role of crystal growth modifiers in the synthesis of zsm-12 zeolite

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Advanced Powder Technology xxx (2014) xxx–xxx

APT 887 No. of Pages 5, Model 5G

1 October 2014

Contents lists available at ScienceDirect

Advanced Powder Technology

journal homepage: www.elsevier .com/locate /apt

Original Research Paper

Role of crystal growth modifiers in the synthesis of ZSM-12 zeolite

http://dx.doi.org/10.1016/j.apt.2014.09.0070921-8831/� 2014 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

⇑ Corresponding author at: Chemical Engineering Department, King Fahd Univer-sity of Petroleum & Minerals, Dhahran 31261, Saudi Arabia. Tel.: +966 13 860 7612.

E-mail address: [email protected] (O. Muraza).

Please cite this article in press as: M.A. Sanhoob et al., Role of crystal growth modifiers in the synthesis of ZSM-12 zeolite, Advanced Powder Tech(2014), http://dx.doi.org/10.1016/j.apt.2014.09.007

Mohammed A. Sanhoob a,b, Oki Muraza a,b,⇑, Eid M. Al-Mutairi a, Nisar Ullah c

a Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabiab Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabiac Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

a r t i c l e i n f o

293031323334353637383940

Article history:Received 9 April 2014Received in revised form 1 September 2014Accepted 19 September 2014Available online xxxx

Keywords:Crystal growth modifiersPolyethylene glycolBrij-35One-dimensional zeolitesZSM-12

a b s t r a c t

The effects of crystallization time, temperature and aging time on the synthesis of pure ZSM-12 in thepresence of polyethylene glycol (PEG) and polyoxyethylene Brij-35 surfactant with fixed ratios of Si/Al(160) and Na/SiO2 (0.144) were systematically investigated. Larger synthesis window and high purityof ZSM-12 were expected. Pure phase of ZSM-12 was obtained when PEG was added to the synthesis mix-ture with PEG/Al2O3 ratio in the range of 0.4–2. Furthermore, the addition of both Brij-35 and PEG in thesynthesis was found to play a significant role in controlling the crystallization rate. The Brij-35/Al2O3 ratiowas varied from 0 to 4.9. Conventional ZSM-12 was typically synthesized at 145 �C for 120 h. Shortercrystallization time of 60 h was achieved when PEG/Al2O3 ratio of 1.22 and a Brij-35/Al2O3 ratio of1.61 was used in the synthesis.

� 2014 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rightsreserved.

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1. Introduction

Zeolites have been used industrially as catalyst and sorbent inshape-selective conversion of hydrocarbon [1,2]. Modification ofzeolite properties can be achieved by varying the synthesis para-meters such as the type of organic structure directing agent(OSDA), the sources of silica or aluminum and by making a combi-nation of different sources [3]. Several types of OSDA have beenused in the synthesis of ZSM-12 (MTW) which have led to differentcrystal morphologies such as cubic, rice, needle and hexagonal[4,5]. The synthesis window of zeolites can be expanded by addingdifferent growth modifiers to the synthesis mixtures [6–8]. Thesecrystal-growth modifiers were mostly solvents, surfactants orpolymers [9,10]. ZSM-12 has a one-dimensional pore system withelliptic opening formed by 12 tetrahedra TO4 (T = Al or Si), withdiameter of 0.56 � 0.62 nm [11]. This pore size is slightly largerthan that of MFI [12]. The acid form of ZSM-12 has been used incatalysis for various transformations of hydrocarbon in refiningof petroleum, such as cracking, hydrocracking, alkylation, and iso-merization [13].

The objective of this study was to investigate the effect of modi-fiers on the purity of ZSM-12. We studied two different growth

modifier; PEG and Brij-35. The optimum condition was obtainedwhen PEG and Brij-35 were added together in the synthesis ofZSM-12. During the course of our studies, widen synthesis windowand reduced synthesis time in obtaining crystalline ZSM-12 wereachieved at the fixed Si/Al and Na/Si ratios.

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2. Experimental

2.1. Reactants

The following chemicals were used. Colloidal silica (40 wt.% inwater, Nissan Chemicals) and aluminum sulfate octahydrate (Al2

(SO4)3�18H2O) were used as silica and aluminum sources respec-tively. Tetraethyl ammonium bromide (TEABr, 98%) was used asan organic structure directing agent (OSDA). Sodium hydroxide(NaOH) was used as alkaline source.

2.2. Synthesis of ZSM-12

In a typical procedure, a solution of 0.97 g of sodium hydroxidein 12.61 g of deionized (DI) water, 0.35 g of aluminum sulfateoctahydrate and 4.45 g of TEABr were added together and the reac-tion mixture was stirred until it became homogeneous. In anotherbeaker, 25.21 g of colloidal silica was added to 10.09 g of DI waterand, after being stirred for 2 min, aluminate solution was added tosilicate drop wise. The reaction mixture was stirred vigorously for

nology

http://dx.doi.org/10.1016/j.apt.2014.09.007

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.u.]

Cristobalite Cristobalite ZSM-5

2 [o]θ

Fig. 1. XRD patterns of zeolites synthesized with different PEG/Al2O3 at 145 �C for90 h, (a) PEG/Al2O3 = 3.1, (b) PEG/Al2O3 = 2.0, (c) PEG/Al2O3 = 1.2, (d) PEG/Al2O3 = 0.4, (e) without PEG.

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y [a

.u.]

2 [o]θ

Fig. 3. XRD patterns of zeolites synthesized with different Brij-35 concentration atPEG/Al2O3 = 1.2 and 145 �C for 90 h, (a) Brij-35/Al2O3 = 4.9, (b) Brij-35/Al2O3 = 3.3,(c) Brij-35/Al2O3 = 1.6, (d) without Brij-35.

Table 1Average crystallite size of ZSM-12 calculated using Scherrerequation.

Brij-35/Al2O3 Crystallite sizea (nm)

0 31.311.6 20.143.3 25.884.9 25.91

a Average crystal size has been determined by Scherrerequation using XRD results.

2 M.A. Sanhoob et al. / Advanced Powder Technology xxx (2014) xxx–xxx


1 October 2014

2 min followed by the sequential addition of 6.34 g of polyethyleneglycol (average molecular weight of 10,000) and 1.0 g of Brij-35 tothe reaction and the mixture was stirred vigorously at 65 �C for90 min. The gel composition was 1 SiO2:0.124 TEABr:0.003Al2O3:0.072 Na2O:12.574 H2O:0.0038 PEG:0.005 Brij-35. In caseof Brij-35 absence, the gel mixture was stirred at ambient tem-perature. The synthesized gel was placed in a sealed 100 mlPTFE-lined stainless steel autoclaves and heated to 145 �C for90 h. The product was repeatedly washed with DI water until theneutralization. The samples were eventually dried in a vacuumoven at 105 �C. The organic template was removed by calcinationunder air flow for 21 h by ramping the temperature from ambienttemperature to 350 �C for 3 h followed by dwelling the samples at350 �C for 3 h. The samples were further heated to 550 �C in 3 husing a heating rate of 1.1 �C per min and were subsequentlydwelled at 550 �C for 12 h.

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2.3. Ion exchange under microwave irradiation

Ion exchange was performed under microwave irradiation. Foreach 1 g of the zeolite, 20 g of 2 M aqueous ammonium nitratesolution was added. The ion exchange was carried out at 85 �Cfor 10 min. Finally, the samples were calcined again at 550 �C.

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2.4. Characterization

The crystallinity and the phase identification were determinedby using X-ray diffraction (XRD) with Cu Ka radiation in the periodof 2h = 5–50�. The scanning step was 0.02 with scanning speed of3� per min. Crystal morphologies and particle sizes wereinvestigated using field-emission scanning electron microscopy(FE-SEM).

1 m 1 m

MCN3-145-90H MCN2-145-9

µ µ

Fig. 2. FE-SEM micrograph of MTW zeolites with Si/Al of 160, NaOH/Si of 0.1

Please cite this article in press as: M.A. Sanhoob et al., Role of crystal growth m(2014), http://dx.doi.org/10.1016/j.apt.2014.09.007

3. Results and discussion

3.1. Effect of polyethylene glycol on phase purity of ZSM-12

Polyethylene glycol was used in ZSM-12 synthesis as a crystalmodifier based on its solubility and hygroscopic properties. PEG,characterized by its long chain due to repeating ethylene glycolunits, is known for its higher flexibility and hydrophilic naturewhich make it useful in various biological, chemical and pharma-ceutical settings. Wider window of framework stability can beobtained by adding PEG. Without PEG, similar composition pro-duced impurities of ZSM-5. This impurity was totally suppressedwhen PEG was added. We expected that the addition of PEG willimprove the zeolite synthesis by enhancing the nucleation rateas well as facilitating the ZSM-12 zeolite synthesis by increasingthe crystallization rate. As a result of this improvement, the nucleiwill be more stable, which means they are ready for the growthstep and this will lead to a shorter time required for the synthesis.From previous experiments, we noticed that ZSM-12 zeolite cannotbe obtained in the absence of the OSDA (TEABr) at the same com-position and at the same temperature.

1 m

0H MCN1-145-90H

µ

44 at 145 �C, (a) PEG/Al2O3 = 2.0, (b) PEG/Al2O3 = 1.2, (c) PEG/Al2O3 = 0.4.

odifiers in the synthesis of ZSM-12 zeolite, Advanced Powder Technology


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MCN4-145-90HMCN7-145-90HMCN8-145-90H

1 m µ I m µ 1 m µ

Fig. 4. FE-SEM micrograph of MTW zeolites at different Brij-35 concentration with Si/Al of 160, NaOH/Si of 0.144 and PEG/Al2O3 = 1.2 at 145 �C, (a) Brij-35/Al2O3 = 4.9, (b)Brij-35/Al2O3 = 3.3, (c) Brij-35/Al2O3 = 1.6.

y [ a

.u.]

M.A. Sanhoob et al. / Advanced Powder Technology xxx (2014) xxx–xxx 3


1 October 2014

Purified PEG is available commercially as mixtures of differentoligomer sizes in broadly or narrowly defined molecular weight(MW) ranges. In our experiments we used a white flake polyethy-lene glycol with average molecular weight of 10,000 g mol�1 [14].

Pure ZSM-12 zeolite can be obtained by tuning the alkalinityand silica to aluminum ratio [15]. Other method to reduce theimpurity can be obtained by adding some modifiers such as poly-ethylene glycol (PEG). Different PEG concentration was added tothe synthesis to study the effect of modifier concentration on theproduced phases of ZSM-12. All experiments were carried out withfixed silica to aluminum ratio (Si/Al) of 160 and alkalinity ratio(NaOH/Si) of 0.144 at 145 �C and with a synthesis time of 90 h.Our objective was to develop a reliable synthesis strategy to obtainpure ZSM-12 without impurity. ZSM-5 and cristobalite are com-mon impurities in ZSM-12. When the PEG/Al2O3 ratio wasincreased from 0 to 0.4, the ZSM-5 impurity was disappeared com-pletely. The pure ZSM-12 phase was also obtained by furtherincreasing PEG/Al2O3 ratio up to 2. However, further increasingthe PEG/Al2O3 ratio to 3.1 enhanced the appearance of cristobaliteas a minor phase. The molar ratio can be represented by 1SiO2:0.124 TEABr:0.003 Al2O3:0.072 Na2O:12.574 H2O:y PEGwhere y was varied between 0 and 0.0094. Our results were inagreement with other previous reports which revealed that cristo-balite minority can be produced at higher alkaline concentration orat higher temperature [4].

XRD patterns for as-synthesized ZSM-12 zeolite with differentconcentration of PEG are shown in Fig. 1. In the absence of PEG, for-mation of ZSM-5 as a minor impurity was observed, indicated bythe small peak at 2-theta of 24.2�. By adding PEG, minor ZSM-5phase was totally removed. At high concentration of PEG (thePEG/Al2O3 ratio of 3.1), clearly an intense peak of cristobalite isobserved at 2-theta of 22.1�. Fig. 2 shows the corresponding FE-SEM micrographs of the MTW zeolite synthesized by adding differ-ent concentration of PEG at 145 �C for 90 h. The morphology of thesecondary particle of ZSM-12 in the presence of PEG is elongatedcross-hexagonal shape crystals with average size of 1200 nm.However, the primary particle size is around 150 nm. Adding PEGto ZSM-12 promoted the agglomeration rate as well as the growthrate (Fig. 2). In the presence of PEG, we observed that the shape ofZSM-12 zeolite particles was uniform.

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2 [o]θ

Fig. 5. XRD patterns of zeolites with different synthesis time with PEG/Al2O3 = 1.2,Brij-35/Al2O3 = 1.6 at 145 �C, (a) 90 h, (b) 70 h, (c) 60 h, (d) 45 h, (e) 40 h, (f) 36 h.

3.2. Effect of polyethylene glycol and surfactant on phase purity ofZSM-12

Surfactants play an important role in zeolite modification. Thetype of surfactants can be ionic and nonionic. Surfactants havetwo molecules groups which represent the hydrophobic andhydrophilic properties [16,17]. In our experiments of ZSM-12zeolite, a nonionic polyoxyethylene glycol alkyl ether


(polyoxyethylene (23) lauryl ether, also called Brij-35), was usedas a crystal modifier beside PEG. The silica to aluminum ratio,the alkalinity ratio (NaOH/Si) and PEG ratio (PEG/Al2O3) were keptconstant, while the Brij-35 to aluminum ratio (Brij-35/Al2O3) wasvaried between 0 and 4.9. With this composition, pure ZSM-12was obtained for the synthesis duration of 90 h. The molar ratioof synthesis gel was 1 SiO2:0.124 TEABr:0.003 Al2O3:0.072Na2O:12.574 H2O:0.0038 PEG:x Brij-35 where x was variedbetween 0 and 0.015. During the addition of Brij-35, the solutionwas stirred at 65 �C to facilitate the dissolving of Brij-35.

Favorable results were observed when both BEG and Brij-35were added to the synthesis mixture as crystal-growth modifiers.A further investigation is required to investigate whether ZSM-12can be produced at different silica to aluminum ratio as well as dif-ferent alkaline cationic concentration which may lead to simplerZSM-12 synthesis with shorter synthesis time and higher purityphase.

XRD patterns for as-synthesized ZSM-12 with different concen-tration of Brij-35 are shown in Fig. 3. It is clear that all peaks rep-resenting pure ZSM-12 are observed in all XRD patterns. There issome mathematical equation can be used to calculate the crystalsize such as Scherrer equation [18]. Scherrer equation was usedto calculate the crystallite size using the XRD data. There was noobservable change when Brij-35 was added to the synthesis mix-ture except the fact that the average of the crystallite size ofZSM-12 was smaller than those prepared with PEG only as shownin Table 1. The corresponding FE-SEM micrographs of the ZSM-12samples with different concentration of Brij-35 synthesized at145 �C for 90 h are shown in Fig. 4. The size of the secondary par-ticle of ZSM-12 in the presence of PEG and Brij-35 is similar to thesize of ZSM-12 particles in the presence of PEG. Still further inves-tigation is required to investigate the best approach to reduce theagglomeration rate.



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Fig. 6. FE-SEM micrograph of MTW zeolites with different synthesis time with PEG/Al2O3 = 1.2, Brij-35/Al2O3 = 1.6 at 145 �C, (a) 90 h, (b) 70 h, (c) 60 h.

ZSM-5

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.u.]

2 [o]θ

Fig. 7. XRD patterns of zeolites with different aging time with PEG/Al2O3 = 1.2, Brij-35/Al2O3 = 1.6 at 145 �C, (a) 24 h, (b) 12 h, (c) 1.5 h.

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3.3. Effect of synthesis time and aging time

Further investigation on the phase purity of ZSM-12 has beenstudied by shortening the synthesis time gradually in the presenceof PEG and Brij-35. Synthesis time was varied between 36 and 90 hwith fixed alkalinity concentration as well as fixed silica to alu-minum ratio. We observed that pure ZSM-12 can be obtained evenif the synthesis time was reduced to 60 h at a silica to aluminumratio of 160, an alkalinity ratio of 0.144, a PEG ratio of 1.22 and aBrij-35 ratio of 1.61. Further reduction in synthesis time to 36 hled to an amorphous material. Longer synthesis time was reportedfor MTW zeolite with synthesis time greater than 120 h [5,12,19–21]. However, this reduced synthesis time was further enhancedin the presence of both PEG and Brij-35, which promote the crys-tallization rate of the zeolite.

1 m µ

Fig. 8. FE-SEM micrograph of MTW zeolites with different aging time with Si/Al


All XRD results for pure ZSM-12 with synthesis time variationare shown in Fig. 5. According to FE-SEM images in Fig. 6, the sizeof ZSM-12 crystals was almost identical for the synthesis time inthe range of 60–90.

Aging time was studied in the range between 1.50 and 24 h.When the aging time was prolonged to 12 h, a trace of impuritywas observed, the 2-theta was between 23.7� and 25�. However,further increase in the aging time from 12 to 24 h had led to thedisappearance of that impurity. XRD patterns of ZSM-12 sampleswith different aging time are shown in Fig. 7. The correspondingFE-SEM micrographs of the samples with aging time variation areshown in Fig. 8. Both particle size and morphology of samplesare almost identical to the one that aged for 90 min. Xu et al.reported the effect of PEG on the synthesis of ZSM-5 (MFI) [10].The addition of PEG promoted nucleation faster crystal growth[10]. This report is in line with our findings for ZSM-12 (MTW)zeolites.

4. Conclusions

Pure ZSM-12 zeolite can be synthesized from a mixture withmolar ratio of 1 SiO2:0.1235 TEABr:0.0031 Al2O3:0.072Na2O:12.574 H2O:0.0038 PEG:x Brij-35 where x is between 0 and0.015. The effects of PEG/Al2O3 molar ratio, Brij-35/Al2O3 molarratio and crystallization time on crystallinity and phase-puritywere investigated. The nucleation of impurities was suppressedby the addition of PEG and Brij-35 altogether. Synthesis time wasreduced from 120 h to 60 h at Si/Al of 160 and Na/Si of 0.144.The effect of prolonged aging time up to 24 h was negligible. Themorphology of the secondary particle of ZSM-12 in the presenceof PEG and Brij-35 is an elongated cross-hexagonal shape crystalswith average size of 1200 nm and the primary particle size isaround 150 nm.

1 m µ

of 160, NaOH/Si of 0.144 and PEG/Al2O3 = 1.2 at 145 �C, (a) 24 h, (b) 1.50 h.



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Acknowledgements

The authors would like to acknowledge the support provided byKing Abdul-Aziz City for Science and Technology (KACST) throughthe Science & Technology Unit at King Fahd University of Petro-leum & Minerals (KFUPM) for funding this work through ProjectNo. 11-NAN2166-04 as part of the National Science, Technologyand Innovation Plan. We also thank Dr. Abbas Hakeem for FE-SEM sessions.

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role of crystal growth modifiers in the synthesis of zsm-12 zeolite

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