Quest for an Efficient 2-in-1 MOF-Based Catalytic System for Cycloaddition of CO2 to Epoxides under Mild Conditions

Abstract

We have devised a straightforward tandem postsynthetic modification strategy for Zr-based metal-organic framework (MOF) materials, which resulted in a series of well-defined 2-in-1 heterogeneous catalysts, cat1-cat8, exhibiting high catalytic activity in the synthesis of cyclic carbonates under solvent-free and co-catalyst-free conditions. The materials feature precisely located co-catalyst moieties decorating the metal nodes throughout the bulk of the MOF and yield cyclic carbonates with up to 99% efficiency at room temperature. We use diffuse reflectance infrared Fourier transform (DRIFT) and solid-state nuclear magnetic resonance (NMR) measurements to elucidate the role of each component in this model catalytic reaction. Establishing a method to precisely control the co-catalyst loading allowed us to observe the cooperativity between Lewis acid sites and the co-catalyst in the 2-in-1 heterogeneous system.

Keywords: carbon dioxide fixation; cycloaddition; heterogeneous catalysis; metal−organic framework; postsynthetic functionalization.

Figure 1

Design of a 2-in-1 MOF-based catalytic system based on the NU-1000 platform: (a) functionalization of inorganic nodes; color code: C (gray), O (red), N (blue), Zr (light green), Br or I (violet), and R group (light gray); (b) the structure of NU-1000; and (c) the schematic representation of tandem postsynthetic functionalization of the 8-connected Zr-nodes.

Figure 2

Structural characterization of representative 2-in-1 catalysts: (a) DRIFT spectra of the representative samples, (b) ¹³C CP-MAS spectra of examined materials, (c) TGA profiles under oxidative conditions (O₂/N₂ = 20/80); the corresponding weight loss is indicated with arrows, and (d) N₂ sorption isotherms at 77 K; filled symbols, adsorption and open symbols, desorption.

Figure 3

(a) Scheme of the cyclic styrene carbonate synthesis in cycloaddition reaction of CO₂ to styrene oxide, (b) catalytic activities of cat1–cat8 in the reaction of CO₂ with styrene oxide at room temperature after 24 h (green), at 60 °C (orange), and at 80 °C (red) after 4 h; (c) time-dependent analysis of cat1 performance at room temperature (25 °C) during 48 h; the inset shows a comparison of cat1 at 25 and 80 °C during the first 4 h.

Figure 4

(a) Time-resolved difference DRIFT spectra of cat1 with styrene oxide at 80 °C under 1 atm of CO₂ and (b) influence of the amount of pyridinium moieties introduced into SALI-4-Py(Zr) by alkylation with methyl iodide on the catalytic activity of cat1 in the formation of styrene carbonate.

Figure 5

Proposed catalytic cycle for the formation of cyclic carbonate catalyzed by cat1; Ar = MeI-4-Py.