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1. Strategies for Designing the Catalytic Environment Beyond the Active site of Heterogeneous Supported Metal Catalysts

   https://doi.org/10.1007/s11244-023-01835-2  

   Bhat, Samiha; Pagán-Torres, Yomaira J.; Nikolla, Eranda (June 2023, Topics in Catalysis)

   Significant emphasis has been placed recently in engineering the catalytic environment beyond the active site for tuning the activity, selectivity, and stability of supported metal catalysts for targeted reactions. The environment around the active site in supported catalysts can be modified by introducing multi-dimensionality through alloying, encapsulation, and surface bound ligands. In this Review, we provide a summary of synthesis strategies that have enabled the design of multifunctionality and multidimensionality in heterogeneous supported catalysts. We specifically discuss alloys, encapsulated/inverted catalytic structures, and ligand capped metal nanoparticle systems. We highlight the effects on catalyst activity, selectivity and stability that arise from modifying the neighboring two-dimensional environment through alloying or three-dimensional environment through encapsulation with porous inorganic films or surface organic moieties. We conclude by providing a short perspective on the promises and remaining challenges associated with engineering the local environment around the active sites of supported heterogeneous catalysts. 

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2. Mechanistic insights into alkene chain growth reactions catalyzed by nickel active sites on ordered microporous and mesoporous supports

   https://doi.org/10.1039/d0cy01186j  

   Joshi, Ravi; Saxena, Arunima; Gounder, Rajamani (November 2020, Catalysis Science & Technology)

   null (Ed.)

   Alkene oligomerization on heterogeneous Ni-based catalysts has been studied for several decades, with recent attention focused on the preparation, structure and function of Ni active site motifs isolated within microporous and mesoporous supports, including zeolites and metal–organic frameworks (MOFs). This mini-review focuses on the active site requirements and the microscopic kinetic and mechanistic details that become manifested macroscopically as activation and deactivation behavior during oligomerization catalysis and that determine measured reaction rates and selectivity among alkene isomer products. The preponderance of mechanistic evidence is consistent with the coordination–insertion (Cossee–Arlman) cycle for alkene oligomerization prevailing on heterogeneous Ni-exchanged zeolites and MOFs, even when external co-catalysts are not present, as they often are in homogeneous Ni-based oligomerization catalysis. Certain mechanistic features of the coordination–insertion route allow catalyst and active site design strategies to influence product selectivity. Our mini-review provides a critical discussion of reported alkene oligomerization data and the challenges in their measurement and interpretation and concludes with an outlook for future research opportunities to improve our kinetic and mechanistic understanding of alkene chain growth chemistries mediated by Ni-based porous catalysts. 

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3. Metallic ion leaching from heterogeneous catalysts: an overlooked effect in the study of catalytic ozonation processes

   https://doi.org/10.1039/c7ew00273d  

   Yang, Wenwen; Vogler, Bernhard; Lei, Yu; Wu, Tingting (January 2017, Environ. Sci.: Water Res. Technol.)

   Heterogeneous catalytic ozonation has been increasingly studied for the degradation and mineralization of refractory organic water pollutants in recent years. Compared with homogeneous catalysts, an important advantage of heterogeneous catalysts is that they can be easily separated from the treated water, making the process economically viable. While many studies have focused on the development and evaluation of metal oxide-based catalytic ozonation, possible leaching of metal ions and the subsequent effect on the contaminants' degradation are sometimes overlooked. Here, we examined metal leaching from several solid catalysts and further investigated the influence of the leached metal ions on the mineralization of two model compounds (oxalate and nitrobenzene) during continuous ozonation. Metallic ion leaching was observed from both commercially-available catalysts and catalysts prepared via wet-chemistry methods in the lab. The water matrix has been demonstrated to play an important role in metal leaching. The homogeneous catalytic effect resulting from the leached metal ions was found to be significant. A mechanism involving the formation of an unstable Cu( iii )/oxalate complex through the reaction between ˙OH and Cu( ii )/oxalate was proposed to explain the experimental observations. Our results indicate that the stability of the solid catalysts and the effects of the leached ions must be carefully examined during the catalytic ozonation of organic contaminants. Through this study we highlight the importance of rigorous, accepted protocols for evaluating and reporting heterogeneous catalyst performance in water/wastewater treatment within the research community. 

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4. Redox-active ligand controlled selectivity of vanadium oxidation on Au(100)

   https://doi.org/10.1039/C7SC04752E  

   Tempas, Christopher D.; Morris, Tobias W.; Wisman, David L.; Le, Duy; Din, Naseem U.; Williams, Christopher G.; Wang, Miao; Polezhaev, Alexander V.; Rahman, Talat S.; Caulton, Kenneth G.; et al (January 2018, Chemical Science)

   Metal–organic coordination networks at surfaces, formed by on-surface redox assembly, are of interest for designing specific and selective chemical function at surfaces for heterogeneous catalysts and other applications. The chemical reactivity of single-site transition metals in on-surface coordination networks, which is essential to these applications, has not previously been fully characterized. Here, we demonstrate with a surface-supported, single-site V system that not only are these sites active toward dioxygen activation, but the products of that reaction show much higher selectivity than traditional vanadium nanoparticles, leading to only one V-oxo product. We have studied the chemical reactivity of one-dimensional metal–organic vanadium – 3,6-di(2-pyridyl)-1,2,4,5-tetrazine (DPTZ) chains with O 2 . The electron-rich chains self-assemble through an on-surface redox process on the Au(100) surface and are characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory. Reaction of V-DPTZ chains with O 2 causes an increase in V oxidation state from V II to V IV , resulting in a single strongly bonded (DPTZ 2− )V IV O product and spillover of O to the Au surface. DFT calculations confirm these products and also suggest new candidate intermediate states, providing mechanistic insight into this on-surface reaction. In contrast, the oxidation of ligand-free V is less complete and results in multiple oxygen-bound products. This demonstrates the high chemical selectivity of single-site metal centers in metal–ligand complexes at surfaces compared to metal nanoislands. 

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5. Covalent organic frameworks in heterogeneous catalysis: recent advances and future perspective

   https://doi.org/10.1039/D3QM00188A  

   Alsudairy, Ziad; Brown, Normanda; Campbell, Allea; Ambus, Abrianna; Brown, Bianca; Smith-Petty, Kayla; Li, Xinle (April 2023, Materials Chemistry Frontiers)

   Catalysis is ubiquitous in ∼90% of chemical manufacturing processes and contributes up to 35% of global GDP. Hence, the development of advanced catalytic systems is of utmost importance for academia, industry, and government. Covalent organic frameworks (COFs) are a rapidly emerging class of crystalline porous materials that precisely integrate organic monomer units into extended periodic networks, offering a propitious platform for heterogeneous catalysis due to salient structural merits of ultralow density, high crystallinity, permanent porosity, structural tunability, functional diversity, and synthetic versatility. The past decade has witnessed an upsurge of interest in COFs for heterogeneous catalysis and this trend is expected to continue. In this review, we briefly introduce COF chemistry concerning the design principles, growth mechanism, and cutting-edge advances in structural evolution, linkage chemistry, and facile synthesis. We then scrutinize four leading design strategies for COF catalysts, namely pristine COFs with catalytically active backbones, COFs as hosts for the inclusion of catalytic species, COF-based heterostructures, and COF-derived carbons for thermo-, photo-, and electrocatalysis. Next, we overview the most recent advances (mainly from 2020 to 2023) of COFs in heterogeneous catalysis, along with their fundamentals and advantages. Finally, we outline the current challenges and offer our perspectives on the future directions of COFs for heterogeneous catalysis. 

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