The present invention’s modifying method for wood elements includes a step for soaking wood elements in one or a mixture of acetic anhydride, acetic acid, or chloroacetic acid; and a step for acetylating the impregnated wood elements in a gaseous phase. 1 to 7 wt % of a structural unit derived from an ethylene-based unsaturated carboxylic acid monomer as the monomer component. An improved training sequence enables time-domain estimation and eliminates the need for correct frequency-domain channel estimation at every frequency. A difference between a zero load voltage and the output voltage is determined to obtain a difference level. In addition, resulting annular unsupported catalysts having a specific pore structure and also the use of such annular unsupported catalysts for the catalytic partial oxidative preparation in the gas phase of acrolein.