Suree Brown

A Superacid-Catalyzed Synthesis of Porous Membranes Based on Triazine Frameworks for CO2 Separation

A general strategy for the synthesis of porous, fluorescent, triazine-framework-based membranes with intrinsic porosity through an aromatic nitrile trimerization reaction is presented. The essence of this strategy lies in the use of a superacid to catalyze the cross-linking reaction efficiently at a low temperature, allowing porous polymer membrane architectures to be facilely derived. With functionalized triazine units, the membrane exhibits an increased selectivity for membrane separation of CO(2) over N(2). The good ideal CO(2)/N(2) selectivity of 29 ± 2 was achieved with a CO(2) permeability of 518 ± 25 barrer. Through this general synthesis protocol, a new class of porous polymer membranes with tunable functionalities and porosities can be derived, significantly expanding the currently limited library of polymers with intrinsic microporosity for synthesizing functional membranes in separation, catalysis, and energy storage/conversion.

Seawater Uranium Sorbents: Preparation from a Mesoporous Copolymer Initiator by Atom-Transfer Radical Polymerization†

The world s oceans, where uranium is found quite uniformly at a concentration of 3.3 mgL , present an alternative source of uranium to terrestrial mining for nuclear fuel. Environmental concerns associated with mining will undoubtedly increase as reserves are depleted, thus increasing the utility of more environmentally friendly feedstocks. Hence, before terrestrial resources become scarce, the development of sorbents designed for seawater extraction is of strategic importance to guarantee future uranium resources. From the first inorganic adsorbents, which showed poor selectivity and mechanical resistance, to the most recent polyethylene-fiberbased sorbents containing amidoxime–carboxylic acid copolymers, and more recently layered metal sulfides and metal– organic frameworks, interest in uranium seawater extractions has continuously increased among governments worldwide. Because the concentration of uranium in the oceans is relatively low, maximization of the adsorption properties of sorbents, for example, through changes in their surface area and pore structure, can greatly improve the kinetics of uranium extraction and the adsorption capacity simultaneously. To facilitate the uptake of uranyl ions with fast kinetics, various sorbents containing the amidoxime group, such as hydrogels, particles and beads, membranes, macroporous fibers, and composites, have been prepared by suspension polymerization, radiation-induced grafting, and even sonochemical functionalization. However, silica beads and most carbon materials have a relatively small accessible surface area for the growth of large polymers or a low number of surface sites available for the grafting of functional groups. Thus, the design of substrates with large numbers of accessible reactive sites for the grafting of polymeric surface groups is necessary for the development of materials with improved uranium-adsorption capacity. Recently, porous polymers based on divinylbenzene (DVB) have been developed for applications in separations and catalysis. For example, the copolymerization of p-styrene sulfonate with divinylbenzene led to a catalytically active porous polymer. This method has the additional advantage that polymers can be obtained with controlled porosity and high surface areas without porogens. It is thus timeand cost-effective, as well as more environmentally friendly than the templated synthesis of carbonaceous materials. Motivated by these findings, we report herein nanoporous polymers based on the vinylbenzyl chloride (VBC) monomer and the DVB cross-linking agent. As well as a well-developed nanoporous structure of microand mesopores, the obtained polymers contain large numbers of accessible chlorine species, which can be used as initiators for atom-transfer radical polymerization (ATRP). These materials are the first examples of ATRP initiators in which the initiator species is located within the framework of the mesoporous support. The accessible framework and surface chlorine species were used to grow polyacrylonitrile chains, which were then converted into polyamidoxime for uranium adsorption from seawater with tailorable adsorption and surface properties. Three copolymer monoliths were synthesized by freeradical polymerization; that is, the monomer 4-vinylbenzyl chloride was cross-linked by divinylbenzene with 2,2’-azobisisobutyronitrile (AIBN, 98%) as the initiator to give copolymers hereafter referred to as p(xDVB-VBC) (in which x stands for the molar ratio of DVB to VBC). By varying the ratio of the monomer and the cross-linking reactant, it was possible to adjust the pore structure, that is, the surface area and pore volume (Figure 1). Since these adjustments arose from changes in the DVB to VBC ratio, the initiator concentration (i.e. the amount of chloride substituents present) was also varied. The nitrogen isotherms measured at 196 8C for the samples show that nonporous materials as well as materials with tailorable mesopore volumes can be [*] Dr. Y. Yue, Dr. R. T. Mayes, Dr. P. F. Fulvio, Dr. X.-G. Sun, Prof. Dr. S. Dai Chemical Sciences Division, Oak Ridge National Laboratory Oak Ridge, TN 37831 (USA) E-mail: dais@ornl.gov

Efficient CO2 Capture by Porous, Nitrogen‐Doped Carbonaceous Adsorbents Derived from Task‐Specific Ionic Liquids

The search for a better carbon dioxide (CO(2) ) capture material is attracting significant attention because of an increase in anthropogenic emissions. Porous materials are considered to be among the most promising candidates. A series of porous, nitrogen-doped carbons for CO(2) capture have been developed by using high-yield carbonization reactions from task-specific ionic liquid (TSIL) precursors. Owing to strong interactions between the CO(2) molecules and nitrogen-containing basic sites within the carbon framework, the porous nitrogen-doped compound derived from the carbonization of a TSIL at 500 °C, CN500, exhibits an exceptional CO(2) absorption capacity of 193 mg of CO(2) per g sorbent (4.39 mmol g(-1) at 0 °C and 1 bar), which demonstrates a significantly higher capacity than previously reported adsorbents. The application of TSILs as precursors for porous materials provides a new avenue for the development of improved materials for carbon capture.

Uranium recovery from seawater: development of fiber adsorbents prepared via atom-transfer radical polymerization

A novel adsorbent preparation method using atom-transfer radical polymerization (ATRP) combined with radiation-induced graft polymerization (RIGP) was developed to synthesize an adsorbent for uranium recovery from seawater. The ATRP method allowed a much higher degree of grafting on the adsorbent fibers (595–2818%) than that allowed by RIGP alone. The adsorbents were prepared with varied compositions of amidoxime groups and hydrophilic acrylate groups. The successful preparation revealed that both ligand density and hydrophilicity were critical for optimal performance of the adsorbents. Adsorbents synthesized in this study showed a relatively high performance (141–179 mg g−1 at 49–62% adsorption) in laboratory screening tests using a uranium concentration of ∼6 ppm. This performance is much higher than that of known commercial adsorbents. However, actual seawater experiment showed impeded performance compared to the recently reported high-surface-area-fiber adsorbents, due to slow adsorption kinetics. The impeded performance motivated the investigation of the effect of hydrophilic block addition on the graft chain terminus. The addition of a hydrophilic block on the graft chain terminus nearly doubled the uranium adsorption capacity in seawater, from 1.56 mg g−1 to 3.02 mg g−1. The investigation revealed the importance of polymer chain conformation, in addition to the ligand and hydrophilic group ratio, for advanced adsorbent synthesis for uranium recovery from seawater.

Synthesis of Porous, Nitrogen‐Doped Adsorption/Diffusion Carbonaceous Membranes for Efficient CO2 Separation

A porous, nitrogen-doped carbonaceous free-standing membrane (TFMT-550) is prepared by a facile template-free method using letrozole as an intermediate to a triazole-functionalized-triazine framework, followed by carbonization. Such adsorption/diffusion membranes exhibit good separation performance of CO2 over N2 and surpassing the most recent Robeson upper bound. An exceptional ideal CO2 /N2 permselectivity of 47.5 was achieved with a good CO2 permeability of 2.40 × 10(-13) mol m m(-2) s(-1) Pa(-1) . The latter results arise from the presence of micropores, narrow distribution of small mesopores and from the strong dipole-quadrupole interactions between the large quadrupole moment of CO2 molecules and the polar sites associated with N groups (e.g., triazine units) within the framework.

Polymer-coated nanoporous carbons for trace seawater uranium adsorption

Polymer-coated mesoporous carbon nanocomposites were prepared from the immobilization of acrylonitrile and acrylic acid copolymers with divinylbenzene as a crosslinker onto a mesoporous carbon framework. High surface areas were maintained after polymerization with accessible porosity. This functional nanocomposite was tested as an adsorbent for uranium from high salinity solutions. Uranium adsorption results have shown that the adsorption capacities are strongly influenced by the density of the amidoxime groups and the specific surface area.

Macroporous monoliths for trace metal extraction from seawater

The viability of seawater-based uranium recovery depends on the uranium adsorption rate and capacity, since the concentration of uranium in the oceans is relatively low (3.3 μg L−1). An important consideration for a fast adsorption is to maximize the adsorption properties of adsorbents such as surface areas and pore structures, which can greatly improve the kinetics of uranium extraction and the adsorption capacity simultaneously. Following this consideration, macroporous monolith adsorbents were prepared from the copolymerization of acrylonitrile (AN) and N,N′-methylene-bis(acrylamide) (MBAAm) based on a cryogel method using both hydrophobic and hydrophilic monomers. The monolithic sorbents were tested with simulated seawater containing a high uranyl concentration (∼6 ppm) and the uranium adsorption results showed that the adsorption capacities are strongly influenced by the ratio of monomer to the crosslinker, i.e., the density of the amidoxime groups. The preliminary seawater testing indicates the high salinity content of seawater does not hinder the adsorption of uranium.

Three-Phase Catalytic System of H2O, Ionic Liquid, and VOPO4–SiO2 Solid Acid for Conversion of Fructose to 5-Hydroxymethylfurfural

Efficient transformation of biomass-derived feedstocks to chemicals and fuels remains a daunting challenge in utilizing biomass as alternatives to fossil resources. A three-phase catalytic system, consisting of an aqueous phase, a hydrophobic ionic-liquid phase, and a solid-acid catalyst phase of nanostructured vanadium phosphate and mesostructured cellular foam (VPO-MCF), is developed for efficient conversion of biomass-derived fructose to 5-hydroxymethylfurfural (HMF). HMF is a promising, versatile building block for production of value-added chemicals and transportation fuels. The essence of this three-phase system lies in enabling the isolation of the solid-acid catalyst from the aqueous phase and regulation of its local environment by using a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]). This system significantly inhibits the side reactions of HMF with H2O and leads to 91 mol % selectivity to HMF at 89 % of fructose conversion. The unique three-phase catalytic system opens up an alternative avenue for making solid-acid catalyst systems with controlled and locally regulated microenvironment near catalytically active sites by using a hydrophobic ionic liquid.

Coordination effect-regulated CO2 capture with an alkali metal onium salts/crown ether system

A coordination effect was employed to realize equimolar CO2 absorption, adopting easily synthesized amino group containing absorbents (alkali metal onium salts). The essence of our strategy was to increase the steric hindrance of cations so as to enhance a carbamic acid pathway for CO2 capture. Our easily synthesized alkali metal amino acid salts or phenolates were coordinated with crown ethers, in which highly sterically hindered cations were obtained through a strong coordination effect of crown ethers with alkali metal cations. For example, a CO2 capacity of 0.99 was attained by potassium prolinate/18-crown-6, being characterized by NMR, FT-IR, and quantum chemistry calculations to go through a carbamic acid formation pathway. The captured CO2 can be stripped under very mild conditions (50 °C, N2). Thus, this protocol offers an alternative for the development of technological innovation towards efficient and low energy processes for carbon capture and sequestration.

A renewable HSO3/H2PO3-grafted polyethylene fiber catalyst: an efficient heterogeneous catalyst for the synthesis of 5-hydroxymethylfurfural from fructose in water

Irradiation-induced co-grafting of acrylonitrile and vinylsulfonic acid (or vinylphosphonic acid) monomers on polyethylene fiber was studied for the heterogeneous catalysis of fructose dehydration into 5-hydroxymethylfurfural (HMF) solely in water. The acidic co-polymer exhibited excellent catalytic activity and maintained a high yield after being regenerated. We attribute these catalytic properties to a branched environment created by grafted chains, hydrophilic enough to interact with fructose in water but collectively dense enough to form a unique local phase mimicking organic solvents.