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Preparation of supported palladium catalyst from hydrotalcite-like compound for dicyclopentadiene resin hydrogenation
Molecular Catalysis xxx (xxxx) xxxx
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Preparation of supported palladium catalyst from hydrotalcite-like compound for dicyclopentadiene resin hydrogenation Zongxuan Baia, Xiao Chena, Chuang Lia, Weixiang Guana, Ping Chenb, Changhai Lianga,*,1 a
State Key Laboratory of Fine Chemicals, Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, PR China b State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pd catalyst Hydrotalcite-like DCPD resin Hydrogenation High stability
Hydrogenated dicyclopentadiene (DCPD) resin has been aroused widely concerned with developing of polymer material science. Due to its cross-linked molecular structure, large steric hindrance, and high activation energy of unsaturated bonds, the design of hydrogenation catalyst with fine structure, high activity, and excellent stability poses great challenges. Herein, basing on the concept of strong metal support interaction, supported palladium catalyst (Pd-MgAlO-HT) derived from thermal decomposition of hydrotalcite-like compound has been synthesized and the related chemical processing conditions have been optimized in a stainless autoclave at 150−270 °C, 1−9 MPa H2 pressure and 0.5−6 h reaction time. The optimal product shows colorless transparent with a degree of hydrogenation over 96.5 %. As comparing with Pd/MgAlO-IM catalyst prepared by conventional impregnation method, the Pd-MgAlO-HT catalyst presents higher activity and stability for DCPD resin hydrogenation. Combined with the XRD, TEM, H2-TPR analysis results, the enhanced catalytic performance of Pd-MgAlO-HT catalyst could ascribe to the small particle size, high dispersion of Pd particles and strong metal support interaction between Pd species and the support.
1. Introduction DCPD petroleum resin is a low molecular weight thermoset polymer manufactured by polymerization of the DCPD monomer [1–3], which is isolated from the C5 fraction products in the ethylene production process [4]. The excellent physical and chemical properties make it suitable for many applications such as tackifiers of adhesive materials, components of elastomers, coating agent for various instruments, and an additive for ink and paint applications [5–7]. However, high content of unsaturated part in the microstructure of DCPD petroleum resin results in many problems such as dark color, foul odor, low thermal stability, poor oxidation resistance and other defects [8], which limit the scope of its application. Hydrogenation of unsaturated groups in DCPD resin is an effective method to improve the physical and chemical properties, including color, thermos-stability [9–13]. However, due to its cross-linked molecular structure, large steric hindrance, and high activation energy of unsaturated bonds [14,15], the design of hydrogenation catalyst with fine structure, high activity, and excellent stability poses great challenge [16]. Traditionally, the majority of the catalysts for the petroleum resin
hydrogenation are based on metal sulfides and transition metals (Table S1) [17–25]. Petrukhina et al. reported polymeric petroleum resins hydrogenation over supported and unsupported sulfide catalysts [17–19]. Yu et al. reported two-stage hydrogenation of C9 resin with NiWS/γ-Al2O3 and PdRu/γ-Al2O3 catalysts [20]. Although these sulfide catalysts can be as candidates for hydrotreating of petroleum resins, the reaction conditions are relative harsh. Besides, in order to maintain the sulfide state of the catalysts, it is necessary to add a sulfur containing agent to the raw material, which may pollute the product. Recently, Nibased catalysts have also been used in the resins hydrogenation under reaction conditions of 250−330 °C and 6–12 MPa [21–23]. Although these non-noble catalysts exhibit desirable catalytic performance, strict conditions (high reaction pressure and temperature) will lead to a decrease in softening point, and the Gardner color No. of the resin after hydrogenation needs to be increased. Noble metal catalysts, especial Pd-based catalysts are considered to be highly effective in the resin hydrogenation reaction. For examples, 10 wt.% Pd/C catalyst presented high activity for aromatic hydrocarbon resin hydrogenation at reaction conditions of 220 °C and 5.5 MPa [24]. 2 wt.% Pd/γ-Al2O3 catalyst was found to be highly active for C9 resin hydrogenation under reaction
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Corresponding author. E-mail address: [email protected] (C. Liang). 1 http://amce.dlut.edu.cn. https://doi.org/10.1016/j.mcat.2019.110728 Received 13 September 2019; Received in revised form 20 November 2019; Accepted 26 November 2019 2468-8231/ © 2019 Published by Elsevier B.V.
Please cite this article as: Zongxuan Bai, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110728
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2.2. Catalyst characterization
conditions of 250 °C and 7 MPa [25]. Nevertheless, high cost and rare reserves limit their large-scale application. Developing an effective approach to prepare well-define supported noble metal nanoparticles is of tremendous interest. However, there are some deactivation problems for Pd-based catalysts due to the fact that Pd particles cannot be effectively stabilized by the supports, resulting in agglomeration and leaching [26–28]. Numerous methods have been developed to optimize the Pd nanoparticles, such as immobilizing Pd on the support surface [29], embedding Pd into the supports [30], and adding lanthanum oxides to the supports [31], all of those processes are relative complicated, limiting their large-scale application. Therefore, it is necessary to optimize the synthesis method to design a highly dispersion and stable Pd-based catalyst for the hydrogenation of DCPD petroleum resin. [M12−+x M3x+ (OH)2 ]x+ Hydrotalcite-like compounds (HTLCs, (An−) x/n ·mH2 O) are a family of lamellar structure inorganic materials composed of positive charge host layers and exchangeable interlayer anions. It has been demonstrated that HTLCs are effective precursors for the synthesis of noble metal catalysts, allowing uniform distribution of M2+ and M3+ in octahedral layers and preferring orientation of anions in the interlayer, which by further calcination can give rise to form highly dispersed and stable metal particles on supports [32–35]. For example, Han and Feng et al. fabricated a supported Pd nanocatalyst from hydrotalcite-like precursor for the selective hydrogenation of citral and acetylene [36,37]. They concluded that catalysts obtained with hydrotalcite-like precursors had strong metal support interaction, promoting the highly dispersion of metal particles on the supports and excellent catalytic performance. In this work, a nanostructured supported Pd-MgAlO-HT catalyst derived from the calcination and then reduction of HTLCs precursors has been exploited and applied on the hydrogenation of DCPD resin. The influence of reaction temperature, H2 pressure, and reaction time on the catalytic performance are determined in terms of the colority, the degree of hydrogenation, the softening point, and the molecular weight distribution. The reasons for the excellent catalytic performances of the Pd-MgAlO-HT catalyst are identified by comparing it with Pd/MgAlO-IM catalyst prepared by traditional impregnation method.
The crystalline phases of the samples were examined by X-ray diff ;raction (XRD) through a D/MAX-2400 diffractometer (Cu Kα1 radiation, λ=0.15418 nm) operated at 40 kV and 100 mA with a scanning speed of 10°/min between 5° and 90°. Thermogravimetric Analyses (TGA) was performed using a TA SDT 650 apparatus, operating in a stream of helium (100 mL/min) with a temperature ramp of 10 °C/min. The specific surface areas of the catalysts were recorded by the Brunauer-Emmett-Teller (BET) method, and the total pore volume and pore size distributions were determined based on nitrogen gas physisorption measurements over Quantachrome Autosorb-iQ instrument. The metal element composition of the catalyst was performed on an inductively coupled plasma-optical emission spectroscopy (ICP-OES) with a Perkinelmer Optima2000DV spectrometer. The distribution of Pd nanoparticles in the catalyst was acquired using a Tecnai G2 F30 transmission electron microscope (TEM). Before analysis, the ultrasound-treated ethanol solution containing powder samples were deposited on holey C/Cu grids. The structural composition and morphology of the catalyst were investigated by scanning electron microscopy (SEM) observations and corresponding energy-dispersive X-ray spectroscopy (EDS) spectra by a FEI Nova Nano SEM 450. H2 temperature programmed reduction (H2-TPR) profiles of the catalyst was carried out using a Micromeritics AutoChem II chemisorption analyzer. The sample was reduced under a continuous 10 % H2/Ar flow (30 mL/min) while heated up from 50 °C to 500 °C (10 °C/ min). 2.3. DCPD resin hydrogenation tests The hydrogenation of DCPD petroleum resin was performed in a 50 mL autoclave, which was equipped with a temperature controller unit and a magnetic paddle agitator. Prior to reaction, the catalyst was reduced at 400 °C with pure H2 for 2 h and then cooled down to room temperature in Ar. A reactant (20 mL cyclohexane solution with 10 wt. % DCPD petroleum resin) and 0.1 g reduced catalyst were placed inside the batch reactor rapidly to avoid prolonged contact with air. After that, the reactor was closed and purged with H2 three times to eliminate air. The whole system was pressurized to the designed H2 pressure and heating was started at 700 rpm. It is recorded as zero time when the system achieved the desired temperature. After the reaction was completed, the autoclave was cooled in a stainless steel sleeve to ambient temperature and excess hydrogen gas in the reactor was released. The product was obtained after removing solvent by vacuum distillation. Nuclear magnetic resonance (both 1H and 13C NMR) was performed to measure the degree of residual unsaturation in hydrogenated DCPD resin. The spectroscopy was operated on a Bruker AVANCE III spectrometer (500 MHz) at 25 °C and chemical shifts were determined with reference to tetramethylsilane. The degree of hydrogenation (DH) in hydrogenated DCPD resin was measured by 1H-NNMR as follows
2. Experimental Section 2.1. Catalyst preparation Tribasic Pd-Mg-Al HTLCs having an Mg2+/Al3+ molar ratio of 2 was prepared by a co-precipitation method at low supersaturating conditions. The synthesis process was as follows: appropriate ratio of Pd (NO3)2·2H2O, Mg(NO3)2·6H2O, and Al(NO3)3·9H2O were dissolved in deionized water to make a mixed solution, and 120 mL NaOH solution (0.12 mol) was used as precipitating agent. Both solutions were added dropwise to 10 mL Na2CO3 solution (0.01 mol) at the same time with a continuously stirring, maintaining the pH at a constant value of 9∼10 under stirring at room temperature. The suspension was kept for ageing at 60 °C and stirred for 12 h. Then the solid precipitate was separated via filtration and rinsed thoroughly using large quantities of deionized water until the pH value decreased to 7. PdMgAl-HTLCs sample can be obtained after drying at 80 °C for 12 h in air. Finally, Pd-MgAlO-HT sample was prepared by calcining the PdMgAl-HTLCs in argon at 500 °C for 2 h and then reducing in pure hydrogen at 400 °C for 2 h. For comparison, catalyst had the same metal ratio with Pd-MgAlOHT was also prepared by a traditional incipient wetness impregnation (IMP) of MgAlO support, which was derived from the MgAl-HTLCs precursor synthesized with a similar method as above. Briefly, 50 mL methanol solution containing Pd(NO3)2·2H2O (0.0126 g) and MgAlO support (1.00 g) was stirred at 25 °C for 2 h and then dried at 70 °C by reduced pressure distillation. The Pd/MgAlO-IM sample can be achieved by calcination at 500 °C for 2 h in flowing argon and reduction with pure hydrogen at 400 °C for 2 h.
DH= (1 −
area of 4.5 − 6.5 ppm for hydrogenated resin ) × 100% area of 4.5 − 6.5 ppm for feedstock
(1)
Wherein the peaks located at 4.5–6.5 ppm of the DCPD resin belongs to the ethylenic unsaturated portion. Before characterization, the resin was dissolved in 0.6 mL CDCl3 and then transferred to a 5 mm diameter NMR tube. The vibration of functional group in the DCPD resin and hydrogenated product was determined by transmission mode Fourier Transformed-Infrared spectroscopy (FT-IR) on a Thermo Fisher 6700 infrared spectroscopy at a resolution of 4 cm−1 in the range 4000–400 cm−1. The molecular weight and polymer dispersity index (PDI, the result of dividing Mw by Mn) of the DCPD resin were measured by Waters1515 gel permeation chromatography. Prior to testing, the sample was dissolved in THF and filtered with a syringe and then 2
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Fig. 2. Thermogravimetric curves of PdMgAl-HTLCs precursor.
desorption of interlamellar water. The third loss occurs at ca. 450 °C, which originates from the removal of the compensating anions, carbonates ions of the interlayer and the dihydroxylation of the brucite like layers [41]. It has been proved by the XRD characterization. It is noticed that the total weight loss is in the range of 42–45 % for the hydrotalcite precursor at ca. 500 °C and there is no obverse weight loss with further increasing temperature in the TGA profiles. N2 physisorption profiles with corresponding pore size distributions (PSD) were obtained for the calcined Pd-containing catalysts, as displayed in Fig. 3. The isotherm of Pd-MgAlO-HT is similar to that of Pd/ MgAlO-IM and shows typical Type IV and standard H2-type hysteresis loop suggesting that both catalysts are mesoporous materials. In the range of relative pressure (P/P0) 0.5–1.0, the adsorption volume of N2 significantly increased due to the capillary condensation of micropores. Nevertheless, the capillary condensation step is steep in the hysteresis loop of Pd-MgAlO-HT catalyst, which indicates that the Pd particles with 0.5 % loading are homogenous dispersed on the mesopores in the matrix. However, for the Pd/MgAlO-IM sample, the condensation steps become less steep, owing to the occupancy of some pores by Pd particles in mixed-metal oxide support during the impregnation process. This may be caused by the aggregation of particles, which can be confirmed by TEM micrographs. Moreover, according to Table 1, the specific surface area of Pd-MgAlO-HT catalyst (240 m2/g) is larger than that of Pd/MgAlO-IM catalyst (214 m2/g), which further confirms that highly dispersion Pd nanoparticles anchors on the mesopores of MgAlO matrix by the calcination and reduction of PdMgAl-HTLCs precursor.
Fig. 1. X-ray diffraction patterns of MgAl-HTLCs, PdMgAl-HTLCs, Pd/MgAlOIM, and Pd-MgAlO-HT samples.
injected at a 1 mL min−1 flow rate at 40 °C. The color was determined through the Gardner method, and the softening point was measured by use of an Automatic Softening Point Tester (SYD-2806 G). 3. Results and discussion 3.1. Catalyst characterization Fig. 1 gives the XRD powder patterns of uncalcined and calcined hydrotalcites. It is noted that the major diffraction peaks of all precursors (MgAl-HTLCs and PdMgAl-HTLCs) appear at 2θ of 12°, 23°, 34°, 39°, 46°, 60°, and 62°, respectively, corresponding to the (003), (006), (009), (015), (018), (110), and (113) reflections of hydrotalcite-like materials (PDF File Card No. 14-0191) [38], which indicates that the lamellar compound with a well-crystalline phase has been synthesized successfully. For the PdMgAl-HTLCs sample, the intensity of peaks decreases obviously compared with that of MgAl-HTLCs, which can be attributed to the lower crystallinity and structural distortion caused by the bigger size of Pd2+ compared with that of Mg2+ [39]. After the calcination, the obtained catalysts lost the XRD peaks of hydrotalcitelike structure due to the dehydroxylation and decomposition of carbonate, which can be proved by thermogravimetric analysis. Both the XRD patterns of Pd-MgAlO-HT and Pd/MgAlO-IM samples exhibit three peaks at 2θ of 35°, 43°, 62°, which are identified as (111), (200), and (220) planes of MgO (periclase) (PDF File Card No. 04-0829) [26], while for the Al-phase, it may be oxidized during calcining to form amorphous Al2O3, and the others present in the lattice of MgO, resulting in no peaks corresponding to alumina or Mg-Al spinel detected in XRD patterns. In addition, these diffraction peaks are broad, indicating the formation of mixed oxides derived from hydrotalcites with small crystallites. Compared with the Pd/MgAlO-IM catalyst, there is no obvious peak at 40° corresponds to the (111) planes of metallic Pd (PDF File Card No. 05-0681) for the Pd-MgAlO-HT sample [40], which can be attributed to highly dispersion of Pd particles throughout the mixedmetal oxide matrix due to the strong metal support interaction (SMSI). It will be further confirmed by TEM and SEM results. Thermogravimetric measurement is performed to study the conversion from hydrotalcite precursor converted to Pd-MgAlO-HT catalyst upon heating in Ar atmosphere. The curve of weight loss as a function of temperature and the corresponding differential signal are shown in Fig. 2. There are three thermal weight loss stages in the sample. The first loss below 100 °C corresponds to the elimination of physically adsorbed water and the second loss at ca. 200 °C involves the
Fig. 3. N2 physisorption profiles and DFT pore size distributions of Pd/MgAlOIM and Pd-MgAlO-HT catalysts. 3
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Table 1 Structure and chemical properties of the as-synthesized catalysts. Samples
Pd/MgAlO-IM Pd-MgAlO-HT
SBET (m2/ g)[a]
214 240
d (nm)[a]
1.7 1.9
Vtotal (cm3/ g)[a]
0.4365 0.5315
Pd loadings (wt.%)[b] Fresh
Spent
0.55 0.50
0.16 0.45
[a] BET surface area (SBET), average pore diameter (d), and total pore volume (Vtotal) as determined by N2 sorption at −196 °C. [b] The loadings of Pd were determined by ICP-OES.
The distribution of Pd particles on the MgAlO support after reductive treatment is clarified by TEM and STEM analysis (as shown in Fig. 4). It can be seen from the STEM images that the Pd particles and the support have a good contrast allowing measurement of individual particles. Significantly, the Pd-MgAlO-HT catalyst gives Pd particles diameter distribution range from 1.4 to 3.0 nm with an average diameter of 2.25 nm (Fig. 4a), which is significantly smaller than that of conventional Pd/MgAlO-IM sample (5.0–6.6 nm with the average diameter 5.73 nm) (Fig. 4d). It is consistent with the XRD results, further confirms that Pd-MgAlO-HT catalyst with SMSI promotes higher dispersion Pd nanoparticles anchoring on the support. In addition, TEM images show that Pd particles are embedded in the MgAlO support and with a high metal dispersion (Fig. 4a-c). While in the case of Pd/MgAlOIM catalyst, a large amount of aggregated spherical particles distributes on the external surface of the support, as presented in Figs. 4d-f. It most likely derives from the confinement eff ;ect during the hydroxide to oxide process, which reduces the possibility of Pd particles agglomerating on the MgAlO support due to the large amount of hydroxyl portion in brucite-like layers [42]. In contrast, Pd particles migrate during calcination and reduction of the Pd/MgAlO-IM catalyst at the same temperature, which can be attributed to the lack of confinement eff ;ect. SEM micrographs and corresponding EDS maps of Pd, Mg, and Al for the Pd containing catalyst are presented in Figure S1. It is clear to see that both Pd, Mg, and Al are homogeneously and continuously distributed in the samples and the mole ratio of Pd, Mg, and Al is very close to the theoretical value in each case, which indicates that Pd particles are indeed distributed in the MgAlO support in the as-prepared catalysts, and the results are in accordance with the XRD and ICP-OES characterization. H2-TPR profiles of the as-prepared catalysts are shown in Fig. 5. The negative peak at ca. 75 °C for both Pd/MgAlO-IM and Pd-MgAlO-HT catalysts, usually assigns to the decomposition of β-PdHx formed by contacting with hydrogen at low temperature [43]. For the Pd-MgAlOHT catalyst, the second peak at ca. 389 °C is ascribed to the reduction processes of Pd2+ to Pd0 [40]. While in the case of Pd/MgAlO-IM
Fig. 5. H2-TPR profiles of Pd/MgAlO-IM and Pd-MgAlO-HT catalysts.
catalyst, a lower reduction peak appears at ca. 366 °C, suggesting there is weaker interaction between PdO species and the MgAlO support [44]. The diff ;erence in the reduction ability can be originated from the interaction between PdO and near groups formed during the decomposition of HTLCs precursors, which seriously hinders the reduction of Pd2+. As a result, the lack of SMSI in Pd/MgAlO-IM sample leads to easy aggregation of the metal particles, which has been confirmed by above TEM and XRD results. 3.2. Catalytic hydrogenation of DCPD resin 3.2.1. Effect of reaction temperatures Since reaction temperature has very high impact on the DCPD resin hydrogenation process, the eff ;ect of temperature in the range 150 °C–270 °C on the degree of hydrogenation (DH) calculated by 1H NMR results has been tested over Pd-MgAlO-HT catalyst, as illustrated in Fig. 6. The peak in the range of 0.4–3.1 ppm corresponds to the aliphatic proton in the DCPD resin, and the double bond peak at 4.5–6.5 ppm mentioned above can be considered to consist of two broad signals, the former (4.5–5.8 ppm) is attributed to cyclopentenetype double bond and the latter (5.8–6.5 ppm) is assigned to norbornene-type double bond. It is noticed that with the reaction temperature increased from 150 °C to 180 °C, the unsaturated bond peaks at 4.5–6.5 ppm decreases significantly, as well as the DH (Fig. 6b) increases rapidly from 59.4%–87.0 %. However, the peak area of norbornene decreases more quickly than that of cyclopentene, indicating
Fig. 4. STEM and TEM micrographs of Pd-MgAlO-HT (a–c) and Pd/MgAlO-IM (d–f) catalysts. 4
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Fig. 6. 1H-NMR (a) and corresponding degree of hydrogenation (DH) (b) of hydrogenated DCPD resin versus various reaction temperature over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, H2 pressure 5 MPa, reaction time 3 h.
groups in DCPD resin at higher hydrogen pressure. However, the crosslinking degree and reticular structure of DCPD resin will decrease at higher H2 pressure, resulting in the decrease of molecular weight of hydrogenated DCPD petroleum resin (as confirmed by Figure S3). Therefore, under the premise of satisfying the DH of hydrogenated DCPD petroleum resin, 5 MPa is the appropriate reaction pressure.
that the ring-tension of the norbornene is more energy-rich than the cyclopentene [8]. When the reaction temperature increased to 210 °C, the double bond corresponding to the cyclopentene almost disappears and the DH of DPCD resin increases to 96.5 %. Further raising the temperature to 270 °C, a plateau in DH can be observed, which indicates that it is beneficial to saturate the DCPD resin at higher temperature. However, the thermal decomposition of resin would happen at higher temperature, leading to the degradation of DCPD resin. As shown in Figure S2, the weight analysis reveales that the molecular weight (Mw) of feedstock decreases from 735 to ca. 700 after the hydrogenation processing at high temperature. Therefore, for balancing the DH and Mw of hydrogenated DCPD petroleum resin, 210 °C is the desired reaction temperature for the hydrogenation of DCPD resin.
3.2.3. Effect of reaction time Under the optimal reaction temperature and H2 pressure, the reaction time has been further investigated (as displayed in Fig. 8a). It is noted that the unsaturated bonds are significantly hydrogenated within only 0.5 h at 210 °C and 5 MPa over Pd-MgAlO-HT catalyst. With increasing to 3 h, the DH of hydrogenated DCPD resin is improved from 82.5%–96.5 %. At even longer reaction time, the increase of DH is not obvious. With the above results achieved, a high DH of hydrogenated DCPD resin can be obtained under a reaction time over 3 h. However, the catalytic degradation is conducted in an identical manner, GPC analysis (Figure S4) shows that hydrogenated DCPD resin has lower Mw compared with the feedstock and the molecular weight decreases as the reaction time increases. As the extension of hydrogenation time, the hydrogenation process is reversible, and degradation easily occurs to lower the Mw. It is clearly that the reduction of molecular weight can lead to the decline of resin quality. Thus, a reaction time of 3 h is considered suitable for the hydrogenation reaction.
3.2.2. Effect of H2 pressures H2 pressure is another important parameter in hydrogenation of DCPD resin. As shown in Fig. 7, it is not conducive to the progress of the hydrogenation reaction at the low H2 pressure and only 76.4 % DH is obtained at 1 MPa. Similarly to the hydrogenation of DCPD resin at lower temperature, norbornene assigned at 5.8–6.5 ppm is first hydrogenated at low pressure (as shown in Fig. 7a). With increasing to 3 MPa, the peak of cyclopentene (4.5–5.8 ppm) decreases accordingly and the DH increases to 87.8 %. However, the DH reaches up to 96.5 % and stays almost constant when continue increasing the H2 pressure (5 MPa - 9 MPa), which indicates that it is benefit to hydrotreat the unsaturated
Fig. 7. 1H-NMR (a) and corresponding degree of hydrogenation (DH) (b) of hydrogenated DCPD resin versus various H2 pressures over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, temperature 210 °C, reaction time 3 h. 5
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Fig. 8. 1H-NMR (a) and corresponding degree of hydrogenation (DH) (b) of hydrogenated DCPD resin versus various reaction time over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, temperature 210 °C, H2 pressure 5 MPa.
3.2.4. Optimal product analysis With the above results obtained, the optimal reaction conditions are 210 °C, 5 MPa H2 pressure and a reaction time of 3 h. Further 13C-NMR and FT-IR analysis for feedstock and optimal hydrogenated product are carried out (as shown in Fig. 9). It can be seen that the peak representing ethylenic unsaturated parts between 120–140 ppm in the 13 C-NMR (Fig. 9a) disappears markedly. Similarly, in the FT-IR spectroscopy shown in Fig. 9b, the peak representing the double bond at 1620 cm−1 and 3040 cm−1 almost disappear, indicating the unsaturated parts have been mostly hydrogenated. Such results are consistent with the 1H-NMR mentioned above. Fig. 10 shows the images of hydrogenated DCPD resin with different DH. It is not hard to find out that the chromogenic functional groups (double bonds with cyclopentene-type and norbornene-type) in DCPD resin have been removed by the hydrogenation processing over Pd-MgAlO-HT catalyst. The optimal product (DH of 96.5 %) exhibits colorless transparent with a Gardner color No. 0 (Table 2), which is much better than that of feedstock (Gardner color No. 11, Table 2). However, it can also be seen from Table 2 that the softening point decreases from 110 °C to 104 °C slightly after the hydrogenation, which may be due to the small amount of hydrogenation pyrolysis and the colloid substance eliminates during the hydrogenation processing. Additionally, the value of PDI for the feedstock and optimal product are close, which indicates that the molecular weight distribution is similar and uniform during the hydrogenation of DCPD resin under the optimal reaction conditions over PdMgAlO-HT catalyst. TGA is carried out to investigate the thermal stabilities of the DCPD
Fig. 10. The images of DCPD resin and the hydrogenated DCPD resin with various DH. Table 2 Physical and chemical properties of the feedstock and optimal product. Samples
Gardner color No.
Softening point / oC
Mw
Mn
PDI
DCPD resin optimal product
11 0
110 104
735 617
396 328
1.85 1.88
resin and optimal product and the results are shown in Fig. 11. The TGA curves are characterized of the decomposition temperatures at 5 %, 10 %, 20 % weight loss (Td,5 %, Td,10 %, Td,20 %) and residual weight at 300 °C (RW) summarized in Table 3. It can be seen that there is a slight decrease in thermal stability of the product compared to the feedstock due to the pyrolysis during the hydrogenation process, which is consistent with the decrease of Mw and the softening point mentioned above.
Fig. 9. 13C-NMR (a) and FT-IR (b) spectra of DCPD resin and optimal product over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, temperature 210 °C, H2 pressure 5 MPa, time 3 h. 6
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Fig. 13. STEM micrographs of spent Pd-MgAlO-HT (a) and spent Pd/MgAlO-IM (b) catalysts.
cycles, indicating its poor stability. The same trend is observed in the color variation of the hydrogenated DCPD resin. As can be seen from Fig. 12b, the products obtained from Pd-MgAlO-HT catalyst are always light, while the products obtained from Pd/MgAlO-IM catalyst show dark color after the first run. In order to clarify the difference in stability between the two catalysts, the spent catalysts are characterized by TEM images and ICP-OES analysis. As presented in Fig. 13a the TEM image of spent Pd-MgAlO-HT catalyst exhibits a good dispersion with the estimated average particle size 3.49 nm, which is larger than that of the fresh catalyst (2.25 nm), which results in a slightly decrease in catalyst activity. However, severe aggregation of Pd particles (9.74 nm) for the spent Pd/MgAlO-IM catalyst (Fig. 13b) is observed and the Pd particles are washed away after the recycling test, which lead to the deactivation of Pd/MgAlO-IM catalyst. As shown in Table 1, the ICP-OES analysis confirms that ca. 70 % of the Pd species in the spent Pd/MgAlO-IM catalyst are leached after 5 recycles. In comparison, the Pd content of the spent Pd-MgAlO-HT catalyst is almost unchanged. The above results demonstrate that Pd-MgAlO-HT is a highly efficient and stable catalyst in hydrogenation of DCPD resin due to the confinement effect of HTLCs precursor, which results in a SMSI between Pd nanoparticles and support after calcination and reduction treatment, thereby preventing the aggregation and leaching of the active metal [33]. While for Pd/MgAlO-IM catalyst prepared by traditional impregnation method have uneven distribution of active metal due to the influence of surface tension of impregnation solution [34]. Furthermore, in the subsequent catalytic reactions, the weak interaction between active metals and support result in the migration and loss of Pd particles.
Fig. 11. TGA of the DCPD resin and optimal product. Table 3 Summary of TGA results for DCPD resin and optimal product. Samples
Td,5%
DCPD resin optimal product
227.3 190.7
a b
a
(oC)
Td,10% 255.3 238.5
a
(oC)
Td,20% 292.3 292.1
a
(oC)
RW
b
(%)
78.4 78.2
Temperature at 5 %, 10 %, 20 % weight loss from TGA, respectively. Residual weight percentage at 300 °C from TGA.
3.2.5. Catalyst stability testing Although the above experiments show that the Pd-MgAlO-HT catalyst has good activity, the stability of the catalyst is also crucial. Hence we carried out five cycle experiments over Pd-MgAlO-HT catalyst under the optimal conditions. For comparison, the same test over Pd/MgAlOIM catalyst was also performed. In the recycling test, the spent catalyst was centrifuged and washed with cyclohexane and dried overnight at 60 °C after the reaction. For the next cycling test, the recycled catalyst is reactivated in the flowing hydrogen. As shown in Fig. 12, Pd-MgAlO-HT catalyst exhibits higher hydrogenation activity than that of Pd/MgAlOIM catalyst in the first reaction run, which may be due to its smaller particle size and more uniform distribution. Furthermore, in the recycling tests, the Pd-MgAlO-HT catalyst keeps a high DH of hydrogenated DCPD resin in the consecutive 5 catalytic recycles with a slight decrease (from 98.0%–94.5 %) of the hydrogenation ability, which indicates that the satisfactory stability of the Pd-MgAlO-HT catalyst. By contrast, the DH decreases significantly from 90.5%–82.2 % after one cycle over Pd/MgAlO-IM catalyst, and finally drops to 77.7 % after 5
4. Conclusions In this work, a well-dispersed supported palladium catalyst with Pd
Fig. 12. Recycling test (a) and corresponding products (b) over Pd/MgAlO-IM and Pd-MgAlO-HT catalysts. Reaction conditions: catalyst 0.1 g, temperature 210 °C, H2 pressure 5 MPa, reaction time 3 h. 7
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loading of 0.5 wt.% over MgAl oxide has been successfully synthesized using hydrotalcite-like compound as precursor. The resulting PdMgAlO-HT catalyst presents a high specific surface area, uniform element distribution and highly dispersed particles with an average size of 2.25 nm. Detailed study of the reaction parameters for the hydrogenation of DCPD resin confirms that the optimal conditions are reaction temperature of 210 °C and 5 MPa H2 pressure within 3 h over PdMgAlO-HT catalyst. The hydrogenated DCPD resin exhibits colorless transparent with a DH of 96.5 %, indicating that chromogenic functional groups (double bonds with cyclopentene-type and norbornenetype) in DCPD resin have been removed by the hydrogenation processing over Pd-MgAlO-HT catalyst. Compared with Pd/MgAlO-IM catalyst prepared by traditional incipient wetness impregnation method, the hydrogenation activity and stability of the catalyst derived from hydrotalcite-like compound is obviously improved due to smaller crystallite size, higher Pd dispersion, and stronger interactions between Pd nanoparticles and MgAl oxide support. Therefore, the Pd-MgAlO-HT catalyst developed in this work is a promising catalyst for DCPD resin hydrogenation.
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Contributions section Zongxuan Bai fabricated the samples, performed the catalytic reactions, and wrote the manuscript. Xiao Chen and Chuang Li designed the sample compositions, characterised the samples. Weixiang Guan assisted with the catalytic tests. All authors reviewed the manuscript. Ping Chen and Changhai Liang supervised the project, discussed the all results and revised the final manuscript.
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Declaration of Competing Interest [21]
The author declare that they have no know competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China [21573031], the Program for Excellent Talents in Dalian City [2016RD09] and Fundamental Research Funds for the Central Universities [DUT18GJ206].
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Appendix A. Supplementary data [26]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110728.
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Preparation of supported palladium catalyst from hydrotalcite-like compound for dicyclopentadiene resin hydrogenation Zongxuan Baia, Xiao Chena, Chuang Lia, Weixiang Guana, Ping Chenb, Changhai Lianga,*,1 a
State Key Laboratory of Fine Chemicals, Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, PR China b State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pd catalyst Hydrotalcite-like DCPD resin Hydrogenation High stability
Hydrogenated dicyclopentadiene (DCPD) resin has been aroused widely concerned with developing of polymer material science. Due to its cross-linked molecular structure, large steric hindrance, and high activation energy of unsaturated bonds, the design of hydrogenation catalyst with fine structure, high activity, and excellent stability poses great challenges. Herein, basing on the concept of strong metal support interaction, supported palladium catalyst (Pd-MgAlO-HT) derived from thermal decomposition of hydrotalcite-like compound has been synthesized and the related chemical processing conditions have been optimized in a stainless autoclave at 150−270 °C, 1−9 MPa H2 pressure and 0.5−6 h reaction time. The optimal product shows colorless transparent with a degree of hydrogenation over 96.5 %. As comparing with Pd/MgAlO-IM catalyst prepared by conventional impregnation method, the Pd-MgAlO-HT catalyst presents higher activity and stability for DCPD resin hydrogenation. Combined with the XRD, TEM, H2-TPR analysis results, the enhanced catalytic performance of Pd-MgAlO-HT catalyst could ascribe to the small particle size, high dispersion of Pd particles and strong metal support interaction between Pd species and the support.
1. Introduction DCPD petroleum resin is a low molecular weight thermoset polymer manufactured by polymerization of the DCPD monomer [1–3], which is isolated from the C5 fraction products in the ethylene production process [4]. The excellent physical and chemical properties make it suitable for many applications such as tackifiers of adhesive materials, components of elastomers, coating agent for various instruments, and an additive for ink and paint applications [5–7]. However, high content of unsaturated part in the microstructure of DCPD petroleum resin results in many problems such as dark color, foul odor, low thermal stability, poor oxidation resistance and other defects [8], which limit the scope of its application. Hydrogenation of unsaturated groups in DCPD resin is an effective method to improve the physical and chemical properties, including color, thermos-stability [9–13]. However, due to its cross-linked molecular structure, large steric hindrance, and high activation energy of unsaturated bonds [14,15], the design of hydrogenation catalyst with fine structure, high activity, and excellent stability poses great challenge [16]. Traditionally, the majority of the catalysts for the petroleum resin
hydrogenation are based on metal sulfides and transition metals (Table S1) [17–25]. Petrukhina et al. reported polymeric petroleum resins hydrogenation over supported and unsupported sulfide catalysts [17–19]. Yu et al. reported two-stage hydrogenation of C9 resin with NiWS/γ-Al2O3 and PdRu/γ-Al2O3 catalysts [20]. Although these sulfide catalysts can be as candidates for hydrotreating of petroleum resins, the reaction conditions are relative harsh. Besides, in order to maintain the sulfide state of the catalysts, it is necessary to add a sulfur containing agent to the raw material, which may pollute the product. Recently, Nibased catalysts have also been used in the resins hydrogenation under reaction conditions of 250−330 °C and 6–12 MPa [21–23]. Although these non-noble catalysts exhibit desirable catalytic performance, strict conditions (high reaction pressure and temperature) will lead to a decrease in softening point, and the Gardner color No. of the resin after hydrogenation needs to be increased. Noble metal catalysts, especial Pd-based catalysts are considered to be highly effective in the resin hydrogenation reaction. For examples, 10 wt.% Pd/C catalyst presented high activity for aromatic hydrocarbon resin hydrogenation at reaction conditions of 220 °C and 5.5 MPa [24]. 2 wt.% Pd/γ-Al2O3 catalyst was found to be highly active for C9 resin hydrogenation under reaction
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Corresponding author. E-mail address: [email protected] (C. Liang). 1 http://amce.dlut.edu.cn. https://doi.org/10.1016/j.mcat.2019.110728 Received 13 September 2019; Received in revised form 20 November 2019; Accepted 26 November 2019 2468-8231/ © 2019 Published by Elsevier B.V.
Please cite this article as: Zongxuan Bai, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110728
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2.2. Catalyst characterization
conditions of 250 °C and 7 MPa [25]. Nevertheless, high cost and rare reserves limit their large-scale application. Developing an effective approach to prepare well-define supported noble metal nanoparticles is of tremendous interest. However, there are some deactivation problems for Pd-based catalysts due to the fact that Pd particles cannot be effectively stabilized by the supports, resulting in agglomeration and leaching [26–28]. Numerous methods have been developed to optimize the Pd nanoparticles, such as immobilizing Pd on the support surface [29], embedding Pd into the supports [30], and adding lanthanum oxides to the supports [31], all of those processes are relative complicated, limiting their large-scale application. Therefore, it is necessary to optimize the synthesis method to design a highly dispersion and stable Pd-based catalyst for the hydrogenation of DCPD petroleum resin. [M12−+x M3x+ (OH)2 ]x+ Hydrotalcite-like compounds (HTLCs, (An−) x/n ·mH2 O) are a family of lamellar structure inorganic materials composed of positive charge host layers and exchangeable interlayer anions. It has been demonstrated that HTLCs are effective precursors for the synthesis of noble metal catalysts, allowing uniform distribution of M2+ and M3+ in octahedral layers and preferring orientation of anions in the interlayer, which by further calcination can give rise to form highly dispersed and stable metal particles on supports [32–35]. For example, Han and Feng et al. fabricated a supported Pd nanocatalyst from hydrotalcite-like precursor for the selective hydrogenation of citral and acetylene [36,37]. They concluded that catalysts obtained with hydrotalcite-like precursors had strong metal support interaction, promoting the highly dispersion of metal particles on the supports and excellent catalytic performance. In this work, a nanostructured supported Pd-MgAlO-HT catalyst derived from the calcination and then reduction of HTLCs precursors has been exploited and applied on the hydrogenation of DCPD resin. The influence of reaction temperature, H2 pressure, and reaction time on the catalytic performance are determined in terms of the colority, the degree of hydrogenation, the softening point, and the molecular weight distribution. The reasons for the excellent catalytic performances of the Pd-MgAlO-HT catalyst are identified by comparing it with Pd/MgAlO-IM catalyst prepared by traditional impregnation method.
The crystalline phases of the samples were examined by X-ray diff ;raction (XRD) through a D/MAX-2400 diffractometer (Cu Kα1 radiation, λ=0.15418 nm) operated at 40 kV and 100 mA with a scanning speed of 10°/min between 5° and 90°. Thermogravimetric Analyses (TGA) was performed using a TA SDT 650 apparatus, operating in a stream of helium (100 mL/min) with a temperature ramp of 10 °C/min. The specific surface areas of the catalysts were recorded by the Brunauer-Emmett-Teller (BET) method, and the total pore volume and pore size distributions were determined based on nitrogen gas physisorption measurements over Quantachrome Autosorb-iQ instrument. The metal element composition of the catalyst was performed on an inductively coupled plasma-optical emission spectroscopy (ICP-OES) with a Perkinelmer Optima2000DV spectrometer. The distribution of Pd nanoparticles in the catalyst was acquired using a Tecnai G2 F30 transmission electron microscope (TEM). Before analysis, the ultrasound-treated ethanol solution containing powder samples were deposited on holey C/Cu grids. The structural composition and morphology of the catalyst were investigated by scanning electron microscopy (SEM) observations and corresponding energy-dispersive X-ray spectroscopy (EDS) spectra by a FEI Nova Nano SEM 450. H2 temperature programmed reduction (H2-TPR) profiles of the catalyst was carried out using a Micromeritics AutoChem II chemisorption analyzer. The sample was reduced under a continuous 10 % H2/Ar flow (30 mL/min) while heated up from 50 °C to 500 °C (10 °C/ min). 2.3. DCPD resin hydrogenation tests The hydrogenation of DCPD petroleum resin was performed in a 50 mL autoclave, which was equipped with a temperature controller unit and a magnetic paddle agitator. Prior to reaction, the catalyst was reduced at 400 °C with pure H2 for 2 h and then cooled down to room temperature in Ar. A reactant (20 mL cyclohexane solution with 10 wt. % DCPD petroleum resin) and 0.1 g reduced catalyst were placed inside the batch reactor rapidly to avoid prolonged contact with air. After that, the reactor was closed and purged with H2 three times to eliminate air. The whole system was pressurized to the designed H2 pressure and heating was started at 700 rpm. It is recorded as zero time when the system achieved the desired temperature. After the reaction was completed, the autoclave was cooled in a stainless steel sleeve to ambient temperature and excess hydrogen gas in the reactor was released. The product was obtained after removing solvent by vacuum distillation. Nuclear magnetic resonance (both 1H and 13C NMR) was performed to measure the degree of residual unsaturation in hydrogenated DCPD resin. The spectroscopy was operated on a Bruker AVANCE III spectrometer (500 MHz) at 25 °C and chemical shifts were determined with reference to tetramethylsilane. The degree of hydrogenation (DH) in hydrogenated DCPD resin was measured by 1H-NNMR as follows
2. Experimental Section 2.1. Catalyst preparation Tribasic Pd-Mg-Al HTLCs having an Mg2+/Al3+ molar ratio of 2 was prepared by a co-precipitation method at low supersaturating conditions. The synthesis process was as follows: appropriate ratio of Pd (NO3)2·2H2O, Mg(NO3)2·6H2O, and Al(NO3)3·9H2O were dissolved in deionized water to make a mixed solution, and 120 mL NaOH solution (0.12 mol) was used as precipitating agent. Both solutions were added dropwise to 10 mL Na2CO3 solution (0.01 mol) at the same time with a continuously stirring, maintaining the pH at a constant value of 9∼10 under stirring at room temperature. The suspension was kept for ageing at 60 °C and stirred for 12 h. Then the solid precipitate was separated via filtration and rinsed thoroughly using large quantities of deionized water until the pH value decreased to 7. PdMgAl-HTLCs sample can be obtained after drying at 80 °C for 12 h in air. Finally, Pd-MgAlO-HT sample was prepared by calcining the PdMgAl-HTLCs in argon at 500 °C for 2 h and then reducing in pure hydrogen at 400 °C for 2 h. For comparison, catalyst had the same metal ratio with Pd-MgAlOHT was also prepared by a traditional incipient wetness impregnation (IMP) of MgAlO support, which was derived from the MgAl-HTLCs precursor synthesized with a similar method as above. Briefly, 50 mL methanol solution containing Pd(NO3)2·2H2O (0.0126 g) and MgAlO support (1.00 g) was stirred at 25 °C for 2 h and then dried at 70 °C by reduced pressure distillation. The Pd/MgAlO-IM sample can be achieved by calcination at 500 °C for 2 h in flowing argon and reduction with pure hydrogen at 400 °C for 2 h.
DH= (1 −
area of 4.5 − 6.5 ppm for hydrogenated resin ) × 100% area of 4.5 − 6.5 ppm for feedstock
(1)
Wherein the peaks located at 4.5–6.5 ppm of the DCPD resin belongs to the ethylenic unsaturated portion. Before characterization, the resin was dissolved in 0.6 mL CDCl3 and then transferred to a 5 mm diameter NMR tube. The vibration of functional group in the DCPD resin and hydrogenated product was determined by transmission mode Fourier Transformed-Infrared spectroscopy (FT-IR) on a Thermo Fisher 6700 infrared spectroscopy at a resolution of 4 cm−1 in the range 4000–400 cm−1. The molecular weight and polymer dispersity index (PDI, the result of dividing Mw by Mn) of the DCPD resin were measured by Waters1515 gel permeation chromatography. Prior to testing, the sample was dissolved in THF and filtered with a syringe and then 2
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Fig. 2. Thermogravimetric curves of PdMgAl-HTLCs precursor.
desorption of interlamellar water. The third loss occurs at ca. 450 °C, which originates from the removal of the compensating anions, carbonates ions of the interlayer and the dihydroxylation of the brucite like layers [41]. It has been proved by the XRD characterization. It is noticed that the total weight loss is in the range of 42–45 % for the hydrotalcite precursor at ca. 500 °C and there is no obverse weight loss with further increasing temperature in the TGA profiles. N2 physisorption profiles with corresponding pore size distributions (PSD) were obtained for the calcined Pd-containing catalysts, as displayed in Fig. 3. The isotherm of Pd-MgAlO-HT is similar to that of Pd/ MgAlO-IM and shows typical Type IV and standard H2-type hysteresis loop suggesting that both catalysts are mesoporous materials. In the range of relative pressure (P/P0) 0.5–1.0, the adsorption volume of N2 significantly increased due to the capillary condensation of micropores. Nevertheless, the capillary condensation step is steep in the hysteresis loop of Pd-MgAlO-HT catalyst, which indicates that the Pd particles with 0.5 % loading are homogenous dispersed on the mesopores in the matrix. However, for the Pd/MgAlO-IM sample, the condensation steps become less steep, owing to the occupancy of some pores by Pd particles in mixed-metal oxide support during the impregnation process. This may be caused by the aggregation of particles, which can be confirmed by TEM micrographs. Moreover, according to Table 1, the specific surface area of Pd-MgAlO-HT catalyst (240 m2/g) is larger than that of Pd/MgAlO-IM catalyst (214 m2/g), which further confirms that highly dispersion Pd nanoparticles anchors on the mesopores of MgAlO matrix by the calcination and reduction of PdMgAl-HTLCs precursor.
Fig. 1. X-ray diffraction patterns of MgAl-HTLCs, PdMgAl-HTLCs, Pd/MgAlOIM, and Pd-MgAlO-HT samples.
injected at a 1 mL min−1 flow rate at 40 °C. The color was determined through the Gardner method, and the softening point was measured by use of an Automatic Softening Point Tester (SYD-2806 G). 3. Results and discussion 3.1. Catalyst characterization Fig. 1 gives the XRD powder patterns of uncalcined and calcined hydrotalcites. It is noted that the major diffraction peaks of all precursors (MgAl-HTLCs and PdMgAl-HTLCs) appear at 2θ of 12°, 23°, 34°, 39°, 46°, 60°, and 62°, respectively, corresponding to the (003), (006), (009), (015), (018), (110), and (113) reflections of hydrotalcite-like materials (PDF File Card No. 14-0191) [38], which indicates that the lamellar compound with a well-crystalline phase has been synthesized successfully. For the PdMgAl-HTLCs sample, the intensity of peaks decreases obviously compared with that of MgAl-HTLCs, which can be attributed to the lower crystallinity and structural distortion caused by the bigger size of Pd2+ compared with that of Mg2+ [39]. After the calcination, the obtained catalysts lost the XRD peaks of hydrotalcitelike structure due to the dehydroxylation and decomposition of carbonate, which can be proved by thermogravimetric analysis. Both the XRD patterns of Pd-MgAlO-HT and Pd/MgAlO-IM samples exhibit three peaks at 2θ of 35°, 43°, 62°, which are identified as (111), (200), and (220) planes of MgO (periclase) (PDF File Card No. 04-0829) [26], while for the Al-phase, it may be oxidized during calcining to form amorphous Al2O3, and the others present in the lattice of MgO, resulting in no peaks corresponding to alumina or Mg-Al spinel detected in XRD patterns. In addition, these diffraction peaks are broad, indicating the formation of mixed oxides derived from hydrotalcites with small crystallites. Compared with the Pd/MgAlO-IM catalyst, there is no obvious peak at 40° corresponds to the (111) planes of metallic Pd (PDF File Card No. 05-0681) for the Pd-MgAlO-HT sample [40], which can be attributed to highly dispersion of Pd particles throughout the mixedmetal oxide matrix due to the strong metal support interaction (SMSI). It will be further confirmed by TEM and SEM results. Thermogravimetric measurement is performed to study the conversion from hydrotalcite precursor converted to Pd-MgAlO-HT catalyst upon heating in Ar atmosphere. The curve of weight loss as a function of temperature and the corresponding differential signal are shown in Fig. 2. There are three thermal weight loss stages in the sample. The first loss below 100 °C corresponds to the elimination of physically adsorbed water and the second loss at ca. 200 °C involves the
Fig. 3. N2 physisorption profiles and DFT pore size distributions of Pd/MgAlOIM and Pd-MgAlO-HT catalysts. 3
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Table 1 Structure and chemical properties of the as-synthesized catalysts. Samples
Pd/MgAlO-IM Pd-MgAlO-HT
SBET (m2/ g)[a]
214 240
d (nm)[a]
1.7 1.9
Vtotal (cm3/ g)[a]
0.4365 0.5315
Pd loadings (wt.%)[b] Fresh
Spent
0.55 0.50
0.16 0.45
[a] BET surface area (SBET), average pore diameter (d), and total pore volume (Vtotal) as determined by N2 sorption at −196 °C. [b] The loadings of Pd were determined by ICP-OES.
The distribution of Pd particles on the MgAlO support after reductive treatment is clarified by TEM and STEM analysis (as shown in Fig. 4). It can be seen from the STEM images that the Pd particles and the support have a good contrast allowing measurement of individual particles. Significantly, the Pd-MgAlO-HT catalyst gives Pd particles diameter distribution range from 1.4 to 3.0 nm with an average diameter of 2.25 nm (Fig. 4a), which is significantly smaller than that of conventional Pd/MgAlO-IM sample (5.0–6.6 nm with the average diameter 5.73 nm) (Fig. 4d). It is consistent with the XRD results, further confirms that Pd-MgAlO-HT catalyst with SMSI promotes higher dispersion Pd nanoparticles anchoring on the support. In addition, TEM images show that Pd particles are embedded in the MgAlO support and with a high metal dispersion (Fig. 4a-c). While in the case of Pd/MgAlOIM catalyst, a large amount of aggregated spherical particles distributes on the external surface of the support, as presented in Figs. 4d-f. It most likely derives from the confinement eff ;ect during the hydroxide to oxide process, which reduces the possibility of Pd particles agglomerating on the MgAlO support due to the large amount of hydroxyl portion in brucite-like layers [42]. In contrast, Pd particles migrate during calcination and reduction of the Pd/MgAlO-IM catalyst at the same temperature, which can be attributed to the lack of confinement eff ;ect. SEM micrographs and corresponding EDS maps of Pd, Mg, and Al for the Pd containing catalyst are presented in Figure S1. It is clear to see that both Pd, Mg, and Al are homogeneously and continuously distributed in the samples and the mole ratio of Pd, Mg, and Al is very close to the theoretical value in each case, which indicates that Pd particles are indeed distributed in the MgAlO support in the as-prepared catalysts, and the results are in accordance with the XRD and ICP-OES characterization. H2-TPR profiles of the as-prepared catalysts are shown in Fig. 5. The negative peak at ca. 75 °C for both Pd/MgAlO-IM and Pd-MgAlO-HT catalysts, usually assigns to the decomposition of β-PdHx formed by contacting with hydrogen at low temperature [43]. For the Pd-MgAlOHT catalyst, the second peak at ca. 389 °C is ascribed to the reduction processes of Pd2+ to Pd0 [40]. While in the case of Pd/MgAlO-IM
Fig. 5. H2-TPR profiles of Pd/MgAlO-IM and Pd-MgAlO-HT catalysts.
catalyst, a lower reduction peak appears at ca. 366 °C, suggesting there is weaker interaction between PdO species and the MgAlO support [44]. The diff ;erence in the reduction ability can be originated from the interaction between PdO and near groups formed during the decomposition of HTLCs precursors, which seriously hinders the reduction of Pd2+. As a result, the lack of SMSI in Pd/MgAlO-IM sample leads to easy aggregation of the metal particles, which has been confirmed by above TEM and XRD results. 3.2. Catalytic hydrogenation of DCPD resin 3.2.1. Effect of reaction temperatures Since reaction temperature has very high impact on the DCPD resin hydrogenation process, the eff ;ect of temperature in the range 150 °C–270 °C on the degree of hydrogenation (DH) calculated by 1H NMR results has been tested over Pd-MgAlO-HT catalyst, as illustrated in Fig. 6. The peak in the range of 0.4–3.1 ppm corresponds to the aliphatic proton in the DCPD resin, and the double bond peak at 4.5–6.5 ppm mentioned above can be considered to consist of two broad signals, the former (4.5–5.8 ppm) is attributed to cyclopentenetype double bond and the latter (5.8–6.5 ppm) is assigned to norbornene-type double bond. It is noticed that with the reaction temperature increased from 150 °C to 180 °C, the unsaturated bond peaks at 4.5–6.5 ppm decreases significantly, as well as the DH (Fig. 6b) increases rapidly from 59.4%–87.0 %. However, the peak area of norbornene decreases more quickly than that of cyclopentene, indicating
Fig. 4. STEM and TEM micrographs of Pd-MgAlO-HT (a–c) and Pd/MgAlO-IM (d–f) catalysts. 4
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Fig. 6. 1H-NMR (a) and corresponding degree of hydrogenation (DH) (b) of hydrogenated DCPD resin versus various reaction temperature over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, H2 pressure 5 MPa, reaction time 3 h.
groups in DCPD resin at higher hydrogen pressure. However, the crosslinking degree and reticular structure of DCPD resin will decrease at higher H2 pressure, resulting in the decrease of molecular weight of hydrogenated DCPD petroleum resin (as confirmed by Figure S3). Therefore, under the premise of satisfying the DH of hydrogenated DCPD petroleum resin, 5 MPa is the appropriate reaction pressure.
that the ring-tension of the norbornene is more energy-rich than the cyclopentene [8]. When the reaction temperature increased to 210 °C, the double bond corresponding to the cyclopentene almost disappears and the DH of DPCD resin increases to 96.5 %. Further raising the temperature to 270 °C, a plateau in DH can be observed, which indicates that it is beneficial to saturate the DCPD resin at higher temperature. However, the thermal decomposition of resin would happen at higher temperature, leading to the degradation of DCPD resin. As shown in Figure S2, the weight analysis reveales that the molecular weight (Mw) of feedstock decreases from 735 to ca. 700 after the hydrogenation processing at high temperature. Therefore, for balancing the DH and Mw of hydrogenated DCPD petroleum resin, 210 °C is the desired reaction temperature for the hydrogenation of DCPD resin.
3.2.3. Effect of reaction time Under the optimal reaction temperature and H2 pressure, the reaction time has been further investigated (as displayed in Fig. 8a). It is noted that the unsaturated bonds are significantly hydrogenated within only 0.5 h at 210 °C and 5 MPa over Pd-MgAlO-HT catalyst. With increasing to 3 h, the DH of hydrogenated DCPD resin is improved from 82.5%–96.5 %. At even longer reaction time, the increase of DH is not obvious. With the above results achieved, a high DH of hydrogenated DCPD resin can be obtained under a reaction time over 3 h. However, the catalytic degradation is conducted in an identical manner, GPC analysis (Figure S4) shows that hydrogenated DCPD resin has lower Mw compared with the feedstock and the molecular weight decreases as the reaction time increases. As the extension of hydrogenation time, the hydrogenation process is reversible, and degradation easily occurs to lower the Mw. It is clearly that the reduction of molecular weight can lead to the decline of resin quality. Thus, a reaction time of 3 h is considered suitable for the hydrogenation reaction.
3.2.2. Effect of H2 pressures H2 pressure is another important parameter in hydrogenation of DCPD resin. As shown in Fig. 7, it is not conducive to the progress of the hydrogenation reaction at the low H2 pressure and only 76.4 % DH is obtained at 1 MPa. Similarly to the hydrogenation of DCPD resin at lower temperature, norbornene assigned at 5.8–6.5 ppm is first hydrogenated at low pressure (as shown in Fig. 7a). With increasing to 3 MPa, the peak of cyclopentene (4.5–5.8 ppm) decreases accordingly and the DH increases to 87.8 %. However, the DH reaches up to 96.5 % and stays almost constant when continue increasing the H2 pressure (5 MPa - 9 MPa), which indicates that it is benefit to hydrotreat the unsaturated
Fig. 7. 1H-NMR (a) and corresponding degree of hydrogenation (DH) (b) of hydrogenated DCPD resin versus various H2 pressures over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, temperature 210 °C, reaction time 3 h. 5
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Fig. 8. 1H-NMR (a) and corresponding degree of hydrogenation (DH) (b) of hydrogenated DCPD resin versus various reaction time over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, temperature 210 °C, H2 pressure 5 MPa.
3.2.4. Optimal product analysis With the above results obtained, the optimal reaction conditions are 210 °C, 5 MPa H2 pressure and a reaction time of 3 h. Further 13C-NMR and FT-IR analysis for feedstock and optimal hydrogenated product are carried out (as shown in Fig. 9). It can be seen that the peak representing ethylenic unsaturated parts between 120–140 ppm in the 13 C-NMR (Fig. 9a) disappears markedly. Similarly, in the FT-IR spectroscopy shown in Fig. 9b, the peak representing the double bond at 1620 cm−1 and 3040 cm−1 almost disappear, indicating the unsaturated parts have been mostly hydrogenated. Such results are consistent with the 1H-NMR mentioned above. Fig. 10 shows the images of hydrogenated DCPD resin with different DH. It is not hard to find out that the chromogenic functional groups (double bonds with cyclopentene-type and norbornene-type) in DCPD resin have been removed by the hydrogenation processing over Pd-MgAlO-HT catalyst. The optimal product (DH of 96.5 %) exhibits colorless transparent with a Gardner color No. 0 (Table 2), which is much better than that of feedstock (Gardner color No. 11, Table 2). However, it can also be seen from Table 2 that the softening point decreases from 110 °C to 104 °C slightly after the hydrogenation, which may be due to the small amount of hydrogenation pyrolysis and the colloid substance eliminates during the hydrogenation processing. Additionally, the value of PDI for the feedstock and optimal product are close, which indicates that the molecular weight distribution is similar and uniform during the hydrogenation of DCPD resin under the optimal reaction conditions over PdMgAlO-HT catalyst. TGA is carried out to investigate the thermal stabilities of the DCPD
Fig. 10. The images of DCPD resin and the hydrogenated DCPD resin with various DH. Table 2 Physical and chemical properties of the feedstock and optimal product. Samples
Gardner color No.
Softening point / oC
Mw
Mn
PDI
DCPD resin optimal product
11 0
110 104
735 617
396 328
1.85 1.88
resin and optimal product and the results are shown in Fig. 11. The TGA curves are characterized of the decomposition temperatures at 5 %, 10 %, 20 % weight loss (Td,5 %, Td,10 %, Td,20 %) and residual weight at 300 °C (RW) summarized in Table 3. It can be seen that there is a slight decrease in thermal stability of the product compared to the feedstock due to the pyrolysis during the hydrogenation process, which is consistent with the decrease of Mw and the softening point mentioned above.
Fig. 9. 13C-NMR (a) and FT-IR (b) spectra of DCPD resin and optimal product over Pd-MgAlO-HT catalyst. Reaction conditions: catalyst 0.1 g, temperature 210 °C, H2 pressure 5 MPa, time 3 h. 6
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Fig. 13. STEM micrographs of spent Pd-MgAlO-HT (a) and spent Pd/MgAlO-IM (b) catalysts.
cycles, indicating its poor stability. The same trend is observed in the color variation of the hydrogenated DCPD resin. As can be seen from Fig. 12b, the products obtained from Pd-MgAlO-HT catalyst are always light, while the products obtained from Pd/MgAlO-IM catalyst show dark color after the first run. In order to clarify the difference in stability between the two catalysts, the spent catalysts are characterized by TEM images and ICP-OES analysis. As presented in Fig. 13a the TEM image of spent Pd-MgAlO-HT catalyst exhibits a good dispersion with the estimated average particle size 3.49 nm, which is larger than that of the fresh catalyst (2.25 nm), which results in a slightly decrease in catalyst activity. However, severe aggregation of Pd particles (9.74 nm) for the spent Pd/MgAlO-IM catalyst (Fig. 13b) is observed and the Pd particles are washed away after the recycling test, which lead to the deactivation of Pd/MgAlO-IM catalyst. As shown in Table 1, the ICP-OES analysis confirms that ca. 70 % of the Pd species in the spent Pd/MgAlO-IM catalyst are leached after 5 recycles. In comparison, the Pd content of the spent Pd-MgAlO-HT catalyst is almost unchanged. The above results demonstrate that Pd-MgAlO-HT is a highly efficient and stable catalyst in hydrogenation of DCPD resin due to the confinement effect of HTLCs precursor, which results in a SMSI between Pd nanoparticles and support after calcination and reduction treatment, thereby preventing the aggregation and leaching of the active metal [33]. While for Pd/MgAlO-IM catalyst prepared by traditional impregnation method have uneven distribution of active metal due to the influence of surface tension of impregnation solution [34]. Furthermore, in the subsequent catalytic reactions, the weak interaction between active metals and support result in the migration and loss of Pd particles.
Fig. 11. TGA of the DCPD resin and optimal product. Table 3 Summary of TGA results for DCPD resin and optimal product. Samples
Td,5%
DCPD resin optimal product
227.3 190.7
a b
a
(oC)
Td,10% 255.3 238.5
a
(oC)
Td,20% 292.3 292.1
a
(oC)
RW
b
(%)
78.4 78.2
Temperature at 5 %, 10 %, 20 % weight loss from TGA, respectively. Residual weight percentage at 300 °C from TGA.
3.2.5. Catalyst stability testing Although the above experiments show that the Pd-MgAlO-HT catalyst has good activity, the stability of the catalyst is also crucial. Hence we carried out five cycle experiments over Pd-MgAlO-HT catalyst under the optimal conditions. For comparison, the same test over Pd/MgAlOIM catalyst was also performed. In the recycling test, the spent catalyst was centrifuged and washed with cyclohexane and dried overnight at 60 °C after the reaction. For the next cycling test, the recycled catalyst is reactivated in the flowing hydrogen. As shown in Fig. 12, Pd-MgAlO-HT catalyst exhibits higher hydrogenation activity than that of Pd/MgAlOIM catalyst in the first reaction run, which may be due to its smaller particle size and more uniform distribution. Furthermore, in the recycling tests, the Pd-MgAlO-HT catalyst keeps a high DH of hydrogenated DCPD resin in the consecutive 5 catalytic recycles with a slight decrease (from 98.0%–94.5 %) of the hydrogenation ability, which indicates that the satisfactory stability of the Pd-MgAlO-HT catalyst. By contrast, the DH decreases significantly from 90.5%–82.2 % after one cycle over Pd/MgAlO-IM catalyst, and finally drops to 77.7 % after 5
4. Conclusions In this work, a well-dispersed supported palladium catalyst with Pd
Fig. 12. Recycling test (a) and corresponding products (b) over Pd/MgAlO-IM and Pd-MgAlO-HT catalysts. Reaction conditions: catalyst 0.1 g, temperature 210 °C, H2 pressure 5 MPa, reaction time 3 h. 7
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loading of 0.5 wt.% over MgAl oxide has been successfully synthesized using hydrotalcite-like compound as precursor. The resulting PdMgAlO-HT catalyst presents a high specific surface area, uniform element distribution and highly dispersed particles with an average size of 2.25 nm. Detailed study of the reaction parameters for the hydrogenation of DCPD resin confirms that the optimal conditions are reaction temperature of 210 °C and 5 MPa H2 pressure within 3 h over PdMgAlO-HT catalyst. The hydrogenated DCPD resin exhibits colorless transparent with a DH of 96.5 %, indicating that chromogenic functional groups (double bonds with cyclopentene-type and norbornenetype) in DCPD resin have been removed by the hydrogenation processing over Pd-MgAlO-HT catalyst. Compared with Pd/MgAlO-IM catalyst prepared by traditional incipient wetness impregnation method, the hydrogenation activity and stability of the catalyst derived from hydrotalcite-like compound is obviously improved due to smaller crystallite size, higher Pd dispersion, and stronger interactions between Pd nanoparticles and MgAl oxide support. Therefore, the Pd-MgAlO-HT catalyst developed in this work is a promising catalyst for DCPD resin hydrogenation.
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Contributions section Zongxuan Bai fabricated the samples, performed the catalytic reactions, and wrote the manuscript. Xiao Chen and Chuang Li designed the sample compositions, characterised the samples. Weixiang Guan assisted with the catalytic tests. All authors reviewed the manuscript. Ping Chen and Changhai Liang supervised the project, discussed the all results and revised the final manuscript.
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Declaration of Competing Interest [21]
The author declare that they have no know competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
[23] [24]
This work was supported by the National Natural Science Foundation of China [21573031], the Program for Excellent Talents in Dalian City [2016RD09] and Fundamental Research Funds for the Central Universities [DUT18GJ206].
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Appendix A. Supplementary data [26]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110728.
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