Groups into Solid Acids
5.4 Discussion
In the standard synthesis. a white. amorphous gel is formed several hours after mixing the silane solution with the aqueous TEAF solution and is then transferred to an autoclave for heating. The fact that a solid phase is obtained as the molecular sieve precursor indicated that it may he possible to synthesize these materials from other amorphous silica sources where the TEAF can make intimate contact with the internal surface area of the silica. Indeed, we show here that a synthetic strategy employing extracted PE-functionalized MCM-41 is also a potential route to OFMS materials. A more general discussion of this approach to preparing molecular sieves is described elsewhere 151.
The properties of the OFMS's can be tailored by choice of synthetic (crystal size) and extraction method (hydrophobicity, pore volume). As illustrated above, materials with a hydrophobicity similar to calcined samples can be synthesized by extraction with aqueous acetic acid at high temperatures (120- 135°C). The high hydrophobicity of the calcined samples is due to the defect-free nature of the material (all Q4 silicon). The
@
silicon centers present in the as-synthesized material are annealed upon calcination. The high hydrophobicity of the materials extracted at high temperatures is likely due to the healing of defect silanols during extraction with aqueous acetic acid. This hypothesis is supported by the "Si MAS NMR evidence that shows essentially no (2i silicon in materials extracted by this method. However, there is the drawback that there is a significant loss in crystallinity when using this approach.By lowering the temperature in the extraction step to 80°C. a material can be produced with good porosity and intermediate hydrophobicity relative to (i) the calcined materials and (ii) the hydrophilie pyridine/lN HCI extracted material.
Results from the SDA extraction techniques have implications for molecular sieves other than OFhfS's. In the case of a pure-silica, beta zeolite synthesized using TEAF as a SDA and u~thout any organos~lane, we shou here that an extremelj
hydrophobic material can be made \rithout combustion of the SDA. Recovery of the TEAF froin the relatively simple three component mixture (acetic acid, water. TEAFi and reuse of the TEAF could significantly reduce the cost of producing hydrophobic large pore molecular sieves for adsorptive and other applications.
XRD patterns are similar for all materials. regardless of extraction method. In contrast, the nitrogen and cyclohexane adsorption data support some loss in
crystallinity (up to 113) associated with extraction with acetic acid at high temperatures.
Thus, as expected, the adsorption methods are more sensitive than XRD when assessing small changes in crystallinity.
Similarities between bulk elemental analysis and surface analysis (XPS) support a uniform distribution of organic functional groups throughout the materials. Thus, there appears to be no zoning of the phenethyl groups. This evidence, coupled with the adsorption results showing a loss of micropore volume attributable to the PE groups is indicative of an even distribution of the organic groups within the micropores at low PE loadings.
As the amount of the PE in the synthesis is increased, there is evidence for the formation of an additional, amorphous phase. In this case (loadings of PE -> 0.075).
the difference in porosity between the extracted and calcined samples can not be attributed to PE solely in the micropores of "BEA. For example, for BetaPE-0.2, the calcined sample has a nitrogen capacity of 0.228 cclg and the extracted sample (method 3, 80°C) has a capacity of 0.148 cclg. Based on the PE content as determined by TGA.
the PE should occupy 0.14 cclg. This volume is not consistent with PE only in the micropores. and hence a significant amount of the PE is likely contained in the amorphous phase.
Clearly, OFhlS's have significant potential for use as shape-selective catalysts, particularly in low temperature liquid phase conversions [ I ] . Sulfonation of the as- synthesized material results in the generation of no sulfonic acids, indicating that
essentially all PE groups are located within the SDA-occluded pores of the as-
synthesized material. Nitrogen adsorption and bulWsurface elemental analysis data also support this view. However. catalysis with extracted. sulfonated materials can give results that are less shape-selective than one would expect from an ideal crystalline material. i.e.. species that would not be expected to enter the OFMS micropores can be converted over these materials. This may be due to some loss in crystallinity or
generation of mesoporosity upon extraction, although no visible difference exists betwen the as-synthesized, extracted, and sulfonated materials by SEM. Catalysts with nearly perfect shape-selective behavior can be generated by surface passivation of the a$-made materials with highly caustic solutions [I], although this harsh treatment is not reliable. While ideal shape-selectivity can be obtained (we have shown this in reference I), the reproducibility of the techniques used to generate this shape-selectivity require further evaluation. Methods for improving reproducibility in the shape-
selectivity of OFMS catalytic materials will be reported at a later date [14].
5.5 Summary
OFMS's with the *BEA structure can be synthesized over a large temperature range (140- 170°C) with numerous organic functional groups, in the presence or absence of non-silicon (Al, B) atoms. The OFMS's can also be prepared using extracted, PE-functionalized MCM-41 as the silicdPE source via a solid state transformation. The extraction techniques used to remove the TEAF from the
1 . .
micropores have a strong influence on the concentration of defect sites ( Q sllrcon) and hence on the hydrophobicity of the structure. Additionally. the porosity of the materials is controlled to a degree by the extraction technique. Bulk elemental and surface
analysis coupled with nitrogen and cyclohexane adsorption results give evidence that the organic functional groups are distributed evenly throughout the micropores of the
material at low PE loadings. The organic PE group is intltct after extraction of the SDA and this group can be e a i l y transformed into a solid acid,