OH Groups In Zeolites Research Articles

New Functional Alkoxysilanes and Silatranes: Synthesis, Structure, Properties, and Possible Applications.

The aza-Michael reaction of 3-aminopropyltriethoxysilane (1) and -silatrane (2) with acrylates affords functionalized silyl-(3-8) and silatranyl-(9-14) mono- and diadducts with up to a 99% yield. Their structure has been proved with IR and NMR spectroscopies, mass spectrometry and XRD analysis. The hydrolytic homo-condensation of triethoxysilanes 3-5 gives siloxanes 3a-5a, which form complexes with Ag, Cu, and Ni salts. They are also able to adsorb these metals from solutions. The hetero-condensation reaction of silanes 4-8 with OH groups of zeolite (Z), silica gel (S) and glass (G) delivers the modified materials (Z4, S7, G4, G5, G7, G8, etc.), which can adsorb ions of noble metal (Au, Rh, Pd: G4 + Au, G5 + Pd, G7 + Rh). Thus, the synthesized Si-organic polymers and materials turned out to be promising sorbents (enterosorbents) of noble, heavy, toxic metal ions and can be applied in industry, environment, and medicine.

Identifying Crystallographically Different Si-OH-Al Brønsted Acid Sites in LTA Zeolites.

A clear understanding of the acidic properties of bridging Si-OH-Al groups containing crystallographically different oxygen atoms in zeolites is a prerequisite for optimizing their performance as industrial solid catalysts and developing new acid-catalyzed reactions, but presents many challenges. Here, we report the direct observation of yet unrecognized bridging Si-OH-Al groups in the LTA zeolite whose oxygen atoms are crystallographically different from those of already known Brønsted acid sites. We also report that the creation of a crystallographically particular type of bridging OH groups in zeolites and its concentration and acid strength can vary strongly with the content and spatial distribution of framework Al atoms, thus being synthetic in nature, which has been rationalized in terms of the secondary building unit concept.

Identifying Crystallographically Different Si−OH−Al Brønsted Acid Sites in LTA Zeolites

AbstractA clear understanding of the acidic properties of bridging Si−OH−Al groups containing crystallographically different oxygen atoms in zeolites is a prerequisite for optimizing their performance as industrial solid catalysts and developing new acid‐catalyzed reactions, but presents many challenges. Here, we report the direct observation of yet unrecognized bridging Si−OH−Al groups in the LTA zeolite whose oxygen atoms are crystallographically different from those of already known Brønsted acid sites. We also report that the creation of a crystallographically particular type of bridging OH groups in zeolites and its concentration and acid strength can vary strongly with the content and spatial distribution of framework Al atoms, thus being synthetic in nature, which has been rationalized in terms of the secondary building unit concept.

Nitrogen as a Probe Molecule for the IR Studies of the Heterogeneity of OH Groups in Zeolites.

One of the methods of IR studies of the heterogeneity of Si–OH–Al groups in zeolites is the investigation of the frequency shift of the band of free OH bands restored upon the adsorption of ammonia and subsequent desorption at increasing temperatures. We extended this method by following the shift of the band of the OH group interacting by hydrogen bonding with nitrogen. The advantage of nitrogen, compared with CO, which has been commonly used as a probe molecule in studies on hydrogen bonding, is that for nitrogen the frequency shift is smaller than for CO and therefore there is no overlapping of shifted OH band with the bands of ammonium ions. For zeolites NaHY, HMFI, and HBEA, the frequency shift of IR bands of both free and hydrogen-bonded Si–OH–Al with the increase of ammonia desorption temperature evidences the heterogeneity of these hydroxyls. On the other hand, in zeolite HFAU of Si/Al = 31, Si–OH–Al were found to be homogeneous. Heterogeneity of OH groups may be explained both by the presence of Si–OH–Al of various number of Al near the bridge and of Si–OH–Al of various geometry.

Impact of proximity between NiMoS and zeolitic HY sites on cyclohexene hydroconversion: An infrared operando study of sulfide catalysts

This work evaluates the effects of sulfide phase location on zeolite-containing catalysts in cyclohexene hydroconversion using the infrared operando technique. Parallels between activity, product yield, and surface species changes with time on stream and under pyridine poisoning were assessed on pure HY zeolite, sulfided NiMo/HY, NiMo/Al2O3, and NiMo/Al2O3 + HY. Isomerization activity could be related to zeolitic acidic OH groups and hydrogenation activity to sulfide phase sites. From the changes due to pyridine pulse injection, intrinsic activities could be calculated for isomerization (1-methyl-1-cyclopentene) and hydrogenation (cyclohexane) routes. The turnover frequency value for isomerization is greater than that for hydrogenation by one order of magnitude. Hence, the poisoning effect of one N-molecule on isomerization will be 10 times higher than that on the hydrogenation route. Whatever the location of the sulfide phase, the coke is mostly formed on the zeolite. As a consequence, on NiMo/HY, the close proximity between acidic and sulfide sites results in lower formation of hydrogenated products, due to blockage of some sulfide sites by coke. In contrast, the hydrogenation activity of NiMo/Al2O3 + HY is less affected by coke, NiMoS being mostly located on alumina. However, the results show that the close proximity between acid and sulfide sites allows a higher degree of hydrogenation of the isomerization product (1-methyl-1-cyclopentene). Thus, the higher proximity has a beneficial effect on methylcyclopentane formation.

Strong and Weakly Acidic OH Groups of HY Zeolite into the Different Routes of Cyclohexene Reaction: An IR Operando Study

IR operando study of cyclohexene reaction on HY zeolite (Si/Al = 15) was performed to determine the role of the OH groups located in the different zeolitic cages and the silanol groups (SiOH). A parallel between the product yield and type and amount of OH groups poisoned by coking and pyridine pulses was established. The role of silanol and zeolitic OH groups during cyclohexene conversion and the coverage rate of surface groups of the dealuminated HY zeolite, were evaluated by FT-IR operando study. At the beginning of the reaction, zeolite exhibited a very intense deactivation due to coke formation. Coke is initially deposited on strong acid sites, inhibiting the H-transfer activity. Further, coke formation decreased. At this stage of the reaction, pyridine addition markedly decreased the isomerization activity. In parallel, it is noted that mainly SiOH, OH groups at hexagonal prisms, and perturbed OH groups in sodalite cages were mainly affected, indicating that these groups are responsible for the isomerization steps. These results showed that even weakly acidic sites are active sites for isomerization reaction. In cyclohexene conversion, hydrogen transfer occurs on supercage OH groups of HY zeolite. In contrast, perturbed sodalite OH and hexagonal prisms OH, as well as SiOH groups, are responsible for isomerization steps, as revealed by pyridine pulse addition during the reaction.

In Situ IR‐ATR Study of the Interaction of Nitrogen Heteroaromatic Compounds with HY Zeolites: Experimental and Theoretical Approaches

AbstractIn the present work, the liquid‐solid interaction of liquid N‐heteroaromatic compounds, commonly present in the petroleum feedstocks of the refineries, with Y zeolites used as hydrocracking catalysts was followed using IR‐ATR spectroscopy. The inhibition of the zeolitic acid sites by strongly basic pyridine and weakly basic indole was highlighted using a continuous flow IR‐ATR cell. Results were assessed by Density Functional Theory calculations to compute the vibrational frequencies of pyridine and indole according to the nature of the interaction sites: silanol groups or acidic OH groups. The study points out that IR‐ATR spectroscopy opens the way for investigating the interaction modes of low vapor pressure molecules (e. g. indole) that present an inherent difficulty to be operated in the gas phase. Moreover, the IR‐ATR makes possible the analysis of the little‐explored low wavenumber zone (<800 cm−1), that presents informative vibrational modes on the adsorption mode of N‐molecules. Hence, this work points out that for pyridine, the bands at 686 and 727 cm−1 are characteristic of pyridinium species formed over zeolitic OH groups, meanwhile, the signals at 703 and 750 cm−1, are associated to pyridine in interaction with silanol groups. The IR‐ATR study reveals that indole, a weakly basic compound, can be protonated on acidic Y zeolites as unambiguously evidenced by the formation of the bands at 1617, 1608, 1543 and 705 cm−1. Findings here exposed are crucial for studying inhibitory effects exerted by weak nitrogenated compounds on acidic materials during hydrocracking processes.

IR observation of activated ether species on acidic OH groups on H-ZSM-5 zeolites

IR spectra of ethers adsorbed on zeolite (H-ZSM-5) were analyzed in detail in comparison with those of water and alcohols. Interpretation of complicated features of IR spectra observed for adsorption of water and alcohols on acidic OH groups of zeolite is summarized in relation with the adsorption structure and the potential energy based on the accepted knowledge. Since the presence of Fermi resonance in vibrational modes between acidic OH groups and adsorbed species can be an estimate for the potential energy of the protonated transition state, IR spectra of adsorption of ethers on zeolite were interpreted by changing acidic OH groups to OD. Adsorption of ethers on silanol and acidic OH groups of zeolite resulted in markedly different IR spectra. In addition, the Fermi resonance between OH stretching of acidic OH (shifted below 2000 cm−1) and CH bending vibrations of adsorbed ethers was confirmed, and the observed adsorbed species were co-existent with protonated transition state in equilibrium at 298 K.

Theoretical Study of the Molecular Complexes between Pyridyne and Acid Sites of Zeolites

The main interaction between pyridine and zeolites leads to form a hydrogen bond between the N atom of pyridine and OH groups of zeolites. The present work reports a theoretical study about the structural, vibrational and topological properties of the charge distribution of the molecular complexes between pyridine and a series of acids sites of zeolites. The calculated structural parameters are the highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE), hardness (η), softness (S), the absolute electronegativity (χ), the electrophilicity index (ω) and the fractions of electrons transferred (ΔN) from zeolites molecules to pyridine. We show N atom of pyridine attacks to the H atom of the OH bridged group of zeolite clusters.

Influence of zeolite acidity on proton conductivity of FAU embedded imidazole

Abstract Imidazole entrapped in H-forms of FAU zeolites with various Si/Al ratios (1, 4, 40) exhibit high and stable proton conductivity (up to ∼10−3 S/cm) in a broad range of temperatures (293–393 K). The conductivity increases with temperature and imidazole loading. The number and strength of zeolite acid sites (resulting from various Si/Al) influence markedly the conductivity of composites. The samples based on zeolites with low Si/Al (1) show markedly higher conduction than these supported on the matrix with medium Si/Al (4), regardless of very similar number of the acid OH groups. The strong acid OH groups in zeolite with Si/Al = 4 result in protonation and immobilization of adsorbed imidazole, which reduce their mobility and subsequently the conduction. The low siliceous with weaker acid sites protonate imidazole and form a chain of H-bonds effective in proton propagation. Almost neutral high silica matrix provides a high mobility of dispersed imidazole, due to weak mutual interaction. The overloaded samples indicate a phase transition effect at elevated temperature due to melting of extra-matrix imidazole, which is reflected in dramatic increase in conductivity.

Technetium removal from the aqueous solution using zeolites A and Y containing transition metal ions Co2+ and Zn2+

The adsorption properties of two zeolite types, faujasite (NaY, ZnY, CoY) and LTA-4A zeolite (NaA, ZnA, CoA), towards technetium were studied in batch static experiments. The Si/Al ratio, acidity, dealumination, chargebalance cations of zeolite, contact time and temperature determined the adsorption efficiency. The maximum removal efficiency of 98.8% and Kd value (2.06 × 10−4 cm3 g−1) was achieved using CoY (zeolite type Y). The kinetics of technetium adsorption followed the pseudo-second order model. The TcO2 adsorption is mainly due to the hydrogen bonds between protons of the zeolitic structural OH groups (Bronsted acid center) and oxygen from the TcO2−.

Rearrangement of Cyclopropylcarbinyl Chloride Over Protonic Zeolites: Formation of Carbocations and Behavior as Solid Solvents

The rearrangement of cyclopropylcarbinyl chloride was studied over protonic zeolites and K-10 Montmorillonite. The energy of activation is lower on zeolites, with K-10 showing almost the same value for the rearrangement in 80% aqueous ethanol solution. HUSY showed the lowest energy of activation, whereas HZSM-5 and HYD [dealuminated with (NH4)2SiF6] presented similar energy of activation. This difference may be due to the presence of extra-framework aluminum species. On the other hand, the entropy of activation is significantly less negative on ZSM-5 and may be associated with the narrower pore structure, providing ionization of the substrate without losing many degrees of freedom. Kinetic isotope effects indicated that ionization is assisted by hydrogen bonding of the zeolite OH groups with the leaving halide, similar to the push–pull mechanism proposed for solution chemistry. Hence, zeolites behave as solid solvents, providing a polar microscopic environment for ionic reactions to take place, and solvating the transition state and the ions formed.

Implications of sepiolite dehydration for earthquake nucleation in the Galera Fault Zone: A thermodynamic approach

A new thermodynamic model for the Mg-phyllosilicate sepiolite was developed and used to calculate its P-T stability conditions and water content for different bulk rock compositions. The standard-state thermodynamic properties, entropy (S°) and enthalpy (H°), were initially estimated by oxide summation taking into account the different entropic and enthalpic contributions of the three types of water in sepiolite: zeolitic water, bound water, and structural OH groups. The starting model was then refined with experimental data. The dehydration process follows a step function that allowed us to define “end-members” with decreasing hydration states in a theoretical solid solution.The stability field of sepiolite is ultimately limited by the reaction sepiolite = talc + quartz + H2O, which is located at about 325 °C at 1–500 MPa. The large thermodynamic stability field of this clay mineral suggests that it could control the mechanical behaviour of crustal faults to 325 °C. The refined model was then applied to the natural case of the Galera Fault Zone (SE Spain) using thermogravimetric analysis and X-ray fluorescence data of the bulk rock composition of the fault core gouge, which is mainly composed of sepiolite. The dehydration of sepiolite at T < 300 °C is modest compared to that occurring during its breakdown, where 62% of water molecules leave the structure to form the association talc + quartz + H2O; a reaction that also involves a reduction of around 31% of the original solid volume. These significant hydration and volume changes of sepiolite can have important implications on the strength and mechanical stability of the Galera Fault and other sepiolite-bearing gouges. We compare triaxial friction experiments on the natural sepiolite-rich gouge with a synthetic gouge to estimate strength changes on the gouge with depth. The synthetic gouge represents the breakdown association above 325 °C, and was constructed following the mineral phases predicted by the thermodynamic model (93 wt% talc and 7 wt% quartz). The contrasting frictional strength and mechanical stability of sepiolite in comparison to talc could be one of the causes of fault instability and earthquake nucleation seen to be concentrated in the upper 13 km of the Galera Fault.

Brønsted acid sites based on penta-coordinated aluminum species.

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (AlIV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-AlIV and -AlV show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of 1H and 27Al sites demonstrates that BAS-AlV co-exists with BAS-AlIV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.

Quantitative infra-red studies of Brønsted acid sites in zeolites: Case study of the zeolite mordenite

Nowadays, curve fitting of unresolved (or poorly resolved) IR bands is a routine tool for quantification of catalytically active Brønsted acid sites (BAS) in zeolite catalysts. A number of experimental studies demonstrated the effect of the local environment on the catalytic properties of BAS in mordenites (MOR); these studies are rooted in IR spectroscopy and curve fitting methods. In this paper we consider the existing IR-methods for BAS quantification in MOR zeolites and identify a number of shortcomings, possible experimental and computational problems. Our results show that fitting precision only cannot prove the validity of the curve fitting method and that consideration of the uncertainty of the results is required. Also, physical criteria, such as compliance with the Beer’s law and the ability of the method to describe the IR spectra of a wide range of zeolite samples, have to be checked. Using both the mathematical and physical criteria, we have proposed a 3-band curve fitting method of MOR IR spectra. Although only MOR zeolite was considered, our discussion and conclusions are relevant to studies of OH groups in other zeolites as well as in metal-organic frameworks.

Comparative evaluation of natural and acid-modified layered mineral materials as rimifon-carriers using UV/VIS, FTIR, and equilibrium sorption study

AbstractThe encapsulation of rimifon on natural (NZ) and acid-modified (AMZ) zeolites was investigated by UV/VIS, FTIR, and equilibrium sorption studies in aqueous medium. The UV/VIS and FTIR spectral investigations provided data on the nature and characteristics of the drug–zeolite complexes. The probable host–guest interactions during rimifon encapsulation in AMZ include van der Waals interactions, as well as H-bonds established between the O-atom from the carbonyl (>C=O) group and N-pyridine/N-hydrazine atoms in rimifon and zeolite OH-groups. The maximum experimental equilibrium sorption capacity of AMZ (qmax = 7.17 mg/g) was approximately 24 times higher than that of NZ. Baudu and Fritz–Schlunder isotherms almost overlapped and seemed to be the best-fitting models with regard to the experimental equilibrium data of rimifon sorption on AMZ. The unique properties of AMZ and the established high extend of rimifon encapsulation proved the possibility of its successful application as rimifon-carrier for en...

Quantitative analysis of acidic OH groups in zeolite by ammonia IRMS-TPD and DFT: Application to BEA

The number and ammonia desorption enthalpy, as a measure of acid strength, of bridging OH groups divided into three portions based on the OH stretching frequency in zeolite β were measured by means of a method of ammonia infrared/mass spectroscopy (IRMS) temperature-programmed desorption (TPD). On the other hand, the optimized structures, wavenumbers and ammonia desorption energies (Edes) of all the geometrically possible acid sites located at crystallographically non-equivalent positions in a BEA structure were calculated with an aid of density functional theory (DFT). The calculated wavenumbers could be classified mainly into three portions. The interaction between H in the AlOHSi unit and oxygen atoms surrounding the unit is suggested to lower the OH frequency. The number, averaged Edes and wavenumber of stretching vibration of each portion were in agreement with the observed values.

Different Brønsted acidity of H-ZSM-5 and D-ZSM-5 zeolites revealed by the FTIR spectra of adsorbed CD3CN

The acidity of the bridging OH/OD groups in high-silica H-ZSM-5 and D-ZSM-5 zeolites was measured using CD 3 CN as an IR probe molecule. We found that the OH groups are more acidic because the ν(CN) modes of the CD 3 CN···HO complexes formed with bridging hydroxyls appeared at frequencies higher by 1.1–1.2 cm − 1 than the respective modes of CD 3 CN···DO complexes. This difference corresponds to ca. 0.2 units of the Hammett acidity function. Similar but less pronounced effect was established with the AlOH/AlOD groups (difference of 0.7 cm − 1 ), i.e., contrary to current opinions, the isotopic effect increases with the acidity strength. Adsorption of CD 3 CN clearly reveals the difference in acidity of the OH and OD groups in ZSM-5 zeolite. • Bridging OH groups in ZSM-5 are more acidic than the corresponding OD groups • The difference in acidity corresponds to 0.2 units of the Hammett acidity function • The isotopic effect decreases with the decrease of the acidity of the OH groups.

Structure and Reverse Hydrogen Spillover in Mononuclear Au0 and AuI Complexes Bonded to Faujasite Zeolite: A Density Functional Study

We have studied the structure of mononuclear gold supported on acidic form of faujasite zeolite in two oxidation states, namely, 0 and +1, using density functional theory. The binding of the gold monomer to the zeolite support is stronger in the oxidation state +1 than in the oxidation state 0. For the oxidation state 0, the hydrogenated clusters AuH/(2H)-FAU, AuH2/H-FAU generated by stepwise reverse hydrogen spillover from bridging OH groups of zeolite are energetically preferred over the Au/(3H)-FAU structure. Reverse hydrogen spillover of all the three acidic protons from the zeolite to the Au monomer did not lead to a stable structure. The calculated reverse hydrogen spillover energy per hydrogen atom for zeolite supported AuH and AuH2 clusters are −10.2 and −5.1 kJ/mol, respectively, in the oxidation state 0, while in the oxidation state +1 it is 20.9 kJ/mol for zeolite supported Au+H cluster.

Role of surface hydroxyl groups of acid-treated natural zeolite on the heterogeneous catalytic ozonation of methylene blue contaminated waters

The effect of surface properties of zeolites on the kinetics of heterogeneous catalytic ozonation of organic pollutants is still under discussion. In this work, the influence of hydroxyl groups (Z-OH2+, Z-OH, Z-O−) of acid-treated natural zeolite on the catalytic ozonation of methylene blue (MB) contaminated waters is analysed. A Chilean natural zeolite was acid-treated using HCl (2.44M). Acid-treated zeolite was chemically and physically characterised by N2 adsorption at 77K, X-ray fluorescence, acidimetric–alkalimetric titration, and by NH3 and CO2 temperature-programmed desorption methods. Fourier transform infrared spectroscopy of pyridine adsorption was used to elucidate the nature and strength of acidic sites resulting from the acid treatment of natural zeolite. Experimental results obtained here are kinetically modelled using a set of two homogeneous and three heterogeneous surface reactions. Moreover, the quantitative effects of single ozonation, adsorption and coupled treatment on MB removal rate, together with the effect of pH and the presence of radical scavengers are analysed. Brønsted acid sites in the form of proton-donating OH groups (Z-OH) of acid-treated zeolite are claimed here to play an important role on the catalytic ozonation of MB in water, acting as active sites for the adsorption of reacting species. The higher catalytic activity is observed at pH above the pH of point of zero charge (pHPZC) and could be related to the presence of surface hydroxyl groups in the deprotonated form.