Tetrahedral Al Species Research Articles

Al-rich Cu-SSZ-13 zeolite synthesized by interconversion from Y: Crystallization process resolution and application in the selective catalytic reduction of NO with NH3

Cu-SSZ-13 zeolite is a promising catalyst in the selective catalytic reduction (SCR) of NOx with NH3 and the interzeolite conversion is considered as a convenient method with relatively low cost to get the Al-rich SSZ-13 zeolite with great hydrothermal stability. However, the crystallization process of interzeolite conversion and its relation to the catalytic performance of Cu-SSZ-13 in the NH3-SCR of NOx are still poorly understood. Herein, the interzeolite conversion process of Y to SSZ-13 was well resolved and the effect of major synthesis factors on the hydrothermal stability and catalytic performance of Cu-SSZ-13 in the NH3-SCR of NOx was investigated. The results indicate that highly crystalline SSZ-13 can be synthesized by the interzeolite conversion from Y, using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as the structure-directing agent (SDA) and with a SDA/Si molar ratio of 0.06–0.2. The Y zeolite of FAU framework is first degraded and decomposed into the oligomeric aluminosilicate fragments quickly in the alkaline circumstance; thereafter, the oligomeric fragments develop into the crystal nuclei that arouse the rapid crystallization of the SSZ-13 zeolite of CHA framework with the help of SDA and solid Al(OH)3, during which the Al(OH)3 source is dissolved and restructured to tetrahedral Al species and incorporated into the CHA framework. The synthesized Cu-SSZ-13 zeolite exhibits excellent catalytic performance in the NH3-SCR of NO, achieving an NO conversion of nearly 90% at a rather low temperature of 150 °C and a gaseous hourly space velocity (GHSV) of 100,000 h−1. Moreover, the Cu-SSZ-13 catalyst displays great hydrothermal stability and remains highly active even after aging at 800 °C. These results help to clarify the mechanism of interzeolite conversion and provide an effective method to synthesize highly crystalline Al-rich SSZ-13 zeolite with great potential for application in the NH3-SCR of NOx.

Nanocrystalline HZSM‐5prepared by supercritical hydrothermal synthesis and its catalytic esterification performances

AbstractThe nanocrystalline HZSM‐5 was synthesized by supercritical hydrothermal synthesis for the first time and investigated in catalytic esterification of pyromellitic dianhydride(PMD)and n‐butanol to tetrabutyl pyromellitate(TBPM). The HZSM‐5 prepared by conventional hydrothermal synthesis was also examined for comparison. The catalysts were studied using XRD, N2 adsorption, NH3‐TPD, Pyridine‐adsorption FT‐IR, SEM,[27] Al MAS NMRandCHNS analysis. According to the results of characterization, the supercritical hydrothermal condition was helpful to reduce the size of crystal particles and improve the crystallinity of HZSM‐5, and shorten the crystallization time. The surface area, especially external surface area, and pore volume were increased, and the total acidity was increased. The strong acid sites were transformed into the weak acid sites, and the strength of weak and strong acid was weakened. The amount of framework tetrahedral Al species and Brönsted acid sites were increased. Therefore, the formation of coke was retarded, and thePMD conversion, the TBPM selectivity and the catalytic stability were obviously improved.

Structure and catalytic activity of highly ordered AlMCM-48 materials with different Si/Al ratios on the degradation of high-density polyethylene

Catalysts of AlMCM-48 with different Si/Al molar rations were synthesized by modified hydrothermal method with respect to previous works. As a consequence, the materials showed different porous structures making them of high adsorption capacities and high efficiency to high-density polyethylene (HDPE). Samples synthesized with various Si/Al molar ratios were characterized by X-ray diffraction (XRD), nitrogen adsorption at – 196 °C, scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared (FTIR) and dispersive energy X-ray spectrometry (EDX). Characterization results suggested the presence of tetrahedral Al species on AlMCM-48(x), giving highly ordered materials that exhibit thermal stability. It is important to highlight that the use pseudoboehmite as a promising Al source for the synthesis of this type of molecular sieves, leading to materials with slight morphological differences due to the formation of interconnected particles spherical. The catalytic performance showed that the high-density polyethylene (HDPE) degradation reaction occurs in complex reaction steps in the heating rates 5, 10 and 20 °C min−1. The pore diameters of the catalysts influence the degradation process under non-isothermal condition. Furthermore, on the conditions studied, the AlMCM-48(x) catalysts exhibited high conversion values and play important roles in the decrease of HDPE degradation temperature, potentially leading to an efficient process in terms of energy expenditure.

Influence of H3O+ on the structure formation of oligomers in aluminium sols prepared from basic aluminium acetate: Experiments and computations

In-depth insight into the structure of oligomers in sols plays an important role in determining the structure and properties of the final materials, such as aluminium borosilicate fibers, flakes, microspheres and composites. In this study, the influence of H3O+ on the structure formation of oligomers in sols prepared from basic aluminium acetate (BAA) was studied by Fourier-transform infrared spectroscopy (FT-IR), 27Al NMR spectroscopy and density functional theory (DFT) calculation. The results indicated that BAA consisting of tetrahedral Al species existed in dimeric form (BAA2-2). The polymerization of BAA2-2 was more thermodynamically favourable than its hydrolysis both in neutral and acidic solutions. The oligomers formed in neutral aqueous solution were beneficial to prepare different polymers with dendritic or three-dimensional structures. The oligomers formed in acidic solution (H+/BAA = 0.5) were mostly linear tetramers composed of three octahedral and one tetrahedral Al species, which were beneficial to the spinnability of sols during the preparation of ceramic fibers. The addition of H+ changed the electronic properties of Al(III) and significantly promoted the coordination ability of water and Al3+, leading to an increase in the octahedral Al species and a great change in the structure of the sol. The DFT results were consistent with the experimental results, which provided a necessary theoretical guidance to improve the properties of aluminium sols and meet the needs of the final materials.

Producing BTX aromatics-enriched oil from biomass derived glycerol using dealuminated HZSM-5 by successive steaming and acid leaching as catalyst: Reactivity, acidity and product distribution

Abstract We have investigated both the GTA (glycerol to aromatics) reaction routes and aromatic products distributions over a series of dealuminated HZSM-5 catalysts to produce BTX aromatics-enriched oil from biomass derived glycerol. As the acid site density in dealuminated HZSM-5 catalysts decreased, the induction period of GTA process became longer. The GTA reaction route was almost unaffected over HNO3 treated HZSM-5 catalyst. However, the liquid route of GTA procedure was enhanced while the gas route was restrained over steaming dealuminated HZSM-5 catalyst. The mild HNO3 treatment could preferentially remove the non-shape selective acid sites from HZSM-5 extra-framework, promoting BTX aromatics formation. In contrast, dealumination by harsh steaming transformed HZSM-5 framework tetrahedral Al species (FAL) into EFAl species, leading to a decrease in the BTX aromatics yields together with an increment in the yields of trimethylbenzene and durene. Furthermore, the steaming treatment created an intramesopore structure, which improved the diffusion property in the GTA process, resulting in an improved life span of dealuminated HZSM-5 catalyst. The successive steaming and HNO3 leaching over HZSM-5 could not only extract the EFAl species formed in 3 h 500 °C/HZSM-5 sample, reopening the EFAl-blocked micropores, but also create intracrystalline mesopores, as a consequence, non-shape selective catalytic side reactions and coking were restrained simultaneously during the GTA process, so the resulting catalyst produced BTX aromatics-enriched oil from biomass derived glycerol and exhibited significantly prolonged catalyst lifetime.

Selective reduction of NO over Cu/Al2O3: Enhanced catalytic activity by infinitesimal loading of Rh on Cu/Al2O3

This study demonstrates that an isolated Cu2+ species formed by interacting with tetrahedral Al species on γ-Al2O3 shows a high catalytic activity for NO reduction with C3H6 and CO under high oxygen concentrations (lean conditions), and that an infinitesimal amount of Rh (0.01wt%, 100ppm) loaded on 0.5wt% Cu/Al2O3 is a more effective catalyst for the reactions, under a wide range of oxygen concentrations, than 0.5wt% Pd/Al2O3 or 1.0wt% Rh/Al2O3. Although 0.5wt% Cu/Al2O3 itself is effective for the reaction at lean conditions, it shows low catalytic activity under stoichiometric and low oxygen concentrations (rich conditions). On the other hand, the loading of 0.01wt% Rh on Cu/Al2O3 dramatically improves catalytic activities under stoichiometric and rich conditions, with maintaining the high NO reduction activity of Cu/Al2O3 itself under lean conditions. Therefore, a Rh species in the Rh/Cu/Al2O3 system must play a role as an active site under stoichiometric and rich conditions. Under lean conditions using Rh/Cu/Al2O3, an isolated Cu2+ species on γ-Al2O3 mainly contributes to the NO reduction properties of the catalyst. No such advantage was observed for Pd/Cu/Al2O3; that is, the catalytic activity of Cu/Al2O3 under lean conditions is diminished by Pd loading. Hence, we reveal that the roles of the Rh and Cu species in the Rh-loaded Cu/Al2O3 catalyst are effectively shared according to oxygen concentration in selective reduction of NO, when loaded with an infinitesimal amount of Rh. These observations provide novel guidelines for the development of catalysts that more beneficially use precious metal resources.

Constraints on aluminum and scandium substitution mechanisms in forsterite, periclase, and larnite: High-resolution NMR

The incorporation of incompatible Al in forsterite is particularly interesting due to its relevance in Al olivine-spinel geobarometry techniques as well as the proposed influence of Al on upper mantle diffusion and water storage capacity. However, determining the site preference and substitution mechanisms of aluminum in forsterite presents considerable challenge, because the incompatibility of Al in the olivine structure results in correspondingly low Al concentrations. In this work we use solid state nuclear magnetic resonance (NMR) to directly observe Al coordination in synthetic samples from which we can constrain site preferences. We investigate Al in MgO and clinoenstatite to confirm that the forsterite spectra do not contain contributions from these impurity phases. To better interpret our results we used the independent component analysis (ICA) algorithm, SIMPLISMA, which accurately deconvoluted complex NMR line shapes, separating spectral components from known impurities and enabling the identification of unknown spectral components that in some cases we can assign to substituting Al. We additionally investigate Al in larnite, which has a tetrahedral environment similar to that of forsterite and that presents an another ICA test case, and investigate Sc in forsterite and periclase to add context to our Al observations. Our 27 Al and 45 Sc MAS NMR spectra place new constraints on the site preferences of Al in synthetic forsterite, clinoenstatite, periclase, and larnite as well as Sc in forsterite and periclase. In forsterite, we identified Al in the tetrahedral site and at least three distinct octahedral environments. The overall ratio of contents of AlO 4 to AlO 6 is about 1:3; the excess of the latter suggesting that at least two substitution mechanisms are necessary for Al 3+ incorporation. In some cases the estimated species abundance for observed features are as low as 70 μg/g Al 3+ , but were readily detected by NMR. Also in forsterite, we identified Sc in a single MgO 6 environment. In larnite (also known as belite or C 2 S in cement chemist notation), a single, ordered tetrahedral Al species is detected. In enstatite samples, primarily composed of clinoenstatite, 27 Al NMR spectra look very similar to previous observations of orthoenstatite, likely indicating a similar coupled AlO 6 and AlO 4 substitution. In periclase, Al is observed in an undistorted MgO 6 site with cubic or nearly cubic point symmetry, in addition to a lower symmetry, sixfold-coordinated site. This work provides new insight into incompatible element substitutions mechanism in minerals, in forsterite’s case indicating complex behavior involving multiple species despite compositional simplicity.

Properties of hierarchical Beta zeolites prepared from protozeolitic nanounits for the catalytic cracking of high density polyethylene

Hierarchical Beta zeolites have been prepared employing two strategies of synthesis for the generation of mesopores with different size distribution. In both cases, the starting gel was subjected to pre-crystallization to induce the formation of protozeolitic nanounits. The first strategy involves the reorganization of the protozeolitic nanounits by incorporation of a surfactant (cetyltrimethylammonium bromide, CTAB), whereas the second one is based on the use of a silanization agent (phenylaminopropyl-trimethoxysilane, PHAPTMS) that it is grafted to the outer part of the nanounits to avoid their total merge during the crystallization process. In addition, conventional Beta zeolite and ordered mesoporous aluminosilicates have been prepared and employed as reference samples. All materials have been characterized by a number of techniques, whereas their catalytic behavior has been evaluated in the cracking of high density polyethylene (HDPE). The h-Beta (CTAB) sample shows a narrower mesopore size distribution, more uniform and stable tetrahedral Al species and a higher acid strength than the material obtained from silanized nanounits, although the latter exhibits a larger accessibility as denoted by its higher mesopore/external surface area (351m2/g in h-Beta (PHAPTMS) versus 228m2/g in h-Beta (CTAB)). While the ordered mesoporous aluminosilicates were almost inactive in HDPE cracking (<5wt% of conversion) due to its weak acidity, high plastic conversions were obtained over the zeolite Beta samples (>50wt% of conversion). The results obtained indicate that the overall catalytic activity depends mainly on the accessibility to the active sites, whereas the product selectivity is rather determined by the acid strength of the zeolite catalysts. Thus, h-Beta (CTAB) sample promoted in a great extension both end-chain cracking and aromatization reactions due to its high acid strength.

Mesoporous and Al-rich MFI crystals assembled with aligned nanorods in the absence of organic templates

Mesoporous zeolites have been paid much attention recently due to their excellent mass transfer and high activity for bulky molecules as well as unique shape selectivity, but their syntheses usually require the presence of organic templates. Here, we report organotemplate-free synthesis of mesoporous and Al-rich MFI zeolite in the presence of the zeolite seeds (MFI-SDS). The MFI-SDS crystals are assembled with rod-like nanocrystals having almost the same aligned direction, confirmed by the sample transmission electron microscopy (TEM). The space between nanocrystals forms the mesoporosity in the sample, supported by N2 sorption isotherms. 27Al MAS NMR spectroscopy and ICP analysis show that the MFI-SDS has abundant tetrahedral Al species in the zeolite framework, giving the Si/Al ratio at 9.5. Catalytic tests in cumene cracking show that the MFI-SDS exhibits higher catalytic activities and longer catalyst life than conventional ZSM-5 zeolites synthesized in the absence (ZSM-5-OTF) or presence of organic template, TPA+ (ZSM-5-TPA). The excellent catalytic performance should be assigned to the combined contribution of the presence of both mesoporosity and rich tetrahedral Al species in the MFI-SDS. These features would be critically important for industrial applications of MFI zeolite catalysts in the future.

Multinuclear Solid-State NMR Studies on the Formation Mechanism of Aluminophosphate Molecular Sieves in Ionic Liquids

In the present work, multinuclear solid-state NMR techniques together with power X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to monitor the crystallization process of aluminophosphate AlPO4-11 molecular sieves in 1-ethyl-3-methylimidazolium bromide ([emim]Br) ionic liquids (ILs). The local environments of the selected solid samples were probed by one-dimensional 27Al, 31P, and 19F MAS NMR experiments, and more information was obtained from two-dimensional 27Al→31P heteronuclear correlation (HETCOR) and 27Al triple-quantum MAS (3Q MAS) experiments. It is found that a large amount of amorphous aluminophosphates was formed with the F–Aloct–O–Ppar and Altet–O–Ppar structures in the initial stage of aging. With increasing crystallization time, the partially condensed framework phosphorus species disappeared gradually and became fully condensed. Meanwhile, the octahedral Al was transformed into the pentahedral and tetrahedral Al species with the structures of F–Alpenta–O–Pful and Altet–O–Pful in the AlPO4-11 frameworks. During the crystallization process, [emim] cations acting as the structure-directing agents were occluded into the channels, and F– anions were connected with pentahedral Al in the final AlPO4–11 phase. All of these findings can allow proposing a possible formation mechanism for the synthesis of AlPO4-11 in ionic liquids.

Synthesis and Structure of an Amino-Linked N-Heterocyclic Carbene and the Reactivity of its Aluminum Adduct

Treatment of [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br·HBr (1) with an excess of NaOH afforded the organic-soluble imidazolium salt 2, [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br, the structure of which was confirmed by X-ray crystallography. Deprotonation of 2 by NaN(SiMe3)2 yielded the thermally unstable amino-linked free carbene, 3. The molecular structure of 3 was determined by single-crystal X-ray analysis, revealing an unexpectedly close intermolecular contact associated with the carbene and amine through an N−H···C interaction. In contrast to compound 3, the reaction of 2 with LiN(SiMe3)2 gave the much more stable LiBr carbene adduct 3-LiBr, which serves as an effective carbene transfer agent for organoaluminum compounds to give the corresponding AlR3-NHC (4a, Me; 4b, Et) in high yield. X-ray diffraction studies of 4a and 4b confirmed the formation of monomeric distorted tetrahedral Al species, in which the NHC binds via conventional σ-donation of the lone pair to the electrophilic metal center. The reaction of benz...

Acid activated montmorillonite: an efficient immobilization support for improving reusability, storage stability and operational stability of enzymes

Three enzymes, a-amylase, glucoamylase and invertase, were immobilized on acid activated montmorillonite K 10 via two independent techniques, adsorption and covalent binding. The immobilized enzymes were characterized by XRD, N2 adsorption measurements and 27 Al MAS-NMR spectroscopy. The XRD patterns showed that all enzymes were interca- lated into the clay inter-layer space. The entire protein backbone was situated at the periphery of the clay matrix. Intercalation occurred through the side chains of the amino acid residues. A decrease in surface area and pore volume upon immobilization supported this observation. The extent of intercalation was greater for the covalently bound systems. NMR data showed that tetrahedral Al species were involved during enzyme adsorption whereas octahedral Al was involved during covalent binding. The immobilized enzymes demon- strated enhanced storage stability. While the free en- zymes lost all activity within a period of 10 days, the immobilized forms retained appreciable activity even after 30 days of storage. Reusability also improved upon immobilization. Here again, covalently bound enzymes exhibited better characteristics than their adsorbed counterparts. The immobilized enzymes could be successfully used continuously in the packed bed reactor for about 96 hours without much loss in activity. Immobilized glucoamylase demonstrated the best results.

｀２７´Ａｌ ＮＭＲ ＭＱＭＡＳ測定による石炭無機成分の構造解析とそれらの存在分布の考察

Structural analysis of inorganic matter in coal is very important, from the viewpoint of both geology and coal utilization. However, it is often difficult to assign inorganic matter from common techniques such as XRD, because some inorganic matter, i.e. clay mineral, in coal has low crystallinity.On the other hand, solid-state NMR techniques are well suited for the analysis of noncrystalline solids such as inorganic matter in coal. Several kinds of minerals were estimated from the percentage of tetrahedral Al species that was determined by 27Al MAS NMR.In this study, we successfully developed the method for characterization of 4 minerals (kaolin, montmorillonite, muscovite, and alumina) in coal by virtue of the combination of a high magnetic field (16.4 T) and a MQMAS (Multiple Quantum Magic Angle Spinning) technique that is capable of cancel the second-order quadrupolar broadening. As a result, the structure of the inorganic matter in 26 natural coals was able to be identified. And 26 natural coals could be grouped into 6 groups according to 1-3 types of minerals combination.

Enzymes immobilized on montmorillonite K 10: Effect of adsorption and grafting on the surface properties and the enzyme activity

The enzymes α-amylase, glucoamylase and invertase were immobilized onto acid activated montmorillonite (K 10) via two independent techniques — adsorption and grafting using glutaraldehyde as spacer. Samples were characterized by XRD, NMR, SEM and BET N 2 adsorption measurements. Activity of the immobilized enzymes was tested in batch and bed reactor and their kinetic parameters were determined. XRD shows an increase of the basal spacing from 1 nm for montmorillonite to about 2.5 nm for immobilized invertase. It indicates that side chains of the enzymes have penetrated between the layers, in case of grafted enzymes, to a higher extent than for the adsorbed enzymes. 27Al-MAS NMR reveals the interaction of octahedral and tetrahedral Al species with the enzyme. Adsorption affects the tetrahedral Al resonance while grafting involves the octahedral sites only. Surface area measurements illustrate that enzyme grafting reduces the porosity than enzyme adsorption. It is proposed that the enzymes are situated at the periphery of the clay mineral particles whereas the side chains of different amino acid residues penetrate between the layers. The immobilized enzymes show substantial activity. The higher activity in the packed-bed reactor indicates a better mass transport. Loss of activity in the batch reactor is due to diffusional mass transfer resistances.

IR and NMR studies of mesoporous alumina and related aluminosilicates

The synthesis of mesoporous alumina and aluminosilicates of Si/Al from 0.2 to 10 was done, and the status of Si and Al was followed by 29Si and 27Al MAS NMR. The properties of acid sites were studied by following the IR spectra of CO, N 2, pyridine and ammonia adsorbed. The mesoporous alumina contained only surface weak Lewis acid sites, the insertion of Si resulted in loss on such Lewis sites and in the formation of Brønsted sites. The Al OH bands decreased (the Al IV OH band related to tetrahedral Al species diminished the first) and Si OH Al one (IR band at 3605 cm −1)—similar as in the case of zeolites appeared as well as Si OH groups (IR band at 3740 cm −1). The aluminosilicates of higher Si content contained also strong Lewis acid sites formed by the dehydroxylation of Si OH Al groups (process similar as in zeolites), which occurred at the activation. The concentration of both Lewis and Brønsted sites was determined by quantitative IR studies of ammonia and pyridine adsorption. The concentration of Brønsted acid sites increased with Si content due to the formation of acidic Si OH Al, and at high Si/Al, it increases also due to the growing surface area. The concentration of Lewis sites decreased with Si content due to the transformation of surface Al species, which are potential Lewis sites into Si OH Al and also to dilution of the material with Si species. The acid strength of Lewis sites increased with Si content due to increasing contribution of strongly acidic sites produced by dehydroxylation. The mesoporous aluminosilicate (Si/Al = 10) studied in this research showed higher concentration and higher acid strength of Brønsted sites than HMCM-41 of the same Si/Al—it may be an potential catalyst in fine chemistry reactions.

Dealumination process of zeolite omega monitored by [formula omitted] 3QMAS NMR spectroscopy

From the 27 Al MAS NMR and 2D 3QMAS NMR experiments, it can be concluded that during the steaming process of zeolite omega, some part of the framework tetrahedral Al species are gradually distorted. The 30 ppm broad hump in the 1D 27 Al spectra is mainly due to appearance of the distorted tetrahedral Al. With the dealumination process going on, penta-coordinated Al species are formed and contribute also to the 30 ppm broad hump. There exists the trend that the formation of penta-coordinated Al occurs with the consumption of the distorted tetrahedral Al species. Because the same phenomenon of the 27 Al NMR signals has also been observed in other zeolites, such as mordenite, ZSM-5 and Y, it could be regarded as a general procedure in the process of dealumination of acidic zeolites.

Shape-selective isopropylation of naphthalene over H–mordenite catalysts for environmentally friendly synthesis of 2,6-dialkylnaphthalene

Recent development of advanced polymer materials such as polyethylene naphthalate, polybutylene naphthalate and liquid crystalline polymers has created an increasing demand for 2,6-dialkylnaphthalene, which has spurred interest in shape-selective naphthalene alkylation. This work deals with mordenite-catalyzed shape-selective naphthalene isopropylation to produce 2,6-diisopropylnaphthalene (2,6-DIPN). Effects of dealumination of mordenite on the structural and acidic characteristics and on the shape selectivity and activity were examined by physicochemical analysis, TPD, solid-state 27 Al and 29 Si MAS NMR, XRD, as well as catalytic alkylation reactions. Dealumination removes octahedral Al species as well as tetrahedral Al species, decreases the unit cell dimensions and reduces the number of strong acid sites in mordenites. Proper dealumination can improve selectivity to 2,6-DIPN from 33 to 61 % and significantly increases 2,6/2,7 ratio. Improved selectivity to 2,6-DIPN upon proper dealumination was attributed to the decrease in mordenite acidity, reduction in unit cell dimension and removal of some strong acid sites. However, neither the change in selectivity nor that in activity is a simple function of dealumination degree or SiO 2 /Al 2 O 3 ratio. Some minor difference in the apparent framework SiO 2 /Al 2 O 3 ratio can result in a major difference in activity or selectivity. There exist optimum conditions of dealumination as well as optimum reaction conditions for achieving higher selectivity to 2,6-DIPN. Version française abrégée – Synthèse respectueuse de l'environnement du 2,6-diisopropylnaphtalène par isopropylation du naphtalène catalysée par la mordénite par effet de sélectivité de forme. Le développement récent de matériaux polymères tels que les polyéthylènes naphthalates, polybutylènes naphthalates et les polymères à cristaux liquides a créé une demande importante en 2,6-dialkylnaphthalène. Ceci a accru l'intérêt pour l'alkylation sélective de forme du naphthalène. Ce travail décrit l'isopropylation du naphtalène catalysée par la mordénite pour produire du 2,6-diisopropylnapthalène (2,6-DIPN). Les effets de la déalumination de la mordénite, sur les caractéristiques structurales, l'acidité, aussi bien que sur les réactions catalytiques d'alkylation (la sélectivité de forme et l'activité) ont été étudiés par les différentes techniques d'analyses, TDP, RMN solide du silicium et de l'aluminium. Les centres Al octaédriques et Al tetraédriques sont éliminés par déalumination, réduisant ainsi les dimensions de la maille cristalline et le nombre de sites à forte acidité de la mordénite. Ceci entraı&#x0302;ne, dans des conditions de déalumination bien choisies, une augmentation de la sélectivité en 2,6-DIPN de 33 à 61 % et du rapport 2,6-DIPN/2,7-DIPN. Cependant, aucune corrélation simple entre la variation de la sélectivité et de l'activité avec le degré de déalumination n'a pu être établie. Les conditions optimales varient pour chaque mordénite.

Synthesis of high surface area boria–alumina mixed oxides characterization by 11B- and 27Al-NMR

Boria–alumina mixed oxides were synthesized by the sol-gel method. MAS-NMR spectroscopy spectra exhibited two boron species, BO 3 and BO 4, the former giving an important second order quadrupolar effect. Then, spectra were interpreted with a simulation software which permitted quantitative evaluation of boron species in both dried and calcined states. Tetrahedral Al species formed in the dried state appear to monitor BO 4 and BO 3 species formation in both dried and calcined states.

Effect of fluoride ions on the acidic and catalytic properties of beta zeolite

Fluorided beta Zeolite samples containing 0–1.6 wt.% F have been obtained by treating the original NH4-beta zeolite with different quantities of NH4F solution followed by drying and calcination. The samples were characterised by X-ray diffraction (XRD), FIR, NMR, X-ray photoelectron spectroscopy (XPS), chemical analysis and catalytic studies. XRD indicated that severe treatment with NH4F results in the formation of (NH4)2SiF6 and (NH4)3AIF6 species that decompose on heating at 723 K to fluorocomplexes of Si and Al and occluded fluoride ions. IR spectroscopy showed that flouridation results not only in an increase and then decrease in the relative intensity of the band at 3610 cm–1, associated with structural Bronsted acid sites (with respect to the 3740 cm–1 band), but also an initial increase and then decrease in the Bronsted-to-Lewis (B/L) ratio of the zeolite. An Al 2p XPS study indicated the presence of two component peaks in fluorided zeolites. The peak at lower binding energy, Eb, corresponds to the zeolitic Al framework atoms (not interacting with fluoride ions) and that at higher Eb to the Al atoms interacting with fluoride ions. However, the F 1s XPS study showed the presence of three component peaks, at low, medium and high Eb, corresponding to fluoride ions interacting with Al atoms, occluded/trapped fluoride ions and fluoride ions interacting with Si atoms. The N 1s XP spectra also exhibited three component peaks in both fluorided and unfluorided beta zeolites that were previously assigned to pyridine chemisorbed on Lewis acid sites and relatively weak and strong Bronsted acid sites. A simultaneous decrease in the relative intensity of the strong Bronsted component and an increase in the relative intensity of the Lewis acid component as a function of fluorine content was observed in the N 1s XP spectra. The 27 Al NMR study showed that octahedral Al species present in the starting material were transformed into tetrahedral Al species by fluoridation suggesting that some octaheral Al atoms belong to the zeolite framework. Catalytic activity data suggested that the beta zeolite with 0.3% F had a higher B/L ratio and higher catalytic activity for n-hexane and cyclohexene conversion reactions. In the light of IR, XPS and NMR data and the catalytic activity study, it is concluded that an optimum fluoride ion concentration is needed to improve the catalytic activity of beta zeolite and the inrease in catalytic activity is mainly due to the increase in the acidic strength of the hydroxy groups remaining after fluoridation.