Structural Insight into Germanium-Containing Silicate Species by

Jul 22, 2009 - The properties of zeolitic and related materials highly depend on their pore dimensions and topology as well as the presence and nature...
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Structural Insight into Germanium-Containing Silicate Species by Electrospray Ionization Mass Spectrometry (ESI-MS) and ESI-MS/MS Bernd Bastian Schaack, Wolfgang Schrader, and Ferdi Schu¨th* Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim a.d. Ruhr ReceiVed: April 5, 2009; ReVised Manuscript ReceiVed: June 29, 2009

The properties of zeolitic and related materials highly depend on their pore dimensions and topology as well as the presence and nature of framework heteroelements. Here, we present detailed insights into prenucleating solutions of zeolitic materials using two different organic structure-directing agents (SDAs) in the presence of fluoride ions. The silicate speciation was analyzed by means of electrospray ionization mass spectrometry (ESI-MS), wherein the structures of the occurring species were proven to depend on the nature of the used organic SDA. In addition, MS/MS experiments were revealed as a powerful tool to obtain detailed structural insight into the occurring silicate oligomers. With this information, we were able to locate heteroelements, namely, germanium, within these species and, furthermore, to determine the structure-directing effects of the used organic SDAs. Introduction Microporous materials, and especially zeolites, are widely used in technologically important processes such as catalysis, separation, and adsorption.1 They are crystalline silicates formed by corner-sharing TO4-tetrahedra, the properties of which are strongly related to their pore topology as well as the presence and location of intra- and/or extraframework heteroelements.2 Thus, the design and tailored synthesis of zeolitic materials with properties for specific applications is one of the challenges for researchers in both industry and academia. Important advances have been made by the development of structure-directing agents (SDAs), mostly organic cations, which can highly influence the structure of the obtained zeolite.3 Furthermore, it is known that Si-O-Si angles in the range of 140-180° are energetically preferred in pure silica materials.4,5 Thus, the introduction of heteroelements into the framework allowed the synthesis of materials with new pore topologies by minimizing the formation energy of smaller Si-O-M angles.6,7 Novel tridirectional large pore zeolites containing four ring (4R) and double four ring (D4R) building units were successfully synthesized by the substitution of framework silicon against germanium.8-11 The obtained materials are of highest interest, since they are suitable to allow access of large molecules, which is crucial in such important processes as oil refining. Although the structuredirecting effect of germanium toward the formation of 4R and D4R motives is well-known, little is known about the underlying chemistry and the distribution of germanium in such solids. This is mostly due to analytical limitations, since germanium in zeolites is elusive to most techniques, such as, for instance, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The fact that such materials are rarely obtained as single crystals further complicates the localization of framework germanium. However, in previous studies, we have shown that electrospray ionization mass spectrometry (ESI-MS) is a suitable tool to study prenucleating12,13 and nucleating14 solutions of zeolitic materials. In particular, we were able to detect oligomeric silicate species, immediately before nucleation, with * To whom correspondence should be addressed. E-mail: [email protected].

characteristic structures of the final solid.15 A detailed structural analysis of such species would yield insight into their germanium distribution and thus contribute to the localization of framework germanium. While conventional MS experiments predominantly give information on the mass to charge ratio (m/z), additional structural information can be obtained by MS/MS studies, which were also performed here. We here present detailed structural investigations on oligomeric silicate species that occur in prenucleating solutions of zeolitic materials, using two different organic SDAs; the N(16)methylsparteinium cation (in the following referred as SDA1), which acts as SDA in the synthesis of ITQ-21,16,17 and the 4-methyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3.2.1-ij]quinolinium cation (in the following referred as SDA-2), whose supramolecular self-assembling effect was used for the synthesis of zeolite A (ITQ-29, LTA topology).18 The composition of the studied solutions is characterized by an increased water amount as compared to the synthesis mixtures of ITQ-21 and zeolite A to achieve long-term stability without the formation of the zeolites. The synthesized prenucleating solutions were analyzed by means of ESI-MS. Structural information on the occurring oligomeric species as well as on their germanium distribution were obtained by MS/MS experiments. Experimental Section Synthesis. The preparation of SDA-119 and SDA-218 in their hydroxide forms followed general synthesis procedures previously described in the literature. The prenucleating solutions were synthesized by dissolving germanium dioxide (GeO2; Aldrich, >99.99%) in an aqueous solution of the organic template. After the addition of hydrofluoric acid (HF; SigmaAldrich, 99.99%), the silicon source (tetraethylorthosilicate, TEOS; Aldrich, 99.999%) was added, leading to prenucleating solutions with the following molar composition: 0.67 TEOS: 0.33 GeO2:0.5 ROH:0.5 HF:100 H2O with ROH:SDA-1 or SDA-2 in its hydroxide form. Characterization. All recorded mass spectra as well as all MS/MS experiments were performed on a Bruker Esquire 3000 mass spectrometer with an electric ion trap, equipped with an Agilent ESI source. The capillary voltage was set to 4 kV, while

10.1021/jp903132b CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

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Figure 1. (a) ESI mass spectrum of a prenucleating solution with the following molar composition: 0.67 TEOS:0.33 GeO2:0.5 SDA-1(OH):0.5 HF:100 H2O. Selected signals are labeled, and corresponding silicate structures are displayed. A complete list of the detected species can be found in the Supporting Information. (b) Structure of ITQ-21. Inset: single 4R entrapped in a [46612]-cage (Crystallographic Information File taken from ref 17).

a flow rate of 120 µL min-1 was applied. For MS/MS experiments, an isolation width of 1.0 Da and a fragmentation amplitude of 1.30 V were chosen. Results and Discussion SDA-1. After GeO2 was dissolved in an aqueous solution of the SDA-1 cation in its hydroxide form containing HF, the silicon source (TEOS) was added. Consequently, it dissolves within minutes, leading to a long-term, stable, equilibrated solution containing oligomeric (germano)silicate species in a mass range of 77-1414 Da. The ESI mass spectrum of this solution can be seen in Figure 1, and a list of all occurring species can be found in the Supporting Information. In general, it is characteristic for all mass spectra that, in addition to the main silicate signals, satellite signals having a reduced mass of n · 18 Da occur. This is due to a loss of n water molecule(s), although it has to be mentioned that these species do not exist in solution as such but are formed in the gas phase of the mass spectrometer.20 The low to middle mass range of the widespread species distribution shown in Figure 1 is characterized by signals that can be attributed to 4R and D4R units. These occur with satellite signals having an enhanced mass of 46 Da, which is due to a substitution of silicon against germanium in such species [∆m/ z(74Ge/28Si) ) 46]. This, together with the characteristic isotopic patterns, proves that up to three silicon atoms are exchanged against germanium. This finding is in line with previous work and theoretical calculations, which predict the incorporation of three germanium atoms in such species as an energetical minimum.21 However, such oligomers are also found to be enlarged by attaching monomeric units to the corner of the square or cube, respectively (see Figure 1). It is noteworthy that in these enlarged species up to four silicon atoms can be substituted against germanium. Furthermore, no oligomeric species in which fluoride ions (F-) are located in D4R units were detected, although this is proposed to be the energetically preferred position in solid zeolitic materials.22,23 MS/MS experiments were performed with all 4R and D4R units that contain more than three germanium atoms to determine whether they are incorporated into the (bi)cyclic silicate species or if they are attached to the corners and thus are not part of the closed cage structures. In such fragmentation experiments, a parent ion

Figure 2. ESI-MS/MS spectra of (a) a single 4R enlarged by condensing a monomeric unit to its corner containing four germanium atoms (m/z ) 531) and (b) a single 4R enlarged by condensing two monomeric units to its corner containing four germanium atoms (m/z ) 575).

is isolated in an electric ion trap. Afterward, its kinetic energy is increased, leading to a fragmentation by collision with the inert background gas (He). The MS/MS spectra for 4R species enlarged by one or two monomeric units, respectively, are shown in Figure 2. The fragmentation pattern of the single 4R at m/z 531, enlarged by one monomeric unit, shows two major signals (Figure 2a). One fragmentation product has a reduced mass of 18 Da, which is due to the loss of one water molecule; the other signal can be attributed to a single 4R containing three germanium atoms (m/z

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Figure 4. ESI-MS/MS spectrum of oligomer 1, containing two germanium atoms (m/z ) 1013).

Figure 3. ESI-MS/MS spectra of (a) a D4R enlarged by condensing two monomeric units to its corner containing four germanium atoms (m/z ) 851) and (b) a D4R enlarged by condensing three monomeric units to its corner containing four germanium atoms (m/z ) 965).

) 409). This fragmentation product is due to splitting of the monomeric unit that was attached to the ring, thus indicating that the parent ion was built up by a single 4R unit containing three germanium atoms and an attached monomeric germanium hydroxo species. It can be excluded that the single 4R contained four germanium atoms, since no signal that can be attributed to an all germanium 4R was detected. Similar results were obtained for the fragmentation of a 4R species enlarged by condensing two monomeric units to the square (Figure 2b). The fragmentation pattern indicates that the parent ion at m/z 575 consists of two isomeric structures: The fragmentation product at m/z 487 is due to a loss of one monomeric unit, which was attached to the ring and can thus be attributed to a 4R enlarged by one monomeric unit. Because this molecule ion contains three germanium atoms, the lost monomeric unit has to be a germanium hydroxo species. The remaining fragmentation products prove whether these three germanium atoms are incorporated into the ring or attached to it. The signal at m/z 409 can be attributed to a single 4R containing three germanium atoms, the one at m/z 383 to a single 4R containing two germanium atoms. Hence, two isomeric structures contributed to the intensity of the parent ion: a 4R enlarged by condensing two monomeric units to the corner of the square in which either two or three germanium atoms are incorporated into the ring. Anyhow, no fragmentation pattern indicating the substitution of more than three silicon atoms in the ring was detected. As mentioned above, D4R units containing up to three germanium atoms were found in the prenucleating solution as well. However, if these oligomers were enlarged by adding two or three monomeric units to the corner of the cube, up to four silicon atoms were found to be substituted against germanium. The MS/MS spectrum of the D4R unit, enlarged by two monomeric units containing four germanium atoms (m/z ) 851), can be seen in Figure 3a. The first fragmentation product at m/z 727 is due to splitting of one monomeric unit. Because this fragment ion contains three germanium atoms, the lost monomeric unit has to be a germanium hydroxo species. However,

whether all of the remaining three germanium atoms are incorporated into the cube, or if one of them is attached to it, can be determined by the analysis of the other fragmentation products. The one at m/z 605 can be attributed to a D4R unit containing two germanium atoms, and the one at m/z 667 can be attributed to one containing three germanium atoms. Both fragmentation products result from splitting off of the two monomeric units, which were attached to corners of the cube. This indicates that the parent ion at m/z 851 consists of two isomeric structures: a D4R unit containing two germanium atoms, enlarged by adding two germanium hydroxo species to the corners of the cube, and a D4R unit containing three germanium atoms, enlarged by condensing both one germanium hydroxo and one silicon hydroxo species to the corners of the cube. The fragmentation of a D4R unit enlarged by adding three monomeric units to the cube, containing four germanium atoms (m/z ) 965), led to similar results as mentioned above. In the MS/MS experiments, three fragment ions were detected (Figure 3b). The first two fragments can both be attributed to a D4R unit enlarged by attaching two monomers to the corner of the cube, one containing four germanium atoms (m/z ) 869) and one containing three germanium atoms (m/z ) 805). This indicates that the parent ion consists of two isomeric structures. However, the fragmentation product at m/z 641 yields information about the germanium distribution in those: This fragment ion can be attributed to a D4R containing two germanium atoms. Furthermore, it represents the basic unit of the parent ion, which is either enlarged by adding two germanium and one silicon hydroxo specie or one germanium and two silicon hydroxo species to the corner of the cube. It is noteworthy that in no MS/MS experiments, fragmentation products indicating an incorporation of more than three germanium atoms into the D4R units were detected. The upper mass range of the spectrum shown in Figure 1 is characterized by oligomeric species that have structural characteristics of ITQ-21, the zeolite of which is obtained by the use of SDA-1 as an organic template. The proposed structure 1 consists of two subunits, a D4R and a 4R, which are linked to each other by one silicate unit; the number of silicon atoms substituted against germanium ranges between two and six. If the germanium atoms would be equally distributed over the two subunits, four fragmentation products should be expected: a single D4R, a 4R, and these structures enlarged by the addition of one monomeric silicate unit to the corner of the cube or square, respectively. If more fragments would occur (n · 4), n isomeric species would contribute to the intensity of the parent signal. Figure 4 shows the MS/MS spectrum of

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Figure 5. ESI-MS/MS spectra of oligomer 1 containing (a) three, (b) four, (c) five, and (d) six germanium atoms. All isomers contributing to the intensities of the parent ions are displayed.

structure 1 containing two germanium atoms (m/z ) 1013). Four fragments were detected, indicating that the germanium atoms are equally distributed over the single and double 4R: The fragment ions can be attributed to a single 4R (m/z ) 339) and D4R (m/z ) 579), both enlarged by one monomeric silicate unit (m/z ) 417, 657). All detected fragments contain only one germanium atom, and no signals were detected, indicating that two germanium atoms are incorporated into one subunit, whereas the other one would be built up only by silicon. The MS/MS spectrum of structure 1 containing three germanium atoms (m/z ) 1058) is shown in Figure 5a. The fragmentation led to eight signals, which can be attributed to either 4R or D4R units, enlarged by one monomeric unit as well. These contain one, respectively, two germanium atoms, indicating two isomeric structures. The fact that all fragments occur in approximately the same intensity proves that germanium is not preferentially built into one of the subunits but rather distributed equally. However, fragments indicating that all germanium atoms are incorporated into one subunit were not detected. Figure 5b shows the fragmentation of an oligomer with structure 1, containing four germanium atoms. Because 12 fragmentation products were detected, three isomers have to contribute to the intensity of the parent ion at m/z 1102: The four main signals can be attributed to a single 4R (m/z ) 383) and D4R (m/z ) 623), both enlarged by condensing a monomeric unit to one of their corners as well (m/z ) 443, 701). Because all of these fragment ions contain two germanium atoms, they belong to the same isomer, containing two germanium atoms in both the 4R and the D4R subunit. The other eight fragmentation products occur in lower intensities (approximately 20% of the main signals). They belong to 4R and D4R units, both enlarged by monomeric species as well.

However, the detected fragments contain either one or three germanium atoms. Thus, they belong to two different isomers, one containing one germanium in the 4R and three germanium atoms in the D4R subunit and the other one containing three germanium atoms in the 4R and only one in the D4R subunit. The fact that these fragments occur in approximately the same intensity proves that germanium is not preferentially built into one of the subunits. However, it is noteworthy that no fragmentation pattern was detected, indicating that the four germanium atoms are completely built into one of the subunits. MS/MS experiments were performed with structure 1 containing five germanium atoms as well. The fragmentation pattern shows that two isomeric structures contribute to the intensity of the parent ion at m/z 1146 (Figure 5c): The eight fragments can be attributed to 4R and D4R units and enlarged equivalents, containing two or three germanium atoms, respectively. No fragments containing less than two or more than three heteroatoms were detected, which proves that one isomer was built up by a 4R containing two and a D4R containing three germanium atoms; the other one was built up by a three germanium atom containing 4R and a two germanium atom containing D4R. Structure 1 was found with a maximum of six incorporated germanium atoms (m/z ) 1154). The fragmentation pattern of such an oligomer can be seen in Figure 5d. Because only four signals were detected, the germanium atoms have to be homogenously distributed over the two subunits. The fragment at m/z 427 can be attributed to a 4R, and the one at m/z 487 can be attributed to an enlarged 4R; the other two fragment ions represent the structure of a D4R (m/z ) 667) and its enlarged equivalent (m/z ) 727), respectively. In all of these species, three silicon atoms are substituted against germanium. As can be seen in Figure 1, oligomers were detected having structure 1 enlarged by attaching a monomeric silicate unit to

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Figure 6. ESI-MS/MS spectra of oligomer 3 containing (a) two (m/z ) 1234) and (b) three germanium atoms (m/z ) 1280).

a corner of the cube or square, respectively. In such oligomers, up to six silicon atoms were found to be substituted against germanium. All MS/MS spectra recorded of these oligomers can be found in the Supporting Information. It was basically shown that the germanium distribution over the subunits is equivalent to the one of oligomer 1. Silicate species that are built up by linking two D4R units over one monomeric silicate unit were found in the presence of the SDA-1 cation as well (oligomer 3, Figure 1). In these species, 2-6 silicon atoms were found to be substituted against germanium. To determine their structure and to yield the germanium distribution over the two subunits, MS/MS experiments were performed with such species. The fragmentation pattern of oligomer 3 with the lowest number of incorporated germanium atoms (m/z ) 1234) is shown in Figure 6a. The fragments prove that no isomer in which both germanium atoms are located in one subunit while the other subunit occurs without incorporation of germanium exist in the prenucleating solution. The obtained fragment ions at m/z 579 and 657 rather represent a D4R unit containing one germanium atom or a D4R unit enlarged by attaching one monomeric silicate unit to its corner, respectively. Thus, the parent ion was built up by two D4R subunits, each containing one germanium atom, connected via one monomeric silicate unit. The fragmentation of oligomer 3, containing three germanium atoms (m/z ) 1280), led to four fragmentation products (Figure 6b). This is due to the fact that the oligomer contains an odd number of germanium atoms. Hence, it had to be built up by two different D4R units; the fragment ions at m/z 579 and m/z 657 can be attributed to a D4R unit containing one germanium atom and to its enlarged equivalent, respectively. The two other fragment ions at m/z 623 and 701 represent the structure of a D4R unit containing two germanium atoms as well as its enlarged equivalent. Thus, the parent ion at m/z 1280 was built up by a D4R unit containing one and a D4R unit containing two germanium atoms, respectively. These findings are in line with MS/MS experiments performed with oligomer 3 containing four, five, and six germanium

Schaack et al. atoms (MS/MS spectra can be found in the Supporting Information). For an even number of incorporated germanium atoms, two fragmentation products were detected, and for an odd number, four fragments were found. That is, oligomer 3 containing four germanium atoms was built up by two D4R units, each containing two germanium atoms. If five germanium atoms were incorporated, two of them were located in one and three in the other D4R, and if oligomer 3 contained six germanium atoms, they were in equal parts distributed over both subunits. As for oligomers 1 and 2, it was thus shown that the substitution of silicon against germanium occurs homogeneously over both subunits. Furthermore, no fragment ions indicating a substitution of more than three silicon atoms against germanium in one subunit were detected. SDA-2. It is known that zeolite A (ITQ-29, LTA-topology) can be synthesized by using the supramolecular self-assembling effect of the SDA-2 cation in the presence of fluoride ions.18 Furthermore, ESI-MS allowed the study of the evolution of silicate species during the synthesis of zeolite A.15 However, detailed insights into the germanium distribution of occurring species are missing, since the speciation in nucleating syntheses solutions usually changes too fast to perform MS/MS experiments. Thus, prenucleating solutions, using the template mentioned above, were synthesized and analyzed by ESI-MS and ESI-MS/MS: After GeO2 was dissolved in an aqueous solution of the organic template in its hydroxide form containing HF, the silicon source (TEOS) was added. Consequently, it dissolves within minutes, leading to a long-term stable equilibrated solution containing oligomeric (germano)silicate species in a mass range of 77-1370 Da. The ESI mass spectrum of such a solution can be seen in Figure 7, and a list of all occurring species can be found in the Supporting Information. It is noteworthy that the detected species are identical to those found during the synthesis of zeolite A using the same template and F- as a mineralizer. The low to middle mass range is characterized by a widespread species distribution, and the occurring species are comparable to those found by the use of the SDA-1 cation. One difference is that single 4R and D4R units enlarged by monomers never contained more than three Ge atoms. However, in the upper mass range, oligomers with structural characteristic of zeolite A were detected; these species are built up by two D4R units; contrary to the ones detected by the use of the SDA-1 cation, they are linked over siloxane bonds directly rather than bridged via monomeric silicate units (oligomer 4, Figure 7). In a MS/MS experiment, this fact would lead to only one fragmentation product, namely, a D4R unit, presuming that oligomer 4 contains one kind of heteroelement or that both subunits contain the same number of germanium atoms. However, oligomer 4 was detected containing 2-6 incorporated germanium atoms. In addition, it is noteworthy that in the presence of SDA-1, no signals indicating the formation of oligomers 1, 2, or 3 have been detected. Figure 8a shows the MS/MS spectrum of oligomer 4, containing two germanium atoms (m/z ) 1136). Given that only one fragment ion was detected (m/z ) 579), the parent ion was built up by two directly linked D4R units, both containing one germanium atom. Thus, the incorporated germanium atoms were homogeneously distributed over both subunits. The fragmentation of oligomer 4, containing three germanium atoms (m/z ) 1182), led to two different fragment ions, which were both a subunit of the parent ion (Figure 8b): a D4R unit, containing one or two germanium atoms, respectively (m/z ) 579, 623). No fragmentation pattern indicating that all germanium atoms are located in only one D4R unit was detected.

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Figure 7. (a) ESI mass spectrum of a prenucleating solution with the following molar composition: 0.67 TEOS:0.33 GeO2:0.5 SDA-2(OH):0.5 HF:100 H2O. Selected signals are labeled, and corresponding silicate structures are displayed. A complete list of the detected species can be found in the Supporting Information. (b) Structure of zeolite A (Crystallographic Information File taken from the International Zeolite Association Structure Database).

Figure 8. ESI-MS/MS spectra of oligomer 4 containing (a) two, (b) three, (c) four, (d) five, and (e) six germanium atoms. All isomers contributing to the intensities of the parent ions are displayed.

The MS/MS spectrum of oligomer 4 with four incorporated germanium atoms shows three fragmentation products (Figure 8c). Thus, two isomers contributed to the intensity of the parent ion (m/z ) 1208): One built up by two D4R units, containing two germanium atoms (m/z ) 605), having thus a homogeneous distribution of the germanium atoms over the subunits. The other one built up by two D4R units containing either one (m/z )

561) or three germanium atoms (m/z ) 631), respectively. These fragment ions occur with lower intensities, though. Figure 8d shows the fragmentation pattern of structure 4 containing five germanium atoms (m/z ) 1270). The two fragment ions prove that the parent ion was built up by a D4R containing two (m/z ) 605) and a D4R containing three germanium atoms (m/z ) 649). No fragments indicating that

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more than three respectively all germanium atoms are located in one D4R unit. Oligomer 4 was detected with a maximum of six incorporated germanium atoms (m/z ) 1370). The only fragmentation product at m/z 667, shown in Figure 8e, can be attributed to a D4R unit containing three germanium atoms. Indeed, no fragment ions that can be attributed to D4R units containing either more or less than three germanium atoms were detected, proving that they are homogeneously distributed over the D4R subunits. Summary and Conclusion Herein, we have shown that ESI-MS is a suitable tool to yield insights into the silicate speciation of prenucleating solutions. Especially MS/MS experiments are a powerful technique, not only to obtain detailed structural information of the occurring oligomeric silicate species but also to determine the location of incorporated heteroelements, namely, germanium, in those. Although the present studies were carried out under conditions too diluted to yield zeolitic materials, the obtained results have direct implications for zeolite synthesis and heteroatom location in zeolitic materials. This is supported by the fact that the speciation of silicate oligomers using the SDA-2 cation and fluoride as mineralizing agent in the presented prenucleating solutions is identical to the one found under real synthesis conditions.15 It was shown that the different organic templates used in this work direct the formation toward silicate oligomers with characteristic structures. Although it is known that independent of the used organic SDA, the low to middle mass range in germanium containing prenucleating solutions is characterized by 4R and D4R units, the speciation of the upper mass range in the presented solutions highly depends on the organic SDA: By using the SDA-1, cation species built up by linking two subunits over one monomeric silicate unit were detected in the upper mass range. These subunits were either two D4R or one D4R and one single 4R unit. The SDA-2 cation directs the formation toward oligomers built up by two D4R units linked directly over a siloxane bond; indeed, oligomers that were detected in the presence of SDA-1 were not observed. Furthermore, we were able, by performing MS/MS experiments, to determine the germanium distribution over these subunits. Independent of the used organic template neither D4R nor 4R units containing more than three germanium atoms were detected. These findings are valid for the subunits of larger oligomeric silicate species built up by D4R and/or 4R units as well. This is in line with studies reporting a preferred incorporation of germanium in such motifs.24,25 Furthermore, the substitution of silicon against germanium occurs rather homogeneously than randomly over the detected subunits. Moreover, it has to be mentioned that in solution no signals were detected, which would indicate fluoride incorporation in the oligomeric silicate species. This must occur during crystal growth, since fluoride was detected in the final crystals from such synthesis systems. Nevertheless, the addition of HF affects the speciation of oligomeric silicate species in prenucleating solutions: As compared to F- free prenucleating solutions,26 larger silicate species were detected, which is most probably due to the lower pH value in solution (almost neutral). In the present work, oligomeric silicate species with structural characteristics of zeolitic materials, which would be obtained from similar solutions at lower water concentration, were detected. However, on the basis of these results, one cannot

Schaack et al. conclude that the formation of these zeolitic materials necessarily proceeds via the assembly of the described structural elements. They can either assemble directly, exchange structural fragments,13 or depolymerize again to feed the silicate pool. Answering the question of the exact zeolite formation pathway is highly challenging and has to be addressed in future work. Nevertheless, the detection and detailed structural analysis of silicate species with characteristic fragments of the final zeolite already in solution are one step forward to understand zeolite formation on a molecular level. Acknowledgment. We acknowledge partial financial support by the Leibniz program of the Deutsche Forschungsgemeinschaft (DFG), in addition to the basic founding of the Max-PlanckGesellschaft (MPG). Supporting Information Available: Lists of all detected silicate species and MS/MS spectra of oligomer 2 (with 2-6 incorporated Ge atoms) as well as oligomer 3 (with 4-6 incorporated Ge atoms). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cejka, J.; Mintova, S. Catal. ReV. 2007, 49, 457–509. (2) Ozin, G. A.; Kuperman, A.; Stein, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 359–376. (3) Barrer, R. M.; Denny, P. J. J. Chem. Soc. 1961, 971–982. (4) Geisinger, K. L.; Gibbs, G. V.; Navrotsky, A. Phys. Chem. Miner. 1985, 11, 266–283. (5) Piccione, P. M.; Laberty, C.; Yang, S.; Camblor, M. A.; Navrotsky, A.; Davis, M. E. J. Phys. Chem. B 2000, 104, 10001–10011. (6) Zwijnenburg, M. A.; Bromley, S. T.; Jansen, J. C.; Maschmeyer, T. Microporous Mesoporous Mater. 2004, 73, 171–174. (7) Sastre, G.; Pulido, A.; Corma, A. Microporous Mesoporous Mater. 2005, 82, 159–163. (8) Corma, A.; Navarro, M. T.; Rey, F.; Rius, J.; Valencia, S. Angew. Chem., Int. Ed. 2001, 40, 2277–2280. (9) Corma, A.; Rey, F.; Valencia, S.; Jorda´, J. L.; Rius, J. Nat. Mater. 2003, 2, 493–397. (10) Paillaud, J.-L.; Harbuzaru, B.; Patarin, J.; Bats, N. Science 2004, 304, 990–992. (11) Corma, A.; Dı´az-Cabanas, M. J.; Jorda´, J. L.; Martı´nez, C.; Moliner, M. Nature 2006, 443, 842–845. (12) Pelster, S. A.; Schrader, W.; Schu¨th, F. J. Am. Chem. Soc. 2006, 128, 4310–4317. (13) Pelster, S. A.; Weimann, B.; Schaack, B. B.; Schrader, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2007, 46, 6674–6677. (14) Pelster, S. A.; Kalamajka, R.; Schrader, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2007, 46, 2299–2302. (15) Schaack, B. B.; Schrader, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2008, 47, 9092–9095. (16) Corma, A.; Diaz-Cabanas, M. J.; Martinez-Triguero, J.; Rey, F.; Rius, J. Nature 2002, 418, 514–517. (17) Blasco, T.; Corma, A.; Dı´az-Cabanas, M. J.; Rey, F.; Rius, J.; Sastre, G.; Vidal-Moya, J. A. J. Am. Chem. Soc. 2004, 126, 13414–13423. (18) Corma, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencia, S. Nature 2004, 431, 287–290. (19) Lobo, R. F.; Davis, M. E. Microporous Mater. 1994, 3, 61–69. (20) Pelster, S. A.; Schu¨th, F.; Schrader, W. Anal. Chem. 2007, 79, 6005– 6012. (21) Sastre, G.; Vidal-Moya, J. A.; Blasco, T.; Rius, J.; Jorda´, J. L.; Navarro, M. T.; Rey, F.; Corma, A. Angew. Chem., Int. Ed. 2002, 41, 4722– 4726. (22) Park, S. S.; Xiao, C.; Hagelberg, F.; Hossain, D.; Pittman, C. U.; Saebo, S. J. Phys. Chem. A 2004, 108, 11260–11272. (23) Caullet, P.; Paillaud, J.-L.; Simon-Masseron, A.; Soulard, M.; Patarin, J. C. R. Chimie 2005, 8, 245–266. (24) Kamakoti, P.; Barckholtz, T. A. J. Phys. Chem. C 2007, 111, 3575– 3583. (25) Vidal-Moya, J. A.; Blasco, T.; Rey, F.; Corma, A.; Puche, M. Chem. Mater. 2003, 15, 3961–3963. (26) Schaack, B. B.; Schrader, W.; Schu¨th, F. Chem.sEur. J. 2009, 15, 5920–5925.

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