Sol-Gel Immobilization

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

As prepared and purified G4-OH(Pt5o) DENs solutions (ca. 20 /xM) can also be used to prepare high surface area sol-gel Pt/silica catalysts. This process can be used to carefully control metal loadings without condensing the solution provided the loadings remain low (less than 0.2 % Pt). The low concentrations do not allow the dendrimer to serve as a porosity template. Eurther, low loadings combined with delayed introduction of the DENs solution to a preformed sol likely lead to the occlusion of metal nanoparticles within the oxide matrix.  

---

Amine catalyzed conversion of P-iodoethyl-benzene to styrene, followed by hydrogenation to ethylbenzene catalyzed by Wilkinson s catalyst Sol-gel immobilization of both catalysts... 

The use of multiple otherwise incompatible catalysts allows multistep reactions to proceed in one reaction vessel, providing many potential benefits. In this chapter, literature examples of nanoencapsulation for the purpose of process intensification have been discussed comprehensively. Current efforts in the literature are mostly concentrated in the areas of LbL template-based nanoencapsulation and sol-gel immobilization. Other cascade reactions (without the use of nanoencapsulation) that allow the use of incompatible catalysts were also examined and showcased as potential targets for nanoencapsulation approaches. Finally, different methods for nanoencapsulation were investigated, thereby suggesting potential ways forward for cascade reactions that use incompatible catalysts, solvent systems, or simply incompatible reaction conditions. 

Blyth D., Poynter S., Russell D., Calcium biosensing with a sol-gel immobilized photoprotein, Analyst 1996 121 1975-1978. 

A.N. Diaz, F.G. Sanchez, M.C. Ramos, and M.C. Torijas, Horseradish peroxidase sol-gel immobilized for chemiluminescence measurements of alkaline-phosphatase. Sens. Actuat. B 82, 176-179 (2002). 

K. Anitha, S.V. Mohan, SJ. Reddy, Development of acetylcholinesterase silica sol-gel immobilized biosensor — an application towards oxydemeton methyl detection. Biosens. Bioelectron. 20, 848—856 (2004). 

Figure 5.14 Sol-gel immobilized TEMPO is an off-the-shelf alcohol oxidation catalyst. In a biphasic reaction system and in organic solvent it yields carbonyls in water it yields carboxylates.

The high catalyst loading typical of sol gel entrapped catalysts ensures a desirably high substrate/catalyst (S/C) ratio as the major part of the heterogeneous catalyst weight originates from the silicate matrix. For example, in a preparative-scale reaction of the alcohol raol-(2-naphthyl)-ethanol only 250 mg of sol-gel CaLB immobilizate could be used per 10 g of substrate. For comparison, all this makes the process based on sol-gel immobilized lipase very competitive with the commercial BASF process using lipase immobilized on Amberlite to produce the amine at a scale of 1000 tons per year. 

Chiral cyanohydrins in fact are high-value building blocks and useful precursors for hydroxyamino acids and amino alcohols. The new sol-gel immobilization technology licensed from Johnson Matthey and named CTIS CACHy (CTIS Chiral Technologies Interface System)32 dramatically improves process economics for large-scale pharmaceutical manufacturing as it increases the turnover number of the catalyst... 

E. F. Murphy, L. Schmid, T. Burgi, M. Maciejewski, A. Baiker, D. Gunther and M. Schneider, Nondestructive Sol-Gel Immobilization of Metal (salen) Catalysts in Silica Aerogels and Xerogels, Chem. Mater., 2001, 13, 1296. 

Deprotonation of AF in M4 is disfavoured because of the lipophilic microenvironment of the immobilized indicator, whereas the apparent pXa of sol gel immobilized dyes depends on the nature of the sol gel matrix. 

B.D. Gupta and D.K. Sharma, Evanescent wave absorption based fiber optic pH sensor prepared by dye doped sol-gel immobilization technique, Opt. Commun., 140(1-3) (1997) 32-35. 

FIGURE 7.5. Infrared spectra of CO adsorbed on two sol-gel immobilized Pt5o/Si02 catalysts. Catalyst A was prepared by adding the DENs solution at the beginning of the sol-gel preparation. For catalyst B, the DENs solution was added after the sols were allowed to form for 24 hours, but before gellation. Both catalysts were oxidized at 300°C for 16 h and reduced at 300°C for 2 h. Reprinted with permission from Appl. Catal. A. 2005, 292, 124-129. Copyright 2005 Elsevier. 

Hussain F, Birch DJ, Pickup JC. Glucose sensing based on the intrinsic fluorescence of sol-gel immobilized yeast hexokinase. Analytical Biochemistry 2005, 339, 137-143. 

Bressler, E., Pines, O., Goldberg, I. and Braun, S. (2002) Conversion of fumaric add to L-malic by sol-gel immobilized Saccharomyces cerevisiae in a supported liquid membrane bioreactor. Biotechnology Progress, 18, 445. 

The extent of CPO immobilized on the sol-gel was determined by the difference between the activity of the initial enzyme solution and that measured in cumulative washes. Based on the cumulative activity lost in six washes, a second preparation of the CPO-bound sol-gel contained 10, 24, and 55 mg of CPO/g of sol-gel for the 50-, 150-, and 200-A CPO sol-gels, respectively. In prior experiments, the total activity was measured and an estimated 80% of the bound CPO was active. The sol-gel immobilization is expected to limit the unfolding of the protein bound inside pores of the sol-gel. Thus, immobilization is expected to affect solvent stability and thermostability. Immobilization would probably not impact peroxide stability, since the mechanism of peroxide inactivation is associated with changes in the redox properties and oxidation state of the heme iron and the active center, which cannot be protected by immobilization. Experimental studies of immobilized CPO were therefore limited to temperature and solvent stability. 

Fig. 5. Effect of solvent on stability of sol-gel-immobilized CPO. The highest stability displayed in 200-A pore sol-gel may be owing to immobilization of the enzyme within the pores of the sol-gel, whereas it may have been immobilized effectively only on the surface in the case of the other pore sizes.

Free CPO lost 95% activity within 3 h when incubated at 40YC, whereas sol-gel CPO lost only 50% activity under this condition (Fig. 6). At an incubation temperature of 50YC, no protection was observed by sol-gel immobilization for CPO. Only the CPO immobilized in 200-A sol-gel retained activity over 3 h at 40AoC. 

Shin MJ, Park JY, Park K et al (2007) Novel sol-gel immobilization of horseradish peroxidase employing a detergentless micro-emulsion system. Biotechnol Bioprocess Eng 12 640-645... 

A sol-gel immobilization of nano an micro size sorbents in poly(dimethyl-siloxane) (PDMS) microchannels for microscale solid phase extraction (SPE). Anal. Chim. Acta 2005, 546, 22-29. 

Hsu, A.-E K.C. Jones T.A. Foglia W.N. Marmer. Optimization of alkyl ester production from grease using a phyllosilicate sol-gel immobilized lipase. Biotechnol. Lett. 2003, 25, 1713-1716. 

Li Juan, Ngin Tan Swee, Ge Hailin. Silica sol-gel immobilized amperometric biosensor for hydrogen peroxide. Analytica Chimica Acta 1996 335 137-145. 

Survival of metal complex architectures during sol-gel immobilization requires either (1) that there are no thermodynamically favorable interactions between the metal centers and any components of the sol-gel medium that will disrupt the desired coordination structure, or (2) that the ligand exchange kinetics at the metal centers are sufficiently slow that the stmcture will not... 

For Class B (substitution labile) metal complexes, reequilibration to more thermodynamically favorable coordination modes will be very rapid relative to immobilization. Such behavior is typical of first-row TM complexes. In addition, these ions are usually very oxophilic, so the metal complexes are typically subject to ICC interactions with oxide materials. Since these metal ions are generally immobilized under conditions of thermodynamic control, all pertinent speciation equilibria, including ICC reactions (Section III.B), must be considered in order to understand or predict the outcome of immobilization reactions. It is essential to understand the relevant equilibria if direct imprinting of active site structures is to be successful. The studies of Klonkowski et al. (210-213), for example, underscore this point Sol-gel immobilization of copper complexes bearing silylated amine and ethylenediamine ligands were shown by EPR to result in multiple copper environments, suggesting competition between immobilization and ICC reactions. 

Jindal, R. and Cramer, S.M., On-chip electrochromatography using sol-gel immobilized stationary phase with UV absorbance detection, Journal of Chromatography A, 1044, 277-285, 2004. 

---