Lewis acidic zeolites are silica-based, crystalline microporous materials containing
tetravalent heteroatoms (M4+=Ti, Sn, Zr, Hf) substituted in framework locations,
and have been reported to catalyze a wide range of reactions involving oxygenates and
hydrocarbons. The synthetic protocols used to prepare Lewis acid zeolites determine
the structures of the active sites and the reaction pockets that confine them, which
in turn influences reactivity, product selectivity, and catalyst stability. Specifically,
aqueous-phase reactions of biomass-derived molecules, such as glucose isomerization,
are sensitive to the hydrophobicity of confining environments, leading to changes in
turnover rates. As a result, precise evaluation of the structure and behavior of reaction
environments and confined active sites among catalysts of varying provenance or
treatment history requires quantitative descriptions of active Lewis acid site densities,
of densities of surface functional groups that determine the polarity of microporous
confining environments, and of the kinetic behavior of these catalytic materials.<div><br></div><div>Methods for quantifying Lewis acid sites and silanol defects are developed here by
analyzing infrared (IR) spectra collected after Lewis base (CD3CN, pyridine) titrations of Lewis acidic zeolite surfaces and are compared to vapor-phase methanol and
water adsorption isotherms. Additionally, IR spectra collected under ex situ (flowing
vapor-phase water) and in situ (aqueous-phase, 373 K, 0-50 wt% glucose) conditions
are used to compare co-adsorbed water densities and structures within hydrophobic
(low silanol density) and hydrophilic (high silanol density) confining environments
within M-Beta zeolites. Under reaction conditions relevant for sugar conversion in aqueous media (353-398 K, 1-50 wt% glucose), hydrophilic reaction pockets stabilize liquid-like extended water structures within microporous environments, while
hydrophobic channels stabilize vapor-phase water at lower intraporous water densities. Higher aqueous-phase glucose isomerization rates (368-383 K, 1-50 wt% glucose,
per kinetically relevant active site) are observed on hydrophobic Ti-Beta (~6-12x, per
Lewis acidic Ti) and Sn-Beta (~50x, per Lewis acidic Sn in open configuration) zeolites over their hydrophilic analogs. Higher turnover rates on hydrophobic M-Beta
zeolites reflect the absence of an extended, hydrogen-bonded network of waters, which
entropically destabilizes kinetically relevant hydride shift transition states by reducing
the flexibility of their primary solvation spheres. These findings suggest catalyst design strategies to minimize the generation of silanol groups within confining reaction
environments would lead to increases in turnover rates.<br></div><div><br></div><div>The methods derived herein can be applied to understanding the role of the confining environment and the associated co-adsorbed water on zeolitic materials of different topology and Lewis acid site identity. For example, the transient formation
of silanol defects under aqueous-phase operating conditions is primarily responsible
for the deactivation of Sn-Beta catalysts observed during aqueous-phase glucose isomerization. Further, quantifying the role of the confining environment geometry and
hydrophobicity on aqueous-phase glucose isomerization rates can be used as guidance for catalyst design to increase reaction rates and selectivities toward specific
isomerization products. These findings show that both the active site identity and
its confining environment, which vary with zeolite topology and micropore polarity,
combine to influence reactivity, selectivity and stability for aqueous-phase glucose
isomerization catalysis.<br></div>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7430771 |
| Date | 16 January 2019 |
| Creators | Michael J. Cordon (5929610) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Consequences_of_the_Hydrophobicity_and_Spatial_Constraints_of_Confining_Environments_in_Lewis_Acid_Zeolites_for_Aqueous-Phase_Glucose_Isomerization_Catalysis/7430771 |
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