title: Nonequilibrium Catalyst Materials Stabilized by the Aerogel Effect: Solvent Free and Continuous Synthesis of Gamma-Alumina with Hierarchical Porosity.
abstract: Heterogeneous catalysis can be understood as a phenomenon which strongly relies on the occurrence of thermodynamically less favorable surface motifs like defects or high-energy planes. Because it is very difficult to control such parameters, an interesting approach is to explore metastable polymorphs of the respective solids. The latter is not an easy task as well because the emergence of polymorphs is dictated by kinetic control and materials with high surface area are required. Further, an inherent problem is that high temperatures required for many catalytic reactions can also induce the transformation to the thermodynamically stable modification. Alumina (Al2O3) was selected for the current study as it exists not only in the stable α-form but also as the metastable γ-polymorph. Kinetic control was realized by combining an aerosol-based synthesis approach and a highly reactive, volatile precursor (AlMe3). Monolithic flakes of Al2O3with a highly porous, hierarchical structure (micro-, meso-, and macropores connected to each other) resemble so-called aerogels, which are normally known only from wet sol-gel routes. Monolothic aerogel flakes can be separated from the gas phase without supercritical drying, which in principle allows for a continuous preparation of the materials. Process parameters can be adjusted so the material is composed exclusively of the desired γ-modification. The γ-Al2O3aerogels were much more stable than they should be, and even after extended (80 h) high-temperature (1200 °C) treatment only an insignificant part has converted to the thermodynamically stable α-phase. The latter phenomenon was assigned to the extraordinary thermal insulation properties of aerogels. Finally, the material was tested concerning the catalytic dehydration of 1-hexanol. Comparison to other Al2O3materials with the same surface area demonstrates that the γ-Al2O3are superior in activity and selectivity regarding the formation of the desired product 1-hexene.
authors.affiliation: University of Konstanz , Department of Chemistry, Universitaetstrasse 10, 78457 Konstanz, Germany.
authors.forename: Kay
authors.initials: K
authors.lastname: Hagedorn
authors.affiliation: University of Konstanz , Department of Chemistry, Universitaetstrasse 10, 78457 Konstanz, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Bahnmüller
authors.affiliation: University of Konstanz , Department of Chemistry, Universitaetstrasse 10, 78457 Konstanz, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Schachtschneider
authors.affiliation: University of Konstanz , Department of Chemistry, Universitaetstrasse 10, 78457 Konstanz, Germany.
authors.forename: Maren
authors.initials: M
authors.lastname: Frei
authors.affiliation: University of Siegen , Department of Chemistry and Biology, Adolf-Reichwein-Strasse 2, 57076 Siegen, Germany.
authors.forename: Wenyu
authors.initials: W
authors.lastname: Li
authors.affiliation: University of Siegen , Department of Chemistry and Biology, Adolf-Reichwein-Strasse 2, 57076 Siegen, Germany.
authors.forename: Jörn
authors.identifier: http://orcid.org/0000-0003-2294-796X
authors.initials: J
authors.lastname: Schmedt Auf der Günne
authors.affiliation: University of Konstanz , Department of Chemistry, Universitaetstrasse 10, 78457 Konstanz, Germany.
authors.forename: Sebastian
authors.identifier: http://orcid.org/0000-0003-1651-4906
authors.initials: S
authors.lastname: Polarz
keywords: aerosol synthesis; heterogeneous catalysis; metal oxides; metastable polymorphs; porous materials