Note: Descriptions are shown in the official language in which they were submitted.
117~796
.
15732-36/KP/io
Haldor Tops~e A/S, Lyngby, Denmark.
Process and catalyst for the preparation of a gas mixture
having a high content of lower olefins, and process for the
preparation of the catalyst.
Field of the invention
The present invention relates to a process for the
preparation of a gas mixture having a high content of lower
olefins, particularly C2-C4 olefins, by the catalytic
conversion of a synthesis gas containing hydrogen and carbon
oxides (carbon monoxide and/or carbon dioxide) and optionally
other gases.
. ~
~1~5796
Background of the invention
Synthesis gas is particularly prepared by the gasification
of coal or heavy petroleum fractions, most often by treatment
with a mixture of steam and oxygen or an oxygen-containiny yas.
Hereby mainly the following reactions take place:
(1) C + H20--~ CO + H2
(2) C + 1/2 2 ~ CO
accompanied, however, by side reactions so that also carbon
dioxide and a little methane are formed. By the gasification
of petroleum fractions the amount of hydrogen in the
synthesis gas becomes higher. Certain carbon gasification
processes cause the formation of larger amounts of methane,
other hydrocarbons, tar, etc. By the gasification oxygen is
normally added in sufficientamount to render the gasification
self-supplying with heat.
By the Fischer-Tropsch synthesis (in the following
abbreviated to the FT-synthesis),synthesis gas may be
converted into me-thane, higher paraffins, olefins, and
possibly also aromatic compounds:
.
( )H2 t CnH2n+2 + nCO2 (paraffin reaction)
(4) 2nCO + nH2 ~ CnH2n + nCO2 (olefin reactlon)
and possibly also
(5) nCO + 2nH2---~CnH2n + nH2 (olefin reaction)
By the socalled shift reaction
(6) CO + H2O~ `CO2 + H2
an equilibrium between carbon monoxide and carbon dioxide
hereby sets itself; by the addition of equation (4) and
equation (5) it is seen that carbon dioxide can also be used
-
~1~757~6
.
as starting point for the olefin production together with
hydrogen:
2 + 3nH2 > CnH2n + 2nH2
A special example of equation (2) is the formation of
methane according to the equation
(8) 2CO + 2H2~ ~CH4 + CO2,
and methane may also be formed from carbon dioxide
according to the scheme
(9) 2 2 CH4 + 2H2o
During the 2nd World War the FT synthesis was used
industrially in Germany and Japan to prepare motor fuel but
at present is has only industrial importance in the Republic
of South Africa for the preparation of hydrocarbons. The
increasing oil prices cause, however, that it is again
attractive to use the FT synthesis for various syntheses. A
condition for rendering it economically profitable is, however,
that the selectivity and activity of the catalysts necessary
to carry out FT-syntheses are improved considerably.
The FT synthesis is a kind of polymerization
reaction wherein the yield structure follows the socalled
Flory distribution (see for instance G.Henrici-Olive et al,
Angew. Chem. 15, 136, 1976, and H. Schultz et al, Fuel Proc.
Technol. 1, 31, 1977), a theoretical distribution of the
various chain lengths which can be deduced mathematically
from simplified kinetic assumptions. It can be shown that the
Flory distribution theoretically can give a maximum yield of
29.63% C2 hydrocarbons (ethane and ethylene), calculated as
% by weight of the carbon (C in CO and CO2) which is converted
into hydrocarbons. By a suitable adjustment of the process
parametres,the yield structure in FT-syntheses may be varied
and according to what one wants to optimize, theoretical
Flory distributions may be for instance 44.44% Cl (methane),
~1757~
.
29.63 C2, 51.0% C2 4, and 4.53% C5- and higher hydrocarbons
if one attempts at maximizing C2; and 25.0% Cl, 25.0% C2,
56.25% C2 4, and 18.75% C5 and higher if one attempts at
maximizing the formation of C2-C4-hydrocarbons. It must be
emphasized, however, that the values in question are theoretical
ones, and in FT-syntheses in practice the C2-content is almost
always substantially below that theoretically expected.
Thus it appears (page 480) from a review of FT-
syntheses in Kirk-Othmer, Encyclopedia of Chemical Technology,
second edition, vol. 4, pp. 446-489, that in the socalled
Hydrocol process it is typically achieved that 60-65~ of
the CO input is conver-ted to C3 and higher hydrocarbons,
15-20% to Cl and C2 hydrocarbons, about 10% to oxygenated
compounds and 10-15% to carbon dioxide. Within the individual
fractions olefins may often be predominant and a substantial
proportion of the product may be obtained as a liquid product,
viz. a highly olefinic gasoline. From the review article by
H. Schultz and J.M. Cronje in 4th edition of Ullmann's
Encyklopadie der technischen Chemie, vol. 14, 32~ (1977) it
is seen that by the socalled Synthol process one typically
obtains that about 13% by weight of the carbon (in CO and
CO2) which is converted to hydrocarbons comes out as methane,
about 10% as C2 hydrocarbons, 39-40% as C2 4 fractions, and
about 47% as C5 and higher fractions. By adjusting the process
so as to maximize C2, typical corresponding results are 50,
17, 41, and 9%, respectively,and by adjusting to maximize the
C2 4 fractions 30, 15, 46, and 24, respectively.
The parameters determining the actual distribution of
the hydrocarbons when converting synthesis gas first and
foremost are the kind of catalyst, the pressure, and the
temperature. The most important catalyst for methanation is
nickel on a support and the most important FT catalysts are
iron and cobalt on supports; however, the majority of metals
have been tried for use in FT syntheses and with very varied
results.
As a general rule a high temperature in the conversion
favours the formation of methane, lower temperature the
formation of higher hydrocarbons, whereas higher pressure
~'7S~6
.
favours the formation of higher hydrocarbons, lower pressure
the formation of methane. A comparatively low activity of a
catalyst in most cases can be increased by increasing both
pressure and tempera~ure, as a rule with consequences on the
yield structure. Moreover it is true that under otherwise equal
conditions there is a tendency that the higher the temperature,
the higher the chance that hydrocarbons of a given chain
length come out as olefins rather than paraffins, in other
words that the increased temperature counteracts
hydrogenation. If a high proportion of olefins and particularly
of short chain olefins is desired,it is accordingly desirable
to operate at a relative high temperature and with a highly
active catalyst having good selectivity for such hydrocarbons.
Prior art
From German Patent Publication No. 2 518 964 it is
known to prepare unsaturated hydrocarbons, particularly
gaseous olefins, by the catalytic hydrogenation of carbon
oxides with hydrogen at 250-350C and 10-30 bar in the presence
of catalysts containing difficultly reducible oxides of the
transition metals of group V or VII in the Periodical Table
in combination with metals of group VIII and also optionally
activators, e.g. alkali metal carbonates, magnesium oxide or
zinc oxide, and support materials such as silicic acid,
kieselguhr, silicates, or aluminium oxide. In experiments
with converting a synthesis gas containing equal parts by
volume of CO and H2 there was obtained, according to the
Examples of the publication, at a conversion of about 85~ CO
a reaction product the components of which were distributed
with about 26-31%w of ethylene, about 1~3-22% propylene, and
about 14-17~ butylene whereas saturated C2-C4 hydrocarbons
constituted about 13-16~, methane about 9.5-12~ and the
remainder was hydrocarbons containing above 4 carbon atoms.
It is difficult to evaluate the figures of the publication
but the yield seems low. As in the majority of FT syntheses
and methanations there must be operated with a feed gas which is
substantially free of sulfur compounds, and although the
publication states that there may be used CO/H2 ratios from
~1757g6
2:1 to 1:2, there should rather be used a feed gas containing
fairly equal a~ounts of the two gases, or preferably a small
excess of carbon oxides.
German Patent Publicat.ion No. 26 53 985 describes
and claims a catalyst for the reduction of carbon monoxide
with hydrogen while forming hydrocarbons having essentially
1-4 carbon atoms. The conversion is carried out at 150-500C,
a pressure up to 100 atm. absolute and a space velocity of
100-3000 Nl gas/l catalyst/hour by the aid of a catalyst
formed by the precipitation in one step and subsequent
thermal decomposition of a precursor in the form of salts
of hydrocyanic acid having the general formula ~eIMeII(CN)x
wherein MeI is Ce, Cu, Co, Ni, Fe, Mn, Zn, Ag, K or mixtures
thereof or Ca and Mg in mixture with (NH4), and MeII is Cu,
Co, Ni, Fe, Mn, Zn, Ag or mixtures thereof and x the sum of
the metal valences. The synthesis gas to be converted by the
aid of such a catalyst only contains H2 and Co, present in a
proportion of 3:1 to 1:2. Also the space velocities usable
in the conversions are fairly low.
German Patent Publication No. 25 46 587 claims and
describes a catalyst having the same use as thatdescribed
in the above German Patent Publication No. 26 53 985. The
catalyst consists of iron or iron and copper and is formed
by the precipitation in one step of a compound of formula
Mex[Fe(CN)6]y where Me is iron and copper, and an after-
treatment. German Patent Publication No. 27 04 575 describes
and claims a catalyst of same kind as that of the
abovementioned Publication No. 25 46 587, only with the
difference that the active catalyst component is in
admixture with a support material. According to these two
publications, the catalysts and precursors are different
from those of the invention, as are the gas to convert and
the space velocity.
~175~16
Objects of the invention
- It is a main object of the invention to provide a
process which can convert a synthesis gas containing carbon
oxides - not only carbon monoxide - and hydrogen at a high
space velocity into a hydrocarbon mixture having a high
content of the gaseous olefins, i.e. ethylene, propylene
and the butylenes, because the C2-C4 olefins are an important
raw material in petrochemical syntheses.
Especially it is an object of the invention to provide
such an embodiment of the FT synthesis that a substantial
proportion of the other hydrocarbons formed by the
conversion of the synthesis gas will be produced as methane
which can be sold or used as fuel, and at the expense of
higher hydrocarbons and other saturated hydrocarbons than
methane.
It is a further object of the invention to provide a
catalyst capable of giving such high yields of C2-C4 olefins
with methane as the predominant by-product (apart from water
or steam inevitable formed in the reaction). A still further
object is to prepare a process and a precursor for preparing
the said catalyst. And finally it is an object of the invention
to provide a process for preparing the said precursor.
It has been found that if a precursor consisting of
or containing one or more metal cyanides of a certain chemical
composition and having a cubic crystal structure with a
lattice parameter of about 5-lOA and a maximum crystallite
size of 200~ is converted in t~o steps into the active
catalyst, then it is possible to convert a synthesis gas
containing only a small amount of CO, at a high space velocity
(about one order of magnitude higher than that employable
according to the abovementioned German Patent Publication No.
26 53 985) into a product gas having a high content of the
desired C2-C4 olefins.
srief description of the invention
According to the present invention a gas mi~ture which
apart from steam formed as a by-product mainly consists of
hydrocarbons and has a high content of C2-C4 olefins can be
~757~6
prepared by the catalytic conversion at a pressure of
l-lS0 bar and a temperature of 200-600C of a synthesis
gas containing carbon oxides, hydrogen and optionally gases
inert in the reactions, if according to the invention the
content of carbon monoxide in the synthesis gas i5 at most
5% by volume and the conversion is carried out at a space
velocity of 1,000-50,000 Nl synthesis gas per kg catalyst
per hour in the presence of a catalyst comprising in intimate
mixture at least two heavy metal components as free metal,
carbides, carbonates and oxides of the metals mentioned under
(b) and (c) below, and optionally an alkali metal carbide,
carbonate or oxide, said catalyst having been prepared by
the reduction and subsequent carburization of a catalyst
comprising at least one complex metal cyanide of (a) an
alkali metal or ammonium, (b) at least one heavy metal
forming difficultly reducible oxides and belonging to group
3 in the Periodical Table, and (c) at least one of the
metals having atomic number 24-30, said metal cyanide or
cyanides having cubic crystal structure with a lattice
parameter of about 5-loA and a crystallite size below 200A;
the precursor - and hence the catalyst - may optionally
contain also a support material which is preferably selected
amongst oxides of the metals which form part of the catalyst
and precursor.
.
Detailed description of process according to the invention
As feed gas there is used a synthesis gas with varying
content of hydrogen and carbon oxides and optionally
containing other gases, e.g. nitrogen and argon originating
from combustion air if the gasification partly or entirely
has been carried out with air as oxygen source; they
do not disturb the process. The volume ratio hydrogen to
carbon oxides will normally be in the range from about
0.4:1 to about 4:1 and most frequently a ratio H2/(CO + CO2)-
between 2 and 4, particularly close to 3 is preferred. If the
content of hydrogen in the synthesis gas is too low, it may
be enriched therewith, e.g. by means of reaction with steam
, ,~.
, .. ...
1175~
according to the shift reaction (6). In most cases this is
desirable in order to obtain a low content of CO in the feed
gas since a high CO-content may cause excessive carbon
deposition on the catalyst. As mentioned the feed gas
contains at most 5% by volume and preferably at most 2%
by volume of CO. The feed gas must be substantially free
of sulfur and sulfur compounds in order to avoid poisoning
of the catalyst; the content thereof must not exceed 1 ppm
and preferably not 0.1 ppm, calculated as volume of H2S on
the volume of the feed gas.
The working pressure at the reaction is not very
critical and will normally be between 5xlO and lOOx105 N/m2
(.e. between about 5 and about 100 bar) although both lower
and higher pressures may come into question, preferably
between 15 and 50 bar and especially of about 25 bar.
Neither is the working temperature very critical but the
reaction is operated near the upper end of the temperature
range normally employed in FT syntehses because a low
temperature favours hydrogenation as mentioned and thereby
the formation of paraffinic rather than olefinic
hydrocarbons. The reaction may advantageously be carried out
at temperatures from 250 to 500C although lower or higher
temperatures may be used in special cases,and preferably the
reaction is operated at 300-400C, for example about 340C.
The process is operated at space velocities (SV) in
the range from 1,000 to 50,000 Nl~kg/h (Nl synthesis gas
per kg catalyst per hour).Compared to what is disclosed in
the abovementioned German Patent Publication No. 26 53 985
this is an increase by a factor of at least 10 since this
publication mentions an SV of 100-3000 Nl/l/h and a liter
of catalyst roughly is the same as a kg of catalyst.
Preferably there is used a space velocity of 3,000 to 20,000
Nl/kg/h. The high SV used according to the present invention
is very advantageous because it involves a much more
efficient utilization of the reactor and accordingly reduced
capital costs for a plant to carry out the process.
57~6
The process may be conducted as a multistep process
with one passage of the gases and with pressurized
condensation between the reactorsj or it may be conducted
with recycling of unconverted feed gas and part of the
lighter fractions of the product gas. In such a recycle
reactor or series of recycle reactors there may be several
cooling sections; cooling gases should preferably contain
much more carbon monoxide than the synthesis gas (feed gas).
The catalyst may be placed as a fluid bed or preferably fixed
bed in one or more reactors. The reaction is exothermal and
it is necessary as mentioned above to limit the increase in
temperature in some way or another. This can be done by using
an adiabatic reactor with recycling andadmixture into the
feed gas of part of the product gas. The reactor may also be
a really cooled reactor wherein the catalyst is placed in
tubes surrounded by a cooling medium such as boiling water,
boilin~ Dowtherm (high boiling heat transfer media) or
flowing gas; the arrangement may also be the reverse.
Possibly one or more adiabatic and one or more cooled
reactors may be combined according to similar principles as
- those described in British patent publication No. 2,039,518.
The actual construction of the reactor will depend to a
high degree of the sources and composition of the synthesis
and the particular product the formation of which one
particularly wants to promote.
The main purpose of the product gas is to utilize
the gaseous ole~ins (i.e. C2 to C4) formed in petrochemical
syntheses. Methane formed may be separated off and sold or
used as fuel. Higher, liquid hydrocarbons may if desired be
used as liquid fuel and other by-products may if desired also
be used in organic syntheses or decomposed into usable product
in various ways.
The main reason for the high yield of the process
is the specific catalyst employed, and especially it is
important that the catalyst has been prepared in two steps -
reduction and carburization - from a precursor comprising
one or more complex cyanides having the cubic structure with
a lattice parameter of about 5-lOA and a crystallite size
not exceeding 200A and preferably being about 100~. This
, , ~
~757~36
11
structure is important for the very intimate mixture of
the metals in the finished catalyst and hence the high
olefin yield. The catalyst, the precursor and the cyanides
will be discussed more fully in the next section of this
specification but is should be mentioned at this spot that
although it may contain the ammonium ion as ion of type
(a), this is preferably an alkali metal and especially a
potassium compound. In practice it will most often be
potassium carbonate because this will be formed by the
carbidization of the precursor following the reduction. The
reason for preferring potassium is the well-known fact that
alkali metals and notably potassium increase the activity
of FT catalysts.
ther aspects of the invention
The catalyst according to the invention comprises
at least two heavy metals in free and/or chemically combined
form and in addition to that preferably an alkali metal
compound, and the catalyst formed by the reduction and
subsequent carburization of a precursor consisting of one or
more complex cyanides as already described and having the
structure likewise described. Optionally, the catalyst may
contain a support.
Preferably there is an alkali metal rather than
ammonium present in the complex cyanide or cyanides
constituting or present in the precursor. The alkali metal
will be carried over into the finished catalyst and under
otherwise e~ual condition will increase the yield of gaseous
olefins. If the precursor contains ammonium ions instead of
alkali metal ions, the former will disappear during the
conversion of the precursor into catalyst so that the latter
will only contain the two components lb) and (c); thus there
may be only two metals/metal compounds present but preferably
two or more metals are present in component (c), both when
component (a) is an alkali metal and when it is the ammonium
ion.
The metal forming difficultly reducible oxides first
and foremost serve at giving the catalyst a good
~1~757~6
12
thermostability. However, conceivably this component also
to some degree may have a similar function as the support
in other catalysts, i.e. contribute to give the catalyst
mechanical strength and large surface area. As stated this
metal must belong to group 3 in the Periodical Table,
which first and foremost means aluminium and the rare earth
metal (the lanthanides), particularly cerium but also
scandium, yttrium, gallium, indium, thallium and the
actinides. As to the Periodical Table reference is made to
53rd edition of Handbook of Chemistry on Physics, CR~
Press, Cleveland, Ohio, 1972-3. According to this, Al
belongs to group 3a and Ce and the other lanthanides to
group 3b.
The metals Nos. 24-30 mainly have importance for the
selectivity of the catalyst with respect to forming lower
olefins, and by the choice hereof it is to some degree
possible to control the process. Preferably more such metals
(in free and/or combined form) are present in the ultimate
catalyst and consequently also in the complex cyanide or
cyanides constituted or comprised by the catalyst. The metals
with numbers 24-30 are chromium, manganese, iron, cobalt,
nickel, copper, and zinc.
Very conveniently the catalyst contains iron as iron
carbide, copper as free metal and optionally a further metal
of type (c). The catalyst thus very conveniently may contain
(a) potassium carbonate, (b) aluminium oxide or an oxide of
one of the rare earth metals, preferably cerium or
lanthanum, and (c) iron carbide. Very good selectivity is
obtained if the catalyst besides this contains at least one
further carbide, preferably cobalt carbide, and metallic
copper.
The invention moreover relates to a process for
preparing the catalyst. In this process one reduces
a precurser containing one or more complex cyanides of (a)
ammonium or an alkali metal, (b) one or more metals forming
difficultly reducible oxides and selected from the metals
of group 3 in the Periodical Table, and (c) one or more
metals selected from the metals having atomic number 24-30,
the cyanide(s) having the cubic crystal structure already
11757~316
13
described, by heating in a stream of hydrogen after which
- the product formed is carbidized at high pressure and
elevated temperature in an atmosphere containing hydrogen
and carbon oxides and being substantially free of free oxyyen.
Before converting the precursor to the catalyst it
must be dried and this must take place at temperatures below
150C in order to avoid substantial premature decomposition
of the complex cyanides. It is important that the precursor
contains complex cyanides containing at least two and
preferably at least three metals as described hereinbefore
because only thereby one obtains a very intimate admixture
of the metals/metal compounds in the ultimate catalyst, so
important for the use of the catalyst.
The dry catalyst is heated in a stream of hydrogen
and thereby a decomposition of the complex cyanides takes
place. The decomposition usually takes place at a temperature
between 300 and 350C and is highly exothermal and
autocatalytic; thus it does not need any added catalyst. It
is most simply-controlled by adjusting the stream of H2.
During the decomposition NH3, CH4 and HCN are evolved
together with small amount of methyl amine. Because of the
toxicity of the hydrogen cyanide and the highly exothermal
nature of the reaction the heating must be carried out very
carefully. When ammonia no more can be ascertained in the
exit gas the reduction can be considered completed.
The product obtained, wherein the two or three metal
components are present in the form of free metal in a very
finely divided form, is highly pyrophoric and extremely
difficult to handle in air or other gas mixture containing
free oxygen; it should therefore be excluded from contact
with free oxygen. The product must be carbidized. This is
most expediently carried out by the aid of a mixture of
hydrogen, carbon monoxide and carbon dioxide with a ratio
H2/(CO + CO2) of between 1:1 and 10:1, preferably between
2:1 and 4:1, i.e. with a gas mixture reminding of that used
as feed gas for the olefin synthesis. However, for the reasons
stated the carbidization gas should be substantially free
~75~16
14
of free oxygen. The temperature at the carburization is
preferably in the range of 300-400C and the pressure must
be rather high, conveniently around 2X106 N/m2 (about 20 bar).
An almost complete carburization can be carried out in about
24 hours. By "complete carburization" is not meant, however,
that all metals are completely converted into carbides. Thus,
alkali metal present in the finished catalyst as a rule will
exist as carbonate, possibly as oxide. The metal of group
3, which as stated is one forming difficultly reducible
oxides, will normally exist in the finished catalyst as oxide,
possibly a mixture of several oxides of one or more metals
because several of these metals have more than one oxydation
states.The metals having atomic numbers 24-30 in the
finished catalyst normally will exist as carbides, copper,
however, as a rule as free metal.
After the carburization the metals/metal compounds
exist as a powder in which, however, all of the components
normally will be present in each powder particle. The powder
is cooled, preferably in a stream of dry nitrogen and
thereafter can be mixed with graphite or ammonium stearate
and pelletized in known manner.
The carburized powder may also be mixed with unreduced
precursor in powdery form and in any proportion, preferably
however such that the amount of unreduced precursor
constitutes 10-80 % by weight of the mixture. Thereafter
another reduction on H2 gas is carried out until the exit
gas no longer contains ammonia. No further separate
carburization is carried out but carburization will take
place in the reactor in which the catalyst is used for an FT
synthesis~
The catalyst does not need any support but may be
supplied with one in known way. Thus, the complex cyanides
may be deposited on a suitable refractory oxidic support
which, however, preferably does not contain oxides of other
metals than those which may be present in the catalyst as
described. The support material is present during the
reduction and carburization processes. It is also possible
11~5~6
]5
to admix the carbidized powder with powdery support material
and, for instance, aluminium stearate or graphite powder,
pelletize the mixture and calcinate it carefully to a porous
catalyst.
The invention moreover relates to a precursor for use
in the preparation of the catalyst described. Thi-s
precursor is composed and has the structure described above
and very conveniently consists of a complex cyanide of (a)
potassium, (b) aluminium or cerium, and tc) iron and
optionally also cobalt and/or copper.
T~e advantage in preparing the catalyst from such a
precursor chiefly is the very intimate mixture of the various
components obtained and the very small particle size and
thereby the very big active catalyst surface achieved.
Preferably the complex cyanide or cyanides have a
crystallite size of about 100~. By crystallite size hereby
the average length of crystallites is meant, determined by
X-ray diffraction according to the method described in
Chapter 9 in "X-Ray Diffraction Procedures" by H.P. Klug
and L.E. Alexander, Chapman and Hall Ltd., London 1959. The
principle of "diffraction line broadening" mentioned therein
is based on Scherrer's method as described in Nachrichten der
Gesellschaft der Wissenschaft, Gottingen 1918, Vol 98.
The invention finally relates -to a process for
preparing the precursor. Fundamentally t it can be prepared
from arbitrary cyanides, especially of an alkali metal,
preferably potassium, and a heavy metal by precipitation in
two or more steps with further metals selected amongst the
categories mentioned, whereby almost any metal may be
introduced into the complex cyanide structure. Thus, as
starting material it is possible to use such complex cyanides
as K4V(CN)6, K3V(CN)6, K4Cr(CN)6, K3Cr(CN)6, K4Mn(CN)6 ,
K3Mn(CN)6, K3Fe(CN)6, K4Co(CN)6, K3Co(CN)6, K4Mo(CN)8,
K Mo(CN)8, K4Rh(CN)6, K3Rh(CN)6, K4W(CN)8, 3 8 4 6
K40s (CN) 6 and K4Fe(CN) 6.
il75~36
16
However, as starting material it is most advantageous
to use ammonium or alkali me-tal salts of ferrocyanic acid,
partly because these compounds are readily available and
partly because the ammonium groups or metal atoms in these
salts comparatively easily can be partly replaced by other
metals. This process therefore according to the invention
is carried out by treatment of an ammonium or alkali metal
salt of ferrocyanic acid in aqueous solution first with a
water-soluble salt of a metal of group 3 in the Periodical
Table forming difficultly reducible oxides, and then with one
or more water soluble salts of one or more metals having
atomic number 24-30. It is observed that it may come into
question to react with an iron salt, namely if there is
desired a Fe-content above 57%.
In this way there is formed a complex cyanide of partly
either ammonium or an alkali metal, partly at least three
heavy metals, viz. a group 3 metal, iron and at least one
further metal having atomic number 24-30.
The first reaction, that of the salt of ferrocyanic
acid with the group 3 metal salt, is carried out
at a temperature near the boiling point of the solution. Even
if one might use solutions in organic solvents in which the
reagents are soluble, it is only realistic to use aqueous
solutions and the process of the invention is limited to
this; the reaction temperature in the first reaction, i.e.
between the starting cyanide and a salt of a group III metal,
is therefore near 100C, e.g. about 90C. The reaction may
be completed in a few minutes up to a few hours, frequently
in about 1/2 hour. The reaction product, which when using
potassium ferrocyanide and an aluminium salt will be a
complex cyanide of potassium, iron and aluminium, usually
remains in solution but will be precipitated in the next step.
In this one or more water-soluble salts of the metal
with atomic numbers 24-30 are added and thereby there is
precipitated a complex cyanide of alkali metal, preferably
potassium, or ammonium, aluminium or other group III metal,
and a further hea~y metal with atomic number 24-30. The
temperature in this reaction may be lower than in the
~5796
17
preceding one and may for instance be from room temperature
- to about 100C, e.g. about 50 C. In this reaction a
precipitation takes place so that a slurry is formed, and
the amount of water relative the amount of reactants
preferably is adjusted so that the slurry becomes relatively
thick, which facilitates the working up.
The slurry is thereafter washed; hereby it is freed
from salts of ammonium or alkali metal with the anion or
anions present in the salts used in the reaction, e.g.
potassium nitrate, since the conversion of the cyanide
consists in replacing parts of the ammonium or alkali metal
kation with one or more of the heavy metals concerned. After
the washing it is expedient to add a little, e.g. 1-10 % by
weight and conveniently 5% graphite, aluminium stearate or
a similar materiale to the filter cake and dry, e.g. spray-
dry it. Thereafter one may optionally pelletize the material
but even without pelletizing the precursor may be considered
finished.
The subsequent conversion of the precursor into
catalyst has been described hereinabove.
Example 1
5000 g of K4Fe~CN)6.3H2O and 1900 g of Al(NO3)3.9H2O
were dissolved in 50 1 of demineralized water. The solution
was heated at 90C for 30 minutes and cooled at 50C.
2400 g of Cu(NO3)2.3H2O and 1200 g of Co(NO3)2.6H2O
were dissolved in 25 1 of demineralized water and the
solution was heated at 50C.
The latter solution was poured into the firstmentioned,
whereby a thick slurry was formed. It was stirred for a few
minutes and thereupon filtered and washed free of KNO3. The
filter cake was collected, about 5% graphite was added,and
it was spray-dried.
Some of the powder thereby formed, which is a catalyst
precursor according to the invention, was pelletized and was
thereupon subjected to reduction for about 72 hours in an
hydrogen atmosphere at 300-350C, after which the reduced
-
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material without coming into contact with free oxygen was
introduced into a reactor through which a mixture of H2,
CO, and CO2 was passed at a temperature of 300-400C and a
pressure of about 2X106 N/m2 for 24 hours. The carbidized
material obtained was mixed with unreduced powder in the
ratio 1:1 and the mixture was pelletized with the aid of
about 5% by weight of graphite.
Example 2
_________
The pellets prepared according to Example 1 were
tested for activity at a pressure of 28 kg/cm2, an inlet
temperature of 340C and a SV of 3000 Nl/l/h in a synthesis gas
containing 0.7% by volume of CO, 21.8% CO2, 75.8% H2 and
1.7% N2. After 70 hours operation there was found a conversion
of C (in CO and CO2) of 51% and carbon converted, calculated
as % by weight of C, was distributed as
21.9~ CH4 8.0% C4H8
10.5% C2H4 2.3% C4Hlo
3.4% C2H6 22.5% C5- and higher hydrocarbons
17.1% C3H6 about 8.0% Cn-OH
202.0% C3H8 about 2.0% Cn~CH
and the remainder products not determined; the yield of
C2 4 hydrocarbons thus was 35.6%, calculated as per cent by
weight of carbon converted. With Cn-OH there is meant alkyl
and alkenyl hydroxy compounds and with Cn-COOH carboxylic
acids.
Sometimes there was found very small amounts of oily
phase. By far the main part of the reaction product is water,
which is connected with the high content of carbon dioxide
in the feed gas, cf. equation (5). Among the alcohols and
acids formed only the lowest boiling have been found, i.e.
ethanol, propanol and butanol and the corresponding acids.
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Example 3
_ _ _ _ _ _ _ _
The pellets prepared according to Example 1 were
tested for activity at a pressure of 20 kg/cm2, an inlet
temperature of 410C and SV of 4000 Nl/l/h in a synthesis
gas containing 2.11% CO, 17% CO2, 79.3% H2 and 1.6% N2.
After about 360 hours operation there was found a conversion
of C (in CO and CO2) of 41%, and, calculated as weight% C,
converted carbon was distributed as
38.5% CH4 6.8% C4H8
1016.9% C2H4 5.0% C5 and higher
3.1% C2H6 about 5.0% Cn - OH
13.2% C3H6 about 1.0% Cn - COOH
1.3% C3H8
and the remainder products not determined.
The yield of C2 4 olefins thus was 36.8%, calculated
as % by weight of converted carbon.
At this higher temperature the amount of the oily
phase was extremely low.
