Unveiling the Active Surface Sites in Heterogeneous Titanium-Based Silicalite Epoxidation Catalysts: Input of Silanol-Functionalized Polyoxotungstates as Soluble Analogues

Abstract : We report on a site-isolated model for Ti(IV) by reacting [Ti(i PrO) 4 ] with the silanol-functionalized polyoxotungstates [XW 9 O 34-x (t BuSiOH) 3 ] 3-(X= P, x=0, 1; X= Sb, x=1, 2) in tetrahydrofuran. The resulting titanium(IV) complexes [XW 9 O 34-x (t BuSiO) 3 Ti(O i Pr)] 3-(X= P, 3; X= Sb, 4) were obtained in monomeric forms both in solution and in the solid state, as proved by diffusion NMR experiments and by X-ray crystallographic analysis. Anions 3 and 4 represent 2 relevant soluble models for heterogeneous titanium silicalite epoxidation catalysts. The POM scaffolds feature slight conformational differences that influence the chemical behavior of 3 and 4 as demonstrated by their reaction with H 2 O. In the case of 3, the hydrolysis reaction of the isopropoxide ligand is only little shifted towards the formation of a monomeric [PW 9 O 34 (t BuSiO) 3 Ti(OH)] 3-(5) species [log K=-1.96], whereas 4 reacted readily with H 2 O to form a -oxo bridged dimer {[SbW 9 O 33 (t BuSiO) 3 Ti] 2 O} 6-(6). The more confined the coordination site, the more hydrophobic the metal complex. By studying the reaction of 3 and 4 with hydrogen peroxide using NMR and Raman spectroscopies, we concluded that the reaction leads to the formation of a titanium-hydroperoxide Ti-( 1-OOH) moiety, which is directly involved in the epoxidation of the allylic alcohol 3-methyl-2-buten-1-ol. The combined use of both spectroscopies also led to understanding that a shift of the acid-base equilibrium towards the formation of Ti( 2-O 2) and H 3 O + correlates with the partial hydrolysis of the phosphotungstate scaffold in 3. In that case, the release of protons also catalyzed the oxirane opening of the in situ formed epoxide, leading to an increased selectivity for 1,2,3-butane-triol. In the case of the more stable [SbW 9 O 33 (t BuSiO) 3 Ti(O i Pr)] 3-(4), the evolution to Ti( 2-O 2) peroxide was not detected by Raman spectroscopy and we performed reaction progress kinetic analysis by NMR monitoring the 3-methyl-2-buten-1-ol epoxidation in order to assess the efficiency and integrity of 4 as precatalyst.
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Teng Zhang, Louis Mazaud, Lise-Marie Chamoreau, Céline Paris, Anna Proust, et al.. Unveiling the Active Surface Sites in Heterogeneous Titanium-Based Silicalite Epoxidation Catalysts: Input of Silanol-Functionalized Polyoxotungstates as Soluble Analogues. ACS Catalysis, American Chemical Society, 2018, 8 (3), pp.2330-2342. ⟨10.1021/acscatal.8b00256⟩. ⟨hal-02151928⟩

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