1. Porous Adsorbents and Catalysts

Microporous zeolites with large internal surfaces of up to 2000 m²/g and pore or cage diameters in the range of 0.5 to 1.2 nm, e.g. for zeolites SAPO-34, SSZ-13, ZSM-5, MCM-22, EU-1, Beta, and Y, are reaction vessels in a molecular scale and play an important role as shape-selective catalysts in research and chemical industry (see "Publications", Topic 1 of "NMR Methods for Heterogeneous Catalysis" and [1]). Substitution of framework silicon by aluminum atoms makes these materials to solid acids, which are suitable to replace polluting liquid acids as catalysts in numerous industrial processes. Due to their well-defined pore systems, these materials are also utilized as molecular sieves in separation processes. During the past decades, routes for the synthesis of mesoporous materials with pore diameters of more than 10 nm were developed. These mesoporous materials, such as MCM-41 ([2], [3], [4], [5]) and SBA-15 ([6], [7], [8], [9]), have frameworks consisting of amorphous walls and ordered mesopores in hexagonal arrangement. Since these materials have no intrinsic catalytic properties, modification of their internal surface is required for developing novel mesoporous solids, which are interesting for applications as adsorbents and catalysts. Dry-gel synthesis routes, the use of zeolite seeds in the synthesis of ordered mesoporous materials, and the introduction of metal species lead to hybrid catalysts with Broensted acidic ([4], [10], [11], [12]), Lewis acidic [5], and redox properties [13]. In further approaches, ordered mesoporous materials were utilized as supports of catalytically active compounds, such as aluminum species ([3], [14], [9]) and noble metals ([8], [15], [16]).

2. Local Structure and Properties of Surface Sites on Porous Adsorbents and Catalysts

Solid-state NMR spectroscopy (NMR: Nuclear Magnetic Resonance) is a very important analytical tool of materials science with high sensitivity for the short-range order of solids. Most of the atoms contributing to the framework of adsorbents and solid catalysts applied in chemical industry consist of isotopes with nuclear spin I. These isotopes open the way for applying solid-state NMR spectroscopy to study framework atoms and surface sites, such as oxygen coordinations, atom-atom distances, bond angles, atom densities, and atom dynamics. Two-dimensional techniques of NMR spectroscopy allow the separation of solid-state interactions and the study of their influence on resonating nuclei. An important solid-state interaction is the quadrupolar interaction of nuclei with spin I > 1/2 with the electric field gradient caused by the charge distribution in their local structure. By analysing the parameters of the quadrupolar interaction, the oxygen coordination and the location of sodium and aluminum species in zeolites are clarified ([17], [18], [19]). By analysis of ¹H NMR broad-line shapes or intensity distributions of ¹H MAS NMR sideband patterns, and by TRAPDOR and REAPDOR experiments, the dipolar coupling between neighbouring atoms is evaluated and atom-atom distances are determined ([20], [21], [22], [23]). In contrast to diffraction techniques, which are suitable for determining the long-range order of crystalline materials, solid-state NMR spectroscopy can also be applied for the investigation of amorphous materials ([24], [25]). A very important topic is the characterization of the chemical properties of catalytically active sites inside the pores and at the outer surface of solids. For this purpose, various methods for the solid-state NMR investigation of surface sites on adsorbents and catalysts without and with adsorption of probe molecules were developed and improved (see "Publications", Topics 2 to 4 of "NMR Methods for Heterogeneous Catalysis" and [26], [27], [28]). The spectroscopic results of these investigations on Broensted and Lewis acidic surface sites as well as supported noble metals are utilized for improving our understanding of the chemical properties of the materials under study in their specific applications.

3. In Situ Solid-State NMR Spectroscopy of Working Catalysts

In situ solid-state NMR techniques are suitable for the investigation of working solid catalysts under batch, semi-batch, stopped-flow, pulsed-flow, and continuous flow conditions (see "Publications", Topic 5 of "NMR Methods for Heterogeneous Catalysis" and [29]). By application of these in situ techniques, catalytically active sites, adsorbate complexes, intermediates, and coke deposits on the surface of working solid catalysts are studied and reaction mechanisms and deactivation processes occurring under industrially relevant conditions are elucidated [30]. Examples are investigations of the methyl tertiary-butyl ether synthesis ([31], [32]) and the conversion of methanol to dimethyl ether and olefins ([33], [34]) on acidic zeolite catalysts, the alkylation of toluene ([35], [36]), aniline ([37], [38]), and phenol [39] on basic and acidic zeolites, and the hydrogenation of olefins [40] on noble metal containing solid catalysts. In situ ¹³C MAS NMR spectroscopy of the conversion of methanol on zeolite H-ZSM-5 and on the crystalline silicoaluminophosphates SAPO-18 and SAPO-34 under continuous-flow conditions support the formation of a catalytically active pool of hydrocarbons in the catalyst pores and cages, which consists of a mixture of branched aliphatic and aromatic compounds [41]. After purging the spent MTO catalysts with pure carrier gas, polymethylaromatics were preferentially found in the pores of these zeolites. These approaches give direct experimental evidence for the hydrocarbon-pool mechanism discussed in literature [42]. According to this mechanism, carbonaceous species exist inside the zeolite pores and cages, which add reactants and split off reaction products, such as light olefins [43].

References

Michael Hunger / Stuttgart, Germany
home