Extra-framework Aluminum Research Articles

Standardization of catalyst preparation using reference catalyst: ion exchange of mordenite type zeolite: 1. Remarkable dealumination accompanying ion exchange

A joint study was organized to establish a standard set of conditions for the ion exchange of zeolite from sodium into proton-form. The sodium-form mordenite with Si/Al 2 = ca. 15 was ion exchanged into an NH 4-form, followed by calcination according to various recipes. Then various advanced techniques for characterization (ICP, TG, XPS, SEM, XRD, N 2 adsorption, 29Si and 27Al NMR, benzene diffusion, IR of OH and adsorbed pyridine and CO, ammonia TPD) were applied, and some test reactions (cracking of cumene, isopropylation of biphenyl, oligomerization of propene and chichibabin condensation of acetaldehyde and ammonia into picoline) were conducted. The ion exchange (removal of sodium) proceeded as expected, but remarkable differences were observed in physicochemical and catalytic properties of the thus-prepared proton-form samples. Exceptionally high temperature (383 K) for the ion exchange resulted in the structural degradation, while most samples exchanged at 333–353 K maintained the crystallinity and pore volume. The use of NH 4Cl slightly changed the crystal morphology. Extra-framework aluminum species was formed on most samples after calcination above 773 K. In contrast, calcination at 673 K maintained the framework aluminum. Therefore, the dealumination is considered to proceed after complete removal of ammonia from the ammonium-form zeolite. However, the proton-form zeolite was stable under dry conditions, so it is speculated that the dealumination was induced by the contact of the proton-form zeolite to atmosphere with humidity. The total and Brønsted acidity decreased with the dealumination, while Lewis acidity increased. On the other hand, rapid heating of the ammonium type zeolite caused narrowing of the micropores. These structural changes seriously affected the catalytic activities for various reactions.

A study of the reaction between zeolite H-ferrierite and Cd or Zn metal

Cd- and Zn-form ferrierites were obtained by reacting the mixture of zeolite H-ferrierite and the powder of cadmium or zinc metal. Hydrogen was detected as reaction product, suggesting that the metal was oxidized by the zeolite protons. The reaction was characterized by the rate of H 2 evolution using the temperature-programmed reaction (TPRc) method. The H 2 -TPRc peak was found to appear at lower temperature for the more volatile Cd metal and for the H-ferrierite, having higher Si-to-Al ratio and containing less extra-framework aluminum (EFAl) species in the zeolite channels. Results suggested that the diffusion rate of the metal vapor within the micropores governed the rate of the reaction.

Infrared Evidence of Three Distinct Acidic Hydroxyls in Defect-Free HY Faujasite

Trimethylamine (TMA) and ammonia (NH(3)) were coadsorbed over a free extraframework aluminum (EFAL) HY zeolite, which allowed us to distinguish at least three different acidic hydroxyls in the faujasite structure.

Room temperature detemplation of zeolites through H2O2-mediated oxidation

Detemplation of zeolite beta has been successfully achieved at low temperature by controlled oxidation of the template using H2O2 and catalyzed by traces of Fe3+. With this approach, unlike calcination, the pristine structure of the material is well preserved; no extra-framework aluminium is formed.

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0= 9.4 to 17.6 T

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.

Mo K-edge EXAFS analysis of Mo/USY zeolite – effects of extra-framework aluminum on the location and dispersion of Mo sulfide catalysts

The local structures around Mo supported on USY zeolites containing different amounts of extra-framework Al species (EFAL) were investigated to elucidate the effects of EFAL on the location and dispersion of Mo sulfide catalysts. Mo K-edge EXAFS analysis indicated that Mo sulfide catalysts were highly dispersed and located, at least in part, in the micropores of USY zeolites, independent of the EFAL amounts. The dispersion of Mo sulfide was found to decrease with increasing amount of EFAL in the zeolites. EXAFS analysis of Mo oxide catalysts, the precursor of the sulfide catalysts, suggested that Mo oxide catalysts deposited on EFAL during the catalyst preparation were reluctant to disperse into micropores and formed larger MoS2 clusters during sulfiding that lowered the dispersion.

Dissociative adsorption of water in CaNaA zeolites studied by TG, DRIFTS and 1H and 27Al MAS NMR spectroscopy

Three CaNaA zeolites, in which, respectively, 49%, 65% and 83% of the Na + cations were exchanged with Ca 2+ ions, along with a NaA sample were studied by thermogravimetry, temperature-programmed DRIFT spectroscopy and MAS NMR. Thermogravimetric desorption profiles of the Ca 2+ exchanged zeolites show a low-temperature peak between 423 and 508 K as well as two high-temperatures peaks at 853 and 973 K, which are missing in case of the NaA sample. Three main bands at 3603–3611, 3495–3510 and 3552–3565 cm −1, of which the first two are due to bridging Si–OH–Al groups located in 8- and 6-rings, respectively, whereas the third corresponds to Ca(OH) + groups dominate the DRIFT spectra of zeolite samples exchanged with Ca 2+ ions. The intensity of the band at 3603–3611 cm −1 decreases significantly until it vanishes almost completely at 823 K. A simultaneous decreases of the intensity of the band of Ca(OH) + groups is indicative of a low-temperature dehydroxylation process between these two species. The intensity of the other Si–OH–Al group bands at 3495–3510 cm −1 shows almost no change up to 823 K, thus suggesting that these groups participate in a high-temperature dehydroxylation process. A weak band in the region 3670–3680 cm −1 is attributed to Al–OH groups of extra-framework aluminum species formed during dehydration of the CaNaA zeolites. Peaks at 4.3 and 5.2 ppm in the 1H MAS NMR spectra are assigned to Si–OH–Al groups in 8- and 6-rings, respectively. 27Al MAS NMR spectra show two main peaks at 57.3 and 78.7 ppm, which are due to tetrahedrally coordinated aluminum and calcium alumosilicate groups. A weak signal between 12 and 13 ppm ascribed to octahedrally coordinated aluminum disappears at higher temperatures.

Influence of the Binder on the n-Octane Hydroisomerization over Palladium-Containing Zeolite Catalysts

The influence of the binder on the properties and performance of palladium-containing zeolite catalysts in the n-octane hydroisomerization was studied. Three different framework zeolites were used as catalysts: mordenite, β, and ZSM-5 with or without binder. To characterize the catalysts, surface area measurements, temperature-programmed desorption of ammonia, atomic absorption spectroscopy, inductively coupled plasma emission spectrophotometry, hydrogen chemisorption, and solid-state 27Al NMR were used. Catalytic performance of the zeolites was strongly influenced by the binder because its presence modified both zeolite acidity and porosity. A decrease in the strong acidity of the zeolites could be observed when the samples were agglomerated, due to a solid-state ion exchange between the zeolite protons and bentonite sodium. The neutralization of some zeolite acid sites caused a decrease in the n-octane conversion for mordenite (from 90.5 to 53.8 mol %) and ZSM-5 (from 78.4 to 71.2 mol %) zeolites. The binder modified the porosity of the zeolite providing meso- and macropores, which allow a higher formation of branched isomers for mordenite (from 27.3 to 48.9 mol %) and ZSM-5 (from 14.3 to 35.4 mol %). Activity of β zeolite was modified by the presence of extraframework aluminum species (EFAL) formed during the agglomeration process, improving both n-octane conversion (from 22.9 to 88.7 mol %) and branched isomer selectivity (from 48.7 to 77.5 mol %).

Catalytic reduction of NO 2 in nanocrystalline NaY zeolite

In this study, in situ transmission FT-IR spectroscopy was employed to investigate the thermal and selective catalytic reduction of NO 2 with propylene in nanocrystalline NaY zeolite. NO was the major product in the thermal reduction of NO 2. In absence of oxygen and water, selective catalytic reduction of NO 2 with propylene at low temperatures ( T ≤ 473 K) resulted in the complete reduction of NO 2 to N 2 and O 2. Isocyanate and nitrile species formed on extra framework alumina sites are found to be important intermediates. Based on kinetic studies, a pathway involving radical reactions appears to be the most plausible mechanism for the complete reduction of NO 2.

Dealumination of proton form mordenite with high aluminum content in atmosphere

A proton form mordenite with Si/Al 2 = 10 was prepared by heating of an ammonium type sample at 813 K in a nitrogen flow and transferred into an NMR (nuclear magnetic resonance) cell in a dry box filled with a controlled atmosphere, and the framework aluminum content was measured by means of 29Si NMR. It was estimated that almost all the aluminum atoms were located in the framework on the sample treated in helium which was purified by passing a liquid nitrogen trap. On the other hand, a considerable amount of extra-framework aluminum was estimated on the sample treated in commercially supplied nitrogen. After the cap of NMR cell was opened for several seconds in atmosphere, also the extra-framework aluminum was observed. These findings point out the readily dealumination of the proton form mordenite at room temperature. The dealumination resulted in the decrease of acidity and catalytic activity for cracking of octane and cumene [(1-methylethyl)benzene]. On the basis of the experiments using mordenite with Si/Al 2 = 15, the extent of dealumination was considered to decrease with decreasing the aluminum content and increasing the alkaline content.

Characterization of extraframework aluminum in H-mordenite dealuminated with ammonium hexafluorosilicate

The status and nature of extraframework Al (EFAl) species in H-mordenite dealuminated by ammonium hexafluorosilicate (i.e., (NH 4) 2SiF 6, AHFS) was investigated. Considerable difference arising from the experimental conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH 4OAc), was observed. Both treatments were not very effective in removing framework aluminum. 27Al and 19F solid-state NMR was used to detect different extraframework aluminum fluoro-complexes resulting from the AHFS dealumination. In the presence of NH 4OAc, the essential part of the dealumination process carried on with AHFS was that most of the extracted Al 3+ reacted with F − to form (NH 4) 3AlF 6. In the absence of NH 4OAc, on the other hand, at least two different forms of aluminum fluoro-complexes other than (NH 4) 3AlF 6 were formed after dealumination. These aluminum fluoro-complexes showed multiple lines located in the range of −144 to −156 ppm in the 19F NMR spectrum. An ab initio method that used both Hartree-Fock and hybrid Hartree-Fock density functional methods at the 6-31+G ∗ basis set level was used to calculate the 19F NMR shifts for discriminating possible aluminum fluoro-complexes. Our results indicated that a combination of NMR and ab initio calculations provided a basis for using them to study EFAl species of unknown compositions and structures.

Incorporation of aluminum into the framework of mesoporous MCM-41: the contribution of diffuse reflectance spectroscopy

Diffuse reflectance spectroscopy (DRS) has been used to investigate structural modification of aluminum-incorporated mesoporous molecular sieves MCM-41. A room temperature synthesis method was used for preparation of pure silica MCM-41 and for aluminum-containing AlMCM-41 materials. Samples with Si/Al ratios of 100, 50, 25 and 10 were synthesized. A post-synthesis modification of a purely siliceous MCM-41 was also carried out using an aqueous aluminum sulfate solution with appropriate concentration to obtain materials with the same Si/Al ratios. The products possess highly ordered structure and high surface area as evidenced from X-ray diffraction (XRD) and BET measurements, respectively. DR spectra of aluminum-containing materials show a distinct and well-defined band at ≈200 nm correlated to framework aluminum. The intensity of this band is proportional to the amount of aluminum incorporated. post-synthesis-modified materials show a slightly lower incorporation of aluminum according to the DR spectra and energy-dispersive X-ray (EDX) analysis. Reflectance spectra of the as-synthesized samples, irrespective of the preparation method, show no significant amount of extra-framework aluminum. A very small amount of extra-framework aluminum was found when samples with a low Si/Al were calcined. These species are detected by producing a broad band at about 240–280 nm in the DR spectra.

A DFT study of the acidity of ultrastable Y zeolite: where is the Brønsted/Lewis acid synergism?

The effect of extra-framework aluminum (EFAL) species on the acidity of a HT6 cluster (T=Si, Al) was studied by density functional calculations. A higher acid strength, compared to an isolated site, was only found with Al(OH)2+. The increase in acid strength was not due to Brønsted/Lewis acid synergism, as previously reported, but to the stabilization of the conjugated base through hydrogen bonding with the EFAL hydroxy groups (see picture).

Factors affecting isomer yield for n-heptane hydroisomerization over as-synthesized and dealuminated zeolite catalysts loaded with platinum

Pt loaded Y, USY, beta, mordenite, ZSM-12, and ZSM-5 zeolites with different Si/Al ratios and a USY and ZSM-12 sample having enhanced activity were studied for the hydroisomerization of n-heptane. All the possible heptane isomers were obtained over Pt/H-ZSM-12 and the product distribution was closer to that over Y and beta catalysts, but very different from that over Pt/H-ZSM-5. Shape-selective effects in ZSM-12 resulted in a significantly higher isomer yield than the beta and Y-zeolite catalysts. For Y, beta, and ZSM-12 samples having very little extraframework aluminum, changing the Si/Al ratio did not affect the isomer yield. However, USY and ZSM-12 samples that showed enhanced activity were very sensitive to the Pt precursor and Pt loading, which caused huge differences in their activity and isomer selectivity. The Pt dispersion in the enhanced activity catalysts was also much lower. A careful optimization of the precursor, amount of Pt loaded, and pretreatment conditions is essential to achieve good balance between the metal and the acid functions in these catalysts. The optimized enhanced activity catalyst is able to provide isomer yields approaching that obtained over the regular catalysts, which suggests that zeolite acidity is not directly affecting the isomer selectivity. These results indicate that the maximum isomer yield for a Pt/H-zeolite catalyst is obtained when the metal and acidic functions of the catalysts are well balanced. Therefore, it may not be possible to increase the isomer yield beyond this maximum by modifying the acidity characteristics of the catalyst. Mordenite and ZSM-5 catalysts showed low isomer yields, but acid dealumination of the parent mordenite improved the isomer yield, possibly due to creation of mesopores.

Factors affecting isomer yield for n-heptane hydroisomerization over as-synthesized and dealuminated zeolite catalysts loaded with platinum

Pt loaded Y, USY, beta, mordenite, ZSM-12, and ZSM-5 zeolites with different Si/Al ratios and a USY and ZSM-12 sample having enhanced activity were studied for the hydroisomerization of n -heptane. All the possible heptane isomers were obtained over Pt/H-ZSM-12 and the product distribution was closer to that over Y and beta catalysts, but very different from that over Pt/H-ZSM-5. Shape-selective effects in ZSM-12 resulted in a significantly higher isomer yield than the beta and Y-zeolite catalysts. For Y, beta, and ZSM-12 samples having very little extraframework aluminum, changing the Si/Al ratio did not affect the isomer yield. However, USY and ZSM-12 samples that showed enhanced activity were very sensitive to the Pt precursor and Pt loading, which caused huge differences in their activity and isomer selectivity. The Pt dispersion in the enhanced activity catalysts was also much lower. A careful optimization of the precursor, amount of Pt loaded, and pretreatment conditions is essential to achieve good balance between the metal and the acid functions in these catalysts. The optimized enhanced activity catalyst is able to provide isomer yields approaching that obtained over the regular catalysts, which suggests that zeolite acidity is not directly affecting the isomer selectivity. These results indicate that the maximum isomer yield for a Pt/H-zeolite catalyst is obtained when the metal and acidic functions of the catalysts are well balanced. Therefore, it may not be possible to increase the isomer yield beyond this maximum by modifying the acidity characteristics of the catalyst. Mordenite and ZSM-5 catalysts showed low isomer yields, but acid dealumination of the parent mordenite improved the isomer yield, possibly due to creation of mesopores .

Verification of extra-framework aluminum in zeolite L by acetylacetone

The removal of the extra-framework aluminum species without destroying the micropore structure and extraction of the species can be achieved by employing complexing agents. The effect of the acetylacetone as a chelating agent for extraction of extra-framework aluminum from zeolite L was investigated by means of diffuse reflectance spectroscopy. The treatment of the as-synthesized zeolite L with acetylacetone at room temperature does not produce any extra-framework aluminum species as evidenced by diffuse reflectance spectra (DRS). However, acetylacetone reacts with extra-framework aluminum species that are formed upon thermal treatment or acid leaching of zeolite L. Removal of aluminum acetylacetone complexes out of the zeolite crystal can be achieved by several successive washing up steps by hot ethanol. DRS monitoring was used to control removal of extra-framework aluminum species because amount and distribution of these species are correlated with catalytic activity and selectivity of zeolites.

Acidic property of modified ultra stable Y zeolite: increase in catalytic activity for alkane cracking by treatment with ethylenediaminetetraacetic acid salt

Temperature-programmed desorption (TPD) of ammonia with improved techniques was applied to quantitative determination of the strength and number of acid sites of a series of modified Y zeolites. A weak acid site was generated by the isomorphous substitution of aluminum for silicon, and the strength was independent of the aluminum concentration. This type of acid site showed almost no activity for cracking of octane at 573 K. Multiple types of extra-framework aluminum were formed on the ultra stable Y (USY) zeolite by steaming to create the cracking activity. Strong Brønsted acid sites were increased by the treatment of USY with a sodium salt of ethylenediaminetetraacetic acid (EDTA). The catalytic activity for alkane cracking showed a positive relationship with the number of this type of acid site, suggesting that the active site of USY for the catalytic cracking is this strong Brønsted acid site.

Influence of Cation Exchange on the 27Al-NMR Spectra of Zeolites

The influence of cation exchange on the ²⁷Al-NMR spectra of NaA-zeolites has been studied by ²⁷Al-MAS- and MQ-MAS-Solid State-NMR. From the ²⁷Al-spectra a characterization of the different Al sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the ²⁷Al-NMR-spectra than exchange with larger cations like Ba²⁺. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites.

Influence of Cation Exchange on the27Al-NMR Spectra of Zeolites

AbstractThe influence of cation exchange on the27Al-NMR spectra of NaA-zeolites has been studied by27Al-MAS- and MQ-MAS-Solid State-NMR. From the27Al-spectra a characterization of the different Al sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the27Al-NMR-spectra than exchange with larger cations like Ba2+. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites.

A DRIFT spectroscopic study of the N 2 adsorption and acidity of H-faujasites

Brønsted acidity of faujasite (FAU) type zeolites, such as, H-Y, dealuminated H-Y, and ultra-stabilized H-Y (USY) was studied by DRIFT spectroscopy using N 2 adsorption at 298 K and at pressures ranging from 1 to 9 bar for probing of the acidic sorption sites. Both the α- and β-cage hydroxyl-groups were found to establish H-bonds with N 2. Dealumination by (NH 4) 2[SiF 6] or ultra-stabilization increased the accessibility of the β-cage protons. The Δ ν OH red shift, induced by the adsorption, was used as a measure of the intrinsic acid strength of the OH-groups. Samples, free of extra-framework aluminum (EFAl) species, had about the same Brønsted acid strength regardless of their Al content. In the EFAl-containing USY sample a fraction of the hydroxyl groups showed enhanced Brønsted acidity.