# Dealuminated Beta Zeolite Modified by Alkaline Earth Metals

Authors: Karolina Chalupka; Renata Sadek; Laetitia Valentin; Yannick Millot; Christophe Calers; Magdalena Nowosielska; Jacek Rynkowski; Stanislaw Dzwigaj
Journal: Journal of Chemistry
 (2018)

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## Abstract

Alkaline Earth metals (Mg, Sr, and Ca) were incorporated into the dealuminated mesoporous beta zeolite (DeAlBeta) by the two-step postsynthesis method. Physicochemical properties of both unmodified and alkaline Earth metal-modified DeAlBeta zeolite were characterized by XRD, DR UV-vis, FTIR, TPD of NH3 and CO2, NMR, and XPS. The dealumination of beta zeolite led to decrease of its acidity and basicity. The incorporation of alkaline Earth metals into the framework of dealuminated beta zeolite did not affect its structure. The modification of DeAlBeta with a small amount of alkaline Earth metals increases the number of acidic centers, which may be related to the formation of framework Mg (Ca or Sr) (II) Lewis acidic sites.


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## Body

## 1. Introduction

The world of science puts special emphasis on the development of new stable materials with the interesting acid-base properties. Due to the unique features (e.g., high specific surface area and strictly defined size of pores) of zeolite, they are among the most promising materials. Moreover, the introduction of various metals into their structures may beneficially change their properties [1–5].Due to the presence of Lewis and Brønsted acidic sites in these materials, they have been used as catalysts in many reactions [6, 7]. Moreover, an introduction of an alkali [8–10] and/or alkaline Earth metal [11–13] into the zeolite structure can lead to modification of their basic properties.The incorporation of alkali metal cations increases the basicity of oxygen atoms, which are present in the framework of the zeolite [14]. The most common methods used for the preparation of such zeolite systems are ion exchange [15] and impregnation [13].Obtained in this way alkali metal-loaded zeolite catalysts have been applied in several reactions such as methylation of toluene [13] and phenol oxidation [16]. As it has been shown, the acid-base properties of alkaline Earth metals-containing zeolites significantly affect the yields of these processes. This is why focusing more attention on a better control of acid-base properties of zeolite catalysts is required. It may allow obtaining more efficient catalysts for different processes, e.g., conversion of ethanol to propylene [17], syngas to dimethyl ether [12], or methanol to olefin [18].In this work, we have used a two-step postsynthesis method, developed earlier by Dzwigaj et al. [19–23] to obtain Mg-, Sr-, and Ca-containing DeAlBeta zeolite with isolated, mononuclear Sr(II), Ca(II), and Mg(II) species. As obtained Mg-, Sr-, and Ca-containing DeAlBeta zeolite catalysts were characterized by XRD, BET, FTIR, XRD, DR UV-vis, NH3-TPD, CO2-TPD, NMR, and XPS.


## 2. Materials and Methods

Tetraethylammonium beta (TEABeta) zeolite provided by RIPP (China) was divided into two fractions. The first one was calcined (air, 15 h, 550°C) in order to obtain an organic-free HAlBeta zeolite (Si/Al = 18). The second fraction of TEABeta was treated in a 13 mol·L−1 HNO3 aqueous solution (4 h, 80°C) to obtain a partially dealuminated DeAlBeta support (Si/Al = 64) with vacant T-atom sites (where T = Al). DeAlBeta was then separated by centrifugation, washed with distilled water, and dried overnight at 80°C. To incorporate Sr2+, Ca2+, or Mg2+ ions in vacant T-atom sites, 2 g of DeAlBeta was stirred under aerobic conditions for 24 h at 25°C in 200 mL aqueous solution of Sr(NO3)2·4 H2O (pH = 2.0), Mg(NO3)2·6 H2O (pH = 2.1), or Ca(NO3)2·4 H2O (pH = 2.0). Then, the suspensions were stirred for 2 h at 80°C until water was evaporated, and the resulting solids were dried in air at 80°C for 24 h. Then, the solids were calcined in air at 500°C for 3 h and labelled Mg2.0DeAlBeta (2.0 wt.% Mg), Sr2.0DeAlBeta (2.1 wt.% Sr), and Ca2.0DeAlBeta (2.2 wt.% Ca)—in parentheses the real content of metals determined by XRF.The elemental composition of tested samples was performed by the X-ray fluorescence (XRF) (SPECTRO XEPOS, AMETEK Materials Analysis Division) at room temperature.Textural characterization of the samples was determined by nitrogen adsorption at −195°C on Micrometrics ASAP 2020 analyzer. Prior to the analysis, ca. 0.08 g of the sample was degassed for 2 h at 350°C. The total surface area was determined using the Brunauer–Emmeet–Teller (BET) method. The micropore volumes and the micropore surface area were obtained by the t-plot method. The Barrett–Joyner–Halenda (BJH) model applied to the adsorption branch of the isotherm was used to obtain the mesopore size distribution.The powder X-ray diffraction patterns (XRD) experiments were performed on a PANalytical Empyrean diffractometer equipped with the Cu Kα radiation (λ = 154.05 pm) in the 2θ range of 5°–90°.The Fourier-transform infrared (FT-IR) spectra of the studied zeolites samples were recorded on a Nicolet iS10 Thermo Scientific equipped with the deuterated triglycine sulfate (DTGS) detector with a resolution of 4 cm−1, at room temperature and ambient atmosphere in the range of 4000–400 cm−1. Before measurements, samples were mixed with potassium bromide at mass ratio 1 : 100 (zeolite : KBr) and then pressed at 1.5–2 tons cm−2 for 2 minutes.In a case of pyridine adsorption, the FT-IR spectrometer Bruker Vector 22 equipped with the DTGS detector with a resolution of 2.0 cm−1 and number of scans 128 was used. Samples were pressed at ∼0.2 tons cm−2 into thin wafers of ca. 10 mg·cm−2 and placed inside the IR cell. After calcination at 450°C for 3 h in O2 (100 Torr) followed by outgassing at 300°C (10−3 Pa) for 1 h, the wafers were contacted at room temperature with gaseous pyridine (1 Torr) via a separate cell containing liquid pyridine. After saturation with pyridine, the samples were outgassed at 150°C (10−3 Pa). FT-IR spectra were recorded at room temperature in the range of 4000–400 cm−1.The temperature-programmed desorption of ammonia (NH3-TPD) measurements were carried out in a quartz reactor using gaseous ammonium. NH3 was adsorbed on zeolite catalysts (c.a. 0.100 g) at 100°C for 10 minutes in flowing He. Before NH3 adsorption, catalysts were dried at 500°C in He flow for 30 minutes (total gas flow: 40 cm3·min−1). The temperature-programmed desorption of NH3 was carried out in the temperature range 25–500°C, after removing physisorbed ammonium from the catalyst with helium flow at 100oC for 10 minutes.The temperature-programmed desorption of CO2 (CO2-TPD) measurements were carried out in a quartz reactor using gaseous carbon dioxide. CO2 was adsorbed on zeolite catalysts (c.a. 0.100 g) at 40°C for 30 minutes in flowing He. Before CO2 adsorption, catalysts were dried at 500°C in He flow for 30 minutes (total gas flow: 40 cm3·min−1). The temperature-programmed desorption of CO2 was carried out in the temperature range 25–500°C, after removing physisorbed ammonia from the catalyst.The diffuse reflectance UV-Vis (DR UV-Vis) spectra were recorded under ambient atmosphere on a Cary 5000 Varian spectrometer equipped with a double integrator with polytetrafluoroethylene as a reference.The X-ray photoelectron spectroscopy (XPS) analyses were performed using an Omicron Argus X-ray photoelectron spectrometer with a monochromated AlKα radiation source (hν = 1486.6 eV) having a 280 W electron beam power. The emission of photoelectrons from the sample was analyzed under ultrahigh vacuum conditions (≤10–10 Torr). Spectra were carried out with 100 eV pass energy for the survey scan and 20 eV pass energy for the other regions. Binding energies were calibrated against the C1s binding energy at 284.7 eV. The spectra were fitted using Casa XPS v.2.3.16 software (Casa Software Ldt, UK) and applying a Gaussian/Lorentzian ratio G/L equal to 70/30.27Al MAS NMR spectra were recorded at 130.33 MHz with 1.3 Ps excitation pulse (S/6), 0.5 s for the recycle delay, and 1024 accumulations. The spectra were acquired at a spinning rate of 12 kHz in a 4 mm zirconia rotor.29Si MAS NMR spectra were recorded at 99.36 MHz with a Bruker Avance 500 spectrometer and 7 mm (external diameter) zirconia rotors, with CP (1H-29Si CP-MAS NMR) and without (29Si MAS NMR). Chemical shifts of silicon were measured by reference to tetramethylsilane (TMS). 29Si MAS NMR spectra were obtained with a 7 mm zirconia rotor at 6 kHz spinning speed, 4 μs excitation pulse duration, and 10 s recycle delay. Polydimethylsilane (PDMS) was used for setting the Hartmann–Hahn condition. The proton π/2 pulse duration, the contact time, and recycle delay were 3.4 μs, 5 ms, and 5 s, respectively.1H MAS NMR spectra were recorded with a Bruker Avance 500 spectrometer and 4 mm zirconia rotors with a 90° pulse duration of 3 μs and a recycle delay of 5 s. The MAS equipment for rotation (12 kHz) was carefully cleaned with ethanol to avoid spurious proton signals. The probe signal was subtracted from the total free induction decay.


## 3. Results and Discussion

### 3.1. Textural and Structure Characterization (BET, XRD, and FTIR)

N2 adsorption-desorption isotherms employed to estimate the textural properties of all samples under study are shown in Figure 1, and the data are listed in Table 1. Exemplary, Figure 2 presents the detailed plots, used for calculations, for the DeAlBeta sample. Neither dealumination nor the introducing of alkaline Earth metal changes the textural parameters of the samples significantly. All of them show the typical type I isotherm with a substantial H4 hysteresis loop from P/P0 = 0.45 with a characteristic step-down in the desorption branch associated with the hysteresis loop closure [24]. Such course of isotherms indicates a micromeso hierarchical porous textural feature. The relatively high micropore volume (ca. 0.22 cm3g−1 indicates the crystalline character of the zeolite framework [25]. The BJH average pore diameter (4.40–4.70 nm) indicates the creation of intracrystalline mesopores. The differences of parameters given in Table 1 between individual samples are not meaningful and do not exceed 7%. However, the dealumination of the zeolite leads to the slight decrease in the total specific surface area and increase in total pore volume. The subsequent introducing of alkaline Earth metal results in an opposite effect, the most pronounced for Mg2.0DeAlBeta, which may be connected with the deep incorporation of the small Mg2+ ions into the zeolite structure.Figure 1
N2 adsorption-desorption isotherms of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.Table 1
Surface characteristic of samples determined by the N2 adsorption-desorption method.
Sample
Total specific surface area, TSSA (m2·g−1)
Micropore area (m2·g−1)
External surface area (m2·g−1)
Total pore volume (cm3·g−1)
Micropore volume (cm3·g−1)
Average pore diameter (nm)
HAlBeta
474
414
60.0
0.317
0.216
4.70
DeAlBeta
498
435
63.0
0.331
0.223
4.68
Mg2.0DeAlBeta
468
407
61.0
0.312
0.212
4.62
Sr2.0DeAlBeta
498
436
62.0
0.328
0.227
4.48
Ca2.0DeAlBeta
472
411
61.0
0.308
0.214
4.40Figure 2
The plots for the DeAlBeta sample: (a) N2 adsorption-desorption isotherm; (b) BJH pore size distribution; (c) t-plot.
(a)
(b)
(c)Figure3 presents the XRD patterns of HAlBeta, DeAlBeta, and Mg-, Sr-, and Ca-containing DeAlBeta samples, in which two main diffraction peaks at 2θ around 7.7°[101] and 22.4°[302] are observed, characteristic for beta materials, in agreement with earlier reports [26–29]. Thus, one can conclude that the presence of the alkaline Earth metal does not affect the structure of DeAlBeta zeolite. A position of the main diffraction peak of the dealuminated DeAlBeta sample (22.30o) is slightly shifted comparing to that of HAlBeta one (22.39°). For the samples modified with alkaline Earth metals, it shows the intermediate values, i.e., 22.34°, 22.35°, and 22.37°, for Sr2.0DeAlBeta, Mg2.0DeAlBeta, and Ca2.0DeAlBeta, respectively.Figure 3
XRD patterns of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.Although the observed changes are very tiny, they may indicate the slight matrix contraction under dealumination of HAlBeta and the expansion under introduction of metals ions into the DeAlBeta structure. The peaks which could be ascribed to alkaline Earth metal species for all tested samples are not observed, what is probably related with good dispersion of the alkaline Earth metals in the zeolite as a result of their reaction with silanol groups of vacant T-atom sites and/or ion exchange with a proton of HAlBeta. The lack of alkaline Earth metal diffraction peaks may also suggest that Mg, Ca, or Sr species have entered the zeolite structure.The FT-IR spectra in the range of T-O (T = Si or Al) of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta are presented in Figure 4(a). The bands attributed to asymmetric and symmetric external O-T-O stretching vibration (at 790–793 and 1227–1231 cm−1), asymmetric internal O-T-O stretching vibration (at 1077–1084 cm−1), and structural vibration (at 521–526, 571–575, and 621–623 cm−1) are very similar for all samples under study. It confirms that the incorporation of alkaline Earth metal ions into the DeAlBeta zeolite does not affect its structure [3, 30, 31].Figure 4
FTIR spectra of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples in the (a) framework vibration region and (b) vibrational range of OH groups.
(a)
(b)The bands at 947 cm−1 on HAlBeta and 955 cm−1 on DeAlBeta spectra may be attributed to the stretching vibrations of Si-O belonging to uncoupled SiO4 tetrahedra, in line with previous works on silica and various siliceous zeolites [21, 32–34]. The band around 950 cm−1 disappears for Mg2.0DeAlBeta and is less intensive for Ca2.0DeAlBeta and Sr2.0DeAlBeta. This phenomenon suggests the reaction of alkaline Earth metals with silanol groups.The difference in the intensity of the discussed band may be related to the differentiated ionic radius of magnesium (86 pm), calcium (114 pm), and strontium (132 pm). The smaller the ionic radius of the alkaline Earth metals, the stronger its incorporation into the vacant T-atom sites, and therefore, the higher the consumption of silanol groups. The FT-IR spectra in the range of O-H stretching vibrations are shown in Figure4(b). If we compare the spectra of HAlBeta with DeAlBeta, one can observe a disappearance of the bands at 3684 and 3743 cm−1 related to the presence of isolated Al-OH and external Si-OH groups in HAlBeta and an appearance of the bands at 3737, 3714, and 3519 cm−1 attributed to the isolated internal, terminal, and hydrogen-bonded SiOH groups, respectively [27, 35, 36]. It suggests that, upon treatment of HAlBEA zeolite with the 13 mol·L−1 HNO3 aqueous solution, aluminum is removed and vacant T-atom sites associated with silanol groups are formed, in line with our previous work on microporous beta zeolite.The introduction of alkaline Earth metals into the DeAlBeta zeolite decreases the intensity of the bands at 3743, 3714, and 3519 cm−1, suggesting that the alkaline Earth metals ions react with silanol groups of vacant T-atom sites. This effect is the most pronounced for the Mg2.0DeAlBeta sample.


### 3.2. Acid and Basic Sites Characterization (Py-IR, NH3-TPD, and CO2-TPD)

To get the information about the Brønsted and Lewis acid sites, FTIR of pyridine adsorption was carried out. The spectra are presented in Figure5. For HAlBeta, the bands at 1548 and 1637 cm−1 correspond to pyridinium ions chemisorbed on Brønsted acidic sites, those at 1455, and 1622 cm−1 to pyridine interacting with strong Lewis acidic centers and that at 1492 cm−1 to pyridine interacting with both Brønsted and Lewis acid sites. The intensity of the appropriate bands on the spectrum of DeAlBeta is significantly lower, and the band at 1455 cm−1 completely disappears. Thus, the dealumination of beta zeolite leads to the decrease of both Lewis and Brønsted acidic sites in beta zeolite [27, 37].Figure 5
FTIR spectra of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples recorded at room temperature after adsorption at room temperature and desorption of pyridine at 300°C.The incorporation of alkaline Earth metals into DeAlBeta zeolite results in the appearance of the band at 1445–1448 cm−1. This band may be attributed to Lewis sites, the population of which increases with decreasing alkaline Earth cation radius [38]. Indeed, the highest intensity of a band at 1448 cm−1 for Mg2.0DeAlBeta is connected with the enhanced incorporation of magnesium ions into the framework of DeAlBeta as well-dispersed Mg(II) Lewis sites. Moreover, for alkaline Earth metal-modified zeolite, the bands around of 1596–1609 cm−1 are observed, which can be ascribed to pyridine interacting with another type of weaker Lewis acidic centers [27, 37].NH3-TPD was carried out to monitor the acid strength and the amounts of acid sites on the samples under study. The results are presented in Figure 6(a). The profile of the initial HAlBeta sample shows the typical course with two unresolved peaks with maxima at around 250 and 414–429°C corresponding to ammonia desorbed from the weak and strong acid sites, respectively [39]. Roughly, the low temperature peak may be described to the Lewis, whereas the high temperature one to the Brønsted acidic sites [37, 40]. For the remaining samples (DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta), the intensity of both peaks diminishes, and the maximum of the low temperature peak is shifted about 30°C towards lower temperatures what suggests that these zeolites possesses lower strength of Lewis acidic sites. Moreover, for alkaline Earth metal-modified zeolites, one can observe lower intensity of the high temperature peak, especially expressed for Mg2.0DeAlBeta. This observation confirms again an incorporation of Mg2+ ions into the zeolite framework connected with the decrease of the amount of strong Brønsted sites.Figure 6
Temperature-programmed desorption profiles of ammonia (a) and carbon dioxide (b) for HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.
(a)
(b)The quantitative analysis of NH3-TPD measurements is shown in Table 2. One can note that dealumination of beta zeolite leads to decreasing of its acidity as a result of removal of the Al from zeolite, in agreement with the earlier report [40]. However, the introduction of a small amount of alkaline Earth metals (c.a. 2 wt.%) into DeAlBeta increases the number of acidic centers. It may be related to an incorporation of these metals into the framework of this material and/or ion exchange with the proton of DeAlBeta zeolite with formation of Lewis acidic sites.Table 2
Surface quantitative analysis of TPD-NH3 and TPD-CO2 measurements on HAlBeta, DeAlBeta, Sr2.0DeAlBeta, Ca2.0DeAlBeta, and Mg2.0DeAlBeta.
Sample
Amount of NH3 adsorbed (μmol·g−1)
Amount of CO2 adsorbed (μmol·g−1)
HAlBeta
1830
2391
DeAlBeta
1181
1761
Mg2.0DeAlBeta
1408
1457
Sr2.0DeAlBeta
1335
1564
Ca2.0DeAlBeta
1345
1550Results of CO2-TPD for all tested samples are presented in Figure 6(b) and Table 2. The profile of HAlBeta depicts one peak with maximum at 250°C, which can be assigned to medium basic sites. The dealumination of beta zeolite leads to the decrease in its basicity what reflects both the shift of the desorption peak’s maximum to about 215°C and the significant decrease in the amount of adsorbed CO2. The introduction of alkaline Earth metals into the zeolite changes its basicity ambiguously. For Sr2.0DeAlBeta and Ca2.0DeAlBeta, the course of CO2-TPD is similar to that of DeAlBeta, although the maxima of desorption peaks are slightly shifted towards higher temperature and the amount of adsorbed CO2 decreases. The most pronounced shift of the maximum to the temperature 300°C is observed for Mg2.0DeAlBeta. Results seem to confirm that the basicity of alkaline Earth metal-modified zeolites depends on both structural type and chemical composition [11, 14].


### 3.3. Spectroscopic Characterization (DR UV-Vis, XPS,27Al,1H, and 29Si MAS NMR)

The DR UV-vis spectra of DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta are shown in Figure 7. For Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta, the broad band are seen at 267, 279, and 263 nm, respectively, related to isolated mononuclear Mg(II), Sr(II), and Ca(II) species well dispersed in the beta structure. The band at around of 267–279 nm could be attributed to pseudotetrahedral Mg(II), Sr(II), and Ca(II) species formed as a results of reaction of magnesium, strontium, and calcium ions with silanol groups of vacant T-atom sites of mesoporous DeAlBeta zeolite, in line with earlier reports on Sr-SAPO-34 [41] and MgAPSO-34 [42]. In the case of the Mg2.0DeAlBeta sample, the very small and broad band at about 300 nm is seen. It is probably related to present of tetrahedral Mg species with 4- and/or 3-coordination in the zeolite framework [4].Figure 7
DR UV-vis spectra recorded at room temperature and ambient atmosphere for DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.To confirm the oxidation state of the prepared samples, XPS analysis was performed, and the results of which are shown in Figure8. The O 1s spectra (not shown here) of all studied samples depict one single peak at binding energy c.a. 533.0 eV that can be attributed to the Si-O-Si bond [43]. The Mg 2s spectrum of Mg2.0DeAlBeta (Figure 8(a)) presents one broad peak at binding energy 90.1 eV, which can be attributed to Mg2+. A similar peak for MgZSM-5 was reported by Cai et al. [44]. The Ca 2p spectrum of Ca2.0DeAlBeta (Figure 8(b)) shows a doublet structure and two peaks with maxima at binding energy 351.7 eV and 348.2 eV corresponding to Ca 2p1/2 and Ca 2p3/2, respectively, and indicates the presence of Ca2+ species in a different environment [45]. The Sr 3d spectrum of Sr2.0DeAlBeta is presented in Figure 8(c). The location of the Sr 3d5/2 spectrum at 136.3 eV and that of Sr 3d3/2 at 134.5 eV proves the existence of Sr2+ [46].Figure 8
X-ray photoelectron spectra recorded at room temperature for (a) Mg 2s of Mg2.0DeAlBeta, (b) Ca 2p of Ca2.0DeAlBeta, and (c) Sr 3d of Sr2.0DeAlBeta.
(a)
(b)
(c)27Al MAS NMR spectroscopy is an efficient probe to determine the coordination and local structure of specific aluminum species in zeolite materials [47–49]. Both tetrahedral and octahedral Al species can be easily distinguished basing on their different chemical shifts. A position of Al resonance gives a good indication of the local environment of the aluminum site. For example, framework aluminum atoms in tetrahedral coordination (AlTd) exhibit a signal at 50–60 ppm, while extraframework aluminum atoms in octahedral coordination (AlOh) usually give a signal at ∼0 ppm.Figure9 shows the 27Al MAS NMR spectra of HAlBeta (Si/Al = 18) and DeAlBeta (Si/Al = 64) samples. The spectra of both samples are similar. Two inseparable signals at about 54.5 and 56.9 ppm suggest the presence of two kinds of AlTd sites in both HAlBeta and DeAlBeta samples [27]. The decrease of the absolute signal intensity for DeAlBeta comparing to HAlBeta indicates the higher amount of aluminum species in the HAlBeta sample and confirms removal of Al from HAlBeta upon treatment with acid nitric [27].Figure 9
27Al MAS NMR spectra recorded at room temperature in a 4 mm (the external diameter) zirconia rotor of as-prepared HAlBeta and DeAlBeta.On the1H MAS NMR spectrum of HAlBeta (Figure 10(a)), one main broad peak is observed at 4.85 ppm due to protons of H-bonded silanol groups present at vacant T-atom sites of HAlBeta zeolite, in line with earlier data on zeolites [50–53]. For DeAlBeta, three main peaks are observed at 1.14–1.25, 3.64–3.74, and 5.45 ppm due to protons of isolated and/or terminal SiO-H and H-bonded silanol groups present at vacant T-atom sites, respectively. The peak at 3.6–3.7 ppm is probably due to protons of H-bonded SiOH groups located in a second kind of crystallographic sites. This shift is comparable with that observed at 3.20 ppm for H-bonded SiO-H groups in silicalite and silica [54, 55]. The disappearance of the peak at 5.45 and at 3.6–3.7 ppm upon incorporation of Mg, Sr, and Ca ions in DeAlBeta evidences the reaction of Mg, Sr, and Ca nitrate precursor with both H-bonded SiO-H groups. The 1H MAS NMR spectra of Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta exhibit the main peak at around 1.23–1.27 ppm due to the protons of isolated SiO-H groups with smaller intensity than that present in DeAlBeta zeolite. It confirms that Mg, Sr, and Ca ions introduced in DeAlBeta react not only with hydrogen bonded SiO-H but also with isolated SiO-H groups.Figure 10
1H MAS NMR (a), 29Si MAS NMR (b), and 1H-29Si CP MAS NMR (c) spectra recorded at room temperature in 4 mm (the external diameter) zirconia rotor of as-prepared HAlBeta and partially dealuminated DeAlBeta.
(a)
(b)
(c)The29Si MAS NMR spectrum of DeAlBeta (Figure 10(b)) shows three resonances at −103.7, 111.2, and −114.8 ppm. The peaks at −111.2 and −114.8 ppm are due to framework Si atoms in the Si (OSi)4 environment, located at different crystallographic sites, in line with the earlier report [56]. The peak at −103.7 ppm is assigned to Si atoms in the Si(OH) (OSi)3 environment as revealed by a strong increase of intensity of the peak at −103.7 ppm when 1H-29Si CP-MAS NMR is applied (Figure 10(c)), the technique which preferentially enhances the signal of the 29Si nuclei close to protons as it is in the case of the Si(OH) (OSi)3 site.After incorporation of Mg, Sr, and Ca ions into the DeAlBeta, the relative intensity of the peak at around −102.1 ppm (compared to the intensity of the peak at around −110 ppm) in1H-29Si CP-MAS NMR spectra is significantly reduced for Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta (Figure 10(c)), confirming the reaction between Mg, Sr, and Ca nitrate precursor and silanol groups of vacant T-atom sites. However, the relatively small decrease of the intensity of the peak at −102.1 ppm suggests that only part of Mg, Sr, and Ca ions is incorporated in the framework position of DeAlBeta zeolite.The change of the Si(OSi)4 environment takes place upon incorporation of Mg, Sr, and Ca ions into the framework of DeAlBeta zeolite as deduced from the appearance of better distinguished 29Si MAS NMR peaks at −110.5 and −114.2 ppm for Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta, respectively (Figure 10(c)). For DeAlBeta, only the shoulder is observed in the 1H - 29Si MAS NMR spectrum at around −114.8 (Figure 10(b)). It suggests that the presence of Mg, Sr, and Ca in the framework of Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta involves some modification of the Si environment.


## 3.1. Textural and Structure Characterization (BET, XRD, and FTIR)

N2 adsorption-desorption isotherms employed to estimate the textural properties of all samples under study are shown in Figure 1, and the data are listed in Table 1. Exemplary, Figure 2 presents the detailed plots, used for calculations, for the DeAlBeta sample. Neither dealumination nor the introducing of alkaline Earth metal changes the textural parameters of the samples significantly. All of them show the typical type I isotherm with a substantial H4 hysteresis loop from P/P0 = 0.45 with a characteristic step-down in the desorption branch associated with the hysteresis loop closure [24]. Such course of isotherms indicates a micromeso hierarchical porous textural feature. The relatively high micropore volume (ca. 0.22 cm3g−1 indicates the crystalline character of the zeolite framework [25]. The BJH average pore diameter (4.40–4.70 nm) indicates the creation of intracrystalline mesopores. The differences of parameters given in Table 1 between individual samples are not meaningful and do not exceed 7%. However, the dealumination of the zeolite leads to the slight decrease in the total specific surface area and increase in total pore volume. The subsequent introducing of alkaline Earth metal results in an opposite effect, the most pronounced for Mg2.0DeAlBeta, which may be connected with the deep incorporation of the small Mg2+ ions into the zeolite structure.Figure 1
N2 adsorption-desorption isotherms of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.Table 1
Surface characteristic of samples determined by the N2 adsorption-desorption method.
Sample
Total specific surface area, TSSA (m2·g−1)
Micropore area (m2·g−1)
External surface area (m2·g−1)
Total pore volume (cm3·g−1)
Micropore volume (cm3·g−1)
Average pore diameter (nm)
HAlBeta
474
414
60.0
0.317
0.216
4.70
DeAlBeta
498
435
63.0
0.331
0.223
4.68
Mg2.0DeAlBeta
468
407
61.0
0.312
0.212
4.62
Sr2.0DeAlBeta
498
436
62.0
0.328
0.227
4.48
Ca2.0DeAlBeta
472
411
61.0
0.308
0.214
4.40Figure 2
The plots for the DeAlBeta sample: (a) N2 adsorption-desorption isotherm; (b) BJH pore size distribution; (c) t-plot.
(a)
(b)
(c)Figure3 presents the XRD patterns of HAlBeta, DeAlBeta, and Mg-, Sr-, and Ca-containing DeAlBeta samples, in which two main diffraction peaks at 2θ around 7.7°[101] and 22.4°[302] are observed, characteristic for beta materials, in agreement with earlier reports [26–29]. Thus, one can conclude that the presence of the alkaline Earth metal does not affect the structure of DeAlBeta zeolite. A position of the main diffraction peak of the dealuminated DeAlBeta sample (22.30o) is slightly shifted comparing to that of HAlBeta one (22.39°). For the samples modified with alkaline Earth metals, it shows the intermediate values, i.e., 22.34°, 22.35°, and 22.37°, for Sr2.0DeAlBeta, Mg2.0DeAlBeta, and Ca2.0DeAlBeta, respectively.Figure 3
XRD patterns of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.Although the observed changes are very tiny, they may indicate the slight matrix contraction under dealumination of HAlBeta and the expansion under introduction of metals ions into the DeAlBeta structure. The peaks which could be ascribed to alkaline Earth metal species for all tested samples are not observed, what is probably related with good dispersion of the alkaline Earth metals in the zeolite as a result of their reaction with silanol groups of vacant T-atom sites and/or ion exchange with a proton of HAlBeta. The lack of alkaline Earth metal diffraction peaks may also suggest that Mg, Ca, or Sr species have entered the zeolite structure.The FT-IR spectra in the range of T-O (T = Si or Al) of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta are presented in Figure 4(a). The bands attributed to asymmetric and symmetric external O-T-O stretching vibration (at 790–793 and 1227–1231 cm−1), asymmetric internal O-T-O stretching vibration (at 1077–1084 cm−1), and structural vibration (at 521–526, 571–575, and 621–623 cm−1) are very similar for all samples under study. It confirms that the incorporation of alkaline Earth metal ions into the DeAlBeta zeolite does not affect its structure [3, 30, 31].Figure 4
FTIR spectra of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples in the (a) framework vibration region and (b) vibrational range of OH groups.
(a)
(b)The bands at 947 cm−1 on HAlBeta and 955 cm−1 on DeAlBeta spectra may be attributed to the stretching vibrations of Si-O belonging to uncoupled SiO4 tetrahedra, in line with previous works on silica and various siliceous zeolites [21, 32–34]. The band around 950 cm−1 disappears for Mg2.0DeAlBeta and is less intensive for Ca2.0DeAlBeta and Sr2.0DeAlBeta. This phenomenon suggests the reaction of alkaline Earth metals with silanol groups.The difference in the intensity of the discussed band may be related to the differentiated ionic radius of magnesium (86 pm), calcium (114 pm), and strontium (132 pm). The smaller the ionic radius of the alkaline Earth metals, the stronger its incorporation into the vacant T-atom sites, and therefore, the higher the consumption of silanol groups. The FT-IR spectra in the range of O-H stretching vibrations are shown in Figure4(b). If we compare the spectra of HAlBeta with DeAlBeta, one can observe a disappearance of the bands at 3684 and 3743 cm−1 related to the presence of isolated Al-OH and external Si-OH groups in HAlBeta and an appearance of the bands at 3737, 3714, and 3519 cm−1 attributed to the isolated internal, terminal, and hydrogen-bonded SiOH groups, respectively [27, 35, 36]. It suggests that, upon treatment of HAlBEA zeolite with the 13 mol·L−1 HNO3 aqueous solution, aluminum is removed and vacant T-atom sites associated with silanol groups are formed, in line with our previous work on microporous beta zeolite.The introduction of alkaline Earth metals into the DeAlBeta zeolite decreases the intensity of the bands at 3743, 3714, and 3519 cm−1, suggesting that the alkaline Earth metals ions react with silanol groups of vacant T-atom sites. This effect is the most pronounced for the Mg2.0DeAlBeta sample.


## 3.2. Acid and Basic Sites Characterization (Py-IR, NH3-TPD, and CO2-TPD)

To get the information about the Brønsted and Lewis acid sites, FTIR of pyridine adsorption was carried out. The spectra are presented in Figure5. For HAlBeta, the bands at 1548 and 1637 cm−1 correspond to pyridinium ions chemisorbed on Brønsted acidic sites, those at 1455, and 1622 cm−1 to pyridine interacting with strong Lewis acidic centers and that at 1492 cm−1 to pyridine interacting with both Brønsted and Lewis acid sites. The intensity of the appropriate bands on the spectrum of DeAlBeta is significantly lower, and the band at 1455 cm−1 completely disappears. Thus, the dealumination of beta zeolite leads to the decrease of both Lewis and Brønsted acidic sites in beta zeolite [27, 37].Figure 5
FTIR spectra of HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples recorded at room temperature after adsorption at room temperature and desorption of pyridine at 300°C.The incorporation of alkaline Earth metals into DeAlBeta zeolite results in the appearance of the band at 1445–1448 cm−1. This band may be attributed to Lewis sites, the population of which increases with decreasing alkaline Earth cation radius [38]. Indeed, the highest intensity of a band at 1448 cm−1 for Mg2.0DeAlBeta is connected with the enhanced incorporation of magnesium ions into the framework of DeAlBeta as well-dispersed Mg(II) Lewis sites. Moreover, for alkaline Earth metal-modified zeolite, the bands around of 1596–1609 cm−1 are observed, which can be ascribed to pyridine interacting with another type of weaker Lewis acidic centers [27, 37].NH3-TPD was carried out to monitor the acid strength and the amounts of acid sites on the samples under study. The results are presented in Figure 6(a). The profile of the initial HAlBeta sample shows the typical course with two unresolved peaks with maxima at around 250 and 414–429°C corresponding to ammonia desorbed from the weak and strong acid sites, respectively [39]. Roughly, the low temperature peak may be described to the Lewis, whereas the high temperature one to the Brønsted acidic sites [37, 40]. For the remaining samples (DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta), the intensity of both peaks diminishes, and the maximum of the low temperature peak is shifted about 30°C towards lower temperatures what suggests that these zeolites possesses lower strength of Lewis acidic sites. Moreover, for alkaline Earth metal-modified zeolites, one can observe lower intensity of the high temperature peak, especially expressed for Mg2.0DeAlBeta. This observation confirms again an incorporation of Mg2+ ions into the zeolite framework connected with the decrease of the amount of strong Brønsted sites.Figure 6
Temperature-programmed desorption profiles of ammonia (a) and carbon dioxide (b) for HAlBeta, DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.
(a)
(b)The quantitative analysis of NH3-TPD measurements is shown in Table 2. One can note that dealumination of beta zeolite leads to decreasing of its acidity as a result of removal of the Al from zeolite, in agreement with the earlier report [40]. However, the introduction of a small amount of alkaline Earth metals (c.a. 2 wt.%) into DeAlBeta increases the number of acidic centers. It may be related to an incorporation of these metals into the framework of this material and/or ion exchange with the proton of DeAlBeta zeolite with formation of Lewis acidic sites.Table 2
Surface quantitative analysis of TPD-NH3 and TPD-CO2 measurements on HAlBeta, DeAlBeta, Sr2.0DeAlBeta, Ca2.0DeAlBeta, and Mg2.0DeAlBeta.
Sample
Amount of NH3 adsorbed (μmol·g−1)
Amount of CO2 adsorbed (μmol·g−1)
HAlBeta
1830
2391
DeAlBeta
1181
1761
Mg2.0DeAlBeta
1408
1457
Sr2.0DeAlBeta
1335
1564
Ca2.0DeAlBeta
1345
1550Results of CO2-TPD for all tested samples are presented in Figure 6(b) and Table 2. The profile of HAlBeta depicts one peak with maximum at 250°C, which can be assigned to medium basic sites. The dealumination of beta zeolite leads to the decrease in its basicity what reflects both the shift of the desorption peak’s maximum to about 215°C and the significant decrease in the amount of adsorbed CO2. The introduction of alkaline Earth metals into the zeolite changes its basicity ambiguously. For Sr2.0DeAlBeta and Ca2.0DeAlBeta, the course of CO2-TPD is similar to that of DeAlBeta, although the maxima of desorption peaks are slightly shifted towards higher temperature and the amount of adsorbed CO2 decreases. The most pronounced shift of the maximum to the temperature 300°C is observed for Mg2.0DeAlBeta. Results seem to confirm that the basicity of alkaline Earth metal-modified zeolites depends on both structural type and chemical composition [11, 14].


## 3.3. Spectroscopic Characterization (DR UV-Vis, XPS,27Al,1H, and 29Si MAS NMR)

The DR UV-vis spectra of DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta are shown in Figure 7. For Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta, the broad band are seen at 267, 279, and 263 nm, respectively, related to isolated mononuclear Mg(II), Sr(II), and Ca(II) species well dispersed in the beta structure. The band at around of 267–279 nm could be attributed to pseudotetrahedral Mg(II), Sr(II), and Ca(II) species formed as a results of reaction of magnesium, strontium, and calcium ions with silanol groups of vacant T-atom sites of mesoporous DeAlBeta zeolite, in line with earlier reports on Sr-SAPO-34 [41] and MgAPSO-34 [42]. In the case of the Mg2.0DeAlBeta sample, the very small and broad band at about 300 nm is seen. It is probably related to present of tetrahedral Mg species with 4- and/or 3-coordination in the zeolite framework [4].Figure 7
DR UV-vis spectra recorded at room temperature and ambient atmosphere for DeAlBeta, Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta samples.To confirm the oxidation state of the prepared samples, XPS analysis was performed, and the results of which are shown in Figure8. The O 1s spectra (not shown here) of all studied samples depict one single peak at binding energy c.a. 533.0 eV that can be attributed to the Si-O-Si bond [43]. The Mg 2s spectrum of Mg2.0DeAlBeta (Figure 8(a)) presents one broad peak at binding energy 90.1 eV, which can be attributed to Mg2+. A similar peak for MgZSM-5 was reported by Cai et al. [44]. The Ca 2p spectrum of Ca2.0DeAlBeta (Figure 8(b)) shows a doublet structure and two peaks with maxima at binding energy 351.7 eV and 348.2 eV corresponding to Ca 2p1/2 and Ca 2p3/2, respectively, and indicates the presence of Ca2+ species in a different environment [45]. The Sr 3d spectrum of Sr2.0DeAlBeta is presented in Figure 8(c). The location of the Sr 3d5/2 spectrum at 136.3 eV and that of Sr 3d3/2 at 134.5 eV proves the existence of Sr2+ [46].Figure 8
X-ray photoelectron spectra recorded at room temperature for (a) Mg 2s of Mg2.0DeAlBeta, (b) Ca 2p of Ca2.0DeAlBeta, and (c) Sr 3d of Sr2.0DeAlBeta.
(a)
(b)
(c)27Al MAS NMR spectroscopy is an efficient probe to determine the coordination and local structure of specific aluminum species in zeolite materials [47–49]. Both tetrahedral and octahedral Al species can be easily distinguished basing on their different chemical shifts. A position of Al resonance gives a good indication of the local environment of the aluminum site. For example, framework aluminum atoms in tetrahedral coordination (AlTd) exhibit a signal at 50–60 ppm, while extraframework aluminum atoms in octahedral coordination (AlOh) usually give a signal at ∼0 ppm.Figure9 shows the 27Al MAS NMR spectra of HAlBeta (Si/Al = 18) and DeAlBeta (Si/Al = 64) samples. The spectra of both samples are similar. Two inseparable signals at about 54.5 and 56.9 ppm suggest the presence of two kinds of AlTd sites in both HAlBeta and DeAlBeta samples [27]. The decrease of the absolute signal intensity for DeAlBeta comparing to HAlBeta indicates the higher amount of aluminum species in the HAlBeta sample and confirms removal of Al from HAlBeta upon treatment with acid nitric [27].Figure 9
27Al MAS NMR spectra recorded at room temperature in a 4 mm (the external diameter) zirconia rotor of as-prepared HAlBeta and DeAlBeta.On the1H MAS NMR spectrum of HAlBeta (Figure 10(a)), one main broad peak is observed at 4.85 ppm due to protons of H-bonded silanol groups present at vacant T-atom sites of HAlBeta zeolite, in line with earlier data on zeolites [50–53]. For DeAlBeta, three main peaks are observed at 1.14–1.25, 3.64–3.74, and 5.45 ppm due to protons of isolated and/or terminal SiO-H and H-bonded silanol groups present at vacant T-atom sites, respectively. The peak at 3.6–3.7 ppm is probably due to protons of H-bonded SiOH groups located in a second kind of crystallographic sites. This shift is comparable with that observed at 3.20 ppm for H-bonded SiO-H groups in silicalite and silica [54, 55]. The disappearance of the peak at 5.45 and at 3.6–3.7 ppm upon incorporation of Mg, Sr, and Ca ions in DeAlBeta evidences the reaction of Mg, Sr, and Ca nitrate precursor with both H-bonded SiO-H groups. The 1H MAS NMR spectra of Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta exhibit the main peak at around 1.23–1.27 ppm due to the protons of isolated SiO-H groups with smaller intensity than that present in DeAlBeta zeolite. It confirms that Mg, Sr, and Ca ions introduced in DeAlBeta react not only with hydrogen bonded SiO-H but also with isolated SiO-H groups.Figure 10
1H MAS NMR (a), 29Si MAS NMR (b), and 1H-29Si CP MAS NMR (c) spectra recorded at room temperature in 4 mm (the external diameter) zirconia rotor of as-prepared HAlBeta and partially dealuminated DeAlBeta.
(a)
(b)
(c)The29Si MAS NMR spectrum of DeAlBeta (Figure 10(b)) shows three resonances at −103.7, 111.2, and −114.8 ppm. The peaks at −111.2 and −114.8 ppm are due to framework Si atoms in the Si (OSi)4 environment, located at different crystallographic sites, in line with the earlier report [56]. The peak at −103.7 ppm is assigned to Si atoms in the Si(OH) (OSi)3 environment as revealed by a strong increase of intensity of the peak at −103.7 ppm when 1H-29Si CP-MAS NMR is applied (Figure 10(c)), the technique which preferentially enhances the signal of the 29Si nuclei close to protons as it is in the case of the Si(OH) (OSi)3 site.After incorporation of Mg, Sr, and Ca ions into the DeAlBeta, the relative intensity of the peak at around −102.1 ppm (compared to the intensity of the peak at around −110 ppm) in1H-29Si CP-MAS NMR spectra is significantly reduced for Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta (Figure 10(c)), confirming the reaction between Mg, Sr, and Ca nitrate precursor and silanol groups of vacant T-atom sites. However, the relatively small decrease of the intensity of the peak at −102.1 ppm suggests that only part of Mg, Sr, and Ca ions is incorporated in the framework position of DeAlBeta zeolite.The change of the Si(OSi)4 environment takes place upon incorporation of Mg, Sr, and Ca ions into the framework of DeAlBeta zeolite as deduced from the appearance of better distinguished 29Si MAS NMR peaks at −110.5 and −114.2 ppm for Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta, respectively (Figure 10(c)). For DeAlBeta, only the shoulder is observed in the 1H - 29Si MAS NMR spectrum at around −114.8 (Figure 10(b)). It suggests that the presence of Mg, Sr, and Ca in the framework of Mg2.0DeAlBeta, Sr2.0DeAlBeta, and Ca2.0DeAlBeta involves some modification of the Si environment.


## 4. Conclusions

For all tested samples, the bands attribute to asymmetric and symmetric external O-T-O stretching vibration, and asymmetric internal O-T-O stretching vibration and structural vibration are observed, which indicate that the structure of beta zeolite was preserved upon introduction of alkaline Earth metals into its framework. This conclusion was confirmed also by low temperature N2 adsorption and XRD studies.On the basis of data obtained from XRD, IR (KBr), IR-pyridine, and NMR, one can suggest that alkaline Earth metal ions are incorporated into framework of beta zeolite.Their incorporation into the framework of DeAlBeta increases its Lewis acidity and changes its basicity. The mesoporous Mg2.0DeAlBeta zeolite is characterized by the most basic character.



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*Source: 7071524-2018-12-02.xml*