Low-temperature hydroformylation of ethylene by phosphorous stabilized Rh sites in a one-pot synthesized Rh-(O)-P-MFI zeolite

Dynamic evolution of metal structures on/in zeolites for catalysis

Dynamic changes of metal species always occur during catalysis, and primarily rely on forming mobile metal species initiated by thermal or chemical conditions. During these processes, a support is important in affecting the catalyst stability and dynamic change pathways. Among several supports, zeolites provide ideal features for regulating the migration of metal species due to their unique pore structures and specific defect sites. This review provides a comprehensive summary of typical cases about dynamic migration of metal species on/in metal-zeolite catalysts, analyzing the mechanisms and driving factors of metal migration under different reaction conditions. We discuss the roles of zeolite supports in the migration process of metal species, particularly their crucial contributions to the stability of metal species and the optimization of active sites. In addition, the potential mechanism of the dynamic migration of metal species, theoretical studies, and practical guidance for designing highly efficient catalysts are also included in this review.

Catalytic Hydroformylation of Alkenes to Branched Aldehydes by Triphenylphosphine-Based Porous Organic Polymer-Anchored Rhodium Cluster Catalysts

Hydroformylation of alkenes to high-value-added aldehydes is one of the most important processes in the chemical industry. The use of heterogeneous hydroformylation catalysts encounters substantial challenges including low catalyst activity and stability as well as difficulties in achieving high selectivity for branched aldehydes. In this work, a porous organic polymer (POP) system with embedded triphenylphosphine ligands was prepared, and through impregnation, reduction, and anchoring, rhodium was immobilized on the PPh3-p-Ph4 material as rhodium clusters. Triphenylphosphine exerts significant steric hindrance toward selective alkene hydroformylation, and using water as a solvent also demonstrated pronounced electronic effects, resulting in a high selectivity toward branched aldehydes with a branched-to-linear ratio of more than ten. Mechanistic studies confirmed that the reaction to the branched aldehydes follows pathways with lower reaction energy, thus achieving a high selectivity and yield (95.4%) of branched products. Importantly, the catalyst can be easily recovered and reused while maintaining its activity and selectivity over five cycles without significant losses. The utilization of triphenylphosphine ligands for the preparation of POP supports for rhodium catalysts not only enables the heterogenization of homogeneous complex catalysts but also presents novel ideas and methodologies for future research on alkene hydroformylation to produce branched aldehydes.

Controlling Reaction Pathways of Ethylene Hydroformylation Using Isolated Bimetallic Rhodium-Cobalt Sites

Designing efficient ligand-free heterogeneous catalysts for ethylene hydroformylation to produce C3 oxygenates is of importance for both fundamental research and practical applications, but it is often hindered by insufficient catalytic activity and selectivity. This work designs isolated rhodium-cobalt (Rh-Co) sites confined within a ZSM-5 zeolite to enhance ethylene hydroformylation rates and selectivity while maintaining catalyst stability. By adjusting the Co/Al ratio in Co-ZSM-5, different sizes of Co are formed; subsequent Rh introduction produces isolated Rh1Cox clusters with different Rh-Co coordination numbers (CNs). In-situ characterizations and density functional theory calculations reveal that a Rh-Co CN of 3, corresponding to an isolated Rh1Co3 site, provides optimal bindings to reaction intermediates and thus achieves the highest hydroformylation rates among supported Rh-based catalysts. This study demonstrates the role of coordination-tuning via a secondary metal in effectively controlling the reaction pathway over single Rh atom catalysts.

Hydroformylation over Zeolite Catalysts with Solvophobic Micropores

Traditional gas-liquid-solid triphase reactions are often limited by gas solubility and diffusion in the liquid phase. We reported that a solvophobic catalyst with gas-filled micropores could enrich enormous amounts of gas molecules to accelerate Rh-catalyzed hydroformylation. This reaction used siliceous MFI zeolite fixed Rh nanoparticles in a mesitylene solvent. Owing to the shape selectivity, zeolite micropores prevent mesitylene from wetting the solid, allowing the rapid transport and efficient enrichment of gaseous reactants. This catalyst catalyzed ethylene hydroformylation with a propanal production rate significantly higher than those of the generally supported catalysts.

Structure-Reactivity Relationship of Zeolite-Confined Rh Catalysts for Hydroformylation of Linear α-Olefins

Substituting the molecular metal complexes used in the industrial olefin hydroformylation process is of great significance in fundamental research and practical application. One of the major difficulties in replacing the classic molecular metal catalysts with supported metal catalysts is the low chemoselectivity and regioselectivity of the supported metal catalysts because of the lack of a well-defined coordination environment of the metal active sites. In this work, we have systematically studied the influences of key factors (crystallinity, alkali promoters, etc.) of the Rh-MFI zeolite catalysts on their performances for the hydroformylation of long-chain α-olefins (LAOs). With the help of comprehensive spectroscopy and electron microscopy characterization results, we can correlate the structural features of various Rh-MFI catalysts and their catalytic performances. The resultant structure-reactivity relationship guides us to prepare a nanosized Rh-MFI catalyst, which exhibits about a 3-fold improvement in specific activity compared to the Rh-MFI catalyst with conventional crystallite sizes and maintains very high regioselectivity for hydroformylation of LAOs.

Elucidation of hemilabile-coordination-induced tunable regioselectivity in single-site Rh-catalyzed heterogeneous hydroformylation

Revealing key factors that modulate the regioselectivity in heterogeneous hydroformylation requires identifying and monitoring the dynamic evolution of the truly active center under real reaction conditions. However, unambiguous in situ characterizations are still lacking. Herein, we elaborately construct a series of Rh-POPs catalysts for propylene hydroformylation which exhibited tunable regioselectivity. Multi-technique approaches reveal the unique microenvironment of the diverse HRh(CO)(PPh3-frame)2 sites with distinct P-Rh-P bite angles ranging from 90° to 120° and 158° to 168°, respectively. In situ time-resolved XAFS, FT-IR, and quasi-in situ Solid-state NMR experiments combined with DFT calculations explain the dynamic evolution of the electronic and coordinate state of the distinct active sites induced by hemilabile PPh3-frame ligands and further disclose the regulatory mechanism of regioselectivity. These state-of-the-art techniques and multiscale analysis advance the understanding of how hemilabile coordination influences regioselectivity and will provide a new thought to modulate the regioselectivity in future industrial processes.

Understanding the role of supported Rh atoms and clusters during hydroformylation and CO hydrogenation reactions with in situ/operando XAS and DRIFT spectroscopy

Supported Rh single-atoms and clusters on CeO2, MgO, and ZrO2 were investigated as catalysts for hydroformylation of ethylene to propionaldehyde and CO hydrogenation to methanol/ethanol with in situ/operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS). Under hydroformylation reaction conditions, operando spectroscopic investigations unravel the presence of both single atoms and clusters and detected at first propanal and then methanol. We find that the formation of methanol is associated with CO hydrogenation over Rh clusters which was further confirmed under CO hydrogenation conditions at elevated pressure. The activity of catalysts synthesized via a precipitation (PP) method over various supports towards the hydroformylation reaction follows the order: Rh/ZrO2 > Rh/CeO2 > Rh/MgO. Comparing Rh/CeO2 catalysts synthesized via different methods, catalysts prepared by flame spray pyrolysis (FSP) showed catalytic activity for the hydroformylation reaction at lower temperatures (413 K), whereas catalysts prepared by wet impregnation (WI) showed the highest stability. These results not only provide fundamental insights into the atomistic level of industrially relevant reactions but also pave the way for a rational design of new catalysts in the future.

Suppressing Metal Leaching and Sintering in Hydroformylation Reaction by Modulating the Coordination of Rh Single Atoms with Reactants

Hydroformylation reaction is one of the largest homogeneously catalyzed industrial processes yet suffers from difficulty and high cost in catalyst separation and recovery. Heterogeneous single-atom catalysts (SACs), on the other hand, have emerged as a promising alternative due to their high initial activity and reasonable regioselectivity. Nevertheless, the stability of SACs against metal aggregation and leaching during the reaction has rarely been addressed. Herein, we elucidate the mechanism of Rh aggregation and leaching by investigating the structural evolution of Rh1@silicalite-1 SAC in response to different adsorbates (CO, H2, alkene, and aldehydes) by using diffuse reflectance infrared Fourier transform spectroscopy, X-ray adsorption fine structure, and scanning transmission electron microscopy techniques and kinetic studies. It is discovered that the aggregation and leaching of Rh are induced by the strong adsorption of CO and aldehydes on Rh, as well as the reduction of Rh3+ by CO/H2 which weakens the binding of Rh with support. In contrast, alkene effectively counteracts this effect by the competitive adsorption on Rh atoms with CO/aldehyde, and the disintegration of Rh clusters. Based on these results, we propose a strategy to conduct the reaction under conditions of high alkene concentration, which proves to be able to stabilize Rh single atom against aggregation and/or leaching for more than 100 h time-on-stream.