Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High‐Surface‐Area Mesoporous CeO₂ with Exceptional Redox Property

Abstract

High‐surface‐area mesoporous CeO2 (hsmCeO2) was prepared by a facile organic‐template‐induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO2 and hsmCeO2 after different thermal treatments were also investigated. XRD, N2 physisorption, UV‐Raman, H2 temperature‐programmed reduction, O2 temperature‐programmed desorption, EPR spectroscopy, and X‐ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO2 calcined at 400 °C shows the highest specific surface area (158 m2 g−1), the highest fraction of surface coordinatively unsaturated Ce3+ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×1015 spins g−1). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO2 catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol gceria-1 h−1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram‐scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO2 catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Tailoring the Reactive Oxygen Species in Mesoporous NiO for Selectivity-Controlled Aerobic Oxidation of 5-Hydroxymethylfurfural on a Loaded Pt Catalyst

“…Generally, three types of oxygen species can be identified (Figure B). The desorption peak below 170 °C can be ascribed to the weakly chemisorbed O 2 molecules . The broad band between 200 and 650 °C can be resolved into two parts.…”

“…The desorption peak below 170 °C can be ascribed to the weakly chemisorbed O 2 molecules. 34 The broad band between 200 and 650 °C can be resolved into two parts. The band at low temperatures (200−470 °C) can be attributed to the desorption of defective O ads.…”

Ceria-Based Materials for Thermocatalytic and Photocatalytic Organic Synthesis

“…The intrinsic properties (e.g., surface oxygen vacancy concentration, Brønsted acid sites, the ratio of Ce 3+ /Ce 4+ ) and morphology (e.g., porous structure) all have significant effects on the oxidation of benzyl alcohol to benzaldehyde, which is the rate-determining step in the direct coupling of alcohol and amine to the corresponding imines. − CeO 2 nanomaterials (e.g., nanoflakes, mesoporous hollow nanostructures, mesoporous structures) have been investigated for optimizing their catalytic performances via tuning the surface oxygen vacancies, Ce 3+ /Ce 4+ ratio and the surface peroxy anion O 2 – ratio. , Besides the intrinsic properties of CeO 2 , the catalytic performance of CeO 2 in the oxidative coupling of alcohols and amines to imines can also be improved by its support, such as the improved dispersion for CeO 2 nanocrystals and the possible reaction between carboxylic and ester sites of the mesoporous carbon support and rich hydroxyl groups of CeO 2 , the promoted oxidation of alcohols to benzaldehyde due to the improved alkalinity at low reaction temperature, and the stabilized Ce 3+ /Ce 4+ pair of CeO 2 due to the generated Lewis basic site by the doped nitrogen in Fe 3 O 4 @CN@CeO 2 …”

“…60−62 CeO 2 nanomaterials (e.g., nanoflakes, mesoporous hollow nanostructures, mesoporous structures) have been investigated for optimizing their catalytic performances via tuning the surface oxygen vacancies, Ce 3+ /Ce 4+ ratio and the surface peroxy anion O 2 − ratio. 63,64 Besides the intrinsic properties of CeO 2 , the catalytic performance of CeO 2 in the oxidative coupling of alcohols and amines to imines can also be improved by its support, 65 such as the improved dispersion for CeO 2 nanocrystals and the possible reaction between carboxylic and ester sites of the mesoporous carbon support and rich hydroxyl groups of CeO 2 , 66 the promoted oxidation of alcohols to benzaldehyde due to the improved alkalinity at low reaction temperature, and the stabilized Ce 3+ /Ce 4+ pair of CeO 2 due to the generated Lewis basic site by the doped nitrogen in Fe 3 O 4 @CN@CeO 2 . 67 Besides the oxidation of alcohols and amine to the imine, CeO 2 is also a versatile, reusable, and optimal catalyst for the conversion of alcohols by the addition of esters, 68 ketones, 69 nitriles, 70 amide, 71 and several molecules together.…”

CeO2 Structure Adjustment by H2O via the Microwave–Ultrasonic Method and Its Application in Imine Catalysis

“…The reaction rates of all the samples were far better than the 0.46 mmol g −1 h −1 reported in Angew ( Tamura and Tomishige, 2015 ). It has been reported earlier that CeO 2 can be used as an easily separable catalyst for imine synthesis at low temperature (<100°C) ( Zhang et al, 2017 ; Zhang et al, 2018 ; Chen et al, 2020 ; Wu et al, 2021 ) and oxygen vacancies (Ce 3+ defects) played an important role during this reaction. The Ce 3+ vacancies can promote the transition of benzyl alcohol to benzaldehyde, and the latter can easily couple with aniline to form an imine compound ( Tamura and Tomishige, 2015 ; Zhang et al, 2017 ; Qin et al, 2019 ).…”