Transient equilibration catalyst

The results of transient experiments by Ballarini et al. (152) showed that the active surface of equilibrated catalysts is different, depending on the reaction conditions and the P/V ratio of the catalyst. At low temperature (320 °C), an active surface forms that is selective and probably is more like VOPO4 than (VO)2P207. However, as the temperature is increased to 380 °C, this material becomes less selective. The active phase formed at temperatures >380 °C was found to be less active than the low-temperature phase but has increased selectivity at this temperature. At these temperatures, the active site is found to imdergo hydrolysis and oxidation, and Ballarani et al. proposed that the active surface is a VO / pol)q5hosphoric acid mixture. The authors speculated that the evolution of different phases at different temperatures (which is also dependent on very minor changes in the P/V ratio) could be the cause of the existence of markedly different surfaces observed in both in situ and ex situ characterizations of the active catalyst. 

Transient siloxanolate anionic catalysts prepared by reacting four moles of D-4 with one of tetramethyl ammonium hydroxide at 80 C are effective for equilibrating "neutral" systems such as the epoxy ( ), "basic" dimethyl-amino (64) or aminopropyl (59,67) end-blockers and D-4. With "acidic" functionality on the end-blocker, we have successfully utilized trifluoroacetic acid for the equilibrations. Further details of the oligomer synthesis and their utilization in segmented copol)nners will be described in future publications. 

Figures 4a-g display plots of the areas of the MA, CO2, and n-butane transient responses versus pulse number when a 3.5/1 CqHio/Ar mixture is pulsed over seven different oxygen-treated catalyst samples at 653 K. The seven catalysts were prepared from the same batch of reactor equilibrated VPO, by oxidizing each sample at a different temperature, and 1 atmosphere of oxygen for 1 hour. The sev en oxidation temperatures, corresponding to the seven sets of MA, CO2, and n-butane data, are in order, 683, 703, 723, 743, 763, 783, and 803 K. The sets of three curves rellect the changes in MA and CO2 production, and in n-butane conversion with pulse number. The MA curves (Figures 2a,b,c,d,e) of catalyst samples oxidized at or below 763 K display a pronounced...

The soluble vanadium catalysts have considerable ligand mobility which permits equilibration to a single, catalytic species. Polaro-graphic measurement of the vanadium valence showed that the ultimate species is divalent (V+ 2). However, there is still disagreement as to whether the active catalyst is this divalent species or is a transient, trivalent entity. Most workers do agree that a low-valence species is essential. 

In the above kinetic expression, the Arrhenius rate constant, 1, is modified by the two adsorption equilibrium constants of methanol and water during the steady state kinetic studies. During the transient TPRS experiments, however, only the Arrhenius rate constant, kj, is measured since there is no vapor phase methanol or water to be equilibrated. The similar TPRS results for the oxidation of the surface methoxy intermediate to formaldehyde over the different supported vanadia catalysts reveal that all the catalysts possess the same kj. Consequently, the dramatic differences in the steady state TOFs during methanol oxidation over the different supported vanadia catalysts must be associated with the methanol and water equilibrium adsorption constants. Both methanol and water can be viewed as weak acids that will donate a proton to a basic surface site, but methanol is more strongly adsorbed than water on oxide surfeces, K, > [16]. Furthermore, the methanol oxidation TOFs... 

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