Faujasite Framework Research Articles

Faujasite catalysis of aromatic nitrations with dinitrogen pentoxide. The effect of aluminium content on catalytic activity and regioselectivity. The nitration of pyrazole †

The reaction of 1-chloro-2-nitrobenzene with dinitrogen pentoxide in dichloromethane catalysed by H-faujasite zeolites F-712, F-720, F-780 and F-901, giving 1-chloro-2,4-dinitro- and 1-chloro-2,6-dinitrobenzene in approximately 30∶1 ratio, is a kinetically first-order process. First-order rate constants are independent of the concentration of N2O5 and proportional to the mass of catalyst used. Specific rate constants, obtained by dividing the first-order rate constant by the mass of faujasite, are constant for a given faujasite. Amongst the faujasites, they increase in approximate proportion to the aluminium content. A mechanism is proposed in which the protons in the faujasite, near aluminium in the faujasite framework, are replaced by nitronium ions derived from N2O5 in a fast pre-equilibrium process. This produces active sites for transfer of nitronium ion from faujasite to aromatic in the rate-determining step. The similar reactions of 2-nitrotoluene, too fast for kinetic study, reveal that the ratio of 2,4- to 2,6-dinitrotoluene in the product increases with the aluminium content of the faujasite.Nitration of nitrobenzene is also catalysed by faujasite. Relative reactivities of nitrobenzene, 1-chloro-2-nitro- and 1-chloro-4-nitrobenzene are compared to those found in mixed-acid nitration.Pyrazole can be nitrated readily with N2O5 over faujasites, yielding 1,4-dinitropyrazole in 80% yield under mild conditions. p

Structure and Bonding of a Site-Isolated Transition Metal Complex: Rhodium Dicarbonyl in Highly Dealuminated Zeolite Y

The structure and bonding of rhodium dicarbonyl bonded to highly dealuminated zeolite Y has been determined by the combined application of extended X-ray absorption fine structure (EXAFS) and infrared spectroscopies and quantum chemical calculations based on density functional theory. The EXAFS and infrared spectra indicate the existence of nearly unique rhodium dicarbonyl species bonded at structurally equivalent positions in the zeolite pores. However, even this anchored structure, one of the simplest known, is not determined fully by the experimental results, and quantum chemical calculations were needed to eliminate the ambiguity. Taken together, the experimental and theoretical results indicate Rh+(CO)2 located at a four-ring of the faujasite framework; the rhodium center is bonded to two oxygen centers of the framework near an aluminum center with a Rh−O distance of 2.15−2.20 A. The results show how spectroscopy and theory used in combination can determine the structure and location of a metal compl...

Selectivity of large pore zeolites in the alkylation of naphthalene with tert-butyl alcohol: analysis of experimental results by computational modelling

Liquid phase alkylation of naphthalene with tert-butanol has been studied using HY and H-beta zeolites with varying silicon to aluminium ratios. Both series of zeolites underwent efficient activities and high selectivities for the mono- and di-( tert-butyl) derivatives. In all cases, 2-( tert-butyl)naphthalene (2-TBN) was the only monoalkylated product (100%,β-selectivity). Over H-beta zeolites, 2-TBN was obtained as the main product with relatively small amounts of dialkylated compounds. Over HY samples, a 2,6-di( tert-butyl)naphthalene (2,6-DTBN) selectivity up to 84% was obtained, with a 2,6-DTBN/2,7-DTBN ratio of 5.6–5.9 and β,β′-selectivity (2,6-+2,7-) of 98–99%. This constitutes the first published observation of such high β,β′ selectivity and 2,6-/2,7- ratio in the liquid phase alkylation of naphthalene. In order to understand such a differentiation in the formation of the two di-( tert-butyl)naphthalenes over the HY zeolites, computational analysis of both energies and molecular dimensions of these derivatives and of the monoalkylated counterparts has been performed, using quantum mechanics (AM1 and PM3) and molecular mechanics (MM +) methods. The energy calculations are in agreement with the experimental results regarding the selective formation of the β and β,β′ isomers. The determination of the kinetic diameters shows that, in their most stable conformation, 2,6-DTBN has a smaller kinetic diameter (7.1 Å) than 2,7-DTBN (7.5 Å), which may explain the selective formation of the 2,6-isomer against the 2,7- within the faujasite framework.

Gallioaluminosilicate Molecular Sieves with the Faujasite Structure

Gallioaluminosilicate hydrogels with composition 0.1 Ga2O3:0.9 Al2O3:10 SiO2:4.0 Na2O:150 H2O were prepared using sodium aluminate, colloidal silica, and a soluble galliosilicate with composition Ga2O3:15 SiO2:10 Na2O:400 H2O as a source of gallia. The hydrothermal transformation of these gels at 95°C yields crystals with the faujasite structure when the crystallization reaction is performed in the absence of stirring and with the gmelinite structure (with minor amount of Pt) when crystallization is completed with stirring of the hydrogel. The crystallization time for Na(Ga,Al)Y can be decreased to 15 h from 48 h by raising the crystallization temperature to 110°C from 95°C. The calcined Na(Ga,Al)Y crystals have surface area in the 600 to 660 m2/g range and, unless La-stabilized, suffer partial lattice degradation when, following an exchange with an NH4NO3 solution, the NH4(Ga,Al)Y crystals are calcined at 500°C in air. This finding is clearly indicated by 27Al, 71Ga, and 29Si MAS NMR spectroscopic results. The 29Si NMR spectra of samples exchanged and calcined in the absence of La are poorly resolved and their quantitative analysis indicates substantial loss of Al and Ga from the faujasite framework. Consistent with this interpretation, 27Al MAS NMR spectra reveal large amounts of octahedrally coordinated species in H(Ga,Al)Y. Moreover, the 71Ga spectra show no evidence of tetrahedrally coordinated framework Ga. In contrast, the multinuclear NMR data of the H(Ga,Al,La)Y crystals indicate only minor Al and Ga losses from the framework, thus revealing the stabilizing influence of La. Thermodesorption of pyridine and microcalorimetry results with NH3 have indicated that the La-stabilized H(Ga,Al,La)Y crystals have a more moderate acidity than the reference HY sample. Model fluid cracking catalyst (FCC)-containing H(Ga,Al,La)Y crystals are more active when cracking gas oil under microactivity testing conditions than similarly prepared FCC-containing HY-type zeolites and a reference equilibrium catalyst containing a rare-earth-stabilized HY, indicating that these crystals should be considered as acidic components in commercial FCC preparations.

Galliosilicate molecular sieves with the faujasite structure

The hydrothermal transformation at 95°C of galliosilicate hydrogels with molar oxide composition Ga 2O 3 : 10SiO 2 : 4.5 Na 2O : 150 H 2O, prepared from colloidal silica and sodium gallate mixtures, yields crystals with the faujasite or natrolite structure if the crystallization reaction is performed in the absence or presence of stirring. Raising the crystallization temperature to 150°C causes the formation of analcime crystals. Moreover, when 10% of the hydrogel Ga 2O 3 content is added in the form of a seeding solution with composition Ga 2O 3 : 15SiO 2 : 10 Na 2O : 400 H 2O, the crystallization time for GaY formation decreases by about 50% and Ga incorporation in the crystals increases. In either case, the crystallization reaction is not stoichiometric and the crystals' silica to gallia molar ratio (SGR) is always less then that in the parent hydrogel. The calcined GaY crystals have surface areas in the 500–550 m 2 g −1 range and, for a given hydrogel composition, the SGR decreases to 3.7 from 4.4 when a seeding solution is used. The thermal stability of GaY can be drastically increased by the introduction of rare earth ions such as La. In fact, 71Ga MAS NMR spectra have shown that in both GaY and H(Ga,La)Y, no extraframework Ga(VI) species are present. In addition, 29Si NMR spectra yield an SGR molar ratio in agreement with chemical analysis. FTIR results from pyridine thermodesorption, together with microcalorimetry experiments with NH 3, indicate that the isomorphous substitution of Al with Ga in the faujasite framework reduces acid site strength, density and distribution.

Faujasite-Supported Ir4Clusters: A Density Functional Model Study of Metal−Zeolite Interactions

The interaction of a metal cluster, Ir4, and a zeolite support was investigated computationally with the aid of a density functional method and a cluster model of a zeolite, i.e., a six-ring consisting of six O atoms and six T (Si or Al) atoms facing a supercage of a faujasite framework. Structural parameters are reported for an Ir4 tetrahedron interacting with the zeolite six-ring. The calculations indicate two Ir−O distances, which match those reported on the basis of EXAFS spectroscopy at about 2.1−2.2 and 2.5−2.7 A for various transition and noble metal clusters on zeolite (and metal oxide) supports, including Ir4 in the supercages of zeolite NaY. The calculations indicate an Ir−Ir distance of about 2.5 A, only a few hundredths of an Angstrom more than the value calculated for the free Ir4 cluster, but about 0.2 A less than the values observed repeatedly by EXAFS spectroscopy for zeolite-supported clusters approximated as Ir4. The experimental distances characterizing the zeolite-supported clusters ar...

Heterogeneity of Brønsted Acidic Sites in Faujasite Type Zeolites due to Aluminum Content and Framework Structure

The effect of Al for Si substitutions in the tetrahedral sites of the faujasite framework with low and high aluminium content on the properties of Brønsted sites is investigated by computational techniques. A combined quantum mechanics−interatomic potential functions approach (QM-Pot.) is applied which uses periodic boundary conditions and treats the whole zeolite structure. Both the Hartree−Fock and the density functional (B3LYP) methods are employed. Energies of deprotonation, O−H stretching harmonic frequencies, and 1H NMR chemical shifts are calculated. In the first approximation, the acidity of Brønsted sites measured by the deprotonation energy is determined by the number of nearest neighbor Al atoms of the Si atom in the Al−O(H)−Si bridge, while the O−H stretching frequency and 1H chemical shift depend on their crystallographic position. Beyond this, the 1H NMR chemical shifts show a strong dependence on the overall lattice composition. There is no correlation between 1H NMR chemical shifts and deprotonation energies; however, a linear relation between 1H NMR chemical shifts and OH vibrational frequencies is supported.

Crystal growth of zincophosphates from conventional media and reverse micelles: mechanistic implications

Zincophosphates can be synthesized with frameworks analogous to aluminosilicate zeolites. Previous studies have shown that reverse micelles, which are microdroplets of water dispersed in an oil-like medium, can serve as nucleation and growth centers for microporous materials. Zincophosphates with the sodalite framework can be readily made from reverse micelles, whereas the more open structure resembling the faujasite framework (ZnPO-X) could not be made. The concentration of sodium ions is found to play an important role in the nucleation of both sodalite and ZnPO-X. At increased sodium ion concentrations under conventional synthesis conditions, sodalite growth is favored. On the other hand, the addition of crown ethers and cryptands which complex sodium ions leads to growth of ZnPO-X. Based on these studies, we propose that the high sodium ion concentrations in reverse micelles cause the formation of sodalite. Infrared spectroscopic studies indicate that zinc- and phosphate-containing reverse micelles also contain significant amounts of disordered water, which can inhibit the nucleation of open-pore frameworks, such as ZnPO-X.

Zeolite ZnY catalysts prepared by solid-state ion exchange

Zeolites ZnY with various overall zinc contents were prepared from mixtures of zeolite NH4Y and crystalline zinc chloride by solid-state ion exchange. The obtained materials were investigated with carbon monoxide, xenon, and nitrogen adsorption as well as with 129Xe NMR and XRF spectroscopy. From the results of these measurements, the zinc cation distributions between the different types of cages of the faujasite framework as well as between the crystallographic positions SIII and SII within the large voids (supercages) were quantitatively determined. The concentrations of zinc cations in the supercages of the presently prepared zeolites are considerably higher than in materials obtained from NaY by conventional wet ion exchange using aqueous zinc salt solutions. Experimental evidence is provided for salt inclusion under certain conditions of preparation.

Interaction of phosphorus trichloride with zeolites

The paper presents a detailed 27Al, 29Si, and 31P n.m.r. and i.r. study of the interaction of zeolites with PCl3. The faujasite framework collapses upon the PCl3 treatment, whereas the mordenite framework is more stable. Phosphorus can be incorporated into zeolites as surface phosphite or phosphate groups but cannot be introduced in a tetrahedral framework position. HM-MOR was dealuminated with PCl3 in one step, whereas the HY-FAU was first dealuminated with SiCl4 followed by a COCl2 treatment, and finally dealumination was completed with PCl3. The HM-MOR sample is only partially dealuminated, as confirmed by 29Si and 27Al n.m.r. data. The dealumination of HY-FAU zeolite is quasi-complete, although a high amount of amorphous aluminosilicate is also formed during the dealumination steps. 31P n.m.r. data show the presence of dihydrogenophosphate SiOP=O(OH)2 and phosphite (SiO)2P=O(H) groups attached to the surface of both zeolites. Infrared data reveal the presence of the phosphite group. Part of the phosphorus is found in a crystalline aluminophosphate phase in the HM-MOR sample, but the aluminophosphate phase of the Y zeolite sample proved to be amorphous with the P/AI molar ratio much lower than unity.

A simulated annealing study of Si,Al distribution in the faujasite framework

Several important properties of faujasite are concerned with the Si,Al distribution in its framework. In the present study, a starting configuration of the distribution is generated with randomly positioned Si,Al. After the simulated annealing procedure, some orderings appear in the optimized configurations of the system and lead to a series of consequences, some of which have already been obtained by pioneer studies concerning ordering schemes. In optimized configurations, rations, a certain degree of disorder reasonably is maintained because of the use of standard Monte Carlo finite temperature techniques, which bring about a realistic employ of the numerical simulation. The calculated Al distribution on the first shell of the Si site is assessed by 29Si MAS n.m.r. experiments to demonstrate the rationality of the present theory. The calculated Al distribution on the second shell of the Al site is related to the acidity, which leads to the proposition that the acid amount of faujasite is in proportion to the number of single Al four rings. The electrostatic energy calculation provides possible explanations of the observed discontinuity in the plot of unit cell parameter against Al content.

Synthesis of faujasite zeolites with crown-ether templates

The function of crown ethers in the synthesis of faujasite zeolites has been studied. Hexagonal faujasite (EMT) was prepared from 18-crown-6 ether and not from 15-crown-5 ether as the organic template under hydrothermal conditions, whereas cubic faujasite (FAU) was obtained from a reaction gel using either 18-crown-6 ether or 15-crown-5 ether. During the early stages of synthesis, 18-crown-6 ether and 15-crown-5 ether were partially occluded in the aluminosilicate hydrogel and EMT or FAU nuclei, but 18-crown-6 ether was only adsorbed on the surface of the hydrogel for FAU crystals. Thermal analysis, Raman spectroscopy and composition analysis measurements indicate that sodium cations play a more important role than 18-crown-6 ether in directing the formation of FAU nuclei and balancing the charge on the framework of FAU when 18-crown-6 ether is used at the organic template. The stacking arrangement and Al content of the synthesized faujasite zeolites were found to depend on the SiO2 : Al2O3 and Na2O : crown ether molar ratios of the reaction mixtures. The structure-directing role of the crown ether in the formation of faujasite zeolites was also studied by theoretical calculations.

The computer modeling of lanthanum in the framework of faujasite: Part 1. Anhydrous lanthanum zeolite

Structures having a La3+ ion assigned to various sites in an idealized faujasite framework have been modeled using potentials developed from the study of oxides and successfully applied to zeolites. An initial structure is relaxed to minimum energy at constant pressure with all species free to move. The resulting lattice energy is used as a measure of the stability of the structure. On this basis, it is shown that distortion of the framework and the combined effect of many relatively long-range electrostatic interactions cause simple analyses based on high symmetry and short-range interactions to be unreliable. In particular, it is shown that the chance of an isolated La3+ ion's occupying an SI' site is comparable to that of its occupying an SI site, and, indeed, the SI' site can be the preferred location. This finding serves to clarify the interpretation of the X-ray diffraction data for single-crystal La-FAU. A further significant finding is that SII sites can be expected to have a higher population of La3+ ions in such structures than they have usually been thought to have.

The computer modeling of lanthanum in the framework of faujasite: Part 2. Hydrated lanthanum zeolite Y

The methodology of Part 1 was used to compute the lattice energies of various structures for hydrated La-ZY. The structure deduced by Cheetham et al. from their 5 K neutron diffraction measurements is shown to be compatible with the potentials used to a high degree of consistency, except for framework hydrogen. However, the modeling studies suggest this is not the only feasible or even the most stable structure type. One variant is the same as the diffraction structure except that the H's of the OH−'s relate differently to the framework. Much more common among the simulated structures, however, is an arrangement in which one of the extraframework H's is on an SII' site, where it interacts closely with three O2's, and the other is more distant from the framework. The La3+ ions remain on SI' sites. An examination of the Madelung and short-range energies of all the species composing the various structures shows that their relative stabilities cannot be understood in terms of a few predominant local interactions. This study also indicates that the conclusions of Cheetham et al. concerning framework hydrogen need to be reviewed.

Coordination sequences of zeolites revisited: asymptotic behaviour for large distances

The coordination sequences of several zeolite frameworks (faujasite, Linde type-A, KFI, sodalite and analcime) have been obtained by direct counting until at least the 23rd coordination sphere. This allows us to study the asymptotic behaviour of the number of sites in the ith shell, Ni, for high values of i. In all cases, Ni adapts well to a parabolic law, Ni=ai2+b, as expected for three-dimensional lattices. The constant a, which ranges from 1.43 for faujasite to 2.40 for analcime, is typical for each framework, and is directly related to the topological density of these materials. For the faujasite framework, we find a modulation superimposed on the parabolic dependence, which is associated with the particular connectivity of this structure.

Statistical thermodynamics of Si,Al ordering in aluminosilicate faujasites.

The Si,Al distribution on the zeolitic framework of faujasite is analyzed by a Monte Carlo method, by following a ``simulated-annealing'' procedure. Equilibrium atom distributions are obtained at the formation temperature of synthetic X and Y zeolites (${\mathit{T}}_{\mathit{F}}$\ensuremath{\sim}350 K). The heat capacity ${\mathit{c}}_{\mathit{v}}$ is calculated from the energy fluctuations in the canonical ensemble, for framework compositions in the range of 48--96 Al atoms per cell (Al atomic fraction from 0.25 to 0.5). ${\mathit{c}}_{\mathit{v}}$ presents two peaks as a function of Al loading, which correspond to two order-disorder phase transitions in the Si,Al distribution. The configurational entropy has been evaluated from the simulation results by thermodynamic integration along a reversible path. The configurational free energy shows characteristic variations for the three structural phases obtained, which are discussed in terms of different arrangements of Si and Al atoms over the building blocks of the zeolitic framework.

Configurational density of states in zeolite frameworks: a Monte Carlo approach

The density of states g( E) associated to the configurational energy of the Si, Al atom distribution on the aluminosilicate framework of faujasite is calculated by a Monte Carlo method. An analysis of g( E) allows to characterize the atom ordering as a function of temperature. The free energy of the system at any temperature is obtained straightforwardly from the density of states. This method can thus be applied to problems of substitutional disorder controlled by long-range interactions.

Energetics of cation ordering in the faujasite framework: Monte Carlo simulations

The energetics of Si,Al ordering in zeolites X and Y is studied by the Monte Carlo method. The microscopic model for the interatomic potential includes long-range Coulomb interactions and oxygen polarization energies. A comparison of Al-O-Si-O-Al group concentrations obtained by simulation and by analysis of earlier 29 Si NMR spectra yields an effective charge difference of 0.32e between Si and Al in this structure. The lattice stabilization due to Si,Al ordering is calculated to go from 560 to 1533 kJ/mol in the range 48-96 Al atoms per unit cell

Zeolite Syntheses from Superalkaline Reaction Mixtures

The synthesis of faujasite-type zeolite from superalkaline reaction mixtures are described. The contribution shows the influence of component K2O added in the system Na2O-Al2O3-SiO2-H2O with H2O/(K2O + Na2O) = 13-15. The reaction course was investigated in the range K2O/(K2O + Na2O) from about 0.07 to 0.5. Under used conditions it is quite possible to isolate faujasite-type zeolite as a single crystalline phase. By means of IR spectroscopy, low SiO2/Al2O3 ratios from 2.0 to 2.1 in the faujasite framework have been indicated. The low SiO2/Al2O3 ratios are interpreted by a higher stability of the Si-O-Al- than the Si-O-Si- bond with increasing hydroxyl ion concentration.

Short-range order of the silicon, aluminum distribution on the faujasite framework

Short-range order of the silicon, aluminum distribution on the faujasite framework