Ionic crystal catalysts

Bulk type I catalysis was found in acid catalysis with the acid forms and some salts at relatively low temperatures. The reactant molecules are absorbed between the polyanions (not in a polyanion) in the ionic crystal by replacing water of crystallization or expanding the lattice, and reaction occurs there. The polyanion structure itself is usually intact. The solid behaves like a solution and the reaction medium is three-dimensional. This is called pseudoliquid catalysis (Sections l.A and VI). The reaction rate is proportional to the volume of the catalyst in the ideal case the rate of an acid-catalyzed reaction is proportional to the total number of acidic groups in the solid bulk. 

This process can explain the detailed events with the help of the electron theory of catalysts advocated by Wolkenstein [25]. Figure 2b shows a conceptual illustration of an ionic crystal consisting of M2+ and O2 ions, e.g., a metal oxide... 

J. Sauer, Molecular models in ab initio studies of solids and surfaces From ionic crystals and semiconductors to catalysts. Chem. Rev. 89, 199-255 (1989)... 

CONTENTS Introduction, Thom H. Dunning, Jr. Electronic Structure Theory and Atomistic Computer Simulations of Materials, Richard P. Messmer, General Electric Corporate Research and Development and the University of Pennsylvania. Calculation of the Electronic Structure of Transition Metals in Ionic Crystals, Nicholas W. Winter, Livermore National Laboratory, David K. Temple, University of California, Victor Luana, Universidad de Oviedo and Russell M. Pitzer, The Ohio State University. Ab Initio Studies of Molecular Models of Zeolitic Catalysts, Joachim Sauer, Central Institute of Physical Chemistry, Germany. Ab Inito Methods in Geochemistry and Mineralogy, Anthony C. Hess, Battelle, Pacific Northwest Laboratories and Paul F. McMillan, Arizona State University. 

The addition of water to hydrophobic ionic liquids can result in the formation of a triphasic system consisting of the ionic liquid + catalyst/water/organic product. The aqueous phase eventually contains the inorganic salt formed during the reaction. This approach has been applied in Pd-catalyzed cross-coupling reactions [84]. The products can also separate after the reaction by perturbation (of a latent biphasic system). This can be realized by tuning the temperature to crystallize the ionic liquid. But the refrigeration cost may be problematic, and some ILs may remain in the product. 

TiCl3 is a typical ionic crystal like sodium chloride. It is a relatively nonporous material with a low specific surface area. It has a high melting point, decomposes to TiCl2 and TiCLj at 450°C, and sublimes at 830°C to TiCLj vapor. It is soluble in polar solvents such as alcohols and tetrahydrofuran but is insoluble in hydrocarbons. The highest specific surface area reported for these catalysts is 100 m /g, but normal values lie in the range of 10-40 m /g. 

Raston has reported an acid-catalyzed Friedel-Crafts reaction [89] in which compounds such as 3,4-dimethoxyphenylmethanol were cyclized to cyclotriveratrylene (Scheme 5.1-57). The reactions were carried out in tributylhexylammonium bis(tri-fluoromethanesulfonyl)amide [NBu3(QHi3)][(CF3S02)2N] with phosphoric or p-toluenesulfonic acid catalysts. The product was isolated by dissolving the ionic liq-uid/catalyst in methanol and filtering off the cyclotriveratrylene product as white crystals. Evaporation of the methanol allowed the ionic liquid and catalyst to be regenerated. 

Ionic liquids, which can be defined as salts that do not crystallize at room temperature [46], have been intensively investigated as environmentally friendly solvents because they have no vapor pressure and, in principle, can be reused more efficiently than conventional solvents. Ionic liquids have found wide application in organometallic catalysis as they facilitate the separation between the charged catalysts and the products. 

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

One typical way to improve the catalyst system was directed at the multi-component bismuth molybdate catalyst having scheelite structure (85), where metal cations other than molybdenum and bismuth usually have ionic radii larger than 0.9 A. It is important that the a-phase of bismuth molybdate has a distorted scheelite structure. Thus, metal molybdates of third and fourth metal elements having scheelite structure easily form mixed-metal scheelite crystals or solid solution with the a-phase of bismuth molybdates. Thus, the catalyst structure of the scheelite-type multicomponent bismuth molybdate is rather simple and composed of a single phase or double phases including many lattice vacancies. On the other hand, another type of multi-component bismuth molybdate is composed mainly of the metal cation additives having ionic radii smaller than 0.8 A. Different from the scheelite-type multicomponent bismuth molybdates, the latter catalyst system is never composed of a simple phase but is made up of many kinds of different crys-... 

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