FTIR spectroscopic and1 H MAS NMR studies of the influence of lattice chemistry and structure on Brønsted acidity in zeolites

The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Brønsted sites in zeolites has been investigated using changes of the vibrational properties of Brønsted OH(OD) groups and a shift change of Brønsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and 1 H MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Brønsted sites in zeolites. Differences in structure or pore size play a much smaller role.     

1H broad-line and MAS NMR: application to the study of acid sites of desilicated zeolite ZSM-5

Broad-line¹H NMR study of desilicated zeolite ZSM-5 was carried out as a function of the number of adsorbed water molecules in amounts lower than or equal to that of the Brønsted acid sites. The dissociation coefficient of the acid OH groups, currently associated with the acid strength, was shown not to be affected by the selective removal of Si from the zeolite ZSM-5 framework which resulted in more Brønsted acid sites per unit surface area. On the other hand, by using MAS NMR, bridging Brønsted acid sites hydrogen-bonded to the zeolite framework were identified on the anhydrous surface of the desilicated ZSM-5. Moreover, MAS NMR spectra of the desilicated zeolite partially rehydrated showed the presence of some Lewis acid sites.     

An NMR study of acid sites on sulfated-zirconia catalysts using trimethylphosphine as a probe

Concentrations of Brønsted and Lewis acid sites on sulfated-zirconia catalysts were determined using the³¹P MAS NMR spectra of adsorbed trimethylphospine. A sample that had been calcined and exposed to air for a long period exhibited only Brønsted acidity; however, treatment of the sample at progressively higher temperatures resulted in the development of at least three types of Lewis acidity, along with a decrease in the concentration of Brønsted acid sites. In a related study the activity of these catalysts for the alkylation of isobutane with 2-butene was determined. The aged catalyst was inactive, but activation of the material at 100°C resulted in the most active catalyst. Thermal treatment at higher temperatures resulted in a loss in activity which paralleled the decrease in the Brønsted acid sites. These results are consistent with a model in which strong Brønsted acidity is a result of the interaction between bisulfate groups and adjacent Lewis acid sites.     

Solid-state NMR characterization of the acid sites in cubic mesoporous Al-MCM-48 materials using trimethylphosphine oxide as a P NMR probe

The acidic properties of Al-MCM-48 with Si/Al ratios ranging from 10 to 67, synthesized with Gemini surfactant as the template, have been characterized by a combination of multinuclear solid-state ¹H, ²³Na, ²⁷Al, ²⁹Si and ³¹P MAS (magic angle spinning) NMR and some double-resonance NMR methods using trimethylphosphine oxide (TMPO) as a probe molecule. XRD and ²⁷Al MAS NMR results indicated that aluminum has been successfully incorporated into the framework of MCM-48 materials up to Si/Al = 17.9 by direct synthesis. ¹H and ³¹P MAS NMR results strongly supported the generation of Brønsted acid sites in the cubic MCM-48 mesoporous material after the incorporation of aluminum, even without ion-exchange treatment. Double-resonance NMR techniques such as ³¹P/²⁷Al TRAPDOR (Transfer Population in Double Resonance) and ²⁹Si/³¹P REDOR (Rotational Echo in Double Resonance) NMR were performed to further correlate the TMPO probe molecule to the Brønsted acid sites in the silica framework. ³¹P/²⁷Al TRAPDOR NMR experiments performed at different temperatures were able to establish the correlation between ³¹P and ²⁷Al spins, further confirms the presence of Brønsted acid sites at 65 ppm in the ³¹P MAS NMR spectrum. Although the assignment of the Lewis acid sites was somehow unambiguous with ³¹P/²⁷Al TRAPDOR NMR, the FT-IR observation of the calcined samples adsorbed with pyridine did reveal the presence of Lewis acid sites. In contrast to the pore size constraints of zeolites, ²⁹Si/³¹P REDOR NMR results indicated that the protonated TMPO was highly mobile inside the mesoporous channels of Al-MCM-48 at the NMR time scale.     

Distribution of acid sites and differential heat of NH3 chemisorption on some aluminas and zeolites

The distribution and the strength of acid sites in some aluminas and zeolites were obtained from microcalorimetry measurements of ammonia adsorption. They were compared to the results of FTIR of adsorbed CO and of¹H²⁷Al cross-polarization (CP) MAS NMR using chemisorbed ammonia as proton reservoir. FTIR of CO adsorption yields the numbers of Lewis sites and CP MAS NMR gives the coordination of surface aluminum. In spite of the variety of catalysts and of acidity encountered, interesting correlations were found with both the energy of adsorption and singularities in the microcalorimetric titration. In particular, in zeolites a clear difference exists in the distribution of the Lewis and Brønsted acid site strengths with respect to the amount of irreversibly adsorbed NH₃.     

Niobium oxide acidity studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

The quantitative study of the Brønsted acidity of niobic acid (Nb₂O₅·xH₂O) using broad-line¹H NMR at 4 K has been performed by interacting niobic acid, pretreated at 573 K under vacuum, with water molecules. The number of oxyprotonated species (H₃O⁺ and H₂O...HO species formed, unreacted acidic OH groups or excess H₂O molecules) deduced from the simulations of the broad-line¹H NMR spectra shows a continuous increase in the number of H₃O⁺ species with adsorbed water molecules. This increase may be due to a classical dilution effect or to a synergistic interaction between Brønsted and Lewis acid sites. These results are compared with those of some HY zeolites with or without framework defects.     

Dinitrogen as a probe of acid sites of zeolites

Nitrogen adsorption on H-ZSM-5 and H-Y zeolites at low temperatures were studied by in situ FT-IR spectroscopy. For each zeolite, two absorption bands were observed at around 2334 and 2352 cm^–1 in thev(NN) region and were assigned to thev(NN) mode of dinitrogen species adsorbed on Brønsted and Lewis acid sites of the zeolites, respectively. These results and previous results for H-mordenite suggest that dinitrogen serves as a probe of acid sites and its advantages as probe are discussed.     

1H mas NMR and IR studies of the acidic properties of realuminated zeolite Y

IR and¹H MAS NMR confirm that extra-framework aluminium present in dealuminated (ultrastable) zeolite Y is reintroduced into the framework by treatment with strongly basic solutions at elevated temperatures. The realuminated sample contains twice as many Brønsted acid sites than the ultrastable precursor and, with an accuracy of 20%, the same number of acid sites as the parent as-prepared zeolite. However, not as many hydroxyl groups associated with framework Al in the product are acccesible to pyridine as in the parent sample.     

Number of isolated aluminum atoms in the framework of dealuminated acid zeolites

Monte Carlo calculations restricted by Loewenstein rule predict correctly the variation of the intensities of the lines contributing to the²⁹Si MAS NMR spectra in dealuminated acid ZSM5, mordenites and Y zeolites covering a domain of ratios silicon/ framework aluminum between 4 and 80. They are also used to calculate the numbers of isolated framework aluminum and the relationship with the Brønsted acidity is briefly discussed.     

In-situ 31P MAS NMR probing of the active centers in Ti silicalite molecular sieve

The investigation of the active centers in titanium zeolite was conducted by a probe reaction, the oxidation of trimethylphosphine. The different sites of TS-1 zeolite can be clearly distinguished through ³¹P solid-state MAS NMR spectra of adsorbed trimethylphosphine. The results showed that only the trimethylphosphine bound to the coordinate-unsaturated framework Ti sites and the distorted tetrahedral Ti sites of TS-1 zeolite could be easily oxidized by urea hydrogen peroxide. Diffuse Reflectance UV–Vis and ²⁹Si MAS NMR spectra further confirmed that the titanium peroxide species (TiOOH), which is formed through the solid–solid interaction between urea–H₂O₂ and TS-1 zeolite, could be catalytic active centers during the oxidation of trimethylphosphine.     

Base and acid sites in alkaline earth cation-exchanged X zeolites

Alkaline earth exchanged X zeolites are characterized by pyrrole chemisorption using FTIR spectroscopy. Lewis basic, Lewis acidic (cations) and also Brønsted acidic sites were detected in these zeolites. Based on a local environmental model, the charges on oxygens (basic sites) and cations are further calculated using the electronegativity equivalence method. The NH stretching frequencies of pyrrole chemisorbed on both basic and acidic sites are well correlated with these charges. This relationship supports the early idea that the alkaline earth cations in faujasite exist in the form of (MOH)¹⁺.     

Stable and Selective Dehydrocondensation of Methane Towards Benzene on Modified Mo/HMCM-22 Catalyst by the Dealumination Treatment

A modified Mo/HMCM-22 catalyst by the dealumination treatment (Mo/HMCM-22-D) exhibited remarkable performance for the catalytic dehydrocondensation of methane with a higher selectivity of benzene and a lower selectivity of coke, in comparison with the same Mo catalyst supported on parent HMCM-22 (Mo/HMCM-22). Excellent catalytic stability as well as a high benzene formation rate of 1500 nmol/(g-cat·s) was obtained on a 6%Mo/HMCM-22-D catalyst at 1023 K, 3 atm and 2700 ml/(g·h) owing to the efficient suppression of coke formation. Dealumination of the HMCM-22 zeolite was characterized by XRD, ²⁷Al and ¹H MAS NMR and NH₃-TPD techniques. It was found that the dealumination treatment of HMCM-22 zeolite resulted in an effective suppression of acid sites, particularly the Brønsted acid sites (proton form in Al--O--Si) owing to the removal of tetrahedral framework aluminum, while the microporous structure and the zeolite framework remained unchanged. It was suggested that the stable and selective dehydrocondensation of methane towards benzene is based on the suppression of coke formation owing to the effective decrease of strong Brønsted acid sites by the dealumination treatment of the HMCM-22 zeolite.     

Realumination of zeolite Y under acidic conditions

Dealuminated zeolites Y were treated with aqueous solutions of various acids and ammonium salts to investigate the realumination behavior under acidic conditions. From the results of ²⁷Al MAS NMR, ²⁹Si MAS NMR and FT-IR measurements, it was found that a part of non-framework aluminum species in the dealuminated zeolite Y is effectively reinserted into the zeolite framework in CH₃COONH₄ and C₆H₅COONH₄ aqueous solutions. Pyridine adsorption experiments also revealed that most of incorporated aluminum species generate tetrahedrally coordinated framework aluminum species, namely Brönsted acid sites. Although the realumination also proceeded in H₂SO₄ and CH₃COOH aqueous solutions, large amounts of incorporated aluminum species were not necessarily responsible for generation of Brönsted acid sites. Framework connected aluminum species, presumably as 3-fold-coordinated Lewis acidic framework aluminum species, were mainly generated. In the TEM image of the realuminated zeolite Y, needle-like crystals with ca. 25–80 nm in length were observed, which are probably due to AlOOH generated from non-framework aluminum species.     

Nature of the active sites in H-ZSM-11 zeolite modified with Zn(2+) and Ga(3+)

H-ZSM-11 zeolite modified with zinc and gallium by ion exchange was investigated using XRD, IR and TPD of ammonia. The modification of the material by zinc produced a lowering of the strong Brønsted acidic sites generating new and strong Lewis sites. Unlike zinc-zeolites, the gallium is localized preferentially on the outer surface of microcrystallites blocking a few Brønsted centers.     

Platinum-sulfated-zirconia. Infrared study of adsorbed pyridine

Pyridine adsorption on sulfated zirconia (SO _2– ⁴ -ZrO₂) provides evidence for infrared bands characteristic of both Brønsted and Lewis acid sites. Samples treated at 100°C retain water and have a higher fraction of Brønsted acidity than when the sample is treated at 400°C. The fraction of Brønsted acid sites observed for SO _2– ⁴ -ZrO₂ is the same in the presence or absence of supported Pt. Based on pyridine adsorption, exposure to gaseous hydrogen at 100 or 150°C did not significantly alter the fraction of Brønsted acid sites following the exposure to hydrogen.     

Surface Acidity of H-Beta and Its Catalytic Activity for Alkylation of Benzene with Propylene

The acidity of H-beta zeolites with SiO₂/Al₂O₃ ratios ranging from 20 to 350 was characterized by NH₃-TPD profiles and FTIR spectra of adsorbed pyridine. As SiO₂/Al₂O₃ ratios of the H-beta zeolites increased, NH₃-TPD acidic amount of the samples is decreased. The IR bands of the adsorbed pyridine on the zeolites are also decreased with the increased SiO₂/Al₂O₃ ratios. The batch reaction of propylene and benzene was carried out in liquid phase at 423 K over H-beta zeolites. The selectivity to isopropylbenzene was high. The catalytic activity of H-beta zeolites is in direct proportion to the acidic amount of the zeolites in general. H-beta zeolite of SiO₂/Al₂O₃=27, which contains the highest amount of Brønsted acid sites as indicated by FTIR spectra of adsorbed pyridine, is the most reactive catalyst in the alkylation reaction. In continuous liquid-phase reactions, high propylene conversion and isopropylbenzene selectivity can be achieved at 413–453 K with benzene to propylene mole ratio from 4 to 8. The catalytic activity and selectivity of the H-beta zeolite do not change after 1100 h of reaction.     

31P and119Sn high resolution solid state CP/MAS NMR study of Al2O3-SnO2 systems

A series of Al₂O₃-SnO₂ catalysts with the mole ratio of Al₂O₃ to SnO₂ equal to 1:1, 1 0.5, 1 0.1, 1 0.05 and 1 0.01 were characterized by³¹P NMR of adsorbed trimethyl phosphine (TMP) and¹¹⁹Sn MAS NMR spectroscopy. It was found from³¹P NMR that no Brønsted acid sites exist in these samples. Pure SnO₂ shows two different types of Lewis acid sites; in the mixed oxide samples a Lewis peak characteristic of pure Al₂O₃ is always seen, together with either one or two other Lewis peaks, depending on the Sn concentration.¹¹⁹Sn CP/MAS NMR spectra of the highest Sn-content sample show one narrow line at –603 ppm superimposed on a very broad line, indicating a strong interaction between Al and Sn oxides.     

Synthesis of aluminum rich MCM-41

The synthesis of MCM-41 mesoporous compounds with Si/Al ratios as low as 2 without observing the presence of octahedral Al in²⁷AlMAS NMR is reported. FTIR spectra of chemisorbed pyridine indicated that MCM-41 materials in their protonated form exhibit both Brønsted and Lewis acid sites.     

A spectroscopic study of amorphous and crystalline Ti-containing silicas and their surface acidity

Different titanium silicalite (TS-1) samples and TiO₂/SiO₂ aerogels samples have been prepared and characterized structurally by XRD, skeletal FT-IR and FT-Raman spectroscopies and diffuse reflectance UV spectroscopy. The surface acidity of all materials has been investigated by FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile. The TS-1 samples are found to be totally free from extra-framework Ti oxide species, while the characteristic features of framework tetrahedrally coordinated Ti cations are well evident. Framework Ti cations substituting for Si into the silicalite MFI structure act as quite strong Lewis acid sites towards acetonitrile. The presence of Ti neither changes significantly the Brønsted acidity of the silicalite silanols, nor results in the formation of new hydroxy groups. TS-1 samples present lower defectivity (smaller amount of clustered hydrolyzed Si–OH OH–Si bonds) than pure silicalite prepared in the same way. Extra-framework TiO₂, instead, is well detectable into aerogel samples, although framework Ti cations are likely also present. The surface of Ti-containing aerogels is covered by large amounts of silanol groups, although these sites do not present enhanced Brønsted acidity. Additionally, Lewis sites are detected whose strength is similar to that observed on TS samples, and is superior to that of the predominant sites on TiO₂. It is suggested that silica tends to cover the TiO₂ core in the case of aerogels. The surface is consequently dominated by silica with Ti sites in substitutional position.     

Isomerization of α-pinene over dealuminated ferrierite-type zeolites

Isomerization of α-pinene was performed on a series of dealuminated ferrierite (FER)-type zeolites in liquid phase at 363 K using a batch reactor. The course of zeolite dealumination was followed in detail using ²⁹Si, ²⁷Al, ¹H MAS NMR, XRD, FTIR, and sorption of nitrogen. The ammonium form of FER was dealuminated with aqueous solutions of HCl. While retaining the crystallinity of the zeolite particles, the treatments removed up to 53% of the tetrahedrally coordinated aluminum atoms from the FER framework. According to ²⁹Si MAS NMR studies, the framework aluminum atoms located at the 10-membered rings in the main channels of FER (T_B sites) were depleted preferentially from their positions. Even relatively mild dealumination of FER led to an active catalyst containing both Brønsted and Lewis centers, yielding up to 97% conversion of α-pinene at 363 K, in contrast to the 72% observed for the parent hydrogen form. Such catalytic behavior was discussed in terms of the conversion of a reactant inside micropores of the zeolite catalyst, on Brønsted acid centers with enhanced strength located probably in the vicinity of Lewis sites. The selectivity toward camphene and limonene changed smoothly with the dealumination level; thus, a higher selectivity toward limonene was observed at the expense of camphene formation with increasing the n_Si/n_Al ratio of the catalysts. The selectivity toward camphene and limonene was close to 85% for all of the materials studied. The initial rates of α-pinene transformations over FER-type materials exceeded those observed for other catalytic systems, heteropoly acid/SiO₂ and H₂SO₄/ZrO₂. This study demonstrates the successful application of a medium-pore zeolite for the catalytic transformation of α-pinene in liquid phase.