Macroporous monoliths

Hedstrom M, Plieva FM, Galaev lY, Mattiasson B (2008) Monolithic macroporous albumin/ chitosan gel a new matrix for enzyme immobilization. Anal Bioanal Chem 390 907-912... 

K. Nakanishi, H. Shikata, N. Ishizuka, N. Koheiya, N. Soga, Tailoring mesopores in monolithic macroporous silica for HPLC. J. High Resolut. Chromatogr., 2000, 23, 106-110. 

In the beginning of 1990s, continuous porous polymer was proposed for a separation medium of HPLC (Svec, 1992). Almost in parallel, Nakanishi et al. found that, by combining transient domain formation due to phase separation and structure freezing due to sol-gel transition, monolithic macroporous silica with continuous thin gel skeletons and... 

Figure 3-5. SEM images of monolithic macroporous sUica, and methyl-modified siloxane gels prepared in capillaries with various inner diameters.

T. (2010) A new route to monolithic macroporous SiC/C composites from biphenylene-bridged polysilsesquioxane gels. Chem. Mater., 22, 2541-2547. 

K.T. Lee, J.C. Lytle, N.S. Ergang, S.M. Oh and A. Stein, Synthesis and rate performance of monolithic macroporous carbon electrodes for lithium-ion secondary batteries, Adv. Fund. Mater. 14,2005,547-556. 

From isotherm measurements, usually earried out on small quantities of adsorbent, the methane uptake per unit mass of adsorbent is obtained. Sinee storage in a fixed volnme is dependent on the uptake per unit volume of adsorbent and not on the uptake per unit mass of adsorbent, it is neeessary to eonvert the mass uptake to a volume uptake. In this way an estimate of the possible storage capacity of an adsorbent can be made. To do this, the mass uptake has to be multiplied by the density of the adsorbent. Ihis density, for a powdered or granular material, should be the packing (bulk) density of the adsorbent, or the piece density if the adsorbent is in the form of a monolith. Thus a carbon adsorbent which adsorbs 150 mg methane per gram at 3.5 MPa and has a packed density of 0.50 g/ml, would store 75 g methane per liter plus any methane which is in the gas phase in the void or macropore volume. This can be multiplied by 1.5 to convert to the more popular unit, V/V. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Svec, E (2004a). Preparation and HPLC applications of rigid macroporous organic polymer monoliths. J. Sep. Sci. 27, 747-766. 

Zhang, B.J., Davis, S.A. and Mann, S. (2002) Starch gel templating of spongelike macroporous silicalite monoliths and mesoporous films. Chemistry of Materials, 14, 1369-1375. 

Fig. 8.5 SEM images of (A) close packed array of latex beads (scale bar= 1 tm) and (B) macroporous aminopropyl-functionalized magnesium phyllosilicate monolith obtained after infiltration and extraction of colloidal template (scale bar= 1 pm).

Monolithic column — The trend to use shorter columns in liquid chromatography means that the resultant lower separation efficiency is of concern. One way to improve HPLC separation efficiency on a shorter column is to reduce the size of the packing material, but at the cost of increased backpressure. Another approach to improve performance is increasing permeability with a monolithic column. Such a column consists of one solid piece with interconnected skeletons and flow paths. The single silica rod has abimodal pore structure with macropores for through-pore flow and mesopores for nanopores within a silica rod8182 (Figure 12.1). 

The monolithic technology was used for CEC by Nilson et al. who introduced superporous imprinted monolithic capillaries in 1997 [125-127]. Isooctane was used as a porogen in order to produce a macroporous structure with large pores without interfering with the imprinting process. These imprinted monoliths were... 

In addition, it has been shown that other enzymes such as trypsin can be successfully immobilized and used for the conversion of substrates with higher molecular masses [76]. Petro et al. [94] compared the activity of trypsin immobilized on macroporous beads and on monolithic supports. They were able to show that the catalytic activity of trypsin bound to a monolith was much higher and resulted in a much higher throughput. Other enzymes such as invertase [76] and... 

The pore size distributions of the molded monoliths are quite different from those observed for classical macroporous beads. An example of pore size distribution curves is shown in Fig. 3. An extensive study of the types of pores obtained during polymerization both in suspension and in an unstirred mold has revealed that, in contrast to common wisdom, there are some important differences between the suspension polymerization used for the preparation of beads and the bulk-like polymerization process utilized for the preparation of molded monoliths. In the case of polymerization in an unstirred mold the most important differences are the lack of interfacial tension between the aqueous and organic phases, and the absence of dynamic forces that are typical of stirred dispersions [60]. 

The polymerization temperature, through its effects on the kinetics of polymerization, is a particularly effective means of control, allowing the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The effect of the temperature can be readily explained in terms of the nucleation rates, and the shift in pore size distribution induced by changes in the polymerization temperature can be accounted for by the difference in the number of nuclei that result from these changes [61,62]. For example, while the sharp maximum of the pore size distribution profile for monoliths prepared at a temperature of 70 °C is close to 1000 nm, a very broad pore size distribution curve spanning from 10 to 1000 nm with no distinct maximum is typical for monolith prepared from the same mixture at 130°C [63]. 

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