Boron Oxide Production Kinetics Using Boric Acid as Raw Material

Article pubs.acs.org/IECR

Boron Oxide Production Kinetics Using Boric Acid as Raw Material Suna Balcı,*,† Naime Aslı Sezgi,‡ and Esin Eren† †

Chemical Engineering Department, Faculty of Engineering, Gazi University, Ankara, Turkey Chemical Engineering Department, Middle East Technical University, Ankara, Turkey

‡

ABSTRACT: Boron oxide with a content of 99.93 wt.% from the dehydration of boric acid was synthesized. Conversion of boric acid to boron oxide was completed within 3 days at low temperature range (T < 130 °C) and approximately 30 min at high temperature range (T > 130 °C) for the isothermal dehydration reaction in the temperature range of 80 and 350 °C. Apparent activation energies were 65 and 28 kJ·mol−1 for low and high temperature ranges, respectively. Thermogravimetric analysis (TGA) results showed that the reactions were nearly completed at around 330 °C, and activation energy for the first temperature region was found to be two-thirds of the isothermal value and the same for the second temperature region. Isothermal data analysis revealed that the apparent reaction order value was around 1.0 at low temperature range and decreased to 0.55 with temperature within the high temperature range.

1. INTRODUCTION Boron compounds have been used in a wide range of traditional industrial applications. Boron compounds have also played an important role in the synthesis of nanotechnology materials.1−9 In nanotechnology, the purity of the raw material has a great effect on the yield and product purity. Preferences for ore containing boron as a source show a shift to direct use of boron oxide since boron oxide is a refined product having the highest boron content that can be obtained from the mineral. Use of the high purity boron oxide in the production technologies provides advantages such as less energy consumption, low cost, and less environmental pollution in addition to its direct effects on the quality of the end product. Boron oxide has great importance in the synthesis of the fine boron compounds having superior properties. Studies related to the utilization of boron oxide, especially in the synthesis of materials such as boron carbide, boron nitride, optic glass, inorganic borates, and composite oxide structure catalyst, have drawn significant interest.1−3,5−14 Boron oxide was produced in muffle furnace, rotary kiln, fluidized bed, microwave oven, and thermobalance using different boron sources (ammonium perborate, ammonium tetraborate tetrahydrate, several boron minerals, borogypsum, etc.).15−24 These studies were based on slow and flash dehydrations. With a slow heating rate, final conversion to boron oxide was reached in a very long period. Although different boron sources were used, the dehydration reaction took place in two temperature regions.15,20,21,23 It was noticed that the product quality was very sensitive to source, temperature, heating rate, and microwave heating energy. The refined boron containing compound with minimum impurity can be produced from the dehydration of orthoboric acid. The dehydration takes place with two reactions in series: orthoboric acid to metaboric acid (Reaction 1) and metaboric acid to boron oxide (Reaction 2). The dehydration of boric acid starts at around 80 °C, and the first reaction is completed at around 130 °C. Reactions slow down at around 450 °C.17,24

HBO2 → 1/2B2O3 + 1/2H 2O (medium and high temp. regions: T > 130 °C (Reaction 2)

In most of the boron oxide production studies, several minerals were used as raw material in which mineral processing, improvement on the production technology, and investigation of the parameter effects (reaction time, temperature, particle size, reactor type) on the production were focused instead of refined product synthesis and rate expression development. The raw material purity affects reaction temperature range so Reaction 2 may start before the completion of Reaction 1. Since the mineral impurities could create inconsistency of the data with the reaction kinetics, use of Merck boric acid as the raw material was preferred in the present study. Kinetic studies in the literature were rare and generally based on the nonisothermal thermogravimetric data.24 Identification of multireaction systems with complex reaction rate expressions and determination of the model parameters are difficult. Therefore, to propose a simple model for such system is very important. In this study, using the nonisothermal and isothermal data, the simple rate expression was developed, and the synthesis conditions for the high purity boron oxide synthesis were also determined via dehydration of boric acid.

2. EXPERIMENTAL SECTION 2.1. Effect of Dehydration Temperature and Waiting Period on the Boron Oxide Content. The dehydration of around 5 g of boric acid (Merck 1.00165.1000) placed in Pt crucibles was performed from room temperature to 450 °C with different heating rates (2 and 5 °C·min−1) by waiting certain periods (0, 30, 60 min) at the selected reaction temperatures (130 and 330 °C) under an air atmosphere in muffle furnace (Table 1). Received: Revised: Accepted: Published:

H3BO3 ↔ HBO2 + H 2O (low temp. region: T < 130 °C)

(Reaction 1)

© 2012 American Chemical Society

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March 14, 2012 August 3, 2012 August 4, 2012 August 4, 2012 dx.doi.org/10.1021/ie300685x | Ind. Eng. Chem. Res. 2012, 51, 11091−11096

Industrial & Engineering Chemistry Research

Article

2.3. Isothermal Dehydration of Boric Acid. Five grams of Merck boric acid samples placed in Pt crucibles was put into the furnace previously heated to the desired temperature in the range of 80−350 °C. At different reaction time intervals, one crucible was taken out from the furnace, quenched in an ice− water mixture, and then weighted. Dwell times for low and high reaction temperature ranges were set as 4 days and 4 h, respectively.

Table 1. Boron Oxide Content of the Samples Synthesized at Different Dehydration Conditions waiting period (min) at heating rate (°C·min−1)

130 °C

330 °C

B2O3 (wt %)

5 5 2

30 30

60 60 60

80.29 97.90 98.52

3. RATE EXPRESSION MODEL Two different methods which were Coast-Redfern25−28 and integral were used to propose the rate expression for the boric acid dehydration reaction. Overall reaction stoichiometry for the dehydration of boric acid (Reaction 1 and Reaction 2) was expressed as follows

The boron oxide content of the dehydrated samples was measured using a volumetric titration method (ISO 1915 and 1916). 2.2. Nonisothermal Dehydration of Boric Acid. Thermal behavior of the samples (Merck and Eti Holding) was determined with a constant heating rate of 5 °C·min−1 from room temperature to 450 °C with an air flow rate of 75 cm3·min−1 using 951 Dupont Thermal Analist Thermogravimetric Analyzer.

H3BO3(s) → 1/2B2O3(s) + 3/2H 2O(g)

(Reaction 3)

Figure 1. Thermal behavior of boric acid samples from (a) Merck and (b) Eti Holding. 11092

dx.doi.org/10.1021/ie300685x | Ind. Eng. Chem. Res. 2012, 51, 11091−11096


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In this study, the temperatures for waiting periods were chosen as 130 and 330 °C. It was noticed that time of waiting at low temperature resulted in a considerable increase in the boron oxide content. For the low heating rate, although small, an increase in the content was also observed (Table 1). 4.2. Nonisothermal Data Analysis. In Figure 1, the thermal behavior of boric acid samples from Merck and Eti Holding is given. The two samples showed a similar thermal trend, and it was in good agreement with the reaction temperature range cited in the literature.20,21 Weight loss started at a temperature higher than 100 °C, consistent with literature. A remarkable weight loss was observed within the temperature range of 130−225 °C and continued up to 330−350 °C. Differential weight loss results showed two peaks at around 160 and 192 °C corresponding to the series reactions (Reaction 1 and Reaction 2). The dehydration was almost completed at a temperature of around 330 °C, and the final solid weight, w∞, and weight fraction, f, at 450 °C were found to be 0.56 and 0.44, respectively. For the proposal of the reaction rate expression, the conversion value of Merck boric acid to boron oxide was calculated from eq 1, and its variation with temperature is shown in Figure 2.

Using the weight loss data, the conversion of boric acid to boron oxide, xA, was defined in the following equation w A 0 − wA w0 − w xA = = wA0 w0(1 − f ) (1) where w0 and wA0 are weight of boric acid and solid weight to be converted to boron oxide at t = 0, respectively, and w and wA are the corresponding values at any reaction time, t. At t→∞ the final solid product was w∞ and the final weight fraction f was defined as f = (w∞/w0). The power law rate expression based on the solid weight for the dehydration reaction (Reaction 3) was used dxA = kw An−0 1(1 − xA)n = kapp(1 − xA)n dt

(2)

3.1. Rate Expression Model for Isothermal Data. Equation 2 was linearized ln

dxA = ln kapp + n ln(1 − xA) dt

(3)

The reaction order n and apparent rate constant kapp were found from the plot of ln(ΔxA/Δt) versus ln(1 − xAav) using the isothermal data. Rate constant k was calculated from the apparent rate constant relation (kapp = kwn−1 A0 ). Activation energy values (Eapp and E) were evaluated from Arrhenius plot. 3.2. Rate Expression Model for Nonisothermal Data. Variation of the dehydration temperature with respect to time was expressed linearly as T = T0 + qt with a constant heating rate of q and an initial temperature of T0. By inserting this linear heating rate equation into eq 2 and integrating it, the following relation was obtained.25 1 − (1 − xA)1 − n T 2(1 − n)

=

ARw An−0 1 ⎡ 2RT ⎤ − E ⎢1 − ⎥e RT qE ⎣ Ea ⎦

(4)

The linearization of the natural logarithm of eq 4 yields

Figure 2. Conversion of boric acid to boron oxide with respect to temperature (from TGA data).

⎡ 1 − (1 − x )1 − n ⎤ ARw An−0 1 ⎡ E 2RT ⎤ A ⎥ ln⎢ ln 1− = − qE ⎢⎣ E ⎥⎦ RT ⎣ T 2(1 − n) ⎦

4.3. Isothermal Data Analysis. Using the isothermal weight loss data, change of the conversion (eq 1) with time is given in Figure 3. With an increase in temperature, the time for the completion of the dehydration reaction decreased dominantly. The final conversion values for each isothermal experiment were reached in around 3 days at lower temperature range; however, the reaction time at higher temperatures was 45 min. While isothermal conversion values at lower temperatures were approximately twice the values of TGA, they reached the TGA values at higher temperatures. In the TGA experiments, the time period necessary for the completion of the reactions at any temperature was not allowed due to the heating rate (5 °C·min−1). Therefore the TGA experiments resulted in lower conversion values than the isothermal ones at low temperature range. For instance, to reach the reaction temperature of 130 °C, 20 s and 200 min were elapsed for the TGA and isothermal experiments, respectively. In other words, at low temperature range, the first reaction might reach equilibrium in the isothermal experiments. 4.4. Dehydration Rate Expression. Reaction Rate Expression Using Isothermal Data. The reaction order n, apparent reaction rate constant kapp, and reaction rate constant

(5)

The second term in brackets on the right side of eq 5 was negligible since 2RT was much much less than the activation energy. By plotting ln[(1 − (1 − xA)1−n)/(T2(1 − n))] versus 1/T the best reaction order and rate constant parameters were predicted. Gas−solid reaction generally takes place as first order with respect to reactive solid.29 For n = 1.0, the left side of eq 5 was turned into ln[(−ln(1 − xA))/T2], and the activation energy value was found from the plot of it with respect to 1/T.

4. RESULTS AND DISCUSSION 4.1. Boron Oxide Content of the Synthesized Materials. Boron oxide content of the samples determined using volumetric titration method at different dehydration conditions is given in Table 1. Since the metaboric acid formation in the dehydration series reaction is reversible, the waiting time at low temperature range is critical for reaching reaction equilibrium. Boron oxide formation reaction (Reaction 2) is generally completed up to a temperature of around 350 °C. This trend was also observed in the TGA data (Figure 1). 11093



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Figure 5. Arrhenius behavior of the boric acid dehydration.

solid content available for the second reaction was unavoidable. Conversion was defined according to the initial weight to be converted to boron oxide (eq 1); therefore, a further increase in the reaction temperature (where the second reaction mainly took place) caused a decrease in the reaction order. Arrhenius plot of the two reaction rate constants is given in Figure 5, and the reaction rate parameters are tabulated in Table 2. Arrhenius plot revealed two different linear segments

Figure 3. Variation of conversion with time for the boric acid dehydration reaction in (a) low and (b) moderate and high temperature ranges.

k (kapp = kwn−1 A0 ) values for each reaction temperature were determined using the plot of ln(ΔxA/Δt) versus ln(1 − xAav) in the temperature interval of 80−350 °C. The reaction order as a function of temperature and the Arrhenius behavior of the reaction rate constants are shown in Figures 4 and 5, respectively.

Table 2. Estimated Reaction Rate Parameters temperature range

n

From the TGA Data low 1 high 1 From the Isothermal Data low ∼1.0

high

1.0 < n < 0.5

activation energy (kJ/mol)

pre-exponential constant

E = 45 E = 24

A = 0.1895 s−1 A = 0.00056 s−1

Eapp = 65 E = 65

Aapp = 35 171 (s−1) A = 24 612 (wt(1‑n).s−1) Aapp = 0.4829 (s−1)

Eapp = 28 E = 27

A = 0.3449 (wt(1‑n).s−1)

at temperatures lower and higher than 130 °C, as seen in the variation of the reaction order with temperature (Figure 4). The temperature ranges were consistent with the ranges in the literature17,20,21,24 which showed the start of the boron oxide formation reaction after the completion of the metaboric acid formation. The apparent activation energies for low and high temperature regions were estimated as 65 kJ·mol−1 and 28 kJ·mol−1, respectively. Since Reaction 1 possessed high activation energy, it was concluded that this reaction played an important role in the completion of the overall reaction. Reaction Rate Expression Using Nonisothermal Data. Using the conversion values in Figure 2, ln[(−ln(1 − xA))/T2] values were plotted against 1/T in Figure 6, and the estimated rate constant parameters were tabulated in Table 2 for the first order kinetics. For temperatures less than 130 °C (1/T > 0.0025 K−1), the slope rose approximately twice the slope for

Figure 4. Variation of reaction order with temperature for the isothermal dehydration.

Boric acid dehydration reactions take place in series. The reaction order was found to be around 1.0 where Reaction 1 mainly took place at a lower temperature range. On the other hand, the order decreased with the further temperature increase. The presence of two different segments in the plot confirmed the fact that dehydration took place with two reactions in series. At high temperature range, a decrease in the 11094



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temperature range it fell to 0.55 with temperature in the isothermal data analysis.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +90 312 582 35 06. Fax: +90 312 230 84 34. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank State Planning Organization of Turkey (Project No:DPT/2003K120470-17) for financial support of this project.

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Figure 6. Coast-Redfern plot for the dehydration of boric acid.

the temperatures higher than 130 °C (1/T < 0.0025 K−1), and the activation energies for Reaction 1 and Reaction 2 were found to be 45 kJ·mol−1 and 24 kJ·mol−1, respectively. The consistency in the activation energies estimated from isothermal and nonisothermal data in both temperature ranges were observed. For the two reaction temperature regions, the average reaction order values were determined from the isothermal data analysis (Figure 4) and used to check the agreement with the nonisothermal data. For the low and high temperature ranges, using the calculated average reaction order values (low temperature range: nav = 1.03; high temperature range: nav = 0.79), the values of ln[(1 − (1 − xA)1−nav)/T2(1 − nav)] were also plotted as a function of reciprocal of temperature in Figure 6. In low temperature range, the two plots were overlapped. However in high temperature range, deviation became significant with an increase in temperature due to the decrease of the solid available for the second reaction. Although kinetic studies related to the dehydration of boric acid were rare, some similarities between the literature and this study were observed especially in nonisothermal analysis. In the study of Sevim and his co-workers,23 the boric acid from industry was used. Two different thermal behavior segments were observed as seen in the present study, but the lower and upper temperatures for the first reaction region were reported as 118 and 162 °C, respectively, in the rate parameter estimation. The lower and upper temperatures in the present study were 80 and 130 °C, respectively. In most of the boric acid dehydration literature studies, it was reported that the first reaction was completed at around 130 °C.17,24

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NOTATION E: activation energy [kJ·mol−1] Eapp: apparent activation energy [kJ·mol−1] f : final weight fraction at time infinity k: reaction rate constant [(weight to be converted to boron oxide)(1‑n)·(time)−1·(initial weight)n‑1)] kapp: apparent reaction rate constant [(time)−1(weight)n‑1] n: reaction order q: linear heating rate [K·min−1] T: reaction temperature [K] T0: initial temperature [K] t: time [min] w: weight of boric acid at any reaction time, t [g] w∞: final solid product at time infinity [g] wA: solid weight to be converted to boron oxide at any reaction time, t [g] wA0: solid weight to be converted to boron oxide at t = 0 [g] w0: weight of boric acid at t = 0 [g] REFERENCES

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5. CONCLUSION Boron oxide product with a content of 99.93 wt.% was synthesized with a heating rate of 5 °C·min−1 and keeping the temperatures at values of 130 and 330 °C for 30 and 60 min, respectively. The dehydration reaction was almost completed with a weight loss of ∼44% with two reactions in series in the temperature range of 80−350 °C. Arrhenius and Coast-Redfern plots revealed two linear segments for the temperatures lower and higher than 130 °C. The activation energy value for the metaboric acid formation estimated from TGA data was found to be two-thirds of the values predicted from the isothermal data analysis. As the reaction order was around 1.00 at the low 11095



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Boron Oxide Production Kinetics Using Boric Acid as Raw Material

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