Effect of pore expansion and amine functionalization of mesoporous silica on CO2 adsorption over a wide range of conditions

Adsorption of CO₂ was investigated over a wide range of conditions on a series of mesoporous silica adsorbents comprised of conventional MCM-41, pore-expanded MCM-41 silica (PE-MCM-41) and triamine surface-modified PE-MCM-41 (TRI-PE-MCM-41). The isosteric heat of adsorption, calculated from adsorption isotherms at different temperatures (298–328 K), showed a significant increase in CO₂–adsorbent interaction after amine functionnalization of PE-MCM-41, consistent with the high CO₂ uptake in the very low range of CO₂ concentration. The CO₂ adsorption isotherm and kinetics data showed the high potential of TRI-PE-MCM-41 material for CO₂ removal in gas purification and separation applications. With TRI-PE-MCM-41, the CO₂ selectivity over N₂ was drastically improved over a wide range of conditions compared to pure mesoporous silica. Moreover, the adsorption was reversible and fast, and the adsorbent was thermally stable and tolerant to moisture.     

Characterization of two single-wall carbon nanotubes samples by Ar and Kr adsorption isotherms

We investigated by Ar and Kr adsorption isotherm techniques for two kinds of carbon single-wall nanotube bundles prepared by different synthesis methods. Despite the difference in the adsorption capacity in the two samples, the adsorption mechanisms are similar, which indicates that the same adsorption sites are involved for Ar and Kr. We have already measured a similar difference in the adsorbed amount in these samples studied by a low-temperature heat-capacity technique, i.e., for the case of ⁴He as adsorbate. These results cannot be easily explained by only taking into account the topology of the bundles if all tubes are closed-ended. A larger spread of effective surface areas among different sources of samples is reported in the literature data.     

Controlling adsorption and release of drug and small molecules by organic functionalization of mesoporous materials

A series of mesoporous nanosphere materials that are functionalized with various terminal and bridging organic groups were synthesized. They have improved adsorption capacity and different release properties for drug and small molecules. The materials contained terminal vinyl, 3-mercaptopropyl, 3-aminopropyl, and secondary amine functional groups and bridging ethane, ethene, and benzene groups within their mesopore channel walls. The samples containing mercaptopropyl and vinyl groups showed greater adsorption capacity and better controlled release behavior for rhodamine 6G molecules. On the other hand, mesoporous matrices containing amine functional groups showed higher adsorption capacity and better release properties for ibuprofen molecules. Further studies revealed that the bridging organic groups in the mesopore channel walls also improved the adsorption capacity and release properties of the materials compared to the corresponding samples containing no bridging organic groups. Such improved adsorption and controlled release properties of molecules by simple changes of functional groups on mesoporous materials are important for the development of nanomaterial drug delivery vehicles and for controlled release of drugs over long time periods at specific targeted sites in the body. By judicious choice of organic groups and by systematic design and synthetic approaches, nanoporous materials having different adsorption capacity and release properties for many other drug molecules can also be achieved.     

Optimizing the separation of gaseous enantiomers by simulated moving bed and pressure swing adsorption

The resolution of racemic gas mixtures by simulated moving bed (SMB) and pressure swing adsorption (PSA) is investigated by dynamic simulation and optimization. Enantiomer separation of inhalation anesthetics is important because there is evidence that the purified enantiomers may have different pharmacological properties than the racemate. The model parameters reported in an experimental investigation performed elsewhere are used to study the feasibility of this separation using SMB and PSA configurations. Both processes were modeled in gPROMS^® as systems of differential algebraic equations. Operating conditions are optimized such that the feed throughput and product recovery for each process were maximized subject to equal constraints on the pressures and superficial gas velocities. SMB was found to be capable of resolving racemic feed mixtures with purity and recovery exceeding 99%. On the other hand, PSA was also able to provide a single purified enantiomer with low recovery of about 30% which may limit its application to enantiomer separation. Nevertheless, PSA consumes less desorbent, and achieves higher throughput at the sacrifice of lower recovery.     

Redox-active silica nanoparticles. Part 4. Synthesis, size distribution, and electrochemical adsorption behavior of ferrocene- and (diamine)(diphosphine)-ruthenium(II)-modified Stöber silica colloidal particles

Stöber silica nanoparticles with a diameter of approximately 800 nm are covalently modified by redox-active ferrocene or (diamine)(diphosphine) ruthenium(II) units attached to a spacer. The particles are characterized by NMR spectroscopic and chemical techniques. Two variants of modification by condensation are compared. Besides an estimation of the size and the particle porosity, the agglomeration behavior in solvents of different polarity is investigated. The adsorption of the particles to an electrode surface is followed.     

2-(p -toluidino)-6-naphthalene sulfonate as a fluorescent probe for flocculation studies of cationic potato amylopectin and nanosized silica particles. 2. Flocculation by binding of nanosized silica particles

2-(p-toluidino)-6-naphthalene sulfonate (TNS) is a probe that fluoresces strongly when bound to certain proteins and polymers, but weakly in aqueous solutions. The reversible association of TNS is used to monitor the binding of anionic nanosized silica particles (NSP) to cationic potato amylopectin starch (CApS) through the decreasing fluorescence emission as TNS is competitively released by the particle binding. Steady-state fluorescence measurements at different mixing ratios of CApS and NSP provide data on the equilibrium binding. The isotherm derived is used to establish the fact that the most efficient flocculation between CApS and NSP occurs when the polymer coils are nearly saturated by NSP, but still have positively charged parts left. This supports a patch-flocculation mechanism. Stopped-flow experiments show that NSP binding to CApS occurs within a few milli seconds. This observation allows turbidity changes which occur on longer timescales to be ascribed to particle-decorated polymers undergoing changes in the conformation or aggregation. Received: 14 August 1998 Accepted in revised form: 4 December 1998     

Redox-active silica nanoparticles. Part 6. Synthesis and spectroscopic and electrochemical characterization of viologen-modified Stöber silica particles with diameters of approximately 125 nm

The synthesis of Stöber silica particles and their redox behavior after viologen modification are reported. The particles were synthesized using high reaction temperatures and a postsynthetic calcination step to yield nonporous silica spheres with d of approximately 125 nm and low polydispersity. As the immobilization strategy, the condensation of surface hydroxyl groups with an alkoxysilane-substituted viologen modifier was chosen. The successful attachment was confirmed by ¹³C-CP/MAS-NMR and diffuse reflectance Fourier transform infrared as well as UV/Vis and energy dispersive X-ray spectroscopy. The surface concentration was estimated to be between 48 and 100  $\upmu$ mol/g. The modified material is redox-active. Differential pulse voltammetry and cyclic voltammetry experiments in nonaqueous solvents at Pt and mercaptopropionic acid-modified gold electrodes reveal two separated and pronounced signals of the first and second reduction of the viologen unit. The observed potentials are in good agreement with those of the free modifier. Furthermore, the treatment of the viologen-modified material with Na₂S₂O₄ leads to the formation of a nanoparticulate silica material with stable free radicals located at the particle surface. In addition, such radicals are generated by the in situ electrolysis at Pt net electrodes in acetonitrile or dimethylformamide. The radical formation was confirmed by EPR and UV/Vis spectroscopy. The reduction is reversed by air oxidation.     

Hydrolysis of mixed monomolecular films of tricaprylin/dilauroylphosphatidylcholine by lipase and phospholipase A₂

The potential of chitosan, a fishery waste-based material, as a soil amendment to clean-up metal contaminated soil was investigated. Chitosan was treated using glutaraldehyde (GLA), epichlorohydrin (ECH) and ethylene glycol diglycidyl ether (EGDE) as cross-linking reagents to enhance its chemical stability in acidic media and to improve its physical properties. Cross-linking treatment had significant effects on chitosan surface area, pore diameter, surface morphology and crystallinity. Interaction with Ag(I), Pb(II) and Cu(II) decreased the crystallinity of the materials and changed their surface morphology significantly. FTIR analysis confirmed that N and O atoms served as binding sites for the metal ions. Chitosan and treated chitosans were able to bind metal ions, even in the presence of K⁺, Cl^? and NO₃^?, which are dominant ions in soil. Therefore, remediation of metal contaminated soil using chitosan and cross-linked treated chitosans as soil amendments is feasible.     

Diffusion of linear paraffins in silicalite studied by the ZLC method in the presence of CO2

The diffusion behavior of C₄–C₁₀ n-alkanes in silicalite-1 has been investigated by using the Zero Length Column method. The diffusivities derived from measurements at different purge rates with different purge gases confirming intracrystalline diffusion control. Data are compared with results reported in the literature for MFI zeolites. The diffusivities were found to be consistent and agree well with data previous obtained by ZLC. However, these data showed a remarkable disagreement with other reported techniques (PFG-NMR, QENS and Permeation). The eventual influence of carbon dioxide (CO₂) adsorption on diffusion properties of n-alkanes in silicalite was also investigated. For this purpose, a series of experiments was performed involving hydrocarbons mixed with CO₂. Data were obtained at 303 K and flow rates between 20 and 80 mL/min. The presence of CO₂ does not seem to influence the intracrystalline transport rate of the investigated light hydrocarbons (n-C₄ and n-C₆). On the other hand, the situation for n-C₈ and n-C₁₀ is more complex. The diffusivity values are higher compared to the previously reported values.     

Preparation of titania hollow particles with independently controlled void size and shell thickness by catalytic templating core–shell polymer particles

Organic/inorganic hybrids were prepared by catalytic hydrolysis and subsequent polycondensation of tetra-n-butyl titanate (TnBT) in shell layers grafted on core particles. The core particles were synthesized by emulsifier-free emulsion polymerization of styrene, N-n-butyl-N-2-methacryloyloxyethyl-N,N-dimethylammonium bromide (C₄DMAEMA), and 2-chloropropionyloxyethyl methacrylate using 2,2′-azobis(2-amidinopropane) dihydrochloride as an initiator. The core diameters were controlled in the range of 70–550 nm by adjusting a C₄DMAEMA feed concentration. The core–shell particles were prepared by surface-initiated activator generated electron transfer–atom transfer radical polymerization of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA). The sizes of core–shell particles were found to increase monotonically with an increase in a DMAEMA concentration. The hybrid particles were fabricated by adding TnBT into a water/ethanol dispersion of core–shell particles. The amounts of titania deposited increased in proportion to the grafted amounts of poly[2-(N,N-dimethylamino)ethyl methacrylate] on the core particles. The X-ray diffraction measurement revealed that the hollow titania particles obtained by heat treatment of hybrids have an anatase crystallographic phase.     

Changes in the intra- and inter-fibrillar structure of lyocell (TENCEL®) fibers caused by NaOH treatment

Some effects of NaOH treatment at concentrations of up to 8 M on (1) the porous structure, (2) the degree of swelling, (3) carboxyl content using methylene blue sorption and 9H-fluoren-2-yl-diazomethane (FDAM) methods, (4) dyeing, (5) the molecular weight distribution measured by gel permeation chromatography (GPC), (6) crystallinity determined by wide angle X-ray diffraction (WAXD) and (7) the tensile properties of lyocell fibers were investigated. The porous structure of fibers was visualised using fluorescence microscopy and transmission electron microscopy (TEM) on fiber cross-sections and was also studied by inversion size exclusion chromatography (ISEC). Mean pore diameter and pore area of fibers were not changed by NaOH treatments. The pore volume increased above 2.5 M NaOH. NaOH-treated samples showed higher dye uptake, higher swelling, but lower carboxyl and moisture content and increased crystallinity. As the NaOH concentration increased, the depth of colour following dyeing with C.I. Direct Red 81 also increased due to deep penetration of alkali into the fiber. In general, fiber properties were distinctly different in the ranges 0 to approximately 3 and 3–8 M NaOH. A part of this paper was presented at the 5th Central European Conference 2007, Fiber Grade Polymers, Chemical Fibers and Special Textiles, Krakow-Bielsko-Biala, Poland, 5–8 September 2007. Hale Bahar ?ztürk, Antje Potthast, Thomas Rosenau, Bill MacNaughtan, John R. Mitchell, Thomas Bechtold—Members of European Polysaccharide Network of Excellence (EPNOE), .     

Capture of CO2 from flue gas streams with zeolite 13X by vacuum-pressure swing adsorption

Vacuum swing adsorption (VSA) capture of CO₂ from flue gas streams is a promising technology for greenhouse gas mitigation. In this study we use a detailed, validated numerical model of the CO2VSA process to study the effect of a range of operating and design parameters on the system performance. The adsorbent used is 13X and a feed stream of 12% CO₂ and dry air is used to mimic flue gas. Feed pressures of 1.2 bar are used to minimize flue gas compression. A 9-step cycle with two equalisations and a 12-step cycle including product purge were both used to understand the impact of several cycle changes on performance. The ultimate vacuum level used is one of the most important parameters in dictating CO₂ purity, recovery and power consumption. For vacuum levels of 4 kPa and lower, CO₂ purities of >90% are achievable with a recovery of greater than 70%. Both purity and recovery drop quickly as the vacuum level is raised to 10 kPa. Total power consumption decreases as the vacuum pressure is raised, as expected, but the recovery decreases even quicker leading to a net increase in the specific power. The specific power appears to minimize at a vacuum pressure of approximately 4 kPa for the operating conditions used in our study. In addition to the ultimate vacuum level, vacuum time and feed time are found to impact the results for differing reasons. Longer evacuation times (to the same pressure level) imply lower flow rates and less pressure drop providing improved performance. Longer feed times led to partial breakthrough of the CO₂ front and reduced recovery but improved purity. The starting pressure of evacuation (which is not necessarily equal to the feed pressure) was also found to be important since the gas phase was enriched in CO₂ prior to removal by vacuum leading to improved CO₂ purity. A 12-step cycle including product purge was able to produce high purity CO₂ (>95%) with minimal impact on recovery. Finally, it was found that for 13X, the optimal feed temperature was around 67°C to maximize system purity. This is a consequence of the temperature dependence of the working selectivity and working capacity of 13X. In summary, our numerical model indicates that there is considerable scope for improvement and use of the VSA process for CO₂ capture from flue gas streams.     

Capture of CO2 from high humidity flue gas by vacuum swing adsorption with zeolite 13X

Capture of CO₂ from flue gas streams using adsorption processes must deal with the prospect of high humidity streams containing bulk CO₂ as well as other impurities such as SO _x , NO _x , etc. Most studies to date have ignored this aspect of CO₂ capture. In this study, we have experimentally examined the capture of CO₂ from a 12% synthetic flue gas stream at a relative humidity of 95% at 30 °C. A 13X adsorbent was used and the migration of the water and its subsequent impact on capture performance was evaluated. Binary breakthrough of CO₂/water vapor was performed and indicated a significant effect of water on CO₂ adsorption capacity, as expected. Cyclic experiments indicate that the water zone migrates a quarter of the way into the column and stabilizes its position so that CO₂ capture is still possible although decreased. The formation of a water zone creates a “cold spot” which has implications for the system performance. The recovery of CO₂ dropped from 78.5% to 60% when moving from dry to wet flue gas while the productivity dropped by 22%. Although the concentration of water leaving the bed under vacuum was 27%(vol), the low vacuum pressure prevented condensation of water in this stream. However, the vacuum pump acted as a condenser and separator to remove bulk water. An important consequence of the presence of a water zone was to elevate the vacuum level thereby reducing CO₂ working capacity. Thus although there is a detrimental effect of water on CO₂ capture, long term recovery of CO₂ is still possible in a single VSA process. Pre-drying of the flue gas steam is not required. However, careful consideration of the impact of water and accommodation thereof must be made particularly when the feed stream temperature increases resulting in higher feed water concentration.     

Kinetic and equilibrium studies of lead(II) adsorption from aqueous media by KIT-6 mesoporous silica functionalized with –COOH

An organic–inorganic hybrid mesoporous silica was synthesized via post-grafting of KIT-6 with 4-(triethoxysilyl)butyronitrile. All samples were characterized using their N₂ adsorption–desorption isotherms, XRD, FT–IR, TEM, SEM, and PT. The adsorption potential of this material for removing Pb(II) from aqueous solutions was investigated via the batch technique, and the effects of pH and contact time were studied. Experimental data showed that the maximum Pb(II) adsorption, 76%, occurred in the pH range around 6. The adsorption equilibrium was reached within 40 min for 10 wt.%COOH/KIT-6. The adsorption data were fitted using the Langmuir and Freundlich isotherms, and the obtained modeling equilibrium adsorption data suggested that the 10 wt.%COOH/KIT-6 sample contained homogeneous adsorption sites that fit the Langmuir adsorption model well. The pseudo-second-order model described well the 10 wt.%COOH/KIT-6 adsorption process. The desorption and regeneration experiments indicated that ≈95% of the metal desorbed and the adsorbent could be regenerated via an acid treatment without altering its properties.     

Gas adsorption process in activated carbon over a wide temperature range above the critical point. Part 2: conservation of mass and energy

Simulations of the thermal effects during adsorption cycles are a valuable tool for the design of efficient adsorption-based systems such as gas storage, gas separation and adsorption-based heat pumps. In this paper, we present simulations of the thermal phenomena associated with hydrogen, nitrogen and methane adsorption on activated carbon for supercritical temperatures and high pressures. The analytical expressions of adsorption and of the internal energy of the adsorbed phase are calculated from a Dubinin-Astakhov adsorption model using solution thermodynamics. A constant adsorption volume is assumed. The isosteric heat is also calculated and discussed. Finally, the mass and energy rate balance equations for an adsorbate/adsorbent pair are presented and are shown to be in agreement with desorption experiments.     

Sol–gel synthesis of tetragonal ZrO2 nanoparticles stabilized by crystallite size and oxygen vacancies

In this letter, we present a facile route to produce metastable tetragonal zirconia (ZrO₂) nanoparticles via pH-controlled precipitation of dilute zirconyl nitrate dihydrate [ZrO(NO₃)₂·2H₂O] solution in liquid NH₃ under ambient conditions and calcination at 500 °C for 2 h. The phase pure tetragonal ZrO₂ nanoparticles are obtained at pH 9. The effect of pH on metastable phase stabilization in precipitated ZrO₂ nanoparticles is demonstrated with the help of XRD, SEM/EDX, and X-ray photoelectron spectroscopy techniques. The stability of tetragonal ZrO₂ phase is attributed to the smaller crystallite size and greater oxygen deficiency in phase-pure tetragonal ZrO₂.     

Synthesis of alginate stabilized gold nanoparticles by γ-irradiation with controllable size using different Au3+ concentration and seed particles enlargement

Gold nanoparticles with pre-selected size in the range 5–40 nm were synthesized by γ-irradiation of Au³⁺ solution containing natural polysaccharide alginate as a stabilizer. The gold nanoparticles with controllable size were prepared by two approaches: (i) varying the concentration of Au³⁺ from 0.25 to 1 mM and alginate from 0.25% to 1% (w/v) and (ii) enlargement of seed particles with double size from 20 to 40 nm at [Au³⁺]/[Au⁰]=6. The obtained gold nanoparticles were characterized by UV–vis spectroscopy and transmission electron microscopy. The results indicated that γ-irradiation method is suitable for production of gold nanoparticles with controllable size and high purity.     

A theoretical study of CO adsorption on gold by Hückel theory and density functional theory calculations

It is crucial to understand the nature of CO adsorption on gold so as to elucidate the mechanism of low-temperature CO oxidation on nanogold catalysts. We performed theoretical analysis of CO adsorption on gold by using Hückel theory and density functional theory (DFT) calculations. Hückel theory indicates that CO adsorption on gold is dominated by the electron distribution at the Au atom, which is greatly affected by neighboring Au atoms, coadsorbed or doping species. The increase of σ-bonding electrons should weaken the CO adsorption, while the increase of π-electrons should strengthen the adsorption. DFT calculations proved this prediction quantitatively for various systems, including CO adsorption on a Au(100)-hex surface with locally varying subsurface configurations and CO coadsorption with acceptor or donor species.