FTIR study of the relation, between extra-framework aluminum species and the adsorbed molecular water, and its effect on the acidity in ZSM-5 steamed zeolite

FTIR study of the relation, between extra-framework aluminum species and the adsorbed molecular water, and its effect on the acidity in ZSM-5 steamed zeolite

Luis Fioravanti Isernia

Laboratorio de Tamices Moleculares, Universidad de Oriente – UO, Maturín, Monagas, Venezuela. E-mail: [e-mail]

The infrared spectroscopy study of zeolite samples, obtained by steam treatment at 560‑960 °C of the ZSM‑5 catalyst (framework Si/Al ratio of 13), suggests an association between adsorbed molecular water and extra‑framework aluminum hydroxyls generated after treatment. Moreover, infrared spectroscopy of adsorbed pyridine shows the reduction of the densities of Brönsted and Lewis sites, when treatment temperature rises, with contradicts the frequently accepted mechanism of the transformation of two bridged Si‑OH‑Al groups for each Lewis site generated. The gradual conversion of the octahedral extra-framework aluminum (Lewis‑associated) in polymeric species with low acidity is the most probable cause of this behavior. On the other hand, the apparent decline of the acid Brönsted strength, with the increase in the temperature of the hydrothermal treatment, has two possible causes: a) the decreasing accessibility, of the pyridine molecular probe to bridged Si-OH-Al groups with the strongest Brönsted acidity, inside the channels, and b) the gradual transformation of these groups into extra framework species of weak acidity.

Keywords: hydrothermal dealumination, infrared spectroscopy, ZSM-5 zeolite, Brönsted and Lewis sites.

Introduction

The zeolite‑based catalysts, subjected to high temperature water steam regeneration cycles, undergo a partial exit of tetrahedral coordinated aluminum, out of the crystalline network and the gradual decrease of the catalytic activity by reduction of the number of acid sites.

In the presence of excess steam the disappearance of two bridged (Si‑OH‑Al) sites for each Lewis site generated is frequently reported. Several authors have propused^1-5 that the interaction between two bridged Si-OH-Al sites, associated⁶ with Brönsted acidity, generate an extra-framework Al(OH)²⁺ (or AlO⁺), which has been considered^7-9 as the true carrier of Lewis acidity. It is generally accepted that the extra-framework aluminum (EFAL) generated after hydrothermal treatment is deposited in the form of 4, 5 and 6 coordinate monomeric species^10,11, where 5 and 6 are related with the Lewis acidity in zeolites. On the other hand, the formation of four-coordinate polymeric aluminum species of low acidity on the surface zeolite crystals, has been reported^1,12.

Zeolite MFI is an interesting shape‑selective solid acid catalyst. However, despite their widespread use in a variety of petrochemical processes, information about the catalytic active centers (bridged Si-OH-Al sites) is not completely understood. This pentasil-type zeolite^13-15, is characterized by a low content of aluminum (Si/Al10), a medium pore size (5.1 × 5.7 Å), great deactivation resistance and high thermal stability.

Fourier transform infrared (FTIR) spectroscopy is widely used in the characterization of catalysts due to its capacity to provide information without destroying the matrix. On the other hand, the use of special cells^16-18, allows tracking the changes occurring in the IR bands of basic probes, adsorbed on the catalyst acid sites^19-21, due to changes in the nature and distribution of these sites. For example, interaction of pyridine with the acid sites of the catalyst, generates a series of signals, among which two are prominent: one signal around 1545 cm^–1 (pyridinium ion bonded to a Brönsted site) and another near 1455 cm^–1 (pyridine adsorbed on Lewis acid sites).

The values of the extinction coefficients ÎB and ÎL, based on the absorbance of these bands, have been reported^22,23, so that they may be used in the estimation of the concentrations of these sites.

The partial disappearance of the Si-OH-Al groups can be followed by checking for changes in intensity and position of the O‑H stretching vibrations²⁴ in the region located between 3700 and 3500 cm^–1. The position and the number of these peaks or bands can vary from one zeolite to another. For example, the bands found at 3640 and 3540 cm⁻¹ due to bridging hydroxyl groups (Si–OH–Al) in faujasite, are replaced in mordenite and ZSM-5 by a single band²⁵ between 3600 and 3615 cm^–1.

The dealumination of a zeolite by hydrothermal treatment^26-28 frequently leads to a decrease of its crystallinity, due to the partial collapse of its microporous structure^24,29 through a mechanism which is not yet fully understood. Fourier transform infrared (FTIR) spectroscopy works complementary to X-ray diffraction^7,24,29,30 in search for changes in the crystallinity³¹, by comparing the peak intensities in the region of 1300 to 300 cm^–1. These bands are grouped in two types of vibrations of the tetrahedral TO4 units (where T = Al or Si):

• Vibrations related to the connections between TO4 tetrahedra, which are therefore sensitive to structural modifications;

• Vibrations into the TO4 tetrahedra, which are insensitive to structural modifications.

For ZSM-5 zeolites²⁴, double ring vibration band at 550‑560 cm^–1 belongs to the type (a), whereas the band at 800 cm^–1 of the symmetric stretching mode of internal TO4 tetrahedra belongs to the type (b). Subsequently, the ratios between the absorbances of these bands can be taken as an acceptable criterion of crystallinity.

In this work, Fourier transform infrared (FTIR) spectroscopy is used to study the nature of adsorbed molecular water, the segregation of aluminum outside the crystal framework, its inter‑conversion between forms with different numbers of coordination, and their influence on the acidity of a ZSM-5 zeolite placed under hydrothermal conditions.

Experimental

Hydrothermal treatment

A ZSM-5 zeolite provided by INTEVEP, underwent hydrothermal treatment, according to the conditions of Table 1. The original zeolite without treatment was labeled as NHT.

The hydrothermal treatment consisted in the application of a constant air flow, dampened by the bubbling of boiling water, on a portion of NHT contained in a quartz reactor, within a tubular furnace (Figure 1), warmed up to 560, 760 and 960 °C. After 120 minutes, the heating was suspended and dry air was injected into the reactor with the zeolite. The resulting three new zeolite samples were labeled HT560, HT760 and HT960, corresponding to the temperature of the hydrothermal treatment.

Fourier transform infrared (FTIR) spectroscopy

In order to study the OH groups on framework and extra‑framework aluminum species, as well as their Brönsted and Lewis acidity, thin self-supporting wafers of each zeolite sample were prepared by the application of 207 MPa of pressure on approximately 15 mg of zeolite. The 1.3 cm diameter wafers were placed in a container that was inserted into a high-temperature vacuum cell (Figure 2). In each trial, the cell containing the sample was subjected to a pressure of 1.33 mPa with a heating ramp rate of 10°C/min, until reaching the 500 °C for one hour. Afterwards, the sample was left to cool down to room temperature, to obtain a FTIR spectrum of the wafer surface without adsorbate. Then pyridine was added, with pressure increment of 133 Pa, until reaching the saturation at approximately 667 Pa. The gas in excess was removed by vacuum suction, until returning to the original pressure of 1.33 mPa. Having done this, it was proceeded to obtain FTIR spectra of each wafer, between 100 and 500 °C, with intervals of 100 °C. Spectra were recorded by a Perkin-Elmer FTIR System Spectrum GX instrument. Scan range: 400‑5000 cm^–1.

In order to study the structural order, dilution of the sample was required by preparing wafers of approximately 0.07 mg of catalyst in 100 mg of KBr, under 103 MPa of pressure. Spectra were recorded by a Perkin Elmer FTIR 1600. Scan range: 450‑4400 cm^–1.

X-Ray diffraction

The XRD study was performed using a Siemens D5005 diffractometer with a q/q geometry in reflection mode, equipped with a graphite secondary monochromator, in the 2θ range of 5°-60° and a step size of 0.02° and a time of 1.5 seconds. Cu Ka (l= 1.54056Å) radiation, working at 40Kv and 30 mA.

Temperature‑Programmed Desorption of Ammonia (TPD‑NH3)

In order to obtain the TPD‑NH3 profiles, a Micromeritics TPD/TPR 2900 system equipped with a thermal conductivity detector (TCD) and its own software, was used. Each one of the samples (150 mg) was previously pulverized in an agate mortar and activated in a quartz reactor at 500 °C, under a 50 mL/min flow of dry air at 101 kPa for one hour. Soon the samples were saturated with NH3 (50 mL/min and 101 kPa) at room temperature for 15 minutes. Excess NH3 was eliminated with a 50 mL/min helium flow (207‑276 kPa) for 15 minutes. The TPD experiments were performed with a continuous helium flow (50 mL/min, 207‑276 kPa) and heating rate of 10°C/min until 600 °C. This temperature was maintained for one hour afterwards.

Results and Discussion

Behavior of the framework OH

Figure 3 shows a small variation of the signal around 3746 cm^–1 of O‑H stretching vibrations of silanols before the addition of pyridine, with the increase of the steam temperature. This contrasts with the strong diminution of the signal at 3613‑3617 cm^–1 which is consistent with the reduction of the number of Si-OH-Al groups and the precipitation of aluminum outside the zeolite framework, after the hydrothermal treatment. The parent zeolite without hydrothermal treatment (NHT) also shows the presence of a weak band at 3785 cm^–1 and a medium and broad band at 3700‑3642 cm^–1, that can be respectively assigned to nOH of terminal Al‑OH and Al‑OH still connected by one or two links to the crystal lattice^34‑36. At a first glance, the results don't support the latest presumption because fast disappearance of these bands, with the increase of the temperature of the hydrothermal treatment, does not agree with the increase of the presence of extra-framework aluminum species, resulting from hydrothermal treatment. A possible explanation for this behavior is the increasing condensation of the extra-framework aluminum hydroxyls to polymeric aluminum species. Such behavior was already been discussed in the literature^1,12. Nevertheless, the difference of the spectra of the solids NHT, HT560 and HT760 minus HT960 (Figure 4) provides evidence of a small signal centered at 1622 cm^–1 that agrees well with the deformation vibration of physically adsorbed molecular water^37‑40; whose intensity decreases with the increasing temperature of the hydrothermal treatment. Similarly, the bands at 3785 cm^–1 and 3700‑3642 cm^–1, reasonably agree with the asymmetric and symmetric stretching vibrations of physisorbed water molecules^32,33. This opens the possibility that the increase of the hydrophobic character, that accompanies the partial exit of tetrahedrally coordinated aluminum out of the crystalline network, by effect of the hydrothermal treatment, causes the diminution of the bands at 3785, 3700‑3642 and 1622 cm^–1. The presence of small amounts of water, after high‑temperature treatment, should not be surprising: Milliken and coworkers⁴¹ found a persistent signal in the 2.7‑3.0 mm region that suggests the presence of structural H2O after heating several solids (including a zeolitic one) under vacuum at 700-850 °C. They appointed several possible causes, including water trapped in inclusions and/or its readsorption from the atmosphere. Moreover, Che and Glotch⁴² and Védrine⁴³, have observed the presence of molecular water, even after pretreatment of solid samples to 500 °C. These results support the possibility of an association between molecular water and extra-framework aluminum hydroxyls, as hydrated species whose disappearance results in the diminution of the last two mentioned bands. Similar observations were made in previous studies^10,44.

The gradual disappearance of the signals at 3785 and 3700‑3642 cm^–1 (Figure 3) with the increase of the temperature during hydrothermal treatment, reveals small signals at 3640 cm^–1 and 3683 cm^–1 (zeolite sample HT760) that finally disappears in HT960. Most likely, the peak around 3640 cm^–1 comes from O‑H stretching vibrations of silanols (Si-OH), hydrogen bonded with other silanols or water molecules, inside the zeolite channels^45,46. Theoretical studies made by Jungsuttiwong and coworkers⁴⁰, allowed assigning the signal at 3683 cm^–1 to "free O‑H" stretching vibrations, pertaining to H5O2⁺ ions adsorbed inside the ZSM-5 zeolite channels.

The temperature at which desorption of pyridine occurs, with the consequent reappearance of to the O‑H stretching vibrations²⁴ in the region located between 3700 and 3500 cm^–1, indicates the strength of acid sites. Reappearance of the signal at 3613‑3617 cm^–1 (O‑H stretching vibrations of Si-OH-Al) in NHT could be observed, when pyridine was desorbed at 400 and 500°C (Figure 5). Nevertheless, the signal at 3700‑3642 cm^–1 did not show important changes. On the other hand, the absence of the band at 3785 cm^–1, could be explained by the "irreversible" adsorption of pyridine on terminals extra-framework aluminum hydroxyls, with the consequent disappearance of its nOH vibration. This suggests that corresponding acid sites are stronger than the Si‑OH‑Al groups. That is incompatible with the position of the nOH signal at a higher frequency than the nOH signal of OH bridged groups. However, the removal of residual trapped water adsorbed on extra‑framework aluminum by pyridine can be an alternative explanation for the absence of the band at 3785 cm^–1. Guisnet and coworkers⁴⁷ reported the irreversible disappearance of the same band under similar circumstances.

On the contrary, the band at 3700‑3642 cm^–1 may be the result of the superposition of the signals of symmetric stretching vibrations of physisorbed water molecules and Al‑OH still connected by one or two links to the crystal lattice, where the latter is dominant. In this case, the removal of water adsorbed on extra‑framework aluminum by pyridine, will not cause a noticeable effect on this signal.

In the FTIR spectra of zeolite sample HT560 (Figure 6), the signal at 3613‑3617 cm^–1 reappears after desorption of the pyridine at 400 and 500 °C, while signals at 3785 and 3662 cm^–1 (this last one being the remaining part of the 3700 3640 cm^–1, band) are absent. The growing condensation of the extra-framework aluminum hydroxyls to polymeric aluminum species, with the increase in the temperature of the hydrothermal treatment, diminishes its contribution to the signal at 3662 cm^–1 promoting water as a main contributor of this signal. In this case, the removal of water adsorbed on extra‑framework aluminum by pyridine, would cause the disappearance of signals at 3785 and 3662 cm^–1.

As expected, the presence of the signal at 3613‑3617 cm^–1 in FTIR spectra of HT760 and HT960 (Figure 7) was not observed, because it was not visible before the adsorption of pyridine (Figure 3). On the other hand, like in the other samples, the signal around 3746 cm^–1 of O‑H stretching vibrations of silanols practically remained inalterable. Moreover, the reappearance of the bands at 3785, 3683, 3662 and 3640 cm^–1 in the zeolite sample HT760 was not observed. Once again, the removal of water molecules by pyridine molecules seems to be the cause of the diminution of these signals below the sensitivity of the FTIR spectrometer.

Brönsted and Lewis acidity

FTIR spectroscopic study of pyridine adsorption at Ts = 200 °C (where Ts is the temperature from which there is no overlap between the signals of the physisorbed and chemisorbed pyridine^22,48‑50) shows changes in the intensity of the bands between 1400 and 1570 cm^–1 with the increase of the temperature of the hydrothermal treatment (Figure 8).

The variability, often caused by very subtle differences in the experimental conditions, between the reported values of the extinction coefficients ÎB and ÎL, based on the absorbances of the bands around 1545 cm^–1 (pyridinium ion bonded to a Brönsted site) and 1455 cm^–1 (pyridine adsorbed on Lewis acid sites) led to use of the ÎL/ÎB ratio (which is less sensible to changes in the experimental conditions). The CB/CL concentration ratio between Brönsted and Lewis acid sites was calculated through the equation: CB/CL = (ÎL/ÎB).(AB/AL), where AB/AL is the absorbance ratio between the bands around 1545 and 1455 cm^–1. Under these considerations, the value of ÎL/ÎB = 0.7, proposed by Robb and coworkers⁵¹, was used as the most adequate value for the determination of the CB/CL ratio, in ZSM‑5 catalysts with Si/Al ratio superior to 7.5.

The decrease of the CB/CL ratio (Table 2) with the increasing temperature of the hydrothermal treatment can be explained by the transformation of tetrahedrally‑coordinated framework aluminum in new species without Brönsted acidity. Another explanation for this behavior is the decreasing accessibility of pyridine molecules inside the zeolite channels (where it is presumed that the majority of the Brönsted acid-sites and a minor portion of the Lewis acid-sites are located) due to accumulation of extra‑framework aluminum species.

The diminution of the intensity of the bands between 1400 and 1570 cm^–1 (Figure 8) indicates that both Lewis and Brönsted acid site densities decrease at the same time. This contradicts the frequently accepted mechanism that underlies the increasing number of Lewis acid sites at the expense of the bridged Si-OH-Al sites^1-5, associated with Brönsted acidity, after hydrothermal treatment. This behavior may be attributed to the decreasing accessibility of pyridine molecules to Lewis acid sites inside the zeolite channels. However, the gradual conversion of the extra‑framework aluminum hydroxyls (Lewis‑associated)^{7-9 in polymeric species of low acidity1,12} may be another possible cause. The results of TPD‑NH3 analyses also seem to agree, when these show, at increasing hydrothermal treatment temperature, the decreasing of the profile areas of zones HT and LT, corresponding to higher and lesser than 300°C desorption temperatures from acid sites (Figure 9). Several authors^52‑56 consider that the peak in the LT zone results of NH3 desorption from non‑acid sites. In order to verify or discard the last affirmation, the sample NHT was put under a superficial dealumination with HCl 1N. The elemental analysis (Table 3) shows that HCl treatment leads to an extraction of a 48% of total aluminum and 5% of total silicon. However, the comparison between TPD‑NH3 profiles before and after treatment (Figure 10) shows that the HT area remains almost equal, whereas the LT diminishes significantly. This confirms the association between the LT peak and aluminum species of low acidity.

The temperature at which desorption starts and therefore, the disappearance of the bands of the adsorbed pyridine between 1400 and 1570 cm^–1 begins, is an indicator of the force of the acid sites in the catalyst. In the zeolite samples NHT and HT560, the disappearance of the signal around 1545 cm^–1 could be observed, when pyridine was desorbed at 400 °C (Figure 11, "a" and "b"). Nevertheless, the signal around 1455 cm^–1 did not show important changes, although its intensity diminished slightly, with the increase of the desorption temperature. However, spectra of HT760 (Figure 11c) showed the disappearance of the signal around 1545 cm^–1 when the pyridine was desorbed between 300 and 500 °C. The presence of this band at 100 and 200 °C, contrasts with the apparent absence in this sample, of the band associated to the Si‑OH‑Al groups, at 3613‑3617 cm^–1 (Figure 7). Due to its low intensity, this last band can be below the resolution of the FTIR spectrometer. On the other hand, in the zeolite sample HT960 spectra, the signal around 1545 cm^–1 is absent at all times (Figure 11d).

The non-disappearance of the band at 1455 cm^–1, in the rank of studied temperatures of desorption, suggests that the Lewis acid sites are stronger than the pyridine accessible Brönsted sites.

At first glance, the results don't support the frequently accepted opinion that the Brönsted acid strength increases with the diminution of the acid‑density^57‑59. Instead, the band at 1545 cm^–1 disappears, in the solids NHT and HT560, at higher desorption temperatures than the solid HT760 (Figure 11), suggesting that Brönsted acid-sites with superior strength occur in samples with the highest densities of acid-sites. This behavior might be caused by the decreasing accessibility of pyridine molecules, due to accumulation of extra-framework aluminum species after the hydrothermal treatment, into the zeolite channels (where is presumed the presence of the strongest Brönsted acid sites) and/or the gradual transformation of tetrahedrally coordinated aluminum, associated with the strongest Brönsted acid sites, in extra-framework species of low acidity.

Study of structural order

Fourier transform infrared (FTIR) spectroscopy, of KBr pellets of zeolite samples NHT, HT560, HT760 and HT960, leads to very similar spectra. However, there are small differences in position and relative intensities of its bands (Figure 12).

The ratio between integrated absorbances of bands around 550 and 800 cm^–1 (double five‑member‑ring and internal tetrahedra TO4 symmetric stretching vibrations, respectively) was used as a measure of crystallinity. The values of the ratio represented in Table 4, show a "volcano" behavior, characterized for an initial increase and subsequent decrease with increasing temperatures of hydrothermal treatment. Moreover, at the same table, the cristallinity, estimated from the peak height at 2θ = 24.3° of X‑ray difractograms³¹ (Figure 13), behave similarly. That suggests a previous step of structural realignment of zeolite sample at treatment temperatures less than 760 °C.

Conclusions

The results suggest an association between adsorbed molecular water and extra-framework aluminum hydroxyls generated after the hydrothermal treatment: The weak band at 3785 cm^–1 can be assigned to asymmetric stretching vibrations of residual trapped water adsorbed on extra‑framework aluminum. Likewise, a medium and broad band in 3700‑3642 cm^–1 can, probably, have its origin in the superposition of two nOH signals: from water (symmetric stretching vibrations) and non‑terminals extra-framework aluminum hydroxyls (where the latter is dominant). In addition, the diminution in the intensity of these signals is consistent with an increase of the hydrophobicity and the gradual condensation of the extra-framework aluminum hydroxyls in polymeric species of weak acidity resulting from the hydrothermal treatment.

The gradual condensation of the extra-framework aluminum species to polymeric aluminum species of weak acidity, with the increasing temperature of the hydrothermal treatment, is the most probable cause of the diminution of the density of Lewis acid sites. On the other hand, decreasing accessibility of pyridine molecules to the strongest Brönsted acid sites into the zeolite channels due to the accumulation of extra-framework aluminum and/or the gradual transformation of these acid sites, into extra-framework species of low acid strength, may be the possible causes of the observed decrease in Brönsted acid site strength after the hydrothermal treatment.

Crystallinity shows a "volcano" behavior, characterized by an initial increase and subsequent decrease with increasing temperatures of hydrothermal treatment that suggests a previous step of structural realignment of zeolite sample at treatment temperatures less than 760 °C.

Acknowledgments

The author thanks the National Research Petroleum Institute of Venezuela (INTEVEP) and the Kinetic and Catalysis Laboratory of "Los Andes" University of Venezuela, for its invaluable support in providing source materials and characterization techniques for this work.

Received: September 21, 2011;

Revised: December 30, 2012

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Publication Dates

• Publication in this collection
  26 Mar 2013
• Date of issue
  Aug 2013

History

• Received
  21 Sept 2011
• Reviewed
  30 Dec 2012