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</sub>
