IR AND SI-29 NMR INVESTIGATION OF THE INFLUENCE OF ALKALINE MODIFICATION ON THE STRUCTURE OF HYDROTHERMALLY DEALUMINATED Y-ZEOLITES .2. HIGHLY DEALUMINATED ZEOLITES

Using IR and Si-29 NMR spectroscopy the influence of alkaline modifica tion on the structure of samples obtained by steaming the ammoniated f orm of NH4NaY zeolite (53 % NH4+, Si/Al = 2.37 pretreated under a mois t atmosphere at 873 K) at 973 and 1073 K has been studied. Highly deal uminated forms of faujasite arise in the course of hydrothermal treatm ent as a result of the transfer of Al from the framework to extraframe work hydroxide species accompanied by the formation of silica-type dis iloxane bonds. The treatment of highly dealuminated samples with an aq ueous solution of KOH at 353 K results in the partial regeneration of the crystallographic positions of Al in the framework. The structure o f the sample modified with the alkali is identical to that of silicaal umina gel (Si/Al approximate to 2.2) ''aged'' in air for a long period of time.