(501d) Quantitative Attenuated Total Reflection Infrared Spectroscopy for Understanding Solvent Effects in Liquid Phase Reactions on Zeolites

Quantitative
Attenuated Total Reflection Infrared Spectroscopy for Understanding Solvent
Effects in Liquid Phase Reactions on Zeolites

Nicholas Gould, Bingjun Xu

Department of Chemical and
Biomolecular Engineering, University of Delaware

Biomass conversion reactions are
frequently conducted in a solvent, due to the highly oxygenated nature of the
feedstock.^1,2 Thus, heterogeneous catalytic
active sites exist at a solid-liquid interface, where the solvent can modify
surface and adsorbate energetics. Even when the solvent does not play a direct
role in the reaction mechanism, it can stabilize or destabilize adsorbates,
intermediates, and transition states, often leading to markedly different rates
and selectivities between solvent choices.^3–5 However, solvent effects are
poorly understood because catalyst characterization techniques, such as probe
molecule adsorption in FTIR, are most often conducted under vacuum or in vapor
phase.^6,7 Further, most studies on solvent
effects focus on screening solvents via catalytic activity testing, where
multiple factors that can influence reactivity exist simultaneously: competitive
adsorption, stabilization of reactants and transition states, and phase
equilibria differences. Thus, there is currently a need for experimental
techniques capable of extracting fundamental thermodynamic properties of
solvents in simple systems, with the end goal of decoupling the effects of
solvent in catalytic activity tests.⁸

Attenuated total
reflection (ATR) fourier transform infrared spectroscopy (FTIR) was used to
characterize zeolites with probe molecules in the presence of solvent. The
ATR-FTIR was further developed into a quantitative technique, with a procedure
for determining extinction coefficients for adsorbed pyridine on zeolites in
the presence of solvent.⁹ This allowed for
quantitative comparisons of the effect of solvent on probe molecule uptake and
protonation in zeolite pores. Ongoing applications of the ATR-FTIR cell include
adsorption isotherms, diffusion measurements, and temperature programmed
desorption (TPD) in porous materials in liquid phase. Further, the effect of
solvent on charge stabilization in zeolite pores was studied using a homemade
TPD set up under back pressurized, flowing solvent. Preliminary pyridine
desorption temperatures from an H/ZSM-5 sample reveal that the ability of a
solvent to stabilize pyridinium ions decreases in the order: water >
acetonitrile > alkane ≈ vacuum. 

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Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044–4098.

2            D.
M. Alonso, S. G. Wettstein and J. A. Dumesic, Green Chem., 2013, 15,
584–595.

3            M.
A. Mellmer, C. Sener, J. M. R. Gallo, J. S. Luterbacher, D. M. Alonso and J. A.
Dumesic, Angew. Chemie - Int. Ed., 2014, 53, 11872–11875.

4            P.
J. Dyson and P. G. Jessop, Catal. Sci. Technol., 2016, 6, 3302–3316.

5            J.
F. Haw, T. Xu, J. B. Nicholas and P. W. Goguen, Nature, 1997, 389,
832–835.

6            F.
Zaera, Chem. Rev., 2012, 112, 2920–2986.

7            H.
Shi, J. Lercher and X.-Y. Yu, Catal. Sci. Technol., 2015, 5,
3035–3060.

8            N.
S. Gould and B. Xu, Chem. Sci., 2018, 9, 281–287.

9            N.
S. Gould and B. Xu, J. Catal., 2017, accepted.