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J. Phys. Chem. B 2000, 104, 4840-4843
Amine Dendrimers as Templates for Amorphous Silicas Gustavo Larsen* and Edgar Lotero Department of Chemical Engineering, UniVersity of Nebraska-Lincoln, Nebraska 68588-0126
Manuel Marquez Los Alamos National Laboratory, Chemical Science & Technology DiVision, CST-DO, Los Alamos, New Mexico 87545, and The Nanotechnology Laboratory (NanoteK) Research & DeVelopment, Kraft Foods Inc., 801 Waukegan Rd., GlenView, Illinois 60025 ReceiVed: January 25, 2000; In Final Form: March 10, 2000
The generation 4.0 PAMAM dendrimer is used as a template for silica gels made from tetraethyl orthosilicate via the sol-gel method. X-ray diffraction and adsorption data show that quasi-spherical dendritic structures can be used as porogens. Used as templates, amine dendrimers are unique in that (I) they have the potential, depending upon dendrimer size, to produce mesopores and micropores with single molecules instead of micelles, and (II) spheroidal pores are the expected pore shape imprinted on inorganic solids.
Introduction Dendrimers are oligomeric molecules with quasi-spherical appearance.1 Typically, these unique molecular micelles have polar groups on both their outer and inner surfaces. The inner/ outer surface ratio is known to increase with generation number.1 In Figure 1, the structure of the so-called Starburst polyamidoamine generation 4.0 (PAMAM) dendrimer is shown. Unlike true micelles, the packing density of a dedrimer’s internal chains is relatively low. Thus, their unusual low-density molecular micelle properties allow them to be employed for rather dissimilar purposes, such as hosts for well-defined metal clusters,2 and precursors for reverse micelles. 3 The behavior of amine dendrimers in aqueous-organic solutions containing silicon sol-gel precursors is expected to resemble to some extent that of alkylamines. In fact, the mesoporous silica and silica-alumina materials originally prepared by Mobil researchers were based on the use of longchain alkylamines as templates.4 Earlier work at Mobil had also focused on pore size control.5,6 In the mid 1970s, Mitchell and Whitehurst5,6 described a method that made use of monoalkylsubstituted silicon alkoxides, as well as alkoxides of transition metals, to affect pore size control (not necessarily pore shape and supramolecular ordering) on silica-based catalytic materials. A priori, there must be at least two key differences between dendrimer and alkylamine template chemistry. Alkylamine micelles should display a larger packing density of their internal hydrocarbon chains relative to that of dendrimers, but should also have greater flexibility in terms of micelle diameter. The swelling of alkylamine micelles with mesitylene was originally used to control the final diameter of mesoporous silicas.4 From a fundamental viewpoint, it appears attractive to determine whether dendrimers can be utilized to imprint cavities via the sol-gel method. During the course of our research, a report on the use of carbosilane dendrimers to produce a sol-gel material with both meso- and microporous structure was presented. 7 The use of carbosilane dendrimers by these authors was primarily aimed at synthesizing hybrid materials with tunable porosity * To whom all correspondence should be addressed.
Figure 1. PAMAM dendrimer.
without template removal. Based on an extensive literature search, we found no reports on the imprinting of (dendrimerfree) cavities in xerogels by means of dendrimer templates. Thus, the purpose of this paper is to report on the use of a commercially available amine dendrimer to obtain templated xerogels. Since dendrimer structures with Si in their skeletons should a priori leave Si atoms upon conventional removal of the template by calcination, we view the use of amine dendrimers as a potentially more practical route to produce dendrimer-templated cavities. Experimental Section We report on the preparation and characterization of one dendrimer-templated material. In brief, the sample was prepared by mixing 4.62 g of the commercial (Aldrich) methanol solution of Starburst PAMAM generation 4.0 dendrimer, with 2.58 g n-butanol and 2.31 g tetraethyl orthosilicate (TEOS), and 0.5 mL HCl 0.12 N. The resulting TEOS/dendrimer molar ratio is approximately 374. However, better insight into this issue is gained if one calculates the number of Si atoms per surface amine group in the PAMAM molecule. In our case, this ratio is roughly 6, which indicates that somewhat dilute packing of voids is expected. The mixture was then left in a closed container for 3 days at 343 K. The resulting white paste was subsequently dried at 373
10.1021/jp000301v CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000
Amine Dendrimers as Templates for Amorphous Silicas
J. Phys. Chem. B, Vol. 104, No. 20, 2000 4841
Figure 2. XRD patterns of Star-1.
K for 3 h, and the fine powder was placed in a quartz U-tube to be further dried under a nitrogen flow for 1.5 h at 803 K. The last synthesis step consisted of calcination under flowing air for 3 h at 833 K. This sample was labeled as Star-1. The selection of both upper temperature limits for the drying and calcination steps resulted from considerable experimentation and was not in any way arbitrary. A complex decomposition pattern takes place during the temperature-programmed drying cycle, which primarily involves evolution of olefins, carbon monoxide, water, and alcohols. Release of gaseous products stops after the 1.5 h temperature plateau (803 K). The temperatureprogrammed oxidation step showed evolution of water and carbon dioxide almost exclusively. The latter ceases immediately after the final temperature (833 K) is achieved. Thus, a pretreatment procedure leading to a carbon/hydrogen free material was carefully established. Other means of template removal (e.g., solvent extraction with Cl2CH2, methanol, and ethanol) proved ineffective. X-ray diffraction patterns were obtained with a computerinterfaced Rigaku instrument using Cu KR radiation. A custombuilt greaseless glass line equipped with a Baratron pressure transducer, mechanical and diffusion pumps, and bakeable threeO-ring Teflon stopcocks was used for the adsorption measurements. Results and Discussion The X-ray diffraction patterns of the calcined and as-prepared Star-1 sample are shown in Figure 2. The as-prepared material (i.e., upon drying at 373 K) showed the same low 2θ reflection (but of significantly lower signal intensity), slightly shifted toward lower angles. The 2θ values for the calcined and asprepared Star-1 sample were 2.7 and 2.5, respectively. This is consistent with the idea that contraction of the X-ray coherent distance in the gel structure occurs upon calcination and removal of the template. No higher order 2θ reflections were observed. The calculated distance for the calcined sample is roughly 32 Å. Interestingly, the experimental radius of the PAMAM generation 4.0 dendrimer is 40 Å,1 which could a priori lead one to believe that successful imprinting of a mesoporous cavity has been achieved. We then proceeded to measure the nitrogen adsorption isotherm of Star-1 at 77 K (see Figure 3). Clearly, in a P/Po range in which mesoporosity for a template the size of the PAMAM dendrimer is generally evident, this material presents an adsorption isotherm characteristic of microporous solids.
Figure 3. Nitrogen adsorption isotherm at 77 K and DubininRadushkevitch plot (inset).
Strong adsorption at low P/Po, coupled with the fact that the data appears to follow the Dubinin-Radushkevitch equation (see Figure 3 inset) is taken as evidence for microporosity. Indeed, when BET analysis is performed in the 0.05-0.35 P/Po range a high surface area (623 m2/g) is derived, but it is accompanied by the occurrence of an unphysical (-39.3) CBET constant. This has long been associated with a strong micropore contribution to the adsorption process, and it is often observed in N2 isotherms of zeolites in which the micropore and (meso+macro)-pore adsorption contributions have not been deconvoluted.8 Furthermore, the fitting of data to the DubininRadushkevitch equation indicates that the micropore volume of Star-1 coincides with that of the plateau adsorption value shown in Figure 3. To determine the nature of microporosity in a temperature range that is closer to that of conventional applications, we proceeded to measure the krypton adsorption uptake of Star-1 at 195 K, and model the adsorption data with a modified Horvath-Kawazoe algorithm.9,10 We adopted the spherical cavity model of Cheng and Yang,10 which appears to be particularly suitable for our material, given the quasi-spherical nature of the dendrimer template. In addition to the issue of selected pore geometry, it must be mentioned that the extrapolated Psat of Kr at the chosen adsorption temperature is extremely high, which made it impossible to correct for coverage effects as proposed by Chen and Yang.10 Thus, we were forced to maintain the Henry’s law type approach used in the original Horvath-Kawazoe (HK) model.9 Calibration of the method with two zeolites, namely 5A and HY, is shown in Figure 4. The
4842 J. Phys. Chem. B, Vol. 104, No. 20, 2000
Larsen et al.
Figure 4. Horvath-Kawazoe plots of zeolite reference materials. Dashed lines are first derivatives.
Figure 5. Horvath-Kawazoe plots of Star-1 and SiO2 blank. First derivatives are shown as solid (SiO2) and dashed (Star-1) lines.
TABLE 1: Physical Parameters for Kr Adsorption at 195 K Adopted for the Modeling of Data under the Horvath-Kawazoe/Spherical Cavity Formalism
species
mag. susceptibility [cm3]
polarizability [cm3]
vapor pressure [Torr]
diameter [nm]
oxide iona Kr
1.3 × 10-29 1.57 × 10-29 b
2.5 × 10-24 4.65 × 10-24
s 27 945c
0.27 0.393d
a Surface density taken as 1.4 × 1015 ions/cm2 (ref 8). b Only free parameter, changed from the experimental value of 2.484 × 10-29 (see ref 8 and refs cited therein) to the value shown in table. c Extrapolated from vapor pressure data (ref 10). d Calculated as DKr ) DAr (RKr/RAr), where DAr is the value used for the HK model in ref 8, and the ratio (RKr/RAr) is the ratio of atomic radii data.
required input parameters are shown in Table 1. With the exception of the Kr polarizability, the method did not require the changing of any other physical parameter, which in turn were obtained from literature.9,10 The full width at halfmaximum of the HK plots for zeolites is finite (about 3 Å) due to coverage effects not taken into account under the Henry’s law approach.10 It should be mentioned that Kr is supercritical at 195 K making the extrapolated Po value for the latter a model convenience. However, the adsorption of supercritical gases has been suggested as useful,10 and formation of zeolite-encaged XenKrm clusters at temperatures as high as 300 K has been reported.11 The Kr adsorption method proved both simple (microporosity is probed in the 1-1000 Torr absolute pressure range with this technique) and accurate (Figure 4 predicts accurate cavity sizes for both 5A and HY). The Kr HK plot for Star-1 is shown in Figure 5. Interestingly, this material has an unusual pore size distribution in the micropore range. For comparison, the HK curve for a blank material (prepared without the dendrimer template) is shown. The latter displays a broad pore size distribution curve with a maximum around 15-16 Å, as expected for microporous (disordered) silicas.7 The X-ray coherent length is different from either pore size maxima observed in Star-1 because such distance must represent some extent of periodicity between voids, rather than cavity diameter. Furthermore, the cavity size that one would expect from a Starburst PAMAM generation 4 dendrimer (64 terminal -NH2 groups) should on the order of 40 Å.1 It appears that contraction upon heating takes place, which is conceivably due to an inward collapse of the low-density dendrimer structure (see Figure 6). This phenomenon leads to templated microcavities that are somewhat ordered in the X-ray sense. A plausible
Figure 6. Proposed mechanism of pore formation: short-range ordering.
explanation for the contraction is that the low-density structure of dendrimers (unlike alkylamine micelles) cannot withstand the high pressure resulting from gel densification during heating. While this material is unlikely to compete with zeolites to produce well-defined microcavities, its amorphous nature makes it an interesting class of its own. There are several questions that remain to be answered. First, it is conceivable that more dense dendrimer templates (e.g., polyether dendrimers1) might be able resist contraction upon heating. In addition, the proposed contraction might well be due to solvent effects in the synthesis medium, which could be controlled. Dendrimers are known to adopt “dense-core” or “dense-shell” structures depending on the type of solvent.13 Second, the issue of pore connectivity is an important one from a practical viewpoint. Note that the cavities are probably quasispherical, and a pore network is unlikely to result unless a more complex supramolecular assembly process takes place during the initial synthesis stages. If rather unrestricted molecular traffic through a pore network is desirable (as in catalysis and adsorption), the tailoring of the latter becomes a crucial issue. Clearly, the dendrimer/TEOS ratio will likely become a critical variable, as progressively thinner walls may ultimately yield to adequate pore connectivity. We do not have a satisfactory explanation for the bimodal pore size distribution observed in Star-1. One possibility is that a lower generation dendrimer impurity leads to the small 11 Å peak in Figure 5. On the other hand, it is less likely that the 11 Å pore size distribution peak is due to the SiO2 natural microporosity. Note that the pore size distribution curve of the blank material is rather broad (Figure 5). The acidity of the synthesis medium is rather modest. The final HCl concentration is roughly 0.006 M, which corresponds to a pH of 2.2. Prolonged heating and higher acidity levels are generally required for decomposition of the amide bond.14 Thus, partial degradation of the dendrimer leading to template species
Amine Dendrimers as Templates for Amorphous Silicas of smaller diameter during synthesis is an unlikely explanation for the bimodal distribution observed. Furthermore, the value for N2 adsorption uptake shown in Figure 3 for our material is approximately equal to that reported for zeolite 5A.15 If a pore volume of 0.3 cm3/g is assumed for zeolite 5A,15 then its Kr uptake, in g/g, appears as physically reasonable (see Figure 4) once the liquid Kr density is taken into account. The latter is 2.4 g/cm3.12 Note that in Figure 4, the Kr uptake for zeolite 5A did not reach its theoretical plateau of 0.72 g/g. This situation is even more pronounced in our material. Even though the inflection points, which are key in pore size determinations, were adequately captured by the Kr adsorption experiment, the portion of the uptake curves covered in this study is still quite far from the adsorption plateau. Thus, a total Kr uptake cannot be inferred from the data shown in Figure 5. The leveling off the Kr uptake in zeolite 5A is much more pronounced than in our material because the former necessarily has a single type of pore. Our material, though templated, might still have some level of broadening in its pore size distribution. It is expected that as the synthetic procedure is improved the X-ray domains might become large enough to allow us to elucidate, via analysis of higher order reflections, the geometric arrangement of the imprinted cavities. This information might be difficult to extract from transmission electron microscopy. Note that even if well-ordered stacks of spherical cavities are formed, their observation (in the absence of very good chaintype connectivity) should be more difficult than in channeltype meso- and microporous materials because the latter can be imaged much more easily along channels. Work with other dendritic structures would reinforce the idea that a template effect is responsible for the observed physical properties of these new materials. With regard to this issue, while our work was under review we preprared similar materials using the so-called DAB-Am-n, all-amine type, dendrimer family. These results will be reported in a separate paper.
J. Phys. Chem. B, Vol. 104, No. 20, 2000 4843 Acknowledgment. We acknowledge support from the National Science Foundation (CTS- CTS-9733756), Kraft Foods Inc., and the Nebraska Research Initiative. Supporting Information Available: Figures of XRD patterns and of the temperature programmed oxidation and pyrolysis of Star-1 gel. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tomalia, A. D.; Naylor, A. M.; Goddard III, W. A.; Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (2) Zhao, M.; Sun, L.; Crooks, R. M.; J. Am. Chem. Soc. 1998, 120, 4877. (3) Chechik, V.; Zhao, M.; Crooks, R. M.; J. Am. Chem. Soc. 1999, 121, 4910. (4) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schnitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L.; J. Am. Chem. Soc. 1992, 114, 10 834. (5) Mitchell, T. O.; Whitehurst, D. D., U. S. Pat. No. 3, 983, 055, assigned to Mobil Oil Corp. (6) Mitchell, T. O.; Whitehurst, D. D., U. S. Pat. No. 4, 086, 261, assigned to Mobil Oil Corp. (7) Kriesel, J. W.; Tilley, T. D.; Chem. Mater. 1999, 11, 1190. (8) Remy, M, J.; Pocelet, G.; J. Phys. Chem. 1995, 99, 773. (9) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470. (10) Cheng, L. S.; Yang, R. T. Chem. Eng. Sci. 1994, 49, 2599. (11) Jameson, C. J.; Jameson, A. K.; Lim, H.-M. J. Chem. Phys. 1997, 107, 4364. (12) 12. CRC Handbook of Chemistry and Physics, 78th ed.; Hide, D. R., et al., Eds., 78th Edition, CRC Press: Boca Raton, FL, 1997-1998. (13) Bosman, A. W.; Janssen, H. M.; Meijer, E. W.; Chem. ReV. 1999, 99, 1665. (14) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms and Structure; McGraw-Hill: New York, 1968; p. 313. (15) Breck, D. Zeolite Molecular SieVes; J. Wiley & Sons: New York, 1974; p. 600.
