Solid State Mechanism

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Solid-state reactions are known from thermal intracrystalline conversions (isomerizations or loss of volatile fragments), photoreactions, gas-solid reactions, and solid-solid reactions. As all of these relate strictly to the crystal packing (unifying solid-state mechanism) and are not separated in the various sections. Also, nontopotactic (normal) and topotactic (very rare) reactions are not separated in different sections. 

Spectroscopic Studies of Zeolite Synthesis Evidence for a Solid-State Mechanism... 

Structural modification of a molecule usually has profound and complex influence on its crystal s packing and mechanical properties. This type of behavior is so pervasive that one usually despairs of being able to apply the kind of linear free energy relationships that are so useful in solution chemistry to studies of solid-state mechanisms. When structural modification is sufficiently subtle, however, analogous techniques can be helpful. 

Fig. 5. Mechanisms of the c-BN to h-BN conversion (CVD mechanism and solid state mechanism) [22]...

As demonstrated in SEM pictures (Fig. 5), the phase conversion starts at the surface of the c-BN crystals, which leads to the formation of h-BN platelets and whiskers (Fig. 5). This clearly shows that a CVD mechanism is involved and the solid state mechanism is not the only reaction pathway [22]. 

Munson E, Schoenich C. Oxidation reactions in the solid state mechanisms and comparison to solution chemistry. Oxidative Degradation and Stabilization Conference. Institute for International Research, Princeton, NJ, July 20-21, 2004. 

Kaupp, G., Solid-state molecular syntheses complete reactions without auxiliaries based on the new solid-state mechanism. CrystEngComm 2003, 5, 117-133. 

Pine has recently focused on the role of vanadium in the destruction of USY zeolite.20 Kinetic studies of the reaction of vanadium and steam (used in catalyst regeneration) have been carried out for vanadium levels of 0-4000 ppm and at temperatures of 740 to 800°C. Kinetic data suggest that vanadium acts as a catalyst for the breakdown of the zeolite lattice via a solid state mechanism. 

Likewise, three-dimensional renderings of MIMs remind us instantly of some of the ordinary objects we encounter in our everyday lives (see Sect. 2.3). Take olympiadane [82], for example (Fig. 13), with its five interlocked rings unmistakably sharing the same topology as the Olympic logo Most catenanes bear resemblance at least to the links of a chain, as their name implies. Regardless of their resemblance to familiar objects, hundreds of beautiful crystal structures of MIMs have been produced since their debut in 1985, when Sauvage [83] published the first solid-state structure of a [2]catenane (Fig. 14a). It would be impossible to do justice to all of the beauty contained in the databank of solid-state mechanically interlocked structures. In Fig. 14 we simply present a few examples that we find noteworthy [84—88]. See Fig. 23 in Sect. 4.2 for more beautiful crystal structures of some particularly novel MIMs. 

The active site on the surface of selective propylene anmioxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an CC-H abstraction component such as Bi3+, Sb3+, or Te4+ an olefin chemisorption and oxygen or nitrogen insertion component such as Mo6+ or Sb5+ and a redox couple such as Fe2+/Fe3+ or Ce3+/ Ce4+ to enhance transfer of lattice oxygen between the bulk and surface of the catalyst. The surface and solid-state mechanisms of propylene ammoxidation catalysis have been determined using Raman spectroscopy (40,41), neutron diffraction (42—44), x-ray absorption spectroscopy (45,46), x-ray diffraction (47—49), pulse kinetic studies (36), and probe molecule investigations (50). 

B.D. McNicol, G.T. Pott, and K.R. Loos. Spectroscopic Studies of Zeolite Synthesis Evidence for a Solid-state Mechanism, in Molecular Series . Adv. Chem. Ser., 1973, 121, 152-161. 

The book comprises 20 chapters that focus on state-of-the-art understanding of solid state mechanisms in heterogeneous catalysis and the relationship between catalytic behavior and solid state structure. The volume contains expanded and updated versions of papers presented on this subject at the ACS symposia in Washington, D.C. (1983) and Las Vegas (1982), and of written contributions from invited participants who could not attend these... 

Much of the understanding of the solid state mechanism of heterogeneous catalysis stems from fundamental studies of single phase model compounds (1-5). In many cases, the role of a metal component in a catalytic process has been discerned through its incorporation into solid solutions of relatively inert host matrices (O. In the case of the selective oxidation and... 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

A single domain ferromagnetic particle suspended in a fluid is subject to two orientational variational mechanisms. The first is that discussed already and takes place inside the particle, it is the solid state mechanism known as Neel [25] relaxation. The second is due to the rotation of the... 

Far-reaching Molecular Migrations in Solid-state Reactions (AFM, GID, SNOM) and Experimental Solid-state Mechanism... 

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