Check out our New Publishers' Select for Free Articles

Adsorption: Journal of the International Adsorption Society (v.18, #3-4)

Amino functionalised Silica-Aerogels for CO₂-adsorption at low partial pressure by K. Wörmeyer; M. Alnaief; I. Smirnova (pp. 163-171).

Effective adsorption of CO₂ at low partial pressures is required for many technical processes, such as gas purification or CO₂ removal in closed loop environmental control systems. Since the concentration of CO₂ in such applications is rather low, a high adsorption capacity is a required property for the adsorbent. Silica aerogels possessing an open pore structure, a high porosity and a high surface area, have a great potential for utilisation as CO₂ adsorbents. Nonetheless in order to reach high adsorption capacities, silica aerogels should be functionalised, for instance by amino functionalisation. In this work, two different functionalisation methods were applied for the generation of amino functionalised aerogels: co-condensation during the sol-gel process and post-treatment of the gel. The co-condensation functionalisation allows the introduction of up to 1.44 wt.% nitrogen into the aerogel structure with minor reductions in surface area, leading however only to minor increases in the adsorption capacity at low partial pressures. The post functionalisation of the gel causes a greater loss in surface area, but the CO₂ adsorption capacity increases, due to the introduction of higher amounts of amino groups into the aerogel structure (up to 5.2 wt.% nitrogen). Respectively, 0.523 mmol CO₂/g aerogel could be adsorbed at 250 Pa. This value is comparable with the adsorption capacity at this pressure of a standard commercially available adsorbent, Zeolite 13X.

Keywords: Carbon dioxide adsorption; Silica; Aerogel; Amino functionalisation; Zeolite 13X; Low partial pressure

Competitive adsorption of the herbicide fluroxypyr and tannic acid from distilled and tap water on activated carbons and their thermal desorption by C. Moreno-Castilla; M. V. López-Ramón; L. M. Pastrana-Martínez; M. A. Álvarez-Merino; M. A. Fontecha-Cámara (pp. 173-179).

A study was conducted on batch and column competitive adsorption of fluroxypyr (FLX) and tannic acid (TA) from distilled (DW) and tap water (TW) on activated carbon cloth (ACC) and granular activated carbon (GAC). Thermal desorption of the adsorbates from the spent ACC was also studied. FLX adsorption was higher from TW than from DW at low FLX equilibrium concentrations, and the inverse was observed at high FLX concentrations. The presence of TA diminished the amount of FLX adsorbed from both solvents due to partial blocking of the microporosity, but the same trends as before were observed at low and high FLX concentrations. Carbon consumption, obtained from the breakthrough curves, was lower as a function of superficial contact time with ACC than with GAC. The presence of TA increased carbon consumption, which was related to the microporosity of the adsorbents. Thermal desorption profiles of the spent ACC showed two peaks and one peak after adsorption from DW and TW, respectively. Desorption peaks shifted to higher temperatures with an increase in the heating rate, allowing the apparent activation energies and pre-exponential factors of the desorption processes to be determined.

Keywords: Batch adsorption; Column adsorption; Fluroxypyr; Tannic acid; Activated carbons; Thermal desorption

Preparation of γ-Al₂O₃ sorbents loaded with metal components and removal of thiophene from coking benzene by Junjie Liao; Yanjun Zhang; Wenbo Wang; Yuanyuan Xie; Liping Chang (pp. 181-187).

Series sorbents of Cu, Zn, Ni, Ce and Ag metal components supported on γ-Al₂O₃ carrier for removing thiophene from benzene were prepared by conventional and ultrasound-assisted incipient-wetness impregnation method. The static adsorption experiments were carried out in the thiophene-benzene solution with thiophene concentration of 500 mg/L. The results show that the desulfurization activity of all γ-Al₂O₃ sorbents modified by different metal components obviously increase, among which the sorbent modified by silver nitrate has the best performance. The active components of sorbents from Cu, Zn, Ni, Ce nitrates loaded on γ-Al₂O₃ carrier are their oxides. Besides Ag₂O, the products of silver nitrate thermal decomposition in sorbent prepared still have Ag⁰ and Ag–O–Al species. The assistant ultrasound in the process of sorbent preparation can not only shorten the impregnation time, but also enrich the pore structure of sorbent and improve the size and distribution of the Ag species, which is favorable to the removal of thiophene from benzene. The desulfurization capacity of sorbent changes with the Ag content loaded. The sorbent with 15 % quality content of Ag prepared by ultrasound-assisted impregnation method has the highest desulfurization efficiency. It could reduce the thiophene concentration to 1.7 mg/L from 500 mg/L at room temperature and ambient pressure, with the desulfurization efficiency of more than 99 %, when the ratio of sorbent to solution was 1:4 (g/mL).

Keywords: Thiophene; Benzene; Ultrasound; γ-Al₂O₃ ; Adsorption

Examination of sorption and desorption of hydrogen on several samples of polish hard coals by Janusz Cygankiewicz; Agnieszka Dudzińska; Mieczysław Żyła (pp. 189-198).

Performed tests showed that at 298 K hard coals sorb relatively small amounts of hydrogen. Those amounts depend on carbon and oxygen content in tested coals. The most considerable amounts of hydrogen are sorbed by coals characterized by strong surface hydrophobicity and high content of aliphatic hydrocarbons. The hydrophilic nature of coal surface does not lead to higher sorption of hydrogen. It was found that the change in amount of sorbed hydrogen is closely related to the moisture. For high moisture coal a significant decrease in hydrogen sorption is observed. Also tests on hydrogen desorption on hard coals were carried out using method of lowering hydrogen pressure above the sorbent. Obtained results showed that tested coals desorb various amounts of hydrogen. Process of sorption is reversible only for some coals, while for the others the desorption isotherm partially lies beneath the sorption isotherm, which indicates that in addition to hydrogen some other chemical substances are emitted from coal.

Keywords: Sorption; Hydrogen; Coal

Effect of CO₂ activation of carbon xerogels on the adsorption of methylene blue by Carlos A. Páez; María S. Contreras; Angélique Léonard; Silvia Blacher; Claudio G. Olivera-Fuentes; Jean-Paul Pirard; Nathalie Job (pp. 199-211).

The effect of physical activation with CO₂ of carbon xerogels, synthesized by pyrolysis of a resorcinol-formaldehyde aqueous gel, on the adsorption capacities of Methylene Blue (MB) was studied. The activation with CO₂ lead to carbon materials with micropore volumes ranging from $0.28 mathrm{to} 0.98~mathrm{cm}^{3},mathrm{g}_{mathrm{C}}^{-1}$ . MB-adsorption isotherm studies showed that the increase of micropore volume and corresponding surface area led to: (i) a significant improvement in the capacity of MB-adsorption at monolayer coverage, from $212 mathrm{to} 714~mathrm{mg},mathrm{g}_{mathrm{C}}^{-1}$ , and (ii) an increase of the binding energy related to Langmuir isotherm constant up to 45 times greater than those of commercial microporous activated carbons used as reference (NORIT R2030, CALGON BPL and CALGON NC35). It is proposed that the increase of the binding energy results from chemical cleaning of the O-groups onto carbon surface as a consequence of CO₂-activation, increasing the π–π interaction between MB and graphene layers of the carbon xerogels. Finally, a series of batch kinetics were performed to investigate the effect of CO₂-activation conditions on the mechanism of MB-adsorption. Experimental data were fitted using pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. From pseudo-second-order kinetic model, one observes an increase in the initial rate of MB-adsorption from 0.019 to 0.0565 min⁻¹, by increasing the specific surface area from $630 mathrm{to} 2180~mathrm{m}^{2},mathrm{g}_{mathrm{C}}^{-1}$ via CO₂-activation. Depending on the activation degree of the carbons, two different mechanisms control the MB-adsorption rate: (i) at low activation degree, the intraparticle diffusion is the rate-limiting phenomenon, whereas (ii) at high activation degree, the reactions occurring at the solid/liquid interface are the rate-limiting steps.

Keywords: Carbon xerogel; CO₂-activation; Methylene blue adsorption; Kinetics; Intraparticle diffusion

MCM-41, MOF and UiO-67/MCM-41 adsorbents for pre-combustion CO₂ capture by PSA: adsorption equilibria by Johanna Schell; Nathalie Casas; Richard Blom; Aud I. Spjelkavik; Anne Andersen; Jasmina Hafizovic Cavka; Marco Mazzotti (pp. 213-227).

Three different adsorbent materials, which are promising for pre-combustion CO₂ capture by a PSA (Pressure Swing Adsorption) process, are synthesized, pelletized and characterized. These materials are USO-2-Ni metal organic framework (MOF), mesoporous silica MCM-41 and a mixed material consisting of UiO-67 MOF bound with MCM-41. On these materials, equilibrium adsorption isotherms of CO₂ and H₂ are measured at different temperatures (25–140 °C) in a wide pressure range (up to 15 MPa). From the experimental data the parameters of different isotherm equations (Langmuir, Sips and Quadratic) are determined, together with the isosteric heats of adsorption. Binary adsorption of CO₂/H₂ mixtures on USO-2-Ni MOF is additionally measured and compared to predicted values using IAST (Ideal Adsorbed Solution Theory) showing a good agreement. The potential of the materials for the application of interest is evaluated by looking at their cyclic working capacity and compared to those of a commercial activated carbon. From this evaluation especially the USO-2-Ni MOF adsorbent looks promising compared to the commercial activated carbon. For the other two materials a smaller improvement, which is limited to lower temperatures, is expected.

Keywords: Pre-combustion CO₂ capture; MCM-41; MOF; USO-2-Ni; UiO-67; PSA; Adsorption equilibria; CO₂ ; H₂

The correlation between structural characteristics of activated carbons and their adsorption of organic solutes from aqueous solutions by Shujuan Zhang; Ting Shao; Tanju Karanfil; Bingcai Pan (pp. 229-238).

Adsorption is controlled by an array of attractive forces between adsorbent, adsorbate, and solvent molecules. Such forces work interactively, making the interpretation and prediction of sorption processes difficult. By carefully designing the experimental matrix, the effects of adsorbent surface chemistry and pore structure on the adsorption of aromatic compounds were isolated from the complicated web of interactions. Two parameters, γ and δ, were created to describe the relative adsorption affinity index of activated carbons to adsorbates and the occupancy rate of activated carbons by active sites that can lead to formation of water clusters. Taking the space availability and the relative adsorption affinity index into account, a correlation between the Freundlich adsorption affinity coefficient and the characteristics of adsorbent was established. With this correlation, if the Freundlich adsorption affinity coefficient of a compound on one carbon is known, its adsorption affinity coefficient on another carbon might be predictable if the surface chemistries and pore structures of both carbons are available.

Keywords: Activated carbon; Adsorption affinity; Physical structure; Surface chemistry; Correlation

Hydrogen adsorption on microporous materials at ambient temperatures and pressures up to 50 MPa by Tyler G. Voskuilen; Timothée L. Pourpoint; Anne M. Dailly (pp. 239-249).

High surface area microporous adsorbents are often proposed as potential hydrogen storage materials, although typically at 77 K and less than 5 MPa. In this study, we focus on conditions more suitable for automotive applications by investigating the storage capacities of microporous materials at 298 K and at pressures up to 50 MPa. In an effort to derive trends within and across material classes, we examined a wide range of materials with varying microstructures including the activated carbons AX-21, KUA-5, and MSC-30; a zeolite templated carbon; a hypercrosslinked polymer; and the Metal Organic Frameworks MOF-177, IRMOF-20, MIL-53, ZIF-8, and Cu₃(btc)₂. The peak excess adsorption of these materials ranged from 0.8–1.8 wt.%, although many did not reach their maximum capacity even at high pressures. However, the total volumetric storage gains over compressed hydrogen gas were quite low and, in many cases, negative. In addressing ambient temperature adsorption at significantly higher pressures than previously reported, our data confirms and extends the range of validity of several existing DFT calculations. Furthermore, our data suggest that, for both activated carbons and MOFs, factors other than specific surface area govern ambient temperature adsorption capacity. Contrary to some reports, the high fractions of sub-nanometer pores in some of the investigated MOFs did not appear to enhance the excess adsorption even at high pressures. For on-board applications with ambient temperature storage, significant enhancements to the attractive force at the materials’ surface are required, beyond merely increasing specific surface area, or for MOFs, tuning of pore sizes.

Keywords: Hydrogen storage; Adsorption; Zeolite templated carbon; Metal organic framework; Activated carbon

A dynamic column breakthrough apparatus for adsorption capacity measurements with quantitative uncertainties by Paul S. Hofman; Thomas E. Rufford; K. Ida Chan; Eric F. May (pp. 251-263).

A dynamic column breakthrough (DCB) apparatus was used to study the separation of CH₄+N₂ gas mixtures using two zeolites, H⁺-mordenite and 13X, at temperatures of (229.2 and 301.9) K and pressures to 792.9 kPa. The apparatus is not limited to the study of dilute adsorbates within inert carrier gases because the instrumentation allows the effluent flow rate to be measured accurately: a method for correcting apparent effluent mass flow readings for large changes in effluent composition is described. The mathematical framework used to determine equilibrium adsorption capacities from the dynamic adsorption experiments is presented and includes a method for estimating quantitatively the uncertainties of the measured capacities. Dynamic adsorption experiments were conducted with pure CH₄, pure N₂ and equimolar CH₄+N₂ mixtures, and the results were compared with similar static adsorption experiments reported in the literature. The 13X zeolite had the greater adsorption capacity for both CH₄ and N₂. At 792 kPa the equilibrium capacities of the 13X zeolite increased from 2.13±0.14 mmol g⁻¹ for CH₄ and 1.36±0.10 mmol g⁻¹ for N₂ at 301.9 K to 3.97±0.19 mmol g⁻¹ for CH₄ and 3.33±0.12 mmol g⁻¹ for N₂ at 229.2 K. Both zeolites preferentially adsorbed CH₄; however, the mordenite had a greater equilibrium selectivity of 3.5±0.4 at 301.9 K. Equilibrium selectivities inferred from pure fluid capacities using the Ideal Adsorbed Solution theory were limited by the accuracy of the literature pure fluid Toth models. Equilibrium capacities with quantitative uncertainties derived directly from DCB measurements without reference to a dynamic model should help increase the accuracy of mass transfer parameters extracted by the regression of such models to time dependent data.

Keywords: Adsorption; Zeolites; Packed bed; Pressure swing adsorption; Petroleum; Gases

Physical and chemical properties of PAN-derived electrospun activated carbon nanofibers and their potential for use as an adsorbent for toxic industrial chemicals by P. Sullivan; J. Moate; B. Stone; J. D. Atkinson; Z. Hashisho; M. J. Rood (pp. 265-274).

A recently developed carbon material, electrospun Activated Carbon nanoFiber (ACnF), exhibits strong potential for use as an adsorbent for toxic industrial chemicals (TICs). As-prepared ACnF contains as much as 9.6 wt% nitrogen, creating a basic surface that enhances acid-gas adsorption. ACnF shows 4–20 times greater HCN adsorption capacities and 2–5 times greater SO₂ adsorption capacities in dry nitrogen, compared to commercially available activated carbon fiber cloth (ACFC) and Calgon BPL™ granular activated carbon, which are considered here as reference adsorbents. ACnF has 50 % of the micropore volume (0.30 cm³/g) of these reference adsorbents, which limits its adsorption capacity at high concentrations for volatile organic compounds (>500 ppm_v). However, at low concentrations (<500 ppm_v), ACnF has a similar capacity to ACFC and about three times the VOC adsorption capacity of Calgon BPL™. ACnF’s small fiber diameters (0.2–1.5 μm) allow for higher mass transfer coefficients, resulting in adsorption kinetics nearly twice as fast as ACFC and eight times as fast as Calgon BPL™. ACnF drawbacks include hydrophilicity and reduced structural strength. The rapid adsorption kinetics and high capacity for acidic TICs warrant further investigation of ACnF as an adsorbent in respiratory protection and indoor air quality applications.

Keywords: Adsorption; HCN; Nanofiber; SO₂ ; Butane; Toxic industrial chemical; Activated carbon

Heuristics for synthesis and design of pressure-swing adsorption processes by Pramathesh R. Mhaskar; Arun S. Moharir (pp. 275-295).

Simulation based synthesis and design of adsorptive enrichment of CO from tail gas having 51 % CO are presented. The adsorption breakthrough curve simulation using this feed gas composition, helped to provide a starting guess of the adsorption step duration in a pressure-swing adsorption cycle for meeting the purity and recovery targets. Use of smaller bed dimensions facilitated the simulation of many cycles. These simulations helped to decide the operating pressure range, operating temperature, constituent steps of the cycle, their sequence, direction of pressurization of the bed, number of beds in the cycle and the composition of the streams to be used for pressurization and/or rinse and/or purge steps. Only an optimally designed pressure-vacuum-swing adsorption cycle achieves the stiff separation targets of getting an extract having 80 % pure CO at 80 % recovery in a single-stage with an adsorbent that uses physical adsorption and offers CO/CH₄ sorption selectivity of just 2.44. Additional simulations are done wherein the bed sizes and velocities are increased to predict the performance of a large-scale unit. These require deciding only the durations of the steps that are finalized from the small-scale unit simulations. These durations were kept fixed and the bed dimensions were varied till the separation targets are obtained for the particular feed rate. The scale-up criteria was matching residence times in the bed. A strategy for treating a feed gas having only 30 % CO is also discussed and a novel concept of cascaded PSA is evaluated using simulations. Some heuristics are evolved from the studies.

Keywords: Heuristics; PSA; Process synthesis; Process design; Simulation

Investigating parameters on the preparation of mesoporous activated carbons by the combination of chemical and physical activations using the Taguchi method by Mohammad Jaber Darabi Mahboub; Ali Ahmadpour; Hamed Rashidi; Neda Jahanshahi (pp. 297-305).

Mesoporous activated carbons were prepared from coconut shell by the combination of chemical and physical activation methods. Zinc chloride and CO₂ were used as chemical and physical agents, respectively. Optimum parameters were obtained from investigating the effect of various factors at different levels on the methane storage of wet activated carbons using the Taguchi experimental design method. Soaking time, carbonization temperature, and carbonization time were found as effective parameters in the methane storage. Finally, after achieving optimum levels for each factors based on the enhancement of methane storage, a confirmation experiment was conducted. Methane uptakes of the activated carbons were measured at temperature of 2 ^∘C up to the pressure of 80 bar and it turned out that the maximum amount of methane storage (241 V/V) had a good agreement with the predicted result from the Taguchi method.

Keywords: Activated carbon; Mesoporous; Methane storage; Adsorbed natural gas (ANG)

Dependence of surface properties of silylated silica on the length of silane arms by Maila Castellano; Enrico Marsano; Antonio Turturro; Lucia Conzatti; Guido Busca (pp. 307-320).

Amorphous precipitated Zeosil 1165 MP silica was silylated with low grafting degrees of organosilicons bearing different alkoxy and hydrocarbon tails, like monomethoxy(dimethyl)octadecylsilane (DMODMS), monomethoxytrimethylsilane (TMMS), trimethoxymercaptopropylsilane (MPTS), and 3-octanoylthio-1-propyltriethoxysilane (NXT^®). Thermogravimetry and Elemental Analysis were used to determine the degree of silane grafting and the final number of free silanol OH groups/nm² on the modified Zeosil surface. Free energy, enthalpy and entropy of adsorption of hydrocarbon probes were determined by Inverse Gas Chromatography at infinite dilution and dispersive component, $gamma_{s}^{d}$ , and specific interaction parameter, I sp, of the surface tension of the silica surface were calculated. Silylation changes the hydrophilic character of Zeosil silica to the hydrophobic one, on increasing the grafting degree and, mainly, the length of hydrocarbon tail of the silane molecule (DMODMS and NXT^®). The long hydrocarbon tails practically shield the silica particle surface and the adsorbed probes preferentially interact with them. In the case of TMMS-Zeosil the adsorbed probes practically interact with the silica surface, with loss of entropy well above that of the bare silica, while being equal the values of the enthalpy of adsorption. All the other modified silicas show loss of entropy lower than that of bare silica. Steric hindrance, played by the presence of methyl groups of TMMS, is suggested to reduce the freedom of translational and rotational movements of the adsorbed probe.

Keywords: Zeosil silica; Silylation; Structure of silanes; Surface properties

Effect of carbon pore structure on the CH₄/N₂ separation by Congmin Liu; Yanyan Dang; Yaping Zhou; Jia Liu; Yan Sun; Wei Su; Li Zhou (pp. 321-325).

The separation between CH₄ and N₂ bears importance in coalbed methane enrichment, and activated carbon is a major adsorbent for industrial PSA (pressure swing adsorption) separation. However, the adsorption of both gases shows supercritical features, and the physicochemical properties are also similar, which results in similar adsorption behavior and renders the separation difficult. To maximize the separation coefficient, the effect of carbon pore structure on the separation was studied and a series of carbons was prepared at different extent of activation. The effect of specific surface area, pore size and pore volume on the separation coefficient was observed and a linear correlation between the separation coefficient and the small pore (0.7–1.3 nm) volume reduced to unit surface area was shown.

Keywords: Adsorption; Separation; CH₄/N₂ ; Methane enrichment; Activated carbon

Sections

Personal tools

Adsorption: Journal of the International Adsorption Society (v.18, #3-4)

Sections

Personal tools

Local resources

Adsorption: Journal of the International Adsorption Society (v.18, #3-4)