Basler, Wolf D.

Bein, Thomas

English

Open Access LMU ( Ludwig-Maximilians-Univ. München)

Communications to the Editor The Journal of Physical Chemistry, Vol. 83, No. 9, 7979 1233 z c U 3 0 0 z E W w L L  0 -1 z VI a Q COMMUNICATIONS TO THE EDITOR A Kinetics of Ammonia Penetrating into the Sodalite Units of Sodium Faujasites Studied by Nuclear Magnetic Resonance Sir: Faujasites and type A zeolites contain two different kinds of intracrystalline voids, Le., a system of large pores interconnected in three dimensions and small pores in the interior of the sodalite building units of the alumosilicate framework. 14s access from large pores into small pores is possible only through six-membered rings of 0.25-nm free aperture, only small molecules such as water or ammonia can penetrate into Ihe sodalite units. It has been found that water molecules in small pores can be detected and measured separately from those in large cavities by pulsed proton NMR, as they give rise to a fast decaying part (T, = 25 ps) in the free induction d e ~ a y . l - ~  By this method the kinetics of water molecules entering4v5 and leaving6 the sodalite units of faujasites has been determined as a function of temperature and activity. Some first experiments with ammonia showed essentially the same effects as in case of water.4 Consequently cor- responding NMR :studies have been performed with ammonia, too. The zeolites used were Na-X (Linde 13X of Union Carbide Corp., Si/A1 = 1.18) and Na-Y (self-made, Si/A1 = 2.36). Afteir outgassing (pressure less 1 mPa for 24 h a t  670 K) the salmples were loaded with purified ammonia from a zeolitic reservoir a t  273 K. After loading no fast decaying part has been observed in the free induction decay a t  a detection limit of 170. Then different parts of the same sample were stored a t  temperatures between 370 and 470 K and the intensity of the fast decaying part of the NMR signal has been measured as a function of storing time. In Figure 1 two typical NMR signals (magnetization vs. time) are shown for Na-Y, signals for ammonia in Na-X can be found in ref 4. As in case of water, the small, fast decaying part of the NMR signal is attributed to intact ammonia molecules in the sodalite units. 'The reasons are as follows: By Fourier transforming the continuous wave spectrum of rotating ammonia molecules, found by Deininger and Reimann for ammonia in the large pores of faujasites a t  77 an oscillating relaxation function (minimum after 35 ALS, maximum after 55 ps, decaying to 10% after 100 ps) is obtained. The fast decaying part of the signal shows well the predicted shape in case of Na-Y a t  295 K, as can be seen in Figure 1, upper curve. In Na-X the relaxation is slower at 295 K, but the predicted shape is found for 'r < 250 K, too. This agrees with the ob- servation that water molecules in the sodalite units of Na-X are more mobile than in those of Na-Y.g 2. The signal cannot be caused by protons by OH groups, as thlese protons do not have an oscillating re- laxation function. 3. The concentration of OH groups or structural defects, where ammonia could be bound strongly, is a t  least one order of magnitude smaller in the sodium faujasites used4 than the intensity of the signal (up to 1 NH, per cage). 4. Binding to OH groups or defects should take place immediately, whereas all zeolites did not show the fast 1. 0022-365417912083- 1233$0 1 .OO/O 1 -4 I 200 40 0 600 TIME /JJS I I 1 1 9  1 Figure 1. Proton NMR free induction decay of 77 mg of NH,/g of Na-Y, SVAI = 2.36, T = 295 K, sweep = 800 ~ s ;  upper curve, after 32 h at 430 K; lower curve, after 6 months at 295 K. TABLE I : Number of Ammonia Molecules Sorbed Per Sodalite Unit and Half-Time of Penetration as Studied by Proton NMRa zeolite Na-X Na-Y TIE mg of NHJg 58 118 172 50 77 P(",), Pa 15a 300 14000  295 fast decaying 9 10 6 10.5 10 moleculesof NH, 0.51 1.17 1.09 0.48 0.73 half-time t , , , ,  h >300  30 >300  370 30 2 <0.2 25 400 5 <0.2 7 430 <0.2 <0.2 <0.2 470 part, % per sodalite unit a Estimated from ref 10. decaying signal when freshly loaded a t  273 K. 5. In faujasites with no cations in the six-membered rings, Le., free access to the sodalite units, the signal has been observed immediately after 10ading.~ The intensity was a t  most 10% of the total signal. Therefore the function of attaining sorption equilibrium could not be determined unambiguously, it  seemed to be exponential. From a plot of the intensity vs. time the time tl to obtain half of the equilibrium intensity was taken w i d  errors up to 50%. All experimental data can be found in Table I. A t  fixed loading and activity a, an Arrhenius law for the rate and hence t I j z  can be assumed: t l p  = tda) exP(EA/R?? (1) As can be seen from Figure 2, an activation energy EA of 120 f 10 kJ/mol is found. This is definitely greater than the value of 90 f 10 kJ/mol found for water in sodium f a ~ j a s i t e s . ~  0 1979 American Ckemica4 Society Communications to the Editor 1234 The Journal of Physical Chemistry, Vol. 83, No. 9, 1979 1000, , 200 160 130 100 "c 0 58 A 118 mg NH3/g NaX x 172 W 22 2. L 2.6 Figure 2. Arrhenius plot of the half-time t,,, of ammonia penetrating into sodalite units of sodium faujasites: (0) 58, (A) 118, (X) 172 mg of NH3/g of outgassed Na-X, (0) 77 mg of NH3/g of Na-Y. Solid lines are plots of eq 1 and 2 with E, = 120 kJ/moi and T~ = s. 1 0 3 ~  / T i I NUX 0 N a y  50 100 150 rng NH3/g oufgossed zeoiife Figure 3. Equilibrium number of ammonia molecules per sodalite unit for different amounts of loading: (0) Na-X, (0) Na-Y. As in faujasites with no cations in the center of the six-membered rings even ammonia passed rapidly into the sodalite units (tljz less 10 min at  300 K),4 the observed activation energy should be connected with the removing of the sodium ion from the six-membered rings by in- teraction with the ammonia molecules sorbed in the large pores. The difference of the activation energy between water and ammonia agrees well with the difference of the energy of interaction of one of these molecules with sodium ions, calculated by Barrer and Bratt as 28 kJ/moLs By comparing the rate at different amounts of loading, the assumption of a first-order reaction with respect to the activity u = p / p o ,  where the vapor pressure of ammonia a t  295 K is taken for po,  i.e. t o ( U )  = To(1.l (2) is seen to be well fulfilled (solid lines in Figure 2 )  with  UT^ = 10-16 s. The extrapolation of the half-times agrees with the observation that a sample of 65 mg of NH,/g of NaX shows no fast decaying part in the signal (<I%)  after 2 years a t  ambient temperature, giving t I j z  > lo5 h (ex- trapolation gives lo7  h). The maximum intensity of the fast decaying part after sorption equilibrium has been reached yielded 1 ammonia molecule per sodalite unit, which is reached for loadings greater 100 mg of NH,/g (Figure 3). As in the case of water, we could not observe any dif- ference of the rate between Na-X and Na-Y (see Table I and Figures 1-3). This is to be expected since all six- membered rings are occupied by sodium cations in this Si/A1 ratio range. References and Notes (1) H. Pfeifer, Surface Sci., 52, 434 (1975). (2) J. S. Murday, R. L. Patterson, H. A.  Resing, J. K. Thompson, and N. H. Turner, J .  Phys. Chem., 79, 2674 (1975). (3) W. D. Basler, H. Lechert, and H. Kacirek, Ber. Bunsenges. Phys. Chem., 80, 451 (1976). (4) W. D. Basler, ACS Symp. Ser., No. 34, 291 (1976). (5) W. D. Basler, ACS Symp. Ser., No. 40, 335 (1977). (6) W. D. Basler, Ber. Bunsenges. Phys. Chem., 82, 1051 (1978). (7) D. Deininger and B. Reimann, Z. Phys. Chem. (Leipzig), 251, 53 (1 972). (8) R. M. Barrer and C. C. Bratt, J. Phys. Chem. Solids, 12, 130 (1959). (9) W. D. Basler, manuscript in preparation. (IO) 0. Grubner, P. Jiru, and M. Ralek, Molekularsiebe, VEB Deutscher Verlag der Wissenschaften, Berlin, 1968, p 96. Institute of Physical Chemistry of the University of Hamburg 2000 Hamburg 13, West Germany Wolf D. Basler" Thomas Bein Received November 22, 1978 I Determination of the Rate Constant of the Reaction CH + 0 - CHO' 4- e- at 295 K Publication costs assisted by the Belg~an "National Fonds voor Wetenschappelijk Onderzoek " Sir: As has been shown extensively in the l i terat~rel-~ the saturation current technique yields very useful information about chemi-ionization phenomena occurring in the oxi- dation process of hydrocarbons. Indeed, this method provides a direct measurement of the chemi-ionization reaction rate U+ which is related to the saturation current i, according to where V is the volume of the entire reaction zone and e the electronic charge. Since reaction 1 is in general CH + 0 ---* CHO' + e- (1) considered to be the primary chemi-ionization reaction in hydrocarbon oxidations, the rate of this reaction U+ is equal to hl[CH] [ O ] .  While the concentrations, the cor- responding saturation currents, and hence k 1  have been measured in a series of low pressure methane and ethylene flames4 a t  temperatures near 2000 K, no such data were available a t  room temperature. We now report the measurement of k l  in a fast flow reactor under the same experimental conditions as in which we have measured is.3 The cylindrical Pyrex reactor has an inner diameter of 16 mm, and a reacting mixture was investigated containing 3 mtorr of acetylene, 5.5 mtorr of atomic oxygen, and 20 mtorr of molecular oxygen with helium as carrier gas at  a total pressure of 2 torr. Concentration profiles of all species were measured by a molecular beam sampling 0022-3654/79/2083-1234$01 .OO/O C 1979 American Chemical Society 

p 96. Institute of Physical Chemistry of the University of Hamburg 2000 Hamburg 13, West Germany Wolf D. Basler&quot; Thomas Bein Received November 22,

Kinetics of ammonia penetrating into the sodalite units of sodium faujasites studied by nuclear magnetic resonance

Universität München: Elektronischen Publikationen

Communications to the Editor The Journal of Physical Chemistry, Vol. 83, No. 9, 7979 1233 
z 
c 
U 
3 
0 
0 
z 
E 
W 
w 
L L  
0 
-1 
z 
VI 
a 
Q 
COMMUNICATIONS TO THE EDITOR 
A Kinetics of Ammonia Penetrating into the Sodalite 
Units of Sodium Faujasites Studied by Nuclear 
Magnetic Resonance 
Sir: Faujasites and type A zeolites contain two different 
kinds of intracrystalline voids, Le., a system of large pores 
interconnected in three dimensions and small pores in the 
interior of the sodalite building units of the alumosilicate 
framework. 14s access from large pores into small pores 
is possible only through six-membered rings of 0.25-nm free 
aperture, only small molecules such as water or ammonia 
can penetrate into Ihe sodalite units. 
It has been found that water molecules in small pores 
can be detected and measured separately from those in 
large cavities by pulsed proton NMR, as they give rise to 
a fast decaying part (T, = 25 ps) in the free induction 
d e ~ a y . l - ~  By this method the kinetics of water molecules 
entering4v5 and leaving6 the sodalite units of faujasites has 
been determined as a function of temperature and activity. 
Some first experiments with ammonia showed essentially 
the same effects as in case of water.4 Consequently cor- 
responding NMR :studies have been performed with 
ammonia, too. 
The zeolites used were Na-X (Linde 13X of Union 
Carbide Corp., Si/A1 = 1.18) and Na-Y (self-made, Si/A1 
= 2.36). Afteir outgassing (pressure less 1 mPa for 24 h a t  
670 K) the salmples were loaded with purified ammonia 
from a zeolitic reservoir a t  273 K. After loading no fast 
decaying part has been observed in the free induction 
decay a t  a detection limit of 170. Then different parts of 
the same sample were stored a t  temperatures between 370 
and 470 K and the intensity of the fast decaying part of 
the NMR signal has been measured as a function of storing 
time. 
In Figure 1 two typical NMR signals (magnetization vs. 
time) are shown for Na-Y, signals for ammonia in Na-X 
can be found in ref 4. 
As in case of water, the small, fast decaying part of the 
NMR signal is attributed to intact ammonia molecules in 
the sodalite units. 'The reasons are as follows: 
By Fourier transforming the continuous wave 
spectrum of rotating ammonia molecules, found by 
Deininger and Reimann for ammonia in the large pores 
of faujasites a t  77 an oscillating relaxation function 
(minimum after 35 ALS, maximum after 55 ps, decaying to 
10% after 100 ps) is obtained. The fast decaying part of 
the signal shows well the predicted shape in case of Na-Y 
a t  295 K, as can be seen in Figure 1, upper curve. In Na-X 
the relaxation is slower at 295 K, but the predicted shape 
is found for 'r < 250 K, too. This agrees with the ob- 
servation that water molecules in the sodalite units of 
Na-X are more mobile than in those of Na-Y.g 
2. The signal cannot be caused by protons by OH 
groups, as thlese protons do not have an oscillating re- 
laxation function. 
3. The concentration of OH groups or structural defects, 
where ammonia could be bound strongly, is a t  least one 
order of magnitude smaller in the sodium faujasites used4 
than the intensity of the signal (up to 1 NH, per cage). 
4. Binding to OH groups or defects should take place 
immediately, whereas all zeolites did not show the fast 
1. 
0022-365417912083- 1233$0 1 .OO/O 
1 
-4 
I 
200 40 0 600 
TIME /JJS 
I I 1 
1 9  
1 
Figure 1. Proton NMR free induction decay of 77 mg of NH,/g of Na-Y, 
SVAI = 2.36, T = 295 K, sweep = 800 ~ s ;  upper curve, after 32 h 
at 430 K; lower curve, after 6 months at 295 K. 
TABLE I : Number of Ammonia Molecules Sorbed Per 
Sodalite Unit and Half-Time of Penetration as 
Studied by Proton NMRa 
zeolite Na-X Na-Y TIE 
mg of NHJg 58 118 172 50 77 
P(",), Pa 15a 300 14000  295 
fast decaying 9 10 6 10.5 10 
moleculesof NH, 0.51 1.17 1.09 0.48 0.73 
half-time t , , , ,  h >300  30 >300  370 
30 2 <0.2 25 400 
5 <0.2 7 430 
<0.2 <0.2 <0.2 470 
part, % 
per sodalite unit 
a Estimated from ref 10. 
decaying signal when freshly loaded a t  273 K. 
5. In faujasites with no cations in the six-membered 
rings, Le., free access to the sodalite units, the signal has 
been observed immediately after 10ading.~ The intensity 
was a t  most 10% of the total signal. Therefore the 
function of attaining sorption equilibrium could not be 
determined unambiguously, it  seemed to be exponential. 
From a plot of the intensity vs. time the time tl to obtain 
half of the equilibrium intensity was taken w i d  errors up 
to 50%. 
All experimental data can be found in Table I. 
A t  fixed loading and activity a, an Arrhenius law for the 
rate and hence t I j z  can be assumed: 
t l p  = tda) exP(EA/R?? (1) 
As can be seen from Figure 2, an activation energy EA of 
120 f 10 kJ/mol is found. This is definitely greater than 
the value of 90 f 10 kJ/mol found for water in sodium 
f a ~ j a s i t e s . ~  
0 1979 American Ckemica4 Society 
Communications to the Editor 1234 The Journal of Physical Chemistry, Vol. 83, No. 9, 1979 
1000, , 200 160 130 100 "c 
0 58 
A 118 mg NH3/g NaX 
x 172 
W 22 2. L 2.6 
Figure 2. Arrhenius plot of the half-time t,,, of ammonia penetrating 
into sodalite units of sodium faujasites: (0) 58, (A) 118, (X) 172 mg 
of NH3/g of outgassed Na-X, (0) 77 mg of NH3/g of Na-Y. Solid lines 
are plots of eq 1 and 2 with E, = 120 kJ/moi and T~ = s. 
1 0 3 ~  / T 
i I 
NUX 
0 N a y  
50 100 150 
rng NH3/g oufgossed zeoiife 
Figure 3. Equilibrium number of ammonia molecules per sodalite unit 
for different amounts of loading: (0) Na-X, (0) Na-Y. 
As in faujasites with no cations in the center of the 
six-membered rings even ammonia passed rapidly into the 
sodalite units (tljz less 10 min at  300 K),4 the observed 
activation energy should be connected with the removing 
of the sodium ion from the six-membered rings by in- 
teraction with the ammonia molecules sorbed in the large 
pores. The difference of the activation energy between 
water and ammonia agrees well with the difference of the 
energy of interaction of one of these molecules with sodium 
ions, calculated by Barrer and Bratt as 28 kJ/moLs 
By comparing the rate at different amounts of loading, 
the assumption of a first-order reaction with respect to the 
activity u = p / p o ,  where the vapor pressure of ammonia 
a t  295 K is taken for po,  i.e. 
t o ( U )  = To(1.l (2) 
is seen to be well fulfilled (solid lines in Figure 2 )  with  UT^ 
= 10-16 s. 
The extrapolation of the half-times agrees with the 
observation that a sample of 65 mg of NH,/g of NaX 
shows no fast decaying part in the signal (<I%)  after 2 
years a t  ambient temperature, giving t I j z  > lo5 h (ex- 
trapolation gives lo7  h). 
The maximum intensity of the fast decaying part after 
sorption equilibrium has been reached yielded 1 ammonia 
molecule per sodalite unit, which is reached for loadings 
greater 100 mg of NH,/g (Figure 3). 
As in the case of water, we could not observe any dif- 
ference of the rate between Na-X and Na-Y (see Table 
I and Figures 1-3). This is to be expected since all six- 
membered rings are occupied by sodium cations in this 
Si/A1 ratio range. 
References and Notes 
(1) H. Pfeifer, Surface Sci., 52, 434 (1975). 
(2) J. S. Murday, R. L. Patterson, H. A.  Resing, J. K. Thompson, and 
N. H. Turner, J .  Phys. Chem., 79, 2674 (1975). 
(3) W. D. Basler, H. Lechert, and H. Kacirek, Ber. Bunsenges. Phys. 
Chem., 80, 451 (1976). 
(4) W. D. Basler, ACS Symp. Ser., No. 34, 291 (1976). 
(5) W. D. Basler, ACS Symp. Ser., No. 40, 335 (1977). 
(6) W. D. Basler, Ber. Bunsenges. Phys. Chem., 82, 1051 (1978). 
(7) D. Deininger and B. Reimann, Z. Phys. Chem. (Leipzig), 251, 53 
(1 972). 
(8) R. M. Barrer and C. C. Bratt, J. Phys. Chem. Solids, 12, 130 (1959). 
(9) W. D. Basler, manuscript in preparation. 
(IO) 0. Grubner, P. Jiru, and M. Ralek, Molekularsiebe, VEB Deutscher 
Verlag der Wissenschaften, Berlin, 1968, p 96. 
Institute of Physical Chemistry 
of the University of Hamburg 
2000 Hamburg 13, West Germany 
Wolf D. Basler" 
Thomas Bein 
Received November 22, 1978 I 
Determination of the Rate Constant of the Reaction 
CH + 0 - CHO' 4- e- at 295 K 
Publication costs assisted by the Belg~an "National Fonds voor 
Wetenschappelijk Onderzoek " 
Sir: As has been shown extensively in the l i terat~rel-~ the 
saturation current technique yields very useful information 
about chemi-ionization phenomena occurring in the oxi- 
dation process of hydrocarbons. Indeed, this method 
provides a direct measurement of the chemi-ionization 
reaction rate U+ which is related to the saturation current 
i, according to 
where V is the volume of the entire reaction zone and e 
the electronic charge. Since reaction 1 is in general 
CH + 0 ---* CHO' + e- (1) 
considered to be the primary chemi-ionization reaction in 
hydrocarbon oxidations, the rate of this reaction U+ is 
equal to hl[CH] [ O ] .  While the concentrations, the cor- 
responding saturation currents, and hence k 1  have been 
measured in a series of low pressure methane and ethylene 
flames4 a t  temperatures near 2000 K, no such data were 
available a t  room temperature. We now report the 
measurement of k l  in a fast flow reactor under the same 
experimental conditions as in which we have measured is.3 
The cylindrical Pyrex reactor has an inner diameter of 16 
mm, and a reacting mixture was investigated containing 
3 mtorr of acetylene, 5.5 mtorr of atomic oxygen, and 20 
mtorr of molecular oxygen with helium as carrier gas at  
a total pressure of 2 torr. Concentration profiles of all 
species were measured by a molecular beam sampling 
0022-3654/79/2083-1234$01 .OO/O C 1979 American Chemical Society 


Wolf D. Basler

Thomas. Bein

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Kinetics of ammonia penetrating into the sodalite units of sodium faujasites studied by nuclear magnetic resonance

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