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.