15N-NMR characterization and quantitative NMR determination of nitrogen adsorbed in MX zeolites

I. Kiricsi, G. Pfil-Borb61y, J.B. Nagy, H.G. Karge (Editors) Porous Materials in Environmentally Friendly Processes Studies in Surface Science and Catalysis, Vol. 125 1999 Elsevier Science B.V.

229

15N-NMRcharactedzztion and quantitative N M R determination o f nitrogen adsorbed in M X zeolites A. Fonseca~, B. Lledosb, P. Ptfllmribib, J. H g n i ~ s b and J. B.Nag~ l_aboratoirede ILM.N., Facult6s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium. b Air Liquide, Centre de Recherche Claude-Delorme, F-78350 Jouy-en-Josas, France. a

NaX and CaX zeolite samples were dehydrated under controlled conditions and 15N2 was adsorbed on the zeolites. The samples contained in Pyrex NMR inserts were sealed, and the water content was determined by quantitative IH-NM1L Afterwards, the gas and adsodxxt nitrogen phases were fully characterized by 15N-NMILThe partition coefficients and relaxation times (Ti and T2*)of the N2 phases were then studied as a fimction of temperam~. ISN-NMR measmements were performed in static and in magic angle spinning (MAS) conditions and it was found that the N2 partition coefficients are different in MAS and in static conditions. Two different types of adsorbed nitrogen molecules were also found on the zeolites. 1. INTRODUC3"ION MX zeolites are good adsoflmats for gases and volatile compounds. If left at open air, they adsorb up to approximately 25 wt% of water. The water in the zeolite pores hydrates the cations and hence the adsorption capacity tremendously decreases towards other compounds [ 1-3]. The dehydration of the MX zeolites is then necessary prior to any adsorption and it must be performed under mild conditions in order to keep the zeolite stmctt~ intact 2. E X P E R I M E N T A L

Zeolite samples of 60 mg were dehydrated in vacuum, at 623 K, in Pyrex NMR sealable inserts connected to a vacutan line equipped with a ttabomolecular pump (10"4Pa). A_Rerwar~, a given volume of ~SN2was adsorbed on the samples at 77 K and the samples were sealed at the same temperature. The SH- and ISN-NMR measurements were performed on a Bruker MSL 400 spectrometer, using a Bruker 7 mm variable t e m ~ probehead spinning at 3.8 kHz. Tetramethylsilane and nitromethane were used as extemal references for the IH and 15N chemical shifts (~5)in ppm, respectively. 99% 15N2 was purchased from ICON. The T I relaxation limes were measured by the inversion-recovery method in MAS and/or in static conditions. The recycling delay used was longer than five times TI. 3. RESULTS AND DISCUSSION The adsorption of gaseous molecules (such as nitrogen, oxygen, etc.) in the zeolite pores depends on the zeolite smaettae and on the nature of the zeolite cations, but also on other pamneters such as temlgrattm~, equilibrium pressure, and coadsodxxl molecules (such as water, oxygen, etc.).

230 The influence of the pmmlaeters affecting the adsorption of nitrogen on the zeolite cations, such as

coadsorbed molecules, temperature, equilibriumpressure and centrifugationforce (caused by the MAS), is investigated in the following paragraphs. 3.1.

Effect of coadsorbed water molecules on the nitrogen molecules adsorbed on the zeolites

In this contribution, the coadsorbed molecules were removed by evacuating the samples at 623 K under vacuum (10.4 Pa) for 18 hours prior to the nitrogen (15N2) adsorption. The lSN2 contents and equilibrium pmssmes of the sealed samples are given in Table 1. The water content of the samples (Table 1) were detem~ed using quantitative results of tH-NMR and assuming that the total proton NMR signal is due to water (0.22 wt% for NaX and 0.57 wt% for CaX). Table 1 Equilibrium pressures, number of adsoltxxt molecules per super cage and results from simulation of the 15N-~'MR~ of 15N2adsorbed on zeolites NaX and CaX at 293K Molec./Cage

N2

Static conditions

MAS conditions I

Zeolite

H20

N2

. . . . . . .

NaX

CaX

/

0.19

0.50

/

2.33

2.21

/

Pcq.

Type

8

(bar),

*

(ppm)

gas

-74.01

45

li

-74.17

li'

6.3

6.9

4.2

AH (Hz) ,

Integral (%)

8

All

Integral

(ppm) j (Hz) ,

(%)

13

-74.16

37

26

18

17

-74.52

13

25

-74.43

23

16

-74.57

8

20

tr

-74.78

25

54

-74.77

11

29

gas

-74.11

55

13

-74.11

50

16

li

-75.97

26

20

-76.25

18

19

li'

-76.24

22

18

-76.34

8

27

tr

-76.62

22

49

-76.56

13

37

gas

-73.99

64

100

-73.99

64

100

gas represents the N2 molecules of the non-adsorbed gas phase, li and li' represent the two nitrogen atoms of the same N2 molecule, adsortxxl on the cation through the nitrogen named li'. tr represents a N2 molecule adsodxxt on a single cation by a p bond or adsorbed between two cations in a symmetrical orientation. The coM~rbed water contents measm~ by IH-NMR are very low and close to the detection limits of the technique. Moreover, the "water content" of the sealed samples give several ~H-NMR lines, that are more characteristic of zeolite acid sites than of water itself [2-11]. In other words, the 0.19 water molecules per zeolite NaX cage represent 0.38 proton per cage stemming from: Si-OH + AI-OH + Si(OH)+-AI + 1-120.For the CaX zeolite, the proton content per cage is equal to 1.0 and "proton" represents: Si-OH + AI-OH + Si-(OH)+-AI + H20 + Ca(HCO3)2. The latter proton containing species (hydrogen carbonate) is formed by the reaction of the hydrated zeolite with cad~on dioxide when left at the open air prior to dehydration. The carbonate content of the hydrated CaX zeolite was also detected by t3C-NMR, but could not by quantified due to the low NMR sensitivity of the 13Cnucleus.

231 The Si/AI ratio of the NaX and CaX zeolites used is 1.24 and 1.23 (determined by 29Si-NMR), respectively, the number of cations per super cage is 10.75 for the NaX and 5.375 for the CaX. As the adsorption of both nitrogen and of water molecules takes place on the cations, they are in competition for the same adsorption sites (cations Na+ or Ca~ in this case). Nevertheless, even if all of the detected protons in the studied zeolites are due to water, only one Na + (Ca++) adsorption site is deactivated in 5 (2) zeolite cages. The ~ r b e d sp~ies, water in this case, can then be neglected in the later comiderations. It is interesting to point out that 12 and 6 wt% of water are enough to have one molecule of water per cation in the zeolites NaX and CaX studied, respectively. 3.2.

Effect of the MAS on the nitrogen mokeules adsorbed on the zeolites at 293K The ISN-NMR measurements were performed on the sealed samples in static and in magic angle spinning (MAS) conditions. The results - original (noisy spectra), individual simulated lines and total simulated spectra- are represented in Figure 1 and the NMR characteristics of. the samples are summarized in Table 1. As seen in Figure 1, the tSN-NMR spectra of the samples could be simulated using 4 lines for each spectrum. The attribution of the gas phase line (gas in Table 1) was done in agreement with the observation of the same signal for a sealed sample containing gas only. The presence of two ~SN-NMR lines of approximately equal intensities in the different spectra (li and li' in Table 1) led us to the conclusion that these lines could arise from the two different nitrogen atoms of the same N2 molecule adsorbed in a linear orientation. The remaining 15N-NMR line (tr in Table 1) was attributed to N2 molecules adsott~ on a single cation by a p bond or adsottxxt between two cations in a symmetrical orientation. In Table 1, it can be observed that the nitrogen atoms of the adsodxxt nitrogen molecules (li, li' and tr) are characterized by a low frequency shift compared to the gas phase (gas). Concerning the nitrogen adsort~ in a linear orientation, giving tlma two NMR lines (li and li') for the same N2 molecule, the attribution of the chemical shift of li' to the nitrogen atom closer to the cation was done considering that the stronger the adsorption, the larger the low frequency shift observed by tSN-NMR. Similar considerations lead to the conclusion that the nitrogen molecules adsort~ on the cations in a tr orientation are more stable because they experience a larger lSN-NMR chemical shift variation. The chemical shift d i f f ~ between the line of nitrogen in the gas phase and the other lines characteristic of adsorbed nitrogen [ 12-15] are very small for NaX zeolite and slightly larger for CaX. If a single line (li*), centered between the li and li' lines, is used to represent the nitrogen molecules adsorbed in a linear orientation, the static and MAS spectra (Figure 1) can be represented as :

NaX static : NaX MAS : CaX static : CaX MAS :

gas J gas - gasgas--

0.29 ppm J li* ~ 0.48 p p m - 0.39 ppm - - li* ~ 0.22 p p m - 1.98 p p m - - l i * - - 0 . 5 3 ppm ~ 2.19 p p m ~ li* m0.26 p p m ~

tr tr tr tr

The low chemical shift difference between gas and li* (0.29 or 0.39 ppm) for NaX means that there is a very low stnacture difference (stabilisafion) from gas to li*. The difference between li* and tr (0.48 or 0.22 ppm) for NaX is of the same magnitude as the one observed from gas to li*, meaning that tr is approximately twice as stable as li*. For the nitrogen adsorbed in the CaX zeolite, the difference between gas and li* is larger (1.98 or 2.19 ppm), meaning that nitrogen experiences a (six times) better stabilisation than when adsorbed on NaX. Concerning the difference between li* and tr (0.53 or 0.26 ppm) for CaX, it is of the same order of magnitude as for nitrogen adsorbed on NaX zeolite.

232

--)3

-44

PPM

36

-+7

-72 -')'3 -')4 -'15 -'76 -'F/ -'fi8 -'79-PPM

d)

L

-43

-44

-45

PPM

-76

.

.

-')7

.

.

.

.

-43

.

-'h

.

.

-45

PPM

-')6

-'77

_

-'78

Figure 1. 40.55 MHz static and MAS spectra of ]SN2 adsorbed on zeolites NaX and CaX at 293 K. a) NaX static; b) NaX MAS; c) CaX static; d) CaX MAS. From the observed chemical shift differences, interpreted as the image of directly related structural stabilisafiom, it can be concluded that : 9 Nitrogen adsorlxxt on Ca~ cations of X zeolite is six times more stable than if adsorbed on Na+ eatiom of the same zeolite. 9 Nitrogen adsorbed on NaX zeolite in a tr orientation is twice as stable as if absorbed in a li* orientation. At low temperature tr will be the preferred adsodxxt configuration on NaX. 9 Nitrogen adsorbed on CaX zeolite in a t r orientation is only 10 to 25% more stable than when absorbed in a li* orientation. At low temtmatute tr will be slightly the preferred adsorl~ configtaation on CaX. 9 ~ of the low stabilimtion of nitrogen molecules adsorbed on NaX zeolite, the centrifugation force will cause a strong desorption. 9 Due to the high stabilisation of nitrogen molecules adsorbed on CaX zeolite, the eentrifugafion force will cause only a low desorptiorL Note that MAS spinning at a high speed (3800 Hz) increases the integral of the ]SN-NMR gas line of the samples (see Table 1: from 13 to 26% for NaX and from 13 to 16% for CaX). Moreover, the observed increase of the lSN-NMR gas lines are in agreement with the conclusiom of the chemical shift consideratiom (a larger increase for the NaX gas line than for the CaX one). This confinm the hypothesis that spinning decreases the appment adsorption energy of the nitrogen molecules. This observation is in

233 agreement with the adsorlxxl N2 molecules behaving more liquid-like and, then being affected by the centrifugation force. The MAS conditions causes a small increase in the ISN-NMR chemical shift for the nitrogen molecules adsoflxxi on the cations in a linear orientation 0i*), equivalent to a lower apparent adsorption energy for these molecules (0.3 ppm for li and 0.1 ppm for li' in Table 1). The chemical shift of the gaseous molecules is of course not affected by the MAS conditions. The tr line is not affected either because the corresponding nitrogen molecules are strongly adsorbed on the cations. Effect of the equilibrium pressure on the nitrogen molecules adsorbed on the zeolites As already mentioned, for the adsodxxt nitrogen molecules an increase in the ISN-NMR chemical shift- becoming closer to the free gas value - suggests a lower adsorption enet~. In Figure 2, it can be observed that the ISN-NMR chemical shift of the nitrogen molecules adsorbed on the zeolites increases - becoming closer to the free gas value - as a ftmction of the equilibrium pressure. The aplxtrent adsorption energy (AH~ ~ ) of the nitrogen molecules being the difference between the energy level of the free gas molecules (E~ and that of the adsorbed ones (F_.a),its exothermic character decreases with ~ i n g the equilibrium pressure (P~.):

3.3.

AH~

= E,- Fa

and

~ = ffP~q.)

The chemical ~ values axis - ppm in Figure 2 - could then be l~placed by an energy values axis in arbitrary units, for the interpretation of the results. Note that the decrease in the chemical shit~ of the tr NMR line cannot be due to the fast exchange ~ the gas site and the coadsorption site, because in all cases both lines are detected showing the slow exchange between the sites. -73 @ . . . .

-74

" .

.

.

.

.

.

.

.

. - - - - - O

......_

-75

o

,-A

A__---

-76 -77

-78

NaX (~)l

1; 7

CaX (t0 Gas

-79

-80 0

1

2

3

4

5

6

7

Equilibrium pressure (Bar)

Figure 2. F_xluih'briumpressure effect on the ISN-NMR chemical shitt of N2 adsodxxt on zeolites NaX and CaX, in static NMR conditions at 293 IC The tr line was chosen to represent the nitrogen adsortxxt on the zeolites. The non-adsorbed nitrogen (gas in Figure 2) tSN-NMR dmnical shift was found to be indetmadent of the equilibrium pressure, in the pressme range studied. When adsorbed in NaX and in CaX zeolites, the nitrogen molecules give tSN-NMR lines 2 and 4 ppm lower than the value of the non-

234 adsort~ nitrogen molecules, respectively. It leads to the conclusion that CaX zeolite is a better adsorbent for nitrogen than NaX zeolite. Hence, when studying the nitrogen adsorption at a given equilibrium pressure for different zeolites, the ISN-NMR chemical shift can give an interesting qualitative idea of the adsorption energies. 3.4.

Effect of the temperature on the nitrogen molecules a d s o ~ on the zeolites The tSN-NMR ~ of the NaX and CaX zeolite were acquired at different temperatures (Figure 3) to study the equilibrium of the nitrogen molecules between the gas phase and the different zeolite adsorpfon sites. In Figure 3, the decrease in the line width of the I~I-NMR total ~ t m as a function of the tempemtme can be observed. In the same figure, it is difficult to distinguish the gas line when the cation is Na+ (Figure 3a) but it becomes evident when the cation is Ca++ (Figure 3b). In the latter figure, the temperature d ~ e n t increase ofthe gas line (at -74 ppm) is evident, mostly from 253 to 313 K.

A

113 133 153 173 193 213 233 253 273 295 313 -65

-70

-75 PPM

-80

-85

-65

-70

-75

-80

-85

PPM

Figta-e 3. Tempetaaa'e detmadent 40.55 MHz static s-pecan of ISN2adsod~ on zeolites NaX and CaX. a) NaX static (Peq = 6.3 bar); b) CaX static (Peq = 6.9 bar). Of very great interest is the apparently abnormal low frequency shift observed on the two series, on cooling from 313 to 273 K for NaX and from 313 to 253 K for CaX (Figure 3). Also apparently abnormal is the small line width decrease observed for the total adsorl~ nitrogen line on cooling from 313 to 273 K for NaX and from 313 to 253 K for CaX. In fact, the apparent low fieqtmacy shifts and line width decreases are due to the migration of the nitrogen molecules from li* sites to more

235 stable tr sites, as the temperature decreases. The low frequem-y shift corresponds to the 0.5 ppm separating the li* line from the tr line in static conditions (Table 1). Concerning the line width decrease (Figure 3), it is cattsed by the conversion of three NMR lines 0i + li' + tr) to a single one (tr). Of course, the narrowest line width (at 372 K for NaX and at 253 K for CaX) is not that of tr in Table 1 because the line width of the tr line has a normal temperature dependence, increasing as the temperattm~ decreases. Note that after the migration of most of the adsodxxt nitrogen molecules to the more stable tr adsorption sites, only the normal temperatta'e dependent line width and chemical shift variations are observed on cooling down to 113 K for NaX and to 153 K for CaX. As the T! and T2~measurements can give interesting informations on the mobility of the nitrogen molecules, these individual tSN-NMR characteristics of the gas and adsorbed nitrogen molecules in the NaX and CaX zeolite were meastn'ed at different temperatures, for the two sealed samples (Figure 4). The samples being sealed at a given equilibrium pressure at room tempemtu~, changing the tempemttre will affect the equilibrium pressures and nitrogen distribution between gas, li* and tr.

a)

b)

3 2.5

1.5

2.5

ppm) ------o--- TI (-73.5 ppm) -- "-~-- -T2 (-74.5 ppm) ~Gas (TI)

. ~'LK

"I:'l (-76 ppm) Tl (-73.5 ppm) - T2* (-76 ppm) Gas (T1)

1.5 & 0.5

0.5

,-a.

~a 0

0.003

I'

'

I

0.005 0.007 l/Temperature; (l/K)

..

0.009

0

0.003

I

I

0.005 0.007 l/Temperature; (l/K)

0.009

Figure 4. T e m ~ effect on the relaxation times Tl(l~q~l)and T2*(I~r) of N2 a d s o ~ on zeolites NaX and CaX. a) NaX static; b) CaX static. In Figure 4 the effect of the t e m ~ on the Ti of the adsorbed (li* + a') and gas nitrogen molecules can be observed. The tempemtm~ dependence for T2* of the adsorbed nitrogen molecules (li* + tr) is also represented in the same figure. The T! value characteristic of a gas only sample (Table 1) is also represented in Figures 4a and 4b. Concerning the line of the gas, it was possible to get its Ti values (9 ms) for the three highest temperatures in the case of CaX. Note that the gas only sample has a Ti value of 3.3 ms. The higher TI value for the gas line in the CaX zeolite compared to the gas only sample may be due to a lower efficiency of the main relaxation mechanism of gas molecules (spin-rotation) in the zeolite cages [ 14-15]. Much higher Ti values (150 to 240 ms) were observed for the -74.5 chemical shift value in the NaX spectra but, as seen in Figure 3, the contribution of the line of the gas to the ~ is very low at 253, 233 and 193 IC At h i ~ temperatures, it was also impossible to distinguish this line from the lines due to the adsorbed species 0i, li' and tr) during Tj measurements for NaX.

236 For the adsorbed nitrogen tSN-NMR lines (li, li' and tr), it was impossible to distinguishthem during Ti and T2* measurements. The Tl and T2* curves given in Figure 4a for-74.5 ppm and in Figure 4b for -76 ppm are characterize of the total adsorbed nitrogen 0i* + tr). The corresponding Tl value varies from 60 to 830 ms for NaX and from 80 to 400 ms for CaX. The presence of a maximum at 173 K for the Tl values of the adsorbed nitrogen in NaX (Figure 4a) is an evidence for the migration of the nitrogen molecules from the less stable adsorption sites 0i*) to more stable ones (tr). In fact, the presence of a maximum or a discontinuity in the Tl curves is characteristic of physical changes when a pure compound is studied. The increasing side of the T~ curve in Figure 4a are characteristic of the nitrogen molecules adsorbed one way (li* + tr) and the decreasing side is typical of the same molecules ~sorlxxt in another way (tr). Similar considerations can also be used to explain the increase in the Tl values, for the nitrogen adsorbed in the CaX zeolite (Figure 4b), followed by a plateau (from 213 to 173 K) and a second increasing step. It can also be the effect on TI of a physical change, again from (li* + tr) to tr. Concerning the T2 values, an abnormal small ~ is observed decreasing the temperature down to 273 K. It is due to the previously explained migration of the nitrogen molecules from the li* to the tr sites. ARerwards, a normal continuous decrease in the T2 value is observed as long as the temperature is decreased. 4. CONCLUSION Different 15N-NMRlines, characteristic of nitrogen molecules in distinct environments of zeolitic cations were observed and structure attributions were suggested. ISN-NMR evidences were found explaining the difference in stability for the nitrogen molecules adsorbed on the different sites. Nitrogen in the non-adsorbed gas phase was also identified and fully characterized by ISN-NMR. REFERENCKS

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