J. Datka

Acid properties of dealuminated beta zeolites studied by IRspectroscopy

The concentration of acid sites in non-dealuminated (Si/Al=10), and dealuminated (Si/Al=20–90) H-beta zeolites has been studied by quantitative IR measurements of pyridine sorption. The number of Bronsted sites was found to be less than that of Al atoms. Most probably this was due to the presence of Al in extraframework positions (Al–OH groups and Lewis acid sites were found). Some positively charged extraframework Al species may neutralize the charge of AlO 4 - and lower the concentration of protonic sites. Dealumination decreased the amount of extraframework Al species and, hence, the difference between the amounts of Al and Bronsted sites. Dealumination also decreased the number of Lewis acid sites. IR studies of pyridine desorption evidenced that, as well as strong Bronsted sites (Si–OH–Al), there were also weak Bronsted sites, not Si–OH–Al; their nature is not clear. The contribution of such weak Bronsted sites decreased with the extent of dealumination. IR studies of pyridine desorption and benzene sorption, evidenced that Si–OH–Al groups were heterogeneous in H-beta zeolites. Dealumination removed the less acidic hydroxy groups first.

Influence of coke on the acid properties of a USHY zeolite

The composition of coke and its effect on the acid sites were determined with samples of a USHY zeolite (with total and framework Si/Al ratios of 3.1 and 5.4, respectively) used for m-xylene transformation at 520 and 720 K. At low coke contents, coke is mainly constituted by methyl substituted polyaromatic compounds with three (520 K) or four (720 K) aromatic rings trapped in the zeolite pores. Coke causes a decrease in the intensity of the IR hydroxyl band. The most acidic hydroxyls, i.e., the bridging hydroxyls in interaction with extra-framework aluminum species are most affected by coke, whereas no interaction is observed between the non-acidic hydroxyls and the coke molecules. Pyridine adsorption shows that, while the number of protonic sites able to retain pyridine adsorbed decreases by coking, this was not the case for the Lewis sites. Pyridine adsorption facilitates the desorption under vacuum of coke molecules from the coked zeolites, the effect being more significant for coking at 520 K. This indicates that the retention of coke molecules is not only due to their low volatility (at 520 K) or to their steric blockage (720 K), but also to their adsorption on the acid sites.

IR spectroscopic studies of the acidic properties of the mesoporous molecular sieve MCM-41

The concentration, acid strength and thermal stability of the Bronsted- and Lewis-acid sites in the proton form of the mesoporous molecular sieve MCM-41 were monitored by IR spectroscopy as function of the Al content. The results were compared with those for zeolite H+-ZSM-5 with the same elemental composition. The spectra of hydroxy groups in purely siliceous [Si]-MCM-41 and H+-[Si, Al]-MCM-41 contain a single band at 3740 cm–1, assigned to the Si—OH groups, and are independent of the Al content. Sorption of ammonia or pyridine generates bands from the ammonium and pyridinium ions. The concentrations of both Bronsted- and Lewis-acid sites in H+-[Si, Al]-MCM-41 are lower than the concentration of four-coordinate aluminium. The Bronsted sites in H+-MCM-41 are more prone to dehydroxylation than those in zeolite H+-ZSM-5, and the effect increases with the Al content. The acid strength of the Bronsted sites in H+-MCM-41 is independent of the Al content and much lower than in zeolite H+-ZSM-5.

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.

Acid properties of NaH-mordenites: Infrared spectroscopic studies of ammonia sorption

Abstract Ammonia was used as a probe molecule to study the acid properties of NaH-mordenites. Ammonia reaches all acid sites in both large and narrow channels. The concentration of Bronsted sites (Si OH Al groups) and of Lewis acid sites was followed as a function of the Na/H exchange degree and the calcination temperature (extent of dehydroxylation). In NaH-mordenites of exchange degrees lower than 80%, the concentration of Bronsted sites was found to be practically the same as the theoretical value (the amount of Al minus the amount of Na). In H-mordenites of exchange degrees of 100%, the experimental value was 20% lower than the theoretical one. This difference was explained by a dehydroxylation that occurs during activation at 820 K. Studies of dehydroxylation have shown that bulk dehydroxylation starts at about 900 K. At 1050 K, most hydroxyls are lost, and the concentration of Lewis acid sites attains the maximal value, 4.5 sites/u.c. This value is comparable to the theoretical one, half of the Al content in H-mordenite (3.6 sites/u.c.). Variation of the O H stretching frequency with the Na/H exchange degree and with the extent of dehydroxylation suggests variation of the acid strength. This is discussed in the terms of the heterogeneity of Si OH Al groups in NaH-mordenites.

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.

Hydroxyl groups and acid sites in NaZSM-5 zeolites studied by i.r. spectroscopy

Abstract The decomposition of TPA (tetrapropylammonium ions) in NaZSM-5 zeolites, the localization of OH groups and acid properties of zeolites were studied by i.r. spectroscopy. NaZSM-5 zeolite in which TPA are not decomposed contain small amount of 3738 cm −1 OH groups. The decomposition of TPA results in the formation of considerable amounts of 3738 cm −1 OH groups situated inside zeolitic channels. These OH groups are not acidic at the reaction with pyridine. NaZSM-5 zeolites contain Bronsted and Lewis acid sites the amounts of which depend on the composition of the zeolite.

FTIR study of hydration of dodecatungstosilicic acid

The dehydration of H4SiW12 O40·15.6 H2O was studied in situ in the IR chamber. On evacuation at room temperature the departure of most loosely bonded water characterized by bands at 3550 and 1616 cm−1 was observed. In the remaining hexahydrate the band at 3445 cm−1 was ascribed to the hydrogen bond between the Od oxygen atom of the Keggin unit and dioxonium H5O2+ ion, the presence of which is manifested by the 1710 and 1100 cm−1 vibrations. All these bands vanish in the case of anhydrous H4SiW12O40, in which the band at 3106 cm−1 ascribed to the hydrogen bond between neighbouring HPA anions Od−H+−Oc is still present. The dehydration of hexahydrate is accompanied by splitting of the W=Od band into 987 and 1010 cm−1 reflecting the change of the kind of hydrogen bond in which the Od oxygen atom is involved. Based on the above results it was concluded that protons forming oxonium ions in hydrated solid heteropoly acid are more strongly bonded than those in anhydrous one which are forming hydrogen bonds between neighbouring Keggin units.

T–O–T skeletal vibration in CuZSM-5 zeolite: IR study and quantum chemical modeling

The location of Cu cations in CuZSM-5, properties of cationic sites and their interaction with guest molecules have been studied by quantum chemical (DFT) modeling and IR spectroscopy based on the frequency shift of antisymmetric T–O–T vibration of oxygen rings. The shift has been found sensitive both to the framework interaction with cations and to the interaction with adsorbed molecules. It has been measured and estimated theoretically from parameters characterising framework distorsion by Cu+ and Cu2+, with MgZSM-5 and NaZSM-5 used as ‘‘reference samples’’. It was found that the ordering of the cation perturbing effect was: Na+<Cu+<Mg2+<Cu2+. NO interaction with Cu cations was much stronger than that of CO and N2 . Divalent copper showed polarized 2-electron covalent bonding with NO strengthening its bond while moderate bonding ability of monovalent copper led to NO bond activation, in accordance with high catalytic activity of Cu+ZSM-5.

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

Abstract 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 Bronsted 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.