ZSM-5
zeolite catalyzed dehydration of biobutanol has been widely
used for the sustainable production of butene. Brønsted acid
sites (BASs) in zeolites usually act as catalytically active sites.
Lewis acid sites (LASs) are also active for alcohol dehydration. Therefore,
extraframework aluminum species have been introduced to zeolites as
LASs. Tricoordinated Al species are the strongest LASs, which can
enhance the acidity of adjacent surface BASs. Here, we combined solid-state
nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared spectroscopy (DRIFTS) to investigate
the local structures and acidity of hierarchical ZSM-5 zeolites with/without
the introduction of Al3+ cations as well as their correlations
with the catalytic performance in biobutanol dehydration. 27Al magic angle spinning (MAS) NMR, 27Al multiple-quantum
(MQ) MAS NMR, and 31P MAS NMR after loading trimethylphosphine
oxide (TMPO) probe molecules showed that the tricoordinated Al species
are dominant acid sites after introducing Al3+ cations
into pure silica hierarchical ZSM-5 zeolites. For BASs-rich hierarchical
ZSM-5 zeolites (SiO2/Al2O3 = 50),
the Al3+ cations tend to stay in proximity to the bridging
Si–OH–Al and thereby strongly enhance the acidic strength
of BASs via the synergy of LASs and BASs. Contributing from the synergy
effect, the ultrastrong BAS has been formed, which gives an obvious
improvement in the biobutanol conversion and butene selectivity. In situ DRIFTS showed that tricoordinated Al3+ cations cooperating with BASs can promote the formation of butoxy
intermediates at a low reaction temperature, which further improves
the butanol dehydration.