Synergy of Extraframework Al³⁺ Cations and Brønsted Acid Sites on Hierarchical ZSM-5 Zeolites for Butanol-to-Olefin Conversion

Abstract

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.

Tailoring and Identifying Brønsted Acid Sites on Metal Oxo-Clusters of Metal–Organic Frameworks for Catalytic Transformation

“…The ZSM-5 was prepared in our lab according to the published method. 37 After 6 h of reaction at 90 °C, 35.5% DHA conversion was achieved with a broad product distribution: 59.0%, 65.9%, and 6.9% as PADA, EL, and PAEA, respectively. The broad product distribution dominated by PADA can be rationalized by the presence of strong Brønsted acid sites in the ZSM-5 catalyst., 38,39 illustrating the advantage of moderate Brønsted acidity with Lewis acid sites in HfNDI in catalyzing a selective DHA-to-EL transformation reaction.…”

“…Lastly, an aluminum-doped zeolite, ZSM-5, was used as a heterogeneous catalyst for DHA conversion. The ZSM-5 was prepared in our lab according to the published method . After 6 h of reaction at 90 °C, 35.5% DHA conversion was achieved with a broad product distribution: 59.0%, 65.9%, and 6.9% as PADA, EL, and PAEA, respectively.…”

The Journal of Physical Chemistry C Virtual Special Issue on Advanced Characterization by Solid-State NMR and In Situ Technology and in Recognition of Michael Hunger’s 65th Birthday

“…The Louis group confirmed that extra-framework aluminum species worked as Lewis acid sites to selectively activate C–H bonds of isobutane at low temperature . The synergy between tricoordinated Al species and bridging OH groups to form ultrastrong acid sites for biobutanol conversion was investigated by Huang and coworkers by 1D and 2D multinuclear NMR …”

“…23 The nature and spectroscopic expression of external surface sites of zeolites such as silanol−Al sites by the combination of MAS NMR and Fourier-transform infrared spectroscopy (FT-IR) with DFT calculations was investigated by the Chizallet group. 24 Koller and coworkers observed additional 1 H− 27 Al dipolar interactions in an extra-large pore zeolite with high aluminum ordering using 1 H{ 27 Al} rotational echo adiabatic passage double-resonance. 25 The Louis group confirmed that extra-framework aluminum species worked as Lewis acid sites to selectively activate C−H bonds of isobutane at low temperature.…”

“…26 The synergy between tricoordinated Al species and bridging OH groups to form ultrastrong acid sites for biobutanol conversion was investigated by Huang and coworkers by 1D and 2D multinuclear NMR. 27 Many articles in the VSI are in the area of in situ NMR characterization. Hartmann and coworkers combined in situ continuous-flow MAS NMR with in situ powder X-ray diffraction (PXRD) to show two phase transformation mechanisms of the local porous structure and the bulk crystalline in porous materials during adsorption and desorption.…”