AC Catalyst for Heterogeneous Methanol

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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La-Stabilized, Single-Atom Ir/AC Catalyst for Heterogeneous Methanol Carbonylation to Methyl Acetate Siquan Feng,†,§ Xiangen Song,*,† Zhou Ren,†,§ and Yunjie Ding*,†,‡ †

Dalian National Laboratory for Clean Energy and ‡State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China § University of Chinese Academy of Sciences, Beijing, 100049, China

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 03/19/19. For personal use only.

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ABSTRACT: Although homogeneous methanol carbonylation is industrialized to produce acetic acid, it still suffers from the problems of difficult separation and metal leaching. Here, we report a heterogeneous single atom Ir−La/AC catalyst to produce methyl acetate in a fixed-bed reactor. A high TOF of 2200h−1 and high selectivity over 90% for methyl acetate with space time yield of 8200 g/(kgcat h) were achieved over the Ir−La/AC catalyst. Furthermore, the effects of reaction temperature, partial pressure of CO and CH3OH, weight ratio of CH3I/CH3OH, total LHSV as well as the role of La on the catalytic performance of Ir/AC were tested in this paper. In addition, we proposed a reasonable mechanism for heterogeneous methanol carbonylation over La stabilized single atom Ir/AC catalyst. Our work provides a reference for the further scale-up of the process and a stable, supported, single-atom system as a substitution for other homogeneous catalyst.

1. INTRODUCTION Homogeneous methanol carbonylation to acetic acid (AA) possesses the global annual manufacturing capacity over 16 million tons now.1 The major of AA is consumed by the downstream products, such as vinyl acetate monomer (VAM), acetic anhydride and acetate esters.2 And the annual demand of AA is increasing with 5% growth rate.3 On the other hand, since the 20th century, the process of methanol production from coal or methane has become more and more mature,4−6 which brought about the large-scale expansion of methanol industry.7,8 Under this background, methanol conversion as a base stock has been extensively studied. Homogeneous methanol carbonylation emerged as an effective way to produce AA. Many technologies have been developed, such as Monsanto process,9 BP Cativa process,10 Celanese & Daidel AO Plus process,11 Chiyoda & UOP Acetica process.12−14 In addition, many interesting and significant works have been reported, such as using iodide compounds and carbonyl complexes to increase the reaction rate,15 adding -N, -P, and -S organic ligand to improve electronic effect.14,16−19 Except for the homogeneous system, heterogeneous fixed-bed methanol carbonylation were © XXXX American Chemical Society

also explored, such as activated carbon supported Ir catalyst,20,21 Rh catalyst,22 Au catalyst,23 Pt catalyst,24 and Ni catalyst;25,26 SiO2 and Al2O3 supported Rh catalyst,27 polymer supported Rh or Ir catalyst,28−31 g-C3N4 supported Rh catalyst,32 heteropoly acid supported Rh catalyst,33 ionic liquid supported Rh catalyst,34,35 as well as other noniodide promoted zeolite supported Cu catalyst.36,37 These works contributed the advance of methanol carbonylation. Among the reported catalyst, some of supported Ir, Rh, and Ni catalysts showed relatively high activity, but the most problem is the unsatisfied stability. Besides, high selectivity of methane for Ni/AC catalyst was often inevitable. Moreover, zeolite catalysts dispensed with the iodide cocatalyst has been attracted more attention, but it also suffers from the problem of poor stability and lower efficiency. Therefore, a better catalytic system with high selectivity and yield to methyl acetate in a fixed-bed reactor is still needed to be developed. Received: Revised: Accepted: Published: A

October 31, 2018 March 4, 2019 March 7, 2019 March 7, 2019 DOI: 10.1021/acs.iecr.8b05402 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

reactor with an inner diameter of 6 mm. Upstream from the reactor, the liquid was vaporized and mixed with CO/H2 in a Hastalloy tube wrapped with heat tape maintained at 240 °C. The products were collected in a condenser cooled by circulating water of 5 °C. The tail gas product was analyzed online using an Agilent 7890B gas chromatography equipped with a Porapak-Q column and a TCD while the liquid products were offline analyzed using FFAP column and FID. The experiment data were analyzed at intervals of 6−12 h. Calculation of methanol conversion and product selectivity was based on the carbon numbers in products and shown as follows:

Here, we report a La-stabilized single atom Ir−La/AC catalyst for the production of methyl acetate (MA) via heterogeneous methanol carbonylation in a fixed-bed reactor. Different from traditional Cativa process of Ir catalyst to produce AA, MA was the main product in our system, with the selectivity higher than 90% and its space time yield (STY) could reached to 8200g/(kgcat h) on Ir−La/AC catalyst with 1 wt % Ir loading, which showed great potential to be further industrially scaled up. Furthermore, the obtained MA could further efficiently converted to ethanol via the mature ester hydrogenation process.38 Ethanol is not only regarded as a bulk chemical, but also as an ideal additive in automotive fuel to reduce COx, NOx and SOx emission.39,40 As the Scheme 1

Con(MeOH) =

2nMA + nAA 100% 2nMA + nAA + nMeOH

(1)

S(MA) =

2nMA 100% + nAA + nCH4

(2)

Scheme 1. Efficient Production Chain from Coal to Ethanol 2nMA

mMA mcat t

(3)

nMA + nAA nIrt

(4)

STY(MA) = TOF =

exhibited, an industrial chain of ethanol production via coal, methanol and methyl acetate (MA) route instead of traditional cereals fermentation is of great significance. Therefore, developing an economic technology of MA production via heterogeneous methanol carbonylation process possesses both academic and industrial values.

Where, Con is the conversion. S is the selectivity. STY(MA) is the space time yield of methyl acetate, TOF is the turnover frequency, and t signifies the reaction time interval of sampling analysis (h); n represents the moles and nMeOH is the mole number in the product of each sampling, and m denotes the mass.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Ir−La/AC catalyst with 1 wt % iridium loading was prepared using incipient impregnation method as follows: first, dissolved 0.0848 g of La2O3 and 0.1724 g of IrCl3 in hydrochloric acid aqueous solution (5 mL 36% HCl & 10 mL H2O); then added 10 g of coconut shell activated carbon (AC) granules of 20−40 mesh into the precursor solution; and then dried it at 65 °C under normal atmosphere. Afterward, the obtained sample was further dried at 120 °C overnight and then calcinated in tube furnace at 300 °C for 2 h under Ar protection. 2.2. The heterogeneous methanol carbonylation. Heterogeneous methanol carbonylation experiment was conducted in a continuous flow fixed-bed Hastelloy C276

3. RESULTS AND DISCUSSION Heterogeneous methanol carbonylation was conducted in a fixed-bed reactor as shown in Scheme 2. As is known to all, heterogeneous tubular reactor has many advantages in comparison with homogeneous batch stirred tank reactor, such as easy product separation, continuous processing, adjustable residence time and constant concentration of reactant. According to homogeneous mechanism of Ir catalyst,10,15,41 methanol carbonylation was triggered by SN2 oxidative addition reaction of CH3I to active species of [Ir(CO)2I2]− to form [MeIr(CO)2I3]−. And then the species of [MeIr(CO)2I3]− underwent the removal of I− and a third

Scheme 2. Schematic Diagram of Heterogeneous Methanol Carbonylation

B

DOI: 10.1021/acs.iecr.8b05402 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 1. Effect of the reaction temperature on the activity of Ir−La/ AC catalyst for heterogeneous methanol carbonylation. Reaction conditions: 2.5 MPa, CO/H2 = 4(vol), CO/CH3OH = 1(mole ratio), CH3I/CH3OH = 1:10(wt %), and LHSV = 10h−1.

Figure 3. Effect of CH3OH pressure on the activity of Ir−La/AC catalyst for heterogeneous methanol carbonylation. Reaction conditions: 240 °C, PCO = 1.0 MPa, PH2 = 0.25 MPa, PCH3I = 0.0232 MPa.

Table 1. Free Energy Changes of the Related Reaction in the Process of Methanol Carbonylation reactions CH3OH + HI → CH3I + H2O CH3OH + CO → CH3COOH CH3COOH + CH3OH → CH3COOCH3 + H2O 2CH3OH + CO → CH3COOCH3 + H2O CH3COI + H2O → CH3COOH + HI CH3COI + CH3OH → CH3COOCH3 + HI

ΔrG⊖ m, 298K (kJ/mol)

ΔrH⊖ m, 298K (kJ/mol)

−51.86 −75.13 −15.85

−52.98 −121.33 −19.98

−90.98

−141.11

−144.31 −160.16

−37.09 −57.07

Figure 4. Effect of the weight ratio of CH3I/CH3OH on the performance of Ir−La/AC catalyst for methanol carbonylation. Reaction conditions: 240 °C, 2.5 MPa, CO/H2 = 4 (vol), CO/ CH3OH = 1 (mole ratio), and LHSV = 10 h−1.

fulfilled the whole reaction cycle. On the basis of the previous homogeneous and heterogeneous works, we discussed our experiment data for heterogeneous methanol carbonylaton over La stabilized single atom Ir/AC catalyst. Figure 1 revealed the influence of reaction temperature on Ir−La/AC catalyst activity for heterogeneous methanol carbonylation. The conversion of methanol (ConMeOH) increased almost linearly from 30% to 80% in the range of 230 to 270 °C, but the SMA decreased from 90% to 75%, suggesting that high conversion of methanol and high selectivity of MA were hardly obtained at the same time. This should be come from the competitive reaction of CH3COI between H2O and CH3OH, shown in Table 1. In addition, the STY of MA increased rapidly from 230 to 250 °C, but the rate increased slowly from 260 to 280 °C, implying that the temperature above 260 °C would have a small promoting effect on catalyst activity. This trend may be influenced by the

Figure 2. Effect of CO pressure on the activity of Ir−La/AC catalyst for heterogeneous methanol carbonylation. Reaction conditions: 240 °C, PCH3OH = 1.0 MPa, PH2 = 0.25 MPa, PCH3I = 0.0232 MPa.

CO coordination. The CO migration insertion reaction subsequently happened for the species of [MeIr(CO)3I2]− to become [MeCOIr(CO)2I2]−, which was believed as the ratedetermining step. Then reductive elimination of CH3COI C


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Figure 5. LHSV effect on the activity of Ir−La/AC catalyst for heterogeneous methanol carbonylation. Reaction conditions: 240 °C, 2.5 MPa, CO/H2 = 4 (vol), CO/CH3OH = 1 (mole ratio), CH3I/ CH3OH = 1:6 (wt %).

Figure 7. Test of Ir/AC and Ir−La/AC for heterogeneous methanol carbonylation. Reaction conditions: 240 °C, 2.5 MPa, CO/CH3OH = 1(vol %), CH3I/CH3OH = 1/10 (wt %), LHSV = 10 h−1.

CO pressure above 10 bar.10 And homogeneous mechanism research verified that the slowest step was the migratory insertion to form iridium-acyl species of [MeCOIr(CO)2I2],15 which involved the elimination of I− and coordination of CO.42 Therefore, in heterogeneous system, a higher CO pressure more than 1.0 MPa was appropriate, not only because it could promote CO migration insertion to form acetyl species, but also avoid the formation of inactive [Ir(CO)2I4]− species.15 In addition, higher concentration of CO would also inhibit CH4 production at high conversion of methanol.43 Therefore, a higher CO partial pressure above 1.0 MPa is beneficial, and 2.5 MPa of total pressure is a better choice to obtain 90% selectivity of MA and less 5% selectivity of CH4 in our heterogeneous system. Except for the positive effect of CO, CH3OH also has important influence, as shown in Figure 3. Homogeneous Cativa process certified that the activity has a strong dependence on the concentration of MA, which acted as the precursor of CH3OH in order to reduce the concentration of H2O.10 In our heterogeneous system, CH3OH partial pressure exhibited positive correlation in promoting the reaction rate of methanol carbonylation, which implying that methanol played an important role in the methanolysis of Ir-acetyl species. G. Zehl et al. also reported that the corresponded reaction order of CH3OH to methyl acetate on Ir/AC catalyst was 0.69, implying that the rate-determining step of heterogeneous methanol carbonylation on Ir/AC is the methanolysis of Iracetyl species.20 Besides, CH3I cocatalyst could be regenerated by the reaction of CH3OH with HI, which was a spontaneous reaction certified by the Gibbs free energy less than zero in Table 1. That guaranteed the sustainability of CH3I and quick consumption of HI in the cycle of reaction. Furthermore, excess HI would react with active species [Ir(CO)2I2]− and transfer it to inactive [Ir(CO)2I4]− species. Additionally, the generation of a large amount of I− from HI would inhibit the elimination of I− from [MeIr(CO)2I3]−.10,15 Therefore, heterogeneous tubular reaction has the advantage of maintaining constant and sufficient CH3OH concentration to react with those excess HI, and then to some extent avoiding the generation of large amount of I− and inactive [Ir(CO)2I4]− species.44

Figure 6. Activity comparison of Ir−La/AC catalyst and other reported catalysts for heterogeneous methanol carbonylation. Reaction conditions: (a) Our work, 240 °C, 2.5 MPa, CO/H2 = 4 (vol), CO/CH3OH = 1 (mole ratio), CH3I/CH3OH = 1:6 (wt %), W/F = 1.92 (gcat h/mol). (b) Ref 49, bubble bed, 190 °C, 5.0 MPa, CH3OH/CH3I = 5.61 (wt %), CH3OH/CH3COOH = 1.79 (wt %). (c) Ref 50, 260 °C, 2.0 MPa, CO/CH3OH = 21.05, CH3OH/CH3I = 4.28 (wt %), W/F = 2.9 (gcat h/mol). (d) Ref 51, 320 °C, 1.0 MPa, CO/CH3OH = 1.05, CH3OH/CH3I = 4.28 (wt %), W/F = 5.0 (gcat h/mol). (e) Ref 37, 270 °C 5.0 MPa, CO/CH3OH = 400, W/F = 290.9 (gcat h/mol). (f) Ref 52, 400 °C, CO/CH3OH = 10, GHSV = 3000 h−1.

high conversion of methanol above 260 °C, and the residual methanol was insufficient to maintain the quick increase in reaction rate. Taking other results into consideration, such as acetic acid and CH4 selectivity, we selected the temperature of 240 °C to investigate other influencing factors. The catalyst activity was closely related to the partial pressure of reactant. As depicted in Figure 2, the TOF (moleacetyl/molIr/h) increased rapidly with the pressure of CO increase from 0.5 to 1.0 MPa, but the increasing rate seemed independent during the range of 1.0 to 2.0 MPa. Besides, it was reported that during the homogeneous BP Cativa process of Ir catalyst, the reaction rate dropped sharply when the CO pressure below 10 bar, because of the formation of inactive species of [Ir(CO)2I4]−, but it was almost independent with D


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Figure 8. (a) HR-TEM image of fresh Ir−La/AC catalyst and (b) the corresponding particle size distribution, (c) the HRTEM image of spent Ir− La/AC catalyst, (d) the HADDF-STEM image of spent Ir−La/AC catalyst (the atom size in red dots region is larger than that of green dots region; red dots represent the atoms of Ir while green ones represent the atoms of La).

active sites were accommodated in the inner micro pores of AC, which tremendously alleviated the problem of corrosion, as HI would be consumed by CH3OH during the process of internal diffusion. Therefore, the Hastelloy reactor was adequate for corrosion resistance in heterogeneous system. As indicated in Figure 4, when the weight ratio of CH3I/ CH3OH (wt %) increased from 1/20 to 1/6, the MeOH conversion and STY of MA increased slowly first in the range of 1/20 to 1/10, and the increase rate speeded up from 1/10 to1/6. It implied that the weight ratio of CH3I/CH3OH in the range of 1/10−1/6 was proper. One concern about the formation of MA is that whether or not it directly originates from methanol carbonylation or second esterification reaction of AA with methanol due to its self-catalysis.48 According to thermodynamic data in Table 1, methanol carbonylation to MA has a ΔrG⊖ m of −90.98 kJ/mol of −141.11 kJ/mol, which were more favorable than and ΔrH⊖ m of −75.13 kJ/mol and ΔrH⊖ the formation of AA with a ΔrG⊖ m m of −121.33 kJ/mol. Accordingly, the formation of MA was prior to AA in terms of free energy change. Besides, in regard of the reactoion between CH3COI with CH3OH, the ΔrG⊖ m of −160.9 kJ/mol is also lower than that of the reaction with H2O (−144.3 kJ/mol). Thus, we concluded that the formation of MA and AA were competing reactions, but the reaction environment in our heterogeneous system is more beneficial to form MA, wherease the homogeneous process favors the formation of AA.

Figure 9. Model distribution of the complex.

Methyl iodide is a necessary cocatalyst for both Rh and Ir catalysts, but its concentration on the activity has distinct difference.41,45 As reported by Forster,45 the reaction rate is proportional to the concentration of CH3I for Rh catalyst, but independent for Ir catalyst when the concentration of CH3I was higher than about 10 wt %.10 In the Monsanto process of Rh catalyst, the mass ratio of CH3I/MA was 1/0.68,46 while in the BP Cativa process of Ir catalyst, the mole ratio of CH3I/ MA was 1/2.36.10 The oxidative addition rate of CH3I to [Ir(CO)2I2]− was 150 times faster than that of [Rh(CO)2I2]−, while the CO migration insertion rate of [MeRh(CO)2I3]− was 1 × 105 to 1 × 106 times faster than that of [MeIr(CO)2I3]−.47 In our heterogeneous methanol carbonylation system, the E


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Figure 10. Proposed mechanism of heterogeneous methanol carbonylation over Ir−La/AC catalyst.

We used HR-TEM and HADDF-STEM characterization to inspect the structure and morphology of Ir−La/AC catalysts in Figure 8. HR-TEM image shows uniformly distributed nanoparticles about 1−2 nm existed for the fresh Ir−La/AC catalyst (Figure 8a, b), but no nanoparticles could be discerned after reaction except the texture of AC (Figure 8c), indicating that the NPs probably been dispersed during reaction. HADDF-STEM image further revealed that Ir and La atoms uniformly distributed on the surface of AC, suggesting the single active sites of Ir−La/AC catalyst. After carefully observing the HADDF-STEM image (Figure 8d), we found that the atoms with different contrast were in close neighbor. The atom contrast was directly related with the brightness in HADDFSTEM picture and the contrast follows a close dependence to Z2 (atomic number).53,54 Therefore, the atom having a bigger atomic number usually emerges as a brighter and larger pot in the HADDF-STEM picture. The brightest and larger atoms correlates to Ir (Z = 77), and the second can be designated to La (Z = 57). After analyzing the atom space (Figures S1 and S2 and Table S1), we attributed the distance of 2.12−3.00 Å to Ir−La model, 3.0−3.8 Å to Ir−I−La model, and 4.4 Å to IrCO-La model respectively (Figure 9).55 Hensley et al. certified the structure with attenuated total reflection Fourier transform infrared spectroscopy and DFT calculation.56,57 Besides, La3+ ion having empty 5d and 6s orbit (La3+, 5s25p65d06s0; La0, 5s25p65d16s2) could serve as iodine store and form Ir−I−La complex. In the case of Ir-CO-La model, La has strong affinity to O atom and acts as lewis acid to accept the lone pair electrons of CO:, then activating CO bond and further promoting the rate-determing step of CO migration insertion. The effect of La maybe was similar to Ru promoter in

STY is usually a key indicator in evaluating the performance of the catalyst especially in industrial application. In the case of a fixed-bed reactor, increasing liquid space velocity (LHSV) is an effective measure to obtain high STY. As depicted in Figure 5, when the LHSV increased from 6 to 16 h−1, the STY of MA and the TOF (mol acetyl/(mol Ir h)) reached the maximum at 12h−1 and decreased from 12 to 16 h−1. Thus, LHSV of 12h−1 was probably an appropriate point, under which the selectivity of MA reached to 90%; and the TOF of Ir atoms could reach to 2200h−1; the conversion of CH3OH reached 50% and the selectivity of CH4 decreased to 5% as well. Most importantly, the STY of MA for Ir−La/AC catalyst was 8200 g/(kgcat h). As a contrast depicted in Figure 6, Chiyoda corp. reported the Rh/resin catalyst with a STY ca. 6000 g/(kgcat h);49 Zehl, G et al. reported the supported Ir/AC catalyst with a STY ca. 3500 g/(kgcat h);50 Fujimoto, K et al. reported the supported Ni/AC with the STY cat. 2050 g/(kgcat h).51 Besides, in the case of noniodide promoted zeolite catalyst, the MA STY of ca. 200 and 156 g/(kgcat h) was reported for Py-MOR37 and Cu/HMOR52 catalysts, respectively. In fact, even the reported noniodide promoted system shows nice prospect to replace the current iodide promoted catalytic system, catalyst stability is still one of the most challenges to be conquered. Therefore, the STY of MA on Ir−La/AC catalyst far surpassed the reported heterogeneous catalytic system. Stability of the catalyst is very critical for industrial scale-up. In the case of BP Cativa process, the formation of an inactive [Ir(CO)I4]− species was inevitable even under high concentration of CO.15 Besides, side reactions like water−gas reaction,39 methanation reaction,43 also occurred to release CO2 and CH4. As shown in Figure 7, promoter La played a crucial role in the stabilization of single atom Ir/AC catalyst. F


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Program of the Chinese Academy of Sciences, Grant XDA21020300 and Grant XDB17020400).

homogeneous system by abstracting an iodine atom from the Ir center and promoting CO insertion.15 On the basis of the discussion above, we proposed a possible mechanism of Ir−La/AC catalyzed heterogeneous methanol carbonylation in Figure 10. During the reaction cycle, [Ir(CO)2I2][LaI2][O-AC]2 was the initial active species, in which Ir species was in +1 valence and La species exhibited +3 valence. Similarly, [Ir(CO)2I2]− was identified as the initial active species in homogeneous Cativa process. Then CH3I oxidative addition with [Ir(CO)2I2][LaI2][O-AC]2 resulted in [MeIr(CO)2I2][LaI3][O-AC]2, similar to the [MeIr(CO)2I3]− species in homogeneous process.15,44,58,59 After migratory insertion of CO, iridium-acetyl complex formed, in which La played a promotion role as Lewis acid. Finally, reductive elimination of acetyl iodide occurred and MA was released to complete the reaction cycle.

Notes

The authors declare no competing financial interest.

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4. CONCLUSION In summary, we reported a heterogeneous methanol carbonylation process to produce MA based on La stabilized single atom Ir/AC catalyst. The active single Ir catalytic sites were accommodated in the internal pore of AC support, which alleviated the problem of corrosion. Various impact factors, such as temperature, pressure of CO and CH3OH, weight ratios of CH3I/CH3OH, and total LHSV were investigated. A high TOF of 2200 h−1 and high selectivity of methyl acetate over 90% with STY of ca. 8200 g/kgcat•h were achieved under mild conditions. In addition, La promoted single atom Ir−La/ AC catalyst exhibited good stability when compared to simple Ir/AC catalyst, and a possible reasonable catalytic mechanism for heterogeneous methanol carbonylation over Ir−La/AC catalyst was also supposed. Our laboratory data are helpful for the further scale-up of the process. In addition, we also provide a reference to develop stable and efficient single atom catalyst for other applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b05402. Figure S1 is the interatomistic distance of atoms in Figure 8d; Figure S2 is the simulation of the structure parameters of distance and bond angle of model Ir-COLa, Ir−La, and Ir−I−La; and Table S1 is distance of the labeled atoms in Figure 8 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]/Fax: +86-411-84379055 (Y.J.D.) *Email: [email protected] (X.G.S.) ORCID

Siquan Feng: 0000-0003-1367-164X Yunjie Ding: 0000-0001-8894-9648 Author Contributions

S.F. conducted the experiment and drafted the first manuscript; Z.R. participated in discussion of the experiment; X.S. and Y.D. conceived the experiment and modified the manuscript. Funding

This work was supported by National Key R&D Program of China (2017YFB0602203), the Strategic Priority Research G


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AC Catalyst for Heterogeneous Methanol

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