Energy & Fuels 1996, 10, 579-586
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Evaluation of Alumina-Aluminum Phosphate Catalyst Supports for Hydrodenitrogenation of Pyridine and Coal-Derived Liquids R. Menon, H. S. Joo, and J. A. Guin* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849
P. J. Reucroft and J. Y. Kim Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506 Received November 28, 1995. Revised Manuscript Received February 23, 1996X
Several alumina-aluminum phosphate (AAP) catalyst supports were prepared by a coprecipitation method. Effect of variations in Al/P atomic ratios on support textural properties were examined. Finished NiMo/AAP catalysts containing nominally 3 wt % Ni and 13 wt % Mo were prepared by incipient wetness and characterized by several methods including elemental, BET, and XPS surface analysis. Initial hydrodenitrogenation (HDN) activities of the catalysts were examined in both pyridine model compound and coal liquid reactions. The AAP supports showed the opportunity to tailor the catalyst pore size by variation of the Al/P ratio. On a per unit surface area basis, the AAP-supported catalysts had initial HDN activities comparable to those of a commercial P-promoted NiMo/Al2O3 catalyst. Because of their unique textural properties, i.e. variable pore sizes, the AAP catalysts may offer advantages when dealing with macromolecular feedstocks where hindered diffusion may slow reaction rates.
Introduction Solid fossil fuels such as coal, which are relatively abundant in comparison with petroleum, could become major fuels sources in the future.1 Toward this goal, a variety of processes for producing coal-derived transportation fuels have been developed in past years. In addition there is a great deal of interest currently in coprocessing coal with waste plastics and/or waste rubber (tires) to produce clean transportation fuels.2-5 The liquid products from processes such as these usually contain relatively high quantities of heteroatoms, especially nitrogen, oxygen, and sulfur. To produce clean transportation fuels, the first-stage liquids must be upgraded by eliminating the heteroatoms through HDN (hydrodenitrogenation), HDS (hydrodesulfurization), and HDO (hydrodeoxygenation) reactions. Traditional hydroprocessing catalysts such as CoMo, NiMo, and NiW supported on Al2O3 often require very low weight hourly space velocities (long residence times) in order to reduce heteroatom contents to acceptable levels . In fact, for coal-derived liquids, it has been estimated that to routinely remove N to acceptable levels for downstream hydrocracking and boiling point reduction at reasonable WHSV’s may require hydrotreating catalyst Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Srivastava, R. D.; McIlvried, H. G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (3), 513. (2) Anderson, L. L.; Tuntawiroom, W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (4), 810. (3) Huffman, G. P.; Feng, Z.; Majajan, V.; Sivakumar, P.; Jung, H.; Tierney, J. W.; Wender, I. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 34. (4) Orr, E. C.; Shi, Y; Liang, J.; Ding, W.; Anderson, L. L.; Eyring, E. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (3), 633. (5) Stiller, A. H.; Dadyburjor, D. B.; Wann, J.; Tian, D.; Zondlo, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 77. X
0887-0624/96/2510-0579$12.00/0
activity to be increased by 1 order of magnitude.6 These circumstances have motivated a search for new and improved hydrotreating catalysts. One avenue of research along these lines is the development of catalyst supports other than the traditional Al2O3. Although less studied than Al2O3, phosphates present intriguing possibilities as catalyst support materials. Phosphorous itself has received considerable attention as a promoter element for CoMo and NiMo hydrotreating catalysts.7-10 Phosphorous is thought to have several beneficial promotional effects including inhibition of the formation of nickel aluminate in the NiMo/ Al2O3 catalyst11 and a reduction in coke deposition thus promoting catalyst longevity.8 Using quinoline as an HDN model reaction, Eijsbouts et al.10 have shown that phosphate itself does not catalyze hydrogenation but that it can catalyze C-N bond hydrogenolysis. Aluminum phosphate has been studied as a catalyst support in many reactions including polymerization, isomerization, and cracking.12,13 Recently, coprecipitated aluminophosphates have received significant attention as (6) Liaw, S. J.; Keogh, R. A.; Thomas, G. A.; Davis B. H. Energy Fuels 1994, 8, 581. (7) Tischer, R. E. Ind. Eng. Chem. Res. 1987, 26, 422. (8) Fitz, C. W.; Rase, H. F. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 40. (9) Mangnus, P. J.; van Langeveld, A. D.; de Beer, V. H. J.; Moulijn, J. A. Appl. Catal. 1991, 68, 161. (10) Eijsbouts, S.; van Gestel, J. N. M.; van Veen, J. A. R.; de Beer, V. H. J.; Prins, R. J. Catal. 1991, 131, 412. (11) Smith, K. J.; Lewkowicz, L.; Oballa, M. C.; Krzywicki, A. Can. J. Chem. Eng. 1994, 72 (Aug), 637. (12) Cheung, T. T. P.; Willcox, K. W.; McDaniel, M. P.; Johnson, M. M.; Bronnimann, C.; Frye, J. J. Catal. 1986, 102, 10. (13) Marcelin, G.; Vogel, R. F.; Swift, H. E. J. Catal. 1983, 83, 42.
© 1996 American Chemical Society
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Table 1. Properties of Commercial Catalysts
catalyst
compositions (wt %)
MoO3, 19.5%; NiO, 4%; P2O5, 8%; Al2O3, balance Harshaw Al-3991 Al2O3, 99.9% (Al2O3-H) SMR 39-2370 Al2O3, 41.1%; P2O5, 58.9% AlPO4 Gel (AlPO4-G)
Shell 424
Table 2. Properties of Coal Derived Liquids
surface pore area volume (m2/g) (mL/g) 155
0.47
230
0.69
172
0.80
possible resid hydrotreating supports11,14,15 and aromatics saturation.16 These porous aluminophosphates are largely amorphous materials produced by coprecipitation of solutions containing appropriate quantities of Al3+ and PO43- ions.12 Even though the mixed oxides are sometimes referred to as alumina-aluminum phosphates or AAP’s, these supports are not simple mixtures of AlPO4 and Al2O3 in that crystallization of these pure phases appears to be mutually inhibited, even at elevated temperatures. In addition, catalytic activities of the AAP’s are not generally a simple average of the respective AlPO4 and Al2O3 activities, but appear to exhibit a synergistic interaction resulting in higher activities.12 The AAP supports have properties which differ from those of alumina catalysts in which P is later added as a promoter.16 The above findings suggest that AAP’s warrant consideration also as potential carriers for coal liquefaction upgrading catalysts. While a variety of reactions occur in upgrading of coal and coal/waste coprocessing liquids, HDN is a major concern as basic N compounds are poisons for hydrocracking catalysts required in downstream processing. HDN assumes additional prominence due to the fact that C-N bond scission in heterocyclic aromatic compounds is more difficult than C-S and C-O scission.17 Because effective N removal activity requires both a hydrogenation and a hydrogenolysis functionality, it seems plausible that combination of the appropriate metal sulfides for hydrogenation activity coupled with the hydrogenolysis activity of surface phosphates might give rise to good HDN activity. While some research11,14,15 has been reported on the use of AAP supports in hydrotreatment of residues with particular attention to HDS, little attention has been given specifically to the use of AAP supports for HDN or for upgrading coal liquids. For these reasons, it was of interest in this study to examine the potential of AAP’s as catalyst supports for HDN reactions. In our study, the HDN activity of laboratory-prepared NiMo/ AAP catalysts were examined in both model compound (pyridine HDN) and coal liquid reactions. The activities of the laboratory-prepared catalysts were compared with those of commercial materials. Experimental Section Materials. To provide for standardization and a basis of comparison, three commercial catalysts with properties listed in Table 1 were used in this work. These commercial catalysts (14) Chen, Y. W.; Hsu, W. C.; Lin, C. S.; Kang, B. S.; Wu, S. T.; Leu, L. J.; Wu, J. C. Ind. Eng. Chem. Res. 1990, 29, 1830. (15) Li, C.; Chen, Y. W.; Tsai, M. C. Ind. Eng. Chem. Res. 1995, 34, 898. (16) Huang, T. C.; Kang, B. C. Ind. Eng. Chem. Res. 1995, 34, 2955. (17) Katzer, J. R.; Sivasubramanian, R Catal. Rev.sSci. Eng. 1979, 20, 155.
elemental compositions (wt %) element
coal distillate
coal-waste coprocessing liquid
carbon hydrogen nitrogen sulfur oxygen (by difference)
88.10 10.40 0.73 0.04 0.73
89.41 10.23 0.17 0.08 0.11
boiling point distribution by simulated distillation (wt %) (ASTM 2887) boiling point distribution (°C)
coal distillate
coal-waste coprocessing liquid
IBP-150 150-205 205-260 260-315 315-425 425-EP
0.2 1.0 10.0 33.3 53.8 1.7
0.1 0.3 1.6 9.7 74.6 13.7
are the following: a hydrotreating catalyst, Shell (now Criterion) 424 phosphorous-promoted NiMo/Al2O3; an aluminum phosphate, AlPO4-G (W. R. Grace & Co. SMR 39-2370 AlPO4 Gel), and a γ alumina, Al2O3-H (Harshaw Chemical Co. Al3991). A coal liquid (courtesy of Exxon) and a coal-waste coprocessing liquid (courtesy of Hydrocarbon Research, Inc.) with properties shown in Table 2 also were utilized. Both liquids were produced from sub-bituminous coal from the Black Thunder mine in Wyoming. The coal distillate is a middle distillate produced in the Wilsonville, AL, coal liquefaction pilot plant in early 1992. The coal-waste coprocessing liquid is a vacuum still overheads blend produced by Hydrocarbon Research, Inc., in Run POC-2 in May-July, 1994. This coalwaste coprocessing liquid was produced by coprocessing 12 tons of mixed plastics (50% high-density polyethylene, 35% polystyrene, and 15% poly(ethylene terphthalate)) and 5 tons of crumb rubber (waste tires) with coal during an 8-day period in an approximate 70%/30% ratio of coal/wastes as detailed by Pradhan et al.18 As seen in Table 2, both liquids contain about the same amount of C/H and fairly low sulfur; however, the coal distillate is higher in N and is of a lower boiling point range, as compared to the coal-waste coprocessing liquid. The SEC chromatogram in Figure 1 indicates that both coal liquids have about the same molecular weight distribution. Chemicals used as received include aluminum nitrate nonahydrate (Aldrich, 98+%), phosphoric acid (Aldrich, 85%), ammonium hydroxide (Aldrich, NH3: 28-30%), 2-propanol (Swan, 99%), ammonium molybdate(VI) tetrahydrate (Aldrich, ACS reagent) and nickel(II) nitrate hexahydrate (Aldrich, crystal) in the preparation of NiMo/AAP catalysts and pyridine (Aldrich, 99%) and naphthalene (Fisher, purified) as model compounds, and hexadecane (Fisher, certified) as a solvent in the HDN activity measurement. Preparation of AAP Supports. The experimental procedures used in this work to prepare the AAP supports were based on methods developed by earlier investigators.12-14 Supports were prepared in duplicate by a coprecipitation method.19 Ammonium hydroxide solution (13%) was added rapidly to the acidic solution prepared from Al(NO3)3‚9H2O and H3PO4, following which precipitation occurred rapidly at 0 °C during a 1-2 min interval in which the pH rose quickly from less than 1.0 to neutrality. Following precipitation and a ripening period of 12 h, the gel was centrifuged and washed with 2-propanol three times. After the 2-propanol was allowed to evaporate, the gel was dried in the vacuum oven for 12 h at 108 °C and calcined at 649 °C for 3 h. The supports prepared were denoted as AAPx on the basis of their nominal Al/P (18) Pradhan, V. R.; Comolli, A. G.; Lee, L. K.; Stalzer, R. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 82. (19) Menon, R. M.S. Thesis, Auburn University, Auburn, AL, 1995.
Alumina-Aluminum Phosphate Catalyst Supports
Figure 1. SEC chromatogram of coal liquids used in HDN reactions: (A) coal distillate; (B) coal-waste liquid. Molecular weight scale is from polystyrene standards. atomic ratios. An Al2O3 support denoted herein as AAP∞ also was prepared for comparison purposes using this same method, but without H3PO4. Preparation of NiMo/AAP Catalysts. Supports AAP1, AAP2, AAP4, AAP8, and AAP∞ (Al2O3) were selected for impregnation with nickel and molybdenum using a sequential incipient wetness procedure. To aid in uniform metals deposition, the supports were ground to -100 mesh prior to impregnation. Two solutions were prepared by dissolving (NH4)6Mo7O24‚H2O and Ni(NO3)2‚6H2O in exact amounts of water required to fill the pore volumes of each support. The supports were impregnated with molybdate first, dried for 16 h at 120 °C in a vacuum oven, and calcined at 500 °C for 5 h in a muffle furnace. Following this, they were impregnated with the nickel solution, dried, and calcined as before. For comparative purposes the commercial Harshaw alumina (Al2O3-H) and the Grace aluminum phosphate (AlPO4-G) (Table 1) were also impregnated with NiMo using the same procedures. Hereafter, the finished catalysts prepared from these two commercial supports, like the AAP’s, are referred to as “laboratory” catalysts. Catalyst Characterization. Surface areas were determined by nitrogen adsorption (BET) using a Quantasorb adsorption unit. Pore volumes were measured by using water as a pore-filling medium. Bulk metal analysis was done by a Jarrell Ash ICAP (inductively coupled argon plasma) 9000. Catalysts were also examined by X-ray diffraction at 40 kV and 40 mA with Cu KR radiation. XPS analysis was used to determine the surface chemical compositions and element depth profiles of the six catalysts employing a Kratos XSAM 800 spectrometer with Mg KR (1253.6 eV) radiation. The samples were mounted on the spectrometer probe tip by means of double-sided adhesive tape. After heating under vacuum (10-5 Torr) at 65 °C for 10 h in a pretreatment chamber to remove volatiles, the samples were inserted into an ultra-highvacuum chamber for surface analysis. The spectrometer was run in a fixed analyzer transmission (FAT) mode at a pass energy of 13 kV and 15 mA. Under these conditions, the full width at half-maximum (fwhm) of the Ag 3d5/2 peak is approximately 1.1 eV. The system pressure was normally maintained below 5 × 10-10 Torr using a 300 L/s ion pump and Ti sublimators to minimize contributions from vacuum contaminants. In situ Ar+ ion sputtering followed by XPS was used to determine element depth profiles and chemical changes that occurred as the exposed surface varied from the initial surface
Energy & Fuels, Vol. 10, No. 3, 1996 581 to the bulk. The incident ion gun was operated at 3.5 keV, and the sample current was kept around 15 µA across a sample area of about 25 mm2 using a differentially pumped and computer-controlled 3M minibeam ion gun. The pressure in the main chamber was kept below 4 × 10-6 Torr during Ar+ ion sputtering. Binding energies were referred to the carbon 1s peak at 285 eV in order to compensate for sample charging. Atomic concentrations were estimated from the XPS element peak areas after applying the atomic sensitivity factors.20 HDN Activity Measurements. All reactions were performed in 20 cm3 316 ss tubing bomb microreactors (TBMR) which were agitated in a temperature-controlled fluidized sand bath. A reactant solution (6 g) containing 2 wt % pyridine in hexadecane was used for model compound studies. The two coal liquids were used in the upgrading experiments at 3 g loadings. The TBMRs were charged with 1000 psig ambient hydrogen pressure, and reactions were performed at 350 °C for 20 min using 0.1 g of catalyst for model compound reactions and at 375 °C for 1 h using 0.4 g of catalyst for upgrading coal liquids. The -100 mesh catalysts were sulfided in situ with dimethyl disulfide (DMDS). The quantity of sulfur added to the TBMR was based upon Ni and Mo contents using 50% excess of the stoichiometric amount needed for bulk sulfidation (MoS2 and Ni3S2) and an additional 1 wt % DMDS in the liquid to maintain catalyst sulfidation. Analysis. Analysis of pyridine reaction products was performed by GC with a 30 m × 0.32 mm × 1.0 µm Restek Stabilwax-DB capillary column using GC-MS and standard additions for products identification. Coal liquid elemental analysis for N content was performed by Huffman Labs, Golden, CO, using the Kjeldahl and Dumas (Leco instrument) methods. Elemental (ICAP) analysis of catalysts and supports was performed by Huffman Labs and by the soils testing laboratory at Auburn University.
Results and Discussion Physical Properties of Supports and Catalysts. Table 3 shows the properties of the supports and catalysts used in this work. Supports AAP1-AAP8 were prepared in duplicate, and the average and standard deviations in surface area are given for the duplicate preparations. While the AAP2-AAP8 preparations agreed very well, the two AAP1 samples had a fairly large difference in surface area, perhaps caused by difficulties in controlling the spontaneous coprecipitations. In producing the final catalyst, the AAP1 preparation with area ) 96 m2/g was used. As Table 3 shows, the BET surface areas of the AAP supports tend to increase as the Al/P ratio increases, that is, as the amount of P in the support is decreased, except for the pure alumina (AAP∞). Overall, there is a fair correlation between the Al/P ratio and the textural properties, i.e. surface area, pore volume, pore diameter, etc., and these properties can be varied by controlling the amount of P in the support. These general trends, e.g. increasing areas and decreasing pore diameter with increasing Al/P ratios, are in agreement with literature results.15 Comparing the surface areas of the original supports with those of the finished catalysts in Table 3 shows that, following impregnation with Mo and Ni, the surface areas of the finished catalysts are substantially reduced as compared to the supports themselves. While detailed experiments were not performed to ascertain the exact cause of this reduction, two possible causes for this effect are plugging of smaller pores by the (20) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray photoelectron spectroscopy; Chastain, J. Ed.; PerkinElmer Corp., Phys. Electronics Div.; Eden Prairie, MN, 1992.
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Table 3. Some Properties of Supports and Finished Catalysts finished catalysts original supports AAP1 AAP2 AAP4 AAP8 AAP∞ Al2O3-H AlPO4-G
surface area (m2/g)
pore volume (cm3/g)
porea diameter (nm)
surface area (m2/g)
118 ( 31.0 155 ( 1.4 228 ( 1.3 259 ( 0.4 194 230 172
1.37d
46.4d
50d
1.69 1.29 0.85 0.42 0.71 1.02
43.6 22.6 13.1 8.7 12.3 23.7
79 123 118 146 176 82
a Pore diameter ) 4 (pore volume/surface area in Table 3). b ICAP analysis. c Atomic ratio. area 96 m2/g.
d
element (wt %)b Al
P
Al/Pc
Mo
Ni
19 20.7 27.1 28.5
19.1 14.9 6.7 5.4
1.1 1.6 4.7 6.1
21.9
25.8
0.97 1
12.2 12.7 12.8 12.6 14.3 13 12.4
3.1 2.8 2.8 2.9 3.1 2.8 3.1
Values for support sample with surface
Figure 2. Typical XPS wide-scan spectra of AAP1 catalyst.
metals themselves and/or partial collapse of the pore structure due to solvent, i.e. water, effects during the impregnation process. Additional work with various impregnation solvents and metals loadings would be needed to ascertain the exact importance of these two effects. Similar reductions of approximately 50% in surface areas following aqueous metal impregnation were also found by Chen et al.14 in preparation of CoMo/ AAP catalysts for HDS studies and by Smith et al.11 A notable point is that the P-containing supports experience around 50% declines in surface areas, while the alumina-only supports (AAP∞ and Al2O3-H) only have around 25% decreases. Thus it appears that the presence of P may increase the water sensitivity of the supports. Although not explored in this study, Kearby 21 suggested that substitution of a solvent such as ethanol in place of water in the metal impregnation steps might tend to mitigate the reduction in surface area. Elemental Analyses. Bulk elemental analyses of the finished catalysts following double impregnations and calcinations are also given in Table 3. The experimentally determined bulk Al/P atomic ratio does not agree exactly with the attempted nominal preparation ratios of 1, 2, 4, and 8, although the order is the same. The Mo and Ni contents vary around the nominal values of 13% and 3%, respectively. (21) Kearby, K. Proceedings of the International Congress on Catalysis, 2nd, Paris, 1960; Technip: Paris, 1961; p 2567.
X-ray Diffraction. The X-ray diffraction patterns of the finished catalysts showed all catalysts to be essentially amorphous, with little or no observable crystallinity. This observation is in agreement with several previous investigators, suggesting that the alumina and aluminum phosphate are not simply admixtures of two separate phases but that the coprecipitated catalysts are a composition in which crystallization of individual phases may be mutually inhibited.11,13 XPS Analysis. Following calcination, the NiMo/AAP catalysts were examined by XPS for surface characterization. Figure 2 shows a typical XPS wide scan spectrum (for the AAP1 catalyst). The initial surface spectrum for AAP1 showed distinct peaks for the major elements, oxygen, aluminum, phosphorus, and molybdenum, with no discernible nickel. A strong carbon peak was observed on the surface of all catalysts which can be ascribed to adsorption of atmospheric contaminants that occurs when the samples were exposed to air before the XPS measurement. Separate narrow survey scans were carried out on each element region to determine the chemical states and the surface elemental concentrations. The binding energies of the Al 2p, P 2p, and Mo 3d peaks were characteristic of the most stable oxidation states of these elements, e.g. MoO3. No significant variation in the chemical state of each element was observed for most of the catalyst samples. The initial surface elemental concentrations determined from the XPS results are given in Table 4,
Alumina-Aluminum Phosphate Catalyst Supports
Energy & Fuels, Vol. 10, No. 3, 1996 583
Table 4. XPS Surface Characterization of Catalysts (at %) element
Al
P
Mo
Ni
AAP1 AAP2 AAP4 AAP8 Al2O3-H AlPO4-G AAP1a AAP4a
10.9 14.8 18.0 17.2 25.3 11.7 10.7 14.5
9.3 10.1 4.1 2.8
1.9 1.8 2.4 2.7 2.1 2.2 1.3 1.5
18 2.00 1.17 1.48 2.92 0.43 0.50
Spent catalyst samples.
Figure 3. Depth profile of AAP1 as a function of AR ion sputtering time.
Figure 4. Depth profile of AAP2 as a function of Ar ion sputtering time.
together with the bulk elemental analysis results. Excellent agreement in Al/P ratio was observed between the surface and bulk values. However, the surface Ni concentration varied from sample to sample resulting in differences between the surface and bulk Mo/Ni atomic ratios. In particular, catalysts AAP1 and AAP2 showed no measurable surface concentration of nickel. Examination of the active metal/support metal ratios in Table 4 gives a qualitative suggestion that the metal dispersion increases for the AAP samples as the Al/P ratio increases. This suggestion is reinforced later by certain HDN reactions carried out. Depth profile results for the AAP1 and AAP2 catalysts as a function of Ar+ ion sputtering time are shown in Figures 3 and 4, respectively, showing the appearance of Ni at 5 min, which corresponds to a depth of 200300 Å from the surface. The reason for the absence of Ni in the initial surface of AAP1 and AAP2 is unknown at this time; however, this was confirmed by repeated analyses on several samples. Various Ni phases including NiO, NiAl2O4, and various mixed Ni oxides are known to exist on the surface of Ni/Al2O3 catalysts, and the coverage of Ni species by an alumina overlayer has been observed under certain experimental, mainly reducing, conditions.22-23 Encapsulation of the metal by the support has also been observed in case of TiO2 supports.24,25 While the above conditions do not correspond exactly to our experimental conditions, they do
suggest the possibility of encapsulation of the Ni by a support layer due to metal support interactions. Depth profile results generally showed a decrease in the Mo/ Ni ratio as a function of Ar+ ion sputtering time, viz. increasing depth from the surface (due to the Ni concentration increasing), and as etching proceeded, the bulk Mo/Ni ratio was approached. Following reaction, two spent catalysts, AAP1 and AAP4, were subjected to XPS analysis, as shown in the last two rows of Table 4. The Al/P ratios are essentially the same as for the fresh catalysts; however, the Ni surface concentration is significantly increased in both spent catalysts. Most notable is the appearance of Ni in AAP1 which previously was absent in the calcined (oxide) form of the catalyst. It appears that, during the reaction process, the Mo/Ni ratio is significantly decreased on the surface, probably due to catalyst sulfidation. In passing, we note another work comparing XPS data for both sulfide and oxide forms of a similar catalyst.10 Pyridine HDN Activity Measurements. For our work, pyridine was chosen as an HDN model compound due to the fact that pyridine-derived compounds are among the simplest N heterocycles found in petroleum and coal liquids and, consequently, the pyridine HDN reaction network involves relatively few intermediates as compared to polycyclic N compounds.26-33 Analysis of our liquid products revealed only five compounds,
(22) Rynkowski, J. M.; Parujczak, T.; Lenik, M. Appl. Catal. 1993, 106, 73. (23) Huang, Y. J.; Schwarz, J. A.; Diehl, J. R.; Baltrus, J. P. Appl. Catal. 1988, 37, 229. (24) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R. J. J. Catal. 1984, 86, 359.
(25) Belton, D. N.; Sun, Y. M.; White, J. M. J. Am Chem. Soc. 1984, 106, 3059. (26) Sarbak, Z. Acta Chim. Hung. 1990, 127 (3), 371. (27) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021. (28) Hadjiloizou, G. C.; Butt, J. B.; Dranoff, J. S. J. Catal. 1991, 131, 545.
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Table 5. Pyridine HDN Product Distributions at 350 °C and 20 min product distribution (wt %)
a
catalyst
pyridine
piperidine
pentylamine
pentylpiperidine
n-pentane
total C5’s
none AlPO4-Ga Al2O3-Ha AAP1 AAP2 AAP4 AAP8 AAP∞ Al2O3-H AlPO4-G Shell 424
1.89 1.62 1.69 0.74 0.63 0.33 0.35 0.26 0.17 0.58 0.14
0.00 0.00 0.00 0.34 0.17 0.07 0.12 0.07 0.04 0.29 0.07
0.00 0.00 0.00 0.10 0.11 0.17 0.24 0.17 0.18 0.12 0.35
0.00 0.00 0.00 0.09 0.09 0.05 0.07 0.03 0.01 0.11 0.03
0.00 0.00 0.00 0.09 0.18 0.38 0.33 0.46 0.64 0.15 0.59
0.00 0.00 0.00 0.21 0.35 0.61 0.56 0.70 0.84 0.30 0.87
Support only, no Ni or Mo added.
namely, pyridine, piperidine, pentylamine, pentylpiperidine, and di-n-pentylamine, as the major N-containing compounds (more than 0.01%). The major terminal product found was n-pentane, although some additional C5’s (indicated by GC-MS) and lower boiling products were also observed at high severities. From the liquid phase products analysis, two measures of HDN activity were derived. The first is based upon the wt % N in the product liquid as calculated from amounts of the five N-containing compounds noted above. Thus, if Nf and Np represent the wt % of nitrogen in the feed and product mixture, respectively,
%HDN ) (Nf - Np)/Nf
(1)
In spite of the apparent simplicity of the above procedure as represented by eq 1, there are some difficulties in application of this method arising from the fact that, in batch reactions, such as performed in this study, some of the liquid phase HDN will occur due to irreversible preferential adsorption of N compounds on the catalyst surface, rather than catalytic reaction per se. Table 5 shows the liquid product distribution from the batch pyridine HDN experiments. Several replicates were run for each experiment, and the typical average standard deviation for %HDN determined from replicate experiments was 3.7% on the scale of 0-100%. The first row in Table 5 shows a blank run for the HDN of pyridine without catalyst. For the blank run, no products were observed although a 6% pyridine loss occurred. The second and third rows show results for two commercial catalyst supports (Table 1) which were not impregnated with NiMo. In the case of the two commercial supports, Al2O3-H and AlPO4-G, no reaction products are formed; however, there is a significant removal of pyridine by adsorption and the pyridine product concentration is less than the feed concentration of 2 wt %. Because of the possible influence of this adsorption effect, we cannot attach significance to small differences in product distribution in the pyridine experiments and attention will only be focused on the major trends. One important point to note from Table 5 is that the alumina and aluminum phosphate supports themselves (rows 2 and 3) possess essentially no pyri(29) Mcllvried, H. G. Ind. Eng. Chem. Process Des. Dev. 1971, 10 (1), 125. (30) Satterfield, C. N.; Cocchetto, J. F. AICHE J. 1975, 21 (6), 1107. (31) Hanlon, R. T. Energy Fuels, 1987, 1, 424. (32) Satterfield, C. N.; Model, M; Wilkens, J. A. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 154. (33) Sonnemans, J.; Van Den Berg, G. H.; Mars, P J. Catal. 1973, 31, 220.
Figure 5. HDN of pyridine as a function of catalyst surface area for seven laboratory catalysts and Shell 424.
dine HDN catalytic activity, as no products are formed. Most likely this stems from the inability of the phosphate and alumina supports, lacking a metallic functionality, to catalyze hydrogenation reactions, specifically the hydrogenation of pyridine to piperidine, which is the first step in the HDN reaction sequence. A second indicator of catalyst activity derived from the pyridine HDN experiments is pentane (or total C5) production. Experiments showed that pentane, unlike the N-containing products, did not adsorb preferentially on the catalyst’s surface. On the basis of its appearance as the major terminal HDN product, the pentane concentration in the liquid product provides a good indicator of overall HDN catalytic activity. Generally, reaction product mixtures with a higher %HDN calculated from eq 1 also had higher wt % of pentane concentrations, showing the two measures of catalyst activity to be internally consistent. The last eight rows of Table 5 show the HDN product distributions obtained using the seven laboratory catalysts listed earlier in Table 3 and the commercial Shell 324 catalyst. The laboratory catalysts have different Al/P atomic ratios and the same nominal Ni/Mo contents and were tested in batch experiments as described earlier at constant catalyst weight loadings. Using the distribution of N-containing products in Table 5, a %HDN can be determined for each catalyst, and this is plotted as a function of catalyst surface area in Figure 5. Corresponding plots of pentane and total C5 production from the same experiments are shown in Figure 6.
Alumina-Aluminum Phosphate Catalyst Supports
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Table 6. Pyridine HDN Product Distributions Using Equal Catalyst Surface Areas (7.75 m2) product distribution (wt %) catalyst
%HDN
pyridine
piperidine
pentylamine
pentylpiperidine
n-pentane
total C5’s
AAP2 AAP4 Shell 424
55 55 52
0.49 0.45 0.45
0.25 0.27 0.26
0.19 0.20 0.30
0.08 0.08 0.07
0.14 0.14 0.17
0.28 0.28 0.34
Figure 6. Pentane (solid line) and total C5 (dashed line) concentrations as a function of catalyst surface area for seven laboratory catalysts and Shell 424.
As shown in Figures 5 and 6, %HDN and pentane/total C5 production activity are correlated well with surface area of catalyst. In general, the %HDN and pentane/ C5 concentrations increased with the increase of Al/P ratio (or surface area of catalyst). Because surface area and Al/P ratio also are related (Table 3, columns 4 and 7), it is difficult to attribute immediately the trends in Figures 5 and 6 to either of these factors independently. The somewhat lower activity of the AAP1 and AAP2 catalysts could be caused by their greater P contents, their lack of surface Ni as revealed by the XPS analysis, or simply their lower surface area. However, the fact that AAP2, which had an absence of surface Ni, has essentially the same activity as AlPO4-G, which did have surface Ni (see Table 4), and the fact that surface Ni is present in the AA1 catalyst following reaction suggest that the lower activities of AAP1 and AAP2 are not caused by the absence of surface Ni. With regard to Ni as a promoter, it is of interest to note that a factorial experiment by Parker et al.34 for croeoste oil hydrotreatment failed to show any significant effect of Ni promotion on a series of NiMo/Al2O3-TiO2 catalysts. Thus it could be that Ni is only a weak promoter of HDN in these experiments. Since the experiments in Table 5 were conducted with equal weights of catalysts, an additional series of pyridine HDN experiments was performed using equal amounts of catalyst surface area (different weights) in each experiment. These results, shown in Table 6 indicate that the AAP catalysts have approximately the same pyridine HDN activity on a per unit surface area basis as the commercial Shell 424 NiMo/Al2O3 catalyst. Again, the lack of surface Ni in AAP2 does not seem to be a significant factor, when compared with the AAP4 catalyst containing surface Ni on an equal area basis in Table 6. The fact that the (34) Parker, J. W.; King, J. A.; McCormick, R. L.; Haynes, H. W. Energy Fuels, 1989, 3, 350.
Figure 7. HDN of coal distillate as a function of catalyst surface area for AAP catalysts and Shell 424: lower solid line, r ) 0.41; upper solid line, r ) 0.68; dashed line, r ) 0.94 (open and solid symbols analyzed by Dumas and Kjeldahl methods, respectively).
activity of AAP4 at a lower weight loading equals that of AAP2 reinforces the suggestion inferred earlier by XPS of higher metal dispersion with increasing Al/P ratio. All factors considered, there does not appear to be any significant effect of the Al/P ratio on the pyridine HDN performance, other than that which can be explained by the surface area variation or, perhaps, by an increasing metal dispersion with increasing Al/P ratio. In this light it is interesting to contrast this result with the similar study by Parker et al.,34 who found a distinct optimum support composition with respect to the Al/Ti ratio in a series of NiMo/Al2O3-TiO2 catalysts. As a final caveat, it should be noted that our batch experiments represent only initial catalyst activity and are not indicative of catalyst performance in long-term runs where deactivation can play a major role. In addition, pyridine is a small molecule and the performance with macromolecular feedstocks, as discussed later, may be somewhat different, in that hindered diffusion effects have been reported with high Al/P ratios (small pores).11,15 Coal Liquid HDN. Batch reaction upgrading experiments were conducted with both the coal distillate and the coal-waste coprocessing liquid in Table 2. With the coal distillate, a total of 22 batch reactions were conducted using the various catalysts. The complete HDN data for all of these reactions are shown in Figure 7 along with three regression lines for the data. Admittedly, there is considerable scatter in the data; however, all data are shown for completeness. The lower solid line in Figure 7 (r ) 0.41) obtained using all the data statistically indicates a 90-95% correlation between HDN and the surface area for all the catalysts. The upper solid line (r ) 0.68) obtained by disregarding the five lowest points, each of which can be rejected with less than a 10% chance of error according to criteria in
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Figure 8. HDN of coal-waste liquid as a function of catalyst surface area for AAP catalysts.
Volk,35 indicates a >99% correlation between HDN and the surface area. On the basis of these considerations, there is a reasonably good correlation between %HDN and surface area. Two AAP catalysts, AAP4 and AAP8, have activity comparable to that of commercial Shell 424. Noting that these 22 HDN experiments were conducted with equal weights of catalyst, we speculate that if corresponding experiments were conducted with equal surface areas, as was done with pyridine in Table 6, the slopes of the lines would likely decrease. The relatively high activity of the Al2O3-H catalyst (no P) indicates that P is not required for good initial HDN activity. The fact that AAP∞ has the lowest activity of the AAP catalysts is possibly caused by diffusional resistance considering that it has the smallest pore size of all the catalysts (Table 3). This observation is reinforced later by the data for the HDN of the coalwaste coprocessing liquid. The HDN data in Figure 7 were determined using two methods of N analysis, viz. the Dumas method (open symbols) and the Kjeldahl method (filled symbols). The Dumas method appears to give less precision. If only the Kjeldahl data (10 experiments) are used, the precision is very good and the plot (dashed line) in Figure 7 results showing an excellent correlation for HDN with surface area, a trend in agreement with the pyridine HDN results shown earlier in Figures 5 and 6. Figure 8 shows the data from the HDN of the coalwaste coprocessing liquid. A total of 11 experiments were performed. The precision of replicated runs, all analyzed by the Kjeldahl method, is excellent in this case. The data again show a good correlation with (35) Volk, W. Applied Statistics for Engineers, 2nd ed.; McGrawHill, Inc.: New York, 1980; p 384.
Menon et al.
surface area, but as was the case with the coal distillate, AAP∞ again falls below the trend. We again speculate that this may be due to the smaller pores of AAP∞ even though, with the pyridine HDN data, AAP∞ fell on the same line as the other catalysts (Figures 5 and 6). In pyridine HDN, hindered diffusion is not likely to be the rate-determining step due to its small molecule size. However, with heavy feeds, hindered diffusion may occur, a phenomenon which has led to optimum pore diameters of 20 nm using AAP catalysts for hydrodemetallization (HDM) of heavy oil and 9 nm for HDS of residues.11,14 With respect to HDN, it has been observed that the nitrogen content increases with increasing boiling point of the fractions in coal-derived liquids36 and a California petroleum.37 It thus seems possible that the lower HDN activity of AAP∞ with the coal liquids might be explained by diffusional resistance encountered by multiring N-containing compounds in entering the smaller pores of AAP∞. Conclusions. NiMo/AAP catalysts have been prepared and tested in batch experiments for the HDN of pyridine and two coal liquids. The textural properties of the AAP supports are influenced by the P content, with higher P tending to give higher pore volumes, lower surface areas, and correspondingly larger pore diameters. Supports with a desired Al/P ratio may be prepared by appropriate selection of reagent quantities. XPS analysis showed that the surface Al/P ratios were very close to the bulk values, while the Mo/Ni ratios were somewhat variable and differed from the bulk values. Following reaction, the Ni concentration on the surface was enriched. The AAP-supported catalysts had activity comparable to that of a commercial P-promoted NiMo/Al2O3 hydrotreating catalyst for pyridine HDN and HDN of coal and coal-waste coprocessing liquids. The initial HDN activity of the AAP-supported catalysts was fairly well correlated with the catalyst surface area, with the exception of the smallest pore catalyst (AAP∞) with coal liquids, a result which could be attributed to diffusional hindrance. HDN performance of AAP-supported catalysts remains to be evaluated in longer term experiments where deactivation plays a major role. Acknowledgment. This work was supported by the Consortium for Fossil Fuel Liquefaction Science. The authors wish to thank Mr. Xiaofeng Yang for obtaining the SEC data for the coal liquids used in this work. EF950244I (36) Wolk, R. H.; Stewart, N. C.; Silver, H. F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1975, 20 (2), 116. (37) Snyder, L. R. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1970, 4 (2), C43.
