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The nature of hydrocarbon pool (HCP) intermediates in the methanol-to-hydrocarbons (MTH) process has been thoroughly investigated, especially for BEA- and CHA-type zeolite catalysts like H-β and H-SAPO-34. Herein, we further reveal the dynamic mechanistic details of the MTH process over the H-ZSM-5 catalyst at 400 °C, based on the dual-cycle mechanism and HCP in this medium-pore zeolite. Application of switching sequences of 13C-labeled and unlabeled methanol pulses over a model H-ZSM-5 catalyst combined with on-line MS analysis and a recently reported technique called “fast scanning-pulse GC analysis” provides a direct and quantitative insight into the MTH reactions under quasi-steady-state conditions. The transient product responses showed the almost instant formation of hydrocarbons upon a small pulse of methanol, followed by secondary formation of light aromatics via HCP decomposition and olefin alkylation-dealkylation, especially in a long catalyst bed when methanol is quickly consumed in the initial reaction zone in the catalyst bed. The isotopic analysis of typical aliphatic C3+ product responses after switching 13C-methanol pulses to the unlabeled methanol pulses showed a fast isotope scrambling in the formation of C3+ species. MS analysis of the light aromatics indicates a complete consecutive but slower isotope incorporation process of 12C into 13C-aromatics. Results provide direct experimental confirmation of the kinetically preferred olefin-based cycle over the aromatic-based cycle. The sequential isotopic incorporation strongly suggests that the paring reaction pathway through aromatic ring contraction and re-expansion steps is operative. In the appearance of aromatics upon pulsing methanol over larger catalyst beds, four processes are directly discerned, involving the displacement of adsorbed species by formed water, isotope incorporation yielding directly labeled and unlabeled products through the paring mechanism and direct aromatization, and HCP conversion through secondary reactions.

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C-H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework ZnEF2+ cations, framework oxygens (OF2−), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C-H bond cleavage performs with a determination coefficient R2 = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R2 = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C-H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property-activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials.

Extraframework cations define the chemical versatility of zeolite catalysts. Addressing their structural complexity and dynamic behavior represents one of the main fundamental challenges in the field. Herein, we present a computational approach for the identification and analysis of the accessible pool of intrazeolite extraframework complexes with a Cu/MOR catalyst as an industrially important model system. We employ ab initio molecular dynamics for capturing the ensemble of reactive isomers with the [Cu3O3]2+ stoichiometry confined in the mordenite channels. The high structural diversity of the generated isomers was ensured by concentrating the kinetic energy along the low-curvature directions of the potential energy surface (PES). Geometrically distinct [Cu3O3]2+ complexes were identified via a series of clustering procedures ensuring that one structure of each local minima is retained. The proposed procedure has resulted in a set of previously unknown peroxo-complexes, which are >50 kJ/mol more stable than the recently hypothesized chair-shaped structure. Our analysis demonstrates that the most stable peroxo-containing clusters can be formed under operando conditions from molecular oxygen and the Cu3O unit, similar to that in methane monooxygenase (MMO) enzymes.

Effective assessment of catalytic performance is the foundation for the rational design and development of new catalysts with superior performance. The ubiquitous screening/optimization studies use reaction yields as the sole performance metric in an approach that often neglects the complexity of the catalytic system and intrinsic reactivities of the catalysts. Using an example of hydrogenation catalysis, we examine the transient behavior of catalysts that are often encountered in activation, deactivation and catalytic turnover processes. Each of these processes and the reaction environment in which they take place are gradually shown to determine the real-time catalyst speciation and the resulting kinetics of the overall catalytic reaction. As a result, the catalyst performance becomes a complex and time-dependent metric defined by multiple descriptors apart from the reaction yield. This behaviour is not limited to hydrogenation catalysis and affects various catalytic transformations. In this feature article, we discuss these catalytically relevant descriptors in an attempt to arrive at a comprehensive depiction of catalytic performance.

Monitoring complex catalytic pathways under industrially-relevant conditions is one of the key challenges in catalysis chemistry and technology. Herewith we describe a direct technique called ‘fast scanning-pulse analysis’ (FASPA) that allows the direct characterization and detailed kinetic analysis of intimately interweaved catalytic pathways. The power and potential of the FASPA approach are demonstrated with an industrially relevant methanol-to-hydrocarbons (MTH) process over H-ZSM-5 zeolite. This reaction proceeds via a hydrocarbon pool (HCP) mechanism producing olefins and aromatics. The HCP is built-up upon exposure to methanol during the induction period, followed by a transition regime to a quasi steady-state MTH operation. This FASPA technique allows (sub-)second resolution of the full temporal products response upon a methanol pulse providing direct and quantitative insights into the MTH reactions. Globally, two consecutive pathways can be discerned: a very fast primary product formation in the presence of methanol in a narrow active MTH reaction zone, followed by a slower formation of light aromatics, which is closely related to the decomposition and release of HCP species and secondary reactions in absence of methanol in the downstream part of the catalyst bed. The time delay between the appearance of inert tracer and primary products represents the time needed to build-up the HCP in the induction period, where methane is observed prior to other products. The primary products (alkanes, olefins, and light aromatics) are nearly instantaneously formed from the pulsed methanol. These results demonstrate the highly dynamic character of the HCP in the MTH process over H-ZSM-5.

The abundance of methane has led to a strong interest to use methane as a feedstock in the chemical industry. One of the main challenges is the initial activation of the methane molecule. This has resulted in the development of several different approaches to utilize methane, some more developed than others. In this work the current status of the different approaches is discussed and the main issues for industrial utilization described. A special focus of this work is the status of catalyst development.

Heterogeneous derivatives of catalysts discovered by Ziegler and Natta are important for the industrial production of polyolefin plastics. However, the interaction between precatalysts, alkylaluminum activators, and oxide supports to form catalytically active materials is poorly understood. This is in contrast to homogeneous or model heterogeneous catalysts that contain resolved molecular structures that relate to activity and selectivity in polymerization reactions. This study describes the reactivity of triisobutylaluminum with high surface area aluminum oxide and a zirconocene precatalyst. Triisobutylaluminum reacts with the zirconocene precatalyst to form hydrides and passivates −OH sites on the alumina surface. The combination of passivated alumina and zirconium hydrides formed in this mixture generates ion pairs that polymerize ethylene., A zirconium precatalyst, alkylaluminum, and oxide support interact to form catalytic sites.

The enhancing effect of extraframework Al (EFAl) species on the acidity of bridging hydroxyl groups in a steam-calcined faujasite zeolite (ultrastabilized Y, USY) was investigated by in situ monitoring the H/D exchange reaction between benzene and deuterated zeolites by 1H MAS NMR spectroscopy. This exchange reaction involves Brønsted acid sites (BAS) located in sodalite cages and supercages. In a reference faujasite zeolite free from EFAl, both populations of BAS are equally and relatively slowly reactive toward C6H6. In USY, in stark contrast, the H/D exchange of sodalite hydroxyl groups is significantly faster than that of hydroxyl groups located in the faujasite supercages, even though benzene has only access to the supercages. This evidences selective enhancement of BAS near Lewis acidic EFAl species, which according to the NMR findings are located in the faujasite sodalite cages.

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives has been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein, we report a Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This reactivity is shown to emerge once the N–H functionality within the Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such masking of the cooperative N–H functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity toward a single positional isomerization was also retained for long-chain alkenes, resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of the reaction between the activated Mn catalyst and olefins points to catalysis operating via a metal–alkyl mechanism─one of the three conventional transposition mechanisms previously unknown in Mn complexes.

α-Ga2O3, β-Ga2O3, and γ-Ga2O3 as well as the silica-supported catalysts γ-Ga2O3/SiO2, β-Ga2O3/SiO2, and Ga(NO3)3-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenat...

Potassium carbonate dispersed over a defective TiO2support (K2CO3/TiO2) is an efficient catalyst for benzene esterification with CO2and CH3OH. Density functional theory calculations reveal that this unique catalytic reactivity originates from the cooperation of the Ti3+/K+surface sites. The K2CO3promotor steers the stabilization of surface intermediates thus preventing catalyst deactivation.

Light aromatic compounds (BTX: benzene, toluene and xylenes) represent an important class of building blocks in the chemical industry. Currently, light aromatics are obtained exclusively from fossil feedstock, whose utilization is associated with serious environmental concerns. Developing new routes for a more sustainable BTX production is, therefore, of high importance. In this work, aromatization of ethylene over well-defined metal-modified HZSM-5 zeolite catalysts is examined. The results show that modification of zeolite with gallium, zinc and silver leads to a significant increase in aromatics production. Metal species are responsible for catalysing dehydrogenation pathways with Ga being the most efficient for BTX production. Increasing temperature and ethylene partial pressure facilitate ethylene aromatization. Employing a combination of isotope labelling with a thorough characterization of zeolite-entrapped species by means of IR and MAS NMR spectroscopy provides evidence for the involvement of intra-zeolite aromatic hydrocarbon species in the catalytic cycle.

Chemistry - a European journal 26(34), 7563 - 7567 (2020). doi:10.1002/chem.202000772, Copper‐oxo clusters exchanged in zeolite mordenite are active in the stoichiometric conversion of methane to methanol at low temperatures. Here, we show an unprecedented methanol yield per Cu of 0.6, with a 90–95 % selectivity, on a MOR solely containing [Cu$_3$($\mu$‐O)$_3$]$^{2+ }$active sites. DFT calculations, spectroscopic characterization and kinetic analysis show that increasing the chemical potential of methane enables the utilization of two μ‐oxo bridge oxygen out of the three available in the tricopper‐oxo cluster structure. Methanol and methoxy groups are stabilized in parallel, leading to methanol desorption in the presence of water., Published by Wiley-VCH, Weinheim

The intermediates formed upon the interaction of methane with Cu-modified ZSM-5 zeolites (Cu/H-ZSM-5) have been analyzed with solid-state NMR spectroscopy and DFT methods. Methane activation by Cu/H-ZSM-5 zeolites gives rise to three distinct surface methoxy-like species (-O-CH3) detected by 13C MAS NMR spectroscopy with specific chemical shifts in the range of 53-63 ppm. DFT calculations on representative cluster models of different sites potentially present in Cu/H-ZSM-5 have been used to assign these signals to (i) methanol adsorbed on two neighboring Cu sites (Cu-(HOCH3)-Cu, 62.6 ppm), (ii) methanol adsorbed on zeolite Brønsted acid sites (52.9 ppm), and (iii) lattice-bound methoxy groups (Si-O(CH3)-Al, 58.6). The formation of these methoxy-like intermediates depends on the Cu loading and, accordingly, the type of Cu species in the Cu/H-ZSM-5 zeolite. For the sample with low (0.1 wt %) Cu loading containing exclusively mononuclear isolated Cu species, only the intermediates ii and iii have been detected. The Cu-bound intermediate (i) is formed upon methane activation by multinuclear Cu sites featuring Cu-O-Cu bridging moieties present in the materials with relatively higher Cu loading (1.38 wt %). The presented results indicate that methane activation by Cu/H-ZSM-5 can be promoted by both mono- and multinuclear Cu species confined in the zeolite matrix.

A new application for bis(cyclometalated) iridium(III) species containing ancillary acyclic diaminocarbene ligands, viz. for sensing of mercury(II) ions, is disclosed. A family of bis(cyclometalated) iridium(III) species supported by both parent isocyanide and acyclic diaminocarbene ligands was prepared, and their electrochemical and photophysical properties were evaluated, revealing efficient blue-green phosphorescence in solution with quantum yields of up to 55%. We uncovered that the photophysical properties of these complexes are dramatically altered by the presence of metal ions and that the complex [Ir(ppy)

The catalytic conversion of glycerol to aromatics (GTA, e.g., benzene, toluene, and xylenes, BTX) over a shaped H-ZSM-5/Al2O3 (60/40 wt%) catalyst was investigated in a continuous fixed-bed reactor to study the addition of the Al2O3 binder in the catalyst formulation on catalyst performance. The experiments were performed under N2 at 550 °C, a WHSV of glycerol (pure) of 1 h−1, and atmospheric pressure. The spent H-ZSM-5/Al2O3 catalysts were reused after an oxidative regeneration at 680 °C and in total 5 reaction-regeneration cycles were performed. Catalyst characterization studies show that the addition of the Al2O3 binder does not affect the surface area and crystallinity of the formulation, but increases the total pore volume (mesopores in particular) and total acidity (Lewis acidity in particular). The H-ZSM-5/Al2O3 (60/40 wt%) catalyst shows a considerably prolonged catalyst life-time (8.5 vs. 6.5 h for H-ZSM-5), resulting in a significant increase in the total BTX productivity (710 vs. 556 mg g−1 H-ZSM-5). Besides, the addition of the Al2O3 binder retards irreversible deactivation. For instance, after 3 regenerations, catalyst performance is comparable to the fresh one. However, after 4 regenerations, some irreversible catalyst deactivation occurs, associated with a reduction in total pore volume, crystallinity, and acidity (Brønsted acidity in particular), and meso-porosity of the Al2O3 binder. This study shows that both the stability and reusability of H-ZSM-5-based catalysts for GTA are remarkably enhanced when using a suitable binder.

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni-CFx bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies.

The electrocatalytic CO2 reduction reaction (CO2RR) is one of the key technologies of the clean energy economy. Molecular-level understanding of the CO2RR process is instrumental for the better design of electrodes operable at low overpotentials with high current density. The catalytic mechanism underlying the turnover and selectivity of the CO2RR is modulated by the nature of the electrocatalyst, as well as the electrolyte liquid, and its ionic components that form the electrical double layer (EDL). Herein we demonstrate the critical non-innocent role of the EDL for the activation and conversion of CO2 at a high cathodic bias for electrocatalytic conversion over a silver surface as a representative low-cost model cathode. By using a multiscale modeling approach we demonstrate that under such conditions a dense EDL is formed, which hinders the diffusion of CO2 towards the Ag111 electrocatalyst surface. By combining DFT calculations and ab initio molecular dynamics simulations we identify favorable pathways for CO2 reduction directly over the EDL without the need for adsorption to the catalyst surface. The dense EDL promotes homogeneous phase reduction of CO2via electron transfer from the surface to the electrolyte. Such an outer-sphere mechanism favors the formation of formate as the CO2RR product. The formate can undergo dehydration to CO via a transition state stabilized by solvated alkali cations in the EDL.

Identification of the catalyst characteristics correlating with the key performance parameters including selectivity and stability is key to the rational catalyst design. Herein we focused on the identification of property-performance relationships in the methanol-to-olefin (MTO) process by studying in detail the catalytic behaviour of MFI, MEL and their respective intergrowth zeolites. The detailed material characterization reveals that both the high production of propylene and butylenes and the large MeOH conversion capacity correlate with the enrichment of lattice Al sites in the channels of the pentasil structure as identified by 27Al MAS NMR and 3-methylpentane cracking results. The lack of correlation between MTO performance and other catalyst characteristics, such as crystal size, presence of external Brønsted acid sites and Al pairing suggests their less pronounced role in defining the propylene selectivity. Our analysis reveals that catalyst deactivation is rather complex and is strongly affected by the enrichment of lattice Al in the intersections, the overall Al-content, and crystal size. The intergrowth of MFI and MEL phases accelerates the catalyst deactivation rate.

The conversion of CO2 to methanol with high activity and high selectivity remains challenging owing to the kinetic and thermodynamic limitations associated with the low chemical reactivity exhibited by CO2. Herein, we report a novel Cd/TiO2 catalyst exhibiting a methanol selectivity of 81%, a CO2 conversion of 15.8%, and a CH4 selectivity below 0.7%. A combination of experimental and computational studies revealed that the unique electronic properties exhibited by the Cd clusters supported by the TiO2 matrix were responsible for the high selectivity of CO2 hydrogenation to methanol via the HCOO* pathway at the interfacial catalytic sites.

Three γ/β-Ga2O3 nanoparticle catalysts that differ in the relative ratio of γ-Ga2O3 to β-Ga2O3 were prepared to evaluate the effect of H2 treatment (500 °C, 2 h) on the coordination environment of bulk and surface Ga sites, Lewis acidity and catalytic activity in propane dehydrogenation (PDH). Independent of the H2 treatment, the initial PDH activity of the γ/β-Ga2O3 catalysts increases with the fraction of the β-Ga2O3 phase. This is explained by the presence of weak Lewis acid sites (LAS) in β-Ga2O3 while such sites are absent in γ-Ga2O3. Treatment with H2 increases the catalytic activity of all three γ/β-Ga2O3 catalysts but for different reasons. For catalysts with higher fractions of β-Ga2O3, H2 treatment increases further the relative abundance of weak LAS, likely by generating coordinatively unsaturated Ga sites (such as tricoordinated Ga sites nearby oxygen vacancies). In contrast, H2 treatment of a catalyst containing a predominant fraction of γ-Ga2O3 phase induces disorder in the sub-surface structure of the nanoparticle, that is, it forms gallium and oxygen vacancies in the bulk and favors migration of gallium, and likely also of oxygen, to the surface. This induces a surface reconstruction that notably increases the fraction of strong LAS (and proportionally decreases the fraction of medium LAS), while creating no weak LAS in γ-Ga2O3-H2. Therefore, the increase in the catalytic activity of H2-treated γ-Ga2O3 is explained by the higher density of surface Ga sites in γ-Ga2O3-H2 relative to calcined γ-Ga2O3. H2-treated catalysts that contain a higher relative amount of weak LAS also feature a higher relative abundance of gallium hydride species associated with a low frequency FTIR band at ca. 1931–1939 cm−1, that is, weak LAS likely give weakly-bound hydrides in β-Ga2O3. Our results highlight that weak LAS in unsupported Ga2O3 catalysts are more active in PDH than mild or strong LAS., Journal of Catalysis, 408, ISSN:0021-9517, ISSN:1090-2694

Metal-modified zeolites are versatile catalytic materials with a wide range of industrial applications. Their catalytic behaviour is determined by the nature of externally introduced cationic species, i.e., its geometry, chemical composition, and location within the zeolite pores. Superior catalyst designs can be unlocked by understanding the confinement effect and spatial limitations of the zeolite framework and its influence on the geometry and location of such cationic active sites. In this study, we employ the genetic algorithm (GA) global optimization method to investigate extraframework aluminum species and their structural variations in different zeolite matrices. We focus on extraframework aluminum (EFAl) as a model system because it greatly influences the product selectivity and catalytic stability in several zeolite catalyzed processes. Specifically, the GA was used to investigate the configurational possibilities of EFAl within the mordenite (MOR) and ZSM-5 frameworks. The xTB semi-empirical method within the GA was employed for an automated sampling of the EFAl-zeolite space. Furthermore, geometry refinement at the density functional theory (DFT) level of theory allowed us to improve the most stable configurations obtained from the GA and elaborate on the limitations of the xTB method. A subsequent ab initio thermodynamics analysis (aiTA) was chosen to predict the most favourable EFAl structure(s) under the catalytically relevant operando conditions.

Supported metal catalysts have shown to be efficient for CO 2 conversion due to their multifunctionality and high stability. Herein, we have combined density functional theory calculations with microkinetic modeling to investigate the catalytic reaction mechanisms of CO 2 hydrogenation to CH 3OH over a recently reported catalyst of Cd 4/TiO 2. Calculations reveal that the metal-oxide interface is the active center for CO 2 hydrogenation and methanol formation via the formate pathway dominates over the reverse water-gas shift (RWGS) pathway. Microkinetic modeling demonstrated that formate species on the surface of Cd 4/TiO 2 is the relevant intermediate for the production of CH 3OH, and CH 2O # formation is the rate-determining step. These findings demonstrate the crucial role of the Cd-TiO 2 interface for controlling the CO 2 reduction reactivity and CH 3OH selectivity.

Activation of homogeneous catalysts is an important step in ensuring efficient operation of any catalytic system as a whole. For the majority of pincer catalysts, the activation step leans heavily on the metal ligand cooperative chemistry that allows these complexes to react with small molecule substrates and engage in catalytic transformations. While the majority of such catalysts require a single activation event to become cooperative, herein we report an exception to this trend. Specifically, we demonstrate that a Ru-PN3P aminopyridine pincer catalyst, which lacks conventional reactivity with hydrogen upon typical one-fold activation, can exhibit this reactivity when a sequential two-step activation is performed. The resulting anionic complexes readily activate molecular hydrogen and react further with CO2 showing the previously unknown reactivity that is critical for CO2 hydrogenation catalysts. While active in CO2 hydrogenation, Ru-PN3Ps are significantly more efficient in hydrogenation of bicarbonates - a likely consequence of the chemistry of these pincers requiring formation of anionic complexes for hydrogen activation.

The electrocatalytic CO

Supported metal catalysts have shown to be efficient for CO

Homogeneous catalytic hydrogenations often operate in dynamic conditions. For example, in hydrogenation of esters reaction medium changes its polarity and becomes protic as reaction proceeds. As a result, the nature of the catalytic and reactive species in catalysis can change throughout the reaction, making it impossible to draw a static picture describing the catalyst performance. Herein we report on the molecular origins of such a complexity in the catalytic hydrogenation of esters. Using a new bis-N-heterocyclic carbene manganese (I) pincer catalyst we perform operando FTIR spectroscopy and kinetic studies to reveal the highly dynamic nature of the Mn intermediates formed in the catalytic mixture. Furthermore, we identify persistent inhibition phenomena caused by the reversible interaction of catalytically competent species with alcohol products. Pronounced strongly in Mn-promoted hydrogenation, this inhibitory pathway can principally affect any transition metal catalyst in ester hydrogenation. Finally, we show that the catalyst inhibition can be suppressed by using basic alkoxide promotors common for the majority of polar hydrogenations. We provide the first experimental and computational evidence that alkoxide promotion can alter the shape of the free energy surface and render catalyst inhibition unfavorable. At large, it implies a marked change to the values of the standard free energies of the key steps in the catalytic cycle that are universally assumed to be constant. While allowing to promote transformations that would otherwise be thermodynamically prohibited, these phenomena make a case for reconsideration of a traditional static viewpoint on homogeneous hydrogenation reactions.

Homogeneous catalytic hydrogenations often operate in dynamic conditions. For example, in hydrogenation of esters reaction medium changes its polarity and becomes protic as reaction proceeds. As a result, the nature of the catalytic and reactive species in catalysis can change throughout the reaction, making it impossible to draw a static picture describing the catalyst performance. Herein we report on the molecular origins of such a complexity in the catalytic hydrogenation of esters. Using a new bis-N-heterocyclic carbene manganese (I) pincer catalyst we perform operando FTIR spectroscopy and kinetic studies to reveal the highly dynamic nature of the Mn intermediates formed in the catalytic mixture. Furthermore, we identify persistent inhibition phenomena caused by the reversible interaction of catalytically competent species with alcohol products. Pronounced strongly in Mn-promoted hydrogenation, this inhibitory pathway can principally affect any transition metal catalyst in ester hydrogenation. Finally, we show that the catalyst inhibition can be suppressed by using basic alkoxide promotors common for the majority of polar hydrogenations. We provide the first experimental and computational evidence that alkoxide promotion can alter the shape of the free energy surface and render catalyst inhibition unfavorable. At large, it implies a marked change to the values of the standard free energies of the key steps in the catalytic cycle that are universally assumed to be constant. While allowing to promote transformations that would otherwise be thermodynamically prohibited, these phenomena make a case for reconsideration of a traditional static viewpoint on homogeneous hydrogenation reactions.

The catalytic asymmetric transfer hydrogenation (ATH) of ketones is a powerful methodology for the practical and efficient installation of chiral centers. Herein, we describe the synthesis, characterization, and catalytic application of a series of manganese complexes bearing simple chiral diamine ligands. We performed an extensive experimental and computational mechanistic study and present the first detailed experimental kinetic study of Mn-catalyzed ATH. We demonstrate that conventional mechanistic approaches toward catalyst optimization fail and how apparently different precatalysts lead to identical intermediates and thus catalytic performance. Ultimately, the Mn-N,N complexes under study enable quantitative ATH of acetophenones to the corresponding chiral alcohols with 75-87% ee.

Homogeneous hydrogenation catalysts based on metal complexes provide a diverse and highly tunable tool for the fine chemical industry. To fully unleash their potential, fast and effective methods for the evaluation of catalytic properties are needed. In turn, this requires changes in the experimental approaches to test and evaluate the performance of the catalytic processes. Design of experiment combined with statistical analysis can enable time- and resource-efficient experimentation. In this work, we employ a set of different statistical models to obtain the detailed kinetic description of a highly active homogeneous Mn (I) ketone hydrogenation catalyst as a representative model system. The reaction kinetics were analyzed using a full second order polynomial regression model, two models with eliminated parameters and finally a model which implements “chemical logic”. The coefficients obtained are compared with the corresponding high-quality kinetic parameters acquired using conventional kinetic experiments. We demonstrate that various kinetic effects can be well captured using different statistical models, providing important insights into the reaction kinetics and mechanism of a complex catalytic reaction.

According to the ISO 14687-2:2019 standard, the water content of H2 fuel for transportation and stationary applications should not exceed 5 ppm (molar). To achieve this water content, zeolites can be used as a selective adsorbent for water. In this work, a computational screening study is carried out for the first time to identify potential zeolite frameworks for the drying of high-pressure H2 gas using Monte Carlo (MC) simulations. We show that the Si/Al ratio and adsorption selectivity have a negative correlation. 218 zeolites available in the database of the International Zeolite Association are considered in the screening. We computed the adsorption selectivity of each zeolite for water from the high-pressure H2 gas having water content relevant to vehicular applications and near saturation. It is shown that due to the formation of water clusters, the water content in the H2 gas has a significant effect on the selectivity of zeolites with a helium void fraction larger than 0.1. Under each operating condition, five most promising zeolites are identified based on the adsorption selectivity, the pore limiting diameter, and the volume of H2 gas that can be dried by 1 dm3 of zeolite. It is shown that at 12.3 ppm (molar) water content, structures with helium void fractions smaller than 0.07 are preferred. The structures identified for 478 ppm (molar) water content have helium void fractions larger than 0.26. The proposed zeolites can be used to dry 400−8000 times their own volume of H2 gas depending on the operating conditions. Our findings strongly indicate that zeolites are potential candidates for the drying of high-pressure H2 gas.

Catalyst passivation refers to the formation of a protective oxide layer on the active metal particles that prevents their oxidation when exposed to air. Common passivation procedures, when applied to Ni/ Al2O3 catalysts, typically result in a significant decrease of the overall Ni surface area and, accordingly, the catalytic activity. Nevertheless, passivation and reactivation is an attractive pre-treatment option for this system. Ni/ Al2O3 typically requires reduction temperatures much higher than the desired reaction temperature, whereas reactivation of passivated samples is a low-temperature reduction. This can be used to avoid temperature limitations of existing systems. Thus, more insight into the passivation process of this system is desirable. In this work we analyzed the impact of passivation on the catalytic performance of a series of Ni/ Al2O3 catalysts in dry reforming of methane. This approach allows for the elimination of scale effects during passivation. We show that changes in conversion and especially of the coke content can be used to track sintering of Ni particles. These metrics allows to identify an adverse effects of catalyst passivation in excess O2, which gives rise to rapid local overheating and, accordingly, Ni sintering even when operating at tens of mg catalyst scale. Our study demonstrates that such problems are not limited to scaling issues and sufficient care must be taken even on a lab-scale when passivating Ni/ Al2O3 catalysts.

International audience; Embryonic X-ray amorphous semi-formed MFI-type zeolite units mixed with the Cu–Zn–Al catalyst have been used for the direct synthesis of dimethyl ether (DME) from syngas. The hybrid catalyst with embryonic zeolite (EZ) with the particle size of 5 nm has demonstrated superior performance in terms of activity, selectivity, and stability compared to the crystalline ZSM-5 zeolite counterpart and the amorphous aluminosilicate material. The FTIR pyridine adsorption uncovered several types of active sites in EZ, including Brønsted acid sites of medium strength. The DFT modeling pointed out the key role of defect sites leading to lower strength of the acid sites in the embryonic MFI zeolite in comparison with the fully crystalline material. The effect of embryonic zeolite on the DME synthesis has been assigned to enhanced transport of methanol from Cu to ultra-small zeolite precursor units with moderate acidity, hence promoting the efficient dehydration to DME with high selectivity and stability.

The reductive amination of benzoic acid and its derivatives would be an effective addition to current synthesis methods for benzylamine. However, with current technology it is very difficult to keep the aromaticity intact when starting from benzoic acid, and salt wastes are often generated in the process. Here, we report a heterogeneous catalytic system for such a reductive amination, requiring solely H-2 and NH3 as the reactants. The Ag/TiO2 or Au/TiO2 catalysts can be used multiple times, and very little noble metal is required, only 0.025 mol % Au. The catalysts are bifunctional: the support catalyzes the dehydration of both the ammonium carboxylate to the amide and of the amide to the nitrile, while the sites at the metal-support interface promote the hydrogenation of the in situ generated nitrile. Yields of up to 92% benzylamine were obtained.

The use of H-ZSM-5 with various binders (Al2O3, SiO2, and kaolinite, 10 wt% on catalyst formulation) for the catalytic conversion of glycerol to bio-based aromatics (GTA) was investigated in a continuous bench-scale unit at a pyrolysis temperature of 450 °C, catalytic upgrading temperature of 500 °C, WHSV of pure glycerol of 1 h−1, and atmospheric pressure, and their performance was compared to H-ZSM-5 (SiO2/Al2O3 molar ratio of 28). The latter gave a peak BTX carbon yield of ca. 31.1C.%, a life-time of ca. 220 min, and a total BTX productivity of ca. 312 mg BTX g−1H-ZSM-5. The introduction of binders affects catalyst performance, which is the most profound and promising for the H-ZSM-5/Al2O3 catalyst. It shows a prolonged catalyst life-time of ca. 320 min and a higher total BTX productivity of ca. 518 mg BTX g−1H-ZSM-5, compared to the H-ZSM-5 without a binder. Catalyst characterization studies show that the addition of the binder does not have a major effect on the specific surface area, total pore volume, and total acidity. Other relevant properties were affected, though, such as micropore volume (SiO2), a reduced Brønsted acidity (Al2O3, and SiO2), and reduced crystallinity (SiO2). Coke formation causes severe catalyst deactivation, ultimately leading to an inactive catalyst for BTX formation. Catalyst characterization studies after an oxidative regeneration showed that the textural properties of the regenerated catalysts were close to those of the original catalysts. However, some dealumination of H-ZSM-5 occurs, resulting in decreased crystallinity and acidity, causing irreversible deactivation, which needs attention in future catalyst development studies.

Formic acid and formate salts are key intermediates along the pathways for CO2utilization and hydrogen storage. Herein we report a highly efficient multiphase catalytic system utilizing a ruthenium PNP pincer catalyst for converting supersaturated bicarbonate solutions and slurries to aqueous formate solutions up to 12 M in molarity. The biphasic catalytic system delivers turnover frequencies up to 73 000 h−1and remains stable for up to 474 000 turnovers once reaction conditions are optimized.

Industrial-scale reforming of methane is typically carried out with an excess of oxidant to suppress coking of the catalyst. On the other hand, many academic studies on dry reforming employ a CO2/CH4 ratio of unity to quickly observe coking which can be reduced by adding a catalyst promoter. In this work, Ni/Al2O3 catalysts were tested for dry reforming of methane (CO2/CH4=1) with additional regeneration steps to test the resistance against an oxidation treatment. Thereby, we wanted to evaluate catalyst stability for industrial relevance. The effects of three promoters, Cr, Mn and Fe, that differ in their degree of CO2 interaction, are compared. A higher iron loading on Ni/Al2O3 leads to higher stability in dry reforming with lower coke formation. However, the higher the concentration of a promoter with high CO2 affinity, the quicker the catalyst is oxidized during regeneration with CO2. Subsequent reduction of a catalyst oxidized with CO2 leads to considerable sintering in all cases. This sintering induces formation of more coke during dry reforming. On such sintered samples only highly effective promoters in large concentrations still have a noticeable effect compared to unpromoted Ni/Al2O3.

The catalytic conversion of pure glycerol to bio-aromatics (bio-BTX) over an un-modified H-ZSM-5 (SiO2/Al2O3 molar ratio of 23) via an ex-situ catalytic pyrolysis approach in a continuous tandem-micro reactor at a scale of 1 g glycerol h−1 was investigated. A BTX peak carbon yield of 28.1 ± 0.2 % was obtained at a pyrolysis temperature of 400 °C, catalytic upgrading temperature of 500 °C, atmospheric pressure and a WHSV of 1 h−1. About 70 % of the bound oxygen in glycerol was converted to water. The latter was mainly formed in the catalytic upgrading unit (70 %), though conversion of glycerol to other oxygenates with water formation was also observed in the pyrolysis unit. Catalyst deactivation occurs at a time scale of hours and is mainly due to coke deposition (12.0 wt.%) on the catalyst surface. An oxidative regeneration procedure to remove coke was applied and 5 cycles of reaction-regeneration were performed successfully, though a drop in activity was observed after each cycle due to irreversible catalyst deactivation. Characterization of the fresh, deactivated and regenerated catalysts by various techniques revealed dealumination of the H-ZSM-5 framework and resulted in a dramatic decrease in Brønsted acidity of the catalyst. Dealumination mainly occurred in the catalytic upgrading reactor and to a by far lesser extent during catalyst regeneration. This information is relevant for a better understanding of the process on a molecular level but also for scale-up studies, e.g. for the design of pilot plants.

Rapid and accurate prediction of reactivity descriptors of transition metal (TM) complexes is a major challenge for contemporary quantum chemistry. The recently-developed GFN2-xTB method based on the density functional tight-binding theory (DFT-B) is suitable for high-throughput calculation of geometries and thermochemistry for TM complexes albeit with moderate accuracy. Herein we present a data-augmented approach to improve substantially the accuracy of the GFN2-xTB method for the prediction of thermochemical properties using pKa values of TM hydrides as a representative model example. We constructed a comprehensive database for ca. 200 TM hydride complexes featuring the experimentally measured pKa values as well as the GFN2-xTB-optimized geometries and various computed electronic and energetic descriptors. The GFN2-xTB results were further refined and validated by DFT calculations with the hybrid PBE0 functional. Our results show that although the GFN2-xTB performs well in most cases, it fails to adequately describe TM complexes featuring multicarbonyl and multihydride ligand environments. The dataset was analyzed with the ordinary least squares (OLS) fitting and was used to construct an automated machine learning (AutoML) approach for the rapid estimation of pKa of TM hydride complexes. The results obtained show a high predictive power of the very fast AutoML model (RMSE ∼ 2.7) comparable to that of the much slower DFT calculations (RMSE ∼ 3). The presented data-augmented quantum chemistry-based approach is promising for high-throughput computational screening workflows of homogeneous TM-based catalysts.

Hydrogenation of levulinic acid to γ-valerolactone is a key reaction in the valorization of carbohydrates to renewable fuels and chemicals. State-of-the-art catalysts are based on supported noble metal nanoparticle catalysts. We report the utility of a bimetallic Fe-Re supported on TiO2 for this reaction. A strong synergy was observed between Fe and Re for the hydrogenation of levulinic acid in water under mild conditions. Fe-Re/TiO2 shows superior catalytic performance compared to monometallic Fe and Re catalysts at similar metal content. The hydrogenation activity of the bimetallic catalysts increased with Re content. H2-TPR, XPS, XANES, EXAFS, Mössbauer spectroscopy, TEM, and low-temperature CO IR spectroscopy show that the bimetallic catalysts contain metallic Re nanoparticles covered by FeOx species and small amounts of a Fe-Re alloy. Under reaction conditions, the partially reduced surface FeOx species adsorb water and form Brønsted acidic OH groups, which are involved in dehydration of reaction intermediates. Under optimized conditions, nearly full conversion of levulinic acid with a 95 % yield of γ-valerolactone could be achieved at a temperature as low as 180 °C in water at a H2 pressure of 40 bar.

The influence of the model and method choice on the DFT predicted

Rapid and accurate prediction of reactivity descriptors of transition metal (TM) complexes is a major challenge for contemporary quantum chemistry. Recently developed GFN2-xTB method based on the density functional tight-binding theory (DFT-B) is suitable for high-throughput calculation of geometries and thermochemistry for TM complexes albeit with a moderate accuracy. Herein we present a data-augmented approach to improve substantially the accuracy of GFN2-xTB for the prediction of thermochemical properties using pKa values of TM hydrides as a representative model example. We constructed a comprehensive database for ca. 200 TM hydride complexes featuring the experimentally measured pKa’s as well as the GFN2-xTB optimized geometries and various computed electronic and energetic descriptors. The GFN2-xTB results were further refined and validated by DFT calculations with the hybrid PBE0 functional. Our results show that although the GFN2-xTB performs well in most cases, it fails to adequately desribe TM complexes featuring multicarbonyl and multihydride ligand environments. The dataset was analyzed with the partial least squares (OLS) fitting and was used to construct an automated machine learning (AutoML) approach for the rapid estimation of pKa of TM hydride complexes. The results obtained show a high predictive power of the very fast AutoML model (RMSE ~ 2.7) comparable to that of the much slower DFT calculations (RMSE ~ 3). The presented data-augmented quantum chemistry-based approach is promising for high-throughput computational screening workflows of homogeneous TM-based catalysts.