Framework Aluminum Atoms Research Articles

27Al MAS NMR characterization of AlPO 4-14 enhanced resolution and information by MQMAS Dr. Hellmut G. Karge on the occasion of his 65th birthday

The crystalline microporous aluminophosphate AlPO 4-14 was studied using 27Al NMR spectroscopy, with both conventional MAS and two-dimensional multiple-quantum MAS (MQMAS) techniques. Data obtained for the four non-equivalent framework aluminum atoms detected in AlPO 4-14 are reported. They are found to occur with the ratio 1:1:1:1 according to the proposed crystal structure. The aim of this paper is also to compare the NMR methods used in this work with those found in literature. It is concluded that the new MQMAS method is by now the best approach to distinguish easily the non-equivalent Al atoms in the structure of such materials. It is also very efficient to obtain rapidly the necessary information about distribution of chemical shifts and quadrupolar interactions.

Molecular shape selectivity of EUO zeolites

The micropores of the EUO zeolite consist of 10-membered ring (10-MR) channels with deep side-pockets circumscribed by 12-MRs. In decane isomerization and hydrocracking EUO zeolites crystallized in the presence of hexamethonium (HM) and dibenzyldimethylammonium (DBDMA) exhibit different molecular shape selective properties. The catalytic sites of EUO specimens crystallized with DBDMA are located in more spacious cavities than those of EUO crystals synthesized with HM. This disparity in molecular shape selectivity is explained by a different location of the framework aluminium atoms in the two materials, viz. by the absence of aluminium in the 10-MR channels in EUO-DBDMA. Such aluminium siting is in agreement with the location of the organic template cations in the pores of the as-synthesized zeolites suggested in literature and revealed by a molecular graphics modeling.

Multinuclear solid-state n.m.r. investigation of zeolite MCM-22

In this paper, the results of the first high-field ( B 0 = 17.6 T) 27Al MAS n.m.r. measurement on zeolite MCM-22 are reported. The 29Si and 27Al MAS n.m.r. spectra of as-synthesized and dealuminated zeolite MCM-22 were interpreted by signals of framework silicon and aluminum atoms located on at least five crystallographically nonequivalent T sites. After ammonium exchange and calcination of zeolite MCM-22, acidic bridging OH groups identified by a 1H MAS n.m.r. signal at 4.2 ppm are formed. Using 29Si CP/MAS n.m.r. spectroscopy these bridging OH groups were attributed to hydroxyl protons bonded via oxygen bridges to Si(1 Al) atoms located on all the at least five crystallographically nonequivalent T sites.

Enhanced resolution in the 1H NMR spectra of zeolite H-ZSM-5 by heteronuclear dipolar-dephasing spin-echo MAS

Dipolar-dephasing with high power 27Al irradiation yields 1H MAS NMR echo spectra of ‘aluminium-unconnected’ hydroxyl groups in dehydrated zeolites. The aluminium-connected SiOHAl and AlOH hydroxyls can be observed as the difference spectra. The technique increases the number of OH lines in the 1H NMR spectra of dehydrated zeolite H-ZSM-5. Two kinds of Brønsted sites can be distinguished and interpreted in terms of their mobility in the vicinity of framework aluminium atoms.

Coking, aging and regeneration of zeolites XV. Influence of the composition of HY zeolites on the mode of formation of coke from propene at 450°C

The formation of coke from propene was investigated at 450°C on a series of HY zeolites differing by the number of framework and extraframework aluminium atoms per unit cell ( N Al and N EFAL ). A constant turnover value for coking was found for samples with low N al while lower values were obtained for high N Al numbers. Extraframework aluminium species participated in coke formation but to a limited extent. The coke composition was established for various coke contents. Whatever the zeolite sample, relatively small polyaromatic (C n H 2n − 26 , C n H 2n − 32 and C n H 2n − 36 ) molecules were formed in the zeolite supercages at low coke contents. These molecules can be transformed into highly polyaromatic molecules either through successive reactions of alkylation, cyclization, hydrogen transfer—or through coupling. The first mode occurs mainly for coke molecules trapped in supercages close to the outer surface of the zeolite crystallites or close to mesopores; the resulting highly polyaromatic molecules overflow onto the outer surface or into the mesopores. The second mode consists of coupling of coke or coke precursor molecules trapped in adjacent supercages. The relative significance of these two modes depends on the rate of coking, hence on the density of the acid sites. The lower the density the more significant the formation of highly polyaromatic molecules through coupling because of the longer residence time of the coke molecules in the pores.

Coking, aging and regeneration of zeolites: XVI. Influence of the composition of HY zeolites on the removal of coke through oxidative treatment

Coke removal by oxidative treatment was studied for a number of HY zeolites with framework Si/Al ratios in the range of 4 to 100, coked to different levels during propene transformation at 450°C. Except for the zeolites with high Si/Al ratios there was practically no effect of the coke content, hence of the nature and of the location of coke molecules, on coke oxidation. Oxidation of pure pyrene trapped in zeolite supercages showed furthermore the same tendency as coke oxidation: The density of framework aluminium atoms, hence of zeolite acid sites, was the determining parameter. Neither extraframework aluminium species nor small quantities of sodium (<0.15 wt.-%) in the zeolites influenced coke removal. However the CO to CO 2 ratio in the oxidation product diminished with increasing sodium content. Oxidation began by the hydrogen atoms with formation of oxygenated compounds and water, and a mechanism involving radical cations as intermediates was proposed to explain this first step of coke oxidation. The subsequent steps of coke removal, comprising decomposition of the oxygenated compounds, would also be catalyzed by acid sites.

Evidence for a participation of zeolite acid sites in the coke removal through oxidative treatment

Coke removal by oxidative treatment was investigated for a number of HY zeolites with framework Si/Al ratios of 4–100, coked at different levels (from 2 to 20 wt%) during propene transformation at 450° C. A strong influence of the density of framework aluminium atoms hence of the density of the acid sites was found. On the other hand the content of the zeolite in extraframework aluminium species as well as the composition of coke were of minor importance for coke removal. A mechanism involving radical carbocations as intermediates is proposed.

Study on the Acidic Properties of LaDAIY With 29 Si MAS NMR and NH 3 -TPD

1 Introduction To study the acidic properties of Y-type zeolites has important meaning in the field of zeolite catalysis. The nature and numbers of active sites of Y-zeolites can be modified by dealumination and rare earth ion exchange to increase the catalytic activity and stability of modified Y-zeolites. In view of the zeolite structure,the origin of strong acid sites is due to the change of the density and distribution of the framework aluminum atoms, which results in the increasing of the isolated framework aluminum atoms or those with no next-nearest

Multinuclear MAS NMR study of the microporous aluminophosphate AlPO 4-17 and the related silicoaluminophosphate SAPO-17

AlPO 4-17 and SAPO-17 in their as-synthesized, calcined, and calcined and subsequently rehydrated forms have been studied by 27Al and 31P MAS NMR. Pronounced structural changes caused by template removal and rehydration can unambiguously be attributed to a change in the coordination number (on calcination: 5 → 4, on rehydration/dehydration: 4 ↔ 6) of part of the framework aluminium atoms. The different resonance lines can be assigned to crystallographically inequivalent sites present in the modified ERI framework. 27Al quadrupolar coupling parameters of the two aluminium sites in the calcined AlPO 4-17 ( C Q = 4.4 MHz and 2.1 MHz) were determined by recording spectra at different field strengths. The isotropic chemical shifts were obtained from rotation sidebands of the (± 3 2 , ± 1 2 ) satellite transitions. 2D 27Al nutation MAS NMR was used to corroborate the line assignment for the as-synthesized and the rehydrated AlPO 4-17. By using 13C MAS NMR it was shown that the occluded template (cyclohexylamine) is present as ions. For the first time, a splitting of the 29Si NMR line caused by crystallographically inequivalent sites in a SAPO-type material is reported. The line splitting of 4.3 ppm is comparable with that observed for isostructural aluminosilicate erionite.

Structure determination and rietveld refinement of aluminophosphate molecular sieve AIPO4-8

The 18-ring pore molecular sieve VPI-5 recrystallizes to the 14-ring sieve AIPO4-8. This recrystallization appears to be reversible under proper conditions, as could be explained on the basis of the structural relationship between the two materials. VPI-5 has an 18-ring channel bounded by six 6-rings and six pairs of adjoining 4-rings. In the conversion to AIPO4-8, two-thirds of the adjoining 4-rings are converted to 6-rings and reconnected to form a smaller, 14-ring channel bounded by eight 6-rings, two pairs of adjoining 4-rings, and two single 4-rings. The space group for AIPO4-8 [Cmc21: a = 33.0894(25), b = 14.6832(13), c = 8.3630(6) Å at room temperature] is a direct subgroup of that for VPI-5 [P63cm: a = 18.549(1), c = 8.404(1) Å for the dehydrated form; orthohexagonal setting: a = 32.128, b = 18.549, c = 8.404 Å]. Interconversion of the two structures may be facilitated by extracoordination of framework aluminum atoms with water molecules. The observed small adsorption capacity of AIPO4-8 can be explained by stacking faults, where, within a single channel, different pairs of adjoining 4-rings are left unbroken.

Isomerization of o-dichlorobenzene over H-mordenite : Effect of the silicon-to-aluminium ratio

The isomerization of o-dichlorobenzene was studied over a series of H-mordenites in which the number of framework aluminium atoms was varied over a large range (4<Si/Al<39.5). The o-dichlorobenzene was converted into m- and p-dichlorobenzene while catalyst deactivation occurred as a result of the formation of coke. The changes in reaction rates with the aluminium content of the catalysts may be directly correlated with the evolution of the acidity of the zeolite. A volcano-type relationship between the activity and the aluminium content of H-mordenite was observed. Deactivation is not related to acidity.

Dealumination of ZSM-5 — Characterization by TPD, DTA/DTG, XPS and Adsorption Measurements

A series of ZSM 5 samples obtained by dealumination is studied by TPD, DTA/DTG, XPS and adsorption measurements. The amount of adsorbed water at p/p0 ≦ 0.1 and the amount of chemisorbed NH3 corresponding to the TPD peak at 693–723 K correlate with the number of framework aluminium atoms per unit cell. At strong dealumination an enrichment of non-framework aluminium in the surface layers of the crystals was found. The HZSM-57 zeolites are transformed into silicalite-like samples during the course of the DTA experiment in the temperature range 973–1273 K.

Physico—chemical characterization of pentasil type materials. IV. Thermal and steam stability, dealumination and aluminium exchange

Thermal and steam stability, dealumination and aluminium exchange are studied on zeolitic samples obtained by the modified Argauer- and Grose-type synthesis procedures. The thermal treatment leads to the orthorhombic—monoclinic transformation for samples of high Si Al ratios, while the orthorhombic form is retained for low Si Al ratios. The steam stability depends on the Na Al ratio. For Na Al > 1 , the zeolite is partially transformed to cristobalite. Steaming of the acidified samples was found to be one of the best dealumination procedures. The tetrahedral framework aluminium atoms are transformed to extra-framework octahedral aluminium species. Surface enrichment in aluminium occurs in samples with Si Al ≈ 24 , while surface impoverishment is detected for samples of Si Al ≈ 80 . Aluminium exchange with Al(NO 3) 3, followed by steaming, leads to framework dealumination and to aluminium migration to the surface.

Dehydroxylation of Hydrogen Zeolites: Conditions for the Preparation of Thermally Stable Catalysts

The elimination of water from hydrogen zeolites proceeds in two steps: (i) between ca. 320 and 550°C water is eliminated from hydroxy groups coordinated to adjacent framework aluminum atoms; (ii) between ca. 550 and 630°C water elimination proceeds via migration of framework aluminum atoms into the intracrystalline pore system. Suitable choice of conditions, especially in the last stage, provides a means of influencing the quality of the resulting zeolite catalysts. Too fast a heat‐up rate between 500 and 600°C causes the crystal lattice to collapse.