Role of Structural Similarity Between Starting Zeolite and Product Zeolite in the Interzeolite Conversion Process

Abstract

GIS- and LTL- (the three capital characters indicate the framework type-code) type zeolites were obtained by organic structure-directing agent free hydrothermal conversion of FAU-type zeolite at 125 degrees C in the presence of NaOH and KOH, respectively. MOR-type zeolite was found coexisting with GIS-type when the hydrothermal conversion with NaOH was carried out at 140 degrees C. There was a common building unit consisting of four-membered ring chain such as d6r, dsc, and dcc (the three characters indicate the composite building unit-code) units present in both the starting zeolite (FAU-type zeolite) and the product zeolites (GIS- and LTL-type zeolites), which was the crucial factor for crystal growth through interzeolite conversion. In the case of severe hydrothermal synthesis conditions such as high temperature, however, the crystallization behavior was similar to that observed in conventional hydrothermal synthesis using amorphous materials because the starting zeolite was excessively decomposed. The hypothesis was confirmed by successful interzeolite conversion of *BEA- to MFI-type zeolite which shared the common composite building unit mor.

High Potential of Interzeolite Conversion Method for Zeolite Synthesis

“…Hydrothermal Conditions Interzeolite conversion of FAU type zeolites was conducted using only alkaline metal hydroxides in the absence of seed crystals ( Table 3) 44) . Using NaOH as both the alkali source and the inorganic SDA, GIS type zeolite was obtained from FAU type zeolite, whereas using KOH, LTL type zeolite was obtained.…”

“…To further confirm the importance of this structural similarity, hydrothermal conversion of FAU type zeolite was carried out in the presence of FER seed crystals with a Si/Al ratio of 25 ( Table 3) 44) . FAU and FER type zeolites have no structural similarities ( Table 2).…”

“…Therefore, the transformation behavior of BEA type zeolite is likely to be different to that of FAU type zeolite. We investigated the transformation behavior of BEA type zeolite in the absence of OSDA under various synthesis conditions 44) . As indicated in Table 4, although BEA type zeolite did not decompose completely even after 7 days, MOR type zeolite phase was observed, suggesting that the starting BEA type zeolite is stable under these hydrothermal synthesis conditions, and that the formation of MOR type zeolite is due to the common composite building unit mor.…”

“…Based on this finding, we investigated the effect of adding seed crystals on the OSDAfree interzeolite conversion of BEA type zeolite 44) . The addition of LEV seed crystals led to the formation of LEV type zeolite.…”

“…On this basis, several researchers have pointed out the high potential of another method for zeolite formation, based on the hydrothermal conversion of one zeolite type into another type (interzeolite conversion) 12) 29) . This study describes our recent investigations of the synthesis of several types of zeolites by interzeolite conversion of FAU (the three characters indicate the framework type code), beta (hereinafter, BEA), and LEV type zeolites in the presence and absence of OSDAs 30) 44) , and reviews the potential of interzeolite conversion as an alternative method for zeolite synthesis.…”