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Title:
PHOTOCHEMICAL METHODS FOR CONVERTING CARBON DIOXIDE INTO OLIGOMERS OR POLYMERS
Document Type and Number:
WIPO Patent Application WO/2017/034525
Kind Code:
A1
Abstract:
A method comprising: irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material (or a colloidal form of the material) which is in contact with at least one liquid solution (e.g., water-based liquid solution) comprising carbon dioxide and at least one polymerization monomer; wherein the carbon dioxide and the polymerization monomer participate in an oligomerization and/or polymerization reaction. Carbon dioxide is photochemically converted at or near a semiconductor material to an oligomer and/or polymer reaction product such as urea-formaldehyde, phenol formaldehyde, or melamine-formaldehyde resin. The semiconductor material can include ZnS and binders can be used such as polytetrafluoroethylene. Acidic or basic conditions can be used. The chain reaction provides for efficient use of the carbon dioxide.

Inventors:
GUARNIERI FRANK (US)
CAMP NICK R (US)
Application Number:
PCT/US2015/046363
Publication Date:
March 02, 2017
Filing Date:
August 21, 2015
Export Citation:
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Assignee:
C2F INC (US)
International Classes:
C08G12/12; C07C273/04; C08F2/48
Foreign References:
US5037525A1991-08-06
Other References:
K ANEMOTO, M ET AL.: "Semiconductor photocatalysis. effective photoreduction of carbon dioxide catalyzed by ZnS quantum crystallites with low density of surface defects.", JOURNAL OF PHYSICAL CHEMISTRY., vol. 96, 1992, XP055366608
WU, C ET AL.: "Polyureas from diamines and carbon dioxide: synthesis, structures and Properties.", PHYSICAL CHEMISTRY CHEMICAL PHYSICS, vol. 14, 2012, pages 464, XP002753219
EGGINS, BR ET AL.: "Factors affecting the photoelectrochemical fixation of carbon dioxide with semiconductor colloids.", JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A: CHEMISTRY., vol. 118, 1998, pages 31 - 40, XP055366609
ZHOU, R ET AL.: "C02 Reduction under Periodic Illumination of ZnS.", JOURNAL OF PHYSICAL CHEMISTRY C., vol. 118, 2014, pages 11649 - 11656, XP055366610
Attorney, Agent or Firm:
RUTT, J. Steven et al. (US)
Download PDF:
Claims:
WHAT IS CLAIMED IS:

1 . A method comprising:

irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide and at least one polymerization monomer;

wherein the carbon dioxide and the polymerization monomer participate in an oligomerization and/or polymerization reaction.

2. The method of claim 1 , wherein the polymerization monomer is selected so that it can undergo a free-radical reaction.

3. The method of claims 1 -2, wherein the polymerization monomer is functionalized with hydroxyl or amino groups.

4. The method of claims 1 -3, wherein the polymerization monomer is functionalized with at least one amino group.

5. The method of claims 1 -4, wherein the polymerization monomer is functionalized with at least one primary amino group.

6. The method of claims 1 -5, wherein the polymerization monomer is functionalized with at least two amino groups.

7. The method of claims 1 -6, wherein the polymerization monomer is functionalized with at least two primary amino groups.

8. The method of claims 1 -7, wherein the polymerization monomer is a urea compound, a melamine compound, or a phenolic compound.

9. The method of claims 1 -8, wherein the polymerization monomer is a urea compound or a melamine compound.

10. The method of claims 1 -9, wherein the polymerization monomer is a urea compound.

1 1 . The method of claims 1 -9, wherein the polymerization monomer is a melamine compound.

12. The method of claims 1 -8, wherein the polymerization monomer is a phenolic compound.

13. The method of claims 1 -12, wherein the liquid is an aqueous liquid.

14. The method of claims 1 -13, wherein the liquid has a pH which is greater than 7.

15. The method of claims 1 -13, wherein the liquid has a pH which is less than 7.

16. The method of claims 1 -15, wherein the solid composite electrode further comprises at least one binder.

17. The method of claims 1 -16, wherein the solid composite electrode further comprises at least one binder and the semiconductor material comprises colloidal ZnS or CdS semiconductor.

18. The method of claims 1 -17, wherein the solid composite electrode further comprises at least one fluoropolymer binder and the semiconductor material comprises colloidal ZnS semiconductor.

19. The method of claims 1 -18, wherein the solid composite electrode further comprises at least one electronically conductive material.

20. The method of claims 1 -19, wherein the ultraviolet light is primarily UV or UV-vis radiation.

21 . The method of claims 1 -20, wherein the irradiating step is carried out without application of an electrical potential to the semiconductor material.

22. A method comprising:

irradiating ultraviolet light on at least one colloidal material comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide and at least one polymerization monomer;

wherein the carbon dioxide and the polymerization monomer participate in an oligomerization and/or polymerization reaction.

23. The method of claim 22, wherein the colloidal material comprises colloidal particles suspended in the liquid solution.

24. A photochemical system adapted to carry out the method according to any of claims 1 -23.

Description:
PHOTOCHEMICAL METHODS FOR CONVERTING CARBON DIOXIDE INTO

OLIGOMERS OR POLYMERS

BACKGROUND

The environmental imperative of reducing greenhouse gas emissions seems to be completely at odds with the growing populations' need for ever greater quantities of inexpensive energy. The population growth over the next 20 years is expected to increase energy utilization and CO 2 emissions by approximately 50% from current levels. Many proposed carbon capture technologies can dramatically increase energy costs, resulting in a clash between the economic demands for increasing supplies of low cost energy and the need to reduce greenhouse gas emissions. One way to address these two seemingly incompatible demands are to capture and incorporate CO 2 into commercially valuable products using a process that requires only a small amount of energy to initiate the chemical reactions.

Prior approaches to address carbon dioxide capture issues include US Patent No. 8,658,016; 8,845,878; and 8,663,447; and US Patent Publications

201 1 /01 14502; 2013/0277209; and 2010/0187123. See also Srinivas et al.,

Photochemistry and Photobiology, 2012, 88, 233-241 .

SUMMARY

The problems of carbon capture and sequestration (CCS) can be understood by enumerating the seemingly contradictory constraints that must be simultaneously satisfied by a system designed to capture and sequester large amounts of CO 2 and produce an economically desirable product. A first precept is little or no burning of fossil fuels, because violation of this principle would be costly and self-defeating since large amounts of CO 2 would be created by a system intended to mitigate CO 2 . Making a product that uses the gas as one of the inputs requires inducing CO 2 to chemically react - so if a chemical reaction is to be started with no heating, then it is logical to conclude that the reaction needs to be initiated with light. Since the amount of CO 2 that needs to be captured is so massive, it was reasoned that it would be highly desirable to create a co-polymer with CO 2 as one of the two reactants. All of these considerations indicate that carrying out a free radical polymerization chain reaction with carbon dioxide would be a very cost-effective means of sequestering the gas if the final product has a high economic value. It is also important that there is continuing demand for the product as C0 2 emissions are expected to grow for the foreseeable future. Finally, because the stack effluent from a power plant emits a gaseous stream that is only 5-15% C0 2 , the polymerization reaction must occur with dilute dissolved C0 2 since concentrating the gas is likely to prove too costly.

Urea-formaldehyde polymers (UFP) are arguably among the most singularly important classes of industrial high molecular weight molecules, because they are used extensively in the wood products and composites industry as coatings and in the agricultural industry as slow-release nitrogen fertilizer. [Minopoulou, E. et al. Use of NIR for Structural Characterization of Urea-Formaldehyde Resins. International Journal of Adhesion & Adhesives 23, 473-484 (2003); Tolescu, C. & lovu, H.

Polymer Conditioned Fertilizer. U.P.B. Sci. Bull., Series B 72, 3-14 (2010)].

They have been extensively studied since the 1920s to the present day with important new inventions on processes of synthesis, curing and control between the viscous liquid and powder form resulting in a continuing stream of patents. See, for example, US Patent Nos. 2,056,457; 3,198,761 ; 4,410,685; 4,640,709; and

5,089,041 . With the demand for UFP projected to grow significantly in both the coatings and fertilizer industry if C0 2 can be used in place of formaldehyde in UFP synthesis, then substantial quantities of this greenhouse gas can be sequestered into a commercially valuable polymer. Interestingly, even though the importance of UFP has been recognized for a century and it has long been established that the polymerization occurs through a process of acid or base catalyzed

polycondensation, the actual reaction mechanism is still a topic of research to this day.

Within this context, embodiments described herein include methods of making materials and using systems and apparatuses, as well as the systems and apparatuses.

A first aspect is a method comprising: irradiating with electromagnetic radiation at least one solid composite electrode comprising at least one

semiconductor material in contact with a liquid solution comprising carbon dioxide and at least one polymerization monomer; wherein the carbon dioxide and the polymerization monomer participate in an oligomerization and/or polymerization reaction. While not limited by theory or reaction mechanism, free radical reaction of carbon dioxide can start the oligomerization or polymerization process in which UV light converts carbon dioxide to a free radical anion.

A second aspect provides for a method comprising: irradiating ultraviolet light on at least one colloidal material comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide and at least one polymerization monomer; wherein the carbon dioxide and the

polymerization monomer participate in an oligomerization and/or polymerization reaction.

In one embodiment, the polymerization monomer is selected so that it can undergo a free-radical reaction. In one embodiment, the polymerization monomer is functionalized with hydroxyl or amino groups. In one embodiment, the

polymerization monomer is functionalized with at least one amino group. In one embodiment, the polymerization monomer is functionalized with at least one primary amino group. In one embodiment, the polymerization monomer is functionalized with at least two amino groups. In one embodiment, the polymerization monomer is functionalized with at least two primary amino groups. In one embodiment, the polymerization monomer is a urea compound, a melamine compound, or a phenolic compound. In one embodiment, the polymerization monomer is a urea compound or a melamine compound. In one embodiment, the polymerization monomer is a urea compound. In one embodiment, the polymerization monomer is a melamine compound. In one embodiment, the polymerization monomer is a phenolic compound.

In one embodiment, the liquid is an aqueous liquid. In one embodiment, the liquid has a pH which is greater than 7 (e.g., about 8-13). In one embodiment, the liquid has a pH which is less than 7 (e.g., about 2-6).

In one embodiment, the solid composite electrode further comprises at least one binder. In one embodiment, the solid composite electrode further comprises at least one binder and the semiconductor material is colloidal ZnS or CdS. In one embodiment, the solid composite electrode further comprises at least one

fluoropolymer binder and the semiconductor material is colloidal ZnS. In one embodiment, the solid composite electrode further comprises at least one

electronically conductive material. In one embodiment, the electromagnetic radiation is primarily UV or UV-vis radiation.

In one embodiment, the irradiating step is carried out without application of an electrical potential to the semiconductor material.

In one embodiment, the colloidal material comprises colloidal particles suspended in the liquid solution.

In one embodiment, a photochemical system is provided which is adapted to carry out a method as described and/or claimed herein.

In a preferred embodiment, for example, it was demonstrated that urea- formaldehyde polymers (UFP) can be readily synthesized from C0 2 and urea via a light driven ZnS mediated free radical chain reaction. Transient exposure to UV radiation is the only energy requirement for converting a fraction of dissolved C0 2 into radical anions, which likely initiates the polymerization chain reaction consuming substantial amounts of or all of the urea and C0 2 in the solution, and thus the gas is sequestered into a polymer with no fossil fuel utilization in a reaction that goes to completion. In a preferred embodiment, it is shown that the process can be easily tuned to produce the viscous UFP resins used in the wood products industry or the UFP powders used as time-release nitrogen fertilizer in the agriculture industry. In this embodiment, by making C0 2 one of the components of the polymer, gram scale quantities of the gas can be captured within minutes using less than 20 Watts of power, demonstrating that large amounts of C0 2 can be sequestered into a commercially valuable product using a negligible amount of energy.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates an example of a photochemical reaction vessel.

Figure 2 shows the compounded semiconductor material.

Figure 3 shows the reaction product under acidic conditions (left) and basic conditions (right).

Figure 4A shows MALDI-TOF mass spectral results for the product produced under basic conditions, and Figures 4B and 4C show proposed structures.

Figure 5A shows electrospray mass spectral results for the product produced under basic conditions. Figure 5B provides a focus on the low molecular weight region, and Figure 5C provides a focus on the high molecular weight region for electrospray mass sprectral results.

Figure 6A shows MALDI-TOF mass spectral results for the product produced under acidic conditions, and Figures 6B and 6C show proposed structures.

Figure 7 shows a proposed reaction mechanism.

Figure 8A shows electrospray mass spectral results for the product produced under acid conditions.

Figure 8B provides a focus on the low molecular weight region for electrospray mass spectral results.

Figure 8C provides a focus on the high molecular weight region for electrospray mass spectral results.

DETAILED DESCRIPTION

INTRODUCTION

References cited herein are incorporated by reference in their entirety.

The term "comprising" or "comprises" as used herein can be replaced in other embodiments with the terms "consisting essentially of" and "consisting of" as known in the art. Basic and novel characteristics of the inventions described herein are described to support use of these phrases.

While not being limited by theory or reaction mechanism, the hypothesis of this work is that a CO 2 free radical anion will behave as a super-reactive

formaldehyde and, in the presence of dissolved CO 2 and urea, it will initiate a free radical chain reaction resulting in the synthesis of UFP. It was shown that this in fact does happen in an extremely efficient manner, demonstrating what is believed to be for the first time that CO 2 can readily be used to initiate a free radical chain reaction that results in the sequestration and capture of CO 2 into an industrially important oligomer or polymer.

Each of the elements and steps summarized above are described in more detail herein.

PHOTOCHEMICAL CELLS AND DEVICES

Photochemical cells or devices can be used to carry out the methods. These are known in the art and include, for example, one or more electrodes as well as the light source and medium for holding the solids, liquids, and gases used in the cell. Observation windows can be built into the cell. Small, medium, and large scale devices and cells can be used. An example is shown in Figure 1 and comprises a lamp and a container for the liquid. The solid composite electrode is also present in a location to be irradiated by the lamp as known in the art. The scale of the cells and devices can be enlarged as known in the art, and one can adapt the methods to be batch, continuous, or semi-continuous as known in the art. The cells and devices can be also adapted for the appropriate temperatures and pressures.

IRRADIATING STEP

Methods and equipment for irradiation and providing electromagnetic radiation, covering the electromagnetic spectrum including UV light, are known in the art. In a preferred embodiment, irradiating primarily with UV light is carried out, using a UV lamp. For example, wavelengths of 200 nm to 1 ,200 nm can be used, extending below and above the wavelength of the visible range (which is

approximately 400-800 nm). However, the focus for initiating the reaction is UV light. Factors such as the power, geometry, and wavelength of the light source such as the lamp can be adapted for a particular application in view of the larger system, as known in the art. The selection of the wavelength of light can be controlled by the selected semiconductor material in the solid composite electrode. Also, methods known in the art such as filters or monochromators can be used to control the wavelength which impacts the electrode.

SOLID COMPOSITE ELECTRODE WITH SEMICONDUCTOR MATERIAL

A UV light receiving solid can be used in various forms, whether a solid, integral mass or solid dispersed particles such as a colloidal dispersion. Solid composite electrodes can be used which include at least one semiconductor material. Multi-layer electrodes can be used. Various geometries can be used including films, which can be flexible films. Film thickness can be adapted to the needs as known in the art.

Semiconducting materials used in, for example, electrodes are known in the art including those used for photochemistry and/or photoelectrochemistry. For example, the semiconductor material can create holes when it is irradiated as known in the art. The semiconductor material can be selected to have a band gap which allows for the creation of holes and reaction with carbon dioxide.

Semiconductor materials include group IV semiconductors, group lll-V semiconductors, group ll-VI semiconductors, and the like, as known in the art.

Examples of ll-VI semiconductors include sulfide, selenide, and telluride materials including, for example, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, MoS 2 , MoSe 2 , MoTe 2 , WS 2 , WSe 2 . Metal dichalcogenides can be used. The semiconductor can be in particle form and compounded with other components to form a solid, integral electrode structure. Average particle size can be, for example, less than about one micron, or from about 5 nm to about 500 nm, or about 10 nm to about 250 nm, or about 25 nm to about 100 nm. The particle size can be of a colloidal nature.

The semiconductor electrode comprising semiconducting material can be, for example, a composite comprising at least one semiconductor, at least one binder, and optional other components such as electronic conductors and/or additives. In one embodiment, for example, the semiconductor electrode comprising

semiconductor material is a composite comprising at least one colloidal ZnS or CdS semiconductor and at least one binder. In another embodiment, the semiconductor working electrode is a composite comprising at least one colloidal ZnS

semiconductor and at least one fluoropolymer binder.

In one embodiment, the ZnS semiconductor can be used in various forms including zinc blende (band gap is 3.58 eV at 300K), wurtzite (band gap is 3.70 eV at 300K), and combinations thereof.

Binders including insulating or conductive binders for use in electrode formation are known in the art. Polymeric materials can be used including

polyolefins, carbon backbone polymers, fluorinated polymers, and perfluorinated polymers and copolymers. Hydrophobic materials and polymers can be used.

Poly(tetrafluoroethylene) ("Teflon") can be used. Additional examples of the binder include a conjugated polymer, whether doped or undoped, which can also serve as an electronic conductor. The binder can be selected to help provide the electrode with flexibility.

In one embodiment, the solid composite electrode further comprises at least one electronically conductive material. The material can be particulate. Electronic conductor for use in electrode formation are well-known in the art including conductive carbon (e.g., graphite) and metal materials including, for example, silver, gold, and copper. Additional examples of electronic conductors include a conjugated polymer, whether doped or undoped.

Other materials can be used as known in the art. For example, additives can be used in forming the working electrode. Solvents can be used in forming the electrode to help with dispersion. Compounded materials can be shaped and dried to form solid composite electrodes with proper shape.

The amount of the semiconductor in the semiconductor material can be, for example, about 5 wt.% to about 95 wt.%. The amount of the binder in the

semiconductor material can be, for example, 5 wt.% to about 95 wt.%. The amount of these and other elements of the solid composite electrode can be selected for a particular system to achieve the desired result.

Colloidal dispersions can be used directly or they can be compounded into solid composite electrodes of various shapes and forms.

LIQUID SOLUTION WITH CARBON DIOXIDE

A liquid solution with carbon dioxide can be prepared which includes one or more solvents such as water and establishes a semiconductor-liquid interface as known in the art. The liquid solution (e.g., based on water) can be subjected to turbulence and mixing so that the carbon dioxide can diffuse in the liquid and participate in the photochemical reactions at the electrode. As known in the art, carbon dioxide can dissolve in water, forming carbonic acid, providing mild acidic pH. Bubbling of carbon dioxide throughout the liquid can be carried out.

Additives can be used in the liquid solution such as, for example, salts, electrolytes, or buffers, to control parameters such as, for example, conductivity or pH, as known in the art. The pH of the liquid solution can be adapted to be higher than 7 or lower than 7 as known in the art.

In one embodiment, the liquid solution is an aqueous solution. The majority liquid component in the liquid solution can be water, and it can be, for example, at least 80 wt.%, or at least 90 wt.%, or at least 95 wt.% water.

The liquid solution can also comprise one or more oligomerization or polymerization monomers, as well as an acid or base component to encourage an oligomerization and/or polymerization reaction, as described more herein. At some point as oligomerization or polymerization starts and progresses, some gelation may occur. Crosslinking or three-dimensional networks can form. In this case, the reaction medium is still a liquid solution as used herein even though a true solution may not be present at some point. For purposes herein, a solution can include situations in which good dispersion is present even if a true solution is not present.

POLYMERIZATION MONOMER AND OLIGOMERIZATION AND/OR

POLYMERIZATION REACTION

Oligomerization and polymerization monomers are known in the art including those monomers capable of undergoing free-radical reactions and reacting with the carbon dioxide under conditions described herein including the photochemical conditions. The monomers can be organic compounds.

As used herein, "oligomerization" produces lower molecular weight materials ("oligomers"), and "polymerization" produces higher molecular weight materials ("polymers"), but no exact demarcation between the two is present, so the terms are used interchangeably. Examples of average number average molecular weight for oligomers and polymers include 50 g/mol to 1 ,000,000 g/mol or 100 g/mol to 100,000 g/mole.

The oligomers and/or polymers can be soluble in the solvent or also can form gels or be insoluble. Crosslinked and/or uncrosslinked polymers and oligomers can form. The materials can be lightly or more heavily crosslinked. Thermosetting types of oligomers and polymers can be produced.

Formaldehyde reactions with other polymerization monomers like, for example, urea, melamine, and phenol are generally known in the art. See, for example, G. Odian, Principles of Polymerization, 4 th Ed., 2004, pp 120-128; and F. W. Billmeyer, Textbook of Polymer Science, 3 rd Ed., 1984, pp 436-442. These reactions can produce, for example, phenolic and amino resins as known in the art.

While not limited by theory or mechanism, in one embodiment, the carbon dioxide described herein can react in a manner analogous to formaldehyde. For example, phenols react with aldehydes, including formaldehyde, to give

condensation products if there are free positions on the benzene ring ortho and para to the hydroxyl group. Acid or base catalysis can be used. The nature of the product can be dependent on the type of catalyst and the mole ratio of reactants, as known in the art. In intermediate steps of the reaction, the formation of addition compounds known as methylol derivatives can be carried out. Addition can occur at the ortho and para positions. As known in the art, further condensation can be carried out to form methylene bridges and ether bridges. Ether bridges can be also converted back to methylene bridges and formaldehyde. Urea and melamine react similarly with formaldehyde to produce amino resins, as known in the art.

Photopolymerization (including photooligomerization) is known in the art and as appropriate the methods of photopolymerization can be adapted for use herein. For example, the photopolymerization reactions can be chain-growth polymerizations which are initiated by the absorption of, for example, visible or ultraviolet light. The light may be absorbed either directly by the reactant monomer (direct

photopolymerization), or else by a photosensitizer which absorbs the light and then transfers energy to the monomer. In general only the initiation step differs from that of the ordinary thermal polymerization of the same monomer; subsequent

propagation, termination and chain transfer steps are unchanged. In step-growth photopolymerization, absorption of light triggers an addition (or condensation) reaction between two comonomers that do not react without light. In step-growth photopolymerization, a propagation cycle is not initiated because each growth step requires the assistance of light.

Hence, the oligomerization or polymerization monomer can be selected so that it can undergo a free-radical reaction, particularly with the activated carbon dioxide and subsequent reaction products as descrbed herein. The monomer can be, for example, functionalized with hydroxyl or amino groups. The monomer can be, for example, functionalized with at least one amino group. The monomer can be, for example, functionalized with at least one primary amino group. The monomer can be, for example, functionalized with at least two amino groups. The monomer can be, for example, functionalized with at least two primary amino groups. The monomer can be, for example, a urea compound, a phenolic compound, or a melamine compound.

ABSENCE OF APPLIED ELECTROCHEMICAL POTENTIAL OR ELECTROCHEMISTRY

In a lead embodiment, the photochemical reaction is carried out without the application of an electrochemical potential to the solid composite electrode or semiconductor material. Hence, in this embodiment, no electrode, working electrode, reference electrode, counter electrode, cathode, anode, or electrolyte need be used as commonly used in electrochemistry. No electrochemical cell or working electrode is used in this embodiment.

WORKING EXAMPLES

Additional embodiments are provided in the following non-limiting working examples. The various parameters in these working examples can be adapted by methods known in the art.

Theory and Construction of the Device

A schematic of a reaction vessel is shown in Figure 1 . A liquid solution containing three grams of urea was added, and a mercury arc lamp providing the light source and glass dispersion tube for bubbling in CO 2 were inserted.

For the purpose of efficiently creating UFP, it is desirable to initiate many simultaneous polymerization reactions with a small amount of photons and thus we created a flexible ZnS-Teflon thin film (Figure 2) that was used to line the inside of the reaction vessel.

Eggins demonstrated that a ZnS colloid exposed to UV light will transfer an electron to CO 2 creating a radical anion [Eggins, B. R., et al. Formation of two- carbon acids from carbon dioxide by photoreduction on cadmium sulphide. J. Chem. Soc, Chem. Commun., 1 123-1 124, (1988); Eggins, B. R., et al. Photoreduction of carbon dioxide on zinc sulfide to give four-carbon and two-carbon acids. J. Chem. Soc, Chem. Commun., 349-350, (1993)]. The goal was that a flexible solid sheet of ZnS completely wrapped around the light source and transiently illuminated with UV radiation, would convert a small population of the dissolved CO 2 into a dispersed set of radical anions. It was further a goal that these free radicals would attack the dissolved urea and the resulting urea-CO 2 radicals would in turn attack other molecules of CO 2 and urea creating UFP. The conceptual purpose of the system is to capture a maximum amount of CO 2 with a minimum amount of input energy into a commercially desirable polymer. Additionally, the system was purposely designed to use only photonic energy so that there is the potential of running it with sunlight. It is desirable in a theoretical limit to not use (or at least minimize use of) any C0 2 emitting fossil fuel energy sources to power the system.

Creation of Various UFP with Free Radical CO?

In the early 1930s, Howald was already describing how complicated making UFP can be by stating in his patent [Howald, A. M., Composite Article and Method of Making. Patent # 2,056,457 (1936)] that "in the presence of any acid, formaldehyde and urea in aqueous solution react violently" and that while he was targeting a UF ratio of 1 :1 .5 "it may be varied within the range from 1 :1 .55 to 1 :1 .05" indicating a wide variation of U-F irregular cross-linking even within the same polymer.

In the present working example, the reaction was run at pH=4 and upon turning on the lamp it was observed immediately that a large amount of a white solid precipitated out of solution (Figure 3A) and the vessel quickly heated up. The speed and exothermic nature of the reaction was generally in accord with Howald's description.

The reaction was also run at pH=8 with the lamp turned on for 5 minutes.

It is believed that acidic conditions favor the creation of the more insoluble methylene urea UFP, while basic conditions favor creating the methylol urea UFP. The ether bonds characteristic of the polymer synthesized under basic conditions often make this form of UFP water soluble. So after shutting off the lamp, this solution was heated for 3 hours at 75 °C in order to evaporate off the water, which revealed the existence of a large quantity of a gel-like substance shown in Figure 3B.

Characterization of the Reaction Products

In order to determine what was synthesized under acidic and basic conditions, samples were sent to a commercial lab for elemental analysis and both MALDI-TOF and electrospray (ES) mass spectrometry were performed. The results of the elemental analysis are shown in Table 1 .

TABLE 1

pH=4 21 7 40 31 99

Theory, 28.92 4.85 42.15 24.07 99.99

Figures 6B

and 6C,

pH=4 sample pH=8 36 6 25 31 98

Theory, 36.78 5.79 26.81 30.62 100.00 Figure 4C,

pH=8 sample

Because the sample created under acidic conditions had a reported elemental analysis that totaled 106%, a fresh sample was resent to redo the analysis, which gave a more reasonable total of 99%.

MALDI-TOF is known to predominantly give parent ion peaks so it is particularly useful for assigning higher molecular weight products. Because MALDI- TOF is unlikely to give urea peaks, electrospray (ES) analysis was also performed explicitly to check how much if any unreacted urea is in the samples in order to see whether the reactions went to completion or not.

Figure 4A shows the MALDI-TOF mass spectrum of a sample created under basic conditions. The MALDI-TOF results using our free radical process is very similar to MALDI-TOF spectra of UFP created under standard base catalyzed polycondensation [Gavrilovic-Grmusa, I., et al. Molar-mass distribution of urea- formaldehyde resins of different degrees of polymerisation by MALDI-TOF mass spectrometry. J. Serb. Chem. Soc. 75, 689-710 (2010)]. The large peak at mass 192.9 corresponds to the structure in Figure 4B, which is the methylene ether linked diurea along with a terminal methanol addition commonly seen in based catalyzed UFP reactions. Simply polymerizing this reaction into a ring structure gives the large peak in the vicinity of mass 522 shown in Figure 4C. The expected elemental analysis is also shown, which is in accord with the experimental results shown in Table 1 .

An important consideration is the degree to which the reaction went to completion. Ferg and coworkers [Ferg, E. E., Pizzi, A. & Levendis, D. C. 13C NMR Analysis Method for Urea-Formaldehyde Resin Strength and Formaldehyde

Emission. J. Appl. Polymer Sci. 50, 907-915 (1993)] have used C 13 NMR to monitor the amount of unreacted urea in a range of acid-base catalyzed UFP syntheses and showed that increased free urea adversely affects the strength of the polymer. An important aspect of the present process is the intrinsic, very high energy of free radical polymerization, which should result in the reactions going to completion. In order to confirm this, we also performed Electrospray mass spectroscopy on the sample specifically to see how much free urea was present after the reaction. Since urea has a MW=60, a urea ES peak should be found at MW=61 . Figure 5A shows the complete ES spectra of the sample created at pH=8. Since it is hard to see the low molecular weight peaks, this region has been isolated, blown up, and presented in Figure 5B, which clearly shows no peak at MW=61 and thus there is no unreacted urea, indicating that the reaction went to completion. Isolation and a blowup of the high molecular weight region is shown in Figure 5C clearly indicating the synthesis of larger polymeric species. The ES was a good compliment to the MALDI-TOF.

Figure 6A shows the MALDI-TOF spectrum for a sample created under acidic conditions. Because acid catalyzed UFP formation favors urea-methylene linkage [Christjanson, P et al., Structure formation in urea-formaldehyde resin synthesis. Proc. Estonian Acad. Sci. Chem. 55, 212-225 (2006)] with removal of the two oxygen atoms from the formaldehyde in a condensation reaction or the CO 2 from the free radical reaction via transformation to water, the proportion of nitrogen in the polymer should increase relative to the base catalyzed reaction. As can be seen from Table 1 , the elemental analysis of our UFP clearly indicates that this is the case as the percentage of nitrogen is above 40% under acidic conditions, while being about 25% under basic conditions. The increase in nitrogen content also implies urea branching as shown in Figures 6B-C in accord with the large peak in the vicinity of mass 332. The large peak at mass 159 in Figure 6A is interesting and somewhat unusual for UFP.

While not limited by mechanism and theory, Figure 7 shows a proposed reaction mechanism, which is a combination of UFP acid catalyzed formation and a Diels-Alder-like free radical cyclization that produces a structure with the required mass. In order to check if the reaction went to completion, this sample was also analyzed with ES. The complete ES spectrum is shown in Figure 8A. A careful look at Figure 8A indicates that there is no detectable peak at 61 and so no unreacted urea. A blow up of the low molecular weight area, however, shows that there is a small peak at 61 indicating that there is a trace amount of unreacted urea and thus the reaction has gone essentially to completion. Figure 8C shows a blowup of the high molecular weight region, which indicates that small amounts of larger polymeric species have been created.

Energy Required to Sequester Carbon Dioxide

Finally, because power plants emit such a vast amount of CO 2 , the great environmental-energy challenge is how to reduce atmospheric levels of this greenhouse gas without invoking prohibitively costly processes. It was reasoned that one way around this seeming paradox is to use a CO 2 free radical

polymerization, because a small unit of input energy in the form of light (e.g., UV light) can initiate a chain reaction resulting in the sequestration of a substantial amount of carbon. Under acidic conditions, the precipitation out of solution occurred so rapidly that one had no specific measure for how much light was input. Under basic conditions, it was chosen to turn the lamp on for 5 minutes as a measure of minimal input energy. Clearly, under the acidic conditions, far less illumination is required. To demonstrate the extreme energy efficiency of this free radical polymerization process, consider the approximate calculations:

1 . The polymer created under acidic conditions is approximately 1 :1 CO 2 :urea

2. The polymer created under basic conditions is approximately 2:1 CO 2 :urea

3. In both cases the reactions went to completion consuming 3 grams of urea

4. The molecular weight ratio of CO 2 :urea is 44:60

5. So approximately 2 grams of CO 2 were sequestered under acidic conditions

6. And approximately 4 grams of CO 2 were sequestered under basic conditions

7. The 450 Watt lamp is 40% efficient and 10% of its emissions are in the UV range

This means that grams of CO 2 are being captured and incorporated into a

commercially valuable polymer using only 18 Watts of power for less than 5 minutes. Shorter reaction times can be used such as, for example 3 minutes, 1 minute, or 30 seconds. One skilled in the art can adapt these and other reaction parameters to the particular needs.