Effect of Preparation Process on Compressive Strength and Hydrogenation Performance of Raney-Ni/Al2O3Catalyst

Luo Guohua; Wang Yanli; Sun Daming; Xu Xin; Jin Haibo

(1. College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617; 2. Beijing Key Laboratory of Fuels Cleaning and Advanced Catalytic Emission Reduction Technology, Beijing 102617;3. Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024)

Effect of Preparation Process on Compressive Strength and Hydrogenation Performance of Raney-Ni/Al2O3Catalyst

Luo Guohua1,2; Wang Yanli3; Sun Daming1; Xu Xin1,2; Jin Haibo1,2

(1. College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617; 2. Beijing Key Laboratory of Fuels Cleaning and Advanced Catalytic Emission Reduction Technology, Beijing 102617;3. Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024)

In this study, Raney-Ni/Al2O3catalysts were prepared from precursors which were calcined in different atmosphere (argon and air) and leached for different time. XRD, SEM, H2-TPR and BET techniques were introduced to characterize the catalysts, and hydrogenation of unsaturated hydrocarbons, including vinyltoluene, dicyclopentadiene and indene, was used as probe reaction to evaluate the catalytic hydrogenation performance of the catalysts. The results demonstrated that the calcination atmosphere had noticeable effects on the compressive strength and hydrogenation performance of Raney-Ni/Al2O3. The catalyst calcined in argon exhibited better compressive strength and catalytic performance than the one calcined in air. The compressive strength and activity stability of catalyst were also determined by the leaching time. A too longer leaching time was not conducive to improving the compressive strength and catalytic performance of catalysts. When the precursor calcined in argon was treated in a 15% (by wt.) sodium hydroxide solution at 343 K for 5 h, a catalyst with higher compressive strength (at 26.10 N/mm) was obtained and the conversion of vinyltoluene, dicyclopentadiene (DCPD), and indene could amount to more than 87.5%, 99% and even 100%, respectively, under the reaction conditions covering a temperature of 493 K, a hydrogen pressure of 2.0 MPa, a LSHV of 3.0 h?1, and a hydrogen/oil ratio of 200:1.

nickel-aluminum alloy; pseudo-boehmite; unsaturated hydrocarbon; hydrogenation

1 Introduction

Raney Ni can be applied in petrochemical and fine chemicals processes including hydrogenation, dehydrogenation, and dehalogenation reactions due to its noticeable catalytic activity[1-4]. However, it can only be applied in batch reactors and slurry bed reactors rather than fixed-bed reactors due to its inferior compressive strength[5]. Besides, since Raney Ni tends to selfignition when it is exposed to air, it has to be stored in water or ethanol, which gives high potential risks in catalyst storage and application[6-7]. Therefore, in order to promote the application of Raney Ni catalyst in the fxed-bed reactor, it is necessary to investigate a method for preparing the Raney-Ni catalyst with excellent compressive strength. Several researchers have reported some preparation methods suited for the fxed-bed Raney Ni catalyst[8-10]. For instance, when the nickel-aluminum alloy powder and pseudo-boehmite were used as raw materials, a supported Raney Ni catalyst with a certain compressive strength and high hydrogenation activity for DCPD was obtained via calcining the precursor in air[11-12]. Nevertheless, there were few researches that had referred to the effect of calcination atmosphere and leaching time on properties of supported Raney Ni catalyst. Therefore, researches on the effects of preparation conditions on the physical and chemical properties of Raney-Ni/Al2O3should be carried out.

2 Experimental

2.1 Catalyst preparation

Nickel-aluminum binary alloy powder (having a Ni/Almass ratio of 47.5/51, and a particle size of 48—74 μm, without molybdenum) and pseudo-boehmite powder were commercial samples supplied by the Dalian Tongyong Chemical Co., Ltd. and the Aluminum Corporation of China, Ltd., respectively. These two kinds of powder at a mass ratio of 1:1 were mixed and kneaded with a 8% nitric acid solution to form a gel. Then the mixture was extruded into clover extrudates (2 mm in diameter) and dried at 120oC for 6 h. Subsequently, the dried extrudates were calcined at 860oC for 4 h in air and argon to obtain the precursors, which were labeled as P-Air and P-Ar, respectively. The precursors were leached in an excess of 15% (wt.) sodium hydroxide solution at 60oC for 5 h, and the obtained Raney-Ni/Al2O3catalysts were marked as C-1 (corresponding to P-Air) and C-4 (corresponding to P-Ar), respectively. Besides, P-Ar was leached for 1 h, 3 h, 5 h, and 7 h, respectively, and the corresponding catalysts were labeled as C-2, C-3, C-4 and C-5, respectively.

2.2 Catalytic activity test

Hydrogenation of indene, dicyclopentadiene, and vinyltoluene was served as the probe reaction to evaluate the catalytic activity, and these three kinds of model compounds were diluted by xylene to about 10% (by weight). The hydrogenation process was performed in a fixed-bed reactor, with the schematic drawing of corresponding experimental apparatus for catalytic activity tests shown in Figure 1. Five mL of catalyst (with a particle size 0.25—0.42 mm) were added and then reduced in a hydrogen fow at 250oC for 3 h prior to commencement of each reaction. Hydrogenation processes were carried out under the following conditions, namely:T= 493 K,P(H2) = 2.0 MPa, WHSV = 3.0 h?1, and hydrogen/oil ratio = 200:1. The product was analyzed by an Agilent 6890N gas chromatograph. The catalytic performance was evaluated by the conversion (X) of unsaturated hydrocarbons:

in whichc0andcwere the mass concentration of unsaturated hydrocarbons in the raw material and in the product, respectively.

Figure 1 Schematic drawing of continuous fixed bed reaction experimental apparatus

2.3 Catalysts characterization

The nitrogen adsorption–desorption isotherms of Raney-Ni/Al2O3catalysts were achieved with an Autosorb-1-MP surface area and pore size analyzer (Quantachrome, USA) using the nitrogen adsorption method at 77 K. The samples were degassed under vacuum at 200oC for 4 h. The BET surface areas were determined by the Brunauere-Emmette-Teller (BET) method, and the total pore volume and pore size distribution of catalyst samples were evaluated by the standard Barrett Joyner Halenda (BJH) treatment.

The compressive strength of catalyst was measured by thefollowing method: 20 catalyst extrudates, 1 cm in length, were picked up randomly and measured by a smart strength tester (Dalian Instrument Corporation, China). Then the maximum and minimum values were excluded and the compressive strength of the catalyst sample was calculated by averaging the remaining values.

The morphology of the catalysts was analyzed by scanning electron microscopy using a Quanta 400F electron microscope (FEI Corporation, USA).

The powder X-ray diffraction patterns of catalyst samples were obtained by XRD (D/max-ⅢA, Shimadzu, Japan) measurements using CuKα radiation operated at 36 kV and 20 mA, performing scanning from 10° to 80° at a scanning speed of 4(°)/min.

The temperature programmed reduction (H2-TPR) was carried out in an automatic equipment (Chemisorb 2750, Micromeritics, USA). The sample (0.1 g) was subjected to a heat treatment (from room temperature to 900oC at a temperature increase rate of 15 °C/min) in a gas flow (at a rate of 20 mL/min) consisting of a mixture of N2:H2(at a volume ratio=95:5). The hydrogen consumption was simultaneously measured by TCD.

3 Results and Discussion

3.1 Effect of precursor calcination atmosphere and leaching time on properties of Raney-Ni/Al2O3

In order to investigate the effects of calcination atmosphere and leaching time on physicochemical properties of catalysts, five catalyst samples and the corresponding precursors were characterized by BET, XRD, H2-TPR techniques, and the relevant results are summarized as follows.

The textural properties of the five catalysts are given in Table 1. The BET surface area of the catalyst C-1 prepared from the precursor P-Air was obviously smaller than the one prepared from the precursor that was calcined in argon (C-4), and the compressive strength of C-4 was apparently higher than C-1.

Table 1 Textural properties of Raney-Ni/Al2O3catalysts prepared under different conditions

The inferior compressive strength of C-1 was mainly attributed to the sintering of precursor calcined in air[13]. According to Figure 2 (A), the diffraction peaks of NiAl3in the precursor that was calcined in air (P-Air) almost disappeared, while the diffraction peaks assigned to α-Al2O3were observed simultaneously. Devred, et al.[4]discovered that Ni2Al3formed in the core of alloy particles during the process for preparation of nickel-aluminum powder, while NiAl3was formed on the outer surface, indicating that more NiAl3species were formed on the surface of alloy particles. When the alloy powder was calcined in air, NiAl3was oxidized and transformed to Ni2Al3and α-Al2O3with simultaneous release of large amount of heat, which could result in the sintering of alloy and expanding the space between particles (Figure 3 (a)). However, when the precursor was calcined in the argon atmosphere, NiAl3would not be transformed to Ni2Al3due to the absence of oxygen so that sintering was avoided and more NiAl3could be retained in the P-Ar sample (Figure 2(A)).

Since the leached Ni2Al3in P-Air during the subsequent leaching process could generate a more loosen structure of Raney-Ni/Al2O3, C-1 exhibited an inferior compressive strength, which was equal to mere 15.92 N/mm. Besides, α-Al2O3could always deposit on the active Ni species, and in consequence the activity of the catalyst would be weakened.

Figure 2 XRD patterns of precursors calcined in different atmospheres and related catalysts.

Figure 3 SEM images of precursors calcined in air (a) and in argon (b)

Being different from P-Air, P-Ar contained more NiAl3and exhibited a slight sintering degree. The porous structure, accompanied by large amount of released hydrogen, could be also formed when the superficial NiAl3species in P-Ar were leached under the same condition. Since NiAl3was more easily removed than Ni2Al3during the leaching process[5], Ni2Al3species with a tight structure in the inner part of alloy were retained after leaching so that the catalyst sample prepared from P-Ar would contain more Ni2Al3than the one prepared from P-Air (Figure 2(B), (C-1), (C-4)), which was also one of the reasons that C-4 exhibited higher compressive strength than C-1.

Figure 4 XRD patterns of catalysts prepared by precursors (P-Ar) leached for different time

Furthermore, the leaching time has a noticeable effect on the compressive strength of Raney-Ni/Al2O3according to the data of C-2, C-3, C-4 and C-5 depicted in Table 1. The compressive strength declined sharply from 31.60 N/mm to 18.70 N/mm with an increasing leaching time. Generally speaking, the dealumination process of NiAl3and Ni2Al3always took place simultaneously, resulting in the generation of more porous skeletal Ni species. The XRD patterns shown in Figure 4 suggest that the diffraction peak located at 2θ= 45° could be attributed to the dispersive skeletal Ni. With an increasing leaching time, not only the peak intensity of NiAl3gradually disappeared, but Ni2Al3species in the inner parts of Ni-Al alloy particles were also leached out, which could result in the decline of peak intensity of Ni2Al3. The compressive strength of C-5 dramatically decreasedto 18.70 N/mm due to the decline of Ni2Al3caused by excessive dealumination. Therefore, the leaching time should be 5 h in order to obtain a proper Raney-Ni/Al2O3that could be used in the fxed-bed reactor.

Figure 5 shows the H2-TPR spectra of the Raney-Ni/ Al2O3prepared from precursors treated at different leaching time. The peaks ranging from 373 K to 473 K were attached to skeletal Ni, indicating that all samples contained active Ni species with high capability to activate hydrogen. The hydrogen-absorption peak area of skeletal Ni increased with an increasing leaching time, suggesting to an increased amount of skeletal Ni in Raney-Ni/Al2O3. Besides, the peaks ranging from 573 K to 773 K were attributed to the NiO that had weak interaction with alumina carrier. The area of these peaks gradually increased with a rising leaching time, and the reduction peak area of NiO dramatically increased when the leaching time reached 7 h. These phenomena indicated that more active skeletal Ni species would be generated, which was accompanied by the decrease of metallic aluminum species in catalyst with an increasing leaching time. Since a part of extremely active skeletal Ni species would be rapidly oxidized to NiO during the drying process of TPR test, the sample obtained with a leaching time of 7 h (C-5) showed the largest reduction peak area of NiO.

Figure 5 H2-TPR spectra of catalysts prepared from precursors leached for different time duration

3.2 Catalytic hydrogenation performance of Raney-Ni/Al2O3

In order to evaluate the catalytic performance of Raney-Ni/Al2O3, the hydrogenation reaction of vinyltoluene, dicyclopentadiene and indene was used as the probe reaction, and the test results obtained at the frst hour and the 20thhour of reaction are summarized in Table 2. Meanwhile, the experimental results on indene hydrogenation over C-1 and C-4 catalysts obtained in 240 hours of reaction are summarized in Figure 6.

Table 2 Catalytic hydrogenation performance of Raney-Ni/Al2O3on unsaturated hydrocarbons

It can be seen from Table 2 that the five catalysts exhibited good hydrogenation activity on conversion of vinyltoluene and DCPD to methyl ethylbenzene and dihydrodicyclopentadiene (DHDCPD) as well as tetrahydrodicyclopentadiene (THDCPD), respectively. The conversion of dicyclopentadiene (DCPD) and vinyltoluene could reach more than 99%, indicating that alkenyl aromatic compound and DCPD were relatively easily to be hydrogenated. However, the catalysts exhibited different catalytic performance on indene hydrogenation. The catalysts (C-2, C-3, C-4, and C-5) prepared from the precursor (P-Ar) showed higher indene conversion than the one (C-1) prepared from P-Air due to the oxidation of NiAl3to Ni2Al3and the covering of active Ni species by α-Al2O3during the calcination process of the precursor in air. Nevertheless, the catalysts (C-2, C-3, C-4, and C-5) prepared from P-Ar contained more Ni2Al3and more active skeletal Ni species afterleaching process so that the catalysts exhibited better catalytic performance. This result can also be confrmed by Figure 6, showing that the conversion rate of indene over C-4 catalyst was higher than that of indene over C-1 catalyst during the 240 hour period of hydrogenation reaction.

Figure 6 Indene hydrogenation over C-1 and C-4 catalysts in a period of 240 h

Moreover, the leaching time has a noticeable effect on catalytic performance of C-2, C-3, C-4, and C-5 on indene hydrogenation. On the one hand, longer leaching time could give higher initial indene hydrogenation activity (in the frst 1 h) since an increasing leaching time could dissolve more aluminum species and generate more active skeletal Ni species, leading to higher catalytic performance. On the other hand, the indene conversion at the twentieth hour was relatively lower than the value achieved in the first hour. Excessive dealumination generated excessively active skeletal Ni species and these Ni species tended to aggregation because of lack of the support of aluminum atoms, which would result in a decreased activity and stability. Thus, the suitable leaching time was important to the catalyst in terms of realizing both a proper compressive strength and a high catalytic performance.

4 Conclusions

Raney-Ni/Al2O3catalyst suitable for the fixed-bed reactor with high compressive strength and excellent catalytic performance on hydrogenation of unsaturated hydrocarbons was acquired by the molding-calcinationleaching method. The excellent catalytic performance and high compressive strength were attributed to affuent active skeletal Ni species and remaining NiAl3and Ni2Al3, respectively, which were determined by the calcination atmosphere and the leaching time. The catalysts prepared from the precursors calcined in argon gas exhibited higher compressive strength and better catalytic performance than that prepared from the precursor calcined in air. When the precursor was leached for 5 h, the catalyst exhibited proper compressive strength (26.10 N/mm) and excellent catalytic performance with the conversion of indene, dicyclopentadiene, and vinyltoluene reaching more than 87.5%, 99% and even 100%, respectively.

Acknowledgement: The authors thank the Beijing Municipal Commission of Education for funding (PXM2013-014222).

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Catalyst RCGP-1 for Boosting Gasoline Yield Passed SINOPEC’s Appraisal

On February, 2017 the project “Development and commercial application of catalyst for boosting gasoline yield used in the residue MIP unit” has passed the technical appraisal organized by the China Petroleum and Chemical Corporation (SINOPEC). The experts attending the appraisal meeting have unanimously recognized that this project has very outstanding economic benefts, with its overall technology reaching the advanced level inside China.

The MIP technology is a technique designed for reducing the olefns content in FCC naphtha to meet the gasoline quality upgrading needs developed by the SINOPEC Research Institute of Petroleum Processing (RIPP). In recent years faced with dramatically reducing Daqing crude supply and ever increasing share of imported crude feed, the SINOPEC Yanshan Branch Company (YBC) has to process FCC feedstock with inferior quality. The research team of RIPP, by taking into account the demand of YBC for increasing the gasoline and propylene output and the specific features of the FCC feed and operating parameters of the #3 FCC unit adopting the MIP technology, has tailor-made the catalyst RCGP-1 for boosting gasoline yield suited to this unit. In comparison with the previous catalyst used by the #3 FCC unit, the catalyst RCGP-1 shows better hydrothermal stability, stronger resistance against heavy metal contamination and stronger residue cracking ability, resulting in a signifcant increase of gasoline yield with comparable dry gas and coke yields.

Beginning from June 2013 the #3 FCC unit at YBC started to use the catalyst RCGP-1 along with optimization of the process parameters. The commercial calibration tests were carried out in July 2015. The calibration results have revealed that after application of the catalyst RCGP-1 in the face of an apparently deteriorating FCC feed the total liquid yield increased by 0.83 percentage points, among which the gasoline yield increased by 2.01 percentage points, with its RON and MON rating increasing by 1.3 units and 0.5 units, respectively. Meanwhile, the diesel yield was reduced by 3.41 percentage points, while the propylene yield was increased by 0.9 percentage points, resulting in an improved product slate and further enhancement of economics on the FCC unit.

date: 2017-02-15; Accepted date: 2017-03-15.

Xu Xin, E-mail: xuxin@bipt.edu.cn.