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Acidity of silicon modified CoAPO-5 molecular sieves
Applied Catalysis A: General, 80 (1992) 59-77 Elsevier Science Publishers B.V., Amsterdam
59
APCAT 2190
Acidity of silicon modified CoAPO-5 molecular sieves R.B. Borade and A. Clearfield* Department of Chemistry, Texas A&M University, College Station, TX 77843, (USA), tel. (+ l-409)845201 I, fax. (+ l-409)8454719, e-mail clearf@ tamxrd. tamu.edn (Received 1 July 1991, revised manuscriptreceived 13 September1991)
Abstract Cobalt substitutedAPO,-5 molecularsievewas synthesizedhydrothermahy.Silicon modified CoAPO5 sampleswere obtained by treatingthe CoAPO-5 samplewith differentquantitiesof ammonium hexafluorosilicatc solution. The modified and unmodified CoAPO-5 samples were characterizedfor acidic propertiesusing pyridine as a probe molecule.The XRD data of modified samplesshowed that the unit cell parametera increasesand c decreaseswith overall increase in the unit cell volume as compared to the unmodified CoAPO-5 sample.The IR spectrain the mid-infraredregionindicatedthat silicon atoms werepresentin the frameworkposition. Temperature-programmeddesorption data showedan increase in the retention capacity for pyridine molecule with increased silicon content. The IR and N,. X-ray photoelectron spectra of chemisorbed pyridine were used to determine the relative concentration of Brensted and Lewis acid sites present in these molecular sieves. Comparison of IR data with XPS, allowed us to conclude that on the surface of the molecular sieve crystallitesthe concentration of the Lewis sites is higher than Brensted acid sites and for the bulk it is vise versa. In view of the above evidence it is concluded that the ammonium hexatluorosilicatetreatment leads to the isomorphous substitutionof phosphorusby silicon in the CoAPO-5 frameworkaccompaniedby a smallbut significant amount of simultaneoussubstitution for aluminumof cobalt plus phosphorus. Keywords: acidity, AlPO,-5, cobalt-APO-5, infrared spectroscopy, temperature-programmeddesorption, X-ray photoelectron spectroscopy.
INTRODUCTION
Molecular sieves based on the aluminophosphate framework structure represent a new and interesting class of porous materials [l-5]. This family of crystalline, microporous materials is designated by AlPO,-n, where n is an integer used to distinguish between structures. Their crystal frameworks consist of alternating AlO, and PO4 tetrahedra, a composition that preserves the electroneutrality of the framework [ 31. Therefore, these materials do not possess Brensted acid sites which are very important for many hydrocarbon transformation reactions. Brensted acid sites in AlPOd-n type materials can be created by one of the following ways: (1)by replacing pentavalent phosphorus by
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60
tetravalent silica (forming SAPO-n type molecular sieves) and/or (2) by replacing the trivalent by divalent ions like Co2+ (forming MeAPO-n type materials). Silicon substitution in AlP04-n can occur for phosphorus, aluminum and or two silicons for an aluminum-phosphorus pair [ 31. There is however, evidence that silicon replaces mainly phosphorus or that two silicons replace an aluminum-phosphorus pair leaving the framework charge unchanged [4]. In the case of cobalt, it is clear that cobalt mainly replaces aluminum [ 6-81. In both cases Brensted acid centers could be treated. Recently, Brinen and White [ 91 modified aluminophosphate type molecular sieves using SiCl, method. They found that Sic&-treated aluminophosphate samples showed high ammonia retention capacity and a three to four fold increase in catalytic activity for toluene methylation but not for the cumene cracking reaction, when compared with the untreated AlPOd- samples. The IR spectra showed Lewis acid bound pyridine only, which was also present in the unmodified AlPOd- sample. These observations led them to conclude that silicon in SAPO-5 (prepared hydrothermally) may play a role in creating Brsnsted acidity whereas the silicon does not play the same role in modified materials. In the present work, substitution of cobalt in the AlPOd- framework was carried out by using the hydrothermal synthesis method and then this cobalt substituted sample (CoAPO-5) was treated with ammonium hexafluorosilicate (AHFS) solution. The CoAPO-5 and CoAPO-5 samples modified with ammonium hexafluorosilicate were characterized by X-ray diffraction (XRD ) , temperature-programmed desorption of pyridine (TPD ) , IR spectra of chemisorbed pyridine, X-ray photoelectron spectroscopy (XPS) and elemental analysis. It is demonstrated that silicon can be introduced in the AlPO,-5 or CoAPO-5 framework by post-synthesis modification such as ammonium hexafluorosilicate treatment. The substitution of silicon in the AlP04-5 or CoAPO5 framework introduces new acidic centers, thus affecting the nature, number and strength of acid sites. These changes in acid sites may have some influence on the activity as well as the product selectivity of the toluene alkylation reaction [ 91.
EXPERIMENTAL
Sample preparation and modification The original CoAPO-5 sample was synthesized using the procedure described by Shiralkar and Clearfield [ 61. In a typical gel preparation, 46.12 g of H,PO, (85%) was diluted with 50 ml of water. To this mixture pseudoboehmite (Catapal B*, Vista Chemical co. ) was added in small quantities until a
61 TABLE 1 Experimental conditions for the ammonium hexafluorosilicate treatments Sample
Reaction medium
CoAPO-5 (A) CoAPO-5
-
AHFS” 0.75 M
(Si/Co) (Molar)
Reaction Temp. (K)
Time (h)
pH (final)
200 ml, 5M AAb
2.26 ml
1.60
353
2
7.1
2g(B) CoAPO-5
200 ml, 5M AAb
4.53 ml
3.20
353
2
6.9
2g (C) CoAPO-5
200 ml, 5M AAb
9.06 ml
6.40
353
2
6.6
2g(D) “Ammonium hexafluorosilicate *Ammonium acetate solution.
solution added at a rate of 1 ml/6 min.
thick gel formed. Then a solution of 15.74 g of CoS04*7Hz0 in 50 ml water was added to the gel with continuous stirring followed by the remainder of the pseudoboehmite. The total amount of pseudoboehmite used was 25.4 g. Finally, 20 ml of water was added and the whole mixture was stirred for 1 h. Then, 22.26 g of triethylamine was added dropwise and the mixture was stirred for 2 h. The gel was then transferred into teflon lined stainless steel autoclaves and kept at 453 K for 21 h. When the reaction was over, the autoclave was quenched under cold water and the content was transferred into a beaker. The dark blue crystals settled rapidly, and were separated from pink material which was slow to settle. The washing of the large blue crystals with deionized water (in beaker) was continued until pink particles were not seen in the wash water. The blue crystals of CoAPO-5 samples were dried at 373 K overnight in an oven. This product was designated as as-synthesized CoAPO-5. The protonated form was obtained by calcination of as-synthesized CoAPO-5 sample at 773 K for about 8 h. This sample was designated as unmodified CoAPO-5. Ammonium hexafluorosilicate (AHFS) treatment has been performed according to the procedure described by Chauvin et al. [lo]. The calcined CoAPO5 sample was dispersed in 200 ml of 0.5 A4 ammonium acetate solution. The ammonium hexafluorosilicate solution was added dropwise while maintaining a vigorous stirring and the pH recorded. When addition was complete, the temperature of the mixture was increased to 353 K and held at this temperature for 2 h. Then the product was separated from the hot suspension by filtration. The solid product was washed with boiling demineralized water and dried in an oven at 373 K overnight. The reaction media and experimental conditions are given in Table 1. Hereafter, CoAPO-5 samples treated with AHFS are referred to modified CoAPO-5 samples.
Pyridine adsorption All samples were activated in a conventional high vacuum system (P B 100~ Torr, 1 Torr= 133.3 Pa) at 673 K for 16 h. The samples were cooled down to 373 K under vacuum and then exposed to pyridine vapors overnight. The excess pyridine was desorbed by evacuating the samples at 373 K for about 16 h. These samples were used for TPD and XPS experiments. TPD of pyridine TPD spectra of pyridine from various samples were obtained by using a Du Pont (model 951) thermal analyzer. The sample, ca. 30 mg, was placed in a quartz bucket and adsorbed pyridine was desorbed by increasing the sample temperature under a nitrogen atmosphere with a heating rate of 10 K/min. The TPD spectrum (derivative plot) was obtained from this curve by plotting the rate of pyridine desorption as a function of temperature. XPS measurements The pyridine chemisorbed sample was pressed into a pellet and then mounted on a gold coated copper sample holder. The sample was evacuated in the pretreatment chamber to ca. 10m3Torr before transferring to the analyzer chamber for XPS measurements. The XPS spectra of all samples were recorded at room temperature using an HP 5950A ESCA spectrometer. The X-ray source used was Al Ka (hv = 1486.6 eV). The residual gas pressure in the spectrometer chamber during the data acquisition was less than 10m8Torr. The measurements were performed in the following sequence: Sipp,Al,, N1,, Cls, Co%, P,, and Oi,. All binding energy values were corrected for charging by assuming the binding energy of the Sisp level to be 103.3 eV for modified samples and the 01, peak at the binding energy value 532.4 eV for unmodified samples. The accuracy of binding energy with respect to this standard value were within 20.2 eV. The intensity of various XPS lines was determined using nonlinear background subtraction and integration of peak areas. N1, peaks were deconvoluted into two or three components by keeping the full width at half maximum (fwhm) constant in a particular spectrum and assuming that each peak has a Gaussian shape. In this deconvolution operation the fwhm value of 2.35 2 0.15 eV was adopted since all other lines recorded (Si,, A&+, P,, Co, and O,,) showed a fwhm values of 2.35 t 0.15 eV for a single component system.
63
Infrared experiments The IR spectra in the mid-infrared region (1300-200 cm-l) were used to establish the substitution of silicon in the CoAPO-5 framework. The spectra were recorded at room temperature using a Digilab FTS-40 spectrometer. The pellets were made by pressing 2-3 mg of the sample with 100 mg of KRr. The IR spectra in the hydroxyl as well as pyridine regions were obtained using the following procedure. The samples were pressed into self supporting wafers containing 9 mg of material. The wafers were mounted on stainlesssteel sample holders and introduced into a Pyrex vacuum cell which was designed to accommodate four samples at a time. The cell was connected to the vacuum system (P= 10m5Torr) and the samples were degassed at 673 K for 16 h. Then the cell was closed, samples were allowed to cool down to room temperature and their IR spectra were recorded. Afterwards, pyridine vapors were admitted to the cell. Then the temperature of the sample was increased to 373 K and pyridine vapor was allowed to react overnight. Finally, excess pyridine was desorbed by evacuating the samples at 373 K for 16 h. The samples were cooled down to room temperature and the spectra were recorded. All the spectra were recorded in the range of 4000-1000 cm-l with a 2 cm-l resolution using a Digilab FTS-40 spectrometer. RESULTS AND DISCUSSION
X-ray diffraction The X-ray diffraction powder pattern of cobalt-substituted AlPO& was very similar to AlPOd- except for some minor changes in the peak intensities. When CoAPO-5 sample was modified with (NH, )&Fe several changes could be observed in the X-ray diffraction patterns (Fig. 1). Some of the more prominent changes are: (1) change in the relative intensities of (210), (002) and (211) reflections, (2) shift in position towards higher 2 19value predominantly for (210) and (220) reflections, (3) the separation of the (210) and (002) reflection was reduced and that of the (002) and (210) increased; and (4) sample D shows overall broadening of peaks. The broadening of peaks and reduction in the peak intensities can be interpreted as a partial disordering of the framework structure. However, the separation of peaks (increase or decrease) can only be explained on the basis of changes in the unit cell dimensions. Shiralkar and Clearfield [ 61 showed that the unit cell parameter c decreases and a increases with substitution of cobalt into the AlP04-5 framework. It was also observed that there is an increase in the unit cell volume. The increase in the unit cell volume was attributed to the fact that the shorter Al-0 bonds (1.54 A) are replaced by longer Co-O (1.97 A) bonds. The unit cell dimensions determined by least squares analysis of 13
64
28 (DEG) -
Fig. 1. XRD powder pattern of unmodified and modifed CoAPO-5 samples. Sample designation as in Table 1. TABLE 2 Unit cell dimensions (with e.s.d.‘s) of parent CoAPO-5 and AHFS treated CoAPO-5 samples Sample
a (A)
c (A)
Unit cell volume (lj3)
A B C D
13.660(4)
8.399(3)
1357
13.633(3) 13.637(6)
8.451(3) 8.473(4)
1360 1364
main reflections between 2 8=5-50” for the samples of the present study are given in Table 2. The data show a very steady increase in the unit cell volume with increase of silicon content. The interesting result here, however, lies in the observation that on the substitution of silicon in the CoAPO-5 framework the unit cell parameter c increases whereas a decreases. In the present case, the increase in the unit cell volume is assumed to be in large measure due to the replacement of shorter P-O bonds (1.54 A) by longer Si-0 bonds (1.61 A). Chenical analysis The analytical data for modified and unmodified CoAPO-5 samples are given in Table 3. According to the mechanism proposed by Flanigen et al. [ 51, metal
65 TABLE 3 Analytical data for ammonium hexatluorosilicate treated CoAPO-5 eamplee Sample
A B C D
Initial (%/Co)
1.60 3.20 6.40
Molar ratio
(Si/Co)M
Si
co
Al
P
0.08 0.22 0.98
0.086 0.073 0.065 0.044
1.0 1.0 1.0 1.0
1.04 0.98 0.94 0.77
1.02 3.46 22.34
(in the present case cobalt) is expected to be incorporated at the aluminum sites. Then, for the CoAPO-5 molecular sieve, the sum of cobalt and aluminum would be expected to be equal to the phosphorus which is not the case. It appears that cobalt occupies sites other that aluminum sites. The substitution of Co’+ for phosphorus i.e. formation of metal-O-Al bond, according to Flanigen et al. [ 51 is very unlikely. Yet another explanation is that cobalt may be present at the cation exchange sites and/or as an occluded oxidic species. By visual appearance (color) of the sample, Chen [ 111 discarded the possibility of the presence of oxidic cobalt species in their as-synthesized CoAPO-5 samples. Further, from the ion-exchange studies he concluded that about 20-30% of the total acid sites (generated by substitution of cobalt in AlPOd- framework) are compensated by Co2+ cations. It is also likely that as-synthesized CoAPO-5 may contain occluded cobalt species that might have come out during the ionexchange treatment. We rule out the possibility of the presence of 20-30% cobalt at the ion-exchange site on the basis of Co, and 01, XP spectra of assynthesized CoAPO-5 sample (see XPS section). Now assuming that the majority of the cobalt in CoAPO-5 is in the framework sites and AHFS treatment leads to the substitution of silicon exclusively for phosphorus sites (mechanism 1), then the sum of cobalt and aluminum should be equal to the sum of the phosphorus and silicon. If silicon incorporation by mechanism (2) had occurred (simultaneous substitution of one aluminum and one phosphorus by two silicon) then the sum of silicon and phosphorus should exceed the sum of cobalt and aluminum. To this end we analyzed product and filtrate (obtained after AHFS treatment ) for sample C and the results are presented in Table 4. It was found that: (a) the amount of silicon incorporated and the amount of phosphorus coming out of the product (in the filtrate) was in the ratio of 1: 1.06 and (b) the sum of (Co + Al) and (P + Si) in the solid was in the ratio of 1: 1.09. The (P/Si) ratio slightly greater than one suggests that the amount of phosphorus released from the sample during the AHFS treatment is in excess than that of silicon incorporated. The (P + Si) / (Co+ Al) ratio greater than one in the solid product is due to the fact that
66 TABLE 4 Product and filtrate material balance for the sample C after AHFS treatment Element
Si co Al P
Starting composition
Mole Product
Filtrate
Total
0.00266 0.00077 0.01163 0.01108
0.00046 0.00029 0.00034 0.00284
0.00312 0.00106 0.01217 0.01390
0.00342” 0.00106 0.01245 0.01245
“Silicon from ammonium hexafluorosilicate solution
some of the aluminum as well as cobalt are extracted from the CoAPO-5 framework by the AHFS treatment (Table 4). Thus, the actual amounts of aluminum in Table 3 for samples A-D are not the same. Based on the elemental analysis of parent CoAPO-5 (sample A) and the product and the filtrate analysis of sample C, it was possible for us to determine the silicon substituted for phosphorus, aluminum and/or cobalt thus providing the total number of cations and anions in the silicon substituted CoAPO-5 framework. The cations/ anions ratio was found to be 1.01: 1 which led us to conclude that substitution of silicon in the CoAPO-5 framework has occurred not only by the mechanism 1 (substitution of phosphorus by silicon) but also to some extent by mechanism 2 (simultaneous substitution of one aluminum or cobalt and one phosphorus by two silicons). Sample B also gave very similar results. The elemental analysis of sample D showed that the amount of silicon present in the solid itself is greater than that of phosphorus. This is an indication that it contains amorphous silica. Therefore, the results for sample D are not discussed in terms of silicon substitution mechanism. Thermal desorption of pyridine The results of thermal desorption of pyridine obtained from modified and unmodified CoAPO-5 samples are shown in Fig. 2. For comparison purposes the pyridine desorption spectra obtained for AlPO,-5, AHFS treated AlPO,-5 and SAPO-5 samples are shown. The position of the peak temperature maximum (or maxima in the case of surface heterogeneity) of such curves is a qualitative indication of the magnitude of pyridine desorption activation energy and consequently the acid strength of the particular site. It is clear that modified and unmodified CoAPO-5 samples show four peaks, AHFS treated AlPOd- and SAPO-5 three peaks and AlPOd- two peaks. In all cases, the first peak (I) that occurs at 243-373 K is due to the desorption of physisorbed water. The water may get adsorbed while transferring the sample from the
67
473
073
873
1073
,
‘3
Temperature (K) -
Fig. 2. Pyridine temperature-programmed desorption spectra of various molecular sieves. Spectrum ( 1) sample A before pyridine adsorption spectra (2-5) correspond to samples A-D of Table 1; (6) AlPO,-5; (7) AHFS-treated AlPO,-5 and (8) SAP04 after pyridine adsorption.
vacuum system to the thermal analyzer (see experimental section). The second peak (II) centered around 510-560 K is due to pyridine desorption from weak acid sites. Choudhary et al. [ 121 studied the acidity of AlPOd- using pyridine TPD. They also found a peak at ca. 573 K due to the desorption of chemisorbed pyridine. The AlPOd- sample of the present study showed the absence of an IR band at 1545 cm- ’ corresponding to the pyridinium ion bonded with Brensted acid sites and the presence of an IR band at 1450 cm-’ due to the pyridine associated with Lewis sites. Therefore, it is reasonable to believe that the acid sites from which pyridine desorbs at about 573 K are Lewis in nature. Indeed, we noticed that Lewis sites present in ZSM-5 zeolites that are formed by dehydroxylation, show a TPD peak at 425-450 K [ 131. The presence of large number of Lewis acid sites may be due to dehydroxylation of these
materials. From comparison of the TPD spectra, it becomes clear that modified and unmodified CoAPO-5 samples contain additional acid sites with higher acid strength. An adsorbate chemisorbed on stronger sites desorbs at relatively higher temperature than that adsorbed on weak sites. The CoAPO-5 and AHFS treated CoAPO-5 and AlPO& samples show the peak (III) at around 773-813 K with a shoulder peak (IV) at about 993 K. A shoulder peak observed at about 993 K clearly indicates that in most cases the sample possesses non-uniform acid sites - some are stronger than others. That the peak at 993 K is not due to dehydroxylation was confirmed by carrying out TGA experiments with a CoAPO-5 sample that was precalcined at 773 K for 8 h (sample A). The TGA curve of this sample showed no weight loss in this temperature region (Fig. 2, curve 1). In an attempt to verify the fact that phosphorus is substituted by silicon, AlPOd- was treated with AHFS under similar conditions to those described for the CoAPO-5 sample. The product was analyzed for Al, P and Si. It was found that the (Al/P) ratio for AlPOd- was 0.96 and after AHFS treatment it became 1.09. Analysis of the filtrate of AHFS treated AlPOd- showed the presence of a significant quantity of phosphorus. From the amount of phosphorus coming out of the sample and the silicon present in the solid it was determined that P/Si= 1.10. The substitution of silicon at the phosphorus sites was also evidenced by TPD of pyridine. The TPD spectrum of SAPO-5 clearly shows that the substitution of silicon in the AlPOd- framework (hydrothermal synthesis) generates acid sites from which pyridine desorbs at about 773 K. The pyridine desorption spectrum obtained from AlPO,-5 shows the absence of a peak at 773 K. However, AHFS treated AlPOd- sample shows the main peak at 773 K along with the shoulder peak at 833 K. Since the peak maximum occurs at the same temperature for AHFS treated AlPOd- and SAPO-5 samples, it is deduced that AHFS treatment also leads to the generation of acid sites having acidic strength similar to those sites present in SAPO5 molecular sieves. The TGA weight loss obtained due to the desorption of pyridine from various samples is given in Table 5. In this study the chemisorption of pyridine is considered as the amount of pyridine desorbed in the temperature range 4331173 K and it is assumed that one acid site is involved in the chemisorption/ desorption of one pyridine molecule. The substitution of cobalt in AlPOd- generates negative charge on the framework which is compensated by protons (for calcined CoAPO-5 sample) thus generating Brensted acid centers. In an ideal situation (no dehydroxylation and no cationic or occluded cobalt species) the (pyridine/cobalt) molar ratio should be= 1. The observed (pyridine/cobalt) molar ratio of 1.22 for the parent CoAPO-5 sample suggests that all cobalt species are in the framework position. It appears that the small amount of extraframework cobalt species identified by XPS (see below) does not have any adverse effect on pyridine
69 TABLE 5 TGA data for CoAPO-5 samples treated with ammonium adsorption Sample
A B C D
hexafluorosilicate
after pyridine
PY/COhd
Wt. loss for peak n0.O (% ) I
II
11+1v
11+111+Iv
1.50 1.64 3.63 2.00
3.40 2.92 3.34 3.38
1.76 2.60 2.03 4.62
5.16 5.52 5.37 8.00
1.22 1.58 1.78 5.32
“See the text for peak assignments.
chemisorption. The pyridine/cobalt ratio > 1 suggests that there exists a considerable amount of acidic sites which are not related with cobalt. The amount of pyridine desorbed from the TPD II peak was found to be 3.20+ 0.2%for all samples. Whereas the amount of pyridine desorbed from the III and IV peaks increased with the silicon content which indicates involvement of silicon in generating acid sites. Considering that all silicon atoms were introduced in the CoAPO-5 framework and each silicon generates one acid site then (Pyridine/Co + Si) ratio should be equal to one. This ratio was found to be much lower than 1.0 which means that not all silicon atoms incorporated in the CoAPO-5 framework are involved in generating acid sites. For example, replacement of one Al-P pair by two silicon atoms does not generate additional charge on the framework. As far as sample D is concerned some comments may be made. Since the XRD pattern of this sample shows an overall broadening of peaks and the chemical analysis data show that silicon is in excess of phosphorus (Table 3) it may be deduced that this sample may contain some extraframework silicon species in the form of amorphous SiOa. Infrared spectroscopy IR spectra of modified and unmodified CoAPO-5 samples are shown in Fig. 3. The most intense absorption band in Fig. 3, which occurs around 1100cm-‘, is assigned to the asymmetric stretching vibrations of T-O bonds (T stands for Al, P, Co or Si) [ 141. This band is shifted to lower frequency for silicon substituted CoAPO-5 samples (1074 cm-’ for sample C) as compared to the unsubstituted CoAPO-5 (1124 cm-’ for sample A). The shift towards lower frequency is in agreement with the replacement of _shorterP-O bonds (1.54 A) in CoAPO-5 sample by longer Si-0 bonds (1.61 A). The infrared spectra in the O-H stretching region did not give much meaningful information. However, the bands in the pyridine region yielded infor-
4000
I
I
35co
3000
I 2500
I 2cm
I l5Ml
V 1MK)
I 500
Wavenumbers, cm-’
Fig. 3. IR spectra of various CoAPO-5 samples in the framework region. Sample designation as in Table 1.
mation about the nature (Brensted and/or Lewis) and the relative concentration of acid sites. This region of the spectrum shown in Fig. 4 is for several samples of interest. In agreement with the literature data, the AlPOd- sample shows the presence of only Lewis acid sites [E&12,15].All other samples show the presence of both bands due to the pyridinium ions (1545 cm-‘) and pyridine associated with Lewis sites (1445 cm-‘) [ 161. The band at 1490 cm-’ can be attributed to pyridine chemisorption on one or the other or both Brensted and Lewis sites. The (B/L)ia ratios were calculated by using the relation: B/ L= (AB/AL) - (Ed/+,) where (AB/AL) stands for IR absorbance ratio and (EL/ En) is the extinction coefficient ratio. Based on the literature data and the amount of silicon incorporated in the present material we choose (CL/en) Vahe 1.40 for B/L determination. This assumes that (&/en) does not vary with silicon incorporation. By using the above relation the Brnrnstedto Lewis ratios determined for samples A-D are 0.83, 1.42, 01.25 and 1.70, respectively. The Brnrnstedto Lewis ratio 0.63 observed for the CoAPO-5 sample suggests that CoAPO-5 molecular sieves undergo dehydroxylation under our experimental conditions. It is, however, seen that there is a small but significant increase in the Brsnsted-to-Lewis ratios as a function of silicon content of the sample. This increase in the Brcansted-to-Lewis ratio can be explained only on the basis of substitution of silicon mainly for phosphorus in the CoAPO-5 framework. If silicon were not substituting in the framework and the AHFS treatment lead to only partial destruction of the framework structure, the (B/L) ratio should either remain the same or decrease since the destruction of the framework should lead to the formation of Lewis sites rather than Brensted sites.
71
I
I
1560
1540
1520
1503
1480
,480
1440
Wavenumbers. cm”
Fig. 4. IR spectra in the region 1425-1575 cm-’ after pyridine adsorption. Spectra (l-4) correspond to samples A-D of Table 1; (5) AlPO-5; (6) AHFS-treated AIPO,-5 and (7) SAPO-5.
X-ray photoelectron spectroscopy
The XPS data for modified and unmodified CoAPO-5 samples are given in Table 6. All BE values were referenced to the Si, level (BE = 103.3 eV) for modified samples and the 01, peak at BE value 532.4 eV for unmodified samples. The C1, peak was not taken as a reference mainly because it was substantially broader than the corresponding Si,, Al+ P,, Ols, and Co, peaks, indicating that it was a composite peak. The BE values of all elements present in our samples agree with those reported in the literature [ 17,181. The XP Spectra of the Co, photoelectron region for as-synthesized, un-
72 TABLE 6 Binding energy (eV) of various components present in pyridine chemisorbed samples Sample
Si,
Al,
P,
01,
NI,
Co2PW2
cozp1/*
A B C D As”
103.3 103.3 103.3 103.3
75.7 75.9 75.8 76.0 76.0
135.0 134.9 134.8 134.8 134.8
532.4 532.4 532.3 532.4 532.4
399.8 400.0 400.7 399.6 401.2b
780.5 780.5 780.7 780.5 780.6
796.0 795.9 796.0 795.9 796.3
“As-synthesized CoAPO-5. bNitrogen of occluded organic molecules.
Binding
Energy
(ev)
Fig. 5. CozpXP spectra of various CoAPO-5 samples. Spectrum (1) as-synthesized CoAPO-5 and spectra (2-5) correspond to sample A-D of Table 1.
modified and modified CoAPO-5 samples are shown in Fig. 5. The binding energy values for CO~,,~,~and Cozp1,2levels are 760.5 ? 0.2 and 795.6 2 0.2 eV, respectively. A 2p1,2-2p3,2 separation of 15.5 + 0.2 eV is observed for all samples. The positions for Co 2p312peak in metallic cobalt and in Co304 phases are
73
778.1 and 781.1 eV, respectively [l&20]. Tan et al. [ 181 and Stencil et al. [ 191 found that the presence of mixed oxide phases ( Co2+ and Co3+ ) decreases the 2Pm -2~~2 separation to 15.0 eV. These reference positions show that the cobalt peak positions are significantly different from those observed for our sample. The Co, peak centered at 780.5 eV is ascribed to the cobalt either in Co0 or the Co (OH), phase [ 18,191. The Co2p spectra of as-synthesized CoAPO-5 sample shows interesting features. It is reported that Co (OH), has strong satellite peaks associated with both Co, peaks [ 181. Those satellite peaks are observed only in the case of as-synthesized CoAPO-5 sample and not for the other samples. The separation between the Cozp main peaks and the satellite peaks matched well the results for the Co ( OH)2 phase reported by Tan et al. [ 181. Stencil et al. [ 191 studied Co, XP spectra of cobalt impregnated ZSM5 zeolite and assigned the XPS shoulder peak at 787.0 eV to Co2+ species pres-
-
Binding Energy (ev)
Fig. 6. 01, XP spectra of various CoAPO-5 molecular sieves. Spsctrum (1) as-synthesized CoAPO5 and spectra (2-5) correspond to sample A-D of Table 1.
-
Binding Energy (ev)
Fig. 7. Nls XP spectra of pyridine adsorbed on various CoAPO-5 samples. Sample designation as in Table 1.
ent ion exchangeable sites of ZSM-5. However, the BE of the Co2p3,2peak of cobalt at cation exchange sites is 783.4 eV. Therefore, it is concluded that the as-synthesized CoAPO-5 contains a mainly cobalt hydroxide like phase as extraframework cobalt and after calcination this hydroxide like phase is transformed into a cobalt oxidic phase. The BE value of 780.5 +_0.2 eV observed for the CO~~,~peak and a 2p1,2-2,,,,separation of 15.5 eV is a clear indication of the divalent state of cobalt. It may be mentioned here that in a detailed XPS investigation of aluminum species present on the ZSM-5 zeolite surfaces, Barr and Lishka [ 211 found that a 50% contribution of extraframework aluminum species to the total aluminum present on the surface is still less than 10% of the bulk value. On similar grounds, the 30% contribution of the extraframework cobalt in our sample is less than 10% of the total cobalt species. The Oi, XP spectrum has been widely used to distinguish metal in lattice
15
positions and associated with an oxidic phase [ 17,18,22-241. The 01, XPS peak due to lattice oxygen occurs at about 532.06 2 0.2 eV and the peak due to oxygen species associated with metal oxide/hydroxide like phases occurs at about 529.02 0.2 eV. Indeed, the 01, XP spectrum of our sample A shows some indication of the presence of a shoulder peak to the main 01, peak confirming the presence of a small amount of extraframework cobalt species (Fig. 6). The contribution of the lower binding energy 01, component peak to the main peak is ca. 10% for sample A and < 7% for other samples. If 20-30% of total cobalt would have been at the cation exchange sites as indicated by Chen [ 111, the 01, XPS component peak associated with oxidic cobalt species should have shown significant increase in the relative intensity upon calcination, due to cobalt present at the cation exchange positions. Such a significant change in the relative intensity is not observed for CoAPO-5 samples before and after calcination. The N1, XP spectra of chemisorbed pyridine has been extensively used by Borade et al. [ 17,25-271 to distinguish various acid sites in zeolites. The broadening of the N1, peak is related to the nature, number and concentration of acid sites present in zeolites. Since all of the N1, peaks of chemisorbed pyridine were less symmetric and the fwhm values were substantially broader than those for the corresponding Si,, A&, Pzp, 01, and Cozp peaks, all the N1, peaks were deconvoluted into two component peaks by keeping a fwhm value of 2.35 eV. This gave perfect fitting and reproducible BE values for all composite peaks. The deconvoluted N1, spectra are shown in Fig. 7. The Ni, XP spectrum of pyridine chemisorbed on the unmodified CoAPO-5 sample is dominated by the peak at 399.6 eV (Fig. 7). When the CoAPO-5 sample was modified with AHFS, the relative intensity of the high binding energy N1, component peak increased. Here, we assign the low binding N1, peak to the pyridine adsorbed on Lewis sites and the high binding energy peak to the nitrogen of pyridinium ions that are formed by the protonation with Brensted acid sites. Based on this peak assignment the (B/L) ratio was found to be 0.39,0.43,0.46 and 0.52 for samples A-D, respectively. Comparison of (B/L) ratios determined
by IR and XPS method
The data in Table 7 show that there are differences in the (B/L) values determined by both methods. IR data, however, clearly show that there is an increases in (B/L) ratio with an increase of silicon content of the sample. The (B/L) ratios determined by the XPS method were always lower than that of the IR method, which suggests that the distribution of the Brensted and Lewis sites is not uniform from bulk to the surface of the crystallites. Since (B/L) values determined by XPS are lower than unity, it may be considered that on the surface the concentration of Lewis sites is higher than Brranstedacid sites in all cases. The TPD peak due to pyridine desorbing from Brensted acid sites
76 TABLE 7 Comparison of (Brenst.ed/Lewis) ratios determined by IR and XPS method Sample
A B C D
B/L IR
XPS
0.83 1.42 1.25 1.70
0.39 0.43 0.46 0.52
(Peak III and IV) is very poorly defined and would be quite arbitrary.Therefore the (B/L) ratios determinedby this method are not compared with the resultsof IR or XPS method. CONCLUSIONS
The resultspresented above may be summarizedas follows: The Co’+ ions in CoAPO-5 sampleare situatedmainlyin latticepositions. XRD data indicate a slight lattice expansion, for AHFS treated samples, suggestingsubstitution of Si*+ for P5+ ions. The shift towards lower IR frequency of the framework T-O vibration is also consistant with this suggestion.However, Co,, and Oi, XP spectraindicatedthe presenceof cobalt hydroxidelike phase in as-synthesizedCoAPO-5 and cobalt oxidic materialin modifiedand unmodifiedCoAPO5 samples.The TPD measurementrevealsthat the total concentrationof acid sites increases with the silicon content of the sample. The IR and Ni, XP spectraof chemisorbedpyridineindicatea smallbut significantincreasein the Brensted to Lewis acid ratio. On the basis of all the experimentalevidence,it is concluded that in the main isomorphous substitutionof Si4+ for P5+ into the CoAPO-5 frameworkhas occurred by the AHFS treatment but a significant amount of simultaneoussubstitutionfor Co2+ or AP+ plus P5+ has also occurred. ACKNOWLEDGEMENT
The financial support of this research by the State of Texas through Advanced Technology Program (Grant No. 32131-70560) is gratefully acknowledged.
77 REFERENCES 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
S.T. WiIson,B.M. Lok, C.A. Messina,T.R. CannonandE.M.Flanigen,ACS Symp. Ser., 218 (1983) 79. ST. Wilson, B.M. Lok, C.A. Messina, T.R. Cannon and E.M. Flanigen, J. Am. Chem. Sot., 104 (1982) 1146. J.M. Bennett, W.J. Dytrych, J.J. Pluth, J.W. Richardson and J.V. Smith, Zeolites, 6 (1986) 349. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, Y.R. Cannon and E.M. Flanigen, J.Am. Chem. Sot., 106 (1984) 6092. E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson, in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (Editors), Studies in Surface Science and Catalysis, Vol. 37, Elsevier, Amsterdam, 1988, p. 13. V.P. ShiraIkar and A. Clearfield, Zeolites, 9 (1989) 474. N.J. Tapp, N.B. Milestone and L:J. Wright, J. Chem. Sot. Chem. Commun., (1985) 1801. C. Montes, M.E. Davis, B. Murray and M. Narayana, J. Phys. Chem., 94 (1990) 6425. J.L. Brinen and M.G. White, J. Cat& 124 (1990) 133. B. Chauvin, M. Boulet, P. Massiani, F. Fajuia, F. Figueras and T.D. Courieres, J. Catal., 126 (1990) 532. Y-T. Chen, Ph.D. Thesis, Texas A & M University, 1990. V.R. Choudhary, D.B. Akolekar, A.P. Singh and S.D. Sansare, J. Catal., 111 (1988) 254. R.B. Borade and A. Clearfield, unpublished results. E.M. FIanigen and R.W. Grose, Adv. Chem. Ser. 101 (1971) 76. S.G. Hegde, P. Ratnasamy, L.M. Kustov and V.B. Kaxanisky, Zeolites, 8 (1988) 137. J.B. Uytterhoeven, R. Schonheydt, B.V. Liengme and W.K. Hall, J. Catal., 13 (1969) 425. R.B. Borade, A. Sayari, A. Adnot and S. Kahaguine, J. Phy. Chem., 94 (1990) 5989. B.J. Tan, K.J. KIabunde and M.A. Sherwood, J. Am. Chem. Sot., 113 (1991) 855. J.M. Stencel, V.U.S. Rao, J.R. Diehl, K.H. Rhee, A.G. Dhere and R.J. DeAngelis, J. Catal., 84 (1983) 109. T.A. Carlson, J.C. Cawer, L.F. Saethre, SF. Garua and G.A. Vernon, J. Elect. Spect. Rel. Phenom., 5 (1974) 247. T.L. Barr and M.A. Lishka, J. Am. Chem. Sot., 108 (1986) 3178. N.S. McIntrye and M.G. Cook, Anal. Chem., 47 (1975) 1975. T.J. Chuang, C.R. BrundIe and D.W. Rice, Surf. Sci., 59 (1976) 413. A. Corma, V. Fornes, 0. Pallot, J.M. Cruz and A. Aye&e, J. Chem. Sot. Chem. Commun., (1986) 333. R.B. Borade, A. Adnot and S. Kaliaguine, Zeolites, 11 (1991) 710. R.B. Borade, A. Adnot and S. Kaliagoine, J. CataI., 26 (1990) 126. R.B. Borade, A. Adnot and S. Kaliaguine, J. Chem. Sot. Faraday Trans., 86 (1990) 3949.
59
APCAT 2190
Acidity of silicon modified CoAPO-5 molecular sieves R.B. Borade and A. Clearfield* Department of Chemistry, Texas A&M University, College Station, TX 77843, (USA), tel. (+ l-409)845201 I, fax. (+ l-409)8454719, e-mail clearf@ tamxrd. tamu.edn (Received 1 July 1991, revised manuscriptreceived 13 September1991)
Abstract Cobalt substitutedAPO,-5 molecularsievewas synthesizedhydrothermahy.Silicon modified CoAPO5 sampleswere obtained by treatingthe CoAPO-5 samplewith differentquantitiesof ammonium hexafluorosilicatc solution. The modified and unmodified CoAPO-5 samples were characterizedfor acidic propertiesusing pyridine as a probe molecule.The XRD data of modified samplesshowed that the unit cell parametera increasesand c decreaseswith overall increase in the unit cell volume as compared to the unmodified CoAPO-5 sample.The IR spectrain the mid-infraredregionindicatedthat silicon atoms werepresentin the frameworkposition. Temperature-programmeddesorption data showedan increase in the retention capacity for pyridine molecule with increased silicon content. The IR and N,. X-ray photoelectron spectra of chemisorbed pyridine were used to determine the relative concentration of Brensted and Lewis acid sites present in these molecular sieves. Comparison of IR data with XPS, allowed us to conclude that on the surface of the molecular sieve crystallitesthe concentration of the Lewis sites is higher than Brensted acid sites and for the bulk it is vise versa. In view of the above evidence it is concluded that the ammonium hexatluorosilicatetreatment leads to the isomorphous substitutionof phosphorusby silicon in the CoAPO-5 frameworkaccompaniedby a smallbut significant amount of simultaneoussubstitution for aluminumof cobalt plus phosphorus. Keywords: acidity, AlPO,-5, cobalt-APO-5, infrared spectroscopy, temperature-programmeddesorption, X-ray photoelectron spectroscopy.
INTRODUCTION
Molecular sieves based on the aluminophosphate framework structure represent a new and interesting class of porous materials [l-5]. This family of crystalline, microporous materials is designated by AlPO,-n, where n is an integer used to distinguish between structures. Their crystal frameworks consist of alternating AlO, and PO4 tetrahedra, a composition that preserves the electroneutrality of the framework [ 31. Therefore, these materials do not possess Brensted acid sites which are very important for many hydrocarbon transformation reactions. Brensted acid sites in AlPOd-n type materials can be created by one of the following ways: (1)by replacing pentavalent phosphorus by
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60
tetravalent silica (forming SAPO-n type molecular sieves) and/or (2) by replacing the trivalent by divalent ions like Co2+ (forming MeAPO-n type materials). Silicon substitution in AlP04-n can occur for phosphorus, aluminum and or two silicons for an aluminum-phosphorus pair [ 31. There is however, evidence that silicon replaces mainly phosphorus or that two silicons replace an aluminum-phosphorus pair leaving the framework charge unchanged [4]. In the case of cobalt, it is clear that cobalt mainly replaces aluminum [ 6-81. In both cases Brensted acid centers could be treated. Recently, Brinen and White [ 91 modified aluminophosphate type molecular sieves using SiCl, method. They found that Sic&-treated aluminophosphate samples showed high ammonia retention capacity and a three to four fold increase in catalytic activity for toluene methylation but not for the cumene cracking reaction, when compared with the untreated AlPOd- samples. The IR spectra showed Lewis acid bound pyridine only, which was also present in the unmodified AlPOd- sample. These observations led them to conclude that silicon in SAPO-5 (prepared hydrothermally) may play a role in creating Brsnsted acidity whereas the silicon does not play the same role in modified materials. In the present work, substitution of cobalt in the AlPOd- framework was carried out by using the hydrothermal synthesis method and then this cobalt substituted sample (CoAPO-5) was treated with ammonium hexafluorosilicate (AHFS) solution. The CoAPO-5 and CoAPO-5 samples modified with ammonium hexafluorosilicate were characterized by X-ray diffraction (XRD ) , temperature-programmed desorption of pyridine (TPD ) , IR spectra of chemisorbed pyridine, X-ray photoelectron spectroscopy (XPS) and elemental analysis. It is demonstrated that silicon can be introduced in the AlPO,-5 or CoAPO-5 framework by post-synthesis modification such as ammonium hexafluorosilicate treatment. The substitution of silicon in the AlP04-5 or CoAPO5 framework introduces new acidic centers, thus affecting the nature, number and strength of acid sites. These changes in acid sites may have some influence on the activity as well as the product selectivity of the toluene alkylation reaction [ 91.
EXPERIMENTAL
Sample preparation and modification The original CoAPO-5 sample was synthesized using the procedure described by Shiralkar and Clearfield [ 61. In a typical gel preparation, 46.12 g of H,PO, (85%) was diluted with 50 ml of water. To this mixture pseudoboehmite (Catapal B*, Vista Chemical co. ) was added in small quantities until a
61 TABLE 1 Experimental conditions for the ammonium hexafluorosilicate treatments Sample
Reaction medium
CoAPO-5 (A) CoAPO-5
-
AHFS” 0.75 M
(Si/Co) (Molar)
Reaction Temp. (K)
Time (h)
pH (final)
200 ml, 5M AAb
2.26 ml
1.60
353
2
7.1
2g(B) CoAPO-5
200 ml, 5M AAb
4.53 ml
3.20
353
2
6.9
2g (C) CoAPO-5
200 ml, 5M AAb
9.06 ml
6.40
353
2
6.6
2g(D) “Ammonium hexafluorosilicate *Ammonium acetate solution.
solution added at a rate of 1 ml/6 min.
thick gel formed. Then a solution of 15.74 g of CoS04*7Hz0 in 50 ml water was added to the gel with continuous stirring followed by the remainder of the pseudoboehmite. The total amount of pseudoboehmite used was 25.4 g. Finally, 20 ml of water was added and the whole mixture was stirred for 1 h. Then, 22.26 g of triethylamine was added dropwise and the mixture was stirred for 2 h. The gel was then transferred into teflon lined stainless steel autoclaves and kept at 453 K for 21 h. When the reaction was over, the autoclave was quenched under cold water and the content was transferred into a beaker. The dark blue crystals settled rapidly, and were separated from pink material which was slow to settle. The washing of the large blue crystals with deionized water (in beaker) was continued until pink particles were not seen in the wash water. The blue crystals of CoAPO-5 samples were dried at 373 K overnight in an oven. This product was designated as as-synthesized CoAPO-5. The protonated form was obtained by calcination of as-synthesized CoAPO-5 sample at 773 K for about 8 h. This sample was designated as unmodified CoAPO-5. Ammonium hexafluorosilicate (AHFS) treatment has been performed according to the procedure described by Chauvin et al. [lo]. The calcined CoAPO5 sample was dispersed in 200 ml of 0.5 A4 ammonium acetate solution. The ammonium hexafluorosilicate solution was added dropwise while maintaining a vigorous stirring and the pH recorded. When addition was complete, the temperature of the mixture was increased to 353 K and held at this temperature for 2 h. Then the product was separated from the hot suspension by filtration. The solid product was washed with boiling demineralized water and dried in an oven at 373 K overnight. The reaction media and experimental conditions are given in Table 1. Hereafter, CoAPO-5 samples treated with AHFS are referred to modified CoAPO-5 samples.
Pyridine adsorption All samples were activated in a conventional high vacuum system (P B 100~ Torr, 1 Torr= 133.3 Pa) at 673 K for 16 h. The samples were cooled down to 373 K under vacuum and then exposed to pyridine vapors overnight. The excess pyridine was desorbed by evacuating the samples at 373 K for about 16 h. These samples were used for TPD and XPS experiments. TPD of pyridine TPD spectra of pyridine from various samples were obtained by using a Du Pont (model 951) thermal analyzer. The sample, ca. 30 mg, was placed in a quartz bucket and adsorbed pyridine was desorbed by increasing the sample temperature under a nitrogen atmosphere with a heating rate of 10 K/min. The TPD spectrum (derivative plot) was obtained from this curve by plotting the rate of pyridine desorption as a function of temperature. XPS measurements The pyridine chemisorbed sample was pressed into a pellet and then mounted on a gold coated copper sample holder. The sample was evacuated in the pretreatment chamber to ca. 10m3Torr before transferring to the analyzer chamber for XPS measurements. The XPS spectra of all samples were recorded at room temperature using an HP 5950A ESCA spectrometer. The X-ray source used was Al Ka (hv = 1486.6 eV). The residual gas pressure in the spectrometer chamber during the data acquisition was less than 10m8Torr. The measurements were performed in the following sequence: Sipp,Al,, N1,, Cls, Co%, P,, and Oi,. All binding energy values were corrected for charging by assuming the binding energy of the Sisp level to be 103.3 eV for modified samples and the 01, peak at the binding energy value 532.4 eV for unmodified samples. The accuracy of binding energy with respect to this standard value were within 20.2 eV. The intensity of various XPS lines was determined using nonlinear background subtraction and integration of peak areas. N1, peaks were deconvoluted into two or three components by keeping the full width at half maximum (fwhm) constant in a particular spectrum and assuming that each peak has a Gaussian shape. In this deconvolution operation the fwhm value of 2.35 2 0.15 eV was adopted since all other lines recorded (Si,, A&+, P,, Co, and O,,) showed a fwhm values of 2.35 t 0.15 eV for a single component system.
63
Infrared experiments The IR spectra in the mid-infrared region (1300-200 cm-l) were used to establish the substitution of silicon in the CoAPO-5 framework. The spectra were recorded at room temperature using a Digilab FTS-40 spectrometer. The pellets were made by pressing 2-3 mg of the sample with 100 mg of KRr. The IR spectra in the hydroxyl as well as pyridine regions were obtained using the following procedure. The samples were pressed into self supporting wafers containing 9 mg of material. The wafers were mounted on stainlesssteel sample holders and introduced into a Pyrex vacuum cell which was designed to accommodate four samples at a time. The cell was connected to the vacuum system (P= 10m5Torr) and the samples were degassed at 673 K for 16 h. Then the cell was closed, samples were allowed to cool down to room temperature and their IR spectra were recorded. Afterwards, pyridine vapors were admitted to the cell. Then the temperature of the sample was increased to 373 K and pyridine vapor was allowed to react overnight. Finally, excess pyridine was desorbed by evacuating the samples at 373 K for 16 h. The samples were cooled down to room temperature and the spectra were recorded. All the spectra were recorded in the range of 4000-1000 cm-l with a 2 cm-l resolution using a Digilab FTS-40 spectrometer. RESULTS AND DISCUSSION
X-ray diffraction The X-ray diffraction powder pattern of cobalt-substituted AlPO& was very similar to AlPOd- except for some minor changes in the peak intensities. When CoAPO-5 sample was modified with (NH, )&Fe several changes could be observed in the X-ray diffraction patterns (Fig. 1). Some of the more prominent changes are: (1) change in the relative intensities of (210), (002) and (211) reflections, (2) shift in position towards higher 2 19value predominantly for (210) and (220) reflections, (3) the separation of the (210) and (002) reflection was reduced and that of the (002) and (210) increased; and (4) sample D shows overall broadening of peaks. The broadening of peaks and reduction in the peak intensities can be interpreted as a partial disordering of the framework structure. However, the separation of peaks (increase or decrease) can only be explained on the basis of changes in the unit cell dimensions. Shiralkar and Clearfield [ 61 showed that the unit cell parameter c decreases and a increases with substitution of cobalt into the AlP04-5 framework. It was also observed that there is an increase in the unit cell volume. The increase in the unit cell volume was attributed to the fact that the shorter Al-0 bonds (1.54 A) are replaced by longer Co-O (1.97 A) bonds. The unit cell dimensions determined by least squares analysis of 13
64
28 (DEG) -
Fig. 1. XRD powder pattern of unmodified and modifed CoAPO-5 samples. Sample designation as in Table 1. TABLE 2 Unit cell dimensions (with e.s.d.‘s) of parent CoAPO-5 and AHFS treated CoAPO-5 samples Sample
a (A)
c (A)
Unit cell volume (lj3)
A B C D
13.660(4)
8.399(3)
1357
13.633(3) 13.637(6)
8.451(3) 8.473(4)
1360 1364
main reflections between 2 8=5-50” for the samples of the present study are given in Table 2. The data show a very steady increase in the unit cell volume with increase of silicon content. The interesting result here, however, lies in the observation that on the substitution of silicon in the CoAPO-5 framework the unit cell parameter c increases whereas a decreases. In the present case, the increase in the unit cell volume is assumed to be in large measure due to the replacement of shorter P-O bonds (1.54 A) by longer Si-0 bonds (1.61 A). Chenical analysis The analytical data for modified and unmodified CoAPO-5 samples are given in Table 3. According to the mechanism proposed by Flanigen et al. [ 51, metal
65 TABLE 3 Analytical data for ammonium hexatluorosilicate treated CoAPO-5 eamplee Sample
A B C D
Initial (%/Co)
1.60 3.20 6.40
Molar ratio
(Si/Co)M
Si
co
Al
P
0.08 0.22 0.98
0.086 0.073 0.065 0.044
1.0 1.0 1.0 1.0
1.04 0.98 0.94 0.77
1.02 3.46 22.34
(in the present case cobalt) is expected to be incorporated at the aluminum sites. Then, for the CoAPO-5 molecular sieve, the sum of cobalt and aluminum would be expected to be equal to the phosphorus which is not the case. It appears that cobalt occupies sites other that aluminum sites. The substitution of Co’+ for phosphorus i.e. formation of metal-O-Al bond, according to Flanigen et al. [ 51 is very unlikely. Yet another explanation is that cobalt may be present at the cation exchange sites and/or as an occluded oxidic species. By visual appearance (color) of the sample, Chen [ 111 discarded the possibility of the presence of oxidic cobalt species in their as-synthesized CoAPO-5 samples. Further, from the ion-exchange studies he concluded that about 20-30% of the total acid sites (generated by substitution of cobalt in AlPOd- framework) are compensated by Co2+ cations. It is also likely that as-synthesized CoAPO-5 may contain occluded cobalt species that might have come out during the ionexchange treatment. We rule out the possibility of the presence of 20-30% cobalt at the ion-exchange site on the basis of Co, and 01, XP spectra of assynthesized CoAPO-5 sample (see XPS section). Now assuming that the majority of the cobalt in CoAPO-5 is in the framework sites and AHFS treatment leads to the substitution of silicon exclusively for phosphorus sites (mechanism 1), then the sum of cobalt and aluminum should be equal to the sum of the phosphorus and silicon. If silicon incorporation by mechanism (2) had occurred (simultaneous substitution of one aluminum and one phosphorus by two silicon) then the sum of silicon and phosphorus should exceed the sum of cobalt and aluminum. To this end we analyzed product and filtrate (obtained after AHFS treatment ) for sample C and the results are presented in Table 4. It was found that: (a) the amount of silicon incorporated and the amount of phosphorus coming out of the product (in the filtrate) was in the ratio of 1: 1.06 and (b) the sum of (Co + Al) and (P + Si) in the solid was in the ratio of 1: 1.09. The (P/Si) ratio slightly greater than one suggests that the amount of phosphorus released from the sample during the AHFS treatment is in excess than that of silicon incorporated. The (P + Si) / (Co+ Al) ratio greater than one in the solid product is due to the fact that
66 TABLE 4 Product and filtrate material balance for the sample C after AHFS treatment Element
Si co Al P
Starting composition
Mole Product
Filtrate
Total
0.00266 0.00077 0.01163 0.01108
0.00046 0.00029 0.00034 0.00284
0.00312 0.00106 0.01217 0.01390
0.00342” 0.00106 0.01245 0.01245
“Silicon from ammonium hexafluorosilicate solution
some of the aluminum as well as cobalt are extracted from the CoAPO-5 framework by the AHFS treatment (Table 4). Thus, the actual amounts of aluminum in Table 3 for samples A-D are not the same. Based on the elemental analysis of parent CoAPO-5 (sample A) and the product and the filtrate analysis of sample C, it was possible for us to determine the silicon substituted for phosphorus, aluminum and/or cobalt thus providing the total number of cations and anions in the silicon substituted CoAPO-5 framework. The cations/ anions ratio was found to be 1.01: 1 which led us to conclude that substitution of silicon in the CoAPO-5 framework has occurred not only by the mechanism 1 (substitution of phosphorus by silicon) but also to some extent by mechanism 2 (simultaneous substitution of one aluminum or cobalt and one phosphorus by two silicons). Sample B also gave very similar results. The elemental analysis of sample D showed that the amount of silicon present in the solid itself is greater than that of phosphorus. This is an indication that it contains amorphous silica. Therefore, the results for sample D are not discussed in terms of silicon substitution mechanism. Thermal desorption of pyridine The results of thermal desorption of pyridine obtained from modified and unmodified CoAPO-5 samples are shown in Fig. 2. For comparison purposes the pyridine desorption spectra obtained for AlPO,-5, AHFS treated AlPO,-5 and SAPO-5 samples are shown. The position of the peak temperature maximum (or maxima in the case of surface heterogeneity) of such curves is a qualitative indication of the magnitude of pyridine desorption activation energy and consequently the acid strength of the particular site. It is clear that modified and unmodified CoAPO-5 samples show four peaks, AHFS treated AlPOd- and SAPO-5 three peaks and AlPOd- two peaks. In all cases, the first peak (I) that occurs at 243-373 K is due to the desorption of physisorbed water. The water may get adsorbed while transferring the sample from the
67
473
073
873
1073
,
‘3
Temperature (K) -
Fig. 2. Pyridine temperature-programmed desorption spectra of various molecular sieves. Spectrum ( 1) sample A before pyridine adsorption spectra (2-5) correspond to samples A-D of Table 1; (6) AlPO,-5; (7) AHFS-treated AlPO,-5 and (8) SAP04 after pyridine adsorption.
vacuum system to the thermal analyzer (see experimental section). The second peak (II) centered around 510-560 K is due to pyridine desorption from weak acid sites. Choudhary et al. [ 121 studied the acidity of AlPOd- using pyridine TPD. They also found a peak at ca. 573 K due to the desorption of chemisorbed pyridine. The AlPOd- sample of the present study showed the absence of an IR band at 1545 cm- ’ corresponding to the pyridinium ion bonded with Brensted acid sites and the presence of an IR band at 1450 cm-’ due to the pyridine associated with Lewis sites. Therefore, it is reasonable to believe that the acid sites from which pyridine desorbs at about 573 K are Lewis in nature. Indeed, we noticed that Lewis sites present in ZSM-5 zeolites that are formed by dehydroxylation, show a TPD peak at 425-450 K [ 131. The presence of large number of Lewis acid sites may be due to dehydroxylation of these
materials. From comparison of the TPD spectra, it becomes clear that modified and unmodified CoAPO-5 samples contain additional acid sites with higher acid strength. An adsorbate chemisorbed on stronger sites desorbs at relatively higher temperature than that adsorbed on weak sites. The CoAPO-5 and AHFS treated CoAPO-5 and AlPO& samples show the peak (III) at around 773-813 K with a shoulder peak (IV) at about 993 K. A shoulder peak observed at about 993 K clearly indicates that in most cases the sample possesses non-uniform acid sites - some are stronger than others. That the peak at 993 K is not due to dehydroxylation was confirmed by carrying out TGA experiments with a CoAPO-5 sample that was precalcined at 773 K for 8 h (sample A). The TGA curve of this sample showed no weight loss in this temperature region (Fig. 2, curve 1). In an attempt to verify the fact that phosphorus is substituted by silicon, AlPOd- was treated with AHFS under similar conditions to those described for the CoAPO-5 sample. The product was analyzed for Al, P and Si. It was found that the (Al/P) ratio for AlPOd- was 0.96 and after AHFS treatment it became 1.09. Analysis of the filtrate of AHFS treated AlPOd- showed the presence of a significant quantity of phosphorus. From the amount of phosphorus coming out of the sample and the silicon present in the solid it was determined that P/Si= 1.10. The substitution of silicon at the phosphorus sites was also evidenced by TPD of pyridine. The TPD spectrum of SAPO-5 clearly shows that the substitution of silicon in the AlPOd- framework (hydrothermal synthesis) generates acid sites from which pyridine desorbs at about 773 K. The pyridine desorption spectrum obtained from AlPO,-5 shows the absence of a peak at 773 K. However, AHFS treated AlPOd- sample shows the main peak at 773 K along with the shoulder peak at 833 K. Since the peak maximum occurs at the same temperature for AHFS treated AlPOd- and SAPO-5 samples, it is deduced that AHFS treatment also leads to the generation of acid sites having acidic strength similar to those sites present in SAPO5 molecular sieves. The TGA weight loss obtained due to the desorption of pyridine from various samples is given in Table 5. In this study the chemisorption of pyridine is considered as the amount of pyridine desorbed in the temperature range 4331173 K and it is assumed that one acid site is involved in the chemisorption/ desorption of one pyridine molecule. The substitution of cobalt in AlPOd- generates negative charge on the framework which is compensated by protons (for calcined CoAPO-5 sample) thus generating Brensted acid centers. In an ideal situation (no dehydroxylation and no cationic or occluded cobalt species) the (pyridine/cobalt) molar ratio should be= 1. The observed (pyridine/cobalt) molar ratio of 1.22 for the parent CoAPO-5 sample suggests that all cobalt species are in the framework position. It appears that the small amount of extraframework cobalt species identified by XPS (see below) does not have any adverse effect on pyridine
69 TABLE 5 TGA data for CoAPO-5 samples treated with ammonium adsorption Sample
A B C D
hexafluorosilicate
after pyridine
PY/COhd
Wt. loss for peak n0.O (% ) I
II
11+1v
11+111+Iv
1.50 1.64 3.63 2.00
3.40 2.92 3.34 3.38
1.76 2.60 2.03 4.62
5.16 5.52 5.37 8.00
1.22 1.58 1.78 5.32
“See the text for peak assignments.
chemisorption. The pyridine/cobalt ratio > 1 suggests that there exists a considerable amount of acidic sites which are not related with cobalt. The amount of pyridine desorbed from the TPD II peak was found to be 3.20+ 0.2%for all samples. Whereas the amount of pyridine desorbed from the III and IV peaks increased with the silicon content which indicates involvement of silicon in generating acid sites. Considering that all silicon atoms were introduced in the CoAPO-5 framework and each silicon generates one acid site then (Pyridine/Co + Si) ratio should be equal to one. This ratio was found to be much lower than 1.0 which means that not all silicon atoms incorporated in the CoAPO-5 framework are involved in generating acid sites. For example, replacement of one Al-P pair by two silicon atoms does not generate additional charge on the framework. As far as sample D is concerned some comments may be made. Since the XRD pattern of this sample shows an overall broadening of peaks and the chemical analysis data show that silicon is in excess of phosphorus (Table 3) it may be deduced that this sample may contain some extraframework silicon species in the form of amorphous SiOa. Infrared spectroscopy IR spectra of modified and unmodified CoAPO-5 samples are shown in Fig. 3. The most intense absorption band in Fig. 3, which occurs around 1100cm-‘, is assigned to the asymmetric stretching vibrations of T-O bonds (T stands for Al, P, Co or Si) [ 141. This band is shifted to lower frequency for silicon substituted CoAPO-5 samples (1074 cm-’ for sample C) as compared to the unsubstituted CoAPO-5 (1124 cm-’ for sample A). The shift towards lower frequency is in agreement with the replacement of _shorterP-O bonds (1.54 A) in CoAPO-5 sample by longer Si-0 bonds (1.61 A). The infrared spectra in the O-H stretching region did not give much meaningful information. However, the bands in the pyridine region yielded infor-
4000
I
I
35co
3000
I 2500
I 2cm
I l5Ml
V 1MK)
I 500
Wavenumbers, cm-’
Fig. 3. IR spectra of various CoAPO-5 samples in the framework region. Sample designation as in Table 1.
mation about the nature (Brensted and/or Lewis) and the relative concentration of acid sites. This region of the spectrum shown in Fig. 4 is for several samples of interest. In agreement with the literature data, the AlPOd- sample shows the presence of only Lewis acid sites [E&12,15].All other samples show the presence of both bands due to the pyridinium ions (1545 cm-‘) and pyridine associated with Lewis sites (1445 cm-‘) [ 161. The band at 1490 cm-’ can be attributed to pyridine chemisorption on one or the other or both Brensted and Lewis sites. The (B/L)ia ratios were calculated by using the relation: B/ L= (AB/AL) - (Ed/+,) where (AB/AL) stands for IR absorbance ratio and (EL/ En) is the extinction coefficient ratio. Based on the literature data and the amount of silicon incorporated in the present material we choose (CL/en) Vahe 1.40 for B/L determination. This assumes that (&/en) does not vary with silicon incorporation. By using the above relation the Brnrnstedto Lewis ratios determined for samples A-D are 0.83, 1.42, 01.25 and 1.70, respectively. The Brnrnstedto Lewis ratio 0.63 observed for the CoAPO-5 sample suggests that CoAPO-5 molecular sieves undergo dehydroxylation under our experimental conditions. It is, however, seen that there is a small but significant increase in the Brsnsted-to-Lewis ratios as a function of silicon content of the sample. This increase in the Brcansted-to-Lewis ratio can be explained only on the basis of substitution of silicon mainly for phosphorus in the CoAPO-5 framework. If silicon were not substituting in the framework and the AHFS treatment lead to only partial destruction of the framework structure, the (B/L) ratio should either remain the same or decrease since the destruction of the framework should lead to the formation of Lewis sites rather than Brensted sites.
71
I
I
1560
1540
1520
1503
1480
,480
1440
Wavenumbers. cm”
Fig. 4. IR spectra in the region 1425-1575 cm-’ after pyridine adsorption. Spectra (l-4) correspond to samples A-D of Table 1; (5) AlPO-5; (6) AHFS-treated AIPO,-5 and (7) SAPO-5.
X-ray photoelectron spectroscopy
The XPS data for modified and unmodified CoAPO-5 samples are given in Table 6. All BE values were referenced to the Si, level (BE = 103.3 eV) for modified samples and the 01, peak at BE value 532.4 eV for unmodified samples. The C1, peak was not taken as a reference mainly because it was substantially broader than the corresponding Si,, Al+ P,, Ols, and Co, peaks, indicating that it was a composite peak. The BE values of all elements present in our samples agree with those reported in the literature [ 17,181. The XP Spectra of the Co, photoelectron region for as-synthesized, un-
72 TABLE 6 Binding energy (eV) of various components present in pyridine chemisorbed samples Sample
Si,
Al,
P,
01,
NI,
Co2PW2
cozp1/*
A B C D As”
103.3 103.3 103.3 103.3
75.7 75.9 75.8 76.0 76.0
135.0 134.9 134.8 134.8 134.8
532.4 532.4 532.3 532.4 532.4
399.8 400.0 400.7 399.6 401.2b
780.5 780.5 780.7 780.5 780.6
796.0 795.9 796.0 795.9 796.3
“As-synthesized CoAPO-5. bNitrogen of occluded organic molecules.
Binding
Energy
(ev)
Fig. 5. CozpXP spectra of various CoAPO-5 samples. Spectrum (1) as-synthesized CoAPO-5 and spectra (2-5) correspond to sample A-D of Table 1.
modified and modified CoAPO-5 samples are shown in Fig. 5. The binding energy values for CO~,,~,~and Cozp1,2levels are 760.5 ? 0.2 and 795.6 2 0.2 eV, respectively. A 2p1,2-2p3,2 separation of 15.5 + 0.2 eV is observed for all samples. The positions for Co 2p312peak in metallic cobalt and in Co304 phases are
73
778.1 and 781.1 eV, respectively [l&20]. Tan et al. [ 181 and Stencil et al. [ 191 found that the presence of mixed oxide phases ( Co2+ and Co3+ ) decreases the 2Pm -2~~2 separation to 15.0 eV. These reference positions show that the cobalt peak positions are significantly different from those observed for our sample. The Co, peak centered at 780.5 eV is ascribed to the cobalt either in Co0 or the Co (OH), phase [ 18,191. The Co2p spectra of as-synthesized CoAPO-5 sample shows interesting features. It is reported that Co (OH), has strong satellite peaks associated with both Co, peaks [ 181. Those satellite peaks are observed only in the case of as-synthesized CoAPO-5 sample and not for the other samples. The separation between the Cozp main peaks and the satellite peaks matched well the results for the Co ( OH)2 phase reported by Tan et al. [ 181. Stencil et al. [ 191 studied Co, XP spectra of cobalt impregnated ZSM5 zeolite and assigned the XPS shoulder peak at 787.0 eV to Co2+ species pres-
-
Binding Energy (ev)
Fig. 6. 01, XP spectra of various CoAPO-5 molecular sieves. Spsctrum (1) as-synthesized CoAPO5 and spectra (2-5) correspond to sample A-D of Table 1.
-
Binding Energy (ev)
Fig. 7. Nls XP spectra of pyridine adsorbed on various CoAPO-5 samples. Sample designation as in Table 1.
ent ion exchangeable sites of ZSM-5. However, the BE of the Co2p3,2peak of cobalt at cation exchange sites is 783.4 eV. Therefore, it is concluded that the as-synthesized CoAPO-5 contains a mainly cobalt hydroxide like phase as extraframework cobalt and after calcination this hydroxide like phase is transformed into a cobalt oxidic phase. The BE value of 780.5 +_0.2 eV observed for the CO~~,~peak and a 2p1,2-2,,,,separation of 15.5 eV is a clear indication of the divalent state of cobalt. It may be mentioned here that in a detailed XPS investigation of aluminum species present on the ZSM-5 zeolite surfaces, Barr and Lishka [ 211 found that a 50% contribution of extraframework aluminum species to the total aluminum present on the surface is still less than 10% of the bulk value. On similar grounds, the 30% contribution of the extraframework cobalt in our sample is less than 10% of the total cobalt species. The Oi, XP spectrum has been widely used to distinguish metal in lattice
15
positions and associated with an oxidic phase [ 17,18,22-241. The 01, XPS peak due to lattice oxygen occurs at about 532.06 2 0.2 eV and the peak due to oxygen species associated with metal oxide/hydroxide like phases occurs at about 529.02 0.2 eV. Indeed, the 01, XP spectrum of our sample A shows some indication of the presence of a shoulder peak to the main 01, peak confirming the presence of a small amount of extraframework cobalt species (Fig. 6). The contribution of the lower binding energy 01, component peak to the main peak is ca. 10% for sample A and < 7% for other samples. If 20-30% of total cobalt would have been at the cation exchange sites as indicated by Chen [ 111, the 01, XPS component peak associated with oxidic cobalt species should have shown significant increase in the relative intensity upon calcination, due to cobalt present at the cation exchange positions. Such a significant change in the relative intensity is not observed for CoAPO-5 samples before and after calcination. The N1, XP spectra of chemisorbed pyridine has been extensively used by Borade et al. [ 17,25-271 to distinguish various acid sites in zeolites. The broadening of the N1, peak is related to the nature, number and concentration of acid sites present in zeolites. Since all of the N1, peaks of chemisorbed pyridine were less symmetric and the fwhm values were substantially broader than those for the corresponding Si,, A&, Pzp, 01, and Cozp peaks, all the N1, peaks were deconvoluted into two component peaks by keeping a fwhm value of 2.35 eV. This gave perfect fitting and reproducible BE values for all composite peaks. The deconvoluted N1, spectra are shown in Fig. 7. The Ni, XP spectrum of pyridine chemisorbed on the unmodified CoAPO-5 sample is dominated by the peak at 399.6 eV (Fig. 7). When the CoAPO-5 sample was modified with AHFS, the relative intensity of the high binding energy N1, component peak increased. Here, we assign the low binding N1, peak to the pyridine adsorbed on Lewis sites and the high binding energy peak to the nitrogen of pyridinium ions that are formed by the protonation with Brensted acid sites. Based on this peak assignment the (B/L) ratio was found to be 0.39,0.43,0.46 and 0.52 for samples A-D, respectively. Comparison of (B/L) ratios determined
by IR and XPS method
The data in Table 7 show that there are differences in the (B/L) values determined by both methods. IR data, however, clearly show that there is an increases in (B/L) ratio with an increase of silicon content of the sample. The (B/L) ratios determined by the XPS method were always lower than that of the IR method, which suggests that the distribution of the Brensted and Lewis sites is not uniform from bulk to the surface of the crystallites. Since (B/L) values determined by XPS are lower than unity, it may be considered that on the surface the concentration of Lewis sites is higher than Brranstedacid sites in all cases. The TPD peak due to pyridine desorbing from Brensted acid sites
76 TABLE 7 Comparison of (Brenst.ed/Lewis) ratios determined by IR and XPS method Sample
A B C D
B/L IR
XPS
0.83 1.42 1.25 1.70
0.39 0.43 0.46 0.52
(Peak III and IV) is very poorly defined and would be quite arbitrary.Therefore the (B/L) ratios determinedby this method are not compared with the resultsof IR or XPS method. CONCLUSIONS
The resultspresented above may be summarizedas follows: The Co’+ ions in CoAPO-5 sampleare situatedmainlyin latticepositions. XRD data indicate a slight lattice expansion, for AHFS treated samples, suggestingsubstitution of Si*+ for P5+ ions. The shift towards lower IR frequency of the framework T-O vibration is also consistant with this suggestion.However, Co,, and Oi, XP spectraindicatedthe presenceof cobalt hydroxidelike phase in as-synthesizedCoAPO-5 and cobalt oxidic materialin modifiedand unmodifiedCoAPO5 samples.The TPD measurementrevealsthat the total concentrationof acid sites increases with the silicon content of the sample. The IR and Ni, XP spectraof chemisorbedpyridineindicatea smallbut significantincreasein the Brensted to Lewis acid ratio. On the basis of all the experimentalevidence,it is concluded that in the main isomorphous substitutionof Si4+ for P5+ into the CoAPO-5 frameworkhas occurred by the AHFS treatment but a significant amount of simultaneoussubstitutionfor Co2+ or AP+ plus P5+ has also occurred. ACKNOWLEDGEMENT
The financial support of this research by the State of Texas through Advanced Technology Program (Grant No. 32131-70560) is gratefully acknowledged.
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