Mesopores in zeolites

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

First-of-its kind results [14,15] with electron tomography for the study of zeolites have been obtained with Ag/Na-Y. The location of the Ag particles of about 10 nm could be unequivocally established with respect to the surface and the interior of the crystals. The moderate resolution of 5 nm of the reconstructed images in this study related to the rather large zeolite crystals (500 nm) involved. Hereafter, we first discuss in more detail the use of electron tomography to study mesopores in zeolite crystals and, secondly, case studies involving metal particles and pore architecture of ordered mesoporous materials. 

ZSM-5 zeolites attracted particular attention due to their ability to catalyze various reactions in petrochemistry, oil refining and fine chemistry [1,2,3]. However, for many useful chemical reactions, size exclusion virtually operates, as the steric requirements of the reactants and products are beyond the pore sizes. There are several methods how to improve accessibility of active sites located inside microporous structure [4,5]. The first possibility how to diminish the role of the molecular transport is a creation of secondary mesopores in zeolite ciystals, which can be done by post-synthesis treatments such as desilication [6] or dealumination [7], however, these treatments cannot provide zeolites having mesopores uniform in size and lattice positions [8]. The second... 

Imaging the mesopores in zeolite Y using three-dimensional transmission electron microscopy... 

A 3D-TEM study on a series of Y zeolites reveals the mesopores (generated by steaming and/or acid leaching) with great clarity. Both the diameters and shapes of the visualised pores correspond very well with nitrogen physisorption measurements of the entire sample. Also cimorphous alumina in the mesopores and on the external surface can be visualised, which is in agreement with results obtained by XPS on these samples. From these results a more detailed model for the formation of mesopores in zeolite Y is proposed. 

The parameters affecting the genesis of mesopores in zeolites during dealutnination have been investigated. The foimation of mesopotes is essentially controlled by the structural defects density. Strucn defects correspond to i) framework vacancies, ii) crystalllographic defects and iii) trivalent elements incorpcHated in the framework. At least two parameters control the structural defects density the initial Si/AJ ratio and the synthesis conditions. 

Two main parameters seem to affect the formation of mesopores in zeolites during dealumination the crystallographic structure and the Si/Al ratio, in addition the synthesis conditions have certainly to be considered as well. In order to determine the effect of each of these parameters, different zeolites with different initial Si/Al ratios were submitted to classical dealumination treatments. 

It thus appears that if the initial Si/Al ratio is a parameter controlling the genesis of mesopores in zeolites during deaiumination, the initial crystallographic distribution of aluminium has also to be considered. 

Figure 2.3 3D-TEM reconstruction of (a) severely steamed and subsequently acid-leached Y zeolite [22] and (b) desilicated ZSM-5 zeolite crystal [24]. The mesopores in the crystal are shown as lighter gray tones.

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

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. 

These data show that, indeed, mesopores dominate except in zeolites and active carbons. Surface areas can be very high viz., up to a few football fields per kg ). 

Capek, L., Kreibich, V., Dedecek, J. et al. (2005) Analysis of Fe species in zeolites by UV-VIS-NIR, IR spectra and voltammetry. Effect of preparation, Fe loading and zeolite type, Microporous Mesoporous Mater., 80, 279. 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Liquid-phase breakthrough experiments were also developed in order to characterize mesopores. The principle of the methodology relied on the analysis of the diffusion and adsorption of molecular probes with various molecular dimensions and adsorption strength. The relative proportion of occluded and accessible mesopores in the studied dealuminated Y zeolite could then be estimated. To allow this estimation, it is necessary to use molecular probes that can or cannot penetrate into the microporosity of the Y zeolite (see Figure 2). 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

The parent zeolites, MOR and BEA, show reversible type-I adsorption/desorption isotherm with a steep rise at pipe, < 0.01, typical for microporous solid while the recrystallized exhibit rather sharp steps at pipe, 0.35, corresponding to the existence of uniform mesopores (typical for MCM-41 phase). According to BJH calculation, the size of the mesopores was about 3.0 nm. The contribution of micro- and mesopores in recrystallized materials was adjusted by variation of alkalinity during recrystallization procedure [2] (Table 1). The formation of mesopores resulted in significant increase of pore volumes of the samples upon recrystallization. 

Partial recrystallization of zeolites into composite micro/mesoporous materials leads to 1,3-2 fold increase of n-octane conversion and 2-3 fold increase of the yield of target products - branched octanes, indicating improved accessibility of active sites and transport of bulky molecules provided by mesopores. In the case of BEA series recrystallization in mild conditions leads to remarkable increase in selectivity to i-octane from 40 to 67%. On the contrary, complete recrystallization results in low catalytic activity, comparable with MCM-41 catalyst. 

Composite zeolite/mesoporous materials show remarkably high catalytic activity, stability and selectivity in the isomerization of n-octane due to high zeolitic acidity combined with improved accessibility of active sites and easier transport of bulky molecules provided by mesopores. The best catalyst performance can be achieved by the optimisation of the contributions of micro- and mesopores in the composite material. 

C. R. Jacob, S. R. Varkey, and R. Ratnasamy, Selective oxidation over copper and manganese salens encapsulated in zeolites, Microporous Mesoporous Mater. 22, 465 74 (1998). 

Cheetham, T., FjeUvag, H., Gier, T.E., Kongshaug, K.O., liUerud, K.P., and Smcky, G.D. (2001) Very open micropo-rous materials from concept to reality, in Zeolites and Mesoporous Materials at the Dawn of the 21st Century (eds... 

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