WO2008017838A1

WO2008017838A1 - Fuel synthesis

Info

Publication number: WO2008017838A1
Application number: PCT/GB2007/003015
Authority: WO
Inventors: Helio Bustamante; Steven Scowcroft
Original assignee: Itm Power (Research) Ltd.
Priority date: 2006-08-08
Filing date: 2007-08-08
Publication date: 2008-02-14
Also published as: GB0822641D0; GB0615731D0; GB2451999B; GB2451999A

Abstract

A method for the manufacture of a hydrocarbon using an electrolysis cell having first and second electrode compartments separated by a membrane across which ions can migrate, comprises forming ions from water in the first electrode compartment, and supplying CO2 to the second electrode compartment, where it can undergo reduction by the ions.

Description

FUEL SYNTHESIS Field of the Invention

This invention relates to a process for the synthesis of hydrocarbon fuels. Background of the Invention At present, research on possible production methods for environmentally friendly liquid fuels is mainly focused on the production of biofuels. In these schemes, biomass captures CO₂ from the atmosphere and is processed either by fermentation or by gasification and chemical synthesis to produce liquid fuels. Because the CO₂ that will be released when these fuels are combusted was initially extracted from the atmosphere, the net effect on the environment is thought to be substantially reduced, compared to the direct use of fossil fuels.

GB2418430 describes a method of permanent sequestration of carbon dioxide. Hydrocarbon fuels are manufactured using CO₂ from the atmosphere and hydrogen generated from a "carbon-free" or "low carbon" process, such as electrolysis. The method of hydrocarbon synthesis exemplified is the well-known Fischer-Tropsch(F-T) process, which converts syngas (a mixture of CO and H₂) into hydrocarbons, according to the equation:

(2n+1)H₂+ nCO -» C_nH_2n+2+ nH₂O

The syngas needed for this reaction can be prepared from CO₂ and H₂: CO₂ + H₂ -«■ *^* CO + H₂O (water gas shift reaction).

An alternative method of CO₂ capture is to use a hydrophobic hollow fibre membrane and KOH as a solvent for the selective absorption of CO₂ from atmospheric air, as described by Stucki et al (1995 International Journal of Hydrogen Energy 20(8), pp. 653-663). The regeneration of KOH can thus be achieved in an electrochemical cell from which a mixture of CO₂ and hydrogen is extracted; this could potentially be used to feed, for example, a conventional methanol synthesis reactor. This reactor also makes use of the water/gas shift reaction and F-T chemistry.

Sullivan et al (1993 Elsevier) describes the following routes to obtain hydrocarbons and oxygenated hydrocarbons from CO₂ via electrochemical reactions:

CO₂ + 2H⁺ + 2e- -» CO(g) + H₂O CO₂ + 4H⁺ + 4e- -> HCHO + H₂O CO₂ + 6H⁺ + 6e- -» CH₃OH + H₂O CO₂ + 6H⁺ + 6e- + CH₃OH(I) -» C₂H₅OH + H₂O CO₂ + 8H⁺ + 8e- -» CH₄ + H₂O Summary of the Invention

The present invention is a method for the manufacture of a hydrocarbon using an electrolysis cell having first and second electrode compartments separated by a membrane across which ions can migrate, which comprises forming ions from water in the first electrode compartment, and supplying CO₂ to the second electrode compartment, where it can undergo reduction by the ions. Description of the Drawings

Figure 1 is a schematic cross-section of a synthesis cell suitable for carrying out the method of the invention. Description of the Preferred Embodiments

An electrolytic cell suitable for use in the invention, comprises a membrane electrode assembly, the membrane electrode assembly comprising a membrane across which ions can migrate, separating catalyst electrode layers at the cathode and anode. In a preferred embodiment, the membrane is a proton exchange membrane. Alternatively, the membrane may be an anionic membrane.

In a preferred embodiment, the membrane across which ions can migrate is a proton exchange membrane. In a cell of this type, water may be oxidised at the anode to form oxygen and H⁺ ions. The H⁺ ions can then migrate through the membrane to the cathode, where they can reduce CO₂. At the cathode, the CO₂, H⁺ ions and electrons can react on the surface of a suitable catalyst to form one or more hydrocarbons. Examples of such reactions are as follows:

CO₂ (g) + 4H₂ (g) = CH₄ (g) + 2H₂O (I) Cathode: CO₂ + 8H+ + 8e^" = CH₄ + 2H₂O;

2CO₂ (g) + 7H₂ (g) = C₂H₆ (g) + 4H₂O (I) Cathode: 2CO₂ + 14H⁺ + 14e^" = C₂H₆ + 4H₂O; 2CO₂ (g) + 6H₂ (g) = C₂H₄ (g) + 4H₂O (I) Cathode: 2CO₂ + 12H⁺ + 12e^' = C₂H₄ + 4H₂O 2CO₂ (g) + 6H₂ (g) = C₂H₅OH (I) + 3H₂O (I)

Cathode: 2CO₂ + 12H⁺ + 12e^" = C₂H₅OH + 3H₂O; CO₂ (g) + H₂ (g) = CO (g) + H₂O (I) Cathode: CO₂ + 2H⁺ + 2e^" = CO + H₂O.

In another embodiment, the membrane across which ions can migrate is an anionic exchange membrane. In a cell of this type, water may be split at the cathode to form hydrogen and OH^" ions. The OH^" ions can then migrate through the membrane to the anode, where they can reduce CO₂. The CO₂, OH^" ions and electrons can react on the surface of a suitable catalyst to form one or more hydrocarbons. Examples of such reactions are as follows: CO₂ (g) + 4H₂O (I) = CH₄ (g) + 2H₂ (g) + 3O₂ (g)

Anode: CO₂ + 4OH^" = CH₄ + 4e^' + 3O₂; CO₂ (g) + 4H₂O (I) = CH₃OH (I) + 2H₂ (g) + 2.5O₂ (g) Anode: CO₂ + 4OH^' = CH₃OH + 4e^" + 2.5O₂. In a preferred embodiment, the membrane is a hydrophilic ionic membrane (e.g. as disclosed in WO03/023890, the content of which is incorporated herein by reference). Membrane materials of this nature make it possible to supply water only one side of the electrolysis cell, whilst still maintaining hydration of the membrane. This is unlike most commercial PEM electrolysers, as water needs to be supplied to both sides of the membrane to maintain hydration. If a hydrophilic ionic membrane is used as a cationic exchange membrane i.e. transport H⁺ ions, water may not need to be supplied to the cathode; the water from the anode can be sufficient to hydrate the entire membrane. If a hydrophilic ionic membrane is used as an anionic exchange membrane, i.e. to transport OH^" ions, water may not need to be supplied to the anode; the water from the cathode can be sufficient to hydrate the entire membrane.

CO₂ is fed to the cathode at an adequate pressure for the synthesis of the desired fuel to take place. One example of a fuel formed by this process is methanol. In another preferred embodiment, a method for the manufacture of hydrocarbons comprises absorbing CO₂ from the atmosphere using of membrane absorber containing a liquid absorbent, regenerating the liquid absorbent by desorbing CO₂ in an electrochemical cell, and supplying CO₂ to an electrolysis cell of the invention. When the process is operated making use of renewable or nuclear energy, the fuel produced will have a neutral effect on the environment in terms of carbon emissions.

The term "membrane absorber", as used herein, is a porous, water- repelling membrane for the transfer of compounds between a gas and a liquid. The membrane forms a gas-permeable barrier between a liquid and a gas, where components can diffuse through the pores and are absorbed by a suitable liquid (liquid absorbent). In a preferred embodiment, the membrane absorber comprises a hydrophobic microporous hollow fibre membrane.

The term "liquid absorbent", as used herein, is any liquid compound that is capable of reversibly binding CO₂. In a preferred embodiment, the liquid absorbent is KOH. In a chemical equilibrium with a CO₂-containing gas, a KOH solution is transformed into a solution of carbonates (including K₂CO₃), the relative amounts of carbonate and bicarbonate depending on the CO₂ partial pressure.

Regeneration of liquid absorbent and CO₂ from CO₂-bound liquid absorbent occurs in an electrochemical cell. For example, when the liquid absorbent is KOH, K₂CO₃ enters the electrochemical at the anode. Here, a net replacement of the potassium ions by hydonium takes place, to form KHCO₃ in a first step, and eventually H₂CO₃, with subsequent stripping of the CO₂ with the oxygen stream. KOH is regenerated in the cathode compartment. In the first stage of a process embodying the invention, atmospheric CO₂ is absorbed by a hydrophobic microporous hollow fibre membrane absorber operating with a KOH solution as the liquid absorber. CO₂ from the air diffuses through the microporous membrane and reacts with the KOH solution to form a K₂CO₃ solution. In the second stage of the process, the K₂CO₃ solution is fed to a membrane electrolyser, where the CO₂ and KOH are regenerated by the electrolysis process. The regenerated KOH is recycled to the membrane absorber, and the CO₂ stream is fed to the final component of the process, a second electrolysis cell. Here, water is oxidised at the anode to form oxygen and hydrogen ions. The hydrogen ions migrate through the membrane to the cathode (the hydrogen-evolving electrode), where they reduce CO₂. Here, the CO₂, H⁺ ions and electrons react on the surface of a suitable catalyst.

The following Example illustrates the invention. Example

A test rig incorporating a cell with the configuration shown in Figure 1 can be constructed. This cell comprises a Proton Exchange Membrane (3) separating a platinum mesh anode (1) and a copper mesh cathode (5).

In the cell, the Proton Exchange Membrane (3), made of a hydrophilic ionic material as described in WO03/023890, divides the structure into two chambers. In an anode chamber, the platinum mesh (1) is mounted and wired to serve as a the current collector for the anode. An electrocatalyst (2) in the anode, for the oxidation of water, is platinum black deposited on a carbon cloth, at a loading of 2 mg/cm². An electrocatalyst (4) used in the cathode, is a mixture of Cu, Zn and Al oxides with an approximate composition 45 wt% CuO, 27.5 wt% ZnO and 27.5 wt% AI₂O₃, deposited on a carbon cloth to obtain a final loading of 1 mg/cm² CuO, and mounted in the cell in intimate contact with the PEM (3). The current collector for the cathode is the copper mesh (5) connected to an external lead. The cell is designed in such way that the assembly of the current collectors (1 ,5), electrocatalysts (2,4) and PEM (3) is compressed together by the cell structure to ensure a good transfer of ions, electrons and molecules to the reaction points on the electrocatalyst.

A cathode chamber (6) filled with a 0.1 M KHCO₃ aqueous solution which acts as a CO₂ carrier whilst facilitating the retention and subsequent analysis of the methanol produced in the cell. CO₂ is bubbled through this solution during the electrochemical synthesis, to ensure that it remains saturated. The CO₂ introduced into the cathode chamber (6) is initially passed through a moisture trap, a hydrocarbons trap and an oxygen trap (Agilent).

Prior to electrosynthesis, water is added to the anode chamber. Following that, 0.5 L/min of CO₂ is fed to the cathode for one hour. The potentiostatic electrosynthesis is then conducted, controlled by a potentiostat/galvanostat, for 3 hours. The liquid electrolyte is at an ambient temperature of 19+2 ⁰C, and the CO₂ at the cathode is at 1 bar of pressure. The electrosynthesis experiments are conducted at total cell potentials in the range of 250 to 800 mV. Electrosynthesis products contained in the liquid electrolyte in the cathode chamber (6) are analysed by gas chromatography.

Claims

1. A method for the manufacture of a hydrocarbon using an electrolysis cell having first and second electrode compartments separated by a membrane across which ions can migrate, which comprises forming ions from water in the first electrode compartment, and supplying CO₂ to the second electrode compartment, where it can undergo reduction by the ions.

2. A method according to claim 1 , wherein the first electrode compartment is the anode and the second electrode compartment is the cathode, and wherein the ions are H⁺ ions.

3. A method according to claim 1 , wherein the first electrode compartment is the cathode and the second electrode compartment is the anode, and wherein the ions are OH^" ions.

4. A method according to any preceding claim, wherein a CO₂ carrier is present in the second electrode compartment.

5. A method according to claim 4, wherein the CO₂ carrier is KHCO₃.

6. A method according to any preceding claim, wherein the membrane comprises a hydrophilic ionic material.

7. A method according to any preceding claim, additionally comprising the steps of: obtaining CO₂ from the atmosphere using a membrane absorber containing a liquid absorbent; regenerating the liquid absorbent by desorbing the CO₂ in an electrochemical cell; and supplying the desorbed CO₂ to the electrolysis cell.

8. A method according to claim 7, wherein the liquid absorbent is KOH.

9. A method according to any preceding claim, wherein the hydrocarbon is methanol.