Reduction of oxides of nitrogen in a gas stream using molecular sieve SSZ-71

Molecular sieve abstract

The present invention relates to new molecular sieve SSZ-71 prepared using a N-benzyl-14-diazabicyclo[2.2.2]octane cation as a structure-directing agent, methods for synthesizing SSZ-71 and processes employing SSZ-71 in a catalyst.

Molecular sieve claims

What is claimed is:

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 produced by the method comprising: (1) preparing an as-synthesized molecular sieve having a composition, as synthesized and in the anhydrous state, in terms of mole ratios as follows: YO.sub.2/WO.sub.d 15 .infin. M.sub.2/n/YO.sub.2 0 0.03 Q/YO.sub.2 0.02 0.05 wherein Y is silicon, germanium or a mixture thereof; W is zinc, titanium or mixtures thereof; d is 1 or 2 (i.e., d is 1 when W is divalent or 2 when W is tetravalent); 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 N-benzyl-14-diazabicyclo[2.2.2]octane cation, the as-synthesized molecular sieve having the X-ray diffraction lines of Table I; (2) thermally treating the as-synthesized molecular sieve at a temperature and for a time sufficient to remove the N-benzyl-14-diazabicyclo[2.2.2]octane cation from the molecular sieve; and (3) optionally, replacing at least part of the zinc and/or titanium with a metal selected from the group consisting of aluminum, gallium, iron, boron, indium, vanadium and mixtures thereof.

2. The process of claim 1 conducted in the presence of oxygen.

3. 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.

4. The process of claim 3 wherein the metal is cobalt, copper, platinum, iron, chromium, manganese, nickel, zinc, lanthanum, palladium, rhodium or mixtures thereof.

5. The process of claim 1 wherein the gas stream is the exhaust stream of an internal combustion engine.

Molecular sieve description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to new molecular sieve SSZ-71 a method for preparing SSZ-71 using a N-benzyl-14-diazabicyclo[2.2.2]octane cation as a structure directing agent and the use of SSZ-71 in catalysts for, e.g., hydrocarbon conversion reactions.

2. State of the Art

Because of their unique sieving characteristics, as well as their catalytic properties, crystalline molecular sieves and zeolites are especially useful in applications such as hydrocarbon conversion, gas drying and separation. Although many different crystalline molecular sieves have been disclosed, there is a continuing need for new zeolites with desirable properties for gas separation and drying, hydrocarbon and chemical conversions, and other applications. New zeolites may contain novel internal pore architectures, providing enhanced selectivities in these processes.

Crystalline aluminosilicates are usually prepared from aqueous reaction mixtures containing alkali or alkaline earth metal oxides, silica, and alumina. Crystalline borosilicates are usually prepared under similar reaction conditions except that boron is used in place of aluminum. By varying the synthesis conditions and the composition of the reaction mixture, different zeolites can often be formed.

SUMMARY OF THE INVENTION

The present invention is directed to a family of molecular sieves with unique properties, referred to herein as "molecular sieve SSZ-71" or simply "SSZ-71". Preferably, SSZ-71 is in its silicate, zincosilicate, aluminosilicate, titanosilicate, germanosilicate, vanadosilicate, ferrosilicate or borosilicate form. The term "silicate" refers to a molecular sieve having a high mole ratio of silicon oxide relative to aluminum oxide, preferably a mole ratio greater than 100 including molecular sieves comprised entirely of silicon oxide. As used herein, the term "zincosilicate" refers to a molecular sieve containing both zinc oxide and silicon oxide. The term "aluminosilicate" refers to a molecular sieve containing both aluminum oxide and silicon oxide and the term "borosilicate" refers to a molecular sieve containing oxides of both boron and silicon.

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 produced by the method comprising: (1) preparing an as-synthesized molecular sieve having a composition, as synthesized and in the anhydrous state, in terms of mole ratios as follows: YO.sub.2/WO.sub.d 15 .infin. M.sub.2/n/YO.sub.2 0 0.03 Q/YO.sub.2 0.02 0.05 wherein Y is silicon, germanium or a mixture thereof; W is zinc, titanium or mixtures thereof; d is 1 or 2 (i.e., d is 1 when W is divalent or 2 when W is tetravalent); 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 N-benzyl-14-diazabicyclo[2.2.2]octane cation, the as-synthesized molecular sieve having the X-ray diffraction lines of Table I; (2) thermally treating the as-synthesized molecular sieve at a temperature and for a time sufficient to remove the N-benzyl-14-diazabicyclo[2.2.2]octane cation from the molecular sieve; and (3) optionally, replacing at least part of the zinc and/or titanium with a metal selected from the group consisting of aluminum, gallium, iron, boron, indium, vanadium and mixtures thereof.

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 OF THE INVENTION

The present invention comprises a family of molecular sieves designated herein "molecular sieve SSZ-71" or simply "SSZ-71". In preparing SSZ-71 a N-benzyl-14-diazabicyclo[2.2.2]octane cation (referred to herein as "benzyl DABCO") is used as a structure directing agent ("SDA"), also known as a crystallization template. The SDA useful for making SSZ-71 has the following structure:

##STR00001##

The SDA cation is associated with an anion (X.sup.-) which may be any anion that is not detrimental to the formation of the molecular sieve. Representative anions include halogen, e.g., fluoride, chloride, bromide and iodide, hydroxide, acetate, sulfate, tetrafluoroborate, carboxylate, and the like. Hydroxide is the most preferred anion.

Benzyl DABCO and a method for making it are disclosed in U.S. Pat. No. 5653956 issued Aug. 5 1997 to Zones.

SSZ-71 is prepared from a reaction mixture having the composition shown in Table A below.

TABLE-US-00001 TABLE A Reaction Mixture Typical Preferred YO.sub.2/WO.sub.d >15 >30 OH--/YO.sub.2 0.10 0.50 0.20 0.30 Q/YO.sub.2 0.05 0.50 0.10 0.20 M.sub. 2/n/YO.sub.2 0 0.40 0.10 0.25 H.sub.2O/YO.sub.2 10 80 15 45

where Y is silicon, germanium or a mixture thereof; W is zinc, titanium or mixtures thereof; d is 1 or 2 (i.e., d is 1 when W is divalent or 2 when W is tetravalent); 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 N-benzyl-14-diazabicyclo[2.2.2]octane cation.

In practice, SSZ-71 is prepared by a process comprising:

(a) preparing an aqueous solution containing sources of at least one oxide capable of forming a molecular sieve and a benzyl DABCO cation having an anionic counterion which is not detrimental to the formation of SSZ-71;

(b) maintaining the aqueous solution under conditions sufficient to form SSZ-71; and

(c) recovering the SSZ-71.

SSZ-71 can be prepared as a zincosilicate or titanosilicate. However, once the SSZ-71 is made, the zinc and/or titanium can be replaced with other metals by techniques well known in the art. Accordingly, SSZ-71 may comprise the molecular sieve and the SDA in combination with metallic and non-metallic oxides bonded in tetrahedral coordination through shared oxygen atoms to form a cross-linked three dimensional crystal structure. The metallic and non-metallic oxides comprise one or a combination of oxides of (1) a first tetravalent element(s), and (2) one or a combination of a divalent element(s), trivalent element(s), pentavalent element(s), second tetravalent element(s) different from the first tetravalent element(s) or mixture thereof. The first tetravalent element(s) is preferably selected from the group consisting of silicon, germanium and combinations thereof. More preferably, the first tetravalent element is silicon. The divalent element, trivalent element, pentavalent element and second tetravalent element (which is different from the first tetravalent element) is preferably selected from the group consisting of zinc, aluminum, gallium, iron, boron, titanium, indium, vanadium and combinations thereof. More preferably, the divalent or trivalent element or second tetravalent element is zinc, aluminum, titanium or boron.

Silicon can be added as silicon oxide or Si(OC.sub.2H.sub.5).sub.4. Zinc can be added as a zinc salt such as zinc acetate. Titanium can be added as Ti(OC.sub.2H.sub.5).sub.4.

A source zeolite reagent may provide a source of metals. In most cases, the source zeolite also provides a source of silica. The source zeolite may also be used as a source of silica, with additional silicon added using, for example, the conventional sources listed above. Use of a source zeolite reagent is described in U.S. Pat. No. 5225179 issued Jul. 6 1993 to Nakagawa entitled "Method of Making Molecular Sieves", the disclosure of which is incorporated herein by reference.

Typically, an alkali metal hydroxide and/or an alkaline earth metal hydroxide, such as the hydroxide of sodium, potassium, lithium, cesium, rubidium, calcium, strontium, barium and magnesium, is used in the reaction mixture; however, this component can be omitted so long as the equivalent basicity is maintained. The SDA may be used to provide hydroxide ion. Thus, it may be beneficial to ion exchange, for example, the halide to hydroxide ion, thereby reducing or eliminating the alkali metal hydroxide quantity required. The alkali metal cation or alkaline earth cation may be part of the as-synthesized material, in order to balance valence electron charges therein.

The reaction mixture is maintained at an elevated temperature until the crystals of the SSZ-71 are formed. The hydrothermal crystallization is usually conducted under autogenous pressure, at a temperature between 100.degree. C. and 200.degree. C., preferably between 135.degree. C. and 160.degree. C. The crystallization period is typically greater than 1 day and preferably from about 3 days to about 20 days.

Optionally, the molecular sieve is prepared using mild stirring or agitation.

During the hydrothermal crystallization step, the SSZ-71 crystals can be allowed to nucleate spontaneously from the reaction mixture. The use of SSZ-71 or SSZ-42 (disclosed in U.S. Pat. No. 5653956 issued Aug. 5 1997 to Zones) crystals as seed material can be advantageous in decreasing the time necessary for complete crystallization to occur. In addition, seeding can lead to an increased purity of the product obtained by promoting the nucleation and/or formation of SSZ-71 over any undesired phases. When used as seeds, as-synthesized SSZ-71 or SSZ-42 crystals (containing the SDA) are added in an amount between 0.1 and 10% of the weight of first tetravalent element oxide, e.g. silica, used in the reaction mixture.

Once the molecular sieve 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 SSZ-71 crystals. The drying step can be performed at atmospheric pressure or under vacuum.