CoAPO redox properties

CoAPO-5 and CoAPO-11 were shown by Peeters et al. [539] via IR spectroscopy (using CD3CN and CO as probes, vide infra. Sects. 5.S.2.6.4 and 5.S.2.6.6) to possess unusual Lewis acidity and redox properties (cf. also [540]). 

Peeters MPJ, van Hooff JHC, Sheldon RA, Zholobenko VL, Kustov LM, Kazansky VB (1993) Spectroscopic investigations of the redox properties of CoAPO molecular sieves. In von Ballmoos R, Higgins JB, Treacy MMJ (eds) Proceedings from the Ninth International Zeolite Conference. Butterworth-Heinemann, Boston London Oxford Singapore Sydney Toronto Wellington, vol I, p 651... 

Framework charge and, hence, cation exchange capacity and acidity in CoAPO molecular sieves are a function of the concentration of incorporated cobalt(II), which can be monitored quantitatively by means of electron absorption spectroscopy. The maximum framework cobalt acceptance as well as the stability depends on the structure type and decreases in the same order CoAPO -5 > CoAPO -11 > CoAPO —16 > CoAPO -34. Less stable structure types such as CoAPO -16 and —34 suffer from a loss in crystallinity and framework cobalt during calcination and reduction experiments, whereas in stable structures framework cobalt can reversibly undergo redox reactions Co(II) <—> Co(III). In conclusion, CoAPOs have potential for both redox catalysis and acidic catalysis. In the latter case, however, fluctuating acidic properties due to changes in the oxidation state can be a serious restriction in many operations. 

Vanadium-cobalt substituted aluminophosphate molecular sieve of AEI structure (VCoAPO-18) was found to be active and selective in the ODH of ethane. Its catalytic behavior can be related to the presence of redox (probably related to and Co " ") and acid sites (related to Co + cations) in addition to its unique structural properties. The conversion and ethene selectivity decreases in the order VCoAPO-18 >VO c/CoAPO-18 > CoAPO-18 [38]. At 873 K, the VCoAPO-18 catalyst showed a 50% ethene selectivity at 60% ethane conversion for an ethane/oxygen molar ratio of 4 8. Acid SAPO-34-based microporous catalysts with chabasite structure have been tested for the ODH of ethane in the temperature range of 823 to 973 K. Pure acid and La/Na containing SAPO-34 were catalytically active and a 75 ethene selectivity for 5% ethane conversion and a 60% ethane selectivity for 30% ethane conversion was observed [39]. 

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