1. N. Davidova, P. Kovacheva: Neftekhimiya,22, 93 (1982).
2. N. Davidova, P. Kovacheva: Bulgarian Pat. 32, 032, 1982.
3. L. Vogt, W. Make, K. Becker, M. Weblr: Z. Chem.,25, 293 (1985).
4. Zeolite Chemistry and Catalysis on Zeolites, Ed. By J. Rabo, Vol. 1. Mir, Moskva 1980.