(583bn) Post-Synthetic Modification of Mesoporous Amino-MOF Through Encapsulation of Guest Species for Reactive Aldehyde Sorbents and Catalysis

Post-synthetic
modification of metal-organic frameworks (MOFs) by the loading of catalytic
species into the pores is a versatile route toward rational design of catalysts
and reactive sorbents. Known catalysts can be incorporated into the MOF
structures, enabling integration of the functionality of well-defined,
single-site catalysts into the MOF micropores and mesopores to perform shape-, size-, chemo-, or enantio-selective reactions.¹ The ability to separate
and reuse a heterogeneous catalyst is highly desirable in large-scale
reactions, where separation and waste disposal can be costly. In the present
work, we based our catalyst and sorbent design on recently introduced aluminum aminoterephthalate MOFs that consist of non-toxic metal and
linker.  The loading of two different catalytic species, aluminum isopropoxide (Al-i-Pro)² and phosphotungstic
acid (PTA),³ into amino-containing, aluminum-based MOFs was studied.

MOFs NH₂-MIL-101(Al)
and NH₂-MIL-53(Al) and their composites with phosphotungstic
acid (PTA) were studied as sorbents of saturated vapors of acetaldehyde, acrolein, and butyraldehyde. MOFs
NH₂-Al-MIL-101 consist of mesopores with un-coordinated amino groups on the linker
molecules and aluminum metal centers. The MOF and MOF/PTA materials exhibited
equilibrium uptake of acetaldehyde from its saturated vapor phase exceeding 50
and 600 wt%, respectively, at 25^oC. The
acetaldehyde vapor uptake occurs through the vapor condensation, pore-filling
mechanism with simultaneous conversion of acetaldehyde to crotonaldehyde
and higher molecular weight compounds resulting from repeated aldol condensation. MOF functionalization by PTA
impregnation from aqueous/methanol solutions resulted in MOF with the original
crystal topology with the presence of an ordered PTA phase in the MOF/PTA
composite. However, SEM/EDX and TEM characterization did not show distinct
macroscopic PTA phases. We will present neutron diffraction data demonstrating
the host/guest structure at a molecular level. 

Composites of NH₂-MIL-101(Al)
and NH₂-MIL-53(Al) with aluminum isopropoxide
(Al-i-Pro) were studied as sorbents of vapors
of volatile aldehydes and catalysts of acetaldehyde dimerization to ethyl
acetate via Tischenko reaction. The BET surface areas
of the MOFs NH₂MIL101(Al) and NH₂MIL53(Al)
of 1950 and 780 m²/g, respectively, were reduced to 280 and 130 m²/g,
respectively, in the corresponding MOF/Al-i-Pro
composites. However, the acetaldehyde and acrolein
uptake by the MOF/Al-i-Pro composites from
saturated vapor atmosphere is comparable to that of their respective parent
MOFs and exceeds the aldehyde uptake of activated carbon or molecular sieves.
The MOF/Al-i-Pro composite catalyzes the
dimerization of acetaldehyde to ethyl acetate due to the activity of Al-i-Pro).

In both these
studies, Al-based amino-MOFs are used as stable host frameworks for the
encapsulation of catalytic species which improve on the adsorption capabilities
of the parent MOF, and enhance their reactivity towards conversion of aldehyde
vapors. The MOF and catalyst composite materials are proven to be recyclable
heterogeneous catalysts.

References:

1.        
Corma,
A.; García, H.; Llabrés i Xamena, F. X., Engineering
metal organic frameworks for heterogeneous catalysis. Chemical
Reviews 2010, 110 (8), 4606-55.

2.        
Bromberg, L; Su, X.; Hatton, T., Heteropolyacid-Functionalized
Aluminum 2-Aminoterephthalate Metal-Organic Frameworks as Reactive Aldehyde
Sorbents and Catalysts. ACS Applied Materials and Interfaces 2013.
In Press.

3.
        Bromberg, L; Su, X.; Hatton, T.,
Aldehyde Self-Condensation Catalysis by Aluminum Aminoterephthalate
Metal-Organic Frameworks Modified with Aluminum Isopropoxide.
Chemistry of Materials. 2013. In Press.