Zeolite reaction-induced

Solid acid zeolites, beside inducing reactions known from superacids are indeed moderate acids " The moderate acidity allows zeolite catalyzed reactions to not suffer from side chains reactions." Enthalpies of reaction compare actually more closely with gas phase reactions than with reactions catalyzed by homogeneous high dipolar acids. However, zeolite micropores have been shown to have a stabilizing effect on the carbocationic transition states with respect to gas phase results, which can reach 10 to 30 % of the activation energies This stabilization has been demonstrated to be mainly of short range electrostatic nature ... 

Typical results for the benzylation of 1-decanol in hexane are shown in Table I. Although an insoluble base such as K2CO3 was ineffective for the reaction, alkali metal cation-exchanged Y-type zeolites efficiently induced the reaction, and the highest yield (73%) was obtained when KY was used. The... 

Recently there have been numerous studies on gas-phase catalytic reactions induced by transition metal ion-exchanged zeolites such as reduction, oxidation, and carbonylation. The transition metal ions in zeolites are held to the zeolite framework by coordinating lattice oxide ions hence, the zeolite framework is considered to be a mono- to polydentate macro ligand for the... 

Reactions of 1,2-Diketones and other 1,2-Dicarbonyl Compounds. - Further details of the control of the photocyclizations of enamides such as (142) affording (143) have been described. The results obtained are influenced by the nature of the chiral auxiliaries used. The best results are obtained using ionic and covalent auxiliaries in the crystalline phase or in zeolites. " Photochemically induced a-cleavage occurs on irradiation of the ketoamides (144) in water. This results in the release of the carboxylic acid moiety. The by-product of the reaction is the hemiacetal (145). " The irradiation of propellane in the presence of... 

A new cholesterol FIA biosensor based on a luminol/H202 ECL reaction induced by a glassy carbon electrode polarized at +425 mV versus a Pt pseudo j reference was introduced. The cholesterol biosensor sensing layer used in this method is based on cholesterol oxidase (COD) immobihzed on either UltraBind membrane or Immunodyne membrane [117]. Exploration to other literature shows that heroin was determined employing Zeolite Y sieves by immobilizing... 

In order for the SSIE process to proceed, there must be some type of driving force making the cations in the ingoing phase mobile enough to break them apart from the solid matrix where they begin, such that they can travel into the zeolite. In this chapter, we break these processes down into three fundamental types (1) moisture induced, (2) high-temperature induced, and (3) reaction induced. 

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

Clennan, E.L. and Sram, J.P. (2000). Photochemical reactions in the interior of a zeolite. Part 5. The origin of the zeolite induced regioselectivity in the singlet oxygen ene reaction. Tetrahedron 56, 6945-6950... 

The choice of appropriate reaction conditions is crucial for optimized performance in alkylation. The most important parameters are the reaction temperature, the feed alkane/alkene ratio, the alkene space velocity, the alkene feed composition, and the reactor design. Changing these parameters will induce similar effects for any alkylation catalyst, but the sensitivity to changes varies from catalyst to catalyst. Table II is a summary of the most important parameters employed in industrial operations for different acids. The values given for zeolites represent best estimates of data available from laboratory and pilot-scale experiments. 

Various ways to modify ZSM-5 catalyst in order to induce para-selectivity have been described. They include an increase in crystal size (15,17,20) and treatment of the zeolite with a variety of modifying agents such as compounds of phosphorus (15,18), magnesium (15), boron (16), silicon (21), antimony (20), and with coke (14,18). Possible explanations of how these modifications may account for the observed selectivity changes have been presented (17) and a mathematical theory has been developed (22). A general description of the effect of diffusion on selectivity in simple parallel reactions has been given by Weisz (23). 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

The ion-exchange reaction of the synthetic zeolites NaX and NaY with cobalt, zinc and nickel ions is shown to be non-stoichiometric at low bivalent-ion occupancy, the hydrolytic sodium loss being about twice as large for NaX ( 5 ions/unit cell) as for NaY. The effect is more pronounced at high temperatures and disappears at high occupancies. Reversibility tests in NaX toward zinc and cobalt ions, as studied by a temperature-variation method, show the temperature history to be an important factor in the irreversibility characteristics. The low-temperature partial irreversibility, induced by a high-temperature treatment (45°C) is interpreted in terms of a temperature-dependent occupancy of the small-cage sites by divalent cations, which become irreversibly blocked at low temperature (5°C). 

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