Dealuminated Beta Zeolite Modified by Alkaline Earth Metals

Alkaline Earth metals (Mg, Sr, and Ca) were incorporated into the dealuminated mesoporous beta zeolite (DeAlBeta) by the twostep 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. ,e dealumination of beta zeolite led to decrease of its acidity and basicity.,e incorporation of alkaline Earth metals into the framework of dealuminated beta zeolite did not affect its structure. ,e 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.


Introduction
e 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][2][3][4][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][9][10] and/or alkaline Earth metal [11][12][13] into the zeolite structure can lead to modification of their basic properties.
e incorporation of alkali metal cations increases the basicity of oxygen atoms, which are present in the framework of the zeolite [14].e 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 metalscontaining zeolites significantly affect the yields of these processes.is 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].

Materials and Methods
Tetraethylammonium beta (TEABeta) zeolite provided by RIPP (China) was divided into two fractions.e first one was calcined (air, 15 h, 550 °C) in order to obtain an organicfree HAlBeta zeolite (Si/Al � 18).e second fraction of TEABeta was treated in a 13 mol•L −1 HNO 3 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 Sr 2+ , Ca 2+ , or Mg 2+ 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 . en, 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.en, the solids were calcined in air at 500 °C for 3 h and labelled Mg 2.0 DeAlBeta (2.0 wt.% Mg), Sr 2.0 DeAlBeta (2.1 wt.% Sr), and Ca 2.0 DeAlBeta (2.2 wt.% Ca)-in parentheses the real content of metals determined by XRF.
e 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.e total surface area was determined using the Brunauer-Emmeet-Teller (BET) method.e micropore volumes and the micropore surface area were obtained by the t-plot method.
e Barrett-Joyner-Halenda (BJH) model applied to the adsorption branch of the isotherm was used to obtain the mesopore size distribution.
e 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 °.
e Fourier-transform infrared (FT-IR) spectra of the studied zeolites samples were recorded on a Nicolet iS10 ermo 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 O 2 (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 .
e temperature-programmed desorption of ammonia (NH 3 -TPD) measurements were carried out in a quartz reactor using gaseous ammonium.NH 3 was adsorbed on zeolite catalysts (c.a.0.100 g) at 100 °C for 10 minutes in flowing He.Before NH 3 adsorption, catalysts were dried at 500 °C in He flow for 30 minutes (total gas flow: 40 cm 3 •min −1 ).e temperature-programmed desorption of NH 3 was carried out in the temperature range 25-500 °C, after removing physisorbed ammonium from the catalyst with helium flow at 100 o C for 10 minutes.
e temperature-programmed desorption of CO 2 (CO 2 -TPD) measurements were carried out in a quartz reactor using gaseous carbon dioxide.CO 2 was adsorbed on zeolite catalysts (c.a.0.100 g) at 40 °C for 30 minutes in flowing He.Before CO 2 adsorption, catalysts were dried at 500 °C in He flow for 30 minutes (total gas flow: 40 cm 3 •min −1 ).e temperature-programmed desorption of CO 2 was carried out in the temperature range 25-500 °C, after removing physisorbed ammonia from the catalyst.
e 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.
e 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.e 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. e spectra were fitted using Casa XPS v.2.3.16software (Casa Software Ldt, UK) and applying a Gaussian/Lorentzian ratio G/L equal to 70/30.

27
Al 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.
e spectra were acquired at a spinning rate of 12 kHz in a 4 mm zirconia rotor.29 Si MAS NMR spectra were recorded at 99.36 MHz with a Bruker Avance 500 spectrometer and 7 mm (external diameter) zirconia rotors, with CP ( 1 H- 29 Si CP-MAS NMR) and without ( 29 Si 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.
e proton π/2 pulse duration, the contact time, and recycle delay were 3.4 μs, 5 ms, and 5 s, respectively.1 H 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. e MAS equipment for rotation (12 kHz) was carefully cleaned with ethanol to avoid spurious proton signals.e probe signal was subtracted from the total free induction decay.

Textural and Structure Characterization (BET, XRD, and FTIR)
. N 2 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.
e relatively high micropore volume (ca.0.22 cm 3 g −1 indicates the crystalline character of the zeolite framework [25]. e BJH average pore diameter (4.40-4.70 nm) indicates the creation of intracrystalline mesopores.e 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.e subsequent introducing of alkaline Earth metal results in an opposite effect, the most pronounced for Mg 2.0 DeAlBeta, which may be connected with the deep incorporation of the small Mg 2+ ions into the zeolite structure.
us, 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 °) 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.e 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.e lack of alkaline Earth metal diffraction peaks may also suggest that Mg, Ca, or Sr species have entered the zeolite structure.
e 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][33][34].e band around 950 cm −1 disappears for Mg 2.0 DeAlBeta and is less intensive for Ca 2.0 DeAlBeta

Sample
Total specific surface area, TSSA ( Total pore volume (cm 3  Journal of Chemistry and Sr 2.0 DeAlBeta.is phenomenon suggests the reaction of alkaline Earth metals with silanol groups.e 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).e 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.e FT-IR spectra in the range of O-H stretching vibrations are shown in Figure 4(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 hydrogenbonded SiOH groups, respectively [27,35,36].It suggests that, upon treatment of HAlBEA zeolite with the 13 mol•L −1 HNO 3 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.e 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.
is effect is the most pronounced for the Mg 2.0 DeAlBeta sample.

Acid and Basic Sites Characterization (Py-IR, NH 3 -TPD, and CO 2 -TPD).
To get the information about the Brønsted and Lewis acid sites, FTIR of pyridine adsorption was carried out.e spectra are presented in Figure 5.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.
e intensity of the appropriate bands on the spectrum of DeAlBeta is significantly lower, and the band at 1455 cm −1 completely disappears.us, the dealumination of beta zeolite leads to the 4 Journal of Chemistry decrease of both Lewis and Brønsted acidic sites in beta zeolite [27,37].e incorporation of alkaline Earth metals into DeAl-Beta zeolite results in the appearance of the band at 1445-1448 cm −1 . is 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 Mg 2.0 DeAlBeta is connected with the enhanced incorporation of magnesium ions into the framework of DeAlBeta as well-dispersed Mg(II) Lewis sites.

Journal of Chemistry
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].NH 3 -TPD was carried out to monitor the acid strength and the amounts of acid sites on the samples under study.
e results are presented in Figure 6(a).e 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, Mg 2.0 DeAlBeta, Sr 2.0 DeAlBeta, and Ca 2.0 DeAlBeta), 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 Mg 2.0 DeAlBeta.is observation confirms again an incorporation of Mg 2+ ions into the zeolite framework connected with the decrease of the amount of strong Brønsted sites.
e quantitative analysis of NH 3 -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.
Results of CO 2 -TPD for all tested samples are presented in Figure 6(b) and Table 2. e profile of HAlBeta depicts one peak with maximum at 250 °C, which can be assigned to medium basic sites.e 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 CO 2 .e introduction of alkaline Earth metals into the zeolite changes its basicity ambiguously.For Sr 2.0 DeAlBeta and Ca 2.0 DeAlBeta, the course of CO 2 -TPD is similar to that of DeAlBeta, although the maxima of desorption peaks are slightly shifted towards higher temperature and the amount of adsorbed CO 2 decreases.e most pronounced shift of the maximum to the temperature 300 °C is observed for Mg 2.0 DeAlBeta.Results seem to confirm that the basicity of alkaline Earth metal-modified zeolites depends on both structural type and chemical composition [11,14]. 27Al, 1 H, and 29 Si MAS NMR).

Spectroscopic Characterization (DR UV-Vis, XPS,
e DR UV-vis spectra of DeAlBeta, Mg 2.0 DeAlBeta, Sr 2.0 DeAlBeta, and Ca 2.0 DeAlBeta are shown in Figure 7.For Mg 2.0 DeAlBeta, Sr 2.0 DeAlBeta, and Ca 2.0 DeAlBeta, 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.e 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 Mg 2.0 DeAlBeta 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].
To confirm the oxidation state of the prepared samples, XPS analysis was performed, and the results of which are shown in Figure 8. e 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].e Mg 2s spectrum of Mg 2.0 DeAlBeta (Figure 8(a)) presents one broad peak at binding energy 90.1 eV, which can be attributed to Mg 2+ .A similar peak for MgZSM-5 was reported by Cai et al. [44].e Ca 2p spectrum of Ca 2.0 DeAlBeta (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 2p 1/2 and Ca 2p 3/2 , respectively, and indicates the presence of Ca 2+ species in a different environment [45].e Sr 3d spectrum of Sr 2.0 DeAlBeta is presented in Figure 8(c).
e location of the Sr 3d 5/2 spectrum at 136.3 eV and that of Sr 3d 3/2 at 134.5 eV proves the existence of Sr 2+ [46].aluminum species in zeolite materials [47][48][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 (Al Td ) exhibit a signal at 50-60 ppm, while extraframework aluminum atoms in octahedral coordination (Al Oh ) usually give a signal at ∼0 ppm.
Figure 9 shows the 27 Al MAS NMR spectra of HAlBeta (Si/Al � 18) and DeAlBeta (Si/Al � 64) samples.e spectra of both samples are similar.Two inseparable signals at about 54.5 and 56.9 ppm suggest the presence of two kinds of Al Td sites in both HAlBeta and DeAlBeta samples [27].e 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].
On the 1 H 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][51][52][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          29 Si MAS NMR (b), and 1 H- 29 Si 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.metals into its framework.is conclusion was confirmed also by low temperature N 2 adsorption and XRD studies.

Conclusions
On the basis of data obtained from XRD, IR (KBr), IRpyridine, and NMR, one can suggest that alkaline Earth metal ions are incorporated into framework of beta zeolite.
eir incorporation into the framework of DeAlBeta increases its Lewis acidity and changes its basicity.e mesoporous Mg 2.0 DeAlBeta zeolite is characterized by the most basic character.

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
. For the samples modified with alkaline Earth metals, it shows the intermediate values, i.e., 22.34 °,22.35

Table 1 :
Surface characteristic of samples determined by the N 2 adsorption-desorption method.

Table 2 :
[56]ace quantitative analysis of TPD-NH 3 and TPD-CO 2 measurements on HAlBeta, DeAlBeta, Sr 2.0 DeAlBeta, Ca 2.0 DeAl-Beta, and Mg 2.0 DeAlBeta.Sr, and Ca ions in DeAlBeta evidences the reaction of Mg, Sr, and Ca nitrate precursor with both H-bonded SiO-H groups.e 1 H MAS NMR spectra of Mg 2.0 DeAlBeta, Sr 2.0 DeAlBeta, and Ca 2.0 DeAl-Beta exhibit the main peak at around 1.23-1.27ppmduetotheprotons 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.e29SiMASNMRspectrum of DeAlBeta (Figure10(b)) shows three resonances at −103.7, 111.2, and −114.8 ppm.e 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].e 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 1 H-Si CP-MAS