Facile Synthesis of Cu/Al2O3with High Copper Dispersion for Direct Synthesis of Dimethyl Ether from Syngas

Wang Yan; Chen Yuexian; Fan Binbin; Zheng Jiajun; Li Ruifeng

(College of Chemistry and Chemical Engineering, Taiyuan University of Technology,Taiyuan 030024)

Facile Synthesis of Cu/Al2O3with High Copper Dispersion for Direct Synthesis of Dimethyl Ether from Syngas

Wang Yan; Chen Yuexian; Fan Binbin; Zheng Jiajun; Li Ruifeng

(College of Chemistry and Chemical Engineering, Taiyuan University of Technology,Taiyuan 030024)

The Cu/Al2O3catalysts were prepared by the solution combustion synthesis method using aluminum nitrate and copper nitrate as oxidants and citric acid as fuel. The Cu/Al2O3catalysts were characterized by XRD, N2adsorptiondesorption, SEM-EDS, and H2-TPR techniques. The test results indicated that this catalyst contained mesoporostity and highly dispersed copper. The Cu/Al2O3catalysts were utilized as bifunctional catalysts in the syngas-to-dimethyl ether (STD) process. The CO conversion and DME selectivity were 70.9% and 58.8%, respectively. Moreover, the CO conversion kept increasing from 260oC to 320oC thanks to the high copper dispersion, which could reduce copper sintering at high reaction temperatures.

syngas; dimethyl ether; solution combustion; bifunctional catalyst

1 Introduction

Dimethyl ether (DME) has been considered as a clean alternative fuel without NOx, SOx, and soot emissions[1]. DME has a high cetane number, which has the potential to replace diesel fuel or can act as the liquefed petroleum gas (LPG)[2]. Therefore, designing more effective catalysts for DME synthesis has been garnering significant attention.

Traditionally, DME has been produced by methanol dehydration using an acid catalyst. Recently, the direct synthesis of DME from syngas (STD) has been attracting a growing interest because of its comparative advantages in the thermodynamic and economic sense[3]. Physically mixed methanol synthesis catalysts (Cu/ZnO or Cr/ZnO) and solid acid catalyst (such as Al2O3or zeolites) are usually employed in the STD processes[4-5].

Alumina is an important support in catalysis[6], especially nowadays when the ordered mesoporous alumina supported metal oxide has emerged as a new group of functional materials with enhanced catalytic activity and selectivity[7]. Alumina has also been widely used as a methanol dehydration catalyst due to its moderate acidity, low cost, and easy availability[8]. Recently, Yuan, et al. reported a facile route to synthesize the ordered mesoporous alumina by the solvent evaporation induced self-assembly (EISA) method[9]. Later, a series of alumina supported metal oxides have been synthesized by this method[10-11]. Compared to the conventional wet impregnation method, the mesoporous alumina supported metals prepared by EISA method were reported to have high-quality mesoporous structures that exhibited strong metal-support interactions and homogeneous distribution of active sites that demonstrated strong metal-support interactions[12].

Jiang, et al. extended the EISA method for the synthesis of mesoporous Cu-Al2O3, and utilized this material as a bifunctional catalyst for one-step synthesis of dimethyl ether from syngas showing excellent catalytic performance[13]. However, this EISA method requires a strict control of experimental conditions and relatively expensive Pluronic P123 as a structure directing agent which would restrict its industrial application. The solution combustion synthesis (SCS) technique is a fast and effective route to synthesize the Al2O3based materials, which is capable of achieving high output synthesis of fne and nanostructured materials[14-15]. Zeng, et al. prepared a Ti doped Ni/γ-Al2O3catalyst by the SCS technique, and this catalyst showed excellent catalyticperformance for CO methanation[16]. The present work attempts to synthesize the Cu/Al2O3catalysts with high dispersion of copper via a facile SCS technique in order to investigate its catalytic performance in the STD process.

2 Experimental

2.1 Synthesis procedure

The starting materials for preparing CuO/Al2O3catalyst were synthesized via a SCS route using analytically pure reagents Al(NO3)3·9H2O and Cu(NO3)2·3H2O as the oxidants and citric acid (C6H8O7) as the fuel. Firstly, 35 mmol of citric acid, 34 mmol of Al(NO3)3·9H2O, and 6 mmol of Cu(NO3)2·3H2O were dissolved in 10 mL of deionized water under magnetic stirring. The total amount of metal species was kept constant (40 mmol), and the copper/aluminum molar ratio was adjusted accordingly. The obtained solution was preheated from room temperature to 800oC at a heating rate of 3oC/min in air for 2 h. The fnal samples were denoted asxCA, in whichxindicates the copper molar percentage, C indicates the copper, and A indicates the aluminum. For example, 5CA refers to the mesoporous aluminum oxides with a 5% molar fraction of copper. The final samples were granulated into grains with the particle size ranging from 0.85 mm to 1.70 mm before the catalytic performance test.

2.2 Characterization

The powder X-ray diffraction patterns were recorded on a Shimadzu XRD-6000 diffractometer using Ni-filtered Cu Kα (0.154 nm) radiation. The transmission electron microscopy (TEM) experiments were performed on a JEOL JEM-2100F microscope operated at 200 kV. The catalysts were characterized in terms of their specific surface area (BET), total pore volume, and average pore diameter by the N2adsorption-desorption technique using a NOVA 1200e gas sorption analyzer. The average pore diameter was calculated using the Barret-Joyner-Halenda (BJH) pore size model applied to the adsorption branch of the isotherm.

The morphology and elemental distribution of the catalyst surface were analyzed by a Hitachi S-4800 scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDS). The temperature programmed reduction (TPR) was performed on a Micromeritics Chemisorb 2720 analyzer loaded with 50 mg of sample. The sample was frst purged with He gas at 300oC for 1 h, then cooled down to 50 °C and switched to a gas mixture of 10% H2in argon at a fow rate of 30 mL/min. The temperature was raised from 50oC to 600oC at a heating rate of 10oC/min.

The NH3-TPD analysis was performed by loading 100 mg of sample, which was activated in helium at 400oC for 1 h and then exposed to 5% NH3in N2gas at 50oC for 0.5 h. The sample was then purged with He for 0.5 h at 50oC to remove physically adsorbed NH3. The TPD measurements were conducted from 50oC to 600oC at a heating rate of 10oC/min in the helium gas fow. The TPD signals were recorded by a TCD detector.

The X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 spectrometer with a mono-chromatized Al Kα source (1 486.6 eV) at a constant analyzer pass energy of 30.0 eV. All binding energies were corrected by using the C 1s (284.6 eV) peak of the contaminant carbon as an internal standard.

2.3 Catalytic activity tests

Before the reaction, 1 g of catalyst was reduced in situ at 280oC in a fxed-bed stainless steel reactor (with a internal diameter of 8 mm, and a length of 120 mm) using a 5% H2in nitrogen gas fow (at a fow rate of 50 mL/min) for 4 h. After reduction, the syngas (H2/CO=2) was introduced into the reactor under a reaction pressure of 5 MPa with a gas hourly space velocity (GHSV) of 1 500 mL/(h·gcat) and at a reaction temperature of 260—320oC. The effluent products were analyzed by an online gas chromatograph (GC) (Shimadzu, TCD). The CO conversion and products selectivity were calculated based on the total carbon balance. All analytical equipment lines and valves were heated to prevent possible condensation of the products before entering the GC to ensure a reliable materials balance.

3 Results and Discussion

Figure 1A presents the small angle XRD patterns of samples with different copper contents. The sample 5CA exhibited a diffraction peak at 1.4°, indicating to the presence of mesoporosity. With the copper content increasing up to 10%, the intensity of this peak decreased,and when the copper content reached 15%, the peak was not detected any more. These results indicated that the increasing copper content caused a loss of the pore structure. The wide angle XRD pattern (Figure 1B) of 5CA indicated that this sample was amorphous. However, the peaks at 31.5°, 37.1°, 45.2°, 55.9°, 59.7° and 65.7° were observed on samples with copper content exceeding 10%, indicating to the formation of copper aluminate spinel (CuAl2O4, ICDD PDF # 33-0448)[17]. No diffraction peaks of CuO were detected in any sample. All the XRD results indicated that copper was incorporated into the alumina matrix, forming a Cu-Al-O crystalline network suggesting that copper was highly dispersed in the samples.

Figure 1 Small angle (A) and wide angle (B) XRD patterns of Cu/Al2O3with different molar fractions of copper and aluminum

Figure 2 shows the nitrogen adsorption-desorption isotherms and the corresponding pore size distribution of Cu/Al2O3with different Cu contents. The isotherms of the samples were type IV, indicating to the formation of mesopores in these samples. The corresponding pore size distribution curves in Figure 2B revealed that the pore size distribution centered at around 3 nm except 25CA, which had a relatively wide pore size distribution with its pore size distribution centering at around 8 nm. With the increase in copper content, the pore size distribution became wider. The pore structure parameters of samples with different copper contents are listed in Table 1. With different copper contents, BET surface area varied from 28.5 m2/g to 81.9 m2/g, while the sample 15CA demonstrated a highest BET surface area among all of the samples. The total pore volume increased from 0.03 cm3/g to 0.13 cm3/g with an increasing copper content.

Figure 2 N2adsorption-desorption isotherms (A) and the corresponding pore size distribution curves (B) of Cu/Al2O3with different copper contents

Table 1 Pore structure parameters of Cu/Al2O3catalyst with different copper contents

The SEM image and surface elemental mapping EDS analysis of 10CA are depicted in Figure 3 in which a homogenous dispersion of Al, Cu and O elements could be visualized. These data indicate the homogeneous distribution of copper throughout the alumina matrix.

Figure 3 SEM image and surface mapping EDS analysis of 10CA

The XPS spectra of Cu2p level for the samples 5CA, 20CA and 25CA are shown in Figure 4. The binding energy at 932.3 eV and 933.9 eV could be attributed to the CuO and Cu2O phases in the samples, respectively[18]. In the sample 5CA, Cu species mainly existed in the form of CuO and Cu2O phases. Along with the increase in Cu content, the samples were mainly composed of CuO according to the XPS results.

Figure 4 XPS Cu 2p spectra of the samples

During the direct synthesis of DME from syngas, the copper species in the Cu/Al2O3catalyst after being reduced are expected to work as the active component for methanol synthesis and Al2O3works as the acid component for methanol dehydration. The H2-TPR profile of the sample 5CA showed no obvious peak (Figure 5), which could be ascribed to its relatively low copper content. According to previous TPR studies, the temperature for reduction of copper oxide to bulk copper aluminum oxide ranged between 200oC to 250oC[19]. In this work, the temperature for reduction of copper oxide to bulk copper aluminum oxide was around 157oC for the samples with copper contents exceeding 10%. The relatively lower reduction temperature suggested a better dispersion of copper in bulk mixed oxides[20]. Moreover, a reduction peak at around 390oC was observed when the copper content was more than 15% in the samples, and this peak increased with an increasing copper content. According to literature reports, copper is incorporated into the alumina matrix to form Cu-Al-O crystalline network which is reduced at 390oC, indicating to a homogeneous copper distribution[13]. The H2-TPR results suggest that a relatively high and homogeneous copper distribution was formed in the Cu/Al2O3catalyst, which was in agreement with the XRD and mapping EDS analysis results. Moreover, more Cu speices were incorporated into the matrix of alumina to form the Cu-Al-O structure in 25CA as compared with other samples.

Figure 5 H2-TPR profiles of Cu/Al2O3with different copper content

Table 2 shows the catalytic performance of Cu/Al2O3catalyst in the STD process. With the copper content increasing from 5% to 20%, the CO conversion dramatically surged from 17.4% to 70.9%. However, when the copper content in the catalyst was equalto 25%, the CO conversion rate dropped to 45.0% because of the increased formation of Cu-Al-O species that replaced the formation of CuO species. Meanwhile, the selectivity of DME was around 58% for the samples 10CA, 15CA and 20CA. The selectivity of DME on 5CA and 25CA was 34.4% and 46.6%, respectively, which was lower than that achieved by other catalysts. The selectivity of methanol was lower than 1% for the Cu/Al2O3catalyst except 5CA. The selectivity of hydrocarbons was more than 5.9% for the Cu/Al2O3catalyst, due to the relatively high reaction temperature of 320oC. The Cu/Al2O3catalyst showed high activity, indicating that the Cu/Al2O3catalyst that was synthesized via the solution combustion method possessed both the CO hydrogenation activity and the acidity for the coupling step in STD reaction. Moreover, the results further confirmed the presence of ZnO, which was previously assumed to be an essential factor comprising the activity in methanol synthesis and now was proved to be not mandatory[13].

Table 2 Catalytic performance of Cu/Al2O3with different copper contents during STD process

The NH3-TPD profiles of samples with different Cu contents are shown in Figure 6. The desorption peak at 125 °C and the shoulder peak at 250 °C could be observed on all samples, which should be attributed to the weak acid sites and medium strong acid sites, respectively. No desorption peak attributed to strong acid sites could be observed in any of the samples. Moreover, the amount of acid sites was almost the same in all the samples. The differences in STD performance were largely dependent on the Cu phase existing in all samples. The sample 20CA showed a highest STD catalytic activity due to the larger amount of the CuO phase as compared with other samples.

Figure 6 NH3-TPD profiles of the samples

The effect of reaction temperature on the catalytic performance of the sample 20CA was investigated, with the results shown in Figure 7. Experiments were carried out by varying the temperature in the range of between 260—320oC while maintaining the pressure at 5 MPa and GHSV at 1500 mL/(h·gcat). As the reaction temperature increased from 260oC to 320oC, the CO conversion increased dramatically from 14.2% to 70.9%. According to literature report[20], the bifunctional catalyst could lose its activity in the STD process due to Cu sintering when the reaction temperature was higher than 280oC. However, the CO conversion over the sample 20CA kept increasing even when the reaction temperature increased to 320oC, which could be attributed to the highly dispersed Cu species that were incorporated into the alumina matrix, which could prevent the Cu species from sintering at a relatively high reaction temperature. Meanwhile, the DME and methanol selectivity remained almost constant in the temperature range investigated thereby. When the reaction temperature increased over 320oC, hydrocracking reactions were favored, which could lead to the increase in hydrocarbon selectivity.

Figure 7 CO conversion and selectivity of DME, methanol, and hydrocarbons over 20CA catalyst at different reaction temperatures

4 Conclusions

The Cu/Al2O3catalysts were prepared by a solution combustion synthesis method and used for direct synthesis of DME from syngas. The preparation procedure could embody a 100% atom economy of metal elements. It is a facile and green route to synthesize catalyst for the STD process. The Cu/Al2O3catalysts were characteristic of high copper dispersion and mesoporosity. During the STD process, the catalytic activity increased when the copper content increased from 5% to 20%, and then decreased with a further increase of the copper content to 25%. The STD activity results indicated that promoters such as ZnO were not essential for attaining high STD activity. The effect of reaction temperature showed that the CO conversion keeps increasing from 14.2% to 70.9% as the reaction temperature increased from 260oC to 320oC. Meanwhile, the DME selectivity remained almost constant. The high activity of Cu/Al2O3catalysts at 320 °C could be attributed to its high copper dispersion that could reduce the copper sintering at high reaction temperatures.

Acknowledgement: This work is supported by the joint funds of the National Natural Science Foundation of China–China Petroleum and Chemical Corporation (the state key program grant No.U1463209), the Shanxi coal based low carbon joint fund of the National Natural Science Foundation of China (the state key program grant No. U1610223), and the Shanxi Province Science Foundation for Youths (201701D221040).

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Successful Preparation of Acetal Hydrogenation Catalyst at FRIPP

The technical achievement relating to the research on acetal hydrogenation catalyst for manufacturing high-purity 1,4-butanediol and its commercial scaleup technique developed by the SINOPEC Fushun Research Institute of Petroleum and Petrochemicals (FRIPP) has passed the technical appraisal organized by the Science and Technology Division of the Sinopec Corp. The experts attending the appraisal meeting, after having unanimously recognized that this hydrogenation catalyst has an appropriate pore structure and acidic property along with good lowtemperature reducibility with all quality indicators reaching or exceeding the level of similar overseas catalysts, have recommended to conduct a commercial scale test as soon as possible.

This acetal hydrogenation catalyst has better hydrogenation activity, selectivity and low-temperature reducibility, in particular its suitability for lowconcentration feedstock. The bench test for catalyst evaluation has revealed that under the proper conditions of temperature, pressure and space velocity the acetal conversion can reach 100%, with the molar yield of 1,4-butanediol equating to nearly 100%. Currently the commercial scale-up tests of this catalyst had been accomplished and the catalyst performance had been assessed in more than 1000 hours. The results have shown that the catalyst prepared in the commercial scale-up tests had its performance similar to that of the catalyst obtained in the bench tests, attesting to the good repeatability and reproducibility of catalyst performance.

date: 2016-12-22; Accepted date: 2017-03-09.

Dr. Wang Yan, Tel./fax: +86-351-6010121; E-mail: wangyan@tyut.edu.cn. Dr. Li Ruifeng, E-mail: rfi@tyut.edu.cn.