Indicadores de trayectoria: mejoras de la última década y persistencia de

diffusivity in contrast to the rigid ion model.

5.12 Discussion

5.12.1 Overview: effect o f oxygen polarization inclusion on diffusion

Here we attempt to draw together some o f the general features obtained from the simulations reported in this chapter. Differences between the rates o f molecular transport within the microporous zeolite channels o f CIT-1 and Faujasite zeolites have been observed for the simulations employing the rigid ion potential and the polarization inclusive shell model.

In both frameworks the consequence o f applying the shell model resulted in self­ diffusion coefficients, which were approximately fifty percent lower than those calculated via the rigid ion model. It must be noted that this decrease in diffusivity was calculated irrespective o f ensemble, loading, temperature or simulation length proving that it was an artefact o f the model. An important relationship exists between

the dynamics o f the walls o f microporous zeolites and diffusing molecules; the results presented in this work provided proof o f that relationship.

5.12.2 Simulation and experimental studies : CIT-1/xylene systems.

Comparison o f the diffusion data generated via molecular dynamics and zero length chromatography (ZLC) are difficult. A direct comparison between the two cannot be made since only the former method utilizes the Einstein relation to evaluate the mean squared displacement. It must also be noted that the sample constituents used during experimental studies, varied from the siliceous systems w hich were simulated. The zeolite CIT-1 is synthesized as a borosilicate and converted to an aluminosilicate after the addition o f aqueous HCL and aqueous aluminium nitrate. This latter form o f Al- CIT-1 has aluminium atoms present in the tetrahedral and octahedral positions o f the framework, giving rise to an Si:Al ratio o f 35. The aluminosilicate version o f CIT-1 was used to carry out the ZLC diffusion studies which yielded diffusivity values o f the order 10'^cm^s'\ The PFG NM R method would be envisaged to generate even higher s e lf diffusion coefficients.

The documented effect o f protonation, cation presence and high Al:Si on m olecule transport into the 10 and 12 MR pores has been to cause a significant decrease in diffusion coefficient. Therefore, because the present values obtained for self-diffusion coefficients are o f the order lO'^cm^s'% a significantly faster transport process has been calculated due to the fact that CIT-1 has been m odelled in a purely siliceous form as is apparent from the data summarized below in table 5.12.

T able 5.12. A comparison o f diffusion coefficients determined theoretically and experimentally for the xylene/CIT-1 system.

rig id ion m odel shell m odel experim ental via F T IR [3 5 0 K ] diffusion coefficient,

D

56.0xl0-^cm V ^ 25.5 xlQ -^cm V 3 . 7 x l O ' W s ‘‘

5,12.3 Simulation an d experimental studies : Faujasite/benzene system s

Turning now to the Faujasite/benzene system, calculated mean squared displacements - see table 5.13, which included polarisation effects were closer to the experimentally determined value o f 4 .4 ± 3 .3 x l0 ’^^m^s'^ obtained by Bull et al}^ who used NM R spin-lattice relaxation experiments on a sample o f purely siliceous Faujasite at 298K. The simulated value obtained from the molecular dynamics study o f Henson et a l was 2.7x10'^m^s'\ This value was larger than our polarization inclusive simulated value o f 0.6x10'^mV^ obtained using the microcanonical ensemble. This is consistent with the greater molecular migration frequency observed during their study. Our simulated value is also plausible when compared to PFG NM R data for Zeolite Y o f IxlO'^^m^s'^ and the earlier simulated values^^ o f 4xlO'^m^s'\ The purely siliceous Faujasite gives rise to the highest diffusivity due to no impediments to the transport process by extra framework cations which benzene is known to form strong attachments to thus impeding its diffusion. Table 5.13 compares the current simulated values with those discussed for previous simulated and experimental work.

T able 5.13. A comparison o f diffusion coefficients determined theoretically and experimentally for the benzene/Faujasite system.

current work Henson e ta l}^ Bull et al. ^

[3 0 0 K ] [2 9 8 K ] [2 9 8 K ]

diffusion 0.6 x lO 'V s * 2.7 x l O ' V s ’ 4.4 x l O ' W s ' coefficient,D

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