<p>The silicoaluminophosphate zeotype ECR-40, which has
the MEI topology, contains linkages of AlO<sub>4</sub> tetrahedra via a common
oxygen atom, thereby violating the famous “Löwenstein’s rule”. Due to the
proven existence of Al-O-Al linkages in this material, it constitutes an ideal
model system to study the acidity and mobility of protons associated with such
unusual linkages. In addition, their properties can be directly compared to
those of protons associated with more common Si-O-Al linkages, which are also
present in ECR-40. In this work, static density functional theory (DFT)
calculations including a dispersion correction were employed to study the
preferred proton sites as well as the Brønsted acidity of the framework
protons, followed by DFT-based ab-initio molecular dynamics (AIMD) to
investigate the proton mobility in guest-free and hydrated ECR-40. Initially,
two different proton arrangements were compared, one containing both H[O6] protons
associated with Al-O-Al linkages and H[O10] protons at Si-O-Al linkages, the
other one containing only H[O10] protons. The former model was found to be
thermodynamically favoured, as a removal of protons from the Al-O-Al linkages causes
a local accumulation of negative charge. Calculations of the deprotonation
energy showed a moderately higher Brønsted acidity of the H[O10] protons, at
variance with previous empirical explanations, which attributed the exceptional
performance of ECR-40 as acid catalyst to the presence of Al‑O‑Al linkages. The
AIMD simulations (<i>T</i> = 298 K)
delivered no appreciable proton mobility for guest-free ECR-40 and for low
levels of hydration (one H<sub>2</sub>O per framework proton). Under saturation
conditions, framework deprotonation occurred, leading to the formation of
protonated water clusters in the pores. Pronounced differences between the two
types of framework protons were observed: While the H[O10] protons were always
removed from the Si-O-Al linkages, the Al-O-Al linkages remained mostly
protonated, but deprotonation did occur to a minor extent. The observation of a
degree of framework deprotonation of Al-O-Al linkages differs from the findings
reported in a recent computational study of hydrated aluminosilicate zeolites
with such linkages (Heard et al., <i>Chem. Sci.</i> <b>2019</b>, <i>10</i>, 5705), pointing to an influence of the overall
framework composition. Further inspection of the AIMD results showed that a
coordination of water molecules to framework Al atoms occurred in many cases,
especially in the vicinity of the Al-O-Al linkages, sometimes resulting in a
pronounced modification of the linkages through additional bridging oxygen
atoms. Given the changes in the local structure, it can be expected that such
modified linkages are especially prone to break upon dehydration. Thus, in addition to elucidating the deprotonation
behaviour of protons associated with different types of linkages, the calculations
also provide insights into possible reasons for the instability of Al-O-Al
linkages, clarifying why Löwenstein’s rule is mostly obeyed in materials that
are formed via a hydrothermal route.</p>
Protonation Equilibrium in the Active Site of the Photoactive Yellow Protein
