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γ-Al2O3 catalysts prepared from Re2(CO)10 precursor
Applied Catalysis A: General 208 (2001) 169–175
Temperature-programmed desorption study of Re/␥-Al2 O3 catalysts prepared from Re2 (CO)10 precursor Jarkko Räty, Tapani A. Pakkanen∗ Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland Received 21 February 2000; received in revised form 21 June 2000; accepted 23 June 2000
Abstract Alumina supported rhenium catalysts were prepared by gas-phase sublimation technique from carbonyl based precursor. Temperature-programmed desorption (TPD) profiles measured at different rhenium loadings suggested the formation of a new rhenium phase when the rhenium content reached a certain level. Acidic properties of the catalysts were investigated by temperature-programmed desorption of ammonia. Total acidity of the samples increased until the metal loading reached 4.1 wt.%, and afterwards decreased. As the metal loading increased, the portion of the middle strength acid sites increased at the expense of the lower strength acid sites which dominated in the pure support sample and samples with low rhenium content. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas-phase preparation; Fluidized bed reactor; Rhenium carbonyl; Re2 (CO)10 ; Alumina; Hydrodesulfurization; TPD; Ammonia TPD
1. Introduction Transition metal carbonyls supported on metal oxide supports such as alumina are capable of catalyzing several different reactions [1]. Supported Re2 O7 particles have proven to be highly active and selective, especially in metathesis reaction [1,2] and they are an important secondary component in Pt–Re-based reforming catalysts [3]. Furthermore, sulfided rhenium particles appear to be more active than conventional Co–Mo catalysts in hydrodesulfurization (HDS) reaction [4,5]. Temperature-programmed techniques are widely used in catalyst characterization [6]. Temperatureprogrammed desorption (TPD) of ligands and probe ∗ Corresponding author. Tel.: +358-13-2513345; fax: +358-13-2513344. E-mail address: [email protected] (T.A. Pakkanen).
molecules provides valuable information about the nature of the adsorbed species [7]. Desorption studies have been carried out on transition metal carbonyls on various supports [7–11]. Hukul and Brenner [7] studied the decomposition behaviour of a number of different metal carbonyl precursors supported on alumina. Temperature-programmed decomposition (TPDE, in principle the same method as TPD) study of alumina supported W(CO)6 indicated a two-stage desorption process in which stable W(CO)3 is formed at about 200◦ C [9]. Metal tricarbonyl Re(CO)3 is formed during the decarbonylation of alumina supported Re2 (CO)10 as was indicated by an IR study in which sample was heated for 10 min at 500◦ C [12]. Similar results were also obtained in a study where Re2 (CO)10 was used as a precursor and acidic NaHY was the support [13]. To our knowledge, detailed analysis of Re2 (CO)10 /␥-Al2 O3 TPD profiles has not been presented.
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Temperature-programmed desorption of simple bases such as ammonia and pyridine has been used as a technique to determine the total number of acid site and acid site distribution on zeolite based catalysts [6]. Ammonia is a small molecule that can detect nearly all the acid sites in such catalysts. As a strong base however, it can give only preliminary indication of the changes in acidic sites. TPD of ammonia is a suitable tool to determine overall acidity. Higher acidity of a hydrotreating catalyst increases the initial activity in HDS reaction [14]. The synergy between the small metal sulfide and the acidic support improves as the acidity of the surface increases. Sarbak [15] found that HDS activity was significantly higher for CoMo hydrotreating modified with fluorine than for same catalysts modified with sodium. The fluorine treated catalysts had Lewis acid sites whereas the sodium modified did not have any. In earlier work we have developed a new controlled gas-phase preparation method for alumina supported Re2 (CO)10 catalysts which showed good activity in thiophene HDS reaction [16]. In this study we present TPD profiles for the Re2 (CO)10 /␥-Al2 O3 catalysts with different rhenium loadings and interpret the new desorption peaks that were recorded for high rhenium content catalysts. Temperature-programmed desorption of ammonia (NH3 -TPD) was carried out to characterize the acidic properties of the samples.
2. Experimental
under nitrogen flow (99.999%, AGA). Reaction time varied between 4 and 17 h. A pulse technique was applied to increase the rhenium content in some of the catalysts. Between deposition cycles the catalysts were partially decarbonylated under N2 flow or totally decarbonylated under H2 (99.999%, AGA) flow. The preparation of catalysts is described in more detail level in a previous paper [16]. Rhenium loading varied between 1.0 and 11.1 wt.%. Metal contents up to 8.3 wt.% were achieved by single deposition pulse with variable deposition time and deposition temperatures. Catalysts with higher rhenium loading were prepared by pulse technique. 2.2. Determination of rhenium content Combined atomic absorption spectrometry (Varian, Spectr AA-400) and energy dispersive X-ray spectrometry (ACAX 300 EDXRF) technique was used to determine rhenium loadings. Calibration of EDXRF was obtained by dissolving some of the catalysts with different metal loading in acidic solvent and determining their rhenium content by AAS. The standard material for AAS calibration was KReO4 (99.99%) supplied by Aldrich. EDXRF technique was used to determine the metal loading of the catalysts. The instrument included a Si–Li detector and Cd (109) radioisotope for the radiation. Measurement time was 100 s and 0.15 keV resolution was obtained. Each catalyst was measured three times and the sample holder was turned between the measurements in order to obtain accurate results.
2.1. Reactants and catalyst preparation 2.3. Temperature-programmed studies Rhenium precursor (Re2 (CO)10 , 99%) was supplied by Strem Chemicals. Aluminium oxide (Brockmann I, standard grade, neutral and activated, 150 mesh, 58 Å, surface area 155 m2 /g) provided by Aldrich was used as a support material. Pre-treatment of the support was carried out at 500◦ C in vacuum for 10 h. Afterwards the support was moved to a nitrogen glove box in air-sensitive manner. Catalysts were prepared by a gas-phase adsorption method. The fluidized–bed reactor system connected to the glove box as described [17]. As soon as the reactor temperature (115 or 120◦ C) was obtained the rhenium precursor was transferred to the system and vaporization of the rhenium carbonyl was started
Temperature-programmed desorption (TPD) and temperature-programmed desorption of ammonia (NH3 -TPD) were performed with a Micromeritics AutoChem 2910 analyser. A thermal conductivity detector (TCD) monitored the difference in conductivity of the sample and the reference lines while the sample temperature was raised in a controlled manner. To obtain information about the desorbed species we connected the outlet of the instrument to a quadrupole mass spectrometer (HP 5920). A typical TPD run began with packing of the sample tube (0.15–0.25 g) into the sample tube in the glove box. The tube was connected to the instrument and the
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program was started. Inert gas (He, 99.9999%, flow rate 10 ml/min, supplied by AGA) flowed through the tube and when the baseline was stable the experiment was started. A ramp rate of 10◦ C/min was applied and the temperature was linearly raised to 500◦ C, where it was held for 15 min. At the end, the samples were cooled at room temperature. The carbon content of the catalysts was determined with an elemental analyzer (CE instrument EA 1110) after TPD run. NH3 -TPD was carried out after all carbonyl groups had been removed from the rhenium particles. A detailed account can be found in our earlier paper [18]. Prior the adsorption sample was heated for 1 h at 550◦ C under helium flow (30 ml/min). The absence of carbon and carbonyl ligands was verified by IR and elemental analyzer. After the sample was cooled at 100◦ C, the adsorption of ammonia (10% NH3 /He, 30 ml/min) was carried out for 1 h and this was followed by a flushing sequence (He) at the same temperature (1 h), so that all the physisorbed ammonia was removed from the surface. The sample was then heated at 550◦ C in a controlled manner in helium flow
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(30 ml/min) and the desorption pattern was established by TCD. Conditions and treatments were same for all catalysts. The instrument reported the amount of ammonia desorbed from the catalyst (ml/g sample) on the basis of an earlier calibration.
3. Results and discussion 3.1. Temperature-programmed desorption Shapes of the desorption profiles differed with the metal loading as can be seen in Fig. 1. Rhenium loading increases upwards direction in the figure (1.1, 3.0 and 8.9 wt.% for Fig. 1c, b and a respectively). In general, carbonyls leave from the surface when the desorption energy reaches a sufficiently high level. The numbers in the figure indicate the temperatures of the desorption peaks. Two peaks at 204 and 464◦ C are clearly distinguishable in the profile (Fig. 1c) of the catalyst with rhenium loading. As the metal content increases to
Fig. 1. TPD profile of Re2 (CO)10 /␥-Al2 O3 catalysts with different rhenium loading (a) 8.9 wt.%, (b) 3.0 wt.% and (c) 1.1 wt.%.
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3.0 wt.% (Fig. 1b) the shape of the pattern remains the same but the temperature maximums decreases to 200 and 411◦ C. The shape of the profile changes markedly when the rhenium content increases further to 8.9 wt.% (Fig. 1a). The profile now contains four desorption maximums at 195, 220, 292 and 369◦ C. In all the profiles the last peak is distinctly broader than the other peaks. The evolution of the last species and the surface reactions take place in this region. The desorption pattern is broader because the surface species evolved at higher temperatures are much more heterogeneous than the adsorbents evolved at low temperature regions. The temperatures of the two desorption maximums that are present in the all three profiles decrease with increase in the rhenium loading, from 204 to 195◦ C for the first peak and more dramatically, from 464 to 369◦ C for the second. A more interesting feature may be the formation of two new maximums at highest metal loading in the temperature region between the two peaks just mentioned. According to Arnoldy et al. [5], the electronic interaction between rhenium particles and the support weakens as the rhenium content increases. The decrease we observed in the temperature of the desorption maximum of higher rhenium loading is in agreement with this suggestions. There is more space around the rhenium precursors at low metal loading and the CO-ligands are able to interact strongly with the surface. The new peaks in Fig. 1a suggest changes in the surface structure. One possibility is the formation of the larger Re2 (CO)10 stacks on the surface. Although the estimated rhenium content at monolayer coverage is approximately 26 wt.%, owing to due the steric hindrance created by rhenium carbonyl precursor and the small pores in the alumina support, the monolayer coverage is achieved at lower rhenium loading. Metal particles are well-dispersed in catalysts prepared by gas-phase adsorption method. As the distance of two Re2 (CO)10 decreases in the surface with increasing rhenium loading, new interactions may form between metal particles leading to the new peaks in the profile. It is interesting that the new maximums at higher rhenium loading are located in the middle temperature region between the existing peaks. The energy required to remove the species represented by the new peaks is higher than that for the peak at 195◦ C.
Fig. 2. Mass spectrometry signal during the TPD run of ␥-Al2 O3 supported Re2 (CO)10 catalyst (8.9 wt.%).
Another explanation for the new peaks may thus be the adsorption of Re2 (CO)10 to different site in alumina surface. The alumina surface provides several different sites for the adsorption of molecules and it may be that after the most suitable sites are occupied, the secondary sites will be filled with a different adsorbed species. Fig. 2 shows the mass spectrometer signal from the catalyst with 8.9 wt.% rhenium loading. The main desorbing component is CO, but some traces of CO2 and methane can be identified as well. The formation of the carbon monoxide is easily explained since the precursor consists of the ten carbonyl ligands, and the carbonyl groups of the precursor interact with the OH-groups of the support. A redox reaction takes place between surface OH-groups and metallic precursor and some CO2 as well as hydrogen is formed [8,9]. In addition, two CO molecules may react on the surface with production of CO2 and carbon (Boudouard reaction) [9]. The carbon and hydrogen left on the surface may react and form methane. The relative proportion of the other desorbing component than CO increases with the temperature of the sample. Evidently the reaction between carbon monoxide and the support is more likely to occur at higher temperature region. Earlier TPD and IR results indicate the presence of CO ligands on the surface after heating of the sample at 200◦ C. The first peak in the TPD profile corresponds to the formation of a Re(CO)3 subcarbonyl species.
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Platero et al. [12] have suggested the formation of Re(CO)3 on the basis of IR studies. Moreover a TPD profile similar to ours at low rhenium loading was recorded in a TPDE study of acidic zeolite supported Re2 (CO)10 [13]. In the first stage, the precursor was decomposed to two Re(CO)3 species and 4 mol CO per mol Re2 (CO)10 was evolved; further heating led to desorption of the remaining carbonyls [13]. IR spectra measured after TPD run showed the absence of CO bands and moreover elemental analysis performed after TPD showed no presence of carbon on the surface. In a previous study [16] we evaluated the HDS activity of Re2 (CO)10 /Al2 O3 catalysts prepared by gas-phase sublimation technique. Thiophene conversion increased up to 8.9 wt.% metal loading, after which it decreased. It was at this content that the two new peaks appeared in our TPD profile. The result suggests that the formation of the new rhenium phase may weaken the activity of the catalyst in thiophene HDS reaction. 3.2. Temperature-programmed desorption of ammonia Table 1 shows the amount of desorbed ammonia as a function of the metal loading. Acidity of the pure alumina support is given in the uppermost row. Overall acidity increases with the rhenium content, with maximum acidity at 4.1 wt.% metal loading. Further increase in the amount of rhenium leads to a decrease in the acidity. Up to a certain limit, overall acidity increases with the rhenium deposition. Since at the same time the free alumina area decreases, the remaining unoccuTable 1 TPD of ammonia from rhenium/␥-Al2 O3 catalysts and pure ␥-Al2 O3 support Rhenium (wt.%)
Desorbed ammonia mmol/g catalyst
␥-Al2 O3 1.03 2.96 3.70 4.1 8.27 8.86 11.10
625 683 741 794 848 812 776 763
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pied sites become more acidic. Eventually, the rhenium species create steric hindrance, with the further decrease in the pure alumina surface, the overall acidity decrease. Schekler-Nahama et al. [19] have suggested that increase of Re2 O7 species on the alumina surface leads to growth of strong Lewis acidity at the expense of weak Lewis acidity and weak Brönsted acidity appears as the rhenium content increases to 14 wt.% level. TPD profiles as well as deconvoluate curves are presented in Fig. 3. The deconvoluation was made in order to find out more information about acid strength populations. The TPD pattern of the pure support is presented in profile 3 a). One clear broad band with minor fine structure is visible in the high temperature region. The band starts below 200◦ C, so that region corresponds to ammonia desorption from the weakest acid sites. The profiles of three rhenium catalysts with increasing rhenium loading are shown in the same figure. The profile changes with increase in the rhenium content. At low rhenium loading the portion of the sites of medium and high acidic strength increases at the expense of the sites of lower acidic strength and further increase in the metal content emphasizes the difference. The percentage amount of the medium strength acid sites increases from 25 to 44% with increasing metal loading and finally they become dominant in the profile. Acidic properties play a significant role in the activity of catalysts. Many studies have attempted to find a correlation between hydrotreating activity and the acidic properties of the catalyst [14,15,20–22]. Fluorine and chloride ions have been added to enhance the acidity of hydrotreating catalysts. Moreover the particular type of acidity has been found to play an important role in hydrodenitrogenation processes [20]. In another study a higher acidity of zeolite based catalysts led to increased thiophene HDS conversion. Evidently, acidity created the synergy effect between the small metal sulfide and the support [14]. Fig. 4 shows our earlier HDS results [16] and overall acidity of the catalysts. The conversion of thiophene increases with increasing metal content and with increasing acidity. Conversion still increases slightly even the overall acidity begins to decrease. The increase of the metal content from 8.9 to 11.1 wt.% decreases both the conversion and the acidity.
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Fig. 3. NH3 -TPD profiles of pure ␥-Al2 O3 support (a) and rhenium catalysts with increasing rhenium loading (b) 1.0 wt.%; (c) 3.7 wt.%; (d) 8.9 wt.%.
At low metal loading catalyst (0.9 wt.%) has good relative molar ratio (conversion of thiophene divided by the amount of metal) in thiophene HDS [16]. The overall acidity of the catalyst increases compared to
pure alumina support clearly so the acidity may have a effect to conversion results in catalysts with low metal loading. Nevertheless clear trend between acidity and conversion results cannot be distinguished.
Fig. 4. Overall acidity and HDS activity [16] results of Re2 (CO)10 /␥-Al2 O3 catalysts with increasing metal content.
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4. Conclusions TPD profiles of Re2 (CO)10 suggests formation of new interactions in the surface with increase in rhenium loading. The new peaks may represent adsorption of rhenium carbonyl to different surface site or, interaction between two rhenium carbonyl groups whose distance shortens with increasing rhenium loading. The NH3 -TPD results suggest a correlation between overall acidity and the rhenium loading. Up to a certain level the total amount of acid sites increased as a function of the rhenium content. However, beyond that level the amount of acidic sites began to diminish probably due to steric hindrance. The deconvoluted TPD patterns showed that the population of the middle strength acid sites increased at the expense of the low strength acid sites. A clear trend between acidity and earlier thiophene HDS results was not observed. References [1] J.C. Mol, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 4, VCH Verlagsgesellschaft, Weinheim, 1997, Section 4.12.2. [2] J.C. Mol, Catal. Today 51 (1999) 289. [3] J.H. Sinfelt, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 4, VCH Verlagsgesellschaft, Weinheim, 1997, Section 3.9.4.
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[4] T.A. Pecoraro, R.R. Chianelli, J. Catal. 67 (1981) 430. [5] P. Arnoldy, E.M. van Oers, V.H.J. de Beer, J.A. Moulijn, R. Prins, Appl. Catal. 48 (1989) 241. [6] B.E. Spiewak, R.D. Cortright, J.A. Dumesic, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 2, VCH Verlagsgesellschaft, Weinheim, 1997, Section 3.2.3.2. [7] D.A. Hukul, A. Brenner, Inorg. J. Phys. Chem. 85 (1981) 496. [8] A. Brenner, D.A. Hukul, S.J. Hardwick, Inorg. Chem. 18 (1979) 1478. [9] A. Brenner, D.A. Hukul, J. Catal. 61 (1980) 216. [10] H.-G. Ang, G.-K. Chuah, S. Jaenicke, W.-L. Loh, J. Chem. Soc., Dalton Trans. (1997) 1243. [11] H.-G. Ang, K.-S. Chan, G.-K. Chuah, S. Jaenicke, S.-K. Neo, J. Chem. Soc., Dalton Trans. (1995) 3753. [12] E.E. Platero, F.R. de Peralta, C.O. Arean, Catal. Lett. 34 (1995) 65. [13] C. Dossi, J. Schaefer, W.M.H. Sachtler, J. Mol. Catal. 52 (1989) 193. [14] W.J.J. Welters, V.H.J. de Beer, R.A. van Santen, Appl. Catal. A 119 (1994) 253. [15] Z. Sarbak, Appl. Catal. A 164 (1997) 13. [16] J. Räty, T.A. Pakkanen, Catal. Lett. 65 (2000) 175. [17] M. Suvanto, T.A. Pakkanen, Appl. Catal. A 166 (1998) 105. [18] M. Suvanto, J. Räty, T.A. Pakkanen, Catal. Lett. 62 (1999) 21. [19] F. Schekler-Nahama, O. Clause, D. Commereuc, J. Saussey, Appl. Catal. A 167 (1998) 247. [20] M. Lewandowski, Z. Sarbak, Appl. Catal. A 156 (1997) 181. [21] M.J.F.M. Verhaak, A.J. van Dillen, J.W. Geus, Appl. Catal. A 105 (1993) 251. [22] M. Nagai, K. Koizumi, S. Omi, Catal. Today 35 (1997) 393.
Temperature-programmed desorption study of Re/␥-Al2 O3 catalysts prepared from Re2 (CO)10 precursor Jarkko Räty, Tapani A. Pakkanen∗ Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland Received 21 February 2000; received in revised form 21 June 2000; accepted 23 June 2000
Abstract Alumina supported rhenium catalysts were prepared by gas-phase sublimation technique from carbonyl based precursor. Temperature-programmed desorption (TPD) profiles measured at different rhenium loadings suggested the formation of a new rhenium phase when the rhenium content reached a certain level. Acidic properties of the catalysts were investigated by temperature-programmed desorption of ammonia. Total acidity of the samples increased until the metal loading reached 4.1 wt.%, and afterwards decreased. As the metal loading increased, the portion of the middle strength acid sites increased at the expense of the lower strength acid sites which dominated in the pure support sample and samples with low rhenium content. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Gas-phase preparation; Fluidized bed reactor; Rhenium carbonyl; Re2 (CO)10 ; Alumina; Hydrodesulfurization; TPD; Ammonia TPD
1. Introduction Transition metal carbonyls supported on metal oxide supports such as alumina are capable of catalyzing several different reactions [1]. Supported Re2 O7 particles have proven to be highly active and selective, especially in metathesis reaction [1,2] and they are an important secondary component in Pt–Re-based reforming catalysts [3]. Furthermore, sulfided rhenium particles appear to be more active than conventional Co–Mo catalysts in hydrodesulfurization (HDS) reaction [4,5]. Temperature-programmed techniques are widely used in catalyst characterization [6]. Temperatureprogrammed desorption (TPD) of ligands and probe ∗ Corresponding author. Tel.: +358-13-2513345; fax: +358-13-2513344. E-mail address: [email protected] (T.A. Pakkanen).
molecules provides valuable information about the nature of the adsorbed species [7]. Desorption studies have been carried out on transition metal carbonyls on various supports [7–11]. Hukul and Brenner [7] studied the decomposition behaviour of a number of different metal carbonyl precursors supported on alumina. Temperature-programmed decomposition (TPDE, in principle the same method as TPD) study of alumina supported W(CO)6 indicated a two-stage desorption process in which stable W(CO)3 is formed at about 200◦ C [9]. Metal tricarbonyl Re(CO)3 is formed during the decarbonylation of alumina supported Re2 (CO)10 as was indicated by an IR study in which sample was heated for 10 min at 500◦ C [12]. Similar results were also obtained in a study where Re2 (CO)10 was used as a precursor and acidic NaHY was the support [13]. To our knowledge, detailed analysis of Re2 (CO)10 /␥-Al2 O3 TPD profiles has not been presented.
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Temperature-programmed desorption of simple bases such as ammonia and pyridine has been used as a technique to determine the total number of acid site and acid site distribution on zeolite based catalysts [6]. Ammonia is a small molecule that can detect nearly all the acid sites in such catalysts. As a strong base however, it can give only preliminary indication of the changes in acidic sites. TPD of ammonia is a suitable tool to determine overall acidity. Higher acidity of a hydrotreating catalyst increases the initial activity in HDS reaction [14]. The synergy between the small metal sulfide and the acidic support improves as the acidity of the surface increases. Sarbak [15] found that HDS activity was significantly higher for CoMo hydrotreating modified with fluorine than for same catalysts modified with sodium. The fluorine treated catalysts had Lewis acid sites whereas the sodium modified did not have any. In earlier work we have developed a new controlled gas-phase preparation method for alumina supported Re2 (CO)10 catalysts which showed good activity in thiophene HDS reaction [16]. In this study we present TPD profiles for the Re2 (CO)10 /␥-Al2 O3 catalysts with different rhenium loadings and interpret the new desorption peaks that were recorded for high rhenium content catalysts. Temperature-programmed desorption of ammonia (NH3 -TPD) was carried out to characterize the acidic properties of the samples.
2. Experimental
under nitrogen flow (99.999%, AGA). Reaction time varied between 4 and 17 h. A pulse technique was applied to increase the rhenium content in some of the catalysts. Between deposition cycles the catalysts were partially decarbonylated under N2 flow or totally decarbonylated under H2 (99.999%, AGA) flow. The preparation of catalysts is described in more detail level in a previous paper [16]. Rhenium loading varied between 1.0 and 11.1 wt.%. Metal contents up to 8.3 wt.% were achieved by single deposition pulse with variable deposition time and deposition temperatures. Catalysts with higher rhenium loading were prepared by pulse technique. 2.2. Determination of rhenium content Combined atomic absorption spectrometry (Varian, Spectr AA-400) and energy dispersive X-ray spectrometry (ACAX 300 EDXRF) technique was used to determine rhenium loadings. Calibration of EDXRF was obtained by dissolving some of the catalysts with different metal loading in acidic solvent and determining their rhenium content by AAS. The standard material for AAS calibration was KReO4 (99.99%) supplied by Aldrich. EDXRF technique was used to determine the metal loading of the catalysts. The instrument included a Si–Li detector and Cd (109) radioisotope for the radiation. Measurement time was 100 s and 0.15 keV resolution was obtained. Each catalyst was measured three times and the sample holder was turned between the measurements in order to obtain accurate results.
2.1. Reactants and catalyst preparation 2.3. Temperature-programmed studies Rhenium precursor (Re2 (CO)10 , 99%) was supplied by Strem Chemicals. Aluminium oxide (Brockmann I, standard grade, neutral and activated, 150 mesh, 58 Å, surface area 155 m2 /g) provided by Aldrich was used as a support material. Pre-treatment of the support was carried out at 500◦ C in vacuum for 10 h. Afterwards the support was moved to a nitrogen glove box in air-sensitive manner. Catalysts were prepared by a gas-phase adsorption method. The fluidized–bed reactor system connected to the glove box as described [17]. As soon as the reactor temperature (115 or 120◦ C) was obtained the rhenium precursor was transferred to the system and vaporization of the rhenium carbonyl was started
Temperature-programmed desorption (TPD) and temperature-programmed desorption of ammonia (NH3 -TPD) were performed with a Micromeritics AutoChem 2910 analyser. A thermal conductivity detector (TCD) monitored the difference in conductivity of the sample and the reference lines while the sample temperature was raised in a controlled manner. To obtain information about the desorbed species we connected the outlet of the instrument to a quadrupole mass spectrometer (HP 5920). A typical TPD run began with packing of the sample tube (0.15–0.25 g) into the sample tube in the glove box. The tube was connected to the instrument and the
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program was started. Inert gas (He, 99.9999%, flow rate 10 ml/min, supplied by AGA) flowed through the tube and when the baseline was stable the experiment was started. A ramp rate of 10◦ C/min was applied and the temperature was linearly raised to 500◦ C, where it was held for 15 min. At the end, the samples were cooled at room temperature. The carbon content of the catalysts was determined with an elemental analyzer (CE instrument EA 1110) after TPD run. NH3 -TPD was carried out after all carbonyl groups had been removed from the rhenium particles. A detailed account can be found in our earlier paper [18]. Prior the adsorption sample was heated for 1 h at 550◦ C under helium flow (30 ml/min). The absence of carbon and carbonyl ligands was verified by IR and elemental analyzer. After the sample was cooled at 100◦ C, the adsorption of ammonia (10% NH3 /He, 30 ml/min) was carried out for 1 h and this was followed by a flushing sequence (He) at the same temperature (1 h), so that all the physisorbed ammonia was removed from the surface. The sample was then heated at 550◦ C in a controlled manner in helium flow
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(30 ml/min) and the desorption pattern was established by TCD. Conditions and treatments were same for all catalysts. The instrument reported the amount of ammonia desorbed from the catalyst (ml/g sample) on the basis of an earlier calibration.
3. Results and discussion 3.1. Temperature-programmed desorption Shapes of the desorption profiles differed with the metal loading as can be seen in Fig. 1. Rhenium loading increases upwards direction in the figure (1.1, 3.0 and 8.9 wt.% for Fig. 1c, b and a respectively). In general, carbonyls leave from the surface when the desorption energy reaches a sufficiently high level. The numbers in the figure indicate the temperatures of the desorption peaks. Two peaks at 204 and 464◦ C are clearly distinguishable in the profile (Fig. 1c) of the catalyst with rhenium loading. As the metal content increases to
Fig. 1. TPD profile of Re2 (CO)10 /␥-Al2 O3 catalysts with different rhenium loading (a) 8.9 wt.%, (b) 3.0 wt.% and (c) 1.1 wt.%.
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3.0 wt.% (Fig. 1b) the shape of the pattern remains the same but the temperature maximums decreases to 200 and 411◦ C. The shape of the profile changes markedly when the rhenium content increases further to 8.9 wt.% (Fig. 1a). The profile now contains four desorption maximums at 195, 220, 292 and 369◦ C. In all the profiles the last peak is distinctly broader than the other peaks. The evolution of the last species and the surface reactions take place in this region. The desorption pattern is broader because the surface species evolved at higher temperatures are much more heterogeneous than the adsorbents evolved at low temperature regions. The temperatures of the two desorption maximums that are present in the all three profiles decrease with increase in the rhenium loading, from 204 to 195◦ C for the first peak and more dramatically, from 464 to 369◦ C for the second. A more interesting feature may be the formation of two new maximums at highest metal loading in the temperature region between the two peaks just mentioned. According to Arnoldy et al. [5], the electronic interaction between rhenium particles and the support weakens as the rhenium content increases. The decrease we observed in the temperature of the desorption maximum of higher rhenium loading is in agreement with this suggestions. There is more space around the rhenium precursors at low metal loading and the CO-ligands are able to interact strongly with the surface. The new peaks in Fig. 1a suggest changes in the surface structure. One possibility is the formation of the larger Re2 (CO)10 stacks on the surface. Although the estimated rhenium content at monolayer coverage is approximately 26 wt.%, owing to due the steric hindrance created by rhenium carbonyl precursor and the small pores in the alumina support, the monolayer coverage is achieved at lower rhenium loading. Metal particles are well-dispersed in catalysts prepared by gas-phase adsorption method. As the distance of two Re2 (CO)10 decreases in the surface with increasing rhenium loading, new interactions may form between metal particles leading to the new peaks in the profile. It is interesting that the new maximums at higher rhenium loading are located in the middle temperature region between the existing peaks. The energy required to remove the species represented by the new peaks is higher than that for the peak at 195◦ C.
Fig. 2. Mass spectrometry signal during the TPD run of ␥-Al2 O3 supported Re2 (CO)10 catalyst (8.9 wt.%).
Another explanation for the new peaks may thus be the adsorption of Re2 (CO)10 to different site in alumina surface. The alumina surface provides several different sites for the adsorption of molecules and it may be that after the most suitable sites are occupied, the secondary sites will be filled with a different adsorbed species. Fig. 2 shows the mass spectrometer signal from the catalyst with 8.9 wt.% rhenium loading. The main desorbing component is CO, but some traces of CO2 and methane can be identified as well. The formation of the carbon monoxide is easily explained since the precursor consists of the ten carbonyl ligands, and the carbonyl groups of the precursor interact with the OH-groups of the support. A redox reaction takes place between surface OH-groups and metallic precursor and some CO2 as well as hydrogen is formed [8,9]. In addition, two CO molecules may react on the surface with production of CO2 and carbon (Boudouard reaction) [9]. The carbon and hydrogen left on the surface may react and form methane. The relative proportion of the other desorbing component than CO increases with the temperature of the sample. Evidently the reaction between carbon monoxide and the support is more likely to occur at higher temperature region. Earlier TPD and IR results indicate the presence of CO ligands on the surface after heating of the sample at 200◦ C. The first peak in the TPD profile corresponds to the formation of a Re(CO)3 subcarbonyl species.
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Platero et al. [12] have suggested the formation of Re(CO)3 on the basis of IR studies. Moreover a TPD profile similar to ours at low rhenium loading was recorded in a TPDE study of acidic zeolite supported Re2 (CO)10 [13]. In the first stage, the precursor was decomposed to two Re(CO)3 species and 4 mol CO per mol Re2 (CO)10 was evolved; further heating led to desorption of the remaining carbonyls [13]. IR spectra measured after TPD run showed the absence of CO bands and moreover elemental analysis performed after TPD showed no presence of carbon on the surface. In a previous study [16] we evaluated the HDS activity of Re2 (CO)10 /Al2 O3 catalysts prepared by gas-phase sublimation technique. Thiophene conversion increased up to 8.9 wt.% metal loading, after which it decreased. It was at this content that the two new peaks appeared in our TPD profile. The result suggests that the formation of the new rhenium phase may weaken the activity of the catalyst in thiophene HDS reaction. 3.2. Temperature-programmed desorption of ammonia Table 1 shows the amount of desorbed ammonia as a function of the metal loading. Acidity of the pure alumina support is given in the uppermost row. Overall acidity increases with the rhenium content, with maximum acidity at 4.1 wt.% metal loading. Further increase in the amount of rhenium leads to a decrease in the acidity. Up to a certain limit, overall acidity increases with the rhenium deposition. Since at the same time the free alumina area decreases, the remaining unoccuTable 1 TPD of ammonia from rhenium/␥-Al2 O3 catalysts and pure ␥-Al2 O3 support Rhenium (wt.%)
Desorbed ammonia mmol/g catalyst
␥-Al2 O3 1.03 2.96 3.70 4.1 8.27 8.86 11.10
625 683 741 794 848 812 776 763
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pied sites become more acidic. Eventually, the rhenium species create steric hindrance, with the further decrease in the pure alumina surface, the overall acidity decrease. Schekler-Nahama et al. [19] have suggested that increase of Re2 O7 species on the alumina surface leads to growth of strong Lewis acidity at the expense of weak Lewis acidity and weak Brönsted acidity appears as the rhenium content increases to 14 wt.% level. TPD profiles as well as deconvoluate curves are presented in Fig. 3. The deconvoluation was made in order to find out more information about acid strength populations. The TPD pattern of the pure support is presented in profile 3 a). One clear broad band with minor fine structure is visible in the high temperature region. The band starts below 200◦ C, so that region corresponds to ammonia desorption from the weakest acid sites. The profiles of three rhenium catalysts with increasing rhenium loading are shown in the same figure. The profile changes with increase in the rhenium content. At low rhenium loading the portion of the sites of medium and high acidic strength increases at the expense of the sites of lower acidic strength and further increase in the metal content emphasizes the difference. The percentage amount of the medium strength acid sites increases from 25 to 44% with increasing metal loading and finally they become dominant in the profile. Acidic properties play a significant role in the activity of catalysts. Many studies have attempted to find a correlation between hydrotreating activity and the acidic properties of the catalyst [14,15,20–22]. Fluorine and chloride ions have been added to enhance the acidity of hydrotreating catalysts. Moreover the particular type of acidity has been found to play an important role in hydrodenitrogenation processes [20]. In another study a higher acidity of zeolite based catalysts led to increased thiophene HDS conversion. Evidently, acidity created the synergy effect between the small metal sulfide and the support [14]. Fig. 4 shows our earlier HDS results [16] and overall acidity of the catalysts. The conversion of thiophene increases with increasing metal content and with increasing acidity. Conversion still increases slightly even the overall acidity begins to decrease. The increase of the metal content from 8.9 to 11.1 wt.% decreases both the conversion and the acidity.
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Fig. 3. NH3 -TPD profiles of pure ␥-Al2 O3 support (a) and rhenium catalysts with increasing rhenium loading (b) 1.0 wt.%; (c) 3.7 wt.%; (d) 8.9 wt.%.
At low metal loading catalyst (0.9 wt.%) has good relative molar ratio (conversion of thiophene divided by the amount of metal) in thiophene HDS [16]. The overall acidity of the catalyst increases compared to
pure alumina support clearly so the acidity may have a effect to conversion results in catalysts with low metal loading. Nevertheless clear trend between acidity and conversion results cannot be distinguished.
Fig. 4. Overall acidity and HDS activity [16] results of Re2 (CO)10 /␥-Al2 O3 catalysts with increasing metal content.
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4. Conclusions TPD profiles of Re2 (CO)10 suggests formation of new interactions in the surface with increase in rhenium loading. The new peaks may represent adsorption of rhenium carbonyl to different surface site or, interaction between two rhenium carbonyl groups whose distance shortens with increasing rhenium loading. The NH3 -TPD results suggest a correlation between overall acidity and the rhenium loading. Up to a certain level the total amount of acid sites increased as a function of the rhenium content. However, beyond that level the amount of acidic sites began to diminish probably due to steric hindrance. The deconvoluted TPD patterns showed that the population of the middle strength acid sites increased at the expense of the low strength acid sites. A clear trend between acidity and earlier thiophene HDS results was not observed. References [1] J.C. Mol, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 4, VCH Verlagsgesellschaft, Weinheim, 1997, Section 4.12.2. [2] J.C. Mol, Catal. Today 51 (1999) 289. [3] J.H. Sinfelt, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Vol. 4, VCH Verlagsgesellschaft, Weinheim, 1997, Section 3.9.4.
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