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Effect of Rare Earth Loading in Y-Zeolite on its Dealumination during Thermal Treatment
Effect of Rare Earth Loading during Thermal Treatment
In
Y-Zeolite on its Dealumination
l.W. Roe1ofsen* H. Mathies*, R.L. de Groot**, P.C.M. van Woerkom** and H. Angad Gaur** *Akzo-Chemie, Ketjen Catalysts, Research Centre Amsterdam P.O. Box 15, 1000 AA Amsterdam, The Netherlands. ~*Akzo Research, Corporate Research Department Arnhem P.O. Box 60, 6800 AB Arnhem, The Netherlands. The dealumination of the framework of Y-zeo1ites exchanged to various degrees with rare earth and ammonium ions and treated thermally in various ways was studied. The Si0 2/A1203-ratio of the framework was determined from the lattice constant, a lattice vibration and by 29 Si MASNMR. Upon calcination no or only moderate dea1umination takes place. Steaming causes an appreciable dea1umination of the zeolite framework. This is concluded from the results based on lattice vibrations and 29 Si MASNMR data. Also according to these techniques the degree of dea1umination does not depend strongly upon the RE 20 (at least up to 15% RE 70 ) . 3-content 3 This is in contrast Eo the information based on the laEt1ce constant, which suggests that dea1umination of the zeolite framework is strongly inhibited by the presence of rare earth ions. INTRODUCTION Rare earth ions (RE) play an important role "in the modificaHon of Y-zeolites. Scherzer et a1.(1) have reported on the influence of La3+-exchange of a NaY-zeolite on the dea1umination of the zeolite framework upon thermal treatment. Their conclusions are based on X-ray diffraction and infrared data. The work presented here compares the information on dea1umination obtained from lattice constants, infrared spectroscopy and 29 Si MASNMR on zeolites exchanged with rare earthand/or ammonium ions after calcination or steaming under various conditions. The Si0 2/AI of the zeolite framework was determined from the lattice 3-ratio constanE by20 using the Breck-F1anigen relation (2), from the lattice vibration by applying the F1anigen, Khatami,Szymansky relation (3) and from 29 Si-MASNMR assuming that the Lowenstein rule holds for these samples. EXPERIMENTAL 1. Sample preparation A NaY-zeolite (Si02/AI?03-ratio of 5.0) was exchanged with a RECl 3-so1ution by stirring the suspension for one hour at 60·C and pH 5.0-5.5. The RE-m1xture consisted of the following elements (expressed as oxides): 55,3 % (m/m) La20 3; 22,8 % /m/m) Nd 13,2 % (m/m)Ce0 8,2 % (m/m) Pr and 0.5 % (m/m)Sm 0 • 2; 2 3 203 203; After filtration and washing the samples were calcined at 450·C for one hour, followed by an NH at 80·C and pH 6.0 using an (NH4)2S04-solution. 4+-exchange After filtration and washing the samples were dried at 120·C. The sample which does not contain RE was exchanged with NH + twice with in 4 the highest between a calcination at 250·C for one hour. The sample hav1ng RE-content was made by exchanging the NaY-zeolite twice with RE3+ with in between a calcination at 450·C for one hour. 337
338 (IM-3-l) Z. Thermal treatment The samples were calcined in a shallow bed configuration (layer thickness 3 mrn). The sample were steamed by passing steam through the sample starting at ZOO°C and heating it up to the appropriate temperature. All samples have been investigated in hydrated form. 3. X-ray diffraction The X-ray diffraction data were obtained using a Philips PW 1710 diffractometer (Cu KDC-radiation). The crystallinity of the starting zeolites was determined relative to a standard NaY-zeolite by comparing the integrated intensities of the reflections in the range Z8 = 9-35 0 and correcting for the background intensity. Also a correction for the mass adsorption due to the presence of rare earth was applied. The lattice constant was determined according to the ASTM-procedure (D 394Z-80). The accuracy is ± O.OOZ nm. 4. Infrared spectroscopy The infrared spectra were obtained from zeolite/KBr"pellets(l% m/m zeolite)usmg a Digilab FTS 15 E spectrometer equiped with a Ge/KBr beamsplitter and a TGS detector. Mostly 500 scans per spectrum ~ire recorded with a 4 cm-1 resolution. The accuracy of the peak maximum is 1 cm. Hydrated samples have been investigated because then disturbing effects due to cationframework interaction may be neglected for at least the peak frequency of the framework adsorption bands (3,4). In the framework region (lZ00-400 cm- 1) of the zeolite spectra, several absorption bands are sensitive to the SiO Z03-ratio. In this work the symmetric stretch vibration at ± 790 cm- 1 has beenZ/AI used to calculate the Si0 2/AI 3-ratio of the thermally treated samples. The choice of Z0 this band arises from tfie fact that the other structure sensitive bands, such as the double ring vibration at 580 em-I, are much more sensitive to type and amounts of cations in the zeolite, the crystallinity and the water content. AccordinR to the literature (3) the peak frequency of the 790 cm-1 band is linear~ proportional to the Al/(Al + Si) ratio. Hence using suitable reference samples (with Si0 known from synthesis) a calibration line can be produced 20 3-ratio 2/A1 Ehe to determ1ne SiO of the thermally treated zeolite samples. Z/AI Z03-ratio 5. Z9Si - Magic Angle Spinning Nuclear Magnetic Resonance The 39.74Z MHzZ9SiMASNMR spectra were obtained using a Bruker CXP ZOO spectrometer. The sp1nn1ng rate was apprOXimately 2500 KHz. For direct experiments 3000-7000 FID's (free induction decays) were accumulated before fourier transformation.90° pulses (duration 6/us ) were applied. The acquisition time was 33 ms, the repetition rate was 8 ms. In the CP-experiments a cross polarization time of 5 ms was used. The duration of the 90 0 pulses was 8 ,us and the pulse repetition rate was 2s. A total of 1200 FID's was accumulated ih each experiment. All experiments were performed at ambient temperature. Lorentz to Gausian transformation was applied to the FID's for resolution enhancement of the spectra. The error on the SiO is estimated to be within 10%. Z/AI Z03-ratio RESULTS AND DISCUSSION 1. Analysis of the fresh samples The composition, crystallinity and lattice cOnstant (ao) of the samples used in this study are given in table 1. For all the RE-containing samples in this table holds that the sum of the equivalents of NA+, NH and RE3+ is higher than necessary to compensate the frame4+ to the formation of RE(OH)+3-n- species upon work charge. This is attributed calcination (5). n
J.W. Roelofsen
339
Table 1. Composition, crystallinitya)and Lat t rce constant of the fresh samples. Zeolite no
1 2 3 4 5
RE
r0 3
%m m
°8.9 9.1
14.2 22.8
a Na20 a 0.20 0.26 0.15 0.12 0.12
b RE
203 c(NH4)20 b c
°0.14 0.13
0.21 0.38
A1
203
0.79 0.59 0.60 0.44
CrystalP Si0 2 linity p %
5.04 5.07 5.07 5.08 5.18
°
115 113 123 113 111
ao nm 2.469 2.473 2.473 2.474 2.472
a. Crystallinity relative to a standard NaY-zeolite. 2. Analysis of the.thermally treated samples 2.1. X-ray diffraction data. The X-ray diffraction data obtained on the thermally treated zeolite samples is given in table 2. Table 2. Crystallinity a)and lattice constant(ao) of calcined and steamed samples Zeolite no
1 2 3 4 5
RE
Calcination at 750°C 3 hours ao crystal- ao linity % nm linity % nm
1 hour r0 3 crystal-
%m m
8.9 °9.1
14.2 22.8
2 93 59 93 105
2.466 78 2.456 53 2.471 85 2.474 109
2.467 2.454 2.471 2.471
Steaming at 750°C 1 hour 3 hours crystalao crystallinity % nm linity % 68 85 78 86 106
2.440 2.453 2.450 2.454 2.466
59 45 78 86 73
ao nm 2.437 2.446 2.448 2.455 2.462
a. Crystallinity is relative to the starting material Steaming of sample 4 at 750°C for 6! hours resulted in a material being 72% crystalline and having a lattice constant of 2.452 nm. The 'data in this table show that the presence of RE influences the crystallinity after calcination and steaming considerably. Also,sample 2 and 3 having the same RE but differing in Na differ appreciable with 203-content 20-content respect to stability against calcination and steaming. As shown in figure 1 the lattice constant of the steamed samples is linearly related to the RE-content. No such a relation was found for the lattice constants of the calcined samples. The Si0 of the framework calculated from the lattice constant is given 2/A1 203-ratio in table 5. 2.2. Infrared spectroscopv The calibration line determined from the spectra of the three samples with known Si0 (5.5,6.0 and 6.5 respectively) is given in figure 2. This line 2/A1203-ratio corresponds fairly well with the data given by Lohse et al (6). The reliability of this line for high SiO IA1 -ratio has been checked by applying it on a SiC1 4 dealuminated Y-zeo!ite.20 lccording to chemical analysis the Si0 of this sample is 20.8. The wavenumber of the symmetric stretch 2/A1 203-ratio vibration was 832 cm-1, which according to the calibration line corresponds with a Si0 of 19.2. 2/A1 203-ratio
340 (IM-3-1)
840 820 800
760 2.430
740
2.420
720 0.1
0.2
0.3
0.4
0.1
b ....
Fig .1. Lattice constant ao plotted versus RE-content (b) X steam 750·C o, steam 750·C 3 hrs. 1 hr • Calcined 750·C 3 hrs.
°
0.2 AI/IAI.SiI
0.3
0.4
Fig.Z. The calibration line of the symmetric stretch vibration as function of Al(Al + Si)-ratio .
A typical example of the effect of calcination or steaming on the infrared spectra of a zeolite is shown in figure 3 for sample 3. Table 3 presents the peak frequencies of the 790 cm-1 band of all samples after thermal tr:atment. Also the SiO determined from the calibration line Z/Al Z03-ratio has been gJ.ven. Table 3. Peakfrequency ( v) and SiO of Z/Al Z03-ratio calcined and steamed samples Zeolite no 1
Z
3 4
5
RE 03 Calcination at 750·C %m~m 1 hour (c~-l) Si0zlAlZ03(c~-1) 8 .9 °9.1
14.Z ZZ.8
788 795 787 788 783
5.Z 6.0 5.1 5.Z 4.7
788 794 787 788 784
Steaming at 750·C 1 hour 3 hours SiOz/AlZ03(C~-1) SiOZ/Alz03
3 hours SiOz/AlZ03(C~-1) 5.Z 5.9 5.Z 5.Z 4.8
8Z5 818 8Z4 818 804
13.8 10.9 13.3 10.9 7.4
tV
831 818 8Z5 818 800
17.5 10.9 13.8 10.9 6.7
After steaming at 750·C for 6! hours sample 4 shows the peak frequency at 8Z3 cm-1 which corresponds with a SiO ratio of 1Z.8. Z/Al Z03 It can be concluded from these data that calcination does not change significantly the SiOZ/Al Z03-ratio except for sample Z (with a somewhat higher ratio) and sample 5 (with a somewhat lower ratio). Steaming of the samples leads to a distinct increase of the SiO which depends on the RE-content of the samples. Z/Al Z03-ratio A higher RE-content has an inhibiting effect on the increase of the SiO Z03ratio. Again sample Z exhibits some deviating behavior, the RE-content Z/Al J.S nearly equal to, but the NaZO-content is higher than in sample 3. The SiO IA1203-ratio however is much lower. Increasing the steaming time increases the Si0 2TAI z0 3-ratio only for sample 1, which does not contain RE. Sample 4 shows a small increase of the SiOZ/AlZ03~ratio
when steaming is prolonged up to 6~ hours.
J.W. Roe1ofsen
341
Sample 5 shows a decrease of the Si0 upon prolonged steaming, but for 2/A1203-ratio this sample the determination of the peaK position was inaccurate. 7B.4
-t
90.Bt<::~~~~~_~~_~~~_
r-~-~~-~~..---~~-~~-;
dO
dO
o
1400
1200
1000
BOO
600
1200
400
Wavenumbers
1000
51.4.'1<-~~--'--'--~~-~~~--t
a.
BOO
600
400
Wavenumbers
c.
dO
1200
1000
800
600
450
Wavenumbers
b. Fig.3. Lattice vibration spectrum of sample 3. (a) as such (b) Calcined 1 hour (c) steamed 1 hour at 750°C at 750°C 2.3. 29Si Magic Angle Spinning Nuclear Magnetic Resonance The results of these experiments are given in table 4. Table 4. Si0 2/A1201-ratio of zeolite framework before and after thermal treatment as determined by 29Si MASNMR. Zeolite no 1 2 3 4 5
RE
203
°8.9 9.1
14.2 22.8
Starting ~.91)
5.2 5.7
Calcination at 75°C 1 hour 3 hours 1)
6.4 7.8 6.2 5.6
Steaming at 750°C 1 hour 3 hour
2) 2) 7.2 6.8 _2)
12.6 10.8 12.2 12.6 8.4
11.~2) 12.8 11. ~2)
1) Not a well resolved spectrum.2) Not measured. Sample 4 steamed at 750°C for 6! hours showed a Si0
2!A1203-ratio
°
of 12.8
Figure 4 shows that the influence of rare earth ions on the spectra of the zeolites is considerable. By comparing the spectra of zeolites with and 22.8% RE 3 (sample 1 and sample 5 respectively) it can be judged that the increase in20 line width is approximately a factor five. Comparison of the spectra of La3+- and Ce3+exchanged zeolites (fig.4) indicates that the paramagnetism of the Ce3+ causes this line broadening.
342 (IM-3-l)
Idl i_~
,-,~O
i
i
-,'20
i
Ie)
'-~o
'
Fig. 4. Z9Si MASNMR spectra of a) sample 1 as such (%RE b) sample 5 as such ZZ.8% RE Z03) Z03 c) sample with 15% Ce d) sample with 15% La Z03 Z03 e) sample 1 steamed 3 hr at 600°C. f) sample 1 steamed 3 hr 600°C with cross polarization. Mere calcination at 750°C hardly (or to a small extent) dealuminates the zeolites. Steaming at 750°C however. results in appreciable dealumination, which is slightly inhibited by the presence of rare earth ions. Steaming of sample 4 between about 1 and 6~ hours has within experimental error no influence on the dealumination process. Table Z shows that the crystallinity of the thermally treated samples may decrease considerably. The resulting material can be regarded as more or less amorphous. Z9Si MASNMR spectra of amorphous silica-alumina show a broad featureless resonance peak. By applying resolution enhancement techniques mainly the resonance of crystalline material is observed. so the SiO 1 Al of only this fraction is obtained. Throughout resolution enhancement Z03-ratioLorentz-Gaussian transformation has been used to resolve structure viz. techniques Si(nAl) sites. The question is whether this influences the intensities of the resonances and therefore the SiO It can be shown that this does not occur. This is also conf~rmed by Z/Al the Z03-ratio. fact that resolution enhancement and deconvolution techniques give similar results when applied on well or partly resolved spectra. In the case of severe overlapping resolution enhancement gives more reliable results. The SiO of the zeolite is influenced by the Z/Al Z03-ratio presence of Si(OH)-and Si(OH~-sites 1n the framework. As was shown by CP-MAS experiments the resonances ot these groups are atthe position of the Si(l Al)-andSi (3 AI) resonances (fig.4). The intensity of these peaks is difficult to calculate in relation to the intensities of the MAS-spectra. So the quantitative contribution of the Si(OH)Z-group to the Si0 2/Al cannot readily be obtained. However. Z03-ratio it can be argued that owing to the presence of these groups in the MAS spectra,the calculated" SiOZI Al are minimum values. Z03-ratio
J.W. Roelofsen
343
Comparison of the Si0 2/AI Z03-ratio of the framework after thermal treatment as determined from lattice constant, lattice vibration and by Z9Si MASNMR. From the lattice constant information on the SiO of the zeolite Z/AIrelation(Z). Z03-ratio framework can be obtained by using the Breck-Flanigen The values for the lattice constant(ao) of the thermally treated samples are given in table ~Z. By using the Breck-Flanigen relation also outside the limits of its determination the SiO as given in table 5 can be calculated. Z/Al Z03-ratio Z~4.
Table 5. The SiO (AI 03-ratio of thermally treated zeolites calculated from ao l) Zeolite
1 Z 3 4 5
RE
Z03 %(m(m)
°8.9 9.1
14.Z Z1.8
Calcination at 750°C
Steaming at 750°C
hour
3 'hours
hour
-Z) 5.0 7.0 4.5 4.Z
-Z) 4.9 7.4 4.5 4.5
14.0 7.8 8.8 7.4 5.0
3 hours 17.0 10.8 9.6 7.Z 5.6
1) Using the Breck-Flanigen relation. Z)Too low crystallinity (see Table Z). The Si02(AI20~-ratio given in table 5 can be compared with those determined from infrarea ana ffAS-NMR as given in table 3 and 4 respectively. The data for the calcined samples appear to be much the same although determined in three different ways. It is apparent that for the high RE-content samples 4 and 5 the value obtained from X-ray measurement is lower than obtained from infrared and NMR. The difference is not big, however, which can be explained by considering the slope of the curve respresentmg the Breck-Flanigen relation. Relatively large var~ations in ao in this range give only relatively slight variations in SiO Z(AI Z03 rat~o. The data for the steamed samples (1 hr at 750°C) are presented graphically in fig.5 . It can be seen that the SiO obtained from X-ray measurements are much lower than the values obta~nedZ(Al Z03-ratio from Infrared and/or MASNMR. The latter values can be regarded as fairly well in agreement with each other. This is also valid for the samples steamed at 750°C for 3 hours. Since the NMR method can be regarded as being the more direct method of the three, it can be concluded that the SiO 203-ratio obtained from ao is too low. It is clear that conclusions as to the Z/Al ~nhiDition of the dealumination by the presence of RE-ions should preferably be drawn form the results of the NMR or infrared measurements. The following conclusions can be drawn : - For the calcined zeolite samples a good correspondence was found for the SiOZ/AIZ-ratio as determined by X-ray, infrared and NMR methods. Calcination causes no or only slight dealurnination of the zeolite framework. - For the RE-containing zeolites which have been dealuminated' by steaming the SiO as calculated by the Breck-Flanigen relation from the lattice Z/AI 20 3-ratio constant ~s too low with respect to the values found from infrared and NMR measurements. - For the NH dealuminated by steaming sucha discrepancy is not found. 4NaY-zeolite After steaming at 750°C for three hours the SiO was 17.0. However Z/AI Z03-ratio this value was not found by NMR. - Dealumination by steaming was found by NMR and infrared methods to occur to an appreciable extent. Up to aRE -content of about 15%(m/m) the influence of Z03 RE 3 on the dea Iumdnat Iorrwas only small. Z0 - For the RE-containing zeolites the dealumination by steaming at 750°C proceeds fast.
344 (IM-3-l)
16 14
~ 12
:! &,10 N
:!
8
~
6
N
~==~~.
-
0
-
4
2 4
6
8 ro U ~ ~ % RE203 in zeolite
~
~
n
~
Fig.5. The Si0 2/A1 3-ratio as function of RE for 203-content samples steamea at 20750°C for 1 hr as determined from ao(e), Infrared (0) and 29Si MASNMR (X).
REFERENCES 1. J. Scherzer, J.L. Bass and F.D. Hunter, J. Phys. Chern. 79 12 (1975) 1194. 2. D.W. Breck and E.M. Flanigen, Mol. Sieves Soc. Chern. Ind. J~68 47. 3. E.M. Flanigen, H.Khatami, H.A. Szymanski,Adv, Chern. Ser vol 101 201 (1971). 4. A.A. Kubasov,Vestn. Mosk. Univ. Ser 2 Khim 1981 22(1) 21. 5. P. Marijnen, A. Maes and A. Cremers, Zeolites 1984 vol 4 287. 6. U. Lohse, E. A11sdorf and H. Stack, Z anorg. A11g. Chern. 447 64 (1978). Acknowledgements. The authors are indebted to Mr. N.G. Bader for preparing the samples, Miss M. Mulders for performing the infrared spectroscopic measurements and Mr. D.J. Pruissen and Mr. H.J. van Toll for performing the X-ray diffraction measurements.
In
Y-Zeolite on its Dealumination
l.W. Roe1ofsen* H. Mathies*, R.L. de Groot**, P.C.M. van Woerkom** and H. Angad Gaur** *Akzo-Chemie, Ketjen Catalysts, Research Centre Amsterdam P.O. Box 15, 1000 AA Amsterdam, The Netherlands. ~*Akzo Research, Corporate Research Department Arnhem P.O. Box 60, 6800 AB Arnhem, The Netherlands. The dealumination of the framework of Y-zeo1ites exchanged to various degrees with rare earth and ammonium ions and treated thermally in various ways was studied. The Si0 2/A1203-ratio of the framework was determined from the lattice constant, a lattice vibration and by 29 Si MASNMR. Upon calcination no or only moderate dea1umination takes place. Steaming causes an appreciable dea1umination of the zeolite framework. This is concluded from the results based on lattice vibrations and 29 Si MASNMR data. Also according to these techniques the degree of dea1umination does not depend strongly upon the RE 20 (at least up to 15% RE 70 ) . 3-content 3 This is in contrast Eo the information based on the laEt1ce constant, which suggests that dea1umination of the zeolite framework is strongly inhibited by the presence of rare earth ions. INTRODUCTION Rare earth ions (RE) play an important role "in the modificaHon of Y-zeolites. Scherzer et a1.(1) have reported on the influence of La3+-exchange of a NaY-zeolite on the dea1umination of the zeolite framework upon thermal treatment. Their conclusions are based on X-ray diffraction and infrared data. The work presented here compares the information on dea1umination obtained from lattice constants, infrared spectroscopy and 29 Si MASNMR on zeolites exchanged with rare earthand/or ammonium ions after calcination or steaming under various conditions. The Si0 2/AI of the zeolite framework was determined from the lattice 3-ratio constanE by20 using the Breck-F1anigen relation (2), from the lattice vibration by applying the F1anigen, Khatami,Szymansky relation (3) and from 29 Si-MASNMR assuming that the Lowenstein rule holds for these samples. EXPERIMENTAL 1. Sample preparation A NaY-zeolite (Si02/AI?03-ratio of 5.0) was exchanged with a RECl 3-so1ution by stirring the suspension for one hour at 60·C and pH 5.0-5.5. The RE-m1xture consisted of the following elements (expressed as oxides): 55,3 % (m/m) La20 3; 22,8 % /m/m) Nd 13,2 % (m/m)Ce0 8,2 % (m/m) Pr and 0.5 % (m/m)Sm 0 • 2; 2 3 203 203; After filtration and washing the samples were calcined at 450·C for one hour, followed by an NH at 80·C and pH 6.0 using an (NH4)2S04-solution. 4+-exchange After filtration and washing the samples were dried at 120·C. The sample which does not contain RE was exchanged with NH + twice with in 4 the highest between a calcination at 250·C for one hour. The sample hav1ng RE-content was made by exchanging the NaY-zeolite twice with RE3+ with in between a calcination at 450·C for one hour. 337
338 (IM-3-l) Z. Thermal treatment The samples were calcined in a shallow bed configuration (layer thickness 3 mrn). The sample were steamed by passing steam through the sample starting at ZOO°C and heating it up to the appropriate temperature. All samples have been investigated in hydrated form. 3. X-ray diffraction The X-ray diffraction data were obtained using a Philips PW 1710 diffractometer (Cu KDC-radiation). The crystallinity of the starting zeolites was determined relative to a standard NaY-zeolite by comparing the integrated intensities of the reflections in the range Z8 = 9-35 0 and correcting for the background intensity. Also a correction for the mass adsorption due to the presence of rare earth was applied. The lattice constant was determined according to the ASTM-procedure (D 394Z-80). The accuracy is ± O.OOZ nm. 4. Infrared spectroscopy The infrared spectra were obtained from zeolite/KBr"pellets(l% m/m zeolite)usmg a Digilab FTS 15 E spectrometer equiped with a Ge/KBr beamsplitter and a TGS detector. Mostly 500 scans per spectrum ~ire recorded with a 4 cm-1 resolution. The accuracy of the peak maximum is 1 cm. Hydrated samples have been investigated because then disturbing effects due to cationframework interaction may be neglected for at least the peak frequency of the framework adsorption bands (3,4). In the framework region (lZ00-400 cm- 1) of the zeolite spectra, several absorption bands are sensitive to the SiO Z03-ratio. In this work the symmetric stretch vibration at ± 790 cm- 1 has beenZ/AI used to calculate the Si0 2/AI 3-ratio of the thermally treated samples. The choice of Z0 this band arises from tfie fact that the other structure sensitive bands, such as the double ring vibration at 580 em-I, are much more sensitive to type and amounts of cations in the zeolite, the crystallinity and the water content. AccordinR to the literature (3) the peak frequency of the 790 cm-1 band is linear~ proportional to the Al/(Al + Si) ratio. Hence using suitable reference samples (with Si0 known from synthesis) a calibration line can be produced 20 3-ratio 2/A1 Ehe to determ1ne SiO of the thermally treated zeolite samples. Z/AI Z03-ratio 5. Z9Si - Magic Angle Spinning Nuclear Magnetic Resonance The 39.74Z MHzZ9SiMASNMR spectra were obtained using a Bruker CXP ZOO spectrometer. The sp1nn1ng rate was apprOXimately 2500 KHz. For direct experiments 3000-7000 FID's (free induction decays) were accumulated before fourier transformation.90° pulses (duration 6/us ) were applied. The acquisition time was 33 ms, the repetition rate was 8 ms. In the CP-experiments a cross polarization time of 5 ms was used. The duration of the 90 0 pulses was 8 ,us and the pulse repetition rate was 2s. A total of 1200 FID's was accumulated ih each experiment. All experiments were performed at ambient temperature. Lorentz to Gausian transformation was applied to the FID's for resolution enhancement of the spectra. The error on the SiO is estimated to be within 10%. Z/AI Z03-ratio RESULTS AND DISCUSSION 1. Analysis of the fresh samples The composition, crystallinity and lattice cOnstant (ao) of the samples used in this study are given in table 1. For all the RE-containing samples in this table holds that the sum of the equivalents of NA+, NH and RE3+ is higher than necessary to compensate the frame4+ to the formation of RE(OH)+3-n- species upon work charge. This is attributed calcination (5). n
J.W. Roelofsen
339
Table 1. Composition, crystallinitya)and Lat t rce constant of the fresh samples. Zeolite no
1 2 3 4 5
RE
r0 3
%m m
°8.9 9.1
14.2 22.8
a Na20 a 0.20 0.26 0.15 0.12 0.12
b RE
203 c(NH4)20 b c
°0.14 0.13
0.21 0.38
A1
203
0.79 0.59 0.60 0.44
CrystalP Si0 2 linity p %
5.04 5.07 5.07 5.08 5.18
°
115 113 123 113 111
ao nm 2.469 2.473 2.473 2.474 2.472
a. Crystallinity relative to a standard NaY-zeolite. 2. Analysis of the.thermally treated samples 2.1. X-ray diffraction data. The X-ray diffraction data obtained on the thermally treated zeolite samples is given in table 2. Table 2. Crystallinity a)and lattice constant(ao) of calcined and steamed samples Zeolite no
1 2 3 4 5
RE
Calcination at 750°C 3 hours ao crystal- ao linity % nm linity % nm
1 hour r0 3 crystal-
%m m
8.9 °9.1
14.2 22.8
2 93 59 93 105
2.466 78 2.456 53 2.471 85 2.474 109
2.467 2.454 2.471 2.471
Steaming at 750°C 1 hour 3 hours crystalao crystallinity % nm linity % 68 85 78 86 106
2.440 2.453 2.450 2.454 2.466
59 45 78 86 73
ao nm 2.437 2.446 2.448 2.455 2.462
a. Crystallinity is relative to the starting material Steaming of sample 4 at 750°C for 6! hours resulted in a material being 72% crystalline and having a lattice constant of 2.452 nm. The 'data in this table show that the presence of RE influences the crystallinity after calcination and steaming considerably. Also,sample 2 and 3 having the same RE but differing in Na differ appreciable with 203-content 20-content respect to stability against calcination and steaming. As shown in figure 1 the lattice constant of the steamed samples is linearly related to the RE-content. No such a relation was found for the lattice constants of the calcined samples. The Si0 of the framework calculated from the lattice constant is given 2/A1 203-ratio in table 5. 2.2. Infrared spectroscopv The calibration line determined from the spectra of the three samples with known Si0 (5.5,6.0 and 6.5 respectively) is given in figure 2. This line 2/A1203-ratio corresponds fairly well with the data given by Lohse et al (6). The reliability of this line for high SiO IA1 -ratio has been checked by applying it on a SiC1 4 dealuminated Y-zeo!ite.20 lccording to chemical analysis the Si0 of this sample is 20.8. The wavenumber of the symmetric stretch 2/A1 203-ratio vibration was 832 cm-1, which according to the calibration line corresponds with a Si0 of 19.2. 2/A1 203-ratio
340 (IM-3-1)
840 820 800
760 2.430
740
2.420
720 0.1
0.2
0.3
0.4
0.1
b ....
Fig .1. Lattice constant ao plotted versus RE-content (b) X steam 750·C o, steam 750·C 3 hrs. 1 hr • Calcined 750·C 3 hrs.
°
0.2 AI/IAI.SiI
0.3
0.4
Fig.Z. The calibration line of the symmetric stretch vibration as function of Al(Al + Si)-ratio .
A typical example of the effect of calcination or steaming on the infrared spectra of a zeolite is shown in figure 3 for sample 3. Table 3 presents the peak frequencies of the 790 cm-1 band of all samples after thermal tr:atment. Also the SiO determined from the calibration line Z/Al Z03-ratio has been gJ.ven. Table 3. Peakfrequency ( v) and SiO of Z/Al Z03-ratio calcined and steamed samples Zeolite no 1
Z
3 4
5
RE 03 Calcination at 750·C %m~m 1 hour (c~-l) Si0zlAlZ03(c~-1) 8 .9 °9.1
14.Z ZZ.8
788 795 787 788 783
5.Z 6.0 5.1 5.Z 4.7
788 794 787 788 784
Steaming at 750·C 1 hour 3 hours SiOz/AlZ03(C~-1) SiOZ/Alz03
3 hours SiOz/AlZ03(C~-1) 5.Z 5.9 5.Z 5.Z 4.8
8Z5 818 8Z4 818 804
13.8 10.9 13.3 10.9 7.4
tV
831 818 8Z5 818 800
17.5 10.9 13.8 10.9 6.7
After steaming at 750·C for 6! hours sample 4 shows the peak frequency at 8Z3 cm-1 which corresponds with a SiO ratio of 1Z.8. Z/Al Z03 It can be concluded from these data that calcination does not change significantly the SiOZ/Al Z03-ratio except for sample Z (with a somewhat higher ratio) and sample 5 (with a somewhat lower ratio). Steaming of the samples leads to a distinct increase of the SiO which depends on the RE-content of the samples. Z/Al Z03-ratio A higher RE-content has an inhibiting effect on the increase of the SiO Z03ratio. Again sample Z exhibits some deviating behavior, the RE-content Z/Al J.S nearly equal to, but the NaZO-content is higher than in sample 3. The SiO IA1203-ratio however is much lower. Increasing the steaming time increases the Si0 2TAI z0 3-ratio only for sample 1, which does not contain RE. Sample 4 shows a small increase of the SiOZ/AlZ03~ratio
when steaming is prolonged up to 6~ hours.
J.W. Roe1ofsen
341
Sample 5 shows a decrease of the Si0 upon prolonged steaming, but for 2/A1203-ratio this sample the determination of the peaK position was inaccurate. 7B.4
-t
90.Bt<::~~~~~_~~_~~~_
r-~-~~-~~..---~~-~~-;
dO
dO
o
1400
1200
1000
BOO
600
1200
400
Wavenumbers
1000
51.4.'1<-~~--'--'--~~-~~~--t
a.
BOO
600
400
Wavenumbers
c.
dO
1200
1000
800
600
450
Wavenumbers
b. Fig.3. Lattice vibration spectrum of sample 3. (a) as such (b) Calcined 1 hour (c) steamed 1 hour at 750°C at 750°C 2.3. 29Si Magic Angle Spinning Nuclear Magnetic Resonance The results of these experiments are given in table 4. Table 4. Si0 2/A1201-ratio of zeolite framework before and after thermal treatment as determined by 29Si MASNMR. Zeolite no 1 2 3 4 5
RE
203
°8.9 9.1
14.2 22.8
Starting ~.91)
5.2 5.7
Calcination at 75°C 1 hour 3 hours 1)
6.4 7.8 6.2 5.6
Steaming at 750°C 1 hour 3 hour
2) 2) 7.2 6.8 _2)
12.6 10.8 12.2 12.6 8.4
11.~2) 12.8 11. ~2)
1) Not a well resolved spectrum.2) Not measured. Sample 4 steamed at 750°C for 6! hours showed a Si0
2!A1203-ratio
°
of 12.8
Figure 4 shows that the influence of rare earth ions on the spectra of the zeolites is considerable. By comparing the spectra of zeolites with and 22.8% RE 3 (sample 1 and sample 5 respectively) it can be judged that the increase in20 line width is approximately a factor five. Comparison of the spectra of La3+- and Ce3+exchanged zeolites (fig.4) indicates that the paramagnetism of the Ce3+ causes this line broadening.
342 (IM-3-l)
Idl i_~
,-,~O
i
i
-,'20
i
Ie)
'-~o
'
Fig. 4. Z9Si MASNMR spectra of a) sample 1 as such (%RE b) sample 5 as such ZZ.8% RE Z03) Z03 c) sample with 15% Ce d) sample with 15% La Z03 Z03 e) sample 1 steamed 3 hr at 600°C. f) sample 1 steamed 3 hr 600°C with cross polarization. Mere calcination at 750°C hardly (or to a small extent) dealuminates the zeolites. Steaming at 750°C however. results in appreciable dealumination, which is slightly inhibited by the presence of rare earth ions. Steaming of sample 4 between about 1 and 6~ hours has within experimental error no influence on the dealumination process. Table Z shows that the crystallinity of the thermally treated samples may decrease considerably. The resulting material can be regarded as more or less amorphous. Z9Si MASNMR spectra of amorphous silica-alumina show a broad featureless resonance peak. By applying resolution enhancement techniques mainly the resonance of crystalline material is observed. so the SiO 1 Al of only this fraction is obtained. Throughout resolution enhancement Z03-ratioLorentz-Gaussian transformation has been used to resolve structure viz. techniques Si(nAl) sites. The question is whether this influences the intensities of the resonances and therefore the SiO It can be shown that this does not occur. This is also conf~rmed by Z/Al the Z03-ratio. fact that resolution enhancement and deconvolution techniques give similar results when applied on well or partly resolved spectra. In the case of severe overlapping resolution enhancement gives more reliable results. The SiO of the zeolite is influenced by the Z/Al Z03-ratio presence of Si(OH)-and Si(OH~-sites 1n the framework. As was shown by CP-MAS experiments the resonances ot these groups are atthe position of the Si(l Al)-andSi (3 AI) resonances (fig.4). The intensity of these peaks is difficult to calculate in relation to the intensities of the MAS-spectra. So the quantitative contribution of the Si(OH)Z-group to the Si0 2/Al cannot readily be obtained. However. Z03-ratio it can be argued that owing to the presence of these groups in the MAS spectra,the calculated" SiOZI Al are minimum values. Z03-ratio
J.W. Roelofsen
343
Comparison of the Si0 2/AI Z03-ratio of the framework after thermal treatment as determined from lattice constant, lattice vibration and by Z9Si MASNMR. From the lattice constant information on the SiO of the zeolite Z/AIrelation(Z). Z03-ratio framework can be obtained by using the Breck-Flanigen The values for the lattice constant(ao) of the thermally treated samples are given in table ~Z. By using the Breck-Flanigen relation also outside the limits of its determination the SiO as given in table 5 can be calculated. Z/Al Z03-ratio Z~4.
Table 5. The SiO (AI 03-ratio of thermally treated zeolites calculated from ao l) Zeolite
1 Z 3 4 5
RE
Z03 %(m(m)
°8.9 9.1
14.Z Z1.8
Calcination at 750°C
Steaming at 750°C
hour
3 'hours
hour
-Z) 5.0 7.0 4.5 4.Z
-Z) 4.9 7.4 4.5 4.5
14.0 7.8 8.8 7.4 5.0
3 hours 17.0 10.8 9.6 7.Z 5.6
1) Using the Breck-Flanigen relation. Z)Too low crystallinity (see Table Z). The Si02(AI20~-ratio given in table 5 can be compared with those determined from infrarea ana ffAS-NMR as given in table 3 and 4 respectively. The data for the calcined samples appear to be much the same although determined in three different ways. It is apparent that for the high RE-content samples 4 and 5 the value obtained from X-ray measurement is lower than obtained from infrared and NMR. The difference is not big, however, which can be explained by considering the slope of the curve respresentmg the Breck-Flanigen relation. Relatively large var~ations in ao in this range give only relatively slight variations in SiO Z(AI Z03 rat~o. The data for the steamed samples (1 hr at 750°C) are presented graphically in fig.5 . It can be seen that the SiO obtained from X-ray measurements are much lower than the values obta~nedZ(Al Z03-ratio from Infrared and/or MASNMR. The latter values can be regarded as fairly well in agreement with each other. This is also valid for the samples steamed at 750°C for 3 hours. Since the NMR method can be regarded as being the more direct method of the three, it can be concluded that the SiO 203-ratio obtained from ao is too low. It is clear that conclusions as to the Z/Al ~nhiDition of the dealumination by the presence of RE-ions should preferably be drawn form the results of the NMR or infrared measurements. The following conclusions can be drawn : - For the calcined zeolite samples a good correspondence was found for the SiOZ/AIZ-ratio as determined by X-ray, infrared and NMR methods. Calcination causes no or only slight dealurnination of the zeolite framework. - For the RE-containing zeolites which have been dealuminated' by steaming the SiO as calculated by the Breck-Flanigen relation from the lattice Z/AI 20 3-ratio constant ~s too low with respect to the values found from infrared and NMR measurements. - For the NH dealuminated by steaming sucha discrepancy is not found. 4NaY-zeolite After steaming at 750°C for three hours the SiO was 17.0. However Z/AI Z03-ratio this value was not found by NMR. - Dealumination by steaming was found by NMR and infrared methods to occur to an appreciable extent. Up to aRE -content of about 15%(m/m) the influence of Z03 RE 3 on the dea Iumdnat Iorrwas only small. Z0 - For the RE-containing zeolites the dealumination by steaming at 750°C proceeds fast.
344 (IM-3-l)
16 14
~ 12
:! &,10 N
:!
8
~
6
N
~==~~.
-
0
-
4
2 4
6
8 ro U ~ ~ % RE203 in zeolite
~
~
n
~
Fig.5. The Si0 2/A1 3-ratio as function of RE for 203-content samples steamea at 20750°C for 1 hr as determined from ao(e), Infrared (0) and 29Si MASNMR (X).
REFERENCES 1. J. Scherzer, J.L. Bass and F.D. Hunter, J. Phys. Chern. 79 12 (1975) 1194. 2. D.W. Breck and E.M. Flanigen, Mol. Sieves Soc. Chern. Ind. J~68 47. 3. E.M. Flanigen, H.Khatami, H.A. Szymanski,Adv, Chern. Ser vol 101 201 (1971). 4. A.A. Kubasov,Vestn. Mosk. Univ. Ser 2 Khim 1981 22(1) 21. 5. P. Marijnen, A. Maes and A. Cremers, Zeolites 1984 vol 4 287. 6. U. Lohse, E. A11sdorf and H. Stack, Z anorg. A11g. Chern. 447 64 (1978). Acknowledgements. The authors are indebted to Mr. N.G. Bader for preparing the samples, Miss M. Mulders for performing the infrared spectroscopic measurements and Mr. D.J. Pruissen and Mr. H.J. van Toll for performing the X-ray diffraction measurements.