Entrapped Molecular Photocatalyst and Photosensitizer in Metal–Organic Framework Nanoreactors for Enhanced Solar CO₂Reduction

Abstract

Herein, we report on a molecular catalyst embedding metal–organic framework (MOF) that enables enhanced photocatalytic CO2 reduction activity. A benchmark photocatalyst fac-ReBr­(CO)3(4,4′-dcbpy) (dcbpy = dicarboxy-2,2′-bipyridine) and photosensitizer Ru­(bpy)2(5,5′-dcbpy)­Cl2 (bpy = 2,2′-bipyridine) were synergistically entrapped inside the cages of the nontoxic and inexpensive MIL-101-NH2(Al) through noncovalent host–guest interactions. The heterogeneous material improved Re catalyst stabilization under photocatalytic CO2 reduction conditions as selective CO evolution was prolonged from 1.5 to 40 h compared to the MOF-free photosystem upon reactivation with additional photosensitizer. By varying ratios of immobilized catalyst to photosensitizer, we demonstrated and evaluated the effect of reaction environment modulation in defined MOF cages acting as a nanoreactor. This illustrated the optimal efficiency for two photosensitizers and one catalyst per cage and further led to the determination of ad hoc relationships between molecular complex size, MOF pore windows, and number of hostable molecules per cage. Differing from typical homogeneous systems, photosensitizerand not catalystdegradation was identified as a major performance-limiting factor, providing a future route to higher turnover numbers via a rational choice of parameters.

Host–Guest Interactions in a Metal–Organic Framework Isoreticular Series for Molecular Photocatalytic CO2 Reduction

“…These results further confirm the TEOAbased experiments with the overall prolonged higher activity indicating system limitation by TEOA radicals. [10,31] This performance compares well to state-of-the-art colloidal MOF systems with TON values in the mid-100s to low 1000s. [9] In a broader context, our systems are competitive to dye-sensitized TiO 2 semiconductor particles with a surfaceanchored ReCl(CO) 3 (bpy)-derivative, which reached a TON value of 435 in organic solvents with BIH.…”

“…For Re-67, and although theoretically possible as TEOA's maximum molecular diameter of 8.6 is smaller than 67's pore size, this is ascribed to limited TEOA diffusion, reducing efficiency as shown in previous reports on immobilized Re catalysts. [10,12,33,34] Results with ReRu-67 samples are in line with disabled electron transfer between distant complexes due to the surface-anchoring of 2 and entrapping of 1. This is supported by decreasing TONs with higher R MOF values, as the probability of having both dye and catalyst surface-anchored decreases with excess 2.…”

“…[8,9] Solar fuel generation strategies involving MOFs include hosting, photoresponsive materials, encapsulation, and scaffolding. [10][11][12] Alongside other hosting materials (e.g., micelles, particles, covalent organic frameworks), MOF-based systems can display an abundance of diversity in pore size, surface area, and topologies (Figure 1 a) that deeply conditions the elec-tronic communication efficiency between the (photo)electroactive species. [13] Although few studies have started careful exploration of MOF parameters on (photo)catalytic performance, e.g., promoting intermediates in engineered MOF pores, the understanding of host-guest effects, specific anchoring sites, and reactive center distances for molecular catalysts in solar fuel production remains limited.…”

Wirt‐Gast‐Wechselwirkungen in einer Serie isoretikulärer Metall‐organischer Gerüststrukturen für molekulare photokatalytische CO2‐Reduktion

“…Solche MOF‐basierten supramolekularen Wirt‐Gast‐Systeme wurden ausgiebig für katalytische Reaktionen, von der Feinchemikalien‐Synthese bis zur Photokatalyse, untersucht [8, 9] . MOF‐involvierende Strategien zur solaren Brennstofferzeugung umfassen photoresponsive Materialien, Verkapselung und Gerüstbildung [10–12] …”

“…Carboxygruppen an den dcbpy‐Liganden wurden gewählt, um 1 und 2 über Knoten und Amin‐modifizierte Linker am MOF zu verankern. Für Letzteres wurde bereits in der Literatur eine stabile Verankerung gezeigt, die zu kolloidalen Systemen führt, in denen photoinduzierter Elektronentransfer von lichtabsorbierenden Einheiten zu benachbarten Katalysatoren in Gegenwart eines OEDs stattfindet [10, 12, 21] . Unsere rationale Wirt/Gast‐Wahl erlaubt Beladungsvariationen von 1 / 2 auf der äußeren MOF‐Partikeloberfläche oder im Inneren der Kavitäten, ermöglicht durch unterschiedliche MOF‐Porendurchmesser und die molekulare Größe von 1 und 2 , und anschließende systematische Studien zur CO 2 ‐Reduktionseffizienz.…”