Rhenium-based Catalysts Research Articles

Rhenium-based catalysts for biomass conversion

This review highlights the advancements in homogeneous and heterogeneous Re-based catalysts for the conversion of biomass into value-added chemicals.

Alkane metathesis over immobilized pincer-ligated iridium complexes: Effect of support nature

In this work, catalytic alkane metathesis has been evaluated as a suitable approach to upcycle hydrocarbons (polyolefins) at moderate temperatures. To this end, a pincer-ligated iridium complex (dehydrogenation catalyst) has been combined with a rhenium-based (metathesis) catalyst, being the effect of immobilizing the Ir complex over different supports deeply investigated. FTIR spectroscopy has been used to confirm the complex grafting and to elucidate the anchoring site to the support. Additionally, the supports have been dehydroxylated at different conditions to evaluate its possible impact in both the complex grafting and the catalytic activity. The influence of the support nature and its participation in the catalytic reaction have been clearly evidenced.

Hydrosilylation Reactions Catalyzed by Rhenium.

Hydrosilylation is an important process, not only in the silicon industry to produce silicon polymers, but also in fine chemistry. In this review, the development of rhenium-based catalysts for the hydrosilylation of unsaturated bonds in carbonyl-, cyano-, nitro-, carboxylic acid derivatives and alkenes is summarized. Mechanisms of rhenium-catalyzed hydrosilylation are discussed.

Visible-Light Photochemical Reduction of CO2 to CO Coupled to Hydrocarbon Dehydrogenation.

Research on the photochemical reduction of CO2 , initiated already 40 years ago, has with few exceptions been performed by using amines as sacrificial reductants. Hydrocarbons are high-volume chemicals whose dehydrogenation is of interest, so the coupling of a CO2 photoreduction to a hydrocarbon-photodehydrogenation reaction seems a worthwhile concept to explore. A three-component construct was prepared including graphitic carbon nitride (g-CN) as a visible-light photoactive semiconductor, a polyoxometalate (POM) that functions as an electron acceptor to improve hole-electron charge separation, and an electron donor to a rhenium-based CO2 reduction catalyst. Upon photoactivation of g-CN, a cascade is initiated by dehydrogenation of hydrocarbons coupled to the reduction of the polyoxometalate. Visible-light photoexcitation of the reduced polyoxometalate enables electron transfer to the rhenium-based catalyst active for the selective reduction of CO2 to CO. The construct was characterized by zeta potential, IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). An experimental Z-scheme diagram is presented based on electrochemical measurements and UV/Vis spectroscopy. The conceptual advance should promote study into more active systems.

Selective Cleavage of C-O Bonds in Lignin Catalyzed by Rhenium(VII) Oxide (Re2 O7 ).

Invited for this month's cover are the collaborating groups of Dr. Changzhi Li from Dalian Institute of Chemical Physics, China, and Dr. Lican Lu, Xiangtan University, China. The cover picture shows the harvest of a renewable supply of aromatic compounds accessible from lignin model compounds and lignin feedstocks through the selective Re2 O7 -catalyzed cleavage of lignin C-O bonds. This study paves the way towards the development of rhenium-based catalysts for the reductive valorization of realistic lignin materials in a controlled manner. Read the full text of the article at 10.1002/cplu.201700547.

New Motifs in Deoxydehydration: Beyond the Realms of Rhenium.

The deoxydehydration (DODH) reaction remains one of the most efficient methods of reducing the oxygen content of biomass while keeping important functionality in place. This reaction is traditionally catalysed by high-valent oxo-rhenium species. Recent research into heterogeneous rhenium-based catalysts for DODH demonstrates their ability to rival and in some cases surpass their homogeneous counterparts. Furthermore, catalysts based on the metals molybdenum and vanadium show great potential as inexpensive alternatives to rhenium catalysts. In this Minireview, we detail the recent developments within the field of DODH with particular emphasis on discoveries outside the realms of rhenium.

In situ-DRIFTS study: influence of surface acidity of rhenium-based catalysts in the metathesis of various olefins for propylene production

Various olefins including 1- and 2-butene, 2-pentene, and ethylene were used as the reactants for producing propylene by self- and cross-metathesis reactions at 60 °C on supported Re-based catalysts (4 wt% Re).

Highly Selective Deoxydehydration of Tartaric Acid over Supported and Unsupported Rhenium Catalysts with Modified Acidities.

The deoxydehydration (DODH) of sugar acids to industrially important carboxylic acids is a very attractive topic. Oxorhenium complexes are the most-often employed DODH catalysts. Because of the acidity of the rhenium catalysts, the DODH products of sugar acids were usually in the form of mixture of free carboxylic acids and esters. Herein, we demonstrate strategies for the selective DODH of sugar acids to free carboxylic acids by tuning the Lewis acidity or the Brønsted acidity of the rhenium-based catalysts. Starting from tartaric acid, up to 97 % yield of free maleic acid was achieved. Based on our strategies, functional polymer immobilized heterogeneous rhenium catalysts were also developed for the selective DODH conversion of sugar acids.

One-pot direct synthesis route to self-assembled highly ordered Zn-decorated mesoporous aluminium oxide toward efficient and sustainable metathesis heterogeneous catalyst design

Zn-modified well-ordered mesoporous alumina successfully synthesizedviaone-pot process and evaluated as support for rhenium-based catalyst for methyl oleate self-metathesis.

Triacylglycerol self-metathesis over highly chemoselective methyltrioxorhenium supported on ZnCl2-promoted mesoporous alumina

Since triolein is the main constituent of most oils such as olive and high oleic sunflower oils, self-metathesis of this triacylglycerol at mild reaction conditions was investigated via a heterogeneous approach, in the presence of ZnCl2-modified mesoporous alumina supported methyltrioxorhenium (MTO) as a catalyst. Both intra and intermolecular metathesis reactions were observed. While, 9-octadecene, dimers and trimers of the triacylglycerol were isolated, intermolecular cyclization metathesis products were also obtained. The MTO-based catalyst was found to be highly chemoselective affording only desired metathesis product formation. In contrast, more than 50% of triolein was converted to undesired solid product in the presence of the homogeneous Grubbs 2nd generation catalyst. Triolein metathesis products were effectively separated by chromatographic techniques and identified using mass spectrometric and NMR analyses. The reported results show that 3%MTO/ZnCl2–Al2O3-meso is an efficient and selective heterogeneous catalyst for bulky functionalized olefins metathesis such as triacylglycerols and edible oils, avoiding the use of costly homogeneous catalysts, or the use other rhenium-based catalysts with toxic promoters such as R4Sn.

ChemInform Abstract: Rhenium Nanochemistry for Catalyst Preparation

AbstractReview: 57 refs.

Rhenium-Catalyzed Dehydration of Nonbenzylic and Terpene Alcohols to Olefins

With the increasing importance of research into biomass as a feedstock for the chemical industry, new methods to reduce the oxygen content of biomass are required. Here, we present our progress in the field of the dehydration reaction, using various rhenium-based catalysts, with rhenium(VII) oxide as the most active catalyst. A wide range of allylic, aliphatic, and homoallylic alcohols have been used as substrates under relatively mild conditions (100–150 °C, 0.5 mol % catalyst loading, technical toluene, ambient atmosphere) as well as various biobased terpene alcohols. Furthermore, we have applied our protocol to tea tree oil as a proof of concept for the catalytic upgrading of essential oils. The reactivity and selectivity of rhenium(VII) oxide is compared with a classical acid catalyst; sulfuric acid; and solid acid catalysts, such as acid resins, zeolites, and mesoporous materials. It was found that rhenium(VII) oxide surpasses all these catalysts in both activity and selectivity and also shows enduring activity after seven recycling runs.

Rhenium Nanochemistry for Catalyst Preparation

The review presents synthetic approaches to modern rhenium-based catalysts. Creation of an active center is considered as a process of obtaining a nanoparticle or a molecule, immobilized within a matrix of the substrate. Selective chemical routes to preparation of particles of rhenium alloys, rhenium oxides and the molecules of alkyltrioxorhenium, and their insertion into porous structure of zeolites, ordered mesoporous MCM matrices, anodic mesoporous alumina, and porous transition metal oxides are considered. Structure-property relationships are traced for these catalysts in relation to such processes as alkylation and isomerization, olefin metathesis, selective oxidation of olefins, methanol to formaldehyde conversion, etc.

ChemInform Abstract: Metathesis of Functionalized Acyclic Olefins

Abstract The metathesis of functionally substituted olefins, i.e olefins containing one or more heteroatoms, affords interesting prospects in the synthesis of valuable organic products. Since the first report on the successful metathesis of certain functional olefins, viz. longchain fatty acid esters, much research has been done into the possibilities of this reaction. It appears that many types of functionalized olefins undergo metathesis in the presence of a suitable catalyst. Despite the fact that many metathesis catalysts are inactive, interesting results have been obtained in the past with two catalyst systems, viz. the homogeneous system WCl6-Sn(CH3)4 and the heterogeneous system Re2O7/Al2O3-Sn(CH3)4. The scope of the reaction of functionalized olefins has been studied for a number of groups. The functional groups suited for metathesis include -COOR, -OCOR, -COR, -OR, -CN, -OSiR3, -Cl etc. Unsaturated carboxylic acids and alcohols can be metathesized by introducing a protecting group, e.g. a trimethylsilyl group. New homogeneous catalyst systems developed in recent years are nearly all based on tungsten compounds. Generally, however, the heterogeneous rhenium-based catalyst system turns out to be more attractive. The catalyst is active even at room temperature and can be recovered easily. In addition it is active towards certain compounds, especially allyl compounds, that are inactive with the tungsten catalysts. An important development was the access to the metathesis of unsaturated nitriles. Here, the system Re2O7/Al2O3-SnR4 in particular gives good results. Higher activities than those observed for the Re2O7/ Al2O3 catalysts can be achieved using modified rhenium catalysts. Thus, noteworthy results were obtained when a third metal oxide was incorporated and when a different support was used, e.g. silica-alumina. These developments are the subject of further study at the present time and will lead to catalysts with activities of practical interest.

Bmim]BF 4-immobilized rhenium-catalyzed highly efficient oxygenation of aldimines to oxaziridines using solid peroxides as oxidants

Various rhenium-based catalysts immobilized in [bmim]BF 4 were found to be efficient for oxygenation of various aldimines to the corresponding oxaziridines in excellent yields under mild conditions using solid peroxides like UHP, SPC and SPB as oxidants. Among the various rhenium-based catalysts studied, MTO was found to be most efficient. The reusability and recyclability of MTO immobilized in [bmim]BF 4 was established by using it for three subsequent cycles for oxygenation of benzylidine- tert-butylamine using UHP as oxidant.

Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands

Trends in methane activation have been explored for rhenium-based catalysts in conjunction with bidentate oxygenated ligands of the form (L1)(L2)Re(OH)(OH2) [L1, L2 = acac, catechol, glycol]. When placed in acidic media, the equilibrium for this reference catalyst shifts to the protonated forms (L1)(L2)Re(OH2)(OH2) in almost all cases. In all cases the activation of the reference complex proceeds through a concerted metathesis type transition state, and only one of the 13 reference complexes proceeds with methane activation through a barrier of less than 35 kcal mol-1. Study of the identity complexes (L1 = L2) revealed that protonation of the ligand oxygens is unfavorable for acac and catechol, but favorable for glycol; however in only one case is the barrier for methane activation improved by this route. Electron density on the central rhenium is the best predictor for the magnitude of the methane activation barrier; namely, increased electron density (obtained by considering lower oxidation states) on the metal leads to lower barriers. Lower oxidation states form weaker Re−O bonds, which increase lability of the leaving groups and decrease the barrier to proton transfer from methane.

Rhenium-Catalyzed Highly Efficient Oxidations of Tertiary Nitrogen ­Compounds to N-Oxides Using Sodium Percarbonate as Oxygen Source

Sodium percarbonate was found to be an ideal and efficient oxygen source for the oxidation of tertiary nitrogen compounds to N-oxides in excellent yields in presence of various rhenium-based catalysts under mild reaction conditions.

Novel rhenium-based catalysts for dehydrocondensation of methane with CO/CO2 towards ethylene and benzene

A new family of rhenium‐based catalysts bearing HZSM‐5 zeolite exhibits remarkable performances for the catalytic dehydrocondensation of methane with CO/CO2 towards ethylene, benzene, and naphthalene in high selectivity of above 90% at 1–3 atm and 973–1023 K. In contrast to Mo/HZSM‐5 catalysts, the EXAFS and TG/DTA/Mass studies reveal that the metallic Re on HZSM‐5 zeolite is a catalytically active and stable phase for the reaction.

Metathesis in Oleochemistry

Metathesis is an attractive reaction for the conversion of oleochemical feedstocks into useful chemical products. Thus, metathesis of unsaturated fatty acid esters (the oleochemical raw materials) provides a convenient route to unsaturated diesters which can be used as intermediates for the production of polymers and specialty chemicals. Cross-metathesis with olefins is a means to change the chain length of fatty acids and derivatives; cross-metathesis with ethene (ethenolysis) gives compounds with terminal double bonds, having a broad range of applications. Ethenolysis of fatty oils or triglycerides allows the transformation of long-chain fatty acid triglycerides into fatty oils of lower molecular weight. Several highly selective catalyst systems - homogeneous as well as heterogeneous - have proven to be successful for the metathesis of unsaturated esters. Especially modified heterogeneous rhenium- and molybdenum-based catalysts have attracted much attention. Considering catalyst activation and regeneration procedures, rhenium-based catalysts are to be preferred.

Mössbauer and solid-state NMR spectroscopic studies of tin-modified rhenium-based metathesis catalysts

Mossbauer and solid-state NMR spectroscopic studies have been carried out on Re2O7/SiO2–Al2O3/SnR4 metathesis catalysts (R = Me, Et or n-Bu). The effects of both the rhenium loading and the SnR4 alkyl group are presented. The amounts of gas released upon the interaction between Re2O7/SiO2–Al2O3 and SnR4 do not correspond to a simple stoichiometry while chemical reactivity suggests the presence of minor amounts of a rhenium–alkylidene species. Mossbauer and NMR data suggest two kinds of tin sites, with the tin atom linked to the surface by one or two Sn—O bonds. The effects of exposure to air and catalyst regeneration on the tin environment have also been investigated. The nature of the tin-containing surface species is discussed.