A silicododecamolybdate/pyridinium-tetrazole hybrid molecular salt as a catalyst for the epoxidation of bio-derived olefins

abstract

The hybrid polyoxometalate (POM) salt (Hptz)(4)[SiMo12O40].nH(2)O (1) (ptz = 5-(2-pyridyl)tetrazole) has been prepared, characterized by X-ray crystallography, and examined as a catalyst for the epoxidation of cis-cyclooctene (Cy) and bio-derived olefins, namely dl-limonene (Lim; a naturally occurring monoterpene found in the rinds of citrus fruits), methyl oleate and methyl linoleate (fatty acid methyl esters (FAMEs) obtained by transesterification of vegetable oils). The crystal structure of 1 consists of alpha-Keggin-type heteropolyanions, [SiMo12O40](4-), surrounded by space-filling and charge-balancing 2-(tetrazol-5-yl)pyridinium (Hptz(+)) cations, as well as by a large number of water molecules of crystallization (n = 9). The water molecules mediate an extensive three-dimensional (3D) hydrogen-bonding network involving the inorganic anions and organic cations. For the epoxidation of the model substrate Cy in a nonaqueous system (tert-butylhydroperoxide as oxidant), the catalytic performance of 1 (100% epoxide yield at 24 h, 70 degrees C) was superior to that of the tetrabutylammonium salt (Bu4N)(4) [SiMo12O40] (2) (63% epoxide yield at 24 h), illustrating the role of the counterion Hptz(+) in enhancing catalytic activity. The hybrid salt 1 was effective for the epoxidation of Lim (69%/85% conversion at 6 h/24 h) and the FAMEs (87-88%/100% conversion at 6 h/24 h), leading to useful bio-based products (epoxides, diepoxides and diol products).

keywords

EPOXIDIZED METHYL OLEATE; CRYSTAL-STRUCTURE; POLYOXOMETALATE; HETEROPOLYMOLYBDATE

subject category

Chemistry, Inorganic & Nuclear

authors

Nunes, MS; Neves, P; Gomes, AC; Cunha-Silva, L; Lopes, AD; Valente, AA; Pillinger, M; Goncalves, IS

Groups

G1 - Porous Materials and Nanosystems
G4 - Renewable Materials and Circular Economy

Projects

Collaboratory for Emerging Technologies, CoLab (EMERGING TECHNOLOGIES)

CICECO - Aveiro Institute of Materials (UIDB/50011/2020)

CICECO - Aveiro Institute of Materials (UIDP/50011/2020)

Conversao catalitica de olefinas derivadas da biomassa (BiOle_AcidOxCat)

acknowledgements

This work was carried out with the support of CICECO - Aveiro Institute of Materials [FCT (Fundacao para a Ciencia e a Tecnologia) Ref. UIDB/50011/2020 & UIDP/50011/2020], REQUIMTE/LAQV (UIDB/50006/2020), and the COMPETE 2020 Operational Thematic Program for Competitiveness and Internationalization (Project POCI-010145-FEDER-030075), co-financed by national funds through the FCT/MCTES and the European Union through the European Regional Development Fund under the Portugal 2020 Partnership Agreement. The position held by P.N. was funded by national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of article 23 of the Decree-Law 57/2016 of 29 August, changed by Law 57/2017 of 19 July. M.S.N. is grateful for a BI(M) grant associated with the funded project POCI-01-0145-FEDER-030075.