Direct synthesis of hybrid layered double hydroxide–carbon composites supported Pd nanocatalysts efficient in selective hydrogenation of citral

Abstract

This present study reports a facile one-pot strategy for the direct synthesis of hybrid layered double hydroxide (LDH)–carbon composites supported palladium nanocatalysts by the in situ reduction of PdCl₄²⁻-intercalated MgAl–LDH combined with amorphous carbon under mild hydrothermal conditions. The results demonstrated that most of the Pd(II) species intercalated in the interlameller space of MgAl–LDH could be reduced in situ to metallic Pd⁰ species, and simultaneously, the hybrid structure of the LDH–C composites facilitated the formation of uniform Pd nanoparticles with small diameter, as well as the strong metal–support interactions. Furthermore, with the decreasing proportion of the LDH component in LDH–C composites, the average diameter of Pd nanoparticles decreased progressively and the metal–support interactions were weakened. The as-formed supported Pd nanocatalyst with Pd loading of 5.5 wt% was found to show a superior catalytic activity in the liquid-phase selective hydrogenation of citral than other supported Pd nanocatalysts, while the one with the Pd loading of 2.7 wt% yielded a much higher yield of citronellal (∼80.0%) at 100% conversion. The catalytic performance of Pd nanocatalysts was proposed to be mainly related to both the metal–support interactions and the compositions of hybrid LDH–C composite supports.

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1. Introduction

Over the last two decades, well-dispersed supported noble-metal (e.g. Pt, Pd, and Au) catalysts have been explored widely in the field of heterogeneous catalysis.^1–4 In general, the nature of the supports is one of the most important factors in the formulation of high-performance supported catalysts,^5–7 because the metal–support interactions can play a significant role in the activity and selectivity of the desired products. Recently, carbon materials (e.g. carbon nanotubes, carbon aerogels, and activated carbon) as excellent catalyst supports have attracted increasing attention,^8–10 due to their high specific surface area, excellent electrical properties, and high mechanical stability. In order to achieve excellent catalytic performance, the enhancement of the metal dispersion and the catalyst stability by controlling the metal–support interfacial structure is highly desired. At present, the most widely used preparation routes for carbon materials-supported catalysts are incipient wetness impregnation,^11,12 deposition–precipitation,^13–15 and ion exchange.¹⁶ In most of the cases of the abovementioned approaches, complex pretreatments of pristine carbon materials are required, such as oxidation by strong acid or functionalization, which usually raise costs as well as cause serious environmental pollution.

Nowadays, the selective hydrogenation of α,β-unsaturated aldehydes is an important step in the production of fine chemicals.^17–21 For example, industrially important unsaturated geraniol/nerol isomers and citronellal can be produced by selectively hydrogenating the C=O functional group and conjugated C=C bond of citral (3,7-dimethyl-2,6-octadienal), respectively.^22–25 Furthermore, the consecutive hydrogenation of the abovementioned intermediates generates citronellol and finally, 3,7-dimethyloctanol. In this regard, palladium, platinum, and ruthenium are tested extensively in the abovementioned hydrogenations.^26,27 Among them, palladium, which favors the hydrogenation of the C=C bond, appears to be the most selective in forming saturated aldehydes.^28–33 As a result, with the aim of the production of desired products, the design of low-cost and efficient supported noble-metal catalysts has become an important issue in terms of economic and environmental sustainability.

In addition, layered double hydroxides (LDHs, [M_1−x²⁺M_x³⁺(OH)₂]^x+(Aⁿ⁻)_x/n·mH₂O) are a class of highly ordered two-dimensional layered clays consisting of positively charged layers with charge-balancing anions between them.^34,35 LDHs can accommodate a large number of tunable divalent and trivalent metal species either in the form of metal cations within the layers or in the form of metal complexes in the interlayer. More interestingly, highly dispersed metal nanoparticles (NPs) on metal oxide matrixes may be attained by controlled calcination and reduction processes for LDH precursors.^35–38 We have explored the generation and stabilization of highly dispersed nickel NPs via either the reduction of NiAl–LDH precursors supported on carbon nanotubes or the self-reduction of hybrid composites of NiAl–LDH and amorphous carbon at high temperatures.^39–41

The abovementioned results have stimulated us to extend our work using wonderful LDH materials as noble-metal catalyst precursors. In this study, a series of hybrid LDH–C composites supported Pd nanocatalysts were directly synthesized via a facile one-step hybridization-reduction route under mild hydrothermal conditions, which involved the assembly of composites of PdCl₄²⁻-intercalated MgAl–LDH and amorphous carbon through the carbonization of glucose and simultaneous reduction of Pd(II) species, without further chemical modification of the supports. The as-formed supported Pd nanocatalysts, in which Pd NPs and hybrid LDH–C composites could form a unique triple-junction structure, were found to be more active or more selective toward the liquid-phase hydrogenation of citral to citronellal compared with the traditional activated carbon supported Pd catalyst. A correlation between the structural characteristics of catalysts and the catalytic performance was investigated.

2. Materials and methods

2.1. Materials

All reagents were of analytical grade and purchased from Beijing Chemical Reagent Ltd. without further purification.

2.2. Synthesis of supported Pd nanocatalysts

PdCl₄²⁻-intercalated MgAl–LDH precursors were synthesized by a coprecipitation method. A salt solution containing Mg(NO₃)₂·6H₂O (0.12 M), Al(NO₃)₃·9H₂O (0.06 M) and H₂PdCl₄ (6 mM) dissolved in decarbonated deionized water was titrated by an alkali solution of NaOH (0.18 M) with vigorous agitation under N₂ atmosphere at room temperature until the pH value of the solution was adjusted to 10.0. The suspension was aged at 70 °C for 24 h, and then centrifuged, washed and redispersed in deionized water for 5 cycles. Finally, the resulting PdCl₄²⁻-intercalated MgAl–LDH precipitate was transferred and dispersed into a Teflon-lined autoclave with 60 mL of glucose solution ([C₆H₁₂O₆] = x[Mg²⁺ + Al³⁺]; x = 0.5, 1.5, 2.5 and 3.5) and aged at 150 °C for 10 h. The resultant suspension was centrifuged and washed 3 times with deionized water. Finally, the obtained solid was dried at 70 °C and denoted as Pd/C_x–LDH. For comparison, an activated carbon (AC) supported Pd catalyst with the Pd loading of 2.9 wt% also was prepared. AC (0.5 g) was added to a Teflon-lined autoclave, in which H₂PdCl₄ aqueous solution (20 mL) and methanol (60 mL) were placed. The autoclave was then tightly sealed and maintained at 100 °C for 10 h. The resultant slurry was dried overnight at 70 °C to obtain the Pd/AC sample.

2.3. Characterization

Powder X-ray diffraction (XRD) patterns of the samples were obtained on a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15418 nm).

Elemental analysis was performed using a Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectroscopy (ICP-AES) after the samples were dissolved in dilute nitric acid at 150 °C. The carbon content was determined by elemental microanalysis (Elementar Vario analyzer).

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were carried out on a JEOL JEM-2010 electron microscope at an accelerating voltage of 200 kV. The size distribution of the particles was determined from more than 200 individual metal particles. The surface area-weighted diameter of particles (D) was calculated by the following equation:

where n_i is the number of particles of diameter d_i.

Low temperature N₂ adsorption–desorption isotherms of the samples were obtained on a Micromeritics ASAP 2020 sorptometer apparatus. The total specific surface areas were evaluated by the multipoint Brunauer–Emmett–Teller (BET) method. The mesopore size and volume were determined by the Barrett–Joyner–Halenda (BJH) method according to desorption isotherms.

The Pd dispersion was determined by H₂–O₂ titration on a Micromeritics ChemiSorb 2720. About 100 mg of sample was loaded in a U-shaped quartz reactor. Before titration, the sample was heated at 150 °C for 2 h in a stream of He gas (40 mL min⁻¹ total flow) and then cooled to room temperature. The sample was then exposed to H₂/He at 120 °C for 60 min to ensure the saturated adsorption of hydrogen on its surface. Subsequently, the sample was purged by He gas for 60 min. Pulses of oxygen were introduced until complete saturation of the sample was achieved. The chemisorbed oxygen was then titrated by hydrogen. The dispersion of Pd (Dis(%)) on the catalysts was calculated by the following equation:⁴²

where M is the formula weight of Pd (M_Pd = 106.42 g mol⁻¹), V^H_T is the volume of H₂ used for the titration of O₂ (mL), W is the mass of sample (g), and P is the weight percentage of Pd in the catalyst as determined by ICP-AES.

X-ray photoelectron spectroscopy (XPS) was recorded on a VG ESCALAB 2201 XL spectrometer with a monochromatic Al Kα X-ray radiation (1486.6 eV photons). Binding energies were calibrated based on the graphite C 1s peak at 284.6 eV.

The adsorption isotherms of phenol dissolved in cyclohexane at 25 °C were used to estimate the surface base properties of the solids. The amount of phenol adsorbed by the solids was measured by UV spectroscopy (λ_max = 271.6 nm).

2.4. Selective hydrogenation of citral

The liquid phase hydrogenation of citral was carried out in a 200 mL stainless steel autoclave. At the beginning of the experiment, 100 mL of citral solution (58.4 mM in isopropanol) was charged with catalyst, in which the citral/Pd ratio was kept at ca. 0.30 mol g⁻¹ through out this study. After that, air was flushed out of the reactor with nitrogen at room temperature, and the reactor was sealed and flushed with 2.0 MPa H₂ at least five times. The reactor was then submerged into a water bath at 100 °C for 1 h under atmospheric pressure. H₂ was fed into the reactor at 1.0 MPa and the reaction was initiated by stirring at a speed of 900 rpm. After reaction, the autoclave was cooled with an ice-water bath, and depressurized carefully. Finally, the liquid products were analyzed by an Agilent GC7890A gas chromatograph equipped with a flame ionization detector and HP-5 capillary column. The injector temperature was 250 °C, and the detector column temperature increased from 100 °C to 150 °C with a ramp rate of 5 °C min⁻¹.

3. Results and discussion

3.1. Characterization of catalysts

The XRD patterns of the supported Pd samples are shown in Fig. 1. It is found that all the samples present the typical characteristic diffractions for (003) and (006) crystal planes of two types of hydrotalcite-like materials, which correspond to the different basal spacings and higher order diffractions.^43,44 In each case, two characteristic lines indexed as (110) and (113) planes of LDH phase also appear at high 2θ of about 61–62°. The intensity of the (003) plane at 2θ of around 11.2° decreases gradually with increasing amounts of glucose added in the course of hybridization, whereas that of another (003) plane at 10.6° increases. The diffractions at 11.2° and 10.6° arise from the basal spacings of approximately 0.847 and 0.771 nm, respectively, which are in good agreement with the reported values for carbonate-intercalated MgAl–LDH (CO₃²⁻–LDH) and PdCl₄²⁻-intercalated MgAl–LDH (PdCl₄²⁻–LDH).⁴⁵ It demonstrates that the proportion of interlayer PdCl₄²⁻ complex anions decreases with the amount of glucose added, and charge-balancing CO₃²⁻ anions easily enter the interlameller space because of its high affinity to hydroxyl groups on the layers through hydrogen bonding. Notably, in each case, the characteristic (111), (200) and (220) diffractions of metallic Pd⁰ (JCPDS 88-2335) appear at about 40.1, 46.7 and 67.9°, respectively. Furthermore, no diffractions related to graphite or other forms of carbon appear in the XRD patterns, which is suggestive of the presence of noncrystalline carbon.

Fig. 1 XRD patterns of Pd/C_0.5–LDH (a), Pd/C_1.5–LDH (b), Pd/C_2.5–LDH (c) and Pd/C_3.5–LDH (d).

In the course of the synthesis of supported Pd samples, the aromatization and carbonization of glucose under hydrothermal conditions can generate amorphous carbon,⁴⁶ and thus the resultant carbon may be assembled with LDH crystallites. Moreover, with the increasing amount of glucose, more interlayer PdCl₄²⁻ anions are reduced in situ to metallic Pd⁰ species by glucose serving as a reducing agent, thereby leading to the decrease in the proportion of PdCl₄²⁻ anions in the interlameller space. Based on the Scherrer formula, X-ray line broadening analyses from the (111) diffraction of the metallic Pd yield average coherent diffraction domain sizes of Pd NPs in the range of 5.6–7.4 nm for the supported Pd samples (Table 1), which are smaller than that in the Pd/AC comparison sample (about 11.2 nm). Small crystallite size of Pd NPs may be associated with the highly hybrid structure of LDH–C composites facilitating the uniform distribution of Pd NPs over the LDH–C matrixes.

Table 1 Analytical, structural and catalytic activity data obtained for different samples

Sample	Content^a (wt%)				D111^c (nm)	SBET^d (m^2 g^−1)	Vp^e (cm^3 g^−1)	dav^f (nm)	D^g (nm)	Dis^h (%)	Specific basicity^i (mmol gcat^−1)	r^j (mmol s^−1 gPd^−1)	TOF^k (× 10^2 s^−1)
	Pd	Mg	Al	C^b
a Determined by ICP-AES.b Determined by elemental analysis.c Crystallite size of metallic Pd based on XRD line broadening of (111) plane for Pd metal.d BET surface area.e Total pore volume.f Average pore size.g Particle size of Pd NPs obtained by TEM analysis.h Degree of Pd dispersion, determined by H2–O2 pulse chemisorption.i Concentration of total basic site, determined by adsorption of phenol.j Initial reaction rate of citral hydrogenation.k Turnover frequency of hydrogenation of citral, which was given as the overall rate of citral conversion normalized by the number of active sites over 15 min.
Pd/C0.5–LDH	5.5	24.4	20.6	39.2	7.4	122	0.38	11.5	7.2	25.6	0.209	406.1	16.9
Pd/C1.5–LDH	3.9	15.9	14.7	55.5	6.9	90	0.40	20.9	6.8	37.7	0.133	380.6	10.7
Pd/C2.5–LDH	2.7	10.5	9.7	67.8	6.0	61	0.36	23.8	6.3	43.3	0.086	295.0	7.2
Pd/C3.5–LDH	1.6	8.6	6.9	72.3	5.6	46	0.38	29.2	5.7	54.6	0.065	227.4	4.4
Pd/AC	2.9	—	—	87.5	11.2	—	—	—	8.7	23.3	—	238.2	10.8

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TEM analyses were conducted to investigate the morphology of the supported Pd samples (Fig. 2). In each case, spheroidal Pd NPs with narrow particle size distributions are uniformly distributed over the support regardless of the compositions of the supports and the Pd loadings, and plate-like LDH crystallites and amorphous carbon interweave together. Correspondingly, Pd NPs and the LDH–C composite support form a unique triple-junction structure. Furthermore, it is observed that the average crystallite size of Pd NPs decreases slightly with the decreasing Pd loading. Therefore, larger Pd NPs, around 7.2 nm with narrow size distribution, exist in the Pd/C_0.5–LDH (inset in Fig. 2a). With the decreasing Pd loading from 5.5 to 1.6 wt%, the average crystallite size of Pd NPs slightly decreases to about 5.7 nm (inset in Fig. 2b–d). This is consistent with that inferred from the XRD results.

Fig. 2 TEM images of Pd/C_x–LDH samples: (a) Pd/C_0.5–LDH, (b) Pd/C_1.5–LDH, (c) Pd/C_2.5–LDH and (d) Pd/C_3.5–LDH. Insets in (a–d) show histograms of the size distributions of Pd NPs.

HRTEM measurements were further carried out to reveal the microstructure of the Pd NPs. As for the representative Pd/C_2.5–LDH sample (Fig. 3), the lattice image displays an interplanar spacing of 0.225 nm, corresponding to the (111) plane of metallic Pd⁰. It is noted that the Pd NPs show a poor contrast with the support matrix, which is suggestive of the presence of the strong metal–support interactions (SMSI). In contrast, severe agglomeration of large globular particles is found in the Pd/AC comparison sample (see Fig. S1†), and the size distribution of particles with an average crystallite size of about 8.7 nm covers a broad range of around 3–11 nm. Such poor dispersion of Pd particles on the AC support suggests the weak metal–support interactions.

Fig. 3 Typical HRTEM images (a and b) of Pd/C_2.5–LDH.

The surface/near-surface chemical states of the samples were determined by XPS (Fig. 4). In the fine spectrum of the Pd 3d core level for Pd/C_0.5–LDH, the peaks at about 335.9 and 340.9 eV are assigned to Pd 3d5/2 and Pd 3d3/2, respectively, which are characteristic of metallic Pd species.^47,48 It can be noted that the binding energies of Pd⁰ species at the 3d region are higher than the reported values for Pd⁰ species (about 335.3 eV at 3d5/2 and 340.5 eV at 3d3/2) in the literature.^49–51 Therefore, as for Pd/C_0.5–LDH, the charge transfer from the surface metallic Pd to the support probably occurs because the higher Pd loading (∼5.5 wt%) may facilitate a more widespread distribution of Pd species over the LDH–C double support. Correspondingly, the electronic interaction between the metallic Pd and the LDH–C support leads to the formation of electron-deficient Pd species. With the decrease in the proportion of MgAl–LDH component in LDH–C composite support, the binding energies of Pd 3d5/2 and Pd 3d3/2 for Pd⁰ species exhibit a small shift to the lower energy side from 335.9 and 340.9 eV for Pd/C_0.5–LDH to 335.2 and 340.4 eV for Pd/C_3.5–LDH, respectively. This may be ascribed to the reduced interphase interaction between the surface Pd⁰ species and positively charged lattices of MgAl–LDH platelets, which mainly originates from the decrease in the proportion of MgAl–LDH component in the composite support. In addition, a small peak in the range of 335.0–338.0 eV is assigned to Pd(II) species in the form of PdCl₄²⁻ anions intercalated in the interlameller space of MgAl–LDH–LDH, as evidenced by the abovementioned XRD results. Clearly, the peak area of Pd(II) species is much smaller than that of Pd⁰ species, indicating that most of the Pd(II) are reduced under hydrothermal conditions and Pd species exist mainly in the form of metallic Pd⁰ species.

Fig. 4 Pd 3d XPS of Pd/C_0.5–LDH (a), Pd/C_1.5–LDH (b), Pd/C_2.5–LDH (c), Pd/C_3.5–LDH (d).

3.2. Catalytic performance of supported Pd nanocatalysts

The liquid-phase catalytic hydrogenation of citral is a typical three-phase catalytic reaction. It is found that the external particle mass transfer limitation in the present hydrogenation system is negligible at an agitation speed of above 900 rpm. Moreover, Weisz–Prater numbers corresponding to Pd/C_x–LDH catalysts were calculated to be lower than 0.15 for both hydrogen and citral, which indicate the absence of the pore transport diffusion limitations.^52,53 Therefore, the intrinsic activity of Pd/C_x–LDH catalysts could be actually determined under the present reaction conditions.

Fig. 5 illustrates the main reactions that probably occur during the hydrogenation of citral. Usually, the catalytic hydrogenation of citral can generate a variety of products. The C=O bond in citral may be hydrogenated to produce unsaturated alcohols (geraniol and nerol), while the conjugated C=C bond can be reduced to citronellal. Subsequently, consecutive hydrogenation of the abovementioned intermediates can produce citronellol and finally saturated alcohol (3,7-dimethyloctanol).

Fig. 5 Reaction paths for liquid-phase hydrogenation of citral.

The dependence of product compositions on time in the liquid-phase hydrogenation of citral over different catalysts at 100 °C is presented in Fig. 6, whereas the initial reaction rates and turnover frequencies (TOF) are summarized in Table 1. It is found that the different supported Pd catalysts exhibit remarkable difference in the catalytic performance. The conversions of citral over Pd/C_0.5–LDH and Pd/C_1.5–LDH increase rapidly within 30 min, and reach approximately 100% after 90 and 120 min, respectively, whereas the conversions of citral over Pd/C_2.5–LDH and Pd/C_3.5–LDH reach 95.7% and 84.6% after 90 min. The selectivity toward geraniol and nerol remains rather low (<10%) during citral hydrogenation, suggesting that the C=C/C=O adsorption competition of citral molecules mainly facilitates the C=C bond on the as-formed supported Pd catalysts under the present experimental conditions. As for Pd/C_0.5–LDH, the hydrogenation of the conjugated C=C bond in citral to citronellal is performed rapidly within the initial reaction time of 15 min, and simultaneously the consecutive hydrogenation of citronellal to citronellol also occurs. Within the reaction time, most citronellal converts gradually into citronellol over the Pd/C_0.5–LDH through the hydrogenation of the C=O bond. Finally, citronellol becomes the dominant product when the reaction is extended to a longer time than 150 min. With the decreasing proportion of LDH component in the composite supports, the hydrogenation of the conjugated C=C bond in citral is dominant at total reaction times, thus leading to the increase in the selectivity toward citronellal (Fig. 6B–D). It is seen from Table 1 that Pd/C_0.5–LDH catalyst yields the highest initial conversion rate of citral (406.1 mmol s⁻¹ g_Pd⁻¹), along with the highest TOF value of 0.169 s⁻¹. With the increasing carbon content in the catalysts, the initial hydrogenation rate drops rapidly by 56.0% over the Pd/C_3.5–LDH catalyst, indicative of a great decline in the activity. For the as-formed supported Pd catalysts, the increase in the carbon content gives rise to the improvement in the selectivity to citronellal, probably due to the reduced activation of the C=O bond.

Fig. 6 Catalytic performance in citral hydrogenation as a function of reaction time over different catalysts: Pd/C_0.5–LDH (A), Pd/C_1.5–LDH (B), Pd/C_2.5–LDH (C), Pd/C_3.5–LDH (D).

In addition, compared with Pd/C_2.5–LDH, the Pd/AC comparison catalyst with a similar Pd loading presents a lower initial reaction rate (Fig. S2†). As shown in Fig. 7, the selectivity towards citronellal can be improved by the appropriate addition of carbon component in catalysts. Among all the catalysts, Pd/C_2.5–LDH exhibits the highest citronellal selectivity of about 87.4% at a reaction time of 45 min (100% conversion of citral).

Fig. 7 Conversion and selectivity for the hydrogenation of citral over different catalysts at a reaction time of 45 min.

In the present catalyst system, the supported Pd/C_x–LDH nanocatalysts present a slight difference in the particle size. It demonstrates that there is no size dependence for hydrogenation activity. Normally, it is believed that the higher catalytic activity is ascribed to a larger amount of accessible active sites on the catalyst surface. Correspondingly, the dispersion of active metal species has a significant effect on the catalytic activity. As listed in Table 1, with the increasing proportion of carbon component in composite supports, the supported Pd catalysts present a decrease in the Pd loading and an improvement in the metal dispersion. Although the metal dispersion of Pd/C_0.5–LDH is lower than other catalysts, it exhibits the highest catalytic activity. It suggests that the metal dispersion is not the only important factor for the reactivity of supported Pd catalysts, and there are other factors affecting the catalyst activity.

As for the hydrogenation catalyzed by metals, the nature of adsorption is the key factor affecting the catalytic activity. According to the abovementioned XPS results, the electron transfer from the metallic Pd species to the support demonstrates the presence of SMSI. With the increasing proportion of MgAl–LDH component in the composite supports, the SMSI is enhanced gradually. Correspondingly, in the case of Pd/C_0.5–LDH, the lower electron density on the metal surface facilitates the adsorption of citral with the electron donating C=C groups, by the lowered repulsive forces existing between d orbitals of Pd and reactants,^54,55 leading to a higher activity for the hydrogenation of the C=C bond.⁵⁶

Moreover, elemental analysis reveals that the content of carbon in Pd/C_0.5–LDH is about 39.2 wt%, demonstrating a dominant MgAl–LDH component in the support, as evidenced by the abovementioned TEM results (Fig. 2). Furthermore, the basic properties of the surfaces of the catalysts were investigated by studying the adsorption isotherms for phenol from cyclohexane solution at 25 °C. As shown in Fig. 8, the overall number of basic sites, calculated on the basis of the data from adsorption isotherms in accordance with the Langmuir equation, decreases with the decreasing proportion of MgAl–LDH component in composite supports (Table 1). As for MgAl–LDH, the basic sites mainly come from hydroxyl groups on the lattice of LDH. Therefore, hydroxyl groups on the surface of the support in intimate contact with metallic Pd⁰ sites can easily interact with the C=O group in the citral molecule through hydrogen bonding, which probably favors the activation of the oxygen electronic doublet of the carbonyl function and thus the improvement of the reactivity of the carbonyl group. As for Pd/C_0.5–LDH, a larger amount of MgAl–LDH in the hybrid composite support inevitably increases the amount of exposed hydroxyl groups on the lattices of the plate-like LDH crystallites, thereby enhancing the catalytic activity for C=O hydrogenation and thus the selectivity towards citronellol after a longer period of reaction time. With the decreasing proportion of MgAl–LDH component, an improvement of the selectivity towards citronellal can be observed due to the reduced reactivity of the carbonyl group. In the case of the Pd/AC comparison catalyst, the relatively low activity should originate from the presence of bulky, large Pd particles, as well as the low metal dispersion. Moreover, large-sized Pd NPs do not favor the adsorption of the C=C bond due to the insufficiency of low-coordinated surface sites, thus resulting in a decline in the selectivity toward citronellal.

Fig. 8 Adsorption isotherms for phenol from cyclohexane solution on different catalysts: Pd/C_0.5–LDH (a), Pd/C_1.5–LDH (b), Pd/C_2.5–LDH (c), Pd/C_3.5–LDH (d).

In the present catalyst system, surface active Pd⁰ species and support composition are two key factors affecting the citral hydrogenation. For the as-formed supported Pd catalysts, the SMSI leads to the formation of electron-deficient metal species, and thus induces an enhanced oriented chemisorption of the C=C bond in the citral molecule on the surface of Pd particles, while the LDH component makes the effective chemical adsorption of citral on the support easier. Therefore, Pd/C_0.5–LDH catalyst remains very active for citral hydrogenation and highly selective towards citronellol after a long reaction time due to the consecutive hydrogenation of citronellal to citronellol. With the decreasing LDH content in the catalysts, the reduced SMSI and hydrogen bonding between the C=O group in the citral molecule and surface hydroxyl groups on the lattices of plate-like LDH crystallites inhibit the hydrogenation of C=C/C=O bonds in citral molecules, thus inducing a negative effect on the activity. Correspondingly, the selectivity to citronellal is improved over catalysts with the higher carbon content. Especially, a high citronellal yield of about 80.0% is achieved over the Pd/C_2.5–LDH at a conversion of 100% (Fig. 6).

Reusability is one of the most attractive hallmarks of catalysts, because a promising catalyst must exhibit fairly stable catalytic performance in multiple successive runs. The reusability of Pd/C_2.5–LDH was investigated in our study. In the cycling tests, the reaction mixture solution was centrifuged to remove the catalyst. The collected catalyst was washed with ethanol and deionized water, and dried for 24 h, then reused in the next reaction cycle. It is found that the catalytic property of Pd/C_2.5–LDH can be well recovered with no detectable loss after recycling 5 times (Fig. 9). Elemental analysis by ICP-AES further reveals that the Pd leaching loss is only about 2.1 wt% for the initial amount of Pd in the Pd/C_0.5–LDH after recycling 5 times, as a result of the unique triple-junction structure of the as-formed supported Pd nanocatalysts.

Fig. 9 The reusability of the Pd/C_2.5–LDH catalyst at a reaction time of 45 min.

4. Conclusions

In summary, we have developed a new method to directly synthesize hybrid LDH–C composites supported Pd nanocatalysts for the liquid-phase selective hydrogenation of citral. Supported Pd nanocatalysts were prepared via a simple in situ reduction of composites of PdCl₄⁻-intercalated MgAl–LDH and amorphous carbon, which were assembled through a carbonization process of glucose. With the increase in the LDH component in LDH–C composite supports, the SMSI was enhanced gradually, which resulted in the enhancement of the adsorption of the C=C bond in the citral molecule on the active Pd⁰ sites and, thus enhanced the catalytic activity. In contrast, with the increase in the carbon component in LDH–C composite supports, the reduced hydrogen bonding interaction between the LDH component and reactants could not facilitate the activation of the C=O bond, thereby leading to the improvement in the selectivity toward citronellal. During citral hydrogenation performed at 100 °C under the hydrogen partial pressure of 1.0 MPa, a high yield of citronellal (∼80.0%) at a conversion of 100% was achieved over the as-formed supported Pd nanocatalyst with the Pd loading of 2.7 wt%.

Acknowledgements

We are grateful for the financial support from National Natural Science Foundation of China, the Program for Beijing Engineering Center for Hierarchical Catalysts, the Fundamental Research Funds for the Central Universities (YS1406) and the Specialized Research Fund for the Doctoral Program of Higher Education (20120010110012).

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Footnote

† Electronic supplementary information (ESI) available: Additional experimental results. See DOI: 10.1039/c5ra03201f

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