Research progress in organic functionalization catalyzed by ceric acid and base

Background of Research

Cerium oxide can catalyze the dehydration of various alcohols to obtain alkenes. Pure phase ceria can be used as a water-resistant Lewis acid-base catalyst for the hydrolysis of ethers, esters and nitriles to produce the corresponding alcohol, acid, amide and other compounds. Heteratom doping can further improve the acid-base properties of ceria, and catalyze the high efficiency of ketoylation, Aldol condensation, Prins condensation and other reactions. In addition, cerium oxide has a unique ability to store and release oxygen, and it is often used as a typical REDOX catalyst in the traditional reaction to efficiently catalyze automobile exhaust gas and water vapor transformation reactions. The acid-base properties of cerium oxide catalysts are closely related to their unique defect properties and can be regulated to a certain extent. The acid-base catalyzed organic functional group reaction has attracted much attention in recent years.

Research Status quo

The organic reactions catalyzed by ceric acid and base include: ceric acid and base catalyzed alcohol dehydration, hydrolysis, ketoylation, Aldol condensation, Prins condensation and other organic reactions.

1. Alcohol dehydration reaction

   
   

   

                                                                                                    Figure 1. Solution based on cerium oxide

                                                                                                    Figure 2. Dehydration of 1, 3-diol to allyl alcohol

2.Hydrolysis reaction

Li et al. used CeO2 bar catalyst to catalyze the hydrolysis of acetonitrile to prepare amide compounds. Wang et al. used cerium oxide as an acid catalyst to catalyze the hydrolysis of dioxane and epoxide compounds to prepare diols.

                                                                                        Figure 3. Hydrolysis reaction catalyzed by CEO2-based catalyst

3.Ketoylation reaction

Snell et al. proposed a ketoylation mechanism requiring α-hydrogen. Ding et al. studied the ketoylation reaction of biomass propionic acid on mixed oxides with different Ce/Zr ratios.

                                                                                                      Figure 4. Ketoylation of carboxylic acids

Glinski et al. studied the MOx/S of different aliphatic esters and aromatic esters with MOx mass fraction of 20% (M=Mn,Ce,Zr,Th; Ketoylation reaction over S=Al2O3,SiO2) catalyst (300 ~ 425℃); Nagashima et al. studied the cycloketoylation of dimethyl caproate at the temperature range of 350 ~ 475℃ on pure CeO2.

                                                                                                              Figure 5. Ketoylation of ester

Sudarsanam et al. used CeZrO2(1:1 metal oxide material) based catalyst for the cycloketoylation of dimethyl adipate. Gaertner et al. proposed the reaction mechanism of ester hydrolyzation-carboxylic acid ketosylation, and carried out the ketosylation of 1-pentahexanoate and 2-pentahexanoate over cerium oxide catalyst. Li et al. gave an example of the conversion of heptanoate into heptanoate according to the above reaction path.

4.Aldol condensation (aldoL condensation) reaction

Gangadharan et al. catalyzed the condensation of propanal with Ce-Zr-O solid solution catalyst. An acid-base balance was found between Ce and Zr. Bhasker-ranganath et al. systematically studied the mechanism of Aldol condensation of acetaldehyde over octahedral cerium oxide catalyst by using in situ diffuse infrared and DFT theoretical calculations.

                                                                                              Figure. 6 Aldol condensation reaction mechanism

5.Prins condensation reaction

                                                                                       Figure. 7 Prins condensation reaction of olefin and formaldehyde

Wang et al. reported for the first time the preparation of 4-methyl-1, 3-dioxane by Prins condensation reaction using propylene and formaldehyde as substrates and pure CeO2 as water-resistant polyphase Lewis acid catalyst. Zhang et al. prepared an efficient and recyclable multiphase water-resistant Lewis acid catalyst through metal ion doped CeO2, which was used to efficiently catalyze Prins condensation reaction between isobutene and formaldehyde aqueous solution to generate 4-dimethyl-1, 3-dioxane.

Figure 8. The relationship between the radius of dopant ions on CeO2 and the Prins condensation activity, the concentration of Lewis acid site and the concentration of oxygen vacancy

Summary and Outlook

The mechanical functional group reaction mainly utilizes the acid-base properties of cerium oxide based catalyst. On the one hand, the acid site generated by the unsaturated Ce ion on CeO2 surface plays a key role in the adsorption and activation of aldehydes, ketones, carboxylic acids and esters, etc. On the other hand, the base site on CeO2 surface plays an important role in the activation of alcohols into alkenes. The acid and base properties of defective ceria can be controlled by roasting conditions, choosing exposed crystal surface and atom doping.

The preparation method of cerium oxide with controllable morphology is relatively simple, and most of them need to be carried out in hydrothermal system with high alkalinity. The introduction of heteroatoms can effectively regulate the catalytic performance of cerium oxide based catalysts, but the fine structure of atom-doped catalysts remains unclear. Most of the structure-activity relationships of acid-base reactions are established under off-site conditions and there is still a lack of effective means to track the dynamic changes of acid-base sites under in-situ reaction conditions. Since the defect property of cerium oxide is associated with its oxidation reducibility and acid-basicity, the regulation of defect structure is often accompanied by changes of multiple factors, which increases the difficulty in clarifying the catalytic nature of cerium oxide.

One of the important research directions in the future is to develop a new method for the preparation of nano-defect ceria. In situ EXAFS, XANS, STEM and other means to clarify the structure of cerium oxide based catalysts is also the future research trend. Plus! A theoretical model of the relationship between catalyst structure and activity was established by taking full advantage of DFT theory, which is helpful to further clarify the catalytic mechanism of CeO2. The above research is expected to broaden the boundaries of traditional acid-base catalytic systems and create more possibilities for the development of green and efficient organic synthesis paths.

Journal author

Journal of Shaanxi Normal University (Natural Science Edition). Online availability: 2022-02

Authors: Jiao Dongxia, Wang Yehong, Zhang Zhixin et al

Unit of Author:

(1) Dalian Institute of Chemical Physics, Chinese Academy of Sciences;

(2) School of Energy, University of Chinese Academy of Sciences