Production of Isocyanic Acid

1. Field of the Invention

This invention relates to a method for preparing isocyanic acid.

2. Description of the Prior Art

An existing method for producing isocyanic acid consists of pyrolyzing cyanuric acid or urea. The method suffers because of the high processing temperatures called for and the need to use relatively costly intermediates as starting raw materials. Another commercially unattractive method involves the extended controlled addition of a mineral acid to potassium cyanate.

Although the current uses of isocyanic acid are rather limited, there is however, the potential usage to prepare aliphatic isocyanates which have extensive industrial application. Accordingly, there is a need for an economical way of producing isocyanic acid particularly if it is to be used as an intermediate in preparing a commodity chemical such as the organic isocyanates.

The genesis of the approach to prepare isocyanic acid in accordance with the chemistry concerned herein is to be found in a number of related papers which began to appear in the early part of the last decade. One of the forerunners of these articles is Unland, M. L., J. Phys. Chem. 77, 1952 (1973). The latter reported work was directed to a study of the reductive removal of nitric oxide from auto exhaust through the agency of a noble metal catalytic converter system. The most significant finding of this study was that carbon monoxide and nitric oxide combined at elevated temperatures to form a complex isocyanate species on the surface of the noble metal catalyst.

The first potential industrial application of the above finding is set forth in U.S. Pat. No. 4,174,377. Therein it is taught that a variety of cyanate compounds, including isocyanic acid, can be prepared directly by contacting a noble metal hydrogenation catalyst at an elevated temperature with a feed stream of nitric oxide, carbon monoxide and a source of hydrogen. The present invention represents an improvement over the foregoing prior art particularly in regards how to enhance the yield of isocyanic acid in carrying out the underlying reaction over supported heterogeneous catalyst.

In accordance with this invention, a method is provided for preparing isocyanic acid by the direct in situ formation thereof from nitric oxide, carbon monoxide and hydrogen. The method comprises contacting a supported iridium or rhodium metal catalyst at an elevated temperature with a feed stream consisting essentially of said reactants. Beyond the specific metal catalysts contemplated, a critical aspect of the invention resides in the nature of the support or carrier component. A plurality of conventional particulate catalyst carriers are useful herein provided the surface area exhibited thereby in terms of square meters per gram does not exceed about one.

It has been documented in the early prior art referred to hereinabove that nitric oxide and carbon monoxide react to form the isocyanate species on the surface of the applicable metal catalysts. It has also been observed by infrared spectroscopy that, where the catalyst metal is supported, the engendered isocyanate species is prone to migrate almost instantly to the surface of the support. This phenomenon has been confirmed herein with the added observation that the support-isocyanate species bond is a comparatively strong one thereby rendering it difficult to free the isocyanate species as isocyanic acid by hydrogenation. When the isocyanate species is attached to the support the hydrogenation products are principally urea and ammonium carbonate.

The concept behind the present invention is that of providing a support matrix wherein the catalyst metal, specifically, iridium or rhodium black, is dispersed throughout in the form of comparatively large clusters or agglomerates. In this manner, migration of the isocyanate species is sufficiently impeded so that the species can be hydrogenated while residing on the surface of the catalyst metal thereby more favorably tending to yield the desired isocyanic acid. While the foregoing description of the modus operandi of the invention is only theoretically founded the yield data associated with a plurality of runs for preparing isocyanic acid set forth in the working example given hereinbelow is credibly supportive of this hypothesis.

When catalyst support of high surface area is used with high contact time of longer than 1 second, the primary product is ammonium carbonate. When the same catalyst is used with short contact time of less than 1 second, the primary product is urea. Isocyanic acid is obtained in the product only when catalyst support of extremely low surface area is used with short contact time of less than 1 second.

As indicated a variety of commercially available catalyst carriers are useful in the practice of this invention. The only requirement in the selection of a suitable carrier particulate is that its surface area characteristic in terms of square meters per gram (m.sup.2 /g) does not exceed about one. The composition of the carrier appears to be of no material consequence. Thus all of the conventional catalyst support materials are applicable. These include silica, .gamma.-alumina, zirconium oxide and silicon carbide. The preferred carriers are silica and .gamma.-alumina.

The procedure for preparing the supported catalyst follows the general practice whereby the carrier is impregnated with an aqueous solution of an iridium or rhodium salt thereupon calcined to provide the metal deposit sites. The latter are then activated by contact with hydrogen at an elevated temperature. Suitable metal contents of the supported catalyst range from about 1 to 10 weight percent. The preferred metal content is in the order of about 3 percent. The preparation of a supported catalyst will be illustrated in the working example given hereinbelow.

The procedure for carrying out the reaction in generating the isocyanic acid is straightforward. The reactants; viz., hydrogen, nitric oxide and carbon monoxide, are introduced directly into a reaction sphere wherein the reaction temperature is maintained between about 200.degree. and 400.degree. C. The preferred operating temperature is from 250.degree. to 300.degree. C. The mole ratio of nitric oxide to carbon monoxide in the reactant feed stream broadly ranges from 1:1 to 1:20, respectively. A preferred molar ratio of the indicated reactants in the order referred to is from 1:2 to 1:16, respectively. The molar amount of hydrogen based on the content of nitric oxide present in the reactant feed stream broadly ranges from 0.5 to 5 and more preferably from 1 to 2. Pressure conditions applicable are essentially ambient with only moderately elevated pressures observed to maintain a reactant flow rate to achieve a desired contact time. Another means to regulate contact time is through the use of an inert diluent gas as a component of reactant feed stream. Applicable contact time is from about 0.05 to 0.5 seconds.

This example illustrates the general procedure observed in preparing a supported catalyst, a variety of which comprise the test catalysts utilized in the succeeding working example. In this connection it is to be noted that a plurality of the test catalysts accordingly outlined do not come within the purview of the subject matter herein claimed. The test results reported for the latter are merely presented to point up the efficacy of the contemplated invention.

A low surface area silica (Carborundum SMC, surface area, 0.3 m.sup.2 /g) was ground to 20-35 mesh. Hexachloroiridic acid in the amount of 1.564 g was dissolved in 13 ml of deionized water and the resultant solution was added to 21 g of silica by an incipient wetness method in a round bottomed flask. The slurry was mixed on the rotary evaporator under atmospheric pressure for even coating at room temperature for 30 minutes. The slurry was then stripped of water under an aspirator vacuum to dryness in a warm water bath. The loaded catalyst was activated in a Vycor tube under the following conditions: slowly heated to 400.degree. C. over a period of 4 hours under vacuum; heating at 400.degree. C. under 100 mm oxygen atmosphere for 90 minutes; evacuating the tube under full vacuum; holding under 100 mm hydrogen atmosphere for 1 hour and then cooling to room temperature under vacuum.

A standardized procedure was observed in conducting the individual test runs of this example. A down-flow glass reactor tube equipped with a frit was utilized. The catalyst consisting of 3% iridium on a support prepared in the general manner described in Example I in the amount of a 3.5 g bed was supported on the frit and surrounded by a glass wool blanket. The individual catalyst samples were preliminarily conditioned under hydrogen flow (80 vol. % H.sub.2, 20 vol. % N.sub.2) at 400.degree. C. for 2 hours. The temperature was lowered to the indicated reaction temperature under a helium flow whereupon the reactants (mole ratio H.sub.2 :NO:CO=1:1:16) were proportioned into the helium flow and the resultant feed stream passed through the catalyst bed. The reaction effluent was passed through a dry ice-isopropyl alcohol cooled trap. Isocyanic acid was collected in a dry ice trap and the purity was determined by melting point and infrared spectroscopy of the trimerized isocyanic acid. Urea and other solid product were recovered before the dry ice trap. Yield figures reported were based on the NO feed not consumed in the reaction. Further details appertaining to the individual test runs and the results obtained are set forth in the following Table I.