A Nano Catalyst of Cofe₂o₄@ B₁₈N₁₈ as A Novel Material

Introduction

CoFe₂O₄ as the Cobalt ferrite crystallizes in a partially inverse spinel position represented as (Co2x⁺⁺Fe³⁺ 1-x)(Co²⁺ 1-xFe³⁺ 1+x)O₄ where x based on thermal¹ condition . It is ferri-magnetic with a Tc approximately 520 degree and exhibits a relatively magnetic hysteresis which distinguishes it from the other of the spinel ferrites^{2, 3}. Magnetic measurements on nano-particles of cobalt ferrite dispersed in various solvents of organic compounds and nano-crystalline² powders prepared by hydroxide precipitation^3-5 have been investigated earlier. In magnetic fluids³, it has been seen that for particles above threenanometer the saturation^3,4magnetization remains constant at about 30 emu g^-1 that is extensively less than the bulk⁵ values.

Magnetic nanoparticles (MNPs) have shown exceptional potential for several biological and clinical applications^{4, 5}. However, MNPs might be coated by the biocompatible shells for such applications. The aim of this study is to understand if and how the surfaces charges and coatings can affect the magnetic and electronic properties of Cobalt ferrite crystallizes. The role of the surfaces on the magnetic moments of a magnetic nano particle such as CoFe₂O₄ is an important issue, and various effects can contribute for making it deviate from the bulk value, including the charges, the nature of the coating, and also the synthetic technique. The electronic properties and ionic distribution of CoFe₂O₄ NPs were probed by X-ray^2-4 absorption spectroscopies X-ray-magnetic-circular-dichroism and X-ray-photoemission-spectroscopy-techniques known as the abbreviation XAC, XMCD and XPS respectively. Magnetite-particles^1-5 is also of interests in medicinal and industries application such as magnetic-resonance-imagingor {MRI}, organic catalyst and nanomaterial synthesize^6-8.

The overall magnetic behavior and the hyper-thermic properties were evaluated by magnetometers and molecular modeling measurements, respectively^7,8. The results show that all of the investigated CoFe₂O₄ NPs have high magnetic anisotropy^6,9energies, and the surfaces charges and coating do not influence appreciably their electronic and magnetic properties. In addition, the citrate shell improves the stability of the NPs in aqueous environment, making CoFe₂O₄ NPs suitable for biomedical applications.Magnetic nanoparticle exhibitsseveral unique properties such as super-para-magnetism compared to bulk^9,6material andparticularly, are used in the field of biology^7-9 and medicines. Magnetic nanoparticle has attracted a great deal of researchinterests due to their distinctive properties and special applicationrecently.

CoFe2O4, is a well-known hard magnetic material with very high cubic magneto-crystalline anisotropy, high coercively, and moderate saturation magnetization. These properties make it a promising material for high-density magnetic recording.

CoFe₂O₄ nanoparticles have been widelysynthesized using severalways, same as sol-gel^9,10, micro-emulsion^11,12, chemical coprecipitation^11,12, hydrothermal synthesisand microwave^11-13 synthesis¹⁴. Among those methods, thesol-gel^9,10routesare very attractive, within the main advantages includingsimple control for chemical composition¹¹. Recently, in thesol­-gel^9,10synthesis of CoFe2O4 particles, the gelsare built-up via physical and chemical binds between the chemical species^11-14. It has been introduced a different sol­-gel^9,10routes@ polyacrylamide gel route for preparing CoFe₂O₄ nanoparticle.Due to lack of controls over the specific transformation of ananoparticle, obviously super-paramagnetic particle has not been prepared from magnetite, i.e. Magnetite-nano-particle whichgenerally loses their permanent magnetic properties in the lake ofthe external magnetic field^6-14.

The most applications require at least a magnetic-particle for dispersing in the non-magnetic matrixes. Thismatrix plays an important role for providing the meaning of particle dispersion for determining a physical property of a composite^10,14

The other important items of these matrixes are to act as a protection for magnetic nano-particlesagainst oxidation or corrosion especially^13,14in the metallic nano-particles^15-17. Among oxide matrixes¹⁵ such as alumina,silica¹⁶, zeolites,titanic oxides, carbon-based, the silica¹⁶ can be a general suitable materials for the matrixes because of inertness ofthe magnetic fields, its non-toxicity and easiness for forming crosslined networks structure^14-17.Silica shells chemically is stable and can be rapidly functionalized in the bio-conjugation purposes, in other words is biocompatible therefore CoFe₂O₄@SiO₂ as a silicacoated magnetite composite nanoparticles have been synthesized by several groups^18-20.

Recently, silica coated magnetite functionalized by γ mercaptopropyl-tri-methoxysilane has been successfully used for extracting Pb²⁺, Cd²⁺_,Na+, K+, Mg²+, Hg²⁺, and Cu²⁺in the wide pH ranges from water^15-20. The complex of metal-CoFe₂O₄@SiO₂–NH₂ nanoparticles^1-4 could be recovered easily from aqueous through magneticseparation and reproducereadily by acid treatment. By this work it has been exhibited the amino-functionalized CoFe₂O₄@B₁₈N₁₈ magnetic nanoparticles compare to CoFe₂O₄@SiO₂is much more effective as recyclable adsorbent for the removal of heavy and alkali/earth metal ions in water and wastewater treatments.

Catalysts has a very sensitivetreatment in technology and modern sciences as they increase reaction yield via reducingthe temperature in synthesisof the chemical product^21,22.There are two basic types of catalysis, (1); “heterogeneous^22-23, where the reaction accomplishes on the surfaces and the catalystsare in the solid phase.(2) “homogeneous²³, where the catalyst is in one phase as reactant²¹.

The heterogeneous²² catalyst might be easily separated from the mixed solvent but because of their limited area of surfacesthe reaction rate is restricted²². Meanwhile homogeneous²³ catalyst might react fast for providing a good rate (conversion rate)for the catalyst, but as they are solvable in the medium reaction, it might be a laboring processingfor removing them of the reaction enviroment²³. The problem in removing homogenous²³, catalysts from the reaction environment leads to problem ofmaintain the catalyzer for repeating²⁴. The bridge between heterogeneous²² and homogeneous²³ catalysts can be attained through the CoFe₂O₄nanoparticle²⁵.CoFe₂O₄exclusively is useful and important as the magnetic nano-particlewhich exhibit strong magnetic-momentand are seldomsustained outside of an external magnetic field²⁵. These kinds of nano-particle might be consist of several materials such as nickel, cobalt, iron oxides, ferrites^26,27and also alloys such as platinum/iron²⁶.CoFe₂O₄ MNPs of silica shells catalytic materialshave the benefit for increasing surface area which causes for any increased reaction rate²⁸. Moreover, nanoparticle might permit additional catalytic functionalitybecause of their unique properties²⁹. Several catalysis of magnetic nano-structure has been investigated up to now^30-32,such as preparation of nano-composite materials consist of magnetic-(core)nano-particle which has been coated by various shells of other catalytically³⁰active nano-material³².

Other type of catalyststhatare interest for organic compounds involves the using of organic molecules which are enabling forpreservationthe materials in the end of any reactions for reuseing^31-32. In this work we have investigated the catalysis’s properties of CoFe₂O₄ nanoparticles @B₁₈N₁₈ instead of SiO₂ for comparing in the area of chemical synthesizes.

Recently extensive theoretical and experimental studies have been accomplished on boron-nitride-fullerenes for understanding their relative stabilities and also sizedependence of important physical properties^33-35. (BN)_n are iso-electronic with carbon species, making them the goal of various research area and enhanced³⁴ by this fact whichB-Ncompounds havea stable crystallinephase near to graphite^36-38. Electronic properties of differentBnNn rings and cages have been investigated theoretically^39-45.Despite those experimental and theoretical works, the structural synthesized ofB_nN_n rings and cages are still unknown.

Boron nitrides exist in several crystalline forms, such as hexagonal and cubic shapes. Due to the closely structures between C-C and B-Nunits, large efforts have been donerecently to “BN” fullerenes, which has excellent properties like structural stability, heat resistance and insulation^46,47 . As the thermodynamic mechanism ofBN growth (from nuclei) is still not wellknown for those nanotubes, a comprehensivetheoretical simulation continue to attractenhanced attention⁴⁸. Although those nanotubes are found to be non-chiral or chiral;however, most interestsin the zigzag and armchair investigation are much less known about the chirality area of hexagonal boron nitride⁴⁹.

In our previous works^50-69, it has been exhibited the BN stabilities, NMR data, electronic properties, chemical phenomenon and the mechanism of tubes generatingfor the various structures of BN especially for the SWBNNT via a multi-walled nanotube including chirality m,n(with n = 3-4 and m=5-7). The diagram generation fornano-ring of the B_nN_n (n = 15, 16, 18 and 20) areshown in the references of^52-54. In addition, we reported a unique stable structures^52-54 with combinations of five and six loops same as a quasi B₃N₃H₆ shape forB₁₅N₁₅ and B₁₈N₁₈, respectively. Those kinds of shapes have significant properties in the nano studies of CoFe2O4@ B₁₈N₁₈ particles. Thesuitable stability and reasonable aromaticity of those structures have been confirmed by thermodynamic data,frequency optimization and NQR^52-54.

Hyperfine parameter and spin density, electrical potentials,electromagnetic properties and isotropic-fermi-coupling-constant^52-55 indicate thestability of those rings through using non-bonded interactionmodel between CoFe₂O₄& B₁₈N₁₈.A novel aminofunctionalized CoFe₂O₄& B₁₈N₁₈ magnetic nano-material with the core-shell structureshas been developed, for removing heavy metal ions from the aqueous media.The elasticity of electron density, electrical properties same aselectron and energy densities, kinetic and potential energiesof densities, ELF or LOL,eta index and ECPforCoFe₂O₄-B₁₈N₁₈ core-shell systems have been calculated and simulated in related reactions for Amino-functionalized CoFe₂O₄@B₁₈N₁₈ core–shell magnetic.The amino-functionalized CoFe₂O₄& B₁₈N₁₈nano-adsorbent exhibited high adsorption affinities for aqueous Fe (iii), Pb (ii), Ni (ii), Cu (ii) and Zn (ii) ions, resulting from complexation by surface amino groups.

Background and Methodology

Magnetic particles are suitable for aqueous transition and heavy metals due to thier unique advantages of quick separation and their high surface area under external magnetic fields^68-71. The surface modification, adsorption affinity, including covalent binding and physical coating, has often been explored^70-75 for enabling specific complexationfor further facilitates^72-76.

Recently it have been exhibited which the amino-functionalized molecules demonstrated outstanding abilities for removing a wide variety of transition heavy metal ions^77-79. Both B₁₈N₁₈& SiO₂are stable under acidic conditions, as compared with someothermaterials and functions for protecting the inner magnetite core^80-82.

Although CoFe2O4@SiO₂ has recently been investigated for potentialbiomedical applications⁸², there is no work about the CoFe2O4@ B₁₈N₁₈.

In this study with the theoretical approaches magneticnanoadsorbent has been developed via covalently grafting amino groupsover the surfaces of CoFe₂O₄@ B₁₈N₁₈nanoparticles. Part of the systems including CoFe₂O₄@ B₁₈N₁₈nanoparticles has been simulated with QM/MM methods and the investigation carried out by the Monte Carlo calculations.In this study, various force fields are done via “Amber” and OPLS for comparing the calculated energy of the CoFe₂O₄@ B₁₈N₁₈ nanoparticles. Furthermore, a Hyper-Chem professional release-7.01program is used for any further calculations.

For the non-covalent forces of B₁₈N₁₈andCoFe2O4, the B3LYP&BLYP methodsare unsuitablefor describing van der Waals^{83, 84} through medium-range interaction⁸⁴. Therefore, the ONIOM* methods withthree levels of tight (H), medium Hamiltonian (M), andlow (L) calculations have been accomplished in these studiesto estimate the non-bonded interaction between B₁₈N₁₈and CoFe₂O₄.

Thedensity functional method is used for the high levelwhile the semi-empirical (pm₆) with pseudo=Lanl₂ and Pm₃MM for both of them respectively. Some accurate studies have indicated that in-accuracy of the low range exchange energies goes tothe large systematic errors for the prediction of molecular properties^85-88.

Geometries optimization and electronic calculations have been accomplished using the m06 functional of DFT. These approachesare based on solution of the Kohn & Sham equation⁸⁹ in the plane-waves sets with projector-augmented-pseudo-potentials⁹⁰. The Perdew&Burke&Ernzerhof PBE⁹⁰exchange-correlation and generalized-gradient-approximation GGA⁹⁰are also used for non-bonding calculation.

The charge transferring and electrostatics potentials derived charges were also estimatedusing the Merz&Kollman&Singh⁹¹, chelpG⁹²or chelp⁹³.The charges calculation methods based on MESP or molecular-electrostatics-potentials fitting are not wellsuited for the larger systems whereas several of the inner-most points are located far away from the centers at which the MESP⁹³are computed.In that position, variation of the inner-most atomic charges would not be towards the changing of the MESP⁹¹ outside of the molecules.

The charge-density profiles of this study have been estimated from the first principle calculation through an averaging process as described in the references^90-95. The interaction energies or adsorbents energies between metals and CoFe2O4@B₁₈N₁₈catalyst were done according to the equation as follows:

That ΔE_s is the adsorbents energies.

Theelectron-localization-function or ELF^96-98, localized-orbital-locator or LOL^96-98  , electron density  of theGradient-norm&Laplacian, values of orbitals wave-functions, electron spin densities, electrostatic-potentials from nuclear-atomic-charges,the exchange-correlation density, as well astotal electrostatic potentials ESP), correlation-holes and correlation-factors, and the average local ionization energies using the Multi-functional-Wave-function analyzer have also been calculated in this study^96-98 .

Density Electron Approach for Interaction Between Mnps and B₁₈N₁₈:

The kinetic energies densitiesare not defined individually, since the expected values of the operators:

Can be estimated by integrating kinetic energy densities from those alternatives definitions. One of the usual used definitionsis as follows:

thelocal kinetic energies given below guarantee^96-98 hence the physical dataare more commonly used. The Lagrangian of kinetic energies densities, G(r)⁹⁷are also known as positive definite kinetic energy densities.

K(r) and G(r) are directly related by Laplacian of electron density

The electrostatic potential from nuclear/atomic charges can be calculated via:

Where R_A and Z_A denote position vector and nuclear charge of atom A, respectively.

Becke and Edgecombe have noted that spherically averaged like-spin conditional pair probability have  correlation with the Fermi hole and it has been suggested which the electron localization function (ELF)⁹⁸.

Where

and

for close-shell system, since

D and D₀ terms can be simplified as

In which the kinetic energies terms in D(r) is replaced by Kirzhnits types second-order gradients expansion, which are

So that ELF is totally independent from the wave-function, and then can be used for  analyzing electron densities from X-ray diffraction data.

Localized orbital locator or LOL^98,99is another item for locating high localization regions likewise ELF, which explained by Schmider&Becke⁹⁹ .

where

D_o (r)

For spin-polarized system and close-shell system are defined in the same way as in ELF⁹⁸. LOL have similar approaches compared to ELF.

Notice that evaluating ESP is much more time-consuming than evaluating other functions. The ESP evaluated under default value is accurate enough in general cases. Reduced density gradient (RDG) RDG are a pair of very important functions for revealing weak interaction region^96-98 for detail. RDG is defined as

^96-98 . Fortunately, it is found that weak interaction analysis under pro-molecular density is still reasonable. Pro-molecular density is simply constructed by superposing electron densities of free-state atoms and hence can be evaluated extremely rapidly.

Where

Is pre-fitted spherically averaged electrons densities of atom A.

Result and Discussion

Thiswork basically focuses on the magnetic properties of CoFe2O4 in the non-bonded systems with B₁₈N₁₈ surfaces including “CoFe2O4@B₁₈N₁₈”. The CoFe2O4@B₁₈N₁₈nano-adsorbent shown high adsorption affinities for aqueousNa+, K+, Mg2+, and the amino-CoFe2O4@SiO2  exhibited high adsorbent for  Fe (iii), Pb (ii), Ni (ii), Cu (ii) and Zn (ii) ions, resulting from complexation of the metals ion with the surface amino groups.The metalloaded CoFe2O4@B₁₈N₁₈ nanoparticles could be recovered easily from aqueous solution bymagneticseparation. The data have shown in ten figures and eight tables.As it is indicated in table1, LOL is low and constant for Both CoFe2O4@silica, CoFe2O4@B₁₈N₁₈ and CoFe2O4@ B₁₈N₁₈.ELF has a similar expression asLOL.

The non-bonded interactions are shown in figs1-3.As it is indicated in tables 1-4, the electrical properties can be obtained from changes in the non-bonded interactions. Potential energy densities, ELF, LOL,electron densities, energy densities,eta index and ECP are shown in tables1-4. The results of ELF and LOL indicate that the surfaces of silica and B18N18are suitable to attach in aromatic and organic compounds in any scale from nano to micro or medium.

Table 1: All Electron Densities of non-bonded interactions for CoFe₂O₄@Silica shell ^(a),CoFe₂O₄@B₁₈N₁₈ shell ^(b) andCoFe₂O₄ (isolate)^C

Atoms of CoFe2O4	Density of all electron (10-3)(a)  (c)  (b)	Density of alpha(10-3)(a) (c) (b)	Density of Beta(10-3)(a) (c) (b)	Spin Density (a) (c) (b)
Co(1)	0.30    0.12     0.36	0.15    0.06   0.18	0.15    0.06   0.18	0.0     0.1    0.0
Fe(2)	0.28    0.10      0.34	0.14    0.05   0.17	0.14    0.05   0.17	0.0     0.1    0.0
Fe(3)	0.14    0.12      0.32	0.07    0.06   0.16	0.07    0.06   0.16	0.0     0.1   0.0
O(1)	0.32    0.16      0.14	0.16   0.08   0.07	0.16   0.08   0.07	0.0     0.0    0.0
O(2)	0.18    0.12      0.24	0.09    0.06   0.12	0.09    0.06   0.12	0.0     0.0    0.0
O(3)	0.30    0.18      0.12	0.15    0.09   0.06	0.15    0.09   0.06	0.0     0.0    0.0
O(4)	0.12    0.18      0.22	0.06    0.09   0.11	0.06    0.09   0.11	0.0     0.0    0.0

Figure 1: the non-bonded interaction between CoFe2O4 and B18N18 shell Click here to View figure

Figure 2: the non-bonded interaction between Fe3O4 and silica shell for binding to 3-aminpropyltrimethoxysilane (APTMS) Click here to View figure

Figure 3: The optimized of CoFe2O4 in B18N18 shell Click here to View figure

 

As it is shown in Figs.5-9, there are clear curves within a decreasing amount for electron density, energy density, ELF and LOL. The interaction energy between two sides of CoFe2O4-silica and CoFe2O4-B18N18 are also calculated. The potential energy difference between the two parts, are depicted in table 2.

Figure 4: Shaded surface map for electron density of CoFe2O4-B18N18 shell Click here to View figure

 

Table 2: All Electron Energies of non-bonded interactions for CoFe2O4@Silica shell ^(a),CoFe2O4@B₁₈N₁₈ shell^(b) and CoFe2O4 (isolate)^C

Atoms of CoFe2O4	Lagrangian kinetic [G(r)]energy (10-3) (a) (c) (b)	Hamiltonian kinetic [K(r)]energy (10-3)(a) (c) (b)	Potential energy Density [U(r)](10-3)(a) (c) (b)
Co(1)	0.32   0.14      0.30	0.60    0.58      0.54	-0.68   -0.52   -0.78
Fe(2)	0.33    0.12      0.31	0.62    0.66      0.62	-0.65  -0.64 -0.63
Fe(3)	0.23    0.16      0.30	0.38    0.72      0.68	-0.34   -0.37   -0.58
O(1)	0.28    0.21      0.14	-0.32  -0.51    -0.43	-0.24   -0.59   -0.50
O(2)	0.24    0.15      0.26	-0.14  -0.40    -0.46	-0.86   -0.68   -0.54
O(3)	0.16    0.15      0.18	-0.44  -0.39    -0.44	-0.33   -0.63   -0.55
O(4)	0.24    0.16      0.18	-0.14  -0.42    -0.42	-0.85   -0.63   -0.51

 

Table 3: Laplacian, ELF, LOL and Local information entropy of non-bonded interactions for CoFe2O4@Silica shell ^(a),CoFe2O4@B₁₈N₁₈ shell ^(b)and CoFe2O4 (isolate)^C

Atoms ofCoFe2O4	Laplacian of electron density (10-3)(a)     (c)      (b)	Electron localization function (ELF) (10-3)(a)     (c)      (b)	Localized orbital locator (LOL) (10-3)(a)      (c)      (b)	Local information entropy (10-3)(a)    (c)   (b)
Co(1)	-0.20      -0.26   -0.22	0.41       0.46    0.65	0.14      0.22      0.13	0.14   0.15  0.25
Fe(2)	-0.25     -0.24   -0.24	0.41       0.48   0.66	0.12      0.24      0.14	0.20   0.34  0.25
Fe(3)	-0.13     -0.27   -0.26	0.13     0.25   0.61	0.63      0.12      0.17	0.49   0.16  0.18
O(1)	0.79      -0.67   -0.55	0.13      0.17    0.25	0.32      0.25      0.25	0.13   0.12  0.10
O(2)	0.18       0.25    0.30	0.13       0.16   0.2 2	0.10      0.15      0.25	0.23   0.32  0.30
O(3)	0.84       0.14   0.25	0.16       0.15    0.26	0.31      0.12      0.25	0.32   0.20  0.31
O(4)	0.23       0.24    0.15	0.16       0.13    0.29	0.16      0.17      0.21	0.32   0.24  0.14

Table 4: Average local ionization energy and RDG of non-bonded interactions for CoFe2O4@Silica shell ^(a),CoFe2O4@B₁₈N₁₈ shell ^(b)and CoFe2O4 (isolate)^C

Atoms of CoFe2O4	Reduced density gradient RDG) (10-3)	Average local ionization energy
	(a)       (c)    (b)	(a)     (c)   (b)
Co(1)	0. 4       0.5       0.5	0.4     0.4      0.5
Fe(2)	0.3       0.4       0.5	0.6     0.5      0.5
Fe(3)	0.4       0.5       0.5	0.4     0.4      0.5
O(1)	0.6       0.4       0.6	0.7     0.6      0.4
O(2)	0.3       0.4       0.5	0.3     0.5      0.4
O(3)	0.6       0.3       0.6	0.7     0.3      0.5
O(4)	0.4       0.3       0.5	0.3     0.4      0.5

Figure 5: LOL versus Position for CoFe2O4@B18N18 Click here to View figure

 

It has been exhibited the BN compounds such as B₁₈N₁₈ and amino-functionalized materials presented an ability for removing a wide range of earth/alkali. In contrast, the most usual used magnetic sorbents are based on iron oxide and unfortunately thussusceptible to leaching under acidic conditions. This problem will removed by replacing the B₁₈N₁₈ instead of Sio2 as a shell of CoFe₂O₄. In addition, functional groups on the coating layer chemically adhering to the MNPs are also assailable to acid treatment.In contrast of SiO2 rings which is stable under acidic conditions the B₁₈N₁₈  rings is independent from acidic situations and hence for B₁₈N₁₈ comparing to SiO₂, the functions is not needed as an ideal shell composite to protect the inner magnetite core. Although CoFe2O4@SiO2 or Silica-coated core–shell magnetite nanoparticles have recently been investigated for potentialbiomedical applications, by this work we exhibit the B₁₈N₁₈ is much useful for removing the earth/alkali metal from the aqueous solution.

Relative adsorption energies of four alkali and alkali/metal ions on CoFe2O4@B₁₈N₁₈have investigated. The adsorption data for Cu, Pb, Cd   and so on were fitted to the Langmuir model according to the equation:

where Ce (mmol/L) and qe (mmol/g) and are the aqueous concentration the adsorbed concentration at equilibrium adsorption, and, b and qm are the coefficient of affinity and the adsorption capacity respectively¹⁰¹.

Conclusion

We think that B₁₈N₁₈ capabilities of magnetic substrates should be explored in the near future. Another interesting development is using the B₁₈N₁₈ on magnetic-nanoparticle enables effective removal of alkali -earth/alkali metals based on catalysts froms important pharmaceutical products in the drug nanotechnology. Our Calculations indicate that the B₁₈N₁₈  are suitable surfaces for CoFe₂O₄ such silica surfaces for removal metal ions such as Na+, Mg++,K+ and Ca++.

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