Jianguo Mi

Li-modified metal–organic frameworks for CO2/CH4 separation: a route to achieving high adsorption selectivity

In this work three Li-modified metal–organic frameworks (MOFs) were constructed from MOF-5, by substituting the H atoms with O–Li groups in the organic linkers. A multiscale approach combining grand canonical Monte Carlo (GCMC) simulation and density functional theory (DFT) calculation was adopted to investigate the separation of CO2/CH4 mixtures in these new Li-modified MOFs, as well as in a previously proposed Li-doped MOF-5 for hydrogen storage and the original MOF-5. The results show that the selectivity of CO2 from CH4/CO2 mixtures in Li-modified MOFs is greatly improved, due to the enhancement of electrostatic potential in the materials by the presence of the metals. One of the new Li-modified MOFs, chem-4Li, shows a higher CO2 selectivity than any other known MOFs. Therefore, this work provides a route to improve the separation performance of MOFs for gas mixtures with components that have large differences in dipole and/or quadrupole moments. In addition, the mechanisms for selectivity enhancement in the Li-modified MOFs were elucidated at the molecular level, and we found that the location of doped metals can change the adsorption sites for CO2, and in turn may change the active sites in MOFs when used as catalysts.

A force field for dynamic Cu-BTC metal-organic framework

A new force field that can describe the flexibility of Cu-BTC metal-organic framework (MOF) was developed in this work. Part of the parameters were obtained using density functional theory calculations, and the others were taken from other force fields. The new force field could reproduce well the experimental crystal structure, negative thermal expansion, vibrational properties as well as adsorption behavior in Cu-BTC. In addition, the bulk modulus of Cu-BTC was predicted using the new force field. We believe the new force field is useful in understanding the structure-property relationships for MOFs, and the approach can be extended to other MOFs.

Structure and effective interactions in polymer nanocomposite melts: An integral equation theory study

The polymer reference interaction site model from integral equation theory is used to investigate the structure and effective interactions in polymer nanocomposite melts where strong nanoparticle-monomer interactions are principally considered in this work. For finite particle volume fraction, the compromise for the interference between polymers and nanoparticles results in an optimum particle volume fraction for nanoparticle dispersion in polymer melts. At constant particle volume fraction, the effects of degree of polymerization become insignificant when it reaches a threshold value, below which quantitative effects on the organization states of polymer nanocomposite melts are found and help nanoparticles to well disperse in polymer. The aggregation of large nanoparticles decreases with the increase of the nanoparticle-monomer attraction strength. These observations may provide useful information for the development of new polymer materials.

Modeling of Drug Release from Bioerodible Polymer Matrices

Models for drug release from bioerodible polymer matrices are proposed in this article. We consider that drug is released continually by diffusion that is influenced by polymer chain degradation, and polymer matrix erosion starts and enhances the drug release at a certain time. The models give excellent reproduction of drug release profiles within the whole release period, and the parameters can be correlated to various factors such as γ-irradiation dose, copolymer composition, and initial drug loading, this correlation indicates that the new models can be used to predict the effects of various factors on drug release profiles based on limited experimental data.

Carbon dioxide adsorbent based on rich amines loaded nano-silica.

An easy strategy to obtain an effective carbon dioxide adsorbent based on rich amines functionalized nano-silica was proposed. Polyacrylic acid (PAA), acted as a multi-functional bridge, was firstly immobilized onto the surface of silica nanoparticles. Each carboxylic acid group was subsequently reacted with an amine group of alkylamines, and plenty of remained amines groups could be coated onto silica nanoparticles. As a result, the rich amines loaded nano-silica was fabricated and applied as CO2 adsorbent. The structures and morphologies of amines modified nano-silica were characterized by FTIR, TGA, TEM, and CHNS elemental analysis. Moreover, the effect of molecular weight of PAA and that of alkylamine on CO2 absorption capacity was discussed. As expected, SiO2-PAA(3000)-PEI(10000) adsorbent possessed remarkably high CO2 uptake of approximately 3.8 mmol/g-adsorbent at 100 KPa CO2, 40°C. Moreover, it was found that the adsorbent exhibited a high CO2 adsorption rate, a good selectivity for CO2-N2 separation, and could be easily regenerated.

Prediction of phase behavior of nanoconfined Lennard-Jones fluids with density functional theory based on the first-order mean spherical approximation

The recently proposed first-order mean spherical approximation (FMSA) [Y. Tang, J. Chem. Phys. 121, 10605 (2004)] for inhomogeneous fluids is extended to study the phase behavior of nanoconfined Lennard-Jones fluids, which is consistent with the phase equilibria calculation of the corresponding bulk fluid. With a combination of fundamental measure theory, FMSA provides Helmholtz free energy and direct correlation function to formulate density functional theory, which implementation is as easy as the mean-field theory. Following previous success in predicting density profiles inside slit pores, this work is focused specially on the vapor-liquid equilibrium of the Lennard-Jones fluids inside these pores. It is found that outside the critical region FMSA predicts well the equilibrium diagram of slit pores with the sizes of 5.0, 7.5, and 10 molecular diameters by comparing with available computer simulation data. As a quantitative method, FMSA can be treated as an extension from its bulk calculation, while the mean-field theory is only qualitative, as its bulk version.

Three-Dimensional Density Functional Study of Heterogeneous Nucleation of Droplets on Solid Surfaces

We present a three-dimensional density functional approach to investigate heterogeneous nucleation behaviors of Lennard-Jones fluid on solid walls. In the theoretical calculation, fast Fourier transforms to compute the convolutions of Euler-Lagrange equation enables a high-efficient algorithm in three-dimensional space. The density distributions of a growing nucleus are presented to account for the nucleation process. Accordingly, the structures of nucleated droplet and surrounding supersaturated vapor on different walls are analyzed, and the corresponding free energy barriers and the critical radii are obtained to evaluate the difficulty of droplet formation. Since the theoretical approach is strictly constructed in three-dimensional space, and the liquid-solid, vapor-solid, and vapor-liquid interfacial tensions as well as the vapor-liquid-solid line tension are entirely integrated into the excess free energy expression, the present approach provides a flexible and efficient tool for studying heterogeneous nucleation.

Control of Uniform and Interconnected Macroporous Structure in PolyHIPE for Enhanced CO2 Adsorption/Desorption Kinetics.

The highly uniform and interconnected macroporous polymer materials were prepared within the high internal phase hydrosol-in-oil emulsions (HIPEs). Impregnated with polyethylenimine (PEI), the polyHIPEs were then employed as solid adsorbents for CO2 capture. Thermodynamic and kinetic capture-and-release tests were performed with pure CO2, 10% CO2/N2, and moist CO2, respectively. It has shown that the polyHIPE with suitable surface area and PEI impregnation exhibits high CO2 adsorption capacity, remarkable CO2/N2 selectivity, excellent adsorption/desorption kinetics, enhanced efficiency in the presence of water, and admirable stability in capture and release cycles. The results demonstrate the superior comprehensive performance of the present PEI-impregnated polyHIPE for CO2 capture from the postcombustion flue gas.

An improved renormalization group theory for real fluids

On the basis of Whites theory, an improved renormalization group (RG) theory is developed for chain bonding fluids inside the critical region. Outside the critical region, the statistical associating fluid theory based on the first-order mean sphere approximation [Fluid Phase Equilibria 171, 27 (2000)] is adopted and all the microscopic parameters are taken directly from its earlier application of real fluids. Inside the critical region, the RG transformation for long-range density fluctuation is derived in the k space, which illustrates explicitly the contributions from the mean-field term, the local density fluctuation, and the nonlocal density fluctuation. The RG theory is applied to describe physical behavior of ten n alkanes (C1-C10) both near to and far from the critical point. With no additional parameters for chain bonding fluids, good results are obtained for critical specific heat and phase coexistence curves and the resulting critical exponents are in good agreement with the reported nonclassic values.

Theoretical Study of Phase Transition, Surface Tension, and Nucleation Rate Predictions for Argon

In this work, a weighted density functional theory has been used to study the equilibrium and metastable processes for argon. In the theoretical approach, the two- and three-body interactions of the fluid molecules are considered simultaneously, and the renormalization group transformation is applied to address the long-range fluctuations inside the critical region. The global phase equilibria, planar and curvature-dependent surface tensions, critical radius, and nucleation rates of argon are investigated systematically. The results are in good agreement with the experimental data. Meanwhile, this work applies a methodology for calculating the curved surface tension in local supersaturated environments, showing that the Tolman length is negligible far from the critical region. Near the critical point, however, the Tolman length becomes positive and appears to diverge.