Shape-selective zeolite process

At the low-molecular-weight end of the spectrum, a process newly commercialized by Mobil for converting methanol into gasoline has significantly expanded opportunities in C-1 chemistry— the upgrading of one-carbon molectrles to mrrlticarbon products. The process involves the use of ZSM-5, a shape-selective zeolite catalyst. (See "Zeolite and Shape-Selective Catalysts" in Chapter 9.)... 

An analogous process has been used industrially for the synthesis of 2,6-di-isopropylnaphthalene from naphthalene and propylene in the presence of H-mordenite, another shape-selective zeolite (Equation 7). Oxidation of... 

The production of gasoline from methanol is a parallel process to the Fischer-Tropsch synthesis of hydrocarbons from syngas (Section 4.7.2). A shape-selective zeolite (ZSM-5) was the catalyst of choice in the process put on stream in 1987 by Mobil in New Zealand however the plant was later closed. The zeolite was used at ca. 400°C in a fluid catalyst reactor, which allows prompt removal of the heat of reaction. 

N. Y. Chen (Mobil Research Development Corp., Princeton, N. J. 08540) It might be of interest to the audience, particularly to those who are not familiar with the application of zeolites in industrial catalytic processes, to mention that since the discovery of catalysis over shape-selective zeolite first published by Weisz and Frilette in I960, a commercial process based on selective hydrocracking reactions similar to that reported in this paper has been in operation on a large scale in more than four of our refineries since 1967. A technical paper describing this process, known as the Selectoforming process, was published in 1968. 

Most industrial shape selective catalytic processes today use medium-pore zeolites from the "pentasil" femily. (The name refers to the five-membered rings in their framework and to their high silicon content.) ZSM-5 is by far the most important member of this family. It has high acid catalytic activity and it is very stable The silica/alumina ratio in ZSM-S varies from the teens to the thousands. High silica/alumina ratios give hydrophobidty, high acid strength, and thermal, hydrothermal, and acid stability. 

In the MTG process, methanol is quantitatively converted to hydrocarbon and water over a shape-selective zeolite with the unique structure of Mobil proprietary ZSM-5 catalyst. The conversion of methanol proceeds at relatively mild conditions following a reaction path well discussed in the literature (ref. 1). The hydrocarbons formed are primarily in the gasoline boiling range suitable for use as high quality automotive fuel. 

Table 1 Processes Utilizing Shape Selective Zeolite Catalysts...

In the first step of the process, propylene and/or n-butenes are converted to species boiling in the gasoline range. The catalyst is a special shape selective zeolite, operating conditions are mild and the space velocity is exceptionally high. 

Multi-stage reactor first reactor for contacting feed with a medium pore shape-selective zeolite catalyst for converting the C3-olefinics to liquid hydrocarbons comprising C5+, in second stage the process conditions should be effective to convert a major amount of ethene... 

One of the first success of zeolites as catalysts, and the first commercial molecular shape selective catalytic process, was the use of erionite in a post-reforming process named selectoforming (39). Ihis 8 MR zeolite was able, based on the principle of size exclusion, to selectively crack the short chain n-parafiins to produce LPG. To avoid the deactivation by coke NiS was deposited on the zeolite. The erionite based catalyst is generally located at the bottom of the last reactor of the reformer unit and operates then at the reformer pressure, and at the temperature of the last reformer reactor. When more flexibility was to be achieved from the selectoforming, the catalyst is introduced... 

Catalytic dewaxing (CDW) was developed by Exxon Mobil in the 1980s. The process employs a shape-selective zeolite called ZSM-5, which selectively converts waxy n-paraffms into lighter hydrocarbons. 

Shape-selective zeolite catalysts were first used in the Selectoforming process by Mobil in 1968. Hydrogen-exchanged natural erionite, containing some nickel,... 

Only a very few selected examples have been discussed. The number of processes based on shape-selective catalysis by zeolites is ever increasing, particularly in the field of speciality and fine chemicals and quite a few have been... 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

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. 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

The microwave technique has been also found to be the best method for preparing strongly basic zeolites (ZSM-5, L, Beta, etc.) by direct dispersion of MgO and KF. This novel procedure enabled the preparation of shape-selective, solid, strongly base catalysts by a simple, cost-effective, and environmentally friendly process [11, 12]. New solid bases formed were efficient catalysts for dehydrogenation of 2-propanol and isomerization of cis-2-butene. 

Zeolites ZSM-5 and ZSM-11 are the most commercially important end-members of a continuous series of intermediate structures belonging to the so-called pentasil family (4,5). The first preparation of ZSM-5 was described in 1972 (6) and since thbn, a number of elaborate synthesis recipes have been reported in the patent literature. Because of the unique and fascinating activity and (shape) selectivity of this material for a variety of catalytic reactions currently processed in chemical industries, increasing attention has been devoted to a better understanding of the various mechanisms that govern the synthesis of ZSM-5 (7-33). 

---