Diffusion in Restricted Geometries. The effect of size and configuration effects on diffusion in polymers and zeolites

Transcription

1 Diffusion in Restricted Geometries The effect of size and configuration effects on diffusion in polymers and zeolites The Middleman Symposium San Diego, June

2 Preface 1960 The John Hopkin Univ University of Rochester Douglas/Laurence 1968 Johns Hopkins revisited Vasudevan 1969 Univ. of Massachusetts Stan finally arrives and eventually a new family...bigg, Calabrese, Pearson, Tirrell, Malone, et alii.

3 The Middleman Family in Massachusetts Ph. D students Robert Cleary, 1971 Donald M. Bigg, 1973 A. Richard Taylor, 1973 Richard Calabrese, 1976 Glen H. Pearson, 1976 Jehuda Greener 1976 Prasad Chintapalli, 1977 John P. Bollmeier, 1977 Matthew Tirrell, 1978 Michael F. Malone, 1979 M.S. Students Gary C. Phillips, 1971 Harold Hopfe, 1972 Thomas Riihimaki, 1972 Gregory Cigal, 1973 Edward W. Pitera, 1973 Anthony Copas, 1973 George Y. Bang, 1975 M.S. with Laurence Therese Bauman, 1980 Ph. D.with Laurence Pradeep Shirodkar, 1981 Tong Chee Hsu, 1986

4 Diffusion in Restricted Geometries The work reported here was done principally by Dominique Arnould (polymers) and Fernão Magalhäes (zeolites) The object of the work is to examine size effects in systems with fixed and mobile diffusion paths.

5 Diffusion of Small Molecules in Polymers Polymers polyvinyl acetate polystyrene polysiloxane polycarbonate polyolefins Tools IGC Sorption Solutes hydrocarbons esters aromatics

6 Diffusion of Cyclics in PVAc 10-7 T ( C) PVAc / Benzene PVAc / Toluene PVAc / Ethylbenzene PVAc / Cyclohexane 10-8 D (cm /s) /(K +T-T ) ( K ) 22 g2-1

7 Inverse Gas (IGC) Chromatograph

8 Diffusion of C-8 Aromatics in PVAc T ( C) p-xylene m-xylene 10-8 o-xylene D (cm /s) /(K +T-T ) ( K ) 22 g2-1

9 Diffusivity of Alkanes in PVAC T ( C) PVAc / Hexane PVAc / Heptane PVAc / Octane PVAc / Nonane D (cm /s) /(K +T-T ) ( K ) 22 g2-1

10 Diffusion of C-6 Alkenes in PVAc T ( C) D (cm /s) ,3,5 Hexatriene c c c c c c 1,4 Hexadiene c c c c c c trans 2 Hexene c c c c c c trans 3 Hexene c c c c c c 1 Hexene c c c c c c /(K +T-T ) ( K ) 22 g2-1

11 Diffusivity of C-6 Alkanes in PVAc 10-7 T ( C) D (cm /s) 10-9 Hexane c c c c c c Methylpentane c c c c c c 2,3 Dimethylbutane c c c c c c c 2,2 Dimethylbutane c c c c c /(K +T-T ) ( K ) 22 g

12 Some Questions Size of the diffusing molecule is one measure useful to describe diffusive behavior, but it is is insufficient? Measures we have examined include size, compactness, and flexibility. Are there others? Is the projected area of the diffusing molecule is a useful notion?

13 Diffusion of Cyclohexane and Alkyl-cyclohexanes in Silicalite To Understand the diffusion of large molecules adsorbed in zeolites. Re-examine results previously reported for diffusion of alkyl-cyclohexanes in silicalite: t-dmch /s) 2 D (cm CH MCH ECH /T x 10 3 (1/K) H. Chon, D. Park, J. Catal. 114 (1988)

14 Adsorbates studied CH MCH ECH t-1,4-dmch c-1,4-dmch

15 Experimental techniques for measurement of diffusion coefficients STA (simultaneous thermal analyzer): Allows for simultaneous collection of gravimetric and calorimetric data. Diffusion coefficients and heats of adsorption can be estimated from a single sample, with the same experimental setup. ZLC (zero length chromatography): Small amount of sample and good contact with carrier gas minimize extraneous mass and heat transfer resistances. Fast diffusing adsorbates can be studied with higher confidence.

16 STA results MCH T = 150 C (m-m ) -m )/(m Heat Flow (cal/s g) time (s)

17 Gravimetric data diffusion model T = 200 C ) -m )/(m (m-m MCH CH ECH time (s)

18 Diffusivity vs. occupancy CH T = 250 C T = 200 C T = 150 C T = 110 C 2 /s) D (cm q (molec./uc) MCH T = 250 C T = 200 C T = 150 C T = 110 C 2 /s) D (cm q (molec./uc)

19 Heats of sorption vs. occupancy ECH MCH H (kcal/mol) CH T = 250 C T = 200 C T = 150 C 5.00 T = 110 C q (molec./uc)

20 Arrhenius plots (gravimetric data) /s) D (cm CH (this work) MCH (this work) ECH (this work) CH (Chon & Park) MCH (Chon & Park) ECH (Chon & Park) t-1,4-dmch (Chon & Park) CH (Cavlc. & Ruthven) /T x 10 3 (1/K)

21 ZLC results 10 0 CH T = 150 C F = 40 cm 3 /min T = 200 C F = 60 cm 3 /min C/C time (s) 10 0 t-dmch T = 90 C F = 100 cm 3 /min 10-1 T = 120 C F = 130 cm 3 /min 0 C/C time (s)

22 Arrhenius plots (ZLC data) /s) D (cm CH (ZLC) t-1,4-dmch (ZLC) CH (Gravimetry) c-1,4-dmch (Gravimetry) /T x 10 3 (1/K)

23 Observed diffusivity trend: t-1,4-dmch >> MCH > CH > ECH, c-1,4-dmch equilibrium transition straight channels zig-zag channels

24 (a) CH MCH H (kcal/mol) ECH t-dmch c-dmch 14 (b) MCH Ea (kcal/mol) ECH c-dmch 10 CH t-dmch 10-3 (c) /s) A (cm MCH ECH t-dmch c-dmch CH 10-5

25 D = λ 2 kt h S* exp R exp E a RT D = λ x 2 2 kt h 1 q * q x q exp V 0x ht q = q y * q * z q * * rot q int q y q z q rot q int q * transition Potential energy E a CH MCH ECH t-dmch c-dmch equilibrium equilibrium Entropy S * transition CH a l k y l - C H

26 Conclusions In Polymer and Zeolite systems, there is an effect ofsize and configuration on diffusion processes. The effects can be more dramatic in the fixed geometry of zeolites, but similar conclusions may be drawn about a polymeric system, i.e., alkenes Qualitative Transition State Theory arguments (based on the measured activation energies, preexponential factors and heats of adsorption) provide a consistent interpretation of the experimental results. Entropic effects apparent in zeolites may be important in the analysis of polymesr. The newer tool (STA ) useful in the study of mass transport in zeolite crystals, could have application in detailing the energetics of small molecule diffusion in polymers.

27 Postface 1979 U.Cal.- San Diego The sun beckons! Shirodkar, Hsu, Bauman 1980 Shirodkar, Bauman finish 1985 Tong Hsu returns 1997 Thanksgiving and Celebration