Switchable Selectivity Research Articles

CO2 Electroreduction on Unsupported Pdpt Aerogels: Effects of Alloying and Surface Composition

The prospect of valorizing anthropogenic CO2 emissions by converting this greenhouse gas into added-value products (e.g., CO, formate) through electrochemical conversion in near-neutral aqueous media holds great research interest. While palladium-based catalysts have been repeatedly shown to display a switchable selectivity between CO and formate at high vs. low overpotentials, respectively, a large fraction of the total current is used to convert water into hydrogen as a parasitic byproduct. This is especially true for carbon-supported materials at high overpotentials (i.e., ≤ − 0.7 V vs. the reversible hydrogen electrode), in which the conversion of water to H2 on carbon and of CO2 to CO on Pd occur at comparable rates. Therefore, the removal of this carbon concomitant to the use of unsupported structures that can maintain a high surface area (e.g., as in aerogels) holds the promise of leading to higher selectivities towards CO2 reduction products. Additionally, alloying of metals is a well-researched approach to modify the adsorption kinetics of intermediate species enabling improvement in activity and the tuning of the selectivity towards a certain product. Specifically, alloys of palladium and platinum have been shown to possess high activities for formate generation from CO2 over a wide potential range [1].With this motivation, in this work, unsupported PdPt-aerogels were synthesized in five alloy compositions (from Pd50Pd50 to Pd90Pt10) and as pure metals (Pd100 and Pt100) at a constant web thickness of 6 ± 1 nm using a novel ethanolic synthesis approach [2]. The aerogels’ structure was studied using high-resolution transmission and scanning electron microscopies (HR-TEM, SEM) confirming the homogeneity of the materials. The alloying-degree of the PdPt aerogels was thereafter investigated using ex-situ X-ray absorption spectroscopy (XAS) measurements from which we derived the Cowley short-range order parameter [3], finding all compositions to be random alloys of Pd and Pt with a stoichiometry in good agreement with the Pt:Pd ratios aimed in the synthesis. Furthermore, energy dispersive X-ray spectroscopy (EDX) measurements in line-scan mode were used to investigate the elemental distribution within the structure of the gel (see Fig. 1A-1F), unveiling a slight enrichment of Pt on the surface of the bimetallic PdPt aerogels.These spectroscopic measurements were complemented by electrochemical characterization using CO-stripping as a means to infer the aerogels’ electrochemical surface area (ECSA) and surface composition. While the inferred ECSA values were all ≥70% of what is the expected value based on the aerogels’ web thickness, we observed significant changes in the shape of the hydrogen underpotential deposition (Hupd) that indicate alloy formation on the surface of the aerogels. Additionally, clear trends in the potential position of the CO-oxidation peak as a function of the alloy composition, and in the CO-oxidation charge depending on the CO-adsorption potential (i.e., 0.1 vs. 0.4 V vs. the reversible hydrogen electrode) were found.Lastly, the CO2-reduction activity of these bimetallic PdPt aerogels was tested in our custom, online gas chromatography setup for CO2-electroreduction. Using this approach, the pure palladium aerogel (Pd100) was shown to display Faradaic efficiencies towards CO and formate exceeding 90% at high or low overpotentials, respectively, proving the beneficial effect of the removal of the carbon support on palladium’s CO2 electroreduction selectivity. Meanwhile, the PdPt aerogel alloys featured formate and CO Faradaic efficiencies much lower than those previously reported in Ref. 1, with hydrogen making up the majority of the Faradaic efficiency (see Fig. 1G). Here, the activity towards CO2 electroreduction products was found to decrease with increasing Pt-content, so that on the pure Pt-aerogel (Pt100) hydrogen was the only product. These results are in line with the above-described Pt-enrichment of the alloys’ surfaces, which explains their high activity for the HER reaction and reduced activity for CO2 electroreduction. Figure 1

Switchable Selectivity in the Asymmetric Rhodium-Catalyzed Hydrothiolation of Cyclopropenes

Switchable Selectivity in the Asymmetric Rhodium-Catalyzed Hydrothiolation of Cyclopropenes

Enantioselective Hydrothiolation: Diverging Cyclopropenes through Ligand Control.

In this article, we advance Rh-catalyzed hydrothiolation through the divergent reactivity of cyclopropenes. Cyclopropenes undergo hydrothiolation to provide cyclopropyl sulfides or allylic sulfides. The choice of bisphosphine ligand dictates whether the pathway involves ring-retention or ring-opening. Mechanistic studies reveal the origin for this switchable selectivity. Our results suggest the two pathways share a common cyclopropyl-Rh(III) intermediate. Electron-rich Josiphos ligands promote direct reductive elimination from this intermediate to afford cyclopropyl sulfides in high enantio- and diastereoselectivities. Alternatively, atropisomeric ligands (such as DTBM-BINAP) enable ring-opening from the cyclopropyl-Rh(III) intermediate to generate allylic sulfides with high enantio- and regiocontrol.

A photoprogrammable electronic nose with switchable selectivity for VOCs using MOF films.

Advanced analytical applications require smart materials and sensor systems that are able to adapt or be configured to specific tasks. Based on reversible photochemistry in nanoporous materials, we present a sensor array with a selectivity that is reversibly controlled by light irradiation. The active material of the sensor array, or electronic nose (e-nose), is based on metal–organic frameworks (MOFs) with photoresponsive fluorinated azobenzene groups that can be optically switched between their trans and cis state. By irradiation with light of different wavelengths, the trans–cis ratio can be modulated. Here we use four trans–cis values as defined states and employ a four-channel quartz-crystal microbalance for gravimetrically monitoring the molecular uptake by the MOF films. We apply the photoprogrammable e-nose to the sensing of different volatile organic compounds (VOCs) and analyze the sensor array data with simple machine-learning algorithms. When the sensor array is in a state with all sensors either in the same trans- or cis-rich state, cross-sensitivity between the analytes occurs and the classification accuracy is not ideal. Remarkably, the VOC molecules between which the sensor array shows cross-sensitivity vary by switching the entire sensor array from trans to cis. By selectively programming the e-nose with light of different colors, each sensor exhibits a different isomer ratio and thus a different VOC affinity, based on the polarity difference between the trans- and cis-azobenzenes. In such photoprogrammed state, the cross-sensitivity is reduced and the selectivity is enhanced, so that the e-nose can perfectly identify the tested VOCs. This work demonstrates for the first time the potential of photoswitchable and thus optically configurable materials as active sensing material in an e-nose for intelligent molecular sensing. The concept is not limited to QCM-based azobenzene-MOF sensors and can also be applied to diverse sensing materials and photoswitches.

A Combined Spectroscopic and Electrochemical Approach to Investigate the Selectivity Determining Factors for CO2 Electroreduction on Pd-Nanoparticles

The electrochemical conversion of carbon dioxide into added-value products keeps on receiving increased interest as a means to valorize CO2 while diminishing its anthropogenic emissions. A broad spectrum of CO2-electroreduction products (e.g., CH4, C2H4) has been reported, but only a few are expected to be economically competitive with regards to industrially established chemical production pathways [1]. Interestingly, Pd-based nanoparticle catalysts are particularly well suited for the production of two of such products, since they feature the unique ability to selectively produce formate and carbon monoxide (CO) at low vs. high overpotentials, respectively [2]. This switching of the selectivity with the applied potential has been linked to a difference in the nature of the Pd catalyst and corresponding reaction mechanism. Specifically, at potentials below − 0.4 VRHE at which CO is the main reduction product [2,3], the reaction is believed to proceed on a fully CO-covered Pd-surface [4]. At higher potentials between − 0.4 and 0 VRHE, though, formate is predominantly produced [2,3] through a hypothesized CO2-hydrogenation mechanism on palladium hydride (PdHx) [5].Chiefly, the design of better-performing catalysts requires a better understanding of the relation between the precise stoichiometry of this hydride phase (i.e., the value of x in PdHx) and the applied potential, reaction duration and corresponding coverage of surface adsorbates (e.g., CO).To determine the relation between the above-described parameters, in this study we used a commercial carbon-supported palladium catalyst (40 % Pd/C) that displayed the above-described switchable selectivity between formate and CO in online gas chromatography measurements in a newly designed parallel plate cell (Figure 1). Specifically, very high selectivities of up to 90 % for formate were found at very low overpotentials (− 0.1 to − 0.3 VRHE), while at high overpotentials (− 0.7 to − 0.9 VRHE) CO is the main product with faradaic efficiencies of over 60 %, and only low amounts of formate are found. The interplay between surface adsorbates and palladium hydride formation was then investigated by quantifying the catalyst’s time and potential-dependent hydride content using chronoamperometry complemented with operando X-ray absorption spectroscopy (XAS) in our group's spectroelectrochemical flow cell [6]. Here, we investigated the influence of CO2 presence in the electrolyte acquiring XAS over 10 minutes of potential hold in both N2- and CO2-saturated potassium bicarbonate.As an example of our results, Figure 2 shows the X-ray absorption near edge structure (XANES) spectra after ten minutes of holding potential at − 0.1 VRHE. While we observed full hydride formation in N2-saturated KHCO3, the palladium in the CO2 saturated electrolyte remained in full metallic state, suggesting that in the presence of carbon dioxide surface adsorbates prevent the formation of PdHx. Additionally, we were able to monitor changes of the palladium hydride stoichiometry in the seconds- to minutes-timescales, thereby enabling us to investigate the hydride formation speeds with an unprecedented time resolution. Finally, the strongly adsorbed surface species formed during CO2 electroreduction were identified and their surface coverage quantified using a novel combined rotating disk electrode (RDE) cyclic voltammetry and in situ external reflection Fourier-transform infrared spectroscopy (FTIR) setup. This allowed us to determine that in the CO2 saturated bicarbonate electrolyte the Pd-surface is covered by a full monolayer of surface-adsorbed CO that prevents hydrogen adsorption and subsequently palladium hydride formation, as unveiled by our XAS results.In conclusion, we have combined electrochemical with spectroscopic methods providing us with the means to improve our understanding of the palladium hydride stoichiometry and adsorbed surface species under CO2 electroreduction conditions. Thus, this contribution will provide novel insight into the relation among the time-dependent bulk and surface composition of Pd-nanoparticles and their link to their CO2-reduction selectivity.Literature:[1] J. Durst et al., Chimia, 2015, 69, 769. [2] M. Rahaman et al., ChemSusChem, 2017, 10, 1733. [3] D. Gao et al., NanoResearch, 2017, 10, 2181. [4] R. Kortlever et al., Catalysis Today, 2015, 244, 58. [5] X. Min et al., J. Am. Chem. Soc., 2015, 137, 4701. [6] T. Binninger et al., J. Elec. Soc., 2016, 163, H906. Figure 1

Tuning the Selectivity of Palladium Catalysts for Hydroformylation and Semihydrogenation of Alkynes: Experimental and Mechanistic Studies

Here, we describe a selective palladium catalyst system for chemodivergent functionalization of alkynes with syngas. In the presence of an advanced ligand L2 bearing 2-pyridyl substituent as a built-in base, either hydroformylation or semihydrogenation of diverse alkynes occurs with high chemo- and stereoselectivity under comparable conditions. Mechanistic studies, including density functional theory (DFT) calculations, kinetic analysis, and control experiments, revealed that the strength and concentration of acidic cocatalysts play a decisive role in controlling the chemoselectivity. DFT studies disclosed that ligand L2 not only promotes heterolytic activation of hydrogen similar to frustrated Lewis pair (FLP) systems in the hydrogenolysis step for hydroformylation but also suppresses CO coordination to promote semihydrogenation under strong acid conditions. This switchable selectivity provides a strategy to design new catalysts for desired products.

A hybrid block consisting of covalent triazine frameworks and GO aerogel with switchable selectivity between adsorption of UV filters and regeneration under sunlight

UV filters, a kind of personal care product (PCP), are intractably degraded using conventional processes because of their trace concentration in the environment. The exploration of high-performance, stable, and reusable materials for the highly efficient removal of UV filters is of great importance. In this study, a series of three-dimensional graphene oxide-covalent triazine-based frameworks aerogels (3D GO-CTF) prepared with different concentration ratios were designed to remove trace concentrations of benzophenone (BP), 2,4-dihydroxybenzophenone (BP-1) and 2-hydroxy-4-methoxybenzophenone (BP-3) from water, which are regenerated by sunlight. 3D GO-CTF not only adsorbs UV filters with high efficiency (0.0311 ± 0.0009 g/(μmol·s)) and capacity (2963.708 ± 4.637 μmol/g), but can also be regenerated with an extremely high percentage recovery of 90–95% in the 4th cycle. These 3D GO-CTF materials exhibit switchable selectivity between the adsorption of the UV filters and regeneration of the 3D GO-CTF by regulating the content of GO to function as the adsorbent and CTF for photocatalytic regeneration. Moreover, the high reusability and extreme stability of the 3D GO-CTF aerogels were verified over seven cycles of night-day testing in a real water matrix, which demonstrated their potential application in the efficient removal of micropollutants from the environment.

Effects of Preparation Parameters of NiAl Oxide-Supported Au Catalysts on Nitro Compounds Chemoselective Hydrogenation.

Effects of preparation parameters of NiAl oxide-supported Au nanocatalysts on their performance in the chemoselective hydrogenation of nitro compounds were investigated. The deposition–precipitation method, low Au loading, and low Ni/Al molar ratio of the support contributed to the generation of small-sized Au nanoparticles. High catalytic properties were related to the small sizes of Au particles and appropriate basicity of supports. Accordingly, the 0.43% Au/NiAlO-2-500 (the Ni/Al molar ratio of the support = 2) showed high activity and excellent selectivity for nitro hydrogenation. It also showed good versatility for other nitro compounds and good recyclability. Interestingly, for the first time, this Au catalyst switched its selectivity to vinyl hydrogenation by mere regulation of the composition of the support (the Ni/Al molar ratio of the support = 4). The observed shift in selectivity was ascribed to the different adsorption behaviors of the nitro and vinyl group on Au nanocatalysts. It provides a novel and facile strategy to construct Au nanocatalysts with high activity and switchable selectivity for hydrogenation of nitro compounds by the fine tuning of preparation parameters.

Aerobic oxidation of primary benzylic amines to amides and nitriles catalyzed by ruthenium carbonyl clusters carrying N,O-bidentate ligands.

Four trinuclear ruthenium carbonyl clusters, (6-BrPyCHRO)2Ru3(CO)8 (R = 4-OCH3C6H4, 1a; R = 4-BrC6H4, 1b) and (2-OC6H4-HC[double bond, length as m-dash]N-C6H4R)2Ru3(CO)8 (R = 4-OCH3, 2a; R = 4-Br, 2b), were synthesized from the reactions of Ru3(CO)12 with the corresponding N,O-bidentate ligands (two pyridyl alcohols and two Schiff bases) respectively in a ratio of 1 : 2. Three new complexes 1b, 2a and 2b have been fully characterized by elemental analysis, FT-IR, NMR and X-ray crystallography. The catalytic activity of these ruthenium complexes for the aerobic oxidation of primary benzylic amines to amides and nitriles in the presence of t-BuOK was investigated, of which the Schiff base complex 2a was found to exhibit the highest activity.

Modelling of Conductive Nanoporous Membranes with Switchable Ionic Selectivity

An interesting area of modern membrane science is the development of “smart” membranes, which can affect the transport properties of selected components via external tuning. When the target components are ions, such a tuning can be realized with the help of electric field created by the conductive pore surface. We have proposed in this work a mathematical model of ions transport in a cylindrical nanopore with the electronic charge at the conductive surface and the chemical charge, which is separated from the surface by the Stern layer. The model is based on one-dimensional equations for potential, ion concentrations, and pressure in the diffuse layer. It is applied for describing the membrane potential at zero current, which characterizes the type and strength of ionic selectivity. It is shown that the change of surface potential in the direction from negative to positive results in the continuous change of pore selectivity from cation to anion. The decrease in the Stern layer capacitance and increase in the pore radius lead to the decrease in ionic selectivity. The presence of positive (negative) chemical charge causes the shift of potential value, at which the selectivity is switched, in the direction of negative (positive) values. At this value of potential, the membrane becomes non-selective, the diffusive flux of ions reaches maximum, and the osmotic flow ceases. The suggested model provides a qualitative and quantitative description of experimental results on switchable selectivity of track-etched membranes modified by the gold coating.

A switchable-selectivity multiple-interface Ni-WC hybrid catalyst for efficient nitroarene reduction

Selective reduction of nitroarenes is extremely valuable in industrial chemical production. The main reduced products are usually aniline derivatives obtained using single-component noble- or transition-metal catalysts; however, other important products such as hydrazobenzene derivatives always involve in harsh conditions and multiple reaction steps. Here, we realize an unexpected switchable reduction of nitroarenes into aniline or hydrazobenzene derivatives with high yield and selectivity just by controlling the molar ratio of nitroarenes to N2H4·H2O with a nickel–tungsten carbide composite nanocatalyst loaded on carbon (Ni-WC/C). A series of control experiments and density functional theory (DFT) calculations indicate that the multiple interfaces between Ni and WC can induce a synergistic effect, significantly modulating the electronic structure of the Ni-WC/C catalyst, and endowing the catalyst with switchable selectivity and high activity for the reduction of nitroarenes by hydrogenation. This synergistic multi-interfacial catalyst may offer a new way to design and explore highly efficient and selective catalysts for the controllable reduction of nitroarenes and similar hydrogenation reactions.

Metal–Organic Capsules with NADH Mimics as Switchable Selectivity Regulators for Photocatalytic Transfer Hydrogenation

Switchable selective hydrogenation among the groups in multifunctional compounds is challenging because selective hydrogenation is of great interest in the synthesis of fine chemicals and pharmaceuticals as a result of the importance of key intermediates. Herein, we report a new approach to highly selectively (>99%) reducing C═X (X = O, N) over the thermodynamically more favorable nitro groups locating the substrate in a metal-organic capsule containing NADH active sites. Within the capsule, the NADH active sites reduce the double bonds via a typical 2e- hydride transfer hydrogenation, and the formed excited-state NAD+ mimics oxidize the reductant via two consecutive 1e- processes to regenerate the NADH active sites under illumination. Outside the capsule, nitro groups are highly selectively reduced through a typical 1e- hydrogenation. By combining photoinduced 1e- transfer regeneration outside the cage, both 1e- and 2e- hydrogenation can be switched controllably by varying the concentrations of the substrates and the redox potential of electron donors. This promising alternative approach, which could proceed under mild reaction conditions and use easy-to-handle hydrogen donors with enhanced high selectivity toward different groups, is based on the localization and differentiation of the 2e- and 1e- hydrogenation pathways inside and outside the capsules, provides a deep comprehension of photocatalytic microscopic reaction processes, and will allow the design and optimization of catalysts. We demonstrate the advantage of this method over typical hydrogenation that involves specific activation via well-modified catalytic sites and present results on the high, well-controlled, and switchable selectivity for the hydrogenation of a variety of substituted and bifunctional aldehydes, ketones, and imines.

Switchable selectivity in Pd-catalyzed [3 + 2] annulations of γ-oxy-2-cycloalkenones with 3-oxoglutarates: C-C/C-C vs C-C/O-C bond formation.

Two complementary [3 + 2] annulation protocols between 3-oxoglutarates and cyclic γ-oxy-2-cycloalkenones, simply differing on the reaction temperature, are disclosed. These domino transformations allow C–C/O–C or C–C/C–C [3 + 2] annulations at will, via an intermolecular Pd-catalyzed C-allylation/intramolecular O- or C-1,4-addition sequence, respectively. In particular, exploiting the reversibility of the O-1,4-addition step, in combination with the irreversible C-1,4-addition/decarboxylation path, the intramolecular conjugate addition step could be diverted from the kinetic (O-alkylation) to the thermodynamic path (C-alkylation) thanks to a simple temperature increase. Crucial for the success of this bis-nucleophile/bis-electrophile [3 + 2] annulation is its well-defined step chronology in combination with the total chemoselectivity of the former step. This [3 + 2] C–C/O–C bond forming annulation protocol could be also extended to 1,3,5-triketones as well as 1,3-bis-sulfonylpropan-2-one bis-nucleophiles.

One Base for Two Shots: Metal-Free Substituent-Controlled Synthesis of Two Kinds of Oxadiazine Derivatives from Alkynylbenziodoxolones and Amidoximes.

Different 1,2-migrations and cyclizations of a variety of alkynylbenziodoxolones (EBXs) and amidoximes under one-base conditions are described. This process provides an efficient protocol for the divergent synthesis of two oxadiazine derivatives via switchable selectivity of EBXs controlled by simply varying the substituents of the amidoximes, which feature transition metal-free conditions, simple execution, and high chemoselectivity.

Visible light-mediated selective α-functionalization of 1,3-dicarbonyl compounds via disulfide induced aerobic oxidation.

A visible light-mediated α-functionalization of 1,3-dicarbonyl compounds with switchable selectivity induced by disulfide is disclosed. Upon irradiation with visible light, the metal- and base-free α-hydroxylation or α-hydroxymethylation reaction proceeded smoothly through a disulfide-catalyzed oxidation under mild conditions. The combination of a continuous-flow strategy could further improve the reaction efficiencies.

Metal-organic framework patterns and membranes with heterogeneous pores for flow-assisted switchable separations

Porous metal-organic-frameworks (MOFs) are attractive materials for gas storage, separations, and catalytic reactions. A challenge exists, however, on how to introduce larger pores juxtaposed with the inherent micropores in different forms of MOFs, which would enable new functions and applications. Here we report the formation of heterogeneous pores within MOF particles, patterns, and membranes, using a discriminate etching chemistry, called silver-catalyzed decarboxylation. The heterogeneous pores are formed, even in highly stable MOFs, without altering the original structure. A decarboxylated MOF membrane is shown to have pH-responsive switchable selectivity for the flow-assisted separation of similarly sized proteins. We envision that our method will allow the use of heterogeneous pores for massive transfer and separation of complex and large molecules, and that the capability for patterning and positioning heterogeneous MOF films on diverse substrates bodes well for various energy and electronic device applications.

Switchable Selectivity in the Pd-Catalyzed Alkylative Cross-Coupling of Esters.

The Pd-catalyzed cross-coupling of phenyl esters and alkyl boranes is disclosed. Two reaction modes are rendered accessible in a selective fashion by interchange of the catalyst. With a Pd-NHC system, alkyl ketones can be prepared in good yields via a Suzuki-Miyaura reaction proceeding by activation of the C(acyl)-O bond. Use of a Pd-dcype catalyst enables alkylated arenes to be synthesized by a modified pathway with extrusion of CO. Applications of this divergent coupling strategy and the origin of the switchable selectivity are discussed.

Additive‐Controlled Switchable Selectivity from Cyanobenzenes to 2‐Alkynylpyridines: Ruthenium(II)‐Catalyzed [2+2+2] Cycloadditions of Diynes and Alkynylnitriles

AbstractA highly efficient additive‐dependent chemoselective protocol for the synthesis of fused cyanoarenes and 2‐alkynylpyridines has been developed by the reaction of 1,6‐diynes with alkynylnitriles using chloro(pentamethylcyclopentadienyl) (cyclooctadiyne)ruthenium(II) as catalyst in dimethoxyethane (DME). The course of the reaction can be drastically altered simply by adding a catalytic amount of AgOTf as an additive resulting in a comprehensive shift in product formation from cyanoarenes to 2‐alkynylpyridines. Theoretical studies clearly indicate that the neutral Ru‐complex is responsible for the formation of cyanobenzenes, whereas the in situ generated cationic Ru‐complex plays a crucial role in the 2‐alkynylpyridines formation.magnified image

Switchable selectivity within a series of boronate esters for dynamic covalent exchange in nonaqueous solvents

The reversible condensation–hydrolysis reactions of boronic acids have proven to be a highly useful class of thermodynamically controlled dynamic covalent process, enabling the construction of sugar sensors, stimuli-responsive materials, and complex covalent architectures. Yet, the common diol or diphenol coupling partners tend to produce relatively unstable condensation products, exhibit oxidative sensitivity, or offer limited options for expanding structural diversity. To address these drawbacks, we explore a series of coupling partners including non-diol salicylate and salicylamide derivatives, in combination with two boronic acids. In nonaqueous solvents, the condensation–hydrolysis equilibria are sensitive to the nature and concentration of Lewis bases, with equilibrium constants that can be tuned across at least five orders of magnitude. Furthermore, differential responses to base concentration can be exploited to create a switchable dynamic covalent system in which a boronic acid can be cycled between expressing each of two condensation products with high fidelity, in response to a simple chemical stimulus.

Ligand controlled switchable selectivity in ruthenium catalyzed aerobic oxidation of primary amines.

A ligand controlled catalytic system for the aerobic oxidation of 1° amines to nitriles and imines has been developed where the varying π-acidic feature of BIAN versus phen in the frameworks of ruthenium catalysts facilitates switchable selectivity.