J. Pfiys. G e m . 1993,97, 12819-12827
12819
Cobalt Phthalocyanine Encapsulated in Y Zeolite: A Physicochemical Study Edgardo PBez-Mozo,t Nyole Cabrimas,? Fabio Lucaccioni, Dwight D. Acosta,: Pasquale Patron03 Aldo La Cmestra,ll Patricio Ruiz,' and Bernard Delmon Catalyse et Chimie des MatCriaux Divisb. UniversitC Catholique de Louvain. Place Croix du Sud, 2-bte 17, 1348 Louvain-la-Neuue, Belgium Received: February 10, 1993; In Final Form: July 22, 1993'
Cobalt phthalocyanine was synthesized within the cavities of Y zeolites and was characterized in all the steps of the synthesis and purification procedures by elemental analysis, Fourier transform infrared spectroscopy, UV-visible spectroscopy, X-ray photoelectron spectroscopy, surface analysis, thermogravimetric analysis, differential thermal analysis, X-ray diffraction, and conventional transmission and high-resolution electron microscopy. Besides the cobalt phthalocyanine (12% in weight), other nitrogen-containing compounds (2% in weight) are associated to the acidic sites of the zeolite and/or axially coordinated to the cobalt complex.
1. Introduction The development of so-called fine chemistry requires the development of new catalysts with sophisticated molecular architectures in order to selectively transform reactants into products with specific structures. Nature shows that microorganisms transform substrates into products with extreme selectivity without wasting reagents and energy. This is accomplished through a complicated interplay of a well-defined geometry of active centers and the concomitant positioning of the chemical species. Enzymes are formed by proteins, with definite and complicated structures. When a metal is part of the active center, it possessesa well-defined coordination and is surrounded by polymeric segments giving a precise shape to the cavity (or at least to the environment) in which it is situated. Many attempts have been made to simulate natural enzymes by encapsulating a coordination compound in the network of a porous solid. An interesting class of enzymescontains a metal coordinated to a porphyrin-typering. Quite logically, several studies mention the encapsulationof such porphyrin rings. Indeed encapsulation of coordination compounds in porous solids like zeolites might combine some characteristics of the support-pore diameter, cavity size, and electrostatic potential-with the electronic and stereochemical properties of the complex and could lead to the highly selective formation of certain products with a precise structure. Recently, the encapsulation of coordination compounds in has attracted much interest and a substantial number of articles have been published, especially concerning their preparation.1-'6 However, in most of the works concerned with encapsulation,there is little characterization of the real state of the encapsulatedcoordination compounds and of the supportused for encapsulation. Results of characterization presented in the literature are generally restricted to two or three physicochemical techniques (principally elementary chemical analysis, Fourier transform infrared spectroscopy (FTIR), or UV-visible spectroscopy (UV-vis). In some cases, X-ray photoelectron spectroscopy (XPS) and electron spin resonance spectroscopy (ESR) were used, but only exceptionally. Due to the complexity of the systems, this is generally not sufficient for unequivocal conclusion. The synthesis and characterization of this kind of compound are not simple tasks. The free aperture of the main channels of t On leave from Universidad Industrial de Santander, Bucaramanga, Colombia. t On leave from lnetituto de mica, UNAM, Mexico. CNR, lstituto Metodologie Av. Inorganiche, Area Ricerca, Monterotondo Scalo, Roma, Italy. I Dipartimento di Chimica, Universita "La Sapienza", Roma, Italy. Abstract published in Aduance ACS Absrracrs, October 15, 1993.
0022-365419312097-128 19$04.00/0
zeolite Y is 7.4 A, and the diameter of the unit cell is 13 A. The cobalt phthalocyanine (CoPc) is a planar molecule with a diameter of 13 A. The only way to incorporate the CoPc into the zeolite is by an in situ synthesis. It is legitimate to wonder whether the in situ (in the cages) synthesis occurs extensively and really in the main cavities, in view of the fact that the phthalocyanines must adopt a distorted shape. On the other hand, insitu synthesis necessitatesthat thereactant diffuseinside theporesofthezeolites, and simultaneously the reaction might thus be hindered. In other fields, such as the introduction of cations in zeolites, it has been shown that a large proportion of the material which is believed to be introduced in the pores of a zeolite actually remains outside the crystallite. A complete characterization of the intercalate is thus mandatory in order to detect possible pitfalls, and many techniquesmust be considered. The results published show that intense efforts have been made to elucidate the principal characteristics of the synthesized compounds. Nevertheless, some aspects are still obscureand not all questions have been adequately addressed. It is therefore not yet possible to draw general conclusions. In the synthesis of well-defined compounds in the restricted space of pores, channels, or cavities, the following questions must be considered: (a) Does the organic material introducedreally form the desired compound? Are there organic remnants, and what is the composition of these remnants? (b) In what yield is the desired compound obtained? (c) How deep inside the porous structure does the desired compound sit? If the special environment of the desired compound in the pores and cavities is supposed to give special properties to the entrapped system, can one rule out that molecules of the desired compound are on the surface of the solid and thus act as a nonentrapped species? (d) Does the in situ synthesis of the desired compound preserve the pore structure of the host solid? In order to give an unequivocal answer to each question, several techniques must be used in conjunction (e.g., comparison between overall nitrogen or cobalt content and phthalocyanine content, comparison between bulk and surface composition). For this reason, the techniques mentioned below were used. Chemical analysis was used in order to obtain information on the bulk composition of samples, in particular to quantify the degree of exchange of the zeolite. X-ray diffraction (XRD) was used to study the diffraction pattern of the zeolite. Due to the small amount of CoPc and other possible organic compounds formed during synthesis, this technique could only detect them in cases of very unsuccessful synthesis. Size and shape of the particle, for sufficiently large-scale features (higher than 100 nm), were studied by conventional transmission electron microscopy (CTEM). The structural analysis of a very small volume of the specimen for which a diffraction pattern can be obtained 0 1993 American Chemical Society
PAez-Mozo et al.
12820 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 has been performed by small area electron diffraction (SAED). Analysis down the scale of the individual lattice planes was performed by high-resolution electron microscopy (HREM). This technique was used in order to study the formation of extra material on, or inside, the Y crystallites and to detect any modification of the framework of the zeolite. The data on molecular structure obtained by FTIR were used to complement structural informationon the zeolite frameworkand to detect the formation of nitrogen-containingorganic compounds (CoPc) or other molecules formed as byproducts. The presence of CoPc was also studied by UV-visiblespectroscopy. This technique gives information on the coordination environment of Co and, hence, on the compounds in which it is present, whether they are formed on the surface or in the bulk of the sample. It is preferentially used for qualitative analysis, but in this work it will also be used to give a quantitative indication of the amount of CoPc formed. The determination of chemical composition of the most external parts of the solid (about 20 monolayers in depth, namely, a few nanometers) as well as the study of the chemical state of surface atoms (cobalt, nitrogen,carbon,and silicon) was realized by XPS. The reactivity of the phases present in the catalysts was studied by thermal analysis. This technique is particularly adequate to detect and to quantify the presence of surface organic phases (in particular CoPc) able to be combusted in presence of air. However, the sensitivityof this techniquedoes not permit analysis when small amounts are prescnt. The Co environmentwas studied by Mbsbauer spectroscopy. Due to the penetrating ability of 7 radiation, this technique detects with equal sensitivity all atoms either in the bulk or on the surface of the sample. Surface area, micropore volume, and pore diameter measurements,which may be altered during synthesis, were used to complete the characterization of the solids. The aim of the present work is to use a systematic program of different characterization techniques during all steps of the synthesis in order to provide detailed information on the encapsulationprocess and to provide arguments to answer these questions, dealing with the case of cobalt phthalocyanine in Y zeolite. A second objective is to show that a correlation between all the techniques used is necessary to correctly assess the results of the solid-state synthesis leading to encapsulation. The synthesis procedure has been conducted as follows: (a) exchangeof the metal in the zeolite, (b) reactionof theexchanged zeolite with dicyanobenzene (DCB), and (c) purification of the material by Soxhlet extractionwith different solvents (methanol, acetone, pyridine, and ethyl ether). In order to get a maximum of information, characterizations were performed at different stages. The following samples were studied: (i) zeolites (Nay and NHdY), (ii) pure cobalt phthalocyanine (CoPc), (iii) freephthalocyaninebase (H~Pc),(iv) metalexchanged zeolite (CoNaY and CoNH4Y), (v) zeolite after reaction with DCB and after purification (reaction blank), (vi) metal-exchanged zeolites after reaction with DCB and after purification (CoPcNaY and CoPcNH4Y). (vii) pyridine extracts of the blank, (viii) pyridine extracts of CoPcNaY and CoPcN&Y, (ix) zeolite impregnated with pyridine ( N a y py), (x) CoPc impregnated with pyridine (CoPc + py), (xi) metal-exchanged zeolite impregnated with pyridine (CoNaY py), and (xii) a mechanical mixture of cobalt phthalocyaninewith zeolite (CoPc + Nay).
+ +
2. Metbods 2.1. Reagents. The zeolite Y Lindemolecular sieves catalysts support were (a) LZY52, sodium form (Nay), with an average diameter between 0.5 and 1 pm (SEM) and (b) LZY62, ammonium form (NH4Y) (extrudate 1/16 in.). Other reagents includecobaltphthalocyanine (dyecontent -97%),cobalt acetate tetrahydrate, dicyanobenzene (98%), and pyridine (99%) and were purchased from Aldrich. Methyl alcohol (99.5%) came
from Janssen Chimica, high-purity acetone (99%) from UCBRPL (Belgium), and sulfuric acid from Merck (pure grade). All the reagents were used without further purification. 2.2. S y n t W i of thecatnlysts. 2.2.1. Preparation ofMeta1Exchanged Zeolites (CoNaY and C o N H J ) . A mixture of 42 g of the sodium or ammonium Y zeolite with 1 L of a 0.025 M solution of cobalt acetate tetrahydrate in demineralized water was stirred for 48 h at room temperature. The slurry was then centrifuged and the pink solid washed with demineralized water and dried overnight at 523 K. 2.2.2. Preparation of Encapsulated Phthalocyanine in the Zeolites (CoPcNaY and CoPcNHJ). A 10-g sample of the driedexchangedzeolitewasmixed with lOgofDCB (6:l excess), introduced in a glass ampule, evacuated to 10-3 Pa for 2 h, and then sealed and heated in an oven at 473 K for 24 h. After heating, the solid color changed from pink to violet blue. The product was purified by Soxhlet extraction, according to the following sequence: acetone, methanol, pyridine, acetone, and finally ethyl ether. Each extraction was performed until the solvent became colorless. The resulting powder was vacuum dried overnight in an oven at 453 K. 2.2.3. Preparation of a Reaction Blank. A blank was prepared by reacting 5 g of NaY with 5 g of DCB and following the same synthesis and purification procedures as described in section 2.2.2. After reaction the solid color changed from white to blue and became colorless after solvent extraction. 2.2.4. Preparation of Pyridine b y ) Impregnated Samples. (a) NaY py: NaY was mixed with pyridine and the mixture left to evaporate in air until dry and then heated for 8 h at 340 K. (b) CoNaY + py: the same procedure as for a was followed. 2.2.5. Preparation of Mechanical Mixture. CoPc + Nay: 0.0749 g of CoPc and 1.0012 g of NaY were mixed in acetone in an ultrasound bath for 3 min and air dried until complete evaporation of the acetone occurred. 2.3. Characterization hocedurea 2.3.2. Elemental Chemical Analyses. The elemental chemical analyses were performed by the Christopher Ingold Lab., University of London, England and by the Unit6 des Proc&ides, Laboratoire d’Analyses Chimiques, UCL, Louvain-la-Neuve, Belgium. 2.3.2. FTZR Analyses. R I R analyses were carried out on a Brqcker IFS 88 spectrometer; the samples were incorporated in KBr pellets for measurement. 2.3.3. W-Visible Spectra. UV-visible spectra were taken in a Philip PU 8700 spectrophotometer. The samples of the extraction solvents obtained during purification procedures were measured directly from the pyridine and methanol solutions. The CoPc in the zeolite was determined by dissolving the sample in concentrated H2SO4. The amount of phthalocyaninewas measured using a calibration curve obtained by dissolving known amounts of CoPc in HzS04and adding zeolite to take into account the matrix effects. 2.3.4. Specific Surface Analyses. Specific surface analyses were performed on a Micromeritics ASAP 2000 with nitrogen as the absorption gas at liquid nitrogen temperature. 2.3.5. Thermogravimetric and Differential Thermal Analyses. Thermogravimetric and differential thermal analyses were performed on a simultaneousTG-DTA thermoanalyzer, Model 78 1 STANTON, with a heating rate of 10 deg/min, in air, with a Pt crucible, Pt/Pt-Rh 13% thermocouples, and flow rate of the controlling gas (air) of 20 mL/min. 2.3.6. X-ray Diffraction. X-ray diffraction of the powdered sampleswas measured on a Siemens D5OO X-ray diffractometer, with CuKa radiation, and the results were treated with Siemens Diffrac I1 Software. 2.3.7. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopic studies were carried out at room temperature with a VG XPS 3 MKII spectrometer equipped with a Tracor Nrothem multichannel analyzer TN 1710, using AlKa radiation (hu =
+
Cobalt Phthalocyanine in Y Zeolite
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12821
TABLE I: Elemental Analyses in Percent by Weight NaY NH4Y CoNaY CONH~Y CoPcNaY COPCNH~Y blank
Si
A1
Na
23.72 22.70 27.05
8.43 17.12 10.11
7.46
23.27 18.48 22.60
8.80 13.95 8.54
Co
C
3.04 1.75 1.15 1.11
0.40
I
N
CoPc
0.00 6.17 4.92 6.28
10.15 13.01 4.23
2.92 3.70 1.01
1486.6 eV), and on a SSI X-probe (SSX-100/206) Fisons spectrometer. The binding energieswere calculated with respect to the C1,peak (C-C, C-H) set at 284.8 eV for pure CoPc sample analysis. As discussed in the literature, ref 20, a dispersion of the binding energy values, probably due to a differential charging effect, has been observed in zeolite studies when CISis chosen as the binding energy reference. Some authors propose the use of the SizPor the Sib peak as an internal reference.21922 Taking into account that the Si-A1 atomic ratio is unchanged in samples containing zeolites and that there is an overlap of the SizPpeak with the C Opeak, ~ ~the Sib peak has been used as a reference for these samples. The charging effect as a function of time was controlled during the measurements. The atomic concentration ratios were calculated by correcting the intensity ratios with the Wagner experimentalsensitivity factor23for the VG system and with the sensitivity factors proposed by the manufacturer for the SSI. Overlapping peaks from the VG and the SSI were both decomposed with the SSI software package on a HP 9000 computer. 2.3.8. Conventional Transmission and High-Resolution Elecfron Microscopy. Two different kinds of microscopy equipment were employed: a JEOL lOOCX microscope equipped with a simple goniometer stage and on EDS system for the CTEM and analytical studies, and a JEOL 4000 EX microscope equipped with a pole piece with a spherical aberration coefficient of C, = 1.O mm for the HREM observations. Powder samples were ground in an agate mortar, dispersed with isopropyl alcohol, and mounted on 200 mesh copper grids covered with carbon film. The samples were kept overnight in the microscope column before the observations, to improve the stability under electron irradiation. 3. Results 3.1. Elemental Analyses. The results of chemical elemental analyses are shown in Table I and give a Si/Al ratio of 2.55 which corresponds to a unit cell of formula' Na54[(A102)54(Si02)138] for NaY and Na~~C0,,~[(A102)~4(Si02)~38] for CoNaY. The A1 and Coin CoNaY give a degree of Na exchange of 27.5%. From this exchanged Co, only 43.4% remain in the CoPcNaY. 3.2. F l l R Spectra. Figure 1 shows the spectra of N a y , CoNaY, CoPcNaY, and CoPc. The principal bands of CoPc are present in CoPcNaY and COPCNH~Y, as given in Table 11. 3.3. Visible-Ultraviolet Analyses. The spectra of CoPc, CoPcNaY, and CoPcNH4Y in concentrated H2SO4 show the characteristic absorption bands at 420,697, and 184 nm. As can be seen in Figure 2, the spectra of pure and encapsulated phthalocyanine are identical. The spectra of the extraction solvents used in the purification of CoPcNaY, CoPcNH4Y,and also the reaction blank show the characteristic absorption bands of the phthalocyanine which were absent in the solution of the purified blank in H2S04. The amounts of CoPc present in the zeolite were found to be 1.18 X 10-4 moles of CoPc/g in CoPcNH4Y (6.74% weight) and 1.48 X 10-4 moles of CoPc/g in CoPcNaY (8.46% weight). 3.4. Specific Surface A ~ l y s e s .The results of specific surface analyses, including BET and Langmuir surface area, micropore volume, and pore diameter,are presented in Table 111. The surface areas of NaY and CoNaY remain unchanged, but the surface
CoPc NaY
Co NaY
NaY 1600
1500
1400
1300
1200
WAVE NUMBERS CH-'
Figure 1. FTIR spectra of N a y , CoNaY, CoPcNaY, and CoPc in KBr pellets.
TABLE Ik FllR Results. Infrared Bands (in cm-1) for CoPc, CoPcNaY, and CoPcNKY in the Region 1650-1200 cm-1 CoPc
CoPcNaY
CoPcNbY
1289 1332 1426 1448 1470 1490 1523 1610
1290 1334 1426 1451 1470 1492 1525 1616
1290 1334 1427 1470 1488 1526 1617
area of CoPcNaY decreases significantly. Similar observations can be made for NH4Y zeolites. Microporous volume and pore diameter decrease after the synthesis of Pc. 3.5. Thermogravimetric and Differential Thermal Analyses. Figure 3 shows the thermogravimetric patterns for CoPc, CoPcNaY, CoNaY, and N a y . Figure 4a shows the differential thermal analysis (DTA) patterns for CoPcNaY and NaY py samples, and Figure 4b presents the DTA patterns for CoPc and CoPcNaY. The comparison of thermogravimetric analysis (TGA) and DTA results for CoPc and CoPcNaY (Figures 3 and 4b) shows the same behavior and combustion temperature for crystalline and encapsulated CoPc. The combustion of at least two organic compounds can be observed in CoPcNaY (14.15% weight loss). The one coincident with the phthalocyanine amounts to a weight loss of 1296, corresponding to 13.3% of the CoPc. The combustion of HzPc, which takes place at T > 500 OC, was not detected in CoPcNaY, but a small peak above 500 OC is present in the blank and corresponds to a 5% weight loss in CoPcNH4Y. 3.6. X-ray Diffraction. Powder diffraction patterns were obtained for Nay, CoNaY, CoPcNaY, NH4Y, CoNHdY,
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12822 The Journal of Physical Chemistry, Vol. 97,No. 49, 1993
Phez-Mozo et al.
L CoPc 75 10
t
li
\CoPcNaY
L
30 I
1
I
L CoPcNa’
1
I
I
I
bo0
500
600
700
800
900
nm
Figure 2. UV-visible spectra of CoPc and CoPcNaY in H2S04.
TABLE III: Specific Surface Analyses Langmuir micropore BET surface surface volume pore area (m2/g) area (m2/g) (cm’/g) diameter (A) NaY 699 943 0.307 72.1 CoNaY 693 934 0.291 61.1 CoPcNaY 378 425 0.126 59.1 NH4Y 645 124 0.196 14.9 CONH~Y 535 0.153 13.1 COPCNH~Y 41 CoPcNHdY, CoPc, and CoPc+NaY samples. No variation was observed in thediffraction patternofthezeoliteafter theexchange and encapsulationprocedures. A unit cell dimensionof a = 24.684 Awasdeterminedfor NH4Y. Thevalueofa = 24.730Aobtained for NaY corresponds to a unit cell formula with 54(A102).17 No CoPc pattern was detected in CoPcNaY or in CoPcNH4Y. 3.7. X-ray Photoelectron Spectroscopy. Figure 5 shows the C0zPphotoelectron spectra of crystalline CoPc, the CoPc + NaY mixture, CoNaY, the CoNaY + py mixture, and CoPcNaY. The satellite structure, well-defined in CoNaY py and CoNaY, is not observed in CoPc or in the CoPc NaY mixture and is present in CoPcNaY, although weaker. The C0zP signal for CoPcNaY and CoPcNH4Y was broad and very weak, requiring a much larger number of scans than for the other samples. In Figure 6, the Nl, lines are shown for H ~ P cpure , CoPc, CoPc + Nay, CoNaY + py, NaY + py, the reaction blank, and CoPcNaY. All the Nl, spectra (except for CoNaY py) were obtained with the SSI equipment. Table IV gives Cozp3/2,and Cozpl/2binding energies together with the Co/Si atomic ratio in the bulk as given by chemical analysis and as obtained by XPS analysis. Table V presents, together with the binding energy for NI,, the Co/N and C/N ratios in the bulk and those calculated by XPS analysis. While the binding energies for Sib, A4, Ols,and Nalsremained unchanged, a small shift toward lower energies for the C Oand ~ ~ NI, binding energies was observed when CoPc was in contact with the zeolite.
+
+
+
10
t‘
0
I
I
I
I
I
I
100
200
300
400
SO0
600
DEG C Figure 3. Thermogravimetric analysis (TGA) results for Nay, CoNaY, CoPc, and CoPcNaY.
The atomic ratio for (Co/Si)xps is higher than for (Co/Si)bulk.
On theother hand, the (N/C)xps and the (N/C)b,lkatomic ratios are about the same. A more detailed quantitative XPS study concerning phthalocyanines of cobalt and of other metals (Mn, Fe, Ni) is in progress.24 All the Nl, spectra (except for the spectrum for CoNaY py) were obtained with the SSI equipment. Analysis of the Nl, peak was very delicate, due to the imprecisions in fitting and deconvolution of the peaks. The results presented in Table V are indicative and do not exclude that other binding energies could be observed,for example, those concerning the presence of possible N-0 type species due to the effect of air exposure. 3.8. CTEM and HREM. Full identification of the sample was performed using small area electron diffraction (SAED) patterns obtained from isolated crystallites in severalorientations for each sample. Figure 7 (HREM) shows a general view with clear resolution. The marked zone shows intensity variation details to be analyzed. Figure 8 shows a sizeable enlargement of this zone and corresponds to a projected potential image of the Y zeolite with the compound in the [ 1 111 direction. Figure 8 shows twoimportant details: (1) the blackarrow corresponds to a tunnel image with a significantlydifferent contrast from the surrounding tunnels. (2) the empty arrow indicates a slight distortion of the surrounding planes.
+
4. Discussion 4.1. Zeolite Framework. The unit cell formula obtained from X-ray diffraction agrees with the one calculated from chemical analyses for Nay. No similar calculations could be made for NH4Y because the probable presence of A1 in the binder used in the fabrication of the zeolite makes it difficult to quantify the Si/A1 ratio exactly. In all samples, no detection of any damage of the main zeolite framework is revealed by the diffraction patterns. From SAED patterns a full identificationof synthetic Y zeolite was carried out. Some nonsystematic variations were observed
Cobalt Phthalocyanine in Y Zeolite
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12823
200
5 0
-5
-
1
v)
c 0
w
0 L
.=
r
2oo
1
-10
-15 -20
v)
150
f
100
.=
0
> 0
L
r
I
1
-25
-301
I
,
!
100
200
300
l
COO
SO0
Deg C
600
200
300
400
500
-50 600
Deg C
+ Py and (b) CoPc and CoPcNaY.
Figure 4. Differential thermal analysis (DTA) results for (a) CoPcNaY and NaY
NaY+py
CoNaY+py
fp
Blank
coNdy+py 20 scans
A
CoPcNaY 115 scans
CoR+NaY
21 scans
6
\
I
I
H2 Pc CoPc 6scans
I
I
I
I
41 0
40 5
400
395
ev
ev
Figure 5. XPS rwults: Colp lines of CoPc, CoPc + Nay, CoPcNaY,
+ py. The vertical scale is not the same for all the
Figure6. XPSresults: N~,linesofH~Pc,CoPc,CoPc+ NaY,CoPcNaY, reaction blank, CoNaY py, and NaY py. The vertical scale is not the same for all the samples.
in some interplanar distances, for instance, values like 8.75 Aand 8.56 A were detected for the d220 reflection. The cause of these
variations has not been investigated, but there is no doubt that structural alterations are present in some Y zeolite crystallites.
CoNaY, and CoNaY samples.
+
+
12024
PQez-Mozoet al.
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE I V CozpBinding Energies and (Co/Si)xps Ratio on the Surface from XPS and Calculated (Co/Si)w BE sample COPC COPC + Py CoPc + NaY CoPcNaY CoNaY + Py CoNaY coo coo0
no. of scans
BE satellite
c02~312
C02D112
satellite
(CO/Si)XPs
(CO/Si)blllk
803.0 803.7
0.0549 0.0455 0.1270 0.1 136
0.01 56b 0.023Y 0.0533b 0.0533c
~~~
6 8 21 185 20 40
780.6 780.8 779.3 779.8 78 1.6 782.1 777.9 780.0
796.0 796.2 794.5 795.0 797.4 797.6 792.9 795.5
787.1 787.1 786.3
802.5
From C. D. Wagner, et al., ref 34. Theoretical calculation. From chemical analysis.
TABLE V
XPS Binding Energies for N1, with the Co/N and C/N Ratios at the Surface (XPS) and in the Bulk
sample H~Pc COPC COPC + Py CoPc + NaY CoPcNaY blank HY + Py' NaY + py CoNaY + Py
(Co/N)xPs 398.9 397.7 398.2
400.5 399.0 398.9 399.9 399.9 399.9 399.7 400.4 399.4
402.0 402.0 401.9 401.7 402.7 401.9
0.14 0.14 0.18 0.16
2.65
(Co/N) bulk 0.1 25b 0.125b 0.095"
("xps
(N/C)blllk 0.24 0.22 0.23 0.18 0.23 0.14
0.25b 0.25b
0.10
0.206
O.2Sb 0.24" 0.15"
Chemical analysis. Theoreticalcalculation. C Boradeet al. (ref 2 9 , corrected for Si2, BE = 102.65 eV. Total C1, peak(quantity of C contamination visible in XPS was negligible compared with C1, peak of the sample).
Figure 7. High-resolutionelectron microscopy image of Y zeolite containing encapsulated CoPc molecules. The arrow shows a small projected potential image of the Y zeolite crystallite in the [ 1111 direction.
No other modification of the framework of the zeolite was observed. 4.2. Formation of Organic Compounds. The formation of nitrogen-containing organic compounds was indicated by the bands present in IR and UV-visible spectra and by the amounts of C and N determined in the elemental analysis and by XPS. The quantity of C contamination visible in XPS was negligible in comparison with the C1, peaks of the samples. The broadening
of the N1, XPS signal in the blank, in NaY + py, and in CoNaY py can be attributed to the adsorptionof the nitrogen-containing compounds on the different acidic sites of the zeolite as found by Borade et al.25126The combustion reactions found in TGA and DTA are typical of organic compounds and correspond to weight losses of 14.2% in CoPcNaY and 22.3% in CoPcNHdY. The large reduction in surface area and micropore volume could be interpreted as the formation of compounds in the pores
+
Cobalt Phthalocyanine in Y Zeolite
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12825
Figure 8. Enlargement of the area indicated by arrows zone in Figure 7. The full black arrow shows localized intensity changes. The empty arrow shows collapsed tunnels probably due to the presence of CoPc in the zeolite structure.
of the zeolite. The variation of intensities and the distortion of planes observed in high-resolutionelectron microscopy also point to the presence of foreign material in the zeolite. 4.2.1. Formation of Cobalt Phthalocyanine. The presence of phthalocyanine in NaY and NH4Y zeolites is confirmed by the characteristic bands found in the IR and UV-vis spectra and by the TGA and DTA patterns. Thermogravimetric analysis and visible spectroscopy permit the quantification of the moles of CoPc present in 1 g of CoPcNaY and give 2.3 X 1V and 1.5 X lW,respectively. The difference found could be rationalized by taking into account the fact that visible spectroscopy gives lower values because the zeolite framework is not completely destroyed by the H ~ S 0 4 . lAlso ~ if, as it is considered in section 4.2.2,there are nitrogen-containing compounds axially bound to the CoPc, the molecular weight would be higher and the weight loss observed in TGA (1 2%) would represent fewer moles of phthalocyanine. Calculations based on the Co content in CoPcNaY found in elemental analysis show that the maximum theoretical amount of CoPc is 11.15% by weight. Comparison of this value with the results mentioned above indicates that at least 76% of the Co present in CoPcNaY is in the form of CoPc. In CoPc, the charge transfer from N to Co increases the electronic density on the Co, giving a calculated atomic charge of 0.31 e2' which explains the lower binding energy determined by XPS for Co in phthalocyanine (780.6 eV) compared with CoNaY (782.1 eV). The low C0Zp binding energy found in CoPcNaY (779.8 eV) is an indication of the presence of Co in the phthalocyanine ring. 4.2.2. Other Compounds Formed during the Synthesis. The larger than expected C/Co and N/Co ratios calculated from elemental analysis, the combustion of other substances prior to phthalocyanine found by thermal analysis (2% by weight), and the large broadening of the N1, XPS signal (more than 4 eV) point to the presence of other nitrogenated organic compounds,
probably due to the great absorption potential of the zeolite13and to the strong affinity of Co for nitrogenated ligands.2* The comparison of the position of the N1, XPS signals (Figure 6) of CoPcNaY with those of NaY + py and the blank suggests the presence of dicyanobenzene and pyridine which could be adsorbed on the ~ e o l i t e . ~ ~ ~ ~ ~ Deconvolutions of Miissbauer spectra of FePcNaY and FePcNH4YZ4show two isomer shiftsand two quadrupolesplittings which could be assigned to FePc axially coordinated to pyridineand CN-containing compounds.29 If this were also the case with CoPcNaY, it would explain the facts that both the C/Co ratio (found in elemental analysis) and the weight loss (found in TGA) are larger than expected. In the diffraction patterns of CoPcNaY and COPCNH~Y, two small peaks were observed that could correspond to the presence of a-H2Pc, but a full identification was not possible due to their very low intensities and the coincidence of a-HzPc and zeolite peaks. Although no evidence of the presence of H2Pc was found in the TGA and DTA of CoPcNaY, in CoPcNH4Y a combustion reaction above 500 OC, contributing to a 5% weight loss, could be tentatively attributed to H2Pc. Even though the formation of the free phthalocyanine is not expected,some observationsin this regard have been reported by Parton et a1.2 4.3. Location of the Cobalt Phthalocyaninein the Zeolite. In CoPcNaY, the XPS N1, signal has been deconvoluted in three different peaks, which could be attributed to the interaction of the nitrogen-containingcompounds with the different acidic sites of the zeolite. By analogy with the spectra of the mechanical mixture, the lower energy peak is assigned to the phthalocyanine nitrogen. Its asymmetry suggestsdifferent kinds of nitrogen and can be decomposed into two peaks at 397.1 and 398.2 eV. The small shift toward lower values of binding energy found for N1, and C0Zpin XPS in encapsulated CoPc samplescompared with pure CoPc indicates a higher electron density which could
Phez-Mozo et al.
12826 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
be due to the impairment of the delocalization of the ?r electrons ofthe ring brought by thelossof planaritycaused by thedistortion of the phthalocyanine when confined in the zeolite.2.6 TheobservedCls,NLs and COZ,XPS signals after encapsulation are weaker, compared to those of the mechanical mixtures made with a similar percentage of CoPc in Nay. This could be an indication that in CoPcNaY, fewer electrons are emitted. Since, with the XPS technique, the inner structure and cavity contents of thesolidsareanalyzed toadepthofabout SO& thisobservation implies that at least part of the CoPc must be within the cavities of the zeolite. Let us note that, on the contrary, if the phthalocyanine was deposited on the most external surface of the zeolite crystallites the signal would be stronger. The fact that the atomic ratio is higher for (Co/Si)xps than for (Co/Si)mimplies that thereissomeenrichment oftheextemal surface of the zeolite in CoPc. This is the case principally for CoPc NaY and CoPcNaY. Deposition of part of the CoPc on the surface of the zeolite, in the mechanical mixtures and in the encapsulated samples, can explain this observation. The broadening of Nl, and COZ,XPS signals in CoPcNaY could be related to the dispersion of the phthalocyaninemolecules. As did Niwa et al.,27.30we have found that while HzPc gives two N1,lines, metal-containing Pc shows a single peak. Coalescence of the N1, signals is due to the strong intermolecular interaction of metal phthalocyanines, as was noted by Goudkov et Since the high dispersion of CoPc within the zeolite prevents intermolecular interactions, it could allow the observance of the nonequivalence of the nitrogens. Although the charge densities on the nitrogen atoms calculated using the extended Huckel molecular orbital method by Niwa et al.3O show that in pure CoPc the central nitrogens are only 0.03 e less negative than the bridging ones (compared to a difference of 0.21 e for H~Pc),this difference could be enhanced by interactions of the bridging N with the zeolite or the distortion of the Pc due to the steric hindrance and the strong electrostatic field of the lattice.32 We have observed that dispersing CoPc on a zeolite, as in the mechanical mixture, broadens the N1, signal and causes a small shift toward lower values in the binding energy, in agreement with the findings of Borade et al.25.26 The absence of CoPc peaks in the X-ray diffraction patterns of CoPcNaY and CoPcNH4Y indicates that the CoPc is not forming crystals of appreciable size and is instead dispersed in the zeolite. Taking into account the content of Na in the zeolite and the fact that only 1% of the cationic exchange sites are on the external surface of the zeolite,17it can be calculated that if all the external ionic exchange sites of the zeolite were completely occupied by Co ions, and if all those cobalt ions would form CoPc, the weight loss due to the combustion of the CoPc present on the surface would be about 0.83%. Since a Pc weight loss of 12% was found, there should be coordination compounds in the internal cavities of the zeolite. HREM images show the presence of marked contrast changes and tunnel size variations, like those shown with arrows in Figure 7. These contrasts might originate from the presence of extra material on, or inside, the tunnels and cages along axial directions in the Y zeolitestructure. Extra material inside the Y crystallites should lead to thickness or crvstalline Dotential variations. which in turn lead to contrast and/& high-rksolution detail changes in Proiected potential Details like those indicatedin Figure ? b; arrows arevisible several high-resolution images obtained at different image formation conditions. In Figure 8, some of the details mentioned are indicated with arrows. This observation is not, alone, a proof of intercalation, but the presence of Pc complexes inside or on the Y zeolite structure could be the cause of localized changes of contrast in HREM zeolite images. If our argument is valid, it can also be deduced from HREM images
+
that Pc complexes are not uniformly distributed through the Y zeolite crystallites. 5. Conclusions Formation of CoPc. The formation of a phthalocyanine ring can be established by the IR and UV-vis spectra. TGA and DTA patterns, as well as the Coz, XPS signal, confirm the presence of CoPc. The amount of CoPc can be quantified with vis-UV and TGA. hesence of Other Organic Compounds. The presence of other organic compounds is shown by the large C/Co ratio (>32) and the large broadening of the N1, XPS peak (>4 eV). These nitrogen-containing compounds held in the acidic sites of the zeolite could also be bound to the axial positions of CoPc and are not eliminated by prolonged extraction with solvents. Their presence contributes to the lowering of the pore volume and could affect the penetration of substrates to the active sites. More drastic or different purification techniques are needed in order to achieve their elimination. Zeolite Structure. The main framework of the zeolite is not damaged, as shown by X-ray diffraction and HREM, although some distortion of planes is observed due to the presenceof included compounds. Location of CoPc. The lowering of the pore volume, the distortion of the zeolite planes shown by HREM, and the considerable weakening of the XPS CozPintensities point to the presence of CoPc within the cavities of the zeolite. Although very promising, these materials constitute a complex system and much work must be accomplished in order to benefit from their high potential by fine tuning their special chemical, physical, and steric properties. Acknowledgment. We thank the Commission of the European Communities (Directorate General - Science, Research and Development - DG XII-G [E.P.M. and N.G.]) for financial support and Mr. M. Genet (UnitCdeChimie des Interfaces, UCL) for his assistance and helpful discussions on XPS. References and Notes (1) Romanovsky, B. V.; Gabrielov, A. G. New Developments inSelective Oxidation by Heterogeneous Catalysis; Ruiz, P., Delmon, B., Us.Elsevier: ; Amsterdam, 1992; Vol. 72. (2) Parton, R. F.; Uytterhoeven, L.; Jacobs, P. 2nd International Sympsium on Heterogeneous Catalysis and Fine Chemicals, Poitiers, France, 1990. (3) Bowers, C.; Dutta, P. J . C a r d 1990, 122, 271. (4) Kowalak, S.; Weiss, R.; Balkus, K. J . Chem. Soc., Chem. Commun. 1986, 57. (5) Herron, H.; Stucky, G.; Tolman, C. J. Chem. Soc.,Chem. Commun. 1986, 1521. (6) Herron, H. J . Coord. Chem. 1988, 19, 25. (7) Tollman, C.; Herron, H. Symposium on Hydroc. Oxidation, 194th National Meeting of the American Chemical Society, New Orleans, LA, Aug. 304ept. 4, 1987. (8) Meyer, G.; WBhrle, D.; Mohl, M.; Schulz-Ekloff, G. Zeolites 1984, 4 , 30. (9) Romanovsky, B. V. Proceedings of the International Symposium on Zeolite Catalysis, Siotok, 1985, p 215. (10) Romanovsky, B. V. Proceedings of the 5th Interational Symposium on Relation between Homogeneousand HeterogeneousCatalysis, Novosibirsk, July 15-19, 1986, p 343. (11) Ozin, G.; Gil, C. Chem. Rev. 1989,89, 1749. (12) Zakharov, A. N.; Romanovsky, B. V.; Luca, D.; Sokolov, V. I. Mefalloys Khim. 1988, I , 119. I 1 31 Zakharov, A. N.: Korol'kova, T. V.; Romanovskv, B. V. Koord. Khim. 19*6 94. (14) Zakharov, A. N.; Romanovsky, B. V. J. Inclusion Phenom. 1985,3, 121
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(1 5) Chan, Y .W.;Wilson, R. B. Prep.-Am. Chem. Soc.,Diu. Pet. Chem. 19889 33*453. (16) Huybrechts, D. R. C.; Parton, R. F.; Jacobs, P. A. Zeolites as partial oxygena~oncatalys~, Chemistryofmicroporoucrystols;Inui,~.,Namb, S., Tatsumi, T., Eds.; Elsevier: Kiodanska, 1991. R. (17) Kriger Breck. Publ,D.co,: W. Zeolite Malabar, molecular FL, 1984. sieves, structure, chemistry and use; (18) Rabo, J. E., Ed. Zeolite Chemistry and Catalysis;ACS Monograph 171; American Chemical Society: Washington, DC, 1976.
Cobalt Phthalocyanine in Y Zeolite (19) Lamen, E.; Jorgensen, K. A. Acru Chem. Scund. 1989,13,259. (20) Madey, T. E.;Wagner, C. D.; Jashi, A. J. ElectronSpecrracc. Relur. Phenom. 1977,lO, 359. (21) Remy, M.J.; Genet, M. J.; Poncelet, G.; Lardinois, P. F.;Nottt, P. P. J. Phys. Chem. 1992, 96,2614. (22) VCdrine, J. C.; Auroux, A.; Dejaifve, A. P.; Ducarme, V.; Hoser, V.; Zhou, S.J. J . Curd 1982, 73, 147. (23) Wagner, C.D.; Davis, L. E.;zefler, M.V.; Taylor, J. A.; Raymond, R. H.;Gale, L. H.SIA, Sur/. Interface A d . 1981, 3 (5), 211. (24) PaCz-Mom, E.; Gabriunas, N.; et al. To be published. (25) Borade, R.; Adnot, A.; Kalliaguine, S. J . Mol. Curul. 1990,61, L711. (26) Borade, R.; Adnot, A.; Kalliaguine, S . J . Cutul. 1990, 126, 26. (27) Niwa, Y.; Kobayashi, H.;Tsuchiya, T. T. Inorg. Chem. 1974, 13, 2891.
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