Micropore Filling and Multilayer Formation in Stöber Spheres upon Water Adsorption

The presence of porosity critically affects the performance of solid systems. The pore accessibility to adsorbate molecules and the corresponding adsorption/desorption behavior are crucial aspects to understand the properties of porous materials but are difficult to address, principally when dealing with narrow micropores. A prominent example is colloidal silica (Stöber) spheres whose microporosity, inaccessible for some adsorbates, can be readily filled by water molecules to a large extent but exhibiting a complex adsorption behavior with unexpected hystereses. Here, we perform water adsorption isotherms on Stöber spheres at different temperatures using an original analysis of the Dubinin–Radushkevich representation to examine both the accessibility to the microporosity and the formation of water multilayers on the outer sphere surface. The micropore filling (and emptying) is found to be limited by the kinetic energy of the water molecules, causing low-pressure hysteresis. We further discover that the (temperature-dependent) completion of the micropore filling delays the onset of multilayer adsorption, leading to hysteresis at a high relative pressure. The number of adsorbed water layers is determined, and the adsorption-induced swelling of the spheres is discussed.This work was funded by Spanish MINECO projects MAT2014-58731-JIN and MAT2016-80285-p and Spanish MCIU project RTI2018-093921-B-C41

Improved thermal management in HKUST-1 composites upon graphite flakes incorporation: Hydrogen adsorption properties

HKUST-1-based composites have been synthesized through the incorporation of synthetic graphite flakes in the MOF synthesis media. The presence of flakes gives rise to high quality HKUST-1 crystals, combining different morphologies (octahedral-shape crystals, cauliflower-shape crystals and truncated pyramids). The incorporation of graphite in the composites improves the structural stability of the embedded nanocrystals upon a conforming step at 377 kg/cm2 (0.5 tons), with limited structural damage (below 10% BET surface area reduction as compared to the 40% observed for pure HKUST-1). Furthermore, composites exhibit a significant improvement in thermal management, associated with the excellent thermal and electrical properties of the graphite microdomains incorporated. The improved stability of the composites is also reflected in the adsorption performance for hydrogen at atmospheric pressure and cryogenic temperatures, with a significant preservation of the adsorption properties (gravimetric capacity, <15% decrease vs powders) in the monoliths containing graphite. The best adsorption performance is achieved with sample HKUST-1@10GF, in monolithic form, with a volumetric excess uptake at 0.1 MPa and −195 °C close to 18.7 g/L. This value is among the best described in the literature for monolithic MOFs under similar adsorption conditions.Authors would like to acknowledge financial support from Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21), Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital, Generalitat Valenciana (project CIPROM/2021/022) and European Union: Horizon Europe (project MOST-H2; Grant agreement no. 101058547). M. R.-C. and M. M. thank the project PID2019-104379RB-C22 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe” for financial support. Authors would like to thank Imerys Graphite & Carbon Ltd. (Dr. Raffaele Gilardi) for the supply of the graphite flakes

Direct Measurement of Microporosity and Molecular Accessibility in Stöber Spheres by Adsorption Isotherms

The microporous nature of monodisperse Stöber silica spheres is demonstrated in the literature, although usually via indirect evidence. Contradictorily, there also exist numerous reports of nonporosity based on conventional N2 adsorption isotherms, leading to a confusing scenario and questioning the evaluation methodology. Thus, there is the strong need of straight measure of microporosity in Stöber spheres, at best by available adsorption techniques, which must be further directly confronted with the standard nitrogen method. Here, for the first time, microporosity detection by N2 and CO2 adsorption is compared in Stöber spheres. We demonstrate that CO2 isotherms at 273 K allow direct detection and quantification of the microporosity (about 0.1 cm3/g in our samples), while N2 at 77 K cannot probe adequately the internal volume. We also show that a large amount of water fills the micropores under usual ambient conditions, also revealing the presence of small mesoporosity. Thus, the porous nature of Stöber spheres is investigated by a simple combination of adsorption isotherms, and the different accessibility of N2, CO2, and H2O molecules is discussed. We emphasize the inadequacy of standard N2 isotherms for micropore detection in Stöber silica, as the access of nitrogen molecules at cryogenic temperatures is kinetically restricted and may lead to erroneous conclusions. Instead, we propose CO2 isotherms as a simple and direct means for evaluation of microporosity.This work was funded by Spanish MINECO Projects MAT2014-58731-JIN, MAT2015-68075-R, MAT2016-80285-p, and SIFE2; Comunidad de Madrid Projects S2013/MIT-2740 and PHAMA_2.0; Generalitat Valenciana Project PROMETEOII/2014/004, and Project EU H2020 MSCARISE-2016/NanoMed

Advanced Removal of Dyes with Tuning Carbon/TiO2 Composite Properties

This study evaluates the removal of several dyes with different charge properties, i.e., anionic (Acid Red 88), cationic (Basic Red 13), and neutral (Basic Red 5) using transition metal-doped TiO2 supported on a high-surface-area activated carbon. Experimental results confirm the successful deposition of TiO2 and the derivatives (Zr-, Cu-, and Ce-doped samples) on the surface of the activated carbon material and the development of extended heterojunctions with improved electronic properties. Incorporating a small percentage of dopants significantly improves the adsorption properties of the composites towards the three dyes evaluated, preferentially for sample AC/TiO2_Zr. Similarly, the photodegradation efficiency highly depends on the nature of the composite evaluated and the characteristics of the dye. Sample AC/TiO2_Zr demonstrates the best overall removal efficiency for Acid Red 88 and Basic Red 5—83% and 63%, respectively. This promising performance must simultaneously be attributed to a dual mechanism, i.e., adsorption and photodegradation. Notably, the AC/TiO2_Ce outperformed the other catalysts in eliminating Basic Red 13 (74%/6 h). A possible Acid Red 88 degradation mechanism using AC/TiO2_Zr was proposed. This study shows that the removal efficiency of AC/TiO2 composites strongly depends on both the material and pollutant.This research was supported by Štefan Schwarz Postdoc Fellowship No. 2022/OV1/010, the Marie Curie Programme H2020-MSCA-RISE-2016-NANOMED No. 734641, and APVV-19-0302 projects. J.S.-A. acknowledges financial support from MCIN/AEI/10.13039/501100011033 and EU NextGeneration/PRTR (Project PCI2020-111968/ERANET-M/3D-Photocat), MCIN (Project PID2019-108453GB-C21), and Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital, Generalitat Valenciana (Project CIPROM/2021/022)

Carbon-based monoliths with improved thermal and mechanical properties for methane storage

A series of activated carbon materials have been prepared from petroleum residue using KOH as activating agent. The gravimetric adsorption capacity for methane of the synthesized samples increases with the activation degree, albeit at a lower packing density of the carbon material. These results anticipate an optimum pitch/KOH ratio (1:3) to achieve an upper limit in the volumetric storage capacity. Activated carbon powders have been conformed into monoliths using a small amount of a binder (5 wt%), either carboxymethyl cellulose or polyvinyl alcohol, with proper mechanical properties. Incorporation of graphite or graphene in the initial formulation does not alter and/or modify significantly the textural properties of the original activated carbon. However, once conformed into monoliths, the presence of graphite or graphene allows to improve i) the packing density of the monoliths (up to 0.52 g/cm3), ii) their mechanical properties (compressive strength ≈ 12.3 MPa) and iii) their thermal conductivity (up to 0.49 W/mK) without compromising the methane storage capacity (ca. 100 V/V).Authors would like to acknowledge financial support from the Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21), MCIN/AEI/10.13039/501100011033 and EU “NextGeneration/PRTR (Project PCI2020-111968 /3D-Photocat) and NATO SPS program (Project G5683)

Direct Measurement of Microporosity and Molecular Accessibility in Stöber Spheres by Adsorption Isotherms

The microporous nature of monodisperse Stöber silica spheres is demonstrated in the literature, although usually via indirect evidence. Contradictorily, there also exist numerous reports of nonporosity based on conventional N2 adsorption isotherms, leading to a confusing scenario and questioning the evaluation methodology. Thus, there is the strong need of straight measure of microporosity in Stöber spheres, at best by available adsorption techniques, which must be further directly confronted with the standard nitrogen method. Here, for the first time, microporosity detection by N2 and CO2 adsorption is compared in Stöber spheres. We demonstrate that CO2 isotherms at 273 K allow direct detection and quantification of the microporosity (about 0.1 cm3/g in our samples), while N2 at 77 K cannot probe adequately the internal volume. We also show that a large amount of water fills the micropores under usual ambient conditions, also revealing the presence of small mesoporosity. Thus, the porous nature of Stöber spheres is investigated by a simple combination of adsorption isotherms, and the different accessibility of N2, CO2, and H2O molecules is discussed. We emphasize the inadequacy of standard N2 isotherms for micropore detection in Stöber silica, as the access of nitrogen molecules at cryogenic temperatures is kinetically restricted and may lead to erroneous conclusions. Instead, we propose CO2 isotherms as a simple and direct means for evaluation of microporosity.This work was funded by Spanish MINECO Projects MAT2014-58731-JIN, MAT2015-68075-R, MAT2016-80285-p, and SIFE2; Comunidad de Madrid Projects S2013/MIT-2740 and PHAMA_2.0; Generalitat Valenciana Project PROMETEOII/2014/004, and Project EU H2020 MSCARISE-2016/NanoMed

Activated carbon materials with a rich surface chemistry prepared from L-cysteine amino acid

A series of activated carbon materials have been successfully prepared from a non-essential amino acid, such as L-cysteine. The synthesized carbons combine a widely developed porous structure (BET surface area up to 1000 m2/g) and a rich surface chemistry (mainly oxygen, nitrogen and sulphur functionalities). These surface functional groups are relatively stable even after a high temperature thermal treatment (O>N∼S). Experimental results show that these samples with a rich surface chemistry exhibit a significant improvement in their hydrophilic character. Although the role of the surface functional groups is less pronounced for the adsorption of non-polar molecules such as CO2, CH4 and C2H4, their adsorption at atmospheric pressure is to some extend conditioned by the characteristics of the adsorbent-adsorbate interactions. The synthesized carbons exhibit an excellent adsorption performance for CO2 (up to 3 mmol/g at 0°C). Furthermore, samples with a low activation degree exhibit molecular sieving properties with very promising CO2/CH4 (up to 4.5) and C2H4/CH4 (up to 6) selectivity ratios. These results anticipate that non-essential amino acids are a versatile platform to obtain carbon materials combining a tailored porous structure and rich multifunctional surface chemistry and with potential application for gas adsorption/separation processes.Authors would like to acknowledge financial support from the MINECO (Projects PID2019-108453GB-C21 and PCI2020-111968/ERANET-M/3D-Photocat) and NATO SPS program (Project G5683)

Surface interactions and mechanisms study on the removal of iodide from water by use of natural zeolite-based silver nanocomposites

In this work a natural zeolite was modified with silver following two different methods to derive Ag2O and Ag0 nanocomposites. The materials were fully characterized and the results showed that both materials were decorated with nanoparticles of size of 5−25 nm. The natural and modified zeolites were used for the removal of iodide from aqueous solutions of initial concentration of 30−1400 ppm. Natural zeolite showed no affinity for iodide while silver forms were very efficient reaching a capacity of up to 132 mg/g. Post-adsorption characterizations showed that AgI was formed on the surface of the modified zeolites and the amount of iodide removed was higher than expected based on the silver content. A combination of experimental data and characterizations indicate that the excess iodide is most probably related to negatively charged AgI colloids and Ag-I complexes forming in the solution as well as on the surface of the modified zeolites

Zr-Porphyrin Metal–Organic Framework as nanoreactor for boosting the formation of hydrogen clathrates

We report the first experimental evidence for rapid formation of hydrogen clathrates under mild pressure and temperature conditions within the cavities of a zirconium-metalloporphyrin framework, specifically PCN-222. PCN-222 has been selected for its 1D mesoporous channels, high water-stability, and proper hydrophilic behavior. Firstly, we optimize a microwave (MW)-assisted method for the synthesis of nanosized PCN-222 particles with precise structure control (exceptional homogeneity in morphology and crystalline phase purity), taking advantage of MW in terms of rapid/homogeneous heating, time and energy savings, as well as potential scalability of the synthetic method. Second, we explore the relevance of the large mesoporous 1D open channels within the PCN-222 to promote the nucleation and growth of confined hydrogen clathrates. Experimental results show that PCN-222 drives the nucleation process at a lower pressure than the bulk system (1.35 kbar vs 2 kbar), with fast kinetics (minutes), using pure water, and with a nearly complete water-to-hydrate conversion. Unfortunately, PCN-222 cannot withstand these high pressures, which lead to a significant alteration of the mesoporous structure while the microporous network remains mainly unchanged.Authors would like to acknowledge financial support from Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21 and PID2022-141034OB-C22), Consejo Superior de Investigaciones Científicas (CSIC) for internal funds (Intramural project, 202280I170), and Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital (Project CIPROM/2021/022). A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory (project IPTS-29742.1)

Controlling the Adsorption and Release of Ocular Drugs in Metal–Organic Frameworks: Effect of Polar Functional Groups

A series of UiO-66 materials with different functional groups (−H, −NH2, and −NO2) have been evaluated for the adsorption and release of a common ocular drug such as brimonidine tartrate. UiO-66 samples were synthesized under solvothermal conditions and activated by solvent exchange with ethanol. Experimental results suggest that the incorporation of surface functionalities gives rise to the development of structural defects (missing linker defects) but without altering the basic topology of the UiO-66 framework. These defects improve the adsorption performance of the parent metal–organic framework (MOF), while the bulkier functionalities infer slower release kinetics, with the associated benefits for prolonged delivery of brimonidine. Among the evaluated MOFs, defective UiO-66-NO2 can be proposed as the most promising candidate due to the combination of a larger brimonidine volumetric uptake (680 mg/cm3), a prolonged delivery (period of up to 25 days), a small particle size, and a larger instability. Contrariwise, at high concentrations UiO-66-NO2 has higher toxicity toward human retinal pigment epithelium cells (ARPE-19) compared to the pure and NH2-functionalized UiO-66.Authors would like to acknowledge financial support from the Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21), H2020 (Project MSCA-RISE-2016/NanoMed) and Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital (Project CIPROM/2021/022)