Influence of temperature inhomogeneity on product profile of reactions occurring within zeolites

In zeolites, diffusion is often accompanied by a reaction or sorption which in turn can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported for the presence of a hot zone presumed to be created by a reaction or chemi- or physi-sorption site. These simulations show that the presence of localized hot regions can alter both kinetic and transport properties such as diffusion. Further, we show that enhancement of diffusion constant is greater for systems with larger barrier height, a surprising result that may be of considerable significance in many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of a hot zone in addition to that which normally exists via concentration. Implications of this coupling for the product profile of a reaction are discussed. We also propose a mechanism by which mobility of ions or diffusion of molecular species within biomembranes may take place. Dedicated to Professor C N R Rao on his 70th birthday     

On the heavy atom on light atom relativistic effect in the NMR shielding constants of phosphine tellurides

The relativistic HALA effect has been shown to depend on the spatial deformation of the lone electron pairs of a heavy atom, as demonstrated for alkyl and alkene phosphine tellurides. It was found that HALA effect on phosphorous nuclear magnetic resonance shielding constant is strongly dependent on the spatial arrangements of light substituents on phosphorus, resulting in the deformation of the lone electron pairs of tellurium.     

Desilication mechanism revisited: highly mesoporous all-silica zeolites enabled through pore-directing agents

The role of pore‐directing agents (PDAs) in the introduction of hierarchical porosity in silicalite‐1 in alkaline medium was investigated. By incorporation of various PDAs in aqueous NaOH, homogenously distributed mesopores were introduced in 2.5 μm silicalite‐1 crystals. It was proven for the first time that framework aluminum is not a prerequisite for the introduction of intracrystalline mesoporosity by desilication. The pore‐directing role is not directly exerted by framework trivalent cations metals, but by species on the external surface of the zeolite. The inclusion of metal complexes (Al(OH)₄^?, Ga(OH)₄^?) and tetraalkyl ammonium cations (tetramethyl ammonium (TMA⁺), tetrapropyl ammonium (TPA⁺)) in the alkaline solution led to distinct mesopore surface areas (up to 286 m² g^?1) and pore sizes centered in the range of 5–20 nm. In the case alkaline treatment was performed in the presence of Al(OH)₄^?, all aluminum partially integrated in the zeolite giving rise to both Lewis and Brønsted acidity. Apart from the concentration and location, the affinity of the PDA to the zeolite surface plays a crucial role in the pore formation process. If the PDA is attracted too strongly (e.g., TMA⁺), the dissolution is reduced dramatically. When the pore‐directing agent is not attracted to the zeolite’s external surface, excessive dissolution occurs (standard alkaline treatment). TPA⁺ proved to be the most effective PDA as its presence led to high mesopore surface areas (>200 m² g^?1) over a broad range of PDA concentrations (0.003–0.1 M ). Importantly, our results enable to extend the suitability of desilication for controlled mesopore formation to all‐silica zeolites.     

Carbonates in zeolites: Formation,properties, reactivity

Two possible schemes of carbonate formation (with and without water participation) in cationic form zeolites are considered. Activation energy for the formation of hydrogen carbonate in NaX zeolite from water and carbon dioxide is calculated at the DFT level with periodic boundary conditions, while the problems of modeling the formation of symmetric carbonate in the same zeolite are discussed. The formation of copper carbonate is studied using binuclear CuOCu clusters from CO₂ where the influence of water on the barrier is discussed. The questions related to DFT application to binuclear copper clusters are also considered by comparison with the data obtained at the MP2 level. The reactivity of copper carbonate is tested in the reaction with methanol. © 2015 Wiley Periodicals, Inc.     

The effect of zeolites on copper uptake by plants growing in contaminated soils

Three-year pot experiments were carried out in a greenhouse with the aim of reducing the copper content in the 2nd link of the soil-plant-animal (man) trophic chain. For this purpose, synthetic zeolites of the 3A, 4A, 5A and 13X type were introduced into copper contaminated soils at levels of 1, 2 and 3% by weight in relation to the soil mass. Mono- and dicotyledonous plants were cultivated in mixtures of soils with the zeolites. The copper content in lettuce grown in the contaminated soil decreased in the presence of zeolites by 29–77%, in grass by 41–78%, in oats by 45–64% and in beets by 21–41%, as compared to the control.     

Role of cations and confinement in asymmetric photochemistry: enantio- and diastereo-selective photocyclization of tropolone derivatives within zeolites

Asymmetric induction in photochemical reactions has been explored using the photochemistry of tropolones as a model. Three approaches have been examined: chiral inductor, chiral auxiliary and [chiral inductor + chiral auxiliary]. All three methods gave excellent asymmetric induction in zeolite and very little or zero induction in solution. Results presented on tropolones clearly illustrate the remarkable influence that a confined space studded with cations can have on asymmetric induction. Tropolone derivatives, upon irradiation undergo 4pi-electron electrocyclization to yield a bicyclic product and a rearranged product. Enantiomeric excess up to 68% has been achieved in the cyclized product. In systems where a chiral inductor has been covalently linked, diastereomeric excess as high as 88% has been achieved within a zeolite while the same system in solution gave 10%.     

A novel concept for photovoltaic cells: clusters of titanium dioxide encapsulated within zeolites as photoactive semiconductors.

Discrete clusters of TiO(2) (of only a few titanium atoms) are prepared within the internal micropore space of zeolite Y (4.8 wt % Ti loading) and characterized by Raman spectroscopy (rutile- and anatase-like structures), electron microscopy combined with elemental analyses (coincident Si and Ti spatial distribution), and X-ray diffraction (minor zeolite crystallinity decrease). The parent TiO(2)@Y sample is modified either by adsorption of acid-organic compounds (benzoic and 4-aminobenzoic acids or catechol) or by nitrogen doping. After modification, the optical UV/Vis spectrum of the parent TiO(2)@Y (onset of the absorption band at wavelengths <300 nm and bandgap of 4.2 eV) changes, and the appearance of new bands expanding to the visible region is observed. In contrast to the inactive zeolite Y matrix, all the zeolite-encapsulated TiO(2) species exhibit a photovoltaic response. The influence of the I(2)/I(3) (-) concentration in the electrolyte solution on the temporal profile of the photovoltage clearly shows that I(2)/I(3) (-) is also a suitable carrier for the positive charge in zeolite-based photovoltaic devices. The photocurrent response and the efficiency of the photovoltaic cell based on zeolite-encapsulated TiO(2) materials depend on the nature of the organic modifier and on the N-doping. The most efficient photovoltaic cell is that based on N-doped TiO(2)@Y, which exhibits a V(OC) (voltage at open circuit) of 270 mV, an I(SC) of 5.8 muA (current at short circuit), and a fill factor (FF) of 0.4. Although these values are low compared to current dye-sensitized TiO(2) solar cells, our findings could open up a promise for a stimulating research on the photovoltaic activity of zeolite-based host-guest solids.     

乙炔、丙烯在多种分子筛上的吸附和脱附性质

在80℃下研究了ZSM5、13X、Y、MOR、5A、SAPO34等多种分子筛和常见载体上乙炔和丙烯的吸附和脱附性能。结果表明,乙炔和丙烯在SiO2、γ-Al2O3上不吸附,而与分子筛存在较强的相互作用。对于同类型的分子筛,两者的饱和吸附量均随分子筛硅铝比的增大而减小。在同一种分子筛上,以摩尔计的丙烯吸附量均明显高于乙炔。在80℃改性β分子筛上乙炔和丙烯饱和吸附量可分别达到0.11mmol/g和4.89mmol/g,该结果明显高于文献报道的结果。     

Photochemical dimerization of acephenanthrylene and the heavy atom effect

The solution-phase photodimerization of acephenanthrylene (1) in cyclohexane, chloroform, dibromomethane, and ethyl iodide with visible light (lambda = 419 nm) occurs to form cis (2) and trans (3) photodimers in ratios that vary with the solvent and with the presence or absence of oxygen. The photodimerization exhibits a significant heavy atom effect and also is sensitive to selective quenching by dissolved oxygen, in which only cis stereoisomers are produced. Irradiation of the dimers (lambda = 254 nm) shows that the trans dimers cleave more rapidly than the cis dimers. This is attributed to stabilization of the excited state in the cis dimers.     

Low-temperature adsorption/storage of hydrogen on FAU, MFI, and MOR zeolites with various Si/Al ratios: effect of electrostatic fields and pore structures

Several zeolites, such as faujasite, mordenite, and ZSM-5, with various aluminum contents have been used to analyze the effect of aluminum or cation concentration (strength of electrostatic field) on hydrogen adsorption at low temperature. Irrespective of the zeolite structure, the adsorption capacity, isosteric heat of adsorption (-DeltaHads), surface coverage, and micropore occupancy increase with increasing aluminum content of a zeolite. Zeolites with a higher amount of aluminum favorably adsorb hydrogen at relatively low pressures. For zeolites with similar aluminum contents, the adsorption capacity, isosteric heat of adsorption, surface coverage, and micropore occupancy are in the order of mordenite>ZSM-5>faujasite, probably due to differing pore sizes and the presence or absence of pore intersections. This work demonstrates that zeolites with strong electrostatic fields and narrow pores without intersections are beneficial for high hydrogen uptake.     

核壳结构的Beta-Y复合分子筛的合成与表征

采用两种不同的水热晶化法合成Beta-Y复合分子筛,并采用XRD、SEM-EDS、FT-IR、N₂ 吸附-脱附技术对其进行表征。结果表明,以Beta分子筛为唯一硅源合成的sample A具有Y和Beta分子筛两种晶型结构,为机械混合形式;而以Beta分子筛母液为前躯体合成的sample B为核壳结构的复合分子筛,具有Y和Beta分子筛两种晶型结构,其核相为Beta分子筛,壳层为Y型分子筛,其晶体中具有介孔结构,并且介孔孔径比较集中。     

Selective conversion of light gasoline into aromatic hydrocarbons over shape selective zeolites, I. Catalytic activity of various zeolites for aromatization of light gasoline, heavy gasoline and coker naphtha

Studies about the development of a new effective aromatization process for converting gasoline rich in light saturated hydrocarbons, using modified molecular shapeselective zeolites, have been carried out. Naphthas with olefins and naphthenes are potential feedstocks. The yield of aromatic hydrocarbons increases with decreasing hydrogen content of the feed. Zn-SABO and Zn-ZSM type zeolites have been shown to be excellent catalysts for the process.     

Alkylation of dichlorobenzene isomers by olefins on zeolites Y and pentasil

Catalytic alkylation of dichlorobenzene isomers by ethylene and propylene on zeolite catalysts has been studied. It has been determined that using catalysts based on zeolite Y makes it possible to change the ratio of 1-alkyldichlorobenzenes formed within a wide range by varying the acidic properties of the catalysts.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1486–1488, August, 1995.     

含钛USY沸石的制备与表征

采用钛酸丁酯/无水乙醇溶液与USY沸石浸渍及采用氟钛酸铵溶液与USY沸石二次合成等方法,制备了一系列含钛USY沸石。通过XRD,FT-Raman光谱,正庚烷吸附等温线,UV-DRS及FT-IR光谱等手段的表征,表明在所制备的含钛USY沸石样品中,当Ti含量较小(≤4.8%TiO~2)时,Ti是以高分散状态分布在沸石表面。不同制备方法所得含钛USY沸石样品中,Ti的状态是各不相同的。UV-DRS和FT-IR证实,以二次合成法制备的含钛USY沸石样品中,Ti主要处于沸石骨架位;而以钛酸丁酯/无水乙醇溶液浸渍法制备的样品,Ti是以单齿(monofunctional)和双齿(bifunctional)形式与沸石的表面羟基相结合。     

Adsorption of 1-olefin/n-paraffin liquid mixtures on NaX and NaY zeolites

The isotherm equation for adsorption of binary liquid mixtures of nonelectrolytes, involving differences in molecular sizes of components and quasi-Gaussian energy distribution of adsorption sites, is used to describe the excess adsorption data for 1-olefin/n-paraffin liquid mixtures on NaX and NaY zeolites at 293 K. This equation gives a good representation of the above adsorption data. Analysis of these data showed that the heterogeneity parameter is slightly dependent on the difference in molecular sizes of both components.     

Thermal analysis of fluorine containing gels and precursors of high silica zeolites

By means of simultaneous DTA-, TG- and DTG-technique, the silicate gels and the MFI crystals obtained from these gels have been investigated. The gels have been prepared in presence and in absence of tetrapropylammonium cation (TPA⁺) and with Li⁺, Na⁺, NH ₄ ⁺ and K⁺ fluorides. In absence of TPA⁺ no thermal effects have been observed in Li⁺- and Na⁺-gels. The effects observed in the NH ₄ ⁺ -gel stem from a decomposition and release of inorganic phases: SiF₄, NH₄F, NH₃. The DTA/DTG effects in the TPA⁺ containing gels and in the MFI crystals of monodisperse size are attributed to the decomposition of TPA⁺ cation. It can be concluded from these effects that the interaction between the gels and the TPA⁺ cation is rather weak. The interaction between TPA⁺ and MFI crystals obtained in fluoride medium is stronger than the interaction with crystals obtained from alkaline media. Similar thermal effects are obtained after grinding the long crystals to those having a large distribution of crystal sizes.

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Zusammenfassung Mittels simultaner DTA-, TG- und DTG-Techniken wurden Kieselgele und die MFI-Kristalle dieser Gele untersucht. Die Gele wurden in und ohne Gegenwart von Tetrapropylammoniumkationen (TPA⁺) und mit Li⁺-, Na⁺-, NH ₄ ^– und Kaliumfluoriden hergestellt. Bei Abwesenheit von TPA⁺ werden für Li⁺- und Na⁺-Gele keine thermischen Effekte beobachtet. Der bei NH ^4– -Gelen beobachtete Effekt ergibt sich aus der Zersetzung und Abspaltung von anorganischen Phasen: SiF₄, NH₄F, NH₃. Die DTA/DTG-Effekte in TPA⁺-haltigen Gelen und in den MFI-Kristallen monodispersen Ausmaes werden der Zersetzung des TPA⁺-Kations zugeschrieben. Daraus kann man schlieen, da die Wechselwirkung zwischen den Gelen und dem TPA⁺-Kation eher schwach ist. Die Wechselwirkung zwischen TPA⁺ und MFI-Kristallen aus fluoridischen Medien ist strker als die Wechselwirkung mit Kristallen aus alkalischen Medien. Ähnliche thermische Effekts erhlt man, nachdem man die langen Kristalle zerkleinert und eine breite Kristallgrenverteilung erhlt.

     

Investigation of the effect of carbon monoxide on the oxidative carbonylation of methanol to dimethyl carbonate over CuX and CuZSM-5 zeolites

Direct synthesis of dimethyl carbonate offers prospects for a “green chemistry” replacement to eliminate use of phosgene for polymer production and other processes. The carbonylation of methanol to produce dimethyl carbonate over Cu⁺X and Cu⁺ZSM-5 zeolites prepared by solid-state ion exchange has been investigated, focusing on the interaction of carbon monoxide with the Cu⁺ zeolites. The methanol carbonylation mechanism reported previously has been extended to account for carbon monoxide adsorption at high pressure. The comparison of the results obtained from Cu⁺X and Cu⁺ZSM-5 show that strong CO adsorption on the catalyst is not related to increased rate of dimethyl carbonate production. The rate limiting step for DMC production is best described as the Eley-Rideal reaction of gas-phase CO with surface methoxide.     

Synthesis of isoamyl acetate usin NaX and NaY zeolites as catalysts

A liquid phase esterification reaction between glacial acetic acid and isoamyl alcohol has been studied using NaX and NaY zeolites as catalysts. The influence of calcination temperature, the amount of catalysts, reaction temperature, time of esterification and the molar ratio of the reactants has been investigated. Water insoluble products have been isolated from the reaction mixture and analyzed for the ester. Both NaX and NaY are found to be active as catalysts in the reaction. However, their catalytic activity varies with the reaction conditions and their calcination temperature. The reaction has been found to be 100% selective to ester formation. Catalytic activity of the zeolites has been correlated with their surface acidity.