Zeolites as Specific Adsorbents for NO.sub.X and CO

As is generally known, zeolites are understood to be a group of naturally occurring silicate minerals, which can also be synthetically produced mainly from sodium silicate, sodium aluminate and sodium hydroxide, which are used to a large extent in industry. A common characteristic of the crystalline structures is a three-dimensional network or matrix made of SiO.sub.4 and AlO.sub.4 tetrahedrons permeated with channels and voids. Excess negative charge loading of the matrix is compensated for by cations, e.g. Na and Ca ions. These ions can be exchanged for other ions. It is known that the suitability of zeolites as an ion exchanger is based on this. Dehydrated zeolites will absorb water in contact with surroundings containing water vapor and therefore can be used as desiccants. It is also possible to retain other, relatively small molecules, such as ammonia, hydrogen sulfide and carbon disulfide, in the voids, separating them in this way from other materials. Based on this property, zeolites can be used as so-called molecular sieves or filters.

Because of their catalytic properties, it is further known to use synthetic zeolites, in particular zeolites loaded with trivalent cations, as catalysts in the petroleum-processing industry. Methods for producing crystalline zeolites, loaded with different cations and usable as catalysts, are known, for instance from U.S. Pat. Nos. 3,702,886 and 4,250,345, British Pat. No. 1 334 243 and many other patents.

It has been surprisingly found that zeolites having an Si:Al ratio of 1:1 to 100:1, preferably 2:1 to 60:1, can also very well be used as specific adsorbents for nitrogen oxides (NO.sub.x) or CO. Depending on the cations present in the network of the zeolites, zeolites with the Si:Al ratio mentioned can be used as specific adsorbents for either nitrogen oxides or carbon monoxide. Because of these properties it is possible, for example, to use such zeolites for producing specific NO.sub.x or CO sensors based on semiconductors which can for instance be used as a specific NO.sub.x or CO sensor for exhaust gases of automotive vehicles or other exhaust gases, because zeolites are stable at temperatures up to approximately 700.degree. C. If the gate of a field effect transistor (FET) is coated with a zeolite which specifically adsorbs NO.sub.x or CO, the current flowing through the Si semiconductor element is influenced by the adsorbents, i.e., adsorbed NO.sub.x or CO, acting on the gate, so that these gases can be separately registered. Because the amount of NO.sub.x is in a direct correlation to the amount of fuel used at the optimum ignition setting, it is also possible to regulate, i.e., minimize, the soot emission by means of this setting.

Zeolites usable in accordance with the invention can be produced according to the usual known methods and processes, such as described in detail e.g. in U.S. Pat. Nos. 3,702,886 and 4,250,345 and in British Pat. No. 1 334 243 and in the patents cited in these patents. This means that according to the invention, synthetic, crystalline zeolites that in accordance with the specification of the mentioned patents can be used as catalysts in the petroleum-processing industry and are also available in the trade can for instance be used as specific adsorbents for NO.sub.x and CO.

The use of those zeolites which are doped with transition metal cations, such as Fe, Co, Ni, Ru and Pt, has proven to be particularly advantageous. However, other zeolites are also usable in accordance with the invention, for example those which are doped with Cu or Ag or alkaline earths, such as Ca.

For use as specific adsorbers of carbon monoxide and nitrogen oxides, zeolites preferably containing the doping metals at concentrations of 0.01 to 10.0 mol %, and more preferably 0.5 to 5.0 mol %.

The zeolites which have been doped with e.g. Ca, Co, Ni, Fe, Cu, Ag, Pt or Ru or other cations can be produced in a simple way from the corresponding "H forms" of the zeolites. This is done in an advantageous way by slurrying the "H form" of the zeolite in the solution of a salt of the doping ion, for example in a solution of cobalt nitrate, ferric nitrate, hexamino-cobalt chloride, tetramino-platinum chloride or ruthenium chloride, by filtering, washing, drying and then calcining at temperatures on the order of 400.degree. to 500.degree. C. over a period of one to three hours.

It was shown that a direct exchange with Na forms is possible in only a few cases, because slurries of these zeolites in water have pH values of 9 to 12 and lead to hydrolysis and the precipitation of the cations to be introduced, such as transition metal cations.

The single FIGURE is a schematic representation of an internal combustion engine with the gas sensor in accordance with the invention, forming part of the exhaust gas sensing system.

The gas sensor 10 is responsive to exhaust gases from an exhaust EX coupled to an internal combustion engine E. The exhaust sensor 10, via suitable control systems, controls the relative emission of fuel and air to the internal combustion engine E, as schematically indicated by the broken lines from sensor 10 to the control arrows in lines A (air) and F (fuel). The fuel-air ratio, thus, can be determined by the reference to the exhaust gas sensor.

The exhaust gas sensor 10 is a semiconductor and, more preferably, a field transistor (FET) 11 which has a gate terminal 13 coated with zeolite coating 12.

Method of preparing zeolites 12 for coating the gate 13 of FET 11:

Gels of NaOH, NaAlO.sub.2, microdispersed silicic acid and tetraalkylammonium hydroxide, e.g. tetraethylammonium hydroxide, tetrapropylammonium hydroxide or tetrabutylammonium hydroxide, in water are prepared, which are maintained in Teflon-lined (polytetrafluoroethylene) or other similar inert material coated autoclaves, for six to twelve days at 140.degree. to 190.degree. C. The Si:Al ratio should be maintained in the 1:1 to 100:1 and preferably 2:1 to 60:1 range. To achieve a practical crystallization, defined molar conditions of the basic components as well as temperature and time must be closely maintained.

During heating in the autoclave, the amorphous gels crystallize into micro-crystalline products which are filtered and are freed of dissolved, non-reacted products by washing in water. Then they are dried and are heated to approximately 500.degree. C. to remove the water bound to the interior of the zeolites. For an NH.sub.4 exchange, the zeolites are slurried three to six times with a 2-molar NH.sub.4 Cl solution at 20.degree. to 90.degree. C. and left standing for a time. After filtering they are washed to remove Cl ions, dried and heated to 520.degree. C. for three hours. The "H forms" of the zeolites thus obtained are used for exchange for transition metal ions, such as Fe, Co, Ru and Pt. Exchange takes place in the same manner as the NH.sub.4 exchange by slurrying in a salt solution, e.g. in a solution of cobalt nitrate, ferric nitrate, hexaminocobalt chloride, tetramino-platinum chloride or ruthenium chloride, by filtering, washing, drying and then calcining at 400.degree. to 500.degree. C.

Direct exchange with the originally obtained Na forms is possible in only a few cases, because slurries of these zeolites in water have pH values of 9 to 12 and lead to hydrolysis and the precipitation of the cations to be introduced.

The "H forms" of zeolites which can be used as the base materials for the production of zeolites used in accordance with the invention are available commercially. Also available commercially are zeolites which are already doped. An example of this is the zeolite type 5A, available in the trade and having the total formula:

Whether a zeolite is suitable as a NO.sub.x or CO adsorbent can be determined in the manner described below:

The adsorption and desorption of CO and NO by the zeolites can be determined by the dynamic method in flowing helium having a constant flow speed by metering a set amount of CO or NO (as a diluted mixture of 20% by volume of NO and 80% by volume of He) into the inert gas flow by means of a gas sample inlet element. The changes in the heat conductivity of the gas caused by desorption can be measured by means of a heat conductivity detector. Through calibration with known amounts of CO and NO, the absorbed amount can be quantitatively measured.

In the above case, the zeolite samples were baked at 420.degree. to 450.degree. C. before the CO or NO was passed over them. Their interior surfaces were determined by the B.E.T. method.

1.0 g of aluminum chips were dissolved in a solution of 3.6 g sodium hydroxide in 10 ml water. Then 12.4 g of SiO.sub.2 (microdispersed silicic acid) were suspended in a (40%) solution of 67.5 g of tetraethylammonium hydroxide in 100 ml water and combined while stirring with the aluminate solution. The gel obtained was stirred for one hour at 20.degree. C. and then maintained for six days in an autoclave lined with Teflon (duPont brand name for polytetrafluoroethylene) at 152.degree. C. The reaction product was washed with plenty of water, dried and then heated for two hours to a temperature of 540.degree. C.

An exchange of the Na ions was performed three times with a 1 molar NH.sub.4 Cl solution (left standing in the NH.sub.4 Cl solution for approximately 10 hours per exchange); then filtering, drying at 120.degree. C. for an hour and baking at 540.degree. C. for two hours was performed. A zeolite of the ZSM5 type with the H form was obtained.

The ZSM5 H zeolite was transformed into the cobalt form by slurrying 1.5 g of the zeolite with 200 mg of cobalt acetate in 30 ml of water, dissolving the cobalt salt by heating and then heating the combined solution to boiling. The cooled solution was left standing overnight. The next day it was washed with water, filtered, dried and baked as described above. A zeolite of type ZSM5, doped with Co, was obtained in this manner.

15 g of a commercially available zeolite 13X in H form (Union Carbide) of the composition [Na.sub.86 (AlO.sub.2).sub.86 (SiO.sub.2).sub.106 ]x H.sub.2 O were suspended in 200 ml of a 2 m NH.sub.4 Cl solution, left standing for one hour, filtered and washed free of chloride. This process was repeated twice. Then drying for one hour at 150.degree. C. and baking at 520.degree. C. for two hours was performed (Zeolite 13X in H form).

1.5 g of the zeolite 13X (H form) obtained were suspended or dissolved in water with 200 mg RuCl.sub.3 and heated to boiling (pH approximately 5). The black precipitate was acidified with diluted hydrochloric acid (pH approximately 2) and heated again. The remaining black precipitate was dried, washed in water and then in acetone, dried at 150.degree. C. and baked at 500.degree. C. for two hours. A zeolite of the type 13X, doped with Ru, was obtained in this manner.

4.1 g of NaAlO.sub.2, obtained from 2.60 g of Na.sub.2 CO.sub.3 and 2.55 g of Al.sub.2 O.sub.3 sintering between 700.degree. C. and 1,150.degree. C., were dissolved 20 ml of water. 1.0 g of NaOH was dissolved in 80 ml of water and 18 g of microdispersed silicic acid (SiO.sub.2) were suspended therein. Both mixtures were combined and maintained for six days in an autoclave at 150.degree. C. They were then filtered, washed with water, dried for one hour at 200.degree. C. and left standing for three periods of 12 hours each in 200 ml of a 2 m NH.sub.4 Cl solution. After drying for two hours at 200.degree. C. and baking for two hours at 520.degree. C., a zeolite A in the H form (inactive) was obtained.

1.34 g of the NaAlO.sub.2, prepared as described above, were dissolved at 100.degree. C. in 15 ml of a solution of (40%) tetraethylammonium hydroxide. Furthermore, 25 g of microdispersed SiO.sub.2 were suspended in 70 ml water and combined with the above solution. The mixture was heated to 178.degree. C. for seven days in an autoclave. It was then slurried in water, washed, dried at 120.degree. C. and baked for two hours at 520.degree. C. A zeolite of the type ZSM8, doped with Na, was obtained in this manner.

0.8 g of NaAlO.sub.2 were slurried in a mixture of 50 ml water and 32 g of (20%) tetrabutylammonium hydroxide solution and partially dissolved. 30 g of SiO.sub.2 were added to this mixture while stirring and the mixture was diluted with water to 100 ml. This mixture was heated in an autoclave to 150.degree. to 170.degree. C. for 10 days.

The reaction product was washed with water, dried and baked one hour at 540.degree. C. The NH.sub.4 exchange was performed three times with a 1 m NH.sub.4 Cl solution with subsequent filtration, drying at 120.degree. C. and baking for 2 hours at 540.degree. C. (ZSM8).

The Ru exchange was performed with [Ru NOCl.sub.3 (H.sub.2 O).sub.2- ]3 H.sub.2 O by slurrying 2 g of the zeolite ZSM8 in 20 ml of a watery solution with 100 mg of the Ru compound and keeping it at a boil for one hour. Filtration, drying at 150.degree. C. and heating to 400.degree. C. for one hour followed.

The adsorption properties of these zeolites are shown in the table below.

Corresponding results in connection with the specific NO adsorption are obtained with the use of the 5A zeolites of Union Carbide together with Ca, with the ZSM11 and ZSM8 zeolites together with Co, Fe, Cu, Ag, Ru or Pt, as well as for the specific adsorption of CO with the 13x zeolite together with Co.