Advanced Materials Research Vols 233-235 (2011) pp 827-832 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.233-235.827
Online: 2011-05-12
Hydrodesulfurization Performances of NiMo Supported on Micro/Mesoporous Beta/MCM-48 Composite Catalyst Aijun Duan1a, Huili Fan1, Zhen Zhao1b, Guiyuan Jiang1, Jian Liu1 1
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P.R. China a
[email protected],
[email protected]
Keywords: Hydrotreating, hydrothermal method
HDS,
micro/mesoporous
composite,
self-assembly,
two-step
Abstract. Using CTAB as the mesostructure directing agent, a novel micro/mesoporous composite material of Beta-MCM-48 was synthesized and self-assembled from nanocrystal Beta solutions by two-step hydrothermal method in this study. The typical physico-chemical properties of material were characterized by the techniques and measurements of BET surface area, pore volume, pore size, XRD and TEM. Based on the characterization results, it could be found that BM48 composite simultaneously possessed BEA microporous structure and cubic Ia3d mesoporous structure, and the degree of mesoporous order was comparably uniform, which construct a suitable gradient of micro/mesopore structures and facilitated the diffusion of large size reactant molecules. The HDS performance in a microreactor indicated that NiMo/ABM48 series catalysts with different BM48 contents and various Mo loadings showed better hydrodesulfurization performances compared with the conventional NiMo/Al2O3 catalyst, and the optimal content of BM48 in the supports was 20 m%, and the suitable loading of Mo in the catalyst was 15 m%. The best HDS efficiency reached 98.3% and the corresponding sulfur content in product was 23.02 µg•g-1, which met the sulfur regulation of Euro IV ultra clean diesel specification. Introduction Porous materials are widely used in the catalytic processes for their eminent characteristics in pore structure and active surface[1~2]. Zeolites are crystalline aluminosilicates with the configurations of uniform pores and channels in molecular dimensions. Zeolites will be good carrier/catalyst candidates for most of catalytic reactions, and the development and synthesis of various zeolites have significantly contributed many efforts in some fields, i.e. catalysis, separation, adsorption and biosynthesis[3~4]. Zeolite beta is one of the most important high silica zeolites related to its interconnected large pore network and tunable acidities with three-dimensional large-pore construction[5~6]. It has been widely used in oxidation, hydroisomerization, aromatic alkylation, disproportionation, hydrocracking, and other processes[7~8]. Marler and his coworkers confirmed the topology of the basic layer as the building unit by refining the superposition structure of zeolite B-beta[9]. They illustrated that the structure of zeolite beta consists of an intergrowth hybrid of two distinct polytypic series of layers noted as polymorphs A and B. Newsam et al. proposed another hypothetical polymorph C, similar to A and B[10]. Corma and the coauthors synthesized the pure polymorph C structure either in the presence of fluoride anions or in a fluoride-free system under alkaline conditions by using germanium to stabilize the structure[11]. Sun et al.[12] used the monocrystalline silicon slice as silica source to replace the conventional colloidal materials (silica sol or fumed silica) and synthesized successfully large zeolite beta crystals. Especially in recent years, the composite micro/meso/macroporous materials with gradient pore structures caught much more attentions in scientific researches. The great diversity in surface functionalization of porous silica combined with different pore sizes, well-ordered pore structures, good connection and accessibility of different pores bestows these materials with excellent advantages in the structural configurations to stimulate the diffusions and reactions of molecules. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 128.42.202.150, Rice University, Fondren Library, Houston, USA-20/05/15,04:09:40)
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Pinnavaia et al.[13] reported the synthesis procedures of Al-MSU-S materials assembled from different zeolite seeds solution involving Y, Beta, and MFI. Xiao and his research groups[14] reported the mesoporous materials of MAS-7, MAS-9 and MTS-9 assembled from preformed nanosized Beta, MFI, and TS-1 precursors, respectively. These kinds of meso/microporous composite materials exhibited highly hydrothermal stability and good activity in catalytic conversion of organic compounds compard with the conventional mesoporous materials of MCM-41. The meso/microporous composites possess micropores and mesopores simultaneously within their crystals, and the pore sizes and acidities of which kept in some gradients. Thus, the special configuration ascribed these composites the screen function and the diffusion promoter of reactant molecules. The widely utilization of porous catalyst supported on the composite micro/mesoporous materials in hydrocarbon conversion processes, i.e., catalytic cracking, isomerization processes, and other adsorption /desorption processes[15]. Few works was related to the catalytic hydrotreating using a Beta/MCM-48 micro/mesoporous composite as the catalyst. Therefore, a full understanding of the pore structure and its HDS performance is very essential to obtain a deep insight from these catalytic processes adopting these composite catalysts, and to set up a sound theoretical base for designing and development of highly active catalysts. In this paper, a series of Beta-MCM-48 novel micro/mesoporous composite materials were synthesized and self-assembled from zeolite Beta nanocrystal solutions by two-step hydrothermal method using CTAB as the mesostructure directing agent. The characterization techniques including XRD, BET and TEM were used to analyze the typical physico-chemical properties and morphology features of different materials. The HDS performance of different catalysts with different BM48 additions and Mo contents were carried out in a microreactor unit to study the effects of BM48 incorporation into the catalysts on the catalytic performance and to investigate the interrelationship between the pore structure and the HDS efficiency. Experimental 2.1 Preparation of Beta/MCM-48 composite materials In the first step, a mixture of aluminasilicate sources derived from NaAlO2 and tetraethylorthosilicate(TEOS) was composed with molar ratios of Al2O3/SiO2/Na2O /TEAOH/H2O of 1.0/100/1.4/15/360 to prepare zeolite Beta nanoemulsion solution at the crystallization conditions of 140 for 12 hours. In the secondary step, using CTAB as the mesoporous directing agent the as-synthesized Beta nanocrystal emulsion was self-assembled with the molar ratio of composite mixtures of Al2O3/SiO2/CTAB/H2O =1/100/13/1130 under the optimal synthesis conditions for crystallization of 3 days. 2.2 Preparation of Beta/MCM-48/Al2O3 supported catalyst The Beta/MCM-48 composite was filtrated, dried and calcined at 550 ℃ in air for 6 h, and ionexchanged with NH4Cl to transform into H-type. The composite supports Beta/MCM-48/Al2O3(ABM48) were prepared by mechanical mixing of different amounts of Beta/MCM-48 and Al2O3, and the corresponding supported catalysts, denoted as NiMo/ABM48-x where x represented as the contents of BM48 composite in the total amounts of supports, were prepared with impregnation of aqueous solutions of ammonium heptamolybdate and nickel nitrate by two-step incipient-wetness impregnation method. Unless otherwise specified the active metal contents of MoO3 is 10 m% and NiO keeps 3.5 m%. 2.3 Characterization of supports and catalysts Powder X-ray diffraction (XRD) patterns were analyzed in the 2θ range of 1.3–50° on an XRD-6000 powder diffractometer using Cu Kα radiation under 40 kV and 30 mA. Nitrogen adsorption and desorption measurements isotherms were performed at -196 on a Micromeritics ASAP 2020 system. The samples need to be outgassed at first under vacuum at 350 before measurement. The surface characteristics of surface area and pore volume were calculated according
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to the BET method and the BJH and t-plot methods. Transmission electron microscopy (TEM) information was taken by using Philips Tecnai G2 F20 equipment operated at 300 kV. 2.4 Activity measurement HDS performances of FCC diesel were tested in a fixed-bed microreactor. The presulfiding conditions and operating conditions are listed in Table 1, and the sulfide agent is 2 m% CS2-cyclohexane liquid mixture. After catalyst presulfurization, the operating conditions were adjusted to evaluate the HDS performance of different catalysts with diesel feedstock. The typical properties of feed are listed in Table 2. Table 2. The typical properties of diesel feedstock Properties Data -3 Density @ 20°C/g·cm 0.8798 1290 S，µg·g-1 -1 920 N，µg·g Distillation/°C IBP 158 30% 212 50% 238 70% 279 EBP 374
Table 1. Typical parameters of presulfiding and operating conditions HDS test Projects Presulfiding Temperature, P, MPa LHSV, h-1 H/O, v/v Time, hr
320
350
4 1 600 4
5 1 600 >13
The total sulfur contents in the feed and products were measured in a LC-4 coulometric sulfur analyzer. The HDS activity was estimated by HDS efficiency, where it is defined as HDS% efficiency =[(Sf−Sp)/Sf]×100%; Sf and Sp stand for the sulfur concentrations in feed and product respectively. Results and Discussion 3.1 Typical physicochemical properties of different materials Table 3 shows the typical physicochemical properties of various materials. As is shown in this table, both BM48 and MCM-48 have high surface areas, which are much higher than that of Al2O3. And BM48 composite has relatively higher value of Vmic/Vmes than MCM-48. Moreover, compared with MCM48, BM48 has a wider pore diameter and thicker pore wall, which may be derived from the assembling of the zeolite primary unit into the mesoporous framework of MCM-48. Table 3. Typical physicochemical properties of MCM-48, BM48 and Al2O3
MCM-48
SBET[1] m2·g-1 928.7
Vt[2] cm3·g-1 0.74
Vmes[3] cm3·g-1 0.76
Vmic[4] cm3·g-1 0.34
Vmic/ Vmes 0.45
a0[5] nm 7.3
dBJH[6] nm 2.5
bw[7] nm 1.3
BM48
907.5
0.66
0.58
0.45
0.78
8.5
2.2
1.6
Al2O3
206.8
0.48
-
-
-
-
-
Samples
Note: [1] Surface area was calculated by the BET method; [2] the total pore volume; [3] mesopore volume; [4] micropore volume; [5] XRD unit cell parameter (a0); [6] dBJH is the mesopore diameter; [7] bw is the wall thickness.
Table 4 lists the typical properties of different catalysts with various BM48 contents in the supports. BM48 composite possesses high surface area and pore volume derived from the combinations of microporous BEA and mesoporous MCM-48 characteristics, and the surface area and pore volume increases Table 4. Textural properties of catalysts with different BM48 contents with the increase of additional Average pore amounts of BM48, while the Catalysts SBET/m2·g-1 V /cm3·g-1 diameter/ nm average pore diameter NiMo/Al2O3 167.4 0.34 8.32 decreases with the BM48 additions due to the increase NiMo/ABM48-10% 248.3 0.40 7.86 of the micropores and 265.9 0.43 7.73 NiMo/ABM48-20% mesopores. The immoderate NiMo/ABM48-30% 286.5 0.47 7.46 incorpor- ations of BM48 will NiMo/ABM48-60% 316.8 0.49 6.85 lead to the rapid shrink of pore
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diameter and result in the inaccessibility of efficient surfaces. Therefore, suitable incorporations of BM48 materials into the supports should be optimized in the following experiments. Table 5 lists the typical Table 5. Textural properties of the catalysts with different Mo loadings physicochemical properties of Average pore different supported catalysts with Catalysts S BET m2/g V /cm3/g diameter/ nm various Mo contents. The BM48 NiMo/Al O 167.4 0.34 8.32 2 3 composite additions in this series NiMo/ABM48-Mo: 5% 226.4 0.37 6.57 supports were kept at the same NiMo/ABM48-Mo: 10% 199.6 0.34 6.93 percentages of 20m%. As is shown NiMo/ABM48-Mo: 15% 174.5 0.31 7.51 in this table, the surface areas and NiMo/ABM48-Mo: 20% 136.4 0.29 8.41 pore volumes show the decreasing Note: BM48 composite additions in the supports were kept at same levels of 20m%. tendencies with the increasing of Mo contents, but the surface areas and volumes decrease sharply when the Mo contents is great than 15%.. The average pore diameters are slightly lower than that of Al2O3, which might be related to the deposition and blockage effects of active metal impregnation to the micropores. Figure 1 shows X-ray diffraction patterns (XRD) of the as-synthesized samples. The typical MCM-48 diffraction peaks were indexed as (211), (220), and others were observed in the small angle XRD patterns of BM48 sample, which confirmed that BM48 material had a well-ordered Ia3d mesoporous structure as MCM-48[16]. But the peak intensities of BM48 composite are relatively lower compared with that of MCM-48 since the order degree of the in-situ assembled BM48 composite from zeolite nanocluster emulsion are not uniform as the organic alumina-silica resources by the mesostructure directing agent[17]. Futhermore, the XRD patterns in the range of wide angle of BM48 showed the characteristics of Beta at 7.6 ˚ and 22.4 ˚, although the peak intensities were not very strong due to the tiny crystal sizes of Beta nanoemulsion. All of these informations demonstrated that BM48 materials possessed BEA microporous structure and cubic Ia3d mesoporous structure simultaneously. (B)
(A)
(a)
2.76
•
MCM-48
• ♦
♣
Intensity / a.u.
Intensity / a.u.
Intensity / a.u.
2.54
X5
(C) • BM48 ·BM48 ♣ Al 33 ♣ Al2O 2O ♦ MO ♦ MoO 3 3
(b)
Beta
♣
Mo:5 m%
d
10 m% c
15 m% b
BM48 X5
♣
20 m%
a
BM48 MCM-48
2
4
6
10
8
15
20
25
30
35
40
45
10
20
2θ /degree
2θ /degree
30
40
50
60
70
80
2θ /degree
Figure 1. XRD patterns of different materials.
dV/dlogD
MCM-48
MCM-48
BM-48
BM48 Al2O3
1
10
100
Pore diameter D/nm
NiMp/ABM48-15%
Figure 2. Pore size distributions of samples
Figure 3. TEM images of different materials
Figure 2 shows the pore size distributions(PSD) of different materials. It can be observed that BM48 composite materials still possess the characteristic dual-pore structure similar to that of
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MCM-48, while the PSD of Al2O3 exhibited the disseminated pore structure。These PSDs indicated that the existence of mesopores still kept after the synthesis of BM-48. The distribution curves of pore sizes indicated the existence of the pores with 2.4 nm and 3.8 nm in diameters, of which the former was ascribed to mesoporous pore of particles and the latter was ascribed to the interparticle pores. The suitable configurations and distributions of micro/mesopores facilitate the diffusions and reactions of reactants. Figure 3 are the TEM images of various materials. From the images of MCM-48 and BM48, it can be found that BM-48 showed well ordered cubic 3D mesoporous channels just like MCM-48 mesoporous material, the as-synthesized catalyst kept the mesoporous structure even after the impregnation process of active metals18. The pore sizes were around 2-3 nm, which were agreed well with the results of N2 adsorption and XRD results. And the existence of beta nanocrystals was also observed in the BM48 image with an average size of 3~5nm. 3.2 Catalytic hydrodesulfurization performance HDS performances of NiMo series catalysts for FCC diesel are shown in Table 6 and Table 7. From the data in Table 6, it is obvious that the addition of BM48 to the catalyst support could improve the hydrotreating performances, and the assessment results indicated that NiMo/ABM48-20% catalyst reached the highest HDS efficiency of 98.1%, and all the other catalysts with BM48 loadings between 10%~40% exhibited better HDS efficiencies than that of the conventional NiMo/Al2O3 catalyst. It might be attributed to the appropriate contents of BM48 improving the surface properties, involving the specific surface area and total pore volume of the catalysts, which could facilitate to the higher dispersion of active metals on the surface of support. Moreover, it is also due to that the suitable incorporation of BM48 could adjust the catalyst acidity, and, thus would benefit to the breakage of C-S bond in sulfides. However, the excessive addition of BM48 into the supports resulted in the decrease of average pore diameter based on the analysis data in Table 4 and impeded the diffusion of large reactant molecules with the steric hindrance, i.e., alkyl-substituted DBTs19. Thus the suitable content of BM48 could be 20 m% in the support, and the sulfur content in product was 26.45 µg•g-1 that can meet the sulfur specification of Euro IV ultra clean diesel standard(
