Effect of Ball-Milling Time on the Performance of Ni-Al2O3 Catalyst for 1,4-Buty

Gao Xianlong; Mo Wenlong; Ma Fengyun; He Xiaoqiang

(Key Laboratory of Coal Clean Conversion & Chemical Engineering Process (Xinjiang Uyghur Autonomous Region), College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046)

Abstract: The Ni-Al2O3 catalyst was prepared by the mechanochemical method in combination with a planetary ballmilling machine. Effect of milling time on the crystal structure, the reduction characteristics and the catalytic performance of Ni-Al2O3 catalyst for hydrogenation of 1,4-butynediol to produce 1,4-butenediol were investigated. The catalysts were characterized by PSD, EDX, XRD, H2-TPR, BET, TEM, and NH3-TPD methods. Results showed that the MCt2.5 catalyst treated at a ball milling time of 2.5 h could form a smallest particle size of 191.0 nm. The evaluation experiments revealed that the activity of the prepared catalyst increased at first and then reached a constant value with the extension of ballmilling time. The BYD conversion, BED selectivity and yield on the MCt2.5 catalyst reached 35.63%, 33.48% and 32.46%,respectively, which were higher than those obtained by other samples. The excellent performance of MCt2.5 sample is mainly related to the following three reasons from characterization results. Firstly, it has a smallest particle size of 191.0 nm;and then, the surface acidity (in terms of strong acids) of the catalyst was weaker than other catalysts; and eventually, the loading amount (23.84%) of the active component Ni exceeded the theoretical value (20%).

Key words: mechanochemical method; Ni-Al2O3 catalyst; 1,4-butynediol; ball-milling time

1 Introduction

1,4-Butenediol (BED), is one of the important chemical raw materials and is widely used in manufacture of pesticides, pharmaceutical intermediates, chemicals,batteries, papermaking, etc[1-4]. The first step for BED production is the formation of propynol from acetylene and formaldehyde; and then propynol reacts with excess formaldehyde to form 1,4-butynediol. The 1,4-butenediol could be obtained from 1,4-butynediol by hydrogenation in the presence of catalyst. Literature reports showed that the Ni-based catalyst has attracted the attention of many researchers because of its high hydrogenation activity and low cost[5-10].

Ball-milling process might affect the structural properties of the catalyst such as particle size, crystal structure, and surface morphology. Liu Z Y, et al.[11]prepared CNT/Al composites by the ball-milling method in order to investigate the microstructure and evolution process of the composites, and the tensile strength was tested for characterizing their properties. With the increase of ballmilling time, the CNTs were gradually dispersed into the Al matrix. The dispersion of the CNT at a ball-milling time of 6 h was more uniform, and serious damage of CNT would be observed with the extension of the ballmilling time. Barkhordarian G, et al.[12]have explored the effect of milling time on the adsorption of hydrogen by magnesium. Results showed that the adsorption activation energy would increase as the milling time was extended.Voicu C, et al.[13]prepared Ni/Al2O3composite powders with a planetary ball-milling machine to investigate the properties of the prepared materials by changing the ball-milling time. Judging from the SEM (scanning electron microscopy) and the OM (optical microscopy)characterizations, high dispersion of Ni and Al2O3could be observed after the ball-milling time was over 90 min.

The grain size of Al2O3and Ni reduced to 196 nm and 30 nm, respectively, when the ball-milling time was increased to 120 min. Wang L, et al.[14]signified the effects of milling time on the electrochemical properties of the composites, which were prepared by mixing La2Mg17and 200% of Ni in aqueous solution. Test results indicated that both the discharge and the cycle stability were improved by increasing the ball-milling time.Therefore, the particle size, the grain size distribution, the active component dispersion, and the phase structure of catalyst could be affected by the preparation process.

Recently, the low-temperature solid phase method[15-16]has attracted much attention of many researchers because of its simple process and easy operation, since it is rarely used in the preparation of catalysts. Mo, et al.[17]has employed planetary ball-milling machine to prepare the Ni-Al2O3catalyst. Results showed that a smallest particle size of 141 nm could be observed for RT60 catalyst prepared at a ball-milling time of 60 min, and the catalyst exhibited good performance, with CO conversion and CH4yield reaching as high as 87.9% and 74.3%, respectively.

In this work, the Ni-Al2O3catalyst was prepared by a mechanochemical method ― one of the low-temperature solid phase methods ― coupled with planetary ballmilling machine, and the prepared catalyst was applied for testing the hydrogenation performance of catalyst for treating 1,4-butynediol to produce 1,4-butenediol. The catalysts were characterized by PSD, EDX, XRD, H2-TPR, BET, TEM, and NH3-TPD techniques. The effect of milling time on the structure and performance of Ni-Al2O3catalyst was investigated.

2 Experimental

2.1 Preparation of catalyst

The Ni-Al2O3catalyst with a Ni loading of 20% was prepared by the mechanochemical method. Ni(NO3)2·6H2O (A.R.,Shanghai Shanpu Chemical Co., Ltd.) and Al(NO3)3·9H2O(A.R., Tianjin Zhiyuan Chemical Reagent Co., Ltd.) were weighed according to their required stoichiometric ratio,and a certain amount of precipitant NH3·H2O (A.R., Tianjin Zhiyuan Chemical Reagent Co., Ltd.) was added to the tank of the planetary ball-milling machine (XQM-2, Changsha Tianchuang Powder Technology Co. Ltd., China). The obtained samples were placed in a fixed bed reactor to be subject to reduction for 3 h at 923 K with a flow of H2introduced at a rate of 40 mL/min. The prepared catalysts were labeled as MCtX according to the grinding time of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 h, respectively. (X represents the ball-milling time, for instance, a catalyst treated at a ball milling time of 1.0 h is labeled as MCt1.0.)

2.2 Characterization of catalyst

The particle size distribution of the Ni-Al alloy powder was measured by a laser particle size distribution (LPSD)analyzer (BT-9300Z, Shanghai Shengke Instrument Equipment Co., Ltd., China), and distilled water was used as the dispersion agent. The X-ray diffraction (XRD) analysis was carried out on a X-ray diffractometer (Rigaku D/Max-2500, Japan) using nickel filtered Cu Kα (λ= 0.15406 nm)radiation. The scan rate, diffraction range, tube voltage,and tube current were 8(°)/min, from 5° to 85°, 40 kV, and 100 mA, respectively. The nitrogen adsorption-desorption profiles at -196 °C were obtained by a Quantachrome automated gas sorption apparatus (Micromeritics ASAP 2020). The energy dispersive X-ray (EDX) analysis was carried out on a German LEO 1530VP spectrometer with an accelerating voltage of 20 kV, a working distance of 15 mm,and an acquisition time of 120 s. The scanning electron microscopy (SEM) images were obtained on a Hitachi H-600 microscope with an accelerating voltage of 100 kV.The transmission electron microscopy (TEM) micrographs were obtained using a JEOL JEM-2100 election microscope operating at 200 kV. Acidic properties of the catalysts were measured via temperature-programmed desorption of ammonia (NH3-TPD) using a Quantachrome Chemisorb instrument. The temperature-programmed reduction with H2(H2-TPR) was carried out on an automated chemisorption analyzer (chem-BET pulsar TPR/TPD, Quantachrome).

2.3 Catalytic performance

The BYD hydrogenation reaction was conducted in a 50-mL high-pressure reactor (Dalian Tongda Reactor Factory, CJF-605, China), and the liquid phase products were analyzed using a gas chromatograph (GC-2014C,Shimadzu Instrument Co. Ltd., Japan). The BYD hydrogenation reaction was carried out at 110 °C under a pressure of 4.0 MPa for 3 h, with a stirring rate of 500 r/min and a catalyst dosage of 0.3 g. The feedstock was comprised of 30 mL of aqueous solution containing 35% of 1,4-butynediol.

1,4-butanediol, 1,4-butene glycol, 1,4-butynediol, and 4-hydroxybutyral are the main substances among the products from catalytic hydrogenation of 1,4-butynediol,which were detected by a SH-Rtx-Wax capillary column.The performance of hydrogenation catalyst was evaluated in terms of the BYD conversion, the BED selectivity, and the BED yield.

3 Results and Discussion

3.1 Catalyst characterization

3.1.1 Particle size distribution

Figure 1 shows the particle size distribution of the catalyst prepared at different ball-milling time. Results showed that the most probable particle size of the sample at first decreased and then increased with an increasing ball-milling time. This might be the reason that the sample was not fully ground when the ball-milling time was less than 2.5 h. Agglomeration of the particles would occur with the extension of milling time, resulting in large particles, while the MCt2.5 catalyst showed a more uniform particle size distribution. Therefore,there was an optimal value for the influence of ball-milling time on the particle size of the prepared catalyst. To illustrate the effect of ball-milling time on the particle size of catalyst,the samples prepared were characterized by SEM technique.

Figure 1 Effect of milling time on the particle size of the catalyst after calcination

3.1.2 Scanning electron microscope

Figure 2 shows the surface morphology of the catalysts obtained at different ball-milling time. The particle size of catalyst firstly grew up, and then trailed off with the increase of milling time. Furthermore, the MCt2.5 catalyst after being turned into finest particles could achieve a more uniform particle size distribution, which was in accordance with LPSD analysis (Figure 2 (f) ). And there was an available value related with the effect of ball-milling time on the particle size of the catalyst. In addition, the ballmilling process made the catalyst particles finer, while the particle agglomeration appeared at a ball-milling time of 3 h. It can be found that a ball-milling time of 2.5 h could make the particles re fined to provide more reaction surface for the hydrogenation of 1,4-butynediol.

Figure 2 SEM image of the catalyst (a) MCt1.0; (b) MCt1.5; (c) MCt2.0; (d) MCt2.5; (e) MCt3.0;(f) the most probable particle size from LPSD characterization

3.1.3 X-ray diffraction analysis

Figure 3 gives the XRD profiles of the catalysts. It can be observed from Figure 3(a) that there are characteristic peaks of Al2O3at 2θ = 37.5°, 45.5°, and 65.9° for the catalysts, and the peak intensity was almost the same for samples treated at different ball-milling time, indicating that Al2O3as a carrier was not damaged by the ballmilling treatment. In addition, when the ball-milling time was less than 1.0 h, the NiO crystal could not found in the XRD pro files because NiO might be highly dispersed on Al2O3or entered the Al2O3lattice. Furthermore, it was obvious that the NiO peak appeared when the ball milling-time reached 1.5 h. The NiO diffraction peak position and the peak intensity of MCt2.0, MCt2.5 and MCt3.0 catalysts did not change with the increase of ballmilling time. Additionally, the crystal phase structure of NiO at 2θ = 43.4° and 63.2° did not change when the ballmilling time exceeded 2 hours.

Figure 3 XRD pattern of the calcinated (a)and the reduced (b) catalysts

It can be seen from Figure 3(b) that the diffraction peak position and intensity of Al2O3demonstrated good stability during the high temperature reduction process, indicating that the crystal structure of the carrier was not damaged. And the characteristic peaks of NiO disappeared, demonstrating that the catalyst was successfully reduced and could be used for hydrogenation of 1,4-butynediol. It can be obviously observed that the characteristic diffraction peak of the active component Ni was only found at 2θ = 51.2° in Figure 3(b), and the diffraction peak intensity did not change when the ballmilling time changed from 1.0 h to 2.0 h. When the ballmilling time was over 2.5 h, the diffuse characteristic peaks of Ni could be observed.

3.1.4 Temperature programmed reduction

Figure 4 shows the H2-TPR results of the catalysts. It can be seen from Figure 4 that there are three types of reduction peaks appearing in the H2-TPR profiles. The first type was attributed to the α type reduction peak,which appeared at about 500 °C, indicating the weak interaction between the active component and the carrier.The second reduction peak could be assigned to the β type with the reduction temperature reaching at about 650 °C, showing the strong interaction between the active component and the carrier than the α-type peak. The last peak might be ascribed to the γ-type NiO species with the reduction temperature reaching 700 °C, corresponding to the strong interaction of Ni species and the carrier,e.g. NiAl2O4spinel, which could be reduced at a higher temperature than the α and β peaks. However, the characteristic diffraction peak of NiAl2O4was not found in the XRD characterization profiles, indicating that the active component precursor might mainly exist in the form of lattice NiO in the catalyst.

Figure 4 H2-TPR results of the calcined samples

It can be also observed from Figure 4 that the reduction peaks of all catalyst samples were mostly the β and γ peaks, and the α peak was less, indicating that the active component Ni of the catalyst had a strong interaction with the carrier, which was beneficial to improving the dispersion and stability of the active metal Ni and preventing the sintering and growth of particle size at high reaction temperature.

3.1.5 NH3temperature programmed desorption

The NH3-TPD patterns for different catalysts are shown in Figure 5. All samples displayed the first distinct NH3desorption peak at around 250 °C and the second peak at around 700 °C. Thus there were two acidic centers on the surface of the catalysts. The former was located at 200-300 °C, corresponding to the weak acid center, and the latter was located at 600-750 °C, corresponding to the strong one[18-19].

Figure 5 NH3-TPD results of the calcined samples

The peak areas of the weak acid for MCt1.5 sample were reduced compared to MCt1.0, indicating the decrease of acid sites. The peak temperature of the strong acid peak of the MCt1.0, MCt1.5, and MCt2.0 catalysts shifted to the low temperature region, indicating the decreased acidity of the catalysts. In addition, the surface acidity of the MCt2.5 and MCt3.0 catalysts would not change significantly when the ball-milling time was over 2.0 h.

3.1.6 N2adsorption-desorption analysis

(1) N2adsorption-desorption characterization

Figure 6 depicts the N2adsorption-desorption isotherms(a) and pore size distribution (b) of the prepared catalysts.All samples had similar trend of the N2adsorption desorption curves at different milling time of catalysts.According to the IUPAC classification, the N2adsorption desorption isotherms of all the samples were attributed to type III, indicating that the prepared catalysts had their adsorption promoted[20].

Figure 6 N2 adsorption-desorption isotherms (a) and pore distribution (b) of the catalysts

Additionally, in the range of p/p0= 0.5—0.8, there was a significant H4 type hysteresis loop in the samples,indicating that the material pores had a slit structure,and it was beneficial to promote the internal diffusion of reactants and products.

Figure 6(b) shows the BJH pore size distribution curves of the catalysts at different ball-milling time. The pore size of all samples was in the range of 40—80 nm,indicating that the prepared catalysts were of typical mesoporous materials, which is mainly attributed to the typical mesoporous carrier Al2O3.

(2) Pore structure parameter

According to the N2adsorption-desorption isotherms, the specific surface area of the samples was calculated by the BET equation, and the pore volume and average pore diameter were calculated by the BJH equation, as shown in Table 1. The specific surface area of all the samples was around 270 m2/g, demonstrating that the ball-milling time had little effect on the specific surface area of the catalysts. The pore volume was about 0.40 cm3/g, and the average pore diameter was in the range of 4.30-4.61 nm,indicating that the prepared catalysts were the type of catalysts with mesoporous pores, which were consistent with the N2adsorption-desorption curves.

Table 1 Specific surface area, pore volume, average pore diameter of the catalysts

3.1.7 Transmission electron microscopy

Figure 7 gives TEM results of the prepared Ni-Al2O3catalysts after reduction. The morphological structure of the catalyst gradually changed from a rod shape to a dipersion layer with ball-milling time ranging from 1.0 h to 1.5 h, and the shape would transform to a flocculent stack as the ball-milling time reached 2.0 h. In addition,the active component Ni was like the cotton particles,which were dispersed in the cotton wool, showing a uniform particle distribution on the carrier Al2O3, which could make the active component distributed stably on the surface of the carrier.

Figure 7 TEM images of the catalysts after reduction

3.1.8 Energy dispersive X-ray

The EDX element quantitative analysis was carried out for the catalysts, with the results presented in Table 2. The EDX method can only test the element content at several nanometer scales on the surface of the sample. The Ni loading was continuously increased from 9.59% for MCt1.0 to 27.14% for MCt2.5. The Ni content of MCt2.5 and MCt3.0 treated at a ball milling duration of 2.5 h and 3.0 h was as high as 23.84% and 27.14%, respectively,which exceeded the theoretical loading value (20%). The reason might be that the surface activity of the obtained carrier was low with less ball-milling time, and the active metal Ni mainly existed in the internal pores of the catalyst. As the ball-milling time was extended, the surface activity of the carrier increased, and the active metal Ni might transfer from the inside pores to the surface of catalyst. Therefore, the ball-milling process would change the distribution of the active component in the carrier, resulting in a large Ni loading difference between the surface and the bulk of the prepared catalyst.

Table 2 Content of elements in samples treated at different milling time

3.2 Catalyst activity evaluation

The activity evaluation results of the catalysts for BYD hydrogenation are presented in Figure 8. It can be seen from Figure 8 that the BYD conversion rates over all the catalyst samples were between 16.15% and 35.63%. The conversion of BYD gradually increased when the ballmilling time was less than 2.5 h, which was ascribed to the gradually changing smaller particle size. The conversion of BYD reached a maximum value of 35.63%as the ball-milling time increased to 2.5 h, with the BED selectivity reaching 91.91% and BED yield equating to 32.64%. And the BYD conversion decreased slightly as the ball-milling time exceeded 2.5 h. The excellent performance of the MCt2.5 catalyst might be mainly related to the following two factors resulted from the ballmilling pretreatment, viz. the small Ni grain size (191 nm)and the active component Ni loading (over 20%).

Figure 8 The conversion of BYD and selectivity and yield of BED achieved by the catalysts

3.3 Characterization of catalyst after hydrogenation process

3.3.1 X-ray diffraction analysis

The catalysts used in hydrotreating process were subjected to XRD characterization, with the results shown in Figure 9. The diffraction peak of the active component Ni still appeared at about 2θ = 52.0°, while the peak intensity and the semi-peak width did not change,indicating that the active component Ni was stable after the hydrogenation process.

Figure 9 XRD patterns of the catalyst samples after hydrogenation reaction

In addition, the positions of Al2O3diffraction peaks were not changed as compared to those of Figure 3(b),indicating that the crystal structure of the carrier was relatively stable in the 1,4-butynediol hydrogenation process.

3.3.2 N2adsorption-desorption analysis

(1) N2adsorption-desorption curve of catalyst after reaction

The used catalysts were characterized by N2adsorption desorption method, as shown in Figure 10. It can be observed that the adsorption isotherms of the catalysts were ascribed to type IV, and the corresponding hysteresis loops belonged to type H4. The hysteresis loop of the used catalyst firstly grew up and then trailed off with the increase of the ball-milling time. Furthermore, the hysteresis loops for the catalysts used in hydrotreating process were closed at a lower relative pressure (p/p0=0.4), which was less than that of the fresh catalysts(p/p0=0.45), which is worthy of further study on these phenomena.

Figure 10 N2 adsorption-desorption isotherms of different Ni content catalysts

(2) Pore structure parameter

Table 3 shows the pore structure parameters of the used catalysts. The specific surface area of each sample was higher than that of the catalysts before hydrogenation reaction. Especially, the surface area of the MCt1.0 catalyst was 378.86 m2/g, which was by 38% higher than the fresh one (274.84 m2/g), and this outcome might be attributed to the less ball-milling time. The increase of specific surface area might be resulted from the large amount of water in the reaction system, leading to the swelling effect on the catalyst particles.

4 Conclusions

The Ni-Al2O3catalyst was prepared by the mechanochemical method. The effect of milling time on the structure of Ni-Al2O3catalyst and its 1,4-butynediol hydrogenation performance was investigated. The surface morphology,the surface acidity, and the crystal phase structure of Ni-Al2O3catalysts presented a significant difference with various ball-milling time. The MCt2.5 catalyst showed good hydrogenation performance, with a BYD conversion of 35.63%, and a BED selectivity of 91.10%, which might be mainly attributed to the high Ni loading (23.84%, which had exceeded the theoretical loading (20%)) to achieve a smaller particle size of 191 nm.

Acknowledgements: This work has been supported by the Xinjiang Uygur Autonomous Region Key R&D Program(2017B02012) and the Xinjiang University Natural Science Foundation Project (BS160221).