Effect of Hydrophobic Carbon Chain Length on the Crystal Structure of MCM-41

Chinese Journal of Chemical Engineering, 16(4) 631ü634 (2008)

Effect of Hydrophobic Carbon Chain Length on the Crystal Structure of MCM-41* ZHANG Guangxu (჆‫ڛ‬༜)1,2,**, TAO Ling (ඈঊ)1 and ZHANG Gaoyong (჆‫غ‬ဇ)2 1 2

School of Chemical Engineering , Wuhan University of Technology, Wuhan 430072, China College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China

Abstract The mesoporous molecular sieve (MCM)-41 using ionic liquid as template has been prepared. The typical template of ionic liquid was [C16mim]X. In this article, the use of 1-alkyl-3-methylimidazolium ([Cnmim]Br, where n˙12,14,16,18,20) salts as templates in the synthesis of MCM-41 is reported. The results showed that the synthesized MCM-41 had uniform pore diameter, high surface area and stable framework. The largest surface area of MCM-41 was the one prepared with [C14mim]Br as template. When using [C18min]Br as template, the narrowest pore distribution sample was obtained and the effect of surface tension of template solution to MCM-41 was first discussed. Keywords ionic liquid, MCM-41, surface tension, single template

1

INTRODUCTION

The discovery of molecular sieves by the Mobil Corporation [1] has attracted considerable interest in both the physical properties of these materials and their potential uses [2], such as sensing [3], catalysis [4] and drug delivery [5]. Mesoporous molecular sieve, MCM-41, possesses a hexagonal array of uniform mesopores structure [1]. Presently, many different templates have been used in the synthesis of MCM-41. During the last decade, interest in the field of ionic liquids has burgeoned. Thus, it has given rise to a wealth of intellectual and technological challenges and opportunities for the production of new chemical and extractive processes [68], fuel cells, batteries [9] and new composite materials [10, 11]. Room temperature ionic liquid (RTIL) has similar structure as that of a surfactant, a hydrophobic carbon chain and a hydrophilic group, with the same molecular directional assembly function. It has unique solvent properties and lacks volatility [1214]. In the hydrothermal synthesis of MCM-41, ionic liquid was used as novel template to improve the ordering of molecular directional assembly. It has been reported that the MCM-41 prepared with RTIL as template showed large surface area, good crystal structure and narrow pore distribution [15]. Adams synthesized MCM-41 using different kinds of ionic liquid and examined the effect of changing templates to the crystal and pore structure of MCM-41. Unfortunately, the reasons for these phenomena were not explored. In this work, MCM-41 zeolite will be synthesized by using [Cnmim]Br (n˙ 12, 14, 16, 18, 20) as template and identified by Infrared Ray spectra, X-ray diffraction and N2 adsorptiondesorption. The effects of alkyl chain length of the template on crystal structure and pore distribution of MCM-41 will be examined.

2 2.1

EXPERIMENTAL Materials

1-Bromododecane(C12H25Br)(AR),1-bromotetrad and 1-bromohexadecane ecane(C14H29Br)(AR) (C16H33Br)(AR) (Sinopharm Chemical Reagent Co., Ltd); 1-bromoctadecane (C18H37Br)(96%) and 1-bromoeicosane (C20H41Br)(98%) (USA); C4H6N2 (CR) (Linhai Kaile Chemical Factory, Zhejiang); tetrahydrofuran (THF)(AR) (Shanghai No.4 Reagent & H.V. Chemical Co., Ltd); tetraethyl orthosilioate (TEOS)(CR) (Tianjin Chemical Reagent Co., Ltd); NaOH(AR) & acetylsalicylic acid(AR) (Tianjin Tianli Chemical Reagent Co., Ltd); ethanol(AR) (Tianjin Guangcheng Chemical Reagent Co., Ltd). 2.2 Instrumentation and conditions The type of IR equipment was CYGNUS-100 made in Thermo Nicolet, America, with the KBr disk. Power X-ray diffraction was operated on D/MAX-rA using Cu KĮ radiation with electric current 20 mA and voltage 30 kV. Nitrogen adsorption-desorption isotherms were measured on ASAP 2000 automatic physics adsorption instrument at approximately ˉ 200°C. Barrett-Joyner-Halenda (BJH) arithmetic was used to analyze the pore morphologies. BrunaureEmmet-Teller (BET) equation was used to calculate the surface areas. The surface tension was collected by wettability tester of Krnjss K100 at 40°C. 2.3

Preparation of [Cnmim]Br

The ionic liquid [Cnmim]Br was synthesized according to Adams’ method [15]. The mixed solution of 1-methylimidazole and CnH2n+1Br(n˙12, 14, 16, 18,

Received 2007-08-06, accepted 2008-03-26. * Supported by the Natural Science Foundation of Zhengzhou Province (064SJZJ23115) and the Key Laboratory of Catalysis and Materials Science of South-Central University for Nationalities Foundation. ** To whom correspondence should be addressed. E-mail: [email protected]

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20) of equivalent mole was heated to 120°C and stirred for 48 h. If the reactant was solid, tetrahydrofuran (THF) was used as solvent. The products were purified by recrystallization with THF [16]. The white crystals were collected by filtration and dried under vacuum at room temperature. The reaction scheme is shown in Fig. 1.

Figure 1

2.4

3.1

ˉ

ˉ

ˉ

The reaction equation of synthesis [Cnmim]Br

Preparation of MCM-41

[Cnmim]Br was first dissolved in NaOH solution (pH 1011). Water was added to make the total volume to 450 ml. The template solution was stirred and heated. When the temperature reached 80°C, TEOS was dropped slowly. After stirring for 2 h at 80°C, the mother gel was aged at 80°C for 24 h. The product was filtered with water and ethanol and dried and calcined at 540°C for 5 h to remove the template. 3

2853cm 1, 721 cm 1, and 648 cm 1 corresponded to almost the same groups compared with the curve (Cƍ) except for slight shifting. Thus, it was shown that the product was [C16mim]Br. Figure 3 shows the IR diagrams of [Cnmim]Br. Compared with [C16mim]Br, most peaks had almost the same position and intensity. Thus, it can be concluded that the products were the right ones.

RESULTS AND DISCUSSION The IR diagrams of RTILs

The production was verified by IR spectra technique at normal pressure and temperature. The IR diagrams of C16H33Br and [C16mim]Br are shown in Fig. 2. ˉ On the curve (Cƍ), the bands at 3479 cm 1 and ˉ1 3431 cm can be attributed to the C N H and ˉ N H stretching vibration. The bands at 3062 cm 1 can be assigned to C H stretching vibration. The ˉ bands at 2916 cm 1 corresponded to the C H of methylene asymmetrical stretching vibration, and the ˉ bands at 2851 cm 1 belonged to the C H symmetriˉ cal stretching vibration. The bands at 1474 cm 1 can be assigned to CH2 deformation vibration. The ˉ (CH2)n (n˚4) rocking vibration at 716 cm 1 could ˉ1 be clearly seen. The bands at 626 cm were assigned to ˉ C Br group. On the curve(C), the bands at 2956 cm 1,

Figure 3 IR diagrams of RTILs Aƍ ü [C12mim]Br; Bƍ ü [C14mim]Br; Cƍ ü [C16mim]Br; Dƍ ü [C18mim]Br; Eƍü[C20mim]Br

3.2 The effect of changing hydrophobic carbon chain to crystal structure Synchrotron X-ray powder diffraction data for the five samples is shown in Fig. 4. They were tested at normal pressure and temperature. On the curve (a), the first peak (100) can be seen but its intensity was weak. The second peak (110) and third one (200) almost disappeared. Sample b’s crystal structure was nearly the same with sample a. Sample c had the most obvious three peaks, whereas the first peak (100) was acute and symmetrical with the strongest intensity, the other two peaks (110), (200) were also clearly seen at right coordinates. Compared with sample c, samples d and e had clear first peaks (100). However, the second (110) and third peaks (200) were combined. It can be inferred that their structure might have more defect lattice than that of sample c. In conclusion, sample c had the best crystal structure. The crystal structures of samples d and e were better than samples a and b. The change of crystal structure with alkyl chain lengths was not linear.

Figure 4 The XRD patterns of samples a, b, c, d, and e template: aü[C12mim]Br; bü[C14mim]Br; cü[C16mim]Br; dü[C18mim]Br; eü[C20mim]Br

3.3 The effect of changing hydrophobic carbon chain to pore distribution and surface area Figure 2 IR diagrams of C16H33Br and [C16mim]Br CüC16H33Br; Cƍü[C16mim]Br

Figure 5 shows nitrogen adsorption isotherm

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Chin. J. Chem. Eng., Vol. 16, No. 4, August 2008 Table 1

Sample

BET surface area ˉ /m2·g 1

a

866.4

0.448

2.081

b

1003

0.887

3.024

Figure 5 Nitrogen adsorption-desorption isotherm curves of samples a to e template: aü[C12mim]Br; bü[C14mim]Br; cü[C16mim]Br; dü[C18mim]Br; eü[C20mim]Br

3.4 curves of samples a to e. The test temperature was ˉ196°C. They all gave a type IV reversible nitrogen adsorption-desorption isotherm, indicating samples a to e were MCM-41 [17, 18]. Fig. 6 shows the pore distributions of samples a, b, c, d, and e. Pore-distributions of MCM-41, which were prepared with different hydrophobic carbon chain as templates, were all narrow. Sample d had the best pore distribution among the five for the highest pore volume and the acute peak (Fig. 6). Moreover, sample a’s pore distribution concentrated from 2 to 2.5 nm, whereas sample e’s pore distribution concentrated from 3.5 to 3.8 nm. It can be seen that pore diameter of MCM-41 gradually increased with the augment of hydrophobic carbon chain. Thus, it is seen that the preparation mechanism of MCM-41 abided by liquid-crystal templating theory [1].

Figure 6 The pore-distribution curves of samples a, b, c, d, and e template: aü[C12mim]Br; bü[C14mim]Br; cü[C16mim]Br; dü[C18mim]Br; eü[C20mim]Br

Table 1 shows the surface area and pore structure of samples a to e. The BET surface area of each samˉ ple exceeded 850 m2·g 1 and sample b’s BET surface area was larger than others. From the data of pore volume (Table 1), it can be seen that sample d had the largest pore volume due to the acute and intensive peak (Fig. 6). This result was in accordance with Trewyn [16]. The pore volume of samples a to c increased, whereas the pore width of the samples augmented gradually. It was also observed that position of the peak had a slight right-moving tendency (Fig. 6). In conclusion, BET surface area and the pore volume did not seem to relate with the alkyl chain lengths consequentially. But, there was a proportional relationship existing between the alkyl chain lengths and the pore width. To explore the microcosmic cause, the surface tension was examined later to study the inner relationship.

The effect of surfactant on surface area and pore structure BJH adsorption cumulative pore volˉ ume/cm3·g 1

BET adsorption average pore width/nm

c

875.8

0.894

3.703

d

978.8

1.105

4.189

e

881.1

1.052

4.488

The surface tension of RTILs solution

After [Cnmim]Br was dissolved in NaOH solution completely, each surface tension by wettability tester of Krnjss K100 at 40°C was measured. Table 2 shows the surface tension of RTILs solution tested at 40°C. The surface tension decreased when the alkyl chain lengths increased. From Table 1 it can be seen that BET adsorption average pore width became larger when alkyl chain lengths increased. The average pore width reached to the largest when using [C20mim]Br as template. At the same time its surface tension was the least. The surface tension originates from clean suction, which depends on gravitation between them [19]. If the surface tension was small, the gravitation would be weak, leading to loose interpacking of the molecule. According to liquid crystal templating theory [1], template molecules’ packing was crucial to the micelle diameter, which would become the pore width after calcining. Thus, pore width would be larger if the packing of template molecules was less tighter. For this reason MCM-41 prepared with [C20mim]Br as template had the biggest pore width. There is, thus, a positive relationship between the MCM-41’ pore width and the alkyl chain lengths. The pore width of MCM-41 would be bigger if the lipophilic group was longer and hydrophilic group was shorter, which means the template solution polarity was smaller macroscopically, in the structure of RTIL molecule. Table 2

The surface tension of RTILs solution

Template

4

ˉ1

Surface tension/mN·m

[C12mim]Br

37.43

[C14mim]Br

37.39

[C16mim]Br

35.04

[C18mim]Br

34.41

[C20mim]Br

34.25

CONCLUSIONS

Mesoporous molecular sieve, MCM-41, could be successfully synthesized using [Cnmim]Br (n ˙ 12,14,16,18,20). They all had narrow pore distribution. The mesoporous molecular sieve, MCM-41, synthesized with [C16mim]Br as template had the best crystal structure, whereas the one obtained with [C14mim]Br as template had the largest BET surface area, and the

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pore width augmented with the increase of the alkyl chain lengths. The surface tension of the template solution was tested. They reduced with the increase of the alkyl chain lengths. This factor plays an important role in the pore width of MCM-41. The main reason for this was the clean suction between the template molecules. REFERENCES 1

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