Organic zeolites

Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) Crown Copyright 9 2005 Published by Elsevier B.V. All rights reserved

37

Organic zeolites D. V. Soldatov a and J. A. Ripmeester b

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia; [email protected]

a

b Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa K 1A 0R6, Canada Organic zeolites are non-silicious solids mimicking physicochemical behavior of zeolites with respect to organic sorbate species. In other words, the shortest definition of "organic zeolites" will be "non-silicious microporous sorbents with hydrophobic porosity". This paper places organic zeolites in a historical context of zeolitic materials and reviews current achievements and problems in their design and identification. 1. HISTORICAL BACKGROUND 1.1. Natural zeolites Zeolites are formed in nature from volcanic glass and saline water, with the time for their formation varying from 50 to 50,000 years. Naturally occurring zeolites are the oldest known family of microporous materials. The release of absorbed water from the natural mineral stilbite was observed by Swedish mineralogist Cronstedt in 1756. [1] Cronstedt introduced the word "zeolites", from the Greek words zeo (to boil) and lithos (stone), to describe unusual behavior of the mineral upon heating it in a blowpipe flame. In fact, the water-containing stilbite was the first representative of the class of inclusion compounds, a type of chemical material identified structurally by Powell only in 1948. [2] Zeolites are crystalline aluminosilicates with a microporous framework formed by linked AlO4 and SiO4 polyhedra. The general formula is Mn[(AIO2)• Pure silica zeolites comprise neutral framework while the introduction of alumina makes it negatively charged and requires the presence of metal cations M residing inside the micropore space along with guest water. Up to the present, about 50 natural zeolites have been identified. [3] The greatest use of natural zeolites (> 1 mln tonnes) has been in radioactive waste encapsulation. [4] 1.2. Synthetic zeolites, zeotypes and related materials The interest in the synthesis of zeolitic materials was stimulated mostly by their utility as catalysts for the production of petrochemicals. Rapid progress in the creation of such materials started in 1950's, after the creation of artificial zeolites by Barter, Milton and others. [5,6] Synthetic zeolites have a high impact on today's industry. For example, between 30 and 50% of all motor fuels (gasoline, jet, and diesel) have been produced world wide with Y zeolite catalysts. [7] The utilization of other tetrahedral centers instead of A1 and Si, such as P, Ge and Fe, resulted in entire families of new microporous materials known as zeotypes. Up to the

38 present, hundreds of synthetic zeolites and zeotypes have been synthesized. [4,7,8] In 1990's, the field of open-framework inorganic materials expanded dramatically to include about 25 elements of the periodic table. [9] Most applications are based on three basic qualities of these inorganic materials: sorption, ion exchange and catalysis. 1.3. Organic zeolites: origin of the term and essential reviews The concept of "organic zeolites" was inspired by "zeolitic" physicochemical behavior of some Werner complexes examined by Barrer and coworkers. [10,11] Inclusion phases of the complexes retained their microporosity in the course of repeatedly performed removal and re-sorption of the guest. The molecules of the complexes, such as [Ni(4-Methylpyridine)4(NCS)2], are formed by octahedral coordination of substituted pyridines and anionic groups to a metal(II) cation. Therefore, the molecules of the first "organic zeolites" neither were truly organic nor were related to zeolites in terms of their composition. The term "organic zeolites" arose in the Barrer's research group as jargon to define any solid able to reversibly and selectively absorb large amounts of hydrophobic (organic) species while showing poor tendency toward sorption of inorganic compounds. [12] In other words, the "zeolites" label was used to mark permanent porosity of the new materials (a characterictics of true zeolites), while the "organic" label was used to stress the hydrophobic nature of the interior pore surface (rather than the organic nature of the host itself). The term passed from the verbal to the literature only in 1980's and 1990's, [13,14] and the first review entitled "Organic Zeolites" appeared as a short sub-section in a book in 1996. [15] The zeolitic behavior of Werner complexes was reviewed by Lipkowski. [16] More recent reviews [17-19] reflected the use of coordination polymers in the design of organic zeolites performed in 1990's. Currently, most organic zeolites fall into one of three groups: glassy or semi-crystalline organic polymers, crystalline organic materials, and crystalline porous coordination polymers also called metal-organic frameworks (MOFs). A very comprehensive review of metal-organic zeolitic materials was done by Kitagawa et al. [20] Finally, a review tracing some recent achievements in the area was published in an encyclopedia. [21] There is a growing number of research papers reporting discoveries and attempts to design organic zeolites rationally; some of them are discussed further in this paper. 2. PHYSICOCHEMICAL BEHAVIOR OF ORGANIC ZEOLITES 2.1. Zeolitic sorption Zeolitic sorption implies the presence of significant and permanently available micropore space in a material. In this case, sorption starts at the lowest guest pressure and continuously rises to a saturation value defined by the sorbent capacity. The sorption process is favored by the formation of a solid solution of guest in host lattice, where the presence of guest lowers the chemical potential of the host units forming the lattice. According to IUPAC classification, porous materials are divided into microporous (< 2 nm), mesoporous (2-50 nm), and macroporous (> 50 nm) [22,23] materials. The sorption inside micropores is the most energetically favorable due to a cumulative effect of many interatomic contacts between sorbate molecule and the internal surface of micropores. By definition, organic zeolites behave similar to real zeolites in the sorption/desorption process. In other words, it may be difficult to guess which of the two types of sorbent is involved until some chemical information about the investigated material is obtained (color, thermal stability, chemical composition, etc.).

39 The ability of the guest sorbate species to enter the pores and, therefore, the availability of the pores in a material, is evident from the sorption isotherm. For the majority of guests, the sorption in both organic and real zeolites will follow a Type I isotherm in classification given by Brunauer [22-24] (Fig. 1). The type I isotherm is characteristic of microporous sorbents: it sharply rises at low guest pressure (energetically favorable sorption in permanently available micropore space) and goes to a plateau (corresponding to a saturated value at which all micropores are filled). III

"o J~ I.. o u~ "o

iv

v

::3 0

E pressure of s0rbate

Fig.1. IUPAC classification of sorption isotherms. First five types were identified by Brunauer [24] When experimental data appear to follow the Type I isotherm, it is useful to redraw the isotherm in Langmuir coordinates: Pguest/X versus Pguest, where Pguest is guest pressure and x is an equilibrium 'absorbed guest' to 'host sorbent' molar ratio. If in such coordinates the experimental data fit a straight line, the data are well approximated with a Langmuir equation X -- Xmax{(K Pguest)/(1 +X Pguest) }

( 1)

transformed into

Pguest/X = ( l/xmax)Pguest + 1/(XmaxK),

(2)

where Xmaxis the maximum value of x (the capacity of the sorbent with respect to the guest used expressed as the guest-to-host molar ratio) and K is a sorption constant. From Eq. (2), as a function Pguest/X (Pguest), the Xmaxand K values can be estimated. The 'specific surface area' of the sorbent may be estimated from the formula (XmaxNa cr)/M, where Na = 6.023x 1023 g tool 1 is the Avogadro constant, cr is the area on the surface occupied by a single guest molecule and M is the molar mass of the host sorbent. Most zeolites are equivalent to adsorbents with specific surface areas of hundreds square meters per gram. Sometimes, other types of isotherms may be observed. A type VI isotherm was found for p-xylene in zeolite H-ZSM-5 (70~ and was ascribed to a change in packing of the guest molecules, at a relative pressure of-0.3, from 4 molecules to -6.5 molecules per unit cell. [25] In organic zeolites, a Type VI isotherm is found more frequently. This fact results from the high flexibility of the host framework in many organic zeolites which makes possible a step-like increase in their sorption capacity with increasing guest pressure. For example, a number of Type VI isotherms were observed for the [M(4-Methylpyridine)4(NCS)2] sorbents (M = Ni, Co) with respect to methane, noble gases and organic guests, where the initial sorption promotes generation of additional sorption sites. [ 10] The hydrophobicity of pores in [Ni(4-Methylpyridine)4(NCS)2] causes the Type V isotherm observed for this sorbent with methanol, a strong polar guest. [10] Finally, Types IV and V isotherms may be observed in experiments with fine-crystalline zeolites due to sorption into the intracrystalline pore volume. 2.2. Sorption versus clathration

Clathration and sorption are two types of basic behavior describing the affinity of host

40 frameworks to supramolecular association with another, the guest species (Fig. 2). The distinction between clathration and sorption has thermodynamic reasons.

clathration

.0

ax

<<<<< +e ]_~/.,,. <<<<< ~ ~ ',.'<)~.< host phase

inclusion

compound

sorption --<>,<>, host matrix

+~

guest pressure

w =0

mclumon compound

~, guest pressure

Fig. 2. Clathration and sorption: the comparison. Structural changes are shown schematically on the left and the dependences of the guest to host ratio as a function of guest pressure are shown on the right Clathration behavior is observed for hosts with unstable porous framework. [26-29] In the absence of guest they exist in a dense modification (host phase) and return to this modification as soon as guest is removed from the inclusion compound they form. The inclusion compounds of such hosts have virtually constant guest-to-host stoichiometry; on a plot of P-X dependence this feature is apparent as a characteristic step change in the guest to host ratio at a certain guest pressure. Sorption behavior is observed for hosts with host lattices which, once formed, become permanent whether empty or filled by guest molecules. [10-12,30-33] Therefore, guest inclusion is not followed by a host structure change and the resulting inclusion compound is a solid solution, that is a compound of variable composition. The P-X dependence for this type of inclusion appears as a smooth curve called the isotherm. It should be noted that the majority of microporous materials are thermodynamically less favorable with respect to a dense form and exist as metastable phases for kinetic reasons. The difference in stoichiometry, which is constant for clathrates and variable for zeolitic inclusions, is explainable in thermodynamical terms by the Gibbs phase rule: F = c - p + 2. The number of components c = 2 (host and guest) for both clathration and sorption but the number of phases in equilibrium p is different, three for clathration (host dense form, inclusion form, and pure guest phase) and two for sorption (host microporous form and pure guest phase). The number of degrees of freedom F for clathration is such that not more than one parameter may vary to retain all the three phases participating in the equilibrium. Therefore, there is only one possible pressure over the clathrate and only one possible composition of the clathrate at a given temperature. In contrast, for sorption, F is two. Therefore, at a given temperature, the guest pressure and the composition of the inclusion phase may change accordingly, following a law expressed as a sorption isotherm.

2.3. Sorption versus encapsulation Encapsulation [34,35] is defined as either inclusion of guest molecules inside a cavity formed by a limited number, usually one, host molecules (molecular inclusion complex), or as

41 confinement of guest molecules inside cage-like cavities of a solid host phase ("clathrate" in the original meaning of this term [2]). The second use of the term overlaps with the term clathration when the host framework is unstable without guest. However, it is often difficult to find out if a host framework can exist in its empty form. Besides, there are intermediate cases, such as amorphous, semi-amorphous and powder phases consisting of single molecular host receptors that can display a wide range of physicochemical behavior. For the host phases that are stable enough to exist in the absence of guest, the distinction between encapsulation and sorption has kinetic reasons. Sorption presumes free inclusion in the open pores of a host framework. Encapsulation implies the confinement of gases or volatile guest species in closed cavities of the crystal structure, with the release of encapsulated guest being either negligible or slow. A classical example of this type of encapsulation is the entrapment of gases in ot and [3 forms of hydroquinone. [36] The c~ [37] and 13 [38] forms have, respectively, one and six cavities per ! 8 hydroquinone molecules and may entrap a range of molecules, such as Ar, Xe, H2S, SO2, HC1, CH3OH, CH3CN. The empty ~ form is a thermodynamically stable polymorph of hydroquinone while the empty [3 form is less favorable but can be isolated as a metastable polymorph. The crystals of [3-hydroquinone inclusions retain encapsulated guest molecules under normal conditions. The release of noble gases may be observed as the formation of bubbles upon dissolution of the inclusion crystals and the release of sulfur dioxide as the appearance of characteristic smell upon grinding the inclusion crystals in a mortar. The distinction between sorption and encapsulation is one of the reasons why organic zeolites cannot be easily distinguished from clathrates using structural information only. An interesting algorithm of the search for potential organic zeolite structures was used by Zass et al. [39] These authors performed a screening of the Cambridge Structural Database and found 33 host organic frameworks that both may accommodate guest molecules and may exist as empty forms. However, without an experiment involving direct or reverse sorption, the ability of these structures to exchange guest with the environment is not clear, and so the sorting of these hosts between organic zeolites and "encapsulators" is hardly possible. It should be noted that the term encapsulation is also used in a different sense, not applicable to bulk materials, to describe molecular inclusion complexes. [34,40] 2.4. Sorption versus dissolution

A number of semi-crystalline and glassy 1D organic polymers, such as syndiotactic polystyrene, exhibit sorption behavior which resembles that of zeolites. [41-45] Sorption in such materials is, in fact, a sum of two processes, zeolitic sorption and dissolution. The first part may be usually described by a Langmuir Equation (1). The second part, dissolution of volatile guest in the polymer, in the simplest case (low pressures and ideal gas) follows Henry's law [46]: X = k Pguest,

(3)

where k is a solubility coefficient. Combining Eqs. (1) and (3), the experimental sorption in such materials will be described by the Eq. (4): x = Xmax{(K/:'guest)/(1+K Pguest)} + k Pguest

(4)

The combined sorption isotherm resembles Type I isotherm at low pressures but, instead of going to a plateau, reveals a monotonic rise at higher pressures. In fact, at high pressures k becomes a function of guest pressure and may acquire a concave shape in this area similar to that of Type II isotherm [47] (Fig. 1).

42 3. METHODS USED TO CONFIRM ORGANIC ZEOLITES Any experiment for confirming zeolitic behavior of an organic or metal-organic material should include a sorption (or desorption) process. The determination of a sorption isotherm is highly desired and sufficient for this task. A number of useful parameters may be derived from the isotherm, such as sorption constant, sorption capacity, total micropore space, size of pores and so on. When sorption follows the Langmuir equation, some parameters may be calculated in a simple way as demonstrated in Section 2.1. However, it should be noted that this equation was developed to describe adsorption on a surface (usually surface chemisorption) and the plateau on the Langmuir isotherm corresponds to the formation of a monolayer on the surface. Zeolitic sorption is driven by filling the micropore space and the plateau corresponds to the filling of all available pores. The analysis of isotherms based on pore-filling mechanism of sorption may be done within the framework of Dubinin's theory. [48,49] Methods of extracting information from the sorption experiments have been discussed [50-55] and standard instruments for the collection and analysis of sorption data are available. [56] Alternatively, the combination of a sorption experiment with structural or spectroscopic methods are used. Some combinations with two or three examples from the literature are listed below. 1) Powder XRD analysis of an inclusion phase before and after the removal of guest. The survival of the microporous framework is evident from the similarity of two powder diffractograms in the positioning of observed reflections. [57,58] 2) Single crystal XRD analysis of inclusion crystals before and after desolvation. [13,33,59] 3) He pycnometry of a microporous material and the comparison of derived "framework body density" with the density calculated from XRD analysis. [57,60,61] 4) Xe NMR of a microporous material placed in atmosphere of Xe (in this case the sorption of Xe atoms takes place). [62,63] 4. RATIONAL DESIGN OF ORGANIC ZEOLITES

4.1. Design principles The design of organic zeolites requires solving two basic problems. The first problem is how to make a material with desired volume and geometry of micropore space. The second problem is how to make the microporous form of the material stable enough to operate in a desired range of experimental conditions. The first problem appears to be different for real and organic zeolites. The main approach to the design of zeolitic aluminosilicates is based on using suitable guest templates and finding appropriate synthetic conditions. These parameters are important for organic zeolites as well but the problem itself becomes more complex because of a much greater choice of building units that may be utilized. The architecture of organic and metal-organic materials is determined to a greater extent by the individual properties of the building elements forming the structure. The second problem is not important for real, but essential for organic zeolites. Although microporous materials of both types are thermodynamically less favorable than the corresponding dense modifications of the materials, the kinetic stability of real zeolites, built upon strong covalent Si-O and A1-O bonds, is significantly higher and they do not collapse. In contrast, organic zeolites, built upon weaker interactions or easier dissociating coordination bonds, reveal a much greater tendency toward the transformation into a thermodynamically

43 stable, dense form. In addition, they typically show lower thermal stability and lower resistance to oxidation. Therefore, the rational design of organic zeolites should be based not only upon choosing right templates and synthetic conditions but also upon appropriate molecular design and principles of crystal engineering. Most earlier organic zeolite materials were discovered by accident. Long-term studies performed by many researchers helped to established several principles useful in such design. According to these principles, the best candidates for making microporous materials are molecules that avoid close packing, molecules that already possess void space, simple or extended analogs of previously known molecular geometries and frameworks, low-density rigid frameworks, especially those that cannot create close packing by interpenetration and/or filling the internal space with other components of the host formula (bulky ligands, side groups, counterions). Several examples illustrating the implication of these principles are given in the following section. 4.2. Examples Macrocycles were found to be very useful building units [59,64-68] (Fig. 3). Metal-organic fragments I, supplemented with pyridine ligands, form a tubular structure by interdigitation of the molecules turned by 60 ~ [64] Rigid organic macrocycles II are hydrogen bonded to each other (R = OH) to form a layer and the layers are aligned to form channels; the desired mode of packing occurs because of reducing repulsive n - n interactions by the electron-withdrawing effect of the triple bonds. [65,66] More flexible octapeptides lII assemble in columns due to eight hydrogen bonds between adjacent molecules. [67] In all of the above structures, the holes of the macrocycles are combined in a channel that facilitates closer packing upon including the guest species into the channel space. R

NH2 O H CH30

o o

~176

HO

~

~ R

~

R

O

C~

N/~C

~

~H N

N

O

R

I

II

III

Fig. 3. Examples of macrocycles used to make microporous structures. Metal-organic (I), organic (II) and peptide (III) macrocycles are shown The idea of using macrocycles as building units for tubular architectures may be advanced to molecular assemblies, both cyclic and spiral (Fig. 4). The formation of cyclic fragments was observed in a microporous copper [3-diketonate I ([CuL2], L - L = CF3COCHCOC(CH3)zOCH3). [31,57,69-71] Three molecules of the complex associate with another three on the next level of the tube by means of intermolecular coordination of methoxy oxygen atoms to copper(II) centers. The channel o f - 6 A diameter is located inside the tube. Recently, zeolitic sorption behavior was confirmed for two materials having a spiral

44

(~

_--

C)

II

I

Fig. 4. The cyclic molecular assembly in [3-[CuL2] (I) and spiral molecular assembly in Ala-Val (II) that forms the channel assembly of peptide dimers. [60] In the structure of alanyl-valine II, the molecules are associated by N-H...O hydrogen bonds; each next molecule is shifted with respect to the other thus forming a spiral rather than a cycle. The spiral enwraps a channel o f - 5 A in diameter. This type of assembly was observed in a number of dipeptides [72,73] and may become one of common principles for "biozeolites", zeolite analogs made up of naturally produced and biologically compatible materials. ~

Fig. 5. "Wheel-and-axle" molecular structure of [Ni(bipy)(DBM)2]n and the packing scheme explaining the formation of permanently available void space Another geometry utilized to create microporosity in a material is a so-called "wheel-and-axle" motif (the same or related motifs are also known as "ladder-and-platform", "shish-kebab" and "bell-shaped"). The first molecules of this type exhibited a good ability to form inclusion architectures. [74-76] However, in the absence of guest, small molecules of this type may rearrange into dense modifications. The increasing stability of microporous architectures may be achieved by restricting the degree of freedom of the host units by

45 combining them into a polymeric structure. This approach yielded a new coordination polymer [Ni(bipy)(DBM)z]n (bipy = 4,4'-bipyridyl, DBM = dibenzoylmethanate) (Fig. 5) demonstrating excellent zeolitic properties. [77]

(H3C)3Si

CH 3

(H3C)3Si

I

k

--' n

II

III

,?.2,vO.,1 C H( C H2)z--C H--( CH 2)2-----I-

/

CH 2x ) ( C F22y ) O(CF 22 ) SO 3H a n

VI

.,c I/ o

t-

/Y--",\ I

IV

R = CH 3

V R = SI(CH3) 3

H3C

VII

Fig. 6. Examples of 1D polymers that form microporous materials .Other molecular geometries that prevent close packing are observed in a number of I D organic polymers. The bulk phases of these polymers, ranging from glassy to crystalline materials, reveal a remarkable zeolitic behavior. The most representative examples are shown in Fig. 6. A common feature of these molecules is the presence of bulky groups attached to the main chain that restrict the conformational freedom of the molecule and prevent close packing of the molecules in a 3D structure. Poly(trimethylsilylpropyne) I (Fig. 6) [78-80] forms amorphous glassy materials with extremely high gas permeability. Alternating double bonds give additional rigidity to the backbone and a diversity of isomeric forms due to cis-trans isomerization. Poly(vinyltrimethylsilane) II [81] has a similar but more flexible molecular structure. Syndiotactic polystyrene III [41-44,82-86] has aromatic side groups alternating on two sides of the backbone. This polymer forms crystalline materials with very high clathration and sorption ability. Inclusion and microporous modifications of the polymer reveal helical chains running parallel and leaving significant amount of cavity space. [85,86] The analogs of III, polymers IV [87] and V [88], polymeric sulfonic acids VI [89] and poly(phenyleneoxide) polymers VII [45] represent other examples of sorbent materials. It should be noted that the molecular structures of the above materials were not deliberately designed as sorbents. An example of purposefully created porous polymers is given by so-called "polymers of intrinsic porosity" (PIMs), complex structures with highly rigid and contorted molecular architectures. [90] The extension to architectures of higher dimensionality illustrates further development of the strategy. The formation of dendritic structures in so-called organic aerogels, [91] cross-linking of 1D polymers [47] and polymerization of synthetic lipids [92] are examples of methods used to create and stabilize microporosity in bulk materials. The creation of rigid frameworks is a fruitful strategy that was extensively utilized for coordination polymers. In the literature, various such architectures are called metal-organic frameworks (MOFs). The rational approach in the design of MOFs implies the use of two or more complimentary types of building units that assemble in a predictable way. In the simplest cases, the knots of a framework are metal cations that dominate in defining topology of the whole architecture due to the specific geometry of coordination sites around each metal center. The spacers connecting metal centers are ligands that dominate in defining dimensions of the framework. In general, more complicated fragments may be used as knots and spacers,

46

so-called secondary building units (SBUs). The main strategies in creation of MOF architectures as well as classifications of MOFs themselves have been extensively reviewed. [20,93-98]

I

11

III

Fig. 7. Examples of MOF topologies. MOF-5 (I), NbO type (II) and two interpenetrating NbO (III) frameworks are shown Rational design through the _creation of extended analogs was demonstrated by Yaghi and coworkers. [99,100] The MOF-5 framework I (Fig. 7) was used as prototype. The knots of the framework are constructed of oxide-centered Zn40 tetrahedra that act as octahedral SBUs for dicarboxilate ligands (doubly negatively charged OOC-R-COO units with aromatic radicals R) acting as spacers. Using ligands with R of various size and geometry, a family of 16 isoreticular (that is having the same topology) frameworks was created. The free space in crystals of these materials varies from 56 to 91% and the materials are stable enough to function as sorbents. The complexity in the application of rational principles to the design of microporous solids is illustrated by the development of MOFs with NbO topology II (Fig. 7). This architecture has large openings and voids and therefore is a desirable topology for porous frameworks. The first organic zeolite with NbO topology of MOF, reported by Gromilov et al. [101], was found by accident. This structure, discussed above in more detail (Fig. 4, I), has a NbO-type framework distorted to hexagonal. Later Eddaoudi et al. [102] deliberately designed another material with MOF of NbO-type. The structure appeared not to be stable as an empty form but survived guest exchange. The choice of NbO topology was motivated by the fact that the NbO structure is not self-dual, that is a perfect self-interpenetration, that could eliminate the available voids, is not possible. Recently Chen et al. [103] reported a material with two NbO-type MOFs interpenetrating asymmetrically that is by displacing the second net by (0.25, 0.25, 0.25) relative to the first (schematically shown in Fig. 7, III). The structure was stabilized with guest solvent molecules but did not survive when the guest was removed. Very recently Bu et al. [ 104] synthesized a new material with interpenetrating NbO nets and this material appeared to be totally stable in empty form and demonstrated organic zeolite behavior. One more example ofNbO-type MOF was reported by Burdukov et al. [105] This architecture is able to host organic molecules and is stable in empty form, but free sorption of guest is impossible because of the too tight openings between the cages. In spite of extensive research in the area, the number of reported NbO-type MOFs remains very limited; as recently reviewed, [69] the building units that could be suitable for this type of architecture mostly form layered structures.

47

4.3. The problem of structure-property relationship Structural design of microporous architectures is based on the assumption that the knowledge of molecular and crystal structure of a material is sufficient to predict its physicochemical behavior. As was illustrated in the previous section, this assumption is valid in many cases. However, there are numerous examples showing that this is not generally true. Zeolitic sorption is a quality of bulk materials. Most of today's studies give an ideal average structure for a particular single crystal. In many cases such features as microimpurities, residual guest quantities and dynamics of the fragments of structure are not or cannot be resolved. Two crystals from the same bulk lot may behave differently even if apparently they have the same structure and composition. A quite illustrative example is the blockage of pores caused either by the presence of bulky or low-volatile species, or by the defects in the host framework itself. This problem is known for such zeolites as mordenite and gmelinite. [106] Dehydrated mordenite has a two-dimensional channel system for small molecules but for larger molecules the channel system is one dimensional and may be subject to diffusion blocks produced by crystal stacking faults or by the presence of amorphous material and cations in the channels. Gases such as nitrogen and oxygen are rapidly absorbed whereas hydrocarbons such as methane and ethane are absorbed slowly. This is inconsistent with channel dimension of 6.7A. Because the main channels run parallel and are isolated from each other, the number of diffusion blocks needed to limit the absorption process is not great. Recently G6rbitz reported [107] that after removal of guest acetonitrile, single crystals of alanyl-valine acquire the ability to absorb, in a matter of seconds, large amounts of organic solvents. The structure of this organic zeolite, discussed above in more detail (Fig. 4, II), has isolated channels and so the diffusion of guest in this material is a one-dimensional process. Our own experience indicates that the sorption properties of alanyl-valine and other dipeptides strongly depend on the history of the preparation of samples. Often samples that appear microporous and empty in the single crystal XRD analysis show no sorption ability at all indicating that the pores are totally blocked. Some crystalline materials have a non-porous structure but show typical sorption behavior of microporous solids. One example is a 1D coordination polymer studied by Takamizawa and coworkers. [108] The crystal structure of the polymer reveals high-density packing without solvent molecules included or cavity space available. Nevertheless, the material readily absorbs gases following a Type I isotherm. The micropore volume in the material estimated from the nitrogen absorption curve is 0.16 cm 3 g~. Presumably, there is a structural change that occurs at an extremely low relative pressure o f - 5 x 10 -6. Other studies [19,109-113] indicate that the transformations of dense forms to microporous forms frequently occur at a low pressure of guest sorbate ("gate pressure"). With respect to real zeolites, this type of behavior is totally new. At the same time, it is common in "soft" materials [114,115] to which many organic zeolites belong; easy structural transformations are facilitated by weak interactions that link building units in a 3D architecture. The dense form of such a material is a state that is able to transform instantly into the microporous form; this state of the host component was referred to as "apohost". [ 19,112,113] The "apohost" is thus a "compressed" empty microporous form showing zeolitic properties in spite of the absence of permanent porosity. Structural analogy also may be misleading. The inclusion compounds of 13-[M(4-Methylpyridine)4(NCS)2] complexes of Ni and Zn as M with 4-Methylpyridine are isostructural but reveal dramatically different physicochemical behavior. [116] The Ni compound retains its microporous structure after removal of guest and serves as a zeolitic

48 sorbent. The Zn compound decomposes as the guest removed, due to insufficient stability of the complex. The detailed studies, which reveal the actual properties of bulk solids with MOF structure, are highly desirable for the development of new, practically useful porous materials. Such studies could help to identify the specific requirements for a sorbent with a particular function. The advantage of narrow pores over large pores for gas absorption was demonstrated by Pan and coworkers. [117] Another study suggested the incorporation of paramagnetic centers in the host framework to monitor the dynamics and disposition of guest species in the micropore space. [71] A good example of design, [118] large-scale synthesis [119] and the development of a mathematical model describing the complex sorption behavior of a material [120] was demonstrated for a copper(II) - benzene-l,3,5-tricarboxylate 3D MOF. 5. ADVANTAGES OF ORGANIC ZEOLITES

5.1. Basic advantages A short comparison between organic and traditional zeolites given in this section underlies new opportunities the new materials may present rather than weaknesses of previously known materials. It appears that each advantage may be associated with new problems, some of which were described in section 4.3. Nevertheless, the increasing interest in the new class of sorbents indicates that organic zeolites may arise as a new generation of microporous materials that will help to solve many existing and future technological tasks. The potential diversity of organic zeolites results from an unlimited number of building units that can be utilized in their creation and an unlimited number of structural motifs in which these units may be assembled. This is the most evident, and the greatest advantage, of organic zeolites over traditional zeolites. The number of known structural zeolitic types is a few hundreds and they are built of 16 identified "secondary structural units". [121] The number of structural types of confirmed organic zeolites is several dozens as of today but almost every one is made of new building units. The variability of organic zeolites implies the possibility of creating series of materials where a desired parameter may be changed in short increments over a wide range. The size of pores in available organic zeolites varies from the smallest to as large as more than two nanometers. The whole series, varying in shape, phobicity, functionality of pores and in crystallinity and chemical composition of host materials were or may be created. The ease of control over self-assembly of building units yields many organic zeolites and creates new advantages in the synthesis and processing of these materials. Such processes may be conducted under very mild conditions, in a short time and are readily controlled with pH and the presence of appropriate solvents. The possibility to induce chirality in MOFs was demonstrated in several studies. [122124] The utilization of lower peptides is another promising strategy in the development of chiral porous frameworks. [60] It should be noted that the occurrence of helical pores in inorganic materials is rare. [125] Many organic zeolites are biocompatib!.e and friendly to the environment. Materials made of natural products and peptides have the highest prospects for the chemistry of future. The flexibility of organic zeolite structures and their transient transformations from one structure to another illustrate the new advantages described below.

5.2. Structural flexibility and dynamic sorption Many organic zeolites show structural flexibility that is revealed during the sorption

49 process. In other words, these materials respond actively and individually to the presence of various guest sorbates. To distinguish such behavior from sorption in totally robust structures, it is referred to as "dynamic sorption". Typically, this type of sorption may be recognized by monitoring structural changes of the sorbent material. Sometimes this behavior is apparent as a Type VI isotherm (Fig. 1). It should be noted that flexible microporous frameworks among purely inorganic materials are possible [126] but are observed extremely rarely. In order to identify "dynamic sorption" as a new phenomenon, Kitagawa & Kondo [127] suggested distinguishing three generations of microporous materials. The first and the second generation materials show clathration and sorption, respectively (see section 2.2). The third generation materials, "dynamic structures, which change their frameworks responding to external stimuli", display the "dynamic sorption" behavior. Flexibility and microporosity are somewhat contradictory properties. Flexibility is facilitated by weaker interactions, while microporosity requires robustness and overall stability of the architecture. Various strategies were utilized to provide an appropriate balance of strong and weak interactions within a desired architecture to make it robust and flexible at the same time. Van der Waals packing of organic and metal-organic 1D polymers, macrocyclic molecules and fragments was mentioned in the section 4.2. A number of flexible microporous architectures of higher dimensionality were reported. For example, a 2.5 % unit cell volume contraction was reported, due to a scissor-like shift in interlocked layers of a 2D coordination polymer. [128] A 3 % contraction was reported for a 3D coordination polymer, due to distortion in the coordination polyhedron of Ag(I). [129] More flexible 3D architectures utilized the angular flexibility of secondary coordination bonds, to give 8.1% [130] and 8.6 % [70] changes in the unit cell volume. The highest flexibility for a crystalline microporous material was observed for the [3 form of the [Ni(4-Methylpyridine)4(NCS)2] Werner complex. This architecture is based on van der Waals packing and shows a different degree of expansion for each individual guest, with the total volume change of 14.5%. [61] An interesting example of a reverse behavior also was reported, where the host framework showed expansion upon the removal of guest. [131 ] In this case the flexibility is possible due to angular distortions occurring in the base of ZnO4N coordination square pyramid.

inclusion

o~ form

<<<<< <<<<< <<<<<

1) te m plate 9

pulse \ ~>~

,),<,<.[.>, Fig. 8. Phase interconversion scheme for a smart sorbent

form ,..~('0'~.,0~...~

50 5.3. Smart sorbents The existence of two conjugated forms for organic zeolite materials (dense and either porous or preorganized) was utilized to create switchable sorbents. [62,113,115,132] Switchable sorbents are functional materials that also may be qualified as smart materials due to their ability to sense changes in external conditions and to respond to the external stimuli with a useful response. [133] Microporosity in such materials may be switched on and off by an operator through the application of certain sequences of external stimuli. The processing scheme demonstrating the functioning of a smart sorbent is shown in Fig. 8. The operation "on" consists of two steps. Step 1: The dense (Gt) form converts into an inclusion compound in atmosphere of a suitable guest sorbate. Step 2: The inclusion compound converts into the microporous (13) form by means of guest sorbate removal. The operation "off" is accomplished through Step 3: The 13 form, which is metastable, collapses into the stable c~ form upon applying a heat pulse, an irreversible step. The application of these operations makes it possible for the operator to "activate" or "deactivate" the sorbent by transforming it to the microporous 13 form or the dense a form, respectively. All of the processes proceed in situ, quantitatively, do not include solvents, and may be controlled to the degree of switching. Future work in this direction might include the development of materials with sorption ability, inclusion selectivity and overall capacity controlled by a sequence of light, electrical or mechanical signals.

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