Thermoanalytical and NMR investigation of NaBH4·2H2O thermolysis process

Article abstract of DOI:10.1007/s10973-017-6821-3

The article describes the thermolysis process of sodium borohydride dihydrate in thermoanalytical experiments. The reaction was carried out without solid catalyst and with catalyst as cobalt boride Co₂B. It has been found out that in both cases the process starts after the peritectic reaction of the starting compound and forms a liquid phase. The enthalpy of peritectic reaction is ΔH_react?=?19?±?2?kJ?mol^?1. When thermolysis proceeds in acetonitrile solution without a catalyst intermediate hydroxyborohydride NaBH₃OH and/or Na(BH₃)₂OH is formed according to the NMR experiment data. The formation of similar complexes in the solid phase is confirmed by experiments on the oxidation of the thermolysis products. Thermolysis process with solid catalyst proceeds with an intense exothermic effect at lower temperatures. The kinetics of the non-catalytic process is described by the model of two consecutive reactions, and reaction with the solid catalyst model is approximated by two parallel reactions.

Full text of DOI:10.1007/s10973-017-6821-3

J Therm Anal Calorim
DOI 10.1007/s10973-017-6821-3
Thermoanalytical and NMR investigation of NaBH Á2H O
4
2
thermolysis process
1
2
3
2
•
I. V. Arkhangelskii
3
•
V. P. Tarasov
3
•
O. V. Kravchenko
3
•
G. Kirakosyan
3
A. V. Shihovzev
M. V. Tsvetkov
•
M. V. Solovev
•
Yu. A. Dobrovolskii
•
Received: 6 July 2017 / Accepted: 1 November 2017
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2017
Abstract The article describes the thermolysis process of
sodium borohydride dihydrate in thermoanalytical experi-
ments. The reaction was carried out without solid catalyst and
Introduction
The hydrolysis of complex alkali metal borohydrides
(NaBH , LiBH , etc.) has long been used to produce
with catalyst as cobalt boride Co B. It has been found out that
2
4
4
in both cases the process starts after the peritectic reaction of
the starting compound and forms a liquid phase. The enthalpy
enough hydrogen for technical purposes [1]. Reasons for
using hydrides as a hydrogen source are high volumetric
density of hydrogen in hydrides and their relative sim-
plicity of obtaining these compounds. Among hydrides,
-1
of peritectic reaction is DHreact = 19 ± 2 kJ mol . When
thermolysis proceeds in acetonitrile solution without a cata-
lyst intermediate hydroxyborohydride NaBH OH and/or
3
sodium borohydride (NaBH ) holds a special place because
4
Na(BH ) OH is formed according to the NMR experiment
3
of the high content of hydrogen (10.8 mass%), affordable
price and stability of its alkaline solutions. Hydrolysis of
sodium borohydride is a promising way of obtaining high-
purity hydrogen. The half of hydrogen is released from the
water [2–5]. Purely formally hydrolysis process can be
represented by the following equation:
2
data. The formation of similar complexes in the solid phase is
confirmed by experiments on the oxidation of the thermolysis
products. Thermolysis process with solid catalyst proceeds
with an intense exothermic effect at lower temperatures. The
kinetics of the non-catalytic process is described by the
model of two consecutive reactions, and reaction with the
solid catalyst model is approximated by two parallel
reactions.
NaBH4 þ 2H2O ! NaBO2 þ 4H2 þ heat
ð1Þ
Unfortunately, the reaction by Eq. (1) is practically not
realized. The main problem is formation of fairly
stable borates, like NaB(OH) ÁH O and NaB(OH) . It
Keywords Sodium borohydride dihydrate Á Peritectic
reaction Á NMR Á Thermal analysis Á Non-isothermal kinetic
4
2
4
should be noted that research into the process of hydrolysis
of sodium borohydride is actively carried out now. In last
2
years not less than 25 papers have been published. In
general, the research deals with search and investigation of
the hydrolysis catalysts. For example, see papers [6, 7].
The use of catalysts enables hydrogen production in the
temperature range from - 40 to ? 85 °C. Catalysts
accelerate the H generating process, and prevent the by-
2
&
I. V. Arkhangelskii
arkh1@yandex.ru
products formation. They also allow easy control of the
hydrogen generation process according to the load, as well
as stop and start it on consumer request.
1
2
Chemical faculty, Moscow State University, Moscow, Russia
It should be noted that sodium borohydride dihydrate
exists as an individual compound in the system sodium
borohydride–water.
Institute General and Inorganic Chemistry RAS, Moscow,
Russia
3
Institute of Problems of Chemical Physics RAS,
Chernogolovka, Russia
123

I. V. Arkhangelskii et al.
The ratio of components in this compound corresponds
to Eq. (1), so this compound can be used in a solid form for
the production of hydrogen during thermolysis. However,
there is hardly any information about this process. There-
fore, the main purpose of this paper is thermoanalytical and
NMR study of the thermolysis NaBH Á2H O, both without
container in the refrigerator. The composition of the sam-
ple was evaluated with respect to X-ray analysis and PMR
spectroscopy (proton magnetic resonance).
X-ray powder patterns of solid samples were registered
with a Siemens Kristalloflex diffractometer (Cu-Ka radia-
tion) with a graphite diffracted beam monochromator and
Ni filter. Data were collected in the range of 2H = 5°–60°
4
2
a catalytic process and using a solid catalyst. Cobalt bor-
ides obtained as described in [8] were used as a catalyst.
The polyterm of solubility binary system NaBH –H O
-
1
with a scan step 2H = 0.02° and scan rate of 18 min
.
Phase identification was performed using ICDD PDF-2
database and data [10].
4
2
[
9] shows that at a temperature of 36.4 °C the anhydrous
sodium borohydride is in equilibrium with a saturated
solution. Invariant point (36.4 °C. 45.2 mass% NaBH4)
corresponds to the peritectic reaction
Preparation of the catalyst
Synthesis of cobalt borides was performed ex situ in
aqueous solution by reduction CoC1 –6H O by NaBH [8].
NaBH4 Á aq ꢀ NaBH4 þ solution
ð2Þ
2
2
4
Consequently, thermolysis process initially proceeds in
a liquid phase.
The process was carried out at 40 °C with constant stirring
of 0.12 M solution of NaBH . The molar ratio of CoC1 –
4
2
The original binary phase diagram of the NaBH –H O
4
2
6H O to hydride was 1:25. The resulting black sample was
2
system copied from Ref. [9] is shown in Fig. 1.
separated from the mother liquor by using a magnet after
completion of gas evolution. It was washed thoroughly
with acetone and was dried under vacuum at 70 °C for 3 h
and stored in an atmosphere of Ar.
Experimental
Synthesis of sodium borohydride dihydrate
Measurements
The starting anhydrous sodium borohydride (99%
‘Acros’’) dissolved in 0.1 N NaOH solution (* 50 g
Thermal gravimetric analysis (TG) and differential scanning
calorimeter (DSC) were used to study thermal behavior,
employing Netsch STA Jupiter 449C Instrumentation. The
aluminum sample holders were used as crucibles Ø-6 mm
and h-8 mm. The sample mass was 3–5 mg. Experiments
were performed at four heating rates of 2.5, 5, 7.5 and 10 °C
min in a stream of dry air at the rate of 70 mL min in
the temperature range 30–550 °C. Thermoanalytical exper-
iments were also carried out in a stream of pure argon. The
thermolysis study using a solid catalyst was carried out as
follows. The catalyst was placed into the crucible
‘
NaBH in 50 mL of 0.1 N NaOH) was stirred at 35–36 °C
4
and then filtered under nitrogen. The clear solution in a
sealed flask was placed in a refrigerator for 24 h at 0 °C.
The precipitated crystals were separated from the mother
liquor by filtration, and well wrung out between sheets of
filter paper. These crystals were used in all studies pre-
sented. The compound was stored in an airtight plastic
-1
-1
t,°C
(
* 0.5 mg Co B), and about 4 mg of NaBH Á2H O was
2
4
2
4
2
0
0
0
NaBH + solution
4
added there. The components were stirred by a magnet. The
catalytic thermolysis was conducted in an atmosphere of dry
air and argon also in the temperature range 30–250 °C. The
experimental data were processed using the programs
package Netzsch company: ‘‘Proteus Analysis’’, ‘‘Peak
Separation’’, ‘‘Thermokinetics’’.
Solution
1
11
NMR spectra of H (300 MHz) and B (96,3 MHz)
were recorded according to standard procedures in the field
of 7.04 Tesla spectrometer Bruker AVANCE 300 to
25–70 °C temperature range in standard ampoules 5 mm
diameter. The sample temperature with an accuracy
of ± 0.10 °C was set using BVT-3200 unit. The direc-
–
–
20
40
NaBH + NaBH ·2H O
4
4
2
Ice + solution
NaBH ·2H O + solution
4 2
Ice + NaBH ·2H O
4
2
0
20
NaBH mass%
40
60
NaBH ·2H O
4
2
-1
4
tional airflow of 400–500 L min was used to eliminate
the temperature gradient along the length of the ampoule.
1
B NMR chemical shifts were measured relative to
Fig. 1 Diagram of the phase composition in binary system NaBH
O at different temperatures and mass% of the borohydride. The
diagram is reprinted from Malceva and Khain [9]
4
–
1
H
2
1
external reference—aqueous solution B(OH) . H NMR
3
1
23

Thermoanalytical and NMR investigation of NaBH
4
Á2H
2
O thermolysis process
shifts relative to TMS. The following samples were stud-
Using the data of [10, 11], the enthalpy value of peri-
tectic decay reaction was calculated, which was equal to
-1
ied: anhydrous NaBH (I) and NaBH Á2H O (II), prepared
4
4
2
as shown above. The main solvent used was CH CN and
3
DHreact = 17 kJ mol . According to DSC measurements,
CD CN. The water content in the main solvent used was
3
the enthalpy of the endothermic effect in the temperature
-1
range of 30–40 °C was 19 ± 2 kJ mol , so the calculated
and experimental values coincide. Heating the sample to
under the spectroscopic detection limit. The formations of
two fractions were observed when dihydrate was dissolved
in acetonitrile at room temperatures. At 50–60 °C, sepa-
ration was not observed, and the solution was regarded as
homogeneous. For anhydrous sample, stratification did not
happen.
5
50 °C in air leads to exothermic effects and significant
mass gain (* 60%) in the temperature range of
50–520 °C (Fig. 3). We assume that the exo-effects in the
3
area of 350–470 °C are characteristic to the process of
oxidation of hydrolysis products. In the temperature range
of 150–350 °C, there is no noticeable change in the mass;
therefore, in the process of thermolysis there is no forma-
tion of hydrates of sodium metaborate, as in the case with
hydrolysis in aqueous solution [11] (Fig. 3). Intensive exo-
effect at 500 °C is caused by oxidation of anhydrous
sodium borohydride, which is confirmed by Fig. 4.
Results and discussion
Thermolysis without solid catalyst
Figure 2 shows the thermoanalytical curves of the heating
process for dihydrate of sodium borohydride in the tem-
perature range 30–150 °C in a dry air atmosphere. At these
temperatures, there is a loss of mass (* 45%), and
endothermic effects are observed in DSC curve. It should
be noted that in argon the process proceeds similarly.
Heating in argon in the temperature range 150–500 °C
does not result in a noticeable change in mass and the
appearance of thermal effects. If the process took place in
accordance with Eq. (1), then significant exothermic effect
could be observed [4, 5]. The process begins with the
peritectic reaction (see Eq. 2). This process can be repre-
sented by the following equation:
It is known [12] that the hydrolysis of borohydride anion
in aqueous solution proceeds according to the scheme:
À
À
À
À
BH À BH OH À BH ðOHÞ ÀBHðOHÞ ÀBðOHÞ
4
3
2
2
3
4
It is believed that the thermolysis NaBH4Á2H2O is
through the formation of hydroxy derivatives after the
appearance of a liquid phase.
NMR experiment was conducted to identify the nature
of intermediates formed in the thermolysis temperature
range of 30–150 °C.
NaBH4Á2H2O (II) is foamed at T C 40 °C due to the
thermolysis. This results in a partial release of material
from the HF band coil and thus to disruption of sample
homogeneity.
NaBH4 Á 2H2O ¼ 0:45NaBH4 solid þ ½0:55NaBH4
ð3Þ
þ 2H O þ DHdissolŠ þDH
2
liq
react
DSC/mW mg–1
TG/%
Complex Peak:
Ex[1
o
↑
Area: –265.4 J g–1
Peak: 39.1 °C
Onset: 34.9 °C
1
1
10
00
0
–
–
–
–
–
1
2
3
4
5
Mass Change: –45.89 %
9
8
7
6
5
4
0
0
0
0
0
0
[1
Peak: 69.7 °C, –3.743 mW mg–1
–6
3
0
40
50
60
70
80
90
100
110
120
Temperature/°C
Fig. 2 Thermoanalytical curves of thermolysis NaBH
4
Á2H
2
O in dry air without solid catalyst, solid line TG, dashed line DSC
123

I. V. Arkhangelskii et al.
–
1
DSC/mW mg
TG/%
Exo
↑
Peak: 502.8 °C, 29 mW mg–1
1
1
1
1
1
80
Peak: 453.5 °C, 14.98 mW mg–1
25
[
4]
60
40
20
00
Peak: 398.4 °C, 21.46 mW mg–1
2
1
1
5
0
0
5
0
Mass Change: 61.74 %
8
0
[
4]
1
00
150
200
250
300
350
400
450
500
Temperature/°C
Fig. 3 Thermoanalytical curves of oxidation of the products of thermolysis NaBH
4
Á2H
2
O in dry air
DSC/mW mg–1
TG/%
↑
Exo
Peak: 495.2 °C, 334 mW mg–1
2
1
1
00
50
00
3
2
2
00
50
00
[
3]
150
1
5
0
00
0
Mass Change: 53.30 %
5
0
0
[
3]
1
50
200
250
300
350
400
450
500
550
Temperature/°C
4
Fig. 4 Thermoanalytical curves of oxidation of NaBH in dry air
Therefore, further temperature measurements were car-
ried out on the NMR samples (I) and (II) in anhydrous
spectra of the sample (II) was observed a quintet with
relative intensities of 1: 4: 6: 4: 1 at - 60 ppm and cou-
1
1
1
pling constants J ( B– H) = 81 Hz due to borohydride
acetonitrile and CD CN.
3
À
À
The signals of BH4 anion in PMR spectra were
anion BH as the main form with more than 97% (Fig. 5).
4
À
11
observed at - 0.12 ppm as a quartet with the coupling
constants J (1H–11B) = 81 Hz. Spectra were recorded on
samples dihydrate solution in acetonitrile and CD2HCN at
In addition to signals from BH4 in NMR B spectra, a
group of multiplets three quartets were recorded. Its indi-
cates the possibility of formation a hydroxyborohydride
-
-
5
0 °C. The water signals at 3.05 ppm and a residual signal
from CD2HCN partially deuterated acetonitrile as a quintet
at * 1.8 ppm with coupling constants (1H-
H) = 2.3 Hz. The ratio of the intensities of proton signals
of water and tetraborohydride was 1: 1, which corresponds
anion BH OH and/or (BH ) OH bridged OH group or a
3 3 2
?
contact ion pair with a cation Na (insert at Fig. 5). For-
mation of such structures is shown in [12, 13]. The singlet
at 17 ppm also appears in this temperature range. This
appearance is due to the formation of anion metaborate
according to reaction (1).
J
2
1
1
exactly to the original composition. At 70 °C in B NMR
1
23

Thermoanalytical and NMR investigation of NaBH
4
Á2H
2
O thermolysis process
11
Fig. 5 NMR spectra B of the
solution of NaBH O in
Á2H
CH
CN at T = 70 °C.
insertion determines the scale
of signals)
4
2
3
(
×
128
–15
–20
–25
–30
–35
–40
–45
–50
–55
–60
ppm
Apparently, similar oxo- and hydroxyforms of borohy-
dride anion are formed during a non-catalytic thermoana-
lytical experiment, which is confirmed by Fig. 3.
Catalytic thermolysis
Carrying out the thermolysis in the presence of a solid
catalyst essentially changes the mechanism of the process
(Fig. 6).
1
1
Summarizing the results of thermal analysis and
B
NMR, the process of non-catalytic thermolysis can be
described by the following equation:
The process, as in the case of non-catalytic thermolysis
reaction, starts with the formation of liquid phase. At this
point, the change in mass, i.e., hydrogen evolution can be
observed. It is possible that partial hydrolysis of borohy-
dride proceeds at this period to form hydroxycomplexes.
Then there is an intensive exothermic peak due, presum-
ably, to the formation of hydrated sodium metaborate
because only in this case, reaction (1) can be highly
exothermic [2, 3].
NaBH Á 2H O ! 0:4NaBH þ 0:01NaBO
4
2
4
2
þ 0:25ðNaBH3Þ O þ 0:09NaBH3OH
ð4Þ
2
þ 0:63H þ 1:64H O
2
2
Non-catalytic thermolysis proceeds mainly through
hydrolysis and dehydration.
Thus, endothermic effects in the temperature range of
3
0–100 °C (Fig. 2) are caused by several processes
Complete dehydration of NaBH4Á2H2O corresponds to
occurring in the reaction system: first, peritectic reaction;
secondly, gradual hydrolysis of borohydride anion; thirdly,
dehydration of solid intermediate reaction products. In the
temperature range 120–140 °C, the process of mass loss is
practically over.
4
9% of mass loss, and complete hydrolysis to 10.8% of
mass loss (Eq. 1). The smaller the mass loss during ther-
molysis, the more hydrogen is released. In the case of
catalytic thermolysis, the mass loss in the temperature
DSC/mW mg–1
TG/%
–
1
Peak: 66.0 °C, 36.28 mW mg
Exo
↑
1
1
05
00
3
3
2
2
1
1
5
0
–
5
0
5
0
5
0
Mass Change: –29.68 %
Mass Change: –22.88 %
9
9
8
8
7
7
6
5
0
5
0
5
0
5
–
1
Peak: 40.3 °C, –4.36 mW mg
[2
[
2
5
50
100
150
200
250
Temperature/°C
Fig. 6 Thermoanalytical curves of catalytic thermolysis NaBH
4
Á2H
2
O in a dry air, solid line TG, dashed line DSC curves
123

I. V. Arkhangelskii et al.
range 40–150 °C is two times less than in the non-catalytic
process (see Eq. 4). Equation (5) reflects this relationship.
By the temperature of 90 °C, the intensive process
practically finished and is transformed into a solid-phase
process of dehydration of metaborate, which is completed
by 250 °C. The process which takes place below 90 °C
may be described by the following equation:
initial estimates for Arrhenius parameters. The kinetic
inverse problem solution revealed several stages of the
process. Therefore, multistage models should be used for
the experimental data fitting. At the next stage, the direct
kinetic problem was studied with the use of multivariate
non-linear regression. As a result, the statistical analysis
revealed the optimal kinetic parameters and the function
type (Fig. 7) as the best data approximation. The two-stage
model (I) gave the best experimental data description
NaBH4 Á 2H2O ! 0:33NaBH4 þ 0:33NaBH3OH
þ 0:33NaBO2 Á H2O þ 1:67H2
ð5Þ
(Fig. 7):
þ 0:67H O
2
A-1 ! B-2 ! C:
In this model, the reaction 1 was described with Eq. (6)
ðIÞ
According to Eq. (5), the mass loss due to evolution of
hydrogen and water vapor is 23%, which corresponds to
the mass loss by 90 °C (Fig. 6). The total mass loss of up to
and can be used for thermogravimetric curve
approximation:
2
50 °C is 29%, which includes mass loss by dehydration of
n
fðaÞ ¼ ð1 À aÞ Áð1 þ K aÞ
ð6Þ
ð7Þ
sodium metaborate. Therefore, Eq. 5 is consistent with the
experimental data (Fig. 6).
1
cat
Reaction 2 was described by Eq. (7):
n
fðaÞ ¼ ð1 À aÞ
2
Thermolysis kinetics
The obtained model parameters and the statistics are
presented in Table 1.
The thermolysis kinetic analysis was made using the
thermogravimetric experiment’s results at different heating
rates. The inverse and direct kinetic problems were solved
using Netzsch Thermokinetics software. Theoretical basis
of the software and practical problems examples are
described in [14]. At first, the inverse kinetic problem was
solved by model-free methods [15] in order to find the
A formal calculation can be interpreted as follows.
Reaction 1 (see Eq. 6) describes processes taking place in
the liquid phase. The hydrolysis changes the pH of the
liquid phase, i.e., hydroxyborohydride anion serves as a
catalyst of the reaction. The calculation demonstrates that
the kinetics of the stage is described by the equation of
autocatalysis (CnB). The hydrogen evolution is the limiting
Fig. 7 The results of modeling
the process of thermolysis of the
dihydrate of sodium
NETZSCH Thermokinetics
Mass/%
borohydride without solid
catalyst in two-stage model (II)
A
-
1
B
-
2
C
100
Step 1: n-th order with autocatalysis by B
Step 2:n-th order
(see Table 1). The correlation
2
coefficient R = 0.999, solid
line—calculation
13.0 K m–
1
9
8
7
6
0
0
0
0
–
–
–
1
1
1
8.8 K m
5.4 K m
2.5 K m
40
60
80
100
120
Temperature/°C
1
23

Thermoanalytical and NMR investigation of NaBH
4
Á2H
2
O thermolysis process
Fig. 8 The results of modeling
the kinetics of thermolysis of
sodium borohydride dihydrate
to a solid catalyst of the two-
stage model with two parallel
processes (see inset in
Figure and Table 2). The
correlation coefficient
2
R = 0.998, solid line—
calculation
NETZSCH Thermokinetics
Mass/%
Step 1: n-th order with autocatalysis by B
Step 2: Prout-Tompkins n-th order,a autocat
100
A
1
2
B
B
96
–1
1
1
0.5 K m
6
.8 K m–
–1
2.7 K m
92
8
8
8
7
8
4
0
6
35
45
55
65
75
Temperature/°C
step of the process. The fraction of reaction 1 (Eq. 6) is
about 71% in the entire process and corresponds to the
reaction in the liquid phase. The next stage is the solid-
phase reaction of nth order. This is the final dehydration of
the reacting system and the formation of hydroxy-
complexes. The products of this reaction oxidized at high
tabular value of the criterion for a given number of degrees
of freedom and 95% probability.
Thus, thermolysis process is due to peritectic decay and
can be adjusted not only by using of the catalyst but also by
changing of the water vapor pressure in the reaction
volume.
temperature in the air together with NaBH (see Fig. 3).
4
The use of other two-stage process schemes did not lead
to a satisfying result. In these cases, the Fisher criterion
value (i.e., the variances ratio statistics for the other
models) exceeded the Fcrit substantially. Fcrit is the
The catalytic thermolysis kinetics
Similar to the previous section, kinetic analysis for the
thermolysis process in the presence of a solid catalyst was
Fig. 9 The change in time of
concentrations of reagents for
the model. A-1 ? B-2 ? C
without catalyst at 70 °C
Isothermal Reactants vs. Time
Cryst - Kr. –0.125
Concentration/%
Temperature 70 °C
100
C
80
60
40
20
0
A B
0
2
4
6
8
10
12
14
Time/min
123

I. V. Arkhangelskii et al.
Table 1 Kinetic parameters obtained by TG data for non-catalytic thermolysis
-1
Kinetic model
E
a
/kJ mol
tÁS
log A
tÁS
n
tÁS
Log K
tÁS
FR 1
0.71
CnB
Fn
43
27
0.4
0.9
4
0.07
0.12
0.73
0.70
0.08
0.07
0.7
–
0.1
–
1.65
CnB Eq. 6, Fn Eq. 7, E
constant, FR 1 reaction 1 fraction in the whole process
a
activation energy, A pre-exponential factor in the Arrhenius equation, tÁS approximation uncertainty, R catalysis
Table 2 Kinetic parameters obtained by the TG data for catalytic
thermolysis
Prout–Tompkins Eq. (8) was chosen as the best fit for
reaction 2:
n
a
model
E
kJ mol
a
/
tÁS log A tÁS
0.4
n
tÁS
a
Log K tÁS
fðaÞ ¼ ð1 À aÞ a
ð8Þ
-
1
2
The model parameter’s values and the statistics are
presented in Table 2.
CnB
Bna
62
80
2
7
7
1.4
–
3
1.7
–
0.1
0.8
2.5 12
0.7 1.6 0.5
This formal approach can be interpreted as follows.
Both functions characterizing reactions 1 and 2 are caused
by the catalysis with sodium metaborate hydrate being the
final solid product in the temperature range of 40–80 °C.
Reaction 1 can be assumed to proceed in the liquid phase.
Reaction 2 can be assumed to proceed with solid NaBH4
participation as the product of peritectic decay. In both
cases, hydrogen evolution occurs.
a
CnB Eq. (5), Bna Eq. (7), E activation energy, A Arrhenius preex-
ponent, tÁS approximation uncertainty, R catalysis constant
carried out. Calculations were performed for the tempera-
ture range of 40–80 °C. The calculations have shown that a
two-stage model (II) with parallel processes gives the best
fit of the experimental data (Fig. 8):
These models give the opportunity to simulate the time
dependence of the concentrations of reactants and products
for various temperatures in the condition of the experiment.
Thus, Fig. 9 shows such dependence for the process at
A
1
2
B
B
ðIIÞ
70 °C without a catalyst (see Fig. 2).
In this model, the function type of reaction 1 (Fig. 7)
was similar to the function (6) of the previous model.
The figure shows that initial substance (liquid phase) is
exhausted in 6 min after the reaction beginning (curve A).
Fig. 10 The change in time of
concentrations of reagents for
two parallel reactions model
with catalyst at 70 °C
Isothermal Reactants vs. Time
Concentration/%
100
NBH_Kr, cryst + Co
Temperature 70 °C
B
80
60
40
20
0
A
0
1.0
2.0
3.0
4.0
Time/min
1
23

Thermoanalytical and NMR investigation of NaBH
4
Á2H
2
O thermolysis process
This process might correspond to reaction 1. Then con-
version reaction (reaction 2) proceeds with an oxoderiva-
tive and sodium metaborate formation. The process
duration is 15 min.
a crystalline NaBH4Á2H2O proceeds like hydrolysis in
solutions.
4. Catalytic thermolysis proceeds as an exothermic process
in the temperature range 40–90 °C, i.e., at much lower
temperatures than non-catalytic thermolysis. Change of
mass (* 23%) indicates an increase in the proportion of
hydrogen in the thermolysis products compared to non-
catalytic process, where the change in mass is about
45%. Consequently, the catalytic thermolysis decreases
the by-products of hydroxyborohydrides.
The situation is dramatically different in the presence of
a solid catalyst (Fig. 10).
The entire process duration is 4 min at the same tem-
perature. Both curves have inflection points in 1 min after
the process starts. This indicates the increasing impact of
the second stage of the process. This might be due to the
increasing rate of the sodium metaborate hydrate forma-
tion. Two minutes after the process starts, it transforms into
a solid phase. Thus, the liquid-phase formation defines the
process behavior in case of the catalytic thermolysis also. It
should be noted that thermolysis process does not occur at
temperatures below 35 °C and under external pressure of
water steam C 7 mm Hg even in the presence of a solid
catalyst.
5. Kinetic calculations showed that the non-catalytic
process proceeds as two successive stages. The
catalytic process had statistically two significant par-
allel processes. The values of the kinetic parameters in
a catalytic process were substantially more, i.e.,
thermolysis speed increases as increasing temperature
becomes much stronger.
Thus, the characteristic feature of thermolysis
NaBH4Á2H2O is formation of the hydroxyborohydrides in
case of non-catalytic process as well in the presence of a
solid catalyst.
Acknowledgements This work was supported by the RFBR under
Grant No. 15-03-0750.
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