Porous Polymers: Enabling Solutions for Energy Applications

Review Porous Polymers: Enabling Solutions for Energy Applications Arne Thomas, Pierre Kuhn, Jens Weber, Maria-Magdalena Titirici, Markus Antonietti* A new generation of porous polymers was made for various energy-related applications, e.g., as fuel cell membranes, as electrode materials for batteries, for gas storage, partly from renewable resources. This review intends to catch this emerging field by reporting on a variety of different approaches to make high performing polymers porous. This includes template techniques, polymers with inherent microporosity, polymer frameworks by ionothermal polymerization, and the polymerization of carbon from appropriate precursors and by hydrothermal polymerization. In this process, we try to not only identify the current status of the field, but also point to open question and tasks to identify the potentially relevant progress. Introduction It is a timely observation to state that the technical and industrial world as we know is currently changing. We are facing a number of game changing problems, e.g., the hunger for energy of developing societies. At the same time, fossil oil resources are getting scarce and expensive, so that free availability of polymer materials turns out to be increasingly coupled to the energy market. As a third aspect, unrestricted growth is however no option, as an industrialization based on fossil fuels brought up the CO2 load of the atmosphere to values where the impact on climate and living conditions are turning serious. Can an article looking into the near future of polymer science ignore this engulfing bigger picture? The answer is ‘‘of course not.’’ Such challenges are however not necessarily paralyzing, but a strong driver for science. A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti Max Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany E-mail: [email protected] Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Such game changes open novel opportunities in polymer and material chemistry. Availability of raw materials and energy will seriously influence the economic use of polymers, and the reach for better performing polymers, more effective polymerization processes, to use less for the same purpose, is certainly one very good response to the altered starting conditions. Besides that, it is the opinion of the authors that a new branch of polymer materials is emerging which could be described as ‘‘energy polymers.’’ These are polymer systems which enable new energy cycles, including better generation, storage, transport, and preservation of energy. Broken down to the level of polymer products, this includes polymers for the fuel cell (FC) membrane, novel porous materials for hydrogen and methane storage, polymer nanofoams providing high thermal insulation, polymer-based catalysts, polymer for light harvesting, etc. The other tightly connected complex is the dawning of the next raw material change or, translated to the problems of our community, how to make synthetic polymers other than from petrochemical monomers, at the beginning at least for some markets only. This includes not only the classical field of biopolymers but also the DOI: 10.1002/marc.200800642 221 A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti Maria-Magdalena Titirici was born in Bucharest, Romania in 1977. She graduated in 1999 from the Department of Chemistry of Bucharest University. After, she conducted her PhD study (with Dr. Börje Sellergren) first at the University of Mainz and later on at the University of Dortmund in the field of Moleculary Imprinted Polymers. In 2005 she joined Prof. Antonietti’s group as a postdoctoral research at the Max-Planck Institute of Colloids and Interfaces. Currently Maria-Magdalena Titirici is currently leading the group of ‘‘Functional Carbonaceous and Polymeric Materials’’ in the same research institute and her scientific interests include sustainable chemistry, carbon materials, molecular recognition and development of novel stationary phases for chromatography. Markus Antonietti is Scientific Member of the Max Planck Society and working in the MPI of Colloids and Interfaces/Golm. He completed his polymer and physical chemistry education in Mainz with H. Sillescu and moved via Marburg to his current occupation. His research is currently covering various aspects of polymer and hybrid materials, but he is also active in higher scientific education and various other aspects of human culture. Arne Thomas received his diploma in chemistry from the Philipps University Marburg and his PhD from the University of Potsdam and the Max Planck Institute of Colloids and Interfaces with Professor Markus Antonietti in 2003. After a postdoctoral fellowship in the group of Professor Galen Stucky at the UCSB he is now heading a group at the Max Planck Institute of Colloids and Interfaces working on meso- and microporous polymers and organic frameworks and is coordinating the project house ‘‘EnerChem - Nanochemical concepts for a sustainable energy supply’’. Pierre Kuhn was born in Mulhouse (France) in 1977. He studied Chemistry at the Université Louis Pasteur in Strasbourg, where he obtained a Diplôme d’Etudes Approfondies in Transition Metal Chemistry and Molecular Engineering in 2002. He received his PhD from ULP in 2006 (Polyolefin synthesis with nickel phosphine complexes). He currently holds a position as a research assistant with Professor Dr. M. Antonietti at the Max Planck Institute of Colloids and Interfaces in Potsdam (Germany). His current research interests focus on the synthesis of micro- and mesoporous functional organic materials. Jens Weber is currently a post doctoral researcher in the group of Lennart Bergström at the Arrhenius Laboratory, Stockholm University. After receiving his diploma in chemistry at the Technische Universität Dresden, he joined the group of Markus Antonietti at the Max Planck Institute of Colloids and Interfaces in 2005 where he finished his PhD thesis on the synthesis, characterisation and application of meso- and microporous high-performance polymers in 2007. His general research interests are the physical chemistry of polymers, with a special emphasis on porous polymers as well as the synthesis of new porous materials. synthetic conversion of biomass to more performing materials and scaffolds which is described here. Fuel Cell Membranes This article starts with the presumably best known case of ‘‘energy polymers,’’ which constitute the membrane of the mobile low temperature fuel cell (FC). FCs are an interesting alternative to existing combustion engine/ energy conversion systems, and some types are already in the market. Generally, FCs combine high efficiency to transfer chemical energy into electric energy, which is not restricted by the Carnot equation, but by electrochemistry, only. In addition, fuels can be used that have the potential to be renewable. Various FC types, differing, e.g., in their operation temperature or proton conducting material, are known. In the present context, the focus lies on the low temperature, polymer exchange membranes fuel cells (PEMFCs), which are interesting for applications in transportation and portable electronics. For a general overview about nearly all types of FCs, and the transport 222 Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim properties of the respective proton conductors we would like to refer the reader to the literature.[1–3] The proton conducting polymer membrane which separates the two electrodes is one key feature defining performance and use. The first polymeric membrane used in a FC was based on sulfonated, cross-linked polystyrene.[4] The ‘‘golden standard’’ of PEMFCs, Nafion, was developed by DuPont and introduced just a few years later.[5,6] PEMFCs have found application already in the GEMINI space project of the NASA as early as in the mid1960s. Since then there have been a lot of research efforts in order to synthesize new polymeric membranes and a variety of different pathways have been suggested. In addition to a number of recent review articles,[3,7–11] special issues of scientific journals were devoted to the progress of research on membranes in FCs.[12,13] As this review is devoted to porous polymers, in the following we will focus on proton conducting membranes with such special architecture, only. The fact that Nafion, although quite old, can still be regarded as the benchmark for new polymeric membranes is due to its favorable nanostructure. The combination of a DOI: 10.1002/marc.200800642 Porous Polymers: Enabling Solutions for Energy Applications perfluorinated backbone and sulfonic acid side groups is responsible for a microphase separation within Nafion membranes upon hydration.[14] These interconnected water channels are finally responsible for the high proton conductivity. A rather complete picture of the microstructure was described by Kreuer in 2001 and is depicted in Figure 1.[15,16] Analysis and optimization of the microstructure of Nafion is still subject to intense research efforts.[17] The drawback of Nafion (and of most other membranes based upon sulfonated aromatics) is its sensitivity against temperature and the coupled degree of hydration. A dramatic decrease in the proton conductivity at high temperatures caused by the breakdown of the microstructure upon dehydration is found. This reduces the usage of Nafion membranes to operating temperatures below 80 8C and requires a technically complex water management in each FC. Besides this sensitivity, higher FC temperatures especially for automotive applications are favored, as problems of efficient cooling, purity of the fuel, and sustainability of the catalyst system are better solved at slightly higher temperatures. Recent attempts to increase the temperature window were Nafion membranes can be used has focused on organic/inorganic hybrids. Thus, silica,[18–21] organosilicas,[22,23] metal oxides,[24] or metal phosphonate[25] have been introduced into Nafion. As example, it has been shown that the incorporation of silica particles into Nafion Figure 1. The relevant mesopore channel structure of a Nafion1 membrane as found to be the key for its extraordinary performance, reprinted from ref. [16]. Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim allows maintaining proton conductivity up to 140 8C.[26] As a porous, yet inorganic analog of Nafion, sulfonic acid functionalized, mesoporous organosilicas have been described. High conductivities could be achieved with fully hydrated samples, while the obstacles of a drop in performance at higher temperatures remained.[27] In a recent report, these two approaches were combined by the synthesis of mesostructured hybrid silica—Nafion membranes.[28] It was shown that the introduction of sulfonated ordered mesoporous silica networks inside the Nafion yields a better dimensional stability and improves the water management, and that enhanced proton conductivities could be observed at higher temperature (95 8C) compared to pure Nafion membranes. The development of alternative FC membranes based on an adduct of poly(benzimidazole) (PBI) and phosphoric acid was put forward since its discovery in 1995.[29] The research efforts were summarized recently and a number of unanswered questions was listed as a task for polymer science.[7] Firstly, it was shown that the operation of PBI/ H3PO4 membranes at high temperatures (well above 130 8C) still requires the presence of water. Based on the knowledge that molecular reorientation is the key issue for formation and rupture of hydrogen bonds and the coupled proton conductivity (structure diffusion),[16] it is not surprising that these reorientations are rather slow for molecules larger than water within the homogeneous PBI matrix. A second problem is the softening of PBI/H3PO4 membranes at higher temperatures and low humidity. Poly(phosphoric acid) is finally a solvent for PBI,[30] implying that cross-linking of the membranes might be beneficial for their mechanical performance. It is a straightforward solution for these problems which illustrates the tasks of ‘‘energy polymers’’ to introduce a Nafion-like, i.e., defined biphasic nanostructure into PBI/ H3PO4 complexes. One approach to enable controlled nanostructures in this system is the use of preformed porous polybenzimidazole, which could be afterwards filled with the second, the phosphoric acid phase. In a first example, a porous PBI has been used generated by leaching out a low-molecular-weight porogen from a preformed PBI/porogen film. Soaking such films in concentrated phosphoric acid yielded proton conducting membranes, with proton conductivity values dependent on the porosity of the prior used pure PBI membrane. However, as stated by the authors, only ill defined macropores were obtained by this method.[31] To achieve a more ‘‘Nafion-like’’ structure, mesoporous PBI networks with 10 nm pores were synthesized, predefining the later membrane morphology. The synthetic strategy was based upon hard-templating of silica nanoparticles.[32,33] After dissolving the silica spheres, homogenous mesoporous PBI networks with pore sizes of around 10–11 nm were obtained (see Figure 2). These pores www.mrc-journal.de 223 A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti achieve phosphonic acid based, phase separated morphologies.[39] Besides chemical issues, also processing innovations have to be brought forward, here via macromolecular engineering. For the design of flat nanostructures with partially complex, multicomponent structure set-up, electrospinning turned out to be a powerful tool. The arrival of electrospinning has enriched polymer science as such,[40] but as also ceramic precursors can be spun,[41,42] and therefore also some valuable applications can be expected in the field of functional polymer resins and carbonFigure 2. (a) TEM micrograph of mesoporous PBI, overlay: chemical structure of the like materials. In the context of energy cross-linked PBI; (b) proton conductivities of phosphoric acid doped mesoporous and polymers, electrospinning was employed non-porous PBI, (b) reprinted from ref. [34]. to create PBI/silica nanocomposite noncould be filled in a second step with phosphoric acid to wovens.[43] These non-wovens could be easily processed yield a highly proton conducting material.[34] It should be into dense membranes by sintering. Based on such noted that both pore collapse and emptying are rather experiments, it can be envisaged that larger parts of a FC improbable because of capillary forces of rather solid could be fabricated in a modular way, e.g., to process the porous materials, while excessive membrane swelling was various parts (membrane, electrodes, catalyst, etc.) of the FC prohibited by the high cross-linking density of the films. stack sequential electrospinning and subsequent sintering, Such complexes show high proton conductivity at zero including pore templates and the anode and cathode humidity and could be easily operated up to high catalysts in the preformed functional polymers. temperatures (200 8C). Their proton conductivity was shown to be one to two orders of magnitude higher than that of a non-porous PBI/H3PO4 complex that was tested The Strive for Superporous Polymers: A New under similar conditions (see Figure 2). Type of Polymer Solid with New Polymer The high proton conductivity can be attributed to the Physics presence of well-defined domains of pure phosphoric acid. In those domains, there is no limitation on the structure Porous polymers are also a relevant classical class of diffusion process, even if condensed species of phosphoric materials, for the application as sorption materials or in acid are present. The effect of the cross-linking density on chromatography.[33,44,45] Very recently, the interest has physical properties will not be discussed here, but details been revamped by the upcoming technological problems can be found in the literature.[34] of the new energy cycles, thus leading to research on the The benefits arise from the approach to transfer key production of energy (e.g., catalysts and catalysts supports) features, i.e., the presence of a microstructure, from one and its storage (gas storage in microporous polymers, polymer system which only works at low temperatures to batteries, etc.). other systems with higher stability which suffered from Controlled microporous materials were up to now the absence of such features. however a domain of inorganic chemistry, as there are The usage of porous polymers would also principally standard recipes for zeolites,[46] activated carbon blacks, allow the post-functionalization of the pore walls with and more recently, the metal-organic frameworks suitable proton conductors. Since the C –P bond is (MOFs),[47] while the development of silica of the Mobil stable against hydrolysis, phosphonic acid functionalized composed matter (MCM)-family[48] and related materipolymers seem for example a promising alternative to als[13]extended those profiles to the mesopore range.[49–51] [35–38] the binary PBI/H3PO4 systems. Also for such Despite the remarkable progress achieved by those functionalized polymers it has been shown that the systems, they have as inorganic materials also serious microstructure plays a significant role, as aggregation of restrictions, such as glass-like brittleness, sensitivity phosphonic acid groups is necessary to obtain good against hot water, acids, and bases, or limitations in the proton transport properties in the dry state.[8] Polymer accessible chemical functionality. post-functionalization strategies like grafting or polymer The search for ‘‘all-organic’’ porous materials made by a reactions also showed to be a versatile pathway to polymerization process from organic monomers therefore 224 Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200800642 Porous Polymers: Enabling Solutions for Energy Applications will lead to extended performances, shapeability and simplified synthesis and processing, but also introduce new opportunities relying on structural or functional properties, such as electronic conductivity or complex ligand functionalities. It is also a sound expectation that these polymer-like materials may overpass the limitations of inorganic materials in terms of pore size and connectivity. Therefore, it is no wonder that actually more and more research groups become interested in the development of microporous organic materials, and several different types of porous polymer networks have been synthesized.[52–54] Hypercrosslinked polymers are the oldest type of microporous polymers, as they were already introduced in the 1970s by Davankov and coworkers[55,56] as a new class of ion exchange resins. The underlying principle of the synthesis of hypercrosslinked polymers is the fixation of the microstructure, which is found in solvent swollen (weakly cross-linked) polymers. While for a long time hypercrosslinked polystyrenes were the only example for this class of polymers, recently this approach has been expanded to microporous, hypercrosslinked polyaniline resins.[57] In contrast, polymers of intrinsic microporosity (PIMs), introduced by Budd and McKeown, preserve porosity using bulky and contorted structure directing motifs, which prevent an efficient packing of polymer chains.[58,59] This concept, adapted from the synthesis of polymers with high free volumes for gas separation membranes, has been extended to microporous polymers using spiro-motifs incorporated into rigid polymer chains. Intriguingly, microporosity in PIMs can also be observed in soluble, that are non-cross-linked polymers, which shows that the porosity is indeed generated during packing of the polymer chains (Figure 3). The concept of PIMs can however also be used for crosslinked polymer networks. This combination yielded in a variety of new organic network materials exhibiting Figure 3. Model of a spirobifluorene derived polyimide [60] showing its highly contorted structure together with a cartoon representations of the space inefficient packing of the polymer chains yielding a polymer with intrinsic microporosity. Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim microporosity and high surface areas. A first example was the introduction of hypercrosslinked poly(arylcarbinoles).[61] Here, dilithiated aryl compounds were reacted with methyl carbonate and after an aqueous workup rigid poly(arylcarbinoles) were obtained. Organic frameworks have been synthesized in an analogous approach using lithiated arylates with tetraethoxysilane yielding siliconlinked networks.[62] Very recently, a series of papers described comparable approaches for the creation of conjugated polymer networks.[63–66] Here the structure directing motif is reduced to simple 1,3,5-functionalized benzene, which is connected using a number of C –C coupling reactions into highly cross-linked, conjugated polymer networks. The 2D architecture of the structure directing motif and the simplicity of the linkers and thus the resulting networks structures even seem to allow for a certain ability to predict and control pore sizes and surface areas of these networks. However, even though the chemical structure of those networks often promises the formation of 2D-layer architectures, which indeed would yield controlled pore structures, the so far reported networks are amorphous, thus rather form 3D, dendritic structures, which complicates characterization and control of the resulting pore size. Indeed, it can be assumed that a periodic polymer framework structure can be just observed under certain requirements. The synthesis of periodic zeolites as well as metal organic frameworks rely on hydrothermal or solvothermal synthesis schemes, which shows that a thermodynamic control of the condensation reaction has to be present, rather than the kinetic polymerization conditions typical for the formation of polymers and applied in the examples above. Thus, the condensation reaction should occur in a reversible or dynamic fashion, that is, chemical bonds of the forming polymer network have to close and open, yielding in consequence the thermodynamically (meta)stable, yet crystalline structure. As for metal organic frameworks the resulting frameworks rely on weaker coordination bonds, such a thermodynamically controlled network formation can be accomplished under rather mild reaction conditions. To create covalently bound organic frameworks, either covalent bonding schemes have to be identified which are rather weak and thus can reopen under mild conditions, or harsher reaction conditions have to be applied. Yaghi and coworkers[67] used the former route and introduced the first periodic, covalent organic frameworks based on the formation of boron oxide (B3O3) rings or boronate esters using either trimerization reactions of diboronic acids or condensation of diboronic acids with alcohols. Very recently this approach has been extended to organic borosilicate frameworks.[68] In analogy to metal organic frameworks, it has been shown that reticular chemistry can be used to control pore sizes of the resulting www.mrc-journal.de 225 A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti Figure 4. (A) Observed PXRD pattern of CTF-1 (black) and calculated PXRD pattern from an optimized structure (eclipsed conformation AAA


) of CTF-1 calculated with MS Modeling (blue). (b) Schematic representation of the structure of CTF-1 (C, gray; N, blue); H atoms are omitted for clarity. Reprinted with permission from Wiley-VCH. materials[69] and that even the formation of 3D periodic frameworks is possible using this approach.[70] Incorporation of alkyl chains has been furthermore used to tailor the pore size of such frameworks.[71] Related networks have been synthesized by trimerization of dicyano-compounds.[72] To enable reversibility, the reaction had to be carried out under much harsher reaction conditions. Indeed, trimerization in molten zinc chloride at 400 8C has shown to yield a covalent triazine-based framework with high chemical and thermal robustness.[72] The resulting polymer network structures plus their small angle scattering behavior underlining their regular structure set-up are shown in Figure 4. Such structures—due to their ability to rearrange at elevated temperatures and in the presence of a catalyst— can still be ripened to other, more beneficial scaffold structures. Higher temperature treatment yielded, depending on conditions, various regular, but non-crystalline polymeric networks, exhibiting micro- and mesoporosity and exceptional high surface areas of up to 3 000 m2
g 1.[73] These polymer structures already showed an extraordinary performance as sorbent, e.g., 1 g of such a hierarchical polymer scaffold can bind more than 1 g of dyestuff from aqueous solutions rather tightly, independent of the specific dye structure.[74] Also, the introduction of various heteroatoms into such networks for metal binding was enabled by this approach.[75] This short summary underlines that in the meantime a variety of microporous polymers are available, exclusively formed from organic compounds, enabling an exquisite control over the chemical nature of the large accessible surface areas as well as the physical properties of the resulting networks. It is not surprising that these purely organic, covalent analogs of the zeolites and metal organic 226 Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim frameworks are now tested for applications complementing or even replacing the latter materials. So far most of the attention has been clearly paid to applications such as hydrogen storage and sorption materials, while other promising applications such as catalyst supports, as thermal insulators or in organic optoelectronic devices still are awaiting their exploitation. In the following, we will therefore report only on the so far described applications of microporous polymers as gas, mainly hydrogen storage materials. Following the purpose of the article, we will, however, also try to give an outlook to the energy applications which can be expected in the near future from this class of polymer materials. Microporous Polymers for Hydrogen Storage and the Storage of Other Gases Hydrogen has exceptional prospects as the future energy carrier. This is due to its chemical abundance, its high energy density (three times that of petrol) and that its conversion just produces water. However, a wider use of hydrogen as an energy carrier, especially in mobile applications, is so far still hampered by its very low density which cannot be by-passed by current technologies. Besides the well-known obstacles regarding the production, distribution, and usage of hydrogen, one major bottleneck for future hydrogen applications is the safe and efficient storage of the gas. The targets set by the U.S. Department of Energy (DoE) for 2010 (storage of 6 wt.-% and 45 kg H2 per m3) and 2015 (storage of 9 wt.-% and 81 kg H2 per m3) seem so far clearly out of reach. Besides compressed and cryogenically stored hydrogen, several classes of materials have been proposed for either chemical or physical storage, and there are several reviews available describing the advantages and drawbacks of these methods.[76–78] The advent of MOFs with their favorable attributes such as high surface areas, tunable pore sizes, and chemical modifications has indeed given hope that a breakthrough for hydrogen storage can be accomplished using these materials as hydrogen containers.[79] It must be mentioned that there is nowadays a broad consent that hydrogen storage via (pure) physisorption will not fulfill the above-mentioned targets. Even though some of the high surface materials described so far, e.g., carbon nanotubes, metal organic frameworks, or microporous polymers, showed indeed very promising hydrogen storage capacities, a simple calculation of DOI: 10.1002/marc.200800642 Porous Polymers: Enabling Solutions for Energy Applications maximum surface areas theoretically possible in such materials[80] shows the fundamental restriction in the amount of hydrogen stored by physisorption, especially at room temperature. Indeed, due to the weak interactions between gas and sorbent at temperatures above the critical temperature of H2 and the related low isosteric heat of adsorption of hydrogen on carbonaceous supports (5–10 kJ
mol 1), most of the measurements carried out so far just describe hydrogen sorption values at 77 K rather than at ambient temperatures.[81] This however has only a restricted value for real life applications. Thus, if microporous polymers are envisioned as potential future materials for hydrogen storage, material design has to learn from the research on other high surface area material. Here, rather an increasing hydrogen adsorption energy is strived for, which can be done again by pore design (theoretical studies indicate that maximum adsorption of hydrogen would occur between two surfaces which are 0.7 nm apart)[82] or by inclusion of open metal sites or heteroatoms into such materials.[79,83] The same approaches can be easily adapted to microporous polymers, where optimization of properties even seems to be much easier. Furthermore, pure organic materials have the distinct advantage that they are composed solely of light elements. Exceptional high hydrogen sorption capacities of polymers have also been reported on non-porous materials, for example HCl treated commercial polyanilines and polypyrrole.[84] However, the hydrogen storage values reported for these polymers seem to be still a matter of debate.[85,86] High surface area microporous polymers have gained more and more attention as hydrogen sorption materials, most prominently represented by hypercrosslinked polymers and PIMs. In a first study on the applicability for hydrogen storage, hypercrosslinked polystyrene was investigated, synthesized by suspension polymerization of vinylbenzyl chloride followed by a Friedel–Crafts type cross-linking.[87] The surface area of this material was determined to be 1 466 m2
g 1. As expected, the H2 sorption capacity measured for this polymer was comparable to that of activated carbons or MOFs with equivalent surface area and at 1 and 15 bar hydrogen pressure at 77 K storage of 1.28 and 3.04 wt.-% hydrogen were achieved, respectively. In this report, also the introduction of custom-designed molecular sorption sites for H2 in order to significantly enhance the potential gas delivery capacity was envisaged. This first study was closely followed by an evaluation of the hydrogen sorption properties of commercial polymer resins.[88] Most of the materials performed in the same range as other high surface area materials, with a best value of 1.3 wt.-% (0.12 MPa, 77.3 K) for a hypercrosslinked polystyrene (Hypersol-Macronet MN200 with a surface area of 840 m2
g 1). Also in this report it was stated that it is unlikely to meet the goals for Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim storage capacities by simply increasing the surface area of such materials, and that therefore a new surface chemistry must be introduced. This finding was supported by a comprehensive investigation of the hydrogen sorption properties of a high number of hypercrosslinked polymers synthesized using the ortho-, meta-, and para-isomers of dichloroxylene (DCX) in different molar ratios plus self-condensation of 4,40 -bis(chloromethyl)-1,10 -biphenyl (BCMBP) and 9,10-bis(chloromethyl)anthracene (BCMA).[89] Resins with BET surface areas of up to 1 904 m2
g 1 have been produced and hydrogen uptakes of close to 1.7 wt.-% (1.13 bar, 77.3 K) and 3.68 wt.-% (15 bar, 77.3 K) could be observed, at this time the highest values reported for microporous polymers. A remarkable step toward a new surface chemistry in hypercrosslinked polymers was enabled by the introduction of the hypercrosslinked polyanilines.[57] Commercial polyaniline was swollen in an organic solvent and hypercrosslinked with either diiodoalkanes or paraformaldehyde. Polymers cross-linked with rigid linker such as diodomethane or paraformaldehyde exhibited permanent porosities and surface areas up to 632 m2
g 1. Even though these polymers displayed a lower overall hydrogen capacity at 77 K compared to some previous systems, it was shown that they offer a very high enthalpy of adsorption of up to 9.3 kJ
mol 1 (for polyaniline cross-linked with paraformaldehyde) and thus a very high affinity for hydrogen. Also PIMs have been studied as possible hydrogen sorption materials. A comparison of the hydrogen capacities of a series of different PIMs, incorporating various subunits such as hexaazatrinaphtylene or porphyrin, was recently presented.[90,91] Furthermore, the results were compared with the capacities observed for a typical hypercrosslinked polystyrene, high surface area activated carbon and several MOF materials. This study proved that PIMs are indeed a promising alternative as a hydrogen sorption material, with the values going well with the BET surface areas. On the basis of this study, the authors envisioned that a PIM material with a surface area of 2 500 m2
g 1 should meet the 2010 DoE target of 6 wt.-% storage at 10 bar/77 K. Most of the above-mentioned publications already state that a straightforward prediction of the hydrogen storage capacities of microporous porous polymers on the basis of BET surface area evaluated by nitrogen sorption is questionable, as this method can afford inflated values for materials containing small pores, and more important, the materials probably swell during nitrogen sorption (as observed by the fact that the desorption isotherms do not close on the adsorption branch). A clear quantification of this effect was provided by the investigation of hydrogen capacities of polyimide and polyamide PIMs generated through the introduction of tetraaminospirobifluorene motifs.[92] While the polyimide network exhibited surface www.mrc-journal.de 227 A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti areas of 982 m2
g 1, no micropore surface area for the polyamide network was detectable via nitrogen sorption measurements. However, hydrogen sorption measured at 1 bar and 77 K showed values of 1.15 and 0.52 wt.-% hydrogen uptake for the two polymer networks, respectively. The SAXS patterns of the polyamide and the polyimide under vacuum indeed showed no remarkable difference pointing to similar local pore structures. Thus, a pore structure of the polyamide network could be assumed which under increasing gas pressure does not open up big enough pores to accommodate the bigger nitrogen molecule, while hydrogen could enter the polymer network. Conjugated polymer networks and covalent organic frameworks have just very recently been evaluated for the purpose of gas storage. Conjugated poly(aryleneethynylene) networks with surface areas ranging from approximately 500–1 000 m2
g 1 showed good hydrogen uptakes (from 0.6 to 1.4 wt.-% at 1.13 bar/77 K)[66] but more importantly enhanced hydrogen adsorption enthalpies, comparable to those observed from hypercrosslinked polyanilines. Microporous poly(phenylene butadiynylene)s published by the same group showed comparable values of hydrogen uptake.[64] In a recent report, the syntheses of 3D covalent organic frameworks (COFs) with surface areas exceeding 4 000 m2
g 1 were described, which promise even more elaborated values in hydrogen uptake and storage applications.[70] Recent calculations on 3D COFs have shown that hydrogen storage values comparable to MOFs with similar surface areas can be expected.[93] Intriguingly, the calculations also proposed that Li and Mg iondecorated COFs would indeed give hydrogen adsorption energies, suitable for practical applications. Indeed, one major advantage of microporous polymers is that the pore walls can be easily functionalized. Thus, metals can be incorporated enabling adsorption sites which are highly accessible, in comparison to most ‘‘metal organic framework’’ (MOF) materials where the metal centers are shielded by the linkers. In this line, a tungsten-based organometallic complex, known to form strong bonds to hydrogen has been incorporated within a polymer support. Storage and release of hydrogen could be triggered by UV light.[94] In this paper, it was already suggested that incorporation of such complexes into high surface area polymers should very much increase the hydrogen storage capacities even at ambient temperatures. A further direction in this research field could be the incorporation of metals,[95] which even can dissociate the hydrogen molecules, yielding a spillover mechanism described for supported metal catalysts.[96] It can be summarized that microporous polymers are indeed promising materials for hydrogen storage applications and that they can certainly compete with other high surface area materials used so far, such as activated 228 Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim carbons and MOFs. Future research will presumably address the question of creating both designed micropores (‘‘ultramicropores’’) and chemically functionalized surface areas to enhance hydrogen adsorption enthalpies on these materials. Furthermore, microporous polymers have certain advantages compared to other materials, as they are often made from cheap monomers, are produced by easy and scalable polymerizations and can be processed in certain morphologies, most importantly monoliths to fill as much space as possible in a future hydrogen tank. While hydrogen storage is still hampered by the very low temperatures involved (77 K), similarly designed materials can be used for the storage of other gases. Indeed, a first commercial application envisaged for MOFs is for propane storage, and it was demonstrated that MOF-5 tablets were able to store three times of the amount of propane compared to an empty container of the same volume.[97] In a recent review, Morris and Wheatley[98] summarized the gas storage application in different nanoporous materials. Besides energy applications medical and environmental applications were also addressed. Interestingly, there is up to now no report for the storage of other gases in microporous polymers, except a very recent one on the storage of methane in microporous hypercrosslinked polymers.[99] Especially the development of polymeric materials for the storage of methane, the major component of natural gas fuels, is a real valuable goal for polymers in energy applications. It should also be mentioned that the main difference from hydrogen is in the interaction energy between methane and the surface of the storage material, which is sufficient to yield reasonable adsorption at room temperature; thus the volumetric targets for methane storage (35 wt.-%)[100] might be reached much faster. As for methane, storage of other gases with energy and environmental impact such as hydrocarbons, CO2, SO2, and NH3 are still most commonly explored for zeolites, activated carbons, and MOFs, which on the other hand means that there is still a large field of discoveries to be made using porous polymers. This is even more surprising as polymer membranes are the classical materials for gas separations, that is, interactions of various gases with polymers are well known and are partly highly discriminating. Gas separation can indeed be seen as energyrelated application, as for example the energy carrier methane has to be separated from natural gas (CH4/CO2) or clean hydrogen has to be generated to be used at the electrodes of FCs. The important features for a gas separation membrane are obviously a high permeability combined with a high selectivity, and comprehensive reviews can be found regarding the topics of gas diffusion and solubilization of gases in polymers.[101] Indeed, PIMs have shown to be very promising materials for gas separation membranes.[102,103] DOI: 10.1002/marc.200800642 Porous Polymers: Enabling Solutions for Energy Applications Scaffolded Polymer Networks as ThreeDimensional Semiconductors for Optoelectronic Applications The realization of the first prototypes of organic field effect transistors (OFET), organic light emitting devices (OLED), and bulk heterojunction solar cells based on conjugated polymers gained much attention and encouraged intensive research in p-conjugated systems. Even though some of these devices have already entered the market, there are still some significant problems regarding their performance. This is mainly due to the fact that any of the devices has special requirements to be solved, such as high electroluminescent efficiency for OLEDs, or optimal absorption properties of the solar irradiation and high absorption coefficient for organic solar cells. Furthermore, a precise control of the highest and lowest molecular orbital (HOMO/ LUMO) for all these devices is needed. For an overview over the task of organic electronics several reviews are available.[104–108] As recently nicely summarized, for such devices materials are needed not only with obviously good electronic properties but also which enable a specific control of molecular interactions and orientation.[109] Indeed, several problems in such devices originate from the onedimensional character of linear p-conjugated systems, provided by linear polymers. For example, anisotropic charge transport and optical properties need an accurate control over the orientation of the conjugated chains on the substrates to provide optimal charge transport. In contrast, 3D organic semiconductors would allow the realization of all kind of electronic or photonic devices without any constraint about molecular orientation. Several research groups were thus striving for organic devices with higher dimensionalities of the organic semiconductors.[109,110] Interestingly, the structure directing motifs enabling such 2D- or 3D-conjugated architectures are similar to those found in the new conjugated polymer networks. Indeed, benzyl, triazine, triphenylamine, tetraphenylmethane, or spirobifluorene motifs have been used to synthesize large, fully conjugated, star-shaped molecules, which were subsequently applied to organic optoelectronic devices, from OLEDS to solar cells.[111–117] Thus, it would not be surprising for us if the two disciplines of molecular engineering toward organic electronics and polymer engineering toward conjugated, microporous polymer networks will soon overlap, as both disciplines apply the same synthetic tools and tectons. Indeed, conjugated polymer networks featuring different structure directing motifs based on polyanilines,[57] paraphenylenes,[63] aryleneethynylene,[65] polythio[95] [64,118] have been reported (see above). phenes, and others Spirobifluorene derived polyparaphenylenes have shown intensive blue emission, interesting for their application in OLED.[63] A further interesting feature for the application of Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim microporous conjugated polymers could be that a second phase (dye, or a corresponding hole or electron conductor) can be introduced after the synthesis of the first film by simple infiltration of the networks, enabling a defined nanostructure of the resulting interpenetrating network. The Polymerization of Carbon: Materials for Battery and Supercapacitor Applications The previous cases of polybenzimidazol for FC membranes or microporous polymers for gas storage and immobilization already prepared the way for the current section: the polymerization of carbon as such. A polymer with ultimate properties will presumably contain larger fragments of allotropes of carbon in the backbone. From a materials point of view, this is obvious: carbon structures (as diamond, graphite, carbon nanotubes) mostly offer the ultimate goal of a property. The highest heat conductivity, the highest mechanical modulus, the highest hardness, the lowest compressibility, the highest electronic conductivity, there is practically no property target which cannot be handled by an appropriate existing or conceptual carbon structure. In organic chemistry, a number of trials can be found to generate all-carbon structures with various connectivities and geometries where hydrogen or other heteroatoms are only used for edge or chain termination.[119–121] In polymer science, the idea to polymerize ‘‘graphite’’ or ‘‘diamond’’ is still out of common consideration. Rare exceptions of exploring a rational chemistry toward extended but defined all-carbon structures can be found in the work of Müllen and coworkers[122] who synthesized large extended graphene sheets by dehydration of polyphenylene dendrimers and in the work of Frauenrath and Jahnke[123] who used preorganized oligoacetylene-multiblock copolymers to generate stacked graphene-like ribbons. Polymerization of a ‘‘graphite-monomer’’ using templates as delineated in the previous sections could be another meaningful option for generating graphitic carbon polymer scaffolds with useful properties. Graphite is a key choice for electrochemical applications, e.g., as electrodes, in the lithium battery or in super capacitors. Here, a comparison between the ordinary mined, mineralic graphite, and a synthetic graphite polymer with structural design features can illustrate the potential of a polymer science devoted to energy applications. In principle, templating toward graphite scaffolds could be done by silica nanoparticles, again, but there are other more benign exotemplates to choose from. Monolithic silica columns for chromatography are commercially available (from Merck KGA) and exhibit a combination of very high surface area and excellent transport properties. These porous silicas are prepared according to the Nakanishi process[124,125] and can easily adopt crack-free www.mrc-journal.de 229 A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti Figure 5. Replication of a continuous silica monolith for chromatography (a) into a polycondensed mesophase pitch (b). The about perfect replication of the three dimension structure in the absence of macroscopic cracking is nicely seen. Reprinted with permission from Wiley-VCH. outer shapes of 30 cm in length and a few centimeters in diameters. Using those monoliths as templates for the Fecatalyzed condensation of mesophase pitch (a highly aromatic ‘‘monomer’’) gave soft, carbonaceous replicas for electrochemical purposes.[126] The quality of this replication process is depicted in Figure 5. Those silica monoliths are empirically optimized for chromatography and exhibit a continuous, sponge-like transport porosity on the 2–5 mm scale, while each of the cross-bars itself is porous and contains the chromatographic active mesoporosity. As only those are filled by capillarity by the pitch monomer, structural inversion occurs on the level of the cross-bars, only, while the structure looks the same at the micron scale. Cross-linking the pitch to a ductile carbon turned out to be essential for the preservation of mesoporosity, while extensive carbonization turned out to be unfavorable for the desired electrochemical properties. This is why a more carbonpolymeric replica treated to 700 8C to stimulate polymer- 230 Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ization but to avoid extensive carbonization turned out to be beneficial. These monoliths made of polymerized mesophase pitch have been tested as an anode material for lithium batteries. The resulting electrochemical cycling behavior is depicted in Figure 6a and b. After the formation of a solid electrolyte interphase in the first four cycles, a remarkably high, fully reversible capacity of > 800 mAh
g 1 is found, clearly exceeding any commercial anode material. The real benefit of the hierarchical monolithic pore structure is reflected in Figure 6b. The materials can be cycled for hundreds of cycles at extreme cycle rates of 60 C, without any loss of battery performance (60 C corresponds to loading with 1 mol electrons/ structural unit in 1 min, i.e., a complete loading and unloading in 1 min each). This example illustrates the extreme benefits for material science when both the chemistry of the polymer scaffold and the pore structure of an active polymer are optimized. Interestingly and also shown in Figure 6b, further graphitization of the polymer indeed lowers the storage capacity and brings the values back to the ordinary behavior of graphene. This is why the extraordinary storage capacitance can speculatively be attributed to polymer properties such as edge terminations or interpolymer free volume. Not surprising, additional covering of the mesoporous polymerized mesophase pitch with a layer of polyaniline yields a material suitable for supercapacitors.[127] The material (PANI grafted on HPCM-1) shows a cycle-stable supercapacitance of above 1 250 F
g 1 PANI even at high current densities and after many cycles, while a corresponding nanocarbon/PANI hybrid of similar chemical composition (PANI on NPCM) only shows a much lower supercapacitance, which in addition detoriates with time (Figure 6c). Again, the advantages of the rational set-up of a polymeric energy-storing system are obvious. Even in those storage applications, there is still much potential for improvement through polymer innovation, especially when potential monomer variation and copolymerization are considered. For instance, nitrogen-rich carbon polymers seem to be very promising for further improving hydrogen and lithium storage,[128,129] and optimization of the electric conductivity of carbons through a monomer approach can open up new vistas in the area of conductive coatings or nanostructures, such as recently demonstrated by Müllen and coworkers[130] for transparent conducting electrodes, based on polymers made of diverse graphite-fragments. Porous Polymers and Carbonaceous Resins Using Biomass as a Raw Material The prospective development of all polymeric materials, also for those used for energy applications, will most DOI: 10.1002/marc.200800642 Porous Polymers: Enabling Solutions for Energy Applications Figure 6. Electrochemical performance of monolithic mesophase pitch in two applications. (a) Lithium insertion behavior at 1 8C, indicating that the material passivates in the first four cycles, is then very stable. (b) Accelerated aging protocol also involving extremely high cycling rates, indicating no loss of performance over 600 load cycles. (c) PANI modified monolith of polymerized mesophase pitch in a supercapacity run. The material exhibits a very high supercapicitance of ca. 1 250 F
g 1 PANI which is in addition practically free of detoriation, at least over 1 200 cycles. Reprinted with permission of Wiley VCH. Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim certainly rely on the availability of their raw materials. Besides the design and synthesis of new or modified polymer structures, it is therefore also a future task of polymer science to implement alternative sources of monomer in the synthesis design. Biomass would provide such a sustainable and cheap alternative to access such monomers, with a number of unexpected benefits coming from the structural preorganization of biomass. In fact, more and more research is currently devoted to a change in the raw material base for polymer chemistry, starting from the traditional use of biopolymers in polymer materials over the creation of new monomers by new chemical pathways from biomass up to white biotechnology. Again, a comprehensive overview on the use of biomass as feedstock for polymer synthesis would be out of scope of this manuscript, and recommendable reviews on that subject can be found in the literature.[131,132] Thus, we will focus on the synthesis of functional porous polymeric and carbonaceous materials using carbohydrate resources, which is a recent developing topic in materials chemistry.[133–139] For example, Clark and coworkers[136] generated a family of mesoporous polymer– carbon materials with surfaces ranging from hydrophilic to hydrophobic. They used the natural ability of the amylase and amylopectin polymer chains within the starch granules to assemble into an organized nanoscale lamellar structure, forming a mesoporous expanded starch. This material was thereafter heated under N2 at different temperatures ranging from 100 to 700 8C in order to produce mesoporous materials varying from starch to activated carbons, including amorphous oxygen containing carbons that have many potential applications due to their surface functionalities.[140] Another method for the production of similar low cost based materials is a polymerization process called ‘‘hydrothermal carbonization’’ (HTC).[133–135,137–139] This represents a simple and environmentally friendly technique through which biomass is first converted into polymerizable monomers, then polymers and finally, following further dehydration/polymerization/condensation processes, into carbon rich resins.[141–143] It also represents an option to sequester carbon and the stored energy from plant material.[135,144] Although this process is already known since 1913 when Bergius[145] described the hydrothermal transformation of cellulose into coal-like materials, the process had been only recently re-discovered and exploited by several groups.[134,137–139] Employing characterization techniques such a 13C solid state NMR and GC-MS, it was found out that the process of HTC mainly takes place via 5-hydroxymethyl-furfural (HMF), while polymerization occurs via aldol condensation, cycloaddition, and polymerization reactions. The process of HTC is essentially a heterophase polymerization reaction, with all the possibilities and demands of it.[146] First the carbohy- www.mrc-journal.de 231 A. Thomas, P. Kuhn, J. Weber, M.-M. Titirici, M. Antonietti drate (e.g., glucose) loses three water molecules and transforms into HMF which separates from water as an emulsion, forming nanosized droplets. These droplets start growing the more biomass is depleted, and the final size is reached when all the sugar is transformed into the carbonaceous material. The coal-like resin is build up from a conjugated sp2[136] backbone and oxygen rich functional hydrophilic side groups.[141,142] These resulting natural monomers obtained from the controlled decomposition of carbohydrates can be used for the generation of porous materials using the nanocasting procedure.[147,148] Thus, using HTC of glucose in the presence of different nanostructured silica templates, a series of porous carbonaceous materials have been produced. For example, SBA-15 could be successfully replicated using carbohydrates precursors, leading to mesoporous hexagonally ordered structures with good crystallographic meso-order, uniform pore sizes, and high surface areas.[148] Some morphologies of previously described materials are shown in Figure 7. Since all these carbohydrate-based nanocasts were generated under hydrothermal conditions, there are still a large number of oxygenated functional groups residing at their surface, also making additional chemical functionalization possible.[148] Contrary to simple prejudice, this ‘‘green,’’ carbohydratebased polymer chemistry also turned out to be extremely useful for the generation of high-end energy materials. The monomers formed in situ can be successfully used for the coating of preformed nanoparticles.[149–151] One particular example is the coating of silicon nanoparticles followed by further carbonization at elevated temperatures to improve conductivity.[150] Due to the hydrothermal conditions, Figure 7. Example of porous materials obtained using HTC of glucose: (a) macroporous cast obtained using totally hydrophobized silica templates; (b) hollow spheres obtained using surface hydrophobized silica templates; (c) mesoporous spheres obtained using calcined mesoporous silica (the inner porosity is not seen in SEM, (d) hexagonally ordered porous carbon structure with ca. 7 nm repeat length obtained by nanocasting of SBA-15. 232 Macromol. Rapid Commun. 2009, 30, 221–236 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim besides the carbon coat, a thin layer of silica was simultaneously formed at the surface of the silicon nanoparticles. The resulting composite material (Figure 8) showed a significantly improved lithium storage performance in terms of a highly reversible lithium storage capacity, very good cycling performance and high rate capability. This can be explained by the elastic buffering of the carbohydrate-based coating, minimizing the mechanical stress induced by huge volume change of active silicon throughout lithium uptake, thus preventing the deterioration of the electrode integrity, and keeping simultaneously the high capacity associated with silicon. The anode capacity of such structures lies rather stable at 1 100 mAh
g 1, five times the lithium-storage capacity of the best current market materials. When using raw biomass plant material (‘‘green waste’’) instead of pure carbohydrates,[135] the material properties Figure 8. TEM images of the Si@SiOx/C nanocomposite for lithium storage, produced by HTC in the presence of Si-nanoparticles; further carbonization at 750 8C under N2: (a) overview of the Si@SiOx/C nanocomposites and higher magnification TEM (in the inset) showing spherical composite particles; (b) HRTEM image displaying details of the silicon nanoparticles coated with SiOx and the polymerized carbon. Reprinted with permission from Wiley-VCH. DOI: 10.1002/marc.200800642 Porous Polymers: Enabling Solutions for Energy Applications ments in the 20–50 nm range (Figure 5b). Such porous carbonaceous resins might find, especially concerning their low price and their CO2-negative character,[144] large scale applications for drinking water purification or to improve soil quality as some type of ‘‘porous superadsorber,’’ or in energy applications, such as for thermal insulation. Conclusion We tried to illustrate in a number of model cases the enabling role of polymer science in the forthcoming years of energy and raw material change, especially focusing on the novel class of micro- and mesoporous polymers. These cases included not only new porous materials for (FC) membranes, but also novel gas storage materials, and conducting polymer carbons for batteries, Figure 9. (a) SEM micrograph of the nanoparticles obtained upon hydrothermal treatelectrodes, solar cells, and supercapaciment of pine needles. (b) TEM picture of a porous bicontinous sponge-like network obtained upon HTC of oak leafs. (c) Adsorption–desorption isotherm of a biomass tors. As a whole, we termed this emersample before and after hydrothermal treatment, underlining the simplified access to ging new subfield of polymer science porous resins. ‘‘energy polymers,’’ and we also gave out visions where the tasks in the field are and how future progress might look like. do not necessarily deteriorate, but sometimes even benefit Finally, we delineated that such a novel polymer science from other enclosed chemical components and the original is not necessarily based on petrochemical monomers, but biological texture of the plant material. A major difference that controlled thermal decomposition of biomass with in the decomposition of pure, simple carbohydrates can be the view of a polymer chemist can bring forward amazing identified in the structural analysis, revealing essentially polymer nanostructures, which are otherwise hardly two different reaction cascades, depending on the plant made. The described cases with their many record material used. Under the applied conditions (16 h and performances also indeed show that a sustainable polymer 200 8C), all mechanically soft biomass (e.g., orange peels, chemistry and high end nanostructured polymer materials pine needles) is completely fluidized before polymerizaare not necessarily contradictions per se, but on the tion, resulting in a dispersion of carbonaceous nanoparcontrary go—‘‘naturally’’—rather well with each other. ticles with interparticular porosity, however being sometimes remarkably smaller than the pure glucose system Received: October 14, 2008; Revised: November 27, 2008; (Figure 9a). 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