Structural And Chemical Properties Of Micro And Mesoporous Materials

Crystalline microporous materials are characterized by structurally well-defined voids, whose diameters exceed typical atomic dimensions. The fact that a molecule has to be able to access these pores puts a lower limit to the pore diameter of approximately 0.25 nm. If these voids are below 2 nm in diameter the material is called microporous, above mesoporous (with an upper limit of approximately 50 nm). It should be noted that in mesoporous materials the pores are also periodically arranged; however, the confining material is typically amorphous.

Particles Gold Example

Fig. 1 Examples for zeolite building blocks and one-, two-, and three-dimensional zeolite structures (T atoms are shown in orange, O atoms in red).

The crystalline materials may be of an inorganic or organic nature. Typical examples are zeolites and related tectosilicates for the inorganic materials[7] or cyclodextrine-type materials[8-10] and metal-organic frameworks.[11-13] Throughout this review, zeotypes (zeolites or aluminum phosphates) will be used as the examples to describe the interactions and catalytic actions. However, when the varying chemistry of the framework materials is kept in mind, the results can be extended to wide variations with respect to metal oxides/phosphates/sulfides or to (metal) organic entities.[14] Common to zeolites and related materials is the principle of structures built by corner sharing TO4 tetrahedra or in some cases octahedra. T indicates the tetrahedrally coordinated cations such as Si4+ and Al3+ for zeolites or Al3+ and P5+ for aluminophos-phates. These building blocks form a three-dimensional structure including a one-, two-, or three-dimensional pore network (see Fig. 1 for examples of zeolite building blocks and one-, two-, and three-dimensional zeolite structures). In some materials larger cages are formed at the intersections of the pores. Currently, about 140 framework types are known. To each framework type a three-letter code (Framework-Type Code) is assigned by the Structure Commission of the International Zeolite Association.^15 The framework type of a zeolite describes the connectivity of the framework tetrahedral atoms in the highest possible symmetry without reference to chemical composition and, therefore, defines the topology of the material with respect to size and shape of the pore openings, the dimensionality of the channels system, the volume and arrangement of the cages, and the type of cation sites available.[1S|

Crystalline molecular sieves are often characterized by their smallest pore opening named after the number of T-atoms in the rings defining the size of the pore. A comparison between the diameters of the pore openings and the size of typical reactant molecules is given in Fig. 2. Small pore zeolites, such as Zeolite A (LTA), Sodalite (SOD), and Zeolite P have pores build-up by 8-membered rings (8 MR pores). Medium pore zeolites, such as ZSM-5 (MFI), have minimum pore openings defined by 10-membered rings, while the pore openings of large pore zeolites, e.g., Zeolite Y (FAU), Mordenite (MOR), and Beta (BEA), are defined by 12-membered ring pores. Materials with pores of a larger minimum diameter are called ultralarge pores materials with 20 T-atom pores being the accepted upper limit. The materials with the largest pores are VPI-5 (18-MR pores)[17] and JDF-20 (20 MR pores).[18] Note that both materials are alumino-phosphates. For siliceous materials UDT-1 (14-MR

Fig. 2 Comparison between the diameters of the pore openings and the size of typical reactant molecules.

What Are The Chemical Properties Gold
Fig. 3 Possibilities of tailoring the zeolite by replacing framework Si4+ or Al3+ with different cations, i.e., P5+.

pore)[19] and CIT-5 (14 MR pores)[20] were among the materials with the largest pore sizes described.

The chemical composition determines the specific properties of microporous molecular sieves. Purely siliceous or aluminum phosphate molecular sieves are chemically inert. Chemical functionality and charge is introduced into such frameworks by substitution of cations with valences differing on the average from the value of four. Thus Al3+ as a T-atom in a silicate framework causes a negative framework charge, which has to be balanced by a proton (forming a hydroxy group, Br0nsted acid site) or by a metal cation (forming a Lewis acid site).[21-23] The bare, negatively charged tetrahedron is the corresponding base anion. The differences in the charge of these tetrahedra determine the theoretically highest concentration of acid and base sites of the material. The possibilities of tailoring molecular sieves by replacing framework Si4+ or Al3+ with different cations, such as P5+, are schematically shown in Fig. 3. Note that only cation exchange molecular sieves appear existing. For detailed information the reader is referred to excellent reviews.[16,24,25]

The chemical composition of the framework determines the polarity of the lattice and, in consequence, the acid-base properties of the material.[21,26-28] Because of the instability of Al-O-Al bonds (proximity of two positively charged atoms), only half of all silicon atoms can be replaced by aluminum in materials based on silica (lowest Si/Al ratio possible of 1). This has been expressed in the Loewenstein rule.[29] Consequently, each Al-O tetrahedron is surrounded by four Si-O tetrahedra. Adjacent Al-O tetrahedra are referred to as next nearest neighbors (NNN). Various models, reviewed by Barthomeuf,[27] relate the acid strength of Br0nsted sites to the concentration of NNN Al atoms. The strongest acid sites are observed in the absence of NNN Al-O tetrahedra. The site strength decreases as the concentration of NNN

Al-O tetrahedra increases. Thus, in addition to longrange effects of the chemical composition, experiments and theoretical calculations[30] show that short-range effects influence the strength of acid sites. Additionally, more subtle factors such as the crystal structure leading to various angles between the tetrahedra (T-O-T bonding angle) and the nature of the metal cations at exchange sites can influence the strength of acid sites.[28,31,32]

Adjusting the combination of chemical composition, (partial) cation exchange, and the molecular sieve structure can lead to a wide variety of chemical properties. Depending upon the charge on the metal cation/proton and the oxygen, the acidic or basic properties of the molecular sieves will dominate and, consequently, the materials will act as a solid acid or base.[33]

Acid/base sites generated by substitution in crystalline materials are generally more stable than sites in mesopo-rous materials with amorphous walls, because the latter material does not face constraints in the relaxation of bonds.[34,35] In contrast to the crystalline materials, mesoporous materials have pores generated by the condensation of (X-ray amorphous) oxides[36] around medium and high molecular weight surfactants.[37] After forming a coherent structure, the surfactants are removed and the regular void structure with pore diameters between 20 and 100 nm remains. The large pores possess many hydroxyl groups,[38] which are well suitable for anchoring functional groups or metal-organic com-plexes.[39] The most prominent members of the family are the MCM41S-type materials (e.g., MCM-41, MCM-48) with hexagonally or cubic structures[40] and materials such as SBA15 synthesized using non-ionic tri-block copolymers.[41,42] For postsynthetic modifications these materials are frequently used in their pure silica form.[43-45] Recently, mesoporous materials have been synthesized with fractions of crystalline materials as building blocks as part of the wall structure[46] or as dela-minated and pillared zeolite entities.[47,48] These materials are able to combine the local structure of crystalline molecular sieve materials and, hence, the presence of stable and strong Br0nsted acid sites with the accessibility for large molecules. The best examples for this are related to delaminated structures of MWW- or FER-type materials. When oxide pillars are used to stabilize the delami-nated layers, widely differing chemical functions can be combined in one material, i.e., acidic and basic sites or acidic and redox sites.[49]

FUNCTIONAL PROPERTIES Bronsted Acid Sites

As described above the strength of Br0nsted acid sites varies markedly with the local and overall environment, i.e., the chemical composition and the structure of the zeolite.[27,28] While the lengths of Al-O and Si-O bonds and the corresponding Al-O-Si and Si-O-Si angles[34] exert a subtle influence, the strongest sites are found in the absence of Al NNN atoms. For these materials Br0nsted acid sites appear to be identical in strength and catalytic activity. This has been shown for proto-lytic cracking of n-hexane[50] and the isomerization of m-xylene[51] both being monomolecular reactions under the chosen reaction conditions. For n-hexane and n-hexene cracking this equivalency was shown also for materials with varying aluminum concentration.[50]

Lewis Acid Sites

Lewis acid sites are electron pair acceptor sites formed by accessible metal cations stemming from lattice defects, oxide nanoparticles formed by lattice degradation, and cation exchange. The strength of Lewis acid sites is proportional to the ratio between the charge of the metal cation and its size,[52] but it can be reduced by limited accessibility (inability to assume minimum bond distance). Lewis acid sites formed by metal cation exchange are weak to moderately strong, because many of the exchangeable cations have a low electronegativity (e.g., alkali or alkali earth metals) or are relatively large (e.g., La3+). Together with the adjacent framework oxygen atoms they act as Lewis acid/base pair and may polarize bonds in reacting molecules.

Lewis acid sites originating from lattice defects or by degradations of the lattice are part of alumina or silica/alumina nanoparticles formed by the extraction of aluminum from the lattice (Fig. 4), which can be charged or neutral.[53,54] Al3+ exists in octahedral and tetrahedral coordination and will have stronger Lewis acid sites than exchangeable metal cations.[26] The extra-framework alumina species (EFAL) are typically extracted from the zeolite lattice by steam treatment at higher temperatures and either block active sites by substituting for exchangeable cations, enhance the acidity by interacting with a Br0nsted acid site, or block the access to micropores by forming voluminous oligomeric species.[55-58]

A third type of Lewis acid site has been recently described as Al3+ reversibly and partially extracted from the lattice using a base molecule such as ammonia.[59,60] The Al3+ cation assumes an octahedral coordination under such conditions. The reversibility is broken, however, once Al-O-Al bonds are formed, i.e., when the aluminum oxide species are completely detached from the lattice. Detailed information on such sites is not available, but it can be speculated that the drastic enhancement of catalytic activity for acid-catalyzed hydrocarbon conversion after mild steaming is associated with such reversible detached Lewis acid sites.[61]

It should be noted here that the interaction of the Lewis acid sites with electron pair donor molecules is very strong. The presence of the acid-base pairs allows polarizing molecules, which can potentially enhance their reactivity.[62,63] Lewis acid sites also act as hydride or anion receptors in a variety of reactions. Their presence seems also to stimulate dehydrogenation of alkanes to some degree.

Basic Sites

Basic sites are proton accepting or electron pair donating oxygen atoms of the lattice. The strength is proportional to the negative charge on the oxygen. To a first approximation, this negative charge can be estimated with the help of the intermediate electronegativity using the Sanderson electronegativity principle^1 or the electronegativity equalization method (EEM).[30] It varies inversely to the intermediate electronegativity, i.e., materials with lower intermediate electronegativity and higher polarity show higher base strength.[65,66] The oxygen 1s binding energy can be used to confirm this trend experimentally.^67-1 Like the acid strength discussed above the base strength of the oxygen atoms in the lattice also depends subtly on T-O-T bond angles and on the local chemical environment (e.g., oxygen atoms in AlO4 tetrahedra are more basic than in SiO4 tetrahedra).[22,68] Additionally, the type of cation influences the polarity of the lat-tice.[69] Even when all cation exchange sites are exchanged with alkali cations, the overall chemical composition allows for the SiO2-based materials only

Chemical Properties Gold
Fig. 4 Hydrothermal extraction of aluminum from the zeolite lattice.

a weak base strength. Stronger base sites are therefore linked to clusters of alkaline or alkali-earth oxide hydroxides in the zeolite pores.[70,71] Very strong basic sites can be created by supporting metallic sodium in the zeolite pores,[25] which interact with framework oxygen atoms or react with trace humidity. Loading zeolites with alkali metal oxides utilizes the micropor-ous material primarily as support.[72] Such materials show high activity in typical base-catalyzed reactions such as alcohol dehydrogenation or toluene side chain alkylation.[73] However, the low rates of base-catalyzed reactions make the materials always very susceptible to traces of catalytically more active protons, which then dramatically change the chemical reactivity.[73]

Redox Sites

Redox sites can be introduced into micro- and mesoporous materials by isomorphous substitution of metal ions into framework positions, ion exchange with metal cations, and grafting metal complexes onto the surface of (mesoporous) molecular sieves. All three options lead to materials with unique properties that may be tailored for specific uses. While the first approach comes closest to mimicking enzyme catalysis, the ion exchanged materials are excellent redox catalysts for gas-phase reactions in environmental applications and the third group brings the wide variety of the metal-organic chemistry to mesoporous materials. For excellent reviews on the materials, see Refs.[43,74-77].

Examples for the first group of materials include Ti-silicalite (TS1-1),[78-80] vanadium silicates with MFI and MEL structure,[81] iron-containing MFI zeolites,[82] and Co and Cr incorporated into the framework of aluminum phosphate with AFI and AEL structure.[83] The uniqueness of these materials lies in the fact that isolated, identical, and stable sites are formed in a matrix that allows the proper adsorption of the reac-tants and provides sufficient space for the chemical transformations (with steric constraints of the molecular sieve directing the reaction). Most importantly, however, the materials can be tailored to have the proper hydrophobic/hydrophilic properties.[84]

Ti-silicalite (TS1-1) is an excellent catalyst for the oxidation of small molecules with H2O2 and was the first commercially used zeolite-based partial oxidation catalyst. Experimental[78-80] and theoretical stu-dies[85,86] suggest that the active sites in TS-1 are isolated and uniformly distributed Ti(IV) atoms in tetrahedral coordination, where each Ti(IV) is separated from other titanyl groups by at least two Si-O tetrahedra. The isolation of the Ti4+ centers is required to obtain high selectivities (low rate of H2O2 decomposition). Next to incorporation into the MFI or MEL structure, incorporation of Ti has also been reported, e.g., for BEA (Ti-Beta), MTW (Ti-ZSM-12), and MWW (Ti-MCM-22).[87-89]

Transition metal cation exchanged materials have seen enormous interest as potential materials for reduction of NOx with hydrocarbons.[77,90-92] Most of the materials investigated are based on the MFI structure, and Cu, Co, and Fe are the most studied materi-als.[93] Such materials cannot be used in liquid-phase reactions as the metal cations at exchange places would be leached. Iron containing ZSM-5 materials are especially remarkable as their catalytic activity differs markedly from that of bulk Fe2O3.[82] Fe-ZSM-5 is unable to activate O2, but shows high activity for the decomposition of N2O[94] and utilizes the oxygen for selective gas-phase oxidation steps. Similar to the microporous Fe-ZSM-5, mesoporous iron-containing silicas with MCM-41-type structure[95,96] can be used to oxidize large hydrocarbons to oxygenates with N2O.

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