Effect of H2O Activity on Zeolite Formation

Claudia Belviso,Francesco Cavalcante

doi:10.3390/ma13214780

Abstract

In an effort to understand the effects of H2O activity on zeolite formation, we have synthesized LTA zeolite using a combination of freezing processes and varying drying temperatures. Sodium aluminate and sodium silicate were used to form LTA zeolite, according to the IZA (International Zeolite Association) protocol. The synthesis steps were modified by adding the precursor frozen process by a rapid liquid nitrogen (−196 °C) treatment or slow conventional freezer treatment (−20 °C). The samples were subsequently sonicated and then dried at 80 °C or 40 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed on the samples immediately after the drying process as well as after 2 weeks and 1 month of aging the solid products. The results indicated that LTA zeolite does not form. The silica-alumina precursor after both freezing processes and after being dried at 80 °C showed the presence of sodalite displaying stable behavior over time. Both sets of samples dried at 40 °C and did not show the presence of zeolite immediately after the drying process. However, after 2 weeks, the liquid nitrogen–frozen precursor was characterized by the presence of EMT whereas zeolites never formed in the −20 °C samples. These results suggest that freezing processes differently control the H2O activity during the drying and aging processes in the solid state. Thus, although the precursor chemical composition is the same, the type of zeolite formed is different.

Highlights

Zeolite Linde Type A (Zeolite LTA or Zeolite A) [Na12 [(AlO2 )12 (SiO2 )12 ] · 27 H2 O [1] can be synthesised using a variety of sources including different types of alumina and silica materials [2,3,4,5,6,7,8,9,10,11,12,13]
The X-ray diffraction (XRD) peaks for the samples dried at higher temperatures (CF1d-80 and LN1d-80 ) were mainly indexed to sodalite with minor aluminium silicate and sodium silicate as well as amorphous material (Figure 2b)
The data show that LTA zeolite does not form when the aluminosilicate precursor is rapidly frozen by liquid nitrogen and dried at 40 or 80 ◦ C nor when it is dried at the same temperatures after conventional freezer treatment

Summary

Introduction

Zeolite Linde Type A (Zeolite LTA or Zeolite A) [Na12 [(AlO2 ) (SiO2 )12 ] · 27 H2 O [1] can be synthesised using a variety of sources including different types of alumina and silica materials [2,3,4,5,6,7,8,9,10,11,12,13]. Many nucleation and crystal-growth mechanisms have been proposed for this type of zeolite [14,15,16,17,18], and many literature data have documented microcrystals zeolite A growth on various substrates to form highly oriented monolayers or multilayers composed of micro crystalline building blocks [19,20,21]. Two processes for LTA synthesis were analysed concluding that rapid zeolite crystallization by sonication treatment ensures rapid transformation into more stable sodalite, whereas the slower double-step mechanism of geopolymer transformation into the crystalline phase by a conventional hydrothermal process is responsible for a very slow transformation of LTA into sodalite [25,26]. Literature data have documented that H2 O confined in mesopores and nanopores shows properties different from those of common bulk water [29,30,31]

Results

Discussion

Conclusion