Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11821−11830
pubs.acs.org/IECR
A Surface-Cofunctionalized Silica Supported Palladium Catalyst for Selective Hydrogenation of Nitrile Butadiene Rubber with Enhanced Catalytic Activity and Recycling Performance Jian Chen,† Zhijie Wu,† Haiyan Liu,† Xiaojun Bao,‡ and Pei Yuan*,‡ †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China National Engineering Research Center of Chemical Fertilizer Catalyst, College of Chemical Engineering, Fuzhou University, Fuzhou 350002, China
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‡
ABSTRACT: A cofunctionalized silica-supported palladium catalyst has been successfully designed and controllably fabricated for the selective hydrogenation of nitrile butadiene rubber (NBR). The amino groups on the surface can anchor and stabilize Pd particles via the strong coordination with diamine ligands and methyl groups can enhance surface hydrophobicity, weakening the adhesion of NBR/hydrogenated NBR on catalyst and accelerating the desorption of product to re-expose the active sites. The cofunctionalized catalyst with small-sized and well-dispersed Pd particles exhibits an enhanced activity and stability, which can be easily recycled by filtration and reused without any treatment. It remains a high activity and 100% selectivity to CC after recycling five times, and only a slight loss of Pd is detected in products. Such catalyst with cofunctionalized surface is very efficient for NBR hydrogenation and also can be applied to other catalytic systems to tune their selectivity, activity, and reusability.
1. INTRODUCTION The selective hydrogenation of the unsaturated olefin groups (CC) with the retention of nitrile groups (−CN) in nitrile butadiene rubber (NBR) is an effective method to obtain high-quality saturated NBR (HNBR) that exhibits improved thermal properties, and oil and chemical resistance over the original polymer.1−8 With the growing demand for HNBR, the hydrogenation of NBR has attracted significant attention in industries. Generally, HNBR is produced by either homogeneous or heterogeneous catalytic hydrogenation of NBR.1 Homogeneous hydrogenation adopts Rh, Ru, and Pd complexes as catalysts, which could eliminate the diffusion problems and allow the catalytic hydrogenation to be realized at much milder conditions,9−13 while the recovery of the expensive catalyst is often difficult, and the residual catalyst would cause HNBR degradation.14−16 Hence, much research has directed attention to the heterogeneous catalysts on account of the easy to separate the recycled catalysts from product. The most widely explored supported catalysts have been developed by immobilizing Rh or Pd nanoparticles (NPs) on various supports, such as C, TiO2, SiO2, and carbon nanotubes, and the hydrogenation degree of 95% could be obtained.6−8,17−19 However, it was found that the supported Pd NPs on bare silica usually exhibited nonuniform distribution of Pd NPs and easily leached from the support during the reaction due to the poor adhesion on the surface, which eventually limited the catalytic activity and recyclability.20−22 Recently, Yue and coauthors immobilized Rh onto the surface© 2019 American Chemical Society
functionalized SiO2 or carbon nanotubes, and Ai’s group immobilized Pd onto the SiO2 with [3-(Trimethoxysilyl)propyl]urea as a modifier through the chemical bonding to improve the adhesion of active metals on the support.18 We also modified the silica surface with N-propylethylendiamine to anchor the Pd particles by the chemical bonding with diamine and Pd, and which were useful for improving the adhesion and reducing the removal of Pd from the support.7 As compared to the metal NPs supported on silica via conversional incipient wetness impregnation, the particles supported on the surfacefunctionalized silica show better distribution and higher catalytic activity. However, although the leaching of active metals can be avoided, it has been found that the used catalyst needs to be washed by a large amount of solvent to restore its activity, or else the activity of the used catalyst without any treatment is very low.17 This is because NBR cannot be desorbed from the catalyst in time after hydrogenation due to the adsorption of −CN groups on the metal particles, resulting in the active sites on the surface being covered, which hinders the contact of CC bonds with the active metals.18 It has been well-known that the hydrogenation of NBR macromolecules is quite different from the small molecules’ reaction. In the NBR hydrogenation, only a part of the unsaturated CC in the NBR chains could be adsorbed and Received: Revised: Accepted: Published: 11821
March 15, 2019 May 8, 2019 June 10, 2019 June 10, 2019 DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
Article
Industrial & Engineering Chemistry Research catalyzed at one time, and the partially hydrogenated NBR needs to be desorbed from the active sites and undergoes a conformational rearrangement to expose other CC bonds adsorbing on active sites for further hydrogenation.19,23 Yet, the polar groups (−CN) in the polymer could enhance the adhesion of polar molecules on the catalyst and suppress the desorption of the product after the reaction with the result that the adsorption and hydrogenation of other NBR macromolecules are restrained due to the occupation of active sites and the steric hindrance of the adsorbed NBR molecules. Therefore, the key to enhancing the desorption of NBR/ HNBR from the active sites is to weaken or eliminate the strong attraction between −CN and catalyst. In the case of selective hydrogenation of p-chloronitrobenzene, it was found that the selectivity of the target product p-chloroaniline could be enhanced due to the methyl groups grafting on the support, which could enhance the hydrophobicity and then accelerate the desorption of the polar molecule of p-chloroaniline.24−27 Inspired by this work, it is necessary to enhance the hydrophobicity of the catalyst to facilitate the desorption of polar molecules (NBR/HNBR). Herein, we report a surface engineering strategy to prepare a cofunctionalized (amino- and methyl-groups) silica supported Pd catalyst for selective hydrogenation of NBR to produce high-quality HNBR. The amino groups on the surface favor to anchor the metal particles steadily on the support through the strong coordination between Pd and diamine ligands to decrease the particle size and improve the dispersion of Pd NPs. The methyl groups can increase the surface hydrophobicity to enhance the repulsion between the polar molecules and catalyst. A comparative study of the Pd supported on pure silica or single amino- or methyl-groupfunctionalized silica was also performed to verify the advantages of the surface-cofunctionalized catalyst in terms of catalytic activity and recycling performance. It is shown that the activity of cofunctionalized catalyst is as high as that of the amino-functionalized catalyst, but much higher than the nonfunctionalized and methyl-functionalized catalysts because the coordination of diamine ligands and Pd could provide the well-dispersed and small-particle sized Pd NPs on the support and guarantee enough active sites exposed to the reactants. However, more polymer molecules are found to be adhered to amino-functionalized catalyst as compared to the cofunctionalized catalyst. Therefore, without any post-treatment for used catalysts, the surface-cofunctionalized catalyst outperformed other catalysts prepared with pure or monofunctionalized silica as support, which could be effectively recycled by centrifugation and only has little loss of activity after being reused five times.
Ltd. H2 and N2 (research grade: >99.99%) were obtained from Beijing AP BAIF Gases Industry Co., Ltd. 2.2. Preparation of Mono- and Cofunctionalized Silica Supports. The functionalization procedure for silica with EDAS and/or TMCS was as follows. The silica was dried at 100 °C for 24 h, cooled, and dispersed in toluene (the weight ratio of silica to toluene is 1:17.5). A certain amount of EDAS and TMCS then was introduced and stirred for 24 h at 80 °C. The weight ratio of silica:EDAS:TMCS was kept to be 1:0.222:0.163. After the reaction, the powders were filtered and washed with toluene, ethanol, acetone, and dichloromethane. The obtained samples were dispersed in ethanol, for 6 h at 60 °C, centrifuged, and dried overtime. The sample was denoted as C−N−SiO2. Meanwhile, the silica singly functionalized with EDAS or TMCS (the weight ratio of silica to EDAS/TMCS = 1:0.222/0.163) was denoted as N−SiO2 or C−SiO2. 2.3. Synthesis of Silica Supported Pd Catalysts. The synthetic procedure has been previously described:7 1.0 g of a support (pure SiO2, N−SiO2, C−SiO2, or C−N−SiO2) was dispersed in 20 mL of toluene, and then the solution (Pd (3 wt %) in 8 mL of CH2Cl2) was removed from the suspension and stirred for 24 h at 60 °C. The products were filtered and dried at room temperature. The as-prepared samples then were reduced under H2 for 2 h at 140 °C to yield the catalysts named as Pd/SiO2, Pd/N−SiO2, Pd/C−SiO2, and Pd/C−N− SiO2. 2.4. Characterization. The Pd contents in different catalysts and the residues of Pd in the produced HNBR were measured through inductively coupled plasma (ICP) performed with the OPTIMA 7300 V (PerkinElmer, U.S.). N2 physisorption isotherms were performed with the ASAP 2020 equipment (Micromeritics, U.S.). The samples to be measured were first degassed at 180 °C overnight under a vacuum of 10−5 Torr. Fourier transform infrared (FT-IR) spectra were collected on a Nexus 470 (NICOLET, U.S.) spectrometer. The morphology of the catalysts and the energy dispersive Xray spectroscopy (EDX) mapping were characterized by field emission scanning electron microscope (FESEM) operated via the FEI Quanta 200F electron microscope. The N and C contents of Pd/SiO2, Pd/N−SiO2, Pd/C−SiO2, and Pd/ C−N−SiO2 catalysts were estimated by elemental analysis carried out on an elemental analyzer (CHN Elementar Vario EL cube) using combustion method. Transmission electron microscopy (TEM) was characterized by a Philips Tecnai G2 F20 electron microscope, operated at 200 kV. Thermogravimetry (TG) spectra were obtained by the Netzsch STA409PC thermogravimetric analyzer with 10 °C/min of a heating rate in oxygen atmosphere. X-ray diffraction (XRD) patterns were measured by the Shimadzu XRD-6000 diffractometer (Cu Kα radiation) from 10° to 90° with a 2θ scanning speed at 2° per minute. Proton nuclear magnetic resonance (1H NMR) spectra were obtained via a JNM-LA300FT-NMR spectrometer. In addition to the FTIR and 1H NMR, the CC amount of NBR was also measured by iodometric analyses.28 H2−O2 titration was carried out at 150 °C under N2 atmosphere. The catalyst was first reduced by H2, and then titrated by O2 pulses until full saturation of the catalyst. The chemisorbed O2 was repeatedly titrated by H2 pulses until a constant signal peak area. The dispersion (D) of Pd NPs and the number of active sites (NPd) were calculated from the volume of H2 used for the titration of O2 by the following simplified equation:29,30
2. EXPERIMENTAL SECTION 2.1. Chemicals. The materials were employed as obtained without any purification. SiO2 was purchased from Fujian Yu Cheng Environmental Protection Technology Co., Ltd. Acetone, toluene, ethanol, and dichloromethane were supported by Beijing Modern Eastern Fine Chemicals Co., Ltd. Palladium acetate (Pd content: 47.4 wt %) was supported by Zhengzhou Alpha Chemical Co., Ltd. N-(2-Aminoethyl-3aminopropyl)trimethoxysilane (EDAS) (95 wt %) and chlorotrimethylsilane (TMCS) (99 wt %) were supported by Aladdin Co., Ltd. Nitrile butadiene rubber (Nancar1052M30) was supported by Taiwan NANTEX Chemical Industry Co., 11822
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
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Industrial & Engineering Chemistry Research Scheme 1. Synthesis Procedure of Pd/C−N−SiO2 Catalyst
D=
2 V ·M ·10−3 3 H2
22.4W ·m
, NPd =
2 V ·N ·10−3 3 H2 A
3. RESULTS AND DISCUSSION 3.1. Characterization for Functionalized SiO2 and Corresponding Catalysts. The preparation strategy of surface-cofunctionalized silica supported Pd catalyst is shown in Scheme 1. First, the amino and the hydrophobic silane coupling agents are simultaneously grafted onto the surface of the silica through the hydrolysis and polycondensation with silanol groups. Pd precursors then are added to the support, which will interact with the diamine ligands due to the strong Pd−N coordination, and, finally, Pd nanoparticles are steadily supported over the cofunctionalized silica after H2 reduction. Figure 1 exhibits FTIR spectra of the pure and functionalized silica. All samples display typical absorbance bands
22.4W
where VH2 is the volume of H2 used for the titration of O2 (mL), W is catalyst mass (g), M = 106.42 g/mol (the relative molecular mass of Pd), m is the Pd mass fraction of the catalyst (%), and NA = 6.02 × 1023 is Avogadro’s constant. 2.5. Catalytic Tests. The experimental conditions have been already described in our previous work.7 A typical procedure for NBR hydrogenation was conducted in an agitated autoclave (500 mL), which contained 1.0 g of NBR dissolved in 100 mL of acetone and 1.0 g of catalyst. The reaction was then stirred vigorously at 800 rpm under 2.0 MPa H2 at 60 °C. The degree of hydrogenation (HD) was calculated by FTIR and 1H NMR.1,28,31 The HD can be calculated according to the strength of three characteristic peaks at 723, 970, and 2236 cm−1 attributed to (CH2)n (n > 4), CC (1,4-trans), and CN groups from FTIR spectrum. The intensity of CN is used as an internal standard to normalize the strength of (CH2)n and CC. The HD is calculated as eq 1: HD (mol %) = 100 − [CC]/[[CC] + [(CH 2)n]] (1)
× 100%
where [CC] and [(CH2)n] are acquired by [CC] = A̅ (970)/[K(970)F] and [(CH2)n] = A̅ (723)/[K(723)F], respectively, A̅ is the peak intensity ratio of A(970 or 723) to A(2236). F is defined as eq 2: −
Figure 1. FTIR spectra of SiO2, N−SiO2, C−SiO2, and C−N−SiO2.
−
F = 1 + A(723)/K(723)+A(970)/K(970)
(2)
around 796 and 1100 cm−1, which represent the bending vibration of Si−O−Si.32 The peaks around 3500 and 1635 cm−1 are assigned to stretching vibration of the O−H bonds and adsorbed water molecules. For N−SiO2, the peak at 960 cm−1 representing the Si−OH groups disappears after the SiO2 functionalized by EDAS, indicating that the amino-silane coupling agent can react with the silanol groups to successfully graft EDAS onto the surface.33 The N−SiO2 samples display four additional peaks at 1484, 2847, 2940, and 1580 cm−1 assigned to the bending, symmetric, and asymmetric stretching vibration of C−H bonds and the bending vibration of N−H bond, respectively.24 Two new absorbance bands appear at 2965 and 830 cm−1 attributed to the C−H stretching vibration of −CH3 group and Si−C bonds in C−SiO2,34−36 indicating the incorporation of −CH3 groups onto the SiO2. The C−N− SiO2 samples display all of the analogous absorbance bands in the N−SiO2 and C−SiO2, and which all confirm the successful surface functionalization of the silica by EDAS and TMCS. Additionally, elemental analysis was conducted to quantify the amounts of amino and methyl groups present in the catalysts. The N and C loadings are detected to be 0.03, 0.78 wt % for
K(970) = 2.3 and K(723) = 0.255 are constants specific to the HNBR polymer. 1 H NMR was further used to identify the HD of the HNBR, and it is calculated as eq 3: | l [8 − 5[CN]] /[2 + 4(A /B)] o o × A / B} HD (%) = m o1 − o o o 1 − [CN] n ~ × 100%
(3)
where [CN] is a mole fraction of CN in HNBR, and A and B are the integral of peaks representing protons of unsaturated CC in HNBR and protons of −CH2− in NBR, respectively. After each reaction, the catalysts were recycled from the hydrogenated NBR solution through centrifugation, and the product HNBR was extracted with ethanol and then dried at 70 °C for 8 h. The catalyst was kept in acetone and then used in the next reaction without any further regeneration treatments. 11823
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
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Industrial & Engineering Chemistry Research Pd/SiO2, 2.36, 5.76 wt % for Pd/N−SiO2, 0.04, 5.49 wt % for Pd/C−SiO2, and 2.16, 9.52 wt % for Pd/C−N−SiO2, respectively. The theoretical values of N and C elements are 2.41 and 5.17 wt %, 0 and 4.73 wt %, 2.20 and 8.96 wt % for Pd/N−SiO2, Pd/C−SiO2, and Pd/C−N−SiO2, respectively. It is indicated that the N and C loadings are in good agreement with the theoretical values. Figure 2 shows the nitrogen adsorption−desorption isotherms of the pure and functionalized silicas, and type IV
Figure 3. XRD patterns of Pd/SiO2 (A), Pd/N−SiO2 (B), Pd/C− SiO2 (C), and Pd/C−N−SiO2 (D).
Figure 4 shows the TEM images and the change in particle size (the insets). It is clearly shown that Pd NPs in Pd/SiO2
Figure 2. Nitrogen physisorption isotherms and pore size distributions (insets) for SiO2, N−SiO2, C−SiO2, and C−N−SiO2.
isotherms with a H3 hysteresis were observed, indicating the presence of abundant mesopores. We could observe that the surface areas, pore volumes, and pore sizes decreased apparently after the silica surface functionalized with EDAS and TMCS (Table 1) because of the loading of functional Table 1. Textural Properties of Support with Different Functionalizations support
BET surface area (m2/g)
pore volume (cm3/g)
pore diameter (nm)
SiO2 N−SiO2 C−SiO2 C−N−SiO2
514 384 424 368
1.1 0.9 1.0 0.7
9.5 8.1 8.9 7.5
Figure 4. TEM images and Pd particle size distributions (insets) of Pd/SiO2 (A), Pd/N−SiO2 (B), Pd/C−SiO2 (C), and Pd/C−N−SiO2 (D).
groups on the wall of mesoporous structure. It was apparent that the C−N−SiO2 exhibits the most significant variation in these textural parameters; such a decrease is suggested as the amino and methyl groups cografting on the inner wall of the mesoporous structure. XRD and TEM were performed on the materials of Pd supported on the above functionalized supports. Figure 3 exhibits the XRD patterns of these catalysts with theoretical Pd loading of 3 wt %. All samples show a broad diffraction peak around 22°, attributed to the reflection of a typical amorphous silica structure. Diffraction peaks indexed to Pd crystalline structure are indiscernible for Pd/N−SiO2 and Pd/C−N− SiO2, indicating the high dispersion of Pd NPs over the support, and the particle sizes are too small to be detected.20,37 Furthermore, Pd/SiO2 and Pd/C−SiO2 with the hydrophobic trimethyl groups show four obvious diffraction peaks at 40.1°, 46.6°, 68.1°, and 82.1°, assigned to the crystal faces (111), (200), (220), and (311) of Pd structure, and the particle sizes can be calculated to be 8.4 and 9.1 nm by the Scherrer equation for Pd/SiO2 and Pd/C−SiO2.38,39
(Figure 4A) and Pd/C−SiO2 (Figure 4C) have a wide size distribution ranging from 4 to 15 nm, and some large particles with severe aggregation can be observed. It is indicated that Pd particles tend to agglomerate over the pure and hydrophobically modified silica due to the poor adhesion between metals and support without the chemical bindings, while the presence of methyl groups could lead to the further aggregation because of the effect of surface hydrophobicity.37,40 Comparatively, Pd NPs in Pd/N−SiO2 (Figure 4B) and Pd/C−N−SiO2 (Figure 4D) are uniformly distributed with much smaller size centered at 2.1 and 3.2 nm, respectively. This could be attributed to the great affinity of the coordinated amino groups with Pd, facilitating the deposition of the Pd precursors and the formation of smaller Pd NPs.41,42 It should be noted that the theoretical Pd loading amounts of all catalysts are 3 wt %, but the actual contents quantified by ICP are 2.14, 2.83, 1.83, and 2.76 wt % for Pd/SiO2, Pd/N−SiO2, Pd/C−SiO2, and Pd/C− N−SiO2, respectively, as shown in Table 2. It is further verified 11824
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
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Industrial & Engineering Chemistry Research Table 2. Catalytic Properties of the Different Catalysts catalysts
actual Pd loading (wt %)a
particle size (nm)b
D (%)c
NPd (×1019 sites/gcat)d
HD (%)e
r0 × 104 (mol/L·min)f
k′ × 104 (/s)g
Pd residue (ppm)h
Pd/SiO2 Pd/N−SiO2 Pd/C−SiO2 Pd/C−N−SiO2
2.14 2.83 1.83 2.76
8.5 2.1 9.4 3.2
18 49 14 43
2.24 8.01 1.52 6.91
88.1 96.5 84.1 96.0
3.37 4.51 3.78 4.70
6.94 10.5 8.63 12.6
426.3 12.6 646.3 27.2
a
Actual loading amount of Pd in different catalysts determined by ICP. bParticle size of Pd NPs obtained from the statistical results from TEM images. cDispersion of Pd NPs. dActive sites determined by H2−O2 titration. eFinal hydrogenation degree of HNBR. fInitial reaction rate obtained by dividing the amount of reacted CC by the reaction time (10 min). The CC amount in 1 g of NBR is measured to be 0.85 mmol by iodometric analyses. gReaction rate constant. hResidual amount of Pd in the HNBR detected by ICP.
that the amino group favors to adsorb and steadily anchor Pd particles on the support due to the strong coordination between Pd and diamine ligands, while the CH3-modified surface is not conducive to the adsorption and deposition of Pd particles because of the repulsive force between hydrophobic surface and Pd precursors.33 The EDX mappings of Pd/C−N− SiO2 are shown in Figure 5. The elements of C, N are
Figure 6. FTIR spectra of NBR and HNBR catalyzed with Pd/SiO2 (a), Pd/N−SiO2 (b), Pd/C−SiO2 (c), and Pd/C−N−SiO2 (d).
96.0%, respectively. The Pd/C−SiO2 catalyst exhibits the lowest NBR conversion attributed to the extremely low Pd contents and the larger Pd particles as mentioned above. The HD of HNBR was further determined by 1HNMR, and the results are displayed in Figure 7. The peaks from 4.9−5.6 Figure 5. SEM EDX mappings of Pd/C−N−SiO2, Si, O, C, N, and Pd, respectively.
thoroughly distributed over the support, indicating the successful surface functionalization with EDAS and TMCS, and Pd element mapping shows the well-dispersed Pd particles, in agreement with the TEM and XRD results. The dispersion of Pd active sites on different catalysts was acquired from H2− O2 titration measurements, and, as shown in Table 2, the higher dispersion and more active sites can be found for Pd/ N−SiO2 (49%, 8.01 × 1019 sites/gcat) and Pd/C−N−SiO2 (43%, 6.91 × 1019 sites/gcat) than for Pd/SiO2 (18%, 2.24 × 1019 sites/gcat) and Pd/C−SiO2 (14%, 1.52 × 1019 sites/gcat), consistent with the TEM observation and further confirming the importance of amino groups. 3.2. NBR Hydrogenation Activity. Figure 6 exhibits typical FTIR characterizations of the pure NBR and the products HNBR produced by the four catalysts. All samples showed a distinct absorbance around 2236 cm−1 (−CN), and no new absorbances assigned to −NH− appeared after the reaction, implying the 100% selectivity to CC hydrogenation. The characteristic absorbances at 970 cm−1 (1,4CC) and 920 cm−1 (1,2-CC) are decreased or even disappeared, and a new peak at 723 cm−1 (−(CH2)n− (n > 4)) appears due to the saturation of CC in HNBR.4,7,8,12 The HD of HNBR produced by these catalysts can be calculated from the FTIR spectrum, which is 88.1%, 96.5%, 84.1%, and
Figure 7. 1H NMR spectra of NBR and HNBR produced by Pd/SiO2 (a), Pd/N−SiO2 (b), Pd/C−SiO2 (c), and Pd/C−N−SiO2 (d).
ppm are attributed to the protons of CC: the small peaks ranging from 4.9 to 5.1 ppm belong to CHCH2 (1,2-unit), while the large peaks ranging from 5.3−5.6 ppm belong to CHCH (cis- and trans-1,4-units). Meanwhile, the peaks from 0.8 to 2.6 ppm are attributed to the aliphatic protons in the CH3, CH2, and CH microstructures.1 In the case of HNBR obtained by Pd/C−N−SiO2 (Figure 7d), it could be clearly seen that most of the peaks in the CC protons region 11825
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
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Industrial & Engineering Chemistry Research disappear and new peaks appear in the aliphatic protons region, which confirms that the unsaturated CC bonds are hydrogenated by H2. The presence of a peak around 2.6 ppm was attributed to −CH(CN)−, and no new peaks for −NH− or −NH2 appear, further suggesting the reaction is 100% toward the CC for NBR hydrogenation. Also, the HD values of the products HNBR obtained by the Pd/SiO2, Pd/N−SiO2, Pd/C−SiO2, and Pd/C−N−SiO2 are calculated via 1H NMR to be 88.3%, 97.2%, 85.4%, and 96.7%, respectively, in accordance with the results obtained from FTIR. Kinetics of the NBR catalytic hydrogenation in the presence of these catalysts were performed with the high-pressure reactor under a constant reaction condition as shown in Figure 8. It is worth noting that all of the reactions can be finished in
Figure 9. −ln(1 − X) versus time of first-order linear fitting over the different catalysts.
the loss of Pd should be controlled as little as possible. Table 2 lists the data obtained by ICP analysis and indicates that as much as 426.3 and 646.3 ppm of Pd were found in the products catalyzed with the Pd/SiO 2 and Pd/C−SiO 2 catalysts, while only 12.6 and 27.2 ppm of Pd residues were detected in HNBR catalyzed with the Pd/N−SiO2 and Pd/C− N−SiO2. It could be concluded that surface functionalization with EDAS not only can guarantee the high Pd actual loading amounts and good dispersion but also can stabilize Pd NPs on the support via the strong chemical adhesion. Because of the residual Pd in the products, HNBR obtained by different catalysts shows different apparent color as provided in Figure 10. It can be clearly seen that the color of HNBR deepens following the order of B < D < A < C, which is in agreement with the results from ICP analysis (Table 2). 3.3. Recycling Performance of Prepared Catalysts. The significant advantage of heterogeneous catalysis is that the catalysts are easy to separate from products and can be recycled and reused, and the reusability is the key to evaluating the quality of catalysts. In our previous work, we have successfully prepared the Pd catalyst supported on EDASmodified silica for the hydrogenation of NBR, which presented a high activity with HD of 96.6%.7 It was found that the used catalysts were easily separated from the HNBR solution after reaction but needed to be washed with acetone several times to recover its activity. Yue’s group also found the similar phenomenon that after the treatment of recovered supported Rh catalyst with chlorobenzene and acetone, the HD was restored from 67.61% to 81.66%.18 The washing treatements with a large number of solvents can indeed restore catalysts’ activity, but this process is time-consuming, environmentally unfriendly, and will increase production costs. Therefore, it is necessary to upgrade the catalyst through surface engineering. Figure 11 shows the recycling activity of the four catalysts with different surface modications for five runs. All of the calalysts were kept in acetone after separation from the HNBR solution and then used in the next reaction immediately without any further regeneration treatments. Pd/N−SiO2 without solvent washing shows a dramatical drease of HD from 96.5% to 63.2% after five times of recycling, results similar to previous reports.17,18 Similarly, Pd/SiO2 and Pd/C−SiO2 catalysts also lose most of their activity from 88.1% to 48.3% and from 84.1% to 40.3% after five continuous cycles, unfortunately. Yet it is surprising and exciting to find that Pd NPs supported on surface-cofunctionalized silica (Pd/C−N−SiO2) exhibit a high
Figure 8. Time−conversion curves of NBR hydrogenation over the different catalysts.
2 h and 90% of the reactions could be completed at the beginning of 30 min, indicating the hydrogenation of NBR on the Pd active sites occurs very fast. Furthermore, Pd/C−N− SiO2 and Pd/N−SiO2 with the amino groups show much higher HD than Pd/C−SiO2 and Pd/SiO2, due to the improved dispersion of Pd NPs and high metallic surface. The values of initial rate of hydrogenation (r0) for Pd/SiO2, Pd/N−SiO2, Pd/C−SiO2, and Pd/C−N−SiO2 are calculated to be 3.37, 4.51, 3.78, and 4.70 × 10−4 mol/(L·min), respectively (Table 2). The hydrogenation of NBR can be regarded as a pseudo-first-order reaction under the high H2 pressure according to eq 4.7,43,44 −
d[CC] = k′[1 − X ] dt
(4)
The k′ and X are the reaction rate constant and the HD of the hydrogenated HNBR. The equation then could be expressed as eq 5: −ln(1 − X ) = k′·t
(5)
The k′ could be obtained from the fitted line of −ln(1 − X) versus time as shown in Figure 9, and which were 6.94, 10.5, 8.63, and 12.6 × 10−4 s−1 for Pd/SiO2, Pd/N−SiO2, Pd/C− SiO2, and Pd/C−N−SiO2, respectively (Table 2). The r0, k′, and final HD of Pd/N−SiO2 and Pd/C−N−SiO2 are all much higher than those other two catalysts, due to the actual Pd loading amounts and the surface active sites. The leaching of Pd is one of the reasons for the decline of the catalytic activity, and, on the other hand, the residual Pd in HNBR could also accelerate the degradation of polymer. Thus, 11826
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
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Industrial & Engineering Chemistry Research
Figure 10. Images of NBR and HNBR produced by different catalysts: Pd/SiO2 (A), Pd/N−SiO2 (B), Pd/C−SiO2 (C), and Pd/C−N−SiO2 (D).
the recovered catalysts are verified to be Pd/SiO2 (1.72 wt %), Pd/N−SiO2 (2.65 wt %), Pd/C−SiO2 (1.33 wt %), and Pd/ C−N−SiO2 (2.55 wt %), respectively, which are 80.4%, 93.5%, 72.7%, and 92.4% of the initial Pd contents in their corresponding fresh catalysts. Thus, this might be another reason for the significant decrease of activity of Pd/SiO2 and Pd/C−SiO2 catalysts after each continuous cycle. However, there is no obvious change in the Pd NPs size and distribution of Pd/N−SiO2, and the content of Pd supported on the Pd/ N−SiO2 also changes little, and thus these are not the reason for the reduced activity in Pd/N−SiO2. Figure 13 shows FTIR spectra of the four new and recycled catalysts. It can be seen that, as compared to the fresh catalysts, the recycled Pd/SiO2 and Pd/N−SiO2 obviously show new absorbance bands at 2236, 2857, and 2927 cm−1 attributed to the −CN stretching vibration and symmetric/asymmetric stretching of C−H,13,45 suggesting a large amount of NBR/ HNBR molecules adsorbed on the recycled Pd/SiO2 and Pd/ N−SiO2 without solvent washing. Yet these characteristic peaks at 2927 and 2857 cm−1 in Pd/C−N−SiO2 and Pd/C− SiO2 are very weak and the peaks at 2236 cm−1 are negligible, indicating a small amount of polymers adsorbing on these two catalysts. To further verify the adsorption of NBR/HNBR macromolecules on the used catalysts, TG analysis was performed, and the results are given in Figure 14. It is clear that the TG curves exhibit an obvious weight loss between 200 and 550 °C due to the thermal decomposition of the TMCS, EDAS, NBR, and HNBR. It is found that the total weight loss follows the trend of Pd/N−SiO2 (53.6 wt %) > Pd/SiO2 (30.5 wt %) > Pd/C−N−SiO2 (23.6 wt %) > Pd/C−SiO2 (13.0 wt %). The theoretical EDAS and TMCS loading amounts are 11.4 and 6.8 wt %. Thus, the adsorbing amounts of polymers on the catalysts are Pd/N−SiO2 (42.2 wt %), Pd/SiO2 (30.5 wt %), Pd/C−N−SiO2 (6.8 wt %), and Pd/C−SiO2 (6.2 wt %), respectively, suggesting a larger amount of macromolecules adsorbing on Pd/N−SiO2 and Pd/SiO2, in good agreement with FTIR results. Pd/N−SiO2 has the most weight loss probably because Pd active sites have an attraction with −CN groups and the polar EDAS also produced a polar attraction with the NBR/HNBR.7 Pd/C−SiO2 has the minimal weight loss, probably due to the nonpolar and hydrophobic surface having a repulsive force with polar NBR/HNBR molecules and facilitating their desorption from the catalyst.46−48 It can be concluded that the reason for the reduced activity in Pd/SiO2 and Pd/C−SiO2 after five continuous cycles might be the migration, aggregation, and the leaching of Pd during the hydrogenation process, while the adhesion of NBR/HNBR macromolecules on the catalyst surface covering the active sites is the reason for the declining activity of Pd/N−SiO2 after five continuous cycles. As illustrated in Scheme 2, because the −CN bonds could have an attraction with Pd NPs and the polar amino groups on the support also might enhance the polar attraction with the polar NBR/HNBR macromolecules,
Figure 11. Hydrogenation degree of HNBR obtained by different catalysts for five recycles.
stability and recyclability, as almost 94% activity is retained after the five cycles. To understand the deactivation mechanism of the recycled catalysts, TEM, IR, and TG analyses were carried out for the four recovered catalysts. TEM images in Figure 12B and D
Figure 12. TEM images of the recycled catalysts: Pd/SiO2 (A), Pd/ N−SiO2 (B), Pd/C−SiO2 (C), and Pd/C−N−SiO2 (D).
show that the used Pd/N−SiO2 and Pd/C−N−SiO2 catalysts have similar Pd particle sizes as fresh ones, and the average particle sizes are around 3−4 nm. However, the particle sizes are obviously enlarged to 15−20 and 20−30 nm in the used Pd/SiO2 and Pd/C−SiO2 catalysts (Figure 12A and C) because of the aggregation of Pd particles and the H2 reduction process. Additionally, through ICP analysis, the Pd amounts of 11827
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
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Figure 13. FTIR spectra of the new and recycled catalysts.
thus HNBR could not easily desorb from the Pd/N−SiO2 and still occupy the Pd active centers. The activity of the used Pd/ N−SiO2 catalyst could be restored only after washing with a large amount of solvent several times to elute the polymers and re-expose the active sites,46−48 or else the recycling performance would be significantly reduced due to the coverage of active metal sites. For the surface-cofunctionalized silicasupported palladium catalyst, the CH3 could improve its surface hydrophobicity and then reduce surface polarity, and the repulsive force between the polar CN bonds and the nonpolar methyl groups could result in the acceleration of the desorption of HNBR macromolecules from the Pd/C−N− SiO2 catalyst after the hydrogenation of CC. Therefore, the Pd active sites could be re-exposed to ensure its high activity and recycling performance in the next reaction without any regeneration treatment. It is demonstrated that the amino
Figure 14. TG analysis of the recycled catalysts after reaction.
Scheme 2. Procedure of NBR Hydrogenation over Pd/N−SiO2 and Pd/C−N−SiO2
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catalyzed by Ru(II) complexes. J. Mol. Catal. A: Chem. 1997, 126, 115−131. (6) Ai, C.; Gong, G.; Zhao, X.; Liu, P. Macroporous hollow silica microspheres-supported palladium catalyst for selective hydrogenation of nitrile butadiene rubber. J. Taiwan Inst. Chem. Eng. 2017, 77, 250−256. (7) Chen, J.; Ma, L.; Cheng, T.; Cai, A.; Hu, Y.; Wu, Z.; Liu, H.; Bao, X.; Yuan, P. Stable and recyclable Pd catalyst supported on modified silica hollow microspheres with macroporous shells for enhanced catalytic hydrogenation of NBR. J. Mater. Sci. 2018, 53, 15064−15080. (8) Cheng, T.; Chen, J.; Cai, A.; Wang, J.; Liu, H.; Hu, Y.; Bao, X.; Yuan, P. Synthesis of Pd/SiO 2 Catalysts in Various HCl Concentrations for Selective NBR Hydrogenation: Effects of H+ and Cl− Concentrations and Electrostatic Interactions. ACS Omega 2018, 3, 6651−6659. (9) Mao, T.-F.; Rempel, G. L. Catalytic hydrogenation of nitrilebutadiene copolymers by cationic rhodium complexes. J. Mol. Catal. A: Chem. 1998, 135, 121−132. (10) Bhattacharjee, S.; Bhowmick, A. K.; Avasthi, B. N. Hydrogenation of nitrile rubber using a new homogeneous Palladium (II) catalyst: Synthesis and characterization. J. Appl. Polym. Sci. 1990, 41, 1357−1363. (11) Ai, C.; Gong, G.; Zhao, X.; Liu, P. Selectively catalytic hydrogenation of nitrile-butadiene rubber using Grubbs II catalyst. Macromol. Res. 2017, 25, 461−465. (12) Liu, Y.; Wei, Z.; Pan, Q.; Rempel, G. L. Hydrogenation of acrylonitrile-butadiene rubber latex using in situ synthesized RhCl(PPh3)3 catalyst. Appl. Catal., A 2013, 457, 62−68. (13) Zhenli, W.; Jialong, W.; Qinmin, P.; L, R. G. Direct Catalytic Hydrogenation of an Acrylonitrile-Butadiene Rubber Latex Using Wilkinson’s Catalyst. Macromol. Rapid Commun. 2005, 26, 1768− 1772. (14) Zhou, W.; Yi, J.; Lin, J.; Fang, S.; Peng, X. Preparation of facile separable homogeneous Rhodium catalyst and its application for the catalytic hydrogenation of nitrile butadiene rubber and styrenebutadiene rubber. Res. Chem. Intermed. 2017, 43, 3651−3662. (15) Yang, L.; Pan, Q.; Rempel, G. L. Development of a green separation technique for recovery of Wilkinson’s catalysts from bulk hydrogenated nitrile butadiene rubber. Catal. Today 2013, 207, 153− 161. (16) Ai, C.; Gong, G.; Zhao, X.; Liu, P. Ureido-modified macroporous hollow silica microspheres for recovery of Wilkinson’s catalyst in hydrogenated nitrile butadiene rubber. Powder Technol. 2017, 318, 501−506. (17) Zou, R.; Li, C.; Zhang, L.; Yue, D. Selective hydrogenation of nitrile butadiene rubber (NBR) with Rhodium nanoparticles supported on carbon nanotubes at room temperature. Catal. Commun. 2016, 81, 4−9. (18) Cao, P.; Ni, Y.; Zou, R.; Zhang, L.; Yue, D. Enhanced catalytic properties of Rhodium nanoparticles deposited on chemically modified SiO2 for hydrogenation of nitrile butadiene rubber. RSC Adv. 2015, 5, 3417−3424. (19) Han, K. Y.; Zuo, H. R.; Zhu, Z. W.; Cao, G. P.; Lu, C.; Wang, Y. H. High Performance of Palladium Nanoparticles Supported on Carbon Nanotubes for the Hydrogenation of Commercial Polystyrene. Ind. Eng. Chem. Res. 2013, 52, 17750−17759. (20) Li, Y.; Liu, H.; Ma, L.; He, D. Influence of Pd precursors and Cl addition on performance of Pd-Re catalysts in glycerol hydrogenolysis to propanediols. Appl. Catal., A 2016, 522, 13−20. (21) Celebi, M.; Yurderi, M.; Bulut, A.; Kaya, M.; Zahmakiran, M. Palladium nanoparticles supported on amine-functionalized SiO2 for the catalytic hexavalent chromium reduction. Appl. Catal., B 2016, 180, 53−64. (22) Yuan, T.; Gong, H.; Kailasam, K.; Zhao, Y.; Thomas, A.; Zhu, J. Controlling hydrogenation selectivity with Pd catalysts on carbon nitrides functionalized silica. J. Catal. 2015, 326, 38−42.
groups could anchor and enhance the well-dispersed Pd NPs over the support and the methyl groups could weaken the adhesion of polar macromolecules with the catalyst to reexpose the active sites, which is of great significance for the hydrogenation of NBR.
4. CONCLUSION Pd/C−N−SiO2 catalyst with a coupling functionalization of coordinated and hydrophobic groups was rationally designed and controllably fabricated. On one hand, the amino groups could effectively anchor and stabilize Pd particles via the strong coordination between diamine ligands and Pd atoms, leading to well-dispersed Pd NPs with small-sized Pd NPs over the support. Also, the methyl groups might enhance the surface hydrophobicity of the support, which could weaken the adhesion of polar NBR/HNBR macromolecules with the support and promote the desorption of the HNBR from Pd/ C−N−SiO2 after reaction and the re-exposure of active sites. Therefore, Pd/C−N−SiO2 catalyst shows a high activity of 96.0% HD with 100% selectivity to CC bonds, and an outstanding stability and recyclability with almost 94% activity remained after 5 cycles due to the synergistic promotional effect. This work clearly reveals the deactivation mechanism of the different catalysts and provides an efficient and recyclable heterogeneous catalyst for NBR hydrogenation. Moreover, this cofunctionalized process could be applied to more catalytic reactions to tune the selectivity, activity, and durability of the supported catalysts and further improve the selectivity of target products.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86 591 22867950. E-mail:
[email protected]. ORCID
Zhijie Wu: 0000-0002-8160-6615 Xiaojun Bao: 0000-0001-7589-5409 Pei Yuan: 0000-0002-2562-6478 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for financial support from the National Natural Science Foundation of China (Grants 21576290, 21776048), the Fujian Province Natural Science Funds for Distinguished Young Scholar (2018J06002), and the Fujian young top-notch innovative talent project.
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REFERENCES
(1) Wang, H.; Yang, L.; Rempel, G. L. Homogeneous Hydrogenation Art of Nitrile Butadiene Rubber: A Review. Polym. Rev. 2013, 53, 192−239. (2) Ai, C. J.; Li, J. G.; Gong, G. B.; Zhao, X. T.; Liu, P. Preparation of hydrogenated nitrile-butadiene rubber (H-NBR) with controllable molecular weight with heterogeneous catalytic hydrogenation after degradation via olefin cross metathesis. React. Funct. Polym. 2018, 129, 53−57. (3) Severe, G.; White, J. L. Dynamically vulcanized blends of oilresistant elastomers with HNBR. J. Appl. Polym. Sci. 2005, 95, 2−5. (4) Liu, Y.; Kim, H.; Pan, Q.; Rempel, G. L. Hydrogenation of acrylonitrile-butadiene copolymer latex using water-soluble Rhodium catalysts. Catal. Sci. Technol. 2013, 3, 2689−2698. (5) Martin, P.; McManus, N. T.; Rempel, G. L. A detailed study of the hydrogenation of nitrile-butadiene rubber and other substrates 11829
DOI: 10.1021/acs.iecr.9b01468 Ind. Eng. Chem. Res. 2019, 58, 11821−11830
Article
Industrial & Engineering Chemistry Research (23) Dong, L. B.; Turgmancohen, S.; Roberts, G. W.; Kiserow, D. J. Effect of Polymer Size on Heterogeneous Catalytic Polystyrene Hydrogenation. Ind. Eng. Chem. Res. 2010, 49, 11280−11286. (24) Li, H.; Xu, Y.; Yang, H.; Zhang, F.; Li, H. Ni-B amorphous alloy deposited on an aminopropyl and methyl co-functionalized SBA-15 as a highly active catalyst for chloronitrobenzene hydrogenation. J. Mol. Catal. A: Chem. 2009, 307, 105−114. (25) Kratky, V.; Kralik, M.; Mecarova, M.; Stolcova, M.; Zalibera, L.; Hronec, M. Effect of catalyst and substituents on the hydrogenation of chloronitrobenzenes. Appl. Catal., A 2002, 235, 225−231. (26) Melero, J. A.; van Grieken, R.; Morales, G. Advances in the Synthesis and Catalytic Applications of Organosulfonic-Functionalized Mesostructured Materials. Chem. Rev. 2006, 106, 3790−3812. (27) Gosselink, R. W.; Xia, W.; Muhler, M.; de Jong, K. P.; Bitter, J. H. Enhancing the Activity of Pd on Carbon Nanofibers for Deoxygenation of Amphiphilic Fatty Acid Molecules through Support Polarity. ACS Catal. 2013, 3, 2397−2402. (28) Singha, N. K.; Bhattacharjee, S.; Sivaram, S. Hydrogenation of Diene Elastomers, Their Properties and Applications: A Critical Review. Rubber Chem. Technol. 1997, 70, 309−367. (29) Feng, J.; Wang, H.; Evans, D. G.; Duan, X.; Li, D. Catalytic hydrogenation of ethylanthraquinone over highly dispersed eggshell Pd/SiO2-Al2O3 spherical catalysts. Appl. Catal., A 2010, 382, 240− 245. (30) He, Y.; Feng, J.; Du, Y.; Li, D. Controllable Synthesis and Acetylene Hydrogenation Performance of Supported Pd Nanowire and Cuboctahedron Catalysts. ACS Catal. 2012, 2, 1703−1710. (31) Bhattacharjee, S.; Bhowmick, A. K.; Avasthi, B. N. Highpressure hydrogenation of nitrile rubber: thermodynamics and kinetics. Ind. Eng. Chem. Res. 1991, 30, 1086−1092. (32) Yu, T.; Yang, R.; Xia, S.; Li, G.; Hu, C. Direct amination of benzene to aniline with H2O2 and NH3·H2O over Cu/SiO2 catalyst. Catal. Sci. Technol. 2014, 4, 3159−3167. (33) Chen, Y.; Guo, Z.; Chen, T.; Yang, Y. Surface-functionalized TUD-1 mesoporous molecular sieve supported palladium for solventfree aerobic oxidation of benzyl alcohol. J. Catal. 2010, 275, 11−24. (34) Demel, J.; Lamač, M.; Č ejka, J.; Š těpnička, P. Palladium Catalysts Supported on Mesoporous Molecular Sieves Bearing Nitrogen Donor Groups: Preparation and Use in Heck and Suzuki CC Bond-Forming Reactions. ChemSusChem 2009, 2, 442−451. (35) Dirè, S.; Ceccato, R.; Facchin, G.; Carturan, G. Synthesis of Ni metal particles by reaction between bis(cyclooctadiene)nickel and sol−gel SiO2 modified with Si-H groups. J. Mater. Chem. 2001, 11, 678−683. (36) Zhang, Z.; Dai, S.; Hunt, R. D.; Wei, Y.; Qiu, S. L. IonImprinted Zeolite: A Surface Functionalization Methodology Based on the “Ship-in-Bottle” Technique. Adv. Mater. 2001, 13, 493−496. (37) Yan, Y.; Chen, Y.; Jia, X.; Yang, Y. Palladium nanoparticles supported on organosilane-functionalized carbon nanotube for solvent-free aerobic oxidation of benzyl alcohol. Appl. Catal., B 2014, 156−157, 385−397. (38) Pinheiro, A. V.; Han, D.; Shih, W. M.; Yan, H. Challenges and opportunities for structural DNA nanotechnology. Nat. Nanotechnol. 2011, 6, 763−772. (39) Smart, O. S.; Horský, V.; Gore, S.; Svobodová Vařeková, R.; Bendová, V.; Kleywegt, G. J.; Velankar, S. Worldwide Protein Data Bank validation information: Usage and trends. Acta Crystallogr. D. Struct. Biol. 2018, 74, 237−244. (40) Sun, H.; Tang, Q.; Du, Y.; Liu, X.; Chen, Y.; Yang, Y. Mesostructured SBA-16 with excellent hydrothermal, thermal and mechanical stabilities: Modified synthesis and its catalytic application. J. Colloid Interface Sci. 2009, 333, 317−323. (41) Chen, T.; Li, D.; Jiang, H.; Xiong, C. High-performance Pd nanoalloy on functionalized activated carbon for the hydrogenation of nitroaromatic compounds. Chem. Eng. J. 2015, 259, 161−169. (42) Pei, Y.; Xiao, C.; Goh, T.-W.; Zhang, Q.; Goes, S.; Sun, W.; Huang, W. Tuning surface properties of amino-functionalized silica for metal nanoparticle loading: The vital role of an annealing process. Surf. Sci. 2016, 648, 299−306.
(43) Bhattacharjee, S.; Bhowmick, A. K.; Avasthi, B. N. Preparation of hydrogenated nitrile rubber using palladium acetate catalyst: Its characterization and kinetics. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 471−484. (44) Mohammadi, N. A.; Rempel, G. L. Homogeneous selective catalytic hydrogenation of CC in acrylonitrile-butadiene copolymer. Macromolecules 1987, 20, 2362−2368. (45) Zhang, L.; Pan, Q.; Rempel, G. L. Hydrogenation of nitrile butadiene rubber in a multistage agitated contactor: Experiments and numerical simulation. Chem. Eng. Sci. 2010, 65, 2027−2036. (46) Kanungo, S.; Keshri, K. S.; van Hoof, A. J. F.; d’Angelo, M. F. N.; Schouten, J. C.; Nijhuis, T. A.; Hensen, E. J. M.; Chowdhury, B. Silylation enhances the performance of Au/Ti-SiO2 catalysts in direct epoxidation of propene using H2 and O2. J. Catal. 2016, 344, 434− 444. (47) Ling, D.; Liu, P.; Cheng, Z. Methanol synthesis in a three-phase catalytic bed under nonwetted condition. AIChE J. 2017, 63, 226− 237. (48) Grosso-Giordano, N. A.; Eaton, T. R.; Bo, Z.; Yacob, S.; Yang, C. C.; Notestein, J. M. Silica support modifications to enhance Pdcatalyzed deoxygenation of stearic acid. Appl. Catal., B 2016, 192, 93− 100.
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