Hydrothermal Synthesis of Molybdenum Based Oxides for The

Hydrothermal synthesis of molybdenum based oxides for the application in catalysis Zur Erlangung des akademischen Grades eines DOKTORS DER NATURWISSENSCHAFTEN (Dr. rer. nat.) Fakultät für Chemie und Biowissenschaften Karlsruher Institut für Technologie (KIT) - Universitätsbereich genehmigte DISSERTATION von Dipl.-Ing. (FH) Kirsten Schuh aus Mainz Dekan: Prof. Dr. Peter Roesky Referent: Prof. Dr. Jan-Dierk Grunwaldt Korreferent: Prof. Dr. Anker Degn Jensen Tag der mündlichen Prüfung: 17. April 2014 Acknowledgements Acknowledgements I owe many thanks to a lot of people who have helped, supported and encouraged me during my doctoral studies, not just scientifically but also personally. First I would like to thank my supervisor Prof. Dr. Jan-Dierk Grunwaldt for the opportunity to complete my doctoral studies in his group and for providing me with a very interesting and diversified topic. I am grateful for the scientific freedom he gave me, the possibility to spend several months at the Technical University of Denmark as well as University of Zurich and for the opportunity to attend international conferences. I am grateful to Dr. Wolfgang Kleist for his scientific help especially with presentations and publications making the manuscripts reader friendly. I would also like to thank Prof. Dr. Anker Degn Jensen for agreeing to be my co- supervisor, for very helpful corrections and suggestions of abstracts, manuscripts and presentations and for giving me the opportunity to spend four months in his group at the Technical University of Denmark (DTU), where I felt very welcome. I am especially grateful for the help of Dr. Martin Høj, who put the selective oxidation set- up at DTU into operation, tested several of my samples for selective oxidation of propylene and performed TEM measurements of my FSP samples. I would like to thank him for the fruitful discussions and for making the start at DTU very easy and the time in Denmark such a positive experience. I also would like to thank Maya, Christina, Amalie, Xiao, Tanja and Christel for the good time in Copenhagen / Lyngby. I would like to thank Prof. Dr. Greta R. Patzke for the possibility to stay one month in her group at the University of Zurich, for getting me started with hydrothermal synthesis and for helpful suggestions especially relating inorganic chemistry. I am grateful to Dr. Franziska Conrad, Dr. Ying Zhou, Min Sheng, Roman Olivier Kontic and Debora Ressnig for their help during my time at University of Zurich and for making it such a good experience. I am very grateful to Dr. Michael Brorson at Haldor Topsøe A/S for the hydrotreating activity measurements, TEM measurements of the spent catalysts and for the scientific Hydrothermal synthesis of Mo based catalysts I Acknowledgements input. I would like to thank Dr. Pablo Beato at Haldor Topsøe A/S for the possibility to measure Raman spectroscopy, for his interest in my study, for discussing my results and for directing me to literature. I would also like to thank Randi Hansen for Raman spectroscopy measurements. I am very grateful to Vanessa Trouillet at the IAM-ESS at KIT who performed and analyzed the XPS measurements and discussed the results with me. I kindly acknowledge Hermann Köhler at IKFT at KIT for the ICP-OES measurements and Dr. Thomas Bergfeldt at the Institute for Applied Materials in the Chemical Analysis group (IAM- AWP) at KIT for the quantitative nitrogen analysis. The European Synchrotron Radiation Facility (ESRF) in Grenoble is acknowledged for providing XAS beamtime at beamline BM01B. I thank Dr. Dmitry Doronkin and Dr. Hudson Carvalho for the XAS measurements and Dr. Wouter van Beek for help and support during the beamtime. I gratefully acknowledge ANKA at KIT for providing PDIFF beamtime, HASYLAB at DESY in Hamburg for providing XAS beamtime and the Center for Electron Nanoscopy (CEN) at DTU for providing microscopy time (SEM and TEM) as well as the help and support during the measurements. I gratefully acknowledge the Danish Council for Strategic Research for financial support in the framework of the DSF proposal “Nanoparticle synthesis for catalysis” and the Karlsruhe House of Young Scientist (KHYS) for financial support during my stay in Denmark (foreign exchange scholarship). I would like to thank Angela Beilmann for performing most of the BET measurements and for good collaboration during my time at KIT. I also owe special thanks to Hans Weickenmeier who keeps everything in building 11.23 running and therefore made working so much easier. I would like to thank Jan Pesek for advices and help with technical problems. During my PhD I met many inspiring people and nice colleagues and definitively made some new friends. I would like to thank Dr. Changbo Lu for the good times sharing an office, for interesting conversations and for making the first year at KIT less lonely. Thanks to Dr. Maria Casapu, who listened to me when I needed it and for the entertaining nightshifts at ANKA. I would like to thank Dr. Henning Lichtenberg for reading some sections of this thesis, for diverting dinners and for the good times in Karlsruhe. Special thanks goes to “the girls” Marina, Melanie, Denise, Karin, Gülperi and Meike for entertaining weekend trips, fun nights out, amusing conversations and for being there. Nice II Hydrothermal synthesis of Mo based catalysts Acknowledgements colleagues and friends make rainy days less grey… It was a pleasure to work in this group and I am grateful for the good work atmosphere. I would like to express my gratitude to my parents, who believed in me, who let me follow my own path and always supported me, no matter what. I am very lucky to have my sister Friederike, my brother Manuel and my friends who listen to me, support me but can also give honest feedback. Family and friends make life so much easier and more enjoyable. Hydrothermal synthesis of Mo based catalysts III Abstract Abstract Shape and structure of a material strongly influences some of its physical and chemical properties and also plays an important role in catalysis. Hence, strong research effort has recently been spent on the preparation of advanced materials with controlled size, shape and composition. Molybdenum oxide based materials find a large variety of applications in catalysis. Hydrothermal synthesis is a typical soft chemistry method giving access to advanced metal oxides with high crystallinity, controlled morphology and good reproducibility. Flame spray pyrolysis is a one-step method leading to homogeneous nanoparticles which are non-porous, have a defined crystallinity and typically a high surface area. In the present work hydrothermal synthesis was applied to prepare MoO3 and mixed molybdenum oxides for the application in catalysis. The influence of the synthesis parameters on the product properties and their catalytic activity was studied in detail. Additionally, flame spray pyrolysis was applied for comparison. The synthesized materials were used for selective oxidation of propylene, hydrodesulfurization and related hydrotreating reactions as target reactions. Orthorhombic molybdenum trioxide (α-MoO3) with various morphologies has been successfully synthesized under hydrothermal conditions using MoO3 ∙ 2H2O and ammonium heptamolybdate as molybdenum precursors, varying the pH value and adding different acids (nitric or acetic acid) to the initial solution. The ammonium containing molybdenumoxide-phase resulting from ammonium heptamolybdate and nitric acid at pH = 1 – 2 was completely transformed into α-MoO3 after calcination at 550 °C. Direct hydrothermal synthesis of α-MoO3-rods from MoO3 ∙ 2H2O was achieved in the presence of acetic or nitric acid and from ammonium heptamolybdate with nitric acid at low pH values at 180 °C. After calcination all samples consisted of α-MoO3. By applying nitric acid during synthesis, the rod-like morphology of the samples could be stabilized during calcination and catalytic activity tests, which was beneficial for the catalytic performance in propylene oxidation. Those samples, which retained their rod-like morphology during the activity tests, yielded the highest propylene conversion, probably due to the large exposure of the (100) facets. IV Hydrothermal synthesis of Mo base catalysts Abstract Although MoO3 is an easy model system, correlation of the particle morphology and the catalytic activity and selectivity in propylene oxidation already requires a detailed characterization of the materials before and after the catalytic test reaction. Extension of the preparation method to mixed transition metal molybdates resulted in a large variety of phases and thus direct correlation between the morphology and the catalytic activity was not possible. Hydrothermal synthesis and flame spray pyrolysis have been used to prepare bismuth molybdate catalysts for the selective oxidation of propylene to acrolein. Application of a high Bi/Mo ratio during hydrothermal synthesis afforded γ-Bi2MoO6 as the main phase, whereas lower initial bismuth contents led to the formation of α- Bi2Mo3O12. The product phase was strongly influenced by the pH value of the initial solution using Bi/Mo = 1:1 but formation of β-Bi2Mo2O9 could not be detected. At low pH values α-Bi2Mo3O12 was the dominant phase, whereas at high pH values γ-Bi2MoO6 was formed. All samples displayed a plate-like morphology but their individual aspect ratios varied with the reaction conditions. Generally, the catalytic performance in propylene oxidation of the samples decreased notably after calcination at 550 °C. The use of nitric acid during hydrothermal synthesis enhanced both propylene conversion and acrolein yield, possibly due to a change in morphology. Flame spray pyrolysis led to phase pure bismuth molybdates with a relatively high surface area, which, however, did not exert a positive influence on the catalytic performance of these samples. In contrast to hydrothermal synthesis, which led to a phase mixture of α- and γ-bismuth molybdate, flame spray pyrolysis provided a single step access to β-Bi2Mo2O9 without thermal post- treatment. This sample showed high catalytic activity in the oxidation of propylene to acrolein at temperatures up to 400 °C.

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