ACIDITY OF ALUMINOPHOSPHATE STRUCTURES .2. INCORPORATION OF COBALT INTO CHA AND AFI BY MICROWAVE SYNTHESIS

Samples of CoAPO/CoAPSO-44 and CoAPO-5 have been prepared using cyclohexylamine as templating agent and a microwave power system for the crystallization. Cobalt substitutes for aluminium. The Co/Al ratios are in the range of 0.05 to 0.3 for the structure 44 and Co/Al = 0.06 for CoAPO-5. The crystallinity of the samples was studied by X-ray diffraction (XRD), IR spectroscopy, optical microscopy, thermal analysis and adsorption measurements and compared with the results for SAPO-44 and SAPO-5. The thermal stability of the samples decreases strongly as a function of the cobalt content. The Co2+ incorporation-generated acid sites were investigated by diffuse reflectance IR Fourier transform (DRIFT) spectroscopy in the OH-stretching vibration region and by the adsorption of probe molecules (NH3 and CO), calorimetric measurements of the differential molar heat of NH3 sorption and temperature-programmed desorption (TPD) of ammonia. From the results of calorimetry and TPD of ammonia it follows that each cobalt atom inserted into a tetrahedrally coordinated framework position creates one acid site. Concerning the nature of acid sites, an equilibrium between bridging hydroxy groups, paired centres of Lewis sites and -POH groups is proposed. The formation of Co3+ in framework positions could not be confirmed. Thermal analysis combined with mass spectrometry indicate the addition of molecular oxygen in calcined CoAPO-44.