Hydrogenation of Bisphenol A Type Epoxy Resin (BE186) over

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Applied Chemistry

Hydrogenation of Bisphenol A Type Epoxy Resin (BE186) over VulcanXC72 Supported Rh and Rh-Pt catalysts in Ethyl Acetate Containing Water Wei-Yuan Lu, Saurav Bhattacharjee, Bo-Xin Lai, An-Bang Duh, Ping-Chieh Wang, and Chung-Sung Tan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02583 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Hydrogenation of Bisphenol A Type Epoxy Resin (BE186) over VulcanXC72 Supported Rh and Rh-Pt catalysts in Ethyl Acetate Containing Water

Wei-Yuan Lua, Saurav Bhattacharjeea, Bo-Xin Laia, An-Bang Duhb, Ping-Chieh Wangb, Chung-Sung Tana*

a

Department of Chemical Engineering, National Tsing Hua University Hsinchu, 30013, Taiwan, ROC b

Chang Chun Plastics Co., Ltd.

Hsinchu Factory, Taiwan 30013 ROC

*

Corresponding author, TEL: +886-3-572-1189, Fax: +886-3-572-1684 E-mail: [email protected]

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ABSTRACT: This study demonstrates the hydrogenation of bisphenol A type epoxy resin (BE186) through solvent selection, development of a catalyst for aromatic ring hydrogenation and control of epoxy loss. A functional greener solvent, water in ethyl acetate was proposed based on solvatochromic parameters, high α and low β. RhOx present in VulcanXC72 supported monometallic Rh and bimetallic Rh-Pt catalysts prepared by polyol method acted as a promoter for hydrogenation. The hydrogenation pathway to control epoxy loss was proposed through changes of reactant to catalyst ratio and temperature. Though Rh-Pt catalysts were superior to Rh for aromatic ring hydrogenation, they led to higher epoxy loss owing to excessive C-O bond cleavage. Using Rh5/VulcanXC72-polyol, 3 wt% water in ethyl acetate and a reactant to catalyst ratio of 100:1, complete hydrogenation of BE186 with 3.4% epoxy loss was achieved at a H2 pressure of 1000 psi and 60 °C for 3.5 h.



1. INTRODUCTION Bisphenol A (BPA) type epoxy resins are in great demand as multi-functional materials because of their mechanical strength, insulation property and light transparency. Commonly, diglycidyl ether of BPA (DGEBA) is used as a sealing agent for light emitting diodes (LEDs) due to its colorless nature. However, on exposure to light, BPAs undergo yellowing and turn brittle due to severe structural degradation of aromatic rings. Use of silicon based epoxy hybrids,1,2 doping with UV resistant additives and hydrogenation of BPA type epoxy resins3-6 have been proposed to abate this structural degradation. Of these solutions, the hydrogenation route offers the best technoeconomic viability as silicon based epoxy resins are typified by complicated synthesis routes and cost prohibitive raw materials while UV resistant additives show weaker resistance to yellowing. The major cause of yellowing in BPAs is the oxidation of the aromatic ring which can be prevented through hydrogenation. Hydrogenated BPA also offers strong weather resistance when used as LED seals.4 There is also tremendous value addition to the tune of 100 USD/kg for hydrogenated BPA from a meagre 2-3 USD/kg for raw BPA. Figure 1 shows two possible BPA type epoxy resin hydrogenation routes. Route 1 follows direct hydrogenation of BPA after base condensation with epichlorohydrin while route 2 illustrates the hydrogenation of BPA first followed by condensation with epichlorohydrin. Route 2 suffers from high epoxy loss and as a result high yellowing in the base condensation process. Route 1, however, calls for more complicated manufacturing process.


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Figure 1. Possible routes for hydrogenation of BPA type epoxy resin.

The severe reaction conditions required for hydrogenation of aromatic rings7 are also likely to affect the highly reactive epoxy groups in BPA type epoxy resins. Thus, the major challenge in epoxy resin hydrogenation is to optimize a process that can maximize aromatic ring hydrogenation with minimum epoxy loss. Epoxy loss negatively impacts the heat resistance and subsequent processing of hydrogenated epoxy resins.4 For the hydrogenation of BPA type epoxy resins, rhodium (Rh) is the preferred choice of metal (Rh > Ru > Pt > Ni > Co), as its superiority towards aromatic ring hydrogenation over C-O bond cleavage opens up the possibility of hydrogenation under mild conditions,8 a key requirement for minimizing epoxy loss in terms of choice of catalyst and controlling the reaction conditions. Being easily dispersible, owing to their strong affinity towards reactants and organic solvents in general, carbon based materials are popular as a support choice in heterogeneous catalysis. Large specific surface area graphite and activated carbon (AC) have often been exploited for this purpose.9 Solvent selection is a very critical factor in hydrogenation reactions. Hydrogen and reactant solubility and dispersion of the catalyst are the prime considerations in solvent selection. Interestingly, several groups have reported significant enhancement in 4 ACS Paragon Plus Environment


reaction rates using water as the solvent, even though reactants are immiscible in water, the phenomenon is called as “on water mechanism”.10 Beattie et al.11 carried out the [4+2] cycloaddition of cyclopentadiene and dimethyl fumarate in different solvents including water. The hydrogen bonding between the aqueous and oil phases under vigorous stirring was responsible for the synergistic effect of water. This H bonding develops as a result of water being a protic solvent and is the driver for the enhancement in the reaction rate. The proton donation ability of water to functional groups of the reactants adsorbed on the active sites of the catalysts, polarizes them, exposing them to the active sites of the catalyst to undergo complete hydrogenation via H2 spillover. As a result, the activation energy of hydrogenation is considerably reduced leading to complete hydrogenation at milder reaction conditions with water acting as a functional solvent. In a previous study by our group, water was successfully used as the solvent for hydrogenation of BPA over a Ru/MCM-41 catalyst.12 The high performance of Ru/MCM-41 was attributed to a good dispersion of hydrophilic MCM-41 in water. Anderson et al.7 compared the hydrogenation of different water miscible benzyl compounds such as benzaldehyde, acetophenone and benzamide over a Pd (oxygen functionalized) catalyst. They reported high aliphatic product selectivity when water was used as the solvent in spite of Pd possessing strong tendency for C-O bond cleavage due to on water mechanism. Van Laar et al.5 showed that a mixture of water and tetrahydrofuran (THF) as solvent exhibited better reactivity than THF alone for the hydrogenation of BPA type epoxy resin. In the present study, an improved process for the hydrogenation of BPA type epoxy resin (BE186) over VulcanXC72 supported Rh and Rh-Pt catalysts in a mixture of ethyl acetate and H2O is proposed with a view of maximizing aromatic ring hydrogenation 5 ACS Paragon Plus Environment

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but minimizing epoxy loss. The role of RhOx as the catalyst promoter is investigated as well. The objectives of this study are to propose the optimal reaction conditions for hydrogenation of BE186 using the proposed catalyst and solvent to obtain higher degree of hydrogenation and less epoxy loss under mild reaction conditions.

2. EXPERIMENTAL 2.1. Materials. Rhodium chloride hydrate (RhCl3.xH2O, Rh content: 38.5 - 45.5%), rhodium acetylacetonate (Rh(acac)3, purity: 99%), chloroplatinic acid hexahydrate (H2PtCl6.6H2O, Pt content: 37.5%) and ethylene glycol (EG, purity: 99%) were purchased from Alfa-Aesar. H2 (purity: > 99.9%) was purchased from Linde Lien Hwa Industrial Gases. BPA type epoxy resin (BE186) was provided by Chang Chun Plastics Co. Ltd (Taiwan). BE186 had a DGEBA purity of > 90% with an epoxide equivalent weight (EEW) of 186.7 g/eq while BPA type epoxy resin impurities in monomer or dimer forms constituted the rest. Commercial 5 wt% Rh/C catalyst was purchased from Sigma Aldrich. Norit GL50 activated carbon and VulcanXC72 were bought from Cabot Co., Ltd. Tetrahydrofuran (THF, HPLC grade), methanol (MeOH, HPLC grade), 1 M sodium hydroxide (NaOH, analytical grade), 1 M hydrochloric acid (HCl, analytical grade), ethyl acetate (EA, HPLC grade) and chloroform-d (CDCl3, purity: 99.8%) were all purchased from Sigma-Aldrich. All chemicals and gases were used as received. 2.2. Catalyst Preparation. Carbon based mono and bimetallic catalysts were prepared using a modified microwave assisted polyol reduction method. In a typical trial to synthesize a bimetallic catalyst, the desired amount of noble metal precursors RhCl3.xH2O and H2PtCl6.6H2O were dissolved into 120 ml of EG/H2O mixture (3/1 v:v) along with 0.2 g of carbon based support. The pH value was then altered to 11 - 12 using 1 M NaOH solution followed by ultra-sonication for 90 min to get a uniform 6 ACS Paragon Plus Environment


distribution of precursor and carbon support. After ultra-sonication, the catalyst slurry was heated using a household microwave oven two times (900 W, 30 seconds each time). After the catalyst slurry cooled down to ambient temperature, the pH value was once again altered to 2 - 3 using 1 M HCl to precipitate metal nanoparticles (NPs) on the carbon support surface. Finally, the catalyst was collected by suction filtration, washed several times by abundant water and dried overnight in an oven at 80 oC. The catalyst prepared by the above method was denoted as MxNy/C-polyol (M and N = Rh and Pt, respectively, C = commercial catalyst support, AC or VulcanXC72, x and y = the theoretical wt% loading of Rh and Pt, respectively). Rh(0)/C, containing Rh in its zero oxidation state and with varying particle sizes were prepared using impregnation and chemical fluid deposition (CFD) methods. The CFD technique for heterogeneous catalyst synthesis using Rh(acac)3 as the metal precursor, THF as the solvent and supercritical CO2 as a binary solvent for achieving uniform dispersion of metal precursors in the support has been described in detail in our previous work.13 The catalysts prepared using impregnation and CFD methods were reduced chemically at 400 oC under H2 and denoted as Rhx/C-impregnation and Rhx/CCFD (x = the theoretical wt% loading of Rh). 2.3. Catalyst Characterization. 2.3.1. Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). The TEM images were taken using a JEOL JEM-2100 high resolution transmission electron microscope at an accelerating voltage of 200 KV. For preparing the TEM samples, the synthesized catalyst powders were diluted with ethanol and then ultra-sonicated for 5 min. Droplets of the prepared sample were put on a copper grid (200 mesh) and dried overnight in a vacuum oven. The particle size distribution of metal NPs was determined by using the imaging analysis software, 7 ACS Paragon Plus Environment

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Image J. 2.3.2. Powder X-ray Diffraction (XRD). XRD was conducted using a Rigaku Ultima IV powder X-ray diffractometer with a Cu-Kα radiation set at 40 kV, 20 mA. The scan rate was 0.5 θ°/min with a 2θ range between 20° - 80°. 2.3.3. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra were recorded on a high resolution X-ray photoelectron spectrometer (ULVAC-PHI XPS, PHI Quantera) with a 180° spherical capacitor analyzer + 32 channel detector. Monochromatic radiation from an Al anode X-ray source was used for excitation. The vacuum degree was fixed at a base pressure lower than 5 × 10−5 torr. 2.3.4. BET Measurements. The BET measurements were based on N2 adsorption using a Micromeritics ASAP 2060 autosorb. The samples were degassed at 300 oC for 3 h before taking the measurements. 2.3.5. Temperature-Programmed Reduction of Hydrogen (H2-TPR) Measurements. H2-TPR was carried out in a quartz tube flow reactor. The TPR profile of each sample was recorded from 35 °C - 500 °C at a ramp rate of 10 oC/min under a 10% H2/Ar flow as the reductive gas. The instrument, Micromeritics ASAP 2920 was equipped with a TCD detector. 2.4. Activity Tests. Hydrogenation of BE186 was carried out in a semi batch system. For a typical trial, 2 g BE186 with different amounts of solvent and 0.05 g of catalyst were added into a high pressure autoclave equipped with a glass vessel and a cross rod magnetic stirrer (Parr). H2 with a pressure of 1000 psi was then charged into the cell at 40 oC for 2 h after several hydrogen purges. The reaction products were collected after the reaction system was depressurized and cooled down to ambient temperature. The reaction products were then centrifuged to remove catalyst traces and the solvent was removed by rotary evaporation (40 oC - 80 oC). The purified products were analyzed 8 ACS Paragon Plus Environment


using 1H-NMR (500 MHz, Bruker, AVANCE-500) with CDCl3 as the d-solvent (δ: 7.26 ppm). The extent of aromatic ring hydrogenation was calculated by the ratio of BPA signal to the aliphatic signal and was defined as percent hydrogenation yield (Yield %). The yield was entirely a function of the extent of aromatic ring hydrogenation irrespective of epoxy loss that might have taken place. The epoxy loss extent of hydrogenated epoxy resin was measured according to ASTM (American Society for Testing and Materials) methods, following the regulations of ASTM-D1652-11 (standard test method for epoxy contents of epoxy resin) and was defined as the percent epoxy loss (Epoxy loss %). For measuring the epoxy loss, the hydrogenated BE186 product was dissolved in a suitable solvent and then titrated directly using hydrogen bromide or in-situ. The quantity of acid consumed was the measure of the epoxy content as hydrogen bromide reacted directly with the epoxy groups to form bromohydrins. The ASTM-D1652-11 document for measuring the epoxy content of epoxy resins maybe referred to for detailed understanding of the procedure employed to measure the epoxy loss. Most experiments were performed at least twice with a relative standard deviation (RSD ≦ 2%) to ensure reproducibility of data.

3. RESULTS AND DISCUSSION 3.1. Solvent Selection for Hydrogenation of BE186. Commercial Rh5/C (SigmaAldrich) catalyst was used for carrying out exploratory studies on solvent selection with regards to hydrogenation of BE186 (Figure 2). The solvent free hydrogenation of BE186 gave only 6.6% hydrogenation yield while the hydrogenation of BE186 in water in which it was insoluble gave 23.7% hydrogenation yield (Figure 3 and entries 1 and 2 in Table S1). This was attributed to the fact that since BE186 is a viscous heavy liquid, its affinity towards VulcanXC72 was very high and unless proper miscibility in the 9 ACS Paragon Plus Environment

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solvent was ensured, it might have been heterogeneously deposited on the catalyst surface blocking the active sites of the catalyst for the purpose of hydrogenation. Water as an effective solvent for aromatic ring hydrogenation has already been reported5 along with other non-aqueous solvent mixtures such as THF+n-hexane.6 Water being a polar solvent behaves differently from non-polar n-hexane, with regards to protic property and H2 solubility. This makes hydrogen bonding a likely driver in hydrogenation process in aqueous medium. Two solvatochromic parameters have been proposed to explain the “on water phenomenon”.14 α and β respectively represent the proton donation and acceptance capability of the solvent or in other words its Brønsted acidity and basicity. Proton donation by the solvent to the functional groups, including aromatic ring and epoxy group, polarizes them, thus exposing the aromatic rings to the active sites of the catalyst and thereby lowering the activation energy for hydrogenation.11,15 Proton acceptance by the solvent, on the other hand, lowers the available active sites in the catalyst for the reactants as some of them are occupied by the solvent accepting spillover H proton from the catalyst. This competitive adsorption between the reactants and the solvent eventually suppresses aromatic ring hydrogenation. However, catalyst poisoning due to the presence of other functional groups in the different solvents used through exchange of OH group for alkoxy groups was not observed. Thus a good hydrogenation solvent is proposed to possess high α and low β values as was observed in the hydrogenation of acetophenone.14 Figure 3 exhibits the hydrogenation results of BE186 in different solvents and solvent mixtures. Compositions of different solvent mixtures are shown in Table S1. Solvent mixture G (EA+H2O) exhibited better efficiency as compared to Solvent A (THF+H2O) and the pure solvents in the order G > EA ≈ A > THF. In agreement with β values for different solvents, shown in the Kamlet-Taft table (Table S2), EA having a 10 ACS Paragon Plus Environment


lower β value when compared to THF, showed better efficiency as a result of lesser competitive adsorption with the reactant. Similarly, for the solvent mixtures G and A, it was observed that the β values of EA and THF had a significant effect on the hydrogenation efficiency, with solvent G proving to be much better than solvent A.

Figure 2. Hydrogenation of BE186 (DGEBA).

Unlike other protic solvents such as MeOH and EtOH, water possesses high α and low β values (1.17 and 0.18, respectively), making it a good solvent and dispersion agent for aromatic hydrogenation. However, it can only act as a minor solvent along with major solvents such as EA and THF due to hydrocarbon reactants being sparingly soluble in water. In the group of pure solvents, EA exhibited better reactivity than THF, which might originate from it having a lower β value (0.45) than THF (0.55), providing for relatively lower competitive adsorption between the solvent and BE186 on the catalyst surface. To confirm the hydrogen bonding effect, MeOH with α of 0.93 and β of 0.62 was used at various concentrations as a minor solvent with major solvents such as THF (Solvents B, C, D, E and F with increasing order of MeOH concentration). MeOH is less protic than H2O but has better reactant solubility. A slight increase in hydrogenation while moving from B to C might be attributed to the greater solubility of the reactants in MeOH. However, there was a steady decrease in the degree of hydrogenation with further increase in MeOH concentration. This clearly established the role of solvent taking part in hydrogen bonding (α effect). With an increase in 11 ACS Paragon Plus Environment

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MeOH concentration, hydrogen bonding by the solvent with the functional groups of the reactant decreased due to an increase in the Brønsted basicity of the minor solvent MeOH (β effect), instead generating a focus on proton acceptance, leading to competitive adsorption with the reactant on the active sites of the catalyst. An EA and MeOH mixture (Solvent H) also resulted in lesser hydrogenation than solvent mixtures containing H2O, once again explained by a predominant β effect as the β values of both EA (0.45) and MeOH (0.62) were higher than that of H2O (0.18). Solvent G (EA+H2O) exhibited the highest hydrogenation ability thus confirming the proposed solvent selection rule of high α and low β. Though water can increase the degree of hydrogenation, it can also lead to an increase in epoxy loss as a result of enhancing the interactions between the epoxy group, oxygen group and active metal sites leading to an increase in acidity. Therefore, the effect of a solvent mixture containing higher amounts of water than solvent G was not investigated while carrying out solvent selection studies for the hydrogenation of BE186 in this study. H2 solubility in solvents indeed plays an important role in hydrogenation reactions as a higher solubility of H2 means more availability of H2 to take part in the reaction. However, since the reaction conditions while carrying out solvent selection for the hydrogenation of BE186 was very mild at 40 ℃ and a H2 pressure of 1000 psi, it was expected that the solubility of H2 in solvents would not play a major role in determining the degree of hydrogenation. Additionally, it has been reported in open literature that the solubility of H2 in EA is higher than that in THF at 323 K,16,17 the two major solvents used in this study. It was therefore assumed that at the mild reaction conditions, even for the solvent mixtures, the solubility of H2 would not be changed too much and therefore was not regarded as a parameter during solvent selection studies. This assumption was backed up by observations that showed that the degree of hydrogenation in solvents containing EA 12 ACS Paragon Plus Environment


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as the major solvent was higher than those containing THF as the major solvent. The hydrogenation yields shown in Figure 3 and Table S1 are representative of a steady increase in the rate of hydrogenation of BE186 over a 2 h reaction time as confirmed through observations made later in the manuscript.

Figure 3. Hydrogenation yield of BE186 using Rh5/C (commercial) catalyst with different solvent mixtures for reaction conditions: 2 g BE186, 2 g solvent, 0.05 g Rh5/C (commercial), H2 pressure of 1000 psi, 40 oC for 2 h, RSD for hydrogenation yield ≦ 2%. (Solvents: A: 94 wt% THF+6 wt% H2O; B: 90 wt% THF+10 wt% MeOH; C: 80 wt% THF+20 wt% MeOH; D: 70 wt% THF+30 wt% MeOH; E: 60 wt% THF+40 wt% MeOH; F: 50 wt% THF+50 wt% MeOH; G: 97 wt% EA+3 wt% H2O; H: 80 wt% EA+20 wt% MeOH)

3.2. Catalysts Characterization. Commercial catalyst Rh5/C that exhibited high potential for BE186 hydrogenation under mild conditions (40 oC) along with our 13 ACS Paragon Plus Environment

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synthesized catalysts were characterized in detail to better understand their catalytic properties. These included N2 physisorption, TEM, XRD, H2-TPR, and XPS. 3.2.1. Textural Properties of Different Carbon Supported Catalysts. Table 1 shows the surface area, pore diameter and pore volumes of the different supports and catalysts as obtained from N2 adsorption. Commercial (Rh5/C) catalyst was mainly consisted of mesopores with a pore diameter between 3 - 4 nm but contained macro and micropores within its structure as well along with a high surface area of 853.8 m2/g. AC exhibited a micro and mesoporous (pore diameter 3 - 4 nm) nature with a surface area of 604.5 m2/g while VulcanXC72 contained both micro and mesopores on its surface18 with 30% of them being micropores. VulcanXC72 also had the highest mesopore diameter of 13.6 nm and the smallest surface area of 232.8 nm among all the carbon supports used in this study. The pore volumes of the different carbon based supports and Rh5/C commercial catalyst are varied in the order: Rh5/C (commercial) > VulcanXC72 > AC. Due to the fact that the BET surface area for the Rh5/VulcanXC72-polyol catalyst was only a 4.0% reduction (223.3 m2/g) over the fresh VulcanXC72 support and there was no obvious change in the pore diameter, it was safely presumed that majority of the metal NPs were located on the outside surface of the support. TEM images of the various catalysts used in this study are shown in Figure 4 and Figure S1. The particle size distribution of the metal NPs calculated using ImageJ are listed in Table 1. The catalysts prepared by the polyol method (Figure 4a, d) revealed a uniform dispersion of Rh NPs within 2 - 4 nm. From the observed TEM images and the negligible change in surface area and pore diameter between the support and the catalysts, it was thus confirmed that the Rh NPs were located mainly on the outside surface of VulcanXC72, indicating that the hydrogenation of BE186 took place without any diffusion resistance. A similar observation could be made for the Rh5/AC-polyol 14 ACS Paragon Plus Environment


catalyst as well, which also demonstrated negligible change in surface area with a uniform dispersion of nanoparticles in a similar size range (3.3 ± 1.4 nm) as that of the VulcanXC72 supported catalysts. It may also be the case that the polyol synthesis method played a role in determining that the Rh NPs be located majorly on the outside surface of VulcanXC72 due to the rapid synthesis of NPs and thus denying them sufficient time to diffuse into the pores of the support. The commercial Rh5/C catalyst (Figure 4e) also possessed a similar NP size as can be seen from Table 1. The metal NP sizes of the impregnation and CFD synthesized catalysts were relatively higher which was attributed to agglomeration of NPs that might take place while using slower synthesis methods. Recent reports have suggested a typical Rh(0) NP size distribution (5-7 nm) to be particularly effective in aromatic ring hydrogenation as different crystal planes of differently sized Rh NPs for adsorption of aromatic ring and activation of hydrogen (e.g. (111) plane favors adsorption of aromatic ring while (100) plane favors H2 spillover).19 Wide angle XRD patterns for the Rh5/C commercial catalyst and all the VulcanXC72 catalysts prepared using the polyol method are shown in Figures 5 and 6, respectively. They, in general, revealed no apparent peaks for Rh (111) face centered cubic (fcc) lattice and only graphitic peaks were observed. This was expected due to the uniform distribution and small size of Rh NPs. However, in catalysts prepared by the impregnation and CFD method, Rh (111) peaks could be observed, which might be due to the agglomeration of metal NPs (Figure 5).


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Table 1. Textural Properties of the Catalysts Prepared by Various Methods (support/catalyst)

SBET (m2/g)

dpa (nm)

Vpb (cm3/ g)

PSDc (nm)

Rh5/C (commercial)

853.8

3.8

0.52

2.9 ± 1.5

AC (Norit GL 50)

604.5

3.7

0.23

--

VulcanXC72

232.8

13.6

0.47

--

Rh5/AC-polyol

618.9

6.1

0.29

3.3 ± 1.4

Rh5/VulcanXC72-polyol

223.3

13.7

0.45

3.1 ± 1.2

Rh5/VulcanXC72-impregnation

243.8

20.8

0.64

5.5 ± 2.1

Rh5/VulcanXC72-CFD

249.9

17.5

0.60

4.0 ± 1.4

Rh2.5Pt2.5/VulcanXC72-polyol

250.5

22.2

0.69

3.1 ± 1.0

a: Pore diameter calculated according to the Barret-Joyner-Halenda (BJH) method, b: Total pore volume, c: Particle size distribution (PSD) calculated from TEM image analysis

HRTEM images of Rh5/VulcanXC72-polyol and Rh2.5Pt2.5/VulcanXC72-polyol catalysts shown in Figure 7 and TEM-EDS of the Rh2.5Pt2.5/VulcanXC72-polyol catalyst (Figure S2) confirmed the fcc structure of the Rh and Rh-Pt catalysts based on the calculated d-spacing of the lattice fringes (0.17 nm, Figure 7a) for Rh and (0.22 nm, Figure 7b) for the Rh-Pt alloys along the (111) plane that is typical of fcc structure of Rh and Pt and agreed well with the observation made by Yu et al.13 It was interesting to note that in Figure 7, a number of disordered carbon metrics were formed on the active Rh and Rh-Pt sites due to a strong coordination between the metal NPs and carbon species. Since the coordination site was from oxygen group, it led to the formation of RhOx on catalyst surface.20



Figure 4. TEM images and metal NP size distribution of: (a) Rh5/VulcanXC72polyol, (b) Rh5/VulcanXC72-impregnation, (c) Rh5/VulcanXC72-CFD, (d) Rh2.5Pt2.5/VulcanXC72-polyol, (e) Rh5/C (commercial) catalysts.


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Figure 5. Wide angle XRD patterns (2θ: 20o - 80o) of Rh/C catalysts.

Figure 6. Wide angle XRD patterns (2θ: 20o - 80o) of Pt, Rh-Pt and Rh/VulcanXC72polyol catalysts. 18 ACS Paragon Plus Environment


Figure 7. HRTEM images of: (a) Rh5/VulcanXC72-polyol, (b) Rh2.5Pt2.5/VulcanXC72-polyol catalysts.

3.2.2. XPS Study of Different Carbon Supported Catalysts. Figure 8 shows the Rh 3d XPS spectra of various catalysts in the 302.0 - 320.0 eV range. The binding energies (BE) in the 302.0 - 311.9 eV and 312.0 - 320.0 eV ranges were attributed to Rh 3d5/2 and Rh 3d3/2, respectively. At least three states that existed for the Rh species. For Rh0, BE was reported to be in a range near 307.9 eV. The BE of Rh1+ was reported to be in a range of 308.0 - 308.9 eV while for Rh3+, it was in the range of 309.0 - 311.9 eV.21 To realize the distribution of electronic states for the active metal, the deconvolution of Rh 3d5/2 signal was conducted and the curve fitting results shown in Table 2 suggested that all the monometallic Rh catalysts except for the impregnation one contained at least 50% of RhOx species while the Rh5/C (commercial) catalyst contained 49.2% RhOx species. In comparison with the monometallic Rh catalysts, the bimetallic Rh2.5Pt2.5/VulcanXC72-polyol and Rh3.75Pt1.25/VulcanXC72-polyol catalysts had 40.5 and 45.2% RhOx species, respectively which was well expected. Also, according to the curve fitting results, the major composition was Rh3+. However, since the Gaussian 19 ACS Paragon Plus Environment

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distribution extended to the Rh1+ BE regime, it was concluded that the overall composition of RhOx was Rh3+ (major) and Rh1+ (minor). It was also worthy to be noted that the exact composition of the monometallic catalysts except for the impregnation one was actually RhOx-Rh(0) and for bimetallic catalyst was RhOx-Rh(0)-Pt(0) (Figure 9). 3.2.3. H2-TPR Study of Different Carbon Supported Catalysts. H2-TPR was carried out from 35 oC - 500 oC. The thermograms for the synthesized and commercial Rh5/C catalysts and for the VulcanXC72 support are shown in Figure 10. An additional hydrogen pick up peak above 400 ℃ was clearly discernible for all the Rh and Rh-Pt containing catalysts as opposed to the fresh VulcanXC72 support. This peak was attributed to H2 spillover, induced by the presence of Rh, leading to the formation of a strong C-H bond. The H2 consumption peak of a C-H bond generally occurs in a range of 500 oC - 600 oC and was consistent in our observation.22 The observed H2 consumption signal below 400 oC was attributed to either RhOx reduction or decomposition of oxygen group that existed inherently on the surface of the carbon based supports. On the VulcanXC72 support they existed as ester (C-O-), carbonyl (C=O) and carbonate (C=O) as seen in the XPS spectra of VulcanXC72 in the C (1s) level (Figure S3). The oxygen group on the catalyst decomposes in the range of 150 oC - 400 oC under a H2 environment, due to the dehydration of –COOH and –COH groups. Besides, the oxygen group on the carbon surface could also act as an acceptor to the H atom that appears due to spillover caused by Rh.21,22 Additionally, there was no evidence of active site blockage due to exchange of COH to COR with alcohol. The low temperature signal (about 57 oC) in the H2-TPR thermograms was attributed to a low coordination and weaker interaction of Rh with the support surface (e.g. Rh1+ species) or to the easy reduction of a thin oxidation layer that might be deposited 20 ACS Paragon Plus Environment


on the catalyst surface when exposed to air. Rh5/VulcanXC72-impregnation catalyst treated under H2 at 400 oC showed no H2 consumption signal, indicating that the Rh species were completely reduced, which was also confirmed via XPS spectra (only Rh(0) species). In comparison, the H2-TPR thermogram of only the fresh VulcanXC72 support showed a H2 consumption peak between 300 oC - 400 oC which was regarded to the decomposition of oxygen group. To confirm the above mentioned observation, the Rh5/C (commercial) catalyst was also subjected to H2 treatment at 400 oC exhibiting no obvious peak between 150 oC - 400 oC range. It was thus concluded that the formation of RhOx mainly took place due to the coordination of the metal species with the oxygen groups present on the surface of the carbon based support and due to incomplete reduction during the synthesis process.

Figure 8. XPS spectra of: (a) Rh5/VulcanXC72-polyol, (b) Rh5/AC-polyol, (c) Rh5/VulcanXC72-impregnation, (d) Rh5/C (commercial) catalysts in the Rh (3d) core level regions. 21 ACS Paragon Plus Environment

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Figure 9. XPS spectra of: (a) Rh2.5Pt2.5/VulcanXC72-polyol catalyst in the Rh (3d), (b) Pt (4f) core level regions.

Table 2. XPS Deconvolution Results of Rh 3d5/2 and Pt 4f7/2 of Various Catalysts catalysts

RhOx

RhOx

Rh0

Rh0

Pt0

Pt0

(% atom)

BE

(% atom)

BE

(% atom)

BE

(eV)

(eV)

RhOx Rh0

(eV)

Rh5/AC-polyol

54.6

309.0

45.4

307.6

--

--

1.20


53.9

309.3

46.1

307.6

--

--

1.17


--

--

100.0

307.6

--

--

--

Rh5/VulcanXC72-CFD

--

--

100.0

307.6

--

--

--

Rh5/C (commercial)

49.2

309.0

50.8

307.5

--

--

0.97


40.5

309.6

59.5

307.7

100.0

71.9

0.68


45.2

309.1

55.8

307.8

100.0

71.9

0.81



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Figure 10. H2-TPR thermograms of VulcanXC72, Rh5/C commercial catalyst and catalysts prepared by polyol and impregnation methods.

3.3. Hydrogenation of BE186 over Different Carbon Supported Catalysts 3.3.1. Hydrogenation of BE186 using Monometallic Catalysts in Different Solvents. Hydrogenation of BE186 was conducted over various carbon supported monometallic catalysts, Rh5/C (commercial) (I), Rh5/AC-polyol (II), Rh5/VulcanXC72-polyol (III) and Rh8/VulcanXC72-polyol (IV). Rh5/C was used for the purpose of comparison. Three solvents were chosen from previous solvent selection studies, namely, THF, EA and solvent G (EA+H2O). The hydrogenation yields for various catalyst and solvent combinations are shown in Figure 11. Quantification of hydrogenation yields can be read from Table S3. The reactivity of different solvents followed the order G > EA > THF, in consonance with solvent selection rule of high α and low β. With reference to hydrogenation

yield,

various

monometallic

catalysts


followed

the

pattern

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Rh8/VulcanXC72-polyol (IV) > Rh5/VulcanXC72-polyol (III) > Rh5/C (I) > Rh5/ACpolyol (II). The catalyst with the higher metal loading (Rh8/VulcanXC72-polyol) was more active than the one with lower loading (Rh5/VulcanXC72-polyol). This was a demonstrative fact that active sites were evenly dispersed in the catalyst with higher metal loading as well. To further understand the difference in the catalytic activity of the various catalysts, the following points such as the extent of RhOx (expressed by the ratio of RhOx and Rh(0) in Table 2), average metal NP size, porosity and number of oxygen groups in the catalysts were looked into. RhOx being strongly electron deficient (Lewis acidity), can act as a promoter for the hydrogenation of aromatic ring. Recently a catalyst regeneration process was explored after the hydrogenation of epoxy resin by introducing an oxygen containing gas at 100 o

C.6 It was reasoned that the regeneration process either eliminated the impurities

deposited on the surface or it formed new RhOx. If this is the case, Rh based catalyst deactivation in hydrogenation process could be viewed as the consumption of RhOx. During the course of XPS characterization, as shown in Table 2, presence of RhOx was confirmed in the polyol catalysts similar to what was found in the Rh5/C (commercial) catalyst. To confirm the purported role of RhOx as a promoter in the hydrogenation of epoxy resins, catalysts without RhOx prepared via impregnation and CFD methods and reduced under H2 at 400 oC were compared with the catalysts containing RhOx for the hydrogenation of BE186, the results of which are shown in Table 3. It was interesting to note that while the average NP size range of Rh(0) (5 - 7 nm) in the impregnation and the CFD catalysts were well suitable for aromatic ring hydrogenation,19,25 hydrogenation yields obtained from the catalysts containing RhOx was much higher as compared to the ones having no RhOx. 24 ACS Paragon Plus Environment


Figure 11. Hydrogenation yield of BE186 over different catalysts and solvents (reaction conditions: 2 g BE186, 2 g solvent, 0.05 g catalyst, H2 pressure of 1000 psi, 40 oC for 2 h, (I) Rh5/C (commercial), (II) Rh5/AC-polyol, (III) Rh5/VulcanXC72polyol, (IV) Rh8/VulcanXC72-polyol), RSD for hydrogenation yield ≦ 2%.

The objective of the hydrogenation of BE186 was not only the complete hydrogenation of the aromatic ring but also to minimize epoxy ring opening, since epoxy ring can be opened by both acidic or basic conditions. Functionality of carbon based materials, decides its acidity or alkalinity.26 In the present study, oxygen was the only functional group in the catalysts based on the support as shown in Table 2 and in the XPS spectra of the VulcanXC72 support (Figure S3) emanating from C-O- (ester), C=O (carbonyl) and C=(O) (carbonate) and was thus used as the index for determining the reactivity of the catalysts. On a macro level, the number of oxygen groups on the catalysts varied in the order Rh5/AC-polyol > Rh5/C (commercial) > Rh5/VulcanXC72polyol based on the support as seen from the XPS results and indicated the oxic 25 ACS Paragon Plus Environment

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environment in which the hydrogenation of BE186 was carried out. Since epoxy ring opening was a competitive reaction with aromatic ring hydrogenation taking place at the same active sites, the desired hydrogenation product yield was less for a catalyst containing more oxygen groups. From our results, it was observed that the Rh5/VulcanXC72-polyol catalyst had the highest hydrogenation yield (entry 3 in Table 3), showing that the catalyst containing the moderate number of oxygen group was best suited for selective hydrogenation of BE186 as a consequence of lower epoxy loss and also lower impedance to the reductive hydrogenation process. The hydrogenation yields for the impregnation and CFD catalysts were the least due to the complete absence of RhOx.

Table 3. Hydrogenation Yield of BE186 using Catalysts Prepared by Different Methods entry

catalyst

oxygen

yield

groupa

(%)

(% atom) 1


--

38.7

2

Rh5/VulcanXC72-CFD

--

28.6

3


4.6

100.0

4

Rh5/C (commercial)

14.0

77.8

5

Rh5/AC-polyol

14.6

59.1

Reaction conditions: 2 g BE186, 2 g solvent G, 0.05 g catalyst, H2 pressure of 1000 psi, 40 oC for 2 h, a: the oxygen group of the catalysts calculated by full image analysis of XPS spectra, RSD for hydrogenation yield ≦ 2%



3.3.2. Hydrogenation of BE186 using Bimetallic Catalysts. Table 4 shows the hydrogenation results for the hydrogenation of BE186 using bimetallic catalysts. Once again monometallic catalyst, Rh5/VulcanXC72-polyol was used as the basis for comparison. A combination of Rh and Pt was selected as the metals for the bimetallic catalyst. Rh-Pt bimetallic catalysts were reported earlier to be effective for the hydrogenation of aromatic ring owing to stronger adsorption ability of aromatic ring onto the active metal sites and effective H2 spillover.13,27,28 In the present study, the actual composition of the synthesized Rh-Pt catalyst was RhOx-Rh(0)-Pt(0) due to the strong coordination of oxygen group with the active metals and incomplete reduction (Table 2). As shown in Table 4, Rh3.75Pt1.25/VulcanXC72 and Rh2.5Pt2.5/VulcanXC72polyol catalysts possessed better hydrogenation ability when compared to the monometallic Rh5/VulcanXC72-polyol catalyst. The enhanced catalytic activity was directly attributed to the synergistic effect of Rh-Pt and enhanced H2 spillover. The H2 spillover in the Rh-Pt bimetallic catalysts took place on the surface of the Rh-Pt alloy that possessed a weaker binding energy for H2 on the (111) surface (2.57 eV) as shown by Pan and Wai27 through first theory DFT calculations when compared to individual Pt (2.72 eV) or Rh (2.81 eV). The weak binding energy for H2 was the driver for an increased H2 dissociation and subsequent spillover of H atoms onto the adsorbed reactants to take part in the reaction. The higher hydrogenation ability of the Rh-Pt bimetallic catalysts over the monometallic catalysts was direct testimony to the fact that effective H2 spillover could take place in a gas-liquid-solid system. The synergistic effect of Rh-Pt arose from the fact that Rh selectively hydrogenated aromatic ring while Pt (111) could adsorb aromatic rings in a bridge manner assisting their rapid hydrogenation by Rh. As can be seen from Table 4, when the total metal loading was kept constant at 5 wt%, the individual metal loadings of Rh and Pt had a 27 ACS Paragon Plus Environment

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big role to play in determining the activity of the bimetallic catalysts. At a Rh and Pt loading of 2.5 wt% each, maximum hydrogenation yield was observed, indicating that the synergistic effects of Rh and Pt plus efficient H2 spillover could be achieved at this catalyst configuration. For catalysts containing higher metal loadings of Pt, lower hydrogenation yields were observed due to the susceptibility of Pt to induce C-O bond cleavage, leading to higher epoxy loss. For catalysts containing higher loadings of Rh, although the hydrogenation ability was better than the monometallic Rh catalysts, it was not as high as the Rh2.5Pt2.5/VulcanXC72-polyol catalyst, as the beneficial effects of having Pt in the system and therefore the synergistic effect of Rh-Pt catalysts was not as pronounced. Thus, a careful control of the individual metal loadings was necessary to maximize the hydrogenation ability of the bimetallic catalysts. 3.4. Control of Epoxy Loss during Hydrogenation of BE186. Epoxy loss negatively impacts the thermal stability and subsequent processing of the hydrogenated epoxy resins. Recently a method was developed to reduce epoxy loss during the hydrogenation of epoxy resins by lowering temperature but increasing reaction time.29 A possible hydrogenation mechanism (Figure 12) was proposed in the present study to provide a solution to this. The products C and H were classified as the desired products according to the literature.29 There are three possible major routes for hydrogenation of epoxy resin as 1). hydrogenation of aromatic ring first and then the subsequent C-O bond cleavage (epoxy ring opening), 2). C-O bond cleavage (ester group first) before the hydrogenation of aromatic ring and the subsequent C-O bond cleavage (epoxy ring opening), and 3). C-O bond cleavage (epoxy ring opening) first and then the subsequent hydrogenation of aromatic ring. A graphical depiction of the various hydrogenation routes can be seen in Figure 12.



Table 5 lists hydrogenation yields and epoxy loss during the hydrogenation of BE186 using Rh5, Rh3.75Pt1.25 and Rh2.5Pt2.5/VulcanXC72-polyol catalysts. The Rh-Pt bimetallic catalysts exhibited better aromatic ring hydrogenation ability albeit with higher epoxy loss when compared to the monometallic Rh catalyst. Since the bimetallic catalysts exhibited much higher epoxy loss than its monometallic counterparts at lower reaction times, reactions to measure the hydrogenation yields of BE186 using the bimetallic catalysts at 2 h of reaction time were not carried out.

Table 4. Hydrogenation Yield of BE186 using Different Composition of Bimetallic Catalysts entry

catalyst

yield (%)

1


66.9

2


77.9

3


69.8

4


89.5

5


65.3

6


62.4

7

Pt5/VulcanXC72-polyol

< 5.0

Reaction conditions: 2 g BE186, 2 g solvent G, 0.05 g catalyst, H2 pressure of 1000 psi, 40 oC for 1 h, concentration is defined as Wreactant/Wreactant+solvent (50 wt%), RSD for hydrogenation yield ≦ 2%

The greater epoxy loss while using the Rh-Pt bimetallic catalysts (entries 6 and 9 in Table 5) was attributed to a stronger C-O bond cleavage ability brought about by Pt. The Rh3.75Pt1.25/VulcanXC72-polyol catalyst gave 15.5% epoxy ring opening for 1 h of reaction when compared to 25.9% given by the Rh2.5Pt2.5/VulcanXC72-polyol catalyst. Nevertheless, 100% hydrogenation was achieved using the Rh5/VulcanXC72-polyol 29 ACS Paragon Plus Environment

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catalyst (entry 4). The epoxy loss was not higher than 30% (entry 10), which meant that at least 70% of epoxy group existed in the hydrogenated product. It is also imperative to note that the undesired intermediate products that might arise from the steps such as C-O bond cleavage (ester group) and epoxy ring opening, in case the hydrogenation of epoxy resin followed the routes 2 or 3, even if observed, were in very minute quantities. From entry 1 in Table 5, it can be seen that for a 0.5 h reaction using the Rh5/VulcanXC72-polyol catalyst, more than 50% hydrogenation yield for BE186 was observed with only 1.6% epoxy loss, which led us to strongly conclude that the major route for hydrogenation of BE186 was the hydrogenation of aromatic ring and then the subsequent epoxy ring opening (route 1 in Figure 12). The epoxy loss of 1.6% was observed as it has been mentioned before that through the Rh5/VulcanXC72-polyol catalysis mechanism, aromatic ring hydrogenation and epoxy ring opening reactions may become competitive to each other due to the presence of oxygen groups in the catalysts based on the support. As a result, some of the hydrogenated products may proceed to epoxy ring opening giving reaction products D and E, shown in route 1 in Fig. 12. The method to therefore lower epoxy loss was in how to avoid the epoxy ring opening reaction after aromatic ring hydrogenation. It is also important to note that the hydrogenation yield of BE186 over both the monometallic Rh and the bimetallic Rh-Pt catalysts increased linearly with time, showcasing that the hydrogenation yields observed were representative of the rate of reaction for the hydrogenation of BE186.



Page 32 of 57

Table 5. Hydrogenation Yield of BE186 using Various Catalysts with Epoxy Loss entry

catalysta

time

yield

epoxy

(h)

(%)

loss (%)

1

Rh5/VulcanXC72

0.5

53.1

1.6

2

Rh5/VulcanXC72

1

66.9

3.7

3

Rh5/VulcanXC72

1.5

81.2

7.5

4

Rh5/VulcanXC72

2

100.0

9.4

5

Rh3.75Pt1.25/VulcanXC72

0.5

61.8

10.1

6


1

77.9

15.5

7


1.5

88.7

20.7

8


0.5

52.0

6.9

9


1

89.5

25.9

10


1.5

96.3

29.1

Reaction conditions: 2 g BE186, 2 g solvent G, 0.05 g catalyst, H2 pressure of 1000 psi, 40 oC, a: the catalysts were prepared by polyol method, concentration is defined as Wreactant/Wreactant+solvent (50 wt%), RSD for hydrogenation yield and epoxy loss ≦ 2% The major reasons affecting epoxy loss were identified as the surface functionality of catalysts and the C-O bond cleavage ability of active metals. While C-O bond cleavage is hard to avoid as both Rh and Pt, inherently catalyze C-O bond cleavage,3032

the surface functionality of the catalysts can be changed by changing the reaction

conditions.


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Figure 12. Possible mechanisms for hydrogenation of BE186.

Though water facilitated an increase in the degree of hydrogenation, water also enhanced interaction between epoxy group, oxygen group and active metal sites, causing epoxy loss due to an increase in acidity.27 Therefore, a solvent or solvent mixture containing no protic solvents was explored to prevent epoxy loss. Increase in the reactant to catalyst ratio with moderate increase in temperature was contemplated for reducing epoxy loss. While moderate increase in temperature could compensate for the loss in surface reactivity due to higher amount of reactant, temperature rise could also help desorb the hydrogenated products rapidly, thus hindering epoxy ring opening. Ideas mooted above were invoked in the hydrogenation of BE186 using Rh5, Rh3.75Pt1.25, Rh2.5Pt2.5/VulcanXC72-polyol catalysts with the objective to find out a combination that would maximize hydrogenation yield and minimize epoxy ring opening. The results are shown in Table 6.



Very impressive hydrogenation yield with low epoxy loss was observed for entry 1 where pure EA was used as the solvent. Under the same reaction conditions, 100% hydrogenation was achieved by solvent G (EA+H2O) albeit with higher epoxy loss (entry 2). While at this stage, it might appear that only solvent EA was superior to solvent G, this scenario, dramatically changed as the reactant to catalyst ratio was increased with moderate rise in reaction temperature. With a reactant to catalyst ratio of 100:1, reaction temperature of 60 ℃ and reaction time of 3.5 h, solvent EA returned only 79.4% hydrogenation yield with further lowering of epoxy loss to 1.7% (entry 5). Under the same reaction conditions, with solvent G (entry 6), epoxy loss was cut down to 3.4% while still maintaining 100% hydrogenation yield, which was at par with entry 1. This observation was of remarkable significance in view of the fact that one of the objectives of the present work was also to arrive at a condition conducive to handling higher amount of reactant with high hydrogenation yield and low epoxy loss. Solvent G scores over solvent EA on this account of higher reactant conversion, once again highlighting the importance of the solvent selection rule. Further lowering of catalyst and solvent EA requirement would also increase the economic viability of BE186 hydrogenation using solvent G. A similar trend was also observed for the bimetallic catalysts Rh3.75Pt1.25, Rh2.5Pt2.5/VulcanXC72-polyol. Change in the reaction conditions by way of increasing reactant to catalyst ratio and moderately enhancing the reaction temperature, could improve hydrogenation yield, but high epoxy loss observed with bimetallic catalysts still remained a niggling issue. Since the C-O bond to aromatic ring ratio in BE186 was about 2:1, the strong C-O bond cleavage capability of Rh-Pt bimetallic catalysts brought about especially by Pt pushed the reaction to follow not only route 1 but also route 3 involving C-O bond cleavage (epoxy ring opening) first and then aromatic ring hydrogenation. The higher epoxy loss observed in Tables 5 and 33 ACS Paragon Plus Environment

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6 when the hydrogenation of BE186 was carried out using the Rh-Pt bimetallic catalysts was indicative of this fact. The monometallic Rh5/VulcanXC72-polyol catalyst was identified as the most promising catalyst for selective hydrogenation of BE186 using solvent G, while controlling the epoxy loss at the same time. Thus, a moderate change of the reaction conditions could effectively control epoxy loss without significantly affecting the hydrogenation system. Although a kinetic study for the hydrogenation of BE186 was not carried out, based on the experimental data obtained over a period of time that were representative of the rates of the competing hydrogenation of BE186 and epoxy loss reactions, it can be seemingly concluded that over the Rh5/VulcanXC72 catalyst and using solvent G, the relative activation energy for the aromatic ring hydrogenation of BE186 was lower than that for epoxy loss. A carbon based commercial Ru/C catalyst was previously tested by our group to be stable over three cycles for the hydrogenation of BPA type epoxy resin in water.12 Based on previous observation, it was expected that the Rh5/VucanXC72-polyol catalyst would also be stable for the hydrogenation of BE186 over at least three cycles. Tests for confirming the stability of the Rh5/VucanXC72-polyol catalyst is currently underway.


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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Table 6. Hydrogenation Yield of BE186 using Different Reaction Conditions and Solvents for Control of Epoxy Loss entry

catalysta

Wreactant/Wcatalyst

solvent

temperature

time

yield

epoxy

(℃)

(h)

(%)

loss (%)

1

Rh5/VulcanXC72

40

EA

40

2

98.8

3.1

2

Rh5/VulcanXC72

40

Solvent G

40

2

100.0

9.4

3


40

EA

40

1

74.9

12.2

4

Rh5/VulcanXC72

100

Solvent G

60

3

85.6

3.5

5

Rh5/VulcanXC72

100

EA

60

3.5

79.4

1.7

6

Rh5/VulcanXC72

100

Solvent G

60

3.5

100

3.4

7


100

EA

60

2

80.7

12.2

8


100

Solvent G

60

3.5

85.8

14.7

Reaction conditions: 2 g solvent, H2 pressure of 1000 psi, a: the catalysts were prepared by polyol method, concentration is defined as Wreactant/Wreactant+solvent (50 wt%), RSD for hydrogenation yield and epoxy loss ≦ 2%


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4. CONCLUSIONS In the present study, several aspects of hydrogenation of BPA type epoxy resin, BE186, including solvent selection, catalyst design and control of epoxy loss were investigated. A viable solvent selection rule for aromatic ring hydrogenation based on hydrogen bonding was proposed as high α and low β for solvents. According to the solvatochromic parameters, a mixture of 3 wt% of H2O in EA and H2O (solvent G) exhibited the best results for the hydrogenation of BE186. The solvent mixture proposed was a greener solvent system when compared to commonly used pure hydrogenation solvents such as EA and THF. The role of RhOx as a promoter was studied with conclusive evidence. Due to its strong electron deficiency, RhOx could be regarded as Lewis acid sites with high affinity for aromatic ring hydrogenation. The formation of RhOx was attributed to a strong coordination between the oxygen groups on the support with the active metal sites. Incomplete reduction was also underlined as a cause for the emergence of RhOx. Amongst the different catalysts studied Rh5/VulcanXC72-polyol was the most superior catalyst, with higher hydrogenation ability than the commercial Rh5/C catalyst. An even distribution of metal nanoparticles on the outside surface of VulcanXC72 and its moderate oxygen group content were the major reasons for its enhanced catalytic activity. Study on Rh-Pt bimetallic catalysts for the hydrogenation of BE186 revealed that although they possessed better aromatic ring hydrogenation ability due to synergistic effects between Rh and Pt, leading to increased H2 spillover, their stronger C-O bond cleavage ability also led to an increase in epoxy ring opening by forcing the reaction to follow unfavorable hydrogenation pathways for BE186. Although not beneficial for the control of epoxy loss in the current study, the use of a cheaper metal along with Rh can effectively reduce the catalyst cost making the process more 36 ACS Paragon Plus Environment


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economically viable and research into bimetallic catalyst pairs pertinent. Bimetallic catalysts also open up the possibility of using greener solvent systems that increases the environmental significance of the developed process. Additionally, the use of bimetallic catalysts can prove to be valuable for the hydrogenation of polymers with higher molecular weights than BE186 as they indeed have better aromatic ring hydrogenation ability. Studies into the same are currently being carried out by our group with encouraging initial results. Epoxy loss during hydrogenation could be effectively controlled by making simple changes to the reaction conditions such as a careful control of the ratio of reactant to catalyst to favorably alter the surface functionality and a moderate increase in the reaction temperature. At the best observed conditions of 5 g BE186, a reactant to catalyst (Rh5/VulcanXC72) ratio of 100, a solvent G of 2 g and a H2 pressure of 1000 psi at 60 °C for 3.5 h, 100% hydrogenation with 3.4% epoxy loss could be achieved.

▪

ASSOCIATED CONTENT

• Supporting Information Hydrogenation yield of BE186 using Rh5/C (commercial) catalyst with different solvent mixtures; Kamlet-Taft table for common solvents (including green solvents) for hydrogenation of BE186; Hydrogenation yield of BE186 using different catalysts and solvents; TEM images of Rh, Rh-Pt and Pt catalysts prepared by microwave

polyol

method;

TEM-EDS

results

of

bimetallic

catalyst

Rh2.5Pt2.5/VulcanXC72-polyol (showing alloy structure); XPS spectra of VulcanXC72 in C (1s) core level (PDF)

▪

AUTHOR INFORMATION 37 ACS Paragon Plus Environment

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Corresponding Author *Email: [email protected]. Phone: +886-3-572-1189. Fax: +886-3-572-1684. ORCID Chung-Sung Tan: 0000-0002-8121-5497 Present Address ||

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, 30013,

Taiwan, ROC Notes The authors declare no competing financial interest.

▪

ACKNOWLEDGEMENTS

This work was supported by the ROC Ministry of Science and Technology (grant number MOST 106-2622-8-007-017) and National Tsing Hua University at Hsinchu, Taiwan, ROC. The authors would also like to acknowledge Mr. Avinash B. Lende for helping with XPS and 1H NMR analysis.

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REFERENCES

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TABLE OF CONTENTS (GRAPHIC)



NUMBERED FIGURES

Figure 1. Possible routes for hydrogenation of BPA type epoxy resin.


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Figure 2. Hydrogenation of BE186 (DGEBA).



Figure 3. Hydrogenation yield of BE186 using Rh5/C (commercial) catalyst with different solvent mixtures for reaction conditions: 2 g BE186, 2 g solvent, 0.05 g Rh5/C (commercial), H2 pressure of 1000 psi, 40 oC for 2 h.


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Figure 4. TEM images and metal NP size distribution of: (a) Rh5/VulcanXC72-polyol, (b) Rh5/VulcanXC72-impregnation, (c) Rh5/VulcanXC72-CFD, (d) Rh2.5Pt2.5/VulcanXC72-polyol, (e) Rh5/C (commercial) catalysts.



Figure 5. Wide angle XRD patterns (2θ: 20o - 80o) of Rh/C catalysts.


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Figure 6. Wide angle XRD patterns (2θ: 20o - 80o) of Pt, Rh-Pt and Rh/VulcanXC72polyol catalysts.



Figure 7. HRTEM images of: (a) Rh5/VulcanXC72-polyol, (b) Rh2.5Pt2.5/VulcanXC72polyol catalysts.


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Figure 8. XPS spectra of: (a) Rh5/VulcanXC72-polyol, (b) Rh5/AC-polyol, (c) Rh5/VulcanXC72-impregnation, (d) Rh5/C (commercial) catalysts in the Rh (3d) core level regions.



Figure 9. XPS spectra of: (a) Rh2.5Pt2.5/VulcanXC72-polyol catalyst in the Rh (3d), (b) Pt (4f) core level regions.


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Figure 10. H2-TPR thermograms of VulcanXC72, Rh5/C commercial catalyst and catalysts prepared by polyol and impregnation methods.



Figure 11. Hydrogenation yield of BE186 over different catalysts and solvents (reaction conditions: 2 g BE186, 2 g solvent, 0.05 g catalyst, H2 pressure of 1000 psi, 40 oC for 2 h, (I) Rh5/C (commercial), (II) Rh5/AC-polyol, (III) Rh5/VulcanXC72-polyol, (IV) Rh8/VulcanXC72-polyol).


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Figure 12. Possible mechanisms for hydrogenation of BE186.


Hydrogenation of Bisphenol A Type Epoxy Resin (BE186) over

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