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Materials World Network Catalyst Materials Synthesis at Accessible High Temperatures and Pressures

Christopher J. Kiely, Lehigh University, DMR 0709887

Project Outline: Complex oxide catalysts are used as heterogeneous catalysts for producing various important organic chemicals. In this project, three types of complex oxide catalysts prepared using novel preparation methods have been studied. Each of them has been evaluated for its catalytic performance, namely (i) the selective oxidation of n-butane to maleic anhydride over vanadium phosphate (V-P-O) materials; (ii) the oxidative dehydrogenation (ODH) of ethane to ethylene over niobium phosphate (Nb-P-O) materials, and (iii) the oxidation of methanol to formaldehyde over iron molybdate (Fe-Mo-O) materials. Analytical electron microscopy, X-ray diffraction and other related characterization techniques have been used to provide useful information regarding the morphology, crystallography and chemical composition of these complex oxide catalysts. The underlying aim of this work is to uncover meaningful synthesis-structure-performance relationships for these three complex catalyst systems.

Materials World Network Collaboration: Researchers from Cardiff University (UK) have prepared complex oxide catalysts by completely new synthesis routes. Lehigh University (USA) have been using state-of-the-art and electron and X-ray microscopies to analyze their micro- and nano-structures.

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Materials World Network Vanadium Phosphorus Oxide (V-P-O) Catalyst Materials

Christopher J. Kiely, Lehigh University, DMR 0709887Results: Three novel synthesis routes have been explored for producing V-P-O catalysts. The first route, involving the addition of various V-P-O ‘seeds’ during the VPD process, was found to have a profound effect on the morphology of the V-P-O precursor and on inducing certain unexpected phase transformations. The second route, namely the use of a di-block copolymer template in the VPO route, was found to generate a more crystalline VOHPO4·0.5H2O precursor with a rhomboidal morphology, which could be activated in a much shorter time period as compared to conventional V-P-O precursors. The third route involved encapsulating the fragile V-P-O rosette-type catalysts within a mechanically protective SiO2 shell. Broader Impact: V-P-O catalysts are used commercially in the manufacture of maleic anyhdride (MA) which is an important constituent in agrochemicals, lubricants, Lycra and even mouthwashes. The ability to develop new structural forms of the V-P-O materials by novel preparation methods may lead to catalysts having a higher productivity.

Images of V-P-O catalyst morphologies(Top) Rosettes from the seeding technique: (Bottom Left) Rhomdoidal platelets from the di-block polymer templating method: (Bottom Right) Rosettes in a mechanically protective porous SiO2 shell.

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Materials World Network Niobium Phosphorus Oxide (Nb-P-O) Catalyst Materials

Christopher J. Kiely, Lehigh University, DMR 0709887

Results: Three different niobium phosphate (Nb-P-O) materials, namely the Nb2P4O15, NbOPO4 and Nb1.91P2.82O12 phases, have been synthesized. Each of them was evaluated for the oxidative dehydrogenation (ODH) of ethane to ethylene and the oxidation of methanol to formaldehyde, respectively. It was found that the Nb1.91P2.82O12 phase is the most desirable structure for ethane ODH, whereas the NbOPO4 phase is more effective for methanol oxidation. The morphological and structural changes induced by both reactions on these Nb-P-O catalysts have been monitored, and correlated to the measured changes in their catalytic performance. Broader Impact: Ethylene (C2H4) is one of the most important chemical building blocks, and it ranks as the highest worldwide volume organic chemical produced. Current olefin demand is achieved commercially through steam cracking of hydrocarbons, mainly naphtha and ethane (C2H6).

Oxidative dehydrogenation (ODH) is a process that potentially offers both energy and economic advantages over traditional cracking technologies.

(Top) An SEM micrograph of an Nb-P-O catalyst showing its general morphology: (Bottom) A high angle annular dark field (HAADF) STEM image showing the atomic structure of the Nb-P-O material.

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Materials World Network Iron Molybdenum Oxide (Fe-Mo-O) Catalyst Materials

Christopher J. Kiely, Lehigh University, DMR 0709887Results: A highly active Fe2(MoO4)3/MoO3 catalyst for methanol oxidation has been prepared using a novel impregnation method. This catalyst consists of MoO3 nanorods on which epitaxial Fe2(MoO4)3 islands are supported. Based on our microstructural characterization studies, a mechanism has been proposed for the formation of this fascinating nanorod/island morphology which involves a thermally induced solid-state diffusive reaction accompanied by the Kirkendall effect. The excellent catalytic performance of this catalyst is thought to result from the synergistic behaviour between the active surface Fe2(MoO4)3 islands and the underlying MoO3 reservoir.

Broader Impact: Formaldehyde, H2C=O is one of the most important basic chemicals, and is used to produce a large number of industrial and consumer products. Large volumes of formaldehyde are employed to make resins with urea, phenol and melamine. The new iron molybdate catalyst morphology described here is highly effective for producing formaldehyde by the oxidation of methanol at low temperatures.

A representative SEM micrograph and EFTEM maps of a new type of Fe-Mo-O methanol oxidation catalyst consisting of epitaxial Fe2(MoO4)3 islands supported on MoO3 nano-rods.

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Materials World Network X-Ray Ultramicroscopy (XuM) of Particulate Materials

Christopher J. Kiely, Lehigh University, DMR 0709887

New Technique Application: X-ray ultramicroscopy (XuM) has been applied for the first time to 1-20mm V-P-O/SiO2 catalyst particles. Imaging the internal structure of particles in this size regime is usually problematical since they are too thick for TEM and SEM gives only surface morphology. We have demonstrated that XuM is an effective non-destructive technique for this solving problem and has general applicability in the field of particulate analysis..

Rosette-type vanadium phosphorus oxide (V-P-O) catalysts for maleic anhydride production have not traditionally been used in circulating reactors because of the damage incurred by mechanical attrition. By encapsulating the V-P-O rosettes in a porous SiO2 shell, a much more robust, but still very active catalyst, can be prepared. Detailed testing and microstructural characterization of these V-P-O/SiO2 catalyst materials has been carried out as part of this Materials World Network program.

A montage of low magnification SEM images (row 1), X-ray ultramicrographs (row 2) and higher magnification SEM images (row 3) of the V-P-O / SiO2 precursor materials (column 1), freshly activated catalyst (column 2) and used catalyst (column 3).