Synthesis And Acidity

Category: Ukrainean-polish sympozium

SYNTHESIS AND ACIDITY

OF AMORPHOUS FLUORINE-CONTAINING

ALUMINIUM BORATES

V. V. Brei1, S. Ya. Brichka2 and R. Leboda2

1Institute of Surface Chemistry of the National Academy of Sciences of Ukraine

Prospekt Nauky 31, 252022 Kyiv, Ukraine

2Department of Chemical Physics, Maria Curie-Sklodowska University

M.C. Sklodowska Sq. 3, 20031 Lublin, Poland

In recent years many scientists devote their attention to studies in the field of heterogeneous catalysis over aluminium borates1, 2. One of the limitations of the materials is considered to be their lower catalytic activity in comparison with g-Al2O3. The crystalline aluminium oxofluorine borates have acid sites whose strength is comparable to that of acid sites of faujasites3.

Experimental

The starting aluminium borate with a composition of Al3BO6 was prepared according to the known procedure1. Then the samples synthesized were modified with F- by the method of the pore volume impregnation with aqueous ammonium fluoride solutions for 20 h at room temperature, following which they were dried at 283 K. The pH value of the reaction mixture is changed from a neutral pH»6.8 to a basic condition pH=7.5-8.6 depending on the solution concentration. The conversion that takes place during the synthesis can be written as follows:

Al3BO6 + nNH4F ® Al3BO6-n/2Fn + nNH3 + n/2H2O

The product samples were characterized by the BET adsorption isotherms of N2, X-ray powder diffraction, IR spectroscopy, TPD of NH3, TPR of dehydration of 2‑propanol, mass-spectrometry, and thermal analysis techniques.

Results and discussion

The above-outlined procedure does not allow one to prepare amorphous fluorine-containing aluminium borates with a ratio of B2O3/HF

thermal analysis data the formation of the crystalline aluminoborate occurs over a temperature range of 823 - 1123 K depending on the content of F- in the sample structure. The mass-spectrometry analysis of the gaseous phase shows that heating of samples leads to release of H2O, (BOF)3 and traces of HF.

The IR spectra of the pyridine adsorbed on the surface of the fluorine-containing aluminium borates have an absorption band for pyridinium ions at 1545cm-1, which points to the presence of Brönsted acid sites. After adsorption of PhCN the IR spectra of the aluminium borate have bands at 2300 and 2310 cm-1 attributed to two Lewis acid sites. On the surface of the all fluorine-containing samples there are three types of L-sites with the PhCN absorption bands at ~2300, ~2312and ~2333 cm-1. With increasing of F‑ content in the samples are observed the following changers in the IR spectra (i) sharp decreasing of the adsorption of PhCN coordinate-bonded at ~2300 and ~2310 cm-1; (ii) increasing the adsorption of PhCN at 2330-2336 cm-1; (iii) the change of the position of bands of PhCN adsorbed on L-sites.

All the synthesized samples are active in the reaction of dehydration of propan-2-ol. In accord with the TPR spectrum for the starting aluminium borate the formation of propylene is characterized by a peak at 453 K. The reactivity of the modified samples is higher. Their Tmax is shifted towards the low-temperature region on 20-70K. According to the TPR spectra the degree of conversion of 2-propanol of some fluorine-containing samples is by 7 - 10 times higher in comparison with the starting aluminium borate.

Conclusions

Following the above experimental data we may conclude that high-surface (180-320 m2g-1) amorphous fluorine-containing aluminium borates can be synthesized by the pore volume impregnation method with NH4F. The amorphous materials with the high content of F- (> 10-15 wt. %) can not be prepared. The acid-base properties of aluminium borates can be regulated by the change of the F- content in the solids. The sample with a nominal F- content in the 1.5-3 wt. % should be the most active as the catalysts.

References

1. Delmastro A., Gozzelino G., Mazza D., Vallino M., Busca G., Lorenzelli V. J. Chem. Soc., Faraday Trans., 1992, vol.88, 14, 2065.

2. Simon S., van der Pol A., Reijerse E.J., Kentgens A.P.M., van MoorselG.J., de Boer E. J. Chem. Soc., Faraday Trans., 1994, vol.90, 18, 2663.

3. Brichka S.Ya. and Brei V.V. Mendeleev Commun., 1997, 3, 125.



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