Process for the manufacture of gasoline

Synthesis gas comprising a mixture of carbon monoxide and hydrogen is derived from fossil fuels and catalytically converted in a first reaction zone to a mixture of methanol and dimethyl ether which in turn is converted in a separate reaction zone in contact with a crystalline aluminosilicate zeolite catalyst having a silica to alumina ratio of at least about 12 and a constraint index of about 1 to 12, and preferably a crystal density in the hydrogen form of not substantially below about 1.6 grams per cubic centimeter to a product which is resolved into a high octane gasoline fraction, a light hydrocarbon gas fraction which may be liquefied and a hydrogen-rich gaseous by-product which is recycled to the conversion of fossil fuels to synthesis gas or may be otherwise used.

BACKGROUND OF THE INVENTION 
This invention is concerned with a process for converting coal and other 
solid, highly viscous liquid, or gaseous fossil fuels to liquid petroleum 
products, particularly hydrocarbon fuels. It more especially is concerned 
with converting such materials to high quality gasoline. 
The increasing demand for high octane gasoline has been met, until now, by 
advanced petroleum refining technology. The two processes which have made 
it possible to satisfy the demand are: catalytic cracking, which serves to 
increase the fraction of crude petroleum which can be brought into the 
gasoline boiling range; and catalytic reforming, which serves to upgrade 
the octane number of low grade gasoline. Obviously, the cracking process 
increases the gasoline yield at the expense of the heavier fuel fractions, 
such as No. 2 fuel oil, which also is subject to growing demand. The 
increasing cost of petroleum, together with the foreseen increase in 
demand for gasoline and other petroleum fractions, makes it necessary to 
seek other fuel sources from which to make high quality gasoline. 
Coal, for example, is an obvious alternative raw material, since there are 
abundant deposits of this fuel. Gasoline has been made from coal by 
gasification and conversion of the gases to gasoline by the 
Fischer-Tropsch process. However, this process, not presently used in this 
country, produces an extremely poor quality of gasoline, with an octane 
number of about 55, which cannot be efficiently upgraded by the known 
catalytic reforming processes because it consists predominantly of 
straight-chain aliphatic hydrocarbons. 
Processes for the conversion of coal and other hydrocarbons to a gaseous 
mixture consisting essentially of hydrogen and carbon monoxide or of 
hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon 
dioxide, are well known. Such a gaseous mixture hereinafter will be 
referred to simply as synthesis gas. 
Although various processes may be employed for the gasification, those of 
major interest for the present invention depend either on the partial 
combusion of the fuel with oxygen, or on the high temperature reaction of 
the fuel with steam, or on a combination of these two reactions. In one 
known variant, for example, coal may be completely gasified by first 
coking, and then subjecting the coke to a cyclic blue water gas process in 
which the coke bed is alternately blasted with air to increase the bed 
temperature and then reacted with steam to produce the synthesis gas. The 
gases generated by the coking step may be used as fuel or steam-reformed 
to additional synthesis gas. In another process coal or coke may be 
reacted with highly superheated steam or with oxygen and steam to produce 
synthesis gas. Regardless of the process variants chosen, oxygen rather 
than air is often used in the chemical step in which the fuel is converted 
to synthesis gas since the use of air would result in a gas that contained 
excessive amounts of inert nitrogen. 
An excellent summary of the art of gas manufacture, including synthesis 
gas, from solid and liquid fuels, is given in Encyclopedia of Chemical 
Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 
353-433, (1966), Interscience Publishers, New York, N.Y., the contents of 
which are herein incorporated by reference. The techniques for 
gasification of coal or other solid, viscous or gaseous fuel are not 
considered to be per se inventive here. 
It is known that raw synthesis gas contains one or more of the following 
impurities: sulfur compounds, nitrogen compounds, particulate matter and 
condensibles. The art of removing these contaminants is known, and is 
described in the above reference and elsewhere. Particular attention is 
called to the sulfur compounds. It is desirable to reduce this contaminant 
below a prescribed level for ecological purposes. 
Purified synthesis gas ordinarily contains a volume ratio of hydrogen to 
carbon monoxide plus carbon dioxide of from as little as about 0.10 to as 
much as 1.1, depending on the particular fuel and process used; in most 
instances, the composition has a volume ratio from about 0.30 to about 
0.65. It is well known that this ratio may be increased by the catalytic 
carbon monoxide shift reaction described by the equation: 
EQU CO + H.sub.2 O .revreaction. CO.sub.2 + H.sub.2 
with subsequent removal of at least part of the produced CO.sub.2 to bring 
said volume ratio into a desired high range. The catalytic carbon monoxide 
shift reaction is commonly conducted with a chromia promoted iron oxide 
catalyst at a flow rate of about 300-1000 standard cubic feet of gas per 
cubic foot of catalyst bed per hours, and at sufficiently elevated 
temperature to allow quasi-equilibration, which is usually about 
700.degree. F. 
Synthesis gas will undergo conversion to form reduction products of carbon 
monoxide, such as alcohols, at from about 300.degree. F. to about 
850.degree. F., under from about 1 to 1000 atmospheres pressure, over a 
fairly wide variety of catalysts. The types of catalyst that induce 
conversion include ZnO, Fe, Co, Ni, Ru, ThO.sub.2, Rh and Os. 
Catalysts based on ZnO are particularly suited for the production of 
methanol and dimethyl ether. Catalysts based on Fe, Co, and Ni, and 
especially Fe, are particularly suited for the production of oxygenated 
and hydrocarbon products that have at least one carbon-to-carbon bond in 
their structure. With the exception of ruthenium, all practical synthesis 
catalysts contain chemical and structural promoters. These promoters 
include copper, chromia, alumina and alkali. Alkali is of particular 
importance with iron catalysts, since it greatly enhances the conversion 
efficiency of the iron catalyst. Supports such as kieselguhr sometimes act 
beneficially. 
The catalyzed reduction of carbon monoxide or carbon dioxide by hydrogen 
produces various oxygenated and hydrocarbon products, depending on the 
particular catalyst and reaction conditions chosen. The products that are 
formed include methanol, dimethyl ether, acetone, acetic acid, normal 
propyl alcohol, higher alcohols, methane, gaseous, liquid, and solid 
olefins and paraffins. It should be noted that this spectrum of products 
consists of aliphatic compounds; aromatic hydrocarbons either are totally 
absent or are formed in minor quantities. 
In general, when operating at the lower end of the temperature range, i.e. 
from about 300.degree. F. to about 500.degree. F., in the reduction of 
carbon monoxide, and with pressures greater than about 20 atmospheres, 
thermodynamic considerations suggest that aliphatic hydrocarbons are 
likely to form in preference to their aromatic counterparts. Furthermore, 
in some catalytic systems it has been noted that aromatic hydrocarbon 
impurities in the synthesis gas inactivate the synthesis catalyst, and one 
may speculate that a number of known synthesis catalysts intrinsically are 
not capable of producing aromatic hydrocarbons. 
The wide range of catalysts and catalyst modifications disclosed in the art 
and an equally wide range of conversion conditions for the reduction of 
carbon monoxide by hydrogen provide considerable flexibility toward 
obtaining selected products. Nonetheless, in spite of this flexibility, it 
has not proved possible to make such selections so as to produce liquid 
hydrocarbons in the gasoline boiling range which contain highly branched 
paraffins and substantial quantities of aromatic hydrocarbons, both of 
which are required for high quality gasoline. A review of the status of 
this art is given in "Carbon Monoxide-Hydrogen Reactions," Encyclopedia of 
Chemical Technology, Edited by Kirk-Othmer, 2nd Edition, Volume 4, pp. 
446-488, Interscience Publishers, New York, N.Y., the text of which is 
incorporated herein by reference. 
Oxygenated compounds and hydrocarbons are produced in varying proportions 
in the conversion of synthesis gas. This is understandable if, as proposed 
by some researchers in the field, the hydrocarbons arise via oxygenated 
intermediates such as alcohols. By selection of less active catalysts such 
as zinc oxide, it is possible to obtain oxygenated compounds as the major 
product. One particular commercial conversion is used to produce methanol 
from synthesis gas with substantially no coproduction of hydrocarbons. 
Suitable catalysts are those comprising zinc oxide, in admixture with 
promoters. Copper or copper oxide may be included in the catalyst 
composition. Particularly suitable are oxide catalysts of the 
zinc-copper-alumina type. Compositions of the type described are those 
currently used in commercial methanol synthesis. Contact of the synthesis 
gas with the methaol synthesis catalyst is conducted under pressure of 
about 25 to 600 atmospheres, preferably about 50 to 400 atmospheres, and 
at a temperature of about 400.degree. F. to 750.degree. C. The preferred 
gas space velocity is within the range of about 1,000 to 50,000 volume 
hourly space velocity measured at standard temperature and pressure. It is 
noted that the conversion per pass is from about 10% of the carbon 
monoxide fed to about 30%, i.e. in this process the unconverted synthesis 
gas must be separated from the methanol product and recycled. 
Crystalline aluminosilicate zeolites have been contacted with methanol 
under catalytic conversion conditions. U.S. Pat. No. 3,036,134 shows a 
98.4% conversion of methanol to dimethyl ether over sodium X zeolite at 
260.degree. C.; 1.6 mole % of the product is a mixture of olefins through 
pentene, with butene the predominant product. Conversion of methanol over 
rare earth exchanged and zinc exchanged X zeolite has been reported to 
produce some hexanes and lighter hydrocarbons (see Advances in Catalysis, 
Vol. 18, p. 309, Academic Press, New York, 1968). It has recently been 
discovered that alcohols, ethers, carbonyl and their analogous compounds 
may be converted to higher hydrocarbons, particularly high octane 
gasoline, by catalytic contact with a special type zeolite catalyst. This 
conversion is described in copending U.S. Patent Applications, Ser. Nos. 
387,224, 387,223, and 387,222 filed on Aug. 9, 1973. 
It is an object of the present invention to provide an improved method for 
converting fossil fuels to high quality gasoline. It is a further object 
of this invention to provide a method for converting a mixture of gaseous 
carbon oxides with hydrogen to high quality gasoline. It is a further 
object of this invention to provide a novel method of converting synthesis 
gas to high octane gasoline. It is a further object of this invention to 
provide a process for the manufacture of substantially sulfur-free liquid 
hydrocarbon fuels. Further objects of this invention will be apparent to 
those skilled in the art. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the stated objects, one aspect of this invention 
provides a process comprising the following steps: coal or other fossil 
fuel is gasified to form synthesis gas; the gas is adjusted by one of 
several means to provide a volumetric ratio of hydrogen to carbon monoxide 
plus carbon dioxide of from 1.0 to 6.0; the adjusted synthesis gas is 
contacted with a carbon monoxide reduction catalyst in a first reaction 
zone to produce a reduction product comprising at least 20 weight percent 
oxygenated products; and, the reduction product is catalytically converted 
by contact with a catalyst which is a crystalline aluminosilicate zeolite 
having a silica to alumina ratio of at least about 12, a constraint index 
of about 1 to 12 and a crystal density, in the hydrogen form, of not 
substantially below about 1.6 grams per cubic centimeter in a second 
reaction zone to form a major fraction of aromatics-rich high octane 
gasoline and a minor fraction of useful products that include a 
hydrogen-rich gaseous mixture that may be recycled to the fossil fuels 
gasifier.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS 
The drawing of FIG. 1 will now be used to illustrate this invention in 
certain of its aspects, without being limited thereto. Coal, shale oil, or 
residua, or a combination thereof, is conveyed via line 1, 2 and 3, resp. 
and thence via line 4 to the synthesis gas plant, 5, where it is converted 
to synthesis gas. If hydrogen sulfide is produced in this plant, it may be 
separated and sent via line 6 to a treatment plant (not shown) for sulfur 
recovery. Synthesis gas, previously treated in a catalytic carbon monoxide 
shift converter and then reduced in carbon dioxide content by selective 
sorption, is conveyed via line 7 to a first reaction zone 8, where it is 
at least partially converted catalytically to produce a carbon monoxide 
reduction product that contains at least 20% by weight of oxygenated 
products. Part or all of the unconverted synthesis gas may be separated 
from such reduction product and recycled via line 10, but it is preferred 
to convey the total mixture via line 9 to the second reaction zone 11 
where catalytic conversion to hydrocarbons and steam occurs. The reaction 
products from the second reaction zone 11 are conveyed via line 12 to a 
cooler, 13, and the cooled products are then conveyed via line 14 to a 
separator 15; note that the cooler 13 and line 14 and separator 15 may be 
one integral unit. Water is removed from separator 15 via line 16, gases 
via line 17, and liquid hydrocarbon products via line 18. The liquid 
hydrocarbon products are conveyed via line 18 to a distillation tower 19. 
Propane and butanes (LPG) are recovered via line 20, and gasoline via line 
21. The gases disengaged in the separator 15 are conveyed via line 17 to 
storage 22, and recycled via lines 23 and 25 to the synthesis gas plant, 
or via lines 23 and 24 and line 7 to the carbon oxides converter 8. 
Fossil fuel, as the term is used in this invention, is intended to include 
anthracite and bituminous coal, lignite, crude petroleum, shale oil, oil 
from tar sands, natural gas, as well as fuels derived from physical 
separation, or more profound transformations, of these materials, 
including coked coal, petroleum coke, gas oil, residua from petroleum 
distillation, and two or more of any of the foregoing materials in 
combination. It is an attribute of this invention that fuels which usually 
contain ecologically undesirable levels of sulfur, i.e. greater than 2% 
organically bound sulfur, may be used to produce products substantially 
free of sulfur. Organically bound sulfur, as distinguished from hydrogen 
sulfide, for example, is sulfur which is chemically bonded to one or more 
carbon atoms, and such sulfur is ordinarily difficult to remove from 
fossil fuels. Particularly preferred for the practice of this invention is 
to use coal. Non-fossil carbonaceous fuels also may be used, however; 
these include wood, cellulosic materials, organic animal waste, and any 
other organic matter characterized by significant fuel value. 
Any of the described fuels may be converted to synthesis gas by techniques 
which are known in the art, and which are not regarded as constituting 
this invention. It is also contemplated to include in gasification 
techniques in situ processes such as the underground partial combustion of 
coal and petroleum deposits. In any case, it is to be understood that the 
gasification art employed shall be selected so as to produce raw synthesis 
gas comprising a mixture of carbon monoxide, carbon dioxide and hydrogen 
as the principal constituents. Synthesis gas, as first produced, contains 
impurities, including hydrogen sulfide and volatile organically bound 
sulfur compounds, including carbonyl sulfide. This mixture shall be 
referred to as raw synthesis gas. 
Raw synthesis gas is next treated to remove impurities. Iron and nickel 
carbonyls, if present, should be removed since they will adversely effect 
the long term behavior of the catalysts used in the subsequent 
conversions. This purification may be carried out, for example, by 
absorption on activated carbon. Particulates and hydrocarbon impurities 
may be removed by sorption processes well known in the art, if so desired. 
It is very important, however to remove a major portion of the sulfur 
which may be present as hydrogen sulfide, organically bound sulfur 
compounds or mixtures of these. The organically bound sulfur compounds may 
be decomposed, for example, over a mixture of alkali metal carbonate and 
sulfurized iron at elevated temperature; the hydrogen sulfide, either 
initially present in the raw synthesis gas, or formed by decomposition of 
the organically bound sulfur compounds, may be reduced in concentration 
and substantially removed by scrubbing under pressure with ethanolamines, 
for example. For the purpose of this invention, it is preferred to remove 
at least 90% of the sulfur present initially in the raw synthesis gas to 
form purified synthesis gas. 
The purified synthesis gas consists essentially of a mixture of hydrogen 
gas, with gaseous carbon oxides including carbon monoxide and carbon 
dioxide. By way of illustration, a typical purified synthesis gas will 
have the composition, in volume percentages, as follows: hydrogen, 51; 
carbon monoxide, 40; carbon dioxide, 4; methane, 1; and nitrogen, 4. 
Depending on the particular fuel and the particular gasification process, 
the hydrogen to carbon oxides ratio may vary widely. It is preferred to 
adjust the hydrogen-to-carbon oxides volume ratio in the synthesis gas to 
from 1.0 to 6.0 prior to use in subsequent conversions. Should the 
purified synthesis gas be excessively rich in carbon oxides, it may be 
brought within the preferred range by the well known water gas shift 
reaction; on the other hand, should the synthesis gas be excessively rich 
in hydrogen, it may be adjusted into the preferred range by the addition 
of carbon dioxide or carbon monoxide. Purified synthesis gas adjusted to 
contain a volume ratio of hydrogen to carbon oxides of from 1.0 to 6.0 
will be referred to as adjusted synthesis gas. 
It is desirable that the adjusted synthesis gas contain not more than 20% 
inert nitrogen since the economic cost for subsequent conversions are 
increased by excess diluent. Low levels of nitrogen are easily achieved by 
supplying essentially pure oxygen gas, in the quantities required, in the 
fossil fuels gasification step. 
It is an essential feature of this invention that the adjusted synthesis 
gas is catalytically converted to oxygenated compounds in a first reaction 
zone and the oxygenated compounds are catalytically converted to liquid 
gasoline boiling range hydrocarbons in a second reaction zone. A number of 
catalysts are known that will cause the carbon monoxide to be reduced by 
the hydrogen of the synthesis gas to form oxygenated compounds, liquid 
hydrocarbons, and mixtures of these in varying proportions, depending on 
the particular catalyst and reaction conditions. 
It is a preferred embodiment of this invention to catalytically convert the 
adjusted synthesis gas in such a manner that substantially all of the 
reaction product from the first reaction zone is oxygenated product, and 
thus obtain maximum benefit from this invention. The conversion to 
methanol, to dimethyl ether, or to mixtures of these and other oxygenated 
compounds is illustrative of this preferred embodiment. 
Conventional methanol synthesis techniques are well suited for the purpose 
of this invention. Contact of the adjusted synthesis gas with a methanol 
synthesis catalyst, preferably under about 50 to 400 atmospheres, at a 
temperature from about 400.degree. F. to 750.degree. F., and at a volume 
hourly space velocity of from about 1000.degree. to 50,000 volumes, serves 
to induce conversion of from about 10% to about 30% of the carbon monoxide 
feed to oxygenates, mainly methanol. Methanol may be formed as the almost 
exclusive product of the reaction, or it may be contaminated by higher 
alcohols such as ethanol, propanol, and butanols. In either case, it is 
possible to pass the gaseous product mixture from the first reaction zone 
through a condenser, separate the crude oxygenated product (methanol), and 
recycle the unconverted synthesis gas to the first reaction zone. However, 
it is preferred to directly convey the mixture of reduction product and 
unconverted synthesis gas to the second reaction zone without separation 
since this provides economies in handling and, in addition, the conversion 
in the second reaction zone is not adversely affected by the presence of 
the unreacted synthesis gas. 
It is to be emphasized that the conversion in the first reaction zone may 
employ catalysts and reaction conditions which lead to improved 
efficiencies of carbon monoxide conversion, notwithstanding that the 
mixture of oxygenated compounds formed may ordinarily be considered 
undesirable for ordinary methanol synthesis because it contains 
substantial or even major fractions of oxygenated compounds other than 
methanol. This is so because the other oxygenated compounds such as 
dimethyl ether, ethanol, propanol and butanol are converted in the second 
reaction zone with an efficiency at least equal to pure methanol. 
Thus, in a preferred embodiment of the present invention, crude methanol 
from the first reaction zone is fed to the second reaction zone without 
separation of oxygenated impurities. In this preferred embodiment full 
advantage is taken of insensitivity of the second reaction zone to these 
impurities, and furthermore the methanol synthesis catalyst and reaction 
conditions in the first reaction zone may be suitably modified to most 
efficiently affect the reduction of carbon monoxide. 
Although maximum benefit from this invention is achieved by catalytic 
conversion of adjusted synthesis gas in a first reaction zone under 
conditions such that substantially all of the carbon monoxide reduction 
product is oxygenated product, substantial benefits will accrue if at 
least 20 percent by weight of the reduction product consists of oxygenated 
compounds. Metallic catalysts of the iron, cobalt and nickel variety are 
suitable for such conversions. Iron promoted by alkali is especially 
useful. By way of example, pure iron, roasted in an oxygen atmosphere in 
the presence of added aluminum and potassium nitrates provides a 
composition that contains 97% Fe.sub.3 O.sub.4, 2.4% Al.sub.2 O.sub.3, and 
0.6% K.sub.2 O with trace amount of sulfur and carbon. This composition 
after reduction with hydrogen at about 850.degree. F. catalyzes the 
conversion of synthesis gas at from 360.degree. F. to 430.degree. F., and 
at 20 Atm. pressure, such that 65% of the carbon monoxide is reduced to a 
mixture consisting of about one third by weight of hydrocarbons boiling in 
the range of 200.degree. F. to about 680.degree. F., and about two thirds 
of oxygenated compounds, mostly alcohols, in the same boiling range. This 
conversion is given by way of illustration only; other catalysts and 
conversion conditions capable of producing at least 20 percent by weight 
oxygenated compounds in the reduction product will be evident to those 
skilled in the art. 
It is a feature of this invention that it is not necessary to separate the 
oxygenated compounds from the liquid hydrocarbons, prior to further 
conversion, when the two are produced simultaneously in the first reaction 
zone. Although the hydrocarbons produced in the first reaction zone are 
likely to be linear paraffins and olefins and therefore undesirable 
components of high octane gasoline, it is a remarkable attribute of this 
invention that these hydrocarbons undergo conversion to highly branched 
paraffins and aromatics along with the alcohols when the mixture is 
converted in the second reaction zone. It is a preferred embodiment of 
this invention to contact the unresolved mixture of hydrocarbons and 
oxygenated compounds with the catalyst in the second reaction zone, 
thereby taking full advantage of the cooperative interaction of the two 
reaction zones to produce maximum high octane gasoline. However, the 
reduction product mixture may be separated from the unconverted synthesis 
gas before contact with the catalyst in the second reaction zone. 
Small quantities of ammonia are sometimes produced in the first reaction 
zone. Although not essential to this invention, it is highly desirable to 
remove these from the product mixture prior to contact with the catalyst 
in the second reaction zone, thereby prolonging the effectiveness of that 
catalyst. This may be done by brief contact with a solid acidic adsorbent, 
such as acid-treated clay, for example. 
An essential step in the present invention is the catalytic conversion of 
the oxygenated compounds to high octane gasoline in a second reaction zone 
in contact with a novel class of zeolite catalysts. This recently 
discovered novel class of zeolites has some unusual properties. These 
catalysts induce profound transformations of aliphatic hydrocarbons to 
aromatic hydrocarbons in commercially desirable yields. Although they have 
unusually low alumina contents, i.e. high silica to alumina ratios, they 
are very active even when the silica to alumina ratio exceeds 30. The 
activity is surprising since the alumina in the zeolite framework is 
believed responsible for catalytic activity. These catalysts retain their 
crystallinity for long periods in spite of the presence of steam at high 
temperature which induces irreversible collapse of the framework of other 
zeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits, 
when formed, may be removed by burning at higher than usual temperatures 
to restore activity. 
An important characteristic of the crystal structure of this class of 
zeolites is that it provides constrained access to, and egress from, this 
intracrystalline free space by virtue of having a pore dimension greater 
than about 5 Angstroms and pore windows of about a size such as would be 
provided by 10-membered rings of oxygen atoms. It is to be understood, of 
course, that these rings are those formed by the regular disposition of 
the tetrahedra making up the anionic framework of the crystalline 
aluminosilicate, the oxygen atoms themselves being bonded to the silicon 
or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred 
type catalyst useful in this invention posess, in combination: a silica to 
alumina ratio of at least about 12; and a structure providing constrained 
access to the crystalline free space. 
The silica to alumina ratio referred to may be determined by conventional 
analysis. This ratio is meant to represent, as closely as possible, the 
ratio in the rigid anionic framework of the zeolite crystal and to exclude 
aluminum in the binder or in cationic form within the channels. Although 
catalysts with a silica to alumina ratio of at least 12 are useful, it is 
preferred to use catalysts having higher ratios of at least about 30. Such 
catalysts, after activation, acquire an intracrystalline sorption capacity 
for normal hexane which is greater than that for water, i.e. they exhibit 
"hydrophobic" properties. It is believed that this hydrophobic character 
is advantageous in the present invention. 
The type zeolites useful in this invention freely sorb normal hexane and 
have a pore dimension greater than about 5 Angstroms. In addition, the 
structure must provide constrained access to larger molecules. It is 
sometimes possible to judge from a known crystal structure whether such 
constrained access exists. For example, if the only pore windows in a 
crystal are formed by eight membered rings of oxygen atoms, then access to 
molecules of larger cross-section than normal hexane is excluded and the 
zeolite is not of the desired type. Windows of ten-membered rings are 
preferred, although excessive puckering of pore blockage may render these 
catalysts ineffective. Twelve-membered rings do not generally appear to 
offer sufficient constraint to produce the advantageous conversions, 
although structures can be conceived, due to pore blockage or other cause, 
that may be operative. 
Rather than attempt to judge from crystal structure whether or not a 
catalyst posesses the necessary constrained access, a simple determination 
of the "constraint index" may be made by passing continously a mixture of 
equal weight of normal hexane and 3-methylpentane over a small sample, 
approximately 1 gram or less, of catalyst at atmospheric pressure 
according to the following procedure. A sample of the catalyst, in the 
form of pellets or extrudate, is crushed to a particle size about that of 
coarse sand and mounted in a glass tube. Prior to testing, the catalyst is 
treated with a stream of air at 1000.degree. F. for at least 15 minutes. 
The catalyst is then flushed with helium and the temperature adjusted 
between 550.degree. F. and 950.degree. F. to give an overall conversion 
between 10% and 60%. The mixture of hydrocarbons is passed at 1 liquid 
hourly space velocity (i.e. 1 volume of hydrocarbon per volume of catalyst 
per hour) over the catalyst with a helium dilution to give a helium to 
total hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a sample 
of the effluent is taken and analyzed, most conveniently by gas 
chromatography, to determine the fraction remaining unchanged for each of 
the two hydrocarbons. 
The "constraint index" is calculated as follows: 
##EQU1## 
The constraint index approximates the ratio of the cracking rate constants 
for the two hydrocarbons. Catalysts suitable for the present invention are 
those having a constraint index from 1.0 to 12.0, preferably 2.0 to 7.0. 
The class of zeolites defined herein include those of the ZSM-5 type and is 
exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-21, TEA mordenite and other 
similar materials. Recently issued U.S. Pat. No. 3,702,886 describing and 
claiming ZSM-5 is incorporated herein by reference. 
ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, the 
entire contents of which are incorporated herein by reference. 
ZSM-12 is more particularly described in West German Offenlagunschrifft No. 
2,213,109, the entire contents of which are incorporated herein by 
reference. 
ZSM-21 is more particularly described in U.S. Application, Ser. No. 
358,192, filed May 7, 1973, the entire contents of which are incorporated 
herein by reference. 
TEA mordenite is more particularly described in U.S. Application Ser. No. 
130,442 filed Apr. 11, 1971, the entire contents of which are incorporated 
herein by reference. 
The specific zeolites described, when prepared in the presence of organic 
cations, are catalytically inactive, possibly because the intracrystalline 
free space is occupied by organic cations from the forming solution. They 
may be activated by heating in an inert atmosphere at 1000.degree. F. for 
one hour, for example, followed by base exchange with ammonium salts 
followed by calcination at 1000.degree. F. in air. The presence of organic 
cations in the forming solution may not be absolutely essential to the 
formation of this type zeolite; however, the presence of these cations 
does appear to favor the formation of this special type of zeolite. More 
generally, it is desirable to activate this type catalyst by base exchange 
with ammonium salts followed by calcination in air at about 1000.degree. 
F. for from about 15 minutes to about 24 hours. 
Natural zeolites may sometimes be converted to this type zeolite catalysts 
by various activation procedures and other treatments such as exchange, 
steaming, alumina extraction and calcination, in combinations. Natural 
minerals which may be so treated include ferrierite, brewsterite, 
stilbite, dachiardite, epistilbite, heulandite and clinoptilolite. The 
preferred crystalline aluminosilicates are ZSM-5, ZSM-11, ZSM-12, ZSM-21 
and TEA mordenite, with ZSM-5 particularly preferred. 
The catalysts of this invention may be in the hydrogen form or they may be 
base exchanged or impregnated to contain ammonium or a metal cation 
complement. It is desirable to calcine the catalyst after base exchange. 
The metal cations that may be present include any of the cations of the 
metals of Groups 1 through VIII of the periodic table. However, in the 
case of Group IA metals, the cation content should in no case be so large 
as to effectively inactivate the catalyst. For example, a completely 
sodium exchanged H-ZSM-5 is not operative in the present invention. 
In a preferred aspect of this invention, the catalysts hereof are selected 
as those having a crystal density, in the dry hydrogen form, of not 
substantially below about 1.6 grams per cubic centimeter. It has been 
found that zeolites which satisfy all three of these criteria are most 
desired because they tend to maximize the production of gasoline boiling 
range hydrocarbon products. Therefore, the preferred catalysts of this 
invention are those having a constraint index as defined above of about 1 
to 12, a silica to alumina ratio of at least about 12 and a dried crystal 
density of not less than about 1.6 grams per cubic centimeter. The dry 
density for known structures may be calculated from the number of silicon 
plus aluminum atoms per 1000 cubic Angstroms, as given, e.g. on page 11 of 
the article on Zeolite Structure by W. M. Meier. This paper, the entire 
contents of which are incorporated herein by reference, is included in 
"Proceedings of the Conference on Molecular Sieves, London, April 1967", 
published by the Society of Chemical Industry, London, 1968. When the 
crystal structure is unknown, the crystal framework density may be 
determined by classical pyknometer techniques. For example, it may be 
determined by immersing the dry hydrogen form of the zeolite in an organic 
solvent which is not sorbed by the crystal. It is possible that the 
unusual sustained activity and stability of this class of zeolites is 
associated with its high crystal anionic framework density of not less 
than about 1.6 grams per cubic centimeter. This high density of course 
must be associated with a relatively small amount of free space within the 
crystal, which might be expected to result in more stable structures. This 
free space, however, is important as the locus of catalytic activity. 
Because the catalyst for the synthesis gas conversion is a hydrogenation 
catalyst, the aromatization catalyst is most preferably maintained in a 
separate reaction zone, where it functions well even in the presence of 
unconverted synthesis gas. 
In the practice of this invention, the conversion in the second reaction 
zone is conducted at a temperature of about 500.degree. F. to 1000.degree. 
F., preferably about 600.degree. F. to 800.degree. F., a pressure of 
subatmospheric to about 50 atmospheres, and at a liquid hourly space 
velocity of about 0.1 to 50 LHSV. 
The conversion with this special, high silica to alumina ratio, catalyst 
produces a sulfur-free, high quality gasoline fraction boiling in the 
range about 82.degree. F. to 415.degree. F. which has a research octane 
number of at least about 80 without the addition of lead. A minor fraction 
of valuable liquefiable petroleum gas (e.g. propane) and a little dry gas 
(e.g. ethane and methane) also are produced. If severe reaction conditions 
are selected, the major fraction of the gasoline is aromatic hydrocarbons, 
and the paraffins are mostly branched. Thus, this total mixture may be 
separated into a small fraction suitably about 1%, of "dry gas" comprising 
methane, ethane, and ethylene, a small fraction, suitably about 26%, of 
liquefiable petroleum gas comprising propane and butanes, and a major 
fraction, suitably the remainder of about 73%, of gasoline with a research 
octane number of at least about 100 without requiring the addition of 
lead. 
The liquefiable petroleum gas may be sold as a sulfur-free fuel, or it may 
be recycled to the gasification plant. In the latter case it need not be 
first separated from the "dry gas" fraction. The dry gas fraction may be 
burned as fuel or sold as such but preferably it is recycled to the fossil 
fuel gasification operation. Hydrogen is sometimes produced in the second, 
or aromatization, reaction zone. This hydrogen is a valuable recycle 
product which can be most useful in carbon oxide hydrogenation. 
It will be seen by a consideration of this entire process that the 
individual steps are quite interrelated and mesh very nicely with each 
other through thermal and/or material conservation and recycle. The 
aromatization reaction is quite exothermic and its heat is most useful to 
generate steam used in other parts of the process.