Note: Descriptions are shown in the official language in which they were submitted.
~o~vzos
Background of the Invention
Sulfuric acid i8 a ~ackbone of industry. It i8
necessary for preparing many industrial products from fertil-
izers to pharmaceuticals and from petrochemicals to steel.
In 1974 approximately 34.5 million tons of sulfuric acid were
produced in the United States.
Sulfuric acid is usually prepared on a commercial scale
by the gas phase oxidation of sulfur dioxide to sulfur trioxide
followed by absorption of the sulfur trioxide in an aqueous
medium. Modern industrial plants for preparing sulfuric acid
usually utilize a supported vanadium contact catalyst for the
oxidation of the sulfur dioxide to sulfur trioxide.
Throughout the history of the "contact sulfuric acid"
process there has been a diligent search for active and durable
sulfur dioxide oxidation catalyst which can be commercially pre-
pared at a reasonable cost. Until this time a practitioner of
the art, on a commercial scale, had a choice between an active
cataly8t or a durable ca~alyst. The virtues of durability and
hi~h activity in a catalyst at a reasonable price have eluded
the diligent searches of catalyst manufacturers.
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108VZI)5
It is the object of the present invention to provide a
durable and active sulfur dloxide oxldation catalyst. A further
object of the present invention is to provide a durable and
active sulfur dioxide oxidation catalyst which can be prepared
at a~reasonable cost. It is a further object of the present
invention to provide a vanadium containing, durable and active ;
sulfur dioxide oxidation catalyst which can be prepared with-
out undue modification of existing equipment and procedures.
Brief Summary of the Invention
According to the present invention, an active and
durable vanadium containing sulfuric acid catalyst is provided
which comprises shaped particles comprising vanadium compo8i-
tions, promoters and activators supported on a calcined and
finely comminuted diatomaceous earth which contains mainly the
fresh water diatom Melosira granulata.
The vanadium compositions, promoters and activators
comprise the active portion of the catalyst (herein noted as
active material) and are well known in the art.
The support material comprises calcined and finely com-
minuted diatomaceous earth containing preferably 50~ of thediatoms by weight of the fresh water diatom Melosira granulata.
Higher proportions of the diatom Melosira granulata provide a
more active and durable catalyst.
The support is calcined at a temperature above about
95GF. and below the temperature at wlnich the structure of the
silica in the diatom changes which is about 1850F. The cal-
cining reduces the amount of organic matter in the diatomaceous
lO~OZ05
earth and at the higher temperatures the chemical composition is
changed. Temperatures between about 1200 and 1800F. are preferred.
The support must be finely comminuted since small
particle size increases both the durability and activity of the
catalyst prepared with the support.
Description of the Drawings
Figure 1 is a diagrammatic drawing of the apparatus
for determining catalyst activity.
Figure 2 is a scanning electron micrograph of the fresh
water diatom Melosira granulata enlarged 3000 diameters.
Figure 3 is a graph of particle size distribution of
the diatomaceous earth supports employed in the examples set
forth herein as determined by a Sharples Micromerograph TM .
Detailed Description of the Invention
In present day sulfuric acid practlce, the sulfur
dioxide oxidation catalyst is generally a supported vanadium
composition which is "promoted" or "activated" by addition of
certain alkali metal moieties and minor amounts of such com-
pounds as cobalt, nickel, calcium, barium, iron and the like.
The composition of such catalysts and the effect of various
promoters and activators on the activity of the catalyst is
well known as is discussed in Topsoe and Nielson, Transactions
_f the Danish Academy of Technical Sciences, No. 1J 1948, and
TandyJ "The Role of Alkali Sulphates in Vanadium Catalysts for
Sulfur Dioxide Oxidation", J. Applied Chemistry, February 6,
1956, Pages 68-74.
108~05
The vanadium composition is usually supported on a
carrier material for several reasons. The amount of expensive
vanadium, the active material, in the catalyst can be reduced
relative to the volume of catalyst required. Additionally, it
5 i9 believed that the active vanadium composition is in the
molten or plastic state at the temperature at which the catalyst
i9 most active for oxidation of sulfur dioxide to sulfur tri-
oxide in the sulfuric acid process. The active vanadium compo-
sition would fuse into a molten mass if it is not supported.
Many materials have been used as supports for vanadium
compositions in sulfur dioxide oxidation catalysts. Materials
such as alumina, pumice, silica gel, fullers earth, diatomaceous
earth, zeolites and mixtures of these materials with various
binders are disclosed in the literature as suitable supports
for sulfur dioxide oxidation catalysts.
The catalyst is usually prepared in the form of dis-
crete shaped particles. The particles can be in the form of
spheres, cylindrical pellets, tablets or irregular particles
graded to a desired particle size range. Shaping and sizing of
the catalyst particles is important to minimize the pressure
drop realized by the gases passing through the catalyst bed in
the operation of the process.
Supported sulfur dioxide oxidation catalysts are
generally prepared by two methods. In one method, a shaped
support particle is prepared and the vanadium containing com-
10~30Z05
position with the promoters and activators is intimately ad-
mixed with the shaped support particle. The shaped catalyst
support can be impregnated with a solution of the catalytically
active materials or the shaped particle can be intimately ad-
mixed with the active catalytic composition and the mixtureheated to a temperature at which the catalytic composition be-
comes a fluid and flows over the surfaces of the shaped support. `~
Another method for preparing shaped, supported
catalysts is to intimately admix the vanadium composition with
the particulate support and subsequently shape the admixture to
form the catalyst particles. The mixture to be shaped can be
prepared by dry mixing the particulate support material with
the active materials followed by the addition of a liquid to
form a mixture of the proper consistency w'.~ich can be shaped
and which will retain the shaped configuration. The support
can be mixed with a slurry of the active material followed by
the addition of a controlled amount of water to obtain a mix-
ture with a consistency suitable for shaping into the desired
catalyst shape. The catalyst can also be prepared by admixing
the catalyst support with a solution of the catalytic composi-
tions and adjusting the amount of water to obtain a mixture of
the consistency suitable for shaping.
Tableting presses, briquetting mills, pellet mills and
~ .
~ the like are suitable means for shaping the mixture of the
;~ 25 catalyst support and the active catalytic compositions into
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.. .
,................... . . . .
1080Z05
particles of the desired siæe and shape. Shaping means are
well known in the art and require no elaboration at this point.
It is preferable to use shaping means which also compact the
mixture to form the shaped particles.
After the mixture containing the vanadium compounds,
promoters and activators, and the support~ is shapedJ it is
dried to remove the water utilized in the preparation.
The dried shaped particles are then contacted with an
oxygen containing gas 9 tream containing small amounts of from
about 2 to about 6~ of sulfur dioxide mixed with sulfur tri-
oxide at a temperature in the range of from about 600 to 900F.
Reaction with the dilute sulfur dioxide containing gas stream
activates the catalyst particles and after a suitable activa-
tion period, the shaped particles are suitable for use as a
catalyst in a sulfur dioxide oxidation process.
The preparation of sulfur dioxide oxidation catalysts
containing vanadium is discussed on Topsoe and Nielson, Trans-
action of the Danish Academy of Technical Sciences, supra.
The activation step which is sometimes called
sulfating and the requirement that the catalyst be reacted with '
dilute sulfur dioxide containing gas streams to accomplish the
activation is discussed in the Topsoe and Nielson article,
supra.
Briefly, the activation is generally utilized since the
unactivated catalyst reacts rapidly and irreversibly with
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~80Z05
sulfur dioxide at the elevated temperature of the sulfur dioxide
oxidation reaction. The reaction of the sulfur dioxide with
the catalyst i9 exothermic and the temperature of the catalyst
can increase to a point at which the catalytic activity and
structural integrity of the catalyst can be severely impaired.
It is` therefore the general practice to activate a vanadium
containing sulfuric acid catalyst by contact with a dilute
sulfur dioxide and sulfur trioxide containing gas stream before
use in a commercial operation. Activation or sulfation of
sulfur dioxide oxidation catalysts is well known in the art.
The catalyst of the present invention iSJ preferably,
prepared by mixing the vanadium containing compositions and
various promoters and activators (active material) with the
novel diatomaceous earth support. The mixture of active mater-
ial with the diatomaceous earth support is then adjusted to aproper consistency by addition or removal of water, and shaped
into the required form. A suitable catalyst particle can also
be prepared by drying a mixture of an aqueous solution or
slurry of the active material with the novel support and crush-
ing and screening the dried mixture to form irregular particlesin a narrow size range. However, it is preferred to prepare
the catalyst in the form of regular shaped particles since the
particles provide for minimum pressure differentials through
the catalyst bed.
,' ' ' . . .
: lO~OZO~
The active materials can be dry mixed with the cata-
lyst support followed by the addition of water to form a mixture
with the desired consistency to be shaped. Another method
suitable for preparing the supported catalyst of the present
invention is to admix a slurry of the active materials with the
catalyst support and adjusting the consistency of the mixture
to that required for shaping.
The catalyst can also be prepared by admixing a solu-
tion of the active materials with the diatomaceous earth support
and adjusting the amount of water in the mixture to provide a
material with a consistency which can be readily formed into
the desired catalytic particle shape.
It is not critical how the active materials are ad-
mixed with the catalyst support as long as an intimate admixture
of the catalytic compositions with the catalyst support is
obtained.
The active materials are well known in the sulfur di-
oxide oxidation catalyst art and the best methods for admixing
the active materials with the diatomaceous earth support would
be dependent upon the particular composition chosen for making
the catalyst. Vanadium pentoxide is a useful vanadium contain-
ing composition for preparing the sulfur dioxide oxidation
catalyst of the present invention. The useful concentrations
of vanadium compositions and the promoters and activators
u~ilized in preparing a sulfur dioxide oxidation catalyst are
well documented in the literature and will not be discussed
herein.
1~30Z05
The improved properties of the catalyst of the
present invention, i.e., the combination of high activity and
high durability, are achieved by use of a particular diatoma-
ceous earth as the catalyst support. The diatomaceous earth
useful in the practire of the present invention is a diatoma-
ceous earth containing a major portion of the diatoms of the
fresh water diatom Melosira granulata. As is known in the artJ
diatomaceous earth is a naturally occurring material which,
after removal of water, contains primarily diatoms mixed with
minor amounts of clay mineral type materials and organic im-
; purities. The diatoms are primarily silicon dioxide. The
amount of alumina in the diatomaceous earth reflects the amount
of clay minerals admixed with the diatoms. The diatomaceous
earth useful in the practice of the present invention generally
contains from about 55 to about 90~ by weight of the calcineddiatomaceous earth of diatoms. The catalyst support should
contain at least about 40~ by weight of the calcined diatoma-
ceous earth of the fresh water diatom Melosira granulata and
most preferably from about 50 to about 80~ of the diatom Melo-
sira ranulata. Higher proportions of the diatom Melosiragranulata in the support increase the durability of the catalyst.
The diatomaceous earth catalyst support preferably contains from
about 2.5 to about 10.5~ aluminum calculated as alumina (Al203).
Figure 2 is a scanning electron micrograph showing the
diatom Melosira ~ranulata enlarged to ~000 diameters.
:. _ g _
lO~VZO~
The support must be finely comminuted. The fine com-
minution increases the activity and also increases the durabil-
ity of the catalyst. It is preferred that the diatomaceous
earth support be comminuted so that at least about 25% by
weight of the particles are smaller than about 10 microns and
preferably at least about 40~ by weight are smaller than 10
microns.
It is preferred that the maximum particles size be no
larger than about 100 microns, but mixtures containing small
amounts of particles over 100 microns can be useful in the
practice of the lnvention.
The diatomaceous earth support can be readily prepared
by grinding coupled with air classification methods.
The support must be calcined. The calcination in-
crea9es the activity of the catalyst. The calcination is
carried out by heating the diatomaceous earth to a temperature
from about 950 to about 1850F. and preferably from about 1200
to about 1800F. The calcination can be carried out in a ro-
tating kiln type apparatus.
The time of calcination is dependent on the temperature.
Times from about 5 minutes to about 8 hours are suitable and
from about 30 minutes to about 4 hours most preferred. Methods
for calcining diatomaceous earth are well known in the art.
It is believed that calcination removes certain
organic impurities from the diatomaceous earth. Calcination can
also decompose carbonate impurities if they are present in the
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- 1080ZOS
diatomaceous earth. The diatomaceous earth must not be heated
to a temperature high enough to change the structure of the
silica which makes up the individual diatoms. Little effect
on the structure of the silica in the diatomaceous earth
occurs at temperatures below about 1850F.
It is preferred to calcine the diatomaceous earth sup-
port before admixture with the active materials and the shaping
of the particles.
The activity of the catalysts shown in the examples
was determined by measuring the conversions obtained by passing
a gas stream containing sulfur dioxide, sulfur trioxide, oxygen
and nitrogen over the catalysts at a controlled rate and temp-
erature.
The activity of the catalyst was determined by measur-
ing the amount o sulfur dioxide converted to sulfur trioxidein a partially reacted sulfur dioxide containing gas stream.
A gas stream containing sulfur dioxide, oxygen and nitrogen is
passed over a sulfur dioxide oxid~tion catalyst to convert from
about 90 to about 97% of the sulfur dioxide to sulfur trioxide.
The sulfur dioxide content of the partially converted gas
stream is determined and the partially converted gas stream is
passed over the catalyst to be tested. The catalyst being
tested is maintained at a controlled temperature and the flow
rate of sulfur dioxide containing gas contacting the catalyst
is carefully controlled. The sulfur dioxide content of the gas
108V~05
stream before and af~er contact with the catalyst being tested
is mea~ured. An activity coefficient for the particular cata-
lyst i8 determined from the concentration of oxygen, sulfur
dioxide and sulfur trioxide in the gas stream entering the test
reactor and the concentration of oxygen, sulfur dioxide and
sulfu`r trioxide in the gas stream leaving the test reactor. The
apparatus for catalyst testing is shown in Figure 1.
Cylinders of dry sulfur dioxide, air and nitrogen
properly pressure-reduced are connected to lines 1, 2 and 3,
respectively. The systems for metering the gases are the same.
The pressure-reduced gases at a pressure of about 10 pounds per
square inch gauge (psig) enter the system and pass through
shut-off valves 5, 6 and 7 in the sulfur dioxide, air and ni-
trogen lines. The gas is metered at a pressure of about 10 psi,g
through rotometers 8, 9 and 10 and low flow control means 11,
12 and 13. The gases are mixed in line 14 at a pressure be-
tween 1 and 2 psig. The pressure in line 14 is monitored by
pressure gauge 15. Vent valve in line 14 permits the mixture
of gases to be passed to the vent until the required mixture of
gases is obtained. The total flow of mixed gases passing
through line 30 is measured by rotometer 19. The mixed gas
stream is passed through line 31 to reactor 17. A sample can
be taken through line 46 and valve 20 to determine the
ratio of sulfur dioxide to oxygen in the gas stream.
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10~UV2V5
Reactors 17 and 18 are electrically heated reactors
and contain a sulfur dioxide oxidation catalyst. Reactors 17
and 18 are utilized to convert a portion of the sulfur dioxide
in the gas stream to sulfur trioxide. One or two reactors are
utilized depending upon the amount of preconversion desired for
a particular run. The preconverted gas leaving reactor 17
passes through line 32 and can pass through line 33 and valve
34 to line 37 and directly to test reactor 41 through valve 42
and line 45. If additional preconversion is required the
partially converted gas stream can be passed through reactor
18, line 35 and valve 36 to line 37 which passes the precon-
verted gas to test reactor 41.
A sample of the preconverted gas in line 37 is removed
from the system through line 44 and valve 43 for analysis. The
15 preconverted gas stream is passed through valve 42 and line 45
to test reactor 41.
Test reactor 41 is immersed in a fluidized sand bath
which is temperature controlled to maintain the proper tempera-
ture in the reactor. The test reactor 41 is 1 inch inside
20 diameter. Fifty cubic centimeters of catalyst is introduced
into the reactor for testing. The fully converted gas stream
is passed through valve 46 in line 39 and passed through valve
40 to the scrubber and the vent. A sample for analysis can be
taken from the system through valve 22.
Reactor 41 can be by-passed by passing the preconverted
gas through line 3~ and valve 21 to line 39.
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1080ZC~S
During testing of a catalyst the sulfur dioxide con-
centration in the gas stream entering the test reactor through
line 45 and leaving the reactor through line ~9 are monitored.
The gas mixture entering the preconverters through line 31 is
adjusted to contain about 9.5~ S0z, about 11.l~ 2 and the
balance, nitrogen. The catalyst being tested is equilibrated
by passing the preconverted gas stream over the catalyst at the
test temperature for 2 hours before sampling the gas stream.
The sulfur dioxide concentration in the preconverted gas stream
and in the gas ~tream after contact with the catalyst being
tested is determined by iodometric titration.
The amount of sulfur dioxide and oxygen in the gas
stream entering and leaving the test reactor is measured. The
rate constant k was computed from the data using the following
rate expression.
rate = k~Ps02/pso3)o-5 Po2/l-(pso3/po2o 5PS02keq) ~ where
keq = equilibrium constant
keq = ~ + 0.611 by TA - 6.7497
TA = temperature, K.
Pso = Partial pressure of S02 in atmospheres.
Pso = Partial pressure of S03 in atmospheres formed by contact
with the catalyst being tested.
P0 = Partial pressure of 2 in atmospheres.
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1080Z05
The rate constant k = R moles S02 converted
atmos X second X gram catalyst
The rate constant k reported herein = k X bulk density of sul-
ated catalyst in grams per cubic centimeter.
The durability of the catalyst (Accelerated Abrasion
Loss) is measured by heating 150 grams of the fresh catalyst
pellets at 1500~F. for 24 hours. The cataly~t pellets after
heating at 1500F. for 24 hours are contacted with a ga8 mixtuPe ~.
containing about 4.0% S02 and about 5.0~ S03 for 2 hours at
810F. A 100 gram sample of the heated and reacted cataly8t i8
shaken over a standard 20 mesh sieve, U.S. Sieve Series, for 1
hour using a Rotap shaker. The loss in weight after shaking
for 1 hour indicates the durability of the cataly~t. The dura-
bility of the catalyst is indicated as the percent 1099 through
the 20 mesh screen. The lower number8 indicate a more durable
catalyst. The Accelerated Abrasion Loss test has been found to
correlate clo8ely with durability experienced with sulfur diox-
ide oxidation catalysts under commercial pr,ocess operating con-
ditions.
The invention will be more fully illustrated by
reference to the following examples.
EXAMPLE 1
,
A test catalyst was prepared by dry mixing 109 parts
,' of vanadium oxide with 302 parts of anhydrous potassium sulfate.
- Diatomaceous earth in an amount of 8~0 parts is admixed with
- 15 -
. :
1080ZU5
the dry vanadium pentoxide and anhydrous potassium sulfate.
Water is added and thoroughly mixed with the solid mixture in
a sufficient amount to form a damp mixture suitable for extru-
sion. The damp mixture is extruded into pellets 7/32 lnch
diameter by 3/8 to 5/8 inch long. The pellets are dried at
250F. for 8 hours. The catalyst pellets are heated at 1000F.
for 4 hours and activated by contact with a dil.ute sulfur diox-
ide-sulfur trioxide containing gas stream for 2 hours at 810F.
before testing.
The diatomaceous earth is a natural product containing
about 63~ by weight of the calcined diatomaceous earth of the
fresh water diatom Melosira granulata which has been calcined
at 1600F. for 1 hour and milled to an average particle size by
weight of about 9 microns. The particle size distribution of
the calcined and milled diatomaceous earth is shown in Table 1.
TABLE 1
PARTICLE SIZE DISTRIBUTION OF DIATOMACEOUS EARTH
CUMULATIVE
SIZE MICRONS DISTRIBUTION
Less than 5 20%
; Less than 10 60~
Less than 20 85%
Less than 50 95%
Less than 100 99%
The plot of a Micromerograph TM distribution of parti-
cle sizes is shown in Figure 3 as curve A. The properties of
- 16 -
10802(15
the diatomaceous earth were as ollows:
Surface Area - meters2/gram 11 - 16
Pore Volume - cubic centimeters/gm. 0.8
Al203 9.3%
~ SiO2 86.8%
Loss on Ignition o.g~
EXAMPLE 2
A catalyst was prepared according to the procedure of
Example 1. The support was the unmilled ca cined diatomaceous
earth of Example 1. The particle size distribution as measured
by Micromerograph TM appears as curve B in Figure 3.
EXAMPLE 3
A catalyst was prepared according to the method of
Example 1. The support is an uncalcined, finely comminuted
fresh water diatomaceous earth. The support contained about 63~
by weight of the diatomaceous earth of the diatom Melosira gran-
ulata ton a calcined basis). The particle size distribution of
the diatomaceous earth as measured by Micromerograph TM is shown
as curve C in Figure ~.
EXAMPLE 4
A catalyst was prepared according to the method of
Example 1 utilizing the diatomaceous earth of Example 3 which
- 17 -
......
'
'
~ 0 80 20 5
wa9 heated at about 1600F. for one hour before admixture with
the active materials.
EXAMPLE 5
A catalyst was prepared according to the method of
Example 1 except the support is an uncalcined, finely comminuted
salt water diatomaceous earth. The particle size distribution
of the diatomaceous earth is measured by Micromerograph TM and -
shown as curve D in Figure 3.
EXAMPLE 6
; 10 A catalyst was prepared according to the method of
Example 1 using the same diatomaceous earth as in Example 1.
Potassium sulfate was used in only 167 parts and 284 parts of
Cesium sulfate were added to the formulation.
The catalysts prepared in Examples 1 through 6 were
tested in an apparatus as shown in Figure 1. The sulfur diox-
ide was preconverted to between 90 and 97% and passed over 50
cubic centimeters of catalyst at the rate of 21 cubic centi-
meters per second. The rate constant was determined at a
temperature of 760F. and 810F.
The Accelerated Abrasion Loss values (AAL) were deter-
mined according to the method disclosed above.
Two samples of commercially available contact sulfur
dioxide catalysts were also tested to show the advantages of
the catalyst of the invention.
~ .
- 18 -
''' ' ., , . .,- ~ , .
1080'~05
The results of the tests of the two commercial cata-
lysts are shown as Examples 7 and 8 in Table 2.
The results of the evaluation tests are set forth in
Table 2.
- 19 -
10802~5
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-- 20 --
1080'~05
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a~
a~ ~ ,~
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oq
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t; .
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C~
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Cq U~ C C
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- 21 -
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10802VS
The test results clearly indicate that it i8 necessary
to finely comminute the fresh water diatomaceous earth consist-
ing primarily of Melosira granulata diatoms and calcine the
.
material to obtain a catalyst with high activity and high dura-
bility.
~ The catalyst prepared with the uncalcined finely com-
minuted diatomaceous earth is a durable catalyst but its O.
activity is relatively low.
The calcined but coarsely ground diatomaceous earth pro-
duces a catalyst with a higher activity but low durability. If
the catalyst support is calcined and finely comminuted, the
catalyst achieves a high activity and maintains a high dura-
bility.
It is not critical that the diatomaceous earth be
calcined before being comminuted as shown by Example 4 in com-
parison to the catalyst of Example 1.
The catalyst of the present invention combines the
durability of commercial catalysts of low activity with the
activity of high activity commercial catalysts.
The catalysts utilized in the examples presented here-
in were prepared using only one concentration of vanadium in
the form of an oxide so that the effects of treatment of the
catalyst support could be clearly shown. Concentration of
active materials in sulfur dioxide oxidation catalysts and
ratios of promoters and activators to vanadium are well known
- 22 -
1080205
in the art. The support of the present invention imparts de-
sirable properties to catalysts within the ranges of concentra-
tion of active materials utilized in the art.
The active materials may be in soluble or insoluble
form~and can be admixed with the novel calcined finely commi-
nuted support in a dry state, as a solution or suspension. The
form of the active materials is not important but intimate ad-
mixture with the catalyst support must be achieved. Vanadium
compounds, activators and promoters suitable for preparing
supported sulfur dioxide oxidation catalysts are well known in
the art.
The examples clearly show that active and durable
catalysts can be prepared by utilizing as a catalyst support,
a calcined finely comminuted fresh water diatomaceous earth
containing primarily diatoms of Melosira granulata.
- 23 -
