JPH09110807A - Production of carbonic diester and inhibition in deterioration of activity of catalyst component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the method for producing a carbonic diester, capable of obtaining the ester in a high yield and in high selectivity in a stable state for a long period and remarkably inhibiting the deterioration of the activity of a catalyst by supplying a specific amount of hydrogen chloride to the reaction system during the reaction. SOLUTION: The method for producing the carbonic diester comprises contact-reacting (A) a nitric ester with (B) carbon monoxide in the presence of a solid catalyst comprising a catalyst component containing a platinum group metal halide (preferably palladium chloride) and supported with a carrier in a gaseous phase. Therein, (D) hydrogen chloride is supplied to the reaction system in an amount required to compensate the amount of the chloride component scattered off from the component C during the reaction. The amount of the component C supported with the carrier is 0.1-10wt.% (converted into the metal body of the platinum group metal) based on the carrier, and the component D is preferably supplied in an amount of 1-50mol%/unit time on the basis of the component C, especially in an amount of 10-500vol.ppm to the raw material gas containing the component A and the component B.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a carbonic acid diester, and more particularly to a method for selectively producing a carbonic acid diester from carbon monoxide and a nitrite ester. The present invention also relates to a method for suppressing a decrease in activity of a catalyst component used in a method for producing a carbonic acid diester during a reaction.

[0002]

BACKGROUND OF THE INVENTION Carbonic acid diesters are very useful compounds as organic synthetic raw materials for medicines, agricultural chemicals, etc., and as intermediates for the production of polycarbonate, urethane, etc.

As a method for producing a carbonic acid diester, a method of reacting phosgene with an alcohol has been practiced for a long time. However, phosgene is extremely toxic and a large amount of hydrochloric acid is produced by the reaction between phosgene and alcohol. Since it is a by-product, there is a problem with the material of the device, and a manufacturing method that does not use phosgene has been desired. Therefore, as a production method without using phosgene, a method of synthesizing a carbonic acid diester from alcohol and carbon monoxide has been studied and proposed in various fields (for example, JP-A-60-7544).
No. 7, JP-A-63-72650, JP-B-63
-38018, WO-87 / 7601 and other publications). The methods described in these publications are
This is a method of synthesizing a carbonic acid diester by using an oxidation reaction of an alcohol and carbon monoxide with oxygen, using copper halide, palladium halide, etc. as a catalyst. However, the selectivity of the carbonic acid diester is low, and purification of the carbonic acid diester is troublesome due to the generation of water.

As this improved method, for example, a solid catalyst in which a nitrite and carbon monoxide are carried on a carrier of a platinum group metal or a compound thereof (eg, a chloride of a platinum group metal such as palladium chloride) is used. A method for producing a carbonic acid diester which is used for a gas phase catalytic reaction is proposed in JP-A-60-181051. In this production method, 10 mol% or more of oxidizer as O ₂ per carbon monoxide is present in the reaction system. However, in this method, in order to suppress the by-production of oxalic acid diester, an oxidant such as oxygen is coexisted with carbon monoxide in the above proportion, but a considerable amount of oxalic acid is present. Diester is a by-product,
The carbonic acid diester selectivity is low and the reaction rate is not sufficient. Furthermore, the use range of nitrite ester in a mixed gas consisting of nitrite ester, carbon monoxide, alcohol, oxygen, etc. used for the reaction exceeds the explosion limit,
It is not always an industrially satisfactory method because of safety issues.

Therefore, as a method for overcoming the above-mentioned problems, the present inventor has used platinum group metals or their compounds (eg, chlorides of platinum group metals such as palladium chloride), iron, copper, bismuth, cobalt and nickel. A method for producing a carbonic acid diester from carbon monoxide and a nitrite ester has already been proposed using a novel solid catalyst in which a carrier carries at least one metal compound selected from the group consisting of tin and tin. (Specification of Japanese Patent Application No. 1-284816 = Japanese Patent Application Laid-Open No. Hei 3)
-141243). This method is an excellent method as an industrial process because it has a high reaction rate of carbon monoxide and nitrite and a high carbonic acid diester selectivity based on carbon monoxide, and is a gas phase reaction.

[0006]

According to the research conducted by the present inventor, nitrite ester is present in the presence of a solid catalyst in which a catalyst component containing the above-mentioned known platinum group metal or its compound is supported on a carrier. When the method for producing a carbonic acid diester by subjecting a carbon monoxide to a carbon monoxide is carried out using a platinum group metal chloride as its catalyst component, the platinum group metal It has been found that there is a drawback that a small amount of chlorine is scattered from the chloride and the catalytic activity gradually decreases.

Therefore, an object of the present invention is to produce a carbonic acid diester with a high selectivity and a high yield under mild reaction conditions by a gas phase method which facilitates separation and recovery of reaction products. By prolonging the life of the catalyst in the gas-phase catalytic reaction system of carbon monoxide and nitrite, it is possible to produce an industrially suitable carbonic acid diester production method that makes it possible to produce carbonic acid diester in a stable state for a long period of time. It is to provide and a method for suppressing the decrease in the activity of the catalyst component.

[0008]

According to the present invention, a nitrite and carbon monoxide are contacted in a gas phase in the presence of a solid catalyst having a catalyst component containing a platinum group metal chloride supported on a carrier. In the method for producing a carbonic acid diester consisting of reacting, an amount of hydrogen chloride necessary for compensating a chlorine content scattered from a chloride of a platinum group metal during the reaction is supplied to the reaction system. There is a method for producing a carbonic acid diester.

The present invention also comprises reacting the nitrite and carbon monoxide in the gas phase in the presence of a solid catalyst having a catalyst component containing a platinum group metal chloride supported on a carrier. When carrying out the method for producing a carbonic acid diester, the platinum characterized by supplying to the reaction system an amount of hydrogen chloride necessary for compensating the chlorine content scattered from the chloride of the platinum group metal during the reaction. There is also a method of suppressing a decrease in activity of a catalyst component containing a chloride of a group metal.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. Examples of the nitrite used in the present invention include methyl nitrite, ethyl nitrite, n- (or i-) propyl nitrite, n- (or i-) butyl nitrite, and nitrite se.
nitrites of lower fatty acid monohydric alcohols having 1 to 4 carbon atoms such as c-butyl, nitrites of alicyclic alcohols such as cyclohexyl nitrite, and alalkyl alcohols such as benzyl nitrite and phenylethyl nitrite. Suitable examples thereof include nitrite, and the nitrites of the lower aliphatic monohydric alcohols having 1 to 4 carbon atoms described above are particularly preferable, and methyl nitrite and ethyl nitrite are most preferable.

The solid catalyst used in the present invention is
The catalyst component is a chloride of a platinum group metal such as palladium, platinum, iridium, ruthenium, rhodium and the like, and the catalyst component is supported on a carrier.
As described in Japanese Patent Application Laid-Open No. 3-141243, a compound of a metal selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin is used in combination as a catalyst component. Is preferred. Specific examples of the chloride of the platinum group metal include palladium chloride, platinum chloride, iridium chloride, ruthenium chloride and rhodium chloride. Among them, palladium, ruthenium or rhodium chloride is particularly preferred, and palladium chloride is preferred. Most preferred.

The chloride of the platinum group metal in the present invention is not limited to those introduced as the above-mentioned chloride, but in the presence of hydrogen chloride, the above-mentioned chloride or chlorine It may be a platinum group metal or a compound of the metal capable of forming a complex that participates in the reaction. Examples of such platinum group metal compounds include halogen compounds such as bromates, iodates, and fluorates of platinum group metals, nitrates,
Examples thereof include phosphates, acetates, oxalates, and benzoates, and more specifically, palladium bromide, palladium iodide, palladium fluoride, palladium acetate, palladium phosphate, palladium acetate, palladium oxalate, benzoate. Palladium acid, ruthenium iodide, rhodium bromide, rhodium iodide, rhodium nitrate, rhodium sulfate, rhodium acetate and the like can be mentioned.

Examples of the above-mentioned compounds of metals such as iron, copper, bismuth, cobalt, nickel and tin are halogen compounds such as chlorides, bromides, iodides and fluorides of these metals,
Examples include nitrates, sulfates, phosphates and acetates,
Among them, the above halogen compounds are particularly preferable.

Suitable examples of the carrier carrying the catalyst component include diatomaceous earth, activated carbon, silicon carbide, titania, alumina and the like, and activated carbon is most preferable. The method of loading the catalyst component on the carrier does not need to be special, and may be any of commonly used methods, such as an impregnation method (immersion adsorption method), a kneading method, a deposition method, and a coprecipitation method. It is desirable to prepare by the method or the kneading method. The amount of platinum group metal chloride supported in the catalyst component supported on the carrier is
In general, 0.1 to 1 per carrier of platinum group metal is calculated for the carrier.
0% by weight, particularly 0.5 to 2% by weight is preferable. iron,
The amount of a compound of a metal such as copper, bismuth, cobalt, nickel or tin supported is 1 to 50 gram atom equivalent based on the amount of these metals, preferably 1 to 10 gram atom equivalent.
It is preferably in gram atom equivalent.

In the present invention, the above-mentioned catalyst is used in the form of powder or granules, but the particle size is not particularly limited, and in the case of powder it is usually 20 to 100 μm. , And 4 for granular
Normally used ones of about 200 mesh are suitable.

When at least one compound selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin is used as the second component, the second component acts as a promoter. By supporting these metal compounds on the carrier in the above amount, the reaction rate of carbon monoxide and nitrite is further significantly improved, as compared with the case where only the platinum group metal chloride is supported. However, there is an advantage that the deactivation rate of the catalyst becomes slow.

The hydrogen chloride used in the present invention is preferably anhydrous. And in this invention,
The method of coexisting hydrogen chloride in the reaction system of carbon monoxide and nitrite is not particularly limited, for example, as described in detail below, a method of continuously adding a trace amount of hydrogen chloride, a moving bed. It is preferable to use a reactor of the type in which a part of the catalyst is continuously extracted from the packed bed and brought into contact with hydrogen chloride, and then returned to the reactor. It is preferable to select an appropriate method based on the type of reaction between the carbon monoxide and the nitrite ester, which is adopted from among the various types of reaction that can be carried out in.

Therefore, first, a method of continuously adding a small amount of hydrogen chloride will be described. The amount of hydrogen chloride added is usually 1 to 5 per unit time with respect to the platinum group metal in the catalyst component.
It is 0 mol%, preferably 5 to 20 mol%. This amount of addition is an amount suitable for compensating for the amount of chlorine scattered from the catalyst during the reaction of carbon monoxide and nitrite, and when added in excess, conversely chloride Excessive adsorption of hydrogen hinders the above reaction, which is not preferable. As a concrete example of an actual reaction, for example, in a fixed bed reactor, gas hourly space velocity (GHSV) is 3000 hr.
^{When the} reaction is carried out at ^-1 , the feed gas (raw material supply gas) usually contains 10 to 500 ppm by volume, preferably 20 to
100 volume ppm of hydrogen chloride is added and continuously supplied to the catalyst layer to supplement the chlorine content scattered from the catalyst.

Next, a method in the case of a reaction in a moving bed system, which involves contacting and adsorbing hydrogen chloride gas with the catalyst extracted from the catalyst packed bed in the reactor to thereby eliminate chlorine scattering. It is a method of returning to the reactor after replenishing. The method of adsorbing hydrogen chloride to the extracted catalyst may be a method usually considered, for example, after filling the extracted catalyst into a suitable container, it is sufficient to pass hydrogen chloride gas diluted with nitrogen under mild conditions. . In this case, as the concentration of hydrogen chloride, 0.1 to 10% by volume is suitable in operation, but it should not be limited to this, and for example, 10% by volume may be used.
0% by volume does not matter. Further, since hydrogen chloride is rapidly adsorbed on the catalyst, the time for contacting hydrogen chloride with the above-mentioned catalyst for extraction is the space velocity (S
Depending on V), 1 to 30 minutes is sufficient. This contacting operation is preferably carried out at room temperature and normal pressure, but the range is not limited, of course, for example, the temperature is 0 to 200 ° C., and the pressure is −0.5 to 5 kg /
It can be sufficiently carried out even in the range of cm ² G.

According to the method of the present invention, the catalytic reaction of carbon monoxide and nitrite can be carried out under extremely mild conditions, which is also one of the features of the present invention.
For example, the reaction is 0 to 200 ° C., preferably 50 to 12
It can be carried out at a temperature of 0 ° C. and normal pressure. Of course, a pressurized system can also be used without any problem, and it can be carried out at a pressure of 1 to ²⁰ kg / cm ² G and a temperature of 50 to 150 ° C.

In the present invention, the nitrite ester as a raw material as described above generates a mixed gas of nitric oxide (NO) and nitrogen dioxide (NO ₂ ) by, for example, decomposing nitric acid or sulfuric acid of an aqueous sodium nitrite solution. Then, a part of NO in this mixed gas is oxidized with molecular oxygen and converted into nitrogen dioxide (NO ₂ ) to obtain NO _X gas of NO / NO ₂ = 1/1 (volume ratio). It can be easily synthesized by contacting alcohol with alcohol. However, considering the synthesis up to this nitrite, the catalytic reaction between the carbon monoxide and the nitrite is 2-3 kg / cm
A slight pressure system of about ² G is particularly desirable.

The reaction between carbon monoxide and nitrite can be carried out in either a batch system or a continuous system as long as it is in a gas phase, but the continuous system is industrially advantageous. is there. The catalyst may be present in the reaction system in any of a fixed bed, moving bed or fluidized bed reactor.

In the present invention, it is desirable that the carbon monoxide and the nitrite ester of the raw material gas are diluted with an inert gas such as nitrogen gas and fed to the above-mentioned reactor. Is not particularly limited. However, from the viewpoint of safety, the concentration of the nitrite is usually 20% by volume or less, preferably 5 to
It is 20% by volume. Accordingly, the concentration of carbon monoxide is
It is economically preferable to set it in the range of 5 to 20% by volume.
That is, considering an industrial manufacturing process, it is preferable to circulate and use a gas such as carbon monoxide or nitrite to be subjected to the reaction, and to purge a part of the circulated gas out of the system. One pass conversion rate is 20-30
%, The loss increases only when the concentration of carbon monoxide is higher than 20% by volume.
If it is lower than the capacity%, there is a problem such as a decrease in productivity. However, ignoring this economy, the concentration of carbon monoxide can actually be up to 80% by volume. That is, the nitrite can be fed in a form diluted with carbon monoxide instead of the inert gas. Therefore, the use ratio of carbon monoxide and nitrite is usually in the range of 0.1 to 10 mol, preferably 0.25 to 1 mol, relative to 1 mol of nitrite.

[0024] In the present invention, the space velocity of the gas containing the reactor is fed to, carbon monoxide and nitrous acid ester in the range of 500～20000Hr ^-1, preferably in the range of 2000～5000Hr ^-1 It is desirable to do in. In the production method of this method, the nitrite ester is decomposed to generate nitric oxide (NO) after participating in the reaction, but from the reaction gas discharged from the reactor,
It is preferable that this NO be recovered, reacted with oxygen and the alcohol corresponding to the nitrite, converted into the nitrite again, and used again.

In this way, in addition to the carbonic acid diester of the desired product, by-products such as oxalic acid diester, unreacted carbon monoxide and nitrite, nitric oxide, carbon dioxide, inert Although a reaction gas containing a gas or the like is derived, for example, after cooling this reaction gas, uncondensed gases such as carbon monoxide, nitrite, nitric oxide, carbon dioxide, and an inert gas are as described above. While purging a part thereof, it is circulated again in the reactor, while the carbonic acid diester is separated and purified from the condensate by a conventional method such as distillation.

The nitrite ester as a raw material is usually prepared by reacting an alcohol and a nitrogen oxide in the presence of molecular oxygen, if necessary, as described above. In addition, unreacted alcohol, nitrogen oxides (particularly nitric oxide), and in some cases trace amounts of water and oxygen are contained. In the present invention, good results are obtained even when such a nitrite-containing gas is used as a nitrite source.

In the present invention, by carrying out the treatments described above, it is possible to prevent the activity of the catalyst from deteriorating in the conventional method, and to produce carbon monoxide in a stable state with high yield and high selectivity for a long period of time. The production of carbonic acid diesters by reaction with nitrites can be continued.

[0028]

EXAMPLES Next, the method of the present invention will be specifically described with reference to Examples and Comparative Examples, but these do not limit the method of the present invention in any way. The space-time yield (STY) Y (g / L · h) in each Example and Comparative Example
r) is the contact reaction time between carbon monoxide and nitrite, θ (hr), and the amount of carbonic acid diester formed during that is a (hr).
(G), and the filling amount of the catalyst into the reaction tube was b (unit: liter (L)), and was determined by the following equation. Y = a / (b × θ)

The selectivity X (%) in each of the examples and comparative examples is based on the supplied carbon monoxide, and the carbonic acid diester, oxalic acid diester and carbon dioxide produced at the above-mentioned θ (hr) are measured. The amount of
(Mol), c (mol) and d (mol) were calculated by the following formula. X = {a / (a + 2 × c + d)} × 100

Example 1 A catalyst (PdCl ₂ -BiCl ₃ / in which PdCl ₂ and BiCl ₃ are supported on granular activated carbon in a gas phase reaction tube (with an outer jacket) having an inner diameter of 20 mm
C) After filling 7 mL, fix the reaction tube vertically,
Circulating the heat medium in the reaction tube jacket, the temperature inside the catalyst layer is 1
The heating was controlled so as to be 00 ° C. From the top of this reaction tube, a mixed gas of carbon monoxide and a gas containing methyl nitrite synthesized from nitric oxide, oxygen and methanol, that is, methyl nitrite: 15% by volume, carbon monoxide: 10% by volume, Nitric oxide: 3% by volume, methanol: 6% by volume,
A mixed gas having a composition of hydrogen chloride: 100 volume ppm and nitrogen: 66 volume% was supplied at a space velocity (GHSV) of 3000 hr ⁻¹ and reacted under normal pressure. Then
The reaction product passing through this reaction tube was passed through ice-cooled methanol and collected. The obtained collected liquid was analyzed by gas chromatography. 1 reaction under these conditions
The reaction was continued for 00 hours, and every 5 hours, the reaction products were sampled and analyzed by gas chromatography as described above.

As a result, the STY of dimethyl carbonate was initially 500 g / L · hr, but decreased to 350 g / L · hr after 10 hours. However, after that, 350 g until the end of the reaction 100 hours after the start of the reaction
/ L · hr did not change. In addition, the selectivity of dimethyl carbonate based on carbon monoxide was 97% in the initial stage, but due to the increase of by-product dimethyl oxalate,
It decreased to 92% 10 hours after the reaction started, but after that,
It was almost constant until the end of the reaction (100 hours after the start of the reaction).

Comparative Example 1 Dimethyl carbonate was produced in the same manner as in Example 1 except that hydrogen chloride was not added to the feed gas to the reaction tube. As a result, the STY of dimethyl carbonate was 500 g / L · hr in the initial stage, but it was 300 g / L by 10 hours after the start of the reaction.
・ Decreased to hr, and 200g / L at 20th hour
・ Hr, 120g / L ・ hr at 30th hour, and 50
It decreased to 60 g / L · hr at the time. Also,
The selectivity of dimethyl carbonate also gradually decreased due to an increase in the amount of dimethyl oxalate produced, reaching 60% at 50 hours.

[Embodiment 2] The same catalyst 2 as in Embodiment 1 was used in a gas phase reaction tube (with a catalyst layer lower extraction tube) having an inner diameter of 40 mm.
00 mL was filled. Separately from this, a catalyst regeneration tube of the same type was prepared and charged with 200 mL of the same catalyst. Therefore,
The gas-phase reaction tube and the catalyst regeneration tube were fixed vertically, and the gas-phase reaction tube had a mixed gas of carbon monoxide and a gas containing methyl nitrite synthesized from nitric oxide, oxygen and methanol from the top. That is, methyl nitrite: 15% by volume, carbon monoxide: 10% by volume, nitric oxide: 3% by volume,
A mixed gas having a composition of methanol: 6% by volume and nitrogen: 66% by volume was mixed with a space velocity (GHS) of 3000 hr ^−1.
V), and reacted under normal pressure. From 1 hour after the start of the reaction, the catalyst was withdrawn from the lower part of the catalyst layer of the reaction tube at a rate of 10 mL per hour through the lower part of the catalyst layer withdrawal tube and supplied to the upper part of the catalyst regeneration tube. At the same time, from the lower part of the catalyst regeneration tube, a catalyst layer lower extraction tube of the regeneration tube was met, and the catalyst was withdrawn at a rate of 10 mL per hour and supplied to the upper part of the reaction tube. And in the catalyst regeneration tube,
From the first hour after the start of the reaction, nitrogen gas containing 100 ppm by volume of hydrogen chloride was caused to flow at a space velocity of 500 hr ⁻¹ .

After the reaction between methyl nitrite and carbon monoxide was carried out as described above, the gas discharged from the reaction tube was collected through ice-cooled methanol and subjected to quantitative analysis by gas chromatography. It was The reaction was continued for 100 hours under these conditions, and every 5 hours, the reaction products were sampled and analyzed by gas chromatography as described above. As a result, the STY of dimethyl carbonate was 500 g / L · hr at the initial stage of the reaction, but it was 370 g / L · h 10 hours after the start of the reaction.
to r. However, thereafter, it was almost constant at 350 to 370 g / L · hr until the end of the reaction 100 hours after the start of the reaction. Also, the selectivity of dimethyl carbonate based on carbon monoxide was 97% at the initial stage of the reaction,
Due to an increase in dimethyl oxalate, which is a by-product, it decreased to 93% at 10 hours after the start of the reaction, but thereafter remained constant at 90 to 93% until the end of the reaction (100 hours after the start of the reaction).

[0035]

EFFECT OF THE INVENTION A method for producing a carbonic acid diester by reacting a nitrite and carbon monoxide in the gas phase in the presence of a solid catalyst having a catalyst component containing a platinum group metal chloride supported on a carrier to react with each other. In carrying out the method, according to the present invention, the amount of hydrogen chloride necessary for compensating the chlorine content scattered from the chloride of the platinum group metal during the reaction is supplied to the reaction system to generate the reaction with the progress of the reaction. Since the decrease in the catalytic activity is significantly suppressed, the carbonic acid diester can be industrially advantageously obtained.

Claims (10)

[Claims] 1. A carbonic acid diester comprising reacting a nitrite ester and carbon monoxide in the gas phase in the presence of a solid catalyst having a catalyst component containing a platinum group metal chloride supported on a carrier. A method for producing a carbonic acid diester, comprising supplying the reaction system with an amount of hydrogen chloride necessary for compensating a chlorine content scattered from a chloride of a platinum group metal during the reaction. 2. The method for producing a carbonic acid diester according to claim 1, wherein the chloride of the platinum group metal is palladium chloride. 3. The amount of the platinum group metal chloride supported on the carrier is 0.1 to 1 based on the platinum group metal metal based on the carrier.
The method for producing a carbonic acid diester according to claim 1, wherein the amount is 0% by weight. 4. The method for producing a carbonic acid diester according to claim 1, wherein hydrogen chloride is supplied to the platinum group metal chloride in an amount of 1 to 50 mol% per unit time. 5. The method for producing a carbonic acid diester according to claim 1, wherein hydrogen chloride is added to a reaction system by adding 10 to 500 ppm by volume to a raw material gas containing a nitrite and carbon monoxide. 6. A carbonic acid diester comprising reacting a nitrite ester and carbon monoxide in the gas phase in the presence of a solid catalyst having a catalyst component containing a platinum group metal chloride supported on a carrier. In carrying out the production method, an amount of hydrogen chloride necessary for compensating the chlorine content scattered from the chloride of the platinum group metal during the reaction is supplied to the reaction system of the platinum group metal. A method for suppressing a decrease in activity of a catalyst component containing chloride. 7. The method for suppressing the activity of a catalyst component according to claim 6, wherein the chloride of the platinum group metal is palladium chloride. 8. The amount of the platinum group metal chloride supported on the carrier is 0.1 to 1 based on the platinum group metal metal based on the carrier.
The method for suppressing the activity of a catalyst component according to claim 6, wherein the content is 0% by weight. 9. The method for suppressing the activity of a catalyst component according to claim 6, wherein hydrogen chloride is supplied to the platinum group metal chloride in an amount of 1 to 50 mol% per unit time. 10. The method for producing a carbonic acid diester according to claim 6, wherein hydrogen chloride is added to the reaction system by adding 10 to 500 ppm by volume to a raw material gas containing a nitrite and carbon monoxide.