Xiaoqian Yao

Understanding Structures and Hydrogen Bonds of Ionic Liquids at the Electronic Level

Due to their unique properties, ionic liquids (ILs) have attracted the academic and industrial attentions. However, recent controversies have focused on what are the main forces to determine the behaviors of ILs. In this work, a detailed DFT calculation was carried out to investigate the intermolecular interactions in two typical ILs, [Emim][BF(4)] and [Bmim][PF(6)]. The results indicate that hydrogen bonds (H-bonds) are the major intermolecular structural feature between cations and anions. Although the electrostatic force remains the major noncovalent force (70% of the total energy by energy decomposition calculation), the interaction energies calculated at different theoretical levels indicate that H-bond and van der Waals interactions cannot be ignored. However, the H-bonded capacities from natural bond orbital (NBO) delocalization energies do not show the consistent changes in the total interaction energies and number of H-bonds. Based on the canonical orbitals analysis, it is found that the σ-type orbital overlap and the partial charges transfer between anion and cation, finally, result in the significant energy reduction and rationalize the preferable location of anion, which is an essential understanding for the interaction and structure in the ion pair. Additionally, the strong agreement between the experimental IR spectra and the calculated vibrations implies that the structures of the larger ion clusters provide a reasonable depiction for bulk ILs at room temperature condition.

Urea as an efficient and reusable catalyst for the glycolysis of poly(ethylene terephthalate) wastes and the role of hydrogen bond in this process

Metal salts and solid super acids are the common catalysts used in the glycolysis of poly(ethylene terephthalate)(PET) wastes to obtain monomer bis(hydroxyethyl) terephthalate (BHET) with high conversion of PET and selectivity of BHET, but there are some drawbacks, such as severe reaction conditions, slow reaction rates and difficult recycling of the catalyst. In this study, urea, as a green, highly active, low-priced, extensive and easily produced catalyst, has been used for the degradation of PET to give BHET. The experimental results show that the catalyst has a high catalytic activity, fast reaction rate and is easily separated from the product. Then the effects of experimental parameters on the conversion of PET and the selectivity of BHET were investigated. The conversion of PET wastes is able to reach up to 100% under the conditions of m(PET wastes) : m(ethylene glycol (EG)) : m(urea) = 1 : 4 : 0.1, atmosphere pressure at 160 °C for 2.5 h. The recycling experiments showed that urea worked efficiently, even though it has been used ten times. In addition, the mechanism was proposed through in situ IR, density functional theory (DFT) calculations and experimental results. It shows that the hydrogen bonds (H-bonds) formed between EG and urea play a key role. Following on from this, five tetraalkylammonium-based amino acid-functionalized ionic liquids were synthesized and were used in the catalytic glycolysis of PET. The experimental results and DFT calculations further verify the important role of H-bonds in the glycolysis process of PET catalyzed by this type of carbonyl-containing catalysts.

The Hydrogen-Bonding Interactions between 1-Ethyl-3-Methylimidazolium Lactate Ionic Liquid and Methanol*

1-Ethyl-3-Methylimidazolium lactate ([EMIM][LAC]) is an environmental friendly ionic liquid with potential industrial applications. Attenuated total reflectance infrared spectroscopy (ATR-IR) and density functional theory (DFT) calculations were employed to investigate the molecular interactions between methanol and [EMIM][LAC]. The infrared spectra were analyzed by two methods: excess spectroscopy and two-dimensional (2D) correlation spectroscopy. In the ATR-FTIR spectra, v(C4,5-H), v(C2-H), v(alkyl), v(-OD), and v(-COO) all show blue shifts upon addition of methanol. 2D correlation analysis indicated that the v(imidazolium ring C-H) band varies before that of v(alkyl C-H) with increasing CD3OD content. The following sequential order of interaction strength is established by DFT calculations: EMIM-methanol -LAC > EMIM-LAC > LAC-methanol > EMIM-methanol.

Deep eutectic solvents as highly active catalysts for the fast and mild glycolysis of poly(ethylene terephthalate)(PET)

Deep eutectic solvents (DESs) have attracted broad attention due to their low cost, easy preparation, low toxicity, good biological compatibility and similar characteristics to those of ionic liquids (ILs). In this study, we found that not only the glycolysis time is sharply shortened under mild reaction conditions, but also the high selectivity of monomer bis(hydroxyalkyl) terephthalate (BHET) is obtained when DESs were used as catalysts. Then, the influences of technological parameters on PET degradation were investigated and the optimization conditions were obtained. Under the optimization conditions of ethylene glycol (EG) (20 g), catalyst (n(urea)/n(ZnCl2) 4/1, 0.25 g), PET (5 g), and atmospheric pressure at 170 °C for 30 min, the conversion of PET and selectivity of BHET were 100% and 83%, respectively. This time is equal to that taken by a supercritical method under 15.3 MPa at 450 °C. In addition, the degradation mechanism of PET wastes catalyzed by DESs is proposed through the experiments and DFT calculations. The high catalytic activity is attributed to the synergetic catalysis of H-bonds and coordination bonds formed between the DES catalyst and EG.

1,3-Dimethylimidazolium-2-carboxylate: a zwitterionic salt for the efficient synthesis of vicinal diols from cyclic carbonates

The development of efficient, cheap and recyclable catalysts for reactions under mild reaction conditions is a very attractive topic in green chemistry. Herein, a series of basic ionic liquids (ILs) were investigated as catalysts for the synthesis of vicinal diols via the hydrolysis of cyclic carbonates in order to improve this kind of synthetic process. The effects of the IL structure, the molar ratio of cyclic carbonate to water, and various reaction parameters on the catalytic performance were investigated in detail. It was found that 1,3-dimethylimidazolium-2-carboxylate, a simple halogen-free zwitterionic catalyst, showed high activity (a space-time yield of 1086 h−1) and excellent selectivity for the preparation of ethylene glycol via the hydrolysis of ethylene carbonate. The catalyst could be reused over six times without obvious loss of catalytic activity. Also, it was applicable to a variety of cyclic carbonates for the production of their corresponding vicinal diols with high yields and selectivities. A possible catalytic cycle for this kind of catalytic process was proposed based on the experimental results, NMR spectroscopy and theoretical calculations. This reaction protocol opens a new possibility for chemical synthesis as a substitution for traditional base or basic ILs.

Boron-doped melamine-derived carbon nitrides tailored by ionic liquids for catalytic conversion of CO2 into cyclic carbonates

A new approach to tailoring edge active groups of graphitic carbon nitrides for catalytic conversion of CO2 into cyclic carbonates was proposed in this work. To improve the catalytic performance, boron-doped melamine-derived graphitic carbon nitrides (MCNB) with numerous exposed edge defects were prepared by using 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) as the soft template and boron source. Different mass ratios of BmimBF4 to melamine were explored for MCNB preparation, MCNB(x) (x is the mass ratio) with different polymerization degree, pore structure and boron doping content were obtained, and the relationship between MCNB properties and the corresponding catalytic activity was then investigated. With low polymerization degree, abundant meso-macroporous structure and small amounts of boron (<1.59 atm%) doped in the skeleton, MCNB(0.01) exhibited better catalytic performance and could be suitable for various epoxide substrates with the yield of cyclic carbonates up to 89.0% at 130 °C in 6 h. According to the XPS analysis and DFT calculation results, the active centers were confirmed to be the partially-condensed amino groups in edge defects, which were enhanced by moderate doping of boron in the skeleton.

Enhanced delignification of cornstalk by employing superbase TBD in ionic liquids

The delignification of cornstalk was efficiently accomplished by using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as an additive in 1-allyl-3-methylimidazolium acetate ([Amim][OAc]). When 1.0 wt% TBD was added to [Amim][OAc], the cellulose and lignin contents of cellulose rich material (CRM) were achieved to be 39.12% and 6.74%, respectively. With the addition of 0.1 wt% TBD to [Amim][OAc], the lignin content of CRM could even be reduced to 2.06% without considering the cellulose content. There could be two possible reasons for the enhanced delignification of cornstalk by adding TBD in [Amim][OAc]. One is the alkalinity and exposed nitrogen atoms of TBD, which make it an efficient dibasic nucleophile and helpful for the lignin β-O-4 ether bond cleavage reaction. The other one is the decreased interaction energy of [Amim]+ and [OAc]− from 99.1 kcal mol−1 to 89.2 kcal mol−1 with the addition of TBD, which makes the [Amim]+ and [OAc]− easier to interact with the cornstalk components. Simultaneously, the CRM regenerated from the system of [Amim][OAc] + TBD was effectively hydrolyzed by cellulase with 98% enzymatic hydrolysis yield, which proved that the cellulose structures were highly disrupted and lignin was significantly removed in the CRM.

Theoretical studies on glycolysis of poly(ethylene terephthalate) in ionic liquids

Ionic liquids (ILs) present superior catalytic performance in the glycolysis of ethylene terephthalate (PET). To investigate the microscopic degradation mechanism of PET, density functional theory (DFT) calculations have been carried out for the interaction between ILs and dimer, which is considered to symbolize PET. We found that hydrogen bonds (H-bonds) play a critical role in the glycolysis process. In this study, 24 kinds of imidazolium-based and tertiary ammonium-based ILs were used to study the effect of different anions and cations on the interaction with PET. Natural bond orbital (NBO) analysis, atoms in molecules (AIM) and reduced density gradient (RDG) approaches were employed to make in-depth study of the nature of the interactions. It is concluded that the interaction of cations with dimer is weaker than that of anions and when the alkyl chain in the cations is replaced by an unsaturated hydrocarbon, the interaction will become stronger. Furthermore, anions play more important roles than cations in the actual interactions with dimer. When the hydrogen of methyl is replaced by hydroxyl or carboxyl, the interaction becomes weak for the amino acid anions and dimer. This work also investigates the interaction between dimer and ion pairs, with the results showing that anions play a key role in forming H-bonds, while cations mainly attack the oxygen of carbonyl and have a π-stacking interaction with dimer. The comprehensive mechanistic study will help researchers in the future to design an efficient ionic liquid catalyst and offer a better understanding of the mechanism of the degradation of PET.

Ionic liquids tailored and confined by one-step assembly with mesoporous silica for boosting the catalytic conversion of CO2 into cyclic carbonates

On the basis of economic and energy-saving criteria, the minimum effective dose of ionic liquids (ILs) for the catalytic conversion of CO2 into cyclic carbonates was first explored by confinement. With one-step assembly of mesoporous silica (mSiO2) by using a fixed amount of silicon source with varying amounts of ILs as templates, certain amounts of 1-ethyl-3-methyl imidazolium bromides (EmimBr) were tailored and confined in mSiO2. The confined ILs (EmimBr@mSiO2) retained the advantages of homo- and heterogeneous catalysts, exhibiting higher performance than bulk EmimBr under identical reaction conditions. Amongst all the prepared materials, EmimBr@mSiO2 with the lowest amount of EmimBr (6.9 wt%) exhibited the biggest improvement in catalytic activity, achieving a TOF of 112.6 h−1 which is almost 1.7 times the bulk phase with good recyclability. The boosted catalytic activity could be attributed to the larger proportion of mesopores, the better dispersity of EmimBr (Si/Br = 25) and the synergistic effect from more exposed silanol groups (Si–OH/Br = 8) in the structure. The good recyclability was then explained by the XPS analysis and density functional theory (DFT) calculation, which confirmed that the compressing effect from Si–OH could enhance the cation and anion interaction to stabilize ILs in the space more firmly when less ILs were confined.