SCIENCE/TECHNOLOGY

Alternative Fuels to Petroleum Gain Increased Attention

Oxygenates are most promising of alternative fuel candidates, and Fischer-Tropsch process gaining new prominence

Joseph Haggin, C&EN Chicago

With the idea of alternative fuels development regaining currency, a substantial change in viewpoint about how and why fuels are pro­duced may be taking place. Eco­nomics remains a principal consid­eration. But national security, the inexorable decline in petroleum re­serves and rise of other hydrocar­bon sources, and the issue of envi­ronmental protection have emerged as being of more than academic im­portance in the search for alterna­tives to petroleum. Problems of de­veloping appropriate fuels for new kinds of vehicles and new fuels for old kinds also exist.

Oxygenates, either neat or as ad­ditives to fuels, appear to be the principal alternative fuel candidates beyond the petroleum refinery, d chemistry, on the back burner fol­lowing the oil price drops of a few years ago, may be restored to some prominence as a means both for new fuels and for organic chemicals.

Promoting the idea of alternatives to petroleum, the U.S. Alternative Fuels Act of 1988 responded to the potential threats from more than 95% of U.S. fuel requirements being satisfied from petroleum and from nearly half of U.S. petroleum being imported. More recently, develop­ment of alternative fuels—methanol for powering automobiles—was ad­vocated by President Bush. Further underscoring interest in this area is a Department of Energy study on

alternative fuels scheduled to be is­sued by the end of this year.

The panic that followed the Arab oil embargo of 1974 produced little in the way of alternative fuels, but it did highlight some of the prob­lems of producing them. Probably the greatest impediment to launch­ing a new era of alternative fuels is the complete dependence of the transport fuels industry on petrole­um as a feedstock. The dominance of petroleum has skewed the eco­nomics of production in its favor.

Nevertheless, over the past few years, significant developments in catalytic chemistry have suggested that alternative fuels from sources other than petroleum are possible and practical. Some of them are even now competing with petroleum-based fuels in some markets.

The results of the forthcoming DOE study are already known, in large part, from preliminary status reports. One authoritative view of the study's conclusions is that of G. Alex Mills, senior scientist in the

Center for Catalytic Science & Tech­nology at the University of Dela­ware. He suggests the report will likely show that methanol may be­come a serious contender as motor fuel and that a number of other alcohols and ethers will also enter the picture. These oxygenates can be produced with high-octane prop­erties, can be produced from vari­ous hydrocarbon sources other than petroleum, and frequently can of­fer environmental advantages.

The oxygenates, says Mills, are the best alternative fuels so far. Con­trary to some opinions, he believes that the problems of changeover from gasoline to oxygenates are fore­seeable and soluble, although there would be problems, and any change would cost something and be irri­tating to many people. Methanol would probably be the alternative fuel most difficult to accommodate. Higher alcohols and ethers would be easier. Alcohol fuels would re­quire higher engine compression ra­tios, and fuel tanks would need to

Current alternative fuels come from synthesis gas

fuels & chemicals

Ci-Ce alcohols *

CH4 «

Isoparaffins <4

[ Gasoline

Diesel fuel +

Chemicals

Gasoline Diesel fuel

Benzene/toluene/xylene

¥ Methyl te/f-butyl ether

Turbine fuel Auto fuel additives

Fuel cell feed • Ethanol * Basic auto fuel

: Near-commercial process

All the products are derived from the catalytic conversion of syngas (carbon monoxide and hydrogen), either directly or via hydrogen.

August 14, 1989 C&EN 25

Science/Technology

Metal-catalyzed F-T reactions generate many products

On the basis of the latest research from South Africa's SASOL opera­tion, the following mechanism has been suggested for the classical Fischer-Tropsch chemistry:

CH3OH -*

RC-C=0 * \ x H

H H RC-C-OH

H H H

RC-COOH H

be larger to accommodate the lower energy density of alcohol fuels.

It is expected that oxygenates would assist in suppressing vehicle pollutants in air. The effects are somewhat seasonal, with higher val­ues in some pollutants being expe­rienced in cold weather. In a trial program, Colorado required that mo­tor fuels containing 1.5% oxygen be used during the past winter. This was done by addition of ethanol and methyl-fert-butyl ether (MTBE). The level of carbon monoxide in the air dropped, and the program is being continued in the coming win­ter with required oxygen content being raised to 2%.

Combustion of methanol is claim­ed to produce undesirable quanti­ties of aldehydes in vehicle engine exhausts. However, it has also been found that the aldehyde concentra­tion can be diminished to accept­able levels by exhaust converters. Mills believes that the problem can be overcome without very much ad­ditional development. The benefits of reduced carbon monoxide and nitrogen oxide concentrations with alcohol fuels are well established.

Of all the oxygenates, MTBE is inherently attractive for technical reasons. It has a low vapor pres­sure, can be blended with other fuels without phase separation, and has desirable octane characteristics. Methyl tert-butyl ether has been used in motor fuels for about 15 years, beginning in Europe but growing rapidly in recent years in the U.S. Present U.S. production of MTBE exceeds 3 million gal per day.

If oxygenates achieve recognition as vehicle fuels, the biggest con­tributor will probably be methanol, production of which is mostly from synthesis gas derived from meth­ane. The process is catalytic, and currenty a great deal of research is in progress to develop a catalyst and process for the direct conver­sion of methane to either methanol or to ethylene, which could sub­sequently be hydrated to methanol. The production economics of meth­anol are peculiar in that imported methanol prices have been drop­ping while domestic prices have ris­en. This is attributed to the very low natural gas prices that a num­

ber of foreign manufacturers take advantage of in producing methanol.

The higher alcohols offer some potential as motor fuels, but their successful production awaits better catalysts. Development of such cat­alysts has mostly been discontinued because of the falling oil prices, but recently interest has been renewed, largely because of some significant developments in catalyst design.

Most of the catalytic development is focused on new active copper-containing catalysts, which can be promoted with rubidium, cesium, and potassium. Potassium is used because of its lower cost, and the catalysts can be made highly selec­tive to methanol over higher alco­hols. Higher alcohols can be pro­duced at temperatures below 300 °C, using copper/zinc oxide/alu­mina catalysts promoted with po­tassium. Although it is currently cheaper to make isobutyl alcohol from by-product isobutylene, it can be synthesized from syngas with alkali-promoted zinc oxide catalysts at temperatures above 400 °C.

Isobutyl alcohol is of particular interest because of its high octane rating, which makes it desirable as a gasoline blending agent. It can also be catalytically reacted with methanol to produce MTBE. In any case, consumption of MTBE is grow­ing steadily whatever its source. There seems to be little doubt that eventually syngas will be the prin­cipal source.

It is the potential value of syngas as a source of motor fuels and chem­icals that is expected to make it the first major challenger of petroleum. Syngas can be made from any car­bon source, but the current choice is methane. Discoveries of major nat­ural gas resources offshore in New Zealand, Australia, South Africa, and the Gulf of Siam, as well as smaller finds offshore in China and the giant onshore gas reserves in Saudi Arabia, have considerably altered the perception of motor fuel feed­stocks worldwide.

Any challenge to petroleum by syngas will be the result of success­ful exploitation of catalytic Q chem­istry, with production of methanol using the active Cu/ZnO/alumina catalysts being the classic example. The structure of these catalysts has

recently been determined to involve copper crystallites dissolved in zinc oxide crystallites in solid solution. The solution forms the active phase suspended in amorphous alumina.

The mechanism is still under de­bate. Some favor initial union of carbon monoxide from the syngas with the catalyst by adsorption of either the carbon or the oxygen. A third possibility is that carbon di­oxide is formed first, before the re­action can proceed.

Carbon monoxide hydrogénation is highly exothermic. This limits per-pass conversion and usually requires high recycle of the reactant feedgas. It also requires separation of un-

26 August 14, 1989 C&EN

reacted feed from some products or staging of the process with inter­mittent heat removal. For these rea­sons, methanol winds up as an en­ergy consumer.

To minimize or eliminate some of these problems, a slurry catalytic process is under development to per­mit better temperature control and greater conversion per pass. The highly proprietary process is ru­mored to be nearly ready for com­mercialization. In the long term, the methanol process may involve syn­gas generation, methanol synthesis, and electricity generation in a com­bined cycle. Union Carbide has pro­posed such an integrated process in connection with its new methanol process.

Higher alcohols and ethers are formed in methanol synthesis with alkali promoters, with appreciable amounts of isobutyl alcohol being produced by the copper catalysts used for methanol. The effects of the usual promoter, potassium, are at­tributed to a basic character that favors formaldehyde condensation. Another octane enhancer is terf-amyl-methyl ether (TAME), which is made from methanol and isoamylene.

Other than with the zinc oxide-based catalysts, methanol can be syn­thesized using alkali alkoxide cata­lysts. However, the economics are currently unfavorable because of separation problems by this route. Of great theoretical interest are tran­sition-metal complexes used in ho­mogeneous systems. Metallic com­plex catalysts may also be used for the reduction of the carbon oxides in the manufacture of hydrocarbons.

Metal clusters have been of inter­est catalytically for a long time. The clusters contain more than one tran­sition metal. An example of this type of chemistry is the production of ethylene glycol from syngas via rho­dium catalysis. Though clusters have yet to be widely accepted as profi­cient industrial catalysts, they are being thoroughly investigated.

The Fischer-Tropsch (F-T) reac­tions and the water-gas shift reac­tion are the basis of today's Q chem­istry. These reactions were devel­oped many years ago, and, with a few exceptions, have been supplant­ed with chemical technology based on petroleum. However, a new era

for this kind of chemistry may be in the making for the manufacture of hydrocarbon fuels.

F-T reactions are catalyzed by iron, cobalt, nickel, and ruthenium. Iron is less active than cobalt in the hy­drogénation of carbon monoxide and produces more olefins and al­cohols. Ruthenium produces high­er hydrocarbons but is very expen­sive. Only iron displays any activi­ty in the water-gas shift reaction, where carbon monoxide and water react to form additional hydrogen. The water-gas shift reaction is es­sential in controlling the composi­tion of syngas as well as for being a source of hydrogen.

Classical F-T reactions have not been selective ones, and much of the recent development in F-T chemistry has been aimed at im­proving catalyst selectivity. In ad­dition to the above-mentioned cata­lysts, nitrides, carbides, and carbo-nitrides have been shown to be po­tential F-T catalysts. These are of particular interest because of their resistance to coking and oxidation. Raney iron-manganese is another active F-T catalyst. The problems of sulfur poisoning in F-T chemistry may be partially overcome by using deliberately sulfided bimetallic clus­ters and alloy catalysts, such as amor­phous nickel/zirconium.

A modern classic chemistry for the production of hydrocarbon fuels is the Mobil MTG (methanol-to-gasoline) process. This process pro­duces high-octane gasoline from methane via methanol intermedi­ates. The hydrocarbons are formed from the methanol using ZSM-5 ze­olites. The first commercial plant, built in New Zealand, utilized fixed beds, but Mobil and a West German partner also have successfully piloted a fluid-bed process. The original MTG process manufactured the methanol as a feedstock for the pro­duction of hydrocarbons in two steps, but there have been several attempts to integrate the syngas/ methanol/hydrocarbon production in a single process train.

The improvement of syngas con­version depends on the development of catalysts that are more selective, more active, and, for economic rea­sons, of less noble character. Meth­ane conversion to syngas is effi­

cient, and there has been consider­able interest in syngas generation from such sources as coal and shales. Gasification of coal is still plagued by high syngas costs. Suitable gas­ification catalysts are under devel­opment, notably at Lawrence Berke­ley Laboratory. However, a long-held desire has been the direct conversion of methane without the necessity of intermediate syngas.

Short of direct conversion of methane, there is a growing con­viction among research chemists that the direct conversion of syngas to fuels, without the necessity of in­termediate methanol, is a closer goal. In this regard, zeolites offer prom­ise as support for conventional cat­alysts in F-T syntheses. Improve­ments in F-T chemistry at South Africa's SASOL coal conversion op­eration produced a slurry reactor that uses finely divided catalyst suspended in oil, which produces a higher conversion per pass than con­ventional reactors. Another varia­tion in F-T chemistry is the up­grading of F-T hydrocarbons with ZSM-5 catalysts. Isoparaffins can be produced in a variation of the F-T synthesis, which operates under more severe conditions and uses a thoria catalyst. The best catalysts are rather expensive rare metals.

Economic studies at SASOL indi­cate that the cost of fuels from syngas via F-T chemistry is domi­nated by syngas production. Some 58% of the cost of production at SASOL is devoted to syngas from coal gasification. The F-T synthesis itself consumes 18%, product sepa­ration 12%, and further refining 10%. This is, obviously, the Achilles' heel of Ci chemistry from coal.

Educated speculation by Mills and others indicate that in the near term the R&D effort in alternative fuels should be channeled to several spe­cific areas. Integration of the F-T/ MTG process, integration of the MTG process itself by developing a multifunctional catalyst to permit the use of a single reactor for both methanol production and conversion to hydrocarbons, and direct conver­sion of methane to Q chemicals are some of the projects of immediate concern. Longer term, the potential of metal complexes and clusters and biocatalysts should be exploited. D

August 14, 1989 C&EN 27