Showing papers on "BET theory published in 2012"

CO2 capture and conversion using Mg-MOF-74 prepared by a sonochemical method

Abstract: Mg-MOF-74 crystals were successfully prepared in 1 h by a sonochemical method (Mg-MOF-74(S)) after triethylamine (TEA) was added as a deprotonating agent. Mg-MOF-74(S) (1640 m2 g−1 BET surface area) displayed similar textural properties to those of a high-quality MOF sample synthesized in 24 h by the solvothermal method (Mg-MOF-74(C), 1525 m2 g−1). However, mesopores were formed, probably due to the competitive binding of TEA to Mg2+ ions, and the average particle size of the former (ca. 0.6 μm) was significantly smaller than that of the latter (ca. 14 μm). The H2O adsorption capacity was 593 mL g−1 at 298 K for Mg-MOF-74(S), displaying higher hydrophilicity than Zeolite 13X. The adsorption isotherms of Mg-MOF-74(S) for CO2 showed high adsorption capacity (350 mg g−1 at 298 K) and high isosteric heats of adsorption for CO2 (42 to 22 kJ mol−1). The breakthrough experiment confirmed excellent selectivity to CO2 over N2 at ambient conditions (saturation capacity of ca. 179 mg g−1). Ten consecutive adsorption–desorption cycles at 298 K established no deterioration of the adsorption capacity, which showed reversible adsorbent regeneration at 323 K under helium flow for a total duration of 1400 min. Mg-MOF-74(S) also demonstrated excellent catalytic performance in cycloaddition of CO2 to styrene oxide under relatively mild reaction conditions (2.0 MPa, 373 K) with close to 100% selectivity to carbonate, which was confirmed by GC-MS, 1H-NMR, and FT-IR. Mg-MOF-74(S) could be reused 3 times without losing catalytic activity and with no structural deterioration.

Cellulose–Silica Nanocomposite Aerogels by In Situ Formation of Silica in Cellulose Gel

Abstract: Aerogels with their low density (0.004–0.500 gcm ), large internal surface area, and large open pores are promising candidates for various advanced applications. The utilization of inorganic aerogels, however, has been hampered by their poor mechanical properties. A prominent example is silica aerogel, which is prepared by an organic sol–gel process, and has unique features, such as ultralow density (the lightest silica aerogel has a density that is similar to the density of air, which is 0.00129 gcm ), near transparency, and low thermal conductivity. However, the extreme fragility of this aerogel necessitates its reinforcement for practical uses. A typical method is hybridization with organic polymers, such as polyurea, polyurethane, poly(methyl methacrylate), polyacrylonitrile, and polystyrene. Other candidates for the reinforcement of inorganic aerogels are insoluble polysaccharides, which are abundantly available and show wide varieties in structure and properties. The useful features of these compounds are hydrophilicity, biocompatibility, hydroxy reactivity, and reasonable thermal and mechanical stabilities. For example, nanofibrillar bacterial cellulose and microfibrillated cellulose gel have been proposed as templates for cobalt ferrite nanoparticles and titanium dioxide. While in the above-mentioned work native cellulose with cellulose I crystallinity was used, cellulose can be prepared as a hydrogel with cellulose II crystallinity through dissolution and coagulation. Some of the resulting aerogels have remarkable mechanical strength and light transmittance. They have high porosity with open structures and thus provide an effective substrate for the synthesis of metallic nanoparticles. To further utilize the regenerated cellulose gel, we herein attempted in situ synthesis of silica in cellulose gels. While a similar attempt has been reported, in which the cellulose gel was obtained from solution in N-methylmorpholine-N-oxide monohydrate, the development of the nanostructure (nitrogen BET surface area of 220–290 mg ) and the level of silica loading (less than 13% w/w) were rather limited. By using the aqueous alkali-based solvent, we obtained the cellulose aerogel with a surface area of 356 mg , and a silica loading of more than 60% w/w resulted in surface areas that exceeded 600 mg . We used the sol–gel synthesis method toward nanostructured silica, which typically starts from tetraethyl orthosilicate (TEOS). The resulting composite gels were dried with supercritical CO2 to give cellulose–silica aerogels with low density, moderate light transmittance, a large surface area, high mechanical integrity, and excellent heat insulation. This method can also lead to fabrication of silica-only aerogels through the removal of cellulose by calcination, that is, the use of cellulose aerogel as sacrificial template. Figure 1 shows the preparation of the aerogel. The cellulose hydrogel is a transparent material that has a water content of 92% and a porosity of 95%. The sol–gel process catalyzed by ammonia converts TEOS to SiO2, which is deposited on the cellulose network (Figure 1b). The composite is converted to an aerogel by drying with supercritical CO2 to maintain the porous structure (Figure 1c), thus resulting in a flexible and translucent cellulose–silica aerogel. Subsequent calcination removes the cellulose matrix to give a silica-only aerogel (Figure 1d and g). The cellulose aerogel is composed of regenerated cellulose fibrils, which are typically less than 10 nm wide (Figure 2a). The BET surface area of 356 mg 1 (determined by

Influence of cerium precursors on the structure and reducibility of mesoporous CuO-CeO2 catalysts for CO oxidation

Abstract: This work investigated the effects of cerium precursors [Ce(NO3)3 and (NH4)2Ce(NO3)6] on the structure, surface state, reducibility and CO oxidation activity of mesoporous CuO-CeO2 catalysts. The catalysts were characterized by TG–DTA, XRD, LRS, N2 adsorption–desorption, HRTEM, XPS, H2-TPR and in situ FT-IR. The obtained results suggested that the precursors exerted a great influence on the properties of CuO-CeO2 catalysts: (1) compared with the catalysts from Ce(III) precursor, the derived Ce(IV) precursor catalysts showed smaller grain size, higher BET surface area, narrower pore size distribution, whereas their reducibility and activities were not enhanced. (2) In contrast, the catalysts from Ce(III) precursor without excellent texture displayed high reducibility and activities for CO oxidation due to the high content of Ce3+, following the redox equilibrium of Cu2+ + Ce3+ ↔ Cu+ + Ce4+ shifting to right to form more stable Cu+ species, which was the origin of synergistic effect. The synergistic effect between copper and cerium was the predominant contributor to the improved catalytic activities of CuO-CeO2 catalysts, instead of structural properties.

Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures

Abstract: TiO 2 supported Mn-Ce mixed oxides (Mn-Ce/Ti) synthesized by an ultrasound-assisted impregnation method were employed to oxidize elemental mercury (Hg 0 ) at low temperatures in simulated low-rank (sub-bituminous and lignite) coal combustion flue gas and corresponding selective catalytic reduction (SCR) flue gas. The catalysts were characterized by BET surface area analysis, X-ray diffraction (XRD) measurement and X-ray photoelectron spectroscopy (XPS) analysis. The combination of MnO x and CeO 2 resulted in significant synergy for Hg 0 oxidation. The Mn-Ce/Ti catalyst was highly active for Hg 0 oxidation at low temperatures (150–250 °C) under both simulated flue gas and SCR flue gas. The dominance of Mn 4+ and the presence of Ce 3+ on the Mn-Ce/Ti catalyst were responsible for its excellent catalytic performance. Hg 0 oxidation on the Mn-Ce/Ti catalyst likely followed the Langmuir–Hinshelwood mechanism, where reactive species on catalyst surface react with adjacently adsorbed Hg 0 to form Hg 2+ . NH 3 consumed the surface oxygen and limited the adsorption of Hg 0 , hence inhibiting Hg 0 oxidation over Mn-Ce/Ti catalyst. However, once NH 3 was cut off, the inhibited mercury oxidation activity could be completely recovered in the presence of O 2 . This study revealed the possibility of simultaneously oxidizing Hg 0 and reducing NO x at low flue gas temperatures. Such knowledge is of fundamental importance in developing effective and economical mercury and NO x control technologies for coal-fired power plants.

Surface area and pore size characteristics of nanoporous gold subjected to thermal, mechanical, or surface modification studied using gas adsorption isotherms, cyclic voltammetry, thermogravimetric analysis, and scanning electron microscopy

Abstract: Nitrogen adsorption/desorption isotherms are used to investigate the Brunauer, Emmett, and Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore size distribution of physically modified, thermally annealed, and octadecanethiol functionalized np-Au monoliths. We present the full adsorption-desorption isotherms for N2 gas on np-Au, and observe type IV isotherms and type H1 hysteresis loops. The evolution of the np-Au under various thermal annealing treatments was examined using scanning electron microscopy (SEM). The images of both the exterior and interior of the thermally annealed np-Au show that the porosity of all free standing np-Au structures decreases as the heat treatment temperature increases. The modification of the np-Au surface with a self-assembled monolayer (SAM) of C18-SH (coverage of 2.94 × 1014 molecules cm−2 based from the decomposition of the C18-SH using thermogravimetric analysis (TGA)), was found to reduce the strength of the interaction of nitrogen gas with the np-Au surface, as reflected by a decrease in the ‘C’ parameter of the BET equation. From cyclic voltammetry studies, we found that the surface area of the np-Au monoliths annealed at elevated temperatures followed the same trend with annealing temperature as found in the BET surface area study and SEM morphology characterization. The study highlights the ability to control free-standing nanoporous gold monoliths with high surface area, and well-defined, tunable pore morphology.

Water adsorption on clay minerals as a function of relative humidity: application of BET and Freundlich adsorption models.

Abstract: Water adsorption on kaolinite, illite, and montmorillonite clays was studied as a function of relative humidity (RH) at room temperature (298 K) using horizontal attenuated total reflectance (HATR) Fourier transform infrared (FTIR) spectroscopy equipped with a flow cell. The water content as a function of RH was modeled using the Brunauer, Emmett, and Teller (BET) and Freundlich adsorption isotherm models to provide complementary multilayer adsorption analysis of water uptake on the clays. A detailed analysis of model fit integrity is reported. From the BET fit to the experimental data, the water content on each of the three clays at monolayer (ML) water coverage was determined and found to agree with previously reported gravimetric data. However, BET analysis failed to adequately describe adsorption phenomena at RH values greater than 80%, 50%, and 70% RH for kaolinite, illite, and montmorillonite clays, respectively. The Freundlich adsorption model was found to fit the data well over the entire range of...

Facile synthesis of multiwall carbon nanotubes/iron oxides for removal of tetrabromobisphenol A and Pb(II)

Abstract: A one-step thermal decomposition strategy, in which a novel reductant participated, was developed to prepare superparamagnetic nearly cubic monodisperse Fe3O4 nanoparticles loaded on multiwall carbon nanotubes (MWCNTs/Fe3O4). Subsequently, the as-prepared MWCNTs/Fe3O4 nanocomposites were modified with 3-aminopropyltriethoxysilane (APTS) (MWCNTs/Fe3O4–NH2). The materials were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM) and the BET surface area method. The results indicated that superparamagnetic Fe3O4 nanoparticles were successfully loaded onto the surface of the MWCNTs, and APTS was also modified on the MWCNTs/Fe3O4 magnetic nanocomposites. The two as-prepared magnetic nanocomposites were used as adsorbents to remove tetrabromobisphenol A (TBBPA) and Pb(II) from wastewater. The adsorption kinetics and adsorption isotherms of TBBPA and Pb(II) on the two as-prepared adsorbents were studied at pH 7.0 and 5.3, respectively. It was revealed that MWCNTs/Fe3O4–NH2 performed better than the MWCNTs/Fe3O4 nanocomposites for the adsorption properties of TBBPA and Pb(II). After adsorption, both adsorbents could be conveniently separated from the media by an external magnetic field within several seconds, and regenerated in 0.1 M NaOH solution.

Preparation and characterisation of activated carbon from waste tea using K2CO3

Abstract: Biomasses and their wastes have been used to produce various materials including activated carbons (ACs). The properties of the ACs mostly depend on the production method and the type of the raw material. Waste tea that is a biomass as a precursor was employed to prepare activated carbon (AC) in accordance with the conventional method using potassium carbonate (K2CO3). The effects of various process parameters such as carbonisation temperature and period, impregnation ratio and period on the characteristics of the final product were determined. The ACs were characterised in terms of the BET surface area, the true density, the pore volumes, chemical structure and surface morphology. The maximum surface area of the AC was 1722 m2/g produced at 900 °C and impregnation ratio of 1.0. The pore volumes of the samples were found out according to the Non Local Density Functional Theory (NLDFT) method. To produce the AC with high micropore volume fraction, 800 °C is the best convenient temperature for the experiments performed at impregnation ratios of 1.0, 1.5 and 2.0. The BET surface area and the mesopore volume fractions of the AC were substantially increased at 900 °C. The results showed that both the carbonisation temperature and impregnation ratio noticeably affected the micro and mesopore volumes as well as the BET surface area.

Control of interpenetration and gas-sorption properties of metal-organic frameworks by a simple change in ligand design.

Abstract: In metal–organic framework (MOF) chemistry, interpenetration greatly affects the gas-sorption properties. However, there is a lack of a systematic study on how to control the interpenetration and whether the interpenetration enhances gas uptake capacities or not. Herein, we report an example of interpenetration that is simply controlled by the presence of a carbon–carbon double or single bond in identical organic building blocks, and provide a comparison of gas-sorption properties for these similar frameworks, which differ only in their degree of interpenetration. Noninterpenetrated (SNU-70) and doubly interpenetrated (SNU-71) cubic nets were prepared by a solvothermal reaction of [Zn(NO3)2]⋅6 H2O in N,N-diethylformamide (DEF) with 4-(2-carboxyvinyl)benzoic acid and 4-(2-carboxyethyl)benzoic acid, respectively. They have almost-identical structures, but the noninterpenetrated framework has a much bigger pore size (ca. 9.0×9.0 A) than the interpenetrated framework (ca. 2.5×2.5 A). Activation of the MOFs by using supercritical CO2 gave SNU-70′ and SNU-71′. The simulation of the PXRD pattern of SNU-71′ indicates the rearrangement of the interpenetrated networks on guest removal, which increases pore size. SNU-70′ has a Brunauer–Emmett–Teller (BET) surface area of 5290 m2 g−1, which is the highest value reported to date for a MOF with a cubic-net structure, whereas SNU-71′ has a BET surface area of 1770 m2 g−1. In general, noninterpenetrated SNU-70′ exhibits much higher gas-adsorption capacities than interpenetrated SNU-71′ at high pressures, regardless of the temperature. However, at P<1 atm, the gas-adsorption capacities for N2 at 77 K and CO2 at 195 K are higher for noninterpenetrated SNU-70′ than for interpenetrated SNU-71′, but the capacities for H2 and CH4 are the opposite; SNU-71′ has higher uptake capacities than SNU-70′ due to the higher isosteric heat of gas adsorption that results from the smaller pores. In particular, SNU-70′ has exceptionally high H2 and CO2 uptake capacities. By using a post-synthetic method, the CC double bond in SNU-70 was quantitatively brominated at room temperature, and the MOF still showed very high porosity (BET surface area of 2285 m2 g−1).