A New Strategy for the Synthesis of Monomethylhydrazine Using the Raschig Process

A new strategy for the development of monomethylhydrazine (MMH) using the Raschig process is discussed in this publication. The determination of optimal conditions for the synthesis of MMH required the development of a kinetic model. In order to validate the results of the calculation, a device for synthesis under pressure (13 bar) by direct injection of the stoichiometric monochloramine, synthesized using microreactor technology, into monomethylamine (MMA) was developed. The experimental data, in accordance with the theoretical results of the kinetic model, make it possible to establish a new process for the synthesis of monomethylhydrazine.


Introduction
The objective of this work is to develop a new strategy for the preparation of monomethylhydrazine (MMH) by intensification of the Raschig process, using microreactor technology. This compound is today a molecule with great potential in the field of propulsion, the pharmaceutical industry and agrochemistry. In the first part of this study already published [1], it was concluded that, given the instability of stoichiometric monochloramine solutions from 36˚C and in order to control heat exchanges, the microreactor technology will be the best choice to avoid any sudden degradation leading to the formation of nitrogen chloride NCl 3 . In the second part [2], the optimal conditions for the synthesis of stoichi-ometric monochloramine were established, using this microreactor technology. This third part concerns the synthesis of monomethylhydrazine (MMH) using the monochloramine synthesized via the microreactor technology. As a first step, it was necessary to establish the different reactions involved in the process. A kinetic model was then formulated. The influence of the monomethylamine/monochloramine initial molar ratio on MMH yield was determined by integrating the differential equation system of this model. Finally, experimental tests of the synthesis of MMH under pressure were carried out.

Specific Device
The experiments were carried out in a specific device, the schema of which is presented in Figure 1 The reactor is heated by a 500 W electric furnace insulated under a collar-shaped stainless steel jacket. It is fixed by flanges allowing it to slide easily along the reactor. The temperature of the reaction medium is maintained at the set temperature by means of a PID-type controller that supplies the furnace via a 25 Ampere solid-state relay. The device is completed by a mechanical agitator to ensure a homogeneous synthesis solution, a bottle of anhydrous MMA and a tank for the chloramine solution, without forgetting a thermometer, a manometer, a balance and a dosing pump.

Experimental Protocol
Before each experiment, the device is dried with nitrogen and then sealed. The tank containing the pre-weighed anhydrous MMA is then connected to the  Figure 1). The transfer of the anhydrous MMA is carried out by cryogenic trapping after quenching the autoclave in a Dewar filled with liquid air. When the temperature reaches −50˚C to −60˚C, the vaporized MMA is introduced slowly at a rate of 1 g•min −1 . The variation of the apparent mass of the MMA tank allows control of the speed of introduction as well as the quantity introduced. At the end of the operation, the MMA container is disconnected from the device, dried and weighed again to determine the exact amount of reagent introduced.
After removal of the cold source from the central reactor, the reactor is placed in the electric furnace and the temperature is set at 70˚C. When thermal equilibrium is reached, the pressure in the reactor is 13.3 bar (saturated vapor pressure of MMA at 70˚C). The stirring system is then switched on. The freshly prepared and alkalized chloramine solution is then introduced into the reactor under pressure using the dosing pump. Agitation is maintained until the reactions are completed.
At the end of the synthesis, the reactor is cooled to room temperature after extraction from the furnace. The synthesis solution is then analyzed by HPLC and UV-Visible.

UV spectrometry
The spectrophotometer used was an Agilent Cary 100 dual-beam spectrophotometer equipped with the Cary WinUV data acquisition system. It allows a repetitive scanning of spectra between 180 nm and 900 nm, programmable as a function of time, and measurements of optical density or its derivatives at a given wavelength. Measurements were made with Hellma ® brand Suprasil ® quartz cells, model 100-QS with a 10 mm optical path, to ensure optimal transmission of UV signals.
As MMH is a molecule transparent to ultraviolet light, its dosage is carried out indirectly, based on the quantitative formation of formaldehyde monomethylhydrazone (FMMH), by condensation of MMH on formaldehyde. Hydrazine Chemical products: The permuted water used is city water purified by passing over an ion exchange resin. The inorganic salts and organic solvents used are of commercial purity (minimum 98%) and are supplied by Acros Organics, Merck and Sigma-Aldrich. They were used without prior purification unless otherwise indicated. Aqueous solutions of sodium hypochlorite NaOCl and sodium hydroxide are supplied by Arkema (Jarrie Plant, Grenoble, France). The aqueous solution, measuring approximately 48 chlorometric degrees (2.4 mol•L −1 ), is stored at 5˚C and systematically titrated before use. Anhydrous MMA is more than 98% pure and is supplied by Sigma-Aldrich.

Monomethylhydrazine Formation
The formation of MMH corresponds to the general scheme of the Raschig process: The constant k 1 was determined in the laboratory from the rate of MMH formation at the time t = 0. It is written as a function of the NH 2 Cl (monochloramine) and CH 3 NH 2 (MMA) initial molar concentrations [4]: where λ 1 and λ 2 are empirical constants for both phenomena. They obey the law of Arrhenius and can be written in the form:

NH2Cl/MMH Reaction
The kinetics of the monochloramine oxidation reaction of MMH was studied as a function of temperature and pH. The main product of the reaction is formaldehyde monomethylhydrazone (FMMH) [4]. to unity and the main product of the reaction is the FMMH. Limited quantities of methanol (CH 3 OH) and methyl-chloride (CH 3 Cl) are also produced.
The reaction appears to be the combination of two reaction mechanisms, one of which is the seat of an acid catalysis phenomenon: The numerical values of the constants 2 λ and 2 λ′ as a function of temperature are as follows: These data allow to describe in a satisfying way the first moments of the MMH synthesis but the gap between calculated and experimental results increases with pH and temperature. This behaviour had already been reported by Ferriol et al. [4]. Taking up their experimental results, Giudice [5] proposed the following empirical expression for the constant k 2 :

Halogen Exchange Reaction between Monochloramine and
Monomethylamine A laboratory study [6] showed that the reaction between chloramine and an amine also led to the formation of an alkylchloramine. This transfer reaction leads to a state of equilibrium.
In a basic medium, the reaction for the formation of methylchloramine is of the first order compared to NH 2 Cl and CH 3 The term C 0 is the reference concentration. It is conventionally equal to 1 mol•L −1 .
-K ai : acidity constant of the species i; The experimental values of λ 3i (i = 0, 1, 2) at 25˚C are as follows: In very basic medium (pH > 13), the constant k 3 follows the Arrhenius law with an activation energy E 3 of 59.8 kJ•mol −1 and a pre-exponential factor A 3 of 6.25 × 10 7 L•mol −1 •s −1 [6].

Formation and Degradation Reactions of Hydrazine N2H4
The reactions involved in the formation and degradation of hydrazine N 2 The overall oxidation reaction of hydrazine N 2 H 4 by monochloramine is also the result of two mechanisms, one of which is pH-independent, the second catalyzed by H + ions. Based on studies carried out in our laboratory several years ago, the following expression for the constant k 5 was obtained: ( )

Decomposition of Monochloramine in Basic Medium
The decomposition of monochloramine in basic medium was studied by Anbar D. M. Le et al. et al. [9]. According to these authors, the process begins with a step of reaction hydrolysis:

Formulation of the Kinetic Model
Based on the above analysis, the main reactions that affect the yield during the synthesis of MMH by the monochloramine route are as follows: The knowledge of the instantaneous concentrations of the main entities and the predictive calculation of the yield as a function of the reagent concentrations, pH and temperature are linked to the resolution of the system of differential equations defined by the laws of reaction rates. By designating x, a, u 1 , s, z, u 2 , b and y respectively the concentrations of monochloramine, MMA, MMH, FMMH, NH 3 , N 2 H 4 , OH − , NaCl, the system is written: with initial conditions (at t = 0): It has been solved numerically by the method of Runge-Kutta of the 7 th order. This method was chosen because it does not require [11] any additional functions, any further differentiation, any additional initial value.
In this work, the calculations were performed using the Maple computer algebra software.

Integration of the System of Differential Equations
The system was integrated according to the [MMA] 0 /[NH 2 Cl] 0 ratio and pH. Figure 2 shows the MMH yield at 70˚C as a function of the ratio of initial concentrations and various final NaOH concentrations. Figure 2 shows that the yield increases with the ratio of initial concentrations [MMA] 0 /[NH 2 Cl] 0 . This phenomenon is related to an increase in the rate of MMH formation at the expense of the oxidation reaction. For a ratio of 10 and a final NaOH concentration of 0.7 mol•L −1 , it is not possible to exceed a yield of 71%. It is therefore necessary to use a ratio close to 20 and a final NaOH concentration higher than 0.7 mol•L −1 .

Synthesis of MMH under Pressure
Experiments were carried out using stoichiometric monochloramine to control the model and verify the absence of hydrazine N 2 H 4 in the crude synthetis solutions.

Results
The kinetic model described above has been integrated to calculate the theoretical MMH and FMMH yield and the final [MMH]/[N 2 H 4 ] ratio. The integration of the system of differential equations was performed using the Maple computer algebra software.
We observe that the computed results are in fairly good agreement with the experimental results since the difference between the calculated and experimental yields is of ±5% as seen in Table 1 All the results of the synthesis and extraction segments allowed the development of a simplified schematic diagram presented in Figure 3. It corresponds to the objectives set, namely, the use of stoichiometric chloramine and a reduction in unit operations.

Conclusion
In the end, the study of the main reactions that affect yield during the synthesis