Showing papers on "BET theory published in 2010"

Facile synthesis of porous carbon nitride spheres with hierarchical three-dimensional mesostructures for CO2 capture

Abstract: Porous carbon nitride (CN) spheres with partially crystalline frameworks have been successfully synthesized via a nanocasting approach by using spherical mesoporous cellular silica foams (MCFs) as a hard template, and ethylenediamine and carbon tetrachloride as precursors. The resulting spherical CN materials have uniform diameters of ca. 4 μm, hierarchical three-dimensional (3-D) mesostructures with small and large mesopores with pore diameters centered at ca. 4.0 and 43 nm, respectively, a relatively high BET surface area of ∼550 m2/g, and a pore volume of 0.90 cm3/g. High-resolution transmission electron microscope (HRTEM) images, wide-angle X-ray diffraction (XRD) patterns, and Raman spectra demonstrate that the porous CN material has a partly graphitized structure. In addition, elemental analyses, X-ray photoelectron spectra (XPS), Fourier transform infrared spectra (FT-IR), and CO2 temperature-programmed desorption (CO2-TPD) show that the material has a high nitrogen content (17.8 wt%) with nitrogen-containing groups and abundant basic sites. The hierarchical porous CN spheres have excellent CO2 capture properties with a capacity of 2.90 mmol/g at 25 °C and 0.97 mmol/g at 75 °C, superior to those of the pure carbon materials with analogous mesostructures. This can be mainly attributed to the abundant nitrogen-containing basic groups, hierarchical mesostructure, relatively high BET surface area and stable framework. Furthermore, the presence of a large number of micropores and small mesopores also enhance the CO2 capture performance, owing to the capillary condensation effect.

Hydrogen storage and carbon dioxide capture in an iron-based sodalite-type metal–organic framework (Fe-BTT) discovered via high-throughput methods

Abstract: Using high-throughput instrumentation to screen conditions, the reaction between FeCl2 and H3BTT·2HCl (BTT3− = 1,3,5-benzenetristetrazolate) in a mixture of DMF and DMSO was found to afford Fe3[(Fe4Cl)3(BTT)8]2·22DMF·32DMSO·11H2O. This compound adopts a porous three-dimensional framework structure consisting of square [Fe4Cl]7+ units linked via triangular BTT3− bridging ligands to give an anionic 3,8-net. Mossbauer spectroscopy carried out on a DMF-solvated version of the material indicated the framework to contain high-spin Fe2+ with a distribution of local environments and confirmed the presence of extra-framework iron cations. Upon soaking the compound in methanol and heating at 135 °C for 24 h under dynamic vacuum, most of the solvent is removed to yield Fe3[(Fe4Cl)3(BTT)8(MeOH)4]2 (Fe-BTT), a microporous solid with a BET surface area of 2010 m2 g−1 and open Fe2+ coordination sites. Hydrogen adsorption data collected at 77 K show a steep rise in the isotherm, associated with an initial isosteric heat of adsorption of 11.9 kJ mol−1, leading to a total storage capacity of 1.1 wt% and 8.4 g L−1 at 100 bar and 298 K. Powder neutron diffraction experiments performed at 4 K under various D2 loadings enabled identification of ten different adsorption sites, with the strongest binding site residing just 2.17(5) A from the framework Fe2+ cation. Inelastic neutron scattering spectra are consistent with the strong rotational hindering of the H2 molecules at low loadings, and further reveal the catalytic conversion of ortho-H2 to para-H2 by the paramagnetic iron centers. The exposed Fe2+ cation sites within Fe-BTT also lead to the selective adsorption of CO2 over N2, with isotherms collected at 298 K indicating uptake ratios of 30.7 and 10.8 by weight at 0.1 and 1.0 bar, respectively.

Evaluation of the BET Method for Determining Surface Areas of MOFs and Zeolites that Contain Ultra-Micropores

Abstract: The BET analysis is commonly used for determining surface areas of metal-organic frameworks (MOFs) and zeolites that contain “ultra-micropores” (<7 A) even though it is often stated that the BET surface areas obtained for such small pores are not really meaningful in an absolute sense. In this study, nitrogen and argon isotherms in MOFs and zeolites (most of them having ultra-micropores) were predicted by grand canonical Monte Carlo (GCMC) simulations and used as pseudoexperimental data to evaluate the BET method for these structures. The BET surface areas calculated from the simulated nitrogen and argon isotherms agree well with the accessible surface areas obtained directly from crystal structures in a geometric fashion. However, this was only true when the BET analysis was performed using the appropriate pressure range based on published “consistency criteria”; the BET analysis underestimates the surface areas of all the selected MOFs and zeolites if it is done in the “standard” BET pressure range. Mor...

Novel Nanocrystalline Ce1−xLaxO2−δ (x = 0.2) Solid Solutions: Structural Characteristics and Catalytic Performance

Abstract: The catalytic CO oxidation results revealed that nanosized ceria-lanthana (CL) solid solution exhibits superior catalytic performance and thermal stability in comparison to ceria-zirconia (CZ) solid solution. The rationale for excellent catalytic behavior of the CL sample was elucidated from the viewpoint of structure, redox behavior, and bulk oxygen mobility. Accordingly, in this investigation CL solid solution has been synthesized by a modified coprecipitation method, and the structural and redox properties have been deeply investigated employing various characterization techniques, namely, X-ray diffraction (XRD), BET surface area, transmission electron microscopy (TEM), Raman spectroscopy (UV-RS and Vis-RS), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), and temperature programmed reduction (TPR). The catalytic efficiency has been evaluated for oxygen storage/release capacity (OSC) and CO oxidation activity. The analyses of the characterization results provided a deep insight on the physica...

Preparation of carbon molecular sieve from lignocellulosic biomass: A review

Abstract: A literature review on preparation of carbon molecular sieve (CMS) from lignocellulosic biomass is presented. The effect of various operation parameters such as pyrolytic temperature, flow rate of the carbonizing agent and time of pyrolysis on the carbonization of the lignocellulosic biomass as a carbon precursor was reviewed. Various physical and chemical processes for the activation of the biomass-based char and their effects on textural properties of the activated char were discussed. Conversion of activated chars to CMS as the final stage of the preparation process through different techniques of chemical vapor deposition (CVD) and controlled pyrolysis was assessed. Survey of literature revealed that production of CMS with BET surface area of 1247 m 2 /g and micropore volume of 0.51 cm 3 /g, under appropriate conditions has been reported. Also, maximum selectivity of 7.6 and 400 for separation of O 2 /N 2 and CO 2 /CH 4 was devoted to palm shell and coconut shell-based CMS, respectively.

Preparation of mesoporous In2O3 nanofibers by electrospinning and their application as a CO gas sensor

Abstract: Mesoporous In 2 O 3 nanofibers with a high surface area were synthesized by calcining electrospun polyvinyl alcohol (PVA)/indium acetate composite fibers. A PVA solution and indium acetate were mixed and electrospun. After calcining the PVA/indium acetate composite nanofiber precursor, mesoporous In 2 O 3 nanofibers were successfully formed. These nanofibers had diameters in the range of 150–200 nm and consisted of cubic indium oxide nanocrystals with a primary particle size of 10–20 nm. The Brunauer–Emmett–Teller (BET) surface area of the In 2 O 3 nanofibers was strongly affected by the calcining temperature. The BET surface area of the fibers calcined at 400 °C was significantly higher than the surface area of the nanofibers calcined at 500 °C or 600 °C and of the commercial In 2 O 3 powder. The response of mesoporous In 2 O 3 nanofibers to CO in air is strongly affected by the surface area. The highly elevated response of In 2 O 3 nanofibers calcined at 400 °C could be attributed to the high surface area, which provides a large amount of surface sites for adsorption and reaction of CO. The results demonstrate that the electrospinning approach is an easy and useful method to synthesize metal oxides with mesopores and high surface area, which may enhance their gas sensing properties.

Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia

Abstract: Esterification of organic acids with alcohols produces an industrially important class of substances with a variety of applications. This work presents an impregnation route to support H3PW12O40 (H3PW) on zirconia (ZrO2) in acidic aqueous solution (HCl 0.1 mol L−1) at different ratios (5, 10, 15, 20, 25, 40 and 60 wt%), which were further applied in the esterification of oleic acid with ethanol. Impregnated samples calcined at 200 °C for 4 h were characterized by FTIR, FT-Raman, XRD, 31P MAS NMR and BET surface area. No decomposition of the Keggin anion structure was observed under these conditions. The XRD results, surface area determination and catalytic tests pointed out that H3PW was well dispersed over the support and only a monoclinic phase of the commercial ZrO2 was detected. An optimum reaction performance (88% of oleic acid conversion) was achieved at 20 wt% loading, 100 °C, 4 h reaction and 1:6 (acid:ethanol) molar ratio. A small leaching of 8 wt% of the initial mass of this catalyst (i.e., the actual loading was 18.4 wt%) was also observed at the end of reaction, which affects the reaction kinetics. Thermal stability studies of 20%H3PW/ZrO2 catalyst, determined by 31P MAS NMR, XRD and FT-Raman revealed that Keggin anion decomposition begins at ca. 500 °C, which was confirmed by the respective decrease of the catalytic activity. A preliminary study of recyclability indicated that a treatment of the spent catalyst involving a sequence of washing with n-hexane, drying at 100 °C and calcining at 300 °C for 4 h, recovered conversion values as high as 70%.

Characteristics and applications of the Lagergren's first-order equation for adsorption kinetics

Abstract: Adsorption kinetic curves of the Lagergren's first-order (LFO) equation were classified into four zones according to their rising characteristics. Of the 85 adsorption systems described by LFO equation, 46% of the kinetic curves belonged to zone II and 29% to zone III, these being good and fast. Activated carbons with a BET surface area of 626–1009 m2/g and a micropore volume fraction of 57.5–88.0% were prepared from plum kernels, pinewood, pistachio shells, and Moso bamboo with steam activation. The adsorption kinetics of methylene blue, tannic acid, humic acid, and phenol on these activated carbons were studied. Normalized standard deviations were shown that the adsorption of methylene blue, tannic acid, and humic acid was better described by LFO equation and that of phenol by PSO equation. Also, the value of k1tref was obviously affected by physical properties and particle sizes of the adsorbents as well as molecular weights of the adsorbates.

Increasing the Oxygen Vacancy Density on the TiO2 Surface by La-Doping for Dye-Sensitized Solar Cells

Abstract: Facilitated by TiO2 particles absorbing La3+ in hydrosol, La-doped TiO2 was prepared by a sol-hydrothermal method. Electron paramagnetic resonance and Brunauer−Emmett−Teller (BET) surface area analysis showed that the obtained La-doped anatase TiO2 surface provided a higher density of oxygen vacancies without a change in the BET surface area. A theoretical calculation was carried out to explain the generation mechanism of the increased oxygen vacancies. The results showed that the La-doped anatase TiO2 (101) surface tends to engender oxygen vacancies. The photoelectric conversion efficiency of dye-sensitized solar cells fabricated from 1 mol % La-doped TiO2 reached 6.72%, which gave an efficiency improved by 13.5% compared with that of the cells fabricated from pure TiO2. The improvement in the efficiency was ascribed to more dye absorbed on the surface of TiO2.

Porous organic polymers: distinction from disorder?

Abstract: An alternative explanation: The new microporous organic polymer framework PAF-1 displays exceptional physicochemical stability along with an extremely high surface area (BET surface area 5640 m2 g−1). The question arises whether this material displays the high degree of crystalline order presumed necessary for this high surface area.

One-step room-temperature synthesis of fibrous polyimide aerogels from anhydrides and isocyanates and conversion to isomorphic carbons

Abstract: Monolithic polyimide aerogels (PI-ISOs) have been prepared by drying wet-gels synthesized via a rather underutilized room-temperature reaction of pyromellitic dianhydride (PMDA) with 4,4′-methylene diphenyl diisocyanate (MDI). The reaction is followed up to the gelation point by liquid 13C-NMR in DMSO-d6 and it proceeds through a seven-member ring intermediate that collapses to the imide by expelling CO2. PI-ISOs are characterized comparatively with aerogels referred to as PI-AMNs, obtained via the classic reaction of PMDA and 4,4′-methylenedianiline (MDA). The two materials are chemically identical, they show similar degrees of crystallinity (30–45%, by XRD) and they both consist of similarly sized primary particles (6.1–7.5 nm, by SANS). By N2-sorption porosimetry they contain both meso- and macroporosity and they have similar BET surface areas (300–400 m2 g−1). Their major difference, however, is that PI-AMNs are particulate while PI-ISOs are fibrous. The different morphology has been attributed to the rigidity of the seven-member ring intermediate of PI-ISOs. PI-AMNs shrink significantly during processing (up to 40% in linear dimensions), but mechanically are much stronger materials than PI-ISOs of the same density. Upon pyrolysis at 800 °C both PI-ISO and PI-AMN are converted to porous carbons; PI-AMNs loose their nanomorphology and more than 2/3 of their surface area, as opposed to PI-ISOs, which retain both. Etching with CO2 at 1000 °C increases the BET surface area of both PI-AMN (to 417 m2 g−1) and PI-ISO (to 1010 m2 g−1), and improves the electrical conductivity of the latter by a factor of 70.

Solvothermal synthesis of Fe-MOF-74 and its catalytic properties in phenol hydroxylation.

Abstract: A Fe-containing metal-organic framework, Fe-MOF-74, was solvothermally synthesized using FeCl2.4H2O and 2,5-di-hydroxy-1,4-benzenedicarboxylic acid. Characterization was conducted by XRD, BET surface area measurement, FT-IR spectroscopy, TGA, and elemental analysis, which confirmed successful preparation of Fe-MOF-74 having an identical framework structure to that reported for MOF-74. Fe-MOF-74 was found to be an effective heterogeneous catalyst for the hydroxylation of phenol using H2O2 as an oxidant; 60% phenol conversion was achieved at 20 degrees C in water with 68 and 32% selectivity to catechol and hydroquinone, respectively. The effect of temperature, phenol/H2O2 mole ratio, catalyst quantity, and solvent on catalytic performance was discussed, and a reaction mechanism is proposed based upon the experimental results.

Preparation of activated mesoporous carbons for electrosorption of ions from aqueous solutions

Abstract: Mesoporous carbon with a narrow pore size distribution centered at about 9 nm, which was prepared by self assembly of block copolymer and phloroglucinol-formaldehyde resin via the soft-template method, was activated by CO2 and potassium hydroxide (KOH). The effects of activation conditions, such as the temperature, activation time, and mass ratio of KOH/C, on the textural properties of the resulting activated mesoporous carbons were investigated. Activated mesoporous carbons exhibit high BET specific surface areas (up to ∼ 2000 m2 g−1) and large pore volumes (up to ∼ 1.6 cm3 g−1), but still maintain a highly mesoporous structure. Heat treatment of mesoporous carbons by CO2 generally requires a moderate to high extent of activation in order to increase its BET surface area by 2–3 times, while KOH activation needs a much smaller degree of activation than the former to reach an identical surface area, ensuring high yields of activated mesoporous carbons. In addition, KOH activation allows a controllable degree of activation by adjusting the mass ratio of KOH/C (2–8), as evidenced by the fact that surface area and pore volume increase with the mass ratio of KOH/C. The electrosorption properties of activated mesoporous carbons were investigated by cyclic voltammetry in 0.1 M NaCl aqueous solutions. Upon activation, the electrosorption capacitance of activated mesoporous carbons was greatly enhanced.