Poison Tolerance to the Selective Hydrogenation of Cinnamaldehyde

Research Article pubs.acs.org/acscatalysis

Poison Tolerance to the Selective Hydrogenation of Cinnamaldehyde in Water over an Ordered Mesoporous Carbonaceous Composite Supported Pd Catalyst Shangjun Chen,† Li Meng,† Bingxu Chen,‡ Wenyao Chen,‡ Xuezhi Duan,‡ Xing Huang,§ Bingsen Zhang,∥ Haibin Fu,† and Ying Wan*,† †

Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials, and Department of Chemistry, Shanghai Normal University, Shanghai 200234, People’s Republic of China ‡ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China § Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4−6, 14195 Berlin, Germany ∥ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China S Supporting Information *

ABSTRACT: A coordination-assisted pyrolysis procedure was adopted to encapsulate palladium (Pd) nanoparticles in a mesoporous carbonaceous matrix. X-ray diffraction and transmission electron microscopy measurements revealed that approximately 2.5 nm nanoparticles were highly dispersed inside the well-ordered porous framework. High-resolution TEM and temperature-programmed hydride decomposition analysis demonstrated the formation of interstitial carbon in the Pd lattice. Diffuse reflectance infrared Fourier transform spectroscopy indicated that carbon species could be deposited on low-coordinated surface sites of the Pd particles. This catalyst exhibited high activity in the selective hydrogenation of cinnamaldehyde (CAL) at 80 °C under an H2 pressure of 1.0 MPa (turnover frequency (TOF) of 2.4 s−1) to produce hydrocinnamyl aldehyde with high selectivity (HCAL; approximately 80%) in water and could be reused eight times with no clear activity loss. A trapping agent poisoning experiment using solid SH-SBA-15 revealed unobvious leaching of Pd into the solution. Exposure to thiourea with a S:Pd ratio of 0.1 resulted in slight activity and undetectable selectivity losses over the current catalyst in the selective hydrogenation of CAL at 80 °C under an H2 pressure of 1.0 MPa. However, a 50% activity loss was observed for commercial Pd/C. Even after an increase in the thiourea concentration to a S:Pd ratio of 3, the TOF remained at 1.9 s−1 with a negligible effect on the HCAL selectivity. Nearly complete deactivation of Pd/C occurred upon high exposure to thiourea. DFT calculations showed that the presence of surface or subsurface carbon can enhance the poison tolerance of the encapsulated Pd catalysts. The enhanced hydrogenation activity and strong poison tolerance are consistent with the interpretation that Pd nanoparticles are modified by carbonaceous deposits. KEYWORDS: C-modified, Pd, hydrogenation, poison tolerance, thermal reduction important intermediate in the preparation of perfumes, flavors, and pharmaceuticals.4 Palladium on activated carbon (Pd/AC) has a long history as a catalyst and exhibits high activity in the selective hydrogenation of α,β-unsaturated aldehydes.5,6 Pd/AC catalyst can be recovered by filtration or centrifugation. However, one of the main drawbacks of noble metals is their strong interaction with sulfur-containing compounds, which are typical environmental contaminants. A decrease of several orders of magnitude

1. INTRODUCTION Heterogeneously catalyzed hydrogenation on noble metals is responsible for the formation and interconversion of some of the most basic molecules and has long been of industrial and academic interest.1 The chemoselective hydrogenation of α,βunsaturated ketones and aldehydes to saturated ketones/ aldehydes or unsaturated alcohols is a critical step in the production of valuable commodity intermediates used in pharmaceuticals, perfumes, and food.2 For example, supported palladium (Pd) nanoparticles are highly selective for the hydrogenation of α,β-unsaturated aldehydes, including hydrogenation of the olefinic group in cinnamaldehyde (CAL) to produce hydrocinnamaldehyde (HCAL), 3 which is an © 2017 American Chemical Society

Received: September 23, 2016 Revised: January 25, 2017 Published: February 2, 2017 2074

DOI: 10.1021/acscatal.6b02720 ACS Catal. 2017, 7, 2074−2087

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ACS Catalysis

mesopores, increasing the effective concentration of these compounds near the active site.28 Here, we could not exclude the involvement of water in the pores, which may behave differently under solvent-free conditions. Therefore, the catalytic activity can be enhanced over active centers, such as Pd.29,30 For example, postgrafting 3 nm Pd on a hybrid carbon (C)−silica support exhibited a high activity as a catalyst for the Heck coupling reaction of chlorobenzene and styrene in water.31 In this study, we encapsulated Pd nanoparticles (approximately 2.5 nm) inside an ordered mesoporous carbonaceous matrix via a coordination-assisted pyrolysis procedure. This Pd/ (SH)MSC (mesoporous silica−carbon composite) catalyst exhibited high activity in the selective hydrogenation of CAL at 80 °C under an H2 pressure of 1.0 MPa (TOF of 2.4 s−1) to produce HCAL with high selectivity (approximately 80%) in water. A trapping agent poisoning experiment using solid SHSBA-15 revealed unobvious leaching of Pd into the solution, confirming that the hydrogenation occurred inside the mesopores. Interestingly, exposure to thiourea at a S:Pd ratio of 0.1 resulted in slight activity and undetectable selectivity losses over the current Pd/(SH)MSC catalyst in the selective hydrogenation of CAL at 80 °C under an H2 pressure of 1.0 MPa. However, 50% activity loss was observed with commercial Pd/C. Higher exposure to thiourea (S:Pd of 3) led to a slight reduction in activity, with a negligible effect on the HCAL selectivity over Pd/(SH)MSC. However, this condition nearly completely quenched the Pd/C activity.

has been observed in olefin hydrogenation reactions in the presence of dibenzothiophene in the feeding gas over a silica− alumina-supported Pd catalyst.7 Pre-exposure of Pd/Al2O3 catalyst to H2S can completely quench the selective hydrogenation of epoxybutene.8 The S-adsorbed structure prevents the dissociative adsorption of hydrogen and poisons the catalyst via formation of subsurface S, which alters both the geometric and electronic structures of the surface.7,9 Thiourea may be able to tune the electronic properties of Pd and Au via backdonation between metal and adsorbed thiourea species.10,11 The introduction of thiourea to the Au/C-catalyzed aerobic oxidation of glucose in water almost completely quenches the reaction with a S:Au molar ratio as low as 0.05.10 Medlin and co-workers reported an improved selectivity for olefin hydrogenation in bifunctional molecules, such as 1-epoxy-3-butene, over Pd/Al2O3 coated with n-alkanethiol self-assembled monolayer (SAM), although the activity was substantially decreased.8,12,13 Very recently, the same authors found the coating of 1-adamantanethiol modifier could improve both the activity and selectivity for the hydrodeoxygenation of benzyl alcohol and the aerobic oxidation of trans-2-hexen-1-ol under low air pressure in comparison to the uncoated Pd catalyst, possibly due to the fact that the coating suppresses accumulation of carbonaceous deposits generated from the reaction.14,15 The development of effective catalysts for conversion processes involving S elements remains challenging. The second drawback to Pd/AC is the reduction in activity after several cycles (e.g., approximately 80% decrease after four cycles with CAL), which may be due to detrimental aggregation as well as detachment during catalysis on the Pd nanoparticles.16 The leaching of metal species into solution during reactions with heterogeneous catalysts leads to an ambiguous understanding of the nature of the catalysis as well as the catalyst contamination of the products. In some cases, the catalyst can be reused with a minimal level of residual Pd in the reactions (i.e., primarily for Pd/AC-catalyzed C−C coupling reactions), and the soluble molecular Pd species are assigned to true active centers.17 The dissolution of Pd in the initial period and redeposition on the solid occur after consumption of the reactants.17 As expected, complete cessation of catalytic activity was observed even after the addition of an adequate amount of S-containing solids, such as QuadraPure TU,18 resin-bound thiols,19 or solid thiol-containing SiO2.20 Encapsulation of the nanoparticles inside a solid porous matrix can result in stability under the reaction conditions.21 However, the intercalation of small Pd nanoparticles inside ordered mesoporous carbonaceous pore walls has been rarely reported. Finally, the hydrogenation of Pd nanoparticles is typically performed using organic solvents, such as dioxane,22 isopropyl alcohol,23 and THF.24 Polar solvents can produce a high HCAL yield. However, CAL is converted to the corresponding diacetal and ether, which results in separation problems due to chemical equilibria.5,25 In nonpolar solvents, no hemi- or diacetals are produced but the reaction rate is significantly lower. Acetalization in water may be inhibited. In addition, in comparison to organic solvents, water is relatively inexpensive, nontoxic, nonflammable, and renewable, providing the opportunity for safer and potentially more sustainable processes.26 One of the major drawbacks to the use of water as a solvent in organic chemistry is the poor water solubility of organic compounds.27 Spatial confinement in the sufficiently large mesopores and the hydrophobic nature of carbonaceous species favor the adsorption of organic compounds inside the

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous Carbonaceous Composite Supported Pd Catalysts. Synthesis of the Pd/(SH)MSC catalysts involved Pd−S coordination in a thiol-containing ordered mesoporous silicate-polymer (SH-MSC) matrix. The SH-MSC solids were prepared according to our previously published procedure.32 In a typical synthesis, 2.0 g of Pluronic F127 (poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), EO106PO70EO106, Mw = 12600, Acros Chemical Inc.) and 5.0 g of ethanol were mixed together to obtain a clear solution. Next, 6.7 mmol of tetraethyl orthosilicate (TEOS, 99%, Acros Chemical Inc.) and 3.33 mmol of 3-thiolpropyltrimethoxysilane (MPTMS, minimum 98 wt %, Acros Chemical Inc.) were prehydrolyzed in the presence of 1.0 g of HCl (1 M) and 5.0 g of ethanol for 30 min to form a mixture, which was subsequently added to the first solution, to which 0.8 g of a low-polymerized resol ethanolic solution (Supporting Information, SI) was added with stirring. The resulting mixture was poured into multiple dishes after 4 h. After evaporation of the ethanol in a hood at 40 °C for 6 h followed by thermopolymerization in an oven at 100 °C for 24 h, the as-prepared transparent films were scraped from the dishes. The composites were refluxed in sulfuric acid (typically, 1.0 g of the as-prepared composite per 100 mL of 48 wt % sulfuric acid) with mechanical stirring at 90 °C for 24 h twice. After filtration and washing with distilled water, the resulting SH-MSC materials were dried at 80 °C under vacuum overnight. Deionized water was used in all the experiments. All chemicals were obtained from the Shanghai Chemical Co. unless specified otherwise and were used as received without further purification. The supported Pd catalysts were synthesized using the following typical synthetic procedure: 3.5 mL of an aqueous solution of PdCl2 (2.0 wt %, 0.01 g/mL) was mixed with 1.0 g 2075


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mL/min) and then reduced at 250 °C in flowing H2 (10 vol % H2 in Ar, 50 mL/min) for 1.5 h. The sweeping gas was switched to a He flow (50 mL/min), and the time was 0.5 h. A cooling procedure to 30 °C was taken under a He stream. The Pd surface area, dispersion, and particle size were calculated from the H2 chemisorption data.36 All of the original Pd particles were assumed to be spherical, and the Pd:H adsorption stoichiometry was presumed to be 1:1. The temperature-programmed hydride decomposition (TPHD) experiments were performed on the same instrument as used for the H2 chemisorption analysis. A 0.05 g portion of the sample was first reduced under a 10% H2/Ar with a flow rate of 30 mL/min at a temperature of 130 °C for 1.5 h, and then the sample was cooled to 20 °C under the same H2/Ar flow and further flushed with He (50 mL/min) for 40 min. After that, it was heated to 150 °C at a temperature ramp of 5 °C/min under 10 vol % H2/Ar with the same flow rate of 30 mL/min.37 Thermal analysis (TG-DTA) measurements were performed on a Mettler-Toledo TG/SDTA 851e apparatus. Samples were heated from 25 to 800 °C at a rate of 10 °C/min in an air flow (50 mL/min). Temperature-programmed desorption (TPD) and temperature-programmed oxidation (TPO) measurements were made on a Cat-Lab instrument (BEL, Japan) coupled to a well-calibrated QIC-200 quadrupole mass spectrometer (InProcess Instruments, GAM 200). Prior to the TPD or TPO measurements, the sample (ca. 50 mg) was purged at 50 °C for 1 h with flowing dry Ar (40 mL/min, >99.999%), and the temperature was then increased to 800 °C at a rate of 10 °C/ min under flowing dry Ar or 20 vol % O2 in Ar (40 mL/min). The temperature was maintained at 800 °C for 20 min to record the signals of mass to charge (m/z) ratios of 18, 28, 44, and 64.38 In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of CO adsorption on the fresh Pd catalyst was performed using a PerkinElmer 100 spectrometer equipped with a modified Harricks Model HV-DR2 reaction cell. A gas mixture of 1.0 kPa of CO in He (20 mL/min, 30 min) was used for adsorbing CO. All of the spectral data are reported in Kubelka−Munk units. The background of the pristine support material was subtracted.39 The UV−vis diffuse reflectance spectra were obtained on a Varian Cary 300 Scan UV−vis spectrophotometer containing an integrating sphere attachment. BaSO4 was used as the background standard. 2.3. Catalytic Tests and Product Analysis. Hydrogenation reactions of CAL were conducted in a high-pressure 25 mL autoclave equipped with a Teflon tube (Parr). The autoclave was loaded with 5 mL of Milli-Q water, 0.5 mL of CAL (AR, Sigma-Aldrich), and 5 mg of catalyst (Pd:CAL molar ratio 1:2836). Before use, the catalyst was dried at 150 °C for 6 h in an oven. Then, the autoclave was purged with ∼0.5 MPa of H2 six times at room temperature, after which the H2 pressure was set to 1.0 MPa. Zero reaction time coincided with the autoclave reaching 80 °C, and the stirring rate (600 rpm) was not switched on. The autoclave was cooled quickly with an ice bath at the end of each reaction. Pd leaching and CAL conversion were determined by hot filtration at different time intervals. After each reaction, the solid catalyst was separated by filtration and further extracted with 50 mL of ethyl acetate in a micro-Soxhlet apparatus. The reaction solution was extracted with 10 mL of ethyl acetate. After washing with water, the separated catalysts were dried at 80 °C under vacuum overnight. The extracted solution was mixed, condensed to 10 mL, and analyzed. The C balance was maintained at a 5% difference. CAL hydrogenation was repeated three times with

of dry mesoporous SH-MSC carrier in an oscillator at 25 °C for 12 h (Pd/(SH)MSC-AS). High-temperature pyrolysis involved a temperature ramp of 2 °C/min from room temperature to a final temperature of 600 °C, which was maintained for 2 h under nitrogen. The final catalyst is referred to as Pd/ (SH)MSC and contained approximately 3.0 wt % Pd unless otherwise specified. An alternative liquid reduction involving the addition of 1.2 mL of an aqueous NaBH4 (0.1 M) solution was also adopted. After 30 min, the solid was separated and thoroughly washed with copious amounts of water, followed by drying at 80 °C under vacuum overnight. The resulting catalyst is referred to Pd/(SH)MSC-BH. For comparison, a Pd/MSC sample was synthesized in the absence of a thiol functional group. All of the procedures were the same as those employed for the synthesis of Pd/(SH)MSC, except that MPTMS was not added in the synthesis of the ordered mesoporous polymer matrix and the total SiO2 molar ratio in the synthetic batch was maintained. A supported Pd catalyst on the ordered mesoporous carbonaceous solid (P-Pd/MSC) was also prepared by postisochoric impregnation (the detailed synthesis procedure is described in the Supporting Information). The Pd content was approximately 3.0 wt %. A commercial Pd/C catalyst was purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. 2.2. Characterization. Transmission electron microscopy (TEM) images were recorded on a JEM 2100 microscope operating at 200 kV. A JEM 2100F microscope equipped with a field emission gun operating at 200 kV was used for scanning transmission electron microscopy−high angle annular dark field (STEM-HAADF) imaging. High-resolution transmission electron microscopy (HRTEM) images for Pd/(SH)MSC were acquired by using a JEOL JEM-ARM200F microscope. Scanning electron microscopy (SEM) observations for the macroscopic grain sizes of the catalyst were performed on a JEOL S4800 instrument. With a probe size of 0.2 nm, the inner and outer semiangles of the HAADF detector were 76 and 203 mrad, respectively. For each catalyst, at least 150 individual Pd nanoparticles were randomly counted to determine the size distribution. Energy dispersive X-ray spectroscopy (EDX) was performed using a Philips EDAX instrument. X-ray diffraction (XRD) measurements were made on a Rigaku D max-3C diffractometer using Cu Kα radiation (40 kV, 20 mA, λ = 0.15408 nm). The estimated metallic Pd sizes were calculated according to Scherrer’s formula as follows: size = 0.89λ/(β cos θ) on the basis of the (111) diffraction peak in the wide-angle XRD patterns.33 For Pd/(SH)MSC and Pd/MSC, this size estimate was rough with an error of ±8%. Inductively coupled plasma−atomic emission spectrometry (ICP-AES, Varian VISTA-MPX) was used to determine the Pd content. X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer PHI 5000CESCA system with a base pressure of 10−9 Torr. N2 adsorption−desorption isotherms were measured at 77 K with a Micromeritics TriStar II 3020 analyzer. The specific surface areas (SBET) were calculated according to the Brunauer−Emmett−Teller (BET) method.34 The BET areas were reproducible to within ±3%, and the mean values are reported in this study. The Barrett−Joyner−Halenda (BJH) model was utilized to calculate the pore volumes and pore size distributions.35 A Micromeritics Auto Chem II 2920 instrument was used for H2 chemisorption analysis by pulsing hydrogen at 30 °C on the catalyst. Prior to the measurement, the samples were heated at 200 °C for 0.5 h under a helium (He) flow (50 2076


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synthesis method for Pd catalyst

Pd content (wt %)

SBET (m2/g)

Vt (cm3/g)

Dp (nm)

dPda (nm)

dPdb (nm)

1.1Pd/(SH)MSC Pd/(SH)MSC 3.9Pd/(SH)MSC Pd/(SH)MSC-Rc Pd/MSC P-Pd/MSC Pd/C pristine SH-MSC

thermal reduction thermal reduction thermal reduction thermal reduction thermal reduction in the absence of thiol groups postimpregnation and reduction in H2 commercial

1.1 3.0 3.9 3.0 2.9 3.0 5.0

618 646 730 590 523 397 796 311

0.56 0.56 0.63 0.52 0.42 0.50 0.52 0.58

4.8 4.7 4.6 4.9 4.3 6.4 5.1 7.2

2.9 2.9 2.9 3.0 npd 3.3 5.5

2.5 2.5 2.6 2.7 npd 2.8 5.0

Particle size was calculated by H2 chemisorption. bParticle size was estimated from the TEM images. cPd/(SH)MSC catalyst after five catalytic cycles. dnp = not provided. a

Figure 1. (A, B) TEM and (C, D) HAADF-STEM images of the (A−C) fresh Pd/(SH)MSC and (D) reused Pd/(SH)MSC-R catalysts viewed along the (A, C, D) [110] and (B) [001] directions. The inset in (B) is the EDX pattern of the fresh Pd/(SH)MSC catalysts. The insets in (C) and (D) are the metal particle size distribution histograms on the basis of counting at least 150 nanoparticles.

the same initial rates, and the total reaction times were within ±8%. An Agilent 7890B gas chromatograph (GC) equipped with a DB-1 capillary column was used to identify and analyze the products by comparison to authentic standards. Each test was repeated at least three times, and the experimental errors were within ±5%. When necessary, the product identification was confirmed using an HP-6890 GC instrument equipped with an HP-5973 mass-selective detector. The catalytic results are reported in terms of the conversion of CAL, selectivity of HCAL, initial reaction rate (molecules of CAL converted per mole of Pd per minute), TOF (molecules of CAL converted per surface atom of Pd per second), and turnover number (TON, millimoles of CAL reacted per millimole of Pd). The TOF and TON were reproducible to within ±8%. The hydrogenation reactions for 3-methyl-2-butenal and 2-pentenal were also carried out, maintaining the same Pd:substrate ratio and reaction conditions.

A soluble Pd trapping experiment was performed according to the protocol reported by Jones and co-workers using thiol group modified mesoporous silica (SH-SBA-15).20 In a standard experiment, 19.7 mg of SH-SBA-15 was placed in the reaction flask prior to the addition of the reaction solution (S:Pd molar ratio 35:1). For the thiourea poisoning experiment, a thiourea solution (AR, Sigma-Aldrich), prepared by dissolving the proper amount of thiourea in Milli-Q water, was added as the solvent. In the standard experiments at 1.0 MPa, the reaction was performed for 0.4−1.5 h to ensure incomplete conversion of CAL for comparison to the activity of the poisoned catalyst.10 For the recycling study, CAL was hydrogenated using the same reaction conditions as previously described, except that the recovered catalyst was used. After each completion of the reaction, the catalyst was recovered by washing thoroughly with copious amounts of ethyl acetate and water. The recovered catalyst was further dried at 80 °C under vacuum overnight, 2077


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ACS Catalysis weighed, and reused. To ensure that the same catalyst amount was used in each cycle, several parallel reactions were also carried out at the same time. The Pd content of the catalyst was estimated in the first and last cycles. After each cycle, the aqueous solution was collected to determine the concentration of leached Pd.

Information) and were clearly highly exposed in the large mesopore space. Figure 2 depicts the HRTEM image of the Pd/(SH)MSC, presenting a very well crystallized solid. The measured lattice

3. RESULTS 3.1. Structural and Textural Properties of Supported Aggregation-Free Pd Nanoparticles over Ordered Mesoporous Silica-C. First, the thiol-functionalized mesoporous polymer materials were synthesized by solvent evaporation induced self-assembly (EISA). The small-angle XRD pattern of the thiol-containing composite contained one distinct diffraction peak and two minor peaks, which were assigned to the 10, 11, and 20 diffractions of the 2-D hexagonal mesostructure (Figure S1A in the Supporting Information). The cell parameter was calculated to be approximately 10.6 nm. After incorporation of Pd into the thiol-modified mesoporous solid and further pyrolysis, the mesostructure was maintained, on the basis of the continued presence of the major peak in the small-angle XRD patterns. The disappearance of the two minor peaks may have been due to the involvement of metals inside the mesopores, which reduces the contrast between the pore walls and space. Similar phenomena have been observed in mesoporous solids with metals, oxides, organic moieties, polymers, and C confined inside pores.40 For all of the supported Pd catalysts with a Pd content of less than 3.9 wt % (determined by ICP-AES), the wide-angle XRD patterns (Figure S1B) contained unresolved peaks, indicating the growth of relatively small Pd nanoparticles 29 or the involvement of amorphous Pd species.41 These results indicate the stability of the Pd nanoparticles resulting from the aggregation-free and ordered mesostructure obtained after high-temperature treatment. The diffuse diffraction peaks at 23° were due to amorphous C and silica, which compose the pore walls, as in the pristine mesoporous silica-C composite in the absence of Pd.32 On the basis of the H2 chemisorption measurements, the metallic Pd size was determined to be approximately 2.9 nm regardless of the studied Pd loading (Table 1). The TEM images of Pd/(SH)MSC show nanoparticles that are well dispersed in typical stripelike and hexagonally arranged pores, confirming the encapsulation of aggregation-free nanoparticles inside the 2-D hexagonal mesostructure (Figure 1). The HAADF-STEM image indicates that the Pd particles were highly distributed inside the matrix with an average size of approximately 2.5 nm. The nanoparticles were smaller than the pore size of the carrier, demonstrating the easily accessible metal sites in the pores. Large particles were not observed and were phase-separated with the ordered domains, indicating that the thermally stable Pd nanoparticles were confined within the carbonaceous matrix. The EDX pattern reveals that undetectable S remained in the sample, implying almost complete elimination of S upon heating. A decrease in the Pd loading to 1.1 wt % did not have a significant effect on the particle size distribution (Figure S2 in the Supporting Information). Nanoparticles with an average size of approximately 2.5 nm were observed inside the matrix. The TEM image of P-Pd/ MSC also shows well-ordered mesopores. Dispersed Pd nanoparticles with sizes of approximately 2.8 nm were observed inside the mesopores (Figure S3 in the Supporting

Figure 2. HRTEM image for Pd/(SH)MSC.

constants were 0.238 and 0.210 nm, assigned to the {111} and {200} planes of an fcc structure. The obvious lattice fringe expansion in comparison with that of bulk Pd demonstrates carbon incorporation into the palladium lattice.42 Carbon diffusion into the Pd lattice had been reported by sweeping hydrocarbons and CO over Pd nanoparticles with heating treatment, which involved deposition of a carbonaceous layer, followed by an activated diffusion of carbon atoms through the metal lattice.43,44 The TPHD curve of the commercial Pd/C catalysts shows the evolution of H2 at 64 °C, representing the decomposition of β-hydride species from the Pd lattice (Figure 3). Similar H2 evolution has been observed over Pd nanocrystals in a welldispersed Pd/carbon catalyst.45 The H2 evolution shifts to lower temperatures for P-Pd/MSC, which is possibly due to the reduction in metal nanoparticle size.45 Notably, the H2 evolution disappeared in the TPHD curve of Pd/(SH)MSC.

Figure 3. TPHD profiles of (a) Pd/C, (b) P-Pd/MSC, (c) Pd/ (SH)MSC, and (d, e) the Pd/(SH)MSC catalyst after reduction by 10% H2/Ar at 400 °C for (d) 1 h and (e) 2 h. 2078


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from 1.1 to 3.9 wt %. The TGA curve (Figure S5 in the Supporting Information) for Pd/(SH)MSC with a Pd content of 3 wt % shows a significant weight loss of 45 wt % in air between 240 and 620 °C due to the combustion of resins and organic moieties. The residues contain silica and palladium oxide. Therefore, the composition for Pd/(SH)MSC is roughly estimated to be Pd:silica:carbon = 3:52:45 in mass content. In addition, the C:H:O molar ratios in carbon ash upon elimination of SiO2 and Pd, as estimated by elemental analysis, is 10.3:1.9:1.0. Sulfur is undetectable. These results are in accordance with the XPS analysis. The DRIFT spectra of CO adsorption on the supported Pd catalysts are shown in Figure 5. The spectrum of the P-Pd/

This phenomenon further demonstrates the carbon deposits on subsurface octahedral sites in Pd/(SH)MSC. The formation of β-hydride species at the same site is blocked. It has been reported that the interstitial carbon is metastable and decomposes at elevated temperature in H2.46 An extremely weak H2 evolution can be observed over the Pd/(SH)MSC catalyst with a further treatment at 400 °C in H2 for 1 h. With a prolongation of the heating time for Pd/(SH)MSC, the H2 evolution peak becomes strong, indicative of the removal of interstitial carbon. Only Pd, C, O, and Si were detected in the XPS spectrum of Pd/(SH)MSC, which confirms the absence of contamination from Cl and S (Figure 4 and Figure S4 in the Supporting

Figure 5. CO−DRIFT spectra of (a) postsynthesized P-Pd/MSC and (b) in situ reduced Pd/(SH)MSC catalysts. Figure 4. XPS spectra in the Pd 3d region of the (a) fresh Pd/ (SH)MSC, (b) reused Pd/(SH)MSC-R, (c) P-Pd/MSC, and (d) asprepared Pd/(SH)MSC catalysts. The colored areas correspond to Pd 3d3/2 and 3d5/2 regions for Pd(0) (green), Pd−O (orange), and Pd(II)−thiolate complexes (blue).

MSC catalyst is dominated by an intense peak centered at approximately 1948 cm−1, which was assigned to the Pd sites on the edges and corners of the particles and to the (100) facets to a lesser extent.49,50 The tail features in the range of 1910 and 1820 cm−1 were associated with the regular (111) facets.51,52 However, the intensities of the IR signals cannot be directly used to indicate the relative abundance of the corresponding sites due to dipole coupling effects.49 For Pd/(SH)MSC synthesized by the carbothermal process, the intense spectral feature was unresolved, and the feature in the low-frequency region persisted. Additionally, a new peak at 2087 cm−1 appeared, potentially due to on-top adsorption sites. Similar phenomena were observed for a C-precovered Pd/Fe3O4 catalyst on which C species were deposited during the TPD of alkenes.49 Therefore, the changes in the CO adsorption over Pd/(SH)MSC may also indicate that C preferentially deposited on the edges, corners, and (100) facets and less contaminated terraces.49,53 The studied nPd/(SH)MSC catalysts exhibited type IV N2 sorption isotherms with distinct capillary condensation steps at relative pressures of 0.4−0.65, which are typical of mesoporous solids with uniform pore sizes (Figure 6). An H2-type hysteresis loop was observed rather than an H1 loop for the pristine mesoporous silica-C carrier,54 which is consistent with that observed for titania-incorporated silica containing mesoporous carbonaceous materials. This result suggests that the pores were approximately cylindrically shaped due to the presence of the Pd nanoparticles.30 The pore size distribution curves calculated by the BJH method for the mesoporous C

Information). The estimated mass percentages of Pd, C, Si, and O are 2.9%, 44.4%, 22.4%, and 30.3%. The Pd 3d3/2 and 3d5/2 peaks in the XPS spectrum of Pd/(SH)MSC were fitted using a model consisting of two signals, and the signals corresponding to Pd(0) were located at 340.9 and 335.6 eV. In addition, signals corresponding to Pd−O were located at 342.6 and 337.4 eV (Figure 4).47 Similar phenomena have been observed for Pd supported on mesoporous CoO-C catalysts. Highly dispersed Pd clusters with low coordination interact with the CoOcontaining interface and form Pd−O bonds rather than Pd oxides.30 In our case, the surface Pd−O bonds were also due to the coordinative unsaturated surface Pd in the nanoclusters. Therefore, the Pd nanoparticles were reduced in situ during the carbothermal process. In comparison, the XPS results for the postimpregnated P-Pd/MSC catalyst contained predominantly Pd(0) signals in the Pd 3d level of the XPS spectrum, along with the Pd−O signals. The contents for the composite catalysts have been analyzed by a combination of ICP, TG, and elemental analysis. The silica component in the catalyst was etched by HF solution. After thorough washing and drying, the carbon support was burned in a crucible and the remaining metals could be dissolved in aqua regia.48 The Pd mass contents estimated by ICP-AES are shown in Table 1, ranging 2079


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the current reaction conditions. To ensure a kinetic regime, three Pd/(SH)MSC catalysts with Pd loadings of 1.1, 3.0, and 3.9 wt % but similar Pd dispersions were used in the Madon− Boudart test (Figure S10 in the Supporting Information).57 A plot of the rate as a function of the surface concentration yielded a straight line, demonstrating that the mass transfer effect was absent. The easy accessibility of the Pd nanoparticles intercalated into the framework was related to the sufficiently large mesopores that facilitated mass transport in the pores.58 Figure 7A shows the activity and selectivity data of Pd/ (SH)MSC in this reaction. The CAL consumption increased

Figure 6. (A) N2 sorption isotherms and (B) pore size distribution curves of fresh and used supported Pd catalysts with different Pd contents: (a) 1.1Pd/(SH)MSC, (b) Pd/(SH)MSC, (c) 3.9Pd/ (SH)MSC, and (d) reused Pd/(SH)MSC-R.

composite supported Pd catalyst exhibit narrow peaks centered at approximately 4.7 nm, which confirms the ordered mesostructure and is consistent with the XRD and TEM results. The catalysts possessed high surface areas (approximately 650 m 2/g, Table 1) and large pore volumes (approximately 0.55 cm3/g). The micropores contributed 10% to the total pore volume. These results indicate the presence of open uniform mesopores and a sufficiently large space for adsorption. The Pd contents show a minor effect on the textural properties (Table 1). For comparison, a P-Pd/MSC catalyst was synthesized by post-treatment. This catalyst possessed tiny Pd nanoparticles with a size of approximately 3.3 nm, estimated by H2 chemisorption (Table 1). N2 sorption isotherms revealed large uniform mesopores 6.4 nm in diameter, a moderately high surface area of 397 m2/g, and a mesopore volume of 0.50 cm3/ g, as well as undetected micropores, similar to the case for the pristine MSC carrier (Figure S6 in the Supporting Information).31 Therefore, the increase in BET surface area and reduction in pore diameter of the Pd/(SH)MSC catalysts may have been due to the carbonization of the matrix catalyzed by the Pd nanoparticles. Catalytic pyrolysis enhances the release of small molecules, such as CO, CH4, and CO2, which results in the generation of micropores and shrinkage of the carbonaceous framework.55 A commercial Pd/C catalyst was also used. The Pd nanoparticle size, estimated from the H2 chemisorption, is approximately 5.5 nm (Table 1), similar to the TEM images (5.0 nm in statistics, Figure S7 in the Supporting Information). This catalyst possesses a high surface area of 796 m2/g. The pore size distribution is wide but shows a high proportion of mesopores with a most probable distribution at 5.1 nm (Figure S8 in the Supporting Information). All studied Pd-containing catalysts are powdered catalysts with a bulk size of approximately 5−40 μm (Figure S9 in the Supporting Information). 3.2. Selective Hydrogenation of CAL in Water. The hydrogenation of CAL could occur at either the CC or C O bond or at both the CC and CO bonds to afford HCAL, cinnamyl alcohol (COL), or hydrocinnamyl alcohol (HCOL).56 Preliminary experiments performed using various stirring speeds (500−1000 rpm) indicated that the selected stirring speed (600 rpm) enabled the reaction over Pd/ (SH)MSC to proceed in the absence of diffusion limits under

Figure 7. Time course plots for (A) the conversion of CAL (−■−, black) and selectivity of HCAL (−●−, red) and HCOL (−▲−, blue) and (B) the conversion of 3-methyl-2-butenal (−□−, black) and 2pentenal (−◇−, red) with the following reaction conditions: catalyst (5 mg Pd/(SH)MSC); substrate (4.0 mmol); water (5 mL); temperature (80 °C); H2 pressure (1.0 MPa).

substantially and reached nearly 100% at 2 h over Pd/ (SH)MSC at 80 °C under an H2 pressure of 1.0 MPa. The Pd activity at 80 °C under an H2 pressure of 1.0 MPa according to the CAL consumption (TOF) was calculated to be 2.4 s−1. CAL-selective hydrogenation was also observed over other Pdcontaining solid catalysts, such as Pd/polymer support,24 Pd/ AC,59 Pd/MWCNT,60 and Pd/hybrid TiO2.61 Almost all of these cases employed organic solvents (Table S1 in the Supporting Information). In a polar solvent, such as 2-propanol or ethanol, the hydrogenation rate is always much higher than that in a nonpolar solvent. However, as previously mentioned, the hydrogenation of CAL to HCAL in ethanol produces a significant amount of byproduct (i.e., HCAL diethyl acetal) that is in equilibrium with hydrocinnamaldehyde.6,25 Therefore, these two compounds are often reported together in reactivity studies. The Pd/(SH)MSC catalyst can convert CAL in water. As a result, a high reaction rate along with undetectable byproducts (i.e., HCAL diethyl acetal) was achieved.62 Unsaturated alcohol COL was not detected under the present conditions, consistent with previous results for Pd/C catalysts demonstrating high selectivity toward the saturated aldehyde in liquid-phase CAL hydrogenation.59 The results 2080


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ACS Catalysis from density functional theory (DFT) calculations indicate that hydrogenation of CC bonds over Pt(111) is significantly easier than hydrogenation of its ketone or aldehyde CO bonds in unsaturated ketones or aldehydes.63 The barrier to the subsequent hydrogenation of the alkyl intermediate to form the saturated alcohol was slightly lower than the barrier to hydrogenation of the CO bond.64 The hydrogenation of CO in an unsaturated aldehyde is not favored, and unsaturated alcohols are predicted to be very minor products. The selectivity for HCOL was initially 35% but decreased to approximately 20% in the final product distribution. This change in selectivity may be due to selective blockage of the active sites by CO generated during the decarbonylation of CAL, which was shown to deactivate this reaction over model Pt nanoparticle catalysts,65 and benzyl alcohol hydrodeoxygenation over Pd catalysts.14 Postimpregnated P-Pd/MSC, commercial Pd/C, and homogeneous Pd(OAc)2 have also been employed as catalysts under the same conditions as above. For rigorous comparison of the catalytic activity of Pd in the supported Pd catalysts, the CAL conversion level was restricted to approximately 20% by adjusting the duration of the catalytic reaction. The TOF values exhibited the following order: Pd/(SH)MSC > P-Pd/MSC ≈ Pd/C > Pd(OAc)2 (Table 2). The order of the selectivity to

Table 3. Catalytic Performance of Various Pd Catalysts for the Selective Hydrogenation of α,β-Unsaturated Aldehydes to Saturated Aldehydesa catalyst

t (h)

conversnb (%)

selb (%)

TOF (s−1)

3-methyl-2butenal

Pd/(SH)MSC

0.167

26

99

4.0

2-pentenal

P-Pd/MSC Pd/C Pd/(SH)MSC P-Pd/MSC Pd/C

0.167 0.167 0.083 0.083 0.167

19 11 18 14 17

98 97 99 99 99

3.2 3.1 5.6 4.8 4.7

substrate

General reaction conditions: catalyst (1.41 × 10−3 mmol of Pd); unsaturated aldehydes (4.0 mmol); water (5 mL); temperature (80 °C); H2 pressure (1.0 MPa). bDetermined by GC. a

CC bonds in both 3-methyl-2-butenal and 2-pentenal and a relatively weak binding energy for the saturated aldehydes on the Pd surface may favor the formation and desorption of saturated aldehyde. The hydrogenation of CO bonds to saturated alcohol is inhibited. The recyclability of the Pd/(SH)MSC catalyst is shown in Figure 8. No obvious changes were detected in the initial

Table 2. Catalytic Performance of Various Pd Catalysts in the Selective Hydrogenation of CALa selb (%) b

entry

catalyst

t (h)

conversn (%)

HCAL

HCOL

TOF (s−1)

1 2 3 4

Pd/(SH)MSC P-Pd/MSC Pd/C Pd(OAc)2

0.25 0.5 0.5 1.0

23 23 14 20

65 52 67 87

35 48 33 13

2.4 1.3 1.3 0.2

a General reaction conditions: catalyst (1.41 × 10−3 mmol Pd); CAL (0.5 mL, 4.0 mmol); water (5 mL); temperature (80 °C); H2 pressure (1.0 MPa). bDetermined by GC.

HCAL was as follows: Pd(OAc)2 > Pd/C ≈ Pd/(SH)MSC > P-Pd/MSC (Table 2). The homogeneous Pd(OAc)2 catalyst displayed a low TOF value. These results suggest that the solid catalysts behave at least somewhat differently from the homogeneous catalyst. The hydrogenation of other α,β-unsaturated aldehydes, including 3-methyl-2-butenal and 2-pentenal, was also tested (Figure 7B and Table 3). The studied solid catalysts exhibit enhanced reaction rates and almost complete yields of saturated aldehydes. The TOF values for Pd/(SH)MSC in the hydrogenation of 3-methyl-2-butenal and 2-pentenal at 80 °C under an H2 pressure of 1.0 MPa reached 4.0 and 5.6 s−1, respectively. Davis and co-workers observed a distinct steric effect of the hydrogenation of CC bonds on the selectivity by substituting the CC group in α,β-unsaturated aldehydes and ketones over Pd/C.66 Although the relative CC to CO hydrogenation selectivity trend cannot be changed, the presence of a large phenyl group at the CC bond in CAL decreases the relative hydrogenation rate of the olefinic bond in comparison with the methyl-substituted CC bond in crotonaldehyde. However, the primary product HCAL may be strongly bound to the surface because of the presence of the phenyl group. Further hydrogenation to a saturated alcohol possibly occurs. As a result, the enhanced hydrogenation of

Figure 8. Reusability in terms of (A) the initial reaction rate and selectivity to produce HCAL at 10 min and (B) the conversion and selectivity to produce HCAL at 2 h for the Pd/(SH)MSC catalyst in the selective hydrogenation of CAL at 80 °C under a H2 pressure of 1.0 MPa.

reaction rates or selectivity to produce HCAL after eight cycles at 80 °C under an H2 pressure of 1.0 MPa. In addition, the complete consumption of CAL after 2 h and the high selectivity to produce HCAL were retained in successive cycles. Therefore, the reaction rate and conversion of the reused catalyst were maintained. The mixture was collected and analyzed by ICP-AES. Some metal leaching did occur during repeated reactions over the same catalyst. After five cycles, the catalyst was characterized. The TEM images were similar to those of the fresh catalyst (i.e., highly dispersed nanoparticles 2081


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ACS Catalysis inside hexagonally arranged pores) (Figure 1), and the average particle sizes were similar. Aggregation of large nanoparticles was not observed, indicating that the heterogeneous catalyst was stable under the reaction conditions. The Pd particle size estimated from H2 chemisorption was approximately 3.0 nm, which is comparable to that measured for the fresh catalyst. The XPS spectrum of the reused catalyst also contained two peaks in the 3d5/2 region, indicating that the coordinatively unsaturated Pd species at the surface and metallic Pd species were retained. The N2 sorption isotherms further confirmed the stability of the catalyst (Figure 6). The uniform mesopores were retained. However, the BET surface area and pore volume decreased slightly due to organic residues inside the mesopores. These results suggest that the Pd/(SH)MSC catalyst is stable and reusable. 3.3. Influence of S-Containing Compounds on Catalytic Performance. The solid trapping agent, thiolcontaining mesoporous silica SH-SBA-15 (Figure S11 in the Supporting Information), was added to the reaction batch to trap the soluble, leached Pd species.20 Near quenching of catalysis by soluble Pd acetate was observed upon addition of the poison in the selective hydrogenation of CAL at 80 °C under an H2 pressure of 1.0 MPa, demonstrating the successful capture of soluble Pd species by SH-SBA-15 (S:Pd = 35). When Pd/C was used as the catalyst, a nearly 55% reduction in the conversion was detected in the presence of SH-SBA-15 (Figure 9A). This phenomenon suggests that soluble Pd species were leached from Pd/C and contribute to the catalytic activity. However, it is important to note that the C carrier has a predominant effect on the dispersion and stability of Pd metal. A suitable carrier with certain chemical properties may stabilize Pd. In contrast, an indistinct change in the solid poison was observed for both the Pd/(SH)MSC and P-Pd/MSC catalysts. Therefore, the activity of these two catalysts did not originate from the soluble Pd in the liquid phase.20 Additionally, thiourea was used as a poisoning agent. Figure 9B shows the normalized conversion of CAL and the selectivity to produce HCAL as a function of the thiourea concentration at 80 °C under an H2 pressure of 1.0 MPa (to maintain a conversion of approximately 50% in the neat reaction, a different reaction time was adopted). The conversion for the hydrogenation of CAL over Pd/(SH)MSC was preserved when the reaction solution contained thiourea with a S:Pd ratio of 0.01. A slight reduction in conversion to approximately 95% occurred with further increase in the thiourea concentration (i.e., S:Pd = 0.1). Even at a high thiourea concentration corresponding to a S:Pd ratio of 3, 80% conversion was maintained. Importantly, the selectivity to produce HCAL remained nearly unchanged in the absence and presence of thiourea. The activity of P-Pd/MSC decreased to approximately 87% of the initial conversion in the presence of thiourea with a S:Pd ratio of 0.01, to 72% with a S:Pd ratio of 0.02, and to 60% with a S:Pd ratio of 3. The selectivity simultaneously decreased from 60% to 41% and then remained nearly constant. The commercial Pd/C catalyst underwent continuous poisoning due to the presence of thiourea. Nearly complete deactivation of Pd/C occurred with high exposure. At the same time, the selectivity to produce HCAL gradually increased as the thiourea concentration increased. For example, the HCAL selectivity substantially increased to >99% upon addition of thiourea with S:Pd = 0.1. Similar behaviors for these three catalysts were also observed in the case of 3-methyl-2-butenal hydrogenation in

Figure 9. (A) Comparison of the conversion of CAL at 80 °C under a H2 pressure of 1.0 MPa in the absence (solid) and presence of SHSBA-15 (shaded) over homogeneous Pd(OAc)2 (green) and supported Pd catalysts Pd/(SH)MSC (red), P-Pd/MSC (blue), and commercial Pd/C (black). (B) Comparison of the normalized conversion of CAL (solid lines and solid symbols) and selectivity to HCAL (dashed lines and open symbols) in the presence of thiourea over Pd/(SH)MSC (red), P-Pd/MSC (blue), and commercial Pd/C (black). The reaction time in (A) was 2 h to obtain a conversion higher than 80%, and in (B), the reaction time was 0.4−1.5 h to maintain a conversion of approximately 50% in the neat reaction.

the presence of thiourea with an S:Pd ratio of 0.1. Decreases of 5%, 21%, and 40% in conversion occur for Pd/(SH)MSC, PPd/MSC, and commercial Pd/C, respectively (Figure S12 in the Supporting Information). These poisoning behaviors imply a different surface nature of the active sites of the supported catalysts.

4. DISCUSSION 4.1. Coordination-Assisted Pyrolysis To Stabilize Pd Nanoparticles with High Metal Dispersion. The binding energy of S 2p in the XPS spectrum for the as-prepared Pd/ (SH)-MSC synthesized in the presence of thiols could not be fit to a single doublet using an approximate 2:1 peak area ratio and 1.2 eV splitting (Figure S4B in the Supporting Information). Two S species are detected for organic thiols that self-assemble as a monolayer onto gold surfaces (i.e., thiols bound and unbound to gold).67 In comparison to the SH-MSC carrier, these species were assigned to bound Pd(II)−thiolate (S 2p3/2 binding energy of 162.0 eV) and unbound thiol (S 2p3/2 binding energy of 163.3 eV).68 The ratio of bound to unbound thiol was estimated to be approximately 1:14. A significant shift toward higher binding energies was observed for Pd 3d3/2 and 3d5/2 (i.e., 342.9 and 337.6 eV, respectively), in 2082


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coordination with thiol moieties were easily reduced to metallic clusters. During a high-temperature heat treatment, resins carbonize and numerous small molecules are continuously released, and a relatively “rigid” interpenetrated framework with an ordered mesostructure was formed by C and silicate.74 Simultaneously, the protection of Pd species by the Scontaining group hinders the decomposition or reduction of Pd. Pd species can decompose or be reduced to a metallic state when the thiol moieties are eliminated. The growth of Pd can be restricted by the matrix, and particles that were approximately 2.5 nm in size homogeneously intercalated into the matrix. The simultaneous formation of a relatively rigid carbonaceous framework and the Pd nanoparticles favors the immobilization of semiexposed Pd onto the pore walls. 4.2. Modification of Surface Sites by Carbonaceous Deposits on Pd Nanoparticles To Improve the Catalytic Activity and Stability. The current Pd/(SH)MSC catalyst exhibited high activity in the conversion of CAL at 80 °C under an H2 pressure of 1.0 MPa in water. The TOF was 1.8 times higher than those of the P-Pd/MSC catalyst with a particle size of 3.3 nm (dispersion of Pd nanoparticles DPd = 0.28) and a commercial Pd/C catalyst with a particle size of 5.0 nm (DPd = 0.17) and either much higher or comparable to literature values (Table 2 and Table S1 in the Supporting Information).24,66 Only HCAL and HCOL were detected in the absence of HCAL diethyl acetal and COL byproducts. Similar suppression of the formation of COL was observed on supported Pd catalysts. This phenomenon is primarily related to their activity in binding and hydrogenating olefins.75 Therefore, mechanistic studies of olefin hydrogenation can provide insight into the activity enhancement of Pd/(SH)MSC. Both the chemisorption results and TEM images demonstrated small Pd particles in Pd/(SH)MSC. A lattice expansion for Pd nanocrystals as evidenced by HRTEM implied the presence of interstitial carbon.43 The presence of interstitial carbon had also been proved by the blockage of the formation of β-PdH.45 The CO adsorption analysis indicated that the edge or corner sites of the Pd nanoparticles in the Pd/ (SH)MSC catalyst were predominantly modified and the regular (111) terraces were significantly less occupied.49 The typical surface and subsurface C-modified Pd surface models (the terraced Pd(111) surface and the stepped Pd(211) surface) were also constructed by DFT calculations (the detailed DFT calculations and parameters of the Pd surfaces are given in Table S2 in the Supporting Information) (Figures S15 and S16 in the Supporting Information). The surface C prefers to locate at the 3-fold hollow hcp site of Pd(111) and the 4-fold site (B5 site) of Pd(211), while the subsurface C prefers to locate at the octahedral subsurface sites of both Pd surfaces. These results are in good agreement with previous studies.76 As discussed above, Pd/(SH)MSC was synthesized via in situ carbonization and reduction. Decomposed carbonaceous species from the polymeric resins may have deposited on the Pd surface and contaminated the surface. At higher temperatures, even the CO bond in CO is possibly broken and deposited on Pd.43 The carbon pool on the surface is available for diffusion into the bulk.43 As a result, the current catalyst possibly consisted of small Pd clusters with low coordination and C modification in surface and subsurface, similar to the C deposition that occurs during olefin hydrogenation.77 In comparison, the supported Pd catalysts synthesized by postimpregnation consisted of Pd nanoparticles with exposed edge and terrace sites. Although C deposits are considered site

comparison to those of PdCl2, which indicates the formation of Pd(II)−thiolate complexes (Figure 4).69 The optical spectrum of the as-prepared Pd/(SH)MSC was similar to that of the pristine Pd-free composite. The two typical absorbance peaks at ca. 350 and 400 nm (Figure S13 in the Supporting Information) of S-containing Pd(II) complexes, which correspond to metal to ligand and ligand to metal chargetransfer bands, respectively, overlapped with the characteristic absorption of the polymeric framework.70 Even when a reducing agent (i.e., sodium borohydride) was added to the as-prepared Pd/(SH)MSC, the UV−vis spectrum did not change, indicating inhibition of the reduction of Pd(II) species to Pd(0). A similar phenomenon was observed for monolayerprotected Pd clusters. Attempts to form Pd NPs starting from thiol:Pd molar ratios higher than 1:1 have failed.71 The TPD-MS and TPO-MS spectra of the as-prepared Pd/ (SH)MSC were recorded to identify the molecules released during carbonization and combustion (Figure S14 in the Supporting Information). Four molecules (i.e., CO, CO2, H2O, and SO2) were detected during heating regardless of the introduction of an oxygen feed. However, the released quantity of CO and CO2 as well as the temperature range differed with/ without O2. Combustion in O2 yielded a large amount of CO until approximately 700 °C, with a major peak at approximately 350 °C, whereas desorption under inert gas produced a relatively higher amount of CO2 until 800 °C and above, with three peaks centered at approximately 350, 515, and 670 °C. The much stronger peak intensity in TPO-MS in comparison to the TPD procedure was possibly due to the complete combustion of the carbonaceous framework in the former and the yield of the residual C matrix in the latter. In particular, S can be fully oxidized in an O2 feed between 325 and 400 °C, with one major peak during TPO. In the presence of an inert gas, three peaks were observed for SO2, similar to the spectra of CO and CO2. Almost complete elimination of S was achieved with prolonged desorption at 800 °C. These results indicate that carbonization of the S-containing, as-prepared Pd/ (SH)MSC polymeric framework leads to the continuous release of CO, CO2, H2O, and SO2 and that S can only be released above 325 °C. Because the stable polymeric framework can form at 350 °C,72 the Pd−S bond can only break down, and in turn, the Pd nanoparticles are reduced within a relatively rigid framework. Therefore, the nonagglomerated Pd nanoparticles with high metal dispersion can be encapsulated into the ordered mesoporous silica-C matrix by coordinationprotected pyrolysis. For comparison, thiol-free mesostructured silica-C was also synthesized for Pd loading. The UV−vis spectrum of the asprepared Pd/MSC contains a red shift in comparison to that of the as-prepared Pd/(SH)MSC, most likely due to the surface plasmon resonance (SPR) of the Pd nanocrystals (Figure S13 in the Supporting Information).73 The XRD pattern of Pd/ MSC exhibits resolved fcc Pd peaks (Figure S1 in the Supporting Information) after pyrolysis at 600 °C, indicating that large particles had aggregated during pyrolysis. Therefore, the coordination between Pd and S may have been due to stabilization of the highly dispersed, lowcoordinated nanoparticles in the carbonaceous framework upon carbonization. The coordination of the −SH group with Pd ions was most likely located either on the pore surface or inside the carbonaceous silicate walls. The mobility and reactive ability may have been inhibited by the formation of a very stable Pd−S complex. In comparison, the Pd ions that lacked 2083


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ACS Catalysis blockers through coke formation,78 the C atoms have been found to be beneficial and lead to enhanced catalyst performance (e.g., olefin hydrogenation) on Pd nanoparticles. Due to the distinctly higher flexibility of the lattice of the nanoparticles, the interior of the particle possibly participates in hydrogenation via hydrogen diffusion, different from that of the bulk material, and in turn, the activity of CAL hydrogenation to HCAL is enhanced to some extent.79 From the viewpoint of selectivity, it has been reported that the (111) orientation of Pd favors simultaneous adsorption on the CC and CO bonds of the CAL molecule with a planar η4 mode,80,81 which could lead to the formation of HCOL as a primary product over Pd/ MSC. On the other hand, the modifiers on the Pd surface may weaken the interaction of the phenyl ring with the surface, hinder decarbonylation of aldehyde in benzyl alcohol hydrodeoxygenation, and in turn enhance the selectivity and activity.14 The presence of interstitial carbon may alter the adsorption model of CAL and/or suppress the decarbonylation of aldehyde to dimmers or oligomers. As a result, the selectivity to HCAL is enhanced. This hypothesis deserves further investigation. The undetectable activity and selectivity losses and Pd leaching over Pd/(SH)MSC during eight catalytic cycles demonstrated the high stability and reusability of this catalyst. The initial reaction rate was maintained, and the cumulative TON in successive cycles was estimated to be 22700. Both intercalation into the C pore walls and C modification may have contributed to the stability of the low-coordinated Pd species. 4.3. Poison Tolerance of S-Containing Solids and Liquids. Solid SH-SBA-15 with a S:Pd ratio of 35 was reported to be an effective and selective poison of homogeneous Pd species.20 Once the thiol concentration is sufficiently high, rapid overcoordination between neighboring S-containing groups and Pd can occur to ensure capture of palladium in solution and avoid release. In our study, CAL conversion in the presence of SH-SBA-15 substantially decreased over commercial Pd/C at 80 °C under an H2 pressure of 1.0 MPa, indicating Pd leaching and low stability. The conversion over Pd/(SH)MSC and PPd/MSC were similar, demonstrating negligible Pd leaching into the solution and tolerance of the solid poison.20 The S-containing compounds in the feed gas or solution have a substantial effect on the catalytic performance of both homogeneous and heterogeneous catalysts. Both the geometric and electronic structures of Pd and Au metals are strongly modified by poisoning agents.7 For example, the desulfurization of dibenzothiophene (DBT) in tetralin hydrogenation feed led to poisoning of the metal sites by the adsorbed S (S:Pd = 3.5:1 in the presence of quinoline in the feed gas).82 The layer of S hinders the adsorption of aromatic molecules, which results in very low activity (several-fold decreases).7 Thiourea can poison hydrogen adsorption. The CS bond is also reactive in the reduction of thiourea:83

The presence of thiourea (S:Pd = 0.1) in the reaction batch resulted in 5%, 30%, and 50% decreases in conversion for Pd/ (SH)MSC, P-Pd/MSC, and commercial Pd/C, respectively, in the CAL hydrogenation at 80 °C under an H2 pressure of 1.0 MPa (similar incomplete conversions of 50% for CAL over pristine supported catalysts were observed). The decreases are 5%, 21%, and 40%, respectively, in the conversion of 3-methyl2-butenal for the above three catalysts under the same conditions. Notably, the selectivity to produce HCAL was unchanged over Pd/(SH)MSC with a poison-containing feed. However, this selectivity substantially increased from 60 to >99% over commercial Pd/C and decreased from 60% to 41% over P-Pd/MSC. Further exposure to thiourea (S:Pd = 3) led to a decrease in the normalized conversion (i.e., 20%, 42%, and 95%) for Pd/(SH)MSC, P-Pd/MSC, and Pd/C, respectively. The supported Pd/C catalyst lost most of its activity due to both the capture of leached Pd by thiourea and the strong adsorption of S to the edge and corner sites, which are active sites. However, metal−thiourea (ligand) interactions, especially for homogeneous species, may be responsible for the enhanced selectivity to produce HCAL. Similar selectivity improvement had also been reported for hydrogenation of α,β-unsaturated ketones: for example, chalcone to dihydrochalcone over Pd/ C.85 After exposure to thiourea, Pd−S bonds could form on the metal surface in P-Pd/MSC due to the fact that thiourea could form a chemisorbed layer of S atoms on the Pd catalyst.83,86 DFT calculations were carried out to investigate the sulfur adsorption on Pd surfaces. The preferential sites of the S adsorption are the 3-fold hollow fcc site of Pd(111) (Figure S17a,b in the Supporting Information) and the 4-fold site (B5 site) of Pd(211) (Figure S18a,b in the Supporting Information), which are in agreement with previous studies.87 The adsorption energies of sulfur on Pd(111) and Pd(211) with respect to atomic S are −4.93 and −5.41 eV, respectively (Table S3 in the Supporting Information). This result is in accordance with the fact that the S−surface interaction is expected to be stronger for coordinatively unsaturated Pd atoms at corner/step sites, in comparison with coordinatively saturated surfaces. 88 Once the corner/edge sites were contaminated by S in P-Pd/MSC, the activity decreased substantially. This result is supported by the calculation that the CC moieties in 2-methyl-3-buten-2-ol are typically being activated by low-coordination sites of Pd clusters in aqueous solution.89 Simultaneously, the HCAL selectivity gradually decreases. The strong poison tolerance of Pd/(SH)MSC is consistent with the interpretation that Pd nanoparticles are modified by carbonaceous deposits. Medlin and co-workers reported that the carbon deposition predominantly affects the rate of ethylene hydrogenation in the case of surface crowding of Pd/Al2O3 by 1-octadecanethiol and 1-adamantanethiol.90 Under Pd surface-crowding conditions where significant coking occurs, the carbon species produce a surface environment with a unique chemical character that favors the adsorption of reactants. Because the reactants produce a layer, the difference between the unmodified and modified surfaces by thiol groups is less distinguishable with respect to reactant adsorption. As a result, the presence of the thiol-coated surface insignificantly affects acetylene hydrogenation over the supported Pd catalyst.90 Therefore, in the present case, the precoverage of carbon on the active center provides an environment that may facilitate reactant adsorption.

M−thioureaads + 4M−Hads → H 2S + H 2C(NH 2)2 + M

The products are methylenediamine and H2S, which are strong poisons. H2S can remain adsorbed, especially because it arises directly from the center (i.e., CS) to which thiourea is adsorbed. The released free gas, such as H2S, would be expelled into the H2 atmosphere. As a result, the site blocking and electronic effect poisoning of the active edge sites of Pd were dominant.84 2084


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ACS Catalysis To understand the enhanced poison resistance of the Pd/ (SH)MSC catalyst, the S adsorption was also calculated on the C-modified Pd surfaces. The most stable adsorption configurations of S on the two C-modified Pd surfaces are shown in Figures S17c,e and S18c,e in the Supporting Information, and the corresponding adsorption energies are summarized in Table S3 in the Supporting Information. In comparison to the clean Pd surfaces, the surface/subsurface C-modified Pd for both Pd(111) and (211) present higher S adsorption energies (less negative), indicating the weaker adsorption of S. As a result, within a relatively low concentration range, the C-modified Pd by the adsorbed S may indistinctly affect the hydrogenation rate of the Pd/(SH)MSC catalyst. The slight decrease in activity upon exposure to a large amount of thiourea may have resulted from the loss of active sites, the change in Pd electronic properties by S adsorption, or formation of subsurface S, which can poison hydrogen adsorption. The unremarkable change in the product selectivity for HCAL accompanying the involvement of thiourea may have also been related to the active edge sites that were not seriously contaminated by S. A detailed investigation of the loss of Pd active sites related to the S concentration and S-containing molecules, formation of subsurface S, and electronic properties of Pd by S adsorption is interesting for future study.

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AUTHOR INFORMATION

Corresponding Author

*Y.W.: tel, 86-21-6432-2516; fax, 86-21-6432-2511; e-mail, [email protected]. ORCID

Ying Wan: 0000-0002-6898-6748 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the State Key Basic Research Program of China (2013CB934102), NSF of China (21322308), Ministry of Education of China (PCSIRT_IRT_16R49 and 20123127110004), International Joint Laboratory on Resource Chemistry of China (IJLRC), and Shanghai Sci. & Tech. and Edu. Committee (14YF1409200).

5. CONCLUSIONS Pd nanoparticles (approximately 2.5 nm in diameter) were encapsulated inside an ordered mesoporous carbonaceous matrix via coordination-assisted pyrolysis. C deposition occurred at the low-coordinated surface sites, such as the edges and corners, on the Pd particles, facilitating subsurface hydrogen diffusion and reactant adsorption and resisting sulfur adsorption. The catalyst exhibited high activity (TOF of 2.4 s−1) in the selective hydrogenation of CAL as well as high selectivity to produce HCAL (approximately 80%) at 80 °C under an H2 pressure of 1.0 MPa in water. Exposure to thiourea with a S:Pd ratio of 0.1 resulted in a slight activity loss and undetectable selectivity loss under the same conditions. Even upon higher exposure to thiourea (S:Pd = 3), the TOF remained at 1.9 s−1, with a negligible effect on the HCAL selectivity. In contrast, the activity decreased by approximately half in the presence of a low concentration of thiourea (S:Pd = 0.1), and nearly complete deactivation was observed for Pd/C with a high concentration of thiourea. The trapping agent poisoning experiments using solid SH-SBA-15 revealed slight leaching of Pd into the solution, confirming that hydrogenation occurred inside the Pd/(SH)MSC mesopores. The Pd/ (SH)MSC catalyst was stable and reusable for eight cycles, with no observable Pd leaching or activity loss. Therefore, a control of the surface structure of the Pd nanoparticles using C deposition is expected to improve the catalytic reactivity and poison resistance in hydrogenation reactions.

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FTIR spectra, TG curves, TEM images, SEM images, UV−vis spectra, TPD-MS, TPO-MS curves, N2 sorption isotherms, and pore size distribution curves of the asprepared and calcined control samples, results of the Madon−Boudart test of the reaction rate for the selective hydrogenation of CAL with the Pd/(SH)MSC, comparison of the normalized conversion of 3-methyl-2-butenal in the presence of thiourea over Pd-based catalysts, computational details of the DFT calculations, and stable adsorption configurations and adsorption energies of S on clean, surface-C and subsurface-C modified Pd(111) and (211) surfaces. (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02720. Preparation and characterization of the low-polymerized phenolic resins, mesoporous silica−carbon composite (MSC), Pd/MSC, P-Pd/MSC, SBA-15, and mercaptofunctionalized SH-SBA-15, comparison of the hydrogenation of CAL using Pd-based catalysts, XRD patterns, 2085


Research Article

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