A protective roasting strategy for preparation of stable mesoporous hollow CeO₂ microspheres with enhanced catalytic activity for one-pot synthesis of imines from benzyl alcohols and anilines

Abstract

A protective roasting strategy can be applied to prepare stable mh-CeO2 microspheres with enhanced catalytic activity and reusability for one-pot synthesis of imines.

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

“…As reported in Table 2 (entries 4, 7, and 8), decreasing the ratio from 1.5/1 to 1/1 brings about a decline in both the conversion of benzyl alcohol (from 99 to 85 %) and imine selectivity (from 99 to 91 %), while benzaldehyde selectivity increases (from 1 to 9 %). This shows that an appropriate excess of aniline is necessary to promote the oxidative coupling reaction, as has been observed on other CeO 2 ‐based catalysts and may be related to the weak basicity of aniline for facilitating selective oxidation of benzyl alcohol to benzaldehyde [14c, 21] . Interestingly, the hsm CeO 2 catalyst can work at 30 °C (Table 2, entry 9).…”

“…In comparison with other ceria catalysts working at 60 °C (Table 3, entries 4–10), the hsm CeO 2 catalyst also exhibits the second highest productivity of imine. Notably, the mh‐CeO 2 and CeO 2 ‐P fa ‐7 catalysts show very high productivity as well, but the mesoporous hollow CeO 2 microspheres [14c] and the organometallic‐precursor‐induced defect‐enriched mesoporous CeO 2 catalysts [10b] must be prepared by complex and well‐defined methods, and the exceptional imine productivity largely depends on the unique morphologies of ceria. On other ceria catalysts, much lower productivities of imine are obtained.…”

“…The highest specific surface area of the hsm CeO 2 catalyst is beneficial to afford more surface Ce 3+ ions and oxygen vacancies, which can adsorb and activate O 2 to generate more reactive O surf. species as the active sites for the oxidative coupling reaction [3b, 14c] . However, CeO 2 and hsm CeO 2 ‐600 catalysts with inadequate redox properties can only exhibit clearly lower catalytic activities.…”

“…developed a mesoporous hollow CeO 2 microsphere (mh‐CeO 2 ) catalyst by a protective roasting strategy. This mh‐CeO 2 catalyst showed superior activity (98 % yield at 60 °C in air) due to its superhigh specific surface area and mesoporous hollow structure [14c] . Nonetheless, the reported catalyst systems still have some obvious shortcomings, such as unsatisfactory conversion or selectivity, difficulty in reuse of catalysts, complicated morphology‐controlled synthesis, and requirement for pure O 2 .…”

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

“…CeO 2 is a new promising catalyst for imine synthesis, which can overcome the problems caused by traditional acid/base catalysts ( Tamura and Tomishige, 2015 ) and bring a lot of advantages, such as mild synthesis conditions and fast separation. Many research studies have focused on CeO 2 application in imine synthesis ( Geng et al, 2016 ; Long et al, 2019a ; Long et al, 2019b ; Cao et al, 2020 ; Rizzuti et al, 2020 ; Tamura and Tomishige, 2020 ), and it has been reported that CeO 2 morphologies can greatly influence the catalytic activity for imine synthesis ( Zhang et al, 2017 ; Yang et al, 2018 ; Zhang et al, 2018 ; Yang et al, 2020a ; Yang et al, 2020b ), which have a close relationship with the CeO 2 specific surface area and Ce 3+ defects.…”