BACKGROUND: Pentanal self-condensation to 2-propyl-2-heptenal is an important process of carbon chain extension for industrial manufacture of 2-propylheptanol. This reaction in industry employs an aqueous caustic alkali catalyst, causing several problems of equipment corrosion, treatment of alkali wastewater, etc. Herein, several acid-base bifunctional ionic liquids were synthesized for catalyzing pentanal self-condensation.
RESULTS: [PEmim]Cl-0.5Zn(CH 3 COO) 2 exhibited a better catalytic performance, and the yield and selectively of 2-propyl-2-heptenal could reach 82.0% and 86.6%, respectively. The recovered [PEmim]Cl-0.5Zn(CH 3 COO) 2 could be reused three times after being regenerated without a significant loss in its catalytic performance.
The structures and charge properties of [PEmim]Cl-Zn(CH 3 COO) 2 and a complex of [PEmim]Cl-Zn(CH 3 COO) 2 and pentanal were simulated by density functional theory calculation. The results show that the action of acid and base sites of [PEmim]Cl-Zn(CH 3 COO) 2 can effectively activate pentanal by enhancing the electrophilicity or nucleophilicity of different pentanal molecules.
CONCLUSION: The acid-base synergetic effect of [PEmim]Cl-Zn(CH 3 COO) 2 could account for its high catalytic performance. An acid-base synergetic catalysis mechanism of [PEmim]Cl-Zn(CH 3 COO) 2 in pentanal self-condensation was established.
Characterization of ABBILsThe 1 H-NMR spectra were recorded in deuterium oxide (D 2 O) or deuterated chloroform (CDCl 3 ) on a Bruker Avance 400 spectrometer (Bruker, Karlsruhe, Germany) operating at 400 MHz with tetramethyl silane (TMS) as an internal standard.Pyridine was used as an infrared (IR) spectroscopic probe molecule to determine the acidity of ionic liquids. 24 The Fourier-transform infrared (FT-IR) spectra of pyridine and ionic J Chem Technol Biotechnol 2020; 95: 710-718
DFT calculation for insight into synergetic effect of [PEmim]Cl-Zn(CH 3 COO) 2Computational methods are useful ways to gain insight about the behaviors of the catalytic systems. 30 To provide evidence for the high catalytic activity of [PEmim]Cl-Zn(CH 3 COO) 2 , DFT calculation was performed for simulating the structure of [PEmim]Cl-Zn(CH 3 COO) 2 and the interaction between pentanal and [PEmim]Cl-Zn(CH 3 COO) 2 .Geometry optimization of Zn(CH 3 COO) 2 , [PEmim]Cl and [PEmim]Cl-Zn(CH 3 COO) 2 was carried out first. The optimized structures of the earlier substances were confirmed as energy minima by frequency calculation. The ESP surfaces for those structures were constructed and shown in Fig. 2, where the regions of more negative and positive charges were represented by red and blue colors, respectively.The ESP surfaces of [PEmim]Cl and Zn(CH 3 COO) 2 are shown in Fig. 2(a, b). There are two four-coordination structures in J Chem Technol Biotechnol 2020; 95: 710-718