7/21/2019 Chemically Controlled Particulate Properties of Zeolites Towards the Face-less Particles of Zeolite a. 2. Influence of… http://slidepdf.com/reader/full/chemically-controlled-particulate-properties-of-zeolites-towards-the-face-less 1/9 Chemically controlled particulate properties of zeolites: Towards the face-less particles of zeolite A. 2. Influence of aluminosilicate batch concentration and alkalinity of the reaction mixture (hydrogel) on the size and shape of zeolite A crystals Sanja Bosnar, Josip Bronic ´, Ður - dica Brlek, Boris Subotic ´ ⇑ Ru - der Boškovic ´ Institute, Division of Materials Chemistry, Laboratory for the Synthesis of New Materials, Bijenic ˇ ka 54, 10000 Zagreb, Croatia a r t i c l e i n f o Article history: Received 6 October 2010 Received in revised form 20 December 2010 Accepted 21 December 2010 Available online 28 December 2010 Keywords: Aluminosilicate hydrogels Crystallization Zeolite A Crystal morphology Crystal size a b s t r a c t Influence of the simultaneous change of batch alkalinity ( A = [Na 2 O/H 2 O] b ) and total batch silica concen- tration (tsc = [SiO 2 /H 2 O] b ) of the reaction mixture, at two characteristic batch molar ratios, y [SiO 2 / Al 2 O 3 ] b (1.3 and 2.0), on the duration of crystallization, reaction yield and the particulate properties (size and shape) of the crystalline end product (zeolite A) was investigated. Sharp decrease of the duration of crystallization process with simultaneous increase of A and tsc is caused by simultaneous increase of the number of nuclei formed in gel matrix, rate of their release from the gel matrix and their growth with the increase of the alkalinity of the reaction mixture. Although the reaction yield, Y R , is proportional to the total batch silica concentration, tsc , the ratio Y R /tsc slightly decreases with increasing alkalinity as a con- sequence of the increasing solubility of zeolite A with the increase of the alkalinity. Specific number of crystals (number of crystals per unit mass of the crystalline end products), N s increases with simulta- neous increase of A and tsc as a consequence of the increase of the number of nuclei formed in the gel matrix with increase of alkalinity of the reaction mixture. On the other hand, a complex dependence of the crystal size distribution on A, tsc and y, is a consequence of influence of the mentioned chemical factors not only on the number of nuclei, but also on their distribution in the gel matrix. Four different morphological forms of zeolite A, namely, (i) regular cubic crystals with ‘‘sharp’’ edges and apexes, (ii) cubic crystals with truncated edges, (iii) cubic crystals with ‘‘rounded’’ edges and apexes and (iv) pseudo-spherical, face-less particles are obtained during hydrothermal treatment of investigated hydro- gels. Influence of the chemical factors on the formation of particular morphological forms of zeolite A are discussed and the possible mechanisms of their formation are considered. 2010 Elsevier Inc. All rights reserved. 1. Introduction Among more than 200 different types of zeolites, zeolite Na-A, also known as zeolite 4A is the most important for industrial applications. Due to applications of zeolite A as absorbents [1] ion-exchangers [2] and especially detergent builders [3], many at- tempts were focused on the investigations of various factors which control the crystallization pathway and particulate properties of the crystalline end product. Numerous studies [4–14] have shown that chemical composi- tion of the reaction mixture (hydrogel) is the mostimportant factor for controlling the particulate properties of zeolite A. In these stud- ies, special attention was paid to influence of alkalinity [8,9,11,13,14] and the batch molar ratio Si/Al [6–8,12–14], respec- tively, of the reaction mixture on both the size and shape of zeolite A crystals. Since the mentioned investigations were conducted un- der very different and incomparable conditions, and mostly fo- cused to only one property of system, the conclusions arising from the analysis of the obtained results are more general, e.g., that alkalinity of reaction mixture has the most expressive effect on the crystal size [8,9,11] and that crystal habit of zeolite A can be mod- ified by the aluminum content in the reaction mixture [6,8]. Thus, although the results of numerous investigations gave very quality and worthy data, however, they do not offer a complete feature on the influence of the chemical composition of the reac- tion mixture on crystallization pathway and the properties of the products, especially the particulate (crystal size distribution, crys- tal morphology) ones. From this reason, we conducted systematic investigation of the influence of the chemical composition of the reaction mixture (hydrogel), entirely defined by the batch molar ratios [Na 2 O/H 2 O] b , [SiO 2 /Al 2 O 3 ] b and [SiO 2 /H 2 O] b , in wide ranges, even these beyond the ‘‘standard’’ chemical conditions of zeolite A synthesis, on the duration of crystallization (detail kinetic 1387-1811/$ - see front matter 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.12.027 ⇑ Corresponding author. Tel.: +385 (0)1 46 80 123; fax: +385 (0)1 46 80 098. E-mail address: [email protected](B. Subotic ´). Microporous and Mesoporous Materials 142 (2011) 389–397 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
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7/21/2019 Chemically Controlled Particulate Properties of Zeolites Towards the Face-less Particles of Zeolite a. 2. Influence of…
Chemically controlled particulate properties of zeolites: Towards the face-lessparticles of zeolite A. 2. Influence of aluminosilicate batch concentrationand alkalinity of the reaction mixture (hydrogel) on the size and shapeof zeolite A crystals
Sanja Bosnar, Josip Bronic, Ður -dica Brlek, Boris Subotic ⇑
Ru -der Boškovic ´ Institute, Division of Materials Chemistry, Laboratory for the Synthesis of New Materials, Bijenic ka 54, 10000 Zagreb, Croatia
a r t i c l e i n f o
Article history:
Received 6 October 2010Received in revised form 20 December 2010Accepted 21 December 2010Available online 28 December 2010
Influence of the simultaneous change of batch alkalinity ( A = [Na2O/H2O]b) and total batch silica concen-tration (tsc = [SiO2/H2O]b) of the reaction mixture, at two characteristic batch molar ratios, y [SiO2/Al2O3]b (1.3 and 2.0), on the duration of crystallization, reaction yield and the particulate properties (sizeand shape) of the crystalline end product (zeolite A) was investigated. Sharp decrease of the duration of crystallization process with simultaneous increase of A and tsc is caused by simultaneous increase of thenumber of nuclei formed in gel matrix, rate of their release from the gel matrix and their growth with theincrease of the alkalinity of the reaction mixture. Although the reaction yield, Y R , is proportional to thetotal batch silica concentration, tsc , the ratio Y R /tsc slightly decreases with increasing alkalinity as a con-sequence of the increasing solubility of zeolite A with the increase of the alkalinity. Specific number of crystals (number of crystals per unit mass of the crystalline end products), N s increases with simulta-neous increase of A and tsc as a consequence of the increase of the number of nuclei formed in the gelmatrix with increase of alkalinity of the reaction mixture. On the other hand, a complex dependenceof the crystal size distribution on A , tsc and y, is a consequence of influence of the mentioned chemicalfactors not only on the number of nuclei, but also on their distribution in the gel matrix. Four differentmorphological forms of zeolite A, namely, (i) regular cubic crystals with ‘‘sharp’’ edges and apexes, (ii)cubic crystals with truncated edges, (iii) cubic crystals with ‘‘rounded’’ edges and apexes and (iv)pseudo-spherical, face-less particles are obtained during hydrothermal treatment of investigated hydro-gels. Influence of the chemical factors on the formation of particular morphological forms of zeolite A arediscussed and the possible mechanisms of their formation are considered.
2010 Elsevier Inc. All rights reserved.
1. Introduction
Among more than 200 different types of zeolites, zeolite Na-A,also known as zeolite 4A is the most important for industrialapplications. Due to applications of zeolite A as absorbents [1]
ion-exchangers [2] and especially detergent builders [3], many at-tempts were focused on the investigations of various factors whichcontrol the crystallization pathway and particulate properties of the crystalline end product.
Numerous studies [4–14] have shown that chemical composi-tion of the reaction mixture (hydrogel) is the most important factorfor controlling the particulate properties of zeolite A. In these stud-ies, special attention was paid to influence of alkalinity[8,9,11,13,14] and the batch molar ratio Si/Al [6–8,12–14], respec-tively, of the reaction mixture on both the size and shape of zeolite
A crystals. Since the mentioned investigations were conducted un-der very different and incomparable conditions, and mostly fo-cused to only one property of system, the conclusions arisingfrom the analysis of the obtained results are more general, e.g., thatalkalinity of reaction mixture has the most expressive effect on the
crystal size [8,9,11] and that crystal habit of zeolite A can be mod-ified by the aluminum content in the reaction mixture [6,8].
Thus, although the results of numerous investigations gave veryquality and worthy data, however, they do not offer a completefeature on the influence of the chemical composition of the reac-tion mixture on crystallization pathway and the properties of theproducts, especially the particulate (crystal size distribution, crys-tal morphology) ones. From this reason, we conducted systematicinvestigation of the influence of the chemical composition of thereaction mixture (hydrogel), entirely defined by the batch molarratios [Na2O/H2O]b, [SiO2/Al2O3]b and [SiO2/H2O]b, in wide ranges,even these beyond the ‘‘standard’’ chemical conditions of zeoliteA synthesis, on the duration of crystallization (detail kinetic
1387-1811/$ - see front matter 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.micromeso.2010.12.027
analysis will be published elsewhere), reaction yield, specific num-ber of crystals, crystal size distribution (by both number and vol-ume) and crystal morphology.
In the first part of our study [15] it was found that depending onthe alkalinity of the reaction mixture, the increase of aluminiumcontent in the reaction mixture (decrease of the batch molar ratio y = [SiO2/Al2O3]b) causes not only appearance of {0 1 1} crystalplanes of zeolite A and thus, formation of cubic crystals with trun-cated edges [6,8], but also a gradual loss the individual crystalplanes and formation of pseudo-spherical, face-less particles of zeolite A (zeolite FLA). Hence it is evident that morphology of zeo-lite A crystals is not determined by only the batch molar ratio y = [SiO2/Al2O3]b, but also by the batch alkalinity A = [Na2O/H2O]b,and total batch silica concentration, tsc = [SiO2/H2O]b or better tosay, by the entire chemical composition of the reaction mixture.
From this reason, the influence of chemical composition of thereaction mixture (hydrogel) on the size (distribution) and mor-phology of zeolite A crystals, obtained by hydrothermal treatment(heating at 80 C) of hydrogels, was investigated. The chemicalcomposition of hydrogels was varied by wide-range variations of the batch molar ratios A (from 0.00507 to 0.0762) and tsc (from0.00255 to 0.03785) for two characteristic values of y (1.3 and 2.0).
2. Experimental
Aluminosilicate hydrogels having the batch chemicalcompositions:
were prepared by pipetting 100 ml of sodium silicate solution of
appropriate concentration with respect to Na2O and SiO2 into aplastic beaker contained 100 ml of stirred (by propeller) sodiumaluminate solution of appropriate concentration with respect toNa2O and Al2O3. Sodium aluminate solutions were prepared by dis-solution of anhydrous NaAlO2 (Riedl de Haen; 54 wt.% Al2O3 and41 wt.% Na2O) in sodium hydroxide solutions of appropriate con-centrations, and sodium silicate solutions were prepared by dilu-tion of water–glass solution (Sigma–Aldrich; 10.54 wt.% Na2O,26.53 wt.% SiO2, 62.93 wt.% H2O) in the sodium hydroxide solu-tions of appropriate concentrations. The solutions were thermo-stated to 25 C prior mixing together. Chemical compositions of hydrogels, determined by the corresponding values of z = [H2O/Al2O3]b, total silica concentration, tsc = [SiO2/H2O]b and alkalinity, A = [Na2O/H2O]b, are shown in Table 1.
The aluminosilicate hydrogels, prepared as described abovewere transferred into stainless-steel reaction vessel preheated atcrystallization temperature (80 C). The reaction vessel was pro-vided with a thermostated jacket and fitted with a water-cooledreflux condenser and thermometer. The reaction mixture washeated at crystallization temperature under stirring with a Tef-lon-coated magnetic bar driven by magnetic stirrer, until the entireamount of the amorphous aluminosilicate (gel) is transformed intozeolite. The end-point of crystallization, t c = t c(end) (t c is the timeof crystallization) was determined by microscopic observation(optical microscope with magnification of 1000 is used for thispurpose) of the samples used from the reaction mixture at varioustimes t c during the synthesis. As t c(end) is used the time t c forwhich some irregularly-shaped particles (gel) are not observed.The absence of the gel phase in the products was additionallychecked by X-ray diffraction.
The reaction yield, Y R , (in wt.%, i.e. grams of zeolite obtainedfrom 100 g of the reaction mixture) was determined as follows: So-lid phase (zeolite A) of the reaction mixture at t c P t c(end) is sep-arated from the liquid phase (supernatant) by centrifugation. Afterremoval of the supernatant, the solid phase was redispersed in dis-tilled water and centrifuged repeatedly until the pH value of the li-quid phase above the sediment was 9–10. The wet washed solidswere dried overnight at 105 C, were cooled in desiccators with sil-icagel and were weighed. From known amounts, mR , of the reactionmixture and, mz, of crystallized zeolite, the reaction yield, Y R , canbe calculated by the simple relation:
Y R ðin grams per 100 g or wt:%Þ ¼ 100 mz=mR ð1Þ
After determination of the reaction yield, the solid sampleswere characterized/analysed by several methods: X-ray diffraction(XRD), scanning-electron microscopy (SEM) and particle size anal-ysis (PSD).
The X-ray diffraction patterns of the samples are taken by a Phi-lips PW 1820 diffractometer with vertical goniometer and Cu Karadiation with graphite monochromator in the corresponding re-
gion of Braggs’s angles (2h = 5–50).Shape of zeolite particles (crystals) was determined from SEM
photographs of appropriate samples. The SEM photographs weretaken by Philips XL 30, scanning-electron microscope.
Particles (crystals) size distribution (PSD) curves of the crystal-line end products were determined with a Malvern Mastersizer2000 laser light-scattering particle size analyzer.
Using the PSD data, the average crystal size, D, and specificnumber of crystals, N s (number of crystals per one gram of thecrystalline end product) were calculated as [16],
D ¼X
N i Di
.XN i ð2Þ
N s ¼X
N i
.ðp=6Þ q
XN i ðDiÞ
3ð3Þ
where N i is the number frequency of the crystals having the size be-tween D and DD, Di = D + DD/2, D is measured equivalent sphericaldiameter and q = 2 g/cm3 is the density of zeolite A.
3. Results and discussion
The solid phases (X-ray amorphous aluminosilicate gels) of allhydrogels (see Table 1) were completely transformed into crystal-line ones during hydrothermal treatment at 80 C. X-ray diffractionanalysis revealed that all the crystalline end products obtained byhydrothermal treatment of hydrogels H1.1.–H1.7. and H2.1.–H2.8.,are pure, fully crystalline zeolite A (see XRD patterns in Supple-ments 1 and 2). However, due to the tendency of spontaneous
transformation of zeolite A into hydrohysodalite (HS) at higheralkalinities [14,17], traces of HS were found in the crystalline end
Table 1
Chemical composition of hydrogels H1.1.–H1.10. in system S1 and H2.1.–H2.10 in
system S2, expressed as the corresponding batch oxide molar ratios. z 1 = [H2O/Al2O3]b
for system S1, z 2 = [H2O/Al2O3]b for system S2. Total silica content, tsc = [SiO2/H2O]b
and alkalinity A = [Na2O/H2O]b are common for both systems.
products obtained from hydrogels H1.8. ( A = [Na2O/H2O]b =0.05500) and H2.9. ( A = 0.06500), while pure HS was the only prod-uct obtained from hydrogels H1.9. ( A = 0.0650), H1.10. and H2.10.( A = 0.0762).
As expected, the reaction yield, Y R , increases with the increaseof the batch molar ratio tcs = [SiO2/H2O]b (see Tables 1 and 2).However, due to simultaneous increase of the total silica concen-tration, tsc , and alkalinity, A, ( A/tsc = constant = 1.99; see Table 1)of hydrogels, Y R is not a linear function of tsc ; the ratio Y R /tsc
slightly decreases with increasing alkalinity as a consequence of the increasing solubility of zeolite A with the increase of the alka-linity [18,19]. Since the molar ratio SiO2/Al2O3 of zeolite A is 2 [20],the ‘‘excess’’ of aluminum ( y = [SiO/Al2O3]b = 1.3) in the reactionmixture cannot directly influence the reaction yield. However,slightly higher values of Y R in system S2 than in the system S1(see Table 2) can be readily explained by the higher total alumino-silicate concentration in the reaction mixtures of system S2 rela-tive to the corresponding reaction mixtures of system S1, andthus by decrease of solubility of the crystallized zeolite A withincreasing supersaturation [21,22].
Since the rate of crystallization and thus, its duration, dependon both the rate of nucleation, dN /dt c and the rate of crystalgrowth, dD/dt c [23–26], the rapid decrease of the duration of crys-tallization with the simultaneous increase of tsc and A (see Tables 1and 2) can be discussed in the terms of the influence of theseparameters on the nucleation and growth processes. Crystalgrowth of zeolites is governed by the reactions of monomericand/or low-molecular weight aluminate, silicate and aluminosili-cate anions from the liquid phase on the surfaces of growing zeo-lite crystals [4,23,26–28] and thus, its rate, dD/dt c, depend on theconcentrations, C Al, of aluminum and, C Si, of silicon in the liquidphase, e.g. [23,27], i.e.,
dD=dt c ¼ kg F ðC Þ ¼ kg ½C Al C AlðeqÞ ½C Si C SiðeqÞ ð4Þ
where C Al(eq) and C Si(eq) are concentrations of aluminum and sili-con in the liquid phase which correspond to the solubility of zeoliteA under the synthesis conditions, and kg is the growth rate constant.Since with increase of alkalinity the solubility of gel increases[19,29] faster than the solubility of zeolite [18,19], the concentra-tion factor F (C ) and thus, the rate of crystal growth increases andconsequently, the duration of crystallization process decreases withincreasing alkalinity of the reaction mixture [4,23,27,30]. On theother hand, the changes of average crystal size, D, and specific num-ber, N s, of crystals in the crystalline end products with simultaneouschange of A and tsc (see Tables 1 and 2) indicates that not only therate of crystal growth, but also the nucleation events influence therate of crystallization and thus, the duration of crystallization pro-cess. This will be discussed latter.
Although the SEM images in Figs. 1 and 2 show a general trendof the decrease of crystal size with simultaneous increase of A and
tsc , the crystal size distributions (CSD) by number (Fig. 3) do notindicate a straightforward changes (e.g., decrease) of the crystalsize with the simultaneous change (e.g., increase) of A and tsc .For example, the CSD curves of the samples s1.1., s1.2., s2.1. ands2.2. (see Fig. 3) show that the crystals formed by hydrothermaltreatment of diluted, low-alkaline hydrogels H1.1., H2.1. ( A =
0.00507; tcs = 0.00255), H1.2. and H2.2. ( A = 0.01058; tcs =
0.00532) have the size in the range from 0.5 to 2l
m with the peaksize of about 0.7 lm and average crystal size (D) of about 1.5 lm(see Table 2). This is, however in contradiction with the observa-tion of relatively large (5–20 lm) crystals in the SEM imagess1.1. and s1.2. (Fig. 1) as well as s2.1. and s2.2. (Fig. 2). In addition,although the SEM images s1.2. and s1.3. (Fig. 1) as well as s2.2. ands2.3. (Fig. 2) show a drastic decrease of the crystal size when A andtsc increase from 0.01058 and 0.00532 (hydrogels H1.2. and H2.2.),respectively, to 0.0165 and 0.00829 (hydrogels H1.3. and H2.3.),respectively, the size range, peak size and average crystal size of the zeolite A crystals obtained from hydrogels H1.3., H2.3. increaserelative to the corresponding size values characteristic for zeolite Acrystals obtained from hydrogels H1.2., H2.2. (see CSD curves s1.2.,s2.2., s1.3. and s2.3. in Fig. 3) and the corresponding data in Table2). Finally, while the increase of A and tsc from 0.01650 and0.00829 (hydrogels H1.3. and H2.3.), respectively, to 0.02291 and0.01151 (hydrogels H1.4. and H2.4.), respectively, does not consid-erably change the size range, peak size and average crystal size (seeCSD curves s1.3., s2.3., s1.4. and s2.4. in Fig. 3 and Table 2), furtherincrease of A (P0.02986) and tsc (P0.0150), causes decrease of thesize range, peak size and average crystal size. Changes of the par-ticulate properties for A > 0.055 and tsc > 0.02761 will be dis-cussed separately.
An apparent disagreement between the size of zeolite crystalsobserved by scanning electron-microscopy (see SEM images s1.1.and s1.2. in Fig. 1 as well as s2.1. and s2.2. in Fig. 2) and those esti-mated from the corresponding CSD curves (see CSD curves s1.1.,s2.1., s1.2. and s2.2. in Fig. 3 and the corresponding values of D
in Table 2) is obviously caused by formation of two separate crystal
populations, as can be clearly shown in the corresponding CSDs byvolume (see CSD curves s1.1., s2.1., s1.2. and s2.2. in Fig. 4). Ananalysis of the CSDs of the zeolite A obtained from hydrogelH1.1. (sample s1.1.) has shown that more than 99% (by number)of crystals have the size lower than 2 lm and that less than 1%of crystals (by number) have the sizes in the range 6–50 lm.While, due to a great disproportion between the numbers of crys-tals of these crystal populations, the bimodal character of crystalsize distribution cannot be observed in the CSD curves by number(see CSD curves s1.1., s2.1., s1.2. and s2.2. in Fig. 3), both the smallpeaks of the smaller crystals and large peaks of the larger crystalscan be clearly distinguished in the CSD curves by volume (see CSDcurves s1.1., s2.1., s1.2. and s2.2. in Fig. 4). On the other hand,although the population of smaller crystals represents more than
Table 2
The end-times, t c(end) of crystallization process and reaction yields, Y R , which correspond to crystallization from hydrogels H1.1.–H1.10 of system S1 and H2.1.–H2.10. of systems
S2 as well as the corresponding average crystal sizes, D, and specific number of crystals, N s, of the crystalline end products obtained by hydrothermal treatment of hydrogels
H.1.1.–H1.10. of system S1 (samples s1.1.–s1.10) and H2.1.–H2.10. of system S2 (samples s2.1.–s2.10.).
Hydrogel (No.) t c(end) (min) Sample (No.) Y R (wt.%) D (lm) N s (/g)
99% (by number) of all the crystals formed from hydrogel H1.1, itsportion in the total mass of the formed crystals is less than 10%;more than 90% of the mass is yielded by less than 1% (by number)of the crystals having the size in the range 6–50 lm. Thus, it isobvious that the CSDs by number of these products are determinedby the large number of crystals having the size in the range 0.5–2 lm (see the CSD curves s1.1., s2.1., s1.2. and s2.2. in Fig. 3) andthat the larger crystals (>6 lm) dominates in the SEM images(see the SEM images s1.1.and s1.2. in Fig 1 as well as s2.1. ands2.2. in Fig. 2).
While the ‘‘excess’’ of aluminium ( y = 1.3) does not markedlyinfluence the CSDs, either by number (see the CSD curves s1.3.–s1.10. and s2.3.–s2.10. in Fig. 3) or by volume (see the CSD curvess1.3.–s1.10. and s2.3.–s2.10. in Fig. 4), of zeolite A obtained from
hydrogels H1.3.–H1.8. and H2.1.–H2.8., the ‘‘excess’’ of aluminiumconsiderably increases specific number, N s, of crystals (see Table 2)
and decreases the size ranges and the peak sizes of the larger-sizecrystal populations (see the CSD curves s1.1., s2.1., s1.2. and s2.2. inFig. 3) of zeolite A obtained from low-alkaline hydrogels H2.1. andH2.2.: For example, the size range and the peak size in the larger-size crystal population of zeolite A obtained from hydrogel H1.1.( y = 2.0) are 2.7–56 lm and 18.7 lm (see the CSD curve s.1.1. inFig. 4), and the corresponding values of zeolite A obtained fromhydrogel H2.1. ( y = 1.3) are 2.7–30 lm and 8.2 lm (see the CSDcurve s.2.1. in Fig. 4).
Now, taking into consideration that:
(i) zeolite nuclei are formed in the gel matrix during its precip-itation [31–36],
(ii) number and distribution of nuclei in the gel matrix consider-
ably depend on the chemical composition of hydrogel andthe mode of its preparation [35–37,9,38,39],
Fig. 1. SEM images of the crystalline end products (zeolite A) obtained by hydrothermal treatment of hydrogels H1.1. (s1.1.), H1.2. (s1.2.), H1.3. (s1.3.), H1.4. (s1.4.), H1.5.(s1.5.), H1.6. (s1.6.), H.17. (s1.7.) and H1.8. (s1.8.).
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(iii) nuclei cannot grow in the gel matrix, but can start to growafter their ‘‘releasing’’ from the gel dissolved during crystal-lization, i.e., when they are in full contact with the liquidphase [4,26,34,37,39,40],
(iv) specific number of crystals, N s, corresponds to the number of active nuclei from which growth the crystals are formed[11,34,9] and
(v) CSD depends on the number and distribution of nuclei in thegel matrix [34,37,39,41,42],
the considerable differences of the particulate properties (specificnumber, N s, of crystals; see Table 2, CSD by volume; see Fig. 4) inthe products obtained from hydrogels H1.1. ( y = 2.0) and H2.1.( y = 1.3) can be explained as follows: Reaction of monomeric
AlðOHÞ
4 anions [43,44] from aluminate solution with higher-poly-merized silicate anions present in low alkaline silicate solutions
[44–46] where AlðOHÞ
4 anions preferentially react with largest sil-icate anions [47], results in the formation (precipitation) of amor-phous phase (gel) having the molar ratio [SiO2/Al2O3]g higher thanis the batch molar ratio y = [SiO2/Al2O3]b [48–50], i.e., for y = 2.0(system S1), [SiO2/Al2O3]g > 2.0. Or in the other words, number of Si–O–Si bonds in the gel matrix increases and at the same timenumber of Si–O–Al bonds in the gel matrix decreases with increaseof y , and vice versa. Since for zeolite A are immanent Si–O–Al andnot Si–O–Si bonds (i.e., Si/Al = 1 in zeolite A framework [20]) zeoliteA nuclei can be rather formed in Si–O–Al than in Si–O–Si environ-ment. From this reason number of the nuclei formed in the gel ma-trix is proportional to the content of aluminum in the gel and thus,determined by the value of [SiO2/Al2O3]g. On the other hand, sincethe value of [SiO2/Al2O3]g decreases with decreasing value of y
[48–50], number of nuclei formed per unit mass of the solid phaseof hydrogels H2.1. and H2.2. ( y = 1.3) are higher than the number of
Fig. 2. SEM images of the crystalline end products (zeolite A) obtained by hydrothermal treatment of hydrogels H2.1. (s2.1.), H2.2. (s2.2.), H2.3. (s2.3.), H2.4. (s2.4.), H2.5.(s2.5.), H2.6. (s2.6.), H2.7. (s2.7.) and H2.8. (s2.8.).
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nuclei formed per unit mass of the solid phase of hydrogels H1.2.and H2.2. ( y = 2.0). Thus, the specific number of crystals, N s, in thecrystalline end products obtained from low-alkaline hydrogelsH2.1. and H2.2. (samples s2.1. and s2.2.) are considerably higherthan the specific number of crystals in the crystalline end productsobtained from low-alkaline hydrogels H1.1. and H1.2. (samples s1.1.and s1.2.; see Table 2).
The formation of two particle (crystal) populations during crys-tallization of zeolite A form low-alkaline hydrogels can be ex-plained by specific distribution of nuclei in the gel matrix; most
of nuclei are formed in the ‘‘centers’’ of the gel particles and lowerpart of nuclei are formed at the surface and subsurface regions of gel particles [37,9,38,39]. Since the nuclei positioned in the surfaceand subsurface regions of gel are released from the gel matrix andstart to grow at the beginning of crystallization process, theirgrowth consumes most of aluminosilicate material before the nu-clei positioned in the ‘‘centers’’ of gel particles are released fromthe gel matrix.
Simultaneous increase of A and tsc ( A/tsc = 1.99 = constant forboth systems) causes at least three effects; (i) increase of specificnumber, N s, of zeolite A crystals in the product of crystallization(see Table 2) and consequently, decrease of crystal size (see Figs.1–4), (ii) decrease in the difference between specific number of crystals in the products obtained by hydrothermal treatment of
corresponding hydrogels of systems S1 ( y = 2.0) and S2 ( y = 1.3.;see Table 2) and consequently, decrease in the difference the be-
tween the corresponding crystal size distributions (see Figs. 3and 4) and (iii) marked change of particle (crystal) morphology(see Fig. 5).
The increase of specific number, N s, of crystals with increase of alkalinity (simultaneous increase of ‘‘free’’ Na+ and OH ions) canbe readily explained by the action of structure-forming Na+ ions[32,51,52] which induces the formation of nuclei in the gel matrix[4,11,32,34,36]. In addition, the increase of alkalinity increases theconcentration of lower-polymerized silicate anions (monomers,dimmers) [44–46]; this governs the reactions between silicate
and aluminate ions to formation of the gels with higher aluminumcontent (decrease in the molar ratio, [SiO2/Al2O3]g, of gel) [50] andthus, additionally facilitates the formation on nuclei in the gel ma-trix. Finally, for A > 0.01 the ‘‘excess’’ of aluminum in the reactionmixture ( y > 2) has not a considerably influence on the molar ratio[SiO2/Al2O3]g [49], so that the vales of N s for both the systems (S1and S2) become close for A P 0.0165 (see Tables 1 and 2). Almostthe same crystal size distributions of the crystalline end productobtained at given alkalinity (0165 6 A6 0.046) for y = 1.3 and y = 2 (see Figs. 3 and 4) indicate that not only number of nuclei,but also their distribution in the gel matrix does not considerablydepend on ‘‘excess’’ of aluminum in the reaction mixture at higheralkalinities of the reaction mixture.
The great influence of the ‘‘excess’’ of aluminum in the reaction
mixture at low alkalinity ( A < 0.01) on the specific number of crys-tals (see Table 2) and crystal size distribution (see Figs. 3 and 4) is
Fig. 3. Crystal size distributions (CSD) by number of the crystalline end productsobtained by hydrothermal treatment of hydrogels H1.1. (s1.1.), H2.1 (s2.1.), H1.2.(s1.2.), H2.2. (s2.2.), H1.3. (s1.3.), H2.3 (s2.3.), H1.4 (s1.4.), H2.4. (s2.4.), H1.5. (s1.5.),H2.5. (s2.5.), H1.6. (s1.6.), H2.6. (s2.6.), H1.7. (s1.7.), H2.7. (s2.7.), H1.8. (s1.8.), H2.8.(s2.8.), H1.9. (s1.9.) H2.9. (s2.9.), H1.10 (s1.10.) and H2.10. (s2.10.). The solid curves(s) corresponds to the CSDs of the products crystallized from system S1 ( y = 2.0) andthe dashed curves (d) correspond to the CSDs of the products crystallized from
system S2 ( y = 1.3). N D is number percentage of crystals having the sphericalequivalent diameter D.
Fig. 4. Crystal size distributions (CSD) by volume of the crystalline end productsobtained by hydrothermal treatment of hydrogels H1.1. (s1.1.), H2.1 (s2.1.), H1.2.(s1.2.), H2.2. (s2.2.), H1.3. (s1.3.), H2.3 (s2.3.), H1.4 (s1.4.), H2.4. (s2.4.), H1.5. (s1.5.),H2.5. (s2.5.), H1.6. (s1.6.), H2.6. (s2.6.), H1.7. (s1.7.), H2.7. (s2.7.), H1.8. (s1.8.), H2.8.(s2.8.), H1.9. (s1.9.) H2.9. (s2.9.), H1.10 (s1.10.) and H2.10. (s2.10.). The solid curves(s) corresponds to the CSDs of the products crystallized from system S1 ( y = 2.0) andthe dashed curves (d) correspond to the CSDs of the products crystallized from
system S2 ( y = 1.3). V D is volume percentage of crystals having the sphericalequivalent diameter D.
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followed by a considerable influence on the crystal morphology(see the SEM images s1.1. and s2.1. in Fig. 5). Based on the previousinvestigations of the influence of y on the crystal morphology of
zeolite A [6,8,15], obtaining the regular cubic crystals with sharpedges and apexes (see the SEM image s1.1. in Fig. 5) for y = 2.0and cubic crystals with truncated edges (see SEM image s2.1. inFig 5) for y = 1.3 was expected. Development of {0 1 1} crystalplanes at the ‘‘excess’’ of aluminum in the reaction mixture indi-cates that, under such condition, the crystal growth in the h0 1 1idirection is suppressed. Knowing that the presence of aluminumatom on a neighboring T atom (Si, Al) site distorts the silicon tetra-hedron, i.e. shortens one of the silicon-oxygen bond [53], it is rea-sonable to assume that just the distortion of the terminal SiO4
tetrahedra caused by the presence of aluminate ions retards thegrowth rate at the points (regions) of distortions, e.g., crystal edgesand/or apexes [15]. On the other hand, appearance of {0 1 1} crys-tal planes on zeolite A crystals obtained from hydrogels H1.2. and
H1.3. ( y = 2; see SEM images s1.2. and s1.3. in Fig 5) as well as in-crease of edge-to-face aspect ratio W e/L (see SEM images s1.1.–
s1.3. and s2.1.–s.2.3. in Fig. 5) with simultaneous increase of alka-linity, A, and total silica concentration, tsc , indicate that not onlythe value of y, but also the values of A and tsc influence the growthrate in h0 1 1i direction relative to the growth rate in h0 0 1i direc-tion. Now, keeping the thesis that the decrease of crystal growth inh0 1 1i direction and thus, developing of {0 1 1} crystal planes of zeolite A is caused by distortion of the terminal SiO 4 tetrahedrain the presence of aluminate ions [15,53], the above mentioned ef-fects can be readily explained by the fact that, the ratio, [Al/Si]L , inthe liquid phase of hydrogel increases with increasing alkalinity[50]. Since, on the other hand, for given values of A and tsc , [Al/Si]L increases with decrease of y [48,49] the higher edge-to-face as-pect ratios of zeolite A obtained from system S2 ( y = 1.3) than of these obtained from the corresponding batches of system S1( y = 2.0), was expected.
Further simultaneous increase of A ( A P 0.02291) and tsc
(tsc P 0.01151) causes ‘‘rounding’’ of the crystal edges and apexes(see SEM images s1.4., s2.4., s1.5 and s1.6. in Fig. 5) following bypartial (for y = 2.0, A P 0.0375 and tsc P 0.01885; see SEM imagess1.5. and s1.6. in Fig. 5) or complete loss of individual crystal faces(for y = 1.3, A P 0.02291 and tsc P 0.01151; see SEM images s2.5–s2.7. in Fig. 5 – also see Fig. 6). The rounding of the crystal edgesand apexes can be explained by surface nucleation [15], as postu-lated by Unemura et al. (see Fig. 2 in Ref. [54]), caused by increaseof supersaturation level with simultaneous increase of A and tsc
[21,22]. On the other hand, since the loss of individual crystal facesis connected with the pseudo-spherical shape of the particles(crystals) of zeolite A (see Fig. 6) one can assume that, under thegiven chemical conditions ( A P 0.02291, tsc P 0.01151 for y = 1.3and A P 0.0375, tsc P 0.01885 for y = 2.0), the growth rate isapproximately the same in all directions. However, since there isnot a theoretical basis or even an experimental evidence for suchan assumption, it is more realistic to assume that a gradual ‘‘round-ing’’ of zeolite A crystals and formation of the pseudo-sphericalparticles of zeolite A is caused by intensive surface nucleation athigh supersaturation characteristic for the above mentioned condi-
tion. On the other hand, the absence of identifiable individual crys-tal planes accompanied by high roughness of particles surfaces (seeFig. 6) indicates that the intense surface nucleation [54] was prob-ably followed by a rapid growth [15] of the formed surface nuclei.In this context, the complete disappearing of individual crystalplanes at lower values of A (0.02291) and tsc (0.01151) for y = 1.3(see SEM image s.2.5 in Fig. 5) relative to only partial disappearing
Fig. 5. SEM images of the crystalline end products obtained by hydrothermaltreatment of hydrogels H1.1. (s1.1.), H2.1 (s2.1.), H1.2. (s1.2.), H2.2. (s2.2.), H1.3.(s1.3.), H2.3 (s2.3.), H1.4. (s1.4.), H2.4. (s2.4.), H1.5. (s1.5.), H2.5. (s2.5.), H1.6. (s1.6.)H2.6. (s2.6.), H1.7. (s1.7.), H2.7. (s2.7.), H1.8. (s1.8.), H2.8. (s2.8.), H1.9. (s1.9.), H2.9.(s2.9.), H1.10 (s1.10.) and H2.10. (s.2.10.).
Fig. 6. Morphological features of the product obtained by hydrothermal treatmentof hydrogel H2.6. (sample s.26.). Magnification: 33,000; size-bar: 100 nm.
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of the individual crystal planes at higher values of A (0.0375) andtsc (0.01885) for y = 2.0 (see SEM image s1.6. in Fig. 5), can be read-ily explained by the fact that for given values of A and tsc , supersat-uration increases with increasing value of y [21,22]. Formation of irregularly-shaped particles of zeolite A at A = 0.04585 andtsc = 0.02315 (see SEM images s1.7, and s2.7. in Fig. 5) is probablycaused by rapid and uncontrolled growth of surface nuclei at in-creased supersaturation.
In addition, for A P 0.055 (which corresponds to P6.1 MNaOH) and tsc = 0.0276 (which corresponds to P1.53 M SiO2),the rapid and uncontrolled growth of surface nuclei is followedby aggregation of individual zeolite A crystals and a tendency of spontaneous transformation of the formed zeolite A into hydroxy-sodalite (HS) [14,17].
Although the number of nuclei formed in the gel matrix for A =
0.055 is assumed to be higher than the number of nuclei formed inthe gel matrix for A = 0.04585, specific number of particles as cal-culated by Eq. (3) using the corresponding CSD data is lower andthe average crystal size, D, is higher for the product obtained athigher alkalinity ( A = 0.055) relative to the corresponding valuesof the product obtained at lower alkalinity ( A = 0.04585) (see Table2). This can be readily explained by a tendency of the formation of crystal aggregates (see the SEM images s1.8. and s2.8 in Fig. 5) athigh alkalinity ( A P 0.055) and high crystal yield (18–19 wt.%;see Table 2) caused by high tsc (=0.02761). However, the differ-ences in the particulate properties of the products obtained fromhydrogels H1.8. and H2.8. (see SEM images s1.8. and s2.8. inFig. 5, CSD curves s1.8. and s2.8. in Figs. 3 and 4, and the corre-sponding data in Table 2) show that the tendency of formation of crystal aggregates is less during the hydrothermal treatment of hydrogel H2.8. (system S2, y = 1.3) than during the hydrothermaltreatment of hydrogel H1.8. (system S1, y = 2.0). This indicates thatthe ‘‘excess’’ of aluminum retards the aggregation processes.
Finally, as expected, at high alkalinity ( A P 0.055) there is atendency of spontaneous transformation of the formed zeolite Ainto HS [14,17]. However, while the products obtained by hydro-
thermal treatment of hydrogels H1.8. and H2.9. contains onlytraces of HS, the products obtained from hydrogels H1.9. andH1.10. are pure HS, as revealed by XRD and the SEM images; thesponge-like features in the SEM images s1.9., s1.10. and s2.10. inFig. 5, are characteristic for HS [55]. The absence of HS in the prod-uct obtained from hydrogels H2.8. and presence of only traces of HS in the product obtained from hydrogel H2.9. indicates thatthe ‘‘excess’’ of aluminum prevents not only the aggregation pro-cesses, but also the transformation of zeolite A into HS even at highalkalinities. This combination of the tendency of transformation of zeolite A into HS and ‘‘protective’’ role of aluminate ions makes theprocesses of crystallization at high alkalinity ( A > 0.055) ‘‘unstable’’and particulate properties of the crystalline end products areunpredictable (see SEM images s1.9., s2.9., s1.10. and s2.10. in
Fig. 5 and CDS curves s1.9., s2.9., s1.10. and s2.10. in Figs. 3 and 4).
4. Conclusions
The presented results show how the pathway of crystallization,reaction yield and especially particulate properties (crystal sizedistribution, crystal morphology) of zeolite A can be controlledby chemical composition of starting aluminosilicate hydrogel, de-fined by the batch molar ratios A = [Na2O/H2O]b, tsc = [SiO2/H2O]b
and y = [SiO2/Al2O3]b at the constant ratio A/tsc = 1.99. Durationof crystallization process sharply decreases with simultaneous in-crease of A and tsc as the consequence of the increase of the num-ber of nuclei formed in gel matrix, rate of their release from the gel
matrix and the growth rate of the released nuclei and crystals withincreasing alkalinity of the reaction mixture. The reaction yield, Y R
(mass of zeolite crystallized from 100 g of the reaction mixture) isproportional to total silica batch concentration tsc ; however, Y R /tsc
slightly decreases with increasing alkalinity as a consequence of the increasing solubility of zeolite A with the increase of thealkalinity.
Specific number, N s, of crystals in the crystalline end productsincreases with simultaneous increase of A and tsc as the conse-quence of the increase of concentration of structure-forming Na+
ions, which induces the formation of nuclei in the gel matrix. How-ever, while the crystalline end products obtained from low alkalinereaction mixtures (hydrogels H1.1., H2.1., H1.2. and H2.2.;0.005076 A6 0.01058) are characterized by two separate crystalpopulations and with a considerable influence of y on the specificnumber of crystals, CSDs of the crystalline end products obtainedfor 0.0165 6 A 6 0.055 are monomodal with no considerable influ-ence of y on either CSD or specific number, N s, of crystals. This wasexplained by specific polycondensation reactions between alumi-nate and (poly)silicate anions during precipitation of amorphousaluminosilicate precursor (gel) under various alkalinities.
Morphology of the crystallized zeolite A depends not only onthe ratio y = [SiO2/Al2O3]b, as it was assumed on the basis of previ-ous studies, but on the overall chemical composition of the reac-tion mixture, and especially on its alkalinity, A = [Na2O/H2O]b.Four different morphological forms of zeolite A, namely (i) regularcubic crystals with ‘‘sharp’’ edges and apexes, (ii) cubic crystalswith truncated edges, (iii) cubic crystals with ‘‘rounded’’ edgesand apexes and (iv) pseudo-spherical particles with no recogniz-able individual crystal planes are obtained during hydrothermaltreatment of investigated hydrogels. The formation of cubic crys-tals with truncated edges (formation of {0 1 1} crystal faces) wasexplained by decrease of crystal growth ion the h0 1 1i directioncaused by distortion of terminal SiO4 tetrahedrons positioned atcrystal edges and apexes in the presence of aluminate ions. Sincefor given values of A and tsc , concentration of aluminate ions in-creases, this effect (decrease of crystal growth in the h0 1 1i direc-tion) increases with increasing alkalinity of the reaction mixture.
On the other hand, formation of cubic crystals with ‘‘rounded’’edges and apexes and even complete loss of recognizable crystalplanes and formation of pseudo-spherical particles is caused by in-tense surface nucleation at increased supersaturation caused byrelatively high alkalinity of the reaction mixture at ‘‘excess’’ of alu-minium in the liquid phase.
Because of simultaneous occurring of several processes, e.g., ra-pid and uncontrolled growth of surface nuclei, crystal aggregationand transformation of the formed zeolite A into hydroxysodalite(HS), the processes of crystallization at high alkalinity ( A > 0.055)is ‘‘unstable’’ and particulate properties of the crystalline end prod-ucts cannot be predicted.
Acknowledgement
The authors gratefully acknowledge funding from BarchemCompany and Ministry of Science, Education and Sport of theRepublic of Croatia.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2010.12.027.
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