Cu(ii)-and Co(ii)-containing metal–organic frameworks (MOFs) as catalysts for cyclohexene oxidation with oxygen under solvent-free conditions

Abstract

Aerobic oxidation of cyclohexene over [Cu 2 (OH)(BTC)(H 2 O)] n ?2nH 2 O (Cu-MOF, BTC = 1,3,5benzenetricarboxylic acid) and [M 2 (DOBDC)(H 2 O) 2 ]?8H 2 O (Co-and Ni-MOF, DOBDC = 2,5dihydroxyterephthalic acid) in the absence of solvent under mild conditions was studied. It is observed that both Cu-MOF and Co-MOF can selectively oxidize cyclohexene to give 2-cyclohexen-1-ol and 2-cyclohexen-1-one as the main products, while Ni-MOF is totally inactive for cyclohexene oxidation. The mechanism of the catalytic oxidation of cyclohexene over Cu-MOF and Co-MOF has been proposed. These MOF-based catalysts are stable and recyclable under current reaction conditions. This study highlights the great potential of developing MOFs as highly stable, molecularly tunable, recyclable and reusable heterogeneous catalysts for alkenes oxidation.

Mixed-Metal Strategy on Metal–Organic Frameworks (MOFs) for Functionalities Expansion: Co Substitution Induces Aerobic Oxidation of Cyclohexene over Inactive Ni-MOF-74

“…8 The similar XRD patterns and comparable Langmuir specific surface area (860 m 2 g −1 ) of the as-prepared Ni-MOF-74, compared with that previously reported, indicated that Ni-MOF-74 with high quality has been obtained ( Figure 1). 7,9 Co-substituted Ni-MOF-74 (denoted as Co/Ni-MOF-74-x, where x represents the incubation time) was prepared by exposing Ni-MOF-74 to a DMF solution containing Co-(NO 3 ) 2 ·6H 2 O for different times.…”

“…12 The successful substitution of Ni by Co was also supported by extended X-ray absorption fine structure (EXAFS) analyses (see Figure 3, and Our previous study has revealed that the cycling of Co between Co 2+ and Co 3+ is essential for the oxidation of cyclohexene over Co-MOF-74, while the difficulty to form Ni 3+ in Ni-MOF-74 makes it inert for this reaction. 8 To study the effect of substituted Co 2+ in the mixed Co/Ni-MOF-74 for this reaction, the catalytic performance of the as-prepared Co/Ni-MOF-74 for cyclohexene oxidation was investigated. As shown in Table 2 (entries 1 and 2), although Ni-MOF-74 is inactive for cyclohexene oxidation, because of the auto-oxidization of cyclohexene, ∼37.2% of cyclohexene was converted over Ni-MOF-74 after reacting for 20 h. This value is comparable to that observed in the system without any catalyst (37.0%).…”

“…7 Our previous work has shown that Co-MOF-74 is active for cyclohexene oxidation while the isostructural Ni-MOF-74 is inactive for this reaction. 8 Herein, we reported that the substitution of Ni 2+ in Ni-MOF-74 framework by active Co 2+ via a post-synthetic metal exchange method can enable the inert Ni-MOF-74 to show catalytic activity for cyclohexene oxidation. The performance over the Co-substituted Ni-MOF-74 (Co/Ni-MOF-74) increased with the amount of incorporated Co 2+ , indicating that the incorporated Co 2+ is responsible for the reaction.…”

“…Ni-MOF-74 was prepared following the previous reported procedures. 8 A solid mixture of H 4 DOBDC (0.2216 g, 1.12 mmol) and Ni(NO 3 ) 2 ·6H 2 O (1.077 g, 3.70 mmol) were added to a 75 mL solution of DMF, EtOH and H 2 O in 1:1:1 ratio. The above mixture was stirred at room temperature to get a homogeneous solution and was transferred into a 100 mL Teflon liner and heated at 120°C for 24 h. After reaction, the solids were separated from the solvent via centrifugation, and then washed with fresh DMF and methanol.…”

Metal-dioxidoterephthalate MOFs of the MOF-74 type: Microporous basic catalysts with well-defined active sites

“…[20,24,29,32] Reports of catalysis using M 2 dobdc are very limited hitherto; they focus on the role of the metal ion as a Lewis acid center, or in accelerating autoxidation reactions. [22,33,34] …”