Microemulsion technique

Figure 2. Scheme of the catalyst preparation via a modified microemulsion technique. 

It has to be underlined that, in comparison to LS containing nucleic acid molecules inside the particle, the production of CLS may be performed obviously without considering the stability problems of nucleic acid molecules. In this view, some preparation procedures are not considered, such as the microemulsion technique [55] that represents a favorable method when working with substances unstable because of the high mechanical stress produced by high-pressure homogenization. 

From our research group Santra et al. [11,41,42] reported the development of novel luminescent nanoparticles composed of inorganic luminescent dye RuBpy, doped inside a sihca network. These dye-doped silica nanoparticles were synthesized using a w/o microemulsion of Tx-lOO/cyclohexane/ n-hexanol/water in which controlled hydrolysis of the TEOS leads to the formation of mono dispersed nanoparticles ranging from 5-400 nm. This research illustrates the efficiency of the microemulsion technique for the synthesis of uniform nanoparticles. These nanoparticles are suitable for biomarker application since they are much smaller than the cellular dimension and they are highly photostable in comparison to most commonly used organic dyes. It was shown that maximum liuninescence intensity was achieved when the dye content was around 20%. Moreover, for demonstration... 

More recently an oil continuous microemulsion technique has been described,16 which allows the study of specific interactions between amino acid side chains and metal ions. Both the metal ion and amino acid are microencapsulated as aqueous droplets in a dispersed phase. The technique is of particular relevance to metalloprotein and metal-membrane interactions where the local dielectric constant can be considerably less than that of bulk water. 

Ritzkopf I, et al. Decreased CO production in methanol steam reforming over Cu/Zr02 catalysts prepared by the microemulsion technique. Appl Catal A Gen. 2006 302(2) 215-23. 

In order to establish that the enhancement was due to the LSPR effect and not to variations in the dye emission when conjugated to the silica shell surface, a separate experiment was performed where the metal NP was replaced by a pure silica NP with the same radius. These NPs were synthesized using a microemulsion technique [18] and the dye was attached as in the case of the metal NPs. The enhancement measurement was repeated and the fluorescence from the dye - silica NP was almost identical to that measured in solution hence confirming the plasmonic nature of the enhancement. 

A novel nanoparticulate lipid-based carrier system was developed by Mumper et al. at the University of Kentucky. ° This carrier system is composed of a lipophilic-emulsifying wax such as cetyl alcohol/ polysorbate 60 and other surfactants such as Brij 72, Brij 78, and Tween 80. The nanoparticles were formed through a warm microemulsion technique where encapsulates have included paclitaxel and plasmid DNA. The emulsification process is spontaneous, and cooling of the emulsion causes solidification of the nanoparticle-containing drug. This novel carrier has shown high efficiency in drug delivery across the blood-brain barrier. 

Agrell, J., Germani, G., Jaras, S.G., and Boutonnet, M. Production of hydrogen by partial oxidation of methanol over ZnO-supported palladium catalysts prepared by microemulsion technique. Applied Catalysis. A, General, 2003, 242 (2), 233. 

One approach to the formation of CEP nanoparticles is through the use of micellar polymerization and microemulsion techniques. The advantage of such an approach is that the particle size can be predehned by establishing the appropriate size and geometry of the templating micelle (Figure 2.16). 

As there are other less sophisticated and less expensive techniques available, surfactant-enhanced aquifer remediation will only be useful for decontamination of LNAPL sites in special cases. However, applicable techniques are still needed for DNAPL sites and microemulsion techniques are really promising. Therefore, most research has concentrated on this type of contaminant in recent years. Integrated concepts have been developed including aspects of soil properties [47, 48, 62, 63], density control [47, 48, 62-64], recovery and reuse of microemulsion components [47], biological degradation of residues of contaminants and injected compounds [48, 65] and costs [47, 48, 64, 65]. Two main approaches have been followed for developing effective surfactant systems which form microemulsions with DNAPL, but do not mobilise the liquid contaminant into deeper... 

Table 1 Preparation of metal catalyst particles Microemulsion technique versus impregnation technique...

We will describe the current state-of-the-art of the microemulsion method for the preparation of metal-based catalysts. First, some general considerations concerning the nature of a microemulsion and its relation to the preparation of particles will be given. Then, both the preparation of solid oxides and metal-supported catalysts by microemulsion will be detailed. When possible, the properties of the solids prepared by microemulsion will be compared with those of their counterparts prepared by traditional techniques. Particular attention will be paid to the description of the catalytic properties of these solids. There is a large body of work in the field of organic synthesis, and enzyme catalysis in which microemulsion techniques play an important role. However this topic is not included in this paper, for that purpose several reviews are available, see for example those by Holmberg and Lawrence Rees... 

For supported catalysts, the nature of the carrier plays an important role in order to obtain a well-dispersed catalyst. Oxide solids, either catalysts or supports, can be prepared by the microemulsion technique. The nature of the precursor and that of the hydrolysis agent determine the final properties of the materials. 

In this chapter, the state-of-the-art of metal oxide preparation by the microemulsion technique is presented. The chapter is divided in two main parts. The first section treats the preparation of catalytically active metal oxides while the second part deals with the preparation of metal oxides mainly used as catalytic support materials. 

Gan et al. have successfully prepared single-phase perovskite-type LaNiOa, La2Cu04 and BaPbOs by the microemulsion technique . Nanosize La-Ni, La-Cu and Ba-Pb oxalates with molar ratios of 1 1,2 1, and 1 1, respectively, could... 

Oxide Materials. - 3.3.1 Silica. Several research groups have investigated the preparation of Si02 by the microemulsion technique. The overall reaction resulting in the formation of silica can be represented as... 

Alumina. Although alumina is one of the most common ceramics used in catalysis as support material, the preparation of alumina by the microemulsion technique has not been investigated to a great extent. 

In this section we will focus on the description of several aspects relevant to the preparation of both catalytically active metal particles and metal-supported catalysts via the microemulsion technique. Regarding the metal supported catalysts, in some cases both the metallic particles and the support were synthesized by microemulsions. However, in general metallic particles prepared from microemulsions were deposited on commercial supports. The catalytic behaviour of these microemulsion-derived materials will be commented and, when possible, compared to catalysts obtained from traditional techniques under similar reaction conditions. Selected results concerning the study of the strong metal-support interaction effect (SMSI) obtained with catalysts prepared by microemulsion will be detailed . Several papers dealing with the preparation of immobilized metal particles on supports have been described although the catalytic behaviour of the solids was not studied. However, their potential catalytic ability led us to include those papers within this chapter. 

This approach was reported independently by Boutonnet et al. and Kishida et al. . First, the Pt particles were prepared in a microemulsion solution according to the microemulsion technique, then the support precursor titanium... 

Binet et al. studied the effect of reduction temperature on the catalytic activity of Pd/Ce02 catalysts . Pd particles were prepared by the microemulsion technique and were used as model catalysts, since they consisted of particles of the same morphology. The behaviour of the catalysts, concerning their accessibility of the metal phase towards CO adsorption, appeared to be independent of Pd loading. 

As stated in this paper as well as is shown in Figure 5, when metal-supported catalysts are prepared by microemulsion technique, the metal particle size is smaller and the particle size distribution is sharper than that observed for analogous catalysts prepared by conventional methods such as impregnation or incipient wetness . 

Recently, the microemulsion technique has been explored as an alternative route to prepare Cu/ZnO catalysts. The Cu/ZnO catalyst was prepared in a microemulsion system using the oxalate route. In spite of the low surface are and poor Cu dispersion, the catalysts obtained by this procedure, when compared to catalysts prepared by a classical co-impregnation method, were more active in H2 production through partial oxidation of methanol. The authors propose this enhancement in the activity to be due either to the presence of some specific... 

Metallic particles displaying a small particle size, around a few nanometres, and narrow size distribution could easily be obtained via the microemulsion technique. Such particles have proved to be active in several classic heterogeneous catalytic processes, in some cases displaying a higher performance than the catalysts prepared following more traditional impregnation methods. Major... 

In this paper, a detailed description of the state-of-the-art of catalyst preparation by the microemulsion technique has been given. It has been shown that one of the major advantages of the microemulsion technique is that it allows the preparation of metal-based catalysts displaying a narrow particle size distribution in the nanosize range. 

In some cases the performance of the microemulsion-based catalysts was superior to that of the traditional catalysts regarding both activity and selectivity. However, for the use of the microemulsion technique in catalyst preparation, the nature of the catalytic process itself should be taken into account. For processes in which the particle size does not play an important role, the election of other techniques for catalyst preparation might be more appropriate. 

S. Rojas, S. Eriksson, and M. Boutonnet (KTH, Stockholm, Sweden) focus on the use of microemulsion techniques for catalyst synthesis. They discuss this as an alternative to traditional methods such as impregnation, ion exchange, and use of organometallic complexes. One specific advantage of the microemulsion method is that it results in a typically narrow particle size distribution. This is true because the metal particle is formed without being influenced by the support. They describe the specific processes used to prepare catalysts with this technique. 

Microemulsion technique is a novel method to prepare ultrafine particles. It has the ability to control the size of particles formed and prevent their aggregation. In a typical study, a water-in-oil (W/0) microemulsion known as Winsor Type II microemulsion has been selected for NS-Ti02 preparation. This technique can provide nanosized particles that are much smaller than an oil-in-water (OAV) or Winsor Type I microemulsion. The procedure to prepare the ultrafine particles by this... 

W/0 microemulsion technique starts with two identical W/0 microemulsions. One system dissolves reactant No. 1 whereas the other one dissolves reactant No. 2 in the aqueous cores. Upon mixing these two microemulsion systems, both reactants react with each other as a result of the collision and coalescence of the tiny liquid droplets (reverse micelles) suspended in the microemulsion. Nanoparticles are then produced in the aqueous cores [88, 89]. 

T. Mishra, Anion supported Ti02-Zr02 nanomaterial synthesized by reverse microemulsion technique as an efficient catalyst for solvent free nitration of halobenzene , Catalysis Communications, 9, 21-26, (2008). 

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