Investigation of the reaction mechanism and kinetics of production of anhydrous sodium metaborate (NaBO2) by a solid-state reaction

Article abstract of DOI:10.1007/s11164-012-0580-3

In this study, the solid-state reaction mechanism and kinetics were investigated for production of anhydrous sodium metaborate (NaBO₂), an industrially and technologically important boron compound. To assess the kinetics of solid-state production of NaBO₂, the chemical reaction between borax (Na₂B₄O₇) and sodium hydroxide (NaOH) was investigated by use of the thermal analysis techniques thermogravimetry (TG) and differential thermal analysis (DTA). DTA curves obtained under non-isothermal conditions at different heating rates (5, 10 and 20 C/min), revealed five endothermic peaks corresponding to five solid-state reactions occurring at 70, 130, 295, 463, and 595 C. The stages of the solid-state reaction used for production NaBO₂ were also analyzed by XRD, which showed that at 70 and 130 C, Na₂B₄O₇ and NaOH particles contacted between the grains, and diffusion was initiated at the interface. However, there was not yet any observable formation of NaBO ₂. Formation of NaBO₂ was initiated and sustained from 295 to 463 C, and then completed at 595 C; the product was anhydrous NaBO ₂. Activation energies (E _a) of the solid-state reactions were calculated from the weight loss based on the Arrhenius model; it was found that in the initial stages of the solid-state reaction E _a values were lower than in the last three steps.

Full text of DOI:10.1007/s11164-012-0580-3

Res Chem Intermed (2013) 39:569–583
DOI 10.1007/s11164-012-0580-3
Investigation of the reaction mechanism and kinetics
of production of anhydrous sodium metaborate
(NaBO2) by a solid-state reaction
•
•
Mehmet Burc¸in Pis¸kin Aysel Kantu¨rk Figen
Hatice Ergu¨ven
Received: 28 March 2012 / Accepted: 26 April 2012 / Published online: 15 May 2012
Ó Springer Science+Business Media B.V. 2012
Abstract In this study, the solid-state reaction mechanism and kinetics were
investigated for production of anhydrous sodium metaborate (NaBO2), an indus-
trially and technologically important boron compound. To assess the kinetics of
solid-state production of NaBO2, the chemical reaction between borax (Na2B4O7)
and sodium hydroxide (NaOH) was investigated by use of the thermal analysis
techniques thermogravimetry (TG) and differential thermal analysis (DTA). DTA
curves obtained under non-isothermal conditions at different heating rates (5, 10 and
20 °C/min), revealed five endothermic peaks corresponding to five solid-state
reactions occurring at 70, 130, 295, 463, and 595 °C. The stages of the solid-state
reaction used for production NaBO2 were also analyzed by XRD, which showed
that at 70 and 130 °C, Na2B4O7 and NaOH particles contacted between the grains,
and diffusion was initiated at the interface. However, there was not yet any
observable formation of NaBO2. Formation of NaBO2 was initiated and sustained
from 295 to 463 °C, and then completed at 595 °C; the product was anhydrous
NaBO2. Activation energies (Ea) of the solid-state reactions were calculated from
the weight loss based on the Arrhenius model; it was found that in the initial stages
of the solid-state reaction Ea values were lower than in the last three steps.
Keywords Sodium metaborate ꢀ Solid-state reaction ꢀ Arrhenius ꢀ
Kinetics ꢀ Mechanism
M. B. Pis¸kin (&)
Department of Bioengineering, Yıldız Technical University, Istanbul 34210, Turkey
e-mail: mpiskin@yildiz.edu.tr
A. K. Figen ꢀ H. Ergu¨ven
Department of Chemical Engineering, Yıldız Technical University, Istanbul 34210, Turkey
e-mail: akanturk@yildiz.edu.tr
H. Ergu¨ven
e-mail: haticeerguven@gmail.com
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M. B. Pis¸kin et al.
Introduction
Since 2005 Turkey has been the world’s leading supplier of boron, with 72 % of the
world’s reserves and a market share of 37 %. The borates of sodium and their
derivatives are the materials mostly used by the chemical and metallurgy industries.
More than 230 boron derivatives are used in the production of [4,000 materials.
Sodium metaborate compounds (NaBO2ꢀ4H2O, NaBO2ꢀ2H2O, NaBO2), especially,
are borax derivatives that can be used both as the powder or in solution for
production of commercial boron compounds [1].
NaBO2 is used in the chemical industry for production of photographic and textile
chemicals, detergents, cleansers, and adhesives. It is also used for production of
sodium perborate compounds (NaBO3ꢀnH2O) as the main material. NaBO2 can also be
incorporated in liquid laundry detergents for pH control, enzyme stabilization, and for
its builder properties. Sodium metaborate tetrahydrate (NaBO2ꢀ4H2O) is incorporated
in many proprietary water-treatment chemicals requiring pH control and corrosion
inhibition; these are used for protection against corrosion in central heating systems
and cooling towers [2].
In recent years, because of the development and popularity of the use of
hydrogen-powered energy, the demand for hydrogen storage with high energy
density has increased. Hydrogen can be stored safely in boron minerals/compounds,
for example as NaBH4. NaBH4, a known reducing agent, has been used as an anodic
fuel in fuel cells or as a hydrogen-storage medium because of its high hydrogen-
storage capacity (10.6 wt%). Use of anhydrous NaBO2 in the energy industry, and
the importance of commercial borates, are, therefore, gradually increasing [3–16].
Solid-state synthesis of boron compounds has been studied. Magnesium–boron
compounds can be produced by solid-state reactions. Mg2B2O5 and Mg3(BO3)2
compounds are synthesized on the basis of solid-state principles [17]. The formation
of boron nitride nanotubes by a solid-state process has also been investigated. Boron
nitride nanotubes have been produced by annealing of ball-milled boron nitride
powder in a nitrogen atmosphere. It has been shown that ball-milling is vital to
formation of disordered boron nitride nanostructures, from which the nanotubes
seem to grow by a solid-state process [18].
Generally, NaBO2 is synthesized by a method from a mixture of borax and
sodium hydroxide by the reaction shown in Eq. (1) [19]:
1
1= Na B O þ = NaOH þ 5=4 H O ! NaBðOHÞ ꢀ2H O
ð1Þ
4
2
2
4
7
2
2
4
When the reaction is complete, the saturated solution is filtered and cooled to room
temperature, and crystals are formed. After synthesis, calcination must be performed
to obtain anhydrous NaBO2, in accordance with the equations:
NaBðOHÞ4ꢀ2H2O ! NaBðOHÞ4þ2H2O
NaBðOHÞ4! NaBO2 þ 2H2O
ð2Þ
ð3Þ
Investigation of the conditions for synthesis of anhydrous NaBO2 from
concentrated tincal (NaBO2ꢀ10H2O) has previously been discussed in detail by
our group. Anhydrous NaBO2 was synthesized, in a single step, from concentrated
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Reaction mechanism and the kinetic investigation
571
tincal. It was decided that concentrated tincal could be used as a starting material for
production of anhydrous NaBO2 without any purification procedure. The hydroxide
phase is transformed into the hydrate structure by further heating, and the anhydrous
form is obtained by heating at 400 °C for 5 h [20, 21].
Sodium metaborate tetrahydrate (NaB(OH)4ꢀ2H2O) was also produced under the
action of ultrasonic irradiation. It was shown that the reaction conditions (amount of water,
temperature, and time) were of crucial importance in the synthesis of NaB(OH)4ꢀ2H2O.
The optimum conditions for the production process were 26 % water by weight, borax
particles of size 250 150 lm, and an irradiation time 60 min at 80 °C [22].
In this study, anhydrous NaBO2 was produced on the basis of the principles
of solid-state synthesis. The purpose of this study was to elucidate the reaction
mechanism and the reaction kinetics for production of anhydrous NaBO2 via a
solid-state reaction. The chemical reaction between borax (Na2B4O7) and sodium
hydroxide (NaOH) was examined by use of thermal analysis (DTA, and TG) and
X-ray diffraction analysis (XRD) to determine the reaction mechanism. Non-
isothermal analysis was adopted to enable understanding of the reaction kinetics and
activation energies (Ea), and the pre-exponential factors (k0) were calculated by
using the Arrhenius kinetic model. By observation of the crystalline phase and study
of the kinetics, the optimum reaction conditions were determined and a solid-state
reaction mechanism has been proposed.
Experimental
Materials and characterization
Anhydrous borax (Na2B4O7) and sodium hydroxide (NaOH) were used as the
starting materials in our experimental studies. Na2B4O7 (ETIBOR-68) containing
68.00 % B2O3 and 30.27 % Na2O was obtained from Bandirma Eti Mine Works,
Turkey. Sodium hydroxide (NaOH, C95 % pure) was purchased from Merck.
The analyses performed for characterization in this study were:
(1) DTA-TG analysis: Thermal analysis of samples was performed by use of a
Perkin Elmer Diamond TG\DTA instrument, which was calibrated by using
the melting points of indium (Tm = 156.6 °C) and tin (Tm = 231.9 °C) under
the same conditions as used for the sample. Samples were ground in an agate
mortar and stored under an inert atmosphere before the analyses. Analyses
were carried out at a heating rate of 10 °/min in an N2 atmosphere at a constant
flow rate of 100 ml/min. The samples were placed in standard Al2O3 crucibles
and heated to 700 °C.
(2) XRD analysis: The crystalline structures of the solids were determined by
XRD. X-ray analysis was conducted at ambient temperature by use of a Philips
Panalytical X’Pert-Pro diffractometer with CuKa radiation (k = 0.15418 nm)
under operating conditions of 40 mA and 45 kV.
(3) FT-IR analysis: Attenuated total reflectance (ATR) FT-IR spectroscopy
(Perkin Elmer Spectrum One) was used for identification of chemical bonds
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M. B. Pis¸kin et al.
of the samples. Before analysis, the crystal area was cleaned and the
background was collected; the solid material was placed over the small crystal
area on the universal diamond ATR top plate. The FT-IR spectrum was
obtained after application of force to the sample, pushing it on to the diamond
surface. The IR spectrum was recorded in the spectral range 4,000–650 cm-1
at ambient temperature, and al a resolution of 4 cm-1
,
.
(4) SEM analysis: Scanning electron microscopy (Cam Scan) was used for
identification of microstructures in the crystals. Before analysis the crystals
were fixed to the sample plate with self-adhesive tape; they were then coated
with gold for conductivity.
(5) ICP–OES analysis: Elemental analysis of the sample was performed by ICP–
OES (Perkin Elmer, Optima 2100 DV). To prepare samples for analysis, the
mineral was digested in hydrochloric acid–nitric acid–hydrofluoric acid–
phosphoric acid (HCI–HNO3–HF–H3PO4) solution. Samples were analyzed at
least three times and mean values were used as a single observation.
Procedure and setup
Thermal analysis of the chemical reaction between borax (Na2B4O7) and sodium
hydroxide (NaOH) was performed to understand the solid-state mechanism of NaBO2
production. A mixture of Na2B4O7 and NaOH was prepared in a molar ratio of 1:2, in
accordance with the NaBO2 productionreactiongiven in Eq. 4. The well-mixed powder
was placed in the Al2O3-based high-temperature crucible of the TG/DTA instrument.
Na2B4O7 þ 2 NaOH ! NaBO2
ð4Þ
Differential thermal analysis (DTA) was performed from room temperature to
700 °C at different heating rates (5, 10 and 20 °C/min) to record any temperature
difference between the sample and the reference. DTA is universally accepted by
mineralogical laboratories as a rapid and convenient means of recording the thermal
effects that occur as a sample is heated [23]. Temperature difference (DT) was
plotted against temperature; the DTA curves obtained are given in Fig. 1. According
to the results from non-isothermal DTA analysis, the solid-state reaction temper-
atures were 70, 130, 295, 463, and 595 °C, so solid-state reactions were performed
at these specific temperatures for 5 h in a calibrated high-temperature furnace.
The standard deviation of the internal temperature of the furnace was determined
as 1 °C. The crystalline phase and the chemical structure of the powders were
examined by use of XRD to determine the optimum solid-state reaction temperature
(Fig. 2). According to the crystalline phase results, the anhydrous NaBO2 solid-state
synthesis reaction occurred at 595 °C. FT-IR analysis were performed to
characterize the molecular structure of the products; the FT-IR spectra obtained
at different production temperatures are given in Fig. 3.
Kinetic analysis of the reaction for solid-state production of NaBO2 was investigated
on the basis of the Arrhenius kinetic model and Eqs. 5, 6, 7, and 8, below. Figure 4
shows the TG curve and the fractional conversion plots for the solid-state reactions. The
order of the solid-state reaction was assumed to be unity (n = 1). According to the
123

Reaction mechanism and the kinetic investigation
573
Fig. 1 Differential thermal analysis curves for the chemical reaction between Na2B4O7 and NaOH
ꢂꢀ ꢁꢀ ꢁꢃ
dW
dt
1
W
Arrhenius method, values of log
were plotted against 1/T (Fig. 5). The
equations for all the solid-state reactions are given in Table 1. The slopes of these
lines, which were used to calculate Ea, were determined from the intercepts (Table 2).
dW
¼ k ꢁ Wn
ð5Þ
ð6Þ
dt
ꢄ
ꢅ
ꢂE
k ¼ Ar:exp
R:T
ꢄ
ꢅ
dW
ꢂE
¼ Ar: exp
ꢁ W
ð7Þ
ð8Þ
dt
R:T
ꢆꢄ
ꢅ
ꢄ
ꢅꢇ
dW
dt
1
E
log
ꢁ
¼ log Ar ꢂ
W
2:303:R:T
Elemental analysis, microstructural analysis, and bulk density measurements were
also conducted to identify all the properties of the final product synthesized under
optimum reaction conditions (595 °C and 5 h). ICP–OES results and SEM images
are given in Table 3 and Fig. 6, respectively.
Results and discussion
Anhydrous sodium metaborate (NaBO2) production by solid-state reaction
The reaction powder obtained from mixing of the starting materials (Na2B4O7 and
NaOH), was heated from the room temperature to 700 °C at heating rates of 5, 10
123

574
M. B. Pis¸kin et al.
Fig. 2 XRD patterns of powders synthesized at different temperatures
123

Reaction mechanism and the kinetic investigation
575
Fig. 2 continued
and 20 °C/min. The corresponding DTA curves are illustrated in Fig. 1. As
indicated by the DTA curves, five endothermic peaks were observed; these peaks
corresponded to different solid-state reactions. Taking into consideration each
heating rate, the profiles of the DTA curves were indicative of similar behavior, and
123

576
M. B. Pis¸kin et al.
Fig. 3 FT-IR spectra of powders synthesized at different temperatures
123

Reaction mechanism and the kinetic investigation
577
(a)
(b)
Fig. 4 Thermogravimetric curve (a) and fractional conversion plots (b) for the solid-state reactions
the maximum peak temperatures (Tm) of the DTA curves were noted, to define the
solid-state reaction temperatures. The assumption that the Tm occurs when the
reaction rate is maximum was supported by the experimental work. It was observed
there was no significant difference in the maximum peak temperature (Tm) of the
DTA curves. It is, therefore, best to use the average maximum peak temperature
(RTm) for the solid-state synthesis reactions. The RTm of solid-state reactions were
123

578
M. B. Pis¸kin et al.
Fig. 5 Arrhenius kinetic model plots for the solid-state reactions
Table 1 Arrhenius model
Reactions
Equations
R2
kinetic equations for the solid-
state reactions
1
2
3
4
5
y = -3458x ? 7.64
0.9559
0.9717
0.9704
0.9774
0.9703
y = -2293x ? 3.91
y = -10622x ? 16.90
y = -30601x ? 39.482
y = -100479x ? 113.06
Table 2 Activation energies
(Ea) of the solid-state reactions
Reaction
Ea (kJ/mol)
1
2
3
4
5
66.21
43.90
203.38
585.92
1923.88
calculated as 70, 130, 295, 463, and 595 °C. At these five temperatures, the reaction
powder was heated for 5 h to determine the reaction mechanism.
The stages of the solid-state reactions were analyzed by XRD. The XRD pattern
of NaBO2 standard is given, as a reference, below the other XRD patterns in Fig. 2,
to enable comparison of the results from the analysis with those of the product,
NaBO2. NaBO2 standard was prepared as indicated in our previous study [20].
123

Reaction mechanism and the kinetic investigation
579
Table 3 ICP–OES elemental
Element
Concentration (ppm)
analysis results for the final
product
S
745.43
140.85
15.425
[0.5
Ca
Fe
Si
As
[0.02
Fig. 6 SEM images of the final product
As indicated by the XRD results, formation of NaBO2 was accompanied by an
increase in the solid-state synthesis temperature. At low temperatures, the main
phase was the hydroxide structure and NaBO2 did not form. Further heating of the
mixture caused the hydroxide phase to be converted into the hydrate phase as the
reaction temperature was increased to 463 °C, and the sodium metaborate structure
began to form. As can be seen from the XRD pattern of the sample synthesized at
595 °C, all the peaks detected were associated with the anhydrous NaBO2 structure;
the sodium boron hydroxide and sodium borate hydride phases were no longer
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M. B. Pis¸kin et al.
present. It was determined that the solid-state synthesis temperature required to
produce NaBO2 was 595 °C.
Figure 3 shows the FT-IR spectra of the products obtained at 70, 130, 295, 463,
and 595 °C. The FT-IR analysis showed the stretching mode of O–H at the
beginning of the reaction, but this disappeared as the temperature increased. The
bands at 1,573 and 1,571 cm-1 were assigned to the bending mode of H–O–H.
The bands at 1,436 and 1,422 cm-1 were assigned to asymmetric and symmetric
stretching, respectively, of B–O.
Kinetic investigation of the solid-state reaction
Figure 4a shows the TG curve obtained during heating of the powder obtained by
mixing of the starting materials (Na2B4O7 and NaOH). In addition, the fractional
conversion values were plotted against reaction temperature for the five basic solid-
state reactions; these are shown in Fig. 4b.
Five stages of mass loss corresponding to the five solid-state reactions were
apparent from the TG curve, as was also apparent from the DTA curves. In the first
two steps the total weight loss was 0.341 and 5.898 % in the temperature ranges
38.01–72.71 and 72.71–156.08 °C, respectively. In the third, fourth, and fifth steps,
corresponding to the temperature ranges 275.55–338.65, 425.16–501.54, and
582.71–603.88 °C, respectively, the weight losses were 8.094, 1.338, and 0.081 %.
Non-isothermal kinetic calculations were performed for all the steps on the
basis of the Arrhenius equation, assuming a first-order reaction. Values of
hꢈ ꢉꢈ ꢉi
dW
dt
1
W
log
were plotted against 1/T (Fig. 5) and the kinetic plots (Table 1)
were used to calculate the kinetic data (Table 2). For the initial stages of the solid-
state reaction (first and second steps), Ea values were lower than for the last three
steps. In the last two steps, Ea values increased because of the formation of NaBO2,
and even more energy was required to produce anhydrous NaBO2.
At 70 and 130 °C, Na2B4O7 and NaOH particles contacted between the grains,
and then diffusion was initiated at the interface. However, there was not yet any
observable formation of NaBO2. Formation of NaBO2 was initiated and sustained
from 295 to 463 °C, and then completed at 595 °C, with formation of anhydrous
NaBO2.
Mechanism of NaBO2 production by solid-state reaction, and structural
observations
Structural characterization of the final product, and confirmation of its synthesis
under the optimum conditions (595 °C, 5 h) were achieved by use of XRD, FT-IR,
SEM, ICP–OES, and the bulk density test (TS 2573).
The microstructure of the final product; NaBO2 was investigated by SEM
analysis, the results for which are shown in Fig. 6. According to the SEM images,
the particles were similar to each other and were distributed homogeneously;
average particle size was calculated as approximately 630.56 lm.
123

Reaction mechanism and the kinetic investigation
581
Fig. 7 Mechanism of formation of anhydrous NaBO2 by the solid-state reaction
ICP–OES was used to determine five elements (S, Ca, Fe, Si, and As) in the final
product, NaBO2. The results are summarized in Table 3. It can be seen that all these
elements were present in trace amounts, originating from the main raw material.
Bulk density measurements were performed as stated in the Turkish Standard
2573 [24]. Bulk density is defined as the mass of many particles of the material
divided by the total volume they occupy. Bulk density is an important property of
solid products taken into account in the packaging of the commercial product [25].
The final product was analyzed at least three times and the mean values were used
as a single observation. The bulk density of the final product, NaBO2, was
calculated as 1.0249 g/cm3. For this reason, the bulk density of NaBO2 could be
reported as ‘‘freely settled’’ and there was no need for a specified compaction
process during packaging.
In view of these analytical results, a solid-state reaction mechanism was
proposed; this is illustrated in Fig. 7. As the synthetic temperature increased,
Na2B4O7 at the surface reacted with NaOH forming a shell of NaBO2 accompanied
by release of H2O. Because of melting of the NaOH, the Na2B4O7 was enclosed.
With increasing synthesis time, the reactive NaOH diffused into the core of the
Na2B4O7, thickening the NaBO2 shell. The diffusion process was completed by
exhaustion of the raw materials, eventually forming the pure anhydrous NaBO2
phase.
Conclusion
In this study, the mechanism of synthesis of anhydrous NaBO2 from Na2B4O7 was
investigated in detail. Anhydrous NaBO2 is an industrially and technologically
important boron compound, specifically used in NaBH4 production. If anhydrous
NaBO2 could be synthesized more economically, storage of hydrogen as NaBH4
could be more applicable.
The following points result from this study:
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M. B. Pis¸kin et al.
(1) A general procedure based on a solid-state reaction mechanism was developed
for synthesis of anhydrous NaBO2. Anhydrous NaBO2 was synthesized from
Na2B4O7 in a one-step, solid-state reaction.
(2) The hydroxide phase was transformed into the hydrate structure by heating,
then the anhydrous form was obtained. The reaction was studied by use of
analytical methods, and the optimum reaction conditions were found to be
reaction at 595 °C for 5 h.
(3) As indicated by kinetic investigation of the solid-state reaction, synthesis of
NaBO2 occurred in five reaction steps. In the initial stage of the solid-state
reaction the Ea values were lower than those in the last steps. Ea values
increased with increasing temperature.
(4) Synthesis of anhydrous NaBO2 from Na2B4O7 was initiated on the surface by
melting of NaOH, which enclosed the Na2B4O7. With increasing calcination
temperature reactive NaOH diffused into the core of Na2B4O7 thickening the
NaBO2 shell.
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