Synthesis of FAU Type Zeolite Y from Natural Raw Materials: Hydrothermal SiO 2 -Sinter and Perlite Glass

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1 The Open Minerlogy Journl, 28, 2, Synthesis of FAU Type Zeolite from Nturl Rw Mterils: Hydrotherml SiO 2 -Sinter nd Perlite Glss George E. Christidis * nd Hr Ppntoni Technicl University of Crete, Deprtment of Minerl Resources Engineering, 731 Chni, Greece Astrct: The first synthesis of pure FAU-type zeolite from SiO 2 -sinter nd perlite glss is presented. FAU-type Zeolite ws synthesized t 95 ± 2 C fter 6-12 hours using gels hving SiO 2 :Al 2 O 3 :N 2 O molr rtios of 8.4:1:4.2 nd using gels with SiO 2 :Al 2 O 3 :N 2 O molr rtios of 1.5:1:4.2 etween 48 nd 12 hours rection time. Except for zeolite, zeolite NP c, gmelinite-n nd the OH-nlogue of sodlite were produced from strting gels with different composition. The ction exchnge cpcity of zeolite ws 3.3 meq.g -1 nd its specific surfce re ws m 2.g -1. Most of the specific surfce re is llocted to micropores. Zeolite synthesized from gels with SiO 2 :Al 2 O 3 molr rtio of 1.5:1 hs lttice prmeter prmeter o = Å nd Si/Al rtio 2:1. It is concluded tht inexpensive rw mterils such s volcnic glsses cn e used for the production of high dded vlue zeolite. Key Words: Zeolite, volcnic glss, perlite, SiO 2 -sinter, synthesis. INTRODUCTION Synthetic zeolites cn e produced from vriety of nturl silic-rich mterils including cidic volcnic glsses [1], such s nturl nd expnded perlite nd pumice [2-9]. Although the synthesis of zeolites from volcnic glsses provides the opportunity for upgrding of minerl resources, which re either not exploited or re utilized in other industril pplictions, it usully yields zeolitic end products with vrile purity [1]. Moreover, in most studies zeolites with smll chnnel dimeters re produced, such s zeolite NP c nd/or the OH-nlogue of sodlite, nd thus cnnot e utilized in the chemicl industry. Also the finl rection products contin impurities such s qurtz, feldsprs nd mics (minly iotite), which do not dissolve during the zeolitiztion process. Giordno et l. [6] proposed the synthesis of pure zeolite A nd X from perlite vi two-step rection process. The first step involved rection of the glss with NOH nd yielded solid phse ssemlge consisting of zeolite NP c ± the OH-nlogue of sodlite ± zeolite V depending on the concentrtion of NOH, the rection time nd the solid:liquid rtio [6,9], nd Si-rich liquid phse, which rected with N-luminte solutions to form n SiO 2 -Al 2 O 3 - rich gel [6]. The ltter ws heted t 9-1 C to form pure zeolite A nd/or zeolite X ccording to the composition of the strting gels. Using similr process, Christidis et l. [9] synthesized pure zeolite A nd zeolite X from perlite nd expnded perlite glsses. In oth cses zeolite crystlliztion from the strting gels ws complete. Although pure FAU-type zeolite X hs een synthesized from nturl volcnic mterils, pure zeolite, which is lso FAU-type zeolite, hs not een synthesized quntittively from nturl mterils yet. Previous ttempts to synthesize *Address correspondence to this uthor t the Technicl University of Crete, Deprtment of Minerl Resources Engineering, 731 Chni, Greece; E-mil: christid@mred.tuc.gr zeolite yielded rection products with zeolite contents lower thn 5% [11-12]. Nevertheless, zeolite is esily synthesized from SiO 2 -Al 2 O 3 gels prepred from pure chemicl regents, with/without ddition of seeds nd/or orgnic molecules, which ct s templtes in the crystlliztion process ([2,13,14] nd references therein). In this contriution, we present the first quntittive synthesis of pure zeolite from SiO 2 -rich nturl mterils such s volcnic glss (perlite) nd hydrotherml SiO 2 -rich sinters. MATERIALS AND EXPERIMENTAL METHODS The SiO 2 -rich source mterils were provided from S&B Industril Minerls SA. The perlite comes from the Provts Perlite Mine, Milos Islnd, Greece nd hs een descried y Christidis et l. [15]. It is fine-grined mteril, which resulted during seprtion of vrious perlite size frctions y ir clssifiction. It is light coloured rhyolite, with 92% of the grins rnging in size etween 2 nd 6 μm nd 8% etween 1 nd 2 μm [15]. The perlite consists principlly of volcnic glss (>95%), qurtz, lite nd iotite nd hs SiO 2 :Al 2 O 3 molr rtio of It hs the following chemicl composition [15]: SiO %, TiO 2.1%, Al 2 O %, Fe 2 O %, MgO.57%, CO 1.15%, N 2 O 3.7%, K 2 O 3.41% nd LOI 3.21%. The SiO 2 -rich sinter is fine-grined white mteril (9% < 25 μm) consisting essentilly of morphous SiO 2 (>97%), the remining eing corse grined qurtz (~2%) nd trces of morphous Fe-oxides. The SiO 2 -sinter ws dissolved t 95 C, fter rection with 5N NOH solution, using 1:1 solid to liquid rtio for 9 minutes. The perlite ws dissolved in n inconel utoclve (Prr Instruments) t 12 C, fter rection with 2N NOH solution nd 1:5 solid:liquid rtio for 2 hours. After dissolution the Si-liquid ws filtered under vcuum nd the Si precipitted in the form of morphous SiO 2 t ph 8 fter neutrliztion with HCl. The SiO 2 ws wshed until free of chloride (AgNO 3 test), dried t 15 C nd stored. The N /8 28 Benthm Science Pulishers Ltd.

2 2 The Open Minerlogy Journl, 28, Volume 2 Christidis nd Ppntoni luminte solution ws prepred y dissolving suitle mounts of Al(OH) 3 (Merck) in NOH t 25 C for 6 minutes in n inconel utoclve. The morphous SiO 2 ws mixed with vrile volumes of N-luminte solution so s to otin gels with the pproprite SiO 2 :Al 2 O 3 :N 2 O:H 2 O molr rtios. Two sets of experiments were performed. The first set involved gels with constnt N 2 O:Al 2 O 3 molr rtio (4.2:1), nd SiO 2 :Al2O 3 molr rtios vrying from 7:1 to 15:1 nd the second set involved gels with constnt SiO 2 :Al2O 3 molr rtio (1.5:1) nd N 2 O:Al 2 O 3 molr rtios vrying from 3.2:1 to 5.2:1. The gels were susequently homogenized for 24 hours t room temperture using continuous stirring (15 rpm). Synthesis of zeolites from the gels ws crried out in teflon-coted sphericl rector t 95 ± 1 C, without stirring. Smples were tken t time intervls of 3-12 hours, wshed with deionized wter to ph 9 nd dried in the tmosphere. All regents were of nlyticl grde. The minerlogy of the rection products ws determined with powder X-ry diffrction (XRD), using Siemens D5 X-ry diffrctometer (CuK rdition, 35 kv nd 35 ma, grphite monochromtor). The morphology of the synthetic zeolite crystls ws oserved on gold-coted powder smples with JEOL 54 SEM. Adsorption-desorption isotherms of the zeolites were otined y N 2 gs dsorption using NOVA 22 surfce re nlyzer (Quntchrome). Smples were degssed overnight t 2 C under vcuum (1-2 Torr). Specific surfce res were determined y the BET eqution [18] using the P/Po rnge -.35 of the dsorption rnch of the isotherms, while the pore size distriution ws determined from the desorption rnch of the isotherms. Infrred spectr of zeolite were otined with Perkin Elmer 1 FTIR spectrometer. The KBr pellet method ws used [17], in the wve numer rnge 4-12 cm -1 in trnsmission mode, y verging 25 scns. 1.5 mg of the zeolite ws mixed with 2 mg KBr nd pressed to 13mm KBr disks, which were susequently dried t 15 C overnight. The ction exchnge cpcity (CEC) of the rection products ws determined fter sturtion with 1N mmonium cette t ph 7. The smples remined in the cette solution for 1 dys nd were wshed with methnol to remove excess slt. The cette solution ws replced every dy. The CEC ws mesured using Kjeldhl microstem distilltion pprtus nd titrtion with dilute sulphuric cid. RESULTS Chrcteriztion of the Synthetic Zeolites The results of the zeolite synthesis from SiO 2 -sinters re shown in (Fig. 1 nd 2). Formtion of zeolite is fvoured from strting gels with SiO 2 :Al 2 O 3 rtios of 8.4:1 nd N 2 O:Al 2 O 3 rtios of 4.2:1. At these conditions, zeolite crystllizes fter less thn 3 hours, s shown y the ppernce of powder diffrction mxim (Fig. 2). After 6 hours, the strting gel ws converted totlly to zeolite. Longer rection times yielded trce mounts of zeolite NP c nd fter 12 hours zeolite NP c constitutes c. 5% of the rection product. The undnce of zeolite NP c increses further with longer rection times. Therefore, it is considered tht zeolite NP c forms t the expense of zeolite. Similr results were otined from perlite strting mterils (Fig. 1 nd 2). In this cse, the undnce of zeolite NPc increses further fter 47 h. Gels with greter SiO 2 :Al 2 O 3 molr rtios re lso suitle for crystlliztion of zeolite. However, crystlliztion egins fter considerly longer rection time. For SiO 2 :Al 2 O 3 rtio of 1.5:1, zeolite egins to crystllize fter 24 h. Also for SiO 2 :Al 2 O 3 rtio of 9.5:1 gmelinite-n is found in the finl products. Gmelinite-N is not stle t higher SiO 2 :Al 2 O 3 rtios. Quntittive formtion of zeolite is oserved fter 48h, lthough minor mounts of morphous mteril is lso present in these syntheses (see elow). Strting gels with SiO 2 :Al 2 O 3 molr rtios greter thn 12:1 do not fvour crystlliztion of zeolite irrespective of the N 2 O:Al 2 O 3 molr rtio. For SiO 2 :Al 2 O 3 molr rtios lower thn 8.4:1 zeolite does not form nd the OH-nlogue of sodlite nd zeolite NP c re the rection products (Fig. 1). However, the degree of conversion of the strting gel is limited, since undnt morphous mteril is present. Gmelinite-N nd zeolite NP c re stle products either t low N 2 O: Al 2 O 3 rtios (3.2:1) or t high N 2 O:Al 2 O 3 rtios (5.2 :1) nd for longer rection times (Fig. 1B). Zeolite is kineticlly fvoured ut rects to more stle phses with incresing rection time. In summry, our results suggest tht zeolite is stle phse t SiO 2 :Al 2 O 3 :N 2 O for molr rtios 8.4:1:4.2 nd with rection times of 6-12 hours or SiO 2 : Al 2 O 3 :N 2 O molr rtios 1.5:1:4.2 nd rection times greter thn 48h. The o unit-cell prmeter for zeolite ws clculted from the XRD dt using Rietveld refinement. The o otined for zeolite produced from perlite nd SiO 2 -sinter with SiO 2 :Al 2 O 3 rtio of 1.5:1 is ±.3 Å, wheres o mesured for zeolite synthesized from SiO 2 - sinter with the SiO 2 :Al 2 O 3 rtio of 8.4:1 is lrger (24.813±.3 Å). In oth cses, o did not vry with rection time. The o prmeter ws used to clculte the Si:Al rtio ccording to [1]. Hence, zeolite produced from oth strting mterils with SiO 2 :Al 2 O 3 rtio of 1.5:1 hs Si:Al rtio of 2:1, wheres the zeolite produced from the SiO 2 -sinter with SiO 2 :Al 2 O 3 rtio 8.4:1 hd Si:Al rtio of 1.68:1. In oth cses these Si:Al rtios re typicl for zeolite, which displys Si:Al rtios in the rnge 1.5:1 3:1. The FTIR powder spectr of zeolite produced t different rection times from oth rw mterils nd strting gels with SiO 2 :Al 2 O 3 :N 2 O molr rtio of 8.4:1: 4.2 nd from SiO 2 -sinter with SiO 2 :Al 2 O 3 :N 2 O molr rtio of 1.5:1:4.2 re shown in Fig. (3). The ssignment of the mjor sorption nds follows [1] nd is listed in Tle 1. The sorption nds recorded re typicl of zeolite. The nds t 74 cm cm -1 (T-O symmetric stretch) nd 573 cm cm -1 (doule ring), were used to clculte the Al mole frction in the frmework of tht zeolite [1]. The nd t 577 cm -1 (perlite nd SiO 2 -sinter with SiO 2 :Al 2 O 3 rtio 1.5:1) nd 573 cm -1 (SiO 2 -sinter with SiO 2 :Al 2 O 3 rtio 8.4:1) yielded Al mole frctions of.26 nd.27, which correspond to Si:Al rtio of 2.85:1 nd 2.7:1 respectively. The nds t 74 cm -1 (perlite nd SiO 2 -sinter with SiO 2 :Al 2 O 3 rtio 1.5:1), nd 695 cm -1 (SiO 2 -sinter with SiO 2 :Al 2 O 3 rtio 8.4:1) yielded Al mole frctions of.3 nd.32 respectively which correspond to Si:Al rtios of 2.33:1

3 Synthesis of FAU Type Zeolite from Nturl Rw Mterils The Open Minerlogy Journl, 28, Volume 2 3 nd 2.13:1 respectively. The position of the sorption nds suggests tht the zeolite synthesized from perlite hs lower tetrhedrl Al-occupncy, lthough the strting gels hd identicl composition. The reson for the slightly different chemicl composition is not fully understood. Moreover the existence of doulet in the nd ttriuted to symmetric stretch of externl linkges of the tetrhedr of the zeolite synthesized from SiO 2 -sinter (765 cm -1 nd 785 cm -1 ), is indictive of vriility of the Si:Al rtio nd suggests locl heterogeneity of the end products. Nevertheless the o prmeter does not vry significntly, ecuse the o mesured reflects the overll Si:Al rtio in these smples. Also it should e noted tht the Si:Al rtios otined from FTIR dt re lrger thn their counterprts clculted from XRD dt. Since clcultion of o using Rietveld refinement is relile, the XRD method is considered more ccurte for clcultion of the Si:Al rtio thn IR spectroscopy. SiO 2:Al 2O 3 N 2O:Al 2O 3 15:1 13:1 1.5:1 9.5:1 8.5:1 8:1 7:1 5.2:1 4.3:1 G 3.2: Gm + P + G +P Fig. (1). Crystlliztion fields of the synthetic zeolites formed in the system: ) SiO 2 :Al 2 O 3 -time plot for N 2 O:Al 2 O 3 molr rtio of 4.2:1 nd ) N 2 O:Al 2 O 3 -time plot for SiO 2 :Al 2 O 3 molr rtio of 1.5:1. = zeolite, P=zeolite NP c, Gm=gmelinite-N, HS=the OH-nlogue of sodlite, G=morphous mteril. The ction exchnge cpcity (CEC) of the synthetic zeolite products vries etween 3.3 nd 3.6 meq.g -1 nd the BET specific surfce re vries etween 41 nd 55 m 2.g - 1. The CEC of pure zeolite is 3.3 meq.g -1. The end products contining gmelinite-n in ddition to zeolite hve higher CEC, in ccordnce with the lower Si:Al rtio of gmelinite-n [16]. The BET specific surfce re (SSA) of G Gm + P Rection time (h) +Gm +Gm+P +P HS+P+G Rection time (h) + P + Gm pure zeolite is m 2.g -1, wheres end products contining other zeolites hve smller SSA. The use of t-plots, Intensity (cps) Intensity (cps) zeolite NP c 47 hours 25 hours Degrees 2θ CuΚα Degrees 2θ CuKα Fig. (2). XRD digrms showing zeolite synthesized ) from SiO 2 -rich sinters nd ) from perlite. 12 = 3 hours =6 hours c=12 hours d=24 hours e=25 hours* f=12 hours** zeolite NP c f e d c Fig. (3). FTIR spectr of synthetic zeolite, -d= from SiO 2 -sinter nd SiO 2 :Al 2 O 3 rtio 8.4:1, e= from perlite nd SiO 2 :Al 2 O 3 rtio 8.4:1, f= from SiO 2 -sinter t SiO 2 :Al 2 O 3 rtio 1.5: Wvenumers cm hours 15 hours 12 hours hours 6 hours 3 hours Trnsmittnce (%)

4 4 The Open Minerlogy Journl, 28, Volume 2 Christidis nd Ppntoni Tle 1. Assignment of IR Lttice Virtion Bnds (Wvenumers [cm -1 ]) for Zeolite from SiO 2 -Sinters nd Perlite (fter [1]) Assignment of Bnds Type of Virtion Zeolite (Sinters) Zeolite (Perlite) Internl tetrhedrl T-O SS T-O end 695,73* 468,5,53* , , Externl virtions of tetrhedrl linkges Doule ring SS AS 573, 577*, 626, 638* 765, 785* 11 *= SiO 2:Al 2O 3 Rtio 1.5:1, Rest Zeolites 8.4:1. SS= Symmetric Bond Stretching, AS= Asymmetric Bond Stretching. which plot the volume of gs dsored versus the sttisticl thickness of n dsored gs film (t) nd which re used for quntittive evlution of microporosity [19], showed tht in ll end products most of the specific surfce re ville to N 2 gs is llocted to micropores. Also the N 2 -dsorption isotherms re type-i isotherms (Lngmuir-type isotherms), typicl of microporous mterils [19] (Fig. 4). No hysteresis ws oserved in the N 2 -desorption rnches. Those end products contining gmelinite-n dsored lower volumes of N 2 gs, in ccordnce with the SSA results. Appliction of the Lngmuir eqution llowed n estimtion of the SSA, ssuming tht meso- nd/or micropores re essentilly sent. The SSA vlues otined re higher thn the BET SSA (dt not shown). 97 h rection products two genertions of crystls re visile: lrger crystls μm in size nd smller crystls c. 1 μm in size. At 12 h the crystl size is more uniform. Frequently the zeolite crystls form ggregtes severl tens of microns in size. Zeolite crystls form in generl cues with rounded edges. Octhedrl crystls re often present (rrows in Fig. 5) nd twinning is lso common (rrow in Fig. 5c). There is evidence tht crystl growth proceeds with formtion of spirls fter nucletion. Spirl growth is etter oserved in long term experiments, ecuse crystl size is lrge enough for oservtion (rrows in Fig. 5d) V dsored (cm 3.g -1 ) P/Po 4 72 hours 5 72 hours 6 23 hours 1 12 hours Fig. (4). Adsorption isotherms of the synthetic zeolites. 1, 4, 5, 6= SiO 2 :Al 2 O 3 :N 2 O molr rtios of strting gel. 1 = 1.5:1:4, 4= 1.5:1:5, 5=9.5:1:4, 6=8.4:1:4. Morphologicl Fetures of the Zeolite Crystls The morphology of zeolite crystls produced from the SiO 2 -sinter t different rection times, using strting gels with SiO 2 :Al 2 O 3 :N 2 O molr rtios 1.5:1:4.2 is shown in the microgrphs in Fig. (5). At 48 h rection time there is morphous mteril which hs not crystllized to zeolite (rrows in Fig. 5). The crystl size of zeolite is μm. With incresing rection time the remining strting gel crystllizes out to zeolite nd the crystl size tends to increse. After 72 h rection time crystl size is c. 2 μm ecoming 3 μm fter 12 h. However in oth the 72 h nd the c Fig. (5). SEM microgrphs of zeolite end products produced from SiO 2 -sinter. ) 48 h rection time. The rrows indicte morphous mtter. ) 72 h rection time. The rrows indicte crystls with octhedrl hit. c) 97 h rection time. The rrow indictes zeolite twins. d) 12 h rection time. The rrows indicte the formtion of growth spirls on the surfce of zeolite DISCUSSION This work hs shown tht oth the SiO 2 -sinter nd perlite glss cn e utilized s rw mterils for synthesis of pure FAU type zeolite. Under the experimentl setup used zeolite cn e formed fter 6-12 hours. Longer rection times yield smll mounts of zeolite NP c. However the undnce of the ltter zeolite is smll so s to consider tht even these experiments yield zeolite quntittively. Compred with previous experiments which yielded other zeolite species, this setup involves complete seprtion of morphous SiO 2 from the originl volcnic glss, ecuse the form of morphous SiO 2 is importnt for synthesis of zeolite. Hence the Si-O linkges in volcnic glsses seem to e unfvourle for the nucletion of zeolite nd so is the SiO 2 -Al 2 O 3 gel d

5 Synthesis of FAU Type Zeolite from Nturl Rw Mterils The Open Minerlogy Journl, 28, Volume 2 5 produced from polymeriztion of the Si-rich solution from the dissolution of volcnic glss [6,9]. In the ltter cse it is considered tht the experimentl setup cnnot produce strting gels with pproprite SiO 2 :Al 2 O 3 :N 2 O:H 2 O molr rtios for crystlliztion of zeolite, due to experimentl constrints. Therefore zeolite A nd/or zeolite X form insted of zeolite. Note tht the perlite used is unsuitle for direct synthesis of zeolite lthough it hs SiO 2 :Al 2 O 3 molr rtio of 9.12 [8], which is comprle to tht necessry for synthesis of zeolite. Direct dissolution of perlite yields zeolite P c, zeolite V nd the OH-nlogue of sodlite [8]. In contrst the morphous SiO 2 produced from the SiO 2 -sinter nd perlite fvours synthesis of zeolite. It is considered tht the onset of crystlliztion of zeolite occurs t considerly shorter time thn 3 hours ecuse t tht time the zeolite yield exceeded 6% (Fig. 2). According to the FTIR spectr nd the SEM microgrphs the zeolite crystls synthesized from the SiO 2 -sinter re inhomogeneous in two spects: ) the Si:Al rtio of the crystls nd therefore their ction exchnge cpcity nd ) zeolite crystl size. With respect to their Si:Al rtio n Al poorer nd n Al-richer vriety hve een recognized in the FTIR spectr. The possiility for n inhomogeneous strting gel is rejected ecuse ll gels were homogenized during ging vi continuous stirring. In n ttempt to explin the compositionl heterogeneity the filtrte solutions fter crystlliztion of representtive zeolite products were nlyzed for Si nd Al. The SiO 2 :Al 2 O 3 rtio of the solutions does not remin constnt with time, ut increses drmticlly with incresing rection time (Fig. 6), suggesting tht zeolite my ecome progressively enriched in Si. Since nucletion of new crystls from the strting gel occurs simultneously with crystl growth for t lest 48 h (Fig. 5) it follows tht the younger smller crystls might e richer in Si, nd tht prt of this Si will e included in the growing crystls. Crystl growth proceeds with spirl growth, vi dissolution of previously formed, unstle crystls, which re richer SiO2:Al2O3 rtio Experimentl run Fig. (6). Evolution of the SiO 2 :Al 2 O 3 molr rtio in the fluid phse of the strting gel during crystlliztion of zeolite from SiO 2 - sinter t vrious experimentl runs. 1, 4, 7=SiO 2 :Al 2 O 3 :N 2 O molr rtios of strting gel. 1=1.5:1:4, 4=1.5:1:5, 7=1.5:1:3. 24 h 48 h 72 h in Al thn their younger smller counterprts formed t lter stge. This Al will counterlnce, t lest prtilly, the excess Si. In this sense the oserved compositionl inhomogeneity of zeolite cn e explined t locl scle, lthough the overll Si/Al rtio does not chnge significntly during synthesis s is indicted y the o prmeter. The finl Si/Al rtio of zeolite is controlled y the vilility of Si. The fct tht the first zeolite to form is reltively richer in Al my explin the formtion of less siliceous zeolites such s zeolite NP c nd gmelinite-n t longer rection times. With incresing crystlliztion time the gel ecomes progressively richer in Si compred to Al (Fig. 6) nd the originl Al-richer zeolite crystls re expected to e unstle nd dissolve, forming Al-richer domins. In such domins the Si:Al rtios my e suitle to form Al-richer zeolites. As expected, the composition of the strting gel lso ffects the composition of zeolite. Hence the zeolite synthesized from the SiO 2 -sinter nd strting gels with SiO 2 :Al 2 O 3 rtio 1.5:1 hve lrger Si:Al rtio s is indicted oth y the o vlues nd the FTIR spectr (spectrum f in Fig. 3). However, these zeolites contin Si-richer nd Si-poorer domins s is indicted y the existence of the doulets t 765 nd 785 cm -1 in the FTIR spectr (Fig. 3), the formtion of which ws explined previously. 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