The synthesis of titanium-containing MCM-41 mesoporous molecular sieves in the presence of dye molecules results in a well developed hexagonal structure if the powder dye is dissolved in a surfactant solution. These materials contain titanium species in tetrahedral and octahedral coordinations simultaneously, indicated (i) by absorption bands at 960 cm−1 in the infrared and Raman spectra, which are assigned to titanium in a silica network, and (ii) by bands in the Raman spectra which are characteristic for rutile. The flat slope of the red flank of the UV bands below 300 nm points also to the presence of fourfold and sixfold coordinated titanium. Zinc phthalocyanine is proposed to be embedded in the micelles as a monomer predominantly, whereas rhodamine B is assumed to be exclusively located outside the micelles or at the external surface as aggregates.
Temperature programmed desorption (TPD) of n-alkanes, ranging from n-hexane to n-hexadecane, from silicalite-1, was conducted between ambient temperature and 500°C on a thermogravimetric (TG) balance at heating rates of 3, 6 and 10 C° min−1. The derivative TG curves all showed two distinctive peaks. An inflection point in the TG profiles was found at about four molecules per unit cell in all cases except hexadecane, which occurred at 1.92 molecules per unit cell. The saturation adsorption volumes of all sorbates was close to the theoretical value of ∼0.19 ml g−1.
Oxygen exchange between adsorbed oxygen and the zeolite X which contains cesium ions in excess of the ion-exchange capacity (cesium-added zeolite X) was studied by TPD of adsorbed O-18(2). The desorbed oxygen molecules were composed mainly of O-18(2) and O-16(2); the fraction of (OO)-O-18-O-16 was small, indicating that a pair of oxygen atoms in the zeolite exchanges with an oxygen molecule. The retention of molecular identity of oxygen during the exchange, together with the ratio of O-18/O-16 in the desorbed oxygen, suggests the existence of the Cs2O particles in the zeolite cavities. A part of the surface layers of the CS2O particles converts into the structures similar to those of the oxides such as Cs(2)O2(,) Cs2O3 and Cs2O4 when oxygen molecules are adsorbed.The catalytic activity of cesium-added zeolite X for 1-butene isomerization was completely poisoned by oxygen. The catalytically active species are concluded to be the Cs2O particles encapsulated in the zeolite cavities. (C) 1997 Elsevier Science B.V.
A comprehensive Xe-129 NMR spectroscopy study on H-ZSM-5 zeolites having different aluminum contents and on cation-exchanged ZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchanged with Group I-III metal ions (K, Ca, Sr, Ba, Al, La) to varying degrees. The chemical shift of adsorbed Xe-129 is seen to be a function of the pentasil structure of ZSM-5, of the number of free Bronsted acid sites and of the number of metal cations in the framework. Differences in the chemical shift of Xe-129 are seen between cations due to their different polarizing forces against xenon. The amount of cations has also an effect on the delta(Xe-Xe) term in Fraissard's equation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework.
Chromia pillared clays differing in pillar density were synthesized from the Li-form of montmorillonite calcined in a controlled manner. These materials were characterized by XRD, TGA, AAS, nitrogen adsorption and their thiophene hydrodesulfurization activity. It is found that clays varying in cation exchange capacity pillared with Cr12 pillar precursor ions can provide a family of catalysts that differ in surface area, porous structure and concentration of active sites. Sulfidation of the heat-treated chromia pillared clay resulted in the formation of chromium sulfide pillared derivatives with high surface area and well-developed porous structure. The activities for thiophene hydrodesulfurization and consecutive butene hydrogenation over chromium sulfide pillared montmorillonite are highest on clays with the largest pillar concentration and correlated with the total surface area. However, the activity per number of pillars increased with decreasing pillar density. Thiophene HDS predominantly results in the formation of butane and butenes, irrespective of the pillar population. The rate of deactivation is somewhat larger for catalysts that have a larger lateral pillar spacing. The chromium sulfide pillared clays exhibit a relatively high stability in thiophene HDS compared to zeolitic catalysts which is most likely due to their low protonic acidity.
Syntheses were performed in the system 10SiO2:1Al2O3:xNa2O:[y wt.%organic compound + (100-y)wt.%H2O] at 150°C. In the presence of dioxane, for Na2O contents between 2 and 3, primarily mazzite-type products were obtained with Si/Al ratios ranging from 3.3 to 4.1. The dioxane content (y=5–85) influences both the availability of the dioxane-Na+ complex, which is essential for MAZ crystallization, and the effective alkalinity. As such, well-crystallized MAZ samples with high and low Si/Al ratios could be obtained at low (y=5–10) and high dioxane content (y=85), respectively, while at intermediate content (y=50), denser phases crystallized. For Na2O contents lower than 2, sodalite phases were formed with the neutral dioxane molecule acting as a templating agent, while above 3, mainly gismondine and analcime were produced. For syntheses with high dioxane content, the cyclic ether was partially replaced by other organic solvents (ethers and alcohols) to study the effect of the solvent properties on zeolite crystallization. The solubility of the organic compound in the aqueous phase and its solvating properties toward anions and cations are of key importance with respect to the rate and the type of zeolite synthesis in organic medium.
The interaction of pyrrole with alkali cation exchanged FAU-type zeolites is studied using a combination of the electronegativity equalization method (EEM) and the Monte Carlo technique. Pyrrole adsorbs at SII cations, the NH-group pointing to the most basic oxygen O4. The NH-bond polarization and the pyrrole framework interaction energy reflect the fundamental and well established trends of the intrinsic framework basicity: an increasing basicity with an increasing Al-content and, at a given Si:Al-ratio, an increasing basicity with increasing size of the exchangeable cations. In contrast to a recent interpretation of the IR-spectra of the NH-stretching vibration of pyrrole (D. Murphy, P. Massiani, R. Franck, D. Barthomeuf, J. Phys. Chem., 100 (1996) 6731; D. Murphy, P. Massiani, R. Franck, D. Barthomeuf, Stud. Surf. Sci. Catal., 105 (1997) 639). our Monte Carlo calculations show that there is only one adsorption site for pyrrole in the FAU-structure at low loadings. An alternative explanation for the observed heterogeneity of the IR-spectra of pyrrole is discussed on the basis of our calculations: the orientation of the pyrrole molecule with respect to the six-ring depends on the Si:Al-ratio of the tetrahedra forming the six-rings.
The aim of this study is to distinguish the effect on the chemical shift of the anisotropy of sample magnetism (shape, chemical composition, external magnetic field B0) from the local effect (binding electron, neighbouring nuclei) for NMR experiments. We calculate the former for an angle θ=54°44″ (magic angle) between the B0 direction and the axis of the rotor. For a small adsorption rate of p-fluoroacetophenone on MFI and FAU-type zeolites, this effect on 19F chemical shift is not negligible, and it depends on the zeolite nature and structure type. The δobs correction varies from +1.8 ppm for MFI with Si/Al ≥35.5 to about +1.4 ppm for FAU zeolites with Si/Al=2.9 and 21.
A meaningful solubility product of a zeolite should remain constant as composition of zeolite mother liquors varies at a given temperature. We identify a solubility product of zeolite A, ∏s=[Si(OH)4][Al(OH−4][Na+]=1.2×10−8M3. from experimental data in highly alkaline aqueous solutions under conditions typical for zeolite A synthesis (1 M ≤ [Na] ≤ 4 M, 353 K ≤ T ≤ 363 K). To accomplish this we develop a thermodynamic model of dilute sodium aluminosilicate solutions representing mother liquors in equilibrium with zeolite A. Proper modeling of the liquid phase adjacent to a growing zeolite is necessary to obtain meaningful supersaturation values and solubility products. Further quantitative modeling of solution mediated zeolite growth and dissolution should include a physically relevant model of aluminosilicate speciation in the liquid phase.
Iron incorporation in VPI-5 occurs within a narrow range of gel composition and hydrothermal treatment conditions. Physico-chemical characterization was carried out by X-ray diffraction, SEM, thermogravimetry and solidstate NMR. The ESR studies showed that Fe 3+ incorporation occurs into lattice sites only; no occluded Fe species are observed. A significant amount of framework iron was removed during the thermal transformation of Fe-VPI-5 to FAPO-8, forming aggregates of occluded iron species in the pores. During the process, part of the Fe 3+ reduced to Fe 2+ with concurrent observations of a signal near zero field.
Iron(III) oxide pillared clays have low micropore volumes (0.04 cm3 g−1) and surface areas (95 m2 g−1). In order to increase the porosity and thereby the adsorption capacity, two different modification procedures were carried out. The first modification is based on the preadsorption of butylammonium ions between the clay sheets prior to the pillaring procedure with the iron precursors. This gives rise to a decrease of the Fe pillar density which results in an increased porosity (Fe-BuA-PILC). Preadsorption of butylammonium ions doubles the adsorption capacity of an Fe-PILC. A second modification is the incorporation of Zr into the Fe precursors to create mixed Fe-Zr pillars with entirely new properties (pillar charge, pillar symmetry, etc.) creating a pillared clay with a different enhanced porosity. These pillared clays were tested with respect to their adsorption and desorption capacities towards CCl4, CHCl3, CH2Cl2 and CH4 at 0 °C. These experiments indicate that the adsorption capacity of the mixed Fe-Zr-PILC is four times higher than pure Fe-PILC. Not only does the capacity differ, but the isotherm type also changes, indicating that adsorbents with different properties can be created by means of simple modification techniques.
Carbon-based highly dispersed Cu and Cu ZnO catalysts were prepared with sawdust impregnated with Cu(NO3)2 and Zn(NO3)2 solution. The hydrogenation of CO2 on these carbon-based Cu and Cu ZnO catalysts was studied and compared with the Cu and Cu-ZnO-containing catalysts prepared by conventional impregnation methods using wood-based active carbon. The carbon-based Cu-ZnO catalyst prepared from Cu or Zn salt-impregnated sawdust displayed high activity for hydrogenation of CO2 to methanol.
A borosilicate beta zeolite was prepared from almost neutral fluoride aqueous gels in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates. The influence of several physical and chemical parameters was studied. In comparison with the (Si,Al) system, this (Si,B) system appears less ‘reactive’ with longer crystallization times. Moreover, the products obtained display a narrower range of compositions. Only boron-rich (Si/B=2) and fluoride-poor media (F/Si=2) led to well-crystallized samples of beta zeolite characterized by a Si/B molar ratio close to 14. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride anions. The crystallization of the boron zeolite beta occurs probably through condensation reactions after hydrolysis of fluoroborate (BF4−), hydroxyfluoroborate (BF3OH− and BF2(OH)2−) and fluorosilicate species. Zeolite beta was characterized by XRD, SEM, chemical analysis, thermal analysis and 11B, 19F and 13C MAS NMR spectroscopy. The latter technique shows that the as-made beta samples contain, beside the dabconium cation, a polymeric compound identified as polyethylene piperazine.
A systematic investigation has been undertaken for tailoring the micropore structure of the pillared clay. Besides the type of metal oxide (e.g. Al2O3 vs. ZrO2) being used as the pillars, the important factors for determining the micropore structure are OH/Al ratio (for Al2O3-pillared clay), calcination temperature and the starting clay. The effect of the cation exchange capacity (CEC) of the clay on the microporous structure (and consequently the adsorption properties) is reported for the first time. Two clays with widely different CECs are used: Arizona montmorillonite (CEC = 1.40 mequiv./g) and Wyoming montmorillonite (CEC = 0.76 mequiv./g). The interlayer spacings of the pillared clays from these different clays are essentially the same, since the interlayer spacing is controlled by the sizes of the oligomers that intercalate between the clay layers. However, the pillar density in the pillared clay is substantially higher with a high CEC in the starting clay, and is shown to be approximately proportional to the CEC. Consequently, the interpillar spacing is substantially lower resulting from the higher CEC. The CH4 adsorption on the pillared clay is nearly doubled by the smaller interpillar spacing, due to the back-to-back overlapping potential in the micropores. The N2 adsorption was not significantly influenced because of its low polarizability (hence low inductive potential). Increasing the calcination temperature of the Al2O3-pillared clay from 400°C to 600°C can decrease the interlayer spacing, but only by 1 (from 8.7 to 7.7 ). The CH4/N2 adsorption ratio of 2.35 is reached on the Al2O3-pillared Arizona clay that is calcined at 600°C. Finally, the surface and pore volume are influenced by the OH/Al ratio (or pH) during pillaring, since this ratio determines the size and charge of the oligomers. A peak surface area is reached at OH/Al = 2.2.
The preparation of microporous silica gels from particulate as opposed to polymeric suspensions of silica using alkylsilicates as precursors is described. The pore structure of this silica gel is determined mainly by the size and the packing geometry of the silica particles. Fast hydrolysis, slow condensation and low solubility all contribute to a high supersaturation level and result in the formation of small particles. Particles formed under acidic conditions are smaller than those obtained under basic reaction conditions due to the slower condensation rate and lower solubility in acid. At the same pH, alkylsilicates having smaller alkyl groups react faster with water leading to smaller primary particles. During the sol to xerogel transformation process, the pH of the sol, together with the time of aging, play important roles in determining the pore structure of the final product.
The shrinkage of the opaque portion of zeolite reaction mixtures during hydrothermal treatment was studied. Experimental results for unstirred, batch zeolite A reaction systems showed that the non-(solid gel) fraction changed during the course of the reaction. The results also showed that the sudden shrinkage of the opaque portion during the reaction did not correlate with the sudden appearance of crystals in the mixture, but was due to crystal growth. Experiments also were performed in which nutrient materials, both with and without a nucleation control agent (triethanolamine), were added to reacting mixtures at various times during reaction. Nutrient additions without nucleation control resulted in only a small increase in size of the zeolite A product. The largest crystal sizes were obtained, however, when the nucleation control agent was present in the original mixture from the beginning of the reaction.
Co(II)saloph (N, N-bis(salicylidene)-1,2-phenylenediamino-cobalt (II)) was encapsulated into VPI-5 and AlPO4-8. Aluminophosphate-hosted cobalt complexes were obtained by impregnating VPI-5 with Co(saloph). Co(saloph) encapsulated into AlPO4-8 was obtained by the structural transformation of VPI-5 occluding Co(saloph) into AlPO4-8. These complexes were characterized by XRD, and IR, 31P NMR and UV-VIS-NIR spectroscopy. The encapsulated complexes do not undergo any significant structural distortions in either case. As the amount of loaded Co(saloph) increases, five-coordinated complexes are formed in VPI-5, but not in AlPO4-8.
A new lamellar mesophase named C16TMA-SnS-Ll was obtained at room temperature in the system C16TMABr SnCl4-Na2S-H2O when the pH of the reactant mixture was 12.0. The transformation of C16TMA-SnS-Ll into C16TMA-SnS-L was observed with the increase of the crystallization time. These two phases were characterized by XRD, SEM, Ar-DTA, TG, 119Sn CPMAS NMR, 13C CPMAS NMR, EDX and bulk chemical analysis. Although the coordination of tint (IV) is tetrahedral in both phases, different environments of tin(IV) are present. The 119Sn shielding tensor of C16TMA-SnS-L is axially symmetric, whereas that of C16TMA-SnS-Ll is asymmetric. A different geometry of the aliphatic chains of CH3(CH2)15N (CH3)3 in both phases was found by 13C CPMAS NMR.
Aluminum, gallium and antimony cations were incorporated into the framework of highly siliceous mordenite to modify the acid strength of Bronsted acid sites by the ‘atom-planting method’ with the corresponding metal chloride vapors at elevated temperatures. The framework incorporation of metal cations was confirmed by IR, MAS NMR and catalytic reactions. The optimum temperature to achieve a maximum incorporation of metal cations into the mordenite framework was demonstrated to be 873 K. Tetrahedral Ga cations exhibited an IR band owing to acidic bridging Si(OH)Ga groups at 3620cm−1 and a characteristic band at 159 ppm in the 71Ga MAS NMR spectrum. The IR band owing to bridging Si(OH)Sb groups was observed at 3663 cm−1. The acid strength of Si(OH)Me (Me = Al, Ga and Sb) groups estimated from their IR frequency, IR spectra of adsorbed pyridine and the activity for toluene disproportionation and cumene alkylation with 2-propanol was in the order Si(OH)Al>Si(OH)Ga> > Si(OH)Sb. The weakest acid sites, Si(OH)Sb, inactive for the toluene disproportionation, showed a significant activity for the cumene alkylation with higher para-selectivity.
Liquid-phase competitive adsorption of three xylene isomers on ZSM-5 zeolites was studied. HZSM-5 zeolites exhibited an extremely high para-selectivity, this para-selectivity decreased with increasing adsorption temperatures. The introduction of Na+ enhanced the para-selectivity. It is apparent that such a high para-selectivity is caused solely by thermodynamic shape selectivity. The complete removal of impurities with small molecular dimensions, such as p-xylene and benzene, from commercial o- and m- xylenes could be attained by the repetition of the shape-selective adsorption on HZSM-5 zeolites.
