Application of zeugmatography to study kinetics of physical adsorption

APPLICATION OF ZEUGMATOGRAPHY TO STUDY KINETICS OF PHYSICAL ADSORPTION W HEINK, J KARGER and H PFEIFER

NMR-Labor, Sektion Physlk der Karl-h&&x-Umversltat. DDR-701 L.e~pzlg,German_Democratlc Repubbc (Recerved

20 December

1976, accepted

m reused form

1 August 1977)

Ahatraet-Spatml resolution of nuclear magneuc resonance smals may be achreved by use of magnetic field gradients Ttus new tecbmque, called “zeugmatography”, has been apphed m the present work to study dynamic processes, especmlly the kmetics of physIcal adsorptlon m muzroporous systems The method provuies mformatlon sundar to the results of Dubmm’s X-ray techmque, but 1s not hmued to X-ray contrast adsorbates After a general dlscussmn, examples of apphcation of “dynarmc zeugmatography” are gven Mass transfer forsorptlon of butane m NaCaA zeobtes of ddferent shape and of water m NaX zeohtes could be obs&ved drrectly

1 INTRODUCTION

The problem of mass transfer m porous media

IS of heterogeneous

considerable merest m adsorption, catalysis and m other branches of chemical physics and

chermcal engmeenng Therefore, attention has been pad to develop the theory of kmettcs of physIcal adsorptlon[lAl, and novel experimental techmques l&e resonance (NMR)[S, 61 neutron nuclear magnetic scattermgL71, IR spectroscopy[8], and chromatographlc methods[91 have been mtroduced m ad&tlon to the “classical” gravunetrtc measurements of sorpuon Using the NMR pulsed field grtient techmqueU, 61 it was possible to show that the true molecular self-dtiuslon coefficients withm zeolite crystallites (mtracrystallme d&uslon) are some orders of magmtude greater than those denved from gravlmetrrc measurements [lo] The latter ones are only apparent dlffuslon coefficients which do not descnbe the real mtracrystallme mass trausfer[ll] Of basic Interest for the kmehcs of adsorption m sohds with a bidlsperse pore structure hke a pelleted zeohte 1s the ratio of the two dlshnct dfiuslonal resistances the macropore resistance of the pellet (mtercrystallme drffusron) and the nucropore resistance of the single zeohte crystalhtes (surface bamer and mtracrystalhue tiuslon) Neglecting surface barners and assummg both the crystalhtes and the pellet as spheres with radius r and R respectively, a characterlstlc quantrty [ 1,2]

are dlstrlbuted over the whole mtercrystallme space of the pellet unmedlately after it has been brought mto contact with the adsorbate These lumtmg cases are called macro-controlled (B * 1) and micro-controlled (Be 1) sorption The spatial and ume dependence of adsorbate concentration for these cases are schematlcally plotted m Fe 1 If there is only a limited amount of

f

u

1 ‘2

t

G

-X

(a)

I

(1) has been defined, where i&. and 4, are the dfluslon coefficients for mtra- and mtercrystallme mass transfer respectively, and pUltordenotes the relative amount of adsorbate m the mtercrystalhne space For j3 * 1 the sorption process IS controlled by the mtercrystallme drffuslon, and the sorbate concentration depends on the spati coordmate For example m the case of rectangular Isotherms, a ste-pwise increase of adsorbate concentration proceeds through the pellet For B 4 1 the uptake of adsorbate by the s&e crystalbtes determmes the total sorption rate and molecules

6

(b)

i,

--

Fig 1 TIIBGdependence (r, > 13> r2> t,) of dmtnbutmn of adsorbetainasalidw&ab&sperse~ muctureUkcapiikted zcohte ta)Thcsurptmpmcee8~cmmeHedbythcM&rcrystalhe Musion macro-cootmlkqf fiorphon (B > 1) (b) The sorpboo process IS co~~trolkdby the uptake rate of the smgie crystalWe mwro-controllcd sorptson (B 4 1) k denotes the total length of the pellet

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1021

Apphcabon of zeugmatography to study kmeucs of physical adsorption

proton-contauug sample with a length of Ax = 2 mm has d), and where been moved along the x duection (a another sample was used which covers the whole range (Ax = 7 5 mm) of constant field gradient On the left hand side the proton datibutions given by the geometrical arrangement and on the r&t hand side the correspondmg zeugmatograms are plotted The computer IS able to accumulate free mductlon decays m 39 dtierent channels The maxlmum number of signals per channel IS 1000, whtie the real value depends on the tune constant of the kmetlc process to be studied After the measurement of 39 dtierent signals each IS Fomer transformed by the computer and the zeugmatograms are plotted automatically on an XY recorder

1

-10

0

-5

5

IO

x. mm (al

4. APPldCATlONS

I

-IO

I

I

I

-5

I

0

1

5

I

IO

x, mm (b)

Fig 4 Spati dependence of resonance sMt (a) and of relative

signal mtenslty (b) of proton magnehc resonance along the xduectron (see Fa 3)

subsequent measurements, the maxunum frequency which can be separated from the free mduction IS 12 5 kHz (samphng theorem, see[20]) Due to unwanted mhomogenetnes of the magnettc fields produced by the electromagnet and by the field gradient cods, the free mductlon decreases to the noise level in about 6 msec Therefore, the lowest frequency to be observed IS about 170 Hz As already mentioned, the spatial resolution depends on the swal-to-noise rat10 of the NMR signal Under the assumpuon that a meduun beat frequency of 4 kHz can be measured with an accuracy of 5%, it follows from the above frequency gradlent that the spatial resolution IS OSfllItI Figure 5 presents results of a test expenment where a

(a)

The dynamic zeugmatography has been used to study the kmetlcs of adsorptlon of butane and of water by vanous zeohte specimens Zeolite without binder was filled mto a glass tube of 7 mm dla its axis bemg onented along the duection of the magnetic field gradlent (xcoordmate) The amount of zeolite corresponds to a length of Ax = 7 mm of the sample and it was placed m the region of constant magnetic field gradient (see Fig 4) Fust of all the zeohte was degassed at 400°C m high vacuum (- lo-* Torr) for 3 hr No proton resonance sunal could be observed after this procedure The butane was supphed to the activated zeohte from a volume of 500 ml contauung nonsaturated gas at 23°C under a pressure of 4OTorr Therefore only a hmlted amount of adsorbate was avadable for the mass transfer into the zeohte In contrast to ths, for the water adsorption expernnents the reservoir was a volume of 5 ml m eqmllbrmm with tridesttied liquid water All sorption experiments were performed at 23°C No special attention was paid to the fact that the temperature of the zeohte may change durmg the adsorption process, since such effects are beyond the scope of this work In F@ 6 the dlstmbutlon of butane m a sample consIstmg of NaCaA zeohte (70% Na’ exchanged by Ca”) crystalhtes with a mean radms r = 20 ~1m IS dlustrated at successive tune intervals of the adsorption process

L

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Fu

-2

0 2 x. mm

4

I

2

I

3 4 Af, ktir

s

5 Proton density III arbtrary units of test samples Oeft hand stde) and experunentaltydetemuned zeug-

matograms(r&t hand side)

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w

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HEINK

et al

Ar, kbiz I

-4

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A/,

4 I

Rg 6 Ehstibufion .of proton concentration (zeugmatograms) for sorption of butane In a sample contamrng NaCaA crystalhtes

t

-4

-2

0 x,

with a brg mean radms (r = 20 Nrn) at tune t, = 3 5 sec. t2 = 10 5 set, tp = 35 set and after reacbmg the equthbnum (1,)

1

1

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A< 4

,.

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0 +

-4

4

2

mm

I 4

1

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Af,

kHz

-2

1 2

(a)

1 I

kHz ,

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kHz 1

0

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4

xa mm

nml (a)

Fw 8 hsmbutum of proton concentration (zeugrnatograms) for sorption of water m a sample contammg NaX crystalhtes w& a small mean radms (I = 2 pm) at tune tl = 9 see, tz = 40 set, tl = 80 sec. r4 = 130 set (FIN &) and-after stoppmg the water suppIy-at tune f4 = 130 set, 8 = 130 mm, t6 = 160 mm, t7 = 20s mm (Fyg 8b)

I

1

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Af. -4

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kHz3

-2

0 x,

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mm (b)

Fe 7 Ihatibutton of proton concentration (zeugmatograms) for sorption of butane m a sample contammg NaCaA crystahtes wth a small mean radms (r=2pm) at tune 2,=45sec, 22= 9sec. t3 = 5osec. t4 = 3 mm (I%g 7a). and at bme t.= 3 mm, IS= 20 mm, ts = 50 mm, r7 = 85 nun (Fs 7b)

Evidently the behavlour IS sundar to that m Fig l(b) and leads to conclusion that the uptake of adsorbate by the smgle crystals determmes the total sorptron rate In agreement with tlus result the tune constant for the adsorption of butane m a monolayer of crystalhtes of the same zeolrte and under equivalent condztlons amounts to some seconds[ll,211 Accordmg to eqn (1) the mtluence of mtercrystalhne ddfuslonal resistance should mcrease with decreasmg radms r of the crystalhtes This 1s demonstrated m Fe 7(a) where zeugmatograms are plotted for the same systems as m Fu 6 but ~th NaCaA zeohte (70% Na’ exchanged by Ca’+) crystal&es havmg a me&urn radms of only 2 pm Smce m thks case the resastance of mtercrystaIlme ddFuslon has Increased conslderably m comparison to the micropore resistance, sorption unmediately leads to a saturation of the crystiillttes in the

Apphcation of zeugmatography to study kmetics of physical adsorption

first layer In such a case gravunetrrc measurements allow no reasoning vvlth regard to mtracrystalhne mass transport The sample has lost Its btporous character The rate of sorption IS less than in the above system where crystal&s wtth a bmer radms have been used This follows from the fact that during the sorption process the gas pressure between the crystalhtes decreases, so that the gas pressure between the crystalhtes decreases, so that the relative amount p,.*=,. of molecules decreases too Since mtercrystalhne mass transport IS strongly determined by p,,, (see eqn 1). a homogeneous dtstrrbutlon of the butane molecules over all crystahtes of the sample 1s reached only after one to two hours (Ftg 7b) Thus IS the situation shown m FQ 2 As can be seen from Fe 8. for the process of water adsorption m NaX zeohtes of small size (f = 2 pm) we have a stmtlar situation as m Rg 7 Due to the smaller value of plntar (in comparison to the nonspecfically adsorbed butane) however, It takes a much longer time to obtam a homogeneous dlsmbution of the water mo.lecules across the zeohte crystaihtes (Fig 7b and Fig 8b) It should be mentioned that the water reservou has been separated from the zeoltte after t4 = 130 set so that the zeugmatograms shown m Fig 8(b) correspond to the steps shown m Fig 2 Acknowledgements-We are Indebted to h B Staudte and mpl-Ing B Knorr for their help m couplmg the pulse spectrometer and the computer, Dr J Care for preparing tbe sorption expenments, and Dr W Schmltz who synthesized the NaCaA zeohte crystalhtes of large diameter

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REFERENCES

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