Steric hindrance effect and kinetic investigation for ionic liquid catalyzed synthesis of 4-hydroxy-2-butanone via aldol reaction
Introduction
4-Hydroxy-2-butanone as a very important chemical intermediate is widely used in both pharmaceutical and food industries (Ichikawa et al., 2005). The current production of this compound is mainly based on aldol condensation between formaldehyde and acetone with catalysis of sodium hydroxide solution at 40 °C, which is the simplest method reported (Ogura et al., 2004). The yield of 4-hydroxy-2-butanone could reach 40–50%, while the side reactions including self-condensation of acetone and disproportionation of formaldehyde proceed heavily in the presence of strong base. In addition, a large quantity of sodium hydroxide are consumed during per batch reaction and alkaline wastewater are also need to deal with. Instead of homogeneous catalytic system, Tanner developed a heterogeneous catalytic process for 4-hydroxy-2-butanone synthesis over acid-base bi-functional vanadium phosphate, while the yield was lower than 60% due to the subsequent dehydration to get methyl vinyl ketone (Tanner et al., 2002). To avoid the use of sodium hydroxide and suppress the byproducts, Huang and Mei conducted the aldol reaction of acetone with formaldehyde under supercritical conditions, which was environmentally but energy-intensive, the generated formic acid via disproportionation of formaldehyde was proved to play a role as catalyst and the highest yield could climb to 90% (Huang et al., 2013, Mei et al., 2015). While Wang designed a type of polystyrene anion exchange resin modified with long chain alkyl groups for synthesis of 4-hydroxy-2-butanone, the yield could be increased from 44.3% to 71.3% by the steric hindrance of cation and synergetic effects between weak and strong basic sites (Wang et al., 2013).
Another alternative route to 4-hydroxy-2-butanone is the selective oxidation of 1, 3-butanediol. Su reported series of W-contained mesocellular silica foams with Si/W ratio of 10–40, revealing the tungsten was well incorporated inside the mesocellular silica foams and the W-mesocellular silica compared with WO3-mesocellular silica foam shown the higher performance of 74.3% yield and 78.1% selectivity to 4-hydroxy-2-butanone in the presence of H2O2 (Su et al., 2006). Kessat used a kind of macroporous resin of 2-methyl-4-poly (styrylmethyl) thiazolium hydrotribromide as stoichiometric oxidation reagent and the attractive yield of 4-hydroxy-2-butanone could be obtained as high as 86% (Kessat et al., 2001). Torresi prepared a silica-supported copper catalyst for dehydrogenation of 1, 3-butanediol to 4-hydroxy-2-butanone, which was promoted by the large Cu0 particles, and the yield could be attained as 25% (Torresi et al., 2013). Díez synthesized a series of single oxides with different acid-base properties and discovered the Cu0 in mono-functional copper-silica, bi-functional copper-acid and copper-base catalysts is to increase the reaction rate and to shift the reaction pathway toward 1, 3-butanediol dehydrogenation with the highest selectivity of 60% (Díez et al., 2013). Sato developed series of copper catalysts modified with ZnO, ZrO2, Al2O3 and MgO and the results indicated the product of 4-hydroxy-2-butanone was efficiently produced by the dehydrogenation of 1, 3-butanediol over Cu/ZnO with 40% yield (Sato et al., 2007). Aside from heterogeneous catalytic process, there is also a homogeneous catalytic system reported by Iwahama, which was based on Co/N-hydroxyphthalimide species for the aerobic oxidation of 1, 3-butanediol to 4-hydroxy-2-butanone with a yield of ca. 50% (Iwahama et al., 2000).
Compared with the selective oxidation of 1, 3-butanediol, the direct aldol reaction between formaldehyde and acetone, which are commercial chemicals with large production, is more feasible for 4-hydroxy-2-butanone synthesis in the view of operation and feedstock supplying. However, the main problems of catalyst recovery and side reactions inhibition should be resolved. In recent years, ionic liquid as novel and green catalyst is widely used in aldol reaction (Aprile et al., 2007, Cordova, 2004, Curnow et al., 2012, Hu et al., 2007, Kong et al., 2013, Lombardo et al., 2008, Mestres, 2004, Wang et al., 2017a, Wang et al., 2019, Wang et al., 2018, Wang et al., 2017b, Wang et al., 2017c, Zhu et al., 2005) and intrigue researchers’ great interests due to the advantages of negligible vapour pressure, chemical and thermal stability, non-flammability and designable structure (Hallett and Welton, 2011, Jessop et al., 2005, Welton, 1999). Stimulated by the discoveries that the steric hindrance of catalyst will affect the selectivity of 4-hydroxy-2-butanone from direct aldol reaction (Wang et al., 2013), series of basic ionic liquids with different sterically hindered cations were prepared for synthesis of 4-hydroxy-2-butanone via aldol condensation of acetone with formaldehyde. The catalytic performance of ionic liquid series with different steric hindrance effect were compared and demonstrated. The reaction pathway was proposed with the product components identified by GC-MS. The reaction kinetic studies was conducted, which would be considered as a reference for process optimization and catalyst development.
Section snippets
Materials
All of the ionic liquids (purity ≥ 99.0%) are purchased from Shanghai Cheng Jie Chemical Co. Ltd. The acetone (purity ≥ 99.5%). Formaldehyde aqueous solution (purity, 36.5–38.0%), 4-hydroxy-2-butanone (purity, 95.0%), methyl formate (purity, 97.0%), methyl vinyl ketone (purity, 99.0%), diacetone alcohol (purity, 99.0%), mesityl oxide (purity, 90%) and isobutanol (purity, 99.5%) are provided by Sigma-Aldrich company. All of these chemicals can be used without further purification.
Synthesis reaction
The typical
Reaction pathway
According to the GC-MS analysis results, the aldol reaction between acetone and formaldehyde with the catalysis of basic ionic liquids in liquid phase yields major product of 4-hydroxy-2-butanone. Otherwise, a small amount of methyl vinyl ketone, diacetone alcohol, mesityl oxide and methyl formate were also detected. The MS information of these components are summarized in Table 1 and the corresponding spectra are provided in Supplementary Material.
Based on the components identified by GC-MS,
Conclusion
In this work, the 4-hydroxy-2-butanone was synthesized from acetone and formaldehyde via direct aldol condensation with catalysis of [Cation]OH-type basic ionic liquid. The increasing steric hindrance of cation of ionic liquid can efficiently promote the selectivity of 4-hydroxy-2-butanone and suppress the self-condensation of acetone. The [N8,8,8,8]OH was identified as the optimal ionic liquid with the selectivity of 91.1% toward 4-hydroxy-2-butanone. The kinetic studies revealed the formation
CRediT authorship contribution statement
Gang Wang: Writing - review & editing. Guangming Cai: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
For this study, we need to thank Prof. C. Li from Institute of Process Engineering, Chinese Academy of Sciences for the guidance and help on basic ionic liquid synthesis.
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