Research Article (change here if your work is review, letter or etc.)

A green chemical synthesis method forproduction of silica gel as sorbent material

Ruken Esra DEMIRDOGENa*, Gozde Ultavb, Tahira PIRZADAc, Syed Sakhawat SHAHd

a Department of Chemistry, Faculty of Science, Cankiri Karatekin University, TURKEYb Department of Chemical Engineering, Faculty of Engineering, Cankiri Karatekin University, TURKEY

c Department of Chemistry, Faculty of Science, Quaid-i-Azam University, Islamabad, Pakistand Department of Chemistry, Hazara University, Mansehra, Pakistan

* Corresponding author’s e-mail address: esrademirdogen@karatekin.edu.tr

A BSTRACT This paper presents a Green Chemical synthesis method for producing silicagel. The ever increasing world population and the demand for continuousdevelopment acquire production of products with better quality inincreasing amounts. However, in order not to encroach on ecologicalsystems, eco-responsible or green vision should be integrated in everyfront of life. Eco-innovative means and the green chemistry (GC) movementappear to be efficient tools for achieving this end. In this study,principles of GC are exploited to develop a new synthesis method forproduction of silica xerogels by using hexadecyl trimethyl ammonium bromide(CTAB), which is a cationic surfactant used as an economic precursor andstructure template, monitoring of experiment at room temperature and theprocess was catalyzed via an alkaline catalyst. Regeneration of the ionexchanger after the experiment rendered the process energy efficient andeco-friendly. The method of synthesis developed provided mesoporous silicaparticles with very small size and almost homogeneous size distribution.The CTAB content in the uncalcined gel was measured via thermogravimetricanalysis, Fourier Transform Infra-Red Spectroscopy and X-Ray DiffractionAnalysis. It was found that CTAB was completely removed from the template.Thus, in this study GC was employed as an eco-efficient and effective wayof producing silica gel.

Keywords: Green chemistry, silica gel, xerogels, ion exchanger

Düzce University Journal of Science &

Düzce University Journal of Science & Technology, ... (2014) …

I. I NTRODUCTION

ILICA GEL offers important advantages as drying chemicals,sorbents, catalysts and in animal husbandry, such as food

preserving, controlling humidity in the animal houses and thus it isused in in many areas.

SThe remarkable economic and population growth of the 20th century,which was based on continuous growth and development rather thansustainability, was closely coupled to substantial increases in theextraction and consumption of natural resources. This causedincreasingly-damaging negative environmental impact. If the economicgrowth rate is not decoupled from the rate of natural resourceconsumption, by 2050, approximately 140 billion tons of minerals,ores, fossil fuels and biomass per year, which is three times thecurrent rate, would be needed [1]. This is far beyond what is likelysustainable if realized at all given finite world resources and itsimpact on our water supply, air quality, forests, climate, biological diversity, and humanhealth would be severe. Today 40% of deaths worldwide are due to water,air and soil pollution [2]. It should be kept in mind that alreadythe world is running out of cheap and high quality sources of someessential materials such as oil, copper and gold, the supplies ofwhich, in turn, require ever- rising volumes of fossil fuels andfreshwater to produce. Therefore, it is a must that the rate ofresource productivity, which is doing more with less, should beimproved faster than the economic growth rate. However, globallythe increase in the economic growth rate was more than the increasein the consumption of natural resources. This indicates thateconomic growth can be realized without depleting the naturalresources and causing negative impact on the environment and on thehealth of humans. As mentioned in the International Resource Panel(UNEP), globally, about 25% less material input was required in 2002compared to 1980 to produce one unit of real Gross Domestic Product(GDP), and some negative environmental impacts have been reduced[1]. In other words, some ‘dematerialization’ of the world economyhas occurred spontaneously. Accelerating this process of decouplingeconomic activity from consumption and environmental impacts is

fundamental to future human well-being. Thus, in order to enabledecoupling to support sustainable development more effectivelyfurther sustainability-oriented innovations are urgently required.Hence, more transformative change and a more resource efficient andGreen Economy, in which economic growth, social equity and humandevelopment are interlinked with environmental security, business andindustry is required to meet the size of the challenge.

Sustainable consumption and production models and patterns should beadopted and devised to promote social and economic developmentwithin the carrying capacity of ecosystems. Therefore, economicgrowth should be delinked from environmental degradation throughimproving efficiency and sustainability in the use of resources andproduction processes; and reducing resource degradation, pollutionand waste. Thus in recent years it is recognized that the science ofchemistry is central to addressing the problems faced today.

Utilization of various sub-disciplines of chemistry and themolecular sciences has increased an appreciation in Green Chemistry(GC) which is a science-based, non-regulatory and economicallydriven approach toward sustainable/green development and isnecessary for designing and attaining sustainable development [3,4]. A central driving force for this increasing awareness is that GCaccomplishes both economic and environmental goals simultaneouslythrough the use of sound, fundamental scientific principles. GCrepresents the pillars that hold up our sustainable future. GC isthe use of chemistry techniques and methodologies that reduce oreliminate the use or generation of feedstocks, products, byproducts,solvents, reagents, etc. that are hazardous to human health or theenvironment.

The drive towards clean technology in the chemical industry, with anincreasing emphasis on the reduction of waste at source, willrequire a level of innovation, new technology and designing clean or‘green’ chemical transformations and processes including synthesis,catalysis, reaction, separation, analysis, and monitoringconditions.

The GC revolution provides an enormous number of opportunities todiscover and apply new synthetic approaches using alternatives,feed-stocks, eco-friendly reaction conditions, energy minimizations

and the design of less toxic and inherently safer chemicals.Briefly, green chemistry is based on 12 basic principles (Fig. 1) ofhazard, energy and waste minimization or prevention which would bethe key driving factors for achieving the necessary decoupling [1,2]. UNEP revealed five major findings for which GC should beexploited to procure the means to realize them:

1) Assessing biofuels: towards sustainable production and use ofresources on;

2) Decoupling natural resource use and environmental impacts fromeconomic growth;

3) Metal Stocks in Society;

4) Recycling Rates of Metals; and

5) Priority products and materials: assessing the environmentalimpacts of consumption and production.

According to these findings as stated in the GC approach whenmaterials and goods are to be designed and produced it should bekept in mind that they are multi functional and multi-purpose andthe approach to be taken should be “cradle to cradle”. A method canbe defined as “less green” if

a chemical used in the method is listed as a PBT (persistent,bioacculumative, & toxic), as defined by the EPA’s TRI,

a chemical used in the method is listed on the TRI or on oneof the RCRA’s D, F, P or U hazardous waste lists,

pH during the analysis is < 2 or > 12, or the amount of waste generated is >50 g

Silica gel is a good and common example of green material. It waswidely used in World War I and II and in many different sectorsincluding industry, husbandry, military, broadcasting, and health.Some of the usage areas of silica gel are indicated in Table 1. Italso provides high quality broadcasting. If excessive moisturebuilds up within a waveguide it can cause arcing inside thewaveguide and this damages the power amplifier which feeds it. Also,the beads of water that form and condense inside the waveguidechange the characteristic impedance and frequency, degrading the

signal. Therefore, small compressed air system is used to circulatethe air inside the waveguide over a jar of silica gel.

Table 1. Some usage areas for silica gel

adsorption of vapors and gases in gas mask canisters protecting military equipment from moisture damage

If excessive moisture builds up within a waveguide it can causearcing inside the waveguide and this damages the power amplifierwhich feeds it. Also, the beads of water that form and condenseinside the waveguide change the characteristic impedance andfrequency, degrading the signal. Therefore, small compressed airsystem is used to circulate the air inside the waveguide over a jarof silica gel.

maintaining the desired relative humidity in humidors, keepingtools or other materials rust-free in damp environments orlong-term storage

preserving dried food. acts as a fluid cracking catalyst for the production of high

octane gasoline acts as a catalyst support for the manufacture of butadiene

from ethanol feedstock for the synthetic rubber program dry the air in industrial compressed air systems adsorbs

moisture from the air, preventing damage at the point of use ofthe compressed air due to condensation or moisture.

dry the compressed air on railway locomotives, wherecondensation and ice in the brake air pipes can lead to brakefailure

animal litter İn keeping medicine dry diagnostic test strips, inhalation devices, syringes, drug test

kits and hospital sanitation kits Separation science: Chromatography columns

can be functionalized to afford specialty silica gels that exhibit unique stationary phase parameters

functionalized silica gels are used in organic synthesis andpurification as insoluble reagents and scavengers

functionalized silica gels are used for removing metal ionsselectively from aqueous media.

Greenness of a material also requires its production process to begreen. However, in the production process of silica gel ammoniumtetrachlorocobaltate (II) (NH4)2CoCl4 or cobalt chloride CoCl2 areused. These substances are toxic and carcinogenic, and in July 2000they were reclassified by the European Union as toxic materials andCoCl2 as carcinogenic. Therefore, development of greener productionprocesses for silica gel have gained much importance.

Sol gel chemistry, which is cost and energy effective and can berealized at room conditions, offers many advantages when comparedwith other synthetic methods. In the conventional sol-gel methodmetal alkoxides are used as precursors. Through catalyzed hydrolysisand condensation they form a sol of metal oxide particles at nanoscale dimensions [5, 6].

The overall process involves hydrolysis and polycondensationreactions of the organometallic (silicon alkoxides) or inorganic(silicic acid) compounds. Polymerization followed by condensation ofsiloxane bonds results in the formation of a gel which is generallycharacterized with a continuous solid frame work embedded in a largevolume fraction liquid phase. Further processing results in theformation of dried gels which are named depending on how the solventis removed from the gel i.e. supercritical drying produces aerogels(aero stands for air), ambient drying yields xerogels (xero standsfor dry) and freeze drying generates cryogels (cryo stands forfreeze). Aerogels are known to possess the highest surface area(above 1000 m2/g) and a very low density which makes them thelightest solids on earth [7]. Synthesis of silica gels involvingsilicon alkoxide has proven to be quite efficient and successful butthere is a problem of some alkoxides being expensive and some arequite sensitive [8], further still, several alkoxides are notcommercially available or are difficult to obtain, thus hinderingthe momentum of research. So a cheaper and easily available materiallike water glass, rice hull ash or clay is needed to synthesizesilica gels. In Figure 1, the synthesis routes of silica gels arepresented.

Figure 1. Sol-gel synthesis routes

When sodium silicate passes through the ion exchangers, thepositively charged Na+ ions are replaced by the positively chargedmuch smaller and more mobile H+ ions of the ion exchanger producingsilicic acid as a result of the reaction (equation 1). pH of thissilica precursor (silicic acid) is reduced to 2 and with over the

time it can condense to form the silica gel but the process is quitetime consuming.

As can be seen in the routes the textural properties of the silicagels can be improved by controlling the precursor concentration,treatment, pH of the precursor mixture, temperature of the reactionmixture, aging time and most importantly evaporation of solventduring the drying stage. Extensive work has been already conductedto improve the textural properties of the silica gels derived fromNa-Si precursor [9-13]. Achieving smaller particle size and highersurface area had been a point of major concern to many groups and itwas observed that Na-Si based gels can have high surface area andsmaller pore size only if an ion-exchanger is used to remove the Naions from the precursor [14]. Another problem faced during thedrying of the gels is the capillary pressure created by theevaporating solid which results in the collapse of the gelstructure. It has been discovered that exchanging water with someorganic solvent having low surface tension can help reduce thegeneration of cracks in the dried gel [14]. In this work, we haveselected water soluble inexpensive inorganic reagent sodium silicateas the silica precursor instead of expensive and more hazardousorganic precursors. To obtain smaller sized particles, it isconverted to silicic acid (H2SiO3) by passing it through a cationexchanger to exchange the Na+ ions in sodium silicate with the H+

ions of the ion exchanger which is later converted to silica gelthrough a process with minimum energy consumption and hazardproduction. Throughout the experiment water is used as a solventwhich has a low boiling point and creates no pollution/hazard.

II. E XPERIMENT

A. MATERIALS

Sodium silicate was purchased from Scharlau, Spain. 35% pureammonium hydroxide was supplied by Fischer Scientific; Hexadecyltrimethyl ammonium bromide (CTAB) was provided by BioWorld whileAmberlite IR 2000 (ion-exchanger) was supplied from Merck. Allchemicals were used without further purification. Throughout theexperiment deionized water was used.

B.METHOD

4% solution of sodium silicate was prepared by dissolving sodiumsilicate in deionized water. Then the solution was slowly passedthrough the ion exchanging column, which is made from a 63.5x 5.08cm pyrex column filled with Amberlite IR-2000. After passing throughthe ion exchange column, the solution was collected in apolypropylene beaker. The pH of the collected silicic acid wasdetermined via pH meter. 17.5 ml of the effluent was drop wise addedto 5 ml of 1% CTAB solution with magnetic stirring. Then 0.05Mammonium hydroxide was slowly added to the mixture with continuousstirring until the pH was 5. Then the silica sol was stirred forabout 30 minutes and was then kept at 50oC for an hour. Then it wascovered with aluminum foil and was cooled to room temperature sothat gelation proceeded smoothly. After the gel was formed within anhour it was left for aging for 2 days so that the gel network becamestronger and stable. Then the solvent was exchanged with ethanol for4 consecutive days replacing ethanol after every 24 hours. On thefourth day, ethanol was allowed to evaporate first at roomtemperature and later at 60oC until the gel was completely dry. Thedry gel was divided into two parts. One was labeled as SXGCT and theother part was calcined in a muffle furnace at 600oC for 3 hoursunder a temperature gradient of 5oC per minute. This sample was namedas SXGCT_C. The overall sequence of reactions is displayed in Fig.6. After the experiment, the ion exchanger was regenerated viawashing with 5% NaOH solution, then with 5% HCl solution and lastlywith de ionized water so that it can be reused. In Figure 2 theroute of synthesis is presented.

Figure 2. Reaction scheme.

III. R ESULTS & D ISCUSSION

To speed up the gelation process, ammonium hydroxide solution isused as a catalyst which speeds up the hydrolysis and condensationreactions of the silicic acid (H2SiO3) to silica. Equation 2, 3 and 4(Scheme I) demonstrate the hydrolysis, condensation andpolycondensation reactions of silica. The siloxane linkage (Si-O-Si)in silica gel keeps networking and strengthening up during theprocess of condensation and gelation. The surface silanol groups (-SiOH) start developing intermolecular bonds with the CTAB present inthe reaction mixture letting it occupy spaces in the silica gel.

A major problem faced by the gels, during the process of evaporationof the solvent is the capillary pressure created by the evaporatingsolvent which results in the creation of cracks on the surface [5].This phenomenon is controlled by using a low boiling point solventwhich evaporates faster than water. In this case, water is exchangedby ethanol as a solvent which helped reduce the creation of cracksduring the evaporation process. Almost the whole process is carriedat room temperature or at a temperature slightly higher than roomtemperature, which proves the energy efficiency of the wholesynthesis.

B.1. SAMPLE CHARACTERIZATION

Characterization Studies were made by the SEM, FT-IR, TGA analysisof the prepared sorbent.

FTIR Studies

The FT-IR studies were carried out in absorbance mode with 28 scanper sample, using Thermo Scientific Nicolet 6700 FTIRspectrophotometer in the range of 4000 to 400 cm-1. IR spectra ofboth the calcined and uncalcined xerogels are shown in Fig. 3.

Fig.3. FTIR spectra of calcined (SXGCT_c) and uncalcined (SXGCT) xerogels.

In Figure 3 the typical peaks shown by silica are labeled in boththe samples while SXGCT shows additional peaks. The main peak in theFTIR spectra of both xerogels appears at 1090 cm-1 and is associatedwith the anti-symmetric stretching mode of Si–O–Si bonds in the SiO2

matrix [13]. The bands at around 800 cm-1 and 460 cm-1 arerespectively assigned to symmetric bond stretching and bond rockingvibration of Si–O–Si bonds in the three-dimensional SiO2 network. Thecalcined silica mainly shows these three prominent peaks whileuncalcined sample shows additional peaks at 1220, 1365 and 1730 cm-1,which arise due to CTAB in the sample.

Thermal Gravimetric Analysis

Thermal properties of the samples were analyzed in Mettler Toledo TGA/SDTA 851e at a temperature range of 0-1000oC at a heating rate of10oC per minute.

Since CTAB is completely removed if heated below 600oC while silicastays stable at temperatures as high as 1000oC; TGA of the as-synthesized xerogels can give an idea about its composition. TGA ofas-synthesized silica shows that almost 20% of the sample weight iscontributed by CTAB which is completely removed at 600oC leavingbehind only silica which stays stable even when heated till 1000oC.Calcined silica, on the other hand, shows a minimal weight loss ataround 200oC, which is may be due to the chemically adsorbed water in

the sample; otherwise the sample stays stable upto 1000oC (Fig. 4).Absence of a prominent weight loss in the calcined gel indicatescomplete removal of CTAB from the sample.

Fig. 4. TGA Thermograms of calcined (SXGCT_c) and uncalcined (SXGCT)xerogels.

As shown in Figure 4, the thermograms show that almost 20% ofthe sample weight is contributed by CTAB which is completelyremoved at 600oC leaving behind only silica which stays stableeven when heated upto 1000oC. On the other hand, the calcinedsilica shows a minimal weight loss at about 200oC.

X-Ray Diffraction (XRD) Analysis

XRD spectrum was recorded using PANalytical X’Pert Prodiffractometerequipped with Nickel-filtered Cu Kα radiations. Data was recordedusing a step size of 0.04oat a scanning rate of 2θ per second and ina scanning range between 10-80o 2θ. Tube voltage was 40 kV and thetube current was 30 mA..

Fig.5. X-ray diffractograms of calcined (SXGCT_c) and uncalcined (SXGCT) xerogels.

In Figure 5 the diffractograms of both samples show a wide peakat 2θ value of around 24o which is a characteristic ofamorphous silica [14]. Both types of XRD patterns indicateamorphous nature of the xerogels[14]. Surprisingly the as-synthesized xerogel does not show any other peak which indicatesexistence of amorphous structure of the xerogel even when thesurfactant is present in the gel (Fig. 5). Some groups have reportedpresence of ordered mesostructures in surfactant templated silica[15]. In this study evidence for the existence of thoroughlyamorphous nature of surfactant templated silica is also observed insome cases [16]. This mainly depends on the reaction conditions.Particularly the pH and catalysis are determinant on the nature ofsilica. Amorphous structures are usually reported when reaction pHis close to neutral conditions [17].

B.2. SURFACE MORPHOLOGY STUDY

A JSM-5910 Scanning Electron Microscope was used to determinethe surface morphology of the gold sputter coated xerogels at 5kV. Surface morphology of the calcined xerogels was studied throughSEM and micrographs of the sample taken at different magnificationsare shown in Fig. 2. The surface looks quite smooth and the tinyparticles are not visible at low magnification (Fig. 2 A) but athigher magnifications, the surface of the xerogel seems to comprise

of very small particles of diameter lesser than 100 nm. Most of theparticles seem to lie in the mesoporous size range i.e. less than

50 nm in size according to IUPAC classification of mesopores.Generation of mesopores can be attributed to the creation of emptyspace which were occupied by the eliminated template (CTAB) beforethe process of calcination. The calcination process also helped tobreak any sort of bonds between silica xerogel and CTAB, whichresulted in the generation of particles with very small diameter.Moreover, the particle size is mostly homogeneously distributed andmost of the particles seem to lie in the same size range i.e. 2-50nm. In Figure 6 SEM analysis results of the silica gels arepresented.

Figure 6. SEM Micrographs of the calcined silica xerogel. (A) at a low magnification, (B) and (C) showthe morphology at high magnification

In Figure 6 it can be seen that the surface is quite smooth and thetiny particles are not visible at low magnification (Fig. 3 A) butat higher magnifications, the surface of the xerogel seems tocomprise of very small particles of diameter lesser than 100 nm. Theparticle size is mostly homogeneously distributed and most of theparticles seem to lie in the same size range i.e. 2-50 nm.

IV. C ONCLUSION

A new green chemical synthesis method was developed for productionof silica gel as a sorbent material for employing in different areasof industry including but not limited to husbandry, environmentalprotection, water industry, pharmaceuticals. Mesoporous silicaxerogels are obtained by using a cost effective green chemistryapproach. Choice of an inexpensive precursor, monitoring ofexperiment at room temperature and regeneration of the ion exchangerafter the experiment made the process not only energy efficient butalso cost effective. Use of catalyst helped to speed up the processwhile using CTAB as a template has resulted in the formation ofparticles with very small size which can be used as sorbents orcatalysts in a variety of processes [17]. Green chemistry caneffectively and efficiently be used to meet the requirements andneeds set forth in the UNEP International Resource Panel and meetthe needs of different industries and people without encroachingneither on the economies nor on the ecology.

V. R EFERENCES

[1] United Nations, Decoupling: Natural Resource Use andEnvironmental Impacts from Economic Growth, UNEP/Earthprint,2011.

[2] West L, Malnutrition, Pollution and Population Growth SpurIncrease of Deadly Diseases,http://environment.about.com/od/healthenvironment/a/malnutrition.htm.

[3] P.T. Anastas., J.C. Warner, Green Chem Theory and Practice,Oxford Univ. Press, New York, 1998.

[4] I.T. Horvath, P.T. Anastas, Chem. Rev., 107 (2007): 2169-2173.[5] J.H. Clark, S. Elings, L.C. Wilson, Catalysis for green

chemistry : ultrahigh loaded mesoporous solid acidsComptesrendus de l'Académie des Sciences Paris, Serie IIc, Chimie:Chemistry, 3 (2000) 399.

[6] C.J. Brinker, G.W. Scherer, Thin Solid Films, In Sol-Gelscience- The Physics and Chemistry of Sol-Gel Processing,Academic Press, San Diego, 1990. I.T. Horvath,

[7] J. Fricke, T. Tillotson, Thin Solid Films, 297 (1997) 212. [8] Q. Tang, T. Wang, J Supercritical Fluids, 35 (2005) 91. [9] A.V. Rao, A.P. Rao, M.M. Kulkarni, J of Non-Crystalline

Solids, 350 (2004) 224. [10] A.V. Rao, A.P. Rao, G.M. Pajnok, Applied Surface Science, 253

(2007) 6032.[11] M.K. Titulaer, M.J. den Exter, H. Talsma, J.B.H. Jansen, J.W.

Genus, J of Non-Crystalline Solids, 2 (170) (1994) 113.[12] M. Park, V.C. Menon, S. Komarneni, J of Sol-Gel Science and

Technology, 12 (1998) 15.[13] P.G. Bird, U.S. Patent No. 2244325 (1941). [14] P. Innocenzi P, J of Non-Crystalline Solids, 316 (2003) 309.[15] J. Estella, J.C. Echeverria, M. Laguna, J.J. Garrido, J of

Non-Crystalline Solids, 353 (2007) 286.[16] D. Baute, D. Frydman, H. Zimmermann, S. Kababya, D. Goldfarb,

J of Physical Chemistry B, 109 (2005) 7807.[17] K. Won-Jong, Y. Seung-Man, Langmuir, 16 (15) (2000) 6084.[18] M.F. Cardinal, M. Lovino, D.L. Bernik, Materials Science and

Engineering C, 27 (2007) 75.[19] A. Sharif, E. Ahmed, M.A. Munawar, S. Jabeen, M.A. Khan, R.

Begum, A. Farrukh, M. Ashraf, S. Arshad, N. Afza, J. of theChemical Society of Pakistan, 33 (2011) 578.