Production of Copper and Cobalt Aluminate Spinels and Their Application As Supports for Inulinase Immobilization

Abstract

Copper and cobalt aluminates were obtained through the use of chitosan as template. In this synthesis route, chitosan is eliminated by heating, and a porous material is produced. These oxides were used as supports for inulinase immobilization by adsorption process. Physical properties of produced particles were analyzed by X-ray diffraction (XRD) and nitrogen adsorption-desorption isotherms. Both oxides presented particles containing mesoporous characteristics and high surface area, which is desirable for applications in enzyme immobilization processes. The results revealed that the copper and cobalt aluminates exhibit high inulinase immobilization efficiencies, which makes them promising supports for enzyme immobilization.

Keywords:
CuAl₂O₄; CoAl₂O₄; synthesis; characterization; inulinase; immobilization

1 Introduction

Spinel structure oxides constitute one of the most interesting classes of advanced ceramic materials due their intrinsic physical and chemical properties. Among the spinel oxides, copper (CuAl₂O₄) and cobalt (CoAl₂O₄) aluminates possess interesting properties for technological application. Specifically, copper aluminate (CuAl₂O₄) possesses important applications in various fields such as gases sensor¹, catalyst in the oxidation reaction of benzyl alcohol^2,3, methanol steam reforming⁴ and hydrogenolysis of glycerol⁵, thermoelectric properties⁶, photocatalyst for degradating the pollutants in aqueous solution^7-10, solar absorber¹¹, oxygen carrier in chemical-looping combustion¹² and optical and dielectric properties^2,13. Cobalt aluminate oxide (CoAl₂O₄) has been employed in various fields such as ceramic pigment^14,15, optical and magnetic properties^16,17, catalysts in reactions such as soot combustion with NO_x/O₂^[18], oxidation of CO^[19], carbon monoxide hydrogenation²⁰, and reforming of methane²¹, and sensors for gas²², humidity²³ and volatile organic compounds^1,24.

The use of immobilized enzymes has been increased in the last years due the many fields application. In the immobilization process enzymes are trapped in a solid matrix, therefore, is possible the reuse and it will not be subjected to denaturation²⁵. Several materials with different characteristics such as natural clay²⁶, grafted alginate beads²⁷, aminated non-porous silica nanoparticles²⁸, chitin²⁹, alginate–chitosan beads³⁰, poly(EGDMA) cryogels³¹ and carbon nanotubes³² have been used as supports for inulinase immobilization by adsorption process. The immobilization process presents some advantages compared with free enzyme, which are the easily removed of the products from immobilized enzymes and possibility the continuous mode of operation³³. Immobilized inulinases have been used for many processes, especially for the sugar production through inulin hydrolysis, yielding fructose and fructooligosaccharides, which could be used as functional ingredient²⁶. In this work a new application for copper and cobalt aluminate oxides was proposed, i.e., as support for inulinase immobilization.

Copper aluminate spinel (CuAl₂O₄) has been prepared by several techniques such as microwave combustion method³, co-precipitation¹⁰, electrochemical process¹¹, solvothermal²⁰, microemulsion method³⁴, sonochemical^9,35, solid state reaction^4,36, sol-gel^7,37, modified sol–gel^2,38, sputtering^13,39, Pechini method⁴⁰ and metallic alkoxide⁴¹. Cobalt aluminate spinel (CoAl₂O₄) has been prepared by reverse microemulsion process¹⁶, sol–gel¹⁷, hydrothermal^42,43, microwave combustion method⁴⁴, sonochemical⁴⁵, via glycine chelated precursors⁴⁶, via polyacrylamide gel method⁴⁷, double decomposition reaction between solid LiAlO₂ and molten KCoCl₃^[48], low temperature combustion⁴⁹, complexation method⁵⁰, reaction-sintering process⁵¹, polymeric precursors method⁵², polymeric-aerosol pyrolysis⁵³, freeze–drying⁵⁴, sol–gel-hydrothermal method⁵⁵, ultrasonic-assisted-hydrothermal method⁵⁶, polymerized complex technique⁵⁷, chemical vapor deposition⁵⁸, and via supercritical water conditions⁵⁹. In this work, both copper and cobalt aluminates were prepared by route using chitosan as template. This technique has as advantages obtain a material with high porosity and surface area⁶⁰. These characteristics are important for enzymes immobilization purposes. Although some aluminum-based spinels have been prepared using chitosan as template^61,62, copper and cobalt aluminates have not yet been prepared by this route.

Hence, in the present study, copper and cobalt aluminates were prepared by the route using chitosan as template. The spinel oxides were prepared at two different temperatures, and their physical properties were determined. Both oxides were evaluated as supports for inulinase immobilization.

2 Experimental

2.1 Synthesis of oxides

Synthesis procedure of copper and cobalt aluminates was based in a previous work⁶², however ultrasonic irradiation was here employed in order to aid the mixing process. CuAl₂O₄ and CoAl₂O₄ powders were prepared from aluminum nitrate (Al(NO₃)₃.9H₂O), cobalt chloride (CoCl₂.6H₂O), copper nitrate (Cu(NO₃)₂.3H₂O) and commercial chitosan (C₆H₁₁O₄N)_n (Purifama, Brazil). All the chemicals (with analytical grade) were used as received. Stoichiometric amounts of metal nitrates (molar ratio Cu:Al = 1:2 and Co:Al = 1:2) were used for preparing CuAl₂O₄ and CoAl₂O₄powders. For preparing the copper aluminate (CuAl₂O₄), aqueous solutions of aluminum nitrate (15 g in 30 mL distilled water) and copper nitrate (4.82 g in 8 mL distilled water) were added into an acetic acid aqueous solution (5% v/v) (80 mL) containing 2.5 g of chitosan previously dissolved, under magnetic stirring. The mixture was ultrasonicated at 30 °C for 30 min in an ultrasonic bath (Unique Inc., model USC 1800A, 35 kHz Brazil). After that, the resulting solution was slowly added to an ammonia aqueous solution (50% v/v, 200 mL) under magnetic stirring. The formed precipitates were separated from the solution and further dried at ambient temperature for 48 h. Then, the solids were calcined in air at 650 °C and 900 °C for 4 h, using a conventional muffle-furnace. Cobalt aluminate powders were prepared by same procedure previously described, but the amounts used for the synthesis were 15 g of aluminum nitrate and 4.76 g of cobalt chloride.

2.2 Characterization techniques of oxides

The phase identification of the samples was carried out by XRD (Rigaku Miniflex 300 diffractometer) using Cu Kα radiation at 30 kV and 10 mA, with a step size (2θ) of 0.03° and a count time of 0.9 s per step. The average crystallite size of each sample was determined through the Scherrer equation⁶³ (silicon powder was used as a standard reference material): D = K.λ /( h_1/2.cos θ), where D is the average crystallite size, K the Scherrer constant (0.9), λ the wavelength of incident X-rays (0.1541 nm), h_1/2 the peak width at half height and θ corresponds to the peak position (2θ = 36.88°, lattice plane of {311} for CuAl₂O₄ and 36.74°, lattice plane of {311} for CoAl₂O₄). N₂ adsorption-desorption isotherms measurements were carried out at 77 K using an ASAP 2020 apparatus. Before analysis the samples were degassed at 200 °C under vacuum. Silica-alumina pellets (Part No. 004-16821-00) were used as a reference material to test instrument performance.

2.3 Enzyme immobilization and activity essays

Adsorption experiments were carried out to investigate the inulinase immobilization from aqueous solution. Commercial inulinase was obtained from Aspergillus niger (fructozyme, exo-inulinase EC 3.2.1.80 and endo-inulinase EC 3.2.1.7) and it was purchased from Sigma-Aldrich (São Paulo, Brazil). The temperature was fixed at 30 °C and the inulinase adsorption was performed using a batch technique. Typically, 0.025 g of oxide was placed in glass flasks containing different inulinase concentrations in a sodium acetate buffer (pH 4.8) solution. The effects of enzyme concentration (0.5; 1.0 and 1.5% v/v) and adsorbent:adsorbate ratio (1:300-1:500) on the adsorption were evaluated. The solution was maintained under agitation at 150 rpm, and then an aliquot of the aqueous solution was collected after reaching the adsorption equilibrium time (90 min), and then filtered through the polyvinylidene difluoride (PVDF) membrane (0.22 μm) before analysis. For the enzyme activity essays, an aliquot of enzyme (0.5 mL) was incubated with sucrose solution (4.5 mL, 2% w/v) in sodium acetate buffer (0.1 M, pH 4.8) at 50 °C. Released reducing sugars were measured by the 3.5-dinitrosalicylic acid method⁶⁴. A separate blank was set up for each sample to correct the non-enzymatic release of sugars. One unit of inulinase activity was defined as the amount of enzyme necessary to hydrolyze 1 μmol of sucrose per minute under the mentioned conditions (sucrose as a substrate). The immobilization efficiency was expressed in terms of inulinase loading capacity (Q_t : U g^–1) through the Equation 1.

${\text{Q}}_{\text{t}}=\frac{({\text{A}}_0-{\text{A}}_{\text{t}})\text{V}}{\text{m}}$ (1)

where A_o and A_t (U L^–1) are the inulinase activities at t = 0 and time t, respectively; V (L) is the volume of solution, and m (g) is the mass of support.

All the immobilization experiments were carried out in duplicate and only the mean values were reported. The maximum deviation observed was about ± 6.0%.

3 Results and Discussion

3.1 Characterization of oxides

The XRD patterns of the copper and cobalt aluminates prepared at different temperatures are shown in Figure 1(profiles a and b, respectively). Copper aluminate samples showed diffraction peaks at 2θ values of 31.20°, 36.88°, 44.85°, 55.73°, 59.42° and 65.40° corresponding to the [220], [311], [400], [422], [511] and [440] diffraction planes, respectively. Cobalt aluminate samples showed diffraction peaks at 2θ values of 31.19°, 36.74°, 44.69, 55.49°, 59.19° and 65.4° corresponding to the [220], [311], [400], [422], [511] and [440] planes, respectively. These planes are associated with the spinel types CuAl₂O₄ and CoAl₂O₄ with cubic structures in agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) cards no. 33-0448 and no. 44-0160, respectively. On the basis of the Scherrer equation, the crystallite size was calculated to be ca. 7.5 for both CuAl₂O₄ samples, and ca. 6.0 and 8.0 nm for the CoAl₂O₄ samples prepared at 650 and 900 °C, respectively.

Figure 1
XRD patterns of (a) copper and (b) cobalt aluminates prepared at 650 and 900 °C. Inset at figure (a): CuAl₂O₄ reference according to JCPDS card no. 33-0448, and inset at figure (b): CoAl₂O₄ reference according to JCPDS card no. 44-0160.

Figure 2 shows the adsorption-desorption isotherms of N₂ of cobalt and copper oxides prepared at 650 and 900 °C. All samples present type-IV isotherms (Figure 2a), in accordance with the International Union of Pure and Applied Chemistry (IUPAC) classification⁶⁵. This adsorption behavior is characteristics of mesoporous structure. The desorption branches present H1-like hysteresis loops at high relative pressures, indicative of the presence of mesoporosity. The corresponding pore size distribution curves (Figure 2b) were measured from the adsorption branches according to the Barrett–Joyner–Halenda (BJH) method. All samples display a similar unimodal distribution, with peaks centered in the mesoporous region (between 20 and 500 Å). These features are of great importance for immobilization purposes because it allows for a greater accessibility of enzymes molecules to the support. Surface physical parameters of all samples are shown in Table 1. It can be observed that specific surface area decreases with increasing calcination temperature. Thus samples prepared at 650 °C present higher surface area. This can occur due to sintering of material when it is treated at higher temperatures, resulting in reducing the void spaces and consequently, its surface area⁶⁶.

Figure 2
(a) N₂ adsorption-desoption isotherms and (b) Barret-Joyner-Halenda (BJH) pore size distributions plots of different samples.

Table 1
Surface physical characteristics of oxide samples prepared at different conditions.

Properties	CuAl2O4/650 °C	CuAl2O4/900 °C		CoAl2O4/650 °C	CoAl2O4/900 °C
BET surface area (m2 g–1)	72.79	23.60	192.27		116.72
Micropore surface area (m2 g–1)	10.26	5.57	3.05		10.02
External surface area (m2 g–1)	62.53	18.03	189.22		106.70
Langmuir surface area (m2 g–1)	99.77	30.16	266.86		161.31
Total pore volume (cm3 g–1)	0.295	0.218	0.369		0.336
Micropore volume (cm3 g–1)	0.0042	0.0232	0.0004		0.0036
Mesopore volume (cm3 g–1)	0.290	0.194	0.369		0.332
Average pore size (Å)	162.13	180.4	96.76		115.14

For comparison purposes, surface area values of CuAl₂O₄ and CoAl₂O₄ powders found in literature are shown in Table 2. From Table 2, it can be seen that each method results in materials with different surface areas. Therefore, the preparation route strongly affects their physical properties. The results revealed that the route presented in this work generates materials with a high surface area. So preparation of single phase spinel with porous structure and high surface area makes this method technically easier, simpler and low cost because it does not need sophisticated procedures and requires inexpensive precursors.

Table 2
Comparing the surface area values of different synthesis methods found in the literature.

CoAl2O4		CuAl2O4
Method	Surface area (m2 g–1)	Method	Surface area (m2 g–1)
Microwave assisted glycothermal18	181	Sol–gel1	10
Hydrothermal42	29	Sol–gel2	10
Sonochemical45	75	Modified sol–gel2	41
Low temperature combustion49	77	Solid-state reaction4	13
Autoignition67	16	Sonochemical9	110
Colloidal sol-gel68	156	Dry soaking74	30
Combustion69	188	Microwave combustion75	87
Microwave70	94	Conventional combustion 75	44
Coprecipitation71	169	Mixed chelates thermolysis76	86
Solution-based combustion72	53	Soft chemical approach77	5
Malonate73	23	Copper-laden sludge thermally treated78	2

As cobalt- and copper-based oxides are colored materials, so this detail was evidenced from visual examination (Figure 3), whose images were obtained by a commercial digital camera (Nikon Coolpix). The powders were compacted on a glass sample holder. These results aimed provide additional information regarding the physical characteristics of produced materials, i.e., their intrinsic color. The samples containing cobalt exhibit a bright blue color whereas samples containing copper are greenish/ yellowish. Cobalt aluminate has been used as blue ceramic pigment^14,15, whereas copper-based oxides as green/yellow/turquoise pigments^79-81.

Figure 3
Images of (a) CoAl₂O₄ and (b) CuAl₂O₄ samples obtained at different temperatures.

3.2 Enzyme immobilization

The enzyme immobilization assays were performed using only the copper aluminate and cobalt aluminate samples at 650 ^oC, because in this temperature was obtained the highest surface area (see Table 1). Table 3 shows the results for inulinase loading capacity using copper and cobalt aluminates. Copper aluminate sample presented a loading capacity of 8,510 U g^–1using an inulinase concentration of 0.5% (v/v) and 1:500 of adsorbent:adsorbate ratio, whereas cobalt aluminate sample presented similar loading capacity (8,343 U g^–1) under the same adsorbent:adsorbate ratio (1:500), but using an inulinase concentration of 1.5% (v/v). The results showed that the cobalt aluminate presents more capacity for inulinase adsorption, which could be explained by the surface area value, being higher when compared to the cupper aluminate. Table 4 shows the inulinase loading capacity of different supports found in literature. According to the results founded it is possible to observe that both aluminates presented a satisfactory inulinase loading capacity. This result could be explained by the high surface area values of both samples prepared in this work.

Table 3
Inulinase loading capacity (Q_t) of CuAl₂O₄ and CoAl₂O₄samples.

Assays	Enzyme concentration (%, v/v)	Adsorbent:adsorbate ratio	CuAl2O4(Qt : U g–1)	CoAl2O4(Qt : U g–1)
1	0.5	1:300	6,642	1,461
2	1.5	1:300	7,337	3,772
3	0.5	1:500	8,510	1,312
4	1.5	1:500	5,623	8,343
5	1.0	1:400	1,329	4,706

Table 4
Inulinase loading capacity (Q_t) of different supports found in literature.

Material	Qt (U g–1)
Grafted alginate beads27	530
Chitin29	291
Alginate-chitosan beads30	668
Carbon nanotubes32	51,047
Aminated non-porous silica82	43
Anion-exchange resins83	11,000
Poly-D-lysine coated CaCO3micro-particles84	32

4 Conclusions

Copper and cobalt aluminates were successfully synthesized using chitosan as template. The powders produced presented high surface area and mesoporous structure. These features are of great importance for immobilization purposes because it allows a greater accessibility of enzyme molecules to the support. Both aluminates presented a satisfactory loading capacity for inulinase immobilization. Therefore, these results revealed that copper and cobalt aluminates can be used as alternative supports for enzymes immobilization.

Acknowledgements

The authors gratefully acknowledge the financial support of the Brazilian research funding institutions CNPq and CAPES.

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Publication Dates

• Publication in this collection
  Sep-Oct 2015

History

• Received
  20 July 2015
• Reviewed
  05 Sept 2015