Molecular sieve abstract
A boron-containing molecular sieve having the CHA crystal structure
and comprising (1) silicon oxide and (2) boron oxide or a combination
of boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium
oxide and mixtures thereof is prepared using a quaternary ammonium
cation derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbornane
as structure directing agent. The molecular sieve can be used for
gas separation or in catalysts to prepare methylamine or dimethylamine,
to convert oxygenates (e.g., methanol) to light olefins, or for
the reduction of oxides of nitrogen n a gas stream (e.g., automotive
exhaust).
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 comprising (1) silicon oxide and (2) boron
oxide or a combination of boron oxide and aluminum oxide, iron oxide,
titanium oxide, gallium oxide and mixtures thereof.
2. The process of claim 1 wherein oxide (2) is more than 50% boron
oxide on a molar basis.
3. The process of claim 1 conducted in the presence of oxygen.
4. 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.
5. The process of claim 4 wherein the metal is cobalt, copper,
platinum, iron, chromium, manganese, nickel, zinc, lanthanum, palladium,
rhodium or mixtures thereof.
6. The process of claim 1 wherein the gas stream is the exhaust
stream of an internal combustion engine.
7. The process of claim 5 wherein the gas stream is the exhaust
stream of an internal combustion engine.
Molecular sieve description
[0001] This application claims the benefit under 35 USC 119 of
Provisional Application No. 60/632005 filed Nov. 30 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. 171962 to Milton
et al. 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-aminonorbornane.
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:
[0006] (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.
U.S. Pat. No. 4544538 does not, however, disclose boron-containing
SSZ-13.
[0007] U.S. Pat. No. 6709644 issued Mar. 23 2004 to Zones et
al., discloses zeolites having the CHA crystal structure and having
small crystallite sizes. It does not, however, disclose a CHA zeolite
containing boron. 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.
SUMMARY OF THE INVENTION
[0008] 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 comprising (1) silicon oxide and (2) boron oxide or a combination
of boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium
oxide and mixtures thereof. The molecular sieve may contain oxide
(2) wherein more than 50% of oxide (2) is boron oxide on a molar
basis. 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
[0009] The present invention relates to molecular sieves having
the CHA crystal structure and containing boron in their crystal
framework.
[0010] Boron-containing CHA molecular sieves can be suitably prepared
from an aqueous reaction mixture containing sources of sources of
an oxide of silicon; sources of boron oxide or a combination of
boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium
oxide and mixtures thereof; optionally sources of an alkali metal
or alkaline earth metal oxide; and a cation derived from 1-adamantamine,
3-quinuclidinol or 2-exo-aminonorbornane. The mixture should have
a composition in terms of mole ratios falling within the ranges
shown in Table A below: TABLE-US-00001 TABLE A YO.sub.2/W.sub.aO.sub.b
>2-2000 OH--/YO.sub.2 0.2-0.45 Q/YO.sub.2 0.2-0.45 M.sub.2/nO/YO.sub.2
0-0.25 H.sub.2O/YO.sub.2 22-80
wherein Y is silicon; W is boron or a combination of boron and
aluminum, iron, titanium, gallium and 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 quaternary ammonium cation derived from 1-adamantamine,
3-quinuclidinol or 2-exo-aminonorbornane (commonly known as a structure
directing agent or "SDA").
[0011] The quaternary ammonium cation derived from 1-adamantamine
can be a N,N,N-trialkyl-1-adamantammonium cation which has the formula:
where R.sup.1R.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 fluoride,
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.
[0012] The quaternary ammonium 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.
[0013] The quaternary ammonium 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.
[0014] 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. Sources
of boron oxide include borosilicate glasses and other reactive boron
compounds. These include borates, boric acid and borate esters.
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, boron oxide and aluminum oxide.
[0015] 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 boron-containing CHA crystals, seeding may
be required. When seeds are used, they can be used in an amount
that is about 2-3 weight percent based on the weight of YO.sub.2.
[0016] 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 boron-containing CHA crystals without
the formation of secondary crystal phases.
[0017] 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.
[0018] Once the boron-containing 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.
[0019] The boron-containing 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:
As-Synthesized Boron-containing CHA Composition
[0020] TABLE-US-00002 TABLE B YO.sub.2/W.sub.cO.sub.d 20-2000
M.sub.2/nO/YO.sub.2 0-0.03 Q/YO.sub.2 0.02-0.05
where Y, W, M, n and Q are as defined above.
[0021] The boron-containing 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 Boron-Containing CHA XRD 2 Theta.sup.(a)
d-spacing (Angstroms) Relative Intensity.sup.(b) 9.68 9.13 S 14.17
6.25 M 16.41 5.40 VS 17.94 4.94 M 21.13 4.20 VS 25.21 3.53 VS 26.61
3.35 W-M 31.11 2.87 M 31.42 2.84 M 31.59 2.83 M .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.
[0022] Table IA below shows the X-ray powder diffraction lines
for as-synthesized boron-containing CHA including actual relative
intensities. TABLE-US-00004 TABLE IA As-Synthesized Boron-Containing
CHA XRD 2 Theta.sup.(a) d-spacing (Angstroms) Relative Intensity
(%) 9.68 9.13 55.2 13.21 6.70 5.4 14.17 6.25 33.5 16.41 5.40 81.3
17.94 4.94 32.6 19.43 4.56 6.8 21.13 4.20 100 22.35 3.97 15.8 23.00
3.86 10.1 23.57 3.77 5.1 25.21 3.53 78.4 26.61 3.35 20.2 28.37 3.14
6.0 28.57 3.12 4.4 30.27 2.95 3.9 31.11 2.87 29.8 31.42 2.84 38.3
31.59 2.83 26.5 32.27 2.77 1.4 33.15 2.70 3.0 33.93 2.64 4.7 35.44
2.53 3.9 35.84 2.50 1.2 36.55 2.46 10.9 39.40 2.29 1.8 40.02 2.25
1.3 40.44 2.23 1.0 40.73 2.21 6.0 .sup.(a).+-.0.10
[0023] After calcination, the boron-containing 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 Boron-Containing CHA XRD 2 Theta.sup.(a) d-spacing
(Angstroms) Relative Intensity 9.74 9.07 VS 13.12 6.74 M 14.47 6.12
W 16.38 5.41 W 18.85 4.78 M 21.07 4.21 M 25.98 3.43 W 26.46 3.37
W 31.30 2.86 W 32.15 2.78 W .sup.(a).+-.0.10
[0024] Table IIA below shows the X-ray powder diffraction lines
for calcined boron-containing CHA including actual relative intensities.
TABLE-US-00006 TABLE IIA Calcined Boron-Containing CHA XRD 2 Theta.sup.(a)
d-spacing (Angstroms) Relative Intensity (%) 9.74 9.07 100 13.12
6.74 29.5 14.47 6.12 4.6 16.38 5.41 14.2 18.85 4.78 22.1 19.60 4.53
2.2 21.07 4.21 32.9 22.84 3.89 2.2 23.68 3.75 0.8 25.98 3.43 13.1
26.46 3.37 8.7 28.27 3.15 1.3 29.24 3.05 1.6 30.32 2.95 1.7 31.30
2.86 14.4 32.15 2.78 9.0 32.56 2.75 0.2 35.26 2.54 2.4 .sup.(a).+-.0.10
[0025] 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 lo is the intensity of the strongest line or
peak, and d, the interplanar spacing in Angstroms corresponding
to the recorded lines, can be calculated.
[0026] 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.
[0027] Boron-containing CHA molecular sieves 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.
[0028] 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. 271981 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.
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