A method of preparing 2-aminopropan-1-ol from hydroxyacetone by reductive amination

Abstract

The present invention relates to a method of preparing 2-aminopropan-1-ol from hydroxyacetone by reductive amination using aqueous ammonia and a co-solvent. The method comprises steps of: mixing hydroxyacetone and aqueous ammonia at a temperature of not more than 20℃ to obtain an oxazoline intermediate product; and adding a solvent alone, or the solvent and a second aqueous ammonia, to the oxazoline product in the presence of a nickel catalyst and hydrogen gas, and performing hydrogenation at a temperature of not more than 90℃ and a pressure of not more than 12 barg to obtain 2-aminopropan-1-ol.

Description

[0001] A method of preparing 2-aminopropan-l-ol from hydroxyacetone by reductive amination

[0002] FIELD OF THE INVENTION

[0003]

[0001] The present invention relates to a method of preparing 2-aminopropan-l-ol from hydroxyacetone by reductive amination using aqueous ammonia and a co-solvent.

[0004] DESCRIPTION OF THE BACKGROUND ART

[0005]

[0002] There are several known processes for preparing 2-aminopropan-l-ol, also known by the names DL-alaninol, DL-2-aminopropan-l-ol. It can be prepared from propylene oxide via chloropropanol by various routes using ammonia, or from commercial monoisopropylamine by fractional distillation and separation from the other isomer, or from glycerol, or from nitroethane, or from alanine or from 2-methyl aziridine. However, manufacturing of 2- aminopropan-l-ol remains challenging to produce on an industrial scale, especially as a commodity chemical. The various routes for preparing 2-aminopropan-l-ol suffer from either low yield, unavailability of raw materials or harsh reaction conditions.

[0006]

[0003] None of the conditions used in the prior art are industrially feasible on a very large scale. Specifically, some routes, as developed by T. Tregner et al., require anhydrous ammonia and produces 2-aminopropan-l-ol with low selectivity. T.Tregner et al. (Chem. Biochem. Eng. Q., 31(4), 455-470 (2017)) teaches that the reaction is to be performed at temperatures of 130- 210°C which is very high.

[0007]

[0004] Reductive amination of hydroxyacetone gives 2-aminopropan-l-ol from commercially available raw material, as taught in US 2007 / 0287868 Al:

[0008] Hydroxyacetone 2) Ni, H2, 35 bar 2-aminopropan-l-oi

[0009] However, this route requires hydrogenation to be performed at high pressure i.e. 1100 psi of hydrogen (35 bar).

[0010]

[0005] Anhydrous ammonia is used in US3448153A which has the disadvantage of producing unwanted intermediates. GB1554176A, granted patent by BASF discloses use of aqueous ammonia in the presence of hydrogen but hydrogenation is carried out at a high pressure of 200 bar.

[0011]

[0006] There is a need for developing a method of preparing 2-aminopropan-l-ol at low-cost and an industrial scale process requiring lower temperatures and lower pressures. SUMMARY OF THE INVENTION

[0012]

[0007] According to an embodiment, the present invention is a method for preparing 2- aminopropan-l-ol by reductive amination, said method comprising steps of: a. mixing hydroxyacetone and aqueous ammonia at a temperature of not more than 20°C to obtain an oxazoline intermediate product; and b. adding a solvent alone, or the solvent and a second aqueous ammonia, to the oxazoline product in the presence of a nickel catalyst and hydrogen gas, and performing hydrogenation at a temperature of not more than 90°C and a pressure of not more than 12 barg to obtain 2- aminopropan- 1 -ol.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014]

[0008] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.

[0015]

[0009] FIG. 1 is a schematic flow representation showing the possible chemical pathways of reaction of ammonia and hydroxyacetone and effect of water in preventing the production of polymeric materials

[0016]

[0010] FIG. 2 is a chromatogram showing the results of gas chromatography of the product of Example 3

[0017] [Oi l] FIG. 3 is a chromatogram showing the results of gas chromatography of the product of Example 7

[0018]

[0012] FIG. 4 is a chromatogram showing the results of gas chromatography of the main cut after fractional distillation of the products of Example 6

[0019]

[0013] FIG. 5 shows the H1NMR spectrum of the main cut after fractional distillation of the products of Example 6

[0020]

[0014] FIG. 6 to FIG. 9 show the 13C NMR spectral data of reaction mixture of 2- aminopropan-l-ol prepared according to Example 3

[0021]

[0015] FIG. 10 to FIG. 11 show the 1H NMR spectral data of reaction mixture of 2- aminopropan-l-ol prepared according to Example 3

[0022]

[0016] FIG. 12a shows a schematic flow chart of the chemical reactions occurring in the present method

[0017] FIG. 12b is a schematic depiction of an embodiment of the present method

[0023]

[0018] FIG. 13 shows gas chromatography results of the hydrogenation reaction of Example 7

[0019] FIG. 14 shows gas chromatography results of oxazoline intermediate of Example 8

[0020] FIG. 15a and FIG. 15b show Mass Spectrometry and NMR results of oxazoline product of Example 8

[0024]

[0021] FIG. 16 shows gas chromatography results of Example 9 reactions of hydrogenation after filtration

[0025]

[0022] FIG. 17 shows gas chromatography results of hydrogenation using recycled skeletal nickel catalyst after filtration

[0026]

[0023] FIG. 18 shows gas induced impeller effect on hydrogenation reaction of Example 11

[0024] FIG. 19a shows effect of pressure and temperature on hydrogenation reactions of Example 12

[0027]

[0025] FIG. 19b shows effect of pressure and temperature on hydrogenation reactions, with and without a gas induced impeller

[0028]

[0026] FIG. 20 shows hydrogenation data of reverse addition on Example 13 reactions

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030]

[0027] In the following detailed description, the embodiments are described in sufficient detail to enable those skilled in the art to practice the invention and it is understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. Accordingly, the description and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present teachings. It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. It should be emphasized that the term “comprises / comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0031]

[0028] According to an embodiment of the invention, there is provided a method for preparing 2-aminopropan-l-ol by reductive amination, said method comprising steps of: a. mixing hydroxyacetone and aqueous ammonia at a temperature of not more than 20°C to obtain an oxazoline intermediate product; and b. adding a solvent alone, or the solvent and a second aqueous ammonia, to the oxazoline product in the presence of a nickel catalyst and hydrogen gas, and performing hydrogenation at a temperature of not more than 90°C and a pressure of not more than 12 barg to obtain 2- aminopropan- 1 -ol.

[0032]

[0029] In step a) or stage I, also called as imination, the oxazoline intermediate product, also referred to herein as imine, can be prepared in presence of the solvent e.g. methanol (to accommodate the recycle streams in continuous mode) with > 90% hydroxyacetone (also known as acetol) conversion. The imine can be stable at temperature >10°C. Step b) or stage II, also called as the hydrogenation step, can be conducted at 5 to 10 barg to get about -90% 2-aminopropan-l-ol yield in the batch time of about 3 to 4 hours.

[0033]

[0030] According to an embodiment, the nickel catalyst is selected from skeletal nickel, Raney® nickel, nickel sponge, recycled nickel catalyst, and any other type of nickel catalyst. According to an embodiment of the method, the temperature in step a) and b) is a value between 4 to 20°C and 40 to 90°C, respectively, and the pressure of hydrogen in step b) is a value between 5 and 12 barg. Higher temperature helps to fasten the reaction kinetics, for instance, at about 85°C the batch time is lower by about 50%.

[0034]

[0031] According to an embodiment of the method hereinabove, in step b) the solvent is selected from methanol, ethanol, propanol, any other C 1-C5 alcohol and a combination thereof.

[0032] According to an embodiment of the method, the ratio of hydroxy acetone : solvent is a value between about 1:0.8 to 1:2. Preferably hydroxyacetone: solvent in a ratio of 1:1 is required to achieve the -90% 2-aminopropan-l-ol yield, lower solvent concentration reduces 2-aminopropan-l-ol yield and a substantially higher solvent concentration does not show any improvement in the 2-aminopropan-l-ol yield.

[0035]

[0033] According to an embodiment of the method, the ratio of hydroxyacetone : aqueous ammonia required, in total, in the method is a value between about 1:3 to 1:5. The total ratio being that used in both steps a) and b).

[0036]

[0034] The present method allows for the aqueous ammonia to be added entirely in a single batch in step a) itself or in two batches between step a) and b). Hence, in step b) when solvent alone is added, the entire amount of aqueous ammonia is added in step a) such that the hydroxyacetone : aqueous ammonia ratio is a value in the range of about 1:3 to 1:5. In a preferred embodiment the ratio is a value between 1:3.5 and 1:4.

[0037]

[0035] Alternatively, in step b) when the second aqueous ammonia is added with the solvent, the amount of aqueous ammonia added in step a) is such that the hydroxyacetone : aqueous ammonia ratio is at least about 1:0.5, then the remainder amount of the second aqueous ammonia added in step b) is that much as required for the total ratio to be a value in the range of about 1:3 to 1:5. In other words, the first aqueous ammonia has to be maintained at a minimum ratio of 1:0.5, more preferably 1: 1, for the reaction to proceed forward. The remainder i.e. second aqueous ammonia is added such that the final ratio of 1:3 to 1:5 is achieved. As during hydrogenation hydroxyacetone : aqueous ammonia required is about 1:3 to 1:5, preferably 1:3.5 to 1:4, to achieve the -90% 2-aminopropan-l-ol yield. According to an embodiment of the method, the aqueous ammonia has a concentration of between 22 to 30%.

[0036] According to an embodiment of the method, the 2-aminopropan-l-ol product is further purified by filtration, vacuum distillation or a combination thereof. Any suitable method may be used for further purification of the 2-aminopropan-l-ol .

[0038]

[0037] According to an embodiment of the method, in step b) the nickel catalyst is recycled by washing with water and thereafter reused. The nickel catalyst may be recycled up to 3 times without any top up. Afterward, an intermediate top up of -1020% is required. The catalyst can be recycled for up to 10 times with an average 2-aminopropan-l-ol yield of about > 80%.

[0039]

[0038] According to an embodiment of the method, the mixing is performed by a gas induced impeller. When mixing is performed by the gas induced impeller the reaction time for hydrogenation of step b) is less than 4 hours.

[0040]

[0039] According to an embodiment of the method, in step b) the solvent is added to the oxazoline product when in step a) about 85% conversion of hydroxy acetone to the oxazoline product is achieved.

[0041]

[0040] Acetol or hydroxy acetone can be prepared by a range of known prior art methods. Commercially available acetol can be prepared via dehydrogenation of 1,2-propyleneglycol (PG), in a process developed by Jefferson Chemical (USA) (Chemical & Engineering News, Dec 20, 1965, p. 38) using metal oxides at elevated temperatures (350-400 °C) with > 96% yield. The reductive amination process of acetol described in US 2007 / 0287868 Al uses acetol synthesized via a different route i.e. from glycerol.

[0042]

[0041] The present method was developed to be an industrially feasible process for manufacturing 2-aminopropan-l-ol at low pressures of 1-15 bar H2. The present method uses aqueous ammonia and C1-C5 alcohols as co-solvent. The alcohol solvent allows for dissolving hydrogen, and, thus, pressure can be reduced when compared to other prior art processes which use 35 bar or 1100 psi pressure and water as a solvent. Hydrogen is less soluble in water than in the alcohol. Since H2 gas in the reaction is much more soluble in the alcohol than in water, the pressure of the system is much lower as compared to the prior art. Less pressure is always beneficial for industrial applications, as the equipment considerations for high pressure processes is significantly more complex and more expensive. The present method can be performed at lower temperatures of 50-90°C as compared to the prior art where temperatures are 85-210°C, thus making the present method even more industrially feasible.

[0043]

[0042] Aqueous ammonia or ammonium hydroxide is used in the present method, which is both easier to work with as compared to gas ammonia, and it also gives good selectivity of 2- aminopropan-l-ol i.e. about 80% selectivity, which is significantly more than in prior art processes. Further, as described below, anhydrous ammonia can lead to undesirable formation of polymeric materials.

[0044]

[0043] FIG. 12a is an embodiment of the present method showing a schematic flow chart of the chemical reactions occurring in the present method where stage I is imination wherein hydroxyacetone [I] reacts with about 25% aqueous ammonia at about 5-10°C to yield oxazoline [II]. The next stage II is called hydrogenation wherein the oxazoline [II] is reacted with solvent methanol, aqueous ammonia and hydrogen gas at a pressure at about 10 bar in the presence of Raney nickel to generate 2-aminopropan-l-ol [III]. FIG. 12b shows a schematic of the embodiment where the aqueous ammonia is added in two batches.

[0045]

[0044] FIG. 1 is a schematic representation of the chemical reaction showing the possible byproducts and effect of water in forming hydroxyacetone aminal and hence preventing the production of by-products I, II and III and polymeric materials. It is hypothesized that water prevents byproducts. The intermediate hydroxyacetone imine formed in the reductive amination can dimerize (e.g. compounds I and III) or trimerize (e.g. compound II) in the absence of water to form a polymer. However, in the presence of water hydroxyacetone aminal may be formed which prevents polymerization. Thus, the present invention discourages the use of anhydrous ammonia in place of aqueous ammonia to prepare 2-aminopropan-l-ol. Further, the present method can be performed as either batch process or continuous process, i.e. tricklebed reactor or continuous stir tank reactor (CSTR), or even a cascade of CSTRs (e.g. two CSTR system in series etc) using the reaction conditions stated herein.

[0046]

[0045] The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

[0047]

[0046] Example: 1 - Nickel Sponge Catalyst Preparation

[0048]

[0047] 2.0 kg of 25% NaOH was taken in a 5 litre glass beaker mounted on magnetic hot plate stirrer. 130 gm of Ni-Al (50%:50%) alloy powder was slowly charged into the beaker at 50- 60°C within 1 hr. The temperature eventually goes up to 80°C. The contents of the beaker are stirred for another 4-5 hours at 50-60°C. The catalyst thus obtained was washed with DI water till its pH became neutral.

[0049]

[0048] Example: 2 - Process of Reductive Amination of Acetol

[0050]

[0049] 200 gm of acetol / hydroxyacetone (abbreviated as HA herein) was added dropwise into a 1 liter round bottom flask containing 590 gm of 28% aqueous ammonia solution below 10°C. The mixture was stirred for 1 hour below 10°C. Progress of the imine conversion was checked by gas chromatography to assess to be >90%.

[0051]

[0050] The above reaction mixture was transferred into a 2-liter autoclave. 400 g methanol was added to the reaction mixture along with 20 g of sponge nickel catalyst as prepared hereinabove at room temperature. The reactor was flushed with hydrogen gas and hydrogen pressure of about 6-10 bar was applied. Though the experiments herein were tested using methanol, a skilled person is well aware that any other Cl -C5 alcohol can be substituted therein.

[0052]

[0051] The reaction mixture was heated to 60 ± 10°C for about 4-8 hours while maintaining hydrogen pressure. The reaction progress was monitored by gas chromatography (abbreviated as GC herein). After >96% complete conversion of imine, the temperature of the contents of the reactor was brought down to room temperature. Selectivity of 2-aminopropan-l-ol is about 80%. The catalyst was separated by filtration.

[0053]

[0052] After filtration of catalyst, crude 2-aminopropan-l-ol (abbreviated as 2AP herein) was isolated as 28.5 w / w % solution that contained also 1.9% of 1,2-propanediol and other impurities as shown in Table 1 below:

[0054] Table 1:

[0055] Ammonia, methanol and water were distilled off under vacuum, to get crude 2-aminopropan- l-ol which was further purified by high vacuum distillation.

[0056]

[0053] Examples: 3 to 6

[0057]

[0054] The process of Example 2 was followed in Examples 3 to 6, except that some reaction parameters were varied as provided in Table 2 below:

[0058] Table 2:

[0059]

[0055] FIG. 2 is a chromatogram showing the results of gas chromatography of the product of Example 3.

[0060]

[0056] In Example 5, the temperature was 40°C for 5hrs, 50°C for 5hrs and 80°C for 5hrs. Similarly, in Example 6, the temperature was 60°C and then 70°C for 4.5hrs each.

[0061]

[0057] FIG. 3 is a chromatogram showing the results of gas chromatography of the reaction mixture of Example 6

[0062]

[0058] Fractional distillation of the contents of the reactor at the end of the process of Example

[0063] 6 provided the following results as shown in Table 3:

[0064] Table 3: 2-aminopropan-l-ol fractional distillation

[0065]

[0059] FIG. 4 is a chromatogram showing the results of gas chromatography of main cut (2AP product fraction) after fractional distillation of the products of Example 6, showing 92% purity of 2-aminopropan-l-ol. FIG. 5 shows the 1H NMR spectrum of the main cut after fractional distillation of the products of Example 6 confirming high purity of 2-aminopropan-l-ol obtained in main cut.

[0066]

[0060] FIG. 6 to FIG. 9 show the 13C NMR spectral data of reaction mixture prepared according to the Example 3. The spectral data confirm formation of 2-aminopropan-l-ol as main product.

[0067]

[0061] FIG. 10 to FIG. 11 show the 1H NMR spectral data of reaction mixture prepared according to the Example 3. The spectral data confirm formation of 2-aminopropan-l-ol as main product.

[0068]

[0062] Next, large scale experiments were conducted to test the method on an industrial level.

[0063] Example 7: Conversion of Hydroxyacetone into 2-aminopropan-l-ol

[0069]

[0064] Hydroxyacetone (50 kg) was transferred into a reaction vessel and was cooled to 10 °C. Then 25% aqueous ammonia (183 kg, 4 mole eq) was added slowly, under gentle stirring while maintaining the internal temperature < 10 °C. After completion of addition the temperature was maintained under 10°C with stirring and hydroxy acetone consumption was monitored by gas chromatography. After 2 hours the intermediate was transferred into a hydrogenation reactor (750 litres) and skeletal nickel (9.5 kg, dry basis, Procat) was charged under nitrogen. The reactor was equipped with a gas inducing impeller. Finally, methanol (50 kg) was charged into the reactor and the reaction mixture was hydrogenated at 10 bar pressure of hydrogen maintaining the temperature at 80°C. After 6 hours, catalyst was filtered off and the crude 2- aminopropan-l-ol solution was first subjected to distillation to remove water, methanol and ammonia. The crude 2-aminopropan-l-ol solution was isolated by filtering of the catalyst to get 84.9% yield of pure 2-aminopropan-l-ol (see Table 4a). In this example all the aqueous ammonia was charged in one batch in step a). [Note: 50 kg batch of hydroxyacetone + 4 eq aqueous ammonia, 9.5 kg catalyst, 80°C, 6 h; supports that hydroxyacetone : aqueous ammonia 1:4 ratio also works]. FIG. 13 shows GC results of the hydrogenation reaction of Example 7. Table 4 shows the GC results of Figure 14. The experiment demonstrates industrial feasibility of the process. The reaction is scalable and reproducible in a typical hydrogenation reactor.

[0070] Table 4:

[0071]

[0065] Table 4a below shows a further scaled up experiment where the 2-aminopropan-l-ol yield was 84.9%.

[0072] Table 4a:

[0073]

[0066] Table 5 below shows gas chromatography results of Table 4 reaction where the percentage yield of oxazoline product at the end of step a) was measured to be 87.91% and 87.41% after 1 hour and 2 hours, respectively. And 2AP selectivity at the end of step b) was measured to be 86.55% together with small quantity (3.28% selectivity) of 1,2-propanediol (PDO) after 6 hours.

[0074] Table 5:

[0075]

[0067] Example 8: Conversion of Hydroxyacetone into Oxazoline

[0068] In this example the aqueous ammonia was charged in two batches over steps a) and b). Hydroxyacetone (85 kg) was charged into a reaction vessel and cooled to 10°C. Then 25% aqueous ammonia (78.1 kg, 1 mol eq) was added slowly with maintaining the internal temperature at 10-12°C under gentle stirring. Addition of ammonia completed in 4 hours and then mixing was continued for additional 2 hours maintaining the internal temperature < 10°C. The progress of the conversion was monitored by gas chromatography. FIG. 14 shows gas chromatography results of oxazoline intermediate of Example 8. An acetol : ammonia ratio of E l is sufficient in step a) to achieve the acetol conversion to oxazoline intermediate by about > 90%. The quantity of aqueous ammonia will be reduced. Table 6 below shows the oxazoline yield to be 87.91 and 87.41%.

[0076] Table 6:

[0077]

[0069] Table 7 shows the GC experimental results of FIG. 14. As seen from the data, an acetol : ammonia ratio of 1 : 1 is sufficient to achieve the acetol conversion to oxazoline intermediate by about > 90%. Then the resulting oxazoline solution was used for the next hydrogenation step of Example 9.

[0078] Table 7:

[0079]

[0070] FIG. 15a and 15b show analysis of the oxazoline intermediate of Example 8 using mass spectroscopy (MS) and nuclear magnetic resonance (NMR). The oxazoline intermediate is formed with high selectivity as confirmed by NMR and MS.

[0080]

[0071] Example 9: Hydrogenation of Oxazoline into 2-aminopropan-l-ol

[0081]

[0072] The above oxazoline solution was transferred into a 750 litre hydrogenation reactor equipped with a gas inducing impeller. Then additional 25% aqueous ammonia (168.1 kg) was transferred into the reactor under nitrogen. The hydrogenator was charged with skeletal nickel (17 kg, dry basis) and methanol (85 Kg). The inerting process of the hydrogenator was performed with nitrogen and the vessel was filled with hydrogen to achieve the pressure of 10 bar. The reaction was heated to 85 ± 5°C maintaining the hydrogen pressure at 10 bar. The reaction progress was monitored by gas chromatography till conversion was achieved to be about >90%. Table 8 below shows the selectivity of 2-aminopropan-l-ol at the end of 2 hours and 3 hours to be 84.21% and 88.62%, respectively, as measured by gas chromatography.

[0082] Table 8:

[0083]

[0073] After 3.5 hours the desired conversion was achieved, temperature of the vessel was brought down to ambient temperature. The catalyst was separated by filtration using a nutsche filter and the filtrate was transferred for distillation. From the crude 2-aminopropan-l-ol mixture, ammonia, water and methanol were distilled of under vacuum to get crude 2- aminopropan-l-ol, which was further purified by high vacuum distillation.

[0084]

[0074] Table 9:

[0085] FIG. 16 and Table 9 show gas chromatography results of Example 9 reactions of hydrogenation after filtration. FIG. 16 and Table 9 results demonstrate high purity of 2-aminopropan-l-ol achieved after ammonia was separated into two additions, in steps a) and b) respectively. [Note: hydrogenation of oxazoline to 2AP completion in 3.5 h, T = 85-90 °C, H2 = 10 psi]

[0086]

[0075] Example 10: Hydrogenation of Oxazoline into 2-aminopropan-l-ol Using Recycled Skeleton Nickel

[0076] From Example 9, the isolated nickel catalyst after filtration was thoroughly washed with water (25 litres x 4 washes) and used for this batch according to the procedure as described for Example 9. The conversion of hydrogenation using recycled skeletal nickel is given below in Table 10:

[0087] Table 10:

[0088]

[0077] The above results indicate that the catalyst can be recycled without any significant loss in the catalytic activity. Thus similar catalytic activity can be achieved by doing 10-20% top up in every 3-4 cycles. FIG. 17 and Table 11 show gas chromatography results of hydrogenation using recycled skeletal nickel catalyst after filtration. The results also indicate that the catalyst can be recycled without any significant loss in the catalytic activity.

[0089] Table 11:

[0090]

[0078] Example 11: Effect of Gas Induced Impeller in the Rate of Hydrogenation

[0091]

[0079] The process of hydrogenation was evaluated to understand the role of gas induced impeller in the kinetics of the hydrogenation reaction. The reactions were performed according to Examples 2-6, except the hydrogenation reaction was carried out using a gas induced impeller and temperature was varied while pressure was set at lObar H2. FIG. 18 shows gas induced impeller effect on hydrogenation reaction of Example 11. Hydrogenation reaction at 82°C and 10 barg pressure is favourable when using a gas induced impeller over a regular impeller as the reaction time is reduced. The gas induced impeller increases the mixing of hydrogen gas in the reaction mixture and thereby increases the gas liquid mass transfer. This effect has significant impact in the rate of hydrogenation. Hydroxyacetone conversion to 2- aminopropan-l-ol at 3 hours is 73% (normal impeller) versus 80.9% (gas induced impeller) at temperature of 75°C. When temperature was increased to 82°C and gas induced impeller was used the conversion further increases to 88.7% at 3 hours. However, at 87°C there is no significant difference between a regular impeller and a gas induced impeller (see FIG. 19a far right).

[0092]

[0080] Example 12: To Study Effect of Pressure and Temperature on 2-aminopropan-l-ol Yield and Oxazoline Consumption in the Hydrogenation Reaction

[0093]

[0081] FIG. 19a shows the effect of pressure at various temperatures on 2-aminopropan-l-ol yield and oxazoline consumption. From FIG. 19a it is evident that the time to achieve >90% conversion is about 5 hours at 75°C. Whereas, the same conversion takes about 3 hours at 87°C. Temperature plays a crucial role in the duration of hydrogenation. Temperature has a crucial influence on kinetics of the reaction. Increasing the pressure of hydrogen from 10 barg up to 20 barg did not impact the reaction profile significantly. However, hydrogenation reaction at 7 barg pressure led to lowering the yield of the reaction. Increasing the temperature beyond 90°C leads to lowering the yield due to potential degradation of 2-aminopropan-l-ol. Hence, the process involving pressure of 10 barg and reaction temperature of 87 °C may be preferred for the scale-up of 2-aminopropan-l-ol. Table 12 shows hydroxyacetone to 2-aminopropan-l-ol conversion after 3 hours at different temperatures and pressures with a gas induced impeller.

[0094] Table 12:

[0095] Ace jxu'&y =

[0096]

[0082] Oxazoline consumption was slower at 75°C irrespective of pressure changes from 10 to 20 barg. Higher temperature of 87°C accelerates the reaction kinetics and ensures minimum oxazoline in the reaction mixture with reaction time of about 4-5 hours. Observed about 91-92 wt % yield of 2-aminopropan-l-ol with fresh catalyst for all pressures at 87°C.

[0083] FIG. 19b shows effect of temperature on 2-aminopropan-l-ol yield and oxazoline consumption with and without gas induced impeller. Gas induced impeller shows effect at 75°C but this effect gets nullified at higher temp of 87°C as seen from FIG. 19b and Table 13 yield values.

[0097] Table 13:

[0098]

[0084] Example 13: Reverse Addition of Hydroxy acetone to Form Oxazoline and Formation of 2-aminopropan-l-ol

[0099]

[0085] In a reverse addition process for the formation of oxazoline, 25% aqueous ammonia (78.1 kg) was charged into a reaction vessel and cooled to 10°C. Then hydroxyacetone (85 kg) was charged slowly with maintaining the internal temperature at 10-12°C under gentle mixing. Addition of hydroxy acetone completed in 2 hours and then mixing was continued for additional 3.5 hours maintaining the internal temperature < 10 °C. The progress of the conversion was monitored by gas chromatography (Table 14). Upon completion, the oxazoline was used for the next step for hydrogenation according to Example 7. The results showed that the sequence of addition (i.e the acetol may be added into ammonia or vice versa) did not significantly affect the reaction.

[0100] Table 14:

[0086] FIG. 20 and Table 15 show hydrogenation data of reverse addition on Example 13 reactions.

[0101] Table 15:

[0102] The results show that the sequence of addition (i.e the acetol may be added into ammonia or vice versa) did not significantly affect the reaction.

[0103]

[0087] The present method is an improved method of preparing 2-aminopropan-l-ol by reductive amination wherein aqueous ammonia is used with a C1-C5 alcohol solvent at a lower pressure than the pressure used in the prior art methods. The process developed in the present invention gives > 96% conversion and 78-93% (Table 13) selectivity of 2-aminopropan-l-ol, as measured by external standards. The remainder is: <4% unconverted hydroxyacetone imine, 1,2-propanediol, 2,4-dimethyl-2-oxazoline-2-methanol (which is a product of condensation of 2-aminopropan-l-ol with hydroxyacetone) as identified by GC / MS and higher molecular weight impurities. The impurities which are formed are easy to separate by fractional distillation, as shown herein.

[0104]

[0088] The above examples are non-limiting. While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

We Claim:

1. A method for preparing 2-aminopropan-l-ol by reductive amination, said method comprising steps of: a. mixing hydroxyacetone and aqueous ammonia at a temperature of not more than 20°C to obtain an oxazoline intermediate product; and b. adding a solvent alone, or the solvent and a second aqueous ammonia, to the oxazoline product in the presence of a nickel catalyst and hydrogen gas, and performing hydrogenation at a temperature of not more than 90°C and a pressure of not more than 12 barg to obtain 2-aminopropan-l-ol.

2. The method as claimed in claim 1, wherein the nickel catalyst is selected from skeletal nickel, Raney® nickel, nickel sponge, recycled nickel catalyst, and any other type of nickel catalyst.

3. The method as claimed in claim 1, wherein in step b) the solvent is selected from methanol, ethanol, propanol, any other C1-C5 alcohol and a combination thereof.

4. The method as claimed in claim 1, wherein the ratio of hydroxy acetone : aqueous ammonia required, in total, in the method is a value between about 1:3 to 1:5.

5. The method as claimed in claim 1, wherein the ratio of hydroxy acetone : solvent is a value between about 1:0.8 to 1:2.

6. The method as claimed in claim 1, wherein in step b) when solvent alone is added, the amount of the aqueous ammonia added in step a) is such that the hydroxy acetone : aqueous ammonia ratio is a value in the range of about 1:3 to 1:5.

7. The method as claimed in claim 1, wherein in step b) when the second aqueous ammonia is added with the solvent, the amount of the aqueous ammonia added in step a) is such that the hydroxyacetone : aqueous ammonia ratio is at least about 1:0.5, then the remainder amount of thesecond aqueous ammonia added in step b) is that much as required for the total ratio to be a value in the range of about 1:3 to 1:5.

8. The method as claimed in claim 1, wherein the 2-aminopropan-l-ol product is further purified by filtration, vacuum distillation or a combination thereof.

9. The method as claimed in claim 1, wherein in step b) the nickel catalyst is recycled by washing with water and thereafter reused.

10. The method as claimed in claim 1, wherein the mixing is performed by a gas induced impeller.

11. The method as claimed in claim 10, wherein when mixing is performed by the gas induced impeller the reaction time for hydrogenation in step b) is less than 4 hours.

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