OH Groups In Zeolites Research Articles

Activation Energies for the Reaction of Ethoxy Species to Ethene over Zeolites

Dehydration of ethanol to ethene on acidic OH groups of zeolites was observed by infrared (IR) spectroscopy and temperature programmed desorption (TPD). The rate of decomposition of surface ethoxy groups was measured by measuring the amount of recovered acidic OH groups by IR, while confirming the evolution of ethene using a mass spectrometer, for the estimation of activation energy. The obtained activation energies for the evolution of ethene from ethoxy groups on hydrogen form mordenite, ZSM-5, and ferrierite zeolites were 161 ± 6, 181 ± 2, and 188 ± 2 kJ·mol−1, respectively. A tendency was found that the activation energy became larger in smaller-pored zeolite. The cause of the difference of three zeolites in activation energy for ethene evolution from ethoxy species is discussed along with an energy diagram described from reported theoretical examinations.

The Influence of Si/Al Ratio on the Distribution of OH Groups in Zeolites with MWW Topology

A series of MWW-type zeolites of increasing Si/Al ratio were investigated with respect to their acidic properties. Concentration of the Brønsted acid centers located at the external crystal surface was invariant for the entire series. Ethanol conversion to ethyl-tert-butyl ether, proceeding only at the external surface, was also constant. The OH groups in MWW zeolites were found to be homogeneous with proton affinity value equal to 1142.7 kJ/mol.

Density Functional Studies on the Structure and Reverse Hydrogen Spillover in Au6 Cluster Supported on Zeolite

We have studied the structure of Au6 cluster supported on acidic form of faujasite zeolite using density functional theory. In the gas phase Au6 minimizes to a triangular structure while on zeolite support it has a three-dimensional structure with three apical centers bending towards the support. The hydrogenated clusters Au6H/(2H)-FAU, Au6H2/H-FAU and Au6H3/FAU generated by stepwise reverse hydrogen spillover from bridging OH groups of zeolite are energetically preferred over the Au6/(3H)-FAU structure. The calculated reverse hydrogen spillover energy per hydrogen atom for zeolite supported Au6H, Au6H2 and Au6H3 clusters are −67.6, −65.6 and −59.1 kJ/mol, respectively.

First Hybrid Embedding Scheme for Polar Covalent Materials Using an Extended Border Region To Minimize Boundary Effects on the Quantum Region.

We present an improved scheme for constructing the border region within a hybrid quantum mechanics/molecular mechanics (QM/MM) embedded cluster approach for zeolites and covalent oxides that ensures proper modeling of adsorption complexes with QM regions of moderate size. The procedure employs a flexible orbital basis set on monovalent oxygen pseudoatoms at the boundary of the QM cluster and introduces a pseudopotential description without explicit representation of valence electrons for their immediate Si neighbors in the MM region. This novel QM/MM border scheme, implemented in the elastic polarizable environment method for polar covalent materials (covEPE), provides an accurate description of the local structure of zeolites and other silica based materials. We assessed the performance of the novel border scheme by comparing calculated and experimental results for structures, vibrational frequencies, and binding energies of CO adsorption complexes at bridging OH groups in zeolites with FAU and MFI structures. In addition, when modeling zeolite-supported metal clusters, the new approach implies considerably reduced corrections due to the basis set superposition error, compared to our previous scheme for treating the border region of the QM partition [J. Phys. Chem. B 2003, 107, 2228].

Reverse Hydrogen Spillover onto Zeolite-Supported Metal Clusters: An Embedded Cluster Density Functional Study of Models M6(M = Rh, Ir, or Au)

A density functional study using isolated cluster models of the zeolite framework (Vayssilov, G. N.; Rosch, N. Phys. Chem. Chem. Phys. 2005, 7, 4019) on supported metal clusters M6/zeo in a hydroxylated faujasite cage showed that for 12 metals M of the groups 8−11 the hydrogenated state, M6(3H)/zeo, is energetically preferred over the bare form M6/zeo(3H). The former state was obtained as result of reverse hydrogen spillover from zeolite OH groups onto the metal particle. In the present work, we reinvestigated this problem of identifying the favorable chemical state of zeolite-supported metal species for selected M6 model clusters (M = Rh, Ir, or Au) using an accurate quantum mechanics/molecular mechanics (QM/MM) approach where a QM partition is embedded in an extended zeolite lattice (MM). The embedding method covEPE, an improved variant of the elastic polarizable environment model EPE, adapted for polar-covalent materials, properly accounts for both the mechanical rigidity of the zeolite framework and t...

Concentration and acid strength of hydroxyl groups in zeolites La,Na-X and La,Na-Y with different lanthanum exchange degrees studied by solid-state NMR spectroscopy

1H MAS NMR spectroscopy was applied for a quantitative investigation of the concentration of hydroxyl groups on lanthanum-exchanged zeolites X and Y as a function of the dehydration temperature. A maximum concentration of Brønsted acidic bridging OH groups (SiOHAl) was reached upon dehydration of zeolites La,Na-X and La,Na-Y at 423 K. Further raise of the dehydration temperature leads to the recombination of the above-mentioned hydroxyl groups, while the dealumination of the framework is negligible. The maximum number of bridging OH groups formed on zeolites La,Na-X and La,Na-Y correlates well with the lanthanum exchange degree. By adsorption of deuterated acetonitrile as a probe molecule, an increase of the acid strength of bridging OH groups in lanthanum-exchanged zeolites X and Y was found. This finding is explained by the polarizing effect of multivalent lanthanum cations, which leads to a withdrawal of electrons from the O–H bonds of Brønsted acid sites in lanthanum-exchanged zeolites with a high concentration of these extra-framework cations.

In situ1H MAS NMR investigations of the H/D exchange of alkylaromatic hydrocarbons on zeolites H-Y, La,Na-Y, and H-ZSM-5

In situ 1H MAS NMR spectroscopy was used to investigate the H/D exchange behavior of benzene- d 6, ethylbenzene- d 10, toluene- d 8, and p-xylene- d 10 with Brønsted acid sites in different zeolite catalysts. In the temperature range of 303 to 393 K, the H/D exchange takes place exclusively between zeolitic bridging OH groups and deuterons bound to the aromatic rings of adsorbed compounds. The activation energies E a of the H/D exchange of same deuterated aromatics adsorbed on different zeolites have the sequence: H-Y ⩾ La,Na-Y > H-ZSM-5. The decrease of the E a values correlates well with the increase of the strength of Brønsted acid sites determined via the low-field shift of the 1H MAS NMR signal of bridging OH groups upon adsorption of deuterated acetonitrile (CD 3CN). The E a values determined for the different deuterated aromatic hydrocarbons adsorbed on the zeolites under study show the sequence: benzene ( E a = 46–76 kJ/mol) > ethylbenzene ( E a = 29–41 kJ/mol) > toluene ( E a = 25–32 kJ/mol) > p-xylene ( E a = 20–27 kJ/mol). Generally, this sequence demonstrates the electron donation effect of alkyl groups bound to the aromatic rings (+I effect). The larger E a value of ethylbenzene in comparison with that of toluene indicates an interaction of ethylbenzene with the zeolite framework via its ethyl group leading to a lower electron density and a decrease of the +I effect of this alkyl group towards the aromatic ring.

FTIR studies of unusual OH groups in steamed HNaY zeolites: Influence of Na insertion

Starting from a prepared series of NaHY zeolites with different number of sodium ion per unit cell (u.c.), the corresponding series of steamed HNaY(S) zeolites were obtained by a modified method of steaming in order to obtain less acidic but nevertheless thermally stable zeolites. All prepared solids exhibit large amounts of unusual OH groups with stretch frequency at 3700 cm−1 which are thermally stable and characterized by their soft acid strength measured by CO adsorption at low temperature: Δν(OH)=ca. 145 cm−1, i.e. H0=−5. All the solids are highly crystalline (80%). Chemical and FTIR analysis of the two series denoted HNaY and HNaY(S) allowed the determination of their molar formula after calcination at 650 K. The concentration of the unusual (3700)OH groups depends of the level of Na exchange and reaches a maximum for 34 Na per u.c. as deduced from the intensity of the corresponding IR absorption band. This study gives precise details about the role of Na in forming these unusual (3700)OH groups.

IR studies of OH groups in mesoporous aluminosilicates

The properties of acidic hydroxyl groups and Lewis acid sites in amorphous mesoporous aluminosilicates was followed by quantitative IR studies of ammonia adsorption and by low temperature studies of CO adsorption. We studied aluminosilicates synthesized by three methods: via atrane complexes, by grafting aluminium onto the siliceous material (HMCM-48) and in aqueous solution following a typical synthesis procedure (HMCM-41). IR spectra of OH groups showed weak but well seen band of Si–(OH)–Al groups at 3600–3610 cm −1. The post-synthesis grafting of Al into MCM-48 was found to be the most effective way to produce mesoporous material with large content of Brønsted acid sites. IR studies have shown that the stoichiometry of dehydroxylation of the amorphous mesoporous materials was different than for zeolites: in mesoporous materials one Brønsted site lost created one Lewis acid site. IR experiments of CO sorption evidenced that generally two kinds of Si–(OH)–Al groups exist in mesoporous aluminosilicates: weakly acidic ones of Δ ν OH⋯CO ≈ 160 cm −1 (similar to that in zeolite NaHX), present in all the materials, hydroxyls of moderate acidity of Δ ν OH⋯CO ≈ 200 cm −1 (acid strength intermediate between that in zeolites NaHX and NaHY) present in HMCM-41 and HMCM-48, or hydroxyls of high acidity of Δ ν OH⋯CO ≈ 300 cm −1 (similar to that in highly siliceous zeolite HZSM-5) present in aluminoslicate prepared via atrane complexes method. This method of synthesis was found to be the most effective in preparation of mesoporous aluminosilicates with the strongest Brønsted acid sites. The weakly acidic hydroxyls in mesoporous aluminosilicates are very prone to dehydroxylation, they are lost at as low temperature as 800 K, hydroxyls of moderate, or high acidity are resistant to dehydroxylation and behave like zeolitic OH groups.

Dry-Gel Synthesis of Mesoporous MCM-41 Materials with Modified Pore Structure

Abstract For the first time, the dry-gel conversion method was applied for the preparation of MCM-41 materials with partially crystalline framework. The X-ray diffraction pattern of the material obtained after a dry-gel conversion time of 36 h consists of reflections of both long-range ordered MCM-41 material and zeolite ZSM-5. The nitrogen adsorption/desorption isotherm confirms the presence of micropores and uniform mesopores with a narrow pore size distribution. By 29Si MAS NMR spectroscopy it was evidenced that a significant portion of the amorphous MCM-41 framework was transformed into a partially crystalline framework. TEM and SEM images indicate the occurrence of mesopores with disturbed arrangements, but no formation of separate zeolite ZSM-5 crystals occurred. By 1H MAS NMR spectroscopy of the above-mentioned material, the formation of hydroxyl groups with a chemical shift of ca. 4 ppm and a concentration of ca. 0.1 mmol/g was observed. The acid strength of these hydroxyl groups, as studied by adsorption of the 13C MAS NMR probe molecule 13C-2-acetone, was found to agree with that of bridging OH groups in zeolite H-ZSM-5. After conversion times of 48 h and 72 h, an extensive to total transformation of the dry gel into zeolite ZSM-5 occurred.

04/02529 Long-term performance of an industrial waterdesalination plant: Al-Bastaki, N. and abbas, A. Chemical Engineering and Processing,2004, 43, (4), 555–558

In situ MAS NMR spectroscopy was applied to investigate the synthesis of methyl tertiary-butyl ether (MTBE) over zeolite HBeta (nSi/nAl=15.8) under continuous-flow conditions. Preliminary catalytic tests with on-line gas chromatographic analysis of the reaction products revealed an activity of zeolite HBeta in the MTBE synthesis comparable with that of Amberlyst-15. By exposing the calcined zeolite, filled into an MAS NMR rotor, to a flow of nitrogen gas loaded with methanol and isobutene, the interaction of these reactant molecules with the zeolite OH groups and the dimerization and oligomerization of isobutene were studied. 13C MAS NMR spectroscopy performed during the conversion of a methanol/isobutene mixture over zeolite HBeta yielded signals at 77 ppm to 90 ppm due to secondary and tertiary carbon atoms of alkoxy complexes formed at the zeolite framework.

Quantitative Determination of Brönsted Acid Sites on Zeolites: A New Approach Towards the Chemical Composition of Zeolites

An original quantitative method based on H/D exchange between H2O/D2O molecules and the OH groups of different zeolites has been developed for the titration of the Bronsted acid sites present on the solid surface. The measured Bronsted acid sites density appears to be in good agreement with the theoretical amount estimated by the Si/Al ratio. In contrary to classical methods, this non-destructive anhydride method titrates the whole quantity of Bronsted acid sites of zeolites.

Heterogeneity of OH groups in HZSM-5 zeolites: splitting of OH and OD bands in low-temperature IR spectra

The IR band of hydroxyl groups in HZSM-5 zeolites is split into five sub-maxima. The splitting was observed in low-temperature (170 K) spectra of HZSM-5 with Si/Al > 50. The OH/OD exchange distinctly improves the quality of the hydroxyl spectrum and makes the splitting more easily seen. We attribute the splitting of the hydroxyl band to the heterogeneity of the hydroxyl groups in HZSM-5 and the presence of hydroxyls of various acid strength. The heterogeneity of OH groups is explained by the presence of (SiO) 3Si–OH–Al(OSi) 3 groups of various bridge geometries.

Characterization of Acidic OH Groups in Zeolites of Different Types: An Interpretation of NH3-TPD Results in the Light of Confinement Effects

Ammonia TPD experiments have been carried out on protonated forms of the zeolites FAU, FER, MFI, and MOR to investigate the acid strength dependence on OH group location and Al content. Effective adsorption energy distributions are derived using a regularization method and corrected to achieve a truer measure of acid strength using nonspecific interaction energies from atomistic Monte Carlo simulations. The correction energies include all interactions not associated with the OH groups and range from 18 to 37 kJ/mol, depending on the region and the zeolite structure. In the FAU structure, we find a bimodal distribution of adsorption energies in both the supercages and the sodalite cages, with the stronger acid sites being more common in the supercages. In contrast, essentially the same acid strength for different regions in MOR is seen, though the acid strength for those sites in the side pockets may be slightly lower. For FER and MFI, the TPD gives unimodal distributions and therefore prohibits distinctio...

Effects of water and ion-exchanged counterion on the FT-IR spectra of ZSM-5: Part III. Cu +(CO) 2–ZSM-5: interaction of Cu +(CO) 2 complex with H 2O and changes in skeletal T–O–T vibrations

The effect of water on Cu +(CO) 2–ZSM-5 ( n Si/ n Al=20, n Cu/ n Al=0.75) has been studied at 293 K by time-resolved FT-IR technique in transmission mode, using self-supporting sample and flow system. After an H 2O pulse addition into the CO/He line, (a) the IR bands of Cu +(CO) 2 di-carbonyls at 2177 (sym) and 2151 cm −1 (as), (b) the skeletal T–O–T vibrations and (c) the OH vibrations considerably changed. In spite of the presence of 7.44 Torr (1 Torr=133.3 Pa) CO, water easily and reversibly displaced one carbonyl ligand [i.e. Cu +(CO) 2+H 2O⇌Cu +(CO)(H 2O)+CO], since the relevant FT-IR spectra showed very similar features to those presented recently for the [(Cu +CO)–ZSM-5+H 2O] system [see, J. Sárkány, Topics in Catalysis 18(2002)271]. Subsequent to H 2O addition, only first one ν( 12 CO) band appeared (stage C), namely at 2111 cm −1 which continuously shifted to 2118 cm −1 with decreasing H 2O concentration, then a new band series was observed (stage B) with gradual shifts from 2132 to 2139 cm −1, while finally the original Cu +( 12 CO ) 2 twin complex (2177 and 2151 cm −1) [and, according to the equilibrium, Cu +( 12 CO ) mono-carbonyl (2157 cm −1)] came back (stage A), via an isosbestic point at 2144 cm −1. Similarly, Cu +( 13 CO )( 12 CO ) mixed di-carbonyls (caused by 1.11% 13 CO being present naturally in CO) showed also three types of interactions with H 2O. Two of them were analogous to [ Cu +( 13 CO)+ H 2 O ] with stages C (2063–2070 cm −1) and B (2083–2089 cm −1), while stage A revealed only one di-carbonyl-related characteristic band at 2111 cm −1 due to low-frequency (LF) [or ν as(CO)] band of Cu +( 13 C O)( 12 C O). The pertinent high-frequency-band (HF-band) [ ν sym(CO)] at 2167–2170 cm −1 could not be observed because of high overlapping with (very) strong ν(CO) bands of Cu +( 12 CO ) 2 and Cu +( 12 CO ) carbonyls. Other frequency regions of the FT-IR spectra also resulted in some useful informations. H 2O reversibly interacted at 293 K with zeolitic OH groups and with the whole zeolite ZSM-5. Large amount of H 2O notably enhanced the intensities of skeletal vibrations.

Characterization of Acidic OH Groups in Y-Type Zeolites by Means of Different Methods of Temperature-Programmed Desorption (TPD) of Ammonia

The acidic properties of HNaY zeolites with different degrees of exchange have been studied by means of two complementary techniques of temperature-programmed desorption (TPD) of ammonia, namely, FTIR-TPD and conventional TPD. In HNaY zeolites with low exchange levels only a single type of OH group is observed. The analysis of the spectra in the region of NH bending modes during NH3-FTIR-TPD on zeolites with higher proton content (40% or higher) shows that ammonia is bound in two different adsorption states. This assessment is further confirmed by evaluation of the OH stretching region. Furthermore, desorption energy distribution functions for the different types of OH groups have been calculated using a numerical regularization method.

The study of heterogeneity of OH groups in zeolites by comparing the half-width of IR band of hydroxyls interacting with adsorbed molecules

We propose a method of the studies of heterogeneity of OH groups in zeolites by comparing the half-width of the band of OH groups interacting with adsorbed molecules with that of homogenous acidic hydroxyls. We determined a correlation between half-width (Δ ν 1/2) and frequency shift (Δ ν) for homogenous OH groups in phenol, acetic acid, trifluoroacetic acid, and on SiO 2 surface as well as in NaHX zeolite (Si/Al=1.0) interacting with various electrodonor molecules ( n-hexane, benzene, toluene, p-xylene, ethene, propene, butene-1, acetonitrile and diethyl ether). The application of our correlation to the problem of heterogeneity of OH groups in zeolites was tested for homogenous OH groups in NaHA and dealuminated H-beta zeolite and heterogeneous OH groups in H-mordenite and H-beta. We used the Δ ν 1/2–Δ ν correlation to study the problem of heterogeneity of OH groups in H-ZSM-5 zeolites in which some previous studies suggested heterogeneity and some others homogeneity of OH groups. We also studied the problem of heterogeneity of P–OH groups in cloverite.

Heterogeneity of OH groups in HZSM-5 zeolites. IR studies of ammonia adsorption and desorption

The studies of ammonia desorption followed by IR spectroscopy suggested that Si–OH–Al groups in HZSM-5 zeolites were heterogeneous. The experiments of ammonia adsorption in HZSM-5 zeolite and in two ‘reference zeolites’, HMordenite and NaHX, showed that ammonia molecules adsorbed up to 480K in HZSM-5, reacted ‘layer-after-layer’ with all the acid sites in a ‘layer’ without selecting the most acidic centres and without redistribution. Redistribution which includes desorption step (activated reaction) may be too slow under the experimental conditions. The adsorption of ammonia layer-after-layer, without selecting the most acidic sites, explains the constant value of adsorption heat of ammonia in microcalorimetric experiments.

FTIR studies of unusual OH groups in steamed HNaY zeolites: preparation and acid properties

We describe a series of steamed HNaY zeolites containing large amounts of unusual (3700)OH groups. The method of steaming and the solids obtained are well described. These hydroxyl groups are thermally stable and characterized by their weak acid strength measured by CO adsorption at low temperature: Δν(OH)≈145 cm −1 i.e. H 0=−5, slightly dependent on the coverage level. Their number is estimated at 11.8 per unit cell. Interesting catalytic properties are expected for these (3700)OH groups since the unusual soft acidity would prevent cracking and coke formation in some acid-catalyzed reactions. Moreover, the (3670)OH groups are obtained in significant amount after soft leaching of the steamed zeolite with NH 4 + solutions. This solid is highly crystalline (≈85%), therefore, after CO adsorption, the shift of the (3670)OH groups is unambiguous: Δν( OH)=185 cm −1 i.e. H 0=−6.

Characterizing Zeolite Acidity by Spectroscopic and Catalytic Means: A Comparison

Adsorption of H2, N2, and CO on four different protonated zeolitesH-ZSM-5, H-β, H-Y, and dealuminated H-Yat low temperatures was studied by transmission Fourier transform infrared spectroscopy. The introduction of the basic probe molecules caused a red-shift of the IR stretching bands of the zeolitic acidic OH groups. This perturbation, which is commonly interpreted as a hydrogen bonding between the acidic OH group and the adsorbate and often taken as a measure of the acidic strength, was then compared with intrinsic activities for the acid-catalyzed cracking of n-hexane previously published for the same zeolite samples. Catalytic and spectroscopic characterization of the acidity is consistent only within the same class of zeolites, e.g. comparison of differently pretreated faujasites. Spectroscopic and catalytic observations for different types of zeolites do not match perfectly, because additional effects, such as interactions of larger molecules with pore walls and the stabilization of transition state...