Organic template removal

Third Concept in Catalyst Design. Fenton Detemplation. Mild Organic Template Removal in Micro- and Mesoporous Molecular Sieves... 

Heng S, Lau PPS, Yeung KL, Djafer M, and Schrotter JC. Low-temperature ozone treatment for organic template removal from zeolite membrane. J Membr Sci 2004 243 69-78. 

When the membrane synthesis requires a template, it has to be removed from the zeolite pores. Chemical leaching or ion exchange with ammonium ions is only possible for big pore sizes (e.g. MOR, Beta, MCM). In most cases, and namely for the MFI structure, the template is removed by thermal treatment in strictly controlled conditions (atmosphere, heating rate, temperature and duration). We have to note that an efficient and gentle low temperature ozone treatment was recently proposed for organic template removal from MFI zeolite membranes [117]. 

Thermal treatments can be applied to modify the properties of a material, for example, dealumination and optimization of crystalHne phases. These techniques do not require oxidants. Oxidative thermal treatments are generally employed to activate molecular sieves, by removing the organic templates employed during synthesis. This is one of the key steps when preparing porous catalysts or adsorbents. In air-atmosphere calcination, the templates are typically combusted between 400... 

Overall the period since the 1980s can be described as a period of explosion in the discovery of new compositions and structures of molecular sieves. This can perhaps be seen most vividly by comparing the numbers of structure types contained in the various editions of the Atlas of Zeolite Structure Types [4]. The first edition (1978) contained 38 structure types, the second edition (1987) 64, the third edition (1992) 85 and the most recent edition (2007) 176. Thus 112 new structure types have been discovered since 1978. However, the reader should be cautioned that a significant number of the structure types included in the Atlas are not truly microporous or molecular sieve materials (i.e., they are not stable for the removal of as-synthesized guest species, typically water or organic templates) and therefore carmot reversibly adsorb molecules or carry out catalytic reactions. Unfortunately, the Atlas gives only limited information on the stability of the structures described. 

In either the transcriptive or synergistic strategy, removal of the organic template by extraction or calcination renders the inorganic mesoporous structure. For synthetic schemes that are not compatible with the formation of stable template assemblies, an alternative approach is to use a preformed, templated inorganic host, such as mesoporous silica, as a mold to nanocast the desired material as an inverse replica of the host, such as that seen in Figure 14. ... 

A possible mechanism of the ammonia hydrothermal treatment for the acid-made sample is shown below. The predominant interaction between the silica wall and the surfactant of the acid-made products is the weak hydrogen bond interaction through an intermediate counterion (i.e. N03). Such weaker interaction eases the removals of organic template by hot water or organic solvent [6], Thus, when the acid-made materials are subjected to the ammonia hydrothermal treatment, the interactions between the surfactant and silicate framework would be transformed as ... 

However, solvent extraction has proven to be more effective. When a sample of 1 g MTS-W was dispersed in 50 mL of ethanol and refluxed for 12 hours. Elemental analysis showed that ca. 50% of the organic templates could be removed without any apparent effect on the stability of the mesostrucutre. We have also found that the mesostructured compounds are stable in concentrated HC1 solution. For example, when MTS-W was stirred in a 12M HC1 solution at room temperature for 12 hours, ca. 30% of the organic templates could be removed without collapsing the mesostructure or decomposing the inorganic walls. 

Powdered, particulate MCM-41 molecular sieves (Si/Al = 37) with varied pore diameters (1.80, 2.18, 2.54 and 3.04 nm) were synthesized following the conventional procedure using sodium silicate, sodium aluminate and C TMAB (n = 12, 14, 16 and 18) as the source materials for Si, A1 and quaternary ammonium surfactants, respectively [13]. Each sample was subjected to calcination in air at 560 °C for 6 h to remove the organic templates. The structure of the synthesized material was confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. Their average pore sizes were deduced from the adsorption curve of the N2 adsorption-desorption isotherm obtained at 77 K by means of the BJH method (Table 1). 

The cracking catalysts were prepared by diluting SAPO-37 samples in a silica matrix. The cracking experiments were carried out in a quartz fixed bed tubular reactor after "in situ" activation. The activation was performed by increasing the temperature up to 823 K at 2K.min ", under N2 flow (240 ml.min"" ). These conditions were maintained for 4 hours, and then N2 was changed to dry air (240 ml.min"" ) for 1 hour. At this point, the temperature was raised (2K.min 1) up to 923 K and maintained overnight. After this treatment all the organic template was removed from SAPO crystals. 

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