J. Phys. Chem. 1993,97, 11373-11375
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ARTICLES Characterization of WOJAlP04 Hydrotreating Catalysts by 'H Magic Angle Spinning NMR Spectroscopy and Low-Temperature Oxygen Chemisorption+ L. Jhansi Lakshmi and P. Kanta Rao' Catalysis Section, Physical and Inorganic Chemistry Division, Indian Institute of Chemical Technology, Hyderabad-500 007, India
V. M. Mastikhin and A. V. Nosov Institute of Catalysis, Novosibirsk 630090, Russia Received: June 24, 1993; In Final Form: August 18, 1993'
The 1H MAS-NMR and low-temperature oxygen chemisorption have indicated the monolayer loading of WOs on AlPO, as 9 wt 7%. The presulfided W03/AlP04 catalysts have shown improved HDS activity compared to conventional y-A1203 supported catalysts.
Introduction Tungsten oxides supportedon y-A1203are the subject of recent investigations because of their significant role in important catalytic reactions like selective oxidation, isomerization, and metathesis of 01efins.l-3 Presulfided Mo and W catalystspromoted by Co or Ni on y-A1203 supports are well-known for their use in hydrotreating of petroleum ~ r u d e s . ~ More J recently, the beneficial effect of the phosphate ion modification of y-A1203 support in the dispersive and hydrodesulfurizationproperties of Co-MoS or Ni-Mo-S sites have been attributed to their interaction with AlP04 formed by the modification of y-A1203 with phosphate ion."1° Aluminum phosphate, thus, has potentialities as a support for hydrotreating catalysts. Though Si02, ZrO2, and y-A1203have been used as supports to disperse active tungsten phase,11-14 the use of AlP04 as a support material has not been reported so far. The efficiency of supported W 0 3 catalysts mainly depends on the dispersion of the active component, which in turn is greatly influenced by the nature of the support material. Supported W 0 3 catalysts have been variously characterized by spectroscopic techniques which include XPS,15J6 EXAFS," ESR,"3J9 laser Raman spectroscopy,2°.21 and ion scatteringspectroscopy.= Recently, the development of the magic angle spinning (MAS) technique has afforded high-resolution NMR spectra of interesting nuclei in solid samples. Thus 'H, 13C, Z7Al, 29Si, and 3lP MAS-NMR spectroscopies have been employed for the structural study in zeolites, aluminas, silicas, The variable temperature lH and aluminum MAS-NMR spectroscopy has afforded characterization of the structure and dynamicsof hydrogen-bonded adsorption complexes in zeolite H ZSM-5.27 In the present investigation amorphous AlPO, has been used as a carrier to synthesizehighly active W 0 3 hydrotreating catalysts. lH MAS-NMR and low-temperature oxygen chemisorptionhave been used to characterizeWOsIAlP04 catalysts. The activities of these catalysts have been estimated toward hydrodesulfurization of thiophene.
Experimental Section Aluminum phosphate was precipitated by adding aqueous ammonia to a solution containing Al(NO3)3.9H20 and 85% Hs7
IICT Communication No. 3088.
* For correspondence.
*Abstract published in Advance ACS Abstracrs, October 1, 1993.
0022-365419312097-11373$04.00/0
PO4 until a pH of -8 was attained. The precipitate was washed with distilled water, dried at 110 "C for 16 h, and finally calcined at 600 OC for 5 h. The X-ray diffraction pattern shows the formation of amorphous AlP04 (BET surface area, 63 m2 g-1). A series of W03/AlP04 catalysts with various W 0 3 loadings (1-2 1 wt W )were prepared by impregnatingthe support material with aqueous solution containing an appropriate amount of ammonium metatungstate. All the catalysts were dried and then calcined at 500 OC for 5 h. The solid-state proton NMR spectra with MAS technique have been recorded on Bruker CXP-300 spectrometer at a frequency of 300.09 MHz. The frequency range was 50 KHz. The ( ~ 1 2 )pulse duration was 5 ps with repetition frequency of 1 Hz. Prior to NMR experiments the samples were placed in special NMR tubes and then evacuated at 250 OC at l e 3 Pa for 24 h and sealed off. The spinning was performed in quartz rotors at a frequency of 3-3.5 kHz using a probe with minimal background signal. The probe head, rotor, and the sample tubes were dried to remove the traces of water from their outside surface. The chemical shifts were measured relative to tetramethylsilane (TMS) as an external standard. The concentration of OH groups was determined by comparing the signal intensity of the catalyst with that of a known standard sample (Si02evacuated at 300 OC for 4 h contained 5 X lO190H groups). Low-temperature oxygen chemisorption experiments were performed on WOs/AlP04 catalysts using a static high vacuum system. Prior to chemisorption measurements the samples were sulfided at 400 OC for 2 h by passing H2 saturated with CS2 and then treated at the same temperature for 2 h with hydrogen. The sample was then evacuated for 1 h at the reduction temperature and cooled slowly to the temperature of oxygen chemisorption (-78 "C). The details of the experimental procedure are described e1~ewhere.l~ The hydrodesulfurization of thiophene to butane and butene was carried out at 450 OC on presulfided catalysts in a continuousmicroflow reactor operating at atmospheric pressure and under differential conditions. Approximately 0.2 g of catalyst was loaded in a tubular glass reactor and sulfided for 2 h at 400 OC by passing H2 saturated with CS2,flushed with N2 for 30 min, and then a reaction mixture consisting of hydrogen (40 cm3 min-l) saturated with thiophene was passed over the catalyst bed. The products of HDS of thiophene, butane, and butene were analyzed by gas-liquid chromatography. 0 1993 American Chemical Society
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Lakshmi et al.
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0
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AI PO4 0
0
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W O j loading, wt .'A ~
Figure 2. Total number of hydroxyl groups plotted as a function of WO3
loading. Figure 1. (A) Solid-state 'HNMR spectra of hydroxylgroups of Alp04 and W03/AlPO4 catalysts. *Spinningside bands. (B) Expanded spectra of Alp04 and 9 wt % WOp/AIPO4.
Results and Discussion The 'H NMR spectra of AlP04 and selected WO3/AlPO4 catalysts are presented in Figure 1A. The expanded spectra of AlP04 and 9 wt % WOs/AlP04 with computer simulation overlaying them are shown in Figure 1B. The spectra of AlP04 consist of several closely spaced overlapping lines from the P-OH group (peaks at 6 = 5.0 and 3.0 ppm).2C2* The line at 6 = -0.5 ppm can be attributed to the A1-OH group coordinated to one octahedral A1 atom, and this is very close to that of the "basic" OH group in A1203. The total number of OH groups as a function of W 0 3loading are shown in Figure 2. The concentration of OH groups decreases with increase in WO3 loading up to 9 wt % and levels off with further increase in loading. This loading can be considered as monolayer coverage of W 0 3on the AlP04support. From the fact that in supported catalysts the monolayer phase is formed by a strong chemical interaction between OH groups of the support surface and the supported metal oxide precursors present in the impregnating s0lution,2~it may be inferred that the drop in the surface concentration of OH groups upon deposition of tungsten oxide and then its leveling off at and beyond a certain loading (Figure 2) provides evidence for interaction between surface OH groups of the AlP04 support and active tungsten species until the completion of the monolayer. However, a smaller decrease in IH N M R signal intensity for WO3/AlP04 catalysts with increase in wt % of W 0 3 may be due to the lack of all the O H groups to interact because some of them may be present in closed pores or interior of the structure. This means that only a part of the total OH groups can be attributed to the surface OH groups. These results are similar to those found earlier on V 2 0 ~ / S n 0 2and ~ ~V ~ O S / S catalyst ~ O ~ ~systems. ~ Low-temperature oxygen chemisorption capacities on partially sulfided WOs/AlP04 catalysts showed a maximum a t 9 wt % loading (Figure 3) which may be postulated as monolayer loading. The 'H N M R results may be considered as supporting this observation. One can depict the monolayer as isolated patches or islands of W 0 3 units attached to the support surface through OH groups by strong chemical interaction. The completion of the monolayer appears to occur when the active component is sufficient to react with all reactive hydroxyl groupson the support surface, while a large fraction of support surface still remains bare. Above a W 0 3loading of 9 wt % crystalline phases of W20058 were detected by X-ray diffraction which was reflected in lower 0 2 uptakes of respective catalysts. The 0 2 uptake per m2 of the
0
0
3
6
9
12
15
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21
W O 3 l o a d i n g , w t . '1.
Figure 3. Oxygen uptake at -78 Alp04 support.
O C
as a function of WO3 loading on
support surface area for partially sulfided WO3/AlP04 catalysts are higher than those of W03/A1203 catalyst^'^ indicating higher dispersion of the active phase in AlP04 support surface than on y-Al203. This can be explained in terms of interaction between phosphate anion and tungsten species which may prevent the quick transport of the latter toward the support surface and subsequent agglomeration during the drying process.32 It can be observed from Figure 4 that 0 2 uptakes measured a t -78 O C directly correlate with the activity of the catalysts toward HDS of thiophene showing that coordinatively unsaturated sites on partially sulfided W03/AlP04 catalysts titrated by 0 2 are responsible for HDS activity. The HDS rates on W03/ AIP04are higher by a factor of 4 than those on W03/-pAI2O3.14 This is a significant effect of the Alp04 support, probably resulting in increase in the dispersion of W03. The improved HDS rates could as well be due to less polarization of the W S bonding in W03/AlP04, since it is known that the decrease in covalent character of the Mo-S bond results in lower a~tivity.'~Phosphate ion modification of y-Al203 has been reported to reduce the polarization of M o S bonding in the molybdena catalysts supported on it.' Conclusions The lH MAS-NMR and low-temperature oxygen chemisorption results show that the monolayer loading of W 0 3on AlP04 is 9 wt %. The amount of W 0 3 necessary to form a monolayer
Characterization of W03/AlP04 Hydrotreating Catalysts
0 ' 0
10
I
I
I
1
20
30
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02 u p t a k e , p mol /g catalrrl
Figure 4. Thiphene HDS rate as a function of 02 uptakes of various WOs/AlP04 catalysts.
depends on the specific surface area of the system under consideration. The Alp04 supported presulfided W 0 3 catalysts have shown higher activity than the conventional 7-A1203 supported catalysts. Acknowledgment. We thank Prof. K. I. Zamaraev, Director, Institute of Catalysis,Novosibirsk, USSR, for providing the NMR facility, and the UniversityGrants Commission, New Delhi, India, for a research fellowship to L.J.L.
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