Membrane silica-based

Figure 6.1. Permeability of several gases through a silica based hollow fiber membrane data taken from Shindo et al. (1983).

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

In the supported systems the catalyst can be coated on the walls of the reactor, supported on a solid substrate or deposited around the case of the light source. Many are the supported materials used in literature, such as glass beads, and tubes [69], silica-based materials [70], hollow beads, membranes [71], optical fibers, zeolites, activated carbon, organic fibers [72], and so on. 

Despite efforts to comply with the limitations on feed water quality, CEDI systems can still foul and scale with microbes, organics, iron and manganese, and calcium- and silica-based scales. This usually occurs due to upsets in the pretreatment system or a deficiency in the system design that result in excursion in feed water quality to the CEDI system. Organics, metals, hardness, and silica problems are usually found on the membranes and sometimes on the resin (as is the case with organics). Biofouling is typically found on the... 

Yoshino, Y., Suzuki, T., Nair, B.N., Taguchi, H., and Itoh, N., Development of tubular substrates, silica based membranes and membrane modules for hydrogen separation at high temperature, Journal of Membrane Science, 267, 8-17, 2005. 

Dense silica-based membranes might be an alternative. A disadvantage of such membranes is the sensitiveness for water while their rather low thermostability can lead to crack formation above 200°C. 

There are two different bonding systems employed with the block a urethane asphalt adhesive/membrane or a special inorganic silica-based mortar. The choice depends on mechanical considerations and on the chemical and thermal environment. 

Foamed glass block laid in its silica-based mortar over a suitable membrane is a good choice for lining these areas. During shutdowns, it is resistant to the wide range of acid that may condense on the surfaces of the ductwork. When the incinerator is operating, the block can withstand the high gas temperatures... 

Digestion can also be achieved using a trypsin IMER, where trypsin is immobilized to a solid support, e.g, macroporous silica [38], on POROS material (Porozyme IMER) [39-40], a PVDF membrane in a microreactor [41], or silica-based [42] or porous polymer monoliths [43-45]. 

The conclusion arising from these experiments was that homogeneous polymeric silica-based binary sols cam be made with the addition of a second component up to 30 mol%. Initially, the fractal dimension (-1.4) and the gyration radii (-2 run) of the polymers were foimd to be low enough to obey the concept of mutual transparency. More details are provided in Chapter 8 on the preparation of such microporous membranes for gas separation. 

High quality microporous membranes are almost exclusively reported for silica or for binary silica-titania or silica-zirconia systems [42,46]. This is due to the very fast hydrolysis and condensation rates of the metal organic precursor of the metals relevant for membrane synthesis (Ti, Zr, Sn, Al). This usually results in too large particles in the precursor solution. Though many authors claim to have produced microporous materials by sol-gel methods (see e.g. Section 8.2.3), only a few have shown the synthesis of membranes of these materials and a still smaller number has characterised them with appropriate separation properties to be reasonably defect free. Therefore in the remainder of Section 8.2.1 a focus will be given to silica-based membranes. 

R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Polymeric silica based sols for membrane modification applications sol-gel synthesis and characterisation with SAXS. /. Non-Cryst. Solids, 191 (1995) 1-16. 

Supported ionic liquid catalysis is one of the main examples of SLPC adopted [120] to take advantage of ionic liquid properties without the drawbacks evidenced in Section 2.3.6. The viability of this concept has been confirmed by several studies that have successfully confined various ionic phases to the surface of support materials and explored their potential catalytic applications. Although most of the evaluated supports were silica based, several studies have focused on polymeric materials, including membranes. These materials were prepared by using two different immobilization approaches. The first involves the covalent attachment of ionic liquids to the support surface whereas the second simply deposits the ionic liquid phases containing catalytically active species on the surface of the support. 

The study of the porous structure as discussed in the previous paragraphs would allow researchers to tailor more durable membranes for H2 separation. Making changes in nanostructure of the silica based porous material could be important in this respect. The partial substitution of Si or O in the pore structure is one possible direction (see Fig. 16.8). For example, it is reported that the partial substitution of O with N could increase the stiffness of the network [44]. This stiffness could change the activation enthalpy required by the gas molecules to diffuse through the network, but will increase the resistance of the network against any degradation. The... 

State-of-the-art micro-porous membranes are based on silica, with sufficiently small pores, 2-10 A, to be selective towards hydrogen separation. One of the major problems with silica membranes under hydrothermal conditions is physical stability. Evaporation of silica-containing species is detrimental to long-term permselectivity and restricts the operation of these membranes to temperatures below 600 °C. Hydrogen permeances of >1 x 10 mol m s Pa with H2/CO2 permselectivity in the range 80-100 have, for instance, been measured with single deadend tubular micro-porous silica membranes for temperatures higher than 300 °C and with 4 bar pressure difference. These membranes were reported to be thermally stable for at least 2000 h at temperatures between 200 and 400 °C [50]. 

---