Size-Controlled Pd Nanoparticles in 2-Butyne-1,4-diol Hydrogenation

Article pubs.acs.org/JPCC

Size-Controlled Pd Nanoparticles in 2‑Butyne-1,4-diol Hydrogenation: Support Effect and Kinetics Study Charline Berguerand, Igor Yuranov, Fernando Cárdenas-Lizana, Tatiana Yuranova, and Lioubov Kiwi-Minsker* Group of Catalytic Reaction Engineering, Ecole Polytechnique Fédérale de Lausanne (GGRC-ISIC-EPFL), CH-1015 Lausanne, Switzerland ABSTRACT: Structured catalyst has been developed for C−C triple bond three-phase hydrogenation. The sintered metal fiber (SMF) coated by different oxides served as support for monodispersed Pd nanoparticles (6.4 ± 0.5 nm). The effect of acid−base properties and reducibility of metal oxide coating on catalytic performance in the liquid phase (T = 303− 348 K; P = 1−20 bar) hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol (B2) has been studied. The oxides MgO, ZnO, Ga2O3, Al2O3, ZrO2, SnO2, and SiO2 and the mixtures of MgO + ZnO + Al2O3, MgO + Al2O3, and ZnO + Al2O3 were tested. The catalyst activity was higher up to 10-fold for Pd0 on acidic supports, like SiO2, but demonstrated lower selectivity to B2 as compared to the basic oxides. The highest yield (∼99%) of the target B2 and stability over four consecutive runs were attained over the 0.2% Pd0/ZnO/SMF catalyst. The high selectivity to B2 was attributed to the formation of an active phase containing intermetallic PdZn alloy as confirmed by XPS. The reaction kinetics was modeled using a Langmuir−Hinshelwood mechanism and found consistent with the experimental data. The developed structured catalyst is suitable for a design of flow multiphase reactors to perform alkyne semihydrogenations in continuous mode.

1. INTRODUCTION The 2-butene-1,4-diol (B2) is an important starting material with a worldwide production ≥5000 tons per year1,2 used for the synthesis of vitamins3 and insecticides.4 The conventional production route involves the partial hydrogenation of 2butyne-1,4-diol (B3) (path 1 in Scheme 1) where high selectivity to the target olefin B2 is the critical issue. Pd/ CaCO3 doped with lead (Lindlar catalyst) is the mostly used catalyst for this reaction. Nonetheless, the presence of toxic lead and low B2 yield (≤90%)5 are the drawbacks that warrant further investigation. B3 catalytic hydrogenation can generate a range of intermediates and byproducts, as shown in Scheme 1.1,2,6−13 Butane-1,4-diol (B1) can be formed at T = 278−328 K and P = 30−200 bar over carbon and alumina supported Pd8 and/or Ni1,9 catalysts as a result of further hydrogenation of B2 (path 2) or by direct hydrogenation of B3 (path 3). A double bond migration, resulting in the formation of γ-hydroxybutyraldehyde (path 4), has been reported for Pd/C catalyst.2,10 This isomer can undergo cyclization to 2-hydrotetrahydrofuran (path 5)13 or hydrogenolysis to 1-butanol (path 6) or crotyl alcohol (path 7). Crotyl alcohol can be further isomerized (to butyraldehyde, path 8) and hydrogenated (to 1-butanol, path 9). These side reactions have been reported over Pt/TiO211 and Pd/C12 being favored in acidic media. The incorporation of NH3 or KOH as basic reaction modifiers to the reaction mixture has proved to increase the yield of B2 (up to 99%)14 by avoiding the hydrogenolysis of γ-hydroxybutyraldehyde (Scheme 1, routes 5 and 6).6 Nonetheless, use of modifiers requires subsequent © XXXX American Chemical Society

separation/purification steps with associated environmental and cost impacts. This issue can be avoided by fine-tuning the properties of the metal active phase through modifications of the support which, in turn, can impact the activity and/or selectivity.12 The use of oxides as catalyst supports is well established due to their combined high surface area and thermal stability.15 The acid−base character of the oxide influences the electronic properties of the supported metal16 by withdrawing/ donating electrons from/to the metal nanoparticles (NPs) affecting the reactant adsorption mode and activation.17 Mixing oxides, although studied to a limited extent, has been proved to control the acid−base character of the carrier, as it was shown in the transesterification reaction to form biodiesel,18 hydrogenation of naphthalene,19 and, of particular relevance to this study, the semihydrogenation of 2-hexyne.20 The last study is only focused on amorphous Si/Ti oxides. Another effective means of controlling a catalytic response is via metal−support interactions observed over reducible oxides. The active metal can be partially/totally encapsulated within a support and/or form an alloy.21 In particular, the active phase containing Zn alloy with Pt-group metals has shown to increase olefin selectivity as compared to the monometallic counter partners.22,23 An evaluation of the kinetic parameters is a key point for catalyst optimization and reactor design. Up to now, only a Received: February 6, 2014 Revised: April 3, 2014

A

dx.doi.org/10.1021/jp501326c | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Scheme 1. Schematic Reaction Network for the Hydrogenation of 2-Butyne-1,4-diol

dried. The SMF were subsequently calcined (823−1373 K, 3 h in air) in order to facilitate the anchoring of the oxide coating in a subsequent step.27 2.2.2. MxOy/SMF Supports. After the pretreatment, the SMF panels were coated with a homogeneous oxide layer and its content was determined gravimetrically. The 5 wt% ZnO/SMF support was synthesized as follows: 65.8 g of Zn acetate dihydrate (Fluka) were dissolved in isopropanol (1000 cm3, Sigma-Aldrich) containing 18.3 g of monoethanolamine (BioChemika) and 12.8 g of acetoin (Fluka) under continuous stirring. Monoethanolamine and acetoin serve as complexing agents and prevent crystallization of Zn acetate during ZnO deposition.28 The ZnO deposition was achieved via a stepwise procedure consisting of (i) dipping, (ii) air-drying at room temperature (0.5 h), and (iii) aircalcination at 873 K (0.5 h). The process was repeated six times to achieve ca. 5 wt% ZnO. Finally, the sample was subjected to a thermal treatment in air at 1173 K (15 min) in order to increase the specific surface area.25 The 4.8 wt% SiO2/SMF support was prepared as follows: 20 g of tetraethoxysilane (ABCR) was dissolved in 30 cm3 of ethanol. Five cm3 of concentrated HCl (37%) and 5 cm3 of distilled water were added to the solution which was stirred for 2 h at 343 K.26 SMF panels were dipped into this solution, dried at room temperature (4 h), and calcined in air at 723 K (2 h). The 5.5 wt% ZrO 2 /SMF support was prepared by impregnating SMF panels with a 15 wt% ZrO2 aqueous dispersion (MEL Chemicals). The impregnated SMF panels were dried at room temperature (4 h) and calcined in air at 823 K (2 h). The other 5 wt % MxOy/SMF (where MxOy = MgO, Ga2O3, Al2O3, SnO2) supports were synthesized by a citrate sol−gel method.29 32 g of Mg(NO3)2·6H2O (Merck), 13.6 g of Ga(NO3)3·H2O (Strem), 36.75 g of Al(NO3)3·9H2O (Fluka), or 6.3 g of SnCl2 (Fluka) were dissolved in 30 cm3 of distilled water containing a calculated amount of citric acid. The M:citric acid molar ratio was always 1:2. Oxide layer deposition was done through a stepwise procedure consisting of (i) dipping, (ii) drying in air at room temperature (0.5 h), and (iii) calcination at 873 K (2 h). The procedure was repeated twice.

handful of studies have been focused on the kinetics of the selective semihydrogenation of B3 concluding a zero order to B3 and a first order to hydrogen.4,14,24 In the present work, we demonstrate the effect of the acid− base and redox properties of the oxide support (for single oxides and mixtures) on the catalytic performance of monodispersed (6.4 ± 0.5 nm) Pd NPs in the selective hydrogenation of B3 to B2. In order to overcome some drawbacks associated with standard powder catalysts (e.g., transport limitations25), a structured support based on sintered metal fibers (SMF) surface coated by oxides is used. The catalytic performance is compared with a commercial Lindlar catalyst. Furthermore, a reaction kinetic model is proposed in order to explain the observed phenomena.

2. EXPERIMENTAL SECTION 2.1. Materials. The reactant 2-butyne-1,4-diol (B3) (99% purity, Aldrich) and solvents 2-butene-1,4-diol (B2) (95%, Aldrich), isopropanol (98%, Aldrich), and ethanol (95%, Aldrich) were used as received without further purification. All the gases employed in this work (H2, N2, Ar, O2, and He) were of ultrahigh purity (>99.99%, Carbagas). 2.2. Catalyst Preparation. Monodispersed Pd nanoparticles were synthesized via the colloidal technique with ethanol and poly(N-vinylpyrrolidone) (PVP) as reducing and stabilizing agents, respectively. Colloidal Pd nanoparticles were deposited (via impregnation) on a structured support of SMF coated by an oxide. The synthesized catalysts were denoted as Pd/MxOy/SMF. Characterization of the supported active phase presents difficulties due to low Pd contents (∼0.2 wt%) and the presence of the SMF metallic support. In an attempt to circumvent these issues, a series of powder catalysts bearing the same metal (Pd)/oxide (MxOy) ratio and synthesized in a similar manner were prepared and denoted as Pd/MxOy. In these samples, the Pd content was between 2.0 and 4.7 wt% depending on the oxide. 2.2.1. Sintered Metal Fibers (SMF). Commercial SMF (Southwest Screens & Filters SA, Belgium) made of an alloy of Cr, Al, Ni, Mo, and Mn with Fe as the major component (67−75 wt%) in the form of a uniform porous panel (0.29 mm thickness, 81% porosity, 20 μm fiber diameter, SSA < 1 m2· g−1)26 were first degreased in boiling toluene (0.5 h) and airB



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areas were reproducible to within ±6%, and the values quoted in this study are the mean. The Pd particle size was determined by transmission electron microscopy (TEM) using a Philips FEI CM12 TEM microscope operated at an accelerating voltage of 120 kV. A Gatan Digital Micrograph 1.82 was employed for data acquisition/ manipulation. The specimens were prepared by dispersion of the Pd/MxOy samples in acetone and deposition on a holey carbon/Cu grid (300 mesh). Up to 500 individual Pd nanoparticles were counted for each catalyst, and an average diameter (d) was calculated from

A series of oxide mixtures (ZnO + Al2O3 (1:1), MgO + Al2O3 (1:1), and MgO + ZnO + Al2O3 (1:1:1)) were prepared by the following procedure: an aqueous solution (70 cm3) containing 20 g of Al(NO3)3·9H2O (Fluka) was heated up to 368 K. 4.35 g of ZnO (Sigma-Aldrich), 2.15 g of MgO (Fluka), or 2.17 g of ZnO + 1.1 g of MgO were added slowly to the solution. The mixtures were kept under constant heating (368 K) and agitation (600 rpm) until complete oxide dissolution. The solutions were then cooled down to room temperature, and oxide layers were deposited on SMF by the same dipping + drying + calcination procedure as described above. 2.2.3. Pd/MxOy/SMF Structured Catalysts. Monodispersed PVP-stabilized Pd nanoparticles (NPs) were prepared by a colloidal technique.30 First, a mixture (600 cm3) of water and ethanol (H2O/C2H5OH = 3/2 v/v) containing 1.0 × 10−2 mol of PVP was heated up to 363 K. An aqueous solution of Na2PdCl4 (5.0 × 10−5 mol·cm−3) prepared by dissolving PdCl2 (Aldrich) + NaCl (Fluka) in water (Pd/Na molar ratio = 1/2; T = 363 K) was then added to the mixture, resulting in a dark brown solution (i.e., formation of Pd0 NPs31) and kept at 363 K under stirring for 3 h. It was demonstrated32 that Pd0 NPs of a 2−10 nm size stabilized PVP are formed in this case. Prior to deposition, Pd NPs were cleaned up from the excess of PVP via flocculation in acetone (colloidal solution/acetone = 1/3 v/v) followed by dispersion in water. Pd NPs were subsequently deposited on the structured support by impregnation. After drying at room temperature, the samples were calcined in air (873 K, 2 h) and reduced by H2 (573 K, 10% v/v H2 in Ar, 475 cm3·min−1, 2 h). In all cases, the final Pd loading was ∼0.2 wt%. 2.2.4. Powder Pd/MxOy Catalysts. A series of five powder 5 wt% Pd/MxOy catalysts were synthesized by deposition of the monodispersed Pd NPs on commercial oxides (ZnO, >99%, Sigma-Aldrich; Al2O3, >99%, Johnson-Matthey; SiO2, >99%, Blazers and MgO, >98%, Fluka) and treated/activated as described in section 2.2.3. 2.3. Catalyst Characterization. The isoelectric point (IEP) was taken as a characteristic of the acid−base properties of the oxide carriers. The IEP values of the pure oxides were taken from the literature.33 The IEPs of the oxide mixtures were calculated as an average of the IEP values of the components following a methodology reported elsewhere.34 The Pd content in the catalysts was determined by atomic absorption spectroscopy (AAS) using a Shimadzu AA-6650 spectrometer with an air-acetylene flame and diluted solutions obtained by dissolving the catalysts in aqua regia (HNO3/HCl = 1/3 v/v). AAS analysis was also used to evaluate any Pd leaching during the catalytic tests. The Pd leaching tests were performed by (i) comparison of the Pd content in the catalysts before and after the catalytic reaction, (ii) Pd AAS analysis of the final reacting mixture, and (iii) carrying out the hydrogenation reaction reusing the final solution after catalyst removal and addition of the substrate. In the last-mentioned case, any detected conversion would indicate Pd leaching during the catalytic reaction. Specific surface area and pore volume were determined using a commercial Sorptomatic 1990 (Carlo Erba). Prior to analysis, the samples were outgassed at 523 K for 2 h under a vacuum (70% of Pd

where MPd represents the Pd atomic mass and D is the metal dispersion (atomPd surface·atomPd total−1). D was calculated from D=

SPd × MPd APd × NA

(5)

where APd is the atomic surface (mPd surface2·atomPd surface−1), SPd is the metal surface area (mPd2·gPd−1), and NA is the Avogadro number. Finally, the Pd surface area was calculated from SPd (mPd 2·g Pd−1) =

6 ρPd × d

(6)

−3

where ρPd = 12.046 g·cm and d is the average mean Pd particle size, as measured by TEM analyses (see section 2.3).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. Pd Colloids. A representative TEM image of PVP stabilized Pd nanoparticles synthesized ex situ is presented in Figure 1A; associated particle size distribution is shown in Figure 1B. Pd nanoparticles present a homogeneous distribution with >50% in the 6−8 nm size range (mean size = 6.1 nm). Alkynol hydrogenations are structure sensitive reactions38 where a mean Pd NP size of 6 nm has been deemed optimum for high activity and B2 selectivity.5 The preformed Pd colloids were deposited on sintered metal fibers covered by an oxide layer. 3.1.2. Pd/MxOy/SMF Structured Catalysts. The degreased panels (before the oxide deposition) have a uniform open macrostructure (Figure 2AI). The metallic network consists of interweaved elementary filaments with a tetragonal-like structure. Up to 100 individual fibers were counted, and a mean filament diameter of ca. 20 μm (Figure 2AII) was D



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Figure 3. Pd NP size distribution obtained from TEM micrographs with (A) Pd/ZnO/SMF and (B) Pd/ZnO (powder). Figure 2. Representative (I) low and (II) high resolution SEM micrographs of SMF (calcined at 573 K) (A) before and (B) after coating by ZnO and (C) after Pd deposition.

after reduction in hydrogen is shown in Figure 4, where the 3d5/2 binding energy peaks at 335.0 and 335.7 eV correspond to metallic Pd0 and PdZn alloy, respectively.22,44 A sampling depth in XPS analysis is typically 3−5 nm.45 However, for the catalysts containing the nonreducible oxides (Al2O3, MgO + Al2O3, and SiO2), the Pd bulk content measured by chemical analysis is close to the surface Pd content obtained by XPS (3.9 ± 0.5 wt%; see Table 2). In contrast, for the ZnO-containing catalysts, the bulk Pd content (3.9 ± 0.5% wt) is significantly higher than the surface Pd concentration (2.2 ± 0.2 wt%). Such an observation can indicate both a metal encapsulation46 and alloy formation.47 A Lindlar catalyst (5 wt% of Pd/CaCO3) selected as a benchmark was also characterized. The measured SSA of 8 ± 1 m2·g−1 is in good agreement with values (5−11 m2·g−1) reported in the literature.48 TEM analysis demonstrates that Pd is present as nanoparticles of 5−40 nm size range with a mean particle size of ∼12.5 nm. In summary, the Pd particle size (d = 6.4 ± 0.5 nm) is independent of the support nature and does not change during the deposition. XPS analysis is consistent with the presence of Pd0 as the main active component in all of the catalytic systems without ZnO. In contrast, the formation of PdZn alloy is confirmed when the support contains ZnO. 3.2. Catalytic Response. 3.2.1. Influence of Oxide Layer. A series of the Pd/MxOy/SMF catalysts with different acid− base properties (different IEPs of MxOy) were tested. The initial B3 consumption rates (−RB3,0) determined from a linear regression of the temporal B3 concentration profiles (not shown) are summarized in Table 3. An increase of the catalyst activity with oxide acidity used for SMF coating is in evidence

NPs supported on ZnO powder have a particle size between 5 and 8 nm. These results extended to all the powder catalysts where a similar Pd dispersion was observed, confirming that both Pd deposition/activation and the nature of the support do not impact significantly Pd dispersion. The specific surface areas (SSA) of the structured and powdered catalysts are shown in Tables 1 and 2. The SSA of the supported and unsupported pure oxides do not differ and correspond to the reported values.40−43 Thus, we can conclude that the powdered Pd/MxOy can be used instead of the corresponding Pd/MxOy/SMF for characterization, since the SMF-based catalyst cannot be assessed because of a low total amount of active phase (∼0.2 wt% Pd) and/or due to the presence of metallic SMF. XPS analysis is used to elucidate the electronic properties of the supported Pd. The binding energies (Pd 3d5/2) for the different powdered catalysts are gathered in Table 2. Two categories of catalysts could be identified. For the catalytic systems without ZnO (Pd/MgO + Al2O3, Pd/Al2O3, and Pd/ SiO2), the spectra contain a single Pd 3d5/2 peak which is shifted (0.2−0.6 eV) to a higher region as compared to bulk metallic Pd, 335.0 eV.44 This suggests an electron transfer from Pd to the support, resulting in a partially positively charged metal phase (Pdδ+). In the case of ZnO-containing catalysts, the XPS spectra could be deconvoluted onto two peaks where the main signal (335.0 eV) is a characteristic of metallic Pd0. The second is shifted (0.7−1 eV) higher and corresponds to a PdZn alloy.42 The Pd 3d spectrum of the Pd/ZnO catalyst measured E



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Table 1. Characteristics of Pd NPs Supported on Metal-Oxide-Coated Sintered Metal Fibers (SMFs) oxide support

ZnO

MgO + Al2O3

ZnO + Al2O3

Al2O3

SiO2

Pd loading (% wt) BEToxide (m2·g−1) dPd (nm)

0.18 8 6.1

0.21

0.16 76 6.0

0.20 153

0.15 451 6.5

Table 2. Characteristics of Pd NPs on Powder Metal Oxides Used as Model Supports oxide Pd loading (% wt)

AAS XPS

binding energies Pd 3d5/2 (eV) BET (m2·g−1) dPd (nm)

ZnO

MgO + Al2O3

ZnO + Al2O3

Al2O3

SiO2

4.7 2.4 335.0, 335.7 7 6.1

3.9 3.9 335.4 180 6.9

4.1 2.0 335.1, 336.0 90 5.9

3.5 3.6 335.6 171 6.4

4.4 4.3 335.2 430 6.7

support.50 The increased activity observed over Pd on an acidic carrier can be linked to the promoted adsorption of the electron rich CC bond51 on Pdδ+ NPs facilitating its attack by hydrogen. The measured transformation rates (see Table 3) fall into the range established for B3 hydrogenation operated under kinetic control (TOF 0.25−158 s−1).5,52 A significantly higher (by up to 12-fold) specific activity was recorded for the synthesized Pd/MxOy/SMF catalysts as compared to the commercial Lindlar catalyst. In terms of selectivity, the catalytic results suggest a correlation between the acidic/basic character of a carrier and the formation of the target B2. The lower B2 selectivity was observed over the more acidic (lower IEP) carrier; i.e., the lowest B2 selectivity (74% at the 99% B3 conversion) was detected over Pd/SiO2/SMF. It was demonstrated elsewhere23 that the alkene selectivity in alkyne hydrogenation depends on the ratio of adsorption strengths of alkyne and alkene. In the case of basic supports, the ratio seems to be higher, avoiding consecutive hydrogenation of alkene. It is worth noting that, though the Lindlar catalyst is within this trend showing enhanced selectivity over basic CaCO3, it contains lead, which blocks the most active sites responsible for unselective hydrogenation.53 The increased yields to B2 (≥95% at the 99% B3 conversion) were also achieved over the ZnOcontaining catalysts. This fact can be attributed to the presence of a PdZn alloy (as demonstrated by XPS), resulting in the modified electronic properties of Pd. A similar effect reported for semihydrogenation of 2-methyl-3-butyn-2-ol22,25 and

Figure 4. XPS spectrum over the Pd 3d region of the Pd/ZnO with independent contributions of Pd(0) (dashed line) and PdZn alloy (dotted line) from peak deconvolution. Note: experimental data are represented by symbols, while lines identify the spectra curve fitting.

with a difference of up to 10-fold between the most acidic (SiO2) and the most basic (MgO) oxide. Zakarina et al.49 studied the activity of Pd supported on different oxides (SiO2, Al2O3, and MgO) in hydrogenation of dimethylethynylcarbinol. No clear tendency was withdrawn. The authors found that the activity was related to the Pd particle size and Pdδ+ charge, which was dependent on the support. We also found that the acidic carrier withdraws electrons from Pd NPs, resulting in the formation of electron deficient Pdδ+ (shown by XPS, see the Catalyst Characterization section). On a basic carrier, the electron density on Pd slightly increases (Pdδ−) due to the electron transfer from the

Table 3. IEP for the Model MxOy Supports and Catalytic Results Obtained in Liquid Phase B3 Hydrogenation over a Series of Pd/MxOy/SMF Catalystsa

a

oxide

isoelectric point

SB2; XB3 = 50% (%)

SB2; XB3 > 99% (%)

initial activity (mol·gPd−1·s−1) × 103

TOF (s−1)

MgO ZnO MgO + ZnO + Al2O3 Ga2O3 MgO + Al2O3 ZnO + Al2O3 Al2O3 ZrO2 SnO2 SiO2 CaCO3 (Lindlar)

12.1 10.3 9.9 9.1 8.9 8.4 7.5 6.7 5.0 3.5 11.0

99.6 99.9 99.6 98.3 98.0 99.6 97.1 92.0 97.4 96.2 96.0

90.2 99.9 98.2 89.0 91.9 99.2 87.9 85.5 92.0 73.9 92.5

5.2 13.1 14.5 19.6 21.2 24.7 29.3 34.6 40.5 52.3 4.5

4.6 10.8 11.9 17.3 19.8 20.0 24.5 28.9 33.8 48.1 7.8

Reaction conditions: T = 353 K; P = 15 bar; B3/Pd = 6500 molB3·molPd−1; solvent - B2. F



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solvent in order to model “solvent-free” reaction conditions. The reactions were carried out over the Pd/ZnO/SMF and Pd/ ZnO + Al2O3/SMF catalysts (Table 4). The calculated initial specific activities are in agreement with the literature where the reaction rates of alkyne hydrogenations are in the same range (0.25−158 s−1).5,52 A high B2 selectivity (>98 ± 0.1%) at full B3 conversion (>99%) was achieved in all solvents. An extremely high B2 yield (YB2 > 99%) was detected in B2 used as a reaction medium (solvent-free conditions). The catalyst activity increases in the order ethanol > isopropanol > water > B2 and follows the hydrogen solubility (Table 4). In spite of the lower activity in B2, the enhanced olefin selectivity as well as the ecological (no solvent is used) and economical (no separation step is required) reasons make B2 the reaction medium of choice for the kinetic study. 3.3. Kinetics Study and Reaction Modeling. The Pd/ ZnO/SMF, Pd/ZnO + Al2O3/SMF, and Pd/Al2O3/SMF catalysts were selected to study the reaction kinetics under solvent-free conditions. The activation energy was determined from the Arrhenius equation using the initial reaction rates over the 353−443 K temperature range. The found values (31 ± 3 kJ·mol−1) are in good agreement with the ones reported in the literature for B3 hydrogenation (17−39 kJ·mol−1).14,59,60 An increase of the rate of B3 transformation observed at higher H2 partial pressures was consistent with the first reaction order with respect to H2 and in line with the literature.14 The concentrations of products as a function of time in a typical run over the Pd/SiO2/SMF catalyst are shown in Figure 6A where during the first minutes the formation of the target B2 is favored (path 1 in Scheme 1). At the same time, B1 is also formed. After passing the maximum, the B2 concentration decreases while the formation of B1 is promoted (path 2 in Scheme 1). This result is consistent with a predominant consecutive reaction pathway. The concentrations of the substrate (B3) and products (B2 and B1) as a function of time are represented in Figure 6B. In this case, even after the full conversion of B3, no transformation of B2 occurs. Temporary dependence of B3 concentration confirms a zero order reaction with respect to B3 up to 80% conversion.60 A Langmuir−Hinshelwood mechanism was adapted for modeling the kinetics assuming one type of Pd active site and dissociative weak adsorption of hydrogen.22 Due to the absence of byproducts, the reaction scheme could be simplified to pathways 1, 2, and 3 in Scheme 1. The rate-determining step was chosen on the basis of the fact that hydrogenations over Pt group metals usually involve dissociative adsorption of H2 and present a first order with respect to H2.61 A two-step hydrogenation for each species involving an intermediate is

pentyne23 hydrogenation was also ascribed to the presence of a PdZn phase. Given the importance of catalyst stability for industrial implementation, the best in terms of selectivity Pd/ZnO/SMF and Pd/ZnO + Al2O3/SMF catalysts were subjected to a stability test. The results presented in Figure 5 demonstrate

Figure 5. Catalyst activity (solid bars) and B2 selectivity (hatched bars) at 99% B3 conversion for four consecutive runs over (A) Pd/ ZnO/SMF and (B) Pd/ZnO + Al2O3/SMF. Reaction conditions: T = 353 K; P = 15 bar; B3/Pd = 6500 molB3·molPd−1; solvent free.

maintenance of both the activity and B2 selectivity over four consecutive runs. The catalyst stability can be explained by strong Pd−support interaction via PdZn alloy formation and preventing Pd from leaching. Hong et al.54 have already proposed the formation of a thin PdZn alloy layer at the metal−support interface, resulting in a strong anchoring of Pd to the support. Uemura et al. reported that Pd0 could activate H2 to reduce ZnO. The reduced Zn atoms migrate into Pd NPs, forming a PdZn alloy.55 3.2.2. Effect of Solvent. The polarity of the reaction medium is known to have an effect on the activity and/or selectivity in hydrogenation reactions.56 This is attributed to the hydrogen solubility and mass transfer.56 Taking into account that B3 is insoluble in nonpolar media, a series of polar solvents (ethanol, isopropanol, and water) was tested. B2 was also used as a

Table 4. Catalytic Results in the Liquid Phase Hydrogenation of B3 to B2 Using Different Solventsa Pd/ZnO/SMF solvent B2 H2O PrOH EtOH

H2 solubility × 103 b

0.9 1.2c 3.9c 5.2c

Pd/ZnO + Al2O3/SMF

SB2; XB3 > 99% (%)


SB2; XB3 > 99% (%)


99 98 98 98

6.0 7.7 12.7 13.4

99 97 99 98

13.1 14.5 18.1 19.0

a Reaction conditions: T = 353 K; P = 8 bar; B3/Pd = 6500 molB3·molPd−1. bFrom ref 57. cx = a·(Pr·Tr) + b, where x is the solubility (in mole fraction), Pr is the reduced pressure defined as the ratio between the critical pressure of hydrogen (Pc = 61.5 bar) and the reaction pressure, and Tr is the reduced temperature (Tc for hydrogen = 513.9 K). From ref 58.

G



Article KB3 **

B3*σ + Hσ ←→ ⎯ B3**σ + σ

KB3 ** =

K***B3

B3***σ + Hσ ←⎯⎯⎯→ B3***σ + σ KB3 *** = k3

B3***σ + Hσ → B1σ + σ

θB3 **θσ θB3 *θ H θB3 ***θσ θB3 **θ H

r2 = k 2θB2 *θH

(15) (16) (17)

We assume a low coverage38 for all of the intermediate species, so 1 = θB3 + θB2 + θB1 + θH + θσ

(18)

where θ represents the coverage. By rearrangement of the variables, the coverage can be determined as follows: θH =

KH2C H2 ·θσ

(19)

θB3 = KB3C B3·θσ

(20)


(21)


(22)

By replacing in eq 18, the coverage of an active site could be defined by θσ = Figure 6. Concentrations of B3 (■), B2 (●), and B1 (▲) as a function of time over (A) Pd/SiO2/SMF and (B) Pd/ZnO/SMF. The lines represent the fitting to the Langmuir−Hinshelwood model. Reaction conditions: T = 353 K; P = 15 bar; B2 as solvent; B3/Pd = 6500 molB3·molPd−1.

KH2

H 2 + 2σ ←→ 2(Hσ )

KB3 = KH2 =

KB3 *

B3σ + Hσ ←→ B3*σ + σ KB3 * = k1

B3*σ + Hσ → B2σ + σ KB2

B2σ ←→ B2 + σ

KB2 =

KB2 *

KB1

=

B1σ ←→ B1 + σ

k 2KB2 *KB2C B2KH2C H2 (1 + KB3C B3 + KB2C B2 + KB1C B1 +

2

θB3 *θσ θB3θ H

=

(8)

k 3KB3 ***KB3 **KB3 *KB3C B3(KH2C H2)2 (1 + KB3C B3 + KB2C B2 + KB1C B1 +

θB2 C B2θσ

θB2 *θσ C B2θ H

KH2C H2 )2 (26)

(9)

By grouping all the constants at the numerator, the previous equations give r1 =

(11)

r2 =

r3 =

(12)

r2 = k 2θB2 *θH (13) KB1 =

KH2C H2 )2 (25)

(7)

θH2

θB1 C B1θσ

KH2C H2 )2

r2 = k 2θB2 *θH

r1 = k1θB3 *θH (10)

B2σ + Hσ ←→ B2*σ + σ KB2 * = B2*σ + Hσ → B1σ + σ

(1 + KB3C B3 + KB2C B2 + KB1C B1 +

(24)

The same procedure can be applied for the path B2 → B1 transformation:

k2

k1KB3 *KB3C B3KH2C H2

r3 = k 3θB3 ***θH

θB3 C B3θσ C H2θσ

(23)

r1 = k1θB3 *θH

proposed. The adsorption equilibrium constants of hydrogenated products are small compared to the B3 constant. This assumption together with weak H2 adsorption on metals61 gives simplified equations where σ and * represent active site(s) and intermediate(s), respectively. Path 1 corresponding to B3 → B2 can be represented as KB3

KH2C H2

With the coverage definition of each species, the rate-limiting steps, r1, r2, and r3, could be adapted:

=

B3 + σ ←→ B3σ

1 1 + KB3C B3 + KB2C B2 + KB1C B1 +

k1′C B3 (1 + KB3C B3 + KB2C B2 + KB1C B1 +

KH )2

(27)

KH )2

(28)

KH )2

(29)

k 2′C B2 (1 + KB3C B3 + KB2C B2 + KB1C B1 +

k 3′C B3 (1 + KB3C B3 + KB2C B2 + KB1C B1 +

To complete the kinetic system, the mass balance is added for the three species.

(14)

dC B3 1 = ( −r1 − r3) dt V

and B3 → B1: H

(30)



Article

dC B2 1 = (r1 − r2) dt V

(31)

dC B1 1 = (r2 + r1) dt V

(32)

and B, respectively, as a function of the IEP. It can be seen that the kinetic constant k1 linearly increases with decreasing IEP. In the case of an acidic carrier, electron deficient Pd will coordinate more easily electron rich compounds such as alkynes, which correlates to the activity constants. The ratio between the alkyne and alkene adsorption equilibrium constants decreases concomitant with basicity. This observation is related to the decrease of selectivity with the increase of the support acidity.23 In the case of a basic support, a higher ratio between alkyne and alkene adsorption constants is obtained, explaining a higher selectivity to B2. The ZnO-containing catalyst shows the highest ratio due to the presence of the PdZn alloy.

The system of equations was resolved using BerkleyMadonna software62 with the Rosenbrock integration method. The results obtained for the Pd/SiO2/SMF and Pd/ZnO/ SMF catalysts are presented in Figure 6 where the overlap between experimental and theoretical values confirms that the model is appropriate for both, i.e., nonselective (Pd/SiO2/ SMF) and selective (Pd/ZnO/SMF) catalysts. In the case of the Pd/ZnO/SMF, the high alkene selectivity can be explained by stronger adsorption of the alkyne compared to that of the alkene (see Figure 7B). The adsorption constant of alkyne is 2

4. CONCLUSIONS Pd nanoparticles (mean size 6.4 ± 0.5 nm) supported on a series of oxides (MgO, ZnO, Ga2O3, Al2O3, ZrO2, SnO2, SiO2, MgO + ZnO + Al2O3, MgO + Al2O3, ZnO + Al2O3) with different acid−base properties were synthesized and studied in the liquid phase selective hydrogenation of 2-butyne-1,4-diol (B3) to 2-butene-1,4-diol (B2). The formation of a PdZn alloy leading to modification of Pd electronic properties and to the strong Pd anchoring to support was established for the ZnOcontaining supports. The acid−base properties of the oxide carrier affect particularly the catalyst activity. The Pd NPs on an acidic carrier show a higher activity (up to 10-fold) as compared to ones supported on a basic oxide. For the first time, ∼99% yield of B2 over 0.2 wt % Pd/ZnO/SMF catalyst has been obtained without any addition of modifiers into the reaction mixture. Moreover, the catalyst maintains its activity and selectivity over at least four consecutive runs without any deactivation or Pd leaching. The reaction kinetics was modeled using a Langmuir− Hinshelwood mechanism with dissociative weak adsorption of hydrogen. In B3 hydrogenation over the ZnO-containing catalysts, the ratio between the alkyne and alkene adsorption constants was greater than 100, being responsible for high B2 selectivity. The study demonstrates the potential of tuning the Pd NPs catalytic properties by changing the acid−base character of the support and increasing catalyst stability, inducing strong metal−support interactions. The developed structured catalyst is suitable for a design of flow multiphase reactors to perform alkyne semihydrogenations in continuous mode.

■

Figure 7. (A) Kinetic constants k1′ (open symbols), k2′ (half open symbols), and k3′ (crossed symbols) and (B) adsorption constants KH (solid symbols), KB3 (half open symbols), KB2 (open symbols), and KB1 (crossed symbols) obtained from the fitting of experimental data to the Langmuir−Hinshelwood mechanism for B3 hydrogenation over a series of Pd/MxOy/SMF catalysts, where MxOy SiO2 (■), Al2O3 (●), ZnO + Al2O3 (▲), MgO + Al2O3 (◆), and ZnO (★). Reaction conditions: T = 353 K; P = 15 bar; B3/Pd = 6500 molB3·molPd−1.

AUTHOR INFORMATION

Corresponding Author

*Fax: (+41) 21-6933667. Phone: (+41) 21-6933182. E-mail: lioubov.kiwi-minsker@epfl.ch. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by the Swiss National Science Foundation (grant 200021-118067) and European Union through the Seventh Framework Program (project POLYCAT; grant CP-IP 246095-2).

53,60

Molnar et orders of magnitude higher than that of alkene. al. assumed that, when the adsorption constant of alkyne is much greater than that of alkene, the readsorption of alkene is inhibited and further hydrogen addition cannot take place.53 The modified Langmuir−Hinshelwood model was applied to the selected Pd/MxOy/SMF catalysts, where MxOy is SiO2, Al2O3, ZnO + Al2O3, MgO + Al2O3, and ZnO. The kinetics constants (k1, k2, and k3) extracted from the modeling and the adsorption equilibrium constants are represented in Figure 7A

■

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Size-Controlled Pd Nanoparticles in 2-Butyne-1,4-diol Hydrogenation

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