Richard D. Noble

Room-Temperature Ionic Liquids and Composite Materials: Platform Technologies for CO2 Capture

Clean energy production has become one of the most prominent global issues of the early 21st century, prompting social, economic, and scientific debates regarding energy usage, energy sources, and sustainable energy strategies. The reduction of greenhouse gas emissions, specifically carbon dioxide (CO(2)), figures prominently in the discussions on the future of global energy policy. Billions of tons of annual CO(2) emissions are the direct result of fossil fuel combustion to generate electricity. Producing clean energy from abundant sources such as coal will require a massive infrastructure and highly efficient capture technologies to curb CO(2) emissions. Current technologies for CO(2) removal from other gases, such as those used in natural gas sweetening, are also capable of capturing CO(2) from power plant emissions. Aqueous amine processes are found in the vast majority of natural gas sweetening operations in the United States. However, conventional aqueous amine processes are highly energy intensive; their implementation for postcombustion CO(2) capture from power plant emissions would drastically cut plant output and efficiency. Membranes, another technology used in natural gas sweetening, have been proposed as an alternative mechanism for CO(2) capture from flue gas. Although membranes offer a potentially less energy-intensive approach, their development and industrial implementation lags far behind that of amine processes. Thus, to minimize the impact of postcombustion CO(2) capture on the economics of energy production, advances are needed in both of these areas. In this Account, we review our recent research devoted to absorptive processes and membranes. Specifically, we have explored the use of room-temperature ionic liquids (RTILs) in absorptive and membrane technologies for CO(2) capture. RTILs present a highly versatile and tunable platform for the development of new processes and materials aimed at the capture of CO(2) from power plant flue gas and in natural gas sweetening. The desirable properties of RTIL solvents, such as negligible vapor pressures, thermal stability, and a large liquid range, make them interesting candidates as new materials in well-known CO(2) capture processes. Here, we focus on the use of RTILs (1) as absorbents, including in combination with amines, and (2) in the design of polymer membranes. RTIL amine solvents have many potential advantages over aqueous amines, and the versatile chemistry of imidazolium-based RTILs also allows for the generation of new types of CO(2)-selective polymer membranes. RTIL and RTIL-based composites can compete with, or improve upon, current technologies. Moreover, owing to our experience in this area, we are developing new imidazolium-based polymer architectures and thermotropic and lyotropic liquid crystals as highly tailorable materials based on and capable of interacting with RTILs.

Designing the Next Generation of Chemical Separation Membranes

New materials can be prepared as membranes that may allow their performance to beat long-standing limits. Synthetic membranes are used in many separation processes, from industrial-scale ones—such as separating atmospheric gases for medical and industrial use, and removing salt from seawater—to smaller-scale processes in chemical synthesis and purification. Membranes are commonly solid materials, such as polymers, that have good mechanical stability and can be readily processed into high–surface area, defect-free, thin films. These features are critical for obtaining not only good chemical separation but also high throughput. Membrane-based chemical separations can have advantages over other methods—they can take less energy than distillation or liquefaction, use less space than absorbent materials, and operate in a continuous mode. In some cases, such as CO2 separations for CO2 capture, their performance must be improved. We discuss how membranes work, and some notable new approaches for improving their performance.

Alumina-Supported SAPO-34 Membranes for CO2/CH4 Separation

SAPO-34 membranes were prepared by in situ crystallization on alpha-Al2O3 porous supports. The crystal size of the seeds was effectively controlled in the 0.7 to 8.5 micron range by employing different structure-directing agents. Seeds smaller than 1 micron produced membranes with CO2/CH4 separation selectivities higher than 170 and unprecedented CO2 permeances as high as 2.0 x 10(-6) mol/m2.s.Pa at 295 K and a feed pressure of 224 kPa. The membranes effectively separated CO2/CH4 mixtures up to 1.7 MPa.

Organics/water separation by pervaporation with a zeolite membrane

Abstract Organic/water mixtures are separated at ambient temperature and pressure by pervaporation through a silicalite zeolite membrane supported on the inner surface of a porous stainless-steel cylindrical tube. Methanol, ethanol and acetone were preferentially separated from aqueous solutions. For methanol/water separations, a relatively constant separation factor between 11 and 14 was obtained over a wide range of methanol feed concentrations. Total mass fluxes of 1 to 2.7 kg/(m 2 h) were obtained. Water and methanol permeances were independent of methanol feed concentration, except at low concentrations. Pervaporation has a higher separation factor than expected for vapor-liquid equilibrium separation. The highest separation factor obtained for acetone/water was 255 at an acetone feed concentration of 0.8 wt% with an acetone flux of 0.20 kg/(m 2 h). The highest acetone flux of 0.95 kg/(m 2 h) was obtained at an acetone concentration of 43 wt%, when the separation factor was 37. Separation factors decreased with increasing acetone concentration. The production indices for the silicalite membrane were much higher then other membranes at similar feed concentrations. The silicalite membrane was unable to selectively remove acetic acid from aqueous solutions at low acid concentrations.

Ceramic-zeolite composite membranes and their application for separation of vapor/gas mixtures

Ceramic-zeolite composite membranes were prepared by in-situ synthesis of a thin (∼ 10μm) polycrystalline silicalite-1 layer on the inner surface of an alumina membrane tube. The inner surface is a λ-alumina coating that has 5-nm diameter pores. X-ray diffraction verified the presence of a pure silicalite phase in the layer, and SEM showed that individual silicalite crystals had grown together to form a continuous silicalite-1 layer. The addition of silicalite to the alumina membrane decreased the N2 permeance by a factor of 5, but it decreased the n-C4H10 permeance by a factor of 190, and n-C4H10 appeared to adsorb on the membrane. At room temperature, the permeance ratio of n-C4H10/i-C4H10 was one for the alumina membrane, but it was 3 for the zeolite membrane. Methanol was separated from H2 and from CH4 at 373 K and pressures from 110 to 1100 kPa by preferentially permeating CH3OH through the zeolite membrane. For some conditions the CH3OH/H2 separation factor was greater than 1000, and the CH3OH/CH4 separation factor was 190. Apparently, CH3OH adsorbs and blocks the pores for H2 or CH4 permeation.

Highly CO2-Selective Organic Molecular Cages: What Determines the CO2 Selectivity

A series of novel organic cage compounds 1-4 were successfully synthesized from readily available starting materials in one-pot in decent to excellent yields (46-90%) through a dynamic covalent chemistry approach (imine condensation reaction). Covalently cross-linked cage framework 14 was obtained through the cage-to-framework strategy via the Sonogashira coupling of cage 4 with the 1,4-diethynylbenzene linker molecule. Cage compounds 1-4 and framework 14 exhibited exceptional high ideal selectivity (36/1-138/1) in adsorption of CO(2) over N(2) under the standard temperature and pressure (STP, 20 °C, 1 bar). Gas adsorption studies indicate that the high selectivity is provided not only by the amino group density (mol/g), but also by the intrinsic pore size of the cage structure (distance between the top and bottom panels), which can be tuned by judiciously choosing building blocks of different size. The systematic studies on the structure-property relationship of this novel class of organic cages are reported herein for the first time; they provide critical knowledge on the rational design principle of these cage-based porous materials that have shown great potential in gas separation and carbon capture applications.

High Density, Vertically-Aligned Carbon Nanotube Membranes

A method is presented to prepare high-density, vertically aligned carbon nanotube (VA-CNT) membranes. The CNT arrays were prepared by chemical vapor deposition (CVD), and the arrays were collapsed into dense membranes by capillary-forces due to solvent evaporation. The average space between the CNTs after shrinkage was approximately 3 nm, which is comparable to the pore size of the CNTs. Thus, the interstitial pores between CNTs were not sealed, and gas permeated through both CNTs and interstitial pores. Nanofiltration of gold nanoparticles and N(2) adsorption indicated the pore diameters were approximately 3 nm. Gas permeances, based on total membrane area, were 1-4 orders of magnitude higher than VA-CNT membranes in the literature, and gas permeabilities were 4-7 orders of magnitude higher than literature values. Gas permeances were approximately 450 times those predicted for Knudsen diffusion, and ideal selectivities were similar to or higher than Knudsen selectivities. These membranes separated a larger molecule (triisopropyl orthoformate (TIPO)) from a smaller molecule (n-hexane) during pervaporation, possibly due to the preferential adsorption, which indicates separation potential for liquid mixtures.

Preparation and separation properties of silicalite composite membranes

Silicalite-alumina composite membranes were prepared by an in situ zeolite synthesis method using an alumina membrane tube with a 5-nm pore diameter, γ-alumina layer as a substrate. Single gas permeances of H2, Ar, n-C4H10, i-C4H10, and SF6 were measured and mixtures of H2i-C4H10 and H2SF6 were separated to characterize the silicalite membrane. These measurements were made from 300 to 737 K, and are compared to an alumina membrane without a silicalite layer. Permeances were lower in the silicalite membrane (a factor of 8 for Ar at 298 K). Permeances for the alumina membrane decreased as the temperature increased, and separation selectivities were lower than values expected for Knudsen diffusion. Transport through the alumina membrane was by Knudsen flow and surface diffusion. The silicalite membrane showed dramatically different behavior, and transport appeared to be controlled by molecular size and adsorption properties. Permeances of all components studied were activated in the silicalite membrane, and activation energies ranged from 8.5 to 16.2 kJ/mol. The ratio of single gas permeances was as high as 136 for H2 to SF6 and 1100 for H2 to i-C4H10 at 298 K. Separation selectivities at elevated temperatures were significantly above Knudsen diffusion selectivity for the silicalite membrane and were larger than ratios of pure gas permeances at the same temperature. The largest permeance ratio for the separation of mixtures was 12.8 for H2SF6 at 583 K. Separation selectivities for both membranes were higher when a pressure drop was maintained across the membrane than when an inert sweep gas was used because of counter diffusion of the sweep gas.

Liquid membrane transport: a survey

Abstract The literature pertaining to facilitated transport and liquid membrane separations is reviewed and summarized, especially work reported since 1977. Liquid membranes of all geometries are discussed, including immobilized liquid membranes and liquid surfactant or emulsion liquid membranes. Emphasis is placed on facilitated, or carrier-mediated transport in both configurations although other mechanisms such as coupled-transport and transport due to solubility differences are discussed. Mathematical modeling and analytical solutions for facilitated transport models are summarized. The possibility of industrial application of liquid membrane technology is mentioned and the most important experimental techniques for liquid membrane research are discussed. Also, directions for future research are recommended.

Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes

Abstract Single gas and mixture permeances of CO 2 and CH 4 were measured as functions of pressure and temperature through three MFI zeolite membranes that have different fractions of their permeances through non-zeolite pores. The effect of pressure on CO 2 permeance, which was different for each membrane, was fit by a modified surface diffusion model. The differences in the pressure behavior of the membranes are attributed to pores with viscous and Knudsen flow. Membranes with the largest permeation through non-zeolite pores have the lowest CO 2 /CH 4 mixture selectivity. The highest CO 2 /CH 4 mixture selectivity is 5.5 at room temperature and decreases with temperature because of a decrease in competitive adsorption. Although increasing pressure at constant pressure drop increases the apparent CO 2 /CH 4 selectivity, the ratio of the CO 2 and CH 4 fluxes decreases.