Nanoscale composition inhomogeneity in silica-aluminas prepared by various methods, страница 7

offered by the purely empirical approach to obtain

materials with uniform acidity. If this had been

achieved, this would have been claimed. A real pessimism

may be expressed that the sol±gel method

could produce amorphous silica±aluminas with uniform

acidity, unless different chemical methods and an

extensive chemical engineering approach pave the

way to a rigorous control of the hydrolysis conditions

at the nanometer scale, or completely new family of

starting reactants is discovered.

We hope that the present work could, together with

other ones like that of Monaco and Ko [17] cited

above, further encourage the development of

advanced methods, in the Æeld both of chemistry

and chemical engineering, which could allow the

obtention of materials with a really homogeneous

acidity.

5. Conclusion

We can conclude that the industrial technologies

and the laboratory methods based on sol±gel processing

at the beginning of the present decennia were still

far from being able to produce silica±aluminas with a

single type of acidic sites, homogeneously distributed

on the surface. The nanoscale chemical composition

inhomogeneities and the presence in many cases of

impurities inhomogeneously distributed in the material,

are mainly responsible for this lack of homogeneity.

The so-called sol±gel methods which were currently

in use in industry until 1991 and which, in

most cases, did not use alkoxides, seem slightly less

efficient than our old laboratory method in reaching a

higher degree of homogeneity.

Acknowledgements

One of the authors, Corneliu SaÃrbu, gratefully

acknowledges the Universite¬ Catholique de Louvain

(Louvain-la-Neuve) for the research grant which

made the present work possible. The contribution

of Mrs. Jocelyn de Kerkhove, who has performed

the test of composition homogeneity of the mineral

samples used for determination of the KAlSi parameter

of our spectrometer, is also gratefully acknowledged.

References

[1] P.O. Scokart, F.D. Declerck, R.E. Sempels, P.G. Rouxhet, J.

Chem. Soc., Faraday Trans. 1 73 (1977) 359.

[2] J.P. Damon, B. Delmon, J.M. Bonnier, J. Chem. Soc., Faraday

Trans. 1 73 (1977) 372.

[3] C. Defosse¬, P. Canesson, P.G. Rouxhet, B. Delmon, J. Catal.

51 (1978) 269.

[4] J.P. Damon, J.M. Bonnier, B. Delmon, Bull. Soc. chim. Fr.

449 (1975).

[5] J.P. Damon, J.M. Bonnier, B. Delmon, J. Colloid Interface

Sci. 55 (1976) 381.

[6] P.G. Rouxhet, P.O. Scokart, P. Canesson, C. Defosse¬, L.

Rodrique, F.D. Declerck, A.J. Leonard, B. Delmon, J.P.

Damon, in: M. Kerker (Ed.), Colloid and Interface Science,

vol. 3, Academic Press, New York, 1976, pp. 81±94.

[7] Y. Matsumoto, K. Mita, K. Hashimoto, T. Tokoroyama, Appl.

Catal. A 131 (1995) L1.

96 C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97

[8] P.G. Menon, B. Delmon, in: G. Ertl, H. Kno»zinger, J.

Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol.

1, VCH, Weinheim, 1997, pp. 100±118.

[9] M.A. Aegerter, D.F. Bozano, in: L.L. Hench, J.K.West (Eds.),

Chemical Processing of Advanced Materials, Wiley, New

York, 1992, pp. 175±186.

[10] J.I. Goldstein, D.B. Williams, Quantitative X-ray analysis, in:

D.C. Joy, A.D. Romig, J.I. Goldstein (Eds.), Principles of

Analytical Electron Microscopy, Plenum Press, New York,

1986, pp. 155±217.

[11] J.K. Park, J. Ardell, J. Microscopy 165(2) (1992) 301.

[12] N. Zaluzec, Private communication.

[13] C. Sarbu, B. Delmon, Proceedings of the Eleventh European

Congress Electron Microscopy EUREM'96, 25±30 August

1996, Dublin, Ireland, vol. 1, s. M2 (CD-version edited by the

organizers).

[14] K.F.J. Heinrich, Electron Beam X-Ray Microanalysis, Van

Nostrand Reinhold Co., Princeton, 1981, p. 156.

[15] J.I. Goldstein, D.B. Williams, Quantitative X-ray Analysis,

in: D.C. Joy, A.D. Romig, J.I. Goldstein (Eds.), Principles of

Analytical Electron Microscopy, Plenum Press, New York,

1986, pp. 188±189.

[16] H.A. Liebhafsky, H.G. Pfeiffer, P.D. Zemany, Anal. Chem. 27

(1955) 1257.

[17] S.J. Monaco, E.I. Ko, Chemtech 28 (1998) 23.

C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 97