Molecular Sieves - Science and Technology - Vol. 6 - Characterization II / 02-Thermal Analysis of Zeolites

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Ohgushi et al. [46] investigated the ion exchange of Na+ for tetramethylammonium and benzyltrimethylammonium ions in ZSM-5. Thermal analysis provided information on the amount and the state (counter cations or ion pairs) of organic ammonium compounds.

Many organic agents promote the synthesis of high-siliceous zeolites. These organic molecules may play a structure-directing role during the nucleation and crystal growth or they can be considered as void fillers which, accommodated in the micropore systems, stabilize the zeolite framework. Thermal analysis is an indispensable tool to characterize the gel precursors and the products obtained during the hydrothermal synthesis.

Thermal analysis of tetramethylammonium (TMA) and tetraethylammonium (TEA) aluminosilicate gels and of the zeolite precursors obtained from them by crystallization was carried out to obtain information on the environment of the organic components [47]. From TMA systems ZSM-5 or ZSM-39 were formed depending on the TMA content. TG and DTG curves of the products demonstrated that the TMA occluded in pure ZSM-5 decomposed in one step at about 813 K, while the decomposition of organic compounds entrapped in ZSM-39 zeolite showed three steps between 743 and 883 K. The different decomposition features made it possible to calculate from TG and DTG curves the relative amount of ZSM-5 and ZSM-39 in products containing both zeolites. From the TEA system the only product was ZSM-5. Thermogravimetry combined with differential thermal analysis supported the presence of TEA+ ions in two different environments in the ZSM-5 framework. The low-temperature step ( 663 K) in the TG curve could be attributed to the release of TEA+ ions compensating the charge of defect sites (SiO–), whereas the high-temperature step ( 763 K) was associated with

the decomposition of TEA+ ions neutralizing the negative framework charges (Si – O – Al–).

Zhao et al. [48] prepared large single crystals of ZSM-5 with n-tripropyl- amine (TRIPA) and TEA in the presence of fluoride ions. TG analysis showed that the numbers of TRIPA and TEA per u.c. are nearly identical (about 4/u.c.) and independent of the aluminum content.

Howden [49] used a thermobalance coupled to a titrator for examination of the thermal behavior of ZSM-5 synthesized with a small amount of tetrapropylammonium cations (0.4 TPA/u.c.) and with different α,ω- diaminoalkanes with alkane chains constituted of 3–8 carbon atoms. It was observed that diaminopropane and diaminobutane could be removed at relatively low temperatures (at around 520 K). The pentane-1,5-diamine probably cracked before leaving the zeolite, i.e. evolution of the nitrogen-bearing (alkaline) fragment of the organic compound took place at a lower temperature than the removal of the bulk of the template molecule. When hexane- 1,6-diamine (HEXDA) was used as a template, the zeolite also released the alkaline nitrogen fraction before the hydrocarbon fraction of the organic molecule, but this process took place at significantly higher temperatures

78 G. Pál-Borbély

(above 673 K). The results indicated that diaminoalkanes with short hydrocarbon chains are not strongly held in the ZSM-5 structure. HEXDA could be regarded as a templating agent. The longer octane-1,8-diamine favored the formation of the straight channels of ZSM-11. The channels were found to be not completely filled. Carbon-to-nitrogen molar ratios determined from TA data indicated that the organic template molecules had undergone partial deamination during the synthesis.

Applying thermoanalytical techniques, Franklin et al. [50] found that silicalite-1 synthesized in the presence of HEXDA contained about 8 HEXDA/ u.c. entrapped in the channel crossings of the silicalite-1 structure. This amount seemed to be sufficient to fill almost all of the void space. A comparative TG study of the thermal behavior of HEXDAand TPA-silicalite-1 showed that the thermal decomposition of HEXDA occurred in three steps over a wide temperature range. In contrast, the weight loss due to removal of tetrapropylamine from TPA-silicalite took place in a much narrower temperature interval.

A much smaller amount of hexane-1,6-diol was incorporated in zeolite ZSM-5. Hexane-1,6-diol is believed to act as a hydrophobic void filler [51].

There are debates in the literature about the structure-directing activity of alcohols and nitrogenor oxygen-containing heterocycles though an effect surpassing that of simple pore filling was attributed to p-dioxane applied in the synthesis of omega zeolite [52, 53].

The amount and decomposition of ethyleneglycol, ethanolamine, and ethylenediamine molecules enclathrated in silica sodalite were investigated by TG coupled to MS [24]. All these organic compounds, notwithstanding their different chemical properties, acted as structure-directing agents for silica sodalite synthesis. This study provided additional support to the thesis of Gies et al. [54] that chemical properties of structure-directing molecules play only a minor role in the crystallization of silica frameworks compared with their size and shape.

Though a wide variety of organic molecules have been successfully applied for synthesis of ZSM-5, the tetrapropylammonium ion proved to be the most powerful structure-directing species. Thermal methods combined with other techniques provided unambiguous evidence for the incorporation of the different organic molecules into the structure of ZSM-5 during the synthesis and gave useful information on the thermal reactivity of the guest and host compounds. Nevertheless, most of the relevant thermoanalytical studies dealt with the investigation of TPA-ZSM-5. TPA fits the ZSM-5 channel structure especially well. The “as-synthesized” sample contains in nearly every channel intersection one TPA species. Applying thermal analysis, Crea et al. [55] found that in silicalite-l prepared with different amounts of template the number of enclosed TPA species per u.c approached in most of the samples, independently of the initial TPABr concentration in the gel, the theoretical value of 4 TPA/u.c expected for full occupation of all channel

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crossings. The authors observed three characteristic endothermal peaks during programmed heating of such a sample in an inert gas flow. On the basis of the data obtained by thermal analysis and 13C-NMR spectroscopy, they assigned in another paper [56] the three DTA peaks to different TPA species. The DTA peak at about 653 K was attributed to the release of “strained” TPA ions located in the outer shell of the crystals. The peak at about 698 K was ascribed to the elimination of “inner-strained” TPA ions occluded in the zeolitic channels. In both cases TPA ions may exist as counter-ions of SiO– defect groups. Finally, the peak at 748 K was attributed to more relaxed TPA ions compensating the charge of SiO– defect groups or, in aluminum-containing samples, i.e. in ZSM-5 zeolites, the framework charges (Si – O – Al–) associated with tetrahedrally coordinated aluminum atoms. It was assumed that the TPA+ ions retained in the solid after heating at about 673 K were in a “relaxed” status contrary to the initial strained form.

Thermal analytical techniques were also applied for the characterization of MFI-type zeolites and borosilicates prepared from TPA-OH and TPA-fluoride containing gels with different Si/Al and Si/B ratios [57]. Besides the quantitative overall estimation of TPA+, useful information could be obtained from the position of the endothermic peaks recorded in the inert gas atmosphere by DTA and DSC with respect to the chemical environment of different types of TPA+ ions. Depending on the Si/M3+ ratio of the zeolite, the template might have been present as an ion pair (TPA+OH– and TPA+F–) and/or as TPA+ counter-ion compensating the negative charge of the framework generated by trivalent elements. Interactions between ≡ Si – O– groups and TPA+ might have also occurred. The ion pairs more loosely bonded to the framework were suggested to decompose at lower temperature than the counter cations. Occluded tetrapropylammonium fluoride proved to be more stable and to decompose at temperatures higher by 45 K than those observed with hydroxide. In zeolites with high Si/Al ratios (> 1000) the observed thermal effects shown in Figs. 6 and 7 (endothermic peaks between 653 and 803 K) arose from the decomposition of ion paired TPA+ and, if lattice defects were present, of SiO–TPA+ species. A single peak at about 753 K in the DTA curve is typical of aluminum-rich (Si/Al 11) samples prepared in alkalineor fluoride-containing media. Thus, this single peak was attributed to the decomposition of TPA+ compensating framework charges. Samples with intermediate Si/Al ratios exhibited a continuous transition of the decomposition peaks, corresponding to the concentration of the different types of TPA+ species.

In the sequel several papers were published on the decomposition of organic templates (tetra-, tri-, diand mono-n-propylammonium cations) incorporated in MFI-type zeolites prepared in the presence of either OH– or F– anions [58–61]. In these papers, thermal analytical methods combined with other techniques (mass spectrometry (MS), gas chromatography (GS), 1H–13C CP MAS NMR, 1H–15N CP MAS NMR and IR spectroscopy) pro-

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Fig. 6 DTA curves of as-synthesized MFI type zeolites prepared in alkaline media (in the presence of TPA-OH). R = Si/Al ratio; heating rate 10 K min–1; argon flow (reproduced from [57])

vided more information about the mechanism of the thermal decomposition of templates by identification of the volatile products. Sometimes earlier expectations were found to be inconsistent with newer results. Here, one of these papers will be reviewed in detail.

In [61] the solids, remaining after partial thermal decomposition of Pr4NF-MFI, Pr3NHF-MFI, and Pr4NOH –MFI precursors, were examined by 1H–13C CP MAS NMR and IR spectroscopy (in these cases the zeolites were heated separately in an inert gas atmosphere in the thermoanalyzer to different temperatures corresponding to characteristic points of the DTA and DSC curves). As has been shown already, the decomposition of the organic cations in a highly siliceous Pr4NF-MFI precursor took place in two temperature ranges (see Fig. 7). After a small endothermic shoulder, both the DTA and DSC curves exhibited a strong peak with a minimum at about 693 K followed by a second broader one, displaying a shoulder at 768 K and a minimum at 793 K. It was proved by a 13C NMR study of the precursor heated previously at 733 K that after the first decomposition step mainly Pr3NH+ remained in the solid. That is in contradiction with an earlier suggestion [57] according to which TPA ions decomposed at about 700 K into triand dipropylamine and propene. Since, in the case of large-size zeolite crystals, the observed weight

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Fig. 7 DSC curves of (a) non-powdered (large crystals) and (b) finely ground siliceous (TPA)-MFI precursor prepared in non-alkaline fluoride-containing medium; argon flow, heating rate 6 K min–1 (reproduced from [61])

loss and DTA and DSC peak areas (Fig. 7, curve a) were greater than those derived from the stoichiometry of the reaction

Pr4N+ –→ Pr3NH+ + C3H6 ,

(2)

it was suggested that the formation of propylene resulted in strong tensions causing fissures inside large crystals from which also part of the template escaped in the form of tripropylamine. In contrast, weight losses and heat effects due to the first degradation step were found to correspond to the stoichiometry of Eq. 2 when a finely ground sample was subjected to the same heat treatment (Fig. 7, curve b). The authors, therefore, assumed that evolution of propene did not cause significant cracks in small crystals. The second peak with a shoulder was ascribed to the overall degradation of tripropylammonium ions. The decomposition of the Pr3NHF-MFI precursor resulted in DTA and DSC peaks similar in shape and temperature range to the second peak of ground Pr4NF-MFI. Among the volatile products, propene was detected in significant amounts by GC and MS which pointed to an initial decomposition mechanism involving Hofmann-type elimination followed by β-elimination. The 13C NMR spectra of the high-siliceous Pr4NOH-MFI precursor proved to be different. In this material the template started to decompose at lower temperatures than Pr4NF-MFI but part of the Pr4N+ remained in the intracrystalline space (in a relaxed state) until completion of the decomposition. In aluminum and gallosilicate precursors, the Pr4N+ ions

82 G. Pál-Borbély

were predominantly counter cations of the negative lattice charges. Thus, the Pr4N+ was bonded to the framework by stronger interactions, and its degradation started at higher temperatures and was characterized by a single peak. Also in these cases, a four-stage Hofmann-type decomposition was suggested, with the first stage as the rate-determining step.

Tavolaro et al. [62] reported on the application of thermal analysis to characterize both the initial gel and the crystalline silicalite-1 obtained in TPA+ and fluoride-containing media. The effect of grinding of the large crystals was shown by TG and DTA. The total weight loss ( 4 TPA/u.c.) was not affected by grinding, however, changes in the DTA curve pointed to alterations in the elimination of the template. According to the interpretation of the authors, the first weight loss step involved, at least partly, also the elimination of the second propylene molecule. In the ground sample the third elimination step was shown to occur easily leading to better-resolved DTA and DTG peaks in the temperature range 733–833 K.

Intense mechanical force (ball milling) exerted on as-synthesized ZSM-5 was found to cause amorphization of the zeolite [63]. The ball-milled sample gradually lost weight upon heating.

Yi and Ihm [64] synthesized high-silica ZSM-5 (Si/Al 175) under atmospheric pressure (at 363 K) in the presence of TPABr in an alkaline medium. Thermogravimetric analysis and differential thermal analysis were carried out (in air) to determine the binding state of TPA+ ions and to monitor the crystallization in this way. As the crystallinity increased, the exothermic peak (at 513 K) due to “free” TPA+ in the intermediate (amorphous) phase disappeared and the intensity of peaks attributed to TPA ions trapped in the channels of ZSM-5 increased.

Quaternary organic ammonium cations other than TPA (TMA+, TEA+, etc.) were used for the synthesis of offretite-erionite type zeolites [65], zeolite omega [16], EU-12 [66], Phi [67], ferrierite [68] and ZSM-20 [17]. In each case, the thermal decomposition of the template was investigated. TG and DTA data confirmed the presence of TMA cations in the gmelinite cages of MAZ and in the sodalite cages of SOD and were used to calculate the amount of these cation species and, in this way, the amount of the zeolitic components in the prepared products [69]. In a more detailed work [70], the incorporation of TMA cations and n-hexane derivatives into the pore system of mazzite-type zeolite was studied by TG and DTA in an oxidative atmosphere. The decomposition of TMA cations occluded in gmelinite cages was found to occur in the temperature range from 770 K to 830 K depending on the nature of the mazzite samples. The product prepared from a sodium aluminosilicate hydrogel (Si/Al = 5) containing TMA+ as an organic template exhibited a sharp weight loss at 810 K assigned to the oxidative decomposition of TMA cations incorporated in the gmelinite cages (Fig. 8A). However, an additional exothermic peak attributed to the decomposition of 1,6-diaminohexane molecules occluded in the 12-MR channels appeared at

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about 610 K in the DTA curve of a pure mazzite sample crystallized in the presence of TMA, 1,6-diaminohexane, and 18-crown-6 ether (Fig. 8B). Neither a weight loss step nor a DTA peak indicative of the decomposition of 18-crown-6 ether was observed. At lower aluminum content of the synthesis gel, SOD was formed as a co-crystallized phase. The third high-temperature weight-loss step observed at about 860 K was ascribed to the decomposition of TMA cations located in sodalite cages of the impurity phase (Fig. 8C). The TG and DTA curves of the zeolite synthesized in the presence of TMA and 1,6- hexanediol reflected only a single exothermal process due to the decomposition of TMA, since the DTA signal appeared at the same temperature as in the case of mazzite prepared in the presence of only TMA. From the absence of

Fig. 8 TG/DTA profiles of mazzite prepared	from sodium aluminosilicate hydrogel
A Si/Al = 5 containing TMA, B Si/Al = 8 and	C Si/Al = 12.5 both containing TMA,

1,6-diaminohexane and 18-crown-6 ether; O2/He (25/75 vol %); heating rate 5 K min–1 (reproduced from [70])

84	G. Pál-Borbély

any additional exothermic DTA peak and TG step the conclusion was drawn that 1,6-hexanediol was not occluded in the pore system. It was evidenced by 13 C CP-MAS NMR spectroscopy that (i) TMA+ was present in the gmelinite cages of all mazzite samples mentioned above; (ii) 1,6-diaminohexane in nonprotonated form was occluded in the 12-MR channels of the respective sample; and (iii) 1,6-hexanediol was not incorporated. TG data were used to quantify the contents of the respective compounds occluded in the pore system.

Thangaraj et al. [71] demonstrated that 13C CP-MAS NMR spectroscopy jointly used with thermal analysis may give valuable information on the location and the possible configuration of dibenzyldimethylammonium ions in zeolite EU-1. In cases where the organic compound acted as a template, a geometrical fit between the configuration of the organic molecule and the topology of the zeolite was anticipated.

Five quaternary ammonium salts were tested in the presence of piperazine as templates for the synthesis of silica molecular sieves [72]. The amounts of the quaternary ammonium cations in the products were calculated from the weight losses between 573–873 K, and thermal analysis was also used to determine the equilibrium water vapor uptakes of the calcined samples.

Different thermal decomposition features were found for silicalite-2 crystals with needle-like and ovate-shaped morphology, prepared in the presence of TBA [73].

Thermogravimetric analysis in combination with N2 adsorption measurements gave information about the state of trimethyl-cetyl-ammonium hydroxide encapsulated in the ZSM-35 structure [74].

A comparative study [75] of the structural and thermal properties of TMA-, calcined Na, H- and H-ferrierite using high-temperature X-ray diffraction and TG measurements showed a correlation between unit cell dimensions and the degree of both the dehydration of the zeolite and the decomposition of the template. The decomposition of TMA resulted in a decrease of all orthorhombic unit cell parameters. In the case of ferrierite, changes (contraction) in the unit cell parameters thermally induced above 850 K were found to be irreversible.

Schreyeck et al. [76] prepared a new highly siliceous layered precursor of FER-type zeolite in a fluoride medium in the presence of 4-amino-2,2,6,6,- tetramethylpiperidine as a template. The dehydration of the precursor and the removal of organic cations compensating ≡ Si – O– groups in the framework were monitored by TGA and DTA.

Ferrierites with different Si/Al ratios were prepared by recrystallization of aluminum-containing kanemites intercalated with piperidine as a template [77]. TG was applied to calculate the total amount of the occluded piperidine and piperidinium ions compensating the negative lattice charges in the products. It was found that the total of piperidine molecules and piperidinium ions in the samples amounted to about 4/u.c., independently of the Si/Al ratio.

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Zeolite Beta is usually crystallized in alkaline media in the presence of Na+ and TEA+ cations as structure-directing agents. Weight losses of the products registered in air at temperatures between 493 K and 623 K have been attributed to the removal of TEA-OH, and those observed above 623 K, to the decomposition of the TEA+ cations. Perez-Pariente et al. [78] found a linear correlation between the content of TEA+ (weight loss above 623 K) and the degree of crystallinity of samples prepared with various techniques. The TG weight loss above 493 K, representing the sum of organics occluded in the samples, proved to be a function of the molar SiO2/Al2O3 ratio of the zeolite. The pores of crystals with high SiO2/Al2O3 ratios (66 and 86) were found to be totally filled with organics (90% of which was TEA-OH). In zeolites with higher aluminum content the amount of TEA+ increased, however, the negative lattice charge could be shown to be never exclusively compensated by TEA+ but partly also neutralized by sodium ions.

Perez-Pariente et al. [79] found that in air the decomposition of TEA in the Beta zeolite occurred in three exothermic steps associated with weight losses, while Hedge et al. [80] observed in the DTA curve of an as-synthesized Beta zeolite four distinct exothermic peaks. In [79] the authors also reported on TA results obtained under nitrogen. The decomposition of the template took place in two endothermic processes corresponding to the pyrolysis of TEAOH and TEA+ cations.

Camblor et al. [81] reported that in the TG curve of as-synthesized [Al]- BEA zeolite four weight loss steps (I–IV) could be clearly distinguished. The endothermic process I was ascribed to the desorption of water and process II proceeding between 423–573 K to the degradation and combustion of TEA+ cations associated with SiO– groups in defect sites. The exothermic hightemperature processes III and IV were attributed to the decomposition and combustion of TEA+ cations balancing the charge of framework Al(OSi)–4 entities and deposited coke, respectively.

Vaudry et al. [82] studied the thermal behavior of as-synthesized Beta zeolites with different Si/Al ratios, the DTG curves of which are shown in Fig. 9. The contents of water, TEA-OH, and charge compensating TEA+ cations were calculated from TG results. Weight loss data and DTG curves were interpreted in the following way:

The amount of occluded tetraethylammonium hydroxide (revealed in Fig. 9 by peak II of sample (a) containing 4.6 Al/u.c.) decreased with increasing aluminum content and approached zero in the case of aluminum-rich zeolites (sample (c) in Fig. 9, containing 8.3 Al/u.c.). The complete filling of the channel space with TEA+ corresponded to six molecules per u.c. In the case of sample (b) containing 5.8 Al/u.c., the amount of TEA+ counterions approximated this value. When the aluminum content of the framework exceeded 6 Al/u.c., hydrated sodium ions occupied a part of the cation sites to preserve charge balance. Consequently, the water content (peak I) of the zeolite increased and, because of the space requirement of the hydrated

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Fig. 9 DTG curves of as-synthesized (TEA)-Beta zeolites containing a 4.6, b 5.8 and c 8.3 Al/u.c.; I, II, III, IV: see text (reproduced from [82])

sodium ions, the amount of the TEA+ ions decreased (peak III, IV). Above 723 K the aluminum-rich sample (c) retained a decreased amount of aminic residues (IV), suggesting a lower strength of the acid sites liberated during the calcination.

Recently, Beta zeolite containing 10.3 Al atoms per tetragonal unit cell has been synthesized [83]. TG data showed that Beta zeolites with Si/Al < 7.5 (Al/u.c. > 7.5) contained only one type of TEA species but two types were found in samples with Si/Al > 7.5.

Usually the interpretation of the various thermal events in as-synthesized Beta zeolite was similar to that proposed by Parker for TEA-MFI [84]. It was based on the hypothesis that tetraalkylammonium ions associated with acid sites of a zeolite are more stable than the other species. Bourgeat-Lami et al. [85] disagreed with such an interpretation of the TG/DTA decomposition patterns of TEA-Beta. They proposed a decomposition mechanism based on the identification of the volatile products by TG/DTA coupled to mass spectrometry and their quantification by titration and on the characterization of the zeolitic product obtained at different decomposition temperatures (13C NMR and IR spectroscopy). According to these authors, TEA ions decomposed in the temperature range 473–623 K, regardless of their nature (occluded ion pairs or counter-ions), in a single step to ethylene and triethylamine. Part of the latter might have remained bound to the acidic sites of the zeolite and decomposed at higher temperatures to lighter amines by sequential Hofmann elimination reactions. A fraction of ethylene might have reacted on the acid sites yielding aliphatic and aromatic hydrocarbons. The desorp-