Qibin Xia

A novel MOF/graphene oxide composite GrO@MIL-101 with high adsorption capacity for acetone

A novel composite material GrO@MIL-101 was synthesized using a solvothermal synthesis method. Then the parent materials (MIL-101 and graphene oxide) and the GrO@MIL-101 were characterized using SEM, TEM, XRD, nitrogen sorption, and Raman. The acetone isotherms on the GrO@MIL-101 and MIL-101 were measured separately. The isosteric heat of adsorption and the desorption activation energies of acetone on the two samples were estimated. The results of characterization confirmed the formation of well-defined GrO@MIL-101 with higher surface area and pore volume compared to the MIL-101, and the crystal size of the MIL-101 in the composite was smaller than that of the parent MIL-101. The acetone isotherms on the GrO@MIL-101 were much higher than those on the MIL-101. The acetone adsorption capacity of the GrO@MIL-101 was up to 20.10 mmol g−1 at 288 K and 161.8 mbar, having an increase of 44.4% in comparison with the MIL-101. The desorption activation energy of acetone on the GrO@MIL-101 was higher than that on the MIL-101, indicating the stronger interaction between acetone molecules and the GrO@MIL-101. Consecutive cycles of acetone adsorption–desorption showed that the desorption efficiency of acetone on the GrO@MIL-101 can reach 91.3%. Acetone adsorption on this composite material was highly reversible.

Experimental and molecular simulation studies of CO2 adsorption on zeolitic imidazolate frameworks: ZIF-8 and amine-modified ZIF-8

ZIF-8 has been rapidly developed as a potential candidate for CO2 capture due to its low density, high surface area, and robust structure. Considering the electron-donating effect of amino functional groups, amino-modification is expected to be an efficient way to improve CO2 adsorption of ZIF-8. In this work, grand canonical Monte Carlo (GCMC) simulation was performed to study the CO2 adsorption isotherm based on ZIF-8, ZIF-8-NH2, and ZIF-8-(NH2)2. ZIF-8 was synthesized and CO2 adsorption isotherms based on ZIF-8 was measured. The experimental surface area, pore volume, and CO2 adsorption isotherm were used to validate the force field. Adsorptive capacity of ZIF-8-NH2, and ZIF-8-(NH2)2 were first estimated. The GCMC simulation results indicated that the order of increasing CO2 capacity of the ZIF-8 in the lower pressure regime is: ZIF-8 < ZIF-8-NH2 < ZIF-8-(NH2)2, and in the high pressure is: ZIF-8 < ZIF-8-(NH2)2 < ZIF-8-NH2. New adsorption sites can be generated with the existence of-NH2 groups. In addition, for non-modified and amino-modified ZIF-8, it was the first time to use density functional theory (DFT) calculations to investigate their CO2 adsorption sites and CO2 binding energies. The present work indicates that appropriate amine-functionalized can directly enhanced CO2 capacity of ZIF-8.

Competitive adsorption of water vapor with VOCs dichloroethane, ethyl acetate and benzene on MIL-101(Cr) in humid atmosphere

It is well-known that water vapor is omnipresent. It would inevitably have a negative influence on VOC adsorption on novel porous materials in actual situations. In this work, the competitive adsorption behavior of water vapor with three VOCs, 1,2-dichloroethane (DCE), ethyl acetate (EA) and benzene, on MIL-101 in a humid atmosphere was investigated by isotherm measurement, breakthrough experiments and TPD experiments. The results showed that adsorption capacities of MIL-101 for DCE, EA and benzene were individually up to 9.71, 5.79 and 3.76 mmol g−1, much higher than those of other conventional adsorbents. Breakthrough experiments indicated that the presence of water vapor in the feed stream resulted in a sharp decrease in the VOCs working capacities of MIL-101 due to competitive adsorption of water vapor on MIL-101 surfaces. The breakthrough times and the working capacities of these VOCs became smaller with an increase in the relative humidity. TPD experiments indicated that the desorption activation energies of water vapor, DCE, EA and benzene on MIL-101 were 72.9, 47.14, 41.9, and 38.16 kJ mol−1, respectively. The stronger interaction of water vapor with MIL-101 formed strong competitive adsorption with VOCs on MIL-101, resulting in the sharp decrease of the VOCs working capacities in a humid atmosphere.

Adsorption performance of a MIL-101(Cr)/graphite oxide composite for a series of n-alkanes

The adsorption performance of the MIL-101@GO composite for a series of linear long chain alkanes (from n-pentane to n-octane) was investigated for the first time. The MIL-101@GO composite based on MIL-101(Cr) and graphite oxide (GO) was prepared, characterized and tested for adsorption and desorption of n-alkanes. Isotherms of a series of n-alkanes on MIL-101@GO were measured. Temperature-programmed desorption (TPD) experiments were conducted to estimate desorption activation energies of n-alkanes on MIL-101@GO. Results showed that the adsorption capacities of n-alkanes on MIL-101@GO increased with the hydrocarbon chain length at regions of low pressure, while the trend was reversed at regions of high pressure. The adsorption capacities of n-alkanes on MIL-101@GO were about 1.6–11 times higher than those of conventional activated carbons and the zeolites. The isotherms of n-alkanes could be fitted favorably by the Langmuir–Freundlich equation. The desorption activation energy increased linearly with the carbon number of the n-alkanes. Consecutive cycle experiments of adsorption–desorption showed the isotherms of n-octane in all five cycles were nearly overlapping, suggesting that MIL-101@GO had excellent reversibility of n-alkane adsorption.

Adsorption Isotherms, Kinetics, and Desorption of 1,2-Dichloroethane on Chromium-Based Metal Organic Framework MIL-101

Adsorption equilibrium and kinetics of 1,2-dichloroethane on a chromium-based metal-organic framework MIL-101 were studied. Desorption activation energies of 1,2-dichloroethane on the MIL-101 were measured using temperature program desorption (TPD) experiments. Results showed that the adsorption capacity of the MIL-101 for 1,2-dichloroethane is 19 mmol/g at 288 K, being much higher than those of some activated carbon, zeolite, and MWCNTs. The isotherms of 1,2-dichloroethane were well fitted by the Langmuir equation. The isosteric heat and diffusion coefficients of 1,2-dichloroethane adsorption on the MIL-101 were separately within the range of 42.0–61.6 kJ/mol and range of 0.854–2.246 × 10−10 cm2/s. TPD spectra exhibited two types of adsorption sites on the MIL-101 with desorption activation energy of 48.6 and 87.6 kJ/mol separately. Multiple recycle runs of 1,2-dichloroethane adsorption-desorption at 298 K (10 mbar for adsorption and 0.05 mbar for desorption) showed the 1,2-dichloroethane adsorption on the MIL-101 is highly reversible, and desorption efficiency is up to 98.42%.

Effect of Textural Properties on the Adsorption and Desorption of Toluene on the Metal-Organic Frameworks HKUST-1 and MIL-101

The adsorption and desorption of toluene on metal-organic frameworks HKUST-1 and MIL-101 were studied. The adsorption isotherms and kinetic curves of toluene on HKUST-1 and MIL-101 were separately measured by gravimetric method. Temperature-programmed desorption experiments were carried out for estimating the interaction of toluene with the surfaces of HKUST-1 and MIL-101. Our experimental results showed that the isotherms of toluene on the adsorbents were well fitted by the Langmuir–Freundlich equation. The maximum amount of toluene adsorbed on MIL-101 was 15.1 mmol/g, which was much higher than that adsorbed on HKUST-1 (6.6 mmol/g). The high adsorption capacity of MIL-101 can be attributed to its higher surface area. However, the isotherm of toluene at P/P0 < 0.09 on HKUST-1 was much higher than that on MIL-101 owing to the smaller pore size of HKUST-1. It is suggested that HKUST-1 had better adsorption performance than MIL-101 at low concentration of volatile organic compounds. The desorption activation energies, Ed, of toluene on HKUST-1 and MIL-101 were 43.8 and 30.2 kJ/mol, respectively, meaning that the interaction of toluene with HKUST-1 was stronger than that with MIL-101. In addition, the diffusion coefficient of toluene within HKUST-1 was smaller than that within MIL-101. Results of thermal desorption experiments showed that the desorption efficiency of toluene from HKUST-1 and MIL-101 can separately reach above 93% and nearly 100% at 393 K, respectively.

Effects of Textural Properties and Surface Oxygen Content of Activated Carbons on the Desorption Activation Energy of Water

This work mainly describes investigations of the effects of pore structure and oxygen content of activated carbons on the desorption activation energy, Ed, of water. First, the textural properties and the surface oxygen content as well as the surface acidities of the activated carbons studied were determined by nitrogen adsorption, XPS and Boehm titration methods. The water vapour isotherms of the samples were then measured and temperature programmed desorption (TPD) experiments were conducted to estimate the desorption activation energy, Ed, of water on the activated carbons. The effects of pore structure and surface oxygen content of the activated carbons on the magnitude of Ed are discussed. The results obtained show that the surface acidities of the activated carbons were in direct proportion to their surface oxygen contents, with the value of Ed for water on the activated carbons increasing with increasing surface oxygen content and decreasing pore size of the activated carbons. When the magnitude of the surface acidity was 0.578, 0.436 and 0.338 mmol/g, respectively, the Ed for water had corresponding values of 48.61, 41.67 and 37.22 kJ/mol, respectively. The amount of water vapour adsorbed at lower relative humidity increased with increasing surface acidity, whilst it was dependent on the pore volume at higher relative humidity due to pore filling, i.e. the larger the total pore volume of the activated carbons, the larger their adsorption capacities towards for water vapour.

Estimation of Activation Energy of Desorption of n-Hexanol from Activated Carbons by the TPD Technique

Activated carbon and five kinds of metal-ion-substituted activated carbons, viz. Ag+-activated carbon, Cu2+-activated carbon, Fe3+-activated carbon, Ba2+-activated carbon and Ca2+-activated carbon, were prepared. A model for estimating the activation energy of desorption was established. Temperature-programmed desorption (TPD) experiments were conducted to measure the TPD curves of n-hexanol and hence estimate the activation energy for n-hexanol desorption from the various activated carbons. The results showed that the activation energies for n-hexanol desorption from the Ag+-activated carbon, the Cu2+-activated carbon and the Fe3+-activated carbon were higher than those from the unsubstituted activated carbon, the Ca2+-activated carbon and the Ba2+-activated carbon.

Effects of loading different metal ions on an activated carbon on the desorption activation energy of dichloromethane/trichloromethane.

The effects of loading Fe(3+), Mg(2+), Cu(2+) or Ag(+) on activated carbons (ACs) on interaction of the carbon surfaces with dichloromethane (DCM) and trichloromethane (TCM) were investigated. Temperature-programmed desorption (TPD) experiments were conducted to measure the desorption activation energy of DCM/TCM on the ACs separately doped with ions Fe(3+), Mg(2+), Cu(2+) and Ag(+). The absolute hardness and electronegativity of DCM and TCM were estimated on the basis of density functional theory. The influence of loading the metal ions on the ACs on the interaction of its surfaces with DCM/TCM was discussed. Results showed that the desorption activation energy of DCM and TCM on the modified ACs followed the order: Fe(III)/AC>Mg(II)/AC>Cu(II)/AC>AC>Ag(I)/AC. Both DCM and TCM were hard base. The loading of ion Fe(3+) or Mg(2+) on the surface of the ACs enhanced the interaction between DCM/TCM and the surfaces due to Fe(3+) and Mg(2+) being hard acid, while the loading of ion Ag(+) on the surface of the AC weakened the interaction between DCM/TCM and the carbon surfaces due to Ag(+) being soft acid.

Activation Energy for Dibenzofuran Desorption from Fe3+/TiO2 and Ce3+/TiO2 Photocatalysts Coated onto Glass Fibres:

In this work, TiO2, Fe3+/TiO2 and Ce3+/TiO2 photocatalytic films were respectively immobilized on glass fibres via the sol—gel technique to prepare supported photocatalysts. Temperature programmed desorption (TPD) experiments were conducted to measure the TPD curves for the removal of dibenzofuran from these photocatalysts, from which the activation energy for dibenzofuran desorption from the photocatalyst surfaces was estimated. The results showed that the activation energies for dibenzofuran desorption from the photocatalysts TiO2, Ce3+/TiO2 and Fe3+/TiO2 coated separately onto the glass fibres were 16.41 kJ/mol, 22.55 kJ/mol and 33.59 kJ/mol, respectively, while the hardness values of the ions Fe3+, Ce3+ and Ti4+ were respectively 13.1 eV, 11.9 eV and 10.6 eV. The data indicated that the use of Fe3+ or Ce3+ ions for doping a TiO2 photo-catalyst increased the local hardness of the doped TiO2 photocatalyst surface. This, in turn, increased the activation energy for the desorption of dibenzofuran from such a TiO2 photocatalyst surface.