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Journal ArticleDOI: 10.1021/ACS.CHEMMATER.0C04832

Interzeolite Conversion and the Role of Aluminum: Toward Generic Principles of Acid Site Genesis and Distributions in ZSM-5 and SSZ-13

02 Mar 2021-Chemistry of Materials (American Chemical Society (ACS))-Vol. 33, Iss: 7, pp 2516-2531
Abstract: The performance of zeolite catalysts depends not only on the strength and number of Bronsted acid (or exchange) sites but also on synergistic effects derived from their proximity, in particular, an...

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Topics: SSZ-13 (52%), ZSM-5 (52%), Zeolite (51%)

5 results found

Journal ArticleDOI: 10.1021/JACS.1C07590
Max L. Bols1, Julien Devos1, Hannah M. Rhoda2, Dieter Plessers1  +4 moreInstitutions (2)
Abstract: α-Fe(II) active sites in iron zeolites catalyze N2O decomposition and form highly reactive α-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these α-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to α-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is α-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV-vis-NIR, FT-IR, and Mossbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in α-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 μmol methanol per gram in a single cycle through H2O/CH3CN extraction and 183 μmol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of α-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored α-Fe(II).

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Topics: Zeolite (51%), Calcination (51%), Stoichiometry (50%)

Open accessJournal ArticleDOI: 10.1039/D1RA02887A
27 Jul 2021-RSC Advances
Abstract: Interzeolite conversion, a synthesis technique for several zeolite frameworks, has recently yielded a large amount of high-performing catalytic zeolites. Yet, the mechanisms behind the success of interzeolite conversion remain unknown. Conventionally, small oligomers with structural similarity between the parent and daughter zeolites have been proposed, despite the fact these have never been observed experimentally. Moreover, recent synthesis examples contradict the theory that structural similarity between the parent and daughter zeolites enhances interzeolite conversion. In this perspective it is proposed that heteroatoms, such as aluminium, are key players in the processes that determine the successful conversion of the parent zeolite. The role of Al during parent dissolution, and all consecutive stages of crystallization, are discussed by revising a vast body of literature. By better understanding the role of Al during interzeolite conversions, it is possible to elucidate some generic features and to propose some synthetic guidelines for making advantageous catalytic zeolites. The latter analysis was also expanded to the interconversion of zeotype materials where heteroatoms such as tin are present.

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Open accessJournal ArticleDOI: 10.1016/J.CEJ.2021.133862
Julien Devos1, Robin Borms1, Sven Robijns1, Gleb Ivanushkin1  +2 moreInstitutions (1)
Abstract: Zeolite activation is the first, but often overlooked, post-synthetic treatment needed to transform an as-made material into an adsorbent or catalyst. Ozonation at low temperatures provides an alternative for conventional calcination treatments, with important consequences for zeolite stability and acidity. While sometimes used in the context of emptying novel less thermally stable structures, a thorough look at ozonation for bulk zeolites is missing. Decomposition of organic structure-directing agents was achieved for three OSDA-zeolite systems yielding empty SSZ-13, ZSM-5 and β zeolites at select ozonation conditions. The effect of various process parameters, as well as synthetic parameters, were investigated. Infrared and mass spectrometry after alternating O2-O3 activation mode experiments provide some mechanistic insights into ozonation. The rate-limiting steps of both low-temperature ozonation and air calcinations can be circumvented by low-temperature ozonation after medium-temperature calcination.

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Topics: Calcination (50%)

Journal ArticleDOI: 10.1039/D1CP03751J
Zhendong Liu1, Anand Chokkalingam1, Shoko Miyagi1, Masato Yoshioka2  +8 moreInstitutions (3)
Abstract: 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.

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108 results found

Journal ArticleDOI: 10.1006/JCAT.1993.1145
C.A. Emeis1Institutions (1)
Abstract: Integrated molar extinction coefficients were determined for infrared absorption bands of pyridine adsorbed on acid sites in Si/Al-based catalysts. The infrared spectra of five zeolites and two amorphous silica-aluminas were recorded during quantitative dosing of pyridine gas at 150°C. The integrated molar extinction coefficients were calculated assuming that they did not depend on the catalyst or the strength of the acid site. The resulting values were 1.67 cm/μmol for the 1545-cm−1 band characteristic of pyridine on a Bronsted acid site and 2.22 cm/μmol for the 1455-cm−1 band of pyridine on a Lewis acid site. The 95% confidence limits were estimated at ±15%. Subsequent analysis of the data provided no evidence for a dependence of the integrated coefficients on the catalyst or the strength of the site. The measurements gave indications for differences between the rates of reaction of Bronsted and Lewis acid sites with pyridine.

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Topics: Pyridine (56%), Lewis acids and bases (52%), Brønsted–Lowry acid–base theory (51%) ... read more

2,034 Citations

Open accessBook
01 Jan 2001-
Abstract: The contributions in this volume can be divided into three categories: updated chapters by the original authors, updated chapters by an expanded or new team of authors, and completely new chapters. This 2nd edition also contains chapters on "Zeolite-based supramolecular assemblies" and on "The use of bulky probe molecules". Finally, the super large pore zeolites and the fast-growing area of ordered mesoporous materials are dealt with by the discoverers of the M41S family of mesoporous materials.

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1,254 Citations

Journal ArticleDOI: 10.1016/J.MICROMESO.2005.02.016
Colin S. Cundy1, Paul A. CoxInstitutions (1)
Abstract: An account is presented of the mechanistic aspects of hydrothermal zeolite synthesis. The introduction provides a historical and experimental perspective and is followed by a summary of proposed mechanisms and associated modelling studies. The central section of the review contains a description of the most probable mechanistic pathways in zeolite formation. In this, the reaction stages of the induction period, nucleation and crystal growth are examined in chronological sequence. Finally, particular aspects of the synthesis process such as the constitution of growth species, template–framework interactions and the nature of zeolite solubility are treated in more detail. Emphasis is placed upon the chemical basis of zeolite synthesis. Fundamental to this are the T O T bond-making and bond-breaking reactions which establish the equilibration between solid and solution components. The consequent generation of order, driven by energy differences and strongly moderated by kinetic limitations, is essentially one of continuous evolution. However, the discreet step of nucleation provides a discontinuity in which isolated regions of local order are superceded by the establishment of a periodic crystal lattice, capable of propagation. Crystal growth occurs through an in-situ, localised construction process from small, mobile species ordered by the participating cations. The process of hydrothermal zeolite synthesis can be most adequately explained by a mechanism based upon the solution–mediation model, whether or not there is a visible liquid phase. The common presence of mobile species emphasises the overall similarity of zeolite synthesis reactions so that the need to distinguish any separate “gel rearrangement” or “solid-phase transformation” mechanism becomes unnecessary.

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1,122 Citations

Open accessJournal ArticleDOI: 10.1126/SCIENCE.AAA6760
31 Jul 2015-Science
Abstract: Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

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1,006 Citations