Today, methanol is one of the most important feedstocks for the chemical industry. Most of the 32 million tonnes of methanol produced yearly are used for the production of a large variety of chemical products and materials, including such basic chemicals as formaldehyde, acetic acid and methyl-tert-butyl ether (MTBE; which is, however, phased out in most of the United States), as well as various polymers, paints, adhesives, construction materials and others. In processes for the production of basic chemicals, raw material feedstocks constitute typically up to 60-70% of the manufacturing costs (Fig. 13.1). The cost of feedstock therefore plays a significant economic role. In the past, for example, acetic acid was predominantly produced from ethylene using the Wacker process, but during the early 1970s Monsanto introduced a process which, by carbonylation of methanol using a Wilkinsons rhodium-phosphine catalyst and iodide (in the form of HI, CH3I or I2), produces acetic acid with a conversion and a selectivity close to 100%. Because of the high efficiency of this process and the lower cost of methanol compared to ethylene, most of the new acetic acid plants built worldwide since then are based on this technology . Taking advantage of its lower cost, methanol is also considered as a potential feedstock for other processes currently utilizing ethylene. Rhodium-based catalysts have been found to promote the reductive carbonylation of methanol to acetaldehyde, with selectiv-ities close to 90%. With the addition of ruthenium as a co-catalyst, the further reduction of acetaldehyde to ethanol is possible, providing a new catalytic route for the direct conversion of methanol into ethanol. The possibility of producing ethy-lene glycol via methanol oxidative coupling instead of the usual process using ethylene as a feedstock is also pursued. Significant advances have also been made on the synthesis of ethylene glycol from dimethyl ether, obtained by methanol dehydration. In addition to acetic acid, acetaldehyde, ethanol and ethylene gly-col, other large-volume chemicals produced currently from ethylene or propylene such as styrene and ethylbenzene may be also manufactured from methanol in the future.
248 | Chapter 13 Methanol-Based Chemicals, Synthetic Hydrocarbons and Materials Methanol Conversion to Olefins and Synthetic Hydrocarbons
While methanol can replace light olefins (ethylene and propylene) in some applications, these compounds will remain indispensable building blocks for synthetic hydrocarbons and other products. Ethylene and propylene are by far the two largest volume chemicals produced by the petrochemical industry. In 2004, about 105 million tonnes of ethylene and 60 million tonnes propylene were consumed worldwide. They are important starting materials in the production of plastics, fibers and chemical intermediates such as ethylene oxide, ethylene dichloride, pro-pylene oxide, acrylonitrile and others. The demand for light olefins however, is primarily driven by the polyolefin production. Today, almost 60% of ethylene and propylene is consumed in the manufacture of polyethylene (low-density polyethylene LDPE, high-density polyethylene HDPE, etc.) and polypropylene resins. The majority of light olefins are currently produced by the petrochemical industry as a byproduct of steam cracking and fluid catalytic cracking (FCC) of naphtha and other gas liquids. The demand for propylene is growing at a faster rate than that for ethylene; about 6% per year compared to 4% per year for ethylene. The reason for the higher growth in propylene demand is the increasing popularity of polypropylene, which is being substituted for many other materials and more expensive polymers, especially for automobile parts. Because of the high cost of transporting light olefins by sea, and the proximity of downstream markets, they are mainly produced in North America and Western Europe. With lower feedstock costs, a large part of the new production capacity for light olefins, however, will be installed in the Middle East. Through steam cracking, light hydrocarbons, especially ethane, produced in association with crude oil or from natural gas in that region, yields predominantly ethylene. Given the higher growth rate in propylene demand, this could lead to supply problems. Steam cracking and fluid catalytic cracking will not be able to cover the expected demand for pro-pylene in the coming decade. Thus, the balance will have to be supplied by other sources including propane dehydrogenation, metathesis, olefin cracking and significantly new methanol to olefins (MTO) technologies. MTO can also provide a part of the demand for ethylene. Because methanol is presently mainly produced from natural gas (through syn-gas), it can decrease the dependence of ethylene on petroleum feedstocks. Considering the very large market for ethylene and propy-lene, these applications will also substantially increase methanol demand, calling for the construction of numerous mega-methanol plants each able to produce between 1 and 3.5 million tonnes of methanol per year. With further new technologies to produce methanol - most significantly via the hydrogenative conversion of CO2 - the methanol to olefins processes already in commercial development will gain increased significance, allowing the production of these light olefins from non-fossil sources and their subsequent conversion to synthetic hydrocarbons and their various products.
More than a century ago, LeBel and Greene first reported the formation of gaseous saturated hydrocarbons (and some hexamethylbenzene) by adding methanol dropwise to "hot" zinc chloride. Under high pressure, significant amounts of light hydrocarbons were formed when methanol or dimethyl ether was reacted over zinc chloride at about 400 °C. It was later also reported that when methanol was reacted with zinc chloride or zinc iodide at 200 °C, a mixture of C4-C13 hydrocarbons containing almost 50% 2,2,3-trimethylbutane (triptane, an excellent high-octane jet fuel) was obtained . Other catalysts, including phosphorus pentox-ide, polyphosphoric acid, and later tantalum pentafluoride and other superacid systems, have also been reported for the synthesis of hydrocarbons from methanol. Most of the described catalysts, however, deactivate rapidly. It was only during the 1970s that researchers at the Mobil Oil Company discovered that an acidic zeolite called ZSM-5 was able to catalyze the practical conversion of methanol to both olefins (MTO process) and hydrocarbons in the gasoline range (MTG process) [231-233].
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