Heterogeneous Ceria Catalyst with Water-Tolerant Lewis Acidic Sites for One-Pot Synthesis of 1,3-Diols via Prins Condensation and Hydrolysis Reactions

Abstract

The use of a heterogeneous Lewis acid catalyst, which is insoluble and easily separable during the reaction, is a promising option for hydrolysis reactions from both environmental and practical viewpoints. In this study, ceria showed excellent catalytic activity in the hydrolysis of 4-methyl-1,3-dioxane to 1,3-butanediol in 95% yield and in the one-pot synthesis of 1,3-butanediol from propylene and formaldehyde via Prins condensation and hydrolysis reactions in an overall yield of 60%. In-depth investigations revealed that ceria is a water-tolerant Lewis acid catalyst, which has seldom been reported previously. The ceria catalysts showed rather unusual high activity in hydrolysis, with a turnover number (TON) of 260, which is rather high for bulk oxide catalysts, whose TONs are usually less than 100. Our conclusion that ceria functions as a Lewis acid catalyst in hydrolysis reactions is firmly supported by thorough characterizations with IR and Raman spectroscopy, acidity measurements with IR and (31)P magic-angle-spinning NMR spectroscopy, Na(+)/H(+) exchange tests, analyses using the in situ active-site capping method, and isotope-labeling studies. A relationship between surface vacancy sites and catalytic activity has been established. CeO(2)(111) has been confirmed to be the catalytically active crystalline facet for hydrolysis. Water has been found to be associatively adsorbed on oxygen vacancy sites with medium strength, which does not lead to water dissociation to form stable hydroxides. This explains why the ceria catalyst is water-tolerant.

Synthesis of 1,3-Diols from Isobutene and HCHO via Prins Condensation-Hydrolysis Using CeO2 Catalysts: Effects of Crystal Plane and Oxygen Vacancy

“…The CeO 2 -rods are about 7.4 nm in diameter and 69 nm in length, and the average sizes of CeO 2 octahedron and CeO 2 -cube are 118 and 61 nm, respectively. Our studies [24,38,39] and others [33,40] have also proved that the nanorods, nanocubes, and nanooctahedrons of CeO 2 selectively expose (110) and (100), (100), and (111) planes, respectively ( Table 1). The theoretical exposed crystalline planes and their ratio are shown in Table 1, as obtained from the literature [37].…”

“…We successfully prepared CeO 2 with different morphologies via the previously reported methods [23,24,36,37], and confirmed their morphology by XRD ( Figure S1) and HRTEM ( Figure S2). The average sizes of CeO 2 with different morphologies were determined by TEM ( Figure S2).…”

“…CeO2 has been proved a highly active and water-tolerant catalyst for the synthesis of 1,3-butanediol from propylene and HCHO via Prins condensation-hydrolysis in water [24]. This work encouraged us to apply the CeO2 catalyst for isobutene-formaldehyde condensation in water to synthesize 3-methyl-1,3-butanediol, which is an important higher-molecule alkanediol for the polymer industries; its synthesis is obviously difficult, but scientifically interesting.…”

“…Our previous work indicates that CeO2 is a water-tolerant Lewis acid catalyst [23,24]. It has been proved that the coordinatively unsaturated Ce cations on CeO2 surface act as the Lewis acid sites [25][26][27][28].…”

“…It has been proved that the coordinatively unsaturated Ce cations on CeO2 surface act as the Lewis acid sites [25][26][27][28]. According to experimental data and theory calculations [24,[29][30][31][32], the concentration of surface coordinatively unsaturated Ce cations is associated with the population of oxygen vacancies over CeO2. Additionally, the formation energy of oxygen vacancies varies greatly among different CeO2 surfaces.…”