Sankaranarayanapillai Shylesh

Magnetically Separable Nanocatalysts: Bridges between Homogeneous and Heterogeneous Catalysis

Recovery and reuse of expensive catalysts after catalytic reactions are important factors for sustainable process management. The aim of this Review is to highlight the progress in the formation and catalytic applications of magnetic nanoparticles and magnetic nanocomposites. Directed functionalization of the surfaces of nanosized magnetic materials is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.

Cooperative acid-base effects with functionalized mesoporous silica nanoparticles: applications in carbon-carbon bond-formation reactions.

Acid-base bifunctional mesoporous silica nanoparticles (MSN) were prepared by a one-step synthesis by co-condensation of tetraethoxysilane (TEOS) and silanes possessing amino and/or sulfonic acid groups. Both the functionality and morphology of the particles can be controlled. The grafted functional groups were characterized by using solid-state (29)Si and (13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy, thermal analysis, and elemental analysis, whereas the structural and the morphological features of the materials were evaluated by using XRD and N(2) adsorption-desorption analyses, and SEM imaging. The catalytic activities of the mono- and bifunctional mesoporous hybrid materials were evaluated in carbon-carbon coupling reactions like the nitroaldol reaction and the one-pot deacetalization-nitroaldol and deacetalization-aldol reactions. Among all the catalysts evaluated, the bifunctional sample containing amine and sulfonic acid groups (MSN-NNH(2)-SO(3)H) showed excellent catalytic activity, whereas the homogeneous catalysts were unable to initiate the reaction due to their mutual neutralization in solution. Therefore a cooperative acid-base activation is envisaged for the carbon-carbon coupling reactions.

Bifunctional Acid–Base Cooperativity in Heterogeneous Catalytic Reactions: Advances in Silica Supported Organic Functional Groups

Emulating nature is a powerful way to realize selective catalytic reactions and to develop structurally complex organic molecules. A well understood fact in biochemistry is that enzymes are capable of accelerating reactions through cooperative interactions between accurately positioned functional groups present in their active sites. Substrate recognition and activation may proceed through electrostatic, hydrogen bonding, or covalent interactions. Following the concept of ‘site isolation’, significant progress has recently been achieved in the immobilization of mutually destructive catalysts for multistep cascade reactions. ‘Site isolation’ or ‘compartmentalization’ is thus considered as one of the paradigms in heterogeneous catalysis especially in the separation of mutually incompatible catalysts, such as acids and bases. This concept, adopted from nature and through which incompatible sites are spatially separated, aids in avoiding undesired interactions. Thereby tedious work up procedures and purification can be greatly minimized. Cascade reactions—defined as two or more reactions occurring in one reactor—are an emerging strategy in modern synthetic chemistry, as a) several bonds can be formed in a single reaction step in the same reaction vessel without isolation of intermediates, b) they allow a rapid and efficient synthesis of complex molecules from simple starting materials, and c) the overall process is ‘green’ in terms of waste and cost reduction. However, it is not trivial for material chemists to try to mimic biological catalysts by fixing antagonist groups in solid supports for various onestep cooperative catalytic reactions. In a classical concept, a support material serves to handle and recover the catalyst, whereas recent systematic investigations suggest that surfaces will often act cooperatively to stabilize specific catalyst structures. Thus during the last years researchers focused their efforts to precisely position bior multifunctional groups on solid supports for cooperative catalytic reactions. Beyond doubt, this concept of multifunctional heterogeneous or heterogenized catalysts is one way to economic and ecologically benign processes. In this context, organic–inorganic hybrid materials have shown their applicability in a broad variety of heterogeneous catalytic reactions. They show high mechanical, thermal, and chemical stability provided by the inorganic components, while the organic compartments can be independently selected and optimized for specific catalytic applications. Organic modifications of structured inorganic solids, for example mesoporous materials, can be performed by three different ways: by the grafting method, by the co-condensation method, and by the formation of an organosilica material (Scheme 1). By employing one of these methods a series of monoor bifunctional organic groups can be successfully fixed inside the channels of silica supports. Heterogeneous bifunctional catalysts can provide a continuous range of functional groups and offer advantages, such as an enhancement of reactivity and stability of antagonist func-

Facile Synthesis of Mesoporous Magnetic Nanocomposites and their Catalytic Application in Carbon–Carbon Coupling Reactions

Inorganic nanoporous materials were widely exploited as catalyst supports for the heterogenization of single-site catalysts. Such catalyst systems can be recovered by filtration or centrifugation when used for batch reactions or they can be applied in continuous processes in packed-bed or slurry reactors. Grafting single-site catalysts on magnetic core-shell nanoparticles is an alternative way for simple catalyst recovery. Such materials combine the magnetic properties of the core with the possibility to further functionalize the surface, for example, by application of well-established silica chemistry. These systems emerged as efficient catalyst supports for C C bond forming reactions, hydrogenation, hydroformylation, oxidation, and organocatalytic reactions. However, the low surface area and rapid aggregation of the magnetic nanoparticles (MNP) cumber practical applications as catalyst supports. To overcome these drawbacks, magnetic nanoparticles have been introduced into various mesoporous silicas, carbons, and other materials with high specific surface areas. Such embedded supports are typically obtained by introducing magnetic nanoparticles into the as-synthesized porous matrix by backfilling the pores or by the synthesis of a porous layer on the surface of pre-made magnetic nanoparticles. 4] These types of magnetic nanoparticle-based materials, combine the advantages of both mesoporous solids and magnetic nanoparticles and are ideal supports for the immobilization of homogeneous catalysts. They posses: a) ordered pore channels in a tuneable size range of 2–50 nm for hosting a variety of catalytic compounds and b) surface Si OH groups inside the pore channels which can easily be modified for diverse applications. The palladium-catalyzed Suzuki–Miyaura cross-coupling of aryl halides with arylboronic acids to form biaryls is an important reaction allowing specific C C bond formation. A myriad of palladium-derived catalysts were reported for this transformation. Most of the reports in the literature focus on homogenous catalysts. The facile and efficient separation of expensive noble-metal catalysts and their consecutive reuse often remains a challenge in terms of economic and environmental considerations. As already mentioned above, covalent grafting of single-site catalysts on high surface area solid supports is an interesting alternative in this context because the catalytically active sites are well dispersed on the surface of the supports. Herein, we describe a “bottom-up” approach to assembe MNPs in mesoporous silica nanoparticles for the generation of a magnetically recoverable material with mesoporous channels. The resulting magnetic mesoporous nanocomposite was then utilized as an easily recoverable support for the grafting of a palladium complex of the type (L)2PdCl2 (L = Si(OMe)3 functionalized PPh3). We have recently found that this system supported on neat silica is an active and efficiently reusable catalyst for the Suzuki–Miyaura cross coupling reaction. The new magnetic hybrid material was systematically characterized by X-ray diffracttion (XRD), high-resolution transmission electron microscopy (HRTEM), field emission-scanning electron microscopy (FE-SEM), nitrogen physisorption analysis, FTIR spectroscopy, and superconducting quantum interference device (SQUID) measurements. For the synthesis, silica-coated magnetic nanoparticles (SMP) were co-condensed with tetraethoxysilane (TEOS) in a medium containing the templating surfactant hexadecyltrimethylammonium bromide (C16TAB), resulting in the formation of a mesoporous hybrid material that exhibits superparamagnetic behavior. Subsequently, the material was calcined at 500 8C to remove the surfactant (Scheme 1).

Covalent Immobilization of Imidazolium Cations Inside a Silica Support: Palladium-Catalyzed Olefin Hydrogenation

Mesoporous organosilica materials with different contents of bistrialkoxysilyl imidazolium salts in the framework were synthesized by a one‐step synthesis. Textural characterization of the materials confirmed that the morphology and surface properties of the imidazolium‐bridged organosilicas depended critically on the amount of organic groups in the framework, whereas solid‐state NMR characterization showed that the imidazolium fragments were integrated covalently into the framework. Further reaction of these materials with Pd(OAc)2, followed by reduction with NaBH4 yielded palladium nanoparticles stabilized in the mesoporous organosilicas. The stabilizing effect of the imidazolium cations and the mesostructure contributed to the high activity, selectivity, and stability of the palladium nanoparticles and allowed olefin hydrogenation under mild reaction conditions.

Large-scale, low-cost fabrication of Janus-type emulsifiers by selective decoration of natural kaolinite platelets.

Bifunctional and/or anisometric Janus particles have proven to be superior to spherical and chemically isotropic colloids for a wide range of promising applications, such as in nanorobots, physical sensors, microrheology, drug delivery, magnetic storage, electronic devices, surfactants, and compatibilizers. Nevertheless, large-scale technical applications have been halted by the restricted accessibility of these polar colloids. Established synthesis methods that allow for breaking the symmetry are laborious and expensive. Previously, microphase separation of triblock terpolymers, the arrangement of symmetrical colloids at an interface followed by modification of the exposed hemisphere by microcontact printing, and different masking techniques 14] have been used. However, nature provides multifunctional spherical nanoparticles, such as proteins. Specific modifications at different functional sites of ferritin or transthyretin have been reported. 16] Recently, Kotov et al. 18] pointed out that semiconductor nanoparticles may resemble such proteins inasmuch as the specific local packing into truncated tetrahedrally shaped morphologies leads to breakage of symmetry of these nanoparticles, which may in turn trigger self-assembling of these nanoparticles on the mesoscale by directional attraction that is mainly of electrostatic nature. Symmetry-breaking in solids and minerals is, however, not limited to the mesoscale. Crystal structures that are inherently polar at the atomistic level are frequently encountered. As a consequence of the polar crystal structure, opposing crystal faces carry different functional groups. Herein, we show that the natural mineral kaolinite [Al2Si2O5(OH)4], an abundant, ubiquitous, and inexpensive (E 200/tonne) mineral, possesses Janus character. Kaolinite is found as anisometric platelets with large aspect ratios that are typically in the range of 20:1–40:1. Consequently, polar basal planes dominate the external surface area. Kaolinite is a dioctahedral layered silicate (Figure 1 a). The octahedral layer and tetrahedral layer are linked to a 1:1 lamella with the basal surfaces confined by differing functional groups (Figure 1b,c). The lamellae are stacked in a polar mode through strong hydrogen bonds (Figure 1a). Furthermore, as no twinning has usually been observed, the two opposing external basal planes of the kaolinite platelets consist of an Al2(OH)4 octahedral layer (octahedral surface, OS), which is capped by m-hydroxy groups at the external surface, and the other side is capped by a SiO4 tetrahedral layer (tetrahedral surface, TS). Although even pristine kaolinite displays a Janus character, the surface tension of the two unmodified external basal surfaces is similar. Although the idealized chemical formula would suggest natural kaolinite to be charge-neutral, it nevertheless possesses a small cation-exchange capacity (CEC) owing to an isomorphous substitution in the tetrahedral layers, which can

Tailoring the Cooperative Acid–Base Effects in Silica-Supported Amine Catalysts: Applications in the Continuous Gas-Phase Self-Condensation of n-Butanal

A highly efficient solid‐base organocatalyst for the gas‐phase aldol self‐condensation of n‐butanal to 2‐ethylhexenal was developed by grafting site‐isolated amines on tailored silica surfaces. The catalytic activity depends largely on the nature of amine species, the surface concentration of amine and silanol groups, and the spatial separation between the silanol and amine groups. In situ FTIR measurements demonstrated that the formation of nucleophilic enamines leads to the enhanced catalytic activity of secondary amine catalysts, whereas the formation of imines (stable up to 473 K) leads to the low activity observed for silica‐supported primary amines. Blocking the silanol groups on the silica support by silylation or cofeeding water into the reaction stream drastically decreased the reaction rates, demonstrating that weaker acidic silanol groups participate cooperatively with the amine groups to catalyze the condensation reaction. This work demonstrates that the spatial separation of the weakly acidic silanols and amines can be tuned by the controlled dehydration of the supporting silica and by varying the linker length of the amine organosilane precursor used to graft the amine to the support surface. A mechanism for aldol condensation was proposed and then analyzed by DFT calculations. DFT analysis of the reaction pathway suggested that the rate‐limiting step in aldol condensation is carboncarbon bond formation, which is consistent with the observed kinetics. The calculated apparent activation barrier agrees reasonably with that measured experimentally.

A Covalently Supported Pyrimidinylphosphane Palladacycle as a Heterogenized Catalyst for the Suzuki–Miyaura Cross Coupling

The amino moiety of an aminopyrimidinyl phosphane allows rapid functionalization of the ligand with a silylated side chain containing a urea linker for catalyst heterogenization. The urea group causes the resulting ligand to undergo spontaneous CH activation at the pyrimidinyl site when reacted with (C6H5CN)2PdCl2 in CH2Cl2. Grafting of the resulting zwitterionic palladacycle complex onto siliceous supports leads to highly active hetereogeneous catalysts for the Suzuki–Miyaura coupling. Leaching tests proved that the catalysts obtained this way are truly heterogeneous.

Bifunctional Mesoporous Materials with Coexisting Acidic and Basic Sites for CC Bond Formation in Co-operative Catalytic Reactions

Nature provides specific catalysts with spatial separation of active functional sites to facilitate multistep reactions. Significant progress has recently been achieved in mimicking such complex reaction cascades. However, the realization of homogeneous multistep catalytic one-pot reactions is relatively rare, since it is difficult to control the activity of each catalyst in the system. Furthermore, unfavorable interactions between reagents and catalysts may cause deactivation. To develop bior multifunctional catalytic systems without mutual deactivation and to enable the catalytic sites to function independently or even in a co-operative manner will surely bring benefits in terms of waste and cost reduction. To immobilize acidic and basic groups together on the surface of one single solid support for the generation of bifunctional catalysts is a considerable challenge due to the highly incompatible nature of the two functional groups. Nevertheless, by combining weak and strong acids, such as silanol groups, ureas, or sulfonic acids, with different organic bases, dual and co-operative activation of electrophiles and nucleophiles was recently achieved. However, the random orientation of the functionalized sites reduces the performance of the whole system, so it is hard to obtain satisfactory catalytic results. Supports used to date for the heterogenization of homogenous basic catalysts have included weakly acid silica, mesoporous Si-MCM-41, amorphous silica–alumina, and crystalline zeolite materials. Asefa and co-workers noted that the spatial arrangement of organoamines and silanol groups in bifunctional mesoporous silicas, developed by the choice of the solvent, has a profound effect on the catalytic activity of C C bond forming reactions. Iwasawa and co-workers noted that organic basic groups supported on acidic supports such as silica–alumina or montmorillonite can function as efficient bifunctional catalysts for various one-pot reaction sequences. Herein we envisage a similar approach; aminopropyl groups are grafted onto a moderately acidic mesoporous aluminosilica by a post-synthesis grafting route. We were looking to take advantage of the unique properties of mesoporous materials compared to unstructured silica–alumina supports, such as high surface areas and pore volumes, as well as tunable pore channels for size/shape-selective catalysis. We have also verified the effect of the solvent (e.g. toluene vs. ethanol) on the textural, compositional, and catalytic behavior of the amine-functionalized mesoporous aluminosilicas. The aluminum-doped mesoporous silica material (Al-MCM41) was prepared according to a procedure published in the literature, using aluminum nitrate and tetraethoxysilane as precursors. The bifunctional acid–base catalyst was subsequently obtained by a simple, one-step grafting procedure with 3aminopropyltrimethoxysilane (3-APTS) as the base precursor in polar ethanol (to afford Al-MCM-NH2-E) and in nonpolar toluene (to afford Al-MCM-NH2-T). Both reactions were carried out at the reflux temperature of the solvent (75 8C and 100 8C, respectively). The catalytic activity of the bifunctional mesoporous materials was evaluated in the nitroaldol (Henry) reaction of substituted benzaldehydes and nitromethane. Si-MCM-41 and USY-zeolite grafted with 3-APTS in the presence of ethanol and toluene were additionally synthesized for comparison. Solid-state NMR spectroscopy was used to obtain spectroscopic evidence for the bulk properties of the support and for the presence of organic functional groups in the mesoporous solids. C cross-polarization magic angle spinning (CP-MAS) NMR spectra confirmed the successful grafting of the aminopropyl groups on all mesoporous supports. The spectra exhibit three peaks at d 43, 25, and 8 ppm, which are typical for the SiCH2CH2CH2NH2 chain (Figure 1). Notably, Al-MCM-NH2-E gave rise to additional peaks at d= 15 and 58 ppm, characteristic of SiOCH2CH3 species arising from incomplete hydrolysis of the silylating agent in the presence of ethanol. The Si CP-MAS NMR spectra show signals at d= 110– 90 ppm (Q sites) and at d= 70– 60 ppm (T sites), the latter originating from the grafting of the silylating agent (Figure 2 A). The presence of T sites further confirms the strong covalent linkage between the organic groups and the silica

From Sugars to Wheels: The Conversion of Ethanol to 1,3‐Butadiene over Metal‐Promoted Magnesia‐Silicate Catalysts

1,3-Butadiene (1,3-BD) is a high-value chemical intermediate used mainly as a monomer for the production of synthetic rubbers. The ability to source 1,3-BD from biomass is of considerable current interest because it offers the potential to reduce the life-cycle greenhouse gas (GHG) impact associated with 1,3-BD production from petroleum-derived naphtha. Herein, we report the development and investigation of a new catalyst and process for the one-step conversion of ethanol to 1,3-BD. The catalyst is prepared by the incipient impregnation of magnesium oxide onto a silica support followed by the deposition of Au nanoparticles by deposition-precipitation. The resulting Au/MgO-SiO2 catalyst exhibits a high activity and selectivity to 1,3-BD and low selectivities to diethyl ether, ethylene, and butenes. Detailed characterization of the catalyst shows that the desirable activity and selectivity of Au/MgO-SiO2 are a consequence of a critical balance between the acidic-basic sites associated with a magnesium silicate hydrate phase and the redox properties of the Au nanoparticles. A process for the conversion of ethanol to 1,3-BD, which uses our catalyst, is proposed and analyzed to determine the life-cycle GHG impact of the production of this product from biomass-derived ethanol. We show that 1,3-BD produced by our process can reduce GHG emissions by as much as 155 % relative to the conventional petroleum-based production of 1,3-BD.