Molecular sieve abstract
A process for the reduction of oxides in a gas stream (e.g., automotive
exhaust) uses a catalyst comprising a molecular sieve having the
CHA crystal structure and having a mole ratio of greater than 50
to 1500 of (1) an oxide selected from silicon oxide, germanium oxide
or mixtures thereof to (2) an oxide selected from aluminum oxide,
iron oxide, titanium oxide, gallium oxide or mixtures thereof.
Molecular sieve claims
1. A process for the reduction of oxides of nitrogen contained
in a gas stream wherein said process comprises contacting the gas
stream with a molecular sieve, the molecular sieve having the CHA
crystal structure and having a mole ratio of greater than 50 to
1500 of (1) an oxide selected from silicon oxide, germanium oxide
or mixtures thereof to (2) an oxide selected from aluminum oxide,
iron oxide, titanium oxide, gallium oxide or mixtures thereof.
2. The process of claim 1 wherein the mole ratio of oxide (1) to
oxide (2) is 200-1500.
3. The process of claim 1 conducted in the presence of oxygen.
4. The process of claim 2 conducted in the presence of oxygen.
5. The process of claim 1 wherein said molecular sieve contains
a metal or metal ions capable of catalyzing the reduction of the
oxides of nitrogen.
6. The process of claim 2 wherein said molecular sieve contains
a metal or metal ions capable of catalyzing the reduction of the
oxides of nitrogen.
7. The process of claim 5 wherein the metal is cobalt, copper,
platinum, iron, chromium, manganese, nickel, zinc, lanthanum, palladium,
rhodium or mixtures thereof.
8. The process of claim 6 wherein the metal is cobalt, copper,
platinum, iron, chromium, manganese, nickel, zinc, lanthanum, palladium,
rhodium or mixtures thereof.
9. The process of claim 1 wherein the gas stream is the exhaust
stream of an internal combustion engine.
10. The process of claim 2 wherein the gas stream is the exhaust
stream of an internal combustion engine.
11. The process of claim 5 wherein the gas stream is the exhaust
stream of an internal combustion engine.
12. The process of claim 6 wherein the gas stream is the exhaust
stream of an internal combustion engine.
Molecular sieve description[0001] This application claims benefit
under 35 USC .sctn. 119 of U.S. Provisional Application No. 60/631715
filed Nov. 29 2004.
BACKGROUND
[0002] Chabazite, which has the crystal structure designated "CHA",
is a natural zeolite with the approximate formula Ca.sub.6Al.sub.12Si.sub.24O.sub.72.
Synthetic forms of chabazite are described in "Zeolite Molecular
Sieves" by D. W. Breck, published in 1973 by John Wiley &
Sons. The synthetic forms reported by Breck are: zeolite "K-G",
described in J. Chem. Soc., p. 2822 (1956), Barrer et al.; zeolite
D, described in British Patent No. 868846 (1961); and zeolite R,
described in U.S. Pat. No. 3030181 issued Apr. 17 1962 to Milton.
Chabazite is also discussed in "Atlas of Zeolite Structure
Types" (1978) by W. H. Meier and D. H. Olson.
[0003] The K-G zeolite material reported in the J. Chem. Soc. Article
by Barrer et al. is a potassium form having a silica:alumina mole
ratio (referred to herein as "SAR") of 2.3:1 to 4.15:1.
Zeolite D reported in British Patent No. 868846 is a sodium-potassium
form having a SAR of 4.5:1 to 4.9:1. Zeolite R reported in U.S.
Pat. No. 3030181 is a sodium form which has a SAR of 3.45:1 to
3.65:1.
[0004] Citation No. 93:66052y in Volume 93 (1980) of Chemical Abstracts
concerns a Russian language article by Tsitsishrili et al. in Soobsch.
Akad. Nauk. Gruz. SSR 1980 97(3) 621-4. This article teaches that
the presence of tetramethylammonium ions in a reaction mixture containing
K.sub.2O--Na.sub.2O--SiO.sub.2--Al.sub.2O.sub.3--H.sub.2O promotes
the crystallization of chabazite. The zeolite obtained by the crystallization
procedure has a SAR of 4.23.
[0005] The molecular sieve designated SSZ-13 which has the CHA
crystal structure, is disclosed in U.S. Pat. No. 4544538 issued
Oct. 1 1985 to Zones. SSZ-13 is prepared from nitrogen-containing
cations derived from 1-adamantamine, 3-quinuclidinol and 2-exo-aminonorbomane.
Zones discloses that the SSZ-13 of U.S. Pat. No. 4544538 has a
composition, as-synthesized and in the anhydrous state, in terms
of mole ratios of oxides as follows: (0.5 to 1.4)R.sub.2O:(0 to
0.5)M.sub.2O:W.sub.2O.sub.3: (greater than 5)YO.sub.2 wherein M
is an alkali metal cation, W is selected from aluminum, gallium
and mixtures thereof, Y is selected from silicon, germanium and
mixtures thereof, and R is an organic cation. As prepared, the silica:alumina
mole ratio is typically in the range of 8:1 to about 50:1 higher
mole ratios can be obtained by varying the relative ratios of reactants.
It is disclosed that higher mole ratios can also be obtained by
treating the SSZ-13 with chelating agents or acids to extract aluminum
from the SSZ-13 lattice. It is further stated that the silica:alumina
mole ratio can also be increased by using silicon and carbon halides
and similar compounds.
[0006] U.S. Pat. No. 4544538 also discloses that the reaction
mixture used to prepare SSZ-13 has a YO.sub.2/W.sub.2O.sub.3 mole
ratio (e.g., SAR) in the range of 5:1 to 350:1. It is disclosed
that use of an aqueous colloidal suspension of silica in the reaction
mixture to provide a silica source allows production of SSZ-13 having
a relatively high silica:alumina mole ratio.
[0007] U.S. Pat. No. 4544538 does not, however, disclose SSZ-13
having a silica:alumina mole ratio greater than 50.
[0008] U.S. Pat. No. 6709644 issued Mar. 23 2004 to Zones et
al., discloses aluminosilicate zeolites having the CHA crystal structure
and having small crystallite sizes (designated SSZ-62). The reaction
mixture used to prepare SSZ-62 has a SiO.sub.2/Al.sub.2O.sub.3 mole
ratio of 20-50. It is disclosed that the zeolite can be used for
separation of gasses (e.g., separating carbon dioxide from natural
gas), and in catalysts used for the reduction of oxides of nitrogen
in a gas stream (e.g., automotive exhaust), converting lower alcohols
and other oxygenated hydrocarbons to liquid products, and for producing
dimethylamine.
[0009] M. A. Camblor, L. A. Villaescusa and M. J. Diaz-Cabanas,
"Synthesis of All-Silica and High-Silica Molecular Sieves in
Fluoride Media", Topics in Catalysis, 9 (1999), pp. 59-76 discloses
a method for making all-silica or high-silica zeolites, including
chabazite. The chabazite is made in a reaction mixture containing
fluoride and a N,N,N-trimethyl-1-adamantammonium structure directing
agent. Camblor et al. does not, however, disclose the synthesis
of all- or high-silica chabazite from a hydroxide-containing reaction
mixture.
SUMMARY OF THE INVENTION
[0010] In accordance with this invention, there is provided a process
for the reduction of oxides of nitrogen contained in a gas stream
wherein said process comprises contacting the gas stream with a
molecular sieve, the molecular sieve having the CHA crystal structure
and having a mole ratio of greater than 50 to 1500 of (1) an oxide
selected from silicon oxide, germanium oxide or mixtures thereof
to (2) an oxide selected from aluminum oxide, iron oxide, titanium
oxide, gallium oxide or mixtures thereof. In one embodiment, the
molecular sieve has a mole ratio of oxide (1) to oxide (2) is 200-1500.
The molecular sieve may contain a metal or metal ions (such as cobalt,
copper, platinum, iron, chromium, manganese, nickel, zinc, lanthanum,
palladium, rhodium or mixtures thereof) capable of catalyzing the
reduction of the oxides of nitrogen, and the process may be conducted
in the presence of a stoichiometric excess of oxygen. In a preferred
embodiment, the gas stream is the exhaust stream of an internal
combustion engine.
DETAILED DESCRIPTION
[0011] The present invention relates to a method of preparing high-silica
molecular sieves having the CHA crystal structure and the molecular
sieves so prepared. As used herein, the term "high-silica"
means the molecular sieve has a mole ratio of (1) silicon oxide,
germanium oxide and mixtures thereof to (2) aluminum oxide, iron
oxide, titanium oxide, gallium oxide and mixtures thereof of greater
than 50. This includes all-silica molecular sieves in which the
ratio of (1):(2) is infinity, i.e., there is essentially none of
oxide (2) in the molecular sieve.
[0012] One advantage of the present invention is that the reaction
is conducted in the presence of hydroxide rather than fluoride.
HF-based syntheses generally require a large amount of structure
directing agent ("SDA"). Typical HF-based reactions will
have a SDA/SiO.sub.2 mole ratio of 0.5.
[0013] High-silica CHA molecular sieves can be suitably prepared
from an aqueous reaction mixture containing sources of an alkali
metal or alkaline earth metal oxide; sources of an oxide of silicon,
germanium or mixtures thereof, optionally, sources of aluminum oxide,
iron oxide, titanium oxide, gallium oxide and mixtures thereof;
and a cation derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbomane.
The mixture should have a composition in terms of mole ratios falling
within the ranges shown in below: TABLE-US-00001 TABLE A YO.sub.2/W.sub.aO.sub.b
220-.infin. (preferably 350-5500) OH--/YO.sub.2 0.19-0.52 Q/YO.sub.2
0.15-0.25 M.sub.2/nO/YO.sub.2 0.04-0.10 H.sub.2O/YO.sub.2 10-50
wherein Y is silicon, germanium or mixtures thereof, W is aluminum,
iron, titanium, gallium or mixtures thereof, M is an alkali metal
or alkaline earth metal, n is the valence of M (i.e., 1 or 2) and
Q is a cation derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbornane.
[0014] The cation derived from 1-adamantamine can be a N,N,N-trialkyl-1-adamantammonium
cation which has the formula: where R.sup.1 R.sup.2 and R.sup.3
are each independently a lower alkyl, for example methyl. The cation
is associated with an anion, A.sup.-, which is not detrimental to
the formation of the molecular sieve. Representative of such anions
include halogens, such as chloride, bromide and iodide; hydroxide;
acetate; sulfate and carboxylate. Hydroxide is the preferred anion.
It may be beneficial to ion exchange, for example, a halide for
hydroxide ion, thereby reducing or eliminating the alkali metal
or alkaline earth metal hydroxide required.
[0015] The cation derived from 3-quinuclidinol can have the formula:
where R.sup.1 R.sup.2 R.sup.3 and A are as defined above.
[0016] The cation derived from 2-exo-aminonorbornane can have the
formula: where R.sup.1 R.sup.2 R.sup.3 and A are as defined above.
[0017] The reaction mixture is prepared using standard molecular
sieve preparation techniques. Typical sources of silicon oxide include
fumed silica, silicates, silica hydrogel, silicic acid , colloidal
silica, tetra-alkyl orthosilicates, and silica hydroxides. Examples
of such silica sources include CAB-O-SIL M5 fumed silica and Hi-Sil
hydrated amorphous silica, or mixtures thereof. Typical sources
of aluminum oxide include aluminates, alumina, hydrated aluminum
hydroxides, and aluminum compounds such as AlCl.sub.3 and Al.sub.2(SO.sub.4).sub.3.
Sources of other oxides are analogous to those for silicon oxide
and aluminum oxide.
[0018] It has been found that seeding the reaction mixture with
CHA crystals both directs and accelerates the crystallization, as
well as minimizing the formation of undesired contaminants. In order
to produce pure phase high-silica CHA crystals, seeding may be required.
When seeds are used, they can be used in an amount that is about
2-3 wt. % based on the weight of YO.sub.2.
[0019] The reaction mixture is maintained at an elevated temperature
until CHA crystals are formed. The temperatures during the hydrothermal
crystallization step are typically maintained from about 120.degree.
C. to about 160.degree. C. It has been found that a temperature
below 160.degree. C., e.g., about 120.degree. C. to about 140.degree.
C., is useful for producing high-silica CHA crystals without the
formation of secondary crystal phases.
[0020] In one embodiment, the reaction mixture contains seeds of
CHA crystals and the reaction mixture is maintained at a temperature
of less than 160.degree. C., for example 120.degree. C. to 140.degree.
C.
[0021] The crystallization period is typically greater than 1 day
and preferably from about 3 days to about 7 days. The hydrothermal
crystallization is conducted under pressure and usually in an autoclave
so that the reaction mixture is subject to autogenous pressure.
The reaction mixture can be stirred, such as by rotating the reaction
vessel, during crystallization.
[0022] Once the high-silica CHA crystals have formed, the solid
product is separated from the reaction mixture by standard mechanical
separation techniques such as filtration. The crystals are water-washed
and then dried, e.g., at 90.degree. C. to 150.degree. C. for from
8 to 24 hours, to obtain the as-synthesized crystals. The drying
step can be performed at atmospheric or subatmospheric pressures.
[0023] The high-silica CHA can be made with a mole ratio of YO.sub.2/W.sub.cO.sub.d
of .infin., i.e., there is essentially no W.sub.cO.sub.d present
in the CHA. In this case, the CHA would be an all-silica material
or a germanosilicate. Thus, in a typical case where oxides of silicon
and aluminum are used, CHA can be made essentially aluminum free,
i.e., having a silica to alumina mole ratio of .infin.. A method
of increasing the mole ratio of silica to alumina is by using standard
acid leaching or chelating treatments. The high-silica CHA can also
be made by first preparing a borosilicate CHA and then removing
the boron. The boron can be removed by treating the borosilicate
CHA with acetic acid at elevated temperature ( as described in Jones
et al., Chem. Mater., 2001 13 pp. 1041-1050) to produce an all-silica
version of CHA.
[0024] The high-silica CHA molecular sieve has a composition, as-synthesized
and in the anhydrous state, in terms of mole ratios of oxides as
indicated in Table B below: TABLE-US-00002 TABLE B YO.sub.2/W.sub.cO.sub.d
Greater than 50-.infin. (e.g., >50-1500 or 200-1500) M.sub.2/nO/YO.sub.2
0.04-0.15 Q/YO.sub.2 0.15-0.25
wherein Y is silicon, germanium or mixtures thereof, W is aluminum,
iron, titanium, gallium or mixtures thereof; c is 1 or 2; d is 2
when c is 1 (i.e., W is tetravalent) or d is 3 or 5 when c is 2
(i.e., d is 3 when W is trivalent or 5 when W is pentavalent); M
is an alkali metal cation, alkaline earth metal cation or mixtures
thereof; n is the valence of M (i.e., 1 or 2); and Q is a cation
derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbornane.
The as-synthesized material does not contain fluoride.
[0025] The present invention also provides a molecular sieve having
the CHA crystal structure and having a mole ratio of greater than
50 to 1500 of (1) an oxide selected from silicon oxide, germanium
oxide or mixtures thereof to (2) an oxide selected from aluminum
oxide, iron oxide, titanium oxide, gallium oxide or mixtures thereof.
In one embodiment, the molecular sieve has a mole ratio of oxide
(1) to oxide (2) is 200-1500.
[0026] High-silica CHA molecular sieves can be used as-synthesized
or can be thermally treated (calcined). By "thermal treatment"
is meant heating to a temperature from about 200.degree. C. to about
820.degree. C., either with or without the presence of steam. Usually,
it is desirable to remove the alkali metal cation by ion exchange
and replace it with hydrogen, ammonium, or any desired metal ion.
Thermal treatment including steam helps to stabilize the crystalline
lattice from attack by acids.
[0027] The high silica CHA molecular sieves, as-synthesized, have
a crystalline structure whose X-ray powder diffraction ("XRD")
pattern shows the following characteristic lines: TABLE-US-00003
TABLE I As-Synthesized High Silica CHA XRD 2 Theta.sup.(a) d-spacing
(Angstroms) Relative Intensity.sup.(b) 9.64 9.17 S 14.11 6.27 M
16.34 5.42 VS 17.86 4.96 M 21.03 4.22 VS 25.09 3.55 S 26.50 3.36
W-M 30.96 2.89 W 31.29 2.86 M 31.46 2.84 W .sup.(a).+-.0.10 .sup.(b)The
X-ray patterns provided are based on a relative intensity scale
in which the strongest line in the X-ray pattern is assigned a value
of 100: W(weak) is less than 20; M(medium) is between 20 and 40;
S(strong) is between 40 and 60; VS(very strong) is greater than
60.
[0028] Table IA below shows the X-ray powder diffraction lines
for as-synthesized high silica CHA including actual relative intensities.
TABLE-US-00004 TABLE IA As-Synthesized High Silica CHA XRD 2 Theta.sup.(a)
d-spacing (Angstroms) Relative Intensity(%) 9.64 9.17 50.8 13.16
6.72 4.4 14.11 6.27 23.1 16.34 5.42 82.4 17.86 4.96 21.7 19.34 4.59
6.1 21.03 4.22 100 22.24 3.99 11.0 22.89 3.88 10.7 23.46 3.79 4.9
25.09 3.55 43.1 26.50 3.36 19.5 28.25 3.16 4.7 28.44 3.14 1.5 30.14
2.96 3.2 30.96 2.89 14.3 31.29 2.86 37.5 31.46 2.84 12.0 33.01 2.71
1.8 33.77 2.65 1.9 34.05 2.63 0.2 35.28 2.54 3.6 35.69 2.51 0.7
36.38 2.47 5.8 39.22 2.30 1.0 39.81 2.26 0.8 .sup.(a).+-.0.10
[0029] After calcination, the high silica CHA molecular sieves
have a crystalline structure whose X-ray powder diffraction pattern
include the characteristic lines shown in Table II: TABLE-US-00005
TABLE II Calcined High Silica CHA XRD 2 Theta.sup.(a) d-spacing
(Angstroms) Relative Intensity 9.65 9.2 VS 13.08 6.76 M 16.28 5.44
W 18.08 4.90 W 20.95 4.24 M 25.37 3.51 W 26.36 3.38 W 31.14 2.87
M 31.61 2.83 W 35.10 2.55 W .sup.(a).+-.0.10
[0030] Table IIA below shows the X-ray powder diffraction lines
for calcined high silica CHA including actual relative intensities.
TABLE-US-00006 TABLE IIA Calcined High Silica CHA XRD 2 Theta.sup.(a)
d-spacing (Angstroms) Relative Intensity(%) 9.65 9.2 100 13.08 6.76
29.3 14.21 6.23 3.9 16.28 5.44 15.2 18.08 4.90 16.1 19.37 4.58 2.3
20.95 4.24 36.8 22.38 3.97 1.9 22.79 3.90 1.9 23.44 3.79 1.5 25.37
3.51 14.1 26.36 3.38 9.5 28.12 3.17 2.0 28.65 3.11 1.9 30.07 2.97
1.0 31.14 2.87 22.0 31.36 2.85 2.9 31.61 2.83 9.3 32.14 2.78 0.9
32.90 2.72 1.0 34.03 2.63 2.1 35.10 2.55 4.3 36.64 2.45 3.3 39.29
2.29 1.3 40.40 2.23 2.6 .sup.(a).+-.0.10
[0031] The X-ray powder diffraction patterns were determined by
standard techniques. The radiation was the K-alpha/doublet of copper
and a scintillation counter spectrometer with a strip-chart pen
recorder was used. The peak heights I and the positions, as a function
of 2 Theta where Theta is the Bragg angle, were read from the spectrometer
chart. From these measured values, the relative intensities, 100.times.I/Io,
where Io is the intensity of the strongest line or peak, and d,
the interplanar spacing in Angstroms corresponding to the recorded
lines, can be calculated.
[0032] Variations in the diffraction pattern can result from variations
in the mole ratio of oxides from sample to sample. The molecular
sieve produced by exchanging the metal or other cations present
in the molecular sieve with various other cations yields a similar
diffraction pattern, although there can be shifts in interplanar
spacing as well as variations in relative intensity. Calcination
can also cause shifts in the X-ray diffraction pattern. Also, the
symmetry can change based on the relative amounts of boron and aluminum
in the crystal structure. Notwithstanding these perturbations, the
basic crystal lattice structure remains unchanged.
[0033] The molecular sieves of this invention may be used for the
catalytic reduction of the oxides of nitrogen in a gas stream. Typically,
the gas stream also contains oxygen, often a stoichiometric excess
thereof. Also, the molecular sieve may contain a metal or metal
ions within or on it which are capable of catalyzing the reduction
of the nitrogen oxides. Examples of such metals or metal ions include
cobalt, copper, platinum, iron, chromium, manganese, nickel, zinc,
lanthanum, palladium, rhodium and mixtures thereof.
[0034] One example of such a process for the catalytic reduction
of oxides of nitrogen in the presence of a zeolite is disclosed
in U.S. Pat. No. 4297328 issued Oct. 27 1981 to Ritscher et
al., which is incorporated by reference herein. There, the catalytic
process is the combustion of carbon monoxide and hydrocarbons and
the catalytic reduction of the oxides of nitrogen contained in a
gas stream, such as the exhaust gas from an internal combustion
engine. The zeolite used is metal ion-exchanged, doped or loaded
sufficiently so as to provide an effective amount of catalytic copper
metal or copper ions within or on the zeolite. In addition, the
process is conducted in an excess of oxidant, e.g., oxygen. |