Anion effect on zeolite X synthesis and its phase transformation

Abstract

• The anion salt significantly impacted both crystallization kinetics and the phase composition of Zeolite X. • Carbonate and phosphate anions accelerated the initial nulceation and crystallization. • Sulphate and nitrate anions inhibited the zeolite X crystallization by promoting the competing zeolites SOD and Na-P1. • The limited anion incorporation for both NaX and Na-P1 against zeolite SOD phase. The growing industrial demand for synthetic zeolite X is an incentive to finding production methods that use cheaper and sustainable feedstock, including industrial byproducts, which often contain various anionic impurities. However, the influence of anionic species on crystallisation and phase transformation of zeolite X has not fully evaluated. This study investigates the effect of four different sodium-type anion salts (Na 2 CO 3 , Na 2 SO 4 , Na 3 PO 4 and NaNO 3 ) on NaX zeolite crystallisation from initial gel mixtures with molar composition of 2.25Na 2 O:0.17Al 2 O 3 :1SiO 2 :86.56H 2 O with salt concentration ranging from 0.1 to 0.2 M. The anion type significantly impacted both crystallisation kinetics and final solid phase composition. Addition of Na 2 CO 3 yielded highly crystalline zeolite X (>98 %), while Na 3 PO 4 at 0.1 M accelerated crystallisation kinetics where 80 % crystallinity was achieved within 2 h. In contrast, Na 2 SO 4 and NaNO 3 inhibited NaX crystallisation by promoting the formation of competing phases of Na-P1 and sodalite respectively. In addition, the acceleration effect of different sodium salts on NaX crystallisation increased in the order of Na 2 CO 3 > Na 3 PO 4 > NaNO 3 > Na 2 SO 4 . This work provides an understanding of the role of different anions in the phase purity and crystallisation kinetics of zeolite NaX, potentially reducing production costs through optimized synthesis conditions enabling sustainable and greener chemical production processes.

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