conversion ot
COAL GASOLINE to
onversion of coal to fluid fuels is approaching a time when synthetic gasoline and pipeline gas will be manufactured in the United States on an enormous scale. Reasons include the growing needs for energy, the large reserves of coal and the limited reserves of petroleum, new technological advances in the production of fluid fuels from coal, and the desire for a secure supply of petroleum based on national security needs and air pollution control made possible by manufacture of low sulfur fluid fuels. From 1975-80 there will be a critically widening gap between supply and demand for petroleum in the United States, although petroleum reserves are thought to be adequate for some time to come (56). However, reserves-to-consumption ratios are declining as new discoveries have not kept pace with demands. Nuclear power will play an increasing role in the energy supply of the United States but it will not fulfill the growing demands for a fluid fuel needed in the transportation sector of energy demand. Here, coal can and will fulfill a unique role. The consumption of energy in the United States has been rising rapidly and is expected to increase from 45 quadrillion Btu in 1960 and 60 in 1969 to a projected 85 in 1980 and 135 in 2000. T o supply these needs there are known recoverable reserves of fuels amounting to, in the same units of quadrillion Btu, 300 for petroleum, 300 for gas, 300 for uranium, and 4600 for coal. These amounts provide a rather limited supply of certain types of fuels; overall, coal represents about 80% of known recoverable fossil fuels. Large quantities of fossil fuel resources are believed to exist but are not yet discovered and are not included in the above reserves. Of coal, tar sands, and oil shale, each offers advantages and disadvantages. One plant manufactures petroleum products from tar sands (28). Shale-togasoline is under active research by both industry and the Bureau of Mines (55). Coal was converted to gasoline on a substantial scale in Germany during World War 11, and one plant is currently in operation in the Republic of South Africa using the Fischer-Tropsch process (47). Coal has certain advantages in terms of its wide geographic distribution and occurrence near large popula-
C
6
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion areas, well-established mining productivity, and the size of known deposits in the United States [2,860,000 million tons at depths of less than 3000 ft, about half of which is recoverable by current mining practices (7); another estimate is 400,000 million tons of recoverable coal at close to today’s prices]. Presently, U . S. coal production (550 million tons/year) is virtually as large on a weight basis as petroleum consumption (11 million barrels/day = 600 million tonslyear), although this amount of coal is about one half that of petroleum on an energy basis. Coal reserves and coal productivity are therefore sufficiently large to be well suited to supply petroleum needs. Recently a major revision in the ownership of coal companies has occurred so that petroleum companies now own coal companies which account for 20 to 25y0 of coal production; their ownership of preferred coal reserves is even larger. It is technically feasible to make highest quality gasoline from coal. Moreover, projected costs are approaching today’s costs for manufacture of gasoline from petroleum. The advent of modern petroleum refining catalysts can contribute much to the more efficient refining of crude oils derived from coal. The most applicable catalysts are cobalt and nickel molybdenaalumina for hydrogenation and hydrodesulfurization, silica-alumina cracking and hydrocracking catalystsparticularly those based on molecular sieves-and platinum-alumina for octane improvement. A further important development is the reduction in the manufacturing cost of hydrogen, although this is currently most applicable to its manufacture from petroleum. The physical disadvantages of coal relati\-e to petroleum are that it is a solid and has a high ash content. The fundamental chemical problem in the manufacture of gasoline from coal is the need to add hydrogen to coal. 0 while for The hydrogen content of coal is about 5 wt 7 gasoline it is about 14y0. Thus, without regard to the technical process used, coal conversion to liquid fuels will require an increase in the hydrogen content of coal. Hydrogenation is a serious problem from three viewpoints: (1) The cost of hydrogen is relatively high. The consumption of 5000 to 10,000 ft3 of hydrogen used to form a barrel of oil from coal corresponds to a cost of
The recent and rapid decrease of U.S. petroleum reserves has produced a need for new methods of fuel production. This paper presents new catalytic conceptsfor the conversion of coal to gasoline and offers a possible solution to a growing problem G. ALEX MILLS
Schematic model of coal structure VOL. 6 1
NO. 7
JULY 1969
7
.
hydrogen of $1.25 to $2.50, based upon a hydrogen cost of 25$/1000 ft3. (2) The investment for a hydrogen production and compression facility is large, amounting in some cases to one third the cost of the entire plant. (3) Hydrogenation occurs with difficulty by known processes. The process used for bituminous coal in Germany during World War I1 required costly operation a t pressures as high as 10,000 psi. Even with modern catalysts and techniques, hydrogenation must be carried out at about 2500 psi. Therefore, processes must be developed to lower the amount of hydrogen needed, the pressure of hydrogenation, and investment and operating costs for hydrogenation of coal to liquids. The alternative Fischer-Tropsch process for synthesis of gasoline from carbon monoxide and hydrogen requires the expensive steps of gasification and purification. Improvements in gasification, the most expensive process step, appear to depend on engineering innovations rather than catalytic inventions and will not be discussed at this time. The projected timing of the need for synthetic fuels has led some experts to predict there will be two types of coalto-gasoline plants: the first type will be an improved form of processes demonstrated in Germany during World War I1 and the U. S. BurMines process that proved operable on U. S. coals. The substantial effort made by the U. S. Office of Coal Research and contracting companies can be considered to be in this category (39). These include “Operation Gasoline” (73), the C.O.E.D. process (ZO), the H-Coal process ( I ) , the de-ashed coal project (48),and Seacoke (4). Fulfilling the need for synthetic fuels on a competitively sound economic basis requires certain technological accomplishments of a somewhat revolutionary nature. Further modification of current technology is likely to bring only minor cost improvements. Therefore, a second generation of coal-to-gasoline plant will depend on new ideas and discoveries which can provide the economic breakthrough required. New catalytic systems hold the most promise for such a breakthrough. They need to be invented especially for coal conversion, although conventional petroleum refining techniques may be employed in preparing the finished gasoline. Fortunately, many recent catalytic concepts have arisen
TABLE I .
C H 0 N S H/C atom ratio
which could contribute significantly to the development of second-generation coal-to-gasoline plants. This paper describes certain new concepts in catalytic chemistry which have potential for a major contribution to coal-to-gasoline technology. Most of these have been investigated at the U. S. Bureau of Mines in its exploratory basic research program and have not yet been developed into practical utility, although work on this phase is actively in progress. Chemical Composition
Structure
CHEMICAL COMPOSITION OF SOME COALS AND PETROLEUM
Anthracite
Medium volatile bit.
High volatile A bit.
High volatile B bit.
Lignite
93.7 2.4 2.4 0.9 0.6 0.31
88.4 5.0 4.1 1.7 0.8 0.67
84.5 5.6 7.0 1.6 1.3 0.79
80.3 5.5 11.1 1.9 1.2 0.82
72.7 4.2 21.3 1.2 0.6 0.69
Coal analysis on moisture- and osh-fTee basis. Ash content of c o d 3 to 15%. C-fraction aromatic = 0.7. Aromatic rings per cluster-not over 3. Hararn/HniiDh = 0.23. H / C atom ratio uf petroleum residua: asphallenes 7.78, resin 1.47, oil 1.67.
8
and
The compositions of the organic matter in several coals are compared with a few petroleum fractions in Table I. The chief distinguishing chemical feature of coal is its relatively low hydrogen content which would require the addition of about 9% hydrogen (5000 ft3/bbl gasoline) for sub-bituminous coal to be of the same composition as typical gasoline. The inclusion of toluene in the table points out its intermediate hydrogen content (8.7%), and by comparison it can be seen what is to be gained by converting coal to aromatic products rather than a “typical” gasoline (14YG hydrogen). The S, N, and 0 contents of coal, are often greater than 1 wt Yo each, and oxygen often is 2070 or higher for lignite. These heteroatoms are expected to cause problems in refining. Knowledge of the structure of coal has benefited enormously from recently developed spectrometric and chemical tools. About 70Ye of all carbon atoms are in aromatic rings, but only about 23Ye of hydrogen atoms are attached to aromatic carbon atoms. The explanation of this apparent discrepancy is that the aromatic structure is highly substituted (17). Consideration of this has led to formulation of the structure of a coal molecule shown in Figure 1. However, the number of aromatic nuclei in a cluster is not large, being a maximum of three (23). These are probably arranged more in the phenanthrene-type structure than the anthracene structure depicted in Figure 1. Figure 2 shows the structure of a resin molecule obtained by low-temperature carbonization and deduced by physical and chemical examination both before and after dehydrogenation (30). I t appears to be a fragment of the original coal with a structure typical of coal.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Petroleum crude 83-87 11-14 0.2 1. o 1.76
Gasoline 86 14
1.94
Toluene 91.3 8.7
1.14
Figure 1. Molecular structure proposed f o r coal (82% C )
Figure 2.
Hypothetical resin molecule
T h e way the molecules are arranged in the solid can also be significant. This has been depicted in the figure on page 7 (26) which illustrates the stacking of aromatic parts of the molecules in layers. The energy and kinetic requirements in unstacking (prerequisite to chemical transformations other than pyrolysis) can be important. Conversion Processes
.-
T h e conversion of petroleum crude to modern highquality gasoline, illustrated in Figure 3, requires three conversion steps to be compared with five steps in conversion of coal to gasoline (Figure 4). First, it is necessary to transform solid coal into a liquid form and second, to remove the ash. I n the third place, S, N, 0 removal and asphaltene transformation are required, usually by means of hydrogenation. Fourth, it is necessary to reduce molecular size-Le., carry out cracking. Frequently, for gasoline manufacture a fifth reforming process is necessary to increase octane. The three steps needed in converting petroleum to gasoline are still required: removal of s, N, 0; cracking (decrease in molecular weight) ; and reforming to convert low-octane to high-octane number hydrocarbons. Bringing the coal to a liquid condition is relatively rapid, but the product
TABLE I I .
Bergius I.C.I. I. G. Farben Brit. Fuel Res. Sta. U. S. BurMines Germany U. S. BurMines Japanese Union Carbide
200 450 435 400 440 440 440 410 500
I II - - - - - -
Figure 3. Conversion of petroleum to gasoline
formed is high in asphaltenes (polynuclear aromatic structures), and it is the hydrogenation of these which is slow. Catalysts that have been used in liquefaction are given chronologically in Table I1 (42). Catalysts used for coal conversion have been reviewed comprehensively (2, 79, 32, 57, 60-62). I n practice some overlapping of process steps of Figure 4 occurs in a single process. For example, the Germans and the U. S. Bureau of Mines carried out an operation in which liquefaction and a substantial
CONDITIONS FOR LIQUEFACTION OF COAL Pressure, atm.
Author or groufi
I _
250 200 215 250 700 700 123 240-420
Vehicle
Catabst
Yield
Date
Heavy recycle oil Hydrocarbon oil Recycle oil Heavy oil Tar oil Recycle oil Recycle oil None Recycle oil
Iron oxide
80
1925
Tin
70 70
1935 1935 1938 1941 1943 1949 1946 1953
... Sn(OH)2
..
SnS Iron oxide
90
Iron oxide ZnClz
..
..
VOL. 6 1
NO. 7 J U L Y 1 9 6 9
9
OCTANE IMPROVEMENT
Figure 4.
Conversion of coal to gasoline
amount of cracking and hydrogenation occurred. The catalyst was iron, although tin and molybdena are also known to be active. The so-called “light oil” (650°F EP) produced was then processed in Germany over a tungsten sulfide catalyst to remove S, N, and 0 followed by a hydrocracking step. Later, catalyst K536 was employed a t 10,000 psi to carry out these two steps in a single operation. I n some instances hydroreforming was performed over MoOS-Al203 catalyst to get higher octane gasoline. Tars derived from coal carbonization have long been refined for production of gasoline and aromatic chemicals. The primary justification of coal carbonization in the TJ. S. has been to produce coke needed by the
I
TABLE I I I.
COMPOSITION OF GASOLINES MADE W I T H VARIOUS CATALYSTS AND PROCESSING TECHN IQU ES. CREOSOTE 01L HYDROGENATION , B I LL I NGHAM
10
Two-stage
Two-stage
Two-stage
5053 and
5058band
Co-Mo-Ale
Co-iMo-Ale
Single stage
Catalqst
648 43 33 7 14
505Bb 3 50 31 16
7 54 33 6
15 48 32 5
80
68
74
76
87
89
Mo-cotolysl.
WSI.
Single stage
Two-stage
Process
Aromaticso Nap ht heneso Branched-chain paraffins0 Straight-chain paraffinsu Octane Number C.F.R. motor method Octane Number Research method 1 . 2 5 ml/TEL gal
w.
metallurgical industry or, in a few instances, to produce a smokeless fuel. T o further the utilization of coal tar, much work has been done to recover chemicals from the tar. The technology of refining oils from coking of coal is significant because the tars represent a product, and refining steps are, if not identical, a t least similar to those obtained by hydrogenolysis of coal. The technology of coal tars obtained by low- and high-temperature carbonization has been reviewed (76, 29, 58). Extensive laboratory and commercial work was carried out in England on hydrogenolysis of creosote oil. The advances made in producing higher octane gasoline using progressively improved catalysts are illustrated in Table 111. Ultimately a two-stage process was adopted
70% WS2 on HF-actrunted Fullers earth.
6434~
Fe on HFacliuated Fullerr earth.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
237d
and 2 3 1 d
and 76791
18
23
91.5
94.5
e Cobalt molybdate on acticated alumina.
”v o n d u a - a l u m n a .
Val.
TABLE
IV.
NEW HYDROGENATION CATALYTIC SYSTEMS
“Nascent”-active hydrogen generated in situ Complexes of transition metals Massive amounts of halide catalysts Organic hydrogen donor solvents Alkali metals (a) With Hz (b) With amines (c) Electrocatalytic Reductive alkylation Miscellaneous
employing a Co-Mo-Al2Os and a Ni-Si02-AI20~ catalyst (76). These catalysts are prototypes of those used today in hydrodesulfurization and hydrocracking of petroleum. Low-temperature carbonization of coal is not used commercially in the U. S. at present, although such plants are in operation in other countries, and the tars so produced are hydrorefined. In recent years, new processes for refining the lower boiling coal tar fractions have been reported and several commercial installations put into operation both in the U. S. and abroad. These are primarily for production of highpurity aromatics. T h e processes consist of hydrogenolysis over a chromia-alumina catalyst which accomplishes desulfurization, hydrocracking of paraffins, and some hydrodealkylation of alkyl aromatics (34). I n addition, thermal-i.e., free radical, hydroprocessing is carried out without a heterogeneous catalyst, for instance, for naphthalene production (53). Refer again to Figure 4-the bottom row describes processes which begin by treatment of coal with a hydrogen donor solvent. A vigorous attack on the problem of coal liquefaction along this line is being supported by the U. s. Office of Coal Research (39). Broadly speaking, extraction can be made as a separate step, as a t Cresap (operation gasoline), or in a kind of combination process in which extraction occurs in the presence of a boiling catalyst bed as in the H-coal process.
perature and pressure. The suggestion has been made that hydrogen, generated in situ by the water gas shift reaction between CO and H20, is in a n activated or nascent form. This accounts for its greater reactivity. Recently, this type of system was investigated using lignite coal in autoclave experiments, and indeed hydrogenation did proceed rapidly a t a relatively low temperature (Figure 5) ( 3 ) . Conversion of the coal to a benzenesoluble product was used to measure degree of hydrogenation. A striking feature of the solubilization of lignite with CO and HzO is the rapidity of the reaction; conversion to benzene-soluble seems essentially completed in 10 min. T h e reaction of lignite with H2 is slower. These reactions were carried out in 1-to-1 phenanthrene-a-naphthol solvent. At larger residence times, the extent of solubilization approaches but does H2O. Comnot reach the value obtained with CO pared with the original lignite, the benzene-soluble tar (87% conversion) has its hydrogen-to-carbon atomic ratio increased from 0.89 to 1.1 and the sulfur reduced from 0.7 to 0.2 wt yo; oxygen content is decreased from 21.7 to 5.6 wt % while the nitrogen content remains about the same. Conditions of hydrogenation were 1-to-1 lignite to water, 1500 psi initial (room temperature) CO pressure, and 2-hr residence a t 38OOC. Complexes of transition metals. Certainly one of the most, if not the most, significant development in the field of catalysis in recent years has been the discovery of a variety of new, and often unusual, catalytic reactions of transition metals and coordination complexes (24, 25, 36, 38). The catalytic properties depend upon the central ion and the number and character of ligands. Some of these catalysts are soluble and have therefore been called homogeneous catalysts. Knowledge of their
+
New Catalytic Concepts
Several novel catalytic systems for addition of hydrogen to coal, which have come to a large extent from the pioneering work a t U. S. Bureau of Mines, offer promise for the second-generation plants. They have not been developed into economical processes but further research work is in active progress. A list of these are given in Table IV. Nascent-active hydrogen generated in situ. A combination of carbon dioxide and steam can hydrogenate coal more rapidly and to a greater degree than does hydrogen itself under the same conditions of tem-
AUTHOR G. Alex Mills is Assistant Director of Coal Research
with the Bureau of Mines, U. S. Defiartment of the Interior, Washington, D. C. 20240.
Figure 5. Hydrogenation of coal by “nascent”-active generated i n situ VOL. 6 1
NO. 7
hydrogen
JULY 1969
11
electronic structure has enabled the establishing of catalytic mechanisms on a molecular basis. Kew and more active catalysts have been discovered, and some have a high degree of selectivity in terms of causing only certain molecules to react from a mixture and then to form only particular and sometimes unusual products. Establishing catalysis on a molecular basis and predicting catalytic properties have long been the objectives of catalytic chemists. Examples of this type of catalysis are found in the hydrogenation of olefins, the hydroformylation of olefins, the stereospecific polymerization of olefins and diolefins, and oxidation of olefins catalyzed by complexes of cobalt, ruthenium, rhodium, palladium, and platinum. T h e capability of such metal-ligands complexes to react with hydrogen to form active species and the existence of coordinatively unsaturated metal complexes are fundamental to the mechanism of their reactivity. One of the earliest and most important examples of their type of catalysts is cobalt carbonyl which, in the presence of CO Hz, is capable of hydroformylating olefins. Cobalt carbonyl is also active in hydrogenating certain aromatic compounds and coal (22). Dicobalt octacarbonyl, in the presence of carbon monoxide and hydrogen, functions as a selective homogeneous hydrogenation catalyst for polynuclear aromatic hydrocarbons (Figure 6). Isolated benzene rings are stable in this system. Naphthalenes are slowly reduced to tetralin. Linearly condensed compounds, such as anthracene, are readily hydrogenated a t the meso positions. Phenanthrene-type compounds are reduced very slowly a t 200" C to dihydro derivatives. More highly condensed systems are reduced to yield phenanthrene derivatives. I n most cases, only one reduction product is obtained. Coal was also treated with CO Hz in the presence of dicobalt octacarbonyl a t the same temperature, 200°C. Both Hz and C O were added to coal under these conditions. Thus, this is one of the most active catalytic systems ever observed for coal hydrogenation. A more recent study (4) confirmed and extended the aromatic model compounds and coal-derived products hydrogenated, as well as the variety of metal carbonyls which are active. Comparative tests were also carried out with Co-Mo-A1203 heterogeneous catalysts. Cobalt carbonyl operated a t a temperature as much as 100°C below that a t which cobalt molybdate was active. I t was less active in removing hetero atoms. Overall, cobalt carbonyl and cobalt molybdate show about equal effectiveness, per unit of hydrogen consumption, in producing a cracking stock from coal-derived tars. Hydrogen transfer agents and phosphine derivatives were ineffective in improving performance of cobalt carbonyl. While the cobalt carbonyl did not, in this study, result in a more effective catalyst than Co-Mo-AlzOa, its
+
+
12
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Figure 6. Hydrogenation with transition metal complex COa(C0)8, CO/H2 mole ratio = I , 3000psi, 752 to 200'C
ability to act a t lower temperatures and the specificity of products indicate this type of catalyst to have much promise in coal-conversion processes. Massive amounts of halide catalysts. The use of halide catalysts in amounts comparable to the coal or polynuclear hydrocarbons used has given some unusual hydrocracking results. Comparison of a SnClz/coal ratio of 0.01 and 1.O (Table V) (27) illustrates that while about 85y0of the coal was converted to benzene solubles in each instance, the asphaltene conversion-the difficult step-was nearly completed with the larger amount of catalyst. Similarly, large amounts of zinc chloride were effective, and this compound was superior to conventional hydrocracking catalysts for coal or coal extract. Zinc chloride gave more rapid reaction, more complete conversion than conventional catalysts, and a very high octane without reforming (54,63, 64). Compared with conventional catalysts, molten zinc chloride more than doubled the conversion and hydrogen consumption when using a hydrocracking residue as feed stock, even though a lower temperature was used. Noteworthy also is the high selectivity of the process for production of gasoline and the high ratio of isoparaffins to normal paraffins (Table VI) (63). -4 commercial process using metal halide catalyst must provide a viable method for regeneration of the catalyst. During the hydrocracking reaction, the zinc chloride is partially converted to ZnS and to ZnClz.Tc"s complexes. T h e regeneration has been investigated with partial success. Regeneration of the melt consists of removal of the bulk of the N, C, and S impurities and return of the melt as relatively pure ZnClz (64). M'hen aluminum chloride is used in high concentrations, it can bring about a high degree of hydrogasifica-
TABLE V.
HYDROGENATION OF BITUMINOUS COAL"
Yield, wt.
a 7
Catabst
Cat/Coal
SnClz SnClz ZnClz
0.01 1.o 1 .o
MAP coal
Coal conversion
Benzeneinsol.
Heavy
L@ht
Asphaltenes
Oil
Oil
HC gaJ
89 83
11 17 10
38 6 2
33 37 13
6 30 45
4 7 13
90
Hr, 425OC, 4000 psi.
tion. With equal weights of high-volatile-type A bituminous coal and AlC13 a t 45OOC and 4000 psi and a residence time of 1 hr, a hydrocarbon gas yield of 68'% and a benzene soluble oil of less than 1% are obtained. At lower temperature the gas decreases, although it is still 27% a t 250°C (37). Hydrogen donor. T h e solution of coal by extraction using organic agents under pressure has long been known, and a vast literature exists. The German chemists, Pott and Brosche, found that a mixture of tetralin, phenol, and naphthalene was the most satisfactory solvent for bituminous coal. A key feature is the presence of tetralin which is a hydroaromatic "donor,') able to transfer hydrogen to coal. This transfer is by a thermal, free-radical mechanism which is not
TABLE VI. COMPARISON OF CONTACT CATALYST W I T H ZnClz MELT FOR HYDROCRACKING 0 , EXTRACT HYDROCRACKING RESIDUE SulJidd
Catahst Temp, O C Catalyst/feed, wt ratio Yields, wt yGMAF feed CH4 C2H6 CaHs i-CdHlo n-C4HlO C S X 150OC dist. 150 X 2OOOC dist. 200 x 400°C dist. MEK-sol 400'C residue MEK-insol 4OOOC residue Conversion, wt % feed H Bconsumed, wt yGfeel
+ +
Total pressure, 4 2 0 0 p . s . i . g . Residence time at temperature, 60 min.
Go-Ma-Ni on S-90 441 0.3
Nickel Molybdate 441 0.3
ZnCl2 Melt 427 1 .oo
1.1 1.2 1.6 0.2 1.o 7.0 1.7 16.9
1.1 1.2 1.6 0.1 1.2 9.1 8.4 16.1
1.2 1.5 4.6 5 .O 0.8 52.5 10.4 6.3
67.5
60.2
19 . O
2.9 29.6 2.64
2.4 37.4 3.14
3.4 77.6 6.85
accelerated by the presence of hydrofining or cracking catalysts. The structure of the donor is important. The function of the phenol is apparently to assist in pulling the hydrogenated coal into solution. This combination effect is demonstrated in Table VI1 where, it is seen, the synergistic effect can be built into a single molecule, o-cyclohexylphenol (47). T h e process mechanism is complicated. For example, much of the oxygen is eliminated in making the coal soluble. T h e maximum amount of hydrogen transferred is significant in establishing the chemistry of the process (72, 75, 33)-2.6 and 2.2 wt yofor a particular coal and its extract, respectively. The transfer of hydrogen by organic agents is important. I n the question of coal hydrogenation, there is the basic problem of getting coal molecules, hydrogen, and the catalyst surface together physically. I t may be necessary to carry out the initial step in such a fashion that a hydroaromatic molecule travels to the coal molecule and a t that location transfers hydrogen. This then permits the partially hydrogenated coal to become mobile (soluble) and it then travels to the catalyst. I n any instance, the process step of hydrogen transfer by an organic solvent is an essential feature where solid heterogeneous catalysts are used and one which is open to improvement. Alkali metals. T h e alkali metals can act in hydrogenation of coal in several related ways: (a) as a direct hydrogenation catalyst (with molecular Hz), (b) with amines, and (c) in a catalytic electrochemical reduction.
TABLEVII. LIQUEFACTION OF BITUMINOUS COAL BY HYDROGEN DONOR SOLVENT"
Vehicle
70
25 32 50 82
Naphthalene Cresol Tetralin o-Cyclohexylphenol
I
a
Liquefaction
0.5 Hr at 4OO0C.
VOL. 6 1
NO. 7
JULY 1 9 6 9
13
CATALYSTS. (a) ALKALIMETALSA S Hz ACTIVATION Although not transition metals, alkali metals are active hydrogenation catalysts (27). Catalytic activity varies from lithium, the least active, to rubidium, the most active. The extent of hydrogenation of aromatic compounds depends upon the temperature and the alkali metal used, time and pressure being constant. At 250°C and 1400 psi, phenanthrene is hydrogenated to 9,lO-dihydrophenanthrene by sodium and to octahydrophenanthrene by sodium-potassium. With sodium-rubidium, dihydrophenanthrene is obtained at 180°C and octahydrophenanthrene a t 200°C. Other polycyclic hydrocarbons behave similarly, giving products containing isolated aromatic nuclei as final products (Figure 7 ) . Sodium is known to act as a stoichiometric reducing agent. When reduction is carried out by sodium and molecular hydrogen, the mechanism may involve formation of sodium hydride which then reacts further with aromatic compounds. However, the reaction is a catalytic one in the sense that each atom of alkali metal brings about hydrogenation of many aromatic molecules. With some oxygen-containing compounds, oxygen Figure 7. Alkali metals as hydrogenation catalysts elimination is achieved. Dibenzothiophene is relatively inert toward both hydrogenation and cleavage in the presence of Na-Rb. The presence of sulfur does not deactivate sodium metal catalysts in marked contrast to the usual behavior of transition metals. With nitrogencontaining heterocyclics, some ring cleavage and deEtnylenediamtne 90-1OOcC 1 IS v o 1s composition are observed. Use of tertiary amines, rather than benzene, as solvents permits hydrogenation at a lower temperature. I t is unnecessary to use the more active metals as such. Mixtures of sodium metal and potassium carbonate produce results equivalent to those obtained with sodium-potassium alloy. Use of alkali metals for hydrogenating coal and coal tar has been explored. A pitch which had originally of 4.26 was reduced to 0.75 when toluene a Harom/Haliph Figure 8. Electrochemical reduction of coal was used as a solvent and to 0.61 without solvent at 350°C. (b) REDUCTION BY ALKALI METALS-AMINES.I n The reduced coal is more soluble in pyridine than the 1958, the reduction of coal by metal-amine systems was starting material, solubility increasing markedly in one first achieved. The chemical reduction of coal by instance as much as 35 fold. One reduced vitrain belithium-ethylenediamine (EDA) proceeds at room temcame 97yc soluble. The reduced vitrains showed perature, adding as much as 55 atoms of hydrogen per changes in the infrared and ultraviolet spectra consistent 100 atoms of carbon (43, 44, 46, 52). This chemical with loss of aromatic rings. reaction consumes the lithium metal according to the In these systems coal is exposed to Li metal (cleaves equation believed to be : C - 0 bonds and adds Hz), Li-EDA (catalyzes isomerization), and EDA (complexes with coal and coal-derived Ar 2Li0 2RNHz + ArHz 2Lif 2RNHsubstances). The reaction is due to a combination of these factors. Extensive experiments with coal (vitrain) (92Yc C on (c) ELECTROCHEMICAL REDUCTION O F COAL. I t is a n ash-free basis) and model aromatic compounds have possible to provide a kind of catalytic reduction of coal provided much information on the course of the reaction. employing lithium in an electrochemical system (Figure With coronene, the product is 27% perhydro and 43% 8) (35, 50, 51). I n this process coal is reduced electrodocosacoronene (the latter contains one double bond).
+
14
+
+
+
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
TABLE VIII. Acid
CATALYSTS IN T H E PETROLEUM INDUSTRY
Catalyst
Function
HzSOa; H F ; H a P 0 4
Polymerization, alkylation, isomerization Cracking
SiO2-Alz03
Hz activation
CrzOa-Al203
Dual function
Co-Mo-AlzOt Pt-Al203 Metal-Si02-AI208
Dehydrogenation, hydrodealkylation Removal of S, N, 0 Naphtha reforming, isomerization paraffin and alkyl aromatic Hydrocracking
chemically in the presence of lithium chloride and ethylenediamine. As much as 53 atoms of hydrogen were added per 100 atoms of carbon. T h e reduced coal is 78% soluble in pyridine and 30y0 soluble in benzene a t room temperature. At present, current efficiency is low, 10 to 20y0. Higher current efficiencies have been obtained for pure compounds. About 80y0 is achieved for benzene reduction although current efficiency for reduction of 1-decene is only 27y0. T h a t hydrogen had indeed been added to coal was substantiated both by chemical analyses and by the fact that all the “added” hydrogen could be removed as an additional amount of hydrogen gas by catalytic dehydrogenation using palladium. Surprisingly, in experiments with vitrain the removal of sulfur takes place only after the more reactive aromatic rings are reduced. I n the electrochemical reduction of aromatic species, e --t Lio is believed to form solvated the reaction Li+ Lio a t the cathode which reacts with the hydrocarbon and amine. Thus, in contrast to the chemical reduction which requires a large excess of metallic lithium, lithium cation is used catalytically in the electrochemical reduction of coal. In the LiC1-EDA-polynuclear aromatics systems, aromatics are reduced by direct electron transfer from cathode to substrate, provided a platinum surface free of surface oxides is used. Benzene is reduced with lithium acting as an electron transfer agent. Electrochemical reduction of coal probably proceeds with rapid addition of hydrogen to reactive aromatic double bonds leading to structures with a relatively high percentage of substituted internal double bonds. Aromatic rings in the reduced coal are probably associated with phenolic groups and are resistant to hydrogenation. T h e relatively high solubility in benzene of reduced vitrains may be due to loss in planarity and the destruction of ether and/or sulfur linkages. Reductive alkylation. T h e formation of aromatic hydrocarbon anions is made possible by reaction with alkali metals. For example, naphthalene, dissolved in
+
Concept Carbonium ion, stable tertiary ion Hydride ion transfer, kinetic control of products Semiconductor, surface unsaturation Dehydrogenation under HZpress. Dual function, balance of 2 functions, dynamic HOH-HC1 Olefin intermediate
hexamethylphosphoramide, reacts with one or two moles of lithium to form the mono- or dianion (Figure 9). I n turn, the dianion can react with C H J to give 9,lO-dimethyl 9,lO-dihydroanthracene. This is called reductive alkylation, since one of the aromatic nuclei is converted to a n alkylated dihydrobenzene (49). Reductive alkylation can also be carried out with coal substance. Although only 3% soluble in HMPA, coal became 90% soluble on addition of lithium to a suspension of coal in HMPA. Alkylation of coal with ethyl iodide yielded an ethylated coal which was 35% soluble in benzene a t room temperature. An additional alkylation now yielded a product which was 85yosoluble in benzene a t room temperature. It was estimatzd that the alkylation corresponds to 1 alkyl to 5 carbon atoms. Reductive alkylation is also a means of adding hydrogen, in that H/C of coal is increased. This alkylation method is much more effective in causing solubility of coal than addition of a n equal number of hydrogen atoms. Moreover, a degree of benzene solubility is achieved which is not possible by hydrogenation.
Figure 9. Reductive alkylation VOL. 6 1
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TABLE IX.
Catdyyst AIR3-TiC13
T h e alkylation of coal was not restricted to use of HMPA. Coal can be readily alkylated in tetrahydrofuran, provided a small amount of naphthalene is added which acts as an electron-transfer agent (Figure 9). Other novel hydrogenation systems. There are a number of other novel hydrogenation systems. These include volatile catalysts such as iodine (78), dehydrogenation of coal to form hydrogen [9000 ft3/ton ( 4 5 ) ] which possibly could be combined in a dehydrogenation-hydrogenation disproportionation process, use of molecular sieves containing metals to bring about hydrogenation of selected molecules, high-energy modification of coal or catalysts, use of ultrasonics to increase coal solubilization. Petroleum Catalysts and Processes
Table VI11 classifies catalysts used in petroleum refining into three groups: acid, Hz-activating, and dualfunction types. I n developing a catalyst for a reaction, it is fundamental to select the correct type corresponding to the type of chemical reaction desired. For example, the concept of a carbonium ion led to processes of polymerization, alkylation, and isomerization. The uniqueness of the carbonium ion lies in the unusual nature of the products produced. The relative stability of the tertiary carbonium ion, the unexpected hydridic nature of H transfer, and the kinetic nonequilibrium control of transfer led to both aromatic products and also paraffins having an isoparaffin-n-paraffin ratio greater than thermodynamic equilibrium. For these reactions, conventional and available mineral acids served as catalysts. An acidactivated bentonite, an existing article of commerce, served as the first commercial cracking catalyst. The advent of catalytic processes which require the activation of molecular hydrogen, however, has required the development of catalysts that did not exist in commerce. Not only were the catalysts novel, but the concept of operating a dehydrogenation process under hydrogen pressure so as to keep the catalyst active (cokefree) was a great step forward. The dual-function catalyst makes possible reactions that are not possible by sequential operation over two beds of the two types of catalysts-in hydrocarbon conversions, only a very small amount of olefin intermediate is thermodynamically limited. Therefore, only a small amount of product is formed through a single passage through the sequential system. I n contrast, in the presence of the dual-function catalyst, the olefin is converted to product (e.g., aromatic or isoparaffin) and more of the olefin is formed and converted so that a high level of conversion to product is achieved in a single reactor. Operational experience with this type of catalyst has contributed greatly to its development, especially relative 16
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
RECENT CATALYSTS FOR HYDROCARBON CONVERSIONS
Function
Concept
Polymerization
Orient monomer, stereo polymer (M~tal)-molecular Selective hydro- Shape-selective sieve genation SbFE/HSOSF Superacid Ala03 Hydrogenation Strain site Pd-C-HaP04 Chemical to Electrically conducting electrical catalysis. Avoid Carnot cycle energy (fuel cell) hf003-.41203 Olefin disproFour-center complex W(C0)S portionation Bi-Mo-PO4 Dehydrogenative Multimolecular multicoupling center (petrochemicals)
to the correct balance between the two functions. Important in this regard was the gradual recognition of the dynamic nature of the composition of the surface of the catalyst. The acidity (responsible for cracking and isomerization activity) can be changed either deliberately or involuntarily by interchange of chloride ion for surface hydroxyl groups introduced by HC1 or HzO from the vapor phase. Processing nonvolatile petroleum residua, notably by hydrodesulfurization, presents certain new or a t least accentuated problems. Specifically these relate to the difficult prevention of catalyst deactivation because of coke and metals deposition. Increases in catalyst surface area, pore volume, and selection of distribution of pore diameters result in greatly improved maintenance of catalyst hydrodesulfurization activity (5, 6 , 70). Such catalyst improvements are of commercial significance. I t has been proposed that the in situ formation of a catalyst having colloidal dimensions could be the basis of a successful process. For example, vanadium acetyl acetonate dissolved in oil and heated in hydrogen could then catalyze desulfurization and hydrogenation of asphaltenes (5). Other recent catalysts active in hydrocarbon transitions and the concepts on which they are based are listed in Table I X . The transition metal-aluminum alkyl combinations are famous as Ziegler-Natta catalysts which can bring about stereospecific polymerization (14). The metal on molecular sieve catalysts can permit selective hydrogenation of molecules-those, and only those, which can enter the pores of the sieve ( 5 9 ) . The superacid catalyst can add a proton even to methane (40). Some substances can have catalytic hydrogenation ability enhanced by strain sites generated by dehydration at high temperature or by exposure to x-rays (37). The fuel cell catalyst is unusual in that direct electron transfer to the “support” is an essential feature ( 8 ) . The discovery of olefin disproportionation has brought interesting knowledge of multicenter sites ( 9 ) . Listed in the last row in Table IX, the reaction of propylene and ammonia with oxygen forming acrylonitrile by oxidative coupling, using a multicentered reaction catalyst, is an
example ( 7 7) of the many petrochemical processes that are coming to dominate the chemical industry even further. Conclusions
Gasoline synthesized from coal will be needed to fulfill the requirements for this essential fluid fuel. Moreover, this time of need is rapidly approaching. Moreover, resources will be conserved if certain technical inventions and developments are made to improve the economics of coal conversion. T h e technical requirements are numerous: coal must be liquefied, ash and s, N, and 0 removed, hydrogen added (aromatic asphaltenes converted), and the tar so produced cracked and reformed for octane improvement. T h e economic manufacture of hydrogen from coal is one of the essential requirements. I n addition to the important features of mechanical operability, process requirements are low hydrogen consumption and relatively low pressure operation. These will require high catalyst selectivity and activity so that large amounts of light paraffin hydrocarbons are not made that consume large volumes of hydrogen. New catalysts especially developed for coal could be of decisive importance. The system, to succeed, probably will contain these important elements: an integral means of separation of ash from catalyst, a superior organic hydrogen transfer agent, and a high activity-high selectivity catalyst. Each of the catalysts discussed here has shown superior features of high activity and selectivity in the laboratory. Each is somewhat unusual. They include alkali metal, transition metal complexes, and “nascent” hydrogen. Reductive alkylation techniques, too, are of importance. The catalysts have not as yet been developed into commercial process systems, but such work is actively in process a t the Bureau of Mines on a bench scale as well as in complementary basic research. REFERENCES (1) American Oil Co., R & D Report No. 32-OCR, Contract 14-01-0001-1188, PB-177068., Dcc. 1967. (2) Anderson, H. C., Wiley, J. L., and Newell, A,, U.S. Bureau of Mines, Washington, D.C., Bulletin 544, I and 11,1954,1955. (3) A pel1 H R and Wender I Division of Fuel Chemistry, 156th Meeting ACE, Atian;ic 6 t y , N.J., Sept: lg68. (4) A R C 0 Chemical Co., R&D Report No. 29, O C R Contract 14-01-0001-473, PB-174926, July 1967. (5) Are W. F., Jr., Blackwell, N. E., and Reichle, A. D., 7th World Petrol. Congr., 4, 16$)(1967). (6) Are,y, W. F., Jr., and Mayer, F. X . , Division of Petroleum Chemistry, 154th Meeting, ACS, Chicago, Ill., Sept. 1967. (7) Averitt, P., U.S. Geological Survey Bulletin 1275, Washington, D.C., 1969. (8) Baker, B. S., Ed., “Hydrocarbon Fuel Cell Technology,” Academic Press, New York (1966). (9) Banks, R. L , and Bailey, G. C., IND. ENO.CHEM.PROCESS DES. DEVELOP., 3, 170 (1964). (10) Beuther, H., and Schmidt, B. K., 6th WorldPetrol. Cong., Section 111, Paper 20 (1963). (11) Callahan, J., Szabo, J. J., and Gertisser, B. (to Standard Oil of Ohio), U S . Patent 3,186,955 (June 1, 1965). (12) Carlson C. S. Langer A. W Jr Stewart J. and Hill R. M Division of Petroleum ChemiLtry, 131h Meetlhg, ’kCS, Miami: Fla., Aprh 1957.”
(13) Consolidation Coal Co., R & D Report 39, Part 1 and 2, O C R Contract 14-01-0001-310, Dec. 1968. (14) . , Cosee. P.. Trans. Furudav, Soc.. , 58. , 1226 (1962). . (15) Curran G . P Struck, R . T., and Gorin, E., IND. ENO. CHEM.PROCESS DES. DEVELOP. )6, 166’?1967). (16) Donath, E. E., “Chemistr of Coal Utilization,” Lowry, H. H., Ed., Sup. Vol. Chap. 22, Wiley, New York h963). (17) Dryden, I. G. C.,ibid., Chap. 6 (1963). (18) Eddinger, R. T., and Friedman, L. D., Fuel, 47,320 (1968). (19) Faragher, W . F., and Horne, W. A., U S . Bureau of Mines, Washington, D.C., Info. Circ. 7368, 1946, (20) F M C Corp., R & D Report No. 11 O C R Contract 14-01-0001-235, Vols. I and I1 and Supplement, PB-169562,169563, and 173916, Jan. 1967. (21) Friedman, S., Kaufman, M. L., and Wender, I., Division of Fuel Chemistry, 148th Meeting, ACS, Chicago, Ill., August 1968. (22) Friedman, S.,Metlin, S., Svedi, A., and Wender, I., J . Org. Chem. 24, 1287 (1959). (23) Given, P. H., Fuel, 39, 147 (1960). (24) Halpern, J., Ann. Rev. Phys. Chem., 16, 103 (1965). (25) Halpern, J., Chem. Enc. News,44 (45), 68-75 (1966). (26) Hirsch, P. B., Proc. Roy. Soc. (London), A226, 143 (1954). (27) Hiteshue, R., U S . Bureau of Mines, Washington, D.C., unpublished results. (28) Innes, E. D., and Fear, J. V. D., 7th WorldPetrol. Congr., 3, 633 (1967). ( 2 9 ) Karr, C., Jr., “Chemistry of Coal Utilization,” Lowry, H. H., Ed., Sup. Vol., Chap. 13,NewYork, 1963. (30) Karr, C., Jr., and Comberiati, J. R., U.S. Bureau of Mines, Washington, D.C., Bulletin 636, 1966. (31) Kawa, W., Friedmann, S., Frank, L. V., and Hiteshue, R., Division of Fuel Chemistry, 156th Meeting, ACS, Atlantic City, N.J., Sept. 1968. (32) Kawa, W., and Hiteshue, R. W., U.S. Bureau of Mines, Washington, D.C., Info. Circ. 8215, 1964. (33) Langer, A. W., Stewart, C. E., Thompson, H . T., White, H. T., and Hill, R . M., Division of Petroleum Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958. (34) Logwinuk, A,, Friedman, L., and Weiss, A. H., IND. EN& CHEM.,56 (4), 20 (1964). (35) Markby, R . E., Wender, I., and Mohilner, D. M., J. Electrochem. SOC.113, 1060 (1966). (36) Mills, G. A “Ene clo edia of Chemical Technology,” Kirk-Othmer, Ed., 2nd ed. Interscience, Jew%ork, 1964. (37) Cornelius E. B., Milliken, T. H., Mills, G. A,, and Oblad, A. G., J. Phys. Ckem., 59, Sd9 (1955). (38) Nyholm, R. S., Proc. 3rd Int. Cong. on Catal., pp 25-87, North-Holland Publishing Co., Amsterdam, 1965. (39) Office of Coal Research, U S . Department of the Interior, Washington, D.C., Annual Report, 1969. (40) Olah, G. A., Pittman, C. U., Namanworth, E., and Comisarow, M. B., J.A m . Chem. Soc., 88, 5571 (1966). (41) Orchin, J., and Storch, H . H., IND.END.CHEM.,40, 1385 (1948). (42) Pinchin, F. J., Bull. Brit. Cool Utilization Research Assoc., 23 (12), 465 (1959). (43) Reggel, L., Raymond, R., Steiner, W. A., Friedel, R . A., and Wender, I., Fuel, 40, 339 (1961). (44) Reggel, L., Wender, I., and Raymond, R., ibid., 43, 74 (1964). (45) Reggel, L., Wender, I., and Raymond, R., ibid., 47, 373 (1968). (46) Reggel, L., Zahn, C., Wender, I., and Raymond, R., U S . Bureau of Mines Bull. 615, Washington, D.C. (1965). (47) Rousseau, P. E., World Power Conf., Tokyo Paper No. 158, IV, 912;(1966). (48) Spencer Chemical Div., R & D Report No. 9, O C R Contract 14-01-0001-275, PB-167809, 1965. (49) Sternber , H. W., and Delle Donne, C. L., D on of Fuel Chemistry, 156th Meeting, A&, Atlantic City, N.J., September 19 (50) Sternberg, H. W., Delle Donne, C. L., Markby, R. E., and Wender, I., Advances $71 Chem. Ser. No. 55, ACS, Washington, D.C., 1966. (51) Sternberg, H . W., Delle Donne, C. L., Markby, R. E., and Wender, I., F u e l , 45, 469 (1966). (52) Sternberg, H. W., Delle Donne, C. L., Reggel, L., and Wender, I., ibid., 43, 143 (1964). (53) Stobauah, R . B., Petrol. Refiner, 45, 149-155 (1966). (54) Struck, R . T., C!ark, W. E., Dudt, P. J., Rosenhoover, W. A., Zielke,,C. W., and Gorin, E., Division of Fuel Chemistry, 156th Meeting, ACS, Atlantic City, N.J., Sept. 1968. (55) U S . Department of the Interior, Washington, D.C., “Prospects for Oil Shale Development,” May 1968. (56) U S . Department of the Interior, “United States Petroleum Through 1980,” 1968. (57) Wainwright, H . W., U.S. Bureau of Mines, Washington, D.C., Info. Circ. ,
I
R054 (19611. _. .. \ - - - - ,
(58) Weiler, J. F.,“Chemistry of Coal Utilization,” Lowry, H. H., Ed., Sup. Vol., Chap. 14, Wiley, New York, 1963. (59) Weisz, P. B., Frilette, V. J., Maatrnan, R. W., and Mower, E. B., J. Cutul. 1, 302 (1962). (60) , , Weller. S. W.. “Catalvsis.” , . Emmett, P. H., Ed., Reinhold, New York, 1956. (61) Wiley, J. L., and Anderson, H . C., U.S. Bureau of Mines Bull. 485 I, 11, and 111, Washmgton, D.C., 1950, 1951, 1952. (62) Wu, W. R. K., and Storch, H. H., ibid., 633, Washington, D.C., 1968. (63) Zielke, C. W., Struck, R . T., Evans, J. M., Costanza, C. P., and Gorin, E., IND.END.CHEM.PROCESS DES.DEVELOP., 5,151, 158 (1966). (64) Zielke, C. W., Struck, R . T., and Gorin, E., Division of Fuel Chemistry, 156th Meeting, ACS, Atlantic City, N.J., Sept. 1968.
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