Synthesis, anti-migration properties and burning rate catalytic properties of ferrocene-based compounds

Article abstract of DOI:10.1016/j.ica.2019.118958

Five ferrocene (Fc)-based compounds (HQ-Fcs) were synthesized by the condensation reaction of ferrocenecarbonyl chloride with corresponding hydroquinone derivatives. Nuclear magnetic resonance (¹H NMR), and Fourier transform infrared (FT-IR) we

Full text of DOI:10.1016/j.ica.2019.118958

InorganicaChimicaActa495(2019)118958
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Inorganica Chimica Acta
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Research paper
Synthesis, anti-migration properties and burning rate catalytic properties of
ferrocene-based compounds
Muhammad Usman, Li Wang⁎, Haojie Yu⁎, Fazal Haq, Ruixue Liang, Raja Summe Ullah,
Amin Khan, Ahsan Nazir, Tarig Elshaarani, Kaleem-ur-Rehman Naveed
State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Burning rate catalyst
Ferrocene-based hydroquinone compounds
Anti Migration behavior
Five ferrocene (Fc)-based compounds (HQ-Fcs) were synthesized by the condensation reaction of ferrocene-
carbonyl chloride with corresponding hydroquinone derivatives. Nuclear magnetic resonance (1H NMR), and
Fourier transform infrared (FT-IR) were used as evidence for the synthesis of HQ-Fcs. The electrochemical
properties of HQ-Fcs were investigated by cyclic voltammetry (CV) and suggested that these compounds ex-
hibited redox behavior due to ferrocene groups. The burning rate catalytic effect of HQ-Fcs was examined by
Thermogravimetric (TG) and differential Thermogravimetric (DTG) techniques. Thermal analysis results in-
dicated that HQ-Fcs were thermally stable and have better burning rate catalytic effects in accelerating thermal
decomposition of AP. Anti-migration behavior of HQ-Fcs in comparison to commonly used 2,2-bis (ethylfer-
rocenyl) propane (catocene) and Fc was evaluated. It was observed that HQ-Fcs delivered excellent anti-mi-
gration performances than catocene and Fc.
1. Introduction
propellants [21], BRCs from Fc still pose some serious disadvantages.
The main shortcoming of these Fc-based BRCs arises from their high
Fc one of most well-known organometallic compounds, is an elec-
tron-rich aromatic complex having ferrous ion sandwiched by cyclo-
pentadienyl rings and has emerged as a subject of intensive research
worldwide in recent times [1–4]. Fc and its derivatives due to their
inherent
migration tendency on prolong storage, sublimation loss during pro-
cessing and phase separation by crystallization [22–24]. These setbacks
leads to the destruction of designed parameters, shortens the service life
of the propellant, reduces resistance to aging by forming sensitive
boundary layers and may cause dangerous explosions [25–27]. To
preclude these drawbacks and also to improve the efficiency of Fc-based
BRCs it is necessary to design low migratory Fc- based BRCs [28,29].
Fc derivatives such as n-butylferrocene (NBF), tert-butylferrocene
(TBF), and 2, 2-bis (ethylferrocenyl) propane (catocene) have been
developed to overcome unwanted migration property. However, these
molecules are volatile, thus exhibit an obvious migration to the pro-
pellant interface [30]. In recent times many efforts have been made
striving for Fc-based BRCs that are less volatile as well as equipped with
high catalytic activities [31]. Anti-migratory research outcomes in-
dicated that grafting of ferrocenyl groups into either the backbones or
the side chains of polymers and oligomers is an effective way for im-
proving burning rates and mitigating migration tendency [32]. Fur-
thermore, the migration problems of Fc-based BRCs can be overcome
by: (i) extending the carbon chain length on Fc ring, (ii) introducing
polar elements like oxygen and nitrogen, (iii) introduction of reactive
groups such as –OH, –NH2, –NCO, etc. which can form cross-linked
thermal stability, aromaticity, low toxicity, lipophilicity, reversible
redox behavior and modification possibilities [5–8] are highly fasci-
nated, therefore they have gained increasing applications in different
fields such as catalysis, medicinal chemistry, non-linear optics, mate-
rials science, electrochemical sensing, memory storage, nano-materials,
biologically active compounds and ion recognition [9–14]. Owing to
excellent catalytic features, numerous ferrocenylated derivatives and
compounds are well recognized BRCs for solid propellant systems
[15,16]. Among all the BRCs, Fc based BRCs from viewpoint of their
microscopic homogeneities in distribution, low pressure exponents,
good compatibility with organic binder, broad range of BR adjustment
and better ignitability in solid propellants are considered to be superior
and have extraordinary effects in elevating the burning rates of solid
propellant [15,17–20]. Although BRCs from Fc family are favorable in
enhancing burning rate and exhibiting catalytic effect for the thermal
decomposition of ammonium perchlorate (AP) in solid composite
⁎ Corresponding authors.
E-mail addresses: opl_wl@dial.zju.edu.cn (L. Wang), hjyu@zju.edu.cn (H. Yu).
https://doi.org/10.1016/j.ica.2019.118958
Received 6 December 2018; Received in revised form 1 April 2019; Accepted 10 June 2019
Availableonline11June2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

M. Usman, et al.
InorganicaChimicaActa495(2019)118958
network and thus prevent volatilization and migration. (iv) introduc-
tion of inorganic groups in Fc ring and (v) synthesis of high molecular
weight ferrocenyl dendrimers, containing large number of polar ele-
ments like oxygen and nitrogen [30,33,34]. Zain and co-workers de-
veloped less migratory Fc-based compounds [27].Ting Li and co-
workers designed low migratory ionic compounds of ferroce-
nylmethyldimethylamine [30]. Luo and co-workers synthesized Fc-
based hyperbranched poly(amine-ester) and hyper-branched poly
(amine-ester) [35]. Although extensive efforts have been oriented to
lower migration tendency and enhance the combustion catalytic ac-
tivity of Fc-based BRCs. However, the problem is still being continued
[36].
Hydroquinones are an important class of compounds and, surpris-
ingly there has been limited number of work entitling the synthesis of
ferrocenyl substituted hydroquinones. Herein, we have reported the
synthesis of low migratory Fc-based compounds in order to solve the
migration problem as well as to improve the burning rate catalytic
activity in AP-based propellants. The success of synthesis was con-
firmed by the 1H NMR, and FT-IR. The electrochemical properties of
HQ-Fcs were studied by CV [37]. TG and DTG techniques were applied
for investigating the thermal catalytic behavior of HQ-Fcs. Anti-mi-
gration behavior in AP-based propellant was investigated at 50 °C and
atmospheric pressure in comparison with catocene and Fc. We in-
vestigated the effect of polar elements (oxygen) and electronegative
halogen group on the anti-migration behavior of small Fc-based hy-
droquinone compounds [27].
The obtained mixture was kept under stirring at room temperature for
15 min. Later to this mixture ferrocenecarbonyl chloride (5.6930 g,
22.909 mmol) in solution form prepared in 35 mL freshly distilled THF
was injected slowly. The resulting solution was then refluxed for 18 h.
The solvent was evaporated on a rotary dryer. The obtained product
was dissolved in 200 mL CHCl3. The resulting solution was first washed
(four times) with 1% NaHCO3 solution prepared in distilled water and
then further washed (three times) with distilled water. The organic
layer was separated, anhydrous Na2SO4 (2.0048 g) was added and was
put overnight. The mixture was filtered for the separation of Na2SO4
and then the obtained filtrate was dried on rotatory evaporator to get
the product. The product was further dried in vacuum oven at 40 °C for
1 day. Similarly, for ferrocenyl tetrafluorohydroquinone (TFH-Fc), fer-
rocenyl dibromohydroquinone (DBH-Fc), ferrocenylhydroquinone-bis
(2-hydroxyethyl)
ether
(BEH-Fc)
and
ferrocenyl
tetra-
cholorohydroquinone (TCL-Fc), above mentioned synthetic procedure
(i.e reacting ferrocenecarbonyl chloride with corresponding hyqui-
nones) was followed [39].
2.3. Characterization
1H NMR spectra of the synthesized Fc-based hydroquinone com-
pounds were recorded on AVANCE NMR spectrometer (600 MHz,
Model DMX-400). For these compounds the chemical shifts were re-
lated to tetramethylsilane (TMS) at δ = 0 ppm by using DMSO or CDCl3
as a solvent. FT-IR spectra were recorded on a Nicolet 5700 infrared
spectrometer by KBr pellet technique. 2% sample and KBr were crushed
to a powder and mixed together and KBr pellet was prepared.
Cyclic Voltammetry was carried out on a CHI-630A electrochemical
analyzer (CH Instruments, Inc., Austin, Texas) in an undivided three-
electrode cell. The concentration of the electrolyte was 0.1 M, the po-
tential scan rates were 0.1–0.5 V/s and concentration of HQ-Fcs were
0.5 m M. The samples for anti-migration studies consisted of three ad-
jacent parts: unloaded part (blank), interface and loaded part. These
three constituents represent insulation, liner and propellant, respec-
tively. The unloaded part consisted of AP, hydroxyl-terminated poly-
butadiene (HTPB) and isophorone diisocyanate (IPDI) and it was pre-
pared by mixing them for 30 min to obtain homogeneous slurry. The
resulting mixture was cast into a mold and cured at 25 °C for three days
before undergoing accelerated aging. The loaded part was prepared by
mixing AP, HTPB, IPDI and a BRC. Typically, sample containing HDQ-
Fc was prepared by mixing AP, HTPB, IPDI and HDQ-Fc for approxi-
mately 1 h. The resulting mixture was subsequently cast on unloaded
layer that had been cured for three days. After curing for three days at
room temperature, the prepared sample was aged in an oven at 50 °C
and atmospheric pressure for 30 days. Burning rate catalytic perfor-
mance of ferrocenylated-amino pyridines and ferrocenylated-amino
thiazoles was investigated with a PerkinElmer Pyris 1 thermogravi-
metric instrument at a heating rate of 5 °C min−1 under nitrogen in the
range 50–600 °C. The required amount (5 wt%) of ferrocenylated-amino
pyridines and ferrocenylated-amino thiazoles and AP was mixed and
ground.
2. Experimental
2.1. Materials
Hydroquinone (HDQ), tetrafluorohydroquinone (TFH), 2,5-di-
bromohydroquinone (DBH), hydroquinone bis(2-hydroxyethyl)ether
(BEH), tetracholorohydroquinone (TCL) and tetrabutylammonium tet-
rafluoroborate (Bu4NBF4) were purchased from J&K Chemical Reagent
Co. Ltd. Dichloromethane (DCM), oxalyl chloride, ferrocenecarboxylic
acid, pyridine, petroleum ether, tetrahydrofuran (THF), triethylamine
(TEA), chloroform, sodium hydride (NaH) and NaHCO3 were also
purchased from Sinopharm Chemical Reagent Co. Ltd.
2.2. Synthesis
2.2.1. Synthesis of ferrocenecarbonyl chloride
Ferrocenemonocarbonyl chloride was prepared in accordance with
the stated procedure in literature, accompanied by slight modifications
[38]. The initial step for this synthesis involved the drying of ferroce-
nemonocarboxylic acid (30.0983 g, 121.904 mmol) at 40 °C for 4 h
under vacuum condition. Afterwards solution of ferrocenemono-
carboxylic acid was formed by using freshly distilled DCM (270 mL).
Pyridine (23 mL, 285.508 mmol) was injected to this ferrocenemono-
carboxylic acid solution and was stirred well under argon (Ar) atmo-
sphere at room temperature for about 15 min. Furthermore drop wise
addition of oxalyl chloride (24 mL, 279.839 mmol) was carried out at
room temperature. The resulting reaction mixture was at first stirred for
30 min at room temperature and then was allowed to reflux for 6 h. The
content of the reaction flask was dried under vacuum and the residue
was extracted using petroleum ether (100 mL) at 90 °C. The obtained
ferrocenemonocarbonyl chloride was kept in Ar atmosphere [38].
3. Results and discussion
3.1. Synthesis and characterization of HQ-Fcs
Ferrocenecarbonyl chloride was prepared from ferrocenecarboxylic
acid by following the reported literature [12]. The synthesis of ferro-
cenecarbonyl chloride is shown in Scheme 1.
2.2.2. Synthesis of HQ-Fcs
The synthetic methodology for HQ-Fcs involved condensation re-
actions of hydroquinones with ferrocenecarbonyl chloride. For the
preparation of ferrocenyl hydroquinone HDQ-Fc, hydroquinone
(1.0985 g, 9.977 mmol) was initially dried under reduced pressure and
then was dissolved in 35 mL freshly distilled THF. Afterwards TEA as a
deacid (3 mL, 21.509 mmol) was injected to the hydroquinone solution.
HQ-Fcs were synthesized by the condensation reaction of ferroce-
necarbonyl chloride with corresponding hydroquinones in freshly dis-
tilled THF. The experimental detail for the synthesis of Fc-based hy-
droquinone compounds is given in Table S1.
The structures of the synthesized compounds were characterized by
1H NMR spectroscopy. On comparing 1H NMR spectra of these HQ-Fcs
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M. Usman, et al.
InorganicaChimicaActa495(2019)118958
Scheme 1. The synthetic route of Fc-based hydroquinone compounds.
with corresponding hydroquinone derivatives, three new peaks
(4.00–5.00 ppm) attributed to Fc protons appeared. 1H NMR spectra of
HQ-Fcs are shown in Fig. S1. The integration of all the peaks including
peaks of Fc was according to the structures, which further supported the
successful synthesis of HQ-Fcs. The spectroscopic data for HQ-Fcs are
given in the supporting information.
S3–S7 and the numerical information is given in Table S2. The elec-
trochemical results indicated that the solvent polarity as well as the
potential scan rate has high impact on redox properties of HQ-Fcs. The
change in polarity of the solvent affected the shape of CV curves and it
has been observed that the increase of solvent polarity resulted in de-
formation of the peak shapes. The peaks were found to be more dis-
torted in case of highly polar DMSO. The high viscosity of DMSO of-
fered greater resistance to ion movement therefor resulted in multiple
redox peaks. Both oxidation and reduction peaks potential was de-
creased with the increase in polarity of the solvent, which showed that
polarity of solvent was proportional to reduction and oxidation pro-
cesses for HQ-Fcs.
The structures of Fc-based hydroquinone compounds were further
confirmed by FT-IR spectroscopy. The strong vibrational and de-
formational bands centered at 1015–1045 cm−1, 801–835 cm−1 and
480–502 cm−1 were the evidence of ferrocenyl groups [40,41]. The
CeH vibrational stretching bands were observed around 3030 cm−1
,
and 2960–2850 cm−1 respectively. The bands around 1725 cm−1 were
assigned to the stretching vibrations of carbonyl esters in HQ-Fcs [42].
Meanwhile the stretching vibrations of esters (CeO) were found around
1073–1150 cm−1 [43]. Peaks located at 1671 cm−1 and 1455 cm−1
were attributed to stretching vibrations of (C]C) in the newly syn-
thesized compounds [44]. FT-IR spectra of HQ-Fcs are shown in Fig. S2.
The most well-defined and sharp peaks were obtained in DCM and
CHCl3 with high current in DCM. The appearance of single oxidation
and reduction peaks indicated the equal behavior of ferrocene in terms
of electronic transfer between Fe2+ and Fe3+
.
HQ-Fcs was also found sensitive to various scan rates in organic
solvents. Increase of scan rate led to increase in peak current values (ipc
and ipa). According to CV theory, these results show that charge
transport of the electrode processes obeyed Fick’s law at room tem-
perature. The rate of electrode reaction was found to be fast in DCM.
The redox reactions were smooth in DCM due to little solvent resistance
and faster charge diffusion rate.
The measurements, carried out in an organic solvent (DMSO), ob-
served to be in the range of 386–443 mV for Ep1/2 and 77–108 mV for
ΔEp seemed adequate, indicative of an quasireversible ferrocene/fer-
rocenium wave. Peak to peak potential separation (ΔEp) was found to
be larger in CHCl3 and smaller in DCM, while the rate of electrode
reaction decreased from DCM to CHCl3.
3.2. Electrochemical behavior of HQ-Fcs
The electrochemical process involves loss and gain of electrons.
Since the combustion process of the solid propellant also involves loss
of electrons, therefore it’s important to investigate the electrochemical
properties of HQ-Fcs for figuring out their potential as BRCs. CV has
been considered as effective technique for exploring redox properties of
Fc based compounds instead of studying their combustion properties
directly [45]. HQ-Fcs showed good solubility with varying polarities in
different solvents, so their electrochemical properties were studied in
DCM, CDCl3, and DMSO with Bu4NBF4 (0.10 M) as a supporting elec-
trolyte at 20 °C. The results of CV curves for HQ-Fcs are shown in Figs.
The ferrocenes substituted compounds showed greater redox
3

M. Usman, et al.
InorganicaChimicaActa495(2019)118958
potentials, indicating the oxidation of new compounds more difficult
than the neutral ferrocene derivatives and exhibit higher anti-oxidative
ability under similar conditions. For TFH-Fc, TCL-Fc and DBH-Fc the
electronic density decreases over the iron center and the 2 + oxidation
state of the iron ion is more stabilized by the electron-withdrawing
effect of the halogen groups. The shielding of the iron atom by the
carbonyl group in HDQ-Fc render the interaction of the iron atom with
the electrode difficult, thereby increasing in the positive charge on the
iron atom and hence reduced. The lesser electron–withdrawing effect of
substituents results in greater stabilization of the 3 + oxidation state of
the metal for BEH-Fc [32,46–48].
3.3. Anti-migration studies of HQ-Fcs
Anti-migration performances of HQ-Fcs were tested against cato-
cene and Fc under the same aging conditions. Sample preparation de-
tails of anti-migration studies are given in Table S3 and the procedure
adopted for the assembly of migration tube is shown in Fig. S8.
For both Fc and catocene obvious migration was noticed as they
start migrating even after 7 days storage, whereas HQ-Fcs showed
much slower migratory behavior than catocene and Fc. After 30 days
negligible migration was found in case of HQ-Fcs. The reason to this
excellent anti-migration behavior was perhaps, the van der Waals forces
between the propellant and HQ-Fcs. These forces were caused by the
presence of oxygen atoms in these compounds. Non-migratory results
observed in case of HQ-Fcs due to their more polar and electronegative
nature is shown in Fig. S9 and their migration distances are plotted in
Fig. 1. Based on the anti-migration trends, migration mechanism was
proposed (Fig. 2).
Fig. 2. Migration mechanism: (a) BEH-Fc with migration model of BEH-Fc and
(b) ferrocene with migration model of ferrocene.
It indicated Fc to be a small and highly volatile molecule as its
migration appeared after short aging period and reached to the ends of
the tube while HQ-Fcs were stable, less volatile and presented decent
anti-migration ability, probably due to their higher molecular weights.
The presence of oxygen atoms attached with carbonyl groups con-
tributed towards the formation of dipole-dipole forces and hydrogen
bonding which binds HQ-Fcs and the propellant providing hindrance
against migration.
and thermally stable BRCs are perceived to encourage the steady
combustion of a solid propellant. Since AP is key component and its
thermal decomposition behavior is related to the combustion behavior
of the propellants in solid propellant. Therefore the burning rate cata-
lytic activity of a Fc-based BRC is often judged by investigating its effect
on the thermal decomposition of AP [21]. The burning rate catalytic
effect of HQ-Fcs for the thermal degradation of AP was determined by
TG and DTG techniques using 5 wt% mixture of the HQ-Fcs with AP.
Fig. 3(a) and (b) presents the results of conducted thermal studies.
The catalytic efficiency of DBH-Fc for the thermal degradation of
AP was also studied by varying the content (1–5 wt%) of DBH-Fc,
shown in Fig. 3(c) and (d). The detail of samples preparation for
thermal analysis and catalytic performance is given in Table S4.
From the TGA results, it was observed that the thermal decom-
position of AP was around 445.27 °C, but the addition of specific
amount of HQ-Fcs (5 wt%) in AP evidently shifted the decomposition
peaks forward towards the left, thereby indicating positive effect in
accelerating HQ-Fcs were found thermally stable up to 220 °C and
presented the best catalytic behavior by lowering the thermal de-
gradation temperature of AP around 351.17 °C. Furthermore Fig. 3(c)
indicated that the increase in wt% of DBH-Fc resulted in decreased
thermal disintegration temperatures of AP. This could be due to in-
creasing concentration of Fc, which is considered to have efficient
catalytic effect in promoting fast decomposition of AP.
DTG curves in Fig. 3(b) showed that pure AP and mixtures of HQ-
Fcs with AP were decayed over two stages. First stage decomposition
was related to low temperature decomposition (LTD) where the partial
decomposition of AP occurred, while second stage where complete
decomposition of AP took place corresponded to high temperature de-
composition (HTD). For pure AP, LTD was observed at 341. 42 °C and
HTD was observed at 432.94 °C, while in the case of HQ-Fcs, LTD was
around 300–330 °C and HTD appeared around 350–390 °C. The max-
imum weight loss was for 5 wt% DBH-Fc (Fig. 3b).With increase in wt%
of DBH-Fc, the decomposition of AP become rapid (Fig. 3d). The drastic
decrease in both LTD and HTD indicated HQ-Fcs to have effective and
3.4. Burning rate catalytic performance and possible catalytic mechanism of
Fc-based compounds
For BRCs thermal stability is considered as significant parameter
g
a
b
c
d
e
f
HDQ- Fc
TFH-Fc
DBH-Fc
BEH-Fc
TCL-Fc
Catocene (Cat)
Ferrocene(Fc)
4
3
2
1
0
g
g
g
f
f
f
e
c
d
ab
e
d
c
c
b
e
b
a
a
d
7 Days
15 Days
30 Days
Time (Days)
Fig. 1. Anti-migration studies of: (a) HDQ-Fc, (b) TFH-Fc, (c) DBH-Fc, (d)
BEH-Fc, (e) TCL-Fc, (f) catocene, and (g) Fc on days 1, 7, 15 and 30 at 50 °C.
4

M. Usman, et al.
InorganicaChimicaActa495(2019)118958
a
b
AP
AP
AP+ 5Wt.% BEH-Fc
AP+ 5Wt.% TCL-Fc
336.54°C
433.14°C
394.41°C
AP + 5% BEH-Fc
AP+ 5Wt.% TFH-Fc
AP+ 5Wt.% DBH-Fc
331.51°C
AP + 5% TCL-Fc
AP + 5% TFH-Fc
AP+ 5Wt.% HDQ-Fc
310.52°C
364.82°C
445.27°C
398.73°C
373.42°C
304.30°C
298.16°C
AP + 5% DBH-Fc
AP + 5% HDQ-Fc
360.10°C
350.36°C
369.25°C
358.23°C
306.13°C
300
351.17°C
400
346.17°C
100
200
300
500
100
200
400
500
Temperature (°C)
Temperature (°C)
c
d
AP
AP
AP + 1% DBH-Fc
AP + 2% DBH-Fc
336.54°C
329.20°C
AP + 1% DBH-Fc
AP + 2% DBH-Fc
433.14°C
AP + 3% DBH-Fc
AP + 4% DBH-Fc
AP + 5% DBH-Fc
386.16°C
325.83°C
321.27°C
AP + 3% DBH-Fc
384.41°C
445.27°C
398.16°C
AP + 4% DBH-Fc
AP + 5% DBH-Fc
376.17°C
361.30°C
392.91°C
384.17°C
291.80°C
371.30°C
298.16°C
300
358.23°C
400
350.36°C
400
100
200
300
500
100
200
500
Temperature (°C)
Temperature (°C)
Fig. 3. (a) TG curves of pure AP and AP with 5 wt% of HQ-Fcs, (b) DTG curves of pure AP and AP with 5 wt% of HQ-Fcs, (c) TG curves of pure AP and AP with 1–5 wt
% of DBH-Fc (d) DTG curves of pure AP and AP with 1–5 wt% of DBH-Fc.
accelerating catalytic effect on thermal decomposition of AP.
The catalytic mechanism of HQ-Fcs in AP-based propellant can be
explained in terms of thermal decomposition of AP. The covalently
bonded hydroquinone structures having polar elements show anti-mi-
gration properties while ferrocene contributes in the burning rate cat-
alytic properties.
electron density from the ring [47,52]. It can be noticed for HDQ-Fc to
have the lower decomposition peak temperatures for AP at around
346.17 °C corresponding to their oxidation potential. The results sug-
gested that the ultimate decomposition temperatures of AP with HDQ-
Fc as additive was lower than the others, implying good effects of HDQ-
Fc as BRC. It has been accepted that ferrocenyl derivatives tend to form
Fe2O3 particles. Dinuclear ferrocenylated compounds are supposed to
be firstly burnt up, producing Fe2O3 nanoparticles and the distance
between the two iron atoms is suitable for the formation of Fe2O3 na-
noparticles, which are strong promoter in the gas phase to accelerate
the decomposition of AP [16,53]. Due to fact that the Fe atoms of HDQ-
Fc are located nearer than that of BEH-Fc, the AP decomposition peak
temperature catalyzed by HDQ-Fc was lower than that of BEH-Fc.
The thermal decomposition mechanism of AP can be presented in
two ways (i) electron transfer process in AP decomposition, which can
be catalyzed by an additive, and (ii) the rupture of the chemical bond in
ClO4 of AP [54]. HQ-Fcs maintain redox properties and initiated the
decomposition by electron transfer process of ferrocene/ferrocenium in
ferrocene groups. The redox potential of these HQ-Fcs presented by
their low oxidation potentials in comparison to ammonium perchlorate
was more helpful to accelerate the decomposition and shifted the AP
peaks towards lower temperatures, corresponding to the possibility that
the Fe2+ in Fc is oxidized into Fe3+ by AP and then reduced into Fe2+
3.5. Possible burning rate catalytic mechanism of ferrocene-based
compounds on the thermal decomposition of AP
Fc-based polymers, and their derivatives have high redox potential
(Ered) and their Ered value depends upon the nature of the substituent on
the ferrocene ring [40,49]. But still the burning rate catalytic me-
chanism of Fc-based polymers, and their derivatives is not clear
[17,50]. Four catalytic mechanisms namely: electron transfer me-
chanism, proton transfer mechanism, forming transition materials me-
chanism and acid–base interaction have been proposed [51]. The
burning rate catalytic mechanism of Fc-based compounds in AP-based
propellant can be explained in terms of thermal decomposition of AP
[15]. The incorporation of ferrocenylated compounds leads to ac-
celerated decomposition of AP through electron transfer mechanism
[17]. The electron-withdrawing halogen substituents of ferrocene
compounds may promote the decomposition process of AP by removing
5

M. Usman, et al.
InorganicaChimicaActa495(2019)118958
by NH3 produced by the degradation of AP at high temperature and the
charge balance of AP is destroyed, which promotes the degradation rate
of AP. Thus, the thermal decomposition of AP takes place through an
electron transfer process below 300 °C. [41]. Ferrocenyl derivatives are
supposed to be firstly burnt up, transforming Fe2O3 particles [28] with
a large surface area above 300 °C [32]. The Fe2O3 particles might ab-
sorb ClO4 in its gas phase to catalyze the rupture of the chemical bond
in AP. Hence, both the electron transfer process and the catalytic de-
composition effect of Fe2O3 particles can play key roles in AP decom-
position. Zain-ul et al. [55], Tong et al. [56] and Povea et al. [16] gave
a more visual description of the burning process. Since combustion is
essentially a drastic oxidation-reduction process and also is a complex
redox reaction, the combustion of composite solid propellants presents
complications, preventing a thorough understanding of its progress.
Based on references [17,28,52–55], the combustion of AP‐based pro-
pellant containing ferrocene‐based compounds results in formation of
nanoscale Fe2O3 fine particles. These nanoscale Fe2O3 fine particles
have larger surface area and show excellent burning rate catalytic ac-
tivity in thermal decomposition of AP [17,28,56,57].
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