Diethyl carbonate (DEC) is a fuel oxygenate additive with the potential to replace methyl tert-butyl ether (MTBE),and is currently thought to be superior to other alternative additives such as dimethyl carbonate (DMC) and ethanol (EtOH) [1]. DEC has also been considered for applications as a solvent [2] and as a component of a lithium cell electrolyte mixture because of its low toxicity [3, 4]. Several commercial processes have been developed for the synthesis of DEC,including; (i) the reaction of highly toxic phosgene gas with EtOH [5],(ii) the oxidative carbonylation of EtOH in the liquid phase [6],(iii) the transesterification reaction of DMC and EtOH [7],(iv) the gas phase oxidative carbonylation of EtOH [1, 8],(v) the reaction between carbon monoxide and ethyl nitrite (C2H5ONO) [9],(vi) the reaction of EtOH with urea over organotin catalysts [10] and (vii) the activation of carbon dioxide [11]. Among these,the oxidative carbonylation of EtOH in the gas phase is the most attractive method because it is environmentally benign by nature [1, 8, 12].
There have been numerous reports concerning the application of chlorine-containing copper catalysts to the synthesis of DEC via the oxidative carbonylation of EtOH [13, 14, 15, 16, 17, 18]. Various catalysts have been investigated for this purpose,all of which are prepared by dispersing CuCl2 or CuCl2 and PdCl2 onto supports. Early studies showed that activated carbon (AC) [9, 12, 14],metal oxides [19] and zeolites [20] may all be used as the support. Subsequently,to reduce problems with equipment corrosion,chlorine-free catalysts were prepared by a solid-state ion exchange method for the synthesis of DEC [21, 22]. However,uncertainties regarding the active species in these catalysts still remain. Thus,in early work with DMC catalytic systems,chlorine-free catalysts were prepared by solid-state ion exchange to explore the issues of equipment corrosion and the uncertainty of the active species [23, 24]. Cu+ ions were studied as the active species through experimental observations [23, 24, 25] as well as theoretical investigations [26, 27]. Later,Cu2O supported on carbon-based materials was also applied as a chlorine-free catalyst to address the above challenges [28, 29]. More recently,nanoscale metal oxides have attracted much attention because of their unique size- and dimensionality-dependent physical and chemical properties [30, 31]. Cupric oxide (CuO) and cuprous oxide (Cu2O) nanostructures are of particular interest because of their interesting properties and promising applications,such as in lithium-ion batteries [32],catalysis [33, 34] and CO oxidation [35]. Therefore,copper-based oxides with different valence states might be efficient supports to catalyze the synthesis of DEC in the gas phase,and studies using these material could also assist in elucidating the active species.
In the present work,the goal was to synthesize DEC from EtOH,CO and O2 in the gas phase over PdCl2-loaded CuO and Cu-Cu2O prepared with different NaOH concentrations. Unfortunately,the initial results did not identify the active species for the synthesis of DEC. To further investigate such species,PdCl2-loaded Cu-Cu2O catalysts having a variety of Cu2O contents were investigated by tuning the glucose concentration. By comparing the catalytic performances of catalysts incorporating different copper species,the active species involved in the gas phase synthesis of DEC were clearly identified.
Copper chloride (> 99%),sodium hydroxide (98%),hexadecyl trimethyl ammonium bromide (> 99%),ethanol (> 99%),PdCl2 (> 99%) were obtained from Sinopharm Chemical Reagent Co,Ltd,China.
CuO and Cu-Cu2O nanoparticles were prepared by hydrothermal synthesis based on literature methods [36, 37]. In a typical synthesis,20 mL of a 1 mol/L CuCl2·2H2O solution was added dropwise into 80 mL NaOH and stirred vigorously at 50 °C. Subsequently,40 mL CTAB (0.25 mol/L) was added and the mixture stirred vigorously at 50 °C for 1 h,after which the reaction mixture was sealed in a stainless steel pressure container and held at 140 °C for 24 h. The resulting product was collected,washed several times with distilled water followed by absolute EtOH and then dried under vacuum to obtain black CuO powder. Different NaOH concentrations (2.5,5 and 10 mol/L) were used during the preparation. In addition,while employing the optimized NaOH concentration,different concentrations of glucose were added during the preparation process after adding the CTAB so as to prepare brick-red Cu-Cu2O nanoparticles containing various Cu2O species. In our previous work,the special roles of Pd and Cu in PdCl2/Cu-HMS catalysts were investigated in detail [16, 18]. Therefore,in the present work,catalysts were also prepared by impregnating the CuO or Cu-Cu2O with a methanol solution of palladium chloride (PdCl2). The total palladium concentration in the final products was 0.25 wt% based on the mass of CuO or Cu-Cu2O.
The composition and phase of each sample was identified by powder X-ray diffraction (XRD) with a D8 X-ray diffractometer (Bruker AXS,German) using Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 2°/min from 2θ = 10° to 80°. Thermogravimetric analysis (TGA) of the samples was performed using a Mettler TGA/SDTA 851E analyzer over the temperature range 25 to 800 °C at a heating rate of 10 °C/min under O2. The morphologies of products were examined by scanning electron microscopy (SEM) with an S-4800 (Hitachi,Japan) scanning electron microscope at an accelerating voltage of 2.0 kV. Inductively couple plasma (ICP) spectrometry using a PE 5300DV was employed to determine the concentrations of metallic palladium in the catalysts. X-ray photoelectron spectroscopy (XPS) was performed with a multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI) as a means of analyzing the various metallic species and their valence states.
Catalytic activities were measured with a computer- controlled continuous micro-reactor system incorporating a stainless steel tubular reactor with an inner diameter of 8 mm. The reaction conditions were as follows: 3 mL catalyst,0.1 mL/min liquid EtOH,10 mL/min O2,80 mL/min CO,50 mL/min N2,reaction temperature of 150 °C and reaction pressure of 0.64 MPa. The analytical methods used during these trials have been previously reported in detail [16, 18].
The catalytic performances during the oxidative carbonylation of EtOH with CO and O2 over PdCl2/CuO and PdCl2/Cu-Cu2O catalysts prepared with different NaOH concentrations (2.5,5 or 10 mol/L) are summarized in Table 1. From these data,it is evident that the product of the catalytic reaction was primarily DEC,together with lesser amounts of acetaldehyde (AH) and ethyl acetate (EA). In comparison with other NaOH concentrations,the catalysts prepared with 5 mol/L NaOH showed better catalytic performances. Neupane et al. [36] pointed out that the use of a strong base in the precipitation reaction played a vital role during the catalyst of synthesis. Both low and high NaOH concentrations could lead to free electrons on the surfaces of the CuO and Cu-Cu2O,possibly resulting in some powder agglomeration and partial coverage of the active sites. In addition,at the same NaOH concentration,the PdCl2/Cu-Cu2O exhibits better catalytic performance than the PdCl2/CuO. As previously reported [8],in the case of Pd(II)-Cu(II) (PdCl2/CuO) catalysts,an induction period during which Pd(II) is reduced to Pd(0) is required prior to electron transfer between Pd and Cu,which might slow the catalytic reaction. Thus the presence of either Cu0 or Cu+ is beneficial with regard to improving the catalytic performance.
The phase compositions and structures of the catalysts were evaluated by XRD,with the results shown in Fig. 1. In Fig. 1(a),the diffraction peaks seen in the XRD patterns can be ascribed to pure CuO,and the principal peaks are indexed to the (111-),(111) and (202-) reflections,corresponding to the crystal structure of CuO (JCPDS 48-1548). In Fig. 1(b),prominent peaks appear at 43.30°,50.43° and 74.13° corresponding,respectively,to the (111),(200) and (220) reflections of cubic Cu (JCPDS 04-0836). Weaker features are also seen at 36.42°,42.30°,61.34° and 73.53° and these are assigned to the (111),(200),(220) and (311) reflections of cubic Cu2O (JCPDS 05-0667). Even when employing different NaOH concentrations,the PdCl2/CuO catalysts exhibit similar XRD patterns,as do the PdCl2/Cu-Cu2O materials,suggesting that the same crystal structures are present,with major crystal planes diffracting at essentially the same 2θ angles.
To assess the thermal stability of the catalysts throughout the reaction,TG analysis was performed under O2 and the resulting data are summarized in Fig. 2. It is evident that there were no mass changes associated with any temperature for the PdCl2/CuO catalysts,while the PdCl2/Cu-Cu2O catalysts exhibit obvious mass increases beginning at 500 °C. Combined with the XRD analysis,these results indicated that the former materials were composed of pure CuO while the latter were a combination of Cu and Cu2O.
The morphologies of the catalysts were observed by means of SEM and EDS,and the results are shown in Fig. 3. These images demonstrated that CuO nanorods with rectangular sections (a‒c) were present in the PdCl2/CuO catalysts,while the PdCl2/Cu-Cu2O catalysts show sphere-like morphologies that might allow greater contact areas during the catalytic reaction. This result suggests one factor affecting the catalytic performance. In conclusion,the PdCl2/Cu-Cu2O exhibited better catalytic performance than the PdCl2/CuO series of catalysts when the synthesis parameters were identical. It has been reported that copper atoms form the main active site during the synthesis of DEC by oxidative carbonylation of EtOH in the gas phase [38‒40],although PdCl2 also plays an important role such that its presence can improve the space time yield (STY) of DEC by a factor of three compared with catalysts without PdCl2 [12]. For this reason,ICP analyses were performed to assess the PdCl2-loadings. The resulting data showed that the PdCl2/CuO and PdCl2/Cu-Cu2O catalysts both had similar PdCl2 concentrations,at 0.15% and 0.16%,respectively.
Based on the above analysis,it was concluded that the PdCl2/Cu-Cu2O catalyst exhibited better catalytic performance than the PdCl2/CuO,although the main factors responsible for the differing catalytic performance between Cu0 and Cu+ remained unknown. Therefore,catalysts with different Cu0 and Cu+ contents were prepared for comparison purposes by varying the glucose concentration applied during synthesis. The catalytic performances over the PdCl2/Cu-Cu2O catalysts are summarized in Table 2. From these data,it is evident that the PdCl2/Cu-Cu2O prepared with 10 mmol glucose showed high catalytic performance. The STY for DEC obtained with this material was 151.9 mg·g−1·h−1,corresponding to an EtOH conversion of 7.2% and DEC selectivity of 97.9% on an EtOH basis. Additionally,it is worth noting that the selectivity for AH based on EtOH increased with the amount of glucose added. Cu0 is the main active center during the synthesis of AH,and so these results could imply that the catalyst prepared with 10 mmol glucose had less Cu0. Ding et al. [25] has reported that a higher ratio of Cu+ can reduce the Pd2+ concentration and limit the CO oxidation side reaction,thus increased the DEC selectivity. This could explain the higher catalytic performance of the PdCl2/Cu-Cu2O catalyst,which had a greater concentration of Cu+ species. From the above results,it can be concluded that the active species in the synthesis of DEC is Cu+.
Figure 4 displays the XRD patterns of the PdCl2/Cu-Cu2O catalysts. It is well known that glucose is a strong reducing agent,and so Cu2O can be expected to be further reduced to Cu in the presence of an excess of glucose,such that a two phase composition is produced. The diffraction peaks at 2θ values of 43.30°,50.43° and 74.13° be assigned to Cu(111),(200) and (220) (JCPDS 04-0836). While the peaks at 36.42°,42.30°,61.34°,73.53° and 77.32° are attributed to the (111),(200),(220),(311) and (222) reflections of Cu2O (JCPDS 05-0667),respectively. The relative intensities of the Cu2O diffraction peaks are seen to decrease with increasing glucose amounts,indicating that the PdCl2/Cu-Cu2O catalyst prepared with 10 mmol glucose contained more of the Cu2O,in accordance with its improved catalytic performance (Table 2). That is to say,the PdCl2/Cu-Cu2O catalyst prepared with 10 mmol glucose had a higher Cu2O content showed better catalytic performance. Accordingly,Cu+ was the more active species in comparison with Cu0.
The thermal stability of each PdCl2/Cu-Cu2O catalyst was assessed by TG between 25 and 800 °C in under O2,with the results shown in Fig. 5. These data allowed the comparison of experimental and theoretical mass gain rates,assuming that the theoretical mass gain rates were limited by the amounts of Cu and Cu2O,respectively. That is to say,supposing that the catalyst was composed purely of Cu2O,the theoretical mass gain rate can be calculated by the chemical equation 2Cu2O + O2→4CuO with the same holding true for Cu,in which case the equation is 2Cu+O2→2CuO. Theoretical weight gain rate of Cu is 25.11% and that of Cu2O is 11.12%. If the theoretical mass gain rate based on Cu2O is closer to the actual rate,this suggests that the catalyst contained a greater proportion of Cu2O. The PdCl2/Cu-Cu2O catalyst prepared with 10 mmol glucose had an experimental mass gain rate of 13.89% which was closer to the calculated rate based on Cu2O of 11.12%,compared with the other catalysts. Thus,the PdCl2/Cu-Cu2O catalyst prepared with 10 mmol glucose had more Cu2O,which resulted in better catalytic performance,as further confirmed by the XRD analysis.
SEM images (Fig. 6(a−c)) of PdCl2/Cu-Cu2O catalysts made with varying amounts of glucose were acquired,and all the catalysts displayed a sphere-like morphology. The low-magnification TEM image in Fig. 6(d) shows the morphology of PdCl2/Cu-Cu2O nanoparticles generated using 10 mmol glucose. Fig. 6(e) is a high-resolution TEM (HRTEM) image taken from the red sphere region in Fig. 6(d),showing that the plane spacings of 0.24 and 0.18 nm correspond to the d spacings of Cu2O(111) and Cu(111),respectively.
The entire XPS spectrum of the PdCl2/Cu-Cu2O catalyst made with 10 mmol glucose is shown in Fig. 7(a),in which both Cu 2p and O 1s peaks are observed. Fig. 7(b) shows a high resolution view that emphasizes the main Cu 2p3/2 and Cu 2p1/2 peaks of the catalyst. The broad Cu 2p3/2 peak has been divided into two peaks,attributed to Cu [41] and Cu2O [42],respectively. It has been reported that PdCl2 plays an important role in catalytic reactions [12],although a metallic Pd peak is not clearly observed in Fig. 7(a) because of the low PdCl2 concentration in the catalyst. Hence,high resolution XPS spectra of the Pd 3d (Fig. 7(d)) and Cl 2p (Fig. 7(e)) regions of the catalyst are presented to illustrate the presence of PdCl2.
Both CuO and Cu-Cu2O nanoparticles were successfully synthesized by a simple hydrothermal method and subsequently used as supports to prepare catalysts for the gas phase synthesis of DEC. The results of this work show that catalysts prepared with 5 mol/L NaOH exhibit better catalytic performances because of their low extent of powder agglomeration. It is noteworthy that the PdCl2/Cu-Cu2O showed better catalytic performance than PdCl2/CuO made at the same NaOH concentration. In Pd(II)-Cu(II) (PdCl2/CuO) catalysts,there is an induction period,during which Pd(II) is reduced to Pd(0),that must occur prior to electron transfer between Pd and Cu,and this can slow the catalytic reaction. To further pinpoint the active species,PdCl2/Cu-Cu2O catalysts with different Cu2O contents were prepared by controlling the dosages of glucose. XRD,TG and catalytic performance data all indicate that the PdCl2/Cu-Cu2O catalyst prepared with 10 mmol glucose had more Cu+ and also showed better catalytic performance. The selectivity for DEC based on EtOH reached 97.9%,together with an EtOH conversion value of approximately 7.2% and a STY for DEC of 151.9 mg∙g-1∙h-1. The higher concentration of Cu+ in the PdCl2/Cu-Cu2O catalyst evidently lowered the Pd2+ concentration and limited the CO oxidation side reaction,thus DEC selectivity was increased and catalytic performance was enhanced. From the above results,it can be concluded that the active species in the synthesis of DEC is Cu+. However,it would still be beneficial to study PdCl2/Cu2O catalysts without Cu in the future to obtain a more complete understanding of this catalyst system.
2015, Vol. 36 Issue (11): 2036-2043
PDF (1201 KB)
2015, Vol. 36 Issue (11): 2036-2043
PDF (1201 KB)