THE ACID-BASE PROPERTIES OF AMORPHOUS SILICA FROM STRAW AND RICE HUSK
Olga D. Arefieva, Polina Dmitriyevna Pirogovskaya, Aleksandr Yevgen'yevich Panasenko, A. V. Kovekhova, Lyudmila A. Zemnukhova
Abstract
The present work shows results of studying acid-base properties of the surface by the methods of pH-metry and Hammett of amorphous silicon dioxide from rice husks and straw obtained by various schemes: oxidative firing; oxidative roasting with preliminary treatment with 0.1 M hydrochloric acid solution; precipitation from alkaline solutions. The samples obtained by the thermal method contain impurities of alkali, alkaline earth metals, aluminum, and aluminum and practically do not contain water. The composition of the deposited samples contains a small fraction of impurities (0.05%) and water - from 8.2 to 10.2%. The pH value of an aqueous suspension of silicon dioxide has a neutral, alkaline or acidic environment depending on the content of impurities of alkali and alkaline earth metals. Distribution of acid-base centers on the surface of the samples is nonmonotonic and heterogeneous, and manifests itself in discreteness with a fairly clear differentiation of sorption bands with maxima of different intensities corresponding to a certain pKa value. Distribution curves of the adsorption centers of the indicators on the surface of the samples of amorphous silicon dioxide are similar to each other. There are four types of active centers on their surface: acidic Lewis (pKa + 16.80), Bransted main (pKa +7.15 and +9.45) and acid (pKa + 2.50). The number of active centers depends on the preparation scheme and is determined by the content of impurity elements and water in the oxide samples.