Arteleo Chemistry Journal

Nickel Catalyst

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Gregory J. Ward and Bryan C. Blanchard

Solutia, Inc., P.O. Box 97, Gonzalez, FL 32560-0097

Abstract

A process for the production of 3-dimethylaminopropylamine (DMAPA) in high (>99%) purity from N,N-dimethylaminopropionitrile (DMAPN) utilizing a low pressure slurry hydrogenation process is described. The basic process comprises contacting the nitrile with hydrogen at low pressure in the presence of a sponge nickel catalyst under conditions sufficient to effect the conversion of the nitrile to the primary amine product. The improvement in the process resides in a combination of carrying out the hydrogenation process at very low pressure in the presence of an optimum amount of caustic and sponge nickel catalyst in order to give an improved selectivity of greater than 99.9 % of DMAPN to DMAPA. This process can be readily adapted to the established Solutia low pressure hydrogenation process, which employs a continuous gas lift slurry reactor. In this process, a decanter is used to perform the bulk catalyst separation, and entrained catalyst and fines in the decanter effluent are removed by hydroclones. Complete removal of catalyst from the crude amine can be obtained with this system with minimal maintenance and high reliability. This system has a variety of advantages over other competing technologies including high selectivity, low catalyst usage, low capital cost, high reliability, established sources of catalyst, and no need to completely recharge catalyst to reactor. Solutia has been practicing this technology for over 30 years for the production of hexamethylenediamine, and considers it the best technology available to convert nitriles to primary amines.

Introduction

The most common and least expensive catalyst for producing primary amines from nitriles is sponge nickel. The generalized reaction, carried out in the presence of sponge nickel catalyst, is the following:

Solutia has been producing hexamethylenediamine via low pressure slurry hydrogenation of adiponitrile since 1973. This process can also been adapted for the production of other amines such as DMAPA. The catalyst employed for this process is a promoted sponge nickel catalyst, and the reaction is carried out in a continuous gas lift reactor as shown in Figure 1.

Recycle Hydrogen

Figure 1 Solutia low pressure amine synthesis process.

Figure 1 Solutia low pressure amine synthesis process.

Recycle hydrogen is injected into the bottom of the upleg. This produces a very high circulation rate of the reaction mass without high maintenance equipment such as pumps or agitators. Unreacted hydrogen is separated from the reaction mass in the separator at the top of the reactor. A compressor is used to recycle the unreacted hydrogen back to the base of the upleg. By maintaining steady recycle hydrogen flow, good circulation rates of the reaction mass are assured, even at low production rates. Nitrile is injected into the reactor leg just above the hydrogen inlet. It reacts quickly with the hydrogen on the surface of the promoted sponge nickel catalyst. The reaction goes essentially to completion producing high purity crude amine product with typically less than one percent organic byproducts. The heat of reaction is removed by tempered water in cooling jackets around the legs.

The reaction mass consists of two liquid phases and one solid phase; no solvent is required. The major liquid phase is the crude amine product itself. The solid phase is promoted sponge nickel catalyst. Surrounding the catalyst is a second liquid phase consisting of concentrated caustic and water. Water and caustic are added continuously to make up for losses leaving in the crude product. The ratios of water, caustic, and catalyst in the reaction mass are controlled to produce high yields of product amine and very low catalyst usages. High catalyst concentrations are employed in the reaction mass to keep the concentration of unreacted nitriles very low; the upper limit on the catalyst concentration is the point where the circulation rate is inhibited.

The crude amine product is taken from the reactor through a decanter as shown in Figure 1. In the decanter, the clarified crude amine product is continuously separated from the catalyst, and the catalyst is returned to the reactor through the decanter under flow line as shown in Figure 1. Entrained catalyst and fines in the decanter effluent are removed by hydroclones. Complete removal of catalyst from the crude amine is important to prevent fouling and product degradation during downstream processing.

Periodically, a portion of the catalyst slurry is purged from the reactor for regeneration. In the catalyst regeneration process, the catalyst is washed with water to remove impurities that accumulate in the caustic phase. Most of the regenerated catalyst is returned to the reactor along with new catalyst. With this configuration, fresh catalyst can be added as required to maintain acceptable catalyst activity without the need to replace the entire reactor charge in a batchwise manner.

Recent publications and patents have suggested selectivity enhancements over the process practiced by Solutia (1, 2) are possible by adjusting the chemical additives used in the process (3, 4) or that fixed bed catalysts are better suited for continuous processes than slurry catalyst (5, 6). These claims have prompted workers at Solutia to re-evaluate the technology employed for production of hexamethylenediamine and to evaluate Solutia low pressure slurry hydrogenation technology for chemistries other than hexamethylenediamine. Specifically, hydrogenation of DMAPN to produce DMAPA (9) was selected to study due to a relatively large number of recent publications pertaining to the production of DMAPA (3, 4, 7, 8).

Experimental Section

Semi-batch hydrogenation involves feeding the nitrile to an autoclave, containing a slurry of catalyst in the reaction product, for a specified time period after which time the nitrile feed is stopped. After the nitrile feed is stopped, the reaction will continue for a short period of time while the residual unreacted nitrile is consumed. Close monitoring of the hydrogen uptake during the time period after the nitrile feed is stopped provides insight into the rate at which catalyst deactivation is occurring, since the hydrogen uptake rate between cycles will decrease noticeably if the catalyst is experiencing significant deactivation. The period from the time the nitrile feed is started until the time the nitrile feed is stopped is referred to as a cycle. After the cycle is completed, a quantity of reaction product equal to the quantity of nitrile fed to the reactor is withdrawn from the autoclave. The feed is then resumed under the same conditions as before. Semi-batch operation is an excellent tool for evaluating catalysts and the effects of process changes.

Figure 2 Experimental apparatus.

The experimental apparatus is shown in Figure 2. The reaction vessel is an Autoclave Engineers one-liter autoclave reactor equipped with double turbine blades, dispersimax-type agitator, a coil extending to the bottom to circulate the transfer fluid from a temperature controlled bath for temperature control, and a fritted, stainless steel metal sample port below the liquid level. Hydrogen is fed from a cylinder equipped with a pressure gauge and a regulator. Hydrogen is continuously added to the reactor as hydrogen is consumed by the reaction to maintain a constant reactor pressure. The hydrogen flows through a mass flow meter. The DMAPN (obtained from Acros®) is pumped to the autoclave with an Isco Model 500D syringe pump.

Caustic preparation begins with obtaining distilled water that has been boiled to remove dissolved carbon dioxide. Caustic solutions are prepared in 100 gram batches containing about 25% caustic by weight. The caustic (KOH,

NaOH, etc.) is added to the degassed water (~ 60 ml) with stirring. After complete dissolution of the caustic, additional water is added to bring the weight of the solution to a total weight of 100 grams. The solution is filtered, and stored in a closed container until use in order to minimize adsorption of CO2 from the air.

The catalyst used for this reaction is sponge nickel catalyst (Degussa MC502) that contains iron and chromium to promote the hydrogenation reaction (the catalyst contains approximately 86% nickel, 10% aluminum, 2% chromium, and 2% iron). 37.5 grams of catalyst is washed 3 times with water and 3 times with DMAPA (Acros; contaminated with 72 ppm TMPDA by GC analysis), each wash consisting of mixed catalyst and material in a 100 ml graduated cylinder, settling the catalyst, and decanting the top 50 ml of clear liquid.

The catalyst slurry in DMAPA is then charged to the autoclave. Additionally, 265 ml of 100% DMAPA and 6 - 8 mL of 25% (wt.) caustic solution in water is charged. The agitator is turned on, and the autoclave heated to 60 °C. The autoclave is then purged three times with nitrogen, and then three times with hydrogen, before being pressurized to 100 psig with hydrogen. The autoclave is then heated to 90 °C, and the pressure is monitored for 5 minutes with no hydrogen addition to confirm there are no leaks.

After the autoclave is charged with catalyst slurry, DMAPA and aqueous caustic solution, the feed of DMAPN is then started to the autoclave at a rate of 5 ml/minute using the syringe pump. Pressure and temperature are maintained at 100 psig and 90 °C, respectively, during the entirety of the run. After 27 minutes, the feed is stopped, and a 150 g sample is withdrawn from the autoclave for analysis. The feed is then resumed under the same conditions as before. This procedure is then repeated for a total of 6 - 8 cycles.

Analytical Section

The reaction mixture is sampled after each reaction cycle and analyzed for purity, reaction progress, and the presence and amount of by-products (if any) formed. Analysis is by gas chromatography (HP 5890 Series II; Phenomenex Zebron ZB-1 capillary column, Phenomenex Cat. No. 7HK-G001-36) with flame ionization detection in order to quantify the by-product impurities. The quantity of the byproducts is determined using an external standard calibration method.

Results and Discussion

In the initial test, 6 ml of 25% (wt.) caustic solution in water was charged to the autoclave with 37.5 grams of catalyst. The caustic used in this run was a blend containing 50 wt. % sodium hydroxide and 50 wt. % potassium hydroxide. The feed of DMAPN containing 0.04 wt. % water was then started to the autoclave at a rate of 5 ml/minute using the syringe pump. Pressure and temperature were maintained at 100 psig and 90 °C, respectively, during the run. After 27 minutes, the feed was stopped, and a 150 g sample was withdrawn from the autoclave for analysis. The feed was then resumed under the same conditions as before. This procedure was then repeated for a total of 7 cycles. The reaction mixture was sampled after each reaction cycle, and analysis of the impurities in the reaction product, with the balance being DMAPA, is given in Table 1.

Table 1 Product analysis.

Cycle	NPA	DAP	DMAPN	TMPDA	2°Amine	Water
	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	Wt. %
1	89	ND	ND	43	200	7.26
2	123	ND	ND	29	267	4.72
3	143	ND	ND	17	284	3.47
4	174	ND	ND	14	308	2.65
5	190	ND	ND	7	265	2.11
6	209	ND	ND	5	236	1.71
7	223	ND	ND	ND	214	NPA = n-propylamine DAP = 1,3-diaminopropane DMAPN = dimethylaminopropionitrile TMPDA = N,N,N',N'-tetramethyl-1,3-propanediamine 2° Amine = 3,3'-iminobis(N,N-dimethylpropylamine) The data in Table 1 shows that the amount of 2° Amine remains generally at or below 300 ppm over the course of the seven reaction cycles indicating that this particular blend of caustic is very effective at suppressing secondary amine formation. NPA is produced via decyanoethylation of the DMAPN followed by hydrogenation of the liberated acrylonitrile. Also, TMPDA is not formed in this process. It is introduced into the system with the DMAPA used to slurry the catalyst charged to the autoclave, and it is purged out by the seventh cycle. Conversion is 100% on all of the cycles as evident by no DMAPN detected in the reaction product. Although not reported in Table 1, no evidence of catalyst deactivation was detected between cycles as measured by the hydrogen uptake after the nitrile feed was stopped. No water was added to the system other than the water that was added with the aqueous caustic solution charged to the autoclave with the catalyst. As the data in Table 1 shows, the water concentration decreased with each cycle as the contents of the autoclave are turned over. If additional cycles had been run, it would have been necessary to add water to the nitrile feed to maintain a constant water concentration in the system since some amount of water is required to maintain good activity and selectivity. Finally, as the data in Table 1 shows, the crude DMAPA product is produced with a molar yield of approximately 99.95%, no TMPDA production, and less than 300 ppm of the secondary amine present in the final product. Next, a series of runs was conducted to determine the effect of various alkali metal hydroxide additions along with the sponge nickel catalyst. The 50 wt. % sodium hydroxide and 50 wt. % potassium hydroxide caustic solution used in the initial test was replaced with an aqueous solution of the alkali metal hydroxide at the level indicated in Table 2. After the reaction number of cycles indicated in Table 2, a sample was removed for analysis. The conditions and results are shown in Table 2. The results reported in Table 2 show the level of 2° Amine in the product from the final cycle. The level of NPA in all of the runs was comparable to the level observed in the initial test. No significant levels of other impurities were detected. Tables 1 and 2 clearly show that the use of such alkali metal hydroxides as KOH, CsOH, and mixtures of KOH/NaOH allowed the reaction to proceed to a high DMAPN conversion with a very high selectivity for the primary amine. These results suggest that the highest selectivity in the hydrogenation of DMAPN to DMAPA is obtained with KOH, and mixtures of KOH/NaOH. Table 2 Effect of alkali metal hydroxide on activity and selectivity. Run Alkali Metal Hydroxide Metal Hydroxide Quantity Temperature (°C) Pressure (PSIG) Number of Cycles 2° Amine (ppm) 1 CsOH 8 ml of 25 % (wt) aqueous CsOH 90 500 6 15 2 LiOH 80 ml of 10 % (wt) aqueous LiOH 90 500 6 2567 3 NaOH 8 ml of 25 % (wt) aqueous NaOH 90 110 6 1440 4 KOH 6 ml of 25 % (wt) aqueous KOH 90 100 6 9 5 RbOH 7 ml of 25 % (wt) aqueous RbOH 90 100 6 1463 6 KOH/NaOH 6 ml of 25 % (wt) aqueous 50/50 NaOH/KOH 90 100 6 9 Another important feature of this reaction is the low pressure at which the reaction proceeds. Unlike hexamethylenediamine or other amines produced with this process, the hydrogenation of DMAPN to DMAPA proceeds at very low pressures. High catalyst activity and high selectivity are obtained at 100 psig for NaOH, KOH, RbOH and blended NaOH/KOH. Testing with CsOH and LiOH was only conducted at 500 psig, and these tests were not repeated at 100 PSIG for CsOH and LiOH due to time constraints. Acknowledgements The authors would like to thank Solutia, Inc. for permission to publish work that is the basis for a US patent (9). We also wish to thank our numerous colleagues at Solutia for developing the methods and establishing the technology that are the basis for this study. We are indebted to those who have worked for many years to establish this technology as the premier technology for converting nitriles to primary amines. References 1. Charles R. Campbell and Arthur D. Hufford, Montifibre-Monsanto Hexamethylenediamine Process, 1984. 2. G. Bartalini and M. Giuggioli, to Montedison Fibre S. p. A., US 3,821,305, June 28, 1974. 3. Thomas A. Johnson and Douglas P. Freyberger, Catalysis of Organic Reaction (82), Marcel Dekker, Inc., New York, 201-227 (2001). 4. Thomas Albert Johnson, to Air Products and Chemicals, Inc., US 5,869,653 February 9, 1999. 5. John D. Super, Catalysis of Organic Reaction (82), Marcel Dekker, Inc., New York, 35-49 (2001). 6. Barbara Bender, Monika Berweiler, Konrad Moebus, Daniel Ostgard, and Gernot Stein, to Degussa-Huels A.-G., US 6,284,703, September 4, 2001. 7. Andreas Ansmann, Christoph Benisch, to BASF AG, US Patent Application 20030120115, June 26, 2003. 8. Jiri Krupka, Josef Pasek, Marketa Navratilova, Coll. Czech. Chem. Commun, Vol. 65 (11), 1805-1819 (2000). 9. Gregory J. Ward and Bryan C. Blanchard, to Solutia Inc., US Patent Application Serial No. 10/327,765, filing date December 23, 2002.

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