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Ind. Eng. Chem. Res. 2010, 49, 11052–11060
A Polymer Electrolyte Hydrogen Pump Hydrogenation Reactor Jay Benziger* and James Nehlsen Department of Chemical Engineering, Princeton UniVersity
Decene and acetone were hydrogenated over Ni, Pd, Pt, Cu, Ag, and Au catalysts in a polymer electrolyte hydrogen pump (PEHP) reactor. Water was oxidized over a Pt mesh anode and protons were pumped to the catalysts supported on porous carbon cathodes in contact with an organic liquid phase. The protons are reduced at the cathode to adsorbed hydrogen atoms which hydrogenate adsorbed olefins and carbonyl groups by heterogeneous catalysis. At low current density decene hydrogenation over Pd and Pt increased with the current density to the 1/2 power, indicating the surface reaction of adsorbed hydrogen with adsorbed decene was the rate limiting step. At high current density the reaction rate decreased linearly with current density, indicating adsorbed hydrogen inhibited decene adsorption and decene adsorption was the rate limiting step. Decene hydrogenation at 50 °C was 100 times slower over Ni, Cu, Ag, and Au catalysts compared to Pd and Pt. Acetone hydrogenation over Pt increased linearly with proton current density and was 10 times slower than decene hydrogenation. Acetone and decene hydrogenation rates at 50 °C were almost the same over Cu catalysts. Data were fit with modified Langmuir-Hinshelwood kinetics; the rate limiting steps were identified as the first hydrogen addition to adsorbed decene and the second hydrogen addition to adsorbed acetone. 1. Introduction Hydrogenation reactions are an integral part of the manufacture of many chemicals, from pharmaceuticals to petrochemicals.1-4 Unsaturated CdC bonds in fats are hydrogenated to improve thermal stability. Aromatics are hydrogenated to make cyclohexane derivatives for nylons. Carbonyl groups in acids and aldehydes are hydrogenated to make alcohols. Hydrogenation of liquid hydrocarbons is essential for new sources of liquid fuels; coal liquefaction involves hydrogenation of aromatic rings. The production of stable liquid fuels from biomass requires catalytic hydrogenation of fats, methyl alkyl esters derived from fats, or hydrogenolysis of polyols produced from cellulose.5-8 Many hydrogenations must be performed in three-phase reactors in which a liquid to be hydrogenated must be contacted with both hydrogen and a solid catalyst, typically Pt-group metals or alloys.9 The reactions are often limited by the low solubility and diffusivity of hydrogen in the liquid phase; as a result, high hydrogen pressure and methods to create small gas bubbles in the liquid to get good interfacial mass transport are required in conventional three-phase reactors.10,11 Because liquid-phase hydrogenation reactions are limited by mass transport of hydrogen to the catalyst surface there is poor control over product selectivity. Competition for adsorption sites can limit the amount of hydrogen adsorbed; the catalyst surface is often covered with the hydrocarbon and starved for hydrogen. Kinetics are almost always first order in hydrogen pressure reflecting hydrogen mass transport to the catalyst surface being the rate limiting step in the overall reaction.1,9-11 The polymer electrolyte hydrogen pump (PEHP) couples heterogeneous catalysis with electrochemistry to improve delivery of hydrogen to the catalyst surface without the need for high pressure and turbulent mixing. The heart of the PEHP is a proton conducting polymer electrolyte membrane (PEM) coated with a catalyst layer,12-17 shown schematically in Figure 1. Protons are generated at the anode by oxidation of water and transported through PEM to the cathode where they are reduced to adsorbed hydrogen. The cathode is the active catalyst for * To whom correspondence should be addressed. E-mail: benziger@ princeton.edu.
hydrogenation. By choice of the catalyst either CdC or CdO bonds can be hydrogenated.17,18 The hydrogen activity at the cathode catalyst is controlled by the current without the need to have a high hydrogen pressure in the gas phase or requiring agitation to disperse hydrogen bubbles to enhance the interfacial transport area. Since adsorbed hydrogen can be formed directly on any cathode surface, metals that do not readily dissociate hydrogen might be activated for hydrogenations in a PEHP reactor. The cathode catalyst layer must be both electrically conductive (to conduct the electrons that reduce the protons) and have a three-phase interface between the catalyst, the proton conducting membrane, and the liquid-phase reactant. As shown in Figure 2 this may be accomplished by having a mixed layer of catalyst and polymer electrolyte (Nafion) supported on a porous conducting electrode. The porous electrode carries the electron current, while the thin polymer layer provides good proton conductivity. The polymer layer must be kept thin enough such
Figure 1. Schematic of the operation of a polymer electrolyte hydrogen pump hydrogenation (PEHP) reactor. A voltage applied between the anode and cathode oxidizes water in the aqueous media at the anode and protons are transported through the aqueous electrolyte and the polymer electrolyte to the cathode catalyst at the interface of the membrane and a liquid organic phase. The protons are reduced to adsorbed hydrogen at the cathode and CdC and CdO bonds can be hydrogenated.
10.1021/ie100631a 2010 American Chemical Society Published on Web 06/11/2010
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010
Figure 2. Schematic of the three-phase interface at the cathode in the PEHP hydrogenation reactor. The catalyst layer is composed of a mixture of metal catalyst particles and polymer electrolyte supported on a porous carbon cloth electrode. The carbon cloth carries the electron current to the metal catalyst particles where the protons are reduced to adsorbed hydrogen. The polymer electrolyte forms a thin film over the catalyst particles to carry the protons to the catalyst surface. Hydrocarbons must diffuse through the thin film of the polymer electrolyte to the catalyst surface to react.
that the diffusional resistance for the organics to get to the catalyst surface is small. The surface coverage of adsorbed hydrogen is controlled by a balance between adsorption and desorption. In the PEHP hydrogen flux to the catalyst surface is equal to current density, which is analogous to the flux from the hydrogen partial pressure in a typical hydrogenation reaction. The analogy between current density and partial pressure is examined with a PEHP reactor for the hydrogenation of decene and acetone over the Pt group metals, known for their hydrogenation activity, and Au group metals, which are typically inactive. We will show that the reactivity trends are the same as seen for traditional heterogeneous catalytic reactions, and the reaction rates can be fit by Langmuir-Hinshelwood kinetics where the surface coverage of hydrogen is determined by a balance of the proton current density and hydrogen atom recombination and desorption. 2. Experimental Section The PEHP reactor uses hydrogen produced by water electrolysis to hydrogenate unsaturated bonds of a hydrocarbon in an organic phase. A Nafion115 polymer electrolyte membrane separates the water from the organic phase while permitting transport of protons. The anode is situated in the aqueous phase. The anode does not need to be in direct contact with the polymer membrane, and we prefer to have the anode removed from the membrane. When the anode is remote from the polymer membrane oxidative damage to the membrane from peroxides and oxygen radicals formed by water oxidation is minimized. However, by removing the anode from direct contact with the membrane it is necessary to have an acid solution at the anode to reduce the resistance for proton transport. The cathode is a metal catalyst on an electrically conductive support (e.g., carbon). The cathode catalyst must be in direct contact with the polymer membrane in the organic phase, forming the essential three-phase interface among the polymer electrolyte membrane (PEM), the catalyst, and the organic phase. Polymer electrolyte is mixed in the catalyst layer to improve proton transport and improve the three-phase interface. Adsorbed hydrogen formed by proton reduction at the cathode either hydrogenates the adsorbed organic molecule or combines with another adsorbed hydrogen atom and desorbs as H2; the proton current is equal to the sum of the rates of these two reactions. The reaction selectivity is the rate of the hydrogenation reaction divided by half the current.
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Catalyst membranes were made employing cathodes made using Pt, Pd, Ni, Au, Ag, and Cu powders supported on a carbon cloth electrode. Powdered metal (50 mg) was suspended in about 20 mL of isopropanol. The nominal metal particle size was 0.3-2.5 µm. A 125 mg portion of 5 wt % Nafion solution (Ion Power, Inc., Liquion 1100) was added to the suspension. The suspension was sprayed onto carbon cloth (E-tek, Inc., Type B-1) using an artist’s airbrush. The metal loadings were ca. 8 mg/cm2 and the Nafion loading was 1 mg/cm2. The resulting layers of metal powder on the electrodes were evenly dispersed. The electrodes were dried at 60 °C for 1 h and then 140 °C for 20 min. The electrodes were pressed onto Nafion 115 membranes (Ion Power, Inc.) for 3 min at 140 °C at 1.7 MPa. A Pt mesh anode was suspended in close proximity to the plain Nafion side of the membrane-electrode assembly. The reactor consists of two graphite plates with a serpentine flow channel. The graphite plates were backed with aluminum blocks with a Teflon spacer between. The Pt mesh anode and the pressed membrane/electrode were sandwiched between the graphite plates and the whole assembly was bolted together, as shown in Figure 3. A Viton gasket sealed the membrane-graphite interface. The total active area of the cell is 5 cm2. The individual electrode potentials were measured with respect to an Ag/AgCl reference electrode (+0.22 V vs SHE) placed into the anode chamber, isolated by a piece of porous Vycor glass. Gas evolution was measured by water displacement in an inverted graduated glass tube connected to the reactor outlet. Temperature was controlled via a programmable temperature controller and cartridge heaters and was held constant at 50 °C for all trials. The number of adsorption sites on the catalysts were estimated by integrating the reduction of adsorbed hydrogen on cyclic voltammograms. The internal resistance of the PEHP was determined before each run by a pulse interrupt measurement. A solution of 40 mL of 1-decene (Fluka, 95%) and 400 mL of cyclohexane (Aldrich, 99%) was prepared for the olefin studies. A solution of 40 mL of acetone (Fisher, 99.5%) and 400 mL of cyclohexane was prepared for the carbonyl hydrogenation studies. Hydrogenation of acetone in aqueous solution was also tested with a 1:10 by volume solution of acetone (Fisher, 99.5%) in deionized water. The reactor system is shown schematically in Figure 3. It was run as a differential reactor, keeping the total conversion sufficiently small so that the reactant concentrations may be considered to be constant. Ten mL of each solution was circulated through the reactor for 1 h, permitting sufficient conversion to effectively measure the products with gas chromatography. The maximum conversion of reactant was 75 mA/cm2 the rate of decene hydrogenation decreased with increasing current density. This suggests a change in the rate-limiting step for reaction; at low current density the rate-limiting step is surface reaction between adsorbed hydrogen and adsorbed decene, while at high current density the rate-limiting step changes to the adsorption of decene on a hydrogen covered surface. Decene hydrogenation was 100 times slower on Ni and Cu, Ag and Au catalysts. Proton current to electrodes with these
metals primarily evolved hydrogen gas; the selectivity for decene hydrogenation was 0
-56.7 -50.6 ∼-13 ∼-32 -31.0 -2.7 >0
0 -11.6 -47.9 -29.3 -30.7 -60.9
