Sponges, mesoporous

There has been some interest in mesoporous sponges of gold [96, 97]. These structures combine the electrical conductivity of gold with a very high surface area. 

Possible applications include optical coatings [98], catalysts [99-101], substrates for Surface Enhanced Raman spectroscopy [102] or biosensor electrodes [103], Mesoporous gold can be prepared by de-aHoying a suitable precursor such as a 

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Figure 7.5 Two topologically distinct types of mesoporous gold sponge, each with 50 volume % gold, (a) Swiss-cheese morphology produced by de-alloying, (b) aggregated particle morphology produced by sintering of nanoparticles.

In any case, it is interesting to note that catalytic efficacy has been observed with nano- or mesoporous gold sponges [99-101, 145] suggesting that neither a discrete particle nor an oxide support is actually a fundamental requirement for catalysis. An alternative mechanism invokes the nanoscale structural effect noted in Section 7.2.2, and proposes that the catalytic effect of nanoscale gold structures is simply due to the presence of a large proportion of lowly-coordinated surface atoms, which would have their own, local electronic configurations suitable for the reaction to be catalyzed [34, 49,146] A recent and readily available study by Hvolbaek et al. [4] summarizes the support for this alternate view. 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Surfactant-based synthesis of mesoporous metal oxides and metal sulfides emerged about four years after the initial report of MCM-41 [21-36]. High surface area and thermally robust mesoporous metal oxides and sulfides represent a new class of materials with diverse opportunities for the development of improved fuel and solar cells, batteries, membranes, chemical delivery vehicles, heavy metal sponges, sensors, magnetic devices and new catalysts. All of these applications could benefit from tailorable Bronsted and Lewis acidity and basicity, flexible oxidation states, and tunable electronic, optical and magnetic properties. 

While MCM-41- and 48-based materials dominate as the primary mesoporous materials explored for gas-phase propylene epoxidation, a recent article examines the reactivity of Au deposited on Ti-TUD containing 3 mol% Ti [57]. Ti-TUD consists of a sponge-like structure with an average pore size of about 13 run. Although the specific surface area of this material is less than that of MCM-41 or MCM-48, the larger pore system allowed for essentially all of the deposited Au to have access to the pore system. A maximum rate of 53.7 gpo kgcat 470 °C... 

Figure 4.1 The sponge-like structure of a typical sample of controlled-pore glass used in sorption experiments. The silica matrix in lighter gray surrounds the mesopores appearing in darker gray.

MCM-41 [hexagonal], (b) Ti-MCM-48 [cubic], (c) large mesoporous Ti-Si02 [sponge-likej. 

Figure 14.8 PO yields as a function of time-on-stream for Au/ Ti-Si02 (sponge-like mesoporous titanium silicate) catalysts with different modifications Catalyst, 0.35 wt%Au/ Ti-SiOj 0.15 g feed gas, CjHs/Oj/Hj/Ar = 10/10/10/70 space velocity, 4000h mlgc. ...

Fig. 2 Selected pSi/polymer scaffolds, (a) Solid PCL cube (3 mm) with mesoporous Si (67 % porosity) on opposite faces (Mukheijee et al. 2006), (b) pSi/porous PCL sponge (Whitehead et al. 2008), (c) pSi/PCL microfibers (Fan...

Li X, Gu M, Hu S, Kennard R, Yan P, Chen X, Wang C, Sailor MJ, Zhang J-G, Liu J (2014) Mesoporous sihcon sponge as an anti-pulverization stmcture for high-performance lithium-ion battery anodes. Nat Common 5. doi 10.1038/ncomms5105... 

Mesoporous gold sponge as a prototype meta-materiar, A. Maaroof, M. B. Cortie, and G. B. Smith, Phys. B, 2007, 394, 167. 

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