Zeotype materials

With rapid development of zeotypic materials and mesoporous solids and their application in heterogeneous catalysis, HRTEM shows its advantages in distinguishing the ultrastructural features [40, 41], Carbon materials are used as support in catalytic reactions due to some of their specific characteristics and many publications report the TEM investigations on various forms of carbon related materials [42-48],... 

Overall Milton s concept of hydrothermal crystallization of reactive gels has been followed with various additions and modifications for most of the molecular sieve, zeolite, and zeotype materials synthesis since the late 1940s. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Highly dispersed surface species, with the limiting form of single-site active centers, play a primary role in a number of catalytic materials because of their peculiar features in terms of activity and selectivity. Both oxo-species and transition metal ions supported on oxides (or in zeotype materials) belonging to these types... 

Finally, in deriving structural information from the features of d-d spectra of TMls, it must be considered that, because of the Laporte selection rule, ion sites with octahedral symmetry can contribute to the spectra only to a very limited extent, and so can escape spectroscopic detection. However, this behavior can be turned into a tool to monitor the distribution of TMIs in sites with different structure as a function of loading, as in the case of CoAPO zeotype materials. In this case, the attainment of a plateau level of the intensity of the d-d bands due to Co ions with tetrahedral symmetry that became inserted in the framework indicated the formation of extra-framework species, containing d-d silent octahedral Co sites with increasing loading (Figure 2.16) [77]. 

Similar to the porosils, the dense, thermodynamically stable Si02 modification a-quartz is also prepared under hydrothermal conditions. However, in the industrial process for the production of quartz, the temperatures are rather high (around 400°C). In this process, NaOH is added as a mineralizer to the aqueous solution to promote dissolution of the silica precursor. The reaction mixtures for the preparation of porosils and other zeotype materials also generally contain a mineralizer, but the reaction conditions are much milder. Synthesis temperatures are below 200°C, typically between 140 and 180°C. Some zeolites can even be prepared from aqueous solutions under reflux at normal pressure. These mild synthesis conditions provide the kinetic control necessary to form metastable products [5-9]. 

Successful syntheses of a wide variety of zeolite and zeotype materials have been developed over the last 50 years. Much is known about the way in which these structures are formed, particularly from recent detailed studies using advanced techniques of microscopy (HRTEM and AFM). However, a number of issues surrounding the mechanism of synthesis remain incompletely resolved. This is partly due to experimental difficulty but is also a reflection of the fact that not all systems arc the same. Nevertheless, in the foregoing account an attempt has been made to review our existing knowledge in terms of basic similarities between one reaction regime and another. In this way, it is hoped to establish some overall principles which, with appropriate modification, may be found to be generally applicable, or at least to provide a framework for further analysis. 

Zeolites, i.e. microporous aluminosilicate materials with pores smaller than 2 nm, play key roles in the fields of sorption and catalysis [134, 135]. The global annual market for zeolites is several million tons. In the past few decades a large variety of zeolites and related zeotype materials have been produced, whereby transition metal incorporation is extensively used to modulate the catalytic characteristics of these materials. Since the catalytic properties depend on the structure and accessibility of the transition metal sites, a lot of effort is put into probing these sites. Nevertheless, the exact nature of the transition metal incorporation is often strongly debated, since most spectroscopic evidence for isomorphous substitution is indirect. 

Framework metal-containing zeotype materials Classification... 

This review is focused on the science and technology of framework metal-containing zeotype materials, a critical and increasingly important class of catalyst. In the following paragraphs, first the structural characteristics of zeo-Htes, which are close and well investigated relatives of zeotype materials, will be briefly summarized. Then the role of the framework metal for the structural and surface chemistry of zeolite and zeotype materials will be introduced. 

The framework metal-containing zeotype materials that are the focus of this review do not contain aluminum and thus do not classify as zeolites. Zeotype materials are characterized by properties that make them alike to... 

Significance of framework metal-containing zeotype materials... 

The invention of framework tin-containing zeotype materials created new opportunities for reactions requiring weak Lewis acid sites. An example is Sn-beta, which has exhibited extraordinary selectivity in partial oxidation reactions like the Baeyer—ViUiger (BV) oxidation of ketones to lactones, and also has been shown to be highly selective in the isomerization of glucose to fructose. 

This chapter focuses on the application of framework metal-containing zeotype materials as catalysts and is a report of the state of the associated science, presenting a summary of current and future challenges. The chapter is organized as follows First, the general synthesis procedures of framework... 

Figure 1.1 The principal synthesis routes used to prepare metal framework-containing zeotype materials. The four major components are a silicon source, a metal source, a template, and a mineralizing agent. These give the overall reagent mixture that then undergoes crystallization under hydrothermal conditions to produce the desired zeotype material.

Tables 1.1—1.5 are summaries of the key synthesis papers together with the detailed synthesis conditions for framework metal-containing (Ti, V, Sn, Ga, Fe) zeotype materials.

Titanium-beta (Ti-BEA) is a large pore zeotype material with intersecting 12 MR pores. These large pores facditate the difiusion of bulky molecules to the active titanium sites. The tetraethylammonium (TEA) ion is used as the SDA in the synthesis of Ti-BEA. A variety of synthesis methods have been reported (135). 

A large number of papers have reported the synthesis of framework iron-containing zeotype materials. The syntheses of Fe-MFI (82,155), Fe-beta (84), Fe-MTT (156), Fe-MOR (157), Fe-TNU-9/-10 (158), Fe-MCM-41 (159), and Fe-SBA-15 (160) have been described, and the properties of these materials have been characterized. 

Iron can be introduced into the frameworks of zeotype materials during their hydrothermal syntheses. Iron isomorphously substitutes for some silicon atoms in the framework if this route is chosen. The Si, Al, and Fe contents depend on the composition of the synthesis gel. The nature and distribution of the iron species in a particular sample strongly depend on the activation treatments that are appHed after the synthesis. 

There are several papers dedicated to the characterization of iron-containing zeotype materials, with an emphasis on determining the locations of the iron. For example, in an investigation of Fe-MFI, Milanesio et al. (161) used synchrotron X-ray powder difiraction in an attempt to determine the location of the Fe(III) in the MFI lattice. The data provided a rather... 

There is stiU a dispute as to whether the catalytic activity of iron-containing zeotype materials, for example, Fe-ZSM-5, should be attributed to isomorphously substituted framework iron or to extra-framework iron oxide or iron hydroxide species that are highly dispersed in the material. These extra-framework iron species are present for two reasons, either because they were not incorporated into the framework during the synthesis or because they were ejected from the framework during postsynthesis treatments (such as calcination or other heat treatments). The unresolved issue of the origin of catalytic activity continues to be the subject of research, whereby state-of-the-art characterization techniques are being applied. 

The success of syntheses designed to incorporate gallium into zeotype frameworks depends on the synthesis conditions, as is true for other framework metal-containing zeotype materials. Several researchers have claimed gaUium incorporation into the frameworks of zeotype structures (5b, 166). 

In this section, the catalytic chemistry of selected framework metal-containing zeotype materials is reviewed, with an emphasis on commercial applications. The catalytic activities of framework metal-containing zeotype materials, especially those containing titanium, vanadium, or tin, have been investigated extensively. The enormous interest in these materials is attributed to their remarkable catalytic activities and especially their selectivities in oxidation reactions. Because hydrogen peroxide is generally used as the oxidant, water is formed as a by-product. Hence, oxidation reactions carried out with these catalysts can be considered environmentally clean processes. Several review articles have been published that summarize the catalytic reactions (2a,3b-d,89). In this section, the focus is on selected industrially relevant reactions. 

A review of the extensive Hterature indicates a diverse array of reactions catalyzed by framework metal-containing zeotype materials. The key reactions are illustrated in Figure 1.4. 

Figure 1.4 Oxidation reactions catalyzed by various metal-containing zeotype materials, with oxidizing agents specified.

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