Epoxidation catalysts prepared by encapsulation of molybdenum hexacarbonyl in UiO-66(Zr/Hf)-type metal-organic frameworks

abstract

The confinement and catalytic behavior of Mo(CO)(6) species inside UiO-66 (zirconium and hafnium) metalorganic frameworks (MOFs) have been investigated by means of multitechnique characterization and catalytic olefin epoxidation. Mo(CO)(6) was encapsulated in nanocrystalline MOFs by solvothermal and vapor phase impregnation methods (STI and VPI, respectively), resulting in Mo loadings of 2.0-8.8 wt% (STI) or 15 wt% (VPI). Powder X-ray diffraction, SEM-EDS, N-2 adsorption, FT-IR and C-13{H-1} CP MAS NMR collectively confirmed immobilization within the pore spaces of the hosts, which maintained their bulk crystallinity and crystal morphologies. CO resonances in the C-13{H-1} CP MAS NMR spectra were assigned to hexacarbonyl guest molecules located in the larger pore spaces, such as the octahedral cavities (for STI and VPI samples), and in more restricted environments such as the tetrahedral cavities (for the VPI sample). FT-IR spectra displayed the characteristic pattern of nu(CO) stretching modes for an octahedral symmetry molecule slightly perturbed by interaction of one of the carbonyl groups with the MOF support. The spectra evidenced the formation of a weak hydrogen bonding interaction of the type mu(3)-OH center dot center dot center dot OCMo(CO)(5) involving the mu(3)-OH groups that are exclusively present in the tetrahedral cavities of the UiO-66 structure. The Mo(CO)(6) loaded MOFs are active and reusable pre-catalysts for cis-cyclooctene epoxidation, exhibiting excellent epoxide selectivity (up to quantitative epoxide yield within 1 h reaction) and tert-butyl hydroperoxide efficiency. Catalyst recycling and retention of structural integrity were demonstrated for UiO-66(Hf) loaded with 8.8 wt% Mo by the STI method. The UiO-66(Hf) host may be a better catalyst support than its Zr counterpart due its smaller crystallite size (similar to 100 nm) and higher defectivity. The substrate scope was broadened for Mo(8.8-STI)/Hf-MOF which promoted the epoxidation of biobased olefins (methyl oleate and DL-limonene) and oxidative dehydrogenation of cyclooctanol to cyclooctanone.

keywords

TERT-BUTYL HYDROPEROXIDE; OLEFIN EPOXIDATION; VIBRATIONAL-SPECTRA; UIO-66; ZIRCONIUM; HAFNIUM; ZR; COMPLEXES; EFFICIENT; ACID

subject category

Chemistry; Science & Technology - Other Topics; Materials Science

authors

Gomes, DP; Silva, AF; Gomes, AC; Neves, P; Valente, AA; Goncalves, IS; Pillinger, M

Groups

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

Projects

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

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

Sistemas ativados por estímulos para a terapia com monóxido de carbono (SASCOT)

Conversao catalitica de olefinas derivadas da biomassa (BiOle_AcidOxCat)

acknowledgements

We acknowledge funding provided within the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, the CENTRO 2020 Regional Operational Programme (project references CENTRO-01-0145-FEDER-028031 and PTDC/QUIQOR/28031/2017) , and COMPETE 2020 Operational Thematic Program for Competitiveness and Internationalization (Project POCI-01-0145-FEDER-030075) , financed by national funds through the FCT (Fundacao para a Ciencia e a Tecnologia) /MEC (Ministerio da Educacao e Ciencia) and when appropriate co-financed by the European Union through the European Regional Development Fund under the Portugal 2020 Partnership Agreement.