Paul R. Coronado

Hydrophobic aerogels for oil-spill clean up – synthesis and characterization

Abstract Hydrophobic aerogels were synthesized, characterized by IR, surface areas, relative pore size distributions and decomposition behavior. The materials were then tested for oil absorbing capacity in oil salt-water mixtures for applications to oil-spill clean up processes. Aerogels containing the CF 3 (CH) 2 -group (CF 3 -aerogel) were successfully synthesized using (CH 3 O) 4 Si and nominally 30, 10, 1.5 mol% CF 3 (CH 2 ) 2 Si(OCH 3 ) 3 in CH 3 OH by a NH 4 OH catalyzed reaction followed by supercritical extraction with CH 3 OH. The aerogels were found to be hydrophobic based on water absorption and sessile drop experiments. By IR, the CF 3 (CH 2 ) 2 -group was found to be intact after the gelation process and after supercritical drying. Si–OH-groups were also found in all cases, with decreasing amounts with increasing amounts of the CF 3 (CH 2 ) 2 -group. Heating under air caused the 30% CF 3 -aerogel to decompose, where the transition temperature from hydrophobic to hydrophilic appeared around 375–400 °C, which corresponded to disappearance of the CF 3 (CH 2 ) 2 -moiety bands in the IR. Treating oil and salt-water mixtures with the aerogel at an oil to aerogel ratio of 3.5 showed that all CF 3 -aerogels cleanly separated the oil from the water, regardless of CF 3 (CH) 2 -group concentration. This indicates that CF 3 -aerogels can absorb oil as much as 237 times their weight, much more than previously determined.

Solvent removal from water with hydrophobic aerogels

Hydrophobic aerogels are shown to be efficient absorbers of solvents from water. Solvents miscible with water are separated from it because the solvents are more volatile than water and they enter the porous aerogel as a vapor across the liquid water/solid interface. Solvents that are immiscible with water are separated from it by selectively wetting the aerogel. Adsorption isotherms are presented for hydrophobic silica aerogels, for several solvents (e.g., toluene, ethyl alcohol, trichloroethylene, chlorobenzene) in water mixtures. The adsorption capacities are compared with the standard activated carbon. Our measurements show that the adsorption capacity of the hydrophobic silica aerogels exceed the capacity of comparable granular activated carbon (GAC), on a gram-per-gram basis, for all of the solvents tested. The improved performance of adsorption capacity by the aerogel over the GAC ranged from factors of ∼30 times for low molecular weight, highly soluble solvents, to factors of 130 times for immiscible solvents.

HYDROPHOBIC AEROGELS FOR OIL-SPILL CLEANUP-INTRINSIC ABSORBING PROPERTIES

A hydrophobic, CF3-functionalized silica aerogel has been applied to water-oil mixtures to determine oil absorbing capacity of the solid aerogel. Preliminary tests on Prudhoe Bay crude oil mixed with salt water, using the aerogel in powder form, show three regimes for absorbing based on oil to aerogel wt ratios (O/A):1) all oil is absorbed and the aerogel is dry, O/A 0?3.5; 2) all oil is involved in an oil?aerogel? water emulsion, O/A 4.6?14; and 3) some oil is involved in an oil?aerogel?water emulsion, but free-phase oil is seen, O/A > 16. For comparison, a silica aerogel that has not been functionalized shows maximum absorbing capacity without the appearance of free-phase oil at O/A < 0.1. Separation and extraction studies showed good aerogel recovery and moderate oil recovery. Extraction studies at O/A of 2.3 showed that the aerogel could be reused at least two times. Infrared studies show that the oil recovered from the absorbing material and the starting oil are structurally similar. These resu...A hydrophobic, CF3-functionalized silica aerogel has been applied to water-oil mixtures to determine oil absorbing capacity of the solid aerogel. Preliminary tests on Prudhoe Bay crude oil mixed with salt water, using the aerogel in powder form, show three regimes for absorbing based on oil to aerogel wt ratios (O/A):1) all oil is absorbed and the aerogel is dry, O/A 0?3.5; 2) all oil is involved in an oil?aerogel? water emulsion, O/A 4.6?14; and 3) some oil is involved in an oil?aerogel?water emulsion, but free-phase oil is seen, O/A > 16. For comparison, a silica aerogel that has not been functionalized shows maximum absorbing capacity without the appearance of free-phase oil at O/A < 0.1. Separation and extraction studies showed good aerogel recovery and moderate oil recovery. Extraction studies at O/A of 2.3 showed that the aerogel could be reused at least two times. Infrared studies show that the oil recovered from the absorbing material and the starting oil are structurally similar. These results indicate that the CF3-functionalized aerogel as a powder has the potential of cleanly separating oil from oil?water mixtures up to 14 times the weight of the aerogel.

Optimization of the rapid supercritical extraction process for aerogels

A partial differential equation is derived that describes the pressure developed in the pores of a gel during the rapid supercritical extraction process. A comparative analysis of the strains caused by syneresis and expansion of the fluid, respectively, suggests that the latter is the dominant effect for this process. Experimental results indicate that the rate of leakage from the mold is equal to the rate of volumetric expansion of the fluid, so this was used as the boundary condition for the calculation. An analytical solution is obtained for the strain produced in a purely elastic gel. The strain is found to develop most rapidly at high temperatures, where the thermal expansion of the fluid increases sharply. The model predicts a temperature dependent heating rate that can be used to avoid irreversible strains by compensating for the increase in thermal expansion coefficient.

Elastic properties of silica aerogels from a new rapid supercritical extraction process

Abstract Silica aerogels were produced by a new process from tetramethoxysilane (TMOS) with ammonia as base catalyst. The process involves pouring the liquid sol in a stainless steel mold and immediately heating to supercritical conditions. Gelation and aging occur during heating and reaction rates are high due to high average temperatures. The gel fills the container completely, which enables relatively fast venting of the supercritical fluid by providing a constraint for swelling and failure of the gel monolith. The whole process can be completed within 1 h. Longitudinal and shear moduli were measured in the dried aerogels by ultrasonic velocity measurements both as a function of chemical composition of the original sol and of position in the aerogel. The sound velocity exhibits marked maxima close to the edges of the cylindrical, where the fluid left during venting. Specimens with high catalyst concentration and high water:TMOS ratio exhibited higher average moduli.

Mixed-metal oxide aerogels for oxidation of volatile organic compounds

Abstract Aerogels of 100% silica, 8 wt% Zr in silica, 5 wt% V in silica and 100% zirconia were synthesized and tested as oxidation catalysts in the temperature range of 300–700°C for destruction of volatile organic compounds. The silica-based aerogels were all amorphous and had surface areas above 600 m2/g after oxidation. The zirconia aerogel was crystalline with a relatively low surface area of 250 m2/g. As catalysts for oxidation (using O2 in He) of CH3OH to CO2, the zirconia aerogel exhibited the highest activity and best selectivity while the silica aerogel exhibited the poorest. Inclusion of Zr at levels as low as 8 wt% into the silica aerogel framework produced activities and selectivities which were very much like the zirconia aerogel. These properties have the impact of producing a Zr based catalyst with high activity, but with thermal stability afforded by Zr–silica mixtures.