Na Hu

Insights into hydrogen bond dynamics at the interface of the charged monolayer-protected Au nanoparticle from molecular dynamics simulation

The structure and dynamics properties of water molecules at the interface of the charged monolayer-protected Au nanoparticle (MPAN) have been investigated in detail by using classical molecular dynamics simulation. The simulation results demonstrated clearly that a well-defined hydration layer is formed at the interface of MPAN and a stable ion wall consisting of terminal NH3 (+) groups and Cl(-) counterions exists at the outmost region of self-assembled monolayer (SAM) where the translational and rotational motions of water molecules slow considerably down compared to those in the bulk owing to the presence of SAM and ion wall. Furthermore, we found that the translational motions of interfacial water molecules display a subdiffusive behavior while their rotational motions exhibit a nonexponential feature. The unique behavior of interfacial water molecules around the MPAN can be attributed to the interfacial hydrogen bond (HB) dynamics. By comparison, the lifetime of NH3 (+)-Cl(-) HBs was found to be the longest, favoring the stability of ion wall. Meanwhile, the lifetime of H2O-H2O HBs shows an obvious increase when the water molecules approach the Au core, suggesting the enhanced H2O-H2O HBs around the charged MPAN, which is contrary to the weaken H2O-H2O HBs around the neutral MPAN. Moreover, the HB lifetimes between water molecules and the ion wall (i.e., the Cl(-)-H2O and NH3 (+)-H2O HBs) are much longer than that of interfacial H2O-H2O HBs, which leads to the increasing rotational relaxation time and residence time of water molecules surrounding the ion wall. In addition, the corresponding binding energies for different HB types obtained from the precise density functional theory are in excellent accordance with above simulation results. The detailed HB dynamics studied in this work provides insights into the unique behavior of water molecules at the interface of charged self-assemblies of nanoparticles as well as proteins.

Tetrabutylphosphonium amino acid ionic liquids as efficient catalysts for solvent-free Knoevenagel condensation reactions

Five tetrabutylphosphonium amino acid ionic liquids ([P4444][AA]) were prepared, characterized, and used as catalysts for solvent-free Knoevenagel condensation reactions. The tetrabutylphosphonium prolinate ([P4444][Pro]) showed excellent catalytic activity and selectivity in Knoevenagel condensation reactions of active methylene compounds with various aromatic aldehydes, and all the yields of corresponding products were more than 85% under mild conditions. Furthermore, a plausible reaction mechanism for the excellent performance of [P4444][Pro] has been proposed, and [P4444][Pro] could be used repetitively at least six times without obvious decrease in activity and quantity.

Kinetics Study of the Esterification of Acetic Acid with Methanol using Low-Corrosive Brønsted Acidic Ionic Liquids as Catalysts

The kinetics for the esterification of acetic acid with methanol was studied systemically in the presence of low-corrosive Brønsted acidic ionic liquids (BAILs) as catalysts. The effect of various parameters such as catalyst type, agitation speed, temperature, catalyst loading, and molar ratio of the reactants on the conversion of acidic acid was studied in detail to obtain the optimal reaction conditions. Moreover, the pseudo-homogeneous (PH) kinetic model was utilized successfully to correlate the experimental data in the temperature range from 323.15 to 343.15 K. Thus, a complete kinetic equation for describing this esterification system was developed that provides to obtain the kinetic information for the simulation and design of catalytic distillation column in the synthesis of methyl acetate.