Conversion of Ammonia to Hydrazine Induced by High-Frequency Ultrasound

Article abstract of DOI:10.1002/anie.202109516

Hydrazine is a chemical of utmost importance in our society, either for organic synthesis or energy use. The direct conversion of NH₃ to hydrazine is highly appealing, but it remains a very difficult task because the degradation of hydrazine is thermodynamically more feasible than the cleavage of the N?H bond of NH₃. As a result, any catalyst capable of activating NH₃ will thus unavoidably decompose N₂H₄. Here we show that cavitation bubbles, created by ultrasonic irradiation of aqueous NH₃ at a high frequency, act as microreactors to activate and convert NH₃ to NH species, without assistance of any catalyst, yielding hydrazine at the bubble–liquid interface. The compartmentation of in-situ-produced hydrazine in the bulk solution, which is maintained close to 30 °C, advantageously prevents its thermal degradation, a recurrent problem faced by previous technologies. This work also points towards a path to scavenge ^.OH radicals by adjusting the NH₃ concentration.

Full text of DOI:10.1002/anie.202109516

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Accepted Article
Title: Conversion of ammonia to hydrazine induced by high frequency
ultrasound
Authors: François Jérôme, Anaelle Humblot, Laurie Grimaud, Audrey
Allavena, Prince N. Amaniampong, Karine De Oliveira Vigier,
Tony Chave, and Stéphane Streiff
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202109516
Link to VoR: https://doi.org/10.1002/anie.202109516

Angewandte Chemie International Edition
10.1002/anie.202109516
COMMUNICATION
Conversion of ammonia to hydrazine induced by high frequency
ultrasound
[
a]
[a]
[a]
[a]
Anaelle Humblot, Laurie Grimaud, Audrey Allavena, Prince N. Amaniampong, Karine De Oliveira
[
a]
[b]
[c]
[a]
Vigier, Tony Chave, Stéphane Streiff, and François Jérôme*
[
a]
Anaelle Humblot, Laurie Grimaud, Audrey Allavena, Dr Prince N. Amaniampong., Prof Karine De Oliveira Vigier., Dr François Jérôme
Institut de Chimie des Milieux et Matériaux de Poitiers,
University of Poitiers, CNRS
1
rue Marcel Doré, 86073 Poitiers, France
E-mail: francois.jerome@univ-poitiers.fr
[
[
b]
c]
Dr Tony Chave
Univ Montpellier, CNRS, UMR 5257, ICSM, CEA, UM, ENSCM, Marcoule, France
Dr Stéphane Streiff
Eco-Efficient Products and Process Laboratory, SOLVAY/CNRS 3966 Jin Du Rd., Xin Zhuang Industrial Zone, Shanghai 201108, China
Supporting information for this article is given via a link at the end of the document
Abstract: Hydrazine is a chemical of outmost importance in our
thermodynamically more favorable (G = -150 kJ/mol) than the
conversion of NH (G = 16.5 kJ/mol) or N /H O (G = 184
society, either for organic synthesis or energy purpose. The direct
3
2
2
conversion of NH
3
to hydrazine is highly appealing but it remains a
kJ/mol) to hydrazine, making the accumulation of hydrazine
scientifically challenging.[5]
very difficult task because the degradation of hydrazine is
thermodynamically more feasible than the cleavage of the N-H bond
Physical activation of the N-H and N-N bond of NH
3
and N
2
[7]
,
respectively, at low temperature with electron beam,[ plasma,
6]
of NH
unavoidably decompose N
created by ultrasonic irradiation of aqueous NH
act as micro-reactors to activate and convert NH
3
. As a result, any catalyst capable of activating NH
. Here we show that cavitation bubbles,
at a high frequency,
to NH species,
3
will thus
H
2 4
light,[8] or by the coupling of these technologies with catalysis such
as electrocatalysis for instance have been explored for the
synthesis of hydrazine.[5a, 9] Although fascinating results were
reported, the in situ decomposition of hydrazine cannot be
avoided, leading to the formation of very diluted feeds of
hydrazine (i.e. micromolar).
3
3
without assistance of any catalyst, yielding hydrazine at the bubble-
liquid interface. The compartmentation of in situ produced hydrazine
in the bulk solution, which is maintained close to 30°C,
advantageously prevents its thermal degradation, a recurrent problem
faced by previous technologies. This work also points towards a path
High frequency ultrasound (HFUS), a technology based on
cavitation phenomenon, is gaining more and more interest in the
field of chemistry.[10] Due to the extreme conditions of pressures
and temperatures existing inside the cavitation bubbles, it was
shown by sonoluminescence spectroscopy that gaseous
to scavenge •OH radicals by adjusting the NH
3
concentration.
2 4
Hydrazine (N H ) is a chemical of outmost importance in the
chemical industry. The hydrazine hydrate market size was 356
million USD in 2015 and is expected to reach 530 million USD in
molecules, with high bond dissociation energies such as N
CO , CH , can be cleaved inside the cavitation bubbles, leading
to the formation of radicals and the occurrence of sonochemical
reactions.[11] So far, the activation of NH
by HFUS has been
investigated sporadically, and the rare previous studies were
mainly focused on the elimination of NH from waste water (by
2 2
, O ,
2
4
2024, mostly boosted by the growing need of our society for the
3
manufacture of polymer foams and agrochemicals, the actual two
biggest market shares of hydrazine.[1] Besides, the rapid
development of hydrazine hydrate fuel cells,[ nowadays
demonstrated with electric vehicles, should also open up new
perspectives for hydrazine in the future. Projections showed that
hydrazine is one of the chemicals expected to record immense
growth prospects across the forecast period of 2020-2030.[3]
3
2]
degassing, pyrolysis).[12] In 2017, a pioneer work of Pflieger
demonstrated, by sonoluminescence spectroscopy, that NH
species can be formed inside the cavitation bubbles, by subjecting
an aqueous solution of NH
at 359 kHz.[13] Inspired by these results, we investigate here the
sonochemical conversion of NH to N . Unlike previous works,
we show here that by adjusting the temperature of the solution
3
(0,17 wt%) to an ultrasonic irradiation
3
Hydrazine is industrially produced by partial oxidation of NH ,
3
2 4
H
either with hypochlorite or with peroxide.[4] In the widest spread
industrial processes (Bayer-ketazine and peroxide process),
hydrazine is trapped in situ under the form of a ketazine, by
reaction with a ketone, to prevent its decomposition (Fig. S1).
Ketazine is then hydrolyzed to release hydrazine and regenerate
and the concentration of NH
decomposition of hydrazine,
3
, it was possible to limit the
a
scientific and technological
problem faced by previous routes.
In a first set of experiments, NH
3
was continuously bubbled (30
3 2 2
the ketone. The direct production of hydrazine from NH or N /H O
is economically and environmentally highly attractive, but it
remains a very difficult task. One of the reason stems from the
mL/min) in 100 mL of water subjected to an ultrasonic irradiation
at 525 kHz, with an acoustic power of 0.17 W/mL. The aqueous
solution of NH was maintained at 30°C using a cooling system
3
coupled to the ultrasonic reactor (Fig. S2). Hydrazine was titrated
by spectrophotometry and results were doubly confirmed by
HPLC titration (Fig. S3-S7). Hereafter, the formation rate of
hydrazine is given with an uncertainty of 5%.
high N-H and N-N bond dissociation of NH
3 2
(435 kJ/mol) and N
(
946 kJ/mol), respectively, requiring harsh conditions of
temperature and pressure, which are not compatible with the
stability of hydrazine. Indeed, the decomposition of hydrazine is
1
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Angewandte Chemie International Edition
10.1002/anie.202109516
COMMUNICATION
reached inside the cavitation bubbles, thus finally quenching
sonochemical reactions.[14]
In order to get more insight on the reaction mechanism, we first
checked any possible role of the metallic reactor wall (316 L
stainless steel) on the hydrazine production. To this end, a similar
experiment was conducted by confining the aqueous solution of
3
0 mL/min 10 mL/min 10 mL/min NH
3
+ 20 ml/min other gases
1
1
1
8
6
4
2
,40E-01
,20E-01
,00E-01
,00E-02
,00E-02
,00E-02
,00E-02
NH NH
3
3
3
NH (5 wt%) in a polyethylene flask (material transparent to the
ultrasonic waves) immerged into the ultrasonic reactor filled with
water. No significant change in hydrazine production rate was
observed, ruling out a possible contribution of the metallic reactor
wall in the reaction mechanism (Fig. S9-S10). Next, the
concentration of hydrazine was plotted as a function of the
0,00E+00
NN HH
3
NN HH
3
HH ee
Gas
NN 2
OO 2
AA rr
3
3
2
2
reaction time (30°C, NH
3
at 30 mL/min, 0.21 W/mL). Very
interestingly, over a period of 20 h, the reaction never plateaued
and hydrazine was continuously produced (Fig. 3). At 20 h of
ultrasonic irradiation at 525 kHz, the concentration of hydrazine
reached 3.3 mmol/L, which is about 1000 times higher than in
previously reported works. These results also suggest that the
formation rate of hydrazine is faster than its decomposition, which
constitute one of the unique examples in the state of the art.
Figure 1. Influence of gases on the initial formation rate of N
2
H
4
(10 ml/min NH
3
+
20 ml/min gas, 525 kHz, 0.17 W/mL, 30°C). All reactions were conducted at
atmospheric pressure.
Pleasingly, under these conditions, hydrazine was continuously
-1
-1
produced at a rate of 0.12 mmol.L .h (Fig. S8). The dilution of
NH in gases such as O , N , Ar, or He resulted in a decrease of
(30°C)
3
2
2
the formation rate of hydrazine (Fig. 1). No correlation was
established between the physicochemical properties of the gas
(
thermal conductivity, heat capacity, etc…) and the formation rate
of N . As a result, we suspect that the formation rate of N is
more related to the solubility of NH in water. To support this claim,
aqueous NH solutions with different concentrations (from 0.1 to
0 wt%) were subjected to an ultrasonic irradiation (525 kHz)
under air (i.e. no bubbling of NH ). The plot of the formation rate
concentration is reported in
2
H
4
2 4
H
(60°C)
3
3
2
3
of hydrazine as a function of the NH
Fig. 2.
3
Figure 3. Concentration of hydrazine as a function of the reaction time (bubbling
of NH at 30 ml/min, 525 kHz, 0.21 W/mL) at 30°C and 60°C.
3
One of the explanations for this result stems from the low
temperature of the aqueous NH solution (30°C), preventing the
thermal decomposition of hydrazine. Indeed, under silent
conditions, hydrazine was found stable in aqueous NH (5 wt%)
3
3
at 30°C, while at 60°C a degradation was observed (Fig. S11). To
support this claim, the temperature of the ultrasonic reactor was
increased from 30 to 60°C. At 60°C, the formation of hydrazine
remained very low and reached only 0.073 mmol/L after 60 min,
(
Fig. 3), confirming the negative impact of the temperature on the
hydrazine production. Although high temperatures are reached in
the cavitation bubbles, the bulk aqueous solution of NH can be
Figure 2. Effect of the NH
kHz, 0.17 W/mL, 30°C).
3
concentration on the formation rate of hydrazine (525
3
maintained at 30°C, which represents a noticeable advantage of
this technology to prevent the thermal degradation of hydrazine.
The second explanation can be understood in terms of •OH
A volcano-type curve was obtained. The formation rate of
hydrazine gradually increased with a concomitant increase of the
radical scavenging by NH
3
. It has been previously demonstrated,
NH
mmol N
the formation rate of hydrazine declined, and was even inhibited
at a NH concentration over 15 wt%. This result can be
understood in terms of thermodynamics. The sonochemical
decomposition of NH is endothermic. Hence, when the
concentration of NH inside the cavitation bubbles is increasing, it
decreases the maximum temperature and pressure that can be
3
concentration from 0.1 to 7 wt%, to reach a maximum of 0.17
by sonoluminescence spectroscopy, that ultrasonic irradiation of
-1
-1
2
4 3 3
H .L .h at 7 wt% NH . At higher concentration of NH ,
an aqueous solution of NH
3
concomitantly generate NH and •OH
radicals.[13] •OH radicals are known to decompose hydrazine.
[15]
3
To assess the reactivity of in situ produced •OH radicals, we first
measured the amount of H
recombination of •OH radicals (Fig. S12, 13). Interestingly, when
the concentration of NH is over 1 wt%, no H was detected,
confirming that •OH radicals are quickly scavenged in situ (Fig.
2 2
O formed, resulting from the
3
3
3
2 2
O
2
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Angewandte Chemie International Edition
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COMMUNICATION
S14). Two scenarios are thus possible: (1) the scavenging of •OH
boiling point (182°C) and a low vapor pressure (0.2 mmHg at
20°C), making it improbable its diffusion inside the cavitation
bubble, as demonstrated in other reports.[18] Hence, phenol is a
suitable radical scavenger to selectively trap radicals at the
bubble-liquid interface. Interestingly, in the presence of phenol
radicals by NH
3
2
, to form •NH radical and water, or (2) the
scavenging of •OH radicals by hydrazine, inducing its partial
decomposition to N and H O. To discriminate between these two
scenarios, a solution of hydrazine (0.1 mmol/L) was subjected to
an ultrasonic irradiation either with or without NH . In line with
previous reports, without NH , hydrazine was nearly completely
decomposed after only 2 h of irradiation by in situ produced •OH
radicals (Fig. 4). In contrast, in an aqueous solution of NH (5
wt%), the concentration of hydrazine kept increasing, indicating
(Fig.4). A similar
2
2
3
3
(0.1 mol/L) in an aqueous NH solution (5 wt%), no hydrazine was
3
formed, suggesting that hydrazine is more likely to be formed at
the bubble-liquid interface. To further support this claim, sodium
benzoate was also tested as another radical scavenger, as it
reacts slower with radical species than phenol, allowing a better
monitoring of the reaction.[16, 19] In addition, as a salt, the vapor
pressure of sodium benzoate is negligible at 30°C. As expected,
an incremental increase of the amount of sodium benzoate led
also a gradual decrease of the amount of hydrazine, confirming
the results obtained with phenol (Fig.S16). Altogether, these
results strongly suggest that the formation of hydrazine occurred
at the bubble-liquid interface. Beside, hydrazine hydrate (64 wt%)
has a boiling point of 114°C and a vapor pressure of 7.5 mmHg
at 30°C. Hence, its diffusion inside the cavitation bubbles is
3
that •OH radicals are scavenged by NH
3
behavior was observed with 1 mmol/L of hydrazine (Fig. S15).
These results can be explained in terms of probability. Indeed, the
concentration of NH
that of hydrazine. As a result, the probability for the •OH radical
to react with NH is much higher than with hydrazine.
3
(1-5 wt% = 0.6-3 mol/L) is much higher than
3
negligible
and
thus
hydrazine
dominantly
remains
compartmented into the bulk solution at 30°C, where its thermal
decomposition is drastically limited. (Fig. 5).
Bulk solution
Bulk solution
NH3
N H
2
4
NH3
●
NH2
NH
Figure 4. Stability of hydrazine under ultrasonic irradiation with and without NH
initial hydrazine concentration = 0.1 mmol/L in water or in 5 wt% ammonia, 525
kHz, 30°C, 0.17 W/mL).
3
H O
2
●
(
Interior
cavitation
bubble
H
2
O + NH
2
●
OH
NH3
●
H
From these data, a plausible reaction mechanism can be
proposed. During the ultrasonic irradiation, NH and vapors of
H
2
3
N H
2
4
water diffuse within the cavitation bubbles to produce NH and •OH
radicals as observed elsewhere.[13] NH species can react either
with NH
OH radicals. These recombination reactions can occur either
inside the cavitation bubbles or at the cavitation bubble-liquid
interface. As the amount of H O is 18 times higher than that of
NH in our reactor (at 5 wt% NH we assume that the main
reaction taking place for NH is the reaction with H O. On the other
hand, and as discussed above, •OH radicals are rapidly trapped
by NH , the second largest abundant chemical in the HFUS
reactor, presumably to form •NH radicals and water, a reaction
which is thermodynamically favored (G = -46 kJ/mol).[16] In terms
of probability, the recombination of •NH radicals with •OH
radicals is less likely to occur, as •OH radicals are rapidly
scavenged by NH . To check this hypothesis, the as-obtained
3
to form two •NH
2
radicals or with H
2
2
O to form •NH and
H O
2
•
Bulk solution
2
3
3 ,
)
Figure 5. Proposed reaction pathway. For the sake of clarity, only reactions
taking place at the cavitation bubble-liquid interface are shown.
2
3
2
As the selectivity to hydrazine is governed by the concentration of
NH and N H into the reactor, one may question the maximum
3 2 4
2
concentration of hydrazine this technology may support. Using a
simple statistical model, and with the approximations that (1) the
rate of each radical reactions is similar and (2) the NH/OH ratio is
1, aqueous solution of hydrazine of 3.5 and 7.3 wt% could be
theoretically obtained with this technology from a 5 and 10 wt% of
3
aqueous solution was fully analyzed by HPLC, in particular to
detect the formation of nitrate. Indeed, it is known that •OH radical
can react with •NH
2
radical to form NH
2
OH which is instable and
aqueous NH
in the SI, Fig. S17-S19). If the NH/OH ratio now varies from 0.01
to 100, the N is expected to be produced within a concentration
range of 2.32-3.91 wt% and 4.72-8.08 wt% from a 5 and 10 wt%
of aqueous NH solution, respectively, which are typical
concentrations routinely handled in industry. The highest amount
of N is obtained at the highest NH/OH ratio (Fig. S20). It is
3
solution, respectively (more information is provided
further rapidly convert to nitrate.[17] As expected, with a bubbling
of NH (30 mL/min), the amount of nitrate remained below the
detection limit of our apparatus, supporting that the recombination
of •NH with •OH is not a dominant reaction. Hence, •NH radicals
has no other option than to dimerize to form N
3
2 4
H
2
2
3
2
4
H .
In an attempt to elucidate whether hydrazine is formed inside the
cavitation bubbles or at the bubble liquid interface, phenol was
added into the solution as a radical scavenger. Phenol has a high
2 4
H
noteworthy that at these concentration ranges, and in our
conditions (atmospheric pressure and 30°C), the risk of
3
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Angewandte Chemie International Edition
10.1002/anie.202109516
COMMUNICATION
detonation/flammability is very unlikely (no flash point of aqueous
hydrazine at these concentrations), water acting as an inerting
agent in this case.[20] More information on safety is discussed in
the supporting information.
With the reactor design in our hands, we are limited by the low
2 4
formation rate of N H , making it very long, and even not realistic,
the formation of 2-8 wt% aqueous solution of hydrazine. However,
the formation rate of hydrazine could be largely increased by
working in a continuous flow reactor, as the acoustic power
density will be locally much higher than in our reactor. To
demonstrate this possibility, the acoustic power density was
varied from 0.1 to 0.23 W/mL (Fig. 6). As expected, the formation
rate of hydrazine was directly impacted by the acoustic power and
the formation rate of hydrazine exponentially increased with the
acoustic power. This last result demonstrates that switching from
a batch to a continuous flow reactor could be a promising
Authors are grateful to the CNRS, the University of Poitiers and
the region Nouvelle Aquitaine for the funding of this study. AH is
also grateful to the CNRS for the funding of her PhD. Authors also
acknowledge Marie Jérôme for the statistic calculations. YingYing
Ma, Bright Kusema (Health Safety and Environment managers
from SOLVAY) and emeritus Prof Charles Kappenstein are also
acknowledged for the discussion on the safety of hydrazine.
Keywords: Ammonia • Cavitation bubbles • Hydrazine •
Radicals • Ultrasound
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bubbles act as micro-reactors to activate NH
3
to hydrazine. Cavitation
[9]
3
, resulting in the
formation of NH species that are further recombined to hydrazine
at the bubble-liquid interface. One of the advantage of this
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solution where the temperature is maintained at 30°C, thus
avoiding its thermal degradation, a recurrent problem faced with
previous technologies. In addition, we showed that adjusting the
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[
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Entry for the Table of Contents
Cavitation bubble
(activation NH
3
)
NH3
Bulk solution
(30 C, Patm
High Temp.
)
High Pressure
●
NH
2
NH
NH
2 4
N H
●
H
3
Cavitation bubbles act as micro-reactors to activate NH
3
, resulting in the formation of NH species that are further recombined to
hydrazine at the bubble-liquid interface. One of the advantages of this technology is the compartmentation of the as-formed
hydrazine in the bulk solution, maintained at only 30°C, which limits its thermal degradation.
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