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H-Beta catalysts: Characterization and reactivity in piperonyl alcohol selective oxidation
Applied Catalysis A: General 359 (2009) 144–150
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Pd/H-Beta catalysts: Characterization and reactivity in piperonyl alcohol selective oxidation Olga P. Tkachenko a, Leonid M. Kustov a, Andrey L. Tarasov a, Konstantin V. Klementiev b, Narendra Kumar c, Dmitry Yu. Murzin c,* a b c
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Leninsky pr. 47, Russia ALBA synchrotron - CELLS, Campus UAB, 08193 Bellaterra, Barcelona, Spain A˚bo Akademi University, Biskopsgatan 8, F1-20500 A˚bo, Turku, Finland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 December 2008 Received in revised form 24 February 2009 Accepted 27 February 2009 Available online 11 March 2009
Pd/H-Beta catalysts with different silica-to-alumina ratio prepared by an impregnation method have been studied by XANES/EXAFS, IR, XRD and TEM. It was shown that palladium in calcined Pd/H-Beta samples exists as palladium oxide that can be easily reduced to Pd metal. The catalysts were tested in selective oxidation of piperonyl alcohol to piperonylaldehyde at 80 8C. The Pd/H-Beta zeolite catalysts with the lowest silica-to-alumina ratio possess the highest catalytic activity in benzylic alcohol oxidation reaction. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Pd/H-Beta zeolites XAS IR TEM Piperonyl alcohol oxidation
1. Introduction Metal nanoparticles formed in various media and stabilized by different mechanisms are the objects of intense studies due to their unique properties, which are significantly influenced by the nanoparticle size, organization of the nanoparticle crystal lattice, nanoparticle surface and the chemical nature of the microenvironment surrounding the nanoparticle [1–4]. The metal nanoparticles exhibit unique properties that differ from the bulk substances, e.g. different heat capacity, vapour pressure and melting point. Moreover, when decreasing the metal particle size sufficiently enough, there occurs the transition of the electronic state from metallic to a non-metallic one. Additionally metal nanoparticles exhibit large surface-to-volume ratio and increased number of edges, corners and faces leading to altered catalytic activity and selectivity. The higher accessibility and concentration of surface metal atoms in nanosized particles makes such catalysts attractive for industrial application. The effect of the support on the metal properties is not very well understood, especially when comparing inorganic and organic matrices having the same metal nanoparticle sizes. The local metal
* Corresponding author. E-mail address: dmurzin@abo.fi (D.Yu. Murzin). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.02.049
environment which can be changed, for instance, by varying the pre-treatment of the catalyst can affect both the metal particle size and the shape/morphology. Moreover the acidity or basicity of the support affects the stabilization of the metal particle. Depending on the type of support materials, in particular acidity, metal particles, for instance Pd0, can exhibit different catalytic activity [5,6]. In the present study, several Pd catalysts supported on Beta zeolite with different acidity were prepared, characterized, and evaluated in selective oxidation of piperonyl alcohol to piperonylaldehyde. Benzilic alcohols oxidation was chosen for several reasons. First, this is the reaction with a focus on the industrial process, such as production of vanilin. Second, the substrate provides several options for the oxidation process and the selectivity may vary depending on the catalyst nature. Third, the palladium-containing catalysts are widely studied in a number of hydrogenation processes and a few oxidation processes, but no information is available on their activity in the oxidation of benzilic alcohols. The patent literature data [7] shows that the existing technologies for the conversion of benzylic alcohols into aldehydes require significant amount of metals (Pt, Pd, Au) and the formation of the side products like salicyclic acid and polymerization materials was noted. Therefore, the field to be studied in this paper opens up new prospects in the application of palladium-containing catalysts in new catalytic processes.
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2. Experimental 2.1. Pd/zeolite preparation Palladium impregnated catalysts Pd-H-Beta-300, Pd-H-Beta150, Pd-H-Beta-25, Pd-H-Beta-22 (the number in the catalyst name denote silica-to-alumina ratio) were synthesized as follows. NH4Beta-300, NH4-Beta-150, NH4-Beta-25 were supplied by Zeolyst International. NH4-Beta-22 zeolite was synthesized as described in [8] with some modifications. A solution was prepared by mixing colloidal silica (Ludox As 40, Aldrich) with distilled water. Another solution was prepared by dissolving sodium aluminate (NaAlO2) in distilled water and adding tetraethylammonium hydroxide ((C2H5)4NOH, Fluka). The two solutions were mixed together and stirred for 30 min. The gel formed was transferred into a Teflon cup, which was inserted into a 300-ml stainless steel autoclave (Parr Instruments). The synthesis was carried out at 150 8C for 96 h. After completion, the autoclave was quenched and the crystalline product was filtered, washed with distilled water, dried at 100 8C for 12 h, and calcined at 550 8C for 9 h in a muffle oven (at a heating rate of 5 8C/min). The sodium form of Beta-22 was ionexchanged with 1 M ammonium chloride (NH4Cl) for 48 h and washed with distilled water to remove any chloride ions. The obtained NH4-Beta-22 zeolite was dried at 100 8C for 12 h. The proton forms of Beta zeolites were obtained by calcination of the NH4 form of the materials in a muffle oven at 500 8C for 4 h (at a heating rate of 5 K/min). All these materials were metal-modified by wetness impregnation in a rotor evaporator (Buchi Rotavapor R114). The source of Pd was an aqueous solution of palladium (II) nitrate dihydrate (Pd(NO3)22H2O, Fluka). The impregnated catalyst was dried overnight at 110 8C and thereafter calcined in an oven at 400 8C for 3 h (at a heating rate of 5 K/min). All catalysts were stored in calcined state. 2.2. Characterization techniques The specific surface areas of the supported metal catalysts were measured with a combined physisorption–chemisorption apparatus (Sorptometer 1900, Carlo Erba Instruments). The Dubinin method was used for the zeolite supported palladium catalysts to calculate the surface areas. The palladium content (wt%) of several zeolites was determined by direct current plasma atomic emission spectrometry (DCP, ARL SpectraSpan7). Prior to the analysis, the samples were dried in an oven at 100 8C overnight and stored in an exicator. 100 mg of the dried catalyst sample was dissolved in a mixture of 3 ml hydrofluoric acid and 3 ml aqua regia in a microwave oven. The dissolved sample was diluted with de-ionized water to 100 ml and analysed by DCP. In order to determine the phase purity and structure, the catalyst samples were pre-treated (dried) at 100 8C in 1 h and stored in acetone, thereafter they were analysed by X-ray powder diffraction (XRD). The patterns from the analyses were compared with the patterns obtained from pure support materials. The measurements were done on a Bragg–Brentano u/2u reflection geometry based Philips PW1820 diffractometer. To determine the metal particle size and dispersion, samples were analysed by CO pulse chemisorption (Autochem 2910, Micromeritics). Prior to the measurements, the samples were dried in an oven at 100 8C overnight and stored in an exicator. 100 mg of the sample was placed into quartz U-tube containing silica wool, the tube was inserted to the system and the sample was further dried in a stream of helium at 50 8C for 30 min. After that, the sample was pre-treated (reduced) at 100 8C for 1 h in hydrogen, using helium as a carrier gas, after which it was
145
flushed with helium (still at 100 8C) for 1 h, cooled to room temperature, placed on a water bath, and subsequently the CO pulses were introduced (10% CO in helium, helium as a carrier gas) until the adsorption was complete. The dispersion was calculated from the amount of CO consumed with Pd:CO stoichiometry presumed to be equal to 2 [9] and atomic crosssectional area to be 0.0787 nm2. The validity of this assumption will be discussed further in the text. The acidity of several impregnated zeolites was measured by Fourier transform infrared spectroscopy (ATI Mattson FTIR) using pyridine as a probe molecule for qualitative and quantitative determination of Brønsted (BAS) and Lewis acid sites (LAS). The catalyst samples were pressed into thin self-supported wafers with a typical weight of 15 mg. The sample was evacuated at 450 8C for 1 h. Pyridine was then adsorbed for 30 min at 100 8C and desorbed by evacuation at different temperatures (250, 350, and 450 8C) to obtain a distribution of acid site strengths. All spectra were recorded at 100 8C. Pyridine is adsorbed on BAS and on LAS to reveal the corresponding vibration frequencies 1545 and 1455 cm1. The concentrations (mmol/gcat) of BAS and LAS were calculated from the intensities of the corresponding bands using the molar extinction coefficients reported by Emeis [10]. XAS (XANES + EXAFS) measurements were carried out at HASYLAB (DESY in Hamburg, Germany) on the beamline X1 (PdK-edge, 24,350 eV) using a double-crystal Si(3 1 1) monochromator, which was detuned to 50% of the maximum intensity to exclude higher harmonics in the X-ray beam. The spectra were recorded in the transmission mode at T = 80 K in order to decrease the Debye–Waller factors. The spectrum of metal Pd-foil was recorded simultaneously between the 2nd and 3rd ionization chambers for energy calibration. Reference spectra were recorded using standard reference compounds (Pd-foil and PdO). All experiments were performed using an in situ EXAFS cell in which a small grained sample was exposed at 120 8C to flowing He (dehydration step) and at 100 and 200 8C to the flowing mixture 5% H2/He (reduction step). Thereafter the samples were flushed by pure helium at the same temperature to avoid the formation of palladium hydride. To extract quantitative information from EXAFS spectra, Fourier-filtered shell contributions were fitted using the standard EXAFS formula in the harmonic approximation:
x ¼ S20
X N j F j ðkÞ j
kR2j
2
expð2s 2j k Þ sin½2kR j þ f j ðkÞ
(1)
with the summation over atomic shells j. The required scattering amplitudes and phase shifts F and f were calculated by the ab initio FEFF8.10 code [11]. The fitting was done in the k- and r-spaces. The shell radius Rj, coordination number Nj, Debye–Waller factor s 2j and adjustable ‘‘muffin-tin zero’’ DEj were determined as fitting parameters. The errors of the fitting parameters were found by decomposition of the statistical x2 function near its minimum, taking into account maximal pair correlations. To evaluate the formal average cluster size (particle radius (R, A˚)) from coordination numbers and Pd–Pd distances in the first coordination sphere, which were obtained by calculating EXAFS data, the following equation was used [12]: 3 1 CN cluster ¼ CN crystal 1 þ 3 4r 16r
(2)
where r = r/R; r is the Pd–Pd interatomic distance in the first shell, A˚; CN cluster is the coordination number of the first shell; and CN crystal = 12 for the fcc lattice of palladium. Analysis of the EXAFS spectra was performed with the software VIPER for Windows [13].
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2.3. Catalytic reaction The piperonyl (3,4-methylenedioxybenzylic) alcohol selective oxidation:
was performed in a sealed tube (20 ml volume) supplied with a gas inlet and outlet. The tube reactor was charged by 1 g of piperonyl alcohol, 10 ml of 1.25 N aqueous solution of NaOH (pH 11), a catalyst activator (0.1 ml of a 0.05 M aqueous solution of lead acetate Pb(OAc)2) and 0.2 g of the catalyst. The reactor was purged with oxygen and the reaction mixture was heated to 80 8C. Under these conditions, dissolution of the starting alcohol takes place. Then oxygen was purged through the reaction mixture with the flow rate of 50 ml/min. The gas flow served also to stir the mixture. The reaction mixture was analysed every hour. For this purpose 1 ml of the reaction mixture was placed in the separation funnel and acidified with HCl until pH 7, then the unreacted alcohol and the aldehyde formed (the target product) were extracted with chloroform (2 ml) and the organic phase was separated from the aqueous phase. In order to check the extraction efficiency it was repeated with a new portion of chloroform followed by quantitative analysis leading to a difference of just 1% compared with the first stage of extraction. Therefore one extraction stage was considered to be efficient enough. The organic phase was analysed by GC using a 3700 model chromatograph with packed SE-54 capillary column (25 m) in the temperature programmed regime: heating to 100 8C for 4 min, then at 20 8C/min heating to 180 8C. 3. Results and discussion
Fig. 1. Analysis of 2 wt% Pd-H-Beta-300 by transmission electron microscopy (a) TEM micrograph and (b) size histogram.
3.1. Characterization Table 1 displays the surface area (Dubinin method), metal content, dispersion, and metal particle size. For the Beta catalysts, the surface area was decreasing in the following order: Pd on HBeta-300 > H-Beta-150 > H-Beta-25 > H-Beta-22. Pd-H-Beta-300 had the lowest metal content, only 2 wt%, while for the others it varied between 3.9 and 4.7 wt%. Pd on H-Beta-300, H-Beta-150 and H-Beta-22 supports were all having approximately the same dispersion. The metal particle size was also obtained from XRD for palladium on H-Beta-25 and H-Beta-22 (Table 1, values in parenthesis). There are limitations with both chemisorption and XRPD methods. For CO chemisorption, one problem is the assumed stoichiometry between metal and CO. The adsorption stoichiometry on particles with the size of several nm (Pd:CO = 2, CO adsorbed in bridged form) is different from small particles (Pd:CO = 1, CO adsorbed in linear form) and when the particle size decreases, an increasing fraction of CO is adsorbed linearly [14]. For XRPD, on the other hand, the detection limit is around 3 nm, which means that small particles are not visible.
For comparison palladium on H-Beta-300 was also analysed by transmission electron microscopy (TEM). A TEM microgram is presented in Fig. 1a and size histogram in Fig. 1b. Large clusters of particles were also visible from TEM. The metal particle diameter varied between 2 and 30 nm and the mean particle size was 7 nm, showing good correspondence with the Pd:CO average chemisorption stoichiometry equal to 2. Table 2 shows the distribution of acid sites for several palladium impregnated zeolites. The acidity at 350 and 450 8C for all the characterized catalysts was negligible, which means that palladium impregnated Beta zeolites do not contain strong acid sites. For comparison the values of acidity for one of the zeolitic materials are presented in Table 2. The concentration of weak acid sites (250 8C) follows the Si/Al ratio. XRD patterns (not shown here) of the synthesized materials with and without palladium clearly demonstrated that the impregnation did not affect the structures of parent materials. Suppression of strong acidity in metalmodified zeolites was previously reported also for other metals [15,16]. Several models have been given for the metal–support interactions [17]. Usually metal–support interactions are operative for small metal particles, while particles larger than 1 nm are
Table 1 Catalyst properties. Catalyst
Dubinin surface area (m2/g)
Metal content (wt%)
Dispersion (%)
Metal particle size (nm)a
Pd/H-Beta-300 Pd/H-Beta-150 Pd/H-Beta-25 Pd/H-Beta-22
862 724 691 575
2.0 3.9 4.7 4.1
18 18 26 16
6.5 6.5 4 (8) 7 (8)
a
Particle size from XRD in parenthesis.
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Table 2 Acidity of catalysts. Catalyst
2.0% Pd/H-Beta-300 3.9% Pd/H-Beta-150 4.7% Pd/H-Beta-25 4.1% Pd/H-Beta-22 H-Beta-150
Lewis acid sites (mmol/g)
Brønsted acid sites (mmol/g) 250 8C
350 8C
450 8C
250 8C
350 8C
450 8C
57 109 161 176 147
0 0 26 20 135
0 0 0 0 114
16 29 75 111 39
0 0 0 8 29
0 0 0 0 16
outside the pores of Beta zeolites and no interactions between the support and metal can be assumed. Therefore, at the present moment the explanation for metal–support interactions leading to suppression of strong acidity remains on a speculative level. XRD is used to identify bulk phases and has one important limitation, namely that clear diffraction peaks are only observed when the sample possesses sufficient long-range order. Therefore, XRD provides information on particles that are sufficiently large (about 10 nm and more), but it does not detect particles that are too small. X-ray absorption method gives the highly valuable information about small particles that cannot be seen either by XRD or by TEM. Figs. 2 and 3 show the Pd K XANES spectra of Pd/H-Beta catalysts after different treatments and palladium references. In the spectra of the catalysts treated in a He flow no shift of Pd 1s electron X-ray absorption relative to the K-edge corresponding to that for PdO reference is observed. Besides in the spectra of the catalysts reduced in a H2 flow no shift relative to that for Pd-foil reference is found. These experimental observations suggest that the Pd electronic state in the Pd/H-Beta catalysts treated in a He flow is similar to Pd2+. On the contrary, the Pd electronic state in the Pd/H-Beta catalysts treated in a H2 flow is similar to Pd0. The nature of these distinctions can be clarified by analysis of the EXAFS data. The FT EXAFS spectra of Pd/H-Beta samples after different treatments (Figs. 4 and 5) show that the local structure of Pd species is different. For the samples treated in a He flow, the FT EXAFS spectra are similar to PdO, whereas for reduced samples they are similar to Pd-foil. Thus, the XAS spectra of Pd/H-Beta samples indicate the presence palladium mostly as PdO bulk phase in the He treated samples and palladium metal particles in the reduced samples. In order to check this hypothesis, model fits of the EXAFS spectra were performed, the results of which are given in Table 3.
Fig. 2. Pd K XANES spectra of calcined Pd/H-Beta catalysts and PdO.
Fig. 3. Pd K XANES spectra of reduced Pd/H-Beta catalysts and Pd-foil.
When using calculated scattering amplitudes and phase shifts (F,f), it is necessary to test their reliability with a reference materials. The results of such a test with a PdO and a Pd-foil whose spectra were recorded under similar conditions are summarized in Table 3. The Pd spectra of all He flow treated samples may be readily fitted in both spaces with a three-shell model – one oxygen and two palladium shells around the central absorbing palladium atom. For fitting the Pd spectra of the reduced samples, a one-shell model was necessary: one palladium shell around the central absorbing palladium atom. Fig. 6 shows the fitting of EXAFS spectra of the 2%Pd/H-Beta-300 sample after He treatment whereas Fig. 7 displays the fitting of EXAFS spectra of the same sample after in situ reduction. The average Pd–O distance in the first coordination shell in a He treated samples (2.029–2.032 0.005–0.004) is identical to that of crystalline PdO (2.030 0.003). The average Pd–O coordination number (3.7 0.2) is identical to that of the bulk PdO (3.8 0.1). Moreover the parameters of the second Pd–Pd and the third Pd–Pd shells (both average distance and CN) likewise are nearly identical to that of the bulk PdO. These results indicate that the most of the Pd in samples treated in He flow at 120 8C is PdO bulk phase mostly located on the surface and the pore openings. It is possible that the small
Fig. 4. FT Pd K EXAFS spectra of calcined Pd/H-Beta catalysts and references.
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Fig. 5. FT Pd K EXAFS spectra of reduced Pd/H-Beta catalysts and references.
amount of palladium (as isolated Pd2+ ions) is located in the pores of H-Beta zeolites. The average Pd–Pd distance in the first coordination shell in an H2 treated samples (2.742 0.003–0.004) is identical to that of Pdfoil (2.744 0.002). The average Pd–Pd coordination number (7.1– 8.9 0.5) is different to that of the bulk Pd metal. The increase of the reduction temperature up to 200 8C does not result in essential differences in the average Pd–Pd distance (2.741–2.743 0.002) whereas the average coordination number increases (9.7–9.8 0.4). Based of the spherical shape of particles the formal size of the Pd metal particles formed upon the reduction of Pd/H-Beta was calculated using both the average Pd–Pd distance and the coordination number. It is necessary to take into account that the size of the metallic particles calculated from EXAFS data can be determined correctly only when the particles have nearly the same size and are rather small (20 A˚). In that case the average formal diameter of Pd particles is 9–19 A˚ and at the same time the size of Pd metal particles formed during reduction at 100 8C in an H2 flow is smaller (almost two-fold) as compared with the samples reduced at a higher (200 8C) temperature.
Fig. 6. Model fits of Pd K EXAFS spectra taken from 2%Pd/H-Beta-300 calcined catalyst in k-space (on top) and r-space (below).
From the formally calculated diameters of the palladium particles one cannot conclude that only highly dispersed particles are present. The long-distance peaks on the radial distribution curves (Fig. 5) point to the presence of large Pd metal particles as
Table 3 Parameters of the best model fit of spectra of Pd/H-Beta catalysts. The values in the parenthesis are the standard errors in the last digit.
s2 103 (A˚2)
DE (eV)
DPd (A´˚ )
(2) (2) (3) (5) (4)
2.4 (5) 2.2 (2) 3 (0.2) 4 (0.3) 3 (0.1)
14 9 8 5 4
(1) (1) (1) (1) (1)
– – 15.8 3.0 22.6 8.2
3.7 3.2 7.1 8.0 9.7
(2) (2) (4) (5) (4)
2.3 (6) 2 (0.2) 3 (0.2) 4 (0.3) 3 (0.2)
15 8 8 5 4
(1) (1) (1) (1) (1)
– – 12.1 1.8 21.4 8.5
(4) (2) (2) (3) (2)
3.7 3.2 6.8 7.4 9.8
(2) (2) (3) (5) (4)
2.4 (5) 2.3 (2) 4 (0.2) 5 (0.3) 3 (0.2)
14 9 8 5 3
(1) (1) (1) (1) (1)
– – 10.5 1.3 22.6 8.2
(4) (2) (2) (4) (2)
3.7 3.1 6.6 7.1 9.7
(2) (2) (3) (5) (4)
2.4 (5) 2.4 (2) 4 (0.2) 5 (0.4) 3 (0.2)
14 9 8 5 3
(1) (1) (1) (1) (1)
– – 9.8 1.2 21.4 8.5
Sample
Treatment
Path
r (A˚)
2%Pd/H-Beta-300
120 8C, He
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.030 3.033 3.416 2.742 2.742
(5) (2) (2) (3) (2)
3.7 3.6 7.6 8.9 9.8
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.029 3.033 3.413 2.742 2.743
(5) (3) (2) (3) (2)
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.032 3.032 3.414 2.742 2.742
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.029 3.029 3.413 2.742 2.741
100 8C, H2 200 8C, H2 3.9%Pd/H-Beta-150
120 8C, He
100 8C, H2 200 8C, H2 4.1%Pd/H-BETA-22
120 8C, He
100 8C, H2 200 8C, H2 4.7%Pd/H-Beta-25
120 8C, He
100 8C, H2 200 8C, H2
CN
PdO
–
Pd–O Pd–Pd Pd–Pd
2.030 (3) 3.042 (1) 3.429 (1)
3.8 (1) 4.2 (1) 7.1 (2)
3 (1) 3 (1) 3 (1)
12 (1) 8 (1) 7 (1)
– – –
Pd-foil
–
Pd–Pd
2.744 (2)
12.0 (5)
3 (1)
3 (1)
–
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Table 4 The catalytic activity of calcined and reduced Pd/H-Beta catalysts in the piperonyl alcohol selective oxidation to piperonal. Catalyst
Time (min)
Alcohol conversion (%)
Rate of alcohol conversion (mmol/gMe h)
4.1%Pd/H-Beta-22 4.7%Pd/H-Beta-25 3.9%Pd/H-Beta-150 2.0%Pd/H-Beta-300
60 60 60 60
17.1/28.1a 13.8/19.5a 5.6/7.2a 2.4/4.0a
136/222a 105/146a 46/59a 40/65a
a
For samples calcined at 300 8C/reduced at 100 8C in H2.
reaction proceeds selectively at these conditions, with piperonal being the only product of this reaction. The catalytic results summarized in Table 4 show the alcohol conversion after 1 h reaction and the initial rate of alcohol conversion. Both the calcined in He and reduced samples under study were active. As mentioned above based on EXAFS data alone it is impossible to state which portion of metallic Pd in the reduced samples has the high/low dispersion. Moreover since calcined samples containing PdO/Pd2+ were also active in the test reaction the initial catalytic activity was calculated per gram of palladium. It was observed that the reduced samples are more active. Besides it was found that the activity of Pd/H-Beta catalysts depends upon the Si/Al ratio in the zeolite. The higher the alumina to silica ratio, the higher the initial catalytic activity in selective piperonyl alcohol oxidation reaction. One exception from this trend was Pd/Beta-300 calcined at 300 8C and reduced at 100 8C, which demonstrated similar activity as Pd/Beta-150. This trend can be explained as follows: obviously the major part of oxidation reaction occurs on the Pd/PdO particles, while at the same time the Brønsted or Lewis acid sites seem to be involved in the process. Fig. 7. Model fits of Pd K EXAFS spectra taken from 2%Pd/H-Beta-300 catalyst reduced in H2 at 100 8C in k-space (on top) and r-space (below).
well. These long-distance peaks are observed in EXAFS spectra of the samples even reduced at 100 8C. The intensity of all peaks increase after reduction at 200 8C and becomes comparable with that of Pd-foil. Besides the intensity of the oscillations observed in XANES spectra of reduced samples are comparable with that of Pdfoil. Thus both XAS data (XANES and EXAFS) certify the presence in Pd/H-Beta samples big Pd particles. These big Pd particles were observed in TEM images and XRD patterns. Interesting to note that according to EXAFS data the higher the silica-to-alumina ratios in the beta zeolite, the larger Pd particles are formed: 2%Pd/H-Beta-300 > 3.9%Pd/H-Beta-150 > 4.7%Pd/HBeta-25. It is known that the largest particles give considerably higher contribution into coordination number calculated from EXAFS data. The increase of the formal average diameter of Pd particles to maximum 30 A˚ after reduction at 200 8C certifies in turn the presence of a large number of small Pd metal particles as well. Thus EXAFS data have shown the presence in reduced Pd/H-Beta Pd metal particles with different dispersion. Based on EXAFS data alone it is impossible to state the fractions of Pd with high and low dispersion. We suppose that the small Pd metal particles are located in the pores whereas the larger ones on the surface of zeolite microcrystals and their pore openings.
4. Conclusions Pd catalysts supported on Beta zeolite with different acidity were synthesized, characterized and studied in piperonyl alcohol oxidation. XAS techniques revealed that Pd in calcined Pd/H-Beta samples with different Si/Al ratio exists as palladium oxide that can be easily reduced to Pd metal particles. It was estimated from EXAFS data that Pd nanoparticles formed during reduction at 100 8C are twice smaller in size as compared with reduction at 200 8C. The study of piperonyl alcohol selective oxidation reaction demonstrated that the Pd/H-Beta catalysts with a lower silica-toalumina ratio possess a higher initial catalytic activity. Additionally, Brønsted or Lewis acid sites can be involved in the process by strongly absorbing the substrate (piperonyl alcohol). It is worthy to note that parent H/Beta zeolite is not active in this reaction. Acknowledgements This work is part of the activities at the A˚bo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2000–2011) by the Academy of Finland. Financial support from European Union through the Sixth Framework Program is gratefully acknowledged. Authors thank HASYLAB (DESY, Germany) for X-ray beam time (project I-20060224 EC). References
3.2. Catalysis The piperonyl alcohol oxidation reaction was performed at 80 8C on the calcined in He and reduced at 100 8C samples. This
[1] [2] [3] [4]
A.T. Bell, Science 299 (2003) 1688. N. Pernicone, Cattech 7 (2003) 196. R. Schlo¨gl, S.B. Abd Hamid, Angew. Chem. Int. Ed. 43 (2004) 1628. G.M. Whitesides, Small 1 (2005) 172.
150
O.P. Tkachenko et al. / Applied Catalysis A: General 359 (2009) 144–150
[5] A.V. Tokarev, E.V. Murzina, P.K. Seelam, N. Kumar, D.Yu. Murzin, Micropor. Mesopor. Mater. 5 (113) (2008) 122. [6] H. Markus, P. Ma¨ki-Arvela, N. Kumar, T. Salmi, D.Yu. Murzin, Catal. Today 121 (2007) 100. [7] US Patent N 6,184,421; US Patent N 3,673,257. [8] R.L. Wadlinger, G.T. Kerr, E.J. Rosinski, US Patent 3,308,069 (1975), to Mobil Oil corporation. [9] P. Canton, G. Fagherazzi, M. Battagliarin, F. Menegazzo, F. Pinna, N. Pernicone, Languir 18 (2002) 6530. [10] C.A. Emeis, J. Catal. 141 (1993) 347.
[11] [12] [13] [14] [15]
A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565. M. Borowski, J. Phys. 4 (1997) 2. K.V.Klementiev, VIPER for Windows, freeware, www.desy.de/klmn/viper.html. L.L. Sheu, Z. Karpinski, W.M.H. Sachtler, J. Phys. Chem. 93 (1989) 4890. ¨ sterholm, N. Kumar, T. Salmi, D.Yu. Murzin, J. J.I. Villegas, D. Kubicka, H. Karhu, H. O Mol. Catal. A: Chem. 264 (2007) 192. ¨ sterholm, D.Yu. [16] D. Kubicka, N. Kumar, T. Vena¨la¨inen, H. Karhu, I. Kubickova, H. O Murzin, J. Phys. Chem. B 110 (2006) 4937. [17] D.C. Koningsberger, J. de Graaf, B.L. Mojet, D.E. Ramaker, J.T. Miller, Appl. Catal. A 191 (2000) 205.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Pd/H-Beta catalysts: Characterization and reactivity in piperonyl alcohol selective oxidation Olga P. Tkachenko a, Leonid M. Kustov a, Andrey L. Tarasov a, Konstantin V. Klementiev b, Narendra Kumar c, Dmitry Yu. Murzin c,* a b c
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Leninsky pr. 47, Russia ALBA synchrotron - CELLS, Campus UAB, 08193 Bellaterra, Barcelona, Spain A˚bo Akademi University, Biskopsgatan 8, F1-20500 A˚bo, Turku, Finland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 December 2008 Received in revised form 24 February 2009 Accepted 27 February 2009 Available online 11 March 2009
Pd/H-Beta catalysts with different silica-to-alumina ratio prepared by an impregnation method have been studied by XANES/EXAFS, IR, XRD and TEM. It was shown that palladium in calcined Pd/H-Beta samples exists as palladium oxide that can be easily reduced to Pd metal. The catalysts were tested in selective oxidation of piperonyl alcohol to piperonylaldehyde at 80 8C. The Pd/H-Beta zeolite catalysts with the lowest silica-to-alumina ratio possess the highest catalytic activity in benzylic alcohol oxidation reaction. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Pd/H-Beta zeolites XAS IR TEM Piperonyl alcohol oxidation
1. Introduction Metal nanoparticles formed in various media and stabilized by different mechanisms are the objects of intense studies due to their unique properties, which are significantly influenced by the nanoparticle size, organization of the nanoparticle crystal lattice, nanoparticle surface and the chemical nature of the microenvironment surrounding the nanoparticle [1–4]. The metal nanoparticles exhibit unique properties that differ from the bulk substances, e.g. different heat capacity, vapour pressure and melting point. Moreover, when decreasing the metal particle size sufficiently enough, there occurs the transition of the electronic state from metallic to a non-metallic one. Additionally metal nanoparticles exhibit large surface-to-volume ratio and increased number of edges, corners and faces leading to altered catalytic activity and selectivity. The higher accessibility and concentration of surface metal atoms in nanosized particles makes such catalysts attractive for industrial application. The effect of the support on the metal properties is not very well understood, especially when comparing inorganic and organic matrices having the same metal nanoparticle sizes. The local metal
* Corresponding author. E-mail address: dmurzin@abo.fi (D.Yu. Murzin). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.02.049
environment which can be changed, for instance, by varying the pre-treatment of the catalyst can affect both the metal particle size and the shape/morphology. Moreover the acidity or basicity of the support affects the stabilization of the metal particle. Depending on the type of support materials, in particular acidity, metal particles, for instance Pd0, can exhibit different catalytic activity [5,6]. In the present study, several Pd catalysts supported on Beta zeolite with different acidity were prepared, characterized, and evaluated in selective oxidation of piperonyl alcohol to piperonylaldehyde. Benzilic alcohols oxidation was chosen for several reasons. First, this is the reaction with a focus on the industrial process, such as production of vanilin. Second, the substrate provides several options for the oxidation process and the selectivity may vary depending on the catalyst nature. Third, the palladium-containing catalysts are widely studied in a number of hydrogenation processes and a few oxidation processes, but no information is available on their activity in the oxidation of benzilic alcohols. The patent literature data [7] shows that the existing technologies for the conversion of benzylic alcohols into aldehydes require significant amount of metals (Pt, Pd, Au) and the formation of the side products like salicyclic acid and polymerization materials was noted. Therefore, the field to be studied in this paper opens up new prospects in the application of palladium-containing catalysts in new catalytic processes.
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2. Experimental 2.1. Pd/zeolite preparation Palladium impregnated catalysts Pd-H-Beta-300, Pd-H-Beta150, Pd-H-Beta-25, Pd-H-Beta-22 (the number in the catalyst name denote silica-to-alumina ratio) were synthesized as follows. NH4Beta-300, NH4-Beta-150, NH4-Beta-25 were supplied by Zeolyst International. NH4-Beta-22 zeolite was synthesized as described in [8] with some modifications. A solution was prepared by mixing colloidal silica (Ludox As 40, Aldrich) with distilled water. Another solution was prepared by dissolving sodium aluminate (NaAlO2) in distilled water and adding tetraethylammonium hydroxide ((C2H5)4NOH, Fluka). The two solutions were mixed together and stirred for 30 min. The gel formed was transferred into a Teflon cup, which was inserted into a 300-ml stainless steel autoclave (Parr Instruments). The synthesis was carried out at 150 8C for 96 h. After completion, the autoclave was quenched and the crystalline product was filtered, washed with distilled water, dried at 100 8C for 12 h, and calcined at 550 8C for 9 h in a muffle oven (at a heating rate of 5 8C/min). The sodium form of Beta-22 was ionexchanged with 1 M ammonium chloride (NH4Cl) for 48 h and washed with distilled water to remove any chloride ions. The obtained NH4-Beta-22 zeolite was dried at 100 8C for 12 h. The proton forms of Beta zeolites were obtained by calcination of the NH4 form of the materials in a muffle oven at 500 8C for 4 h (at a heating rate of 5 K/min). All these materials were metal-modified by wetness impregnation in a rotor evaporator (Buchi Rotavapor R114). The source of Pd was an aqueous solution of palladium (II) nitrate dihydrate (Pd(NO3)22H2O, Fluka). The impregnated catalyst was dried overnight at 110 8C and thereafter calcined in an oven at 400 8C for 3 h (at a heating rate of 5 K/min). All catalysts were stored in calcined state. 2.2. Characterization techniques The specific surface areas of the supported metal catalysts were measured with a combined physisorption–chemisorption apparatus (Sorptometer 1900, Carlo Erba Instruments). The Dubinin method was used for the zeolite supported palladium catalysts to calculate the surface areas. The palladium content (wt%) of several zeolites was determined by direct current plasma atomic emission spectrometry (DCP, ARL SpectraSpan7). Prior to the analysis, the samples were dried in an oven at 100 8C overnight and stored in an exicator. 100 mg of the dried catalyst sample was dissolved in a mixture of 3 ml hydrofluoric acid and 3 ml aqua regia in a microwave oven. The dissolved sample was diluted with de-ionized water to 100 ml and analysed by DCP. In order to determine the phase purity and structure, the catalyst samples were pre-treated (dried) at 100 8C in 1 h and stored in acetone, thereafter they were analysed by X-ray powder diffraction (XRD). The patterns from the analyses were compared with the patterns obtained from pure support materials. The measurements were done on a Bragg–Brentano u/2u reflection geometry based Philips PW1820 diffractometer. To determine the metal particle size and dispersion, samples were analysed by CO pulse chemisorption (Autochem 2910, Micromeritics). Prior to the measurements, the samples were dried in an oven at 100 8C overnight and stored in an exicator. 100 mg of the sample was placed into quartz U-tube containing silica wool, the tube was inserted to the system and the sample was further dried in a stream of helium at 50 8C for 30 min. After that, the sample was pre-treated (reduced) at 100 8C for 1 h in hydrogen, using helium as a carrier gas, after which it was
145
flushed with helium (still at 100 8C) for 1 h, cooled to room temperature, placed on a water bath, and subsequently the CO pulses were introduced (10% CO in helium, helium as a carrier gas) until the adsorption was complete. The dispersion was calculated from the amount of CO consumed with Pd:CO stoichiometry presumed to be equal to 2 [9] and atomic crosssectional area to be 0.0787 nm2. The validity of this assumption will be discussed further in the text. The acidity of several impregnated zeolites was measured by Fourier transform infrared spectroscopy (ATI Mattson FTIR) using pyridine as a probe molecule for qualitative and quantitative determination of Brønsted (BAS) and Lewis acid sites (LAS). The catalyst samples were pressed into thin self-supported wafers with a typical weight of 15 mg. The sample was evacuated at 450 8C for 1 h. Pyridine was then adsorbed for 30 min at 100 8C and desorbed by evacuation at different temperatures (250, 350, and 450 8C) to obtain a distribution of acid site strengths. All spectra were recorded at 100 8C. Pyridine is adsorbed on BAS and on LAS to reveal the corresponding vibration frequencies 1545 and 1455 cm1. The concentrations (mmol/gcat) of BAS and LAS were calculated from the intensities of the corresponding bands using the molar extinction coefficients reported by Emeis [10]. XAS (XANES + EXAFS) measurements were carried out at HASYLAB (DESY in Hamburg, Germany) on the beamline X1 (PdK-edge, 24,350 eV) using a double-crystal Si(3 1 1) monochromator, which was detuned to 50% of the maximum intensity to exclude higher harmonics in the X-ray beam. The spectra were recorded in the transmission mode at T = 80 K in order to decrease the Debye–Waller factors. The spectrum of metal Pd-foil was recorded simultaneously between the 2nd and 3rd ionization chambers for energy calibration. Reference spectra were recorded using standard reference compounds (Pd-foil and PdO). All experiments were performed using an in situ EXAFS cell in which a small grained sample was exposed at 120 8C to flowing He (dehydration step) and at 100 and 200 8C to the flowing mixture 5% H2/He (reduction step). Thereafter the samples were flushed by pure helium at the same temperature to avoid the formation of palladium hydride. To extract quantitative information from EXAFS spectra, Fourier-filtered shell contributions were fitted using the standard EXAFS formula in the harmonic approximation:
x ¼ S20
X N j F j ðkÞ j
kR2j
2
expð2s 2j k Þ sin½2kR j þ f j ðkÞ
(1)
with the summation over atomic shells j. The required scattering amplitudes and phase shifts F and f were calculated by the ab initio FEFF8.10 code [11]. The fitting was done in the k- and r-spaces. The shell radius Rj, coordination number Nj, Debye–Waller factor s 2j and adjustable ‘‘muffin-tin zero’’ DEj were determined as fitting parameters. The errors of the fitting parameters were found by decomposition of the statistical x2 function near its minimum, taking into account maximal pair correlations. To evaluate the formal average cluster size (particle radius (R, A˚)) from coordination numbers and Pd–Pd distances in the first coordination sphere, which were obtained by calculating EXAFS data, the following equation was used [12]: 3 1 CN cluster ¼ CN crystal 1 þ 3 4r 16r
(2)
where r = r/R; r is the Pd–Pd interatomic distance in the first shell, A˚; CN cluster is the coordination number of the first shell; and CN crystal = 12 for the fcc lattice of palladium. Analysis of the EXAFS spectra was performed with the software VIPER for Windows [13].
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2.3. Catalytic reaction The piperonyl (3,4-methylenedioxybenzylic) alcohol selective oxidation:
was performed in a sealed tube (20 ml volume) supplied with a gas inlet and outlet. The tube reactor was charged by 1 g of piperonyl alcohol, 10 ml of 1.25 N aqueous solution of NaOH (pH 11), a catalyst activator (0.1 ml of a 0.05 M aqueous solution of lead acetate Pb(OAc)2) and 0.2 g of the catalyst. The reactor was purged with oxygen and the reaction mixture was heated to 80 8C. Under these conditions, dissolution of the starting alcohol takes place. Then oxygen was purged through the reaction mixture with the flow rate of 50 ml/min. The gas flow served also to stir the mixture. The reaction mixture was analysed every hour. For this purpose 1 ml of the reaction mixture was placed in the separation funnel and acidified with HCl until pH 7, then the unreacted alcohol and the aldehyde formed (the target product) were extracted with chloroform (2 ml) and the organic phase was separated from the aqueous phase. In order to check the extraction efficiency it was repeated with a new portion of chloroform followed by quantitative analysis leading to a difference of just 1% compared with the first stage of extraction. Therefore one extraction stage was considered to be efficient enough. The organic phase was analysed by GC using a 3700 model chromatograph with packed SE-54 capillary column (25 m) in the temperature programmed regime: heating to 100 8C for 4 min, then at 20 8C/min heating to 180 8C. 3. Results and discussion
Fig. 1. Analysis of 2 wt% Pd-H-Beta-300 by transmission electron microscopy (a) TEM micrograph and (b) size histogram.
3.1. Characterization Table 1 displays the surface area (Dubinin method), metal content, dispersion, and metal particle size. For the Beta catalysts, the surface area was decreasing in the following order: Pd on HBeta-300 > H-Beta-150 > H-Beta-25 > H-Beta-22. Pd-H-Beta-300 had the lowest metal content, only 2 wt%, while for the others it varied between 3.9 and 4.7 wt%. Pd on H-Beta-300, H-Beta-150 and H-Beta-22 supports were all having approximately the same dispersion. The metal particle size was also obtained from XRD for palladium on H-Beta-25 and H-Beta-22 (Table 1, values in parenthesis). There are limitations with both chemisorption and XRPD methods. For CO chemisorption, one problem is the assumed stoichiometry between metal and CO. The adsorption stoichiometry on particles with the size of several nm (Pd:CO = 2, CO adsorbed in bridged form) is different from small particles (Pd:CO = 1, CO adsorbed in linear form) and when the particle size decreases, an increasing fraction of CO is adsorbed linearly [14]. For XRPD, on the other hand, the detection limit is around 3 nm, which means that small particles are not visible.
For comparison palladium on H-Beta-300 was also analysed by transmission electron microscopy (TEM). A TEM microgram is presented in Fig. 1a and size histogram in Fig. 1b. Large clusters of particles were also visible from TEM. The metal particle diameter varied between 2 and 30 nm and the mean particle size was 7 nm, showing good correspondence with the Pd:CO average chemisorption stoichiometry equal to 2. Table 2 shows the distribution of acid sites for several palladium impregnated zeolites. The acidity at 350 and 450 8C for all the characterized catalysts was negligible, which means that palladium impregnated Beta zeolites do not contain strong acid sites. For comparison the values of acidity for one of the zeolitic materials are presented in Table 2. The concentration of weak acid sites (250 8C) follows the Si/Al ratio. XRD patterns (not shown here) of the synthesized materials with and without palladium clearly demonstrated that the impregnation did not affect the structures of parent materials. Suppression of strong acidity in metalmodified zeolites was previously reported also for other metals [15,16]. Several models have been given for the metal–support interactions [17]. Usually metal–support interactions are operative for small metal particles, while particles larger than 1 nm are
Table 1 Catalyst properties. Catalyst
Dubinin surface area (m2/g)
Metal content (wt%)
Dispersion (%)
Metal particle size (nm)a
Pd/H-Beta-300 Pd/H-Beta-150 Pd/H-Beta-25 Pd/H-Beta-22
862 724 691 575
2.0 3.9 4.7 4.1
18 18 26 16
6.5 6.5 4 (8) 7 (8)
a
Particle size from XRD in parenthesis.
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Table 2 Acidity of catalysts. Catalyst
2.0% Pd/H-Beta-300 3.9% Pd/H-Beta-150 4.7% Pd/H-Beta-25 4.1% Pd/H-Beta-22 H-Beta-150
Lewis acid sites (mmol/g)
Brønsted acid sites (mmol/g) 250 8C
350 8C
450 8C
250 8C
350 8C
450 8C
57 109 161 176 147
0 0 26 20 135
0 0 0 0 114
16 29 75 111 39
0 0 0 8 29
0 0 0 0 16
outside the pores of Beta zeolites and no interactions between the support and metal can be assumed. Therefore, at the present moment the explanation for metal–support interactions leading to suppression of strong acidity remains on a speculative level. XRD is used to identify bulk phases and has one important limitation, namely that clear diffraction peaks are only observed when the sample possesses sufficient long-range order. Therefore, XRD provides information on particles that are sufficiently large (about 10 nm and more), but it does not detect particles that are too small. X-ray absorption method gives the highly valuable information about small particles that cannot be seen either by XRD or by TEM. Figs. 2 and 3 show the Pd K XANES spectra of Pd/H-Beta catalysts after different treatments and palladium references. In the spectra of the catalysts treated in a He flow no shift of Pd 1s electron X-ray absorption relative to the K-edge corresponding to that for PdO reference is observed. Besides in the spectra of the catalysts reduced in a H2 flow no shift relative to that for Pd-foil reference is found. These experimental observations suggest that the Pd electronic state in the Pd/H-Beta catalysts treated in a He flow is similar to Pd2+. On the contrary, the Pd electronic state in the Pd/H-Beta catalysts treated in a H2 flow is similar to Pd0. The nature of these distinctions can be clarified by analysis of the EXAFS data. The FT EXAFS spectra of Pd/H-Beta samples after different treatments (Figs. 4 and 5) show that the local structure of Pd species is different. For the samples treated in a He flow, the FT EXAFS spectra are similar to PdO, whereas for reduced samples they are similar to Pd-foil. Thus, the XAS spectra of Pd/H-Beta samples indicate the presence palladium mostly as PdO bulk phase in the He treated samples and palladium metal particles in the reduced samples. In order to check this hypothesis, model fits of the EXAFS spectra were performed, the results of which are given in Table 3.
Fig. 2. Pd K XANES spectra of calcined Pd/H-Beta catalysts and PdO.
Fig. 3. Pd K XANES spectra of reduced Pd/H-Beta catalysts and Pd-foil.
When using calculated scattering amplitudes and phase shifts (F,f), it is necessary to test their reliability with a reference materials. The results of such a test with a PdO and a Pd-foil whose spectra were recorded under similar conditions are summarized in Table 3. The Pd spectra of all He flow treated samples may be readily fitted in both spaces with a three-shell model – one oxygen and two palladium shells around the central absorbing palladium atom. For fitting the Pd spectra of the reduced samples, a one-shell model was necessary: one palladium shell around the central absorbing palladium atom. Fig. 6 shows the fitting of EXAFS spectra of the 2%Pd/H-Beta-300 sample after He treatment whereas Fig. 7 displays the fitting of EXAFS spectra of the same sample after in situ reduction. The average Pd–O distance in the first coordination shell in a He treated samples (2.029–2.032 0.005–0.004) is identical to that of crystalline PdO (2.030 0.003). The average Pd–O coordination number (3.7 0.2) is identical to that of the bulk PdO (3.8 0.1). Moreover the parameters of the second Pd–Pd and the third Pd–Pd shells (both average distance and CN) likewise are nearly identical to that of the bulk PdO. These results indicate that the most of the Pd in samples treated in He flow at 120 8C is PdO bulk phase mostly located on the surface and the pore openings. It is possible that the small
Fig. 4. FT Pd K EXAFS spectra of calcined Pd/H-Beta catalysts and references.
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Fig. 5. FT Pd K EXAFS spectra of reduced Pd/H-Beta catalysts and references.
amount of palladium (as isolated Pd2+ ions) is located in the pores of H-Beta zeolites. The average Pd–Pd distance in the first coordination shell in an H2 treated samples (2.742 0.003–0.004) is identical to that of Pdfoil (2.744 0.002). The average Pd–Pd coordination number (7.1– 8.9 0.5) is different to that of the bulk Pd metal. The increase of the reduction temperature up to 200 8C does not result in essential differences in the average Pd–Pd distance (2.741–2.743 0.002) whereas the average coordination number increases (9.7–9.8 0.4). Based of the spherical shape of particles the formal size of the Pd metal particles formed upon the reduction of Pd/H-Beta was calculated using both the average Pd–Pd distance and the coordination number. It is necessary to take into account that the size of the metallic particles calculated from EXAFS data can be determined correctly only when the particles have nearly the same size and are rather small (20 A˚). In that case the average formal diameter of Pd particles is 9–19 A˚ and at the same time the size of Pd metal particles formed during reduction at 100 8C in an H2 flow is smaller (almost two-fold) as compared with the samples reduced at a higher (200 8C) temperature.
Fig. 6. Model fits of Pd K EXAFS spectra taken from 2%Pd/H-Beta-300 calcined catalyst in k-space (on top) and r-space (below).
From the formally calculated diameters of the palladium particles one cannot conclude that only highly dispersed particles are present. The long-distance peaks on the radial distribution curves (Fig. 5) point to the presence of large Pd metal particles as
Table 3 Parameters of the best model fit of spectra of Pd/H-Beta catalysts. The values in the parenthesis are the standard errors in the last digit.
s2 103 (A˚2)
DE (eV)
DPd (A´˚ )
(2) (2) (3) (5) (4)
2.4 (5) 2.2 (2) 3 (0.2) 4 (0.3) 3 (0.1)
14 9 8 5 4
(1) (1) (1) (1) (1)
– – 15.8 3.0 22.6 8.2
3.7 3.2 7.1 8.0 9.7
(2) (2) (4) (5) (4)
2.3 (6) 2 (0.2) 3 (0.2) 4 (0.3) 3 (0.2)
15 8 8 5 4
(1) (1) (1) (1) (1)
– – 12.1 1.8 21.4 8.5
(4) (2) (2) (3) (2)
3.7 3.2 6.8 7.4 9.8
(2) (2) (3) (5) (4)
2.4 (5) 2.3 (2) 4 (0.2) 5 (0.3) 3 (0.2)
14 9 8 5 3
(1) (1) (1) (1) (1)
– – 10.5 1.3 22.6 8.2
(4) (2) (2) (4) (2)
3.7 3.1 6.6 7.1 9.7
(2) (2) (3) (5) (4)
2.4 (5) 2.4 (2) 4 (0.2) 5 (0.4) 3 (0.2)
14 9 8 5 3
(1) (1) (1) (1) (1)
– – 9.8 1.2 21.4 8.5
Sample
Treatment
Path
r (A˚)
2%Pd/H-Beta-300
120 8C, He
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.030 3.033 3.416 2.742 2.742
(5) (2) (2) (3) (2)
3.7 3.6 7.6 8.9 9.8
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.029 3.033 3.413 2.742 2.743
(5) (3) (2) (3) (2)
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.032 3.032 3.414 2.742 2.742
Pd–O Pd–Pd Pd–Pd Pd–Pd Pd–Pd
2.029 3.029 3.413 2.742 2.741
100 8C, H2 200 8C, H2 3.9%Pd/H-Beta-150
120 8C, He
100 8C, H2 200 8C, H2 4.1%Pd/H-BETA-22
120 8C, He
100 8C, H2 200 8C, H2 4.7%Pd/H-Beta-25
120 8C, He
100 8C, H2 200 8C, H2
CN
PdO
–
Pd–O Pd–Pd Pd–Pd
2.030 (3) 3.042 (1) 3.429 (1)
3.8 (1) 4.2 (1) 7.1 (2)
3 (1) 3 (1) 3 (1)
12 (1) 8 (1) 7 (1)
– – –
Pd-foil
–
Pd–Pd
2.744 (2)
12.0 (5)
3 (1)
3 (1)
–
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149
Table 4 The catalytic activity of calcined and reduced Pd/H-Beta catalysts in the piperonyl alcohol selective oxidation to piperonal. Catalyst
Time (min)
Alcohol conversion (%)
Rate of alcohol conversion (mmol/gMe h)
4.1%Pd/H-Beta-22 4.7%Pd/H-Beta-25 3.9%Pd/H-Beta-150 2.0%Pd/H-Beta-300
60 60 60 60
17.1/28.1a 13.8/19.5a 5.6/7.2a 2.4/4.0a
136/222a 105/146a 46/59a 40/65a
a
For samples calcined at 300 8C/reduced at 100 8C in H2.
reaction proceeds selectively at these conditions, with piperonal being the only product of this reaction. The catalytic results summarized in Table 4 show the alcohol conversion after 1 h reaction and the initial rate of alcohol conversion. Both the calcined in He and reduced samples under study were active. As mentioned above based on EXAFS data alone it is impossible to state which portion of metallic Pd in the reduced samples has the high/low dispersion. Moreover since calcined samples containing PdO/Pd2+ were also active in the test reaction the initial catalytic activity was calculated per gram of palladium. It was observed that the reduced samples are more active. Besides it was found that the activity of Pd/H-Beta catalysts depends upon the Si/Al ratio in the zeolite. The higher the alumina to silica ratio, the higher the initial catalytic activity in selective piperonyl alcohol oxidation reaction. One exception from this trend was Pd/Beta-300 calcined at 300 8C and reduced at 100 8C, which demonstrated similar activity as Pd/Beta-150. This trend can be explained as follows: obviously the major part of oxidation reaction occurs on the Pd/PdO particles, while at the same time the Brønsted or Lewis acid sites seem to be involved in the process. Fig. 7. Model fits of Pd K EXAFS spectra taken from 2%Pd/H-Beta-300 catalyst reduced in H2 at 100 8C in k-space (on top) and r-space (below).
well. These long-distance peaks are observed in EXAFS spectra of the samples even reduced at 100 8C. The intensity of all peaks increase after reduction at 200 8C and becomes comparable with that of Pd-foil. Besides the intensity of the oscillations observed in XANES spectra of reduced samples are comparable with that of Pdfoil. Thus both XAS data (XANES and EXAFS) certify the presence in Pd/H-Beta samples big Pd particles. These big Pd particles were observed in TEM images and XRD patterns. Interesting to note that according to EXAFS data the higher the silica-to-alumina ratios in the beta zeolite, the larger Pd particles are formed: 2%Pd/H-Beta-300 > 3.9%Pd/H-Beta-150 > 4.7%Pd/HBeta-25. It is known that the largest particles give considerably higher contribution into coordination number calculated from EXAFS data. The increase of the formal average diameter of Pd particles to maximum 30 A˚ after reduction at 200 8C certifies in turn the presence of a large number of small Pd metal particles as well. Thus EXAFS data have shown the presence in reduced Pd/H-Beta Pd metal particles with different dispersion. Based on EXAFS data alone it is impossible to state the fractions of Pd with high and low dispersion. We suppose that the small Pd metal particles are located in the pores whereas the larger ones on the surface of zeolite microcrystals and their pore openings.
4. Conclusions Pd catalysts supported on Beta zeolite with different acidity were synthesized, characterized and studied in piperonyl alcohol oxidation. XAS techniques revealed that Pd in calcined Pd/H-Beta samples with different Si/Al ratio exists as palladium oxide that can be easily reduced to Pd metal particles. It was estimated from EXAFS data that Pd nanoparticles formed during reduction at 100 8C are twice smaller in size as compared with reduction at 200 8C. The study of piperonyl alcohol selective oxidation reaction demonstrated that the Pd/H-Beta catalysts with a lower silica-toalumina ratio possess a higher initial catalytic activity. Additionally, Brønsted or Lewis acid sites can be involved in the process by strongly absorbing the substrate (piperonyl alcohol). It is worthy to note that parent H/Beta zeolite is not active in this reaction. Acknowledgements This work is part of the activities at the A˚bo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2000–2011) by the Academy of Finland. Financial support from European Union through the Sixth Framework Program is gratefully acknowledged. Authors thank HASYLAB (DESY, Germany) for X-ray beam time (project I-20060224 EC). References
3.2. Catalysis The piperonyl alcohol oxidation reaction was performed at 80 8C on the calcined in He and reduced at 100 8C samples. This
[1] [2] [3] [4]
A.T. Bell, Science 299 (2003) 1688. N. Pernicone, Cattech 7 (2003) 196. R. Schlo¨gl, S.B. Abd Hamid, Angew. Chem. Int. Ed. 43 (2004) 1628. G.M. Whitesides, Small 1 (2005) 172.
150
O.P. Tkachenko et al. / Applied Catalysis A: General 359 (2009) 144–150
[5] A.V. Tokarev, E.V. Murzina, P.K. Seelam, N. Kumar, D.Yu. Murzin, Micropor. Mesopor. Mater. 5 (113) (2008) 122. [6] H. Markus, P. Ma¨ki-Arvela, N. Kumar, T. Salmi, D.Yu. Murzin, Catal. Today 121 (2007) 100. [7] US Patent N 6,184,421; US Patent N 3,673,257. [8] R.L. Wadlinger, G.T. Kerr, E.J. Rosinski, US Patent 3,308,069 (1975), to Mobil Oil corporation. [9] P. Canton, G. Fagherazzi, M. Battagliarin, F. Menegazzo, F. Pinna, N. Pernicone, Languir 18 (2002) 6530. [10] C.A. Emeis, J. Catal. 141 (1993) 347.
[11] [12] [13] [14] [15]
A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565. M. Borowski, J. Phys. 4 (1997) 2. K.V.Klementiev, VIPER for Windows, freeware, www.desy.de/klmn/viper.html. L.L. Sheu, Z. Karpinski, W.M.H. Sachtler, J. Phys. Chem. 93 (1989) 4890. ¨ sterholm, N. Kumar, T. Salmi, D.Yu. Murzin, J. J.I. Villegas, D. Kubicka, H. Karhu, H. O Mol. Catal. A: Chem. 264 (2007) 192. ¨ sterholm, D.Yu. [16] D. Kubicka, N. Kumar, T. Vena¨la¨inen, H. Karhu, I. Kubickova, H. O Murzin, J. Phys. Chem. B 110 (2006) 4937. [17] D.C. Koningsberger, J. de Graaf, B.L. Mojet, D.E. Ramaker, J.T. Miller, Appl. Catal. A 191 (2000) 205.