11, 4 (44), 2010
R. Alibeyli
NEFT KİMYASININ VƏ NEFT EMALININ EKOLOJİ PROBLEMLƏRİ
ECOLOGICAL PROBLEMS OF PETROCHEMISTRY AND OIL REFINING
ЭКОЛОГИЧЕСКИЕ ПРОБЛЕМЫ НЕФТЕХИМИИ И НЕФТЕПЕРЕРАБОТКИ
PRODUCTION, HYDROLYSIS AND APPLICATIONS OF
SODIUM BOROHYDRIDE
R. Alibeyli
Gebze Institute of Technology, Department of Materials Science and Engineering
41400, Gebze, Kocaeli / TURKEY
In this study, technologically and economically more effective sodium borohydride
(SBH) synthesis process in a single step without using expensive sodium metal from anhydrous borax was developed. In a autoclave type fixed bed reactor (V=1000cm 3), the effect of process parameters including reaction temperature, reaction time and hydrogen
pressure on SBH efficiency was investigated. Optimum synthesis conditions were stated
as T= 550-560oC, PH2= 40-50 bar and t=4-6 hours. In these conditions, the production efficiency of SBH was found to be 85-90% by mole and 97-99 % in purity. It was observed
that SBH efficiency depends on synthesis conditions whereas its purity is not. Besides,
technological principles of process were developed. Different effective homogeneous
and inexpensive heterogeneous catalysts without using Pt, Ru, etc. for the hydrolysis of
SBH were developed. Homogeneous (liquid) catalysts were more active compared to solid catalysts at the same conditions. The effects of temperature, concentrations of both
SBH and NaOH on hydrolysis reaction were studied. Different principles for SBH hydrolysis were investigated to apply for fuel cells
INTRODUCTION
One of the main problems of hydrogen not to be used widely as a most clean and endless
energy resource in various areas is based on its storage. For the storage of hydrogen in world-wide
range many different methods have been used such as liquefaction; storage in tanks under high
pressure; storage as a hydride in metals, alloys and; storage in chemical compounds (boron hydrides etc.); and storage in carbon nanotubes etc. In terms of security and economical aspects, each
of these methods has advantages and disadvantages. According to recent researches in terms of
easy transportation and usability, storage of hydrogen in chemicals, particularly in boron hydrides
has been showing one of the most important methods. Boron hydrides have two main categories;
boranes (BnHn+4 or BnHn+6 ) and metal borohydrides (MeBH4). Boranes’ main drawback is being
toxic. For this reason, metal borohydrides attain more importance for hydrogen storage. Recently,
there are various metal boron hydrides and sodium borohydride (SBH) is the most important one
among them due to their physicochemical properties and their safety.
Sodium borohydride is a very effective reducing agent and is widely used in various industrial fields including: the textile, glass, ceramic and porcelain; in the pharmaceutical and perfume
industries; in the production of cellular plastics; as an additive in rocket solid fuels and as a fuel in
high energy jet motors and rockets.
SBH is also widely used for the separation of heavy metals from industrial wastes and in
reclamation processes. NaBH4 is used, in particular, for the removal of metals which are environmental pollutants such as lead and mercury from waste water.
One of the most important characteristics of sodium borohydride is that it is able to store
significant amount of hydrogen: 1 mol of sodium borohydride includes 10.6% wt. of hydrogen.
The release of the hydrogen contained in NaBH4 occurs during its hydrolysis reaction with water
in the presence of homogeneous or heterogeneous catalysts (1-5). During hydrolysis of NaBH4, the
2 moles of hydrogen of water in the reaction is also released. Consequently, hydrolysis of 1 mole
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of sodium borohydride with water produces 4 moles of hydrogen:
NaBH4 + 2H20  4H2 + NaBO2 + Heat (300 kJ)
As a result of the current development and spread of fuel cell technology, application of sodium borohydride has becoming
more important around the world as a hydrogen energy resource.
Sodium borohydride can be synthesized from various boron compounds by using many different methods (6-11). These methods can be carried out in different conditions and each of them
has more important advantages and disadvantages in point of technological and economical aspects.
Although there are many methods for synthesis of SBH, NaBH4 has been fabricated nowadays mainly in two methods in industrial scale.
One of industrial methods, known as the ‘Schlesinger’ process (12), consists of the production of sodium borohydride from boric acid, methyl alcohol, sodium metal and hydrogen by means
of a three stage method:
2Na + H2  2NaH
H3BO3 + 3CH3OH  B(OCH3)3 + 3H2O
B(OCH3)3 + 4NaH  NaBH4 + 3NaOCH3
The process is carried out under conditions: T=250-300ºC, P=3-10 bar.
The second industrial method, known as the ‘Bayer’ method, uses anhydrous borax, sodium
metal, quartz sand and hydrogen (13):
Na2B4O7 + 16Na + 7SiO2 + 8H2  4NaBH4 + 7Na2SiO3
The method is carried out at 450-500°C and at a pressure of 3 bar hydrogen. To reach the
reaction rate up to the necessary level, the method is applied in two stages.
The ‘Schlesinger’ and ‘Bayer’ methods have the serious disadvantages:
 in these methods, 4 moles of expensive Na metal are used to produce 1 mol SBH;
 more than 70 % of used Na metal is converted to a less important byproduct NaOCH3 or
Na2SiO3;
 the process is multistage;
 significantly high SBH producing cost.
One of the main reasons why sodium borohydride is not used widely either in the energy
field or another important industrial fields are that the existing industrial production cost is very
high (approximately 50-55 US $/kg). Therefore, research is currently being carried out into the
development of new technologies and methods of synthesis which might significantly reduce the
cost of production of sodium borohydride.
The aim of the present study is developing a more effective method of producing sodium
borohydride from anhydrous borax without using of sodium metal and in a single stage. In addition, to develop effective inexpensive catalysts for SBH hydrolysis and their application principles
in fuel cells are aim of the study as well.
EXPERIMENTAL
Single stage of SBH synthesis process was investigated at laboratory scale and its experimental principal scheme is shown in Fig.1. Experimental system works periodically with reference
to solid inputs but continuously reference to hydrogen.
Autoclave type tubular stainless steel reactor with fixed bed consists of 1000 cm3 general
volume and 100 mm internal diameter. The pressure of pure hydrogen (%99.9) fed by tank in the
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system has been controlled by regulator and special valve. The reactor is located within an electric
oven, the temperature of which is automatically adjusted by PID controller.
Formed SBH extracted from solid reaction products via various solvents. The removing of
solvent was done under vacuum at temperature of 40-60°C and the solid SBH was obtained. The
purity of SBH was analyzed by XRD diffraction technique.
Solid catalysts were prepared by impregnation method of metal salt solutions on the support
surface for the SBH hydrolysis.
Fig. 1. Laboratory scale experimental scheme for SBH synthesis
1-Hydrogen tank; 2-Regulator; 3-Fixed bed reactor; 4- Hydrogen distributor;
5-Thermocouple 6- Temperature controller (PID); 7-Gas wash unit;
8- Gas flow meter; 9-Special valve; 10:Valve; 11-Furnace
RESULTS AND DISCUSSION
1. Synthesis of SBH
In order to improve more effective sodium borohydride method economically, there have
been extensive investigations. A more economical technological novel method without using sodium metal in a single stage was developed. In this method, sodium borohydride has been synthesized from anhydrous borax in a single stage as follows:
Na2B4O7 + M(metal) + MOH(metal hydroxide) + H2 + Catalyst  NaBH4 + MO + Catalyst
This technological method has more advantages to available industrial methods like ‘Schlesinger’ and ‘Bayer’ as follows:
 no usage of expensive Na metal;
 single stage and can be performed by simple technological method ;
 used all inputs are cheap and produced in industry;
 cost of obtained pure NaBH4 is much cheaper (10-15 US $/kg).
To find optimum working conditions, the effect of SHB synthesis main parameters (temperature, reaction temperature, hydrogen pressure and reaction time) on SHB yield and purity was
studied.
In different reaction temperatures and hydrogen pressures, the effect of reaction time on
SBH yield are given respectively in Fig. 2 and Fig. 3.
In general, as reaction time and temperature increase, SBH yield increases as well as it is
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given in Fig. 2. Moreover, by rising reaction temperature maximum (%100 by mole) SBH yield is
reached in a shorter time: to reach maximum SBH yield at 570oC it takes 5 hours whereas it
takes about 7 hours at 560oC.
Fig. 2. The effect of reaction temperature and time on SBH efficiency (P H2=40bar)
Increasing hydrogen pressure results in SBH efficiency rise without depending on generally
reaction time, Fig. 3. Particularly, increasing of hydrogen pressure from 10 bar to 40 bar affects
seriously conversion from borax to SBH. Increasing of hydrogen pressure over 40 bar loses its
effect on SBH efficiency.
To state purity of synthesized solid SBH, its crystal structure searched by XRD analysis
method. It was found that the purity of SBH generally does not depend on synthesis parameters,
but it mainly depends on solvent type used for SBH removing and applied physical method.
Fig.3. The effect of hydrogen pressure and reaction time on SBH efficiency (T=550oC)
As a result, depending on obtained data the optimum parameters of SBH synthesis process
has stated as:
 Reaction temperature - 550-560 oC;
 Reaction time - 4-6 hours
 Partial pressure of hydrogen- 40-50 bar;
Main results of the process found at the optimum parameters:
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 SBH efficiency (by mole to anhydrous borax) - 85-90 %;
 SBH purity - 97-99 %.
2. Technological Principles of SBH Synthesis
Technological principles of single stage SBH synthesis process from borax was prepared
and demonstrated in Fig. 4, based on data obtained at laboratory conditions.
As it is seen from Fig. 4, SBH synthesis process contains mainly four sections:
 Preparation of solid input (raw materials, etc.);
 Synthesis of SBH;
 Extraction of SBH from reaction products via solvent;
 Acquisition of pure SBH and solvent returning to process.
To get anhydrous borax, to grind all solid inputs into 10-15µm size powder and to mix all of
them have been done in solid input preparation section. Then, solid inputs have been transferred
into reactor where SBH synthesis has been done in optimum conditions under presence of hydrogen. After that, SBH has been extracted from reaction products by various solvents (alcohols,
amines etc.) have been used. Finally, after removal of solvent under vacuum, pure solid SBH has
been obtained.
Fig.4. Block diagram of single stage SBH production from anhydrous borax
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3. Hydrolysis of SBH and Production of Hydrogen
In this study, homogenous (liquid) and heterogeneous (solid) catalysts have been developed
for the hydrolysis of SBH besides. Various organic and inorganic acids or their various mixtures’
liquid solutions have been used as homogenous catalysts. For the preparation of heterogeneous
catalysts, various oxides (Al2O3, SiO2 etc.) and natural zeolites were used.
Catalysts were prepared from mainly Ni, Co etc. like cheap metals’ salt solutions or their
mixtures’ absorption on the support by different methods.
Hydrolysis of SBH with water process has been investigated by using prepared homogeneous and heterogeneous catalysts. It was noticed that homogenous catalysts were more active in
hydrolysis process than heterogeneous catalysts at the same conditions according to obtained data.
With the help of selected heterogeneous catalyst, concentrations’ effect of NaBH4 and NaOH in
the solution on SBH hydrolysis were investigated and obtained data are shown in the Fig. 5 (a,b)
and 6 (a,b).
As it is seen from Fig. 5a, as SBH concentration increases from % 5 to % 40 by weight,
formed total hydrogen amount in the hydrolysis reaction increases in all reaction times.
Besides, at the same SBH concentration, as reaction duration increases from 5 min to 30
min total hydrogen amount increases considerably as well.
In addition, as SBH concentration increases hydrogen formation increases as well, Fig. 5b.
However, as reaction time increases hydrogen formation rate decreases considerably independently from SBH concentration in the solution.
Fig. 5. Concentration of SBH in the solution effect on hydrolysis reaction:
a) Effect on formed total H2 amount; b) Effect on H2 formation rate
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In Fig. 6a and Fig. 6b, concentration of NaOH effect on formed total hydrogen amount and
hydrogen formation rate respectively in different reaction durations in SBH solution in hydrolysis
reaction have been shown.
As it is seen, as NaOH concentration increases from %2.5 to %15 by weight formed total
H2 amount Fig. 6a, and H2 formation rate Fig. 6b decreases independently from reaction time.
At the same time, as it is seen from Fig. 6b, in general as reaction time increases hydrogen
formation rate decreases independently from NaOH concentration in the solution.
Fig. 6. Effect of NaOH concentration on SBH hydrolysis reaction in the solution:
a)
Effect on formed total H2 amount; b) Effect on H2 formation rate
4. Hydrogen Production Principles from SBH in Fuel Cells
SBH as a hydrogen storing has been used mainly in fuel cells, particularly in Proton Exchange Membrane (PEM) Fuel Cells. Various sodium borohydride hydrolysis methods can be
applied to depending on fuel cell application area and capacity of electrical power to be produced.
For this reason, different application principles of SBH in fuel cells were investigated in this study
and some of them are shown in Fig. 7.
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a) Transfer of NaBH4 solution onto liquid catalyst
b)Transfer of liquid catalyst onto solid NaBH4
c) Transfer of liquid catalyst onto NaBH4 solution
d) Transfer of NaBH4 solution onto solid catalyst
Fig.7. Various SBH hydrolysis methods and application principles in fuel cells
As it is understood from Fig. 7, each hydrolysis method contains mainly three sections:
Fuel (SBH) section, catalyst section, and fuel cell section. In Fig. 7a and 7d, fuel solutions have
been transferred onto liquid or solid catalyst. However, in Fig. 7b and 7c, liquid catalyst solutions
have been transferred onto either solid or liquid SBH.
It has been observed after some investigations that SBH fuel concentration decreased as
time passes in Fig. 7 b and 7 c hydrolysis methods. That is why depending on produced H2 amount
electrical energy obtained from fuel cell has been changing. This situation may prevent system’s
constant working. These drawbacks have been solved in Fig. 7a and Fig. 7d hydrolysis methods
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and according to 7b and 7c methods have the following superiorities:
 to keep constant concentrations of sodium borohydride and liquid catalyst solutions
along the work;
 to keep constant produced hydrogen amount along the work;
 to keep constant the energy to be produced in fuel cell along the fuel cell;
 secure and continuous working system along the work.
CONCLUSIONS
Sodium borohydride (SBH) synthesis process was developed directly from anhydrous borax
in a single stage without using expensive sodium metal. It was observed that SBH efficiency depends on mainly reaction temperature, reaction time and hydrogen pressure. However, synthesis
conditions were stated that they did not affect SBH efficiency considerably.
SBH synthesis process optimum parameters were found. Technological principles of the
process were developed.
For the hydrolysis of SBH with water, different effective homogeneous (liquid) and heterogeneous (solid) catalysts not containing expensive metals (Pt, Ru etc.) were developed.
At the same hydrolysis conditions, liquid catalysts have been observed more active than solid catalysts. It was stated that increasing hydrolysis temperature and increasing SBH concentration, hydrogen increases hydrogen efficiency whereas increasing NaOH concentration decreases
hydrogen efficiency. Different principles of SBH hydrolysis methods were studied to apply for
fuel cells.
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