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The effect of ion exchange on adsorption properties of a 13X molecular sieve
Vacuum/volume Printed in Great
42/number
The effect properties Sun Da-Ming. received
13 July
13lpages
845 to 848/I
0042-207x/91
991
of ion exchange on adsorption of a 13X molecular sieve
Department 1990 and in
$3.00+.00
$3 1991 Pergamon Press plc
Britain
of Physics, Anhui
final form
74 November
University,
Hefei, P R China
1990
The paper sets out to show that by exchanging Na’ ions in a sodium 13X molecular sieve by various divalent ions (Ca2+, Sr2+ and Ba2’), the adsorption capacities for N2, Ar and air are greatly improved at low temperature and low pressure. The objective is to improve the performance of 13X molecular sieve as a vacuum pump. By analysing the residual gas by mass spectroscopy, the efficiency with which the various components of atmospheric air are removed is estimated.
1. Introduction At low temperature and low pressure, the 13X molecular sieve has evident adsorption properties and a pumping action’ ‘, which results from the capture of molecules up to I nm molecular diameter (such as vacuum pump oil)4. Our aim is, through the use of the ion exchange method, to change the adsorption properties and desorption rate of the 13X molecular sieve, whilst keeping the vacuum chamber pressure constant and the vacuum system pumping.
measured within 30 min, the average value evaluated, and the adsorption speed isotherms of the molecular sieve in the specimen chamber under this average pressure of sorption is obtained using the formula : S=
(1)
(inlgs-‘)
C(Pi-Ps)/P*‘W
where S is adsorption speed, P, is pressure in the gas filling chamber, P, is the average pressure in the sorption chamber, C is the conductance ofcapillary and Wis the mass of the molecular sieve.
2. Experiment 3. Experimental results and analysis The samples used in the experiment are Ca’+, Sr’+ and Ba’+ exchanged Na+ in a 13X molecular sieve by means of the solution method described in refs 5 and 6 in order to obtain CaNaX, SrNaX and BaNaX types of molecular sieve. The ion exchange rate of Sr’+ and Ba’+ can be higher than 95% while the Ca*+ ion exchange rate is about 80%. The ion exchanged 13X molecular sieve was then washed and baked in order to obtain the desired product. The pH value of water after washing is I I-12. The adsorbates are N, (99.99%) Ar (99.99%) and dry-air. A schematic diagram of the experimental apparatus and a description of the method is presented in ref 3. Figure I shows the experimental apparatus for measuring the adsorption speed isotherms of a 13X molecular sieve. This is a high vacuum system made of glass. The high vacuum part, marked by dotted lines. can be baked out. There are small holes of I mm diameter bored in the wall of the tube in specimen chamber 7, which is used to allow gas to reach the specimen. Capillary 8 is 3.21 mm in diamcter, 102.2 mm in length. The conductance is 3.92 x IO-’ I s ’ at molecular flow. The IG-3H ionisation gauge 9, IO, can be calibrated by direct comparison with a reference gauge in air urd N,; the accuracy of calibration is 5%. After the specimen Izhamber and high vacuum system have been baked out for IO h
3.1. Desorption rate. Employing the apparatus and method described in ref 3 and using 28 g of the CaNaX. SrNaX and
___-
r----
2
----
3
1
Figure 1. The experimental device. (I) Mechanical
pump; (2) diffusion pump; (3) dryer; (4) liquid nitrogen trap; (5) sorption chamber (500 ml) : (6) filling chamber (500 ml) : (7) specimen chamber : (8) capillary ; (9, 10) IG3H ultrahigh vacuum ionisation gauge ; (I I) DL-3 thermocouple
gauge; (12, 13) JB-type high vacuum valve; (14) high vacuum microvalve ; (15) high vacuum valve ; (16) three-way valve ; (17) Omegatron type mass spectrometer. 845
Sun Da-Ming:
Adsorption
propertles of a 13X molecular sieve
(b)
10 10-5
-a*...-.-....“.-.*.z.%
4
R
12
20
pressure
Adsorption
-.-..
16
10-z
10-S
10-q
10-I
1
1PO1
Figure 3. The adsorption isotherms of ion exchanged molecular sieves. Curve A is SrNaX : curve B is CaNaX ; curve C is BaNaX ; and curve D is the non-ion exchanged 13X molecular sieve (see ret” 3). Adsorbate air: adsorption temperature : 77 K.
Time ihl 101CoNaX
(bl SrNaX
(cl EoNaX
Id) 13X (ref 31
Figure 2. The &sorption rate of the molecular sieves at various temperatures. (a) CaNaX : (b) SrNaX : (c) BaNaX : and (d) 13X (ref 3).
BaNaX
types of molecular
ambient
temperature
sieves each, the dcsorption
was obtained.
This desorption
rate at
rate. as well
temperatures, is shown in Figure 2. In Figure 2 the dcsorption rate of the CaNaX. SrNaX and BaNaX types of molecular sieves at ambient temperature is seen to be low and almost constant over more than 20 h. The desorption rate at 100 C is initially one order ofmagnitude larger and drops linearly with pumping time. The desorption rates at 200 and 300 C start two orders of magnitude higher than at ambient temperature and drop exponentially with the pumping time. This shows that the best conditions for outgassing CaNaX. SrNaX and BaNaX types of molecular sicvcs arc at a 300 C for 4 6 h. It is disadvantageous to outgas them at 350 C bccausc this may product powder in the molecular sieves. Their desorption rates are similar to those of 13X molecular sieve [see Figure 2(d)]. as those at higher
3.2. Adsorption isotherms. Using the apparatus and method described in ref 3, 10 g of CaNaX. SrNaX and BaNaX types of molecular sieves each was put separately into the specimen chamber (the volume is 0.25 I for CaNaX, 0.20 I for SrNaX and 0.23 I for BaNaX). After baking and pumping, the adsorption isotherms for N,, Ar and air at low temperature (77 K) and low pressure were measured and shown in Figures 3, 4 and 5. We can see from the adsorbed amount in Figures 3 and 4 that curve A is the largest, followed by curve B. Both arc larger than that of the 13X molecular sieve without ion cxchangc. Curve C is the adsorption isotherm for Ba’+ ions exchanged 13X molecular sicvc. which is smaller than curve D. Meanwhile. the Ba’- ion exchanged 13X molecular sieve is destroyed after being baked and this shows that the adsorption properties of Ba” ion exchanged 13X molecular sicvc becomes worse. In Figure 5 it is shown that the adsorption properties of the ion exchanged molecular sicvc do not change significantly in comparison with the I3X molecular sicvc. 846
IO
1
’ ’ “““’
10-5
’ ’ “““’
10-q
’ ’ “““’
10-Z
Adsorption
Figure 4. The adsorption Curve A is SrNaX ; curve is the non-ion exchanged adsorption tcmperaturc:
’ ’ “““’
10-z
pressure
’ ’ “‘LM
10-I
1
(Pa)
isotherms of ton exchanged molecular sieves. B is CaNaX ; curve C is BaNaX ; and curve D 13X molecular Steve (see ref 3). Adsorbate: N2; 77 K.
IO" ’ ““I”’ ’ “““” ’ “““” “““” ’ “““” lo-' 1om2 1om3 10-5 10-4 Adsorpt,on
PreSSure
1
(PO)
Figure 5. The adsorption isotherms of ion exchanged rnolccular sieves. Curve A is SrNaX ; curve B is CaNaX : curve C is BaNaX ; curve D is nonion exchanged 13X molecular sieve. Adsorbate : Ar: adsorption temperature: 77 K.
Adsorption
Sun Da-Ming:
properties
of a 13X molecular
sieve
Room
0
’
’ 1”““’
’ ““I”1
10-4
10-5
Average
’ “““”
10-S
adsorption
temperature
’ “““U
10-Z
pressure
10-1
(Pa) D
Figure 6. The adsorption
speed isotherm of ion exchanged molecular sieves. Curve A is SrNaX : curve B is CaNaX : curve C is BaNaX : curve D is non-ion exchanged 13X molecular sieve. Adsorbate : air: adsorption temperature : 77 K,
B A
10-S' ’ 012345678
Adsorption speed isotherms. Using the apparatus shown in Figure 1, IO g of the CaNaX, SrNaX, BaNaX types of molecular
’
’
’
’
’
’
’
’ 9
J 10
3.3.
sieves
each
and
the
13X molecular
sieve were
put
into
the 0.25.
0.23 and 0.25 1 specimen chambers, respectively. During pumpdown the system and specimen chamber are baked at 350°C for IO h. The adsorption speed isotherms of the CaNaX, SrNaX, BaNaX types of molecular sieves and 13X molecular sieve obtained from equation (1) arc shown in Figure 6. When P, is smaller than 5 x 10m4 Pa, the adsorption speed of curve A is smaller than curve B; when P, is larger than 6 x IO ’ Pa, adsorption speed of curve A is larger than curves D and B, curve B is larger than curve D. The adsorption speed ofcurve C is obviously smaller than curve D. 0.20.
3.4. The adsorption pumping experiment. In the high vacuum system shown in Figure I. we used 200 g of each of the ion exchanged CaNaX. SrNaX and BaNaX types of molecular sieves to carry out adsorption pumping experiments in a vacuum chamber of I.4 I volume. The system was first pumped down with a mechanical pump. At the same time, the system and molecular sieve were baked for 6 h at 350 C. The system pressure was2x IO ’ Pa. Next, valve I3 was closed stopping the pumping and the molecular sieve left to cool down naturally to ambient temperature. Then after the liquid nitrogen (LN2) Dewar was placed around the molecular sieve trap, the system pressure dropped rapidly. After 2-3 h, the pressure had dropped to the limiting pressure. Figure 7 shows the relation between the adsorption pumping pressure and pumping time of the molecular sieve. The closeness of curves A and B in Figure 7 shows that their adsorption pumping properties are similar. only the final pressure of curve A is lower (7 x 10 ‘). The adsorption pumping properties of curve C are different, and its final pressure is about 9 x IO ’ Pa. Comparing the adsorption pumping propertics of curves A and B with that of the 13X molecular sieve given in ref 3. with their load rate V: W being equal, we can see that for the ion exchanged CaNaX and SrNaX types of molecular sieves the adsorption pumping properties have improved. 3.5. Mass spectroscopy analysis exchanged 13X molecular sieve change 13X molecular sieve were for pumping. When the lowest
of residual gas. The Sr’+ ion (SrNaX) and the non-ion cxput into the high vacuum system total pressures were reached,
Pumping
time
Ch)
Figure 7. The pumping
curves of the ion exchanged molecular sieves. Curve A is SrNaX ; curve B is CaNaX ; curve C is BaNaX : curve D is non-ion exchanged 13X molecular sieve. The pumped gas is air.
7 x 10~ ’ and 1.7 x IO-’ Pa, respectively. the composition and partial pressure of residual gas in the system were measured using an Omegatron type mass spectrometer. Table I shows the results of the residual gas analysis of the SrNaX type molecular sieve and Table 2 the residual gas analysis of the 13X molecular sieve. The composition and partial pressure shown in Table I are smaller than those in Table 2, but the water content of the two cases is very close. 4. Discussion We know from the experimental results. that after divalent ion exchange, various kinds of I3X molecular sieve change their gas
Table 1. Final pressure type molecular 7.5 x IO-’ Pa
and residual gas composition of the SrNaX sieve adsorption pumping. The Omegatron sensitivity is ’ and the collector current is 3 x IO ” A
~~~ Adsorption temperature
(K)
77
Residual gas composition (ion c_uyyt x IO ” A)
Final total pressure (Pa)
H,
Hc
N,
H1O
Ne
CO,
7.0x
32
IO
6.8
IX
48
2
10 ’
Table 2. Final pressure and residual gas composition of the 13X molecular sieve adsorption pumping. The Omegatron sensitivity is 7.5 x IO ’ Pa and the collector current is 3 x IO ’ A Residual gas composition (ion current x 10 ” A) Adsorption temperature 77
(K)
Final total pressure (Pa)
H,
He
N,
H,O
NC
CO,
1.7x lo~l
72
26
16
I4
102
5
847
’
Sun Da-Ming:
Adsorption properties of a 13X molecular sieve
adsorption properties at low tcmpcrature and low pressure. The whole adsorption speed and adsorbed amount of the Cal+ and St-‘+ ion exchanged molecular sieves improve over the whole pressure range, but those of the Ba’* ion exchanged molecular sicvc arc clearly lowered. We shall explain the reason for this from the effect that ion exchange has on the molecular sieve. It is known that the adsorption properties of the molecular sieve arc mainly due to their anion structure which determines their crystal cage, crystal window and surface area. When we use Ca’+. Sr’+ and Ba’+ ions for ion exchange. the radius, charge. electron structure and hydration of the cation will all influence the physical and chemical qualities. For example, the radius of the NaX type molecular sieve with Na’ cations is 0.095 nm. while the radius of Ca’+ . Sr” and Ba’+ divalent ions used for ion exchange are 0.09. 0.113 and 0. I35 nm. respectively. When ion exchange proceeds, Ca”. St-‘+ or Ba’+ divalcnt ions can exchange with two Na+ cations. So after ion exchange, the crystal cage, crystal window and surface area of the molecular sieve either increase or decrease and, thcrcforc. the gas adsorption propertics are either affected positively or negatively. Ion exchange bctwcen a Ca’+ divalcnt ion and the 13X molecular sieve turns the sodium type 13X molecular sieve into the calcium X(CaX) type molecular sieve (10X). The crystal window becomes smaller and the surface area larger. So the adsorption properties of the CaNaX type molecular sieve are better than the
848
13X molecular sieve. When we use a Ba” divalent ion to cause ion exchange with the 13X molecular sicvc, bccausc the radius of Ba’- is too large (0.135 nm), the 13X molecular sicvc’s structure is destroyed after ion exchange ; especially after baking out at high temperature (400’ C). the stability is clearly lowered. and this causes the drop of adsorption properties. According to all the analysis, WCcan draw the conclusion that ion exchange can change the adsorption propcrtics of molcculat sieve either way : positively or negatively. So in practice, WCcan choose different cations for ion exchange, to meet the different needs of manufacturing and experimental technology.
Acknowledgement The author is grateful for the help of Chen Ying-Shan, Wen-Ging and Hua Xing.
Wang
References ’ S A Stern and F S Diapolo. J Vuc Sci Twlrnol. 4, 347 (1967). P/z.,s, 28, I53 (I 980). ‘Sun Da-Ming, J Ph>..vE: Sc,i I~7strun7. 22, 159 (1989). ‘A Luches and A Zecca. J Cbc Sci Tdwd, 9, 1237 (1972). ‘C K Hersh, Moleculu~ Sierrs. Reinhold Publishing Corporation (1961). “Zhu Chong-Ye and Cm Fei-Xian et ~1. VW Sci Tcchol (China). 6(6),
’ M JIckcl and R Bartel, Esp Tdznol
6 (1986).
42/number
The effect properties Sun Da-Ming. received
13 July
13lpages
845 to 848/I
0042-207x/91
991
of ion exchange on adsorption of a 13X molecular sieve
Department 1990 and in
$3.00+.00
$3 1991 Pergamon Press plc
Britain
of Physics, Anhui
final form
74 November
University,
Hefei, P R China
1990
The paper sets out to show that by exchanging Na’ ions in a sodium 13X molecular sieve by various divalent ions (Ca2+, Sr2+ and Ba2’), the adsorption capacities for N2, Ar and air are greatly improved at low temperature and low pressure. The objective is to improve the performance of 13X molecular sieve as a vacuum pump. By analysing the residual gas by mass spectroscopy, the efficiency with which the various components of atmospheric air are removed is estimated.
1. Introduction At low temperature and low pressure, the 13X molecular sieve has evident adsorption properties and a pumping action’ ‘, which results from the capture of molecules up to I nm molecular diameter (such as vacuum pump oil)4. Our aim is, through the use of the ion exchange method, to change the adsorption properties and desorption rate of the 13X molecular sieve, whilst keeping the vacuum chamber pressure constant and the vacuum system pumping.
measured within 30 min, the average value evaluated, and the adsorption speed isotherms of the molecular sieve in the specimen chamber under this average pressure of sorption is obtained using the formula : S=
(1)
(inlgs-‘)
C(Pi-Ps)/P*‘W
where S is adsorption speed, P, is pressure in the gas filling chamber, P, is the average pressure in the sorption chamber, C is the conductance ofcapillary and Wis the mass of the molecular sieve.
2. Experiment 3. Experimental results and analysis The samples used in the experiment are Ca’+, Sr’+ and Ba’+ exchanged Na+ in a 13X molecular sieve by means of the solution method described in refs 5 and 6 in order to obtain CaNaX, SrNaX and BaNaX types of molecular sieve. The ion exchange rate of Sr’+ and Ba’+ can be higher than 95% while the Ca*+ ion exchange rate is about 80%. The ion exchanged 13X molecular sieve was then washed and baked in order to obtain the desired product. The pH value of water after washing is I I-12. The adsorbates are N, (99.99%) Ar (99.99%) and dry-air. A schematic diagram of the experimental apparatus and a description of the method is presented in ref 3. Figure I shows the experimental apparatus for measuring the adsorption speed isotherms of a 13X molecular sieve. This is a high vacuum system made of glass. The high vacuum part, marked by dotted lines. can be baked out. There are small holes of I mm diameter bored in the wall of the tube in specimen chamber 7, which is used to allow gas to reach the specimen. Capillary 8 is 3.21 mm in diamcter, 102.2 mm in length. The conductance is 3.92 x IO-’ I s ’ at molecular flow. The IG-3H ionisation gauge 9, IO, can be calibrated by direct comparison with a reference gauge in air urd N,; the accuracy of calibration is 5%. After the specimen Izhamber and high vacuum system have been baked out for IO h
3.1. Desorption rate. Employing the apparatus and method described in ref 3 and using 28 g of the CaNaX. SrNaX and
___-
r----
2
----
3
1
Figure 1. The experimental device. (I) Mechanical
pump; (2) diffusion pump; (3) dryer; (4) liquid nitrogen trap; (5) sorption chamber (500 ml) : (6) filling chamber (500 ml) : (7) specimen chamber : (8) capillary ; (9, 10) IG3H ultrahigh vacuum ionisation gauge ; (I I) DL-3 thermocouple
gauge; (12, 13) JB-type high vacuum valve; (14) high vacuum microvalve ; (15) high vacuum valve ; (16) three-way valve ; (17) Omegatron type mass spectrometer. 845
Sun Da-Ming:
Adsorption
propertles of a 13X molecular sieve
(b)
10 10-5
-a*...-.-....“.-.*.z.%
4
R
12
20
pressure
Adsorption
-.-..
16
10-z
10-S
10-q
10-I
1
1PO1
Figure 3. The adsorption isotherms of ion exchanged molecular sieves. Curve A is SrNaX : curve B is CaNaX ; curve C is BaNaX ; and curve D is the non-ion exchanged 13X molecular sieve (see ret” 3). Adsorbate air: adsorption temperature : 77 K.
Time ihl 101CoNaX
(bl SrNaX
(cl EoNaX
Id) 13X (ref 31
Figure 2. The &sorption rate of the molecular sieves at various temperatures. (a) CaNaX : (b) SrNaX : (c) BaNaX : and (d) 13X (ref 3).
BaNaX
types of molecular
ambient
temperature
sieves each, the dcsorption
was obtained.
This desorption
rate at
rate. as well
temperatures, is shown in Figure 2. In Figure 2 the dcsorption rate of the CaNaX. SrNaX and BaNaX types of molecular sieves at ambient temperature is seen to be low and almost constant over more than 20 h. The desorption rate at 100 C is initially one order ofmagnitude larger and drops linearly with pumping time. The desorption rates at 200 and 300 C start two orders of magnitude higher than at ambient temperature and drop exponentially with the pumping time. This shows that the best conditions for outgassing CaNaX. SrNaX and BaNaX types of molecular sicvcs arc at a 300 C for 4 6 h. It is disadvantageous to outgas them at 350 C bccausc this may product powder in the molecular sieves. Their desorption rates are similar to those of 13X molecular sieve [see Figure 2(d)]. as those at higher
3.2. Adsorption isotherms. Using the apparatus and method described in ref 3, 10 g of CaNaX. SrNaX and BaNaX types of molecular sieves each was put separately into the specimen chamber (the volume is 0.25 I for CaNaX, 0.20 I for SrNaX and 0.23 I for BaNaX). After baking and pumping, the adsorption isotherms for N,, Ar and air at low temperature (77 K) and low pressure were measured and shown in Figures 3, 4 and 5. We can see from the adsorbed amount in Figures 3 and 4 that curve A is the largest, followed by curve B. Both arc larger than that of the 13X molecular sieve without ion cxchangc. Curve C is the adsorption isotherm for Ba’+ ions exchanged 13X molecular sicvc. which is smaller than curve D. Meanwhile. the Ba’- ion exchanged 13X molecular sieve is destroyed after being baked and this shows that the adsorption properties of Ba” ion exchanged 13X molecular sicvc becomes worse. In Figure 5 it is shown that the adsorption properties of the ion exchanged molecular sicvc do not change significantly in comparison with the I3X molecular sicvc. 846
IO
1
’ ’ “““’
10-5
’ ’ “““’
10-q
’ ’ “““’
10-Z
Adsorption
Figure 4. The adsorption Curve A is SrNaX ; curve is the non-ion exchanged adsorption tcmperaturc:
’ ’ “““’
10-z
pressure
’ ’ “‘LM
10-I
1
(Pa)
isotherms of ton exchanged molecular sieves. B is CaNaX ; curve C is BaNaX ; and curve D 13X molecular Steve (see ref 3). Adsorbate: N2; 77 K.
IO" ’ ““I”’ ’ “““” ’ “““” “““” ’ “““” lo-' 1om2 1om3 10-5 10-4 Adsorpt,on
PreSSure
1
(PO)
Figure 5. The adsorption isotherms of ion exchanged rnolccular sieves. Curve A is SrNaX ; curve B is CaNaX : curve C is BaNaX ; curve D is nonion exchanged 13X molecular sieve. Adsorbate : Ar: adsorption temperature: 77 K.
Adsorption
Sun Da-Ming:
properties
of a 13X molecular
sieve
Room
0
’
’ 1”““’
’ ““I”1
10-4
10-5
Average
’ “““”
10-S
adsorption
temperature
’ “““U
10-Z
pressure
10-1
(Pa) D
Figure 6. The adsorption
speed isotherm of ion exchanged molecular sieves. Curve A is SrNaX : curve B is CaNaX : curve C is BaNaX : curve D is non-ion exchanged 13X molecular sieve. Adsorbate : air: adsorption temperature : 77 K,
B A
10-S' ’ 012345678
Adsorption speed isotherms. Using the apparatus shown in Figure 1, IO g of the CaNaX, SrNaX, BaNaX types of molecular
’
’
’
’
’
’
’
’ 9
J 10
3.3.
sieves
each
and
the
13X molecular
sieve were
put
into
the 0.25.
0.23 and 0.25 1 specimen chambers, respectively. During pumpdown the system and specimen chamber are baked at 350°C for IO h. The adsorption speed isotherms of the CaNaX, SrNaX, BaNaX types of molecular sieves and 13X molecular sieve obtained from equation (1) arc shown in Figure 6. When P, is smaller than 5 x 10m4 Pa, the adsorption speed of curve A is smaller than curve B; when P, is larger than 6 x IO ’ Pa, adsorption speed of curve A is larger than curves D and B, curve B is larger than curve D. The adsorption speed ofcurve C is obviously smaller than curve D. 0.20.
3.4. The adsorption pumping experiment. In the high vacuum system shown in Figure I. we used 200 g of each of the ion exchanged CaNaX. SrNaX and BaNaX types of molecular sieves to carry out adsorption pumping experiments in a vacuum chamber of I.4 I volume. The system was first pumped down with a mechanical pump. At the same time, the system and molecular sieve were baked for 6 h at 350 C. The system pressure was2x IO ’ Pa. Next, valve I3 was closed stopping the pumping and the molecular sieve left to cool down naturally to ambient temperature. Then after the liquid nitrogen (LN2) Dewar was placed around the molecular sieve trap, the system pressure dropped rapidly. After 2-3 h, the pressure had dropped to the limiting pressure. Figure 7 shows the relation between the adsorption pumping pressure and pumping time of the molecular sieve. The closeness of curves A and B in Figure 7 shows that their adsorption pumping properties are similar. only the final pressure of curve A is lower (7 x 10 ‘). The adsorption pumping properties of curve C are different, and its final pressure is about 9 x IO ’ Pa. Comparing the adsorption pumping propertics of curves A and B with that of the 13X molecular sieve given in ref 3. with their load rate V: W being equal, we can see that for the ion exchanged CaNaX and SrNaX types of molecular sieves the adsorption pumping properties have improved. 3.5. Mass spectroscopy analysis exchanged 13X molecular sieve change 13X molecular sieve were for pumping. When the lowest
of residual gas. The Sr’+ ion (SrNaX) and the non-ion cxput into the high vacuum system total pressures were reached,
Pumping
time
Ch)
Figure 7. The pumping
curves of the ion exchanged molecular sieves. Curve A is SrNaX ; curve B is CaNaX ; curve C is BaNaX : curve D is non-ion exchanged 13X molecular sieve. The pumped gas is air.
7 x 10~ ’ and 1.7 x IO-’ Pa, respectively. the composition and partial pressure of residual gas in the system were measured using an Omegatron type mass spectrometer. Table I shows the results of the residual gas analysis of the SrNaX type molecular sieve and Table 2 the residual gas analysis of the 13X molecular sieve. The composition and partial pressure shown in Table I are smaller than those in Table 2, but the water content of the two cases is very close. 4. Discussion We know from the experimental results. that after divalent ion exchange, various kinds of I3X molecular sieve change their gas
Table 1. Final pressure type molecular 7.5 x IO-’ Pa
and residual gas composition of the SrNaX sieve adsorption pumping. The Omegatron sensitivity is ’ and the collector current is 3 x IO ” A
~~~ Adsorption temperature
(K)
77
Residual gas composition (ion c_uyyt x IO ” A)
Final total pressure (Pa)
H,
Hc
N,
H1O
Ne
CO,
7.0x
32
IO
6.8
IX
48
2
10 ’
Table 2. Final pressure and residual gas composition of the 13X molecular sieve adsorption pumping. The Omegatron sensitivity is 7.5 x IO ’ Pa and the collector current is 3 x IO ’ A Residual gas composition (ion current x 10 ” A) Adsorption temperature 77
(K)
Final total pressure (Pa)
H,
He
N,
H,O
NC
CO,
1.7x lo~l
72
26
16
I4
102
5
847
’
Sun Da-Ming:
Adsorption properties of a 13X molecular sieve
adsorption properties at low tcmpcrature and low pressure. The whole adsorption speed and adsorbed amount of the Cal+ and St-‘+ ion exchanged molecular sieves improve over the whole pressure range, but those of the Ba’* ion exchanged molecular sicvc arc clearly lowered. We shall explain the reason for this from the effect that ion exchange has on the molecular sieve. It is known that the adsorption properties of the molecular sieve arc mainly due to their anion structure which determines their crystal cage, crystal window and surface area. When we use Ca’+. Sr’+ and Ba’+ ions for ion exchange. the radius, charge. electron structure and hydration of the cation will all influence the physical and chemical qualities. For example, the radius of the NaX type molecular sieve with Na’ cations is 0.095 nm. while the radius of Ca’+ . Sr” and Ba’+ divalent ions used for ion exchange are 0.09. 0.113 and 0. I35 nm. respectively. When ion exchange proceeds, Ca”. St-‘+ or Ba’+ divalcnt ions can exchange with two Na+ cations. So after ion exchange, the crystal cage, crystal window and surface area of the molecular sieve either increase or decrease and, thcrcforc. the gas adsorption propertics are either affected positively or negatively. Ion exchange bctwcen a Ca’+ divalcnt ion and the 13X molecular sieve turns the sodium type 13X molecular sieve into the calcium X(CaX) type molecular sieve (10X). The crystal window becomes smaller and the surface area larger. So the adsorption properties of the CaNaX type molecular sieve are better than the
848
13X molecular sieve. When we use a Ba” divalent ion to cause ion exchange with the 13X molecular sicvc, bccausc the radius of Ba’- is too large (0.135 nm), the 13X molecular sicvc’s structure is destroyed after ion exchange ; especially after baking out at high temperature (400’ C). the stability is clearly lowered. and this causes the drop of adsorption properties. According to all the analysis, WCcan draw the conclusion that ion exchange can change the adsorption propcrtics of molcculat sieve either way : positively or negatively. So in practice, WCcan choose different cations for ion exchange, to meet the different needs of manufacturing and experimental technology.
Acknowledgement The author is grateful for the help of Chen Ying-Shan, Wen-Ging and Hua Xing.
Wang
References ’ S A Stern and F S Diapolo. J Vuc Sci Twlrnol. 4, 347 (1967). P/z.,s, 28, I53 (I 980). ‘Sun Da-Ming, J Ph>..vE: Sc,i I~7strun7. 22, 159 (1989). ‘A Luches and A Zecca. J Cbc Sci Tdwd, 9, 1237 (1972). ‘C K Hersh, Moleculu~ Sierrs. Reinhold Publishing Corporation (1961). “Zhu Chong-Ye and Cm Fei-Xian et ~1. VW Sci Tcchol (China). 6(6),
’ M JIckcl and R Bartel, Esp Tdznol
6 (1986).