Atomically Dispersed Rh Active Sites on Oxide Supports with

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12632−12641

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Atomically Dispersed Rh Active Sites on Oxide Supports with Controlled Acidity for Gas-Phase Halide-Free Methanol Carbonylation to Acetic Acid Ji Qi and Phillip Christopher* Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93117, United States

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ABSTRACT: Acetic acid (AA) is an important bulk commodity chemical produced primarily via methanol carbonylation using homogeneous organometallic catalysts in liquid phase reactors with halides that participate in each catalytic turnover. Here we report a heterogeneous, halidefree, gas-phase process for methanol carbonylation to AA using a catalyst consisting of atomically dispersed rhodium (Rh) active sites on an acidic support. It is demonstrated that active site pairs consisting of atomically dispersed Rh and support acid sites enable highly selective AA production, whereas Rh clusters drive methanol decomposition to CO and CO2. Through a comparison of methanol carbonylation over atomically dispersed Rh species on γ-Al2O3, ZrO2, and Na-modified ZrO2, it was identified that by decreasing the concentration of acidic sites, the production of byproduct dimethyl ether could be minimized. After the acidity of the support was tuned by depositing 5 wt % Na on ZrO2 that contained atomically dispersed Rh, AA selectivity was promoted to 54% in a stable-gasphase, halide-free process operating at a stoichiometric methanol to CO feed ratio. Mechanistic analysis suggested that AA formation proceeded through a bifunctional reaction mechanism where CO adsorbed to Rh inserted into methoxy species on adjacent acid sites in the kinetically relevant step. This work illustrates the role of Rh structure and support acidic sites on methanol carbonylation and provides insights into how paired sites of atomically dispersed metals and acidic sites can be manipulated to drive important catalytic processes.

1. INTRODUCTION Acetic acid (AA) is one of the most important bulk commodity chemicals with a worldwide production of more than 10 million tonnes per year, and is used as a precursor for the production of vinyl acetate, acetic anhydride, and acetic esters, among others.1 Monsanto commercialized a Rh-based homogeneous catalyst for the carbonylation of methanol to produce AA, which exhibits excellent activity and selectivity under relatively mild reaction conditions (180−220 °C, 2−4 MPa).2 BP Chemicals has since developed the Cativa process, which uses a Ru-promoted Ir catalyst and is similar in process operation to the Monsanto process. Although both of these processes require methyl-iodide to participate in each catalytic turnover3 and are operated in the liquid phase, the Cativa process exhibits higher catalytic activity at lower water content and produces less by products as compared to the Monsanto process.4 The current methanol carbonylation processes to produce AA are effective and selective, but the required separation of catalysts and water, in addition to the use of halides, drives increased process costs.5,6 With the motivation of decreasing separation costs, there have been many attempts to develop efficient heterogeneous catalysts by immobilizing organometallic complexes on solid supports.7−9 However, these systems still require the use of halides and water in the catalytic process, which minimizes the potential benefit of © 2019 American Chemical Society

the immobilized catalysts. Reports have also demonstrated that halide free, gas phase methanol carbonylation to AA can be achieved using zeolites, such as H-mordenite (H- MOR).10 In these systems, methanol adsorbs on Brønsted acid sites to form methoxy groups, which can react directly with gas phase CO to form an acylium cation that can be quenched by H2O to produce AA.11,12 Further promotion of reactivity can be achieved by using Cu-modified H-MOR (Cu-MOR), where CO adsorbed to Cu cations promote the rate of CO insertion into methoxy species on nearby acid sites through a bifunctional mechanism.13 However, methoxy species on acidic sites can also react with gas phase methanol to produce dimethyl ether (DME) and Cu promotes carbonylation of DME to produce methyl acetate.14 As a result of these alternate reaction pathways, promoting high selectivity to AA requires process operation at high CO to methanol ratios (>50:1), which would necessitate the use of very large recycle streams in industrial processes.6 Heterogeneous Rh catalysts were recently reported for use in ethanol carbonylation, which suggests that heterogeneous Rh Received: Revised: Accepted: Published: 12632

April 27, 2019 June 21, 2019 June 25, 2019 June 25, 2019 DOI: 10.1021/acs.iecr.9b02289 Ind. Eng. Chem. Res. 2019, 58, 12632−12641

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Industrial & Engineering Chemistry Research active sites may also be useful in methanol carbonylation.15,16 Oxide-supported atomically dispersed Rh species have recently been demonstrated to exhibit distinct catalytic properties compared to Rh nanoparticles for reactions such as CO, C3H6 and C3H8 oxidation reaction.17−19 Further, atomically dispersed Rh species in zeolites and on oxide supports have also been shown to exhibit interesting properties for mild methane oxidation to methanol and AA, albeit in liquid water and at quite low rates.20,21 In these cases, it was explicitly demonstrated that the pathway for AA production did not occur through carbonylation of methanol intermediates. Because the processes were operated in liquid water, it is likely that active sites required for bifunctional methanol carbonylation to AA reaction pathways, similar to what has been observed in the Cu-MOR systems, would be poisoned by adsorbed water. We hypothesized that atomically dispersed Rh-acidic site pairs could potentially be selective for methanol carbonylation to AA under dry conditions, as methanol could adsorb on acidic sites to form methoxy species and atomically dispersed Rh should promote CO insertion, while minimizing methanol decomposition that would occur on larger Rh domains.22−24 Thus, it was envisioned that atomically dispersed Rh and supports with controlled acidity could provide a selective pathway for methanol carbonylation to AA. Here, we demonstrate the influence of Rh structure and support acidity on the gas phase, halide-free methanol carbonylation to AA. This was achieved by preparing catalysts containing predominantly atomically dispersed Rh species or Rh clusters on γ-Al2O3, ZrO2, and ZrO2 modified by varying weight loadings of Na. It was observed that atomically dispersed Rh species predominantly promoted the AA formation pathway and further that decreasing the concentration of support acid sites effectively decreased the rate of DME formation. Ultimately, a catalyst consisting of 5%Na deposited on ZrO2 that contained atomically dispersed Rh species exhibited stable reactivity over the course of 50 h with 54% selectivity to AA when operating at 300 °C and 1:1 methanol to CO molar feed ratio. Mechanistic analysis using methanol and CO partial pressure dependent product formation rate and in situ Fourier Transform infrared (FTIR) measurements suggested that the reaction proceeded through a dual site mechanism where CO bound to Rh inserted into methoxy species on the support acid sites. This work provides evidence that heterogeneous catalysts consisting of atomically dispersed Rh-acid site pairs offer a promising route toward gas-phase, halide-free methanol carbonylation to AA.

dissolved in 5 mL of high-performance liquid chromatography (HPLC) grade water and the pH of the precursor solution was adjusted to the same value of the support solution through dropwise addition of NH4OH. The 5 mL Rh precursor solution was mixed with the 60 mL support suspension under magnetic stirring with syringe pump at a 4 mL/hour injection rate. For the synthesis of catalysts containing a predominance of Rh clusters, 3% weight loading Rh was deposited on each support by using a simple impregnation technique. Fifteen mg of rhodium(III) chloride was dissolved in an evaporation dish with 300 μL of HPLC grade water. The aqueous Rh solution was added onto 0.5 g of support drop by drop and a stirring rod was used to mix the paste until homogeneous in appearance. All samples were dried at 60 °C in an oven overnight, ground up with a mortar and pestle, and calcined in a tube furnace at 350 °C in air for 3 h. 2.2. Catalyst Characterization. 2.2.1. Fourier Transform Infrared Spectroscopy (FTIR). The catalysts were characterized using CO probe molecule FTIR spectroscopy to elucidate the Rh structure and identify the predominant surface species under reaction conditions. In all experiments catalysts were loaded into a Harrick Praying Mantis low-temperature reaction chamber with ZnSe windows mounted inside of a Thermo Scientific Praying Mantis diffuse reflectance adapter set inside of a Thermo Scientific Nicolet iS10 FT-IR spectrometer with a mercury cadmium telluride (MCT) detector cooled by liquid nitrogen. In CO probe molecule experiments, gases were fed through an isopropyl alcohol/liquid nitrogen cold trap and a desiccant to remove trace moisture. Before characterization, catalysts were oxidized in situ at 350 °C for 1 h under an O2 flow, followed by in situ reduction using a 10% H2 flow balanced with Ar at 200 °C for 1 h. After pretreatment, the catalyst was cooled to subambient temperature (−125 °C) under Ar, and then a baseline spectrum was taken before CO introduction. CO probe molecule FTIR experiments were executed at cryogenic conditions to minimize the process of CO induced Rh cluster fragmentation and enable characterization of the catalysts in the as synthesized condition.30,31 A 10% CO/90% Ar mixture was then introduced to the catalyst until the spectra stabilized (∼10 min) for saturation CO adsorption, and then the cell was purged with 100 sccm Ar for 10 min. In all measurements, spectra were obtained by averaging 64 sequentially collected scans at a resolution of 4 cm−1. The spectra were obtained in absorbance units. For pseudo in situ FTIR measurements, catalysts were oxidized at 350 °C for 1 h and then exposed to methanol and CO in a molar ratio of 1:1 at a given temperature for 30 min. Ar was introduced to purge the cell of gas phase methanol and CO at each temperature. Spectra were then collected under Ar by averaging 64 sequentially collected scans at a resolution of 4 cm−1. Methanol and CO were then reintroduced to the cell and the temperature was increased to the next desired temperature and the same procedure as described above was followed. 2.2.2. Temperature-Programmed Ammonia Desorption (NH3-TPD). NH3-TPD measurements were performed on a Micromeritics AutoChem 2920 instrument to quantify the characteristics and concentration of support acid sites.32,33 In a typical experiment, 0.2 g of catalyst was loaded into a U-shaped, flow-thru, quartz sample tube. Prior to measurements, the catalyst was pretreated in He (30 cm3 /min) at 350 °C for 1 h to remove adsorbed water. A mixture of 10% NH3/90% He was flown over the catalyst (30 cm3 /min) at 50 °C for 1 h. Then the sample was flushed with pure He (30 cm3 /min) at 100 °C for 1

2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. Twenty nanometer diameter ZrO2 (US Research Nanomaterials, US3659) and five nanometer diameter γ-Al2O3 nanoparticles (US Research Nanomaterials, US3007) were used as supports in these studies. The ZrO2 support existed in the monoclinic phase,25 which has predominantly (111) exposed facets.26,27 Rh was deposited using rhodium(III) chloride (Sigma-Aldrich, 307866) precursors. Rh catalysts with a predominance of atomically dispersed Rh species were prepared by a strong electrostatic adsorption (SEA) method.28,29 The weight loading of Rh was chosen as 0.2% to promote the formation and stability of atomically dispersed Rh. In these syntheses, 0.5 g of support was suspended in 60 mL of HPLC grade water (JT4218−3, J.T. Baker) under magnetic stirring and the pH of the suspension was adjusted with the use of NH4OH to 10 and 11.5 for γ-Al2O3 and ZrO2 solution, respectively. One mg of rhodium(III) chloride was 12633

DOI: 10.1021/acs.iecr.9b02289 Ind. Eng. Chem. Res. 2019, 58, 12632−12641

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Industrial & Engineering Chemistry Research h. The TPD measurements were carried out in the range 100− 400 °C at a heating rate of 10 °C/min. NH3 concentration in the effluent was monitored with a gold-plated filament thermal conductivity detector. The amount of desorbed NH3 was determined on the basis of the calibrated area under the desorption curve. 2.3. Reactivity Measurements. The catalytic activity for methanol carbonylation was evaluated in a fixed-bed quartz reactor in the temperature range of 200−300 °C operating at atmospheric pressure. The corresponding rates of product formation were observed to be independent of superficial velocity (Figure S1), demonstrating that mass transfer effects were negligible. A 350 mL min−1 total flow rate was selected for all kinetic experiments. To prevent heat transfer and/or pressure drop effects on kinetic measurements, we further diluted 300 mg of the catalyst by physically mixing with 1 g of SiO2 sand (SigmaAldrich, acid purified, lot # BCBS5184 V, CAS: 60676−86−0). All gas flows were controlled by mass flow controllers (Teledyne Hastings) and an in-line bubbler was used to deliver gas-phase methanol (Fisher Scientific, HPLC grade, lot 177964) to the catalyst. Ar (Airgas, UHP, 99.999%) was used to bubble methanol and used as a diluent to control the methanol partial pressure. The reaction effluent was quantified with online mass spectrometry (HALO 201, Hiden Analytical Inc.). The following m/z values were used to detect each product: m/z = 28 for CO, m/z = 31 for MeOH, m/z = 44 for CO2, m/z = 45 for DME, and m/z = 60 for acetic acid. The concentrations were calibrated to the signal intensity at each mass and Ar was used as an internal standard. The mass spectrometry results were compared to calibrated analysis via gas chromatography, which showed quantitative agreement at all explored conditions. Prior to reactivity measurements, catalysts were pretreated by oxidation at 350 °C for 1 h with pure O2 at 50 mL min−1. Catalysts were then exposed to 33 mbar methanol and 33 mbar CO for ∼4−5 h to stabilize reactivity, followed by measurements at varying temperatures. The system was allowed a couple hours at each temperature to ensure steady state was achieved. For methanol and CO partial-pressure-dependent measurements, catalysts were maintained at a constant temperature of 300 °C, and the methanol partial pressure was successively controlled from 20 mbar to 100 mbar by adjusting the flow rates of Ar that bubbled through the methanol, while holding the CO partial pressure constant. Similarly, in CO partial-pressure-dependent measurements, an additional Ar stream was used as a diluent and to maintain a constant total flow rate. The conversion of the limiting reagent was kept below ∼5% in all kinetic measurements, except with the 3 wt % Rh catalysts operating at 300 °C, where reactant conversion reached ∼20%.

Figure 1. Comparison of NH3-TPD results for (a) γ-Al2O3 and ZrO2; (b) ZrO2, 1%Na- ZrO2, 5%Na-ZrO2, and 10%Na-ZrO2.. (c) Linear relationship between the amount of NH3 desorbed during TPD and the Na weight loading on ZrO2 is shown, demonstrating a systematic blocking of acidic sites. All catalysts were pretreated in He at 350 °C for 1 h. A mixture of 10% NH3/90% He was flown over the catalyst at 50 °C for 1 h. The TPD measurements were carried out in the range 100−350 °C at a heating rate of 10 °C/min under He.

shape NH3-TPD spectrum as for γ-Al2O3, but the total amount of desorbing NH3 was ∼2-fold lower for ZrO2 as compared to γAl2O3, see Table 1.37,39,40 These results demonstrate that γTable 1. Amount of NH3 Desorption As Determined from the Peak Area of the NH3-TPD catalyst

NH3 desorption (1 × 1016 molecules /m2)

γ-Al2O3 ZrO2 1 wt % Na-ZrO2 5 wt % Na-ZrO2 10 wt % Na-ZrO2

22 9.7 8.1 6.5 2.4

Al2O3 exhibited a higher concentration of and stronger acidic sites than ZrO2, consistent with previous reports.41−43 In Figure 1b, c it is shown that the addition of Na to ZrO2 with varying weight loading of 1, 5, and 10% systematically decreased the concentration of acidic sites. Quantification of the decrease in concentration of acidic sites with increasing Na loading showed a close to linear dependence, see Figure 1c and Table 1. The amount of Na required to neutralize ZrO2 was much larger than required for monolayer coverage. This suggests that the Na deposition approach used here resulted in Na clusters being deposited, rather than 1:1 neutralization of acid sites on the oxide surface. CO probe molecule FTIR was used to study the structure of as-synthesized Rh species.44 It is known that atomically dispersed Rh on oxide supports adsorbs two CO ligands and forms a Rh gem-dicarbonyl species, Rh(CO)2, which exhibits two unique vibrational stretches that can be differentiated from CO adsorbed linearly or in a bridge bound configuration to Rh clusters.24,45−47 However, it is also well-known that CO can induce fragmentation of Rh clusters to form atomically dispersed species.30,31,48 Thus, CO probe molecule FTIR analysis of the

3. RESULTS 3.1. Characterization of Support Acidity and Rh Structure. Previous work has demonstrated that acidic sites are necessary to facilitate methoxy formation in halide-free methanol carbonylation processes, which enables CO insertion and AA formation.14 To study the influence of support acidity on methanol carbonylation reactivity, γ-Al2O3, ZrO2, and Namodified ZrO2 were explored, as they contain varying acidic site concentration and strength.34−38 Ammonia temperature-programmed desorption (NH3-TPD) experiments were used to characterize and quantify acidic sites on the supports.25 As shown in Figure 1a, γ-Al2O3 exhibited a broad NH3 desorption peak in the temperature region 100−400 °C, which is attributed to desorption from surface acid sites. ZrO2 showed a similar 12634


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Industrial & Engineering Chemistry Research as-synthesized materials was executed at −125 °C to kinetically trap Rh clusters from fragmenting in the CO environment. Figure 2a, c shows CO probe molecule FTIR spectra for 0.2 wt % Rh/γ-Al2O3 and 0.2 wt % Rh/ZrO2, respectively. The two

previously been assigned to an atomically dispersed Rh species that is simultaneously coordinated to CO and an additional O.50 Detailed analysis and assignment of the CO stretch at ∼2130 cm−1 is outside the scope of this study, but this likely derives from Rh species that were not completely reduced by the H2 treatment. There is also some CO stretching intensity between 2095 and 2025 cm−1, which may suggest the existence of a small concentration of Rh clusters. Regardless, the CO FTIR spectra suggest that Rh predominantly existed as atomically dispersed species on both γ-Al2O3 and ZrO2 catalysts at 0.2 wt %. In the case of the 3 wt % Rh samples, two new prominent CO stretches at 2061 and 1840 cm−1 were observed, in addition to CO stretches corresponding to gem-dicarbonyl species, as shown in Figure 2b, d. The stretches centered at 2061 and 1840 cm−1 were assigned to CO adsorbed in linear and bridge bound geometries on the surface of Rh clusters.47,51 On the basis of the strength of the CO stretches associated with adsorption on Rh clusters, and their lower extinction coefficients compared to Rh gemdicarbonyl species,52 it is concluded that for the 3 wt % Rh samples, a significant portion of Rh resided in the form of clusters. It is important to note that the results presented in Figure 2 reflect only the structure of Rh species in the assynthesized materials. It is shown later in the text that following exposure to reaction conditions, the low Rh loading samples exhibit signatures associated with Rh being essentially exclusively in the atomically dispersed Rh(CO)2 state. 3.2. Influence of Rh Structure and Support Acidity on Methanol Carbonylation. On the basis of the NH3-TPD results and the CO FTIR analysis, the influence of support acidity and Rh structure on methanol carbonylation reactivity and selectivity could be examined by comparing 0.2 and 3% Rh on ZrO2 and γ-Al2O3. Catalysts were preoxidized at 350 °C in O2 and then exposed to a gas mixture of methanol and CO at a molar ratio of 1:1 for reactivity measurements. We start by discussing the reactivity of the γ-Al2O3-based catalysts. Acidic sites on oxide supports are known to be active for methanol conversion at these temperatures and as a result the inherent reactivity of the support was also studied.53,54 Figure 3a and Figure S2 show the rate and selectivity for AA and DME

Figure 2. In situ IR spectra of CO adsorbed at −130 °C and at saturation coverage on (a) 0.2 wt % Rh loaded on γ-Al2O3, (b) 3 wt % Rh loaded on γ-Al2O3, (c) 0.2 wt % Rh loaded on ZrO2, and (d) 3 wt % Rh loaded on ZrO2. All catalysts were pretreated identically by 350 °C oxidation in O2 followed by 200 °C reduction in H2 for 1 h. Insets in the figure panels show proposed structure assignments associated with the observed CO stretches. The spectra are presented in normalized absorbance, where the highest intensity stretch was normalized to 1.0.

strong bands centered at ∼2094 and ∼2022 cm−1 for Rh/γAl2O3 and ∼2095 and ∼2025 cm−1 for Rh/ZrO2 were assigned to the symmetric and asymmetric stretches of the Rh(CO)2 gem-dicarbonyl species, which is uniquely associated with atomically dispersed Rh species.46,49 For Rh/ZrO2, Figure 2c, another CO stretch at 2130 cm−1 was observed, which has

Figure 3. Product selectivity and production rates for methanol carbonylation catalytic performance at 33 mbar methanol and 33 mbar CO as a function of temperature on (a) γ-Al2O3, (b) 3 wt % Rh/γ-Al2O3, and (c) 0.2 wt % Rh/γ-Al2O3. Before reactivity experiments, catalysts were oxidized at 350 °C for 1 h. For clarity in (b) only the selectivity to CO is shown. Solid lines are used for selectivity data, whereas dashed lines are used for production rate data; the arrows are only meant to guide the eye. 12635


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Figure 4. Product selectivity and production rates for methanol carbonylation catalytic performance at 33 mbar methanol and 33 mbar CO as a function of temperature on (a) ZrO2, (b) 3 wt % Rh/ZrO2, and (c) 0.2 wt % Rh/ZrO2. Before reactivity experiments, catalysts were oxidized at 350 °C for 1 h. For clarity in b, only the selectivity to CO is shown. Solid lines are used for selectivity data, whereas dashed lines are used for production rate data; the arrows are only meant to guide the eye.

Figure 5. Product selectivity and production rates for methanol carbonylation catalytic performance at 33 mbar methanol and 33 mbar CO as a function of temperature on (a) 0.2 wt % Rh loaded on 1%Na-ZrO2, (b) 0.2 wt % Rh loaded on 5%Na-ZrO2, and (c) 0.2 wt % Rh loaded on 10%NaZrO2. Before reactivity experiments, catalysts were oxidized at 350 °C for 1 h. Solid lines are used for selectivity data, whereas dashed lines are used for production rate data; the arrows are only meant to guide the eye.

Al2O3 and further that the primary reaction pathway on Rh clusters is methanol decomposition.56 The addition of 0.2 wt % Rh to γ-Al2O3 resulted in methanol conversion rates and selectivity that were more similar to the bare γ-Al2O3 than observed for the 3 wt % Rh case, see Figure 3c. It was observed that the rate of DME formation increased only slightly (15−20% at maximum) and the rate of AA formation increased by ∼3-fold depending slightly on temperature, due to the addition of 0.2 wt % Rh to γ-Al2O3, see Figure S2. This demonstrated that atomically dispersed Rh species on γ-Al2O3 are selective toward AA formation, distinct from that of Rh clusters, but that the reactivity of γ-Al2O3 acidic sites were responsible for a majority of the observed methanol conversion on the 0.2 wt % Rh/γAl2O3 catalyst.

formation (the only two detectable products on this sample) over the bare γ-Al2O3 support. We note that product formation rates are all reported on a per gram of catalyst basis (rather than per Rh basis) because the support itself has significant inherent reactivity. The bare γ-Al2O3 support exhibited DME selectivity of >99% for the entire explored temperature range with increasing production rate from 0.0025 to 0.012 mmol s−1 gcat−1 as temperature was increased from 230 to 300 °C. The addition of 3 wt %Rh to γ-Al2O3, primarily in the form of Rh clusters, significantly promoted the rate of methanol consumption by ∼10-fold compared to the bare support and switched the selectivity to primarily CO, in addition to CO2 and DME, see Figure 3b. This suggested that the inherent reactivity of Rh cluster surfaces, with extended Rh ensembles, for methanol conversion is significantly higher than that of γ12636


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acidic sites blocked by Na in these catalysts, this hypothesis cannot be substantiated. Before analyzing the AA formation mechanism, the stability of the 0.2 wt %Rh/5%Na-ZrO2 catalyst was examined. This catalyst was chosen as it exhibited the highest selectivity of all systems studied. The stability was examined at 300 °C in a stoichiometric feed of methanol and CO, see Figure 6. No

The influence of the strength of acidic sites on the oxide support for methanol carbonylation was analyzed by comparing the reactivity of γ-Al2O3 catalysts to ZrO2 catalysts. The bare ZrO2 catalysts exhibited methanol conversion rates that were almost 2 orders of magnitude smaller than observed for bare γAl2O3. Given the surface area difference of only ∼5-fold, this suggests that the strong acid sites on γ-Al2O3 imparted the higher observed reactivity. For ZrO2, the AA selectivity increased from just a couple percent to almost 10% at a temperature of 300 °C, shown in Figure 4a. The addition of 3 wt % Rh to ZrO2 in the form of Rh clusters promoted methanol decomposition to CO and CO2, similar to what was observed on γ-Al2O3. Alternatively, the 0.2 wt % Rh/ZrO2 catalyst, containing predominantly atomically dispersed Rh, showed almost exclusively a promotion in AA production as compared to the bare ZrO2 catalyst. The AA production rate increased ∼6fold compared to bare ZrO2, whereas the DME formation rate remained almost unchanged. This demonstrated that the atomically dispersed Rh species active sites were highly selective to AA formation under stoichiometric methanol to CO feed conditions. The AA selectivity on the 0.2 wt % Rh/ZrO2 catalyst reached >30% at 300 °C. From the analysis of the results in Figures 3 and 4, it is clear that atomically dispersed Rh species are selective for AA formation when deposited on acidic supports, whereas Rh clusters drive methanol decomposition. Interestingly, the production rate of AA was similar on the 0.2 wt % Rh/ γAl2O3 and 0.2 wt % Rh/ZrO2, catalysts suggesting that locally the dispersed Rh active sites act similarly on both supports. Further, it was observed that the weak acid sites on ZrO2 minimized the production rate of DME and promoted the overall AA selectivity as compared to γ-Al2O3. Thus, to further promote AA selectivity, it is necessary to minimize the contribution of acidic sites to the DME formation rate. 3.3. Tuning Support Acidity to Promote Acetic Acid Selectivity. To further minimize the contribution of acidic sites to the DME formation rate, we modified the 0.2 wt % Rh/ZrO2 atomically dispersed catalyst with 1, 5, and 10 wt % Na.57 NH3 TPDs shown and analyzed in Figure 1 and Table 1 demonstrate that Na deposition systematically decreased the number of acidic sites. Methanol carbonylation was executed over these catalysts under the same condition as described above. The addition of 1 wt % Na to 0.2 wt % Rh/ZrO2 caused the AA production rate to decrease by ∼1.2-fold, whereas the DME reaction rate decreased 2.5-fold compared to 0.2 wt %Rh/ZrO2, resulting in a selectivity to AA at 300 °C of 48%, Figure 5a. An increase in Na loading to 5 wt % decreased the AA and DME production rates by 1.5- and 3.6-fold, respectively, as compared to 0.2 wt %Rh/ZrO2, resulting in an AA selectivity of ∼54% at 300 °C shown in Figure 5b. Finally, when the Na loading increased to 10 wt %, the AA and DME production rates decreased by 4.4 and 5.3-fold, respectively, as compared to 0.2 wt % Rh/ZrO2, resulting in an AA selectivity of ∼40% at 300 °C, Figure 5c. It is clear from these results that the lower loadings of Na caused a more significant decrease in DME production rate as compared to AA production rate. Under the assumption that AA production occurred through a bifunctional mechanism involving Rh and directly adjacent acidic sites (this is discussed more below),14 this may suggest that the lower loadings of Na more preferentially blocked acidic sites remote from Rh where DME formation was the primary reaction pathway. However, without direct evidence probing the distance between Rh and

Figure 6. Methanol carbonylation stability test at 33 mbar methanol and 33 mbar CO partial pressures at 300 °C over 0.2 wt %Rh/5%NaZrO2 pretreated by 350 °C oxidation for 1 h.

measurable change in activity was observed during the course of a 50-h experiment suggesting that the active sites associated with AA and DME formation were quite stable under these conditions. Methanol conversion in this experiment was 0.5%, which would have to be significantly increased for any analysis of industrial viability of this process. 3.4. Mechanistic Analysis of Methanol Carbonylation to AA on Atomically Dispersed Rh. To provide insights into the AA formation mechanism on atomically dispersed Rh active sites, we executed CO and methanol partial-pressure-dependent reactivity measurements at 300 °C on 0.2 wt % Rh/ZrO2. When the methanol partial pressure was increased from 0.056 to 0.25 bar (while holding the CO partial pressure constant at 0.033 bar), the production rate of AA increased ∼4-fold and the DME production rate increased by ∼5-fold, which resulted in decreased AA selectivity, Figure 7a, b. Alternatively, increasing the CO partial pressure from 0.12 to 0.32 bar (while holding the methanol partial pressure constant at 0.033 bar) promoted the AA production rate more than the DME production rate (∼3fold versus ∼2-fold, respectively), which promoted the AA selectivity from ∼54 to 57%, see Figure 7c, d. It was observed that the AA formation rate was first order with respect to CO partial pressure and ∼0.7 order with respect to methanol over the explored ranges. The slightly nonlinear behavior in methanol partial pressure and less than 1 reaction order suggests that the coverage of methanolderived reactive intermediates on acidic sites is increasing in this pressure range and blocking sites associated with the kinetically relevant step for AA formation. Before discussing the mechanism of AA formation, pseudo in situ FTIR experiments are presented in Figure 8 to provide insights into the adsorption sites and stability of methanol and CO derived intermediates. These experiments were performed by exposing bare ZrO2 or 0.2 wt % Rh/ZrO2 to methanol and CO at 33 mbar at a given temperature, then sweeping the reactants out with Ar and collecting IR spectra to identify remaining adsorbed species. For bare ZrO2, following exposure to methanol and CO at 100 and 200 °C, two stretches at 2920 and 2812 cm−1 were observed, Figure 8a. These stretches correspond to methoxy on ZrO2, which forms via methanol 12637


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2 support derived oxygens.62,63 This suggests that atomically dispersed Rh species under reaction conditions were not particularly mobile, although further analysis is needed to substantiate this. From this analysis, it was concluded that under reaction conditions, methanol primarily interacted with ZrO2, whereas CO primarily interacted with Rh. To summarize the experimental results: 1. Atomically dispersed Rh species were selective for methanol carbonylation, whereas Rh clusters promoted methanol decomposition. 2. Acidic sites on the support remote from Rh species were responsible for the byproduct DME formation, and the overall process selectivity to AA could be promoted by poisoning acidic sites. Stable and selective (∼54%) AA formation was observed on 0.2 wt %Rh/5%Na-ZrO2 catalysts at stoichiometric methanol to CO conditions. 3. Partial-pressure-dependent and FTIR measurements suggest that AA formation occurs through a dual-site mechanism where methanol adsorbed on ZrO2, whereas CO adsorbed on Rh.

Figure 7. (a, c) Methanol carbonylation selectivity and (b, d) AA production rates over 0.2 wt % Rh/ZrO2 as a function of (a, b) methanol partial pressure and (c, d) CO partial pressure. In both sets of experiments, the nonvaried reactant partial pressure was held constant at 33 mbar. Before reactivity experiments, the catalyst was oxidized at 350 °C for 1 h.

4. DISCUSSION We start the discussion by considering why atomically dispersed Rh and Rh clusters exhibit unique selectivity in methanol carbonylation. The reason why atomically dispersed Rh exhibits higher selectivity to AA than Rh clusters is because on Rh clusters methoxy species can successively dehydrate due to the local availability of Rh ensemble sites.55 Alternatively, atomically dispersed Rh will preferentially form bonds with CO rather than methoxy.21,24,64 Also, methoxy will not decompose on atomically dispersed Rh because there is no ensemble of sites to accept produced hydrogens.23,24 Thus, Rh clusters show higher selectivity to CO because the Rh ensembles enable methoxy decomposition, while atomically dispersed Rh shows higher selectivity to AA because these sites cannot promote methoxy decomposition. From pseudo in situ FTIR results on ZrO2 and 0.2 wt % Rh/ ZrO2, the AA formation mechanism can be proposed where methoxy groups are formed on acidic sites of the support and CO adsorbs on atomically dispersed Rh. The preferential adsorption of CO to Rh, as compared to methanol or methanolderived intermediates, is consistent with previous density functional theory calculations for atomically dispersed Rh species.21,24,64 The localization of Rh(CO)2 species next to adjacent acidic sites is hypothesized to promote the rate of CO insertion into methoxy species bound to the oxide support, resulting in enhanced rates of AA formation as compared to the gas phase CO attack of methoxy species that occurs when no Rh is present. The production rates of AA and DME on ZrO2 and 0.2 wt % Rh/ZrO2 support the hypothesis of the dual site mechanism. Since methoxys only are primarily formed on support acidic sites, DME production rates on both ZrO2 and 0.2 wt % Rh/ ZrO2 were quite similar, whereas CO preferentially adsorbed on atomically dispersed Rh, which promoted the CO insertion reaction and as a result the AA production rate on 0.2 wt % Rh/ ZrO2 was ∼6 times higher than ZrO2, without any influence on the DME formation rate. An analysis of the CO and methanol reaction orders for AA production over the considered range of pressures provides further insights into the reaction mechanism. The increasing rate of DME and AA production due to increasing methanol pressure suggests that the concentration of

Figure 8. Pseudo in situ FTIR spectra for (a) ZrO2 and (b) 0.2 wt % Rh/ZrO2. The sample were oxidized at 350 °C before being exposed to methanol and CO at molar ratio 1:1 at a given temperature. The spectra were collected under Ar at each temperature, which means that only stably adsorbed species were being measured. The spectra are presented in absorbance and they are scaled on the top of each other.

dehydration.57 As the temperature of reactant exposure was increased to 300 and 400 °C, two additional stretches at 2959 and 2866 cm−1 were increasingly present. These two stretches are assigned to formate species that are produced from dehydration of methoxy species.58−60 No prominent peaks were observed in the CO stretching region of the spectra for pure ZrO2, suggesting that any AA formed for pure ZrO2 occurred through gas phase attack of methoxy species by CO.61 For 0.2 wt %Rh/ZrO2, the C−H vibration region was indistinguishable from the bare ZrO2 species, suggesting that methanol did not interact significantly with atomically dispersed Rh species. Alternatively, for 0.2 wt %Rh/ZrO2 prominent stretches at 2086 and 2013 cm−1 were observed, which are consistent with the symmetric and asymmetric stretches of the Rh(CO)2 species. The observed stretches associated with Rh(CO)2 suggests that under reaction conditions the significant CO concentration kept Rh in a highly dispersed state. We also comment that the symmetric and asymmetric stretches of the Rh(CO)2 exhibited similar intensities. This is consistent with an ∼90° angle between the carbonyls, which forms when Rh binds to the support in a close to square planar geometry with bonds to 12638


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Industrial & Engineering Chemistry Research surface methoxy species is dependent on pressure at the explored conditions.61,65,66 The first-order dependence of AA production on CO partial pressure could be explained by either the increase in concentration of CO on Rh, or due to the gasphase attack of methoxy species by CO.61,67 Future work will focus on separating the role of the gas phase CO attack on methoxy versus CO insertion derived from Rh(CO)2 species. Further, the positive order of both reactants on the AA production rate suggests that both adsorbed species are involved in the rate-limiting step. The decreasing reaction order in methanol with increasing methanol pressure suggests that the active site responsible for the kinetically relevant step is being increasingly blocked. Alternatively, the CO pressure dependence was consistent with a reaction order of 1 across the explored pressure region. Within the hypothesis that methanol adsorption and dehydration primarily occurs at acidic sites and CO adsorption occurs primarily on Rh, the results suggest that the kinetically relevant step occurs on the acidic support sites. On this basis, it is hypothesized that CO migration from Rh and insertion into methoxy species on the acidic support is the rate-limiting step in AA formation on the atomically dispersed Rh active sites, although future work will examine this in more detail. From the discussion above and previous work,14 it is clear that during heterogeneous methanol carbonylation without halide promoters, acidic sites are needed to initiate the reaction by producing methoxy group. Because acidic sites are required to react with methanol and produce methoxy species, water will likely inhibit the carbonylation reaction by competing with acidic sites, or by blocking adjacent sites needed for CO insertion.67,68 Furthermore, water will likely react with methoxy to reform methanol. Thus, water inhibition of methanol carbonylation is hypothesized to explain why methanol did not serve as an intermediate for AA production in methane oxidation processes executed in liquid water.20,21 This work demonstrated that atomically dispersed Rh-acidic sites pairs were an effective active site structure for driving selective methanol carbonylation. Other recent work has also demonstrated the idea that creating paired sites of atomically dispersed Rh and localized Brønsted acidic sites on zeolites could promote other interesting chemistries, including ethylene dimerization and selective methane carbonylation.20,69,70 Combining these reports and our report here, it is suggested that unique catalytic properties can be achieved by controlling the local environment of atomically dispersed metal catalysts.71

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Phillip Christopher: 0000-0002-4898-5510 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.C. acknowledges funding from the Air Force Office of Scientific Research MURI Grant FA9550-15-10022 and support from University of California, Santa Barbara.

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REFERENCES

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5. CONCLUSIONS This study demonstrated that selective methanol carbonylation to AA could be achieved over a heterogeneous catalyst in a gasphase, halide-free process by tuning Rh structure and support acidity. Atomically dispersed Rh species paired with acidic sites provide selective active sites for AA production, which likely operate in a bifunctional reaction mechanism. By poisoning acidic sites native to the support, the overall process selectivity could be promoted to ∼54%. This work demonstrates how cooperative actions involving atomically dispersed metal and acidic sites pairs enable selective processes.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02289. 12639


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