(196e) Identification of Distinct Sn Sites in Siliceous Zeolites for Mpvo and C-C Bond Formation Catalysis

Identification of Distinct Sn Sites in Siliceous
Zeolites for MPVO and C-C Bond Formation Catalysis

John R. Di Iorio, Jennifer D. Lewis,and Yuriy
Román-Leshkov 

Department of Chemical Engineering, Massachusetts Institute
of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

Lewis acidic Sn sites confined
within siliceous *BEA zeolites have garnered significant attention as catalysts
for oxygenate upgrading due to their high selectivity and stability for Meerwein-Ponndorf-Verley
reduction and Oppenauer oxidation (MPVO), Baeyer-Villiger oxidation (BVO), and
aldol condensation reactions in condensed media. IR spectra of Sn-*BEA zeolites
titrated with d₃-acetonitrile (CD₃CN) reveal two distinct
Lewis acidic Sn sites (ν = 2316 cm^-1 and 2308 cm^-1)
that have been assigned to partially-hydrolyzed open ((SiO)₃-Sn-(OH))
and closed ((SiO)₄-Sn) Sn sites, respectively, by a combination of ¹¹⁹Sn
magic-angle spinning (MAS) NMR [1] and DFT calculations [2]; the former of
which appears to be the active site for BVO [2] and glucose isomerization
reactions [3]. Moreover, ³¹P MAS NMR spectra of adsorbed
trimethylphosphine oxide (TMPO) on Sn-*BEA zeolites shows distinct ³¹P
chemical shifts for TMPO bound to different Sn sites that have been correlated with
rates of aldol condensation (δ ~ 58 ppm) and glucose isomerization (δ
~ 55 ppm) [4]. Here, we use IR spectra of adsorbed CD₃CN and ³¹P
MAS NMR spectra of adsorbed TMPO to quantify the concentration of distinct Sn
sites on a series of hydrophobic, synthesized hydrothermally in F-media, and
hydrophilic, prepared via post-synthetic grafting of Sn into defective
frameworks, Sn-*BEA zeolites. Rates of glucose isomerization (373 K) and
benzaldehyde-acetone aldol condensation (383 K) are used to unify existing
observations regarding the catalytic role of different Sn sites quantified via ex-situ
CD₃CN and TMPO site titrations. These results further highlight how
different Sn sites confined within siliceous *BEA zeolites facilitate distinct
chemical transformations important for the upgrading of biomass-derived
oxygenates.

References:

[1] Bermejo-Deval et al., ACS
Catal., 2014, 4, 2288

[2] Boronat et al., J. Catal.,
2005, 234, 111

[3] Harris et al., J. Catal.,
2016, 335, 141

[4] Lewis et al., ACS Catal.,
2018, 8, 3076