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 3d5/2 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.
Highlights
Solid superacids always have attracted considerable interest as promising catalysts for the isomerization of C4-7 n-paraffins and alkylbenzenes, and the acylation, nitration and rearrangement of aromatic compounds, where strong acid sites (H0 < À12) are required
The acidity of mixed ZrSiAl oxide depends on the Zr:Si:Al ratio
At Zr:Si:Al 1⁄4 35:53:12, this mixed oxide is characterized by the highest strength (M0 1⁄4 À14.52) and content (1.4 mmol/g) of acid sites
Summary
Solid superacids always have attracted considerable interest as promising catalysts for the isomerization of C4-7 n-paraffins and alkylbenzenes, and the acylation, nitration and rearrangement of aromatic compounds, where strong acid sites (H0 < À12) are required. We have found that at a doping of ZrO2–SiO2 with Al3þ ions, the strength of acid sites of ternary ZrO2–SiO2–Al2O3 oxide increases on three order to H0 1⁄4 À14.52. Acid strength for all 56 synthesized ZrSiAl samples was determined, and the results are plotted in the ternary diagram (Figure 1).
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