Control Of Product Selectivity Research Articles

A Dual CQD-Catalysis and H-Bond Acceptor for Controlling Product Selectivity and Regioselectivity in Symmetric/Unsymmetric Azoxy Arenes.

Azoxy arenes are valuable compounds in different areas of chemistry, such as organic chemistry, medicinal chemistry, and natural product chemistry. Despite their value, the regioselective synthesis of unsymmetric azoxybenzenes has remained a real challenge in the field. Herein, the product selectivity in oxidative homocoupling of anilines into symmetric azoxybenzenes was first achieved by an asparagine-functionalized CQD catalyst. Subsequently, in the cross-coupling of anilines into the unsymmetric azoxybenzenes via an ortho H-bond acceptor (HBA) on one of the coupling anilines, the regioselectivity was effectively controlled. It was demonstrated that ortho-HBA could mechanistically establish a six-membered intramolecular hydrogen-bonded ring on an N,N'-dihydroxy intermediate. The formed hydrogen bond makes the nearby nitrogen eminently suitable for the slow dehydration step. As a result, the functional oxygen of the azoxy compound is placed far from the HBA. The o-HBA mechanism also controls the regioselectivity ratio in which 1:0 (with an intramolecular H-bonded hexagonal ring), 2:1 (with an intramolecular H-bonded pentagonal ring), and 1:1 (without an ortho-HBA) isomeric mixtures could be achieved. The HBA mechanism was exploited by different substituted anilines, and various unsymmetric azoxybenzenes were synthesized. Finally, with the aid of mechanistic studies, a plausible mechanism for the reaction was proposed.

(Invited) Adsorbate Coverage Steers Selectivity during Catalytic Nitrate Reduction

Nitrate (NO3 -) is a pervasive contaminant in groundwater that is harmful to human health. In this talk, we will discuss strategies for developing (electro)catalysts that selectively reduce NO3 - to either dinitrogen (N2), which is the desirable product if the goal is to remove NO3 - from drinking water, or to ammonia (NH3), which is the desirable produce for resource recovery. Using density functional theory (DFT), we found that product selectivity to NH3 versus N2 is controlled by competition between N-H bond formation and N-N bond formation on Cu and Pd surfaces. On the Cu(100) surface, N-H bond formation is favorable in the hydrogenation step of surface-bound NO*. This leads to the formation of an HNO* intermediate that is readily reduced to NH3. On the Pd(100) surface, NO* is hydrogenated to form an NOH* intermediate that is readily reduced to N*. N2 then is generated through a N*-NO* coupling step. Importantly, we found that the N-N bond formation barrier on Pd is highly dependent on the NO* surface coverage, where a high NO* coverage lowers the N*-NO* coupling barrier. As such, increased NO* coverage enhances N2 selectivity. Using this concept, we worked with collaborators to design a Pd nanocube electrocatalyst with partial Cu coverage that achieves high NO3 - reduction activity and N2 selectivity by increasing NO* coverage through a Cu-to-Pd spillover mechanism (ACS Catal. 2023, 13, 1, 87–98). This work highlights the importance of NO* surface coverage for controlling product selectivity, explains the different selectivity observed for Cu and Pd, and demonstrates strategies for boosting N2 selectivity through NO* spillover.

Vacancy-Engineered 1D Nanorods with Spatially Segregated Dual Redox Sites for Visible-Light-Driven Cooperative CO2 Reduction.

Cooperative CO2 photoreduction with tailored organic synthesis offers a potent avenue for harnessing concurrently generated electrons and holes, facilitating the creation of both solar fuels and specialized chemical compounds. However, controlling the crystallization and morphologies of metal-free molecular nanostructures with exceptional photocatalytic activities toward CO2 reduction remains a significant challenge. These hurdles encompass insufficient CO2 activation potential, sluggish multielectron processes, delayed charge-separation kinetics, inadequate storage of long-lived photoexcitons, unfavorable thermodynamic conditions, and the precise control of product selectivity. Here, melem oligomer 2D nanosheets (MNSs) synthesized through pyrolysis are transformed into 1D nanorods (MNRs) at room temperature with the simultaneous engineering of vacancies and morphology. Transient absorption spectral analysis reveals that vacancies in MNRs trap charges, extending charge carrier lifetimes. Additionally, carbon vacancies enhance CO2 adsorption by increasing amine functional centers. The photocatalytic performance of MNRs for CO2 reduction coupled with benzyl alcohol oxidation is approximately ten times higher (CH3OH and aromatic aldehyde production rate 27 ± 0.5 and 93 ± 0.5 mmol g-1 h-1, respectively) than for the MNSs (CH3OH and aromatic aldehyde production rate 2.9 ± 0.5 and 9 ± 0.5 mmol g-1 h-1, respectively). The CO2 reduction pathway involved the carbon-coordinated formyl pathway through the formation of *COOH and *CHO intermediates, as mapped by in situ Fourier-transform infrared spectroscopy. The superior performance of MNRs is attributed to favorable energy-level alignment, enriched amine surfaces, and unique morphology, enhancing solar-to-chemical conversion.

Product Selectivity Control in the Brønsted Acid-Mediated Reactions with 2-Alkynylanilines.

Brønsted acid-catalysed/mediated reactions of the 2-alkynylanilines are reported. While metal-catalysed reactions of these valuable building blocks have led to the establishment of robust protocols for the selective, diverse-oriented syntheses of significant heterocyclic derivatives, we here demonstrate the practical advantages of an alternative methodology under metal-free conditions. Our investigation into the key factors influencing the product selectivity in Brønsted acid-catalysed/mediated reactions of 2-alkynylanilines reveals that different reaction pathways can be directed towards the formation of diverse valuable products by simply choosing appropriate reaction conditions. The origins of chemo- and regioselectivity switching have been explored through Density Functional Theory (DFT) calculations.

Mechanistic and Kinetic Insights into H2O Effects in the Conversion of C1‐C3 Hydrocarbons to Value‐added Products

The direct conversion of cheap and widely available C1‐C3 alkanes in natural gas/shale gas into building blocks for the chemical industry is highly attractive from an environmental perspective as a replacement for current oil‐based processes. Due to the high chemical inertness of these alkanes, but the high reactivity of the desired reaction products, which are easily involved in non‐selective sequential reactions, ongoing research activities are focused on controlling product selectivity through catalyst design and/or reactor operation. In this context, we have critically analyzed research studies dealing with the effect of steam or liquid water on catalyst activity and, in particular, on selectivity in the conversion of CH4, C2H6 and C3H8 to C2+‐hydrocarbons, formaldehyde, methanol, ethylene, acetic acid, and propene. In addition, our personal views on possible future developments are also given.

Hydrogen‐Bond‐Mediated Formation of C−N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets

AbstractReductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C−N or C=N bond readily manipulated through a solvent‐induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive‐coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C−N bonds via hydrogen‐bonding activation. Experimental and theoretical study reveals that the building of the hydrogen‐bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C−N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

Hydrogen-Bond-Mediated Formation of C-N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets.

Reductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C-N or C=N bond readily manipulated through a solvent-induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive-coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C-N bonds via hydrogen-bonding activation. Experimental and theoretical study reveals that the building of the hydrogen-bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C-N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

Non-Photochemical Origin of Selectivity Difference between Light and Dark Catalytic Conditions.

The interest in introducing light into heterogeneous catalysis is driven not only by the urgent need of replacing fossil energy but also by the promise of controlling product selectivity by light. The product selectivity differences observed in recent studies between light and dark reactions are often attributed to photochemical effects. Here, we report the discovery of a non-photochemical origin of selectivity difference, at essentially the same CO2 conversion rate, between photothermal and thermal CO2 hydrogenation reactions over a Ru/TiO2-x catalyst. While the presence of the photochemical effect from ultraviolet light is confirmed, it merely enhances the catalytic activity. Systematic investigation reveals that the gradual formation of an adsorbate-mediated strong metal-support interaction under catalytic conditions is responsible for the variation in the catalytic selectivity. We demonstrate that differences in product selectivity under light/dark reactions do not necessarily originate from photochemical effects. Our study refines the basis for determining photochemical effects and highlights the importance of excluding non-photochemical effects in mechanistic studies of light-controlled product selectivity.

Hybrid mesoporous electrodes evidence CISS effect on water oxidation.

Controlling product selectivity is essential for improving the efficiency of multi-product reactions. Electrochemical water oxidation is a reaction of main importance in different applications, e.g., renewable energy schemes and environmental protection, where H2O2 and O2 are the two principal products. In this Communication, the product selectivity of electrochemical water oxidation was controlled by making use of the chiral induced spin selectivity (CISS) effect at mesoporous-TiO2 on the molecule-modified Au substrate. Our results show a decrease in H2O2 formation when using chiral hetero-helicene molecules adsorbed on the Au substrate. We propose a mechanism for this kinetic effect based on the onset of CISS-induced spin polarization on the Au-helicene chiral interface. We also present a new tunable substrate to investigate the CISS mechanism.

CO2 Hydrogenation to Hydrocarbons over Fe‐Based Catalysts: Status and Recent Developments

AbstractTo control anthropogenic CO2 emissions worldwide, it is necessary not only to align the chemical industry and energy sector with renewable resources but also to implement large‐scale utilization of CO2 as a feedstock. The Fe‐catalyzed CO2‐modified Fischer‐Tropsch Synthesis (CO2‐FTS) is one of the most promising options for efficient CO2 utilization, as it can be used to synthesize desired higher hydrocarbons (C2+), including lower olefins (C2=‐C4=), the main building blocks of the chemical industry, and long‐chain hydrocarbons (C5+), which can be used as fuels. To optimize catalyst and process design for the purpose of developing an economically viable industrial process, the reaction mechanism and the factors controlling product selectivity need to be fully understood. This article discusses the current state‐of‐the‐art in catalyst design and approaches for making effective progress in addressing these challenges.

Phosphonium-Catalyzed Monoreduction of Bisphosphine Dioxides: Origin of Selectivity and Synthetic Applications.

Controlling product selectivity in successive reactions of the same type is challenging owing to the comparable thermodynamic and kinetic properties of the reactions involved. Here, the synergistic interaction of the two phosphoryl groups in bisphosphine dioxides (BPDOs) with a bromo-phosphonium cation was studied experimentally to provide a practical tool for substrate-catalyst recognition. As the eventual result, we have developed a phosphonium-catalyzed monoreduction of chiral BPDOs to access an array of synthetically useful bisphosphine monoxides (BPMOs) with axial, spiro, and planar chirality, which are otherwise challenging to synthesize before. The reaction features excellent selectivity and impressive reactivity. It proceeds under mild conditions, avoiding the use of superstoichiometric amounts of additives and metal catalysts to simplify the synthetic procedure. The accessibility and scalability of the reaction allowed for the rapid construction of a ligand library for optimization of asymmetric Heck-type cyclization, laying the foundation for a broad range of applications of chiral BPMOs in catalysis.

Nitrogen-Doped Titanium Dioxide for Selective Photocatalytic Oxidation of Methane to Oxygenates.

Photocatalytic conversion of methane (CH4) to value-added chemicals using H2O as the oxidant under mild conditions is a desired sustainable pathway for synthesizing commodity chemicals. However, controlling product selectivity while maintaining high product yields is greatly challenging. Herein, we develop a highly efficient strategy, based on the precise control of the types of nitrogen dopants, and the design of photocatalysts, to achieve high selectivity and productivity of oxygenates via CH4 photocatalytic conversion. The primary product (methanol) is obtained in a high yield of 159.8 μmol·g-1·h-1 and 47.7% selectivity, and the selectivity of oxygenate compounds reached 92.5%. The unique hollow porous structure and substituted nitrogen sites of nitrogen-doped TiO2 synergistically promote its photo-oxidation performance. Furthermore, in situ attenuated total reflectance Fourier transform infrared spectroscopy provides direct evidence of the key intermediates and their evolution for producing methanol and multicarbon oxygenates. This study provides insights into the mechanism of photocatalytic CH4 conversion.

Controlling product selectivity and catalyst lifetime by altering acid strength, cavity size of SAPO, and diffusion rate of methanol in the MTO reaction: DFT and MD calculations.

The initiation mechanisms of the MTO process over silicoaluminophosphate (SAPO) catalysts with zeolite-like structures using first-principles calculations have been investigated. The supramolecular system of silicoaluminophosphates consisting of inorganic cages with Brønsted acid sites and trapped organic compounds was used as a catalyst in the MTO reaction. To study the structure-property relationship in more detail, the effect of acidity and cage size of different types of SAPOs (SAPO-18, SAPO-34, and SAPO-17 with CHA, AEI, and ERI structures, respectively) in the aromatic cycle of hydrocarbon pool mechanism was investigated. The differences in reaction barriers can be explained by the cage size, pore topology, and environment of framework protons of materials. Product selectivity was controlled by using cavity-type zeolite, the steric constraint of the cavity for the formation of critical intermediates, and acidic strength. The results show that ethylene selectivity increases as the cavity size decreases, and the elliptical pore size of the structures decreases, thereby decreasing the acidity of the zeolite structure, leading to an increase in propylene selectivity. SAPO-18 exhibits the longest reaction lifetime and has the highest amount of carbonaceous material after reaction completion. SAPO-17 with small pore and cavity size is selective to ethylene, although it shows a rapid catalyst deactivation rate.

Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base.

The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent -OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N'-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(ii) center being the rate-determining step. Upon addition of the conjugate base, N,N'-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O-O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.

Recent advances in the selective semi-hydrogenation of alkyne to (E)-olefins.

Considering the potential importance and upsurge in demand, the selective semi-hydrogenation of alkynes to (E)-olefins has attracted significant interest. This article highlights the recent advances in newer technologies and important methodologies directed to (E)-olefins from alkynes developed from 2015 to 2023. Notable features summarised include the catalyst or ligand design and control of product selectivity based on precious and nonprecious metal catalysts for semi-hydrogenation to (E)-olefins. Mechanistic studies for various catalytic transformations, including synthetic application to bioactive compounds, are summarised.

Light switching for product selectivity control in photocatalysis.

Artificial switchable catalysis is a new, rapidly expanding field that offers great potential advantages for both homogeneous and heterogeneous catalytic systems. Light irradiation is widely accepted as the best stimulus to artificial switchable chemical systems. In recent years, tremendous progress has been made in the synthesis and application of photo-switchable catalysts that can control when and where bond formation and dissociation take place in reactant molecules. Photo-switchable catalysis is a niche area in current catalysis, on which systematic analysis and reviews are still lacking in the scientific literature, yet it offers many intriguing and versatile applications, particularly in organic synthesis. This review aims to highlight the recent advances in photo-switchable catalyst systems that can result in two different chemical product outcomes and thus achieve a degree of control over organic synthetic reactions. Furthermore, this review evaluates different approaches that have been employed to achieve dynamic control over both the catalytic function and the selectivity of several different types of synthesis reactions, along with the remaining challenges and potential opportunities. Owing to the great diversity of the types of reactions and conditions adopted, a quantitative comparison of efficiencies between considered systems is not the focus of this review, instead the review showcases how insights from successful adopted strategies can help better harness and channel the power of photoswitchability in this new and promising area of catalysis research.

Highly efficient PCCP/Cr(III) ethylene tetramerization catalyst designed using artificial neural network approach

Chromium complexes are feasible for inducing ethylene selective oligomerization to generate valuable linear α-olefins, such as 1-hexene and 1-octene. The current theoretical calculation techniques can effectively accelerate the design of molecular catalysts. In this work, we combined density functional theory (DFT) and artificial neural network (ANN) methods to model vinyl-bridged PCCP/Cr active species and used an ANN-based model to predict and screen the designed novel PCCP ligands. DFT was used to optimize the structure of active species and calculate electronic properties. The datasets were established using steric descriptors, electronic descriptors, and experimental data. The ANN-based models were trained based on the established datasets. We found that the optimized ANN-based models ANN-HT and ANN-OT, respectively for the prediction of 1-hexene and 1-octene selectivity, provide satisfactory predictive performance, with R2 values of 0.962 for ANN-HT and 0.976 for ANN-OT. We investigated the contribution of steric and electronic descriptors on catalytic performance, and the results showed that steric descriptors have a greater contribution to controlling product selectivity compared to electronic descriptors. The similarity between the ethylene oligomerization test results of the experimentally prepared L28/Cr catalyst and the ANN-based model prediction results proved the accuracy of the models. The experimentally prepared L28/Cr exhibited catalytic activity of 2181 kg∙g−1∙h−1 and 65.8 % 1-C8 selectivity under suitable reaction conditions.

Oxygen vacancy-driven strong metal-support interactions on AuPd/TiO2 catalysts for high-efficient air-oxidation of 5-hydroxymethylfurfural

The regulation ofstrong metal-support interactions (SMSI) has emerged as a powerful strategy in boosting catalytic activity and controlling product selectivity. However, directly tuning the metal-supportinteractions in oxide supported metal nanocatalysts remains challenging. Herein, citric acid (CA)‑assisted synthesized TiO2 layer on halloysite nanotubes (HNTs) with varied oxygen vacancies (Ov) concentrations was designed for loading AuPd nanoparticles. SMSI between the AuPd nanoparticles and the TiO2-xCA@HNTs support was successfully induced by adjustable Ov concentrations. The synthesized catalyst was employed for air-oxidation of bio-based 5-hydroxymethylfurfural (HMF) to bioplastic monomer 2,5-furandicarboxylic acid (FDCA) in water. A satisfactory FDCA yield of 98.4 %, accompany with an outstanding FDCA formation rate of 900.9 mmol·g−1·h−1 and an excellent TOF value at 441.2 h−1 was afforded. Based on catalysts characterizations and experimental studies, the catalytic performance was significantly affected by Ov concentrations. The kinetic study showed that the supported gold nanoparticles in alkaline environment had strong aldehyde oxidation activity but weak alcohol oxidation ability. On the contrary, supported palladium nanoparticles were more conducive to alcohol oxidation. This imbalance was overcome by the loading of AuPd nanoparticles, which enhanced the activity for air-oxidation of HMF. Mechanistic studies demonstrated that the stronginteractions between noble metal nanocatalysts and supports were regulated by Ov, which not only increased the adsorption capacity of the substrate and crucial intermediate, but also facilitated the adsorption and activation of molecular oxygen to generate oxygen active species of superoxide radicals. The possible catalytic mechanism for air-oxidation of HMF over Ov-driven SMSI on the obtained catalysts was proposed. This work sheds lights on the design of supported metal catalysts with SMSI for the catalytic upgrading of biomass-derived platform chemicals.

Electrochemical carbonyl reduction on single-site M–N–C catalysts

Electrochemical conversion of organic compounds holds promise for advancing sustainable synthesis and catalysis. This study explored electrochemical carbonyl hydrogenation on single-site M–N–C (Metal Nitrogen-doped Carbon) catalysts using formaldehyde, acetaldehyde, and acetone as model reactants. We strive to correlate and understand the selectivity dependence on the nature of the metal centers. Density Functional Theory calculations revealed similar binding energetics for carbonyl groups through oxygen-down or carbon-down adsorption due to oxygen and carbon scaling. Fe–N–C exhibited specific oxyphilicity and could selectively reduce aldehydes to hydrocarbons. By contrast, the carbophilic Co–N–C selectively converted acetaldehyde and acetone to ethanol and 2-propanol, respectively. We claim that the oxyphilicity of the active sites and consequent adsorption geometry (oxygen-down vs. carbon-down) are crucial in controlling product selectivity. These findings offer mechanistic insights into electrochemical carbonyl hydrogenation and can guide the development of efficient and sustainable electrocatalytic valorization of biomass-derived compounds.

Revealing the anti-sintering phenomenon on silica-supported nickel catalysts during CO2 hydrogenation

The CO2 catalytic hydrogenation represents a promising approach for gas-phase CO2 utilization in a direct manner. Due to its excellent hydrogenation ability, nickel has been widely studied and has shown good activities in CO2 hydrogenation reactions, in addition to its high availability and low price. However, Ni-based catalysts are prone to sintering under elevated temperatures, leading to unstable catalytic performance. In the present study, various characterization techniques were employed to study the structural evolution of Ni/SiO2 during CO2 hydrogenation. An anti-sintering phenomenon is observed for both 9% Ni/SiO2 and 1% Ni/SiO2 during CO2 hydrogenation at 400°C. Results revealed that Ni species were re-dispersed into smaller-sized nanoparticles and formed Ni0 active species. While interestingly, this anti-sintering phenomenon leads to distinct outcomes for two catalysts, with a gradual increase in both reactivity and CH4 selectivity for 9% Ni/SiO2 presumably due to the formation of abundant surface Ni° from redispersion, while an apparent decreasing trend of CH4 selectivity for 1% Ni/SiO2 sample, presumably due to the formation of ultra-small nanoparticles that diffuse and partially filled the mesoporous pores of the silica support over time. Finally, the redispersion phenomenon was found relevant to the H2 gas in the reaction environment and enhanced as the H2 concentration increased. This finding is believed to provide in-depth insights into the structural evolution of Ni-based catalysts and product selectivity control in CO2 hydrogenation reactions.