Hydrocarbon processing, zeolite

This chapter discusses the synthesis, characterization and applications of a very unique mesoporous material, TUD-1. This amorphous material possesses three-dimensional intercoimecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1 however, many versions of TUD-1 using different metal variants have been prepared, characterized, and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules, and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this chapter. 

Figure 4.11 Different types of selectivity in zeolite catalysts. (Reprinted from Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, Vol. 137, I.E. Maxwell, W.J.H. Stork, Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, Vol. 137, Hydrocarbon Processing with Zeolites, pp. 747-819. Copyright 2001. With permission from Elsevier.)...

Understanding the adsorption, diffusivities and transport limitations of hydrocarbons inside zeolites is important for tailoring zeolites for desired applications. Knowledge about diffusion coefficients of hydrocarbons inside the micropores of zeolites is important in discriminating whether the transport process is micropore or macropore controlled. For example, if the diffusion rate is slow inside zeolite micropores, one can modify the post-synthesis treatment of zeolites such as calcination, steaming or acid leaching to create mesopores to enhance intracrystalline diffusion rates [223]. The connectivity of micro- and mesopores then becomes an... 

Many books, reviews and treatises have been pubUshed on related subjects [1-7]. Thus the objective of this chapter is the deUneation of the key features of the catalytic surface and the process conditions which enable better control of the reaction pathways for more efficient and environmentally friendly processes and minimal utiHzation of precious natural resources. As it stands today, hundreds of known framework types have been synthesized and scaled-up [8], but only a handful have found significant application in the hydrocarbon processing industries. They are zeolite Y and its many variants, ZSM-5, Mordenite and zeohte Beta. Other very important crystalline materials (including aluminophosphates (ALPOs),... 

ENSORB [ExxoN adSORBtion] A process for separating linear from branched hydrocarbons, using a zeolite molecular sieve. The adsorbed gases are desorbed using ammonia. It operates in a cyclic, not a continuous, mode. Developed by Exxon Research Engineering Company, and used by that company on a large scale at the Exxon refinery in Baytown, TX. Asher, W. J., Campbell, M. L., Epperly, W. R., and Robertson, J. L., Hydrocarbon Process., 1969, 48(1), 134. 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

High sensitivity, fast response, and well-defined flow patterns make the TEOM an excellent tool for determining diffusivities of hydrocarbons in zeolites. Moreover, the TEOM has provided a unique capability for gaining knowledge about the effects of coke deposition on adsorption and diffusion under catalytic reaction conditions. An application of the TEOM in zeolite catalysis by combining several approaches mentioned above can lead to a much more detailed understanding of the catalytic processes, including the mechanisms of reaction, coke formation, and deactivation. 

Diffusional motion. Many rotational and translational diffusion processes for hydrocarbons within zeolites fall within the time scale that is measurable by quasielastic neutron scattering (QENS). Measurements of methane in zeolite 5A (24) yielded a diffusion coefficient, D= 6 x lO" cm at 300K, in agreement with measurements by pulsed-field gradient nmr. Measurements of the EISF are reported to be consistent with fast reorientations about the unique axis for benzene in ZSM-5 (54) and mordenite (26). and with 180 rotations of ethylene about the normal to the molecular plane in sodium zeolite X (55). Similar measurements on methanol in ZSM-5 were interpreted as consistent with two types of methanol species (56). 

Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. 

Bonardet, J.L., Barrage, M.C. and Fraissard, J. (1995) NMR techniques for studying the coking of zeolite-based catalysts, in Deactivation and Testing of Hydrocarbon-Processing Catalysts (eds... 

The Fischer-Tropsch catalysis of converting syngas to predominantly linear aliphatic hydrocarbons has been extensively reviewed (350-354). A related process, converting syngas to oxygenates and hydrocarbons over zeolite-supported Pd catalysts, will be briefly described here. 

In hydrocarbon processing the A-type zeolites are frequently used as adsorbents for purifying feeds and separating products but never as catalysts due to some inappropriate characteristics (e.g. stability and pore aperture). However, the simplicity of its structure and its well characterized ion-exchange properties make zeolite A suitable to models for investigating the redistribution of cations after various treatments and for studying adsorbent-adsorbate interactions (which finally may lead to chemical transformations). 

CO2 hydrogenation to methanol is one of the most favorable valorization of the carbon dioxide considering that the methyl alcohol can easily be converted into hydrocarbons on zeolite type catalysts [1,2] even in a "one step" process using composite material. 

Maxwell, I. E. and W. H. J. Stork. Hydrocarbon Processing with Zeolites. Stud. Surf. Sci. Catal., 1991, 58, 631-726. 

If it can be shown that the photooxidation of hydrocarbons in zeolites is a general method, then the shape and size-selective properties of zeolites may potentially be used to control the selectivity of specific oxidation reactions (2,3). For example, ZSM-5 is an important shape-selective catalyst in many reactions, such as the disproportionation of toluene (4). Para-xylene is the dominant product because the transport of the other isomers, ortho- and meta-xylene, is restricted due to the pore size of ZSM-5. Thus, stereochemical aspects of selective photooxidation reactions may also be influenced by the zeolite and may be used to design environmentally benign processes for the synthesis of industrially useful molecules. 

Figure 12.13 Coke-forming process occurring during the methanol-to-hydrocarbon process over H-ZSM-5 zeolites, and a corresponding in situ UV-vis spectrum measured under reaction conditions. (Reproduced with permission from Ref. [121].)...

The transformation of methanol to hydrocarbons over zeolite H-ZSM-5 is the basis of several industrially important reactions, such as the MTG or the MTO processes [20,21]. The mechanism of the reaction, particularly as concerns the formation of the first C-C bond and the nature of the interactions between the CH3OH molecules and the zeolitic framework has been the subject of controversy [22,23]. H NMR has been used [24-26] to study the chemistry of methanol adsorbed on H-ZSM-5. 

The grounds of catalysis by zeolites were clearly and conclusively presented in four chapters included in the book (actually, a handbook for zeoUte scientists) entitled Introduction to zeolite science and practice [2]. These chapters were focused on (1) acid-catalyzed hydrocarbon conversion (by Martens and Jacobs), (2) hydrocarbon processing (industrial processes) (by Maxwell and Stork), (3) organic syntheses by zeolite catalysts (by Hoelderich and van Bekkum), and (4) selective oxidation reactions by transition metals introduced... 

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