Joanna Górka

Sonochemical functionalization of mesoporous carbon for uranium extraction from seawater

Extracting uranium from seawater is challenging due to its low concentration (3.3 ppb) and the myriad of competing ions. Mesoporous carbon materials provide a high surface area alternative to the traditional polymeric fiber braids developed for seawater extractions, specifically uranium extraction. In this work, sonochemical grafting of acrylonitrile onto the pores of soft-templated mesoporous carbons followed by its conversion to amidoxime functionalities was used to prepare an effective sorbent material with a high density of binding sites. Pore blockage, often observed for free radical polymerization, leads to poor adsorbent performance but can be easily overcome by the use of ultrasound during polymerization. Parameters such as surface area and surface pre-treatment, sonication intensity, solvent system, and monomer/initiator ratios were varied to optimize the polymerization and uranium adsorption capacity while not blocking the porosity, a significant hurdle in the utilization of functionalized porous materials. The results show that neither the surface oxidation with nitric acid nor CO2 activation alone is sufficient to cause significant improvement in grafting and uranium uptake. However, when coupled together, a greatly enhanced performance of the adsorbent materials was observed.

Amidoxime-modified mesoporous silica for uranium adsorption under seawater conditions

Amidoxime-modified ordered mesoporous silica (AO-OMS) materials were prepared by a two-step process involving: (1) co-condensation synthesis of cyanopropyl-containing ordered mesoporous silica (CP-OMS), and (2) conversion of cyanopropyl into amidoxime groups. The aforementioned co-condensation synthesis is simple and less time consuming as compared to the post-synthesis grafting and assures high loading of organic groups. The intermediate CP-OMS exhibited ordered mesoporosity, high specific surface area, and narrow pore size distribution. Interestingly, conversion of CP-OMS to AO-OMS further improved its properties by enhancing the specific surface area and porosity and achieving high loading of amidoxime groups. High affinity of these groups towards uranium species makes the AO-OMS material an attractive sorbent for uranium recovery as evidenced by very high uranium uptake reaching 57 mg of uranium per gram of AO-OMS under seawater conditions.

Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with benzene- and thiophene-bridging groups

Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with aromatic benzene- and thiophene-bridging groups were synthesized using 1,4-bis(triethoxysilyl)benzene and 2,5-bis(triethoxysilyl)thiophene as organosilica precursors in the presence of a F127 PEO–PPO–PEO triblock copolymer under acidic conditions. Highly ordered 3D cubic (Im3m) mesostructure was confirmed by synchrotron small angle X-ray scattering and transmission electron microscopy. Nitrogen adsorption analysis of the aforementioned samples (extracted and additionally calcined in nitrogen at 375–400 °C) revealed their high surface area (460–630 m2 g−1) and accessible ordered large pores (in the range of 6–9 nm). High thermal stability of these benzene- and thiophene-PMO samples (up to 500 and 400 °C, respectively) was confirmed by thermogravimetric analysis, while their framework chemistry was studied by solid-state 13C and 29Si MAS NMR.

Adsorption Properties of Micro-/Meso-porous Carbons Obtained by Colloidal Templating and Post-synthesis KOH Activation

Nitrogen adsorption studies have been employed to examine the structural properties of micro-/meso-porous carbons obtained by colloidal templating and post-synthesis KOH activation. These carbons were prepared by polymerizing phenol and formaldehyde in the pores of a colloidal silica template followed by carbonization, silica dissolution and post-synthesis KOH activation. Colloidal silica was used to create spherical mesopores, while KOH activation was performed to create additional microporosity and to enlarge the surface area of the resulting carbons. Nitrogen adsorption studies showed that a single impregnation of the colloidal silica template with phenolic resin precursors afforded carbons with relatively thin pore walls which did not withstand the post-synthesis KOH activation; however, a double impregnation generated mesoporous carbons with thicker pore walls. Activation of these latter carbons with KOH created micropores in the mesopore walls without significant deterioration of the mesopore structure. The resulting mesoporous carbons possessed high specific surface areas (ca. 800 m2/g) and large single-point pore volumes (ca. 1.8 cm3/g). The post-synthesis KOH activation enlarged the surface area up to 1500 m2/g, which was achieved via the creation of microporosity. Hence, the carbon obtained by double impregnation and activation exhibited a single-point pore volume of ca. 2 cm3/g with a relatively high microporosity which attained 17% of the total porosity. High-surface-area micro-/meso-porous carbons are attractive for adsorption, catalysis and energy-related applications such as capacitors and batteries.

Adsorption by Soft-Templated Carbons

The main focus of this chapter will be placed on the preparation of well-ordered mesoporous carbons by self-assembly of phenolic resins in the presence of triblock copolymers used as soft templates. Since the soft-templating approach was elaborated not long ago, the methods established to monitor carbon adsorption and structural properties will be discussed, including procedures that exclusively address tuned mesoporosity as well as the ones leading to largely enhanced microporosity and specific surface area. Next, surface modification of polymer-templated carbons will include: i) direct functionalization during the synthesis by using different carbon precursors or the addition of inorganic salts to introduce metal/metal oxide nanoparticles to the carbon matrix and ii) the postsynthesis functionalization. Also to complete the view on the versatility of carbons obtained by this method, the issues of possible macroscopic morphologies of these carbons will be addressed.