Difference Of Isosteric Heats Research Articles

A hydrophobic and hierarchical porous resin-based activated carbon modified by g-C3N4 for toluene capture from humid conditions

The synthesis of hydrophobic and hierarchical porous activated carbon (AC) used for toluene capture from a humid atmosphere still remains a challenge. In this work, a novel hydrophobic and hierarchical porous AC was prepared from resorcinol-formaldehyde resin spheres using g-C3N4 as a new pore-forming agent. By the proper participation of g-C3N4, the obtained adsorbent GNHRC-1.5 was endowed with an excellent toluene adsorption capacity (10 mmol/g) under low pressure of P = 0.2 kPa, which was 54% higher than that of nonmodified resin-based AC. By DIH (Difference of Isosteric Heats) prediction, the adsorption selectivity of GNHRC-1.5 to toluene/water was further enhanced to 139.55, higher than that of other reported adsorbents. Toluene breakthrough tests and kinetic analysis showed that GNHRC-1.5 possessed a faster intraparticle diffusion and outstanding reusability, which was ascribed to its a certain amount of mesopores. Grand Canonical Monte Carlo and molecular dynamics simulation were applied to explore the competitive adsorption mechanism of toluene and water vapor from the perspective of pore size and surface functional groups. The results indicated that the competitive adsorption was controlled by Van der Waals energy, intramolecular energy and electrostatic energy. Both the higher content of narrow micropores below 1 nm and carboxyl are not beneficial for toluene adsorption under humid conditions. For GNHRC-1.5, an appropriate micropore distribution, a certain amount of mesopores, and a lower content of oxygen functional groups, afford it excellent adsorption selectivity for toluene and fast adsorption kinetics under humid conditions.

Toluene and water vapor adsorption characteristics and selectivity on hydrophobic resin-based activated carbon

The competitive adsorption between volatile organic compounds (VOCs) and water vapor seriously decreases the adsorption capacity of adsorbents. In this paper, a series of hydrophobic spherical resorcinol-formaldehyde resin-based activated carbons (CRFs) were prepared and characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmet-Teller (BET) techniques. The adsorption equilibrium and selectivity of toluene and water vapor on CRFs were investigated systematically. Compared to other adsorbents, HCRF800 treated by hydrogen reduction exhibited the highest toluene uptake (8.64 mmol/g at 293 K) and the strongest hydrophobicity, which was much superior to commercial activated carbon. The results of isosteric heats showed that toluene on HCRF800 was higher than that of water vapor, indicating a stronger interaction between the adsorbent and toluene than water vapor. The adsorption selectivity of toluene/water vapor on CRFs was estimated by the Difference of Isosteric Heats (DIH) equation, and the results suggested that HCR800 possessed the highest selectivity, and the value was in the range of 9.5–52.3. Furthermore, HCRF800 also displayed good toluene uptake at different humidities and faster intra-particle diffusion behavior due to its lower content of surface oxygen groups. Temperature-programmed desorption experiments and Density Functional Theory calculations further revealed that surface oxygen groups had a negative effect on the selectivity of toluene/water, and the negative effect followed the order as O-CO > C-O > CO. Moreover, HCRF800 possessed excellent reversibility. These results demonstrated that HCRF800 would be a superior adsorbent for VOCs recycling from a humid atmosphere.

Superhigh selective capture of volatile organic compounds exploiting cigarette butts-derived engineering carbonaceous adsorbent

Herein, we develop cost-efficient superhigh-performance of engineering carbonaceous adsorbent from cigarette butts using combined wet-impregnated and re-dispersed method of KOH, which optimizes the implant approach of activator, breaking the restriction of selective capture of toluene using traditional activated carbon. The Brunauer-Emmett-Teller (BET) surface area and pore volume of targeted adsorbent can attain 3088 m2·g−1 and 1.61 cm3·g−1, respectively, by optimizing the temperature-dependent synthetic factor effect of the adsorbent. The adsorption capacity of resultant adsorbent for presenting volatile benzene and toluene shows a positive correlation with increasing carbonization temperature of carbon precursor. Besides, we demonstrated the unsmoked and smoked butts derived adsorbents afford feeble difference in saturated adsorbed capacity of volatile organic compounds (VOCs). The highest adsorption capacity of sample CF-800 for benzene and toluene in CF group is as high as 1268.1 and 1181.6 mg·g−1 respectively, slightly higher than that of sample UF-800, but far outperforming reported other adsorbents. The predicted adsorption selectivity of CF-800 and UF-800 for C7H8/H2O (g) using the DIH (Difference of Isosteric Heats) equation reach up to ca. 3800 and 7500 respectively, indicating the weak adsorbability of water vapor on the developed adsorbent and greater superiority of the smoked butts derived adsorbents in selective capture of VOCs at low relative humidity in the competitive adsorption process for practical mixed VOCs.

Carbon dioxide adsorption over amine modified silica: Effect of amine basicity and entropy factor on isosteric heats of adsorption

SBA-15 mesoporous silica was modified with 3-aminopropyl (AP), N-propyl ethylenediamine (DA) and N-propyl diethylenetriamine (TA) ligands. The prepared materials were characterized by thermogravimetry (TGA), small angle X-ray scattering (SAXS), nitrogen adsorption/desorption and transmission electron microscopy (HRTEM). The carbon dioxide adsorption/desorption experiments were performed at 273 K, 293 K, 313 K and 333 K. The adsorption capacity of CO2 at pressures below 2 kPa well correlates with number of amine centers in the respective ligand. Based on the adsorption isotherms, measured at different temperatures, the isosteric heats of adsorption (Qst) were calculated. The Qst values correlate with the basicity of the nitrogen atoms of respective amine ligands and their steric availability. The AP and DA ligands with the higher basicity of amine nitrogen showed the higher isosteric heats of adsorption at zero coverage (Qst = 72 kJ/mol), while TA ligand, with the lower basicity and steric availability of nitrogen atoms exhibited at zero coverage the lowest value of isosteric heat of adsorption (Qst = 40 kJ/mol). This low value shows, that in addition to amine basicity, also the entropic factor may influence the differences in isosteric heats of adsorption. While the mono- and diamine ligands loaded in the pores of SBA-15 are flexible enough to react with the carbon dioxide, the lower isosteric heats of adsorption of carbon dioxide over the TA ligands may be due to unfavorable entropic factor associated with filling of the pores of SBA-15 by bulkier TA ligands. Steric hindrance among TA ligands may result in lower flexibility of TA ligands and unfavor the formation of carbamates through intramolecular reaction in the pores of SBA-15.

Benzene/toluene/water vapor adsorption and selectivity of novel C-PDA adsorbents with high uptakes of benzene and toluene

A series of novel porous carbon-based material C-PDA (Carbonized Polydopamine Adsorbent) was prepared using one-step synthesis method for VOCs adsorption, and then characterized. The BET surface area and pore volume of C-PDAs can reach as high as 3291 m2/g and 1.78 cm3/g, respectively. FTIR spectra suggested the presence of N/O functionalities on the surfaces of C-PDAs, and its contents decreased with the increasing KOH/C ratio at which the samples were prepared. Adsorption capacity of C-PDAs for VOCs at low pressure increased with the surface N and O contents of C-PDAs. And the adsorption capacity under a high relative pressure depends on the pore volume of the materials. Adsorption capacities of C-PDA for C6H6 and C7H8 reached as high as 19.1 and 15.8 mmol/g (1491.9 and 1455.8 mg/g) at P/P0 of 0.6, much higher than many other adsorbents. Isotherms of C-PDAs for water vapor exhibited S-shaped type of isotherms, indicative of weak adsorption of water vapor on the sample surfaces at low relative humidity. TPD experiments revealed adsorption mechanism of VOCs on C-PDAs. The isosteric heats of benzene and toluene adsorption on C-PDA-4 were significantly higher than that of water vapor, which made C-PDA-4 possess a characteristic of favorable adsorption of benzene or toluene over water vapor. The adsorption selectivities of the C-PDA-4 for C6H6/H2O(g) and C7H8/H2O(g) were estimated by means of DIH (Difference of Isosteric Heats) equation, which reached as high as 99 and 13.6, respectively, much higher than those of MIL-101(Cr). The resultant C-PDA-4 is a promising adsorbent for adsorption of VOCs.

Selective capture of trace sulfur gas by porous covalent-organic materials

We have systematically investigated adsorption and separation of the sulfur gas H2S and SO2 in covalent-organic materials (COMs) by using molecular simulation. Results indicate that among these materials studied, PAF-302 shows the highest excess uptake of H2S (51.94mmol/g) and SO2 (50.69mmol/g) due to its large pore volume and high BET surface area. Both maximum excess uptakes of H2S and SO2 follow the order of PAF-302>COF-102>COF-10>COF-5>COF-8>COF-6, which is entirely consistent with the orders of the pore volumes. For gas mixtures, the selectivity of these two-dimensional (2D) COM materials is obviously better than the three-dimensional (3D) COM materials, especially for COF-6 with smaller pore size. In short, PAF-302 is an excellent candidate for H2S and SO2 adsorption, while COF-6 is an excellent material for sulfur gas separation. In addition, we also find that the modified equation based on the difference of isosteric heats (i.e. DIH equation) can predict the selectivity of trace sulfur gas mixtures at zero pressure satisfactorily.

Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Analysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts.

A DIH-based equation for separation of CO2–CH4 in metal–organic frameworks and covalent–organic materials

We develop a new S(DIH) equation based on the difference of isosteric heats (DIH) to calculate the selectivity for CO2 over CH4 in metal–organic frameworks (MOFs) and covalent–organic materials. Using the S(DIH) equation to predict the selectivity requires only the adsorption isotherms of pure components and the DIH of the two components. By comprehensive comparison with the GCMC data in different types of porous materials, including MOFs, ZIFs, COFs and PAFs, it is found that the new S(DIH) equation can predict with high accuracy the selectivity of different types of porous materials for CO2 over CH4 at the low pressure of p = 0–1 bar. Therefore, the new S(DIH) can serve as an efficient tool for the selectivity predictions of porous materials for CO2 over CH4 at p = 0–1 bar, especially for the cases in which experiments can measure the adsorption isotherms and adsorption heats of pure components (such as CO2, CH4, N2 and H2) because the new S(DIH) requires only the adsorption isotherms and adsorption heats of pure components as inputs. In short, the new S(DIH) equation can be considered as a valuable screening tool for obtaining an estimation about the selectivity of a porous material for a certain component of the gas mixture.

Carbon Dioxide Capture by PAFs and an Efficient Strategy To Fast Screen Porous Materials for Gas Separation

We have systematically investigated the CO2 storage and separation in a new class of porous aromatic frameworks (PAFs) with diamond-like structure by molecular simulations. Because of the small pore size of 5.2 A, PAF-301 exhibits not only much higher CO2 uptakes at low pressure (275 mg/g at 298 K and 1 bar) but also much higher selectivities for the CO2/H2, CO2/N2, CO2/CH4, and CH4/H2 mixtures than the other three PAFs. The uptakes of CO2 in PAF-303 and PAF-304 reach 3432 and 3124 mg/g at 298 K and 50 bar, respectively, which are larger than MOF-200 (2437 mg/g) and MOF-210 (2396 mg/g), suggesting the two PAFs are promising candidates for high-capacity CO2 storage. Interestingly, the selectivity of PAFs is closely related to the difference of isosteric heats (DIH) of two components and is independent with the molar fraction at zero pressure. Therefore, we proposed an efficient DIH approach to screen porous materials for separation of mixture gases and derived the relation between the selectivity and the d...