transformation of iron nitrate into iron … · a smart mesoporous silica molecular sieve...

Maria Ulfa, Teguh Endah Saraswati, Bakti Mulyani, Didik Prasetyoko

709

TRANSFORMATION OF IRON NITRATE INTO IRON NANOPARTICLES EMBEDDED IN MESOPOROUS SILICA MOLECULAR SIEVE SBA-15 NANOSTRUCTURES

BY ULTRASONICATION AND MICROWAVE IRRADIATION

Maria Ulfa1, Teguh Endah Saraswati2, Bakti Mulyani1, Didik Prasetyoko3

ABSTRACT

Regular iron nanoparticles embedded in mesoporous silica molecular sieve SBA-15 walls are synthesized using classic impregnation modified by ultrasonication and a microwave procedure. Iron nanoparticles in mesoporous silica molecular sieve of a regular honeycomb structure are prepared by using mesoporous silica molecular sieve SBA-15 as a support and iron nitrate as an iron precursor. The characteristics of the material are investigated using several methods such as energy nitrogen adsorption-desorption, small angle X-ray diffraction (SA-XRD), energy dispersive X-ray (EDAX), Fourier transformation infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The results show that the iron-incorporation process does not significantly change the structure of the mesoporous silica molecular sieve materials after calcination at 700ºC. However, ultrasonication and microwave treatment during the impregnation process enhance the affinity between the silica and the functional iron groups leading to silanol formation. The addition of iron after ultrasonication leads to a large content of iron in the mesoporous silica molecular sieve and silanol groups during calcination. The microwave treatment increases the iron content with the temperature increase during the calcination. A regular mesostructures’ destruction occurs due to fast thermal reactions between silica and oxygen during the calcination at 700ºC.

Keywords: iron nanoparticle, mesoporous silica molecular sieve, ultrasonication, microwave.

Received 15 March 2018Accepted 18 March 2019

Journal of Chemical Technology and Metallurgy, 54, 4, 2019, 709-714

1 Department of Chemistry Education, Faculty of Teacher Training and Education Sebelas Maret University, Ir. Sutami 36A Surakarta Center of Java 57126 Indonesia 2 Department of Chemistry, Faculty of Science and Mathematics Sebelas Maret University, Ir. Sutami 36A Surakarta, Center of Java 57126 Indonesia 3 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Institute Technology Sepuluh Nopember, Jl Keputih, Surabaya, East Java Indonesia E-mail: [email protected]

INTRODUCTION

Industry requires porous materials as adsorbents, catalysts, separators, drug delivery carrier and energy storage [1, 2, 21]. Silica is one of the classic materials of many specific characteristics used for this purpose. Mesoporous silica molecular sieve SBA-15 is the most interesting silica because of its unique characteristics such as the large pore diameter, the ordered structure, the high pore volume, and the high specific surface area

[2 - 4, 21]. The upgrading mesoporous silica molecular sieve SBA-15 has been a focus of research in the last few years. Magnetization is one of the methods used leading to an increase of nano-silica applications for magnetic-resonance imaging as well as for production of carriers, separators, and magnetic material based storage tools [5, 6]. On the other hand, the nano-silica-based magnetiza-tion process has rarely been studied.

The magnetization material generated by incorporat-ing a metallic source into the corresponding support is

Journal of Chemical Technology and Metallurgy, 54, 4, 2019

710

in fact a new material that can be separated by a magnet [6], which is more applicable, effective, and efficient than the classic separation processes [7]. The choice of the metal is an important step in the magnetization process. Some materials are developed by using various aromatic precursors and transition-metal compounds such as iron nitrate, nickel nitrate, and cobalt chloride [8, 9]. The iron nitrate is the most common metal source for this purpose due to the fact that it is nontoxic, has a low cost and requires a simple preparation [9]. Com-mon impregnation is used to incorporate a metal into a mesoporous silica molecular sieve. Several approaches are applied aiming to avoid the pore-blocking during the metal incorporation. They refer to stirring, choos-ing a proper solvent and a dispersion-metal preparation. The treatment required for the proper metal distribution resulting in pore minimization includes techniques such as self-assembly, metal-particle adsorption, microwave treatment, coating, and ultrasonication [10,11].

The microwave irradiation provides heating which is much faster than that of the conventional method and a more homogeneous nucleation. Furthermore, it provides the best conditions for supersaturation and crystallization time minimization [11]. It is efficient and economical for the preparation of a mesoporous material as it maintains the particle dispersion and controls the particle size during the pore formation. The ultrasonica-tion homogenizes the organic–inorganic molecules in the course of nanomaterial generation [12].

A simple chemical approach should be advanced for a smart mesoporous silica molecular sieve preparation. This paper reports a detailed study on the synthesis of iron-mesoporous silica molecular sieve (IMS) through ultrasonication and microwave treatment during the impregnation process. Iron nitrate is directly embedded into mesoporous silica molecular sieve SBA-15 as an iron precursor prior to the microwave irradiation. The mesoporous silica molecular sieve SBA-15 particles are considered a hard support and a microwave absorber. An ultrasonic technique is applied to control the particle size prior to the calcination process. The chemical transforma-tion is followed by observing the change of the elements present and the characteristics of the IMS material. The results of this study can potentially be used as a reference to gain in-depth knowledge of the influence of ultrasoni-cation and microwave irradiation on incorporating iron into mesoporous silica molecular sieve pores.

EXPERIMENTAL

Materials Mesoporous silica molecular sieve SBA-15 was

purchased from SIX-C0014, China and used as a sup-porting material. All chemicals, including HCl as a hydrolysis agent and ethanol as a washing agent, were purchased from Merck. Iron nitrate, (Fe(NO3)2·9H2O), used as an iron precursor, and double-distilled deionized water, used in the synthesis process, were purchased from Fluka.

Synthesis of iron nanoparticles in mesoporous silica molecular sieve (IMS)

The iron nanoparticles in mesoporous silica mo-lecular sieve were prepared by gradually adding 1 g of mesoporous silica molecular sieve SBA-15 to a stirred solution containing 0.05 g of iron nitrate Fe(NO3)2.9H2O, 0.14 ml of HCl, and 10 ml of water. The mixture was placed in a microwave digestion system of an operat-ing power of 400 watts and 110 psi for 30 min at 70ºC. The resulting solution was placed for ultrasonication homogenization within 20 min. The resulting yellow sample was dried in an oven for 30 h at 110ºC. By the end of this step the sample became dark yellow in color. It was additionally heated under flowing air in a furnace for 7 h at 750ºC at a heating rate of 5ºC/min. The resulting iron-mesoporous silica molecular sieve solid was filtered, then washed with double-distilled deioni-zation water and dried for 30 h at 110ºC. The obtained iron-silica was labeled as IMS. The mesoporous silica molecular sieve SBA-15 used as a support was labeled as MS. Finally, the iron was impregnated varying the iron nitrate quantity. The different samples were labeled as IMS-n % (where n indicated the weight of iron nitrate compared to the total weight of the MS).

Characterization The SA-XRD patterns of the mesoporous samples

were recorded with a Bruker D8-Advance powder diffractometer using Ni-filtered Cu-Ka radiation (k = 1.54056 Å) over a 2-h range at 0.5º - 6º with a 0.04º step size and a 2 s step time. The IMS sample morphol-ogy was recorded by a TEM instrument (STEM-JEOL 2010) operating at 120 kV. The dispersion of the IMS samples was determined using ethanol and dropping each sample on a Cu grid coated with a carbon film. The


711

adsorption-desorption nitrogen isotherms were followed at 196ºC using Nova instrument. Prior to the adsorption measurement, the IMS materials were degassed for 3 h at 310ºC in a vacuum. The specific surface areas were determined via the BET method. The pore-size distribu-tions were derived from the isotherm curves using the Barett–Joyner–Halenda (BJH) model. The total pore volumes were obtained from the amount of the nitrogen uptake at P/P0 = 0.99. The FTIR spectra were recorded by a FTIR spectrometer at 36ºC (Bruker Shimadzu 70). The samples were thoroughly mixed with KBr powder (3 mg of a sample per 300 mg of KBr). The FTIR spectra were recorded in the range from 400 cm-1 to 4000 cm-1. The iron-nitrate solutions and the mesoporous silica molecular sieve SBA-15 were mixed by using a micro-wave digestion system (MARS-5, CEM Products) and ultrasonication homogenization (Nuve model ST402).

RESULTS AND DISCUSSION

Fig. 1 shows the nitrogen adsorption-desorption isotherms of IMS-2 % and IMS-5 % together with that of mesoporous silica molecular sieve SBA-15 (MS) for a comparison. The IMS isotherms match type IV with pronounced hysteresis loops indicating that the pores of the IMS materials are of a mesosize. The inflection of IMS in P/P0 range from 0.45 to 0.75 indicates the condensation of the capillary channels within the silica pores. The IMS samples adsorb more nitrogen at relative pressure, which explains the ordered regular pores of 8.0 nm (Fig. 2). The shift of IMS hysteresis loop from high to low pressure indicates a decreasing IMS pore size compared to that of the MS sample due to the existence of iron nanoparticles. The capillary condensation of IMS refers to the adsorption by two pore types (discussed later) imaged in the TEM. The iron-mesoporous silica molecular sieve shows a regular pore diameter of 6.5

nm (Fig. 2). The similar pore sizes of the iron and silica pore systems (as observed in the TEM image in Fig. 4) cannot be distinguished by BJH analysis. The nitrogen-adsorption isotherm of IMS samples at P/P0 ranging from 0.0–0.3 shows high adsorption due to micropores presence (Fig. 3). IMS-2 % shows a higher surface area (479 m2/g) compared to that of IMS-5 % (347 m2/g) due to the multimodal pore system and greater pore spaces, making IMS-2 % more efficient for application studies. A considerable fraction of the large surface area of IMS arises from its microporous wall structure. This material, with its hollow-type framework configuration, provides many more sites for magnetization of big molecules. The structural transformation can not only be observed by the nitrogen adsorption-desorption isotherms but also by the change of the elements in energy dispersive X-ray (EDAX) spectrum prior to and after the nanoparticle-impregnation process (Table 1).

Fig. 1 shows N2 adsorption and desorption isotherms of iron nanoparticles on a mesoporous silica molecular sieve synthesized via microwave irradiation and ultra-sonication. The samples of IMS exhibit hysteresis loops

Fig. 1. Nitrogen adsorption-desorption isotherms of iron-mesoporous silica molecular sieve samples.

Table 1. Elemental analysis by EDAX of iron-mesoporous silica molecular sieve samples.

A sample An element (wt %) MS IMS 2 % IMS 5 % Si 80.10 77.1 74.1 O 18.10 20.2 19.2 Fe 0.00 1.98 4.38 Other 0.70 0.90 2.32


712

at P/P0 = 0.6, indicating that mesoporous structures are obtained [13 - 16]. The amount of the iron embedded in the mesoporous silica molecular sieve depends on the amount of the iron nitrate added. The volume of the porous iron on the mesoporous silica molecular sieve prepared with a high amount of iron is much less than that of the sample prepared with a lower amount of iron. This phenomenon shows that the iron particles under microwave irradiation and ultrasonication do not significantly contribute to creation of mesopores within the silica. However, this does occur during the prolonged calcination process, which could possibly increase the iron aggregation. Fig. 3 shows that iron aggregation occurs in the range of 0.3 nm - 0.5 nm due to the mesoporous silica role played in the course of microwaves absorbance connected with enhancement of iron-silica interaction. This is confirmed by TEM image in Fig. 4, which shows a distribution of narrow particles larger than the iron one in IMS samples ranging from 5 nm to 10 nm.

Fig. 2 illustrates the pore-size distribution of the iron-mesoporous silica molecular sieve samples. The iron embedded in MS can cause a small structural destruction. The stability of IMS-2 % after calcination at 750ºC is confirmed by the changes of the pore size (from 8 nm to 6 nm) and the surface area (from 550 m2/g to 497 m2/g). When the iron concentration increases from 2% to 5%, the surface area decreases rapidly from 459 m2/g to 374 m2/g, indicating a destruction of a part of the uniform mesopore rod caused by iron-particle incorporation. This destruction is confirmed by XRD and EDAX data as shown in Fig. 3

and Table 1, respectively. It reveals that the incorporat-ing iron particles can decrease the stability of the silica framework under air-flow treatment at 750ºC because they cover up some porous sections of the mesoporous silica molecular sieve. Moreover, XRD and BET data shows that the regular nano-iron particles in the mesoporous silica structure of IMS-2 % can not be preserved after an incorporation treatment for 3 h at 750ºC.

The IMS samples’ morphology is investigated us-ing TEM, and the results are presented in Fig. 4. The TEM images are in agreement with the XRD data. IMS shows a honeycomb structure that is separated by small mesoporous pipes confirming the presence of a multi-modal pore system. Fig. 2 exhibits the multiporous types corresponding to the covering silica surface created by incorporating iron particles during the iron aggregation of IMS tubes. The TEM images of MS (A) and IMS-2 % (B) in Fig. 4 show the honeycomb pore arrange-ments of the samples. The surface-to-surface distance between the adjacent carbon nanopores is 3.4 nm. The synthesized IMS-2 % shows also a highly ordered hex-agonal structure - an inverse replica of MS consisting of cylindrical silica tubes separated by ordered arrays of iron particles (the black sphere in the white circle). The silica nano pipes have a diameter of ca 6.7 nm, while the centers of the adjacent rods are 9.5 nm apart with a surface-to-surface distance of ca 2.8 nm. A greater iron-particle distribution appears in IMS-5 % than that in IMS-2 % sample. This indicates that the amount of iron nitrate penetrating the pores corresponds to the amount that enters during the impregnation process. It is worth

Fig. 2. Pore-size distributions in iron-mesoporous silica molecular sieve samples.

Fig. 3. Powder XRD patterns of iron-mesoporous silica molecular sieve samples.


713

noting that some of the pores in IMS-5 % are damaged, which possibly occurs because of the large amount of the iron metal bound to the surface and pores of MS. This phenomenon is confirmed by BET data which shows 25 % decrease of surface area of IMS-5 %.

The iron distribution on the surface framework of the mesoporous silica molecular sieve is investigated using FTIR spectroscopy. The FTIR spectra shown in Fig. 5 exhibit two characteristic absorption bands of medium-pore silica, namely at 3743 cm-1 (silanol groups) and 1610 cm-1 (bridging Si–O–Si groups). The FTIR spectra of IMS samples after microwave treat-ment show a sharp peak at 1680 cm-1 and 1300 cm-1 that confirms the presence of iron and a small amount of nitrogen in the silica framework [15 - 18]. The iron and nitrogen contained in the nitrate compound are located in mesoporous silica molecular sieve environments of IMS framework. The absorption band at 1300 cm-1 dis-appears after calcination, indicating the loss of nitrate

ions during the heating process under air flow. In addi-tion, the broadening XRD peak of IMS-2 % and IMS-5 % in Fig. 3 shows the asymmetrical nature of the iron particles. The increasing amounts of iron particles leads to a small destruction during the microwave treatment (silanol group decrease) due to the unbalanced iron posi-tion. It is well recognized that silica plays an important role as a microwave absorber minimizing structural destructions [19-21]. Moreover, after the calcination process, the silanol group presence increases (confirmed in FTIR spectra) as the iron concentration decreases due to the iron crystallization. Finally, the absorption band of IMS after microwave irradiation and ultrasonication is quite different from that of IMS after calcination, in-dicating that the transformation from iron nitrate to iron nanoparticles in the mesoporous silica occurs during the preparation procedure via impregnation.

CONCLUSIONSIron incorporation into mesoporous silica molecular

sieve is successfully achieved by microwave treatment and ultrasonication. The transformation of iron nitrate to iron nanoparticles occurs during the heating process. Mesoporous silica molecular sieve particles act as a hard support for iron and microwave absorbers. The metal dispersion can minimize the mesoporous silica destruction during the preparation by controlling the weight of the iron precursor. The BET, TEM, XRD, and FTIR analyses verify that iron aggregates cover the silica surface and that the characteristics observed can vary based on the iron concentration used in the microwave-irradiation synthesis. IMS-2 % has a regular mesopore-size distribution, which provides its applica-tion as a potential magnetization material.

Fig. 4. A transmission electron microscopy image of iron-mesoporous silica molecular sieve samples.

Fig. 5. FTIR spectra of iron nanoparticles on mesoporous silica molecular sieve in the course of preparation.


714

Acknowledgements The authors would like to thank Directorat General of

High Education (DIKTI), Indonesia, for the financial sup-port through a Post Doctoral Research 2019 scheme grant with number contract of grant 718/UN27.21/PN/2019 (DIKTI Number contract 092/SP2H/LT/DRPM/2019).

REFERENCES1. G. Chandrasekar, W.J. Son, W.S. Ahn, Synthesis of

mesoporous materials SBA-15 and CMK-3 from fly ash and their application for CO2 adsorption, J. Porous Mater., 16, 2009, 545-551.

2. H. Dabwan, D. Imai, S. Kaneco, S. Isamu, N. Kat-suyuki, K. Hideyuki, S. Tohru, O. Kiyohisa, Water purification with sintered porous materials fabri-cated at 400 degrees C from sea bottom sediments., J. Environ. Sci. (China), 20, 2, 2008, 172-176.

3. D.B. Dadyburjor, A.H. Stiller, C.D. Stinespring, A. Chadha, D. Tian, S.B. Martin, Use of an Aerosol Technique to Prepare Iron Sulfide Based Catalysts for Direct Coal Liquefaction, Adv. Catal. Nano-structured Mater., 5, 3, 5, 1996, 567-581.

4. H. Dai, X. Gao, E. Liu, L.Yang, J. Fan, X. Hu, Syn-thesis and characterization of graphitic carbon nitride sub-microspheres using microwave method under mild condition, Diam. Relat. Mater., 38, 2013, 109-117.

5. S. Deng, W. Chu, H. Xu, L. Shi, L. Huang, Effects of impregnation sequence on the microstructure and performances of Cu-Co based catalysts for the synthesis of higher alcohols, J. Nat. Gas Chem., 17, 4, 2008, 369-373.

6. J. Roggenbuck, T. Waitz, and M. Tiemann, Synthesis of mesoporous metal oxides by structure replication: Strategies of impregnating porous matrices with metal salts, Microporous Mesoporous Mater., 113, 1-3, 2008, 575-582.

7. A.Y. Khodakov, V.L. Zholobenko, R. Bechara, D. Durand, Impact of aqueous impregnation on the long-range ordering and mesoporous structure of cobalt con-taining MCM-41 and SBA-15 materials, Microporous Mesoporous Mater., vol. 79, 1-3, 2005, 29-39.

8. J.B. Koo, N. Jiang, S. Saravanamurugan, Direct syn-thesis of carbon-templating mesoporous ZSM-5 using microwave heating, J. Catal., 276, 2, 2010, 327-334.

9. J. Lee, S. Jin, Y. Hwang, J. Park, H. Min, T. Hyeon, Simple synthesis of mesoporous carbon with magnetic nanoparticles embedded in carbon rods, Carbon , N. Y., 43, 2005, 2536-2543.

10. Y.P. Lin, H.P. Lin, D.W. Chen, H.Y. Liu, H. Teng, C.Y. Tang, Using phenol-formaldehyde resin as

carbon source to synthesize mesoporous carbons of different pore structures, Mater. Chem. Phys., 90, 2-3, 2005, 339-343.

11. T. Cai, M. Zhou, G. Han, S. Guan, Phenol-formal-dehyde carbon with ordered/disordered bimodal mesoporous structure as high-performance electrode materials for supercapacitors, J. Power Sources, 241, 2013, 6-11.

12. D. Srinivas, P. Ratnasamy, Spectroscopic and catalyt-ic properties of SBA-15 molecular sieves function-alized with acidic and basic moieties, Microporous Mesoporous Mater., 105, 1-2, 2007, 170-180.

13. Y.H. Taufiq-Yap, Sudarno, U. Rashid, Z. Zainal, CeO2-SiO2 supported nickel catalysts for dry reform-ing of methane toward syngas production, Appl. Catal. A Gen., 468, 2013, 359-369.

14. K. Ariga, A. Vinu, J.P. Hill, T. Mori, Coordina-tion chemistry and supramolecular chemistry in mesoporous nanospace, Coord. Chem. Rev., 251, 21-24, 2007, 2562-2591.

15. A. Vinu, M. Miyahara, K. Ariga, Preparation and pore size control of cage type mesoporous carbon materials and their application in protein adsorption, Stud. Surf. Sci. Catal., 5, 5, 2005, 971-978.

16. J.L. White, M.J. Truitt, Heterogeneous catalysis in solid acids, Prog. Nucl. Magn. Reson. Spectrosc., 51, 2, 2007, 139-154.

17. L. Xiang, S. Royer, H. Zhang, J.M. Tatibouët, J. Barrault, S. Valange, Properties of iron-based mesoporous silica for the CWPO of phenol: A com-parison between impregnation and co-condensation routes, J. Hazard. Mater., 172, 2-3, 2009, 1175-1184.

18. D.-D. Xu, Y.-L. Deng, C.-Y. Li, Y. Lin, H.-W. Tang, Metal-enhanced fluorescent dye-doped silica nanopar-ticles and magnetic separation: A sensitive platform for one-step fluorescence detection of prostate specific antigen, Biosens. Bioelectron., 87, 2017, 881-887.

19. H. Yan, Y. Chen, S. Xu, Synthesis of graphitic car-bon nitride by directly heating sulfuric acid treated melamine for enhanced photocatalytic H2 production from water under visible light, Int. J. Hydrogen Energy, 37, 1, 2012, 125-133.

20. H. Zhou, S. Zhu, I. Honma, K. Seki, Methane gas storage in self-ordered mesoporous carbon (CMK-3), Chem. Phys. Lett., 2, 4, 2004, 165-173.

21. M. Ulfa and D. Prasetyoko, Infrared Spectroscopic and Scanning Electron Microscopy Study of Ibupro-fen Loading onto the Molecular Sieve Mesoporous Silica SBA-15 Material, Orient. J. Chem., 34, 5, 2018, 2631-2636.

transformation of iron nitrate into iron … · a smart mesoporous silica molecular sieve...

Documents