METHOD AND APPARATUS FOR CONVERTING CARBON DIOXIDE

Abstract

The invention relates to a method for preparing a hydrocarbon by reducing CO.sub.2, wherein CO.sub.2 is reduced to a hydrocarbon with the aid of a directly heated electrode. A device for carrying out a corresponding method, a corresponding power plant and a system comprising said power plant and a vehicle with a combustion engine are also objects of the invention. The method and device may, e.g., be used as a micro-energy system for decentralized energy supply.

Claims

1. A method for producing a hydrocarbon by reducing CO.sub.2, comprising reducing CO.sub.2 to a hydrocarbon with the aid of a directly heated electrode.

2. The method according to claim 1, wherein the reduction occurs enzymatically.

3. The method according to claim 1, wherein the reduction is carried out in several steps, wherein the reduction in at least one, preferably, in all steps, is catalyzed by an enzyme associated with a directly heated electrode.

4. The method according to claim 3, wherein a plurality of steps, preferably, all steps, are catalyzed by enzymes which are each associated with an electrode directly heated to a temperature optimal for the respective reaction.

5. The method according to claim 3, wherein a plurality of steps, preferably, all steps, are catalyzed by enzymes which are associated with the same directly heated electrode.

6. The method according to claim 1, wherein the reduction is carried out in several steps, wherein the reduction in at least one, preferably, in all steps, is catalyzed by an enzyme which at the same time oxidizes a cofactor which is regenerated at a directly heated electrode, wherein the cofactor is selected from the group comprising NADH, NADPH, and FADH.

7. The method of claim 1, wherein the reduction is catalyzed by formate dehydrogenase, aldehyde dehydrogenase and/or alcohol dehydrogenase.

8. The method of claim 1, wherein the CO.sub.2 is transformed to bicarbonate by a carboanhydrase, wherein, optionally, the carboanhydrase is associated with a directly heated electrode.

9. The method according to claim 1, wherein the reduction occurs non-enzymatically at a heated electrode, wherein said electrode preferably comprises a material selected from the group comprising platinum, copper, titan, ruthenium and combinations thereof.

10. The method of claim 1, wherein the directly heated electrode has the form of a spiral or a helix or net or plane.

11. The method of claim 1, wherein the directly heated electrode consists of an electrode material selected from the group comprising carbon, in particular, vitreous carbon or graphite, a precious metal, in particular, gold or platinum, an optically transparent conductive material, in particular stannic oxide doped with indium, copper, stainless steel and nickel.

12. A device in which a method of claim 1 is carried out or which is suitable for carrying out said method, said device comprising two electrodes and a membrane for separating the anodic and cathodic reaction.

13. The device according to claim 12, wherein a plurality of reaction vessels are run in parallel, which can together produce the reaction product.

14. The device according to claim 12, which is constructed as a single use reactor or a reactor suitable for recycling.

15. A device for preparing a hydrocarbon by fixing CO.sub.2, comprising a) a directly heated electrode, with which, preferably, at least one enzyme capable of catalyzing a step in the reduction of CO.sub.2 to a hydrocarbon is associated, or, preferably, at least one cofactor capable of interacting with an enzyme capable of catalyzing a step in the reduction of CO.sub.2 to a hydrocarbon is associated, and b) a device for introducing gaseous CO.sub.2, suitable for introducing the CO.sub.2 into a reaction compartment in which it can contact the directly heated electrode, wherein the device optionally is a device according to claim 12.

16. A power plant for providing energy in the form of electric energy and/or a hydrocarbon, comprising i) an energy source, preferably a regenerative energy source, e.g., based on photovoltaics, ii) the device according to claim 12, wherein the energy required for the preparation of a hydrocarbon is derived from the energy source i), iii) a hydrocarbon storage device, and iv) optionally, a hydrocarbon fuel cell for producing electric energy, or v) optionally, a device for burning hydrocarbon for preparation of warm water or thermal energy for heating of buildings or apartments.

17. A system comprising a power plant according to claim 16 and a vehicle selected from the group comprising car, bus and motorcycle, wherein the vehicle is equipped with an engine suitable for, preferably, optimized for, combustion of a hydrocarbon, preferably, methanol.

18. The method of claim 1, device, power plant or system according to any of the preceding claims wherein the hydrocarbon is selected from the group comprising methanol and methane and formic acid and formaldehyde.

19. The device of claim 1, wherein the hydrocarbon is selected from the group comprising methanol and methane and formic acid and formaldehyde.

20. The power plant of claim 1, wherein the hydrocarbon is selected from the group comprising methanol and methane and formic acid and formaldehyde.

21. The system of claim 1, wherein the hydrocarbon is selected from the group comprising methanol and methane and formic acid and formaldehyde.

Description

[0046] In the context of the invention, “a” means “one or more”, unless specified otherwise.

[0047] FIG. 1 Scope of the project.

[0048] FIG. 2 Planned result of the project activity: Improvement of productivity and stability of the enzyme

[0049] FIG. 3 Core technology—Electro-enzymatic reactor, in which CO.sub.2, H.sub.2 from H.sub.2O and electricity are transformed into high-value fuel such as CH.sub.3OH (methanol).

EXAMPLES

Project Description

[0050] 1. Core Technology Development Activities I—Electro-Enzymatic Reactions

[0051] Our key reaction will be the electrochemical formation of a C1-organic molecule, like methanol (CH.sub.3OH), in an enzyme-cascade over several steps, carried out at conducting and directly heated electrodes.

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[0052] This approach is new, e.g., due to the following features: [0053] mild conditions—no high pressure or high temperatures needed, [0054] high selectivity due to enzymatic conversion; [0055] No purification/concentration of high volume streams of ambient air [0056] No electrolysis in H.sub.2O-conversion to produce H.sub.2 directly from water (producing O.sub.2) [0057] Higher reaction velocity, higher turnover without significant or without degradation of enzymes through the use of directly heated electrodes in comparison to non-heated electrodes; and/or [0058] Direct control of turnover and enzyme activity through integrated electrochemical measurements of turnover and temperature is possible

[0059] These features distinguish our approach from further CO.sub.2 utilization techniques and make it particularly suitable for applications at a small scale.

[0060] Key activities in this project will target the enhancement of the enzymes' yield and stability, i.e, elongation of lifetime. The current status and planned project result are shown in FIG. 2.

[0061] The planned activities to attain these aims are: [0062] Increase enzyme stabilities and activity (by factors of 100 and 30, respectively) [0063] Decrease enzyme manufacturing costs (aim: <10 £/g)

[0064] Similar measures have proven to be successful in prior projects of 3 years duration.

[0065] There are examples of scientifically proven developments, where enzymes were enhanced by using heated electrodes or heated reaction media. These enzymes are typically derived from thermophilic organisms and catalyse the partial reaction in the NADH cycle [McPherson, I. J. und Vincent, K. A.; Electrocatalysis by hydrogenases: lessons for building bio-inspired device. Journal of the Brazilian Chemical Society, 2014].

[0066] 2. Core Technology Development Activities II—Electro-Enzymatic Reactor

[0067] The key reaction cascade is carried out in a specifically designed reactor. Due to our intended business model, the main aim of this project is the development and realisation of a disposable electro enzymatic reactor in a cost-effective manner (assembly, placement of enzymes, wiring). However, a recyclable reactor may also be used, in which, e.g., after a decrease in the efficiency, after a purification, new enzymes are associated with the electrodes.

[0068] The reactor will contain directly heated electrodes onto which the enzymes will be immobilized in a way that electrons from the electrical energy source can be transferred to the electro-biocatalytic reaction. This will be achieved by using big area electrodes in beaker glasses or by modifying the inside of the tube-reactors.

[0069] 3. Core Technology Development Activities III—System Integration

[0070] According to the project's overall aim, the core technologies will preferably be integrated into a standalone system which can be integrated into existing domestic heating infrastructure.

[0071] The basic functional system: an optimized enzyme-cascade is able to produce x gram product within y hours by using z milligram enzyme-catalyst. It is our goal to produce 5 kg of methanol per day for use in existing infrastructure, e.g., heating systems. Owing to the complexity of scaling up bio-catalytic reactions, we will focus on a discrete scale-up strategy: just to increase the number of (disposable) parallel-running electro-biocatalytic reactors. According to current designs, 1.000 parallel reactors will be capable of yielding the intended amount of energy/fuel per day.

[0072] The reaction-medium is separated from the product methanol, e.g., by using a pervaporation unit. The reaction medium will be pumped in a cycle, while the methanol is produced and stored inside the device.

[0073] 4. Comparative Experiments for Bioelectrocatalysis with a Non-Heated Electrode

[0074] Before an optimization bioelectrocatalysis was carried out with the HF Thermalab® (Gensoric, Rostock, DE), preliminary experiments for reducing CO.sub.2 to formic acid were carried out with a nonheatable enzyme alginate electrode. For preparing this electrode, 75 mg processed preparation of a formate dehydrogenase from Candida spp. (Candida boidinii) was used (Sigma Aldrich). The enzyme was immobilized in alginate on a carbon fabris used as a working electrode for reducing CO.sub.2 to formic acid (Wagner A. Enzyme Immobilization on Electrodes for CO.sub.2 Reduction. 2013. Institute of Physical Chemistry). As a reference electrode, a silver silver chloride electrode (Ag/AgCl) with 3 M KCl and as counter electrode a 2 mm graphite rod were used. All reactions were carried out at room temperature in 20 mL of a water based buffered electrolyte solution (0.05 M TRIS, pH 7.7) in a 100 mL reactor. For the bioelectrocatalytic synthesis of formic acid, the reactor was continually supplies with gaseous CO.sub.2. Control experiments were carried out in argon saturated electrolyte solutions in the absence of CO.sub.2. The functionality of the enzyme alginate electrode was first tested by cyclovoltammetry, and the reduction peak of CO.sub.2 was determined to be at about −0.8 V.

[0075] Then, the synthesis of formic acid was carried out with chronoamperometry (Table 1).

TABLE-US-00001 TABLE 1 Bioelectrocatalytic synthesis of formic acid from CO.sub.2 by means of chronoamperometry. voltage Concentration of Amount of (vs. Ag/AgCl, formic acid formic acid Experiment 3M Cl.sup.−) (after 9 h) (after 9 h) No. 1   −1 V 0.16 mM 0.15 mg No. 2 −0.8 V 0.15 mM 0.14 mg

[0076] In a first preliminary experiment, with a voltage of −1 V and at ambient temperature, in total 0.15 mg formic acid could be prepared from CO.sub.2 in a bioelectrocatalytic manner. The quantification was carried out by an enzymatic assay of the sample and via HPLC. In a second preliminary experiment, a voltage of −0.8 V was applied. A similar yield of 0.14 mg formic acid from CO.sub.2 was obtained.

[0077] 5. Optimization of Bioelectrocatalysis at Heated Electrodes

[0078] In the further course of the experimental series, the bioelectrocatalytic reduction of CO.sub.2 at heated electrodes was to be optimized. In this, by targeted heating of the electrodes, the effect of the temperature on the catalytic characteristics of the immobilized enzyme was to be analyzed and thus, optimal parameters for the enzyme catalyzed reaction were to be found. The system HF Thermalab™ from Gensoric was used for the experiments.

[0079] 1 mg enzyme (formate dehydrogenase from Candida sp.) was immobilized through alginate suspension on a heated microelectrode. With this, a series of chronoamperometric measurements at different temperatures (22° C. 30° C., 35° C., 40° C., 45° C.) was carried out. As reference- and counter electrode, the electrodes form the comparative experiments were used. Before the experiments, 1 mg of regenerated cofactor NADH was added and a test of the enzyme microelectrode for bioelectrocatalytic activity with CO.sub.2 was carried out at ambient temperature. In every case, this test was between −320 μA and −330 μA of a chronoamperogram carried out at −0.8 V for 2-3 minutes. During the experiments, the temperature of the electrolyte in the reactor was measured. As a negative control, an experiment with alginate without enzyme on the microelectrode was carried out.

[0080] In the comparative experiments, voltages of −1.0 V and −0.8 V were used. As the difference in yield was less than 10%, and the input of energy into the reactor was to be minimized to avoid overpotentials, a voltage of −0.8 V (vs. Ag/AgCl, 3 M Cl.sup.−) was used.

[0081] In all chronoamperometric measurements, an increased electrical current flow could be observed at the start of the measurements, which decreased after about 3 hours, respectively, to a constant level. The electrical current flow of the reaction at an electrode temperature of 40° C. was highest in comparison to other experiments, both for the start and during the course of the further reaction, which allows the conclusion that there are comparatively high reduction rates. In contrast, the chronoamperogram at 22° C. had the lowest electrical current flow of the enzyme electrodes. The course without enzyme for control hardly showed electrical current flow with about −30 μA, further decreasing to about 0 μA in the course of the reaction. During the chronoamperometric measurements, samples were taken from the reactor after 3 h and after 9 h, respectively, which were analyzed for synthesized formic acid via HPLC (Table 2). In all experiments with heated enzyme microelectrodes, formic acid from the reactor continually supplied with CO.sub.2 was already detected after 3 h, wherein the amount approximately doubled after 9 h. In this, at an electrode temperature of 40° C., the highest amount of formic acid was detected, whereas in experiments with other electrode temperatures (22° C., 30° C., 35° C., 45° C.), less formic acid was produced. With the exception of the experiment at 45° C., the yield of product continually increased proportionally to the temperature of the electrode. During the experiments, the temperature of the electrolyte solution in the reactor was continually monitored. Even at the highest applied heating power, at 45° C. electrode temperature, the electrolyte solution in the reactor was constantly at 22° C. (Table 2).

TABLE-US-00002 TABLE 2 Bioelectrocatalytic synthesis of formic acid from CO.sub.2 at different temperatures on heated electrodes. Formic acid Formic acid Electrolyte Electrode after 3 h after 9 h temperature temperature of reaction of reaction in the reactor 22° C. (with enzyme) n.d.* 0.05 mg 22° C. 30° C. (with enzyme) 0.09 mg 0.19 mg 22° C. 35° C. (with enzyme) 0.13 mg 0.29 mg 22° C. 40° C. (with enzyme) 0.14 mg 0.30 mg 22° C. 45° C. (with enzyme) 0.12 mg 0.22 mg 22° C. 22° C. (control n.d.* n.d.* 22° C. without enzyme) *n.d.: not detected

DISCUSSION

[0082] In comparative experiments, the synthesis of 0.15 mg formic acid from CO.sub.2 by chronoamperometry (−1 V; vs. Ag/AgCl, 3 M Cl.sup.−) in the course of 9 h at room temperature in a 100 mL reactor could be shown. In this, the selective catalytic characteristics of the electro enzyme allowed for application of a low voltage (−0.8 V; vs. Ag/AgCl, 3 M Cl.sup.−) for nearly the same power of synthesis (0.14 mg formic acid in 9 h). In following experiments, the successful synthesis of 0.05 mg formic acid was possible under similar conditions. One essential difference between comparative experiments and the first following experiment at ambient temperature was in the amount of enzyme used as well as the nature of the electrode material. Whereas in the comparative experiment, in total 75 mg of enzyme were immobilized on the carbon textile as bioelectrocatalyst, in the following experiments, a heatable microelectrode from Gensoric was used, wherein, due to the much smaller electrode surface, only 1 mg of enzyme was immobilized.

[0083] Accordingly, the yield with the heatable microelectrode in relation to the use of catalyst was clearly higher with 0.05 mg formic acid per mg enzyme in comparison to the comparative experiment with about 0.002 mg formic acid per mg enzyme.

[0084] The advantage of electrodes heated for use is that temperature for optimal reaction conditions can be directly adjusted at the electrode surface, and it is not required to maintain temperature in the complete electrolyte solution of each reactor, which improves the balance of energy of electrocatalytic processes of every kind. The temperature in the reactor was continuously monitored during the bioelectrocatalytic synthesis. In this, the temperature was constantly maintained at 22° C. This could either be due to a continuous mixing because of the continuous gas supply to the electrolyte, favoring a transport of warmth to the surroundings. On the other hand, a part of the warmth from the heated electrodes could be directly channeled to the immobilized enzymes, further stimulating their conformation changes.

[0085] In further experiments, the rate of synthesis of formic acid was optimized by direct heating of the enzyme microelectrodes. The highest rates of synthesis, with 0.02-0.03 mg/h (in relation to a constant rate of synthesis according to the chronoamperogram after the first 3 hours of the reaction) occurred at 35° C. and 40° C. In comparison, the rate of synthesis of formic acid decreases both with a decrease of the electrode temperature to 22° C. and an increase to 45° C. Probably, both uptake of substrate and delivery of product at the enzyme microelectrode and changes of conformation status of the immobilized enzyme required for execution of the catalytic reaction mechanism were optimal between 35° C. and 40° C., leading to an increase of conversion by the factor 6 compared to ambient temperature (22° C.). It is interesting to note that in the literature, reaction optima of the same enzyme in solution are described to be about 60° C. (Tishkov V et al., Catalytic mechanism and application of formate dehydrogenase. Biochemistry (Moscow), 2004, 69(11):1252-1267).

[0086] In total, at the beginning of each experiment, respectively, the strongest flow of current was measured, which indicates that the majority of the reactions at the electrodes occurs in the first hours. This could be confirmed by measuring the concentration of formic acid. After 3 h, about half of the formic acid present in the further experiment after further 6 h had accordingly already been synthesized. The decrease of the reaction rates can be explained with the increasing product concentration in the bioelectrocatalysts. According to this, diffusion effects are responsible for the reaction taking place more quickly in the beginning. Probably, furthermore, the regeneration of NADH is a limiting factor for the bioelectric catalysis. As, in the beginning, there was enough cofactor for reduction of CO.sub.2, the reactions all took place the fastest. In the further course, the cofactor was reduced to isomers which could not anymore be used for the enzymatic reaction, which led to a decreased speed of the reaction. An effective regeneration of the cofactor, e.g., also enzymatic regeneration, can thus increase efficiency of the reaction.

[0087] As the experiments with different temperatures were always carried out with the same enzyme microelectrode, it is possible to start from the assumption that the bioelectrocatalytic synthesis of formic acid further constantly rises even beyond the 9 h, as, in the tests before each experiment, degeneration of the immobilized material over the duration of one week was not observed.