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Ir and Pt+Ir catalysts
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Factors influencing selectivity in the liquid-phase phenol hydrodeoxygenation over ZSM-5 supported Pt/Ir and Pt+Ir catalysts B. Paweleca,*, C.V. Loriceraa, C. Geantetb, N. Motaa, J.L.G. Fierroa, R.M. Navarroa,* a b
Institute of Catalysis and Petroleum Chemistry, CSIC, Cantoblanco, 28049 Madrid, Spain Research Institute on Catalysis and Environment Lyon (IRCELYON), CNRS: UMR5256-University Claude Bernard-Lyon (UCBL), France
A R T I C LE I N FO
A B S T R A C T
Keywords: Bio-oils Phenol Hydrodeoxygenation Pt/Ir ZSM-5
The effect of partial Pt substitution by Ir on the catalytic response of bimetallic Pt-Ir/ZSM-5 (Pt/Ir = 1) catalyst in the liquid-phase hydrodeoxygenation (HDO) of phenol was investigated. To evaluate the catalyst intrinsic activity, the ZSM-5 supported Pt, Ir and Pt + Ir catalysts were prepared with similar total metal loading (ca. 3 wt. %) and acidity. The catalysts were characterized by several physical and chemical techniques and tested in the HDO of phenol reaction carried out in a batch reactor at T = 200 °C under H2 pressure of 3.0 MPa. For all the catalysts, the phenol transformation mainly proceeds via hydrogenation + dehydration (HYD reaction route) leading to cyclohexane as the main product (selectivity 60–96%). Partial substitution of Pt by Ir in catalyst formulation (Pt50-Ir50/ZSM-5) enhances both activity and selectivity toward O-free products. This was associated to Ir surface enrichment (from HRTEM). The Ir/ZMS-5 and Pt50-It50/ZMS-5 catalysts having well dispersed Ir nanoparticles were more selective toward dehydration than Pt/ZSM-5 having Pt species with a broad size distribution range. The catalyst activity-structure correlation suggests that the dehydration of cyclohexanol (intermediate product) can occur at the interface between the metal and acid sites of the support.
Introduction Bio-oils are alternative feed stocks for fossil fuels. These bio-oils are chemically and thermally unstable due to the high content of oxygenated compounds. In addition, they exhibit an acidic nature and a tendency to polymerization. To solve these problems, bio-oils have to be upgraded and traditional hydrodeoxygenation (HDO) process already existing within the petroleum refining infrastructure can be employed [1–5]. In this process the oxygenated compounds can be removed via hydrogenation in the presence of a catalyst. As the content of phenolic compounds in bio-oils represent nearly one quarter of the total O-containing compounds, phenolic compounds are often used as a model compounds of bio-oils [4]. A general overview of the catalysts [1–6], reaction conditions [7] and mechanisms of HDO reactions of biooils are summarized in a few excellent reviews [1–8]. The catalysts tailored for the HDO reaction are bifunctional; this means that they exhibit metal and acid sites both needed for hydrogenation of aromatic ring and for O-removal via dehydration, respectively [2]. Thus, there is significant experimental evidence that the acidic supports promote HDO reaction [8–11]. For example, it was found that the support acidic function of the HBeta zeolite catalyzed the reaction of trans alkylation “during the anisole conversion to gasoline⁎
range products over bifunctional Pt/HBeta [9]. Similarly, in the case of Pd catalysts, the acidity of parent HBeta and ZSM-5 zeolites affected the initial rates of HDO of benzophenone to the desired diphenylmethane [10]. However, the acid function should be moderate because strong acidity favors catalyst deactivation via coking [11]. The synergy of the support acid function and the metal function in the catalytic HDO of mcresol was observed [11]. Considering the metal function, the first catalysts studied for the HDO reaction were based on the metal sulphides, which were previously used by the petroleum refining industry for the removal of sulphur compounds from crude oil [7]. However, the reactivity of metal sulphides is lowered in the presence of water due to formation of surface oxides [12]. To maintain the catalyst stability, sulphur species should be introduced into reactor [12]. By contrast, noble metals are more resistant toward deactivation by H2O and they exhibit better hydrogenation activity at a lower hydrogen pressure and temperatures [8]. Thus, the catalysts based on noble metals seem to be a good option for the substitution of metal sulphides in the catalyst formulation. In this sense, our previous study demonstrated that the incorporation of a low amount of Pt or Ir to RuS2/SBA-15 catalyst improved the activity for phenol HDO [13]. Many noble metals (Ru, Pt, Ir, Rh, Pd) supported on reducible
Corresponding authors. E-mail addresses: [email protected] (B. Pawelec), [email protected] (R.M. Navarro).
https://doi.org/10.1016/j.mcat.2019.110669 Received 8 July 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: B. Pawelec, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110669
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wetness impregnation method using an aqueous solution of IrCl3 ·H2O and H2PtCl6 ·6H2O, respectively. The bimetallic Pt50-Ir50/Z catalyst was similarly prepared by simultaneous impregnation with an aqueous solution of both precursors. The nominal metal loadings of the monometallic Pt/Z and Ir catalysts were 3 wt. %, whereas of the bimetallic Pt50-Ir50/Z catalyst were 1.5 wt. % Ir and 1.5 wt. % Pt. Then, impregnates were dried overnight in air at 110 °C and finally calcined at 400 °C for 3 h.
oxides (CeO2, TiO2, ZrO2) and non-reducible substrates (zeolites, silica, alumina, carbon) have been tested in the HDO of phenolic model compounds [11,14–29]. Among the different noble metals, Pt and Rubased catalysts are the most studied [11,15–20,26] whereas the HDO activities of the Ir-based catalysts are scarcely reported [21–25]. For example, a homogeneous Ir catalyst was successfully employed in the HDO of 2,5-hexadione [22] confirming that Ir can be active for CeO bond hydrogenolysis [21]. However, a heterogeneous Ir/ZrO2 catalyst tested in HDO of guaiacol exhibited dominant cyclohexanol formation suggesting that only partial deoxygenation occurs [23]. Therefore, it seems that there is some confusion in literature concerning the Ir intrinsic deoxygenation activity. To clarify this inconsistency, the aim of this work was to study the effect of partial substitution of Pt by Ir (Pt50Ir50) on the catalyst selectivity in phenol HDO reaction. Generally, the HDO activity can be enhanced using bimetallic catalyst formulation, as it was demonstrated with carbon supported Pt-Cr and Pt-V catalysts when tested in the selective phenol transformation to cyclohexanone [20]. The Ir-Re/γ-Al2O3 catalyst tested in the HDO of isoeugenol was found to be more effective than the Pt-Re/γ-Al2O3 [24]. This was linked not only to the high iridium dispersion but also to formation of the IrRe active phase [24]. Similarly, the Ir–ReOx/SiO2 catalyst was active and selective toward glycerol hydrogenolysis [25] and hydrogenation of unsaturated aldehydes to unsaturated alcohols under low H2 pressure (0.8 MPa) and low temperature (30 °C) [26]. An update of the heterogeneous catalysts employed for bio-oil and model compounds upgrading has been recently reviewed by Ruddy et al. [6]. Concerning the support, recent studies have shown that zeolites are better substrates than alumina or silica in hydrodeoxygenation of phenol due to their regular pore system and moderate acidity [26,27]. Among the various zeolites tested for the conversion of biomass, the best results were obtained with medium pore zeolites, such as acidic ZSM-5 zeolite [9]. This is due to the ability of this zeolite to allow the entrance of the molecules present in the bio-liquids, and also to its medium acidity, and high hydrothermal stability [27,29,30]. In addition, ZSM-5 zeolite exhibits an appropriate 2D pore network comprising interconnecting straight (0.53 nm x 0.56 nm) and sinusoidal channels (0.55 nm x 0.51 nm), which allows the shape selectivity [9]. After mesopore incorporation, the hierarchical mesoporous structure can be obtained [30]. Pt and Pd catalysts supported on mesoporous ZSM-5 zeolite were found to be active in the gas-phase dibenzofuran hydrodeoxygenation [29] and liquid-phase HDO of m-cresol [30] respectively, whereas mesoporous ZSM-5 zeolite was active in the HDO of dibenzofuran [26]. In line with the above, this work was undertaken with the aim to enlarge our previous investigation on the ring-opening with Pt-Ir catalysts [31,32] by comparing the catalytic behaviour of ZSM-5-supported bimetallic (Pt50-Ir50/Z) with the monometallic Pt/Z and Ir/Z catalysts in the liquid-phase phenol HDO. To the best of our knowledge, the use of the bimetallic Pt-Ir/ZSM-5 catalyst for O-removal from biooils or phenol HDO reaction has not been reported yet. This is probably because the preparation of bimetallic Pt-Ir catalysts having a homogeneous dispersion of both metals is difficult due to the Ostwald ripening during the catalyst thermal activation treatment [31,33]. In order to study the migration of Pt/Ir particles, the bimetallic Pt50-Ir50/ Z catalyst was prepared with the same nominal Pt and Ir loadings (Pt/ Ir = 1) by wet co-impregnation of the zeolite. Structural and surface studies have been conducted with the aim to establish some connections between catalyst structure and HDO performance.
Characterization techniques The elemental analysis of the oxide precursors was determined by Total Reflection X-ray Fluorescence (TRXF) technique on an EXTRA-II TXRF Spectrometer (Rich & Seifert, Germany). Their textural properties of the calcined catalysts were determined from the N2 adsorption-desorption isotherms at −196 °C on a Micromeritics TriStar 3000 apparatus. The catalysts were previously degassed at 270 °C for 5 h. Temperature-programmed reduction (TPR) was carried out in a Micromeritics TPR/TPD 2900 instrument. Details of the procedure can be found elsewhere [34]. The X-ray diffraction of the fresh reduced catalysts was performed on a Seifert 3000 diffractometer, using Ni-filtered CuKα radiation (40 kV, 40 mA). The acidity of the calcined catalysts was evaluated by the temperature-programmed desorption of NH3 technique (TPD-NH3) using the same Micromeritics TPR/TPD 2900 instrument. The X-ray Photoelectron Spectroscopy (XPS) analysis of the spent catalysts was carried out in a VG Escalab 200R spectrometer equipped with a Mg Kα X-ray source (hν = 1253.6 eV) and hemispherical electron analyser. The binding energy of C1s peak at 284.8 eV was used as reference. Transmission electron microscopy (TEM) combined with energy-dispersive X-ray (EDX) microanalysis of spent catalysts was performed using a JEOL JEM 2000FX microscope working at 200 kV. Catalytic activity Phenol HDO reaction was studied in a batch reactor (300 mL, T = 200 °C, reaction time: 5 h and total hydrogen pressure: 3.0 MPa). The autoclave reactor was provided with a mechanical stirrer (900 rpm) for the maximum dispersion of gas in a liquid system. The hydrogen pressure was maintained constant through the experiment by means of a pressure regulator valve. The reagent (10.6 mmol phenol), was dissolved in a non-polar tetralin solvent (80 mL). Hexadecane was used as the internal standard. Prior to reaction, the catalyst (100 mg) was activated ex-situ by reduction in a flow of H2 in N2 (10:1 M) at 350 °C for 3 h. For the experiments, first the catalyst was added to phenol-tetralin solution and carefully mixed. After the phenol-catalyst solution was loaded in the evacuated autoclave, it was swept by N2 to remove all air in the reactor. Then, the N2 was replaced by H2 and the reactor was pressurized to 3 MPa. Then, the temperature was raised to the final reaction temperature of 200 °C and the experiment was continued for 5 h. The products of the reaction were analysed with a Hewlett-Packard 5890A chromatograph equipped with a flame ionization detector (FID) and a DB-1 capillary column (30 m x0.450 mm x2.55 μm thick) to a temperature of 250 °C. More details of the procedure can be found elsewhere [35]. Results and discussion Characterization of the calcined catalysts
Experimental The Pt and Ir loadings of the oxide precursors, determined by XRXF spectrometry, are presented in Table 1. The metal content of all the catalysts is lower than of the theoretical one (3 wt. %). Considering the total metal loading, the observed trend is: Pt/Z (2.7 wt. %) > Pt50Ir50/Z (2.0 wt.%) > Ir/Z (2.3 wt. %). The textural properties of bare ZSM-5 zeolite before and after metal
Catalyst preparation Commercial ZSM-5 zeolite (labelled hereafter as Z; Si/Al = 19; Akzo Nobel; Sweden) was used to support Ir, Pt and Pt50-Ir50 catalysts. The monometallic Pt/Z and Ir/Z catalysts were prepared by the incipient 2
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combination of a type I and type IV isotherm which are typical for hierarchical materials having both micro- and mesoporous structure. Similar type of isotherm was reported for alkaline-treated ZSM-5 zeolite by Groen et al. [39]. In contrast to the parent ZSM-5 (Fig. 1A), the N2 adsorption-desorption isotherms of the Pt50-Ir50/Z catalyst shows one hysteresis loop characteristic of a percolation effect caused by ink-bottle type pores [40,41]. The presence of mesopores is inferred from the strong increase in N2 uptake at higher pressures [40], which is supported by the t-plot data (Table 1). External surface and surface roughness might contribute to mesoporosity [41]. The formation of mesopores probably occurs during zeolite drying before impregnation that would introduce intracrystal mesoporosity. Fig. 2B shows the BJH adsorption pore-size distribution for all ZSM5 supported catalysts. A narrow peak at about 1.5 nm is associated to the tensile strength effect [36,39,42]. This is due to the fact that the BJH method is only suited for mesoporous materials having pore sizes larger than ca. 2 nm [42], consequently, this peak is not further considered. For all calcined catalysts, the broad pore size distribution in the 10–30 nm range (centered at 10.9 nm) suggests the presence of mesopores. The contribution of mesoporosity is a consequence of the external surface and roughness [41]. Noticeably, the contribution of the mesoporosity to overall catalyst porous structure is small being all catalysts mainly microporous, as deduced from the comparison of the Smicro and Vmicro values derived from t-plot (Table 1). A non-homogeneous active phase dispersion on the support surface was confirmed by HRTEM (vide infra).
Table 1 Metal loading and textural properties of the calcined catalysts. Catalyst
Pta (wt.%)
Ira (wt. %)
SBETb (m2/g)
Smicroc (m2/g)
Vtotalb (cm3/g)
Vmicroc (cm3/g)
Z Pt/Z Pt50-Ir50/Z Ir/Z
– 2.7 1.1 –
– – 0.9 2.3
358 309 319 323
74 209 213 213
0.22 0.22 0.23 0.23
0.04 0.11 0.11 0.11
a
As determined by Total Reflection X-Ray Fluorescence. Specific surface area determined by BET method and total pore volume adsorbed at P/P0 = 0.98 (from N2 physisorption at −196 °C). c As determined by N2 physisorption at −196 °C using t-plot method. b
loading were evaluated using N2 physisorption. N2 nitrogen adsorption–desorption isotherms and pore size distributions (PSDs) for parent ZSM-5 are shown in Fig. 1A. The N2 isotherms exhibits two hysteresis loops; the low pressure hysteresis in the relative pressure ranges from 0.1 to 0.3 and the high-pressure hysteresis above relative pressure of 0.5. Similar isotherm was observed for ZSM-5 with high Si/ Al ratio by Thommes et al. [36]. Taking into account that the pore filling occurs at relative pressure > 0.35, it was hypothesized that such hysteresis behavior indicates phase transition of the zeolite framework (between its monoclinic and orthorhombic) [37]. Since our ZSM-5 zeolite contains a moderate Si/Al atomic ratio of 19, this explanation is no so convincing. Instead it can be associated to the so-called “breathing effect” frequently reported for MOFs materials. At the low pressure region when adsorbate molecules are introduced, the framework of ZSM5 starts contracting while inhaling, yielding decreasing framework stress. As the pressure increases, the framework stress is increased causing the elastic expansion of the sample [38]. This stress-based model implies that two distinctive states of framework might exist in the parent ZSM-5. Pore size distributions, derived from adsorption branch of isotherm by BJH method, yielded 2 peaks at approximately 13.5 and 30.6 nm (inlet of Fig. 1A). The morphology of the bare ZSM-5 zeolite was confirmed by TEM. Fig. 1(B) displays the HRTEM image of this zeolite showing its [011] lattice planes (JCPDS 00-042-0024). The specific BET surface area (SBET) of bare ZSM-5 zeolite was high (358 m2/g). The metal loading into the ZSM-5 substrate resulted in a very small decrease in the specific surface area suggesting that the main location of the noble metals is on the external part of the zeolite crystals. This point will be discussed here after. Fig. 2A displays N2 isotherms of the representative Pt50-Ir50/Z catalyst. As can be seen, the isotherm of this bimetallic catalyst is a
Reduction behavior from TPR studies The reduction behaviour of the Pt and Ir oxide species on the surface of the ZSM-5 zeolite was studied by a temperature-programmed reduction technique. The TPR profiles of the oxide precursors are illustrated in Fig. 3. The TPR profile of Ir/Z catalyst exhibit a small peak at 148 °C which is likely originated by the reduction of small clusters of IrOx deposited on the zeolite surface, whereas the intermediate strong peak at around 216 °C could be attributed to the iridium oxide species interacting strongly with the support. Finally, a small broad peak with maxima around 350 °C comes likely from the reduction of the Ir ions located at exchanged positions within the zeolite structure [43]. With regard to the Pt/Z catalyst, one intense peak at 452 °C originates from the reduction of the Pt2+ species strongly interacting with the support, whereas a broad peak with maxima around 784 °C is usually attributed to the reduction of platinum species placed at exchange positions of ZSM-5 zeolite [44]. A shoulder observed at about 367 °C could be attributed to the platinum oxide species interacting weakly with the
Fig. 1. Nitrogen adsorption-desorption isotherm (-195 °C) (A) and HRTEM image obtained on pure ZSM-5 zeolite (B). The BJH adsorption pore-size distribution is plotted as an inlet of Fig. 1(A). 3
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Fig. 2. N2 adsorption-desorption isotherms of the calcined Pt50-Ir50/Z (A) and BJH adsorption pore-size distributions of all ZSM-5 supported catalysts (B).
trichloride (IrCl3·3H2O) needs to be heated and vigorously stirred to solubilize it. Under these conditions the anion PtCl62− and polar IrCl3 precursor species are hydrated and thus surrounded by water ligands held in place by ion–dipole and dipole–dipole interactions, respectively [47]. When zeolite is contacted with the impregnant aqueous metal solutions, a lower interaction between zeolite and PtCl62− anion is weaken that in the case of Clδ− when using the IrCl3 precursor. As a consequence, the TPR peak corresponding to the reduction of the Pt species is shifted towards lower temperature, which is generally considered as proof of a lower metal-support interaction.
Investigation of catalyst acidity The acidity of calcined catalysts was evaluated by NH3-TPD experiments. The ammonia desorption profiles, displayed in Fig. 4, shown main peaks at about 181 °C and 391 °C which can be ascribed to weak and strong Brønsted acid sites, respectively. The latter acid sites are associated with framework aluminium atoms whose distribution throughout the zeolite lattice is often no uniform [48]. It is likely that the weak acid sites are located on the surface, whereas the strong acid sites are placed in the deepest part of the zeolite crystals [49]. As seen
Fig. 3. TPR profiles of oxide precursors: (a), Pt/Z: (b), Pt/50-Ir50/Z) and (c), Ir/ Z.
support. As expected, the binary Pt50-Ir50/Z catalyst exhibits many more peaks than the monometallic ones, which are observed at temperature maxima of 99, 230, 367 and 443 °C (shoulder). The two former peaks are due to the reduction of IrOx species whereas the two latter peaks can be associated with the reduction of Pt(II) species. It can be seen that the peak associated with the reduction of Pt2+ species strongly interacting with support (443 °C) is downward shifted with respect to that of the monometallic Pt/Z catalyst (452 °C). This shift suggests that the addition of iridium promotes the reduction of the platinum species in the binary Pt50-Ir50/Z catalyst, as it was observed for the Pt50-Ir50/γ-Al2O3 catalysts prepared by wet impregnation [45]. It is likely that the interaction between Pt and Ir metals originated just during the incipient wetness impregnation when the metal precursors are spatially well-distributed on the substrate [46]. In addition, this could be explained by taking into consideration that hexachloroplatinic acid (H2PtCl6·6H2O) is easily dissolved in water, whereas iridium
Fig. 4. NH3-TPD profiles of the calcined catalysts. 4
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Fig. 5. Wide-angle X-ray diffraction patterns of the fresh reduced catalysts: (a), pristine zeolite; (b), Pt50-Ir50/Z; (c), Pt/Z and (d), Ir/Z. Fig. 6. Ir 4f-Al 2p core level spectra of the spent Pt50-Ir50/Z (a) and Ir/Z (b) catalysts.
in Fig. 4, all ZSM-5 supported catalysts exhibited very similar acidity and acid sites distribution. Thus, considering the experimental error of the TPD-NH3 measurements (5%), the total acidity value remains almost unchanged upon metal loading. This is expected because all catalysts are supported on the same support material and their total metal loading is very similar (ca. 3 wt.%).
composition and oxidation state of the elements of the spent catalysts. Fig. 6 displays the XPS spectra of Ir(Pt) 4f–Al 2p. As expected, all the catalysts showed Si 2p and Al 2p peaks with a BE at 103 ± 0.1 eV and 75.0 eV respectively, corresponding to the framework Si and Al species of the ZSM-5 zeolite [53]. The binding energies (BE) of core electrons and surface atomic ratios are summarized in Tables 2 and 3, respectively. For the Pt-containing catalysts, the Pt 4d5/2 core level was also recorded. The BE of Pt 4d5/2 level at 314.7 eV is indicative of the presence of metallic Pt species only [54]. For the Pt50-Ir50/Z catalyst, the Pt 4f7/2 level at ca. 71.1 eV is associated with Pt0 [55] while for the Ir/Z and Pt50-Ir50/Z catalysts the Ir 4f7/2 level at a BE of 61.0 eV and 60.6 eV, respectively, are close to that reported for Ir0 species (60.9 eV) [56]. A similar BE value was reported previously for a freshly reduced Pt-Ir/Al2O3 catalyst (61.2 eV) [57]. The downward shift in the BE of Ir 4f7/2 peak in Pt50Ir50/Z catalyst with respect to that of Ir/Z one would suggests the formation of a Pt-Ir alloy in the bimetallic system. However, taking into account the quite similar electronegativity of both elements, Ir (2.20) and Pt (2.28), the charge transfer from iridium to platinum is hardly possible. Considering the quantitative Pt/Si and Ir/Si atomic ratios (Table 3), total metal exposure follows the trend: Ir/Z > Pt50-Ir50/Z = Pt/Z. Noticeably, Ir/Z catalyst exhibits higher Ir surface exposure than both the Pt-containing catalysts. By comparing surface Pt/Si and Ir/Si ratios with the bulk ratios in Fig. 7, it appears that Ir is location on the outer surface of both the Ir/Z and Pt50-Ir50/Z catalysts. In contrast, the larger bulk Pt/Si atomic ratios with respect to surface Pt/Si ratios for both Pt and Pt50-Ir50/Z catalysts strongly suggest the preferential Pt location within the inner crystalline structure of ZSM-5. For the binary catalyst, the XPS Ir/Pt atomic ratio of 6 clearly suggests a six-fold larger Ir than Pt surface exposure. The location of metal particles in the spent catalysts was examined also by HRTEM. Fig. 8 displays HRTEM images of the ZSM-5 supported catalysts. The binary Pt50-Ir50/Z catalyst exhibits globular shape particles of a size much larger than the pore diameter together with very
Crystallite structure Fig. 5 shows the wide-angle XRD patterns of the fresh reduced catalysts and pristine ZSM-5 zeolite. The presence of the low-angle diffraction peaks (2θ < 10°) suggest that metal incorporation does not affect crystallinity of the base zeolite to a great extent. The pristine zeolite and all the ZSM-5-supported catalysts exhibited typical diffraction lines at 2θ angles of 7.9°, 8.8°, 14.8°, 15.6°, 23.3° and 23.7°, which are characteristic of the zeolite substrate (JCPDS 00-042-0024). For the Ir/Z catalyst, only the diffraction lines of the zeolite substrates were observed indicating that the size of the Ir crystallites were below the XRD detection limit (< 4 nm) or its dispersion is very high. Conversely, both Pt/Z and Pt50-Ir50/Z catalysts show additional diffraction lines at ca. 39.8° and 81.3° originated by metallic Pt with a cubic crystallite structure (JCPDS 00-004-0802). For the binary Pt50-Ir50/Z catalyst, the planes (111), (200) and (220) suggest the presence of a single-phase disordered structure. However, the formation of binary Pt-Ir alloy is hardly possible because the temperature miscibility limit for the Pt50Ir50 composition is very high (close to 960 °C) [50]. The average Pt0 crystallite sizes of Pt/Z catalyst, calculated by applying the Debye-Scherrer's equation to the most intense diffraction line at 2θ angle of 39.8°, was much larger than that of the Pt50-Ir50/Z catalyst (44.5 nm vs. 35.1 nm). This calculation allows concluding that Pt dispersion on the binary catalyst is enhanced with respect to its monometallic Pt/Z counterpart. As the fresh reduced catalysts were characterized by XRD, it is more likely that the growing of Pt metal particles could occur during catalyst calcination and/or reduction. In line with this, there are literature reports showing that agglomeration and size change of Pt nanoparticles on these supports occur during the reaction conditions [51]. To obtain Pt particles without changing the nanoparticle’s size and morphology during the catalyst pretreatment and/or reaction conditions, the colloid deposition method could be more effective method for catalyst preparation, as it was argued by Kaidanovych et al. [52].
Table 2 Binding energy (eV) of core electrons of the spent catalysts.
Surface analysis of spent catalysts Photoelectron spectroscopy was used to evaluate both surface 5
Catalyst
Si 2p
Al 2p
Pt 4d5/2
Pt 4d7/2
Ir 4f7/2
Ir/Z Pt50-Ir50/Z Pt/Z
103.0 102.9 103.1
75.0 74.9 75.0
– 314.7 314.7
– 71.1 –
61.0 60.6 –
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having active phase well dispersed on the support surface. In contrast, the other monometallic Pt contains large Pt0 crystallites located on the support surface together with the small Pt0 particles located within the zeolite pores. Thus, it can be pointed out that Pt particles show higher mobility than Ir ones on the zeolite carrier, which can be linked to their different metal-support interaction (from TPR). These results agree with those obtained previously from Ir-Pd/SiO2-Al2O3 [31] and Ir-Pd/γAl2O3 [33] systems showing that Pd species exhibit a greater mobility than Ir over SiO2-Al2O3 and γ-Al2O3 substrates.
Table 3 Surface atomic ratios of the spent catalysts. Catalyst
Si/Al at XPS
Ir/Si XPS (Bulk)
Pt/Si XPS(Bulk)
(Pt + Ir)/Si XPS
Ir/Z Pt/Z Pt50-Ir50/Z
17.0 16.5 16.9
0.016 (0.008) – 0.006(0.003)
– 0.007(0.009) 0.001(0.004)
0.016 0.007 0.007
Catalyst activity The performance of the catalysts was evaluated in the liquid-phase phenol HDO reaction in the presence of a non-polar solvent (tetralin). Activity data, in terms of HDO conversion, are presented in Table 4. The HDO conversion follows the trend: Pt50-Ir50/Z (36%) > Ir/Z (32%) > Pt/Z (23%). However, taking into consideration the catalyst intrinsic activity expressed as the moles of phenol transformed per moles of metal and per second, this trend is different: Pt50-Ir50/ Z > Pt/Z > Ir/Z. The reaction products detected by GC analysis were: cyclohexanol, cyclohexane, benzene and methylcyclohexane. The main product with all the ZSM-5-supported catalysts was cyclohexane (selectivity of 60–96 %), which is the most desired product. Considering the products formed, the possible reaction scheme is shown in Fig. 9. As seen in this figure, the reaction scheme for phenol hydrogenolysis includes two reaction routes: (1) hydrogenation of aromatic ring of phenol (HYD route) to cyclohexanol + dehydration of cyclohexanol to cyclohexane; and (2) direct hydrogenolysis (DDO route) of C–O bond of phenol to benzene + hydrogenation of benzene to cyclohexane. Phenol can be converted to benzene by the direct deoxygenation (DDO) reaction pathway, and/or can be converted to cyclohexanol via the aromatic ring hydrogenation (HYD route). For both Pt/Z and Pt50-Ir50/Z catalysts, the formation of cyclohexanol suggests that the phenol HDO reaction proceeds mainly via a hydrogenation pathway. As compared to both Ir/Z and Pt50-Ir50/Z, Pt/Z catalyst shows partially inhibited dehydration reaction, as deduced from its highest selectivity toward cyclohexanol (Table 4). In contrast to the Ir/Z, the Ir/ZrO2 catalyst tested in HDO of guaiacol exhibited dominant cyclohexanol formation suggesting that only partial deoxygenation occurs [23]. Noticeably, the partial substitution of Pt by Ir in the catalyst formulation (Pt50-Ir50/Z) enhanced cyclohexanol dehydration to cyclohexane. Both Ir/Z and Pt/Z catalysts were the only ones able to generate a small amount of methylcyclopentane (4% and 2%, respectively) suggesting the involvement of the acid function in the reaction mechanism. Taking into account the similar acidity and acid sites distribution of all catalysts, the highest formation of methylcyclopentane on the Ir/Z catalyst could be explained considering the high hydrogenation/dehydrogenation activity of Ir [56] and the large metal surface exposure
Fig. 7. Bulk Pt(Ir)/Si atomic ratios (from TXRF) against XPS surface Pt(Ir)/Si atomic ratios of spent catalysts.
small Ir0/Pt0 particles. The local EDS analysis (not shown here) with electronic spot of 10 and 20 nm pointed out to a non-homogeneous Pt and Ir dispersion, in agreement with XPS (vide supra). The mean particle size calculated from various HRTEM images of Pt50-Ir50/Z, Ir/Z and Pt/Z catalysts are 9.1, 3.1 and 2.7 nm, respectively. As expected, much smaller crystalline particles was obtained from TEM images than from XRD data. This is because the smallest crystalline particles are not detected by XRD (detection limit below 4 nm) but still visible in TEM images. However, both XRD and HRTEM techniques strongly suggests the formation of large Pt0 crystallites on the surface of the bimetallic catalyst. In contrast with this catalyst, the monometallic Ir/Z one exhibits small Ir crystallites homogeneously dispersed on the support surface. Finally, the TEM image of the Pt/Z catalyst indicates the nonhomogeneous Pt0 species dispersion and a much lower density of Pt nanoparticles on the support surface, as also revealed by XPS results. The schematic representation of the Pt and Ir species location within the zeolite body and on its surface deduced from the overall catalyst characterization is visualized in Fig. 9. As seen, the bimetallic Pt50Ir50/Z exhibits smallest Ir0 particles and largest Pt0 crystallites located on the outer zeolite surface together with small Pt0 particles located within the zeolite pores. On the other hand, the Ir/Z is unique catalyst
Fig. 8. TEM micrographs of the ZSM-supported spent catalysts. 6
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Fig. 9. Schematic representation of the Pt and Ir species location within the ZSM-5 zeolite body and on its surface.
catalyst with respect to Pt/Z counterpart is the interesting feature of this work because this reaction is generally considered as an acid-catalyzed reaction [11]. The selectivity results strongly suggest that the well dispersed Ir nanoparticles can be more selective toward dehydration reaction than Pt species with a broad size distribution. This is because hydrogen can be easily activated and cleaved at the metalsupport interface region, as it was demonstrated in the field of SRO for the binary Pt-Ir/HY system [60]. Thus, in contrast to the work by Postman et al. [61], Ir/Z catalyst having well dispersed Ir nanoparticles was found to be more effective toward dehydration reaction than Pt/Z counterpart. Indeed, one might expect that the well-dispersed Ir atoms possessing electron donor/acceptor properties could facilitate the hydrogen spillover mechanism [62]. Thus, both hydrogen spill over mechanism and a larger intrinsic activity of Ir than Pt for cleavage of unsubstituted CeC bonds of the naphthenic compounds would explain the largest selectivity of Ir/Z catalyst toward metylcyclopentane in agreement with literature reports [32. 62, 63]. The results presented here agree with those reported previously for ring-opening of Pt5-Ir95/ CeO2-Al2O3 catalyst indicating that activity of this catalyst is governed by the accessibility of metallic Ir sites on the surface [32]. Taking into consideration the literature reports [64,65] and the enhanced hydrogen activation originated by a largest Ir surface exposure (from XPS), a low coke formation on the Ir/Z catalyst can be expected. Finally, the lowest selectivity of Pt/Z catalyst toward CO-bond scission can be explained considering the absence of the formation of Pt (211) phase, as confirmed by XRD, and the steric inhibition of hydrogen adsorption on Pt0 sites located deeply in the channels of the Pt/Z catalyst, as deduced from Fig. 7. Pt(211) plane having step sites is known to play a crucial role in C–O bond cleavage, as it was recently confirmed by a combined density functional theory (DFT) calculation and microkinetic model for Pt (211) and Ir (211) metals tested in ethanol HDO reaction [66]. Similarly, the DFT calculation indicated that Rh (211) crystal plane was more active than Rh (111) for CO-bond cleavage in the HDO of phenol [67].
Table 4 HDO conversion and selectivity (at 60% phenol conversion) for HDO of phenol over ZSM-5-supported catalysts (Reaction conditions were: batch reactor, T = 200 °C, PH2 = 3.0 MPa).
a
HDO conv. (%) rate (molPhenol molMetal−1 s−1)c Methylcyclopentaneb (%) Benzeneb (%) Cyclohexaneb (%) Cyclohexanolb (%) a b c
Ir/Z
Pt/Z
Pt50-Ir50/Z
32.0 0.028 4 0 96 0
23.0 0.054 2 6 60 32
36.0 0.066 0 0 95 5
At reaction time of 5 h. Reaction conditions were: a batch reactor, T = 200 °C, PH2 = 3.0 MPa. At 5 h per metal surface exposure (from XPS).
(Table 3). For all catalysts studied, the preferential selectivity towards HYD route products (94–100% selectivity) strongly suggests participation of the metallic function rather than the acidic function. Relationship between activity and structure It is well established that bifunctional catalysts having both acid and metal sites are suited for HDO reactions: the metal sites are needed for hydrogenation/dehydrogenation, whereas the acid sites of the support are required for dehydration, bond breaking, and isomerization [57]. There is synergy between the support acid function and the metal function, as it was confirmed recently for the HDO of m-cresol over Pt catalyst supported on γ-Al2O3 modified by acid treatment [11]. In this work, all catalysts exhibit similar acidity, as confirmed by TPD-NH3 (Fig. 4). Thus, the catalyst acidity did not mask the effect of its metal function. However, the similar acidity of the catalysts did not preclude the existence of synergy between their acid and metal functions. The best catalytic response of the bimetallic Pt50-Ir50/Z catalyst in the liquid-phase hydrodoxygenation with respect its monometallic Pt/Z and Ir/Z counterparts having the same total metal loading and the same acidity evidenced the positive ‘geometric’ effect of the simultaneous presence of both metals on the surface. Catalysts characterization by combined XRD, XPS and HRTEM strongly suggest that the main factors influencing on catalytic behavior are the intrinsic activities of Ir and Pt phases and their dispersion. Taking into account the ability of acidic supports to contribute to the activity of a catalyst for both aromatic hydrogenation [58] and dehydration reactions [59], it is hypothesized that reaction might occur at the interface of metal and acidic support. In such case, the HDO reaction mechanism might involve phenol adsorption on the interface between Ir(Pt) metal surface and support whereas the heterolytic dissociation of H2 into H+ and H– take place on the metal nanoparticles. However, further kinetic studies are needed for clarify the reaction mechanism. The larger extent of dehydration of cyclohexanol observed with Ir/Z
Conclusions The results presented in this work underline the factors influencing the performance of noble metals (Pt, Ir) supported on ZSM-5 zeolite in the liquid-phase hydrodeoxygenation of phenol. It was found that the catalyst activity was strongly influenced by the metal species exposure being acidic catalyst function similar for all catalysts studied. The use of a binary Pt50-Ir50/Z catalyst formulation was shown to have some advantage with respect to the monometallic Pt/Z and Ir/Z catalysts. Partial substitution of Pt by Ir in the catalyst formulation (Pt50-Ir50/ ZSM-5) enhances activity and selectivity toward O-free products which was associated with the surface enrichment of Ir species (from XPS and HRTEM). Pt species exhibited higher mobility than the Ir ones over the zeolite carrier. As a consequence, the Ir/Z catalyst having a larger metal 7
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surface exposure exhibited a greater activity and selectivity toward HDO products than its Pt/Z homolog. Ir was more effective for dehydration reaction than Pt indicating that this reaction might occur in the interface between Ir nanoparticles and acidic support.
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Factors influencing selectivity in the liquid-phase phenol hydrodeoxygenation over ZSM-5 supported Pt/Ir and Pt+Ir catalysts B. Paweleca,*, C.V. Loriceraa, C. Geantetb, N. Motaa, J.L.G. Fierroa, R.M. Navarroa,* a b
Institute of Catalysis and Petroleum Chemistry, CSIC, Cantoblanco, 28049 Madrid, Spain Research Institute on Catalysis and Environment Lyon (IRCELYON), CNRS: UMR5256-University Claude Bernard-Lyon (UCBL), France
A R T I C LE I N FO
A B S T R A C T
Keywords: Bio-oils Phenol Hydrodeoxygenation Pt/Ir ZSM-5
The effect of partial Pt substitution by Ir on the catalytic response of bimetallic Pt-Ir/ZSM-5 (Pt/Ir = 1) catalyst in the liquid-phase hydrodeoxygenation (HDO) of phenol was investigated. To evaluate the catalyst intrinsic activity, the ZSM-5 supported Pt, Ir and Pt + Ir catalysts were prepared with similar total metal loading (ca. 3 wt. %) and acidity. The catalysts were characterized by several physical and chemical techniques and tested in the HDO of phenol reaction carried out in a batch reactor at T = 200 °C under H2 pressure of 3.0 MPa. For all the catalysts, the phenol transformation mainly proceeds via hydrogenation + dehydration (HYD reaction route) leading to cyclohexane as the main product (selectivity 60–96%). Partial substitution of Pt by Ir in catalyst formulation (Pt50-Ir50/ZSM-5) enhances both activity and selectivity toward O-free products. This was associated to Ir surface enrichment (from HRTEM). The Ir/ZMS-5 and Pt50-It50/ZMS-5 catalysts having well dispersed Ir nanoparticles were more selective toward dehydration than Pt/ZSM-5 having Pt species with a broad size distribution range. The catalyst activity-structure correlation suggests that the dehydration of cyclohexanol (intermediate product) can occur at the interface between the metal and acid sites of the support.
Introduction Bio-oils are alternative feed stocks for fossil fuels. These bio-oils are chemically and thermally unstable due to the high content of oxygenated compounds. In addition, they exhibit an acidic nature and a tendency to polymerization. To solve these problems, bio-oils have to be upgraded and traditional hydrodeoxygenation (HDO) process already existing within the petroleum refining infrastructure can be employed [1–5]. In this process the oxygenated compounds can be removed via hydrogenation in the presence of a catalyst. As the content of phenolic compounds in bio-oils represent nearly one quarter of the total O-containing compounds, phenolic compounds are often used as a model compounds of bio-oils [4]. A general overview of the catalysts [1–6], reaction conditions [7] and mechanisms of HDO reactions of biooils are summarized in a few excellent reviews [1–8]. The catalysts tailored for the HDO reaction are bifunctional; this means that they exhibit metal and acid sites both needed for hydrogenation of aromatic ring and for O-removal via dehydration, respectively [2]. Thus, there is significant experimental evidence that the acidic supports promote HDO reaction [8–11]. For example, it was found that the support acidic function of the HBeta zeolite catalyzed the reaction of trans alkylation “during the anisole conversion to gasoline⁎
range products over bifunctional Pt/HBeta [9]. Similarly, in the case of Pd catalysts, the acidity of parent HBeta and ZSM-5 zeolites affected the initial rates of HDO of benzophenone to the desired diphenylmethane [10]. However, the acid function should be moderate because strong acidity favors catalyst deactivation via coking [11]. The synergy of the support acid function and the metal function in the catalytic HDO of mcresol was observed [11]. Considering the metal function, the first catalysts studied for the HDO reaction were based on the metal sulphides, which were previously used by the petroleum refining industry for the removal of sulphur compounds from crude oil [7]. However, the reactivity of metal sulphides is lowered in the presence of water due to formation of surface oxides [12]. To maintain the catalyst stability, sulphur species should be introduced into reactor [12]. By contrast, noble metals are more resistant toward deactivation by H2O and they exhibit better hydrogenation activity at a lower hydrogen pressure and temperatures [8]. Thus, the catalysts based on noble metals seem to be a good option for the substitution of metal sulphides in the catalyst formulation. In this sense, our previous study demonstrated that the incorporation of a low amount of Pt or Ir to RuS2/SBA-15 catalyst improved the activity for phenol HDO [13]. Many noble metals (Ru, Pt, Ir, Rh, Pd) supported on reducible
Corresponding authors. E-mail addresses: [email protected] (B. Pawelec), [email protected] (R.M. Navarro).
https://doi.org/10.1016/j.mcat.2019.110669 Received 8 July 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: B. Pawelec, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110669
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wetness impregnation method using an aqueous solution of IrCl3 ·H2O and H2PtCl6 ·6H2O, respectively. The bimetallic Pt50-Ir50/Z catalyst was similarly prepared by simultaneous impregnation with an aqueous solution of both precursors. The nominal metal loadings of the monometallic Pt/Z and Ir catalysts were 3 wt. %, whereas of the bimetallic Pt50-Ir50/Z catalyst were 1.5 wt. % Ir and 1.5 wt. % Pt. Then, impregnates were dried overnight in air at 110 °C and finally calcined at 400 °C for 3 h.
oxides (CeO2, TiO2, ZrO2) and non-reducible substrates (zeolites, silica, alumina, carbon) have been tested in the HDO of phenolic model compounds [11,14–29]. Among the different noble metals, Pt and Rubased catalysts are the most studied [11,15–20,26] whereas the HDO activities of the Ir-based catalysts are scarcely reported [21–25]. For example, a homogeneous Ir catalyst was successfully employed in the HDO of 2,5-hexadione [22] confirming that Ir can be active for CeO bond hydrogenolysis [21]. However, a heterogeneous Ir/ZrO2 catalyst tested in HDO of guaiacol exhibited dominant cyclohexanol formation suggesting that only partial deoxygenation occurs [23]. Therefore, it seems that there is some confusion in literature concerning the Ir intrinsic deoxygenation activity. To clarify this inconsistency, the aim of this work was to study the effect of partial substitution of Pt by Ir (Pt50Ir50) on the catalyst selectivity in phenol HDO reaction. Generally, the HDO activity can be enhanced using bimetallic catalyst formulation, as it was demonstrated with carbon supported Pt-Cr and Pt-V catalysts when tested in the selective phenol transformation to cyclohexanone [20]. The Ir-Re/γ-Al2O3 catalyst tested in the HDO of isoeugenol was found to be more effective than the Pt-Re/γ-Al2O3 [24]. This was linked not only to the high iridium dispersion but also to formation of the IrRe active phase [24]. Similarly, the Ir–ReOx/SiO2 catalyst was active and selective toward glycerol hydrogenolysis [25] and hydrogenation of unsaturated aldehydes to unsaturated alcohols under low H2 pressure (0.8 MPa) and low temperature (30 °C) [26]. An update of the heterogeneous catalysts employed for bio-oil and model compounds upgrading has been recently reviewed by Ruddy et al. [6]. Concerning the support, recent studies have shown that zeolites are better substrates than alumina or silica in hydrodeoxygenation of phenol due to their regular pore system and moderate acidity [26,27]. Among the various zeolites tested for the conversion of biomass, the best results were obtained with medium pore zeolites, such as acidic ZSM-5 zeolite [9]. This is due to the ability of this zeolite to allow the entrance of the molecules present in the bio-liquids, and also to its medium acidity, and high hydrothermal stability [27,29,30]. In addition, ZSM-5 zeolite exhibits an appropriate 2D pore network comprising interconnecting straight (0.53 nm x 0.56 nm) and sinusoidal channels (0.55 nm x 0.51 nm), which allows the shape selectivity [9]. After mesopore incorporation, the hierarchical mesoporous structure can be obtained [30]. Pt and Pd catalysts supported on mesoporous ZSM-5 zeolite were found to be active in the gas-phase dibenzofuran hydrodeoxygenation [29] and liquid-phase HDO of m-cresol [30] respectively, whereas mesoporous ZSM-5 zeolite was active in the HDO of dibenzofuran [26]. In line with the above, this work was undertaken with the aim to enlarge our previous investigation on the ring-opening with Pt-Ir catalysts [31,32] by comparing the catalytic behaviour of ZSM-5-supported bimetallic (Pt50-Ir50/Z) with the monometallic Pt/Z and Ir/Z catalysts in the liquid-phase phenol HDO. To the best of our knowledge, the use of the bimetallic Pt-Ir/ZSM-5 catalyst for O-removal from biooils or phenol HDO reaction has not been reported yet. This is probably because the preparation of bimetallic Pt-Ir catalysts having a homogeneous dispersion of both metals is difficult due to the Ostwald ripening during the catalyst thermal activation treatment [31,33]. In order to study the migration of Pt/Ir particles, the bimetallic Pt50-Ir50/ Z catalyst was prepared with the same nominal Pt and Ir loadings (Pt/ Ir = 1) by wet co-impregnation of the zeolite. Structural and surface studies have been conducted with the aim to establish some connections between catalyst structure and HDO performance.
Characterization techniques The elemental analysis of the oxide precursors was determined by Total Reflection X-ray Fluorescence (TRXF) technique on an EXTRA-II TXRF Spectrometer (Rich & Seifert, Germany). Their textural properties of the calcined catalysts were determined from the N2 adsorption-desorption isotherms at −196 °C on a Micromeritics TriStar 3000 apparatus. The catalysts were previously degassed at 270 °C for 5 h. Temperature-programmed reduction (TPR) was carried out in a Micromeritics TPR/TPD 2900 instrument. Details of the procedure can be found elsewhere [34]. The X-ray diffraction of the fresh reduced catalysts was performed on a Seifert 3000 diffractometer, using Ni-filtered CuKα radiation (40 kV, 40 mA). The acidity of the calcined catalysts was evaluated by the temperature-programmed desorption of NH3 technique (TPD-NH3) using the same Micromeritics TPR/TPD 2900 instrument. The X-ray Photoelectron Spectroscopy (XPS) analysis of the spent catalysts was carried out in a VG Escalab 200R spectrometer equipped with a Mg Kα X-ray source (hν = 1253.6 eV) and hemispherical electron analyser. The binding energy of C1s peak at 284.8 eV was used as reference. Transmission electron microscopy (TEM) combined with energy-dispersive X-ray (EDX) microanalysis of spent catalysts was performed using a JEOL JEM 2000FX microscope working at 200 kV. Catalytic activity Phenol HDO reaction was studied in a batch reactor (300 mL, T = 200 °C, reaction time: 5 h and total hydrogen pressure: 3.0 MPa). The autoclave reactor was provided with a mechanical stirrer (900 rpm) for the maximum dispersion of gas in a liquid system. The hydrogen pressure was maintained constant through the experiment by means of a pressure regulator valve. The reagent (10.6 mmol phenol), was dissolved in a non-polar tetralin solvent (80 mL). Hexadecane was used as the internal standard. Prior to reaction, the catalyst (100 mg) was activated ex-situ by reduction in a flow of H2 in N2 (10:1 M) at 350 °C for 3 h. For the experiments, first the catalyst was added to phenol-tetralin solution and carefully mixed. After the phenol-catalyst solution was loaded in the evacuated autoclave, it was swept by N2 to remove all air in the reactor. Then, the N2 was replaced by H2 and the reactor was pressurized to 3 MPa. Then, the temperature was raised to the final reaction temperature of 200 °C and the experiment was continued for 5 h. The products of the reaction were analysed with a Hewlett-Packard 5890A chromatograph equipped with a flame ionization detector (FID) and a DB-1 capillary column (30 m x0.450 mm x2.55 μm thick) to a temperature of 250 °C. More details of the procedure can be found elsewhere [35]. Results and discussion Characterization of the calcined catalysts
Experimental The Pt and Ir loadings of the oxide precursors, determined by XRXF spectrometry, are presented in Table 1. The metal content of all the catalysts is lower than of the theoretical one (3 wt. %). Considering the total metal loading, the observed trend is: Pt/Z (2.7 wt. %) > Pt50Ir50/Z (2.0 wt.%) > Ir/Z (2.3 wt. %). The textural properties of bare ZSM-5 zeolite before and after metal
Catalyst preparation Commercial ZSM-5 zeolite (labelled hereafter as Z; Si/Al = 19; Akzo Nobel; Sweden) was used to support Ir, Pt and Pt50-Ir50 catalysts. The monometallic Pt/Z and Ir/Z catalysts were prepared by the incipient 2
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combination of a type I and type IV isotherm which are typical for hierarchical materials having both micro- and mesoporous structure. Similar type of isotherm was reported for alkaline-treated ZSM-5 zeolite by Groen et al. [39]. In contrast to the parent ZSM-5 (Fig. 1A), the N2 adsorption-desorption isotherms of the Pt50-Ir50/Z catalyst shows one hysteresis loop characteristic of a percolation effect caused by ink-bottle type pores [40,41]. The presence of mesopores is inferred from the strong increase in N2 uptake at higher pressures [40], which is supported by the t-plot data (Table 1). External surface and surface roughness might contribute to mesoporosity [41]. The formation of mesopores probably occurs during zeolite drying before impregnation that would introduce intracrystal mesoporosity. Fig. 2B shows the BJH adsorption pore-size distribution for all ZSM5 supported catalysts. A narrow peak at about 1.5 nm is associated to the tensile strength effect [36,39,42]. This is due to the fact that the BJH method is only suited for mesoporous materials having pore sizes larger than ca. 2 nm [42], consequently, this peak is not further considered. For all calcined catalysts, the broad pore size distribution in the 10–30 nm range (centered at 10.9 nm) suggests the presence of mesopores. The contribution of mesoporosity is a consequence of the external surface and roughness [41]. Noticeably, the contribution of the mesoporosity to overall catalyst porous structure is small being all catalysts mainly microporous, as deduced from the comparison of the Smicro and Vmicro values derived from t-plot (Table 1). A non-homogeneous active phase dispersion on the support surface was confirmed by HRTEM (vide infra).
Table 1 Metal loading and textural properties of the calcined catalysts. Catalyst
Pta (wt.%)
Ira (wt. %)
SBETb (m2/g)
Smicroc (m2/g)
Vtotalb (cm3/g)
Vmicroc (cm3/g)
Z Pt/Z Pt50-Ir50/Z Ir/Z
– 2.7 1.1 –
– – 0.9 2.3
358 309 319 323
74 209 213 213
0.22 0.22 0.23 0.23
0.04 0.11 0.11 0.11
a
As determined by Total Reflection X-Ray Fluorescence. Specific surface area determined by BET method and total pore volume adsorbed at P/P0 = 0.98 (from N2 physisorption at −196 °C). c As determined by N2 physisorption at −196 °C using t-plot method. b
loading were evaluated using N2 physisorption. N2 nitrogen adsorption–desorption isotherms and pore size distributions (PSDs) for parent ZSM-5 are shown in Fig. 1A. The N2 isotherms exhibits two hysteresis loops; the low pressure hysteresis in the relative pressure ranges from 0.1 to 0.3 and the high-pressure hysteresis above relative pressure of 0.5. Similar isotherm was observed for ZSM-5 with high Si/ Al ratio by Thommes et al. [36]. Taking into account that the pore filling occurs at relative pressure > 0.35, it was hypothesized that such hysteresis behavior indicates phase transition of the zeolite framework (between its monoclinic and orthorhombic) [37]. Since our ZSM-5 zeolite contains a moderate Si/Al atomic ratio of 19, this explanation is no so convincing. Instead it can be associated to the so-called “breathing effect” frequently reported for MOFs materials. At the low pressure region when adsorbate molecules are introduced, the framework of ZSM5 starts contracting while inhaling, yielding decreasing framework stress. As the pressure increases, the framework stress is increased causing the elastic expansion of the sample [38]. This stress-based model implies that two distinctive states of framework might exist in the parent ZSM-5. Pore size distributions, derived from adsorption branch of isotherm by BJH method, yielded 2 peaks at approximately 13.5 and 30.6 nm (inlet of Fig. 1A). The morphology of the bare ZSM-5 zeolite was confirmed by TEM. Fig. 1(B) displays the HRTEM image of this zeolite showing its [011] lattice planes (JCPDS 00-042-0024). The specific BET surface area (SBET) of bare ZSM-5 zeolite was high (358 m2/g). The metal loading into the ZSM-5 substrate resulted in a very small decrease in the specific surface area suggesting that the main location of the noble metals is on the external part of the zeolite crystals. This point will be discussed here after. Fig. 2A displays N2 isotherms of the representative Pt50-Ir50/Z catalyst. As can be seen, the isotherm of this bimetallic catalyst is a
Reduction behavior from TPR studies The reduction behaviour of the Pt and Ir oxide species on the surface of the ZSM-5 zeolite was studied by a temperature-programmed reduction technique. The TPR profiles of the oxide precursors are illustrated in Fig. 3. The TPR profile of Ir/Z catalyst exhibit a small peak at 148 °C which is likely originated by the reduction of small clusters of IrOx deposited on the zeolite surface, whereas the intermediate strong peak at around 216 °C could be attributed to the iridium oxide species interacting strongly with the support. Finally, a small broad peak with maxima around 350 °C comes likely from the reduction of the Ir ions located at exchanged positions within the zeolite structure [43]. With regard to the Pt/Z catalyst, one intense peak at 452 °C originates from the reduction of the Pt2+ species strongly interacting with the support, whereas a broad peak with maxima around 784 °C is usually attributed to the reduction of platinum species placed at exchange positions of ZSM-5 zeolite [44]. A shoulder observed at about 367 °C could be attributed to the platinum oxide species interacting weakly with the
Fig. 1. Nitrogen adsorption-desorption isotherm (-195 °C) (A) and HRTEM image obtained on pure ZSM-5 zeolite (B). The BJH adsorption pore-size distribution is plotted as an inlet of Fig. 1(A). 3
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Fig. 2. N2 adsorption-desorption isotherms of the calcined Pt50-Ir50/Z (A) and BJH adsorption pore-size distributions of all ZSM-5 supported catalysts (B).
trichloride (IrCl3·3H2O) needs to be heated and vigorously stirred to solubilize it. Under these conditions the anion PtCl62− and polar IrCl3 precursor species are hydrated and thus surrounded by water ligands held in place by ion–dipole and dipole–dipole interactions, respectively [47]. When zeolite is contacted with the impregnant aqueous metal solutions, a lower interaction between zeolite and PtCl62− anion is weaken that in the case of Clδ− when using the IrCl3 precursor. As a consequence, the TPR peak corresponding to the reduction of the Pt species is shifted towards lower temperature, which is generally considered as proof of a lower metal-support interaction.
Investigation of catalyst acidity The acidity of calcined catalysts was evaluated by NH3-TPD experiments. The ammonia desorption profiles, displayed in Fig. 4, shown main peaks at about 181 °C and 391 °C which can be ascribed to weak and strong Brønsted acid sites, respectively. The latter acid sites are associated with framework aluminium atoms whose distribution throughout the zeolite lattice is often no uniform [48]. It is likely that the weak acid sites are located on the surface, whereas the strong acid sites are placed in the deepest part of the zeolite crystals [49]. As seen
Fig. 3. TPR profiles of oxide precursors: (a), Pt/Z: (b), Pt/50-Ir50/Z) and (c), Ir/ Z.
support. As expected, the binary Pt50-Ir50/Z catalyst exhibits many more peaks than the monometallic ones, which are observed at temperature maxima of 99, 230, 367 and 443 °C (shoulder). The two former peaks are due to the reduction of IrOx species whereas the two latter peaks can be associated with the reduction of Pt(II) species. It can be seen that the peak associated with the reduction of Pt2+ species strongly interacting with support (443 °C) is downward shifted with respect to that of the monometallic Pt/Z catalyst (452 °C). This shift suggests that the addition of iridium promotes the reduction of the platinum species in the binary Pt50-Ir50/Z catalyst, as it was observed for the Pt50-Ir50/γ-Al2O3 catalysts prepared by wet impregnation [45]. It is likely that the interaction between Pt and Ir metals originated just during the incipient wetness impregnation when the metal precursors are spatially well-distributed on the substrate [46]. In addition, this could be explained by taking into consideration that hexachloroplatinic acid (H2PtCl6·6H2O) is easily dissolved in water, whereas iridium
Fig. 4. NH3-TPD profiles of the calcined catalysts. 4
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Fig. 5. Wide-angle X-ray diffraction patterns of the fresh reduced catalysts: (a), pristine zeolite; (b), Pt50-Ir50/Z; (c), Pt/Z and (d), Ir/Z. Fig. 6. Ir 4f-Al 2p core level spectra of the spent Pt50-Ir50/Z (a) and Ir/Z (b) catalysts.
in Fig. 4, all ZSM-5 supported catalysts exhibited very similar acidity and acid sites distribution. Thus, considering the experimental error of the TPD-NH3 measurements (5%), the total acidity value remains almost unchanged upon metal loading. This is expected because all catalysts are supported on the same support material and their total metal loading is very similar (ca. 3 wt.%).
composition and oxidation state of the elements of the spent catalysts. Fig. 6 displays the XPS spectra of Ir(Pt) 4f–Al 2p. As expected, all the catalysts showed Si 2p and Al 2p peaks with a BE at 103 ± 0.1 eV and 75.0 eV respectively, corresponding to the framework Si and Al species of the ZSM-5 zeolite [53]. The binding energies (BE) of core electrons and surface atomic ratios are summarized in Tables 2 and 3, respectively. For the Pt-containing catalysts, the Pt 4d5/2 core level was also recorded. The BE of Pt 4d5/2 level at 314.7 eV is indicative of the presence of metallic Pt species only [54]. For the Pt50-Ir50/Z catalyst, the Pt 4f7/2 level at ca. 71.1 eV is associated with Pt0 [55] while for the Ir/Z and Pt50-Ir50/Z catalysts the Ir 4f7/2 level at a BE of 61.0 eV and 60.6 eV, respectively, are close to that reported for Ir0 species (60.9 eV) [56]. A similar BE value was reported previously for a freshly reduced Pt-Ir/Al2O3 catalyst (61.2 eV) [57]. The downward shift in the BE of Ir 4f7/2 peak in Pt50Ir50/Z catalyst with respect to that of Ir/Z one would suggests the formation of a Pt-Ir alloy in the bimetallic system. However, taking into account the quite similar electronegativity of both elements, Ir (2.20) and Pt (2.28), the charge transfer from iridium to platinum is hardly possible. Considering the quantitative Pt/Si and Ir/Si atomic ratios (Table 3), total metal exposure follows the trend: Ir/Z > Pt50-Ir50/Z = Pt/Z. Noticeably, Ir/Z catalyst exhibits higher Ir surface exposure than both the Pt-containing catalysts. By comparing surface Pt/Si and Ir/Si ratios with the bulk ratios in Fig. 7, it appears that Ir is location on the outer surface of both the Ir/Z and Pt50-Ir50/Z catalysts. In contrast, the larger bulk Pt/Si atomic ratios with respect to surface Pt/Si ratios for both Pt and Pt50-Ir50/Z catalysts strongly suggest the preferential Pt location within the inner crystalline structure of ZSM-5. For the binary catalyst, the XPS Ir/Pt atomic ratio of 6 clearly suggests a six-fold larger Ir than Pt surface exposure. The location of metal particles in the spent catalysts was examined also by HRTEM. Fig. 8 displays HRTEM images of the ZSM-5 supported catalysts. The binary Pt50-Ir50/Z catalyst exhibits globular shape particles of a size much larger than the pore diameter together with very
Crystallite structure Fig. 5 shows the wide-angle XRD patterns of the fresh reduced catalysts and pristine ZSM-5 zeolite. The presence of the low-angle diffraction peaks (2θ < 10°) suggest that metal incorporation does not affect crystallinity of the base zeolite to a great extent. The pristine zeolite and all the ZSM-5-supported catalysts exhibited typical diffraction lines at 2θ angles of 7.9°, 8.8°, 14.8°, 15.6°, 23.3° and 23.7°, which are characteristic of the zeolite substrate (JCPDS 00-042-0024). For the Ir/Z catalyst, only the diffraction lines of the zeolite substrates were observed indicating that the size of the Ir crystallites were below the XRD detection limit (< 4 nm) or its dispersion is very high. Conversely, both Pt/Z and Pt50-Ir50/Z catalysts show additional diffraction lines at ca. 39.8° and 81.3° originated by metallic Pt with a cubic crystallite structure (JCPDS 00-004-0802). For the binary Pt50-Ir50/Z catalyst, the planes (111), (200) and (220) suggest the presence of a single-phase disordered structure. However, the formation of binary Pt-Ir alloy is hardly possible because the temperature miscibility limit for the Pt50Ir50 composition is very high (close to 960 °C) [50]. The average Pt0 crystallite sizes of Pt/Z catalyst, calculated by applying the Debye-Scherrer's equation to the most intense diffraction line at 2θ angle of 39.8°, was much larger than that of the Pt50-Ir50/Z catalyst (44.5 nm vs. 35.1 nm). This calculation allows concluding that Pt dispersion on the binary catalyst is enhanced with respect to its monometallic Pt/Z counterpart. As the fresh reduced catalysts were characterized by XRD, it is more likely that the growing of Pt metal particles could occur during catalyst calcination and/or reduction. In line with this, there are literature reports showing that agglomeration and size change of Pt nanoparticles on these supports occur during the reaction conditions [51]. To obtain Pt particles without changing the nanoparticle’s size and morphology during the catalyst pretreatment and/or reaction conditions, the colloid deposition method could be more effective method for catalyst preparation, as it was argued by Kaidanovych et al. [52].
Table 2 Binding energy (eV) of core electrons of the spent catalysts.
Surface analysis of spent catalysts Photoelectron spectroscopy was used to evaluate both surface 5
Catalyst
Si 2p
Al 2p
Pt 4d5/2
Pt 4d7/2
Ir 4f7/2
Ir/Z Pt50-Ir50/Z Pt/Z
103.0 102.9 103.1
75.0 74.9 75.0
– 314.7 314.7
– 71.1 –
61.0 60.6 –
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having active phase well dispersed on the support surface. In contrast, the other monometallic Pt contains large Pt0 crystallites located on the support surface together with the small Pt0 particles located within the zeolite pores. Thus, it can be pointed out that Pt particles show higher mobility than Ir ones on the zeolite carrier, which can be linked to their different metal-support interaction (from TPR). These results agree with those obtained previously from Ir-Pd/SiO2-Al2O3 [31] and Ir-Pd/γAl2O3 [33] systems showing that Pd species exhibit a greater mobility than Ir over SiO2-Al2O3 and γ-Al2O3 substrates.
Table 3 Surface atomic ratios of the spent catalysts. Catalyst
Si/Al at XPS
Ir/Si XPS (Bulk)
Pt/Si XPS(Bulk)
(Pt + Ir)/Si XPS
Ir/Z Pt/Z Pt50-Ir50/Z
17.0 16.5 16.9
0.016 (0.008) – 0.006(0.003)
– 0.007(0.009) 0.001(0.004)
0.016 0.007 0.007
Catalyst activity The performance of the catalysts was evaluated in the liquid-phase phenol HDO reaction in the presence of a non-polar solvent (tetralin). Activity data, in terms of HDO conversion, are presented in Table 4. The HDO conversion follows the trend: Pt50-Ir50/Z (36%) > Ir/Z (32%) > Pt/Z (23%). However, taking into consideration the catalyst intrinsic activity expressed as the moles of phenol transformed per moles of metal and per second, this trend is different: Pt50-Ir50/ Z > Pt/Z > Ir/Z. The reaction products detected by GC analysis were: cyclohexanol, cyclohexane, benzene and methylcyclohexane. The main product with all the ZSM-5-supported catalysts was cyclohexane (selectivity of 60–96 %), which is the most desired product. Considering the products formed, the possible reaction scheme is shown in Fig. 9. As seen in this figure, the reaction scheme for phenol hydrogenolysis includes two reaction routes: (1) hydrogenation of aromatic ring of phenol (HYD route) to cyclohexanol + dehydration of cyclohexanol to cyclohexane; and (2) direct hydrogenolysis (DDO route) of C–O bond of phenol to benzene + hydrogenation of benzene to cyclohexane. Phenol can be converted to benzene by the direct deoxygenation (DDO) reaction pathway, and/or can be converted to cyclohexanol via the aromatic ring hydrogenation (HYD route). For both Pt/Z and Pt50-Ir50/Z catalysts, the formation of cyclohexanol suggests that the phenol HDO reaction proceeds mainly via a hydrogenation pathway. As compared to both Ir/Z and Pt50-Ir50/Z, Pt/Z catalyst shows partially inhibited dehydration reaction, as deduced from its highest selectivity toward cyclohexanol (Table 4). In contrast to the Ir/Z, the Ir/ZrO2 catalyst tested in HDO of guaiacol exhibited dominant cyclohexanol formation suggesting that only partial deoxygenation occurs [23]. Noticeably, the partial substitution of Pt by Ir in the catalyst formulation (Pt50-Ir50/Z) enhanced cyclohexanol dehydration to cyclohexane. Both Ir/Z and Pt/Z catalysts were the only ones able to generate a small amount of methylcyclopentane (4% and 2%, respectively) suggesting the involvement of the acid function in the reaction mechanism. Taking into account the similar acidity and acid sites distribution of all catalysts, the highest formation of methylcyclopentane on the Ir/Z catalyst could be explained considering the high hydrogenation/dehydrogenation activity of Ir [56] and the large metal surface exposure
Fig. 7. Bulk Pt(Ir)/Si atomic ratios (from TXRF) against XPS surface Pt(Ir)/Si atomic ratios of spent catalysts.
small Ir0/Pt0 particles. The local EDS analysis (not shown here) with electronic spot of 10 and 20 nm pointed out to a non-homogeneous Pt and Ir dispersion, in agreement with XPS (vide supra). The mean particle size calculated from various HRTEM images of Pt50-Ir50/Z, Ir/Z and Pt/Z catalysts are 9.1, 3.1 and 2.7 nm, respectively. As expected, much smaller crystalline particles was obtained from TEM images than from XRD data. This is because the smallest crystalline particles are not detected by XRD (detection limit below 4 nm) but still visible in TEM images. However, both XRD and HRTEM techniques strongly suggests the formation of large Pt0 crystallites on the surface of the bimetallic catalyst. In contrast with this catalyst, the monometallic Ir/Z one exhibits small Ir crystallites homogeneously dispersed on the support surface. Finally, the TEM image of the Pt/Z catalyst indicates the nonhomogeneous Pt0 species dispersion and a much lower density of Pt nanoparticles on the support surface, as also revealed by XPS results. The schematic representation of the Pt and Ir species location within the zeolite body and on its surface deduced from the overall catalyst characterization is visualized in Fig. 9. As seen, the bimetallic Pt50Ir50/Z exhibits smallest Ir0 particles and largest Pt0 crystallites located on the outer zeolite surface together with small Pt0 particles located within the zeolite pores. On the other hand, the Ir/Z is unique catalyst
Fig. 8. TEM micrographs of the ZSM-supported spent catalysts. 6
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Fig. 9. Schematic representation of the Pt and Ir species location within the ZSM-5 zeolite body and on its surface.
catalyst with respect to Pt/Z counterpart is the interesting feature of this work because this reaction is generally considered as an acid-catalyzed reaction [11]. The selectivity results strongly suggest that the well dispersed Ir nanoparticles can be more selective toward dehydration reaction than Pt species with a broad size distribution. This is because hydrogen can be easily activated and cleaved at the metalsupport interface region, as it was demonstrated in the field of SRO for the binary Pt-Ir/HY system [60]. Thus, in contrast to the work by Postman et al. [61], Ir/Z catalyst having well dispersed Ir nanoparticles was found to be more effective toward dehydration reaction than Pt/Z counterpart. Indeed, one might expect that the well-dispersed Ir atoms possessing electron donor/acceptor properties could facilitate the hydrogen spillover mechanism [62]. Thus, both hydrogen spill over mechanism and a larger intrinsic activity of Ir than Pt for cleavage of unsubstituted CeC bonds of the naphthenic compounds would explain the largest selectivity of Ir/Z catalyst toward metylcyclopentane in agreement with literature reports [32. 62, 63]. The results presented here agree with those reported previously for ring-opening of Pt5-Ir95/ CeO2-Al2O3 catalyst indicating that activity of this catalyst is governed by the accessibility of metallic Ir sites on the surface [32]. Taking into consideration the literature reports [64,65] and the enhanced hydrogen activation originated by a largest Ir surface exposure (from XPS), a low coke formation on the Ir/Z catalyst can be expected. Finally, the lowest selectivity of Pt/Z catalyst toward CO-bond scission can be explained considering the absence of the formation of Pt (211) phase, as confirmed by XRD, and the steric inhibition of hydrogen adsorption on Pt0 sites located deeply in the channels of the Pt/Z catalyst, as deduced from Fig. 7. Pt(211) plane having step sites is known to play a crucial role in C–O bond cleavage, as it was recently confirmed by a combined density functional theory (DFT) calculation and microkinetic model for Pt (211) and Ir (211) metals tested in ethanol HDO reaction [66]. Similarly, the DFT calculation indicated that Rh (211) crystal plane was more active than Rh (111) for CO-bond cleavage in the HDO of phenol [67].
Table 4 HDO conversion and selectivity (at 60% phenol conversion) for HDO of phenol over ZSM-5-supported catalysts (Reaction conditions were: batch reactor, T = 200 °C, PH2 = 3.0 MPa).
a
HDO conv. (%) rate (molPhenol molMetal−1 s−1)c Methylcyclopentaneb (%) Benzeneb (%) Cyclohexaneb (%) Cyclohexanolb (%) a b c
Ir/Z
Pt/Z
Pt50-Ir50/Z
32.0 0.028 4 0 96 0
23.0 0.054 2 6 60 32
36.0 0.066 0 0 95 5
At reaction time of 5 h. Reaction conditions were: a batch reactor, T = 200 °C, PH2 = 3.0 MPa. At 5 h per metal surface exposure (from XPS).
(Table 3). For all catalysts studied, the preferential selectivity towards HYD route products (94–100% selectivity) strongly suggests participation of the metallic function rather than the acidic function. Relationship between activity and structure It is well established that bifunctional catalysts having both acid and metal sites are suited for HDO reactions: the metal sites are needed for hydrogenation/dehydrogenation, whereas the acid sites of the support are required for dehydration, bond breaking, and isomerization [57]. There is synergy between the support acid function and the metal function, as it was confirmed recently for the HDO of m-cresol over Pt catalyst supported on γ-Al2O3 modified by acid treatment [11]. In this work, all catalysts exhibit similar acidity, as confirmed by TPD-NH3 (Fig. 4). Thus, the catalyst acidity did not mask the effect of its metal function. However, the similar acidity of the catalysts did not preclude the existence of synergy between their acid and metal functions. The best catalytic response of the bimetallic Pt50-Ir50/Z catalyst in the liquid-phase hydrodoxygenation with respect its monometallic Pt/Z and Ir/Z counterparts having the same total metal loading and the same acidity evidenced the positive ‘geometric’ effect of the simultaneous presence of both metals on the surface. Catalysts characterization by combined XRD, XPS and HRTEM strongly suggest that the main factors influencing on catalytic behavior are the intrinsic activities of Ir and Pt phases and their dispersion. Taking into account the ability of acidic supports to contribute to the activity of a catalyst for both aromatic hydrogenation [58] and dehydration reactions [59], it is hypothesized that reaction might occur at the interface of metal and acidic support. In such case, the HDO reaction mechanism might involve phenol adsorption on the interface between Ir(Pt) metal surface and support whereas the heterolytic dissociation of H2 into H+ and H– take place on the metal nanoparticles. However, further kinetic studies are needed for clarify the reaction mechanism. The larger extent of dehydration of cyclohexanol observed with Ir/Z
Conclusions The results presented in this work underline the factors influencing the performance of noble metals (Pt, Ir) supported on ZSM-5 zeolite in the liquid-phase hydrodeoxygenation of phenol. It was found that the catalyst activity was strongly influenced by the metal species exposure being acidic catalyst function similar for all catalysts studied. The use of a binary Pt50-Ir50/Z catalyst formulation was shown to have some advantage with respect to the monometallic Pt/Z and Ir/Z catalysts. Partial substitution of Pt by Ir in the catalyst formulation (Pt50-Ir50/ ZSM-5) enhances activity and selectivity toward O-free products which was associated with the surface enrichment of Ir species (from XPS and HRTEM). Pt species exhibited higher mobility than the Ir ones over the zeolite carrier. As a consequence, the Ir/Z catalyst having a larger metal 7
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surface exposure exhibited a greater activity and selectivity toward HDO products than its Pt/Z homolog. Ir was more effective for dehydration reaction than Pt indicating that this reaction might occur in the interface between Ir nanoparticles and acidic support.
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