Infrared spectroscopic studies of acid properties of NaHZSM-5 zeolites

Abstract

Acid properties of NaHZSM-5 zeolites of various exchange degrees were studied by quantitative infrared (IR) studies of pyridine adsorption. It has been verified that pyridine may be used as adsorbate for such studies. Two series of NaHZSM-5 zeolites were prepared by exchange in HCl and NH 4NO 3 solutions. Two kinds of Brønsted acid sites were detected in NaHZSM-5 zeolites, OH groups vibrating at 3609 cm −1 being strong acid sites and other protonic sites of lower acidity. The concentrations of these two kinds of Brønsted site and also of Lewis acid sites were determined as a function of exchange degree and also of mode of preparation. It has been found that first protons introduced into zeolite form weak Brønsted acid sites and a small amount of 3609-cm −1 OH groups. The amount of weak acid sites attains a constant value after the exchange of 30–40% of Na + ions and the next-introduced protons form 3609-cm −1 OH groups. The mode of introduction of protons do not influence either the sequence of sites formation or the proportion of strong and weak acid sites. The amounts of acid sites determined by IR spectroscopy were found to be equal to the amounts of protons introduced into zeolites by the ion exchange, thus indicating that the introduction of protons into the zeolite do not “acidify” weaker sites preexisting in the zeolite before ion exchange. The protonic acid sites in NaHZSM-5 zeolite are resistant to dehydroxylation (the amount of Lewis acid sites is low), which may be due to the high Si Al ratio of zeolites studied in the present research.