Effect of Configuration Addition of Precursors on Structure and

Aug 3, 2017 - Ceramics Department, National Research Centre, El-Bohouth Street, 12622 Cairo, Egypt. ∥ PetroChina Petrochemical Research Institute, ...
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Effect of Configuration Addition of Precursors on Structure and Catalysis of Cu/SiO2 Catalysts Prepared by Ammonia Evaporation− Hydrothermal Method Zheng Chen,†,‡ Juan Zhang,† Mohamed Abbas,†,§ Yingying Xue,†,‡ Jiaqiang Sun,† Kefeng Liu,∥ and Jiangang Chen*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Ceramics Department, National Research Centre, El-Bohouth Street, 12622 Cairo, Egypt ∥ PetroChina Petrochemical Research Institute, Beijing 100195, China S Supporting Information *

ABSTRACT: The effect of a precursor’s configuration addition on the texture, composition, morphology, and catalytic performance of Cu/SiO2 catalysts, prepared by the ammonia evaporation−hydrothermal method (AE−H), was investigated. Different configuration additions of the precursor have significant impacts on the chrysocolla (Cu2Si2O5(OH)2) content, which determines the metal copper specific surface area due to the differing copper particle size and reducibility. The Lewis acid and basic sites of the reduced catalyst surface are linearly dependent on the copper specific surface area. In gas-phase hydrogenation of ethyl acetate to ethanol, the Lewis basic and acid sites have a synergistic effect on ethanol selectivity. The proper configuration addition of the precursors gave rise to the highest ethanol selectivity of 96% at reaction conditions of P = 2.40 MPa, T = 553 K, H2/AcOEt (molar ratio) = 40, liquid hourly space velocity (LHSV) = 1 h−1, and AcOEt/H2O/EtOH (mass ratio) = 90/6/4.

1. INTRODUCTION

Generally, for ester hydrogenation, transition metals (e.g., Fe, Co, Ni, Cu) supported on various oxides (e.g., SiO2, ZrO2, Cr2O3, etc.) have been extensively used.5−7 Typically, copperbased catalysts were considered to be suitable for selective hydrogenation of ester due to better selective hydrogenation of CO bonds and relatively less inactivity for C−C bond hydrogenolysis.8 Among all copper-based catalysts, Cu/SiO2 catalysts stand out in ester hydrogenation owing to high dispersion of copper species and strong metal−support interaction.2 The reported works have confirmed that high metal dispersion of Cu/SiO2 is very essential to high activity and stability in the vapor-phase ester hydrogenation to alcohols, since the sintering of copper is rather facile due to low THüttig and TTammann.2,6,9,10 Not surprisingly, ammonia evaporation (AE), as a kind of homogeneous deposition−precipitation method, has been extensively used to prepare Cu/SiO2 catalysts due to high copper species dispersion and the formation of chrysocolla.11−14 Chrysocolla (Cu2Si2O5(OH)2), being a kind

Although fossil fuels are considered to be the major source for providing us with required energy, they are also counted as one of the main causes of atmospheric pollution as a serious worldwide problem. However, in contrast to the conventional fossil fuels, ethanol (EtOH) is an interesting candidate as an alternative synthetic fuel to be used in automobiles with little environmental hazard. Nowadays, EtOH is produced through two major approaches, and the primary method is based on the fermentation process of sugars coming from sugarcane, potato, manioc (cassava), and corn.1 The second approach for the production of EtOH is through the catalytic conversion of syngas (CO + H2), which comes from the gasification of coal or biomass. However, the required enormous research and development effort and cost before the accomplishment of commercial scale for the first approach, and the low yield, fast chain growth of C2 intermediates, and poor selectivity in the second approach have limited their wide and practical application.2,3 Alternatively, the hydrogenation of esters (e.g., oxalates and ethyl acetate) as a very interesting and fundamental reaction in organic chemistry is employed for the production of EtOH and has attracted tremendous interest of scientists due to the clean utilization of coal and biomass.4 © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

May 17, 2017 August 2, 2017 August 3, 2017 August 3, 2017 DOI: 10.1021/acs.iecr.7b02034 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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min. The initial pH of all suspensions was 10−11, and all the above operations were performed at room temperature. All suspensions were transformed to an oil bath preheated at 80 °C for 90 min, to allow for the evaporation of ammonia. When the evaporation process was terminated, the pH of all suspension decreased to 7−8. After that, 70 mL of the resultant mixture was sealed in a 100 mL Teflon-lined autoclave and kept at 150 °C for 24 h. The precipitates were washed several times with water and ethanol, collected using centrifugation, and then dried for 4 h at 80 °C. Thereafter, the catalysts precursors were calcined at 400 °C in a muffle furnace for 4 h, pelletized, crushed, sieved to 40−60 meshes, and marked as AE-1, AE-2, AE-3, and AE-4, respectively. The catalyst preparation procedures are summarized in Table 1.

of copper phyllosilicate with a lamellar structure that consists of layers of SiO4 tetrahedra sandwiched between discontinuous layers of CuO6 octahedra, is beneficial for antisintering ability of Cu/SiO2 due to strong interaction between copper and silica.2,10,15 In order to optimize the catalytic performance of Cu/SiO2 prepared by AE, the preparative parameters were studied in previous publications.11,14 Chen et al. have investigated that the AE temperature has a profound effect on the hydrogenation activity of Cu/SiO2 due to the different texture and composition of Cu/SiO2.11 Toupance et al. showed tht chrysocolla content depended on the pH of precursor solution, the concentration of metal precursor, and the solution/silica contact time for AE.14 Furthermore, the ammonia evaporation method is followed by a hydrothermal process to avoid the low copper loading due to disadvantages of −OH group deficient on silica surface.7,12 Nonetheless, to the best of authors’ knowledge the effect of the precursor’s configuration addition on the structure, morphology, and catalytic performance of Cu/SiO2 in AE has not reported up to now. In the present work, Cu/SiO2 catalysts are prepared by the ammonia evaporation−hydrothermal method (AE−H) focusing on the effect of the precursor’s configuration addition on Cu/SiO2 catalysts. The physicochemical properties of Cu/SiO2 were examined by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, temperature-programmed reduction (TPR), N2O titration, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energydispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The ethyl acetate (AcOEt) hydrogenation to ethanol is chosen as a testing reaction to investigate the effect of different precursor configuration additions on catalytic performance. After that, our study aims at understanding the relationships among the precursor’s configuration addition, the surface structure, and the catalytic performance of the Cu/SiO2 catalysts prepared by AE−H to optimize the preparative condition for prepared high efficiency catalysts.

Table 1. Configuration Addition of Precursors and Change of pH in Preparation Processa sample

mix 1

mix 2

AE-1 pH AE-2 pH AE-3 pH AE-4 pH

2/3B + A 10−11 B+A 10−11 B 11−12 B+C 11−12

1/3B + C 11−12 C 2−3 C+A 4−5 A 4−5

configuration addition add add add add add add add add

mix mix mix mix mix mix mix mix

2 2 2 2 2 2 2 2

into into into into into into into into

mix mix mix mix mix mix mix mix

1 1 1 1 1 1 1 1

dropwise dropwise dropwise dropwise dropwise dropwise dropwise dropwise

A, 250 mL of 0.1 mol/L Cu(NO3)2·3H2O aqueous; B, 30 g of 25% (wt %) NH3 (aqueous); C, 14.76 g of 30% (wt %) silica sol. A + B means A is added into B dropwise. a

2.3. Hydrogenetion of Ester Reaction. The activity test of Cu/SiO2 was performed on a continuous flow unit equipped with a vertical stainless steel tubular reactor. The H2 flow was controlled by a Brooks 5850E mass flow controller (MFC). The pressure was always maintained at 2.4 MPa via a backpressure regulator, during the whole evaluation period. After in situ reduction at 300 °C for 4 h with a H2 flow rate of 30 mL min−1, the catalysts were cooled to room temperature. Then, the fixed bed reactor was heated to 280 °C at steps of 2 °C min−1. The liquid reactant (mass ratio AcOEt/H2O/EtOH = 90/6/4) was injected by a double-plunger pump, enabling a tunable liquid hourly space velocity (LHSV). After vaporizing by a preheater, the vapor was mixed with H2. The reaction products were collected in a 5 °C cooling cool trap and analyzed by gas chromatography (Shimadzu GC-14C) using a capillary column (19091n-213, HP-INNOWAX, 0.25 mm × 30 m) with an flame ionization detector (FID) and a packed column (TDX-101) with a thermal conductivity detector (TCD). The gas products were analyzed by online gas chromatography (Shanghai Haixin GC-950) with a packed column filled with a carbon molecular sieve. 2.4. Characterization. The powder X-ray diffraction (XRD) pattern of the prepared catalysts was recorded on a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 1.5418 Å) in the 2θ scanning range between 5 and 85°. Fourier transform infrared (FT-IR) spectroscopy was performed in transmission mode from 4000 to 500 cm−1 using a Bruker Vector 22 spectrometer equipped with a DTGS detector and a KBr beam splitter. Then, the TPR experiments were performed as follows: the fresh catalysts (20 mg) were placed in a quartz reactor and were reduced by a 5% H2/N2 gas mixture with a flow rate of 50 mL min−1, ramping at 5 °C

2. EXPERIMENTAL SECTION 2.1. Materials. Copper(II) nitrate hydrate (Sinopharm Chemical Reagent Co., Ltd., AR), ammonia solution (25 wt %, Sinopharm Chemical Reagent Co., Ltd., AR), and colloidal silica aqueous solution (Qingdao Ocean Chemical Co., Ltd., 30 wt %) were used as received. All reagents used in this work were of analytically pure grade and used without further purification. 2.2. Catalyst Preparation. The 25% Cu/SiO2 catalysts were prepared by the ammonia evaporation−hydrothermal method (AE−H) described as follows. Typically, 6.04 g of Cu(NO3)·3H2O was dissolved in 250 mL of deionized water, marked as “A”; 30 g of 25% ammonia aqueous solution is marked as “B”; 14.76 g of silica sol is marked as “C”. For more detailed information, (1) 2/3B was added into A drop by drop and stirred for 5 min. Then, 1/3B was added into C drop by drop and stirred for 5 min. Last, the second aqueous solution was added into the first aqueous solution drop by drop and stirred for 30 min. (2) B was added into A drop by drop and stirred for 5 min. Then, C was added into the above aqueous solution drop by drop and stirred for 30 min. (3) C was added into A drop by drop and stirred for 5 min. Then, it was added into B drop by drop and stirred for 30 min. (4) B was added into C drop by drop and stirred for 5 min. Then, A was added into the above aqueous solution drop by drop and stirred for 30 B

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Figure 1. (a) XRD spectra and (b) Fourier transform infrared spectra of Cu/SiO2 catalysts prepared by AE−H with different configuration additions.

min−1 to the final temperature. Meanwhile, hydrogen consumption was recorded using a thermal conductivity detector (TCD). The dispersion and specific surface area of Cu0 (DCu0 and SCu0) were measured by N2O−H2 redox titration at 50 °C. For more details, see the Supporting Information. The morphologies of the samples were characterized by means of a high-resolution transmission electron microscope (HRTEM, JEM 2100F). SEM−EDS mapping analysis was performed with a JEOL JSM-7800F. X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD) was carried out to analyze the elemental valence of the surface. Temperatureprogrammed desorption of adsorbed CO2 (CO2-TPD) on the reduced catalysts was analyzed using a quadrupole mass spectrometer (HIDEN, Hpr-20; Pfeiffer Vacuum Technology AG) equipped with the same apparatus employed for the TPR experiment. Prior to CO2 adsorption, the sample (120 mg) was reduced in a H2 (30 mL min−1) gas flow at 300 °C for 4 h. After reduction, the sample was swept at 300 °C for 1 h and subsequently cooled to room temperature by a He gas flow (50 mL min−1). Several CO2 pulses (1.2 mL of CO2 of one pulse) were then introduced to the sample to obtain a saturated adsorption of CO2. The sample was purged by He for 1 h at 100 °C to eliminate the physically adsorbed CO2; then, the sample was heated to 800 °C at 10 °C min−1 in He and the effluent was continuously monitored by a mass spectrometer. The mass number used was 44 for the CO2 species. Temperature-programmed desorption of adsorbed NH3 (NH3-TPD) on the reduced catalysts was also analyzed, and the procedure is similar to that of CO2-TPD which is just CO2 gas replaced by NH3 gas.

3.2. Crystal Structures and Morphologies of the Catalysts after Calcination. The crystal structures and phases of all the prepared catalysts were characterized by Xray diffraction techniques, as shown in Figure 1a. All samples exhibited a main peak at around 2θ of 22°, which corresponded to amorphous silica. Additionally, no obvious peaks corresponding to CuO and Cu2O were detected, and only weak and broad characteristic peaks of chrysocolla (PDF 027-0188) were observed (Figure 1a). This indicated that copper species are highly dispersed and present in the extremely disordered or amorphous states.2,10,15 In order to further determine the components of the prepared catalysts, FT-IR spectroscopy was performed. As shown in Figure 1b, the spectrum clearly displayed an absorption peak at approximately 670 cm−1 assigned to the δOH band, the νSiO shoulder peak at 1040 cm−1 ascribed to chrysocolla, and the peak at 1100 cm−1 attributed to the ν SiO asymmetric stretching band of SiO2.2,11,14 The relative amount of chrysocolla in Cu/SiO2 can be roughly qualitatively estimated by I670/I800, which is the integrated intensity of the δOH band at 670 cm−1 normalized to the integrated intensity of the νSiO asymmetric band of SiO2 at 800 cm−1.11,14 The inset in Figure 1b indicates that the relative chrysocolla content in Cu/SiO2 is varying as the precursor’s configuration addition changes. As a consequence, the chrysocolla content is in the sequence AE-1 > AE-4 > AE-3 > AE-2. More unambiguous evidence supporting the different physicochemical properties of Cu/SiO2 prepared by AE−H with different precursor configuration additions are given below. To further confirm our proposal for the effect of the changing the precursor’s configuration addition on the properties of the prepared catalysts, H2-TPR and N2O−H2 redox titration were performed. Figure S1 compares the H2TPR profiles, and it can be seen that all the catalysts display reduction profiles composed of a main peak at ca. 259 °C which contains several shoulder peaks (Figure 2) . The high reduction temperature suggests a strong interaction between copper species and silica support and might be related to good dispersion of copper species.10 However, the reduction temperature is different for the catalysts with different configuration additions. Gaussian fitting is performed to distinguish each component of copper species in catalysts, as shown in Figure 2. The reduction peaks are fitted by two or three curves, and the relative parameters are shown in the inset. As a result, the fitting peak was found to be around 245 °C for the surface Cu2+ reduction to Cu0 and 256 °C for the bulk of Cu2+ to Cu0.6,10,16 The last fitting peak at 263 °C is supposed to

3. RESULTS AND DISCUSSION 3.1. pH Monitor of the Process in the Preparation. The preparation of the Cu/SiO2 catalysts was performed by the ammonia evaporation−hydrothermal method (AE−H). As previously stated, the effect of pH of the precursor solution14 has a remarkable impact on copper loading and chrysocolla (Cu2Si2O5(OH)2) content. In this work, it is worth noting that the precursor’s configuration addition has an impact on the pH of mix 1 and mix 2 solutions (Table 1) and the contact sequence which can cause the formation of different intermediates,14 which may be a main reason giving rise to different chrysocolla contents and further effect on the catalytic performance. Table 1 shows the different precursor configuration additions and the pH changing during the preparative process. C

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the reducibility of catalysts. In order to measure the property of surface copper, N2O titration is performed. The typical TPR profiles before and after N2O oxidation are shown in Figure S2, and the results are tabulated in Table 2. For Figure S2, the peak area of TPR1 is equivalent to all of Cu2+ in the catalysts, and that of TPR2 corresponds to surface Cu+ generated by N2O oxidation. Thus, the dispersion (DCu0), the specific surface area (SCu0), and the average volume−surface diameter of metal copper (dvs Cu0) can be calculated on the basis of the peak areas of TPR1 and TPR2.17,18 From Table 2, it can be found that the prepared catalysts by AE−H have a good dispersion with a value of beyond 39%, but there are differences for DCu0, SCu0, and dvs Cu0 due to the different chrysocolla contents caused by the different configuration additions of the precursor. It is found that SCu0 is increased with the increment of chrysocolla content due to its special filamentous morphology (Figure 3),11,12 as shown in Figure S3. However, if the chrysocolla content is too high, SCu0 will decrease due to the difficult reduction of Cu2+ in chrysocolla. As a consequence, DCu0 and SCu0 of the prepared catalysts follow the order AE-1 < AE-2 < AE-3 < AE-4, and the order of dvs Cu0 is in the opposite direction. The TEM images of calcined catalysts are shown in Figure 3. The light gray pattern is identified as silica supports, and the dispersed dark dots are attributed to Cu species. It is apparent that Cu species are highly dispersed onto the silica support in all samples, no matter what configuration of the precursor’s addition is. As shown in Figure 3, chrysocolla exhibits filamentous morphology, as is disclosed in other works.11 Moreover, the filamentous shape is more popular in Figure 3a,d than in Figure 3b,c, which is in accord with chrysocolla content as measured by FT-IR spectroscopy and TPR. In addition, though no particles can be observed in TEM experiment at initial time (see Figure 3e), 5 nm of average size copper nanodots is observed after image taking for a while, as shown in Figure 3f, indicating the sintering of copper particles due to exposure to electron beam. Therefore, the growth of the copper particle exposed to electron beam as a counterevidence indicated that the copper species evenly distributed on the silica support initially. In order to confirm high dispersion of

Figure 2. Gaussian curve fitting of TPR profiles of prepared catalysts with different configuration additions.

be the reduction of Cu2+ in chrysocolla. It is found that AE-1 > AE-4 > AE-3 > AE-2 on the basis of area of the last fitting peaks (24.02, 23.04, 6.58, and 0, respectively) which is in agreement with the results from FT-IR spectroscopy. Therefore, the Cu2+ in chrysocolla is difficult to reduce due to the strong interaction between Cu2+ and SiO2. The different chrysocolla contents lead to different reduction degrees (DRs), which can be calculated by the integrated TPR peaks before 263 °C normalized to all integrated TPR peaks, as shown in Table 2. It can be found that Table 2. Physicochemical Properties of the Calculated Catalysts with Different Configuration Additionsa catalyst

dvs Cu0 (nm)

SCu0 (m2 g−1)

DCu0 (%)

DR (%)

AE-1 AE-2 AE-3 AE-4

2.53 2.44 2.43 2.29

267.9 277.1 278.2 295.9

39.6 40.9 41.1 43.7

92.1 100.0 97.9 93.0

Determined by N2O chemisorption. DCu0 = (2Y/X) × 100%; S Cu0 = 1353Y/X (m2 of Cu/g of Cu); dvs Cu0 = 0.5X/Y (nm). X = area of TPR1 peak and Y = area of TPR2 peak. a

the order of DR is similar to the order of chrysocolla content. Therefore, it is deduced that chrysocolla content can regulate

Figure 3. TEM images of calcined catalysts with different configuration additions: (a) AE-1, (b) AE-2, (c) AE-3, and (d) AE-4. TEM images of AE-1 catalyst at initial time (e) and after exposure to electron beam (f). D

DOI: 10.1021/acs.iecr.7b02034 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Cu/SiO2 catalysts prepared by the AE−H method, EDS mapping analysis was performed, as shown in Figure 4 and Figure S4. Figure 4b−d displays the elemental maps of Cu, O, and Si separately. The good distributions of the elements are disclosed in elemental mapping images.

Table 3. Copper Species Particle Size of the Catalysts before and after Reactiona

a

catalyst

dBR Cu (nm)

dAR Cu (nm)

dBR Cu2O (nm)

dAR Cu2O (nm)

AE-1 AE-2 AE-3 AE-4

4.28 7.83 5.15 4.60

6.20 10.00 9.00 6.85

3.98 3.31 4.30 3.68

3.97 3.82 3.88 4.54

Determined by the Scherrer formula.

the reduced catalysts, which have a synergistic effect for the hydrogenation of ester reaction.3,11,15 Therefore, the prepared catalysts by the AE−H method with different configuration additions will have good hydrogenation activity, as will be discussed below. Figure 5b shows the XRD patterns of the Cu/SiO2 catalysts after hydrogenation reaction. It can be found clearly that there are no additional copper phases appearing after reaction. However, the intensity of the characteristic peaks about Cu or Cu2O is different for the catalysts with different configuration additions. Therefore, the crystallite sizes before and after reaction for both Cu and Cu2O are calculated by the Scherrer formula, as shown in Table 3. The size of the metal Cu particles (dAR Cu) after reaction increased due to low melting points, THüttig and TTammann, of metal copper and the redox of copper species in ester hydrogenation. Besides, it is found that the size of the metal copper particle is smaller for high chrysocolla content than for low chrysocolla content, no matter whether the catalyst is being reduced or reacted. Herein, the chrysocolla content is responsible for preventing the Cu particle from sintering. In order to determine the component changes during reaction, the FT-IR spectra of prepared catalyst after reduction and reaction were performed in comparison with that after calcination, as shown in Figure 5c, and the FT-IR spectra of all prepared catalysts after reduction and reaction can be found from Figure S5. It is found that the supporting evidence of chrysocolla formation, i.e., the peaks at 670 and 1040 cm−1, has disappeared after reduction and reaction, suggesting that the chrysocolla phases have been reduced. After reduction and reaction, in all the FT-IR spectra appear new weak vibrating peaks at 980 and 1230 cm−1 which can be assigned to surface Si−O− groups and asymmetric Si−O stretching, respectively,

Figure 4. SEM−EDS mapping analysis of AE-1 after calcination. (a) SEM image of AE-1. (b−d) Element EDS mapping images of AE-1: (b) Cu, (c) O, and (d) Si.

3.3. Crystal Structures and Morphologies of Catalysts after Reduction and Reaction. The XRD patterns of the reduced Cu/SiO2 catalysts are shown in Figure 5a, with a relatively strong diffraction peak at 43.3° along with two weak peaks at 50.4 and 74.1°, and all them are characteristic of metal Cu (JCPDS 04-0836).11,12 Additionally, the obvious diffraction peak at 37° is assigned as characteristic of Cu2O (JCPDS 050667), indicating that a portion of copper species exist as Cu+ after reduction in hydrogen at 300 °C.11,12 The content of those species is an indicator of the amount of chrysocolla. The less chrysocolla content, the sharper the peak. It can be found that the minimum size of metal Cu particles before reaction (dBR Cu) is achieved by AE-1 with a value of 4.28 nm and the maximum of that is achieved by AE-2 with a value of 7.83 nm, as shown in Table 3. As a consequence, Cu0 and Cu+ coexist in

Figure 5. XRD patterns of (a) reduced Cu/SiO2 catalysts and (b) Cu/SiO2 catalysts after reaction. (c) FT-IR spectra of AE-1 catalyst after calcination, reduction, and reaction. Reaction conditions: T = 280 °C, P = 2.4 MPa, n(H2)/n(AcOEt) = 40 (molar ratio), LHSV of AcOEt = 1 h−1, and reaction time 200 h. E

DOI: 10.1021/acs.iecr.7b02034 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research indicating a high degree of polycondensation of SiO2.19,20 Therefore, reduction of chrysocolla leads to the formation of Si−O−Si bonds via a polycondensation.20 Meanwhile, it is found that a new weak vibrational peak at 645 cm−1, corresponding to Cu−O bond, appeared only after reaction.21 This indicates the existence of Cu−O bonds in catalysts after reaction due to redox reaction in hydrogenation of ester, which also explains that synergistic effect of Cu0 and Cu+ in the hydrogenation. Figure 6 illustrates the Cu LMM Auger electron spectra in order to distinguish Cu0 and Cu+. The Auger kinetic energy

acidic sites is also calculated, signed as I193/I657. The results of calculations indicated that the relative amounts of Lewis basic and acidic sites follow the order AE-1 < AE-2 < AE-3 < AE-4, which obeys the order of SCu0. Therefore, the different SCu0 values lead to different contact surfaces between metal copper and silica, which has a direct impact on Lewis basic and acid sites of reduced catalysts. 3.4. Gas-Phase Hydrogenation of AcOEt. The catalytic performance of the Cu/SiO2 catalysts prepared by AE−H with different configuration additions is investigated in the gas-phase hydrogenation of AcOEt. At a reaction temperature of 280 °C, the conversion of AcOEt for all the catalysts reaches 95%. Therefore, the reaction temperature is kept for 200 h to study the catalytic performance of the catalysts, and the results are shown in Figure 7. It can be seen that the conversion of AcOEt

Figure 6. Cu LMM Auger spectra of reduced catalysts.

peaks can be deconvoluted into two symmetrical peaks centered at ca. 912.5 eV and ca. 916.0 eV which correspond to Cu+ and Cu0, respectively. The Cu+/(Cu0 + Cu+) intensity ratios are 43.1, 43.8, 44.2, and 49.3%, respectively, calculated by the deconvolution results. It is found that all catalysts in this work have a high Cu+/(Cu0 + Cu+) intensity ratio which may endow their high hydrogenation performances.3,11,15 The order of Cu+/(Cu0 + Cu+) ratio is the same as that of SCu0, suggesting that chrysocolla content has an effect on the proportion of Cu+ to Cu0. CO2-TPD and NH3-TPD of the reduced catalysts with different configuration additions were performed, as shown in Figure S6. As verified from Figure S6a, these catalysts show different profiles regarding their interactions with CO2. Several obvious CO2 desorption peaks (at ca. 133, 330, and 472 °C) were observed over the whole temperature range. It is inferred that the temperature of desorption peaks at 133 °C is related to the weak basic sites, the ones at 330 °C are strong basic sites, and, finally, the peaks above 472 °C are due to the release of CO2 from the support.6 Therefore, the CO2-TPD profiles of the prepared catalysts disclose a clear concentration of varied basic sites on the surface. There are three category desorption peaks for all catalysts, with each peak being different in intensity for different catalysts, indicating the configuration addition has an effect on the surface basic sites of the prepared catalysts. As stated, the third peak is due to the release of CO2 from the support, so it does not work for the hydrogenation reaction. As a consequence, the related amount of basic sites is calculated by considering the integrated intensity of desorption peaks at 133 and 330 °C, signed as I330/I133. Figure S6b shows the NH3-TPD profiles, which indicate a clear concentration of varied acidic sites on the surface. Similar to CO2-TPD, there are two desorption peaks at 193 and 657 °C over the entire temperature range corresponding to weak and strong acidic sites, respectively, and the related amount of

Figure 7. Catalytic performance of catalysts at reaction conditions T = 280 °C, P = 2.4 MPa, n(H2)/n(AcOEt) = 40 (molar ratio), and LHSV of AcOEt = 1 h−1.

has a high stability as well as hydrogenation activity during the whole reaction, and it is ascribed to the cooperative effect of Cu0 and Cu+ and the strong interaction between copper species with silica. However, the selectivity of EtOH is dramatically different for different catalysts. Remarkably, the maximum of the selectivity is obtained by AE-1 catalyst with a value of 96%, and the minimum of that by AE-4 catalyst with a value of 89%. Furthermore, the selectivity of EtOH follows the order AE-1 > AE-2 > AE-3 > AE-4. In other words, with ethane as the main byproduct, its selectivity follows the order AE-1 < AE-2 < AE-3 < AE-4, as shown in the inset of Figure 7, which obeys the order of SCu0. As stated, the chrysocolla content has a direct effect on SCu0, which further has an influence on Lewis basic and acid sites of reduced catalysts due to different contact surfaces between metal copper and silica. Therefore, the different Lewis basic and acid sites may be the main reason for different EtOH selectivities, which have been correlated with EtOH selectivity as shown in Figure 8. It can be concluded that the smallest relative amount of basic and acidic sites corresponds to the highest selectivity of EtOH and vice versa. Previous studies have reported a remarkable relationship between the amount of ammonia chemisorbed on the reduced catalyst and the ethane selectivity, suggesting the formation of ethane is on the acidic sites for Cu/SiO2.8,22 This is because the acidic sites are beneficial for ethanol dehydration to olefin. Olefin formation is the predominant reaction on acidic oxides, and olefin will further hydrogenate to ethane in our reaction F

DOI: 10.1021/acs.iecr.7b02034 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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vital factor for several hydrogenation reactions and have great implications for industrial applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02034. Physicochemical property characterization of Cu/SiO2 with different configuration additions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 0351-4040290. ORCID

Mohamed Abbas: 0000-0001-6268-6069 Jiangang Chen: 0000-0001-9230-6338

Figure 8. Relative relationship between ethanol selectivity and relative basic and acid sites.

Notes

The authors declare no competing financial interest.

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condition.23 However, a few studies have reported the effect of the basic sites on ethane selectivity in the hydrogenation reaction for Cu/SiO2. From this work, it is found that the basic sites have also an obvious effect on the formation of ethane. This may be because basic characteristics favor ethanol dehydrogenation but inhibit dehydration. Therefore, low basic characteristics are beneficial for ethanol dehydration but inhibit ethanol dehydrogenation, which leads to formation of ethane as the main byproduct.24 Therefore, it is inferred that the evolution of the catalytic performance was determined by the ratio of acid and basic sites for the hydrogenation of AcOEt to EtOH due to the synergistic effect between the acidic and basic sites. As a consequence, the different configuration additions for Cu/SiO2 catalysts prepared by AE−H can regulate the ratio of basic and acidic sites on the surface of the catalyst which have a great effect on the selectivity of EtOH. In the present work, the proper configuration addition provided the optimized ratio of acid and basic sites like the AE-1 catalyst, giving rise to the highest ethanol selectivity of 96% under the reaction conditions.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21673272, 21373254, and 21503256) and PetroChina (Project Nos. PRIKY14006, PRIKY15038, PRIKY15039, and PRIKY15042).

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REFERENCES

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4. CONCLUSION The present work demonstrated that the different precursor configuration additions have profound effects on the texture, composition, and structure of Cu/SiO2 prepared by the AE−H method. The texture, composition, and structure of Cu/SiO2 have been characterized by XRD, FT-IR spectroscopy, TPR, N2O titration, TEM, EDS, and XPS after reduction or reaction. It is found that the different precursor configuration additions have a direct effect on the chrysocolla content, which can regulate the reducibility and dispersion of copper, yielding different specific surface areas of metal copper. Also, both the Lewis basic and acid sites on the reduced catalyst surface are correlated with the specific surface area of metal copper due to different contact surfaces between metal copper and silica. In gas-phase hydrogenation of AcOEt to EtOH, the basic and acidic sites on the surface of Cu/SiO2 have a synergistic effect on EtOH selectivity. In the present work, the proper precursor configuration addition provided the optimized amount of acid and basic sites like AE-1 catalyst, giving rise to the highest ethanol selectivity of 96% under the reaction conditions. Therefore, the effect of different precursor configuration additions on preparation of catalyst could be a promising G

DOI: 10.1021/acs.iecr.7b02034 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.7b02034 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX