WO2015137816A1

WO2015137816A1 - Nanoporous adsorbent material for removing species from an aqueous solution

Info

Publication number: WO2015137816A1
Application number: PCT/NL2015/050167
Authority: WO
Inventors: Albertus Petrus Hendrikus Johannes Schenning; Dirk Jan Broer; VAN Hubertus Petrus Cornelis KURINGEN
Original assignee: Dutch Polymer Insitute
Priority date: 2014-03-14
Filing date: 2015-03-16
Publication date: 2015-09-17

Abstract

The invention relates to process for removing ionic species and/or species capable of forming hydrogen bridges from an aqueous mixture wherein at least part of the species are dissolved, in which process an adsorbent material is used that comprises a polymeric network of monomers having two or more polymerizable groups, the monomers being arranged as a smectic liquid crystalline phase, wherein the polymerization of the monomers has locked the smectic liquid crystalline phase into a smectic liquid crystalline network.

Description

Nanoporous adsorbent material for removing species from an aqueous solution

The invention relates to a nanoporous adsorbent material, to a process for preparing such material, to a use of such material and to a process for removing charged species from an aqueous solution.

Waste water treatment sites receive high volumes of industrial wastes, human wastes and runoff from rainwater. These waste streams may contain a variety of contaminants that are undesirable, e.g. heavy metals, organic matter such as gasoline, oil, paint, pesticides and medicines, and in particular degradation products of such organic matter. Unwanted species present in aqueous streams are usually charged or are at least capable of forming hydrogen bridges. The removal of such species from the aqueous waste streams is an important challenge for waste water treatment sites.

Many methods exist for the removal of ionic species or species capable of forming hydrogen bridges from aqueous solutions, many of which make use of the principle of adsorption to a solid phase, i.e. to adsorbent materials (in accordance with this term, the ionic species or species capable of forming hydrogen bridges may be characterized as "adsorbates").

However, adsorption materials used in these methods usually do not have a satisfactory adsorption capacity. This means that the number of adsorption spots present per gram of material is undesirably low. In addition, due to the lack of an open structure in the adsorption material, it is difficult for the adsorbates to reach sufficient adsorption spots. This means that the molar adsorption ratio is also undesirably low. For many adsorbent materials, this ratio does not even exceed 50%. A further problem is that many conventional adsorbent materials exhibit a lack of selectivity towards adsorbates. For example, it appeared difficult to selectively remove only one particular species amongst a plurality of other species, especially in cases wherein the particular species of which the removal is desired (for example because it is highly toxic) is present at a low concentration compared to that of the other species. Thus, it may be difficult to remove particularly harmful species that represent only a minor part of the total amount of species present. It is therefore an object of the invention to provide a material that has a higher adsorption capacity and/or a higher molar adsorption ratio than adsorption materials known in the art. It is in particular an object to provide a material with a molar adsorption ratio of 80% or more, more in particular of 90% or more.

It is also an object to provide a material that has a higher selectivity towards particular ionic species and/or species capable of forming hydrogen bridges, for example towards toxic heavy metals or particular organic matter. It is in particular an object to provide a material wherein the pore size can be tuned so that that particular material can be made selective for any of a plurality of different adsorbates.

It is a further object to provide a method for treating waste water that is more efficient and/or effective than methods known in the art. It is a further object that the method is selective towards particular ionic species and/or species capable of forming hydrogen bridges, for example towards toxic heavy metals or particular organic matter.

One or more of these objects have been reached by using a

particular nanoporous adsorbent material.

Accordingly, the invention relates to a process for removing ionic species and/or species capable of forming hydrogen bridges from a first aqueous mixture wherein at least part of the species are dissolved,

comprising

- contacting the first aqueous mixture with an adsorbent material comprising a polymeric network of monomers having two or more polymerizable groups, the monomers being arranged as a smectic liquid crystalline phase, wherein the polymerization of the monomers has locked the smectic liquid crystalline phase into a smectic liquid crystalline network, the monomers comprising at least

1 ) a first type of monomers wherein the polymerizable groups are at least connected via a sequence of covalent bonds, which sequence lacks non-covalent bonds; 2) a second type of monomers wherein at least two polymerizable groups are connected via a sequence of covalent bonds, which sequence also contains at least one non-covalent bond; the smectic liquid crystalline network comprising pores wherein tethers with charged end-groups are present, which pores and tethers are formed by disruption of at least part of the non-covalent bonds, and which pores are capable of adsorbing the ionic species and/or species capable of forming hydrogen bridges; thereafter

- contacting the adsorbent material with a second aqueous mixture capable of reconstituting non-covalent bonds in the smectic liquid crystalline network between the end-groups of the tethers, so that the ionic species and/or the species capable of forming hydrogen bridges are removed from the adsorbent material.

The adsorbent material of the present invention and that used in a method of the invention is a polymeric network of monomers having two or more polymerizable groups. With a polymerizable group is meant a group capable of participating in a polymerization reaction. When two monomers are connected in a polymerization, a polymerizable group of each monomer participates in forming the connection. For example, a hydoxyl group may react with a carboxylic acid group to form an ester moiety connecting the two monomers. Also, an amine group may react with a carboxylic acid to form an amide connection or a hydroxyl group may react with an isocyanate to form a urethane connection. A connection between monomers may also be

accomplished by reaction between two of the same polymerizable groups, such as between ethylenically unsaturated groups or epoxy groups. For example, an ethylenically unsaturated group may be selected from the group of acrylate groups, methacrylate groups, acrylamide groups, methacrylamide groups and vinyl ether groups. The monomers in an adsorbent material of the invention may have two or more polymerizable groups selected from the group of ethylenically unsaturated groups and epoxy groups. In particular, the monomers may have two or more polymerizable groups selected from the group of acrylate groups, methacrylate groups, acrylamide groups,

methacrylamide groups, vinyl ether groups and epoxy groups. In a specific embodiment, a monomer comprises two or more acrylate groups or two or more methacrylate groups. A monomer may also comprise two or more epoxy groups.

Two main characteristics of the polymeric network are the following:

1 ) The monomers are thermotropic hydrogen-bridged smectic liquid crystalline molecules that form a large monolithic structure by self- organization (see the left structure represented in Figure 1 ). This smectic liquid crystalline phase is locked into a smectic liquid crystalline network upon polymerization of the monomers (see the middle structure represented in Figure 1 ). The result is that the network comprises stacked layers of monomers.

2) At least two types of monomers are present in the network. There is a first type wherein the polymerizable groups are linked to each other via a sequence consisting of covalent bonds, and a second type wherein the linkage between two polymerizable groups comprises at least one non-covalent bond (and in the event that there is a plurality of linkages, each of these linkages comprises at least one non-covalent bond). The non-covalent bonds in the monomers of the second type are capable of being disrupted while the monomers of the first type remain intact. This gives the material its unique properties, which will be elaborated hereinbelow.

In a particular embodiment, the first type of monomers has two or more acrylate groups as polymerizable groups and/or the second type of monomers has two or more acrylate groups as polymerizable groups.

The at least one non-covalent bond connects the two polymerizable groups in a monomer of the second type, thus it connects a first part to a second part in the monomers of the second type. The non-covalent bonds in the second type of monomers are typically hydrogen bridges. In such case, one of the two parts comprises at least one hydrogen bond donor and the other part comprises at least one hydrogen bond acceptor. For example, the second type of monomers may be a dimer wherein the two parts are connected via hydrogen bridges between two facing carboxylic acid groups or two facing cytosine moieties. In the case the second type of monomers is a dimer, the two parts are the same. The two parts may however also be different. They may for example differ in the moieties that form the non- covalent bond. In case the non-covalent bond is a hydrogen bridge, the two parts may differ in the hydrogen bond donor and hydrogen bond acceptor.

Conditions under which the non-covalent bonds in the network are disrupted (e.g. at a high or low pH) do normally not lead to disintegration of the network, because there is a backbone in the network consisting of covalent bonds that is not disrupted under these conditions. That backbone is a structure formed by polymerization of monomers of the first type. Therefore, when polymerized, the monomers of the first type can be regarded as cross-links in the network. The cross-links and the disrupted monomers can be seen in the right structure represented in Figure 1 , wherein the cross-links are shown as white rectangles with a black border, and the disrupted monomers of the second type are shown as black rectangles.

As can also be seen in Figure 1 , the monomers of the first type usually represent a minor part of the total amount of monomers. The amount is usually 50 mol% or lower, 40 mol% or lower, 30 mol% or lower, 20 mol% or lower, 15 mol% or lower or 10 mol% or lower. The amount is usually 1 mol% or higher, 2 mol% or higher, 3 mol% or higher, 5 mol% or higher or 10 mol% or higher. The amount is in particular in the range of from 1 -40 mol%, 2-25 mol%, 3-20 mol% or 5-15 mol%. When the amount of monomers of the first type is too low, the network may disintegrate upon disrupting the non-covalent bonds. This lower limit is dependent on the nature of the different monomers used, but lies usually in the range of from 1 -5 mol%.

When the non-covalent bonds are disrupted, two tethers are formed each having one free end. As can be seen in the right picture of Figure 1 , these free ends can move away from each other so that an opening is created. A plurality of such disruptions forms a pore that extends through the material. However, it extends only in the respective layer and can thus be regarded as a "two-dimensional" pore.

Since the free end of the tethers contain charged groups such as carboxylate or ammonium, the interior of the pores is an ionic environment that is capable of adsorbing ionic species and/or species capable of forming hydrogen bridges, in particular from an aqueous solution with which the adsorbent material is contacted.

In view of the molecular structure as described hereinabove, the attachment (sorption) of the species to the material of the invention can be seen as adsorption, because there is a physical adherence of the species onto the inner surface of the pores. However, when a bulk sample of the material is considered, the sorption can be seen as absorption because the species are truly incorporated in the material, namely in the pores. Thus, in principle any of the two terms would apply when describing the invention. For the sake of clarity, however, the term adsorption is used rather than the term absorption.

Further, in case the adsorption in a process of the invention concerns the absorption of ions, the adsorption is in fact part of an ion exchange process. An adsorption material having deprotonated carboxylic acid groups in its pores may for example comprise sodium or potassium counter ions. In order to retain an overall neutrality, these ions need to be exchanged when adsorption of ions occurs in the pores.

When the ionic species to be adsorbed are cations, the charged groups at the end of the tethers that cover the interior of the pores need to be groups that bear a net negative charge. These groups may be selected from the group of carboxylates, phosphates and sulfonates. Accordingly, for the adsorption of cations, the adsorbent material is exposed to an acidic, neutral or basic environment, depending on the pK_a of the groups at the end of the tethers. For desorption to occur, the tethers of the adsorbent material need to (re)connect to each other by forming a non-covalent bond, thereby releasing the cationic species. This desorption is usually achieved by exposure to a more acidic environment. In case carboxylates, phosphates or sulfonates are used to adsorb the cations, the protonated forms of these groups will participate in the (re)constitution of hydrogen bridges (i.e. as carboxylic acids, phosphonic acids and sulfonic acids, respectively). In an alternative method, desorption can also be achieved by providing the pores with groups that bind stronger to the charged groups at the end of the tethers than the species that are to be desorbed.

Preferably, a group involved in hydrogen bridges in an adsorbent material for adsorbing cations is a carboxylic acid group, in particular a 4- substituted benzoic acid group. A corresponding monomer for manufacturing the adsorbent material may be the dimer of 4-(6-acryloyloxylhexyloxy)benzoic acid. The synthesis and characterization of a material which can be used in a method of the invention and which is prepared with this particular monomer and with 4-((4-(6-(Acryloyloxy)hexyloxy)phenoxy)carbonyl)phenyl 4-(6- (acryloyloxy)hexyloxy)benzoate as the monomer that provides the cross- linker, is described by Carmen Luengo Gonzalez et al. in "Advanced

Materials, 20 (2008) 1246-1252".

When the ionic species to be adsorbed are anions, the charged groups at the end of the tethers that are present in the pores need to be groups that bear a net positive charge. These groups are usually protonated groups, in particular groups having a protonated nitrogen atom. Accordingly, for the adsorption of anions, the adsorbent material is exposed to a neutral or acidic environment. For desorption to occur, the tethers of the adsorbent material need to (re)connect to each other by forming a non-covalent bond, thereby releasing the anionic species. For this desorption, a more basic environment is required. In case a group having a protonated nitrogen atom is used to adsorb the anions, the deprotonated forms of these groups will participate in the (re)constitution of hydrogen bridges (i.e. as an amine group).

The pH values for adsorption and desorption of ionic species and/or species capable of forming hydrogen bridges may vary for each specific combination of ion and adsorbent material, because they in particular depend on the nature of the ion and on the type of charged groups (in particular on their pK_a or pKb values) in the interior of the pores.

For cations, the pH for their uptake is usually 8 or higher, in particular it is 9 or higher, 10 or higher or 10.5 or higher. The pH for their release the pH is usually 6 or lower, in particular 5 or lower, 4 or lower or 3.5 or lower.

For anions, the pH for their uptake is usually 6 or lower, in particular 5 or lower, 4 or lower or 3.5 or lower. The pH for their release the pH is usually 8 or higher, in particular it is 9 or higher, 10 or higher or 10.5 or higher.

The monomers with a non-covalent bond are in principle polymerized while their non-covalent bonds are still in intact. Thus, the network of the adsorbent material is in principle formed with intact non-covalent bonds. When the adsorbent material is applied in a method of the invention, it has pores resulting from the disruption of the non-covalent bonds so that it can adsorb the ionic species and/or the species capable of forming hydrogen bridges. The disruption of the non-covalent bonds usually occurs under either acidic or basic conditions, depending on the nature of the non-covalent bond. Usually, the aqueous solution containing the species to be removed is used to disrupt the non-covalent bonds. In such case the aqueous solution therefore has a pH that effects the disruption, for example a pH of 8 or higher in case the non-covalent bond is a hydrogen bridge between two facing carboxylic acid groups (see Figure 1 ). Usually, the pH used for the disruption is higher than that used to achieve the adsorption.

Once the ionic species and/or the species capable of forming hydrogen bridges of the first solution are adsorbed to the adsorbent material (and, preferably, when the degree of adsorption remains at a constant level), a second solution is required to remove the species from the adsorbent material. This allows reuse of the adsorbent material and recovery of the adsorbed species of the first solution. The second solution may be in a different container than the first solution. In this case, the adsorbent material is removed from the first solution and contacted with the second solution. It may however also be that the second solution is passed as a stream along the adsorbent material, following the stream of the first solution. The adsorbent material may for example be present as a coating at the inner side of a tube.

The adsorbent material may also be present as a plurality of particles, in particular as spherical, nearly spherical or sphere-like particles. In particular, the shape of the particles is such that any of the three dimensions is not more than ten times larger than any of its other two dimensions. The particles may consist of the particular adsorbent material, or they may be a composition of the adsorbent material with another material. Particles comprising adsorbent material may in particular be obtained by emulsion polymerization, more in particular by emulsion photopolymerization.

Particles of adsorbent material may also be present as flakes. For example, the dimensions of a particle are such that two of its three

dimensions are more than three times larger than the third dimension. They may also be more than 5, more than 10 or more than 20 times larger.

The synthesis of adsorbent particles by emulsion polymerization can be carried out according to literature procedures, e.g. Ramon-Gimenez, Laura; Wilson, Jonathan Henry; Parri, Owain Llyr; Goulding, Mark John;

Kemp, Roger; Farrand, Louise Diane, PCT Int. Appl. (2012), WO2012/152409 A1 20121 1 15].

It has surprisingly been found that when the particles are prepared by emulsion polymerization (and in particular by emulsion photopolymerization), that there also is a high degree of ordering in the material. This means that also in this polymerization process the smectic liquid crystalline phase is locked into a smectic liquid crystalline network. It has in particular been found that there is a concentric alignment of smectic layers in the particles, wherein the liquid crystals are radially aligned (Figure 1 1 ).

In addition, a nanoporous material can be prepared from the obtained particles by disrupting the non-covalent bonds of the monomers of the second type. It was also found that that further treatment of the obtained particles by exposing them to different conditions (for example a different aqueous solution leading to the disruption of the non-covalent bonds), leads to an anisotropic swelling. This makes the particles also suitable for other applications such as optics. An advantage is that with this method the particles can easily be prepared in large quantities and that the properties of the particles can be tuned easily.

When the method of the invention is carried out with adsorbent material that is present as a plurality of particles, the particles can be separated from the first aqueous solution by filtration, they can then eventually be washed and they can then be contacted with the second aqueous mixture so as to allow the recovery of the adsorbed species (i.e. the ionic species and/or species capable of forming hydrogen bridges). This makes the process operation more convenient.

In addition, this allows easy reuse of the adsorbent material. It appeared possible to run a plurality of cycles wherein - in each cycle - adsorption of methylene blue was followed by desorption. Figure 12 shows the occupation (%) during a sequence of three cycles.

To investigate the maximum adsorption capacity for the materials of the invention, they were exposed to solutions with increasing concentrations of methylene blue. The methylene blue to carboxylate molar ratios were chosen in the range of 0.04 - 2. The total volume of the dye solution was kept constant. The solutions having a methylene blue to carboxylate molar ratio of up to 0.88 all became almost colorless, indicating (nearly) complete

adsorption of the methylene blue. The solutions with a higher ratio became less blue but not colorless. The films became darker blue when they had adsorbed more methylene blue and at a certain point they even appeared black, which is most likely caused by combination of the high dye

concentration in the film and the absorbance of methylene blue in the visual area. A plot of the occupation degree of carboxylate moieties in the adsorbent at different methylene blue to carboxylate ratios is shown in Figure 13. A straight line is observed up to a ratio of almost 1 , indicating full adsorption, which was expected since these solutions became colorless at equilibrium. Saturation is obtained at an occupation degree of 100% when all carboxylate moieties are occupied with a methylene blue molecule. Surprisingly, only a small excess of dye is necessary to reach complete occupation, meaning that the affinity between methylene blue and the adsorbent is high.

At 100% occupation, the adsorption capacity is 980 mg of methylene blue for 1 g of material. Thus, a material of the invention is capable of adsorbing 30 times more than the well-known adsorbent activated carbon. On the other hand, poly(acrylic acid) based hydrogels (see e.g. Y. Liu, Y. A.

Zheng, A. Q. Wang, J. Environ. Sci. China 2010, 22, 486 ) are able to adsorb more adsorbent than a material of the invention, which is due to the lower molecular mass of acrylic acid with respect to the benzoic acid derivative in the embodiment of the invention. The adsorption equilibrium data can be fitted well to the Langmuir adsorption isotherm, which isotherm is based on the assumption that the adsorbent has a specific number of independent active sites that can bind a finite number of molecules. The best fit was obtained with an adsorption capacity of 1 .00 mole methylene blue per mole of carboxylate. This means that all sites are reachable for methylene blue, and to achieve this the material has to be sufficiently porous and there should not be steric limitations. In contrast, the adsorption capacity for the poly(acrylic acid) based hydrogel (see reference above) is only 0.45 mole/mole.

The Langmuir adsorption constant obtained for a material of the invention is 3.42 χ 10^"4 L mg^-1 (0.363 L mol^-1), which is approximately 5 times higher than that of the poly(acrylic acid) based hydrogel (6.36 χ 10^"5 L mg^"1 ).

The adsorption kinetics were investigated by measuring the absorbance of the solution over time (Figure 14). The absorbance of the reference methylene blue solution did not significantly change, and therefore (photo-)reduction of methylene blue can be neglected. The data were fitted to both pseudo-first order and pseudo-second order kinetics. The pseudo- second order equation appears to fit the adsorption kinetics well, having a correlation coefficient R² of 0.9987. The pseudo-second order kinetic is often found when studying the adsorption of methylene blue, and supports the view that ionic interactions between the liquid crystalline network and methylene blue are occurring. The rate constant of a material of the invention is 2.89 χ 10^-3 g adsorbent per mg of methylene blue per minute. Such a high rate constant has not been reported before for materials which also have a high adsorption capacity. The rate constant is 5 times higher than that of a poly(acrylic acid) based hydrogel (see reference above) (0.575 χ 10^-3 g/(mg min)) and nearly 2000 times higher than that of a supramolecular hydrogel (see e.g. 1 .5 10^-6 g/(mg min)) (B. O. Okesola, D. K. Smith, Chem.

Commun. 2013, 49, 1 1 164).

Thus, it has surprisingly been found that adsorbent materials of the invention have an adsorption capacity that surpasses that of most materials known in the art. Moreover, it has been found that adsorbent materials of the invention combine this property with a molar adsorption ratio that surpasses that of many materials known in the art. It may even be nearly 100%. And a further beneficial property that is combined in a material of the invention is the very high adsorption rate. This unexpected combination of properties is unprecedented for adsorption materials

Further, when a mixture of species is present in the first solution, an adsorbent material of the invention is capable of removing one of them with a selectivity that surpasses that of many materials known in the art. The combination of a high adsorption capacity, a high molar adsorption ratio and a high selectivity is an improvement over adsorbent materials known in the art.

In addition, it has been found that with an adsorption process of the invention, the adsorption can be charge selective (allowing the separation of ions with a different valency, for example by selectively adsorbing divalent ions) and/or size selective (allowing the separation of species with a different size, for example by selectively adsorbing smaller species rather than larger species).

Further, it has been found possible to tune the pore size by changing the spacing between the layers of the adsorbent material. This is achieved by changing the length of the monomer of the first type in the network. In this way, the adsorbent material can be made selective for any of a plurality of different ionic species and/or species capable of forming hydrogen bridges.

With a process according to the invention and/or an adsorbent material according to the invention it is possible to have a controlled desorption of the adsorbed species. Therefore, the invention can not only be used for removing ionic species and/or species capable of forming hydrogen bridges from an aqueous solution, but also for collecting such species. For example, it may be used to recover particular species such as metal ions from certain waste streams.

The invention further relates to an adsorbent material for use in a process of the invention, which is a material comprising a polymeric network of monomers having two or more polymerizable groups, the monomers being arranged as a smectic liquid crystalline phase, wherein the polymerization of the monomers has locked the smectic liquid crystalline phase into a smectic liquid crystalline network, the monomers comprising at least

1 ) a first type of monomers wherein the polymerizable groups are at least connected via a sequence of covalent bonds, which sequence lacks non-covalent bonds;

2) a second type of monomers wherein at least two polymerizable groups are connected via a sequence of covalent bonds, which sequence also contains at least one non-covalent bond; the smectic liquid crystalline network comprising pores wherein tethers with charged end-groups are present, which pores and tethers are formed by disruption of at least part of the non-covalent bonds, and which pores are capable of adsorbing the ionic species and/or species capable of forming hydrogen bridges.

In a preferred embodiment, the first type of monomers is a type of monomers wherein the polymerizable groups are at least connected via a sequence of covalent bonds, which sequence lacks non-covalent bonds and comprises an -N=N- moiety. More preferably, the -N=N- moiety is an azobenzene moiety (a -Ph-N=N-Ph- moiety), in particular one wherein the two substituents at the phenyl rings are at the 1 and the 4 position of the rings (i.e. the connection of the phenyl ring to the -N=N- group and the connection to the rest of the network are in para-position). In this way, an adsorbent material is obtained wherein the pore size can be changed under the influence of light and/or wherein the adsorption capability can be influenced by light. This is due to the property of an -N=N- moiety such as the

azobenzene group that conversion between the two geometric isomers may occur under the influence of light. Thus, the moiety is capable of undergoing a cis-trans isomerization under the influence electromagnetic radiation, in particular light. When an azobenzene moiety is exposed to UV-radiation of e.g. 365 nm, a photostationary state is reached within a few minutes, while the back isomerization is slow; at room temperature in the dark it takes many hours. The back isomerization can be accelerated by increasing the

temperature and/or by illumination at 450 nm. The c/s-isomer has a shorter effective length than the frans-isomer (see Figure 2). Accordingly, in a material wherein cross-linkers have a cis- geometry, the distance between two stacked layers is smaller than in a material wherein cross-linkers have a frans-geometry, leading to a material with smaller and larger pores, respectively. Thus, a material of the invention has the property of a photo-controllable pore size.

Also, the dipole moment is different for each of the two states. Upon changing from the trans to the cis conformation of azobenzene, for example, the dipole moment changes from ~ 0 D to ~ 3 D. As a result, the interior of the pores changes, which results in modified adsorption capabilities. Thus, a material of the invention has the property of a photo-controllable formation of binding sites. This is demonstrated in Figure 15. Figure 15a shows two vials. The vial on the left contains a colored solution (methylene blue) and a yellow piece of adsorption material on the bottom (in the circle). The vial on the right represents the state after irradiation with UV. This vial contains a colourless solution and a colored piece of adsorption material on the bottom, indicating that the irradiation with UV changes the binding sites to such extent that they become capable of adsorbing the methylene blue. A representation of this process at the molecular level is schematically demonstrated in Figure 15b, showing the conversion of hydrogen bridged binding sites to binding sites wherein a potassium ion serves as a counterion for the carboxylate groups.

It has in particular been found that - under the appropriate

circumstances - the isomerization may result in the disruption of at least part of the hydrogen bridges so that the polymer salt is formed. This may occur when the pH of the solution is just below the pH at which the disruption would normally occur (i.e. the normal pH-induced disruption during which the -N=N- moiety would remain unchanged in the trans conformation).

In a material wherein the pore size can be tuned by an external stimulus such as light, it is not only possible to convert a material with smaller pores into a material with larger pores and vice versa, but it is also possible to create a material wherein both pore sizes are present. For example, in certain regions of the material the pore size can be made different from that in other regions of the material. This may be achieved by simultaneously exposing different regions of the material to different external stimuli, for example by illumination with a plurality of wavelengths in a particular pattern with e.g. a mask.

In particular, the different external stimuli (e.g. the exposure pattern) can be changed in time. This makes it possible to create waves of larger pores that propagate in a region of smaller pores. In case the properties of adsorbates (e.g. size, dimensions and charge) are such that the adsorbates have a preference for residing in the larger pores rather than in the smaller pores, this principle would allow a net movement of adsorbates through the adsorbing material in a particular direction, i.e. it would allow transportation of adsorbates through the adsorbing material. In this way, a continuous process for the removal of charged species from an aqueous solution can be realized.

In a preferred embodiment, the surface of the adsorption material (in particular a material having a sheet-like shape) is exposed to light of the appropriate wavelength for isomerization of the -N=N- groups. When these - N=N- groups at surface have been isomerized, the light of the appropriate wavelength is not absorbed anymore by these groups, because of the different light absorption properties of the other isomer. This allows that the light penetrates deeper into the material and can isomerize -N=N- groups that lie further away from the surface. When the light exposure is interrupted (or stopped), the isomerized bonds will revert to their original state by thermal relaxation. In this way, first the pores at the material's surface can be squeezed (made smaller) by the light exposure, thereby locking species into the material. When the squeezing propagates through the material

(substantially perpendicular to its surface), the species are forced to follow the same path. This allows the separation of the species, their active transport through the material and, eventually, their recollection at the other surface. When the light exposure is stopped, the squeezed pores relax (become larger) and a new cycle of pore squeezing and propagation of species through the material can be started. This method is more convenient than a method wherein a mask is used. Responsive polymer membranes may play a role in a diverse range of applications, such as biosensors, actuators, drug delivery, ultrafiltration, gas filtration and stimuli responsive adsorbents.

The invention further relates to a process for preparing an adsorbent material of the invention comprising copolymerizing (in particular

photocopolymerizing) a monomer of the first type as defined in claim 1 and a monomer of the second type as defined in claim 1 .

The invention further relates to an adsorbent material obtainable by copolymerizing (in particular photocopolymerizing) a monomer of the first type as defined in claim 1 and a monomer of the second type as defined in claim 1 .

A monomer of the first type is in particular a monomer wherein the polymerizable groups are at least connected via a sequence of covalent bonds, which sequence lacks non-covalent bonds. More in particular, the monomer comprises an -N=N- moiety. Even more in particular, the monomer comprises a -Ph-N=N-Ph- moiety.

A monomer of the second type is in particular a monomer wherein at least two polymerizable groups are connected via a sequence of covalent bonds, which sequence also contains at least one non-covalent bond.

The invention further relates to an article comprising an adsorbent material as defined in claim 1. The article may for example be a tube, pipe or container. In a particular embodiment, the adsorbent material may be present on a support. The support may be a porous support.

The invention further relates to the use of an adsorbent material according to the invention (i.e. as defined in claim 1 and as defined

hereinabove) for removing ionic species and/or species capable of forming hydrogen bridges from an aqueous mixture wherein at least part of the charged species are dissolved. EXAMPLES

Adsorbent materials used in the method of the invention were prepared according to methods described by Carmen Luengo Gonzalez et al. in "Advanced Materials, 20 (2008) 1246-1252".

Adsorption of methylene blue (MB)

Two dyes were investigated, cationic methylene blue (MB) and anionic methyl orange (MO). The adsorbent material was prepared by copolymerization of a monomer of the first type (6-OBA; 4-(6-acryloyloxy hexyloxy)benzoic acid) and a monomer of the second type as a cross-linker (C6H; 4-((4-(6-(Acryloyloxy)hexyloxy)phenoxy)carbonyl)phenyl 4-(6- (acryloyloxy)hexyloxy)benzoate) in a 90/10 ratio, followed by a base treatment for 15 minutes in 0.05 M KOH solution to open (or activate) the pores. This resulted in a nanoporous material with a negatively charged carboxylate interior.

Aqueous 10μΜ MB solution was prepared (pH = 7) and the colorless transparent activated membrane film was immersed in the blue colored solution. After 4-8 hours of stirring the solution was completely colorless and the film was blue. The membrane has adsorbed the cationic dye from the solution. In contrast, an adsorption material that has not been treated with 0.05 M KOH did not get colored (Figure 3b). The absorbance spectra were recorded with UV-VIS spectrometry, which confirmed the absence of dye in the solution with the nanoporous membrane (Figure 3a). The solution with the untreated ("closed") film had nearly the same absorbance as the initial solution.

Adsorption of methyl orange (MO) The experiments with an activated adsorbent were also performed with an aqueous 10μΜ MO solution (pH = 7). Herein, no color change has been observed (Figure 4b). This was confirmed by UV-VIS spectroscopy measurements, since no adsorption of this anionic dye in the nanoporous membrane was observed (Figure 4a).

Selective adsorption of MB from a solution of MB and MO

A 1 : 1 mixture was prepared with both the blue cationic dye MB and the yellow anionic dye MO, which resulted in a green solution. The UV-VIS spectrum of the solution revealed two peaks, one with a peak maximum at 465 nm belonging to MO and the second absorbance peak at 665 nm belonging to MB. The nanoporous adsorption material was added to this mixture and after approximately 30 minutes of stirring a color change was already observed with the naked eye. After 4 hours of stirring the color of the solution was yellow and the film was blue (Figure 5b). In the UV-VIS spectra of the solution the MO peak has the same absorbance as the initial solution, while the MB peak is nearly invisible (Figure 5a). This evidences that with the adsorbent material it is possible to have a selective adsorption of MB.

Quantitative adsorption of MB. A solution was prepared wherein the amount of dye is twice the amount of carboxylate anions in the adsorbent material (concentration MB: 0.325 mM). After a few hours of stirring the solution already became less blue. After a few days UV-VIS spectra were recorded from the initial solution and from the solution with the adsorbent material, which revealed a decrease of 50% of the absorbance of the solution at 665 nm. Thus, the molar adsorption ratio is 100% or close thereto, because every carboxylate anion is occupied with one dye molecule.

The adsorption capacity of the adsorbent material was calculated to be 970 mg of MB per g of membrane. This is substantially higher than adsorption capacities of many known MB adsorbing materials. That of zeolites is 5-15 mg/g, that of activated carbon is 32-50 mg/g, that of

(activated) functional biological waste streams is 5-915 mg/g and that of clay polymer composites is 78-370 mg/g. Although poly(acrylic acid) based hydrogels have a higher adsorption capacity (1600-2000 mg/g), the molar adsorption ratio of these materials lies in the range of 40-50%. Thus, adsorbent materials of the invention exhibit improved properties over that of known materials.

Size selective adsorption of MB from a solution with MB and a dye-labeled cationic dendrimer.

A mixture was prepared with both the blue cationic dye MB (0.0064 mg/mL) and a yellow FITC labeled 4^th generation cationic poly (propylene imine) dendrimer (0.2 mg/mL), which resulted in a green solution (pH 7). The UV-VIS spectrum of the solution revealed two peaks, one with a peak maximum at 503 nm belonging to the dye labeled dendrimer and the second absorbance peak at 665 nm belonging to MB. The nanoporous adsorption material was added to this mixture and after approximately 15 minutes of stirring the material became blue (see Figure 6b). After two days of stirring the UV-VIS spectrum of the solution was recorded, which revealed the absence of the MB absorbance peak. The absorbance peak of the dye labeled dye was still present (Figure 6a). The slight decrease of this signal is caused by the absence of the tail of the MB peak. This evidences that with an adsorbent material of the invention it is possible to have size selective adsorption.

Selective adsorption of MB from a solution of MB and Rhodamine B

A solution was made with zwitterionic rhodamine B (RhoB) and MB. RhoB has a permanent positive charge and under the conditions used the acid group is deprotonated, which results in negative charge as well. This zwitterion was dissolved in distilled water (5.0 g mL^"1, 10.4 μΜ) together with 5.0 g mL^-1 MB (15.6 μΜ) and excess of adsorbent was added. This resulted in a purple solution which gradually became pink and the adsorbent became blue/purple (Figure 7b). The UV-vis spectrum reveals the disappearance of MB signal, while the RhoB absorbance peak at 554 nm is only reduced (Figure 7a). This reduction is caused by the disappearance of the overlapping MB absorbance peak and the slight adsorption of RhoB in the adsorbent. A single MB absorbance peak was subtracted from this spectrum and revealed an RhoB adsorption of 33%, while MB is adsorbed for at least 99%.

Photoresponsive adsorbent material with switchable pore size

Photoresponsive adsorbent materials (AZO-LCN) were prepared by in situ photo-polymerization (at the smectic phase temperature) of a mixture of monomer 1 (95%wt) and the azo compound 2 (5%wt), which are shown in Figure 8. A layered and well-organized polymer network was obtained. The photoswitchable nanoporous membranes were formed by disrupting the hydrogen bridges through a treatment at basic pH (pH>8). The

photoisomerization kinetics of the system in the dry (AZO-LCN) and the swollen state (azo-nanomembrane) were studied to demonstrate the reversibility of the system under UV exposure. The swollen azo film is irradiated with UV light (365nm) through a mask to bring half of the sample in the 'closed' cis state and keep the remaining area in the Open' trans state (Figure 9). The dye appeared to diffuse fast in the trans area of the sample and slow in cis area. Further, it has been observed that in-diffusion of the dye molecule Nile Blue A into the adsorbing material occurred only in the larger pores, from which it was concluded that the pore size can be switched from an open to a closed state, under UV exposure (see Figure 10; the c/s-area represents slow diffusion, the frans-area fast diffusion).

Claims

1 . A process for removing ionic species and/or species capable of forming

hydrogen bridges from a first aqueous mixture wherein at least part of the species are dissolved, comprising

2) a second type of monomers wherein at least two polymerizable groups are connected via a sequence of covalent bonds, which sequence also contains at least one non-covalent bond; the smectic liquid crystalline network comprising pores wherein tethers with charged end-groups are present, which pores and tethers are formed by disruption of at least part of the non-covalent bonds, and which pores are capable of adsorbing the ionic species and/or species capable of forming hydrogen bridges; thereafter

2. Process according to claim 1 , wherein the monomers have two or more

polymerizable groups selected from the group of ethylenically unsaturated groups and epoxy groups.

3. Process according to claim 1 or 2, wherein the first type of monomers has two or more acrylate groups as polymerizable groups and the second type of monomers also has two or more acrylate groups as polymerizable groups. Process according to any of claims 1-3, wherein the monomers of the first type constitute from 5 to 15 mol% of the sum of the amount of monomers of the first type and the amount of monomers of the second type.

Process according to any of claims 1-4, wherein

- the ionic species is a positively charged species;

- the second type of monomers comprises a dimer of a compound having one ethylenically unsaturated group and one carboxylic acid group, in which dimer the two carboxylic acid groups are connected to each other via hydrogen bridges;

- the pH of the first aqueous mixture is at least 10; and wherein

- the pH of the second aqueous mixture is 4 or less.

Process according to claim 5, wherein the second type of monomers comprises a dimer of a compound having one acrylate ester group and one 4-substituted benzoic acid group such as 4-(6-acryloyloxy hexyloxy)benzoic acid.

Process according to any of claims 1-4, wherein

- the ionic species is a negatively charged species;

- the second type of monomers comprises a dimer of a compound having one ethylenically unsaturated group and one basic group, in which dimer a hydrogen bridge connects at least the two basic groups;

- the pH of the first aqueous mixture is 4 or less; and wherein

- the pH of the second aqueous mixture is at least 10.

Process according to claim 7, wherein the basic group is an amine group. An adsorbent material for use in a process of any of claims 1-8, comprising a polymeric network of monomers having two or more polymerizable groups, the monomers being arranged as a smectic liquid crystalline phase, wherein the polymerization of the monomers has locked the smectic liquid crystalline phase into a smectic liquid crystalline network, the monomers comprising at least

1 ) a first type of monomers wherein the polymerizable groups are at least connected via a sequence of covalent bonds, which sequence lacks non-covalent bonds; 2) a second type of monomers wherein at least two polymerizable groups are connected via a sequence of covalent bonds, which sequence also contains at least one non-covalent bond; the smectic liquid crystalline network comprising pores wherein tethers with charged end-groups are present, which pores and tethers are formed by disruption of at least part of the non-covalent bonds, and which pores are capable of adsorbing the ionic species and/or species capable of forming hydrogen bridges.

10. An adsorbent material according to claim 9, wherein the first type of

monomers comprises an -N=N- moiety.

1 1 . An adsorbent material according to claim 10, wherein the -N=N- moiety

comprises a -Ph-N=N-Ph- moiety wherein the two substituents at the phenyl rings are at the 1 and the 4 position of the rings.

12. An article comprising a composition of an adsorbent material according to any one of claims 9-1 1 and a support.

13. An article according to claim 12, wherein the article is selected from the group of tubes, pipes and containers.

14. An adsorbent material according to any one of claims 9-1 1 which is spherical or nearly spherical, in particular wherein any of the three dimensions is not more than ten times larger than any of its other dimensions.

15. Use of an adsorbent material according to any one of claims 9-1 1 and 14 for removing ionic species and/or species capable of forming hydrogen bridges from an aqueous mixture wherein at least part of the species are dissolved.