Framework Reduction of GeO2 Zeolites During Calcination

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Framework Reduction of GeO2 Zeolites During Calcination Luis A. Villaescusa, and Miguel A. Camblor Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03682 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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Chemistry of Materials

Framework Reduction of GeO2 Zeolites During Calcination Luis A. Villaescusa∗,†,‡ and Miguel A. Camblor∗,¶ †Instituto Interuniversitario de Investigaci´ on de Reconocimiento Molecular y Desarrollo Tecnol´ ogico (IDM); Departamento de Qu´ımica, Universitat Politècnica de València (UPV), Camino de Vera s/n, 46022 Valencia, Spain. ‡CIBER de Bioingenier´ıa, Biomateriales y Nanomedicina (CIBER-BBN). ¶Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Cient´ıficas (CSIC), Sor Juana Inés de la Cruz 3, 28039 Madrid, Spain. E-mail: [email protected]; [email protected]

Abstract Calcination in air of GeO2 zeolites with AST-topology causes reduction of the framework, hence structural destruction, in a notable extension. This is due to the impossibility for the organic cation occluded inside to react with ambient oxygen. As the temperature increases, reoxidation in air of the reduced framework causes episodes of weight-gain. When calcination is carried out in N2 , weight losses close to 70% imply a loss of Ge due to sublimation of germanium monoxide, GeO. The nature of the organic cation occluded determines the nature and oxidation state of the final residue after calcination in N2 : for tetramethylammonium the residue is GeO2 while trimethylterc-butylammonium, thanks to its larger C content, yields metallic Ge. Framework organoreduction is not unique to GeO2 –AST zeolites, but also occurs during the calcination in air of other germanium-containing zeolites with larger pore openings.

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Introduction Zeolites are crystalline microporous materials with a wide field of applications, typically dependant on its structure and composition. In the search for new zeolite structures, the introduction of germanium, with a claimed structure-direction effect towards topologies that contain double 4-ring (D4R) units, 1,2 has proven much fruitful. 1,3–5 Applications may, however, be compromised by a poor thermal, hydrothermal or chemical stability because the undercoordinated tetrahedral Ge in the calcined zeolite framework may react with water, with hydrolysis eventually degrading the framework. It is in fact known, but scarcely recognized, that GeO2 zeolites with a high Ge content are moisture sensitive and severely damaged if, after calcination to remove the organics occluded during crystallization, water is allowed into their pores. 1 This degradation process apparently relies on the ability of Ge to adopt coordination numbers above 4, so that coordination to water molecules may trigger subsequent rearrangements and, eventually, amorphization. The instability of Ge in zeolites is, hence, a topic of significant interest and may not necessarily be detrimental, as demonstrated by the development of a strategy in which germanium extraction followed by recondensation of the resulting layers, with or without additional linkers, produced a number of new 3D zeolite frameworks (the Assembly, Disassembly, Organization and Reassembly, or ADOR, approach). 6 We will show here that structural degradation of germanium zeolites may already occur during the calcination step without intervention of water, because Ge may also change its oxidation state. In this respect, Su and coworkers very recently showed that framework reduction and reoxidation of the interrupted germanate framework PKU-10 synthesized without fluoride is possible. 7 An interrupted framework contains abundant ”defects” (or, more strictly speaking, not fully connected framework nodes) which could be blamed for the reported instability. Additionally, PKU-10 has a very low framework density of 11 Ge/1000 ˚ A−3 that may contribute to a decreased stability. Surprisingly, no detailed account has been so far reported for the more general and 2


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interesting case of a fully connected germanate zeolite, which could be anticipated to be more robust but for which, nonetheless, the literature had already shown strong indications of a poor thermal stability (although these indications generally passed uncommented, see below). In this work, we demonstrate for the first time that calcination in air of a fully connected germanium oxide zeolite may involve complex redox processes affecting the GeO2 framework itself, which may be first reduced and then reoxidized, as demonstrated by weight gaining steps in the TGA traces. Such steps have been previously observed but, with up to our knowledge a unique exception, have passed unnoticed and uncommented in the literature, and hence had remained unexplained so far. Here we show that, under non-oxidizing conditions (N2 atmosphere), even full reduction of GeO2 to metallic Ge may occur in certain instances, while in other cases amorphous GeO2 is formed. Extensive sublimation of GeO also occurs, explaining low amounts of calcined residues due to the loss of Ge. Our work suggests that TGA data of Ge–containing zeolites should be analyzed with much care, and previous reports may need to be revised. The most typical observation that the weight loss in the thermal analysis of germanium zeolites is close to the theoretical values considering only the contents of organics and fluoride, are surprising in the light of our findings.

Experimental Section Two zeolites with the AST Framework Zeolite Type and with the same framework composition (GeO2 ) but occluding two different organic cations were synthesized by methods reported before. 8 The samples will be denoted according to the occluded cation as TMTBA– GeO2 or TMA–GeO2 , where the acronyms stand for trimethyl-terc-butylammonium and tetramethylammonium, respectively. Full structural, chemical and physicochemical characterization of both materials were provided in our prior work. 8 The ideal chemical composition per unit cell of both samples, C14 H18 N2 F2 Ge20 O40 and C8 H12 N2 F2 Ge20 O40 , respectively, is defined by charge balance and full occupancy of the available cavities: per unit cell, two

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large [46 612 ] cavities able to hold one organic cation each and two [46 ] cages where fluoride ends up occluded. Thermogravimetric (TGA) and diferential thermal analyses (DTA) were performed on a Mettler Toledo thermogravimetric analyser by heating the samples from 25 to 1000 ◦ C at a rate of 10◦ Cmin−1 followed by an additional 30 min. isothermal step at 1000 ◦ C. Experiments were run in either air or nitrogen atmospheres at a flux of 80 mLmin−1 . X-ray diffraction (XRD, Bruker D8 Advance X-Ray diffractometer, Cu Kα anode (λ = 1.5418 ˚ A) operating at 40 kV and 30 mA) was used for identification of the calcined products. FTIR spectra were recorded on a BRUKER ISS 66V-S, using the KBr technique in the 4000-250 cm−1 range. To further investigate calcination under non-oxidizing conditions, we calcined a larger amount of solids under a flow of N2 using a cold finger to trap some of the evolved volatile products. These experiments were carried out in a horizontal furnace provided with an alumina tube closed with airtight water–refrigerated caps at both ends. At the downstream cap, we placed a cold finger, essentially a stainless steel jacket through which a coolant (an ethylenglycol/water mixture in a 40:60 volume ratio) was passed. The coolant was kept at -10 ◦ C by means of a Buchi F-105 recirculating chiller. A N2 flow (50 mLs−1 ) was passed along the tube through ad hoc holes drilled in both the upstream cap and the cold finger. Farther downstream the gases were passed through a concentrated NaOH hydroxide solution. The furnace was programed to reach a temperature of 1000 ◦ C in 100 min, which was maintained for 120 min before allowing to cool down.

Results and discussion TGA in air of both as-made GeO2 samples, TMTBA–GeO2 and TMA–GeO2 , are shown in Figure 1 and the corresponding differential thermal analyses in Figure 2. The first process detected in both samples is endothermic and associated with a similar weight loss, 9.35 and 9.50 % respectively. Moreover, they occur at a rather similar temperature (548 and 566 ◦ C,

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respectively, measured at the mid point of the TG step, or 554 and 578 ◦ C, respectively, in the DTA peak maxima). Due to the topology of the framework, which lacks apertures larger than 6MR, and the associated difficulty for oxygen diffusion, this step could be initially assigned to the thermal decomposition, rather than oxidation, of the organics and subsequent loss of degradation products. This would agree with the endothermic nature of this process, despite the oxidative conditions of the experiment (flowing air). However, inspection of the TGA traces beyond this first step reveals in both cases several episodes of weight gaining at higher temperatures, the first of them starting right after a sharp exothermic process detected in the DTA of both samples around 680 ◦ C and associated with just a very small weight loss in the TGA trace. A second, more gentle, weight gaining process is only apparent in TMTBA–GeO2 , starting above 925 ◦ C. Additionally, the maximum weight loss expected according to the ideal stoichiometry (two SDA+ F – per unit cell) should be 11.44 % for TMTBA–GeO2 and 8.18 % for TMA–GeO2 . These values are significantly smaller than the registered total weight loss at 1000 ◦ C (13.14 and 10.5%, respectively). Thus, in addition to the organics and fluoride, something else, necessarily related to the framework, has been lost during calcination in air. A literature search allows to conclude that weight gaining processes are not uncommon in the thermal analyses of Ge–containing zeolites. However, and as far as we know, they have passed apparently unnoticed, with a unique exception, and remained so far unexplained. For instance, in reports of a Si,Ge,Al–MFI (containing a 2D system of 10MR channels, 10+10MR), 9 IM–14 (8+10+10MR), 10 PKU–9 (8+10+10/10MR) 11 and PKU–15 (7+10+12MR) 12 weight gaining processes could be observed in the corresponding thermal analyses in air. These reports all concern frameworks with significantly larger pores than AST (all contain at least two different 10MR or larger pores). The exception noted above corresponds to a report on IM–10, where a gaining step was noticed and tentatively attributed to the reoxidation of a GeO phase previously formed. 13 To explain the behaviour observed in Figure 1, we hypothesized that the first weight

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Figure 1: Thermogravimetric analysis under flowing air of TMTBA–GeO2 and TMA–GeO2 AST zeolites.

Figure 2: Differential thermal analysis under flowing air of TMTBA–GeO2 and TMA–GeO2 AST zeolites (exothermic goes up).

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loss does not correspond, or not exclusively, to a thermal decomposition of the organics, but also encompased to a certain extent an organothermal reduction of GeO2 . If this is the case, after production of some GeO or metallic Ge0 with destruction of the framework and partial release of the organic degradation products, the reduced species could be reoxidized in the flowing air, producing the observed weight gaining steps in the thermograms. Further processes of weight loss may then be due to either removal of residual organics and/or to sublimation of GeO. We note here that, with temperature, a mixture of metallic Ge and GeO2 reacts to yield GeO, which sublimates at temperatures above 480-490◦ C. 14,15 Thus, at the temperatures of the first step in the TGA (well over 500 ◦ C for both samples) formation and sublimation of GeO may already occur, explaining the larger than expected weight loss at that temperature.

Figure 3: Thermogravimetric analysis under flowing N2 of TMTBA–GeO2 and TMA–GeO2 AST zeolites. Note that the amount of solid residues (around 30%) are much smaller than the amount of Ge in the zeolites (over 60%, see text). Our hypothesis hence imply that a reduction of the framework germanium oxide mediated by the organic species may occur under conditions of a shortage of the oxygen supply to the zeolite interior. To test the hypothesis we made further experiments in the absence of oxygen, 7



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Figure 4: Differential thermal analysis under flowing N2 of TMTBA–GeO2 and TMA–GeO2 AST zeolites by recording the thermal analysis under a N2 flow. As a strongly conclusive evidence for the existence of redox processes affecting the GeO2 framework itself, at the end of the thermal analysis of TMTBA–GeO2 in N2 , the solid residue was a small pearl with a metallic luster, in which we identified by XRD a cubic form of metallic Ge (see below). The residue in the case of the TMA–GeO2 lacked such a metallic luster and appeared as a glassy residue strongly sticked to the TGA crucible. In a separate experiment in a horizontal oven (see below), we were able to scrape a bit of the residue and identified it by XRD as mainly quartz-type GeO2 , with a small amount of metallic Ge (see below). As shown in Figure 3, the weight loss after thermal analysis of both TMTBA–GeO2 and TMA–GeO2 dramatically exceed the expected values for SDA and fluoride: instead of the expected 11.4 and 8.2%, the recorded values are 69.0 and 68.5%, respectively. The residues then amount to merely 31.0 and 31.5%, respectively, which is even far less than the expected content of elemental Ge in both samples (61.5 and 63.7%, respectively), implying that there must have been significant sublimation of GeO, 14–16 to account for the additional weight loss. The first weight loss has a similar magnitude and occurs at the same temperature in both air and N2 for each zeolite, 8


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suggesting it is essentially independent of the presence of oxygen: i.e. it is either a pyrolisis of the SDA or an organoreduction of the framework mediated by the SDA. However, a pyrolisis of TMTBA or TMA to yield pure carbon could amount to merely 4.33 and 3.96% weight loss for each zeolite, respectively and, hence, cannot explain the observed first weight loss (of around 8%). The next step is exothermic in air (Figure 4) and associated to only a small weight loss. We think at that temperature, and with the zeolite already destroyed, diffusion of oxygen may already be possible, so organic remains are burned off, which is inmediately followed by a weight gain, resulting from the oxidation of metallic with ambient oxygen. By contrast, at the same temperature, thermal analysis in N2 shows only a continuous and much larger loss. For TMTBA-GeO2 there are at least four additional thermal processes occurring at around the same temperatures in air and N2 . These are all exothermic in air and, surprisingly, two are exothermic and two endothermic in N2 . In the TMA–GeO2 sample, the initial weight losing steps also occur at approximately the same temperature in air and in N2 , although for this sample no exothermic processes are detected under N2 . The nature of every one of these higher temperature processes is not clear. In air, the initial organoreduction of GeO2 must be followed by reoxidation to explain weight gaining. However, at some point GeO is formed by an antidismutation reaction between Ge and GeO2 , and its sublimation explains the higher than expected weight loss. While these phenomena warrant further investigation to understand the details, our experiments establish for the first time and beyond doubt that during the thermal treatment of as-made zeolite germanates, even under air, the GeO2 framework itself may undergo redox processes and that sublimation of GeO also occurs. Our results challenge the conventional interpretation of thermogravimetric analyses, in both N2 and air, of organic containing germanates. For instance, if a very large weight loss needs to be assigned to ’volatile germanium oxides’ 17–22 it is important to understand that the volatile oxide is germanium monoxide, GeO, which can only be formed by the organic-mediated reduction of framework GeO2 (or, more properly, by the antidismutation reaction of GeO2 and Ge). This implies that preceding weight losses

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cannot be attributed solely to the removal of guests (organics, water, solvent, fluorine) but also to the reduction of GeO2 . Similarly, TG analyses in air or O2 need to be interpreted with care: a weight gaining step implies a prior framework reduction during organics removal but such a step may not be seen when it is coincidental with further removal of organic residues. Further, weight losses associated with sublimation of GeO, that occurs easily in N2 , may also occur in O2 , as discussed above. To investigate in more detail the formation and sublimation of GeO during the calcination of GeO2 zeolites, we calcined around 1g TMTBA–GeO2 or TMA–GeO2 AST zeolites up to 1000 ◦ C under flowing N2 in a horizontal furnace equipped with a cold finger (see experimental section for details). In the case of TMTBA–GeO2 , after calcination the solid residue in the crucible consisted in dark gray/black pearls with an XRD pattern corresponding with cubic metallic Ge◦ (Figure 5). In the case of TMA–GeO2 , the residue was identified as quartz-type GeO2 with a small fraction of metal Ge, as mentioned above. For both materials, the cold finger, but also the furnace alumina tube close to it were covered by powder with a range of colors (from white to yellow to orange and brown). The recovered materials generally contained broad peaks corresponding to Ge on an amorphous background, sometimes also with quartz type GeO2 , and in some cases there were additional peaks that could not be identified (Figure 6). This agrees with the formation and sublimation of GeO: GeO vapours are reported to deposit in cold surfaces and disproportionate into metal Ge and GeO2 phases, 23,24 and their crystallinity and crystal size depend on the thermal history of the sample. At low temperature both phases are amorphous but if annealed at increasing temperature crystallization occurs: first nanocrystalline Ge embbeded in a vitreous GeO2 phase is formed and then this phase also crystallizes. Further increases in temperature make the size of both Ge and GeO2 domains grow. 25 The IR spectra of residues recovered from the alumina tube and at the cold finger present bands in similar regions as quartz-like GeO2 (Figure 7). The main absorption band centered at 879 cm−1 in quartz-like GeO2 is assigned to the asymmetric Ge-O strecthing band

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Figure 5: XRD powder patterns of the residues obtained after calcination at 1000 ◦ C of TMTBA–GeO2 (magenta) and TMA–GeO2 (green) in N2 flow. The cian and the red patterns are simulations of cubic metallic Ge and quartz-type GeO2 , respectively.

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Figure 6: XRD powder patterns of several solids collected at the alumina tube (blue, yellow) or the cold finger (magenta, green) after calcination at 1000 ◦ C of TMTBA–GeO2 (yellow, magenta) and TMA–GeO2 (blue, green) in N2 flow. The cian and the red patterns are simulations of cubic metallic Ge and quartz-type GeO2 , respectively.

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and it appears at higher wavenumbers in amorphous GeO2 . 26 In the collected solids, this band always appears at higher wavenumber (893 to 908 cm−1 ), suggesting the existence of amorphous GeO2 , in agreement with XRD results (Figure 6). In the 500-600 cm−1 region, hexagonal GeO2 presents three characteristic bands at 585, 549, 516 cm−1 , which are shifted to higher wavenumbers in the collected solids (TMTBA–GeO2 ) or are overlapped in a broader and featureless envelop (TMA–GeO2 ). All this, together with the absence of a clearly resolved band close to 970 cm−1 would suggest the GeO2 phase is mainly amorphous. 27 Moreover, Shaw et al. refer that devitrification of amorphous GeO2 causes two bands at 1538 and 1439 to appear, while in our solid they don’t appear and there are instead two bands at markedly different positions, 1426 and 1401 cm−1 , with an uncertain assignation. As a whole, the XRD and IR results indicate that, after deposition in relatively cold surfaces, the germanium monoxide vapours disproportionate into nanocrystalline Ge metal and GeO2 that is mainly vitrous but may also contain some crystalline phases. Coming back to the residue of the calcination, the fact that two very similar phases (in structure and composition) show very similar TG traces (in wt. % as well as in temperature of the processes) but, nonetheless, give different residues (either metal Ge or GeO2 ) appears at first sight surprising. Here we note that, for both TMTBA–GeO2 and TMA–GeO2 the amount of oxygen in the framework (40 per u.c.) would not be enough to completely oxidize the organics to the fully oxidized products but they could be at least enough to produce CO, H2 O, NH3 and HF (28 or 16 O per uc required for TMTBA and TMA respectively). However, since a significant portion of Ge sublimates as GeO and in the case of TMA–GeO2 the final residue is the dioxide, not all the available O is used for the combustion of the SDA. The large TMTBA cation contains a larger amount of C than TMA so its reducing capacity should exceed that of TMA, justifying that in this case the residue is metallic Ge. It is possible to propose some chemical equations that justify the stoichiometry of the process from the point of view of the final oxidation state of Ge:

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1 Figure 7: IR spectra of the solids recovered at the alumina tube (blue, yellow) and at the cold finger (magenta, green) after calcination at 1000◦ C under a N2 stream of TMTBA– GeO2 (yellow, magenta) and TMA–GeO2 (blue, green). The red spectrum corresponds to a commercial quart-type GeO2 sample (Aldrich)

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T

[C7 H18 NF]2 [GeO2 ]20 −−→ 12 CO ↑ + 2 CO2 ↑ + 14 H2 O ↑ + 2 NH3 ↑ + 2 HF ↑ + 5 GeO2 + 15 Ge N2

(1)

T

5 GeO2 + 15 Ge −−→ 10GeO ↑ + 10 Ge N2

(2)

The equations are not intended to reproduce every aspect of the thermochemistry of this process (particularly with regard to the products of the oxidation of the guests, eq. 1), but only to account for the fact that the final solid residue amounts to 31% of the starting solid (10.1 mole/mole of uc, eq. 2). In the case of TMA–GeO2 , since the residue is GeO2 (31.5%, or 6.9 moles per mole of unit cell), the fraction of oxygen used in the combustion of the cation has to be smaller. Assuming, hence, the formation of some formaldehyde (eq. 3), the following equations account for the nature and amount of the residue found (eq. 4):

T

[C4 H12 NF]2 [GeO2 ]20 −−→ 5 CO ↑ +3H2 CO ↑ +5 H2 O ↑ +2 NH3 ↑ +2 HF ↑ + N2

13 27 T GeO2 + Ge −−→ 13GeO ↑ + 7 GeO2 N 2 2 2

13 27 GeO2 + Ge 2 2 (3)

(4)

In other words, the nature of the final residue (metal Ge or GeO2 ) depends on the extent of the initial reduction to metallic Ge: if more than half of the framework is reduced, the solid residue will be metallic Ge, while if it is less than that, the residue will be GeO2 . It can be thus foreseen that, given the right organic cation, it may be possible to find a germanate zeolite that completely ’vaporizes’ upon calcination in N2 . This would occur when the initial reduction of the framework produces equimolar amounts of Ge and GeO2 . Finally, we would like to highlight the relevance of our findings for zeolite chemistry. 15



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First, it is important to realize that, although we studied the thermal behaviour up to the very high temperature of 1000◦ C and the striking weight gaining step in air also occurs at a pretty high temperature (typically close to 700 ◦ C) this process is a reoxidation and, hence, needs to be preceded by a framework reduction. Since framework reduction bears a weight loss and requires something to be oxidized it necessarily occurs during the weight loss step associated with the removal of the organics at a much lower temperature (around 500◦ C in our case). Framework reduction during calcination in air at conventional temperatures, which is the most important process revealed in this work, may be thus difficult to detect because it is necessarilly coincidental with the typical weight loss step associated with organics removal. The most clear signature of framework reduction is the subsequent weight gaining reoxidation, which seems to typically require a much higher temperature to reveal itself and which may get obscured when occurring at temperatures of further removal of organic residues. It is also important to stress that the reported reduction of the zeolite framework is not limited to small pore zeolites or to pure germanate phases, as the ones shown here. As stated above, examples of weight gaining steps during thermal analysis in air, implying prior reduction processes, have appeared in the literature for several Ge-containing zeolites with medium (MFI, IM–14, PKU–9) 9–11 or even large pores (PKU–15) 12 and with various chemical compositions (frameworks containing also Si and Al in addition to Ge). We have collected literature examples of reoxidation processes of Ge-containing zeolites in Table 1. We have included in the list CIT–13, a new germanosilicate with medium (10MR) and extralarge pores (14MR), because a very recent report on this material shows that, for one particular organic SDA (1-methyl-3-(3,5-dimethylbenzyl)imidazolium), there is a small but neat weight gaining towards the end of the thermogram. 28 Additionaly, we have found in our lab some other examples of germanosilicate zeolites with medium pores (Ge–HPM–1) or with unknown structures (Ge–HPM–4) showing reduction/reoxidation, and they will be reported in due time. Finally, we would like to warn about what, in the light of our findings,

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may likely be an abundance of incomplete or erroneous interpretations of thermograms in the prior literature. Table 1: Literature examples of Germanium-containing open materials showing weight gaining steps during thermal analysis under air or O2 flow Phase Metal Poresa Weight lossb Weight gainb Reference IM–14 Ge 8x10x10 200 (400) 680 (720) Figure 8 10 Ge–MFI Ge, Si, Al 10x10x10 150 (400) 500 (500) & 800(800) Figure S2 9 PKU–9 Al,Ge 8x10x10 330 (450) 485 (505) & 690 (760) Figure S4 11 PKU–10 Ge 13x13x13 240 (320) 650 (755) Figure 5 7 PKU–15 Ge,Si 7x10x12 400 (450) 686 (770) Figure 4a 12 SU–79 Ge,Ni 10x10x11 300 (350) & 700 800/(–c ) Figure S5 29 c CIT–13 Ge,Si 10x14 260 (340) & 450 (570) 700(– ) Figure 5b 28 a number of framework metal polyhedra limiting diffusion in each pore; b Onset temperature (temperature at the mid of the step between parentheses) in ◦ C c Undetermined because the step is not finished at the end of the thermogram.

Conclusions During calcination in air of zeolite germanates, thermal processes may include reduction of the GeO2 framework and oxidation and removal of the organic guest, followed by reoxidation of the framework. This results in weight gaining steps in the thermogravimetric analysis. Sublimation of GeO also occurs, accounting for larger than expected total weight loss values. In the absence of oxygen, the reduction processes are dramatically notorious, and a large portion of Ge is lost by sublimation of GeO. The nature of the final residue depends on the nature of the occluded cation. In the case of TMTBA, with a much larger C content (7C/cation) the solid residue is metallic Ge, while for TMA (4C/cation) is GeO2 .

Acknowledgement The authors thank the Spanish Ministery of Economy and Competitiveness (Projects MAT201231759, MAT2015-71117-R and MAT2012-38429-C04) and Generalitat Valenciana (PROMETEOII/2014/047). 17



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Framework Reduction of GeO2 Zeolites During Calcination

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