Covalent organic frameworks (COFs) are organic porous materials with many potential applications, which very often depend on the presence of chemical functionality at the organic building blocks. Functionality that cannot be introduced into COFs directly via de novo syntheses can be accessed through post-synthetic modification (PSM) strategies. Current strategies for the post-synthetic modification of COFs involve (i) incorporation of a variety of active metal species by using metal complexation through coordination chemistry, (ii) covalent bond formation between existing pendant groups and incoming constituents and (iii) chemical conversion of linkages. (iv) The post-synthetic modification is sometimes assisted by a monomer truncation strategy for the internal functionalization of COFs. (v) Even more intriguing methods that go beyond PSM are herein termed building block exchange (BBE) which encompasses framework-to-framework transformations taking advantage of the fact that reversible bond formation is a characteristic feature of COFs. This strategy allows the use of protoCOF structures (i.e., the utilization of a parent COF as a template) for the evolution of new COF structures with completely new components.

Post-synthetic modification of covalent organic frameworks

We synthesized a readily accessible molybdenum-doped covalent-organic framework catalyst (Mo-COF) linked by a hydrazine linkage via a facile two-step bottom-up approach. This Mo-COF catalyst, as an open nanochannel-reactor, exhibited promising catalytic activity for the selective oxidation reaction.

Bottom-up approach to engineer a molybdenum-doped covalent-organic framework catalyst for selective oxidation reaction

The development of new functional catalytic materials prepared via appropriate chemical modification of mesoporous silica SBA-15, MCM-41 or carbon nanomaterials CMK-3, are presented. Their synthesis has been carried out via two synthetic approaches: (a) a two steps procedure which includes grafting of the Schiff base ligand 1,3-bis[3-aza-3-(1-methyl-3-oxobut-1-enyl)-prop-3-en-1-yl]-2-(4-hydroxy-phenyl)-1,3-imidazolidine (L) onto the suppors and subsequent metalation of the so-formed hybrid material, and (b) an one step procedure which allows covalent grafting of the entire [Mn II -Schiff base] catalyst onto the carbonaceous support. The resulting single-site heterogeneous catalysts were characterized and evaluated for alkene epoxidation with H 2 O 2 in the presence of CH 3 COONH 4 as additive. They are efficient and selective towards formation of epoxides. The highest TONs have been achieved by L@MCM-41-Mn II and Mn II -L@CMK-3 . Moreover, Mn II -L@CMK-3 is operative for a second use and kinetically very fast, demonstrating remarkably high TOFs 65–634 h −1 that is correlated to its practically zero porosity. Based on the present data, the textural features of the obtained catalysts are discussed in correlation with their catalytic performance.

Mn-Schiff base modified MCM-41, SBA-15 and CMK-3 NMs as single-site heterogeneous catalysts: Alkene epoxidation with H2O2 incorporation

Molybdenum(VI) complex with a tridentate Schiff base ligand immobilized on SBA-15 as effective catalysts in epoxidation of alkenes

The cavities of SBA mesoporous silica materials can be used as nanoreactors for embedding catalytic species such as nanoparticles, complexes and heteropolyacids etc. The encapsulated catalysts are renowned as superior catalytic systems regarding their catalytic activity, selectivity and reusability as well as suppressed leaching. An efficient method to improve the encapsulation of these catalysts, is in creating anchors for capturing catalytic species. It has been performed by functionalization of the surface of SBA mesoporous silica materials with various groups including sulfur, phosphorus and amine-based functional groups. The aim of this article is to review the recent advances (2014–2017) in the field of the utility of functionalized SBA for developing encapsulated catalysts. In most cases the reusability of the catalyst and the leaching of the catalytic active species were discussed to disclose the role of functionalized SBA in heterogenizing and suppressing the leaching. Moreover, to show the superiority of these developed encapsulated catalysts, with a couple of examples we compared their catalytic activities with those of the conventional ones.

/pdf/current-advances-in-the-utility-of-functionalized-sba-56sst67xlu.pdf

Current advances in the utility of functionalized SBA mesoporous silica for developing encapsulated nanocatalysts: state of the art

A novel organic–inorganic hybrid heterogeneous catalyst system was obtained from the reaction of the molybdenum(VI) complex of salicylidene 2-picoloyl hydrazone with mesoporous silica containing 3-chloropropyl groups prepared by a direct synthetic approach involving hydrolysis and co-condensation of tetraethylorthosilicate (TEOS) and 3-chloropropyltrimethoxysilane in the presence of the triblock copolymer P123 as template under acidic conditions. Characterization of the functionalized materials by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), N 2 adsorption/desorption, FT-IR and UV–Vis spectroscopy, and thermogravimetric analysis (TGA) reveals that the molybdenum complex is successfully grafted into the pores of the host silica structure. Furthermore, the resulting hybrid material was found to be highly active catalyst in the liquid-phase epoxidation of olefins with t -BuOOH as the oxygen source. Leaching tests and metal analysis of reaction solutions show that the catalytic activity stemmed from the immobilized species, not from the leaching of active species into solution.

Immobilization of dioxomolybdenum(VI) complex bearing salicylidene 2-picoloyl hydrazone on chloropropyl functionalized SBA-15: A highly active, selective and reusable catalyst in olefin epoxidation

Novel mesostructured hybrid materials containing a molybdenum Schiff base complex grafted on the internal surface of SBA-15 pores were prepared by introducing MoO 2 (acac) 2 into a mesoporous silica functionalized with Schiff base ligands. The SBA-15 supports modified by an amine or salicylaldehyde were obtained by co-condensation of tetraethylorthosilicate and the corresponding organosilane in the presence of Pluronic P123 surfactant as a structure directing agent using bis[3-(trimethoxysilyl)propyl]amine or 5-chloromethylsalicylaldehyde coupled with bis[3-(trimethoxysilyl)propyl]amine as precursor. The molybdenum (VI) complexes immobilized on the internal surface of SBA-15 pores were employed as catalyst in the epoxidation of various alkenes using tert-butylhydroperoxide as oxidant. These immobilized molybdenum complexes are highly active and selective catalysts in liquid phase olefin epoxidation in dichloroethane at 84 °C. Leaching tests and metal analysis of reaction solutions showed that the kinetically competent catalyst is the molybdenum complex immobilized on the internal surface of SBA-15 pores and there is no molybdenum species in the solution.

Taghi Salemnoush

Papers

Immobilization of dioxomolybdenum(VI) complex bearing salicylidene 2-picoloyl hydrazone on chloropropyl functionalized SBA-15: A highly active, selective and reusable catalyst in olefin epoxidation

Epoxidation of olefins catalyzed by a molybdenum-Schiff base complex anchored in the pores of SBA-15