高手话不多Si MAS NMR spectrum is shown in Fig.8.The areas under each 29Si MAS NMR peakat )93,)97.8,)98.9and )102.3correspond to the respective Si coordination environments Si(3Al),Si(2Al),Si(1Al)and Si(0Al).These areas were used to determine the SiO 2/Al 2O 3ratio to be 4.36.This was in good agreement with the SiO 2/Al 2O 3?4.75ratio obtained from ICP OES.The technique for performing this calculation is described thor-oughly in the workof Thomas and Klinowsk i [33].
TG analyses were carried out to verify the TMA content calculated using ICP OES analyses.Fig.9shows TG analysis results for batch T3.6.At a
temperature lower than 200°C,weight loss of nanocrystals is ascribed to water desorption.The DTG peakat 308°C is generally attributed to the decomposition or volatilization of TMA cations in the supercage,while the peakat 460°C is attrib-uted to decomposition of the TMA occluded in the zeolite Y sodalite cages [22].Good agreement was achieved between the TMA content results
ob-
Table 3
Zeolite Y nanocrystal chemical content based on ICP OES analyses and TG studies in air Batch T2.4T3.6Al 2O 3,wt%a 20.320.0SiO 2,wt%a 56.456.0Na 2O,wt%a 5.7 5.0TMA 2O,wt%a ;b 17.619.0TMA 2O,wt%a ;c 17.4
19.8
SiO 2/Al 2O 3 4.70 4.75Na/Al 0.46
0.41
Unit cell formula
Na 26TMA 31Al 57-Si 135O 384
Na 23TMA 34Al 57-Si 135O 384
a On an anhydrous basis.
b
All aluminum counterbalanced by either Na tor TMA t.c
Obtained with TG analyses.
24 B.A.Holmberg et al./Microporous and Mesoporous Materials 59(2003)13–28
tained using the ICP OES data with the Al-cation balance assumption and the TMA content data obtained with TG analyses(Table3).The zeolite Y TMA content,shown in Table3,was slightly higher for samples synthesized in the presence of a 50%higher concentration of TMA cations in the reaction mixture.3.5.Surface area and pore volume of zeolite Y nanocrystals
The N2adsorption–desorption isotherm for as-synthesized T3.6is shown in Fig.10.The initial adsorption step at low relative pressure indicates complete?lling of the micropores even
though
there has been no template removal attempted on the sample.The hysteresis of the isotherm in the range of relative pressure of0.45–0.85strongly suggests mesoporosity in the sample.The as-syn-thesized zeolite Y nanocrystals from batch T3.6 exhibited a BET surface area as high as448m2gà1.
A micropore volume of0.12cm3gà1was deter-mined by the t-plot method.The mesopore volume of the sample was estimated to be0.21cm3gà1, and the mesopore diameter was determined to be 39 A using BJH method.This small mesopore size is obviously due to the close packing of the small zeolite Y nanocrystals that occurs during the drying process[4–8].
4.Conclusions
We were able to successfully minimize the size of zeolite Y nanocrystals while simultaneously in-creasing their yield by using TMABr as an addi-tional source of organic template.Addition of TMABr as a second template source was shown to increase nucleation while promoting crystal size stabilization at nanocrystal sizes smaller than40 nm.Varying the TMABr/TMAOH ratio in the synthesis mixture gives excellent control over the product?nal crystal size.XRD and TEM show that the zeolite Y nanocrystals obtained are highly crystalline.By combining ICP OES,29Si MAS NMR and TG analysis,structural formulae were found to be a little di?erent for samples synthe-sized upon adding di?erent amounts of TMABr. The as-synthesized zeolite Y nanocrystals show high N2adsorption;and BET surface area and micropore volume are determined to be448m2gà1 and0.12cm3gà1respectively. Acknowledgements
We would like to thank California Energy Commission,Riverside Public Utilities,and UC-SMART for the funding of this project.We would also like to thank Dr.Leonard Mueller and Douglas Elliott for the NMR workthey per-formed.The authors would also like to thank Dr. Anupam Mitra,Dr.Zhengbao Wang and Dr. Limin Huang for their guidance and support. References
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