A process for continuous and selective MONO-nitration of aromatic hydrocarbons without using mixed acid

Abstract

Disclosed is a continuous-flow process for selective mono-nitration of aromatic hydrocarbons and polyaromatic hydrocarbons without using mixed acid. The process contacts a stream of aqueous nitric acid, preferably at or near the nitric-acid / water azeotrope (about 68–69 wt.% HNO3), with a stream containing an aromatic substrate in a continuous-flow reactor under controlled temperature and back-pressure to provide a residence time sufficient to achieve high conversion while suppressing di- and poly-nitration. The reactor effluent is phase-separated to recover (i) an organic phase comprising a mono-nitro aromatic product and optionally unreacted substrate, and (ii) an aqueous phase comprising unreacted and / or diluted nitric acid. The aqueous phase is optionally reconcentrated (e.g., by distillation) to a near-azeotropic nitric acid composition for recycle to the reactor feed. The process provides high selectivity to mono-nitro products with improved safety and reduced acidic waste relative to sulfuric-acid-based mixed-acid nitration.

Description

[0001] TITLE OF THE INVENTION

[0002] A PROCESS FOR CONTINUOUS AND SELECTIVE MONO-NITRATION OF AROMATIC HYDROCARBONS WITHOUT USING MIXED ACID

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to aromatic nitration. In particular, it relates to a continuous process for selective mono-nitration of aromatic and polyaromatic hydrocarbons using aqueous nitric acid without sulfuric acid or other mixed-acid systems, and with optional recycle of nitric acid and unreacted substrate.

[0005] BACKGROUND

[0006] Nitration reactions are widely used to introduce nitro groups into aromatic substrates to produce intermediates for explosives, dyes, pharmaceuticals, agrochemicals, and other products. Industrial nitration is traditionally carried out using a mixed-acid system comprising concentrated nitric acid and concentrated sulfuric acid. While effective, mixed-acid nitration presents significant drawbacks, including highly corrosive conditions, large volumes of spent sulfuric acid, NOx formation, energy-intensive acid recovery, and substantial safety risks due to the strongly exothermic nature of nitration and the potential for runaway reactions. Alternative approaches have been investigated, including solid-acid catalysis, ionic liquids, and microreactor-based nitration. Many reported approaches still rely on sulfuric acid, require expensive catalysts, use fuming nitric acid, or provide insufficient conversion and / or selectivity at conditions compatible with industrial scale. Accordingly, there remains a need for a conversion-efficient, selective, and safer nitration process that avoids mixed acids and facilitates recycle of nitric acid to minimize waste.

[0007] SUMMARY

[0008] In one aspect, a continuous process is provided for selectively mono-nitrating an aromatic substrate without using mixed acid. The process comprises: (a) feeding an aqueous nitric acid stream to a continuous-flow reactor;(b) feeding a stream comprising an aromatic substrate to the reactor; (c) contacting the streams in the reactor at a controlled temperature and back-pressure to provide a defined residence time and to produce a reactor effluent comprising an organic phase containing a mono-nitro aromatic product and an aqueous phase containing nitric acid; (d) phase separating the effluent to recover the organic phase and the aqueous phase;

[0009] (e) optionally washing and / or purifying the organic phase to obtain a mono-nitro aromatic product; and (f) optionally recycling at least a portion of the aqueous phase (and optionally unreacted substrate) to the reactor feed, optionally after reconcentrating the aqueous phase to a near-azeotropic nitric acid composition.

[0010] In certain embodiments, the nitric acid concentration is in the range of about 5-80 wt.% HNO3 in water, preferably about 60-80 wt.% and more preferably about 68-69 wt.% HNO3. In certain embodiments, the reactor temperature is 0-150 °C, the back-pressure is 1-50 bar (e.g., 3-50 bar), and the residence time is 0.5-10 minutes (e.g., 30 seconds to 7 minutes). In certain embodiments, the process provides >99% selectivity to mono-nitrated products with substantially complete conversion for selected substrates.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a schematic process flow diagram illustrating an example continuous-flow setup including feed pumps, heat exchangers, a continuous-flow reactor with a back-pressure regulator, phase separation, washing, and recycle of nitric acid and unreacted substrate.

[0013] DETAILED DESCRIPTION

[0014] Unless otherwise indicated, percentages are by weight (wt.%) and pressures are gauge pressures relative to atmospheric pressure.Definitions

[0015] • “Continuous-flow reactor” refers to a reactor configured for continuous introduction of reagents and continuous withdrawal of reaction mixture, including microreactors and millimetric reactors, such as tubular, plate-type, and coil-type reactors, optionally including static mixing elements.

[0016] • “Aqueous nitric acid” includes nitric acid solutions from about 5 wt.% to about 80 wt.% HNO3 in water, including near-azeotropic nitric acid-water compositions (about 68-69 wt.% HNO3).

[0017] • “Aromatic substrate” includes unsubstituted or substituted aromatic hydrocarbons and polyaromatic hydrocarbons that undergo electrophilic nitration, including benzene, toluene, xylenes, ethylbenzene, cumene, naphthalene, and halo-substituted aromatics such as chlorobenzene.

[0018] In an embodiment, a first feed stream comprising aqueous nitric acid is delivered to a continuous-flow reactor by a pump compatible with nitric acid. A second feed stream comprising an aromatic substrate is delivered to the reactor by a separate pump. The streams may be preconditioned (heated or cooled) prior to contacting. The streams are contacted in the reactor (optionally via a micromixer) and reacted at a selected temperature and back-pressure to control the reaction rate and to maintain the reaction mixture in the desired phase condition. The residence time is selected to maximize mono-nitration while suppressing further nitration.

[0019] Typical operating ranges include: temperature from 0 to 150 °C (e.g., 40 to 150 °C), back-pressure from 1 to 50 bar (e.g., 3 to 50 bar), and residence time from 0.5 to 10 minutes (e.g., 30 seconds to 7 minutes). The molar ratio of nitric acid to aromatic substrate may be selected from about 10:1 to about 1:10 depending on substrate reactivity and target conversion.

[0020] Phase Separation and Product Workup The reactor effluent is cooled as required and depressurized through a back-pressure regulator. The effluent, typically a biphasic liquid-liquid mixture, is separated in a phase separator (e.g., decanter, centrifuge, or membrane separator) into (i) anorganic phase comprising mono-nitro aromatic product and optionally unreacted substrate, and (ii) an aqueous phase comprising nitric acid and water. The organic phase may be washed with water and / or brine and further purified (e.g., by distillation) to obtain a mono-nitro aromatic product. Unreacted substrate may be recovered and recycled.

[0021] Nitric Acid Recycle In certain embodiments, the aqueous phase is reconcentrated by distillation to return the nitric acid composition to a near-azeotropic composition (about 68-69 wt.% HNO3). Water removed overhead may be discarded or reused. The reconcentrated nitric acid stream may be recycled to the nitric acid feed, reducing consumption of fresh acid and minimizing waste.

[0022] Reactor and Mass / Heat Transfer Considerations In certain embodiments, the continuous-flow reactor is selected to provide enhanced heat transfer and mixing, including a heat exchange coefficient in the range of about 1 to 1000 kW / m3K and a liquid-liquid volumetric mass transfer coefficient (kLa) in the range of about 0.1 to 10 s '. The reactor may include passive mixing features and a heat-transfer jacket to maintain the desired reaction temperature.

[0023] Optional Solvents When a solvent is used to deliver the aromatic substrate or to manage phase behavior, the solvent is preferably resistant to hydrolysis, nitration, and oxidation under the reaction conditions. Non-limiting examples include ethylene dichloride, methylene dichloride, o-dichlorobenzene, carbon tetrachloride, chloroform, and tetrachloroethane .

[0024] Industrial Applicability The disclosed process is suitable for continuous manufacture of mono-nitro aromatic compounds with improved safety and reduced acidic waste relative to mixed-acid nitration, and is compatible with industrial scale-up using continuous-flow equipment.Examples

[0025] The following examples illustrate the invention and are not intended to limit the scope of the claims.

[0026] Example 1 - Representative Mono-Nitration Conditions for Various Substrates

[0027] Using a continuous-flow setup generally as shown in Figure 1, a stream of aqueous nitric acid (~68 wt.% HNO3) and a stream comprising the aromatic substrate were pumped to a continuous-flow reactor operated at the conditions shown in Table 1.

[0028] The effluent was phase-separated and the organic phase was washed and purified.

[0029] Conversion and mono-nitration selectivity were determined by GC-FID.

[0030]

[0031] Table 1: Representative operating conditions and results.

[0032] Example 2 - Effect of Operating Parameters on Benzene Mono-Nitration

[0033] Benzene was nitrated as described above while varying temperature, back-pressure, molar ratio, and residence time. Representative results are shown in Table 2.

[0034]

[0035] Table 2: Representative benzene results illustrating effects of operating parameters.Example 3 - Mono-Nitration of Toluene

[0036] Toluene was nitrated under continuous-flow conditions using aqueous nitric acid (~68 wt.%). Representative results are shown in Table 3. The mono-nitrated products comprised predominantly o -nitrotoluene.

[0037]

[0038] Table 3: Representative results for toluene mono-nitration.

[0039] Example 4 - Recycle of Nitric Acid and Substrate

[0040] After phase separation, the aqueous nitric acid stream was reconcentrated by distillation to a near-azeotropic composition and recycled. Unreacted benzene was recovered by distillation and recycled. Representative results are shown in Tables 4 and 5.

[0041] <

[0042]

[0043]

[0044] Advantages

[0045] 1. Elimination of sulfuric acid from the nitrating system simplifies chemical handling and materials-of-construction requirements, while reducing operationalhazards associated with highly corrosive mixed-acid inventories, accidental releases, and acid-mist / fiime evolution from spent acid mixtures.

[0046] 2. Removal of the sulfuric acid co-acid eliminates generation of a dedicated spent sulfuric acid (or mixed-acid) effluent stream, thereby substantially reducing downstream neutralization, treatment, and disposal burdens.

[0047] 3. Continuous-flow nitration under controlled hydrodynamics reduces batch-related downtime (charging, temperature stabilization, reaction hold, and workup), enabling higher space-time yield, improved throughput, and stable steady-state operation for aromatic hydrocarbon nitration.

[0048] 4. Intensified mixing and near-isothermal operation improve mass- and heattransfer coefficients, suppressing concentration / temperature gradients that promote side reactions. The system further enables rapid quench and / or instantaneous heat removal, limiting post-reactor nitration, over-nitration, and oxidative degradation pathways.

[0049] 5. Precisely defined residence-time distribution (typically 1-10 min) allows tight control of kinetic exposure of the aromatic substrate to the nitrating species, improving selectivity toward the mono-nitrated product and mitigating uncontrolled progression to di- and tri -nitrated derivatives.

[0050] 6. Reduced waste intensity relative to conventional mixed-acid nitration: conventional processes generate large volumes of sulfate-containing spent acid that are costly and environmentally complex to neutralize, regenerate, or recycle. A nitric-acid-only configuration reduces or eliminates sulfate-derived waste and associated treatment infrastructure.

[0051] 7. Use of azeotropic or near-azeotropic HNO3-H2O feeds (-68-69 wt.% HNO3) enables procurement of widely available industrial nitric acid and permits efficient reconcentration of the diluted acid phase (e.g., via simple distillation) for recycle. This improves atom economy and reagent utilization, reduces fresh acid consumption, and enhances overall process sustainability and economics.

[0052] 8. Improved environmental and regulatory profile: a more contained process with fewer and simpler waste streams (notably absence of sulfate-rich effluents)supports compliance with stringent discharge norms and can reduce regulatory complexity associated with hazardous waste generation and off-site disposal.

Claims

We claim;1. A continuous process for selectively mono-nitrating an aromatic substrate without using mixed acid, the process comprising: (a) feeding an aqueous nitric acid stream comprising about 5 to about 80 wt.% HNOs in water to a continuous-flow reactor; (b) feeding a stream comprising an aromatic substrate to the continuous-flow reactor;(c) contacting the streams in the continuous-flow reactor at a temperature of about 0 to about 150 °C and a back-pressure of about 1 to about 50 bar for a residence time of about 0.5 to about 10 minutes to form a reactor effluent comprising an organic phase containing a mono-nitro aromatic product and an aqueous phase containing nitric acid; and (d) phase separating the reactor effluent to recover the organic phase and the aqueous phase, wherein the process is configured to provide predominant mono-nitration of the aromatic substrate.

2. The process of claim 1, wherein the aqueous nitric acid stream comprises about 60 to about 80 wt.% HNO3 in water.

3. The process of claim 1, wherein the aqueous nitric acid stream comprises a near-azeotropic nitric acid-water composition comprising about 68 to about 69 wt.%HNO3.

4. The process of claim 1, wherein the temperature is about 40 to about 150 °C.

5. The process of claim 1, wherein the back-pressure is about 3 to about 50 bar.

6. The process of claim 1, wherein the residence time is about 30 seconds to about 7 minutes.

7. The process of claim 1, wherein a molar ratio of nitric acid to aromatic substrate is about 10: 1 to about 1:10.

8. The process of claim 1, wherein the continuous-flow reactor is a tubular reactor, a plate-type reactor, or a coil-type reactor.

9. The process of claim 1, wherein the continuous-flow reactor provides a heat exchange coefficient of about 1 to about 1000 kW / m3K.

10. The process of claim 1, wherein the continuous-flow reactor provides a liquid-liquid volumetric mass transfer coefficient (kLa) of about 0.1 to about 10 s '.

11. The process of claim 1, wherein the aromatic substrate is selected from benzene, toluene, xylene, ethylbenzene, cumene, naphthalene, and chlorobenzene.

12. The process of claim 1, further comprising washing the organic phase with water and / or brine to reduce residual acidity.

13. The process of claim 1, further comprising purifying the organic phase by distillation to recover unreacted aromatic substrate for recycle and to obtain the mono-nitro aromatic product.

14. The process of claim 1, further comprising reconcentrating at least a portion of the aqueous phase by distillation to provide a recycle nitric acid stream having a near-azeotropic nitric acid composition.

15. The process of claim 1, further comprising recycling at least a portion of the aqueous phase to the aqueous nitric acid stream fed to the continuous-flow reactor.

16. The process of claim 1, further comprising recycling at least a portion of unreacted aromatic substrate recovered from the organic phase to the stream comprising the aromatic substrate fed to the continuous-flow reactor.

17. The process of claim 1, wherein contacting the streams comprises mixing the streams in a micromixer upstream of the continuous-flow reactor.

18. The process of claim 1, wherein the back-pressure is maintained using a back-pressure regulator downstream of the continuous-flow reactor.

19. The process of claim 1, wherein the stream comprising the aromatic substrate further comprises a solvent resistant to hydrolysis, nitration, and oxidation under the reaction conditions.

20. The process of claim 19, wherein the solvent is selected from ethylene dichloride, methylene dichloride, o-dichlorobenzene, carbon tetrachloride, chloroform, and tetrachloroethane.

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