Al2O3 based Co-Schiff Base complex catalyst in hydrogen generation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 3 9 1 e2 8 4 0 1

Available online at www.sciencedirect.com

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Al2O3 based Co-Schiff Base complex catalyst in hydrogen generation € Dilek Kılınc¸ a,*, Omer S‚ahin b a b

Department of Chemistry, Faculty of Science and Letters, Siirt University, Turkey Department of Chemical Engineering, Faculty of Engineering, Siirt University, Turkey

highlights
Al2O3 supported Co(II)-Schiff Base complex catalyst was synthesized.
Al2O3 supported complex worked as an excellent catalyst for NaBH4 hydrolysis.
Initial rates (Ro) were 106540 and 147193,3 mL H2/gcat..min at 30  C and 50  C, respectively.
The catalyst displayed exceptional stability up to eight recycle.

article info

abstract

Article history:

This work presents the study of the catalytic activity of aluminum oxide supported Co-

Received 26 November 2018

Schiff

Received in revised form

ditertbutylsalicylaldimine-Co-Schiff Base complex in sodium borohydride hydrolysis.

6 August 2019

This catalyst is characterized with XRD, FT-IR, SEM, TEM, and BET. The respective reaction

Accepted 8 August 2019

kinetics have been calculated. With the catalyst condition, maximum reaction (initial) rate

Available online 5 September 2019

is 106540 and 147193,3 mL H2/gcat..min. at 30  C and 50  C. For this reaction apparent

Base

complex

derived

from

4,40 -Methylenebis(2,6-diethylaniline)-3,5-

activation energy is 44,7792 kJ.mol1 with 20e50  C. The reaction order value (n) for this Keywords:

catalytic system is 0,31. Additionally when Al2O3 supported Co-Schiff Base complex

Aluminum oxide

compared with pure Co-Schiff Base complex, the experimental results show that the

Schiff base

aluminum oxide support exhibits enhancing effect with 106540 and 64147 mL H2/gcat. min

Cobalt complex

respectively in sodium borohydride hydrolysis to Hydrogen production.

Catalyst

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hydrogen generation

Introduction Due to the consuming of energy and renewable energy sources in over time, the global world will soon encounter various energy problems. Considering the fossil fuels, hydrogen is the most important basic and technological alternative energy source owing to its excellent energy density and perfect energy conversion efficiency [1].

Nowadays B(N)H type chemicals have been a major area for H2 storage [2]. Though a lot of work have been made to explore powerful storage materials, neither have reached to relivable levels for applications. Between the varied materials, the complex hydrides, like LiBH4, Mg(BH4)2, NaBH4, NaAlH4, NH3BH3 are appealing hydrogen storage materials [3]. Chemical hydrides are shown as a new sources of hydrogen because of many advantages that they contain. Between these, NaBH4 is the most preferred H2 storage

* Corresponding author. E-mail address: [email protected] (D. Kılınc¸). https://doi.org/10.1016/j.ijhydene.2019.08.053 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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compound due to the higher H2 capacity, the higher purity of the produced hydrogen and having a controllable hydrogen release property [4]. NaBH4 can carry energy in two ways, with indirect and direct, which was first seen as a hydrogen producer, also serves as an energy carrier because it can produce electrons at the same time. NaBH4 hydrolysis reaction proceeds [5] as equation (1): Catalyst

NaBH4 þ ð2 þ XÞH2 O





!NaBO2 þXH2 O þ 4H2 þ 210kJ

(1)

NaBH4 may show anodic state. In the presence of a catalyst and in an alkaline environment, it may be oxidized [6] as equation (2): NaBH4 þ 8OH/NaNO2 þ6H2 O þ 8e

(2)

With respect to Schlesinger et al. [7], sodium borohydride reacts with water to give four moles of hydrogen per mole sodium borohydride at room temperature. This reaction can proceed faster with increasing temperature or with the addition of acid/catalyst. Although NaBH4 is not effective in cold water and unreacted borohydride is partially collected as dihydrate NaBH.42H2O [8], in hot water H2 generation continues rapidly by hydrolysis reaction [9]. It is necessary to use a suitable catalyst in order to achieve this reaction by itself at room temperature and to ensure efficient hydrogen production. At room temperature sodium, borohydride is able to deliver the clean hydrogen with controllable hydrolysis system by existence of catalyst. Brown [10] has found that the most heavy metals have very strong activities in NaBH4 hydrolysis reaction. Based on this, they showed that the Pt group metals were active at 25  C in the form of Ru, Rh > Pt > Co > Ni > Os > Ir > Fe [Pd]. Although noble metal-containing catalysts such as Rh [11,12], Ru [13], Pt [14], Pd [15] exhibit excellent activity when they used in the hydrolysis reaction of NaBH4, due to the fact that the prices of these metals are higher and it is difficult to obtain them. Therefore metallic catalysts such as Co [16,17], Ni [18] have been studied extensively as an alternative to noble metal catalysts. As a competitor to noble metals, cobalt-containing catalysts have high potential catalytic activity in the hydrolysis of NaBH4 [19,20]. There are many records about the catalytic activity of Co based catalyst, such as Co3O4 and LiCoO2 [21e23] and CoeB [19], Co@BaOb(OH)g [24], CoeMoePdeB [25], Co/KF/Al2O3 [26], etc... catalysts using in NaBH4 hydrolysis reaction for hydrogen generation in literature. However, there is not any research related to Co-Schiff Base complex other than our works [27e29] with high efficiency to hydrogen generation in this area. For this reason, it is important to synthesis and bring these type metal-organic catalysts to literature for hydrogen generation from sodium borohydride hydrolysis. Recently, aluminum oxide can be used as a support material to increase the surface area and therefore its activity of the metal catalyst [30,31]. Between the metal nanoparticles and the support material such as ZrO2 [32], SiO2 [33], TiO2 [34] or Al2O3 [35,36]. Interactions thoroughly affect the activity of heterogeneous catalysts in hydrolysis reaction of NaBH4 [37]. The support material is effective in the surface of the active phase contribution in catalytic reaction; therefore, the choice of the suitable support is quite important factor [38].

Schiff Bases that was first reported by the German chemist with Hugo Schiff, and it was formed the reaction of primary amines of carbonyl compounds. The most important property of these compounds is the azomethine group and their structure was expressed as RHC ¼ N-R general formula [39]. Bis-Schiff base ligands and their complexes have a significant part in science, stereochemistry, spectroscopy and magnetic fields. Recently, Schiff base ligands and their metal complexes have become very popular because of their activities in medicine as antitumor, antibacterial, anticancer, antifungal, anti HIV, antiphrastic, antiviral experiments [40e46]. Schiff baseCobalt complexes are also highly studied catalyst by scientists in the field of coordination chemistry and biochemistry [47]; especially cobalt-Schiff base complexes derived from salicylaldimine have been very popular structures [48]. However, there was a limited record for Schiff Base-metal complexes as catalysts for using in engineering and hydrogen energy area [27,28,49e52]. In present work, 4-40 -methylene bis(2,6-diethyl)aniline-3,5di-tert-butilsalisilaldimin-Co(II) complex that we earlier synthesized [27], was used to supported on Al2O3 particles to obtain a new catalyst and to test its catalytic behavior in H2 production from NaBH4 hydrolysis with depend on temperature, concentration of sodium borohydride, concentration of Co complex and catalyst amount and concentration of sodium hydroxide factors. Additionally the Al2O3 supported complex catalyst was re-used for eight times in this reaction with strong stability. Moreover, the catalyst was characterized with SEM, TEM, XRD, BET, and FT-IR.

Experiment Preparation of catalyst Al2O3 supported Co-Schiff Base complex catalysts were prepared one by one according to the literature [37,53] with different percentages (% 1, %5, % 7, % 10) of 4-40 -methylene bis(2,6-diethyl)aniline-3,5-di-tert-butilsalisilaldimin-Co(II) complexes at 30  C. Al2O3 supported-Co (II)-Schiff Base complex synthesize reaction was displayed in Fig. 1.

Hydrogen production experiments The activity of aluminum oxide supported-Co (II)-complex catalyst was evaluated based on the hydrogen volume measurement from the hydrolysis of NaBH4. The experiments were realized with the same way of our previous works [54]. The produced H2 gas, measured by the displacement level of water in a cylinder at 30  C. With this conditions according to equation (1), the calculated hydrogen gas volume should be just about 560 mL.

Result and discussion The effects of NaOH concentration The effects of NaOH concentration in NaBH4 hydrolysis reaction as shown in Fig. 2. Namely, H2 generation rates changed

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Fig. 1 e Preparation of Al2O3 based-Co (II) -Schiff Base complex catalyst.

600

Hydrogen Volume (ml)

500 400 300 200 100

0% NaOH

5% NaOH

7% NaOH

10% NaOH

0 0

20

40

60

80

Time (min.)

100

120

140

Fig. 2 e The effects of NaOH concentration on H2 production. with different sodium hydroxide percentage, that 0%, 5%, 7%, 10%. Even though the amount of produced H2 is the same, under the same reaction conditions, but the H2 production rate was faster than the other concentration for 10% NaOH, at about 42 min. Whereas the same amount of H2 was produced in 7% NaOH about 50 min, with 5% NaOH about 55 min. Nevertheless, without NaOH (0% NaOH) the expected H2 volume was not provided due to the incomplete reaction. The calculation results of initial rate (Ro) for NaBH4 hydrolysis reaction indicated the increased values as 24598; 68420; 78913,33; 106540 mL H.2(g.cat.min.)1 respectively, with growing NaOH percentage (0%, and 5%, 7%, 10%). Thus, it can be said that in the same reaction conditions, by rising NaOH concentration, the NaBH4 hydrolysis reaction completed with

shorter time, and higher initial rate, due to the best controllable reaction system.

The effects of Co(II)-complex percentage in all Al2O3 supported catalyst The effects of Co(II)-complex concentration was tested based on 1%, 5%, % 7, %10 Co(II)-Schiff Base complex in all catalyst for H2 production, and results were shown in Fig. 3. When using 1% Co(II) Schiff Base complex, unexpectedly the best initial rate Ro was obtained with 106540 mL H2/(g.cat..min.) for this reaction system under the same conditions. On the other hand with using 5%, % 7, and %10 Co(II) Schiff Base complexes as a catalyst, Ro values were decreased at 23905,33; 20343,81;

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600

Hydrogen Volume (ml)

500 400 300 200 100

1%Co comp

5%Co comp

7%Co comp

10%Co comp

0 0

10

20

30

40

Time( min.)

50

Fig. 3 e The effects of Co(II)-Schiff Base complex concentration. 15190,67 mL H2/(g.cat.min.) respectively as shown at Table 1. And reaction completed times were close to each other like 45 min, 45 min, 40 min and 38 min, in order of 1%, 5%, % 7, and %10 Co(II)-complex. We are able to comments, using 1% Co(II)Schiff Base complex catalyst in this reaction the catalyst surface is covered with Co-complex completely and with increasing complex concentration, it become excess for catalyst surface additionally a new layer is formed on the catalyst surface which reduces the catalytic activity.

catalyst including 1 %-Co-Schiff Base complex at 30 BC. With respect to increasing NaBH4 concentrations from 2% to 10%, generated H2 was increased and initial reaction rate (Ro) were increased 106540; 131333; 154800; 159733 mL H2/(g..catmin.) respectively. Therefore said that in NaBH4 hydrolysis reaction, NaBH4 concentration changing is not effect to Al2O3 supported-Co Schiff Base complex catalytic activity.

The effects of catalyst amounts

With 20, 30, 40, 50 BC as different temperatures towards the hydrogen generation rates changing graphic was shown in Fig. 6 by using 15 mg Al2O3 supported-Co Schiff Base complex catalyst including 1 %-Co-Schiff Base complex in 10% NaOH solutions and 2% NaBH4 at 30 BC. depending on the increasing temperature values, the reaction completion times were decreased meanwhile the initial rates were increased. The kinetic calculations were done according to several reaction parameters at different temperature (20, 30, 40, 50  C) in sodium borohydride hydrolysis by 15 mg Al2O3 supported Co(II)-Schiff Base complex including 1% Co(II)-Schiff Base complex. The Al2O3 supported complex catalyzed sodium borohydride hydrolysis kinetics were described by the following equations;

Effects of catalyst amounts were worked with 5 mg, 15 mg, 25 mg, and 50 mg of Al2O3 supported-Co-Schiff Base complex on hydrogen generation and they were shown in Fig. 4. Amounts increasing of Al2O3 supported-Co-Schiff Base complex catalyst have made the enhancing effect for the initial rates (Ro) from 5 mg to 15 mg as 92500e106540 ml H2/(g..catmin.) respectively, but with using 25 mg and 50 mg the initial rates (Ro) were decreased to 87192 and 83000 respectively and was exhibited at Table 2. with decreasing hydrogen generation time as 110 mine45 min from 5 mg to 15 mg Al2O3 supported complex.

Effects of NaBH4 concentration Different NaBH4 concentrations (2%, 5%, 7%, 10%) towards to the hydrogen generation rates changing graphic was shown in Fig. 5. These experiments were carried out in 10% NaOH solution by 15 mg Al2O3 supported-Co-Schiff Base complex

Effects of the temperatures

rNaBH4 ¼ 

CZ NaBH4

CNaBH40

Co-Schiff Base complex concentration % % % %

1 5 7 10

Initial Rate (R0) (mL H2/(g.cat.min.)) 106540 23905,333 20343,810 15190,670

(3)

When separating and integrating:



Table 1 e Initial Rates changing with different Co-Schiff Base complex concentration.

dCNaBH4 ¼ k:CnNaBH4 dt

dCNaBH4 ¼ CnNaBH4

Zt k

dt

(4)

0

1 1 1  n1 ðn  1Þ Cn1 CNaBH40 NaBH4

! ¼ kt

1 1 ¼ ðn  1Þk:t þ n n1 CNaBH40 CNaBH4

(5)

(6)

As shown in Fig. 7a, the plot of the graph against time 1 at different temperatures gives the linear lines against Cn1 NaBH4

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600

Hydrogen Volume (ml)

500 400 300 200 100

5 mg cat.

15 mg cat.

25 mg cat.

50 mg cat.

0 0

20

40

60

80

100

120

Time (min.) Fig. 4 e Effects of catalyst amounts on H2 production.

Table 2 e Comparison in different supported complex amounts. Initial Rate (R0) (mL H2/(g.catmin.))

Amount of catalyst 5 mg 15 mg 25 mg 50 mg

92500 106540 87192 83000

and the slopes were able to use for calculation of the reaction rate constant k. Equation (6) is estimated by the accuracy coefficient with the value of n, which gives the most appropriate right equation (R2). The n value in the hydrolysis reaction that catalyzed Al2O3-supported Co(II) complex was calculated as 0,31. The rate constants obtained from Fig. 7a were used to find the apparent activation energy from the Arrhenius equation. k ¼ Aeð-Ea:=RTÞ

(7)

As given in Fig. 7b. In this study, at 20e50  C the apparent activation energy (Ea.) of the hydrolysis reaction of sodium borohydride in existence of Al2O3-supported Co(II)-Schiff base complex was calculated 44,779 kJ/mol.

The reaction mechanism of Al2O3 supported Co(II) complex catalyzed NaBH4 hydrolysis reaction that proposed by us is shown in Fig. 8. It is supposed to based on the suggested mechanism by Penea-Alonso et al. [55]. Here, BH4 is recommended to apply reversible adsorption on two Co complexes with high surface area provided by Al2O3 support, and which lead to BH3 adsorbed to Co metal on the Co complex. The negative charge that on to BH3 is carried to the hydrogen. Then Co complex adsorbed hydride reacts with the hydrogen of the free aqua to release the H2. The remaining hydroxyl group reacts with the BH3, and this BH3 begins to transfer the remaining hydrides to the second Co complex that presence on Al2O3 support and it creates the structure of BH2-(OH). Finally this reaction proceeds in the same steps until tetrahydroxyborate (B(OH)4) and a total of 4 mol of hydrogen gases (H2) is released.

Reusability of aluminum oxide supported Co(II)-Schiff Base complex The supported complex catalyst was reused eighth times in all reaction recycle. For this purpose, the catalyst was washed with deionized water to take out all of the undesired materials

Hydrogen Volume (ml)

2500 2000 1500 1000 500

2% NaBH4

5% NaBH4

7% NaBH4

10% NaBH4

0 0

50

100

150

200

Time (min.) Fig. 5 e Effects of NaBH4 concentration on H2 production.

250

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Fig. 6 e Effect of temperature on the hydrogen production.

0

20

40

Time (min.) 60 80

100

20

30

40

50

120

140

0

1/CNaBH4 (0,31-1)

-0.1

a

-0.2 -0.3 -0.4 -0.5 -0.6 -0.7 0 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 -1 y = -5386x + 13,064 R² = 0,9838

Lnk

-2

b

Ea=44,7792 kJ / mol -3 -4 -5 -6

1/ T Fig. 7 e Linear regression based on n-order at 20  C, 30  C, 40  C, 50  C (a) and the activation energy of NaBH4 hydrolysis (b).

from the catalyst surface. Then, the catalyst was dried at room temperature with protected from any pollution. The results illustrate that at the end of the eighth experiment, the Al2O3 supported Co(II) complex catalyst continues its catalytic

performance with 96% in the hydrolysis of NaBH4 as shown in Fig. 9. This results shows that Al2O3 supported cobalt complex catalyst maintains its stability at the time of using in the NaBH4 hydrolysis reaction.

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Fig. 8 e Proposed reaction mechanism of Al2O3 supported-Co (II) complex catalyzed NaBH4 hydrolysis reaction.

Characterization of catalyst Scanning electron microscopy (SEM) Some selected SEM images of Al2O3 supported-Co(II) complex catalyst was exhibited in Fig. 10. Al2O3 supported-Co(II) complex was appeared powdery and indented structure in 1 mm (a), and complex is displayed bigger dimension particles shapes with 10e20 mm (b-d). In addition in 10 (b), 20 mm (c) there is able to seen from the micrographs that the external surface of the Al2O3 supported has some grains in different formal. In 100 mm (e) and 200 mm (f) it seems more clearly that there was Al2O3 particles and Co(II)-Schiff Base complex separately on catalyst surface. All these represented that supported complex catalyst is fully formed.

XRD measurements The XRD measurement of Al2O3 supported-Co (II) complex is exhibited at Fig. 11. These graphic was showed that Al2O3 supported-Co(II) complex display a crystalline structure. In addition some amorphisms were observed in the section between 5-15 and 70e80 that on the axis 2-Theta is due to the own structure of the Al2O3. The significant peaks of Al2O3 supportedCo (II) complex were occurred with 15,854; 17,736; 18,962; 20,660; 21,800; 22,298; 26,044; 30,180; 31,321; 33,181; 34,202; 35,281; 36,979; 43,038; 44,960; 46,321; 47,380; 49,881; 51,619;

55,161; 66,141 the crystalline peak was at 15,854; 17,736; 20,660 and 31,321 that shown in Fig. 11. The diffraction peaks, which belongs to Al2O3 supported-Co (II)-Schiff Base complex, is narrowed and sharper and it demonstrates enhance on crystal structure. In addition, after the hydrolysis reaction, the XRD analysis of Al2O3 supported-Co (II) complex catalyst was done. It was observed that to have the same measured values and the results showed that our catalyst has good stability and there was no change on its structure after the hydrolysis reaction.

BET analysis The average diameter, surface area, and the pore volume that owns Co(II) complex and Al2O3 supported-Co (II) complex are expressed in Table 3. Measurement values show that the surface area that belongs to the Co-complex is increased by using Al2O3 supported material. This situation noticed that the bigger surface area of Al2O3 supported-Co(II) complex. Additionally the pore volume is also bigger than Co(II) complex for supported complex as 0,1989 cm3/g and 0,1625 cm3/g. As the Co(II) complex surface area was 88,967 m2/g, correspondingly for Al2O3 supported-Co(II) complex this value increased to 119,089 m2/g. There is a direct proportional relationship between the surface areas of the catalysts and the catalytic activity. Therefore, we can say that the reason for the higher catalytic performance of Al2O3-supported-Co(II) complex catalyst than the Co(II) complex catalyst is due to higher surface area.

Hydrogen volume (mL)

600 500 400 300 200

1. usage

2. usage

3. usage

4. usage

5. usage

6. usage

7. usage

8. usage

100 0 0

10

20

30

40

50

60

Time (min.) Fig. 9 e The reusability of Al2O3 supported-Co (II) complex catalyst in NaBH4 hydrolysis reaction at the end of eighth trial.

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Fig. 10 e SEM images Al2O3 supported Co (II)-Schiff Base complex (a: 1 mm. b: 10 mm, c: 20 mm, d: 10 mm, e: 100 mm, f: 200 mm).

Fig. 11 e XRD patterns of Al2O3 supported Co(II)-Schiff Base complex.

FT-IR spectra In the FT-IR spectra, the typical absorptions peaks of Al2O3 supported-Co(II) complex catalyst and Co complex was shown

in Table 4 with some difference to each other. But some significant frequencies of the Co complex and Al2O3 supportedCo(II) complex catalyst were same. Several basic peaks of Co complex shifted with supported on Al2O3. The too sharp peak

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Table 3 e BET measurements of complex and supported complex. Catalyst Co-comp./Al2O3 Co-comp.

SBET (m2/g)

Apore (nm)

VPore (cm3/g)

119,089 88,967

18,0210 10,2475

0.1989 0,1625

at 1618,52 cm1 were uncovered own to eCH]N- stretching vibrations. For Al2O3 supported-Co(II) complex, it turned to 1619,36 cm1 with lower and broadly frequency (see Table 4). As seen that between 2970,87 and 29372,90 cm1 peaks that belong to Co complex and Al2O3 supported-Co(II) complex that caused of the intramolecular OH…N or free eOH. The peak at 3441,55 and 3360,13 cm1 mean that M  O bond for Al2O3 supported-Co(II)-Schiff Base complex catalyst that came from AleO bonding, which is not exist on Co complex. At 1100 cm1 the vibration of Co complex turned to 1027,56 cm1 for Al2O3 supported-Co(II)-Schiff Base complex own that was due to -CO- stretching vibrations. In addition, there are no changes in

the Co(II) complex and the Al2O3-supported-Co(II) complex at 508, 398 cm1 peaks that M-O and M-N bonds which demonstrate complexation. Additionally, after the hydrolysis reaction, the FT-IR analysis of Al2O3 supported-Co (II) complex catalyst was done again, and it was observed that to have the same values. So this result supported that our catalyst has a good stability and there was no change on its structure after the hydrolysis reaction.

Transmission electron microscopy images (TEM) Fig. 12 displays TEM images of Al2O3 supported Co(II)-Schiff Base complex. Dark cobalt complex particles with a round shape and diameter of 5e50 nm are observed on the Al2O3 support. The TEM data allow a conclusion that the dispersed metal particles on the Al2O3 surface are homogeneous distribution. Thus, TEM analysis results supported the formation of Co (II)-Schiff Base complex supported by Al2O3 with the other analysis technique.

Table 4 e The important infrared bands of the synthesized Al2O3 supported-Co(II)-Schiff Base complex catalyst and CoSchiff Base complex. Complex Co(II)-Schiff Base complex Al2O3 supported-Co(II)-Schiff Base complex (Before Hydrolysis Reaction) Al2O3 supported-Co(II)-Schiff Base complex (After Hydrolysis Reaction)

ῡM-O (forAl2O3)

ῡC

e 3360,13 3441,55 3360,13 3441,58

1618,52 1619,36

¼N d(OeH) ῡCeO ῡMeO ῡMeN 2874 2900,32

1100 1027,56

508 508

398 398

1619,36

2900,33

1027,60

508

398

Table 5 e Comparison of Co catalysts with Al2O3 supported Co(II)-Schiff Base complex in sodium borohydride hydrolysis. Catalyst

Temperature

Co/CsWPA CoeB CoeP Co/KF/Al2O3 CoeCeeB CoeNieB CoeMoeB CoeMoePdeB Co(II)-Schiff Base complex Al2O3 supported-Co(II)-Schiff Base complex

303 298 303 303 303 298 303 298 303 303

K K K K K K K K K K

Activation energy (kJ/mol)

Initial Rate (mL H2/(g.catmin))

e 30 22 e 33,1 34 29 36.36 21,165 44.779

4039 [21] 4330 [34] 3750 [19] 233 [21] 4760 [56] 1175 [17] 2400 [17] 6023 [20] 64147 [27] 106540 (this work)

Fig. 12 e TEM images of the Al2O3 supported Co(II)-Schiff Base complex (a:5 nm, b:50 nm).

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Conclusions [12]

We have presented to synthesize a new and highly active organic catalyst that named Al2O3 supported Co(II)-Schiff Base complex for NaBH4 hydrolysis. It was prepared at room temperature in ethanol solution to sodium borohydride hydrolysis to hydrogen production. Al2O3 supported Co(II)-Schiff Base complex catalyzed hydrogen generation process was studied based on the NaOH concentration, Co-complex in total Al2O3 supported-Co (II)-Schiff Base complex catalyst, NaBH4 concentration, Al2O3 supported Co(II)-Schiff Base complex amount and temperature effects. The optimal conditions were provided with 1% Co(II)-Schiff Base complex with using 10% NaOH, % 2 NaBH4 and 15 mg Al2O3 supported Co(II) complex catalyst at 303 K. In addition, the kinetic calculations were done for this reaction at 20e50  C. The initial rates were obtained at 30 BC and 50 BC in order of 106540 mL H2/(gcat.min) and 147193,3 mL H2./(gcat.min). The apparent activation energy was 44,779 kJ/mol. Additionally the Al2O3 supported complex catalyst reused for eight times in sodium borohydride hydrolysis reaction and seen that the catalyst displayed exceptional stability up to (eight) recycle with 96% stability. When it was examined the literature survey to compare the Al2O3 supported-Co (II)-Schiff Base complex catalyst with the other Co-catalysts in sodium borohydride hydrolysis reaction as seen Table 5, it showed that Al2O3 supported-Co(II)Schiff Base complex exhibited more higher catalytic performance than the others.

[13]

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