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CeO2–Al2O3 catalyst in the sulfur-resistant methanation
Effect of boron addition on the MoO 3 /CeO2 -Al2 O3 catalyst in the sulfurresistant methanation Baowei Wang, Wenxia Yu, Weihan Wang, Zhenhua Li, Yan Xu, Xinbin Ma PII: DOI: Reference:
S1004-9541(17)30502-5 doi:10.1016/j.cjche.2017.08.011 CJCHE 909
To appear in: Received date: Revised date: Accepted date:
27 April 2017 16 August 2017 16 August 2017
Please cite this article as: Baowei Wang, Wenxia Yu, Weihan Wang, Zhenhua Li, Yan Xu, Xinbin Ma, Effect of boron addition on the MoO3 /CeO2 -Al2 O3 catalyst in the sulfur-resistant methanation, (2017), doi:10.1016/j.cjche.2017.08.011
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Catalysis, Kinetics and Reaction Engineering
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sulfur-resistant methanation
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Effect of boron addition on the MoO3/CeO2-Al2O3 catalyst in the
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Baowei Wang*, Wenxia Yu, Weihan Wang, Zhenhua Li, Yan Xu, Xinbin Ma
Key Laboratory for Green Chemical Technology of the Ministry of Education, School
Supported by the National High Technology Research and Development Program of
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of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
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China (863 Project) (2015AA050504) and the National Natural Science Foundation of China (21576203).
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* Email addresses: [email protected] (B.W. Wang)
Abstract
The effect of born on the performance of MoO3/CeO2-Al2O3 catalysts, which were was prepared with impregnation method, was investigated. The catalysts were characterized with N2 adsorption-desorption, XRD, H2-TPR, and NH3-TPD, and were tested
in
sulfur-resistant
methanation.
The
results
indicated
that
the
MoO3/CeO2-Al2O3 catalysts modified by born showed higher catalytic performance in sulfur-resistant methanation. The CO conversion increased from 47 % to 62 % with 0.5 wt% boron content. When the content of born was under 0.5 wt%, the results
ACCEPTED MANUSCRIPT suggested there was an increase in the amorphous form of MoO3 caused by the generation of weak and intermediate acid sites, which had weakened the interaction
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between the active components and supports. While, the catalyst added 2.0 wt% born
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showed the strong acid sites and the largest crystalline size resulting in the uneven distribution of ceria.
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Keywords: born; sulfur-resistant; methanation; Mo-based catalyst; composite support
1 Introduction
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In recent years, the crises of energy and environment problems have been frequently
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mentioned. Many efforts were made to improve the present situation, including the
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effective utilization of the extant resources, researching the reproducible, clean energy
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and so on. Combined with the energy reserves of China (rich coal, meager oil and little gas) [1], it is deserved to put into action to improve utilization efficiency of coal.
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It has been proved in many excellent studies that synthetic nature gas (SNG) is a clean energy which is synthesized from the coal. This process includes the coal gasification, the purification of gas, and methanation. and The methanation is a critical step for the synthesis of SNG [2]. It not only takes full advantage of coal but also reduces the emission of greenhouse gas. Nowadays, there are mainly two kinds of catalysts for methanantion including the Ni catalysts and Mo catalysts. The Ni-based catalysts have been applied commercially in industry because of their higher CO conversion and CH4 selectivity. However, the feed gas have to be higher H2/CO (e.g. H2/CO=3) ratio and the content of H2S need to be under 0.1 ppm with the traditional Ni based
ACCEPTED MANUSCRIPT catalysts. In this point, the novel catalysts should be developed to overcoming the above mentioned limits. Recently, Mo-based catalysts have attracted more and more
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attention due to their good property of resistant sulfur and developed catalytic activity
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of methanation. It is mentioned that MoS2 catalyst has been used in hydrotreating reaction, such as HDS and water-shift-gas [3-5]. In the sulfur-resistant methanation,
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the CO hydrogenation reaction on Mo-based catalysts was carried out according to the following reaction (1), described as direct methanation [6]. CH4 + CO2
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2 CO +2 H2
(1)
It is important to highlight that MoO3 instead of MoS2 is well-dispersed on the
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supports in our study, and molybdenum trioxide has no catalytic activity for methanation. Hence, it is necessary for MoO3 catalysts to be presulfided with the
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H2S/H2 before the evaluation of catalysts. The catalytic activity in the sulfur-tolerant methanation is lower than that of Ni catalysts, which is the mainly challenge for the
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Mo-based catalysts. Therefore, the great efforts have been focused on improving the catalytic activity, including the employment of CeO2-Al2O3 composite supports [7], diversification of the Mo precursors [8], and using some promoters like Co [9,10], citric acid [11]. Currently, the catalysts modified with born has been demonstrated in plenty of study that the addition of born could improve the monolayer loading over the alumina and weaken the interaction between the active component and supports [12]. Morihige et al. [13] have reported the effect of born on MoO3/Al2O3 catalysts, indicating that the born addition actually promoted the Mo dispersion on the surface
ACCEPTED MANUSCRIPT of γ-Al2O3 and reduced the amount of the crystalline MoO3. In addition, the suitable contents of B2O3 would cause the little changes of morphological parameters on the
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supported-Mo catalysts [14], as well as NiMo/Al2O3 catalysts [15,16]. It was proposed
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that the addition of born could weaken the interactions between the molybdenum oxides and Al2O3 [14,17]. The intrinsic activity of cobalt-molybdenum catalysts
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improved in the TOF on CoMoS phase when the catalysts presulfided at 673 K for the hydrodesulfurization of thiophene. Similarly, Chen et al. [18] studied the supported
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molybdenum catalysts modified by boron acid in the HDS, the results suggested the acidity on the alumina surface had changed with the means of infrared spectroscopy,
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which have a positive influence on the active phase, and these are in agreement with Yu et al.[19]. There were also numerous studies about the born modified catalysts in
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the hydrodenitrogenation [20], hydrogenation [21,22], dry-reforming of methane [23], and the oxygen-free conversion of methane [24], The addition of B2O3 could improve
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actually the catalytic activity. There is little study for the impact of B2O3 in the sulfur-resistant methanation so far. This study would try to modify the MoO3/CeO2-Al2O3 catalysts with B2O3. In present work, it was devoted to the influence of the B2O3 modified catalysts on the dispersion of active component, acidic sites on the surface, and catalytic properties.
2 Experimental 2.1 Catalysts preparation The preparation of composite supports: Several CeO2-Al2O3 composite supports with
ACCEPTED MANUSCRIPT different concentrations of B2O3 were prepared by deposition precipitation, as described in our previous study [7]. The concentration of the CeO2 in the support 25
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wt% and the loading of MoO3 were 20 wt%. The high CO conversion and selectivity
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of CH4 were confirmed by pre-experiment for sulfur-resistant methanation. First, Al(NO3)3·9H2O and Ce(NO3)3·9H2O (Kemiou chemical reagent Co. Lit., Tianjin,
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China) [c(Ce+3+Al+3)=1 mol•L-1] was dissolved in deionized water. Then, the 15 wt% ammonia solution was added to continuously stirring. The suspension was firstly aged
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with agitation for about 30 min as adding the desired boron acid, followed the mixture standed for 60 minutes. The products were filtrated, washed with deionized water
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until the pH of the filtrates was close to 7. These products were first dried at room temperature for 24 h. After dried at 120 ℃ overnight, the solids were heated to
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600 ℃, and keep for 4 h in muffle furnace. Finally, the samples were crush and sieved to granules (≤ 180 meshes) for synthesis of catalysts. of
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Preparation
catalysts:
MoO3/CeO2-Al2O3
catalysts
were
prepared
with
impregnation of the composite support with a solution of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) (Kemiou chemical reagent Co. Lit., Tianjin, China), as the precursor of MoO3. The impregnated samples were dried at room temperature, dried at 120 ℃ for 4 h, and calcined at 600 ℃ for 4 h. The 20 % MoO3/ 25 %CeO2-Al2O3 catalysts with different contents of B2O3 were denoted as Mo/CeAl-x, where x was B2O3 content in mass percent. 2.2 Catalysts characterization
ACCEPTED MANUSCRIPT The prepared catalysts were characterized by N2-physisorption analysis, X-ray diffraction patterns (XRD), NH3-TPD, and H2-TPR.
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N2-physisorption analysis was performed on Tristar-3000 apparatus (Micromeritics,
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USA), in order to acquire the textural properties, such the BET surface and pore size. The trace water and the foreign gas were removed out from the samples (about
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0.1~0.2 g) at 300 °C for 3 h, before any measurement was carried out. XRD analysis was characterized by a RigakuD/max-2500 X-ray diffractometer
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(Rigaku, Japan), which was with a Ni-filtered Cu-Kα radiation source (λ=0.154 nm). The sweep angle was form 5 (°)·min-1 to 90 (°)·min-1. The crystallite sizes of all
the XRD spectra.
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catalysts were calculated by the Scherrer equation and the values of the full-wide in
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The temperature programmed reduction (TPR) analysis was conducted by an AutoChem 2910 analyzer (Micromeritics, USA). Before the measurements were
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collected, the sample (approximately 200 mg) was swept with nitrogen at 200 °C, aimed to remove the water traces and some foreign gas absorbed on the surface of catalysts, then cooled to 60 °C. The reduction temperature of samples was increased up to nearly 1000 °C at the heating rate of 10 °C•min-1, this process was kept in a gas mixture of 10 vol % H2/Ar. NH3-TPD analysis was performed using TP-5076 TPD/TPR dynamic adsorption instrument (Xianquan, Tianjin, China). The same goals of the pretreatment samples with N2-physisorption analysis was swept with argon (99.999%) at 400 °C for 60 min
ACCEPTED MANUSCRIPT and cooled to 100 °C, before the measurements were taken out. Then, the flow of ammonia was passed through the product at 100 °C for 60 min, and the gas was
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replaced by argon, in order to get rid of the adsorbed NH3. The process of desorption
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was performed by heating the products from 100 °C to 700 °C at the rate of 10 °C•min-1. The quantity of desorbed ammonia was tested by TCD detector.
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The raman analysis of all spent Mo/CeAl-x catalysts was performed by DXR Raman spectrometer (Thermo Fisher, USA) and the emission line at 532 nm was used, in
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order to learn the imformation about the MoS2 species after methanation reaction. 2.3 Evaluation of catalytic activity
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The catalytic activity for sulfur-methanation had been evaluated in a fixed-bed continuous-flow reactor, whose inner diameter was 12 mm and length was 700 mm.
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The loading of catalysts was 3 ml (pellets, 20-40 meshes). Before catalytic evaluation, all catalysts were firstly sulfide with 3 vol% H2S/H2, which was described by Jiang
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et al [25]. And then the sulfide gas was switched into the syngas. The catalytic activity measurements were carried out under the following conditions: syngas (H2/CO ratio=1.0) including 0.27 vol% H2S and 10 vol% N2, p= 3.0 MPa, GHSV=5000 h-1, T= 550 °C. The out gas was analyzed on-line using a GC equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). Finally, the CO conversion and selectivity to hydrocarbon were calculated with the following equation: X (CO )
n(CO )in n(CO )out n(CO )in
100%
(2)
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nC2 H6 ,out nC2 H6 ,in nCO,in nCO,out
nC3 ,out nC3 ,in nCO,in nCO,out
(3)
100%
(4)
100%
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SC2 H6 4
100%
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nCO,in nCO,out
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nCH4 ,out nCH4 ,in
SCH4 2
3 Results
(5)
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3.1 Textural properties of the Mo/CeAl-x catalysts
BET Surface
Pore
Pore
DXRD/n
/m2•g-1
volume/cm3•g-1
size/nm
m
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Table 1 Textural properties and DXRD for the Mo/CeAl-x catalysts
119
0.19
5.97
6.5
103
0.22
6.67
8.2
Mo/CeAl-0.5
123
0.23
6.53
7.6
Mo/CeAl-2.0
61
0.24
12.6
13.2
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Sample
Mo/CeAl
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Mo/CeAl-0.2
DXRD：calculated with the scherrer equation using values of the XRD peak of CeO2 at 2θ =28.5°
As shown in Table 1, the information about the BET surface areas, pore volume and pore size of all the samples can be learned. The textural properties of all catalyst samples were changed by adding boron into CeO2-Al2O3 composite supports. It can be seen from Table 1 that the BET surface area slightly changed from 103 m2•g-1 to 123 m2•g-1 with the loading of B2O3 under 0.5 wt%, but decreased significantly to 61
ACCEPTED MANUSCRIPT m2•g-1 with a further increase of B2O3 to 2.0 wt%. The pore size of Mo/CeAl-2.0 catalysts was larger than that of other catalysts, and it was mainly caused by
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penetrations of the dispersed superfluous B2O3 into the pores of support, resulting in
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getting through some hole and collapsing partly of structure.
CeOl2O3B2O3
c b a
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20
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Intensity (a. u.)
d
10
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3.2 XRD
30
40
50
60
70
2 /
Fig. 1 XRD spectra of MoO3/CeO2-Al2O3 with and without boron: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
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The XRD spectra of the Mo/CeAl-x catalysts are shown in Fig. 1. All catalysts revealed the diffraction peaks at 2θ=28.5°, 33.3°, 47.6°, and 56.5°, which were assigned to CeO2 species [25], and one peak of γ-Al2O3 was found at 67.1° (PDF: 08-0384). Compared to MoO3/CeO2-Al2O3, the diffraction peak of crystalline MoO3 was not detected at 23.1°. Thus, we could consider that the molybdenum species were dispersed easily on the CeO2-Al2O3 supports after adding B2O3. It should be pointed out that the diffraction peak of B2O3 at 45.6° (PDF: 80-0338) was detected when the B2O3 loading was 2.0 wt%. This indicated that the addition of B2O3 was presented as
ACCEPTED MANUSCRIPT highly dispersed on the surface of support when B2O3 content was below 2.0 wt%.
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wt% born was superfluous in agreement with BET analysis.
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The crystalline B2O3 was found in Mo/CeAl-2.0 catalysts. This suggested that 2.0
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The CeO2 crystallite sizes of the series of Mo/CeAl catalysts were calculated from their XRD FWHMs, which were summarized in Table 1.
K (Scherrer equation) B c o s
(6)
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As shown in Table 1, the CeO2 crystallite size of catalysts was larger than that of the Mo/CeAl-x (x = 0, 0.2, 0.5 wt%) catalysts. This indicated that the 2.0 wt% loading of
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B2O3 may cause the more aggregation of CeO2, compared with the catalysts with the addition of B2O3 less than 0.5 wt%. And the CeO2 particles were dispersed unevenly
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3.3 H2-TPR
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resulting from the growth of CeO2 crystallite size.
Intensity (a.u.)
d
490C c b
570C
a
300
400
500
600
700
800
900
Temperture /oC
Fig. 2 TPR profiles of Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
ACCEPTED MANUSCRIPT TPR measurements were performed in order to investigate the dispersion and state of Mo species on the surface of composite supports, which were modified with boron
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addition. Fig. 2 shows the TPR patterns of the MoO3/CeO2-Al2O3 with various B2O3
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contents. Generally, the TPR patterns of Mo catalysts showed the lower temperature peak at 490 °C or the higher temperature peak at about 600 °C. For the reduction of
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Mo species, the H2 consumption peaks contained the reduction of octahedrally coordinated Mo+6 to Mo+4, which were well-dispersed on the surface of supports
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between 350-560 °C, whereas the reduction of crystalline of MoO3 occurred at about 600 °C [25]. The reason for the migration of H2 absorption peak was the stronger
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interaction of crystalline MoO3 than that of amorphous form. As for Mo/CeAl catalysts, there was only one peak at 600 °C, indicating that plenty of crystalline of
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MoO3 existed on the surface of catalysts without B2O3. The temperature of H2 consumption peak decreased from 600 °C to 490 °C with an increase in B2O3
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concentration from 0.0 to 0.5wt %, which indicated that the higher amounts of amorphous Mo species were dispersed on the supports in the absence of boron. It also suggested that the addition of boron in the MoO3/Al2O3 catalysts weakened the interaction between active component and the supports [12]. As for the Mo/CeAl-2.0 catalysts, the H2 consumption peak was at 570 °C which was higher than that of Mo/CeAl-0.5 catalysts. In the combination with the results of XRD, the crystallite size of CeO2 sharply increased for Mo/CeAl-2.0 catalysts, resulting that the CeO2 scattered unevenly on the Al2O3 surface. This result led to the form of Al2(MoO4)3,
ACCEPTED MANUSCRIPT which caused the stronger interaction between active component and the supports [26].
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3.4 NH3-TPD
c Intensity (a.u.)
b
a
200
300
400
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100
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d
500
600
Temperature / C
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Fig. 3 NH3-TPD profiles of Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
weak acid/μmol•g-1
strong acid/μmol•g-1
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Table 2 Acidity amount of Mo/CeAl-x catalysts
Mo/CeAl
537
127
Mo/CeAl-0.2
2280
---
Mo/CeAl-0.5
3067
--
Mo/CeAl-2.0
1275
813
sample
The NH3-TPD profiles of the Mo/CeAl catalysts with various B2O3 contents were shown in the Fig. 3, which were divided into the low-temperature and the hightemperature region, respectively. These were corresponded to weak and strong acid
ACCEPTED MANUSCRIPT sites. According to previous studies [27,28], ammonia could be adsorbed on the acid sties of catalysts, and the peaks of NH3 desorption increased by increasing the number
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of acid sites on the surface of catalysts. It can be seen in Figure 3 and Table 2 that the
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total acidities of Mo/CeAl-x catalysts with B2O3 were higher than those of catalysts without B2O3. The Mo/CeAl catalysts exhibited two desorption peaks, one at range of
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200-400 °C and another at above 500 °C which indicated that the surface of Mo/CeAl catalysts exited the weak and strong acid centers. As for the catalysts with the loading
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of B2O3 under 0.5 wt%, the higher temperature peak ascribed to strong acid center almost disappeared and the amount of weak acid centers increased obviously, which
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was revealed in Table 2. However increasing of the addition of B2O3, the amount of weak acid sites of the Mo/CeAl-2.0 catalysts sharply decreased and the strong acid
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sites were found again. These indicated that the addition of B2O3 could actually change the proportion of weak and strong acid sites.
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It was proved that the acidity on the catalysts had a direct influence on the properties of molybdenum sulfide [18], which was in favor for hydrogenation activity of the active phase. Thus in the sulfur-resistant methanation, the number of weak acid of Mo/CeAl catalysts with B2O3 loading under 0.5 w% was most in those of the Mo/CeAl catalysts, resulting in the better-dispersed active Mo species which was prove by the results of H2-TPR and better catalytic performance. Although the number of strong acid sites of the Mo/CeAl-2.0 catalysts and crystalline size of CeO2 were more than those of the Mo/CeAl-x (x=0.2, 0.5) catalysts, suggesting the superfluous
ACCEPTED MANUSCRIPT boron was harmful to the catalytic performance, the correlation between the contents of acidity and catalytic performance was reported in study [20, 28]. The complete
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details of the catalytic activity caused by boron would be discussed below.
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3.5 Raman
402
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Intensity (a.u.)
379
c b
D TE 300
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250
350
d
a 400
450
Raman Shift / cm
500
550
600
-1
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Fig. 4 Raman profiles of spent Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
The raman profiles of spent Mo/CeAl-x catalysts with various B2O3 contents were displayed in Fig. 4. It was seen that all spent catalysts with and without boron revealed peaks found at 379 cm-1 and 402 cm-1, which were assigned to MoS2 species according to the literature [29]. In addition, the intensity of peaks of sulfide Mo changed significantly as the catalysts were added into boron and these raman peaks intensity of Mo/CeAl-0.5 catalysts were higher than those of another catalysts in the Fig.4, it was indicated that the amount of MoS2 on the surface of Mo/CeAl-x catalysts
ACCEPTED MANUSCRIPT increased clearly when the contents of B2O3 increased up to 0.5 wt%, followed by decrease with a further increase of B2O3. Compared with the results of XRD, H2-TPR
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and NH3-TPD, the addition of boron had an positive effect on the dispersion of active
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catalytic precursor and interaction between support and Mo species which could improve the formation of disulfide Mo.
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3.6 Effect on the catalytic performances of Mo/CeAl catalysts
100
a
80
c
d
70 60 50 40
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CO Conversiom /%
b
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90
30
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20 10
0
2
4
6
8
12
SCH 4
100
14
16
18
20
SC2H6
SC3+
80
Selectivity %
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10
Time (h)
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0
60
40
20
0
a
b
c
d
Fig.5 CO conversion and selectivity to hydrocarbon of Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
The effect of boron on the CO conversion and selectivity to hydrocarbon over the
ACCEPTED MANUSCRIPT Mo/CeAl-x catalysts is displayed in Fig. 5. As shown in Fig. 5, the catalytic activity on the sulfur-resistant methanation increased from 47 % to 62 % as the B2O3 contents
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rose up to 0.5 wt %, however the conversion of CO decreased with increasing the
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boron adding to 2.0 wt%. In other words, the catalysts modified by 0.5 wt % B2O3 showed the highest catalytic activity. It was noted that the catalysts with 0 wt % and
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0.2 wt % B2O3 exhibited fairly similar activity toward methanation. However, the catalysts modified by 2.0 wt% showed a drastic decline as seen from Fig. 5, which
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showed the lowest CO conversion, about 35 %. This indicated that the addition of boron can improve the CO conversion because of the well-dispersed of Mo oxides
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and the weak interaction between active phase and supports. The reason for the sharp decrease on the catalytic activity was that the interaction between the Mo species and
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supports was enhanced and the crystalline MoO3 was formed. The CeO2 layer had not been formed, maybe resulting the formation of Al2(MoO4)3. This was in agreement
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with the results of H2-TPR and XRD. Further, it was shown the addition of B2O3 could make no significant effect on the selectivity to hydrocarbons. As for all Mo/CeAl-x catalysts, the selectivity of CH4 maintained a high level about 95% while trace of C2H6 and C3+ was formed.
4 Conclusions MoO3/CeO2-Al2O3 catalysts with various contents of B2O3 in the sulfur-resistant methanation were investigated. The study found that the addition of B2O3 could change the percentage of acid sites on the surface of catalysts. As for sulfur-resistant
ACCEPTED MANUSCRIPT methanation, the Mo/CeAl-0.5 catalysts showed the highest catalytic activity and the CO conversion could increase up to 62 %. The catalysts with the B2O3 loading under
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0.5 wt% increased the proportion of weak acid sites while the strong acid sites nearly
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disappeared, resulting in weakening the interaction between the active components and CeAl supports. However for Mo/CeAl-2.0 catalysts, the strong acid sites on the
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surface of catalysts were found again and the largest crystalline size of CeO2 leaded to the uneven distribution of CeO2 and the formation of Al2(MoO4)3. Thus, these
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indicated that a small number of B2O3 could increase percentage of weak acid sites strength which was in favor of the higher sulfur-resistant methanation activity, and
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[12] U. Usman, M. Takaki, T. Kubota, Y. Okamoto, Effect of boron addition on a MoO3/Al2O3
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catalyst. Appl. Catal. A Gen. 286 (1) (2005) 148-154.
[13] H. Morihige, Y. Akai, Effect of boron addition on the state and dispersion of Mo supported
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[14] Usman, T. Kubota, Y. Araki, K. Ishida, Y. Okamoto, The effect of boron addition on the
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hydrodesulfurization activity of MoS2/Al2O3 and Co–MoS2/Al2O3 catalysts. J. Catal. 227 (2)
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[15] D. Ferdous, A. K. Dalai, J. Adjaye, A series of NiMo/Al2O3 catalysts containing boron and phosphorus Part II. Hydrodenitrogenation and hydrodesulfurization using heavy gas oil derived
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cobalt-tungsten
and
cobalt-molybdenum
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hydrodesulfurization of thiophene. Bull. Chem. Soc. Jpn. 79 (4) (2016) 637-643. [18] W. B. Chen, F. Mauge, J. V. Gestl, H. Nie, D. D. Li, X. Y. Long, Effect of modification of the alumina acidity on the properties of supported Mo and CoMo sulfide catalysts. J. Catal. 304 (2013) 47-62.
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Vatutina, O. V. Klimov, A. Nadeina, I. G. Danilova, E. Y. Gerasimov, I. P. Prosvirin,
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[21] D. W. Zhuang, Q. Kang, S. S. Muir, X. D. Yao, H. B. Dai, G. L. Ma, P. Wang, Evaluation of a cobalt-molybdenum-boron catalyst for hydrogen generation of alkaline sodium borohydride
Zhuang, J. J. Zhang,i H. B. Da, P. Wang, Hydrogen generation from hydrolysis of
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[24] M. W. Ngobeni, A. F. Carley, M. S. Scurrell, C. P. Nicolaides, The effects of boron and silver on the oxygen-free conversion of methane over Mo/H-ZSM-5 catalysts. J. Mol. Catal. A Chem. 305 (1-2) (2009) 40-46. [25] M. H. Jiang, B. W. Wang, J. Lv, H. Y. Wang, Z. H. Li, X. B. Ma, S. D. Qin, Q. Sun, Effect of sulfidation temperature on the catalytic activity of MoO3/CeO2-Al2O3 toward sulfur-resistant methanation. Appl. Catal. A Gen. 466 (2013) 224-232. [26] M. H. Jiang, B. W. Wang, Y. Q. Yao, Z. H. Li, X. B. Ma, S. D. Qin, Q. Sun, A comparative study of CeO2-Al2O3 support prepared with different methods and its application on
ACCEPTED MANUSCRIPT MoO3/CeO2-Al2O3 catalyst for sulfur-resistant methanation. Appl. Surf. Sci. 285 (2013) 267-277. [27] P. Berteau, B. Delmon, Modified aluminas relationship between activity in 1-Butanol
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Graphic abstract
S1004-9541(17)30502-5 doi:10.1016/j.cjche.2017.08.011 CJCHE 909
To appear in: Received date: Revised date: Accepted date:
27 April 2017 16 August 2017 16 August 2017
Please cite this article as: Baowei Wang, Wenxia Yu, Weihan Wang, Zhenhua Li, Yan Xu, Xinbin Ma, Effect of boron addition on the MoO3 /CeO2 -Al2 O3 catalyst in the sulfur-resistant methanation, (2017), doi:10.1016/j.cjche.2017.08.011
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Catalysis, Kinetics and Reaction Engineering
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☆
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sulfur-resistant methanation
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Effect of boron addition on the MoO3/CeO2-Al2O3 catalyst in the
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Baowei Wang*, Wenxia Yu, Weihan Wang, Zhenhua Li, Yan Xu, Xinbin Ma
Key Laboratory for Green Chemical Technology of the Ministry of Education, School
Supported by the National High Technology Research and Development Program of
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of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
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China (863 Project) (2015AA050504) and the National Natural Science Foundation of China (21576203).
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* Email addresses: [email protected] (B.W. Wang)
Abstract
The effect of born on the performance of MoO3/CeO2-Al2O3 catalysts, which were was prepared with impregnation method, was investigated. The catalysts were characterized with N2 adsorption-desorption, XRD, H2-TPR, and NH3-TPD, and were tested
in
sulfur-resistant
methanation.
The
results
indicated
that
the
MoO3/CeO2-Al2O3 catalysts modified by born showed higher catalytic performance in sulfur-resistant methanation. The CO conversion increased from 47 % to 62 % with 0.5 wt% boron content. When the content of born was under 0.5 wt%, the results
ACCEPTED MANUSCRIPT suggested there was an increase in the amorphous form of MoO3 caused by the generation of weak and intermediate acid sites, which had weakened the interaction
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between the active components and supports. While, the catalyst added 2.0 wt% born
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showed the strong acid sites and the largest crystalline size resulting in the uneven distribution of ceria.
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Keywords: born; sulfur-resistant; methanation; Mo-based catalyst; composite support
1 Introduction
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In recent years, the crises of energy and environment problems have been frequently
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mentioned. Many efforts were made to improve the present situation, including the
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effective utilization of the extant resources, researching the reproducible, clean energy
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and so on. Combined with the energy reserves of China (rich coal, meager oil and little gas) [1], it is deserved to put into action to improve utilization efficiency of coal.
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It has been proved in many excellent studies that synthetic nature gas (SNG) is a clean energy which is synthesized from the coal. This process includes the coal gasification, the purification of gas, and methanation. and The methanation is a critical step for the synthesis of SNG [2]. It not only takes full advantage of coal but also reduces the emission of greenhouse gas. Nowadays, there are mainly two kinds of catalysts for methanantion including the Ni catalysts and Mo catalysts. The Ni-based catalysts have been applied commercially in industry because of their higher CO conversion and CH4 selectivity. However, the feed gas have to be higher H2/CO (e.g. H2/CO=3) ratio and the content of H2S need to be under 0.1 ppm with the traditional Ni based
ACCEPTED MANUSCRIPT catalysts. In this point, the novel catalysts should be developed to overcoming the above mentioned limits. Recently, Mo-based catalysts have attracted more and more
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attention due to their good property of resistant sulfur and developed catalytic activity
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of methanation. It is mentioned that MoS2 catalyst has been used in hydrotreating reaction, such as HDS and water-shift-gas [3-5]. In the sulfur-resistant methanation,
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the CO hydrogenation reaction on Mo-based catalysts was carried out according to the following reaction (1), described as direct methanation [6]. CH4 + CO2
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2 CO +2 H2
(1)
It is important to highlight that MoO3 instead of MoS2 is well-dispersed on the
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supports in our study, and molybdenum trioxide has no catalytic activity for methanation. Hence, it is necessary for MoO3 catalysts to be presulfided with the
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H2S/H2 before the evaluation of catalysts. The catalytic activity in the sulfur-tolerant methanation is lower than that of Ni catalysts, which is the mainly challenge for the
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Mo-based catalysts. Therefore, the great efforts have been focused on improving the catalytic activity, including the employment of CeO2-Al2O3 composite supports [7], diversification of the Mo precursors [8], and using some promoters like Co [9,10], citric acid [11]. Currently, the catalysts modified with born has been demonstrated in plenty of study that the addition of born could improve the monolayer loading over the alumina and weaken the interaction between the active component and supports [12]. Morihige et al. [13] have reported the effect of born on MoO3/Al2O3 catalysts, indicating that the born addition actually promoted the Mo dispersion on the surface
ACCEPTED MANUSCRIPT of γ-Al2O3 and reduced the amount of the crystalline MoO3. In addition, the suitable contents of B2O3 would cause the little changes of morphological parameters on the
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supported-Mo catalysts [14], as well as NiMo/Al2O3 catalysts [15,16]. It was proposed
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that the addition of born could weaken the interactions between the molybdenum oxides and Al2O3 [14,17]. The intrinsic activity of cobalt-molybdenum catalysts
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improved in the TOF on CoMoS phase when the catalysts presulfided at 673 K for the hydrodesulfurization of thiophene. Similarly, Chen et al. [18] studied the supported
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molybdenum catalysts modified by boron acid in the HDS, the results suggested the acidity on the alumina surface had changed with the means of infrared spectroscopy,
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which have a positive influence on the active phase, and these are in agreement with Yu et al.[19]. There were also numerous studies about the born modified catalysts in
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the hydrodenitrogenation [20], hydrogenation [21,22], dry-reforming of methane [23], and the oxygen-free conversion of methane [24], The addition of B2O3 could improve
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actually the catalytic activity. There is little study for the impact of B2O3 in the sulfur-resistant methanation so far. This study would try to modify the MoO3/CeO2-Al2O3 catalysts with B2O3. In present work, it was devoted to the influence of the B2O3 modified catalysts on the dispersion of active component, acidic sites on the surface, and catalytic properties.
2 Experimental 2.1 Catalysts preparation The preparation of composite supports: Several CeO2-Al2O3 composite supports with
ACCEPTED MANUSCRIPT different concentrations of B2O3 were prepared by deposition precipitation, as described in our previous study [7]. The concentration of the CeO2 in the support 25
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wt% and the loading of MoO3 were 20 wt%. The high CO conversion and selectivity
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of CH4 were confirmed by pre-experiment for sulfur-resistant methanation. First, Al(NO3)3·9H2O and Ce(NO3)3·9H2O (Kemiou chemical reagent Co. Lit., Tianjin,
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China) [c(Ce+3+Al+3)=1 mol•L-1] was dissolved in deionized water. Then, the 15 wt% ammonia solution was added to continuously stirring. The suspension was firstly aged
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with agitation for about 30 min as adding the desired boron acid, followed the mixture standed for 60 minutes. The products were filtrated, washed with deionized water
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until the pH of the filtrates was close to 7. These products were first dried at room temperature for 24 h. After dried at 120 ℃ overnight, the solids were heated to
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600 ℃, and keep for 4 h in muffle furnace. Finally, the samples were crush and sieved to granules (≤ 180 meshes) for synthesis of catalysts. of
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Preparation
catalysts:
MoO3/CeO2-Al2O3
catalysts
were
prepared
with
impregnation of the composite support with a solution of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) (Kemiou chemical reagent Co. Lit., Tianjin, China), as the precursor of MoO3. The impregnated samples were dried at room temperature, dried at 120 ℃ for 4 h, and calcined at 600 ℃ for 4 h. The 20 % MoO3/ 25 %CeO2-Al2O3 catalysts with different contents of B2O3 were denoted as Mo/CeAl-x, where x was B2O3 content in mass percent. 2.2 Catalysts characterization
ACCEPTED MANUSCRIPT The prepared catalysts were characterized by N2-physisorption analysis, X-ray diffraction patterns (XRD), NH3-TPD, and H2-TPR.
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N2-physisorption analysis was performed on Tristar-3000 apparatus (Micromeritics,
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USA), in order to acquire the textural properties, such the BET surface and pore size. The trace water and the foreign gas were removed out from the samples (about
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0.1~0.2 g) at 300 °C for 3 h, before any measurement was carried out. XRD analysis was characterized by a RigakuD/max-2500 X-ray diffractometer
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(Rigaku, Japan), which was with a Ni-filtered Cu-Kα radiation source (λ=0.154 nm). The sweep angle was form 5 (°)·min-1 to 90 (°)·min-1. The crystallite sizes of all
the XRD spectra.
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catalysts were calculated by the Scherrer equation and the values of the full-wide in
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The temperature programmed reduction (TPR) analysis was conducted by an AutoChem 2910 analyzer (Micromeritics, USA). Before the measurements were
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collected, the sample (approximately 200 mg) was swept with nitrogen at 200 °C, aimed to remove the water traces and some foreign gas absorbed on the surface of catalysts, then cooled to 60 °C. The reduction temperature of samples was increased up to nearly 1000 °C at the heating rate of 10 °C•min-1, this process was kept in a gas mixture of 10 vol % H2/Ar. NH3-TPD analysis was performed using TP-5076 TPD/TPR dynamic adsorption instrument (Xianquan, Tianjin, China). The same goals of the pretreatment samples with N2-physisorption analysis was swept with argon (99.999%) at 400 °C for 60 min
ACCEPTED MANUSCRIPT and cooled to 100 °C, before the measurements were taken out. Then, the flow of ammonia was passed through the product at 100 °C for 60 min, and the gas was
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replaced by argon, in order to get rid of the adsorbed NH3. The process of desorption
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was performed by heating the products from 100 °C to 700 °C at the rate of 10 °C•min-1. The quantity of desorbed ammonia was tested by TCD detector.
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The raman analysis of all spent Mo/CeAl-x catalysts was performed by DXR Raman spectrometer (Thermo Fisher, USA) and the emission line at 532 nm was used, in
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order to learn the imformation about the MoS2 species after methanation reaction. 2.3 Evaluation of catalytic activity
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The catalytic activity for sulfur-methanation had been evaluated in a fixed-bed continuous-flow reactor, whose inner diameter was 12 mm and length was 700 mm.
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The loading of catalysts was 3 ml (pellets, 20-40 meshes). Before catalytic evaluation, all catalysts were firstly sulfide with 3 vol% H2S/H2, which was described by Jiang
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et al [25]. And then the sulfide gas was switched into the syngas. The catalytic activity measurements were carried out under the following conditions: syngas (H2/CO ratio=1.0) including 0.27 vol% H2S and 10 vol% N2, p= 3.0 MPa, GHSV=5000 h-1, T= 550 °C. The out gas was analyzed on-line using a GC equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). Finally, the CO conversion and selectivity to hydrocarbon were calculated with the following equation: X (CO )
n(CO )in n(CO )out n(CO )in
100%
(2)
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nC2 H6 ,out nC2 H6 ,in nCO,in nCO,out
nC3 ,out nC3 ,in nCO,in nCO,out
(3)
100%
(4)
100%
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SC2 H6 4
100%
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nCO,in nCO,out
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nCH4 ,out nCH4 ,in
SCH4 2
3 Results
(5)
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3.1 Textural properties of the Mo/CeAl-x catalysts
BET Surface
Pore
Pore
DXRD/n
/m2•g-1
volume/cm3•g-1
size/nm
m
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Table 1 Textural properties and DXRD for the Mo/CeAl-x catalysts
119
0.19
5.97
6.5
103
0.22
6.67
8.2
Mo/CeAl-0.5
123
0.23
6.53
7.6
Mo/CeAl-2.0
61
0.24
12.6
13.2
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Sample
Mo/CeAl
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Mo/CeAl-0.2
DXRD：calculated with the scherrer equation using values of the XRD peak of CeO2 at 2θ =28.5°
As shown in Table 1, the information about the BET surface areas, pore volume and pore size of all the samples can be learned. The textural properties of all catalyst samples were changed by adding boron into CeO2-Al2O3 composite supports. It can be seen from Table 1 that the BET surface area slightly changed from 103 m2•g-1 to 123 m2•g-1 with the loading of B2O3 under 0.5 wt%, but decreased significantly to 61
ACCEPTED MANUSCRIPT m2•g-1 with a further increase of B2O3 to 2.0 wt%. The pore size of Mo/CeAl-2.0 catalysts was larger than that of other catalysts, and it was mainly caused by
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penetrations of the dispersed superfluous B2O3 into the pores of support, resulting in
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getting through some hole and collapsing partly of structure.
CeOl2O3B2O3
c b a
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20
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Intensity (a. u.)
d
10
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3.2 XRD
30
40
50
60
70
2 /
Fig. 1 XRD spectra of MoO3/CeO2-Al2O3 with and without boron: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
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The XRD spectra of the Mo/CeAl-x catalysts are shown in Fig. 1. All catalysts revealed the diffraction peaks at 2θ=28.5°, 33.3°, 47.6°, and 56.5°, which were assigned to CeO2 species [25], and one peak of γ-Al2O3 was found at 67.1° (PDF: 08-0384). Compared to MoO3/CeO2-Al2O3, the diffraction peak of crystalline MoO3 was not detected at 23.1°. Thus, we could consider that the molybdenum species were dispersed easily on the CeO2-Al2O3 supports after adding B2O3. It should be pointed out that the diffraction peak of B2O3 at 45.6° (PDF: 80-0338) was detected when the B2O3 loading was 2.0 wt%. This indicated that the addition of B2O3 was presented as
ACCEPTED MANUSCRIPT highly dispersed on the surface of support when B2O3 content was below 2.0 wt%.
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wt% born was superfluous in agreement with BET analysis.
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The crystalline B2O3 was found in Mo/CeAl-2.0 catalysts. This suggested that 2.0
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The CeO2 crystallite sizes of the series of Mo/CeAl catalysts were calculated from their XRD FWHMs, which were summarized in Table 1.
K (Scherrer equation) B c o s
(6)
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As shown in Table 1, the CeO2 crystallite size of catalysts was larger than that of the Mo/CeAl-x (x = 0, 0.2, 0.5 wt%) catalysts. This indicated that the 2.0 wt% loading of
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B2O3 may cause the more aggregation of CeO2, compared with the catalysts with the addition of B2O3 less than 0.5 wt%. And the CeO2 particles were dispersed unevenly
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3.3 H2-TPR
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resulting from the growth of CeO2 crystallite size.
Intensity (a.u.)
d
490C c b
570C
a
300
400
500
600
700
800
900
Temperture /oC
Fig. 2 TPR profiles of Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
ACCEPTED MANUSCRIPT TPR measurements were performed in order to investigate the dispersion and state of Mo species on the surface of composite supports, which were modified with boron
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addition. Fig. 2 shows the TPR patterns of the MoO3/CeO2-Al2O3 with various B2O3
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contents. Generally, the TPR patterns of Mo catalysts showed the lower temperature peak at 490 °C or the higher temperature peak at about 600 °C. For the reduction of
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Mo species, the H2 consumption peaks contained the reduction of octahedrally coordinated Mo+6 to Mo+4, which were well-dispersed on the surface of supports
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between 350-560 °C, whereas the reduction of crystalline of MoO3 occurred at about 600 °C [25]. The reason for the migration of H2 absorption peak was the stronger
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interaction of crystalline MoO3 than that of amorphous form. As for Mo/CeAl catalysts, there was only one peak at 600 °C, indicating that plenty of crystalline of
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MoO3 existed on the surface of catalysts without B2O3. The temperature of H2 consumption peak decreased from 600 °C to 490 °C with an increase in B2O3
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concentration from 0.0 to 0.5wt %, which indicated that the higher amounts of amorphous Mo species were dispersed on the supports in the absence of boron. It also suggested that the addition of boron in the MoO3/Al2O3 catalysts weakened the interaction between active component and the supports [12]. As for the Mo/CeAl-2.0 catalysts, the H2 consumption peak was at 570 °C which was higher than that of Mo/CeAl-0.5 catalysts. In the combination with the results of XRD, the crystallite size of CeO2 sharply increased for Mo/CeAl-2.0 catalysts, resulting that the CeO2 scattered unevenly on the Al2O3 surface. This result led to the form of Al2(MoO4)3,
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3.4 NH3-TPD
c Intensity (a.u.)
b
a
200
300
400
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100
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d
500
600
Temperature / C
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Fig. 3 NH3-TPD profiles of Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
weak acid/μmol•g-1
strong acid/μmol•g-1
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Table 2 Acidity amount of Mo/CeAl-x catalysts
Mo/CeAl
537
127
Mo/CeAl-0.2
2280
---
Mo/CeAl-0.5
3067
--
Mo/CeAl-2.0
1275
813
sample
The NH3-TPD profiles of the Mo/CeAl catalysts with various B2O3 contents were shown in the Fig. 3, which were divided into the low-temperature and the hightemperature region, respectively. These were corresponded to weak and strong acid
ACCEPTED MANUSCRIPT sites. According to previous studies [27,28], ammonia could be adsorbed on the acid sties of catalysts, and the peaks of NH3 desorption increased by increasing the number
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of acid sites on the surface of catalysts. It can be seen in Figure 3 and Table 2 that the
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total acidities of Mo/CeAl-x catalysts with B2O3 were higher than those of catalysts without B2O3. The Mo/CeAl catalysts exhibited two desorption peaks, one at range of
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200-400 °C and another at above 500 °C which indicated that the surface of Mo/CeAl catalysts exited the weak and strong acid centers. As for the catalysts with the loading
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of B2O3 under 0.5 wt%, the higher temperature peak ascribed to strong acid center almost disappeared and the amount of weak acid centers increased obviously, which
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was revealed in Table 2. However increasing of the addition of B2O3, the amount of weak acid sites of the Mo/CeAl-2.0 catalysts sharply decreased and the strong acid
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sites were found again. These indicated that the addition of B2O3 could actually change the proportion of weak and strong acid sites.
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It was proved that the acidity on the catalysts had a direct influence on the properties of molybdenum sulfide [18], which was in favor for hydrogenation activity of the active phase. Thus in the sulfur-resistant methanation, the number of weak acid of Mo/CeAl catalysts with B2O3 loading under 0.5 w% was most in those of the Mo/CeAl catalysts, resulting in the better-dispersed active Mo species which was prove by the results of H2-TPR and better catalytic performance. Although the number of strong acid sites of the Mo/CeAl-2.0 catalysts and crystalline size of CeO2 were more than those of the Mo/CeAl-x (x=0.2, 0.5) catalysts, suggesting the superfluous
ACCEPTED MANUSCRIPT boron was harmful to the catalytic performance, the correlation between the contents of acidity and catalytic performance was reported in study [20, 28]. The complete
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details of the catalytic activity caused by boron would be discussed below.
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3.5 Raman
402
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Intensity (a.u.)
379
c b
D TE 300
CE P
250
350
d
a 400
450
Raman Shift / cm
500
550
600
-1
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Fig. 4 Raman profiles of spent Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
The raman profiles of spent Mo/CeAl-x catalysts with various B2O3 contents were displayed in Fig. 4. It was seen that all spent catalysts with and without boron revealed peaks found at 379 cm-1 and 402 cm-1, which were assigned to MoS2 species according to the literature [29]. In addition, the intensity of peaks of sulfide Mo changed significantly as the catalysts were added into boron and these raman peaks intensity of Mo/CeAl-0.5 catalysts were higher than those of another catalysts in the Fig.4, it was indicated that the amount of MoS2 on the surface of Mo/CeAl-x catalysts
ACCEPTED MANUSCRIPT increased clearly when the contents of B2O3 increased up to 0.5 wt%, followed by decrease with a further increase of B2O3. Compared with the results of XRD, H2-TPR
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and NH3-TPD, the addition of boron had an positive effect on the dispersion of active
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catalytic precursor and interaction between support and Mo species which could improve the formation of disulfide Mo.
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3.6 Effect on the catalytic performances of Mo/CeAl catalysts
100
a
80
c
d
70 60 50 40
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CO Conversiom /%
b
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90
30
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20 10
0
2
4
6
8
12
SCH 4
100
14
16
18
20
SC2H6
SC3+
80
Selectivity %
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10
Time (h)
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0
60
40
20
0
a
b
c
d
Fig.5 CO conversion and selectivity to hydrocarbon of Mo/CeAl-x catalysts: Mo/CeAl(a), Mo/CeAl-0.2(b), Mo/CeAl-0.5(c), Mo/CeAl-2.0(d)
The effect of boron on the CO conversion and selectivity to hydrocarbon over the
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rose up to 0.5 wt %, however the conversion of CO decreased with increasing the
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boron adding to 2.0 wt%. In other words, the catalysts modified by 0.5 wt % B2O3 showed the highest catalytic activity. It was noted that the catalysts with 0 wt % and
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0.2 wt % B2O3 exhibited fairly similar activity toward methanation. However, the catalysts modified by 2.0 wt% showed a drastic decline as seen from Fig. 5, which
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showed the lowest CO conversion, about 35 %. This indicated that the addition of boron can improve the CO conversion because of the well-dispersed of Mo oxides
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and the weak interaction between active phase and supports. The reason for the sharp decrease on the catalytic activity was that the interaction between the Mo species and
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supports was enhanced and the crystalline MoO3 was formed. The CeO2 layer had not been formed, maybe resulting the formation of Al2(MoO4)3. This was in agreement
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with the results of H2-TPR and XRD. Further, it was shown the addition of B2O3 could make no significant effect on the selectivity to hydrocarbons. As for all Mo/CeAl-x catalysts, the selectivity of CH4 maintained a high level about 95% while trace of C2H6 and C3+ was formed.
4 Conclusions MoO3/CeO2-Al2O3 catalysts with various contents of B2O3 in the sulfur-resistant methanation were investigated. The study found that the addition of B2O3 could change the percentage of acid sites on the surface of catalysts. As for sulfur-resistant
ACCEPTED MANUSCRIPT methanation, the Mo/CeAl-0.5 catalysts showed the highest catalytic activity and the CO conversion could increase up to 62 %. The catalysts with the B2O3 loading under
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0.5 wt% increased the proportion of weak acid sites while the strong acid sites nearly
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disappeared, resulting in weakening the interaction between the active components and CeAl supports. However for Mo/CeAl-2.0 catalysts, the strong acid sites on the
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surface of catalysts were found again and the largest crystalline size of CeO2 leaded to the uneven distribution of CeO2 and the formation of Al2(MoO4)3. Thus, these
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indicated that a small number of B2O3 could increase percentage of weak acid sites strength which was in favor of the higher sulfur-resistant methanation activity, and
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Graphic abstract