Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02432200 2003-06-18
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CATALYST WITH BIMODAL PORE RADIUS DISTRIBUTION
The present invention relates to catalysts having a bimodal pore
radius distribution and comprising a) zirconium dioxide and, if
desired, b) aluminum oxide, titanium dioxide and/or silicon
oxide and c) at least one element of main group I or II, an
element of transition group III, an element of transition group
VIII, of the Periodic Table of the Elements, lanthanum and/or
tin.
US-A-5,220,091 discloses catalysts comprising Pt/Sn as active
component on a Zn spinel support for the dehydrogenation of
small hydrocarbon molecules such as isobutane using steam as
diluent. The performance of these catalysts is in need of
improvement since, despite the high dilution of the feed with
steam (ratio 4:1), only relatively low yields and selectivities
are achieved at high reaction temperatures of 600 C. Likewise
deserving of improvement is the operating life of the catalysts,
since they have to be regenerated after an operating time of
only 7 hours.
US-A-4,788,371 discloses Pt/Sn/Cs/A1203 catalysts for the
dehydrogenation of hydrocarbons using steam dilution (e.g.
steam/propane = 10:1). Despite the high degree of dilution, only
low conversions of 21% are achieved.
WO-A-94/29021 discloses catalysts based on mixed oxides of
magnesium and aluminum and further comprising a noble metal of
group VIII, a metal of group IVa and, if desired, an alkali
metal of group Ia, of the Periodic Table of the Elements for the
dehydrogenation of, for example, a gas mixture of
H20/propan/H2/N2 in a ratio of 8:7:1:5. A drawback of these
catalysts in industrial applications is their low hardness,
which makes industrial use difficult. Furthermore, the
performance of these catalysts, in particular at low reaction
temperatures, is in need of improvement. A further disadvantage
is the complicated operating procedure which, to maintain the
performance, requires the addition of hydrogen to the feed and
the mixing in of nitrogen for further dilution.
it is an object of the present invention to remedy the
abovementioned disadvantages.
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We have found that this object is achieved by new and improved catalysts
having a bimodal pore radius distribution and comprising:
a) from 30 to 99.9% by weight of zirconium dioxide and
b) from 0 to 60% by weight of aluminum oxide, silicon dioxide and/or
titanium dioxide; and
c) from 0.1 to 10% by weight of at least one element of main group I and II,
an element of transition group III, an element of transition group VIII, of
the Periodic Table of the Elements, lanthanum and/or tin,
with the proviso that the sum of the percentages by weight is 100, a process
for
the dehydrogenation of C2-C16-hydrocarbons and the use of these catalysts for
this purpose and also a process for producing these catalysts.
The catalysts of the present invention as claimed, more specifically consist
of a
catalyst having a bimodal pore radius distribution and consisting essentially
of:
a) from 30 to 99.9% by weight of zirconium dioxide of which from 50 to
100% by weight is in the monoclinic modification and;
b) from 0 to 60% by weight of aluminum oxide, silicon dioxide and/or
titanium dioxide; and
c) from 0.1 to 10% by weight of at least one element selected from among
main groups one and two and transition groups three and eight of the
Periodic Table of the Elements and tin,
with the proviso that the sum of the percentages by weight is 100.
The catalysts of the present invention, preferably consist of,
a) from 30 to 99.9% by weight, preferably from 20 to 98% by weight,
particularly preferably from 30 to 95% by weight and even more
preferably from 64 to 95% by weight, of zirconium dioxide of which from
50 to 100% by weight, preferably from 60 to 99% by weight, particularly
preferably from 70 to 98% by weight, is in the monoclinic and/or
tetragonal modification and
b) from 0.1 to 30% by weight, preferably from 0.5 to 25% by
weight, particularly preferably from 30 to 20% by weight, of
silicon dioxide and
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c) from 0 to 60% by weight, preferably from 0.1 to 50% by
weight, particularly preferably from 1 to 40% by weight, in
particular from 5 to 30% by weight, of aluminum oxide,
silicon dioxide and/or titanium dioxide in the form of
rutile or anatase and
d) from 0.1 to 10% by weight, preferably from 0.2 to 8% by
weight, particularly preferably from 0.5 to 5% by weight, of
at least one element of main group I or II, an element of
transition group III, an element of transition group VIII,
of the Periodic Table of the Elements, lanthanum and/or tin,
where the sum of the percentages by weight is 100.
Preferably, the catalysts of the present invention comprise potassium or
cesium
as an element of main group I of the Periodic Table.
The amount of a noble metal present in the catalysts of the present invention
is
generally from 0.01 to 5% by weight, preferably from 0.1 to 1% by weight,
particularly preferably from 0.2 to 0.5% by weight.
According to another preferred embodiment, the catalysts preferably contain
from 0.1 to 5% by weight of potassium and/or cesium.
Moreover the catalysts advantageously contain from 0.05 to 1% by weight of
platinum and from 0.05 to 2% by weight of tin.
In the catalysts of the present invention, from 70 to 100%,
preferably from 75 to 98%, particularly preferably from 80 to
95%, of the pores are smaller than 20 nm or in the range from 40
to 5000 nm.
To produce the catalysts of the present invention, use can be
made of precursors of the oxides of zirconium, titanium, silicon
and aluminum (forming the support) which can be converted by
calcination into the oxides. These can be prepared by known
methods, for example by the sol-gel process, precipitation of
the salts, dehydration of the corresponding acids, dry mixing,
slurrying or spray drying. For example, a Zr02 = xA1203 = xSiOz
mixed oxide can be prepared by first preparing a water-rich
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zirconium oxide of the formula Zr02 = xH2O by precipitation of a
suitable zirconium-containing precursor. Suitable zirconium
precursors are, for example, Zr(N03)4, ZrOC12 or ZrC14. The
precipitation itself is carried out by addition of a base such
as NaOH, KOH, Na2CO3 and NH3 and is described, for example, in
EP-A-849 224.
To prepare a Zr02 = xSiOz mixed oxide, the Zr precursor obtained
as above can be mixed with an Si-containing precursor. Well
suited Si02 precursors are, for example, water-containing sols
of Si02 such as LudoxTM. The two components can be mixed, for
example, by simple mechanical mixing or by spray drying in a
spray dryer.
when using mixed oxides, it is possible to influence the pore
structure in a targeted way. The particle sizes of the various
precursors influence the pore structure. Thus, for example,
macropores can be generated in the microstructure by use of
A1203 having a low loss on ignition and a defined particle size
distribution. An aluminum oxide which has been found to be
useful for this purpose is Puralox (A1203 having a loss on
ignition of about 3%).
To prepare a Zr02 = xSi02 = xA1203 mixed oxide, the Si02 = xZr02
powder mixture obtained as described above can be admixed with
an Al-containing precursor. This can be carried out, for
example, by simple mechanical mixing in a kneader. However, a
Zr02 = xSi02 = xA1203 mixed oxide can also be prepared in a
single step by dry mixing of the individual precursors.
Compared to pure Zr02, the mixed oxides have the advantage,
inter alia, that they can be shaped easily. For this purpose,
the powder mixture obtained is admixed in a kneader with a
concentrated acid and can then be converted into a shaped body,
e.g. by means of a ram extruder or a screw extruder.
A further possible way of producing the support having a
specific pore radius distribution for the catalysts of the
present invention is to add, during the preparation, various
polymers which can be partly or completely removed by
calcination so as to form pores in defined pore radius ranges.
The mixing of the polymers and the oxide precursors can, for
example, be carried out by simple mechanical mixing or by spray
drying in a spray dryer.
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More preferably, the polymers are selected from the group consisting of
polyamines, polyacrylates, polyalcohols, polysiloxanes, carbohydrates,
polyvinylpyrrolidone or mixtures thereof.
The use of PVP (polyvinylpyrrolidone) has been found to be
particularly advantageous for producing the supports having a
bimodal pore radius distribution. If PVP is added during a
production step to one or more oxide precursors of the elements
Zr, Ti, Al or Si, macropores in the range from 200 to 5000 nm
are formed after calcination. A further advantage of the use of
PVP is that the support can be shaped more readily. Thus,
extrudates having good mechanical properties can be produced
easily from freshly precipitated water-containing Zr02 = xHZO
which has previously been dried at 120 C when PVP and formic
acid are added, even without further oxide precursors.
The mixed oxide supports of the catalysts of the present
invention generally have higher BET surface areas after
calcination than do pure Zr02 supports. The BET surface areas of
the mixed oxide supports are generally from 40 to 300 m2/g,
preferably from 50 to 200 m2/g, particularly preferably from 60
to 150 m2/g. The pore volume of the catalysts of the present
invention is usually from 0.1 to 0.8 ml/g, preferably from 0.2
to 0.6 ml/g, and even more preferably from 0.25 to 0.5 mi/g. The mean pore
diameter of the catalysts of the present invention, which can be detertmined
by
Hg porosimetry, is from 5 to 20 nm, preferably from 8 to 18 nm. Furthermore,
it
is advantageous for from 10 to 80% of the pore volume to be made up by pores
> 40 nm.
The calcination of the mixed oxide supports is advantageously
carried out after the application of the active components and
is carried out at from 400 to 700 C, preferably from 500 to
650 C, particularly preferably from 560 to 620 C. The
= 0050/50549 CA 02432200 2003-06-18
calcination temperature should usually be at least as high as
the reaction temperature of the dehydrogenation for which the
catalysts of the present invention are used.
5 The catalysts of the present invention have a bimodal pore
radius distribution. The pores are mostly in the range up to 20
nm and in the range from 40 to 5000 nm. Based on the pore
volume, these pores make up at least 70% of the pores. The
proportion of pores less than 20 nm is generally from 20 to 60%,
while the proportion of pores in the range from 40 to 5000 nm is
generally likewise from 20 to 60%.
The doping of the mixed oxides with a basic compound can be
carried out either during their preparation, for example by
coprecipitation, or subsequently, for example by impregnation of
the mixed oxide with an alkali metal compound or alkaline earth
metal compound or a compound of transition group III or a rare
earth metal compound. Particularly suitable dopants are K, Cs
and lanthanum.
The application of the dehydrogenation-active component, which
is usually a metal of transition group VIII, is generally
carried out by impregnation with a suitable metal salt precursor
which can be converted into the corresponding metal oxide by
calcination. As an alternative to impregnation, the
dehydrogenation-active component can also be applied by other
methods, for example spraying the metal salt precursor onto the
support. Suitable metal salt precursors are, for example, the
nitrates, acetates and chlorides of the appropriate metals, or
complex anions of the metals used. Preference is given to using
platinum as H2PtCl6 or Pt(N03)2. Solvents which can be used for
the metal salt precursors are water and organic solvents.
Particularly suitable solvents are lower alcohols such as
methanol and ethanol.
Further suitable precursors when using noble metals as
dehydrogenation-active component are the corresponding noble
metal sols which can be prepared by one of the known methods,
for example by reduction of a metal salt with a reducing agent
in the presence of a stabilizer such as PVP. The preparation
technique is dealt with comprehensively in the German Patent
Application DE-A-195 00 366.
The catalyst can be used as a fixed bed in the reactor or, for
example, in the form of a fluidized bed and may have an
appropriate shape. Suitable shapes are, for example, granules
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(crushed material), pellets, monoliths, spheres or extrudates
(rods, wagon wheels, stars, rings).
As alkali metal and alkaline earth metal precursors, use is
generally made of compounds which can be converted into the
corresponding oxides by calcination. Examples of suitable
precursors are hydroxides, carbonates, oxalates, acetates or
mixed hydroxycarbonates of the alkali metals and alkaline earth
metals.
if the mixed oxide support is additionally or exclusively doped
with a metal of main group III or transition group III, the
starting materials in this case should be compounds which can be
converted into the corresponding oxides by calcination. If
lanthanum is used, suitable starting compounds are, for example,
lanthanum oxide carbonate, La(OH)3, La3(C03)2, La(N03)3 or
lanthanum compounds containing organic anions, e.g. lanthanum
acetate, lanthanum formate or lanthanum oxalate.
The dehydrogenation of propane is generally carried out at
reaction temperatures of from 300 to 8000C, preferably from 450
to 7000C, and a pressure of from 0.1 to 100 bar, preferably from
0.1 to 40 bar, and at a WI3SV (weight hourly space velocity) of
from 0.01 to 100 h-1, preferably from 0.1 to 20 h-1. Apart from
the hydrocarbon to be dehydrogenated, the feed may further
comprise diluents such as C02, N2, noble gases and/or steam,
preferably N2 and/or steam, particularly preferably steam.
A specific feature of the catalysts of the present invention is
that they are active in the dehydrogenation of hydrocarbons in
the presence of steam and it is therefore possible to utilize
the advantages associated therewith, for example removal of the
equilibrium limitation, reduction in carbon deposits and
lengthening of the operating life.
If desired, hydrogen can be added to the hydrocarbon feed, in
which case the ratio of hydrogen to hydrocarbon is generally
from 0.1:1 to 100:1, preferably from 1:1 to 20:1. The
dehydrogenation of hydrocarbons using the catalysts of the
present invention is preferably carried out without use of
hydrogen.
Apart from the continuous addition of a gas, in particular of
steam, it is possible to regenerate the catalyst by passing
hydrogen or air over it from time to time. The regeneration
itself takes place at from 300 to 9000C, preferably from 400 to
8000C, using a free oxidizing agent, preferably air, or in a
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reducing atmosphere, preferably hydrogen. Regeneration can be
carried out at subatmospheric pressure, atmospheric pressure or
superatmospheric pressure. Preference is given to pressures in
the range from 0.5 to 100 bar.
Hydrocarbons which can be hydrogenated by means of the catalysts
of the present invention are, for example, C2-C16-hydrocarbons
such as ethane, n-propane, n-butane, iso-butane, n-pentane,
iso-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane,
n-undecane, n-dodecane, n-tridecane, n-tetradecane,
n-pentadecane, n-hexadecane, preferably C2-C8-hydrocarbons such
as ethane, n-propane, n-butane, iso-butane, n-pentane,
iso-pentane, n-hexane, n-heptane, n-octane, particularly
preferably C2-C4-hydrocarbons such as ethane, n-propane,
n-butane and iso-butane, in particular propane and iso-butane.
Propylene is a sought-after product, particularly for the
synthesis of polypropylene or for the synthesis of
functionalized monomers and their polymerization products. An
alternative to the preparation of propylene by steam cracking of
light naptha is the dehydrogenation of propane.
isobutene is an important product, particularly for the
preparation of MTBE (Methyl tert-butyl ether). It is used,
particularly in the USA, as a fuel additive for increasing the
octane number. isobutene can be prepared by dehydrogenation of
isobutane in a process analogous to that for producing
propylene.
Examples
Catalyst production
Example 1
A solution of 0.7793 g of SnC12 = 2H20 and 0.5124 g of H2PtC16 =
6H20 in 400 ml of ethanol was poured over 67.03 g of Zr02 = xSiOZ
= xA1203 (MEL, product designation XZO 747/03, 1.6-2 mm
granules). The excess solution was removed under a reduced
pressure of 28 mbar on a rotary evaporator over a period of 30
minutes. The composition was dried at 100OC for 15 hours and
calcined at 5600C for 3 hours. A solution of 0.5027g of CsN03
and 1.7668g of KN03 in 166 ml of water was then poured over the
catalyst. The supernatant solution was removed under a reduced
pressure of 30 mbar over a period of 30 minutes. The catalyst
,
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was dried at 100 C for 15 hours and calcined at 560 C for 3
hours.
The catalyst had a BET surface area of 92 m2/g. Mercury
porosimetry measurements gave a pore volume of 0.29 ml/g, a pore
area of 67 m2/g and a mean pore radius of 4.9 nm. Based on the
pore volume, about 31% of the pores had a diameter of less than
nm and about 57% had a diameter in the range from 200 and
4000 nm.
The composition of the catalyst is shown in Table 1.
Example 2
186.73 g of ZrOC12 = 8H20 were dissolved in 800 ml of water. At
room temperature, 347 ml of 5 M NaOH were added dropwise to this
solution at a rate of 1 ml/min. After a time of about 6 hours,
the precipitation was complete and the pH was 14. The
precipitated material was aged for 15 hours at 100 C. The
suspension was subsequently filtered, the solid was washed with
3000 ml of a 5% strength NH4NO3 solution and subsequently with
pure water until free chloride could no longer be detected. The
solid was dried at 100 C for 16 hours and was then heated at a
heating rate of 1 C/min to 600 C and calcined at this
temperature for 12 hours.
110 g of a Zr02 powder prepared in this way were pretreated with
3.3 g of walocel in 40 ml of water and the mixture was kneaded
for 2 hours, then extruded under a pressure of 30 bar to form
3 mm extrudates and subsequently crushed.
A solution of 0.465 g of SnC12 = 2H20 and 0.306 g of H2PtC16 =
6H20 in 245 ml of ethanol was poured over 40 g of the crushed
material produced as described above (sieve fraction: 1.6-2 mm).
The excess solution was removed under a reduced pressure of
28 mbar on a rotary evaporator over a period of 30 minutes. The
composition was dried at 100 C for 15 hours and calcined at
560 C for 3 hours. A solution of 0.299 g of CsN03 and 0.663 g of
KN03 in 105 ml of water was then poured over the catalyst. The
supernatant solution was removed under a reduced pressure of
30 mbar over a period of 30 minutes. The catalyst was dried at
100 C for 15 hours and calcined at 560 C for 3 hours.
The catalyst had a BET surface area of 107 mz/g. Mercury
porosimetry measurements gave a pore volume of 0.46 ml/g, a pore
area of 102 m2/g and a mean pore radius of 7.7 nm. Based on the
ti
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pore volume, about 37% of the pores had a diameter of not more
than 10 nm and about 40% had a diameter in the range from 200
and 5000 nm.
The composition of the catalyst is shown in Table 1..
Example 3
373.46 g of ZrOCl2 = 8H20 were dissolved in 3200 ml of water. At
room temperature, 694 ml of 5 M NaOH were added dropwise to this
solution at a rate of 1 ml/min. After a time of about 6 hours,
the precipitation was complete and the pH was 14. The
precipitated material was aged for 15 hours at 100 C. The
suspension was subsequently filtered, the solid was washed with
6000 ml of a 5% strength NH4NO3 solution and subsequently with
pure water until free C1- could no longer be detected. The solid
was dried at 100 C for 16 hours. 6 g of PVP
(polyvinylpyrrolidone) and 6 g of concentrated formic acid in
70 ml of water were added to 200 g of the precipitated material
prepared in this way. The mixture was kneaded for 2 hours and
extruded under a pressure of 20 bar to form 3 mm extrudates
which were subsequently crushed.
A solution of 0.639 g of SnC12 = xH2O and 0.421 g of H2PtC16 =
6H20 in 337 ml of ethanol was poured over 40 g of the crushed
material produced as described above (sieve fraction: 1.6-2 mm).
The excess solution was removed under a reduced pressure of
28 mbar on a rotary evaporator over a period of 30 minutes. The
composition was dried at 100 C for 15 hours and calcined at
560 C for 3 hours. A solution of 0.411 g of CsN03 and 0.725 g of
KN03 in 144 ml of water was then poured over the catalyst. The
supernatant solution was removed under a reduced pressure of
30 mbar over a period of 30 minutes. The catalyst was dried at
100 C for 15 hours and calcined at 560 C for 3 hours.
The catalyst had a BET surface area of 102 m2/g. Mercury
porosimetry measurements gave a pore volume of 0.32 ml/g, a pore
area of 101 m2/g and a mean pore radius of 7.8 nm. Based on the
pore volume, about 50% of the pores had a diameter of not more
than 10 nm and about 25% had a diameter in the range from 200
and 2000 nm.
The composition and the performance of the catalyst are shown in
Table 1.
0050/50549 CA 02432200 2003-06-18
Example 4
A solution of 0.384 g of SnC12 = 2H2O and 0.252 g of H2PtCl6 =
6H20 in 196 ml ethanol was poured over 32 g of a crushed Zr02 =
5 xSiOZ mixed oxide from Norton (# 9816590; sieve fraction
1.6-2 mm).
The excess solution was removed under a reduced pressure of
28 mbar on a rotary evaporator over a period of 30 minutes. The
10 composition was dried at 1000C for 15 hours and calcined at
5600C for 3 hours. A solution of 0.247 g of CsN03i 0.435 g of
KN03 and 3.147 g of La(N03)3 = 6H2O in 120 ml of H20 was then
poured over the catalyst. The supernatant solution was removed
under a reduced pressure of 30 mbar over a period of 30 minutes.
The catalyst was dried at 1000C for 15 hours and calcined at
5600C for 3 hours.
The catalyst had a BET surface area of 82 m2/g. Mercury
porosimetry measurements gave a pore volume of 0.27 ml/g, a pore
area of 65 m2/g and a mean pore radius of 11.7 nm. Based on the
pore volume, about 58% of the pores had a diameter of not more
than 20 nm, about 18% of the pores had a diameter of from 40 to
100 nm and about 30% had a diameter of more than 40 and less
than 5000 nm.
The composition of the catalyst is shown in Table 1.
Comparative Example 1 (Comp. 1)
A catalyst was prepared for comparison using the method in
WO-A-94/29021, Example l (Pt/Sn/Cs/Mg(Al)O).
The composition of the catalyst is shown in Table 1.
Comparative Example 2(Comp. 2)
The catalyst was produced using a method analogous to
Comparative Example 1.
The composition of the catalyst is shown in Table 1.
Catalyst test
20 ml of a catalyst produced as described above were installed
in a tube reactor having an internal diameter of 22 mm. The
catalyst was treated with hydrogen at 5800C for 30 minutes. The
catalyst was then exposed to a mixture of 80% of nitrogen and
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20% of air (lean air) at the same temperature. After a flushing
phase of 15 minutes using pure nitrogen, the catalyst was
reduced with hydrogen for 30 minutes. 20 standard 1/h of propane
(99.5% pure) and H20 in a molar ratio of propane/steam of 1:1
were then passed over the catalyst at a reaction temperature of
580 C or 610 C. The pressure was 1.5 bar and the GHSV was
1000 h-1. The reaction products were determined by gas
chromatography.
The results using the catalysts of Examples 1 to 4 and the
Comparative Examples are shown in Table 1.
Table 1: Performance of the catalysts of Examples 1 to 4 and
Comparative Examples 1 and 2 in the dehydrogenation of propane*
Conversion Selecti-
[~] after vity [%]
after
Example Pt Sn K Cs Zr02 Si02 A1203 1 h 17 h 1 h 17 h
No./[ C] [%] [%] [%] [%] [%] [%l [%]
1 /580 0.3 0.6 1.0 0.5 85.6 2.1 12.0 38 36 85 91
2 /580 0.3 0.6 0.5 0.5 98.1 --- --- 41 34 89 85
3 /580 0.3 0.6 1.0 0.5 97.6 --- --- 38 32 92 86
4 /610 0.3 0.6 0.5 0.5 90.8 4.5 --- 49 45 93 95
Comp. 1 0.3 0.3 --- 0.5 --- --- --- 33 29 92 95
/580
Comp. 2 0.3 0.6 --- 0.5 -47 38 93 93
/610
*) Test conditions: 20 ml of catalyst, granule size = 1.6 -
2 mm; 580 C or 610 C; propane/H20 = 1:1 (mol/mol); 20
standard 1/h of propane; GHSV = 1000 h-i; 1.5 bar.
**) Comparative catalyst: Pt/Sn/Cs/Mg(Al)O from WO-A-94/29021
Example 1.
45
