Hydrodesulfurization, Hydrodenitrogenation and ... - J-Stage

Journal of the Japan Petroleum Institute, 60, (6), 301-310 (2017)

301

[Regular Paper]

Hydrodesulfurization, Hydrodenitrogenation and Hydrodearomatization over CoMo/SAPO-11-Al2O3 Catalysts Thanh Tung NGUYEN, Akira SHINOZAKI, and Eika W. QIAN＊ Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology (TUAT), Nakacho 2-24-16, Koganei, Tokyo 184-8588, JAPAN (Received January 24, 2017)

Several SAPO-11-Al2O3 supports with different amounts of SAPO-11 were used to prepare a series of CoMo/ SAPO-11-Al2O3 catalysts by the impregnation method with or without addition of citric acid. Hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene, hydrodenitrogenation (HDN) of acridine, and hydrodearomatization (HDA) of o-xylene, 1-methylnaphthalene and phenanthrene were carried out to evaluate the catalytic activity of the catalysts. The catalysts were characterized by BET, XRF, NH3-TPD, XPS and TEM. The acidity of the supports strongly affected the active slabs structure and activity of the catalysts. 20 wt% SAPO-11 added catalyst showed lower HDA, HDS and HDN activities compared to CoMo/Al2O3 catalyst. The length and stacking number of CoMoS active slabs on the former increased because of decreased dispersion of Mo species due to the reduction in number of strong acid sites on the support surface in the presence of SAPO-11. Addition of citric acid during active metal impregnation decreased HDA activity but increased HDS and HDN activities due to improvement of Mo species dispersion, resulting in formation of shorter length and higher stacking of CoMoS active slabs. Keywords SAPO-11 alumina support, Hydrotreating, Metal support interaction, Light cycle oil, Support acidity, Citric acid effect

1.

Introduction

The fluidized catalytic cracking (FCC) process is a method for the catalytic conversion of heavy oil into lighter products, and is very important in the petroleum refinery. The main product of the FCC process is FCC gasoline and the by-product is light cycle oil (LCO), which mainly consists of bicyclic or polycyclic aromatic compounds and small amounts of sulfur and nitrogen compounds. Conversion of large content of polyaromatic compounds in LCO to mono-aromatic compounds would yield high added value products for p r o d u c i n g B e n z e n e - To l u e n e - X y l e n e ( B T X s ) . However, conventional hydrotreating catalysts, as well as catalysts for the removal of sulfur compounds and nitrogen compounds cause the hydrogenation of polyaromatic compounds, so no mono-aromatic compounds are obtained. Therefore, a specific catalyst is needed for desulfurization and denitrogenation, and selective hydrogenation of bicyclic or tricyclic aromatic compounds to monocyclic aromatic compounds1). DOI: doi.org/10.1627/jpi.60.301 ＊ To whom correspondence should be addressed. ＊ E-mail: [email protected]

Several models for the structure of the active phase2)∼4) of cobalt promoted MoS2 catalysts have been proposed, but the CoMoS phase consisting of cobalt atoms on the edge of MoS2 slabs is widely accepted5)∼7). The RimEdge model with two types of active sites, the Rim site and the Edge site, can explain the differences in the hydrogenation (HYD) and hydrodesulfurization (HDS) active sites8). The Rim sites are located only on the top and bottom of the MoS2 slabs, and have both hydrogenation and desulfurization activities. In contrast, the Edge site located on the edge of each layer of the slabs only has desulfurization activity. Based on this model, a catalyst for selective hydrogenation, i.e. higher HDS and/or hydrodenitrogenation (HDN) but lower hydrodearomatization (HDA), could be developed by controlling the proportions of the Rim and Edge sites. The acidity of the support is expected to affect the hydrogenation activity9),10). Higher support acidity enhanced hydrogenation activity of catalyst9). Low acidity zeolite materials such as the SAPO series or SBA series could be used mixed with Al2O3 to decrease support acidity and control hydrogenation catalytic activity. Evaluation of various zeolite types and alumina for hydrocracking catalysts concluded that zeolite with both small crystal size and mesoporous alumina

J. Jpn. Petrol. Inst., Vol. 60,

No. 6, 2017

302

resulted in higher activities for hydrogenation and hydrocracking11). Use of chelating agents could also improve the catalytic performances of cobaltmolybdenum (CoMo) catalysts. Increased HDS activity of CoMo and NiMo catalysts can be achieved by using nitrile triacetic acid12), and other chelating agents such as ethylene diamino tetraacetic acid13) or citric acid are now used widely to improve the performance of hydrotreating catalysts. However, the effect of adding zeolites or chelating agents to the active slabs structure directly corresponding to catalytic activity remains unclear. In the present paper, CoMo catalysts supported on various supports with various acidities consisting of Al2O3, 5 %-SAPO-11-95 % Al2O3, 20 %-SAPO-1180 % Al2O3 were prepared to investigate the relationship between metal-support interaction and support acidity, and between active phase structure and catalytic activity. This study also investigated the effect of addition of citric acid added by the impregnation method on metal dispersion and the structure of the active sites.

samples was determined by NH3 adsorption on a ChemBET Pulsar temperature-programmed reduction/ temperature-programmed desorption (TPR/TPD) instrument (Quantachrome Instruments). The X-ray photoelectron spectroscopy (XPS) spectra were acquired using an ESCA-3200 (Shimadzu Corp.) spectrometer with monochromatic Mg Kα radiation (240 W, 8 kV, E ＝1253 eV). Transmission electron microscope (TEM) was performed on a JEOL 2100 microscope (JEOL Ltd., Japan), operating at 200 kV. 2. 3. Catalytic Activity Test The activity tests were carried out using a highpressure fixed-bed continuous-flow stainless steel reactor (i.d. 8 mm), operated in the down-flow mode, under the 300 mL/min flowing H2 at pressure of 5 MPa, with a weight hourly space velocity (WHSV) of 39 h –1 at 280 to 380 ℃. The concentration of phenanthrene, 1-MN and o-xylene in HDA feedstock were 2, 10 and 10 wt% in decalin, respectively. For HDS and HDN feedstocks, 0.4 wt% of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and acridine in decalin were used.

2.

3.

Experimental

2. 1. Preparation of Support and Catalysts SAPO-11 powder was mixed with alumina powder with the addition amount of 5 wt% and 20 wt% mechanically, then the mixture was added by the boehmite. The pellet SAPO-11 added alumina supports were prepared from this mixture by extrusion molding method, sieved to yield 425-800 μm particles and dried at 120 ℃ in 1 h. The prepared support samples are denoted as AlSAX (SA: added SAPO-11, X: percentage). The cobalt and molybdenum supported catalysts (CoO: 3.3 wt%, MoO3 : 16.6 wt%) were prepared by conventional incipient wetness impregnation [26]. After each impregnation, the sample was dried at 120 ℃ in 1 h and at last calcined at 450 ℃ for 16 h. The prepared added SAPO-11 catalyst samples were named as CoMoAlSAX (SA: added SAPO-11, X: percentage). The citric acid was added to ammonium heptamolybdate impregnating solution with the molecular ratio Mo : citric acid＝1 : 1 and then the sample was not calcined but only dried at 120 ℃ in 1 h. The synthesized catalyst was denoted as CoMo(CA)AlSAX. Prior to analysis or reaction, the prepared catalyst was activated by sulfidation with a mixture of 5 vol% H2S in H2 (50 mL/min) at 400 ℃ for 3 h. 2. 2. Characterization of Supports and Catalysts The textural properties of the supports and catalysts were determined by the nitrogen adsorption-desorption isotherms using a Belsorp-mini II automated sorption system (BEL Japan, Inc.). The loading amount of MoO3 and CoO were determined by X-ray fluorescence (XRF) measurement using Rayny EDX-700/800XRF analyzer (Shimadzu Corp.). The acidity of support

Results

3. 1. Characrerization of Supports and Catalysts The textural properties of prepared supports and catalysts are shown in the Table 1. By addition of SAPO-11, surface area and pore volume of alumina support decreased slightly from 256 to 214 m2/g and 0.79 to 0.64 cm3/g, respectively, while pore diameter was not changed. The surface area and the pore volume of each catalyst sample decreased about 15-20 % after metal impregnation due to the form of active phase. The average pore diameter and volume of citric acid added catalyst were slightly smaller than those for the catalyst without adding citric acid while the BET surface area was not changed. Chemical compositions obtained by XRF analysis indicated that the amount of active metals loaded was almost as expected. For all catalysts, the cobalt oxide loading varied within 3.6 to 3.9 wt%, and the molybdenum oxide loading ranged from 16.3 to 17.8 wt%. The molybdenum oxide loading was slightly lower for SAPO-11 added catalyst. Therefore, active metal loading on the SAPO-11 added support might be more difficult than on Al2O3 because the former has fewer acidic sites and smaller pore diameter. In the impregnating solution, molybdenum species were mainly present as Mo7O246– and cobalt species as Co2＋, which are larger and smaller than SAPO-11 pore diameter, respectively. Therefore, with added SAPO-11, the loading amount of Mo species slightly decreased due to the hindered diffusion inside the pores of SAPO-11. On the other hand, no significant difference was observed in the loading of Co species. The surface acid properties of the supports were


No. 6, 2017

303

investigated via NH3-TPD as shown in Fig. 1. The profiles were divided into three areas of desorption temperature corresponding to weak (below 300 ℃), medium (from 300 to 400 ℃) and strong (above 400 ℃) acid sites. SAPO-11 supports showed a very sharp peak at ca. 281 ℃ and a shoulder peak at ca. 395 ℃ corresponding to weak and medium acid sites. The alumina support had a sharp peak at ca. 371 ℃ and a smaller peak at ca. 250 ℃. The big peak in the medium and strong acidity area of the alumina support became blunt and shifted to the smaller peak with higher SAPO-11 added amount. To compare the acidity distribution between these catalysts, the NH3-TPD profiles were fitted to the Gaussian function, and area of each peak and total acidity of various supports were calculated as shown in Table 2. The surface total acidities of Al, AlSA5, AlSA20 and SA were 462.9, 465.8, 469.1 and 496.9 μmol/g, respectively. Strong acidity of alumina supports decreased with the addition of 5 wt% and 20 wt% SAPO-11 from 151.9 to 144.9 and 102.9 µmol/ g, respectively, possibly caused by the fewer strong acid sites of SAPO-11. XPS measurement was conducted to determine the oxidation states of the metals on sulfided catalysts. Table 3 shows the binding energy (BE) and relative concentrations of various Mo and Co species obtained from the area percentage of Mo 3d and Co 2p. The binding energy of the main contributions agreed well with reported values14). The Mo 3d XPS spectra were decomposed into three sets of doublets, corresponding

to Mo6＋, Mo5＋, and Mo4＋ species. The Mo6＋ species were obviously Mo oxide species that were not completely sulfided. The Mo5＋ species may be a Mo oxy-sulfide species, and the Mo4＋ species were MoS215). The concentration of Mo4＋ species was not changed by addition of 5 wt% SAPO-11, but increased from 58.7 to 74.1 % by addition of 20 wt% SAPO-11. Presumably the addition of SAPO-11 had weakened the metalsupport interaction which facilitated molybdenum sulfidation from Mo6＋ to Mo4＋. Moreover, addition of citric acid improved the dispersion of molybdenum and less Mo4＋ was formed. Co occurred as surface cobalt

Fig. 1●N H 3- T P D P r o f i l e s o f A l 2O 3, 5 % - S A P O -11- A l 2O 3, 20 %-SAPO-11-Al2O3 and SAPO-11

Table 1 Textural and Structural Characteristics of Prepared Supports and Catalysts Catalysts

CoOa) [wt%]

MoO3a) [wt%]

SBETb) [m2/g]

dpc) [nm]

Vpd) [cm3/g]

Al AlSA5 AlSA20 CoMoAl CoMoAlSA5 CoMoAlSA20 CoMoAl(CA)SA5 CoMoAl(CA)SA20

3.6 3.9 3.9 3.8 3.7

17.8 17.8 16.3 17.1 16.7

256 235 214 200 185 167 179 167

12 12 12 12 11 12 11 10

0.79 0.72 0.64 0.60 0.53 0.48 0.52 0.43

a) Chemical composition determined by XRF. b) Specific surface area. c) Average pore diameter. d) Pore volume. Table 2 Acidity of Supports Determined by NH3-TPD

Catalysts

SAPO-11 content [wt%]

Al AlSA5 AlSA20 SAPO-11

0 5 20 100

Acidity [μmol/g] Weak (＜ 300 ℃)

Medium (300-400 ℃)

Strong (＞ 400 ℃)

Total

98.5 137.4 269.2 256.2

212.5 183.5 97.0 157.3

151.9 144.9 102.9 83.5

462.9 465.8 469.1 496.9


No. 6, 2017

304 Table 3●Percentage of Each Species Evaluated from the Decomposition of Mo 3d, Co 2p XPS Spectra for the Different Sulfide Catalysts Mo 3d concentration [%]

Co 2p concentration [%]

Catalysts

Mo4＋ 228.8 eV

Mo5＋ 229.9 eV

Mo6＋ 232.7 eV

Co2＋ 781.9 eV

CoMoS 779.0 eV

Co9S8 778.2 eV

CoMoAl CoMoAlSA5 CoMoAlSA20 CoMo(CA)AlSA20

58.7 60.3 74.1 61.6

3.3 17.5 6.6 9.0

38.0 22.2 19.3 29.4

61.4 38.9 46.0 32.2

38.6 61.1 54.0 67.8

0.0 0.0 0.0 0.0

Average binding energy (eV) is given for each species.

oxide Co2＋ and CoMoS in all samples. CoMoS generally has much more activity than MoS2. The percentage of CoMoS increased from 38.6 to 61.1 % and 54.0 % after addition of 5 wt% and 20 wt% SAPO-11, respectively. Therefore, addition of SAPO-11 made formation of CoMoS much easier. However, large amount of SAPO-11 also reduced the dispersion of active metals which inhibit the formation of CoMoS. CoMoS concentration increased dramatically from 54.0 to 67.8 % with the addition of citric acid. Therefore, citric acid enhanced dispersion of active metals and formation of CoMoS active slabs. TEM observation of sulfided catalysts was performed to investigate the effect of SAPO-11 and citric acid on the morphology of CoMoS active slabs. Figure 2 shows representative TEM micrographs and the distributions of CoMoS slab length and stacking number on prepared catalysts. The black, long lines indicate the presence of CoMoS slabs. The distributions of length and stacking number of CoMoS slabs were calculated from at least 400 samples of each catalyst and mean values are summarized in Table 4. The slab lengths of CoMoAl, CoMoAlSA5 and CoMo(CA)AlSA20 ranged from 3 to 4 nm and the slab length of CoMoAlSA20 ranged from 4 to 5 nm. Moreover, the active slabs of catalyst with citric acid addition mainly contained 4 or 5 stacked layers, 1 or 2 layers more than catalysts without citric acid addition. The average slab length and stacking number increased from 2.9 to 3.6 nm or 4.5 nm and from 2.6 to 3.3 nm or 3.4 nm with addition of 5 wt% and 20 wt% of SAPO-11, respectively. Addition of citric acid further influenced the morphology of CoMoS slabs, resulting in greater mean length and highest stacking number compared to other catalysts. This result implies that adding citric acid improves the dispersion of active metal Mo species. Adding citric acid together with molybdenum impregnation solution improved the dispersion of Mo on the support surface. Therefore, more Mo-based active slabs were formed. High dispersion of Mo also led to smaller length and higher stacking number of CoMoS active slabs. These findings agree well with previous reports of sulfided CoMo/Al2O3 catalyst16).

3. 2. Hydrotreating Test 3. 2. 1. Hydrodearomatization The HDA conversions of o-xylene, 1-MN and phenanthrene in the temperature range of 320-380 ℃ over prepared catalysts are shown in Figs. 3 to 5. All catalysts showed low conversions below ca. 1 % of the monocyclic aromatic compound o-xylene at all temperatures, with the highest with CoMoAl and the lowest with CoMoAlSA20, and other catalysts with the same activity. Therefore, hydrogenation of one-ring aromatics can be ignored. In contrast, the HDA conversions of 1-MN were higher with all catalysts as shown in Fig. 4. Addition of 5 wt% SAPO-11 to CoMoAl catalyst slightly increased the HDA activity. HDA conversion was in the order of CoMoAlSA5 ≈ CoMoAl ≥ CoMo(CA)AlSA5 ≈ CoMo(CA)AlSA20 ＞ CoMoAlSA20. HDA conversion of 1-MN was slightly increased about 2 to 4 % by adding 5 wt% SAPO-11 but dramatically decreased by adding 20 wt% SAPO-11. Addition of citric acid caused no significant change in HDA conversion with CoMo(CA)AlSA5 but significantly increased HDA activity with CoMo(CA)AlSA20. HDA conversion of phenanthrene over prepared catalysts showed similar changes to 1-MN. The main products of 1-MN hydrogenation were 1-methyltetralin (1-MT) and 5-methyltetralin (5-MT). Two polycyclic aromatic compounds, tetrahydrophenanthrene (THP) and dihydrophenanthrene (DHP), and the monocyclic aromatic compound, octahydrophenanthrene (OHP), were detected as phenanthrene hydrogenated products. However, no fully hydrogenated compounds, perhydrophenanthrene and methyldecalin, could be detected. These findings and the low o-xylene HDA conversion revealed that prepared catalysts had very low activity for the hydrogenation of monocyclic aromatic compounds. To evaluate the selective hydrogenation activity of prepared catalysts, the formation of the valuable target monocyclic aromatic compound OHP was investigated as shown in Fig. 6. No significant change in the selectivity of OHP was observed with the addition of SAPO-11, but a slight increase of about 4-7 % was observed with the addition of citric acid in the case of


No. 6, 2017

305

Fig. 2●TEM Micrographs and Distributions of Slab Length and Stacking Number of Sulfided (a) CoMoAl, (b) CoMoAlSA20, (c) CoMoAlSA5, (d) CoMo(CA)AlSA20 Catalysts

CoMo(CA)AlSA20. Therefore, the addition of citric acid improved the yield of monocyclic aromatic compounds. The selectivities of phenanthrene and 1-MN HDA are shown in Tables 5 and 6. The selectivities of hydrogenation from bicyclic aromatic 1-MN to monocyclic aromatic compounds 1-MT or 5-MT did not change with the addition of SAPO-11 and citric acid. These results showed the same behavior as for OHP. The selectivity of hydrogenation from tricyclic phenanthrene to bicyclic aromatic compounds THP and DHP did not

change with the addition of SAPO-11 but slightly decreased by about 5 % with the addition of citric acid. 3. 2. 2. Hydrodesulfurization 4,6-DMDBT HDS conversion over various catalysts is shown in Fig. 7. The HDS activity of all catalysts increased with higher temperature but showed less difference at high reaction temperature of 380 ℃. Therefore, HDS conversion at low temperature of 320 ℃ were evaluated to investigate the catalytic activity for HDS over prepared catalysts. HDS activity decreased from 61 to 47 % over 5 wt% SAPO-11 added


No. 6, 2017

306 Table 4 Average Length and Stacking Number of CoMoS Slabs Calculated from TEM Micrographs Catalysts

Average length [nm]

Average stacking number

CoMoAl CoMoAlSA5 CoMoAlSA20 CoMo(CA)AlSA20

2.9 3.6 4.5 3.5

2.6 3.3 3.4 4.0

Fig. 3●Conversion of o-Xylene in Hydrodearomatization over Various Catalysts

Fig. 5●Conversion of Phenanthrene in Hydrodearomatization over Various Catalysts

Fig. 4●C o n v e r s i o n o f 1 - M e t h y l n a p h t h a l e n e i n Hydrodearomatization over Various Catalysts

Fig. 6●S e l e c t i v i t y o f O c t a h y d r o p h e n a n t h r e n e i n Hydrodearomatization of Phenanthrene over Various Catalysts

catalyst. Moreover, HDS activity decreased from 61 to 41 % over 20 wt% SAPO-11 added catalyst. HDS conversion increased dramatically from 38 to 85 % with addition of citric acid. HDS reaction of 4, 6-DMDBT has two possible pathways, direct desulfurization (DDS) reaction and indirect desulfurization including hydrogenation (HYDS)17). In DDS, 4,6-DMDBT is directly desulfided to form dimethylbiphenyl (DMBP). In HYDS, 4,6-DMDBT is firstly hydrogenated to form hydrogenated intermediates which are then desulfided to form methylcyclohexyltoluene (MCHT) and dimethylbicyclohexyl

(DMBCH). Thus, the HYDS pathway includes hydrogenation. Figure 8 and Table 7 show the selectivities for products of the indirect desulfurization reaction that slightly decreased in the order of CoMoAl ＞ CoMoAlSA20 ＞ CoMoAlSA5 ≈ CoMo(CA)AlSA5 ≈ CoMo(CA)AlSA20. These findings indicate that SAPO-11 and citric acid inhibited the HYDS activity of CoMoAl. In addition, the change in selectivity of hydrogenation from 4,6-DMDBT to MCHT and DMBCH showed the same trend as phenanthrene HDA. In particular, the selectivity for MCHT and DMBCH showed


No. 6, 2017

307 Table 5

Selectivity for Products in HDA of Phenanthrene over Various Catalysts

320 ℃

Temperature

340 ℃

360 ℃

380 ℃

Catalysts

DHP [%]

THP [%]

OHP [%]

DHP [%]

THP [%]

OHP [%]

DHP [%]

THP [%]

OHP [%]

DHP [%]

THP [%]

OHP [%]

CoMoAl CoMoAlSA5 CoMoAlSA20 CoMo(CA)AlSA5 CoMo(CA)AlSA20

59.0 57.9 57.5 56.2 53.9

24.0 23.1 22.9 20.3 20.3

17.0 19.0 19.6 23.5 23.8

41.4 41.2 42.5 46.2 38.5

24.6 24.7 26.0 21.2 22.5

34.0 34.1 31.4 32.6 38.0

29.6 24.4 27.2 43.1 25.0

29.7 29.6 31.0 22.5 26.8

40.7 46.0 41.8 34.4 48.2

18.5 18.4 17.2 19.3 20.3

40.0 39.0 38.4 38.1 34.6

41.5 42.6 44.4 42.5 44.7

Table 6

Selectivity for Products in HDA of 1-Methylnaphthalene over Various Catalysts 320 ℃

Temperature

340 ℃

360 ℃

380 ℃

Catalysts

1-MT [%]

5-MT [%]

1-MT [%]

5-MT [%]

1-MT [%]

5-MT [%]

1-MT [%]

5-MT [%]

CoMoAl CoMoAlSA5 CoMoAlSA20 CoMo(CA)AlSA5 CoMo(CA)AlSA20

31.9 31.6 30.7 32.1 32.1

68.1 68.4 69.3 67.9 67.9

34.5 33.0 33.2 33.5 33.5

65.5 67.0 66.8 66.5 66.5

34.1 34.1 34.4 5.6 34.4

65.9 65.9 65.6 37.6 65.6

33.9 34.4 34.6 65.4 34.6

66.1 65.6 65.4 41.4 65.4

Fig. 7●Conversion of 4,6-DMDBT in Hydrodesulfurization over Various Catalysts

Fig. 8●S e l e c t i v i t y f o r M C H T a n d D M B C H i n I n d i r e c t Hydrodesulfurization of 4,6-DMDBT over Various Catalysts

no significant change over SAPO-11 added catalysts and slightly decreased by about 9 % over citric added catalyst. 3. 2. 3. Hydrodenitrogenation Very high overall HDN conversions of acridine HDN over 99.8 % were obtained over all catalysts because of the ease of acridine hydrogenation. The real HDN activity of prepared catalysts was investigated using the HDN ratio, defined by the following equation.

Table 7●Selectivity for Products in HYDS of 4,6-DMDBT over Various Catalysts

{

HDN ratio = 1 -

Nitrogen compounds in products [ mol ] Acridine in feedstock [ mol ]

}

The HDN ratios over prepared catalysts are shown in Fig. 9. The HDN ratio did not change much from 320 to 340 ℃ but increased dramatically from 320 to 380 ℃. The HDN ratio order of catalysts was the same as that for HDS of 4,6-DMDBT. The HDN ratio

Catalysts CoMoAl CoMoAlSA5 CoMoAlSA20 CoMo(CA)AlSA5 CoMo(CA)AlSA20

[%] [%] [%] [%] [%]

320 ℃

340 ℃

360 ℃

380 ℃

75 66 75 66 67

70 62 68 61 62

64 58 60 57 57

60 53 54 51 51

was unchanged for 5 wt% SAPO-11 added catalysts but was slightly decreased for 20 wt% SAPO-11 added catalyst. CoMo(CA)AlSA20 showed the best HDN activity with the highest conversion of 13 % at 380 ℃. HDN of acridine occurs through complicated reaction pathways18),19). HDN of acridine is divided into two processes: firstly acridine is quickly hydrogenated


No. 6, 2017

308

to investigate the effect of SAPO-11 and citric acid on HDN activity as shown in Fig. 10. Formation rates of OHA and PHA both decreased over SAPO-11 added catalysts, indicating that SAPO-11 inhibits hydrogenation activity in HDN of acridine. In contrast, formation rates of OHA and PHA increased dramatically over citric acid added catalyst. These findings and high HDN conversion over CoMo(CA)AlSA20 indicate that addition of citric acid enhances both hydrogenation and denitrogenation activities in HDN of acridine. Overall, HDA activity of CoMoAlSA20 was lower compared to CoMoAl but HDS and HDN activity also decreased. HDA activity was slightly increased and HDS and HDN activity were dramatically increased by the addition of citric acid. CoMo(CA)AlSA20 catalyst had low HDA and high HDS and HDN activity compared to CoMoAl catalyst.

to produce hydrogenated intermediates and then denitrogenated to remove the nitrogen atom and produce hydrocarbons. Large amounts of hydrogenated intermediates of acridine HDN such as dihydroacridine (DHA), tetrahydroacridine (THA), octahydroacridine (OHA) and perhydroacridine (PHA) were generated at all reaction temperatures. Cyclohexylphenylmethane (CHPM), dicyclohexylmethane (DCHM) and perhydrofluorene (PHF) were formed by sequential HDN. OHA and PHA are the main hydrogenated intermediates which are denitrogenated to produce CHPM, DCHM and PHF. Selectivity for those products in HDN of acridine are shown in Table 8. The presence of large amounts of hydrogenated intermediates indicates that the denitrogenation reaction was the ratelimiting reaction for all prepared catalysts. The formation rates of OHA and PHA were calculated

4.

The HDA activity of 20 wt% SAPO-11 added catalyst was about 20 % lower than the base CoMoAl catalyst, and the HDS and HDN activities were also lower. Addition of citric acid did not change the HDA activity whereas the HDS and HDN activities increased. The effect of SAPO-11 and citric acid addition on active slab morphology and catalytic activity was investigated assuming the formation of active slab CoMoS based on the Rim-Edge model. Based on the Rim-Edge model and TEM results, the active slab model of SAPO-11 added catalyst was proposed as shown in Scheme 1 (a). The NH3-TPD and catalytic test results showed that the HDA activity decreased in the same order as the number of strong acid sites as follows: Al ≈ AlSA5 ＞ AlSA20. Moreover,

Fig. 9●HDN Ratio of Acridine in Hydrodenitrogenation over Various Catalysts

Table 8

Selectivity for Products in HDN of Acridine over Various Catalysts 320 ℃

Temperature CoMo catalysts Al DCHM CHPM PHF PHA OHA THA DHA

[%] 0 [%] 0 [%] 0 [%] 33 [%] 64 [%] 3 [%] 0

DCHM CHPM PHF PHA OHA THA DHA

[%] 3 [%] 1 [%] 1 [%] 45 [%] 47 [%] 3 [%] 0

340 ℃

AlSA5

AlSA20

Al(CA)SA5

Al(CA)SA20

Al

AlSA5

AlSA20

Al(CA)SA5

Al(CA)SA20

0 0 0 31 65 3 1

0 0 0 29 26 42 3

1 2 0 55 13 28 2

1 2 0 51 45 1 0

1 1 0 46 47 4 1

1 1 0 45 50 3 0

1 0 0 40 29 26 4

1 2 0 58 25 11 1

1 3 0 59 36 1 0

360 ℃

Temperature CoMo catalysts Al

Discussion

380 ℃

AlSA5

AlSA20

Al(CA)SA5

Al(CA)SA20

Al

AlSA5

AlSA20

Al(CA)SA5

Al(CA)SA20

3 1 1 44 48 3 0

3 1 1 39 42 10 4

3 3 0 44 42 6 2

3 4 0 49 43 1 0

6 2 1 21 66 4 0

7 2 1 24 62 4 0

7 1 1 21 60 6 4

6 4 0 15 66 4 5

8 6 0 25 59 2 0


No. 6, 2017

309

Fig. 10●Effect of Addition of SAPO-11 and Citric Acid on Formation Rates of PHA and OHA in HDN of Acridine over Various Catalysts at Different Temperatures

Scheme 1●Activity Slabs Model Showing the Effects of Addition of SAPO-11 and Citric Acid

the increasing selectivity for DMBP and decreasing selectivity for MCHT and DMBCH in HDS with the addition of SAPO-11 suggested that the hydrogenation activity has a strong relationship with the support acidity, especially the number of strong acid sites. The decrease in amount of strong acid sites on the catalyst surface in the presence of SAPO-11 caused increases in the length and stacking number of CoMoS active slabs, resulting in an increase proportion of Edge sites. Based on the Rim-Edge model, high stacking slabs have fewer Rim sites and more Edge sites which have only desulfurization and denitrogenation activities without HYD. The XPS results also showed that the reduction of Mo6＋ to Mo4＋ increased in SAPO-11 added catalysts and that caused the increased formation of CoMoS. On the other hand, the low proportion of Rim sites also led to high HDS and HDN activities based on the Rim-Edge model. However, CoMoAlSA20 had very low HDS and HDN activities compared with other catalysts. Possibly the acidity of the support is not the only factor to influence the HDS and HDN activities. The TEM results showed that the average size and stacking number of CoMoAlSA20 were increased compared to CoMoAl, so such low activities could be ascribed to the low dispersion of active sites. In particular, SAPO-11 weakened the interaction between the support and active metal leading to low dispersion of

activity slabs. Thus, the numbers of activity slabs decreased and so the catalytic activity was lower. Citric acid added catalysts showed higher HDS and HDN activities, and lower HDA activity compared to CoMoAl. Citric acid added catalysts also achieved better direct desulfurization and enhanced direct denitrogenation. Similar findings for NiMo supported SBA-15 catalysts in an investigation of HDS of DBT showed that citric acid addition resulted in better dispersion of Mo species using powder X-ray diffraction (XRD) and UV-vis diffuse reflectance spectroscopy (DRS) characterization20). Higher Mo4＋ content as demonstrated by XPS also revealed that citric acid promoted Mo atom mobility and reduction from Mo6＋ to Mo4＋. As a result, citric acid added catalysts show better CoMoS formation. Moreover, TEM experiments exhibited shorter slabs length and higher stacking number in citric acid added catalysts as shown in the proposed activity slabs image in Scheme 1 (b), resulting in more Rim sites sharing and high HDS and HDN activities. Our findings show that citric acid is very important in enhancing active metal dispersion, CoMoS active phase formation and hydrogenation catalytic activity of sulfide catalysts. In conclusion, the preparation method of weakening support̶active metal interaction by controlling the support surface acidity combined with improving active metal dispersion by adding chelating agent could develop novel catalysts with selective hydrogenation activity. 5.

Conclusions

CoMo supported on Al2O3 catalysts with the addition of various amounts of SAPO-11 were prepared with and without citric acid during impregnation of the active metals. Compared to CoMoAl, 5 wt% SAPO-11 added catalyst showed less change in catalytic activity. The HDA activity of CoMoAlSA20 decreased by 20 % and the HDS and HDN activities also decreased in the


No. 6, 2017

310

same order of CoMoAlSA20 ＞ CoMoAl. The acidity of the supports was found to strongly affect the catalytic activity. Possibly the length and stacking number of CoMoS active slabs on the former catalyst increased because the dispersion of Mo species decreased due to the lower number of strong acid sites on the support surface in the presence of SAPO-11. Moreover, the HDA activity did not change and the HDS and HDN activities increased over citric acid added catalysts in the order of CoMoAl ≈ CoMo(CA)AlSA5 ≈ CoMo(CA) AlSA20. By adding citric acid during active metal impregnation, the HDS and HDN activities increased and the HDA activity decreased due to the improvement of Mo species dispersion and formation of shorter length and high stacking of CoMoS active slabs. References 1) Daage, M., Chianelli, R. R., J. Catal., 149, 141 (1994). 2) Voorhoeve, R. J. H., Stuiver, J. C. M., J. Catal., 23, 228 (1971). 3) Farragher, A. L., Cossee, P., Hightower, J. W., (ed.), “Proceedings of the Fifth International Congress Catal.,” vol. 2, North Holland, Amsterdam (1973), p. 1301. 4) Delmon, B., Froment, G. F., Catal. Rev. Sci. Eng., 38, 69 (1996). 5) Clausen, B. S., Mørup, S., Topsøe, H., Candia, R., J. de Physique, 37, 249 (1976).

6) Topsøe, H., Candia, R., Topsøe, N. Y., Clausen, B. S., Bull. Soc. Chim. Belg., 783, 93 (1984). 7) Topsøe, H., Clausen, B. S., Topsøe, N. Y., Pedersen, E., Ind. Eng. Chem. Fundam., 25, 25 (1986). 8) Berhault, G., Rosa, M. P. D., Mehta, A., Yácaman, M. J., Chianelli, R. R., Appl. Catal. A: General, 345, 80 (2008). 9) Ding, L., Zhang, Z., Zheng, Y., Ring, Z., Chen, J., Appl. Catal. A: General, 301, 241 (2006). 10) Pawelec, B., Fierro, J. L. G., Montesinos, A., Zepeda, T. A., Appl. Catal. B: Environmental, 80, 1 (2008). 11) Ishihara, A., Itoh, T., Nasu, H., Hashimoto, T., Doi, T., Fuel Proc. Tech., 116, 222 (2013). 12) Thompson, M. S., 1986, European Pat. EP 0181035 A2. 13) Gonzalez-Cortés, S. L., Xiao, T. C., Costa, P. M. F. J., Fontal, B., Green, M. L. H., Appl. Catal. A: General, 270, 209 (2004). 14) Gandubert, A. D., Krebs, E., Legens, C., Costa, D., Guillaume, D., Raybaud, P., Catal. Today, 130, 149 (2008). 15) Li, D., Sato, T., Imamura, M., Shimada, H., Nishijima, A., J. Catal., 170, 357 (1997). 16) Castillo-Villalón, P., Ramirez, J., Vargas-Luciano, J. A., J. Catal., 320, 127 (2014). 17) Wang, H., Prins, R., J. Catal., 264, 31 (2009). 18) Kabe, T., Ishihara, A., Qian, W., “Hydrodesulfurization and Hydrodenitrogenation,” Kodansha and Wiley-VCH, Tokyo/ Weinheim (1999). 19) Girgis, M. J., Gates, B. C., Ind. Eng. Chem. Res., 30, 2021 (1991). 20) Valencia, D., Klimova, T., Appl. Catal. B: Environmental, 129, 137 (2013).

要旨 CoMo/SAPO-11-Al2O3 触媒上での水素化脱硫，水素化脱窒素および水素化脱芳香族反応 Thanh Tung NGUYEN，篠崎晃，銭衛華東京農工大学生物システム応用科学府，184-8588 東京都小金井市中町2-24-16 異なる量の SAPO-11 を添加した SAPO-11-Al2O3 担体を調製

20 wt% SAPO-11 を添加した触媒の HDA，HDS および HDN 活

し，これらの触媒を用いて，4,6-ジメチルジベンゾチオフェン

性はすべて低くなった。これは SAPO-11 の添加より担体表面

の水素化脱硫反応（HDS），アクリジンの水素化脱窒素反応

の強酸点が減り，Mo 種の分散度が減少し，形成された CoMoS

（HDN），o-キシレン，1-メチルナフタレンおよびフェナントレ

活性相の長さおよび積層数が増大したと考えられる。一方，

ンの水素化脱芳香族反応（HDA）を行った。触媒のキャラク

Co および Mo 種を担持する際，クエン酸を添加したことにより，

タリゼーションには BET，XRF，NH3-TPD，XPS と TEM を用

Mo 種の分散が改善され，CoMoS 相の長さが減少し，HDA 活

いた。これらの結果より，担体の酸性は触媒構造および活性に

性は抑制されたが，HDS および HDN 活性は向上された。

強く影響を及ぼすことが分かった。無添加触媒と比べると，


No. 6, 2017