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Molecular sieve effect of chemically modified Na-A type zeolite and its molecular dynamic simulation
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V. All rights reserved.
2315
MOLECULAR SIEVE EFFECT OF CHEMICALLY MODIFIED Na-A TYPE ZEOLITE AND ITS MOLECULAR DYNAMIC SIMULATION JUN IZUMI*, AKINORI YASUTAKE*, NARIYUKI TOMONAGA*, NOBUKI OKA*, HIROMITSU OTA**, NOBUO AKUTSU**, SATOSHI UMEDA**, MOTOAKI TAJIMA** * Mitsubishi Heavy Industries, Ltd., Nagasaki R & D Center, 5-717-1, Fukahori-machi, Nagasaki 851-03, Japan ** Tokyo Electric Power Company, Energy and Environment R & D Center, Global Environment Dept., 4-1, Egasaki-cho, Tsurumi-ku, Yokohama 230, Japan SUMMARY In this study, the precise window shrinkage of Na-A type zeolite (Na-A) in the order of 0.1 A by calcination after rehydration, a partial K exchange and low temperature adsorption was evaluated[I],[2],[3]. It was understood that the precise window shrinkage of thermally and chemically modified Na-A was related to the behavior of Na located in the center of 8 oxygen members ring. Following the adsorption evaluation of oxygen selectivity based on an oxygen/nitrogen binary system and CO2 selectivity based on CO~/nitrogen binary system, a more precise relationship between the window shrinkage and the behavior of Na located at the center of 8 oxygen members ring was studied with an MAS-NMR, a single crystal X-ray diffraction (SCXD) and a molecular dynamic simulation (MD). 1. INTRODUCTION As the window diameter of Na-A is about 4 A, the adsorption rates of molecules which are smaller than the window diameter such as CO2 (3.2A) or oxygen (3.8 A), are faster and that of larger molecules such as nitrogen (4.2A) are slower. This phenomena is well known as the molecular sieve effect and it defines how Na located at the center of 8 oxygen members ring determine the window diameter. Furthermore, according to our previous study, it was confirmed that the window can be shrunk in the order of 0.1A by 1) calcination after rehydration, 2) a partial K exchange with Na, 3) low temperature adsorption, etc.[I],[2],[3]. With these modifications, it is possible to control the window diameter very precisely and also it is expected to prepare the high selective adsorbent as a result of this window shrinkage. In this study, after the molecular sieve effect was evaluated with the adsorption experiment of chemically and thermally modified Na-A based on CO2/nitrogen and oxygen/nitrogen binary systems, the behavior of Na at the 8 members ring was analyzed with the SCXD and the MAS-NMR and the mechanism of the molecular sieve effect with the abovementioned precise window shrinkage was studied with the crystal structure of both of thermally and chemically treated Na-A. In this study, the behavior of Na at the center of the 8 members ring was simulated with the MD (MXDORTO) produced by Kawamura. The simulation conditions were chosen in correspondence with calcination, partial K exchange, low temperature adsorption and the window shrinkage from result of the SCXD measurement. 2. EXPERIMENT 2.1 13reparationof samples (1) Na-A Preparation of the Na-A pellet is shown in the left branch of the left column in Fig.1. Na-A powder, pore-enricher and lubricant, kaolin as a binder are homogeneously mixed with a kneeder, and then formed into pellets of 1.6mm using an extruder. After drying, the green pellets are calcined at a high temperature (above 600~ In the calcination process, the poreenricher (a kind of cellulose) mixed with the binder is bumt out to keep a macropore for gas to flow in or out of. The area, from which the crystal water was desorbed, was remained as the cavity and it becomes adsorption active sites. These samples are further used as Na-A samples.
2316
(2) Na-A with calcination The preparation procedure of Na-A with calcination is shown in the right branch of the left column. As shown in Fig.l, this sample is prepared by re-calcining the prepared Na-A. Here, the calcination primarily for forming the Na-A pellet is first done at a temperature of approximately 600~ followed by the calcination at a higher temperatures in range of 720 to 760~ so to control the precise window shrinkage of Na-A. These samples are further used as Na-A with calcination. (3) Partially K-exchanged Na-A As the authors reported, the adsorption selectivity of Na-A can be controlled by partially exchanging Na ions with K ions. The preparation procedure of the samples is shown in the fight column of Fig.1. When Na-A powder is suspended in KCe aqueous solution (0.1 mol%), part of Na in Na-A is exchanged with K: Na12-(Ae Oz),2- (Si02),2 + nKC e ---, Na12-n 9 Kn- (Ae Oz)12 9 (8iO2)12 + NaC e
I Na-A Powder I ---[ Water I Na-A Slurry I J J INa--Klon I Exchange '--J KCL
I
Filtration Forming I
"~ Binder I I I
'l Binder I
Forming
J
[
Drying
I
I
!
! I Calcination ! I [ Cooling I
i
! N.-A I
I
D~ing 1
I
I
I Calcination I
I
I Cooling
I
I
Na-K-A
[
I
[
Recalcination I
Cooling I Na-A
With Calcination
Fig.1 Preparation of Thermally and The pellet forming is the same as that for Na-A in Chemically Modified Na- A (1). The calcination of the Na-K-A green pellet was made at higher temperatures in the range of 720 to 760~ so to control the precise window shrinkage.
2.2 Evaluations of Na-A, Na-A with calcination and Na-K-A (1) Measurement of adsorption capacity The oxygen capacity and the selectivity of each sample were evaluated with the small column (10 gram pellets loaded) test apparatus shown in Fig.2. The same evaluation method was also used for CO2. In Fig.2, valves V-I, V-2 and V-3 were opened and air flowed through the column at the specified adsorption pressure until the breakthrough of air. The column temperature was controlled by refrigeration. Oxygen and a small volume of nitrogen were adsorbed and unadsorbed nitrogen enriched gas flowed out. The outlet flow rate Gl(m e N/cycle) and the oxygen concentration Cl(vol.%) were measured. At the desorption step, only the valve V-3 was opened and the'adsorbed oxygen was recovered with a vacuum pump. The desorbed gas G~(m e N/cycle) and its oxygen concentration C2(vol.%) were also measured. The dead volume of the column GDV (me N/cycle) was measured and the oxygen concentration CDV (vol.%) was assumed to be Co and the capacity:of oxygen and-nitrogen, qO, qN, (m e N/g) at specific temperatures can be described as follows: q0 = (C2- G3 - CDV. GDV) / W qN = {(1 - C2)- C,a - (1 - CDV). GDV} / W
(1) (2)
(2) MAS-NMR measurement The analyzer used was ARX400 (Superconductive magnet 9.4 Tcsla coil) made by BRUKER. Since the high sensitive amplifier and the magic angle spinner arc usually used for solid samples, Na poses a problem of the side band and it had to be measured without the spinner.
2317
And also as zeolite adsorbs moisture in air, it had to be sealed into the sample tube Refrigerator inside a glove box filled with dry nitrogen. The experimental procedure is shown below. 1)For preparation of the sample, the Cold Box [ /k~l sample was a fully ground and then, PCV ! V ~i [--I I 250 sealed in a zirconia rotor. This work I ~ was done in the glow box. 2)Tuning and matching of the signal i oo. oo / / i detector. FL1 I Small Column I 3)The reference peak for chemical shift is 'V-3 ' measured with the standard sample. ( ~ 7.21ppm of 23Na NaC/? was used as the standard peak for the Na element. 4)The sample is placed into the magnet and set to the magic angle. 5)Measurement conditions were as follow; Inlet Gas Vacuum Rotatory
{}
the
observed
~equency
23Na"
105.847MHz, the single pulse program, the number of rotations 0Hz, the pulse
Cylinder
Pump
Mass Flow Meter
r--I II
'
!!
o,.
j
Oxygen Monitor
Oxygen
Flow Meter
Monitor
Fig.2 Schematic Diagram of Small Column
duration 1 tasec, the cycling time 0.5 see
Apparatus
and the counting number 1000 counts. (3) Single crystal X-ray diffraction Na-A single crystals of which the diameters were more than 801xm were prepared by Channel method. The large crystal was sampled to a glass capillary under the microscopic observation. When the crystal was calcined, the sample capillaz3' was installed at the electric furnace with the nitrogen flow at the temperature between 680-800~ If the actuated Na-A was recalcined for the window diameter control the humid air was supplied to the capillary and the sample capillary was recalcined. When the Na-A crystal was K partially exchanged, NaC ~ -KC e binary solution was injected to the capillary and Na-K-A was prepared and then calcined at the temperature between 680-800~ with the nitrogen flow. Each of these three sample crystals (Na-A, Na-A with calcination, K-Na-A) was set on the Table 1 MD Claculation Conditions rotatory table and the X-ray diffraction pattern Calculation Software Mxdorto was measured. These data were converted to the Numerical Integration Varlet Method atoms' positions" of each sample to determine the Calculation of Ewald Method structure. Electrostatic Interaction Potential
3.
MOLECULAR DYNAMIC SIMULATION Na-A exhibits a molecular sieve effect, showing nitrogen selectivity at higher temperatures and oxygen selectivity at lower temperatures in oxygen/nitrogen binary system. However, to date the mechanisms at atomic and molecular levels of gas separation in zeolite still remains to be unclarified and the detailed structural changes that cause this mechanisms in zeolite also have not yet been fully clarified. The molecular dynamics (MD) method was used to
Integral Time Interval At [fs] Step Number Ensemble Particle Number in Unit Cell 'Particle Number in Basic' Cell is Always Rxed as Shown on the Right According to 3 - D :Periodical Boundary ,Condition
Morse Type Two Body Center Force Potential 2 5000
NPT Constant O
384
Si
96
Al
96
Na
96
2318
analyze the dynamic behavior of zeolite. MXDORTO, which was produced by Kawamura and workable on the personal computer, was used in this study. The calculation conditions are listed in Table 1. 4. RESULTS AND DISCUSSION In oxygen/nitrogen binary system, Na-A showed nitrogen selectivity at room temperature and came to exhibit oxygen selectivity increase with decreasing temperature, and this trend was more prominent in Na-A with calcination and Na-K-A. The precise window shrinkage mechanism of thermally or chemically modified Na-A can be interpreted as follows: when oxygen as a relatively small molecular with a molecular size of 2.8 x 3.8 J~ and nitrogen as a relatively large molecule with a molecular size of 3.2 x 4.2 J~ are adsorbed to NaAhaving a window size of approximately 4j~, steric hindrance due to Na located at the 8 members ring of the window site had more influence on nitrogen as a larger molecule and less influentially on oxygen as a smaller molecule, causing the adsorption rate of oxygen to exceed that of nitrogen and thus Na-A to behave as a rate type adsorbent. Takaishi[4] proposes an explanation that Na in 8 members ring has weak bonding with A ~ at the window and is pushed out by oxygen and nitrogen during their adsorption at the room temperature, causing both oxygen and nitrogen to be adsorbed whereas the bonding force between A ~ in the 8 members ring increases with decreasing temperature, gradually reducing the adsorption of nitrogen, which causes oxygen selectivity to appear. This mechanism has been proposed for ordinary Na-A, but it offers no suggestion for improvement of oxygen selectivity of Na-A with calcination and Na-K-A as described above. Therefore, we attempted to analyze the window structure using SCXD to understand how crystal structures of Na-A and Na-K-A change with calcination. Furthermore, Na-A at the temperature of 450~ showed no significant change, compared with standard Na-A. The adsorption amount of oxygen and nitrogen in oxygen/nitrogen binary system are shown in Fig.4. The transition temperature from nitrogen selectivity to oxygen selectivity of standard Na-A is 100 ~
i
•
+,
O,
+-"
-"
i` P~
.+ ,.:, ,
+
'-~' ,.
z
809
"+',q
v
N.,.,...,.;,I 41b
~
41
O : 02 Z~ : N= [ ] : 02 O:N2 9 :02 A :~ ==:02 4 k : N2
Adsorption Adsorption Adsorption Adsorption Adsorption Adsorption Ad,~rption Adsorption
by by by by by by by by
N a - K - A Pellet N a - K - A Pellet N a - A Pellet N a - A Pellet Na-A Powder N a - A Powder N a - A Calcined Na-A Calcined
,=.
c~ <
60
.~,
Fig.3 a)Standard Na-A
g
mo Si mA(
40
i \\
9
\
z "o e-
~
20
~---~.,.
el 0
m Na mK
-
0 150
- 100
- 50
0
Temperature ('C)
Calcined at 720 ~ lh, 1Torr
Fig.3 b ) N a - A
Fig.4 02 and N2 Isobar of Na-A, N a - A - Calcined and N a - K - A
50
2319
-100~ whereas Na-A with calcination has a transition temperature of approximately -45~ and Na-K-A, a transition temperature of-30~ The SCXD data of Na-A calcined at 720 ~ for one hour in 1 Torr is shown in Fig.3. [6]Na at the 8 members-rhrgand Na at the 6 members ring have partially been lost. Possible cause of Na loss in SCXD are as follows; 1) Na at the window moves out of the zeolite crystal when calcined at a high temperature. 2) The long range order of Na at the window can no longer be retained and a presence probability of Na at the same position is below the detection limit of SCXD though Na still remains in crystal. If we adopt the hypothesis 1) that Na at the window site moves out of crystal, it would imply that the window size would increase while oxygen and nitrogen adsoiption amount reduces, this would contradict the experimental results that oxygen selectivity improves at 720~ without showing any reduction in oxygen adsorption. In this respect, if 2) Na at the window site still remains within the window provided that its long range order can be no longer retained, this hypothesis supports the oxygen selectivity increase at low temperatures without showing any reduction in oxygen adsorption. In this case of Na-K-A (7% of the Na was exchanged with K), Na in the 8 members ring and the 6 members ring sites has been lost. However, K which was substituted with Na due to the ion exchange was always stoichiometrically located in the 6 members ring. Regarding the selectivity during the ion exchange with K, Takaishi[4] explained that K is first exchanged with Na in the 8 members ring, thus shrinking the window, whereas K exchanged with Na in the 6 members ring takes place with a higher ion exchange ratio. As shown in Fig.4, the adsorption test results show that the transition temperature at which Na-KA (K exchange 7% and calcination temperature of 720'~) shifted from nitrogen selectivity to oxygen selectivity increased (-30~ According to the SCXD measurement, any K exchanged are located only in the 6 members ring and does not seem to contribute to the window shrinkage. As the authors thought that the Na behavior at the window played an important role in the appearance of oxygen selectivity, the Na behavior at the window was analyzed using MAS-NMR. Fig.5 shows the Na MAS-NMR measurements of Na-A with calcination. Veeman et al. assigned the peak appearing in the center near 0ppm related to Na in the 4 members ring or in the 8members ring, explaining that this can be one peak due to the quadra pole moment of Na in the asymmetric potential gradient in the 4 members ring or in the 8 members ring. While, the two peaks appearing --120ppm away have been assigned as Na in the 6 members ring, explaining that they canbe two split peaks due to the quadra pole moment of Na in the symmetrical potential gradient of the 6 members ring. Vccman ct al. also Calcination conducted In-Situ measurements at higher Temperature temperatures and showed that the peak intensity assigned to Na in thi~ 4 members ring and in the 8 8 0 0 " C x lh members ring decreased with increasing temperature and completely disappeared at 230 ~ However, this xlh process was said to be reversible and peaks at both shoulders re-appeared with decreasing temperature. Veeman et al. propose an explanation on this mechanism that Na in zeolite resides at its site at ~ ~ - . ~ ~ ~ x lh lower temperatures but comes to move easily in i i i I i 200 0 -- 200 ppm crystal with increasing temperature. For this reason, Na became homogeneous by inter-exchanging sites or potential energy, which eventually unified to one peak Fig.5 NMR Profiles of Na at Respective near Oppm. Calcination Temperatures
2320
The spectrum from Na in Na-A with calcination measured by the authors was Vei-y similar to the high temperature NMR data of Veeman et al., and Na on the shoulder disappears with the calcination temperatures increase and the spectra unified to one peak near 0ppm. The difference was that Veeman et al.[7] measured the loss of peaks_fxom the shoulder in their In-Situ measurements at high temperatures, which reversibly re-appeared with decreasing temperature whereas in the adsorbent of this study, peaks from the shoulder disappeared when the adsorbent treated at higher temperature was measured at the room temperature. Also, the spectral band of the peak in the center became smaller with Na-A at the higher temperatures. However, a significant difference from Na-A measurement by Veeman was that the temperature dependency of thermally and chemically modified Na-A in this study was irreversible. For this reason, there is strong evidence suggesting that the calcination at higher temperatures caused Na retained by Na-A to be dislocated from its regular crystal site both in the 6 members ring and in the 8 members ring, which in turn, caused an irregular arrangement of Na in the 8 members ring to provide the window shrinkage corresponding to the calcination temperature during the adsorption of nitrogen as a larger molecule, thus leading to the appearance of oxygen selectivity. The structural change of Na-A during its calcination can also be estimated from the molecular dynamics. According to the simulation of Jhon[8] et al. on the thermal decomposition process of NaA from 273K to 1,000K, it has been noted that dislocation of Na in the 8 members ring first takes place, then followed by deformation of the 8 members ring. The temperature dependence of the trajectory of Na in the 8 and 6 members ring based on the MD calculation are shown in Table 2 a), Na's mobility at the temperature between -30-800~ were evaluated with the mean square distance. (MSD) Na was very stable at -30~ and 25~ near 800~ individual atoms became active and it was suggested that the high temperature calcination to interfere adsorption. The trajectory of Na relating to the temperatures in the 8 members ring have been well interpreted and the MD simulation produced closely correlating results. Thus, the authors' simulation using the MD software of Kawamura et al. provided results similar to those of Jhon et al., and evaluation of the trajectory of Na at room temperature showed that motion of Na in the 8 members ring exhibits more active than that of Na in the 6 members ring. For the simulation of the oxygen appearance at the lower temperature, the MSD of the adsorbed oxygen and nitrogen at the temperature between 25 - -30~ were calculated. These results were shown in Table 2 b). At room temperature, the MSD of oxygen and nitrogen were almost the same. However, when the temperature decreased to -30~ only the MSD of nitrogen decreased by 50%. With the knowledge of the stability of Na in the 8 members ring at the lower temperature, it is considered that the Na becomes more stable to give a hindrance to adsorbed gasses and the larger molecule of Table 2 a) Mean Squared Displacement of Na in the 6 and 8 Members Ring [A =] rature('c) ~
Na Location
Na in t:hie8 membersring
-30
25
780
1.367
1.391
30.259
members ring
3.089
4.020
45.183
Na in the 4 members ring
9.564
9.187
57.495
Na in the 6
Table 2 b) Mean Squared Displacement of Oxygen and Nitrogen in Na-A Crystals ~t ~ ~ ~ ~ t u r e (
*(3) -30
0
25
Oxygen(Oz)
49.470
72.526
85.634
Nitrogen(Nz)
47.067
44.566
55.421
[A =]
2321
nitrogen are more interfered by Na in the 8 members ring than the smaller molecule of oxygen. As a result, the oxygen selectivity appeared as a nature of rate type adsorption. (In the case of NaA, at -110~ or higher, from the equilibrium point of view, the nitrogen adsorption amount is larger than that of oxygen at the constant partial pressure.) The precise window shrinkage in this study also affected the selectivity of CO2 adsorption and. as shown in Fig.6, the CO2 selectivity was improved by the calcination temperatures increase.
8 ~>
The Value of Na-X is 15.3
~_ -~
T145
7
s
'
690 Temperature ( ~ )
1 ~"
The Value of Na-X is 0.86
0.943
0.95
5. CONCLUSION
~
0.925
The results discussed above are summarized in Table 3 for the oxygen adsorbent and in Table 4 for COz adsorbent. From Table 3, jt is shown that Na-A exhibits
co o
o 0.9
I
650
I
670 690 Temperature (=C)
oxygen selectivity at-45~ or below but also it Fig.6 Small Column Test Results can exhibit oxygen selectivity even -45~ or upper N a - A Type Zeolite : Dependency on Heat by providing calcination. The K ion exchange also Treatment Temperature greatly enhanced the oxygen selectivity at all the temperatures, but below-45~ its adsorption rate became slow and this type of adsorbent would thus be unsuitable as an oxygen adsorbent below -45~ Table 3
O=, N2 Adsorption Behavior of Thermally and Chemically Modified Na-A mp.
Less Than
- 45"(3
Na-A
Strong Oxygen Selective
- 4 5 = C - 0~C Weak Nitrogen Selective
0"C- 30=C Strong Nitrogen Selective
Na- A With Calcination
Strong Oxygen Selective
Weak Oxygen Selective
Weak Nitrogen Selective
K-Na-A
Strong Oxygen Selective
Strong Oxygen Selective
Weak Oxygen Selective
Table 4
CO2 Adsorption Behavior of Thermally and Chemically Modified Na-A
~I t' ~e m~ ~ s o r b~e n t CO2/N2 Selectivity
Na- A
Na- X
Very High
High
CO2 Adsorption Amount
Small
Large
CO2 Adsorption Rate
Slow
Fast
In summary of the CO2 adsorption performance based on Table 4, Na-A has a particularly high C(h selectivity but its CCh adsorption amount and adsorption rate is smaller than that of Na-X type zeolite (Na-X). As Na-X is inferior to Na-A with respect to CO2 selectivity, it can be said in Na-X is a good CO2 adsorbent for the improvement of the CO2 recovery ratio and reduction of the recovery energy. We greatly appreciated Dr. Serf (University of Hawaii) to measure our Na-A samples and also Dr. Kim Yang (Pusan National University) to give us a good suggestion about the structure determination of chemically modified Na-A.
2322 REFERENCES [1] Izumi J., Shirakawa S., et al., Japan Patent, 55-147149, (1980) [2] Izumi J., Shirakawa S., et al., Japan Patent, 1511319, (1989) [3] lzumi J. et al., 64th CAIX3J Meeting No.2 AID, (1989) [4] Takaishi T., Koubutugaldcaishi vo1.17, No.3, (1985) [5] Barrer R.M., Hydrothermal Chemistry of Zeolites, Academic Press, New York (1982) [6] Serf K., Sun T., Master thesis of Hawaii U. (1992) [7] VeemanW.S., et al., J. Phys. Chem., vol.93, No.2, (1989) [8] Jhon S.M., et al., J. Phys. Chem., vol.93, No.13, (1989)
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V. All rights reserved.
2315
MOLECULAR SIEVE EFFECT OF CHEMICALLY MODIFIED Na-A TYPE ZEOLITE AND ITS MOLECULAR DYNAMIC SIMULATION JUN IZUMI*, AKINORI YASUTAKE*, NARIYUKI TOMONAGA*, NOBUKI OKA*, HIROMITSU OTA**, NOBUO AKUTSU**, SATOSHI UMEDA**, MOTOAKI TAJIMA** * Mitsubishi Heavy Industries, Ltd., Nagasaki R & D Center, 5-717-1, Fukahori-machi, Nagasaki 851-03, Japan ** Tokyo Electric Power Company, Energy and Environment R & D Center, Global Environment Dept., 4-1, Egasaki-cho, Tsurumi-ku, Yokohama 230, Japan SUMMARY In this study, the precise window shrinkage of Na-A type zeolite (Na-A) in the order of 0.1 A by calcination after rehydration, a partial K exchange and low temperature adsorption was evaluated[I],[2],[3]. It was understood that the precise window shrinkage of thermally and chemically modified Na-A was related to the behavior of Na located in the center of 8 oxygen members ring. Following the adsorption evaluation of oxygen selectivity based on an oxygen/nitrogen binary system and CO2 selectivity based on CO~/nitrogen binary system, a more precise relationship between the window shrinkage and the behavior of Na located at the center of 8 oxygen members ring was studied with an MAS-NMR, a single crystal X-ray diffraction (SCXD) and a molecular dynamic simulation (MD). 1. INTRODUCTION As the window diameter of Na-A is about 4 A, the adsorption rates of molecules which are smaller than the window diameter such as CO2 (3.2A) or oxygen (3.8 A), are faster and that of larger molecules such as nitrogen (4.2A) are slower. This phenomena is well known as the molecular sieve effect and it defines how Na located at the center of 8 oxygen members ring determine the window diameter. Furthermore, according to our previous study, it was confirmed that the window can be shrunk in the order of 0.1A by 1) calcination after rehydration, 2) a partial K exchange with Na, 3) low temperature adsorption, etc.[I],[2],[3]. With these modifications, it is possible to control the window diameter very precisely and also it is expected to prepare the high selective adsorbent as a result of this window shrinkage. In this study, after the molecular sieve effect was evaluated with the adsorption experiment of chemically and thermally modified Na-A based on CO2/nitrogen and oxygen/nitrogen binary systems, the behavior of Na at the 8 members ring was analyzed with the SCXD and the MAS-NMR and the mechanism of the molecular sieve effect with the abovementioned precise window shrinkage was studied with the crystal structure of both of thermally and chemically treated Na-A. In this study, the behavior of Na at the center of the 8 members ring was simulated with the MD (MXDORTO) produced by Kawamura. The simulation conditions were chosen in correspondence with calcination, partial K exchange, low temperature adsorption and the window shrinkage from result of the SCXD measurement. 2. EXPERIMENT 2.1 13reparationof samples (1) Na-A Preparation of the Na-A pellet is shown in the left branch of the left column in Fig.1. Na-A powder, pore-enricher and lubricant, kaolin as a binder are homogeneously mixed with a kneeder, and then formed into pellets of 1.6mm using an extruder. After drying, the green pellets are calcined at a high temperature (above 600~ In the calcination process, the poreenricher (a kind of cellulose) mixed with the binder is bumt out to keep a macropore for gas to flow in or out of. The area, from which the crystal water was desorbed, was remained as the cavity and it becomes adsorption active sites. These samples are further used as Na-A samples.
2316
(2) Na-A with calcination The preparation procedure of Na-A with calcination is shown in the right branch of the left column. As shown in Fig.l, this sample is prepared by re-calcining the prepared Na-A. Here, the calcination primarily for forming the Na-A pellet is first done at a temperature of approximately 600~ followed by the calcination at a higher temperatures in range of 720 to 760~ so to control the precise window shrinkage of Na-A. These samples are further used as Na-A with calcination. (3) Partially K-exchanged Na-A As the authors reported, the adsorption selectivity of Na-A can be controlled by partially exchanging Na ions with K ions. The preparation procedure of the samples is shown in the fight column of Fig.1. When Na-A powder is suspended in KCe aqueous solution (0.1 mol%), part of Na in Na-A is exchanged with K: Na12-(Ae Oz),2- (Si02),2 + nKC e ---, Na12-n 9 Kn- (Ae Oz)12 9 (8iO2)12 + NaC e
I Na-A Powder I ---[ Water I Na-A Slurry I J J INa--Klon I Exchange '--J KCL
I
Filtration Forming I
"~ Binder I I I
'l Binder I
Forming
J
[
Drying
I
I
!
! I Calcination ! I [ Cooling I
i
! N.-A I
I
D~ing 1
I
I
I Calcination I
I
I Cooling
I
I
Na-K-A
[
I
[
Recalcination I
Cooling I Na-A
With Calcination
Fig.1 Preparation of Thermally and The pellet forming is the same as that for Na-A in Chemically Modified Na- A (1). The calcination of the Na-K-A green pellet was made at higher temperatures in the range of 720 to 760~ so to control the precise window shrinkage.
2.2 Evaluations of Na-A, Na-A with calcination and Na-K-A (1) Measurement of adsorption capacity The oxygen capacity and the selectivity of each sample were evaluated with the small column (10 gram pellets loaded) test apparatus shown in Fig.2. The same evaluation method was also used for CO2. In Fig.2, valves V-I, V-2 and V-3 were opened and air flowed through the column at the specified adsorption pressure until the breakthrough of air. The column temperature was controlled by refrigeration. Oxygen and a small volume of nitrogen were adsorbed and unadsorbed nitrogen enriched gas flowed out. The outlet flow rate Gl(m e N/cycle) and the oxygen concentration Cl(vol.%) were measured. At the desorption step, only the valve V-3 was opened and the'adsorbed oxygen was recovered with a vacuum pump. The desorbed gas G~(m e N/cycle) and its oxygen concentration C2(vol.%) were also measured. The dead volume of the column GDV (me N/cycle) was measured and the oxygen concentration CDV (vol.%) was assumed to be Co and the capacity:of oxygen and-nitrogen, qO, qN, (m e N/g) at specific temperatures can be described as follows: q0 = (C2- G3 - CDV. GDV) / W qN = {(1 - C2)- C,a - (1 - CDV). GDV} / W
(1) (2)
(2) MAS-NMR measurement The analyzer used was ARX400 (Superconductive magnet 9.4 Tcsla coil) made by BRUKER. Since the high sensitive amplifier and the magic angle spinner arc usually used for solid samples, Na poses a problem of the side band and it had to be measured without the spinner.
2317
And also as zeolite adsorbs moisture in air, it had to be sealed into the sample tube Refrigerator inside a glove box filled with dry nitrogen. The experimental procedure is shown below. 1)For preparation of the sample, the Cold Box [ /k~l sample was a fully ground and then, PCV ! V ~i [--I I 250 sealed in a zirconia rotor. This work I ~ was done in the glow box. 2)Tuning and matching of the signal i oo. oo / / i detector. FL1 I Small Column I 3)The reference peak for chemical shift is 'V-3 ' measured with the standard sample. ( ~ 7.21ppm of 23Na NaC/? was used as the standard peak for the Na element. 4)The sample is placed into the magnet and set to the magic angle. 5)Measurement conditions were as follow; Inlet Gas Vacuum Rotatory
{}
the
observed
~equency
23Na"
105.847MHz, the single pulse program, the number of rotations 0Hz, the pulse
Cylinder
Pump
Mass Flow Meter
r--I II
'
!!
o,.
j
Oxygen Monitor
Oxygen
Flow Meter
Monitor
Fig.2 Schematic Diagram of Small Column
duration 1 tasec, the cycling time 0.5 see
Apparatus
and the counting number 1000 counts. (3) Single crystal X-ray diffraction Na-A single crystals of which the diameters were more than 801xm were prepared by Channel method. The large crystal was sampled to a glass capillary under the microscopic observation. When the crystal was calcined, the sample capillaz3' was installed at the electric furnace with the nitrogen flow at the temperature between 680-800~ If the actuated Na-A was recalcined for the window diameter control the humid air was supplied to the capillary and the sample capillary was recalcined. When the Na-A crystal was K partially exchanged, NaC ~ -KC e binary solution was injected to the capillary and Na-K-A was prepared and then calcined at the temperature between 680-800~ with the nitrogen flow. Each of these three sample crystals (Na-A, Na-A with calcination, K-Na-A) was set on the Table 1 MD Claculation Conditions rotatory table and the X-ray diffraction pattern Calculation Software Mxdorto was measured. These data were converted to the Numerical Integration Varlet Method atoms' positions" of each sample to determine the Calculation of Ewald Method structure. Electrostatic Interaction Potential
3.
MOLECULAR DYNAMIC SIMULATION Na-A exhibits a molecular sieve effect, showing nitrogen selectivity at higher temperatures and oxygen selectivity at lower temperatures in oxygen/nitrogen binary system. However, to date the mechanisms at atomic and molecular levels of gas separation in zeolite still remains to be unclarified and the detailed structural changes that cause this mechanisms in zeolite also have not yet been fully clarified. The molecular dynamics (MD) method was used to
Integral Time Interval At [fs] Step Number Ensemble Particle Number in Unit Cell 'Particle Number in Basic' Cell is Always Rxed as Shown on the Right According to 3 - D :Periodical Boundary ,Condition
Morse Type Two Body Center Force Potential 2 5000
NPT Constant O
384
Si
96
Al
96
Na
96
2318
analyze the dynamic behavior of zeolite. MXDORTO, which was produced by Kawamura and workable on the personal computer, was used in this study. The calculation conditions are listed in Table 1. 4. RESULTS AND DISCUSSION In oxygen/nitrogen binary system, Na-A showed nitrogen selectivity at room temperature and came to exhibit oxygen selectivity increase with decreasing temperature, and this trend was more prominent in Na-A with calcination and Na-K-A. The precise window shrinkage mechanism of thermally or chemically modified Na-A can be interpreted as follows: when oxygen as a relatively small molecular with a molecular size of 2.8 x 3.8 J~ and nitrogen as a relatively large molecule with a molecular size of 3.2 x 4.2 J~ are adsorbed to NaAhaving a window size of approximately 4j~, steric hindrance due to Na located at the 8 members ring of the window site had more influence on nitrogen as a larger molecule and less influentially on oxygen as a smaller molecule, causing the adsorption rate of oxygen to exceed that of nitrogen and thus Na-A to behave as a rate type adsorbent. Takaishi[4] proposes an explanation that Na in 8 members ring has weak bonding with A ~ at the window and is pushed out by oxygen and nitrogen during their adsorption at the room temperature, causing both oxygen and nitrogen to be adsorbed whereas the bonding force between A ~ in the 8 members ring increases with decreasing temperature, gradually reducing the adsorption of nitrogen, which causes oxygen selectivity to appear. This mechanism has been proposed for ordinary Na-A, but it offers no suggestion for improvement of oxygen selectivity of Na-A with calcination and Na-K-A as described above. Therefore, we attempted to analyze the window structure using SCXD to understand how crystal structures of Na-A and Na-K-A change with calcination. Furthermore, Na-A at the temperature of 450~ showed no significant change, compared with standard Na-A. The adsorption amount of oxygen and nitrogen in oxygen/nitrogen binary system are shown in Fig.4. The transition temperature from nitrogen selectivity to oxygen selectivity of standard Na-A is 100 ~
i
•
+,
O,
+-"
-"
i` P~
.+ ,.:, ,
+
'-~' ,.
z
809
"+',q
v
N.,.,...,.;,I 41b
~
41
O : 02 Z~ : N= [ ] : 02 O:N2 9 :02 A :~ ==:02 4 k : N2
Adsorption Adsorption Adsorption Adsorption Adsorption Adsorption Ad,~rption Adsorption
by by by by by by by by
N a - K - A Pellet N a - K - A Pellet N a - A Pellet N a - A Pellet Na-A Powder N a - A Powder N a - A Calcined Na-A Calcined
,=.
c~ <
60
.~,
Fig.3 a)Standard Na-A
g
mo Si mA(
40
i \\
9
\
z "o e-
~
20
~---~.,.
el 0
m Na mK
-
0 150
- 100
- 50
0
Temperature ('C)
Calcined at 720 ~ lh, 1Torr
Fig.3 b ) N a - A
Fig.4 02 and N2 Isobar of Na-A, N a - A - Calcined and N a - K - A
50
2319
-100~ whereas Na-A with calcination has a transition temperature of approximately -45~ and Na-K-A, a transition temperature of-30~ The SCXD data of Na-A calcined at 720 ~ for one hour in 1 Torr is shown in Fig.3. [6]Na at the 8 members-rhrgand Na at the 6 members ring have partially been lost. Possible cause of Na loss in SCXD are as follows; 1) Na at the window moves out of the zeolite crystal when calcined at a high temperature. 2) The long range order of Na at the window can no longer be retained and a presence probability of Na at the same position is below the detection limit of SCXD though Na still remains in crystal. If we adopt the hypothesis 1) that Na at the window site moves out of crystal, it would imply that the window size would increase while oxygen and nitrogen adsoiption amount reduces, this would contradict the experimental results that oxygen selectivity improves at 720~ without showing any reduction in oxygen adsorption. In this respect, if 2) Na at the window site still remains within the window provided that its long range order can be no longer retained, this hypothesis supports the oxygen selectivity increase at low temperatures without showing any reduction in oxygen adsorption. In this case of Na-K-A (7% of the Na was exchanged with K), Na in the 8 members ring and the 6 members ring sites has been lost. However, K which was substituted with Na due to the ion exchange was always stoichiometrically located in the 6 members ring. Regarding the selectivity during the ion exchange with K, Takaishi[4] explained that K is first exchanged with Na in the 8 members ring, thus shrinking the window, whereas K exchanged with Na in the 6 members ring takes place with a higher ion exchange ratio. As shown in Fig.4, the adsorption test results show that the transition temperature at which Na-KA (K exchange 7% and calcination temperature of 720'~) shifted from nitrogen selectivity to oxygen selectivity increased (-30~ According to the SCXD measurement, any K exchanged are located only in the 6 members ring and does not seem to contribute to the window shrinkage. As the authors thought that the Na behavior at the window played an important role in the appearance of oxygen selectivity, the Na behavior at the window was analyzed using MAS-NMR. Fig.5 shows the Na MAS-NMR measurements of Na-A with calcination. Veeman et al. assigned the peak appearing in the center near 0ppm related to Na in the 4 members ring or in the 8members ring, explaining that this can be one peak due to the quadra pole moment of Na in the asymmetric potential gradient in the 4 members ring or in the 8 members ring. While, the two peaks appearing --120ppm away have been assigned as Na in the 6 members ring, explaining that they canbe two split peaks due to the quadra pole moment of Na in the symmetrical potential gradient of the 6 members ring. Vccman ct al. also Calcination conducted In-Situ measurements at higher Temperature temperatures and showed that the peak intensity assigned to Na in thi~ 4 members ring and in the 8 8 0 0 " C x lh members ring decreased with increasing temperature and completely disappeared at 230 ~ However, this xlh process was said to be reversible and peaks at both shoulders re-appeared with decreasing temperature. Veeman et al. propose an explanation on this mechanism that Na in zeolite resides at its site at ~ ~ - . ~ ~ ~ x lh lower temperatures but comes to move easily in i i i I i 200 0 -- 200 ppm crystal with increasing temperature. For this reason, Na became homogeneous by inter-exchanging sites or potential energy, which eventually unified to one peak Fig.5 NMR Profiles of Na at Respective near Oppm. Calcination Temperatures
2320
The spectrum from Na in Na-A with calcination measured by the authors was Vei-y similar to the high temperature NMR data of Veeman et al., and Na on the shoulder disappears with the calcination temperatures increase and the spectra unified to one peak near 0ppm. The difference was that Veeman et al.[7] measured the loss of peaks_fxom the shoulder in their In-Situ measurements at high temperatures, which reversibly re-appeared with decreasing temperature whereas in the adsorbent of this study, peaks from the shoulder disappeared when the adsorbent treated at higher temperature was measured at the room temperature. Also, the spectral band of the peak in the center became smaller with Na-A at the higher temperatures. However, a significant difference from Na-A measurement by Veeman was that the temperature dependency of thermally and chemically modified Na-A in this study was irreversible. For this reason, there is strong evidence suggesting that the calcination at higher temperatures caused Na retained by Na-A to be dislocated from its regular crystal site both in the 6 members ring and in the 8 members ring, which in turn, caused an irregular arrangement of Na in the 8 members ring to provide the window shrinkage corresponding to the calcination temperature during the adsorption of nitrogen as a larger molecule, thus leading to the appearance of oxygen selectivity. The structural change of Na-A during its calcination can also be estimated from the molecular dynamics. According to the simulation of Jhon[8] et al. on the thermal decomposition process of NaA from 273K to 1,000K, it has been noted that dislocation of Na in the 8 members ring first takes place, then followed by deformation of the 8 members ring. The temperature dependence of the trajectory of Na in the 8 and 6 members ring based on the MD calculation are shown in Table 2 a), Na's mobility at the temperature between -30-800~ were evaluated with the mean square distance. (MSD) Na was very stable at -30~ and 25~ near 800~ individual atoms became active and it was suggested that the high temperature calcination to interfere adsorption. The trajectory of Na relating to the temperatures in the 8 members ring have been well interpreted and the MD simulation produced closely correlating results. Thus, the authors' simulation using the MD software of Kawamura et al. provided results similar to those of Jhon et al., and evaluation of the trajectory of Na at room temperature showed that motion of Na in the 8 members ring exhibits more active than that of Na in the 6 members ring. For the simulation of the oxygen appearance at the lower temperature, the MSD of the adsorbed oxygen and nitrogen at the temperature between 25 - -30~ were calculated. These results were shown in Table 2 b). At room temperature, the MSD of oxygen and nitrogen were almost the same. However, when the temperature decreased to -30~ only the MSD of nitrogen decreased by 50%. With the knowledge of the stability of Na in the 8 members ring at the lower temperature, it is considered that the Na becomes more stable to give a hindrance to adsorbed gasses and the larger molecule of Table 2 a) Mean Squared Displacement of Na in the 6 and 8 Members Ring [A =] rature('c) ~
Na Location
Na in t:hie8 membersring
-30
25
780
1.367
1.391
30.259
members ring
3.089
4.020
45.183
Na in the 4 members ring
9.564
9.187
57.495
Na in the 6
Table 2 b) Mean Squared Displacement of Oxygen and Nitrogen in Na-A Crystals ~t ~ ~ ~ ~ t u r e (
*(3) -30
0
25
Oxygen(Oz)
49.470
72.526
85.634
Nitrogen(Nz)
47.067
44.566
55.421
[A =]
2321
nitrogen are more interfered by Na in the 8 members ring than the smaller molecule of oxygen. As a result, the oxygen selectivity appeared as a nature of rate type adsorption. (In the case of NaA, at -110~ or higher, from the equilibrium point of view, the nitrogen adsorption amount is larger than that of oxygen at the constant partial pressure.) The precise window shrinkage in this study also affected the selectivity of CO2 adsorption and. as shown in Fig.6, the CO2 selectivity was improved by the calcination temperatures increase.
8 ~>
The Value of Na-X is 15.3
~_ -~
T145
7
s
'
690 Temperature ( ~ )
1 ~"
The Value of Na-X is 0.86
0.943
0.95
5. CONCLUSION
~
0.925
The results discussed above are summarized in Table 3 for the oxygen adsorbent and in Table 4 for COz adsorbent. From Table 3, jt is shown that Na-A exhibits
co o
o 0.9
I
650
I
670 690 Temperature (=C)
oxygen selectivity at-45~ or below but also it Fig.6 Small Column Test Results can exhibit oxygen selectivity even -45~ or upper N a - A Type Zeolite : Dependency on Heat by providing calcination. The K ion exchange also Treatment Temperature greatly enhanced the oxygen selectivity at all the temperatures, but below-45~ its adsorption rate became slow and this type of adsorbent would thus be unsuitable as an oxygen adsorbent below -45~ Table 3
O=, N2 Adsorption Behavior of Thermally and Chemically Modified Na-A mp.
Less Than
- 45"(3
Na-A
Strong Oxygen Selective
- 4 5 = C - 0~C Weak Nitrogen Selective
0"C- 30=C Strong Nitrogen Selective
Na- A With Calcination
Strong Oxygen Selective
Weak Oxygen Selective
Weak Nitrogen Selective
K-Na-A
Strong Oxygen Selective
Strong Oxygen Selective
Weak Oxygen Selective
Table 4
CO2 Adsorption Behavior of Thermally and Chemically Modified Na-A
~I t' ~e m~ ~ s o r b~e n t CO2/N2 Selectivity
Na- A
Na- X
Very High
High
CO2 Adsorption Amount
Small
Large
CO2 Adsorption Rate
Slow
Fast
In summary of the CO2 adsorption performance based on Table 4, Na-A has a particularly high C(h selectivity but its CCh adsorption amount and adsorption rate is smaller than that of Na-X type zeolite (Na-X). As Na-X is inferior to Na-A with respect to CO2 selectivity, it can be said in Na-X is a good CO2 adsorbent for the improvement of the CO2 recovery ratio and reduction of the recovery energy. We greatly appreciated Dr. Serf (University of Hawaii) to measure our Na-A samples and also Dr. Kim Yang (Pusan National University) to give us a good suggestion about the structure determination of chemically modified Na-A.
2322 REFERENCES [1] Izumi J., Shirakawa S., et al., Japan Patent, 55-147149, (1980) [2] Izumi J., Shirakawa S., et al., Japan Patent, 1511319, (1989) [3] lzumi J. et al., 64th CAIX3J Meeting No.2 AID, (1989) [4] Takaishi T., Koubutugaldcaishi vo1.17, No.3, (1985) [5] Barrer R.M., Hydrothermal Chemistry of Zeolites, Academic Press, New York (1982) [6] Serf K., Sun T., Master thesis of Hawaii U. (1992) [7] VeemanW.S., et al., J. Phys. Chem., vol.93, No.2, (1989) [8] Jhon S.M., et al., J. Phys. Chem., vol.93, No.13, (1989)