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Calorimetric study of the absorption of n-butane and but-l-ene on a highly dealuminated Y-type zeolite and on silicalite
Short papers
Calorimetric study of the absorption of n-butane and but-l-ene on a highly d e a l u m i n a t e d Y-type zeolite and on silicalite H. Thamm, H. Stach and W. Fiebig
Acaderny of Sciences of the GDR, Central Institu te of Physical Chemistry, GDR-1199 Berlin, Rudower Chaussee 5, GDR (Received 2 August 1982) Differential heats of adsorption have been determined calorimetrically for n-butane and but-l-ene on molecular sieves US-Ex and silicalite. It is shown that on US-Ex, but-l-ene isomerizes whereas on silicalite it is physically adsorbed. The heat of adsorption for but-l-ene on silicalite was found to be less than the heat of adsorption for n-butane. Keywords: Calorimetry; heat of adsorption; n-butane; but-l-ene; dealuminated zeolite; silicalite
INTRODUCTION In an earlier publication I we investigated the adsorption equilibrium of benzene, cyclohexane and n-hexane on zeolite Na-Y and on US-Ex (almost fully dealuminated Na-Y zeolite 2) by means of adsorption calorimetry. It has been shown that the 'SiO2-molecular sieve' US-Ex behaves like a nonspecific microporous adsorbent with a nearly energetically homogeneous inner surface with respect to the C6-hydrocarbons studied. A more adequate test for polarity, energetic homogeneity and also catalytic activity of adsorbents is the comparison of the heats of adsorption for nparaffins and n-olefines of the same chain length. It was therefore of interest to investigate calorimetrically the behaviour of US-Ex towards the adsorption of n-butane and but-l-cne and to extend this examination to the recently synthesized SiO2-molecular sieve silicalite 3. EXPERIMENTAL The differential molar heats of adsorption were measured by means of a Calvet-type microcalorimeter (Setaram) at 301 K. The adsorption equilibrium was controlled by the thermokinetic curve of the calorimeter as well as by recording the equilibrium pressure (Baratron pressure meter). With the exception of but-1-ene/US-Ex the adsorption equilibrium was reached within two hours for all systems examined. Synthesis of silicalite was accomplished on the basis of data published by Flanigen et al. 3. The first silicalite batch synthesized exhibited a reduced adsorption capacity 4 (compared with Ref. 3) which could be accounted for by the presence of a minor part of amorphous SiOv For the silicalite sample used in the present paper, the adsorption capacity towards n-butane is 0144-2449/83/030095-03503.00 © Bunerworth & Co. (Publishers) Ltd.
in agreement with the value given in Ref. 3 as well as X-ray analysis data of silicalite s. The preparation of US-Ex is described in Ref. 2. In Table 1 the unit cell compositions and the pore volumes of the molecular sieves investigated are given. Before application all adsorbents were activated in high vacuum ( < 1 0 - 3 p a ) for 24 h. The purity of the adsorbates was better than 99 mol% (Fluka AG). RESULTS AND DISCUSSION
Figure 1 shows the differentiM molar heats of adsorption for n-butane and but-l-ene on molecular sieves Na-Y and US-Ex. The comparison of the curves for the paraffin and the olefine on Na-Y exhibits the expected behaviour. The heat of adsorption of but-l-ene is substantially higher than the heat for the n-butane/Na-Y system in the entire range of coverage. At zero coverages (a ~ 0) the difference between the Q-values amounts to Q0(butene)--Q0(butane) = 56.5-41.4 =15.1 kJ mo1-1 and may be explained by the specific interaction or the rr-bonds in but-l-ene with the cations of the zeolite. By comparing the heats of adsorption for n-butane on molecular sieves Na-Y and US-Ex it can be seen that dealumination leads to a distinct decrease in the energy of the adsorption interaction. The corresponding heat difference for nbutane is equal to Q 0 ( N a - Y ) - - Q 0 ( U S - E x ) = 41.433.1 = 8.3 kJ mo1-1. This value which may be Table 1 Characteristics of the investigated adsorbents Adsorbent
Unit cell composition
Pore volume (cm3g-t)
Na-Y US-Ex Silicalite
Na54(AIO2) 5~(SiO2),3s Nao.~(AIO2) t-s(Si02 ) ~9~-~s7 (Si0=)96
0.29 0.56 0.19
ZEOLITES 1983, Vol 3, April 95
11C ~ 10C 9C
26 ~
24 ~x
22 20
80
":
"\
E 7C .~
16
6C
14
x
10 4C
" 8
3C
I
05
I
10
I
I
J
15 20 25 e (mmol g-~)
I
30
Figure 1 Differential molar heats of adsorption on molecular sieves Na-Y and US-Ex at 28°C. (v) n-butane/Na-Y, (A) but-l-ene/ Na-Y, (+) n-butane/US-Ex, (X) but-l-ene/US-Ex, filled symbols desorption
interpreted as the influence of the electrostatic field in Na-Y on the heat of adsorption 6 amounts to about 20% of the heat of adsorption on Na-Y. The adsorption of but-l-ene on US-Ex reveals a quite different picture. Whilst the nonpolar nbutane as well as n-hexane, cyclohexane and even the aromatic benzene x are physically adsorbed and the highly dealuminated US-Ex can be regarded as a h o m o g e n e o u s microporous adsorbent with respect to adsorption of these molecules, but-l-ene is irreversibly adsorbed. The heat effect of but-1erie adsorption exceeds the heats of adsorption of all the other systems investigated, and in contrast to them, falls with coverage. Adsorption equilibrium has not been reached. A decrease in the vapour pressure has been noticed even after a week when the heat flow was less than the sensitivity of the calorimeter. This behaviour gives evidence of a catalytic reaction. The gas chromatographic analysis of the desorbed p r o d u c t (Table 2) shows the dominating formation of isomerization species (trans-but-2-ene and cis-but-2-ene) and to a minor part polymerization and disproportionation products respectively. (Desorption was accomplished at r o o m temperature by means of a liquid nitrogen cold trap.) It is interesting to note that the conversion of but-l-ene already is nearly complete at r o o m temperature although the n u m b e r and strength of acid centres in US-Ex is relatively small compared with zeolitic catalysts such as rare earth exchanged Na-Y or N a H - Y zeolites 7 and the OHgroups are mainly of the terminal type 8.
96
ZEOLITES 1983, Vol 3, April
In Figure 2 the dependences of the heats of adsorption on coverage are given for n-butane and but-1ene on silicalite. In contrast to US-Ex and N a - Y the heat of adsorption of but-l-ene is less than the heat of adsorption for n-butane (lowest coverages a < 0.1 m m o l g-X excluded) and desorption measurements coincide with adsorption. The difference between the Qwvalues amounts to Q 0 ( b u t e n e ) - Q0(butane) = 49.0-52.3 = - - 3.3 kJ mol -a. The nature and location of the adsorption sites (probably OH-groups 3) which are responsible for the heat rise with falling adsorption at a < 0.3 m m o l g-1 cannot be finally explained at present. If we neglect this coverage range silicalite may be regarded as a nonpolar, energetically h o m o g e n e o u s adsorbent with respect to physical adsorption of but-l-ene. The surplus of two Hatoms in the structure of the butane molecule compared with b u t e n e is clearly reflected in the measured heats of adsorption whereas the higher electron density of the double b o n d in butene (electron configuration sp 2) in contrast to butane (sp 3) has no significant influence on the interaction energy, i.e. the adsorbate-adsorbent dispersion interaction is the dominating factor for the but-l-ene/silicalite system as well as for n2 Gas chromatographic composition (%) of the desorbed product from but-l-ene/US-Ex system. (Column: n-octane/Porasil C, 175X0.15 cm, 22°C) Table
propane i-butane n-butane but-1 -erie trans but-2-ene cis but-2-ene >C4
D
k
1.0 4.6 3.5 2.9 71.8 13.2 3.0
)
~
/
~
;14 ~"
10
40
8 3O I
0
0.5
I
I
10 15 o (rnmol g "~) Figure 2 Differential molar heats of adsorption on silicalite at 28°C. (o) n-butane/silicalite, (o) but-l-ene/silicalite, filled symbols -desorption
butane/silicalite. Similar high degrees of the energetic homogeneity of adsorbents have up to now only been shown on flat surfaces, e.g. graphitized thermal carbon black 9. The comparison of the heats of adsorption for n-butane on the molecular sieves investigated demonstrates the influence of pore size on the heat of adsorption. The heats of adsorption diminish in the following order: Q0(silicalite) > Q0(Na-Y) > Q0(US-Ex). This result is understandable if one keeps in mind the comparable sizes of the effective n-butane molecule diameter (~ 0.49 nm) and the pore diameters in silicalite ( ~ 0 . 5 1 - 0 . 5 7 nm) which favour an optimum molecule-adsorbent interaction and on the other hand hinders molecule interaction as can be seen from the relatively small rise of the heat curve with coverage on silicalite compared with Na-Y and US-Ex. The small difference between the Q-value at the maximum of the heat curve and Q0 on silicalite (7.2 kJ mo1-1) compared with Na-Y (16.8 kJ mol -l) and US-Ex (11.7 kJ mol-:) supports the assumption of an 'end-to-end' con-
figuration of n-butane molecules in silicalite channels as proposed for the adsorption of nalkanes in H-ZSM-51°. REFERENCES 1
Schirmer, W., Thamm, H., Stach, H. and Lohse, O. 'The properties and applications of zeolites', (Ed. R. P. Townsend) Special Publication No. 33, The Chemical Society, London 1980, p. 204 2 Lohse, U., Alsdorf, E. and Stach, H. Z. Anorg. AIIg. Chem. 1978, 447, 64 3 Flanigen, E. M., Bennett, J. M. Grose, R. W., Cohen, J. P., Patton, R. L., Kirchner, R. M. and Smith, J. V. Nature 1978, 271,512 4 Thamm, H., Stach, H., Schirmer, W. and Fahlke, B. Z. Phys. Chemie (Leipzig) 1982, 263, 461 5 Fichtner, H. Krista/I und Technik in preparation 6 Schirmer, W., Stach, H., Thamm, H., Dubinin, M. M., Issirikjan, A. A., Regent, N. I. and Anaktschjan, E. Ch. Z. Phys. Chemie (Leipzig) 1980, 261, 1129 7 Bremer, H., Lohse, U., Reschetilowski, W. and Wendlandt, K.-P. Z. Anorg. AI/g. Chem. 1981,482, 235 8 Bos~ek, V., Patzelov$, V., Tvaru}kova, Z., Freude, D., Lohse, U., Schirmer, W., Stach, H. and Thamm, H. J. Cata/. 1980, 61, 435 9 Kiselev, A . V . Zh. Fiz. /(him. 1967,41,2470 10 Jacobs, P. A., Beyer, H. K. and Valyon, J. Zeo/ites 1981, 1, 161
Examination of the surface structure of zeolites by fast atom bombardment mass spectrometry (FABMS). Loewensteins rule j . Dwyer*, I. S. Elliott, F. R. Fitch, J. A. van den Berg and J. C. Vickerman
Chemistry Department, UMIST, Manchester, P.O. Box 88, UK (Received 13 August 1982) Fast atom bombardment mass spectrometry (FABMS) is used, in the negative ion mode, to investigate the presence of paired aluminium species in zeolites and in aluminas. Results do not support the systematic breaking of Loewensteins rule in zeolite A. Keywords: Zeolites, surface composition; Loewenstein; fast-atom bombardment
INTRODUCTION Fast atom b o m b a r d m e n t mass spectrometry is a recently developed technique in which species sputtered from surfaces by a beam of argon atoms are analysed mass spectrometrically. This technique, which is a development of secondary ion mass spectrometry (SIMS) was recently used to study the surface composition of zeolites I. Currently there is controversy concerning the arrangement of silicon and aluminium atoms in zeolites. In particular, on the basis of 295i n.m.r. results it has been suggested that Loewensteins rule, which forbids pairing of aluminiums, is systematically brokcn in zeolite A, in some forms of sodalite mad in other zeolites 2. Decomposition * To whom correspondence should be sent
0144-2449/83/030097-03503.00 © Butterworth & Co. (Publishers) Ltd.
of zeolites by silanation suggests that Loewensteins rule is also broken in faujasites 3 but this is not supported by 29Si n.m.r, a or by detailed X-ray studies s. If the structure of zeolite A is such that all the aluminiums are paired via oxygen bridges, whereas in zeolites X and Y Loewensteins rule holds completely*, then it is to be expected that surface species sputtered from zeolite A would include more paired aluminiums than X or Y. Since zeolites A and X can be synthesized with Si/A1 close to unity, we should also expect more paired silicons to be sputtered from A than from X. Similarly we would expect more Si-O-A1 pairs in X and in A. With this in mind samples of zeolites and of reference materials were examined by FABMS. Zeolites A and Y were kindly provided by Laporte
ZEOLITES 1983, Vol 3, April 97
Calorimetric study of the absorption of n-butane and but-l-ene on a highly d e a l u m i n a t e d Y-type zeolite and on silicalite H. Thamm, H. Stach and W. Fiebig
Acaderny of Sciences of the GDR, Central Institu te of Physical Chemistry, GDR-1199 Berlin, Rudower Chaussee 5, GDR (Received 2 August 1982) Differential heats of adsorption have been determined calorimetrically for n-butane and but-l-ene on molecular sieves US-Ex and silicalite. It is shown that on US-Ex, but-l-ene isomerizes whereas on silicalite it is physically adsorbed. The heat of adsorption for but-l-ene on silicalite was found to be less than the heat of adsorption for n-butane. Keywords: Calorimetry; heat of adsorption; n-butane; but-l-ene; dealuminated zeolite; silicalite
INTRODUCTION In an earlier publication I we investigated the adsorption equilibrium of benzene, cyclohexane and n-hexane on zeolite Na-Y and on US-Ex (almost fully dealuminated Na-Y zeolite 2) by means of adsorption calorimetry. It has been shown that the 'SiO2-molecular sieve' US-Ex behaves like a nonspecific microporous adsorbent with a nearly energetically homogeneous inner surface with respect to the C6-hydrocarbons studied. A more adequate test for polarity, energetic homogeneity and also catalytic activity of adsorbents is the comparison of the heats of adsorption for nparaffins and n-olefines of the same chain length. It was therefore of interest to investigate calorimetrically the behaviour of US-Ex towards the adsorption of n-butane and but-l-cne and to extend this examination to the recently synthesized SiO2-molecular sieve silicalite 3. EXPERIMENTAL The differential molar heats of adsorption were measured by means of a Calvet-type microcalorimeter (Setaram) at 301 K. The adsorption equilibrium was controlled by the thermokinetic curve of the calorimeter as well as by recording the equilibrium pressure (Baratron pressure meter). With the exception of but-1-ene/US-Ex the adsorption equilibrium was reached within two hours for all systems examined. Synthesis of silicalite was accomplished on the basis of data published by Flanigen et al. 3. The first silicalite batch synthesized exhibited a reduced adsorption capacity 4 (compared with Ref. 3) which could be accounted for by the presence of a minor part of amorphous SiOv For the silicalite sample used in the present paper, the adsorption capacity towards n-butane is 0144-2449/83/030095-03503.00 © Bunerworth & Co. (Publishers) Ltd.
in agreement with the value given in Ref. 3 as well as X-ray analysis data of silicalite s. The preparation of US-Ex is described in Ref. 2. In Table 1 the unit cell compositions and the pore volumes of the molecular sieves investigated are given. Before application all adsorbents were activated in high vacuum ( < 1 0 - 3 p a ) for 24 h. The purity of the adsorbates was better than 99 mol% (Fluka AG). RESULTS AND DISCUSSION
Figure 1 shows the differentiM molar heats of adsorption for n-butane and but-l-ene on molecular sieves Na-Y and US-Ex. The comparison of the curves for the paraffin and the olefine on Na-Y exhibits the expected behaviour. The heat of adsorption of but-l-ene is substantially higher than the heat for the n-butane/Na-Y system in the entire range of coverage. At zero coverages (a ~ 0) the difference between the Q-values amounts to Q0(butene)--Q0(butane) = 56.5-41.4 =15.1 kJ mo1-1 and may be explained by the specific interaction or the rr-bonds in but-l-ene with the cations of the zeolite. By comparing the heats of adsorption for n-butane on molecular sieves Na-Y and US-Ex it can be seen that dealumination leads to a distinct decrease in the energy of the adsorption interaction. The corresponding heat difference for nbutane is equal to Q 0 ( N a - Y ) - - Q 0 ( U S - E x ) = 41.433.1 = 8.3 kJ mo1-1. This value which may be Table 1 Characteristics of the investigated adsorbents Adsorbent
Unit cell composition
Pore volume (cm3g-t)
Na-Y US-Ex Silicalite
Na54(AIO2) 5~(SiO2),3s Nao.~(AIO2) t-s(Si02 ) ~9~-~s7 (Si0=)96
0.29 0.56 0.19
ZEOLITES 1983, Vol 3, April 95
11C ~ 10C 9C
26 ~
24 ~x
22 20
80
":
"\
E 7C .~
16
6C
14
x
10 4C
" 8
3C
I
05
I
10
I
I
J
15 20 25 e (mmol g-~)
I
30
Figure 1 Differential molar heats of adsorption on molecular sieves Na-Y and US-Ex at 28°C. (v) n-butane/Na-Y, (A) but-l-ene/ Na-Y, (+) n-butane/US-Ex, (X) but-l-ene/US-Ex, filled symbols desorption
interpreted as the influence of the electrostatic field in Na-Y on the heat of adsorption 6 amounts to about 20% of the heat of adsorption on Na-Y. The adsorption of but-l-ene on US-Ex reveals a quite different picture. Whilst the nonpolar nbutane as well as n-hexane, cyclohexane and even the aromatic benzene x are physically adsorbed and the highly dealuminated US-Ex can be regarded as a h o m o g e n e o u s microporous adsorbent with respect to adsorption of these molecules, but-l-ene is irreversibly adsorbed. The heat effect of but-1erie adsorption exceeds the heats of adsorption of all the other systems investigated, and in contrast to them, falls with coverage. Adsorption equilibrium has not been reached. A decrease in the vapour pressure has been noticed even after a week when the heat flow was less than the sensitivity of the calorimeter. This behaviour gives evidence of a catalytic reaction. The gas chromatographic analysis of the desorbed p r o d u c t (Table 2) shows the dominating formation of isomerization species (trans-but-2-ene and cis-but-2-ene) and to a minor part polymerization and disproportionation products respectively. (Desorption was accomplished at r o o m temperature by means of a liquid nitrogen cold trap.) It is interesting to note that the conversion of but-l-ene already is nearly complete at r o o m temperature although the n u m b e r and strength of acid centres in US-Ex is relatively small compared with zeolitic catalysts such as rare earth exchanged Na-Y or N a H - Y zeolites 7 and the OHgroups are mainly of the terminal type 8.
96
ZEOLITES 1983, Vol 3, April
In Figure 2 the dependences of the heats of adsorption on coverage are given for n-butane and but-1ene on silicalite. In contrast to US-Ex and N a - Y the heat of adsorption of but-l-ene is less than the heat of adsorption for n-butane (lowest coverages a < 0.1 m m o l g-X excluded) and desorption measurements coincide with adsorption. The difference between the Qwvalues amounts to Q 0 ( b u t e n e ) - Q0(butane) = 49.0-52.3 = - - 3.3 kJ mol -a. The nature and location of the adsorption sites (probably OH-groups 3) which are responsible for the heat rise with falling adsorption at a < 0.3 m m o l g-1 cannot be finally explained at present. If we neglect this coverage range silicalite may be regarded as a nonpolar, energetically h o m o g e n e o u s adsorbent with respect to physical adsorption of but-l-ene. The surplus of two Hatoms in the structure of the butane molecule compared with b u t e n e is clearly reflected in the measured heats of adsorption whereas the higher electron density of the double b o n d in butene (electron configuration sp 2) in contrast to butane (sp 3) has no significant influence on the interaction energy, i.e. the adsorbate-adsorbent dispersion interaction is the dominating factor for the but-l-ene/silicalite system as well as for n2 Gas chromatographic composition (%) of the desorbed product from but-l-ene/US-Ex system. (Column: n-octane/Porasil C, 175X0.15 cm, 22°C) Table
propane i-butane n-butane but-1 -erie trans but-2-ene cis but-2-ene >C4
D
k
1.0 4.6 3.5 2.9 71.8 13.2 3.0
)
~
/
~
;14 ~"
10
40
8 3O I
0
0.5
I
I
10 15 o (rnmol g "~) Figure 2 Differential molar heats of adsorption on silicalite at 28°C. (o) n-butane/silicalite, (o) but-l-ene/silicalite, filled symbols -desorption
butane/silicalite. Similar high degrees of the energetic homogeneity of adsorbents have up to now only been shown on flat surfaces, e.g. graphitized thermal carbon black 9. The comparison of the heats of adsorption for n-butane on the molecular sieves investigated demonstrates the influence of pore size on the heat of adsorption. The heats of adsorption diminish in the following order: Q0(silicalite) > Q0(Na-Y) > Q0(US-Ex). This result is understandable if one keeps in mind the comparable sizes of the effective n-butane molecule diameter (~ 0.49 nm) and the pore diameters in silicalite ( ~ 0 . 5 1 - 0 . 5 7 nm) which favour an optimum molecule-adsorbent interaction and on the other hand hinders molecule interaction as can be seen from the relatively small rise of the heat curve with coverage on silicalite compared with Na-Y and US-Ex. The small difference between the Q-value at the maximum of the heat curve and Q0 on silicalite (7.2 kJ mo1-1) compared with Na-Y (16.8 kJ mol -l) and US-Ex (11.7 kJ mol-:) supports the assumption of an 'end-to-end' con-
figuration of n-butane molecules in silicalite channels as proposed for the adsorption of nalkanes in H-ZSM-51°. REFERENCES 1
Schirmer, W., Thamm, H., Stach, H. and Lohse, O. 'The properties and applications of zeolites', (Ed. R. P. Townsend) Special Publication No. 33, The Chemical Society, London 1980, p. 204 2 Lohse, U., Alsdorf, E. and Stach, H. Z. Anorg. AIIg. Chem. 1978, 447, 64 3 Flanigen, E. M., Bennett, J. M. Grose, R. W., Cohen, J. P., Patton, R. L., Kirchner, R. M. and Smith, J. V. Nature 1978, 271,512 4 Thamm, H., Stach, H., Schirmer, W. and Fahlke, B. Z. Phys. Chemie (Leipzig) 1982, 263, 461 5 Fichtner, H. Krista/I und Technik in preparation 6 Schirmer, W., Stach, H., Thamm, H., Dubinin, M. M., Issirikjan, A. A., Regent, N. I. and Anaktschjan, E. Ch. Z. Phys. Chemie (Leipzig) 1980, 261, 1129 7 Bremer, H., Lohse, U., Reschetilowski, W. and Wendlandt, K.-P. Z. Anorg. AI/g. Chem. 1981,482, 235 8 Bos~ek, V., Patzelov$, V., Tvaru}kova, Z., Freude, D., Lohse, U., Schirmer, W., Stach, H. and Thamm, H. J. Cata/. 1980, 61, 435 9 Kiselev, A . V . Zh. Fiz. /(him. 1967,41,2470 10 Jacobs, P. A., Beyer, H. K. and Valyon, J. Zeo/ites 1981, 1, 161
Examination of the surface structure of zeolites by fast atom bombardment mass spectrometry (FABMS). Loewensteins rule j . Dwyer*, I. S. Elliott, F. R. Fitch, J. A. van den Berg and J. C. Vickerman
Chemistry Department, UMIST, Manchester, P.O. Box 88, UK (Received 13 August 1982) Fast atom bombardment mass spectrometry (FABMS) is used, in the negative ion mode, to investigate the presence of paired aluminium species in zeolites and in aluminas. Results do not support the systematic breaking of Loewensteins rule in zeolite A. Keywords: Zeolites, surface composition; Loewenstein; fast-atom bombardment
INTRODUCTION Fast atom b o m b a r d m e n t mass spectrometry is a recently developed technique in which species sputtered from surfaces by a beam of argon atoms are analysed mass spectrometrically. This technique, which is a development of secondary ion mass spectrometry (SIMS) was recently used to study the surface composition of zeolites I. Currently there is controversy concerning the arrangement of silicon and aluminium atoms in zeolites. In particular, on the basis of 295i n.m.r. results it has been suggested that Loewensteins rule, which forbids pairing of aluminiums, is systematically brokcn in zeolite A, in some forms of sodalite mad in other zeolites 2. Decomposition * To whom correspondence should be sent
0144-2449/83/030097-03503.00 © Butterworth & Co. (Publishers) Ltd.
of zeolites by silanation suggests that Loewensteins rule is also broken in faujasites 3 but this is not supported by 29Si n.m.r, a or by detailed X-ray studies s. If the structure of zeolite A is such that all the aluminiums are paired via oxygen bridges, whereas in zeolites X and Y Loewensteins rule holds completely*, then it is to be expected that surface species sputtered from zeolite A would include more paired aluminiums than X or Y. Since zeolites A and X can be synthesized with Si/A1 close to unity, we should also expect more paired silicons to be sputtered from A than from X. Similarly we would expect more Si-O-A1 pairs in X and in A. With this in mind samples of zeolites and of reference materials were examined by FABMS. Zeolites A and Y were kindly provided by Laporte
ZEOLITES 1983, Vol 3, April 97