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Supermicroporous alumina–silica zinc oxides
Microporous and Mesoporous Materials 90 (2006) 81–86 www.elsevier.com/locate/micromeso
Supermicroporous alumina–silica zinc oxides Boris G. Shpeizer, Vladimir I. Bakhmutov, Abraham Clearfield
*
Texas A&M University, Department of Chemistry, P.O. Box 30012, College Station, TX 77842-3012, USA Received 21 July 2005; received in revised form 1 October 2005; accepted 15 October 2005 Available online 15 December 2005 Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract ˚ range have been prepared. TEOS acts as a solvent and as a A new family of porous mixed oxides with pores largely in the 8–20 A source of silica to which aluminum butoxide and transition metal acetates are added. Neutral amines are added as templates and to effect hydrolysis. This paper describes the ZnO–Al2O5–SiO2 system but similar results have been obtained with other transition metal oxides. An interesting feature of the technique is that the larger the amine template the greater is the surface area of the mixed oxide with only a slight increase in the average pore diameter. Both NMR and atomic pair distribution functional methods have been used to prove the homogeneity of the mixed oxide products. This preparative method complements our earlier report in Chemical Communications on mixed oxides prepared with ZrO2 and TiO2 incorporating transition metal oxides. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Porous oxides; Supermicroporous oxides; Alumina–silica NMR; Microporous zinc–alumina–silica; Amine templated oxides
1. Introduction Zeolites are among the most important class of known porous materials, finding valuable uses as catalysts, sorbents, ion exchangers and in a host of other applications [1,2]. Their limitation is that their pore sizes are relatively small. This is a blessing as well as a limitation in that their small pore or cavity size results in strong interactions between the cavity or tunnel with the sorbed substrates. Nevertheless, several experts have discussed the need for zeolites with larger pore sizes [3,4] and many efforts have been expended in this direction. Although zeolites with larger pores have been synthesized they have not found commercial usage because of lattice instability or pore blockage. A turning point was reached with the discovery of FSM16 [5] and MCM-41 [6]. These compounds provided a means of producing silica based materials of high surface ˚ . Since areas and pores with diameters from 20 to 100 A *
Corresponding author. Tel.: +1 979 845 2936; fax: +1 979 845 2370. E-mail address: Clearfi[email protected] (A. Clearfield).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.10.023
then, a great effort into the synthesis, properties and application of the silica and aluminosilica based materials has been expended. These topics have received attention in several comprehensive reviews [7–9]. Soon after, similar mesoporous materials were synthesized for non-silica oxides for which a comprehensive review to 2001 is available [10]. Recently we reported preparing porous mixed oxides of chromia–zirconia and NiO–SiO2 [11]. These mixed oxides were prepared by dissolving their respective metal acetates in propanol using hexyl or octylamine as templates and hydrolysis agents. Addition of water with or without heating resulted in hydrolysis to form a gel. The gel was refluxed or treated hydrothermally and the solids recovered by centrifugation and washed with 95% ethanol until free of solvent and acetate ion. Calcination at 260–400 °C yielded amorphous mixed oxides with regular pores in ˚ desired size range. the 10–20 A In this paper we describe a second method of preparing ˚ diameter mixed oxides with pores generally in the 10–20 A range. It involves the use of tetraethylorthosilicate (TEOS) as the medium for gel formation.
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2. Experimental 2.1. Synthesis The general method is to place a fixed amount of TEOS in a beaker to which is added about 5 mol% of aluminum isobutoxide. An amount of amine was added that was two-thirds the number of moles of TEOS. Then the desired amount of first row transition element is added in the desired amount and the whole is stirred for 45 min. Water is then added in an amount equal to or slightly more than the number of moles of TEOS. This mixture is initially a clear solution that forms a clear gel in 1–2 hours. The gel was left to age for 20 h and then evaporated to dryness in an oven at 120 °C. The glassy looking particles were pulverized and heated to 400–500 °C. As a specific example we use zinc acetate as the source of transition metal additive. 24.45 g of TEOS (120 mmol) was placed into a 100 ml beaker and 1.56 g of aluminum isobutoxide (6.3 mmol) added. This addition was followed by adding 4.75 g of propylamine (80.4 mmol) to the mix. Then 5.47 g of zinc acetate, Zn(OOCCH3)2 Æ 2H2O (25 mmol), was added and the mixture stirred for 45–50 min. This treatment was followed by addition of 2.54 g (141 mmol) of ddi H2O (distilled and deionized) to the beaker and the whole was aged for 20 h. The beaker was then placed into a preheated (120 °C) oven and heated until completely dry. The mole ratio of reactants is SiO2:Al2O3:ZnO:H2O:amine 1:0.026:0.20:1.2:0.688. The product was heated to 400 °C overnight in air to remove organic matter as determined by a lack of any 13C resonance in a SS MAS NMR spectrum. CHN analysis yielded 0.65%, 0.61%, <0.02%, respectively. A second set of samples was heated to 500 °C overnight and yielded almost identical sorption isotherms. This experiment was repeated for a series of n-alkylamines using the same mole ratio of amine as given above except for octadecylamine where 0.0678 mol was added. 2.2. Chemicals and instrumental Aluminum tri-sec-butoxide, TEOS, propyl-, amyl- and octadecyl amine were from Aldrich, tetra-, hexyl-, and octylamine (Lancaster), heptylamine (Fluka AG, Buchs SG) and zinc acetate (Fisher). Thermogravimetric analyses were carried out on a TA Q500 instrument at a heating rate of 10 °C/min under nitrogen or under air as required. X-ray diffraction powder data (XRPD) were taken with a Bruker Avance unit at 40 kV and 40 mA from 1 to 40 °C 2h. CHN analyses were carried out by Robertson Microlit Laboratories. 2.3. NMR The room-temperature solid-state NMR experiments were performed on a Bruker Avance-400 spectrometer at the frequency of 400 MHz for 1H nuclei. 29Si nuclei were observed using direct ({1H}) or cross-polarization (CP)
from the neighboring protons with relaxation delays of 20 and 5 s, respectively. The maximum H–Si polarization transfer was achieved at a contact time of 6 ms. The CP 1 H-spin-lock experiments were performed with a locking time of 10 ms. 27Al nuclei are registered by direct polarizations with relaxation delays of 0.4 s. 1H MAS spectra were collected at relaxation delays of 10 s. In all cases, 4 mm zirconia rotors were applied. Calibration standards were external with Al2O3 and TMS for 27Al and 29Si, respectively. 2.4. Porosity and texture Sorption–desorption isotherms were obtained by N2 sorption at 77 K and by Ar at 87 K. Micropore distribution curves were obtained from a plot of pore volume as a func˚ ) by the MP method. The tion of mean pore diameter (A instrument used was an Autosorb-6 from Quantachrome. Samples were outgassed at 324 °C for 24–48 h. 3. Results and discussion 3.1. Results X-ray patterns showed that the solids were completely amorphous. Fig. 1 presents the N2 sorption–desorption isotherms of the ZnO–Al2O3–SiO2 preparations in the ratio 0.2:0.05:1.0. It is striking that the surface area increases rapidly as the chain length of the n-alkyl amine increases. However, the increase is stepwise, being almost identical for propyl and butylamine, amyl and hexylamine and heptyl and octylamine. The surface area is seen to increase from 420 m2/g for propylamine to about 870 m2/g for octadecylamine (Table 1). However, the pore size increase is remarkably small. It should be added that about 10% of the pore volume is due to larger pores up to about ˚ . As the surface area increases so does the percentage 25 A of larger pores. However, even these larger pores do not increase in pore diameter. Fig. 2 shows three representative TGA weight loss curves. These data were taken on samples that had been dried at 130 °C. The weight loss data and the yields of products are collected in Table 2 from which an interesting conclusion can be derived. For all the samples the mole ratios of the ingredients were constant, which for the amines averaged 0.0808 mol (actual values given in the table). For octadecylamine it was less, 0.0678 mol. The yields of the gel dried at 130 °C are seen to increase to more than double that of the propylamine for the octadecylamine templated preparation. The TGA results show that the recorded weight losses parallel the weight increase of the samples. From this data we obtain the total weight of oxides recovered which for 100% recovery amounts to 9.4 g. The percent oxide recovered is less than 100% due to transfer losses and also to the fact that some of the dried propyl and butyl gel clung to their containers and were not removed completely. Subtraction of the oxide weight from the dried gel weight in column one yields
B.G. Shpeizer et al. / Microporous and Mesoporous Materials 90 (2006) 81–86
83
Hexylamine
Propylamine
Butylamine
Amylamine
Heptylamine
Octylamine
Octadecylamine
330
Volume (cc/g)
280
230
180
130
80 0.00
0.25
0.50
0.75
1.00
Relative pressure (P/Po) Fig. 1. Nitrogen adsorption/desorption isotherms on [ZnOAl2O3/SiO2] vs. size of the amine employed.
Table 1 Surface area/pore volume summary of [ZnO in Al2O3/SiO2] Surface area (m2/g) BET method
Propylamine Butylamine Amylamine Hexylamine Heptylamine Octylamine Octadecylamine (369 °C)
Pore volume (cm3/g) t-Method micropore
Total pore volume
Ar
N2
Ar
N2
Ar
N2
Ar
431.5 454.5 580.1 585.9 763.1 765.9 872.5
411.7 438.7 550.69 558.5 728.9 734.6 –
428.7 451.8 577.1 582.5 756.8 757.7 860.01
408.6 436.7 547.3 554.3 721.6 726.2 –
0.183 0.185 0.249 0.250 0.356 0.364 0.449
0.169 0.169 0.232 0.234 0.337 0.344 –
0.178 0.181 0.243 0.245 0.347 0.352 0.430
0.164 0.166 0.226 0.227 0.327 0.331 –
Weight (%)
100
Propylamine
60
Octylamine
40
Octadecylamine
20 0 0
100
200
300
t-Method micropore volume
N2
120
80
Average pore size ˚) Distribution (A
400 500 600 Temperature (˚C)
700
800
900
9–10 8–11 9–11 9–11 10–12 11–13 10–13
that the amount of amine retained by the gel increases in a stepwise fashion and becomes a larger percentage of the organic matter retained by the gel. The pore volume required to contain 1 mol of octadecylamine is much larger than that to contain 1 mol of propylamine, but surprisingly the pore sizes increase only slightly. If we think of the pores having a cylindrical shape, then the same number of small n-amine molecules as larger ones are required to fill a cross-section of the pore. The difference is that the length of the pore and thus the volume and surface area must increase accordingly. Additional evidence for this hypothesis will be sought, however, the increase in the percent of larger pores also needs to be accounted for.
Fig. 2. Thermogravimetric analysis data for representative samples of gel dried at 120 °C.
3.2. NMR spectra
the total organic matter retained by the solid. CHN analyses were obtained for all the samples (Table 3) which showed
As a baseline comparison a sample was prepared without ZnO with an Si/Al ratio of 20 using hexylamine as the base and added acetic acid (20.3 mmol). This sample
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Table 2 Total recovery of the oxides and the amount of amine retained in the dried gel Amine template
Yield of gel at 120 °C (g)
% Wt. loss 1000 °C
Total oxide recovered (g)
% Yield oxides
Total organic (g)
Moles of encapsulated amine
Propyl Butyl Pentyl Hexyl Heptyl Octyl Octadecyl
12.12 12.40 13.95 14.79 17.20 17.79 26.92
31.22 33.10 36.73 38.47 48.06 48.71 68.88
8.34 8.30 8.83 9.10 8.93 9.12 8.38
88.66 88.12 93.81 96.49 94.99 96.85 87.53
3.78 4.10 5.12 5.69 8.27 8.67 18.54
0.0159 0.0185 0.0288 0.0293 0.0416 0.0402 0.0452
Table 3 Carbon, hydrogen and nitrogen analysis of the ZnO–Al2O3–SiO2 samples Amine template
%C
%H
%N
Propyl Butyl Pentyl Hexyl Heptyl Octyl Octadecyl
13.45 17.64 20.63 23.89 30.50 32.86 52.21
3.46 3.69 4.37 4.72 5.52 5.96 8.70
1.84 2.09 2.89 2.77 3.39 3.16 2.35
was investigated as a dried gel (4 G) and calcined at 234 °C and 450 °C (4C-234; 4C-450). 13C spectra were obtained by the CP MAS NMR method. The spectrum of the dried gel clearly shows the presence of the amine (resonances at 13.3, 22.3, 26.3, 31.1, and 40.0 ppm), ethyl alcohol from hydrolysis of the ethoxide groups (17.4 and 58.3 ppm) and acetic acid (22.2 and 173.3 ppm). After heating at 234 °C some organic material is still present as shown by 13C resonances. However, heating at 450 °C removes all of the organic. The 27Al spectrum of the dried gel gave a single resonance with d of +38 ppm referred to external Al2O3
Fig. 3. 27Al solid state MAS NMR (A) for Al2O3–SiO2 gel heated at 120 °C, (B) reheated at 450 °C, (C) ZnO–Al2O3–SiO2 heated at 400 °C.
(Fig. 3A). The resonance represents the +1/2 M 1/2 transition for tetrahedral Al that is incorporated into the silica lattice [12]. The heated sample 4C-450 yielded a spectrum with three broad resonances with maxima at 38, 14 and 16 ppm (Fig. 3B). Because the same pattern of resonances is observed in the 2D 27Al 3Q MAS spectrum, where second-order quadrupolar effects are reduced [13], the resonances with d of +14 and 16 may be assigned to five and six coordinate Al, respectively. Fitting deconvolution procedures carried out for the 27Al spectrum led to 45%, 15% and 40% for the three peaks as populations of the above states. So roughly 45% of the aluminum is retained within the silica lattice on the assumption that the more highly coordinate Al probably results from its migration to the surface or to the pores and interacts with pore water. The 29Si {1H} MAS NMR spectrum of 4C-450 (Fig. 4A) exhibits three strongly overlapped peaks centered at 92, 100 and 108 ppm. These resonances can be better resolved by the 29Si CP MAS NMR experiments (at long contact time, 6 ms, to observe 29Si nuclei with relatively long Si–H distances), particularly when an 1H spin-lock section [14] is incorporated into the H–Si CP sequence (Fig. 4B). Since the CP pulses better excite Si nuclei when located within 2–3 bond lengths from the nearest hydrogen atoms, dramatic changes in the relative line intensities are observed. This allows us to attribute the resonances at 92, 100, and 108 ppm to Q2[Si(OH)2], Q3[SiOH] and 4 Q (Si) environments, respectively [15–17]. Fitting deconvolution procedures for the 29Si{1H} spectrum led to a ratio Q4/(Q3 + Q2) of 1.6. Because the Q3 and Q2 are probably situated on the surface of the silica lattice [15–17], these ratios correspond to silanol groups on the surface of the pore walls. Thus, the thickness of the walls consists of 3–4 layers of condensed silica units. It is important to note that the 29Si CP MAS NMR spectrum of the dried gel and the gel calcined at 450 °C are practically identical. Thus, the silica lattice is already formed in the dried gel and heating does not lead to extensive condensation of silanol groups to give largely Q4 Si species. At the same time the transformation of four coordinate Al on heating was not detectable in the 29Si spectra, probably due to its small concentration and the broadness of the 29Si peak. However, we are synthesizing samples with up to 20 mol% Al to determine the effect of 27Al on the 29Si spectra.
B.G. Shpeizer et al. / Microporous and Mesoporous Materials 90 (2006) 81–86
85
Fig. 5. The 1H solid state MAS NMR of Al2O3–SiO2 sample 4C-450.
Fig. 4. 29Si solid state MAS NMR of (A) Al2O3–SiO2 (5 mol% alumina) heated at 450 °C, hydrogen decoupled, (B) the same sample with 1H–29Si cross-polarization and 1H spin-lock, (C) 29Si spectrum in a similar sample containing 20 mol% ZnO heated at 400 °C, hydrogen decoupled.
provided a fitting deconvolution procedure which led to two lines in a 4/3 ratio located at 102 and 109 ppm. The resonance at 102 ppm with a relatively high intensity was not observed in the spectrum of 4C-450. Since the Al concentration in the Zn containing samples is small, the peak could be assigned to Si–O–Zn sites in the silica lattice. Similar shifts have been reported for Si–O–Ti linkages in Si/Ti materials [13]. Another interesting feature of these NMR spectra is that the 27Al resonance does not shift to upfield positions indicating the absence of coordination numbers greater than four in the ZnO containing samples. 3.3. Discussion
Finally, the 1H MAS NMR spectrum for 4C-450 is shown in Fig. 5. It was recorded at 14 kHz spinning rate and shows a wide and intense line at 6 ppm due to hydrogen bonded water. The spectrum also shows two shoulders of the peak at 8.0 and 8.7 ppm and two weak but sharp resonances at 2.0 and 3.03 ppm. These latter peaks could be attributed to isolated (or vicinal) and H-bonded silanol groups, respectively [18]. Microprobe analysis did not reveal the presence of any impurities. The more intense signals at 8.0 and 8.7 belong to acidic protons [18]. This assignment suggests the existence of acid sites on the surface of the silica lattice. The NMR spectra of three zinc oxide containing samples templated with propylamine, butylamine and hexylamine, respectively, and calcined at 393–400 °C were obtained. They are all quite similar and similar to the spectra of 4C-450. The differences observed in the 29Si CP MAS spectra are that the peaks due to Q2[Si(OH)2], Q3[SiOH] and Q4 are more broadened. Nevertheless, the maxima of the lines observed in the 29Si{1H} MAS NMR spectra are high-field shifted to 108 ppm compared to their CP spectra. The sample prepared with hexylamine showed slightly better resolution in both the 29Si CP and {1H} MAS NMR spectra. The latter spectrum (Fig. 4C)
In our previous paper [11] we described the synthesis of Cr2O3–ZrO2 and NiO–SiO2 by dissolving their acetates or TEOS for silica in propanol and adding hexylamine as a neutral base template. In that paper we showed that addition of alumina as aluminum tributoxide increased the surface area and stability of the resultant oxide mix. We have prepared a number of such oxide systems containing transition element oxides and will report on these later. A major question this study raised is whether the amine was acting as a template or just a base to effect the hydrolysis of the reactants. We felt it was serving both functions and particularly the template role because the precipitated oxide mix always contained a high proportion of the added amine. In our present study, using no solvent other than TEOS, we found that the length of the amine alkyl chain has a marked influence on the surface area. Also the amine trapped in the pores increases as the size of the amine chain increases suggesting a similar role for the amine regardless of its chain length. We intend to explore the effect of different types of amines on the pore structure. Another concern is whether the oxide product is uniform. The NMR data indicates that the oxides form a complete solid solution based on the effect they have on each
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others surroundings. We have also carried out an atomic pair distribution (APDF) study on a mixed oxide sample containing 12 mol% of MnO. Not only could the interatomic Si–O, Mn–O, Si–Si and Mn–Mn be found in the
was always fun to be with. We shall miss them both. I was pleased that we could contribute to this issue in their honor.
spectra but there was clearly a
Acknowledgement
O
distance present. Mn
Si
Furthermore, the 29Si NMR spectrum was greatly affected by the magnetic field of the Mn unpaired electrons in that it exhibited numerous spinning side bands whereas no side bands were evident when manganese was absent. The combined NMR and APDF study is preliminary to a more extended study in progress but indicates that the transition metal forms an oxide layer lining the pores. The APDF results will be published in the near future separately along with microprobe and electron micrographs. Finally it is of great importance that acidic hydrogens are contained in the Zn–Si–Al samples. The ZnO keeps the aluminum in a four coordinate state. Either some of the zinc balances the charge or in the absence of a second cation, H+ must act in this way. Further examination of the proton content and acidity will be undertaken. In summary we have demonstrated new methods of preparing porous mixed oxide systems based on ZrO2, SiO2, Al2O3–SiO2 and TiO2 (paper in preparation) containing transition metal oxides over broad composition ranges [11]. The surface areas can be varied from low (100– 200 m2/g) to high (700–900 m2/g) depending on the tem˚ to near plate and pores can be controlled from about 7–8 A ˚ 20 A. The amount of transition metal loaded in the mixture can be very high up to 40–50% in most cases. Potential for catalytic activity would appear to be very positive and this is a path we are pursuing. Epilogue I knew Denise and George very well. I met Denise at several meetings she attended in the US and when in Paris I always stopped to see her. We had many fruitful discussion on zeolites and pillared clays. George and I are both Philadelphia boys. He always sought me out to confide his latest discoveries and thoughts. I was impressed by his knowledge and enthusiasm for zeolite research and he
This material is based upon work supported by the National Science Foundation under Grant No. DMR0332453, for which grateful acknowledgement is made. We wish to thank Dr. Lev Zakharov for results dealing with APDF and acknowledge with thanks the use of the facilities at the Advanced Photon Source, Argonne National Laboratory (BESSRC/XOR 11-1D-C) beamline supported by the US Department of Energy. References [1] S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Zeolite Science and Technology, Marcel Dekker, Inc., New York, 2003. [2] Proceedings of the International Zeolite Conferences, vols. 1–12. [3] M.E. Davis, Acc. Chem. Res. (1994) 111. [4] A. Corma, M.E. Davis, Chem. Phys. Chem. 5 (2004) 304. [5] T. Yanagiwawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988. [6] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [7] U. Ciesla, F. Schu¨th, Micropor. Mesopor. Mater. 27 (1999) 131. [8] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56. [9] M. Linden, S. Schacht, F. Schu¨th, A. Steel, K.K. Unger, J. Porous Mater. 5 (1998) 177. [10] F. Schu¨th, Chem. Mater. 13 (2001) 3184. [11] B.G. Shpeizer, A. Clearfield, J.M. Heising, Chem. Commun. (2005) 2396. [12] M.J. Duer, in: M.J. Duer (Ed.), Solid-State NMR Spectroscopy: Principles and Applications, Blackwell Science, Oxford, 2002, p. 450. [13] S. Ganapathy, K.U. Gore, R. Kumar, J.-P. Amoureax, Solid State Nucl. Magn. Reson. 24 (2003) 184. [14] M.J. Duer, in: M.J. Duer (Ed.), Solid-State NMR Spectroscopy: Principles and Applications, Blackwell Science, Oxford, 2002, p. 98. [15] R. Mokaya, Micropor. Mesopor. Mater. 119 (2001) 44. [16] R. Mokaya, W. Zhou, W. Jones, J. Mater. Chem. 10 (2000) 1139. [17] R. Simonutti, A. Comotti, S. Bracco, P. Sozzani, Chem. Mater. 13 (2001) 771. [18] T.V. Kovalchuk, H. Sfihi, A.S. Korchev, A.S. Kovalenko, V.G. Ilin, V.N. Zaitsev, J. Fraissard, J. Phys. Chem. B 109 (2005) 13948.
Supermicroporous alumina–silica zinc oxides Boris G. Shpeizer, Vladimir I. Bakhmutov, Abraham Clearfield
*
Texas A&M University, Department of Chemistry, P.O. Box 30012, College Station, TX 77842-3012, USA Received 21 July 2005; received in revised form 1 October 2005; accepted 15 October 2005 Available online 15 December 2005 Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract ˚ range have been prepared. TEOS acts as a solvent and as a A new family of porous mixed oxides with pores largely in the 8–20 A source of silica to which aluminum butoxide and transition metal acetates are added. Neutral amines are added as templates and to effect hydrolysis. This paper describes the ZnO–Al2O5–SiO2 system but similar results have been obtained with other transition metal oxides. An interesting feature of the technique is that the larger the amine template the greater is the surface area of the mixed oxide with only a slight increase in the average pore diameter. Both NMR and atomic pair distribution functional methods have been used to prove the homogeneity of the mixed oxide products. This preparative method complements our earlier report in Chemical Communications on mixed oxides prepared with ZrO2 and TiO2 incorporating transition metal oxides. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Porous oxides; Supermicroporous oxides; Alumina–silica NMR; Microporous zinc–alumina–silica; Amine templated oxides
1. Introduction Zeolites are among the most important class of known porous materials, finding valuable uses as catalysts, sorbents, ion exchangers and in a host of other applications [1,2]. Their limitation is that their pore sizes are relatively small. This is a blessing as well as a limitation in that their small pore or cavity size results in strong interactions between the cavity or tunnel with the sorbed substrates. Nevertheless, several experts have discussed the need for zeolites with larger pore sizes [3,4] and many efforts have been expended in this direction. Although zeolites with larger pores have been synthesized they have not found commercial usage because of lattice instability or pore blockage. A turning point was reached with the discovery of FSM16 [5] and MCM-41 [6]. These compounds provided a means of producing silica based materials of high surface ˚ . Since areas and pores with diameters from 20 to 100 A *
Corresponding author. Tel.: +1 979 845 2936; fax: +1 979 845 2370. E-mail address: Clearfi[email protected] (A. Clearfield).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.10.023
then, a great effort into the synthesis, properties and application of the silica and aluminosilica based materials has been expended. These topics have received attention in several comprehensive reviews [7–9]. Soon after, similar mesoporous materials were synthesized for non-silica oxides for which a comprehensive review to 2001 is available [10]. Recently we reported preparing porous mixed oxides of chromia–zirconia and NiO–SiO2 [11]. These mixed oxides were prepared by dissolving their respective metal acetates in propanol using hexyl or octylamine as templates and hydrolysis agents. Addition of water with or without heating resulted in hydrolysis to form a gel. The gel was refluxed or treated hydrothermally and the solids recovered by centrifugation and washed with 95% ethanol until free of solvent and acetate ion. Calcination at 260–400 °C yielded amorphous mixed oxides with regular pores in ˚ desired size range. the 10–20 A In this paper we describe a second method of preparing ˚ diameter mixed oxides with pores generally in the 10–20 A range. It involves the use of tetraethylorthosilicate (TEOS) as the medium for gel formation.
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B.G. Shpeizer et al. / Microporous and Mesoporous Materials 90 (2006) 81–86
2. Experimental 2.1. Synthesis The general method is to place a fixed amount of TEOS in a beaker to which is added about 5 mol% of aluminum isobutoxide. An amount of amine was added that was two-thirds the number of moles of TEOS. Then the desired amount of first row transition element is added in the desired amount and the whole is stirred for 45 min. Water is then added in an amount equal to or slightly more than the number of moles of TEOS. This mixture is initially a clear solution that forms a clear gel in 1–2 hours. The gel was left to age for 20 h and then evaporated to dryness in an oven at 120 °C. The glassy looking particles were pulverized and heated to 400–500 °C. As a specific example we use zinc acetate as the source of transition metal additive. 24.45 g of TEOS (120 mmol) was placed into a 100 ml beaker and 1.56 g of aluminum isobutoxide (6.3 mmol) added. This addition was followed by adding 4.75 g of propylamine (80.4 mmol) to the mix. Then 5.47 g of zinc acetate, Zn(OOCCH3)2 Æ 2H2O (25 mmol), was added and the mixture stirred for 45–50 min. This treatment was followed by addition of 2.54 g (141 mmol) of ddi H2O (distilled and deionized) to the beaker and the whole was aged for 20 h. The beaker was then placed into a preheated (120 °C) oven and heated until completely dry. The mole ratio of reactants is SiO2:Al2O3:ZnO:H2O:amine 1:0.026:0.20:1.2:0.688. The product was heated to 400 °C overnight in air to remove organic matter as determined by a lack of any 13C resonance in a SS MAS NMR spectrum. CHN analysis yielded 0.65%, 0.61%, <0.02%, respectively. A second set of samples was heated to 500 °C overnight and yielded almost identical sorption isotherms. This experiment was repeated for a series of n-alkylamines using the same mole ratio of amine as given above except for octadecylamine where 0.0678 mol was added. 2.2. Chemicals and instrumental Aluminum tri-sec-butoxide, TEOS, propyl-, amyl- and octadecyl amine were from Aldrich, tetra-, hexyl-, and octylamine (Lancaster), heptylamine (Fluka AG, Buchs SG) and zinc acetate (Fisher). Thermogravimetric analyses were carried out on a TA Q500 instrument at a heating rate of 10 °C/min under nitrogen or under air as required. X-ray diffraction powder data (XRPD) were taken with a Bruker Avance unit at 40 kV and 40 mA from 1 to 40 °C 2h. CHN analyses were carried out by Robertson Microlit Laboratories. 2.3. NMR The room-temperature solid-state NMR experiments were performed on a Bruker Avance-400 spectrometer at the frequency of 400 MHz for 1H nuclei. 29Si nuclei were observed using direct ({1H}) or cross-polarization (CP)
from the neighboring protons with relaxation delays of 20 and 5 s, respectively. The maximum H–Si polarization transfer was achieved at a contact time of 6 ms. The CP 1 H-spin-lock experiments were performed with a locking time of 10 ms. 27Al nuclei are registered by direct polarizations with relaxation delays of 0.4 s. 1H MAS spectra were collected at relaxation delays of 10 s. In all cases, 4 mm zirconia rotors were applied. Calibration standards were external with Al2O3 and TMS for 27Al and 29Si, respectively. 2.4. Porosity and texture Sorption–desorption isotherms were obtained by N2 sorption at 77 K and by Ar at 87 K. Micropore distribution curves were obtained from a plot of pore volume as a func˚ ) by the MP method. The tion of mean pore diameter (A instrument used was an Autosorb-6 from Quantachrome. Samples were outgassed at 324 °C for 24–48 h. 3. Results and discussion 3.1. Results X-ray patterns showed that the solids were completely amorphous. Fig. 1 presents the N2 sorption–desorption isotherms of the ZnO–Al2O3–SiO2 preparations in the ratio 0.2:0.05:1.0. It is striking that the surface area increases rapidly as the chain length of the n-alkyl amine increases. However, the increase is stepwise, being almost identical for propyl and butylamine, amyl and hexylamine and heptyl and octylamine. The surface area is seen to increase from 420 m2/g for propylamine to about 870 m2/g for octadecylamine (Table 1). However, the pore size increase is remarkably small. It should be added that about 10% of the pore volume is due to larger pores up to about ˚ . As the surface area increases so does the percentage 25 A of larger pores. However, even these larger pores do not increase in pore diameter. Fig. 2 shows three representative TGA weight loss curves. These data were taken on samples that had been dried at 130 °C. The weight loss data and the yields of products are collected in Table 2 from which an interesting conclusion can be derived. For all the samples the mole ratios of the ingredients were constant, which for the amines averaged 0.0808 mol (actual values given in the table). For octadecylamine it was less, 0.0678 mol. The yields of the gel dried at 130 °C are seen to increase to more than double that of the propylamine for the octadecylamine templated preparation. The TGA results show that the recorded weight losses parallel the weight increase of the samples. From this data we obtain the total weight of oxides recovered which for 100% recovery amounts to 9.4 g. The percent oxide recovered is less than 100% due to transfer losses and also to the fact that some of the dried propyl and butyl gel clung to their containers and were not removed completely. Subtraction of the oxide weight from the dried gel weight in column one yields
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83
Hexylamine
Propylamine
Butylamine
Amylamine
Heptylamine
Octylamine
Octadecylamine
330
Volume (cc/g)
280
230
180
130
80 0.00
0.25
0.50
0.75
1.00
Relative pressure (P/Po) Fig. 1. Nitrogen adsorption/desorption isotherms on [ZnOAl2O3/SiO2] vs. size of the amine employed.
Table 1 Surface area/pore volume summary of [ZnO in Al2O3/SiO2] Surface area (m2/g) BET method
Propylamine Butylamine Amylamine Hexylamine Heptylamine Octylamine Octadecylamine (369 °C)
Pore volume (cm3/g) t-Method micropore
Total pore volume
Ar
N2
Ar
N2
Ar
N2
Ar
431.5 454.5 580.1 585.9 763.1 765.9 872.5
411.7 438.7 550.69 558.5 728.9 734.6 –
428.7 451.8 577.1 582.5 756.8 757.7 860.01
408.6 436.7 547.3 554.3 721.6 726.2 –
0.183 0.185 0.249 0.250 0.356 0.364 0.449
0.169 0.169 0.232 0.234 0.337 0.344 –
0.178 0.181 0.243 0.245 0.347 0.352 0.430
0.164 0.166 0.226 0.227 0.327 0.331 –
Weight (%)
100
Propylamine
60
Octylamine
40
Octadecylamine
20 0 0
100
200
300
t-Method micropore volume
N2
120
80
Average pore size ˚) Distribution (A
400 500 600 Temperature (˚C)
700
800
900
9–10 8–11 9–11 9–11 10–12 11–13 10–13
that the amount of amine retained by the gel increases in a stepwise fashion and becomes a larger percentage of the organic matter retained by the gel. The pore volume required to contain 1 mol of octadecylamine is much larger than that to contain 1 mol of propylamine, but surprisingly the pore sizes increase only slightly. If we think of the pores having a cylindrical shape, then the same number of small n-amine molecules as larger ones are required to fill a cross-section of the pore. The difference is that the length of the pore and thus the volume and surface area must increase accordingly. Additional evidence for this hypothesis will be sought, however, the increase in the percent of larger pores also needs to be accounted for.
Fig. 2. Thermogravimetric analysis data for representative samples of gel dried at 120 °C.
3.2. NMR spectra
the total organic matter retained by the solid. CHN analyses were obtained for all the samples (Table 3) which showed
As a baseline comparison a sample was prepared without ZnO with an Si/Al ratio of 20 using hexylamine as the base and added acetic acid (20.3 mmol). This sample
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Table 2 Total recovery of the oxides and the amount of amine retained in the dried gel Amine template
Yield of gel at 120 °C (g)
% Wt. loss 1000 °C
Total oxide recovered (g)
% Yield oxides
Total organic (g)
Moles of encapsulated amine
Propyl Butyl Pentyl Hexyl Heptyl Octyl Octadecyl
12.12 12.40 13.95 14.79 17.20 17.79 26.92
31.22 33.10 36.73 38.47 48.06 48.71 68.88
8.34 8.30 8.83 9.10 8.93 9.12 8.38
88.66 88.12 93.81 96.49 94.99 96.85 87.53
3.78 4.10 5.12 5.69 8.27 8.67 18.54
0.0159 0.0185 0.0288 0.0293 0.0416 0.0402 0.0452
Table 3 Carbon, hydrogen and nitrogen analysis of the ZnO–Al2O3–SiO2 samples Amine template
%C
%H
%N
Propyl Butyl Pentyl Hexyl Heptyl Octyl Octadecyl
13.45 17.64 20.63 23.89 30.50 32.86 52.21
3.46 3.69 4.37 4.72 5.52 5.96 8.70
1.84 2.09 2.89 2.77 3.39 3.16 2.35
was investigated as a dried gel (4 G) and calcined at 234 °C and 450 °C (4C-234; 4C-450). 13C spectra were obtained by the CP MAS NMR method. The spectrum of the dried gel clearly shows the presence of the amine (resonances at 13.3, 22.3, 26.3, 31.1, and 40.0 ppm), ethyl alcohol from hydrolysis of the ethoxide groups (17.4 and 58.3 ppm) and acetic acid (22.2 and 173.3 ppm). After heating at 234 °C some organic material is still present as shown by 13C resonances. However, heating at 450 °C removes all of the organic. The 27Al spectrum of the dried gel gave a single resonance with d of +38 ppm referred to external Al2O3
Fig. 3. 27Al solid state MAS NMR (A) for Al2O3–SiO2 gel heated at 120 °C, (B) reheated at 450 °C, (C) ZnO–Al2O3–SiO2 heated at 400 °C.
(Fig. 3A). The resonance represents the +1/2 M 1/2 transition for tetrahedral Al that is incorporated into the silica lattice [12]. The heated sample 4C-450 yielded a spectrum with three broad resonances with maxima at 38, 14 and 16 ppm (Fig. 3B). Because the same pattern of resonances is observed in the 2D 27Al 3Q MAS spectrum, where second-order quadrupolar effects are reduced [13], the resonances with d of +14 and 16 may be assigned to five and six coordinate Al, respectively. Fitting deconvolution procedures carried out for the 27Al spectrum led to 45%, 15% and 40% for the three peaks as populations of the above states. So roughly 45% of the aluminum is retained within the silica lattice on the assumption that the more highly coordinate Al probably results from its migration to the surface or to the pores and interacts with pore water. The 29Si {1H} MAS NMR spectrum of 4C-450 (Fig. 4A) exhibits three strongly overlapped peaks centered at 92, 100 and 108 ppm. These resonances can be better resolved by the 29Si CP MAS NMR experiments (at long contact time, 6 ms, to observe 29Si nuclei with relatively long Si–H distances), particularly when an 1H spin-lock section [14] is incorporated into the H–Si CP sequence (Fig. 4B). Since the CP pulses better excite Si nuclei when located within 2–3 bond lengths from the nearest hydrogen atoms, dramatic changes in the relative line intensities are observed. This allows us to attribute the resonances at 92, 100, and 108 ppm to Q2[Si(OH)2], Q3[SiOH] and 4 Q (Si) environments, respectively [15–17]. Fitting deconvolution procedures for the 29Si{1H} spectrum led to a ratio Q4/(Q3 + Q2) of 1.6. Because the Q3 and Q2 are probably situated on the surface of the silica lattice [15–17], these ratios correspond to silanol groups on the surface of the pore walls. Thus, the thickness of the walls consists of 3–4 layers of condensed silica units. It is important to note that the 29Si CP MAS NMR spectrum of the dried gel and the gel calcined at 450 °C are practically identical. Thus, the silica lattice is already formed in the dried gel and heating does not lead to extensive condensation of silanol groups to give largely Q4 Si species. At the same time the transformation of four coordinate Al on heating was not detectable in the 29Si spectra, probably due to its small concentration and the broadness of the 29Si peak. However, we are synthesizing samples with up to 20 mol% Al to determine the effect of 27Al on the 29Si spectra.
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85
Fig. 5. The 1H solid state MAS NMR of Al2O3–SiO2 sample 4C-450.
Fig. 4. 29Si solid state MAS NMR of (A) Al2O3–SiO2 (5 mol% alumina) heated at 450 °C, hydrogen decoupled, (B) the same sample with 1H–29Si cross-polarization and 1H spin-lock, (C) 29Si spectrum in a similar sample containing 20 mol% ZnO heated at 400 °C, hydrogen decoupled.
provided a fitting deconvolution procedure which led to two lines in a 4/3 ratio located at 102 and 109 ppm. The resonance at 102 ppm with a relatively high intensity was not observed in the spectrum of 4C-450. Since the Al concentration in the Zn containing samples is small, the peak could be assigned to Si–O–Zn sites in the silica lattice. Similar shifts have been reported for Si–O–Ti linkages in Si/Ti materials [13]. Another interesting feature of these NMR spectra is that the 27Al resonance does not shift to upfield positions indicating the absence of coordination numbers greater than four in the ZnO containing samples. 3.3. Discussion
Finally, the 1H MAS NMR spectrum for 4C-450 is shown in Fig. 5. It was recorded at 14 kHz spinning rate and shows a wide and intense line at 6 ppm due to hydrogen bonded water. The spectrum also shows two shoulders of the peak at 8.0 and 8.7 ppm and two weak but sharp resonances at 2.0 and 3.03 ppm. These latter peaks could be attributed to isolated (or vicinal) and H-bonded silanol groups, respectively [18]. Microprobe analysis did not reveal the presence of any impurities. The more intense signals at 8.0 and 8.7 belong to acidic protons [18]. This assignment suggests the existence of acid sites on the surface of the silica lattice. The NMR spectra of three zinc oxide containing samples templated with propylamine, butylamine and hexylamine, respectively, and calcined at 393–400 °C were obtained. They are all quite similar and similar to the spectra of 4C-450. The differences observed in the 29Si CP MAS spectra are that the peaks due to Q2[Si(OH)2], Q3[SiOH] and Q4 are more broadened. Nevertheless, the maxima of the lines observed in the 29Si{1H} MAS NMR spectra are high-field shifted to 108 ppm compared to their CP spectra. The sample prepared with hexylamine showed slightly better resolution in both the 29Si CP and {1H} MAS NMR spectra. The latter spectrum (Fig. 4C)
In our previous paper [11] we described the synthesis of Cr2O3–ZrO2 and NiO–SiO2 by dissolving their acetates or TEOS for silica in propanol and adding hexylamine as a neutral base template. In that paper we showed that addition of alumina as aluminum tributoxide increased the surface area and stability of the resultant oxide mix. We have prepared a number of such oxide systems containing transition element oxides and will report on these later. A major question this study raised is whether the amine was acting as a template or just a base to effect the hydrolysis of the reactants. We felt it was serving both functions and particularly the template role because the precipitated oxide mix always contained a high proportion of the added amine. In our present study, using no solvent other than TEOS, we found that the length of the amine alkyl chain has a marked influence on the surface area. Also the amine trapped in the pores increases as the size of the amine chain increases suggesting a similar role for the amine regardless of its chain length. We intend to explore the effect of different types of amines on the pore structure. Another concern is whether the oxide product is uniform. The NMR data indicates that the oxides form a complete solid solution based on the effect they have on each
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others surroundings. We have also carried out an atomic pair distribution (APDF) study on a mixed oxide sample containing 12 mol% of MnO. Not only could the interatomic Si–O, Mn–O, Si–Si and Mn–Mn be found in the
was always fun to be with. We shall miss them both. I was pleased that we could contribute to this issue in their honor.
spectra but there was clearly a
Acknowledgement
O
distance present. Mn
Si
Furthermore, the 29Si NMR spectrum was greatly affected by the magnetic field of the Mn unpaired electrons in that it exhibited numerous spinning side bands whereas no side bands were evident when manganese was absent. The combined NMR and APDF study is preliminary to a more extended study in progress but indicates that the transition metal forms an oxide layer lining the pores. The APDF results will be published in the near future separately along with microprobe and electron micrographs. Finally it is of great importance that acidic hydrogens are contained in the Zn–Si–Al samples. The ZnO keeps the aluminum in a four coordinate state. Either some of the zinc balances the charge or in the absence of a second cation, H+ must act in this way. Further examination of the proton content and acidity will be undertaken. In summary we have demonstrated new methods of preparing porous mixed oxide systems based on ZrO2, SiO2, Al2O3–SiO2 and TiO2 (paper in preparation) containing transition metal oxides over broad composition ranges [11]. The surface areas can be varied from low (100– 200 m2/g) to high (700–900 m2/g) depending on the tem˚ to near plate and pores can be controlled from about 7–8 A ˚ 20 A. The amount of transition metal loaded in the mixture can be very high up to 40–50% in most cases. Potential for catalytic activity would appear to be very positive and this is a path we are pursuing. Epilogue I knew Denise and George very well. I met Denise at several meetings she attended in the US and when in Paris I always stopped to see her. We had many fruitful discussion on zeolites and pillared clays. George and I are both Philadelphia boys. He always sought me out to confide his latest discoveries and thoughts. I was impressed by his knowledge and enthusiasm for zeolite research and he
This material is based upon work supported by the National Science Foundation under Grant No. DMR0332453, for which grateful acknowledgement is made. We wish to thank Dr. Lev Zakharov for results dealing with APDF and acknowledge with thanks the use of the facilities at the Advanced Photon Source, Argonne National Laboratory (BESSRC/XOR 11-1D-C) beamline supported by the US Department of Energy. References [1] S.M. Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of Zeolite Science and Technology, Marcel Dekker, Inc., New York, 2003. [2] Proceedings of the International Zeolite Conferences, vols. 1–12. [3] M.E. Davis, Acc. Chem. Res. (1994) 111. [4] A. Corma, M.E. Davis, Chem. Phys. Chem. 5 (2004) 304. [5] T. Yanagiwawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988. [6] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [7] U. Ciesla, F. Schu¨th, Micropor. Mesopor. Mater. 27 (1999) 131. [8] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56. [9] M. Linden, S. Schacht, F. Schu¨th, A. Steel, K.K. Unger, J. Porous Mater. 5 (1998) 177. [10] F. Schu¨th, Chem. Mater. 13 (2001) 3184. [11] B.G. Shpeizer, A. Clearfield, J.M. Heising, Chem. Commun. (2005) 2396. [12] M.J. Duer, in: M.J. Duer (Ed.), Solid-State NMR Spectroscopy: Principles and Applications, Blackwell Science, Oxford, 2002, p. 450. [13] S. Ganapathy, K.U. Gore, R. Kumar, J.-P. Amoureax, Solid State Nucl. Magn. Reson. 24 (2003) 184. [14] M.J. Duer, in: M.J. Duer (Ed.), Solid-State NMR Spectroscopy: Principles and Applications, Blackwell Science, Oxford, 2002, p. 98. [15] R. Mokaya, Micropor. Mesopor. Mater. 119 (2001) 44. [16] R. Mokaya, W. Zhou, W. Jones, J. Mater. Chem. 10 (2000) 1139. [17] R. Simonutti, A. Comotti, S. Bracco, P. Sozzani, Chem. Mater. 13 (2001) 771. [18] T.V. Kovalchuk, H. Sfihi, A.S. Korchev, A.S. Kovalenko, V.G. Ilin, V.N. Zaitsev, J. Fraissard, J. Phys. Chem. B 109 (2005) 13948.