Infrared Investigation of Dynamic Behavior of Brønsted Acid Sites on Zeolites at High Temperatures

Temperature-dependent behavior of acidic OH groups on zeolites was observed by infrared (IR) spectroscopy. While the IR band of acidic OH groups appeared the same in frequency and intensity below 300 K, gradual shifts in the peak-top position to lower frequencies and decreases in integrated intensity were recognized when samples were heated at higher temperatures. These changes were completely reversible and only dependent on the temperature. Based on the assumption that there is an equilibrium between undissociated and dissociated states of OH groups, a model is proposed in which the intensity decrease is attributed to the dissociation of OH groups to form IR inactive species at high temperatures. The enthalpy difference between the two states was estimated using the van’t Hoff equation, leading to two different values in two temperature ranges (about 398–548 and 573–773 K) for zeolites with various topologies (<b>MFI</b>, <b>MOR</b>, and <b>CHA</b>). Based on the presence of two different types of enthalpy values, different mechanisms were proposed for these two situations. Liberated protons may move across four lattice oxygen atoms around the Al site at lower temperatures (around 550 K or below). At high temperatures, the protons may move in wider regions over the framework. DFT calculations show that the frequency of the OH band varies depending on which of the four different oxygen atoms around the Al site acts as the proton acceptor. The experimentally observed peak-top shift to the lower frequency side is explained by assuming that the population of protons changed with temperature such that more protons reside at low-frequency sites at higher temperatures. This process is interpreted as localized hopping or limited delocalization. At the higher temperature range (573–773 K), the enthalpy difference was independent of the Al amount but was only dependent on the zeolite topology. This supports the free hopping of protons over the framework. The enthalpy difference at higher temperatures increased in the order <b>CHA</b> < <b>MFI</b> < <b>MOR</b>, indicating that a zeolite with smaller pores tends to generate more protons at the same temperature. In other words, zeolites with small pores function as stronger acid catalysts for reactions at high temperatures. This proposition was supported by experimental results of a monomolecular reaction taking place at a single site, that is, H/D isotope exchange between acidic OD groups and CH<sub>4