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liatentediluly 26,. 1938
2,124,567
PATENT OFFICE
UNITED 1 STATES;
'
‘
'
'
‘
REISSUEi-J
2424.501
CONVERSION OF HYDBOGABBONS
Ariatidiv. Grease, Chicago, 11]., assignor to Uni
,veraai Oil Products Company. Chicago. 111., a
corporation of Delaware
-
'No'lh'awing. Application October 15, 1936. 80118]
4. JUN 25 1940
no. 105.111
4
(cl. zoo-.168)
' This invention relates‘ particularly to the con-‘_ hydrocarbon into an aromatic hydrocarbon of the
version of straight chain ' hydrocarbons into
closed chain or cyclic hydrocarbons. .
,4
.
' . More speci?cally, it ‘is concerned with a process
5 involving the use of special catalysts and speci?c
conditions of. operation in regard to temperature,‘
’ pressure ‘and time of reaction whereby aliphatic
hydrocarbons 'can be e?iciently, converted into
aromatic hydrocarbons.
10
'
' In the straight pyrolysis. of pure hydrocarbons
or hydrocarbon mixtures such as those encoun
tered in fractions from petrolemor other natur
ally occurring or synthetically produced hydro
carbon mixtures the reactions involved which
15 produce aromatics from paraiiins and ole?ns are
of an exceedingly-complicated character and con
not be very readily controlled.
—
_ It is generally recognized that, in the thermal‘
decomposition of hydrocarboncompounds or by
V 20 drocarbon mixtures of relatively narrow range
that whatever intermediate reactions are involved,
there is an overall loss of hydrogen, a tendency
to carbon ~separatlon'and a generally wider boil
ing range in the total liquid products as com
25 pared with the original chargez. Under mild
cracking conditions involving relatively low tem
peratures and pressures and short times of ex
posure to cracking conditions it is possible to
same number of carbon atoms by way of the pro
gressive steps shown. If this is done it‘ is usually
with very low yields which are of very little or no
practical signi?cance.
‘
'.
_
5
The search for catalysts to speci?cally control
and
accelerate
desired conversion reactions
'amonghydrocarbons has been attended with the -
usual difficulties encountered in ?nding catalysts
forv other types of reactions since there are no 10
basic laws or rules for predicting the effectiveness
of catalytic materials and the a as a whole is in
a more or less empirical state. 11 using catalysts
even in connection with conversion reactions
among pure hydrocarbons and_particularly in 15
connection‘ with theconversion of the relatively
heavy distillates and residua which are available
for cracking, there is a general tendency for the
decomposition reactions to proceed at a very rapid
rate, necessitating the use of extremely short time 20
factors and very accurate control of temperature
and pressure to avoid too extensive decomposition.
There are further di?lculties encountered. in,
maintaining the e?iciency of catalysts. employedv
in pyrolysis since there is usually a rapid depo- 25
sition of carbonaceous materials on their sur- ‘
faces and in their pores.
_
- _
The foregoing brief review of the art of hy-
'
some extent to control crackingreactions so that . drocarbon pyrolysis is given to furnish a general
30 they are limited ‘to primary decompositions and. background for indicating the improvement .in 30
there is a minimum loss of hydrogen and a max‘ such processes which is embodied in the present
imum production oi’ low. boiling fractions con
sisting of compounds representing the fragments
of the original high molecular weight compounds.
.35
.As the conditions of pyrolysis are increased in
severity using higherv temperatures and higher
times of exposureto 'pyrolytic' conditions, there ‘
is .a progressive increase in loss othydrogen and
a large amount of secondary reactions involving
40 recombination of primary radicals to form poly
mers and some cyclization toi'orm naphthenes
and aromatics, but the mechanisms involved in
invention, which may be applied to the treatment
of pure para?in or ole?n hydrocarbons, hydro
carbon mixtures containing substantial percent- .
vages oi’ para?ln hydrocarbons such as relatively 35
close cut fractions producible by distilling petro
leum, and analogous fractions which contain un
saturated as well as saturated straight chain hy- -
drocarbons, such fractions resulting from crack
ing operations upon the heavier fractions of pe- 40
troleum.
‘
'
In- one speci?c embodiment the present in
these cases are of so complicated a nature that veution comprises the conversion of aliphatichy
very little positive information has been evolved drocarbons including para?in and ole?n hydro
45 in spite of the large amount of experimentation ' carbons into aromatic hydrocarbons by subject- 45
which has been done and the large number of ing them at elevated temperatures of the order
theories proposed; In general, however, it may of 400-700° vC. to contact for de?nite times of the
be said that, starting with para?ln hydrocarbons order of 6-50 seconds with catalytic materials
representing the highest degree of saturation, comprising major proportions of aluminum oxide ‘ ,
these compounds are changed progressively into ,
ole?ns, naphthenes, aromatics, and ?nally into
of relatively low vcatalytic activity supporting 50
minor proportions vof oxides of elements selected '
carbon and hydrogen and other light ?xed gases. vfrom those occurring in the lefthand columns of"
It is- not intended to infer from this statement Group VI of the periodic table, these oxides hav
' that any particular success has attended the con‘
55 version of any given para?in or other aliphatic
ing relatively high catalytic activity. -
.
According to the 'present invention aliphatic or 55
2
2,124,007 '
_It will be seen from the foregoing that the scope
straight chain hydrocarbons having 6 or more
of the present invention is preferably limited to
carbon atoms in chain arrangement in their ' the treatment of aliphatic hydrocarbons which
structure are specifically dehydrogenated in such contain at least 6 carbon atoms in ‘straight chain
a way that the chain of carbon atoms undergoes arrangement. In the case of para?‘in hydrocar
ring' closure vwith the production in the simplest. ' bons containing less than 6 carbon atoms in linear
case of‘ benzene from n-hexane or n-hexene and arrangement, some formation of aromatics may
in the case of higher molecular weight paraiiins take place due to primary isomerization reactions
of various alkyl derivatives of benzene.‘ Under
although obviously the extent of this will vary
properly controlled conditions of- times of con-" considerably with the type of compound and the
10 tact, temperature and pressure, very high yields
of the order of 75 to 90% of the benzene or aro
matic compounds are obtainable which are far in
conditions of operation. The process is readily
applicable to parafiins from hexane up to dodec
excess of any previously obtained in the art either
with or without catalysts. For the sake of illus
crease in molecular weight beyond this point the
10
ane and: their corresponding oleflns. With in
percentage of undesirable side reactions tends to
trating and exemplifying the types of hydrocar
increase and yields of the desired alkylated aro
bon conversion reactions which are specificallyv matics decrease in proportion.‘ _
4
accelerated underthe preferred conditions by the
According to the present invention'composite
present types of catalysts, the following structural catalytic materials are employed which comprise
equations are introduced:
in general major proportions by weight of granu 20
15
lar activated aluminum oxide as a base catalyst or
CH
20
/
CH:
v
G1?‘
on’
CH1
‘
on
on
CH
CH
1:!
CH:
+411,
CH:
4 supporting material for minor proportions of
oxides of the elements in the lefthand column of
CH
n-hexane
of relatively low catalytic activity while» the ox- '
benzene
ides of the elements mentioned are of relatively
C-CH:
/
CH1
\
0st.
30
cal-on.
on,
on
a.
on;
on,
»_
on
on +411’
v
on
n-heptane
on
toluene
'
CH
1
'
/
,
\
CH:
CHI-CH:
0
CHI-‘CH:
a v
high catalytic activity and furnish by far the
'
on
CH:
n-octane
on
C-CH;
CH
C-CHs
:2
Group VI of 'the periodic table'comprising the
elements chromium, molybdenum, and tungsten.
The base material comprising aluminum oxide is
‘+411,
C
o-xylene
greater proportion of the vobserved catalytic ef 30
fects. The oxides of these several elements vary
somewhat in catalytic activity in any given re-‘
action comprised within the scope 'of the inven
tion and this variation may further vary in the. . v
case of different types of dehydrogenation and 35
cyclization reactions. Some of. the properties of
these catalytically active oxides, which are de
veloped on the surface and in the pores of the
alumina particles will be described in succeeding
‘paragraphs.
40
-
It should be emphasized that in the ?eld of
catalysis there have been very few rulesv evolved I
In the foregoing table the structural formulas. which would enable the prediction of what ma
of the primary para?ln hydrocarbons have been ~ terials would catalyze a given reaction. Most
represented as a‘ nearly closed ring instead of by , of the catalytic work has been done on a purely
empirical basis, even ‘though at times certain
45 the usual linear arrangement for the sake of indi
cating the possible mechanisms involved. No at ‘groups of" elements or compounds have been
found to be'more or less equivalent in accelerating
tempt has been made to indicate the possible in
termediate existence of mono-ole?nsf'diole?ns, certain types of reactions. is generally‘ preferable
hexamethylenes or "'alkylated ~ hexamethyienes'
so
Aluminum oxide which‘
which"mi“ghtjresult from the loss of various .as a base material for the manufacture of cata- '
amounts of hydrogen.v It is not known at the I lysts for the process may be obtained from natural ' 4'
present time whether ring closure occurs at the aluminum oxide minerals or ores such- as bauxite
loss of one hydrogen molecule or whether dehy
or carbonates such as dawsonite byfproper cal-'
cination,'or it may be prepared by precipitation
- drogenation of the chain carbons occurs so that
55 the ?rst ring compound formed is an aromatic of aluminum hydroxide from solutions of alumi
such as benzene or one of its derivatives. ‘The num sulfate or different alums, and dehydration
above three equations are of. a relatively simple
‘character indicating generally the type of reac
of the . precipitate of aluminum hydroxide by
heat. Usually it ‘is desirable ‘and advantageous
tions involved but in the case of n-para?lns or ' to further treat it with air or other gases, or by
60
mono-ole?ns of higher molecular weight than
the octane shown and in the case of branched
, chain. compounds which contain various alkyl
substltuent groups indifferent positions along
the six-carbon atom chain, more complicated re
65 actions will be' involved. ' For example, in the
case of such a primary compound as 2,3-dimethyl
" :hexane the principal! resultant product is appar
ently o-xylene although there are concurrently
produced‘
e?nite
yields 'of- such‘ compounds
as _
_
.
ethyl benzene indicating an isomerization of
two‘ substituent methyl groups. 'In the case of
other means to'activ'ate it prior to use.
so
_
Two hydrated oxidesof aluminum occurin na
ture, to-wit:- bauxite having the formul'aAhOa.v ' ' '
21-120 and, diaspore A12Oa.H2O. _ In both of' these
oxides iron sesqui-oxide may partially replace the
alumina. ‘These twominerals or corresponding
oxides produced from precipitated aluminum hy- ' e '
droxide ‘ are particularly suitable, ‘for the manu
facture of the present typev of catalysts and in
some instances have given the best results of any
of the base compounds whose .use is at present
nonanes which are represented bythe compound; contemplated:v > The mineral dawsonite having
" 2,3,4-trimethyl hexane, there is formation not'.. the formula Na:Al(COa) 3.2A1(OH)3 is another
only of mesitylene'but also of such compounds as
75 methyl ethyl benzol and various propyl benzols. i
mineral which may be used as a source'of alumi
num oxide.
-
'
75
-
2,124,507
3‘
It is best practice in the ?nal steps of preparing
to drive off’ water and leave a
aluminum oxide‘as a base catalyst to ignite it for heated
of oxides on the carrier particles. '. some time at temperatures within the approxi
In regard to uranium,,which is the heaviest‘
mate range of from 800-900“ C. This probably member
of the present natural group of elementsdoes not correspond to complete dehydration of whose oxides
are preferred as catalysts, it may‘
the hydroxide but apparently gives a catalytic merely be stated‘that while this element furnishesi
material of good strength and porosity so that it is catalytic oxides having some order of catalytic _
able to resist for a long period of time the deteri
orating effects of the service and regeneration
10. periods to which it is subjected.
_. _
,My investigations have also de?nitely demonstrated that the catalytic e?lciency of alumina,
activity, its scarcity and cost naturally pres,
cludes its extensive use in practice,
.
'rIt has been found essential to. the production 10
of high yields of aromaticsfrom aliphatic. hy
drocarbons when using the preferredtypes of‘
catalysts-that depending upon the aliphatic hy
which has some catalytic potency in itself. is
greatly improved by the presence of oxides of the ' drocarbon or mixture of
preferred elements in relativelyminor amounts,
hydrocarbons ‘ being '
treated, temperatures from 400Jl00° 0., should
usually of the order of less than 10% by weight be employed. contact times of approximately 6
of the carrier. It is most common practice to
i5 -
to 50 seconds and pressures approximating at
utilize catalysts comprising 2 to 5% by weight of mospherie, The use of subatmospheric pressures these oxides, particularly the lower oxides.
of the order of V4 atmosphere may be bene?cial ..
20
The oxides which constitute the principal ac-‘ in that reduced pressures generally favor selec 20.
tive catalytic materials may be deposited upon
tive dehydrogenation reactions but on the other
the surface and in the pores of the activated hand ‘moderately supe'ratmospheric pressures
alumina granules by' several ‘alternate methods‘ usually of the order of less than 100 lbs. per
such as for example, the ignition of nitrates squarevinch tend to increase the capacity ofcom
25 which have been adsorbed or ‘deposited from
mercial plant equipment so that in‘practice a_ .
aqueous solution by evaporation or by a similar balance is struck between these two factors. The
ignition of precipitated hydroxides. As an al
times of contact most commonly employed with
ternative method though 'obviouslyless prefer
n-para?lnic or mono-ole?nic hydrocarbons hav- ' '
ing from 6-12 carbon atoms tothe molecule are
able, the ?nely divided oxides may be mixed‘me
30 chanically with the alumina granules ‘either in of the'order of 8-20 seconds. - It will be appreci
30'
the wet or the dry condition. The point- oi’v - ated by‘ those familiarwlth the art of hydrocar-'
‘achieving the most uniform practical distrlbu- bon conversion in the presence of catalysts that
the factors of temperature, pressure and .‘time
tion of the oxides on the alumina should con
stantly be borne in mind since the observed: cata , will frequently have to be adjusted from there
35 lytic effects evidently depend principally upon a sults of preliminary experiments to produce the
surface action.
.
‘_
best results in any given instance. The criterion,
The element chromium has three oxides, the of the yield of aromatics will serve to'?x the
trioxide CI'OQ, thedioxide CrO; andv the sesqui- - best conditions of operation. -‘ In ageneral sense
oxide OM03, the last-named being readily pro
duced. by heating the trioxide in hydrogen or
. hydrocarbon vapors at a temperature of 250°‘
the relations between ,time, temperature and
pressure are preferably. adjusted so that rather
intensive conditions are employed of su?l'cient
severity to insure a maximum amount of the
cyclization reactions with a minimum of
equimolecularv mixture of the trioxide and the;_ desired
undesirable side reactions. If too short times of
sesquioxide.
The
oxides
are
readily
developed
45
on the surfaces and pores of alumina granules contact are employed the conversion reactions
C. The dioxide has been considered‘ to be an
50
by utilizing primary solutions of chromic acid 'will not proceed‘ beyond those of simple de
HaCrO; or chromium nitrate‘ Cr(NO=) a. The hydrogenation and the. yields of ole?ns and di-‘
ignition of the chromic acid, the nitrate or a ; ole?ns will predominate over those of aromatics.
While the present process is particularly ap
precipitated trihydroxide produces primarily the _ plicable
to the production of the corresponding
trioxide which is then reduced tov the sesquioxide
to furnish an active catalyst for use in reactions
' of the present character.
The two most important oxides of molybdenum
-55 which are alternatively employed as catalysts ac'-.
cording to the present invention ‘are the dioxide
M00: and sesquioxide M0203. Since the reduc "
tion of the trioxide by hydrogen begins at.300°
C. (572° F.) and the reduction is rapid at 450°‘
60 C. (842° F1). the e?’ective catalytic material is
‘
principally the sesquioxide. The trioxide may
aromatics from‘an aliphatic hydrocarbon or a
45
50.
mixture of aliphatic hydrocarbons, the invention
may also be employed to produce aromaticsfrom '
aliphatic hydrocarbon mixtures such as'distillates
from para?'inic or mixed base crude petroleum.
In this case the aromatic character of the dis 55
tillates will-.be increased and as a rule the octane
If desired and found '
number. will be higher.
feasible on a basis of concentration, the aromatics
produced in' the hydrocarbon mixtures may be v60
recovered as such by' distillation into fractions
be added-to the active alumina carrier from a -of
proper ,boiling range followed by chemical
.of ammonium molybdate which are added in treatment with reagents capable of reacting se
65 amounts just requisite to wet the carrier granules lectively with them. Another method of aromatic
uniformly and the mass is then dried and ignited. concentration willinvoive the ‘use of selective sol 65
solution in aqueous ammonia or from a solution
The'jelement tungstenhas- three oxides: the
trioxide W03, the dioxide WOzand the sesqui
oxide W203. The trioxide. is readily soluble in
vents such as liquid sulfur dioxide, alcohols,’ fur
fural, chlorex, etc;
'
_
.
_
In operating the process the general procedure
is to. vaporize hydrocarbons or mixtures of hy-.
70 aqueous ammonia from which it may be de
posited upon active alumina granules and it is drocarbons and after heating the, vapors to a
' ordinarily reduced preliminary to service by the vsuitable temperature within the ranges previously
action’ of hydrogen at a red heat. Tungstic acids speci?ed, to pass them through'stationary masses
may be precipitated from aqueous solution to of granular :catalytic materialin- vertical cylin
75 form the hydrated oxides’ and these may be . dricah treating columns 0 vbanks ‘of catalyst
containing tubes in parallel connection. ' Since
4-.
2,124,567
‘
i the reactions vare» endothermic it
,
which had been developed on the carrier-particles I '
may be neces
by the ignition of chromium nitrate and the‘ re
sary to apply some heat externally to maintain duction of the primary trioxide by hydrogen at
the best reaction temperature. After passing a temperature of_250-3G0° C. The hexane frac-‘
through the catalytic zone theproducts are sub
tion was passed through a bed of this cat'al'ystat
mitted to fractionationvto recover cuts or frac
a temperature of 525° C., atmospheric pressure
tions containing the desired aromatic ‘product and a time of contact of 20 seconds to produce
‘with the separation of. ?xed gases, unconverted _ a once-through yield -of about 47%. The ?nal
hydrocarbons and. heavier residual materials,
yield after recycling unconverted-hexane several
which may be disposed of in any suitable manner.
10
depending upon their composition. The overall
yield of aromatics may be increased by recycling
the unconverted straight chain hydrocarbons to
further treatment with fresh material, although
'
10,
times was above 90% as previously stated.
Example '11 f
In this case n-heptane was converted to toluene
this is a more or less obvious expedient and not
speci?cally characteristic of the/present invention.
utilizing a catalyst supporting molybdenumpxe
ides on the preferred alumina base. The catalyst 15
was made by utilizing a solution of ammonium"
It is an important feature of the present proc- . molybdate in an excess of ammonia and adding
ess that the vapors undergoing dehydrogenation the concentrated solution to about three times its _
should be free from all but traces of water vapor
since the presence of any substantial amounts of
weight of granular alumina particles followed by
steam ‘reduces the catalytic selectivity of the
and ammonia and leave a residue of the trioxide.
Before ‘service the particles were treated with
hydrogen at about 450° C. to reduce a material
composite catalyst to a marked degree. . In. view
of the empirical state of the catalytic art, it is
not intended to submit a complete explanation
‘of the reasons for the deleterious in?uence of
water'vapor on the course of the ‘present type
of catalyzed reactions, but it may be suggested
that the action of‘the steam may be to cause a
partial hydration of alumina and some of the
catalytic oxides due to preferential adsorption‘ so
that in effect the hydrocarbons are prevented
from reaching or being adsorbed by the catalyti
cally active surface‘.
.
careful mixing and calcining to drive o? water v20
portion of the trioxidev to lower'oxides such as
the sesquioxide.
effective in removing hydrogen from chain com
pounds in such a way that cyclization may be
promoted without removal of hydrogen from end
-
.
25
.
particles at a temperature of 555°v C'., atmospheric
pressure and 13 seconds contact time to produce
a yield of approximately 50% of toluene on a
once-through basis, this yield being ?nally raised
so
to about 80% by complete recycling of uncon
verted material.
_
The present types of catalysts are particularly
.
, n-I-Ieptane.was passed over a'bed of the catalyst
Example III -
v This example is given to illustrate the direct
formation of toluene from n-heptene, which con
version was accomplished using the catalyst simi
lar to that described under-Example II, a temper
as
carbon atoms so that both end and side alkyl
groups may appear as substituents in benzene ’ ature of 510° C., atmospheric pressure and a time
rings and it has been found that under proper of contact of approximately 20 seconds. The 40
operating conditions they do not tendto promote once-through yield of toluene was ‘76% and the
any great amount of undesirable side reactions ultimate yield was in the neighborhood of 93
- leading to' the'deposition of carbonor carbona
ceous materials and for this reason show reac
tivity over relatively ‘long periods of time. ‘When
their activity begins to diminish‘ after a period
of service, it is readily regenerated by the simple
expedient of oxidizingwith air or other oxidizing
gas at
a_
moderately elevated
temperature, usually
within the range employed in the dehydrogena
.0 tion and cyclizationreactions. This oxidation
effectively removes traces of carbon deposits
which contaminate the surface of the particles
and decrease their e?iciency. It is characteristic
of the present types of catalysts that they may
95% by recycling unconverted ole?n.
_ Example v1V
To prepare thecatalyst'an ammonlacal aqueQ 45
ous solution of tungsten trioxide was used to 'de- .
posit the trioxide upon an activated alumina.
After reduction with hydrogen, analyses showed
there was. present from 4-5% of mixed tungsten
,
oxides.
,
1
50'
‘
'
Using the above catalystthevapors of n-hep- tane were treated at a temperature of 560° ‘0.,
substantially atmospheric pressure, and 15 seg/
onds contact time to produce aHyieldsofW46%' of
toluene on a once throughbasi's which was ?nally 55
.- be repeatedly regenerated with only a very grad
brought to'about a r16% ultimate yield after sev
ual loss of catalytic efficiency.
recyclings of unconverted charge.
During oxidation with air or other/oxidizing eral
The foregoing speci?cation and examples show
gas mixture in regenerating partly spent matew ,
clearly the character of the invention ‘and the
rial, there is evidence to indicate that the lower res'ults'to be expected in its application to all 60
oxides are to a large extent, if not completely,
60
- oxidized to higher oxides which combine with
phatic. hydrocarbons, ~although neither section
is intended to be unduly limiting.
aluminum oxide to form aluminum salts of vari
I claim as-my invention:
'
_
able composition. Later. these salts are decomé
l. A process for the production of aromatic
posed by contact with reducing gases in the ?rst \hydrocarbons
from aliphatic hydrocarbons’ of 65
stages of service ‘to reform the loweroxides and
regenerate the real catalyst and hence thecata-' from six to twelve carbon atoms, which comprises .
lytic activity.
'
'
'
Example I
In this example an ultimate yield of 'over 90%
benzol was produced by the catalytic conversion
,of an n-hexane. fraction‘ obtained-from a highly
dehydrogenating and cyclicizing the aliphatic
hydrocarbon byssubjectionto a temperature of
the order of 400 to ‘700° C. for a period of about _ 6 to 50 seconds, in the presence of, an aluminum 70 a
oxide catalyst containing a relatively small amountof an oxide of a metal fromthe left. hand
para?ln'ic crude petroleum by close fractionation.’ ,column of Group 'VI' of -the periodic table and
. selected from the class consisting. of chromium,‘
The catalyst comprised an alumina base support75 ing about 4% by weight of chromium sesquioxide
'
molybdenum,v tungsten ‘and uranium. , .
‘
'
75
2,124,007
2. A~process for the production of aromatic
hydrocarbons from aliphatic hydrocarbons of
from six to twelve carbon atoms, which c0m-,
prises dehydrogenating and cyclicizing theoali
phatic hydrocarbon by subjection to a tempera
ture of the order of 400 to 700° C. for a period oi.’
about 6 to 50 seconds, in the presence of an
aluminum oxide catalyst containing a relatively
small amount of an oxide oi.’ chromium.
10
3. A process for the production of aromatic hy
drocarbons from aliphatic hydrocarbons of from
six to. twelve carbon atoms, which comprises de
hydrogenating and cyclicizlng the aliphatic hy
drocarbon by subjection to a temperature otthe
v
5
order 01' 400 to 700° C. for a period'o! about 6
to 50 seconds,‘ in the presence 0! an aluminum
oxide catalyst containing, a relatively small
amount of an oxide of molydenum.
4. A process for the production of aromatic hy
drocarbons irom aliphatic hydrocarbons of from
six to twelve carbon atoms, which comprises def
hydrogenating and cyclicizing the aliphatic hy
drocarbon by subjection td a temperature of the
order ot’400 to IZ00“ C. for a period or about 6
to 50 seconds, in the presence of an aluminum
oxide catalyst containing a relatively vsmall
amount of an oxide of tungsten.
-
' '
,ARISTID V. GROSSE,
.
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