Time Evolution of an Aluminogermanate Zeolite Synthesis

Article pubs.acs.org/cm

Time Evolution of an Aluminogermanate Zeolite Synthesis: Segregation of Two Closely Similar Phases with the Same Structure Type Luis A. Villaescusa*,†,‡ and Miguel A. Camblor*,§ †

Instituto Interuniversitario de Investigación de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Departamento de Química, Universitat Politècnica de València (UPV), Camino de Vera s/n, 46022 Valencia, Spain ‡ CIBER de Bioingenierı ́a, Biomateriales y Nanomedicina (CIBER-BBN) § Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Sor Juana Inés de la Cruz 3, 28039 Madrid, Spain S Supporting Information *

ABSTRACT: A synthesis of zeolite-like aluminogermanates using N,N,N-trimethyl-terc-butylammonium (TMTBA) and fluoride evolved from an initially crystallized zeolite with the AST Zeolite Framework Type and very little Al in it into an Alfree GeO2 phase with the same structure but a slightly smaller unit cell. In spite of both zeolites having the same framework type, same symmetry, a much similar unit cell size and almost the same framework composition, phase segregation occurred at intermediate crystallization times as the initial AST phase was gradually replaced by the new one. For reaction mixtures with a high Al content complete replacement of the large cell by the smaller cell phase was accomplished. The solids have been characterized by powder X-ray diffraction (XRD), Rietveld refinement from synchrotron diffraction data, multinuclear Nuclear Magnetic Resonance (NMR) of both intact and disolved solids and infrarred spectroscopy. The study reveals TMTBA can degrade under the crystallization conditions to initially yield triethylamine (tMA), whereas transmethylation between TMTBA and tMA produces tetramethylammonium (TMA), as probed by liquid NMR. The TMA cation can also structure-direct to AST zeolites but with a smaller unit cell volume and no Al in the framework. Phase segregation occurs despite the lack of structural mismatch because both crystallization events are decoupled, occurring at different moments and from largely different solutions. When both TMTBA and TMA cations are intentionally added at the start, no phase segregation but crystallization of a solid solution occurs instead.

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INTRODUCTION Zeolites are tectoaluminosilicates with open frameworks built from SiO4/2 and [AlO4/2]− tetrahedra that share oxygen atoms at the vertices. Frequently, tetrahedral atoms (T atoms) different from Si and Al can also be incorporated within these frameworks, both at doping levels as well as entirely substituting for Si and/or Al in the inorganic framework. Zeolites are found in nature as minerals and may be hydrothermally synthesized in the lab as well.1 The factors determining which zeolite framework type crystallizes in the lab are globally referred to as structure-directing factors. These include: (a) organic agents, generally cationic, specifically known as structure-directing agents (SDA),2 (b) T atoms such as Ge, which has been claimed to favor topologies that contain double 4-ring (D4R) units,3,4 allowing its use in the discovery of new zeolite topologies,3,5−7 as well as Zn and Be,8,9 which apparently favor zeolites with 3 membered rings (3MR) units, (c) F− anions, primarily used as a mineralizer but that has also been reported to play a significant structuring directing role © 2016 American Chemical Society

toward the formation of zeolites containing small cages, where F anions end up occluded,10,11 and (d) the degree of concentration of the synthesis mixture, which at least for pure silica zeolites prepared with F− as a mineralizer direct toward less dense phases as the concentration increases.11 In addition to these structure-direction effects, the preparation of materials with germanium populating D4R has recently given rise to a strategy in which germanium extraction followed by recondensation of the resulting layers, with or without previous treatment with additional linker species, has produced a number of new 3D zeolite frameworks (the assembly, disassembly, organization and reassembly, or ADOR, approach).12 One of the most representative examples of the structuredirecting role of F− anions is the formation of neutral Received: February 3, 2016 Revised: April 12, 2016 Published: April 12, 2016 3090

DOI: 10.1021/acs.chemmater.6b00507 Chem. Mater. 2016, 28, 3090−3098

Chemistry of Materials framework zeolites (SiO2, GeO2, AlPO4, etc.) containing double 4-ring units (D4R), also called [46] cages, with fluoride occluded inside balancing the positive charge of the organic cation.13−18 An [nmn′m′...] unit is a cage formed by m windows of n Si−Si edges, so a [46] cage is a cube like unit containing six rings of 4 tetrahedra each. The incorporation of a TIII element at framework tetrahedral sites contributes a net negative charge (in the form of[TIIIO4/2]− units) so that it may be formally considered as substituting for one tetravalent element (Si, Ge) plus one F−.4,19 The framework of the zeolite AST can be described as a clathrate compound containing large [46612] cages that share their hexagonal faces, leaving much smaller [46] cages (D4R units) between them.20,21 This arrangement yields a material with the same number of large and small cages. The framework presents two topologically distinct T atoms: T1 builds the D4R units, whereas T2 links four D4R units. For a SiO2 composition one organic cation typically resides in the large cavity, with a relatively distant fluoride anion occluded in the D4R cages (two cations and two anions per tetragonal cell of 20 Si atoms). Both the organic and inorganic cations can be removed by calcination.13 The AST zeolite, because of its simplicity, may be used as a model to investigate complex phenomena occurring during zeolite synthesis. In this work, we intended to find out whether incorporation of Al in the AST framework occurs with any preferential occupation by Al of a particular T site. Because Al and Si have similar X-ray scattering factors,22 we decided to work in the germanate system. A number of aluminogermanate zeolites have been reported (including FAU,23,24 SOD,25,26 RHO,27 JBW,28 GIS29) and there is even an aluminogermanate with a new zeolite framework type (PKU-9 (PUN)).30 There are also some few reports on the simultaneous substitution of Ge and Al for Si,31−33 as well as on the postsynthetic substitution of Al for Ge.34,35 However, as far as we know, there are no reports of true isomorphous substitutions of Ge by Al in otherwise fully germanate zeolites. In this work, we used N,N,N-trimethyl-tertbutylammonium (TMTBA) as structure-directing agent together with fluoride because they made possible the preparation of both the (alumino)silicate and aluminophosphate AST versions in a wide range of compositions and temperatures.13,36 Unexpectedly, the aluminum-substituted GeO2 system evolved with time in a very peculiar way: from an initially crystallized AST zeolite with very little Al in it into an Al-free GeO2 AST zeolite with a slightly smaller unit cell. Segregation occurred at intermediate crystallization times despite the fact that both zeolites have the same AST framework type, the same symmetry, a very similar unit cell size and almost the same framework composition (with Al/(Al+Ge) in the 0−0.02 range). For reaction mixtures with a high Al content, complete replacement of the large cell by the smaller cell phase was accomplished. Our study reveals that degradation of the SDA cation is followed by a transmethylation reaction that produces a new cation that acts also as an SDA for AST and that phase segregation occurs despite the lack of any structural mismatch because both crystallization events are decoupled: they occur at different moments and from largely different solutions. The replacement of one phase by the other tends to produce hollow crystals.

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EXPERIMENTAL SECTION

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RESULTS AND DISCUSSION

Synthesis. TMTBA was synthesized as previously described.13 For the synthesis of zeolites, gels were prepared by dissolving GeO2 (Aldrich) in a solution of TMTBA hydroxide. When the effect of aluminum was investigated, it was added as Al(NO3)3.9H2O. Because Al hydrolyzes according to Al3+ + 4 OH− → AlO2− + 2H2O an extra amount of TMTBAOH (four times the amount of Al) was added in order to compensate for the OH− anions consumed in the hydrolysis. Once dissolved, water was allowed to evaporate at RT until the desired amount of water was achieved. Finally, a solution of HF (48% by weight) was added to the paste and this was kneaded by hand with a spatula. The paste was then distributed among several 22 mL Teflon lined autoclaves and allowed to react under static conditions in an oven at 150 °C. Final overall molar compositions were: (1 − x)GeO2: xAl(NO3)3:(1/2 + 4x)TMTBAOH:1/2HF:7H2O where the Al molar fraction, x = Al/(Ge + Al), was chosen to be 0, 0.048, and 0.1 in different runs. At selected time intervals the autoclaves were quenched and the pH of their mother liquors measured (around 7 in all cases). Solids were isolated by filtration and dried at 60 °C for several hours. Characterization. X-ray diffraction (XRD, Bruker D8 Advance Xray diffractometer, Cu Kαanode (λ = 1.5418 Å) operating at 40 kV and 30 mA) was used to check the purity of the samples. For Rietveld refinement, data was collected at the SpLine BM25A at the ESRF, Grenoble, in capillary mode (0.8 mm) using synchrotron radiation with λ = 0.82548 Å. The aluminum content was determined by ICP/ MS after dissolution in NaOH 0.1 M using an Agilent 7500cx equipment with 45Sc as internal standard. The infrared spectra was recorded using the KBr pellet technique using a Bruker Tensor 27 FTIR-ATR spectrophotometer with a resolution of 2 cm−1. Multinuclear Solid State MAS NMR for 1H, 13C, 27Al, and 19F were collected at room temperature in a Bruker AV-400-WB equipment using a triple channel probe with 4 mm ZrO2 rotors and Kel-F lids spinning at 10 kHz for 1H, 13C and 27Al and a double channel probe with 2.5 mm ZrO2 rotors and Vespel lids spining at 25 kHz for 19F. The resonance frequencies were 400.13, 100.61, 104.26, and 376.45 MHz, respectively. 1H NMR spectra in solution were recorded using a Bruker AV400 spectrometer after dissolving the solid samples in NaOD/D2O in the presence of tetraethylammonium bromide (≥99%, Merck) used as an internal standard.

Synthesis. To investigate the isomorphous substitution of Ge by Al in the AST framework, we first synthesized the pure GeO2−AST using TMTBA. Two synthesis at 1 and 30 days were carried out, both resulting in pure AST zeolite. The conventional XRD data of both samples were indexed as tetragonal with unit cell edges a = 9.47 and c = 14.11 Å, similar to the values obtained for GeO2-AST by Wang et al. (a = 9.271, c = 14.349 Å or a = 9.371 and c = 14.154 Å for the phases prepared with dimethyldiethylammonium, DMDEA, or with isopropyltrimethylammonium, iPTMA, respectively)37 and by Liet al. (a = 9.2314, c = 14.096 Å for a sample prepared with 1,4-diazabicyclo[2,2,2]-octane, DABCO).38 The unit-cell volume of our sample (1265.40 Å3) is, however, significantly larger than those reported by the other authors (1233.32, 1242.94, and 1201.24 Å3, respectively), reflecting the larger volume of the SDA: as shown in Figure S1, the unit cell volume of all these GeO2−AST phases correlate weakly but clearly with the Connolly volumes of the occluded cations (Connolly solvent excluded volumes: 140.0, 128.2, 128.3, and 111.4 Å3 for TMTBA, DMDEA, iPTMA, and DABCO, respectively, calculated using a 1.4 Å probe). For this pure GeO2 synthesis, there was no change in phase or in unit-cell parameters along the reaction time, demonstrating a high stability of the GeO2− AST zeolite under these synthesis conditions. 3091


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and in fact both phases have the same reflection conditions. The results are summarized in Table 1. Apparently, given the close similarity between both phases, the initial AST structure has been transformed into another AST phase with a smaller unit cell, and this occurred with no significant change in yield along time within this series (see Table 1), implying the phase with smaller unit cell grows at the expense of the initially formed larger unit cell phase. The same type of transformation also occurs in the preparation with lower aluminiun content, GeAl20-y, again with no change in yield. Unlike the former case, however, 30 days of reaction is not enough to complete the transformation (Figure S2). The cell parameters of both the large and small unit-cell phases appeared to be largely independent of the Al content of the synthesis mixtures (see Table 1). A tempting explanation could be that the isomorphous substitution of Ge by Al simply resulted in a phase segregation with the all-GeO2 AST framework crystallizing at the first stages of the synthesis and then dissolving as the Al containing phase crystallized. Thus, the large cell would correspond to the allGeO2 phase and the small one, to the AST framework containing two Al per cell. However, phase segregation does not appear to be warranted given that there is not a large structural and compositional mismatch between the small and large cell AST phases (see below). Generally, isomorphous substitutions in zeolites give rise to solid solution series with a gradual and frequently linear change in unit-cell parameters as the degree of substitution increases.39−42 Phase segregation of two well-defined structures without a significant structural or compositional mismatch thus deserved a better explanation. Multinuclear NMR. We have run multinuclear solid-state NMR for three key samples: the large cell GeO2-AST and the two pure large and small unit cell AST end members prepared at Ge/Al = 9 (GeAl9-1 and GeAl9-30, respectively). 1H and 13C NMR in Figures 2 and 3 reveal that the organic species hosted into frameworks with different unit-cell volume are different in nature. The two large unit cell materials (GeO2-AST and GeAl9-1) host intact TMTBA cations. This is established by 1H NMR showing two signals integrating the same area at 1.6 and 3.1 ppm, assigned to methyl hydrogens in the t-butyl moiety and to hydrogens in methyl groups bonded to N, respectively. It is also shown by 13C NMR showing the three expected signals (25.3 and 50.7 ppm for the methyl groups in the t-butyl moiety and the methyl groups bonded to N, respectively, as well as a less intense signal at 71.8 ppm for the quaternary carbon). By contrast, the sample with a small unit cell (GeAl9-

We then introduced Al into the synthesis mixture in order to produce the Al substituted GeO2−AST. Charge balance imposes a limit to the isomorphous incorporation of TIII atoms into the AST framework, since F− plus [TIIIO4/2]− must equal the amount of organic cation, which cannot exceed the number of large cavities (two per unit cell of 20 T atoms). Thus, the maximum amount of Al in the zeolite would occur for a composition [NR4]2[AlO2]2[GeO2]18. Hence, two compositions were chosen, one with the maximum amount of Al (Ge/Al = 9) and another one with an intermediate value (Ge/Al = 20). The synthesis pastes were allowed to react for different times, covering the full range, 1 to 30 days, in which the pure GeO2−AST was observed to be stable. Samples obtained in the Al-containing experiments were designated as GeAlx-y, according to the Ge/Al molar ratio (x) and days of heating (y). XRD patterns of the resulting solids for the GeAl9y series are included in Figure 1, wheras the GeAl20-y series is

Figure 1. Powder XRD of the GeAl9-y series (from bottom to top: y = 1, 9, 16, and 30). The bottom and top tics mark allowed reflections in space group I4m for the pure AST phases with large and small cells (GeO2-AST and sGeAl9-30, respectively).

shown in Figure S2. Pure crystalline AST is obtained within a day for both aluminogermanate preparations. As time goes by, however, additional peaks appear, whose relative intensity increases with time. After a month of reaction, the initial AST phase has completely disappeared for the Ge/Al = 9 mixture. The new phase was indexed as tetragonal with unit cell parameters a = 9.19 and c = 14.15, and we noticed that the patterns were closely similar to those of the starting GeO2-AST, Table 1. Summary large u.c. (Å) sample

time (days)

yield (%)

GeO2 GeAl20-1 GeAl20-9 GeAl20-15 GeAl20-30 GeAl9-1 GeAl9-9 GeAl9-16 GeAl9-30

30 1 9 15 30 1 9 16 30

33.4 32.0 32.0 31.5 28.8 29.4 27.3 28.8

a

c

9.464 9.473 9.471 9.470 9.470 9.465 9.458 9.467

14.102 14.116 14.112 14.112 14.114 14.134 14.128 14.123

small u.c. (Å) a

c

9.193 9.193

14.153 14.150

9.194 9.193 9.187

14.151 14.15 14.149

TMA molar fraction (%)a

s/(L+s) (%)b

0 2 1 30 67 3 7 34 97

0 0 0 26 65 0 5 33 100

a

100*TMA/(TMA+TMTBA), by quantitative 1H NMR of the solids dissolved in NaOD/D2O bPercentage of the small uc volume phase, determined from the integrated area of the [101] and [112] XRD reflections 3092


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tMAH+ cations could comfortably occupy one cavity, while other cavities had to be filled with only one cation, so the 1/3 excess suggested that the organic occluded in the small cell phase could be tetramethylammonium (TMA+) and not tMAH+. To solve this question, we prepared two solutions containing the same ratios of GeO2 and NaOD in D2O and added tMA hydrochloride or TMACl to each one, as well as TEABr as a reference (methyl and methylene protons at 1 and 3 ppm, respectively) . The 1H NMR of the TMA solution showed a single resonance at 3 ppm, while the resonance was at 1.9 ppm in the tMA solution (Figure S4). This definitely demonstrates the species occluded in the small cell phase is TMA and not tMA. Further, we were able to quantify the amount of both cations in each of the samples (see Table 1) and found in each case that the molar fraction of TMA, [TMA]/([TMA]+[TMTBA], correlates well with the small/(small+Large) ratio of phases obtained from the integrated area of the [101] and [112] reflections (see last two columns in Table 1). Thus, large and small unit cell AST phases are differentiated mainly because of the different occluded organics (TMTBA and TMA, respectively), and their different unit cell volumes appear to be directly and primarily related to the size of the occluded organics (see Figure S1). The 27Al NMR spectra of two samples prepared with the highest aluminum content in the reaction mixture, GeAl9−1d and GeAl9−30, are shown in Figure 4. Surprisingly, only the

Figure 2. 1H MAS NMR of (from bottom) as-made GeO2-AST, GeAl9-1, and GeAl9-30.

Figure 3. 13C MAS NMR of (from bottom) as-made GeO2-AST, GeAl9-1, and GeAl9-30.

30) mainly shows a single resonance at 3.2 and 57.9 ppm in the 1 H and 13C NMR spectra, respectively, showing that this phase does not occlude TMTBA but some other, simpler, species. We next measured 1H NMR in solution for all samples, after dissolution in NaOD/D2O, with addition of tetraethylammonium bromide (TEABr) as an internal quantitation standard (Figure S3). For GeO2-AST only two singlets, assigned as above, confirmed TMTBA as the occluded organic species. However, samples that contained increasing amounts of the small unit cell phase also showed a resonance around 3.2 ppm with increasing relative intensity. Hoffmann degradation of TMTBA by elimination of β hydrogens shall produce trimethylamine (tMA) and isobutene and, at the synthesis pH of around 7, trimethylamine should be predominantly protonated (tMAH+). Thus, it could be that tMAH+ was able to structure-direct the crystallization of an AST phase with a smaller unit cell because of its smaller volume. However, the quantitative analysis of the small cell phase GeAl9-30 showed an excess of cations over large cavities very close to one-third if the cation was tMAH+. We found very unlikely that two

Figure 4. 27Al MAS NMR of (from bottom) as-made GeAl9-1 and GeAl9-30, * mark spinning side bands.

AST material synthesized at short time evidence the presence of some Al (0.365 Al per unit cell, i.e., a Ge/Al molar ratio of 54). The single resonance at 62.3 ppm is at the expected chemical shift for a tetrahedral environment of aluminum, Al(OGe)4.25 Thus, it can be inferred that the AST framework incorporates very little Al by isomorphic substitution of Ge and the incorporation occurs only as long as the organic cation occluded is TMTBA, unlike the samples containing TMA, which rather unexpectedly do not incorporate any aluminum. Figure 5 shows the 19F NMR spectra of the three relevant samples. The three of them present one single signal in the chemical shift range typically assigned to F anions occluded in germanium-based double 4 ring units (F-D4R), (from −9 to −16 ppm).43 The samples hosting TMTBA (GeO2-AST and 3093


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chemical composition. All the spectra are rather alike, apart from a weak band at around 710 cm−1, which is clearly present in the sample with the highest amount of aluminum (GeAl9-1), hardly seen in the sample with an intermediate amount (GeAl20-1) and completely absent in the all-GeO2-AST sample. We hypothesize that this band may be related to the void D4R units, similarly to the observation of a band around 562 cm−1 in silica AST materials upon calcination or after introduction of Al.44 Because Al balances the positive charge of a fraction of organic cations, its introduction causes the fluoride content to decrease,4 causing some D4R units to be void and thus the appearance of the 562 or 710 cm−1 IR band in SiO2 and GeO2-based materials, respectively. As expected in light of the NMR results, the small cell pure AST sample hosting TMA, i.e., GeAl9-30, does not show the bands corresponding to the original organic cations. Comparison of the bands of the organic species clearly shows a higher correspondence with TMA rather than with the tMA hydrochloride. Also, the spectrum shows meaningful blueshifts of 10−20 cm−1 in bands characteristic of the zeolitic structure in the 400−600 cm−1 region. The opposite shift is observed in the prominent asymmetric stretching band around 860 cm−1, which is red-shifted by about 12 cm−1. Finally, this Al-free sample lacks any band similar to the 710 cm−1 observed in the TMTBA-containing aluminogermanate samples, further supporting its relation to the Al incorporation (see above). As a summary at this point, the expected substitution of Ge by Al occurs at the 1 day sample (GeAl9-1), as deduced by 27Al MAS NMR and without large significant changes in unit cell parameters. This suggests the isomorphous substitution of Ge by Al is difficult to characterize just by the shifting in XRD reflections or change in unit cell parameters, which it is likely due to the small Al content and to the closely similar ionic radii of Ge and Al. On the contrary, the lack of any 27Al MAS NMR resonance indicates no isomorphous substitution of Al for Ge occurred at the 30 day sample, GeAl9-30, despite the large change in unit-cell parameters, which is rather due to the change of the occluded organics. That little TMTBA is occluded in that sample is strongly supported also by the quantitative 1H NMR of the dissolved solids and the infrared study. In further support to the idea that TMTBA decomposition as the concentration of Al (and hence, TMTBA, see Experimental Section) increases, we note that when the Ge/Al ratio was further decreased to 7.5, SOD zeolites were obtained. Because sodalite lacks cavities large enough to host TMTBA, it is highly likely that its formation is mediated by TMA. As a final proof, we synthesized a pure GeO2−AST phase using TMA, and its XRD reflections appeared at identical 2θ positions as those of GeAl9-30 (Figure S5). TMTBA Degradation and Rearrangement. Our understanding of this system relies on a rather peculiar rearrangement of TMTBA to finally yield, among other things, TMA. The formation of TMA is well-established by 13C and 1H MAS NMR of the small cell phase (Figures 2 and 3) and by 1H NMR of its solution in NaOD/D2O (Figure S3). It is also supported by IR spectroscopy (Figure 6), and by the close correspondence between the proportion of small cell phase and the TMA fraction deduced by quantitative 1H NMR (see Table 1). There is also an indirect evidence from the correspondence between Connolly volumes of several organic cations and the cell volume of the AST phase in which they reside (Figure S1), where TMA falls within the correlation while tMA is well off.

Figure 5. 19F MAS NMR of (from bottom) as-made GeO2-AST, GeAl9-1, and GeAl9-30.

GeAl9-1) show a resonance at −13.3 ppm with, however, a clear asymmetry in the sample with a small amount of Al. On the other hand, the sample hosting TMA also shows a resonance assigned to the presence of the F-D4R substructure but at a different chemical shift (−16.3 ppm). IR Spectroscopy. The IR spectra of as-made GeO2-AST and several GeAlx-y are shown in Figure 6. The IR spectrum of halide salts of the organic TMTBA, TMA and tMAH cations have been also included for comparison. In the samples with the large unit-cell phase no shift is detected in any of the bands typically considered to be sensitive to the framework composition, presumably due to the too small differences in

Figure 6. Infrared spectra of (from bottom to top) TMTBAI, GeO2AST, GeAl20-1, GeAl9-1, tMAHCl, TMACl, and GeAl9-30. The arrows point to the band assigned to void D4R units at 710 cm−1, whereas the vertical lines mark the position of GeAl9-30 (solid) and GeO2 bands (dotted). 3094


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of tMA, ethyl acetate and DMTBA (not shown). We thus propose that the initial degradation of TMTBA to yield tMA is followed by reaction of these two compounds, producing TMA as the product of the nucleophilic attack of tMA on a methyl group of TMTBA, with DMTBA as the leaving group. Amine exchange reactions between a quaternary ammonium and a tertiary amine to yield the corresponding tertiary amine and quaternary ammonium compounds are known to occur thermally,45 but their importance in the synthesis of zeolites has been hitherto unreported. The fact that the extend of the transformation of the large cell AST phase into the small one is dependent on the Ge/Al ratio (after 30 days it does not occur when there is no Al, is not complete when Ge/Al = 20, and is complete for Ge/Al = 9) can now be undestood, because the larger the Al content the higher the TMTBA concentration (see Experimental Section) and hence the faster the degradation and rearrangement of TMTBA. Phase Segregation. Rietveld refinement of three relevant phases was accomplished using synchroton diffraction data: the TMTBA-containing GeO2−AST phase prepared from Al-free reaction mixtures, and the GeAl9-1 and GeAl9-30 phases prepared from gels with Ge/Al = 9 at 1 and 30 days. We recall that GeAl9-1 contains some Al (in a much limited amount, Ge/ Al = 54 in the zeolite) and TMTBA, whereas GeAl9-30 is a pure GeO2 phase and occludes TMA. Experimental details, Rietveld plots, and other crystallographic data are provided as Tables S1−S5 and Figures S8−S13, whereas Figure 8 shows

Because the expected Hoffmann degradation of TMTBA via elimination of β hydrogens would produce tMA and isobutene, we hypothesized that re-reaction of tMA with TMTBA could produce TMA by transmethylation/amine exchange (see Scheme 1).45 We thus followed by 1H NMR the evolution at Scheme 1. Proposed Formation of TMA from TMTBA, by Transmethylation between TMTBA and tMA Following Hoffman Degradation of TMTBAa

a

Nu represents a nucleophile such as OH−, F− or H2O.

150 °C of a equimolar solution of TMTBA and tMAHCl in water at close to neutral pH (reached by adding concentrated NaOH) in brand new Teflon liners. The NMR was recorded at basic pH by further addition of NaOD. The initial solution contains TMTBA (resonances at 1.5 and 3.1 ppm with equal intensities) and tMA (one single resonance at 2.2 ppm) very close to the 1:1 ratio used (Figure 7 and Figure S6). After 2

Figure 8. Comparison of the frameworks of the three AST phases, aproximately along (110), with color codes as indicated. The origin of the three cells at (000) have been made coincident at the center of the figure, which spans ±0.6 cell units along each crystallographic axis.46

Figure 7. Time evolution at 150 °C of a 1:1 TMTBA:tMA aqueous solution at neutral pH, obtained by integration of the corresponding 1 H NMR signals.

that the framework of the three phases are closely similar to each other and there is no significant mismatch to warrant phase segregation. Table S5 allows us to compare distances and angles between the large and small cell AST GeO2 frameworks, according to Rietveld refinement with no distance or angle restrains. Both tetrahedral angles and Ge−O distances may play a role in explaining the different unit cell volume of both GeO2 phases. When replacing TMTBA by TMA the a and b cell edges decrease by 3% while the c edge instead increases by 0.3%. The Ge−O distances decrease only 1.1% (Ge1) or 2.1% (Ge2), but, at the same time there are also changes in the angles. The variations in tetrahedral angles are complex so that while the average tetrahedral angles are almost exactly identical and very close to the tetrahedral angle (average O−Ge1−O 109.40 and 109.43°, and O−Ge2−O 109.49 and 109.53°, respectively), some specific angles significantly increase while others decrease. For instance, two O−Ge−O angles around Ge2 decrease 10° (113.3 to 103.6°), whereas the other four

days at 150 °C, there is a very large increase in the resonance of tMA relative to those of TMTBA, following Hoffmann degradation. Additionally, a singlet at 3.2 ppm reveals the formation of TMA, whereas two smaller resonances at 1.1 and 2.2 also develop and are temptatively assigned to dimethylterbutylamine, DMTBA. Some few signals could not be assigned. After 5 and 7 days, the same resonances are still apparent, with relative intensities that indicate the concentration of tMA largely increases, that of TMTBA decreases (both resonances of this cation keep their equal intensities), TMA slightly increases and the rest hardly change. The final solution was also analyzed by 13C NMR (Figure S6), confirming the presence of tMA (45.9 ppm), TMTBA (23.0, 49, and 71.2 ppm), TMA (55.3 ppm) and DMTBA (24.4, 36.7, and 69.8 ppm). Some additional small resonances could not be assigned. After basification of the final solution and extraction with ethyl acetate GC-MS revealed the presence in this nonpolar fraction 3095


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Chemistry of Materials increase 5° (107.58 to 112.49°). Intertetrahedral Ge−O−Ge angles are very similar in each phase. FE-SEM microscopy reveals interesting differences in the crystal morphology of the different phases. The pure GeO2− AST zeolite prepared in the absence of Al consists of submicrometer crystals with blunt edges (Figure S7). By contrast the GeAl9-1 phase is made of neatly defined tetragonal bipyramids with sharp edges and a larger size (generally in the 1−2 μm range, occasionally larger, Figure 9). After 9 days of crystallization and despite the fact that the fraction of the small cell phase is only around 5% (see Table 1), the crystals no longer have neat and smooth faces, as terraces and overgrowths are clearly observed and some truncated vertices become also apparent. In the final end member of the series (GeAl9-30, which is essentially the small cell phase, Table 1), the crystals, which tend to be large, have the pyramid tips truncated at a much lower relative height, so they look flatter. And, surprisingly, many of the crystals appear to be hollow and have holes, most frequently squared centered on, and with edges parallel to, the base of the bypyramids (see additional standard FE-SEM (Figure S14) and FIB-FE-SEM at different levels of sputtering by focused ion bombardment (Figure S15)). We propose that after crystallization of the initial large cell phase formed with TMTBA (with little Al), the formation of TMA in solution leads to the crystallization of the new small cell TMA−containing phase mainly as layers overgrown on the large cell crystals, which is justified by the lack of any mismatch between both phases (see Figure 8). This new crystallization event occurs at the expense of the large cell phase which has to act as a GeO2 reservoir and, critically, from largely different conditions (when the concentration of TMA has increased and TMTBA is essentially depleted), which explains the phase segregation and lack of a solid solution series (with a varying concentration of TMTBA and TMA). As the new phase forms and the old crystals dissolve, hollow crystals with squared holes, where the pyramids tips of the old crystals were placed, may develop. To validate our hypothesis that the occurrence of phase segregation in this system is not due to a mismatch but to a decoupling (in time and chemical composition) in the crystallization of both phases, we have also made synthesis experiments with both TMTBA and TMA added to the gel at start, keeping the total SDA/GeO2 ratio at 0.5 and varying TMA/(TMA+TMTBA) ratios. The crystallizations were carried out at the relatively short time of 1 day in order to prevent artifacts due to organics degradation and rearrangement. The zeolites were characterized by XRD and their TMA/ (TMA+TMTBA) ratios were determined by 1H NMR after dissolution of the zeolites in NaOD/D2O. As seen in Figure S16, in this case we found no phase segregation but a solid solution series, i.e., the XRD patterns could be indexed as single AST phases with unit-cell parameters that change smoothly and close to linearly as the TMA fraction in the zeolite changes, see Figure 10. This confirms there is no mismatch between the TMTBA− and TMA-containing phases that could justify phase segregation, hence validating our hypothesis. Quite interestingly, the incorporation of cations to the zeolite is largely favored for TMTBA compared to TMA (see Figure 11). This suggests a higher stability of the GeO2-TMTBA phase, although as expected for germanate zeolites, after calcination at the temperature needed to remove the organics (550 °C in both cases), the materials are amorphous by XRD (not shown).

Figure 9. FE-SEM pictures of the crystals formed at Ge/Al = 9 after (from top to bottom) 1, 9, 16, and 30 days of crystallization.

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CONCLUSIONS The isomorphous substitution of Al for Ge in the AST framework occurs in a very limited extent in the presence of TMTBA and without significant changes in the unit cell 3096


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GeO2, GeAl9−1 and GeAl9−30; additional FE-SEM and FIBFE-SEM of the GeAl9−30 phase. This material is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00507. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding from the Spanish Ministery of Science and Competitiveness (Projects MAT2012-31759 and MAT2012-38429-C04) and Generalitat Valenciana (PROMETEOII/2014/047). We also thank A. Valera for technical expertise (FESEM) and Ms. Caramelitorl for help with the chemical analysis. Thanks are also due to the BM25 Spline staff at ESRF in Grenoble (France), particularly to G. Castro, and also to C. Belver for help in collecting the synchrotron XRD data and for helpful discussions.

Figure 10. a, c, and volume unit-cell changes in GeO2−AST phases prepared using both TMA and TMTBA cations as a function of the TMA/(TMA+TMTBA) fraction occluded.

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Figure 11. Fraction of TMA incorporated in GeO2−AST as a function of the TMA fraction added to the crystallization gel.

parameters. Prolonged heating promotes the formation of another AST zeolite with a smaller unit cell, because of the occlusion of a smaller cation, TMA, which results from the reaction of TMTBA with its decomposition product tMA. This phase grows mainly as overgrowths on and at the expense of the initially formed crystals and, for the gel with the highest Al content, completely replaces it in 30 days yielding abundant hollow crystals. Surprisingly, in the TMA-containing phase the isomorphous substitution of Al for Ge is not realized. In this system, phase segregation cannot be the result of a structural mismatch but of the occurrence of two different and decoupled crystallization events at different times and from chemically different solutions.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Powder XRD patterns of the GeAl20-y series; 1H NMR of the dissolved solids; 1H NMR of TMA and tMAH; unit-cell volume of several GeO2−AST phases vs Connolly volume of the occluded cation; XRD pattern of a GeO2−AST phase directly synthesized with TMA; evolution of 13C NMR of a mixture of TMTBA and tMA treated at 150 °C; FE-SEM images of the GeO2 phase; details of Rietveld refinements, crystallographic data, fractional coordinates, Rietveld plots and structures of 3097


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Time Evolution of an Aluminogermanate Zeolite Synthesis

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