Catalytic Performance of Zr‐Based Metal–Organic Frameworks Zr‐abtc and MIP‐200 in Selective Oxidations with H₂O₂

Abstract

The catalytic performance of Zr‐abtc and MIP‐200 metal–organic frameworks consisting of 8‐connected Zr6 clusters and tetratopic linkers was investigated in H2O2‐based selective oxidations and compared with that of 12‐coordinated UiO‐66 and UiO‐67. Zr‐abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron‐deficient C=C bonds in α,β‐unsaturated ketones. The significant predominance of 1,2‐epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr‐abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr‐abtc. Electrophilic activation of H2O2 can also be realized, as evidenced by the high activity of Zr‐abtc in epoxidation of the electron‐rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr‐abtc structure after the catalysis. The low activity of MIP‐200 in H2O2‐based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H2O2.

Activation of H2O2 over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates

“…This contrasts drastically with thioether oxidation over Zr-MOFs, for which sulfone was the predominated product (99% selectivity at 49% MPS conversion). 57,58 This suggestion was further confirmed by the oxidation of the well-known test substrate thianthrene-5-oxide (SSO), the molecule of which possesses both nucleophilic and electrophilic sulfur (Scheme S1). The ratio of thianthrene 5,5-dioxide (SSO 2 ) and 5,5,10-trioxide (SOSO 2 , product of further oxidation of both oxides) to all products, including thianthrene 5,10-dioxide (SOSO), gives the so-called nucleophilic parameter X Nu [X Nu = (SSO 2 + SOSO 2 )/(SOSO + SSO 2 + 2SOSO 2 )].…”

“…Predomination of sulfoxide over sulfone indicates the formation of electrophilic oxidizing species upon the interaction of Zr-POM and H 2 O 2 . This contrasts drastically with thioether oxidation over Zr-MOFs, for which sulfone was the predominated product (99% selectivity at 49% MPS conversion). , …”

“…It is noteworthy that a similar trend was observed recently for Nbcontaining Lindqvist-type POMs 92 and Zr-containing MOFs. 56,58 Also, for Ti-containing POMs, the addition of acidic protons accelerated the reaction and increased the selectivity toward the heterolytic products in alkene oxidation with H 2 O 2 . 99 Even minor (0.1 equiv per Zr) additives of acid caused a pronounced effect on the rate and selectivity of CyH oxidation in the presence of 2 (Figure 1a, Table S1).…”

“…In recent years, metal–organic frameworks (MOFs) have induced a great interest owing to their prospective applications in gas separation and storage, molecular recognition, drug delivery, and heterogeneous catalysis. − Zr-containing MOFs turned out to be active and recyclable catalysts for oxidative transformations using H 2 O 2 as an oxidant. − Recently, some of us have discovered a notable effect of acid additives on the selectivity of alkene oxidation with H 2 O 2 catalyzed by Zr-based MOFs. , It was shown that protons favor heterolytic activation of the oxidant and suppress its unproductive decomposition over Zr-MOFs, leading to the epoxidation of the CC bond and avoiding allylic oxidation of C–H bonds. However, the structure of the active peroxo zirconium intermediates operating in Zr-MOFs remained unclear.…”

“…48−58 Recently, some of us have discovered a notable effect of acid additives on the selectivity of alkene oxidation with H 2 O 2 catalyzed by Zr-based MOFs. 56,58 It was shown that protons favor heterolytic activation of the oxidant and suppress its unproductive decomposition over Zr-MOFs, leading to the epoxidation of the CC bond and avoiding allylic oxidation of C−H bonds. However, the structure of the active peroxo zirconium intermediates operating in Zr-MOFs remained unclear.…”