Halide-free carbonylation of methanol with H-MOR supported CuCeO_x catalysts

Abstract

Heterogeneous halide-free carbonylation of methanol to acetates, including methyl acetate (MA) and acetic acid, using non-precious metal catalysts has been a topic of interest for decades. The key issue is that the water produced by methanol dehydration inhibits the formation of acetyl species and reduces the MA selectivity. Here, we report that CuCeO_x/H-mordenite (H-MOR) catalyst can nearly eliminate the inhibiting effect of water on carbonylation by a water-gas shift reaction (WGSR) on-site, and can thus achieve 96.5% methanol conversion with 87.4% MA selectivity for the halide-free carbonylation of methanol. The results of powder X-ray diffraction, transmission electron microscopy, and scanning electron microscopy show that the Cu and Ce species are highly dispersed on H-MOR even when the CuCeO_x contents are as high as 29 wt-%. Fourier transform infrared spectroscopy and CO chemisorption analysis reveal that a small portion of Cu species can migrate into the channel of H-MOR when CuCeO_x/H-MOR is calcined at 500 °C and these Cu species are converted into Cu⁺ sites upon reduction. The Cu⁺ sites facilitate the WGSR and are also active sites for methanol carbonylation. The introduction of Ce benefits the inhibition of coke deposits and thus enhances the catalyst stability.

Key words： methanol carbonylation; halide-free; methyl acetate; H-mordenite; copper and cerium oxide

1 Introduction

Homogeneous carbonylation of methanol using carbonyl Rh (Monsanto) [1] or Ir (BP Cativa^TM) [2] as catalysts to prepare acetic acid (AA) and methyl acetate (MA) has been industrialized on a large scale. However, Rh and Ir catalysts cannot activate methanol directly; hence, corrosive iodomethane (CH₃I) must be used as an accelerator [3,4]. The process itself has problems such as difficulty in separating products from catalysts, loss of precious metals, requirements for a large investment in equipment and high energy consumption [5]. Thus, a number of catalysts, such as Ni/AC [6,7], Cu/TiO₂-SiO₂ [8], Cu/H-MOR [9,10], Ir-La/AC [11,12], Au/TiO₂ [13], Au/AC [14], and Rh-I copolymeric catalysts [15], have been tested for heterogeneous methanol carbonylation. Kwak et al. [11] discovered a heterogeneous Ir-La/C catalyst that is highly active, selective, and stable for the carbonylation of methanol to produce AA, achieving a very high productivity of approximately 1.5 mol acetyl·mol_Ir^–1·s^–1 with>99% selectivity toward acetates. Feng et al. [12] also reported a heterogeneous single atom Ir-La/AC catalyst to produce MA in a fixed-bed reactor. The catalyst showed a turn-over-frequency (TOF) value as high as 2200 h^–1, a selectivity to MA greater than 90% and an MA space time yield of 8200 g·kg_cat^–1·h^–1. These works contributed to the advancement of heterogeneous methanol carbonylation. Unfortunately, systems with high MA selectivity require CH₃I as an accelerator. However, halides are not ideal raw materials because of their extreme corrosiveness that results in a need for very expensive anti-corrosive materials. The serious influence and potential harm of various toxic halides on human health, water, soil, and atmospheric environments have attracted considerable attention. Eliminating or reducing the use of halides to decrease pollutant emissions and protect human health is in line with the aims of green chemistry and sustainable development. Therefore, the development of catalysts with competitive activity and selectivity comparable to those of homogeneous catalysts for halide-free heterogeneous methanol carbonylation has become a research hotspot in recent decades.

Fujimoto et al. [16] first reported in 1984 that zeolites (H-Y, H-zeolite socony mobil 5 (H-ZSM-5) and H-mordenite (MOR)) and Cu-modified zeolites (Cu/H-ZSM-5 and Cu/H-MOR) can catalyze heterogeneous methanol carbonylation in the absence of a halogen promoter. The process used in their study produced very low (<1%) AA and MA yields, but demonstrated that methanol can undergo carbonylation on zeolites. Calafat and Laine [17] discovered in 1995 that a sulfided MoCo/C catalyst is effective for methanol vapor carbonylation under a nonrhodium and nonhalogen system. MA was the main product (S_MA>60 mol-%) in the reaction of methanol with CO at pressures of up to 7.5 MPa and a temperature of 250 °C. Catalysts need to be sulfided with a H₂S/H₂ mixture (1/10) at 400 °C prior to catalysis. However, the reaction conditions and catalyst preparation conditions are very harsh and not conducive to industrial applications. Peng and Bao [18] discovered in 2004 that NiCl₂-CuCl₂/AC catalysts exhibit satisfactory catalytic performance for methanol vapor-phase carbonylation without the addition of any promoter in the feed. A methanol conversion of 34.5% and a carbonylation selectivity of 94.7 mol-% were achieved under optimum conditions (reaction temperature= 300 °C; methanol concentration= 14.5 mol-%; CO space velocity= 3000 L·kg_cat^–1·h^–1). The average atomic ratio of Cl/(Ni+ Cu) decreased from 1.8 before the reaction to 0.7 after 6 h of reaction. This outcome showed that a remarkable loss of chlorine after several hours of reaction deactivates the catalyst.

On the other hand, Cheung et al. [19] found in 2006 that the carbonylation rate of dimethyl ether (DME) on zeolites (H-MOR, H-FER and H-ZSM-5) is much higher than that of heterogeneous methanol carbonylation. In particular, H-MOR catalyzed the carbonylation of DME into MA at low temperatures (150–190 °C) with high selectivity (>99%) and catalyst stability. Since then, carbonylation of DME has attracted considerable attention in the field of heterogeneous catalysis [3,20]. In 2017, a DME-to-ethanol conversion process with a production capacity of 100000 tons of ethanol/year, developed by the Dalian Institute of Chemical Physics, was commercially operated with great success in China [20]. The carbonylation of DME is superior to that of methanol because the water produced by methanol dehydration competes with CO for adsorption sites and inhibits the insertion of CO into the methoxy group to form an acetyl group; thus, methanol dehydration reduces the MA selectivity. Therefore, improving the selectivity of MA through the halide-free carbonylation of methanol is still challenging. The key technical problem in the halide-free system is the elimination of the influence of on-site water during the carbonylation reaction.

We previously constructed a combined halide-free Cu-based catalyst system composed of Cu-H-MOR+ CuCeO and Cu-H-MOR (labeled Cu-H-MOR+ CuCeOǁCu-H-MOR) for the heterogeneous conversion of methanol into MA in the presence of CO [21]. Cu-H-MOR catalyzed the dehydration of methanol to DME and the subsequent carbonylation of DME to MA. CuCeO converted the water obtained from methanol dehydration into H₂ and CO₂ through a water-gas shift reaction (WGSR) at relatively low temperatures. This process produced H₂ and almost eliminated the influence of water, thus achieving high MA selectivity. However, the need to separately prepare each functional catalyst and combine several catalysts increases the cumbersomeness and complexity of the operation. Most importantly, it is inconvenient to characterize the structure of the combined catalyst.

In this work, we provided an integral copper-based catalyst, CuCeO_x/H-MOR, for the synthesis of MA through heterogeneous halide-free carbonylation of methanol. This catalyst can simplify the operation process and maintain high MA selectivity. It also avoids the use of precious metals and halides, reduces production costs, and avoids equipment corrosion and environmental pollution; at the same time, it is favorable to correlate the structure and performance of the catalyst. Therefore, CuCeO_x/H-MOR may have high academic research importance and industrial application value for MA production via a halide-free non-noble metal route from methanol.

2 Experimental

2.1 Catalyst preparation

H-MOR (SiO₂/Al₂O₃ = 12, specific surface area (S_BET) = 355 m²·g^–1) was purchased from Yangzhou Zhonghe Petroleum Chemicals Institute Co., Ltd., and calcined at 550 °C. Analytical-grade copper(II) nitrate trihydrate [Cu(NO₃)₂·3H₂O], cerium(III) nitrate hexahydrate [Ce(NO₃)₃·6H₂O], and aqueous ammonia (NH₃·H₂O, 25 wt-%) were purchased from Sinopharm Chemical Reagent Co., Ltd., and were used without further purification.

The CuCeO_x/H-MOR catalyst was prepared by the deposition-precipitation method as follows: H-MOR (4.8 g) was ultrasonically dispersed in 100 mL of deionized water and added to 2.41 g of Cu(NO₃)₂·3H₂O and 8.68 g of Ce(NO₃)₃·6H₂O (Cu/Ce molar ratio= 1/2) to form a blue mixture. Approximately 4 mL of 25 wt-% ammonia solution was added dropwise into the mixture at room temperature under strong magnetic stirring for 10 min. The resulting slurry was kept at 70 °C and stirred for another 2 h. Subsequently, the obtained precipitate was filtered, washed with deionized water until neutral, and dried overnight at 100 °C. The dried product was calcined at 500 °C in static air for 4 h by ramping at 2 °C·min^–1 to obtain the CuCeO_x/H-MOR catalyst with 9.2 wt-% Cu loading and 19.6 wt-% Ce loading (S_BET = 304 m²·g^–1, shown in Table S1 and Fig. S1, cf. Electronic Supplementary Material, ESM). Single-metal catalysts (Cu/H-MOR and Ce/H-MOR) were prepared in the same way, but only Cu(NO₃)₂·3H₂O or Ce(NO₃)₃·6H₂O was used. The catalysts discussed herein were all calcined at 500 °C unless otherwise stated.

2.2 Catalyst characterization

The powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku Ultima-IV powder diffractometer equipped with a graphite monochromator and Cu-Ka radiation (35 kV and 15 mA) by scanning over the 2q range from 10° to 80°. The collected diffraction data were identified and compared with reference patterns in the Powder Diffraction File database.

The Ar adsorption-desorption isotherms of the catalysts were measured by static Ar physisorption at –186 °C using a Micromeritics ASAP 2020 surface area and pore analyzer equipped with a turbo molecular pump. All catalysts were outgassed at 300 °C for 4 h to remove physically adsorbed impurities before each measurement. S_BET was calculated using the Brunauer-Emmett-Teller (BET) method by adopting the isotherm data in a relative pressure (P/P₀) range of 0.05–0.2. The pore size distributions of micropores and mesopores were derived from the adsorption branch of the isotherm using the Horvath-Kawazoe and Barett-Joyner-Halenda methods, respectively.

A Bruker S8 TIGER X-ray fluorescence spectrometer was used to determine the chemical composition. The powder catalyst was pressed into a small circle with a diameter of 13 mm prior to testing by using a Shimadzu tablet pressing machine.

Transmission electron microscopy (TEM) was performed on a Philips Analytical FEI Tecnai 30 electron microscope operating at an acceleration voltage of 300 kV. The catalysts for TEM analysis were ground and ultrasonically dispersed in ethanol. Then, the catalyst suspension was collected on a carbon-coated copper grid.

Scanning electron microscopy (SEM) images were obtained on a Phenom LE field emission scanning electron microscope operating at 15 kV. SEM-energy-dispersive X-ray spectroscopy (EDS) was performed using the same spectrometer operating at 15 kV in SEM mode to determine the elemental compositions of the catalysts. The powder catalyst was sprayed with Pt, directly adhered to the conductive paste, and observed under an electron microscope.

The hydrogen temperature-programmed reduction (H₂-TPR) profiles of the catalysts were measured on a Micromeritics Autochem II 2920 instrument. First, the catalyst (100 mg) was treated under Ar flow (30 mL·min^–1) in a quartz U-tube reactor at 300 °C for 2 h. A flow of 5% H₂/Ar (30 mL·min^–1) was then introduced to pass through the catalyst bed after cooling to 50 °C in high-purity Ar. The temperature was ramped from 50 °C to 800 °C at a rate of 10 °C·min⁻¹, and the hydrogen consumption rate was monitored by a thermal conductivity detector (TCD). CO temperature-programmed desorption (CO-TPD) was measured on the same instrument. Briefly, 50 mg of catalyst was reduced at 300 °C for 1 h under a 5% H₂/Ar flow at a rate of 30 mL·min^–1. The system was purged with He for 20 min to remove excess H₂ when the temperature decreased to 50 °C, and then CO adsorption was performed by passing a high-purity CO stream (30 mL·min^–1) through the catalyst bed for 1 h. Then, He was introduced to remove the gas phase and weakly adsorbed CO until the baseline was stable, and the curve was recorded from 50 °C to 800 °C at a ramp rate of 10 °C·min^–1. A mass spectrometer was used to monitor the corresponding CO desorption profiles by m/z = 28.

Fourier transform infrared (FTIR) spectra were collected on a Nicolet 6700 spectrometer, and all the spectra were determined by accumulating 36 scans at a resolution of 4 cm^–1. A pyridine adsorption experiment was conducted on a stainless steel cell with KBr windows by allowing the in situ treatment of the catalyst at up to 600 °C under different atmospheres or high vacuum. Briefly, 30 mg of catalyst was pressed into a self-supported wafer and placed in an in situ infrared (IR) cell before testing. The catalyst was pretreated under vacuum at 300 °C for 2 h and then cooled to 50 °C. Pyridine was adsorbed at 50 °C for 5 min after background correction by degassing under vacuum. IR spectra were collected after exposure to vacuum for 5 min. CO adsorption experiments were performed on the same instrument. The catalyst (approximately 30 mg) was compressed into a self-supporting wafer and carefully loaded into the in situ cell. The catalyst wafer was reduced for 1 h under a flow of 5% H₂/Ar (30 mL·min^–1) at 300 °C, cooled to 50 °C, and evacuated using a molecular pump for 10 min to completely remove the chemisorbed hydrogen species. The catalyst wafer was exposed to high-purity CO (20 mL·min^–1) for 30 min and then evacuated at 50 °C. IR spectra were collected at different evacuation times and referenced to the background spectrum on a 50 °C reduced catalyst before CO soaking under high vacuum at the same temperature.

Electron spin resonance (ESR) spectra were obtained on an X-band Bruker EMX-10/12 device at –173 °C. For each test, a 40 mg of powder catalyst in a quartz tube was analyzed, and the instrument was operated at the X-band frequency of 9.47 GHz with a field modulation of 100 kHz.

Thermogravimetric (TG) analysis was performed on a Netzsch TG209F1 analyzer to evaluate the weight loss of the catalysts upon calcination. During the measurement, approximately 5 mg of the catalyst was heated from 35 °C to 970 °C at a heating rate of 10 °C·min^–1 under air flow (50 mL·min^–1).

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were carried out using a Qtac-100 LEISS-XPS instrument with an Al-Ka radiation source for in situ catalyst characterization, and the binding energies were calibrated using the C 1s peak (284.5±0.2 eV) as a reference. Briefly, the catalyst was pressed into a small disk and placed on the sample holder. The catalyst tablet was reduced in the pretreatment chamber with 5% H₂/Ar (30 mL·min^–1) at 300 °C for 1 h. After cooling, the catalyst was introduced into the ultra-high vacuum chamber for XPS measurement at room temperature.

2.3 Catalytic performance evaluation

The catalytic performance of the CuCeO_x/H-MOR catalyst for heterogeneous methanol carbonylation was tested in continuous flow mode using a stainless-steel fixed-bed tubular reactor equipped with a computer-controlled system. A schematic diagram of the apparatus was presented in our previous work [21]. The catalyst (0.7 g) was reduced in situ under 5% H₂/N₂ flow (50 mL·min^–1) at atmospheric pressure prior to reaction. The reduction temperature was programmed to increase from room temperature to 300 °C at a ramping rate of 2 °C·min^–1 and maintained for 6 h. The catalyst bed was cooled to the target reaction temperature (190–220 °C), and then CO was introduced with methanol by bubbling. The flow rates of the gases were set by a mass flow meter. The pressure was maintained at 1.0 MPa using a back pressure valve. The products were analyzed using two online gas chromatographs (Ruimin GC2060) equipped with a flame ionization detector for CH₄, DME, acetone, MA, CH₃OH, and AA and a TCD for CO and CO₂. The correction factor of the product was measured relative to methanol, and the methanol conversion and product selectivity were calculated by the normalization method.

3 Results and discussion

3.1 Catalyst characterization

XRD was performed to investigate the crystal structure of different catalysts (H-MOR, Cu/H-MOR, Ce/H-MOR and CuCeO_x/H-MOR). Figure 1 shows that the single-metal catalysts (Cu/H-MOR and Ce/H-MOR) give rise to obvious characteristic diffraction peaks of metal oxides (CuO and CeO₂, respectively) [22]. However, the CuCeO_x/H-MOR catalyst shows only the characteristic diffraction peak of CeO₂ and almost no diffraction peak of CuO. This finding indicated that the diffraction peaks of metal oxides, especially CuO, are remarkably less intense than that of the single-metal catalyst Cu/H-MOR. The CuO diffraction peak broadened and almost disappeared because the Cu in the catalyst was highly dispersed on the zeolite support or dispersed by CeO₂ after Ce addition. Another important reason for the inability to observe the XRD peak of CuO is that its diffraction peak may overlap with that of the zeolite support. We found that the crystallinity of the H-MOR zeolite decreased after metal loading because the introduction of metals diluted the sample and reduced the crystallinity of the zeolite to a certain extent, as manifested by the XRD characterization results. This phenomenon has also been observed in another work [23].

Fig.1 XRD patterns of different zeolite catalysts: (a) H-MOR; (b) Cu/H-MOR; (c) Ce/H-MOR; (d) CuCeO_x/H-MOR (Cu: 9.2 wt-%; Ce: 19.6 wt-%).

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The TEM and high-resolution (HR)-TEM images of the CuCeO_x/H-MOR catalyst are shown in Fig. 2. Figures 2(A) and 2(B) show that the darker black spots are the active metal components (Cu and Ce) of the catalyst that are dispersed on the zeolite support. Some agglomerates inevitably formed on the surface of H-MOR, as the total metal loading was as high as 29% (Fig. 2(C)). However, the agglomerates were composed of numerous metals with small particle sizes; hence, the active components, Cu and Ce, of the catalyst have a high degree of dispersion. Obvious lattice fringes were observed, as shown in Fig. 2(D). The measured lattice spacings of the metal particles were observed to be 0.316, 0.271 and 0.195 nm, which correspond to the CeO₂ (111), (200), and (220) planes, respectively [24]. However, the lattice fringes of CuO were not observed in the HR-TEM image (Fig. 2(D)) because the CuO was too small to detect or because CuO existed in an amorphous state, which makes the lattice fringes difficult to observe. This result is also consistent with the XRD results.

Fig.2 (A–C) TEM images and (D) HR-TEM image of the CuCeO_x/H-MOR catalyst (Cu: 9.2 wt-%; Ce: 19.6 wt-%).

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The catalysts were characterized by SEM to observe their surface morphologies, which are shown in Fig. 3. The rectangular material with a smooth surface is the H-MOR zeolite carrier, and the irregular agglomerated particles dispersed on the carrier are the Cu and Ce metal oxides (Figs. 3(A) and 3(B)). Many CuCe species were deposited on the outer surface of the zeolite because of their high content, but some surfaces of the zeolite were very smooth, which suggests that no metal was deposited on these outer surfaces of H-MOR. However, the SEM-EDS analysis of the elemental composition of the catalyst (Figs. 3(C–H)) showed the distribution of metals in the smooth region as well; therefore, the metal species may also exist in the interior of the zeolite and not only on the outer surface. The site of the carbonylation reaction is in the zeolite rings [25,26], and the entrance of metal species into the zeolite is one of the basic conditions for the high activity of the catalyst.

Fig.3 (A and B) SEM images of the CuCeO_x/H-MOR catalyst; (C–H) elemental EDS maps of the CuCeO_x/H-MOR catalyst that correspond to the image (C) and five maps for O (D), Si (E), Al (F), Cu (G), and Ce (H) (Cu: 9.2 wt-%; Ce: 19.6 wt-%).

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3.2 Catalytic performance

The performance of catalysts loaded with different metals was evaluated in methanol heterogeneous carbonylation reactions under the same reaction conditions. The catalytic performance of Cu/H-MOR, Ce/H-MOR, and CuCeO_x/H-MOR catalysts is displayed in Table 1. The H-MOR catalyst achieved 86.3% methanol conversion with selectivity to DME and MA of 97.4% and 1.2%, respectively, showing a performance much different from that of the catalysts containing Cu species. These findings indicate that H-MOR contributes to the methanol dehydration to DME and that the active Cu sites are important for the formation of carbonylation products. The MA selectivity was 49.1% when Cu/H-MOR was used as the catalyst, and almost no carbonylation product (0.9% MA selectivity) was produced when only Ce was loaded. Surprisingly, the CuCeO_x/H-MOR catalyst dramatically increased the MA selectivity to 87.4%. We found that the MA selectivity was closely and positively correlated with CO₂ selectivity. CuO/CeO₂ was an excellent catalyst for WGSR [22,27] and allowed water to react with CO to generate CO₂ and H₂. A large amount of water was generated in the heterogeneous methanol carbonylation reaction because of the dehydration of methanol. The inhibition effect of water on carbonylation could not be eliminated owing to the absence of the active site of WGSR in the single-metal catalyst; therefore, the MA selectivity was very low. However, the CuCeO_x/H-MOR catalysts had the ability to convert water into CO₂ by the WGSR. This process effectively removed water and improved the carbonylation efficiency. In addition, we found that Cu was the active site for the carbonylation reaction and played an essential role in the WGSR. Hence, Cu served as a bridge connecting the WGSR and carbonylation.

In addition, we performed experiments with different metal loadings to determine whether the water was consumed. Figure S2 (cf. ESM) reveals the influence of the metal loadings on the CuCeO_x/H-MOR catalyst on methanol carbonylation. The selectivity to CO₂ was 5.9%, 9.5%, 12.1%, and 11.6% over the CuCeO_x/H-MOR catalysts with 12, 20, 29 and 37 wt-% CuCe loadings, respectively. Clearly, the CO₂ value of 5.9% over the CuCeO_x/H-MOR catalyst with 12 wt-% CuCe implied that the WGSR was incomplete, and the presence of water made complete carbonylation impossible and resulted in low MA selectivity. The highest carbonylation selectivity was 87.4% over the CuCeO_x/H-MOR catalyst with 29 wt-% CuCe loading, where 12.1% CO₂ selectivity was achieved. However, the catalytic performance declined when the loading was increased to 37 wt-% and the CO₂ selectivity did not increase, indicating that water was almost consumed by the WGSR when the metal loading likely at 29 wt-%.

In the present case, it was difficult to accurately calculate the TOF value because part of Cu was sacrificed for WGSR, and thus, the selectivity was used as a measurement standard.

Tab.1 Catalytic performance of different catalysts for heterogeneous methanol carbonylation^a)

Catalyst	CH3OH conversion/%	CO2 selectivity/%	Selectivity/%
			MA	AA	DME	Acetone	CH4
H-MOR	86.3	0.9	1.2	0.5	97.4	0.5	0.4
Cu/H-MOR	93.6	6.1	49.1	2.5	47.8	0.3	0.2
Ce/H-MOR	84.4	0.3	0.9	0.7	97.9	0.2	0.3
CuCeOx/H-MOR	96.5	12.1	87.4	0.2	1.0	11.1	0.3

a) Reaction conditions: T = 200 °C; Pco = 1.0 MPa; CH3OH/CO= 2.15 mol-%; F = 10 mL·min–1; time on stream= 3 h.

Different treatment temperatures cause differences in the final catalysts when the precursor is treated in air. For instance, Wang et al. [28] reported that the reduction degree decreased with increasing temperature when treating the precursor and was related to the distributions of Co²⁺ ions in the zeolite. Some Co²⁺ ions may migrate from supercages to small sodalite cages and hexagonal prisms with increasing treatment temperatures and may hinder access to NaBH₄. This similar phenomenon occurred in our experiments as shown below.

The effect of the calcination temperature on the catalytic performance of the CuCeO_x/H-MOR catalyst for heterogeneous methanol carbonylation was investigated. The results are depicted in Fig. 4. Almost no MA was produced during carbonylation, and the main product was DME by methanol dehydration when the calcination temperature was 300 °C. The MA selectivity increased substantially from 1.1% to 70.8% when the calcination temperature was increased to 350 °C, and the MA selectivity continued to increase and reached its highest value at 500 °C. A further increase in the calcination temperature resulted in a decrease in the MA selectivity. In particular, the MA selectivity was only 23.5%, and methanol conversion dropped to 89.8% at 700 °C. The XRD pattern in Fig. 5 shows that the diffraction peak of the catalyst carrier H-MOR had no visible difference after calcination at different temperatures; hence, the skeleton of the zeolite was not destroyed even at 700 °C. However, high-temperature calcination had an important influence on the metal distribution and particle size. Figure 5 shows that the diffraction peak of the metal oxide (CuO) was not obvious at low temperature (<500 °C) but was remarkably enhanced at 700 °C. This enlargement of the metal particles may decrease the catalytic performance.

Fig.4 Effect of the calcination temperature on the CuCeO_x/H-MOR catalyst for heterogeneous methanol carbonylation (reaction conditions: T = 200 °C; P_co = 1.0 MPa; CH₃OH/CO= 2.15 mol-%; F = 10 mL·min^–1; time on stream= 3 h).

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Fig.5 XRD patterns of the CuCeO_x/H-MOR catalyst treated in air at different calcination temperatures: (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 500 °C, (e) 600 °C, and (f) 700 °C (Cu: 9.2 wt-%; Ce: 19.6 wt-%).

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3.3 Catalyst structure-performance correlation

We speculate that calcination can promote a small portion of metal migration from the outside to the inside of the zeolite channel for the metal active sites to be located in the best position for the reaction. The radii of Cu and Ce are listed in Table S2 (cf. ESM). We found by comparing the atomic radius of Ce and the pore size of zeolite that, in theory, Ce ions could not enter the 8-membered ring (2.6 Å × 5.7 Å) or the side-pockets (3.4 Å × 4.8 Å) of the 8-membered ring and could only stay in the 12-membered ring (6.5 Å × 7.0 Å) or the outer surface of the zeolite. Thus, we think that the WGSR takes place in the non-8-membered ring. However, the optimal site for carbonylation is the 8-membered ring [8,29]. Accordingly, Cu ions must enter the 8-membered ring to carry out the carbonylation reaction. The low carbonylation selectivity at 300 °C may be because the Cu ion did not enter the 8-membered ring but entered the 12-membered ring or the outer surface of H-MOR. Cu ions migrated from the outer surface of the zeolite or the 12-membered ring to the 8-membered ring when the calcination temperature increased. Therefore, calcination was the driving force for Cu ions to enter the 8-membered ring. The migration of the formed Cu⁺ from the 12-membered ring to 8-membered ring channels has been previously reported [30].

Tables S3 and S4 (cf. ESM) show the element composition determined by SEM-EDS in the CuCeO_x/H-MOR catalyst before and after calcination. The atomic ratio of Cu in the smooth area (the smooth area represents that area with little metal deposition on the outer surface of the zeolite) of the catalyst before calcination was 1.47%, and the content of Cu after calcination at 500 °C increased to 1.94%. This result indicated that the metal indeed migrated to the inner part of the zeolite after calcination. However, we were unable to determine whether Cu was in the 12-membered ring or the 8-membered ring because the migration from the 12-membered ring to the 8-membered ring could not be distinguished by SEM alone.

Two sets of tests were performed to prove that calcination can promote a small portion of metal migration and change the catalytic performance. The first test was to calcine the CuCeO_x precursor at different temperatures (300, 350, 400 and 500 °C) and the resultant was recorded as CuCeO_x-T, which was then mechanically mixed with H-MOR to form the CuCeO_x-T + H-MOR catalysts. The synthesis of the CuCeO_x precursor was the same as that of CuCeO_x/H-MOR, except no H-MOR was added during the preparation. This method was labeled C&M (calcination and then mixing). The other set of tests was performed by mixing and calcination (M&C). The CuCeO_x precursor and H-MOR were mechanically mixed and then calcined at different temperatures to form a (CuCeO_x + H-MOR)-T catalyst. According to the results (Fig. S3, cf. ESM), the temperature rise had little effect on the performance of the CuCeO_x-T + H-MOR catalyst but had an obvious promotion effect on the (CuCeO_x + H-MOR)-T catalyst. For example, CuCeO_x-500+ H-MOR had only 3.7% MA selectivity, whereas the MA selectivity on (CuCeO_x + H-MOR)-500 considerably increased to 50.2%. We think that the M&C method will promote the migration of the metal into the pores of the zeolite during thermal treatment, whereas the C&M method will not allow the metal to enter the pore because of the independent thermal treatment and therefore will cause failure of the active center to exert the carbonylation reaction. Furthermore, the CuCeO_x-500+ Cu-H-MOR catalyst (Cu-H-MOR prepared by the ion exchange method) had an MA selectivity of 77.4%, which was remarkably higher than that of the CuCeO_x-500+ H-MOR and (CuCeO_x + H-MOR)-500 catalysts. The results showed that prior incorporation of Cu into H-MOR considerably improved the MA selectivity. Therefore, an appropriate calcination temperature will promote more metal ions to enter the H-MOR ring to improve the performance of the carbonylation reaction.

We indirectly proved through carrier effect experiments that the migration of metals into the pores of H-MOR is a necessary condition for the carbonylation reaction (Fig. 6). Methanol conversion remained high, almost no MA was found, and DME was the main product when H-ZSM-5 was used as the carrier; methanol conversion was reduced by half and DME was still the main product when the carrier was replaced with titanium silicalite-1; and catalytic activity was the lowest when activated carbon was the carrier. This finding shows that a special catalyst carrier is needed to obtain a high MA selectivity in the case of the same metal loading. Only H-MOR provides appropriate sites for the carbonylation reaction; in other words, the active center must be in the H-MOR zeolite channel to have a high MA selectivity.

Fig.6 Catalytic performance of CuCeO_x/H-MOR catalysts with different carriers for the methanol heterogeneous carbonylation reaction (reaction conditions: T = 200 °C; P_co = 1.0 MPa; CH₃OH/CO= 2.15 mol-%; F = 10 mL·min^–1; time on stream= 3 h. Cu: 9.2 wt-%; Ce: 19.6 wt-%).

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We subsequently characterized the catalysts calcined at different temperatures through FTIR CO-chemisorption, H₂-TPR, and CO-TPD (Fig. 7). The migration of the Cu species into the zeolite channel will lead to different cupreous sites. CO bound to various surface cupreous sites displays different chemisorption behaviour. The IR peak position and the adsorption strength of the chemisorbed CO can be used to distinguish Cu⁰, Cu⁺, and Cu²⁺ species [31]. CO adsorbed on Cu⁰ and Cu²⁺ species is not stable at room temperature, whereas CO interacts with Cu⁺ cations preferentially and strongly [30,32]. The catalysts were subjected to hydrogen reduction at 300 °C for 1 h and subsequently exposed to pure CO. The IR spectra of the catalysts after evacuation using a molecular pump are shown in Fig. 7(A). All the catalysts calcined at different temperatures gave rise to an obvious IR absorption band with a peak position at 2155 cm^–1 after an evacuation time of 25 min. The IR band observed at 2155 cm^–1 after long-term evacuation was typically assigned to the Cu⁺-CO complex in the 8-membered ring of H-MOR [10,31,33]. We observed no peak at 2155 cm^–1 in the CO-IR spectrum of CuCeO_x and H-MOR (Fig. S4, cf. ESM), which suggests that the peak at 2155 cm^–1 is indeed caused by Cu⁺-CO in H-MOR containing Cu. The intensity of the stable Cu⁺-CO IR bands at 2155 cm^–1 initially increased and then decreased when the calcination temperature for the catalyst was increased from 300 to 700 °C. The maximum intensity was observed on the 500 °-Ccalcined catalyst, which exhibited the best activity for heterogeneous methanol carbonylation. The trend was in good agreement with the performance of the catalyst and indicated that the catalytic performance was directly proportional to the content of Cu⁺. The catalyst calcined at 500 °C had the most Cu⁺ after reduction, which was favorable for CO adsorption and heterogeneous methanol carbonylation into MA. This phenomenon revealed the migration of Cu species during calcination. An appropriate calcination temperature will promote more Cu metal ions to enter the 8-membered ring to form active Cu⁺ cation sites after reduction to improve the performance of the carbonylation reaction.

To further verify the valence state of the metal species, we performed in situ XPS analysis on the catalyst calcined at 500 °C (Figs. S5 and S6, cf. ESM). The Ce 3d XPS is presented in Fig. S5. The peaks centered at 879.8, 895.7, 898.4 and 913.9 eV are attributed to Ce⁴⁺. The peak observed at 882.6 eV can be assigned to Ce³⁺ [34]. This finding reveals that Ce⁴⁺ is partly reduced to Ce³⁺ by a reductive gas, resulting in the coexistence of Ce⁴⁺ and Ce³⁺ on the catalyst surfaces. Fig. S6(A) shows the Cu 2p XPS of the catalyst after in situ reduction. The Cu 2p_3/2 and Cu 2p_1/2 peaks are at binding energies of 929.6 and 949.5 eV, respectively [35], and no satellite peaks at approximately 940 eV belonging to Cu²⁺ were observed. This finding indicates that the copper species on the catalyst have been completely reduced. As shown in Fig. S6(B), AES was used to distinguish Cu⁺ and Cu⁰ species. Two asymmetric and partially overlapping auger peaks were obtained by split-peak fitting of the Cu LMM AES. The kinetic energy was approximately 916.5 and 920.7 eV, corresponding to Cu⁺ and Cu⁰ species, respectively. The content of Cu⁺ fluctuated around 30%–32%.

The redox properties of the catalysts were characterized by H₂-TPR. As shown in Fig. 7(B), a well-defined two-step reduction profile (designated as a and b) was observed for all CuCe catalysts in the range of 120–270 °C. The profile indicates that at least two copper species were present in the catalysts. The a and b peaks are ascribed to the reduction of finely dispersed CuO species and the reduction of larger CuO particles, respectively [35,36]. The intensities of the a peaks increased initially and then decreased, and reached the maximum with the catalyst calcined at 500 °C as the calcination temperature increased from 300 °C to 600 °C. These changes were consistent with those of the catalyst performance. The reduction peak temperature of this catalyst was much higher than those of the others when the calcination temperature increased up to 700 °C, and the reduction peak temperature probably represents the reduction of bulk CuO with an obvious diffraction peak according to the XRD. The CO-TPD spectrum in Fig. 7(C) shows two desorption peaks at 150 and 600 °C. There was no low temperature peak in the CO-TPD results of CuCeO_x and H-MOR (Fig. S7, cf. ESM), indicating that the CO desorption at 100–300 °C was not due to physisorption. The low-temperature peak may be the desorption peak of Cu⁺-CO, and the peak area was positively correlated with Cu⁺ content. In addition, the formation of CO₂ was tracked by mass spectrometry (Fig. S8, cf. ESM), which indicates that the high-temperature peak is likely due to the dissociative desorption of carbonates.

Fig.7 (A) FTIR spectra of the chemisorbed CO on the as-reduced CuCeO_x/H-MOR catalyst with different calcination temperatures after evacuation for 25 min; (B) H₂—TPR and (C) CO-TPD profiles of the CuCeO_x/H-MOR catalyst treated in air at different calcination temperatures: (a) 300 °C; (b) 350 °C; (c) 400 °C; (d) 500 °C; (e) 600 °C; (f) 700 °C (Cu: 9.2 wt-%; Ce: 19.6 wt-%).

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In addition to the metal sites, the acidity of H-MOR may also be an important factor. High-temperature calcination is prone to cause dealumination of the zeolitic framework, resulting in a decrease in the acid content and number of acid sites, thereby affecting the catalytic activity [37,38]. By pyridine FTIR spectra (Fig. S9, cf. ESM), it was found that with an increase in the calcination temperature, B acidic sites are gradually transformed into L acidic sites, and few B acidic sites remain in the 700 °C-calcined catalyst. During the carbonylation of methanol, B acidic sites were essentially active for methanol dehydration to DME, while the carbonylation of DME to MA was largely related to the active metal sites. As we can see in Figs. 4 and S9, when the catalyst was calcined at temperatures above 500 °C, the methanol conversion decreased gradually, most likely due to the significant reduction in the number of B acidic sites.

The foregoing characterization results provide strong evidence that calcination can promote a small amount of metal migration and affect the content of Cu⁺ on catalysts after reduction. These Cu⁺ sites play important roles in binding and activating CO and forming acetyl groups during heterogeneous methanol carbonylation reactions, but their precise functions in the catalytic reaction are still unclear.

3.4 Ketonization reaction and catalytic mechanism

To study the origin of the byproduct acetone, we examined the effect of the reaction temperature and methanol concentration on the catalytic activity for heterogeneous methanol carbonylation, as shown in Fig. S10 (cf. ESM). The conversion of methanol increased almost linearly from 93.8% to 99.1% in the range of 190–220 °C, whereas the MA selectivity initially increased, then decreased, and finally reached an optimal value 87.4% at 200 °C. The selectivity of acetone increased with increasing reaction temperature. This phenomenon may be related to ester ketonization [39,40], which is a typical endothermic reaction. Therefore, the rate of the ketonization reaction was enhanced when the reaction temperature increased to allow MA consumption, which resulted in an increase in acetone selectivity and a decrease in MA selectivity. The MA selectivity presented a volcanic-type variation with increasing methanol concentration (Fig. S11, cf. ESM). Interestingly, the MA selectivity was not the highest, but acetone had the highest selectivity at a low methanol concentration of 1.66%; therefore, the ketonization reaction is likely to occur after the carbonylation reaction. Therefore, we used MA instead of methanol as the raw material for the experiments to demonstrate the ketonization reaction. Interestingly, the selectivity of acetone reached 89.7% (the other byproducts were methanol and DME), indicating that MA is susceptible to ketonization to form acetone in the presence of CO, which is consistent with our conjecture (the reaction mechanism of ketonization is shown in Fig. 8). Increasing the methanol concentration was not conducive to the occurrence of the ketonization reaction. Therefore, the ketonization reaction is related to the reaction temperature and methanol concentration.

Fig.8 Proposed mechanism for the ketonization reaction.

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On the basis of the experimental phenomena described above, we proposed a possible reaction mechanism for how CuCeO_x/H-MOR catalyzes heterogeneous methanol carbonylation without the addition of CH₃I, as shown in Fig. 9. First, methanol reacts with the B acid sites (the pyridine-IR results are shown in Fig. S9) on the H-MOR zeolite framework to form methyl groups and release H₂O simultaneously. Then, CO activated by Cu reacts with the lattice oxygen in CeO₂ to produce CO₂, and CeO₂ is reduced to Ce₂O₃. Ce₂O₃ with oxygen vacancies easily captures oxygen from H₂O to regenerate CeO₂, and H₂ is generated [24,41]. The removal of water from the reaction by the WGSR is a critical step in the cycle and is the main reason for the increase in the carbonylation selectivity. Later, CO inserts into the C–O bond between the methyl group and the framework oxygen of the zeolite to form an acetyl group, which is the rate-limiting step of the reaction [10,29,42,43]. Finally, the acetyl group reacts with another molecule of methanol to form MA to achieve circulation.

Fig.9 Proposed reaction mechanism of heterogeneous methanol carbonylation over the CuCeO_x/H-MOR catalyst.

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3.5 Catalyst stability

Catalyst stability is crucial for catalytic research. The lifetime of a catalyst can be extended by metal atom modification [42], selective dealumination of H-MOR [44], and selective poisoning with pyridine [45] and nanomordenites [23,46], but the challenge remains. The results of the stability tests of the CuCeO_x/H-MOR catalyst for heterogeneous methanol carbonylation are shown in Fig. 10. The activity of the CuCeO_x/H-MOR catalyst was very stable with constant methanol conversion; little deactivation was observed, and the deactivation rate constant was 0.005 h^–1. The stabilization time of the CuCeO_x/H-MOR catalyst was substantially enhanced compared with that of our previously combined Cu-H-MOR+ CuCeO||Cu-H-MOR catalyst [21], as reflected by the decrease in the deactivation rate constant from 0.03 to 0.005 h^–1. The stability of several catalysts (Cu-H-MOR+ CuCeO||Cu-H-MOR, Cu/H-MOR, and Ce/H-MOR) is shown in Fig. 10. For the dual-bed catalyst system composed of Cu-H-MOR+ CuCeO||Cu-H-MOR, the selectivity toward MA decreased gradually after 18 h on stream, which has been shown to be due to carbon deposition on the catalyst [21]. The Cu/H-MOR and Ce/H-MOR catalysts showed a lower MA selectivity, and their deactivation was relatively gentle but also occurred earlier than that of CuCeO_x/H-MOR.

Fig.10 Results of carbonylation stability tests (A) methanol conversion and (B) MA selectivity over different catalysts: (a) CuCeO_x/H-MOR; (b) Cu-H-MOR+CuCeO||Cu-H-MOR; (c) Cu/H-MOR; (d) Ce/H-MOR (reaction conditions: T = 200 °C; P_co = 1.0 MPa; CH₃OH/CO= 2.15 mol-%; F = 10 mL·min^–1).

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The increase in CuCeO_x/H-MOR catalyst stability may be related to the introduction of Ce. Ce species entered the 12-membered ring and adsorbed onto the acid site, which was not selective for carbonylation but beneficial for carbon deposition; thus, Ce species inhibited the formation rate of hard coke and improved the stability. According to the results of TG analysis (Fig. S12, cf. ESM), we found that the weight loss of the catalysts after reaction for 30 h was basically similar to that of the as-calcined catalyst, but a small weight loss occurred at 242 °C, which might be attributed to soft coke (e.g., surface methyl group and MA). This phenomenon is different from the combined catalysts reported previously [21]. The results show that the introduction of Ce could inhibit the formation of coke, but the mechanism remains to be further explored. Relevant conclusions have been reported for bimetallic catalysts. For example, Reule et al. [47] reported that adding zinc to copper-exchanged MOR catalysts (Cu-Zn/H-MOR) dramatically improved the DME carbonylation. The catalyst with zinc maintained>90% MA selectivity, which led to a 6-fold increase in product yield and a 4-fold increase in lifetime even during deactivation versus 60% for the Cu/H-MOR. The enhanced stability of the Cu-Zn/H-MOR catalyst was ascribed to blockage of the T4 sites, which are assumed to be responsible for the formation of the coke precursor [48]. Further research needs to be done to prove the improvement in stability because Ce and Zn are different metals with different properties.

The spent catalyst was characterized to determine the reason for catalyst deactivation. It can be seen from the XRD patterns (Fig. S13, cf. ESM) that the diffraction lines of H-MOR and CeO₂ do not change significantly before and after the reaction. The as-reduced CuCeO_x/H-MOR catalyst shows a typical diffraction line due to metallic Cu species and the intensity of this line increases after the reaction. After regeneration by burning coke, the weak diffraction lines ascribed to CuO appear accompanied by the disappearance of metallic Cu species, indicating the oxidation of metallic Cu to CuO. However, the particle size of CuO becomes larger than that of the as-prepared catalyst. This phenomenon can also be seen in the ESR results. The as-calcined catalyst (Fig. S14, cf. ESM) shows four-line hyperfine splitting with a g factor of 2.39. According to the literature [49], this signal can be attributed to isolated Cu²⁺ ions. The spectrum also exhibited a peak with a g factor of 2.08, which was correlated with interacting Cu²⁺ species. Moreover, no ESR signals due to crystallized or bulk CuO (Fig. S14) were observed. Therefore, the interacting Cu²⁺ species were considered to be small clusters of copper oxide [50]. The ESR signals of the regenerated catalyst obtained by burning coke (Fig. S14) were weaker than that of the as-calcined catalyst, suggesting that CuO particles become larger in the catalyst after regeneration. The catalyst performance could not be fully recovered by burning coke because the selectivity toward MA was reduced to 60.8% over the regenerated catalyst. By combining the results of XRD, TG, and ESR, we consider that the main reason for the deactivation of the catalyst is not due to carbon deposition but rather to the growth of the metal particles. Therefore, further exploration is necessary to realize complete regeneration of the spent catalyst.

4 Conclusions

The CuCeO_x/H-MOR catalyst synthesized by a simple deposition-precipitation method without halogen and precious metals afforded satisfactory activity, selectivity and stability in a heterogeneous methanol carbonylation reaction. The presence of Cu and Ce and the occurrence of the WGSR effectively overcame the adverse effect of water produced by methanol dehydration. The MA selectivity was only 49.1% without Ce addition but reached 87.4% with the CuCeO_x/H-MOR catalyst. Calcination can promote a small amount of metal migration, and FTIR CO chemisorption revealed that the concentration of a unique Cu⁺ site in H-MOR that could stably adsorb CO molecules reached a maximum on a CuCeO_x/H-MOR catalyst, which was calcined at 500 °C and had the best catalytic activity. In addition, the Ce in the CuCeO_x/H-MOR catalyst played a vital role in improving the stability of the reaction. This work achieved heterogeneous carbonylation of methanol to MA through halide-free catalysis with the assistance of the WGSR. The use of nontoxic and harmless raw materials and conditions to prepare environmentally friendly products is in accordance with the aims of green chemistry, and the results are valuable for the development of a stable and efficient catalyst for further applications.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grant Nos. 2018YFB0604703, 2017YFA0206801, and 2018YFB0604701), the National Natural Science Foundation of China (Grant Nos. 21972113 and 91545115), the Program for Innovative Research Team in Chinese Universities (Grant No. IRT_14R31), and the Fundamental Research Funds for the Central Universities (Grant No.20720190039).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11705-020-2019-5 and is accessible for authorized users.

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