Nanofiltration advantages

A number of potential methods for homogeneous catalyst separation and recovery have been discussed in the preceding chapters. This chapter addresses the separation of homogeneous catalysts by means of advanced filtration techniques. Separation of homogeneous catalysts by size exclusion (ultra- or nanofiltration, defined in detail in Section 4.3.1) offers several advantages ... 

We will discuss here applications of polyelectrolyte-modified electrodes, with particular emphasis on layer-by-layer self-assembled redox polyelectrolyte multilayers. The method offers a series of advantages over traditional technologies to construct integrated electrochemical devices with technological applications in biosensors, electrochromic, electrocatalysis, corrosion prevention, nanofiltration, fuel-cell membranes, and so on. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

For example, POPAM dendrimers of 1,3-diaminopropane type have been used in membrane reactors as supports for palladium-phosphine complexes serving as catalysts for allylic substitution in a continuously operated chemical membrane reactor. Good recovery of the dendritic catalyst support is of advantage in the case of expensive catalyst components [9]. It is accomplished here by ultra-or nanofiltration (Fig. 8.2). 

A practically useful predictive method must provide quantitative process prediction from accessible physical property data. Such a method should be physically realistic and require a minimum number of assumptions. A method which is firmly based on the physics of the separation is likely to have the widest applicability. It is also an advantage if such a method does not involve mathematics which is tedious, complicated or difficult to follow. For the pressure driven processes of microfiltration, ultrafiltration and nanofiltration, such methods must be based on the microhydrodynamics and interfacial events occurring at the membrane surface and inside the membrane. This immediately points to the requirement for understanding the colloid science of such processes. Any such method must account properly for the electrostatic, dispersion, hydration and entropic interactions occurring between the solutes being separated and between such solutes and the membrane. 

The first awaited advantage of the process is to be able to work with a high permeate flux because of the low viscosity of SC CO2 (10 times lower than for water). Indeed the membrane used in the process is a hybrid nanofiltration element, constituted from an inorganic substrate (Ti02 with a mean pore diameter of about 10 run) on which a nation layer had been deposited, the prevailing mass transfer mechanism of which is convection. This fact could be checked through experiments conducted successively with water and SC CO2, flows obtained being in the opposite ratio of fluid viscosities within less than 10%. 

Nanofiltration or reverse osmosis treated water is needed in the most demanding places of the null such as for the high-pressure showers in a paper machine. Surface water used in the mill as intake freshwater may also need NF or RO treatment. Permeation of monovalent ions, in particular chloride ions, is both an advantage and a disadvantage of NF compared to RO. Monovalent salts cause significant osmotic pressure when retained in RO. In NF their permeation keeps the osmotic pressure lower and thus the transmembrane pressure needed to overcome the osmotic pressure is lower. The permeation of chloride ions in NF may restrict the reuse of the permeate because of concerns regarding corrosion caused by chloride. On the other hand, the concentrate then contains less chloride and its reuse or incineration is safer. 

Besides the use of homogeneously soluble polymethacrylates or poylstyrene, as for the examples described above, other soluble supports may be used in order to yield a catalyst which can be retained by ultra- or nanofiltration membranes. Several groups have introduced catalysts (chiral and nonchiral) coupled to dendrimers and dendrimer-like structures [54, 59-76]. Compared with catalysts coupled to polymers, such complexes offer the advantage of a more defined structure. Thus, the number of active sites can be controlled more accurately. As these will be present at the surface of a globular structure they will be easily accessible. 

It is noted that a growing number of dendrimers and hyperbranched polymers have been synthesized that have metal-metal bonded dimers or metal clusters attached to the surface of the dendrimer. These materials are analogous to polymers that have metal-metal units appended to the polymer backbone rather than incorporated into the backbone chain, and as such they are not covered in this chapter. The field was recently reviewed by Rossell and co-workers.54 The interest in these materials stems from the catalytic properties of the metal complexes attached to the dendrimer surface. In particular, these dendrimer species offer the opportunity to combine the advantages of homogeneous and heterogeneous catalysis, because the materials can be recycled by nanofiltration. 

Kragl et al.100 described the retention of diaminopropyl-type metallodendrimers bearing palladium phosphine complexes on ultra- or nanofiltration membranes and their use as catalysts for allylic substitution in a continuously operating chemical membrane reactor. Their results demonstrated a viable procedure for catalyst recovery, because these metallodendrimers acting as catalyst supports offered an advantage in that the intrinsic viscosity of the solution is smaller, facilitating filtration. 

K. DeSmet, S. Aerts, E. Ceulemans, l.F.J. Vankelecom, and P.A. Jacobs, Nanofiltration-coupled catalysis to combine the advantages of homogeneous and heterogeneous catalysis, Chem. Commun. 7 (2001) 597-598. 

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