Guido P. Pez

New facilitated transport membranes for the separation of carbon dioxide from hydrogen and methane

New facilitated transport membranes which selectively permeate carbon dioxide from hydrogen and methane have been prepared and examined under a variety of conditions. Membranes consist of melts of the salt hydrates tetramethylammonium fluoride tetrahydrate, [(CH3)4N]F·4H2O, or tetraethylammonium acetate tetrahydrate, [(C2H5)4N]CH3CO2·4H2O, immobilized in films of Celgard 3401®. Operating at 50°C, both membranes exhibited CO2 permeabilities which increased with decreasing feed partial pressure of CO2, characteristic of a facilitated transport membrane. Selectivities of CO2H2 and of CO2CH4 increased with decreasing feed pressure since H2 and CH4 permeances were independent of feed pressure. Selectivities of CO2H2 were disappointingly low due to permeation of H2 through the dense phase of Celgard®. With some difficulty, the microporous support was eliminated by construction of membranes consisting of liquid [(CH3)4N]F·4H2O on the surface of a film of poly (trimethylsilylpropyne). Selectivities of CO2H2 as high as 360 were then observed at low feed partial pressures of CO2. Modeling of membrane properties is consistent with permeation of CO2 by a facilitated transport mechanism with reasonable derived diffusivities for the chemically bound CO2 carrier species.

Hydrogen spillover in the context of hydrogen storage using solid-state materials

Hydrogen spillover has emerged as a possible technique for achieving high-density hydrogen storage at near-ambient conditions in lightweight, solid-state materials. We present a brief review of our combined theoretical and experimental studies on hydrogen spillover mechanisms in solid-state materials where, for the first time, the complete mechanisms that dictate hydrogen spillover processes in transition metal oxides and nanostructured graphitic carbon-based materials have been revealed. The spillover process is broken into three primary steps: (1) dissociative chemisorption of gaseous H2 on a transition metal catalyst; (2) migration of H atoms from the catalyst to the substrate and (3) diffusion of H atoms on substrate surfaces and/or in the bulk materials. In our theoretical studies, the platinum catalyst is modeled with a small Pt cluster and the catalytic activity of the cluster is examined at full H atom saturation to account for the essentially constant, high H2 pressures used in experimental studies of hydrogen spillover. Subsequently, the energetic profiles associated with H atom migrations from the catalyst to the substrates and H atom diffusion in the substrates are mapped out by calculating the minimum energy pathways. It is observed that the spillover mechanisms for the transition metal oxides and graphitic carbon-based materials are very different. Hydrogen spillover in the transition metal oxides is moderated by massive, nascent hydrogen bonding networks in the crystalline lattice, while H atom diffusion on the nanostructured graphitic carbon materials is governed mostly by physisorption of H atoms. The effects of carbon material surface curvature on the hydrogen spillover as well as on hydrogen desorption dynamics are also discussed. The proposed hydrogen spillover mechanism in carbon-based materials is consistent with our experimental observations of the solid-state catalytic hydrogenation/dehydrogenation of coronene.

On the relative power of electrophilic fluorinating reagents of the NF class

Abstract Electrochemical measurements have been employed as a measure of the relative chemical reactivity of a series of NF class electrophilic fluorinating reagents. A correlation has been found between the potential for the first one-electron reduction of the reagents and their observed reactivity in synthetic fluorination reactions. Comparative electrochemical data in acetonitrile and dimethylformamide are reported.

Polyelectrolyte-salt blend membranes for acid gas separations

Abstract The CO 2 CH 4 and CO 2 H 2 permselectivity of poly(vinylbenzyltrimethylammonium fluoride), PVBTAF, polyelectrolyte membranes can be significantly improved by blending in certain fluoride-containing organic and inorganic salts. For example, the CO 2 permeance of a PVBTAF-4CsF (4 mol CsF/mol repeat unit) composite membrane was more than four times that of a simple PVBTAF composite membrane while CO 2 CH 4 and CO 2 H 2 selectivities were comparable. Surprisingly, the blends are at least macroscopically homogeneous even with as much a 6 mol salt/mol PVBTAF repeat unit. The optimal salt loading appears to be approximately 4 mol CsF/mol polyelectrolyte repeat unit. Membrane performance is strongly dependent on the relative humidity of the gas streams and is maximized in the range of 30–50% relative humidity. Membranes containing choline fluoride exhibited improved membrane performance at relative humidities below 30%. Permselective data suggests that CO 2 transport is kinetically limited in 10 μm thick films. The blends are stable in CO 2 /CH 4 /H 2 streams for more than 30 days of continuous operation, however, the membranes suffer an irreversible degradation due to reaction with trace level sulfur-containing contaminants common to cylinder H 2 S.

Amination of benzene and toluene with hydroxylamine in the presence of transition metal redox catalysts

Abstract The amination of benzene and toluene to aniline and toluidines with hydroxylamine sulfate has been investigated in water–acetic acid and water–acetic acid–sulfuric acid media in the presence of transition metal compounds as catalysts. The process yields are strongly dependent on the temperature, added sulfuric acid and the composition of catalyst. For the amination of benzene, the soluble catalysts, NaVO3 and Fe(III) salts produce high yields of aniline without addition of H2SO4, whereas Na2MoO4 and FeSO4 exhibit substantial activity only in 5 M H2SO4. Amination is accompanied by a disproportionation of hydroxylamine catalyzed by the redox active transition metal ions. The favorable effect of H2SO4 on the amination is due mostly to the greater stability of hydroxylamine in the strongly acidic medium. Mixed oxides containing V(V) and Mo(VI) are active amination catalysts when suspended in 5 M solution of H2SO4 in acetic acid. Introduction of metallic Pd into these oxide catalysts improves performance increasing the yield and selectivity of amination with respect to the aromatic substrate. Toluene exhibited a close to benzene reactivity in amination giving approximately equal yields of o-, m-, p-toluidines. Mechanistic considerations based on literature data and results of ab initio quantum mechanics calculations suggest that the aminating species is the protonated amino radical ⋅NH3+, which in the rate-determining step reacts with benzene and toluene to yield the corresponding aminocyclohexadienyl and aminomethylcyclohexadienyl radical intermediates. These are then oxidatively aromatized to give, respectively, aniline and a non-regiospecific mixture of toluidines.

Hydrogen sulfide separation from gas streams using salt hydrate chemical absorbents and immobilized liquid membranes

The salt hydrate tetramethylammonium fluoride tetrahydrate, [(CH3)4N]F·4H2O, in the liquid state reversibly absorbs large quantities of hydrogen sulfide, for example 0.30 mol H2S per mole of salt at 50°C and 100 kPa. Gas absorption likely occurs by deprotonation of H2S to form bisulfide, HS− , and bifluoride, HF2 −. The equilibrium constant for the reaction of H2S with [(CH3)4N]F·4H2O is 1.4 × 10− 2 kPa− 1 at 50°C as determined from absorption/desorption data. The heat of H2S absorption was surprisingly low, −0.78 kcal mol− 1. A second salt hydrate, tetraethylammonium acetate tetrahydrate, [(C2H5)4N]CH3CO2·4H2O, also reversibly absorbs H2S but with a lower affinity. However, at higher pressures, its H2S capacity increases more than does that of [(CH3)4N]F·4H2O making it more suitable for use as a pressure swing absorbent. Absorption/desorption data are consistent with reaction of one mole of [(C2H5)4N]CH3CO2·4H2O for each mole of H2S and an equilibrium constant of 6.4 × 10− 4 kPa− 1 mol H2S per mole of salt. Membranes consisting of liquid [(CH3)4N]F·4H2O immobilized in a microporous support exhibit selective permeation of H2S from CH4 and CO2. Permeabilities of H2S increased with decreasing feed pressure, consistent with facilitated transport of H2S. The H2S/CH4 selectivities ranged from 140 to 34 and decreased with increasing feed pressure while H2S/CO2 selectivities were 8–6. The presence of H2S in the feed tends to suppress permeation of CO2, implying that both gases compete for the same carrier species.

Molten salt facilitated transport membranes. Part 1. Separation of oxygen from air at high temperatures

Abstract A new approach to the fabrication of facilitated transport (FT) liquid membranes for the separation of gases is described. The membranes are prepared by imbibing a thin, porous flat sheet support with a molten salt that reacts reversibly with the desired permeant gas, and which thus acts as a facilitated transport carrier of the gas. As an illustration of this concept, membranes consisting of molten lithium nitrate and sodium nitrate immobilized in thin porous metal supports were constructed and tested for the separation of oxygen from dried zero grade air at 350–524°C. These immobilized molten salt (IMS) membranes were shown to selectively permeate oxygen via a FT mechanism, which is based on the reversible reaction of oxygen with nitrite ions in the melts: M+NO−2 + 1 2 O2 ⇄ M+ NO−3, M  Li, Na For a NaNO3-IMS membrane oxygen permeabilities (POO2) ranging from 58 to 1110 barrer with corresponding oxygen to nitrogen selectivities ranging from 4 to 80 were observed, at 452°C to 524°C and specified experimental conditions. There is a strong dependence of PoO2 on operating temperature, which is indicative of a nitrate ion dissociation rate determining step for the FT of oxygen through the membrane. The advantages and limitations of immobilized molten salt versus conventional immobilized liquid FT membranes are discussed.

Selective Permeation of Ammonia and Carbon Dioxide by Novel Membranes

Abstract Experimental results are presented on membranes of novel composition which selectively permeate ammonia and carbon dioxide from mixtures containing hydrogen. The CO2-selective membrane, which consists of a thin liquid film of the salt hydrate tetramethylammonium fluoride tetrahydrate, exhibits a CO2 permeance of 4-1 × 10−5 cm3/cm2·s·cmHg with selectivity, α(CO2/H2), ranging from 360-30. The NH3-selective membrane, poly(vinylammonium thiocyanate), displays a high NH3 permeance, 5−20 × 10−5 cm3/cm2·s·cmHg, with α(NH3/N2) as high as 3600 and α(NH3/H2) as high as 6000. Such membranes, which retain H2 at pressure in the feed stream, may offer new opportunities in the design of separation processes.

Bis(2-methoxyethyl)aminosulfur trifluoride: a new broad-spectrum deoxofluorinating agent with enhanced thermal stability

Bis(2-methoxyethyl)aminosulfur trifluoride (Deoxo-FluorTM) is effective for the conversion of alcohols to alkyl fluorides, aldehydes/ketones to the corresponding gem-difluorides and also for the transformation of carboxylic acids to their trifluoromethyl derivatives; it is a less thermally sensitive, broader-spectrum alternative to the traditional dialkylaminosulfur trifluoride (DAST) deoxofluorination reagents.

Molten salt facilitated transport membranes. Part 2. Separation of ammonia from nitrogen and hydrogen at high temperatures

Abstract Immobilized molten salt (IMS) membranes have been prepared and shown to be effective for the separation of NH3 from mixtures with N2 and H2 at the high temperature encountered in the recycle loop of ammonia synthesis plants. The membranes were prepared by immobilizing molten LiNO3 and ZnCl2, salts which react reversibly with NH3 but not with N2 or H2, in thin porous supports. Ammonia permeabilities of 103 to 105 barrer and NH3/N2 and NH3/H2 selectivities of at least 1000 were observed at 250–300°C. The extraordinary NH3 permeability through the ZnCl2 IMS membrane is explained by a carrier-mediated facilitated transport model wherein zinc ammoniate complexes function as mobile carriers.