Insight into the Tunable CuY Catalyst for Diethyl Carbonate by

Mar 6, 2014 - Three different methods were used to prepare CuY catalysts, which play an important role in Cu loading and ion-exchange level during the...
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Insight into the Tunable CuY Catalyst for Diethyl Carbonate by Oxycarbonylation: Preparation Methods and Precursors Shouying Huang, JiJie Zhang, Yue Wang, Pengzhen Chen, Shengping Wang, and Xinbin Ma* Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: Three different methods were used to prepare CuY catalysts, which play an important role in Cu loading and ionexchange level during the oxidative carbonylation of ethanol to synthesize diethyl carbonate. Of the prepared CuY catalysts, those synthesized by the ammonia evaporation method exhibited a significantly enhanced activity compared to those obtained by the other two methods. In addition, under optimized conditions, four different copper precursors were applied to adjust the textural properties and chemical states of the CuY catalysts. To obtain a deep understanding of their structure−performance relationship, XRD, XPS, CO adsorption, DRIFTS, and NH3 TPD were conducted to characterize the CuY catalysts. The experimental results indicated that the catalytic performances were in line with the proportions of Cu+ in CuY catalysts, which can be regulated by cupric precursors. In addition, the textural structures of the catalysts and the acidity and type of Cu+ species influenced by the precursors were all responsible for the activity and product distribution.

1. INTRODUCTION Dialkyl carbonates, as ecofriendly chemicals, are gaining popularity because of burgeoning applications in organic synthesis, solvents, and electrolytes of lithium batteries due to their unique physicochemical properties, low toxicity, and low bioaccumulation. Recently, oxygen-containing fuel additives have been used in engines because they can decrease emissions of particulate matter (PM) and suppress the human respiratory problems caused by particles with sizes of less than 10 and 2.5 μm (PM10 and PM2.5, respectively).1 Dimethyl carbonate (DMC) and diethyl carbonate (DEC) have the simplest structures and most extensive utilization among dialkyl carbonates. In comparison with dimethyl carbonate and ethanol, DEC has a lower volatility, more favorable fuel/ water partitioning coefficient, more benign hydrolysis products, and similar blending octane properties, all of which make DEC a preferable alternative to methyl tert-butyl ether (MTBE). Despite the high yield of DEC obtained by the phosgenation method, this synthesis approach suffers from the main drawbacks of using toxic and hazardous chemicals such as phosgene. Considering the prohibitions and restrictions on phosgene use, the direct synthesis of dialkyl carbonates offers a green alternative for the production polycarbonates and other processes.2 Oxidative carbonylation in the vapor phase rather than in the liquid phase is a promising alternative. CuCl2 and CuCl2−PdCl2 supported on active carbon or mesoporous silica catalysts exhibit excellent initial catalytic performances. However, these catalyst systems still have shortcomings of deactivation and reactor corrosion due to the loss of chlorine.3−7 King first reported that Cu-exchanged zeolite exhibits good performance in the oxidative carbonylation of methanol with little deactivation, suggesting that the presence of chlorine is unnecessary in this reaction system.8,9 Since then, Cu-doped zeolites have been proposed as a series of potential catalysts by several research groups.9−15 Li et al. © 2014 American Chemical Society

described the interaction between CuCl and HY zeolite under an inert atmosphere.10 Anderson and Root reported that the better activity of CuX compared to CuZSM-5 can be attributed to the weaker adsorption of CO.16 Bell and co-workers investigated the effects of zeolite structure and composition on catalytic performance.12,14 Previously, our group established a quantitative relationship between the partial Brønsted acidity of zeolites and their catalytic performance in oxidative carbonylation.17 Although residual chlorine is very limited in Cuexchanged zeolite prepared by solid-state ion exchange (SSIE), the use of chloride as CuCl and CuCl2 still causes corrosion problems. Therefore, chloride-free Cu-doped zeolites have recently been investigated. Richter et al. prepared CuY catalyst from NaY with Cu(II) salts, completely avoiding the use of chloride, which allowed a high selectivity and yield of DMC.18,19 Dang et al. introduced Cu into the silicoaluminophosphate structure SAPO-37 by SSIE using copper(II) acetylacetonate and observed a catalytic behavior comparable to those of CuY zeolite catalysts.20 In Cu-doped zeolites, the nature of the Cu species is a crucial factor affecting catalytic activity and stability. King reported that Cu(I) can participate in the redox cycle through the formation of an adsorbed (CH3−Cu)+ species, whereas the stable Cu2+ can resist reduction. 9 Drake et al. explored the local environment of the Cu+ ion in CuY prepared by SSIE and the relationship between the cation location and catalytic performance.12 Unlike Cu zeolites prepared by SSIE, which contain only well-defined isolated Cu+ species, the heterogeneity of copper species in catalysts prepared by conventional ion exchange using aqueous solutions of cupric ions makes it Received: Revised: Accepted: Published: 5838

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2.2. Catalyst Characterization. X-ray diffraction patterns were obtained using Cu Kα radiation (γ = 1.54056 Å) on a Rigaku C/max-2500 diffractometer at 40 kV and 200 mA. Specific surface areas and pore volumes of the samples were determined by nitrogen adsorption at 77 K on a Micromeritics ASAP-2020 apparatus. Prior to analysis, all of the samples were outgassed at 573 K for 24 h. Surface areas were determined by the Brunauer−Emmett−Teller (BET) method. In situ DRIFTS of CO adsorption was carried out with a Thermo Nicolet 6700 FTIR spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector and a KBr beam splitter. The experiments were performed using a high-temperature vacuum chamber with ZnSe windows connected to a gas-dosing and evacuation system. Before adsorption experiments, all samples were placed in the holder and pretreated at 673 K for 60 min under He (18 mL/min) to remove impurities from the sample surface and then cooled to 413 K (the optimized reaction temperature). Background spectra were collected before the introduction of CO. Then, CO was flowed over the samples for 30 min until saturation, after which the chamber was purged with He gas for 30 min at the same temperature. Spectra were recorded at a resolution of 4 cm−1 by collecting 32 scans for a single spectrum. The O−H stretching region of the IR spectra of HY and CuY were also recorded using the MCT detector. In this case, 15-mg samples were pressed into 13-mm-diameter self-supporting pellets under 15 MPa and then placed in an in situ cell with CaF2 windows. Before measurements, the samples were pretreated in a vacuum at 673 K for 1 h and then cooled to room temperature. Spectra were obtained using 32 scans with a resolution of 4 cm−1 in the range of 4000−1100 cm−1. As a reference, NaY was transformed to HY according to the procedure in our previous work.17 The NH4 form of Y zeolite was obtained by exchanging NaY with a 0.5 M NH4NO3 solution twice at 333 K and drying at 393 K for 4 h under a vacuum. The resulting NH4Y was calcined at 773 K for 3 h in air at 2 K/min. The copper loadings in the samples were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (VISTA-MPX, Varian). Before measurements, the samples were digested in HCl and HF aqueous solution. H3BO3 was used for complexation of HF. The same concentration of HCl, HF, and H3BO3 solution was used for blank experiments. Irreversible CO adsorption isotherms were obtained to quantify the ratio of Cu+. The isotherms were collected with the chemisorption module of a Micromeritics ASAP 2020 instrument. Fresh catalysts (∼150 mg) were first degassed, and a total adsorption isotherm was collected between 50 and 500 mmHg CO. After evacuation for 30 min, the measurement was repeated to determine the reversible adsorption isotherm. The difference between the total and reversible adsorption curves was used to determine the irreversible isotherm. The irreversible adsorption was determined as the asymptotic value obtained by averaging the irreversible isotherm points between 150 and 500 mmHg CO. The contents of Cu+ were calculated assuming that one molecule of CO was adsorbed on one Cu+ ion. The surface properties of activated CuY-AE catalysts were determined by X-ray photoelectron spectroscopy (XPS) on a Perkin-Elmer PHI 1600 ESCA instrument equipped with a monochromatic Mg Kα source (hν = 1253.6 eV). The binding

difficult to elucidate the structural and catalytic properties of Cu species. In this work, we have investigated the effects of the preparation method on the physicochemical properties and catalytic performance of CuY catalysts in the oxidative carbonylation of ethanol. The ammonia evaporation (AE) method was utilized to improve the exchange level and structural model of Cu cations in CuY for the oxycarbonylation reaction. Four different salts (copper nitrate, chloride, acetate, and sulfate) were also used during the ammonia evaporation method to adjust the textural properties and chemical states of the CuY catalysts. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO adsorption, diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and NH3 temperature-programmed desorption (TPD) were used to characterize the CuY catalysts for a deeper understanding of their structure−performance relationship.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. 2.1.1. Different Methods of Preparing CuY Catalysts. CuY-I was prepared by ion exchange in Cu(NO3)2 solution. Commercial zeolite NaY (SiO2/Al2O3 molar ratio of 5, Nankai Catalyst, Tianjin, China) was added to a Cu(NO3)2 solution. The slurry was stirred vigorously for 5 h at room temperature and then filtered, washed, and dried at 353 K under a vacuum overnight. CuY-AI was prepared as follows: First, a desired amount of Cu(NO3)2 was dissolved in 100 mL of deionized water, and 25 wt % aqueous ammonia solution (Sinopharm Chemical Reagent Co.) was added dropwise to adjust the pH to 11 under stirring. Next, 9 g of commercial NaY was added to the copper ammonia complex solution, and the mixture was stirred vigorously for 5 h at room temperature. Afterward, the obtained precipitates were filtered, washed with deionized water and ethanol sequentially, and dried at 353 K under a vacuum overnight. Compared to the synthesis of CuY-AI, an ammonia evaporation step was added to the synthesis of CuY-AE(N). After the slurry had been stirred for 5 h, subsequent heating to 353 K in a water bath allowed for the evaporation of ammonia, which caused a color change from blue to brown. When the pH value of the suspension decreased to 6−7, the ammonia evaporation process was terminated. The subsequent procedure of washing, filtering, and drying was the same as for CuY-AI. 2.1.2. CuY Catalysts Prepared from Different Copper Precursors. To investigate the effects of precursors, four CuYAE catalysts were prepared from different copper salts [Cu(NO3)2, CuCl2, CuSO4, Cu(OAc)2, Sinopharm Chemical Reagent Co.] by the ammonia evaporation (AE) method. The loading in terms of metallic copper was nominally set as 10 wt %. The procedure was the same as mentioned above. All of the solids resulting from the above three methods are denoted as as-synthesized catalysts. 2.1.3. Catalyst Activation. Each as-synthesized catalyst was placed in a tube furnace and exposed to N2 (99.999%) flowing at 60 mL/min. The temperature was ramped at 2 K/min to 773 K, held for 4 h, and then decreased slowly to room temperature. All activated CuY-AE catalysts were obtained and stored in a drybox. Before catalytic measurements for each run, the catalyst was pressed and sieved to 20−40 mesh. The CuY-AE catalysts obtained using Cu(NO3)2, CuCl2, CuSO4, and Cu(OAc)2 as precursors are labeled CuY-AE(N), CuYAE(C), CuY-AE(S), and CuY-AE(A), respectively. 5839

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CuY catalyst. Compared to CuY catalysts obtained by the other two methods, CuY-AE(N) showed the highest STY of DEC due to the improved ethanol conversion. Hence, for the same Cu loading, preparation methods showed a significant influence on the activities of CuY catalysts in DEC synthesis through oxidative carbonylation. 3.1.2. Effect on Cu Loading by Different Methods. The preparation method of Cu-doped zeolites influences the ionexchange level of Cu ions directly, which had an effect on their physicochemical properties and corresponding catalytic performance. The contents of Cu in the CuY samples were measured by ICP-OES and are summarized in Table 2.

energies were normalized to the C 1s peak at 284.6 eV. The spectra were obtained after ion-beam sputtering at 25 mA for 1 min. After activation ex situ, the catalysts were cooled to room temperature under N2. Thereafter, the catalysts were transferred into the XPS sample holder using a glovebox and then into the XPS chamber without exposure to air. For acidity evaluations, NH3 TPD measurements were performed on a Micromeritics Autochem II 2910 instrument equipped with a thermal conductivity detector (TCD). The sample (∼50 mg) was pretreated at 473 K for 1 h in a flow of Ar and then subjected to NH3 adsorption until saturation at room temperature. Desorbed NH3 was monitored at a heating rate of 10 K/min from room temperature to 823 K. 2.3. Catalytic Measurements. Measurements of catalyst activity and selectivity were performed using 1 mL of catalyst loaded into an 8-mm-i.d. quartz tubular microreactor system controlled by a computer (WFS-3015). The reaction pressure was controlled and maintained at 0.7 MPa by a back-pressure regulator (26-1700 series, Tescom). Before being exposed to reactants, the catalyst was heated to 413 K at 3 K/min in a stream of high-purity N2. Ethanol was controlled by a syringe pump (Elite, P230) and mixed with the flow of CO and O2, which were controlled by mass flow meters. The gas mixture was fed into the catalyst bed through an evaporator. The feed consisted of ethanol/CO/N2/O2 at a molar ratio of 2.6/10/ 6.5/1 and a total flow rate of 80.5 mL/min. The temperature of the catalyst during the reaction was set to 413 K and measured with a thermocouple placed inside the inner quartz tube. The reaction products were analyzed online using a gas chromatograph (7890A GC, Agilent Technologies), equipped with DB-624 capillary column connected to a flame ionization detector (FID) and two Porapack Q and one molecular sieve 13X columns connected to a thermal conductivity detector (TCD). Organic compounds (ethanol, DEC, acetal, and acetaldehyde) were detected with the FID, whereas gases (CO, O2, CO2) were detected with the TCD. The ethanol conversion and DEC yield were calculated as described previously.17

Table 2. Peak Areas in the OH Stretching Region of CuY and Calculated Levels of Ion Exchange sample

peak area

level of ion exchange

content of Cu element (wt %)

HY CuY-I CuY-AI CuY-AE(N)

152.7 139.4 86.4 11.0

− − 43.4 92.8

− 5.3 7.3 11.0

Obviously, the Cu loading was very low when Cu(NO3)2 was utilized as the precursor. After the addition of ammonium hydroxide to Cu(NO3)2 solution, the Cu loading on the zeolite was improved appreciably. In comparison, the content of Cu in CuY prepared by the ammonia evaporation method was high at up to 10%, which was equal to the amount added. In the preparation processes of CuY-AI and CuY-AE, a large excess of NH4+ ions exchanged with the Na+ ions of the starting NaY and were converted to Brønsted acid sites by the removal of NH3 in the subsequent calcination step. Therefore, quantitative information about the ion-exchange levels and acidities of the two samples can be obtained by in situ IR spectroscopy in the range of the OH vibration. Figure 1

3. RESULTS AND DISCUSSION 3.1. Effects of Preparation Methods. 3.1.1. Catalytic Activity. Table 1 shows the reports the distributions and space Table 1. Catalytic Performances of CuY Samples Prepared by Different Methods method

SDECa (%)

SACEa (%)

SDEEa (%)

STYDEC(mg·g−1·h−1)

CuY-I CuY-AI CuY-AE(N)

10.4 45.5 41.1

89.6 53.9 58.9

0 0.6 0

1.3 15.9 36.8

a

ACE, acetaldehyde; DEC, diethyl carbonate; DEE, 1,1-diethoxyethane; S, selectivity; STY, space time yield. Figure 1. FTIR spectra in the OH stretching region of CuY catalysts prepared by different methods.

time yields (STYs) of DEC over CuY samples prepared by different methods under the same reaction conditions. The CuY-I, which was synthesized through conventional ion exchange in a Cu(NO3)2 solution, exhibited a rather low activity in the oxidative carbonylation of ethanol (STYDEC, 1.3 mg·g−1·h−1). The product was mainly acetaldehyde, whereas 1,1-diethoxyethane (DEE) was not observed. Upon the addition of ammonia to the Cu(NO3)2 solution, both the STY and selectivity of DEC were facilitated. Furthermore, the ammonia evaporation (AE) method was first used to prepare

compares the normalized infrared spectra for the hydroxyl stretching region of HY, CuY-AI, and CuY-AE. HY has three OH stretching vibration bands at 3550, 3640, and 3740 cm−1. The first two bands are assigned to the Brønsted acid located in the sodalite cage and supercage, and the last is attributed to the terminal silanol groups in zeolite Y.21 After preparation by different methods, the intensities of these three bands for the CuY catalysts changed. As a reference, NaY was transformed to 5840

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Table 3. Textural Properties and Catalytic Performances of CuY-AE Samples Prepared from Different Precursors before and after Activation catalyst CuY-AE(N) CuY-AE(C) CuY-AE(S) CuY-AE(A) NaY

SDECa,b (%)

SACEa,b (%)

SDEEa,b (%)

STYDECa (mg·g−1·h−1)

total Cu content (wt %)

specific surface area (m2/g)

t-plot micropore volume (cm3/g)

41.1 56.7 42.4 35.9 −

58.9 41.3 54.0 64.1 −

0 (0) 2.0 (0) 3.6 (0) 0 (0) −

36.8 46.1 27.1 15.8 −

11.0 10.1 9.3 9.4 −

654.1 585.9 594.8 551.3 697.3

0.28 0.30 0.24 0.23 0.31

(0) (0) (0) (0)

(100) (100) (100) (100)

a

ACE, acetaldehyde; DEC, diethyl carbonate; DEE, 1,1-diethoxyethane; S, selectivity; STY, space time yield. bValues in parentheses present the results on CuY-AE catalysts without activation.

Figure 2. XRD patterns of CuY-AE catalysts synthesized from various precursors (a) before and (b) after inert thermal activation.

Cu ions migrated into the zeolite channels from the outer surface areas and eventually fixed on the zeolite framework,19 whereas CuO might partially reduce to CuOx species, both of which supplied more active sites for oxidative carbonylation. For the same content of Cu, different precursors were used to control the nature of Cu species and textural properties of CuYAE catalyst, offering a deeper understanding of the structure− performance relationship of CuY catalyst in the oxidative carbonylation of ethanol. 3.2. Effects of Precursors in AE Method. 3.2.1. Catalytic Performance. Different copper precursors were used for the preparation of CuY-AE catalysts by the AE method to clarify the effects of copper precursors on the catalytic performance of DEC synthesis. Table 3 reports the activityies and selectivities of oxidative carbonylation of ethanol over CuY-AE catalysts. The contents of Cu element in CuY-AE catalysts derived from different precursors were measured by ICP-OES and are listed in the last column of Table 3. The Cu loading was close to the designed value, indicating that the AE method prevents the loss of Cu element during preparation process. The values in parentheses represent the results for CuY-AE catalysts without activation. For unactivated CuY-AE catalysts, acetaldehyde was the only product. In contrast, the target product DEC formed after activation at high temperature in an inert gas flow. Acetaldehyde was detected as the main byproduct, although small amounts of acetal were also formed over CuY-AE(C) and CuY-AE(S). As shown in Table 3, differences in terms of catalytic activity can be observed, giving the following sequence of DEC production: CuY-AE(C) > CuY-AE(N) > CuY-AE(S) > CuY-AE(A). Under the same reaction conditions, CuY showed the best selectivity and yield of DEC when CuCl2 was used as the precursor, higher than the values for its Cu(NO3)2

proton-type zeolite by ion exchange in NH4NO3 solution followed by calcination according to the procedure in our previous work.17 The reduction of the total area of these three peaks reflects the exchange level between copper ions and protons on the zeolites. The quantitative information calculated from Figure 1 is listed in Table 2. It is noteworthy that ammonia evaporation promoted almost complete ion exchange in comparison with CuY-AI. The difference of Cu loading and ion-exchange levels was in line with the variation trend of activity over CuY catalysts for DEC synthesis. Because it was difficult to obtain a high copper exchange level using only one cycle of exchange at ambient temperature,22 several repeated ion-exchange processes must be necessary. Iwamoto and co-workers reported that overexchanged Cu zeolites can be obtained through ion exchange of Cu2+ in basic ammonium hydroxide (NH4OH) solution.23 Schreier et al. proposed that another mechanism of interaction between Cu ammine complexes and zeolites, namely, electrostatic adsorption at silanol groups, occurs at a pH of about 10−11, in addition to ion exchange at the Al exchange sites.24 Both of these reactions are pH-dependent and responsible for the formation of overexchanged metal zeolites, especially the former. This provides a rational explanation for the appreciable improvement in Cu loading for CuY-AI. For CuY-AE(N), the solution pH decreased as the ammonia evaporated during catalyst preparation. At lower pH, a copper ammine complex transformed to one or more copper hydrates, such as Cu(OH)+, Cu2(OH)22+, which took part in ion exchange.25 It was demonstrated that the formation and electrostatic deposition of the cationic Cu2(OH)22+ complex results in a higher surface density of Cu in zeolites.24 In addition, Cu(OH)2 precipitate was formed and further converted into dispersed CuO with decreasing pH. During the heating post-treatment, 5841

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date, two mechanisms have generally been accepted, as proposed by Sárkány et al.31 and Larsen et al.:28 One pathway invokes the existence of copper pairs and the loss of both oxygen and water (Supporting Information, eqs 2−4), whereas the other is based exclusively on water elimination (Supporting Information, eqs 5 and 6). Because the activation with temperature employed herein was similar to that used in these references, we deduced that the appearance of Cu+ anchored to the framework followed these two mechanisms. Among the samples, Cu0 diffraction peaks were much less intense in CuY-AE(C) compared to the others, indicating the low content or high dispersion of the metallic Cu phase. Chloride precursors interact with hydroxyl groups on the surface of solid acid supports, hindering the coalescence of Cu particles.32 Therefore, we inferred that weaker interactions of Cu species with surface hydroxyls in other precursors would result in easing of the reduction and aggregation of Cu species, leading to the lower selectivity of DEC. On the other hand, the size of anions influences the incorporation of metal ions into supports through shielding effects.33 In this work, the sizes of the anions follow the order Cl− < NO3− < SO42− < OAc−. Considering steric hindrance and anion size during the ionexchange process, the larger anions such as OAc− would cause greater shielding effects, leading to much harder incorporation of Cu2+ at the exchange sites. This mechanism can also be used to explain the changes in micropore volume occurring after the introduction of different Cu precursors into the parent NaY samples. XPS was performed to analyze the valence states of Cu species on the surfaces of the samples. All CuY-AE samples showed only two peaks centered at 932.7 and 952.8 eV, which were assigned to Cu 2p3/2 and Cu 2p1/2 peaks, respectively. Together, the absence of Cu 2p shakeup satellite peaks at 942− 944 eV in the XPS data (Figure 3) and the appearance of two

and CuSO4 counterparts. CuY-AE(A) exhibited the lowest conversion of ethanol and selectivity of DEC. 3.2.2. Textural Properties. N2 adsorption−desorption was performed to determine the textural properties of CuY-AE prepared from different precursors. All of the CuY-AE catalysts exhibited typical type-I profiles, which are characteristic for microporous structures. Table 3 summarizes the BET surface areas and micropore volumes obtained by the t-plot method. In comparison with the parent NaY sample, a moderate loss of surface area and pore volume was observed after precipitation of the copper salt. Among these catalysts, CuY-AE(A) and CuY-AE(S) showed relatively obvious reductions in micropore volume, indicating that the introduction of Cu phases blocked part of the micropores in zeolite Y. Yin et al. attributed the lower BET surface area to the presence of OAc− when using Cu(OAc)2 as the precursor, although no detailed explanation was given.26 In comparison, CuY-AE(N) and CuY-AE(C) exhibited larger pore volumes. Nevertheless, the data from physical adsorption confirmed that no extensive deterioration of the zeolite structure or pore blocking occurred during catalyst preparation by the AE method and following activation under high temperature. 3.2.3. Nature and Local Environment of Cu Species. XRD was employed to acquire information about the Cu species on CuY catalysts synthesized from various precursors. To clarify the changes in the catalysts occurring during the activation process under an inert gas, the XRD patterns of CuY-AE both before and after activation are shown in panels a and b, respectively, of Figure 2. Before activation, the CuY-AE samples exhibited the characteristic diffraction peaks of Y zeolite, all of which maintained the crystallinity well. The dark diamonds indicate peaks due to metallic Cu, whereas the blue triangles show peaks attributable to CuO. The samples contained both crystalline metallic Cu and copper oxides, suggesting the occurrence of the precipitation and reduction of Cu2+ during ammonia evaporation, where the color of the slurry changed from blue to gray and then brown (Supporting Information, eq 1). The presence of CuOx might be due to the reduction of CuO by NH3 resulting from the thermal decomposition of NH4+ ions of the zeolite, as proposed by Richter et al.19 On the other hand, isolated Cu2+ ions bonding to oxygen atoms by ion exchange might also exist in the samples but be undetectable by XRD because of their high dispersion. After thermal activation in a N2 flow, changes in intensity of the diffraction peaks, especially for the first diffraction peak (2θ = 6.18°) corresponding to the (110) crystal plane, revealed a slight deterioration of zeolite crystallinity at high temperature. At the same time, the diffraction intensity of the (311) plane (2θ = 15.6°) increased in each CuY-AE sample. Changes in these two peaks were observed that were ascribed to the interaction between these modified metals and Y zeolite.27 As illustrated in Figure 2, we concluded that a strong interaction between Cu and Y zeolite would exist in CuY-AE(C) before heat treatment. On the contrary, only CuY-AE(A) maintained the diffraction intensity of the (110) and (311) planes after activation treatment, indicating a weaker interaction than in the other samples. In comparison with the XRD patterns of assynthesized CuY samples, the disappearance of the CuO phase paralleled the enhancement in metallic Cu, indicating that calcination initiates the reduction and agglomeration of finely dispersed Cu species. The autoreduction of Cu2+ to Cu+ on solid acid supports by thermal treatment under a vacuum or in a flow of inert gas has been discussed and accepted.28−30 To

Figure 3. Cu 2p XPS spectra of CuY-AE samples prepared from various precursors.

overlapping peaks at 335.0 and 338.2 eV in the Cu LMM X-rayexcited Auger spectroscopy (XAS) spectra of the CuY-AE catalysts (Figure 4) successfully confirm the complete transition from Cu2+ to Cu0 and Cu+ species during the inert thermal treatment.34 In other words, the presence of Cu2+ species on the surface of the catalysts can be excluded, as all of these species were autoreduced to Cu+ and Cu0. To obtain quantitative information on the chemical state distribution of Cu species, we calculated the Cu+/(Cu0 + Cu+) ratios from the Cu LMM XAS spectra. As listed in Table 4, the ratios of the 5842

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samples, we applied in situ IR spectroscopy using CO as a probe molecule to discriminate not only the chemical states but also the localization of Cu ions in the zeolite,35 which would affect their catalytic activity and stability. Because CO complexes with Cu2+ are stable only at very low temperature, adsorbed CO preferentially interacts with Cu+ cations.36 Figure 5 shows the CO adsorption behavior on CuY-AE samples

Figure 4. Cu LMM XAS spectra of CuY-AE samples prepared from various precursors.

Figure 5. CO adsorption IR spectra of CuY-AE samples prepared from different precursors.

Table 4. Surface Cu Components of CuY-AE Catalysts Based on Cu LMM Deconvolution and Irreversible CO Adsorption Isotherms

synthesized from various precursors. The bands in the range of 2140−2160 cm−1 remained after the samples had been purged with helium for 30 min, demonstrating the formation of strong bonds between Cu+ sites and CO. This indicates that the Cu+ ions were fixed on exchange sites of the zeolite. The intensities of these two bands reflect the contents of Cu+ in the CuY-AE catalysts. The order was in line with the trend of XPS and irreversible CO adsorption isotherms. In addition to the chemical states of the Cu species, the IR spectra also provided more specific information about the local environment of Cu+. As shown in Figure 5, two types of Cu+−CO adducts were observed, characterized by two overlapping bands at 2145 and 2160 cm−1, that formed on sites II* and II, respectively. Furthermore, a shoulder at 2110 cm−1 was also detected on these samples, especially CuY-AE(A), which was assigned to Cu(I) species in oxide-like environments.37 Based on the intensities of these three bands, Cu+ located on the zeolite framework existed in CuY-AE as the main species, compared to CuOx aggregates. Investigations of the the active components in Cu zeolites for the oxidative carbonylation of methanol have been reported in the literature. King found that Cu2O dispersed and stabilized on zeolite could catalyze the formation of DMC.9 When prepared by SSIE, isolated Cu+ ions located at exchange sites in Cu zeolites were recognized as active sites for oxidative carbonylation.11,12,38 Two possible pathways are proposed: (I) formation of monomethyl carbonate followed by insertion of CO and (II) insertion of CO into dimethoxide species. Recently, the participation of lattice oxygen of CuOx aggregates was demonstrated to promote the oxidation of methanol to methoxy, pointing to a Mars−van Krevelen mechanism.37,39 Therefore, monomethyl carbonate, as the reaction intermediate, was reported to have formed on both isolated Cu+ ions and CuOx species in the published works.11,12,37−39 Considering the similarity of the homologues, we considered that a small amount of partly reduced CuO species might promote the formation rate of DEC. However, a great deal of CuOx

Cu+/total Cua (%) sample CuYAE(N) CuYAE(C) CuYAE(S) CuYAE(A)

total Cu content (wt %)

calcined

uncalcined

Cu+/(Cu0 + Cu+)b (%)

11.0

56.7

0.286

73.0

10.1

86.0

0.028

86.6

9.3

48.9

0.170

63.0

9.4

39.7

0. 104

55.4

a Calculated from irreversible CO adsorption isotherms. bCalculated from Cu LMM XAS spectra.

catalysts varied in the order CuY-AE(C) > CuY-AE(N) > CuYAE(S) > CuY-AE(A), in accordance with the catalytic activities for oxidative carbonylation. Because XPS can obtain information about only valence state and chemical composition on the surface of catalysts, irreversible CO adsorption isotherms were used to confirm the valence-state distribution of Cu for supplemental corroboration of the data from XPS. Table 4 also includes the results from irreversible CO adsorption isotherms. Before inert thermal activation, the content of Cu+ was very low, indicating that only minimal reduction of Cu2+ occurred during the impregnation and ammonia evaporation procedures. However, the activation process led to an obvious change in the chemical states of Cu species. A significant proportion of Cu2+ was reduced to Cu+, and the order followed a trend similar to the XPS data, when we assumed that the content of Cu0 and Cu+ was equal to the total content of Cu element determined by ICP-OES. By combining the above two characterizations, we concluded that cuprous species were active centers for DEC formation. To obtain a deep understanding of Cu species on the CuY-AE 5843

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DEC synthesis. The activities decreased in the order CuYAE(C) > CuY-AE(N) > CuY-AE(S) > CuY-AE(A). During the ammonia evaporation process, Cu2+ precipitated and was partly reduced on the zeolite. Thermal treatment in an inert flow led to the autoreduction of Cu2+ and eventual fixation of Cu+ to the exchange sites of zeolite, which was essential to obtain notable catalyst activity. The dispersion and state of Cu species in the CuY-AE catalysts were found to be sensitive to the precursor, which was influenced by the strength of the interactions between the precursors and the supports. DEC formation increased monotonically with the ratio of monovalent Cu species in the CuY-AE samples obtained using various precursors. Among these samples, CuY-AE(C) and CuYAE(N), which had large surface areas and pore volumes and high contents of Cu+ located on the zeolite, provided better activities and selectivities for DEC synthesis. Moderate acid sites in CuY-AE(C) and CuY-AE(S) contributed to DEE formation. When Cu(OAc)2 was used as the copper precursor, excess CuOx aggregates existed in the CuY-AE catalyst that covered and decreased the active sites of the catalyst, finally leading to the lowest activity and selectivity.

aggregates would block the cages and channels of Y zeolite as evidenced by the N2 adsorption data. This phenomenon does not favor the accessibility of active sites and molecular transport to achieve good catalytic performance. On the other hand, based on the same Cu loading, large amounts of CuOx aggregates imply less Cu+ exchanged with Na+ ions, which has been confirmed as the main active sites for oxidative carbonylation. Therefore, excess CuOx aggregates in CuY-AE catalysts would result in an obvious decrease in catalytic activity for DEC production. 3.2.4. Acidity of CuY. The nature of the acidity in CuY-AE catalysts dominates the decomposition of dialkyl carbonate and the formation of byproducts and, hence, influences the selectivity of the target product.2 To check the nature of the acid sites influenced by different precursors, NH3 TPD measurements were performed. The normalized patterns are presented in Figure 6. An overlapping of two or more peaks

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ASSOCIATED CONTENT

S Supporting Information *

Equations of the precipitation and reduction of Cu2+ during ammonia evaporation and activation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Figure 6. NH3 TPD patterns of CuY-AE samples prepared from different precursors.

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was observed for each of the samples. Unlike the other catalysts prepared from inorganic precursors, the CuY-AE(A) sample exhibited the largest global acidity because of its lowest level of ion exchange, as confirmed by the CO adsorption IR and XPS measurements. In our previous study, we found that an increase in moderately acidic sites greatly improved the yield of DMM (dimethoxymethane), as well as the conversion of methanol, because of the enhanced condensation reaction of formaldehyde with methanol.40 Hence, it can be concluded that the shoulder peak at 500 K in the middle of the three NH3 desorption peaks presented on samples CuY-AE(C) and CuYAE(S) was responsible for DEE formation.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (20876112, 20936003), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (Grant 20090032110021), and the Program of Introducing Talents of Discipline to Universities (B06006) is gratefully acknowledged. REFERENCES

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4. CONCLUSIONS In this article, different methods were investigated to prepared CuY catalysts for the oxidative carbonylation of ethanol. An ammonia evaporation step was found to promote the Cu loading and ion-exchange level appreciably, which is consistent with an improvement in activity. To adjust the physicochemical properties of the CuY catalyst prepared by the ammonia evaporation method, different Cu precursors were used. As catalytic measurements and characterizations demonstrated, the different Cu precursors exerted profound effects on the textural properties, chemical states, and acidities of the CuY-AE catalysts, which influenced their catalytic performances for 5844

dx.doi.org/10.1021/ie500288g | Ind. Eng. Chem. Res. 2014, 53, 5838−5845

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dx.doi.org/10.1021/ie500288g | Ind. Eng. Chem. Res. 2014, 53, 5838−5845