- Email: [email protected]
Synthesis and characterization of aluminum-containing MCM-48
MESOPOROUS MOLECULARSIEVES 1998 Studies in Surface Science and Catalysis, Vol. 117 L. Bonneviot, F. B61and,C. Danumah, S. Giasson and S. Kaliaguine (Editors) 9 1998 Elsevier Science B.V. All rights reserved.
249
Synthesis and characterization o f aluminum-containing M C M - 4 8 Martin Hartmann and Christian Bischof Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany
Pure siliceous and aluminum containing mesoporous MCM-48 materials were synthesized from gels containing CmTAB (m = 14,16), tetraorthosilicate (TEOS), water and diluted NaOH. The synthesis for the all silica materials, performed in a 51 autoclave, was monitored by characterization of small samples, taken every two hours, employing X-ray diffraction and 29Si MAS NMR spectroscopy. A number of mesoporous aluminosilicate molecular sieves with MCM-48 structure has been synthesized in 250 ml and 1 I autoclaves using different sources of aluminum and characterized in detail by XRD, 298i and 27A1 MAS NMR and nitrogen adsorption. By using soluble aluminum salts, viz. aluminum nitrate, hydroxide or sulfate, MCM-48 can be prepared with all aluminum in tetrahedral coordination. However, high quality MCM-48 materials can only be obtained to ansi /nA~-ratio of 80. 1. INTRODUCTION Since the discovery of ordered mesoporous molecular sieves with pore sizes between 2 and 10 nm, much research work has been devoted to this new class of mesoporous materials, denoted M41S. However, most studies focused on the hexagonal member of this group, MCM-41 [1,2]. The same synthesis principles can be applied for the synthesis of another member of this family, the cubic phase MCM-48, although, only little information is available on the synthesis of this material [3]. The incorporation of aluminum into MCM-41, which is interesting with respect to catalytic applications, is discussed in numerous papers, while only Schmidt et al. [4] report on the synthesis of AIMCM-48. Using aluminum sulfate, they succeeded in preparation of an AIMCM-48 (nsi/n/u = 20) with aluminum exclusively in tetrahedral coordination. Zhao and Goldfarb have also reported the synthesis of an aluminumcontaining MCM-48 (nsi/n~u = 46) without, however, stating the details of the synthesis or reporting any characterization data of this material [5]. Furthermore, Ryoo et al [6] described a post synthesis method to incorporate aluminum into all silica MCM-48. Despite of this initial success, aluminum incorporation into MCM-48 yielding high quality AIMCM-48 is still a challenge. In our study, pure siliceous and aluminum-containing MCM-48 materials were synthesized in 250 ml, 1 1 and 5 1 autoclaves with and without agitation. Different sources of aluminum and variation of other experimental parameters were checked for their potential yielding high quality AIMCM-48 materials.
250
2. EXPERIMENTAL SECTION
2.1. Synthesis Mesoporous all silica MCM-48 was synthesized in a 5 1 autoclave using hexadecyltrimethylammoniumbromide (C~6TAB, Aldrich), tetraorthosilicate (TEOS, Aldrich) and diluted NaOH. First, a gel with the molar composition nsi: ncr~: nr~,2o:nmo = 1 . 0 : 0 . 6 : 0 . 5 : 6 3 was prepared. A typical synthesis procedure is outlined as follows: 840 g of hexadecyltrimethylammoniumbromid (C~6TAB) were diluted in 2.5 1 of distilled water under stirring at 50 ~ to give a 25 wt.-% solution. Then 800 g of TEOS were slowly added and the resultant solution was stirred for 5 min. After addition of 1.25 1 of a 1 M NaOH solution, the gel was stirred for another 5 min. Thereafter, the resultant synthesis gel was charged into a 51 autoclave and heated to 100 ~ under agitation. To monitor the progress in MCM-48 formation, small samples were taken every 2 h from the autoclave. Each sample was washed with 100 ml of ethanol, 250 ml of destilled water and subsequently dried at 80 ~ for 24 h to secure comparable conditions for further characterizations. After 40 h the synthesis was stopped and the resultant material was filtered, washed with ethanol and water and air dried. To remove the surfactant, the as-synthesized material was calcined for 10 h at 540 ~ in a flow of nitrogen and air. A similar procedure was used to synthesize MCM-48 with a smaller pore diameter using Ct4TAB as a surfactant. For the incorporation of aluminum into MCM-48 materials in tetrahedral coordination, different synthesis strategies and aluminum sources were tested. A1MCM-48 was synthesized using the same general procedure as described above. As sources, aluminum sulfate (Merck) and nitrate (Merck), sodium aluminate, aluminum triisopropylate (Fluka) and aluminum hydroxide were explored. The molar composition, pH-value and temperature of the synthesis gel, the aluminum source and the mixing time was varied in a large number of experiments. 2.2. Characterization The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). X-ray powder diffraction pattern were recorded after synthesis and template removal on a Siemens D5000 diffractometer using CuK~ radiation. After calcination, nitrogen adsorption and desorption isotherms were measured on a Micromeritics ASAP 2010 sorption analyzer. 27Al and 29SiMAS ~ spectra were recorded on a Bruker MSL 400 spectrometer using single pulse excitation with standard 4 mm (27A1) and 7 mm (29Si) rotors. The resonance frequencies were c00/2~ = 104.31 and 79.49 MHz for 27A1 and 29Si, respectively. A 1.0 M solution of aluminum nitrate in water and tetramethylsilane (TMS) were employed as chemical shift references. 3. RESULTS AND DISCUSSION Figure 1 exhibits the X-ray diffraction patterns for a all silica MCM-48 synthesis as a function of the reaction time. For a better view, the diffraction patterns of samples taken every 4 h were displayed. Hence, it appears that the formation of the MCM-48 structure is completed after 22 h. No further increase in the X-ray diffraction intensity is observed between 22 and 40 h. Between 6 and 18 h of the reaction time a broad reflection is observed at 20 = 3.7 o. With increasing synthesis time, this signal becomes lower in intensity, indicating
251
that the cubic MCM-48 is probably build over an intermediate phase. Probably, this intermediate phase is a disordered three dimensional arrangement and is transformed during the synthesis into a well ordered MCM-48 structure. The 29Si MAS NMR spectra of the as synthesized samples show, that during the synthesis of the MCM-48 condensation of the silanol groups takes place [7]. The Q4/Q3-ratio increases with increasing reaction time. The Q4/Q3-ratio in the synthesis gel (t = 0) is 0.93 and after a reaction time of 12 h the Q4/Qa-ratio increases to 1.58. During this condensation, the structure is probably transformed via a disordered three dimensional phase into the cubic MCM-48. Similar results were found by Huo et al [8], who investigated the formation of MCM-41 using X-ray diffraction and 29Si MAS NMR. In contrast to the formation of the MCM-48 structure, the characteristic pattern of the MCM-41 phase is already recognizable in the synthesis gel. Further crystallization only results in intensity enhancement of the X-ray reflections. This indicates that the hexagonal structure of MCM-41 is already pre-arranged in the synthesis gel.
. ~ r
//lrl!/lll84 L
03 Z uJ IZ
r
26 h 22 h ~'18h 14h 1
h 6h ANGLE 20 / degree
9
Figure 1. X-ray diffraction patterns for samples taken from an all silica MCM-48 synthesis batch ~ e r different reaction times. While the synthesis and scale up of all silica MCM-48 with CmTAB (m = 14,16) denoted as MCM-48/14 and MCM-48/16 was successful, the synthesis of MCM-48 with C~TAB (m = 10,12,18) did not yield in the formation of well ordered MCM-48. A comparison between MCM-48/14 and MCM-48/16 samples is given in Table 1. As expected, the unit cell parameter ao, the BET-surface, the pore volume and the pore diameter decrease with decreasing length of the alkyl chain.
252 Table 1 Unit cell parameters and results from the nitrogen adsorption experiments for MCM-48/14 and MCM-48/16 unit cell parameter / n m
pore volume') / pore diameter") /
BET-surface /
i
as-synthesiz~
calcined
m2g"t
cm3g"l
nm
MCM-48/14
8.43
7.07
1030
0.56
1.7
MCM-48/I 6
8.91
7.61
1350
0.86
2.0
i
a) The pore volumes 'and diameters were calculated from the desorption branch of the adsorption isotherms using the BJH-model. A collection of the X-ray diffraction patterns of AIMCM-48 materials synthesized with different aluminum sources is shown in Figure 2. In contrast to the results obtained by Schmidt et al. [4], the use of aluminum sulfate does not lead to the formation of AIMCM-48 with a well resolved X R pattern regardless of the A1~(SO4)3 concentration. Only a disordered phase is obtained (Figure 2a), which was also observed by Pu et al. [9] and denoted AlMCM-48 (nsi/hal = 32). A well ordered cubic MCM-48 phase exhibiting 5 - 8 reflections in the rang of 20=2-9~ obtained with aluminum nitrate, aluminum hydroxide and aluminum triisopropylate as aluminum sources (Figures 2b-d). In all cases, the sample with the lowest achievable nsi/nArratio leading to a well resolved XRD pattern is displayed.
..=-
C-L_
= = =
r z w l-Z
c
9
2
I
3
.
I
9
4 ANGLE
I
5
9
I
di
6
|
7
,
i
|
9
8
26/ d e g r e e
Figure 2. X-ray diffraction patterns of the AIMCM-48 materials synthesized with different aluminum sources: a) aluminum sulfate (nsi / hA== 17), b) aluminum triisopropylate (nsi / nm = 5), c) aluminum hydroxide (nsi/n~u = 80), d) aluminum nitrate (nsi/n~a = 100).
253 Well ordered MCM-48 materials with nsi/nAj = 100 and nsi/nAj = 80 were synthesized using aluminum nitrate and aluminum hydroxide, respectively. A disordered phase is observed, when the nsi/nA~-ratios were decreased with aluminum nitrate and aluminum hydroxide as aluminum sources in the synthesis gel. In these cases, variation of synthesis parameters did not result in the formation of high quality AIMCM-48. When aluminum triisopropylate is used as an aluminum source, the nsi/nAj-ratio can be reduced to 5. The nitrogen adsorption and desorption isotherms for the AIMCM-48 materials synthesized with different aluminum sources are displayed in Figure 3. All samples show adsorption isotherms typical for mesoporous materials with small pore size distribution [ 10]. The results of the nitrogen adsorption experiments and the chemical compositions of these samples are summarized in Table 2. 700
-
600
b
('0
E o
500
O ILl nn n," o 400 O9 a < ILl ~E~
a
300
d
_,1
0 > 200
i001,, 003
,
,,,i 0.1
....... ~
i
,,
0.2
I 0.3
.... ,
, I .... 0.4
,,
I 0.5
P/Po
Figure 3. Nitrogen adsorption (open symbols) and desorption isotherms (closed symbols) for AIMCM-48 materials synthesized with different aluminum sources: a) aluminum triisopropylate, b) aluminum sulfate, c) aluminum hydroxide, d) aluminum nitrate.
254 Table 2 Unit cell parameters, ns/nArratios and results from the nitrogen adsorption experiments for different AlMCM-48 materials. Aluminum
unit cell parameter / nm
nsi/nm BET-surface/ pore volume') / pore diameter.) /
|,
source
as-synthesized
talc.
hydroxide
8.40
8.15
80
1380
1.0
2.2
nitrate
8.45
7.60
100
1080
0.69
1.9
sulfate
-
-
17
1000
0.74
2.3
9.14
8.08
5
940
0.76
2.6
triisopropylate
m~g1
r
!
nm
a) The pore volumes and diameters were calculated from the desorption branch of the adsorption isotherms using the BJH-model. The low quality of AlMCM-48 synthesized with aluminum sulfate is also reflected in the adsorption data. An unusually broad pore size distribution (not shown) indicates an ordering which is reduced in comparison with all silica MCM-48 and other AlMCM-48 samples. The nitrogen adsorption is significantly enhanced in AIMCM-48 synthesized with aluminum hydroxide in comparison with the material prepared with aluminum nitrate. The pore volume and the surface area of the latter material are substantially lower, although the nsi/nArratio is in the same order of magnitude. AlMCM-48 synthesized with nsi / hal = 5 using aluminum triisopropylate has a pore diameter of dp = 2.6 nm, which is larger than dp of the all silica MCM-48.
I
-150
i
I
i
-100
I
-50
a
I
0
i
I
50
I
I,
100
Chemical Shift 6 / ppm Figure 4. 27A1MAS NMR spectra of the AlMCM-48 materials prepared using a) aluminum triisopropylate (calcined material), b) aluminum sulfate (calcined material), c) aluminum nitrate and d) aluminum hydroxide.
255 The 27Al MAS ~ spectrum of the sample synthesized with aluminiumtriisopropylate, displayed in Figure 4a, consists of two lines at 8 = 53 ppm and 6 = 0 ppm. While the latter line corresponds to aluminum in octahedral coordination, the signal at 5 = 53 ppm is typical for tetrahedrally coordinated aluminum in mesoporous alumosilicates [11]. A similar 27Al MAS NMR spectrum is observed for AIMCM-41, when aluminum triisopropylate is used as a aluminum source. Samples with aluminum exclusively in tetrahedral coordination are synthesized with water soluble aluminum sources, viz. A12(SO4)3, AI(NO)3 and AI(OH)3, (Figures 4b,c and d). The NMR spectra give a sharp resonance at 8 = 53 ppm originating from tetrahedrally coordinated aluminum. Upon template removal, the sharp 27Al resonance broadens, but does not decrease in absolute intensity. The broadening is a result of decreased symmetry of some aluminum sites in the calcined sample. Attempts to synthesize AIMCM-48 with low nsi/n~-ratios using aluminum nitrate and aluminum hydroxide did not result in the formation of a well ordered cubic phase. In that cases, the resulting X-ray diffraction patterns were similar to the one displayed in Figure 2a. However, the 27A1MAS NMR spectra still only consist of a single line at 8 = 53 ppm. The appearance of not well resolved XRD patterns in MCM-41 type materials, when introducing aluminum is explained by Corma [12] not as a result of a less ordered material, but due to the formation of smaller crystallites of MCM-41. However, a literature survey shows that different synthesis strategies and aluminum sources result in materials of widespread quality and properties, which holds also true in for the cubic MCM-48 phase. More work is necessary to understand the influences of synthesis parameters and the nature of the aluminum source. 4. CONCLUSIONS All silica and aluminum containing MCM-48 samples have been synthesized in 250 ml, 1 1 and 5 1 autoclaves. The synthesis of a larger quantity and slow agitation was found to have a promoting effect on the synthesis of high quality materials. Formation of the cubic phase is achieved after 22 h via condensation of a less well ordered three dimensional structure. However, the synthesis of well ordered aluminum-containing MCM-48 materials with low nsi/n~u-ratios is still a challenge. An nsi/n~-ratio as low as 5 can be obtained using aluminum triisopropylate, but not all aluminum is in tetrahedral coordination. Exclusively tetrahedrally coordinated aluminum is found in samples synthesized with water soluble aluminum sources, viz. AI(OH)3, AI(NO)3, and A12(SO4)3. Unfortunately, the synthesis of high quality MCM-48 is limited to nsi/n~a = 80 and aluminum hydroxide as aluminum source. Further investigations aiming at a substantial reduction of the nsi/n~-ratio are in progress. ACKNOWLEDGMENTS Financial support from Deutsche ForschungsgemeinschaR (DFG) and Fonds der Chemischen Industrie is gratefully acknowledged. M.H. acknowledges generous support from Prof. J. Weitkamp.
256 REFERENCES
[1]
C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359, 710(1992). [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C. T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992). [3] A.A. Romero, M.D. Alba, W. Zhou, J. Klinowsky, J. Phys. Chem. B, 101, 5294 (1997). [4] R. Schmidt, H. Junggreen and M. St6cker, Chem. Commun., 875 (1996). [5] D. Zhao and D. Goldfarb, in: Zeolites a refined tool for designing catalytic sites, L. Bonneviot and S. Kaliaguine (eds.), Stud. Surf. Sci. Catal. Vol. 97, Elsevier, Amsterdam: 1997, pp. 181-188. [6] R. Ryoo, S. Jun, J. M. Kim and M.J. Kim, Chem. Commun., 2225 (1997). [7] C. Bischof and M. Hartmann, in preparation. [8] Q. Huo, D.I Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Siegler, A. I. Firouzi, B.F. Chmelka, F. Sch0th and G.D. Stuck),, Chem Mater., 6, 1176 (1994). [9] S.B. Pu, J.R. Kim, M. Seno and T. Inui, Microporous Mater., 10, 25 (1997). [10] P.L. Liewellyn, Y. Cillet, F. Sch0th, H. Reichert and K.K. Unger, Microporous Mater., 3,345 (1994). [ 11] Z. Luan,, C.F. Cheng, W. Zhou and J. Klinowski, J. Phys. Chem., 99, 10 ! 8 (1995). [12] A. Corma, Chem. Rev., 97, 2373 (1997).
249
Synthesis and characterization o f aluminum-containing M C M - 4 8 Martin Hartmann and Christian Bischof Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany
Pure siliceous and aluminum containing mesoporous MCM-48 materials were synthesized from gels containing CmTAB (m = 14,16), tetraorthosilicate (TEOS), water and diluted NaOH. The synthesis for the all silica materials, performed in a 51 autoclave, was monitored by characterization of small samples, taken every two hours, employing X-ray diffraction and 29Si MAS NMR spectroscopy. A number of mesoporous aluminosilicate molecular sieves with MCM-48 structure has been synthesized in 250 ml and 1 I autoclaves using different sources of aluminum and characterized in detail by XRD, 298i and 27A1 MAS NMR and nitrogen adsorption. By using soluble aluminum salts, viz. aluminum nitrate, hydroxide or sulfate, MCM-48 can be prepared with all aluminum in tetrahedral coordination. However, high quality MCM-48 materials can only be obtained to ansi /nA~-ratio of 80. 1. INTRODUCTION Since the discovery of ordered mesoporous molecular sieves with pore sizes between 2 and 10 nm, much research work has been devoted to this new class of mesoporous materials, denoted M41S. However, most studies focused on the hexagonal member of this group, MCM-41 [1,2]. The same synthesis principles can be applied for the synthesis of another member of this family, the cubic phase MCM-48, although, only little information is available on the synthesis of this material [3]. The incorporation of aluminum into MCM-41, which is interesting with respect to catalytic applications, is discussed in numerous papers, while only Schmidt et al. [4] report on the synthesis of AIMCM-48. Using aluminum sulfate, they succeeded in preparation of an AIMCM-48 (nsi/n/u = 20) with aluminum exclusively in tetrahedral coordination. Zhao and Goldfarb have also reported the synthesis of an aluminumcontaining MCM-48 (nsi/n~u = 46) without, however, stating the details of the synthesis or reporting any characterization data of this material [5]. Furthermore, Ryoo et al [6] described a post synthesis method to incorporate aluminum into all silica MCM-48. Despite of this initial success, aluminum incorporation into MCM-48 yielding high quality AIMCM-48 is still a challenge. In our study, pure siliceous and aluminum-containing MCM-48 materials were synthesized in 250 ml, 1 1 and 5 1 autoclaves with and without agitation. Different sources of aluminum and variation of other experimental parameters were checked for their potential yielding high quality AIMCM-48 materials.
250
2. EXPERIMENTAL SECTION
2.1. Synthesis Mesoporous all silica MCM-48 was synthesized in a 5 1 autoclave using hexadecyltrimethylammoniumbromide (C~6TAB, Aldrich), tetraorthosilicate (TEOS, Aldrich) and diluted NaOH. First, a gel with the molar composition nsi: ncr~: nr~,2o:nmo = 1 . 0 : 0 . 6 : 0 . 5 : 6 3 was prepared. A typical synthesis procedure is outlined as follows: 840 g of hexadecyltrimethylammoniumbromid (C~6TAB) were diluted in 2.5 1 of distilled water under stirring at 50 ~ to give a 25 wt.-% solution. Then 800 g of TEOS were slowly added and the resultant solution was stirred for 5 min. After addition of 1.25 1 of a 1 M NaOH solution, the gel was stirred for another 5 min. Thereafter, the resultant synthesis gel was charged into a 51 autoclave and heated to 100 ~ under agitation. To monitor the progress in MCM-48 formation, small samples were taken every 2 h from the autoclave. Each sample was washed with 100 ml of ethanol, 250 ml of destilled water and subsequently dried at 80 ~ for 24 h to secure comparable conditions for further characterizations. After 40 h the synthesis was stopped and the resultant material was filtered, washed with ethanol and water and air dried. To remove the surfactant, the as-synthesized material was calcined for 10 h at 540 ~ in a flow of nitrogen and air. A similar procedure was used to synthesize MCM-48 with a smaller pore diameter using Ct4TAB as a surfactant. For the incorporation of aluminum into MCM-48 materials in tetrahedral coordination, different synthesis strategies and aluminum sources were tested. A1MCM-48 was synthesized using the same general procedure as described above. As sources, aluminum sulfate (Merck) and nitrate (Merck), sodium aluminate, aluminum triisopropylate (Fluka) and aluminum hydroxide were explored. The molar composition, pH-value and temperature of the synthesis gel, the aluminum source and the mixing time was varied in a large number of experiments. 2.2. Characterization The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). X-ray powder diffraction pattern were recorded after synthesis and template removal on a Siemens D5000 diffractometer using CuK~ radiation. After calcination, nitrogen adsorption and desorption isotherms were measured on a Micromeritics ASAP 2010 sorption analyzer. 27Al and 29SiMAS ~ spectra were recorded on a Bruker MSL 400 spectrometer using single pulse excitation with standard 4 mm (27A1) and 7 mm (29Si) rotors. The resonance frequencies were c00/2~ = 104.31 and 79.49 MHz for 27A1 and 29Si, respectively. A 1.0 M solution of aluminum nitrate in water and tetramethylsilane (TMS) were employed as chemical shift references. 3. RESULTS AND DISCUSSION Figure 1 exhibits the X-ray diffraction patterns for a all silica MCM-48 synthesis as a function of the reaction time. For a better view, the diffraction patterns of samples taken every 4 h were displayed. Hence, it appears that the formation of the MCM-48 structure is completed after 22 h. No further increase in the X-ray diffraction intensity is observed between 22 and 40 h. Between 6 and 18 h of the reaction time a broad reflection is observed at 20 = 3.7 o. With increasing synthesis time, this signal becomes lower in intensity, indicating
251
that the cubic MCM-48 is probably build over an intermediate phase. Probably, this intermediate phase is a disordered three dimensional arrangement and is transformed during the synthesis into a well ordered MCM-48 structure. The 29Si MAS NMR spectra of the as synthesized samples show, that during the synthesis of the MCM-48 condensation of the silanol groups takes place [7]. The Q4/Q3-ratio increases with increasing reaction time. The Q4/Q3-ratio in the synthesis gel (t = 0) is 0.93 and after a reaction time of 12 h the Q4/Qa-ratio increases to 1.58. During this condensation, the structure is probably transformed via a disordered three dimensional phase into the cubic MCM-48. Similar results were found by Huo et al [8], who investigated the formation of MCM-41 using X-ray diffraction and 29Si MAS NMR. In contrast to the formation of the MCM-48 structure, the characteristic pattern of the MCM-41 phase is already recognizable in the synthesis gel. Further crystallization only results in intensity enhancement of the X-ray reflections. This indicates that the hexagonal structure of MCM-41 is already pre-arranged in the synthesis gel.
. ~ r
//lrl!/lll84 L
03 Z uJ IZ
r
26 h 22 h ~'18h 14h 1
h 6h ANGLE 20 / degree
9
Figure 1. X-ray diffraction patterns for samples taken from an all silica MCM-48 synthesis batch ~ e r different reaction times. While the synthesis and scale up of all silica MCM-48 with CmTAB (m = 14,16) denoted as MCM-48/14 and MCM-48/16 was successful, the synthesis of MCM-48 with C~TAB (m = 10,12,18) did not yield in the formation of well ordered MCM-48. A comparison between MCM-48/14 and MCM-48/16 samples is given in Table 1. As expected, the unit cell parameter ao, the BET-surface, the pore volume and the pore diameter decrease with decreasing length of the alkyl chain.
252 Table 1 Unit cell parameters and results from the nitrogen adsorption experiments for MCM-48/14 and MCM-48/16 unit cell parameter / n m
pore volume') / pore diameter") /
BET-surface /
i
as-synthesiz~
calcined
m2g"t
cm3g"l
nm
MCM-48/14
8.43
7.07
1030
0.56
1.7
MCM-48/I 6
8.91
7.61
1350
0.86
2.0
i
a) The pore volumes 'and diameters were calculated from the desorption branch of the adsorption isotherms using the BJH-model. A collection of the X-ray diffraction patterns of AIMCM-48 materials synthesized with different aluminum sources is shown in Figure 2. In contrast to the results obtained by Schmidt et al. [4], the use of aluminum sulfate does not lead to the formation of AIMCM-48 with a well resolved X R pattern regardless of the A1~(SO4)3 concentration. Only a disordered phase is obtained (Figure 2a), which was also observed by Pu et al. [9] and denoted AlMCM-48 (nsi/hal = 32). A well ordered cubic MCM-48 phase exhibiting 5 - 8 reflections in the rang of 20=2-9~ obtained with aluminum nitrate, aluminum hydroxide and aluminum triisopropylate as aluminum sources (Figures 2b-d). In all cases, the sample with the lowest achievable nsi/nArratio leading to a well resolved XRD pattern is displayed.
..=-
C-L_
= = =
r z w l-Z
c
9
2
I
3
.
I
9
4 ANGLE
I
5
9
I
di
6
|
7
,
i
|
9
8
26/ d e g r e e
Figure 2. X-ray diffraction patterns of the AIMCM-48 materials synthesized with different aluminum sources: a) aluminum sulfate (nsi / hA== 17), b) aluminum triisopropylate (nsi / nm = 5), c) aluminum hydroxide (nsi/n~u = 80), d) aluminum nitrate (nsi/n~a = 100).
253 Well ordered MCM-48 materials with nsi/nAj = 100 and nsi/nAj = 80 were synthesized using aluminum nitrate and aluminum hydroxide, respectively. A disordered phase is observed, when the nsi/nA~-ratios were decreased with aluminum nitrate and aluminum hydroxide as aluminum sources in the synthesis gel. In these cases, variation of synthesis parameters did not result in the formation of high quality AIMCM-48. When aluminum triisopropylate is used as an aluminum source, the nsi/nAj-ratio can be reduced to 5. The nitrogen adsorption and desorption isotherms for the AIMCM-48 materials synthesized with different aluminum sources are displayed in Figure 3. All samples show adsorption isotherms typical for mesoporous materials with small pore size distribution [ 10]. The results of the nitrogen adsorption experiments and the chemical compositions of these samples are summarized in Table 2. 700
-
600
b
('0
E o
500
O ILl nn n," o 400 O9 a < ILl ~E~
a
300
d
_,1
0 > 200
i001,, 003
,
,,,i 0.1
....... ~
i
,,
0.2
I 0.3
.... ,
, I .... 0.4
,,
I 0.5
P/Po
Figure 3. Nitrogen adsorption (open symbols) and desorption isotherms (closed symbols) for AIMCM-48 materials synthesized with different aluminum sources: a) aluminum triisopropylate, b) aluminum sulfate, c) aluminum hydroxide, d) aluminum nitrate.
254 Table 2 Unit cell parameters, ns/nArratios and results from the nitrogen adsorption experiments for different AlMCM-48 materials. Aluminum
unit cell parameter / nm
nsi/nm BET-surface/ pore volume') / pore diameter.) /
|,
source
as-synthesized
talc.
hydroxide
8.40
8.15
80
1380
1.0
2.2
nitrate
8.45
7.60
100
1080
0.69
1.9
sulfate
-
-
17
1000
0.74
2.3
9.14
8.08
5
940
0.76
2.6
triisopropylate
m~g1
r
!
nm
a) The pore volumes and diameters were calculated from the desorption branch of the adsorption isotherms using the BJH-model. The low quality of AlMCM-48 synthesized with aluminum sulfate is also reflected in the adsorption data. An unusually broad pore size distribution (not shown) indicates an ordering which is reduced in comparison with all silica MCM-48 and other AlMCM-48 samples. The nitrogen adsorption is significantly enhanced in AIMCM-48 synthesized with aluminum hydroxide in comparison with the material prepared with aluminum nitrate. The pore volume and the surface area of the latter material are substantially lower, although the nsi/nArratio is in the same order of magnitude. AlMCM-48 synthesized with nsi / hal = 5 using aluminum triisopropylate has a pore diameter of dp = 2.6 nm, which is larger than dp of the all silica MCM-48.
I
-150
i
I
i
-100
I
-50
a
I
0
i
I
50
I
I,
100
Chemical Shift 6 / ppm Figure 4. 27A1MAS NMR spectra of the AlMCM-48 materials prepared using a) aluminum triisopropylate (calcined material), b) aluminum sulfate (calcined material), c) aluminum nitrate and d) aluminum hydroxide.
255 The 27Al MAS ~ spectrum of the sample synthesized with aluminiumtriisopropylate, displayed in Figure 4a, consists of two lines at 8 = 53 ppm and 6 = 0 ppm. While the latter line corresponds to aluminum in octahedral coordination, the signal at 5 = 53 ppm is typical for tetrahedrally coordinated aluminum in mesoporous alumosilicates [11]. A similar 27Al MAS NMR spectrum is observed for AIMCM-41, when aluminum triisopropylate is used as a aluminum source. Samples with aluminum exclusively in tetrahedral coordination are synthesized with water soluble aluminum sources, viz. A12(SO4)3, AI(NO)3 and AI(OH)3, (Figures 4b,c and d). The NMR spectra give a sharp resonance at 8 = 53 ppm originating from tetrahedrally coordinated aluminum. Upon template removal, the sharp 27Al resonance broadens, but does not decrease in absolute intensity. The broadening is a result of decreased symmetry of some aluminum sites in the calcined sample. Attempts to synthesize AIMCM-48 with low nsi/n~-ratios using aluminum nitrate and aluminum hydroxide did not result in the formation of a well ordered cubic phase. In that cases, the resulting X-ray diffraction patterns were similar to the one displayed in Figure 2a. However, the 27A1MAS NMR spectra still only consist of a single line at 8 = 53 ppm. The appearance of not well resolved XRD patterns in MCM-41 type materials, when introducing aluminum is explained by Corma [12] not as a result of a less ordered material, but due to the formation of smaller crystallites of MCM-41. However, a literature survey shows that different synthesis strategies and aluminum sources result in materials of widespread quality and properties, which holds also true in for the cubic MCM-48 phase. More work is necessary to understand the influences of synthesis parameters and the nature of the aluminum source. 4. CONCLUSIONS All silica and aluminum containing MCM-48 samples have been synthesized in 250 ml, 1 1 and 5 1 autoclaves. The synthesis of a larger quantity and slow agitation was found to have a promoting effect on the synthesis of high quality materials. Formation of the cubic phase is achieved after 22 h via condensation of a less well ordered three dimensional structure. However, the synthesis of well ordered aluminum-containing MCM-48 materials with low nsi/n~u-ratios is still a challenge. An nsi/n~-ratio as low as 5 can be obtained using aluminum triisopropylate, but not all aluminum is in tetrahedral coordination. Exclusively tetrahedrally coordinated aluminum is found in samples synthesized with water soluble aluminum sources, viz. AI(OH)3, AI(NO)3, and A12(SO4)3. Unfortunately, the synthesis of high quality MCM-48 is limited to nsi/n~a = 80 and aluminum hydroxide as aluminum source. Further investigations aiming at a substantial reduction of the nsi/n~-ratio are in progress. ACKNOWLEDGMENTS Financial support from Deutsche ForschungsgemeinschaR (DFG) and Fonds der Chemischen Industrie is gratefully acknowledged. M.H. acknowledges generous support from Prof. J. Weitkamp.
256 REFERENCES
[1]
C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359, 710(1992). [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C. T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992). [3] A.A. Romero, M.D. Alba, W. Zhou, J. Klinowsky, J. Phys. Chem. B, 101, 5294 (1997). [4] R. Schmidt, H. Junggreen and M. St6cker, Chem. Commun., 875 (1996). [5] D. Zhao and D. Goldfarb, in: Zeolites a refined tool for designing catalytic sites, L. Bonneviot and S. Kaliaguine (eds.), Stud. Surf. Sci. Catal. Vol. 97, Elsevier, Amsterdam: 1997, pp. 181-188. [6] R. Ryoo, S. Jun, J. M. Kim and M.J. Kim, Chem. Commun., 2225 (1997). [7] C. Bischof and M. Hartmann, in preparation. [8] Q. Huo, D.I Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Siegler, A. I. Firouzi, B.F. Chmelka, F. Sch0th and G.D. Stuck),, Chem Mater., 6, 1176 (1994). [9] S.B. Pu, J.R. Kim, M. Seno and T. Inui, Microporous Mater., 10, 25 (1997). [10] P.L. Liewellyn, Y. Cillet, F. Sch0th, H. Reichert and K.K. Unger, Microporous Mater., 3,345 (1994). [ 11] Z. Luan,, C.F. Cheng, W. Zhou and J. Klinowski, J. Phys. Chem., 99, 10 ! 8 (1995). [12] A. Corma, Chem. Rev., 97, 2373 (1997).