Commercial processes involving zeolites

Table IV. Main commercial processes involving zeolites...

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.)... 

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4 ) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps, as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1). 

Cyclohexane, produced from the partial hydrogenation of benzene [71-43-2] also can be used as the feedstock for A manufacture. Such a process involves selective hydrogenation of benzene to cyclohexene, separation of the cyclohexene from unreacted benzene and cyclohexane (produced from over-hydrogenation of the benzene), and hydration of the cyclohexane to A. Asahi has obtained numerous patents on such a process and is in the process of commercialization (85,86). Indicated reaction conditions for the partial hydrogenation are 100—200°C and 1—10 kPa (0.1—1.5 psi) with a Ru or zinc-promoted Ru catalyst (87—90). The hydration reaction uses zeolites as catalyst in a two-phase system. Cyclohexene diffuses into an aqueous phase containing the zeolites and there is hydrated to A. The A then is extracted back into the organic phase. Reaction temperature is 90—150°C and reactor residence time is 30 min (91—94). 

These processes involve a multistep transformation from the carbohydrate fraction to the value-added products which makes most of them far from commercialization. Hence, intensive efforts are stiU required to enable scale up of synthetic protocols developed on a lab-scale into industrial processes. Some of the current drawbacks might be overcome by the one-pot transformation of lignocellulose carbohydrates in value-added chemicals without isolation of the intermediate platform molecules (Delidovich et al., 2014). Moreover, nanoporous materials, such as acidic, basic or metallic catalysts (zeolites, mesoporous silicas, microporous/mesoporous carbons, resins, metal oxides, etc.), wUl play a crucial role in this biomass transformation (Wang and Xiao, 2015). 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

After a short description of the main features of zeolites, the significant contribution of zeolite catalysts in green chemistry will be shown in examples of commercial or the potential processes of refining, petrochemicals, and fine chemicals involving acid or metal acid bifunctional catalysts. 

Recently, the Sumitomo Chemical Co., Ltd. developed the vapour-phase Beckmann rearrangement process for the production of 8-caprolactam. In the process, cyclohexanone oxime is rearranged to e-caprolactam by using a zeolite as a catalyst instead of sulfuric acid. EniChem in Italy developed the ammoximation process that involves the direct production of cyclohexanone oxime without producing any ammonium sulfate. The Sumitomo Chemical Co., Ltd. commercialized the combined process of vapour-phase Beckmann rearrangement and ammoximation in 2003 ". 

The specifically formulated CGP-1 catalyst plays a vital role in the MIP-CGP process. Unique catalyst design, such as metal promoted MFl zeolite, phosphorus modified Y zeolite, and a novel matrix with excellent capability to accommodate coke [12] were involved to ensure the primary cracking and secondary reactions to proceed within a defined path. The commercial trial results of the MIP-CGP process in SINOPEC Jiujiang Company showed that, in combination with CGP-1 catalyst, the propylene yield was 8.96 wt%, which increased by more than 2.6% as compared with FCC process. The light ends yield and slurry yield are basically equal. The olefin content of the gasoline produced by MIP-CGP process was 15.0 v%, which was 26.1% lower than that of FCC gasoline. The sulfur content of gasoline was decreased from 400 to 270 pg/g. 

Concentrated sulfuric acid and hydrogen fluoride are still mainly used in commercial isoalkane-alkene alkylation processes.353 Because of the difficulties associated with these liquid acid catalysts (see Section 5.1.1), considerable research efforts are still devoted to find suitable solid acid catalysts for replacement.354-356 Various large-pore zeolites, mainly X and Y, and more recently zeolite Beta were studied in this reaction. Considering the reaction scheme [(Eqs (5.3)—(5.5) and Scheme 5.1)] it is obvious that the large-pore zeolitic structure is a prerequisite, since many of the reaction steps involve bimolecular bulky intermediates. In addition, the fast and easy desorption of highly branched bulky products, such as trimethylpentanes, also requires sufficient and adequate pore size. Experiments showed that even with large-pore zeolite Y, alkylation is severely diffusion limited under liquid-phase conditions.357... 

The commercially available catalytic processes for NOx abatement involve the use of transition metal ions such as Cu etc. on a titania (anatase) or zeolite support. The metal... 

NOx (typically 1-3%), present in the stack gas of nuclear waste process plants, is removed in die WINCO Process by two primary reactors to 300-1000 ppm by selective catalytic reduction (SCR) with NH3 over a commercial zeolite catalyst at 300-500°C followed by reduction to low ppm levels in a third cleanup reactor. This study involved laboratory tests on advanced SCR zeolite catalysts, NC-301, ZNX, and Cu-ZSM-5, for the primary SCR reactors over a range of anticipated process conditions using gas mixtures containing 500-5000 ppm NO+NO2, 500-5000 ppm NH3, 1-2% CO, 14% O2, and 20% steam in He. All three catalysts have acceptable levels of performance, i.e. selectively reduce >80% the NOx with NH3 to N2 over the temperature range of 400-500°C at a space velocity of 30,000 h- The Cu-ZSM-5 catalyst is the most active and selective catalyst converting >95% NOx d NH3 (at 500 - 5000 ppm of each) to N2. 

Alkylation of Aromatics with Liquid Catalysts. Forty years ago, ethylbenzene, cumene, and dodecyl benzenes were produced by alkylation reactions of benzene with liquid catalysts. Although some production processes still involve these catalysts, solid catalysts such as zeolites are now often the preferred catalysts. Olefins are generally employed for commercial alkylation reactions. The chemistry discussed next will involve liquid catalysts that are protonic acids or Friedel-Crafts catalysts. 

The synthesis of crystalline molecular sieve zeoHtes in hydrothermal systems involves the combination of the appropriate amoimts of aliuninates and sih-cates, usually in basic media, and usually in an aqueous medium. Syntheses generally will proceed at ambient or moderate temperatures, however, crystallization rates generally are much faster at elevated temperatures, approaching 100 °C, if pressures below one atmosphere are desired, and temperatures up to about 180 °C, if high pressure vessels are used. Most zeolites of commercial interest are metastable phases, requiring that synthesis processes be terminated at some predetermined time to avoid contamination of the solid product with denser undesirable phases. 

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