Pillaring Species Research Articles

Sulfamic acid well dispersed in the micropores of Al-pillared α-ZrP as efficient heterogeneous catalyst for synthesis of structurally diverse 1,4-dihydropyridines under mild conditions

In this study, α-zirconium phosphate (α-ZrP) has been prepared via reflux method. The α-ZrP material was pillared with [Al13O4(OH)24(H2O)12]7+ polycations to improve its surface area and thermal stability. The aluminum oxyhydroxy clusters were synthesized by in situ partial base hydrolysis of the aluminum salt precursor using sodium hydroxide as base. The microporous inorganic matrix of Al-pillared α-ZrP (AZP) was used for molecular dispersion of sulfamic acid to prepare novel composite materials. The sulfamic acid modified AZP materials were characterized by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) specific surface area analysis. XRD study indicated an expansion in the interlayer space as a result of bilayer intercalation of Al137+ pillaring species. The pillared interlayer space is retained in the composite material. FTIR study indicated the structural integrity of the sulfamic acid. The composite materials exhibited enhanced microporosity with surface area and pore volume in the range 80–120 m2/g and 0.1–0.2 cc/g. FESEM and HRTEM studies indicated morphological reorganization of α-ZrP particles as a result of pillaring and subsequent dispersion of sulfamic acid. Elemental mapping study suggested well dispersion of the sulfamic acid in the composite material without any sign of local agglomeration. The catalytic activity of sulfamic acid loaded AZP materials has been evaluated for the synthesis of 1,4-dihydropyridines by multi component condensation reaction of ethyl acetoacetate, arylaldehydes/chalones and ammonium acetate. The composite materials were highly active for synthesis of structurally diverse 1,4-dihydropyridines in high yield and purity under mild conditions.

Iron aluminium mixed pillared montmorillonite and the rare earth exchanged analogues as efficient catalysts for phenol oxidation

Iron aluminium mixed pillared montmorillonite prepared by partial hydrolysis method was exchanged with lanthanum, cerium and thorium metal salts. The pillared montmorillonite shows considerable increase in basal spacing and surface area. 27Al and 29Si nuclear magnetic resonance spectra show the presence of iron substituted Keggin cation as the pillaring species. Energy dispersive X-ray analysis shows the presence of about 2% rare earth metals. The prepared systems are excellent catalysts for hydroxylation of phenol underlining the use of eco-friendly clay catalysts for effective removal of pollutants. A detailed study of reaction variables suggests that oxidation of phenol increases with temperature and maximum conversion is obtained at 90°C. Hydroquinone selectivity increases with phenol concentration and time but decreases with temperature.

La Complex@Fe–PILM Offering Resilient Option for Efficient and Green Processing toward Epoxidation of Cyclohexene

An overview focused on the synthesis, structural features, and catalytic applications of lanthanum salen complex heterogenized in iron pillared montmorillonite is described here. In this approach the pillaring species was introduced in the interlayers of montmorillonite by a simple cation-exchange mechanism. The material on subsequent calcination resulted in the formation of iron pillared montmorillonite. Consequently, lanthanum salen complex was intercalated into the porous structure of iron pillared montmorillonite. The prima facie evidence of pillaring and complex anchorage inside the pillared clay was established by various spectroscopic and analytical characterization techniques. The heterogeneous catalyst was identified to promote the cyclohexene epoxidation reaction very efficiently with tert-butylhydroperoxide (TBHP) as the oxidant, affording the desired product with a yield up to 95%. Again the reusability of the catalyst and its stability were investigated from its postcatalytic XRD and FT-IR an...

A study on Al and Al–Ce–Fe pillaring species and their catalytic potential as they are supported on a bentonite

Abstract This work describes the synthesis and characterization by XRD of pillaring species in solid state, obtained from Al or Al–Ce–Fe polyhydroxocationic solutions, the modification of a bentonite via pillaring by ionic exchange with such solutions and the catalytic activity of the pillared clays in the phenol oxidation reaction in diluted aqueous medium with hydrogen peroxide. Results indicate the formation of boehmite from the Al 3+ polyhydroxocationic solution and the presence of three different crystalline structures (boehmite, α-Fe 2 O 3 and CeO 2 ) into the solid synthesized from the Al–Ce–Fe polyhydroxocationic solution. The EPR analysis confirms the formation of iron oxide particles and the likely inclusion of isolated Fe 3+ species into the alumina matrix. On the other hand, the clay modified with Al–Ce–Fe is a very active (100% of phenol conversion in the first hour of reaction) and selective (55% of selectivity to CO 2 ) catalyst in the total oxidation of phenol, at 20 °C and atmospheric pressure.

Effect of various pillaring oxides on adsorption behaviour of novel MCM-36 derivatives

For MCM-36 derivatives pillared with various mixed oxides, adsorption isotherms of nitrogen, argon, benzene, ethanol, and water have been measured and discussed. The dimensions, specific surface area, and volume of the mesopores depend on the reagents used for intercalation. Pure SiO 2 pillars mechanically support the expanded layers while other metal oxides also create new adsorption centres in the materials. Two factors influence the adsorption properties in a synergy: the mesopore dimensions and interactions between specific adsorption sites and adsorbates. All the isotherms and pore size distributions show a clear division of the materials into two groups without and with silica in the pillars. The former group shows a higher stability. The latter one exhibits distinctly higher sorption capacities at smaller mesopore sizes and broader distributions of sizes due to more complex pillaring species. The measurements revealed a tendency for an increase in sorption capacities (with some exceptions and excluding ethanol): no pillars (MCM-22) < single < two < three metal oxide composite pillars, although the reasons for this effect remain unclear.

Models of (ZrO2)n Complexes Intercalated into Montmorillonite

The article discusses the properties of several model zirconium dioxide complexes (ZrO(2))(n)() intercalated into the interlayer space of montmorillonite clay. Grand canonical Monte Carlo simulation was used in a series of numerical experiments during analysis of the low-temperature nitrogen adsorption in the micropores thus generated. The goal of such experiments was to determine the geometrical parameters of introduced molecular complexes of different types inside micropores of various widths. The obtained information was used to characterize textural and structural properties of three pillared interlayer materials prepared by using pillaring species synthesized via aging of zirconyl chloride solutions containing as additives chlorides of Ca, Sr, or Ba. It was found that in the cases of Ba and Ca the interlayer micropores are filled with isolated tetramers (ZrO(2))(4). Meanwhile, the presence of Sr in the pillaring solution, most likely, favors the preservation of larger sheetlike complexes (ZrO(2))(8).

Pillaring of Layered Perovskites, K1−xLaxCa2−xNb3O10, with Nanosized Fe2O3 Particles

Nanosized Fe2O3 clusters are pillared in the interlayer spaces of layered perovskites, H1−xLaxCa2−xNb3O10 (0≤x≤0.75) by a guest-exchange reaction using the trinuclear acetato-hydroxo iron cation, [Fe3(OCOCH3)7 OH·2H2O]+. The interlayer spaces of niobate layers are pre-expanded with n-butylammonium cations (n-C4H9NH+3), which are subsequently replaced by bulky iron pillaring species to form Fe(III) complex intercalated layer niobates. Upon heating at 380°C, the interlayered acetato-hydroxo iron complexes are converted into Fe2O3 nanoclusters with a thickness of ca. 3.5 Å irrespective of the interlayer charge density (x). The band-gap energy of the Fe2O3 pillars (Eg∼2.25 eV) is slightly larger than that of bulk Fe2O3 (Eg∼2.20 eV) but is smaller than that expected for such a small-sized semiconductor, which can be assigned to the pancake-shaped Fe2O3 pillars of 3.5 Å in height with comparatively large lateral dimension. X-ray absorption spectroscopic measurements at the Fe K-edge are carried out in order to obtain structural information on the Fe2O3 pillars stabilized between the niobate layers. XANES analysis reveals that the interlayer FeO6 octahedra have coordination environments similar to that of bulk α-Fe2O3, but noncentrosymmetric distortion of interlayered FeO6 is enhanced due to the asymmetric electric potential exerted by the negatively charged niobate layers. Scanning electron microscopic observation and nitrogen adsorption–desorption isotherm measurement suggest that the pillared derivatives are nanoporous materials with the highest BET specific surface area of ca. 116 m2/g.

Characterization and acidic properties of silica pillared titanates

Reaction of alkylammonium titanates with tetraethylorthosilicate (TEOS) resulted in the intercalation and polymerization of TEOS between the titanate sheets. After heating to 500 °C, the basal spacing of pillared silica titanates increased with increasing length of the alkylamine chains and the amount of silicon intercalated. Although the layers of pillared titanates were expanded by the silica species, and the specific surface area increased, only negligible micropores were formed, suggesting that the interlayer region was “stuffed” with the pillaring species. The adsorption of pyridine indicated the formation of strong Brönsted acid sites up to 300 °C. In addition some basic Lewis sites were detected on silica pillared titanates.

Structure of Zirconia Nanoparticles used for Pillaring of Clay

ABSTRACTSAXS and EXAFS were applied to study genesis of polynuclear zirconium hydroxyspecies in pillaring solutions as dependent upon the zirconium concentration, addition of alkaline-earth chlorides and aging. After the montmorillonite clay pillaring, the structure of zirconium nanopillars was characterized by applying X-ray structural analysis, UV-Vis, FTIRS of adsorbed CO and nitrogen adsorption isotherms. Main pillaring species appear to be nanorods comprised of several Zr4 tetramers. Basic structural features of the tetramers are preserved in zirconia nanoparticles fixed between alumosilicate layers in pillared clays. In calcined samples, those nanoparticles contain only bridging hydroxyls and/or oxygen anions responsible for bonding within pillars and between pillars and clay sheets.

Factors affecting the preparation of alumina pillared montmorillonite employing ultrasonics

In the preparation of alumina pillared clays, the intercalation step is shown to be completed within a few minutes using ultrasonics, and such materials show enhanced textural properties and improved thermal stability. The role of ultrasonics and various preparative parameters that govern the preparation of alumina pillared montmorillonite are presented. This includes the effect of exchangeable cations (Na, Ca, La), three different pillaring precursors, the concentration of pillaring species, and the concentration of the clay in suspension. It is shown that the exchangeable cations and their mode of binding influence the diffusional rates of the pillaring species. When different pillaring precursors are employed, not much variation in the textural properties of pillared clays is noticed. However, their optimum concentration is essential. It is also observed that this method can handle a concentrated clay suspension for pillaring with alumina. From the kinetics of calcium exchange experiments during pillaring, it was observed that the exchange under conventional conditions is completed instantaneously, while under ultrasonic treatment only 26% of Ca 2+ exchange took place instantaneously and further exchange occurred only after ultrasonic treatment. From these observations, it is proposed that the role of ultrasonics in the present synthesis is the acceleration of diffusion of the intercalating aluminium pillaring species followed by an ion exchange process.

Theoretical evaluation of the microporosity of pillared layered double hydroxides

Over the last ten years, the concept of pillaring has frequently been applied on layered double hydroxides (LDHs). Due to the variety of possible anionic pillaring species and the adjustable layer charge density, LDHs offer good perspectives with regard to the creation of porous adsorbents and catalysts. But despite these possibilities, their porosity properties can still not compete with those of industrially applicable materials like zeolites. In this study, theoretical calculations based on geometrical models and performed on both Fe(CN)6-MgAl-LDHs (A) and [PV2W10 O40]-ZnAl-LDHs (B) were reported. Properties such as the micropore volume and the interpillar distance were calculated, and compared to experimental data. For a M(II)/M(III) ratio in the layers of 3, the theoretical maximum micropore volumes were 0.3843 cm3/g (A) and 0.1497 cm3/g (B), respectively. By implementing parameters like the stack size, pillars on the outside of the stacks and the possibility of collapse, the model was adjusted in order to create a realistic picture of the microstructure of pillared LDHs. This led to a better understanding of the limiting factors, and gave an explanation for the relatively low micropore volumes of pillared LDHs. For the Fe(CN)6-MgAl-LDHs, small interpillar distances were responsible for the partial inaccessibility of the interlayer regions by N2. This effect was the most pronounced for high charge density LDHs. The situation for the [PV2W10O40]-ZnAl-LDHs is more complex. Probably due to an incomplete pillaring process, the theoretical maximum values are not reached.

Systematic Comparison of a Saponite Clay Pillared with Al and Zr Metal Oxides

Alumina and zirconia pillared Ballarat saponites have been prepared by cation exchange of the saponite in aluminum chlorhydrate and zirconyl chloride solutions, respectively. The nature of the pillaring species and the method of preparation have been varied in order to examine their effect on the amount of occluded oxide, as well as the thermal stability, surface area, and acidic properties of the resulting pillared clays. Higher reaction (cation exchange) temperature (80 °C) enhances the degree of polymerization, increases the mobility of the pillaring species, and results in greater incorporation between the sheets of the saponite. In general, it is observed that the alumina-pillared saponites have a higher thermal stability than their zirconia-pillared analogues and retain an ordered layered structure even after calcination at 700 °C. Infrared spectroscopy of adsorbed pyridine suggests the presence of Bronsted acid sites and two types of Lewis acid sites (of different strength) for the alumina-pillared materials. Only single types of Bronsted and Lewis acid sites, however, were detected for the zirconia-pillared saponites. The thermal desorption of cyclohexylamine and dehydration of pentan-1-ol were used as further probes to monitor the acidity of the different pillared saponites.

The relation between the synthesis of pillared clays and their resulting porosity

Clays can be converted to stable microporous materials by the incorporation of pillars. The type of pillars determine to a large extent the porosity features of the pillared clays. Different pillaring species (Al, Ti, Zr, Fe) were investigated and discussed in terms of pore volume, pore size and pore size distribution. The porosity induced by pillaring can be further modified by small modifications during the synthesis. More meso- and macroporosity can be achieved by choosing clays with smaller clay layer sizes (laponite compared to montmorillonite) or by the way of drying (air or freeze drying). Incorporation of Zr and Cr in the Fe-pillars creates mixed oxide pillared clays and pillared clays with new properties were obtained. The adsorption strength strongly increased resulting in higher adsorption capacities as demonstrated for gases (O 2, N 2 , CO 2, CO and CH 4) and chlorinated hydrocarbons. Pre-adsorption of amines effectively reduces the pillar density (Fe-PILL) and distributes the pillars more homogeneously between the layers (Al- and Ti-PILC) creating higher pore volumes and higher adsorption strength. Amines were also useful for the orientation of laponite clay layers to more regularly ordered face-to-face stacked pillared laponite clays which exhibit significant increased pore volumes.

Controlling the chemistry of the micropore volume in pillared clays and micas

Pillared clays are microporous materials formed by propping apart clay layers with robust inorganic polyoxocations. The chemistry of the micropore space can be tailored by choosing suitable pillaring species, by adding various functionality to the pillar surfaces, and by incorporating small metal particles within the micropores. We have found that by using a commercially available zirconyl acetate solution as the zirconia polyoxocation precursor, zirconia-pillared micas with superior properties of crystallinity and microporosity can be produced. Catalytic tests have shown that treatment of a zirconia-pillared montmorillonite with sulfate increases the acid site strength and density of the zirconia pillars. In addition to materials with enhanced acidity, it is desirable to produce materials with little or no acidity for use as metal supports for catalysts in applications such as light alkane dehydrogenation, where support acidity leads to undesirable side reactions. Tetrasilicic fluoromica can be pillared by silsesquioxane oligimers derived from the in situ hydrolysis of an aminosiloxane reagent. After a two step calcination, the silica-pillared fluoromica produced has a high surface area and crystallinity, and the combination of the inert fluoromica layers with the non-acidic silica pillars makes this new material an interesting nonacidic support for noble metal catalysts.

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 A), compared to the Zr-PILC, which had the smallest spacing between the layers (max, 4 A). 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.

Characterization of acidity of pillared clays by proton affinity distribution and DRIFT spectroscopy

Diffuse reflectance FTIR (DRIFT) spectra and potentiometric titration data have been obtained for montmorillonite samples pillared with different polycations. The IR spectra are interpreted after deconvolution into Voigt band profiles. The titration data are analysed using a stable numerical method for the solution of the adsorption integral equation (SAIEUS). The pK distributions obtained are used to characterize the acidic properties of the modified minerals to demonstrate the relationship between these properties and the coordination of cations in the pillaring species. Heat treatment enhances the acidity of the materials which may be the result of the rearrangement of pillars (dehydroxylation). The approach presented enables one to assess both qualitatively and quantitatively the number and strength of acidic sites present on the surface of pillared clays.