Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids

Reactive capture and electrochemical conversion of CO2 with ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture and conversion (RCC) of CO2 due to their wide electrochemical stability window, low volatility, and high CO2 solubility. There is environmental and economic interest in the direct utilization of the captured CO2 using electrified and modular processes that forgo the thermal- or pressure-swing regeneration steps to concentrate CO2, eliminating the need to compress, transport, or store the gas. The conventional electrochemical conversion of CO2 with aqueous electrolytes presents limited CO2 solubility and high energy requirement to achieve industrially relevant products. Additionally, aqueous systems have competitive hydrogen evolution. In the past decade, there has been significant progress toward the design of ILs and DESs, and their composites to separate CO2 from dilute streams. In parallel, but not necessarily in synergy, there have been studies focused on a few select ILs and DESs for electrochemical reduction of CO2, often diluting them with aqueous or non-aqueous solvents. The resulting electrode-electrolyte interfaces present a complex speciation for RCC. In this review, we describe how the ILs and DESs are tuned for RCC and specifically address the CO2 chemisorption and electroreduction mechanisms. Critical bulk and interfacial properties of ILs and DESs are discussed in the context of RCC, and the potential of these electrolytes are presented through a techno-economic evaluation.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Ethene Hydroformylation Catalyzed by Rhodium Dispersed with Zinc or Cobalt in Silanol Nests of Dealuminated Zeolite Beta

Catalysts for hydroformylation of ethene were prepared by grafting Rh into nests of ≡SiOZn-OH or ≡SiOCo-OH species prepared in dealuminated BEA zeolite. X-ray absorption spectra and infrared spectra of adsorbed CO were used to characterize the dispersion of Rh. The Rh dispersion was found to increase markedly with increasing M/Rh (M = Zn or Co) ratio; further increases in Rh dispersion occurred upon use for ethene hydroformylation catalysis. The turnover frequency for ethene hydroformylation measured for a fixed set of reaction conditions increased with the fraction of atomically dispersed Rh. The ethene hydroformylation activity is 15.5-fold higher for M = Co than for M = Zn, whereas the propanal selectivity is slightly greater for the latter catalyst. The activity of the Co-containing catalyst exceeds that of all previously reported Rh-containing bimetallic catalysts. The rates of ethene hydroformylation and ethene hydrogenation exhibit positive reaction orders in ethene and hydrogen but negative orders in carbon monoxide. In situ IR spectroscopy and the kinetics of the catalytic reactions suggest that ethene hydroformylation is mainly catalyzed by atomically dispersed Rh that is influenced by Rh-M interactions, whereas ethene hydrogenation is mainly catalyzed by Rh nanoclusters. In situ IR spectroscopy also indicates that the ethene hydroformylation is rate limited by formation of propionyl groups and by their hydrogenation, a conclusion supported by the measured H/D kinetic isotope effect. This study presents a novel method for creating highly active Rh-containing bimetallic sites for ethene hydroformylation and provides new insights into the mechanism and kinetics of this process.

Ionic Liquid-Modified Porous Organic Polymers as Efficient Metallocene Catalyst Supports

Porous organic polymers (POPs) are widely used in various areas such as adsorption, separation and catalysis. In the present work, ionic liquid-modified porous organic polymers (IL-POPs) synthesized by dispersion polymerization were applied to immobilize metallocene catalysts for olefin polymerization. The prepared IL-POPs were characterized by Fourier transform infrared spectrometer (FT-IR), nitrogen sorption porosimetry, X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), inductively coupled plasma atomic emission spectrometer (ICP) and scanning electron microscope (SEM) analysis. The IL-POPs obtained pores with surface specific area (SSA) ranging from 16.9 m2/g to 561.8 m2/g, and total pore volume (TPV) ranging from 0.08 cm3/g to 0.71 cm3/g. The supported catalysts Zr/MAO@IL-POPs exhibit great activity (3700 kg PE/mol·Zr·bar·h) in ethylene polymerization, and the GPC-IR results show that the polyethylene has narrow molecular weight distribution (2.2 to 2.8). The DSC results show that the melting point of prepared polyethylene was as high as 138 °C, and the TREF analysis results indicate that they have similar chemical composition distribution with elution temperature at 100.5–100.7 °C.

Influence of ionic liquids on the electronic environment of atomically dispersed Ir on (MgO) (100)

Recently, ionic liquids (ILs) have been used as ligands for single-site Ir(CO)2 complexes bound to metal-oxide supports because of their electron-donor/acceptor capacities. The combined effects of supports and ILs as...

Mechanistic and Electronic Insights into a Working NiAu Single-Atom Alloy Ethanol Dehydrogenation Catalyst

Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multipronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective nonoxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge that increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogeneous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step toward developing structure-function relationships that inform the design of new catalysts.

Topological engineering of two-dimensional ionic liquid islands for high structural stability and CO2 adsorption selectivity

Ionic liquids (ILs) as green solvents and catalysts are highly attractive in the field of chemistry and chemical engineering. Their interfacial assembly structure and function are still far less well understood. Herein, we use coupling first-principles and molecular dynamics simulations to resolve the structure, properties, and function of ILs deposited on the graphite surface. Four different subunits driven by hydrogen bonds are identified first, and can assemble into close-packed and sparsely arranged annular 2D IL islands (2DIIs). Meanwhile, we found that the formation energy and HOMO–LUMO gap decrease exponentially as the island size increases via simulating a series of 2DIIs with different topological features. However, once the size is beyond the critical value, both the structural stability and electrical structure converge. Furthermore, the island edges are found to be dominant adsorption sites for CO₂ and better than other pure metal surfaces, showing an ultrahigh adsorption selectivity (up to 99.7%) for CO₂ compared with CH₄, CO, or N₂. Such quantitative structure–function relations of 2DIIs are meaningful for engineering ILs to efficiently promote their applications, such as the capture and conversion of CO₂.

Multi-scale simulations reveal the structure and properties of the two-dimensional ionic liquid islands supported by graphite, and the island edges show an ultrahigh adsorption selectivity for CO₂ compared with CH₄, CO, or N₂.

Interaction between Ionic Liquids and a Pt(111) Surface Probed by Coadsorbed CO as a Test Molecule

We used temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) to study the desorption behavior of CO on Pt(111) coadsorbed with four kinds of ionic liquids (ILs), namely 1-butyl-1-methyl-pyrrolidinium-bis(trifluoromethylsulfonyl)imide ([C₄C₁Pyr][NTf₂]), 1-ethyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide ([C₂C₁Im][NTf₂]), 1-butyl-1-methyl-pyrrolidinium-trifluoro-methanesulfonate ([C₄C₁Pyr][OTf]), and 1-butyl-1-methyl-pyrrolidinium-hexafluorophosphate ([C₄C₁Pyr][PF₆]). We found that CO desorbs earlier from a Pt(111) surface with coadsorbed ILs than without. In addition, the CO desorption temperature varies between different types of coadsorbed ILs, which follows the order: [C₄C₁Pyr][PF₆] (365 K) > [C₄C₁Pyr][NTf₂] (362 K) > [C₂C₁Im][NTf₂] (352 K) > [C₄C₁Pyr][OTf] (348 K). We ascribe the difference in CO desorption temperature to the different interaction strength between ILs and the Pt(111) surface. A stronger IL-Pt(111) interaction leads to a lower CO desorption temperature. We suggest that TP-IRAS experiments of CO coadsorbed with ILs can be a useful method to aid the characterization of the interaction strength between ILs and metal surfaces such as Pt(111).

Prototype Atomically Dispersed Supported Metal Catalysts: Iridium and Platinum

When metal nanoparticles on supports are made smaller and smaller-to the limit of atomic dispersion-they become cationic and take on new catalytic properties that are only recently being discovered. The synthesis of these materials is reviewed, including their structure characterization-especially by atomic-resolution electron microscopy and X-ray absorption and infrared spectroscopies-and relationships between structure and catalyst performance, for reactions including hydrogenations, oxidations, and the water gas shift. Structure determination is challenging because of the intrinsic nonuniformity of the support surfaces-and therefore the structures on them-but fundamental understanding has advanced rapidly, benefiting from nearly uniform catalysts consisting of metals on well-defined-crystalline-supports and their characterization by spectroscopy and microscopy. Recent advances in atomic-resolution electron microscopy have spurred the field, providing stunning images and deep insights into structure. The iridium catalysts have typically been made from organoiridium precursors, opening the way to understanding and control of the metal-support bonding and ligands on the metal, including catalytic reaction intermediates. Platinum catalysts are usually made with less precision, from salt precursors, but they catalyze a wider array of reactions than the iridium, typically being stable at higher temperatures and seemingly offering rich prospect for discovery of new catalysts.

Cooperativity in supported metal single atom catalysis

The discussion concerning cooperativity in supported single-atom (SA) catalysis is often limited to the metal-support interaction, which is certainly important, but which is not the only lever for modifying the catalytic performance. Indeed, if the interaction between the SA and the support, which can be seen as a solid ligand presenting its own specificities that fix the first coordination sphere of the metal, plays a central role as in homogeneous catalysis, other factors can strongly contribute to modification of the activity, selectivity and stability of SAs. Therefore, in this mini-review, we briefly summarize the importance of the support (oxide, carbon or a second metal) in SA photo- electro- and thermal-catalysis (support-assisted operation), and concentrate on other types of cooperativities that in some cases enable previously impossible reaction pathways on supported metal SAs. This includes topics that are not specific to SA catalysis, such as metal-ligand or heterobimetallic cooperativity, and cooperativity which is SA-specific such as nanoparticle-SA or mixed-valence SA cooperativity.

Ligand-coordinated Ir single-atom catalysts stabilized on oxide supports for ethylene hydrogenation and their evolution under a reductive atmosphere

Effective, stable, durable, and tunable Ir-ligand single-atom catalysts for ethylene hydrogenation, studied in situ for structural evolution of Ir single-atoms under a reducing atmosphere.

Towards complete elucidation of structural factors controlling thermal stability of IL/MOF composites: effects of ligand functionalization on MOFs

In this work, we incorporated an ionic liquid (IL), 1-n-butyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO4]) into two different metal organic frameworks (MOFs), UiO-66, and its amino-functionalized counterpart, NH2-UiO-66, to investigate the effects of ligand-functionalization on the thermal stability limits of IL/MOF composites. The as-synthesized IL/MOF composites were characterized in detail by combining x-ray diffraction, scanning electron microscopy, Brunauer-Emmett-Teller analysis, x-ray fluorescence, infrared spectroscopies (FTIR), and their thermal stability limits were determined by thermogravimetric analysis (TGA). Characterization data confirmed the successful incorporation of the IL into each MOF and indicated the presence of direct interactions between them. A comparison of the interactions in [BMIM][MeSO4]-incorporated UiO-66 and NH2-UiO-66 with those in their 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])-incorporated counterparts showed that the hydrophilic IL, [BMIM][MeSO4], interacts with the 1,4-benzenedicarboxylate (BDC) ligand of the UiO-66, while the hydrophobic IL, [BMIM][PF6], is interacting with the joints where zirconium metal cluster coordinates with BDC ligand. The TGA data demonstrated that the composite with the ligand-functionalized MOF, NH2-UiO-66, exhibited a lower percentage decrease in the maximum tolerable temperature compared to those of IL/UiO-66 composites. Moreover, it is discovered that when the IL is hydrophilic, its hydrogen bonding ability can be utilized to designate an interaction site on MOF's ligand structure, leads to a lower reduction in thermal stability limits. These results provide insights for the rational design of IL/MOF composites and contribute towards the complete elucidation of structural factors controlling the thermal stability.

Outstanding catalytic performance in the semi-hydrogenation of acetylene in a front-end process by establishing a "hydrogen deficient" phase

Pd-[Bmim][Cl] phase was immobilized onto Al2O3 to neutralize the excessive hydrogen in the gas phase to prevent the over-hydrogenation of acetylene, thereby achieving a high selectivity for ethylene.

MIL-53(Al) as a Versatile Platform for Ionic-Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium-based ionic liquids (ILs) were incorporated into a metal-organic framework (MOF), MIL-53(Al), to investigate the effect of IL incorporation on the CO2 separation performance of MIL-53(Al). CO2 , CH4 , and N2 adsorption isotherms of the IL/MIL-53(Al) composites and pristine MIL-53(Al) were measured to evaluate the effect of the ILs on the CO2 /CH4 and CO2 /N2 selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF6 ]/MIL-53(Al) exhibited the largest increase in CO2 /CH4 selectivity, 2.8-times higher than that of pristine MIL-53(Al), whilst [BMIM][MeSO4 ]/MIL-53(Al) exhibited the largest increase in CO2 /N2 selectivity, 3.3-times higher than that of pristine MIL-53(Al). A comparison of the CO2 separation potentials of the IL/MOF composites showed that the [BMIM][BF4 ]- and [BMIM][PF6 ]-incorporated MIL-53(Al) composites both showed enhanced CO2 /N2 and CO2 /CH4 selectivities at pressures of 1-5 bar compared to composites of CuBTC and ZIF-8 with the same ILs. These results demonstrate that MIL-53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas-separation applications.

Improving CO2 Separation Performance of MIL-53(Al) by Incorporating 1-n-Butyl-3-Methylimidazolium Methyl Sulfate

1-n-Butyl-3-methylimidazolium methyl sulfate is incorporated into MIL-53(Al). Detailed characterization is done by X-ray fluorescence, Brunauer-Emmett-Teller surface area, scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. Results show that ionic liquid (IL) interacts directly with the framework, significantly modifying the electronic environment of MIL-53(Al). Based on the volumetric gas adsorption measurements, CO2, CH4, and N2 adsorption capacities decreased from 112.0, 46.4, and 19.6 cc (STP) gMIL-53(Al) -1 to 42.2, 13.0, and 4.3 cc (STP) gMIL-53(Al) -1 at 5 bar, respectively, upon IL incorporation. Data show that this postsynthesis modification leads to more than two and threefold increase in the ideal selectivity for CO2 over CH4 and N2 separations, respectively, as compared with pristine MIL-53(Al). The isosteric heat of adsorption (Qst) values show that IL incorporation increases CO2 affinity and decreases CH4 and N2 affinities. Cycling adsorption-desorption measurements show that the composite could be regenerated with almost no decrease in the CO2 adsorption capacity for six cycles and confirm the lack of any significant IL leaching. The results offer MIL-53(Al) as an excellent platform for the development of a new class of IL/MOF composites with exceptional performance for CO2 separation.

Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports

Single-site Ir(CO)2 complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)2(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.

The conversion of α-pinene to cis-pinane using a nickel catalyst supported on a discarded fluid catalytic cracking catalyst with an ionic liquid layer

The concept of a solid catalyst coated with a thin ionic liquid layer (SCILL) was applied to the stereoselective hydrogenation of α-pinene. Nickel, a non-noble metal, was supported on a discarded fluid catalytic cracking catalyst (DF3C) and then modified with different loadings of the ionic liquid 1-ethanol-3-methylimidazolium tetrafluoroborate ([C2OHmim][BF4]). The resulting catalysts showed a range of conversions and selectivities for the hydrogenation of α-pinene. The SCILL catalysts afforded cis-pinane with high selectivity and their activity depended on the ionic liquid loading. For an ionic liquid loading of 10 wt%, although the catalytic activity was suppressed, the selectivity and conversion could reach above 98% and 99%, respectively. In addition, the catalyst remained stable after 13 runs and the activity was almost unchanged with the conversion maintained at approximately 99%. Thus, the ionic liquid layer not only improved the selectivity for cis-pinane but also protected the active site of the catalyst and prolonged the service lifetime of the catalyst. The SCILL catalytic system provides an example of an ionic liquid catalytic system which eliminates organic solvents from the catalytic process.

Harnessing the Wisdom in Colloidal Chemistry to Make Stable Single-Atom Catalysts

Research on single-atom catalysts (SACs), or atomically dispersed catalysts, has been quickly gaining momentum over the past few years. Although the unique electronic structure of singly dispersed atoms enables uncommon-sometimes exceptional-activities and selectivities for various catalytic applications, developing reliable and general procedures for preparing stable, active SACs in particular for applications under reductive conditions remains a major issue. Herein, the challenges associated with the synthesis of SACs are highlighted semiquantitatively and three stabilization techniques inspired by colloidal science including steric, ligand, and electrostatic stabilization are proposed. Some recent examples are discussed in detail to showcase the power of these strategies in the synthesis of stable SACs without compromising catalytic activity. The substantial further potential of steric, ligand, and electrostatic effects for developing SACs is emphasized. A perspective is given to point out opportunities and remaining obstacles, with special attention given to electrostatic stabilization where little is done so far. The stabilization strategies presented herein have a wide applicability in the synthesis of a series of new SACs with improved performances.

Toward Rational Design of Ionic Liquid/Metal-Organic Framework Composites: Effects of Interionic Interaction Energy

One of the structural factors controlling the extent of interactions between ionic liquids (ILs) and metal-organic frameworks (MOFs) in IL/MOF composites is elucidated. Results showed that the thermal stability limits and adsorption performances of the IL/MOF composites can be tuned by the interionic interaction energy of bulk ILs, which can be probed spectroscopically via C2H infrared stretching frequency.