Small-pore zeolites oxidation

Minachev et al. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300° and 450°C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84%), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42), which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites. 

Solid state ion exchange is a versatile tool for the fast and easy preparation of metal containing small pore (i. e., 8-membered ring) zeolites. Therefore it offers a valuable alternative to the crystallization inclusion method with its limited applicability. The introduction of noble metals into small pore zeolites via solid state ion exchange results in highly shape selective catalysts over which the hydrogenation of the linear alkene out of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) is strongly preferred. This indicates that the major part of the metal is located in the intracrystalline voids of the zeolites. Preliminary fUrther experiments in our laboratory surest that the new method is not restricted to noble metal chlorides, but also works with other salts, e. g., oxides and nitrates. 

Since the initial discovery, much work has gone into improving the catalyst. The original zeolite contained small pores that limited oxidations to relatively small molecules with shapes that allowed them to move in and out of that pore system. One modification has been to isolate titanium in zeolites with larger pores so larger molecules can be oxidized. Another modification has been to incorporate other metal ions into the frameworks of different zeolites with... 

Mn impregnated into MCM-4i, a silicalite containing uniform mesopores of approximately 22 A, catalyzes TBHP epoxidation of alkenes.88 Over Mn-MCM-41, both cis- and trans-stilbene yield trans-stilbene oxide, which the authors conclude signals a radical mechanism.88 In contrast, over Ti—MCM-41, trans-stilbene cannot be oxidized, only cis-stilbene is epoxidized to the cis-stilbene oxide, which suggest a radical-free mechanism.89 Finally, emphasizing the shape selectivity possibilities, only trans-stilbene (not cis-stilbene) can be epoxidized over Mn-ZSM-5, a zeolite with relatively small pores of 5.1 x 5.4 A (Fig. 6.14).88... 

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

In recent years, modification of zeolites, such as HZSM-5, by phosphoric compounds or metal oxides has been extensively studied, but little information is available on the modification of zeolites by diazomethane, which is an excellent methylating agent for protonic acidic sites. It is capable of entering into the small pores of zeolites because of its small molecular size and linear molecular structure. Yin and Peng (1,2) reported that the acidity and specific surface area of the inorganic oxide supports (AljOs, SiOj) and zeolite catalysts... 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Ti-containing zeolites have recently appeared as selective oxidation catalysts, in particular the TS-1 catalyst.[58] TS-1 has a MFI structure with small pore dimensions so that its used is not suitable for the oxidation of carbohydrates. Several attempts have been made to incorporate Ti in MCM-41 materials in order to perform oxidation of carbohydrates.[59,60]... 

The previous section discussed the use of crystalline metal silicalites as catalysts for the liquid-phase oxidation of organic substrates with peroxygens. The unfortunate feature of many of the crystalline catalysts is their relatively difficult synthesis, requiring specialized expertise, equipment and the use of proprietary knowledge. Also the small pores of the zeolite limit access to the catalytic sites by large organic substrates, which are often more prevalent in the... 

The blank test in liquid phase yields less than 1% conversion. Unlike non-zeolitic catalysts, except for H-Nafion, most zeolites yield complete conversion. A high yield of the keto aldehyde 24 up to 81%, was surprisingly attained by using H-FER as a heterogeneous catalyst. For the rearrangement of isophorone oxide, the presence of acidic sites is necessary for the catalytic activity. The reactivity of H-FER can be explained by the acidic outer surface of the catalyst Molecular modeling showed that the isophorone epoxide is too bulky for the small pore size of ferrierite. 

Both Ti-beta and TS-1 show a high activity towards the oxidation of small size substrates such as ethyl sulfide in methanol as solvent, which can be the proof that the efficiency of the active Ti sites is the same for the two catalysts. In this context, the poor activity of the TS-I catalyst in the case of the large molecules can be attributed both to a restricted transition state shape selectivity and to a difliisivity effect of reagents and products. These effects are not as important for a large pore zeolite, such as Ti-beta. A similar result was... 

In fact, the small pore opening of the MFI and MEL structures (= 5.5 A) makes that hydrogen peroxide is the single stable peroxide capable of entering the zeolite channels and, therefore, permitting the reaction. Yet, tert-butyl hydroperoxide (TBHP) has proved to be an efficient oxidant in the homogeneous epoxidation of olefins catalyzed by titanium alkoxides [4]. 

The most important class of solid-state enzyme mimics is based on zeolites. Zeolites are solid materials composed of Si04 or AIO4 tetrahedra linked at their corners, affording a three-dimensional network with small pores of molecular dimensions. They possess a unique feature of a strictly uniform pore diameter. In particular, zeolites with encapsulated metal complexes are used as inimics of cytochrome P-450.An efficient enzyme mimic was obtained by encapsulating an iron phthalocyanine complex into crystals of zeolite Y, which were, in turn, embedded into a polydimethylsiloxane membrane acting as a mimic of the phospholipid membrane.With t-butylhydroperoxide as the oxidant, the system hydroxyl-ates alkanes at room temperature with rates comparable to those for the enzyme. It shows similar selectivity (preference oxidation of tertiary C-H bonds) and a large kinetic isotope effect of nine. 

Furthermore, a very active research field in zeolites with great potential is the reduction of nitrogen oxides by selective catalytic reduction, especially when focused on small-pore, hydrothermaUy stable zeohtes. Table 8.1 illustrates some interesting applications of zeohtes in the production of chemicals and fine chemicals and in emerging energy and environmental sustainable applications. 

Conversion of nitrogen oxides in the presence of NH3 to yield N2 (selective catalytic reduction, SCR) Cu-ZSM-5, Cu-ZSM-ll,Cu-ZSM-12, Cu-P, Cu-SSZ-13, Cu-SAPO-34 Small-pore, highly hydrothermally stable zeolites are currently of increasing interest in the SCR process [58, 71]... 

Many oxidation reactions have been carried out using hydrogen peroxide and the titanosilicate, TS-1. However, this catalyst has relatively small pores and is therefore not an efficient catalyst for the oxidation of large molecules. This problem has been solved by the successful generation of a medium-pore titanium zeolite Beta-Ti [136]. Cyclododecane and cyclohexane are both oxidised selectively by H2O2 in the presence of the new titanium zeolite, favouring the ketone product. 

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