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Catalytic properties of molecular sieves MCM41 type doped with heteropolyacids for ethanol oxidation
Applied Surface Science 255 (2008) 1830–1835
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Catalytic properties of molecular sieves MCM41 type doped with heteropolyacids for ethanol oxidation A. Popa a,*, V. Sasca a, J. Halasz b a b
Institute of Chemistry Timis¸oara, Bl.Mihai Viteazul 24, Timisoara 300223, Romania University of Szeged, Department of Applied and Environmental Chemistry, Rerrich ter 1, Szeged H-6720, Hungary
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 February 2008 Received in revised form 15 May 2008 Accepted 10 June 2008 Available online 21 June 2008
Keggin type heteropolyacids (HPAs) have been used in acid-catalysed reactions as well as oxidation reactions both in the heterogeneous and homogeneous systems. In order to be more effective for catalytic reactions, HPAs are usually impregnated on different porous materials with high surface area. The structure and texture of H3PMo12O40 and H4PVMo11O40 supported on Al-MCM41 and Fe-MCM41 were studied by XRD, low-temperature nitrogen adsorption and thermal analysis. Catalytic properties of prepared samples were studied by using a reactant pulse method at different temperatures. The ethanol conversion proceeds by two main pathways: an oxidehydrogenation reaction on redox catalytic centers, and a dehydration reaction on acidic centers, respectively. The favourable effect of HPAs deposition on AlMCM41 and Fe-MCM41 supports for oxidehydrogenation pathway to acetaldehyde results from the examination of apparent activation energies Ea and reaction rate values. An evidenced decreasing of Ea was obtained for acetaldehyde formation in the case of Fe-MCM41-supported HPM (25 kJ/mole) comparatively with pure HPM (56.6 kJ/mole). For all reaction temperatures, the reaction rates of acetaldehyde formation are with one order of magnitude higher for supported samples than unsupported ones. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Heteropolyacids Molecular sieves Heterogeneous catalysis Ethanol conversion
1. Introduction The catalytic function of heteropolyacids (HPAs) with Keggin structure have been attracted much attention because their acidic and redox properties can be controlled at atomic and molecular level. As a consequence they have been widely used in acidcatalysed reactions as well as oxidation reactions both in the heterogeneous and homogeneous systems [1–6]. Pure HPAs generally show low catalytic reactivity owing to their small surface area. In order to be more effective for catalytic reactions, HPAs are usually impregnated on different porous materials with high surface area (silica, titania, molecular sieves). Among different mesoporous molecular sieves, MCM41 seems to be the most used as support because it has a high thermal stability (up to 1198 K), larger surface area (over 700 m2/g) and a good adsorption capacity for organic molecules [7–20]. From preparation method and reactants types, the pore dimensions of MCM41 can be engineered in the range from 15 to 100 A˚, which allow the easy introduction of HPAs molecules (12 A˚ diameter).
* Corresponding author. Tel.: +40 256 491818; fax: +40 256 491824. E-mail address: [email protected] (A. Popa). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.040
In the literature very few references have been reported concerning H3[PMo12O40] and H4[PMo11VO40] supported on MCM41 mesoporous silicate, majority of the studies have been focused on investigation the most acidic HPAs in the series, namely H3PW12O40. Ali et al. [13] have studied Rh-HPAs supported on MCM41 prepared by impregnation of Rh complexes with H3PW12O40 and H3PMo12O40 for the hydroformylation of styrene derivatives. The choice of the solvent of impregnation was very important in order to minimize the leaching of the catalyst during reaction. The presence of HPAs with Rh(I) or Rh(III) on the support increased the catalytic activity of rhodium catalysts by improving the conversions of styrene and the recycling of the supported catalyst. Jalil et al. [16,17] showed that HPW/Si-MCM41 composites obtained by impregnation from methanol present a dispersion of active phase more homogeneously distributed within the pores of the support than in the case of aqueous impregnation solution. Upon impregnation, an interaction between the protons of HPW and hydroxyl groups of Si-MCM41 appeared and as a consequence, thermal stability of composites decreased to 400 8C, when they decompose to tungsten oxides. The loading effect on both acidity and catalytic activity for HPW/ Si-MCM41 composites was investigated using the conversion of
A. Popa et al. / Applied Surface Science 255 (2008) 1830–1835
1,3,5-triisopropylbenzene [19]. Whilst an important increase in acidity and catalytic activity is observed at low loading (up to 23 wt.%), a decrease in both is found at higher loading (over 33 wt.%), the activity of composites decays to that of HPW acid. The optimum loading calculated from the 1H NMR spectrum agrees with that found in catalytic studies. From 1H NMR and 31P NMR spectra result that at the loading of 23 wt.% HPW only ca. 80 wt.% is located inside the pores, whereas the rest of active phase is located at the entrance of the pores or at the external surface of support grains. In the selective oxidation of unsaturated aldehydes and of low alcohols most frequently used are molybdophosphoric acid H3[PMo12O40]xH2O (HPM) and 1-vanado-11-molybdophosphoric acid H4[PMo11VO40]yH2O (HPVM) in consequence of their high catalytic activity. In order to obtain highly dispersed heteropolyacids species, HPM and HPVM were supported on Al-MCM41 and Fe-MCM41. The goal of this work was to characterise the texture, structure and catalytic activity of these heteropolyacids supported on modified MCM41molecular sieves in reference to the bulk solid heteropolyacids. 2. Experimental H4[PMo11VO40]12H2O was prepared by two methods: the Tsigdinos method (ether extraction from HPVM solution) and hydrothermal method (the obtaining of HPVM from the constituent acids at 353 K) [21,22]. In both cases HPAs were crystallized slowly from aqueous solutions at room temperature. H3[PMo12O40]13H2O was purchased from Merck. The as-received material was recrystallized prior to use. They are stable at room temperature with 12–14 H2O molecules. The (Al,Fe)-MCM41 molecular sieves were synthesized according to the procedure of Refs. [23,24] and details are described therein. The HPAs active phase deposition on Al-MCM41 (Si/Al = 20) and Fe-MCM41 (Si/Fe = 50) supports was performed by impregnation from water: ethanol = 1:1 solution. The HPM and HPVM acids were deposited in the concentration of 15 and 30 wt.% loading. The structure and texture of HPM and HPVM supported on molecular sieves were studied by XRD, low-temperature nitrogen adsorption and scanning electron microscopy with EDS analysis. Thermal stability of both HPAs supported on Al-MCM41 and FeMCM41 supports was studied by TG and DSC measurements. Textural characteristics of the outgassed samples were obtained from nitrogen physisorption using a Quantachrome instrument, Nova 2000 series. The specific surface area SBET, mean cylindrical pore diameters dp and adsorption pore volume VpN2 were determined (Table 1). Prior to the measurements the samples were degassed to 105 Pa at 250 8C. The BET-specific surface area was calculated by using the standard Brunauer, Emmett and Teller method on the basis of the adsorption data. The pore size distributions were calculated applying the Barrett–
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Joyner–Halenda (BJH) method to the desorption branches of the isotherms. The IUPAC classification of pores and isotherms were used in this study. Microstructure characterisation of the catalyst particles was carried out with a JEOL JSM 6460 LV instrument equipped with an OXFORD INSTRUMENTS EDS analyser. Powder materials were deposited on adhesive tape fixed to specimen tabs and then ion sputter coated with gold. Powder X-ray diffraction data were obtained with a XD 8 Advanced Bruker diffractometer using the Cu Ka radiation in the range 2u = 5–608. Thermal stability of HPAs supported on Si-MCM41 Fe-MCM41 and Al-MCM41 supports was studied by TG and DSC measurements. Thermogravimetric analysis was performed on a Thermal analysis system Star TG-DTA, Mettler Toledo. The working conditions were as follows: samples amount 30–50 mg for the supported samples; platinum crucible, heating rate 2.5, 5 and 10 K/ min; air or nitrogen atmosphere. Catalytic experiments were carried out using a pulse microreactor, with 1.0 ml of catalyst, in the temperature range 483– 563 K. The volume of catalyst was maintained constant for all bulk or supported samples in the catalytic experiments. In function of material density, 1 ml of pure HPAs has 0.5 g weight, while 1 ml of Al-MCM41-supported HPAs has 22 mg of active phase (15% HPAs/ Al-MCM41) and 1 ml of Fe-MCM41-supported HPAs has 53 mg of active phase (30% HPAs/Fe-MCM41). The experiments were performed in an oxygen-free atmosphere, under an inert gas atmosphere. The carrier gases (Ar or N2) flows were kept constant at 30 mL/min. Six pulses of 3 ml of C2H5OH (2,4 103 g ethanol corresponds to 58.25 mmol) were introduced within 20 min. The products were analyzed by gas chromatographs with columns filled with Porapak QS. The reoxidation process was realized in an airflow during 2 h, at the same temperature used for fresh catalysts test. Prior to catalytic reactions, all catalysts were activated in situ at the reaction temperature, for 2 h in carrier gas flow (argon) (30 ml/min). 3. Results and discussion The XRD data of Fe-MCM41, Al-MCM41 and the HPAs supported samples are presented in Table 1. The patterns show the characteristics of a typical mesoporous MCM41 structure (not shown). The d100 diffraction peaks of calcined Al-MCM41 are shifted towards higher values compared to its Si-MCM41 analogue. As in the case of HPW/MCM41 [10–14], the amount of HPM or HPVM has an important effect on the intensity of the main reflection peaks of the Al-MCM41 and Fe-MCM14 supports, and the peaks height is inversely proportional to the amount of loaded HPAs. It can be observed that the long-range order of Fe-MCM41 is decreased evidently for loading of 30 wt.% HPAs. The XRD diffraction patterns of supported HPAs did not show any bulk
Table 1 Textural properties of molecular sieves and supported HPAs Sample
XRD d1 0 0 d-spacing (A˚)
Unit cell, ao (nm)
Surface area (m2/g)
Pore size BJHDes (A˚)
Pore volume BJHDes (cm3/g)
Si-MCM41 15% HPM/Si-MCM41 15% HPVM/Si-MCM41 Fe-MCM41 15% HPVM/Fe-MCM41 30% HPVM/Fe-MCM41 30% HPM/Fe-MCM41 Al-MCM41 15% HPM/Al-MCM41 15% HPVM/Al-MCM41
37.40 35.88 36.03 36.78 36.48 36.18 36.18 42.58 41.25 42.04
4.32 4.14 4.16 4.25 4.21 4.18 4.18 4.92 4.76 4.85
1104 860 888 917 769.5 655 439 559.3 408.5 364.3
25.02 25.01 25.07 26.8 24.8 24.2 24.2 28.8 26.9 26.9
1.06 0.776 0.770 0.968 0.649 0.685 0.562 0.82 0.56 0.44
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HPAs crystal phases, indicating that active phase is finely dispersed on the Fe-MCM41 support (not shown). The nitrogen adsorption isotherms of heteropolyacids supported on Al-MCM41 and Fe-MCM41 indicated that all samples were mesoporous. For supported samples, one observed a type IV isotherm with a type H1 hysteresis loop in the high range of relative pressure. For the values of relative pressure higher than 0.8, condensation take place giving a sharp adsorption volume increase. This behaviour indicates that these samples have a mesoporous character. Hysteresis loop type shows that supported samples consist of agglomerates or compacts of approximately uniform spheres in fairly regular array [25,26]. The pore size distributions were calculated by Barret–Joyner– Halenda (BJH) method applied to the desorption branches of the isotherms. The pore size distribution curves of HPM and HPVM supported on Fe-MCM41 show a maximum at 2.4 nm and HPAs supported on Al-MCM41 show a maximum at 2.7 nm (Table 1). The morphology of pure HPM and HPVM illustrated by SEM showed that it consists of agglomerates of irregular crystallites varying in size and habit. The surface morphology of the supported heteropolyacids was practically identical to that of molecular sieves supports and was composed of relatively well formed, spherical crystallites with diameter varying from 50 to 100 nm (not shown). Thermal behaviour of the pure HPM and HPVM heteropolyacids, respectively, HPM and HPVM supported on Al-MCM41 and Fe-MCM41 was studied comparatively. The two pure HPA was stable in form of anhydrous species within the range of temperature 423–553 K for HPVM and 423–653 K for HPM. Above these temperature, HPAs decomposed losing the constitutive water and transform into correspondent oxides [27–30]. The HPAs acids deposited on the support have had a different behaviour than that of the pure HPAs in the range of high temperatures. On TG curves of HPM and HPVM decomposition was observed a continuous weight change up to 773 K without any plateau that indicates the absence of a steady state during the thermal decomposition of both HPAs supported on molecular sieves. The endothermic peak at 648 K (DTG and DTA curves) was associated with the loss of a part of constitutional water from supported HPM and HPVM and the exothermic peak over 823 K (DTA curve) was associated with complete decomposition of HPAs and MoO3 crystallization. The catalytic properties of supported and unsupported HPAs were studied for ethanol conversion using a pulse method. This method is very often used to test acid–base properties of the catalyst surface. The reaction products obtained on acid (dehydration) catalytic centres were ethylene (ET) and diethyl ether (DEE), and acetaldehyde (ACA) which was obtained on redox (dehydrogenation) catalytic centres. In the pulses leaving the reactor were detected also unreacted alcohol and especially at 543 and 563 K minor quantities of COx. The neat Al-MCM41 and Fe-MCM41 molecular sieves showed no activity for ethanol conversion up to the temperature of 543 K. The reaction products obtained from the ethanol conversion on Al and Fe-MCM41 supports were only water and unreacted ethanol. Ethanol conversion over both pure HPAs catalysts decreases with pulse number and the conversion over HPM has higher values than for HPVM catalyst in the whole temperature range 483– 563 K. For Al-MCM41- and Fe-MCM41-supported catalysts, the values of ethanol conversion are higher in comparison with pure HPAs ones in the temperature range 503–543 K. After reoxidation, values of ethanol conversion are comparable with those obtain after first cycle of reduction for all tested catalysts. The DEE and ethylene quantities over both pure heteropolyacids remain nearly constant with the increasing of pulse number
after the second pulse. The values of DEE quantities reach 10 mmoles at 523 K, while for ethylene the maximum of quantity (20 mmoles) is reached at 563 K. The acetaldehyde amount increased gradually with pulse number from 0.7 to 9 mmoles at 543 K and from 3 to 9.8 mmoles at 563 K for pure HPVM. In the case of Fe-MCM41-supported catalysts the ethyl ether amount increases with pulse number, and reaches a maximum at last pulses. Comparably with supported HPM, the amounts for ethyl ether are quite similar over supported HPVM. After second pulse, constant levels are reached in the case of ethylene quantity for both supported catalysts but the yield of ethylene is higher over supported HPM. Acetaldehyde (ACA) which is known to be produced on redox centres resulted in significant amount on both Al-MCM41- and FeMCM41-supported catalysts in comparison with pure HPAs ones. However, supported HPVM led to higher quantities of ACA, especially at 523 and 543 K. ACA quantities increased with the number pulses for supported catalysts and after the third pulse reached a relatively constant level (Fig. 1b). Thus, the very high dispersion of the HPAs on high surface area molecular sieve yielded an active catalyst for ethanol conversion and especially for acetaldehyde formation. The reaction rates corresponding to the transformation of ethanol and formation of acetaldehyde, respectively increased on fresh Al-MCM41- and Fe-MCM41-supported HPAs comparatively with unsupported HPAs ones in the whole range of temperatures: 503–543 K (Table 2). In comparison with the fresh catalysts, after reoxidation, the values of reaction rates of acetaldehyde formation for both Al-MCM41- and Fe-MCM41-supported and
Fig. 1. Quantities of acetaldehyde for ethanol conversion on HPVM (a) and FeMCM41-supported HPVM (b).
A. Popa et al. / Applied Surface Science 255 (2008) 1830–1835
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Table 2 Average reaction rates of reaction products formation (106 mole/g s) at different temperature for molecular sieves-supported and unsupported heteropolyacids Sample
rACA
HPM HPVM HPM-reox HPVM-reox HPM/Fe-MCM41 HPVM/Fe-MCM41 HPM/Fe-MCM41-reox HPVM/Fe-MCM41-reox HPM/Al-MCM41 HPVM/Al-MCM41 HPM/Al-MCM41-reox HPVM/Al-MCM41-reox
rET
rDEE
503 K
523 K
543 K
503 K
523 K
543 K
503 K
523 K
543 K
6.19 2.98 6.50 5.76 50.17 73.40 59.56 74.86 47.60 41.73 43.08 36.89
13.67 7.09 12.23 8.73 68.69 81.72 70.74 80.88 63.19 56.20 62.60 58.75
10.67 8.76 11.74 9.48 79.70 97.76 99.46 99.25 95.53 102.2 88.88 125.5
7.50 6.71 5.31 5.80 10.01 10.00 9.15 7.67 12.94 20.52 15.58 18.28
12.41 14.97 12.32 12.11 22.35 14.88 15.16 11.76 30.61 50.65 27.94 30.43
23.58 20.99 19.84 15.38 35.33 28.25 42.86 27.31 101.2 78.26 78.76 89.05
6.57 7.07 4.68 6.12 10.69 15.73 13.61 12.35 19.74 25.42 19.83 23.25
10.59 12.13 10.07 10.76 17.04 15.35 13.84 13.23 26.85 31.87 22.76 25.80
7.87 9.87 7.65 9.66 9.17 13.22 16.67 16.07 44.04 73.37 32.96 92.59
unsupported HPAs catalysts are increased as a result of enriching catalyst surface with oxygen. An exception was observed for AlMCM41-supported HPAs at 503 K, when the values of reaction rates of acetaldehyde were lower after reoxidation (Table 3). An explanation could be that, probably at lower temperature (503 K), the degree of oxidation (the surface enriching in oxygen) during the process of reoxidation with air is lower that the same process at temperature higher than 523 K. In contrast, the reaction rates of ethylene and diethyl ether, respectively present lower values after catalyst reoxidation for all catalysts at 503 and 523 K. In this case, a different behaviour was observed for Fe-MCM41-supported HPAs and for HPVM/Al-MCM41 at 543 K, when the values of reaction rates of ET and DEE were higher after reoxidation (Table 2). An explanation could be that, probably at higher reaction temperature, the acidic active centers of HPAs (which are responsible for ET and DEE formation) are still very active after initial six pulses, and could became more active following the reoxidation process. Also, it is known that the amount of ET increase with reaction temperature of the conversion of ethanol [31,32].
The catalytic activities of supported and unsupported HPAs, fresh and reoxidated samples were compared on the basis of reaction rates per gram catalyst [(mol reaction product) (gcat)1 s1]. The resulting reaction rates for acetaldehyde formation are presented in Table 3 and as ln r versus 1/T in Figs. 2 and 3. It could be observed that reaction rates values show that Al-MCM41and Fe-MCM41-supported catalysts, both fresh and after reoxidation appeared to be far more active than bulk HPM and HPVM catalysts in dehydrogenation reaction. For bulk catalysts, the values of apparent activation energies for all reaction products formation are smaller for HPVM than for HPM one. On the other hand apparent activation energies in case of FeMCM41-supported catalysts have lower values as the unsupported HPAs for acetaldehyde and diethyl ether formation and almost the same values for ethylene formation. Al-MCM41-supported catalysts show values of apparent activation energies almost the same with the values of bulk catalysts ones. For all the studied catalysts, the activation energy of ethylene formation (dehydration route) is higher than for acetaldehyde formation (dehydrogenation route). This is typical for low alcohols conversion [5].
Table 3 The kinetic data for acetaldehyde formation on Al-MCM41- and Fe-MCM41-supported and unsupported heteropolyacids Sample
Temperature (K)
Fresh catalysts 6
rACA 10 (mol g
After reoxidation 1
1
s
)
Ea (kJ/mol)
rACA 106 (mol g1 s1)
1
A (s
) 6
Ea (kJ/mol)
A (s1)
3.82 5.38 6.50 12.23 14.11
38.2
5.15 104
HPM
483 503 523 543 563
2.37 3.56 6.19 13.67 14.89
56.6
3.14 10
HPVM
483 503 523 543 563
2.48 2.98 7.09 8.76 10.55
44.9
1.8 105
4.07 5.76 8.73 9.48 16.18
36.6
3.82 104
HPM/Fe-MCM41
503 523 543
50.17 68.7 77.8
24.9
2.05 104
59.50 70.74 79.46
28.8
5.87 104
HPVM/Fe-MCM41
503 523 543
60.33 81.72 95.24
25.8
3.09 104
74.86 80.88 99.25
15.8
3.29 103
HPM/Al-MCM41
503 523 543
47.6 63.19 95.53
39.2
5.97 105
43.8 62.6 88.9
40.9
7.98 105
HPVM/Al-MCM41
503 523 543
41.73 56.20 102.20
50.3
7.28 106
36.89 58.75 125.46
68.9
5.36 108
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A. Popa et al. / Applied Surface Science 255 (2008) 1830–1835
Fig. 4. Correlation between log A vs. activation energy for acetaldehyde formation: (A) the dots correspond to: HPM, HPVM fresh and reoxidated, (B) the dots correspond to HPM/Fe-MCM41, HPVM/Fe-MCM41 fresh and reoxidated, and (C) the dots correspond to HPM/Al-MCM41, HPVM/Al-MCM41 fresh and reoxidated.
Fig. 2. Arrhenius plots of specific reaction rates of Fe-MCM41-supported and unsupported heteropolyacids series for acetaldehyde formation.
of Fe-MCM41-supported HPAs and Ti = 522 K for Al-MCM41supported HPAs. 4. Conclusion
Fig. 3. Arrhenius plots of specific reaction rates of Al-MCM41-supported and unsupported heteropolyacids series for acetaldehyde formation.
A comparison of the apparent activation energies of tested catalysts for acetaldehyde formation reveals an important decrease in the case of Fe-MCM41-supported HPAs. Their values amounted to ca. 25 kJ mole1 for supported HPAs, while for parent acids the values are 57 kJ/mole for HPM and 45 kJ/mole for HPVM, respectively (Table 3). A ‘‘compensation effect’’ was evidenced from the relationship between pre-exponential factor A and apparent activation energies Ea for acetaldehyde (Fig. 4). On the basis of compensation effect, it was calculated the isokinetic temperature Ti. At this temperature all kinetic parameters of a reaction are equal. Isokinetic temperature is calculated by the equation: 1 Ti ¼ 2:303Ra where ‘‘a’’ is the slope of the linear plots. The calculated isokinetic temperature for acetaldehyde formation was Ti = 547 K in the case
HPAs anions preserved their Keggin structure on the Fe-MCM41 and Al-MCM41 surface and forms finely dispersed HPAs species. No HPAs detectable crystal phase is observed at 30 wt.% loading. The Fe-MCM41 host material suffers structural distortions at higher loadings, which leads to a loss in long-range order. Both FeMCM41 and Al-MCM41-supported HPAs exhibit differential pore size distribution in the mesoporosity range. The surface area and pore size diameter of supported HPAs decreases with increase in HPAs loading due to the partial blockage of the mesopores of the support by HPAs particles. The ethanol conversion proceeds by two main pathways: an oxidehydrogenation reaction on redox catalytic centres, and a dehydration reaction on acidic centres, respectively. Al-MCM41- and Fe-MCM41-supported HPAs have showed mainly redox properties and therefore the reaction product obtained in more important yield is acetaldehyde. The favourable effect of HPAs deposition on Fe-MCM41 and Al-MCM41 supports for oxidehydrogenation pathway to acetaldehyde results from the apparent activation energies and reaction rate values. An evidenced decreasing of Ea was obtained for acetaldehyde formation in the case of supported HPM (24.9 kJ/mol) comparatively with pure HPM (56.6 kJ/mol). A ‘‘compensation effect’’ was evidenced from the relationship between pre-exponential factor A and apparent activation energies Ea for acetaldehyde, ethylene and ethyl ether formation. Acknowledgement These investigations were financed by the Roumanian Ministry of Education and Research, Grant CNCSIS No. 78GR/2007. References [1] [2] [3] [4]
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Catalytic properties of molecular sieves MCM41 type doped with heteropolyacids for ethanol oxidation A. Popa a,*, V. Sasca a, J. Halasz b a b
Institute of Chemistry Timis¸oara, Bl.Mihai Viteazul 24, Timisoara 300223, Romania University of Szeged, Department of Applied and Environmental Chemistry, Rerrich ter 1, Szeged H-6720, Hungary
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 February 2008 Received in revised form 15 May 2008 Accepted 10 June 2008 Available online 21 June 2008
Keggin type heteropolyacids (HPAs) have been used in acid-catalysed reactions as well as oxidation reactions both in the heterogeneous and homogeneous systems. In order to be more effective for catalytic reactions, HPAs are usually impregnated on different porous materials with high surface area. The structure and texture of H3PMo12O40 and H4PVMo11O40 supported on Al-MCM41 and Fe-MCM41 were studied by XRD, low-temperature nitrogen adsorption and thermal analysis. Catalytic properties of prepared samples were studied by using a reactant pulse method at different temperatures. The ethanol conversion proceeds by two main pathways: an oxidehydrogenation reaction on redox catalytic centers, and a dehydration reaction on acidic centers, respectively. The favourable effect of HPAs deposition on AlMCM41 and Fe-MCM41 supports for oxidehydrogenation pathway to acetaldehyde results from the examination of apparent activation energies Ea and reaction rate values. An evidenced decreasing of Ea was obtained for acetaldehyde formation in the case of Fe-MCM41-supported HPM (25 kJ/mole) comparatively with pure HPM (56.6 kJ/mole). For all reaction temperatures, the reaction rates of acetaldehyde formation are with one order of magnitude higher for supported samples than unsupported ones. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Heteropolyacids Molecular sieves Heterogeneous catalysis Ethanol conversion
1. Introduction The catalytic function of heteropolyacids (HPAs) with Keggin structure have been attracted much attention because their acidic and redox properties can be controlled at atomic and molecular level. As a consequence they have been widely used in acidcatalysed reactions as well as oxidation reactions both in the heterogeneous and homogeneous systems [1–6]. Pure HPAs generally show low catalytic reactivity owing to their small surface area. In order to be more effective for catalytic reactions, HPAs are usually impregnated on different porous materials with high surface area (silica, titania, molecular sieves). Among different mesoporous molecular sieves, MCM41 seems to be the most used as support because it has a high thermal stability (up to 1198 K), larger surface area (over 700 m2/g) and a good adsorption capacity for organic molecules [7–20]. From preparation method and reactants types, the pore dimensions of MCM41 can be engineered in the range from 15 to 100 A˚, which allow the easy introduction of HPAs molecules (12 A˚ diameter).
* Corresponding author. Tel.: +40 256 491818; fax: +40 256 491824. E-mail address: [email protected] (A. Popa). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.040
In the literature very few references have been reported concerning H3[PMo12O40] and H4[PMo11VO40] supported on MCM41 mesoporous silicate, majority of the studies have been focused on investigation the most acidic HPAs in the series, namely H3PW12O40. Ali et al. [13] have studied Rh-HPAs supported on MCM41 prepared by impregnation of Rh complexes with H3PW12O40 and H3PMo12O40 for the hydroformylation of styrene derivatives. The choice of the solvent of impregnation was very important in order to minimize the leaching of the catalyst during reaction. The presence of HPAs with Rh(I) or Rh(III) on the support increased the catalytic activity of rhodium catalysts by improving the conversions of styrene and the recycling of the supported catalyst. Jalil et al. [16,17] showed that HPW/Si-MCM41 composites obtained by impregnation from methanol present a dispersion of active phase more homogeneously distributed within the pores of the support than in the case of aqueous impregnation solution. Upon impregnation, an interaction between the protons of HPW and hydroxyl groups of Si-MCM41 appeared and as a consequence, thermal stability of composites decreased to 400 8C, when they decompose to tungsten oxides. The loading effect on both acidity and catalytic activity for HPW/ Si-MCM41 composites was investigated using the conversion of
A. Popa et al. / Applied Surface Science 255 (2008) 1830–1835
1,3,5-triisopropylbenzene [19]. Whilst an important increase in acidity and catalytic activity is observed at low loading (up to 23 wt.%), a decrease in both is found at higher loading (over 33 wt.%), the activity of composites decays to that of HPW acid. The optimum loading calculated from the 1H NMR spectrum agrees with that found in catalytic studies. From 1H NMR and 31P NMR spectra result that at the loading of 23 wt.% HPW only ca. 80 wt.% is located inside the pores, whereas the rest of active phase is located at the entrance of the pores or at the external surface of support grains. In the selective oxidation of unsaturated aldehydes and of low alcohols most frequently used are molybdophosphoric acid H3[PMo12O40]xH2O (HPM) and 1-vanado-11-molybdophosphoric acid H4[PMo11VO40]yH2O (HPVM) in consequence of their high catalytic activity. In order to obtain highly dispersed heteropolyacids species, HPM and HPVM were supported on Al-MCM41 and Fe-MCM41. The goal of this work was to characterise the texture, structure and catalytic activity of these heteropolyacids supported on modified MCM41molecular sieves in reference to the bulk solid heteropolyacids. 2. Experimental H4[PMo11VO40]12H2O was prepared by two methods: the Tsigdinos method (ether extraction from HPVM solution) and hydrothermal method (the obtaining of HPVM from the constituent acids at 353 K) [21,22]. In both cases HPAs were crystallized slowly from aqueous solutions at room temperature. H3[PMo12O40]13H2O was purchased from Merck. The as-received material was recrystallized prior to use. They are stable at room temperature with 12–14 H2O molecules. The (Al,Fe)-MCM41 molecular sieves were synthesized according to the procedure of Refs. [23,24] and details are described therein. The HPAs active phase deposition on Al-MCM41 (Si/Al = 20) and Fe-MCM41 (Si/Fe = 50) supports was performed by impregnation from water: ethanol = 1:1 solution. The HPM and HPVM acids were deposited in the concentration of 15 and 30 wt.% loading. The structure and texture of HPM and HPVM supported on molecular sieves were studied by XRD, low-temperature nitrogen adsorption and scanning electron microscopy with EDS analysis. Thermal stability of both HPAs supported on Al-MCM41 and FeMCM41 supports was studied by TG and DSC measurements. Textural characteristics of the outgassed samples were obtained from nitrogen physisorption using a Quantachrome instrument, Nova 2000 series. The specific surface area SBET, mean cylindrical pore diameters dp and adsorption pore volume VpN2 were determined (Table 1). Prior to the measurements the samples were degassed to 105 Pa at 250 8C. The BET-specific surface area was calculated by using the standard Brunauer, Emmett and Teller method on the basis of the adsorption data. The pore size distributions were calculated applying the Barrett–
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Joyner–Halenda (BJH) method to the desorption branches of the isotherms. The IUPAC classification of pores and isotherms were used in this study. Microstructure characterisation of the catalyst particles was carried out with a JEOL JSM 6460 LV instrument equipped with an OXFORD INSTRUMENTS EDS analyser. Powder materials were deposited on adhesive tape fixed to specimen tabs and then ion sputter coated with gold. Powder X-ray diffraction data were obtained with a XD 8 Advanced Bruker diffractometer using the Cu Ka radiation in the range 2u = 5–608. Thermal stability of HPAs supported on Si-MCM41 Fe-MCM41 and Al-MCM41 supports was studied by TG and DSC measurements. Thermogravimetric analysis was performed on a Thermal analysis system Star TG-DTA, Mettler Toledo. The working conditions were as follows: samples amount 30–50 mg for the supported samples; platinum crucible, heating rate 2.5, 5 and 10 K/ min; air or nitrogen atmosphere. Catalytic experiments were carried out using a pulse microreactor, with 1.0 ml of catalyst, in the temperature range 483– 563 K. The volume of catalyst was maintained constant for all bulk or supported samples in the catalytic experiments. In function of material density, 1 ml of pure HPAs has 0.5 g weight, while 1 ml of Al-MCM41-supported HPAs has 22 mg of active phase (15% HPAs/ Al-MCM41) and 1 ml of Fe-MCM41-supported HPAs has 53 mg of active phase (30% HPAs/Fe-MCM41). The experiments were performed in an oxygen-free atmosphere, under an inert gas atmosphere. The carrier gases (Ar or N2) flows were kept constant at 30 mL/min. Six pulses of 3 ml of C2H5OH (2,4 103 g ethanol corresponds to 58.25 mmol) were introduced within 20 min. The products were analyzed by gas chromatographs with columns filled with Porapak QS. The reoxidation process was realized in an airflow during 2 h, at the same temperature used for fresh catalysts test. Prior to catalytic reactions, all catalysts were activated in situ at the reaction temperature, for 2 h in carrier gas flow (argon) (30 ml/min). 3. Results and discussion The XRD data of Fe-MCM41, Al-MCM41 and the HPAs supported samples are presented in Table 1. The patterns show the characteristics of a typical mesoporous MCM41 structure (not shown). The d100 diffraction peaks of calcined Al-MCM41 are shifted towards higher values compared to its Si-MCM41 analogue. As in the case of HPW/MCM41 [10–14], the amount of HPM or HPVM has an important effect on the intensity of the main reflection peaks of the Al-MCM41 and Fe-MCM14 supports, and the peaks height is inversely proportional to the amount of loaded HPAs. It can be observed that the long-range order of Fe-MCM41 is decreased evidently for loading of 30 wt.% HPAs. The XRD diffraction patterns of supported HPAs did not show any bulk
Table 1 Textural properties of molecular sieves and supported HPAs Sample
XRD d1 0 0 d-spacing (A˚)
Unit cell, ao (nm)
Surface area (m2/g)
Pore size BJHDes (A˚)
Pore volume BJHDes (cm3/g)
Si-MCM41 15% HPM/Si-MCM41 15% HPVM/Si-MCM41 Fe-MCM41 15% HPVM/Fe-MCM41 30% HPVM/Fe-MCM41 30% HPM/Fe-MCM41 Al-MCM41 15% HPM/Al-MCM41 15% HPVM/Al-MCM41
37.40 35.88 36.03 36.78 36.48 36.18 36.18 42.58 41.25 42.04
4.32 4.14 4.16 4.25 4.21 4.18 4.18 4.92 4.76 4.85
1104 860 888 917 769.5 655 439 559.3 408.5 364.3
25.02 25.01 25.07 26.8 24.8 24.2 24.2 28.8 26.9 26.9
1.06 0.776 0.770 0.968 0.649 0.685 0.562 0.82 0.56 0.44
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HPAs crystal phases, indicating that active phase is finely dispersed on the Fe-MCM41 support (not shown). The nitrogen adsorption isotherms of heteropolyacids supported on Al-MCM41 and Fe-MCM41 indicated that all samples were mesoporous. For supported samples, one observed a type IV isotherm with a type H1 hysteresis loop in the high range of relative pressure. For the values of relative pressure higher than 0.8, condensation take place giving a sharp adsorption volume increase. This behaviour indicates that these samples have a mesoporous character. Hysteresis loop type shows that supported samples consist of agglomerates or compacts of approximately uniform spheres in fairly regular array [25,26]. The pore size distributions were calculated by Barret–Joyner– Halenda (BJH) method applied to the desorption branches of the isotherms. The pore size distribution curves of HPM and HPVM supported on Fe-MCM41 show a maximum at 2.4 nm and HPAs supported on Al-MCM41 show a maximum at 2.7 nm (Table 1). The morphology of pure HPM and HPVM illustrated by SEM showed that it consists of agglomerates of irregular crystallites varying in size and habit. The surface morphology of the supported heteropolyacids was practically identical to that of molecular sieves supports and was composed of relatively well formed, spherical crystallites with diameter varying from 50 to 100 nm (not shown). Thermal behaviour of the pure HPM and HPVM heteropolyacids, respectively, HPM and HPVM supported on Al-MCM41 and Fe-MCM41 was studied comparatively. The two pure HPA was stable in form of anhydrous species within the range of temperature 423–553 K for HPVM and 423–653 K for HPM. Above these temperature, HPAs decomposed losing the constitutive water and transform into correspondent oxides [27–30]. The HPAs acids deposited on the support have had a different behaviour than that of the pure HPAs in the range of high temperatures. On TG curves of HPM and HPVM decomposition was observed a continuous weight change up to 773 K without any plateau that indicates the absence of a steady state during the thermal decomposition of both HPAs supported on molecular sieves. The endothermic peak at 648 K (DTG and DTA curves) was associated with the loss of a part of constitutional water from supported HPM and HPVM and the exothermic peak over 823 K (DTA curve) was associated with complete decomposition of HPAs and MoO3 crystallization. The catalytic properties of supported and unsupported HPAs were studied for ethanol conversion using a pulse method. This method is very often used to test acid–base properties of the catalyst surface. The reaction products obtained on acid (dehydration) catalytic centres were ethylene (ET) and diethyl ether (DEE), and acetaldehyde (ACA) which was obtained on redox (dehydrogenation) catalytic centres. In the pulses leaving the reactor were detected also unreacted alcohol and especially at 543 and 563 K minor quantities of COx. The neat Al-MCM41 and Fe-MCM41 molecular sieves showed no activity for ethanol conversion up to the temperature of 543 K. The reaction products obtained from the ethanol conversion on Al and Fe-MCM41 supports were only water and unreacted ethanol. Ethanol conversion over both pure HPAs catalysts decreases with pulse number and the conversion over HPM has higher values than for HPVM catalyst in the whole temperature range 483– 563 K. For Al-MCM41- and Fe-MCM41-supported catalysts, the values of ethanol conversion are higher in comparison with pure HPAs ones in the temperature range 503–543 K. After reoxidation, values of ethanol conversion are comparable with those obtain after first cycle of reduction for all tested catalysts. The DEE and ethylene quantities over both pure heteropolyacids remain nearly constant with the increasing of pulse number
after the second pulse. The values of DEE quantities reach 10 mmoles at 523 K, while for ethylene the maximum of quantity (20 mmoles) is reached at 563 K. The acetaldehyde amount increased gradually with pulse number from 0.7 to 9 mmoles at 543 K and from 3 to 9.8 mmoles at 563 K for pure HPVM. In the case of Fe-MCM41-supported catalysts the ethyl ether amount increases with pulse number, and reaches a maximum at last pulses. Comparably with supported HPM, the amounts for ethyl ether are quite similar over supported HPVM. After second pulse, constant levels are reached in the case of ethylene quantity for both supported catalysts but the yield of ethylene is higher over supported HPM. Acetaldehyde (ACA) which is known to be produced on redox centres resulted in significant amount on both Al-MCM41- and FeMCM41-supported catalysts in comparison with pure HPAs ones. However, supported HPVM led to higher quantities of ACA, especially at 523 and 543 K. ACA quantities increased with the number pulses for supported catalysts and after the third pulse reached a relatively constant level (Fig. 1b). Thus, the very high dispersion of the HPAs on high surface area molecular sieve yielded an active catalyst for ethanol conversion and especially for acetaldehyde formation. The reaction rates corresponding to the transformation of ethanol and formation of acetaldehyde, respectively increased on fresh Al-MCM41- and Fe-MCM41-supported HPAs comparatively with unsupported HPAs ones in the whole range of temperatures: 503–543 K (Table 2). In comparison with the fresh catalysts, after reoxidation, the values of reaction rates of acetaldehyde formation for both Al-MCM41- and Fe-MCM41-supported and
Fig. 1. Quantities of acetaldehyde for ethanol conversion on HPVM (a) and FeMCM41-supported HPVM (b).
A. Popa et al. / Applied Surface Science 255 (2008) 1830–1835
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Table 2 Average reaction rates of reaction products formation (106 mole/g s) at different temperature for molecular sieves-supported and unsupported heteropolyacids Sample
rACA
HPM HPVM HPM-reox HPVM-reox HPM/Fe-MCM41 HPVM/Fe-MCM41 HPM/Fe-MCM41-reox HPVM/Fe-MCM41-reox HPM/Al-MCM41 HPVM/Al-MCM41 HPM/Al-MCM41-reox HPVM/Al-MCM41-reox
rET
rDEE
503 K
523 K
543 K
503 K
523 K
543 K
503 K
523 K
543 K
6.19 2.98 6.50 5.76 50.17 73.40 59.56 74.86 47.60 41.73 43.08 36.89
13.67 7.09 12.23 8.73 68.69 81.72 70.74 80.88 63.19 56.20 62.60 58.75
10.67 8.76 11.74 9.48 79.70 97.76 99.46 99.25 95.53 102.2 88.88 125.5
7.50 6.71 5.31 5.80 10.01 10.00 9.15 7.67 12.94 20.52 15.58 18.28
12.41 14.97 12.32 12.11 22.35 14.88 15.16 11.76 30.61 50.65 27.94 30.43
23.58 20.99 19.84 15.38 35.33 28.25 42.86 27.31 101.2 78.26 78.76 89.05
6.57 7.07 4.68 6.12 10.69 15.73 13.61 12.35 19.74 25.42 19.83 23.25
10.59 12.13 10.07 10.76 17.04 15.35 13.84 13.23 26.85 31.87 22.76 25.80
7.87 9.87 7.65 9.66 9.17 13.22 16.67 16.07 44.04 73.37 32.96 92.59
unsupported HPAs catalysts are increased as a result of enriching catalyst surface with oxygen. An exception was observed for AlMCM41-supported HPAs at 503 K, when the values of reaction rates of acetaldehyde were lower after reoxidation (Table 3). An explanation could be that, probably at lower temperature (503 K), the degree of oxidation (the surface enriching in oxygen) during the process of reoxidation with air is lower that the same process at temperature higher than 523 K. In contrast, the reaction rates of ethylene and diethyl ether, respectively present lower values after catalyst reoxidation for all catalysts at 503 and 523 K. In this case, a different behaviour was observed for Fe-MCM41-supported HPAs and for HPVM/Al-MCM41 at 543 K, when the values of reaction rates of ET and DEE were higher after reoxidation (Table 2). An explanation could be that, probably at higher reaction temperature, the acidic active centers of HPAs (which are responsible for ET and DEE formation) are still very active after initial six pulses, and could became more active following the reoxidation process. Also, it is known that the amount of ET increase with reaction temperature of the conversion of ethanol [31,32].
The catalytic activities of supported and unsupported HPAs, fresh and reoxidated samples were compared on the basis of reaction rates per gram catalyst [(mol reaction product) (gcat)1 s1]. The resulting reaction rates for acetaldehyde formation are presented in Table 3 and as ln r versus 1/T in Figs. 2 and 3. It could be observed that reaction rates values show that Al-MCM41and Fe-MCM41-supported catalysts, both fresh and after reoxidation appeared to be far more active than bulk HPM and HPVM catalysts in dehydrogenation reaction. For bulk catalysts, the values of apparent activation energies for all reaction products formation are smaller for HPVM than for HPM one. On the other hand apparent activation energies in case of FeMCM41-supported catalysts have lower values as the unsupported HPAs for acetaldehyde and diethyl ether formation and almost the same values for ethylene formation. Al-MCM41-supported catalysts show values of apparent activation energies almost the same with the values of bulk catalysts ones. For all the studied catalysts, the activation energy of ethylene formation (dehydration route) is higher than for acetaldehyde formation (dehydrogenation route). This is typical for low alcohols conversion [5].
Table 3 The kinetic data for acetaldehyde formation on Al-MCM41- and Fe-MCM41-supported and unsupported heteropolyacids Sample
Temperature (K)
Fresh catalysts 6
rACA 10 (mol g
After reoxidation 1
1
s
)
Ea (kJ/mol)
rACA 106 (mol g1 s1)
1
A (s
) 6
Ea (kJ/mol)
A (s1)
3.82 5.38 6.50 12.23 14.11
38.2
5.15 104
HPM
483 503 523 543 563
2.37 3.56 6.19 13.67 14.89
56.6
3.14 10
HPVM
483 503 523 543 563
2.48 2.98 7.09 8.76 10.55
44.9
1.8 105
4.07 5.76 8.73 9.48 16.18
36.6
3.82 104
HPM/Fe-MCM41
503 523 543
50.17 68.7 77.8
24.9
2.05 104
59.50 70.74 79.46
28.8
5.87 104
HPVM/Fe-MCM41
503 523 543
60.33 81.72 95.24
25.8
3.09 104
74.86 80.88 99.25
15.8
3.29 103
HPM/Al-MCM41
503 523 543
47.6 63.19 95.53
39.2
5.97 105
43.8 62.6 88.9
40.9
7.98 105
HPVM/Al-MCM41
503 523 543
41.73 56.20 102.20
50.3
7.28 106
36.89 58.75 125.46
68.9
5.36 108
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Fig. 4. Correlation between log A vs. activation energy for acetaldehyde formation: (A) the dots correspond to: HPM, HPVM fresh and reoxidated, (B) the dots correspond to HPM/Fe-MCM41, HPVM/Fe-MCM41 fresh and reoxidated, and (C) the dots correspond to HPM/Al-MCM41, HPVM/Al-MCM41 fresh and reoxidated.
Fig. 2. Arrhenius plots of specific reaction rates of Fe-MCM41-supported and unsupported heteropolyacids series for acetaldehyde formation.
of Fe-MCM41-supported HPAs and Ti = 522 K for Al-MCM41supported HPAs. 4. Conclusion
Fig. 3. Arrhenius plots of specific reaction rates of Al-MCM41-supported and unsupported heteropolyacids series for acetaldehyde formation.
A comparison of the apparent activation energies of tested catalysts for acetaldehyde formation reveals an important decrease in the case of Fe-MCM41-supported HPAs. Their values amounted to ca. 25 kJ mole1 for supported HPAs, while for parent acids the values are 57 kJ/mole for HPM and 45 kJ/mole for HPVM, respectively (Table 3). A ‘‘compensation effect’’ was evidenced from the relationship between pre-exponential factor A and apparent activation energies Ea for acetaldehyde (Fig. 4). On the basis of compensation effect, it was calculated the isokinetic temperature Ti. At this temperature all kinetic parameters of a reaction are equal. Isokinetic temperature is calculated by the equation: 1 Ti ¼ 2:303Ra where ‘‘a’’ is the slope of the linear plots. The calculated isokinetic temperature for acetaldehyde formation was Ti = 547 K in the case
HPAs anions preserved their Keggin structure on the Fe-MCM41 and Al-MCM41 surface and forms finely dispersed HPAs species. No HPAs detectable crystal phase is observed at 30 wt.% loading. The Fe-MCM41 host material suffers structural distortions at higher loadings, which leads to a loss in long-range order. Both FeMCM41 and Al-MCM41-supported HPAs exhibit differential pore size distribution in the mesoporosity range. The surface area and pore size diameter of supported HPAs decreases with increase in HPAs loading due to the partial blockage of the mesopores of the support by HPAs particles. The ethanol conversion proceeds by two main pathways: an oxidehydrogenation reaction on redox catalytic centres, and a dehydration reaction on acidic centres, respectively. Al-MCM41- and Fe-MCM41-supported HPAs have showed mainly redox properties and therefore the reaction product obtained in more important yield is acetaldehyde. The favourable effect of HPAs deposition on Fe-MCM41 and Al-MCM41 supports for oxidehydrogenation pathway to acetaldehyde results from the apparent activation energies and reaction rate values. An evidenced decreasing of Ea was obtained for acetaldehyde formation in the case of supported HPM (24.9 kJ/mol) comparatively with pure HPM (56.6 kJ/mol). A ‘‘compensation effect’’ was evidenced from the relationship between pre-exponential factor A and apparent activation energies Ea for acetaldehyde, ethylene and ethyl ether formation. Acknowledgement These investigations were financed by the Roumanian Ministry of Education and Research, Grant CNCSIS No. 78GR/2007. References [1] [2] [3] [4]
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