Hydroxyapatite prepared from biomineral calcium carbonate

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Hydroxyapatite prepared from biomineral calcium carbonate resources: a Ru-catalyst support for

hydrogen generation

NAKAZATO Tsutomu†, MURATA Yuma, KAI Takami

Department of Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima, 890-0065 Japan

(Received August 17, 2015, Accepted June 16, 2016)

In this study, we hydrated calcines of biomineral calcium carbonate resources such as eggshells and

scallop shells and converted them to hydroxyapatite (HAp) under different conditions for use as Ru catalyst supports. The catalytic activity of Ru/HAp in the hydrolysis of sodium borohydride was investigated by measuring the volume of hydrogen gas generated from the alkaline solution at 298 K. The results clarify that, when different calcium sources are used as starting materials, high Ru content does not always produce high catalytic activity. The hydrogen generation rate is closely correlated with average pore size of the catalyst. High-performance catalysts obtained in the present study have average pore sizes of 20 nm or less and a molar ratio Ca/P of approximately 1.60.

Keywords: biomineral carbonate resource, hydroxyapatite, catalyst support, ruthenium, sodium

borohydride hydrolysis

1. Introduction

Domestic shipments of chicken eggs in Japan now reach over 2.4 million tons per year, 14% of which are used as ingredients in processed foods such as confectionery, fish paste, and mayonnaise. Because the eggshell (ES) accounts for 11.6% of an egg’s total weight, nearly 40 thousand tons of eggshells are discarded by the food-processing industry. Approximately 20% of this material is recycled for use as chalks, line powders, and soil-improvement agents, but disposing of the rest of the material is done at a significant cost. The same situation applies to scallop shell (SS), whose annual domestic discharge exceeds 0.2 million tons per year.

The main component of eggshells and scallop shells is calcium carbonate, which is more porous than limestone. Several studies have focused on the conversion of calcined eggshell and sea shell to more useful products such as hydroxyapatite (HAp; Ca10(PO4)6(OH)2)1–5).

Although HAp is mostly used in medical and dental biomaterials, it has also been used as a catalyst or catalyst support for various reactions6–8) because of its acid–base properties, ion-exchange ability, and adsorption capacity. Exploiting hydroxyapatite’s stability under alkaline conditions, Jaworski et al.9)

recently demonstrated that Ru-ion-exchanged HAp (Ru/HAp) provides an efficient catalytic activity and excellent durability at room temperature for hydrogen generation via hydrolysis of sodium borohydride (NaBH4), which is an excellent hydrogen-storage material with high storage capacity (10.6% in mass in H capacity)10). The overall reaction is

NaBH4 + 2H2O 4H2 + NaBO2,

ΔH = –216.7 kJ/mol-NaBH4. (1) To prevent self-hydrolysis, the storage of NaBH4 is implemented in a high-pH NaOH solution. The characteristic feature of reaction (1) is that half of the hydrogen comes from the chemical decomposition of H2O, which means that the use of NaBH4 as a hydrogen carrier can contribute to the goal of reaching a low-carbon society.

Based on the facts mentioned above, the development of HAp catalyst support from biomineral calcium carbonate resources can lead to a new recycling model for eggshells and scallop shells. The aim of this study is to investigate how the type of calcium source (reagent-grade CaO, eggshell or scallop shell), hydration condition of the corresponding calcines, and aging condition during the synthesis of HAp affect the catalytic performance

†E-mail: [email protected]

Journal of Ecotechnology Research, 18[1], 11- 16 (2016) ©2016 International Association of Ecotechnology Research

Original Article

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Journal of Ecotechnology Research, 18[1], 11- 16 (2016)©2016 International Association of Ecotechnology Research

Original Article

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of Ru/HAp for the hydrolysis of NaBH4 in alkaline solution at 298 K.

2. Materials and Methods

2.1 Preparation of HAp ES and SS were used as CaCO3 sources. The

materials were ground into particles with an average diameter of 5 μm. To remove organic substances for long-term room-temperature storage, these powders were first heated to 773 K for 2 h in an air atmosphere in a muffle furnace. The pretreated powders were then calcined at 1173 K for 2 h under nitrogen-gas flow to convert CaCO3 to CaO. For comparison, high-purity reagent-grade CaO (Kantokagaku) was also used.

To obtain Ca(OH)2, 2.8 g of CaO was hydrated in 100 mL of distilled water at room temperature under rigorous or mild stirring (300 or 120 rpm) for 1 h. Then, 60 mL of 0.5 M phosphoric acid solution prepared by diluting 85% phosphoric acid (JIS Special Grade, Wako Pure Chemical Industries) with distilled water was added dropwise to the Ca(OH)2 suspension at a titration rate of 2 mL/min up to the stoichiometric Ca/P molar ratio of 1.67 for HAp while under stirring at 300 rpm. The suspension was aged to form HAp at the same stirring rate but at either 373 K for 1 h or at 298 K for 72 h. The conditions for hydration and aging are summarized in Table 1.

After aging, the suspension was subjected to vacuum filtration to efficiently remove water. The precipitate obtained was then dried at 343 K for 12 h, crushed, and then sieved to yield HAp powder with grains of 180 μm or less in diameter. Table 1 Conditions for hydration and aging.

Preparation ID Hydration Aging A 300 rpm, 1 h 373 K, 1 h B 300 rpm, 1 h 298 K, 72 h C 120 rpm, 1 h 373 K, 1 h D 120 rpm, 1 h 298 K, 72 h

2.2 Ru-ion exchange on HAp

Ru-ion exchange at the HAp surface was conducted according to the work by Jaworski et al.9). Ruthenium trichloride n-hydrate (0.04149 g, no grade, Wako Pure Chemical Industries) was first dissolved in 20 mL of distilled water. The RuCl3 solution was then mixed with 2.0 g of the HAp powder and stirred at 800 rpm for 10 min, followed by 10 min of sonication and 20 min of stirring at 300 rpm. The treated HAp powder was filtered and washed twice with 200 mL of distilled water. The powder sample was then dried overnight at 343 K and then sieved to obtain a grain diameter of 180 μm or less.

The Ru-solution-treated HAp powder (Ru/HAp) was labeled xx-y, where xx is the identification (ID) of the starting material of the calcium source (ID = ES, SS, CaO) and y gives the preparation conditions of HAp, as shown in Table 1 (A, B, C, D). For example,

SS-B corresponds to a Ru/HAp powder prepared under condition B by using scallop shell-derived Ca.

2.3 Characterization

The Ru/HAp powder was characterized by powder X-ray diffractometry (XRD, X’Pert PRO MPD, PANalytical), Brunauer–Emmett–Teller (BET) analysis with a nitrogen adsorbent (BELSORP-18, BEL, Japan), laser-diffraction particle-size analysis (SUCELL&RODOS, Sympatec GmbH), and inductively coupled plasma–atomic emission spectrometry (ICP-AES) (Optima 3100 RL, PerkinElmer).

Powder XRD patterns were measured by using Cu-Kα radiation with a generator voltage of 45 kV and a tube current of 40 mA. The BET specific surface area was determined by multipoint analysis in the relative pressure range of 0.05–0.35. An average pore diameter Dp [nm] was calculated by the following equation:

Dp = 4 Vp × 103/SBET (2) where Vp is the pore volume [cm3/g] up to the relative pressure of 0.990 (corresponding to 190 nm in pore diameter) and SBET is the BET specific surface area [m2/g]. The mean particle size was calculated based on five measurements of average particle size. The quantities of Ru, Ca, and P of the catalyst sample were measured by ICP-AES analysis of the solution in which 0.03 g of the powder was dissolved in 30 mL of a nitric acid solution.

Fig. 1 Schematic diagram of experimental setup for

hydrogen-generation measurements.

2.4 Hydrogen generation from NaBH4 The catalytic activities of the Ru/HAp catalysts

were determined by measuring the amount of H2 generated during the hydrolysis of NaBH4.

Figure 1 shows the experimental setup used for the hydrogen-generation measurements. First, 0.2 g of NaBH4 was dissolved in 20 mL of a 3% aqueous NaOH solution. The mixture was vigorously stirred in a sealed flask at a constant temperature of 298 K by using a cooling plate (SCP-85, As One). The reaction was then triggered by adding 0.1 g of the Ru/HAp

NaBH4 solution

Three-necked flask

Magnetic stirrer300

25 ºCCooling plate

Catalyst inletThermometer

Water

Silicone rubber tube

Measuring cylinder

NAKAZATO Tsutomu et al Hydroxyapatite prepared from biomineral calcium carbonate resources: a Ru-catalyst support for hydrogen generation

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catalyst to the flask. The volume of H2 gas produced during the reaction was measured by the downward displacement of water in a cylinder.

The catalytic activity and induction period were determined, respectively, by the linear gradient of the hydrogen-generation curve between about 100 and 400 mL-H2 evolution, and the zero-volume evolution time, which was obtained by extrapolating the above-mentioned linear gradient of the hydrogen-generation curve to zero volume.

3. Results and Discussion Visual observation during the procedure of Ru-ion

exchange on HAp confirmed that the color of the powder sample changed to black, implying that Ru-ion exchange with the HAp surface was successful in forming the Ru/HAp catalyst. The Ru content in the sample was estimated to be near 1 wt.% during the preparation because the number n of water molecules in the reagent RuCl3·nH2O was considered to be in the range of 1–3.

10 20 30 40 50 60 70 80

Inte

nsity

[a. u

.] CaO-A

CaO-B

CaO-C

CaO-D

(a)

10 20 30 40 50 60 70 80

Inte

nsity

[a. u

.] ES-A

ES-B

ES-C

ES-D

(b)

10 20 30 40 50 60 70 80

Inte

nsity

[a. u

.]

2 [o ] (Cu, K)

SS-A

SS-B

SS-C

SS-D

(c)

Fig. 2 XRD patterns of Ru/HAp catalysts prepared

from various calcium sources: (a) CaO, (b) ES, and (c) SS.

3.1 XRD patterns Figure 2 shows XRD patterns of Ru/HAp catalysts

prepared under various conditions and by using different types of calcium sources. All peaks are consistent with the main peaks of HAp, irrespective of the type of calcium source and the preparation conditions. No peaks for ruthenium oxide are observed because of low metal content in the catalysts.

For condition A, the HAp peaks are the highest for CaO-A, followed by SS-A and ES-A, whereas for condition B, the HAp peaks are rather small for all catalysts. Overall, the intensities of the HAp peaks for conditions A and C are greater than those for conditions B and D, respectively, because of rapid aging.

0 50 100 150 200 250

CaO-ACaO-BCaO-CCaO-D

0

100

200

300

400

500

Vol

ume

of H

2 gen

erat

ed [m

L] (a)

0 50 100 150 200 250

ES-AES-BES-CES-D

0

100

200

300

400

500

Vol

ume

of H

2 gen

erat

ed [m

L] (b)

0 50 100 150 200 250

SS-ASS-BSS-CSS-D

0

100

200

300

400

500

Vol

ume

of H

2 gen

erat

ed [m

L]

Time [min]

(c)

Fig. 3 Volume of H2 generated as a function of time for

Ru/HAp catalysts prepared from various calcium sources: (a) CaO, (b) ES, and (c) SS.

3.2 Hydrogen generation rate

Figure 3 shows the experimental results of hydrogen-generation measurements made by using the catalysts shown in Fig. 2. The theoretical volume of generated hydrogen is about 470 mL, but slightly smaller volumes of generated hydrogen are ultimately seen in some cases because NaBH4 solution undergoes gradual autohydrolysis during the time spent in

Journal of Ecotechnology Research, 18[1] (2016)

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preservation before the measurement. The results clearly indicate that the hydrolysis

follows a zero-order reaction11). Each case developed a hydrogen generation rate with a stable slope, which depends both on the type of calcium source and on the conditions for hydration of CaO and the aging of HAp.

Table 2 summarizes the induction periods and catalytic activity of Ru/HAp catalysts. For HAp particles prepared under condition A, the highest catalytic activity is for ES-A (7.20 mL-H2/min), whereas the lowest is for CaO-A (4.37 mL-H2/min). The activity of the SS-A catalyst (5.90 mL-H2/min) is moderate. In contrast, the aging under condition B resulted in the highest catalytic activity for CaO-B (7.05 mL-H2/min), followed by ES-B (5.75 mL-H2/min) and SS-B (5.37 mL-H2/min), both of which exhibit a H2 generation rate comparable to that of SS-A, although with a longer induction period.

Comparing condition A with C (or B with D) reveals that using higher stirring speed during the preparation of Ca(OH)2 (hydration) gives higher catalytic activity, suggesting that the conversion of CaO can be enhanced by stirring at higher speed during hydration.

The results shown in Fig. 2 and Table 2 imply that the crystallinity of HAp is not always a key factor to enhance the catalytic activity.

Table 2 Induction period, catalytic activity, Ru content,

and Ca/P molar ratio of Ru/HAp catalysts. Catalyst

ID Induction

period [min]

Catalytic activity

[mL-H2/min]

Ru [wt.%]

Ca/P [-]

CaO-A 22.5 4.37 – – CaO-B 20.1 7.05 0.896 1.597 CaO-C 23.5 4.18 – – CaO-D 37.7 3.79 – – ES-A 11.4 7.20 0.831 1.598 ES-B 21.7 5.75 0.651 1.797 ES-C 47.9 2.63 0.612 1.543 ES-D 50.6 4.42 – – SS-A 15.8 5.90 – – SS-B 30.5 5.37 0.902 1.630 SS-C 19.3 5.89 – – SS-D 53.4 2.37 – –

3.3 Effect of physical properties Figure 4 shows the relation between the catalytic

activity and the BET specific surface area of the catalysts for conditions A and B, in which the hydration is believed to be sufficient.

Condition B, which corresponds to mild aging during the preparation of HAp, results in large BET specific surface areas (of more than 85 m2/g) all starting materials. However, condition A, which corresponds to rapid aging, leads to lower BET specific surface areas except for SS-A. The reason for the different tendency observed for SS can be due to a noticeable decrease in mean particle size when

prepared under condition A (3.89 μm) rather than condition B (5.36 μm), as shown in Table 3.

0

2

4

6

8

60 70 80 90 100 110

CaO-AES-ASS-ACaO-BES-BSS-B

Cat

alyt

ic a

ctiv

ity [m

L-H

2/min

]

BET specific surface area [m2/g] Fig. 4 Relation between catalytic activity and BET

specific surface area for Ru/HAp catalysts prepared under conditions A and B.

Table 3 Mean particle size of Ca(OH)2 after hydration

and those of HAp prepared under conditions A and B.

Starting material

Hydration [μm]

Condition A [μm]

Condition B [μm]

CaO 5.17 4.78 5.27 ES 6.53 6.90 6.36 SS 9.31 3.89 5.36

Figure 5 shows the relation between pore volume

up to 190 nm in pore diameter and BET specific surface area of the catalysts. Changing the aging condition from B to A hardly affects the total pore volume of the prepared Ru-supporting catalysts derived from CaO and SS, although the trend differs for changes in the BET specific surface. For the catalyst derived from ES, however, the BET specific surface area and total pore volume of the catalyst both decrease significantly. These differing trends imply that the reaction in the preparation of Ru/HAp catalyst depend on the type of starting calcium materials.

0

0.1

0.2

0.3

0.4

0.5

0.6

60 70 80 90 100 110


Pore

vol

ume

[cm

/g]

BET specific surface area [m /g] Fig. 5 Relation between pore volume and BET specific

surface area for Ru/HAp catalysts prepared under conditions A and B.

Figure 6 shows the relation between the catalytic

activity and the average pore diameter of the catalysts calculated by Eq. (2). The catalytic activity clearly


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increases upon decreasing the average pore size, irrespective of the starting material. Condition A is advantageous, especially for biomineral calcium carbonate resources (ES and SS), because it produces smaller average pore diameter (around 20 nm or less), which eventually leads to higher catalytic activity.

0

2

4

6

8

10

15 20 25 30 35


Cat

alyt

ic a

ctiv

ity [m

L-H

2/min

]

Average pore diameter [nm] Fig. 6 Relation between catalytic activity and average

pore diameter for Ru/HAp catalysts prepared under conditions A and B.

Equation (1) indicates that the catalyst pores may

fill with hydrogen gas as the reaction proceeds. Smaller pores are believed to more effectively release the generated hydrogen into the bulk liquid phase by introducing the reactants from the liquid phase with higher capillary forces.

A byproduct produced in our experimental conditions is liquid-phase NaBO2

10), so the catalyst is not deactivated by surface coverage of NaBO2 because it can diffuse into the bulk liquid phase. 3.4 Effect of Ru content and Ca/P molar ratio

The effect of Ru content and Ca/P molar ratio of several catalysts is shown in Table 2. For ES, the catalytic performance seems to improve with increasing Ru content. ES-A exhibits the best catalytic activity, although its Ru content is not so high (0.831 wt.%) as compared with CaO-B (0.896 wt.%), showing a comparable catalytic performance. In fact, SS-B had the highest Ru content (0.902 wt.%), but did not work as well as CaO-B and ES-A.

Table 2 also indicates that the Ru content reaches its maximum near a nearly stoichiometric Ca/P molar ratio of HAp (1.67), possibly due to its excellent ion-exchange capacity. For calcium-deficient HAp (Ca/P < 1.67), a lower Ca/P molar ratio correlates with a lower Ru content, which corresponds to longer induction periods and smaller catalytic activity, as recognized for ES. This may also be true for CaO and SS. However, high Ru content does not always produce high catalytic activity when using different starting materials as calcium sources. Other factors may affect the catalytic performance, but further investigation is needed to clarify the relation of these factors with catalytic performance.

The nature of the deficiency in nonstoichiometric HAp (Ca/P < 1.67) is also important. The study of

Joris and Amberg on 1-butanol dehydration over HAp contends that incompletely coordinated cations in the HAp lattice form stronger acid sites than the HPO4

2− groups and, through interaction with water molecules, provide the necessary catalytic centers for the reaction12). If the same functional matter is working, it is reasonable to consider that not only Ru content but also Ca/P molar ratio (preferably around 1.60) can play an important role for the performance of Ru/HAp catalysts.

4. Conclusion

Hydroxyapatite was synthesized via a wet process under four different conditions by using calcines of eggshell and scallop shell as well as reagent-grade CaO. Hydroxyapatite-supported Ru catalysts were then prepared for H2 generation via hydrolysis of sodium borohydride in alkaline solution at 298 K. The following conclusions are obtained: (1) The synthesis route involving rapid aging

produces high-intensity peaks of hydroxyapatite in the XRD analysis. Such conditions lead to low BET specific surface area except when using scallop shell as a starting material. This result is possibly due to the reduction of particle size.

(2) Ru content is not the only factor affecting the catalytic activity when using different starting materials of calcium sources and fixed amounts of Ru salt to prepare the catalyst.

(3) Average pore diameters around 20 nm or less and a Ca/P molar ratio of about 1.60 are effective for the catalyst used in sodium borohydride hydrolysis.

Acknowledgments

The authors acknowledge Takahashi Industrial and

Economic Research Foundation for their financial support of this work. Special thanks are due to Professor Akira Ohki, Associate Professor Tsunenori Nakajima, and their laboratory members at Kagoshima University for ICP-AES analysis. The authors would also like to thank Enago (www.enago.jp) for the English language review.

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8) Zhang, D., Zhao, H., Zhao, X., Liu, Y., Chen, H. and Li, X.; Application of hydroxyapatite as catalyst and catalyst carrier, Progress in Chemistry, 23, 687-694 (2011).

9) Jaworski, J. W., Kim, D., Jung, K., Kim, S., Jung, J. H., Jeong, J. O., Jeon, H. S., Min, B. K. and Kwon, K.-Y.; Surface modification of hydroxyapatite for hydrogen generation, Journal of Colloid and Interface Science, 358, 598-603 (2011).

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Hydroxyapatite prepared from biomineral calcium carbonate