Light-switchable catalyst for the hydrogen production from para-formaldehyde

Abstract

Disclosed is a method of producing hydrogen from formaldehyde, the method comprising obtaining an aqueous mixture having a basic pH and comprising formaldehyde, an iron containing photocatalyst, and a base, and subjecting the aqueous mixture to light to produce hydrogen (H.sub.2) gas from the formaldehyde.

Claims

1. A method of producing hydrogen from formaldehyde, the method comprising: (a) obtaining an aqueous mixture having a basic pH and comprising formaldehyde, an iron containing photocatalyst, and a base; and (b) subjecting the aqueous mixture to light to produce hydrogen (H2) gas from the formaldehyde; wherein the iron containing photocatalyst is an Fe(II) containing photocatalyst comprising at least one reversibly dissociable ligand and can undergo reversible dissociation of the at least one reversibly dissociable ligand upon irradiation with visible light.

2. The method of claim 1, wherein the molar ratio of formaldehyde to base is 0.5:1 to 1.5:1.

3. The method of claim 1, wherein the iron containing photocatalyst and the formaldehyde are each homogenously present in the aqueous mixture.

4. The method of claim 1, wherein the formaldehyde is para-formaldehyde, hydrated formaldehyde, or a combination thereof.

5. The method of claim 1, wherein the formaldehyde is para-formaldehyde.

6. The method of claim 1, wherein the Fe(II) containing photocatalyst comprises ferrocyanide (Fe(CN).sub.6).sup.4 or a salt thereof.

7. The method of claim 1, wherein the mixture has a pH from 10 to 14.

8. The method of claim 1, wherein the method further produces formic acid, and wherein H.sub.2 gas is further produced from the formic acid.

9. The method of claim 1, wherein the temperature of the mixture in step (b) ranges from 10 C. to 40 C.

10. The method of claim 1, wherein an external bias is not used to produce H.sub.2 gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings.

(2) FIG. 1 is a schematic of an embodiment of a reaction system of the present invention.

(3) FIG. 2 are graphs of the amount of products formed or consumed over time during irradiation.

(4) FIG. 3 are graphs of the hydrogen production and change of pH of the non-catalyzed reaction and the catalyzed reaction over time.

(5) FIG. 4 is a graph of the change of hydrogen evolution versus the amount of p-formaldehyde added at a constant amount of NaOH.

(6) FIG. 5 is a graph of the effect of illumination on hydrogen evolution from ferrocyanide catalysis over time.

(7) FIG. 6 are graphs of the hydrogen flow in ml/min versus time in minutes at various p-formaldehyde and NaOH concentrations.

(8) FIG. 7 are graphs of the hydrogen production versus time in minutes at various p-formaldehyde and NaOH concentrations.

(9) FIG. 8 are graphs of hydrogen production in mL versus time in min of the production of hydrogen using various types of water.

(10) FIG. 9 are graphs of hydrogen evolution versus time using different light sources.

(11) While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

(12) The present invention provides for an efficient and scalable process for producing hydrogen gas from formaldehyde. The process includes subjecting an aqueous basic solution having an iron containing photocatalyst, formaldehyde (e.g., methanediol or para-formaldehyde or a combination thereof), and a base to light (e.g., natural or artificial light or a combination thereof), and producing hydrogen gas from the formaldehyde. As illustrated in non-limiting embodiments in the examples, this process can have large turn-over numbers, be operated at relatively low temperatures (e.g., room temperatures such as 15 C. to 30 C., preferably from 20 C. to 25 C.) and under a variety of conditions, thereby allowing for the efficient and scalable production of hydrogen gas. In certain instances, production of unwanted by-products such as carbon dioxide can be avoided.

(13) These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

(14) A. Iron Containing Photocatalyst

(15) The iron containing catalyst can be a saturated 18-electron complex with Fe(II) in an octahedral, strong ligand-field. The iron containing catalyst can undergo reversible dissociation reaction of at least one ligand upon irradiation with visible light. Without wishing to be bound by theory it is believed that the dissociation of at least one organic ligand can produce a transient penta-coordinated 16-electron species isolobal with an organic carbocation. Such an electrophilic species can react with nucleophiles. A non-limiting example of such an iron(II) complex is ferrocyanide ([Fe(CN).sub.6].sup.4). Iron ferrocyanide is available from many commercial manufacturers, for example, Sigma Aldrich (USA), as sodium ferrocyanide decahydrate ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10). A non-limiting example of an iron containing catalyst, ferrocyanide, undergoing a reversible dissociation reaction is shown in equation (9) below.
[Fe(CN).sub.6].sup.4[Fe(CN).sub.5].sup.3+CN.sup.(9)

(16) The iron containing catalyst can react with nucleophiles in the reaction mixture, for example, hydroxide ion as shown in equation (10) below.
[Fe(CN).sub.5].sup.3+OH.sup.[Fe(CN).sub.5OH].sup.4(10)

(17) Without wishing to be bound by theory, it is believed that the [Fe(CN).sub.5OH].sup.4 species is responsible for the reaction with small organic molecules (e.g., formaldehyde in either intact or hydrated form), followed by reductive elimination of hydrogen and consequent formation of the formate anion as shown in the reaction pathway (A) below. Alternatively, the partly deprotonated form of methanediol (CH.sub.2(OH).sub.2), as obtained from the attack of hydroxide ion to p-formaldehyde, may also directly coordinate to the 16-electron [(CN).sub.5Fe].sup.3 intermediate to form the same species as shown in reaction pathway (A) below, where p-FA represents para-formaldehyde.

(18) ##STR00003##
B. Reactants and Medium

(19) 1. Reactants

(20) The reactants can include any small organic molecule capable of dehydrogenation and a base. The small organic molecule can be an organic compound with a terminal aldehyde (RHCO) or carboxylic acid (RCOOH), where R is H or an alkyl group having 1 to 3 carbons. In a preferred instance, the aldehyde is formaldehyde. The carboxylic acid can be formic acid. Formaldehyde can be formaldehyde, aqueous formaldehyde solutions (for example 37% in water), para-formaldehyde, or combinations thereof para-Formaldehyde is the polymerization of formaldehyde with a typical degree of polymerization of 1 to up to 100 units. Aqueous formaldehyde (methanediol) and para-formaldehyde are available from many commercial manufacturers, for example, Sigma Aldrich (USA). The basic reagent includes a metal hydroxide (MOH or M(OH).sub.2), where M is a alkali or alkaline earth metal. Non-limiting examples of alkali or alkaline earth metals include lithium, sodium, potassium, magnesium, calcium, and barium. In a preferred embodiment, the base is sodium hydroxide (NaOH). The molar ratio of small organic molecule (e.g., formaldehyde) to base is equal to or less than 2:1, 1.9:1, 1.8:1, 1.7:1, 1.6:1, 1.5:1, 1.2:1, 1.1:1, 1:1, 0.5:1 or any range there between.

(21) 2. Medium

(22) The generation of hydrogen can be performed in any type of medium that can solubilize the catalyst and reagents. In a preferred embodiment, the medium is water. Non-limiting examples of water include de-ionized water, salt water, river water, canal water, city canal water or the like.

(23) C. Generation of Hydrogen

(24) As illustrated in the Examples section, hydrogen can be produced by irradiating, with light, an aqueous composition having a basic pH, formaldehyde, and an iron containing photocatalyst. In preferred instances, the iron-containing catalyst and the small organic molecule are partially or fully solubilized within the aqueous composition. FIG. 1 is a schematic of an embodiment of the reaction system 100. Hydrogen generating system 100 includes container 102, light source 104, and aqueous mixture 106. Container 102 can be transparent, translucent, or even opaque such as those that can magnify light (e.g., opaque container having a pinhole(s) or those that include a light source within the container). The aqueous homogeneous mixture 106 includes the aqueous formaldehyde (methanediol), an iron containing catalyst, and a base described throughout the specification. Light source 104 can be natural sunlight or an artificial light source such as light from a xenon lamp, a fluorescent light, a light emitting diode (LED), an incandescent light, an ultraviolet (UV) light, or any combination thereof. In certain instances, a combination of natural and artificial light can be used. The iron containing photocatalyst can be used to catalyze the production of hydrogen from the formaldehyde as shown in reaction pathway (A) above. When equimolar solutions of p-formaldehyde and sodium hydroxide are combined a slow Cannizzaro's disproportionation to MeOH and (HCOO)Na can occur as shown in equation (11) below. The addition of a catalytic amount of the iron containing photocatalyst does not appear to inhibit this disproportionation.

(25) ##STR00004##
When the aqueous mixture 106 is exposed the light source 104, H.sub.2 (gas) 108 is produced. Notably, hydrogen is only evolved when the solution containing the catalyst is exposed to light. No hydrogen is evolved when aqueous formaldehyde and sodium hydroxide solution are exposed to light. Thus, it should be understood that you can either illuminate and then add the catalyst or add the catalyst and then illuminate the solution. In addition to the production of H.sub.2, formate (e.g., sodium formate) can be produced as illustrated in the reaction pathway (A) above and equation (12) below. Without wishing to be bound by the theory, the production of hydrogen 108 is in the homogeneous phase of the aqueous mixture. The formate (or formic acid), which is also dissolved in the solution, can then further react as shown in pathway (A) with the iron intermediate (e.g., [Fe(CN).sub.5OH].sup.4) to form additional hydrogen.
CH.sub.2O(l)+NaOH(aq).fwdarw.H.sub.2(g)+HCOONa(aq)(12)
Gf.sup.0=91 kJ/mol

(26) Notably, no carbon dioxide is formed during the production of hydrogen. Thus, the process can be considered a green process. Furthermore, system 100 does not require the use of an external bias or voltage source, although one can be used if so desired. Further, the efficiency of system 100 allows for one to use small organic molecules as a hydrogen storage agent.

EXAMPLES

(27) The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1

Materials and Testing Procedures for Production of Hydrogen from Formaldehyde

(28) Materials.

(29) Paraformaldehyde, 37% formaldehyde solution, and sodium ferrocyanide decahydrate, acetamide were purchased from Sigma-Aldrich (USA). Formic acid was purchased from Acros Organics (BELGIUM). Sodium thiosulfate was purchased from Oakwood Chemicals (USA). Iodine was purchased from Strem Chemicals, Inc. (USA). Citric acid was purchased from Fisher Scientific (USA). Acetic anhydride was purchased from VWR International (USA). Chemicals were used without further purification. If not specifically mentioned, all reactions were carried out in distilled water without degassing or other modifications.

(30) Analytical Equipment.

(31) pH measurements were taken with a Hanna HI 2210 benchtop pH meter with a general purpose combination pH electrode, both purchased from Sigma-Aldrich. Powder XRD diffractograms were obtained on a Rigaku Ultima IV diffractometer set to 2 2o/min from 10-70 2o. UV-Vis spectra were obtained on a Specmate UV-1100 spectrometer. Infrared spectra were obtained on a Nicolet 6700 FTIR with diamond ATR between 650-4000 cm.sup.1, at 128 scans with a resolution of 4 cm.sup.1.

(32) Product Analysis.

(33) H.sub.2, CO.sub.2, CO and O.sub.2 gas identification and detection was carried out with an Agilent 7820A GC equipped with a thermal conductivity detector (TCD), using an Agilent GS-CarbonPlot column (for CO.sub.2) or Agilent HP-Molesieve column (for all other gasses).

(34) Determination of Reaction Kinetics.

(35) The gaseous outflow of the reaction mixture was hooked up to a Restek ProFLOW 6000 Electronic Flow-meter connected to a computer.

(36) Determination of pH.

(37) Two identical solutions of 66.6 mmol of p-formaldehyde and 375 mmol of NaOH were prepared simultaneously and were measured to have identical pH values. Both solutions were then illuminated and to one solution, 2 mmol of sodium ferrocyanide was added, and the pH values were measured at regular intervals for 300 minutes.

(38) Determination of Formate Concentration.

(39) Concentration of dissolved formate was determined according to a modified colorimetric procedure by Sleat et al. (Appl. Environ. Microbiol. 1984, 47, 884). An aliquot of the reaction mixture (0.5 mL) was added to acetamide (10%, 2 mL) and citric acid (0.05%) dissolved in a 1:1 mixture of isopropanol and water. To the test mixture, sodium acetate (0.1 mL of 30%) and of acetic anhydride (7 mL) were added. The test mixture was shaken and incubated at room temperature for 60 minutes and measured spectrophotometrically at 510 nm. The concentration was determined against a standard curve.

(40) Determination of Formaldehyde Concentration.

(41) Formaldehyde concentrations were determined through iodine/sodium thiosulfate titrations. To an aliquot of the reaction mixture (10 mL), de-ionized water (20 mL), iodine (25 mL, 0.05M/L in methanol) and sodium hydroxide (10 mL, 1.0M) were added and stirred for 10 minutes followed by the addition of sulfuric acid (15 mL, 1.0M). The sample solution was then titrated with sodium thiosulphate, with addition of a 1% starch solution as an indicator once the solution turned light yellow. The concentration of formaldehyde was then calculated by a standard curve.

(42) Isolation of Iron Oxide.

(43) Iron oxide was collected after allowing a standard reaction to continue for 5 days with a continuous addition of para-formaldehyde and sodium hydroxide. A brown-red precipitate slowly formed which was centrifuged, washed and dried.

Example 2

Generation of Hydrogen from Para-Formaldehyde

(44) Formaldehyde (50 mmol of p-formaldehyde or 37% formaldehyde solutions) was added to NaOH (250 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (500 mol (1 mol %)) was added to the solution. The reaction mixture was illuminated with a 300 W Xe arc-lamp and the evolution of hydrogen was monitored. In this experiment 825 mL of hydrogen was generated over a 300 minute time period. The disappearance of formaldehyde was monitored by the titration method described above, while the formation of sodium formate was monitored by the colorimetric method described above. FIG. 2 are graphs of formation of products versus reagent consumption during irradiation. Data 202 is the amount of formaldehyde over time. Data 204 is the amount formate formed over time, and data 206 is the amount of hydrogen produced over time. As shown in FIG. 2, hydrogen and formate were both produced, which indicated that formaldehyde and hydroxide ion were both consumed in the reaction process.

Example 3

Catalytic Versus Non-Catalytic Generation of Hydrogen from Formaldehyde

(45) Non-Catalyzed Procedure.

(46) Formaldehyde (66.6 mmol of p-formaldehyde) was added to NaOH (375 mmol) in H.sub.2O. The reaction mixture was illuminated with a 300 W Xe arc-lamp. The change in pH was measured. No hydrogen evolution was detected. No catalyst was added to this solution.

(47) Catalyzed Procedure.

(48) Formaldehyde (66.6 mmol of p-formaldehyde) was added to NaOH (375 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (2 mmol) was added to the solution (total volume 300 mL). The reaction mixture was illuminated with a 300 W Xe arc-lamp, the evolution of hydrogen was monitored, and the change in pH was measured.

(49) FIG. 3 are graphs of the change of pH of the non-catalyzed reaction and the catalyzed reaction over time. Data 302 is the non-catalyzed reaction (blank). Data 304 is the catalyzed reaction of the present invention. Data 306 is the hydrogen production from the catalyzed reaction of the present invention. As shown in FIG. 3, the pH in both a catalyzed standard H.sub.2 evolving reaction and a non-catalyzed reaction decrease over time. The pH of the catalyzed reaction had a faster rate with respect to H.sub.2 production versus the non-catalyzed reaction (time 0 until no more hydrogen evolution was detected, about 30 min, See, FIG. 3), but overtime the rate of pH change of the catalyzed reaction became similar to the rate of pH change of the non-catalyzed reaction. Without wishing to be bound by theory it is believed that decrease in the pH after no more hydrogen evolution was detected (about 30 min.) is due to the Cannizzaro reaction (See, equation 11). Also, without wishing to be bound by theory, it is believed that the initial rate of pH change at the beginning of the catalyzed reaction indicated that hydroxide ion is also required to activate the catalyst.

Example 4

Variation of Formaldehyde Concentration

(50) p-formaldehyde in the amounts listed in Table 1 were added to NaOH (375 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (0.5 mmol) was added to the solution (total volume 250 mL). The amount of hydrogen evolution was measured. The reaction mixture was illuminated with a 300 W Xe arc-lamp and the evolution of hydrogen was monitored. FIG. 4 is a graph of the change of hydrogen evolution versus the amount of p-formaldehyde added at a constant amount of NaOH. From the data in FIG. 4, at low initial concentrations of p-formaldehyde, the conversion to hydrogen was deemed to be as high as 100%, but decreased when the concentration levels of the p-formaldehyde increased. The maximum total productivity was reached when the amount of p-formaldehyde was approximately equimolar with NaOH (p-formaldehyde/NaOH=1.2). At higher ratios hydrogen production was not as pronounced.

(51) p-formaldehyde in concentrations listed in Table 1 was added to 300 mL of 1.125 M NaOH (about 0.34 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (0.6 mmol) was added to the solution and the evolution of hydrogen was monitored. The reaction mixture was illuminated with a 300 W Xe arc-lamp. The amount of hydrogen evolution was measured. Table 1 is a listing of the amount of hydrogen evolved and the catalyst turnover.

(52) TABLE-US-00001 TABLE 1 Formaldehyde Hydrogen (mmoles) evolved (mmoles) Yield (%) Turnovers 0.33* 0.33 100% 0.56 3.33* 3.15 94% 5.25 6.67* 5.23 78% 8.72 33.33* 19.62 59% 32.70 66.67* 34.13 51% 56.88 333.33* 69.90 21% 116.49 375.00** 59.35 16% 98.92 *Commercial para-formaldehyde. **Commercial Formalin solution.

Example 5

Effect of Radiation

(53) Formaldehyde (66.6 mmol of p-formaldehyde) was added to NaOH (325 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (2 mmol) was added to the solution (total volume 300 mL). The reaction mixture was illuminated with a 300 W Xe arc-lamp and the evolution of hydrogen was monitored. FIG. 5 is a graph of the effect of illumination on hydrogen evolution from ferrocyanide catalysis over time. The portion of the lines that have a slope (data 502, ) represents periods when catalyst is illuminated and the substantially flat portions of the line (data 504, .circle-solid.) represents periods when catalyst is in the dark. From the data in FIG. 5, it was determined that when irradiation with visible light was interrupted, the hydrogen evolution stopped and it was either restarted or arrested by intermittently turning the light on and off. In other words, the catalytic system of the present invention is light-switchable.

Example 6

Hydrogen Rate of Formation and Production

(54) Formaldehyde (50 mmol of p-formaldehyde) was added to NaOH (250 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (0.5 mmol) was added to the solution (total volume 300 mL). The reaction mixture was illuminated with a 300 W Xe arc-lamp and the evolution of hydrogen was monitored. An initial 0.25 moles of NaOH was added to both solutions to ensure that the pH was suitable for deprotonation of methanediol. Every 30 minutes, 50 mmol p-formaldehyde and either 50 mmol or 200 mmol NaOH were added. The hydrogen rate of formation (FIG. 6) and productivity (FIG. 7) were monitored by regularly adding p-formaldehyde and NaOH in both a 1:5 and 1:1 molar ratio. FIG. 6 are graphs of the hydrogen flow in ml/min versus time in minutes at various p-formaldehyde and NaOH concentrations. Data 602 is at 50 mmol of NaOH and data 604 is at 200 mmol of NaOH. FIG. 7 are graphs of the hydrogen production versus time in minutes at various p-formaldehyde and NaOH concentrations. Data 702 is at 50 mmol of NaOH and data 704 is at 200 mmol of NaOH. Elevated hydrogen evolution rates were observed when the ratio of p-formaldehyde to NaOH was 1:5, which slowed before the next sample was added. However, with each addition of p-formaldehyde and NaOH, the maximum rate decreased rapidly and after the 10.sup.th addition of regents, the rate was zero. This drop was attributed to decomposition of the catalyst to Fe.sub.2O.sub.3 in highly basic conditions. As shown in FIG. 7, addition of 1:1 p-formaldehyde to NaOH portions (data 602) resulted in a steady productivity being reached, with no decline of catalytic activity for 350 minutes. Within each addition, a reduction in the initial spike of hydrogen production is observed, which stabilizes into a nearly continuous release of 10 mL of hydro-gen per minute.

Example 7

Effect of Water Purity on Catalytic Activity

(55) Formaldehyde (66.6 mmol of p-formaldehyde) was added to NaOH (375 mmol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (3 mmol) was added to the solution (total volume 300 mL). Three types of water de-ionized water, river water and salt water were evaluated. FIG. 8 are graphs of hydrogen production in mL versus time in min of the production of hydrogen using various types of water. Data 802 is hydrogen production using de-ionized water, data 804 is hydrogen production using city canal water, and data 806 is hydrogen production using salt water. As determined from the data in FIG. 8, the reaction rates and final production from the three sources were nearly identical. Thus, the choice of water source (i.e., distilled water, 3.5% NaCl solutions (to match the average salinity of the ocean), or water taken directly from city canal water), had no significant effect on the activity of the catalyst.

Example 8

Long Range Catalytic Runs

(56) Formaldehyde (0.5 mol of p-formaldehyde) was added to NaOH (0.25 mol) in H.sub.2O. The photocatalyst, sodium ferrocyanide ([(CN).sub.6Fe]Na.sub.4(H.sub.2O).sub.10), (120 mg) was added to the solution (total volume 300 mL). The reaction mixture was run twice with two different light sources, a 300 W Xe arc-lamp and a Hg lamp. The evolution of hydrogen was monitored. FIG. 9 are graphs of hydrogen evolution versus time using different light sources. Data 902 is the generation of hydrogen using the method of the present invention using a Xenon arc-lamp as a light source. Data 904 is the generation of hydrogen using the method of the present invention using the Hg lamp as a light source. From the data in FIG. 9, it was determined that the reactions were uninterruptedly carried out for 16 h with periodical additions of NaOH/p-formaldehyde every 30 min. Over time the catalytic system slowly decayed after having produced about 4.8 L of pure hydrogen (14.5% based on p-formaldehyde). At this stage a significant amount of Fe.sub.2O.sub.3 become visible, which was isolated and characterized as described in Example 1.