Marco Ranocchiari

Catalysis by metal–organic frameworks: fundamentals and opportunities

Crystalline porous materials are extremely important for developing catalytic systems with high scientific and industrial impact. Metal-organic frameworks (MOFs) show unique potential that still has to be fully exploited. This perspective summarizes the properties of MOFs with the aim to understand what are possible approaches to catalysis with these materials. We categorize three classes of MOF catalysts: (1) those with active site on the framework, (2) those with encapsulated active species, and (3) those with active sites attached through post-synthetic modification. We identify the tunable porosity, the ability to fine tune the structure of the active site and its environment, the presence of multiple active sites, and the opportunity to synthesize structures in which key-lock bonding of substrates occurs as the characteristics that distinguish MOFs from other materials. We experience a unique opportunity to imagine and design heterogeneous catalysts, which might catalyze reactions previously thought impossible.

Metal organic frameworks for photo-catalytic water splitting

Metal organic frameworks (MOFs) have recently debuted as participants and solid supports in catalytic water splitting. Their porosity and structural versatility offer a tantalising consolidation of the components needed for solar light harvesting and water splitting. Herein, we describe a selection of relevant contemporary investigations that employ electrocatalysis, chemically introduced redox partners, and photo-catalysts to generate dioxygen and dihydrogen from water. The role of semiconducting MOFs in these systems is addressed, in tandem with band gap control by linker functionalisation and doping. Considered holistically, MOFs offer an impressive physical, spatial and chemical versatility with which to support and sustain water splitting reactions. Major challenges toward practical implementation do remain, but opportunities for development are evidently numerous.

Selective anaerobic oxidation of methane enables direct synthesis of methanol

A two-step protocol oxidizes methane to methanol using a copper zeolite that is reoxidized by water. A watery route from methane to methanol Methanol production is an expensive, energy-intensive process that initially overoxidizes methane to carbon monoxide. Sushkevich et al. used copper sites in a zeolite to oxidize methane to methoxy intermediates; they then added water to release methanol and hydrogen while reoxidizing the copper. This inexpensive process could prove useful at gas well sites for producing an easily stored and transported liquid from excess gas that at present is burned away. Science, this issue p. 523 Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (~97%) over a copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH3OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at CuII oxide active centers, followed by CuI reoxidation by water with concurrent formation of hydrogen.

Synthesis of Water-Soluble Phosphine Oxides by Pd/C-Catalyzed P–C Coupling in Water

Cross-coupling between diphenylphosphine oxide and halogenated benzoic acids catalyzed by Pd/C in water is a green, simple, and fast protocol to obtain water-soluble tertiary phosphine oxides without the addition of ligands and additives. Low reaction times and microwave irradiation make this method general and excellent for laboratory and large-scale synthesis without the need to use organic solvents in reactions and workup.

Direct Conversion of Methane to Methanol under Mild Conditions over Cu-Zeolites and beyond

In the recent years methane has become increasingly abundant. However, transportation costs are high and methane recovered as side product is often flared rather than valorized. The chemical utilization of methane is highly challenging and currently mainly based on the cost-intensive production of synthesis gas and its conversion. Alternative routes have been discovered in academia, though high temperatures are mostly required. However, the direct conversion of methane to methanol is an exception. It can already be carried out at comparably low temperatures. It is challenging that methanol is more prone to oxidation than methane, which makes high selectivities at moderate conversions difficult to reach. Decades of research for the direct reaction of methane and oxygen did not yield a satisfactory solution for the direct partial oxidation toward methanol. When changing the oxidant from oxygen to hydrogen peroxide, high selectivities can be reached at rather low conversions, but the cost of hydrogen peroxide is comparably high. However, major advancements in the field were introduced by converting methane to a more stable methanol precursor. Most notable is the conversion of methane to methyl bisulfate in the presence of a platinum catalyst. The reaction is carried out in 102% sulfuric acid using SO3 as the oxidant. This allows for oxidation of the platinum catalyst and prevents the in situ hydrolysis of methyl bisulfate toward the less stable methanol. With a slightly different motif, the stepped conversion of methane to methanol over copper-zeolites was developed a decade ago. The copper-zeolite is first activated in oxygen at 450 °C, and then cooled to 200 °C and reacts with methane in the absence of oxygen, thus protecting a methanol precursor from overoxidation. Subsequently methanol can be extracted with water. Several active copper-zeolites were found, and the active sites were identified and discussed. For a long time, the process was almost unchanged. Lately, we implemented online steam extraction rather than off-line extraction with liquid water, which enables execution of successive cycles. While recently we reported the isothermal conversion by employing higher methane pressures, carrying out the process according to prior art only yielded neglectable amounts of methane. Using a pressure <40 bar methane gave higher yields under isothermal conditions at 200 °C than most yields in prior reports. The yield, both after high temperature activation and under isothermal conditions at 200 °C, increased monotonously with the pressure. With this account we show that the trend can be represented by a Langmuir model. Thus, the pressure dependence is governed by methane adsorption. We show that the isothermal and the high temperature activated processes have different properties and should be treated independently, from both an experimental and a mechanistic point of view.

Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature

Direct partial oxidation of methane into methanol is a cornerstone of catalysis. The stepped conversion of methane into methanol currently involves activation at high temperature and reaction with methane at decreased temperature, which limits applicability of the technique. The first implementation of copper-containing zeolites in the production of methanol directly from methane is reported, using molecular oxygen under isothermal conditions at 200 °C. Copper-exchanged zeolite is activated with oxygen, reacts with methane, and is subsequently extracted with steam in a repeated cyclic process. Methanol yield increases with methane pressure, enabling reactivity with less reactive oxidized copper species. It is possible to produce methanol over catalysts that were inactive in prior state of the art systems. Characterization of the activated catalyst at low temperature revealed that the active sites are small clusters of copper, and not necessarily di- or tricopper sites, indicating that catalysts can be designed with greater flexibility than formerly proposed.

Continuous-Flow Microwave Synthesis of Metal-Organic Frameworks: A Highly Efficient Method for Large-Scale Production.

Metal-organic frameworks are having a tremendous impact on novel strategic applications, with prospective employment in industrially relevant processes. The development of such processes is strictly dependent on the ability to generate materials with high yield efficiency and production rate. We report a versatile and highly efficient method for synthesis of metal-organic frameworks in large quantities using continuous flow processing under microwave irradiation. Benchmark materials such as UiO-66, MIL-53(Al), and HKUST-1 were obtained with remarkable mass, space-time yields, and often using stoichiometric amounts of reactants. In the case of UiO-66 and MIL-53(Al), we attained unprecedented space-time yields far greater than those reported previously. All of the syntheses were successfully extended to multi-gram high quality products in a matter of minutes, proving the effectiveness of continuous flow microwave technology for the large scale production of metal-organic frameworks.

AuI Catalysis on a Coordination Polymer: A Solid Porous Ligand with Free Phosphine Sites

Heterogeneous catalysts are extremely important for the chemical industry. They are employed in the most important processes, which range from bulk to fine-chemical synthesis, because they present major economical and practical advantages, such as easy catalyst separation and recycling. Traditional heterogeneous catalysts are mainly constituted from metal particles that are deposited onto an oxide support, but, more often than not, only a few percent of the metal acts as the “real” active site, whereas the remaining majority is a spectator species. For this reason, the design of traditional heterogeneous catalysts on the atomic level for selective processes is quite difficult. On the other hand, homogeneous catalysts are well-defined and allow fine-tuning of the electronic and steric properties of the catalyst, thereby increasing the efficiency and selectivity of this process. However, their recycling and separation from the reaction mixture is expensive and presents more than a few engineering issues. Many researchers have tried to combine the separation and recycling properties of heterogeneous catalysts with the ability to have well-defined single sites, as in homogeneous catalysts. One such strategy is to develop a material that allows the facile coordination of single sites by designing a homogeneous-like catalyst on a solid support. However, few materials have the ability to coordinate single atoms without sintering, which is one of the main reasons why this chemistry is still at such an early stage. To overcome this problem, materials that are functionalized with atoms that are able to form relatively strong bonds with transition metals, with retention in a confined space, are required. Such materials, which are termed solid porous ligands (SPL), are normally quite difficult to obtain, owing to the limited chemical reactions that are known for standard solid supports. Following the discovery of coordination polymers (CPs), that is, materials that are constituted from a combination of multidentate organic building blocks with inorganic units, and their derivatives with a defined metal–organic framework (MOF) structure, new perspectives for the synthesis of functional solids that are able to coordinate single atoms through post-synthetic modification (PSM) have been reported. Nowadays, CPs that contain functionalities such as amino, alcohol, and imidazolium salts exist, but their synthesis is often not straightforward and their applications are limited. Surprisingly, materials with free organophosphorus functionalization are extremely rare, even though phosphines are among the most widely employed ligands in industrial homogeneous catalysis. Herein, we report the synthesis, characterization, and catalytic application of a new highly microporous P-functionalized CP that allows the coordination of single metal atoms (in this case, Au). This material was applied in catalysis by employing experimental and theoretical concepts that were developed in homogeneous catalysis. Very high metal loading was achieved. The concept and ability to heterogenize homogeneous catalysts that are based on phosphine SPLs at high weight loading opens up many new opportunities for the further application of coordination polymers and metal–organic frameworks as catalysts. Our design of a P-functionalized coordination polymer began by choosing an appropriate organic linker and inorganic unit ; as an organic building block, we selected 4,4’,4’’-phosphinetriyltribenzoic acid (H3ptba; Figure 1), owing to its facile synthesis from commercially available tris(4-methylphenyl)phosphine. Our metal of choice was zirconium because: 1) it has a low affinity with phosphine ligands and, therefore, the coordination of the P atom of H3ptba to it is minimized; and 2) it can form highly stable MOFs, such as UiO-66. An initial synthesis screen was performed under analogous conditions to those used for the production of UiO-66, which afforded an amorphous material (see the Supporting Information, Figure S1a) with a relatively low surface area (414 mg ). Because the addition of an acid, such as benzoic and acetic acid, in the synthesis of the UiO-66 series increased the size and surface area of the crystallites, we performed the synthesis in the presence of acetic acid (150 equiv; Figure 1, step 1). Under such conditions, the PXRD pattern of the product showed a crystalline material that featured several reflections of 2q between 38 and 408 (see the Supporting Information, Figure S1b). This compound was named LSK-1, in which LSK represents the German acronym of the Laboratory for Catalysis and Sustainable Chemistry at the Paul Scherrer Institute. [a] M. Servalli, Dr. J. Szlachetko, Dr. M. Ranocchiari, Prof. J. A. van Bokhoven Laboratory of Catalysis and Sustainable Chemistry Paul Scherrer Institute CH-5232, Villigen PSI (Switzerland) E-mail : jeroen.vanbokhoven@chem.ethz.ch marco.ranocchiari@psi.ch [b] J. V clav k, Dr. C. Lothsch tz, Prof. J. A. van Bokhoven Department of Chemistry and Applied Biosciences Laboratory of Chemical and Bioengineering ETH Zurich, CH-8093, Z rich (Switzerland) [c] Dr. J. Szlachetko Institute of Physics University of Kielce 25-314 Kielce (Poland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200844.

Single-atom active sites on metal-organic frameworks

Single-site heterogeneous catalysis has been recently accepted as a novel branch of heterogeneous catalysis. Catalysts with single-atom active sites (SAHCs) allow the design and fine-tuning of the active moiety, and can potentially combine the advantages of heterogeneous and homogeneous catalysis. This study illustrates how porous metal-organic frameworks (MOFs) can be synthesized with homogeneous distribution of SAHCs. The catalytic potential of MIXMOFs is shown. A short overview of catalysis with mesoporous silica materials is described to demonstrate their importance in SAHC.

The Direct Catalytic Oxidation of Methane to Methanol—A Critical Assessment

Despite the large number of disparate approaches for the direct selective partial oxidation of methane, none of them has translated into an industrial process. The oxidation of methane to methanol is a difficult, but intriguing and rewarding, task as it has the potential to eliminate the prevalent natural gas flaring by providing novel routes to its valorization. This Review considers the synthesis of methanol and methanol derivatives from methane by homogeneous and heterogeneous pathways. By establishing the severe limitations related to the direct catalytic synthesis of methanol from methane, we highlight the vastly superior performance of systems which produce methanol derivatives or incorporate specific measures, such as the use of multicomponent catalysts to stabilize methanol. We thereby identify methanol protection as being indispensable for future research on homogeneous and heterogeneous catalysis.