Rückblick GEFTA Jahrestagung 1999

Vanadium-phosphorous-oxides are well known catalysts for the selective oxidation of n-butane to maleic anhydride. During the oxidation of n-butane in air/n-butane an equilibrium oxidation state of vanadium is reached depending on the redox properties of the catalyst. Only a few surface layers are reduced and reoxidised during the reaction, which proceeds according to a Mars & van Krevelen mechanism. The rate of the reoxidation step is an important parameter, which is supposed to depend on the morphology of the activated catalyst.

We studied in more details the relationship between the morphology and the oxidation state of various catalytic materials. Thermal analysis was proven to be a useful method for this study because only small amounts of catalyst are needed and information on oxidation state and oxidation/reduction of the phases can be obtained.

Activity tests were made in laboratory scale reactors on 0.5 to 2.0g samples. The catalysts were prepared from precursors containing mainly V⁴⁺-phases. The precursors were transformed into the active phase, (VO)₂P₂O₇, plus some V⁵⁺-containing phases by calcination in the test reactor in n-butane/air/N₂ mixtures at temperatures around 400°C for different lengths of time. The type of precursor and the activation procedure determine the activity and the selectivity of the catalyst, which was tested at 400°C immediately after the activation procedure.

Examination of catalytic material after microreactor test yields important information in the development of a more active catalyst. After test the catalyst was cooled in N₂ to maintain its active state. XRD and SEM were used for studying phases and morphology. Surface area was determined by N₂ BET. Simultaneous TG/DSC was used for determining the oxidation state by heating the sample in air to 825°C.

The onset of the reoxidation reaction varies between 500 and 600°C depending on the type of catalyst. At 700°C the sample is fully oxidised to VOPO₄. From 760°C the thermal reduction of vanadium begins and the sample melts at ~800°C. The oxidation state of V was calculated from the mass increase between 450°C and 725°C. Two groups of oxidation states were found: One group below 4.2 and one group around 4.5. The second group is moreover more easily oxidised in the STA.

The line broadening of some XRD reflections revealed that two groups of catalyst morphology were obtained, having different crystallite size in different directions. Some were platelike with (100) being the dominant crystal plane. Others were prismatic having no dominant crystal habits. The prismatic catalysts showed the highest oxidation states of V and the highest rate of reoxidation. This confirms that oxidation ability and thus the catalytic activity strongly depends on the morphology of the (VO)₂P₂O₇ crystals.

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