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Based on the advantages offered by the LPDME process, dimethyl ether has begun to attract considerable attention in the petrochemical industry for a variety of applications. The usage of DME as an alternative fuel is being tested in Europe, Asia and North America. DME can also be used as a starting material towards the synthesis of various hydrocarbon and oxygenate products. Here, we present an overview of our research dealing with DME conversion to targeted hydrocarbons and value-added chemicals. DME conversion to hydrocarbons, lower olefins in particular, is studied using ZSM-5 catalysts with varying SiO2/Al2O3 ratios, whereas the DME carbonylation reaction to produce methyl acetate is studied over a variety of group VIII metal substituted heteropolyacid catalysts.
An overview of the single-stage DME synthesis from syngas as well as its rationale is provided followed by the subsequent conversion of DME into value-added chemicals as follows:
The liquid phase methanol synthesis process was originally developed by Chem Systems, Inc. In the liquid phase methanol LPMeOHTM process, syngas (typically a mixture of carbon monoxide, hydrogen, carbon dioxide) reacts over the active catalyst (Cu/Zn/Al2O3) dispersed in an inert oil medium . This process offers considerable advantages over the conventional vapor phase synthesis of methanol in the areas of heat transfer, exothermicity, and selectivity toward methanol. However, this process suffers from the drawback that the methanol synthesis reaction is a thermodynamically governed equilibrium reaction. Methanol concentration in the liquid phase in the vicinity of the catalytic sites is quite high due to its low solubility. Thus, the productivity of the liquid phase methanol synthesis as well as the conversion of syngas could be limited by the chemical equilibrium barrier caused by high local methanol concentration in the liquid phase. One of the routes to alleviate this limitation is the in-situ dehydration of methanol into dimethyl ether (DME). Co-production of DME along with methanol significantly improves the methanol reactor productivity. Two functionally different yet compatible catalysts are used in this dual catalytic mode of operation. This single-step, liquid phase synthesis of DME from coal or natural gas based syngas is extremely significant from both scientific and commercial perspectives. Several key advantages of this process over methanol synthesis include higher methanol reactor productivity, higher syngas conversion, and lesser dual catalyst deactivation and crystal growth.
Synthesis of dimethyl ether from syngas (the LPDME process) can be carried out in the liquid phase at moderate temperature and pressure, 250 oC and 1000 psi. This single-stage process involves dual catalysts slurried in a liquid oil medium (Witco-40). The bi- functional catalyst consists of a mixture of methanol synthesis catalyst (Cu/ZnO/Al2O3) and methanol dehydration catalyst (gamma-Alumina). The process chemistry is shown in Equations (1-3).
The single-stage, liquid phase DME synthesis process is very significant from both scientific and commercial perspectives. The LPDME process reduces the chemical equilibrium limitation that could be encountered in methanol synthesis from syngas, especially in the areas of catalyst activity, per-pass conversion and reactor productivity. Thus, the usefulness of DME as a starting material for other industrially important petrochemicals stems, in part, from the inherent advantages in its synthesis from syngas. DME can be effectively converted to gasoline-range hydrocarbons, lower olefins, and other oxygenates.
The process details are published in:
Dimethyl ether (DME) has been increasingly used as a propellant in aerosol formulations to replace chlorofluorocarbons, which are found to destroy the ozone layer of the atmosphere. DME is nontoxic and easily degrades in the troposphere. Although about 90% of the major current U.S. aerosol industry uses hydrocarbon-based propellants (mostly iso-butane and propane), DME could become a more widely used propellant in the next five years. Several aerosol-based household products include colognes, hair sprays and dyes, personal care mousses, antiperspirants, and room air fresheners.
DME has very promising uses as an ultra-clean transportation fuel as well as a fuel for power generation. DME has a high cetane value of about 55-60 and can be directly and effectively used for DME diesel engines. Burning DME in diesel engines results in a lower NOx with no SOx, thus also contributing to the societal air quality. Serious efforts in these areas are seen in Europe and Japan.
DME is an useful intermediate for the preparation of many important chemicals, including methyl sulfate. Dimethyl sulfate is an important commercial commodity as a solvent and also as an electrolyte in high energy density batteries.
It may be used directly as a transportation fuel in admixture with methanol or as a fuel additive. In particular, dimethyl ether is shaping up as an ultraclean alternative fuel for diesel engines. The advantages of using DME are ultralow emissions of nitrogen oxides (NOx), reduced engine noise or quiet combustion, practically soot-free or smokeless operation and hence no exhaust aftertreatment, and high diesel thermal efficiency.
Dimethyl ether is also an essential intermediate in the synthesis of hydrocarbons from coal or natural gas derived syngas. Lower olefins like ethylene and propylene or higher molecular weight compounds such as gasoline range boiling hydrocarbons are produced from syngas using dimethyl ether as an intermediate. A variety of specialty industrial chemicals such as oxygenates, acetaldehyde, acetic acid, ethylene glycol intermediate like DMET, etc. can be formed using dimethyl ether as a feedstock.
A DME Conversion Experimental Process Unit has been designed and set up which includes a fixed bed reactor as well as a fluidized bed reactor. Analyses are carried out on feed and product gases as well as liquids using gas chromatography and mass spectrometry.
A process that can convert dimethyl ether to gasoline-range hydrocarbons or to lower olefins over zeolite catalysts has been developed. This process, which received a U.S. Patent U.S. 5,459,166, is very efficient, when coupled with a single-stage DME synthesis process, such as the one developed by this Laboratory [EPRI Report TR-100246]. The conversion of dimethyl ether to hydrocarbons consists of the olefin formation reaction, ethylene and propylene in particular, and the olefin conversion reaction to form aromatics and paraffins by cyclization and hydrogen transfer reactions respectively as shown below:
CH3OCH3 ---------> C2-C4 Olefins ---------> Aromatics+ Paraffins
Selectivity towards light olefins can be enhanced by using low acidity catalysts (high SiO2/Al2O3 ratios) and optimum operating conditions such as temperature, partial pressure and space velocity of DME. Zeolite catalysts have pores and channels of molecular dimensions that impose spatial constraints on reactants/products of the reaction. Shape selectivity is an important property in terms of product distribution as well as the catalyst activity. Zeolites exhibit product shape selectivity, which involves the limitation of diffusion of some of the hydrocarbon products out of the pores thereby enabling a tailored product spectrum. Another important type of selectivity is the transition state shape selectivity that deals with constraints toward the formation of transition states based on molecular size and orientation. This property hinders the formation of bulky molecular compounds that are the coke precursors, thereby hindering catalyst deactivation.
ZSM-5 is the primary catalyst used in this study, with varying SiO2/Al2O3 ratios of 30, 50, 80, and 150. These ratios are indicative of acidity of the catalyst, which is also an important property in determining the product distribution and catalyst lifetime. In particular, the focus is on the H-ZSM-5 with a SiO2/Al2O3 ratio of 150, since it has been previously determined that this is the best choice of catalyst for maximum olefin selectivity. This catalyst has a surface area of 425 m2/ g and its internal structure consists of straight channels of size 5.1 x 5.5 Angstroms as well as sinusoidal channels of size 5.4 x 5.6 Angstroms. At an operating temperature of 400 oC, atmospheric pressure, and a DME space velocity of 20 hr-1, selectivities towards C2-C4 olefins as high as 60 wt.% are observed. These reactions were carried out for 200 hours using the same catalyst without any noticeable change in the product distribution, thereby proving the resistance of small pore sized zeolite towards deactivation by coking.
The DME-to-Hydrocarbons process as well as the single-stage DME synthesis process can be ingeniously coupled with a variety of biogas generation processes, resulting in a process that may be classified as a “Green Gasoline Technology.? Recent efforts by this Laboratory in this area include the utilization of low-quality syngas as well as carbon dioxide rich feed streams. The technology can be ideally adopted for gasoline-range hydrocarbon synthesis starting from biological, mixed, or even waste feedstocks.
The significance of the present study of DME carbonylation is multi-faceted. First, it examines the potential for long-term production of methyl acetate at conditions, which are favorable from an economic, safety and process engineering standpoint. This reaction is carried out at a low temperature of 225 oC and atmospheric pressure. Also, it makes use of a relatively new catalyst which is multi-functional, containing both acidic and carbonylation activity. Finally, an alternate route to methyl acetate would be highly valuable to the chemical industry. Methyl acetate may be further carbonylated to acetic anhydride, which can be converted to acetic acid, a highly valuable chemical for the production of other petrochemicals. Methyl acetate may also be converted to ethylidene diacetate (EDA), a precursor to vinyl acetate and polyvinyl acetate. The DME carbonylation reaction is shown below:
CH3OCH3 + CO --------->CH3C(O)OCH3
In the present study, conversion of dimethyl ether and carbon monoxide to methyl acetate is investigated over a variety of group VIII metal substituted phosphotungstic acid salts. Experimental results of this catalytic reaction using rhodium, iridium, ruthenium and palladium catalysts are evaluated and compared in terms of selectivity toward methyl acetate. The effects of active metal, support types, multiple metal loading, and feed conditions on carbonylation activity are examined. Finally, the differences in the reaction pathway for methyl acetate production from dimethyl ether versus methanol are compared. It has been observed that iridium metal substituted phosphotungstic acid supported on Davisil type 643 (pore size 150 Angstrom) silica gel possesses the highest activity for DME carbonylation. An important feature of the Ir based catalyst, which makes it superior to the other ones, is its higher CO conversion. In this case, CO conversion levels start at 10% and drop steadily over time to around 2% over 5.5 hours. The formation of hydrocarbons is pronounced as well. Methyl acetate selectivity starts off lower at a value of 80% due to the increased hydrocarbon production, but also drops rapidly over the 5.5 hour time on stream.
Dimethyl ether has evolved and blossomed into a very promising building block in the fuel and petrochemical industry. Its attractive features as a fuel, both in terms of energy content and clean burning characteristic, very candidly proclaim that the utilization of DME is here to stay. Also, ongoing research directed towards the consumption of DME for the synthesis of value-added chemicals is only the tip of the iceberg. DME has very realistic potential to be a major raw material in the chemical industry, significant enough to be harnessed. As with any other feedstock, DME's actual usage will ultimately be dictated by economics. But, it is heartening to note that the increasing utilization and market for DME will favor it's cheaper production in a single-stage from syngas. Further, DME is the closest companion chemical to methanol in a scenario of "Methanol Economy"