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A study was carried out on the synthesis of ethyl tert-butyl ether from ethanol and 2-methylpropene on silica gel samples modified by the addition of ZrO 2 and Al 2 O 3 . A decrease in the turnover frequency (TOF) of the reaction is observed with increasing acidity of surface of the modified silica gels. A relationship was found between the TOF of the reaction and the fractal dimension of the catalyst. The TOF of the reaction decreases with increasing fractal dimension of the catalyst.Key words: silica gel, fractal dimension, synthesis of ethyl tert-butyl ether, acidity, turnover frequency.Some of the most important factors determining catalytic properties of a material are the chemical nature of the active sites and the surface morphology. Catalysts, as a rule, have irregular structure, which is difficult to describe quantitatively. A fractal approach has recently begun to be used for the quantitative description of catalyst morphology [1]. In particular, the structural sensitivity of heterogeneous catalytic reactions may be due to change in the fractal dimension of the catalyst [1]. It has also been shown that the fractal dimension may affect the rate of heterogeneous catalytic reactions [1]. Conversion and selectivity for such reactions as the oxidation of formaldehyde on Pt/TiO 2 [2], petroleum reforming on PtRe/Al 2 O 3 [3], and the hydrodesulfurization of thiophene on MoP-Al 2 O 3 [4] depend on the fractal dimension of the catalyst. In previous work [5], we have shown that change in the preexponential factor of the rate constant of an oxidation-reduction reaction results from change in the fractal dimension of the catalyst.The fractal approach, as a rule, is used to examine oxidation-reduction reactions. However, it is often difficult in such processes to relate the catalytic activity to the active surface sites. The nature of the active surface sites can be examined more concretely in reactions proceeding through an acid-base interaction. In this case, the activity of the catalysts is determined by the acid or basic surface sites, whose concentration may be established rather exactly using reported methods [6]. The clear understanding of the nature of the active sites makes the examination of acid-base reactions a promising and interesting area for the application of the fractal approach. Hence, in the present work, we extended the fractal approach for analyzing acid-base catalysis processes. 328One such acid-base reaction is the synthesis of ethyl tert-butyl ether (ETBE) from ethanol and 2-methylpropene in the presence of acid catalysts [7]:Side-reactions may include the dimerization of 2-methylpropene to give diisobutylene, the dehydration of ethanol to give diethyl ether, and the hydration of 2-methylpropene by water present in the starting raw material to give 2-methylpropan-1-ol.The rate of the reaction leading to ETBE depends significantly on the chemical nature and type of acid-base catalyst used. Such catalysts may be ion-exchange resins, aluminas, oxides, and zeolites [7]. In our previous work [8]...