Methanol to Olefins and Aromatics

Methanol to Olefins and Aromatics 525 Table 15.2 Comparison of SAPO-34 and SSZ-13 (chabazite) catalysts for MTO. 

When methanol is reacted on HZSM-5 zeolites, and hydrogen transfer and cyclization is avoided, then instead of producing aromatic gasoline, the process is directed to the production of light olefins (213,214). Different 8 MR zeolites (215,216) have been used to convert methanol to olefins and the results obtained (217) are compared in Table 13 with those obtained with ZSM-5 zeolite. 

Dimethyl ether is an important intermediate in several proeesses eonverting Cl feedstoeks to liquid fuels or ehemieals. DME may also be used as an alternative propellant for aerosols [67]. It is well known that DME ean be produced from methanol over acid dehydration catalysts under relatively mild conditions. Most of the investigations reporting DME formation are related to olefin and oliie production via the Mobile methanol-to-olefins and methanol-to- oline processes, discussed in Chapter 4. In both cases, elevated pressures (13 MPa) and temperatures above 300°C are used to maximize olefin and aromatic yield. In this section DME production from methanol is considered in the context of producing DME as a useful chemical rather than as an intermediate in a Mobil process. 

Both Lewis and Brdnsted acidity are involved in the dehydration reactions over acid catalysts, and selectivity control to limit the dehydration of DME to olefins and aromatics requires that the surface acidity not be too hi and the reaction temperature be below 300°C [65]. The olefins are generally thou t to be produced by a consecutive reaction in which methanol is first converted to DME, which in turn is converted to olefins and aromatics. Reaction mechanisms for DME formation have been proposed by various investi tors. According to Kubelkova et aL [78], the mechanism over Si-Al zeolites involves protonation of the hydroxyl group of methanol on a Bronsted acid site to form a skeletal methoxyL This methoxyl group reacts with a -phase methanol molecule to form DME at 180300°C and C2C5 aliphatics and aromatics above 300°C. According to these authors, Lewis acid sites (Al -OH), associated with nonskeletal alumina, can also form methoxyls according to the reaction... 

One of the most important challenges in the modern chemical industry is represented by the development of new processes aimed at the exploitation of alternative raw materials, in replacement of technologies that make use of building blocks derived from oil (olefins and aromatics). This has led to a scientific activity devoted to the valorization of natural gas components, through catalytic, environmentally benign processes of transformation (1). Examples include the direct exoenthalpic transformation of methane to methanol, DME or formaldehyde, the oxidation of ethane to acetic acid or its oxychlorination to vinyl chloride, the oxidation of propane to acrylic acid or its ammoxidation to acrylonitrile, the oxidation of isobutane to... 

Since pyrolizing gasifiers yield olefins and aromatics in the raw gas, these compounds tend also to occur in the acid gas stream. Physical solvent processes for acid gas extraction, such as cold methanol wash, especially tend to take hydrocarbons into the acid gas stream water solutions have less tendency to do so. 

The methanol transformations discussed precedingly can be modified to produce high amounts of light alkenes.437 454 474 475 The key to achieve this change is to prevent C2-C4 olefinic intermediates to participate in further transformations. Such decoupling of alkene formation and aromatization can be done by the use of small-pore zeolites or zeolites with reduced acidity. Reduced contact time and increased operating temperature, and dilution of methanol with water to decrease methanol partial pressure, are also necessary to achieve high alkene selectivities. This approach has led to the development of the MTO (methanol-to-olefin) process, which yields C2-C5 alkenes with about 80% selectivity. 

Many substances can be partially oxidized by oxygen if selective catalysts are used. In such a way, oxygen can be introduced in hydrocarbons such as olefins and aromatics to synthesize aldehydes (e.g. acrolein and benzaldehyde) and acids (e.g. acrylic acid, phthalic acid anhydride). A selective oxidation can also result in a dehydrogenation (butene - butadiene) or a dealkylation (toluene -> benzene). Other molecules can also be selectively attacked by oxygen. Methanol is oxidized to formaldehyde and ammonia to nitrogen oxides. Olefins and aromatics can be oxidized with oxygen together with ammonia to nitriles (ammoxidation). 

Substrates which can undergo partial oxidation are characterized by a 7T-electron system or unshared electrons olefins and aromatics contain the first, methanol, ammonia and sulphur dioxide the second. Alkanes do not contain such electrons. Their selective oxidation appears to demand (thermal or catalytic) dehydrogenation to alkenes as the initial process. 

Metals such as Fe, Co, Ni, or Ru on alumina or other oxide supports convert CO and H2 to hydrocarbons. Using different catalysts and reaction conditions either CH4, liquid hydrocarbons, high molecular weight paraffins, methanol, higher alcohols, olefins, and aromatics can be obtained, though rarely (with the exception of CFL, and methanol) with high selectivity. Hydrocarbons typically exhibit a Schulz-Flory type molecular weight distribution. 

All workers agree that the products are formed by an initial dehydration step and Derouane et al used n.m.r. and g.l.c. to study the products obtained from methanol and ethanol interaction with ZSM-5 zeolite in the range 250-400 °C. At temperatures <300 C the products were the usual respective ether and olefin, but when temperatures > 300 °C were used Ca-Cs-olefins and aromatics were obtained, the overall reaction being expressed as (Scheme 12) ... 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

Principal Characteristics. - Molecular sieves with pore openings of about 0.45 nm show very interesting shape-selectivity properties for the conversion of methanol to olefins (MTO process). The small-pore molecular sieves studied in the MTO process are chabazite, erionite, zeolite T, ZK-5, ZSM-34, zeolite A, SAPO-17, SAPO-34, and SAPO-44. All of them can sorb only straight chain molecules, e.g. primary alcohols and linear paraffins and olefins, but no branched isomers and aromatics the pore opening is smaller than the kinetic diameter of branched and aromatic molecules, but large enough to permit the access of linear molecules. 

Marchi and Froment observed that on dealuminated mordenite the selectivity changed with time on stream. The yield of C2-C4 olefins increased, while the yields of paraffins and aromatics dropped, even if the methanol conversion was maintained at 100% (see Figure 10). Coke would already be formed at early stages of the methanol conversion and would cover the strong acid sites, thus reducing the conversion of olefins to paraffins and aromatics. 

While the conversions were relatively high (up to 35%), the hydrocarbon selectivities were rather low (<42%). A similar study was carried out by Anderson and Tsai using H-ZSM5 and other metal and metal oxide catalysts over a wide range of conditions. They found that CO2 and CO were the only carbon products when Oj was used and methanol, olefins, and aromatics were produced when N2O was used. The best C2+ selectivity was 32% at 0.1% conversion. 

Table 6 shows the results for dehydration of methanol to olefins over MAPO-n (n=5, 11, 36), SAPO-11, and ZSM-5. The selectivity in Table 6 was calculated based on the carbon number. MAPO-5 and MAPO-11 form few hydrocarbons, and a large part of the product on these catalysts is dimethyl ether. On the other hand, hydrocarbons are the main products over SAPO-11 and ZSM-5 however, the product distribution is largely different for SAPO-11 and ZSM-5. The main product on SAPO-11 was aliphatic hydrocarbons with a carbon number higher than 5. In contrast to SAPO-11, the main product over ZSM-5 is lower paraffins and aromatics. These differences are also attributed to the low hydride transfer over SAPO-11 resulting from its mild acidity. 

Recent advances have shown zeolites are effective in catalysing the direct conversion of synthesis gas to motor fuels. The MTO (methanol-to-olefins) process converts MeOH to C2-C4 alkenes and is also catalysed by ZSM-5. The development of a gallium-modified ZSM-5 catalyst (Ga-ZSM-5) has provided an efficient catalyst for the production of aromatic compounds from mixtures of C3 and C4 alkanes (commonly labelled LPG). 

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