Hydroxyl Nests Research Articles

Nature and reactivity of Co species in a cobalt-containing beta zeolite: an FTIR study

Co–Al–BEA samples have been prepared in an attempt to introduce Co at tetrahedral lattice sites. No positive evidence of insertion has been gained, and no new Brønsted acidity is developed. The sample, instead, contains at least three families of Co 2+ surface sites, CoA, CoB and CoC. CoA ions, the dominating species, are at extraframework positions (carbonyl species at 2208 cm −1; mononitrosyl at 1939 cm −1: a large fraction forms dinitrosyl species). CoB and CoC are two types of Co 2+ ions grafted to the surface in defective situations (hydroxyl nests or stacking faults) and similar to Co species grafted to amorphous silica (carbonyl species at 2196 and 2184 cm −1; mononitrosyl species at 1915 and 1895 cm −1, respectively, form dinitrosyl species readily). Co 3+ is also present in small amounts only after NO contact, and is responsible for a mononitrosylic species at 1954 cm −1. Dinitrosyl species for CoA, CoB and CoC share the same modes at 1815 and 1900 cm −1: Co 3+(NO) 2 probably has the asymmetric mode at 1850 cm −1. Interaction with ammonia of irreversibly adsorbed NO species suggests the possible occurrence of cation pairs. The availability of two coordinative unsaturations on most Co species (or the occurrence of cation pairs) allows a reversible disproportionation of NO 2, consisting in the simultaneous adsorption of two molecules with a partial electron transfer from one ligand to the other, with formation of NO 2 δ+ and NO 2 δ− species: at a later stage of interaction, a proper disproportionation yielding nitrate species is observed. The presence of Co 2+ ions at extraframework positions together with some dealumination of the samples accounts for the negligible presence of Brønsted acidity.

Neutron powder diffraction study of orthorhombic and monoclinic defective silicalite

The crystal structure of silicalite (SiO2) with a substantial amount of structural hydroxyl groups [Si(1-x)O(2-4x)(OH)4x, with 0.08<x<0.10] has been refined from neutron powder diffraction data measured using the HRPD instrument at the ISIS pulsed neutron source. Powder data were collected on the as-synthesized orthorhombic sample at 298 K, and on the deuterated monoclinic sample at 100 K. Preferential location of Si-atom vacancies was found on four out of 12 independent T sites in the orthorhombic silicalite [Si(6), Si(7), Si(10) and Si(11)], although the H atoms of the substituting hydroxyl groups could not be located because of the low statistical site occupancy on multiple sites. No significant population of D atoms or of Si vacancies was found in the tetrahedral sites of the monoclinic sample. The detected long-range order of adjacent Si atoms in defective orthorhombic [MFI] structures is compatible with a mechanism of Si vacancy clustering and with the model of hydroxyl nests assumed in the literature on the basis of IR spectroscopic evidence. Crystal data: orthorhombic, Pnma, Z = 8, a = 20.0511 (1), b = 19.8757 (1), c = 13.36823 (9) A, V = 5327.62 (5) A3, Dx = 1.798 g cm(-3), Mr = 721.01; monoclinic, P2(1)/n, Z = 4, a = 19.8352 (2), b = 20.0903 (2), c = 13.3588 (1) A, beta = 90.892 (1), V = 5322.78 (6) A3, Dx = 1.799 g cm(-3), Mr = 1442.02.

A model for the formation of point defects in zeolites

Using ab initio DFT periodic and molecular cluster models, we investigate transformations of Brønsted acid sites (Al substitutionals compensated by H) in aluminium silicates upon steaming and calcination, possibly assisted by gamma-irradiation. These treatments can result in formation of (i) hydroxyl nest (hydrogarnet) defects; (ii) oxygen vacancies coupled with Al impurities; or (iii) over-coordinated Al substitutionals. We concentrate on process (ii) which involves either spin or charge separation at the defect site. For zeolites with high Si/Al ratios, we show that two paramagnetic centres are formed preferentially: an electron bound to Si adjacent to the O vacancy site, the E' centre (trigonal Si+ being a counterpart), and an electron hole localised on oxygens at the dehydrogenated Brønsted site (the V1.2, or X1.2 centre). Energetics, geometries and electron and hole distributions are calculated.

Quantitative Characterization of "Hydroxyl Nests"Concentration for the Dealuminated Y Zeolite

Quantitative Characterization of "Hydroxyl Nests"Concentration for the Dealuminated Y Zeolite

UV−Visible Diffuse Reflectance Spectroscopy of Zeolite-Hosted Mononuclear Titanium Oxide Species

Mononuclear titanium oxide species in faujasites of different Si/Al ratios are prepared by reaction of dehydrated zeolites with TiCl4 at 100 °C. The analysis of UV−vis diffuse reflectance spectra in combination with Raman spectroscopy and X-ray diffraction reveals the presence of three different types of titanium oxide species in the faujasite structure exhibiting excitation energies for the charge transfer from the 2p orbitals of oxygen ligands to the 3d orbitals of Ti of around 36 000, 45 000 and 49 000 cm-1. A reliable evaluation of diffuse reflectance spectra requires the calculation of the Kubelka−Munk spectra from the absolute reflectance of the sample. The different titanium oxide types are proposed to result from monofunctional, bifunctional, and multifunctional bonding of TiCl4 to isolated or neighboring OH groups or to hydroxyl nests with coordination numbers ranging from four to six. The blue shifts in the UV−vis spectra of mononuclear titanium oxide species can be discriminated from those of quantum-sized titania particles.

Characterization of Titanium Species Incorporated into Dealuminated Mordenites by Means of IR Spectroscopy and 18O-Exchange Technique

Titanium mordenite, Ti-M, was prepared by the solid−gas reaction (atom-planting method) between highly dealuminated mordenites and TiCl4 vapor at elevated temperatures (473−873 K), and the coordination state of the incorporated Ti was studied comparing with that in titanosilicate with MFI structure (TS-1). The amount of Ti incorporated leveled off at 1.7 kPa of TiCl4 vapor for a process time of 1 h, and only trace amount of Al and Si was released during the treatment. IR spectra confirmed that TiCl4 reacted with internal SiOH groups in hydroxyl nests. The concentration of oxygen atoms bound to the framework Ti atoms in both TS-1 and Ti-M zeolites was measured by the 18O-exchange reaction between C18O2 and the zeolite. The number of oxygen atoms bound to a Ti atom was ca. 4 for TS-1 zeolites and varied from 5 to 4 for Ti-M zeolites with increasing temperature of the TiCl4 treatment. From these results, a reaction mechanism was proposed for the reaction of TiCl4 with hydroxyl nests in highly siliceous mordenites. Ti-M exhibited an IR band at 963 cm-1, while it was observed at 960 cm-1 for TS-1. The relationship between the relative intensity of the 963 cm-1 band and the Ti content also depended on the treatment temperature. The 18O exchange with C18O2 shifted the 963 cm-1 band for Ti-M to 928 cm-1. On the basis of the 18O exchange and IR measurements, we assign the characteristic IR band around 960 cm-1 for titanium zeolites to the stretching vibration of Si−O−Ti-bonds.

FT-IR and solid-state NMR investigation of surface hydroxyl groups on dealuminated ZSM-5

H-ZSM-5 (Si/Al = 14.6) was progressively dealuminated by steaming at 350°C (1–72 h) and then characterized by transmission infra-red and solid-state NMR spectroscopies ( 29Si, 27Al, and 1H). The structural hydroxyl groups on the activated zeolite surface were examined by FT-IR before and after pyridine adsorption in the range 400–4000 cm -1. The number of Brönsted acid sites decreases as the steaming time increases and Brönsted sites of varying acid strength are shown to be present on the surface. Moderately steamed (25–50 h at 350°C) ZSM-5 contains three distinct types of hydroxyl groups, namely, bridged Si-OH-Al species of strong Brönsted acidity, non-acidic terminal SiOH groups, and weakly acidic hydroxylated aluminum species, which was also shown by 1H NMR. Two distinct types of Lewis acid sites were observed. The number of Si-OH defects estimated by 29Si and 27Al NMR is at maximum in a sample steamed for 25 h and then decreases due to the healing of hydroxyl nests during further dealumination.

Effect of H2O on dielectric properties of berlinite. I. Dielectric constant

The dielectric constants of a series of hydrothermally grown berlinite crystals containing increasing amounts of H2O were determined using a two-terminal method and empirically determined edge corrections with kappa 11T ranging from 4.67 to 4.86 and kappa 33T from 4.54 to 4.76. The extrapolated values for 'dry' berlinite are kappa 11S= kappa 33S=4.55. Infrared studies of these crystals show bands at 4400 and 5200 cm-1 which are attributed respectively to hydroxyl ions and macroscopic and microscopic inclusions of the 6-7 M HCl/H3PO4 growth medium. The hydroxyl ions are believed to result from terminal P-OH groups at the surfaces of inclusions or as (4H)Al or (3H)Al hydroxyl nests. The OH- groups have a negligible influence on the dielectric constant, whereas the inclusions are responsible for the increase in kappa aaS and kappa 33T to 4.8 over the dry value of 4.55.

Dealumination of zeolite Y by H4EDTA

Solid-state NMR shows that the treatment of zeolite Y by H4EDTA dealuminates the zeolite framework. Since no extra-framework aluminium is found, all the Al removed from the framework is subsequently washed out of the crystallites in the form of an H4EDTA complex. Unlike with hydrothermal dealumination or treatment with SiCl4, H4EDTA gives defective crystals in which framework vacancies created by the expulsion of aluminium are not healed by silicon from other parts of the framework. The likely reason for this is the slow intracrystalline diffusion of Si(OH)4 at low temperatures.29Si CP/MAS NMR and infrared spectra confirm the creation of hydroxyl nests in the zeolitic framework. A small amount of disordered siliceous phase responsible for a low intensity29Si MAS NMR shoulder at −110 ppm is produced in the process.

Influence of synthesis conditions and postsynthesis treatments on the nature and quantity of structural defects in highly siliceous MFI zeolites: A high-resolution solid-state 29Si n.m.r. study

The formation and annealing of various kinds of defects in the framework of highly siliceous MFI-type zeolites, as evidenced by high-resolution solid-state 29 Si n.m.r., can be explained by considering the systematic influence of synthesis parameters and postsynthesis treatments. High pH values and high concentrations of alkali metal and fluoride ions prevent a complete polycondensation of the silicate species and generate materials that contain significant levels of nonbonding point defects. By contrast, a high crystallization temperature results in the ready hydrolysis of the terminal Si-O − groups, leading to structures containing fewer defects. A large number of T vacancies (hydroxyl nests) are created in zeolites crystallized at low temperatures in the presence of high Pr 4 N + concentations and in the absence of F − or Na + ions. Such conditions favor the formation of double-5-ring silicate anions. The hypothesis, that such species may further condense, conducting to frameworks with an important number of empty T-sites, is proposed. On the basis of these findings, ideal synthesis conditions can be proposed to produce (Si) MFI zeolites that contain a minimum number of defects. Various postsynthesis treatments, namely, different calcination conditions followed, or preceded, by selected ionic (akali cation) exchanges, result in a partial or a nearly total healing of these defects in the final materials.

Defect sites in highly siliceous HZSM-5 zeolites: a study performed by alumination and IR spectroscopy

The concentration of oxygen atoms on defect sites in a highly siliceous HZSM-5 type zeolite was estimated by the {sup 18}O-exchange reaction between C{sup 18}O{sub 2} and the zeolite. The concentration of oxygen atoms on defect sites could be controlled by means of changes of the gel composition and of the use of various silica sources in the hydrothermal synthesis. The relationship between the concentration of oxygen atoms on defect sites in a highly siliceous HZSM-5 and the concentration of aluminum introduced into the framework of the HZSM-5 by an alumination was examined. The concentration of the framework aluminum was the same as one-fourth that of the oxygen atoms on defect sites. These results suggest that the defect sites into which aluminum atoms are introduced tetrahedrally can be identified with hydroxyl nests that consist of four silanol groups. The existence of hydroxyl nests could be confirmed by IR spectroscopy. From the {sup 18}O-exchange reaction and IR measurements, the authors conclude that the sharp band at 3,740 cm{sup {minus}1} can be attributed to both isolated SiOH groups on the external surface and intracrystalline isolated SiOH groups and that the broad band at 3,505 cm{sup {minus}1} can be attributed to the SiOHmore » groups in hydroxyl nests.« less

Effect of dealumination defects on the properties of zeolite Y

Two series of zeolite Y samples with different amounts of defect structure or hydroxyl nest have been prepared and investigated using various physicochemical and spectroscopic techniques. The unit cell size and the IR asymmetric stretch frequency of the zeolites are affected by the defect structure, because the hydroxyl nest is larger than the SiO 4 tetrahedron but smaller than the AlO 4 tetrahedron and the presence of hydroxyl groups in the nest weakens the SiO bonds in the framework. In consequence, the thermal and hydrothermal stabilities of the zeolites are lowered due to the presence of the defect structure. The defect structure also influences the acid site distribution of the zeolites. The strong acidity of the samples with a low defect content displays a volcano curve as the aluminium content is reduced due to the change in the environment of the aluminium tetrahedral sites. For samples with high defect concentration the strong acidity is reduced and the medium acidity increases significantly because of the decomposition of the zeolites during heat pretreatment. The catalytic activities of the two types of zeolites toward model acid-catalysed reactions are in good agreement with the acidity measurement.

Acid site promotion of mordenite

The surface acidity of H-mordenite and partially dealuminated H-mordenite has been characterized by gravimetric and spectroscopic measurements of ammonia adsorption. The acidic properties of the mordenite samples are compared with those of other acidic solids. An increase in the number of strongly acidic sites was observed upon dealumination of mordenite up to SiO 2 Al 2O 3 ratios of 16. Evidence is presented for the formation of hydroxyl nests in dealuminated mordenite. The results suggest that Bronsted acid site strength is promoted by the hydroxyl nests, acting through the lattice.

Inoperative protonic sites in highly exchanged L-zeolites

It has been shown that highly exchanged L zeolites have hydroxyls vibrating at 3740, 3730, 3680 and 3630 cm–1 and a large number of strongly interacting hydroxyls. The structural 3630 cm–1 hydroxyls, stable under the conditions of catalytic experiments, are strongly acidic toward pyridine. Their inactivity in catalysis for less basic molecules such as hydrocarbons suggests that protons in these highly exchanged samples become inoperative either because they form hydroxyl nests or because they move to inaccessible cavities.

On the existence of hydroxyl nests in acid-extracted mordenites

On the existence of hydroxyl nests in acid-extracted mordenites

47. The adsorption of water vapor on quartz and calcite.: N. Hackerman and A. C. Hall, J. Phys. Chem., 62, 1212–1214, Oct. 1958

Titanium mordenites (Ti-M) with different titanium and aluminum content have been prepared by the solid-gas reaction at elevated temperatures (atom-planting method) between highly dealuminated mordenites and TiCl4 vapor. Ti-M samples were characterized with MAS NMR, IR and UV spectroscopies, which indicates that Ti atoms have been incorporated tetrahedrally into the mordenite framework through the reaction of TiCl4 with hydroxyl nests composed of SiOH groups. The catalytic properties of Ti-M for the liquid phase hydroxylation of various aromatic substrates having different molecular sizes with H2O2 were studied by comparing with those of MFI-type titanosilicate (TS-1). Ti-M showed comparable specific activity to that of TS-1 for the hydroxylation of smaller substrates which can diffuse easily into the pores of both catalysts. However, Ti-M was more active than TS-1 in the hydroxylation of bulkier aromatics, because Ti-M has larger window size of pore than TS-1.