Stabilizing Role of Linear Polyamines as Pore Fillers During the

Chapter 12

Downloaded by UNIV OF LEEDS on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch012

Stabilizing Role of Linear Polyamines as Pore Fillers During the Crystallization of Zeolite NU-10 1,2

Concettina Pellegrino, Rosario Aiello, and Zelimir Gabelica

Dipartimento di Chimica, Universitàdella Calabria, 87030 Arcavacata di Rende (CS), Italy We report a first example showing a straightforward correlation between the synthesis efficiency of a zeolite in terms of Al framework incorporation and the pore filling ability of a template. Zeolite Nu-10, prepared from gels of variable Si/Al ratios and using different alkali cations and tetraethylenepentamine (TEPA) as template, incorporates Al in variable amounts governed by the gel composition, provided its framework is stabilized y a complete filling by the polyamine during the growth phase. When TEPA or other less appropriate linear diaminoalkanes are used in such experimental conditions that the resulting Nu-10 framework is only partly filled, a further specific Al incorporation is needed to stabilize the lattice, so that the composition of the zeolite is no longer governed by the initial Si/Al ratio. In each case, structural Si-O defect groups, imposed by the actual framework Al content, are created and charge compensated by the protonated polyamines and /or by the alkali cations. The latter, incorporated in low and non sequential amounts, play a negligible stabilizing role and only affect the nucleation and growth rates, or sometimes favour crystallization of S i O polymorphs, as side phases. -

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Quaternary ammonium ( A l k N ) ions have been recognized to direct the structure of a wide range of zeolite materials (7,2). The (Si, A1)0 oxide tetrahedra organize around the organic molecule and provide the initial building blocks of a particular structure. 4

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Address correspondence to this author. Current address: Department of Chemistry, Laboratory of Catalysis, Facultés Universitaires Notre Dame de la Paix, Namur, 61 Rue de Bruxelles, B-500 Namur, Belgium

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0097-6156/89A)398-0161$06.00/0 ο 1989 American Chemical Society Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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ZEOLITE SYNTHESIS

These primary silicate or aluminosilicate complexes then form, along with the inorganic cations and water molecules, the first zeolite nuclei. At that stage, the guest molecules that especially well fit to the framework zeolite void space, are referred to as templates (1) and they usually tend to further act as structure directing agents. It is also presumed that the positively charged centers of the cation specifically interact with A1C> ~ tetrahedra so that the templating effect is further enhanced by such electric-dipole interaction. The resulting structure is completely filled by the organic molecules and these latter are usually found intact in the pore volume of the zeolite. 2

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Zeolites (Pr N )-ZSM-5 and ( B u N ) - Z S M - l l are among the most famous examples illustrating this concept (3-5). Perfectly stabilized by the template, the zeolitic lattice is little influenced by the gel chemistry and can be built over a wide range of Si/Al ratios. Downloaded by UNIV OF LEEDS on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch012

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In the presence of minor amounts of P r N , poorly crystalline ZSM-5 is obtained (6) while in complete absence of template, only limited amounts of ZSM5 appear after several days of crystallization (7, 8). In that case, hydrated sodium ions constitute a poor replacement as template or pore filling agents for P r N (8). The crystallization rate of such systems can be considerably increased by addition of amines that then primarily act as pore fillers (3, 9, 10) but not necessarily as specific structure directors during nucleation (3). 4

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Conversely, in many other cases studied, the stabilization of hydrophobic clathrasils, zeosils and very high silica zeolitic frameworks is induced by neutral guest molecules that only fill the channels and cavities (11, 12). They are thought to form a solid solution on the growing crystals and thereby lower the chemical potential of the framework (13). The energy required to stabilize such a framework mostly derives from weak Van der Waals bonds between the guest molecule and the siliceous framework (12). In that respect, the templation of ZSM-5 with alcohols (14) or ethers (15) would also have to be interpreted in terms of a pore filling model. Between these two extremes, a whole range of guest species may participate in the stabilization of a zeolitic lattice among which (poly)alkylene(poly)amines are the most widely used (1). The efficiency of such molecules to direct specifically a structure can be strongly related to their geometry. Slight changes either in the (poly)amine chain length (16-19) or in the length and position of the side chains (13, 19) often produce zeolites with different structures. However, a given zeolitic structure can also be directed by polyamines with variable chain lengths (18-20). They then stabilize to a certain degree the framework, partly aspore fillers and partly by interacting electrostatically in their protonated forms with the framework A l O ^ " centers or with SiO~ defect groups. The increment of the framework stability can be brought by other factors, the best known being the A l lattice incorporation. Indeed, isomorphic substitution of A l for Si in a framework results in its net destabilization (21) with respect to aluminosilicate lattice stabilized by counterions (22) or charge compensating protons (23).

Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

12. PELLEGRINO ET AL.

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Linear Polyamines as Pore Fillers

The preference of a zeolitic matrix for A l or Si can be qualitatively estimated by deriving "synthesis efficiency curves" that compare the Si/Al atomic ratio of the gel, plotted against that of the zeolite. The efficiency can be good over a wide [e.g.(Pr N )ZSM-5 (24)] or restricted [e.g., siliceous mordenite synthesized in presence of trioctylamine (25)] compositional range, or very poor [e.g. faujasite-type zeolites (26, 27)], indicating that in this latter case, other factors than templating effect must play a stabilizing role (27). Generally, however, a good measure for the potency of a template is the range of Si/Al ratios over which it is effective. This is generally interpreted by considering that A10 ~ negative centers do not request specifically the positive charges from the template to be neutralized. To our knowledge, a straightforward relation between the synthesis efficiency and the solely pore filling ability of a template has never been established. In the present paper, we describe a situation where the synthesis efficiency of a zeolite can be controlled and optimized by using nonspecific templates, that, under given experimental conditions, fill to various extents the pore volume during growth. We present the selected example of zeolite Nu-10 whose structure involves parallel, one dimensional noninterconnecting channels. This material can be synthesized in presence of various linear polyamines that act as templates with variable efficiency (20, 28—32). Typical preparations demonstrate that, when achieving a complete filling under specific synthesis conditions, molecules of appropriate length such as TEPA sufficiently stabilize the framework as to allow A l to be incorporated in variable amounts, governed by the gel composition. In contrast, a partial filling achieved by TEPA under different conditions or by other less efficient linear diamines results in a need of A l incorporation to stabilize the lattice by interacting with a positive charge, so that the zeolite composition is no * longer modulated by the initial Si/Al ratio. +

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Experimental

Zeolite Nu-10 was prepared from starting gels having the molar composition in the following range: 2-3 M 0 , 50-80 M ' C l , A 1 0 , 40-100 S i 0 , 30-50 R, 3880 H 0 , where M and / or M ' = Li, Na, K, Rb, Cs; R = tetraethylenepentamine (TEPA) or 1,3 diaminopropane (DP). Reagents were alkali hydroxydes and chlorides (all A.R. from Merck or Carlo Erba), Aluminum "trihydrate gel Dry Pharm U S P X X " (Serva), Precipitated silica (BDH), TEPA, purum, 20% soin (Merck), DP, (A.R., Fluka). A l l chemicals were used without further purification. A typical synthesis consists in adding a solution prepared by mixing A l and the alkali hydroxydes to an aqueous solution of the organic template in which solid silica has been dispersed. A third solution containing the alkali chloride dissolved in the remaining distilled water was added to the mixture. The so formed gel was homogenized under stirring for 0.5 h at 25 C prior to heating at 180 C in Teflon-coated stainless steel autoclaves, underautogeneous pressure, for various periods of time. A large number of syntheses were run in order to achieve a systematic study of the synthesis of Nu-10 under various conditions, and to describe the appropriate crystallization fields (30, 32). Even under improved crystallization conditions [optimized M / O H ' ratios and use of M chlorides (30)]. Nu-10 very 2

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164

ZEOLITE SYNTHESIS

often co-crystallizes with a variety of other zeolitic materials, mostly mordenite and ZSM-5. In the present study, we have selected a limited series of preparations in the monocationic systems (Μ = M ' ) , from which an accurate chemical analysis of the as-formed Nu-10 was possible (Table I, samples 1 to 8). The as made solids either consisted of pure Nu-10 having various crystallinities, from which the zeolite crystals could be separated from the remaining amorphous phase by ultrasonic treatment (33), or of Nu-10 contaminated with known amounts of cristobalite. Syntheses in the bicationic mixed systems (Μ £ Μ') are original and the final composition of the 4 as made solids is given for completeness. Two of them (samples 9 and 11) allowed the determination of Nu-10 composition. All the solid phases were identified and characterized for crystallinity by X-ray powder diffraction (Philips PW 1730/10 diffractometer, Cu K radiation equipped with a PW 1050/70 vertical goniometer and connected to a P.C. computer for quantitative analyses). Crystallinities for Nu-10 and cristobalite were computed by comparing the intensity of the most characteristic diffraction peaks of each sample to that of the corresponding pure 100% crystalline phases used as standards. In some cases calibration curves derived from Nu-10/cristobalite mechanical mixtures were used. Si, A l , and alkali contents were determined either on precursors or calcined samples (900 C , air flow, 4h) by atomic absorption, using a Perkin-Elmer 380 A A instrument after digestion and dissolution of the samples in H^S0 /HF solutions and further elimination of H F by gentle heating at 60 C for 12 n. The amounts of occluded template were determined from weight losses measured by heating the pure Nu-10 precursors in air flow from 25 to 900 C (10 C min" ) and maintaining the residues isothermally at 900 C for 4-6 h, in situ, into a Netzsch STA 429 computer controlled thermobalance (combined TG-DTADTGA). D T A confirmed the previous findings of Araya and Lowe (20), namely that the organics start to decompose above 200 C , temperature at which almost all the water was released. Amounts of hydration water and of the occluded organics were calculated accordingly.


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Results Syntheses

Samples 1 to 12 of Table I were synthesized in monocationic or bicationic systems according to the recently optimized conditions, namely by using TEPA as template and different alkali chlorides as mineralizers (30, 32). Diaminoalkanes proved less efficient in yielding pure Nu-10, in agreement with the results of Hogan et al. (29), who showed that 1,6 diaminohexane (DH) and 1,8 diaminooctane (30) were the only organics directing the formation of pure Nu-10, all other diaminoalkanes yielding poorly crystalline Nu-10, often contaminated with cristobalite or kenyalte, 1,3 diaminopropane also yielded only 68% crystalline Nu-10 in the Na system in our conditions, but the absence of other dense phases might be due to the shorter crystallization time (sample 3). Indeed, in most of the cases and even with TEPA, the Na^O-NaCl system yielded Nu-10 contaminated with cristobalite (sample 4) but most often with ZSM-5. In fact, it was shown that although pure highly crystalline Nu-10 is usually formed under a markedly wide Si/Al range, extending from 15 to



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lSi0 (tetrametaxysilane) 4DEA 56^0 (Al free)

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214Na 0 48NaCl 84Si0 25TEPA 3421H^D

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• Rb 0 2 Cs 0 2

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Stabilizing Role of Linear Polyamines as Pore Fillers During the

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