394 Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors)

© 2004 Elsevier B.V. All rights reserved.

PREPARATION OF ORDERED MESOPOROUS ALUMINOSILICATE USING CARBON MESOPOROUS

MATERIALS AS TEMPLATE

Sakthivel, A.\ Huang, S.\ Chen, W.\ Kim, T.^ Ryoo, R.^ Chiang, A.S.T.^ Chen, K.̂ and Liu, S}

^ Institute of Atomic and Molecular Sciences, Academia Sinica, PO Box 23-166, Taipei, Taiwan 106, R.O.C. Fax: +886-2-23620200. E-mail: sbliu@sinica.edu.tw

^National Creative Research Initiative Center for Functional Nanomaterials, Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea.

^Department of Chemical and Materials Engineering, National Central University, Chung-Li, Taiwan 320, R.O.C.

ABSTRACT

A novel method for preparing ordered replicated mesoporous-microporous composite materials (RMM) using carbon mesoporous molecular sieves as template is reported. In particular, the novel aluminosilicate material (RMM-1), synthesized by replication of carbon mesoporous molecular sieve CMK-1 using ZSM-5 zeolite precursor, is found to possess a structure analogous to MCM-48 molecular sieve.

INTRODUCTION

Mesoporous molecular sieves, whose structure consists of uniform channels with tuneable pore size, wall thickness and morphology, are known to possess high surface area, high hydrocarbon sorption capacity, and high thermal and hydrothermal stabilities [1-3]. These unique properties render their applications in a wide range of areas, for examples, as catalysts or supports in sorption/separation processes [1-2]. Nonetheless, the lack of strong acid sites in most conventional mesoporous molecular sieves normally limits their applications as solid acid catalysts [1]. Recent developments in synthesizing mesoporous-microporous composite materials, which possess mesoporous pore structure with microporous walls, have drawn much research attention [4-10]. A recent breakthrough in improving hydrothermal stability and acidity of mesoporous molecular sieves has been made by one-step synthesis using nano-sized zeolite seeds [7-10]. The fact that most of the aforementioned one-step preparation methods invoke low pH synthesis conditions in addition to the difficulties in introducing the hetero-atoms onto the matrix clearly put additional constraints in terms of their mass production and commercial applicability. Thus, the development of novel routes to synthesize three-dimensional meso-/microporous composites is a demanding task. Jacobsen et al. demonstrated the synthesis of zeolite single crystals having a uniform size, high BET surface area and good acidity under confined space of either carbon black [11] or multi-wall carbon nanotubes [12]. More recently, the syntheses of mesoporous ZSM-5 monolith and mesoporous zeolite crystals by using carbon aerogel [13] and colloid imprinted carbon [14], respectively, have also been reported. Alternatively, the syntheses of highly ordered mesoporous silicates (SBA-15 and HUM), regenerated by reversible replication of carbon mesoporous molecular sieve, such as CMK-3 [15, 16] and CM48T-C [17], have also been illustrated. In the present work, we report the first successful synthesis of ordered replicated mesoporous-microporous aluminosilicate molecular sieves (RMM-1) with zeolite secondary building units through replication of carbon mesoporous molecular sieve CMK-1.

EXPERIIMENTAL

Mesoporous carbon molecular sieve CMK-1 was synthesized according to the procedure described elsewhere [18]. Replicated meso-/microporous composite materials (RMM-1) with varied Si/Al ratio of 50, 100, 150 were prepared by the following steps: (1) ca. 1 g of carbon mesoporous molecular sieve CMK-1 (supplied by KAIST) [18] was first activated in air at 373 K for 1 h; (2) ca. 10 mL of ethanolic solution containing tetraethyl orthosilicate (TEOS; 4 mL) and aluminium isopropoxide (Al(ip)3) were introduced into the activated carbon material. The amounts of Al(ip)3 used in preparing samples with Si/Al ratio of 50, 100,

395

and 150 were 0.075, 0.037 and 0.025 g, respectively; (3) the resultant gel was subjected to aging at 313 K for 24 h to form aluminosilicate/CMK-1 composite; (4) ca. 25 wt% tetrapropyl ammonium hydroxide (TPAOH; 7 mL) were added before further aging for one day at the same temperature; (5) the excess amount of TPAOH was filtered and washed by distilled water then by ethanol; (6) the resultant composite was then allowed to crystallize in the presence of saturated steam at 373 K for 1-5 days in an autoclave; (7) finally, the as-synthesized CMK-l/RMM-1 composite (RMM-la) was calcined in air at 853 K for 6 h to obtain carbon-free sample (denotes as RMM-lc). Note that unlike most mesoporous materials synthesized via alkali route, using TPAOH as hydrolysis agent not only warrants the formation of zeolite secondary building units in the framework but also leads to immediate creation of acid sites after calcination. Samples are designated as RMM-l(x)-y, where x indicates the number of days steamed and y represents the Si/Al molar ratio in the synthesis gel.

Several experimental techniques have been used to characterize the structural and physical/chemical properties of the synthesized samples, including XRD, SEM, FTIR, BET surface area analysis, hyperpolarized (HP) ^^^Xe-NMR and ^̂ P MAS NMR of adsorbed phosphine oxide probe molecules [19].

RESULTS AND DISCUSSION

All of the as-synthesized samples have the same appearance as black powder but tuned into pure white colour indicating that the aluminosilicate composite materials are present in the channel of the CMK-1 carbon molecular sieve before calcinations. Whereas, when the synthesis is carried out under the presence of excess TEOS or a prolonged aging time, the as-synthesized sample normally reveals a grey colour. This is most likely due to excess silica condensation occurring on the external surface of the mesoporous carbon. FT-IR spectra of RMM-1 samples with Si/Al = 100 synthesized under different steaming periods are shown in Figure 1 together with that of ZSM-5 and MCM-48. While the spectra of RMM-Is are in close resemblance to Al-MCM-48 with the same Si/Al ratio, an additional weak band occurring at ca. 550 cm'̂ indicating the presence of zeolite secondary building unit [14] in the framework. Furthermore, that the observed weak band shift towards higher wavenumber compare to that of ZSM-5 suggests the secondary building units are in nano-scale [20, 21]. Similar observations were also found for samples with different Al contents. Figure 2 displays the XRD profiles of CMK-1 and RMM-l(x)-100 samples prepared under different steaming period of 1 -5 days. It is clear that all samples show characteristics of mesoporous nature with 29 angle centring at ca. 2.7° and a weaker shoulder peak near 3.2° analogous to mesoporous MCM-48 structure. The weak broad signal in the 26 angle of 20-28° is likely due to the presence of nano-sized (2.7 x 1.0 X 1.3) zeolite (ZSM-5) building units [21]. However, as exemplified by RMM-1(5)-100 sample, prolonged steaming (> 3 days) leads to a decrease in mesoporosity and, consequently, the formation of ZSM-5 crystallites. This is also evidenced by the notable increase in microporosity observed in BET analyses (see below). SEM pictures of RMM-l(x)-100 (x = 1-3) are depicted in Fig. 3. A steady decrease in uniformity of crystalline size with steaming period is evident.

ZSM-5-140

15DD 1DD0

Wavenumber (cm ''j

Figure 1. FT-IR spectra of RMM-1 (Si/Al = 100) synthesized under different steaming periods (2, 3, and 5 days). Spectra of ZSM-5 (Si/Al - 140) and Al-MCM-48 (Si/Al = 100) are also depicted for comparison.

396

3

i CMK-1

Two theta (degree)

Figure 2. XRD patterns of parent CMK-1 and RMM-1 samples synthesized under different steaming periods.

Figure 3. SEM picttires of (a) RJV[M-1(1)-100, (b) RMM-1 (2)-100 and (c) RMM-1 (3)-100.

Figure 4a and 4b shows the XRD pattern and pore size distribution of RMM-1 (2)-y, where y represents the Si/Al ratio of the initial gel. All samples show the typical of cubic mesoporous pattern and reveal broad distribution of mesopores (ca. 3-6 nm) as well as micropores (ca. 0.5-0.6 nm). BET surface area data derived from N2 adsorption/desorption measurements (at 77 K) are summarized in Table 1. A typical BET surface area of more than 500 m^g'̂ is found for all samples except RMM-1 (3)-100, in which partial distortion of meso-structure may present due to prolonged steaming treatment. A notable increase in the relative microporosity (and hence decrease in crystallinity of meso phase) of the samples with increasing steaming time is evident.

-1(2)-50

3 6 9 12 15 18

Pore diameter (nrr^

Two theta (degree)

Figure 4. (a) XRD patterns and (b) pore size distribution (insert) of RMM-1 synthesized with varied Si/Al ratio.

397

Table 1. Surface area analysis of replicated meso-/microporous composites RMM materials.

Sample

RMM-1(1)-100 RMM-1(2)-100 RMM-1(3)-100 RMM-1(2)-150 RMM-l(2)-50

BET

628 554 422 632 638

BET*

(m'g"^)

147 154

146 82 66

BET*/BET (%)

23.4 27.8 34.6 13.0 10.4

Vtotal

(cm^g-^)

0.68 0.77 0.36

0.93 0.89

V » micropore

(cm^g-^) 0.060 0.074 0.069

0.040 0.030

D ^ ^--'mesopore

(nm) 2-6 3-6 3-5 4-6 4-5

* Surface area of micropores calculated using t-plot analysis; ^ Pore size distribution calculated by BJH method.

Wi>iiit^»*iM>W<» ^ ^ ^ ^

(b)

i»iM<>»W**<i A A_

AI-MCM-48

ZSM-5

PKPIIWWWW

|^-MCM-48:ZSM-6 (1:1)

Physical mijcture

RMM-1C2>10D

iUi»f^irtikMmm\fim 300 -100

300 200 100 0

^ ^ e Chemical Shift (ppm)

-100 200 100 0

'•"Xe ClKraral Shift (ppnO

Figure 5. HP ̂ ^̂ Xe NMR spectra of (a) RMM-1(2)-100 obtained at various temperature and (b) various samples obtained at 180 K.

The porous nature of RMM-1 is further characterized by continuous flow, hyperpolarized (HP) ^̂ ^Xe NMR technique [22, 23]. Unlike conventional ^̂ ^Xe NMR spectroscopy technique whose chemical shift is largely dictated by the Xe-Xe interactions, HP ^̂ ^Xe NMR can be accomplished at dilute Xe condition and hence the observe chemical shift directly reflect the interactions between Xe atom and inner surface of the porous adsorbent [23]. Variations of HP '̂ ^Xe NMR spectrum with temperature for the RMM-1(2)-100 sample are shown in Fig. 5a. The spectrum obtained at ambient temperature revealed a broad, asymmetric resonance in the downfield direction relative to the reference (or dilute gaseous Xe) peak at 0 ppm. However, upon decreasing temperature, the broad peak eventually split into a doublet indicating the presence of two different pore environments. Specifically, for a given Xe partial pressure of ca. 15 Torr, the spectrum at 180 K exhibits two distinct peaks at 160 and 113 ppm (Fig. 5b), which can be assigned due to Xe adsorbed in micro- and mesopores, respectively. The HP '̂ ^Xe NMR spectra of ZSM-5 (Si/Al = 140), Al-MCM-48 (Si/Al = 100) and 1:1 physical mixture of ZSM-5 and Al-MCM-48 are also shown in Fig. 5b for comparison. Both microporous ZSM-5 zeolite and mesoporous Al-MCM-48 molecular sieve show a single resonance peak indicating a uniform pore distribution. On the other hand, two distinct peaks are evident for 1:1 physical mixture sample indicating that at the given NMR time-scale, a fast exchange of Xe between two types of pores exists even at low temperature, analogous to that of RMM-1. That the chemical shift difference between the two major peaks in RMM-1 being smaller than that in the physical mixture sample provides additional support to the co-existence of meso- and micropores [5] in the RMM-1 material.

398

The acidic properties of the novel repUcated meso-/microporous composites were followed by diffuse reflectance infrared spectroscopy (DRIFT) at 673 K. Figure 6a shows the DRIFT spectra of RMM-1 synthesized under different steaming periods. A slightly higher acid density in RMM-1 materials compare to mesoporous Al-MCM-48 is evidenced by the sharper and much broader peak around 3650 cm"\ which can be ascribed due to the presence of strong Bronsted acid sites. The DRIFT spectra obtained from RMM-1 samples having different Si/Al ratio in Fig. 6b also reveal the expected increase in Bronsted acidity with increasing sample Al content. Separate experiments using ^̂ P MAS NMR of adsorbed trimethylphosphine oxide (TMPO) as the probe molecule [19] were also performed and is exemplified by the spectrum taken obtained from RMM-1 (2)-100 sample shown in Fig. 7. Results obtained from Gaussian deconvolution (dashed curves) confirmed the existence of three different acid sites corresponding to the three distinct resonance peaks at chemical shifts of 61.7, 66.7 and 70.7 ppm. A higher observed value of ^̂ P chemical shift would reflect acid site with stronger acid strength [19]. Similar ^̂ P MAS NMR experiments using adsorbed trimethyl phosphine (TMP) as probe (not shown) further revealed the absence of Lewis acidity in the matrix of the novel RMM-1 material.

RMM-1(2>1S0

2d00 'LOOO

Wave number (cm ) Wavenumber (cm )

Figure 6. DRIFT spectra of RMM-1 synthesized by (a) different steaming time and (b) various Si/Al ratios.

66.7

CiTstalline TMPO

3*-v

149 - I T 1— \—r-

109 S9 60 49 T r 29

Chemical shift (ppm)

Figure 7. ^̂ P MAS-NMR of TMPO adsorbed on novel replicated meso / micro porous molecular sieve composites (RMM-1 (2)-100).

399

CONCLUSIONS

In summary, a novel route has been developed to synthesize replicated meso-Zmicroporous composite material (RMM-1) using carbon mesoporous molecular sieve CMK-1 as template and zeolite ZSM-5 as precursor. For the latter, the use of TPAOH helps to create microporosity on the framework while in the meantime provokes direct generation of acid sites. Thus unique synthetic method of preparing replicated aluminosilicate mesoporous molecular sieves should be advantageous in incorporating hetro species, for example transition metals onto the tetrahedral frameworks.

ACKNOWLEDGEMENT

The financial support of this work by National Science Council of Taiwan (NSC91-2113-M-001-030 to SBL) is greatly acknowledged.

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