Tetrapropylammonium cations

Recent single crystal X-ray diffraction studies (although currently limited to those materials for which single crystal specimens S 50 im are available) have determined benzene locations in Na-X [31], and, in ZSM-5, the geometry of the tetrapropylammonium cation used as a template in synthesis [32] and the location of sorbed p-xylene [33]. Some success using powder X-ray diffraction (PXD) has also been noted [34, 35]. 

Numerous organic species are known to lead to the crystallization of the MFI-type structure (7). but the tetrapropylammonium cations can be considered to be the most specific. To our knowledge no thermodynamic data such as standard formation enthalpies (AfH°) and stabilization energies attributed to the organic species have been published to corroborate this experimental observation. The published thermodynamic data (AfG°, AfH°, AfS°. Cp) are for natural zeolites (8-11) or for organic-free synthetic zeolites. However. some data have been obtained by calculations using lattice energy minimization and extended Hiickel theory (1 2) or by semi-empirical methods based on addition of the thermodynamic functions of the oxide compo-... 

Figure 1. Yield of pentasil zeolites (percentage crystallization) as a function of time. Abbreviations are C, for mono-n-butylair ne, CU for mono-n-propylamine, C2 for mono-n-ethylamine, C. for mono-n-methylamine, di-C, for di-n-propylamine, di-C, for di-n-propylamine,+di-C2 for di-n-ethylamine, di-n-C.. for di-n-methylamine, TPA Al-free for the aluminium-free synthesis with the tetrapr pylammonium cation, TPA for tetrapropylammonium cation, TEA Al-free for the+aluminium-free synthesis with the tetraethylammonium cation, TEA for tetraethylammonium cation and tri-C for tri-n-propylamine.

In work not yet published the laser ablation method has also been applied to zeolites containing adsorbed species. It has been shown that the technique is able to generate molecular ions from strongly adsorbed species such as hexamethylbenzene in NaY, the tetrapropylammonium cation in as synthesized MFI zeolites, and polyethylene oligomers generated from ethylene in HZSM-5. 

The state of the template tetrapropylanunonium ion [TPA] was also monitored with Raman spectroscopy during various stages of ZSM-5 synthesis. It was found that this cation is trapped into the amorphous solid phase at the earliest stages of the synthesis in the all-trans configuration. Upon crystallization of the zeolite, there is a forced change in the conformation of the trapped tetrapropylammonium cation such that it can fit into the zig-zag zeolite channels. The template [TPA]" was decomposed by... 

Titanium-substituted silica-based molecular sieves, in particular TS-1 (MFI), have been the most intensively studied [6,7,9]. This generally involves controlled hydrolysis of a mixture of Si(OEt)4 and Ti(OEt)4 in the presence of the template, the tetrapropylammonium cation in the case of TS-1. Many workers have experienced problems in TS-1 synthesis and the various pitfalls haven been reviewed [9]. Small amounts of Na or orginating from commercial samples of the template suffice to prevent the substitution of Ti into the framework. Similarly, the presence of F leads to the formation of octahedral extra framework titanium. 

A series of ZSM-5 samples with differing framework aluminium contents (containing tetrapropylammonium cations, [TPA]+) have been characterised by Raman spectroscopy. Difference Raman spectra reveal evidence for two distinct occluded species in samples with non-zero framework aluminium content. These species have been Identified as [TPA]+ cations associated with framework anionic sites and non-framework anions such as Br or OH-, on the basis of correlations between the integrated intensities of difference spectra and zeolite aluminium content. The relative abundance of the two forms have been determined semi-quantitatlvely and empirical evidence for [TPA]+ disordering is reported. 

The AlP04-5 synthesis takes place starting with phosphoric acid, aluminum hydroxide (pseudoboehmite) and an organic template such as tetrapropylammonium cation. The pH starts out on the acid side, going gradually to neutral. A wide variety of templates may be used. Conditions such as temperature, dilution and agitation may be varied widely. 

Both well-ordered tetrapropylammonium cations in the asymmetric unit occupy special positions of site symmetry 2 in space group C222i atoms N(5) and N(6) are situated at Wyckoff positions 4(b) at (1/2, y, 1/4) and 4(a) at (., 0, 0), respectively. Figure 20 shows a honeycomb-like double layer at z = 1/4 with large octagonal windows and (ti-C3H7)4N(5) cations trapped within it. When the crystal structure of 2.3 is viewed parallel to the a axis, the ( -C3H7)4N(6) cations are seen to be concentrated about the (002) planes and sandwiched between adjacent double layers. 

The method has already been used with success (e.g. ref. 40-43) for sorbates as n-hexane, tetrapropylammonium cations, benzene, p-xylene at low (<4 mol. per u.c.) or high coverages (4-8 mol. per u.c.) as shown in fig. 1 in ref. 40 and in fig. 5 to 8 in ref. 44. This shows clearly that the extent of hydration or of adsorption influences the unit cell parameters. It follows that if one uses XRD pattern to determine if a given element is substituted for silicon in a lattice the environmental atmosphere during XRD pattern recording has to be careful defined. An example is given in fig. 2 and table 1 from ref. 44. 

The molar enthalpy of hydration is seen to be strongly dominated by the combined electrostatic terms for all the ions, and increases (becomes more negative) sharply with the ionic charge, Z. Except for the tetraalkylammonium ions the neutral and water- structural contributions tend to cancel each other to an appreciable extent. The hydrophobic effect AHst is very marked for the tetrapropylammonium cation, and bulky ions have fairly important contributions from the cavity formation term, A//Nt. The model does not distinguish between cations and anions, since the charge enters the expressions squared or as the absolute value. 

The clusters in Figs. 8.14 and 8.15 have been assembled from oligomers as shown in Fig. 8.13, by a process which is driven by the specific stabilizing interaction of the tetrapropylammonium cation and silicate cluster. The Sisa cluster apparently has an optimum interaction between the tetrapropyl ion and the silicalite cage. It is the result of self assembly... 

MCM-41S with partially zeolitized walls have been tentatively prepared in alkaline medium by post-synthesis recrystalhzation of MCM-41S in the presence of tetrapropylammonium cations [122] and via simultaneous synthesis of ZSM-5 and MCM-41 using mixtures of Ce and Cm alkyltrimetylammonium templates [123]. The results show that varying amounts of the two structures can actually be obtained by adjusting the synthesis parameters, but the zeo-hte forms generally as discrete nanocrystals, 2-3 nm in size, segregated from the mesophase. The formation of composite materials and the lack of for-... 

In the solid state, lb afforded planar [1 + l]-type pentacoordinated Cl /Br complexes by using pyrrole NH, bridging CH, and aryl ortho-CH units (Fignre 4.3a). Planar lb Cl /Br anions stacked along with tetrapropylammonium cations (TPA+) to form charge-by-charge columnar strnctnres with distances between the halides of 8.54 and 8.67 A for Ck and Br, respectively, and those between the receptor-halide complexes of 7.29 and 7.44 A, respectively. Similarly, (J-substituted 2b and 3b exhibited the formation of [1 + l]-type complexes. 

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