Zeolites partial

As-made zeolite Partially detemplated Calcined zeolite... 

Metal oxides were also chirally modified and few of them showed a significant or at least useful e.s. Thus, while Al203/alkaloid [80] showed no enantiodifferentiation, Zn, Cu, and Cd tartrate salts were quite selective for a carbene addition (45% e.e.) [81] and for the nucleophilic ring opening of epoxides (up to 85% e.e.) [82], Recently, it was claimed that /(-zeolite, partially enriched in the chiral polymorph A, catalyzed the ring opening of an epoxide with low but significant e.s. (5% e.e.) [83], All these catalysts are notyet practically important but rather demonstrate that amorphous metal oxides can be modified successfully. 

This work has shown that such a catalyst must possess very high cracking activity that is stable in the presence of steam and high temperature, but must have good selectivity as well. This has been achieved by employing Ultrastable Y zeolite, partially enhanced in activity by addition of a small amount of rare earth, both in the zeolite and in the matrix. 

Procedure. Partially Hydrated Zeolites. Partially hydrated zeolites are made from samples previously dehydrated by evacuation at 400°C in the conductivity cell, by adsorbing known amounts of water. For comparison, adsorption isotherms were determined independently at the same temperature and pressure. After each adsorption of water, the pellet is allowed to equilibrate for 3 hours. Capacity and conductivity are measured at several frequencies in the range 200-107 Hfc. Regular, checks are made on this equilibrium period with overnight and weekend equilibration times. No appreciable changes of conductivity and capacity were observed after 3 hours. 

Transalkylation reactions are observed in Y zeolites partially exchanged with ethyl-, diethyl-, and triethylammonium cations (EA, DEA, and TEA, respectively) heated above 150° C in air or under vacuum, in the presence of residual water molecules. The main reactions may be depicted schematically as follows (EA) Y — (DEA)Y > (NHt+)Y, (DEA)Y - (EA)Y > (TEA)Y, and (TEA)Y - (DEA)Y > (EA)Y, iAc main constituent in the gas phase being CJh. They are similar to those observed in montmorillonite in the presence of a water vapor pressure of a few torr. It is proposed that in both cases the transalkylation processes are acid catalyzed, the residual water molecules and the surface oxygen being the active spots recycling the protons in montmorillonite and zeolite, respectively. 

The formation of these dinuclear complexes can be impeded by entrapment of the Mn(BPY)22+ complexes in the structure of zeolite Y. Preferably, Mn(BPY)2+ is assembled via ship-in-a-bottle synthesis in zeohte Y, through BPY adsorption on a NaY zeolite partially exchanged with Mn2+. Because a single zeolite Y supercage can contain only one Mn(BPY)2+ complex, the formation of dinuclear complexes is impossible for steric reasons. The reaction of H2C>2 with the zeolite-entrapped Mn(BPY)2+ complex does not lead to the same vigorous peroxide decomposition as occurs in solution. Instead, H2O2 is heterolytically activated on the Mn complex with civ-bipyridine ligands to form a Mn(IV)=0 or Mn(V)=0 species. The latter is a... 

Industrial samples of HY zeolites partially dealuminated (HYD) then acid washed (HYDW) were studied. Coke was produced during cracking of n-hexane (4.7 % (HYDW4.7H) and 8.6% (HYDW8.6H) of coke w/ w) or ortho-xylene (8.7% (HYDW8.7X) of coke w/w) at 673K, Partial or total oxidation was carried out in a... 

The adjacent step represented by values of 20-21 kcal/mole leads to the assumption that the first molecules of ammonia are adsorbed at the 4 Ca " cations in the cage. This is shown by measurements of the adsorption of ammonia in several NaX zeolites partially containing tran-... 

Sodium Y zeolites partially exchanged with Co have been studied by 129-Xe NMR spectroscopy. Large chemical shifts due to paramagnetic effects of Co ions are observed when the water concentration is less than 6 H20/Co. The (Jq term (5 value for zero xenon concentration) derived from a second-order polynomial expansion is characteristic of the Co " " - Xe interactions. This term increases exponentially when [FI2O] decreases. 

The active acid sites in ZSM-5 can also be characterized spectroscopically in a less direct manner by observing their interaction with basic molecules. For example, the infrared spectrum of chemisorbed pyridine shows bands characteristic of the pyridinium cation formed by reaction with Brtfnsted (protonic) acid sites, and pyridine coordinated to Lewis acid sites (coordinatively unsaturated A1 ). Figure 3(a) shows the spectrum of pyridine chemisorbed in a freshly activated ZSM-5, compared with the corresponding spectrum in Figure 3(b) of pyridine in the same zeolite partially coked after methanol conversion (ref. 

Baba T, Tohjo Y, Takahashi T, Sawada H and Ono Y (2001), Properties of chemisorbed hydrogen species on Ag-A zeolite partially reduced with hydrogen as studied by H MAS NMR , Catal Today, 66, 81. 

Wolf, Pilchowski, and Karch have studied the adsorption of n-hexanol from toluene by NaA zeolite partially ion exchanged with Ca and Mg. The isotherms rise to a sharp plateau, which is reached at a concentration of about 15 mg hexanol/g toluene (x = 0.013). Pure NaA exhibits no adsorptive capacity for hexanol but, as divalent ions are introduced, this rises, initially slowly, then more sharply to reach, at about 30% degree of ion exchange, a maximum value that remains unchanged from 40% upwards. The paper also deals with the kinetics of the adsorption process, and presents calculations of effective diffusion coefficients and break-through curves under dynamic adsorption conditions. Some experiments with butan-l-ol and ethanol are also reported. 

The rate of coke burning for coke deposited on a zeolite-containing catalyst has been reported to be first order with respect both to coke concentration and oxygen partial pressure (23) ... 

Waters of intermediate hardness frequently contain fair amounts of other constituents and there is often a tendency for the scale to be loosely attached, permitting corrosion to occur irregularly underneath. In most waters the bicarbonate content is less than the hardness, but a few natural waters are known where the reverse is the case. These waters have been partially softened by the zeolite process which occurs underground, and then contain sodium bicarbonate which, together with the high concentration of chloride and other minerals, may accelerate attack. 

Figure 2.7 Schematic representation of the partial detem-plation and desilication treatment to tailor mesoporosity development in zeolite crystals.

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

Partial oxidation reactions are usually carried out over transition metal oxides capable of changing their valent state during their interaction with reacting molecules. Naturally, zeolites with their alumina-silicate composition did not prove themselves as good oxidation catalysts. They failed also to serve as efScient catalyst supporters, since transition metals being introduced into the zeolite matrix lose their ability to activate dioxygen [3,4],... 

This discovery was quite unexpected, since iron oxide has been never reported as an active catalyst in either partial or full oxidation. The studies of two simplest reactions, i.e. O2 isotopic exchange and N2O decomposition, revealed a dramatic change of Fe properties in the ZSM-5 matrix compared to Fe203 [4]. Fe atoms lose their ability to activate O2 but gain remarkably in their ability to activate N2O. It gives rise to a great effect of the oxidant nature in the reaction of benzene oxidation over the FeZSM-5 zeolite (Table 1). Thus, in the presence of N2O benzene conversion is 27% at 623 K, while in the presence of O2 it is only 0.3% at 773 K. And what is more, there is a perfect change of the reaction route. Instead of selective phenol formation with... 

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