ROBUST CATALYST FOR HYDROGEN PRODUCTION FROM P-FORMALDEHYDE

Abstract

Disclosed is a method of producing hydrogen from formaldehyde. The method includes mixing an aqueous base, formaldehyde, and a transition metal complex having a transition metal-halide bond to form a homogenous aqueous solution having a basic pH. The halide dissociates from the transition metal complex in response to the basic pH of the solution to produce hydrogen from the formaldehyde present in the homogeneous aqueous solution.

Claims

1. A method of producing hydrogen from formaldehyde, the method comprising: (a) mixing an aqueous base, formaldehyde, and a transition metal complex having a transition metal-halide bond to form a homogenous aqueous solution having a basic pH; and (b) producing hydrogen (H.sub.2) gas from the formaldehyde present in the homogeneous aqueous solution at a temperature from greater than 0 C. to less than 50 C.

2. The method of claim 1, wherein the molar ratio of formaldehyde to base is equal to or less than 2:1.

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

4. The method of claim 1, wherein the base is NaOH and the transition metal complex reacts with the OH.sup. of the NaOH to form a transition metal complex having a transition metal-hydroxyl bond, and wherein the transition metal complex having the transition metal-hydroxyl bond reacts with the formaldehyde to produce H.sub.2 gas.

5. The method of claim 1, wherein the halide is fluorine (F), chlorine (Cl), bromine (Br), iodine (I), or astatine (At), preferably Cl.

6. The method of claim 1, wherein the transition metal is iron (Fe), ruthenium (Ru), iridium (Ir), copper (Cu), or silver (Ag).

7. The method of claim 1, wherein the transition metal complex is an Fe complex, a Ru complex an Ir complex, a Cu complex, or an Ag complex.

8. The method of claim 1, wherein the base is NaOH.

9. The method of claim 1, wherein the mixture has a pH from 8 to 14.

10. 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.

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

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

13. A homogeneous aqueous solution having a basic pH, the solution comprising an aqueous base, formaldehyde, and a transition metal complex having a transition metal-halide bond wherein the temperature of the solution ranges from greater than 0 C. to less than 50 C.

14. The aqueous solution of claim 13, wherein the molar ratio of formaldehyde to base is equal to or less than 2:1.

15. The aqueous solution of claim 13, wherein the formaldehyde is para-formaldehyde, hydrated formaldehyde, or a combination thereof.

16. The aqueous solution of claim 13, wherein the halide is fluorine (F), chlorine (Cl), bromine (Br), iodine (I), or astatine (At), preferably Cl.

17. The aqueous solution of claim 13, wherein the transition metal is iron (Fe), ruthenium (Ru), iridium (Ir), copper (Cu), or silver (Ag).

18. The aqueous solution of claim 13, wherein the transition metal complex is an Fe complex, a Ru complex, a Ir complex, a Cu complex, or an Ag complex.

19. The aqueous solution of claim 13, wherein the base is NaOH, and wherein the mixture has a pH from 8 to 14.

20. The aqueous solution of claim 13, wherein the temperature of the solution is from 10 C. to 40 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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.

[0026] FIG. 1 is a schematic of an embodiment of a reaction system of the present invention.

[0027] FIG. 2 depicts graphs of the amount of hydrogen formed over time using a RuCl.sub.3 catalyst of the present invention.

[0028] FIG. 3 depicts graphs of the amount of hydrogen formed over time using a IrCl.sub.3 catalyst of the present invention.

[0029] 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

[0030] The present invention provides for an efficient and scalable process for producing hydrogen gas from formaldehyde. The process includes mixing an homogeneous aqueous basic solution having a transition metal catalyst (e.g., a transition metal catalyst having a metal-halide bond), formaldehyde (e.g., methanediol or para-formaldehyde or a combination thereof), and a base 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.

[0031] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Transition Metal Complex Catalyst

[0032] In some instances, a transition metal complex having a coordination bond between the transition metal and a leaving group acts as a catalyst for the production of hydrogen (H.sub.2) and, in some cases formate, from formaldehyde. The transition metal complex can undergo a reversible dissociation reaction of at least one leaving group. Without wishing to be bound by theory, it is believed that the dissociation of at least one leaving group can produce a transient electrophilic species. A non-limiting example of a transition metal complex catalyst undergoing a dissociation reaction is shown in equation (3) below:

[M.sup.a(Z.sub.n).sup.b(L.sub.o).sup.x].sup.y.Math.[M.sup.a(Z.sub.n).sup.b].sup.y+(L.sub.o).sup.x(3)

where M is a transition metal having a charge a, Z is one or more ligands bonded to the metal with a total charge of b, L is one or more leaving group with total charge of x, and a is a positive integer from 0 to 6, preferably 0 to 3, b is an negative integer from 0 to 5, x is a negative integer from 1 to 2, y is the total charge of the transition metal complex, and n and o are the atomic ratio relative to M, where n ranges from 0 to 6 and o ranges from 1 to 3. In some instances y is 0, 1, 2, 3, 4, 5, or 6.

[0033] The transition metal complex can react with nucleophiles in the reaction mixture, for example, hydroxide ion as shown in equation (4) below.

[(M).sup.a(Z.sub.n).sup.b(L.sub.o).sup.x].sup.y+(OH.sup.).sub.p.Math.[(M).sup.a(Z.sub.n).sup.b(OH.sup.).sub.p].sup.y(4)

where M is a transition metal having a charge a, Z is one or more ligands bonded to the metal with a total charge of b, L is one or more leaving group with total charge of x, and a is a positive integer from 0 to 6, preferably 0 to 3, b is an negative integer from 0 to 5, x is a negative integer from 1 to 2, y is the total charge of the transition metal complex, and n, o, and p are the atomic ratio relative to M, where n is ranges from 0 to 6, o ranges from 1 to 3, and p ranges from 0 to 1. In some instances, y is 0, 1, 2, 3, 4, 5, or 6.

[0034] Without wishing to be bound by theory, it is believed that the [(M).sup.a(Z.sub.n).sup.b(OH.sup.).sub.p].sup.y species can react 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 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 [(M).sup.a(Z.sub.n).sup.b (OH.sup.).sub.p].sup.y intermediate to form the same species.

##STR00002##

[0035] where M is a transition metal having a charge a, Z is one or more ligands bonded to the metal with a total charge of b, L is one or more leaving group with total charge of x, and a is a positive integer from 0 to 6, or 0, 1, 2, 3, 4, 5, 6, preferably 0 to 3, b is an negative integer from 0 to 5, or 0, 1, 2, 3, 4, 5, x is a negative integer from 1 to 2, y is the total charge of the transition metal complex, and n, o, and p are the atomic ratio relative to M, where n is ranges from 0 to 6, or 0, 1, 2, 3, 4, 5, 6, o ranges from 1 to 3, or 1, 2, or 3, and p ranges from 0 to 1. In some instances y is 0, 1, 2, 3, 4, 5, or 6. In a preferred instance y is 0.

[0036] The transition metal in the transition metal complex catalyst can be iron (Fe), ruthenium (Ru), rhodium (Rh), iridium (Ir), copper (Cu), or silver (Ag), zinc (Zn) or any combination thereof. Preferably, the transition metal is Fe(II), Ru(III), Ir(III), Cu(I), or Ag(I). In some instances, at least one of leaving groups (L.sub.o) can include a halide. Non-limiting examples of halides including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), or astatine (At), preferably, chlorine (Cl). Ligand Z can be the same or different than leaving group L. In some embodiments, Z can be an inorganic ligand, an organic ligand or a combination thereof. Non-limiting examples of organic groups include aromatic groups, a cyano group, a substituted cyano group, acetate thiocyanate, aminidate, nitrate, or combinations thereof. Non-limiting examples of inorganic groups include a halide, phosphate, or both. In some complexes Z is not necessary (e.g., when M has a charge of +1). In a preferred instance, the transition metal complex is a metal halide (e.g. a transition metal complex having the structure MZL, where Z and L are both halides).

B. Reactants and Medium for Production of Formate and Hydrogen

[0037] 1. Reactants

[0038] The reactants in the step of producing formate and H.sub.2 can include formaldehyde, paraformaldehyde, or other organic molecules that release formaldehyde in aqueous solution. 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 can include a metal hydroxide (MOH or M(OH).sub.2), where M is an 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.

[0039] 2. Medium

[0040] The production of formate and hydrogen from formaldehyde 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.

C. Generation of Hydrogen

[0041] As illustrated in the Examples section, hydrogen can be produced by mixing an aqueous composition having a basic pH, formaldehyde, and a transition metal complex catalyst. In preferred instances, the catalyst and the formaldehyde are partially or fully solubilized within the aqueous composition. FIG. 1 is a schematic of an embodiment of a reaction system 100 for producing formate and hydrogen from formaldehyde. System 100 is particularly suited to methods that use a transition metal complex catalyst having a leaving group that dissociates from the transition metal complex in response to basic pH. System 100 includes container 102, aqueous mixture 104, and mixer 106. Container 102 can be transparent, translucent, or opaque. The aqueous homogeneous mixture 104 includes the aqueous formaldehyde (methanediol), a transition metal complex catalyst, and a base described throughout the specification. Mixer 106 can agitate the mixture to assist in dissolution of the formaldehyde, transition metal catalyst, and base. The transition metal complex catalyst can be used to catalyze the production of formate and 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 (5) below. The addition of a catalytic amount of the transition metal catalyst of the present invention does not appear to inhibit this disproportionation.

##STR00003##

[0042] The production of formate (e.g., sodium formate) can be as illustrated in the reaction pathway (A) above and equation (6) below.

CH.sub.2O(l)+NaOH(aq).fwdarw.H.sub.2(g)+HCOONa(aq) Gf.sup.0=91 kJ/mol(6)

Without wishing to be bound by the theory, the production of hydrogen is in the homogeneous phase of the aqueous mixture. The spent transition metal complex (e.g., (M).sup.a(Z.sub.n).sup.b) can precipitate, or be precipitated, from the solution by addition of acid to increase the pH of the solution. The resulting precipitate can be removed, or substantially removed, through known solid/liquid filtration methods (e.g., centrifugation, filtration, gravity settling, etc.). In some embodiments, the transition metal complex is not removed or is partially removed from the solution. The formate (or formic acid), which is also dissolved in the solution, can then be used as a carbon source for production of other compounds (e.g., oxalate and/or monoethylene glycol.

[0043] Notably, and in one non-limiting embodiment, no carbon dioxide is formed during the production of hydrogen and, optionally formate. 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 formaldehyde as a hydrogen storage agent and formate as a carbon source for homologation reactions.

D. Methanol Feedstock for Production of Monoethylene Glycol

[0044] In some instances, methanol can be used as a feedstock for the production of hydrogen. Methanol can be oxidized to form formaldehyde by, for example, the Formox (Formox AB, Sweden) process. In this process, methanol and oxygen react in the presence of a catalyst such as silver metal or a mixture of an iron oxide with molybdenum and/or vanadium to form formaldehyde. When the catalyst is a mixture of iron oxide with molybdenum and/or vanadium, methanol and oxygen react at about 300 to 400 C., or 325 to 375 C., or 330 C. to 360 C., or any value there between to produce formaldehyde according to equation (19) below:

CH.sub.3OH+O.sub.2.fwdarw.CH.sub.2O+H.sub.2O(19)

The formaldehyde can then be used as described above in the production of hydrogen

EXAMPLES

[0045] 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

[0046] Materials.

[0047] Paraformaldehyde, 37% formaldehyde solution, and sodium ferrocyanide decahydrate, acetamide were purchased from Sigma-Aldrich (USA). Formic acid was purchased from Acros Organics (BELGIUM). Ruthenium chloride (RuCl3) and iridium chloride (IrCl.sub.3) were purchased from Sigma-Aldrich (USA). 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.

[0048] Analytical Equipment.

[0049] 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.

[0050] Product Analysis.

[0051] H.sub.z, 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).

[0052] Determination of Reaction Kinetics.

[0053] The gaseous outflow of the reaction mixture was hooked up to a Restek ProFLOW 6000 Electronic Flow-meter connected to a computer.

[0054] Determination of Formate Concentration.

[0055] 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.

[0056] Determination of Formaldehyde Concentration.

[0057] 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, 1.0 M) were added and stirred for 10 minutes followed by the addition of sulfuric acid (15 mL, 1.0 M). 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.

Example 2

Generation of Hydrogen from Para-Formaldehyde-RuCl.SUB.3 .Catalyst

[0058] Formaldehyde (2 g, of p-formaldehyde or 37% formaldehyde solutions) was added to NaOH (3 g) in H.sub.2O. The transition metal catalyst, RuCl.sub.3 (1.33 mmoles) was added to the solution. The reaction mixture was stirred at room temperature for seven (7) days with additions of formaldehyde (2 g) and sodium hydroxide (3 g) each day. On each day, hydrogen generation was determined over a period of 0 to 450 minutes. FIG. 2 are graphs of hydrogen production versus time for 7 days. Data lines 202-214 represent data for days 1-7, respectively. As shown in FIG. 2, the transition metal complex with a metal-halide bond was effective at dehydrogenating formaldehyde to produce hydrogen. Specifically, the most hydrogen (greater than 160 mL) was generated day 1, with days 2-7 producing about the same amount of hydrogen.

Example 3

Generation of Hydrogen from Para-Formaldehyde-RuCl.SUB.3 .Catalyst

[0059] Formaldehyde (2 g, of p-formaldehyde or 37% formaldehyde solutions) was added to NaOH (3 g) in H.sub.2O. The transition metal catalyst, IrCl.sub.3 (0.66 mmoles) was added to the solution. The reaction mixture was stirred at room temperature for five (5) days with additions of formaldehyde (2 g) and sodium hydroxide (3 g) each day. On each day, hydrogen generation was determined over a period of 0 to 450 minutes. FIG. 3 are graphs of hydrogen production versus time for 5 days. Data lines 302-310 represent data for days 1-5, respectively. As shown in FIG. 3, the transition metal complex with a metal-halide bond was effective at dehydrogenating formaldehyde to produce hydrogen.