Process for Production of Acrylamide

1. Field of the Invention

This invention relates to an improvement in a process for producing acrylamide by reacting acrylonitrile with water in the presence of a copper-based catalyst.

2. Description of the Prior Art

Acrylamide has been used in acrylamide polymers which are useful as papermaking chemicals, flocculents, oil recovery additives, soil hardeners, etc., and also has found extensive use as a comonomer for other polymers. Early production of acrylamide for these uses was by the so-called sulfuric acid process. Recently, a catalytic hydration process which comprises reacting acrylonitrile with water in the presence of a copper-based catalyst was developed and has now superseded the sulfuric acid process in industrial production.

Various copper-based catalysts to be described hereinbelow are used in a liquid phase fixed bed or suspended bed in the hydration of acrylonitrile. In either case, however, the catalysts undergo degradation and lose activity with the lapse of time. In an attempt to remedy this defect, there have been proposed many methods which include the pretreatment of the starting acrylonitrile with an acidic aqueous solution or a cation exchange resin (Japanese Laid-Open Patent Publications Nos. 29154/1975 and 108916/1977), the addition of salts such as NaCl to the acrylonitrile hydration system (U.S. Pat. No. 3,869,511), and the regeneration of the degraded catalyst by a reducing treatment (U.S. Pat. No. 3,645,913).

These prior methods are effective, and as required, some of them may be used in combination as a series of improved methods. In order to increase the utility of this new manufacturing process, it has been desired to develop a more effective method for maintaining the activity of copper-based catalysts.

It is an object of this invention to provide an improved process for producing acrylamide by catalytic hydration of acrylonitrile with water in the presence of a copper-based catalyst.

Another object of this invention is to increase the economic advantage of acrylamide production by this process by maintaining the activity of the copper-based catalyst over a long period of time.

These objects of the invention are achieved, in a process for production of acrylamide by reacting acrylonitrile with water in the presence of a copper-based catalyst, by the presence of 30 to 10,000 ppm, preferably 30 to 4,000 ppm, of acetone of the reaction system.

Examples of the copper-based catalyst used in the process of this invention include:

(A) a combination of a copper ion and metallic copper in the form of copper wire or powder,

(B) reduced copper obtained by reducing a copper compound with a reducing agent,

(C) decomposed copper obtained by decomposing a copper compound by heat, etc. and

(D) Raney copper obtained by leaching a Raney alloy of copper with an alkali, etc. Reduced copper is produced, for example, by

(1) a method which comprises reducing copper oxide in the gaseous phase with hydrogen, carbon monoxide or ammonia,

(2) a method which comprises reducing a salt or hydroxide of copper in aqueous solution with formaldehyde, hydrazine or sodium borohydride, and

(3) a method which comprises reducing a salt of hydroxide of copper in aqueous solution with elemental aluminum, zinc or iron. The main catalytic ingredient of the products of these methods is considered to be elemental copper. The decomposed copper is produced, for example, by

(1) a method which comprises thermally decomposing in aqueous alkaline solution copper hydride obtained by treating a copper compound with sodium hypophosphite, etc.,

(2) a method which comprises thermally decomposing copper formate or copper oxalate,

(3) a method which comprises thermally decomposing cluster copper, as shown in Japanese Laid-Open Patent Publication No. 108015/1974, and

(4) a method which comprises adding copper acetylide or copper nitride directly to the acrylonitrile hydration system. The main catalytic ingredient of the products obtained by these methods, including the method (4), is considered to be elemental copper. Production of Raney copper is by such methods as

(1) a method which comprises leaching a copper-aluminum alloy substantially completely with sodium hydroxide, sulfuric acid, organic amines, etc., and

(2) a method which comprises partially leaching a copper-aluminum alloy with sodium hydroxide, sulfuric acid, water, organic amines, etc. to leave part of aluminum together with copper. The main catalytic ingredient in these products is considered to be elemental copper. These copper-based catalysts may be supported on usual carriers such as silica, or may contain metals other than copper, for example chromium or molybdenum.

The copper-based catalysts described above are obtained by the respective methods of preparation, and differ from each other in catalytic activity. It has been ascertained however that with regard to their tendency toward interaction with acetone and impurities and other materials present in the reaction mixture, these copper-based catalysts do not differ from each other with the methods of their preparation, for example the methods (A) to (D) described above. Desirably, contact of these copper-based catalysts with oxygen and oxygen-containing gases should be avoided before and during use, because oxygen impairs their catalytic activity and increases by-products such as ethylene cyanohydrin.

Generally, acrylonitrile produced by ammoxidation of propylene is used as the starting acrylonitrile in the hydration reaction of this invention. This acrylonitrile generally contains acetone, acrolein, oxazole, acetonitrile, propionitrile, methacrylonitrile and cis- and trans-crotonitriles as impurities, and hydroquinone monomethyl ether as a stabilizer.

The catalytic hydration reaction in accordance with this invention is carried out in the following manner using such acrylonitrile in the presence of the aforesaid copper-based catalyst. The reaction is carried out in a suspended or fixed catalyst bed in the liquid phase by passing the reactants through the catalyst bed by a flowing or batchwise method. The weight ratio between acrylonitrile and water used in the hydration reaction is substantially optional. Preferably, the weight ratio of acrylonitrile to water is from 60:40 to 5:95, especially from 50:50 to 10:90. The reaction temperature in the hydration reaction is preferably 70.degree. to 150.degree. C., especially 90.degree. to 140.degree. C. The conversion of acrylonitrile is preferably 10 to 98%, especially preferably 30 to 95%.

At the aforesaid acrylonitrile/water weight ratios, reaction temperatures and acrylonitrile conversions, the unreacted acrylonitrile, the unreacted water and the resultant acrylamide sometimes do not form a uniform solution. To avoid this, the prepared acrylamide may be again added to the reaction mixture as a co-solvent. The inside of the reactor is maintained at a pressure consisting of the vapor pressure at the aforesaid reaction temperatures and reactant ratios with or without the pressure of an inert gas such as nitrogen added thereto. Usually, this pressure is from atmospheric pressure to 10 atmospheres. Since dissolved oxygen contained in the catalyst, acrylonitrile, water, acrylamide as a co-solvent, etc. impairs the activity of the catalyst and increases by-products as stated hereinabove, it is desirable to remove oxygen fully before these materials are fed into the reactor. For the same reason, the inside of the reactor is desirably maintained in an oxygen-free atmosphere. The reaction solution withdrawn from the reactor after the hydration reaction consists of the unreacted acrylonitrile, the unreacted water and acrylamide, and contains small amounts of copper and by-products such as ethylene cyanohydrin.

The reaction solution obtained by the above reaction is subjected to an ordinary evaporation or distillation operation to obtain a concentrated aqueous solution of acrylamide and to recover substantially all of the unreacted acrylonitrile and a part of the water. The recovered acrylonitrile and water are generally re-used as starting materials although they may be used in other applications. In the aforesaid process for producing acrylamide by hydrating acrylonitrile in the presence of a copper-based catalyst, acetone as an impurity in the starting acrylonitrile is slightly lost by the reaction, etc. but the remainder is recovered by distillation together with the unreacted nitrile and recycled to the reaction system. If, therefore, this operation is continued, the concentration of acetone in the reaction system increases. However, this concentration does not increase infinitely. Generally, the amount of acetone lost increases with an increase in its concentration, until finally the amount of acetone in the reaction system is equilibrated with that of acetone fed together with acrylonitrile from outside. Thus, a certain fixed concentration is attained, and there is no further increase. The equilibrium concentration of acetone can be adjusted intentionally by adding or removing acetone. The present invention is based on the discovery that this equilibrium concentration of acetone has to do with the activity and life of the catalyst. Specifically, it has been found in accordance with this invention that if the concentration of acetone in the reaction solution is adjusted to 30 to 10,000 ppm, preferably 30 to 4,000 ppm, good catalytic activity and a long catalyst life can be obtained and the process is economical.

If the amount of acetone in the reaction solution is smaller than the lower limit specified above, the activity of the copper-based catalyst decreases rapidly and its lifetime is short. Even when the amount of acetone is larger than the upper limit specified above, the catalyst has low activity and a short lifetime. The mechanism of interaction of the copper catalyst with acetone is not clear, but presumably, it is complexly related to the polymerization of acrylonitrile, acrylamide, etc. in the reaction solution, the presence of a copper ion, etc.

The concentration of acetone in the reaction solution can be adjusted to the range specified above in the following manner. To increase the acetone concentration in the reaction solution, acetone is added to the starting acrylonitrile and water, the reaction solution, the distillate to be recycled, etc. The concentration of acetone in the reaction solution can be decreased, for example, by subjecting the starting acrylonitrile or the recovered distillate to such a treatment as distillation or extraction before use, or by separating a minor portion of the recovered distillate and keeping this portion from re-use.

In order to convert the acrylamide so synthesized to a final industrial product, it is treated in the following manner. The aqueous solution of acrylamide obtained by subjecting the above reaction solution to a concentrating treatment such as evaporation or distillation is then purified by various purifying methods such as a cation exchanging treatment, treatment with a chelate resin, an anion exchanging treatment, treatment with air or oxygen gas, or treatment with activated carbon. There can also be used a so-called synthetic adsorptive resin (e.g., ADSORPTIVE RESIN, a tradename for a product of Hokuetsu Tanso Kogyo Co., Ltd.) which can be used by a similar method to that used for activated carbon or ion exchange resins. The aqueous solution of acrylamide may be subjected to the aforesaid concentrating treatment during or after this purifying step, or it may again be concentrated. The acrylamide so purified is marketed in the form of an aqueous solution or after it is crystallized.

The following Examples and Comparative Examples illustrate the present invention further.

A Raney copper alloy having a size of 80 mesh or smaller was leached with sodium hydroxide and washed in a customary manner to prepare a Raney copper catalyst. Contact of it with air was avoided during its manufacture and subsequent handling. This catalyst (150 parts by weight) was charged into a stainless steel reactor having a stirrer and a catalyst separator built therein, and acrylonitrile and water from which dissolved oxygen had been removed by using nitrogen gas were fed into the reactor at a rate of 300 and 700 parts by weight per hour, respectively, and reacted at 110.degree. C. The acrylonitrile used was synthesized by ammoxidation of propylene and contained less than 0.1 ppm of acetone.

The reaction solution was stirred together with the catalyst and became a suspension. The suspension was passed through the catalyst separator, and withdrawn from the reactor as a solution substantially free from the catalyst. The resulting solution was introduced into a reduced-pressure concentrating device consisting of a concentrator, a packed tower and a condenser. An aqueous solution of acrylamide having a concentration of about 40% by weight was obtained from the condenser, and nearly all of the unreacted acrylonitrile, a part of the water and acetone were recovered from the condenser. The recovered acrylonitrile, water and acetone were recycled to the reactor while avoiding their contact with air. In the above process, the ratio of the freshly fed acrylonitrile to the recycled acrylonitrile was about 1:1 at the initial stage. In continuing the above reaction, the same catalyst was additionally fed at suitable times so as to make up for the reduced activity of the catalyst, and the above acrylonitrile ratio was maintained at about 1:1. When the above reaction was continued for 60 days, the concentration of acetone in the reaction solution and the cumulative amount of the catalyst additionally supplied during this time were as shown in Table 1.

The same reaction as in Comparative Example 1 was carried out except that the concentration of acetone in the reaction solution was adjusted to 30, 280, and 3,500 ppm, respectively, by adding a small amount of acetone to acrylonitrile. The reaction was continued for 60 hours in each run. The cumulative amounts of the additionally supplied catalyst were as shown in Table 2.

It is seen from Table 2 that the amounts of the catalysts additionally supplied in these Examples can be decreased from that in Comparative Example 1.

One hundred parts by weight of the same catalyst as used in Comparative Example 1 was charged into a 2-liter stainless steel reactor having a stirrer and a catalyst separator built therein. Acrylonitrile and water from which dissolved oxygen had been removed by using nitrogen gas were fed into the reactor at a rate of 400 and 600 g/hr., respectively, and reacted. The acrylonitrile was synthesized by ammoxidation of propylene and contained 0.1 ppm of acetone. Acetone was added to this acrylonitrile in an amount of 0.9, 2.7 and 10% by weight. The reaction solution was stirred together with the catalyst and became a suspension. The suspension was passed through the catalyst separator, and withdrawn from the reaction as a solution substantially free from the catalyst. This reaction was continued for 14 days, and the conversion of acrylonitrile to acrylamide and the concentration of acetone in the reaction solution were measured. The results are shown in Table 3.

Copper oxide in small pellets was packed into a stainless steel reactor, and reduced with a gaseous mixture of hydrogen and nitrogen at about 200.degree. C. to form reduced copper. Using this catalyst, the same catalytic hydration of acrylonitrile as in Example 1 was carried out. The amount of the catalyst initially charged was 250 parts by weight, and the concentration of acetone in the reaction solution was adjusted to 208 ppm.

The cumulative amount of the additionally supplied catalyst in this reaction was 700 parts by weight 30 days later, and 1330 parts by weight 60 days later.