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Abstract

Activation and deactivation behavior of heteropoly acids as catalyst precursors O.Timpe, H. Noack, und G. Mestl§ Fitz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin Heteropoly acids (HPA) are important catalysts for selective partial oxidations due to their acid and redox active properties. Thee Keggin-type phosphorus molybdo acid H4[PMo12O40] is the basis for an industrially used catalyst in the methacrylic acid synthesis which contains vanadium too. The protons of the free HPA are partially exchanged by Cs+ ions. The insufficient stability of this class of substances does not allow their optimum use as catalysts. A continuous degradation of their catalytic properties and thus of the possible yields is observed under stationary reaction conditions. Due to their characteristic instability, it is unlikely that intact HPA´s are the active catalysts, although frequently stated in the literature. This correlates with the fact that the HPA has to be activated in the industrial application prior to the actual catalysis. The structural and chemical properties of the precursor-HPA are well characterized, correlate however only indirectly with the catalytic data. The oxides of the constituting transition metals are the final state of the HPA-decomposition, whose stoichiometry or degree of oxidation is determined by the surrounding atmosphere. The reaction pathway to these final decomposition products can be understood as a condensation process followed by complete structural reorganization. The loss of water occurs in two steps. The irreversible loss of structural water at elevated temperatures follows after the reversible loss of crystal water at temperatures below 400K. Oxygen defects are introduced into the Keggin anions with the loss of structural water. These defects are presumably the reason for the low reaction barriers of the subsequent structural reorganization of the defective Keggin anions. It could be shown by comparing thermogravimetric measurements of different molybdenum HPA, containing P as well as Si as the hetero atom, in different atmospheres that the reduction of the HPA occurs only after the loss of structural water. This reduction is the consequence of the loss of structural water because these HPA systems do not have a stability plateau after this step of water removal. Under oxidative conditions, the formation of MoO3 is observed after this step of loss of water. The comparison of HPA´s reveals a continuous mass loss of the P-containing samples after the loss of structural water under isothermal conditions in hydrogen atmosphere, which is qualitatively identified as the loss of P in form of reduced P-compounds, e.g. PH3. Directly after this mass loss, the TG curves show a step which is attributed to the reduction of the transition metal ions. The incorporation of V into the Keggin leads to an enhanced destabilizing effect in case of Si-HPA as compared to P-HPA. Investigations of the catalytic activity (TPRS) can be correlated with the TG data. After the loss of structural water, all investigated HPA are active for the partial oxidation reactions. This catalytic activity decreases with time on stream. Under catalytic conditions, the active fragments are not stable which were formed from the Keggin anions after loss of structural water. Total oxidation increases with the reactor temperature. In case of the Si-HPA, the total oxidation is the major reaction path already after the loss of structural water. It can be concluded from a comparison of the catalytic activities and selectivities of the different HPA that P stabilizes the fragments of the decomposed Keggins, which are characterized by a high degree of partial oxidation, in contrary to Si containing catalysts. Olefins react already at room temperature with the crystal water of the acidic HPA under water addition to alcohols. In this way the loss of water of the HPA samples is considerably increased and occurs already at room temperature. In contrast, HPA are not reduced by hydrogen at room temperature. The loss of structural water is shifted to lower temperatures in a similar way in presence of olefins. HPA preparations in which all protons of the HPA were exchanged by Cs+ ions exhibited an considerably enhanced stability toward reduction. These salts cannot desorb structural water. The decomposition and condensation of the Keggin anion fragments therefore is suppressed. Hence, these Cs salts are catalytically inactive for the partial oxidation. In contrast, more unstable tetraalkyl ammionum salts are catalytically active already at much lower temperatures which correlate with the redox potentials of the cations. It can be concluded that structural intact Keggin anions are catalytically inactive and have to be considered as the precursors of the actual active compounds.