Ionic liquid platform for the preparation of catalytically active porous cellulose–TiO2 membranes and spheres

Alexandra Wittmar, Chair for Technical Chemistry II, University of Duisburg-Essen, CENIDE – Center for Nanointegration Duisburg-Essen, NETZ – NanoEnergieTechnikZentrum, Essen, Germany
Mathias Ulbricht, Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen and CENIDE – Center for Nanointegration Duisburg-Essen, NETZ – NanoEnergieTechnikZentrum, Essen, Germany

Processing of cellulose (renewable material) with ionic liquids (green solvents) and functional nanoparticles like metal oxides (TiO2, ZnO) may open new rational and environmentally friendly pathways towards a new generation of porous nanocomposites which may find direct applications in water purification, catalysis and self-cleaning materials [1]. The advanced control of the non-solvent induced phase separation process for the generation of the desired mesoporous polymer structure as well as the homogenous distribution and accessibility of the nanoparticles within the matrix are necessary conditions for the functionality of the resulting nanocomposites.

Porous flat sheets (membranes) and spheres from cellulose doped with commercial metal oxide nanoparticles have been prepared by non-solvent induced phase separation from polymer solutions in ionic liquids. The work evaluates the possibilities to control the membrane structure formation during the phase separation from polymer solution through the addition of a polar organic co-solvent and during the drying step. The complex relation between the catalytic activity of the TiO2-cellulose nanocomposites with different geometries and obtained in different processing conditions has been studied, in relation with their structure, during the photodegradation of model organic dyes like rhodamine B and methylene blue. It was observed that the collapse of the pores during the drying under ambient conditions is responsible for a drastic reduction of the membrane catalytic activity. For the uncollapsed membranes, the addition of a polar co-solvent has a minimal influence both on the membranes structure and on their corresponding photocatalytic activities.

Shaping of the porous material as a flat sheet (membrane) was observed to confer it a significantly better catalytic activity, which can be explained by the fact that a larger fraction of nanoparticles do not receive UV irradiation in the case of the spheres. However, the porous nanocomposite spheres may have great potential as intermediates for the preparation of carbon based porous composites by appropriate stabilization and carbonization processes. The increase of the TiO2 concentration in the membrane was observed to result in a significant improvement of the photocatalytic activity.

[1] A. Wittmar, H. Thierfeld, S. Köcher, M. Ulbricht, RSC Advances 2015, 5, 35866.

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