Synthesis and characterization of Al-MCM-48 type materials using coal fly ash

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1229

Synthesis and characterization of A 1 - M C M - 4 8 type materials using coal fly ash P. Kumar*, N.K. Mal, Y. Oumi 1, T. Sano I and K. Yamana Ceramic Section of Chemistry & Food Department, Industrial Research Institute of Ishikawa Kanazawa, Ishikawa 920-0223, Japan. 1School of Materials Science, Japan Advanced Institute of Science & Technology, Tatsunokuchi, Ishikawa 923-1292, Japan.

Supernatant of the coal fly ash solution was used to prepare aluminum containing MCM48 (A1-MCM-48). It was found that most of the Si and A1 components in the fly ash could be effectively transformed into MCM-48 when a surfactant mixture containing cationic cetyltetramethylammonium bromide, CTMABr and tetraoxyethylene dodecyl ether, C12(EO)4 were used as templates. Alkali fusion was found to be necessary as it improves the hydrothermal condition for synthesis of the mesoporous materials. High degree of aluminum incorporation into the tetrahedral positions was revealed when A1-MCM-48 was prepared under controlled pH condition. 1. INTRODUCTION MCM-48 (cubic, space group Ia3d) with its highly branched and interwoven threedimensional networks of the mesopore channels is one of the most interesting mesoporous materials among many mesoporous silica molecular sieves [1]. It is believed that MCM-48 is much more resistant to pore blockage than one-dimensional channel system with a hexagonal MCM-41 while being used as absorbents and catalyst supports [1-6]. Although the discovery of the MCM-48 materials was reported simultaneously with the hexagonal MCM-41 in 1992 [1-2], research reports until now on synthesis and application of these materials have been severely biased to MCM-41 [3-4]. The bias may be attributed largely to the fact that the synthesis of MCM-48 required very specific synthesis conditions [5]. Presently however, both the economic and environmental costs for large-scale manufacture of these materials are high due to the cost and toxicity of both templates and preferred silica source. A variety of silica sources are generally used to prepare these materials including fumed silica and silicon tetraethoxide. The industrial manufacture of mesoporous materials is likely to be economically prohibitive if silicon alkoxides and fumed silica in particular are selected. * Corresponding author. Tel: + 49-241/80-20115; Fax: + 49-241/8022-291 E-mail: pnt67 @hotmail.com Present address: Chemical Technology and Heterogeneous Catalysis University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany.

1230 Since the synthesis of MCM-48 requires some very specific condition, a variety of synthesis routes have been developed in order to overcome the synthesis shortcomings [7]. These synthesis results demonstrated that the crystallinity of the MCM-48 pass through an optimum as a function of time. The MCM-48 products were obtained as an intermediate between a hexagonal or disordered surfactant-silica mesophase and a more stable lamellar mesophase [8]. Similarly, one report suggested that the transformation of the MCM-48 mesophase to lamellar can be quenched by adjusting the pH of the reaction mixture [9]. Another report indicated that the mixed surfactant approach resulted into high quality MCM48 as an energetically favored mesophase [10]. Very recently, it was reported that the use of gemini surfactants induce the formation of cubic structure even using fumed silica as silicon source [11]. All these studies indicate that the formation of MCM-48 type materials is possible under certain synthesis conditions. Coal combustion, which accounts for about 37% of the world's electricity production generates, about 600 million Tons, coal fly ash per year as a by-product [12]. Current applications of this vast amount of coal fly ash (only 15%) is not enough and requires further attention to utilize this waste material [13-16]. Since fly ash contains mainly amorphous aluminosilicates (glassy phase) and some crystalline minerals (quartz, mullite, etc.), it can be used as a raw material for the synthesis of porous materials. Very recently we have reported our studies on the synthesis of aluminum containing MCM-41 (A1-MCM-41) and SBA-15 type of materials and their characterization as well as the catalytic properties [17-18]. To further extend this synthesis regime, we have carried out the studies on the preparation condition of A1-MCM-48 type materials using coal fly ash as the silicon and aluminum source [19]. In this report various characterization techniques such as 27A1MAS NMR, FF-IR, TEM, N2 adsorption and cumene cracking reaction are used to further evaluate the materials obtained.

2.

EXPERIMENTAL

2.1.

Materials Coal fly ash used in this study was obtained from Nanao-Ota power plant, Hokuriku and used as obtained. The chemical composition of fly ash revealed apart from the main constituents such as silica (67.5%) and alumina (18.7%), the other impurities such as Fe203, CaO, MgO, K20, TiO2, Cr203, P205 Na20, K20 and SO3 with 3.6%, 2.0%, 0.7%, 0.9%, 0.8%, 0.9%, 0.3%, 0.2%, 0.4%, 0.7%, respectively. The specific surface area (BET) and cation exchange capacity (CEC) of the coal fly ash were found to be 4.5 mE/g and 0.8 meq/100g, respectively. 2.2. Synthesis of AI-MCM-48 The supernatant obtained from fused fly ash powder was used as the silica and aluminum source [17]. The concentrations of Si, A1 and Na measured in supernatant were 11,000, 380 and 35,000 ppm, respectively. The detail synthesis procedure for MCM-41 was followed from our previous study [ 18]. Different samples of MCM-48 type materials with varying Si/A1 ratio were prepared using both single surfactant and a surfactant mixture of CTMABr and C12(EO)4 (Aldrich) [19]. In brief all batches were prepared using a synthesis gel with the following molar composition: CTMABr/C12(EO)4]I-I20/Si = 0.35-0.55/0.15-0.25/100/1. The Si/A1 ratio

1231 was also varied from 60 to 14. To remove the surfactant in the mesoporous materials, the assynthesized sample was calcined in air under static conditions at 813 K for 6 hours, with a linear temperature ramp of 0.5K / min and two plateaus of 60 minutes each at 423 and 623 K.

2.3. Analysis and characterization Powder X-ray diffraction (XRD) patterns obtained from CuK~ radiation were measured by using MAX18X. cE The chemical composition was analyzed by the LilEB404 method using the X-ray fluorescence (XRF) technique (Philips PW2400). BET specific surface area was determined from NE-adsorption at liquid nitrogen temperature (Belsorp 28SA). Transmission electron microscope (TEM) image was obtained by using JEOL 2010. FI'-IR spectra of the self supporting wafers were measured by JEOL JIR-7000. 27A1 MAS NMR spectra were obtained on a Varian VXP-400.

2.4.

Catalytic activity

The cumene cracking was performed in an atmospheric pressure flow system. The sample placed in the quartz tube reactor of a 10mm inner diameter was dehydrated at 673 K for 1 h in a nitrogen stream. The temperature was then brought into a reaction temperature (623 K). The reactant was fed into the catalyst bed with micro-feeder. Nitrogen was used as a carrier gas (40 ml/min), the contact time (W/F) was 0.20 h, and the partial pressure of the cumene was 7.9 kPa. On line product analysis was done on a Shimadzu GC-17A gas chromatograph (FID) with a GL-Science TC-1 capillary column (30 m).

3. RESULTS AND DISCUSSION 9

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9

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9

3.1. AI-MCM-48 prepared by direct thermal synthesis from supernatant Figure 1 shows the XRD patterns of different MCM phases of calcined samples prepared under different surfactant/silica ratio. It can be seen that the low concentration of surfactant (CTMABr) results into MCM-41 type materials as suggested from the XRD pattern (Fig. la) with four peaks that are consistent with indexing to a hexagonal cell, typical of an MCM-41 type product. The observation of three higher angle reflections other than the dl00 indicates that the product is likely to possess the symmetrical hexagonal pore structure of MCM41. A further increase in surfactant concentration resulted into mesophases, poor in hexagonal structural order as indicated from the gradual disappearance of diffraction peaks assigned to (110), (200) and (210) reflections (Figure lb, lc). By increasing the concentration of CTMABr in the synthesis gel, a phase transitions from hexagonal to lamellar passing

211

4

-_~" --=

110 2

a-~ 4

6

8

20/degree Figure 1. XRD profiles of the different calcined MCM type materials. CTMABr/SiO2 9 a = 0.22, b = 0.35, c, d and e = 0.55; C12(EO)4/SIO2: d = 0.15 and e = 0.18

1232 Table 1 Physical properties of the raw material and the calcined mesoporous Sample

/SiO2

/SiO2

SBET/ m 2 g-1

Fly ash A1-MCM-41 (a)

0.20

-

4.5 761

Si/A1 Pore d 100 volume /nm / c m 3 g-1 2.9 . . . 14.0 0.57 4.24

A1-MCM-41 (b)

0.35

-

738

18.5

0.57

A1-MCM-41 (c)

0.55

-

731

65.0

A1-MCM-48 (d)

0.55

0.15

639

A1-MCM-48 (e)

0.55

0.18

848

A1-MCM-48 (f)

0.55

0.18

A1-MCM-48 (g)

0.55

0.18

1 2 3 4

Surf 1 Surf 2

d 211 /nm .

ao 3

/nm

Pore size4 /nm

. -

4.9

2.8

3.56

-

4.1

2.9

0.57

3.56

-

4.1

2.7

62.3

0.55

-

3.17

7.8

2.5

59.4

0.82

-

3.04

7.4

3.0

760

18.2

0.76

-

2.98

7.3

3.0

756

14.0

0.74

-

2.98

7.3

3.0

cetyltrimethyl ammonium bromide tetraoxyethylene dodecyl ether unit cell parameter, using 2d100/~/3 for MCM-41 and d211~/6 for MCM-48 Dollimore-Heal method

through an intermediate state of cubic structure is reported [ 1-4]. But using the supernatant as a silica source it was not observed, in other words MCM-48 formation was not facilitated under the synthesis condition using CTMABr alone. Figure l d and l e shows the XRD patterns of materials the surfactant-silica mesophase obtained from the starting mixtures of CTMABr/C12(EO)4 = 0.55/0.15 and 0.55/0.18, respectively. It can be seen that the presence of neutral surfactant has resulted into mesophase, identical to the cubic MCM-48. We observed that the optimum condition for MCM-48 using the supernatant as a silica source was CTMABr/ClE(EO)4 = 0.55/0.18 as it showed the sharpest XRD pattern. From the XRD pattern in Figure le, a highly ordered MCM-48, without any trace of lamellar phase peaks was obtained. The high ordered array of these materials could be inferred from the presence of a well defined set of diffraction peaks between 3 ~ and 6 ~ in the XRD patterns assigned to the (211), (220), (321), (420), (422) and (431). Two more samples A1-MCM-48 (f) and A1MCM-48 (g) (XRD not shown) with high aluminum concentration was then prepared using the similar composition. Table 1 summarizes characteristics of the calcined mesoporous materials obtained. The gel representing higher than 0.18 of C 1 2 ( E O ) 4 resulted either into unidentified mesophase or didn't show any XRD pattern. The (211) reflection is found at approximately 3.6 nm for all the as-synthesized samples. This correspond to a unit cell size of -- 8.7 nm. For the calcined samples the same reflection occurs at 3.1 nm, a unit cell length of --7.5 nm. This shrinkage of the unit cell (--13%) during calcinations probably is due to silanol condensation. This magnitude of unit cell shrinkage was in the range of values normally reported in the literature using other silicon source, approximately in the 5-15% range [14-17]. The same tendency is observed for the (220) reflection, suggesting that the supernatant of coal fly ash containing dissolved silica species could be used as the source materials for the preparation of such kind of materials.

1233 The N~ adsorption-desorption isotherms of 600 different samples (c, d and e) are shown in ,,.-:,. Figure 2. It belongs to a reversible type IV n 500 isotherm, characteristic for mesoporous o~~:)o c} materials. An inflection point is observed at g o relative pressures between 0.25 and 0.3. din400 (5) This corresponds to the filling of the mesopores and the sharp increase in the 300 ~=,~'~" [] " adsorbed volume indicates a uniform poresize distribution. It can be seen (Table 1) "~ 200 @ that the presence of neutral surfactant 0 M C M 4 8 (mixed,e) facilitates the formation of MCM-48. The ::3 [] MCM-48 (mi~Ex:l, d ) presence of a small hysteresis loop in sample c, indicates the formation of lamellar A M C M 4 8 (single. c ) phase which is very similar to the studies 0,,. I that has been reported at the high surfactant/ 0 0.5 1 silica ratio [20]. TEM image of microRelative pressure (P/Po) sectioned sample (Figure 3) also showed well developed pores arranged on the cubic Figure 2. N2 isotherms of different samples. plane (sample g), confirming that the materials possess the pore system symmetries that are inferred from XRD and N2 isotherms. Another factor that affected the formation of cubic phase was the pH of the supematant-surfactant mesostructure. Generally, a high pH condition is a major driving force for the transformation to lamellar [21]. In our case the pH adjustment to 10.2 during the synthesis arrested this transformation and also helped to improve the product yields. This is in agreement with the report where the pH adjustment was mentioned as a means for quenching the transformation of the MCM-48 mesophase to lamellar [ 10]. A mixed surfactant approach has been reported in the literature for the preparation of mesoporous materials [20,22]. In many cases, two different surfactants are completely miscible and form liquid crystalline misceller mesophase cooperatively. This phase behavior becomes more complicated when silica and alumina sources are present in the form of supernatant of coal fly ash. Supematant is a highly alkaline solution of silicate and aluminate (anions) and are strongly attracted by electrostatic interaction surrounding the head groups of the CTMABr, which may lead to the high concentration of the anions on the surface of the surfactant micelles. The neutral surfactant has no strong interaction with the ~;~:i? " 50nm , ,~,f~.i anions, and consequently its incorporation to the micelles will bring a dilution of the anions at the surface. This low surface Figure 3. TEM image of sample g. concentration may further lead to a certain

1234 contraction of the micelles surface, resulting in a phase transition from hexagonal to cubic. At this stage we are not advancing any explanation about the complexities of phase behavior of the supernatant-surfactant mesostructures in the aqueous solution, however we believe that C12(EO)4 acts more as a diluents and based on our observation facilitated the formation of MCM-48 structure.

3.2. Acidity of various AI-MCM-48 samples One of the most important features of our study using coal fly ash is the aluminum incorporation into the framework of the synthesized materials [17-18]. We found in the previous study on A1-MCM-41 that although there is no clear explanation for a large amount of tetrahedrally (Ta) coordinated framework aluminum in A1-MCM-41 derived from the supernatant, the supernatant is very effective for preparation of A1-MCM-41 without any Oh nonframework (0 ppm) aluminum. Very similar results we have also observed for the different MCM-48 samples. In Figure 4 the 27A1MAS NMR spectra of A1-MCM-48 (samples e, f and 100 50 0 -50 g) are presented. Chemical shift is referenced to 1 M Al(NO3)3 aqueous solution and the peak 27A1MAS-PPN~ spectra for Alintensity was normalized based on 1 g of Figure4. material. A single peak at ca 54 ppm, without MCM-48 prepared from supernatant of any evidence of any Oh aluminum can be seen fused fly ash powder. Si/A1 ratio; e=59.4, f=18.2, g=14.0 in all three samples, the intensity for which increased with low Si/A1 ratio. This is H H interesting and suggests the formation of acid sites in the mesoporous system. To further authenticate this, the samples were tested for pyridine adsorption using FT-IR. Aluminum in tetrahedral position creates ion exchange site associated with the charge compensating Na § ions. Figure 5 shows IR spectra of pyridine < adsorbed on the samples (Si/A1 = 59.4, 18.2 and 14.0 for e, f and g, respectively) after degassing at 423 K for 30 min. The samples did not show any acidity as expected, the weak bands at 1446 and 1598 cm -1 are probably due to pyridine 1600 1500 1400 adsorbed via H-bond interaction. When the samples were ion-exchanged twice with the Wave number (cm-1) NH4+ salt and calcined (protonation), a clear Figure 5. Fr-IR spectra of adsorbed pattern of acidity generated on the samples can be seen in Figure 6. Intense bands were pyridineonA1-MCM-48 samples before measured around 1456 cm 1 and 1623 cm 1 protonation. !

i .

.

.

.

1235 (Lewis acid sites), 1556 c m "1 (BrCnsted acid sites) and 1494 cm -1 (overlapping BrCnsted and Lewis L B+L acid sites). The intensity of these bands increases with the A1 content of the samples, showing a corresponding increase in the number of acidic sites. However, majority of acid sites generated on the samples were found to be Lewis acid sites (Figure 6) and the peaks arising from BrCnsted acid sites disappeared after evacuation at 523 K for 1 h, suggesting that the acidic strength of the BrCnsted acid sites in the A1-MCM-48 synthesized is very weak. Nevertheless, it is I I 1600 1500 1400 interesting to observe the acidity in the A1-MCM48 derived from coal fly ash, which confirms the Wave number (cm-1) aluminum incorporation suggested by the 27A1 MAS NMR measurement. Figure 6. FF-IR spectra of adsorbed pyridine on protonated A1-MCM-48 Catalytic activity of the A1-MCM-48 samples. prepared was further evaluated using the cumene cracking reaction at different time on stream and compared with the A1-MCM-48 prepared from 25 pure chemicals. The initial activity of coal fly ash derived materials (Si/AI= 18.2) was lower compared to the initial activity of A1-MCM-48 9 (Si/AI= 22.0) prepared from pure chemicals. Taking into account the fact that the cracking reaction require medium to strong BrCnsted acid r sites and the peaks derived from BrCnsted acid o [] a sites disappeared after evacuation at 523 K, it is suggested that the acidity of A1-MCM-48 prepared from the supernatant of coal fly ash is = "0" b o o o very weak. In other words, all the aluminums I I present in the A1-MCM-48 prepared from fly ash are catalytically not active. A number of reports 0 1 2 3 Time on stream (h) provides sufficient evidence for the partial inaccessibility of aluminum due to its Figure 7. Conversion profile of cumene incorporation in separate aluminum phases or on protonated A1-MCM-48 samples deeply imbedded in the porous walls if an prepared from (a) pure chemicals and (b) coal fly ash. aluminum source is added to the initial synthesis gel [26-27]. Our observation in this study provides further support for this point that the catalytic active sites are not connected with the total aluminum concentration but linked only to the amount of accessible aluminum, preferably on the surface.

12

4. CONCLUSIONS Supernatant of coal fly ash can be used as a raw material for the synthesis of aluminum containing MCM-48. The use of surfactant mixture has greatly facilitated the synthesis of

1236 MCM-48 performed under controlled pH condition. A high aluminum incorporation in tetrahedral position is revealed in the mesoporous materials which in turn generate ionexchange sites as well as acid sites when measured by pyridine adsorption using FF-IR. The experimental data produced here suggest that the coal fly ash could be a suitable source of silicon/aluminum with a low economy and environmentally friendly reagent for the preparation of well ordered mesoporous materials. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680. 3. A. Corma, Chem. Rev., 97 (1997) 2373. 4. A. Sayari, Y. Yang, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 103 (1999) 3651. 5. J..M. Kim, S. K. Kim and R. Ryoo, J. Chem. Soc., Chem. Commun., (1998) 259. 6. C.L. Landry, S. H. Tolbert, K. W. Gallis, A. M. Monnier, G, D. Stucky, P. Norby and J. C. Hanson, Chem. Mater., 12 (2001) 1600. 7. M.L. Pena, Q. Kan, A. Corma and F. Rey, Microporous Mesoporous Mater., 44-45 (2001) 267. 8. A. Corma, Q. Kan, and F. Rey, J. Chem. Soc., Chem. Commun., (1998) 579. 9. J. Xu, Z. Luan, H. He, W. Zhou and L. Kevan, Chem. Mater., 10 (1998) 3690. 10. R. Ryoo, S.H. Joo and J.M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 11. P. Van Der Voort, M. Mathieu, F. Mees and E. F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. 12. C. Zevenbergen, J.P. Bradley, L.P.V. Reeuwijk, A.K. Shyam, O. Hjelmar and R.N.J. Comans, Environ. Sci. Technol., 33 (1999) 3405. 13. G. Belardi, S. Massimilla and L. Piga, Resource, Conservation and Recycling, 24 (1998) 167. 14. A. Singer and V. Berkgaut, Environ. Sci. Technol., 29 (1995) 1748. 15. S. Rayalu, N. K. Labhasetwar and P. Khanna, U.S. Patent No. 6027708 (22 February 2000). 16. N. Shigemoto, S. Sugiyama, H. Hayashi and K. Miyaura, J. Mater. Sci., 30 (1995) 5777. 17. P. Kumar, Y.Oumi, K. Yamana and T. Sano, J. Ceram. Soc. Japan, 109 (2001) 968. 18. P. Kumar, N. K. Mal, Y.Oumi, K. Yamana and T. Sano, J. Mater. Chem., 11 (2001) 3279. 19. P. Kumar, Y. Oumi, K. Yamana and T. Sano, accepted to Nanoporous Materials III, June 12-15 th 2002, Canada. 20. G. Oye, J. Sjoblom and M. Stocker, Microporous Mesoporous Mater., 27 (1999) 171. 21. R. Ryoo and J.M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 22. J. L. Palous, M. Turmine and P. Letellier, J. Phys. Chem. B, 102 (1998) 5886. 23. K.R. Kloetstra, H.W. Zandergen and H. van Bekkum, Catal. Lett., 33 (1995) 157. 24. A. Jentys, K. Kleestofer and H. Vinek, Microporous Mesoporous Mater., 27 (1999) 321.