Investigation of the interaction between Cu(acac)₂ and NH₄Y in the preparation of chlorine-free CuY catalysts for the oxidative carbonylation of methanol to a fuel additive

Abstract

The high temperature anhydrous interaction between copper(II) acetylacetonate Cu(acac)₂ and NH₄Y was investigated to prepare a chlorine-free CuY catalyst for the oxidative carbonylation of methanol to dimethyl carbonate. When a physical mixture of Cu(acac)₂ and NH₄Y is heated from ambient temperature to 230 °C, Cu(acac)₂ firstly sublimates and then is adsorbed immediately onto the surface of the Y zeolite. Simultaneously the ion exchange between Cu(acac)₂ and NH₄Y occurs at about 174 °C. During the activation process from 230 to 500 °C, the exchanged Cu²⁺ is reduced to a Cu⁺ active center, and the adsorbed and unreacted Cu(acac)₂ on the NH₄Y surface decomposes to nano-CuO. For NaY zeolite, no solid state ion-exchange occurs between Cu(acac)₂ and NaY during the heat treatment and only CuO exists on the Cu/NaY catalyst surface. While for HY zeolite, there is less ion-exchanged Cu⁺ in the supercages. The Cu/NaY catalyst has no catalytic activity and the Cu/HY catalyst exhibits lower activity than the Cu/NH₄Y catalyst. Strong evidence is provided that during heat treatment, a solid state ion-exchange between Cu(acac)₂ and NH₄Y occurs and makes more of the Cu⁺ located in the supercages accessible to reactants.

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Introduction

In the last few decades, nontoxic and easily biodegraded dimethyl carbonate (DMC) has been found to have huge potential applications. Compared with the conventional octane enhancing additive methyl tert-butyl ether (MTBE), DMC is a more suitable component of reformulated fuels owing to its good blending properties, high oxygen content and a significant reduction in pollutants in exhaust gases.^1–4 DMC is also more valuable as a green organic intermediate to substitute the toxic phosgene dimethyl sulfate.^5,6 Of course, many innovative strategies for the synthesis of DMC have been developed.^7,8 Among them, the oxidative carbonylation of methanol was considered as an environmentally friendly synthesis of DMC and has been widely investigated.^8,9 As early as the 1980s, the Enichem Company (Italy) had demonstrated the oxidative carbonylation of methanol to DMC using cuprous chloride CuCl catalyst.⁷ But during the catalytic process, the catalyst suffers from loss of chlorine to form HCl, which results in catalytic deactivation and the equipment seriously corrodes. Then, supported Cu-based catalysts were considered to reduce the influence of chlorine and have been widely explored.¹⁰

King^11,12 reported that when heating a physical mixture of CuCl and HY zeolite at high temperature, the solid state ion exchange (SSIE) of Cu⁺ of CuCl with H⁺ of HY occurred, and the formed Cu⁺Y catalyst catalyzed the oxidative carbonylation of methanol to DMC with a higher than 5% conversion of methanol and 80% DMC selectivity. It means that without chlorine, Cu⁺ also has catalytic activity. After that, many solid acids, such as MCM-41,^13,14 Hβ zeolite,¹⁵ HZSM-5 zeolite,¹⁶ SiO₂–Al₂O₃,¹⁷ SiO₂–ZrO₂,¹⁸ and SiO₂–TiO₂,¹⁹ were employed to prepare Cu⁺ supported catalysts through SSIE. Among them, HY is of great interest to many researchers due to its special supercage microstructure and large surface area.^20–25 However, a little chlorine inevitably remains and then forms HCl which corrodes the equipment.

To avoid the negative effects of chlorine, intensive efforts have been made to prepare a chlorine-free CuY catalyst using non-chloric copper salts, such as Cu₂O,¹² Cu(NO₃)₂ (ref. 26–31) and Cu(CH₃COO)₂.³² A CuY catalyst prepared through heating the powder mixture of Cu₂O and HY zeolite at 650 °C for 70 h with a helium purge was active for producing DMC, but its activity was very low and quickly decreased.¹² In our previous work,^28–30 it was shown that when a Cu²⁺Y zeolite material prepared through liquid solution ion exchange (LSIE) of NaY zeolite with Cu(NO₃)₂ and cuprammonia was heated at high temperature, Cu²⁺ in the supercages was auto-reduced to active Cu⁺ and the activated materials showed superior activity. Furthermore, during high temperature activation of the Cu(NO₃)₂/Y zeolite prepared through incipient wetness impregnation (IWIM), HY undergoes easier ion-exchange with Cu²⁺ than NaY and the former exhibited a better catalytic activity. Richter et al.³² also found that for heat treatment above 700 °C of the CuY zeolite prepared through deposition–precipitation (DP) of Cu(CH₃COO)₂ on NH₄Y zeolite in aqueous solution, ion exchange does occur to form Cu²⁺ on the surface of Y zeolite and the released NH₃ is favorable to reduce Cu²⁺ to Cu⁺.

Recently, an organometallic compound Cu(acac)₂ has demonstrated a unique charm in the field of copper material preparation due to its properties of sublimation and decomposition at low temperatures.^33–36 Goel et al.³³ reported that, through chemical vapor deposition of a Cu(acac)₂ precursor, Cu₂O and CuO were generated at temperatures of 195 °C to 430 °C respectively, but at 285 °C, both Cu₂O and CuO coexisted. Nasibulin et al.³⁵ found that when the oxygen concentration of the oxygen/nitrogen carrier gas was 0.5%, through Cu(acac)₂ vapor decomposition, nanoparticles of Cu₂O and CuO were prepared at 432 °C and 705 °C respectively; when the oxygen concentration was above 10.0%, above 596 °C only CuO nanoparticles were generated; when the oxygen concentration was below 0.0003%, only Cu nanoparticles were generated. What’s more, oxygen plays a very important role in removing impurities.

Therefore, similar to the process of SSIE between CuCl and NH₄Y zeolite, it is also expected that when a physical mixture of NH₄Y zeolite and Cu(acac)₂ is heated in a muffle furnace, Cu(acac)₂ would sublimate and be adsorbed onto the NH₄Y surface several times and become dispersed uniformly, then the NH₄Y zeolite would decompose to HY zeolite and Cu(acac)₂ would decompose to Cu₂O, and simultaneously SSIE between HY zeolite and Cu₂O would form the Cu⁺Y catalyst. Decomposition gases such as CO, NH₃, N₂, CO₂ form the reducing atmosphere in the heating process, which is favorable for retaining Cu⁺. To the best of our knowledge, this novel process for the preparation of a chlorine-free Cu⁺ catalyst has not been reported previously.

In our recent work,³⁷ chlorine-free CuY catalysts were successfully prepared through high temperature anhydrous interaction (HAI) between Cu(acac)₂ and NH₄Y and were used in the oxidation carbonylation of methanol. It was found that the catalytic activity depends highly on the heating temperature. In this study, the interaction process of heating the mixture of Cu(acac)₂ and NH₄Y under anhydrous conditions and the formation of the Cu catalytic active centres were investigated in more detail using TG/DSC, XRD, H₂-TPR, XPS, TEM, etc.

Experimental section

Materials

NaY zeolite with a Si/Al molar ratio of 2.7 was purchased from the Nankai Catalyst Factory. Reagents such as NH₄NO₃ (AR, Tianjin Chemical Reagent no. 3 Factory), Cu(acac)₂ (AR, Sinopharm Chemical Reagent Co. Ltd), CH₃OH (AR, Tianjin Kemiou Chemical Reagent Company) as well as carbon monoxide (99.99%, Beijing Haipu Beifen Gas Industry Co., LTD), oxygen, nitrogen and hydrogen (99.99%, TISCO) were used.

Catalyst preparation

Firstly, 30.0 g of NaY zeolite was added to 300 mL of a 0.5 mol L⁻¹ NH₄NO₃ aqueous solution and stirred to ion exchange for 4 h at 30 °C, and dried at 100 °C. Then the procedures were repeated once and the final solid material was NH₄Y zeolite.

Then, some of the prepared NH₄Y zeolite was packed in a crucible and then put into a muffle furnace in order to calcinate NH₄Y zeolite to HY zeolite. The furnace temperature was firstly raised from the ambient temperature to 200 °C at 3 °C min⁻¹, then raised from 200 °C to 500 °C at 1 °C min⁻¹ and held constant at 500 °C for 6 h. Finally, the cooled solid material was HY zeolite.

The Cu/NH₄Y catalyst was prepared through heat treatment of a mixture of Cu(acac)₂ and NH₄Y according to the following two stages. In the first stage, 2.045 g of Cu(acac)₂ and 5.000 g of NH₄Y were mixed and ground well with a mortar and pestle to form an intimate gray-blue mixture. Then the mixture was heated in a muffle furnace from the ambient temperature to 180 °C at 3 °C min⁻¹, and then held at 180 °C for 4 h to disperse Cu(acac)₂ very well on the surface of the NH₄Y zeolite to form the precursor. In the second stage, the precursor material was activated to form the Cu/NH₄Y catalyst through heating at temperatures of 300, 400, or 500 °C for 4 h from the ambient temperature at a heating rate of 3 °C min⁻¹. The activated Cu/NH₄Y catalysts were denoted as Cu/NH₄Y-300, Cu/NH₄Y-400 and Cu/NH₄Y-500 respectively.

According to the above procedure, NaY and HY were also used to substitute NH₄Y to prepare Cu/NaY-400 and Cu/HY-400 catalysts respectively. The activation temperature was 400 °C. All catalysts were stored in a dryer, and pressed to 40–60 mesh particles before catalytic tests.

Characterization techniques

Thermogravimetric analyses (TG/DSC) were performed on a SETSYS Evo 16/18 thermogravimetric analyzer (Setaram) under an air flow rate of 100 mL min⁻¹, employing a heating rate of 10 °C min⁻¹ up to a final temperature of 800 °C. The evolved gas analysis of the decomposition reaction was monitored with an omnistar quadrupole Pierre mass spectrometer (MS) linked to the thermobalance. The MS operated with an ionization energy of 40 eV, using a Channeltron detector (1000 V).

Temperature-programmed reduction (H₂-TPR) was performed on a Micromeritics Autochem II 2920 chemical adsorption instrument. 20 mg of sample was loaded in a U-shaped quartz tube, and then heated at 10 °C min⁻¹ to 300 °C with 30 mL min⁻¹ argon for 30 min to remove adsorbed moisture. After cooling to room temperature, the gas was switched to 10 vol% H₂ in an argon flow, and the sample was heated to 1000 °C. The hydrogen consumption was monitored through thermal conductivity.

X-ray diffraction (XRD) data were collected by using CuKα radiation (λ = 0.154056 nm) on a Rigaku D/max 2500 diffractometer at a 40 kV target voltage and 100 mA tube current in the 2θ range from 5° to 80° at a scanning rate of 8° min⁻¹.

Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100F electron microscope operating at 200 kV. Exposition times were confined to about 2 s due to the beam sensitivity of the samples. Sample preparation was carried out by dispersing 5 mg of ground powder in 5 mL of ethanol using ultrasonication, followed by dropping the suspension on a rounded carbon-coated Cu-Grid mesh and drying at room temperature.

X-ray photoelectron spectroscopy (XPS) data were collected on an AXIS ULTRA DLD electron spectrometer by using a 150 W monochromatic Al Kα radiation source operating at 1486.6 eV. The binding energy and the Auger kinetic energy scales were referenced to the C 1s line at 284.8 eV from adventitious carbon.

Atomic absorption spectroscopy (AAS) was used to determine the Cu loading of the prepared catalyst by using VARIAN AA240FS equipment. The testing solution was prepared through the following steps: 0.2 g of fresh catalyst was calcined with 2.0 g of potassium hydroxide at 500 °C for 6 h, digested in a hydrochloric acid solution, and diluted through making the volume up to 250 mL in a volumetric flask with deionized water. Then 5.0 mL of this solution was transferred to a 100 mL volumetric flask, diluted with deionized water to the volume, and mixed.

Catalytic tests

Catalytic measurements were performed in a continuous fixed bed micro reactor (Φ 6 mm × 450 mm) under atmospheric pressure with an on-line Agilent 6890 gas chromatograph (GC). 0.45 g of catalyst (about 1.0 mL) was packed in the tubular reactor and the reactor was positioned in the furnace. 0.02 mL min⁻¹ of methanol was introduced using a constant-flux pump (2PB-05) and vaporized in the pre-heater. In the meantime, 28.0 mL min⁻¹ of carbon monoxide and 2.8 mL min⁻¹ of oxygen were introduced into the pre-heater and mixed well with the vaporized methanol, and then the mixed reactants were fed into the catalyst bed to catalyze the oxidative carbonylation of methanol. The temperature and time of the catalytic reaction were 140 °C and 10 h respectively.

The thermal gas products were analyzed using an on-line GC with three valves and four columns every 20 minutes. The products flowed through the HP-PLOT/Q capillary column (30 m × 0.53 μm × 40 μm), the organic products were held back. Then the expelled inorganic products flowed through a Propack-Q packed column: CO₂ firstly flowed out and entered into a thermal conductivity detector (TCD) and was detected; secondly, CO and O₂ were separated using an HP-PLOT Molesieve/5A capillary column (30 m × 0.53 μm × 25 μm) and detected using TCD. The products flowed through an HP-INNOWax (30 m × 0.53 μm × 1 μm) capillary column and entered into a flame ionization detector (FID), thus MeOH, methyl formate (MF), dimethoxymethane (DMM), dimethyl ether (DME) and DMC were detected.

Results and discussion

Thermal analysis

It is well known that when NH₄Y zeolite is heated at 400 °C, NH₃ is dissociated, and the NH₄Y zeolite converts to HY zeolite.³⁸

Then the dehydroxylation of silicol on the surface of the HY zeolite takes place at heating temperatures above 450 °C.^38,39 The TG/DTG-DSC curves of the as-prepared NH₄Y zeolite are illustrated in Fig. 1(a). It is clearly shown that when the temperature is below 200 °C, the desorption of zeolite-adsorbed water causes an obvious weight loss and endothermic phenomenon;⁴⁰ between 200 and 440 °C, the dissociation of NH₃ from the NH₄Y zeolite causes a slight weight loss with endothermic phenomenon; above 440 °C the dehydroxylation of silicol on the surface of the HY zeolite causes another slight weight loss with endothermic phenomenon.

Fig. 1 TG/DTG-DSC curves of the samples. Curve 2 is the TG resulting from the addition of the TG of Cu(acac)₂ and NH₄Y.

For the Cu(acac)₂ sample, only one obvious weight loss peak in the DTG at around 250 °C and only one strong exothermic peak on the DSC at 247 °C are observed in Fig. 1(b). It strongly indicates that Cu(acac)₂ decomposed at 150–350 °C, because there is no further weight loss and no heat changes above 350 °C. The final material from Cu(acac)₂ decomposition should be CuO, and the theoretical weight loss is 69.4%. But the actual weight loss of the TG in Fig. 1(b) is as high as 93.7%. If the DSC curve is enlarged as shown in Fig. 1(b), a weak endothermic peak at 220 °C is clearly observed before the strong exothermic peak at 247 °C. This must be caused by the sublimation of Cu(acac)₂. Therefore, it is concluded that Cu(acac)₂ firstly sublimates and then decomposes to CuO at temperatures from 150 °C to 350 °C.

For a well physical mixture of NH₄Y with 28.6 wt% Cu(acac)₂, the thermal behavior is so complicated that it is not the simple addition of two raw materials, and the TG of the mixture and the simple addition are labeled as curve 1 and curve 2 respectively, shown in Fig. 1(c). It is clear that the weight loss undergoes four steps according to the TG/DTG curves of the mixture. In the first step, zone I, at temperatures below 120 °C, the weight loss of 8.8% for the peak at 75 °C must be attributed to the loss of adsorbed water in the NH₄Y zeolite. This water desorption peak temperature on curve 1 is obviously lower than that of pure NH₄Y and on curve 2. It indicates that some of the Cu(acac)₂ would have adsorbed onto the surface of the zeolite and facilitated the release of zeolite-adsorbed water. In the second step, zone II, at temperatures between 120 °C and 230 °C, the peak at 182 °C is for a weight loss of 10.1% for curve 1 which is lower than the loss of 13.1% for curve 2, which is perhaps because Cu(acac)₂ begins to sublimate and was partly absorbed on NH₄Y. In the third step, zone III, at temperatures between 230 °C and 500 °C, the rate of weight loss for curve 1 is slower and lower than that for curve 2, so the weight loss of 13.7% with a broad peak at 356 °C is assigned to the mild decomposition of Cu(acac)₂. In the fourth step, zone IV, at temperatures above 500 °C, nearly no weight loss is observed. Over the total temperature range, the actual weight loss is 32.6% for curve 1 and 45.4% for curve 2. Moreover, endothermic and exothermic peaks on the DSC curve of the mixture are overlapping and the largest exothermic peak also shifts from 247 °C to 349 °C. This shows that a complicated interaction exists between Cu(acac)₂ and NH₄Y during heat treatment.

To obtain more detailed information during heat treatment of the mixture of Cu(acac)₂ and NH₄Y, TG-MS measurements were performed and the results are shown in Fig. 2. The signals at m/z = 17, 18 and 44 are assigned to the main volatile products NH₃, H₂O and CO₂ respectively, and the signals at m/z = 58, 60 and 100 are ascribed to the trace organic products acetone, propyl alcohol and acetylacetone respectively.⁴¹ During heat treatment of the mixture, water is released at 76 °C below the water desorption peak temperature (121 °C) of pure NH₄Y, which has been demonstrated from TG/DTG. With the increase of the heat treatment temperature, the release of NH₃ proceeds in two steps from the curve of m/z = 17. In the first step, NH₃ is released at 174 °C, simultaneously, a trace amount of organic products are also observed. Additionally, only above 200 °C, the NH₄Y zeolite decomposes and releases NH₃. So it was proven that the ion exchange between the Cu²⁺ of Cu(acac)₂ and NH₄⁺ of NH₄Y does occur at 174 °C. In the second step, at the temperature of 417 °C, the release of NH₃ comes from the residual NH₄⁺ decomposition of NH₄Y or the desorption of adsorbed NH₃ from the ion exchange. When the heat treatment temperature reaches about 360 °C, the organic products acetone, propyl alcohol and acetylacetone were observed in addition to CO₂ and H₂O.

Fig. 2 MS spectra of gaseous products of the physical mixture of NH₄Y and Cu(acac)₂.

According to the above analysis, during heat treatment of the mixture, (a) in temperature zone I, the zeolite-adsorbed water of NH₄Y is released; (b) in temperature zone II, Cu(acac)₂ sublimates and Cu(acac)₂ vapor is mostly adsorbed on the Y zeolite, and in the meantime, ion exchange between Cu(acac)₂ and NH₄Y occurs; (c) in temperature zone III, there is decomposition of adsorbed Cu(acac)₂, and residual NH₄⁺ decomposition of NH₄Y.

Structural properties of CuY catalysts

The XRD patterns of the samples are compared in Fig. 3. For the mixture, the diffraction peaks of Cu(acac)₂ and NH₄Y are simultaneously observed. However, the diffraction peaks of the precursor and the Cu/NH₄Y catalysts are similar to those of Y zeolite and no diffraction peaks of the corresponding Cu species are observed. It proves that Cu species are well dispersed on the surface of the Y zeolite during the preparation of the CuY catalyst. In addition, these results also indicate that the preparation of the CuY catalyst does not have significant effects on the framework structure of the zeolite, only the diffraction peak intensity is slightly weakened.

Fig. 3 XRD patterns of Cu(acac)₂ (a), NH₄Y (b), the mixture (c), the precursor (d), Cu/NH₄Y-300 (e), Cu/NH₄Y-400 (f) and Cu/NH₄Y-500 (g).

Owing to the ion-exchanged ions substituting the Na⁺ of NaY and undergoing rearrangement, there is alteration of the relative intensities of the (220) and (311) crystal planes of Y zeolite.⁴² As can be seen in Fig. 3, compared with NH₄Y, I₁₁₁ of the precursor decreases obviously, I₂₂₀ and I₃₁₁ also decrease slightly, but I₂₂₀ decreases more greatly than I₃₁₁. It illustrates that Cu²⁺ substitutes the NH₄⁺ of the NH₄Y zeolite through the ion exchange between the Cu²⁺ of Cu(acac)₂ and the NH₄⁺ of NH₄Y. I₁₁₁, I₂₂₀ and I₃₁₁ also decrease for the Cu/NH₄Y catalysts, yet compared with the precursor, I₁₁₁ and I₂₂₀ increase slightly. This difference indicates that adsorbed Cu(acac)₂ decomposes to copper oxide and copper ions migrate from supercages to small cages during activation, so the particles of copper species in the Cu/NH₄Y catalysts become smaller and more uniform, resulting in less impact on the zeolite structures.

Fig. 4 presents the TEM images of the Cu/NH₄Y-400 catalyst. Richter et al.²⁶ reported that the CuY samples suffered major modifications during TEM imaging. In order to suppress the influence of the electron beam, imaging was carried out immediately after the beam was focused. From Fig. 4(A), the faujasite structure of zeolite Y is unbroken, nanosized copper particles are well dispersed on the surface of zeolite Y, and the particle sizes are in a range of 2–6 nm. From Fig. 4(B), light grey Cu species of 1–2 nm are observed in the channel of zeolite Y. Meanwhile, some larger black Cu species also exist, caused by incident electron beam.

Fig. 4 TEM images of the Cu/NH₄Y-400 catalyst.

Analysis of copper valence and state

To determine the distribution of the reducible copper species on the CuY catalyst, H₂-TPR was carried out and the profiles are shown in Fig. 5. The H₂-TPR profile of the Cu/NaY-400 catalyst only appears as a sharp reduction peak at around 224 °C. However, the profiles of the Cu/NH₄Y and Cu/HY-400 catalysts exhibit more than one reduction peak from ambient temperature to 1000 °C. In order to investigate the content and location of the reducible copper species, the H₂-TPR profiles were studied by means of peak-differentiating analysis (dotted lines in Fig. 5).

Fig. 5 H₂-TPR profiles: Cu/NH₄Y-200 (a), Cu/NH₄Y-220 (b), Cu/NH₄Y-250 (c), Cu/NH₄Y-280 (d), Cu/HY-400 (e), Cu/NH₄Y-300 (f), Cu/NH₄Y-400 (g), Cu/NH₄Y-500 (h), Cu/NaY-400 (i).

The reduction of the reducible copper species on CuY has been discussed by many researchers and the following conclusions^{26,28–30,43–45} are accepted: (i) the reduction of exchanged Cu²⁺ to Cu⁺ occurs from 200 °C to 300 °C, and the reduction temperature depends highly on the location and in the order: supercages < sodalite cages < hexagonal prisms. (ii) The reduction of dispersed CuO to Cu⁰ occurs at around 250 °C. (iii) The exchanged Cu⁺ is reduced at distinctly higher temperatures. So for the H₂-TPR of the as-prepared CuY catalyst in Fig. 5, the peaks at 192 °C, 284 °C and 347 °C are attributed to the reduction of Cu²⁺ to Cu⁺ located in supercages, sodalite cages and hexagonal prisms of the Y zeolite, respectively. The peaks at 224 °C of the Cu/NaY-400 catalyst and at 245 °C of the Cu/NH₄Y and Cu/HY-400 catalysts belong to the reduction of CuO to Cu⁰ in one step. The reduction peaks above 700 °C are attributed to the reduction of Cu⁺ to Cu⁰. Thus it can be seen that only CuO exists on Cu/NaY-400, while there is ion-exchanged Cu²⁺ and Cu⁺ besides CuO on Cu/NH₄Y and Cu/HY-400. The hydrogen consumption and the relative content of reducible Cu species were calculated from the area of the hydrogen consumption peak and are listed in Table 1.

Table 1 Quantitative analysis of the H₂-TPR profiles of the catalysts

Catalyst	H2 consumption (cm^3 g^−1)								Cu mass (%)			Cu^+/Cusum (%)
	H2sum	Cu^2+ → Cu^+			CuO → Cu^0	Cu^+ → Cu^0			Cu^2+	Cu^+	CuO
		Sup.	Sod.	Hex.		Sup.	Sod.	Hex.
Cu/NaY-400	28.4	0	0	0	28.4	0	0	0	—	—	8.1	0
Cu/NH4Y-250	19.8	8.9	1.1	0	0	9.8	0	0	—	—	0	0
Cu/NH4Y-280	25.8	9.1	1.3	0	0.7	5.7	9.0	0	5.9	2.5	0.2	32.9
Cu/NH4Y-300	26.3	8.7	0.6	0	1.4	3.3	12.3	0	5.3	3.6	0.4	38.7
Cu/NH4Y-400	25.5	7.4	0.4	0	2.4	5.2	10.1	0	4.5	4.3	0.7	45.3
Cu/NH4Y-500	27.3	6.3	0.7	0	7.4	2.5	10.4	0	4.0	3.4	2.1	35.8
Cu/HY-400	26.7	3.2	3.1	2.1	7.0	2.5	4.6	4.2	4.8	1.7	2.0	20.0

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From Fig. 5, for the Cu/NH₄Y catalysts, until the activation temperature reaches 280 °C, the hydrogen consumption peaks of the CuO and Cu⁺ species appear on the H₂-TPR profile of Cu/NH₄Y-280. When the activation temperature is lower than 280 °C, the hydrogen consumption peaks of the CuO and Cu⁺ species are not observed on the corresponding H₂-TPR profile of Cu/NH₄Y, and there are some undetermined hydrogen consumption peaks from 300 °C to 700 °C. This suggests that the initial temperature of exchange for Cu²⁺ being reduced to the Cu⁺ active center is 280 °C. When the activation temperature is 400 °C, the CuY catalyst has the highest Cu⁺ content of 4.3 wt% (Table 1). Comparing Cu/NH₄Y-400 with Cu/HY-400, it is found that the content of CuO and Cu²⁺ of Cu/NH₄Y-400 are both lower than for Cu/HY-400, but the content of Cu⁺ in the supercages of Cu/NH₄Y-400 is more than double that of Cu/HY-400.

It’s worth mentioning that many works have shown that the exchanged copper ions on CuY were populated preferentially in small cages.^46,47 However, Cu(acac)₂ cannot enter into small cages because of steric hindrance, so up to 95% of the Cu²⁺ ions on the Cu/NH₄Y-400 catalyst located in the supercage are accessible to reactants. This is an advantage of preparing the CuY catalyst with high catalytic activity for the oxidation carbonylation of methanol to DMC.

XPS is a surface analysis technique, however, due to the large voids of zeolite Y, the sampling depth is higher in zeolites and the effective depth is about 5–10 zeolite unit cells below the surface.⁴⁸ Fig. 6 presents the Cu 2p_3/2 XPS spectra and the Cu LMM Auger spectra of Cu/NH₄Y. It was reported that the kinetic energies of the Cu LMM Auger electrons at 913.7 eV and 916.6 eV are ascribed to Cu⁺ and Cu²⁺,³² and the kinetic energy of Cu⁰ is in the range from 918 to 920 eV.²⁹ As can be seen in Fig. 6, the Cu LMM Auger spectra strongly supports the presence of Cu⁺ with a kinetic energy of 912.8 eV and Cu²⁺ with a kinetic energy of 915.9 eV, and there is no Cu⁰ peak in the range from 918 to 920 eV. The characteristics of divalent copper can be further confirmed by the Cu 2p_3/2 binding energy within the range of 933.0–936.0 eV and the shake-up satellite peak. Monovalent copper can be assigned to the binding energy of the XPS peaks ranging from 932.0 to 933.0 eV without any shake-up satellite peaks.^49,50 The XPS spectra of Cu 2p (Fig. 6) shows an overlapping Cu 2p_3/2 signal, and from curve-fitting of the Cu 2p_3/2 spectra, it can be fitted into two peaks at 933.4 eV attributed to Cu⁺ and 935.7 eV attributed to Cu²⁺ respectively. From the proportion of area percentages, the calculated Cu⁺ content percentages of the catalysts are 46.9%, 52.5%, and 39.1% respectively when the activation temperatures are 300 °C, 400 °C and 500 °C. The change trend coincides with that of the Cu⁺ content percentage which is shown by the H₂-TPR results (Table 1).

Fig. 6 Cu 2p XPS and Cu LMM Auger spectra of Cu/NH₄Y-300 (a), Cu/NH₄Y-400 (b), and Cu/NH₄Y-500 (c).

As indicated above, it is deduced that the main reactions are as follows: during heat treatment, firstly Cu(acac)₂ sublimates and is adsorbed onto the NH₄Y surface, and in the meantime the ion exchange between the Cu²⁺ of Cu(acac)₂ and the NH₄⁺ of NH₄Y to form Cu²⁺Y takes place.

At the activation stage, the adsorbed unreacted Cu(acac)₂ starts to decompose, and at the same time, NH₄Y decomposes to form HY and releases NH₃. Meanwhile ion exchanged Cu²⁺ is auto-reduced to form the Cu⁺ active centre for the oxidative carbonylation of methanol to DMC.

In addition, Lu et al.⁵¹ reported that CuY is the most active catalyst for NH₃ oxidation from 200 °C to 300 °C. Also, organic ligands and their products react with oxygen in the air and generate CO₂ and H₂O. So the following reactions are possible during the preparation of the CuY catalyst through HAI.

Catalytic properties of CuY catalysts

Catalytic properties of the as-prepared CuY catalysts in the direct gas phase oxidative carbonylation of methanol to DMC are listed in Table 2. For comparison, some CuY catalysts in the literature^24,28,32 are also listed in Table 2 for reference. The as-prepared Cu/NaY-400 catalyst has no catalytic activity, and the catalytic activity of the Cu/NH₄Y-400 catalyst with 153.0 mg g⁻¹ h⁻¹ of STY_DMC is better than that of the Cu/HY-400 catalyst with 65.4 mg g⁻¹ h⁻¹ of STY_DMC. The above analysis of the results has shown that the NH₄⁺ of NH₄Y can introduce Cu²⁺ into extraframework cation locations of zeolite Y through ion exchange between the Cu²⁺ of Cu(acac)₂ and the NH₄⁺ of NH₄Y, and the decomposed NH₃ from NH₄Y facilitates the reduction of Cu²⁺ to Cu⁺ to form more active centres. Compared with the Cu/HY catalyst, more copper ions on the Cu/NH₄Y catalyst located in the supercages are accessible to the reactants. However, the Na⁺ of NaY can’t exchange with the Cu²⁺ of Cu(acac)₂ under the same experimental conditions, and there was no Cu⁺ active center on the Cu/NaY-400 catalyst, only CuO species.

Table 2 Catalytic activity of the catalysts in the oxidative carbonylation of methanol^a

Catalysts	Preparation	Support/activation temperature (°C)	Cu (wt%)	STYDMC (mg g^−1 h^−1)	XCH3OH (%)	SDMC (%)	Ref.
a Reaction conditions: VCH3OH : VCO : VO2 = 4 : 10 : 1, GHSV = 2520 h^−1, T = 140 °C, and P = 0.1 MPa.b Reaction conditions: VCH3OH : VCO : VO2 = 6 : 8 : 1, 1% Ar (He balance), GHSV = 3000 h^−1, T = 140 °C, and P = 0.1 MPa. The Cu/NH4Y(D)-400 catalyst prepared through DP had an NH4Y support and was activated at 400 °C. The naming rules are used to name the other catalysts from the literature.c The actual Cu loading was determined using atomic absorption spectrometry.d The theoretical Cu loading was calculated through copper addition.
Cu/NaY-400	HAI	NaY/400	8.2^c	0	0	0
Cu/NH4Y-300	HAI	NH4Y/300	9.5^c	137.2	7.5	61.2
Cu/NH4Y-400	HAI	NH4Y/400	9.5^c	153.0	8.2	61.9
Cu/NH4Y-500	HAI	NH4Y/500	9.5^c	113.3	6.2	60.9
Cu/HY-400	HAI	HY/400	8.5^c	65.4	3.5	60.7
Cu/NaY(L)-600	LSIE	NaY/600	6.3^c	131.4	6.5	68.5	28
Cu/HY(I)-600	IWIM	HY/600	10.0^d	95.6	5.1	62.0	28
Cu/NaY(I)-600	IWIM	NaY/600	10.0^d	16.3	1.1	50.7	28
Cu/HY(S)-650	SSIE	HY/650	12.2^c	97.3	4.4	74.6	24
Cu/NH4Y(D)-400^b	DP	NH4Y/400	10.0^d	0	0	0	32
Cu/NH4Y(D)-750^b	DP	NH4Y/750	10.0^d	84.4	8.1	53.5	32

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As for Cu/NH₄Y catalysts with different activation temperatures (300 °C, 400 °C, 500 °C), when the activation temperature was 300 °C, the conversion of methanol (X_CH3OH) and the space-time yield of DMC (STY_DMC) were 7.5% and 137.2 mg g⁻¹ h⁻¹, respectively, for Cu/NH₄Y-300. However, the Cu/NH₄Y-400 catalyst activated at 400 °C exhibited an excellent catalytic activity with 8.2% X_CH3OH and 153.0 mg g⁻¹ h⁻¹ STY_DMC. When the activation temperature was 500 °C, X_CH3OH and STY_DMC decreased to 6.2% and 113.3 mg g⁻¹ h⁻¹, respectively, for Cu/NH₄Y-500. The H₂-TPR and XPS characterisation have shown that the Cu/NH₄Y-400 catalyst has the highest Cu⁺ active center content.

In addition, it was also found that the catalytic activity of the Cu/NH₄Y-400 catalyst is better than those of the chloride-free CuY catalysts prepared through LSIE,²⁸ IWIM²⁸ or DP³² and the Cu/HY(S)-650 catalyst prepared through SSIE.²⁴ For example, the STY_DMC of the Cu/NH₄Y-400 catalyst with a 9.5 wt% Cu loading reaches 153.0 mg g⁻¹ h⁻¹, which is higher than those of the CuY catalysts with a 10.0 wt% Cu loading prepared through IWIM,²⁸ including the Cu/HY(I)-600 catalyst with an STY_DMC of 95.6 mg g⁻¹ h⁻¹ and the Cu/NaY(I)-600 catalyst with an STY_DMC of 16.3 mg g⁻¹ h⁻¹. Even the STY_DMC of the Cu/NH₄Y-400 catalyst is higher (57.2%) than that of the Cu/HY(S)-650 catalyst with a 12.2 wt% Cu loading prepared through SSIE.²⁴ The Cu/NH₄Y(D)-400 catalyst has no catalytic activity, and the Cu/NH₄Y(D)-750 catalyst has an STY_DMC of 84.4 mg g⁻¹ h⁻¹ which is also lower (44.8%) than that of the Cu/NH₄Y-400 catalyst. For the Cu/NaY(L)-600 catalyst with an STY_DMC of 131.4 mg g⁻¹ h⁻¹ prepared through LSIE,²⁸ the 6.3 wt% Cu loading is the maximum permissible loading for LSIE, and the STY_DMC is lower (14.4%) than that of the Cu/NH₄Y-400 catalyst.

According to previous studies,²³ the proposed mechanism for the oxidative carbonylation of methanol to dimethyl carbonate can be concluded as follows. Firstly, CH₃OH with oxygen adsorbs on Cu⁺ Lewis sites through formation of either mono- or di-methoxide species bound to the Cu⁺ cations. Then, adsorption and insertion of carbon monoxide into the methoxide species leads to the formation of DMC directly or monomethyl carbonate species (MMC) as an intermediate. Last, the MMC species react with methanol to form DMC. The reaction of carbon monoxide into methoxide is the rate-limiting step. The Cu⁺ of CuY has been demonstrated to improve the adsorption energy of co-adsorbed CO in the co-adsorbed CO/CH₃O system, and stabilizes the transition state for the CO insertion reaction to produce monomethyl carbonate (MMC) species.^52–54 In our experiments, the content of Cu⁺ attained the highest value for the Cu/NH₄Y-400 catalyst, which exhibits the best catalytic performance. Besides, Engeldinger et al.^27,31 also conclude that the formation of DMC is initiated by the reaction of molecularly adsorbed methanol on Cu⁺ cations. More significantly, they found that CuO_x agglomerates in the supercages favor the oxidation and oxocarbonylation reactions of methanol and enhances the formation of DMC. According to our studies, the Cu/NaY catalyst, which only contains CuO species, has no catalytic activity. When the Cu loading is too high, excessive amounts of CuO_x agglomerates cover the Cu⁺ active centers, resulting in a decrease in the catalytic activity of the CuY catalyst.³⁷ Therefore, we suggest that the Cu⁺ ions on the CuY catalyst are the active centers for the oxidation carbonylation of methanol to DMC and a modest amount of CuO_x agglomerates contribute lattice oxygen to the formation of DMC and improve the catalytic performance.

Conclusions

During heat treatment of a physical mixture of Cu(acac)₂ and NH₄Y, from 120 °C to 230 °C, Cu(acac)₂ sublimates and Cu(acac)₂ vapor is mostly adsorbed and well dispersed on the surface of Y zeolite. In the meantime, ion exchange between Cu(acac)₂ and NH₄Y occurs in the supercages. Above 230 °C, unreacted Cu(acac)₂ decomposes to form copper oxides, and simultaneously NH₄Y decomposes and releases NH₃, which facilitates the reduction of Cu²⁺ to Cu⁺. Above 500 °C, no physical or chemical processes occur.

For the NaY zeolite, no solid state ion-exchange occurs between Cu(acac)₂ and NaY during heat treatment, and the as-prepared Cu/NaY catalyst has no catalytic activity. For the HY zeolite, there is less ion-exchanged Cu⁺ in the supercages and the as-prepared Cu/HY catalyst exhibits lower activity than the Cu/NH₄Y catalyst. The distribution of Cu²⁺, Cu⁺ and CuO on the Cu/NH₄Y catalyst depends highly on the activation temperature. When the activation temperature is 400 °C, the content of Cu⁺ attains the highest value, the corresponding Cu/NH₄Y-400 catalyst also shows the best catalytic activity, and the conversion of methanol, selectivity of DMC and the DMC space-time yields are 8.2%, 61.9% and 153.0 mg g⁻¹ h⁻¹ respectively.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (21276169).

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