Mechanistic insight into the roles of anions and cations in the degradation of poly(ethylene terephthalate) catalyzed by ionic liquids.

Ionic liquids (ILs) have shown high catalytic activity in the degradation of poly(ethylene terephthalate) (PET), but the effects of the anions and cations, as well as the mechanism, remain ambiguous. Glycolysis is an important recycling method that converts waste PET into monomers through various chemical reactions. To reveal the role of ILs and the molecular mechanism of the glycolysis of PET, density functional theory (DFT) calculations have been carried out for the possible pathways for the generation of bis(hydroxyethyl)terephthalate (BHET) catalyzed by isolated anions/cations and ion pairs at different sites. The pathway with the lowest barrier for the glycolysis of PET is the cleavage of the C-O ester bond, which generates the BHET monomer. The synergistic effects of the cations and anions play a critical role in the glycolysis of PET. The cations mainly attack the carbonyl oxygen of PET to catalyze the reaction, and the anions mainly form strong H-bonds with PET and ethylene glycol (EG). In terms of the mechanism, the H-bonds render the hydroxyl oxygen of EG more electronegative. The cation coordinates the carbonyl oxygen of the ester, and the hydroxyl oxygen of EG attacks the ester group carbon of PET, with proton transfer to the carbonyl oxygen. A four-membered-ring transition state would be formed by PET, EG, and the IL catalyst, which regularly accelerates the degradation of PET. These results provide fundamental help in understanding the roles of ILs and the mechanism of IL-catalyzed PET degradation.

Influence of the auxiliary acceptor and π-bridge in triarylamine dyes on dye-sensitized solar cells.

Four metal-free organic dyes (AFL7-AFL10) based on triarylamine donors and different π-bridges with or without a benzothiadiazole auxiliary acceptor have been synthesized. The dyes' photophysical and electrochemical properties, theoretical calculations, and dye-sensitized solar cell (DSSC) performances have been investigated. Electrochemical measurement data indicate that varying the π-conjugacy system can tune the HOMO and LUMO energy levels. A high molar extinction coefficient combined with a broad absorption spectrum helps to reinforce the dyes' light harvesting ability; therefore, it could help to increase the JSC of the DSSCs. Among the four dyes, AFL7 bearing the bithiophene bridge achieves the best photocurrent performance with a JSC value of 16.94 mA cm-2, corresponding to an overall conversion efficiency η of 7.92% under standard AM 1.5 G conditions (that of N719 dye was 8.53% under the same conditions). The results show that the dyes based on the triarylamine donor containing fluorenyl and biphenyl moieties are promising candidates for improving the performance of DSSCs.

Practical Asymmetric Synthesis of Sitagliptin Phosphate Monohydrate.

Optically pure sitagliptin phosphate monohydrate is efficiently and practically synthesized through a chiral hemiacetal as the key intermediate in 54% overall yield starting from (E)-4-(2,4,5-trifluorophenyl)but-2-enal and N-boc-protected hydroxylamine. The chiral hemiacetal fragment is constructed by a tandem aza-Michael/hemiacetal reaction catalyzed by an organocatalyst and the influence of acidity of Brønsted acid on tandem aza-Michael/hemiacetal reaction is researched in detail.

Probing the mechanism of the conversion of methyl levulinate into γ-valerolactone catalyzed by Al(OiPr)3 in an alcohol solvent: a DFT study.

Biomass-derived γ-valerolactone (GVL) is a versatile chemical that can be used in various fields. As an efficient, cheap, and sustainable catalyst, Al(OiPr)3 has been successfully used in the conversion of methyl levulinate (ML) to GVL in the solvent isopropanol (IPA). However, the molecular mechanism of this conversion catalyzed by Al(OiPr)3 remains ambiguous. To investigate the mechanism of the conversion of ML to GVL catalyzed by Al(OiPr)3, the reaction pathways, including the transesterification, Meerwein-Ponndorf-Verley (MPV) hydrogenation, and ring-closure steps, were probed using density functional theory (DFT) calculations at the M062X-D3/def2-TZVP level. Among the elementary steps, it is found that ring-closure is the rate-determining step and that Al3+ can coordinate with the oxygen of 2-hydroxy-isopropyl levulinate (2HIPL) to catalyze the last ring-closure step. A four-centered transition state can be formed, and Al(OiPr)3 shows a strong catalytic effect in the two steps of the ester exchange reaction. The center of Al(OiPr)3 mainly coordinates with the carbonyl oxygen atom of the ester to catalyze the reaction. The present study provides some help in understanding the conversion mechanism of ML to GVL and designing more effective catalysts for use in biomass conversion chemistry.

Cellulose Regeneration in Imidazolium-Based Ionic Liquids and Antisolvent Mixtures: A Density Functional Theory Study.

Cellulose can be dissolved in ionic liquids (ILs), and it can be recovered by adding antisolvent such as water or alcohol. In addition, the regenerated cellulose can be used for textiles, degradable membranes, hydrogels/aerogels, etc. However, the regenerated mechanism of cellulose remains ambiguous. In this work, density functional theory (DFT) calculation is reported for the cellulose regeneration from a cellulose/1-n-butyl-3-methylimidazolium acetate (BmimOAc)/water mixture. To investigate the microscopic effects of the antisolvents, we analyzed the structures and H-bonds of BmimOAc-nH2O and cellobiose-ILs-nH2O (n = 0-6) clusters. It can be found that when n ≥ 5 in the BmimOAc-nH2O clusters, the solvent-separated ion pairs (SIPs) play a dominant position in the system. With the increasing numbers of water molecules, the cation-anion interaction can be separated by water to reduce the effects of ILs on cellulose dissolution. Furthermore, the BmimOAc-nH2O and cellobiose-ILs (n = 0-6) clusters tend to be a more stable structure with high hydration in an aqueous solution. When the water molecules were added to the system, H-bonds can be formed among H2O, the hydroxyl of cellulose, and the oxygen of OAc. Therefore, the interactions between cellulose and ILs will be decreased to promote cellulose regeneration. This work would provide some help to understand the mechanism of cellulose regeneration from the view of theoretical calculation.

Fischer-Helferich glycosidation mechanism of glucose to methyl glycosides over Al-based catalysts in alcoholic media.

The Fischer-Helferich glycosidation reaction is generally the initial step in the conversion of glucose to levulinate in alcohol media. However, the relevant molecular mechanism catalyzed by Al-based catalysts is still not well understood. In this work, the reaction mechanism of the glycosidation from glucose to methyl glycosides catalyzed by Al3+ coordinated with methanol/methoxyl was investigated through density functional theory (DFT) calculations. The whole reaction process includes ring-opening, addition, and ring-closure events. The addition of methanol to the ring-opening structure of glucose makes the electronegativity of C1 site stronger to proceed with the following ring-closure reaction. Among the 28 kinds of ways of ring-closure reaction, the most preferred way is to close the loop through the six-membered ring (O5-C1) to generate methyl glucoside (MDGP). The rate-determining step is the ring-closure and the Al3+ shows a great catalytic effect which is mainly reflected in coordinating with the solvents to transfer protons. The results would be helpful to understanding the Fischer-Helferich glycosidation mechanism catalyzed by Al-based catalysts and comprehend the conversion of glucose to high value-added chemicals.