The effect of Na2SO4 salt on the synthesis of ZSM-5 by template free crystallization method

Microporous and Mesoporous Materials 118 (2009) 361–372

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The effect of Na2SO4 salt on the synthesis of ZSM-5 by template free crystallization method Na Young Kang a, Bu Sup Song a, Chul Wee Lee a, Won Choon Choi a, Kyung Byung Yoon b,*, Yong-Ki Park a,* a b

Division of Advanced Chemical Technology, Korea Research Institute of Chemical Technology (KRICT), Jang-dong 100, Daejeon 305-600, Republic of Korea Department of Chemistry, Sogang University, Seoul 121-742, Republic of Korea

a r t i c l e

i n f o

Article history: Received 20 May 2008 Received in revised form 3 September 2008 Accepted 6 September 2008 Available online 23 September 2008 Keywords: Effect of Na2SO4 ZSM-5 Sodium silicate Template-free Oxyanion

a b s t r a c t ZSM-5 was synthesized by template-free method using sodium silicate as a silica source and the content of Na2O in mother liquid was controlled by H2SO4. To see the effect of Na2SO4 generated by added H2SO4, hydrothermal crystallization was carried out with two types of mother liquids having low and high Na2SO4 contents at 170 °C. The physical and chemical properties of ZSM-5s prepared in two different conditions were compared by XRD, EDS, FTIR, micropore analyzer and particle size analyzer. High crystalline, isometrical shaped and uniform 1–2 lm sized ZSM-5s with relative crystallinity around 100% could be obtained successfully from the mother liquids having the molar range of (11.0–21.0) Na2O:(21.4–11.4) Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O. Through the crystallization in the absence and presence of extra Na2SO4, it was found that the condensation rate of aluminosilicate was increased by the Na2SO4 and resulted in the formation of amorphous phase instead of crystalline ZSM-5. The formation of crystalline HZSM-5 was prohibited critically if the total amount of Na2SO4 in mother liquid was higher than 24.4 mole based on 100.0 mole of SiO2. The enhanced condensation reaction and the preferential formation of agglomerated amorphous aluminosilicates in the presence of Na2SO4 could be explained by the oxyanion effect of SO2 4 generated by Na2SO4. Crown Copyright Ó 2008 Published by Elsevier Inc. All rights reserved.

1. Introduction Because the ZSM-5 has extremely high thermal and acid stability, shape selectivity and activity in catalytic reactions, which are very critical for industrial application, it has been used widely in many petrochemical catalytic processes such as cracking, isomerization, aromatization and alkylation process [1]. Since the Mobil Oil Corporation, in 1972, issued a first patent on the synthesis of a pentasil aluminosilicate zeolite and termed ZSM-5 by using TPA+ ion (tetrapropylammonium ion) as templates [2], there has been a lot of approaches to synthesize it in more mild conditions and from the cheaper raw materials. Generally, high crystalline ZSM-5 could be synthesized easily via a hydrothermal method from a mother liquid containing silica sol, sodium aluminate, and sodium hydroxide together with organic template cations such as TPA+ and TEA+. These raw materials are quite efficient precursors to obtain high crystalline ZSM-5 but it has some drawbacks in economics and environment due to relatively high raw material cost and toxicity. Therefore, there have

* Corresponding authors. Tel.: +82 42 860 7672; fax: +82 42 860 7388 (Y.-K. Park). E-mail addresses: [email protected] (K.B. Yoon), [email protected] (Y.-K. Park).

been a lot of efforts to synthesize ZSM-5 with or without seed from cheaper silica sources such as sodium silicate and silicic acid in the absence of template [3–14]. For the template-free synthesis of ZSM-5, the synthetic conditions such as types of raw material, molar composition, mixing procedure, aging time and crystallization temperature should be controlled precisely because the working windows for each parameter are much narrower than those in templated systems. The selection of silica source is a most important one and only after its decision the other parameters such as Al2O3/SiO2 ratio, Na2O/ SiO2 ratio, mixing procedure and time of reactants, and the aging time are optimized. The silica source should be chosen based on two guidelines of material cost and quality of obtained ZSM-5. Among the several types of silica sources such as silica sol, amorphous silica and water glass, the water glass could be considered as one of attractive silica sources because high crystallinity of ZSM-5 is guaranteed without the loss of product yield from this most cheap and easy market accessible water glass. One difficulty encountered in water glass system can be ascribed to its high content of alkali ingredients more than that required for ZSM-5 synthesis. That is, the minimum Na2O/SiO2 mole ratio of commercially accessible sodium silicate is 0.33 as shown in Eq. (1) but the Na2O/SiO2 molar ratio of mother liquid required for the synthesis of high crystalline pure ZSM-5 is in the

1387-1811/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.09.016

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range of 0.1–0.25. To remove the excess Na2O, it should be neutralized with acids such as sulfuric acid, nitric acid and hydrochloric acid. Among the various types of acids, sulfuric acid is a preferred one due to its low corrosiveness and price. During the neutralization process, considerable amount of Na2SO4, which corresponds to 1/3 – 2/3 of Na2O included in water glass, is generated as shown in Eq. (2), That is, the formation of Na2SO4 salt is unavoidable if water glass is used as a silica source.

2. Experimental 2.1. Synthesis of ZSM-5 zeolite For the synthesis of ZSM-5, sodium silicate (30.0 wt.% SiO2), aluminum sulfate (8.0 wt.% Al2O3) were used as a silica and alumina source. Finally, to adjust Na2O/SiO2 ratio, 25 wt.% H2SO4 was added and Na2SO4 was formed during this procedure according to Eq. (2). The detail mole composition of mother liquids is described in Table 1. ZSM-5 was synthesized according to the overall experimental procedure described in Fig. 1. The composition of the mother liquid was changed in the molar range of 30–80 SiO2/ Al2O3, 0.10–0.22 Na2O/SiO2 and 40 H2O/SiO2. After mixing silica source and alumina source vigorously for 30 min, it was introduced into a 100 ml autoclave lined with Teflon and then crystallized hydrothermally in a rotating oven maintained at 170 °C and 200 rpm for 6–24 h. After completion of the crystallization, the synthesized product was cooled to room temperature, rinsed with distilled water, and then dried at 120 °C for 12 h. From the solid obtained after drying, solid product yield was calculated by using Eq. (3)

Sodium silicate : 2NaOH þ 3SiO2 ! 3Na2=3 SiO2 1=3 þ H2 OðNa2 O=SiO2 ¼ 0:33Þ

ð1Þ

Mother liquid : 3Na2=3 SiO2 1=3 þ H2 SO4 ! Na2 SO4 þ 3SiO2 þ H2 O

ð2Þ

This study was motivated after finding the fact that the crystallization time was much shorter in the case of ZSM-5 synthesis with sodium silicate than that with silica sol even though the synthetic procedure and molar composition of mother liquid were same. That is, when the ZSM-5 was synthesized from the mother liquid having the same molar composition (18 Na2O:100 SiO2:2 Al2O3:4000 H2O), high crystalline one was obtained within a day from the sodium silicate while more than two days from the silica sol. The main difference is the presence of Na2SO4 in the mother liquid. So, we were curious about the role of Na2SO4 salt which is generated inevitably during the hydrothermal crystallization of ZSM5. Even though the presence of Na2SO4 in mother liquid brings so big changes in crystallization rate and morphology, there have been no studies to see the effect of Na2SO4 systematically in the template-free synthesis of ZSM-5 using sodium silicate as a silica source. However, to see the effect of Na2SO4 independently is not easy because the content of Na2SO4 and Na2O change simultaneously by the adding of sulfuric acid. Therefore, to see the effect of Na2SO4 more clearly, two different synthetic compositions which depend sensitively on the content of Na2SO4 were chosen and then extra Na2SO4 was added; one is in low edge of Na2O/SiO2 and the other is in high edge of Na2O/SiO2 in which ZSM-5 phase could be obtained. That is, firstly, the crystallization composition windows depending on the ratio of Al2O3/SiO2, Na2O/SiO2 and H2O/SiO2 were determined by controlling molar compositions of water, aluminum sulfate and sulfuric acid. Secondly, based on these crystallization windows, two types of mother liquid having different Na2O/SiO2 ratio were prepared after fixing the Al2O3/SiO2 and H2O/SiO2 ratio and the Na2SO4/SiO2 was changed by adding extra Na2SO4.

Solid product yield ðwt:%Þ ¼

Weight of solid obtained after crystallization Weight of solid in mother liquid

ð3Þ

To change the prepared zeolites into H-form, they were ionexchanged with 500 ml of 0.5N NH4NO3 at 60 °C for 6 h and it was repeated two times. Finally, they were calcined at 550 °C and air atmosphere for 6 h. 2.2. Characterization Synthesized ZSM-5 zeolite products were characterized by X-ray diffraction (XRD; Rigaku, model D/Max-2200V) using Cu Ka radiation operated with a 2h scanning speed of 4° min1. The relative crystallinity was determined from the peak area between 2h = 22– 25° using peak area of highly crystalline ZSM-5 (Zeolyst, CBV 5524G) as a reference. The degree of the crystallinity was calculated from the following Eq. (4), as previously done by Ping Wang [15].

Relative crystallinity ð%Þ ¼

Peak area of product Peak area of reference sample

ð4Þ

Crystal morphology and size were identified by SEM (Akasi Alpha-25A) equipped with image analyzer (Oyang system Co.). The pore size and surface area were measured by micropore analyzer (Micromeritics Co. ASAP2010) using Argon adsorption at 77K. Elemental analysis was conducted by EDS to confirm SiO2/Al2O3 molar ratio of the obtained solid products. To monitor the effect of Na2SO4

Table 1 Crystallization field of zeolite depending on Na2O/SiO2, Na2SO4/SiO2 and Al2O3/SiO2 ratio x

y

z (SiO2/Al2O3 ratio) 2.50 (40)

2.00 (50)

1.25 (80)

Molar composition = x Na2O:y Na2SO4:100 SiO2:z Al2O3:4000 H2O 22.0 10.4 Mordenite

3.30 (30)

Mordenite

Mordenite

Amorphous

21.0 20.0 19.0 18.0 17.0 16.0 14.0 13.0 12.0 11.0 10.0

Mordenite Mordenite Mordenite Mordenite Mordenite + ZSM-5 Mordenite + ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5

Mordenite Mordenite Mordenite + ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 Amorphous

ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5 Amorphous Amorphous

11.4 12.4 13.4 14.4 15.4 16.4 18.4 19.4 20.4 21.4 22.4

Mordenite Mordenite Mordenite Mordenite Mordenite Mordenite Mordenite + ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5

N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372

(1) Sodium silicate+ H 2 O

(2) Aluminum sulfate + H 2SO4 + H 2O + (Salt)

Mix ing: (1) + (2)

Hydr other mal Cr ystallization o

at 170 C, 200 r pm for 6~ 24h

Filtr ation and Washing

o

Dr ying at 120 C for 12 hr s

Ion Ex change with NH 4 NO3

o

Calcination at 550 C for 6 hr s Fig. 1. Synthetic procedure of ZSM-5 from sodium silicate without templating agent.

on nucleation and crystal growth of zeolite, the particle size change and Si–O bond formation rate of mother liquid were monitored by Particle size analyzer (ELS-Z2, Otsuka) and FTIR in ATR mode (Avartar 360, Nicolet), respectively, depending on aging time. 2.3. Cracking activity test To evaluate catalytic activity of ZSM-5s synthesized in content of Na2SO4, naphtha cracking reaction, one of representative reactions to evaluate catalytic activity of ZSM-5, was carried out in a fixed-bed flow reactor made of Inconel under atmospheric pressure. 0.5 g of catalyst was put into the reactor and then pretreated in helium flow at 500 °C for 1 h before reaction. The cracking reaction was done in the following condition; naphtha/steam(wt/ wt) = 2, WHSV = 2 h1, temperature = 675 °C. The gas and liquid products were separated by cooling to 2 °C and then analyzed by an GC (HP 6890) equipped with NP-1(Alltech) and ATTMPETRO(Alltech) columns, respectively. 3. Results and discussion 3.1. Crystallization field of zeolite depending on Na2O content and SiO2/Al2O3 ratios A series of hydrothermal synthesis of zeolite was carried out by using sodium silicate as a silica source at 170 °C for 24 h while the

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molar composition of Na2O and Na2SO4 was changed by adding H2SO4 at four different SiO2/Al2O3 ratios of 30, 40, 50 and 80 which correspond to the Al2O3 mole fraction of 3.3, 2.5, 2.0 and 1.25 (Table 1). Initially, a mother liquid containing 32.4 mole of Na2O based on 100 SiO2 was prepared and then its content was reduced by neutralizing with sulfuric acid. During this step, stoichiometric amount of Na2SO4 was formed according to Eq. (2). The content of Na2SO4 was decided by the amount of added sulfuric acid and the sum of Na2O and Na2SO4 was maintained to a constant value of 32.4. Two interesting things were found in this experiment. Firstly, the Na2O composition window for ZSM-5 phase was widened as the molar ratio of SiO2/Al2O3 increased. That is, ZSM-5 could be synthesized in the range of 10–13 mole of Na2O at SiO2/Al2O3 molar ratio of 30 but it was widened to 12–21 mole at SiO2/Al2O3 molar ratio of 80. Secondly, Mordenite phase was obtained if the content of Na2O is too high while an amorphous phase was obtained if its content was too low. As can be seen in Table 1, quite predictable crystallization field was obtained depending on the contents of Na2O and Na2SO4 and SiO2/Al2O3 ratio even though the absolute values are different from those reported by Ping Wang [15]. The trend in crystallization field could be explained as the stability of aluminosilicate depending on the alkalinity of mother liquid. If the aluminosilicate is exposed at high alkaline condition, it becomes more soluble with decrease of its condensation rate. Thereby, it will lead to the transformation of comparatively less amount of aluminosilicate to the crystalline solid product. As reported by Jacobs, this may be the reason for the formation of Mordenite which is more metastable than ZSM-5 with decreased solid product yield [16]. Oppositely, if the aluminosilicate is exposed to a low alkaline condition, it becomes less soluble and condensation will occurs more rapidly. The increased condensation rate will lead to the formation of amorphous phase with high solid product yield because the aluminosilicate does not have enough time to form a crystalline phase such as ZSM-5 and Mordenite. This trend could be seen clearly from the XRD patterns of solid products obtained at different Na2O contents (Fig. 2). The XRD patterns were obtained after hydrothermal crystallization at 170 °C for 24 h with mother liquids having molar composition of (10.0–20.0) Na2O:(22.4–12.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O. In the range of 11.0–18.0 mole of Na2O, the XRD patterns of high crystalline ZSM-5 were obtained. However, out of this range, two different types of solid products were obtained. At 20 mole of Na2O (high alkaline condition), mixed XRD patterns of ZSM-5 and Mordenite were obtained while at 10 mole of Na2O (low alkaline condition), an amorphous one was obtained (Fig. 2a and f). Another evidence for the dependence of Na2O could be seen from the changes of crystallinity and solid product yield (Table 2). As same as in the XRD patterns, almost 100% relative crystallinity of ZSM-5s with was obtained in the range of 12.0 - 18.0 mole of Na2O but they decreased rapidly out of this range. That is, below 12 mole of Na2O, poor crystalline ZSM-5s were obtained. The solid product yield calculated by Eq. (3) was also monitored depending on the content of Na2O (Table 2). It depends linearly on the content of Na2O. The higher the content of Na2O, the less solid product yield was obtained. This means that the solubility of aluminosilicate strongly depends on the content of Na2O in the mother liquid. Therefore, to obtain high solid product yield, it is required to maintain the content of Na2O as low as possible. Fig. 3 shows the SEM images of solid products synthesized from the same mother liquid described in Table 2. In the range of (10.0–20.0) Na2O and (22.4–12.4) Na2SO4, 1–2 lm uniform-sized, isometrical-shaped and high crystalline ZSM-5s were obtained (Fig. 3b–e). However, out of this range, irregular-shaped and 0.2–0.3 lm sized fine spheroidal particles were obtained. From

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ing to Eq. (2) in introduction section. If sodium silicate having Na2O/SiO2 molar ratio of 0.33 is used as a silica source and SiO2/ Al2O3 ratio is fixed to 50, then the molar compositions of Na2O and Na2SO4 in mother liquid are decided by the added sulfuric acid and the sum of Na2O and Na2SO4 becomes 32.4 mole according to Eq. (5).

: Mordenite

(a)

(b)

xNa2 O þ yNa2 SO4 ¼ 32:4 and yNa2 SO4 ¼ wH2 SO4

Where wH2 SO4 = H2SO4 mole, xNa2 O = Na2O mole, yNa2 SO4 = Na2SO4 mole) If sulfuric acid is used as a neutralizing agent, then it is impossible to change independently the content of Na2SO4 in mother liquid without changing that of Na2O. Therefore, to see the effect of Na2SO4, the content of Na2SO4 was changed by Na2SO4 itself instead of the sulfuric acid. That is, two mother liquids having 12 and 18 mole of Na2O, which correspond to the regions of phase transformation ‘‘from ZSM-5 to Mordenite” and ‘‘from ZSM-5 to amorphous phase,” were prepared and then desired amount of extra Na2SO4 was added to these mother liquids.

(c) Intensity (cps)

ð5Þ

(d)

(e)

(f)

5

10

15

20

25

30

35

40

45

50

2Theta (deg.) Fig. 2. XRD patterns of solid products synthesized from the mother liquids described in Table 2 (a) x = 20.0, y = 12.4; (b) x = 18.0 y = 14.4; (c) x = 16.0, y = 16.4; (d) x = 14.0, y = 18.4; (e) x = 12.0, y = 20.4; (f) x = 10.0, y = 22.4, where the molar composition = x Na2O:y Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

Table 2 Relative crystallinity and yield of ZSM-5 depending on Na2O/SiO2, Na2SO4/SiO2 (Silica source = sodium silicate, Al2O3/SiO2 = 2) x

y

Relative Crystallinity (%)

Solid Yield (wt%)

Molar composition = x Na2O:y Na2SO4:100 SiO2:2 Al2O3:4000 H2O 22.0 10.4 Mordenite 42.5 21.0 20.0 19.0 18.0 17.0 16.0 14.0 13.0 12.0 11.0 10.0

11.4 12.4 13.4 14.4 15.4 16.4 18.4 19.4 20.4 21.4 22.4

Mordenite Mordenite Mordenite 100.2 106.2 96.6 109.6 93.9 93.1 74.5 21.3

45.4 49.6 46.9 58.4 60.3 66.2 69.78 64.9 68.1 71.4 78.1

the results of relative crystallinity, solid product yield, XRD and SEM, it was confirmed that cubical-shaped and uniform sized high crystalline ZSM-5 could be synthesized successfully even from sodium silicate without templating agent if the content of Na2O is controlled properly in a certain range. 3.2. Effect of Na2SO4 salt As can be seen in the column 1 and 2 of Table 2, if sodium silicate is used as a silica source, the formation of Na2SO4 is unavoidable because sulfuric acid has to be added to neutralize excess Na2O. This means that the contents of Na2SO4 and Na2O in mother liquid are decided automatically by the added sulfuric acid accord-

3.2.1. Effect of Na2SO4 at 0.18 Na2O/SiO2 molar ratio To see the effect of Na2SO4 near the region of phase transformation from ZSM-5 to Mordenite, 0–14 mole of extra Na2SO4 were added additionally to the mother liquid having the molar composition of 18.0 Na2O:14.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O and then synthesized at 170 °C for 24 h. The details of mother liquid composition and the results of relative crystallinity and solid product yield are described in Table 3a. If the content of Na2SO4 was less than 8 mole, which corresponds to the total Na2SO4 content of 22.4 mole, both of relative crystallinity and solid product yield were not influenced so much. However, further addition of Na2SO4, higher than 10 mole, led to the rapid decrease of relative crystallinity together with some increase of solid product yield. This means that the condensation of aluminosilicate is more favorable in the presence of Na2SO4 even though it is not helpful for the formation of crystalline phases such as ZSM-5 and Mordenite. From this result it can be induced that the Na2SO4 plays an important role to enhance dehydration of aluminosilicate and leads to the formation of more polymeric aluminosilicate. Moreover, the crystal phase stability and morphology change depending on the content of Na2SO4 were also monitored by XRD patterns and SEM images. As can be seen in Fig. 4, no other crystal phases except ZSM-5 were observed even though the content of Na2SO4 was higher than 10 mole. The role of Na2SO4 is much different from that of Na2O which plays an important role as a structure directing agent. That is, different from the phase from ZSM-5 to Mordenite in excess Na2O condition, no phase transformation was occurred by the addition of Na2SO4 even its content is higher than 10 mole. Therefore, it could be suggested that the Na2SO4 just interrupts the formation of crystalline ZSM-5 by the enhanced condensation reaction of aluminosilicate. It is a good comparison with the Na2O which plays an important role as a structure directing agent to provide a strong driving force for the formation of certain crystal phases. The morphology changes were also observed on the products prepared in excess Na2SO4 condition. As can be seen in the SEM images of Fig. 5, the addition of Na2SO4 leads to the formation of more tightly aggregated particles together with reduced particle size. If the content of Na2SO4 is higher than 10 mole (Fig. 5d), the particles aggregated with crystalline ZSM-5 and amorphous phase could be seen clearly. Therefore, to obtain high crystalline ZSM-5, it is recommended to reduce the content of Na2SO4 in the mother liquid as low as it can be. To see how much the physical properties are influenced by the Na2SO4, micropore and EDS analysis were carried out for the three solid products prepared from mother liquids having 0, 8 and

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Fig. 3. SEM images of solid products synthesized from the mother liquids described in Table 2 (a) x = 20.0, y = 12.4; (b) x = 18.0 y = 14.4; (c) x = 16.0, y = 16.4; (d) x = 14.0, y = 18.4; (e) x = 12.0, y = 20.4; (f) x = 10.0, y = 22.4, where the molar composition = x Na2O:y Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

Table 3 Relative crystallinity and solid yield of ZSM-5 depending on the added amount of Na2SO4 (Silica source = sodium silicate) x (Na2O) (mol)

y (Initial Na2SO4) (mol)

z (Added Na2SO4) (mol)

Molar composition = 18 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O (a) 18 (Na2O/SiO2 = 0.18) 14.4 4 6 8 10 12 14 20.4 2 (b) 12 (Na2O/SiO2 = 0.12) 4 6

[y + z ] (Total Na2SO4) (mol)

Relative Crystallinity (%)

Solid yield (wt%)

18.4 20.4 22.4 24.4 26.4 28.4 22.4 24.4 26.4

106.8 98.7 103.5 78.2 75.1 50.5 113.2 76.7 11.7

58.0 53.1 55.8 59.3 62.3 67.0 74.3 76.4 81.1

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different from that in the region of phase transformation from ZSM-5 to Mordenite. That is, the relative crystallinity decreased more rapidly as the content of Na2SO4 increased. It decreased to 76.7% and 11.7%, respectively, even by the addition of 4 and 6 mole of extra Na2SO4. As can be seen in Figs. 6 and 7, coincided dependence of Na2SO4 was also observed in SEM images and XRD patterns. When 6 mole of Na2SO4 was added to the mother liquid, the solid product agglomerated only with amorphous fine particles was obtained instead of crystalline ZSM-5. The reason for the more sensitive dependence of Na2SO4 could be explained as follows. The high alkaline mother liquid in the region of phase transformation from ZSM-5 to Mordenite contains only 14.4 mole of Na2SO4 and the relative crystallinity began to decrease rapidly after addition of 10 mole of extra Na2SO4, which corresponded to the total amount of 24.4 mole of Na2SO4. Different from the high alkaline mother liquid, the low alkaline mother liquid in the region of phase transformation from ZSM-5 to amorphous contains already 20.4 mole of Na2SO4 and the relative crystallinity began to decrease rapidly even after addition of 4 mole of extra Na2SO4, which corresponded to the total amount of 24.4 mole of Na2SO4. When the relative crystallinities, XRD patterns and micropore analysis are compared based on the total amount of Na2SO4, quite well coincided results are obtained. This means that if the total amount of Na2SO4 in mother liquid is higher than 24.4 mole based on 100 mole of SiO2 then the formation of ZSM-5 is severely interrupted and amorphous aluminosilicate is obtained.

(a)

Intensity (cps)

(b)

(c)

(d)

(e)

(f) 5

10

15

20

25

30

35

40

45

50

2Theta (deg.) Fig. 4. XRD patterns of solid products synthesized from the mother liquids described in Table 3a; (a) x = 4, (b) x = 6, (c) x = 8, (d) x = 10, (e) x = 12, (f) x = 14, where the molar composition = 18.0 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

14 mole of extra Na2SO4. As shown in Table 4, the ZSM-5s prepared with 0 and 8 mole of Na2SO4 revealed high BET surface area (about 430 m2/g) with high crystallinity but the ZSM-5 prepared with 14 mole of Na2SO4 revealed very low BET surface area (164 m2/g) with poor crystallinity. The pore volume also showed same dependence on Na2SO4 as that of BET surface area. If 14 mole of extra Na2SO4 was added to the mother liquid, the pore volume of solid product decreased rapidly from 0.16 to 0.60 cm3/g. Different from the pore volume, however, all the solid products revealed constant pore diameter independent of the amount of Na2SO4. This provides further evidence that the solid product is a physical mixture of crystalline ZSM-5 and amorphous phase. Chemical composition change was also monitored. Different from the relative crystallinity and BET surface area, it didn’t show any dependence on the content of Na2SO4. All of the solid products revealed constant SiO2/Al2O3 ratio of around 25 regardless of the added Na2SO4. This means that the condensation of silica and alumina occurred equally proportional to the mole composition of mother liquids independent of the Na2SO4 content. 3.2.2. Effect of Na2SO4 at 0.12 Na2O/SiO2 molar ratio To see the effect of Na2SO4 near the region of phase transformation from ZSM-5 to amorphous, that is, in the region of low Na2O content, 0–6 mole of Na2SO4 were added additionally to the mother liquid having the molar composition of 12.0 Na2O:20.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O and then synthesized at 170 °C for 24 h. The details of mother liquid composition and the results of relative crystallinity and solid product yield are described in Table 3b. Comparing Table 3a and b, it could be seen that the dependence of relative crystallinity on the Na2SO4 content is

3.3. Effect of Na2SO4 on crystallization time As can be seen in the results of previous section, the crystallinity of ZSM-5 is strongly influenced by the Na2SO4 in mother liquids. Therefore, the crystallinity and morphology of solid products were examined depending on crystallization time in the absence and presence of extra Na2SO4 (Table 5, Figs. 8 and 9). For this experiment, two types of mother liquids having low Na2SO4 (18.0 Na2O:14.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O) and high Na2SO4 (18.0 Na2O:28.4 Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O) were prepared and then crystallization was carried out at 170 °C for 6–24 h. Table 5 and Fig. 8 show the dependences of relative crystallinity, solid product yield and XRD patterns on the crystallization time. The crystallization rate at 28.4 mole of Na2SO4 was much faster than that at 14.4 mole of Na2SO4. Especially, the induction period of crystallization in high Na2SO4 is much shorter than that in low Na2SO4. That is, it took only 18 h to obtain high crystalline ZSM-5 having relative crystallinity of 99.7% in high Na2SO4 while it took 24 h to obtain high crystalline one in low Na2SO4. As shown in Fig. 9a and b, they also showed some differences in morphology. The more agglomerated solid product was obtained at high content of Na2SO4. The extent of agglomeration could be estimated easily from the SEM images at 12 h. The agglomerated particles were formed from the beginning of crystallization and they maintained their form continuously over the whole crystallization period. The condensation behavior could be inferred from the pattern of solid product yield depending on crystallization time. Both of the products revealed high solid product yield even at the beginning of crystallization time before the crystalline ZSM-5 was formed. Then their yields decreased rapidly when the crystalline ZSM-5 began to be formed. This means that amorphous precipitates are formed initially through the condensation reaction and then transformed to the crystalline ZSM-5 after having induction period for nucleation. According to Kalipcila [14], two types of crystallization routes have been proposed; one is a direct transformation of amorphous precipitates into a crystalline zeolite and the other is a mediated crystallization. In the latter case, the crystallization proceeds

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367

Fig. 5. SEM images of solid products synthesized from the mother liquids described in Table 3a; (a) x = 4, (b) x = 6, (c) x = 8, (d) x = 10, (e) x = 12, (f) x = 14, where the molar composition = 18.0 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

Table 4 The physical and chemical properties of ZSM-5 synthesized with different amount Na2SO4 Sample

x (Added Na2SO4, mol)

Relative crystallinity (%)

SiO2/Al2O3 ratio by EDS

BET (m2/ g)

Horvath–Kawazoe Pore diameter (Å)

Pore volume (cm3/g)

Molar composition = 18 Na2O:(x + 14.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O (a) 0 97.0 25.0 437.0 6.1 0.163 (b) 8.0 103.5 25.6 431.8 6.0 0.161 (c) 14.0 50.5 25.6 163.9 6.3 0.060

through two reaction steps; firstly amorphous precipitates dissolved into soluble species and then nucleation and crystal growth proceed separately from these soluble species. As can be seen in Fig. 9, only homogeneous species were found without any mixing of amorphous and crystalline phases even though their morphologies changed depending on the crystallization time. From this result, it could be suggested that the crystallization proceeds through direct transformation of amorphous precipitates into a crystalline zeolite in template-free synthesis of ZSM-5 from sodium silicate. That is, the observed agglomerated precipitates are true chemical or structural precursors to the final solid products as suggested by Cundy [17].

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Intensity (cps)

(a)

(b)

(c)

5

9

13

17

21

25

29

33

37

41

45

49

2Theta (deg.) Fig. 6. XRD patterns of solid products synthesized from the mother liquids described in Table 3b; (a) x = 2, (b) x = 4, (c) x = 6, where the molar composition = 12.0 Na2O:(x + 20.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

The reduced crystallization time in the presence of high Na2SO4 could be explained by the effect of oxyanions provided by Na2SO4 salt as suggested by Kumar et al. [18]. They evaluated the effect of  3 oxyanions such as ClO4 , PO3 4 and AsO4 in the synthesis of MFItype microporous materials. According to their results, the crystallization time decreased linearly until an optimum concentration of oxyanions reached, after which constant crystallization time remained. Luo and co-workers also observed decreased crystallization time in hydrothermal synthesis of EMT zeolite by the addition of sodium phosphate (Na3PO4  12H2O). When 0.2 mole of Na3PO4  12H2O was added to the mother liquid having the molar composition of (2.1 Na2O:10.0 SiO2:1.0 Al2O3:0.3 (18-crown-6): 140.0 H2O), the crystallization time decreased from 25 days to 6 days. This unusual result is ascribed to the phosphate acting as an emulsifying and water-structure-breaking agent [19]. Another thing to be concerned is that the relative crystallinity of solid product decreased rapidly from 99.8% to 52.4% at high concentration of Na2SO4 as the crystallization time increased from 18 to 24 h. The similar phenomenon was also observed in template-free synthesis of ZSM-5 with silicic acid and colloidal silica by Nastro et al. [20]. They explained that this decrease is caused by the decomposition of ZSM-5 because its stability range is narrower in template- free system relative to the template containing system. Shiralkar and Clearfield reported that eventually they transformed to more stable phases like Quartz and Mordenite type zeolites in the extend times of crystallization [7]. 3.4. Effect of Na2SO4 on condensation rate Even though the detail data is not included in this paper, an interesting thing was found during the aging time of mother liquid before autoclaving for hydrothermal synthesis. The mother liquids having more Na2SO4 gelated more rapidly than that of less one. If the mother liquid contained 28.4 mole of Na2SO4, then it became a hard gel and could not be stirred within an hour even at room temperature. This phenomenon motivated to monitor the particle size change of mother liquids depending on aging time of mother liquid. Because the concentration of original mother liquid is too high to measure particle size by light scattering method, it was di-

Fig. 7. SEM image of solid products synthesized from the mother liquids described in Table 3b; (a) x = 2, (b) x = 4, (c) x = 6, where the molar composition = 12.0 Na2O:(x + 20.4) Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

luted 4 times with water; that is, the content of water increased from 4000 mole to 16,000 mole. Two types of diluted mother liquids having low (14.4 mole) and high (28.4 mole) contents of Na2SO4 were prepared and then their particle size was monitored at room temperature. As shown in Fig. 10a and b, from the series of data, differences in particle size and its growing rate could be seen clear depending on the quantity of Na2SO4 in the mother liquids. In the case of the mother liquid having low Na2SO4, initially 7–8 nm sized fine particles were

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sumed that the Na2SO4 induces high nucleation then more fine particles have to be formed on the mother liquid with high Na2SO4 at the initial stage of crystallization. However, completely opposite result was obtained. That is, bigger particles, even the fraction is not so high, were obtained from the mother liquid with high Na2SO4. It is thought that the bigger particles are formed through the agglomeration of primary particles and it is mainly caused by the Na2SO4. In the SEM images of previous section, it was also discussed that the agglomeration of crystal was more pronounced in the presence of Na2SO4. Because the agglomeration of primary particles is closely related with condensation reaction, it could be suggested clearly that the addition of salt leads to the increase of condensation reaction. To see how much the condensation rate of silica is influenced by the Na2SO4, three different types of mother liquids having 14.4, 21.4 and 28.4 mole of Na2SO4 based on 18.0 Na2O:100.0 SiO2:2.0 Al2O3:4000.0 H2O were prepared, respectively and then the Si–O stretching bands were monitored by a FTIR in ATR mode. As can be seen in Fig. 11, the IR band intensities at 1050 and 1220 cm1 showed different time dependence depending on the concentration of Na2SO4. According to Kulkarni et al. and Chao et al. [21,22], these bands could be assigned to the symmetric and asymmetric stretching vibration of Si–O bonds, respectively, located at tetrahedral site of silica (the T–O linkage). As the concentration of Na2SO4 increased from 14.4 mole to 28.4 mole, the increasing rate of these two bands depending on time became more pronounced. However, the dependency of intensity change on the content of Na2SO4 was different. In the case of 14.4 mole to 21.4 mole of Na2SO4 where high crystalline ZSM-5 could be obtained, the increase of intensities were not so high even after 20 h of aging (Fig. 11a, b). However, in the presence of 28.4 mole of Na2SO4 where poor crystalline ZSM-5 was obtained, the high increase of intensity was observed (Fig. 11c). This means that there exists a

Table 5 Effect of Na2SO4 on crystallization time of ZSM-5 Time (hrs)

x = 14.40 (low Na2SO4)

x = 28.40 (high Na2SO4)

Relative crystallinity (%)

Relative crystallinity (%)

Solid yield (wt%)

Solid yield (wt%)

Molar composition = 18 Na2O:x Na2SO4:100 SiO2:2 Al2O3:4000 H2O 6 Amorphous 62.7 Amorphous 12 Amorphous 61.0 Amorphous 18 19.88 59.8 99.77 24 93.11 54.4 52.35

72.1 75.2 58.1 64.1

formed. Once formed, they grew very slowly and reached to 10 nm even after 7.5 h (Fig. 10a). Different from the low Na2SO4, the growing rate of particle in high Na2SO4 was quite high. Even at 0.5 h, a shoulder peak corresponding to 20 nm in size was detected together with main peak at 7–8 nm. The main peak disappeared completely after 2.5 h and the shoulder peak at 20 nm peak became a dominant one. The particles grew continuously and reached to about 50 nm in size at 7.5 h (Fig. 10b). The growing rate of particle in high Na2SO4 content was 5 times higher than that in low Na2SO4. Generally, the growing rate of particle in colloidal solution is controlled by two factors; nucleation rate and condensation rate, which have the same meaning as initiation and propagation in polymerization reaction. Usually, the nucleation step involves in early stage of crystallization and more fine particles are obtained if its rate is high. Different from the nucleation, the condensation reaction involves over the whole process of crystallization and much more related with particle size growing. Based on these concepts, the growing of particle size depending on the content of Na2SO4 could be explained. At 0.5 h, corresponding to the initial stage of crystallization, fine particles with narrow size distribution were obtained regardless of content of Na2SO4. If it is as-

a

b Time = 6 hrs

Time = 12 hrs

Time = 12 hrs

Time = 18 hrs

Time = 18 hrs

Time = 24 hrs

Time = 24 hrs

Intensity (cps)

Time = 6 hrs

5

10

15

20

25

30

35

2 Theta (deg.)

40

45

50 5

10

15

20

25

30

35

40

45

50

2 Theta (deg.)

Fig. 8. XRD patterns of solid products depending on crystallization time; the mother liquids have molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O, and (b) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

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Fig. 9. SEM images of solid products depending on crystallization time; the mother liquids have molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O, and (b) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O.

critical concentration of Na2SO4 in which the condensation rate of silica speeds up rapidly. As shown in crystallization field diagram of Table 1, it is thought the 24.4 mole of Na2SO4 is the critical concentration under which high crystalline ZSM-5 could be obtained. These results also well coincide with that of the particle size change where the aggregation of particle is more pronounced in the presence of 28.4 mole Na2SO4 (Fig. 10b). In the view point of chemistry, however, still it is not clear why condensation rate of alluminosilicate increased by the Na2SO4.

Therefore, more fundamental studies are required to verify the role of Na2SO4. 3.5. Cracking activity of ZSM-5s synthesized in different Na2SO4 concentration For the three types of ZSM-5s synthesized in three different content of Na2SO4, naphtha cracking reaction, one of representative reactions to evaluate catalytic performance of ZSM-5, was car-

371

Weight Fraction (%)

Weight Fraction (%)

Weight Fraction (%)

Weight Fraction (%)

N.Y. Kang et al. / Microporous and Mesoporous Materials 118 (2009) 361–372

50

a

Table 6 Naphtha cracking activity of over the ZSM-5s synthesized in different concentration of Na2SO4

b

40

Time = 0.5 hr

Time = 0.5 hr

30 20 10 0 50 40

Time = 2.5 hr

Time = 2.5 hr

30 20 10 0 50 40

Time = 5.0 hr

Time = 5.0 hr

30 20

Na2SO4 (mol)

Commerciala ZSM-5

14.4 (low)

22.4 (medium)

28.4 (high)

SiO2/Al2O3 (mol ratio) Relative crystallinity (%) Product yield (wt%) H2 CO CO2 CH4 C2= C2 C3= C3 C4 C5 BTX Others C2= + C3= C2=/C3=

29 100.0

50 97.0

50 103.5

50 50.5

0.1 0.8 0.6 9.1 18.7 8.1 15.9 7.8 8.2 3.5 21.1 3.0 34.6 1.2

0.1 1.0 0.5 9.7 19.4 8.9 15.6 8.8 7.5 2.6 20.8 1.4 35.0 1.2

0.1 0.9 0.5 9.3 20.2 7.4 16.7 6.3 7.1 3.1 24.2 1.0 36.9 1.2

0.1 0.6 0.2 7.2 14.9 4.9 15.3 4.0 7.1 6.0 18.0 5.0 30.2 1.0

Reaction condition: Naphtha/Steam = 2, WHSV = 2 h1, Temp.= 675 °C. a Commercial ZSM-5 produced by Albermarle corp. and having SiO2/Al2O3 of 29.

10 0 50 40

(Table 6). Light olefins yield (C2= + C3=) and Ethylene/Propylene ratio (C2=/C3=) are good barometers to decide catalytic properties of ZSM-5. 35.0% and 36.9% light olefin yields were obtained over the ZSM-5s synthesized in the presence of 14.4 mole and 22.4 mole of Na2SO4 while 30.2% yield was obtained over the ZSM-5 synthesized in the presence of 28.4 mole. That is, regardless of the content of Na2SO4, once ZSM-5s with high crystalline and high surface area are obtained their cracking activities are guaranteed. This means that the Na2SO4 in mother liquid only influences the degree of ZSM-5 crystallization and does not influence the physical and chemical properties of prepared ZSM-5. Through the naphtha cracking activity test, it could be suggested that the Na2SO4 in the mother liquid involves only in hydrothermal crystallization process and does not affect the properties of prepared ZSM-5.

Time = 7.5 hr

Time = 7.5 hr

30 20 10 0 1

100 1

10

10

Particle size (nm)

100

Particle size (nm)

Fig. 10. Particle size distribution depending on aging time of mother liquids having the molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:16000 H2O, and (b) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:16000 H2O.

ried out and then the results were compared with that of commercial ZSM-5 produced by Albermarle. As shown in Tables 4 and 6, the ZSM-5s synthesized in the presence of 14.4 mole and 22.4 mole of Na2SO4 revealed high crystallinity and surface area and is comparable to that of commercial ZSM-5 while that synthesized in the presence of 28.4 mole of Na2SO4 revealed low crystallinity and surface area. Depending on the crystallinity and surface area of prepared ZSM-5s, quite different cracking activities were observed

60

(a) 14.4 mole Na 2SO 4

4. Conclusion High crystalline ZSM-5 could be synthesized successfully using sodium silicate as a silica source by template-free method. For the synthesis of high crystalline ZSM-5, the content of Na2O in mother

50

Reflectance (%)

(c) 28.4 mole Na 2SO 4

(b) 21.4 mole Na 2SO 4 0 hr 1 hr 5 hr 20 hr

40

30

20

10

1300

1200

1100

Wavenumber (cm-1)

1000 1300

1200

1100

Wavenumber (cm-1)

1000 1300

1200

1100

1000

Wavenumber (cm-1)

Fig. 11. IR spectra of mother liquid having the molar composition of (a) 18.0 Na2O:14.4 Na2SO4:100 SiO2:2 Al2O3:4000H2O, (b) 18.0 Na2O:21.4 Na2SO4:100 SiO2:2 Al2O3:4000 H2O and (c) 18.0 Na2O:28.4 Na2SO4:100 SiO2:2 Al2O3:4000H2O depending on aging time at room temperature.

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liquid had to be controlled precisely by sulfuric acid and stoichiometric amount Na2SO4 corresponding to the added sulfuric acid was generated. The Na2SO4 in mother liquid influenced the properties of final product such as relative crystallinity, morphology, BET surface area and pore volume. It was found that the Na2SO4 prohibits the formation of crystalline ZSM-5 by the enhanced condensation reaction of silicate and it becomes critical if the total amount of Na2SO4 in mother liquid is higher than 24.4 mole based on 100 mole of SiO2. The enhanced condensation reaction could be explained by the oxyanion effect of SO2 4 generated by Na2SO4. Acknowledgements This research was supported by a grant from Carbon Dioxide Reduction & Sequestration Research Center funded by the Ministry of Science and Technology of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2008.09.016. References [1] N.Y. Chen, W.E. Garwood, F.G. Dwyer, Shape Selective Catalysis on in Industrial Applications, 2nd ed., Marcel Dekker, Inc., New York, 1996.

[2] R.J. Arauer, G.R. Landolt, US Patent 3,702,886, 1972. [3] R.W. Grose, E.M. Flanigen, US Patent 4,257,885, 1981. [4] F.Y. Dai, M. Suzuki, H. Takahashi, Y. Saito, Stud. Surf. Sci. Catal. 28 (1986) 223. [5] F.Y. Dai, M. Suzuki, H. Takahashi and Y. Saito, in: M.L. Occelli, H.E. Robson (Eds.), ACS Symp., Los Angeles, Ser. No. 398, 1989, p. 244. [6] W. Schwieger, K.H. Bergk, D. Freude, M. Hunger and H. Pfeifer, in: M.L. Occelli, H.E. Robson (Eds.), ACS symp., Los Angeles, Ser. No. 398, 1989, p. 274. [7] V.P. Shiralkar, A. Clearfield, Zeolites 9 (1989) 363. [8] A. Tissler, P. Polanek, U. Girrbach, U. Müller, K.K. Unger, in: H.G. Karge, J. Weitkamp (Eds.), Stud. Surf. Sci. Catal., vol. 46, 1989, p. 399. [9] C.J. Plank, E.J. Rosinski, US Patent 5,102,644, 1992. [10] N.P. Martinez, J.A. Lujano, N. Alvarez, F. Machado and C.M. Lopez, US Patent 5,254,327, 1993. [11] W.J. Kim, C.W. Lim, S.A. Lee, M.C. Lee, C.Y. Jeong, J. Korean Ind. Eng. Chem. 4 (1993) 776. [12] D.J. Klocke, US Patent 5,240,892, 1993. [13] M. Otake, Zeolites 14 (1994) 42. [14] H. Kalipcilar, A. Culfaz, Cryst. Res. Technol. 36 (2001) 1197. [15] P. Wang, B. Shen, J. Gao, Catal. Today 125 (2007) 155. [16] P.K. Jacobs, J.A. Marten, Stud. Surf. Sci. Catal. 28 (1986) 223. [17] C.S. Cundy, P.A. Cox, Micropor. Mesopor. Mater. 82 (2005) 1–78. [18] R. Kumar, P. Mukherjee, R.K. Pandey, P. Rajmohanan, A. Bhaumik, Micropor. Mesopor. Mater. 22 (1998) 23. [19] Y. Luo, J. Sun, W. Jhao, J. Yao, Q. Li, Chem. Mater. 14 (2002) 1906. [20] A. Nastro, F. Crea, D.T. Hayhurst, F. Testa, R. Aiello, L. Toniolo, Stud. Sci. Catal. 49 (1989) 321. [21] S.B. Kulkarni, V.P. Shiralkar, A.N. Kotasthane, R.B. Borade, P. Ratnasamy, Zeolites 2 (1982) 313. [22] K.J. Chao, T.C. Tasi, M.S. Chen, I.J. Wang, J. Chem. Soc. Faraday Trans. I 77 (1981) 547.