Hydroxyl Nests Research Articles

On existence of hydroxyl nests in acid dealuminated zeolite Y

The removal of Al with acid solution from a zeolite framework is customarily associated with formation of framework defects known as hydroxyl nests, but their existence has not been unambiguously confirmed thus far. TPD and FTIR techniques were used here to identify such hydroxyl nests in a 22% acid dealuminated zeolite Y by comparing it to its non-dealuminated parent counterpart. Both zeolites, parent and dealuminated, were analyzed in fully alkaline Cs, Na ion exchanged form. TPD analysis showed that hydroxyl nests cannot be present in samples calcined above 200°C. FTIR study did not find any evidence for the existence of such units even in hydrated samples which had never been subjected to modification at temperatures higher than 80°C.

A defect-based strategy for the preparation of mesoporous zeolite Y for high-performance catalytic cracking

Mesoporous ultra-stable Y zeolite with outstanding performance for heavy oil cracking was prepared through the combination of desilication and dealumination. Atomic removal of silicon atoms from the starting NaY zeolite framework during desilication was reported. Nucleation and propagation of mesopores in the following dealumination process was studied. Si(0Al) and Si(1Al) atoms are selectively removed from zeolite framework by desilication. Consequently, silanol nests and unique hydroxyl nests with framework associated Al-OH were created in the framework of the desilicated zeolite. The artificial defects effect the initiation of mesopores in crystal parts where used to be rich in framework silicon and unfavorable for mesopore formation and promote the propagation and coalescence of mesopores. Catalytic studies revealed improved activity and lifetime in the crack of 1,3,5-triisopropylbenzene and showed increased gasoline and light cycle oil selectivity when the conversion of a heavy oil was compared.

Evidence for controlled insertion of Fe ions in the framework of clinoptilolite natural zeolites

This work is focused on the spectroscopic characterization of a set of materials obtained by hydrothermal treatments from a natural clinoptilolite zeolite, NZ. OPAZ (orthophosphoric acid treatment on NZ), Fe2+-OPAZ (ferrous sulphate acid treatment on OPAZ) and Fe3+-OPAZ (ferric sulphate acid treatment on OPAZ) samples were characterized by HRTEM, UV–Vis, temperature dependent EPR and Infra Red spectroscopies in order to describe the structure, nuclearity and distribution of iron ions and iron oxides clusters and particles. Complementary pieces of information were obtained from the different techniques. In particular, HRTEM analysis suggested an effect of the hydrothermal treatments not only on the Fe speciation but also on the zeolite particles morphology and surface. The same technique could not clearly detect the presence of iron oxide phases, suggesting their very small size and high dispersion. A more detailed description of the isolated and clustered species could be in turn obtained by UV–Vis spectroscopy. This was confirmed by temperature dependent EPR spectroscopy, suggesting the presence of nanosized ferromagnetic magnetite-like particles. Finally, IR spectroscopy gave clear cut evidence about the insertion of Fe3+ ions in framework positions, with consequent generation of distinct Brønsted acidity, after hydrothermal treatments with iron salts. This transformation could be explained through the reaction with defective hydroxyl nests formed by mildly acid hydrothermal treatment with isolated Fe3+ ions.

Postsynthesis and Selective Oxidation Properties of Nanosized Sn-Beta Zeolite

Nanosized Beta zeolites were postsynthetically modi-fied through the solid−gas reaction of highly dealuminated Beta zeolite with SnCl4 vapor at elevated temperatures. The incorporation mechanism of Sn ions and the physicochemical properties of resultant Sn-Beta-PS were characterized by various techniques. Its catalytic performance in Baeyer−Villiger oxidation was compared with the micrometer-sized Sn-Beta-F hydrothermally synthesized by conventional fluoride method. The Sn species were inserted into the framework via the reaction of the SnCl4 molecules with the silanols in the hydroxyl nests that were created by dealumination and thus occupied predominately the tetrahedral coordination sites. The Sn content gained by postsynthesis reached up to 6.2 wt %, corresponding to a Si/Sn ratio of ca. 35. The isolated Sn species exhibited Lewis acidity useful for the Baeyer−Villiger oxidation of ketones. Containing higher Sn contents and more importantly proposing less diffusion limitations to the substrates with a large molecular dimension, nanosized Sn-Beta-PS was superior to Sn-Beta-F in the selective oxidation of 2-adamantanone with hydrogen peroxide.

Preparation of Ti incorporated Y zeolites by a post-synthesis method under acidic conditions and their catalytic properties

Ti incorporated Y zeolites were prepared by post-synthesis treatment of a dealuminated USY zeolite with an aqueous solution of (NH 4) 2TiF 6, and characterized by XRD, 27Al MAS NMR, FT-IR, SEM, TEM, UV–vis, and XANES techniques. The results showed that Ti species were tetrahedrally incorporated into the hydroxyl nests in the zeolite framework under acidic conditions. Furthermore, the catalytic performance of Ti incorporated Y zeolites in epoxidation of cycloalkenes using an aqueous solution of hydrogen peroxide was higher than that of TS-1 zeolite.

Adsorption properties of Fe-containing dealuminated BEA zeolites as revealed by FTIR spectroscopy

Adsorption properties of Fe-containing dealuminated BEA zeolites were investigated by FTIR spectroscopy of adsorbed CO and NO. Two Fe-containing SiBEA zeolite samples were prepared by a two-step post-synthesis method: creation of vacant T-atom sites (T = Si, Al) by dealumination of tetraethylammonium BEA zeolite with nitric acid followed by impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO 3) 3. The two samples differed in iron content (0.9 and 4.2 wt.%, for Fe 0.9SiBEA and Fe 4.2SiBEA, respectively). The parent SiBEA sample was characterized by IR bands at 3735 cm −1 (isolated internal silanols), 3705 and 3515 cm −1 (associated with hydroxyl nests at vacant T-atom sites). Upon the impregnation step, the bands at 3705 and 3515 cm −1 practically disappeared, indicating consumption of the corresponding hydroxyls and incorporation of iron into the framework of SiBEA zeolite (also confirmed by XRD). In agreement with this, the IR spectra of the two samples revealed acidic bridging hydroxyls of a Fe 3+–O(H)–Si type characterized by a band at 3632 cm −1 in higher concentration for Fe 4.2SiBEA. The 3632 cm −1 band shifted to 3352 cm −1 after low-temperature CO adsorption (Δ ν ∼ 280 cm −1) evidencing a high acidity of the bridging OH groups. Low-temperature CO adsorption experiments revealed the presence of mainly two families of Fe n + sites, evidenced by carbonyl bands at 2215 and 2186 cm −1, respectively. The latter sites were in higher concentration for Fe 4.2SiBEA. In addition, a minor fraction of iron sites were found to be able to form tricarbonyls (bands at 2155, 2123 and 2115 cm −1). It was also deduced that the majority of iron introduced was in a Fe 3+ state and the majority of these ions did not interact with probe molecules. Adsorption of NO leads to appearance of different mononitrosyls (1901, 1869 and 1842 cm −1). With time and in the presence of NO, polynitrosyls (1920 and 1815 cm −1) were also formed. Experiments on coadsorption of CO and NO reveal that the iron sites producing the 2215 cm −1 carbonyls form nitrosyl species absorbing at 1901 cm −1. It is suggested that highly electrophilic Fe 3+ ions act as adsorption sites in this case. Treatment of the samples with CO at 673 K generated new Fe 2+ sites monitored by CO at 2183, 2174 and 2166 cm −1. NO adsorption revealed different mono-, di- and polynitrosyl species. A peculiarity in this case was that interconversion between poly- and dinitrosyl species was well observed. The amount of reduced iron was much higher for Fe 4.2SiBEA than for Fe 0.9SiBEA.

High-temperature dehydrogenation of defective silicalites

It has been previously shown that upon heating samples of silicalite-1 containing hydroxyl nests to temperatures exceeding 873 K, the IR bands characteristic of these nests (∼3400 cm −1) decrease in intensity. The disappearance of these signals has been attributed to dehydration of the silanol groups. Using temperature-programmed desorption (TPD), we show that the product of the dehydroxylation of hydroxyl nests in silicalite-1 is hydrogen and not water. The samples are characterized using N 2 adsorption isotherms, 29Si-MAS-NMR spectroscopy and optical spectroscopy. We propose that the disappearance of hydrogen from the hydroxyl nests is accompanied by the formation of bisperoxysilyl groups (Si–O–O–Si). Ultraviolet–visible light spectroscopy (UV–vis) measurements support this model due to an increase in an UV absorption band at ∼260 nm with increasing temperatures. Electronic structure calculations are also employed to support the energetic feasibility of this reaction mechanism. Silicalite-1 samples made with tetraethyl-orthosilicate show a number of differences in their IR and UV–vis spectra compared to samples made with colloidal silica. It appears that the source of silica plays an important role in the structure of the internal defects.

Structure, acidity and redox properties of MCM-22 ferrisilicate

Aluminum-free MCM-22 type ferrisilicate, [Fe]MCM-22 (Si/Fe = 19.2) and, for comparison, MCM-22 aluminosilicate, [Al]MCM-22 (Si/Al = 13.1), were synthesized. The materials and their precursors were characterized by X-ray diffractometry (XRD), thermogravimetry (TG), transmission and diffuse reflectance Fourier-transform infrared spectroscopy (FT-IR and DRIFTS), temperature-programmed ammonia evolution (TPAE), in situ Mössbauer spectroscopy and temperature-programmed reduction with hydrogen (H 2-TPR). The presence of Fe 3+ T atoms (Fe FW) were indicated by the appearance of a ν OH IR-band at 3634 cm −1. The red shift of the ν OH band, induced by the H-bonding of bridged hydroxyls and N 2, was smaller for the [Fe]MCM-22 (64 cm −1) than for the [Al]MCM-22 (86 cm −1). The smaller shift evidences sites of weaker intrinsic acid strength. The rate and temperature of the TG-release of hexamethyleneimine template showed relation with the acidity of the sample. The amount of Fe 3+ species, present in highly distorted tetrahedral coordination state, was in good correspondence with the concentration of the Brønsted acid sites. After reduction with H 2 or CO, Fe 2+ appeared in a novel coordination state. Results suggest that these Fe 2+ species are bound to the framework via oxygen atoms. Reducibility of the framework iron is related to the high concentration of hydroxyl nests generated in both the wide (12-MR) and the narrow (10-MR) channels of [Fe]MCM-22 during the synthesis and/or template removal.

Influence of steaming and acid-leaching treatments on the hydrophobicity of HBEA zeolite determined under static conditions

A series of HBEA dealuminated samples was prepared by treatment with HCl or steaming of a parent sample. Samples were characterized by XRD, elemental analysis, nitrogen adsorption and pyridine adsorption–desorption followed by IR spectroscopy. The hydrophobicity coefficient, determined by separated static adsorption of water and toluene was used as a quantitative measurement of the hydrophobic properties. It is shown that dealumination causes drastic increase of the hydrophobicity of the different samples. The increase of the hydrophobic character is rather higher for steamed samples than for the acid-leached ones. For the same number of Brønsted sites, the water uptake on the steamed samples corresponds to 11 molecules by unit cell whereas it is 17 for the acid-leached samples. The low hydrophobic character of acid-leached samples is attributed to the creation of hydroxyl nests during acid treatment.

Use of mesoporous SBA-15 for nanostructuring titania for photocatalytic applications

SBA15–TiO 2 samples prepared by introducing titanium with a grafting method and having TiO 2 loadings below 15 wt.% have been characterized by XRF, XRD, IR, porosimetry, SEM, HRTEM, and UV–Visible diffuse reflectance. Differently from the samples reported in the literature characterized by a high TiO 2 loading, no evidences have been found for the presence of titania particles inside or outside the mesopores of SBA-15. Three different titanium species were instead evidenced to be present. The first two derive from the reaction of titanium with silanol groups in the corona area of inner SBA-15 walls leading to the formation of either TiO 4 tetrahedral sites (by reaction by hydroxyl nests of surface defect sites) and/or pseudo-octahedral surface sites anchored by two (or more) Si or Ti ions through bridging oxygens. The third species derives from the reaction of titanium in the regions with high sylanol density, e.g. in the micropores located in the corona of SBA-15 channels, leading to the formation of TiO 2-like nanoareas (probably Si-doped) with dimensions of around 1–2 nm maximum. The potential interest of these materials as photocatalysts, for the presence of a TiO 2-like nanoareas highly accessible by reactants, is discussed.

Performances of Fe-[Al, B]MFI catalysts in benzene hydroxylation with N 2O : The role of zeolite defects as host sites for highly active iron species

The catalytic performances of Fe-[Al, B]MFI zeolites in benzene hydroxylation with N 2O are studied as a function of the nature of the T atom (Al, B) and compared with the characterization of the nature of iron species determined by UV–vis diffuse reflectance spectroscopy. The results indicate that the mechanism for generation of iron species highly active in this reaction is the creation of hydroxyl nests in the zeolite framework due to framework to extra-framework migration of the T atom. The process is favoured when B is present as T atom, but occurs also in the case of Fe and Al as T atoms. Iron atoms migrate to these zeolite defect sites forming highly active Fe 3+ isolated species in square pyramidal or highly distorted octahedral coordination. The process occurs during the catalyst pre-treatment as well as during the initial half an hour of time on stream, leading to an increase in the phenol productivity during this step. A model of these active sites is presented. The effect of the silica source in the preparation of these catalysts is also reported.

Two Polymorphs of a Tetramethylammonium Zincophosphate with Diamond Net‐Related Frameworks Containing [H3O4(P)4] Hydroxyl Nests: Solution‐Mediated and Mechanochemical Phase Transformations and Crystal Structures.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Transformation of hydroxyl nests in microporous aluminosilicates upon annealing

Computer modelling techniques based on density functional theory are appliedto modelling the structures and energies of key defect species in microporousaluminosilicates. We focus on dehydroxylation and dehydrogenation reactions of‘hydroxyl nest’ defects (silicon vacancies saturated by hydroxyl groups). Ourcalculations chart the mechanisms and energetics of the transformation of thesedefects, which are introduced during the synthesis and processing of these materials.

Characterisation of Lewis and Brønsted acidic sites in H-MFI and H-BEA zeolites: a thermodynamic and ab initio study

Adsorption enthalpies of N 2, CO, CH 3CN and NH 3 on H-BEA and H-MFI zeolites have been measured calorimetrically at 303K in order to assess the energetic features of the various interactions occurring within the zeolite nanocavities, namely: (i) specific adsorption on Lewis and Brønsted acidic sites; (ii) H-bonding interactions with hydroxyl nests; (iii) dispersive forces interactions with the walls of the cavities ( confinement effects). Confinement effects have been investigated on an all-silica MFI zeolite (silicalite). The interaction of the molecular probes with model clusters mimicking Lewis and Brønsted sites has been simulated at ab initio level. The combined use of the two different approaches allowed to discriminate among the different interactions contributing to the measured heat of adsorption (−Δ ads H). Whereas CO and N 2 single out contributions from Lewis and Brønsted acidic sites, CH 3CN and NH 3 are not preferentially adsorbed on Lewis sites, suggesting that the adsorption on Brønsted sites is competitive with Lewis sites. The zero-coverage heats of adsorption for the different probes on the various systems correlate well with the proton affinity (PA) of the molecular probes.

Two polymorphs of a tetramethylammonium zincophosphate with diamond net-related frameworks containing [H 3O 4(P) 4] hydroxyl nests: solution-mediated and mechanochemical phase transformations and crystal structures

The formation and crystal structures of two polymorphs of the tetramethylammonium zincophosphate (ZnPO) (NMe 4)[ZnP 2O 8H 3] have been reinvestigated. Methods employed were single-crystal and powder X-ray diffraction, thermogravimetic and difference thermal analysis and 13C and 31P solid state NMR spectroscopy. The following new findings are reported: The polymorphs formulated as (NMe 4)[Zn(HPO 4)(H 2PO 4)] ( I) and (NMe 4)[Zn(H 1.5PO 4) 2] ( II) crystallize from clear solutions near room temperature. A solution-mediated reversible phase transformation readily occurs with I and II being the thermodynamical stable phase at room temperature and 50 °C, respectively. In the solid state these transformations are very sluggish. Solid I can be mechanochemically transformed into II (grinding at room temperature for ca. 30 min). Both polymorphs possess interrupted 3D open-framework structures that are closely related to the diamond net and contain different kinds of distorted tetrahedral [H 3O 4(P) 4] groupings (hydroxyl nests). Formal substitution of the three protons in each hydroxyl nest by a trivalent cation results in fully four-connected frameworks with a novel topology (in the case of I) and the gismondine topology ( II). ZnPO and cobalt(II) phosphate (CoPO) materials with framework structures that contain [H x O 4(P) 4] hydroxyl nests ( x=3 or 4) and/or being related to the diamond net are briefly surveyed.

Structural consequences of mild oxidative template removal in the synthesis of modified MCM-41 silicates

Results concerning the structural consequences of template removal from MCM-41 mesoporous materials are described using ozone, N 2O and NO 2 as oxidants in comparison with the conventional method applying oxygen. Si, TiSi-, VSi and ZrSi-MCM-41 samples were synthesized by the usual methods. For characterization of the as-synthesized and treated samples XRD, nitrogen adsorption, 29Si MAS NMR-, IR- and UV–Vis spectroscopic methods were used. The catalytic activity of the samples was tested in the Friedel–Crafts alkylation of toluene by benzyl chloride. The comparison of template removal agents showed that ozone was the most active at low temperature (423 K), and the treatment was less destructive than burning off the template in oxygen. Nitrogen oxide treatment (NO 2 and N 2O) resulted in template removal at relatively low temperature (573–623 K), and structure deterioration was small. Si NMR spectroscopic data and IR spectra taken in the framework vibration range revealed that more original –SiOH groups remained as hydroxyl nests, furthermore, the heteroatom remained in tetrahedral coordination after ozone and nitrogen oxide treatment compared to burning off the template by oxygen. The results proved the advantages of ozone or nitrogen oxide treatments: (i) gentler to heteroatoms situated in the framework of the materials, probably leaves them intact, (ii) does not result in the formation of secondary micropores, which would decrease the uniform arrangement of the original pore systems, (iii) by preserving the active centers in their original coordination more uniform product distribution may be expected in catalytic reactions.

Benzene Selective Oxidation with N 2 O on Fe/MFI Catalysts: Role of Zeolite and Iron Sites on the Deactivation Mechanism

The catalytic performances of Fe-zeolites having MFI structures and in which the Fe introduced either by ion exchange or during the hydrothermal synthesis has undergone partial framework to extra-framework migration induced by controlled heat treatment are reported. In particular, the catalytic behavior as function of time-on-stream and the formation of carbonaceous species were studied. The results suggest that only a small fraction of the iron is active in the selective oxidation of benzene to phenol in the presence of N2O. It is suggested that the active fraction is formed by isolated iron ions in a pseudo-octahedral configuration with the sites positioned in hydroxyl nests (defects) of the zeolite and is selective in phenol formation as a result of in situ reduction during the catalytic tests. Two possible pathways of carbonaceous species were identified, the first through the intermediate further hydroxylation of phenol and the second through the coupling of phenol with benzene or another phenol molecule. This second pathway is the dominant mechanism of formation of carbonaceous species, although the relative rate of the two pathways depends on the zeolite characteristics and iron loading. It is also suggested that the second pathway depends on the strong chemisorption of phenol, probably on Lewis acid sites, which hinders the fast back-desorption of phenol out from the zeolite channels and thus favors the formation of carbonaceous species. Catalysts prepared by hydrothermal treatment show a lower rate of deactivation than those prepared by ion exchange, although the latter show a comparable productivity to phenol for amounts of iron in extra-framework positions around 20 to 30 times lower. The results also indicate that the presence of Al in the zeolite framework is beneficial for reducing the rate of deactivation as compared to that of Fe-silicalite samples.

Two polymorphs of a piperazinium zincophosphate with different interrupted SOD-type frameworks

Two polymorphous forms of the piperazinium zincophosphate (ZnPO) (C 4N 2H 12)[Zn 2P 3O 12H 3] have been obtained under mild hydrothermal conditions from mixtures of ZnCl 2, H 3PO 4 piperazine and H 3BO 3 in ethylene glycol/water solvents. Single-crystal X-ray structure analyses revealed that both polymorphs possess an open-framework structure with sodalite (SOD) topology. While polymorph I (formulation (C 4N 2H 12)[Zn 2(HPO 4) 3], space group P2 1/c, Z=8) is reported for the first time, polymorph II (formulation (C 4N 2H 12)[Zn 2(PO 4)(H 1.5PO 4) 2], space group C2/c, Z=4) is a known phase. Its structure has been reinvestigated in order to unambiguously determine the hydrogen bonds. The interruptions in the ZnPO host structures possess the form of distorted tetrahedral [H 3O 4(P) 4] groupings (hydroxyl nests). A diprotonated piperazine molecule is enclosed in every [4 66 8] cage. The polymorphs differ in the host–guest N–H⋯O and C–H⋯O interactions, the degree of condensation of the distinct phosphate tetrahedra, the hydrogen-bonding systems in the interruptions, and the long-range ordering of interruptions in the host frameworks. Considerable differences are seen for the donor–acceptor O⋯O distances (ranging from 2.47 to 3. 14 Å) in the total of three distinct [H 3O 4(P) 4] groupings.

Local States in Microporous Silica and Aluminum Silicate Materials. 1. Modeling Structure, Formation, and Transformation of Common Hydrogen Containing Defects

We develop a theoretical approach to point defects in zeolites as catalytically active framework sites, advance structural models for the most important defects and, by calculating the energy minimum configurations, study the main stages of the defect transformation in synthesis, postsynthetic treatment, and aging processes. Calculations have been focused on common hydrogen containing defects in zeolites including bridging hydroxyl and complexed silanol groups. The Brønsted acid site [AlH]Si, the vicinal disilanols and hydroxyl nests (or hydrogarnet defect) have been investigated using a periodic density functional theory approach employing numeric atomic orbital basis sets, as implemented in the Dmol3 code. We propose a new mechanism for the modification of zeolite frameworks based on the silanol inversion. We identified a novel, low energy path (with reaction energy ca. 21 kcal/mol) to an elusive trigonal aluminum species, which does not involve dehydroxylation processes. The inversion in the vicinal disilanol structure reported previously has been confirmed to lead to a lower energy configuration with two loosely bound single silanol groups, which we suggest cannot be associated with 1H NMR shifts of hydrogen bonded silanols. The inversion of the hydroxyl nest is shown to preserve a close trisilanol ring structure with extensive hydrogen bonding while opening this defect to an interaction with guest molecules in the pores of zeolites. On the basis of a low energy of ca. 0.5 kcal/mol calculated for the inversion of the hydroxyl nest, we propose that in zeolite materials T-site (silicon or aluminum) vacancies are realized in two major conformations: a traditional nest structure and a pair consisting of a trisilanol ring and a single silanol group.

Calorimetric and IR spectroscopic study of the interaction of NH3 with variously prepared defective silicalites

Abstract Nature, population and acidic properties of the hydroxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicalites have been characterised by the adsorption of NH 3 , monitored through the combined use of microcalorimetry and IR spectroscopy. The experimental data (enthalpy of adsorption, stretching frequency of the O–H bonds perturbed by the interaction with NH 3 ) have been compared with ab initio computational results (binding energy, red shift of the stretching frequency) and were found to be in good agreement. The presence of rings of mutually interacting silanols has been postulated (on the basis of both experimental and computational results) in the case of chemically very pure, defective silicalites. The dimension of the rings depends on the preparation conditions. A perfect silicalite sample, taken as hydrophobic reference material, was found to be virtually unreactive towards NH 3 , in that it interacts very scarcely with ammonia, showing a heat of adsorption much lower than the heat of liquefaction of NH 3 itself.