Temperature programmed desorption of benzene from NaY,CeNaY and CeNaX zeolites

Temperature-programmed desorption (t.p.d.) of benzene from NaY, CeNaY (with different degrees of Ce⁺³-exchange), and CeNaX in a helium stream from 353 to 673 K at different heating rates (2.5–25 K min⁻¹) and initial sorbate loadings has been investigated using a gas chromatographic technique. The heat of sorption of benzene, estimated from the t.p.d. data for the zeolites is in the following order: NaY > Ce(33)NaY > Ce(72)NaY > Ce(90)NaX > Ce(98)NaY (the value in the parentheses indicates the percentage of Ce⁺³-exchange). The heat of sorption was found to decrease almost linearly with an increase in the degree of Ce⁺³-exchange of CeNaY. This shows that benzene interacts more strongly with Na⁺ than with the acidic sites (i.e., the hydroxyl groups responsible for the protonic acidity). Thus, sorption of benzene in the zeolites is not very specific and is the consequence of interaction of benzene molecules with both the acidic (protonic) and nonacidic (Na⁺ cations) sites. The increase in the peak maximum temperature of the t.p.d. curve with a decrease in the initial sorbate loading indicates that benzene sorption sites of different energies are present in the CeNaY and CeNaX zeolites, whereas, in case of NaY, the benzene sorption sites do not differ significantly from each other in their energy.     

Effect of two methoxy groups bound to an amino-benzene donor unit for thienyl-di-vinylene bridged EO chromophores

We report the molecular first hyperpolarizability (β) and absorption and ¹H NMR spectra of electro-optic (EO) chromophores comprised of amino-benzene with two methoxy groups at meta-positions as the donor units, thienyl-di-vinylene as the π-electron bridges, and 2-(dicyanomethylene)-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF), 2-(dicyanomethylene)-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (CF₃-TCF) or 2-(dicyanomethylene)-3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-2,5-dihydrofuran (CF₃-phenyl-TCF) as the acceptor units. Improvement in linear and nonlinear optical properties was found in EO chromophores with two methoxy groups compared with benchmark EO chromophores without these groups and EO chromophores with one methoxy group. The data obtained from measurements of hyperpolarizability (β) and absorption and ¹H NMR spectra indicate a possible mechanism that can contribute to improvement – double intra-molecular hydrogen bonding.     

Factors influencing the conversions of alkylaromatic hydrocarbons on high-silica zeolites: Part II. Presence of extralattice Al

The effect of different amounts of dislodged Al present in hydrothermally treated Y-type zeolites has been studied in the conversions of toluene, m-xylene, and alkylation of benzene with ethylene. T.p.d. of NH₃ is used to explain changes in the catalytic activity and modification of the strength and number of acid sites occurring in relation to the extralattice Al content. No indications for direct participation of the nonframework Al in the alkylaromatic conversions have been observed.     

Changes in the vicinity of impurity Fe3+ ions in various zeolites as studied by proton n.m.r. relaxation of adsorbed molecules

In commercial zeolites, the proton relaxation time T₁ of adsorbed molecules is controlled by impurity Fe³⁺ ions. A considerable number of various zeolite/adsorbate systems show an enhancement of T₁ by about one order of magnitude, corresponding to a decrease in the number of accessible Fe³⁺ ions. This indicates changes in the vicinity of the Fe³⁺ ions. In these zeolites, bridging hydroxyl groups were produced and water was present during or after this process.     

On the unusual mechanism of Lewis acidity manifestation in HZSM-5 zeolites

The adsorption of CD₃CN and NH₃ on HZSM-5 zeolites containing prechemisorbed olefins were studied using diffuse reflectance i.r. spectroscopy. It is shown that in the presence of strong bases the unusual manifestation of Lewis acidity in these zeolites takes place as a result of fission of the Al←O bond in bridged alkoxyl groups by adsorbed base molecules. Nonempirical quantum-chemical calculations confirm this experimental finding and define the requirements of such Lewis acidity generation.     

An inelastic electron tunneling study of barriers formed in ethylene and benzene glow discharges

Inelastic electron tunneling was used to study the molecular structure of barriers formed in a glow discharge of ethylene or benzene. The results indicated the presence of aliphatic CH₃ and CH₂ groups which were formed by the break-up of the vapor molecules. Barriers formed with benzene also contained aromatic rings, and those formed in ethylene contained olefinic CC bonds. A similar molecular structure was found for benzene barriers grown on aluminum and magnesium films.     

Zeolite synthesis in the system pyrrolidine-Na2OAl2O3SiO2H2O

Crystallization of zeolites from the system pyrrolidine-Na₂OAl₂O₃SiO₂H₂O was investigated. Five zeolites, ZSM—5, ZSM—35, ZSM—39, ZSM—48 and KZ—1, were synthesized as single phases, whose crystallinity was very high. In this work zeolite ZSM—48 was prepared from the pyrrolidine system for the first time. It was suggested that pyrrolidine might stabilize certain aluminosilicate polyanions which have building units of oxygen five-membered rings. It was also presumed that all zeolites, except ZSM—39, have a pore opening of oxygen ten-membered rings.     

Formation of Na-A and -X zeolites from waste solutions in conversion of coal fly ash to zeolites

Na-A and -X zeolites were synthesized from waste solutions in conversion of coal fly ash (Fa) to zeolite. The amorphous SiO₂ and Al₂O₃ of Fa were completely dissolved to form Po, Pt, and Pc type zeolites in NaOH solutions at 85°C. Only 24% of Si⁴⁺ eluted from Fa were converted to the zeolites and the remaining waste solutions contained high Si⁴⁺ concentrations. When molar ratio SiO₂/Al₂O₃ of the waste solutions was modified at 1.0≤SiO₂/Al₂O₃≤2.0 by addition of NaOH-NaAlO₂ solutions and the solutions were agied at 85°C, a single phase of Na-A zeolite was formed. The Na-X zeolite was formed at SiO₂/Al₂O₃≥2.5 and its crystallinity was increased with increasing the SiO₂/Al₂O₃ ratio, whereas the crystallinity of Na-A zeolite was decreased. At SiO₂/Al₂O₃=7.3, a single phase of Na-X zeolite was produced.     

Effect of SiO2Al2O3 ratio on the activity of HZSM-5 zeolites in the different steps of methanol conversion to hydrocarbons

The conversion of methanol to hydrocarbons over HZSM-5 zeolites of different SiO₂Al₂O₃ ratio was studied at 450°C. The initial zeolite activity in the different steps of the reaction (alkene formation from methanol and dimethyl ether, alkene aromatization, coke formation) was strictly proportional to the aluminum content of the zeolites. Under reaction conditions, catalytic properties of HZSM-5 of different aluminum content were modified in the initial period of catalyst performance in different ways. Enhanced activity sites, which are generated in HZSM-5 of relatively high aluminum content, were shown to be the reason for enhanced aromatization and coking activity of these zeolites.     

Conversion of toluene and trimethylbenzene over NaHY zeolites

The conversion of toluene, 1,2,4-trimethylbenzene (TMB), and other C₉ aromatics on NaHY zeolites was investigated in a fixed-bed, continuous flow reactor at 460°C and a pulse microreactor at 400°C. It was found that the product distribution and reaction selectivity are dependent on the alkali-ion content in NaHY and the composition of feedstocks. Disproportionation was the main reaction in the conversion of toluene or TMB. In the reaction of toluene with 1,2,4-TMB, the amount of xylene produced from TMB dealkylation and TMB-toluene transalkylation was greater than that from toluene disproportionation; in turn, the product xylene to benzene mole ratio was found to be 7–27. The selectivity of TMB dealkylation increased with increasing Na⁺ content in NaHY or increasing 1,2,4-TMB concentration in feed. Among all the C₉ aromatics studied, the reaction of toluene with TMB produced the greatest amount of xylene.     

On the role of enhanced activity sites in the different steps of methanol transformation into olefins over HZSM-5

The activities of parent and mildly steamed HZSM-5 (SiO₂/Al₂O₃ = 34) were studied in the different steps of methanol-to-olefin conversion at 370°C. It was found that enhanced activity sites, which cause the increased cracking activity of the steamed zeolites, do not participate in the steps of initial CC bond formation from methanol. For the steps of methanol interaction with olefins, these sites are more active than are the normal catalytic sites of HZSM-5 (bridging hydroxyl groups).     

Advances in principal factors influencing carbon dioxide adsorption on zeolites

Abstract

We report the advances in the principal structural and experimental factors that might influence the carbon dioxide (CO₂) adsorption on natural and synthetic zeolites. The CO₂ adsorption is principally govern by the inclusion of exchangeable cations (countercations) within the cavities of zeolites, which induce basicity and an electric field, two key parameters for CO₂ adsorption. More specifically, these two parameters vary with diverse factors including the nature, distribution and number of exchangeable cations. The structure of framework also determines CO₂ adsorption on zeolites by influencing the basicity and electric field in their cavities. In fact, the basicity and electric field usually vary inversely with the Si/Al ratio. Furthermore, the CO₂ adsorption might be limited by the size of pores within zeolites and by the carbonates formation during the CO₂ chemisorption. The polarity of molecules adsorbed on zeolites represents a very important factor that influences their interaction with the electric field. The adsorbates that have the most great quadrupole moment such as the CO₂, might interact strongly with the electric field of zeolites and this favors their adsorption. The pressure, temperature and presence of water seem to be the most important experimental conditions that influence the adsorption of CO₂. The CO₂ adsorption increases with the gas phase pressure and decreases with the rise of temperature. The presence of water significantly decreases adsorption capacity of cationic zeolites by decreasing strength and heterogeneity of the electric field and by favoring the formation of bicarbonates. The optimization of the zeolites structural characteristics and the experimental conditions might enhance substantially their CO₂ adsorption capacity and thereby might give rise to the excellent adsorbents that may be used to capturing the industrial emissions of CO₂.     

Effect of different modifications of BEA-zeolites on operational characteristics of conductometric biosensor

Effect of different modifications of zeolite Na⁺-BEA on working characteristics of urease-based conductometric biosensor was studied. As the biosensor sensitive elements were used bioselective membranes based on urease and various zeolites immobilised with bovine serum albumin on the surface of conductometric transducers. Influence of zeolites on sensitivity of urea biosensor was investigated as well as reproducibility of biosensor signal and reproducibility of activity of the bioselective element after different variants of urease immobilisation on the surface of conductometric transducer. The biosensors based on zeolites (NH₄⁺-BEA 30 and H⁺-BEA 30) were shown to be the most sensitive. Concentration of these zeolites in the bioselective membrane was optimized. Use of zeolites modified with methyl viologen and silver was ascertained to be of no prospect for urea conductometric biosensors. It was demonstrated that characteristics of urea biosensors can be regulated, varying zeolites modifications and their concentrations in bioselective membranes.     

Towards a Green Synthesis of LAB's: Effect of Rare Earth Metal Ions on the Benzene Alkylation with 1-Dodecene over NaFAU-Y Zeolites

Chemically modified microporous materials can be prepared as robust catalysts suitable for application in vapor phase processes such as Friedel-Crafts alkylation. In the present paper we have investigated the use of rare earth metal (Ce³⁺, La³⁺, RE³⁺, and Sm³⁺) exchanged Na-Y zeolites as catalysts for the alkylation of benzene with long chain linear 1-olefin; 1-dodecene. Thermodesorption studies of 2,6-dimethylpyridine adsorbed catalysts (in the temperature range 573 to 873 K) show that the rare earth zeolites are highly Brönsted acidic in nature. A perfect correlation between catalyst selectivity towards the desired product (2-phenyldodecane) and Brönsted acid sites amount has been observed.     

Reduction behaviour of (AgNa)A and (AgM2+Na)A zeolites

The reduction of silver-exchanged A zeolites proceeds in at least two distinct steps. At lower reduction temperatures up to 500 K about 67% of the silver ions present in the lattice of the unreduced zeolites are reduced and charged silver clusters (Ag₃⁺) are formed. This process is accompanied by a redistribution of the remaining cations resulting in characteristic changes of the adsorption properties. The position of silver ions in (AgNa)A and (AgM²⁺Na)A zeolites does not influence their reduction behaviour in the first reaction step, The further reduction of the Ag₃⁺ clusters at higher temperatures depends on the Ag-exchange and on the kind of M²⁺ ion, It seems that the low temperature reduction proceeds over either Ag₉⁺ or Ag₉²⁺ species depending on the silver content.     

On the e.s.r. line widths of hydrated Cu2+ in zeolites

In this study, the electron spin resonance (e.s.r.) of Cu(H₂O)₆P²⁺ ions were used to investigated the mobility and the freezing properties of water adsorbed on the synthetic zeolites of types 3A, 4A, 5A, and 13X with pore diameters of 3, 4, 5, and 10 Å, respectively, and the natural zeolites, heulandite and clinoptilolite. The temperature dependencies of the e.s.r. line widths of the zeolites with pore diameters of 3, 4, and 5 Å indicated that the first layers of water molecules from the surfaces are restricted in motion even at room temperature, and beyond these, the water molecules are mobile. The water molecules in the 13X zeolite, heulandite, and clinopotilolite behave like liquid water.     

Base properties of zeolite catalysts

Distributions of base strengths were determined using indicator techniques and i.r. spectroscopic studies of acetic acid adsorbed on samples of faujasite-type zeolites. The results show that base sites of variou zeolites differ significantly in both their strength and nature. Among sodium forms of the zeolites, NaX appeared to exhibit higher strength and concentrations of base sites than NaY. For hydrogen-sodium forms, the situation was reversed: HNaY prevailed over HNaX. The total basicities of the hydrogen—sodium zeolites were considerably higher than those of the sodium forms, due to contributions from both hydroxyl groups and oxygen ions of (AlO₄)⁻ tetrahedra; whereas the latter exhibited Lewis basicity alone. Nonetheless, the higher basicities of the HNaY and HNaX zeolites do not correlate with their catalytic activity for decomposition of diacetone alcohol to acetone, a model base-catalysed reaction. The relatively poor conversions to acetone are examined within the context of base strengths and the high acidity of HNaX and HNaY since another reaction pathway, acid-catalysed formation of mesityl oxide, was also observed.     

Infrared studies on nitrogen oxides adsorbed on alkali metal ion-exchanged ZSM-5 zeolites

The adsorption of nitrogen oxides (NO_x) on a series of alkali metal ion-exchanged ZSM-5 zeolites at 226 K was investigated by in situ infrared spectroscopy. In the low NO pressure region, the main species found on each zeolites was an NO monomer linearly adsorbed onto the cation through an O atom (M⁺ ··· ON), but an M⁺ ··· NO species was not detected. On the other hand, a part of adsorbed NO disproportionated into N₂O and N₂O₃ under high NO pressures (higher than 1.3 kPa). The formation of N₂O₃ was enhanced at a high density of nitrogen oxides (NO_x) in the ZSM-5 pore.     

Highly conductive single molecular junctions by direct binding of π-conjugated molecule to metal electrodes

We have investigated electrical conductance of the single C₆₀ and benzene molecules bridging between metal electrodes. The single C₆₀ and benzene molecular junctions were prepared in ultra high vacuum. The single molecular junctions showed the high conductance values (around 0.1–1 G₀: G₀ = 2e²/h), which were comparable to that of the metal atomic contact. The highly conductive single molecular junctions could be prepared by direct binding of the π-conjugated organic molecule to the metal electrodes without the use of anchoring groups. For comparison, the single 1,4-benzenediamine molecular junction was investigated in solution. The benzene molecule was bound to the Au electrodes via amine (anchoring group) for the single 1,4-benzenediamine molecular junction. The conductance of the single 1,4-benzenediamine molecular junction was 8 × 10⁻³G_0. It was suggested that the anchoring groups acted as resistive spacers between the molecule and metal.     

Crystal structures of zeolites with the general formula CaAl<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>16</Subscript>·<Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O

Analysis of the structures of goosecreekite and yugawaralite indicates that, like in the zeolites with the general formula CaAl₂Si₄O₁₂ · nH₂O (chabazite, laumontite, and wairakite), the structural framework of goosecreekite is made up of alternating six-and four-membered rings. In the structural frameworks of goosecreekite, chabazite, and wairakite, such chains are linked so as to form eight-membered rings. In contrast, the structural framework of yugawaralite is made up of chains of two five-membered rings and one four-membered ring. The salient features of the zeolites with the general formula CaAl₂Si₆O₁₆ · nH₂O are the presence of calcium-water chains in the structure of goosecreekite and a calcium-water network in the structure of yugawaralite. Analysis of calcium-oxygen, calcium-water, and oxygen-hydrogen bonds has made it possible to reveal features common to the structures of yugawaralite and wairakite.