Metal framework-containing zeotype materials

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.

Further metal framework-containing zeotype or ordered mesoporous materials... 

Table 1.10 List of processes for propylene oxide manufacture with metal framework-containing zeotype or other porous materials as catalysts...

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... 

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... 

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. 

The principal reactions for each of the framework metal-containing zeotype materials are the following ... 

In the commercially relevant reports of benzene oxidation catalyzed by framework metal-containing zeotype materials hydrogen peroxide or nitrous oxide (N2O) are used as oxidants. Thus phenol is produced without major by-product formation. 

The activation of phenol is much easier than that of benzene because of the high aromaticity of benzene and the corresponding charge delocalization. TS-1 has been used primarily as the catalyst for phenol hydroxylation, and the conditions have been thoroughly optimized (215). TS-2 (216) and Ti-MCM-41 (217) were also evaluated, but they do not perform as well as TS-1. Ramaswamy et al. (52) compared various framework metal-containing zeotype materials with MEL structure for phenol hydroxylation the incorporated metals were aluminum, tin, titanium, and vanadium. The... 

In this section, the commercial applications involving framework metal-containing zeotype catalysts are summarized. Notwithstanding the intriguing properties, only a limited number of framework metal-containing zeotype materials, among them TS-1, Ga-MFI, and some mesoporous catalysts, are currendy used for commercial applications. Framework metal-containing catalysts are no different than other catalysts in that the activity, stability, lifetime, manufacturability, and catalyst cost must meet certain requirements for a new catalytic process to be implemented. 

Other manufacturers offering TS-1 for sale include PQ, which is offering its PQ-TS-1, and CleanScience PUNE, India (252). Both companies manufacture TS-1 in the form of extrudates and microspheres. The listed process apphcations are aromatic hydroxylation, oxidation of primary alcohols to aldehydes, olefin epoxidation, and alkane oxidations. Presumably, there are more companies, especially in China, producing framework metal-containing zeotype materials. 

Large pore materials, hydrophobic framework metal-containing zeotype materials, hybrid materials, and materials with unique Lewis acidity are in the current pipeline for commercialization. Some new developments that are close to commercialization will be also be reviewed. 

Iron incorporation into MFI resulted in the development of another important application for framework metal-containing zeotype materials that is, the abihty to synthesize phenol directly frombenzene using N2O as oxidant. 

However, there is still opportunity for the commercialization of new processes using framework metal-containing zeotype materials. It is remarkable that in this particular research field the successful commerciafization has been achieved partly as a result of cooperation between scientists in academic groups and industrial researchers. For example, such cooperation played a role in the development of the PO processes. However, the costs involved in the syntheses of metal-containing zeotype materials (mainly due to SDAs and other reagents), and frequently the costs associated with both scale-up and manufacture, have tended to inhibit wide application of these new materials. As a result, new materials are finding initial catalytic applications only in specialty apphcations (i.e., production of high value-added products) or niche areas. 

We beheve that future developments of environmentally friendly processes using framework metal-containing zeotype materials will only be achieved when the following challenges are met by academia and industry. Primarily for Academia ... 

Better understanding of the chemistry and limitations of framework metal-containing zeotype materials New materials with close to 100% selectivity New materials for biomass conversion... 

Both academic and industrial research and development, as well as cooperation between industries, has resulted in a large effort in the discovery, synthesis, and catalytic application of framework metal-containing zeotype materials. These collaborations have led to new appHcations of metal-containing zeotype materials for the large-scale production of chemicals without major by-product formation. New environmentally friendly processes such as propylene epoxidation, phenol hydroxylation, ammoximation of cyclohexanone, and aromatization of light paraffins have been commer-ciahzed. Many new developments are in the pipehne, and they will Hkely be commerciahzed when both the economics and the environmental requirements become favorable. 

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.

Zeotype materials containing metal cations, for example ions of titanium, vanadium, chromium, iron, or tin, in the tetrahedral positions of their frameworks have been explored as solid Lewis acid catalysts (Ig). Such materials have been shown to be active in the Meerwein—Pormdorfr-Verley reduction of carbonyl compounds (151,233), and the BV oxidation (also called BV rearrangement) (234). 

---