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Abstract

Strongly acidic ZrO₂–SiO₂–Al₂O₃ mixed oxide with different Zr:Si:Al ratios have been synthesized by sol–gel method. It was found that superacid (H₀ = −14.52) sites are formed at 12 ≤ Zr⁴⁺≤ 39, 48 ≤ Si⁴⁺≤ 72, 3 ≤ Al³⁺≤ 31 at.%. According to the infrared spectra of adsorbed pyridine and acetonitrile-D₃, acidic B- and strong L-sites are present on ZrO₂–SiO₂–Al₂O₃ surface. Observed high-energy shift of Zr 3d_5/2 peak in the X-ray photoelectron spectrum and low-field shift of ²⁹Si nuclear magnetic resonance signal of superacid Zr₃₅Si₅₃Al₁₂ oxide indicate that the electron density transfers from zirconium to silicon atom. The generation of superacidity in ZrO₂–SiO₂–Al₂O₃ matrix could be explained by formation of coordinatively unsaturated Zr⁴⁺ ions which are strong L-sites. It was shown that superacid ZrO₂–SiO₂–Al₂O₃ efficiently catalyzes polymerization of tetrahydrofuran in the presence of acetic anhydride at 40℃ with 68% yield of polytetramethyleneoxide acetate.

Keywords

Solid superacids mixed oxides zirconia acid catalysis polymerization of tetrahydrofuran

Introduction

Solid superacids always have attracted considerable interest as promising catalysts for the isomerization of C_4-7 n-paraffins and alkylbenzenes, and the acylation, nitration and rearrangement of aromatic compounds, where strong acid sites (H₀ < −12) are required. After discovery of sulfated and tungstated zirconia (Hino and Arata, 1980, 1988), the peak on the synthesis and study of similar materials was observed in 1980–2000. The synthesis and properties of superacids based on ZrO₂ were in details described in the reviews (Arata, 1990; Brei, 2005; Corma, 1995; Ivanov and Kustov, 2000; Reddy and Patil, 2009; Yadav and Nair, 1999). Recently, a new class of solid superacids, based on the sulfated metal-organic framework, have been prepared and studied (Jiang et al., 2014; Jiang and Yaghi, 2015).

It is known (Tanabe et al., 1989) that numerous mixed oxides possess higher acidity than corresponding individual oxides. In our opinion, binary mixed oxides, in particular ZrO₂–SiO₂, can be a good basis for a synthesis of ternary strong acidic oxides. The maximal acidity of ZrO₂–SiO₂ with H₀ = −11.35 is achieved at Si/Zr = 2 atomic ratio (Bosman et al., 1994; Inshina et al., 2012). Recently, we have found that at a doping of ZrO₂–SiO₂ with Al³⁺ ions, the strength of acid sites of ternary ZrO₂–SiO₂–Al₂O₃ oxide increases on three order to H₀ = −14.52.

In this communication, the data on acidity of ZrO₂–SiO₂–Al₂O₃ samples with different Zr:Si:Al ratios are presented. The formation of superacid sites in ZrO₂–SiO₂–Al₂O₃ matrix is discussed. The results on catalytic activity of obtained oxide in tetrahydrofuran (THF) polymerization are also presented.

Experimental

The ZrO₂–SiO₂–Al₂O₃ samples with different Zr:Si:Al ratios were synthesized by a sol–gel method. Zirconyl nitrate ZrO(NO₃)₂·10H₂O, tetraethylorthosilicate (TEOS) and aluminum nitrate Al(NO₃)₃·9H₂O were used as starting chemicals. In a typical procedure, calculated amounts of zirconyl and aluminum nitrates were dissolved in water. Besides, the solution of silicic acid oligomers was obtained by hydrolysis of TEOS in aqueous ethanol solution (n_C2H5OH/n_TEOS = 0.7 and n_Н2O/n_TEOS = 65) in the presence of nitric acid. Then, two obtained solutions were mixed, and urea was added under stirring. The sol was aged for two days at 93℃. The gel was washed, dried at 120℃ and calcined at 750℃ for 2 h. The ZrO₂–SiO₂–Al₂O₃ samples were denoted as Zr_xSi_yAl_z, where x, y and z represented the atomic percentage of cations.

Total number of acid sites was determined by reverse titration using n-butylamine solution in cyclohexane with bromthymol blue as an indicator. The acid strength and concentration-strength acid site distribution were examined by the Hammett indicators method (Tanabe et al., 1989) using 0.1% solution of the corresponding indicator in cyclohexane.

The X-ray powder diffraction analysis was performed using DRON-4-07 diffractometer (CuKα). Specific surface area, total pore volume and average pore radius of the samples were calculated from N₂ adsorption–desorption isotherms determined on a Quantachrome Nova 2200e Surface Area and a Pore Size Analyzer. The IR spectra of pyridine and acetonitrile-D₃ adsorbed on ZrSiAl sample were recorded on a Specord IR-75 spectrometer, according to procedure described in Inshina et al. (2012). The XPS spectra were registered on ЕC-2402 spectrometer with PHOIBOS-100_SPECS energy analyzer (E_MgK _α = 1253.6 eV, P_max = 200 W, BE calibration with C 1 s = 285.0 eV). ²⁷Al and ²⁹Si magic–angle–spinning (MAS) NMR spectra were recorded on a Bruker Avance 400 spectrometer at 104.2 MHz for ²⁷Al and 79.5 MHz for ²⁹Si.

Catalytic activity of ZrSiAl samples was estimated in oligomerization of THF in the presence of acetic anhydride (THF:AA = 8:1, mole). The experiments were carried out in a vertical down-flow glass reactor with a fixed bed of catalyst at 30–60℃ under atmospheric pressure. Liquid hourly space velocity (LHSV) was varied from 0.5 to 5.7 h⁻¹ using an Orion M 361 syringe pump. The number molecular weight (M_n) of polytetramethyleneoxide acetate (PTMA) was measured by a Waters gel permeation chromatography system equipped with a Waters 2414 refractive index detector and a set of Waters Styragel HR3 column.

Results and discussion

Acid strength for all 56 synthesized ZrSiAl samples was determined, and the results are plotted in the ternary diagram (Figure 1). High acidic sites with H₀ = −13.16 in ZrSiAl are generated at addition to silica matrix even 10 at.% of Zr⁴⁺ and 10 at.% of Al³⁺ ions. Superacid region with H₀ = −13.75 is limited to the cation concentrations of 8 ≤ Zr⁴⁺≤ 40, 20 ≤ Si⁴⁺≤ 73, 3 ≤ Al³⁺≤ 70 at.% and runs along the ratio of Si:Zr = 2:1 towards increasing aluminum content to 70 at.% (Figure 1). Maximum strength of acid sites (H₀ = −14.52) is observed at 12 ≤ Zr⁴⁺≤ 39, 48 ≤ Si⁴⁺≤ 72, 3 ≤ Al³⁺≤ 31 at.%. The highest acid sites concentration of 1.4 mmol/g is present on the Zr₃₅Si₅₃Al₁₂ surface. The concentration-strength acid site distribution for Zr₃₅Si₅₃Al₁₂ shows a wide range of acid strength (Figure 2). About 66% of acid sites correspond to superacid range −12.14 ≥H₀ ≥ −16.04 and 17% to medium acid strength (−8.2 ≥ H₀ ≥ −12.14).

Figure 1.

Effect of Zr:Si:Al ratio on acid strength of ZrO₂–SiO₂–Al₂O₃.

Figure 2.

Concentration-strength acid site distribution on Zr₃₅Si₅₃Al₁₂ surface.

According to the X-ray analysis, ZrSiAl samples with content of Zr⁴⁺ and Al³⁺ less than 40 at.% are amorphous. These samples have high-specific surface area (305-430 m²/g), medium pore diameter of 3.2–4.4 nm and acid site concentration in the interval of 1.1 – 1.4 mmol/g.

It is known (Bosman et al., 1994; Inshina et al., 2012) that the maximal content of acid sites in ZrO₂–SiO₂ mixed oxide was observed at Si⁴⁺/Zr⁴⁺= 2. The acid site strength was sharply decreased at changing this ratio (Inshina et al., 2012). As regards ZrO₂–SiO₂–Al₂O₃ mixed oxide, rather wide superacid region is observed, and area along Si/Zr ≈ 2 (Figure 1) indicates that the formation of acid sites in these two mixed oxides is similar.

The infrared (IR) spectra of pyridine adsorbed on Zr₃₅Si₅₃Al₁₂ reveal both L- and B-acid sites (Figure 3(a)). The band at 1445 cm⁻¹ is the result of coordinately bonded pyridine and the band at 1547 cm⁻¹—of protonated pyridine with Brønsted sites on the surface of ZrO₂–SiO₂ (Bosman et al., 1994; Corma, 1995; Lopez et al., 1995; Tarafdar et al., 2005). These bands are observed even after evacuation of the sample at 300℃. In IR spectrum of adsorbed acetonitrile-D₃, the band at 2330 cm⁻¹ is detected (Figure 3(b)) that confirms the existence of strong Lewis acid sites (Pelmenschikov et al., 1993). The other observed ν_C≡N bands at 2322 and 2265 cm⁻¹ correspond to medium L-sites and vibrations of liquid CD₃CN (Pelmenschikov et al., 1993).

Figure 3.

IR spectra of pyridine (a) and acetonitrile-D₃ (b) adsorbed on Zr₃₅Si₅₃Al₁₂ after evacuation at 150℃ and 300℃.

The formation of acid sites in binary ZrO₂–SiO₂ oxide is associated with different coordination state ^VIIIZr⁴⁺ and ^IVSi⁴⁺ ions in mixed oxide matrix and can be explained by Tanabe’s rule (Tanabe et al., 1989). Obviously, the formation of bridging ≡Zr–O(H)–Si≡ and ≡Al–O(H)–Si≡ groups as B-sites in ternary ZrO₂–SiO₂–Al₂O₃ oxide also may be based on this rule because silica is the main oxide.

²⁹Si MAS NMR spectrum of Zr₃₅Si₅₃Al₁₂ sample shows a broadening and low-field shift on 4 ppm of the Q⁴ signal to −106 ppm comparing to the peak of pure SiO₂ at −110 ppm (Figure 4). Similar shift is observed in ²⁹Si MAS NMR spectra of aluminosilicates (Fyfe et al., 1983) that indicates on the increase of electron density on silicon nucleus.

Figure 4.

²⁹Si MAS NMR spectra of Zr₃₅Si₅₃Al₁₂ and SiO₂.

Comparison of the ²⁷Al MAS NMR spectra of γ-Al₂O₃ and Zr₃₅Si₅₃Al₁₂ shows that in ZrSiAl matrix the content of ^IVAl³⁺ ions (53 ppm) is higher in relation to γ-Al₂O₃ (Figure 5). However, in amorphous Zr₃₅Si₅₃Al₁₂ matrix, more ^VAl³⁺ (28 ppm) and ^VIAl³⁺ ions are observed than in zeolites with four coordinated aluminum ions (Fyfe et al., 1983).

Figure 5.

²⁷Al MAS NMR spectra of Zr₃₅Si₅₃Al₁₂ and γ-Al₂O₃.

The XPS spectra of Zr 3 d-, Si 2p- and Al 2p-levels of the Zr₃₅Si₅₃Al₁₂ and pure ZrO₂, SiO₂ and Al₂O₃ oxides are shown in Figure 6. The corresponding binding energies and the full width at half maximum values are summarized in Table 1. The obtained binding energy of Zr 3 d, Si 2p and Al 2p correspond to known values for individual ZrO₂ (Briggs and Seach, 1983; Guittet et al., 2001), SiO₂ (Barr and Lishka, 1986; Guittet et al., 2001; Wagner et al., 1979) and Al₂O₃ (Barr and Lishka, 1986; Hinnen et al., 1994).

Figure 6.

Photoelectron lines of Zr₃₅Si₅₃Al₁₂, ZrO₂, SiO₂ and Al₂O₃ (a – Zr 3d, b – Si 2p, c – Al 2p).

Table 1.

Measured binding energies (E_b) and FWHM of Zr 3d, Si 2p and Al 2p photoelectron lines in Zr₃₅Si₅₃Al₁₂, ZrO₂, SiO₂ and Al₂O₃.

Sample	Zr 3d_5/2		Si 2p		Al 2p
Sample	E_b, eV	FWHM, eV	E_b, eV	FWHM, eV	E_b, eV	FWHM, eV
Zr₃₅Si₅₃Al₁₂	183.4	2.2	102.5	2.3	74.9	1.9
ZrO₂	182.1	1.8	–	–	–	–
SiO₂	–	–	103.3	2.2	–	–
Al₂O₃	–	–	–	–	75.5	1.8

FWHM: full width at half maximum.

The Zr 3d_5/2 peak position for Zr₃₅Si₅₃Al₁₂ is shifted towards higher binding energies comparing to that of pure ZrO₂, while binding energy of the Si 2p and Al 2p are smaller in Zr₃₅Si₅₃Al₁₂ than in pure SiO₂ and Al₂O₃ oxides, respectively (Figure 7, Table 1). Similar high-energy shift of Zr 3d_5/2 peak in the XPS spectra of zircon (ZrSiO₄) was detected by Guittet et al. (2001). Observed shifts indicate that the electron density transfers from zirconium to silicon and aluminum atoms in Zr₃₅Si₅₃Al₁₂ sample that is agreed with low-field shift of signal in the ²⁹Si MAS NMR spectrum (Figure 4). Thus, induced positive charge on zirconium ions in ZrSiAl matrix could form strong L-sites with H₀ = −14.52. Adsorption of CD₃CN molecules on these sites results in the shift of ν_C≡N from 2265 to 2330 cm⁻¹ (Figure 3(b)).

Figure 7.

Conversion (X) and space–time–yield (STY) of PTMA over Zr₃₅Si₅₃Al₁₂ and WO₃–ZrO₂–SiO₂ at different feed rates (40 ℃).

The catalytic activity of Zr₃₅Si₅₃Al₁₂ sample in liquid-phase THF polymerization in the presence of AA was compared to high acidic WO₃–ZrO₂–SiO₂ oxide (H₀ = −11.35; S = 270 m²/g, d = 2.8 nm) which was synthesized according to the described procedure (Prudius et al., 2010). Strong acids are needed for protonating of AA (pK_BН⁺= −11.0) with formation of acetyl cations which initiate the polymerization of THF (Setoyama et al., 2009). Superacid Zr₃₅Si₅₃Al₁₂ catalyst provides significantly higher yield of oligomers compared with WO₃–ZrO₂–SiO₂ (Figure 7). So, 64% yield of PTMA is observed over Zr₃₅Si₅₃Al₁₂ sample at mass feed rate of 1.4 g_THF g_cat⁻¹ h⁻¹ and 40℃, what is about three times higher than over WO₃–ZrO₂–SiO₂ (22%). The formed macromolecules have number-average molecular weight (M_n = 550–750) and polydispersity (M_w/M_n = 1.7), that is acceptable for practical use.

The maximum PTMA yield of 68% has been obtained over Zr₃₅Si₅₃Al₁₂ at 40℃ and at load on a catalyst of 0.8 g_THF g_cat⁻¹ h⁻¹. Also, Zr₃₅Si₅₃Al₁₂ catalyst works under much higher feed rates with space–time–yield of 1.2 g_PTMA g_cat⁻¹ h⁻¹ at 55.1% yield of oligomers (Figure 7). The high activity of Zr₃₅Si₅₃Al₁₂ compared with WO₃–ZrO₂–SiO₂ can be explained by both its higher acidity and specific surface area.

Conclusions

The acidity of mixed ZrSiAl oxide depends on the Zr:Si:Al ratio. At Zr:Si:Al = 35:53:12, this mixed oxide is characterized by the highest strength (Н₀ = −14.52) and content (1.4 mmol/g) of acid sites. The IR pyridine and acetonitrile adsorption spectroscopy indicates on Brønsted and strong Lewis acid sites on the ZrSiAl surface. According to the ²⁷Al MAS NMR spectra, four, five and six coordinated Al³⁺ ions are present in ZrSiAl matrix. Observed high-energy shift of Zr 3d_5/2 peak in the XPS spectrum indicates on the electron density transfer from zirconium to silicon and aluminum atoms in Zr₃₅Si₅₃Al₁₂ sample. Superacidity generation in ZrSiAl matrix could be explained by the formation of coordination-unsaturated Zr⁴⁺ ions as strong Lewis acid sites. It was determined that superacid ZrSiAl mixed oxide efficiently catalyzes the polymerization of THF with 68% yield of PTMA at 40℃.

Footnotes

Acknowledgements

First presented at the 15th Ukrainian–Polish Symposium on Theoretical and Experimental Studies of Interfacial Phenomena and their Technological Applications, Lviv, Ukraine, 12–15 September 2016.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Synthesis and study of superacid ZrO 2 –SiO 2 –Al 2 O 3 mixed oxide

Abstract

Keywords

Introduction

Experimental

Results and discussion

Conclusions

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References