomen MB series using a Jasco DR 81 diffuse reflectance accessory. About 0.5 g of Si0 2/A120 3 was previously degassed at 103 torr and furthermore submitted to a pyridine vapor for few minutes, and the excess eliminated under vacuum for 4 h at 473 K. RESULTS AND DISCUSSION The specific surface area of the untreated silica is 460 m2.g", and that of Si0 2/Al 20 3 not submitted to heat treatment is 416 m2 .g . There is an area decrease upon chemical modification of the surface, presumably due to the smallest pores being blocked by the reagent. Figure 1 shows the variation of the specific areas as the samples were heat treated from room temperature up to 1473 K. A large area decrease is observed for sample treated at 1023 K, and above 1100 K the matrix becomes non-porous. Table I summarizes the XPS results. Two oxygen 0 Is peaks can be distinguished: one between 531.1 and 532.3 eV due to oxygen bonded to silicon, Si-O (designated as 0') [10], and other between 532.6 and 534.2 eV due to oxygen bonded to aluminum, AI-O (designated as 0OI) [11]. The numbers in pare
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