Intrazeolite Chemistry

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Downloaded by 80.82.77.83 on June 7, 2017 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.pr001

PREFACE ZEOLITE AND INTRAZEOLITE CHEMISTRY are the subjects of two symposia dealing with a dynamic and rapidly growing field in which a broad range of new chemistry, from solid state to organic, is being explored. Zeolites introduce both unusual activity and selectivity into heterogeneous adsorption and catalytic processes and are the inorganic analogues of enzymes in living systems. The 27 papers in this volume present a cross section of synthesis, characterization, and chemical studies by academic and industrial researchers. Although zeolite research activity has been predominantly industrially based, increasing effort and results are now emanating in academic laboratories. The synergism and mutual stimulation of these efforts can be expected to increase the quality and quantity of current and future research involving microporous crystalline molecular sieves. The search for solid state materials that have all or a large portion of their framework atoms available as adsorption sites is leading to new classes of substances that require a redefinition of the word "zeolite." The etymology of zeolite begins with Greek words "zein" and "lithos," which have been condensed to the Swedish "zeolit" or "boiling stone." J. V . Smith in the American Mineral Society Spec. Paper (1963) expanded this

description to "an aluminosilicate framework structure enclosing cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration" with formula M [(A1O ) (A1O ) ]mH O. Cartraud, Cointot, and Renaud (J. Chem. Soc., 1981) discuss the zeolitic properties of the hexacyanoferrate(II), K Zn [Fe(CN) ] X H O , so that in fact these materials need not contain aluminum, silicon, or oxygen and may be transition metal based. In this book a new class of microporous crystalline inorganic molecular sieves that contain no silicon is described. The scope of future "zeolite" symposia will undoubtedly include an increasing number of materials that have framework structures made up of atomic elements from a large portion of the periodic table. The synthesis of zeolites has long been regarded as an art with procedures and reactants reminiscent of those described by Chaucer for the synthesis of the philosopher's stone. x/n

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Unslekked lyme, chalk, and glayre of an ey, Poudres dyvers and asshes, dong, and cley . . . xi Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by 80.82.77.83 on June 7, 2017 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.pr001

The importance of recognizing and dealing with zeolite synthesis as a kinetic process that involves the isolation of metastable phases is pointed out in this book in a variety of ways. A n examination of the extensive scientific and patent literature on zeolite synthesis rapidly convinces one that a lack of understanding of this point has been a major bottleneck in the characterization of zeolite chemical and physical properties. The zeolite properties are defined not only by synthesis parameters, but also by treatment following synthesis; for example, most synthesis treatment of zeolites withfluorinecan be used to modify hydrophobicity drastically and increase catalytic activity for n-butane cracking. The second section of the book is concerned with characterization. The development and use of powerful, new analytical techniques, such as highresolution solid state nuclear magnetic resonance, electron microscopy and microanalysis, and powder neutron diffraction structural analysis have made it possible to define the phase inhomogeneity, lattice defects, and framework disorder that one would expect from metastable phases. Zeolite A is the simplest zeolite that has been structurally characterized with a Si/Al ratio of 1, the minimum value necessary to satisfy Pauling, Loewenstein, and Dempsey and Olsen's rules. The first structural analysis of zeolite A was reported by Reed and Breck (J. Amer. Chem. Soc., 1956). Differentiation of Si and A l sites is difficult by x-ray analysis and this difficulty, plus the flexibility inherent in the zeolite framework, which is displayed on dehydration or supported ion migration, results in a structural problem that was not resolved for over 25 years by numerous investigators. The symposia upon which this book is based were the first in which a mutually satisfactory solution to the structure of zeolite A was reported. The resolution of the structure of zeolite A has pointed out the usefulness of high-resolution solid state N M R and of powder neutron diffraction. These studies provide the groundwork for deciphering more complicated zeolite structures and their framework order and/or disorder. Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, E X A F S , and other novel spectroscopic techniques. This research is described in the second part of this text. Diffusion, absorption, and mass transport properties, of course, also define microporous molecular sieve selectivity. No other physical properxii Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ties are more important to understand and to define for the characterization of zeolites. This area of research is summarized in a manuscript by Douglas Ruthven. It is worth noting the suggestion of one reviewer that the presentation and topic deserves to be expanded into a text on its own. The ability of zeolites to differentiate between nearly identical molecules such as O and N is examined by D . T. Hayhurst and M . D . Sefcik. The catalytic activity of zeolites in alkane to olefin reactions, photochemical conversion reactions, Fischer-Tropsch hydrogenation, isocyanation, carbonylation, and related chemistry make up the last theme. A n important focus of this is to explore the utility of zeolites as selective heterogeneous catalysts for reactions that involve Group VIII metals. The mechanistic nature of some of this chemistry is presented, along with the characterization of supported organometallic transition metal complexes. One of the most exciting aspects of zeolite chemistry is our increasing understanding of the functionality of framework structure, chemisorption, and mass transport as related to chemical behavior. The research in this area has resulted in numerous advances during the past 2 years, many of which are presented in this text. It is expected that the applications and interest in crystalline molecular sieves will continue to expand rapidly.

Downloaded by 80.82.77.83 on June 7, 2017 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.pr001

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G A L E N D . STUCKY

E. I. du Pont de Nemours and Company December 16, 1982

xiii Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.