Mesopores surface diffusion

Modification of mesoporous membranes can result in (i) a decreased pore size which increases the contribution of surface diffusion, and (ii) a change in the nature of the pore surface and consequently a change in all types of interaction energies with the gas phase. Both phenomena have an effect on permeation and separation. 

The gas is applied as a mixture to the retentate (high pressure) side of the membrane, the components of the mixture diffuse with different rates through the membrane under the action of a total pressure gradient and are removed at the permeate side by a sweep gas or by vacuum suction. Because the only segregative mechanisms in mesopores are Knudsen diffusion and surface diffusion/capillary condensation (see Table 9.1), viscous flow and continuum (bulk gas) diffusion should be absent in the separation layer. Only the transition state between Knudsen diffusion and continuum diffusion is allowed to some extent, but is not preferred because the selectivity is decreased. Nevertheless, continuum diffusion and viscous flow usually occur in the macroscopic pores of the support of the separation layer in asymmetric systems (see Fig. 9.2) and this can affect the separation factor. Furthermore the experimental set-up as shown in Fig. 9.11 can be used vmder isobaric conditions (only partial pressure differences are present) for the measurement of diffusivities in gas mixtures in so-called Wicke-Callenbach types of measurement. 

The nitrogen adsorption isotherm of the carbon nanofibers (Fig. 2) is of Type II in the BDDT classification, which is normally obtained for carbons, which are predominantly mesoporous. The BET surface area (calculated with the data between 0.05 < p/po < 0.2) equals 91 m g and the BJH mesopore surface area equals 79 m g. The generd view is that a low surface area of the support would limit the metal loading if the purpose is to finely disperse the metal catalyst [22]. As was discussed in the introduetion the highly mesoporous structure of the carbon nanofibers significantly reduces diffusion limitations. 

Dvoyashkin et al. (2009) studied surface diffusion of n-heptane in two mesoporous adsorbents with different morphologies of the pore network (Vycor random porous glass and porous silicon with linear pores) using PFG-NMR (Figure 1.238). The obtained diffusivities revealed increasing... 

FIGURE 1.238 Diffusivities of n-heptane in Vyeor porous glass (open symbols) and in porous silicon (solid symbols) as a function of surface coverage measured at different temperatures. (Adapted from Micropor. Mesopor. Mater., 125, Dvoyashkin, M., Khokhlov, A., Naumov, S., and Valiullin, R., Pulsed field gradient NMR study of surface diffusion in mesoporous adsorbents, 58-62, 2009. Copyright 2009, with permission from Elsevier.)... 

Dvoyashkin, M., Khokhlov, A., Naumov, S., and Valiullin, R. 2009. Pulsed field gradient NMR study of surface diffusion in mesoporous adsorbents. Micropor. Mesopor. Mater. 125 58-62. 

Mesoporous multi-/ monolayer, low-level porosities 20/40 % 1,000 °C - reduced pressure eonditions Enhanced surface diffusion, enhanced stmctural changes close to the top surface in reduced pressure conditions and preconditioning with H2 Kuzma-Fihpek (2010) Kuribayashi et al. (2004) Sato et al. (2000)... 

An industrial DMTO fluidized bed catalyst pellet is basically composed of SAPO-34 zeofite particles and catalyst support (or matrix). The pores of zeolite particles and matrix are interconnected as a complex network. The pores inside zeofite particles are typically micropores (less than 2 nm) and the matrix normally has either mesopores (2-50 nm) or macropores (>50 nm), or both (Krishna and Wesselingh, 1997). The bulk diffusion coefficients in the meso- and macropores might be several orders of magnitude larger than surface diffusion coefficients in the micropores. Kortunov et al. (2005) found that the diffusion in macro- and mesopores also plays a crucial part in the transport in catalyst pellets. Therefore, other than a model for SAPO-34 zeofite particles, a modeling approach for diffusion and reaction in MTO catalyst pellets, which are composed of SAPO-34 zeofite particles and catalyst support, is needed. 

Liquid and gaseous molecules have been known to exhibit characteristic transport behaviors in each type of porous material. For example, mass transport can be obtained via viscous flow and molecular diffusion in a macroporous material, through surface diffusion and capillary flow in a mesoporous material and by activated diffusion in a microporous material. 

Rao and Sircar [5-7] introduced nanoporous supported carbon membranes which were prepared by pyrolysis of PVDC layer coated on a macroporous graphite disk support. The diameter of the macropores of the dried polymer film was reduced to the order of nanometer as a result of a heat treatment at 1,000°C for 3 h. These membranes with mesopores could be used to separate hydrogen-hydrocarbon mixtures by the surface diffusion mechanism, in which gas molecules were selectively adsorbed on the pore wall. This transport mechanism is different from the molecular sieving mechanism. Therefore, these membranes were named as selective sitrface flow (SSF ) membranes. It consists of a thin (2-5 pm) layer of nanoporous carbon (effective pore diameter in the range of 5-6 A) supported on a mesoporous inert support such as graphite or alumina (effective pore diameter in the range of 0.3-1.0 pm). The procedures for making the selective surface flow membranes were described in [5, 7]. In particular, the requirements to produce a surface diffusion membrane were shown clearly in [7]. 

High-flux, limited-selectivity gas separations rely on (small) differences in Knudsen diffusion fluxes. Mesoporous 7-alumina membranes have also been reported to exhibit a substantial CO2 flux by surface diffusion on the internal pore surface (Uhlhom et al., 1992). This mechanism might be of use for the separation of CO2 and other polar molecules. 

Generally, however, the aim is to avoid conditions leading to film diffusion control. This means that the focus is shifted towards transport processes that occur at the intermediate level (that is, in the mesopores and macropores within the macroparticle or pellet itself) and those which occur at the smallest dimensional level (viz., in the very micropores of the molecular sieve) [45, 89]. Within the mesopores and macropores between the primary zeolite crystallites transport will be dominated by molecular and ionic intercrystalline diffusion possibly coupled to surface diffusion processes, while, in the zeolite micropores themselves, intracrystalline diffusion occurs, also possibly coupled... 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

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