Reaction selectivity zeolites

Shape-selective zeolites can also be used to discriminate among potential products of a chemical reaction, a property called product shape selectivity. In this case, the product produced is the one capable of escaping from the zeolite pore structure. This is the basis of the selective conversion of methanol to gasoline over... 

Zeolite chemistry is an excellent example of how a three-dimensional surface can alter the course of chemical reactions, selecting for one product out of a host of potential candidates. In addition to the many commercial applications that they have found, shape-selective zeolites have provided the basis for a rich new area of catalytic science and technology, one expected to spawn yet more materials, knowledge, and applications. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Clennan, E.L. and Sram, J.P. (1999). Photooxidation in zeolites. Part 2 a new mechanistic model for reaction selectivity in singlet oxygen ene reactions in zeolitic medsia. 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

Na - K - Co exchange in Y zeolite. Heterovalent exchange reactions in zeolites generally show an even more pronounced dependency on loading (116-118). Rees (116) observed variations of the selectivity coefficient by a factor 1000 for the Na-Ca and Na-llg exchange in zeolite A at 25 °C. An+exajnple of e treipe variations is shown in fig. 9 for the K -Co and Na -Co selectivities in zeolite Y at 45 °C (117). The exchange... 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

Frei and co-workers also extended this reaction to other zeolites showing that almost identical behavior was observed in BaY, BaX, and in the K+ and Ba " forms of zeolite L [45,46]. Xiang et al. [47] have also studied the photooxidations of a series of 1-alkenes in the more acidic BaZSM-5 [48] and Ba- 3. The extensive polymerization of propylene in these zeolites demonstrates the detrimental effect of Bronsted acid sites on the reaction selectivity. These workers also used ex situ nuclear magnetic resonance (NMR) allowing more detailed... 

Moreover, contrary to alkyne hydration where no adsorption of the carbonyl compound was detected, the problem is complicated here by the saturation of the strong acidic sites by the formed amide, the concentration of which shows a rapid stabilization against time (Fig.3). Consequently the reaction selectivity greatly depends on the ester percentage. The behaviour of the amide itself over the studied zeolites confirms this observation the conversion of the amide into ester goes faster on the HY2 g zeolite than on the Hg and on the HMg zeolites. This later point, together with the comprehension of the different mechanisms in relation with the zeolite properties, will be discussed in a further paper. 

The key to the process was the development by Mobil of a size-selective zeolite catalyst, whose geometry and pore dimensions have been tailored so that it selectively produces hydrocarbon molecules within a desired size range. This is a highly exothermic reaction and the major problem in any plant design is the reactor system to effect the necessary heat removal. A plant completed in 1985 in New Zealand uses about 4 million m3/day of natural gas as the feedstock to produce the methanol by the ICI procedure described above. In 1990 the plant produced about 16,000 bbl/day of gasoline, which is somewhat above its design output. 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

N. Y. Chen, T. F. Degman, Jr., C. M. Smith, Molecular Transport and Reaction in Zeolites - Design and Application of Shape Selective Catalysts, VCH, New York, 1994. 

Thus it is evident that zeolites offer considerable potential for steering reaction selectivity on the basis of differences in molecular shape. These possibilities extend far beyond the more familiar molecular sieve effects where bulky molecules are simply excluded entry into the zeolite cavities due to pore diameter restrictions. 

For a variety of methyl-substituted cyclopropene molecules, and a variety of zeolites (e.g., NaCaA, NaA, CaA, NaX, and HY), the reaction selectivity to dimer, diene, or polymer was found to be determined by at least three factors, namely ... 

In order to slow down the overall reaction we added acetic anhydride as a diluent, thereby giving the zeolite a better chance to exert an influence over the reaction. Indeed, this led to a slower reaction and zeolite HP then exerted a greater influence over both the rate and selectivity. The yield after 2 h at -10 °C in the absence of the zeolite was only 16 % and the 2 3 ratio was the usual 2 1, but with the zeolite present the yield increased to 99 % and the ratio to 17 1. p-Nitrotoluene could also be nitrated with the optimised system used for o-nitrotoluene, but the reaction was much slower. Therefore, for direct dinitration of toluene it would be necessary to minimise the amount of the diluent used. 

It has been concluded that, in most cases, catalytic reactions over zeolites occur within their intracrystalline cages and channels. Zeolite catalysts can therefore be considered as a succession of nano or molecular reactors. The consequence is that the activity, selectivity, but also the stability of all the reactions carried out over zeolite catalysts, depend (slightly or significantly) on the shape and size of cages, channels and of their apertures, hence that shape selectivity is a general characteristic of zeolite catalyzed reactions. 

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