This review paper deals with proven and potential applications of mesoporous molecular sieves in catalysis. In addition to introduction and conclusion, the text is divided into two parts, respectively, dedicated to the design of solid catalysts and catalyst supports and to some relevant examples of catalytic processes.

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Perspectives in catalytic applications of mesostructured materials

Ligand exchange offers an effective way to modify the properties of the recently prepared quantum clusters of gold. To tune optical and photoluminescence properties of one of the most stable quantum clusters of gold, Au25SG18 (SG-glutathione thiolate), we functionalized it by the exchange of −SG with functionalized -SG and with an altogether different ligand, namely, 3-mercapto-2-butanol (MB). The products were characterized by various techniques such as optical absorption (UV−vis), Fourier-transform infrared (FT-IR), nuclear magnetic resonance (NMR), X-ray photoelectron (XPS), and luminescence spectroscopies, mass spectrometry, and thermogravimetry (TG). Analyses of the TG data helped to establish the molecular composition of the products. Ligand exchange reaction was monitored by NMR spectroscopy, and it was found that the exchange reaction follows a first order kinetics. The XPS study showed that after the exchange reaction there was no change in the chemical nature of the metal core and binding energy...

http://www.dstuns.iitm.ac.in/listpdf/172_SI.pdf

Ligand Exchange of Au25SG18 Leading to Functionalized Gold Clusters: Spectroscopy, Kinetics, and Luminescence

A novel interfacial route has been developed for the synthesis of a bright-red-emitting new subnanocluster, Au(23), by the core etching of a widely explored and more stable cluster, Au(25)SG(18) (in which SG is glutathione thiolate). A slight modification of this procedure results in the formation of two other known subnanoclusters, Au(22) and Au(33). Whereas Au(22) and Au(23) are water soluble and brightly fluorescent with quantum yields of 2.5 and 1.3 %, respectively, Au(33) is organic soluble and less fluorescent, with a quantum yield of 0.1 %. Au(23) exhibits quenching of fluorescence selectively in the presence of Cu(2+) ions and it can therefore be used as a metal-ion sensor. Aqueous- to organic-phase transfer of Au(23) has been carried out with fluorescence enhancement. Solvent dependency on the fluorescence of Au(23) before and after phase transfer has been studied extensively and the quantum yield of the cluster varies with the solvent used. The temperature response of Au(23) emission has been demonstrated. The inherent fluorescence of Au(23) was used for imaging human hepatoma cells by employing the avidin-biotin interaction.

http://www.dstuns.iitm.ac.in/listpdf/191.pdf

Bright, NIR‐Emitting Au23 from Au25: Characterization and Applications Including Biolabeling

We explore, with the use of extensive molecular dynamics simulations, several principal issues pertaining to the energetics of formation of superlattices made through the assembly of passivated nanoclusters, the interactions that underlie the cohesion of such superlattices, and the unique mechanical, thermal and structural properties that they exhibit Our investigations focus on assemblies made of crystalline gold nanoclusters of variable sizes, passivated by monolayers of alkylthiol molecules An analytic optimal packing model that correlates in a unified manner several structural characteristics of three-dimensional superlattice assemblies is developed The model successfully organizes and systematizes a large amount of experimental and simulation data, and it predicts the phase-boundary between different superlattice structural motifs that evolve as a function of the ratio between the chain-length of the extended passivating molecules and the radius of the underlying gold nanocluster The entropic contribution to the formation free energy of the superlattice assembly is found to be large and of similar magnitude as the potential energy component of the free energy The major contribution to the cohesive potential energy of the superlattice is shown to originate from van der Waals interactions between molecules that passivate neighboring nanoclusters The unique mechanical, thermal, thermomechanical, and thermostructural properties of passivated nanocluster assemblies, are discussed

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Small is different: energetic, structural, thermal, and mechanical properties of passivated nanocluster assemblies

An alloy cluster containing a 13-atom core, with a composition Ag 7 Au 6 (H 2 MSA) 10 (H 2 MSA=mercaptosuccinic acid) was synthesized from silver clusters by a galvanic exchange reaction. The clusters were characterized by several spectroscopic and microscopic methods. The alloy cluster shows luminescence with a quantum yield of 3.5×10 -2 at room temperature. Theoretical calculations for Ag 7 Au 6 (SCH 3 ) 10 suggest a distorted icosahedral core.

Ag7Au6: A 13-Atom Alloy Quantum Cluster

Alkanethiol-protected silver clusters of average diameter 4.0 ± 0.5 nm form single-phase superlattice solids, and their X-ray powder diffractograms have been fully indexed to single cubic unit cells. Whereas alkanethiols with five or more carbon atoms form superlattices, the corresponding cluster with four carbons yield only separated clusters. The superlattice solids can be recrystallized from nonpolar solvents. No such superlattices are seen for the corresponding gold clusters. The superlattice collapses upon heating, but the solid retains the structure even at 398 K, much above the melting point of crystalline alkanes and the corresponding self-assembled monolayer. In situ variable-temperature X-ray diffraction investigations did not show any solid-state phase transitions in the superlattice. Temperature-dependent infrared spectroscopy reveals the melting of the alkyl chain, and it is seen that the chain as a whole achieves rotational freedom prior to the collapse of the superlattice. Calorimetric inve...

https://www.dstuns.iitm.ac.in/listpdf/79.pdf

Monolayer-Protected Cluster Superlattices: Structural, Spectroscopic, Calorimetric, and Conductivity Studies

Melting of crystalline solids (superlattices) of octadecanethiol and octanethiol protected silver clusters has been studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. These solids have been compared with the silver thiolate layered compounds in view of their similarity in alkyl chain packing and x-ray diffraction patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silver reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggregation of clusters during heating/cooling cycles. DSC shows two distinct transitions, first corresponding to alkyl chain melting and the second corresponding to superlattice melting. Only alkyl chain melting is observed in variable temperature IR and increased order is manifested upon repeated heating/cooling cycles. Alkyl chain assembly shows strong interchain coupling leading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche defects, whereas significant increase in defect structures is not seen in superlattices. Repeated heating/cooling cycles increase interchain interactions within a cluster and the superlattice order collapses.

http://www.dstuns.iitm.ac.in/listpdf/80.pdf

G. Panneer Selvam

Papers

Monolayer-Protected Cluster Superlattices: Structural, Spectroscopic, Calorimetric, and Conductivity Studies

Melting of monolayer protected cluster superlattices