Organization of Cooperating Aluminum Pairs in Ferrierite Evidenced by Luminescence Quenching.

We show that four cooperating Al atoms located at the two neighboring six-membered (6-MR) rings in the ferrierite framework can be readily discerned by luminescence studies. Thus, luminescent Zn(II) cations accommodated by one aluminum pair of the 6-MR ring can be effectively quenched by neighboring Co(II) ions stabilized by the second ring. Quenching occurs via the energy transfer mechanism and allows estimation of the critical radius of Zn(II)-Co(II) interactions. This points to the appropriate geometry and distance of the transition metal ions accommodated within zeolite, providing direct evidence of the four-aluminum atom arrangement in the ferrierite framework.

Formation and local structure of framework Al Lewis sites in beta zeolites.

Framework AlFR Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by 27Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by 1H MAS, 27Al WURST-QCPMG, and 29Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of AlFR Lewis sites corresponds to an AlTRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This AlTRI-OH2 complex is reflected in 27Al NMR resonance with δiso = 70 ± 5 ppm and CQ = 13 ± 2 MHz. In addition, the AlTRI atom with adsorbed acetonitrile-d3 (the probe of AlFR Lewis sites in FTIR spectroscopy) exhibits a similar 27Al NMR resonance. We suggest that these AlFR Lewis sites are formed from Al-OH-Si-O-Si-O-Si-OH-Al sequences located in 12-rings (i.e., close unpaired Al atoms).

Analysis of NH3 -TPD Profiles for CuSSZ-13 SCR Catalyst of Controlled Al Distribution - Complexity Resolved by First Principles Thermodynamics of NH3 Desorption, IR and EPR Insight into Cu Speciation*.

NH3 temperature-programmed desorption (NH3 -TPD) is frequently used for probing the nature of the active sites in CuSSZ-13 zeolite for selective catalytic reduction (SCR) of NOx . Herein, we propose an interpretation of NH3 -TPD results, which takes into account the temperature-induced dynamics of NH3 interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first-principles thermodynamic (FPT) modeling of NH3 adsorption on single Cu2+ , Cu+ , [CuOH]+ centers, dimeric [Cu-O-Cu]2+ , [Cu-O2 2- -Cu]2 species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH3 desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low-, medium- and high-temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH3 -TPD experiment. Thus, a rigorous interpretation of the NH3 -TPD profiles of CuSSZ-13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers.

Dioxygen splitting at room temperature over distant binuclear transition metal centers in zeolites for direct oxidation of methane to methanol.

Here we demonstrate for the first time the splitting of dioxygen at RT over distant binuclear transition metal (M = Ni, Mn, and Co) centers stabilized in ferrierite zeolite. Cleaved dioxygen directly oxidized methane to methanol, which can be released without the aid of an effluent to the gas phase at RT.

Ultrasonic Pretreatment as a Tool for the Preparation of Low-Defect Zeolite Mordenite.

The effects of the ultrasonic (US) pretreatment of synthesis gel for the preparation of mordenite zeolite were studied in comparison with the classical stirring method. Even though the US pretreatment was performed before the hydrothermal crystallization, it significantly affected the properties of the obtained mordenite crystals. The US-assisted procedure resulted in a material with improved textural characteristics, in particular, the micropore volume accessible for nitrogen molecules in the as-made form. On the other hand, mordenite prepared with the classical stirring method demonstrated comparable sorption properties only after a postsynthetic treatment. Moreover, in the case of US-pretreated mordenite, altered crystal shape and more homogeneous morphology were observed. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) demonstrated that the US pretreatment introduced structural changes on the atomic level, resulting in fewer defects (reflected in the number of silanol groups) and less pore blockage (affected by Na+ cations) for the as-made sample.

Effect of Alkali-Free Synthesis and Post-Synthetic Treatment on Acid Sites in Beta Zeolites.

Beta zeolites with Si/Al around 14 were prepared using three new alkali-free synthesis methods based on the application of amorphous aluminosilicate precursor and calcined in ammonia or air. All samples exhibit structural and textural properties of standard beta zeolite. Comprehensive study by 27Al and 29Si MAS NMR, together with FTIR adsorption of d3-acetonitrile and pyridine were used to characterize the influence of both the synthesis and calcination procedure on the framework Al atoms and related Brønsted and Lewis acid sites. While calcination in ammonia preserves all framework Al atoms, calcination in air results in 15% release of framework Al, but without restrictions of the accessibility of the beta zeolite channel system for bulky pyridine molecules. Terminal (SiO)3AlOH groups present in the hydrated zeolites were suggested as a precursor of framework Al-Lewis sites. Surprisingly, the mild dealumination of the air-calcined zeolites result in an increase of the concentration of Brønsted acid sites and a decrease of the total concentration of Lewis sites with the formation of the extra-framework ones.

Dioxygen dissociation over man-made system at room temperature to form the active α-oxygen for methane oxidation.

Activation of dioxygen attracts enormous attention due to its potential for utilization of methane and applications in other selective oxidation reactions. We report a cleavage of dioxygen at room temperature over distant binuclear Fe(II) species stabilized in an aluminosilicate matrix. A pair of formed distant α-oxygen species [i.e., (Fe(IV)═O)2+] exhibits unique oxidation properties reflected in an outstanding activity in the oxidation of methane to methanol at room temperature. Designing a man-made system that mimicks the enzyme functionality in the dioxygen activation using both a different mechanism and structure of the active site represents a breakthrough in catalysis. Our system has an enormous practical importance as a potential industrial catalyst for methane utilization because (i) the Fe(II)/Fe(IV) cycle is reversible, (ii) the active Fe centers are stable under the reaction conditions, and (iii) methanol can be released to gas phase without the necessity of water or water-organic medium extraction.

The proximity of aluminium atoms influences the reaction pathway of ethanol transformation over zeolite ZSM-5.

The organization of aluminium atoms in zeolites affects their catalytic properties. Here we demonstrate that the aluminium distribution is a key parameter controlling the reaction pathway of acid catalysed reactions over ZSM-5 zeolites. We study ethanol transformation over two ZSM-5 samples with similar Si/Al ratios of ~15, and with aluminium atoms located mainly at the channel intersections but differently distributed in the framework. One of the samples contains mostly isolated aluminium atoms while the other has a large fraction of two aluminium atoms located in one ring. The FT-IR time-resolved operando study, supported by catalytic results, reveals that the reaction pathway in ethanol transformation over ZSM-5 is controlled by the proximity of aluminium atoms in the framework. ZSM-5 containing mostly isolated Al atoms transforms ethanol in the associative pathway, and conversely ZSM-5 containing a dominating fraction of two aluminium atoms in one ring transforms ethanol in the dissociative pathway.

Mechanochemical Pretreatment for Efficient Solvent-Free Synthesis of SSZ-13 Zeolite.

The economical and environmentally benign synthesis of SSZ-13 zeolite was possible due to the mechanochemical activation of dry reagents by planetary mill. Contrary to manual grinding in a mortar, the proposed automatized approach is scalable and reproducible. This solvent-free process provided a huge gain in product/gel ratios, significantly minimized reaction space and organic structure-directing agent use, and allowed for the elimination of agitation. Obtained materials were comparable to the product of "classical" syntheses. The use of different silica sources resulted in SSZ-13 zeolites with various characteristics: different Si/Al ratio and crystal size.

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid-Catalyzed Reactions.

The organization of Al atoms in the framework of Si-rich zeolites is very important and includes two classes: (i) the Al siting that determines which individual, crystallographically distinguishable framework T sites are occupied by Al atoms and (ii) the Al distribution, which describes the relation of two or more Al atoms in the framework, their distances, and the possibility of neighboring Al atoms to cooperate in the formation of active sites. The organization of Al significantly affects the catalytic properties of Si-rich, zeolite-based catalysts in acid and redox catalysis. Herein, what is known about the organization of Al in the framework of industrially very important pentasil-ring Si-rich zeolites (ZSM-5, beta zeolite, mordenite, ferrierite, MCM-22, and TNU-9), as well as the very promising SSZ-13 Si-rich zeolite with the CHA structure, is summarized.

TNU-9 Zeolite: Aluminum Distribution and Extra-Framework Sites of Divalent Cations.

The TNU-9 zeolite (TUN framework) is one of the most complex zeolites known. It represents a highly promising matrix for both acid and redox catalytic reactions. We present here a newly developed approach involving the use of 29 Si and 27 Al (3Q) MAS NMR spectroscopy, CoII as probes monitored by UV/Vis and FTIR spectroscopy, and extensive periodic DFT calculations, including molecular dynamics, to investigating the aluminum distribution in the TUN framework and the location of aluminum pairs and divalent cations in extra-framework cationic positions. Our study reveals that 40 and 60 % of aluminum atoms in the TNU-9 zeolite are isolated single aluminum atoms and aluminum pairs, respectively. The aluminum pairs are present in two types of six-membered rings forming the corresponding α and ß (15 and 85 %, respectively, of aluminum pairs) sites of bare divalent cations. The α site is located on the TUN straight channel wall and it connects two channel intersections. The suggested near-planar ß site is present at the channel intersection.

27Al-27Al double-quantum single-quantum MAS NMR: Applications to the structural characterization of microporous materials.

In this paper, we review and illustrate applications, reported in the literature or used in our group, of 27Al-27Al double-quantum single-quantum (DQ-SQ) MAS NMR experiments for the structural characterization of Al-containing microporous solids, namely zeolites, aluminophosphates and metal-organic frameworks. Information regarding the periodic frameworks or the localization of the various aluminum species in the materials are obtained from the analysis of the two-dimensional NMR spectra, which allows getting local structural details sometimes inaccessible from other characterization technique. An application of 27Al-27Al of the DQ-SQ experiment for the detection of aluminum pairing in zeolite is shown.

Interface Induced Growth and Transformation of Polymer-Conjugated Proto-Crystalline Phases in Aluminosilicate Hybrids: A Multiple-Quantum (23)Na-(23)Na MAS NMR Correlation Spectroscopy Study.

Nanostructured materials typically offer enhanced physicochemical properties because of their large interfacial area. In this contribution, we present a comprehensive structural characterization of aluminosilicate hybrids with polymer-conjugated nanosized zeolites specifically grown at the organic-inorganic interface. The inorganic amorphous Al-O-Si framework is formed by alkali-activated low-temperature transformation of metakaoline, whereas simultaneous copolymerization of organic comonomers creates a secondary epoxide network covalently bound to the aluminosilicate matrix. This secondary epoxide phase not only enhances the mechanical integrity of the resulting hybrids but also introduces additional binding sites accessible for compensating negative charge on the aluminosilicate framework. This way, the polymer network initiates growth and subsequent transformation of protocrystalline short-range ordered zeolite domains that are located at the organic-inorganic interface. By applying an experimental approach based on 2D (23)Na-(23)Na double-quantum (DQ) MAS NMR spectroscopy, we discovered multiple sodium binding sites in these protocrystalline domains, in which immobilized Na(+) ions form pairs or small clusters. It is further demonstrated that these sites, the local geometry of which allows for the pairing of sodium ions, are preferentially occupied by Pb(2+) ions during the ion exchange. The proposed synthesis protocol thus allows for the preparation of a novel type of geopolymer hybrids with polymer-conjugated zeolite phases suitable for capturing and storage of metal cations. The demonstrated (23)Na-(23)Na DQ MAS NMR combined with DFT calculations represents a suitable approach for understanding the role of Na(+) ions in aluminositicate solids and related inorganic-organic hybrids, particularly their specific arrangement and clustering at interfacial areas.

Location of Framework Al Atoms in the Channels of ZSM-5: Effect of the (Hydrothermal) Synthesis.

(27) Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co(II) ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM-5. Furthermore, the effect of Na(+) ions together with tetrapropylammonium cation (TPA(+)) in the ZSM-5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA(+) as a structure-directing agent (i.e., in the absence of Na(+) ions) led to 55-90% of Al atoms located at the channel intersection, regardless the presence or absence of Al pairs [Al-O-(Si-O)2 -Al sequences in one ring] in the zeolite framework. The presence of Na(+) ions in the synthesis gel did not modify the Al location at the channel intersection (55-95% of Al atoms) and led only to changes in i) the distribution of framework Al atoms between Al pairs (decrease) and single isolated Al atoms (increase), and ii) the siting of Al in distinguishable framework tetrahedral sites.

Structure of framework aluminum Lewis sites and perturbed aluminum atoms in zeolites as determined by 27Al{1H} REDOR (3Q) MAS NMR spectroscopy and DFT/molecular mechanics.

Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework AlFR Lewis acid sites are often found in their H-forms. The formation of AlFR Lewis sites in zeolites is a key issue regarding their selectivity in acid-catalyzed reactions. The local structures of AlFR Lewis sites in dehydrated zeolites and their precursors--"perturbed" AlFR atoms in hydrated zeolites--were studied by high-resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO)3AlOH groups and are characterized by a broad (27)Al NMR resonance (δi = 59-62 ppm, CQ = 5 MHz, and η = 0.3-0.4) with a shoulder at 40 ppm in the (27)Al MAS NMR spectrum. Dehydroxylation of (SiO)3AlOH occurs at mild temperatures and leads to the formation of AlFR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)3Al Lewis sites exhibit an extremely broad (27)Al NMR resonance (δi ≈ 67 ppm, CQ ≈ 20 MHz, and η ≈ 0.1).

Biaxial Q-shearing of 27Al 3QMAS NMR spectra: insight into the structural disorder of framework aluminosilicates.

In this contribution, we present the application potentiality of biaxial Q-shearing of (27)Al 3QMAS NMR spectra in the analysis of structural defects of aluminium units in aluminosilicates. This study demonstrates that the combination of various shearing transformations of the recorded (27)Al 3QMAS NMR spectra enables an understanding of the broadening processes of the correlation signals of disordered framework aluminosilicates, for which a wide distribution of (27)Al MAS NMR chemical shifts and quadrupolar parameters (i.e., second-order quadrupolar splitting and quadrupole-induced chemical shifts) can be expected. By combining the suitably selected shearing transformation procedures, the mechanisms of the formation of local defects in aluminosilicate frameworks, including Al/Si substitution effects in the next-nearest neighbouring T-sites, variations in bond angles, and/or variations in the physicochemical nature of charge-balancing counter-ions, can be identified. The proposed procedure has been extensively tested on a range of model aluminosilicate materials (kyanite, γ-alumina, metakaolin, analcime, chabazite, natrolite, phillipsite, mordenite, zeolite A, and zeolite Y).

Aluminium siting in the ZSM-5 framework by combination of high resolution 27Al NMR and DFT/MM calculations.

The Al siting in the ZSM-5 zeolite was investigated by (27)Al 3Q MAS NMR spectroscopy and QM/MM calculations. It was found that the occupation of the framework T-sites by Al and the concentration of Al in these T-sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. At least 12 out of the 24 distinguishable framework T-sites of ZSM-5 are occupied by Al in the set of the investigated zeolite samples. A partial identification of the Al sites is possible. The calculated (27)Al NMR shielding values were converted to (27)Al isotropic chemical shifts using the experimental isotropic chemical shift of 60.0 ppm referenced to the aqueous solution of Al(NO(3))(3) and the corresponding calculated NMR shielding of 490.0 ppm of a silicon rich (Si/Al 38) chabazite structure zeolite as a secondary internal standard. The observed (27)Al isotropic chemical shifts of 50.0 and 54.7 ppm correspond to Al atoms in the T20 and T6 sites, respectively. The pair of measured isotropic chemical shifts of 52.9 and 53.7 ppm can be assigned to the T4, T8 pair. At the low-shielding end, two assignments are plausible. The smallest deviations between the calculated and observed isotropic chemical shifts are reached for the assignment as follows: T24 (64.8 ppm) is not occupied in the samples and that the observed isotropic chemical shifts 63.6, 62.8, and 60.0 ppm belong to T1, T17, and T7, respectively. It follows then that T-sites T12 (60.8 ppm), T3 (61.7 ppm), and T18 (62.0 ppm) are most likely not occupied by Al in our ZSM-5 samples. If we assume that the calculated isotropic chemical shifts are systematically larger than the observed ones then we can assign the largest observed isotropic chemical shifts of 63.6 and 62.8 ppm to the least shielded T24 and T1 sites, respectively, and 60.0 ppm to T12. Then the sites T3 (61.7 ppm), T18 (62.0 ppm), and T17 (62.5 ppm) would be unoccupied by Al in our ZSM-5 samples. It was further shown that there is no simple linear relationship between the observed (27)Al isotropic chemical shifts and the average Al-O-Si angles.

Control of Al distribution in ZSM-5 by conditions of zeolite synthesis.

Concentration of Al-O-(Si-O)1,2-Al sequences located in one ring and forming cationic sites for divalent cations in ZSM-5 can be controlled in a wide range of their concentrations by the variation of the source of aluminium and silicon used for synthesis of this zeolite.