Thermally reversibly crosslinkable polymers for use as sorbent materials in the area of carbon capture

Abstract

The invention relates to a method for reversibly storing CO2 (carbon dioxide) by means of the Diels-Alder reaction and retro-Diels-Alder reaction. The invention further relates to the two starting compounds and use thereof for this purpose. It is envisaged that the following two starting compounds are used as a mixture at a temperature of < 70°C, so that a thermally reversible sorbent material is formed by means of the Diels-Alder reaction: - 1,3-diene compound and a - dienophile compound, wherein each of these two compounds comprises a polymer chain having amine groups. Upon contact with CO2, it is reversibly stored in the thermally reversible sorbent material.

Description

[0001] Description

[0002] Thermally reversibly crosslinkable polymers for application as sorbent materials in the field of carbon capturing

[0003] The invention relates to a process for the reversible storage of CO2 (carbon dioxide) by means of the Diels-Alder reaction and the retro-Diels-Alder reaction.

[0004] The need to slow global climate change caused by greenhouse gas emissions is very urgent. Above all, the increase in atmospheric CO2 levels must be sustainably prevented. In addition to the prevention and reduction of CO2, technologies for adsorbing CO2 from the ambient air, known as "direct air capture" (DAC), are suitable for reducing the proportion of CO2 in the atmosphere through "negative carbon emissions." The development of suitable adsorption materials should enable efficient and energetically viable CO2 capture. One challenge is the development of efficient sorption materials / sorbent materials that have a high CO2 adsorption capacity and require low energy for desorption, as this is particularly important in large-scale industrial settings.

[0005] Classic sorbent materials for CO2 storage include, for example, metal-organic frameworks (MOFs), amine-functionalized adsorbers, functionalized activated carbon materials, zeolites, and other polymers. Polymer-based adsorbers, in particular, offer a good opportunity to combine high adsorption capacity with economic advantages.

[0006] KR 2017009796 A discloses a CCh absorber made of 3D-porous graphene nanoribbon, in which the CO2 is enclosed in the carbon network. A nanoribbon with defined 3D pores is constructed from carbon repeat units using a Diels-Alder reaction. During the Diels-Alder reaction, CO2 is released from one of the starting compounds and escapes. Consequently, this Diels-Alder reaction is irreversible. Unfortunately, the CO2 release occurs at elevated temperatures, which leads to degradation processes. For dissolved substances, such as pharmaceutical active ingredients, US 2013 / 0048853 A1 describes a heterocyclic copolymer adsorber. The copolymers are composed of divinylbenzene and triallyl cyanurate.

[0007] WO 2019 / 101160 A1 discloses a gas adsorber made of 2,5-furan-guanidine as an acidic gas adsorber and an anionic precipitant. A disadvantage is that the chemical compound must be dissolved in a solvent to later separate a yellow CO2 adduct (carbonate 2,5-furan-guanidine tetrahydrate). This can subsequently release CO2 and H2O at great expense. The disclosed method is not suitable for large-scale CO2 storage because of its highly complex process.

[0008] Polyethylenimine polymers (PEI) are also established; they are branched and cross-linked to ensure good adsorption capacity through intermolecular interactions between the carbon dioxide and the amine polymer. However, a major disadvantage of these materials is their tendency to degrade over time. Aging mechanisms include, for example, oxidative degradation at elevated temperatures with the formation of ammonium salts, hydroxylamine structures, amine oxides, nitroso compounds, and nitro compounds. Other degradation mechanisms include, for example, amine degradation with the formation of urea derivatives and the formation of cyclic urea structures from the polymer chain. For CO2 storage in polymers, however, the most significant degradation mechanism is thermal degradation, as this occurs during the temperature-induced CO2 desorption process.This type of degradation, for example, leads to Hofmann elimination (Hofmann degradation) with the formation of quaternary ammonium compounds. Further thermal degradation leads to the formation of cyclic imidazole structures and the formation of numerous urea derivatives, disrupting the linear chain structure in the polymer chain. The disruption of the polymer chains and the associated loss of aminic structures leads to a reduction in the amine density of the polymer and thus to a steady reduction in the adsorption capacity for CO2.

[0009] A further disadvantage of such known polyethyleneimine polymer networks, in which the polyethyleneimines themselves are irreversibly cross-linked, is the presence of tertiary and quaternary alkyl amine groups (i.e., nitrogen atoms to which no hydrogen is bonded) formed during cross-linking between the polyethyleneimine chains. In particular, the quaternary alkyl amine groups no longer have a free electron pair available on the nitrogen. This leads to a low sorption capacity for CO2. Another disadvantage is that this cross-linking in the polyethyleneimine polymers leads to a lower mobility of these polymer chains and thus tends to result in longer diffusion times for CO2.

[0010] The object of the invention is to provide a process and / or starting materials that enable the reversible storage of gaseous CO2 by sorption. The process should be simple, and the starting materials should be easy to use. Furthermore, it is important that it be applicable on an industrial scale. The sorbent material should be long-lasting and not subject to significant degradation processes, as its CO2 absorption capacity should remain consistently high over a long period of time.

[0011] The invention relates to a 1,3-diene compound and / or a dienophile compound, each comprising a polymer chain containing amine groups. In particular, the repeating units of the polymer chain contain the amine groups. Naturally, both compounds may also comprise several such polymer chains.

[0012] The invention further relates to a sorbent material for the sorption of CO2, which comprises a mixture of the 1,3-diene compound and the dienophile compound.

[0013] “1,3-diene compound” is known to mean that within this chemical compound two CC double bonds are arranged next to each other, as for example in 1,3-butadiene, which has the following formula: or furan with the following formula:

[0014] A "dienophile compound" is a chemical compound that is an alkene, i.e., one that contains at least one double bond. The compound is a suitable reaction partner for 1,3-dienes in a Diels-Alder reaction. Electron-withdrawing substituents on the double bond are known to withdraw electrons. This enables the reaction with a 1,3-diene compound. Those skilled in the art recognize these electronic properties from the double bond and know which compounds are or could be suitable for this reaction. A well-known example of a dienophile is maleic anhydride, which has the following formula: or maleimide with the following formula:

[0015] “Polymer chain” in the sense of the invention includes both linear and branched polymers, because it is known to the person skilled in the art that amine groups can interact with CO2 and that the degree of branching of the polymer chain, comprising the amine groups, can be variable. However, the branched polymers as a polymer chain are not fully cross-linked. This means that they are still soluble in solvents, for example in water. In particular, the amine groups are secondary and tertiary amine groups, i.e. those that still have a free electron pair on the nitrogen atom, which can interact with the CO2. In particular, a polymer chain is recommended as a homopolymer, i.e. constructed from identical repeating units, with each repeating unit having such an amine group. However, Copolymers are also possible, in which case at least one of the monomers used has amine groups.Within the meaning of the invention, it is also encompassed that coupling groups are contained between the polymer chain and the respective compound, which can be different depending on whether the polymer chain has been linked to the 1,3-diene compound or to the dienophile compound.

[0016] The invention utilizes the concept of the Diels-Alder reaction of a 1,3-diene with a dienophile. This concept has been modified to create a thermoreversible, polymeric sorbent material for CO2 at low temperatures and the possibility of initiating a retro-Diels-Alder reaction by increasing the temperature. The CO2 to be bound interacts with the amine groups. However, it can only be stored for extended periods if the 1,3-diene compound according to the invention and the dienophile compound have been linked via the Diels-Alder reaction, with at least one polymer chain attached to the amine groups on each of the compounds, thus creating a 3D network of polymer chains (i.e., a cross-linked polymer network). Only in this way can the CO2 be stored for extended periods.The invention allows the cross-linked polymer network to be dissolved by increasing the temperature above 70 °C, which leads to the separation of the polymer chains according to the invention, so that the CO2 is released again over time.

[0017] The invention further relates to a method for the reversible storage of CO2 comprising the steps: a) Providing a mixture containing a

[0018] • 1,3-diene compound and a

[0019] • Dienophile compound, wherein each of these two compounds comprises (at least) one polymer chain having amine groups, at a temperature of <70 °C, to form a thermoreversible sorbent material, b) contacting CO2 with the thermoreversible sorbent material from step a).

[0020] The thermoreversible sorbent material thus comprises the adduct of a 1,3-diene compound and a dienophile compound with the respective polymer chains containing amine groups. It also encompasses the inclusion of other substances; in particular, unavoidable substances such as solvents, catalysts, etc., in amounts of a maximum of 10 wt.%, in particular a maximum of 2 wt.%, or even <1 wt.%, ideally <0.3 wt.%. Contact with CO2 can occur simultaneously with step a) or downstream of the step. If downstream, the temperature in step b) is the same as in step a).

[0021] Finally, the invention also relates to the use of a 1,3-diene compound and / or a dienophile compound, each comprising a polymer chain having amine groups, for the reversible storage of CO2, in particular the use of the two compounds according to the invention in the process according to the invention.

[0022] The advantage of the invention is that the low temperatures in the Diels-Alder reaction (this is the reaction that takes place in step a) and upon contact of the 1,3-diene compound with the dienophile compound at <70 °C) and also the still relatively low temperature of the retro-Diels-Alder reaction (this is the optional reaction for re-releasing the CO2 from the sorbent material at temperatures >70 °C) prevent the sorbent material from degrading. Thus, advantageously, there is no or only minimal degradation of the sorbent material, in particular of the polymer chains containing the amine groups.

[0023] A further advantage of the invention is that the "limit temperature" in step b) is low at 70 °C, meaning that a particularly high temperature is not required for the subsequent desired desorption. The invention is therefore suitable for the reversible storage of CO2 on a large-scale industrial scale.

[0024] The advantage of the invention is that the polymer chains with the amine groups do not have to be irreversibly cross-linked to absorb the CO2. This is because the CO2 is bound by the Diels-Alder reaction of the two compounds according to the invention (1,3-diene and dienophile), which forms the actual polymer network, which allows the CO2 to remain bound. The amine groups can therefore remain free, i.e., they retain at least one free electron pair, and even better, an H substituent, so that the CO2 sorption capacity does not necessarily have to be reduced.

[0025] In this context, a significant advantage is that, due to the possibility of keeping the amine groups in the polymer chain free, the diffusion of CO2 into the formed sorbent material can occur more quickly than if the polymer chains themselves had to be irreversibly cross-linked via the amine groups in order to bind the CO2.

[0026] A key advantage of the invention is that it makes it possible to reversibly store and release CO2 in a simple way, simply by changing the temperature, which is very easy to implement on an industrial scale. No solvent change is necessary.

[0027] In a preferred embodiment of the invention, the polymer chain is a branched polymer chain with at least 5% branched repeat units. This means that a maximum of 95% of the repeat units of the polymer chain are unbranched. However, the polymer chain is not yet cross-linked in a way that would prevent the polymer chains from being soluble in a solvent. This means that the branched polymer chains in this embodiment are still soluble in a solvent, particularly in water. Particularly preferably, at least 20% are branched, and in particular even at least 70%. This is particularly effective for the reversible storage of CO2. In particular, this degree of branching allows for a balanced balance between effective storage in the (reversibly cross-linked) sorbent material and rapid CO2 release upon "dewetting" of the sorbent material by increasing the temperature (via the retro-Diels-Alder reaction).

[0028] In one embodiment, where the polymer chain is polyethyleneimine, the branching lies in a range obtained when ethyleneimine (aziridine) is polymerized to polyethyleneimine by ring-opening polymerization. This degree of branching is difficult to measure, but is very characteristic and uniform in this preparation variant.

[0029] In one embodiment, the temperature in step a), i.e. during formation of the thermoreversible sorbent material, is even < 65 °C, in particular also < 60 °C.

[0030] In a preferred embodiment of the invention, re-release of the CO2 is achieved by the step: c) heating the thermoreversible sorbent material (after step b) of the process) to a temperature of > 70 °C to release the CO2 (preferably > 75 °C, in particular even > 80 °C). This temperature preferably does not exceed 90 °C, but preferably remains below or equal to 85 °C, because the sorbent material should suffer little or no thermal degradation.

[0031] Advantageously, this temperature for desorption of CO2 is relatively low, so that on the one hand the sorbent material is not subject to thermal degradation even during CCh release and on the other hand the operating costs remain manageable on a large industrial scale.

[0032] It has been shown that the entire thermoreversible sorbent material does not have to decompose into its individual components; rather, it is sufficient for the pores to expand due to the dissolution of individual cross-linking points. Therefore, a temperature of > 70 °C (up to a maximum of 90 °C) is advantageously sufficient to effectively achieve this and the associated re-release of CO2 without initiating significant thermal degradation.

[0033] In a preferred variant of this embodiment with a release step by heating the thermoreversible sorbent material after step b), the temperature in step a) is < 65 °C and that in release step c) is then > 75 °C. Even more preferably, the temperature in step a) is < 60 °C and in the release step > 80 °C.

[0034] In a particular variant of the above-mentioned embodiment with step c), in addition to heating, a vacuum is also applied to accelerate the release of CO2. In a preferred embodiment of the invention, the 1,3-diene compound is a furan compound, i.e., it comprises a furyl unit. Preferably, it is a furfuryl alcohol, with the polymer chain according to the invention particularly preferably being attached to the alcohol group of the furfuryl alcohol. Advantageously, such compounds are readily available on an industrial scale, thus inexpensive and available in large quantities. Furthermore, furfuryl alcohol can advantageously be produced entirely from renewable raw materials (starting from furfural from agricultural residues).

[0035] In a preferred embodiment of the invention, the 1,3-diene compound is a compound obtained by substitution of 2-[(oxiranyl-methoxy)methyl]furan (also called 2,3-epoxypropyl 2-furylmethyl ether), that is to say the following chemical structure or hydrates or salts thereof with the polymer chain having the amine groups on one of the two carbon atoms of the epoxy unit.

[0036] In a preferred embodiment of the invention, the dienophile compound is a maleimide compound, such as maleimide, which has been functionalized at the nitrogen atom with the polymer chain containing the amine groups, or such as / V-hydroxymaleimide, which has been functionalized at the OH group with the polymer chain. For example, in the case of maleimide, this corresponds to the structure: where R2 represents the polymer chain containing the amine groups.

[0037] The / V-hydroxymaleimide is expediently linked to the polymer chain at the OH group. In a preferred embodiment of the invention, the 1,3-diene compound is a compound according to formula I and / or the dienophile compound is a compound according to formula II, in which R 1 and R 2 the polymer chain containing amine groups are:

[0038] This is particularly well-suited for the reversible storage of CO2. In particular, the -CH2-O- link in the compound according to formula I is advantageous for achieving a certain degree of flexibility in the (reversibly cross-linked) sorbent material according to the invention, which improves CO2 storage.

[0039] In a further embodiment of the invention, the polymer chain has a chain length of at least 5 repeat units, with each of the repeat units containing at least one of the amine groups. This means, in particular, that the polymer chain according to the invention also has at least this number of amine groups. In the case of a copolymer, these repeat units with the amine groups can also be distributed throughout the entire copolymer. This is particularly effective for the reversible storage of CO2. In particular, this minimum polymer size allows for a balanced balance between effective storage in the (reversibly crosslinked) sorbent material and rapid CO2 release upon "dewetting" of the sorbent material by increasing the temperature (via the retro-Diels-Alder reaction).

[0040] The polymer chain particularly preferably has at least 10 repeat units, most preferably at least 300, in particular 300-600, or even 400-500. Here, too, the polymer chain preferably has this as the minimum number of amine groups.

[0041] In a preferred embodiment of the invention, the polymer chain is a polyethyleneimine chain. It is particularly preferably a branched polyethyleneimine. This exhibits a particularly strong interaction with CO2, which has proven particularly advantageous for reversible storage via the Diels-Alder and retro-Diels-Alder reaction.

[0042] In a particularly preferred variant of this embodiment of the invention, the polyethyleneimine chain has a molecular weight of 14,000 g / mol to 21,000 g / mol. The molecular weight is determined by gel permeation chromatography and light scattering detection as the mean of the number-average molecular weight Mn and the weight-average molecular weight Mw. Mn is in particular 8,000-12,000 g / mol (very particularly preferably 10,000 ± 500 g / mol) and Mw is 20,000-30,000 g / mol (very particularly preferably 25,000 ± 1,000 g / mol). The polydispersity is therefore particularly preferably (Mw / Mn) 2.5 (±20%), or even 2.2-2.8. Finally, the molecular weight (as the average of Mn and Mw) is preferably 14,000 - 21,000 g / mol, in particular 17,500 ± 2,500 g / mol or even 17,000 - 18,000 g / mol.These molecular masses have proven to be particularly effective for the reversible storage of CO2, because the molecular distance of the polymer chains between the crosslinking points (which result from the two compounds: 1,3-diene compound and dienophile compound) is designed so favorably that penetration through the carbon dioxide is possible and interactions with the amine groups can take place.

[0043] Further preferred embodiments of the invention emerge from the remaining features mentioned in the subclaims.

[0044] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0045] The invention is explained below in exemplary embodiments with reference to the accompanying drawings.

[0046] Fig. 1 schematically shows the operation of the invention, where R1 and R2 are the polymer chains containing the amine groups. It illustrates the sorption of CO2 when the temperature T2 falls below 70 °C (because a cross-linked polymer network has formed due to the Diels-Alder reaction) and the desorption of CO2 when the temperature T2 also exceeds 70 °C due to the retro-Diels-Alder reaction.

[0047] Example 1:

[0048] The two starting materials used to absorb CO2 were PEI-functionalized maleimide (maleimide) and PEI-functionalized 2-[(oxiranyl-methoxy)methyl]furan.

[0049] Both substances were dissolved in suitable solvents. CO2 was bubbled through the solution at room temperature (15 °C - 45 °C). To release the solution, the liquid solution was heated to 82 °C and treated under a slight vacuum of 500 mbar for 1 hour. The polymer chains used on the two starting materials were PEI (polyethyleneimine) (branched) with a molecular weight of 25 kDa (Mw) with an Mw of approximately 25,000 (determined by light scattering detector) and an Mn of approximately 10,000 determined by GPC. The calculated degree of polymerization is approximately 43.

Claims

Claims 1,3-diene compound and / or dienophile compound, each comprising a polymer chain containing amine groups. Method for the reversible storage of CO2, comprising the steps: a) Providing a mixture containing a • 1,3-diene compound and a • Dienophile compound, wherein each of these two compounds comprises a polymer chain having amine groups, at a temperature of < 70 °C, to form a thermoreversible sorbent material, b) contacting CO2 with the thermoreversible sorbent material from step a). The process according to claim 2, wherein the release of the CO2 is achieved by the step: c) heating the thermoreversible sorbent material after step b) to a temperature of > 70 °C to release the CO2. The process according to any one of claims 2 or 3, wherein the 1,3-diene compound is a furan compound. The process according to any one of claims 2 to 4, wherein the dienophile compound is a maleimide compound. The process according to any one of claims 2 to 5, wherein the 1,3-diene compound is a compound of formula I and / or the dienophile compound is a compound of formula II, in which R 1 and R 2 the polymer chain containing amine groups are: (I) (II) The process according to any one of claims 2 to 6, wherein the polymer chain has a chain length of at least 5 repeat units, each of the repeat units having at least one of the amine groups. The process according to any one of claims 2 to 7, wherein the polymer chain is a polyethyleneimine chain. The process according to claim 8, wherein the polyethyleneimine chain has a molecular weight of 14,000 g / mol to 21,000 g / mol. Use of a 1,3-diene compound and / or a dienophile compound, each comprising a polymer chain having amine groups, for the reversible storage of CO2.

Images