Photochemical reforming of hydrogen vectors

Abstract

A process for the production of hydrogen from a small molecule hydrogen vector by reforming such small molecule hydrogen vector under light in the presence of a light-absorbing carbon enabler.

Description

PHOTOCHEMICAL REFORMING OF HYDROGEN VECTORS Field of Invention

[0001] The present invention relates to the production of hydrogen from a hydrocarbon with saturated C-H bonds and in particular to a process of reforming one or more members of a class of abundant small molecule hydrogen vectors exemplified by gaseous alkanes and water vapor, in the presence of a light-absorbing carbon enabler, to generate hydrogen and a value-added co-product. Background of Invention

[0002] Hydrogen is the only clean burning and non-toxic fuel for a sustainable future. Combustion of hydrogen results in the formation of water as the only by-product of combustion. However, transportation and storage of hydrogen in both gaseous or liquid forms pose safety challenges. This difficulty can be circumvented with the use of more stable and abundant small molecule hydrogen vectors exemplified by, but not limited to, CH4 and H2O, as the source of hydrogen.

[0003] Conventional methods of reforming these small molecules to hydrogen requires some form of non-renewable or renewable energy enabled by heat, electricity, or light.

[0004] Natural gas is a major source of methane and is one of the most abundant fossil fuels currently available on the earth. In addition to its use as a fuel, natural gas is an important source of methane and light alkanes which can be processed to produce valuable chemicals including hydrogen and unsaturated hydrocarbons. The alkanes may be processed by, for example, thermally powered catalytic dehydrogenation togenerate alkenes and hydrogen. However, these processes typically require extreme temperatures, which are usually achieved by burning fossil fuels, resulting in significant greenhouse gas emissions.

[0005] The direct conversion of methane under mild reaction conditions to produce carbon and hydrogen is a “holy grail” in chemistry, mainly because of the large energy (434 kJ mol-1) required to break the C-H bond. SUMMARY OF THE INVENTION

[0006] The present invention provides an alternate to the use of the high energy consuming methods of the prior art to reform hydrogen vectors to produce hydrogen. In the process of the present invention, one or more small molecule hydrogen vectors are introduced into a reactor in which a light absorbing carbon enabler is present. The small molecule hydrogen vectors are brought into contact with the light absorbing carbon enabler, while the carbon enabler is exposed to a light source, which may be a Xenon or LED light source, resulting in the generation of hydrogen and a value added co- product from the small molecule hydrogen vector.

[0007] Artificial lighting used in the process of the present invention may be powered by renewable energy. The intensity of the light may be adjusted by optical or electronic means, such as by the use of lenses, or by adjusting the power to the Xenon or LED lighting. Compared to traditional thermal strategies, using solar energy, or artificial light generated from renewable energy, to trigger reformation of hydrogen vectors can break the thermodynamic limitation and access the reactions at milder conditions, as well as to mitigate greenhouse gas emissions.

[0008] The light absorbing carbon enablers of the present invention are exemplified by, but not limited to coal, biochar, char, carbon black, and biomass. These carbon enablers react with a hydrogen vector, whereby both carbon and said hydrogen vectors are consumed in the process, according to the following reaction equations: C + CH4 + light → 2C + 2H2 C + H2O + light → CO+ H2 Brief Description of Drawings

[0009] The following drawings only specific examples of the present disclosure and should not be considered as limiting its scope; they may be used to understand the processes disclosed herein and the results of the experimental work discussed below.

[0010] Figure 1 is a bar graph detailing the hydrogen evolution activity of a tested carbon material from methane under simulated light, dark conditions, and natural light at various surface temperatures with and without external heating.

[0011] Figure 2 details the Raman spectra of the carbon material before and after reaction with methane.

[0012] Figure 3 details the rate of production of carbon monoxide and hydrogen gas from the photoreforming of a carbon material and water at 40-60 ^C and 15.8, 23.6, and 35.4 W cm-2light intensities from a Xe lamp used to simulate solar light.Detailed Description of the Invention

[0013] The present invention provides a method for reforming hydrogen vectors to produce hydrogen and useful co-products. The hydrogen vectors of the present invention include small molecules like methane, gaseous alkanes, other saturated hydrocarbons with C-H bonds similar to those in methane, and water. In the use of the present invention, the high temperatures and consequent high energy requirements of the prior art methods can be avoided by the use of light absorbing carbon enablers in the presence of light to reform these hydrogen vectors to produce hydrogen and useful co-products.

[0014] In the process of the present invention, one or more small molecule hydrogen vectors are introduced into a reactor, which may be a batch reactor or a continuous flow reactor in which a light absorbing carbon enabler is present. The small molecule hydrogen vectors are brought into contact with the light absorbing carbon enabler, while the carbon enabler is exposed to a light source.

[0015] The light souce may be solar light, or a Xenon or light emitting diode light source, or other artificial light source. The intensity of the light source may range from about 0.1 to about 300 W cm-2, and preferably from about 10 to about 60 Wcm-2. The intensity of the light may be adjusted by optical or electronic means, such as by the use of lenses, or by adjusting the power to the xenon, LED, or other artificial lighting used. Use of light together with the use of one or more light absorbing carbon enablers can resulting in the generation of hydrogen and a value added co-product from the small molecule hydrogen vector at lower temperatures than those required in the prior art, with consequent lower energy consumption.

[0016] Artificial lighting used in the process of the present invention may be powered by renewable energy, which may be generated concurrently with its use or stored in batteries or other energy storage means until required. Compared to traditional thermal strategies, using solar energy, or artificial light generated from renewable energy, to trigger reformation of hydrogen vectors can break the thermodynamic limitation and access the reactions at milder conditions, as well as to mitigate greenhouse gas emissions.

[0017] The light absorbing carbon enablers of the present invention are exemplified by, but not limited to coal, biochar, char, carbon black, and biomass. These carbon enablers react with a hydrogen vector, whereby both carbon and said hydrogen vectors are consumed in the process in the process of the invention, according to the following exemplary reaction equations: C + CH4 + light → 2C + 2H2 C + H2O + light → CO+ H2

[0018] The process of the present invention may be carried out at temperatures as low as 20 degrees Celcius, although the rate of production of hydrogen may increase as temperatures in the reactor are increased, Notwithstanding this, the use of light and light absorbing carbon enablers permits the generation of hydrogen at temperatures substantially lower than those of the prior art.

[0019] The process of the present invention may be carried out in a batch or flow process at pressures from about ambient pressure to 30 psi, or at other pressures within the skill and knowledge of a person skilled in the art.

[0020] The present invention permits the replacement of an energetically inefficient thermochemical steam gasification process with a more efficient photochemical one, operating under ambient conditions powered by renewable sunlight, with low activation energies in the range Ea = 2-7 kJ mol-1.

[0021] The hydrogen production described herein carries a zero-carbon footprint, uses eco-friendly and renewable precursors, such as but not limited to coal, biochar, char, carbon black, biomass, and is readily scalable using fixed or fluid bed photoreactor technology. Moreover, solar cells and light-emitting diodes can drive the process continuously and circumvent the intermittency of sunlight. One of the products of the process of the present invention, namely syngas, a mixture of H2 / CO can be used to make hundreds of thousands of commodity chemicals.

[0022] To demonstrate / determine the utility of the process of the present invention in reforming small molecule hydrogen vectors, one or more are small molecule hydrogen vectors were introduced into a reactor in which a light absorbing carbon enabler was present, as described in more detail in the following examples.

[0023] The reactor comprised a stainless steel reactor body with one or more quartz glass windows disposed on its circumference, and valves controlling the inflow and outflow of gas to and from the reactor body. The quartz glass windows in the reactor allowed light, which could be solar, or LED or Xenon lighting, simulating solar light, to irradiate the surface of the light absorbing carbon enablers which had been placed within the reactor and to promote the generation of hydrogen in the reactor. The reactor may be heated by way of, for example, a heating cartridge located inside or outside thereactor body. The reactor is readily scalable using fixed or fluid bed photo reactor technology to create a reactor suitable for commercial use.

[0024] Either solar power, magnified by the use of optical lenses, Xenon lighting, or four combined or separated LED lights (UV, blue, green and red)were used as the light source for the reaction. Manually varying the power to the Xenon or LED lights yielded varying light intensities. An OMEGA temperature controller was attached to a heating cartridge inserted into a copper block along with a thermocouple inserted into the reactor for measuring and controlling the temperature in the reactor. The reactant gas, comprising a small molecule hydrogen vector, was introduced into the reactor with a flow rate controlled by massflow controllers. The surface area of the carbon enabler that was illuminated by the light could be controlled by optical lenses. New samples were used for every test condition. The effluent gas was periodically sampled and analyzed.

[0025] In all these examples, the carbon and light are necessary facilitators to allow these photochemical reforming reactions, where the carbon is consumed or changed in the process, and the light powers the conversion of the hydrogen vector to hydrogen and a value-added carbon or carbon monoxide co-product. The process of the present invention is unique in that the carbon enabler is not a catalyst and is changed or consumed during the generation of hydrogen.Example 1: Carbon-Enabled Photoreforming of Alkanes to Green Hydrogen under Ambient Conditions

[0026] To confirm the utility of replacing the thermochemical methane pyrolysis process typically operating in the temperature range 800-1100°C with a solar-powered photochemical process operating under ambient conditions, methane was passed through a laboratory scale reactor as described above. The same carbon enabler was used for each test. Four tests were carried out under “dark” conditions, at the reactor temperatures shown in Figure 1. Four tests were carried out under a 300 watt Xenon light, at a light intensity of 10 W cm-2, and at the temperatures shown in Figure 1. One test each was conducted under solar and Xenon lighting, at light intensities of 54.8 and 35 W cm-2, respectively, but without additional heating. The spot size of the lighting was adjusted as shown in Figure 1.

[0027] Hydrogen generation was detected and measured by gas chromatography equipped with both thermal conductivity (TCD) and flame ionization (FID) detectors.

[0028] The activation energies shown in Figure 1 were determined by an Arrhenius plot where k = Ae(-Ea / T). Example 2: Change in carbon material morphology before and after photoreforming reaction with CH4 to produce H2 and C

[0029] Raman analysis of the carbon material utilized in Example 1 was conducted on the carbon enabler both before and after the reaction to confirm changes in its morphology and chemical construction during the reaction. The results are shown in Figure 2.

[0030] Raman spectrums of the carbon material were collected using a red laser (532 nm) in a Bruker Senterra Raman microscope at 1 mW.The results are shown in FIG.2; the differences in ratios between the D and G bands (as can be seen in the left and right bumps at 1350 and 1600 wavenumbers, respectively) and height of the bands signal changes in carbon material morphology. The increase in height of the D-band in the second spectra indicates changes in the morphology of the carbon from a more ordered to less ordered state, Example 3: Carbon Enabled Photoreforming of Water to Green Syngas under Ambient Conditions

[0031] The utility of the process of the present inventio in reforming water vapour into hydrogen and carbon monoxide (CO) was demonstrated as follows. Water vapour was passed through a laboratory scale reactor as described above. The same carbon enabler was used for each test. Four tests were carried out under a 300 watt Xenon light, at a light intensity of 15.8 W cm-2, and at the temperatures, ranging from 40°C to 60°C as shown in Figure 1. One test was conducted under Xenon lighting, at light intensities of 23.6 W cm-2, and another at 35.4 W cm-2,respectively, each at 60°C.

[0032] Production rates of hydrogen and carbon monoxide under each of these conditions were measured by periodic sampling of the outflow gas using GC-FID and GC-TCD are shown in Figure 3, indicating that the use of light and light absorbing carbon materials allows production of hydrogen and carbon monoxide at ambient or slightly above ambient temperatures, well below the temperature range 800-1000 °C required by the processes of the prior art.

[0033] The activation energies for H2 and CO, namely 7.0 and 2.2 kJ mol-1, were well below the activation enerygies of 180-240 kJ mol-1required by the prior art processes.

[0034] While the embodiments of the present disclosure have been described in detail, it is to be understood that, unless otherwise specified, the present disclosure is not limited to particular materials, reaction materials, processes, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequences where this is logically possible.

Claims

Claims 1. A process for the reforming of small molecule hydrogen vectors to produce hydrogen and one or more co-products, said process comprising: a. introducing said small molecule hydrogen vectors into a reactor containing a light absorbing carbon enabler; b. exposing said light absorbing carbon enabler in said reactor to a light source having an intensity of at least about 0.1 W cm-2; and c. optionally, separating the hydrogen from on or more co-products of the reaction; wherein the light absorbing carbon enabler is at least partially consumed during the reaction.

2. A process for photochemical reforming of hydrogen vectors in a reactor containing a light-absorbing carbon enabler, said process comprising: a. introducing said small molecule hydrogen vectors into said reactor; b. exposing said light absorbing carbon enabler in said reactor to a light source having an intensity of at least about 0.1 W cm-2; and c. optionally, separating the hydrogen from on or more co-products of the reaction; wherein the carbon is consumed or changed in the process.

3. The process of claims 1 or 2 wherein the small molecule hydrogen vector is selected from the group consisting of gaseous alkanes, methane and water,4. The process of claims 1 or 2 wherein the small molecule hydrogen vector is selected from the group consisting of methane and water.

5. The process of any one of claims 1 to 4 wherein the light absorbing carbon enabler is one or more of coal, biochar, char, carbon black, or biomass.

6. The process of any one of claims 1 to 5 in which the intensity of the light source is greater than about 0.1 W cm-2.

7. The process of any one of claims 1 to 5 in which the intensity of the light source is greater than about 2 W cm-2.

8. The process of any one of claims 1 to 5 in which the intensity of the light source is between about 0.1 W cm-2and about 300 W cm-2.

9. The process of any one of claims 1 to 5 in which the intensity of the light source is between about 2 W cm-2and about 60 W cm-2.

10. The process of any one of claims 1 to 5 in which the intensity of the light source is between about 10 W cm-2and about 60 W cm-2.

11. The process of any one of claims 1 to 5, wherein a co-product of the reaction is carbon or carbon monoxide (CO).

12. The process of any one of claims 1 to 5 wherein the reaction is powered by solar or artificial light.

13. The process of any one of claims 1 to 5, wherein the reaction occurs under ambient conditions.

14. The process of any one of claims 1 to 5, wherein the reaction occurs at a temperature of between 20 and 60 degrees Celsius.

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