Lattice Aluminum Research Articles

Acidity of H-Y Zeolites: Role of Extralattice Aluminum

The nature of extralattice aluminum and its effect on acidity in zeolites was studied. A series of zeolites (some commercial), dealuminated by various methods, were obtained having Si/Al ratios between 1.5 and 20. These were characterized using volumetric sorption, XRD, AA, NMR, and reaction studies. Samples dealuminated using ammonium hexafluorosilicate were found to contain little or no extralattice aluminum. In contrast, steam-dealuminated zeolites had large amounts of extralattice aluminum. In some cases a significant portion of the extralattice aluminum was unobservable by 27Al NMR, suggesting the existence of an aluminum species of low symmetry. Proton NMR indicated that all the protons were associated with the lattice aluminum atoms. Depending on the preparation history, different groups of H-Y zeolites exhibited different maxima in catalytic activity as a function of aluminum content. The maxima in catalytic activity for n-pentane cracking was found to be a function of both lattice and extralattice aluminum contents.

Toluene disproportionation over ZSM-5 zeolite: Effects of crystal size, silicon-to-aluminum ratio, activation method and pelletization

The catalytic activity and selectivity of ZSM-5 zeolite in toluene disproportionation were studied as a function of crystal size, silicon-to-aluminum ratio, acidity, activation method and pelletization of zeolite. Although para-xylene formation was slightly favoured by increasing the crystal size, high para-selectivity was not achieved. At the lowest silicon-to-aluminum ratio, catalytic activity was enhanced with a drop on the selectivity to disproportionation, suggesting the presence of “super acid sites” related with extra lattice aluminum species. From IR spectroscopy measurements with adsorption/desorption of pyridine it was observed that during the activation step, a high H + concentration in the cation exchange increases the strength of Brensted sites, increasing the number of acid sites which are active in toluene disproportionation. Subsequent calcination completes zeolite activation, increasing the number of Brønsted acid sites active in that reaction. In the pelletization there is a cation exchange between Na + from the montmorillonite and H + from the zeolite when both are mixed and suspended in water. Nevertheless, sodium montmorillonite proved to be a suitable binder of ZSM-5 provided that the catalysts are reactivated after pelletization.

Relation between properties and performance of zeolites in paraffin cracking

The effects of zeolite structure, acid site density, and reaction temperature on the mode of n-hexane cracking have been studied. The results can be consistently explained in terms of the occurrence of two different cracking routes, viz. the classical β-scission route and the monomolecular protolytic pathway. The relative contribution of each of the two cracking routes is expressed by the “cracking mechanism ratio” (CMR): a high value of this index points to a relatively high contribution of the protolytic cracking route, whereas a low value indicates that the classical β-scission route is the main cracking pathway. With increasing temperature, decreasing aluminium content, and decreasing pore dimensions the relative contribution of the (monomolecular) protolytic cracking route increases as compared to the (bimolecular) classical route. On the basis of these results the variation of the constraint index (i.e., the ratio of the first-order rate constants for conversion of n-hexane and 3-methylpentane) with aluminium content and reaction temperature can be easily rationalized. Furthermore, the present results also provide an explanation for the change in the activation energy of n-hexane cracking with aluminium content in zeolite ZSM-5 (MFI). Finally, it is tentatively concluded that the classical cracking route is favoured by the presence of two adjacent acid sites and by low reaction temperatures. Consequently, the relation between the reaction rate constant and the lattice aluminium content changes with the reaction temperature; whereas at 811 K the cracking constant varies linearly with the aluminium content, at 623 K the order in the aluminium content increases to about 2.

The assay of acid sites on zeolites as measured by ammonia poisoning

The decomposition of neopentane was studied with and without selective site poisoning with NH 3. The lethal dose, which was just sufficient to kill the catalytic activity, was estimated for each of a series of H-mordenites, HY and HZSM-5 zeolites, all with Si Al ratios higher than 7.0, i.e., over silica-rich compositions where ideally all the sites are supposedly identical. It was found that poisoning no more than 10% of the sites (counted as lattice aluminum ions) was sufficient to eliminate the activity. Some ramifications of these observations are discussed. Unpoisoned and partially poisoned zeolites were investigated using 13C-MASNMR following adsorption of isopropanol-2- 13C. These findings clearly show the efficacy of adsorbed NH 3 in repressing formation of polymeric surface residues.

A microcalorimetric study of NH 3-sorption onto differently modified and pretreated ZSM-5 surfaces

A comparative study of NH 3-sorption onto several ZSM-5 samples differently modified by acid-dealumination, boron incorporation in the zeolite (by synthesis), as for lattice aluminum and boron impregnation as H 3BO 3, has been carried out at 423 K for differently pretreated (1.33 mPa/673 K/12 h and 1.33 mPa/1073 K/12 h) surfaces. Various volumetric, calorimetric and thermokinetic parameters have been found to correlate well, revealing that (i) acid treatment, whilst it eliminates a portion of the zeolite aluminum of potential acidity, creates new surface Brønsted sites of moderate acidity, (ii) boron replacement for a portion of the zeolite aluminum, however, does not alter the number of exchangeable acid sites, decreases markedly the overall zeolite acidity and enhances heterogeneous distribution of surface acidity, and (iii) boron impregnation, though it neutralizes proportionally the zeolite Brønsted acidity for samples pretreated at 673 K, appears to neutralize the zeolite Lewis acidity as well for samples pretreated at 1073 K, which results in a drastic loss of the overall zeolite acidity. The mechanistic implications illustrating the effect of such modifications are described.

Effect of thermal treatments on shape selectivity and acidity of ZSM-5 type zeolite catalysts

Thermally treated H-ZSM-5 samples were investigated as catalysts of toluene disproportionation and xylene isomerization and they were characterized by i.r. and TPD techniques. Thermal pretreatment enhanced selectivity of p-xylene formation and decreased activity. These effects can be partially reversed by subsequent treatment of the sample with HCl. It is suggested that extra lattice aluminium, originating from thermal dealumination, forms diffusion barriers in the pores, which are responsible for the catalytic effects and that HCl treatment partially dissolves these barriers.

ChemInform Abstract: Catalytic and Acidic Properties of Boron Pentasil Zeolites

Pentasil-type zeolite with boron isoraorphously substituted for silicon in the pentasil zeolite framework and aluminum pentasil (ZSM-5 type) zeolite impregnated with a boron compound have been prepared and studied. Lattice boron is shown to present trigonal or tetrahedral environment depending upon ligand adsorbates and to induce a weak acid strength and subsequently negligible catalytic activity for acidic-type reactions as methanol conversion to hydrocarbons, toluene alkylation with methanol and toluene disproportionation. Some activity was observed only if lattice aluminum was present or if aluminum based binder was used. Lattice boron was shown not to modify appreciably shape selectivity. Boron compound impregnated on aluminum pentasil sample was shown to decrease the acidity and subsequently the catalytic activity for acid type reactions but to sharply enhance shape selectivity, i.e. the yield in the less bulky para isomers of aromatics with respect to the other isomers. Boron of the impregnated compound (H3BO3) was shown to partly isomorphously subsitute for Si or Al upon calcination at 773K while calcination at 1073K results in the partial formation of a glassy borosilicate compound at the surface of the zeolite particles. These findings are discussed in light of binder-zeolite interactions.

Active sites on ZSM—5 zeolites for toluene disproportionation

The acidic and catalytic properties of HZSM—5 zeolites of varying SiO 2 Al 2O 3 ratios have been investigated. Three types of acid sites, weak, medium and strong have been identified by temperature programmed desorption of ammonia. The weak and medium strength acid sites are due to terminal silanol groups and occluded extra lattice species of aluminium respectively, while the strong acid sites originate from the lattice aluminium atoms. It has been found that the number and strength of acid sites increase with an increase in aluminium content in the ZSM—5 zeolite. It is suggested that the strength of the strong acid sites increases due to formation of ‘super acid sites’ with increasing aluminium content. Among the three types of acid sites, the strong acid sites are active for toluene disproportionation.

Catalytic and acidic properties of boron pentasil zeolites

Abstract

Investigation of the dealumination of high silica zeolite A (ZK-4) by 29Si magic-angle-spinning n.m.r. spectroscopy

The high-field (11.7 T)29Si magic-angle-spinning n.m.r. spectrum of the completely siliceous analogue of zeolite A, prepared by hydrothermally dealuminating zeolite ZK-4, which consists of a single sharp sharp resonance at –112.9 p.p.m. from Me4Si is indicative of the removal of all the lattice aluminium and retention of the crystalline framework; powder X-ray diffraction measurements confirm that this novel material retains the framework topology of zeolite A.

Aluminum distribution in four-rings and six-rings of the faujasite lattice

The distribution of four-rings containing 0, 1, or 2 aluminum ions and the distribution of six-rings containing 0, 1, 2, or 3 lattice aluminum ions as a function of aluminum content was calculated on the basis of the computer procedure for randomly siting aluminum ions in the faujasite lattice. Qualitative agreement with available test data was obtained.

X, Y, aluminum-deficient, and ultrastable faujasite-type zeolites: III. Catalytic activity

The catalytic properties of faujasite-type zeolites have been studied in the cracking of iso-octane. The selectivity of the reaction depends mainly on the sodium ion content. The catalytic activity decreases with the number of strong acid sites. The active sites are linked to the aluminum sites which are related to strong acidity. They are removed only if the aluminum extraction exceeds 30% of the total aluminum content. Inactive and weak acid sites are related to the more easily removable fraction of the aluminum atoms. This distinction of aluminum atoms into two types correlates very well with the results obtained for zeolites subjected to deep-bed calcination or heated at 870 °C. The results point out the importance of AlO 4 − lattice aluminum with regard to the ultrastability.

Hydrogenolysis of ethane over nickel catalysts in continuous flow systems

Studies of the selective catalytic reduction (SCR) of nitrogen oxide with methane over H-mordenites were made to develop a basic understanding of the role of acid sites in this reaction and the mechanism of deactivation of these solids. A series of H-mordenites with varying Si/AI ratios (5.9-16.9) were prepared by acid leaching. They were thoroughly characterized before and after reaction by 27AlMAS NMR, XRD, FTIR spectroscopy, N2adsorption and ammonia TPD. A linear correlation was found between the conversion of NO to N2, the lattice aluminum content and the concentration of strong acid sites measured by ammonia chem isorption. The catalysts were partially deactivated after being on stream at 650°C, subsequently reaching a stable behavior. The origin of this phenomenon could be traced back to the additional dealwnination occurring at high temperatures in the reacting stream. This additional expulsion of aluminum from the lattice caused a proportional decrease in the concentration of strong acid sites.

On the Mechanism of Clay Particle Linking by Anionic Polyelectrolyte

In our previous paper, the flocculating effect of anionic polyelectrolyte on clay particles suspended in water was studied in relation to the mechanism of the formation of polyanion linkage among the clay particles. A new theory on the bridging mechanism has been proposed. The purpose of this paper is to provide satisfactory evidences for the theory.Aqueous suspension of clay (a mixture of quartz and pyrophillite) used in this work was treated with sodium polyacrylate, a typical polyanion, and one of the following reagents: sodium hydroxide, aluminium chloride, hydrochloric acid, or positively charged colloid of ferric hydroxide. For each suspension the pH value, the turbidity, and the amount of waterstable floc formed by the polymer linkage were determined.The experimental results on clay suspension with aluminium chloride showed that, in the range of pH 5.6-7, clay flocculation occurred through the interparticle bridging by polymer chains. The maximum amount of water-stable floc was obtained at pH of about 6.2 Above pH 7, the clay particles became deflocculated owing to the disappearance of the polymer linkage and the electrostatic repulsion among the particles. At the acid pH, below 5.5, polyanions could not participate in the bridging, because they were combined with free aluminium, resulting into a firmly coiled state inactive to the linkage. However, in the clay suspension treated with hydrochloric acid, it was found that the increase of acidity increased the formation of polymer linkage, unless the polymer was curled by the free aluminium produced by the dissociation of the clay lattice.In the case of the interparticle bridging, the polymer chain could be expected to be anchored by positively charged sites on the clay surface through chemical bond, maybe a co-ordinated sort. From the result on the clay suspension treated with colloidal ferric hydroxide, such positively charged sites were estimated to be the colloids of ferric-and/or aluminium-hydroxide fixed on the clay surface, as well as the lattice aluminium on the brocken edges of the clay.Further experiments were carried out concerning the order of addition of sodium polyacrylate and another reagent, aluminium chloride or colloidal ferric hydroxide. These results might also be interpreted by the above mentioned mechanism.

Adsorption of Hydrolyzed Polyacrylonitrile on Kaolinite

AbstractAdsorption experiments were conducted by mixing homoionic kaolinite and C14-labeled hydrolyzed polyacrylonitrile (HPAN) in water and solutions of electrolytes. Langmuir type adsorption isotherms resulted. Exchange cations on kaolinite increased adsorption in approximately the same order as such cations reduce zeta potential, i.e. Th4+ > Ca2+ > Ba2+ > H+ > NH4+ > K+ > Na+. The anomalous increase in adsorption in the presence of polyvalent exchange cations suggested adsorption at the site of base exchange salt formation. Sorbed anions increased adsorption of HPAN in the order of electronegativity, i.e. F- > OH- > H2PO4- > Cl- > CH3COO- > NO3-.An increase in the concentration of electrolyte and acidity of the adsorbate medium increased adsorption of HPAN. This effect was apparently due to the reduction of electrostatic repulsion between HPAN and kaolinite and a reduction in size of the polymer coil. Divalent cations, especially the transition metals capable of being complexed by HPAN, were more effective than univalent cations. Dissolution of the clay lattice caused desorption of HPAN.HPAN adsorption reduced the intensity of the OH and O—Al—OH bands in infrared absorption spectra of the kaolinite. Preadsorption of aurintricarboxylic acid blocked adsorption of HPAN, suggesting that “positive spots” due to exposed lattice aluminum on edges of the clay platelets were adsorption sites.

Adsorption of Hydrolyzed Polyacrylonitrile on Kaolinite: II. Effect of Solution Electrolytes

AbstractAdsorption experiments were conducted by mixing NaCl‐kaolinite and C14‐labelled hydrolyzed polyacrylonitrile (HPAN) in water solutions of various electrolytes. An increase in the concentration of electrolyte caused an increase in extent of HPAN adsorption, the effect apparently being due to reduction of electrostatic repulsion between HPAN and kaolinite and reduction in size of the HPAN coil. Divalent cations, especially the transition metals capable of being complexed, were more effective than univalent cations. Maximum adsorption took place in the presence of H+ ions.Anions capable of complexing lattice aluminum and/or decomposing the clay crystal reduced adsorption of HPAN. There was some evidence of competition for adsorption sites.