Abstract
One of the fundamental issues in catalysis is to identify the catalytic active site. Due to its prominent pore topology and acidity, ferrierite (FER) zeolite has attracted extensive interest in various catalytic reactions such as isomerization of butenes. However knowledge on the active Brönsted acid site is still absent. In the present study, we perform extensive density functional theory calculations to explore the distribution and strength of the Brönsted acid sites and their potential catalytic activity for the double-bond isomerization of 1-butene to 2-butene. We employ a two-layered ONIOM scheme (our Own N-layered Integrated molecular Orbital + molecular Mechanics) to describe the structure and energetic properties of FER zeolite. We find that the hydrogen bond could improve the stability of Brönsted acid sites effectively, and, as a result, Al4-O6-Si2 and Al4-O-(SiO)2-Al4 are the most stable sites for 1-Al substitution and 2-Al substitution, respectively. We further find that the Brönsted acid strength tends to decrease with the increase of Al contents and increase when the distance between the Al atoms is increased in 2-Al substitution. Finally it is demonstrated that the strength of acid sites determines the catalytic activity for the double bond isomerization of 1-butene to 2-butene.
Highlights
Silicon-rich zeolites have been proved efficient catalysts in the solid-catalyzed reactions such as isomerization, alkylation, and etherification [1,2,3]
Al4-O6-Si2 site, which is formed from an Al atom occupied with the T4 site, is most the stable location for Brönsted acid in 1-Al model
The Al4‐OH‐()‐Al4‐OH site exhibits the lowest acidity, suggesting that the located between Al atoms exhibits stronger acidity than that with only one proton located between Al presence of hydrogen bonds causes the acid strength to decrease
Summary
Silicon-rich zeolites have been proved efficient catalysts in the solid-catalyzed reactions such as isomerization, alkylation, and etherification [1,2,3]. The experiment research on these issues was mentioned a little, several DFT calculations were carried out to investigate the distribution of Al in FER zeolite [23,24] Their cluster model was so extremely small that it could not represent the various types of Al/Si distributions, which are believed to be significant in determining the catalytic reactivity in butene isomerization. To further investigate the catalytic properties of FER zeolite, the double-bond isomerization of 1-butene to 2-butene at various Brönsted acid sites was studied. We revealed the corresponding relationship of the catalytic performance between the distribution and strength of Brönsted acid sites on zeolite These theoretical results are contributed to guide the experiment methods for designing the appropriate catalyst for reaction and understand the properties of the catalyst
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