Orientation of the acidity probes benzene, acetonitrile and pyridine in SAPO-5 and GaAPO-5 molecular sieves: an FTIR microscopy study

The adsorption and desorption behaviour of benzene, acetonitrile (CD₃CN) and pyridine for large crystals of SAPO-5 and GaAPO-5 is studied using FTIR microscopy with polarised IR radiation. From the analysis of the polarised adsorption bands it is concluded that the adsorbates are ordered in the molecular sieves at 373 K. The orientation of molecules interacting with Brönsted or Lewis acid sites and of physisorbed molecules are discussed. Benzene adsorbed at Brönsted sites is oriented with the aromatic ring parallel to the crystallographic c-axis of the molecular sieve, whereas physisorbed benzene is oriented with the aromatic ring parallel to the crystallographic a,bplane. Acetonitrile can adsorb on top of the Brönsted sites, resulting in an orientation with the CN-axis in the a,bplane. Physisorbed acetonitrile has the CN-axis parallel to the c-axis. Pyridinium ions have their aromatic rings parallel to the a,b-plane. Pyridine molecules at Lewis sites are oriented parallel to the a,b-plane in SAPO-5 and perpendicular to this plane in GaAPO-5. Physisorbed pyridine is oriented with its C₂-axis parallel to the c-axis.