Reversible interaction of NH3 with the framework of template-free zeolite-type SAPO-34

Abstract

The interaction of ammonia with the template-free SAPO-34 framework is studied by XRD, ²⁹Si, ²⁷Al and ³¹P magic angle spinning (MAS) NMR and IR spectroscopy and temperature-programmed desorption (TPD). A reversible loss of crystallinity upon NH₃ adsorption–desorption at room temperature is accompanied by the reversible formation of new ²⁷Al and ³¹P NMR spectroscopic peaks, and the displacement of the chemical shifts of ²⁹Si, ²⁷Al and ³¹P nuclei. Rupture of some AI—O—P bonds is proposed. Upon NH₃ desorption, the broken Al—O—P bonds are reformed. Lewis acidity, detected by IR spectroscopy, is assumed to proceed from the addition of ammonia on Al partly or fully connected to the framework. Such an interaction may affect acidity measurements made using ammonia adsorption.