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Ind. Eng. Chem. Res. 2009, 48, 2774–2780
Effect of Order of Impregnation of Mo and Ni on the Hydrodenitrogenation Activity of NiO-MoO3/AlMCM-41 Catalyst S. J. Sardhar Basha, P. Vijayan, C. Suresh, D. Santhanaraj, and K. Shanthi* Department of Chemistry, College of Engineering, Anna UniVersity, Sardar Patel Road, Guindy, Chennai - 600025, India
The influence of order of the impregnation of NiO and MoO3 on the structure and catalytic behavior of H-AlMCM-41 supported Ni-Mo catalysts for hydrodenitrogenation (HDN) has been investigated. These catalysts were well characterized by TEM, XRD, N2 adsorption-desorption, XPS, and FT-IR spectra of CO adsorption and tested for hydrodenitrogenation (HDN) of o-toluidine and cyclohexylamine. It was found that the catalyst prepared by sequential impregnation of NiO and MoO3 was more active than the catalyst prepared by coimpregnation. Among the sequentially impregnated catalysts, the catalyst in which NiO deposited first followed by MoO3 shows high HDN activity. TEM, FT-IR spectra of CO adsorption, and estimation of total sulfur of sulfide catalyst results clearly show that the NiO and MoO3 are well dispersed over H-AlMCM-41 on reverse order impregnated catalyst. The very low activity of coimpregnated catalyst is attributed to poor dispersion of metal oxides as well as due to the formation of stable stoichiometric oxide and aggregates of metal particle over the support surface. 1. Introduction Catalytic hydrotreating is one of the important secondary processes developed to improve the quality of petroleum crude for hydrocracking. In recent years, hydrotreating has received considerable interest from researchers to meet the stringent environmental regulations with regard to reduction of sulfur and nitrogen level.1 Hence, there is a need for the development of a very efficient catalyst to meet the environmental regulations. Over the years, many supports that include oxides, mixed oxides, zeolites, and mesoporous materials have been tried for Co (Ni) promoted Mo catalysts, with the view to reduce the sulfur content by hydrodesulfurization (HDS) and nitrogen content by hydrodenitrogenation (HDN).2-6 Research has shown that the proper choice of support is of great importance for the enhancement of catalytic hydrotreating in general and to work under less severe reaction conditions in particular. After the discovery of mesoporous materials, MCM41 has received increasing attention due to its very high surface area and regular pore dimensions. MCM-41 alumino silicate favors a high dispersion of the active species while increasing the accessibility of the large molecules of the gas feed containing heteroatoms to the catalytic active sites.7 Despite the extensive use of MCM-41 in catalysis, there has been relatively limited application as support for hydrotreating processes until recently. The catalytic activities of Ni (Co)-Mo sulfided catalysts supported on MCM-41 have been examined for HDS of dibenzothiophene (DBT),8 LCO (light cycle oil), 9 and hydrocracking of VGO (vacuum gas oil).10 There is evidence in the literature to show that the catalytic performance is not only due to the high surface area of the support but also due to the other changes that occur in their textural properties while modifying the preparation procedure.11-13 Shanthi et al.11 found that changing the order of impregnation, significantly affects the physical properties and hence the HDN activity of Al2O3 supported NiOMoO3 catalysts. Cordero and Agudo12 showed that a Mo-Ni catalyst prepared by the sequential impregnation method reduces * To whom correspondence should be addressed. Tel.: +91 44 22203158. Fax: +91-44 22200660. E-mail: kshanthiramesh@ yahoo.com.
more readily than coimpregnation catalyst. Recently, HDS, HDN, HDA, and hydrocracking of model compounds have been carried out over Mo-Ni with various acidities.14 Landau et al.13 applied ultrasonic treatment to deposit higher loading (up to 60 wt %) of CoO-MoO3 on AlMCM-41, subjected to HDS of DBT and reported that at 45 wt % of MoO3 loading there is monolayer coverage on this mesoporous surface. Also, activity of the catalyst is found to be 1.7 times higher than the commercial catalyst for HDS of DBT. Recently, Salerno et al.15 studied the effect of the impregnation method (coimpregnation and sequential impregnation) and the impregnation order of the active phases, Ni and Mo on the structure and catalytic activity of NiMo hydrotreating catalysts supported on an Al-pillared montmorillonite. The catalyst prepared by coimpregnation of Ni and Mo was more active than those prepared by sequential impregnation. The HDS activity of CoMo/γ-Al2O3 catalyst prepared by depositing Mo species via EDF (equilibrium deposition filtration) and then the Co species by dry impregnation is reported to be more active than the catalyst prepared by the conventional method.16 However, there has been no focus on the study of the effect of the order of impregnation of Ni and Mo on HDN of AlMCM41 supported NiO-MoO3 catalysts. In the previous communication,17 the authors studied the effect of MoO3 content on the hydrodenitrogenation activity of H-AlMCM-41 supported catalyst and reported that the 24 wt % of MoO3 loading is optimum for the fine dispersion of active Mo species on H-AlMCM-41 at constant NiO loading (7 wt %) and for maximum activity. It has been seen that metal dispersion at this loading was better than other catalysts for HDN of o-toluidine. In this report, the results of the study made on the effect of the order of impregnation of active metal oxides on the structure and HDN activity of NiO-MoO3/AlMCM-41 catalysts are presented. For this purpose, pre- reduced catalyst has been used instead of presulfide catalyst. It is to be noted that the as synthesized catalyst will undergo sulfidation under the industrial (trivial) conditions due to the presence of traces of sulfur compounds in the petroleum feed by way of HDS. The functional catalyst will only be the sulfide form under operational conditions. It has also been reported in the literature that the function of the
10.1021/ie800932u CCC: $40.75 2009 American Chemical Society Published on Web 02/18/2009
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SH-group in sulfide form is expected to be fulfilled by OHgroup in prereduced state.18 Hence, performance of the pre reduced Ni-Mo/MCM-41 catalysts is tested for HDN of simple amines like o-toluidine (OT) and cyclohexylamine (CHA). A correlation of catalytic activity was made with physicochemical properties. 2. Experimental Section 2.1. Synthesis of Support. AlMCM-41 was synthesized as described in detail in the previous paper.17 2.2. Preparation of Catalysts. The Ni-promoted catalysts were prepared by incipient wetness coimpregnation and sequential impregnation method. H-AlMCM-41 was impregnated with a solution containing a mixture of nickel nitrate (SRL India) and ammonium paramolybdate (SRL India) to give (NiO.MoO3)/ H-AlMCM-41 (designated as A). Two other catalysts were prepared by impregnating H-AlMCM-41 with a solution of ammonium paramolybdate followed by nickel nitrate to give NiO-MoO3/ H-AlMCM-41 (designated as B); reversing the order of impregnation led to MoO3-NiO/H-AlMCM-41 (designated as C) catalysts. The catalysts were dried at 373 K and calcined at 773 K in air for 6 h. All catalysts had nominal contents of 24 wt % MoO3 and 7 wt % NiO. The impregnated samples were prereduced prior to every catalytic run by passing H2 for 2 h at a rate of 50 cm3/min at 723 K. 2.3. Catalytic Studies. HDN activity of the prereduced NiOMoO3 catalysts was tested in a fixed bed flow reactor. The reaction set up used in this study has been described in the previous paper.17 The reactions were carried out using the feed containing 2.5 wt % of o-toluidine (OT) and cyclohexylamine (CHA) in cyclohexane, which was chosen to simulate the actual industrial feed composition. The HDN of simple amines (AR grade) were carried out at several reaction temperatures viz., 673, 698, and 723 K in the presence of ultrahigh pure grade hydrogen (50 cm3/min) at atmospheric pressure. The HDN of o-toluidine under these conditions led to only toluene and NH3, whereas cyclohexylamine led to cyclohexane and NH3. No other products were observed under these experimental conditions. The collected liquid products were analyzed by Shimadzu 17A gas chromatograph using 20% SE-30 column. Ammonia formed as a result of nitrogen removal from the amine was estimated by titration with 0.01 N hydrochloric acid. The experiments were repeated at least three times for reproducibility. The accuracy of the analytical results was with in (0.5%. After every catalytic run the catalyst was regenerated by passing CO2 free air over the catalyst at 773 K for 6 h. 2.4. Characterization. The physicochemical properties of synthesized catalysts were characterized using different techniques like TEM, XRD, N2 adsorption-desorption studies, XPS, and FT- IR spectroscopy of adsorbed CO and chemical analysis of sulfur content. X-ray diffraction (XRD) patterns of NiO, MoO3 impregnated on H-AlMCM-41 catalysts were recorded on a Rich Seifert X-ray diffractometer using Cu KR radiation (λ ) 1.5406 Å). The diffractograms were recorded in the 2θ range 1.5° to 60° with a step size of 0.05°. The count time was 4 s over each step. The textural characteristics of the impregnated H-AlMCM41 materials were analyzed by nitrogen adsorption method. Nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP2010 apparatus analyzer. All the samples were initially degassed at 473 K for 2-3 h under vacuum in the degas port of the analyzer. The specific surface area was calculated following the BET procedure. The pore size distribu-
tions of the materials were calculated from the desorption branch of the nitrogen isotherm following the BJH method. Transmission electron microscopy measurement for the reduced catalysts was carried out in JEOL 200 kV electron microscope operating at 200 kV. Catalyst sample powders were dispersed on to “holy carbon” coated grids, which were then introduced to the microscope column, which was evacuated to less than 1 × 10-6 Torr. Specimens were enlarged using thin photographic paper. The size of metal particles visible in the photograph were measured manually and averaged. The state of metals in the prereduced catalyst was analyzed using XPS. An ESCALAB MK II spectrometer was used to record X-ray photoelectron spectra using Mg KR radiation (1253.6 eV). Positioning the signal C1s, linked with the contaminating carbon, at 284.6 eV, a value generally admitted, makes energy recalibration of the spectra. The catalyst powders were placed in a container and were mounted on a sample probe. Calcination and reduction were carried out in the catalyst preparation chamber. So, the catalysts could be moved to the analysis chamber without exposure to air. During spectral acquisition the pressure of the analysis chamber was maintained at lower than 1 × 10-9 Torr. FT-IR spectra of the adsorbed CO were recorded using a Nicolet Magna 550 IR spectrometer equipped with a MCT detector-using 256 scans at a resolution of 4 cm-1. The catalyst powder was pressed into a self-supported wafer (∼15 mg) and placed into Spectra tech insitu IR cell, equipped with CaF2 windows and double walls with space for the cooling agent. The catalyst was reduced with H2 at 450 °C for 2 h. After the reduction, the catalyst was flushed with He for 30 min and sulfidation was done by passing thiophene vapor for 1 h with He flow and then the temperature was lowered to room temperature. The background IR spectrum was recorded. CO was passed over the catalyst at a flow rate at 30 mL/min for 2 h. Then, the physisorbed CO was removed by flushing with He for 30 min and the CO adsorption modes were recorded in the region of 1900-2300 cm-1. The total metal (Mo and Ni) content was estimated by an indirect qualitative chemical analysis.11 It is seen from the literature that the amount of total sulfur present on the catalyst provides information about total available metal on the surface. For this purpose, a simple method was devised to estimate sulfur content by chemical analysis. Ni-Mo catalysts were presulfided by passing H2/H2S mixture (1:10 v/v ratio) at 673 K for 2 h. After flushing with nitrogen gas, the sulfur on the catalyst was oxidized to SO3/SO2 and the oxides were estimated volumetrically by absorption in aqueous H2O2 (3 vol %), followed by titration with dilute NaOH. 3. Results and Discussion 3.1. Hydrodenitrogenation Activity. Polycyclic N-containing compounds have a carbocyclic as well as heterocyclic ring, and therefore, there are two routes to remove nitrogen atom from such molecules. One route goes via aniline type molecules and the other via pyridine and piperidine type molecules.19 As model compound for aniline type, o-toluidine (OT) was chosen, while cyclohexylamine (CHA) was chosen as the representative of piperidine type compounds. Pure molybdena and H-AlMCM41 individually showed no activity for the HDN of amines. HDN of OT and CHA were carried out on the Ni-Mo catalysts at least three times and the values agreed to within 2%. The original activity is maintained for appreciable number of cycles. The effect of temperature on the activity of catalysts A, B, and C for the HDN of OT and CHA was studied at 673, 698,
2776 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009
Figure 1. Effect of temperature on the catalytic activity for o-toluidine HDN: Influence of the impregnation method. Reaction conditions: Activity between 150-210 min; LHSV ) 0.75 h-1; o-toluidine ) 4.2 × 10-4 mol/h; H2 flow rate ) 50 cm3/min (1 atm). Catalysts: A, (NiO · MoO3)/H-AlMCM41; B, NiO-MoO3/H-AlMCM-41; C, MoO3-NiO/H-AlMCM-41.
Figure 3. Effect of time on stream in the catalytic activity for o-toluidine HDN: Influence of the impregnation method. Reaction conditions: Temperature ) 723 K; LHSV ) 0.75 h-1; o-toluidine ) 4.2 × 10-4 mol/h; H2 flow rate ) 50 cm3/min (1 atm).
Figure 4. Effect of time on stream in the catalytic activity for cyclohexylamine HDN: Influence of the impregnation method. Reaction conditions: Temperature ) 723 K; LHSV ) 0.75 h-1; cyclohexylamine ) 4.5 × 10-4 mol/h; H2 flow rate ) 50 cm3/min (1 atm). Figure 2. Effect of temperature on the catalytic activity for cyclohexylamine HDN: Influence of the impregnation method. Reaction conditions: Activity between 150-210 min; LHSV ) 0.75 h-1; cyclohexylamine ) 4.5 × 10-4 mol/h; H2 flow rate ) 50 cm3/min (1 atm). Catalysts: A, (NiO · MoO3)/HAlMCM-41; B, NiO-MoO3/H-AlMCM-41; C, MoO3-NiO/H-AlMCM-41.
and 723 K, and the results are illustrated in Figures 1 and 2, respectively. In the HDN of OT and CHA, the activity of the catalyst increased with an increase in temperature. The maximum activity was observed at 723 K over all the catalysts. If HDN of OT and CHA are compared, the activity of the catalysts for the former is lower than the latter. The delocalization of nitrogen lone pair on the aromatic ring of OT is suggested to impart double bond characteristics to C-N bond and hence become much stronger than the C-N bond in aliphatic ring. Consequently, C-N bond in aromatic ring can be broken less readily than in aromatic ring.20 The effect of time on the activity of catalysts A, B, and C was examined for 330 min. The results are illustrated in Figures 3 and 4. The activities of all the catalysts increased with an increase in time on stream (TOS). There was a nonlinear response between 90-330 min. The activity slowly increased between 90-150 min and rapidly between 150-210 min. The increase in activity with time clearly demonstrates time dependence of activity. Although OT and CHA have different chemical natures, the maximum activity is observed between 150-210 min. In addition, the activity variation with time also appears similar for both OT and CHA. An interesting observation was made on the overall activity of the catalysts. The activity of the catalyst C, prepared by impregnating NiO first followed by MoO3, was the highest among the three catalysts A, B, and C under all the experimental
conditions [Figure 1-4]. The activity also follows the trend: 24% MoO3-7% NiO/H-AlMCM-41 (C) > 7% NiO-24% MoO3/H-AlMCM-41 (B) > (7% NiO · 24% MoO3)/H-AlMCM41 (A). This trend is a clear indication for the dependence of the activities on the order of impregnation of NiO and MoO3 on the H-AlMCM-41 surface. Catalyst (C), prepared by impregnating Ni first and Mo second, appeared to generate more number of active sites on the catalyst surface than A and B. It can be very well understood that the order of impregnation of the catalytic component significantly affects the structural characteristics of the catalysts.11 From the evaluation of catalytic activity it is concluded that the impregnation of Ni first and Mo second might be a better way to confer higher activity to a catalyst. The physicochemical properties of all the catalysts were systematically studied to account for the observed trend in catalytic activity of the catalysts. 3.2. Study of Physico-chemical Properties. The XRD pattern of the catalyst C recorded from 0 to 10 2 θ (deg) is shown in Figure 5A. This confirms that mesoporous nature of MCM-41 is retained after loading the active components. The XRD patterns of catalysts (NiO · MoO3)/H-AlMCM-41 (a), NiOMoO3/H-AlMCM-41 (b), and MoO3-NiO/H-AlMCM-41 (c), recorded from 0 to 60 2 θ (deg), are shown in Figure 5Ba, Bc, and Bb, respectively. The effect of order of impregnation on the crystallization and growth of different phases is clearly evident from the patterns of the XRD. Figure 5Ba shows the characteristic patterns due to MoO3 (2 θ ) 26-27°), NiMoO4 (2 θ ) 28.8°), and NiO phases. In Figure 5Bb the patterns due to MoO3 and NiMoO4 phases are evident. Figure 5Bc is similar
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 2777
Figure 5. (A) XRD pattern of catalyst C 0 to 10 2 θ (deg). (B) XRD patterns of (a) catalyst A, (b) catalyst C, and (c) catalyst B.
to that of Figure 5Bb. From the intensities of the patterns due to the different phases it can be depicted that the phases have large dimensions in the catalyst prepared by coimpregnation method. Hence, the NiMoO4 phase, instead of getting well dispersed over H-AlMCM-41, appears to grow to a large dimension. This might be responsible for its low catalytic activity. In B and C, the pattern due to NiMoO4 is not as resolved and as intense as that of A. Hence, in the catalysts B and C, this phase must be well dispersed with smaller dimensions. The BET surface area, pore volume, and pore size of the catalysts are presented in Table 1. It is seen from the results that the surface area and the pore size have been retained with a small change after loading the metal. This indicates the integrity of H-Al-MCM-41 structure. Among the three catalysts designed, the catalyst C has the lowest surface area. Catalysts A and B appear to have almost the same surface area. The low surface area of sample C suggests that there is more deposition of metal oxide phases in the pores of H-AlMCM-41. The particles dispersed in the mesopores are to be of nano dimensions. But in the catalysts A and B there might be a greater number of particles outside the mesopores. However, these particles might have dimensions even bigger than those in sample C. The typical TEM images of reduced NiO-MoO3/H-AlMCM41 catalysts are shown in Figure 6. The black dots seen on the support matrix are supposed to be NiO-MoO3 particles on the surface of the support. Figure 6a shows the TEM picture of (NiO · MoO3)/H-AlMCM-41 catalyst, prepared by coimpregnation method. There are aggregates of metal particles without any definite shape. Hence, the dispersion of NiO-MoO3 particles on H-AlMCM-41 support is not uniform. In Figure 6b it appears that tiny particles are present with sharp edges dispersed over the support. It therefore, indicates that the method of impregnation of Mo first and Ni second is a better way than the coimpregnation procedure. The TEM picture of MoO3-NiO/H-
AlMCM-41 is shown in Figure 6c. There are large number of impregnated crystallites seen in Figure 6c, showing that this method of impregnation appears to be better than the previous methods. It indicates that loading NiO first and the MoO3 leads to the formation of large number of active metal particles on the surface of H-AlMCM-41, which is considered to be beneficial for the HDN activity. The XPS data of catalysts A, B, and C are given in Table 2. The assignment of peak maximum to different oxidation states of Mo and the corresponding binding energies are also given in Tables 2 and 3. From the spectral data, two different oxidation states of Mo are immediately evident for both the catalysts. The absence of Mo6+ species in both the catalysts, namely, (7 wt % NiO · 24 wt % MoO3)/H-AlMCM-41 and 24 wt % MoO3-7 wt % NiO/H-AlMCM-41 shows that Mo6+ is completely reduced to lower oxidation states. However, it was seen from the spectra that the percentage peak intensities corresponding to Mo5+ and Mo4+ differ in these two catalysts. This distinct difference between these two catalysts is due to the order of impregnation of Ni and Mo. Because NiO was added first it could very well disperse over the entire surface of H-AlMCM-41. This facilitates avoidance of formation of any free Mo grains without association of NiO. Hence, the probability for complete reduction of Mo6+ to lower valence state is rather high, thus preventing any residual Mo6+ after reduction. Based on the peak percentage values, it can be reported that Mo5+ is more abundantly available than Mo4+ on reverse order impregnated catalyst 24 wt % MoO3-7 wt % NiO/H-AlMCM-41 than on normal order impregnated catalyst. The XPS data of catalyst 7 wt % NiO-24 wt % MoO3/HAlMCM-41 (B) is presented in Tables 2 and 3. A broad envelope covering the range 228-240 eV was seen in the XPS of Mo. This broadband is deconvoluted to assign different oxidation states of Mo, based on the values reported in the literature.21 The binding energy (BE) values corresponding to the maximum of each peak, the oxidation states and the percentage of peak area corresponding to each maximum are indicated in Table 2 and Table 3, respectively. Mo4+, Mo5+, and Mo6+ species are evident from XPS data. The percentage corresponding to each oxidation state is in the order Mo4+ < Mo5+ ≈ Mo6+. Hence, this observation establishes that the reduction of Mo6+ species is incomplete. Because the process of impregnation of 24% Mo first and 7% Ni second could leave sufficient Ni covering of MoO3 surface, the grains of MoO3 might have less probability of getting reduced to a lower valence state. The BE values and percentages of the peak area corresponding to each oxidation state are also given in Table 2. Catalyst B shows three signals corresponding to nonstoichiometric NiO (BE 853.3 eV),21 Ni2O3 (BE 855.3 eV), and Ni in NiMoO4 (BE 857 eV). Similar observations were also made for the catalyst C. The BE equal to 853.0 eV was observed for catalyst A which is very close to the binding energy of the metallic Ni (852.7) Ni metal: 852.3 eV.21 The Ni in low oxidation state may not contribute to the promoting action of Mo, unlike the other two catalysts, which have more of Ni in high oxidation states. The tendency of Ni2+ to reduce to Ni0 in catalyst A might not have beneficial effect for the promoting action of Mo. Hence, catalyst A exhibits lower HDN activity than catalysts B and C. The atomic surface concentration of Mo determined by XPS is presented in Table 2. For the most active HDN catalyst, namely, reverse order impregnated catalyst (C), the atomic surface concentration was relatively low when compared with the other catalysts A and B. This is in contradiction with the results
2778 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 Table 1. Physical Characteristics of H-AlMCM-41 and its Supported Catalysts catalysts
surface area (m2/g)
pore volume (cm3/g)
H-AlMCM-41 24% MoO3/H-AlMCM-41 7% NiO/H-AlMCM-41 7% NiO-24% MoO3/H-AlMCM-41(A)b 7% NiO-24% MoO3/H-AlMCM-41(B)c 24% MoO3-7% NiO/H-AlMCM-41(C)d
1209 653 1002 586 566 499
1.08 0.44 0.87 0.19 0.07
a Total weight of sulfur present on the presulfided catalysts;. Reverse order impregnated catalyst.
b
A: Simultaneous impregnated catalyst.
c
pore size (Å)
amount of sulfura (g)
30.41 28.20 30.73 27.39 34.62 29.30
2.68 × 10-2 4.34 × 10-2 4.7 × 10-2
B: Normal order impregnated catalyst.
d
C:
Figure 6. TEM images of reduced (a) catalyst A, (b) catalyst B, and (c) catalyst C. Table 2. XPS Binding Energies (eV) and Its Corresponding Percent of Peak Area and the Atomic Surface Concentration of Mo (CMo) and Ni (CNi) binding energy (eV) catalyst (reduced) a
A
Bb Cc
Mo 3d5/2
%
Mo 3d3/2
%
Ni 2p
%
CMo (% atoms)
CNi (% atoms)
230.6
30.1
233.5
15.3
7.27
38.0 10.0 24.1 27.1 12.4
235.0 233.3 235.1 236.4 234.1
16.6 7.8 12.2 18.7 8.1
3.96
2.16
3.15
5.98
53.1
235.7
26.4
36 26 38 23.8 56.5 19.7 25 48.8 26.2
4.37
231.8 230.4 231.9 233.6 230.1
853.0 854.5 856.4 853.3 855.3 857.0 853.4 855.2 856.9
232.4 a
b
A: Simultaneous impregnated catalyst. B: Normal order impregnated catalyst. c C: Reverse order impregnated catalyst.
Table 3. Various Oxidation States Present in NiO-MoO3/ H-AlMCM-41 Catalysts oxidation states of catalysts (reduced)
Mo
Ni
(7% NiO-24% MoO3)/H-AlMCM-41(A)a Mo5+, Mo4+ Ni2+, N0 7% NiO-24% MoO3/ H-AlMCM-41(B)b Mo6+, Mo5+, Mo4+ Ni2+ 24% MoO3-7% NiO/ H-AlMCM-41(C)c Mo5+, Mo4+ Ni2+ a A: Simultaneous impregnated catalyst. b B: Normal impregnated catalyst. c C: Reverse order impregnated catalyst.
order
obtained from TEM, which strongly suggests better dispersion for the supported Mo phase on the catalyst C. However, a similar finding was reported by Papadopoulou et al.16 for Mo dispersed over alumina catalyst which was accounted in terms of the atomic concentration values obtained from the XPS technique, which are indicative of repartition rather that the dispersion of
the supported Mo phase. Hence, on catalyst A, the corresponding increase in XPS signal is attributed to the redistribution of Mo atoms enriching the surface concentration as explained by Papadopoulou et al.16 Hence, from this XPS analysis, it is clearly evident that the order as well as the method of impregnation is important in the design of highly active catalyst, and it is established that the impregnation of NiO first followed by MoO3 is more beneficial for the process of HDN. FT IR spectra were recorded for CO adsorbed Ni-Mo catalyst A, B, and C to measure qualitatively the amount of Mo dispersed on the H-AlMCM-41surface. The results obtained on the catalysts of reduced NiO-MoO3 catalysts are presented in Figure 7. The band at ∼2090 cm-1 is attributed due to the CO adsorbed on Mo sites (un promoted and free Mo) and ∼2070
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Figure 7. FT-IR spectra of CO adsorbed on reduced (a) catalyst A, (b) catalyst B, and (c) catalyst C.
cm-1 is due to CO adsorbed on Ni promoted Mo sites on the basis of literature in which similar spectra were observed on sulfide Mo catalysts.22,23 Both reduced and sulfide form is expected to give same IR spectra corresponding to only different type of Mo sites when CO is adsorbed. The spectra of impregnated catalysts show the characteristic features NiOMoO3 catalysts. A clear difference arises in the intensity of the adsorbed bands when the order and method of impregnation are changed. The intensity of CO adsorbed band was found to be the highest for catalyst C (reverse order impregnated catalyst) among the three catalysts A, B, and C. This indicates the presence of more number of finely dispersed Mo species on the surface of support. From the intensity of the CO adsorbed band, the following trend is proposed for the dispersion of Mo on the H-AlMCM-41 surface. Reverse order impregnated catalyst (C) > normal order impregnated catalyst (B) > coimpregnated catalyst (A). The total sulfur present on all the three sulfided catalysts was estimated by chemical analysis, which can give an estimate of metal species on the support surface (Table 1). It is estimated to be 4.7 × 10-2 g on reverse order impregnated catalyst (C) and 4.34 × 10-2 g on normal order impregnated catalyst (B). The simultaneous impregnated catalyst (A) had 2.68 × 10-2 g. The large amount of sulfur present on the reverse order impregnation catalyst supports strongly for the fact that there are more number of finely dispersed metal species available on the surface for the HDN reaction which is sulfided during sulfidation. This analytical data clearly confirms the results obtained from sophisticated techniques such as TEM and FTIR adsorbed CO. It is concluded from the characterization techniques that the order of impregnation of NiO and MoO3 and method of preparation significantly influence the dispersion of metal particles over the support H-AlMCM-41 surface. 3.3. Correlation of Activity with Surface Properties. The activities of the catalysts have been correlated with the physicochemical characteristics. Among the three catalysts studied, the coimpregnated catalyst A exhibited lowest activity (Figures 1 and 2). Though the surface area of this coimpregnated catalyst
was high, its low activity was due to poor dispersion, formation of stable stoichiometry oxide, and aggregates of metal particles over the support surface as evidenced from TEM, total sulfur content, CO adsorption by FT-IR and XRD studies. Again, the XPS analysis showed complete reduction of Mo6+ to lower oxidation states with Ni2+ tending to form of Ni0, which might not have beneficial effect for promoting the action of Mo. However, sequentially impregnated catalysts show the formation of tiny particles with sharp edges, well dispersed over the support surface. Therefore, the activity of the sequentially impregnated catalyst is better than coimpregnated catalyst. The activity of normal order impregnated catalyst B was lower than the reverse order impregnated catalyst C (Figures 1 and 2). From the characterization results it is clear that, on catalyst C, there are more numbers of finely dispersed Mo species on the support as observed from the intensities of CO adsorbed FT-IR band and total sulfur content, which is a reflection of metal species on the surface. It is also understood from the XPS analysis that there is complete reduction of Mo6+ with Mo5+ prevailing more than Mo4+, which is beneficial for high HDN activity. In general, it can be concluded that, the order of impregnation significantly influence the surface properties and correspondingly the activity of the catalysts. Under these experimental conditions, the catalyst prepared by reversing the order of impregnation of the active components MoO3 and NiO appears to be best for HDN. 4. Conclusions From this study it is concluded that order in which the promoter oxide NiO and active metal oxide MoO3 are impregnated is an important factor in designing supported catalysts. Impregnation of NiO first and MoO3 second over H-AlMCM41 provides catalyst of high activity when compared with the catalyst prepared by other methods. Both XRD and TEM analyses indicate fine dispersion of the active phases over reverse order impregnated catalyst. BET surface area measurements illustrate the formation of active phases of nano dimensions in the pores of H-AlMCM-41. XPS analysis evidence the presence of Mo5+ in abundance on the surface of calcined and reverse
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order impregnated catalyst and the results on the FT-IR of CO adsorption band due to Mo in the reduced catalyst and chemical analysis of total sulfur content of presulfided catalysts, indicate the presence of more number of finely dispersed active molybdenum species in reverse order impregnated catalyst than normal order and coimpregnated catalysts, thus accounting for the high activity of reverse order impregnated catalyst. Acknowledgment The authors would like to thank the Defence Research and Development Organization (DRDO) of India for providing financial support. Literature Cited (1) Lewandowski, M.; Sarbak, Z. The effect of boron addition on hydrodesulfurization and hydrodenitrogenation activity of NiMo/Al2O3 catalysts. Fuel 2000, 79, 487–495. (2) Datye, A. K.; Srinivasan, S.; Allard, L. F.; Pedan, C. H. F.; Brenner, J. R.; Thompson, L. T. Oxide supported MoS2 catalysts of unusual morphology. J. Catal. 1996, 158, 205–213. (3) Segawa, K.; Satho, S. TiO2-coated on Al2O3 support prepared by CVD method for HDS catalysts. Stud. Surf. Sci. Catal. 1999, 127, 129– 136. (4) Li, D.; Nishijma, A.; Morris, D. E. Zeolite-supported Ni and Mo catalysts for hydrotreatments: I Catalytic activity and spectroscopy. J. Catal. 1999, 182, 339–348. (5) Reddy, K. M.; Wei, B.; Song, C. Mesoporous molecular sieve MCM41 Supported Co-Mo catalyst for hydrodesulfurization of petroleum resides. Catal. Today. 1998, 43, 261–272. (6) Wang, H.; Liang, C.; Prins, R. Hydrodenitrogenation of 2-methylpyridine and its intermediates 2-methylpiperidine and tetrahydro-methylpyridine over sulfided NiMo/γ-Al2O3. J. Catal. 2007, 251, 295–306. (7) Ying, J.; Mehnert, C.; Wong, M. Synthesis and applications of supramolecular-templated mesoporous materials. Angew. Chem., Int. Ed. 1999, 36, 56–77. (8) Song, C.; Reddy, K. Mesoporous molecular sieve MCM-41 supported Co-Mo catalyst for hydrodesulfurization of dibenzothiophene in distillate fuels. Appl. Catal., A 1991, 176, 1–10. (9) Turaga, U. T.; Song, C. MCM-41-supported Co-Mo catalysts for deep hydrodesulfurization of light cycle oil. Catal. Today. 2003, 86, 129– 140. (10) Corma, A.; Martinez, A.; Martinez-Soria, V.; Monton, B. Hydrocracking of vacuum gas oil on the novel mesoporous MCM-41 aluminosilicate catalyst. J. Catal. 1995, 153, 25–31.
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ReceiVed for reView June 13, 2008 ReVised manuscript receiVed November 17, 2008 Accepted December 2, 2008 IE800932U
