Pore confinement effect

Through covalent and noncovalent bonding methods, different kinds of molecular catalysts could be incorporated into MSs and MOFs. These porous materials with the incorporated molecular catalyst could catalyze various kinds of chemical reactions. A review of all the related works is impossible and not necessary in this chapter. We only review some representative examples for demonstrating the unique properties of the nanoreactor for catalytic reactions, including the pore confinement effect, the enhanced cooperative activation effect, and the isolation effect, as well as the microenvironment and the porous structure engineering of the nanoreactor and the catalytic nanoreactor engineering. 

The above examples show that the asymmetric reaction in nanopores, compared to that on the surface and in a homogeneous system, can improve the enantioselectivity for some asymmetric reactions. When the nanopore size of the support or the tether length is tuned to a suitable value, the chiral catalysts in the nanopores can show higher ee values for some cases. Moreover, the hydrophobic modification of the inner wall of MSs can also result in improved catalytic activity and enantioselectivity [81]. 

Modification of nanopores with methyl groups can further improve the TOP and ee values. Reprinted with permission from Ref. [80]. Copyright 2006 Elsevier. 

For investigating the pore confinement effect, the chiral Mn(Salen) catalyst was immobilized in MCM-41 and MCM-48 with different pore sizes [84]. In the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as oxidant, it was found that the conversion and enantioselectivity were closely correlated with the pore size of the supports. The catalysts immobilized on MS with large pore sizes exhibited higher conversion. For the MCM-41-supported catalyst, the enantioselectivity increased with increasing pore size. However, for MCM-48-supported catalysts, the compatible pore size of the support with the substrate was found to be beneficial for obtaining higher enantioselectivity in olefin epoxidation. 

Besides Mn(Salen) forasymmetric epoxidation reactions, other types of molecular catalysts immobilized in the nanopore also display higher ee values than their homogeneous counterparts. One interesting example was reported by Thomas 

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In this chapter, we summarize the recent advances in the development of nanaoreactors based on porous solid materials for chemical reactions, including the general methods for the fabrication of typical porous materials, (mesoporous silicas (MSs), carbon nanotubes (CNTs), and the MOFs), the assembly of the molecular catalysts in the cavities and pores of the porous materials, the chemical reactions in the porous-material-based nanoreactors, and some important issues concerning the porous-material-based nanoreactor, such as the pore confinement effect, the isolation effect, and the cooperative activation effect We close this chapter with an outlook of the future development of the nanoreactors. 

Selectivity (chemo-, regio-, and stereoselectivity) control is a key issue for organic synthesis. In addition to controlling the selectivity by developing appropriate catalytic systems, the selectivity of a chemical reaction could be controlled by the restriction of the reaction in a confined nanospace. For example, we discussed the enhancement of enantioselectivity by the pore confinement effect in Section 10.4.1. In this section, we will discuss the selectivity control of a chemical reaction by the isolation of the substrates and the restriction on the rotational and translational motions of the substrates in a confined nanospace. 

A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

Chapter 15 gives an extensive and detailed review of theoretical and practical aspects of macromolecular transport in nanostructured media. Chapter 16 examines the change in transport properties of electrolytes confmed in nanostructures, such as pores of membranes. The confinment effect is also analyzed by molecular dynamic simulation. 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

The confinement effects of the narrow pore on the ILs and the ionic interactions between [BMIM] favor the open pore while the anion, [PF6], was attached to the open metal sites, was observed in a simulation study. It was ascertained that C02 was favorably attached to the [PF6] anions sites. The study demonstrated that IL/MOF composites are a potential candidate for C02 adsorption and have displayed significantly high CO2/N2 selectivity. To the best of our... 

AG is always negative, and the decrease in free energy can be due to adsorption effects (change in AH) or entropic interactions (change in AS). AS is always operating when the polymer chain cannot occupy all possible conformations in a pore (confined space) due to the limited size of the pore relative to the size of the macromolecule. In a real... 

In this chapter we will focus on molecular ordering and confinement effects in pores. Diffusion experiments with the pulse-field gradient method ([162-165] and references therein) and characterization of the surface properties using NMR of noble gases such as 129Xe ([166-171] and references therein), or 83Kr [172], will be omitted due to excellent reviews that have appeared quite recently in these areas. 

The studies of Thomas and Raja [28] showed a remarkable effect of pore size on enantioselectivity (Table 42.3). The immobilized catalysts were more active than the homogeneous ones, but their enantioselectivity increased dramatically on supports which had smaller-diameter pores. This effect was ascribed to more steric confinement of the catalyst-substrate complex in the narrower pores. This confinement will lead to a larger influence of the chiral directing group on the orientation of the substrate. Although pore diffusion limitation can lead to lower hydrogen concentrations in narrow pores with a possible effect on enantioselectivity (see Section 42.2), this seems not to be the case here, because the immobilized catalyst with the smallest pores is the most active one. 

The smallest pores that can be formed electrochemically in silicon have radii of < 1 nm and are therefore truly microporous. However, confinement effects proposed to be responsible for micropore formation extend well into the lower mesoporous regime and in addition are largely determined by skeleton size, not by pore size. Therefore the IUPAC convention of pore size will not be applied strictly and all PS properties that are dominated by quantum size effects, for example the optical properties, will be discussed in Chapter 7, independently of actual pore size. Furthermore, it is useful in some cases to compare the properties of different pore size regimes. Meso PS, for example, has roughly the same internal surface area as micro PS but shows only negligible confinement effects. It is therefore perfectly standard to decide whether observations at micro PS samples are surface-related or QC-related. As a result, a few properties of microporous silicon will be discussed in the section about mesoporous materials, and vice versa. Properties of PS common to all size regimes, e.g. growth rate, porosity or dissolution valence, will be discussed in this chapter. 

The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

Pore-confinement of the catalytically active center may have favorable effects as demonstrated for (i) protection of the reactive sites from deactivation processes... 

The surface force apparatus (SFA) has been used extensively over the past 30 years to measure the force directly as a function of separation between surfaces in liquids and vapors. If the force-measuring spring is replaced with a mechanically more rigid support, the two opposing surfaces become an ideal model pore for the study of confinement effects on phase behavior [16], A detailed review can be found in reference ]. Briefly, the shift of the melting temperature AT can be related to the size h of the condensate measured with SFA according to... 

This concept of zeolites as enzyme mimics was used by Derouane and Vanderveken (59) to explain the selective aromatization of n-hexane on Pt/LTL catalysts confinement effects combined with the unique pore structure of LTL zeolite would be responsible for the fast and selective conversion of n-hexane to benzene. 

It has been shown that single ring aromatic alkylation reactions such as benzene to ethylbenzene take place primarily within the 12- ring (12-MR) system, and that the 10-ring (10-MR) system contributes little to the ethylbenzene reaction. A key feature of MCM-22 is its ability to operate stably at low benzene-to-ethylene ratios with minimal production of polyethylbenzenes (PEBs) or ethylene oligomers. The excellent ethylbenzene selectivity of the MCM-22 catalyst is likely due to confinement effects within this 12-MR pore system and to the very facile desorption... 

A zero order with respect to phenol was found, which can be related to the strong physical adsorption of phenol into the zeolite micropores (confinement effect). This explains the pronounced increase in phenol conversion with the methanol/phenol ratio. This strong retention of phenols and phenolic products within the zeolite micropores is responsible for a large part of the high apparent ratio of secondary reactions and especially of coking , i.e. of formation of heavy products which remain trapped in the zeolite micropores. This fast coking of zeolites is responsible for their rapid deactivation by pore blockage. 

The possibility of obtaining different pore sizes and geometries allows studying the specific role of the pore diameter and interconnection in confinement effects. However, the main problem in the use of MCM materials in radiolysis is the poor definition of the silica-based walls. The presence of micropores and a high content in non-condensed silica (silanols groups) has been evidenced in some cases. 

As shown in Fig. 4, for pore diameter larger than 150 nm, the HO production in confined solutions is similar to that obtained in homogenous solutions, whereas bellow 100 nm, the HO yields decrease with the pore size. The amount of HO available for reaction with coumarin is decreased by almost a factor of 6 for 8 nm pores compared to bulk water and by a factor of 1.5 for a pore as large as 50 nm. For such large pores, this decrease cannot be explained by a pore-cage effect, but rather by an interpore decreased diffusion that limits the amount of radicals that escape their production site. 

When people consider confinement effects, they consider mainly an increase in the encounter probability inside a single pore and therefore, expect an acceleration of the reaction. Such in-pore acceleration has been quantified by Tachiya and co-workers for diffusion-limited reactions through the so-called confinement factor [see Eq. (11.58) in Ref. 40]. From this treatment, confinement effects are expected to disappear when the reaction radius is less than one tenth of the confinement radius. Considering the reaction radii of radiolytic species, no acceleration by confinement should be expected for pore diameter larger than 10 nm. For smaller pore size, acceleration of the recombination reactions within spurs would be critical in the determination of radiolytic yields in the nanosecond time range. However, the existence of such an acceleration of radiolytic reactions has not been suggested in the nanosecond pulse radiolysis of zeolites and has still to be assessed using picosecond pulse radiolysis. 

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