Heterogeneity Of OH Groups Research Articles

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.

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.

IR study of heterogeneity of OH groups in zeolite HY-splitting of OH and OD bands

We have been studying the problem of heterogeneity of OH groups in zeolites HY for a long time. The heterogeneity was suggested by the shift of the IR band of OH groups restoring upon ammonia desorption and also by the fact that the band of OH groups forming hydrogen bonds was relatively broad (broader than for homogeneous acidic OH). In the present study we present another important argument for heterogeneity: the splitting of the IR band of free OH and OD groups in a zeolite of Si/Al=8.3 dealuminated by (NH 4) 2SiF 6 treatment. Such a splitting is the best seen in low temperature spectra of OD groups. We found less acidic 3640 cm −1 (AlO)(SiO) 2Si O 1 HAl(OSi) 3 and more acidic 3625 cm −1 (SiO) 3Si O 1 HAl(OSi) 3 groups. The presence of these two kinds of hydroxyls corresponds to the presence of Si(2Al) and Si(1Al), respectively, detected in 29Si MAS NMR spectra. We also found a small amount of strongly acidic 3599 cm −1 hydroxyls interacting with extraframework Al species.

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.

Heterogeneity of OH groups in mordenites

Hydroxyl groups in H-mordenites are localized both in broad channels and in side pockets. Heterogeneity of OH groups in large channels was studied by IR experiments of benzene and CO sorption. It has been found that the band of hydroxyls interacting with both probe molecules was asymmetric and split. The submaxima of lower Δν (206 cm−1 for CO and 240 cm−1 for benzene) were assigned to less acidic (SiO)2(AlO)Si–OH–Al(OSi)3 groups, and submaxima of higher Δν (300 cm−1 for CO and 360 cm−1 for benzene) were assigned to more acidic (SiO)3Si–OH–Al(OSi)3. The presence of Si(1Al) and Si(2Al), which are indispensable for the formation of the above proposed hydroxyls, was confirmed by our 29Si MAS NMR experiments. The values of deprotonation energies (PA) of both kinds of hydroxyls were estimated from Δν of hydroxyls interacting with CO. Analysis of the second derivative diagram of IR bands of more acidic (SiO)3Si–OH–Al(OAl)3 hydroxyls interacting both with benzene and CO suggested that these bands were also split and that these hydroxyls were also heterogenous.

Heterogeneity of OH Groups in H-Mordenites TPD and IR studies of ammonia desorption

We have investigated the temperature-programmed desorption (TPD) of ammonia during the activation of NH4Na-mordenites of different exchange degrees. Using a regularization method, desorption energy distribution functions have been calculated. The obtained results indicate the heterogeneity of the bridging Si-OH-Al groups in HNa-mordenites. This was concluded from the width of the distribution functions and from the presence of submaxima. For HNa-mordenites of exchange degrees below 50%, containing only hydroxyls in the broad channels, two distinct submaxima are present, thus suggesting the presence of at least two kinds of bridging hydroxyls of various acid strengths. In HNa-mordenites of exchange degrees above 50%, the hydroxyls appear in narrow channels and the distribution of ammonia desorption energy broadens on the side of higher energies. This may be related to a strong stabilization of ammonium ions inside narrow channels. The maximum concentrations of hydroxyls of desorption energies between 95 and 135 kJ mol-1 and between 135 and 165 kJ mol-1 calculated from TPD data were 3.9 and 3.3 OH per unit cell (u.c.). These values agree well with our previous IR results of concentrations of hydroxyls in broad and in narrow channels (3.7 and 2.8 OH per u.c.). The TPD data obtained for the heterogeneity of OH groups in HNa-mordenites are in accordance with the IR data concerning ammonia desorption. The IR band of OH groups restoring upon saturation of all the hydroxyls with ammonia and subsequent step-by-step desorption at increasing temperatures shifts to lower frequencies indicating that there are hydroxyls of various acid strengths and the less acidic hydroxyls restore first at lower desorption temperatures.

Microcalorimetric and IR spectroscopic studies of pyrindine sorption in NaH-mordenites

The adsorption of pyridine in NaH-mordenites of various Na/H exchange degrees was followed by adsorption microcalorimetry and IR spectroscopy. IR data evidenced that practically only Brönsted acid sites (OH groups) were present in our mordenites. The concentration of Lewis sites was very low. All the results obtained in this study concern therefore the properties of acid hydroxyls in large channels, which are active sites in reactions catalyzed by mordenites. The amounts of such hydroxyls determined volumetrically were found to be close to the amounts determined in a previous IR study. The average acid strength determined microcalorimetrically increased with exchange degree up to 47% and then was constant. It indicates that as long as new hydroxyls are formed in large channels, their average acid strength increases (probably more strongly acid hydroxyls are formed at higher exchange degrees). Above 47% of exchange, the acid strength of hydroxyls in large channels was constant, indicating that the formation of hydroxyls in side pockets (which takes place above 50% of cation exchange) did not influence the properties of hydroxyls in large channels. The differential heat of pyridine adsorption with most of acid sites (OH groups) did not depend on pyridine loading. Our earlier IR and TPD results of ammonia desorption suggested heterogeneity of OH groups in large channels of mordenites. However, the constant value of adsorption heat suggests that pyridine molecules reacted with all the accessible hydroxyls without selecting the most acid ones. This hypothesis was confirmed by IR studies of pyridine adsorption: the stretching frequency of hydroxyls consumed by pyridine was independent on the amount of pyridine adsorbed.

Heterogeneity of OH groups in NaH-mordenites: Effect of Na/H exchange degree

Our earlier results and the results of other authors gave evidence that OH groups in NaH-mordenites may be localized both in the 12- and 8-ring channels. In this study we determined (by quantitative i.r. studies of pyridine and ammonia sorption) the concentration and acid strength of both kinds of hydroxyls. The first protons introduced into mordenite (at low exchange degrees) form HF hydroxyls in the 12-ring channels. Their concentration was close to the theoretical values (amount of AI minus amount of Na). At an exchange degree of about 50% the concentration of these hydroxyls reached the maximal value, about 4 OH/u.c), and the next protons introduced form LF hydroxyls in the 8-ring channels. This limit value of 4 OH/u.c. may be related to the number of 4.1 Na+ ions in D and E sites in the 12-ring channels in dehydrated Na-mordenites. The acid strength of HF and LF hydroxyls was studied by comparing the values of stretching frequencies and extinction coefficients of OH bands as well as in pyridine (or ammonia) thermodesorption experiments. It has been found that the acid strength of both kinds of hydroxyls increased with the exchange degree. It is discussed by assuming the heterogeneity of OH groups in NaH-mordenites, and assuming that in each population of hydroxyls (HF and LF) there are more and less acidic ones. It may be assumed that in each population of hydroxyls the first protons introduced form less acidic hydroxyls and the next protons more acidic ones. It increases the average acid strength of each of these two populations of hydroxyls. It has been found also that the formation of LF hydroxyls did not influence the acid strength of HF hydroxyls. Opposite results were obtained earlier with NaHY zeolites in which the formation of O3H in hexagonal prisms increased distinctly the acid strength of O1H inside supercavities.

Heterogeneity of OH groups in H-mordenites: Effect of dehydroxylation

Quantitative i.r. studies of ammonia and pyridine sorption in H-mordenite showed that the amount of Br\/onsted acid sites (acidic hydroxyls) 6.5 H+/u.c. was close to the theoretical value calculated from the chemical analysis (7.2 H+/u.c.). Approximately half of this amount (3.0 H+/u.c.) was situated inside the 12-ring channels (the main channels) and half (3.5 H+/u.c.) inside the 8-ring channels. Hydroxyls in both kinds of channels were found to be prone to dehydroxylation to the same extent. The number of Lewis acid sites (created by dehydroxylation) detected by pyridine was much lower than detected by ammonia. This observation, together with the fact that in dehydroxylated H-mordenite most of the acidic hydroxyls are inaccessible to pyridine, suggests that dehydroxylation results in a narrowing of pores, making them inaccessible to pyridine (also to other bulky reactant molecules). We studied the acid strength of OH groups by comparing the stretching frequencies and extinction coefficients of free OH bands and in ammonia thermodesorption experiments. We found that in the nondehydroxylated H-mordenite, the acid strength of the OH groups inside the 12-ring channels was higher than inside the 8-ring. Dehydroxylation decreases the acid strength of hydroxyls. This concerns both the whole population of OH groups and the population of hydroxyls accessible to pyridine only. These effects are discussed considering the heterogeneity of OH groups in H-mordenite and removal of the most acidic hydroxyls first.

Application of Principal Component Analysis to the Studies of OH Groups in NaHY Zeolites

The Si-OH-Al bridging hydroxyls in zeolites are active sites in many important reactions catalyzed by zeolites. Their amount and their acid strength have been extensively studied in many laboratories (for reviews see Refs. 1 and 2). We undertook studies of the heterogeneity of Si-OH-Al groups in zeolites, i.e., the presence of a few kinds of bridging hydroxyls of various acid strength in the same zeolite structure. We studied the heterogeneity of OH groups in NaHZSM-5 zeolites, NaHA zeolite, and NaH faujasites.

Heterogeneity of OH Groups in Faujasites Studied by IR Spectroscopy

Heterogeneity of OH groups in faujasites was studied by analysis of the IR absorption band of free hydroxyls and hydroxyls engaged in hydrogen-bonding with chlorobenzene. The Si-O 1H-Al groups were found to be homogeneous in NaHX zeolite (Si/Al = 1.06) and heterogeneous in NaHY zeolite (Si/Al = 2.56). Experiments of pyridine sorption and desorption have shown that the more acidic hydroxyls desorb pyridine at higher temperatures than the less acidic hydroxyls. The IR spectroscopic results agree well with previously published 29Si MAS NMR results. Only one Si(4A1) signal is present in the NMR spectrum of X zeolite (Si/Al ≅ 1), but there are four signals (Si(3Al), Si(2Al), Si(1Al), and Si(OAl)) in Y zeolite. As Si(OAl) cannot create bridging hydroxyls, this implies three kinds of hydroxyls of various acid strengths in NaHY zeolite and one kind in NaHX, as actually observed in the present IR study. The hydroxyl groups in NaHX zeolite can be assigned to (AlO) 3Si-OH-Al(SiO) 3, and the three kinds of hydroxyls in NaHY to (AlO) 2(SiO)Si-OH-Al(SiO) 3, (AlO)(SiO) 2Si-OH-Al(SiO) 3, and (SiO) 3Si-OH-Al(SiO) 3, respectively. The elimination of a small amount of the most acidic S-O 1H-Al by pyridine significantly reduces the catalytic activity of NaHY zeolite in butene-1 isomerization.

ChemInform Abstract: Heterogeneity of OH Groups in NaH‐ZSM‐5 Zeolite Studied by IR Spectroscopy.

AbstractWhen homogeneous OH groups (in organic acids, phenol, or SiO2) engage in H‐bonding with aromatic compounds or interact with n‐hexane a sym. and nonsplit IR band is observed.

Heterogeneity of OH groups in NaHZSM-5 zeolite studied by infrared spectroscopy

The properties of the OH groups in NaHZSM-5 zeolite absorbing at 3609 cm −1 were studied with respect to their hydrogen bonding to weak electron donor molecules ( n-hexane, fluorobenzene, chlorobenzene, benzene, toluene, p-xylene, ethene). The observed band splitting was attributed to the heterogeneity of the OH groups. A comparative study of systems with homogeneous OH groups did not show such a splitting. The Bellamy-Hallam-Williams (BHW) relation was obeyed for all kinds of OH groups, and the proton affinities (PA) were calculated from this relation. Two probable explanations of the heterogeneity of OH groups in NaHZSM-5 zeolite are considered. One assumes the presence of OH groups with various numbers of Al atoms in the immediate neighborhood, and the other (more plausible) assumes dependence of OH acid strength on SiO and AlO bond lengths and AlOHSi bond angles.