Thermo‐Enhanced Photocatalytic Activity in Aerobic Oxidative Coupling of Amines to Imines over Cu‐Doped MIL‐125‐NH₂

Abstract

Photocatalytic oxidation of raw organic molecules to value-added chemicals provides a cost-effective and sustainable strategy in organic synthesis. In this work, a series of Cu-doped MIL-125-NH 2 with narrower band gaps were synthesized by a one-pot hydrothermal method and used as photocatalysts for thermo-enhanced aerobic oxidative coupling of amines to imines under visible light irradiation. With optimal doping content, the conversion of benzylamine over MIL-125-NH 2 /Cu 2% can reach 100% at 60 °C under visible light irradiation for 8 h using molecule oxygen as oxidant. It is proved that doping MIL-125-NH 2 with a suitable Cu content can broaden the visible light absorption range, promote the separation of photo-induced charge carriers and improve the photocatalytic performance. Moreover, the increase of reaction temperature of the photocatalytic system promotes the performance. Free radical capture experiments and ESR tests show that holes and superoxide free radicals are the main active species. Such an excellent photocatalytic performance can be attributed to the strong light absorption and more efficient electron-hole separation efficiency via the Cu-doping strategy in MIL-125-NH 2 catalysts.

Effect of Metal Substitution on the Optical Band Gap of MIL‐125‐NH2

“…For the metal‐doped samples, the morphologies of Fe‐doped and Co‐doped MIL‐125‐NH 2 are similar to that of the original sample, while the morphology of Cu‐doped MIL‐125‐NH 2 appeared to be tetragonal, and the particle size is larger. It has been reported that the size of the Cu‐doped sample which is obtained by using small amount of Cu(NO 3 ) 3 ⋅3H 2 O dissolved in DMF and methanol mixture solution and synthesizing at 150 degrees for 20 hours is similar to the original sample [26] .We synthesized Cu‐doped sample using more metal salts at lower temperature, which the metal salts would be less soluble in organic solvents and the nucleation rate became slower, thus, larger size particles were obtained [27] . The results of ICP‐OES analysis for the concentration of metal elements are shown in Table 1.…”

Selective photo-oxidative coupling of amines to form C–N bonds using post synthetic modification of MIL-68-NH2 with metal acetylacetonate

“…32 The conversion of benzylamine to imine over MIL-125-NH 2 /Cu2% can approach 100% at 60 °C under visible light irradiation for 8 hours, utilizing molecular oxygen as an oxidant. 33 Bipyridyl-containing Cd-MOFs are very active and selective for the photo-oxidation of benzylamine to N -benzylbenzaldimine utilizing atmospheric air as the oxidant. 34 A multi-component MOF (Co-MIX) has been created by combining two different photoactive compounds, tricarboxytriphenylamine (H 3 TCA) and tris(4-(pyridinyl)phenyl)amine (NPy 3 ).…”

Preparation of CuNi/NH2-MIL-125(Ti) for the photocatalytic synthesis of 1,4-dihydropyridines and β-acetamido ketones

“…9) and H 2 , 10 photodegradation of organic pollutants 11 and heavy metals, 12 and photo-driven synthesis of organic compounds. 13,14 Numerous organic compounds, for instance, pyrrolo[2,1- a ]isoquinoline, pyranopyrazoles, pyridines, imines, and β-acetylamino acrylosulfones have been synthesized using various MOF photocatalysts, such as JNU-204(Zn), 15 MIL-53(Fe), 16 H 3 PW 12 O 40 @PCN-222(Zr), 17 NH 2 -MIL-125/Cu, 18 and UiO-67(Zr)–Ru, 19 respectively. Among different kinds of MOFs, a titanium-based MOF (NH 2 -MIL-125) offers high BET surface area (1350 m 2 g −1 ) and pore volume (0.58 cm 3 g −1 ), high thermal stability (up to 350 °C), and narrow energy bandgap (2.75 eV).…”