Types Of Catalytic Sites Research Articles

Integrated in situ spectroscopic characterization of bi-functional nanoporous hybrid catalysts

Bi-functional catalysts possess various catalytic sites and can catalyze different types of reactions in a single-pot cascade manner. Herein, we report the synthesis and characterization of mono- and bifunctional silica-based mesoporous hybrid catalysts involving acid and base active sites. The ability for cooperative catalysis has been investigated using a multi-technique approach involving powder X-ray diffraction, FT-IR, and multinuclear MAS NMR spectroscopy, as well as thermogravimetric analysis. To elucidate the nature and strength of multifunctional catalytic sites, different types of probe molecules were employed and studied using spectroscopic techniques. The results show that the activity of the mesoporous surface-grafted acid and/or base sites is directly related to the intimacy criterion, the separation between the different types of catalytic sites. The presence or absence of mutual interactions between the different catalytic sites dictates the selectivity and yield of the reactions.

Metal–Organic Framework Supported Low‐Nuclearity Cluster Catalysts for Highly Selective Carbon Dioxide Electroreduction to Ethanol

AbstractIt is still a great challenge to achieve high selectivity of ethanol in CO2 electroreduction reactions (CO2RR) because of the similar reduction potentials and lower energy barrier of possible other C2+ products. Here, we report a MOF‐based supported low‐nuclearity cluster catalysts (LNCCs), synthesized by electrochemical reduction of three‐dimensional (3D) microporous Cu‐based MOF, that achieves a single‐product Faradaic efficiency (FE) of 82.5 % at −1.0 V (versus the reversible hydrogen electrode) corresponding to the effective current density is 8.66 mA cm−2. By investigating the relationship between the species of reduction products and the types of catalytic sites, it is confirmed that the multi‐site synergism of Cu LNCCs can increase the C−C coupling effect, and thus achieve high FE of CO2–to–ethanol. In addition, density functional theory (DFT) calculation and operando attenuated total reflectance surface‐enhanced infrared absorption spectroscopy further confirmed the reaction path and mechanism of CO2–to–EtOH.

Metal-Organic Framework Supported Low-Nuclearity Cluster Catalysts for Highly Selective Carbon Dioxide Electroreduction to Ethanol.

It is still a great challenge to achieve high selectivity of ethanol in CO2 electroreduction reactions (CO2RR) because of the similar reduction potentials and lower energy barrier of possible other C2+ products. Here, we report a MOF-based supported low-nuclearity cluster catalysts (LNCCs), synthesized by electrochemical reduction of three-dimensional (3D) microporous Cu-based MOF, that achieves a single-product Faradaic efficiency (FE) of 82.5 % at -1.0 V (versus the reversible hydrogen electrode) corresponding to the effective current density is 8.66 mA cm-2. By investigating the relationship between the species of reduction products and the types of catalytic sites, it is confirmed that the multi-site synergism of Cu LNCCs can increase the C-C coupling effect, and thus achieve high FE of CO2-to-ethanol. In addition, density functional theory (DFT) calculation and operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy further confirmed the reaction path and mechanism of CO2-to-EtOH.

In-depth exploration of catalytic sites on amorphous solid water: I. The astrosynthesis of aminomethanol.

Chemical processes taking place on ice-grain mantles are pivotal to the complex chemistry of interstellar environments. In this study, we conducted a comprehensive analysis of the catalytic effects of an amorphous solid water (ASW) surface on the reaction between ammonia (NH3) and formaldehyde (H2CO) to form aminomethanol (NH2CH2OH) using density functional theory. We identified potential catalytic sites based on the binding energy distribution of NH3 and H2CO reactants, on a set-of-clusters surface model composed of 22 water molecules and found a total of 14 reaction paths. Our results indicate that the catalytic sites can be categorized into four groups, depending on the interactions of the carbonyl oxygen and the amino group with the ice surface in the reactant complex. A detailed analysis of the reaction mechanism using Intrinsic Reaction Coordinate and reaction force analysis, revealed three distinct chemical events for this reaction: formation of the C-N bond, breaking of the N-H bond, and formation of the O-H hydroxyl bond. Depending on the type of catalytic site, these events can occur within a single, concerted, albeit asynchronous, step, or can be isolated in a step-wise mechanism, with the lowest overall transition state energy observed at 1.3 kcal mol-1. A key requirement for the low-energy mechanism is the presence of a pair of dangling OH bonds on the surface, found at 5% of the potential catalytic sites on an ASW porous surface.

Quantifying the fraction and activity of catalytic sites at different surface densities of aminosilanes in SBA-15 for the aldol reaction and condensation

The complexity of aminosilica materials manifests as catalysts with multiple types of active sites. Whereas previous work has assumed that each site contributes equally to the observed catalytic behavior, this work rigorously investigates the catalytic activity on a per site basis using site quantification experiments. These experiments reveal that aminosilica materials have four types of catalytic sites, including high, medium, and low activity sites as well as inactive sites. The fraction of each of these sites depends on the surface density. The low activity sites are associated with amines clustered on the surface. Additional experiments reveal that the high activity sites are likely not associated with amines grafted in pairs or amines located on the external surface, leading us to speculate that the difference is amines cooperatively interacting with different types of silanols. Overall, the results demonstrate the potential to create highly active catalytic materials using cooperative interactions.

The proteasome - structural aspects and inhibitors: a second life for a validated drug target

The proteasome is the central component of the adaptable ubiquitin proteasome system (UPS) discovered in the 1980's. It sustains protein homeostasis (proteostasis) under a large variety of physiological and pathological conditions. Its dysregulation has been often associated to various human diseases. Its potential regulation by modulators has emerged as promising avenue to develop treatments of various pathologies. The FDA approval in 2003 of the proteasome inhibitor bortezomib to treat multiple myeloma, then mantle lymphoma in 2006, has considerably increased the clinical interest of proteasome inhibition. Second-generation proteasome inhibitors (carfilzomib and ixazomib) have been approved to overcome bortezomib resistance and improved toxicity profile and route of administration. Selective inhibition of immunoproteasome is a promising approach towards the development of immunomodulatory drugs. The design of these drugs relies greatly on the elucidation of high-resolution structures of the targeted proteasomes. The ATPase-dependent 26S proteasome (2.4 MDa) consists of a 20S proteolytic core and one or two 19S regulatory particles. The 20S core contains three types of catalytic sites. In recent years, due to technical advances especially in atomic cryo-electron microscopy, significant progress has been made in the understanding of 26S proteasome structure and its dynamics. Stepwise conformational changes of the 19S particle induced by ATP hydrolysis lead to substrate translocation, 20S pore opening and processive protein degradation by the 20S proteolytic subunits (2β1, 2β2 and 2β5). A large variety of structurally different inhibitors, both natural products or synthetic compounds targeting immuno- and constitutive proteasomes, has been discovered. The latest advances in this drug discovery are presented. Knowledge about structures, inhibition mechanism and detailed biological regulations of proteasomes can guide strategies for the development of next-generation inhibitors to treat human diseases, especially cancers, immune disorders and pathogen infections. Proteasome activators are also potentially applicable to the reduction of proteotoxic stresses in neurodegeneration and aging.

A Characterization Study of Reactive Sites in ALD-Synthesized WOx/ZrO2 Catalysts

A series of ZrO2-supported WOx catalysts were prepared using atomic layer deposition (ALD) with W(CO)6, and were then compared to a WOx/ZrO2 catalyst prepared via conventional impregnation. The types of sites present in these samples were characterized using temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements with 2-propanol and 2-propanamine. Weight changes showed that the WOx catalysts grew at a rate of 8.8 × 1017 W atoms/m2 per cycle. Scanning transmission electron microscopy/energy-dispersive spectroscopy (STEM-EDS) indicated that WOx was deposited uniformly, as did the 2-propanol TPD-TGA results, which showed that ZrO2 was completely covered after five ALD cycles. Furthermore, 2-propanamine TPD-TGA demonstrated the presence of three types of catalytic sites, the concentrations of which changed with the number of ALD cycles: dehydrogenation sites associated with ZrO2, Brønsted-acid sites associated with monolayer WOx clusters, and oxidation sites associated with higher WOx coverages. The Brønsted sites were not formed via ALD of WOx on SiO2. The reaction rates for 2-propanol dehydration were correlated with the concentration of Brønsted sites. While TPD-TGA of 2-propanamine did not differentiate the strength of Brønsted-acid sites, H–D exchange between D2O and either toluene or chlorobenzene indicated that the Brønsted sites in tungstated zirconia were much weaker than those in H-ZSM-5 zeolites.

An enzyme-like imprinted-polymer reactor with segregated quantum confinements for a tandem catalyst.

This study was aimed at addressing the present challenge in tandem catalysts, as to how to furnish catalysts with tandem catalytic-ability without involving the precise control and man-made isolation of different types of catalytic sites. This objective was realized by constructing an enzyme-like imprinted-polymer reactor made of a unique polymer composite inspired from the compartmentalization of cells, a composite of a reactive imprinted polymer (containing acidic catalytic sites), and encapsulated metal nanoparticles (acting as catalytic reduction sites). The compilation of two types of catalytic sites with admissible access allowed this reactor to behave like compartments of cells for enzymatic reactions and hence catalytically constituted two quantum interaction-segregated domains, which led to the occurrence of catalytic tandem processes. Unlike the reported functional reactors that run tandem catalysis by largely depending on the precise control and man-made isolation of different types of catalytic sites, tandem catalysis in this reactor run naturally with segregated quantum confinements, which does not involve the precise control and isolation of different types of catalytic sites. This protocol presents new opportunities for the development of functional catalysts for complicated chemical processes.

Reactions of 2-octanol on Amberlyst® 15: concurrent Eley–Rideal and Langmuir–Hinshelwood reactions on two different types of active sites

On Amberlyst® 15 (dry) at 75 °C, 2-octanol undergoes concurrent esterification and dehydration in the presence of heptanoic acid in the liquid phase. Kinetic data and 18O labelling studies show that the esterification reaction most likely occurs by an Eley–Rideal (ER) mechanism between adsorbed carboxylic acid and bulk phase alcohol, while the dehydration reaction occurs by a Langmuir–Hinshelwood (LH) mechanism requiring interaction of adsorbed alcohol with a vacant site. The two reactions occur on two different types of catalytic sites: while all the sites are sulfonic acid groups, kinetic data suggest that not all of the acidic sites are positioned appropriately to a neighboring site to assist with dehydration. A kinetic model fits the observed data for the simultaneous ER and LH reactions over at least 23 h of observation.

Tungstate-loaded triazine-based magnetic poly(Bis-imidazolium ionic liquid): An effective bi-functional catalyst for tandem selective oxidation/Knoevenagel condensation in water

A novel bi-functional polymeric catalyst was synthesized by immobilization of tungstate anions onto the nitrogen rich poly(ionic liquid)/magnetic nanocomposite. The resulting catalyst has two types of catalytic sites: (i) immobilized WO4 anions with bis-imidazolium ionic liquid cation for selective oxidation of alcohols and (ii) basic amine groups for Knoevenagel condensation between produced aldehyde and malononitrile. Due to the polymeric nature of the catalyst, large amounts of tungstate and basic nitrogen groups were presented on the solid surface which led to a decrease in the applied catalyst mass for catalytic reaction. High catalytic activity and excellent selectivity of catalyst in water medium make this protocol a green way for production of fine chemicals.

Substrate processing in intramembrane proteolysis by γ-secretase - the role of protein dynamics.

Intramembrane proteases comprise a number of different membrane proteins with different types of catalytic sites. Their common denominator is cleavage within the plane of the membrane, which usually results in peptide bond scission within the transmembrane helices of their substrates. Despite recent progress in the determination of high-resolution structures, as illustrated here for the γ-secretase complex and its substrate C99, it is still unknown how these enzymes function and how they distinguish between substrates and non-substrates. In principle, substrate/non-substrate discrimination could occur at the level of substrate binding and/or cleavage. Focusing on the γ-secretase/C99 pair, we will discuss recent observations suggesting that global motions within a substrate transmembrane helix may be much more important for defining a substrate than local unraveling at cleavage sites.

Nanostructured copper-zirconia composites as catalysts for methanol decomposition

Nanostructured mesoporous ZrO2 was obtained by hydrothermal synthesis using surfactant assisted approach. Copper modifications of thus obtained ZrO2 (4–25wt.% copper) were prepared by incipient wetness impregnation with the corresponding nitrate or acetyl acetonate precursors and further treatment in oxidative or reduction atmosphere. The obtained materials were characterized by N2 physisorption, XRD, UV–vis, FTIR, XPS and temperature programmed reduction with hydrogen and tested as catalysts in methanol decomposition to hydrogen and CO. Selected samples were investigated after partial coverage of the surface with 11C-labeled methanol and 11C-labeled methyl formate. The formation of monolayer of finely dispersed copper oxide species on zirconia at about 4wt.% copper loading and bulk copper nanoparticles at higher one is observed. It was assumed that zirconia support provides the formation of two types of catalytic sites: the first ones strongly adsorb methanol and exhibit predominantly dehydrogenation activity, while the others, possess acidic function. The modification of zirconia with copper significantly improves the catalytic activity and selectivity in methanol decomposition via (i) creation of additional surface acidic sites which contribute to generation of methoxy intermediates; (ii) stabilization of Cu2+-Cu+ and Cu+-Cu0 redox pairs where the electron transfer is facilitated by zirconia support and (iii) assisting the recombination and release of hydrogen during the transformation of produced on zirconia surface intermediates.

The effect of coke deposition on the activity and selectivity of the HZSM-5 zeolite during ethylbenzene alkylation reaction in the presence of ethanol

The alkylation of ethylbenzene (EB) with ethanol over HZSM-5 zeolite catalysts was carried out using a riser simulator reactor at different reaction temperatures and contact times. It was observed that the amount of coke deposited over the zeolite has a great influence on the reaction pathway. At higher temperature (400 °C), the spent catalyst was found to exhibit a much higher activity towards the alkylation products (DEB) as compared to the fresh catalyst. This difference between spent and fresh samples became less significant at lower temperature (250 °C). The highest yield of DEB products over the spent catalyst was obtained for intermediate temperatures (300–350 °C). The coke deposition was further analyzed using terahertz time-domain spectroscopy (THz-TDS), 27Al MAS NMR spectroscopy, TPO measurements and pyridine-FTIR. THz-TDS and TPO results revealed that the structure of coke formed on all catalysts is essentially the same, while pyridine-FTIR studies revealed that coke leads to a reduction in the acidity of the catalyst. 27Al MAS NMR results of spent samples suggested a relationship between alkylation activity and extra-framework aluminium species, which is possibly associated with the formation of very active Lewis sites. This work shows that an understanding and control of different types of catalytic sites over zeolite surfaces may improve and optimize reaction performances during alkylation of aromatics.

Porphyrin based porous organic polymer as bi-functional catalyst for selective oxidation and Knoevenagel condensation reactions

Porphyrin based microporous organic polymer Fe-POP-1 has been synthesized through a facile solvothermal method involving extended aromatic substitution of pyrrole and terephthaldehyde in the presence of Fe(III). This material has very high BET surface area and exhibits two types of catalytic sites: iron-free porphyrin moieties for base catalysis as well as Fe(III)-bound sites for slective oxidation reactions. Due to the presence of basic porphyrin macrocyclic site in Fe-POP-1, it catalyzes Knoevenagel condensation of aromatic aldehydes with malononitrile at room temperature, whereas the Fe(III)-bound site catalyzes selective oxidation of alcohols to the respective aldehyde/ketones in the presence of tert-butyl hydroperoxide (TBHP) as oxidant. Good reusability and excellent selectivity makes this Fe-POP-1 as a promising and efficient bi-functional heterogeneous catalyst for the production of organic fine chemicals through the environmentally benign liquid phase catalytic reactions.

ChemInform Abstract: The Effects of Carbon Dioxide, Methanol, and Alkali Promoter Concentration on the Higher Alcohol Synthesis Over a Cu/ZnO/Cr2O3 Catalyst.

The behavior of the methanol and higher alcohol (HAS) syntheses as a function of catalyst promoter concentration, C02 feed gas concentration, and methanol feed gas concentration has been investigated over a Cu/ZnO/Cr203 catalyst at 10 MPa and 285–315°C. In the presence of C02 in the feed gas, the methanol and HAS yields are greater for the unpromoted catalyst than the 0.5% K2CO3-promoted catalyst. The methanol and HAS yields also reach a maximum as the C02 concentration increases for both catalysts. In the absence of C02, both the methanol and HAS yields reach a maximum as the K2CO3 promoter concentration increases. The complex behavior results from the ability of these catalysts to incorporate CO, C02, and methanol into higher alcohols, on at least two different types of catalytic sites. It is suggested that C02 participates directly in HAS on copper sites while alkali/copper interfacial sites are involved in converting CO. Higher alcohol production is therefore very sensitive to the feed gas composition and the components and composition of the catalyst.

Gas-phase phenol methylation over Mg/Me/O (Me = Al, Cr, Fe) catalysts: mechanistic implications due to different acid–base and dehydrogenating properties

This contribution reports about an in situ FT-IR investigation and the catalytic reactivity of Mg/Me(3+) mixed oxides (Me = Cr, Fe, or Al; Mg/Me = 2, atomic ratio) in the gas-phase methylation of phenol with methanol. It is the second of two papers concerning the mentioned systems, and its purpose is twofold: to confute the classic and not accurate theory concerning the reaction mechanism, and to propose a novel interpretation based on the combined use of catalytic tests and in situ molecular spectroscopy. Results here reported highlight that: (i) the reaction mechanism in phenol methylation, when catalysed by basic systems, is not a classical electrophylic substitution, as generally reported in the literature, but proceeds through the formation of formaldehyde as an intermediate, and (ii) the catalytic behaviour in respect to both methanol and phenol reactants is strictly dependent on catalyst features. Although all investigated systems exhibit a basic-type behaviour with regard to phenol, which dissociates to yield an adsorbed phenolate species, the distribution of phenolic compounds obtained with the Mg/Al/O catalyst was that typically observed with acid catalysts, with prevailing formation of anisole when the reaction was carried out below 350 degrees C and of mono and poly-C-alkylated compounds when the reaction temperature was above 350 degrees C. On the contrary, the reactivity shown by both Mg/Fe/O and Mg/Cr/O systems was that reported in the literature as typical of mixed oxides possessing basic features. The extent of methanol decomposition into light compounds was maximum in the case of Mg/Fe/O catalysts, because of the pronounced redox behaviour typical of Fe(3+) species, whereas neither methanol dehydrogenation nor decomposition were ever observed with Mg/Al/O up to 400 degrees C. Reactivity tests and spectroscopic experiments hinted for methanol dehydrogenation to formaldehyde as the first step in the ring-methylation of phenol with Mg/Cr/O and Mg/Fe/O: in that case, o-cresol and 2,6-xylenol were the only reaction products. But, with Mg/Al/O systems, for which no methanol dehydrogenation occurred, the formation of anisole was due to the synergistic effect of stronger basic features and the presence of Lewis acidic sites, that facilitate the reaction between phenol and methanol after activation over the two different types of catalytic sites.

Mesoporous silica–alumina as support for Pt and Pt–Mo sulfide catalysts: Effect of Pt loading on activity and selectivity in HDS and HDN of model compounds

The potential of mesoporous silica–alumina (MSA) material as support for the preparation of sulfided Pt and Pt–Mo catalysts of varying Pt loadings was studied. The catalysts were characterized by their texture, hydrogen adsorption, transmission electron microscopy, temperature programmed reduction (TPR) and by activity in simultaneous hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyridine. Sulfided Pt/MSA catalysts with 1.3 and 2wt.% Pt showed almost the same HDS and higher HDN activities per weight amounts as conventional CoMo and NiMo/Al2O3, respectively. The addition of Pt to sulfided Mo/MSA led to promotion in HDS and HDN with an optimal promoter content close to 0.5wt.%. The results of TPR showed strong positive effect of Pt on reducibility of the MoS2 phase which obviously reflects in higher activity of the promoted catalysts. The activity of the MSA-supported Pt–Mo catalyst containing 0.5wt.% Pt was significantly higher than the activity of alumina-supported Pt–Mo catalyst. Generally, Pt–Mo/MSA catalysts promoted by 0.3–2.3wt.% Pt showed lower HDS and much higher HDN activities as compared to weight amounts of CoMo and NiMo/Al2O3. It is proposed that thiophene HDS and pyridine hydrogenation proceed over Pt/MSA and the majority of Pt–Mo/MSA catalysts on the same type of catalytic sites, which are associated with sulfided Pt and MoS2 phases. On the contrary, piperidine hydrogenolysis takes place on different sites, most likely on metallic Pt fraction or sites created by abstraction of sulfur from MoS2 in the presence of Pt.

The roles of gallium hydride and Brønsted acidity in light alkane dehydrogenation mechanisms using Ga-exchanged HZSM-5 catalysts: A DFT pathway analysis

We have used density functional theory (DFT) to study light alkane dehydrogenation by Ga-exchanged HZSM-5 by considering two types of catalytic sites: a mono-Al site of the form Z−[HGaX]+ (X=H, CH3, OH, Cl) and a di-Al site of the form Z2−[GaH]2+. For the mono-Al site, we report a new, direct one-step dehydrogenation mechanism; however, we conclude in general that mono-Al sites in ZSM-5 are not likely responsible for alkane dehydrogentation, as calculated activation energies are too high compared to experimental values (∼60kcal/mol versus 39kcal/mol). Instead, we propose [GaH]2+ residing near di-Al sites (Z2−[GaH]2+) are more active sites for dehydrogenation. We report a three-step mechanism for di-Al sites consisting of (1) C–H activation, followed by (2) alkene desorption and (3) H2 removal. We find that as Al–Al distance increases, the activation barrier for C–H activation decreases (ranging from 85.72 to 38.38 to 19.69kcal/mol), while the barrier for H2 removal increases (ranging from 15.49 to 36.71 to 47.38kcal/mol)—resulting in an optimal Al–Al separation distance of 4.53Å arising from these competing trends. As a result, we propose a simple ‘structure-to-activity’ correlation based on the Sabatier principle, which could be used to model and design the catalyst with required dehydrogenation activity.

Kinetic behavior of ethylene/1‐hexene copolymerization in slurry and solution reactors

AbstractThe copolymerization of ethylene and 1‐hexene over a spherical polymer/MgCl2‐supported TiCl4 catalyst was studied as a function of the polymerization temperature from 40 to 100 °C in a slurry reactor and from 120 to 200 °C in a solution reactor with triethylaluminum (TEA) as a cocatalyst (1.0–6.8 mmol). The activities increased from 40 to 80 °C and then declined monotonically with increases in the temperature during the slurry and solution polymerizations. The kinetic behavior in the slurry and solution operations was described by the same rate expression. The modeling results indicated that the catalyst had at least two different types of catalytic sites; one site was responsible for the acceleration–decay nature of the activity profiles, whereas the second site resulted in long‐term activity. The apparent activation energy for site activation in the slurry operation was 69.9 kJ/mol; no activation energies for site activation could be estimated for the solution operation because the activation process was essentially instantaneous at the higher temperatures. The activation energies for deactivation were 100.3 kJ/mol for the slurry operation and 31.2 kJ/mol for the solution operation. The responses to TEA were similar for the slurry and solution operations; the rates increased with increasing amounts of TEA between 1.0 and 3.4 mmol and then decreased with larger amounts of TEA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2248–2257, 2005

Supported copper triflate as catalyst for the cationic polymerization of styrene

Silica-supported copper triflate catalysts have been prepared and used in the heterogeneous cationic polymerization of styrene in an attempt to reduce the environmental hazards associated with the traditional vinyl polymerization catalyzed by homogeneous Lewis acid. Faster reactions and higher molecular weight polymers were observed compared to the homogeneous catalyst. However, the presence of active sites both within the pores and on the external surface, and the possible availability of different types of catalytic sites, leads to broad polymer molecular weight distribution. The catalysts exhibited both Lewis and Brønsted acidities due to the presence of residual moisture within Cu(OTf) 2, as well as the surface silanol groups of the silica and the methanol used as solvent in the catalyst preparation. Initiation occurs via a catalyst/cocatalyst process and water has proven to play a major role as the cocatalyst. Proton transfers from the growing polymer molecules to the initiating species or to the monomer represent the principal termination mechanism. The reuse of the catalyst was limited by the blocking of the pores by the polymer chains, reducing the diffusion of the monomer units to the active sites.