Original Research Paper
Synthesis of sustainable silica xerogels/aerogels using inexpensive steel slag and bean pod ash: A comparison study

https://doi.org/10.1016/j.apt.2019.12.013Get rights and content

Highlights

  • Silica xerogels/aerogels were synthesized from steel slag and bean pod ash.

  • The effects of drying conditions on the properties of materials were investigated.

  • Properties of silica xerogels were compared with that of the silica aerogels.

  • Silica aerogels showed more porous structure than silica xerogels.

  • Lower density and thermal conductivity made silica aerogels unique materials.

Abstract

Low cost silica xerogels/aerogels were synthesized from steel slag and bean pod ash by sol–gel method. Comparison study showed differences between structural, morphological, textural, thermal and physical properties of the silica xerogels and aerogels. Formation of amorphous structure and silica network was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analyses, respectively. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses revealed that silica xerogels had smaller interlinked network in contrast to silica aerogels. Typical type IV isotherm was observed for all samples in N2 adsorption-desorption isotherms. The highest surface area was determined as 371 m2 g−1 for silica aerogel synthesized from steel slag. Particle size of silica aerogels was lower than that of the silica xerogels. The more porous structure made silica aerogels desirable materials with lower bulk density and thermal conductivity when compared to silica xerogels. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) exhibited high thermal stability of the silica xerogels/aerogels. Although silica xerogels had highly hydrophilic structure, contact angle of silica aerogels synthesized from steel slag and bean pod ash was 60° and 74°, respectively. The comparison study will give a new point of view about differences between silica xerogels and aerogels synthesized from by-products or inorganic/organic waste instead of silicon alkoxides.

Introduction

Silica aerogels, cross-linked network structure, are one of the ultra-low density solid-state materials which are generally synthesized by low temperature sol-gel method [1], [2], [3]. Silica aerogels show high porosity (80–99%) and surface area (~500–1500 m2 g−1), low thermal conductivity (0.005–0.1 W m−1 K−1), dielectric constant (1–2) and sound velocity (100 m s−1) etc. [4], [5], [6], [7]. The combination of unique properties makes them promising materials in many applications such as thermal and sound insulation, fire retardation, air and liquid purification [8], [9], sensors, catalysis, electronics and so on [10].

Synthesis of silica aerogels consists of three steps: (1) gel formation, (2) aging and (3) drying of the gel [11]. In the first step, an organic solvent is mixed with silicon alkoxide like tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), polyethoxydisiloxane (PEDS), methyltriethoxysilane (MTES), propyltrimethoxysilane (EDAS) [12], [13]. Following the addition of stoichiometric amount of water, hydrolysis of Si-O-C bonds occurs by acid, base or two-stage catalyzed reactions. Polycondensation reactions which provide formation of silica gel start simultaneously along hydrolysis [14]. Gelation time changes considerably depending on water:silica precursor ratio, hydrolysis time, temperature, pH, solvent type, concentration and amount of catalyst, additives etc. [12]. In the aging process, an improvement in strength of silica network is purposed by immersing the silica gel in water/alcohol or siloxane solution. During aging, two mechanisms can take place at different rate: (1) neck growth originated in transportation of silica dissolved from particle surfaces to the neck region between silica particles [15] and (2) accumulation of dissolved smallest particles on the more active network sites related with inequality in surface energy which is known as Ostwald ripening [16]. In the last step, different drying techniques involving ambient pressure drying, supercritical drying and freeze drying are carried out to remove solvent in the silica gel. Silica xerogels are obtained using ambient pressure drying in which capillary stresses are inevitable. The process results in formation of dense and brittle materials in addition to irreversible shrinkage [17]. Supercritical drying which is performed above critical point of the solvent is used to avoid surface tension between liquid and vapor that provides crack free materials, so-called silica aerogels. The drying method also enables the production of high porosity materials without excessive shrinkage [18]. The silica cryogels are synthesized through freeze drying that includes sublimation process in pursuit of freezing entrapped liquid in the gel pores [19]. When compared the drying techniques, ambient pressure drying comes into prominence due to inexpensive, simple and safe method. However, more than one solvent exchange can be required during the aging step that leads to run out of too much solvent [20], [21]. The main limitations of supercritical drying for large scale productions are high cost and energy consumption, difficult to handle as well as safety need. In spite of the disadvantages, silica aerogel monoliths with desirable porosity are easily obtained by supercritical drying [10], [22]. Freeze drying method is commonly utilized in medical applications which require operation in low temperatures. When examining drawbacks of this method, specific equipment is necessary and porous structure of the gel can be damaged in some instances [23].

Silicon alkoxides for synthesis of silica based materials are expensive and hazardous [24]. So, utilization of various industrial by-products, clays, biomass ash and agricultural waste has received great attention in silica xerogel/aerogel production recently. For example, fly ash [25], oil shale ash [26], kaolin [27], rice husk waste [28], bagasse ash [29], sago waste ash [30], cellulose [31], wheat starch [32], corn stalk ash [33] etc. have been used to prepare silica network structure by an alkali extraction. Sodium silicate solution (Na2O.xSiO2) is easily obtained from the materials which are rich in silica in basic medium such as NaOH with the following reaction:xSiO2intheprecursor+2NaOHNa2O.xSiO2+H2O

After the addition of acid like HCl to the solution, the following reaction takes place at pH ≤ 10 [29]:Na2O.xSiO2+2HClxSiO2+2NaCl+H2O

In this step, presence of Na+ ions in the solution that increases surface tension adversely affects porous structure of the silica gel. To eliminate Na+ ions, excessive washing with water is performed or ion exchange resins are commonly used [25]. Silica xerogels/aerogels are produced with using similar aging and drying steps as mentioned above.

Several studies about silica xerogel/aerogel synthesis from commercial silicon alkoxides have been reported over the last two decades. However, as far as we know, there is no comparison study of silica aerogel and xerogel obtained from by-products or biomass. Starting from this point, novel silica xerogels/aerogels were successfully synthesized from basic oxygen furnace steel slag and bean pod for the first time in this study. Steel slag is a by-product of conversion process of iron to steel. The amount of steel slag is approximately 10–15% of steel production [34], [35]. In spite of some applications of steel slag in hot metal dephosphorization, road and hydraulic construction, production of cement and concrete, waste water treatment, CO2 capture and flue gas desulfurization, and agriculture, utilization of steel slag is very low [36]. Accumulation of steel slag has considerably increased that causes serious environmental problems. So, improvement in utilization ratio of steel slag is required immediately. Bean pod is an agricultural waste which is produced through processing of bean seeds [37]. Recycling of the abundant waste at different forms has gained great attention to provide clean environment. Up to now, bean pod ash has been used as adsorbent for removal of heavy metal ions, filler for polymer composites, additive in soap making, and so on [38], [39]. The structural, morphological, textural, thermal and physical properties of the materials were investigated in details. This study provides not only synthesis of low cost, sustainable and environmental friendly materials, but also a decrease in accumulation of by-product/waste.

Section snippets

Raw materials and chemicals

Steel slag (140 mesh) was obtained from Iskenderun Iron and Steel Works Co. in Turkey. The chemical composition of steel slag is SiO2 37%, CaO 34%, Al2O3 10%, MgO 10% and small amounts of Fe2O3, K2O and Na2O. Bean pod (230 mesh) with elemental composition of C 40%, O 31%, H 11%, Si 6%, N %1 and trace amount of K, S, Ca, Na and Fe was easily supplied from Kayseri region. Hydrochloric acid (HCl, Sigma-Aldrich), sodium hydroxide (NaOH, Sigma-Aldrich), isopropanol (C3H8O, Merck) and n-hexane (C6H14

XRD analysis

XRD patterns of the silica xerogel/aerogel synthesized from steel slag and bean pod ash are shown in Fig. 1a and b, respectively. In all materials, typical broad peak was observed at about 23° which is indication of amorphous structure formation [40]. It can be clearly seen that no peak corresponded to NaCl appeared in the XRD patterns. In other words, removal of NaCl compound was successfully provided with sufficient washing for all materials synthesized from steel slag and bean pod ash.

FTIR analysis

FTIR

Conclusions

In this study, low cost silica xerogels/aerogels were synthesized from inexpensive inorganic by-product/organic waste. A detailed comparison between properties of silica xerogels and aerogels were performed by different analyses. XRD analyses indicated that all materials were amorphous structure without NaCl compound. FESEM and TEM images showed higher porous structure of silica aerogels than that of the silica xerogels. DLS analyses revealed lower particle size of silica aerogels than that of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors are very grateful to the Academic Staff Training Program (2016-ÖYP-071), TUBITAK for 2211-E Direct PhD Scholarship Program and Konya Technical University. All the authors would like to thank Prof. Dr. Metin Guru (Gazi University) for providing supercritical drying system.

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