The relationship between sub-micrometer sized ZSM-5, slice-like (lamellar) keatite and hollow...

CrystEngCommwww.rsc.org/crystengcomm Volume 15 | Number 25 | 7 July 2013 | Pages 4983–5190

PAPERRen, Subotić et al. The relationship between sub-micrometer sized ZSM-5, slice-like (lamellar) keatite and hollow a-quartz particles: a phase transformation study

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Cite this: CrystEngComm, 2013, 15,5032

The relationship between sub-micrometer sized ZSM-5,slice-like (lamellar) keatite and hollow a-quartzparticles: a phase transformation study3

Received 28th November 2012,Accepted 14th January 2013

DOI: 10.1039/c3ce26941h

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Nan Ren,*a Josip Bronic,b Sanja Bosnar,b Maja Dutour Sikiric,c Tatjana Antonic Jelic,b

Jian-Jiang Maoa and Boris Subotic*b

Crystallization–transformation processes in the reaction mixture: (4Na2O–0.1667Al2O3–100SiO2–4000H2O)

seeded with 4 wt% of 260 nm silicalite-1 seeds, at 170 uC were investigated by various experimental

methods such as, X-ray diffraction (XRD), electron diffraction (ED), scanning-electron microscopy (SEM),

transmission electron microscopy (TEM), particle size analysis (PSD), pH measurement, atomic absorption

spectrometry (AAS) and dynamic light scattering (DLS). Analysis of the obtained results have shown that

the crystallization–transformation takes place by a chain of processes: (i) dissolution of amorphous silica

(used as silica source), and formation of 10–50 nm precursor species (PSs), (ii) spending of the PSs for both

the limited growth of zeolite ZSM-5 on the surfaces of silicalite-1 seed crystals and formation of worm-like

particles (WLPs), (iii) formation of condensed aggregates (CAs) by aggregation/condensation of the WLPs;

a part of the composite particles, composed of the silicalite-1 core and ZSM-5 shell are incorporated into

CAs, (iv) nucleation of keatite followed by solution-mediated transformation of the condensed WLPs into

lamellar keatite, (v) nucleation of a-quartz followed by solution-mediated transformation of keatite into

a-quartz; a part of the nuclei of a-quartz are formed on the surface of the core–shell silicalite-1–ZSM-5

composite species and another part of the nuclei are formed in the vicinity of the dissolving keatite and (vi)

dissolution of the silicalite-1 core and a major part of the ZSM-5 ‘‘interlayer’’ of the composite particles

(a-quartz shell-ZSM-5 ‘‘interlayer’’-silicalite-1 core) and formation of hollow a-quartz particles with a thin

ZSM-5 layer on the inner surfaces of the hollow a-quartz particles. Occurrences of all the mentioned

processes are rationally analyzed and explained on the basis of the existing and newly obtained

knowledge.

Introduction

Owing to the medium-sized, abundant micro-porosity, specificmolecular sieving effect, strong/adjustable acidity and highthermal/hydrothermal stability, zeolite ZSM-5 represents oneof the most important type of zeolites which has wide andconsiderable applications in a broad field, especially in energy-relevant catalytic processes.1–4 Due to the diffusion-controlled

character of the participating reactions, the efficiency of suchkinds of zeolitic catalysts relies on the optimal selection ofcrystal size, e.g., small crystals ensure the long ‘time-on-stream’ behavior, while large crystals enable enough diffusionpath length for guest molecules for good selectivity.5

The conventional procedures for the synthesis of zeoliteZSM-5 adopts a large amount of organic ammonium com-pounds as structure directing agents (SDAs),6,7 whichincreases the cost of both the production and the wastedisposal in post-treatment calcinations. Thus, there arepermanent attempts in the development of environmentallyfriendly (SDA-free) and cheap syntheses8,9 of zeolite ZSM-5with controllable crystal size,5,10 especially in the nano-sizedrange.5

However, in the absence of SDA, the successful productionof the desired crystalline framework becomes very sensitivetowards slight changes of conditions, e.g., the batch alkalinity(A = [Na2O/H2O]b), batch silica to alumina ratio ([Si/Al]b) andeven sodium ion content ([Na/Si]b).11 The impurity phases,including low silica zeolites and dense non-porous crystalline

aDepartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and

Innovative Materials, Institute of Catalysis, and Laboratory of Advanced Material,

Fudan University, Shanghai, 200433, P. R. China. E-mail: [email protected];

Fax: +86-21-65641740bRud–er Boskovic Institute, Division of Materials Chemistry, Laboratory for the

Synthesis of New Materials, Bijenicka 54, 10000, Zagreb, Croatia.

E-mail: [email protected]; Fax: +385-14680098; Tel: +385-14680123c

Rud–er Boskovic Institute, Division of Physical Chemistry, Laboratory for Synthesis

and Processes of Self-assembling of Organic Molecules, Bijenicka 54, 10000 Zagreb,

Croatia

3 Electronic supplementary information (ESI) available: Morphologicalparticularities of the solid phase of SRM (SI-1); description of the processesdetails (SI-2) and process control for pure phase slice-like keatite or hollowa-quartz crystals (SI-3). See DOI: 10.1039/c3ce26941h

CrystEngComm

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5032 | CrystEngComm, 2013, 15, 5032–5044 This journal is � The Royal Society of Chemistry 2013

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SiO2 (keatite and quartz),12–14 normally accompany theformation of MFI zeolites, which evidently lower the efficiencyof their application.

Thus, the key for overcoming the mentioned barriers forthe low-cost and controllable synthesis of MFI-type zeolites isthe knowledge of the mechanism of the formation/transforma-tion of the impurity phases during the synthesis. Althoughsome relevant studies on the conditions of formation of low-silica zeolites during the batch synthesis of MFI-type zeoliteshave already been carried out,15 the processes of the formationof high-silica impurities either from amorphous gel or fromMFI-type zeolites, formed during hydrothermal treatment, arenot yet clarified. In this context, investigation of the mechan-ism(s) of formation of high-silica impurities is currently quiteimportant for at least two reasons, (i) high-silica impuritiesnormally form from the batches relevant for the synthesis ofzeolites ZSM-5 with high silica to aluminum ratios. The ZSM-5candidates with isolated strong acid sites (high frameworksilica to aluminum ratio) are crucial for catalytic reactions, e.g.,methanol transformation16 and cracking reactions,17 in whichthe phase purity is very critical for successful applications and(ii) keatite and a-quartz are very important functionalmaterials, of which useful applications can be found intailoring of electronic and optical devices.18–20 While theordinary candidates were fabricated mostly from thermal/hydrothermal transformation processes at very high tempera-tures (400–1000 uC) and pressures (0.1–40 GPa),21–28 most ofthe keatite and a-quartz products obtained from batches ofzeolites, recognized as impurities, are obtained from mildhydrothermal conditions. Since the application of the materialis strictly correlated with its properties such as chemicalcomposition, crystal size and morphology, it could be believedthat the keatite and a-quartz obtained at mild conditions beardifferent characteristics, compared with their counterpartsobtained under strict conditions, which definitely endowsthem with new and interesting behaviors in their applications.

Following requirements for the controllable and low-costsynthesis of MFI-type zeolites, we have recently developed aTPA-free (TPA = tetrapropyl ammonium), seed surface crystal-lization (SSC) procedure for the synthesis of zeolite ZSM-5 withadjustable submicron crystal size and a specific structure withan all-silica core and aluminum containing shell.29–32 Bycareful studies on the influence of the relevant parametersincluding size and amount of silicalite-1 seed crystals,29,32

batch alkalinity, A = [Na2O/H2O]b,30 content of sodium31 andtime of room-temperature ageing31 of reaction mixture, it wasfound that crystallization takes place by a chain of processes,from the formation of precursor species by dissolution ofamorphous aluminosilicate, through the deposition of theseprecursors onto the silicalite-1 seed crystals to obtain a sub-micrometer sized specific structure with all-silica core andaluminum containing shell. The results of these studies alsoshowed that fully crystalline zeolite ZSM-5, grown on thesilicalite-1 seed crystals, can be successfully obtained for 0.006¡ A ¡ 0.01, 40 ¡ [Si/Al]b ¡ 70 and 0.13 ¡ [Na/Si]b ¡ 0.20,after a short (¡2 h) hydrothermal treatment (heating at 210

uC) under static conditions. The batches having compositionsout of these relatively narrow ranges33 often yield impurityphases, such as un-reacted amorphous solids,34 quartz,35

mordenite,36 analcime,37 etc. Thus, the above synthesis systemis selected for further study of the correlations between sub-micron-meter sized MFI zeolites, keatite and a-quartz, in theirparticulate properties and morphological features. By under-standing these processes, the formation/transformationmechanisms of keatite and a-quartz could be clarified tocontrol their formation with aimed phase purity and morphol-ogies from one side and to shed more light on the criticalprocesses which occur during crystallization of ZSM-5 zeolitesfrom the other side.

Hence, in this work, a detailed study of the changes ofchemical composition of the liquid phase and structural(phase composition) and particulate (size, shape) properties ofthe solid phase during hydrothermal treatment (at 170 uC,under static conditions) of the chosen reaction mixture iscarried out in order to define the pathway of the crystal-lization–transformation process, to explain its mechanism(s)and to control the process for aimed crystalline products.

Experimental

Synthesis of silicalite-1 seed crystals

The silicalite-1 seed crystals, having the size of 260 nm, weresynthesized from clear solution as follows:29 tetraethylorthosilicalite (TEOS, analytical grade, Shanghai ChemicalCo. China) was added into the mixture of aqueous solution oftetrapropylammonium hydroxide (TPAOH, analytical grade, 40wt% aqueous solution, Alfa Aesar) and sodium hydroxide(NaOH, analytical grade, Shanghai Chemical Co. China). Thesystem was stirred at ambient temperature for 24 h. Then, thereaction mixture (0.1Na2O–25SiO2–4.4TPAOH–1048H2O–100EtOH) was transferred to a polypropylene bottle andhydrothermally treated at 100 uC for 1 day. The obtainedproduct (silicalite-1 nanocrystals suspended in the motherliquor) was directly used as the seed suspension, without anycentrifugation or washing process.

The prepared seed crystals were fully crystalline silicalite-1having a typical shape29,30 and a size in the range from 130 to600 nm and an average size (equivalent spherical diameter) of268 nm, i.e., almost the same as determined from the SEMimage (260 nm), as shown in our previous publications.29

Phase transformation study

Reaction mixture (hydrogel) having the batch molar composi-tion: 4Na2O : 0.1667Al2O3 : 100SiO2 : 4000H2O was preparedby dissolving of sodium aluminate (NaAlO2, analytical grade,Shanghai Chemical Co. China NaAlO2) in the solution ofsodium hydroxide (NaOH, analytical grade, ShanghaiChemical Co. China), followed by the addition of colloidalsilica (SiO2, analytical grade, 40 wt% aqueous dispersion, AlfaAesar) under stirring. After stirring for 15 min, the suspensionof silicalite-1 seed crystals, having the size of 260 nm, wasadded into reaction mixture. The added amount of seed

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crystals was 4 wt% of the total silica contained in the reactionmixture. The seeded reaction mixture was additionally stirredat room temperature for 1 h. Thereafter, a part of the seededreaction mixture (SRM) was immediately centrifuged for 20min at 12 000 rpm, with a relative centrifugal force (RCF) of17 000g (the rotor’s friction coefficient value, k, equals to 1350)to separate the solid from the liquid phase. The rest of theSRM was divided among the needed number of autoclaves.The autoclaves containing the SRM were put into an ovenpreheated at crystallization temperature (170 uC). The momentwhen the autoclaves were poured into the preheated oven wasrecorded as the zero time (tc–t = 0) of the crystallization–transformation process.

At predetermined crystallization–transformation times, tc–t,the selected autoclaves were taken out from the oven andcooled to the ambient temperature. After cooling the auto-clave, the SRM was centrifuged at 12 000 rpm for 20 min toseparate the solid from the liquid phase. The clear liquidphase (above sediment) was carefully removed without thedisturbance of the solid phase (sediment) and used fordetermination of Si and Al concentrations in the liquid phase,for measurement of pH of the liquid phase, as well as for DLS(Dynamic Light Scattering) analysis of the particles present inthe liquid phase. After removal of supernatant, the solid phasewas redispersed in deionized water and centrifuged repeat-edly. The procedure was repeated until the pH of the liquidphase above the sediment was about 9. The washed solidphase was dried at 80 uC for 10 h and then cooled to roomtemperature and stored in desiccators over dry silica gel.

Characterization

Concentrations of aluminium and silicon in the solutionsseparated from the SRM at various crystallization–transforma-tion times, tc–t, were determined by atomic absorptionspectroscopy (AAnalyst 200, Perkin-Elmer).

pH values of the solutions, separated from the SRM atvarious crystallization–transformation times, tc–t, were mea-sured using Corning Pinnacle 555 pH/ion meter. The pH meterwas calibrated with pH 9.0 and pH 12.0 buffer solutions at 25uC. The accuracy of the pH meter was ¡0.01 pH units.

Size distribution curves of the particles present in thesolutions separated from the SRM at various crystallization–transformation times, tc–t, were measured by DLS approachusing Malvern Zetasizer Nano-ZS analyzer.

X-ray diffraction (XRD) patterns of the solids separated fromthe SRM at various crystallization–transformation times, tc–t,were recorded on a Rigaku D/Max-rB 12 kW diffractometer (CuKa). The content of the amorphous phase and/or differentcrystalline phases (MFI-type zeolites, keatite, a-quartz) in thesolid samples, separated from the SRM at different crystal-lization–transformation times, tc–t, was calculated by theHermans–Weidinger method.38

Scanning electron microscopy (SEM) measurements of thesolids separated from the SRM at various crystallization–transformation times, tc–t, were performed by a Philips XL30D6716 instrument at an operating voltage of 25 kV.

Transmission electron microscopy (TEM) experiments withselected area electron diffraction (SAED) were carried out witha JEOL JEM-2010 instrument at an operating voltage of 200 kV.

Particles size distribution (PSD) curves of the solidsseparated from the SRM at various crystallization–transforma-tion times, tc–t, were determined with a Malvern Mastersizer2000 laser light-scattering (LLS) particle size analyzer.

Results

Changes in the fractions, fa, of amorphous phase, fMFI, of MFI-type zeolite(s), fk, of keatite and, fq, of a-quartz in the solidphase of SRM (Fig. 1A), concentrations, CSi(L), of Si in theliquid phase of SRM (Fig. 1B), CAl(L), of Al in the liquid phaseof SRM (Fig. 1C) and pH of the liquid phase of SRM (Fig. 1D) atvarious crystallization–transformation times, tc–t, show thatthe crystallization–transformation process strictly follows theOstwald rule of stages.39 The thermodynamically most stableand the least soluble phase (a-quartz, the third crystallinephase) forms by the expense of MFI-type zeolites (firstcrystalline phase), keatite (second crystalline phase) and/or

Fig. 1 Changes in: (A) the fraction, fx = fa of amorphous phase ($), fx = fMFI, ofMFI-type zeolites (#), fx = fk, of keatite (&) and fx = fq, of a-quartz (%), (B)concentration, CSi(L), of Si in the liquid phase, (C) concentration, CAl(L), of Al inthe liquid phase and (D) pH of the liquid phase of the reaction mixture RMSduring its hydrothermal treatment (heating at 170 uC). tc–t is the time ofcrystallization–transformation.


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the un-reacted amorphous residue, at the end of the process.Thus, the crystallization–transformation processes, canroughly be divided into three particular time intervals (stages):

Stage I. From tc–t = 0 to tc–t # 30 h during which timeinterval the silicalite-1 seed crystals’ growth is limited and thesolid phase of SRM contains only amorphous phase and MFI-type zeolites (silicalite-1, ZSM-5) (Fig. 1A-I).

Stage II. From tc–t # 30 h to tc–t # 115 h during which timeinterval the remaining (unexpended) amorphous phase almostcompletely transforms into keatite (Fig. 1A-II).

Stage III. From tc–t # 115 h to tc–t # 190 h during whichtime interval the formed keatite, remaining amorphous phaseand MFIs transforms into a-quartz (Fig. 1A-III).

Due to particularities and complexities of the events in thementioned reaction stages, each of them is consideredseparately.

Stage I of the crystallization–transformation process

Immediately after preparation of the seeded reaction mixture(SRM; t = ta = 0) as well as after its room temperature aging (t =ta = 1 h), the solid phase of SRM is composed of added seedcrystals and the particles of amorphous colloidal silica, usedas a silica source. However, while the added seed crystals tendto sediment to the bottom of the vessel during roomtemperature ageing,29,30 the particles of colloidal silica, havingthe size 2.6–6.8 nm (Fig. 2A) remains dispersed in the liquidphase (solution) containing dissolved sodium hydroxide (Na+

and OH2 ions) and sodium aluminate (Na+ and Al(OH)42

ions). The starting pH of the liquid phase (pH = 12.02 at tc–t =0, Fig. 1D-I) is determined by the concentration of OH2 ionswhich arise from both the added sodium hydroxide/sodiumaluminates and the pre-hydrolysis of silica precursors.

Heating of SRM causes formation of an (alumino)silicate gelby aggregation of colloidal silica particles used as silica source,so that at tc–t = 2 h the solid phase of SRM is composed ofsilicalite-1 seed crystals surrounded by gel particles (Fig. 3A;see also Fig. SI-1.1 in the ESI,3 SI-1). According to DLSmeasurements, the liquid phase of SRM contains sub-colloidalparticles having a size in the range from 10–50 nm and averagesize, Dav, of 18.6 nm (Fig. 2B and Table 1) at tc–t = 2 h. Thementioned processes (formation of gel in the solid phase andformation of sub-colloidal particles in the liquid phase) arefollowed by a sudden decrease of both the concentrations,CSi(L), of silicon (Fig. 1B-I) and, CAl(L), of aluminum (Fig. 1C-I)in the liquid phase and decrease of pH of the liquid phase(Fig. 1D-I).

Further heating of SRM causes a growth of 260 nm silicalite-1 seed crystals (Fig. 3A) and formation of well developed,regular MFI crystals having the size in the range betweenabout 500 and 650 nm (Fig. 3C, 3D, 4A and 4B). The growth ofMFI-type crystals is also revealed by its corresponding typicalXRD (Fig. 5) and electron diffraction patterns (inset in Fig. 4B).The crystal growth process is followed by a gradual decrease ofCSi(L) from 0.22 mol dm23 at tc–t = 2 h to 0.14 mol dm23 at tc–t

= 19 h (Fig. 1B-I) and the slight increase of pH from 11.20 to11.40 in the same time interval (Fig. 1D-I).

In addition, the heating of the SRM causes ‘‘restructuring’’of the solid phase; while the amorphous part of the solid

phase of SRM exists in the form of small (about 10–50 nm; seeFig. SI-1.1C in ESI,3 SI-1) aggregates of colloidal silica(Fig. 3A),30–32 at tc–t = 2 h, the amorphous part of the solidphase of SRM appears in the form of worm-like particles(WLPs) at tc–t = 30 h (Fig. 3D, 4C and 4D). Previousinvestigation40 has shown that, upon formation, the WLPs

Fig. 2 Size distribution (by number) of the particles present in the liquid phaseof the reaction mixture hydrothermally treated for tc–t = 0 (A), tc–t = 2 h (B), tc–t =19 h (C), tc–t = 30 (D), tc–t = 42 h (E), tc–t = 90 h (F), tc–t = 137 h (G) and tc–t = 187 h(H). ND is number percentage of the particles having the spherical equivalentdiameter D.

Fig. 3 SEM images of the solid phases separated from SRM at tc–t = 2 h (A), tc–t =10 h (B), tc–t = 19 h (C) and tc–t = 30 h (D). The inset of D displays the TEM imageof the SRM at tc–t = 30 h. Scale bars represent 1 mm in A–D and 200 nm in theinset of D.


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are ‘‘stable’’, i.e., they do not change their size, shape andchemical composition for a long time – at least to tc–t = 30 h inthis case.

PSD curve by number of the solid phase separated from SRMat tc–t = 2 h show a particle population having the size in therange from 0.7 mm to about 5 mm (solid curve in Fig. 6A); thissize is considerably higher than the size of the added silicalite-1 seed crystals (0.1–0.7 mm; dashed curve in Fig. 6A). On theother hand, the PSD curve by volume (mass) of the samesample (Fig. 6A9) shows that most of the solid phase exists in

the form of aggregates having the size in a broad range from#10 mm to #600 mm.30–32 Thus, it is reliable to assume thatthe smaller particles (solid curve in Fig. 6A) represent the seedcrystals probably covered by small amorphous silica particles(Fig. 3A and Fig. SI-1.1B in ESI,3 SI-1.1)30–32 and that the largerparticles (Fig. 6A9) represent different kinds of aggregates ofthe amorphous silica particles used as a silica source (Fig. 3Aand Fig. SI-1.1C in ESI,3 SI-1).30–32,40

As already mentioned, prolonged heating of SRM causes notonly the limited growth of silicalite-1 seed crystals, but also theredistribution of the particles present in the solid phase. Thelater is manifested by a decrease in the particle size of smallerparticle population (solid curve in Fig. 6B–6D) from the oneside and appearance of two distinct particle populations in thePSD curves by volume (Fig. 6B9–6D9) from the other side. Thedecrease in the size of smaller particles (solid curves in Fig. 6Aand 6B) is caused by detachment/dissolving of the smallparticles of colloidal silica from/on the seed crystals duringheating of the SRM. Hence it is evident that the decrease in thesize of seed crystals caused by detachment/dissolving ofcolloidal silica is more expressed than the increase of the sizeof seed crystals caused by their growth (compare Fig. 6A and6B). Thus, it is certain that the populations of smaller particles(0.25–1.5 mm; see Fig. 6B–6D and 6B9–6D9) are relevant for thealready-grown MFI crystals, and that the populations of largerparticles (D > 1.5 mm; see Fig. 6B9–6D9) are relevant for theamorphous phase (gel, in the form of WLPs ‘‘network’’ – see

Fig. 4 TEM images with different magnifications of the MFI crystals (A and B)with relevant ED patterns (inset of B) and WLPs (C and D) in the solid phaseseparated from SRM at tc–t = 30 h.

Fig. 5 XRD patterns of the solid phases separated from SRM at tc–t = 2 h (A), tc–t

= 10 h (B), tc–t = 19 h (C) and tc–t = 30 h (D).

Fig. 6 Particle size distributions by number (A–D) and by volume (A9–D9) of thesolid phases separated from the reaction mixture at tc–t = 0 (A, A9), tc–t = 2 h (B,B9), tc–t = 19 h (C, C9) and tc–t = 30 h (D, D9). The dashed curves in A–D representthe PSD of the silicalite-1 seed crystals. ND is number percentage and VD isvolume (mass) percentage of the crystals having the equivalent diameter D.


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Fig. 4C and 4D). Although the PSD by number does not changeits profile (size range: 0.25–1.5 mm; ‘‘peak’’ size: 0.36 mm;average size: 0.4 mm) in the time interval from tc–t = 10 h to tc–t

= 30 h (solid curves in Fig. 6B–6D), the relative mass of thesmaller particles (0.25–1.5 mm) gradually increases from 9.5wt% at tc–t = 2 h, through 18 wt% at tc–t =19 h to 40 wt% at tc–t =30 h (Fig. 6B9–6D9). At the same time, maximum size of theamorphous aggregates decreases from #600 mm at tc–t = 2 h to#150 mm at tc–t = 10 h and to #100 mm at tc–t = 19 h and 30 h,respectively (Fig. 6A9–6D9). Since the fraction, fx = fMFI, ofcrystalline phase (MFI) is constant (fx = fMFI # 0.34, asdetermined from the corresponding XRD patterns – Fig. 5C–5D) in the time interval from tc–t > 10 h to tc–t = 30 h (Fig. 1A-I),the increase of the relative mass of the smaller particles (0.25–1.5 mm) in the same time interval indicates that the MFIcrystals have been grown in the gel matrix29,31,41 and thengradually released from the gel matrix during subsequentheating of the reaction mixture.41 The releasing of MFIcrystals, which are possible ‘‘connective elements’’ of thenano-sized constituents (MFI crystals, WLPs) of the ‘‘net-work’’,40 is a possible reason for the decrease of maximum sizeof the amorphous particles during stage I of crystallization–transformation process (Fig. 6A9–6D9). Thus, the solid phase ofSRM at tc–t = 30 h is composed of the grown (with limitations)discrete MFI crystals (Fig. 3D, 4A, 4B and 5D) and remaining(unexpended) amorphous phase (Fig. 3D) in the form ofrickety aggregates of WLPs (Fig. 3D, 4C and 4D) having a sizein the range from #1.5 mm to #100 mm (Fig. 6D9).

In contrast to complex changes occurring in solid phase, theDLS analysis of the liquid phase show that the size of the sub-colloidal particles present in the liquid phase graduallyincreases from Dav = 18.6 nm at tc–t = 2 h, through Dav = 36nm at tc–t = 19 h to Dav = 40 nm at tc–t = 30 h (Fig. 2B–2D andTable 1). The constancy of CAl(L) (Fig. 1C-I) and complexchanges of CSi(L) (Fig. 1B-I) and pH (Fig. 1D-I), during stage I,will be discussed later.

Stage II of the crystallization–transformation process

Prolonged heating of SRM causes intense changes of morpho-logical (Fig. 7), structural (Fig. 1A-II and 8) and particulateproperties (Fig. 9) of the solid phase of SRM. The SEM imagesA and A9 in Fig. 7 indicate that the solid phase, separated fromSRM at tc–t = 37 h, is composed of discrete MFI crystals

surrounded by amorphous material (Fig. 7A) as well as ofaggregates of amorphous phase occluded by discrete MFIcrystals and slice-like features (Fig. 7A9 and Fig. SI-1.2 in ESI,3SI-1).

Since the corresponding XRD pattern (Fig. 8A) shows thatthe solid phase contains MFI-type zeolites and only anunexpended amorphous phase, it is evident that the numberof slice-like features is below the detection limit by XRD at tc–t

= 37 h. While the aggregates are more or less irregularlyshaped (Fig. SI-1.3 in ESI,3 SI-1), and have the size in the rangefrom 1.5 mm to 70 mm (Fig. 9A9) at tc–t = 37 h, the aggregates,with the size in the range from 1.5 mm to 20 mm (Fig. 9B9),acquire more regular, near-spherical habits at tc–t = 42 h(Fig. 7B and 7B9 and Fig. SI-1.4 in ESI,3 SI-1).

This redistribution of size and shape, respectively, of theaggregates is followed by the appearance of the new XRD peakat 2h = 26.03u (marked by asterisk in Fig. 8B–8F), which

Table 1 Minimum (Dmin), peak (Dp), maximum (Dmax) and average (Dav) sizes(spherical equivalent diameters) of the particles present in the liquid phase ofthe reaction mixture SRM hydrothermally treated for various times tc–t

tc (h) Dmin (nm) Dp (nm) Dmax (nm) Dav (nm)

acsa 2.33 3.62 8.72 4.102 10.10 15.69 50.75 18.6219 18.17 32.67 122.40 35.8930 21.04 32.67 141.80 40.1242 24.36 43.82 141.80 47.3290 24.36 37.84 190.10 45.58137 21.04 37.84 220.20 46.89187 24.36 37.84 220.20 43.70

a acs = water suspension of amorphous colloidal silica.

Fig. 7 SEM images of the solids separated from SRM at tc–t = 37 h (A, A9), tc–t =42 h (B, B9), tc–t = 48 h (C, C9), tc–t = 70 h (D, D9) and tc–t = 90 h (E, E9). The scalebars represent: 1 mm (A, A9, B9, C9, D9 and E9) and 5 mm (B–E).


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belongs to keatite – a crystalline modification of silicon(IV)dioxide.42 Hence, it is evident that the slice-like features inFig. 7 can be identified as keatite. TEM images (Fig. 10) andcorresponding electron diffraction patterns (inset of Fig. 10D)of the surface of near spherical aggregates at tc–t = 90 h showthat keatite crystallizes in the form of well crystalline sheet-likefeatures (Fig. 10B and 10C), are probably composed of smallflakes having sizes in the nanometer range (Fig. 10D).

The slice-like features arranged as the spherical assembly,having sizes of about 5–10 mm (Fig. 7C–7E and 7C9–7E9), arethe dominant morphological forms in the time interval fromtc–t = 48 h to tc–t = 90 h. Although the general morphologicalcharacteristics of the solid phase (a spherical assembly of theslice-like features with attached MFI crystals and smallparticles of the unexpended amorphous phase) do not changein the time interval from tc–t = 48 h to tc–t = 90 h (Fig. 7C–7Eand 7C9–7E9; see also Fig. SI-1.5 in the ESI,3 SI-1), the XRDpatterns B–E in Fig. 8 and Fig. 1A-II, respectively, show that fx =fMFI does not change, the fraction, fa, of amorphous phasedecreases and the fraction, fk, of keatite simultaneouslyincreases in the time interval from tc–t # 30 h to tc–t # 110h. Thus, it is evident that keatite is formed directly from theamorphous phase. Transformation of amorphous phase intokeatite is followed by a slight decrease of CSi(L) in the timeinterval from tc–t # 30 h to tc–t # 110 h (Fig. 1B-II) while thepH of the liquid phase is near constant (Fig. 1D-II) in the sametime interval. These changes will be, discussed later in detail.

PSD curves by the number of solids separated from SRM atvarious crystallization–transformation times, in the timeinterval from tc–t = 37 h to tc–t = 90 h (solid curves inFig. 9A–9E), show that the size (0.25–1.5 mm) of MFI crystalsdid not change in this time interval. On the other hand, thePSD curves by volume show that after an expressive increase ofthe relative mass of discrete (individual) MFI crystals in thetime interval from tc–t = 2 h to tc–t = 30 h (Fig. 6A9–6D9), theirrelative mass decreases from 40 wt% at tc–t = 30 h (Fig. 6D9) to25 wt% at tc–t = 37 h (Fig. 9A9); thereafter the relative mass ofindividual MFI crystals continuously decreases (Fig. 9A9–9E9)and reaches the value of 14 wt% at tc–t = 90 h. This decrease isevidently caused by the re-attachment of part of the individualMFI crystals on the surfaces of amorphous aggregates duringtheir structural and morphological changes. This is consistentwith the observation of a large number of MFI crystals at thesurfaces of aggregates (Fig. SI-1.3–SI-1.5 in the ESI,3 SI-1).

In contrast to a slight decrease in the size of sub-colloidalparticles in the liquid phase in stage I (Fig. 2B–2D andTable 1), their size is approximately constant (Dav # 40 nm) instage II (Fig. 2E and 2F and Table 1).

Fig. 8 XRD patterns of the solids separated from SRM at tc–t = 37 h (A), tc–t = 42h (B), tc–t = 48 h (C), tc–t = 70 h (D), tc–t = 90 h (E) and tc–t = 113 h (F). The XRDpeak at 2h = 26.03u, belonging to keatite, is marked by asterisks in B–F.

Fig. 9 Particle size distributions by number (A–E) and by volume (A9–E9) of thesolid phases separated from SRM at tc–t = 37 h (A, A9), tc–t = 42 h (B, B9), tc–t = 48h (C, C9), tc–t = 70 h (D, D9) and tc–t = 90 h (E, E9). The dashed curves in A–Erepresent the PSD of the silicalite-1 seed crystals. ND is number percentage andVD is volume (mass) percentage of the crystals having the equivalent diameter D.


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Stage III of the crystallization–transformation process

Fig. 1A-III shows that both keatite and MFI-type zeolites startto transform into a-quartz at tc–t # 110 h. However, it isevident that the rate of transformation of keatite is consider-ably faster than the rate of transformation of MFI-type zeolites;almost half of keatite has already been transformed intoa-quartz when a slight decrease of fMFI is observed (Fig. 1A-III).Although the SEM image shows the existence of slice-likefeatures characteristic for keatite at tc–t = 137 h (aggregates-I inFig. 11A; see also Fig. SI-1.6A and SI-1.6B in ESI,3 SI-1), thecorresponding XRD pattern (Fig. 12A) shows that the solidphase of SRM contains only a-quartz and MFI-type zeolites attc–t = 137 h. This indicates that the amount of keatite presentin the solid phase at tc–t = 137 h is below the XRD detectionlimit and thus, almost an entire amount of keatite has beentransformed into a-quartz till tc–t # 137 h. The appearance ofaggregates-II (Fig. 11A; see also Fig. SI-1.6A in ESI,3 SI-1) showthat transformation of keatite into a-quartz causes destructionof the slices into small irregularly shaped particles (flakes;Fig. 10D, 11A9 and 11B9; see also Fig. SI-1.6A, SI-1.6C and SI-1.7in the ESI,3 SI-1). Prolonged heating of the reaction mixturecauses complete transformation of remaining MFI-type zeo-lites into a-quartz (Fig. 1A-III and 12) and formation of nearspherical crystal aggregates of a-quartz at tc–t = 187 h (Fig. 11Cand 11C9).

The processes of transformation of keatite and MFI-typezeolites into a-quartz are followed by the continuous increaseof pH value in the liquid phase (Fig. 1D-III). The possiblemechanism of transformation, formation of hollow crystals ofa-quartz (Fig. 11C9 and 13; see also Fig. SI-1.8 in the ESI,3 SI-1)as well as preserving of PSDs in both the solid (Fig. 14) and the

liquid phase (Fig. 2G and 2H and Table 1) during thetransformation, will be discussed later.

Discussion

Heating of the SRM, after its homogenization (Scheme 1a),triggers several processes, (i) incorporation of aluminate ionsonto the surfaces of colloidal silica particles (Schemes SI-2.1aand SI-2.1b in the ESI,3 SI-2),30,31,40,43 (ii) dissolution ofcolloidal silica particles (Schemes SI-2.1b and SI-2.1c2 in theESI,3 SI-2)29–31,40,44 and (iii) condensation reactions throughthe terminal –OH groups on the surfaces of colloidal silica44

(Schemes SI-2.1c1 and SI-2.1c2 in the ESI,3 SI-2).This results in the formation of 10–50 nm aggregates of

‘‘primary’’ amorphous silica particles (APASPs; Fig. SI-1.1 inthe ESI,3 SI-1); their further agglomeration results in theformation of 10–600 mm ‘‘gel’’ particles, around the seedcrystals, in the solid phase (Fig. 3A, 6A9 and Scheme 1b; see

Fig. 10 TEM images, with different magnifications, of keatite present in thesolid phase of SRM at tc–t = 90 h. The relevant ED pattern is presented in theinset of D.

Fig. 11 SEM images of the solids separated from SRM at tc–t = 137 h (A, A9), tc–t

= 146 h (B, B9), and tc–t = 187 h (C, C9). Meaning of I, II and III is explained in thetext of paper. The appearance of hollow particles in B9 is indicated by arrows.The scale bars represent: 1 mm (A9–C9); 5 mm (A) and 20 mm (B and C).

Fig. 12 XRD patterns of the solids separated from SRM at tc–t = 137 h (A), tc–t =146 h (B) and tc–t = 187 h.


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also Fig. SI-1.1 in the ESI,3 SI-1.1). However, a part of theAPASPs remains dispersed in the liquid phase of SRM (Fig. 2Band Scheme 1b; see also Scheme SI-2.1d in the ESI,3 SI-2).Thus, it is obvious that a dynamic equilibrium between theAPASPs in solid and in liquid phase of SRM is establishedduring its heating.45,46 A rapid decrease of pH (Fig. 1D-I) iscaused by spending of OH2 ions for both dissolution ofcolloidal silica (Scheme SI-2.1b in the ESI,3 SI-2) and thecondensation reactions through the terminal –OH groups onthe surfaces of colloidal silica40,44 (Scheme SI-2.1c in the ESI,3SI-2). On the other hand, the overall decrease of theconcentration, CSi, of silicon in the liquid phase from CSi(L)= 0.449 mol L21 at tc–t = 0 to CSi(L) = 0.265 mol L21 at tc–t # 2 h(Fig. 1B-I) indicates that the decrease of CSi(L), caused byprecipitation of gel by agglomeration of the colloidal silica

particles dispersed in the liquid phase, is larger than theincrease of CSi(L) caused by the starting dissolution ofcolloidal silica. Finally, the disappearance of soluble alumi-num species in the liquid phase, expressed as the decrease ofthe concentration, CAl(L), of aluminum to near zero (Fig. 1C-I),is caused by already mentioned irreversible incorporation ofaluminate ions from the liquid phase onto the surfaces ofcolloidal silica particles43,47 (Scheme SI-2.1a in the ESI,3 SI-2).

Prolonged heating of SRM causes formation of differentkinds of soluble silicate and/or aluminosilicate species by

Fig. 13 TEM images, with different magnifications, of hollow a-quartz particlespresent in the solid phase of SRM at tc–t = 187 h. The corresponding ED patternis displayed in the inset of B.

Fig. 14 Particle size distributions by number (A–C) and by volume (A9–C9) of thesolid phases separated from SRM at tc–t = 137 h (A, A9), tc–t = 146 h (B, B9) and tc–

t = 187 h (C, C9). The dashed curves in A–E represent the PSD of the silicalite-1seed crystals. ND is number percentage and VD is volume (mass) percentage ofthe crystals having the equivalent diameter D.

Scheme 1 Schematical illustration of the phase-transformation processes.


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further dissolution of both the separate (individual) APASPsdispersed in the liquid phase and the APASPs assembledinto 10–600 mm ‘‘gel’’ particles48–51 (Scheme SI-2.1d in theESI,3 SI-2). The formed silicate and/or aluminosilicatespecies gradually assemble into active growth precursorspecies29–31,41,46,52,53 (Scheme SI-2.1e in the ESI,3 SI-2)which then participate in both the growth of silicalite-1seed crystals29,30,41,54,55 and formation of a ‘‘network’’ ofworm-like particles (WLPs)40 (Fig. 3C, 3D, 4C and 4D; seealso Fig. SI-2.1 and Schemes SI-2.1e–SI-2.1g in the ESI,3SI-2). The consumption of the precursor species for both thegrowth of zeolite ZSM-5 on silicalite-1 seed crystals and theformation of WLPs causes a gradual decrease ofthe concentration, CSi(L), of silicon in the liquid phase inthe time interval from tc–t # 4 h to tc–t # 20 h (Fig. 1B-I).This, together with the ‘‘stability’’ of the formed WLPs,40 isthe reason for the cease of the growth of MFI crystals at thevery early stage of the crystallization–transformation pro-cess, i.e., at tc–t # 20 h (Fig. 1A-I). Hence, the solid phase ofSRM at tc–t # 30 h is composed of MFI crystals surroundedby WLPs (Fig. 3D and Scheme 1c). On the other hand,aggregation of the precursor species formed by dissolutionof APASPs in the liquid phase results in the formation of theamorphous aggregates40 having the size of about 35–40 nm(Fig. 2C, 2D, Table 1 and Scheme 1c; see also Schemes SI-2.1g and SI-2.1f in the ESI,3 SI-2), which are in dynamicequilibrium with the particles in the solid phase.

A slight increase of pH in the liquid phase from pH = 11.2 attc–t = 2 h to pH = 11.44 at tc–t # 30 h (Fig. 1D-I) is caused by bothgrowth of zeolite ZSM-5 on silicalite-1 seed crystals29–31 andnucleophilic attack of low molecular weight silicate speciesfrom the liquid phase on the aluminate centers at the WLPssurfaces (see ESI,3 SI-2).40,47 Since both the processes take placesimultaneously, they cannot be strictly discriminated.

Formation of condensed aggregates (CAs) at tc–t # 37 h(Fig. 7A9) is in accordance with previous findings40 and thus,can be explained by condensation reactions of the silanolgroups47 of neighboring WLPs and formation of WLP-Si–O–Si-WLP links (Scheme 1d; see also Scheme SI-2.2a in the ESI,3 SI-2). Appearance of keatite slices on the surface of aggregatesimmediately after their formation (at tc–t # 37 h; see Fig. 7A9)indicates that transformation of the amorphous phase intokeatite (Fig. 1A-II) is conditioned by formation of CAs(Schemes 1d, 1e and Scheme SI-2.2b in the ESI,3 SI-2).Namely, high concentration of silicate material45,56,57 in CAsis favorable for spontaneous nucleation and crystal growth ofkeatite in the amorphous matrix of CAs. (Scheme 1e; see alsoScheme SI-2.2b in the ESI,3 SI-2). Thereafter, the remainingamorphous phase transforms into keatite are in the timeinterval from tc–t # 37 h to tc–t # 90 h (Fig. 1A-II, 8 andSchemes 1e and 1f). Since the shape (Fig. 7) and size (Fig. 9) ofthe near-spherical aggregates do not considerably changeduring the same time interval, it is evident that transformationof the amorphous phase into keatite occurs not only at thesurface of CAs, but also in their interiors (Schemes 1e and 1f;see also Scheme SI-2.2b in the ESI,3 SI-2).

The decrease of Si concentration in the liquid phase fromCSi(L) # 0.14 mol dm23 at tc–t # 50 h to CSi(L) # 0.1 moldm23 at tc–t # 90 h (Fig. 1B-II) indicates that the transforma-tion of thermodynamically less stable (more soluble) amor-phous phase into thermodynamically more stable (lesssoluble) keatite is more solution-mediated than the solid-stateprocess, i.e., the transformation is in accordance withOstwald’s rule of stages,39 takes place, by dissolution ofamorphous phase and formation of the silicate ions neededfor nucleation and crystal growth of keatite. This, at the sametime, shows that there is intense exchange of material betweenthe liquid phase and CAs, not only from their surfaces, butalso from their interiors (Schemes 1e and 1f).

When the entire amount of amorphous phase has beentransformed into keatite (at tc–t # 115 h), the solid phase (MFIcrystals and formed keatite) is in a pseudo-equilibrium withthe liquid phase containing Na+, OH2 and various silicate andaluminosilicate species,45,46 including about 45 nm particlesin the liquid phase (Fig. 2F and 2G, Table 1 and Scheme 1f).

Thereafter, both keatite and MFI-type zeolites start totransform into a-quartz (Fig. 1A-III, 11 and 12). Assuming thattransformation is a solution-mediated process (dissolution ofkeatite and MFI-type zeolites and formation of silicate ionsneeded for nucleation and growth of a-quartz), the slower rateof transformation of MFI-type zeolites relative to the rate oftransformation of keatite (Fig. 1A-III) indicates that the rate ofdissolution of MFI-type zeolites is slower than the rate ofdissolution of keatite. This can be readily explained by the factthat keatite is an all silica compound and that MFI-typezeolites represent a core–shell composite with a silicalite-1core and zeolite ZSM-5 shell.30–32,58–60 Thus, taking intoconsideration the inertness of surface Si–O–Al bonds (zeoliteZSM-5) of the MFI composite on hydroxide attack,30–32,61–63 itis reliable to assume that the rate of dissolution of the ZSM-5shell of the MFI composite is slower than the rate ofdissolution of keatite. This causes that at tc–t = 137 h thesolid phase of SRM contains the formed a-quartz (73 wt%),untransformed MFI-type zeolites (23 wt%) and only traces ofkeatite (Fig. 1A-III and 12A). The continuous increase of the pHof the liquid phase in the time interval from tc–t # 110 h to tc–t

# 180 h (Fig. 1D-III) and intense decrease of the concentra-tion, CSi(L), of Si in the liquid phase in the time interval fromtc–t # 170 h to tc–t # 200 h (Fig. 1B-III), support theassumption that, in accordance with Ostwald’s rule of stages,39

the transformation of keatite into a-quartz is a solution-mediated process (see also the ESI,3 SI-2). In this context, it canbe concluded that the small irregularly shaped particles on thesurface of aggregate II (Fig. 11A9; see also Fig. SI-1.7 in the ESI,3SI-1) are formed by destroying slices of keatite (aggregates I inFig. 11A) due to their dissolution. SEM images A9–C9 in Fig. 11as well as Fig. SI-1.7 and SI-1.8 in the ESI,3 SI-1 show thatdestroying slices of keatite is followed by the appearance ofhollow crystals at tc–t = 146 h (marked by arrows in Fig. 11B9)and an increase of their relative number during furtherheating of SRM. The corresponding XRD pattern (Fig. 12C)shows that the solid phase at tc–t = 187 h is almost pure


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a-quartz with only traces of MFI-type zeolites and thus, it isevident that both ‘‘normal’’ and hollow crystals representmostly a-quartz. The constancy or only slow change in the size(Fig. 9B9–9E9 and 14A9–14C9) and shape (Fig. 7B–7E and 11) ofaggregates, as well as the changes of CSi(L) (Fig. 1B-III) and pH(Fig. 1D-III) indicate that not only transformation of theamorphous phase into keatite, but also transformation ofkeatite and MFI-type zeolites, respectively, into a-quartz occurby the solution-mediated processes in the interiors of theaggregates (Schemes SI-2.3a and SI-2.3b in the ESI,3 SI-2).However, the narrowing of the size distribution of aggregatesin the time interval from tc–t = 90 h (Fig. 9E9) to tc–t = 146 h(Fig. 14B9) is probably caused by the transformation ofaggregates I (Fig. 7E and 7E9) to aggregates II (Fig. 11B and11B9). The corresponding data (see Fig. 3B–D, 6, 7A9–7E9, 9 and15) show that the size and shape of crystals in aggregates aswell as of the size distribution of individual crystals (Fig. 6 and9) are constant or change a little during stages I and II of thecrystallization–transformation process. On the other hand,although the shape of the crystals does not considerablychange, their size slightly increases in stage III of thecrystallization–transformation process (Fig. 15).

On the basis of these findings, the processes of transforma-tions of keatite and MFI-type zeolites into a-quartz may besuggested as follows: spontaneous dissolution of thermody-namically less stable (more soluble) keatite produces lowmolecular weight silicate ions which are precursors fornucleation and subsequent growth of thermodynamicallymore stable (less soluble) a-quartz (Scheme 1g; see alsoSchemes SI-2.3a and SI-2.3b in the ESI,3 SI-2). A part of thenuclei is formed on the surfaces of MFI crystals and anotherpart of the nuclei can be formed near the surfaces of thedissolving keatite, i.e., at the places where the concentration ofsilicate ions is highest (Scheme 1g1; see also Scheme SI-2.3a inthe ESI,3 SI-2). Growth of the nuclei formed at MFI crystals,causes formation of a layer of a-quartz on the surfaces of MFIcrystals (Scheme 1g2: see also Scheme SI-2.3b in the ESI,3 SI-2).

On the other hand, the growth of nuclei formed near thesurfaces of dissolving keatite results in the formation ofdiscrete (individual) crystals of a-quartz (Scheme 1g2 andScheme SI-2.3b in the ESI,3 SI-2). The formation of a layer ofa-quartz on the surfaces of MFI crystals (Scheme 1g2; see alsoScheme SI-2.3b in the ESI,3 SI-2) increase the particle (crystal)size (Fig. 15) and, at the same time, additionally preventsdissolution of MFI crystals, so that the further growth ofa-quartz takes place only on account of the dissolution ofkeatite. After almost an entire amount of keatite has beentransformed into a-quartz (at tc–t # 140 h; see Fig. 1A-III),action of OH2 ions at the part of the surface of the a-quartz–ZSM-5–silicalite-1 composite particles, probably uncovered bya-quartz, causes dissolution of MFI-type zeolites (Scheme 1h;see also Scheme SI-2.3c in the ESI,3 SI-2). Again, due to theinertness of surface Si–O–Al bonds on hydroxide attack,57–59

the dissolution does not start at the zeolite ZSM-5 interlayer,but in the silicalite-1 core (Scheme 1h; see also Scheme SI-2.3cin the ESI,3 SI-2). This is consistent with the recent resultswhich show that an alkaline treatment of silicalite-1–ZSM-5core–shell composites results in the formation of hollow ZSM-5 particles.29,32 The described process (Schemes 1g and 1h)results in the formation of hollow particles of a-quartz(Fig. 11C9 and Scheme 1i; see also Fig. SI-1.8 in the ESI,3 SI-1). However, the XRD pattern of the final product (Fig. 12C)shows the presence of trace amounts of MFI-type zeolite (ZSM-5). This most probably arises from a thin layer of un-dissolvedAl-rich zeolite layer on the inner part of the hollow particles ofa-quartz (Scheme 1i). At the same time, just the existence ofvery small amounts of zeolite ZSM-5 indicate that the‘‘compact’’ crystals (without holes) have not MFI core andthat these crystals are formed by the direct growth of thea-quartz nuclei positioned near the surface of the dissolvingkeatite (Schemes SI-2.3a and SI-2.3b in the ESI,3 SI-2). Finally,since most of the (alumino)silicate material participating inthe crystallization–transformation processes are ‘‘concen-trated’’ in the aggregates (Fig. 7 and 11), the final product(a-quartz) appears in the form of polycrystalline aggregates(Fig. 11C, 11C9 and Scheme 1i; see also Scheme SI-2.3c in theESI,3 SI-2) with hollow crystals observable at their surfaces.

By understanding the above transformation process, theformation of lamellar keatite can be regarded as the result ofthe direct transformation of un-reacted amorphous precursor(condensed aggregates) and the subsequent formation ofa-quartz crystals with special morphology (hollow box) isrelated to the transformation of as-formed sub-micrometersized ZSM-5 crystals. The principle can be further adopted as aguide to obtain either the lamellar keatite aggregates or hollowa-quartz crystals with high phase purity just through changingthe gel compositions (see ESI,3 SI-3 for examples).

Conclusions

By careful analysis of the changes of chemical, structural,and particulate properties in both the solid and the liquid

Fig. 15 Change of the average size, Lav, of crystals (MFI during stages I and IIand MFI-type zeolites + a-quartz during stage III) during the crystallization–transformation process. The values of Lav, correspond to the average lineardimensions of crystals determined from the corresponding SEM images.


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phase during hydrothermal treatment (heating at 170 uCunder static conditions) of the reaction mixture:4Na2O : 0.1667Al2O3 : 100SiO2 : 4000H2O, seeded with 260nm silicalite-1 crystals, it is concluded that the crystal-lization–transformation process takes place by a chain ofevents, through three stages:

Stage I: (i) dissolution of Al-enriched amorphous silica andformation of different silicate species, including 10–50 nmparticles in the liquid phase. (ii) Further dissolution of the Al-enriched amorphous silica causes formation of y36 nmparticles in the liquid phase and formation of growthprecursor species in the solid phase (gel). (iii) Simultaneousdeposition of the growth precursor species onto the surfaces ofsilicalite-1 seed crystals (crystal growth of MFI-type zeolites)and formation of the ‘‘network’’ of worm-like particles (WLPs),probably by aggregation of amorphous precursor species. Dueto limited dissolution of Al-enriched amorphous silica at lowalkalinity and spending of the growth precursor species for theformation of WLPs, the growth of silicalite-1 seed crystalsstops at the early stage of the process, i.e., at tc–t # 20 h.

Stage II: (iv) formation of condensed aggregates (CAs) bylinking the WLPs through the terminal MSi–OH (silanol)groups on the surfaces of WLPs particles and furthercoalescences of the linked WLPs. (v) It is suggested thatnucleation and crystal growth of keatite at both the surfacesand interiors of CAs. Formation of keatite is most possibly asolution-mediated process which takes place by dissolution ofthe remaining amorphous phase (condensed WLPs) andformation of silicate ions needed for nucleation and crystalgrowth of keatite. The formed keatite appears in the form ofspherical aggregates composed of slice-like (lamellar) sheets.

Stage III: (vi) nucleation and crystal growth of a-quartz onthe surfaces of MFI crystals. Starting formation of a-quartztakes place by dissolution of keatite and formation of silicateions needed for crystal growth of nuclei formed at the specificsites on the surfaces of MFI crystals as well in the vicinity ofdissolving keatite. After an entire amount of keatite has beentransformed into a-quartz, the crystalline product exists in twoforms, (a) the ‘‘compact’’ a-quartz crystals formed by thegrowth of nuclei developed in the vicinity of dissolving keatiteand (b) the complex ‘‘core–shell’’ particles with a-quartz shelland silicalite-1 core; a-quartz shell and silicalite-1 core are‘‘separated’’ by a thin layer of zeolite ZSM-5. (vii) Furthertransformation of MFI-type zeolites from the form (b) intoa-quartz by dissolution of the silicalite-1 core. The silicate ionsobtained by dissolution of silicalite-1 participate in the furthergrowth of a-quartz crystals in the shell. This process results inthe formation of hollow a-quartz crystals having a thin layer ofzeolite ZSM-5 on the inner surfaces of a-quartz hollow crystals.

Besides the scientific significance in the explanation of acomplex crystallization–transformation process which takesplace during seed-induced crystallization of zeolite ZSM-5, theresults of this study have also a potential practical significancein using the obtained product either as high temperaturestable support for catalytic purposes (hollow quartz crystals) or

as potential advanced electronic–photonic devices (lamellar,sheet-like keatite crystals).

Acknowledgements

This work is supported by NSFC (20803010), ‘‘Chen Guang’’project supported by Shanghai Municipal EducationCommission and Shanghai Education DevelopmentFoundation (09CG02), ‘Brain Gain’ Post-Doc project (I-668–2011) supported by Croation Science Foundation and project098–0982904–2953, financially supported by the Ministry ofScience, Education and Sport of the Republic of Croatia.

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The relationship between sub-micrometer sized ZSM-5, slice-like (lamellar) keatite and hollow...

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