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

Thermal Analysis of Zeolites

87

tion of the so-called hydrocarbon residues required a temperature higher than 773 K.

Kanazirev and Price [86] found that oxygen essentially influences the formation of residues during the thermal decomposition of TEA in zeolite Beta precursors. In a comparative study of this process monitored by thermoanalytical techniques, the experiments were carried out in both He and He/O2 atmospheres. Up to about 650 K, thermal effects were found to be not influenced by the gas atmosphere but at higher temperatures, significant differences between thermoanalytical curves measured in inert and oxidative atmosphere were observed (see Fig. 10). In Fig. 10 the weight losses are related to the sample weight at 403 K at which desorption of weakly bound water was complete. Stable starting conditions were much better met by this reference point than by the initial weight, since the materials lost adsorbed water very rapidly when placed in a dry gas flow. The TG curves measured in inert and oxidative atmosphere began to diverge near 730 K and the organic residue retained in the oxidative atmosphere at this temperature was relatively stable up to about 800 K. At about 730 K, O2 was obviously involved in processes which led to the formation of organic residues (the mechanism is not yet clear). The amount of the residue was found to depend on the concentration of oxygen in the atmosphere surrounding the sample and probably on the Si/Al ratio. In the final stage of the thermal treatment the organic residues burned off. Also, the thermal decomposition of 1-propylamine (1-PA) adsorbed on calcined BEA exhibited similar features. In contrast, in the absence of O2 the formation of residue was strongly suppressed. In He flow the weight losses distinguished in the TG curves of as-synthesized (TEA)-Beta and 1-PA on Beta zeolite were assigned to

Fig. 10 Thermogravimetric analysis of zeolite Beta synthesized in the presence of TEAOH from a gel with SiO2/Al2O3 ratio = 20. Heating rate 10 K min–1 in a flow of pure He

(—) and 25% O2 in He (- - -) (reproduced from [86])

88	G. Pál-Borbély

–desorption/decomposition of weakly bound TEA or desorption of amine which is either occluded in channels or bound as a ligand in second level solvation shells;

–decomposition of TEA cations or alkylammonium ions;

–decomposition of organic residues, a step which seems to be negligible with regard to the above-mentioned relevant observations of the authors.

It should be noted that the interpretations, given by Kanazirev and Price [86] and by Bourgeat-Lami et al. [85] to explain the decomposition feature of TEABeta, are contradictory in some regards.

5

Characterization of Isomorphously Substituted Zeolites

A great number of papers deal with synthesis and physico-chemical properties of isomorphously substituted zeolites. The synthesis of such materials generally requires the presence of organic compounds (templates) in the reaction mixture. Thus, it is evident that thermoanalytical techniques offer a variety of possibilities for the investigation of such materials. Kosslick et al. [87] presented excellent examples of how to get reliable information even on framework characteristics from thermoanalytical data. They carried out TG and DTA measurements of [Ga]-ZSM-5 prepared in an alkaline medium with TPA bromide and observed in the as-synthesized precursor material during heating in air four thermal events which were assigned to the following processes:

I.water desorption from zeolite pores at about 373 K;

II. decomposition (oxidation) of the embedded template between 633–723 K; III. oxidation of coke resulting from the template above 773 K; and

IV. dehydroxylation.

Process II was found to proceed in [Ga]-ZSM-5 at somewhat higher temperatures than in the pure silica analogue of ZSM-5, and this difference in the maximum decomposition temperature proved to be correlated, at least up to 4 Ga/u.c., with the gallium content of the sample. This temperature shift illustrated by DTA data in Fig. 11 was regarded as an indication of incorporation of trivalent gallium into tetrahedral framework sites, since such an isomorphous substitution of silicon by gallium must result in the creation of negative lattice charges and, hence, in stronger ionic interactions between the template and the zeolitic framework. Obviously, the formation of coke originating from the catalytic cracking of template decomposition products, e.g., propene, on strongly acidic Brönsted sites also evidences the incorporation of gallium into framework positions. Furthermore, it was shown (as illustrated in Fig. 12) that onset and course of the crystallization process of galliumvarieties of ZSM-5 could be easily detected and monitored by the intensity of

Thermal Analysis of Zeolites

89

Fig. 11 DTA curves of as-synthesized (1) gallosilicate analogue of ZSM-5 (Si/Ga = 20) and

(2) silicalite-1 in air. Heating rate: 5 K min–1 (reproduced from [87])

Fig. 12 Differential thermal analysis of TPA-[Ga]-ZSM-5 after different time of crystallization. Si/Ga = 25; heating rate: 5 K min–1; atmosphere: air (reproduced from [87])

the DTA peak at about 743 K associated with process II. For samples isolated from alkaline crystallization mixtures at increasing crystallization times, this intensity reflected the amount of embedded template which is a function of the zeolite content (degree of crystallinity).

90	G. Pál-Borbély

Gabelica et al. [6] found also for ZSM-5 samples a good correlation between the weight loss due to decomposition of intracrystalline TPA and XRD crystallinity.

The amount of the TPA+ ions, trapped in the crystals during the synthesis of MFI-type zeolites prepared in fluorine-containing media with up to 3 Ga/u.c. in the framework, was measured by TG [88]. DTA results indicated that under N2 in the gallium-rich sample the decomposition of TPA cations occurred in one step at about 753 K. In contrast, two-step decomposition (at 693 and 753 K) was found in the case of samples containing less Ga. Similar to the assignment of DTA peaks of Al-MFI [57], the high-temperature peak was attributed to TPA+ (Si – O – Ga)– ion pairs and the low-temperature peak to TPA+ ions linked to SiO– defect sites or TPA+ F– (or OH–) ion pairs. However, the authors pointed out that decomposition of TPAF might contribute to the high-temperature step, since plots of TPA involved in the high-temperature decomposition step vs. the Ga content deviated at small Ga values somewhat from the straight line towards higher amounts of TPA. A dependence of the temperature of DTA peaks on the gallium content was not found.

Studying the thermal behavior of ferrisilicate analogues of ZSM-5, Borade [89] found a linear relationship between the X-ray crystallinity on the one hand and the weight losses due to desorption of water (dehydration) and decomposition of TPA ions on the other hand. With increasing crystallinity of the sample the weight loss due to dehydration decreased, while that due to decomposition of organic cations increased.

The amount and the decomposition of hexamethyleneimine as the template in [Fe,Al]-MCM-22 were investigated by TA [90].

Thermoanalytical curves (DTA/TG) of pure Ga-substituted Nu-23 zeolite prepared in the presence of cetyltrimethylammonium bromide indicated that the template was occluded in the pores of [Ga]-Nu-23 and decomposed around 973 K [91].

Crystalline gallosilicate (Si/Ga ≈ 10) [92] and iron-silicate (Si/Fe = 9) [93] analogues of mordenite have been synthesized using tetraethylammonium bromide as a template. Substitution of gallium or iron for aluminum resulted in a shift of the exothermic weight changes, associated with the loss of occluded TEA-bromide and TEA+ lattice cations in air, to lower temperatures. In line with these observations, the exothermic DTA peak was found [94] to be slightly shifted towards lower temperatures upon substitution of iron for aluminum in the ZSM-11 framework.

In DTA curves of [Ga]-ZSM-22 [95] and [Fe]-ZSM-22 [96], both prepared in the presence of 1-ethyl-pyridinium bromide as the template, the peaks associated with the oxidative decomposition of the occluded organic material and of organic ions strongly interacting with sites created by incorporation of Ga3+ and Fe3+ ions were shifted to lower temperatures compared to their position in the thermogram of the respective Al analogue, i.e. organic species were held less strongly in the isomorphously substituted zeolite varieties. Ac-

Thermal Analysis of Zeolites

91

cordingly, IR spectroscopy and TPD of NH3 showed that Brönsted acid sites present in the ferrisilicate analogue of ZSM-22 are weaker than those present in the proper zeolite [97].

Also, in Ga- [98] and Fe-varieties [99] of zeolite Beta the interactions between TEA+ cations and the framework were found to be significantly weaker, i.e. thermal effects due to TEA+ decomposition were observed at lower temperatures than in the case of the Al-containing zeolite. Other kinds of TEA species (TEA hydroxide occluded in the pore system or TEA+ associated with defect sites) could not be distinguished by thermal analysis. The total amounts of organic material, found to be between 5.5 and 6.5 TEA per u.c., were determined by thermogravimetry. The decomposition of TEA+ ions in [Ga]-Beta in air and N2 were compared in [100].

Borade and Clearfield [101] described a modified synthesis route (crystallization in methanolic medium) for ferrisilicate with the framework topology of zeolite Beta and reported detailed results concerning the physical-chemical properties of samples with various Si/Fe ratios (7.5–30). Valuable information with respect to the incorporation of iron into the Beta framework could be obtained from TGA experiments. The assignment of weight losses to different TEA species was based on earlier findings [78, 80, 102]. The most important observations were that the weight loss between 323–633 K related to desorption of occluded TEAOH species decreased and that due to the decomposition of TEA+ species (in the temperature range 633–823 K) increased with increasing Fe content in the solids. The DTG and TG curves obtained in the presence of nitrogen clearly showed that the pyrolysis of TEA+ cations interacting with the zeolite framework occurred at lower temperatures in the case of [Fe]-Beta samples compared to [Al]-Beta. The shift (40 K) in the DTG peak maximum towards lower temperatures points to the presence of negatively charged FeO4 tetrahedra in the zeolite framework and is indicative of weaker interaction of charge-compensating TEA+ cations with the framework of [Fe]-Beta than with that of [Al]-Beta. The acidity of [Fe]-Beta and [Al]-Beta zeolites was investigated by TGA of NH4+-exchanged samples. In [Fe]-Beta, NH4+-ions are decomposed at lower temperatures than in [Al]- Beta zeolite which points, in accordance with the electronegativity concept, to a lower acid strength of the Brönsted sites in [Fe]-Beta.

Camblor et al. [103] concluded from the thermal behavior of zeolitic varieties of the Beta-type containing various amounts of Ti that this element occupied tetrahedral framework positions.

Tuel and Ben Taarit [104] compared the thermal behavior of a titaniumsilicalite with MEL (silicalite-2)-structure synthesized in the presence of tetrabutylphosphonium (TBP) ions with that of silicalite-2 prepared with TBAhydroxide as the template. The numbers of TBA+ and TBP+ ions per u.c. were estimated from thermoanalytical data. The TG and DTG curves clearly revealed that the elimination of TBP requires higher temperatures than that of TBA. The thermal decomposition of organic molecules occluded in titanium silicate with

92	G. Pál-Borbély

MFI structure prepared with mixtures of TBP-hydroxide and tetraethylphosphonium (TEP) hydroxide was found to occur in only one step [105]. Though, as revealed by 13C and 31P NMR spectroscopy, both TBP+ and TEP+ cations were incorporated in the zeolite structure during crystallization.

6

Characterization of Zeolites by Thermal Analysis of Adsorbed Alkylamines and Alcohols

Parillo et al. [106] examined the adsorption features of various alkylamines in H-ZSM-5 (Si/Al = 35) using thermal analysis combined with TPD and IR spectroscopy. Simultaneous TPD (MS) and TG measurements (under vacuum) showed that ethylamine, 1- and 2-propylamine adsorbed on the zeolite in excess of the aluminum content (expressed in molarities) desorbed unreacted below 500 K (MS signals characteristic of alkylamines were detected), while at higher temperatures alkenes and ammonia were formed simultaneously as decomposition products. Their amounts corresponded to the decomposition of one alkylamine molecule per bridged hydroxyl associated with framework aluminum atoms. The absence of the IR band at about 3605 cm–1 provided evidence for the protonation of the alkylamines. The decomposition of the protonated species was found to occur, depending on the nature of the alkyl group, between 500–575 K for t-butylamine, 575–650 K for 2-propylamine, 650–700 K for 1-propylamine, and 650–725 K for ethylamine.

A detailed study of adsorption and desorption processes in the 2-pro- pylamine/zeolite and 2-propanol/zeolite systems using combined TPD-TGA methods was published by Kofke et al. [107, 108]. It was established that 2-propanol and 2-propylamine formed well-defined 1 : 1 adsorption complexes with acidic framework sites not only in H-ZSM-5 [107] but also in H-ZSM-12 and H-mordenite [108]. Proton transfer was found to be the driving force for strong binding of the adsorbate.

Kofke et al. [109] examined the adsorption of 2-propanol and 2-propyl- amine on H-[Ga]-ZSM-5 using simultaneous TPD(MS)-TGA in high vacuum. 1 : 1 adsorption complexes of adsorbates with the hydroxyl sites associated with tetrahedrally coordinated framework Ga ions were identified. The TPD-TGA results concerning the thermal behavior of 2-propylamine in H-[Ga]-ZSM-5 are shown in Figs. 13A and B. The weakly adsorbed 2-propylamine desorbed unreacted in a well separated low-temperature step. The weight loss between 570–650 K corresponded to a coverage of one alkylamine molecule/Ga, and TPD provided evidence for the unimolecular acidcatalyzed decomposition of the alkylamine on Brönsted sites under formation of propene and ammonia. No second thermal effect was observed for 2-propylamine on ZSM-5 with a SiO2/Al2O3 ratio of 880.

Thermal Analysis of Zeolites

93

Fig. 13 TG (A full line) and TPD-MS (B) curves of 2-propylamine on H-[Ga]-ZSM-5. Adsorption of 2-PA at 295 K; heating rate: 10 K min–1 under vacuum. Signals m/e = 44, m/e = 41 and m/e = 17 correspond to the fragments [CH2 – CH2 – NH2], [CH3 – CH = CH], and NH3 indicative of 2-propylamine, propene, and ammonia, respectively. For comparison, the TG curve of 2-propylamine on high-siliceous ZSM-5 is also shown (A, dashed line) (reproduced from [109])

Kanazirev et al. [110] applied other experimental conditions for thermal analysis of adsorbed propylamines than Gorte et al. [106–109]. The experiments were carried out not in high vacuum but under atmospheric pressure in a He flow. Despite this alteration of the procedure similar TA features were observed for both 1- and 2-propylamine in H-MFI zeolite. The weight losses corresponding to the plateau of TG curves between 550–640 K were indicative of the formation of stoichiometric 1 : 1 complexes of propylamines with zeolitic protons. However, on comparison of the results with data published earlier in [107, 108] it became evident that additional to the high-temperature process not one but two thermal effects, reflected by weight loss steps and DTA peaks, occurred at low temperatures below 550 K. It was suggested that in high vacuum the very weakly bound propylamine was removed before the TA measurement was started. In accordance with previous data reported in [107] it was found that nearly two 1-propylamine (1-PA) molecules per framework Al were retained in H-ZSM-5 (Si/Al = 20) at 400 K. To explain this adsorption feature of 1-PA, Kanazirev et al. [110] assumed that a second 1-PA molecule was attached to each propylammonium ion formed in the zeolite,

94	G. Pál-Borbély

constituting a primary “solvation shell”, the breakup of which gave rise to the second low-temperature DTA peak.

Thermal analysis (TG, DTG) was applied in a detailed study of alkylamine guests ordering in the H-form of zeolite hosts [111]. Thermoanalytical curves showing typical features of 1-PA adsorbed in H-MFI samples with different Si/Al ratios are presented in Fig. 14. The “plateau” region (575–625 K) in the TG curves is interpreted as to be associated with primarily adsorbed species (PAS), anchored directly to the active sites (protons) in the host. The two effects at lower temperatures are attributed to the elimination of

–nearest neighbor species (NNS) attached to PAS; and

–weakly bound species (WBS).

Quantification of the three adsorbed 1-PA species by TG showed that for the MFI materials with framework SiO2/Al2O3 ratios ≥ 40 roughly equal numbers of 1-PA molecules were bound as PAS and NNS, i.e. each PAS interacted with another amine molecule to form a NNS. With increasing Al content in the zeolite, the amount of both PAS and NNS rose at the expense of that of WBS. When the SiO2/Al2O3 ratio was < 40, steric constraints in the pore system evoked by high pore filling began to limit the number of NNS.

Biaglow et al. [112] used TGA and temperature-programmed desorption (TPD) to compare the adsorption and desorption behavior of 2-propylamine and 2-propanol on steamed and chemically dealuminated faujasites. The 2-propylamine adsorbed on Brönsted sites decomposed in the same temperature range as observed in the case of H-ZSM-5. The quantity of amine decomposed in the high-temperature region was regarded as a measure of

Fig. 14 TG and DTG curves of 1-propylamine (1-PA) adsorbed on H-ZSM-5 with SiO2/ Al2O3 ratios of 25.9 (—), 39.5 (- · -), 48.7 (· · · · ·), 127 (- - -), and 329 (-··-··). The SiO2/ Al2O3 ratios were obtained from the weight loss due to the decomposition of 1-PA above 625 K. Heating rate: 5 K min–1 in He flow. PAS: primarily adsorbed species, NNS: nearest neighbor species, WBS: weakly bound species (reproduced from [111])

Thermal Analysis of Zeolites

95

the Brönsted acid site concentration. In the temperature range between 450 and 600 K, unreacted 2-propylamine was only desorbed from faujasite but not from high-silica zeolites. It was supposed that this 2-propylamine interacted with the less strongly acidic hydroxyls giving rise to the low-frequency band at 3540 cm–1 in the IR spectrum [113].

Two different adsorption/desorption features of 1-propylamine on H-MFI (Si/Al = 20) zeolite were observed by combined DTA/TG/MS depending on the temperature (323 or 593 K) at which 1-PA was contacted with the zeolite [114]. At 593 K, conversion of 1-PA to dipropylamine (DPA) occurred which was retained in cationic form. Consequently, the major desorption feature shifted to a higher temperature compared to the run following the adsorption at 323 K. In addition, the sample weight in the plateau region (between 550 and 675 K) of the TG curve was found to correspond to 1.7 1-PA molecules/framework Al instead of one 1-PA/Al as generally obtained. The conversion of 1-PA to DPA was supported by the similarity of TA curves measured after contact of H-ZSM-5 with the mono-alkylamine at 593 K and with DPA at 323 K.

Recently, the interaction of mono-alkylamines (C1 –C5) with ETS-10, a zeolite-like titanosilicate with Ti(IV) in an octahedral environment, was studied by TG combined with other techniques [115].

These examples demonstrate that TGA alone gives information about the temperature ranges in which different processes (reactions) take place in zeolitic samples, but additional “complementary” methods (MS, GC, IR, NMR) are in general indispensable for the assignment of weight losses to particular desorbed molecules or decomposed organic constituents. Thus, TA offers possibilities for the quantification of organic compounds weakly adsorbed in zeolites or associated with acidic framework sites and, hence, of the Brönsted acid sites themselves.

7

Thermal Analysis of Alkylamines Adsorbed on Metal Ions Located in Cationic Positions of the Zeolite Framework

TPD/TGA measurements of 2-propylamine adsorbed on copper-exchanged ZSM-5 (Si/Al = 35) and silicalite samples were performed by Parrillo et al. [116]. (The H-form of the zeolite and the silicalite-1 were treated with cupric acetate solutions.) The results were compared with those obtained on H-ZSM-5. 2-propylamine adsorbed on H-ZSM-5 showed a two-step desorption feature. Below 500 K, only unreacted 2-propylamine desorbed from the sample, between 575 and 650 K the protonated alkylamine decomposed to propene and ammonia. In the TPD/TGA curves of 2-propylamine adsorbed on a partially (Cu = 0.186 mmol g–1) and a highly (Cu = 0.743 mmol g–1) ionexchanged Cu-ZSM-5 new TPD peaks and weight-loss steps were obtained,

96 G. Pál-Borbély

one at 500–550 K due to desorption of unreacted 2-propylamine, the other at 650–800 K. The high-temperature process was ascribed to the decomposition of 2-propylamine strongly interacting with those copper cations which compensated the negative framework charges of the zeolite associated with tetrahedrally coordinated aluminum. This process was not observed when copper was introduced into silicalite-1 in a smaller (but not negligible) amount than in the case of H-ZSM-5 (Fig. 15). The release of unreacted 2-propylamine observed in the case of all Cu-exchanged samples between 500–550 K was suggested to be due to copper species not associated with negative charges generated by framework aluminum. TPD/TGA proved to be a useful combination of thermal analytical methods for the characterization of the nature of copper in zeolites.

High-temperature treatment of a mechanical mixture of CuO and H-MFI zeolite in the absence of oxygen resulted in the incorporation of copper ions into cation positions of the zeolite. Thermal analysis (TG combined with MS) of 1-propylamine adsorbed on mixtures of CuO/H-MFI previously treated under different conditions was applied to determine the degree of consumption of zeolitic protons during the ion exchange process [117]. CuO/H-ZSM-5 previously heated in helium up to only 848 K and kept at this temperature for 0.5 h showed a desorption feature for 1-propylamine very similar to

Fig. 15 TG (A) and TPD-MS (B) curves of 2-propylamine adsorbed (1) on highly copperexchanged ZSM-5 and (2) on Cu-silicalite-1. Heating rate: 20 K min–1 under vacuum. Signals m/e = 44, m/e = 41, and m/e = 17 correspond to the fragments [CH2 – CH2 – NH2 ], [CH3 – CH =CH], and NH3 indicative of 2-propylamine, propene and ammonia, respectively (reproduced from [116])