Advanced Heterogeneous Catalysts For Size- And Shape-selective Reactions

Metal-organic frameworks (MOFs) are a unique class of porous materials constructed using metal nodes and organic linkers, they exhibit many noticeable features which make them as strong candidates for the incorporation of active phases to enable catalytic applications.

In particular, heterogeneous catalysts, which are composed of nanoparticles (NPs) incorporated into MOFs, have attracted significant attention across the scientific community. As unique host matrices, MOFs not only maintain the stability of NPs but also provide outstanding size- or shape-selectivity in catalytic reactions.

Great efforts have been made to fabricate the NPs@MOF composites for heterogeneous catalysis. Nevertheless, most of the incorporated MOFs currently used in heterogeneous catalysis are on the order of hundreds of nanometers or even several micrometers. Despite the excellent catalytic selectivity, MOFs have unfavorably long and narrow channels from their surface to the active sites of the encapsulated NPs, which results in low diffusion rates of the reactants and decreases the catalytic efficiency. As molecular diffusion is a key factor in enhancing the catalytic efficiency of the relevant reactions of NPs@MOF catalysts, the synthesis of smaller MOF crystals, which contain NPs, is favored. The fabrication of nanosized MOF crystals can shorten the diffusion path of the reactants and products to and from the catalytic sites, which increases the diffusion rate of molecules in heterogeneous catalysis.

Firstly, NPs@MOF crystals as small as 50 nm were synthesized by regulating the nucleation and growth of the MOFs around pre-synthesized catalytic-active NPs; in addition, the resulting crystal size could be further controlled. This strategy is suitable for synthesizing a variety of NPs and MOF crystals because well dispersed Pt or Pd NPs can be incorporated in several types of nanosized MOFs (UiO-66, UiO-66-NH₂, MOF-801, ZIF-8, and ZIF-7).

Secondly, to verify the advantage of the NPs@nano-MOFs in heterogeneous catalysis. The synthesis of imines with nitrobenzene as the starting reactant was used as a model reaction to investigate the size effect of the Pt@UiO-66 and demonstrate the catalytic efficiency of the Pt @nano-UiO-66 compared to that of its larger counterparts. The results show that as the size of the Pt@UiO-66 composites decreased, the conversion rate of the reactants increased. Pt@UiO-66~30 nm exhibited the highest catalytic activity compared to the Pt@UiO-66~125 nm or Pt@UiO-66~380 nm catalysts.

Importantly, the enhancement of catalytic activity and the molecular sieving behavior of the NPs@nano-MOFs in heterogeneous catalysis was further demonstrated by the liquid-phase hydrogenation reactions of n-hexene and cis-cyclooctene using Pt@ZIF-8 composites with a size of 40 nm, 850 nm and 1.5 μm as the catalysts.

No conversion was observed in three Pt@ZIF-8 composite samples when cyclooctene was used as the substrate, but all these samples exhibited hydrogenation activities for linear n-hexene, which indicated their excellent size-selectivity. Moreover, the Pt@ZIF-8~40 nm-catalyzed reaction clearly exhibited a higher conversion rate (95.3 %) comparing with the Pt@ZIF-8~850 nm (23.8 %)and Pt@ZIF-8~1.5μm (7.6 %). The results further indicated that the nano-MOFs provided a shortened diffusion distance for the molecules and facilitated the catalytic reactions.

Based on this strategy can be used to encapsulated nanostructures with various NPs and different types of nano-MOFs, it is believed that it would broaden the potential for creating optimized catalysts with high reactivity and selectivity.

The work is led by Prof. Junfeng Liu and her Ph.D. student Bingqing Wang at Beijing University of Chemical Technology. The study Nanoparticles@nanoscale metal-organic framework composites as highly efficient heterogeneous catalysts for size- and shape-selective reactions was recently published in the journal Nano Research.