Process for the manufacture of omega nitrooxy-1 -alkanols

The invention relates to a safe and efficient process for the manufacture of -nitrooxy-C ₃ - ioalkane-1-ols.

Global temperature is increasing, a process referred to as global warming or climate change. One of the main focuses to reduce this warming effect is to reduce the amount of greenhouse gases emitted into the atmosphere. Greenhouse gases are emitted from several different sources, both natural and anthropogenic; however, the two sources with the biggest impact are the agricultural and fossil fuel industries. Within agriculture, ruminants and in particular cattle are the major contributors to the biogenic methane formation, and it has been estimated that the prevention of methane formation from ruminants would almost stabilize atmospheric methane concentrations. 3- Nitrooxypropanol (3-NOP, also known as 3-nitrooxy-propan-1-ol or 1 ,3-propanediol mononitrate) has been reported to be highly efficient in reducing the formation of methane in ruminants without affecting microbial fermentation in a way that would be detrimental to the host animal (W 0-2012/084629).

The mononitration of polyols is generally poorly selective and quickly leads to formation of di- or polynitrated alcohols since the kinetics are not easy to control. Moreover, by carrying out the same reaction on short chain alkanediols strong decomposition reactions are easily triggered.

Thus, WO-2012/084629 discloses the preparation of 3-nitrooxypropanol by reacting 3-bromopropanol in acetonitrile with silver nitrate, a process which is, however, not economical in industrial scale production. Zikas et al (Bioorganic & Medicinal Chemistry, 13 (2005) 6485-6492) discloses the mononitration of alkanediols with acetyl nitrate (generated from nitric acid, acetic acid and acetic anhydride) at room temperature in the presence of ethyl acetate. Next to the necessity of an organic solvent, another drawback of this process is the use of acetyl nitrate which, even though an excellent nitration agent, is very explosive and thus not feasible for industrial scale production.

WO-2004043898 discloses the batch-wise mononitration of alkanediols with stabilized nitric acid in a water-immiscible chlorinated organic solvent at a reaction temperature lower than room temperature (RT), such as preferably equal to or lower than 0°C. The stabilized nitric acid consists of fuming nitric acid diluted in water to a concentration of about 83 to 85% w/w and is substantially free from nitrous acid and nitrogen oxides. The weight ratio of "stabilised" nitric acid to the alkanediols ranges from about 10:1 to about 15:1.

Next to the use of a chlorinated solvent, which is highly unwanted in chemical production, a significant drawback of this process is the requirement of high acid equivalents which leads to a high salt load. Furthermore, the process uses a non-commercial nitric acid dilution. Also, the reaction requires low temperatures (i.e. < RT) to control the selectivity and to avoid thermal runaways or even explosions.

Thus, there is an ongoing need to develop an efficient, economic and safe industrial process for the direct nitration of a, -C3-ioalkanediol such as in particular of 1 ,3-propanediol, which can be carried out batch-wise and in the absence of an organic solvent, requires low amounts of nitric acid equivalents and is readily amenable to scale- up.

Surprisingly and in contrast to the common knowledge in the art, it has now been found that (o-nitrooxyC ₃ -ioalkane-1-ols such as in particular 3-nitrooxypropanol can also be obtained in high yields in a batch-wise, ‘solvent-free’, one-pot process at elevated temperatures using significantly less nitric acid equivalents than reported in literature while at the same time maintaining the reaction stability.

Thus, the invention relates to a process for the preparation of -nitrooxyC ₃ -ioalkane-1-ols, comprising the nitration of an a, -C3-ioalkanediol with a nitration agent, characterized in that the nitration agent is a mixture consisting of 50 to 75 wt.-% of nitric acid in water and wherein the nitric acid is used in an amount selected in the range from 3 to 8 mol equivalents based on the a, -C3-ioalkanediol and wherein the nitration is carried out at a temperature selected in the range from 25-100 °C (nitration temperature). The term‘a, -C3-ioalkanediols’ as used herein refers to the linear a,w-alkanediols having 3 to 10 carbon atoms such as 1 ,3-propanediol, 1 ,4-butanediol, 1 ,5-pentanediol, 1 ,6-hexanediol, 1 ,7-heptanediol, 1 ,8-octanediol, 1 ,9-nonanediol and 1 , 10-decanediol. Particularly preferred in all embodiments according to the present invention is 1 ,3-propanediol.

The term’ -nitrooxyC ₃ -ioalkane-1-ols’ refers to the linear w-nitrooxyalkane-l-ols having 3 to 10 carbon atoms such as 3-nitrooxypropane-1-ol (also referred to as 3-nitrooxypropanol), 4-nitrooxybutane-1 -ol, 5-nitrooxypentane-1-ol, 6-nitrooxyhexane-1 -ol, 7-nitrooxyheptane-1-ol, 8-nitrooxyoctane-1-ol, 9-nitrooxynonane-1-ol and 10-nitrooxydecane-1-ol. Particularly preferred in all embodiments according to the present invention is 3-nitrooxypropanol.

In all embodiments of the present invention, preferably 3 to 6 mol equivalents, more preferably 3.5 to 5 mol equivalents, more preferably 3.5 to 4.5 mol equivalents, most preferably 3.5 to 4 mol equivalents of nitric acid based on the a, -C3-ioalkanediol such as in particular of 1 ,3-propanediol is used.

In all embodiments of the present invention, the concentration of the nitric acid in water is preferably selected in the range from 60 to 72 wt.-%, even more preferably in the range from 62 to 68 wt.-%. Such nitric acid is readily commercially available (e.g. at Lonza or Yara). Alternatively, regenerated nitric acid can be used.

It is well understood that in all embodiments of the present invention nitric acid is used as sole nitration agent, i.e. in the absence of any sulfuric acid or agents leading to the formation of an activated nitrate (e.g. acetyl nitrate) such as acetic anhydride.

In an advantageous embodiment, the reaction is carried out in the presence of an agent able to remove nitrous acid and nitrogen oxides (i.e. in the presence of a suitable nitrite trapping agent) such as in particular in the presence of urea and / or sulfamic acid, most preferably in the presence of urea.

In all embodiments of the present invention, the amount of the agent able to remove nitrous acid and nitrogen oxides (i.e. the nitrite trapping agent) is preferably selected in the range from 0.1 to 0.7 mol equivalents, preferably in the range from 0.2 to 0.5 mol-equivalents, most preferably in the range from 0.2 to 0.4 mol-equivalents, based on the amount of the a, -C3-ioalkanediol such as in particular based on the amount of 1 ,3-propanediol.

Furthermore, in all embodiments of the present invention, the nitration is preferably carried out in the absence (i.e. without the addition) of any organic solvent but carried out solely in water, i.e. the nitration is carried out in a reaction mixture consisting essentially of nitric acid, water, the a, -C3-ioalkanediol and optionally the nitrite trapping agent.

The term‘organic solvent’ as used herein is well known to a person skilled in the art and refers to carbon-based solvents which are capable of dissolving other substances and which are generally liquid at room temperature. Typical organic solvents encompass aliphatic-chain compounds e.g. hexane, halogenated hydrocarbons e.g. dichloromethane, esters e.g. ethyl acetate, ethers e.g. diethyl ether, aromatic hydrocarbon e.g. toluene or xylene, alcohols e.g. methanol or ethanol as well as ketones acetone without being limited thereto.

In a particular advantageous embodiment, the present invention relates to a process for the preparation of -nitrooxyC ₃ -ioalkane-1-ols comprising the nitration of an a,(o-C3-ioalkanediol with a nitration agent in a reaction mixture consisting essentially of nitric acid, water, an a, -C3-ioalkanediol and a nitrite trapping agent, characterized in that the nitration agent is a mixture consisting of 50 to 75 wt.-% of nitric acid in water wherein the nitric acid is used in an amount selected in the range from 3 to 8 mol equivalents based on the a, -C3-ioalkanediol, and wherein the nitration is carried out at a temperature selected in the range from 25 - 100°C (nitration temperature) with the proviso that all the preferences and definitions as given herein also apply to said process.

Advantageously, in all embodiments of the present invention, the reaction temperature during the nitration (nitration temperature) is selected in the range from 25-90°C, more preferably in the range of 25-80°C, such as in the range from 30 to 80°C, even more preferably in the range from 40-75°C, and most preferably in the range from 50 to 65°C, such as in particular in the range from 55 to 65°C or in the range from 50 to 60°C or 55 to 60 °C or even 60 to 65 °C.

In a particular advantageous embodiment, the nitration temperature is selected in the range from 25 to 60°C, when more than 5 mol equivalents (i.e. from 5 to 8 mole equivalents) of nitric acid based on the amount of the a, -C3-ioalkanediol, in particular based on the amount of 1 ,3-propanediol, are used and the nitration temperature is selected in the range from 60 to 100°C, preferably 60 to 90°C, most preferably 60 to 80°C when equal or less than 5 mole equivalents (i.e. from 3 to 5 mole equivalents) of nitric acid based on the amount of a, -C3-ioalkanediol, in particular based on the amount of 1 ,3-propanediol, are used as such process conditions lead to a minimization of undesired by-products (in particular dinitration) while still providing good yields allowing an economical industrial scale production.

The mononitration of the a, -C3-ioalkanediol, preferably of 1 ,3-propanediol is preferably carried out for reaction times ranging from about 30 to about 300 minutes, preferably from about 60 to about 120 minutes.

The progress of the reaction can be monitored with conventional analytical methods and the optimal reaction time can thus be determined. Thus, in a preferred embodiment, the present invention relates to a process according to the present invention, wherein the reaction is monitored and the reaction is terminated at an in-process yield of about 40- 65 %, preferably at an in-process yield of about 40 to 55 %, most preferably at an in- process yield in the range from about 40 to 50 % or even 45 to 55 % as determined by HPLC or GC, e.g. by RP-HPLC based on an external standard.

It is well understood, that the process according to the present invention is designed to be carried out batchwise (not continuously) in a safe and economical manner, thus in a preferred embodiment, the present invention is directed to processes for the batchwise nitration of -nitrooxyC ₃ -ioalkane-1 -ols with all the definitions and preferences as given herein.

In a particular advantageous embodiment, the process according to the present invention with all the definitions and preferences as given herein is a process (I) comprising the following consecutive steps:

i. Provision of the nitration agent (e.g. by loading of the reactor vessel with the nitric acid water mixture), followed by

ii. Addition of the agent able to remove nitrous acid and nitrogen oxides (i.e. the nitrite trapping agent) and stirring of the obtained reaction mixture, preferably for 10 to 60 minutes, more preferably for 15 to 45 minutes, followed by

iii. Addition of the a, -C3-ioalkanediol, preferably the 1 ,3-propanediol (actual nitration reaction) In a particular advantageous embodiment, the process according to the present invention is a process (II) which is a process (I) comprising next to the consecutive steps (i) to (iii), furthermore the subsequent step of

iv. Quenching of the reaction mixture obtained in (iii) with water or a base to obtain an aqueous reaction mixture comprising the -nitrooxyC3-ioalkane-1-ol, preferably the 3-nitrooxypropanol.

Optionally, the aqueous reaction mixture obtained in (iv) may be further neutralized with a suitable base, in particular in case of quenching with water (i.e. the process contains a subsequent neutralization step (v)).

Suitable bases in all embodiments of the present invention include alkali or earth alkali bases such as alkali or earth alkali hydroxides or carbonates as well as amines without being limited thereto.

Preferably in all embodiments of the present invention the base the base used in step (iv) and in the optional neutralization step (v) is selected from the group of NaOH, KOH, Ca(OH)2 or ammonia, more preferably an aqueous solution thereof is used. Most preferably in all embodiments of the present invention the base is aqueous NaOH or ammonia (NH4OH).

In case of quenching with water preferably a neutralization step (v) is included into the process (II).

In a further preferred embodiment, the process according to the present invention furthermore includes an extraction step (vi).

The temperature in step (i) and the starting temperature in step (ii) may already be the nitration temperature. It is however preferred that the temperature in step (i) and the starting temperature in step (ii) is selected in the range from 18-30°C, such as at ambient temperature (generally about 20-22°C) and the reaction mixture obtained in (ii) is subsequently heated up to the desired nitration temperature with all the preferences and definitions as given herein prior to the addition of the a,w-03-ioalkanediol, preferably of the 1 ,3-propanediol (i.e. before step (iii)).

In a further advantageous embodiment, the quenching of the reaction is performed either with cold water, such as with water having a temperature selected in the range from 0 to 20°C, more preferably in the range from 5 to 15 °C or with aqueous inorganic base such as in particular aqueous NaOH, KOH, Ca(OH)2 or NH4OH having a temperature selected in the range from (-20) to 15°C, more preferably in the range from (-10) to 10°C.

In a particular advantageous embodiment, the pH of the aqueous reaction mixture comprising the -nitrooxyC3-ioalkane-1-ol such as in particular 3-nitrooxypropanol in the quenching respectively neutralization step is adjusted to a pH of 8-12, more preferably to a pH of 9 to 1 1 , most preferably with aqueous sodium hydroxide (NaOH), potassium hydroxide (KOH) or aqueous ammonia (NH ₄ OH).

Preferably, in all embodiments of the present invention, during work-up, the w-nitrooxyalkane-l -ol, more preferably the 3-nitrooxypropanol is separated from residual starting material (i.e. the a, w-03-ioalkanediol, preferably the 1 ,3-propanediol) and from the by-product (i.e. a,w-03-ioalkanedioldinitrate, preferably the 1 ,3-propanediol dinitrate) by selective extraction.

Thus, the present invention also relates to a process (III) which is a process (I) or (II), which comprises an additional extraction step (vi) either after step (iii), step (iv) or step (v).

Particularly preferred in all embodiments of the present invention is a process (IV) comprising a selective extraction step (vi) after the quenching step (iv) respectively the neutralization step (v).

The pH of the aqueous mixture comprising the -nitrooxyC3-ioalkane-1-ol such as in particular 3-nitrooxypropanol prior to the selective extraction is preferably adjusted to a pH of 8-12, more preferably to a pH of 9 to 1 1 , most preferably with aqueous sodium hydroxide (NaOH), potassium hydroxide (KOH) or aqueous ammonia (NH4OH).

The selective extraction according to the present invention comprises the extraction of the aqueous reaction mixture obtained in step (iii), (iv) or (v) comprising the -nitrooxyC3- ioalkane-1-ol, preferably the 3-nitrooxypropanol, first with an apolar solvent, such as preferably an aliphatic or aromatic hydrocarbon solvent, more preferably an aromatic solvent such as xylene or toluene, most preferably toluene (to remove impurities/ by products such as e.g. the dinitrated a,w-03-ioalkanediols) followed by extraction with an ether solvent (e.g. a cyclic ether such as tetrahydrofuran or dioxane or an alkyl ether such as diethyl ether, dibutyl ether, diisobutyl ether or methyl tert.-butylether) such as in particular methyl tert-butylether (MtBE) (to extract the -nitrooxyC ₃ -ioalkane-1-ol such as in particular the 3-nitrooxypropanol).

It is furthermore preferred to wash the obtained ether phase with water to ensure complete removal of the a, -C3-ioalkanediol, preferably the 1 ,3-propanediol. Following evaporation of the ether solvent (optionally after a drying step with e.g. with MgSC ), pure (o-nitrooxyC ₃ -ioalkane-1-ol such as in particular the pure 3-nitrooxypropanol is obtained. Even more preferably, the apolar solvent used in the selective extraction such as in particular the hydrocarbon, even more in particular the aromatic hydrocarbon solvent phase is re-extracted with an aqueous solvent, such as e.g. the aqueous solution obtainable from washing the ether phase to further increase the yield of the -nitrooxyC ₃ - ioalkane-1-ol.

The reaction of the process according to the invention can in principle be carried out in any reactor suitable for the respective reaction type. Without restricting generality, the following are mentioned by way of example: suspension reactor, stirred tank, stirred tank cascade, tubular reactor, tubular reactors containing mixing elements like static mixers, shell-type reactor, shell and tube reactor, reactive distillation column. The aqueous reaction mixture obtained in step (iv) or (v) as outlined above may be supplemented directly to ruminants such as in particular cattle due to economic reasons.

As such aqueous solutions of the -nitrooxyC ₃ -ioalkane-1 -ol, preferably the 3-nitrooxypropanol are still novel, the present invention is also concerned with an aqueous solution obtainable from step (iv) or (v) as outlined above as well the use thereof to supplement ruminants such as in particular cattle to reduce methane formation in said ruminants. Such aqueous solutions are characterized by a high stability, a low toxicity and an increased cost efficiency as the -nitrooxyC3-ioalkane-1 -ol does not need to be isolated before being supplemented to the ruminant.

In a particular advantageous embodiment such aqueous solutions (all wt.-% are all based on the total weight of the aqueous solution) consist essentially of

(a) 1 to 30 wt.-%, preferably 1 to 15 wt.-%, more preferably 2 to 10 wt.-%, most preferably 5 to 9 wt.-% of a a, -C3-ioalkanediol, preferably of 1 ,3-propandiol, (b) 2.5 to 40 wt.-%, preferably 5 to 30 wt.-%, more preferably 5 to 20 wt.-%, most preferably 7.5 to 20 wt.-% of a -nitrooxyC ₃ -ioalkane-1-ol, preferably of 3-nitrooxypropanol, (c) 0.1 to 5 wt.-%, preferably 0.3 to 3 wt.-%, most preferably 0.5 to 2 wt.-% of a a,w - dinitrooxyC3-ioalkane, preferably of 1 ,3 dinitrooxypropane,

(d) 0.1 to 10 wt.-%, preferably 0.1 to 6 wt.-%, more preferably 0.3 to 4 wt.-%, most preferably 0.5 to 2 wt.-% of a nitrite trapping agent, preferably of sulfamic acid or urea,

(e) 5 to 30 wt.-%, preferably 10 to 25 wt.-%, most preferably 15 to 20 wt.-% of a nitrate, preferably sodium nitrate,

(f) 0.1 to 5 wt.-%, preferably 0.3 to 3 wt.-%, most preferably 0.5 to 2 wt.-% of a base, preferably sodium hydroxide, and

(g) 30 to 80 wt.-%, preferably 40 to 70 wt.-%, most preferably 50 to 65 wt.-% of water.

Preferred aqueous solutions (all wt.-% are all based on the total weight of the aqueous solution) according to the present invention consist essentially of

(a) 5 to 10 wt.-% of 1 ,3-propanediol,

(b) 5 to 20 wt.-% of 3-nitrooxypropanol,

(c) 0.5 to 3 wt.-% of 1 ,3 dinitrooxypropane,

(d) 0.5 to 4 wt.-% of urea,

(e) 15 to 20 wt.-% of sodium nitrate,

(f) 0.5 to 3 wt.-% of sodium hydroxide, and

(g) 50 to 65 wt.-% of water.

Most preferred aqueous solutions (all wt.-% are all based on the total weight of the aqueous solution) according to the present invention consist essentially of

(a) 5 to 9 wt.-% of 1 ,3-propandiol,

(b) 5 to 20 wt.-% of 3-nitrooxypropanol,

(c) 0.5 to 2 wt.-% of 1 ,3-dinitrooxypropane,

(d) 0.5 to 2 wt.-% of urea,

(e) 15 to 20 wt.-% of sodium nitrate,

(f) 0.5 to 2 wt.-% of sodium hydroxide, and

(g) 50 to 65 wt.-% of water.

The term‘consist essentially of as used according to the present invention means that the amounts of the ingredients (a) to (g) sum up to 100 wt.-%. It is, however, not excluded that small amount of impurities or additives may be present which are, for example, introduced via the respective raw materials of the ingredients (a) to (g). The following examples are used to further illustrate the invention without being limited thereto.

Example 1 : Preparation of 3-nitrooxypropanol (3-NOP)

A ) Nitrate ester formation

The nitration was carried out in a double-jacket reactor that was kept open, preferably with a light nitrogen flow above the reaction during the whole nitration process. Nitric acid (3-14 equivalents based on the 1 ,3-propanediol as indicated in table 1 ) was loaded into the reactor and stirred for 15 min at 20 °C. Urea (0.3 eq) was added and stirring was continued for 30 min. Meanwhile, the jacket temperature was set to the reaction temperature (20-90 °C). Then 1 ,3-propanediol (1 eq) was dosed into the acid during 15 min via a dropping funnel and the reaction was stirred for 60-120 min. Then the reaction was quenched by pouring it onto water cooled to 10-15 °C in an amount to dilute the nitric acid to -20%. The quenched reaction solution was slowly adjusted to pH 9-11 with 30 % aq. NaOH. Alternatively, the reaction was quenched by pouring it onto 20 % aqueous NaOH cooled to (-10)-10 °C such as to obtain an aqueous solution of pH 9-11.

B) Work-up (Selective Extraction)

The aqueous solutions obtained in step A were subsequently extracted with toluene followed by MtBE. The MtBE-phases were washed with water and evaporated to dryness to obtain 3-nitrooxypropanol (1 ,3-propanediol mononitrate) as a colorless to slightly yellow oil.

Table 1

* extensive dinitration, reaction slightly instable during 1 ,3-propanediol addition

* isolated yield ~ 1-10% lower compared to in-process yield

Synthesis of 3-nitrooxypropanol (PDMN) as outlined above using the conditions in entry Table 1 , entry 5 (4 eq. 65% HNO3, 70 °C, 60 min) with a quench onto 20 % NaOH resulted in 52 % yield on a 25 g scale.

C.) Composition of the neutralized reaction mixture

(conditions: Table 1 , entry 5, quench onto 20 % NaOH)

Example 2

Synthesis of 3-nitrooxypropanol (PDMN) as outlined above (4 eq. 65% HNO ₃ , 60 °C, 90 min) with 0.3 eq. sulfamic acid instead of urea resulted in 34 % isolated yield. Example 3

The example as outlined above (4 eq 65% HNO ₃ , 60 °C, 90 min) was repeated with 1 ,4-butanediol instead of 1 ,3-propanediol resulting in 19% isolated yield of 4-nitrooxybutanol.