Martin Dieterle

Thermally and Chemically Induced Structural Transformations of Keggin-Type Heteropoly Acid Catalysts

Abstract Raman characterization revealed that the Keggin anion structure of H 4 PVMo 11 O 40 is inherently unstable upon heat treatment and loss of water. Vanadyl and molybdenyl species are expelled from the Keggin cage and defective Keggin structures are formed. These defective structures further disintegrate to presumably Mo 3 O 13 triads of the former Keggin. These Keggin fragments oligomerize at later stages to molybdenum oxygen clusters comparable to hepta- or octamolybdates. The final disintegration and structural reorganization product is MoO 3 . This disintegration and recondensation process seems to be strongly affected by the heating rate and hence the presence of water in the sample. Only partial expulsion of V occurred under moderate dehydration conditions. The absence of water during heat treatments stabilizes the intermediate defective structures. Raman spectroscopy proved that free polyacids are unstable under catalytic partial oxidation conditions. Therefore, it can be suggested that intact Keggin anions are not the active species within an operating partial oxidation catalyst. From this Raman spectroscopy study it may be inferred that the structurally reorganized intermediates are relevant for the catalytic action. The Raman investigations of the HPA decomposition additionally revealed a dependency of the decomposition process on the reactive atmosphere and the presence of Cs. The presence of Cs led to a partial stabilization of the structural disintegration products of PVMo 11 and to the formation of the thermodynamically stable, but catalytically inactive Cs 3 -salt. Cs also inhibited the condensation of MoO 3 -type oxides. O 2 present in the gas phase also led to stabilization of the structural reorganization intermediates. Importantly, the presence of water did not lead to a stabilization of the intact Keggin structure. In contrast, hydrolysis of the Keggin anions seemed to be enhanced compared to the water-free situation. This observation is of high importance because water is added to the feed in industrial partial oxidation reactions. Hence, under industrial conditions, HPA-derived catalysts are inherently unstable and cannot contain intact Keggin anions at their active surface. Catalytic partial oxidation conditions even led to a more pronounced structural reorganization and amorphous suboxides of the MoO 3− x type seemed to be formed. Hence, heteropolyacids have to be understood only as defined molecular precursor compound.

Mixed molybdenum oxide based partial oxidation catalyst 2. Combined X-ray diffraction, electron microscopy and Raman investigation of the phase stability of (MoVW)5O14-type oxides

Thermal activation of a nanocrystalline Mo5O14-type Mo0.64V0.25W0.09Ox catalyst leads to enhanced catalytic performance in the partial oxidation of methanol, propylene and acrolein. This thermal activation process was invest igated by X-ray diffraction, transmission electron microscopy and Raman microspectroscopy. Ther mal activation of the nanocrystalline Mo0.64V0.25W0.09Ox precursor oxide in inert atmospheres induces partial crystallization of a Mo5O14-type oxide only in a narrow temperature range ending at 818 K. The Raman spectrum of the crystalline Mo5O14 oxide was identified by statistical analysis and by comparison with XRD and TEM results. The observed Raman bands in the M=O stretching mode regime were attributed to the different Mo=O bond distances in Mo 5O15. A fraction of the precursor oxide remains nanocrystalline after activation as shown by Raman spectroscopy. HRTEM identified amorphous surface layers on top crystalline cores. Above 818 K, the Mo5O14-type structure disproportionates into the stable phases MoO 2 and MoO3. This disproportionation occurs via an intermediate state which is formed by bundles of molybdenum oxide chains exhibiting structural order in only one dimension as shown by HRTEM. These results from the combined structural analysis suggest that the improvement of the catalytic performance of the MoVW oxide catalyst in the partial oxidation of methanol is related to the formation of the Mo5O14 type mixed oxide.

Molybdenum oxide based partial oxidation catalyst: 1. Thermally induced oxygen deficiency, elemental and structural heterogeneity and the relation to catalytic performance

Abstract A mixed oxide catalyst containing Mo, V, and W was used for the partial oxidation of methanol. The relation between the structure and the degree of reduction of this mixed oxide catalyst and its catalytic performance was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Rutherford backscattering (RBS), XPS, ion scattering spectroscopy (ISS), UPS, X-ray diffraction (XRD) and Raman microspectroscopy. Thermal activation of the MoVW mixed oxide led to an increase in the methanol conversion by a factor of 3 and an increase in selectivity to formaldehyde from 66% to 80%. SEM–EDX characterization of the untreated catalyst revealed the presence of at least two different phases in the sample on a micron range, one with a high V concentration, and another with all three metals present. TEM–EDX showed a homogeneous element distribution in the submicron regime. The thermally activated mixed oxide revealed an inhomogeneous element distribution in the micron and submicron regime as shown by SEM–EDX and TEM–EDX. The activation led to a reduction of the bulk oxide as determined by RBS and its surface as shown by XPS, ISS, and UPS. The formation of Mo 4+ and V 4+ centers was detected at the catalyst surface upon thermal activation. XRD of the starting material showed the presence of nanocrystalline material which was identified as being a mixture of a majority of Mo 5 O 14 and a minority of MoO 3 -type MoVW oxides. Confocal Raman microspectroscopy confirmed the presence of two different components. The major component could be identified as amorphous Mo 5 O 14 -type MoVW mixed oxide. The second, minor component was similar to an amorphous MoO 3 -type MoVW oxide. XRD showed that the thermally activated mixed oxide consisted of a mixture of a majority of crystalline Mo 5 O 14 -type oxide and of small amounts of crystalline MoO 3 -type and MoO 2 -type oxides. The Raman spectrum of the Mo 5 O 14 -type phase could be identified by statistical data evaluation of 1000 spectra and by comparison with the XRD result. Raman microscopy confirmed the presence of a minority of MoO 3 - and MoO 2 -type oxide. The formation of Mo 5 O 14 -type oxide upon loss of oxygen is discussed with respect to the remarkable increase in the catalytic activity and selectivity.

Molybdenum oxide based partial oxidation catalyst: Part 3. Structural changes of a MoVW mixed oxide catalyst during activation and relation to catalytic performance in acrolein oxidation

The activation of a Mo9V3W1.2Ox catalyst was investigated in the partial oxidation of acrolein as function of reaction temperature and atmosphere. The activity and selectivity to acrylic acid considerably increased during activation in the acrolein oxidation reaction comparable to the recently reported activation after thermal pretreatment in inert gas. The activation during the catalytic acrolein oxidation, however, proceeds at about 200 K lower temperatures as compared to the inert gas pretreatment. The initial nanocrystalline catalyst structure changed during operation in the acrolein oxidation, as shown by X -ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy and energy dis- persive X-ray (EDX) analysis. A (MoVW)5O14-type mixed oxide was found to be the main crystalline phase in the active and selective catalysts. Hence, this (MoVW)5O14 phase crystallizes already during catalysis at the low acrolein oxidation reaction temperatures. The evolution of the catalytic performance is directly related to this low temperature formation of the (MoVW)5O14 phase. It is suggested that this (MoVW)5O14 phase has to be present in a detected, specific ordering state which is vital for optimum selective oxidation properties. In addition, other minority phases were identified by transmission electron microscopy (TEM) in the operating catalyst. The so-called bundle-type, and a new corona-type texture was detected, which show ordering in only one or two dimensions, respectively. These disordered structures are also relevant candidates for active catalyst phases as they are detected during the activation period of the MoVW mixed oxide.