Acidity of aluminophosphate structures. Part 2.—Incorporation of cobalt into CHA and AFI by microwave synthesis

Abstract

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.