Bimetallic Catalyst Catalyzed Carbonylation of Methanol to Acetic Acid

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Article Study on Rh(I)/Ru(III) Bimetallic Catalyst Catalyzed Carbonylation of Methanol to Acetic Acid

Shasha Zhang 1, Wenxin Ji 1,2,*, Ning Feng 2, Liping Lan 1, Yuanyuan Li 1,2 and Yulong Ma 1,2

1 College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China; [email protected] (S.Z.); [email protected] (L.L.); [email protected] (Y.L.); [email protected] (Y.M.) 2 State Key Laboratory of High-eﬃciency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China; [email protected] * Correspondence: [email protected]; Tel.: +86-135-1957-9989; Fax: +86-951-206-2323

Received: 13 July 2020; Accepted: 3 September 2020; Published: 11 September 2020

Abstract: In this study, a Rh(I)/Ru(III) catalyst with a bimetallic space structure was designed and synthesized. The interaction between the metals of the bimetallic catalyst and the structure of the bridged dimer can eﬀectively reduce the steric hindrance eﬀect and help speed up the reaction rate while ensuring the stability of the catalyst. X-ray photoelectron spectroscopy (XPS) results show that rhodium accepts electrons from chlorine, thereby increasing the electron-rich nature of rhodium and improving the catalytic activity. This promotes the nucleophilic reaction of the catalyst with methyl iodide and reduces the reaction energy barrier. The methanol carbonylation performance of the Rh/Ru catalyst was evaluated, and the results show that the conversion rate of methyl acetate and the yield of acetic acid are 96.0% under certain conditions. Furthermore, during the catalysis, no precipitate is formed and the amount of water is greatly reduced. It can be seen that the catalyst has good stability and activity.

Keywords: carbonylation; acetic acid; bimetallic catalyst

1. Introduction As an important organic raw material, acetic acid is widely used in chemical, pharmaceutical, pesticidal, and food industries, among others. During the early 1990s, BP Chemicals developed an iridium complex catalyst that was in competition with the rhodium complex catalyst, and the new technology was commercialized as the Cativa process in 1995. The Monsanto process was commercialized in 1966 using an improved Rh-based homogeneous catalyst for a liquid-phase methanol carbonylation with a methyl iodide promoted rhodium catalyst [1–3]. It is an important raw material for the synthesis of a vinyl acetate monomer and acetic anhydride. At present, Monsanto’s methanol carbonylation to acetic acid process with (Rh(CO)2I2)− as the catalytic active center has become the mainstream process for global acetic acid production due to mild reaction conditions and suﬃcient raw material sources [4–7]. In the Monsanto rhodium/iodine catalyst catalyzed carbonylation to acetic acid reaction, Forster [8] proposed that the reaction mainly undergoes four elementary reactions: CH3I oxidative addition, ligand migration, CO coordination, and CH3COI reduction. Among them, the ﬁrst step in the CH3I oxidation addition reaction is the rate-determining step in the entire catalytic process [9]; therefore, to improve the activity of the catalyst, it is necessary to reduce the reaction barrier of the CH3I oxidation addition. However, Rh(I) is unstable during the reaction of the Monsanto rhodium/iodine catalyst. When the CO partial pressure is low, (Rh(CO)2I2)− interacts with hydroiodic acid to form dimers to form Rh(III) precipitates [10–13], causing the deactivation of the precious metal catalyst. Therefore, a large number of polar solvents such as H2O and acetic acid are added to the system to increase the solubility of the

Materials 2020, 13, 4026; doi:10.3390/ma13184026 www.mdpi.com/journal/materials Materials 2020, 13, 4026 2 of 12 catalyst, thereby improving the stability of the catalyst. In the industrial processing of methanol to acetic acid, water and acetic acid are added to the system to prevent the catalyst from deactivating. Therefore, under the premise of ensuring the activity of the catalyst, selecting a highly stable catalyst will effectively increase the production efficiency of acetic acid and reduce the production cost, which has also become a research hotspot in the field of carbonylation to acetic acid. Bimetallic complex catalysts with heteronuclear structures are often used to show better performance than single-metal catalysts through metal–metal interactions and their ligand effects [14–16]. As a result, the catalyst has become a research hotspot [17–22]. The bridged ligand and Rh coordinate to form a bidentate coordination structure, and after the ligand is coordinated with Rh, it can firmly grasp Rh. The bimetallic complex with a stable structure can use the steric effect to prevent catalyst deactivation. A catalyst with a bimetallic structure (Rh(I)/Ru(III)) was designed and synthesized. The catalytic evaluation of catalysts using methanol and CO as raw materials for carbonylation to produce acetic acid was compared with the (Rh(CO)2I2)− catalyst in the Monsanto process. Through theoretical calculations, the structural characteristics and the catalytic reaction mechanism of Rh(I)/Ru(III) were studied at the molecular level, and the internal relationship between the catalyst structure and catalytic performance was discussed [23,24]. The Rh(I)/Ru(III) catalyst was synthesized under theoretical guidance, and its structure was confirmed through characterization. The performance of the catalyst for the preparation of acetic acid by methanol carbonylation was evaluated. This provides new ideas for the design of carbonylation catalysts.

2. Materials and Methods

2.1. Raw Materials and Reagents

Methanol (CH3OH), acetic acid (CH3COOH), and sodium carbonate (Na2CO3) were analytical-grade products from Shanghai Aladdin Reagent Co. Ltd. (Shanghai, China). Lithium iodide (LiI) and potassium iodide (KI) were analytical grade products from Shanghai Macklin Biochemical Technology Co. Ltd. (Shanghai, China). Methyl iodide (CH3I) was an analytical-grade product from Shandong West Asia Chemical Co. Ltd. (Linyi, China). Rhodium chloride hydrate (RhCl 3H O) and 3· 2 ruthenium chloride hydrate (RuCl 3H O) were both analytical grade products from Shanxi Dongwal 3· 2 Chemical Co. Ltd. (Taiyuan, China). Carbon monoxide (CO) and nitrogen (N2), with a volume fraction of 99.999%, were both from the Ningxia Guangli Gas Plant (Yinchuan, China).

2.2. Theoretical Calculation Method Based on Lanl2dz, the mechanism of methanol carbonylation of the catalyst was studied theoretically using density functional theory (DFT) and the B3LYP method. At the molecular level, the catalytic reaction mechanism was studied and the structure and energy of intermediates and transition states were obtained [25]. All energies were corrected at zero, while the reaction energy was compared with the catalytic performance of the Monsanto iodine catalyst. All calculations were performed using the Gaussian 09 quantum chemistry program.

2.3. Catalyst Preparation

2.3.1. Synthesis of Precursor Rh2(CO)4Cl2 To begin, 0.2 g of rhodium chloride hydrate (RhCl 3H O) was weighed, and the particles were 3· 2 ground into to a powder state. The powder was transferred to a U-shaped glass tube with a sand core device at the bottom, in which the air had been removed. Then, 0.5 MPa of CO passed, and the tube was allowed to react in an oil bath at a constant temperature of 80–90 ◦C for 6 h. As the reaction progressed, orange-red crystals appeared in the glass tube. After the reaction was over, the orange-red crystals were collected into a brown reagent bottle where they were weighed and stored to dry. Materials 2020, 13, 4026 3 of 12

2.3.2. Synthesis of Rh(I)/Ru(III) Bimetallic Catalyst Rh (CO) Cl and RuCl 3H O were weighed at a molar ratio of 1:2 and dissolved in 10 mL 2 4 2 3· 2 of distilled water. This solution was mixed by stirring at room temperature for 30 min, and the mixture was then slowly added dropwise to 50 mL of 0.1 mol/L Na2CO3 solution. The resulting mixture was placed in a water bath at a constant temperature of 50 ◦C and reacted for 10 h until black precipitate appeared. After cooling, centrifuging, and drying the mixture at 50 ◦C, black solid particles were obtained.

2.3.3. Evaluation of Catalyst Performance

The total mass of reactants was 25 g. According to the H2O:CH3OH:LiI:CH3COOH:CH3I mass ratio of 6:24:4:54:12, H2O, CH3OH, LiI, CH3COOH, and CH3I were added to a 100 mL autoclave. Then, 0.03 g of the Rh(I)/Ru(III) bimetallic catalyst was added. The initial CO pressure was 3.5 MPa, and the reaction temperature was 190 ◦C. The stirring rate was 500 r/min, and the reaction time was 60 min. After the reaction was completed, a sample was taken for product analysis.

2.4. Characterization of the Catalyst X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Fisher ESCALAB 250 Xi (Symerﬁsh Technologies, New York, NY, USA) equipped with an Al Ka radiation X-ray source. FTIR measurements were carried out by an Alpha FTIR spectrometer (Brucker Optics, Ettlingen, Germany) equipped with exchangeable sampling modules. The spectra were collected as 1 1 the average of at least 200 scans at a resolution of 4 cm− in the frequency range of 2500–400 cm− . Inductively coupled plasma optical emission spectroscopy (ICP-OES) experiments were performed on an Agilent 725 optical emission spectrometer (Agilent Technology Limited, Santa Clara, CA, USA). Quantiﬁcation was performed by an external standard method.

2.5. Catalyst Performance Analysis Method The product was quantitatively analyzed using a Shimadzu gas chromatograph (GC-2014), an Intercap FFAP polar capillary column, and a hydrogen ﬂame ionization detector (FID). The injection volume was 1 µL, and the heating rate was 5 ◦C/min from 32 to 60 ◦C. This was held for 2 min, then the temperature was increased to 150 ◦C at a 10 ◦C/min heating rate and held for 2 min. Finally, the product was quantiﬁed by an external standard method. The qualitative analysis was performed using a Shimadzu gas chromatograph mass spectrometer (GCMS-QP2010, Shimadzu Production Centre, Kyoto, Japan) under the same conditions. Taking the conversion rate of methanol (x, %), the selectivity of acetic acid (sHAc, %), the selectivity of methyl acetate (sMeOAc, %), the yield of acetic acid (yHAc, %) and the yield of methyl acetate (yMeOAc, %) is used as the evaluation index of catalyst catalytic performance, and the calculation formula is as follows: n x = 2 100% n1 ×

n3 sHAC = 100% n2 × n4 sMeOAc = 100% n2 × y = x s 100% HAC × HAc × y = x s 100% MeOAc × MeOAc × reaction times TOF = (number o f active sites reaction time) × Materials 2020, 13, x FOR PEER REVIEW 4 of 13

reaction times TOF = Materials 2020, 13, 4026 ()number of active sites× reaction time 4 of 12

reaction reaction times times TONTON== numbernumber o f active of active sites site s where n1 is the amount of methanol added, n2 is the amount of methanol consumed, n3 is the amount Where n1 is the amount of methanol added, n2 is the amount of methanol consumed, n3 is the amount of acetic acid produced, and n4 is the amount of methyl acetate produced; the unit is mol. of acetic acid produced, and n4 is the amount of methyl acetate produced; the unit is mol. The productproduct was was qualitatively qualitatively analyzed analyzed by GC-MS.by GC-MS. Figure Figure1 shows 1 thatshows the that peaks the of thepeaks spectrum of the spectrumcorresponded corresponded to methyl to iodide, methyl methyl iodide, acetate, methyl methanol, acetate, methanol, and acetic and acid. acetic acid.

Figure 1. GC-MSGC-MS diagram of carbonylation reaction product. 3. Results 3. Results 3.1. Molecular Design 3.1. Molecular Design It can be seen from Figure2d that Rh(I) /Ru(III) is a four-coordinate bimetallic space structure, and theIt can distance be seen between from Figure the central 2d that metal Rh(I)/Ru(III) atom Rh is and a four-coordi Ru is 3.5846nate Å. Thebimetallic dimer space usually structure, has the andmost the stable distance planar between structure the and central lowest metal energy. atom We Rh assume and Ru the isplanar 3.5846 structureÅ. The dimer of the usually Rh/Ru catalyst.has the Themost calculation stable planar results structure show and that lowest the distance energy. between We assume Rhand the planar Ru in the structure planar of structure the Rh/Ru is3.6832 catalyst. Å, Theand calculation the molecular results energy show of thethat space the distance structure between is 54.67 Rh kJ/ moland lowerRu in thanthe planar that of structure the planar is 3.6832 structure. Å, Thisand the reduced molecular energy energy mainly of the comes space from structure the interaction is 54.67 kJ/mol between lower Rh than and that Ru, therebyof the planar improving structure. the Thisstability reduced of the energy catalyst. mainly comes from the interaction between Rh and Ru, thereby improving the stability of the catalyst. 3.2. Catalytic Reaction Mechanism The mechanism of Rh(I)/Ru(III) catalyzing methanol carbonylation is similar to that of the Monsanto process (see Figure3a). Figure4 shows the geometric structure of reactants, transition states, intermediates, and products in methanol carbonylation. As shown in Figures3 and4, during the Rh(I)/Ru(III) catalysis process, the oxidation addition reaction of CH3I passes through the transition state (TS1), while the Rh(I)/Ru(III) bimetallic catalyst catalytic reaction is from the original four-coordinate structure to a stable six-coordinate four-pyramid double-cone structure (TN1). During the oxidative addition of CH3I, as shown in Figure2d, the Rh /Ru catalyst was obtained through structural optimization. The bond length of Cl(1)/Rh (2.5141 Å) is longer than that of Cl(2)/Rh (2.5065 Å); the bond energy is also lower and easier to extend. As shown in Figure4, the Rh /Cl(1) bond was ﬁrst elongated (from 2.5141 to 3.0090 Å) and then recovered. The spatial structure of the catalyst was slightly deformed. It can be seen that this deformation can cooperate with the addition of the rate-determining step CH3I, eﬀectively reducing the energy barrier of the rate determination reaction and thereby improving the activity of the catalyst. After carbonyl migration and carbonyl coordination,

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it is ﬁnally eliminated by CH3COI reduction and hydrolysis to form acetic acid, thus completing the entire catalyticMaterials cycle 2020, 13 process., x FOR PEER REVIEW 5 of 13

Figure 2. Geometric structure of Rh(I)/Ru(III) bimetallic catalysts: (a,c) plane structure of Rh(I)/Ru(III) bimetallic catalysts; (b,d) stereo structure of Rh(I)/Ru(III) bimetallic catalysts (bond length in Å, bond angle in degrees).

3.2. Catalytic Reaction Mechanism The mechanism of Rh(I)/Ru(III) catalyzing methanol carbonylation is similar to that of the Monsanto process (see Figure 3a). Figure 4 shows the geometric structure of reactants, transition states, intermediates, and products in methanol carbonylation. As shown in Figures 3 and 4, during the Rh(I)/Ru(III) catalysis process, the oxidation addition reaction of CH3I passes through the transition state (TS1), while the Rh(I)/Ru(III) bimetallic catalyst catalytic reaction is from the original four-coordinate structure to a stable six-coordinate four-pyramid double-cone structure (TN1). During the oxidative addition of CH3I, as shown in Figure 2d, the Rh/Ru catalyst was obtained through structural optimization. The bond length of Cl(1)/Rh (2.5141 Å) is longer than that of Cl(2)/Rh (2.5065 Å); the bond energy is also lower and easier to extend. As shown in Figure 4, the Rh/Cl(1) bond was first elongated (from 2.5141 to 3.0090 Å) and then recovered. The spatial structure of the catalyst was slightly deformed. It can be seen that this deformation can cooperate with the addition of the rate-determiningFigure 2. Geometric step structure CH3I, of effectively Rh(I)/Ru(III) reducing bimetallic the catalysts: energy ( abarrier,c) plane of structure the rate of determination Rh(I)/Ru(III) Figure 2. Geometric structure of Rh(I)/Ru(III) bimetallic catalysts: (a,c) plane structure of Rh(I)/Ru(III) reactionbimetallic and thereby catalysts; improving (b,d) stereo the structure activity ofof Rh(I)/Ru(III) the catalyst. bimetallic After carbonyl catalysts migration(bond length and in Å, carbonyl bond bimetalliccoordination,angle catalysts; in it degrees). is finally (b,d ) eliminated stereo structure by CH3COI of Rh(I)reduction/Ru(III) and bimetallichydrolysis to catalysts form acetic (bond acid, lengththus in Å, bondcompleting angle in degrees).the entire catalytic cycle process. 3.2. Catalytic Reaction Mechanism The mechanism of Rh(I)/Ru(III) catalyzing methanol carbonylation is similar to that of the a b Monsanto process (see Figure 3a). Figure 4 shows the geometric structure of reactants, transition states, intermediates, and products in methanol carbonylation. As shown in Figures 3 and 4, during the Rh(I)/Ru(III) catalysis process, the oxidation addition reaction of CH3I passes through the transition state (TS1), while the Rh(I)/Ru(III) bimetallic catalyst catalytic reaction is from the original four-coordinate structure to a stable six-coordinate four-pyramid double-cone structure (TN1). During the oxidative addition of CH3I, as shown in Figure 2d, the Rh/Ru catalyst was obtained through structural optimization. The bond length of Cl(1)/Rh (2.5141 Å) is longer than that of Cl(2)/Rh (2.5065 Å); the bond energy is also lower and easier to extend. As shown in Figure 4, the Rh/Cl(1) bond was first elongated (from 2.5141 to 3.0090 Å) and then recovered. The spatial structure of the catalyst was slightly deformed. It can be seen that this deformation can cooperate with the addition Materialsof 2020 the , rate-determining13, x FOR PEER REVIEW step CH 3I, effectively reducing the energy barrier of the rate determination6 of 13 reaction and thereby improving the activity of the catalyst. After carbonyl migration and carbonyl FigureFigurecoordination, 3. Diagram 3. Diagram it ofis finally theof the reaction eliminatedreaction mechanism mechanism by CH3COI for reduction methanol and carbonylation: carbonylation: hydrolysis to ( aform) (cyclea) acetic cycle for acid,Monsanto for Monsantothus catalyst catalystcompleting catalyzed catalyzed the carbonylation entire carbonylation catalytic of cycle methanol of process.methanol to aceticto acetic acid; acid; (b) cycle(b) cycle for Rh(I)for Rh(I)/Ru(III)/Ru(III) bimetallic bimetallic catalyst catalyzedcatalyst carbonylation catalyzed carbonylation of methanol of methanol to acetic acid.to acetic acid. a b

FigureFigure 4. Geometric 4. Geometric structure structure of of reactants, reactants, transitiontransition st states,ates, intermediates, intermediates, and and products products in methanol in methanol carbonylation (bond length in Å, bond angle in degrees): (a) CH3I addition reaction product (TN1); carbonylation (bond length in Å, bond angle in degrees): (a) CH3I addition reaction product (TN1); (b) CH3I addition reaction state (TS1); (c) ligand migration reaction state (TS2); (d) ligand migration (b) CH3I addition reaction state (TS1); (c) ligand migration reaction state (TS2); (d) ligand migration reaction product (TN2); (e) CO addition reaction product (TN3); (f) CH3COI reductive elimination reaction product (TN2); (e) CO addition reaction product (TN3); (f) CH3COI reductive elimination reaction state (TS3); (g) CH3I; (h) CH3COI; (DFT, B3LYP/Lanl2dz). reaction state (TS3); (g) CH3I; (h) CH3COI; (DFT, B3LYP/Lanl2dz).

It can be seen from Figure 5 that compared with the Monsanto catalyst, the CH3I oxidation reaction of methanol carbonylation catalyzed by the Rh(I)/Ru(III) bimetallic catalyst reduces the energy barrier by 23.88 kJ/mol. At this time, the elimination reaction has become the rate-determining step. Compared with Monsanto’s rhodium iodide catalyst, the Rh(I)/Ru(III) bimetallic catalyst has a higher stability due to the interaction between Rh/Ru metal and its bidentate structure. In the process of ligand migration, the Rh/Cl (1) bond is elongated, but Cl (2) and Rh are still firmly grasped to prevent the decomposition and inactivation of the precipitate. As a result, it is advantageous to increase the rate of the carbonylation reaction and the selectivity and activity of the catalyst.

Figure 5. Potential energy diagram for the reaction pathway. Black line: The reaction barrier diagram of the Monsanto catalyst catalyzing methanol carbonylation to acetic acid. Blue line: The reaction barrier diagram of Rh(I)/Ru(III) bimetallic catalyst catalyzing methanol carbonylation to acetic acid.

Figure 6 shows a comparison of the infrared image of Rh2(CO)4Cl2 with the infrared image of RhCl3·3H2O. At 2007 and 1900 cm−1, there are two rhodium terminal carbonyl peaks. It can be seen that Rh2(CO)4Cl2 was successfully synthesized. In the infrared spectrum (Figure 6), the characteristic absorption peaks of the terminal carbonyl group of the Rh(I)/Ru(III) catalyst appear at 1600 and 1700

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Figure 3. Diagram of the reaction mechanism for methanol carbonylation: (a) cycle for Monsanto catalyst catalyzed carbonylation of methanol to acetic acid; (b) cycle for Rh(I)/Ru(III) bimetallic catalyst catalyzed carbonylation of methanol to acetic acid.

Figure 4. Geometric structure of reactants, transition states, intermediates, and products in methanol

carbonylation (bond length in Å, bond angle in degrees): (a) CH3I addition reaction product (TN1); Materials 2020, 13, 4026 6 of 12 (b) CH3I addition reaction state (TS1); (c) ligand migration reaction state (TS2); (d) ligand migration

reaction product (TN2); (e) CO addition reaction product (TN3); (f) CH3COI reductive elimination

reaction state (TS3); (g) CH3I; (h) CH3COI; (DFT, B3LYP/Lanl2dz). It can be seen from Figure5 that compared with the Monsanto catalyst, the CH 3I oxidation reaction of methanol carbonylation catalyzed by the Rh(I)/Ru(III) bimetallic catalyst reduces the energy It can be seen from Figure 5 that compared with the Monsanto catalyst, the CH3I oxidation barrierreaction by 23.88 of methanol kJ/mol. Atcarbonylation this time, the catalyzed elimination by the reaction Rh(I)/Ru(III) has becomebimetallic the catalyst rate-determining reduces the step. Comparedenergy withbarrier Monsanto’s by 23.88 kJ/mol. rhodium At this iodide time, the catalyst, elimination the Rh(I) reaction/Ru(III) has become bimetallic the rate-determining catalyst has a higher stabilitystep. due Compared to the with interaction Monsanto’s between rhodium Rh /iodideRu metal catalyst, and the its Rh(I)/Ru(III) bidentate structure. bimetallic catalyst In the processhas a of ligandhigher migration, stability the due Rh to/ Clthe (1)interaction bond is between elongated, Rh/R butu metal Cl (2) and and its Rh bidentate are still structure. ﬁrmly grasped In the process to prevent the decompositionof ligand migration, and inactivationthe Rh/Cl (1) ofbond the is precipitate. elongated, but As Cl a result, (2) and it Rh is advantageousare still firmly grasped to increase to the rate ofprevent the carbonylation the decomposition reaction and and inactivation the selectivity of the andprecipitate. activity As of a the result, catalyst. it is advantageous to increase the rate of the carbonylation reaction and the selectivity and activity of the catalyst.

FigureFigure 5. Potential 5. Potential energy energy diagram diagram for for the the reactionreaction pathwa pathway.y. Black Black line: line: The The reaction reaction barrier barrier diagram diagram of theof Monsantothe Monsanto catalyst catalyst catalyzing catalyzing methanol methanol carbonylationcarbonylation to to acetic acetic acid. acid. Blue Blue line: line: The Thereaction reaction barrierbarrier diagram diagram of Rh(I) of Rh(I)/Ru(III)/Ru(III) bimetallic bimetallic catalyst catalyst catalyzing methanol methanol carbonylation carbonylation to acetic to acetic acid.acid.

Figure 6 shows a comparison of the infrared image of Rh2(CO)4Cl2 with the infrared image of Figure6 shows a comparison of the infrared image of Rh 2(CO)4Cl2 with the infrared image of −1 RhCl3·3H2O. At 2007 and 1900 cm1 , there are two rhodium terminal carbonyl peaks. It can be seen RhCl3 3H2O. At 2007 and 1900 cm− , there are two rhodium terminal carbonyl peaks. It can be seen that· Rh2(CO)4Cl2 was successfully synthesized. In the infrared spectrum (Figure 6), the characteristic that Rh (CO) Cl was successfully synthesized. In the infrared spectrum (Figure6), the characteristic Materials 2 20204 , 132, x FOR PEER REVIEW 7 of 13 absorption peaks of the terminal carbonyl group of the Rh(I)/Ru(III) catalyst appear at 1600 and 1 1700absorption cm− . Comparing peaks of the the terminal infrared carbonyl images group of Rh of2 (CO)the Rh(I)/Ru(III)4Cl2, it can catalyst be seen appear that dueat 1600 to and the 1700 influence of thecm ligand,−1. Comparing the peak the ofinfrared the terminal images carbonylof Rh2(CO) rhodium4Cl2, it can in be the seen Rh(I) that/ Ru(III)due to the bimetallic influence catalyst of the has a redligand, shift. Atthe thepeak same of the time, terminal due carbonyl to the influence rhodium ofin thethe centralRh(I)/Ru(III) metal bimetallic Rh, its vibration catalyst has frequency a red is shift. At the same time, due to the influence of the central metal Rh, its vibration frequency is significantly reduced. significantly reduced.

FigureFigure 6. FTIR 6. FTIR spectra spectra of of di differentﬀerent catalysts. catalysts. Blue line: line: FT FTIRIR spectra spectra of Rh(I)/Ru(III) of Rh(I)/Ru(III) bimetallic bimetallic catalyst; catalyst; greengreen line: line: FTIR FTIR spectra spectra of of RhCl RhCl3H3·3HO;2O; redred line:line: FTIR FTIR spectra spectra of of RuCl RuCl3·3H3H2O; blackO; black line: line:FTIR FTIRspectra spectra 3· 2 3· 2 of (Rh2(CO)4Cl2). of (Rh2(CO)4Cl2).

Figure 7 shows the XPS spectrum of the Rh(I)/Ru(III) bimetallic catalyst and precursor. XPS requires calibration since charging of the sample will influence the kinetic energies. Spectra from samples have been charge-corrected to give the adventitious C1s spectral component (C–C, C–H) a binding energy of 284.8 eV. This process has an associated error of ± 0.1–0.2 eV [26]. As shown in Figure 7, compared with Rh3d5/2 (308.9 eV) in Rh2(CO)4Cl2, the binding energy (308.5 eV) of the Rh(I)/Ru(III) bimetallic catalyst is reduced by 0.4 eV, while the binding energy of Ru3d3/2 (284.4 eV) is reduced by 2 eV compared to RuCl3/3H2O and Ru3d3/2 (286.4 eV). The Cl2p in the Rh(I)/Ru(III) bimetallic catalyst is 199.7 eV. After coordination with Rh and Ru, the binding energy of Cl2p in Rh2(CO)4Cl2 and RuCl3/3H2O increased by 1.2 eV and 1.1 eV, respectively. It can be seen that during the coordination process, a Cl/Rh coordination bond and a Cl/Ru coordination bond are formed. The binding energy of rhodium and ruthenium is reduced, and the binding energy of chlorine is increased, indicating that the electrons of chlorine are transferred to rhodium and ruthenium. The electron-rich nature of rhodium increases the activity of the catalyst and increases the reaction rate. The composition of the Rh(I)/Ru(III) bimetallic catalyst was investigated using inductively coupled plasma optical emission spectrometry (ICP-OES, AGILENT 725). The mole ratio of Rh/Ru was 1.15. Based on the characterization results of XPS and ICP-OES, we believe that the experimentally obtained Rh(I)/Ru(III) bimetallic catalyst configuration conforms to the theoretical design configuration.

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Figure7 shows the XPS spectrum of the Rh(I) /Ru(III) bimetallic catalyst and precursor. XPS requires calibration since charging of the sample will influence the kinetic energies. Spectra from samples have been charge-corrected to give the adventitious C1s spectral component (C–C, C–H) a binding energy of 284.8 eV. This process has an associated error of 0.1–0.2 eV [26]. As shown in Figure7, ± compared with Rh3d5/2 (308.9 eV) in Rh2(CO)4Cl2, the binding energy (308.5 eV) of the Rh(I)/Ru(III) bimetallic catalyst is reduced by 0.4 eV, while the binding energy of Ru3d3/2 (284.4 eV) is reduced by 2 eV compared to RuCl3/3H2O and Ru3d3/2 (286.4 eV). The Cl2p in the Rh(I)/Ru(III) bimetallic catalyst is 199.7 eV. After coordination with Rh and Ru, the binding energy of Cl2p in Rh2(CO)4Cl2 and RuCl3/3H2O increased by 1.2 eV and 1.1 eV, respectively. It can be seen that during the coordination process, a Cl/Rh coordination bond and a Cl/Ru coordination bond are formed. The binding energy of rhodium and ruthenium is reduced, and the binding energy of chlorine is increased, indicating that the electrons of chlorine are transferred to rhodium and ruthenium. The electron-rich nature of rhodium increases the activity of the catalyst and increases the reaction rate. The composition of the Rh(I)/Ru(III) bimetallic catalyst was investigated using inductively coupled plasma optical emission spectrometry (ICP-OES, AGILENT 725). The mole ratio of Rh/Ru was 1.15. Based on the characterization results of XPS and ICP-OES, we believe that the experimentally obtained Rh(I)/Ru(III)Materials bimetallic 2020, 13, x FOR catalyst PEER REVIEW configuration conforms to the theoretical design configuration.8 of 13

Figure 7. XPS spectra for Rh(I)/Ru(III) bimetallic catalyst and precursors: (a) XPS spectra of Rh (Rh2(CO)4Cl2); (b) XPS spectra of Ru (RuCl3 3H2O); (c) XPS spectra of Rh (Rh(I)/Ru(III) bimetallic Figure 7. XPS spectra for Rh(I)/Ru(III) bimetallic· catalyst and precursors: (a) XPS spectra of Rh catalyst); (d) XPS spectra of Ru (Rh(I)/Ru(III) bimetallic catalyst); (e) XPS spectra of Cl2p (Rh2(CO)4Cl2); (Rh2(CO)4Cl2); (b) XPS spectra of Ru (RuCl3·3H2O); (c) XPS spectra of Rh (Rh(I)/Ru(III) bimetallic (f) XPS spectra of Cl2p (Rh(I)/Ru(III) bimetallic catalyst); (g) XPS spectra of Cl2p (RuCl3 3H2O). catalyst); (d) XPS spectra of Ru (Rh(I)/Ru(III) bimetallic catalyst); (e) XPS spectra of Cl2p (Rh2(CO)4Cl·2); (f): XPS spectra of Cl2p (Rh(I)/Ru(III) bimetallic catalyst); (g) XPS spectra of Cl2p (RuCl3·3H2O).

3.3. Comparison of Catalyst Performance Table 1 shows the experimental results of methanol carbonylation catalyzed by Rh(I)/Ru(III) catalyst and the Monsanto catalyst under the same conditions. It can be seen from Table 1 that when Rh(I)/Ru(III) catalyzes the carbonylation reaction, its catalytic performance is significantly better than that of the Monsanto catalyst. At the end of the reaction, some precipitation of RhI3 appeared in the reactor in the Monsanto catalytic system, but no precipitation occurred in the Rh(I)/Ru(III) catalytic system, indicating that Rh(I)/Ru(III) has a better stability [27–30]. As a result of the catalytic performance of RuCl3/3H2O, it has no catalytic activity, but it stabilizes the catalyst structure and forms a folded structure, which reduces the overall energy of the catalyst system.

Table 1. Catalytic performance of Rh(I)/Ru(III), RuCl3·3H2O, and Rh2(CO)4Cl2 for methyl acetate carbonylation.

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3.3. Comparison of Catalyst Performance Table1 shows the experimental results of methanol carbonylation catalyzed by Rh(I) /Ru(III) catalyst and the Monsanto catalyst under the same conditions. It can be seen from Table1 that when Rh(I)/Ru(III) catalyzes the carbonylation reaction, its catalytic performance is signiﬁcantly better than that of the Monsanto catalyst. At the end of the reaction, some precipitation of RhI3 appeared in the reactor in the Monsanto catalytic system, but no precipitation occurred in the Rh(I)/Ru(III) catalytic system, indicating that Rh(I)/Ru(III) has a better stability [27–30]. As a result of the catalytic performance of RuCl3/3H2O, it has no catalytic activity, but it stabilizes the catalyst structure and forms a folded structure, which reduces the overall energy of the catalyst system.

Table 1. Catalytic performance of Rh(I)/Ru(III), RuCl 3H O, and Rh (CO) Cl for methyl 3· 2 2 4 2 acetate carbonylation.

1 Catalyst x/% sHAc/% yHAc/% TOF/h− TON Materials 2020, 13, xRh(I) FOR/ PEERRu(III) REVIEW 94.9 85.2 80.9 3.2 107 3.2 107 9 of 13 × × Rh (CO) Cl 90.3 49.6 44.8 7.6 106 7.6 106 2 4 2 × × T = 190 °C; pRuCl = 3.5 MPa;3H O t = 60 min; 0 w(CH3COOH) 0 = 54 wt%; 0w(H2O) = 6 wt%; 0 x/%: Conversion 0 rate of 3· 2 methanol;T = 190 ◦C; ps=HAc3.5/%: MPa; thet =selectivity60 min; w(CH of 3aceticCOOH) acid;= 54 wt%;yHAc/%: w(H 2theO) =yield6 wt%; of x /%:acetic Conversion acid; TOF rate of: turnover methanol; frequency,sHAc/%: the selectivityi.e., the number of acetic acid;of molecule yHAc/%:s the formed yield of per acetic active acid; site TOF: per turnover second; frequency, TON: total i.e., the number number of of molecules formed per active site per second; TON: total number of products formed molecules per active site. products formed molecules per active site. 3.4. Catalyst Performance Evaluation 3.4. Catalyst Performance Evaluation In the Monsanto catalyst, to avoid the deactivation of the catalyst active metal Rh when the CO In the Monsanto catalyst, to avoid the deactivation of the catalyst active metal Rh when the CO partial pressure is low, a large amount of water is usually added. We investigated the effect of water partial pressure is low, a large amount of water is usually added. We investigated the effect of water content on the performance of the bimetallic catalyst in the methanol carbonylation reaction [31–33]. content on the performance of the bimetallic catalyst in the methanol carbonylation reaction [31–33]. Figure8 shows the performance of Rh(I) /Ru(III) bimetallic catalyst catalyzed carbonylation of methanol Figure 8 shows the performance of Rh(I)/Ru(III) bimetallic catalyst catalyzed carbonylation of under different H2O mass fractions. Figure8 shows that the selectivity of acetic acid first increases methanol under different H2O mass fractions. Figure 8 shows that the selectivity of acetic acid first with increasing water content and then decreases. At a water content of 6 wt%, acetic acid has the increases with increasing water content and then decreases. At a water content of 6 wt%, acetic acid highest selectivity, 95.90%, and no other by-products. has the highest selectivity, 95.90%, and no other by-products.

Figure 8. 8. CatalyticCatalytic performance performance of ofRh(I)/Ru(III) Rh(I)/Ru(III) bimetallic bimetallic catalysts catalysts at different at diﬀerent contents contents of water of water (T =

190(T = °C;190 p ◦=C; 3.5p =MPa;3.5 MPa;t = 60 tmin;= 60 w(CH min; w(CH3COOH)3COOH) = 54 wt%;= 54 x wt%;/%: conversion x/%: conversion rate of ratemethanol). of methanol).

As can be seen from Figure 99,, thethe selectivityselectivity ofof aceticacetic acidacid alsoalso graduallygradually increasesincreases withwith thethe increaseincrease of the amount of solvent. When When the the solvent solvent co contentntent is 54 wt%, the acetic acid selectivity is as high as 95.90%. 95.90%. Considering Considering the the production production and and prod productionuction costs costs of of by-products, by-products, the optimal optimal amount amount of solvent in this experiment was 54 wt%. It can be seen from Figure 10 that in the Rh(I)/Ru(III) bimetallic catalytic system, the methanol conversion rate is 100.00%. With the increase of the system pressure, the selectivity of acetic acid increases ﬁrst and then decreases. The selectivity of methyl acetate gradually decreases and then

Figure 9. Catalytic performance of Rh(I)/Ru(III) bimetallic catalyst in different solvent contents (T =

190 °C; p = 3.5 MPa; t = 60 min; w (H2O) = 6 wt%; x/%: conversion rate of methanol).

It can be seen from Figure 10 that in the Rh(I)/Ru(III) bimetallic catalytic system, the methanol conversion rate is 100.00%. With the increase of the system pressure, the selectivity of acetic acid increases first and then decreases. The selectivity of methyl acetate gradually decreases. At 3.5 MPa, the acetic acid selectivity is 95.90%. When the pressure continues to increase, by-product methyl

Materials 2020, 13, x FOR PEER REVIEW 9 of 13

T = 190 °C; p = 3.5 MPa; t = 60 min; w(CH3COOH) = 54 wt%; w(H2O) = 6 wt%; x/%: Conversion rate of

methanol; sHAc/%: the selectivity of acetic acid; yHAc/%: the yield of acetic acid; TOF: turnover frequency, i.e., the number of molecules formed per active site per second; TON: total number of products formed molecules per active site.

3.4. Catalyst Performance Evaluation In the Monsanto catalyst, to avoid the deactivation of the catalyst active metal Rh when the CO Materialspartial2020 pressure, 13, 4026 is low, a large amount of water is usually added. We investigated the effect of water9 of 12 content on the performance of the bimetallic catalyst in the methanol carbonylation reaction [31–33]. increases.Figure 8 Atshows 3.5 MPa, the theperformance acetic acid of selectivity Rh(I)/Ru(III) is 95.90%. bimetallic When cataly thest pressure catalyzed continues carbonylation to increase, of methanol under different H2O mass fractions. Figure 8 shows that the selectivity of acetic acid first by-product methyl acetate appears in the product, and the acetic acid selectivity begins to decrease. increases with increasing water content and then decreases. At a water content of 6 wt%, acetic acid Therefore, 3.5 MPa is the optimal reaction pressure of the system. has the highest selectivity, 95.90%, and no other by-products.

Materials 2020, 13, x FOR PEER REVIEW 10 of 13 Materials 2020, 13, x FOR PEER REVIEW 10 of 13 acetate appears in the product, and the acetic acid selectivity begins to decrease. Therefore, 3.5 MPa is theacetate optimal appears reaction in the pressure product, of and the thesystem. acetic acid selectivity begins to decrease. Therefore, 3.5 MPa isFigure theFigure optimal 9. 8. Catalytic reaction performance performance pressure ofof oftheRh(I)/Ru(III) Rh(I) system./Ru(III) bimetallic bimetallic catalysts catalyst at different in diﬀ contentserent solvent of water contents (T =

(T190= 190 °C;◦ pC; = p3.5= MPa;3.5 MPa; t = 60t =min;60 min;w(CH w3COOH) (H2O) == 654 wt%; wt%; xx/%:/%: conversionconversion rate of of methanol). methanol).

As can be seen from Figure 9, the selectivity of acetic acid also gradually increases with the increase of the amount of solvent. When the solvent content is 54 wt%, the acetic acid selectivity is as high as 95.90%. Considering the production and production costs of by-products, the optimal amount of solvent in this experiment was 54 wt%.

FigureFigure 10. 10.Catalytic Catalytic performanceperformance performance ofof Rh(I)ofRh(I)/Ru(III) Rh(I)/Ru(III)/Ru(III) bimetallic bimetallic bimetallic catalysts catalysts catalysts at at diat ﬀdifferentdifferenterent pressures pressures (T (T=(T 190= = 190 190◦C; t = 60 min; w(CH COOH) = 54 wt%; w(H O) = 6 wt%; x/%: conversion rate of methanol). °C; t°C; = 60 t = min; 60 min; w(CH 3w(CH3COOH)3COOH) = 54 = 54wt%; wt%; w(H w(H2 2O)2O) = =6 6wt%; wt%; x x/%:/%: conversion conversion raterate of methanol).

As shown in Figure 11, with the increase of reaction temperature, the selectivity of acetic acid shows As shownAs shown in Figurein Figure 11, 11, with with the the increase increase of of reac reactiontion temperature, temperature, the selectivity ofof acetic acetic acid acid an increasingshows an increasing trend. At 180,trend. 190, At and180, 200190, ◦andC, the 200 selectivity °C, the selectivity of acetic of acid acetic reached acid reached over 90%. overAt 90%. 190 At◦C , shows anFigure increasing 9. Catalytic trend. performance At 180, 190,of Rh(I)/Ru(III) and 200 °C, bimetallic the selectivity catalyst in of different acetic acid solvent reached contents over (T =90%. At the highest190 °C, the acetic highest acid acetic selectivity acid selectivity is 96.32%, is and 96.32% no other, and by-products no other by-products are produced. are produced. Therefore, Therefore,190 ◦Cis 190 °C, 190the °C; highest p = 3.5 acetic MPa; t acid = 60 min;selectivity w (H2O) is = 96.32% 6 wt%; x, /%:and conversion no other by-productsrate of methanol). are produced. Therefore, 190the 190 optimal°C is°C the is reactiontheoptimal optimal temperaturereaction reaction temperature temperature for this reaction. for for this this reaction. reaction. It can be seen from Figure 10 that in the Rh(I)/Ru(III) bimetallic catalytic system, the methanol conversion rate is 100.00%. With the increase of the system pressure, the selectivity of acetic acid increases first and then decreases. The selectivity of methyl acetate gradually decreases. At 3.5 MPa, the acetic acid selectivity is 95.90%. When the pressure continues to increase, by-product methyl

Figure 11. Catalytic performance of Rh(I)/Ru(III) bimetallic catalysts at different temperatures (p = 3.5 FigureMPa; 11. 11. t Catalytic= 60Catalytic min; w(CHperformance performance3COOH) of = Rh(I)/Ru(III)54 of wt%; Rh(I) w(H/Ru(III)2 bimetallicO) = 6 bimetallic wt%; catalysts x/%: catalystsconversion at different at rate di ﬀ temperaturesoferent methanol). temperatures (p = 3.5

MPa;(p = 3.5 t = MPa 60 min;; t = w(CH60 min;3COOH) w(CH3 COOH)= 54 wt%;= 54w(H wt%;2O) w(H= 6 wt%;2O) = x6/%: wt%; conversion x/%: conversion rate of methanol). rate of methanol). 4. Discussion 4. DiscussionUnder the guidance of theory, the Rh(I)/Ru(III) catalyst with bimetallic space structure was synthesized.Under the Throughguidance theoretical of theory, calculations, the Rh(I)/Ru(III) it is found catalyst that thewith bimetallic bimetallic catalyst space with structure a 3D space was synthesized.structure hasThrough a shorter theoretical Rh/Ru spacin calculations,g, forming it is a foundfolded thatstructure, the bimetallic reducing catalyst the system with energy, a 3D space and structureincreasing has athe shorter catalyst Rh/Ru stability. spacin Ing, addition, forming the a folded presence structure, of steric reducing hindrance the can system also preventenergy, theand increasingdeactivation the catalyst of the Rh stability. center ofIn the addition, catalyst. the XP Spresence results show of steric that hindrancerhodium accepts can also electrons prevent from the deactivationchlorine, whichof the promotesRh center itsof electronthe catalyst. enrichment XPS results and is show beneficial that rhodium to the improvement accepts electrons of catalyst from activity. This is conducive to the nucleophilic reaction of the catalyst with methyl iodide and reduces chlorine, which promotes its electron enrichment and is beneficial to the improvement of catalyst the reaction energy barrier. The Rh/Ru bimetallic catalyst has a bridged dimer space structure. During activity. This is conducive to the nucleophilic reaction of the catalyst with methyl iodide and reduces the oxidation addition of CH3I, one bridge Cl seizes Rh and the other bridge Cl/Rh is elongated. After the reaction energy barrier. The Rh/Ru bimetallic catalyst has a bridged dimer space structure. During the addition, the bridge Cl/Rh bond-length is restored. This synergistic effect can reduce the reaction the oxidation addition of CH3I, one bridge Cl seizes Rh and the other bridge Cl/Rh is elongated. After energy barrier of CH3I addition and ensure the stability of the catalyst. the addition, the bridge Cl/Rh bond-length is restored. This synergistic effect can reduce the reaction energy barrier of CH3I addition and ensure the stability of the catalyst.

Materials 2020, 13, 4026 10 of 12

4. Discussion Under the guidance of theory, the Rh(I)/Ru(III) catalyst with bimetallic space structure was synthesized. Through theoretical calculations, it is found that the bimetallic catalyst with a 3D space structure has a shorter Rh/Ru spacing, forming a folded structure, reducing the system energy, and increasing the catalyst stability. In addition, the presence of steric hindrance can also prevent the deactivation of the Rh center of the catalyst. XPS results show that rhodium accepts electrons from chlorine, which promotes its electron enrichment and is beneficial to the improvement of catalyst activity. This is conducive to the nucleophilic reaction of the catalyst with methyl iodide and reduces the reaction energy barrier. The Rh/Ru bimetallic catalyst has a bridged dimer space structure. During the oxidation addition of CH3I, one bridge Cl seizes Rh and the other bridge Cl/Rh is elongated. After the addition, the bridge Cl/Rh bond-length is restored. This synergistic effect can reduce the reaction energy barrier of CH3I addition and ensure the stability of the catalyst. A Rh(I)/Ru(III) bimetallic catalyst was synthesized, and its performance catalyzing methanol carbonylation to produce acetic acid was investigated. The Rh(I)/Ru(III) bimetallic catalyst optimizes the reaction system of methanol carbonylation to acetic acid. The results show that at a reaction temperature of 190 ◦C, the initial CO pressure is 3.5 MPa, the solvent is 54 wt% acetic acid, and the water content is 6 wt%. The carbonylation reaction is the best, and the selectivity of acetic acid is 96.32%. No precipitation occurs in the system, and the catalyst maintains good stability. The amount of solvent water is significantly lower than that of industrial water (~13–16 wt%), no by-products are produced, industrial production costs are reduced, and energy consumption for subsequent separation is saved.

Author Contributions: W.J. conducted investigation, reviewing and editing; S.Z. conducted writing—original draft preparation; N.F. and L.L. conducted investigation; Y.L. and Y.M. conducted supervision. All authors have read and agreed to the published version of the manuscript. Funding: The National Science Foundation of China (No. 21663019) and The East-West Cooperation Project of Ningxia Key R & D Plan (2019BFH02014). Acknowledgments: This study is supported by the National Science Foundation of China (No. 21663019) and the East-West Cooperation Project of Ningxia Key R & D Plan (2019BFH02014). Conﬂicts of Interest: There are no conﬂicts of interest to declare.

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