Pd nanoparticles supported on amino-functionalized magnetic mesoporous silica nanotubes: a highly selective catalyst for the catalytic hydrodechlorination reaction

Abstract

Highly dispersed Pd nanoparticles supported on amine-functionalized magnetic mesoporous silica nanotubes (NH₂-MSNTs) have been successfully prepared and tested in the aqueous phase catalytic hydrodechlorination (HDC) of chlorophenols (CPs). The as-synthesized Pd@NH₂-MSNTs catalyst is investigated by transmission electron micrograph (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), Fourier-transfer infrared spectroscopy (FT-IR) and N₂ adsorption–desorption. The synthesized MSNTs material exhibits a large surface area of 966 m² g⁻¹, high pore volume of 0.81 cm³ g⁻¹, highly open mesoporous size of 3.4 nm and high saturation magnetizations of 19.65 emu g⁻¹. For the HDC of CPs, the catalyst shows 100% conversion with 99% selectivity for cyclohexanone in aqueous solution at mild conditions of temperature (60 °C) under ambient pressure. It is astonishing that the Pd@NH₂-MSNTs catalyst shows better catalytic performance, and it can synthesize cyclohexanone in one pot, in comparison with other heterogeneous catalysts for which phenol is the only reaction product. The novel heterogeneous magnetic catalyst exhibits excellent catalytic stability and can be used seven times without an obvious decrease of product selectivity. In addition, our study further reveals that the synergetic effect between Pd nanoparticles and NH₂-MSNTs support plays a key role in the HDC of CPs.

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Introduction

Chlorophenols (CPs) have been widely applied as intermediates and industrial products in the synthesis of disinfectants, pesticides, dyes and wood preservatives.¹ However, as priority pollutants, CPs are known to exhibit high toxicity, low bio-degradability and a strong bioaccumulation ability.² Therefore, it is necessary to put forward effective treatment methods to degrade the contaminants from foul water. Recently, various abatement methods, such as incineration,³ adsorption,⁴ photochemical catalysis,⁵ advanced oxidation processes,⁶ wet catalytic oxidation⁷ and catalytic hydrodechlorination,^8,9 have been proposed. Especially, catalytic hydrodechlorination (HDC) is recognized as a promising environmentally-friendly technique and thus transforming toxic CPs into useful products like cyclohexanone (C-one) which is a key precursors in the synthesis of nylon 6 and nylon 66.¹⁰

A wide variety of catalysts based on noble metals like Pd,^11,12 Pt¹³ and Rh¹⁴ have been studied in aqueous phase HDC of CPs. According to previous research, Pd presents high catalytic activity and selectivity for hydrogenation CPs to C-one.¹⁵ The catalyst support also plays an important role in catalytic properties and has been widely investigated on activated carbon,¹⁶ alumina,¹⁷ pillared clays,¹⁸ γ-alumina¹⁹ and silica nanospheres.²⁰ Recently, magnetic mesoporous silica nanotubes (MSNTs) have gained particular interest because of their anisotropic structures, hollow nanoscale morphology, high aspect ratio and ultra-large specific surface area.²¹ Meanwhile, compared with the conventional filtration and centrifugation methods, the magnetic property of MSNTs can make catalyst more efficiently to separate from reaction system.²² Hitherto, the MSNTs material has not been used for HDC reactions, to the best of our knowledge, only by this work, although they have offered a wide range of applications in drug delivery,²³ separation systems²⁴ and functional channels.²⁵ Nanotubular structures of materials are always synthesized by soft-template-based routes,²⁶ hard template methods²⁷ and template-free syntheses.²⁸ CNTs are used as a hard template for preparing MSNTs material by a template-directed approach; the as-prepared MSNTs material is used as the support for Pd nanoparticles (Pd NPs), on which the Pd NPs are highly dispersed. Furthermore, mesoporous structures consist of the outer surface, the inner surface of the central tubular cavity and the internal surface of the tubule sheath. Their increased surface area, pore volume and catalytic activity respect to other supports make them interesting candidates as catalysts for the HDC of CPs in aqueous phase.

In this study, CNTs are chosen as the hard template because of their hollow structures and ultralarge specific surface areas. CNTs, iron(III) acetylacetonate (Fe(acac)₃) and tetraethyl orthosilicate (TEOS) are used to prepare the MSNTs material. The prepared MSNTs material has a high specific surface area, large pore volumes and abundant NH₂ groups on the surface via amino-functionalization, which is used as support to prepare Pd NPs based catalyst (Pd@NH₂-MSNTs). Thus the Pd@NH₂-MSNTs demonstrates outstanding catalytic activity and selectivity for the HDC and further hydrogenation of CPs to a useful industrial product C-one in aqueous phase at mild conditions under atmospheric pressure. Meanwhile, the Pd@NH₂-MSNTs has excellent magnetic property, thus it could be easily separated from reaction and recycled several times without obvious deterioration in catalytic activity. The excellent catalytic properties and superior recyclability of the obtained catalysts indicate that the amine-functionalized magnetic mesoporous silica nanotubes is a promising candidate for construction of efficient heterogeneous catalysts in the future.

Experimental section

Materials

Triethylene glycol (TREG, AR), tetraethyl orthosilicate (TEOS, AR), 3-aminopropyltriethoxysilane (3-APTES, AR), iron(III) acetylacetonate (Fe(acac)₃, >99%) were purchased from Sigma-Aldrich. Palladium chloride (PdCl₂), chlorophenols (CPs, 99%), ethanol (≥99.5%), ammonia solution (NH₄OH, 28 wt%) were purchased from Shanghai Chemical Reagent Co., Ltd. CNTs (>95%) were obtained from Shenzhen Nanotech Port Ltd Co (China). All the chemicals were used without further purification.

Preparation of CNTs/Fe₃O₄ composites

The synthesis of CNTs/Fe₃O₄ composite is similar to the previous reported literatures.²⁹ In the first step, 0.1 g of CNTs and 0.4 g of Fe(acac)₃ were added to 60 mL of triethylene glycol followed by ultrasonication for 60 min. After that, the mixture was heated at 278 °C for 30 min in N₂ atmosphere with a heating rate of 3 °C min⁻¹. Then, after cooling to room temperature, the obtained CNTs/Fe₃O₄ composites were magnetically separated by a magnet and washed with ethanol several times and dried at 30 °C in a vacuum oven.

Synthesis of silica-coated CNTs/Fe₃O₄ (CNTs/Fe₃O₄@SiO₂)

The CNTs/Fe₃O₄@SiO₂ were fabricated via a simple method according to the Stöber's method.³⁰ In a typical synthesis, 0.1 g of CNTs/Fe₃O₄ and 0.7 g of CTAB were dispersed into 60 mL of deionized water and the mixture was sonicated for 40 min. After that, 200 mL of anhydrous ethanol was poured into the above mixture and sonicated for 10 min to form a distributed system, followed by the addition of 1.2 mL of NH₃·H₂O. Then, the TEOS solution (0.4 mL TEOS in 20 mL ethanol) was added dropwise to the above mixture under ultrasonication, and the mixture was held at room temperature for 12 h with continuous stirring. Finally, the obtained black precipitate was collected by centrifugation and washed with deionized water and ethanol for several times, respectively. The product was dried overnight at 30 °C.

Synthesis of magnetic mesoporous silica nanotubes (MSNTs)

In order to synthesize the MSNTs material, the as-synthesized CNTs/Fe₃O₄@SiO₂ magnetic composites were calcined at 550 °C for 6 h in air at a rate of 3 °C min⁻¹ to remove the CTAB and CNTs.

Synthesis of the amino-modified magnetic mesoporous silica nanotubes (NH₂-MSNTs)

The amino-functionalization of the MSNTs material with NH₂ groups was prepared by adding 0.1 mL of 3-APTES to 50 mL of dry toluene containing 0.1 g of MSNTs material.³¹ The resulting solution was heated to 110 °C and refluxed for 15 h with N₂ protection. After cooling to room temperature, and then washed with ethanol to remove the remaining 3-APTES and dried at 30 °C in a vacuum oven.

Loading of Pd on NH₂-MSNTs support (Pd@NH₂-MSNTs)

The Pd@NH₂-MSNTs were fabricated using K₂PdCl₄ as the Pd source through an aqueous impregnation method. In a typical synthesis, 0.1 g of NH₂-MSNTs support was first dissolved in 50 mL of deionized water, followed by the addition of 1 mL of K₂PdCl₄ (10 mM) solution under stirring for 4 h, then 8 mL of NaBH₄ (30 mM) solution was slowly dropped into the above homogeneous suspension under magnetic stirring. After reduction of 4 h, the products were separated by a magnet and washed thoroughly with deionized water and ethanol for several times and dried in a vacuum oven at 30 °C. The final product named as Pd@NH₂-MSNTs was obtained. The content of Pd was identified by atomic absorption spectroscopy as 4.83 wt%.

Hydrodechlorination experiments

The HDC experiments were carried out in a 50 mL three-necked flask under continuous stirring at atmospheric pressure. Typically, the Pd@NH₂-MSNTs (20 mg, Pd content, 1.82 mol% relative to CPs), H₂O (30 mL), NaOH solution (0.5 mmol) and CPs (5 mL, 0.1 M) were mixed in the flask and it was stirred for 5 min allowing for CPs adsorption onto the catalyst. After stabilization of temperature, H₂ was fed to the flask and this moment was considered as the initial reaction time. Samples were withdrawn from the reaction medium at 5 min, 10 min, 30 min and every half hour until completing 180 min. To track the reaction progress, the reaction product was collected with a specimen tube with a certain time interval, and the catalyst was separated by filtration using a PTFE filter (pore size 0.22 μm). The filtrate was extracted by ethyl acetate, after that the reaction conversion of CPs and product selectivity were evaluated by GC-MS. To observe the reusability of the Pd@NH₂-MSNTs, after each HDC reaction, the Pd@NH₂-MSNTs was recovered by a magnet, then washed with deionized water and dried in vacuum at 30 °C for the next catalytic run.

Characterization

TEM and high resolution TEM (HRTEM) images were taken on a FEI Tecnai G²F30 electron microscope operating at 300 kV (FEI Company). FT-IR (Bruker IFS66/S) was also utilized to characterize the samples. The composition and chemical state of Pd@NH₂-MSNTs were studied by energy dispersive X-ray (EDX) and XPS. EDX was prepared by a Tecnai G² microscope and XPS analysis was recorded on a Perkin-Elmer PHI-5702 X-ray photoelectron spectrometer. XRD measurements were performed on a Rigaku D/max-2400 PC diffractometer employing Cu-Kα radiation as the X-ray source in the 2θ range of 10–80°. N₂ adsorption–desorption isotherms of the support and catalyst were calculated at 77 K on a micromeritics apparatus of model ASAP-2050 system. Magnetic characterization of samples were investigated with a Quantum Design VSM at 25 °C. The metal contents in the Pd@NH₂-MSNTs were obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The conversion of CPs and selectivity of product were measured using GC-MS (Agilent 5977E).

Results and discussion

Catalysts characterization

The process of synthesis of Pd@NH₂-MSNTs is shown in Scheme 1. Firstly, CNTs/Fe₃O₄ composite was manufactured by a solvothermal method. Secondly, a thin layer of silica was coated onto the CNTs/Fe₃O₄ to fabricate CNTs/Fe₃O₄@SiO₂, which was then converted into MSNTs material via calcinations to remove the CNTs and CTAB templates. Thirdly, the amino-functionalization of MSNTs material was carried out by adding 1 mL of 3-APTES into dry toluene containing MSNTs material. Surface silanol groups can easily react with silane coupling agents, which not only to serve as a linker for organic moieties, but also to stabilize and disperse Pd NPs. Lastly, K₂PdCl₄ was supported on the surface of NH₂-MSNTs support and reduced to Pd NPs by NaBH₄.

Scheme 1 Preparation of Pd@NH₂-MSNTs catalyst.

TEM and HRTEM are used to visualize the morphologies and structural features of the samples. Fig. 1a represents the pure CNTs without introducing other groups for functionalization are intact, maintain their original smooth sidewall and tubular structure. The Fe₃O₄ NPs with an average diameter of 5.7 nm were successfully loaded on the surfaces of CNTs and no detectable local aggregation is observed despite the coverage density being very high (Fig. 1b). From the HRTEM image (Fig. 1c), the lattice fringes are clearly visible with spacing of 0.49 nm and 0.25 nm which corresponds to the d-spacing of (111) and (311) crystal plane in the Fe₃O₄ NPs. Fig. 1d shows a thin SiO₂ layer was successfully coated onto CNTs/Fe₃O₄ composites, the thickness of thin SiO₂ layer is approximately 15 nm. By calcining at 550 °C in air to remove CTAB and CNTs to prepare MSNTs material (Fig. 1e). The SiO₂ layer still maintains the integrity of the tubular morphology even after calcination and avoids aggregation of the Fe₃O₄ NPs during high-temperature calcination as well. Simultaneously, the Fe₃O₄ NPs were transformed into magnetic γ-Fe₂O₃ NPs after this procedure.³² A representative HRTEM image of MSNTs material is shown in Fig. 1f, the lattice spacings with 0.248 nm assigns to the spacing of (311) crystal plane of γ-Fe₂O₃. Well-dispersed Pd NPs were successfully loaded onto the NH₂-MSNTs support to obtain Pd@NH₂-MSNTs catalyst (Fig. 1g). As shown in the inset picture of Fig. 1g, the particle size distribution indicates that most of the Pd NPs fall in the size range 4–6 nm and the average particle diameter is about 4.7 nm. The Pd@NH₂-MSNTs HRTEM image (Fig. 1h) shows that the lattice fringes of the (111) crystal face (d = 0.223 nm) is attributed to the Pd NPs. The EDX analysis confirms that the Pd@NH₂-MSNTs consists of the elements C, N, O, Fe, Si, Pd and Cu (Fig. 1i). C and Cu peaks are influenced by the copper network support films in these elements, Fe, O, Si, N and Pd signals result from the NH₂-MSNTs support and Pd NPs.

Fig. 1 TEM images of (a) CNTs, (b) CNTs/Fe₃O₄, (c) HRTEM of Fe₃O₄ NPs, (d) CNTs/Fe₃O₄@SiO₂, (e) MSNTs material, (f) HRTEM of γ-Fe₂O₃ NPs, (g) Pd@NH₂-MSNTs (inset pictures: size distribution histograms of the Pd NPs crystal structures on Pd@NH₂-MSNTs), (h) HRTEM of Pd NPs and (i) EDX analysis.

Wide-angle XRD (WAXRD) patterns of the MSNTs material and the Pd@NH₂-MSNTs catalyst are shown in Fig. 2a, solid blue square displays a broad characteristic diffraction peak at approximately 2θ = 23.1° derived from the amorphous mesoporous SiO₂ nanotube and the diffraction peaks at 2θ values of 30.4°, 35.7°, 43.5°, 53.7°, 57.4° and 63.1° in the WAXRD patterns are assigned to the (220), (311), (400), (422), (511) and (440) planes of γ-Fe₂O₃ NPs.³³ WAXRD patterns of Pd@NH₂-MSNTs show two characteristic diffraction peaks at 2θ = 40.6° and 46.7° corresponding to Pd (111) and Pd (200), respectively. Besides, it can be seen that WAXRD patterns have no obvious change after the Pd NPs were immobilized on the NH₂-MSNTs support, indicates that the crystalline structures of the composite nanoparticles and the NH₂-MSNTS support are well maintained.³⁴ Unexpectedly, the Pd@NH₂-MSNTs shows very weak reflection corresponding to Pd metallic phase due to their lower concentrations and excellent dispersion on the support than γ-Fe₂O₃ NPs.³⁵ The XRD results are also in agreement with the TEM images Fig. 1d–g.

Fig. 2 (a) WAXRD and (b) SAXRD patterns of MSNTs material and the Pd@NH₂-MSNTs catalyst.

Fig. 2b shows the small-angle XRD (SAXRD) patterns of MSNTs and Pd@NH₂-MSNTs. There is a typical reflection peak in two samples, which confirmed that the support has ordered mesoporous structure.³⁶ The diffraction intensity of the Pd@NH₂-MSNTs displays a slightly decrease compared with MSNTs support, which can be attributed to the immobilizing Pd on surface of the support, so the ordered mesoporous structure of MSNTs support could be slightly destroyed.³⁷

The valence state of O, C, N, Si, Fe and Pd in the Pd@NH₂-MSNTs is investigated by XPS (Fig. 3). All the binding energy values are corrected for the C 1s = 284.6 eV. The existence of the N 1s (399.4 eV) provides a direct evidence for the successful introduction of NH₂ groups on the surface of MSNTs material. The Pd 3d levels spilt into 3d_3/2 and 3d_5/2 states (Fig. 3b) because of spin–orbital splitting. As for the Pd@NH₂-MSNTs, the binding energy values at 335.5 eV and 340.8 eV are attributed to Pd⁰ 3d_5/2 and Pd⁰ 3d_3/2, respectively. While two more peaks at 338.1 eV and 343.3 eV prove the presence of Pd²⁺ species.³⁸ XPS data indicates that the ratio of Pd⁰ is as high as 52.7% and the ratio of Pd²⁺ is 47.3% (Pd²⁺/Pd⁰ ≈ 1) on the surface of the Pd@NH₂-MSNTs. The existence of Pd²⁺ is mainly attributed to the coordination of Pd NPs with the NH₂ groups on the surface of the MSNTs material.³⁹

Fig. 3 (a) XPS spectrum of the Pd@NH₂-MSNTs, (b) high-resolution spectrum of Pd 3d.

Fig. 4 illuminates the N₂ adsorption–desorption isotherms and corresponding pore-size distribution curves for MSNTs material, NH₂-MSNTs and Pd@NH₂-MSNTs. All isotherms show type IV BET isotherms with a H3-type hysteresis loop (P/P₀ > 0.4), implying that the typical mesoporous structures exist.⁴⁰ As shown in Fig. 4b, the BJH pore size distribution curve was calculated by the BJH method. As a result, the BET surface area and the cumulative pore volume of MSNTs material are 966 m² g⁻¹ and 0.81 cm³ g⁻¹, respectively. After amino-functionalization, the BET surface area and the cumulative pore volume of NH₂-MSNTs are 192 m² g⁻¹ and 0.44 cm³ g⁻¹, respectively. The Pd@NH₂-MSNTs have a large surface area of 178 m² g⁻¹ and pore volumes of 0.35 cm³ g⁻¹ as displayed in Table 1. After the modification, the isotherm shape has not changed, indicating that the Pd NPs do not block or alter the pore system.^41,42 However, the BET surface area of catalyst is generally decreased upon post-modification reaction with 3-APTES.⁴³

Fig. 4 (a) N₂ adsorption–desorption isotherms of the MSNTs material, NH₂-MSNTs and Pd@NH₂-MSNTs, (b) the corresponding pore diameter distribution.

Table 1 Textural parameters and elements content of the prepared samples

Samples	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)	Dpore (nm)	Pd (wt%)
MSNTs	966	0.81	3.4	—
NH2-MSNTs	192	0.44	8.9	—
Pd@NH2-MSNTs	178	0.35	7.9	4.83

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Fig. 5a shows the FT-IR spectra of MSNTs material and the NH₂-MSNTs support. The IR absorption bands at 462 and 572 cm⁻¹ are assigned to the Fe–O bonds of γ-Fe₂O₃, which illustrated Fe₃O₄ NPs transformed thoroughly into γ-Fe₂O₃ after calcination at 550 °C.^44,45 Two bands at approximately 1083 and 803 cm⁻¹ are assigned to v_as(Si–O–Si) and v_s(Si–O–Si), respectively. The band at 966 cm⁻¹ is attributed to the bending vibration of the Si–O–H.⁴⁶ In the hydroxyl region, the weak band at 1633 cm⁻¹ and the broad high-intensity band at 3439 cm⁻¹ can be attributed to the H–O–H stretching mode of physisorbed water.⁴⁴ The NH₂-MSNTs support after surface modification displayed the methylene (–CH₂) groups stretching bands at 2936 cm⁻¹, which confirmed the MSNTs material surface was successfully modified by 3-APTES.⁴⁷ In addition, another band at 1494 cm⁻¹ is assigned to the N–H stretching bands.⁴⁶ The nitrogen, hydrogen and carbon contents obtained from elemental analysis are 2.34%, 2.29%, and 9.19%, respectively. The FT-IR spectra and elemental analysis result proved that the MSNTs material surface was aminofunctionalized by 3-APTES, thus enabling them to act as robust anchors for connecting metal nanoparticles.

Fig. 5 (a) FT-IR spectra of MSNTs material and Pd@NH₂-MSNTs, (b) the magnetic hysteresis loops of CNTs/Fe₃O₄, MSNTs material, Pd@NH₂-MSNTs and CNTs/Fe₃O₄@SiO₂.

The room temperature magnetization curves reveal that the magnetic nanoparticles is superparamagnetic with no coercivity and remanence. The hysteresis loops of CNTs/Fe₃O₄, MSNTs material, Pd@NH₂-MSNTs and CNTs/Fe₃O₄@SiO₂ are shown in Fig. 5b, their saturation magnetizations are 19.65, 15.95, 11.88 and 10.22 emu g⁻¹, respectively. The magnetic saturation value of CNTs/Fe₃O₄@SiO₂ is lower than the Pd@NH₂-MSNTs, and the reason of saturation magnetization decrease is the magnetic nanoparticles wrapped completely by SiO₂ layer. Compared to CNTs/Fe₃O₄ and MSNTs material, the decreased saturation magnetization of Pd@NH₂-MSNTs shows the presence of NH₂ groups and Pd NPs on the surface of the magnetic mesoporous support. Even with this reduction in the saturation magnetization, the catalyst can still be efficiently separated from the reaction system by external magnetic force.

Hydrodechlorination and further hydrogenation of CPs to C-one

The as-prepared Pd@NH₂-MSNTs were added into the HDC reaction under mild conditions. We firstly investigated the effect of different catalyst dosages (20 mg, 40 mg and 60 mg) on the concentration of 4-CP and reaction products varying with time in aqueous solution (Fig. 6a–c). As can be seen from diagram, the 4-CP could be completely catalytically converted to Ph within 10 min, increasing the catalyst dosage could improve the catalyst activity for HDC and further hydrogenate Ph originated from first step.⁴⁸ Compared with the effect of different dosages catalyst on selectivity, the 40 mg catalyst exhibited as much as 90% selectivity for C-one. Thus, we focused our attention on the 40 mg catalyst for HDC of CPs. Based on Fig. 6, we proposed the reaction pathways as shown Scheme 2. In HDC reaction, the H₂ amount was excessive compared with raw material (4-CP) and can be treated as constant, hence we could consider the linear relationship of HDC reaction follows the pseudo-first-order kinetics.¹⁵

Fig. 6 HDC of 4-CP with different amounts of Pd@NH₂-MSNTs. (a) 20 mg (Pd, 1.82 mol% to 4-CP), (b) 40 mg (Pd, 3.64 mol% to 4-CP) and (c) 60 mg (Pd, 5.46 mol% to 4-CP). (d) Plots of ln(C₀/C) versus reaction time for the generation of C-one over different amounts of Pd@NH₂-MSNTs.

Scheme 2 Reaction pathways for 4-CP HDC with the Pd@NH₂-MSNTs.

The reaction rate constant was computed from the slope of the straight line by reaction kinetics ln(C₀/C) = kt. The values of k₁ and k₂ at different catalyst dosages were given in Fig. S1† and 6d. The reaction rate constant and the catalyst-mass normalized reaction rate k′ = k/m_Pd were summarized in Table 2. In fact, Table 2 displays the dechlorination process and further hydrogenation under the influence of different catalyst dosages. It could be seen that with the increasing of catalyst dosage, the reaction rate constant increases due to the increase of active sites.⁴⁹ But the two catalyst-mass normalized reaction rates of 60 mg catalyst were lower than that of the other two doses. It could be explained that after the catalyst dosage were improved to 60 mg, the catalyst active sites were excessive and not all the catalyst were involved in the reaction.⁵⁰ There was a quite different the catalyst-mass normalized reaction rate (k₂) for dosage of 20 mg compared with that of 40 mg or 60 mg, which was due to the less of the active sites in the same concentration of reactants. Therefore, the appropriate Pd@NH₂-MSNTs dosage for 4-CP HDC in solution was 40 mg by compromising HDC efficiency and reagent cost.

Table 2 Values of the first-order rate constants of Fig. 6 for HDC of 4-CP

Catalyst dosage (mg)	k1 (min^−1)	k1/mPd (min^−1 g^−1)	k2 (min^−1)	k2/mPd (min^−1 g^−1)	r^2
20	0.196	202.9	0.003	3.11	0.99
40	0.388	200.8	0.009	4.66	0.98
60	0.497	171.5	0.013	4.49	0.99

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Fig. 7a–c reveal the effect of reaction temperature for the Pd@NH₂-MSNTs (Pd content, 3.64 mol% to 4-CP) in the HDC of 4-CP. When reaction continued to 160 min, the selectivity of C-one catalyzed by 40 mg Pd@NH₂-MSNTs at 50 °C, 60 °C and 70 °C are 49.5%, 97.6% and 91.4%, respectively, along with a little C-ol generating. A high selectivity toward C-one was desirable from an environmental perspective, since it is the useful industrial raw materials among the HDC products. At the end of the experiment (180 min), the selectivity toward C-one at 60 °C was almost 99%, so we selected the 60 °C as the reaction temperature for HDC of CPs. By calculating, the reaction rate constant k₁ increases with increasing the reaction temperature (Fig. S2†). It turns out that properly increasing the reaction temperature could speed up the dechlorination process of 4-CP HDC. The reaction rate constant k₂ at different temperatures are 3 × 10⁻³ min⁻¹, 9 × 10⁻³ min⁻¹ and 1.1 × 10⁻² min⁻¹, respectively (Fig. 7d). The calculated reaction rate constant per unit mass k′ is 4.66 min⁻¹ g⁻¹ for the further hydrogenation of Ph catalyzed at 60 °C. Compared with the previously reported Pd/NH₂-MCM catalyst and the Pd/DMSNs catalyst, their k′ are 3.39 min⁻¹ g⁻¹ and 0.60 min⁻¹ g⁻¹, respectively.^51,52 The synthesized catalyst, Pd@NH₂-MSNTs, exhibits higher catalytic activity in HDC and further hydrogenation of Ph to C-one.

Fig. 7 Effect of reaction temperature on the HDC of 4-CP and its further hydrogenation to C-one over 40 mg of Pd@NH₂-MSNTs (Pd, 3.64 mol% to 4-CP). (a) 50 °C, (b) 60 °C and (c) 70 °C. (d) Plots of ln(C₀/C) versus reaction time for the generation of C-one over different temperatures.

During the HDC of CPs, previous studies have proved that the HCl formed could poison the Pd NPs and reduce the reaction rate.⁸ In the liquid-phase system, the presence of alkali acts as a proton scavenger, maintaining Pd in a reduced state and limiting combination Cl⁻ and H⁺.⁵³ Therefore, different bases were tested in the water system (Fig. 8a), we observe that the selectivity of C-one is greatly affected by the basicity of neutralizer for HCl. When using a strong base (NaOH), the selectivity of C-one is the highest. In contrast, the selectivity is sluggish when the base is weak (NH₄OH, (C₂H₅)₃N, Na₂CO₃ and CH₃COONa). In order to investigate the recyclability of the catalyst in HDC reactions, the catalyst was separated from the reaction system after completion of every reaction by the external magnet. As shown in the Fig. 8b, the conversion of 4-CP has not conspicuously decrease after the catalyst recycled seven times. ICP-AES measurements reveal the Pd content in catalyst is 4.78 wt% after reuse seven times, thus proving the catalyst is very stable in the reaction.

Fig. 8 (a) Effect of different alkalis for HDC reaction and (b) the reusability of Pd@NH₂-MSNTs for catalytic HDC of 4-CP.

Hot filtration test

We have also carried out the hot filtration test to investigate the leaching of Pd during the process of HDC reaction. In a typical reaction, a mixture solution of Pd@NH₂-MSNTs (40 mg), 4-CP (0.5 mmol), NaOH (0.5 mmol), in H₂O (30 mL) was heated at 60 °C for 180 min. The Pd@NH₂-MSNTs catalyst was separated from the hot reaction mixture after 90 min using magnetic separation technique. Then, it was evaluated by using GC-MS that C-one selectivity of only 56% was achieved (Fig. S3†). The reaction was continued with the filtrate for another 90 min at the same reaction temperature. But, the reaction did not proceed, indicating that no catalytically active Pd remained in the filtrate. After completion of the reaction, no Pd could be detected in the filtrate by AAS analysis.

In order to compare the potential textural properties of different catalyst supports, the HDC of 4-CP and further hydrogenation of Ph to C-one were carried out at 60 °C in 180 minutes (1 atm). The time evolution of the product selectivity are shown in Fig. S4.† At first, the reaction was performed only using support without any catalyst at 60 °C in water (Table 3, entry 1 and 2), there was no any product. Then, the reaction was performed using different catalyst systems (Table 3, entry 3–6), including Pd/C, Pd/mSiO₂, CNTs/Fe₃O₄@SiO₂–NH₂–Pd and Pd@NH₂-MSNTs. To our delight, the synthesized Pd@NH₂-MSNTs displayed remarkably catalytic activity with a conversion of 100% as well as a greatly increased selectivity (>99%) toward C-one (Table 3, entry 6). This would highlight the effectiveness of magnetic mesoporous silica nanotubes in HDC reaction. From the consequences, we could establish the supports relative sequence regarding their selectivity of catalysts: NH₂-MSNTs > CNTs/Fe₃O₄@SiO₂ > mSiO₂ > AC. According to this, the NH₂-MSNTs support which has a high surface area and a mesoporous structure was very beneficial for the HDC reaction. In addition, these results indicated that the synergetic effect between the Pd NPs and NH₂-MSNTs support played a key role for the HDC of CPs.

Table 3 Hydrogenation of 4-CP to C-one over different catalysts^a

Entry	Catalysts	Conv. (%)	Selectivity^b (%)
			Ph	C-one
a Conditions: 4-CP (0.5 mmol), base (0.5 mmol), solvent (30 mL), Pd (3.64 mol% relative to 4-CP), 1 atm H2, 60 °C, 180 min.b Phenol indicates Ph and cyclohexanone indicates C-one.
1	None	0	0	0
2	NH2-MSNTs support	0	0	0
3	Pd/C	100	92.8	7.2
4	Pd/mSiO2	100	85.4	14.6
5	CNTs/Fe3O4@SiO2–NH2–Pd	100	23.2	76.8
6	Pd@NH2-MSNTs	100	0	>99

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Fig. S5† shows the time evolution of the product selectivity. Table 4 summarizes the performance of Pd@NH₂-MSNTs for the HDC of CPs, and shows Pd@NH₂-MSNTs owned higher selectivity under mild condition in aqueous solution. It was noteworthy that the hydrogenation of 3-CP, 2,4-DCP and 2,4,6-TCP (entries 2–4) over the Pd@NH₂-MSNTs also occurred with excellent selectivity (>99%) during the reactions (60 °C, ≤300 min), except 2-CP (71.1%, entry 1). Despite the C-one could be further hydrogenated to C-ol, this result indicated that C-one is stable on Pd@NH₂-MSNTs under the conditions employed. Therefore, the Pd@NH₂-MSNTs can not only catalyze the HDC reaction of CPs, but also can further hydrogenate of Ph to generate the much useful product (C-one).

Table 4 Catalytic HDC and further hydrogenation of different CPs over the Pd@NH₂-MSNTs^a

Entry	CPs	Time (min)	Conv. (%)	Selectivity^b (%)
				Ph	C-one	C-ol
a Conditions: CPs (0.5 mmol), base (0.5 mmol), solvent (30 mL), Pd (3.64 mol% relative to CPs), 1 atm H2, 60 °C.b Phenol indicates Ph, cyclohexanone indicates C-one and cyclohexanol indicates C-ol.
1	2-CP	180	100	19.1	71.1	9.8
2	3-CP	180	100	0	>99	1
3	2,4-DCP	300	100	0	>99	1
4	2,4,6-TCP	240	100	0	>99	1

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Conclusions

In this work, we successfully synthesized multicomponent Pd@NH₂-MSNTs with large surface areas and high saturation magnetizations. The Pd@NH₂-MSNTs shows excellent catalytic performance for the HDC of CPs and further hydrogenation of Ph to a very useful industrial product (C-one), with a 100% conversion of CPs and high C-one selectivity (>99%) under mild conditions in aqueous solution. The unique magnetic nanostructure exhibits more convenient separability and excellent reusability in comparison to many of the highly active Pd-based catalysts reported to date. Therefore, this unique support is promising as a novel candidate for other catalysts synthesis. What's more, the design concept for the magnetic mesoporous tubular materials can be extended to the fabrication of other multifunctional nano-systems with integrated and enhanced properties for the hydrodechlorination and selective hydrogenation of CPs in practical application, leading to their environmentally friendly disposal.

Acknowledgements

This research was supported by the National Science-technology Support Plan Projects (no. 2014BAK16B01) and the Key Laboratory of Catalytic engineering of Gansu Province and Resources Utilization, Gansu Province for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13807a

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