Scaling-Up Microwave-Assisted Synthesis of Highly Defective Pd@UiO-66-NH2 Catalysts for Selective Olefin Hydrogenation under Ambient Conditions

Abstract

The need to develop green and cost-effective industrial catalytic processes has led to growing interest in preparing more robust, efficient, and selective heterogeneous catalysts at a large scale. In this regard, microwave-assisted synthesis is a fast method for fabricating heterogeneous catalysts (including metal oxides, zeolites, metal-organic frameworks, and supported metal nanoparticles) with enhanced catalytic properties, enabling synthesis scale-up. Herein, the synthesis of nanosized UiO-66-NH₂ was optimized via a microwave-assisted hydrothermal method to obtain defective matrices essential for the stabilization of metal nanoparticles, promoting catalytically active sites for hydrogenation reactions (760 kg·m^-3·day^-1 space time yield, STY). Then, this protocol was scaled up in a multimodal microwave reactor, reaching 86% yield (ca. 1 g, 1450 kg·m^-3·day^-1 STY) in only 30 min. Afterward, Pd nanoparticles were formed in situ decorating the nanoMOF by an effective and fast microwave-assisted hydrothermal method, resulting in the formation of Pd@UiO-66-NH₂ composites. Both the localization and oxidation states of Pd nanoparticles (NPs) in the MOF were achieved using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively. The optimal composite, loaded with 1.7 wt % Pd, exhibited an extraordinary catalytic activity (>95% yield, 100% selectivity) under mild conditions (1 bar H₂, 25 °C, 1 h reaction time), not only in the selective hydrogenation of a variety of single alkenes (1-hexene, 1-octene, 1-tridecene, cyclohexene, and tetraphenyl ethylene) but also in the conversion of a complex mixture of alkenes (i.e., 1-hexene, 1-tridecene, and anethole). The results showed a powerful interaction and synergy between the active phase (Pd NPs) and the catalytic porous scaffold (UiO-66-NH₂), which are essential for the selectivity and recyclability.

Keywords: UiO-66-NH2; defect engineering; gram-scale; microwave-assisted synthesis; palladium; selective hydrogenation.

Figure 1

Density contour plots of (a) crystal size (estimated by using the Scherrer equation from PXRD); (b) decomposition temperature (first derivative of TGA curves); (c) number of defects (TGA, compared to theoretical weights at different temperatures; in terms of missing linker units); and (d) BET surface area (using nitrogen sorption isotherms), as a function of modulator concentration and reaction time. Red indicates the highest values, while blue indicates the opposite.

Figure 2

HAADF-STEM images of UiO-66-NH₂ (a, b) and Pd(1.7%)@UiO-66-NH₂ (c, d); magnification ×50,000 and ×150,000, respectively, and (e) particle size distribution of Pd(1.7%)@UiO-66-NH₂.

Figure 3

XPS spectra of UiO66-NH₂ and Pd(1.7%)@UiO-66-NH₂ (a) survey, (b) Zr 3p and Pd 3d regions, (c) Zr 3d region, and (d) N 1s region.

Scheme 1. Hydrogenation of a Complex Mixture of Alkenes using Pd(1.7%)@UiO-66-NH₂

Figure 4

(a) Hydrogenation cycles of the Pd(1.7%)@UiO-66-NH₂ catalyst and TEM images of (b) fresh Pd(1.7%)@UiO-66-NH₂ catalyst and (c) Pd(1.7%)@UiO-66-NH₂ catalyst after four cycles.