Patent Application
20060131179
This invention employs Non Faradaic Electrochemical Modification of Catalytic Activity and Selectivity (NEMCA) to electrochemically activate catalytic reactions at low temperature by use of a catalytic converter employing NEMCA.
The NEMCA effect can be described as an electrochemically induced and controlled promotion effect of catalytic surfaces generated by electrolyte charge carrier (ion) spillover to/from the electrolyte onto the catalyst surface. This process is generated when a catalyst layer, in the form of a polarizable electrode, is interfaced with a solid electrolyte and a galvanic cell is made by using a counter electrode of inert catalytic material and by applying current of voltage between such electrodes. During NEMCA the spill-over ionic species acts as an electro-promoter for the catalytic reaction, thus creating an effective double layer at the gas-metal catalyst interface and inducing pronounced and reversible changes in the chemisorptive bonding state of reactants and intermediates. During the electrochemically-induced ion spillover, the catalyst layer is polarized by a magnitude equal to the cell potential and this effect yields improved catalytic reaction rate and product selectivity. The NEMCA effect has been demonstrated in reactor configurations that are fuel cell type (i.e., the catalyst and counter electrodes are in different compartments and therefore exposed to different reactants), and in single-chamber type, where the reactants and products are all in contact with the catalyst and counter electrodes.
Traditionally catalytic processes use classical promoters, typically added during catalyst preparation, to activate a catalytic process. Another option is the use of metal-support interactions, which activates the catalytic function by using an active support. None of these approaches allows for accurate and “on demand” dosage of promoters during reaction conditions. Furthermore, classical promotion is restricted to the use of “permanent” promoters (such as Na+, K+).
Use of NEMCA technology allows for the precise dosing of electropromoters to a catalyst surface during reaction conditions, by adjusting the flux of ions (promoters) to the catalyst surface by controlling the applied current or voltage to the cell. Furthermore, NEMCA allows for the incorporation of “sacrificial” promoters (O2−, H+, OH−) to the catalyst/electrode surface during reaction. This aspect is not possible with classical promotion.
EP 0480116 teaches the use of NEMCA technology on cells comprising a catalyst layer and a second counter electrode interfaced with the solid electrolyte and the induction of this effect by applying current or voltage between these two electrodes. Unlike the present invention which comprises a catalytic converter incorporating a NEMCA cell employing a third electrode in the form of a reference electrode, a reference electrode is used in EP 0480116 to control the level of voltage applied to the catalyst layer and the level of catalytic conversion. The third reference electrode used in the present invention allows the better controlling and modulation of the activating voltage of the catalyst electrode, thus better controlling the performance of the catalyst layer and catalytic converter in comparison to a two electrode architecture where the voltages per electrode or cell component cannot be isolated. Various references, such as the below articles, discuss the existing art:
“Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-Support Interactions” C. G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, and D. Tsiplakides, Kluwer/Plenum Press, New York (2001)) Book
“Non-Faradaic Electrochemical Modification of Catalytic Activity 1. The Case of Ethylene Oxidation on Pt” S. Bebelis dn C. G. Vayenas J. Catal. 118, 125-146 (1989).
“In Situ Controlled Promotion of Pt for CO Oxidation via NEMCA Using a CaF2 as the solid electrolyte” I. V. Yentekakis and C. G. Vayenas J. Catal. 149, 238-242 (1994).
“Electrochemical Promotion in Heterogeneous Catalysis” C. Vayenas adn S. Bebelis Catal. Today 51 581-594 (1999) “Dependence of catalytic rates on catalyst work function” C. G. Vayenas, S. Bebelis and S. Ladas Nature 343, No 6259 625-627 (1990)
“Rules and Mathematical Modeling of Electrochemical and Chemical Promotion” C. G. Vayenas, S. Brosda and C. Pliangos J. Catal. 203, 329-350 (2001).
The reactions such as H2S deep oxidation to SO3 and H2O with subsequent formation of H2SO4, SCO and CS2 conversion, SO2 reduction/decomposition to elemental sulfur, olefin and paraffin isomerization and alkylation, paraffin to mono-olefin dehydrogenation and oxydehydrogenation, paraffin hydrocracking and dehydrocylization to aromatics are traditionally carried out in conventional catalyzed non-electrochemical processes and some of them are classically promoted. However, there is demand in the industry for more efficient and product-selective methods for these reactions.
The present invention employs Non Faradic Electrochemical Modification of Catalytic Activity (NEMCA) or Electrochemical Promotion in Catalysis (EPOC) that allows for carrying out the above reactions and other reactions more efficiently, at higher rates, with better selectivity, and at lower (or improved) operation temperatures than the incumbent standard catalytic processes, by use of a catalytic converter employing NEMCA.
Described is a catalytic converter comprised of non-impermeable metal-solid electrolyte catalysts and porous conductive (metallic or oxidic) films in contact with a solid electrolyte, connected to external or internal applications of an electrical potential or current between the conductive catalyst layer and a second counter electrode layer, also in contact with the solid electrolyte and having a third electrode as a reference electrode. The reference electrode is used to control the level of voltage applied to the catalyst layer and the level of catalytic conversion.
Disclosed herein is a method for the low temperature, high activity and/or selective catalytic conversion, by the use of a catalytic converter employing NEMCA. The converter comprises a catalyst layer electrode, charge conducting solid phase, counter electrode and third electrode as reference electrode for controlling current or voltage applied to the converter, such as to modulate the level of converter catalytic activity and selectivity.
One of the embodiments of the invention can be explained by
In an another embodiment of the invention, NEMCA cell for use in a single chamber reactor containing an K+-conducting β″(K+)Al2O3 solid electrolyte, Pt catalyst forming the working electrode, and Au electrocatalyst forming the counter and reference electrodes,
In a further embodiment of the invention, the NEMCA cell is used in a fuel cell or single chamber reactor containing an H+-conducting solid electrolyte, catalyst forming the working electrode, and electrocatalyst forming the counter and reference electrodes,
In an another embodiment as described in
In an another embodiment as recited in
In an another embodiment as recited in
In an another embodiment of the invention the three-electrode single chamber NEMCA reactor used for electrochemical activation of chemical processes can be setup as shown in
Use of NEMCA or Electrochemical Promotion in Catalysis is not restricted to any type of solid electrolyte or electrolyte form (solid or liquids). Typically, the solid electrolyte to be used for NEMCA studies is selected based on: 1—Compatibility with the reaction temperature. For example, ceramic conductors like doped Zirconia or Ceria, O2− conductors, for “high” reaction temperature 300-800° C., or low temperature polymeric proton conductors like Nafion® or Flemion® for low temperature or/and liquid NEMCA operation. 2—Catalyst/electrolyte ion/reaction species adsorbate electropromotion rules, as defined by Vayenas on “Reaction classification and promotional rules”, C. G. Vayenas, S. Brosda and C. Pliangos, J. Catal., 203, 329-350 (2001).
Examples of solid electrolytes that can be used in NEMCA processes include oxide ion conductors (O2−), which are defect ceramics such as ZrO2 doped with Y2O3 (typically ZrO2-8% Y2O3 or YSZ) and ZrO2 doped with CaO, MgO, Sc2O3, Yb2O3, etc. In addition, CeO2 doped with La2O3 or CaO and HfO2 doped with MgO or Y2O3 can be used for O2− transport. The O2− or O□− ion is known as a “sacrificial” promoter as it will eventually react (during oxidation, for example) or desorb after recombination from the catalyst surface.
Alkaline conductors are also used for NEMCA processes that require the use of a “permanent” promoter (instead of the “sacrificial O2− electropromoter) and include Na+ and K+ β and β″-aluminas. These materials can be ion exchanged to Cs+, Rb+, NH4+, H+ and Ag+, thus allowing different varieties of cation conductors.
Proton conductors (which also yield sacrificial electropromoters in the form H+ or Hδ+) are also important for use in NEMCA cells. Examples of proton conductors include CsHSO4, some zeolites, some forms of silico-aluminates, SrCeO3, polyfluorosulfonic acid-based polymeric materials, such as Nafion® and Flemion®, and polybenzimidazole (PBI)-based polymers. Other miscellaneous conductors that can be used in NEMCA include F-conductors, such as CaF2 and PrF2, Ag+ conductors, like Agl, etc.
For the type of NENCA cell used in an embodiment (single-chamber mode exposing all electrodes to the reacting mixture and products) the counter and reference electrodes should be made of metals or electronic conductive oxides that are not catalytically active or not active at the desired operation temperature, but are electrocatalytically active for the redox process involving the charge carrier of the solid electrolyte. For example, the counter/reference electrode should be electrocatalytically active for the process O2+4e−→2O2− when using YSZ or Na++e−→NaO when using β″(Na+) Al2O3 electrolytes. Typically Au, Ag, Ni, Cu are used as counter/reference electrodes for NEMCA cells. Perovskite-type oxides, which are mixed O2− and electronic conductors, can also be used as cathodes for the counter electrode. Finally, the electrode for counter and reference operation should be non-polarizable i.e., very reactive for the redox of choice and yielding plenty three-phase boundary or electrocatalytic active sites, thus offering minor resistance to charge transport.
The working electrode is prepared from metals or electronic-conducting metal oxides that are catalytically active for the reaction that is being electropromoted. In addition, this catalyst should be a good electrocatalyst active for the redox process involving the conversion of the charge carrier of the solid electrolyte into the electropromoter species: O2−→[Oδ−,δ+]+2e− for YSZ, K++e−→[K67 +,d−] for β″(K+) Al2O3, and H++e−→[Hδ+,δ−] for Nafion®, for example. This working/catalyst electrode should be polarizable, i.e., its magnitude of three-phase boundary and concomitant exchange current density should yield an appropriate resistance to charge transfer during the electrocatalytic process generating overvoltage at the catalyst/electrolyte interface. Although the selection of catalyst is very specific of the reaction, typical catalysts used for NEMCA include the platinum metals group: Pt, Pd, Rh, Ru, Os, Ir and Re. Other metal catalysts include Fe, Cu, Ag, Ni, Co, Au. Electronic conducting metal oxides, such as IrO2, PtO2, Ru oxides, Pd oxides and Rh oxides can also be used for NEMCA working electrodes. All these metals and metal oxides are also good electrocatalysts for the redox process involving generation of the electropromoter species.
The present process various applications for carrying out catalytic reactions in chemical reactors. Examples of applications for which NEMCA technology is useful include but are not limited to the following:
H2S direct oxidation to SO3/H2SO4 is useful for sulfur management and recovery operations in refinery and chemical plants (especially for Claus tail gas post-treatment) and for the production of sulfuric acid from hydrogen sulfide. The main reaction is: H2S+2O2→H2SO4, which takes place in two steps by the catalytic, low temperature deep oxidation of H2S to SO3: H2S+2O2→SO3+H2O, followed by the gas phase absorption (reaction) of SO3 and H2O forming sulfuric acid: SO3+H2O→H2SO4.
H2S direct oxidation to SO. This process is of technological environmental and economical interest for sulfur management and recovery in refinery and chemical plants (especially for Claus tail gas post-treatment). The catalytic reaction is: H2S+½O2→SO+H2O.
SCO and S2C conversion. Carbonyl sulfide (COS) and carbon disulfide (CS2) are undesirable side reactions of the Claus process. Its conversion includes oxidation to elemental sulfur: COS+½O2→SO+CO2 and CS2+O2→2SO+CO2. The conversion of this products also involve its hydrolysis to H2S: COS+H2O→H2S+CO2 and CS2+2H2O→2H2S+CO. SO2 reduction to elemental sulfur. This process is also of technological environmental and economical interest for sulfur management and recovery operations in refinery and chemical plants and for the production of elemental sulfur from sulfur oxides and hydrogen sulfide (Claus process and Claus tail gas unit post-treatment). The main reaction is the decomposition/reduction of sulfur oxides to sulfur: SO2→SO+O2. Under the scope of the present invention this decomposition process can be carried out electrochemically via NEMCA activation of this catalytic process SO2→SO+O2 and electrocatalytically via SO2+4e−→SO+2O2−.
H2S decomposition to elemental sulfur. The decomposition reaction is H2S→SO+H2.
Isomerization of normal and cyclic paraffins. This reaction also forms part of catalytic reforming and includes the isomerization of n-paraffins to isoparaffins and alkyl cyclo-paraffins to cyclic paraffins. Examples are the skeletal isomerization of n-hexane to isoparaffins: n-C6H14→iso-C6H14 and the skeletal isomerization of methylcyclopentane to cyclohexane: CH3—C5H9→c-C6H12.
Dehydrogenation of paraffins to olefins. The technological importance of catalytic reforming is to process gasoline-range hydrocarbon fraction to increase its octane number, but without changing the molecular weight, and to process low carbon number C2-C4 fractions for olefin production.
One important class of reforming reactions includes dehydrogenation of paraffins to olefins and naphtalenes to aromatics. As examples for the gasoline range we can include dehydrogenation of n-hexane to mono-olefins: n-C6H14→C6H12+H2, dehydrogenation of cylohexane to benzene: c-C6H12→C6H6+3H2 and dehydrogenation C10-C14 paraffins to olefins.
As examples for the C2-C4 hydrocarbon range we can include dehydrogenation of ethane to ethylene: C2H6→C2H4+H2, dehydrogenation of propane to propylene: C3H8→C3H6+H2, and butane to butenes: C4H10→C4H8+H2.
Catalytic activity, product selectivity and resistance to coking (deactivation) can be improved and reversibly controlled implementing NEMCA-activated reforming and selective hydrogenation. In addition, the posterior dehydrogenation of mono-olefins to diolefins and triolefins can also be controlled. Electrochemically-activated dehydrogenation NEMCA cells contain dual catalyst functionality, metallic and acidic sites, at their working electrode/catalyst surface.
Oxydehydrogenation of paraffins to olefins. This process includes the selective dehydrogenation of paraffin in the presence of O2. Oxygen is used to oxidize the H2 co-product to force the dehydrogenation equilibrium to the right, thus increasing the conversion of the process towards olefin production. For example, the ethane oxydehydrogenation in the presence of O2 to ethylene and H2O: C3H8+½O2→C3H6+H2O.
Dehydrocyclization of normal paraffins to aromatics. This reaction is essential in catalytic reforming and includes the formation of aromatic compounds from paraffins. This is a complex, combined process involving dehydrogenation, isomerization, cracking, ring formation and aromatization. Example is the catalytic dehydrocyclization of n-hexane to benzene: n-C6H14→C6H6+4H2.
Hydrocracking of paraffins. This reaction is also essential in catalytic reforming. Example is the hydrocracking of n-hexane to olefins and paraffins. H2S direct oxidation to SO. This process is of technological environmental and economical interest for sulfur management and recovery in refinery and chemical plants (especially for Claus tail gas post-treatment). The catalytic reaction is: H2S+½O2→SO+H2O.2e−H2S reduction to elemental sulfur. As the prior reaction, this process is also of technological environmental and economical interest for sulfur management and recovery operations in refinery and chemical plants and for the production of elemental sulfur from hydrogen sulfide. The decomposition reaction is H2S→SO+H2.
The following reactions can be carried out in a single chamber NEMCA reactor fed by paraffins saturated in (or injected with) an inert carrier stream and H2, in the temperature range of 300-500° C.
The following examples illustrate certain embodiments and aspects of the invention. However, these examples should not be construed as limiting in any way.
This example describes how to electrochemically activate the conversion of SO2 to using a Pt|YSZ|Ag NEMCA cell.
Cell Preparation, Reactor Assembly and Testing Setup:
An yttria-stabilized zirconia (ZrO2 8 mol % Y2O3, or YSZ) electrolyte disk (from Ceramatec, Solon, Ohio), Zycron composition 1373 of 1 mm thickness and 2.1 cm diameter was used as the electrolyte for the NEMCA cell. This ceramic electrolyte is an O2− (oxide) ion conductor at the temperature of the experiment.
The Pt working electrode was applied onto one side of the electrolyte disk via standard brushing using Pt metal resinate solution A1121 (12% Pt) from Engelhard (Newark, N.J.), followed by drying and calcination in air at 400° C. for 2 h and 830° C. for 0.4 h respectively. Following this procedure, the entire side of the disk was transformed as the catalyst or working electrode (WE). The Pt loading of this catalyst electrode is ca. 1 mg cm−2. The counter and reference electrodes (CE and RE, respectively) were applied onto the opposite side of the YSZ electrolyte using a suspension of metallic silver in butyl acetate (Silver Print 842, MG Chemicals B. C., Canada) following by drying at 80° C. for 2 h and calcination in air at 300° C. for 0.4 h. The CE is applied to cover ca. 70% of the surface of the cell. The RE size is prepared to be meaningfully smaller than the CE and RE (ca. 1/10) and it is applied adjacent to the CE (same plane), but opposite to the WE. This reference electrode will be used to properly and efficiently control the catalyst working electrode cell potential (and therefore the catalyst layer reactivity) during electrochemical bias. The Ag loading of these counter and reference electrodes are ca. 10 mg cm2−. Each one of the electrodes is interfaced with 0.25 mm silver wires (interfacing done using the Ag Silver Print paste) and this catalyzed cell is then introduced into the testing reactor.
For the catalytic reactions of interest and the low temperatures of operation (350-400° C.) of the present embodiment only Pt is catalytically (but also electrocatalytically) active for SO2 conversion, while Ag is only electrocatalytically active for the O2 redox process O2+4e−→2O2−.
The reactor is a ca. 30 cm3 volume CSTR-type differential reactor that until 1 liter/min flow behaved as a well mixed reactor. The head is stainless steel and the body is made of quartz. The head of the reactor is fitted with a Conax fitting (Troy, N.Y.) that allows sealing the three electrode leads through quartz capillaries. The reactor's head has water cooling capability to allow the elastomer o-ring to seal against the quartz reactor body. A thermocouple was placed inside the reactor body to control the heating through a ceramic radiant heater. The Pt|YSZ|Ag cell is then placed inside the reactor (with this reactor configuration all three electrodes are exposed to the same reactant/product mixture), the reactor is sealed and it is ready for testing.
The testing is done in a reaction rig equipped with ultra high pure (UHP) blend gases of SO2/He and O2/He. A stream of pure He is also injected to adjust the partial pressure of reactants. Upon establishing the desired reacting mixture composition and achieving the desired reactor temperature, the reactant stream is pre-heated at 150° C. and injected into the reactor. A bypass loop is used to establish a composition baseline. On line FTIR and MS is used to quantify the level of conversion of SO2 and O2. The cell is electrochemically driven using a Princeton Applied Research Versastat galvanostat potentiostat. The galvanostat potentiostat is used in potentiostatic mode, controlling the potential of the working (catalyst) electrode versus the reference electrode (UWR), thus inducing a current IWC between working and counter electrodes. The galvanostat-potentiostat is also used in galvanostatic mode, controlling the current IWC and measuring UWR. Fluke 87 voltmeters are also used to measure cell voltage and cell resistance.
The NEMCA or electrochemically activated conversion of sulfur dioxide (that allows low temperature-high activity catalytic reduction/decomposition and oxidation) was carried in the above NEMCA reactor loaded with this Pt|YSZ|Ag cell at the low temperatures of Tcell=350 and 400° C. for dilute streams containing 1005 ppmv SO2 and 0.5 and 0.05% O2 in He balance at 1 atm total pressure and 30 cm3(STP)min−1 total flow. Mass balances of SO2 and O2 are also conducted while monitoring reactant and reaction products.
This example describes how to electrochemically activate the reversible conversion of SO2 to SO using an Rh|YSZ|Au NEMCA cell.
The Rh working electrode was applied onto one side of the electrolyte disk via standard brushing using Rh metal resinate solution A8826 (10% Rh) from Engelhard (Newark, N.J.), followed by drying and calcination in air at 400° C. for 2 h and 850° C. for 6 h respectively. Following this procedure, the entire side of the disk was transformed as the catalyst or working electrode (WE). The Rh loading of this catalyst electrode is ca. 1 mg cm−2. The counter and reference electrodes were applied onto the opposite side of the YSZ electrolyte also via brushing using Au metal resinate solution A1118 (24% Au) from Engelhard (Newark, N.J.), followed by drying and calcination in air at 400° C. for 2 h and 850° C. for 0.5 h respectively. As before, The CE is applied to cover most of the surface of the cell. The RE size is prepared to be meaningfully smaller than the CE and RE and it is applied adjacent to the CE (same plane), but opposite to the WE. This reference electrode will be used to properly and efficiently control the catalyst working electrode cell potential (and therefore the catalyst layer reactivity) during electrochemical bias. The Au loading of these counter and reference electrodes are ca. 1 mg cm−2. Each one of the electrodes is interfaced with 0.25 mm silver wires (interfacing done using the Ag Silver Print paste) and this catalyzed cell is then introduced into the testing reactor.
The same reactor used in example I is used for this test. The reactor is now biased potentiostatically by setting and controlling UWR (i.e., potential of the catalyst of working electrode versus the Au reference electrode) and reading IWC.
As before, for the catalytic reactions of interest and the low temperatures of operation (500° C.) of the present embodiment only Rh is catalytically (but also electrocatalytically) active for SO2 conversion, while Au is only electrocatalytically active for the O2 redox process O2+4e−→2O2−.
The NEMCA or electrochemically activated low temperature decomposition and oxidation of sulfur dioxide was carried in the NEMCA reactor described in
It should be pointed out that this mode of operation: potentiostatic with reference electrode is preferred, as allows precise application of a desired (NEMCA-triggering) activation overpotential to the catalyst layer (as it is referenced to a catalytically inert, voltage stable, reversible reference electrode with no current transport through it). This mode of operation then offers improved tunning and control of the cell reaction rate NEMCA enhancement level regardless ohmic and concentration overpotential effects (i.e., regardless electrolyte thickness and conductivity IR effects, counter electrode polarization losses and mass transport effects).
This example describes the use of proton and potassium ion conducting solid-electrolyte NEMCA cells to electrochemically activate the catalytic dehydrogenation of propane to propylene:
A Pt|β″(K+)Al2O3|Au NEMCA cell of the type shown in
The cell configuration used in the prior example, formed by working electrode and opposed counter and reference electrodes, is again used. The cell is wired and is placed in the 30 cm3 CSTR single chamber reactor depicted in
The reactor is heated to the reaction temperature of 450° C. in He. A mixture of C3H8 in He (pC3=10 kPa, Ptotal=1 atm, balance He) with total flow of 30 cm3(STP)/min is then passed through the reactor. An on line FTIR/MS setup can be used to follow the reactants and products of the reaction. A galvanostat-potentiostat is used to electrochemically bias the cell to induce NEMCA on the Pt catalyst-working electrode.
During the experiment, the galvanostat-potentiostat is used to apply a catalyst potential (UWR) more negative than the open circuit potential (UOWR) to this NEMCA cell. When catalyst overpotentials of UWR−UOWR between −500 mV up to −1 V are obtained, negative transient ionic currents (μA level) yielding flux of K+ from the electrolyte to the working electrode take place. This process deposits potassium promoter on the Pt catalyst (in the form of the electropromoter [Kδ+,δ−]) at equilibrium coverages of several percents of monolayer. This negative potentiostatic bias of the cell, and concomitant electrochemically-induced potassium dosing of the catalyst surface (electropromoter dosing), causes a non-Faradaic enhancement in the catalytic rate of propane dehydrogenation to propylene versus the open circuit value. The magnitude of this enhancement ratio in dehydrogenation activity (ρ=rNEMCA/ropen circuit) increases with negative catalyst potential.
This process is reversible as when the cell is potentiostatically biased to catalyst potential (UWR) more positive than the open circuit potential (UOWR), then the rate of C3H8 dehydrogenation decreases (due to the removal of potassium from the catalyst surface). The rate of dehydrogenation can then be increased or decreased by changing the magnitude and polarity of the catalyst potential.
In a second test, a Pt|β″(H+)Al2O3|Au NEMCA cell of the type shown in
The cell configuration used in the prior example, formed by working electrode and opposed counter and reference electrodes, is again used. The cell is wired and is placed in the 30 cm3 CSTR single chamber reactor depicted in
The reactor is heated to the desired reaction temperature of 350° C. in He. A mixture of C3H8, H2 and H2O in He (pC3=10 kPa, pH2=20 kPa, pH2O=5 kpa Ptotal=1 atm, balance He) with total flow of 30 cm3(STP)/min is then passed through the reactor. The He balance is saturated in a water bubbler at temperature to achieve the desired partial pressure (water is added to the reacting mixture to stabilize the proton conduction of the alumina). As before, on line FTIR/MS setup is used to follow the reactants and products of the reaction. A galvanostat-potentiostat is used to electrochemically bias the cell to induce NEMCA on the Pt catalyst-working electrode.
Using the galvanostat-potentiostat positive and negative (steady state) currents are applied to this NEMCA cell. Typical currents are 100-1000 μA. Negative currents pump H+ to the working electrode, while positive currents remove hydrogen from the catalyst surface. Application of positive and negative currents induce an enhancement in the catalytic rate of propane dehydrogenation to propylene versus the open circuit value. This NEMCA enhancement in dehydrogenation activity is due to the dosage (or removal) of the electropromoter species [Hδ+,δ−] to or from the catalyst surface. The magnitude of this enhancement ratio in dehydrogenation activity (ρ=rNEMCA/ropen circuit) increases with applied cell current.
This example describes the use of polymeric and ceramic-based proton conducting solid-electrolyte NEMCA cells in liquid and gas phase to electrochemically activate the alkylation of paraffins and olefins, specifically the NEMCA alkylation of isobutane and 1-butene to 2,2,4 trimethylpentane (isooctane).
For example, a NEMCA cell for electrochemically-activated alkylation can be prepared by interfacing (bonding, via hot pressing at temperatures close to the glass transition temperature of the ionomer) a Nafion® 117 membrane in proton form (from DuPont, Wilmington Del., a polymeric sulfonic-acid based superacid proton conductor) with a working electrode containing Pt/C catalyst bonded with PTFE and counter & reference electrodes containing Pt/C electrocatalyst (gas diffusion electrodes, E-Tek, Somerset N.J.). This Pt|Nafion®|Pt NEMCA cell in the form of a membrane-electrode assembly (MEA) is placed in a 5 cm2 electrode active area fuel cell-type reactor with reference electrode (fuel cell fixture/reactor from Electrochem Inc., Woburn Mass.). The MEA is assembled in the fuel cell fixture using fiberglass coated PTFE gaskets and it is torqued until sealed. This basic cell design is depicted in
This fuel cell-type reactor is used in electrolytic mode, with external voltage applied to the NEMCA MEA cell using a galvanostat-potentiostat. In this cell, the working electrode is the paraffin/olefin side of the cell. The counter and reference electrodes are the H2 side of the cell. The cell fixture is heated using pad heaters located at the side of the steel casing of the cell. The cell is operated and controlled at 37° C. (close-to-room temperature) and at atmospheric pressure.
The working electrode of the cell is fed with a mixed feed consisting of 1 ml/min of 0.1 M H2SO4 (support electrolyte) and a gaseous mixture consisting of isobutane and 1-butene in He (5% i-C4H10, 1% 1-C4H8 isoparaffin/olefin ratio=5/1, balance He). The gas mixture is injected in the liquid acid stream and is mixed using an on-line static mixer prior entering the working electrode feed of the reactor. The counter/reference electrode of the cell is fed with a stream of 0.5% H2 in He. An on line FTIR/GC setup connected to the working electrode exit stream can be used to follow the reactants and products of the reaction. A galvanostat-potentiostat is used to electrochemically bias the cell to induce NEMCA on the Pt catalyst-working electrode and electrochemically activate the alkylation reaction.
Using the galvanostat-potentiostat negative (steady state) currents are applied to this NEMCA cell. Current in the range of 100-1000 μA (with cell voltages UWR up to 1 V) are applied during the experiment. Negative currents pump H+ to the Pt/Nafion® working electrode, which is exposed to the isobutane, 1-butene reactants. Application of negative currents induces, via NEMCA, an enhancement in the catalytic rate of alkylation of isobutane and butene versus the open circuit rate. The magnitude of this enhancement ratio in alkylation reaction rate (ρ=rNEMCA/ropen circuit) increases with applied cell current. Furthermore, the use of Nafion®, NEMCA and dilute acid minimizes the formation of secondary reactions such as olefin oligomerization.
This NEMCA enhancement/electrochemical activation in the rate of formation of 2,2,4 trimethylpentane under negative galvanostatic bias is due to the combined effect of 1-electrochemically-induced dosage of the acid electropromoter species [Hδ+,δ−] to the working electrode catalyst surface (followed by the concomitant addition of spillover proton to the Pt-adsorbed 1-butene forming a carbonium ion (n-butoxy), which by further reaction with butene and hydride transfer by isobutane, forms isooctane), and 2- by the electrochemical activation of the Nafion® electrolyte portion of the Pt/electrolyte double layer, improving the activity of the polymer's sulfonic acid sites for protonation of the 1-butene.
NEMCA then allows carrying out the alkylation at higher reaction rates with better selectivity (at reasonable close-to-room temperatures) and yields by controlling the magnitude of catalyst potential UWR or cell current IWC externally applied to the NEMCA reactor.
In another embodiment, we use NEMCA to enable and activate the alkylation process in gas phase, without the need of liquid support electrolytes.
A NEMCA cell formed by Pd|β″(H+)Al2O3|Au (
The cell is wired electrochemically to the flow-type reactor depicted in
Using the galvanostat-potentiostat negative (steady state) currents are applied to this NEMCA cell. Current in the range of 100-1000 μA (with cell voltages UWR up to 1 V) are applied during the experiment. Negative currents pump H+ to the Pt/β″(H+)Al2O3 working electrode where the isobutane 1-butene alkylation reaction takes place. Application of negative currents induces, via NEMCA, an enhancement in the catalytic rate of alkylation of isobutane and butene versus the open circuit rate. The magnitude of this enhancement ratio in the 2,2,4 trimethylpentane formation rate (ρ=rNEMCA/ropen circuit) increases with applied cell current. Furthermore, the use this gas phase alkylation process minimizes the formation of secondary reactions such as olefin oligomerization. In addition, this process has the advantage of not requiring separation of the alkylate from the electrolyte.
