Catalytic characterization of pillared clays through toluene methylation reaction

Pillared clays (PILCs) with Al and Zr oxide pillars were studied in terms of their structure and texture. The catalytic properties of the samples were evaluated through toluene methylation reaction. As comparison material, a commercial zeolite HZSM-5 was used. The toluene conversion at short time-on-stream over PILCs, although smaller than the value obtained with HZSM-5, attains values of 37 (molar %). In all cases the reaction products are a mixture of xylene, ethyltoluene and trimethylbenzene isomers. All samples present higher selectivity to xylene isomers, which are the primary products. For Zr pillared clay, the percentage of these isomers is the highest.     

Characterization of cationic pillared layer clays

Na-montmorillonite and its Al-, Ti-, Fe-, Cr-, Zr- and Sn-pillared derivatives (pillared layer clays) were prepared and characterized by instrumental methods and a chemical reaction. Structural characterization involved X-ray fluorescence (XRF) analysis, X-ray diffractometry (XRD),²⁹Si and²⁷Al MAS NMR spectroscopies, thermogravimetric (TG) and BET measurements. By the distinctly characteristic shapes of the d₀₀₁ vs. temperature curves two groups could be distinguished, (i) Al-PILC, Ti-PILC, Cr-PILC, Zr-PILC and (ii) Fe-PILC, Sn-PILC, Na-montmorillonite. On montmorillonite isomerization of cyclopropane only occurred while oligomerization followed isomerization over Al-PILC. This latter reaction was attributed to Lewis and (the considerably smaller number of) Br?nsted acid sites whose number, strength and accessibility increased due to pillaring.     

Realisation of truly microporous pillared clays

When pillaring a well crystalline synthetic hectorite using molecular pillars, we obtained a truly microporous material for the first time that displays long range order of the pillars and consequently a narrow pore size distribution.     

X-ray diffraction study of zirconia pillared clays

X-ray powder diffraction (XRPD) and X-ray radial electronic distribution density (RED) of initial and zirconia-pillared interlayered clays (Zr-PILC) were studied. After pillaring, the basal (001) spacing was found to increase from 11 Å in the initial clay kept under air to 17.7 Å in Zr-PILC. The structure of zirconium nanopillars was characterized. The interatomic distances with corresponding coordination numbers obtained from the RED curves were close to those in zirconium tetramers.This revised version was published online in December 2005 with corrections to the Cover Date.     

Adsorption and characteristics of base-treated pillared clays

The effect of base treatment on the cation exchange capacity (CEC) of pillared clays and their adsorption isotherms for Cu²⁺, Cr³⁺ and Pb²⁺ have been investigated. Results indicate that although the CEC of pillared clays are only about 15% of that of the parent clays, a large fraction of the native clays CEC may be recovered by treatment with base. The fraction of the CEC recovered depends upon the base strength, its concentration, and the temperature. Contrary to previous suggestions the mechanism of recovery is related to the destruction of pillars which is accompanied by the loss of surface area. It is possible under conditions specified to prepare these base treated pillared clays as a new class of useful, regenerable adsorbent for heavy metal adsorption from aqueous solution.     

Synthesis,characterization, and challenges faced during the preparation of zirconium pillared clays

In this study, Zr-pillared montmorillonite clays (Zr-PILCs) were synthesized using two different precursor materials: raw montmorillonite (CM) and sodium ion-saturated montmorillonite (Na-CM) at different Zr/clay ratios (2.5, 5 and 10 mmol/g). To study the effect of Zr concentration and clay pre-treatment with NaCl on pillaring, the modified clay samples were characterized in detail using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (STEM-EDX). The XRD analysis showed the increase of basal spacing of Zr-PILCs prepared from both precursor materials: from 1.26 to 1.74 nm in the case of CM, and from 1.13 to 1.93 nm for Na-CM. Results from FT-IR revealed new bands ascribed to Zr-O bonds in the range of 400–500 cm^?1 in Zr-pillared samples obtained from Na-CM at Zr/clay ratios of 2.5 and 5 mmol/g. The distribution and nature of Zr species in between the silicate layers were studied using STEM-EDX and HAADF imaging. They were found to be separated by a distance of 1.5–3 nm and their thickness lies in the range of 1–2 nm. Pillared clays prepared from pre-treatment with NaCl were more thermally stable at higher temperatures.     

Controlled gas adsorption properties of various pillared clays

Microporous pillared clays (PILC) were prepared by the intercalation of montmorillonite with particles of titania (Ti-PILC), zirconia (Zr-PILC), alumina (Al-PILC), iron oxide (Fe-PILC) and mixed lanthania/alumina (LaAl-PILC). Nitrogen adsorption isotherms (77 K) and XRD data provided information on the porosity, surface area, micropore volume and interlayer distance of these samples. The surface area varied between 198 and 266 m²/g for Ti- and Fe-PILC, respectively. The titania pillared clay had also the highest micropore volume (0.142 cc/g) and interlayer spacing (16–20 Å), compared to the Zr-PILC, which had the smallest spacing between the layers (max, 4 Å). Despite this fact, Zr-PILC always showed a high adsorption capacity for gases such as N₂, O₂, Ar or CO₂, due to its high adsorption field in the very small micropores.From gas adsorption experiments on these various PILCs, it became clear that their adsorption properties depend on the pillars in three ways: (i) the pillar height, (ii) the distribution of the pillars between the clay layers and (iii) the nature of the pillaring species.The incorporation of other elements in the pillars leads to specific adsorption sites in the pores. This was demonstrated by the preparation of mixed Fe/Cr and Fe/Zr pillared clays. Compared to the parent Fe-PILC, the incorporation of chromium and zirconium in the iron oxide pillars had a positive influence on the adsorption capacity. Also the modification of a PILC with cations increases both capacity and selectivity for gases. This was confirmed by the increased adsorption of N₂, O₂ and CO₂ at 273 K on a Sr²⁺ exchanged Al-PILC.     

Depollution of uranyl polluted waters using pillared clays

This paper reports on the use of clays pillared with copper polyhydroxycations in the depollution of waters polluted with radioactive elements. The pillared clays have very good surface properties which recommend them for use in cationic depollution. The article includes a kinetic study on the depollution of radioactive polluted waters, studying the influence of temperature on the process. The pillared clays are prepared from Romanian clay originally from Valea Chioarului deposits; the experiments have used an autochthonous material in an unconventional performant depollution technology.     

Arsenic adsorption onto pillared clays and iron oxides

Arsenic adsorption was carried out on simple materials such as goethite and amorphous iron hydroxide, and more complex matrices such as clay pillared with titanium(IV), iron(III), and aluminum(III). These matrices were synthesized from a bentonite whose montmorillonitic fraction was pillared according to optimized parameters. These sorbents were characterized by various methods: XRD, FTIR, BET, DTA/TGA, surface acidity, and zetametry. Elimination of arsenite and arsenate as a function of pH was studied. Arsenate elimination was favored at acidic pH, whereas optimal arsenite elimination was obtained at 4相似文献   

Characterisation of pillared clays by contrast-matching small-angle neutron scattering

The contrast-matching SANS technique has been utilised to determine inter-pillar distances (and surface texture) in montmorillonite and beidellite pillared smectite clays; they lie in the range 1.40-1.80 nm, reflecting different inter-pillar orderings.     

Uptake of cesium and strontium radioisotopes onto pillared clays

Pillared clays were characterised by thermal analysis and small-angle X-ray scattering. They were then examined for their ability to take up¹³⁷Cs and⁹⁰Sr/⁹⁰Y isotopes as a function of concentration and competing cations (Na⁺, K⁺, NH₄ ⁺, Ca²⁺, Mg²⁺) in the concentration range 10⁻¹ to 10⁻⁴ M. The radioisotope uptakes were quantified byK _d (ml/g) measurements.     

Propene adsorption and reaction on zeolites and pillared clays

Comparative IR and UV-Vis spectroscopic studies of propene adsorption and reaction on H-mordenite, dealuminated H-mordenite, dealuminated mazzite, montmorillonite and Al₁₃-pillared montmorillonite have been carried out. On all systems propene is first transformed into polymeric species (C_nH_2n+1 ⁺). On HMOR (both as such and dealuminated), allylic carbocations are successively produced by loss of H₂, the monoenic species (C_nH_2n?1 ⁺) being formed at room temperature and the dienic (C_nH_2n?3 ⁺) and trienic (C_nH_2n?5 ⁺) species at higher temperatures. These species are not observed on the other systems, although they are presumably formed as unstable intermediates. In fact, on all the zeolites studied here two cyclic penta-atomic and hexa-atomic allylic carbocations have been observed for the first time. On all systems, the final products of reaction are polyaromatic species which, on the basis of their reaction with NH₃ still exhibit unsaturated carbocation behaviour. The activity of the various samples depends on their pore dimensions and on the nature of acidic sites involved: the larger the available pore space, the more branched is the polymer and the more difficult it is to observe allylic carbocations. Evidence is provided for a Brønsted-induced mechanism.     

Tailoring micropore dimensions in pillared clays for enhanced gas adsorption

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.     

Development of novel pillared clays for the encapsulation of inorganic complexes

Several pillared clays were prepared by using a polyalcohol (ethylene glycol or poly(vinyl alcohol)) or a poly(ethylene oxide) surfactant as an interlayer gallery template and an aluminum oligomer species as the pillaring agent. The use of polyalcohols or nonionic surfactants, such as Tergitol, gave materials which, in general, presented larger basal spacing than those found for the solids prepared by a similar procedure but without additives. The initial positive effect in the expansion of the clay interlayers was not totally retained after calcination of the materials; most probably, at the end, the basal spacing is still ruled by the intercalating aluminum species. The pillared clay with the largest basal spacing and specific surface area was used to encapsulate copper(II) complexes with pentadentate N3O2 Schiff base ligands derived from copper(II) acetylacetonate by in situ synthesis. The characterization made (X-ray diffraction, X-ray photoelectron spectroscopy, FTIR spectroscopy, chemical analysis, and low-temperature N2 adsorption) provided evidence that copper(II) complexes with pentadentate N3O2 Schiff base ligands were efficiently entrapped within the lower dimension pores of the pillared clay and that they interact strongly with the pillared clay matrix.     

Selective formation of cumene on pillared clays by isopropylation of benzene

Summary Cumene is prepared selectively through an eco-friendly route using naturally available montmorillonite clay as the green catalyst. Pillaring of montmorillonite is done to improve the structural stability for long-term use. Alumina pillaring and the influence of mixed pillaring with zirconia and chromia on the structural as well as textural properties are characterized. Montmorillonite shows very high conversion, selectivity and regenerability upon pillaring towards isopropylation of benzene.     

Sorption of basic dyes onto granulated pillared clays: thermodynamic and kinetic studies

Effect of the granulation process onto the thermodynamic and kinetic sorption parameters of two basic dyes (Basic Yellow 28-BY 28 and Basic Green 4-BG 4) was evaluated in the present work. The charge surface properties of the surfactant-modified aluminium-pillared clay (CTAB-Al-Mont-PILC) particles were not modified, and the isoelectric point remains constant after high shear wet granulation. The Gibbs free energy of both BY 28 and BG 4 sorption was negative and decreased with the granulation; the endothermic nature of the sorption process was confirmed by the positive values of ΔH°. Adsorption kinetics of the two dyes, studied at pH 6 and 150 mg L(-1), follow the pseudo-first order kinetic model with observed rate constants of 2.5-4.2×10(-2) min(-1). The intraparticle diffusion model, proposed by Weber and Morris, was applied, and the intraparticle plots revealed three distinct sections representing external mass transfer, intraparticle diffusion and adsorption/desorption equilibrium. Diffusion coefficients, calculated from the Boyd kinetic equation, increased with the granulation and the particle size. Pseudo-first order kinetic constants, intraparticle diffusion rate constants and diffusion coefficients were determined for two other initial concentrations (50 and 100 mg L(-1)) and include in a statistical study to evaluate the impact of granulation and initial concentration on the kinetic parameters. Kruskal-Wallis tests, Spearman's rank order correlation and factor analysis revealed a correlation between (i) the diffusion coefficients and granulation, and between (ii) the intraparticle diffusion rate constants and initial concentration.     

Characterization of a delaminated clay and pillared clays by adsorption of probe molecules

Characterization of the textural and structural properties of a sodium form of a delaminated calcic montmorillonite, and of aluminium pillared materials prepared with and without amine pre-adsorption, was made using the adsorption of different probe molecules (nitrogen, toluene, methyl ethyl ketone and 1,1,1-trichloroethane). Due to the delaminated character of the prepared solids, the characterization by X-ray diffraction of the pillared materials was not possible. In this context, the adsorption of probe molecules revealed to be informative since, although the prepared materials were mainly mesoporous solids in consequence of their delaminated nature, when the amine pre-adsorption was used before the pillaring, microporosity was also formed.     

Spectroscopic studies on organic molecules intercalated into clays

An overview of clay structures and characterization methods is followed by a discussion of some adsorption reactions of organic molecules which have been studied spectroscopically. These can be divided into two main classes: adsorption with and without reaction.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August 1995.     

Spectroscopic characterization of some metal complexes derived from 4-acetylpyridine nicotinoylhydrazone

Coordination compounds of Mn^II, Cu^II, Fe^III and Zn^II ions with 4-acetylpyridine nicotinoylhydrazone (4-APNH) were synthesized and characterized by elemental analyses, molar conductivity, magnetic moments, i.r., u.v./vis., m.s., ¹H-n.m.r. and thermal analyses. I.r. spectra show that the ligand can act either in the enol form as monovalent bidentate or in the keto form as neutral bidentate depending on the metal salt used. Octahedral structures are proposed for Fe complex and square – planar for the Cu complex, while tetrahedral structures were suggested for Zn and Mn complexes on the basis of magnetic and spectral evidences. Semi-empirical calculations ZINDO/1 have been used to study the molecular geometry and the harmonic vibrational spectra with the purpose to assist the experimental assignment of the complexes. In memory of the late Professor Dr. Abdel Hamid M. Shallaby 2/5/2006     

Catalytic activity and acidity of Al pillared clays and zeolites in different hydrocarbon reactions

In comparison with zeolites, a variety of Al pillared clays did not prove as promising cracking catalysts, possibly because their Br?nsted sites are hidden. In the less demanding double bond shift isomerization, both catalyst types were comparable probably due to the presence of Lewis sites on both catalysts.