Heterogenization of molecular catalysts within porous solids: the case of Ni-catalyzed ethylene oligomerization from zeolites to metal-organic frameworks

The last decade has seen a tremendous expansion of the field of heterogenized molecular catalysis, especially with the growing interest in metal-organic frameworks and related porous hybrid solids. With successful achievements in the transfer from molecular homogeneous catalysis to heterogenized processes come the necessary discussions on methodologies used and a critical assessment on the advantages of heterogenizing molecular catalysis. Here we use the example of nickel-catalyzed ethylene oligomerization, a reaction of both fundamental and applied interest, to review heterogenization methodologies of well-defined molecular catalysts within porous solids while addressing the biases in the comparison between original molecular systems and heterogenized counterparts.

Unravelling the Molecular Structure and Confining Environment of an Organometallic Catalyst Heterogenized within Amorphous Porous Polymers

The catalytic activity of multifunctional, microporous materials is directly linked to the spatial arrangement of their structural building blocks. Despite great achievements in the design and incorporation of isolated catalytically active metal complexes within such materials, a detailed understanding of their atomic-level structure and the local environment of the active species remains a fundamental challenge, especially when these latter are hosted in non-crystalline organic polymers. Here, we show that by combining computational chemistry with pair distribution function analysis, 129 Xe NMR, and Dynamic Nuclear Polarization enhanced NMR spectroscopy, a very accurate description of the molecular structure and confining surroundings of a catalytically active Rh-based organometallic complex incorporated inside the cavity of amorphous bipyridine-based porous polymers is obtained. Small, but significant, differences in the structural properties of the polymers are highlighted depending on their backbone motifs.

ZIF-8 thin films by a vapor-phase process: limits to growth

Metal-organic frameworks are a class of porous materials that show promising properties in the field of microelectronics. To reach industrial use of these materials, gas phase techniques are often preferred and were recently introduced. However, the thicknesses achieved are not sufficient, limiting further development. In this work, an improved gas phase process allowing ZIF-8 layer formation of several hundreds of nm using cyclic ligand/water exposures is described. Then, by a combination of in-depth surface analyses and molecular dynamics simulations, the presence and role of hydroxyl defects in the ZIF-8 layer to reach this thickness are established. At the same time, this study unveils an inherent limit of the method: thickness growth is consubstantial with defect repairing upon the crystallites ripening; such defect repairing eventually leads to the decrease of the pore window below the diffusion radius of the incoming linker, thus apparently capping the maximum MOF thickness observable for this type of material topology through this growth method.

Nickel-Catalyzed Direct Arylation Polymerization for the Synthesis of Thiophene-Based Cross-linked Polymers

An earth-abundant nickel(II) bipyridine catalyst, combined with lithium hexamethyldisilazide as base, demonstrates its wide applicability in the direct arylation polymerization of di- and tri-thiophene heteroaryls with poly(hetero)aryl halides. With a nickel catalyst loading of 2.5 mol%, a series of twenty highly cross-linked organic polymers is obtained in 34 to 99 % yields. Using mixed polytopic coupling partners allows obtaining alternating and optically active thiophene-based solids with intrinsic porosity.

Finding the Sweet Spot of Photocatalysis─A Case Study Using Bipyridine-Based CTFs

Covalent triazine frameworks (CTFs) are a class of porous organic polymers that continuously attract growing interest because of their outstanding chemical and physical properties. However, the control of extended porous organic framework structures at the molecular scale for a precise adjustment of their properties has hardly been achieved so far. Here, we present a series of bipyridine-based CTFs synthesized through polycondensation, in which the sequence of specific building blocks is well controlled. The reported synthetic strategy allows us to tailor the physicochemical features of the CTF materials, including the nitrogen content, the apparent specific surface area, and optoelectronic properties. Based on a comprehensive analytical investigation, we demonstrate a direct correlation of the CTF bipyridine content with the material features such as the specific surface area, band gap, charge separation, and surface wettability with water. The entirety of these parameters dictates the catalytic activity as demonstrated for the photocatalytic hydrogen evolution reaction (HER). The material with the optimal balance between optoelectronic properties and highest hydrophilicity enables HER production rates of up to 7.2 mmol/(h·g) under visible light irradiation and in the presence of a platinum cocatalyst.

Rhodium-Based Metal-Organic Polyhedra Assemblies for Selective CO2 Photoreduction

Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. Metal-organic polyhedra (MOP) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organized around an inner cavity, making them appealing candidates for catalytic applications. Here, we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within MOP (Rh-MOP) and their three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in the Rh-metal-organic framework (Rh-MOF) and yields turnover frequencies of up to 60 h^-1 and production rates of approx. 76 mmole formic acid per gram of the catalyst per hour, unprecedented in heterogeneous photocatalysis. The enhanced catalytic activity is investigated by X-ray photoelectron spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favorable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis. These results demonstrate the high potential of MOP as catalysts for the photoreduction of CO₂ and open a new perspective for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels.

A Disruptive Innovation for Upgrading Methane to C3 Commodity Chemicals : Technical challenges faced by the C123 European consortium

C123 is a €6.4 million European Horizon 2020 (H2020) integrated project running from 2019 to 2023, bringing together 11 partners from seven different European countries. There are large reserves of stranded natural gas waiting for a viable solution and smaller scale biogas opportunities offering methane feedstocks rich in carbon dioxide, for which utilisation can become an innovation advantage. C123 will evaluate how to best valorise these unexploited methane resources by an efficient and selective transformation into easy-to-transport liquids such as propanol and propanal that can be transformed further into propylene and fed into the US$6 billion polypropylene market. In C123 the selective transformation of methane to C3 hydrocarbons will be realised via a combination of oxidative conversion of methane (OCoM) and hydroformylation, including thorough smart process design and integration under industrially relevant conditions. All C123 technologies exist at TRL3 (TRL = technology readiness level), and the objectives of C123 will result in the further development of this technology to TRL5 with a great focus on the efficient overall integration of not only the reaction steps but also the required purification and separation steps, incorporating the relevant state-of-the-art engineering expertise.

Synthetic and computational assessment of a chiral metal-organic framework catalyst for predictive asymmetric transformation

Understanding and controlling molecular recognition mechanisms at a chiral solid interface is a continuously addressed challenge in heterogeneous catalysis. Here, the molecular recognition of a chiral peptide-functionalized metal–organic framework (MOF) catalyst towards a pro-chiral substrate is evaluated experimentally and in silico. The MIL-101 metal–organic framework is used as a macroligand for hosting a Noyori-type chiral ruthenium molecular catalyst, namely (benzene)Ru@MIL-101-NH-Gly-Pro. Its catalytic perfomance toward the asymmetric transfer hydrogenation (ATH) of acetophenone into R- and S-phenylethanol are assessed. The excellent match between the experimentally obtained enantiomeric excesses and the computational outcomes provides a robust atomic-level rationale for the observed product selectivities. The unprecedented role of the MOF in confining the molecular Ru-catalyst and in determining the access of the prochiral substrate to the active site is revealed in terms of highly face-specific host–guest interactions. The predicted surface-specific face differentiation of the prochiral substrate is experimentally corroborated since a three-fold increase in enantiomeric excess is obtained with the heterogeneous MOF-based catalyst when compared to its homogeneous molecular counterpart.

Understanding and controlling molecular recognition mechanisms at a chiral solid interface has been addressed in metal–organic framework catalysts for the asymmetric transfer hydrogenation reaction.

Immobilization of a Full Photosystem in the Large-Pore MIL-101 Metal-Organic Framework for CO2 reduction

A molecular catalyst [Cp*Rh(4,4'-bpydc)]²⁺ and a molecular photosensitizer [Ru(bpy)₂ (4,4'-bpydc)]²⁺ (bpydc=bipyridinedicarboxylic acid) were co-immobilized into the highly porous metal-organic framework MIL-101-NH₂ (Al) upon easy postsynthetic impregnation. The Rh-Ru@MIL-101-NH₂ composite allows the reduction of CO₂ under visible light, while exhibiting remarkable selectivity with the exclusive production of formate. This Rh-Ru@MIL-101-NH₂ solid represents the first example of MOFs functionalized with both a catalyst and a photosensitizer in a noncovalent fashion. Thanks to the coconfinement of the catalyst and photosensitizer into the cavity's nanospace, the MOF pores are used as nanoreactors and enable molecular catalysis in a heterogeneous manner.

Enhanced formation of >C1 Products in Electroreduction of CO2 by Adding a CO2 Adsorption Component to a Gas-Diffusion Layer-Type Catalytic Electrode

The addition of a CO₂ -adsorption component (substituted imidazolate-based SIM-1 crystals) to a gas-diffusion layer-type catalytic electrode enhances the activity and especially the selectivity towards >C1 carbon chain products (ethanol, acetone, and isopropanol) of a Pt-based electrocatalyst that is not able to form products of CO₂ reduction involving C-C bond formation under conventional (liquid-phase) conditions. This indicates that the increase of the effective CO₂ concentration at the electrode active surface is the factor controlling the formation of >C1 products rather than only the intrinsic properties of the electrocatalyst.

Systematic study of the impact of MOF densification into tablets on textural and mechanical properties

Densification process of MOF powders (HKUST-1, UiO-66, UiO-66-NH₂, and UiO-67) into mechanically resistant pellets with maintained microporosity and enhanced volumetric uptake.

Origin of highly active metal–organic framework catalysts: defects? Defects!

This article provides a comprehensive review of the nature of catalytic sites in MOFs.

Photocatalytic carbon dioxide reduction with rhodium-based catalysts in solution and heterogenized within metal-organic frameworks

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.

Structure–property relationships of water adsorption in metal–organic frameworks

Among porous solids, MOFs exhibit a wealth of water adsorption behaviors.

Water adsorption in MOFs: fundamentals and applications

MOF and water, friend or enemy?

Nickel-catalyzed C-H arylation of azoles with haloarenes: scope, mechanism, and applications to the synthesis of bioactive molecules

Novel nickel-based catalytic systems for the C-H arylation of azoles with haloarenes and aryl triflates have been developed. We have established that Ni(OAc)(2)/bipy/LiOtBu serves as a general catalytic system for the coupling with aryl bromides and iodides as aryl electrophiles. For couplings with more challenging electrophiles, such as aryl chlorides and triflates, the Ni(OAc)(2)/dppf (dppf = 1,1'-bis(diphenylphosphino)ferrocene) system was found to be effective. Thiazoles, benzothiazoles, oxazoles, benzoxazoles, and benzimidazoles can be used as the heteroarene coupling partner. Upon further investigation, we discovered a new protocol for the present coupling using Mg(OtBu)(2) as a milder and less expensive alternative to LiOtBu. Attempts to reveal the mechanism of this nickel-catalyzed heterobiaryl coupling are also described. This newly developed methodology has been successfully applied to the syntheses of febuxostat (a xanthine oxidase inhibitor that is effective for the treatment of gout and hyperuricemia), tafamidis (effective for the treatment of TTR amyloid polyneuropathy), and texaline (a natural product having antitubercular activity).

Influence of pregnancy on the evolution of background retinopathy. Preliminary results of a prospective fluorescein angiography study

Previous reports have suggested that pregnancy does not influence the development of diabetic retinopathy. Since 1979 a prospective study using fluorescein angiography has been in progress. Preliminary results include 22 diabetic women with or without moderate background retinopathy studied until delivery. They were maintained under good metabolic control (mean HbA1: 7.68%). Five clinical and fluorescein angiography examinations were performed at prefixed intervals (before the 15th week, at the 28th and 35th weeks of pregnancy, and 6 and 15 months after delivery). The analysis of fluorescein angiography which did not induce any side effect showed that the mean number of microaneurysms increased from 42.7 before pregnancy to 56.7 at the 28th week and to 79.7 at the 35th week. Six months later the number was 62.7, and 15 months later 60.3, still higher than at the initial examination. This study suggests that pregnancy has an unfavourable influence on the background retinopathy in diabetic patients even when a good metabolic control is achieved and retinopathy is minimal.

Binding of 125I-insulin to the human histiocytic lymphoma cell line U-937: effect of differentiation with retinoic acid

The human histiocytic lymphoma line U-937 consists of cells having characters of immature monocytes. We have demonstrated that these cells possess highly specific insulin receptors with binding properties similar to that found for mature human blood monocytes. 125I-insulin binding increased progressively with time to reach a maximum at 90 min at 22 degrees C and was proportional to the number of cells in the incubation medium. Insulin degradation as assessed by TCA precipitation was negligible. Scatchard analysis of the binding data was curvilinear and the total number of insulin binding sites was around 13,500. The average affinity profile gave an 'unoccupied site' affinity constant of 1.34 nM-1. When the U-937 cells were induced to differentiate into morphologically and functionally monocyte-like cells, after incubation with retinoic acid, the total number of binding sites decreased significantly with no change in the affinity of the hormone for its receptor.

[Prevalence of smoking among diabetics and influence of tobacco on diabetic retinopathy]

Among 647 patent diabetics regularly followed who attended the out-patient clinic of a hospital Diabetology Department between December 1st, 1979 and June 30th, 1980, 35% were habitual smokers, 20% ex-smokers and 45% non-smokers. Insulin-treated diabetics did not smoke more than non insulin-treated diabetics. Only one-half of the smokers declared that they had been advised by a doctor to stop smoking. This study failed to demonstrate any effect of tobacco on the prevalence or severity of diabetic retinopathy.

[Effect of long-term administration of clonidine on growth hormone secretion in hypertensive diabetics]

The anti-hypertensive effect of clonidine is closely related to its central alpha-agonist action. In acute administration, the drug provokes a significant increase in the plasma concentrations of growth hormone (GH). In chronic administration, the effects of clonidine on GH secretion are not well documented. Clonidine was administered at a daily dose of 0,15 to 0,30 mg to 10 non-obese diabetic hypertensive male subjects for at least 3 months. Blood glucose and GH plasma concentrations were determined 15 times during the 24-hour cycle. Blood glucose and GH values recorded before the intake and after stopping the drug, in the basal state, after meals, after measured muscular exercise and during the first stage of sleep could be superimposed when on and off clonidine. The effects of prolonged administration of clonidine on GH secretion thus differ markedly from the effects of acute administration as in non diabetic subjects. The long-term use of clonidine does not induce a chronic increase of GH plasma concentrations which might worsen the evolution of the diabetic microangiopathy.

[Mediation by opioid peptides of anterior pituitary response to insulin-induced hypoglycemia (author's transl)]

A 1.6 mg dose of the specific opiate antagonist naloxone was administered to normal volunteers, either separately or in conjunction with insulin-induce hypoglycemia, in order to study the possible mediation by opioid peptides of the release of adrenocorticotrophin (ACTH), lipotropins (LPH), human growth hormone (GH) and prolactin (PRL). Administered separately, naloxone was associated with a significant fall in PRL levels (p less than 0.01), a significant and unexpected rise in GH levels (p less than 0.02), and a suppression of the circadian decrease of ACTH and LPH levels. The association of naloxone with insulin-induced hypoglycemia significantly reduced the PRL peak (p less than 0.05), did not affect the rise of GH and lowered the ACTH peak, without altering the LPH peak. These data suggest the existence of a positive opioid tone to PRL secretion, as well as an opioid peptide role in the PRL response to hypoglycemia. They argue against the likelihood of an opioid pathway in the GH response to hypoglycemia. Furthermore, the data favor a paradoxical effect of naloxone on ACTH release during insulin-induced hypoglycemia.

Rapid identification of diabetic patients with essential hypertension sensitive to acebutolol

The antihypertensive effect of 2,000 mg of acebutolol investigated with an acute 48 hr test in 60 diabetic and 60 non-diabetic in-patients with essential hypertension. In hypertensive diabetic patients, acebutolol was induced a significant fall in blood pressure similar to that observed in non-diabetics. The acute antihypertensive effect of acebutolol was not uniform in hypertensive subjects: a significant decrease of blood pressure was observed in 34 diabetics and 31 non-diabetic patients. Fifteen out of the 34 diabetic responders to the 48 hr test were treated by acebutolol alone for six months; a highly significant correlation between the acute and the chronic antihypertensive effect of the beta-blocker was observed. As long-term results paralleled those of the short-term experiment, acute acebutolol administration appears to be a rapid means to select hypertensive diabetics sensitive or resistant to betablockers. Plasma renin activity was not found to give, in hypertensive diabetics, a reliable predictive index of the response to acute administration of acebutolol.

[Long-term results of surgical hypophysectomy for diabetic retinopathy (author's transl)]

Between 1969 and 1974, ten insulin-dependent diabetic patients underwent surgical hypophysectomy for rapidly progressive proliferative retinopathy endangering sight. Six to 11 years later, the 7 patients who survived had useful visual acuity and were leading a normal life. The long-term results of hypophysectomy on the eye could be observed in this series of patients with exceptionally long follow up: oedema had subsided, haemorrhagic exsudates and newly formed retinal vessels had disappeared, and the retinal had become hypovascular. However, argon laser photocoagulation has now considerably reduced the indications of surgical hypophysectomy.

[A study of the knowledge of diabetics (author's transl)]

One hundred and forty five diabetics regularly attending a hospital clinic were submitted to a questionnaire assessing their knowledge of diabetes. The patients had previously been instructed in urine glucose analysis, insulin injections, diet and hypoglycaemic attacks. Despite this, they appeared to be neither better controlled nor more painstaking with their disease than before education. The replies to the questionnaire revealed that the patients had poor understanding of the symptoms and of the long-term complications of diabetes, of the criteria of good control, and of the proportion of carbohydrate in common food. Moreover, they held many fallacious ideas with regard to these concepts. The results suggest that although these diabetics had acquired sufficient knowledge to avoid the acute metabolic disturbances of diabetes and to lead a day-to-day existence with a minimum of harmful effects, they were not properly aware of the long-term consequences of diabetes and of the overall strategy for the good management of their disease. An improved educational strategy appears necessary to enable diabetics to take better care of themselves.