High Micropore Volume Research Articles

Surface and related studies in sulphated oxides of zirconium

Zirconium oxide superacids with different sulphate ion contents were prepared from Zr(SO4)2 using urea as precipitating agent. These oxides were subjected to different heating temperatures. Characterization was carried out by means of infrared spectroscopy, X-ray diffraction and nitrogen adsorption studies. The surface acidity was determined by the method described by Boehm [1]. Infrared results confirmed the presence of the sulphate ions and suggested the gradual lowering through heat-treatment of the symmetry of the sulphate ions by complex formation. Surface acidity was favored when the sulphate ions were in a bidentate state of ligation. The latter ligands seemed in turn to be stabilized by water. The strongest acidity was displayed in systems where a high concentration of bidentated sulphate ions was present in conjunction with high surface areas and micropore volumes.

Template Assisted Design of Microporous Gallium Nitride Materials.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Powdered cellulose as excipient for extrusion–spheronization pellets of a cohesive hydrophobic drug

This study compared a powdered cellulose (PC) and a microcrystalline cellulose (MCC) as sole excipients in the preparation of furosemide pellets by extrusion–spheronization. Pellets prepared with PC and 25 or 50% furosemide showed smaller mean size, a broader particle size distribution, similar sphericity, greater surface roughness and higher friability than equivalent pellets prepared with MCC. Furosemide release rate was markedly higher from PC pellets, which may be attributable to their higher micropore volume.

Template assisted design of microporous gallium nitride materials

For the first time the synthesis of a porous gallium nitride material (GaN1.15H1.18O0.06C0.04) with narrow pore size distribution in the micropore regime using a nonylamine assisted sol-gel route is reported. The yellow solid has a very high micropore volume of 0.176 cm3 g(-1) comparable to that of zeolitic solids.

Understanding chemical reactions between carbons and NaOH and KOH: An insight into the chemical activation mechanism

Direct mixing of an anthracite with hydroxides (KOH or NaOH) and heat treatment up to 730 °C has shown to be a very good activation procedure to obtain activated carbons with very high surface areas and high micropore volumes. The reactions involved during the heat treatment of these hydroxide/anthracite mixtures have been analysed to deep into the fundamental of the knowledge of this chemical activation process, that has not been studied before. For this purpose, the present paper analyses the drying process, the atmosphere during the carbonisation, the chemical state of the activating agents (NaOH, KOH and Na 2CO 3) and the chemical reactions occurring during the heat treatment which have been followed by FTIR and TPD. The analysis of our results allows us to conclude that steam is a good atmosphere for the carbonisation process, alone or joined with nitrogen, but not as good as pure nitrogen. On the other hand, during the activation process, the presence of CO 2 should be avoided because it does not develop porosity. The reactions, and chemical changes, involved during this chemical process are discussed both from a thermodynamical point of view as well as identifying the reaction products (H 2 by TPD and Na 2CO 3 by FTIR). As a result, this paper helps to cover the present lack of understanding of the fundamentals of the reactions of an anthracite with hydroxides which are necessary to understand the activation of the material.

Usefulness of chemically activated anthracite for the abatement of VOC at low concentrations

A previous paper showed a very effective and simple chemical activation method for the preparation of activated carbons from an anthracite that allowed to obtain different ranges of porosity. In addition, these materials were suitable for the VOC adsorption because this activation method renders AC with high volumes of narrow micropores. In this paper, the method has been successfully extended to some other precursors, such as subbituminous and lignite coals and lignocellulosic materials. The developed method permits the easy preparation of AC with very well-developed porosities and with high narrow micropore volumes. These characteristics of the AC allow to reach high adsorption capacities to VOC (benzene and toluene) at low concentrations, which are much higher than previously reported in the literature.

Adsorption of Volatile Organic Compounds on Activated Carbon Fiber Preparedby Carbon Dioxide

Viscose rayon fabric impregnated with (NH 4 ) 2 HPO 4 were carbonized in 200 cm 3 /min N 2 at 5°C/min with a residence time of 1 h at 850°C and then activated with 200 cm 3 /min CO 2 at 850°C for different burnoffs. The pore structure of all samples was characterized by N 2 adsorption at 77 K. Adsorption of methy-ethyl-ketone (MEK) and benzene vapor on the adsorbents was measured by gravimetric method. The results show that viscose rayon-based activated carbon fiber (ACF) exhibited narrower microporosity and higher micropore volume as compared with that activated by steam. ACFs prepared in this way have higher adsorption capacity for low concentration VOCs.

Preparation of activated carbons from Spanish anthracite: II. Activation by NaOH

This paper complements the previous one (activation with KOH) analysing the development of porosity of a Spanish anthracite by chemical activation with NaOH. The preparation method has been optimised through the analysis of diverse experimental variables. Among them, activating agent/coal ratio, drying process, method of mixing of the activating agent and coal, nitrogen flow during pyrolysis and mineral matter content of the coal, have been studied. The results obtained confirm the importance of the activating agent to coal ratio and the flow of gas during carbonisation on the development of porosity. In addition, it shows that chemical activation with NaOH can be successfully used to develop activated carbons with high surface area and micropore volumes (i.e., up to 2700 m 2/g and 1 cm 3/g). Comparing chemical activation using an impregnation method and physical mixing we conclude that physical mixing of NaOH and coal, which is a very easy preparation method, renders the best results. Therefore physical mixing has been also applied in this work to KOH with rather interesting results. Although impregnation produces a higher development of porosity for KOH, activated carbons with high micropore volumes can be synthesised through a much simpler method. Physical mixing produces larger pore volumes by KOH activation than by NaOH activation.

Preparation and characterization of microporous 18 Å Al-pillared structures from natural phlogopite micas

A new method to prepare Al-pillared phlogopite micas with thermally resistant 18 Å spacings is reported. They are obtained in a two-step process, one consisting of a preliminary treatment of mica in controlled conditions, which allows a permanent reduction of the layer charge density, and brings the charge-reduced mica to a homoionic Na, or Ca form. Pillaring is achieved in a second step by bringing the Na-exchanged phlogopite in contact with a base-hydrolyzed Al solution or commercial Al-chlorhydrol. Al-pillared phlogopites exhibit textural and structural features similar to those of pillared smectites (temperature resistant spacings, high surface areas and micropore volumes). As to the difference with Al-pillared smectites, Al-pillared micas withstand temperature treatments up to 800°C, owing to the higher intrinsic thermal stability of the host structure. They also have a higher content of Brønsted acid sites.

Microporosity of activated carbons produced from heat-treated and fractionated pitches

Activated carbons with degrees of burn-off ranging from 12 wt.% to 60 wt.% have been prepared from toluene-insoluble (T.I.) fractions of a heat-treated A240 petroleum pitch. After toluene fractionation, the resulting pitches have been stabilized, carbonized and then activated using carbon dioxide at 1173 K. Characterization by high pressure carbon dioxide adsorption has been performed in order to determine the properties of activated carbons and to evaluate their performances for adsorption refrigerating machines. Microporous properties are influenced by pitch composition resulting from toluene fractionation and residual γ-resins inhibit the microporosity development. Due to their high microporous volume, activated carbons prepared from extensively γ-resins extracted pitches appear appropriate for cooling applications.

Gas adsorption-desorption behavior of activated carbon spheres derived from phenol resin

Adsorption and desorption behavior of carbon spheres prepared from phenol resin under various heat treatment conditions were studied by measuring isotherms of N2 at 77 K and CO2 at 201 K. The pyrolysis of the phenol resinwhich occurs up to 800°C gave the carbon with a BET surface area of about 500 m2/g and the micropore volume ofabout 0.4 cc/g, and no effect of heating atmosphere was observed on gas adsorption-desorption behavior. Thecarbonization above 800°C in a flow of high-purity Ar resulted in slight decreases in surface area and micropore volumeof the resultant carbons, due to shrinkage accompanied by carbonization, and gave irreversible adsorption of N2. Theheat treatment in CO2 above 800°C led to high surface area and large micropore volume which were caused by activation. Linear relations between percentage of weight loss during heat treatment and specific surface areas calculated fromboth N2 and CO2 adsorption were found irrespective of heat treatment atmosphere.

Adsorption onto Fibrous Activated Carbon: Applications to Water Treatment

The adsorption of polluted waters is performed by activated carbon fibers (ACF). This new material is characterized by scanning electronic microscopy. BET surface areas and pore volumes are determined. Adsorption of natural organics (humic substances) and micropollutants (aromatic compounds such as benzene and toluene) is carried out in a batch or dynamic reactor. Classical models are applied and kinetic constants calculated. The results show that the performance of ACF is significantly higher than that of granular activated carbon (GAC) in terms of adsorption velocity and selectivity for micropollutants. These higher performances are due to some ACF physical properties, such as their high BET surface area and micropore volume. Moreover, the micropores are directly connected on the external surface area of fibers, which allows smaller mass transfer resistance. In a dynamic reactor, the breakthrough curves obtained with ACF beds are particularly steep, suggesting a smaller mass transfer resistance than tha...

Natural gas storage with activated carbon from a bituminous coal

Granular activated carbons (−20 + 100 mesh; 0.149−0.84 mm) were produced by physical activation and chemical activation with KOH from an Illinois bituminous coal (IBC-106) for natural gas storage. The products were characterized by BET surface area, micropore volume, bulk density, and methane adsorption capacities. Volumetric methane adsorption capacities ( V m V s ) of some of the granular carbons produced by physical activation are about 70 cm 3/cm 3 which is comparable to that of BPL, a commercial activated carbon. ( V m V s ) values above 100 cm 3/cm 3 are obtainable by grinding the granular products to −325 mesh (<0.044 mm). The increase in ( V m V s ) is due to the increase in bulk density of the carbons. Volumetric methane adsorption capacity increases with increasing pore surface area and micropore volume when normalizing with respect to sample bulk volume. Compared with steam-activated carbons, granular carbons produced by KOH activation have higher micropore volume and higher methane adsorption capacities (g/g). Their volumetric methane adsorption capacities are lower due to their lower bulk densities.

Passive CO 2 removal using a carbon fiber composite molecular sieve

Manufacture and characterization of a carbon fiber composite molecular sieve (CFCMS), and its efficacy as a CO 2 gas adsorbent are reported. The CFCMS consists of an isotropic pitch derived carbon fiber and a phenolic resin derived carbon binder. Activation (selective gasification) of the CFCMS creates microporosity in the carbon fibers, yielding high micropore volumes (>0.5 cm 3/g) and BET surface areas (>1000 m 2/g). Moreover, the CFCMS material is a rigid, strong, monolith with an open structure that allows the free-flow of fluids through the material. This combination of properties provides an adsorbent material that has several distinct advantages over granular adsorbents in gas separation systems such as pressure swing adsorption (PSA) units. The results of our initial evaluations of the CO 2 adsorption capacity and kinetics of CFCMS are reported. The room temperature CO 2 adsorption capacity of CFCMS is >120 mg of CO 2 per g of CFCMS. A proposed project is described that targets the development, over a three-year period, of a demonstration separation system based on CFCMS for the removal of CO 2 from a flue gas slip stream at a coal-fired power plant. The proposed program would be conducted jointly with industrial and utility partners.

New porous Ga/Cr mixed oxide pillared α-zirconium phosphate materials

Mixed oligomeric solutions of gallium(III) and chromium(III), with Ga:Cr mole ratios between 10:90 and 90:10, have been prepared by adding n-propylammonium acetate and n-propylamine to the mixed nitrate solutions at pH 4.4–4.5. The mixed oxyhydroxyacetate oligomers thus obtained were intercalated into colloidal α-zirconium phosphate or precipitated with an excess of n-propylamine. Whilst the mixed precipitates show X-ray diffraction (XRD) reflections typical of mixed GaxCr2–xO3 oxides after calcination above 400 °C, most of the mixed intercalates were amorphous between room temperature and 800 °C, and only after calcination at 1000 °C were XRD reflections corresponding to zirconium pyrophosphate and Ga/Cr mixed oxides observed. These results suggest that mixed Ga/Cr oxide pillared α-zirconium phosphate materials are formed at 400–800 °C. The mixed pillared materials with Ga:Cr rations <60:40 have high BET surface areas (200–300 m2 g–1) and micropore volumes (0.08–0.11 cm3 g–1)1. Higher Ga:Cr ratios lead to materials with BET surface areas lower than 100 m2 g–1 and low micropore volumes. The mixed oxide powders also present high BET surface areas (184–236 m2 g–1) and micropore volumes (0.08–0.10 cm3 g–1), probably due to a templating effect of the acetate ion. A comparison of the bulk and surface compositions of pillared materials, obtained by chemical analysis and X-ray photoelectron spectroscopy (XPS), respectively, reveal that, at least up to a Ga:Cr ratio of 60:40, the intercalated species into the phosphate are mixed oligomers.

Effect of non-ionic surface active agents on TEOS-derived sols, gels and materials

The sol-gel process, starting from tetraethylorthosilicate precursor, is a suitable technique for the preparation of silica thin films. The use of specific organic additives, like non ionic surface-active agents, drastically modifies the gelation process and allows the preparation of microporous materials with a high microporous volume. The effects of additives on the sol, gel and material characteristics have been investigated by several methods such as 29Si NMR, QELS, SAXS (for sols and gels), and N2 adsorption, FESEM (for fired materials). It appears that the interactions of surface active agents with TEOS derived species limit condensation reactions and particle growing. A brittle gel structure is generated which leads to highly porous microporous silica after the elimination of organic chains by thermal treatment at 450°C. The material porous texture (specific surface area, pore size distribution and porous volume) can be varied especially by varying the surface active agent chain length and quantity. This kind of sol-gel system is suitable to prepare microporous silica membranes candidate for gas separation or catalytic reactor applications.

Controlled gas adsorption properties of various pillared clays

Microporous pillared clays (PILC) were prepared by the intercalation of montmorillonite with particles of titania (Ti-PILC), zirconia (Zr-PILC), alumina (Al-PILC), iron oxide (Fe-PILC) and mixed lanthania/alumina (LaAl-PILC). Nitrogen adsorption isotherms (77 K) and XRD data provided information on the porosity, surface area, micropore volume and interlayer distance of these samples. The surface area varied between 198 and 266 m2/g for Ti- and Fe-PILC, respectively. The titania pillared clay had also the highest micropore volume (0.142 cc/g) and interlayer spacing (16–20 A), compared to the Zr-PILC, which had the smallest spacing between the layers (max, 4 A). Despite this fact, Zr-PILC always showed a high adsorption capacity for gases such as N2, O2, Ar or CO2, due to its high adsorption field in the very small micropores. From gas adsorption experiments on these various PILCs, it became clear that their adsorption properties depend on the pillars in three ways: (i) the pillar height, (ii) the distribution of the pillars between the clay layers and (iii) the nature of the pillaring species. The incorporation of other elements in the pillars leads to specific adsorption sites in the pores. This was demonstrated by the preparation of mixed Fe/Cr and Fe/Zr pillared clays. Compared to the parent Fe-PILC, the incorporation of chromium and zirconium in the iron oxide pillars had a positive influence on the adsorption capacity. Also the modification of a PILC with cations increases both capacity and selectivity for gases. This was confirmed by the increased adsorption of N2, O2 and CO2 at 273 K on a Sr2+ exchanged Al-PILC.

Mixed alumina–chromia pillared layered α-zirconium phosphate

Mixed aluminium(III)–chromium(III) oligomers have been intercalated into colloidal α-zirconium phosphate by in situ, polymerisation from buffered mixed Al–Cr solutions at the reflux temperature. Single mixed species are formed in a wide range of added Al : Cr ratios, between 10 : 90 and 70 : 30. All materials have high basal spacings between 25.7 and 42.1 A at room temperature. 27Al MAS NMR studies suggest that the mixed Al–Cr oligomeric species intercalated at high Al : Cr ratios are higher polymers than the tridecamer [AlO4Al12(OH)24(H2O)12]7+. On calcination at 400 °C most intercalation compounds maintained free heights between 13.5 and 24.8 A, suggesting that true pillared materials are formed that are thermally stable up to 700–800 °C, after which they began to decompose into Cr2O3 and collapsed structures of aluminium zirconium phosphate. The surface of zirconium phosphate shows strong affinity for aluminium at high Al : Cr ratios as indicated by XPS analysis. The mixed alumina–chromia pillared α-zirconium phosphate materials presented high BET surface areas (158–634 m2 g–1) and high micropore volumes (0.072–0.322 cm–3 g–1) up to an Al : Cr ratio of 70 : 30. Above this ratio, the materials are barely porous with low surface areas.

Adsorption of carbon dioxide on activated carbons from diluted ambient environments

The influence of the microporosity of activated carbons on their CO 2 adsorption capacity and on the kinetics of the CO 2 adsorption process from ambient environments at low CO 2 concentrations has been studied. The experimental results show that the micropore size distribution is more important than the micropore volume or the surface area alone. Thus, activated carbons with low degree of activation, and therefore with a greater number of narrow micropores, have a higher adsorption capacity than those with a higher micropore volume but with wider micropores. The diffusion parameter and the apparent activation energy of the process were also related with the size of micropores

A microporous carbon produced by arc-evaporation

A carbon soot formed by arc evaporation and activated by heating under carbon dioxide is found to have a surprisingly high internal micropore volume (> 0.25 ml g–1) and an apparent BET surface area of ca. 700 m2 g–1, a large proportion of the pores are ca.⩽5 Å; transmission electron microscopy shows a highly disordered microstructure, which electron irradiation readily transforms into quasi spherical concentric nanoparticles of diameter of ca. 60 Å, the carbon material, which is highly absorbent to methane, shows molecular sieving properties and is more inert to oxidation than other forms of high surface area carbon.