Revealing scenarios of interzeolite conversion from FAU to AEI through the variation of starting materials.

Interzeolite conversion, which refers to the synthesis of zeolites using a pre-made zeolite as the starting material, has enabled promising outcomes that could not be easily achieved by the conventional synthesis from a mixture of amorphous aluminum and silicon sources. Understanding the mechanism of interzeolite conversion is of particular interest to exploit this synthesis route for the preparation of tailor-made zeolites as well as the discovery of new structures. It has been assumed that the structural similarity between the starting zeolite and the target one is crucial to a successful interzeolite conversion. Nevertheless, an image as to how one type of zeolite evolves into another one remains unclear. In this work, a series of dealuminated FAU zeolites were created through acid leaching and employed as the starting zeolites in the synthesis of AEI zeolite under various conditions. This experimental design allowed us to create a comprehensive diagram of the interzeolite conversion from FAU to AEI as well as to figure out the key factors that enable this kinetically favourable crystallization pathway. Our results revealed different scenarios of the interzeolite conversion from FAU to AEI and pinpointed the importance of the structure of the starting FAU in determining the synthesis outcomes. A prior dealumination was proven effective to modify the structure of the initial FAU zeolite and consequently facilitate its conversion to the AEI zeolite. In addition, this strategy allowed us to directly transfer the knowledge obtained from the interzeolite conversion to a successful synthesis of the AEI zeolite from dealuminated amorphous aluminosilicate precursors. These results offer new insights to the design and fabrication of zeolites via the interzeolite conversion as well as to the understandings of the crystallization mechanisms.

Toward Efficient Synthesis of Chiral Zeolites: A Rational Strategy for Fluoride-Free Synthesis of STW-Type Zeolite.

The STW-type zeolite is attractive for developing novel enantioselective syntheses/separation of chiral compounds because it is the only chiral zeolitic microporous material whose enantioenriched synthesis has been achieved. In addition to the conventional industries in which zeolites are used, STW should have diverse industrial applications in the pharmaceutical and food industries. However, the toxic and caustic fluoride required for synthesizing STW severely hinders its commercialization by mass production. Herein, we report the first example of fluoride-free STW synthesis, in which the two roles of fluoride-formation of a zeolitic framework rich in tetravalent T-atoms and promotion of double 4-membered ring unit formation-were substituted by dry gel conversion and Ge addition, respectively. The STW obtained was highly crystalline, with a similar micropore volume and thermal stability as those of original fluoride-based STW. Our approach is promising not only for the fluoride-free synthesis of enantiomeric STW but also for general fluoride-free syntheses.

An Isomorphously Substituted Fe-BEA Zeolite with High Fe Content: Facile Synthesis and Characterization.

An isomorphously substituted Al-free Fe-BEA zeolite with extremely high Fe content (~7 wt.%) was synthesized by a facile and industrially friendly method using an excess amount of NaOH. The obtained Fe-BEA zeolite was highly crystalline and showed a well-facetted bipyramidal morphology, similarly to beta zeolites synthesized in a fluoride medium. The chemical states of Fe in the above zeolite were investigated by diffuse reflectance ultraviolet-visible (UV-Vis) absorption spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. The observed chemical states were similar to those of the Fe-BEA zeolite synthesized in the presence of fluoride (Fe-BEA-F). Considering the fact that more than 88% of the micropore volume of the calcined Fe-BEA zeolite was retained after hydrothermal treatment at 1000 °C for 5 h, and 53% of the tetrahedrally coordinated Fe3+ was retained after hydrothermal treatment at 700 °C for 20 h, the obtained Fe-BEA zeolite was concluded to be highly hydrothermally stable. The synthesized zeolite was evaluated in the selective catalytic reduction of NOx by ammonia (NH3-SCR), exhibiting greater catalytic activity than Fe-BEA-F throughout the reaction temperature range. Moreover, the potential of this catalyst as a hydrocarbon adsorbent for cold-start emission control was characterized by dynamic adsorption-desorption of toluene. Interestingly, only 66% of adsorbed toluene was desorbed from the Fe-BEA zeolite (cf. 96% for commercial beta zeolite), even though the gas stream did not contain oxygen, suggesting that hydrocarbon oxidation involved oxygen stored inside the Fe-BEA zeolite.

Tracking the crystallization behavior of high-silica FAU during AEI-type zeolite synthesis using acid treated FAU-type zeolite.

During AEI zeolite synthesis using acid treated FAU (AcT-FAU), we found the recrystallization of high-silica FAU with high crystallinity and Si/Al ratio of 6.1 using N,N-dimethyl-3,5-dimethylpiperidinium hydroxide (DMDMPOH) after 2 h, followed by the crystallization of AEI via FAU-to-AEI interzeolite conversion at a longer synthesis time. In order to understand the formation mechanism of high-silica FAU and generalize its direct synthesis, we have investigated this synthesis process. An analysis of the short-range structure of AcT-FAU revealed that it has an ordered aluminosilicate structure having a large fraction of 4-rings despite its low crystallinity. The changes in the composition of the products obtained at different synthesis times suggested that DMDMP+ plays a certain role in the stabilization of the FAU zeolite framework. Moreover, the results of thermogravimetric analysis showed that the thermal stability of DMDMP+ changed with the zeolite conversion. To the best of our knowledge, this is the first study to clarify the structure-directing effect of DMDMP+ on FAU zeolite formation.