Selective removal of thiophene from aromatic components is one of the key challenges facing the petrochemical industry. The adsorption and separation mechanism from the molecular viewpoint can guide and upgrade the relative adsorption based technology. Therefore, we performed the grand canonical ensemble Monte Carlo (GCMC) simulation to investigate the adsorption performance and mechanism of competitive adsorption. Density distribution and radial distribution functions (RDF) analysis give a more detailed description of the adsorption sites. For pure component adsorption, donut-shaped adsorption sites were obtained for both benzene and thiophene from the straight channel point. From the viewpoint of the zigzag channel, the sorbates follow the straight line shape distribution at low loading and the S shape distribution at high loading. As for the binary component adsorption, more benzene adsorbs in the zeolite than thiophene at low pressure; however, thiophene competes successfully at high pressure. This can be explained by the key factor: at low pressure, the size effect plays an important role. While the pressure increases, the interaction energy dominates the process. Analyzing RDFs of the binary adsorption, we observed that when benzene competes with thiophene, the preferential adsorption sites do not change; however, the emergence possibility of benzene gets smaller.

•Alkylation mechanism of o-xylene with styrene catalyzed by AlCl3-ionic liquid system was studied in DFT method.•The reaction pathway is consisted of CC coupling and a hydrogen shift, and their corresponding transition states are located.•The reactive energy catalyzed by superelectrophilic AlCl2+ is distinctly lower than that by AlCl3 in ionic liquid.To explore sustainable catalysts with innovative mechanisms, the alkylation mechanism of o-xylene with styrene was studied using DFT method in AlCl3-ionic liquid catalytic system. The reaction pathway was consisted of CC coupling and a hydrogen shift, in which two transition states were found and further discussed. The reactive energy catalyzed by superelectrophilic AlCl2+ (12.6 kcal/mol) was distinctly lower than AlCl3 (43.0 kcal/mol), which was determined as the rate-determining step. Mulliken charge along IRC gave a comprehensive understanding of charge distribution and electron transfer in dynamic progress. Bond orders and AIM theory were used to study the nature of chemical bonds and the driving forces in different reaction stages.Alkylation mechanism of o-xylene with styrene is studied using DFT method in AlCl3-ionic liquid catalytic system. Two transition states corresponding to CC coupling and hydrogen shift are located and further investigated. The reactive energy catalyzed by AlCl2+ (12.6 kcal/mol) is lower than AlCl3 (43.0 kcal/mol). The nature of reactions is also studied in different stages.Download high-res image (153KB)Download full-size image

Multiple techniques were used to study the reversibility of a series of imido-based ionic liquids (ILs). DFT (density functional theory) modeling originally indicated that methyl transfer favorably took place at the unsaturated CX bond in the N–CX (N, O and S) fragment. A series of imido-based ILs derived from the N–CX (N, O and S) fragment were studied and characterized using theoretical and experimental methods. Seven imido-based ILs were facilely synthesized in the experiment, which is consistent with the lower energy barriers obtained in the potential energy surface (PES) compared to the other ILs. Their structures were measured in nuclear magnetic resonance (NMR) spectra. The thermal stabilities were further studied by thermogravimetric analysis (TGA). The bond order results indicated that non-covalent interactions were the major driving force in the methyl transfer process. Non-covalent interactions in these ILs were investigated and characterized using atoms in molecules (AIM), reduced density gradient (RDG) and natural bond orbital (NBO) methods.

•Cellulose dissolution mechanism in ILs was studied in experiment and DFT modeling.•NMR spectra indicated hydrogen bonds were the driving force of dissolution.•Hydrogen bonds were studied and characterized in AIM, RDG and NBO method.Cellulose dissolution mechanism in acetate-based ionic liquids was systematically studied in Nuclear Magnetic Resonance (NMR) spectra and Density Functional Theory (DFT) methods by using cellobiose and 1-butyl-3-methylimidazolium acetate (BmimAc) as a model system. The solubility of cellulose in ionic liquid increased with temperature increase in the range of 90–140 °C. NMR spectra suggested OAc− preferred to form stronger hydrogen bonds with hydrogen of hydroxyl in cellulose. Electrostatic potential method was employed to predict the most possible reaction sites and locate the most stable configuration. Atoms in molecules (AIM) theory was used to study the features of bonds at bond critical points and the variations of bond types. Simultaneously, noncovalent interactions were characterized and visualized by employing reduced density gradient analysis combined with Visual Molecular Dynamics (VMD) program. Natural bond orbital (NBO) theory was applied to study the noncovalent nature and characterize the orbital interactions between cellobiose and Bmim[OAc].NMR spectra suggested hydrogen bonds are the driving force of cellulose dissolution in acetated-based ionic liquids, which was further studied and characterized by DFT modeling.

•Proton transfer mechanisms in ionic liquids were studied in DFT and ATR-IR spectra.•Transition state obtained in DFT modelling well showed the proton transfer process.•NBO and AIM methods were applied to further study bond features.In this study, the proton transfer of three protic ionic liquids (PILs) pyrrolidinium acetate ([Pyrrol]OAc), diethylammonium acetate ([DEA]OAc) and bis-(2-methoxyethyl)-ammonium acetate ([BMOEA]OAc) were investigated. At first, the structures of the ion-pairs and molecular pairs of these PILs were optimized at B3LYP/6-311 ++G(d,p) level. The interaction energy between anions and cations was also obtained. The proton transfer processes were verified by intrinsic reaction coordinate (IRC) pathways tracing to the energy profiles connecting the transition state (TS) to the two desired minima, i.e. ion pair and molecular pair. The experimental attenuated total reflection (ATR) FTIR spectra of these PILs at room temperature were determined and compared with the results calculated at B3LYP/6-311 ++G(d,p) level. Vibrational mode analyses (VMA) for [Pyrrol]OAc found that δ(NH) has an imaginary frequency (− 147.3 cm− 1), which is accounted for proton transfer from [NH2]+ to OAc−. Natural bond orbital (NBO) analyses pointed out that second order perturbation stabilization energy of (E(2)) of LP(N1) → σ*(O2-H5) was much larger than that of other orbitals, and should be the symmetrical matching with the maximum overlap and the minimum gap (0.73 au). The hybridized index of N atom is varied from sp3.65 in ionic pair to sp4.49 in TS. The constituent of s orbital decreases 3.3% and the length of N1-H5 increases from 1.02 Å in ionic pair to 1.65 Å in TS, and the symmetric stretching vibration takes place the red shift. It could be explained that the N1-O2-H5 played an important role in the stabilization of molecular pair. The electron density ρ(r) and the Laplacian of the electron density ∇2ρ(r) derived from atoms in molecules (AIM) analyses were used to describe the intensity and characteristic of a bond. The results indicate that a very strong interaction of the hydrogen bonds exists in the ion-pair geometries and the bonds are the covalent bond.The proton transfer process in protic ionic liquids was investigated in density functional theory and ATR-IR spectra.

Carbon dioxide capture by amine-functionalized ionic liquids (IL), 1,2-dimethyl-(3-aminoethyl)imidazolium fluoride ([aEMMIM][F]), [aEMMIM][Cl], [aEMMIM][Br], and [aEMMIM][I] were synthesized and characterized by both DFT simulation and experimental methods. The most stable geometrical parameters of structures in this study were optimized at the B3LYP/6-311++G(d,p) level by employing the Gaussian09 program. The results showed that CO2 can be chemically captured in ILs by forming carbamic acid with a 1:1 molar ratio stoichiometry. DFT simulations were performed to investigate the configuration variations of the reactants, intermediates, transition states and products, as well as energy barriers and vibration frequency changes in the gas phase using the conductor-like polarizable continuum model (CPCM) in an aqueous solution. The vibration frequency obtained in DFT simulation was consistent with the experimental result by employing a scaling factor. AIM and NBO analysis were also carried out to investigate the nature and features of the studied structures at the molecular level.

•Radioiodine removal mechanism by bare anions and corresponding 1-butyl-3-methyl-imidazolium cation ([Bmim]+) based ILs are investigated in both theoretical and experimental methods.•[Bmim][Br], [Bmim][I] and [Bmim][Cl] are better candidates for radioiodine removal for their removal efficiencies of over 80% in 5 h.•Investigation indicates intramolecular interaction in IL will weaken its radioiodine removal capacity.In order to remove and store radioactive substances effectively, studies on the mechanisms of radioiodine captured by ionic liquids (ILs) with a fixed cation (1-butyl-3-methyl-imidazolium cation [Bmim]+) were carried out in experimental and theoretical methods. Fourier transform infrared attenuated total reflectance (FT-IR ATR) spectra of 2BP8HQ and ultraviolet–visible (UV/vis) spectroscopy were used to investigate the kinetic process of radioiodine removal by ILs in experiment. Corresponding theoretical investigations on the structures and formation mechanisms of ILs, bare anions and complexes as well as hydrogen bonds was carried using density functional theory. The electrostatic potential was used in configuration design and construction. Charge distribution was used to show the variation of atom charge density, Interaction energy and vibration frequency change were performed to explore possible mechanisms on the halogen bond formation between radioiodine molecule and bare anion or anion in ILs when radioiodine captured by ILs. In order to characterize halogen bonds both natural bond orbital analysis and atoms in molecules analysis were performed. Both experimental and computational results showed that radioiodine could be captured by ILs with a 1:1 mol stoichiometry. It was noteworthy that [Bmim][Br], [Bmim][I] and [Bmim][Cl], containing high radioiodine capture efficiency anions, were better candidates in removal and reliable storage of radioiodine for their capture efficiencies of over 80% in 5 h.The electrostatic potential surface (ESP) of the complexes of I2 with [Bmim][Br] illustrates halogen bond are generated when radioiodine is captured by [Bmim][Br]. The efficiency of iodine capture by ILs depends on halogen bond strength, which has been confirmed by both theoretical and experimental methods. [Bmim][Br], [Bmim][I] and [Bmim][Cl], containing the high radioiodine removal efficiency anions ([Br]−, [I]− and [Cl]−), are better candidates for removal and reliable storage of radioactive iodine for their removal efficiencies of over 80% in 5 h.

We use density functional theory to examine structure–activity relationships of small vanadia clusters supported on anatase TiO2(001) and rutile TiO2(110) surfaces. A thermodynamic analysis indicates that the vanadia monomer cluster can be stabilized on the anatase TiO2(001) surface in a catalytically relevant oxygen environment. On the other hand, vanadia clusters tend to aggregate into dimers on the rutile TiO2(110) surface because this surface binds the monomer less strongly as compared to anatase. Hydrogen adsorption is found to be exothermic on the vanadia monomer adsorbed on both supports, enhanced by a charge transfer between the adsorbate and the substrate. There is no such charge transfer on vanadia dimers and tetramers, where the hydrogen adsorption energies are similar to that on the single crystal V2O5(001) surface. The improved catalytic performance of the anatase support can be attributed to the ability of this surface to stabilize the catalytically active vanadia monomer clusters.

The rearrangement mechanisms of the novel Baeyer–Villiger oxidation (BVO) of benzaldehyde and acetaldehyde have been studied using density functional theory methods. All structures associated with the product formation step of the new Criegee intermediate, α-hydroxyalkoxy-λ3-bromane, are reported. B3LYP/6-31++G** calculations give a good description for the group shift of these two typical reactants: phenyl shift is easier than hydrogen shift for benzaldehyde; hydrogen migration is more favorable than methyl migration for acetaldehyde. Different mechanisms and various conformers of the novel BVO reaction have been considered for the migration step. Solvent effects and rate constants are also taken into account. The calculated and experimentally observed branching ratios are in good agreement with each other.

Alkaline earth-metal cation-exchanged faujasite zeolites have received widely attention, especially in catalyzing the reaction of oxygen-containing organic compounds. As a common one-carbon electrophile in the organic synthesis, preservation and reaction of formaldehyde were limited by its low boiling point and polymerization tendency. In this paper, a classical carbonyl-ene reaction between formaldehyde and propylene was examined on the MgY zeolite catalyzed system and the bare system by ONIOM2 (B3LYP/6-31G(d,p):UFF) method and density-functional (B3LYP/6-31G(d,p)) calculation, respectively. It was found that reaction energy barrier on the MgY zeolite was 15.4 kcal/mol, lower than the bare system (26.1 kcal/mol). And the adsorption energy of formaldehyde on the MgY zeolite have reached 27.5 kcal/mol which illustrate that formaldehyde can be successfully preserved by MgY zeolite in a monomer at the ambient temperature. In addition, some thermodynamic constants (ΔH and ΔG) and rate constants (k) were also analyzed.Graphical abstractHighlights► Adsorption of formaldehyde on MgY zeolite is stable. ► The energy barrier of carbonyl-ene reaction between formaldehyde and propylene on MgY is lower than the bare system. ► MgY zeolite can successfully preserve formaldehyde and catalyze carbonyl-ene reaction. ► All calculation by DFT/ONIOM2.

A systematic computational study was carried out to characterize the hydrogen bonding of complexes formed between formamide and cytosine by DFT calculations. The computations were performed mainly with the B3LYP/6-311++G(d,p) level. Seven stable cyclic structures are found on the potential energy surface, in which four structures have two normal hydrogen bonds and the others have only one normal hydrogen bond with a very weak hydrogen bond that can be neglected. In the four structures with two normal hydrogen bonds, two have seven-membered rings, and the others have an eight-membered ring. The eight-membered ring is preferred to the seven-membered one by analyzing the hydrogen bond lengths and the interaction energies. The infrared spectrum frequencies, vibrational frequency shifts and charge number are also reported.

The rearrangement mechanisms of the novel Baeyer–Villiger oxidation (BVO) of benzaldehyde and acetaldehyde have been studied using density functional theory methods. All structures associated with the product formation step of the new Criegee intermediate, α-hydroxyalkoxy-λ3-bromane, are reported. B3LYP/6-31++G** calculations give a good description for the group shift of these two typical reactants: phenyl shift is easier than hydrogen shift for benzaldehyde; hydrogen migration is more favorable than methyl migration for acetaldehyde. Different mechanisms and various conformers of the novel BVO reaction have been considered for the migration step. Solvent effects and rate constants are also taken into account. The calculated and experimentally observed branching ratios are in good agreement with each other.