Concept: BET theory
We report the template-free, low-temperature synthesis of a stable, amorphous, and anhydrous magnesium carbonate nanostructure with pore sizes below 6 nm and a specific surface area of ∼ 800 m(2) g(-1), substantially surpassing the surface area of all previously described alkali earth metal carbonates. The moisture sorption of the novel nanostructure is featured by a unique set of properties including an adsorption capacity ∼50% larger than that of the hygroscopic zeolite-Y at low relative humidities and with the ability to retain more than 75% of the adsorbed water when the humidity is decreased from 95% to 5% at room temperature. These properties can be regenerated by heat treatment at temperatures below 100°C.The structure is foreseen to become useful in applications such as humidity control, as industrial adsorbents and filters, in drug delivery and catalysis.
A model of Langmuir monolayer liquid adsorption into effective pores was used to study the monolayer adsorption capacity by theory calculation. The activated carbon (AC) from rice husk with NaOH activation was selected as adsorbent to uptake phenol and methylene blue (MB). Materials were characterized by N(2) adsorption, infrared spectroscopy (IR), and ultraviolet spectrophotometer (UV). In adsorption kinetics, it was observed that the experimental data were well explained by the pseudo second-order equation. Moreover, the Langmuir isotherm was more suitable to explicate results than Freundlich isotherm, implying the monolayer adsorption. Basing on the molecule diameter of 0.72 nm and cross-sectional area of 0.414 nm(2) for phenol, the theoretical adsorption capacities were close to the actual values with small relative error (<7%). Due to the large molecule size of MB (0.98 nm, 0.749 nm(2)), the sample with wider pores activated at 900°C exhibited the higher adsorption capacity than AC at 700°C. And the capillary condensation increased the adsorption capacities, consistent with the order of average pore diameter for ACs. From the model, it indicated that the adsorbent was suitable for adsorption when its critical pore width was two times of adsorbate molecule diameter.
- Proceedings of the National Academy of Sciences of the United States of America
- Published almost 3 years ago
Strength and other mechanical properties of cement and concrete rely upon the formation of calcium-silicate-hydrates (C-S-H) during cement hydration. Controlling structure and properties of the C-S-H phase is a challenge, due to the complexity of this hydration product and of the mechanisms that drive its precipitation from the ionic solution upon dissolution of cement grains in water. Departing from traditional models mostly focused on length scales above the micrometer, recent research addressed the molecular structure of C-S-H. However, small-angle neutron scattering, electron-microscopy imaging, and nanoindentation experiments suggest that its mesoscale organization, extending over hundreds of nanometers, may be more important. Here we unveil the C-S-H mesoscale texture, a crucial step to connect the fundamental scales to the macroscale of engineering properties. We use simulations that combine information of the nanoscale building units of C-S-H and their effective interactions, obtained from atomistic simulations and experiments, into a statistical physics framework for aggregating nanoparticles. We compute small-angle scattering intensities, pore size distributions, specific surface area, local densities, indentation modulus, and hardness of the material, providing quantitative understanding of different experimental investigations. Our results provide insight into how the heterogeneities developed during the early stages of hydration persist in the structure of C-S-H and impact the mechanical performance of the hardened cement paste. Unraveling such links in cement hydrates can be groundbreaking and controlling them can be the key to smarter mix designs of cementitious materials.
Nanotechnology receives a widespread application in semiconductor, manufacturing, and biotechnology industries . Its biggest societal impact in pharmaceutical application is related to its use in design of nanomedicine with the aim to improve medical efficacy via resolving the poor drug bioavailability status. Pharmaceutical nanoparticles can be described as solid colloidal particles with sizes below 1000 nm [1-6]. Examples of nanocarrier are liposome, cocheates, polymeric micelle, dendrimer, nanosuspension, nanoemulsion, nanosphere and nanotube [4, 7-10]. The nanoparticles can be used to deliver polypeptides, proteins, nucleic acids, genes and vaccines . Active pharmaceutical ingredients can be adsorbed, encapsulated or covalently attached to the surface/into the matrix of nanoparticles [1, 11-14]. Owing to small physical size and large specific surface area, they can improve the dissolution of poorly water-soluble drugs, enhance transcytosis across epithelial and endothelial barriers, enable drug targeting, enhance bioavailability, reduce dose and associated toxicity [1, 6, 14, 15]. Nanoparticles can enhance drug stability and efficacy, and enable sustained delivery [16, 17]. They can avoid or encounter rapid clearance by phagocytes thereby leading to prolonged or reduced drug circulation in the body, as a function of particle size and surface characteristics [8, 18, 19]. The nanoparticles can penetrate cells and target organs such as liver, spleen, lung, spinal cord and lymph. Its drug targeting element is mainly exploited in the treatment of solid tumors, cardiovascular diseases, and immunological diseases [15, 20-22]. Nonetheless, manufacturing of nanomedicine can be complex and additional hurdles are expected in the development for clinical usage. Spray drying is commonly used in the pharmaceutical industry to convert a liquid phase into a dry, solid powder. Both microparticles and nanoparticles can be produced/processed by means of spray drying technology. A thorough review of patents with reference to the value of spray drying technology in nanoproduct development and commercialization has been provided by Beck et al. (2012) and Patel et al. (2014) in the late issues of Recent Patents on Drug Delivery and Formulation [23,24]. Principally, the nanoparticles are obtainable via three approaches: i) spray drying solutions to obtain nanoparticles, ii) spray drying emulsions/dispersions to obtain nanoparticles and iii) spray drying pre-formed nanoparticles. The main challenges encountered by the existing spray drying or the latest nanospray drying technology are low production throughput, long production duration, and limited flexibility in processing operation when two or more reactive substances are required to be co-sprayed in situ, use of protein drugs that are prone to be lost via adsorption onto the processing device with time, and need of complex decoration of nanoparticles to enable drug targeting are concerned. Inferring from these shortfalls, it indicates that there is still an ample room for nanospray drying technology development in order to meet the therapeutic and commercial demands of the nanomedicine.
A simple method is presented to synthesize micro/nano-structured Fe-Ni binary oxides based on co-precipitation and subsequent calcination. It has been found that the Fe-Ni binary oxides are composed of the porous microsized aggregates built with nanoparticles. When the atomic ratio of Fe to Ni is 2 to 1 the binary oxide is the micro-scaled aggregates consisting of the ultrafine NiFe2O4 nanoparticles with 3-6nm in size, and shows porous structure with pore diameter of 3nm and a specific surface area of 245m(2)g(-1). Such material is of abundant surface functional groups and has exhibited high adsorption performance to As(III) and As(V). The kinetic adsorption can be described by pseudo-second order model and the isothermal adsorption is subject to Langmuir model. The maximum adsorption capacity on such Fe-Ni porous binary oxide is up to 168.6mgg(-1) and 90.1mgg(-1) for As(III) and As(V), respectively, which are much higher than the arsenic adsorption capacity for most commercial adsorbents. Such enhanced adsorption ability for this material is mainly attributed to its porous structure and high specific surface area as well as the abundant surface functional groups. Further experiments have revealed that the influence of the anions such as sulfate, carbonate, and phosphate, which commonly co-exist in water, on the arsenic adsorption is insignificant, exhibiting strong adsorption selectivity to arsenic. This micro/nano-structured porous Fe-Ni binary oxide is hence of good practicability to be used as a highly efficient adsorbent for arsenic removal from the real arsenic-contaminated waters.
Terrestrial mosses are commonly used as bioindicators of atmospheric pollution. However, there is a lack of standardization of the biomonitoring preparation technique and the efficiency of metal adsorption by various moss species is poorly known. This is especially true for in vitro-cultivated moss clones, which are promising candidates for a standardized moss-bag technique. We studied the adsorption of copper and zinc on naturally grown Sphagnum peat moss in comparison with in vitro-cultivated Sphagnum palustre samples in order to provide their physico-chemical characterization and to test the possibility of using cloned peat mosses as bioindicators within the protocol of moss-bag technique. We demonstrate that in vitro-grown clones of S. palustre exhibit acid-base properties similar to those of naturally grown Sphagnum samples, whereas the zinc adsorption capacity of the clones is approx. twice higher than that of the samples from the field. At the same time, the field samples adsorbed 30-50% higher amount of Cu(2+) compared to that of the clones. This contrast may be related to fine differences in the bulk chemical composition, specific surface area, morphological features, type and abundance of binding sites at the cell surfaces and in the aqueous solution of natural and cloned Sphagnum. The clones exhibited much lower concentration of most metal pollutants in their tissues relative to the natural samples thus making the former better indicators of low metal loading. Overall, in vitro-produced clones of S. palustre can be considered as an adequate, environmentally benign substitution for protected natural Sphagnum sp. samples to be used in moss-bags for atmospheric monitoring.
Capturing CO2 from humid flue gases and atmosphere with porous materials remains costly because prior dehydration of the gases is required. A large number of microporous materials with physical adsorption capacity have been developed as CO2-capturing materials. However, most of them suffer from CO2 sorption capacity reduction or structure decomposition that is caused by co-adsorbed H2O when exposed to humid flue gases and atmosphere. We report a highly stable microporous coppersilicate. It has H2O-specific and CO2-specific adsorption sites but does not have H2O/CO2-sharing sites. Therefore, it readily adsorbs both H2O and CO2 from the humid flue gases and atmosphere, but the adsorbing H2O does not interfere with the adsorption of CO2. It is also highly stable after adsorption of H2O and CO2 because it was synthesized hydrothermally.
Microporous carbon compartments (MCCs) were developed via controlled carbonization of wheat flour producing large cavities that allow CO2 gas molecules to access micropores and adsorb effectively. KOH activation of MCCs was conducted at 700 °C with varying mass ratios of KOH/C ranging from 1 to 5, and the effects of activation conditions on the prepared carbon materials in terms of the characteristics and behavior of CO2 adsorption were investigated. Textural properties, such as specific surface area and total pore volume, linearly increased with the KOH/C ratio, attributed to the development of pores and enlargement of pores within carbon. The highest CO2 adsorption capacities of 5.70 mol kg(-1) at 0 °C and 3.48 mol kg(-1) at 25 °C were obtained for MCC activated with a KOH/C ratio of 3 (MCC-K3). In addition, CO2 adsorption uptake was significantly dependent on the volume of narrow micropores with a pore size of less than 0.8 nm rather than the volume of larger pores or surface area. MCC-K3 also exhibited excellent cyclic stability, facile regeneration, and rapid adsorption kinetics. As compared to the pseudo-first-order model, the pseudo-second-order kinetic model described the experimental adsorption data methodically.
Metal-organic frameworks (MOFs) have emerged as an effective platform for the rational design of multifunctional materials, combining large specific surface areas with flexible, periodic frameworks that can undergo reversible structural transitions, or “breathing”, upon temperature and pressure changes, and through gas adsorption/desorption processes. Although MOF breathing can be inferred from the analysis of adsorption isotherms, direct observation of the structural transitions has been lacking, and the underlying processes of framework-reorganization in individual MOF nanocrystals is largely unknown. In this paper, we describe the characterization and elucidation of these processes through the combination of in situ environmental transmission electron microscopy (ETEM) and computer simulations. This combined approach enables the direct monitoring of the breathing behavior of individual MIL-53(Cr) nanocrystals upon reversible water adsorption and temperature changes. The ability to characterize structural changes at the lattice level provides fundamental insights into the relationship between pore size/shape and host-guest interactions.
Metal-organic frameworks (MOFs) have a high internal surface area and widely tunable composition, which make them useful for applications involving adsorption, such as hydrogen, methane or carbon dioxide storage. The selectivity and uptake capacity of the adsorption process are determined by interactions involving the adsorbates and their porous host materials. But, although the interactions of adsorbate molecules with the internal MOF surface and also amongst themselves within individual pores have been extensively studied, adsorbate-adsorbate interactions across pore walls have not been explored. Here we show that local strain in the MOF, induced by pore filling, can give rise to collective and long-range adsorbate-adsorbate interactions and the formation of adsorbate superlattices that extend beyond an original MOF unit cell. Specifically, we use in situ small-angle X-ray scattering to track and map the distribution and ordering of adsorbate molecules in five members of the mesoporous MOF-74 series along entire adsorption-desorption isotherms. We find in all cases that the capillary condensation that fills the pores gives rise to the formation of ‘extra adsorption domains’-that is, domains spanning several neighbouring pores, which have a higher adsorbate density than non-domain pores. In the case of one MOF, IRMOF-74-V-hex, these domains form a superlattice structure that is difficult to reconcile with the prevailing view of pore-filling as a stochastic process. The visualization of the adsorption process provided by our data, with clear evidence for initial adsorbate aggregation in distinct domains and ordering before an even distribution is finally reached, should help to improve our understanding of this process and may thereby improve our ability to exploit it practically.