Lanthanum-exchanged Zeolites Research Articles

Negative thermal expansion and cationic migration in zeolite Y used in FCC catalysts

The influence of extra-framework cations on the intrinsic negative thermal expansion (NTE) of calcined ultrastable and lanthanum-exchanged zeolites Y was analysed. High-temperature thermal behaviour, cationic migration and dealumination were examined by in situ high temperature X-ray powder diffraction and thermogravimetric (TG) analyses. We analysed protonated ultrastable zeolite Y and zeolite Y containing 13% \(\hbox {La}_{2}\hbox {O}_{3}\) (LaY), and investigated how the presence of \(\hbox {La}^{3+}\) influences thermal behaviour of zeolite Y. Uncalcined LaY was also analysed to observe the \(\hbox {La}^{3+}\) migration before calcination. Mass loss up to \(990{^{\circ }}\hbox {C}\) was accompanied by TG analyses. X-ray powder diffraction data were analysed by Rietveld method. Our data show that lanthanum migration from supercages to sodalitic cavities is not reversible. However, some \(\hbox {La}^{3+}\) migration occurs above \(400{^{\circ }}\hbox {C}\) and we observe emptying of site \(\hbox {I}{'}\), migration to site I and to some extent to site \(\hbox {II}{'}\). Furthermore, we examined the dealumination process as well. Both samples show two distinct thermal behaviours, positive or near-zero thermal expansion up to \(500{^{\circ }}\hbox {C}\) and NTE above this temperature. Together with intrinsic NTE in faujasite type zeolites, the dealumination process is largely responsible for NTE at high temperatures.

Lanthanum-exchanged zeolite and clay as anticorrosive pigments for galvanized steel

A wide variety of inhibitive pigments is now being offered as possible alternatives to chromate and lead compounds for painted metals protection. Unfortunately, the most wide spread of these substitute pigments, zinc phosphate, has, at present, raised some environmental concern because phosphate causes the eutrophication of water courses and zinc itself is toxic. The aim of this research was to study the anticorrosive performance of a mixture consisting of zinc phosphate, modified zeolite and clay (bentonite) in order to diminish phosphate content in paints. The zeolite and the clay were exchanged with La(III) ions, as inorganic green inhibitor. In the first step, the anticorrosion protection by La(III) ions in solution was assessed by electrochemical tests. In the second step, an epoxy-polyamide paint formulated with the pigment mixture applied on galvanized panels was studied by salt spray test and electrochemical noise measurements (ENM). The results showed that it was possible to replace part of the zinc phosphate content in the paint with the exchanged zeolite and the clay.

Relationship between two characteristic diffractions and the status of cationic lanthanum species in zeolite LaNaY

By employing ex situ and in situ powder X-ray diffraction (XRD) studies, the evolution of two characteristic diffractions at 11.9° and 12.4° (2θ) with the real-time migration of cationic lanthanum species in hydrated, dehydrated, and rehydrated lanthanum-exchanged zeolite NaY (LaNaY) have been discussed thoroughly. We found that the [La(H2O)m]3+ ions in the supercages can strengthen the (222) diffraction and reduce the (311) diffraction dramatically, and such effects disappear when [La(H2O)m]3+ ions migrate into the sodalite cages and the hexagonal prisms. The relationship obtained shows the capability to monitor the content/location of lanthanum species in LaNaY sample by monitoring the diffractions between 11.6° and 12.8° (2θ) in its XRD pattern, and may be useful in the routine characterization of LaNaY samples in petrochemical industry, avoiding long time collection and the structure refinement of XRD patterns. Moreover, carefully designed in situ XRD characterization proves that H2O molecules play an important role in the reversible migration of cationic lanthanum species in zeolite Y. 27Al MAS NMR was also performed for validation.

Concentration and acid strength of hydroxyl groups in zeolites La,Na-X and La,Na-Y with different lanthanum exchange degrees studied by solid-state NMR spectroscopy

1H MAS NMR spectroscopy was applied for a quantitative investigation of the concentration of hydroxyl groups on lanthanum-exchanged zeolites X and Y as a function of the dehydration temperature. A maximum concentration of Brønsted acidic bridging OH groups (SiOHAl) was reached upon dehydration of zeolites La,Na-X and La,Na-Y at 423 K. Further raise of the dehydration temperature leads to the recombination of the above-mentioned hydroxyl groups, while the dealumination of the framework is negligible. The maximum number of bridging OH groups formed on zeolites La,Na-X and La,Na-Y correlates well with the lanthanum exchange degree. By adsorption of deuterated acetonitrile as a probe molecule, an increase of the acid strength of bridging OH groups in lanthanum-exchanged zeolites X and Y was found. This finding is explained by the polarizing effect of multivalent lanthanum cations, which leads to a withdrawal of electrons from the O–H bonds of Brønsted acid sites in lanthanum-exchanged zeolites with a high concentration of these extra-framework cations.

Low-Temperature Activation of Branched Octane Isomers over Lanthanum-Exchanged Zeolite X Catalysts

Adsorption and surface chemistry of octane isomers on La-exchanged zeolite X was explored under near-ambient conditions. At low coverage, the sorption constants depend mainly on dispersion forces. However, very strong polarization of the C−H bonds is indicated by an unusually high extinction coefficient of the C−H vibrations. Bi- and tribranched alkanes react under these conditions, and the reactivity increases with the degree of branching. The activation proceeds via hydride abstraction forming alkoxy groups, which subsequently isomerize, and crack. Cracking products desorb primarily via hydride transfer from mobile alkanes leading predominantly to isobutane and isopentane. The surface chemistry and alkanes produced show that adsorption and desorption, cracking and alkylation, as well as hydride-transfer reactions already occur at near ambient conditions in these zeolites.

Disproportionation of hydrogen peroxide into singlet oxygen catalyzed by lanthanum-exchanged zeolites

Lanthanum(III) was immobilized on various zeolite supports by simple cation exchange. The resulting materials were tested in the disproportionation of H 2O 2 into singlet molecular oxygen ( 1O 2). The rate of H 2O 2 disproportionation and the reaction of the formed 1O 2 with citronellol as a typical olefinic substrate were strongly dependent on the zeolite topology and composition. Zeolites with a large crystal size, small pores, or a one-dimensional pore system showed low activity in the disproportionation of H 2O 2. In contrast, zeolites with a small crystal size or zeolites possessing large intersecting pores were active 1O 2 generators. La supported on ultrastable Y zeolite (USY) and zeolite Beta were identified as the most active and efficient catalysts. Typically, within 24 h at 40 °C, 100 mol of citronellol hydroperoxides is produced per mol of La supported on USY zeolite. Within this time, 800 mol of H 2O 2 is disproportionated into 1O 2. In comparison with unsupported La-hydroxide, La-USY shows a significantly higher activity and a slightly more efficient use of H 2O 2. The heterogeneous nature of the catalysis by La-USY was verified by filtration tests, and its stability was proved by X-ray diffraction and recycle experiments. Finally, the catalyst was used in the peroxidation of various olefinic compounds. Compared with conventional molybdate catalysts, La-USY is particularly useful for the selective peroxidation of allylic alcohol derivatives, with very little competitive epoxidation and no alcohol oxidation being observed.

Lanthanum-exchanged zeolites as active and selective catalysts for the generation of singlet oxygen from hydrogen peroxide

Lanthanum(III)-exchanged zeolites Beta and USY are active and selective catalysts for the generation of singlet oxygen from H2O2 showing superior activity and oxidant efficiency compared to unsupported La-catalysts, e.g. La(OH)3.

Influence of the Nature of Residual Alkali Cations on the Catalytic Activity of Zeolites X, Y, and EMT in their Brønsted Acid Forms

Lanthanum-exchanged zeolites X, Y, and EMT, containing either sodium or potassium as residual cations, were used as catalysts in two acid-catalyzed hydrocarbon reactions, viz. the disproportionation of ethylbenzene and the isomerization of n-octane (for the latter reaction, Pd/γ-Al2O3 was admixed to the acid zeolites to make the catalysts bifunctional). All pairs of LaM-type zeolites (where M stands for Na or K) showed remarkable activity differences, the LaK form being always less active than the LaNa form. IR and 1H NMR spectroscopy revealed that, for a given zeolite type and degree of lanthanum exchange, the concentration of bridging hydroxyl groups in the large cages was at least 20% lower than that for the LaK form. 139La NMR spectroscopy indicated that the concentration of lanthanum ions in the large cages of the as-exchanged LaK forms was always lower than that in the corresponding LaNa forms. Surprisingly, already in the as-exchanged LaK forms, some La3+ migration into the small cages appears to occur, and this was corroborated by framework strains indicated by 29Si and 27Al NMR. It is proposed that potassium cations in the large cages cause an enhanced migration of lanthanum cations into the small cages and a preferential formation of Brønsted acid sites in these small cages during the thermal dehydration of lanthanum cations (Hirschler–Plank mechanism). A similar effect was found in HNaY and HKY zeolites: The presence of the bulkier potassium cations causes a significant diminution of the accessible Brønsted acid sites in the large cages and of the catalytic activity. Overall, one must conclude from these results that in zeolites possessing both large and small cages, the nature of the residual alkali cations can exert a pronounced influence on the local distribution of the Brønsted acid sites, regardless of how these sites were generated. Copyri ght 2001 Academic Press.

Deactivation of solid acid catalysts during isobutane alkylation with C4 olefins

Coke formation on solid acid catalysts during isobutane alkylation with C4 olefins was studied. Y-zeolite, mordenite and L-zeolite were investigated, as well as sulfated zirconia catalysts. Zeolites were used in protonic form or after ion exchange with lanthanum nitrate. Studies were carried out in liquid phase in a fixed-bed reactor. It was found that Y-zeolite exchanged with lanthanum, being the catalyst with the best stability, is the catalyst that forms higher quantities of carbonaceous deposits. The amount of coke on this catalyst can be as high as 13–14%. This coke requires temperatures higher than 500°C to be completely eliminated. Temperature-programmed analyses indicated that the amount of coke eliminated from the catalyst during partial regeneration, strongly depends on the heating rate. This is due to a competition between coke gasification and its modification in structure leading to an aromatic type of coke, which then requires higher temperatures to be burnt. Pore and surface area measurements carried out on deactivated catalysts suggest that a pore plugging mechanism takes place during the reaction. Catalysts with too strong acidity (e.g. protonic form of Y-zeolite or sulfated zirconia) have no activity for trimethylpentane production at 80°C, most probably due to a very fast deactivation, even though the amount of coke is notably lower than in the lanthanum-exchanged zeolites.

Cation location and migration in lanthanum-exchanged zeolite NaY studied by X-ray powder diffraction and MAS NMR spectroscopy

The population of cation sites in a lanthanum-exchanged zeolite NaY (LaNaY-73) in the course of calcination and at conditions similar to catalysis (T= 623 K, continuous flow of m-xylene and nitrogen over calcined zeolite LaNaY) has been studied by ex situ and in situ X-ray powder diffraction. Ex situ magic-angle spinning (MAS) NMR spectroscopy at room temperature complemented the diffraction results. During calcination of LaNaY, a cation migration process (lanthanum from supercage to sodalite cage, sodium from sodalite cage to supercage), after pretreatment at 363 K, was observed. In the calcined sample all lanthanum cations are located at position SI′ in the sodalite cage while 10 of the 16 sodium cations per unit cell occupy position SII in the supercage. A sodium migration from the sodalite cages to position SII in the supercages was observed by ex situ temperature-dependent experiments on samples calcined between 298 and 623 K. X-Ray powder diffraction, as well as multinuclear solid-state NMR spectroscopy, has shown that adsorption of m-xylene induces sodium migration from the sodalite cages to position SII in the supercage, whereas the positions of lanthanum are not influenced. Furthermore, m-xylene could be located by in situ X-ray powder diffraction at conditions similar to catalysis at the preferred adsorption site in front of the six-ring window of the supercage, as determined by ex situ diffraction experiments at low temperature. The positions and populations of the cations derived at 623 K agree with the results obtained from ex situ diffraction experiments carried out at this temperature.

Conductivity variations in hydrated lanthanum-exchanged zeolites X, Y and A

Sodium-form zeolite X, Y and A parent materials have been lanthanum-exchanged to varying degrees. Temperature dependences (−40°C ⪕ T ⪕ 80°C) of ionic conductivities have been determined using impedance techniques for materials stored at a fixed relative humidity (RH ≈ 30% at ambient temperature) and approximate to Arrhenius behaviour in all cases at T ⪕ 30°C. Conductivities decrease on lanthanum exchange. The dominant mobile charge carrier in these systems is hydrated Na+. A minimum in the variation of apparent activation energy (E) with sodium content is apparent for both X and Y zeolites.

Role of the amorphous matrix in the hydrothermal ageing of fluid catalytic cracking catalysts

The influence of hydrothermal ageing on a matrix-embedded lanthanum-exchanged zeolite Y has been reinvestigated using several physical techniques (IR, XRD, NMR and STEM). It was found that the crystallinity of LaY zeolite was totally preserved when the zeolite was incorporated into the amorphous matrix, while the same zeolite, if not embedded, lost 55% of its initial crystallinity upon steaming. In addition, a significant dealumination of the zeolite structure was observed. Direct local measurements of the chemical composition of the steamed catalyst using a STEM analytical microprobe showed that severe steaming induced material transportation between the zeolite and the matrix. Isotopic 29Si labelling of the matrix combined with 29Si MAS-NMR provided the decisive proof for a silicon transportation from the matrix toward the zeolite component and its subsequent reincorporation into the zeolite framework in the place of the expelled aluminium atoms. This phenomenon is thought to be responsible for the “healing” of matrix-embedded zeolites and therefore to explain their increased resistance against severe steaming conditions.

Dynamics of acetonitrile crystals and clusters

Three crystalline phases of acetonitrile have been investigated by high-resolution neutron inelastic scattering spectroscopy for 20 < T/K < 300 over the energy-transfer range 1 < ΔE/cm–1 < 4000. Clear differences are seen between the phonon densities of states for the α and β phase crystals at 20 K as well as between each of these and the approximately five unit cell clusters of acetonitrile encaged in lanthanum-exchanged zeolite Y. For the intramolecular region corresponding frequencies are found in all phases. Using the excitation energies and the intensities of scattering the most consistent intramolecular force field is defined and the coupling between internal and lattice modes, as represented by combination bands, studied over a wide frequency domain for the α and β phases. Experimental and calculated neutron scattering intensities agree well for the two bulk solid phases and are consistent with strongly bound acetonitrile dimers as a persistent dynamical unit, an aspect which also strongly determines the rotational dynamics. The low-energy maximum in the density of vibrational states for clusters is strongly shifted to low energies, indicating an appreciable effect of the cluster size on the rotational dynamics.

The role of polyvalent cations in developing strong acidity: A study of lanthanum-exchanged zeolites

A series of faujasite and ZSM-20 zeolites was prepared such that they had a minimum amount of extraframework aluminum, but a range of Si/Al ratios. These materials were relatively inactive for the hexane cracking reaction, which was used as a test of strong acidity. When La 3+ ions had been exchanged into the zeolites, the cracking activity first increased with respect to lanthanum loading and then went through a maximum. When the maxima were plotted as a function of the framework aluminum content, Alf, for the series of zeolites, the activities increased until Alf was ca. 35 per u.c. and then decreased below a measurable level with zeolite X. The change in activity with respect to Alf paralleled that observed with steam-dealuminated zeolites, but the absolute activities of the La-exchanged samples were less. These results are consistent with a model of strong acidity in which both the Alf distribution and the presence of polyvalent cations in the β /OHM ,n+ cages are important. Lanthanum ions in the form of ▪ or La(OH) 2+ species are believed to be responsible for the withdrawal of electrons from the framework hydroxyl groups, thus making the protons more acidic.

Study of lanthanum ion-exchanged zeolite NaY by solid-state nuclear magnetic resonance spectroscopy

The 29Si chemical shifts of dehydrated and rehydrated lanthanum-exchanged zeolite NaY are found to shift to up-field with increasing lanthanum content. The spectrum of a dehydrated sample can be rationalized by assuming that each of the four Si(nAl) lines splits into two lines with an intensity ratio equal to the Na/La equivalent ratio. This result shows that the effect of the La ions on the Si nuclei can be distinguished from that of the Na ions.