Zeolites selectivity using

Figure 5. Range of Selective Oxidation Reactions Catalyze by the TS-1 Zeolite System Using Aqueous H202 as Oxidizing Agent...

When a microporous material, e.g. a zeolite, is used as a catalyst, only those molecules whose diameters are small enough to enter or pass through the pores can react and leave the catalyst. This is the basis for so-called shape-selectivity (Fig. 3.40). Reactant selectivity is encountered when a fraction of the feed molecules can enter the zeolite, whereas the other fraction cannot. For the molecules produced in the interior the same reasoning applies. The favoured products are the less bulky ones, i.e. those with diameters smaller than the pores of the zeolites. For instance, in the zeolite represented in Fig. 3.40 the production of p-xylene is favoured over the production of o- and m-xylenes. Also the bulkiness of the transition state can lead to a different. selectivity, as shown in the last example in Fig 3.40. 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

Nickel containing MCM-36 zeolite was used as new catalyst in the ethylene oligomerization reaction performed in slurry semi-batch mode. This catalyst, with micro-mesoporous structure, mild acidity and well balanced Ni2+/acid sites ratio, showed good activity (46 g of oligomers/gcataLh) and selectivity (100% olefins with even number of carbon atoms). The NiMCM-36 behaviour was compared to those obtained with NiMCM-22, NiY, NiMCM-41 and NiMCM-48 catalysts. 

Subsequently, Goncalves cl al.97 reported the acetylation of glycerol with acetic acid performed over different solid acids, including montmorillonite K-10 and such acid zeolites as HZSM-5 and HUSY. Among the siliceous porous materials examined, montmorillonite K-10 gave the best performance, with 96% conversion into the mono-, di- and tri-acetylated derivatives. When zeolites were used, the conversion was lower than with the other catalysts, giving a 30% conversion for HZSM-5 and only 14% for HUSY. However, selectivity for the primary monoacetylated product,... 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

Non-noble metals such as Ni, Co, Mo, W, Fe, Ag and Cu have been added to zeolites for use in catalysis. In addition to CO, nitric oxide (NO) has been shown to be a good adsorbate for probing the electronic environment of these metals. When NO chemisorbs on these metals, it can form mononitrosyl (M-NO) and dinitrosyl species (ON-M-NO). The monontrosyl species has a single absorption band and the dinitrosyl species has two bands due to asymmetric and symmetric vibrational modes of the (ON-M-NO) moiety. Again, there have been many studies reported in the literature on the use of NO and/or CO adsorption on non-noble metals supported on zeolites and they are too numerous to list here. Several examples have been selected and summarized to provide the reader with the type of information that can be provided by this method. 

Zeolite membranes have also been employed for organic-organic separations where selectivity is based on adsorption and diffusion differences of non-aqueous mixtures. NaX and NaY zeolite were used in the separation of methanol from MTBE and benzene (800 < a< 10000) exploiting the more polar nature of methanol which is attracted to the electrostatic poles of the high A1 content zeolites [38]. Other separations include (i) separation of n-hexane from 2,2-DMB using ZSM5, (ii) benzene from p-xylene using MOR/FER and (iii) xylene isomers [34]. 

Oxidation of cyclic ketones by H2O2 in the presence of an acid zeolite was used for the preparation of lactones and u-hydroxycarboxylic acids [124]. Thus, for c-pentanone oxidation in the presence of H-ZSM-5, -valerolactone is obtained with 62.3% selectivity at 40% conversion, whereas 5-hydroxy-pentoic acid is obtained with 34% yield in the presence of Zeolon M. 

Olefin Separation. U.O.P. s Olex Process. U.O.P. s other hydrocarbon separation process developed recently—i.e., the Olex process—is used to separate olefins from a feedstock containing olefins and paraffins. The zeolite adsorbent used, according to patent literature 29, 30), is a synthetic faujasite with 1-40 wt % of at least one cation selected from groups I A, IIA, IB, and IIB. The Olex process is also believed to use the same simulated moving-bed operation in liquid phase as U.O.P. s other hydrocarbon separation processes—i.e., the Molex and Parex processes. 

As a test reaction, we selected the bleaching of phenolphthalein at pH 10 with H2O2. Apart from the Mo-LDH, several redox-active zeolites were used, such as Mn2t-exchanged Y, or Y zeolites with entrapped Mn... 

As shown in Figure 1, the equilibrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equilibrium can be achieved when zeolites are used, owing to shape selectivity (see Molecular sieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

Peng et a/.[45] reported the nitration of toluene with liquid nitrogen dioxide and oxygen in the presence of a variety of zeolites, and using toluene as reactant and solvent. The reactions were performed at room temperature over 22 h. In the absence of catalyst the reaction was highly unselective giving a mixture of MNTs, dinitrotoluenes, phenylnitromethane and benzaldehyde. In the presence of zeolites, HZSM-5 and HBeta, the selectivity to the p-nitrotoluene was enhanced. In contrast with the results reported by Smith et a/.,[14] HZSM-5 zeolite exhibited better selectivity than Beta zeolite (57 % and 46 %, respectively), which can be attributed to the absence of mediation of the chlorinated solvent molecules, as well as to the higher Si/Al of the HZSM-5 sample (Si/Al =1000). It was found that zeolites... 

Recently Milczak et al.[57] have reported the nitration of o-xylene using 100% nitric acid over silica supported metal oxide solid acid catalysts with high yields (up to 90 %) but low selectivity to 4-o-NX (40-57 %). Choudary et a/. 5X 591 performed the nitration of o-xylene and other aromatic hydrocarbons by azeotropic removal of water over modified clay catalysts achieving low yields of 4-o-NX and a selectivity of 52%. Better results were obtained when HBeta zeolite was used as catalyst, performing the reaction in dichloromethane at reflux temperature.[60] Conversions of 40 % and maximum selectivity 68 % of 4-o-NX were obtained. Similar conversions and higher selectivities for 4-o-NX (65-75 %) were reported by Rao et al M 1 using a nanocrystaUine HBeta sample and working at 90 °C in the absence of solvent. 

In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening by replacement of Ca2-with Na+ or another ion. When dehydrated they have important catalytic applications, promoted by the Brensted acid sites, and by the large area of internal surface. They are used for the cracking of petroleum and for the isomerisation of hydrocarbons, where limited pore size exerts a shape selectivity, which allows one desirable product to be formed in high yield. 

Conversion of n-butane into isobutene over theta-1 and ferrierite zeolites was studied in a continuous flow microreactor at 530°C and 100% n-butane as a feed. The zeolites were used as catalysts in the H- and Ga-forms. Insertion of Ga into the zeolites resulted in improved isobutene selectivities due (i) to an increase in the dehydrogenation activities and (ii) to a decrease in the cracking activities of the catalysts. The highest selectivities to isobutene (-27%) and butenes (-70%) were obtained with the Ga-theta-1 catalyst at n-butane conversions around 10%. These selectivities decreased with increasing conversion due to olefin aromatisation, which was enhanced considerably by the Ga species present in the catalysts. 

Dinitration of Toluene. We attempted to carry out a single step nitration of toluene (Scheme 2) under conditions similar to those developed for selective nitration of o-nitrotoluene, but using two equivalents of nitric acid and a smaller quantity of acetic anhydride. The reaction produced dinitrotoluenes in almost quantitative yield. For a 17.5 mmol reaction the selectivity was exceptional (2 3 = 25 1) using 1 g zeolite, and as high as has been reported heretofore (2 3 = 14 1) even when only 0.5 g of zeolite was used. 

We then carried out similar reactions in which various zeolites were used as catalysts instead of Fe(acac)3 in an attempt to determine which zeolite, if any, would be most applicable to para-selective aromatic nitration. Zeolites HB and NaB produced the greatest selectivity for para-chloronitrobenzene (85 %) and the highest yields (90 and 96 %). Therefore, zeolite HB was tested with a range of other substrates (Scheme 4). The results are shown in Table 1. 

Since the side reaction is a consecutive one, higher diffusivity certainly benefits selectivity of the main product. With the same zeolite also used in LP alkylations, when operated under SCFP reaction conditions, no xylenes are found. 

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