Energy & Fuels 1992,6, 300-307
300
decreased by approximately 25% relative to thiophene. A gradual increase of a peak at a high-energy position was attributed to the formation of sulfonic acid as a result of oxidation of the disulfidic sulfur form. Acknowledgment. Support for this study by the Office
of Exploratory and Applied Fiesearch of the Electric Power Research Institute under EPRI Contract RP 8003-20 is gratefully acknowledged. Registry NO.Pyrite, 1309-36-0;pyrrhotite, 1310-50-5;sulfate,
14808-79-8.
A Comparative Gas Oil Hydroprocessing Study of Alumina, Carbon, and Carbon-Covered Alumina Supported Ni-Mo Catalysts: Effect of Quinoline, Thiophene, and Vanadium Spiking P. M. Boorman* and K. Chong Department of Chemistry, University of Calgary, Calgary, Alberta, Canada, T2N 1N4 Received September 19, 1991. Revised Manuscript Received January 13, 1992
Catalysts with 3-15 w t % NiO-Mo03 supported on alumina, carbon-covered alumina, and carbon were used to hydroprocess gas oil spiked with quinoline, thiophene, and vanadium to give an insight into the relationship between the support and deactivation. Experiments were carried out in a batch reactor at 410 "Cand lo00 psi initial H2 pressure. It was found that N, S, and V compounds in the feed suppress the hydrogen uptake by gas oil hydroprocessed with alumina or carbon-covered alumina supported catalysts but carbon supported catalysts are insensitive to these compounds. Quinoline HDN and thiophene HDS do occur thermally to a small extent but a catalyst is required for efficient HDN and HDS. Under the conditions used for our experiments, quinoline is rapidly hydrogenated to 1,2,3,4-tetrahydroquinolineand it is the subsequent C-N bond hydrogenolysis that is the ratedetermining step. For hydroprocessing the high nitrogen content quinoline spiked gas oil,the carbon supported catalyst is comparable to the alumina supported catalyst for sulfur removal but is superior for quinoline HDN. Basic nitrogen compounds, such as quinoline, have an affiiity for acidic catalyst surfaces and undergo coking reactions thereby deactivating metal centers and reducing the total surface area. The use of a carbonaceous support decreases the amount of coke deposited on the catalyst surface.
Introduction In addition to high catalytic activity and selectivity, a successful catalyst must also have an acceptable lifetime, resisting deactivation, if it is to be any of any industrial value. This will be the focus and the challenge in hydroprocessing catalyst research and development due to the shift in feedstock quality to heavier feeds' combined with the implementation of stricter environmental regulations.2 With feedstocks becoming heavier, Le., more aromatic content, higher levels of sulfur, nitrogen, and metals such as vanadium, catalysts need to be able to withstand the deleterious effects imposed by these compounds in the feed. There are many causes of dea~tivation.~Generally, the catalyst can be deactivated by the condensation of olefins and aromatics to form heavier polynuclear species (coke) or by the irreversible adsorption of metal species and/or other compounds in the feed. These compounds can deactivate the catalyst by having an affinity for catalyst active sites, e.g., basic nitrogen compounds poisoning acidic sites, a chemical interaction, or by physically plugging the pore structure preventing the reactants from accessing the active sites. Both result in a decrease in the catalyst's active surface area. It has been reported that catalyst
* To whom correspondence should be addressed. 0887-0624/92/2506-0300$03.00/0
deactivation by coke formation is several times greater than that caused by metal deposition' for the conversion of high boiling feedstocks to lower boiling distillates. However, the exact mechanism by which coke attenuates the catalytic activity is unclear and is complicated by the fact that deactivation is dependent on feedstock composition. For example, a naphtha hydrodesulfurization catalyst may remain active for years, whereas a residuum hydroprocessing catalyst may last only a few months.6 Furthermore, it is generally accepted that catalyst deactivation by coke and metals involves acid sites and that the basicity of a compound will strongly influence its propensity to form coke? It is apparent then that catalyst deactivation is not a simple process; rather, it is a complex network of reactions involving physical and chemical processes. ~~
(1) 'Energy Alberta 1990"; Energy Resources Conservation Board Review of Alberta Energy Resources in 1990 (ERCB 91-40). (2) Skubnik, M.; Wilson, M. F.; McCann, T. J. AOSTRA J . Res. 1990, 6, 1. (3) Richardson, J. T. In Principles of Catalyst Deuelopment; Twigg, M. V., Spencer, M. S., Eds.; Plenum Press: New York, 1989; p 185. (4) Ternan, M.; Furimsky, E.; Parsons, B. I. Fuel Process. Technol. 1979, 2, 45. (5) Diez, F.; Gates, B. C.; Miller, J. T.; Sajkowski, D. J.; Kukes, S. G. Ind. Eng. Chem. Res. 1990,29, 1999. (6) Adkins, B. D.; Milburn, D. R.; Goodman, J. P.; Davis, B. H. Appl. Catal. 1988, 44, 199.
0 1992 American Chemical Society
Energy & Fuels, Vol. 6, No. 3, 1992 301
Hydroprocessing Study of Ni-Mo Catalysts Table I. ComDosition and Surface Areas of Catalysts surface designawt % wt % area, SUDDort tion NiO MOO? m2/g 7-alumina A1 0 0 167 A2 3 15 153 carbon-covered CA1 0 0 135 alumina' CA2 3 15 104 carbon c1 0 0 1357 c2 3 15 792 'The carbon-covered alumina support has 11.28 w t % carbon.
In our previous work, we explored the role of the support by doing a comparative s t u d y of alumina, carbon and carbon-covered alumina supported Ni-Mo catalysts in model compound' and gas oil hydroprocessine studies. Alumina is the conventional support material, but in recent years, there has been growing interest in carbon and carbon-covered alumina as a catalyst support. A potential advantage of a carbon support is its reduced coking propensity which has been reported? This is consistent with our findings8 where, in general, less coke was deposited on carbon-covered alumina supported catalysts than on the alumina supported catalysts. The intent of the present s t u d y is to extend this work b y investigating the deactivation properties of these supports for Ni-Mo catalysts using gas oil spiked with nitrogen, sulfur, and vanadium compounds. Experimental Section Catalyst Preparation. The catalysts were prepared by the incipient wetness method with the additives, Ni(N03)z.6H20and (NH4)6MqOu4Hz0,dissolved in distilled water to give 3 wt % NiO and 15 w t % Moo3, respectively. The order of addition was Mo followed by Ni with no drying steps in between. The catalyst was mulled for 1-2 min after each addition and was not subjected to a calcination between additions. The catalyst was left to dry in air a t room temperature overnight followed by drying a t 100 O C overnight. At this stage, the alumina supported catalyst was calcined by heating at 4 OC/min to 500 O C and maintaining this temperature for 4 h under a stream of air. The carbon and carbon-covered alumina catalysts were not calcined because oxidative degradation would occur. The y-alumina was prepared by taking 50 g of boehmite (aAlOOH) (Alfa Products, 40 pm, 90% AlzO3,9% HZO) and mulling for 3-5 min with 16 mL of distilled water. A further 10 mL of distilled water was added in 5-mL portions with mulling for 3-5 min after each addition. The alumina was left to dry at room temperature, and then at 100 "C overnight before b e i i subjected to a calcination following the same procedure as for the alumina supported catalyst described above. The activated carbon used in this study was Darco KB from the Aldrich Chemical Co. It was used as supplied following drying a t 100 O C for 24 h. The carbon-covered alumina was prepared as by Vissers et al.l0 by pyrolyzing cyclohexene a t 600 O C . Approximately 2 g of y-alumina was placed in a calcination tube and heated to 600 OC a t a rate of 10 OC/min under a dry N2 flow of 20 mL/min. It was held a t 600 OC for 1/2 h after which time the Nz was bubbled through cyclohexene held at room temperature before passing over the alumina for 6 h a t 600 OC. The sample was allowed to cool under a flow of dry Nz.The catalyst compositions and surface areas are listed in Table I. (7) Boorman, P. M.; Chona, K.; Kydd, R. A.; Lewis, J. M. J. Catal. 1991,128,537. (8) Boorman, P. M.; Kydd, R. A.; J. M.; Bell, W . S. Fuel 1992, 71, 87.
A.
Sorensen, T. S.; Chong, K.; Lewis,
(9) deBeer, V. H. J.; Derbyahire, F. J.; Groot, C. K.; Prins, R.; Scaroni,
W.:Solar.J. M. Fuel
1984.63.
1095.
(10) Vim&, J. P. R.; Mercx, F.'P. M.; Bouwens, S. M. A. M.; deBeer, V. H. J.; Prins, R. J. Catal. 1988, 114, 291.
Table 11. Properties of Hydroprocessed Gas Oil, Unspiked' 90 density, wt% catalyst H:C aromatics g/cm3 viscosityb sulfur feed' 1.40 37 0.9917 100 4.79 noned 1.47 34 0.9534 13.1 2.24 A1 1.35 42 0.9529 8.7 3.12 A2 1.59 25 0.9015 8.6 0.51 CA1 1.45 40 0.9506 9.5 2.78 CA2 1.56 30 0.9198 9.3 0.80 c1 1.32 42 0.9484 10.0 3.59 c2 1.59 31 0.9169 9.9 0.74 @Fromprevious work (ref 8). All experimenta of %hour dura* Percent viscosity relative to untreated gas oil. tion. 'Unprocessed Syncrude combined gas oil feed. Contains 0.43 wt % nitrogen. No catalyst present, under H2 only (thermal hydroprocessing). Gas Oil Hydroprocessing Procedure. The Syncrude combined gas oil was supplied by the Alberta Research Council. Its properties are described in Table II. The gas oil was spiked with nitrogen, sulfur, or vanadium compounds and then was used as the feed for the experiments. The compounds and concentratione used for spiking the gas oil were 4.11 x lo4 mol of quinoline/g of feed (5.3 wt % ), 5 wt % thiophene and 500 ppm vanadium (0.05 wt %) as the tetraphenylporphine vanadium(IV) oxide. All are representative of the types of compounds found in heavy oil. Experiments were carried out in an Autoclave Engineers 300-mL Hastalloy C batch reactor. Each experiment consists of three stages. Firstly, 1g of catalyst is sulfided by 2 mL of CSz under 6.9 MPa (lo00 psig) initial Hz pressure at 440 O C for 3 h (includingthe approximately 1 h to reach 440 OC). The sulfded catalyst is then cooled to room temperature and all gases are vented from the system. Secondly, removal of excess sulfur is effected by repressurizing to 6.9 MPa with H2 under the same conditions as for sulfiding. Finally, the hydroprocessing experiment is carried out by adding 22 0.5 g of spiked gas oil under positive argon pressure to prevent the catalyst from b e i i expoeed to air. The system is then pressurized to 6.9 0.35 MPa (loo0 50 psig) with Hz at room temperature. The reaction is continuously stirred during the experiment and is maintained a t approximately 410 O C for the duration of the experiment. The H2 preasure is not maintained at a constant value during the c o w of the reaction. For a 3-h experiment, the drop in H2 pressure (measured before and after the experiment a t room temperature) is in the order of 0-100 psi for the thermal experiment and if blank supports are used and between 200 and 300 psi if a Ni-Mo catalyat is employed. Since gas samples were not collected and analyzed, the change in pressure may not be a true reflection of hydrogen uptake and hence AP was not considered in the discussion of results. After the hydroprocessing experiment, the reactor is cooled rapidly to room temperature. All aspects of the hydroprocessing procedure (heating rate, reaction time, reaction temperature, stirring rate, cooling rate, etc.) are computer controlled, ensuring repeatability between experiments. A combination of the reactor furnace and cooling coils in the reactor are used to maintain constant temperature. The typical temperature range during a reaction is 408 3 OC. Analysis of Oil Products. The hydroprocessed gas oil was analymd to investigate the influence of nitrogen, sulfur, and vanadium compounds in the feed on the catalyst's ability to hydrogenate, crack larger molecules into smaller ones, and remove sulfur and nitrogen. The carbon, hydrogen, and nitrogen contenta were determined by the Analytical Services Lab at the University of Calgary. The densities a t 15-16 OC and viscosities a t 25 "C were determined using an Anton Paar DMA40 density meter and a Brookfield DV-I cone/plate shear viscometer, respectively. The viscosity in centipoise was obtained with a rotational speed of 100 rpm and a multiplication factor of 2.56. The viscosities are reported relative to the untreated, unspiked gas oil assigned the value of 100. Sulfur was analyzed by X-ray fluorescence using a Princeton Gamma-Tech chemical analyzer. The quinoline hydrodenitrogenation ability of the catalyst was determined by gas chromatography using tetrahydronaphthalene as an internal standard. A 1 0 1 mixture of gas oil samp1e:inted standard was diluted in acetone and -0.2 pL injected directly into a H P 5890
*
as
*
302 Energy & Fuels, Vol. 6,No. 3, 1992
catalyst feedC noned A1 A2 A2
A2 A2 CA1 CA2 CA2 CA2 CA2
c1 c2 c2 c2 c2
Boorman and Chong
Table 111. ProDerties of Quinoline-Spiked HydroDrocessed Gas Oil reaction time, h H:C % aromatics density, g/cm3 viscosity wt 1.30 45 0.9972 64.8 44 1.39 0.9563 6.6 3 42 1.39 0.9511 6.1 3 1.51 0.9028 3.6 3 26 1.50 0.9098 4.6 2 29 1.55 0.9305 7.8 1 32 1.55 0.9447 12.4 33 112 1.46 0.9519 6.6 38 3 1.51 0.9198 32 4.9 3 1.50 0.9317 31 6.1 2 1.47 0.9429 34 10.2 1 1.44 0.9623 36 16.1 112 1.34 0.9500 43 6.3 3 1.56 0.8951 26 2.3 3 1.56 27 2 0.9103 4.0 1.51 0.9295 30 1 10.6 1.48 0.9412 32 11.7 1/2
% sulfur
% quinoline HDNb
4.28 3.07 2.97 0.47 0.60 1.00 1.51 2.93 1.13 1.42 1.91 2.35 2.98 0.47 0.72 1.17 1.53
21
30 71 66 49 37 29 52 49 38 30 26 83 74 66 57
"Percent relative viscosity; untreated, unspiked gas oil = 100%. "Quinoline HDN = (mol/g of quinoline in feed - of all nitrogen compounds in the reaction network (excluding PCHA)) + mol/g of quinline in feed. c4.11 X 10"' mol of quinolinelg of spiked gas oil feed, not hydroprocessed. No catalyst present, under Hz only (thermal hydroprocessing). Table IV. Properties of Sulfur- and Vanadium-Spiked" Hydroprocessed Gas Oilb % density, wt% catalyst H:C aromatics d c m 3 viscositv sulfur sulfur spiked feedd 1.44 39 0.9958 61.6 6.21 nonee 1.38 41 0.9440 10.4 3.47 A2 1.63 28 0.8957 12.0 0.40 c2 1.61 26 0.9013 9.5 0.51 vanadium spiked feedd 1.45 39 0.9930 109.7 4.60 A2 1.60 32 0.9003 12.9 0.53 c2 1.60 30 0.9103 9.1 0.68 ~
Figure 1. Quinoline HDN reaction network (adapted from ref 11).
gas chromatograph/HP3392A integrator. A flame ionization detector and a HP1 methyl silicone gum column (10 m X 0.53 mm i.d. x 2.65 pm film thickness) was used. Injection and detector temperatures of 200 and 220 "C, respectively, were used while the oven was temperature programmed to hold at 50 "C for 2 min, then heat at 10 "C/min to 200 "C and hold for 8 min. Figure 1 shows the quinoline hydrodenitrogenation (HDN) reaction network." The quinoline HDN activity was calculated by subtracting the sum of the concentrations of all the nitrogen-containing compounds in the quinoline reaction network (excluding PCHA) from the concentration of quinoline in the feed and then dividing by the concentration of quinoline in the feed. The concentrations were all corrected by multiplying by their respective relative response factors which were experimentally determined by us. Propylcyclohexylamine (PCHA) was ignored since we did not have a pure sample for retention time/RRNpurposes and due to the fact that it has been reported that PCHA is not detected as a quinoline HDN product, but it is a plausible intermediate that is very rapidly denitrogenated.lZ The aromatic content of the products was determined by 13C NMR (Bruker ACE-200 instrument) using an inverse gated decoupling pulse sequence with ) ~a relaxation agent. a short delay time of 1s utilizing C r ( a ~ a cas The spectra were recorded as 40% v/v solutions in CDC13 containing 0.05 M Cr(acac)% A minimum of 2000 scans was collected. The spectrum was phased three times and the integral values averaged. The % aromatic carbon was determined from the integrated aromatic (160-110 ppm) and aliphatic (60-5 ppm) regions. The chemical shifts were measured using the middle peak (11) Satterfield,C. N.; Yang, S. H.Ind. Eng. Chem. Process. Des. Deu. 1984, 23, 11.
(12) Satterfield, C. N.; Cochetto, J. F. Ind. Eng. Chem. Process Des. Deu. 1981, 20, 53.
~~~
" Gas oil spiked with either 5 wt % thiophene or 500 ppm vanadium as tetraphenylporphine vanadium(1V) oxide. *All experiments of 3-h duration. Percent relative viscosity; untreated, unspiked gas oil = 100%. dFeed properties, not hydroprocessed. e No catalyst present, under H2 only (thermal hydroprocessing). of the CDC13 triplet a t 77.0 ppm as the reference. There will undoubtedly be differences in feed properties between identical gas oil hydroprocessing experiments. T o check the repeatability, three thermal experiments were performed and the products analyzed for density, wt % sulfur, and % aromatica. Each property was measured a minimum of three times for each experiment. Deviations from the average value for the three runs for density and wt % sulfur were quite small, f0.0073 and fO.O1, respectively. A larger deviation of f 3 exists for % aromatics determination. These are the errors associated with these properties and hence very small differences, especially for % aromatics, may not be significant. We have done our utmost to minimize errors to ensure validity. For example, each analytical method is performed for all the samples in succession, i.e., on the same day by the same operator which ensurea the error aeaociated with the measurement is constant for all the samples. In addition, all the trends in our results are self-consistent; e.g., H:C data supports changes in % aromatics, which suggests then that our results, especially when looking a t them in a relative sense, are valid. Catalyst Characterization. The surface areas of the catalysts were measured by the BET method, with Nz as the adsorbent, using a Micromeritics high-speed surface area analyzer. The wt % carbon analyses were determined by the Analytical Services Lab a t the University of Calgary. The spent catalysts after gas oil hydroprocessing were characterized after washing with 10 mL of CH2Clzand drying.
Results and Discussion Catalyst Composition. The catalyst compositions and surface areas are listed in Table I. The wt % carbon
Hydroprocessing Study
catalyst none' A1 A2 A2 A2 A2 CA1 CA2 CA2 CA2 CA2
c1 c2 c2 c2 c2
of
Ni-Mo Catalysts
Energy & Fuels, Vol. 6, No. 3, 1992 303
Table V. Quinoline HDN: Gas Chromatography Analysis amount," mol/g x lo-' reaction time, h Q 1THQ 5THQ DHQ OPA 0.15 0.038 3 0.97 1.91 0.19 1.65 0.16 0.020 3 0.83 0.21 0.043 0.082 0.0 3 0.0 1.07 0.15 0.15 0.0 1.08 2 0.0 0.70 0.32 0.038 1.05 1 0.0 1.42 0.32 0.071 0.80 0.5 0.0 1.78 0.22 0.0 0.22 3 0.72 0.30 0.0 3 0.0 1.14 0.54 0.0 0.42 1.32 0.34 2 0.03 1.81 0.28 0.075 0.31 1 0.07 2.20 0.20 0.047 0.19 0.5 0.23 3 1.44 1.21 0.19 0.020 0.19 0.21 0.022 0.017 0.44 3 0.028 0.56 0.09 0.0 0.44 2 0.0 0.81 0.16 0.0 0.41 1 0.0 0.11 0.60 0.33 0.73 0.5 0.0
T
Q HDN? %
3.26 2.87 1.19 1.38
21 30 71 66 49 37 29 52 49 38 30 26 83 74 66 57
2.11
2.61 2.94 1.99 2.11
2.55 2.87 3.05 0.72 1.09 1.38 1.77
See Figure 1 for a description of the quinoline HDN reaction network and definition of acronyms. *Quinoline feed concentration = 4.11 mol of quinoline/g of feed. Q HDN = (mol Q/g feed - of all nitrogen compounds in the reaction network (excluding PCHA)) + catalyst present, under H2 only (thermal hydroprocessing). mol of Q/g of feed. a
X
lo-'
coverage of the carbon-covered alumina support and the decrease in surface areas upon carbon deposition and loading with metals are consistent with and described in detail in our previous study'. Hydrogen Uptake. Tables 11,111, and IV compile the results for hydroprocessed gas oil that is unspiked, spiked with quinoline, and spiked with sulfur and vanadium compounds, respectively. Table I1 summarizes the results from our previous stud9 for unspiked gas oil. It was found by using the hydrogen to carbon ratio and % aromatic carbon content of the hydroprocessed feed as a measure of hydrogen uptake that (1)thermal hydrogen uptake does occur to a small extent under the conditions of our experiment, (2) the supports alone promote dehydrogenation to give aromatics, and (3) the addition of a Ni-Mo catalyst causes a dramatic increase in hydrogen uptake capability for all three supports and Ni-Mo supported on alumina is the superior catalyst for the reduction of aromatic content. However, it should be noted that subsequent thermal experiments (without a blank support or catalyst present) have shown the % aromatic carbon present in the thermally hydroprocessed feed to increase slightly. In either case, the change in % aromatic carbon is small and reflects the opposing reactions that are going on under thermal conditions. Thermal cracking of aliphatic side chains would tend to increase the % aromatic carbon whereas sulfur removal (hydrogenolysis reactions) and hydrogenation would decrease the % aromatic content. The influence of nitrogen, sulfur, and vanadium compounds added to the gas oil on the hydrogen uptake capability is presented in Tables 111 and IV. Treating the gas oil without a blank support or catalyst under the conditions used for our experiments (410 OC, 1000 psi initial hydrogen pressure, 3 h) shows a small increase in hydrogen uptake (H:C, % aromatics) for the quinolinespiked gas oil but a decrease is observed when the gas oil is spiked with thiophene. The addition of quinoline to the gas oil does not appear to impede hydrogen uptake by thermal processes. However, the addition of sulfur as thiophene at a concentration of 5 w t % appears to inhibit thermal hydrogenation of the gas oil. A possible explanation for this may be the following. It has long been recognized that HDN is more difficult than HDS.13714This (13) Ho, T. C. Catal. Reu. Sci. Eng. 1988,30, 117. (14) Perot, G.; Brunet, S.; Canaff, C.; Toulhoat, H. Bull. SOC.Chim. Belg. 1987, 96,865.
may be due to the fact that the C-N bond is stronger than the C-S bond. Carbon-heteroatom bond hydrogenolysis is likely the determining factor since the resonance energies of nitrogen and sulfur heterocycles are comparable; thus hydrogenation of nitrogen heterocycles should be no more difficult than hydrogenation of sulfur heter0cyc1es.l~ Furthermore, it has been shown with a real feedstock that nitrogen removal is largely catalytic whereas sulfur removal has a significant thermal ~0mponent.l~Our results support this since under our thermal hydroprocessing conditions: we observe 18% quinoline HDN (Table 111)for the quinoline spiked gas oil whereas a 56% reduction in sulfur (Table IV) is achieved with the thiophene-spiked feed. It is not surprising that there is more total sulfur removal than quinoline HDN since some sulfur will be in thiol, thioether, and disulfide type structures which are more thermally labile than sulfur in aromatic structures. However, if the thiophene used to spike the feed is readily hydrogenated to tetrahydrothiophene and thermally cracked under the conditions of our experiment, the thiophene-derived radical species would then compete with the feed molecules for hydrogen and possibly abstract hydrogen from the feed molecules. If the thiophene-derived species were preferentially hydrogenated to yield butane and H2S, hydrogenation of thermally cracked gas oil molecules would be retarded and subsequent condensation reactions would result in an increase in aromatic content and decreased hydrogen to carbon ratio. This mechanism giving rise to an increase in aromaticity is not observed for quinoline-spiked gas oil. Although quinoline is readily hydrogenated to lTHQ, significant amounts of lTHQ remain after the thermal experiment (Table V) indicating that the C-N bond is not readily cleaved to give rise to species which would then abstract or compete for the available hydrogen. This supports the premise that HDN is more difficult than HDS due to the stronger C-N bond. The vanadium spiked feed was not hydroprocessed in the absence of a catalyst. With just the blank supports, it appears that improved hydrogenation characteristics are obtained when quinoline-spiked gas oil is hydroprocessed for all three supports in contrast to the situation for unspiked gas oil. However, (15) Sanford, E. C. The effect of catalyst deactivation on the mechanism of catalytic hydrocracking of resid from Athabasca bitumen. Presented at the Spring National A.1.Ch.E. Meeting, Symposia on Resid Upgrading Processes, Houston, TX, April, 1991.
Boorman and Chong
304 Energy & Fuels, Vol. 6, No. 3, 1992
the quinoline-spiked feed results for hydrogen to carbon ratio and % aromatic carbon are very close to those obtained when unspiked gas oil is processed with blank supports (for example, compare A1 in Tables I1 and 111). It appears that thermal hydrogenation of quinoline in the spiked feed may be responsible for the apparently improved hydrogenation properties for the spiked feed. In this instance, a more appropriate comparison would be to compare the blank support % aromatic results for the spiked feed to the 37% aromatics present for the unspiked, unprocessed feed (Table 11) which would then show the blank supports to have an inhibitory effect on feed hydrogenation. The similar results obtained for spiked and unspiked feed when blank supports are used suggest that the blank supports behave in a similar fashion, i.e., produce an inhibitory effect by providing surface sites for freeradical chain termination reactions.8 The blank supports were not subjected to the sulfur- or vanadium-spiked feeds. The addition of metals to all three support systems results in a dramatic increase in hydrogen uptake by the gas oil. The changes over time are given for the quinolinespiked feed in Table III. For all three support systems, a steady improvement in aromatics reduction is observed with time. Changes in hydrogen to carbon ratio are more subtle and trends are not as evident. Comparing A2, CA2, mol and C2 in Tables I1 and 111,the effect of 4.11 X of quinolinelg of feed is interesting. We observe that, for A2 and CA2, the quinoline has an inhibitory effect on hydrogen uptake as evidenced by the lower hydrogen to carbon ratios and slightly higher (possibly unchanged) aromatic carbon content when quinoline is added. However, for C2, an improvement in aromatic reduction is observed. The reason for this may be the following. Quinoline has a six-membered pyridinic ring with an unshared pair of electrons on nitrogen giving it basic character. Thus, quinoline can interact with and deactivate the acid sites which are present for A2 and CA2. This suggests that catalyst acidity plays a role in the hydrogenation of gas oil feedstocks. Carbon, on the other hand, is not an acidic surface and no such deactivating effect is observed; in fact, an improvement is observed. The effect of sulfur and vanadium on A2 and C2 is presented in Table IV. For A2, the hydrogen to carbon ratios are comparable to the unspiked feed but poorer aromatic reduction is observed (32% aromatic reduction for the unspiked feed vs 28% for the thiophene-spiked feed and 18% reduction for the vanadium-spiked feed). Again, an inhibitory effect for the alumina supported Ni-Mo catalyst is observed, this time due to thiophene and vanadium. However, as before, the carbon supported catalyst appears to be insensitive to sulfur and vanadium and an actual improvement in aromatic reduction is observed (16% aromatic reduction for the unspiked feed vs 33% for the thiophene-spiked feed and 23% for the vanadiumspiked feed). It is perplexing as to why an improvement is seen for the carbon supported catalyst. For both A2 and C2, vanadium has a more severe deactivating effect. Density and Viscosity Changes. One of the objectives in hydroprocessing is to crack and hydrogenate the large feed molecules into smaller ones. Typically, cracking and hydrogenation reactions are facilitated by thermal and/or catalytic mechanisms. The use of density and viscosity measurements provides a probe by which we can estimate the degree of cracking and hydrogenation. The influence of added quinoline, thiophene, and vanadium on density and viscosity is presented in Tables I11 and IV and can be compared to the unspiked feed (Table 11). Previously, it was found that thermal treatment results in a significant
drop in density and viscosity. The presence of the blank supports results in a further decrease, especially in viscosity. This is attributed to cracking reactions since the supports alone have an inhibitory effect on hydrogenation as previously described and the extent of viscosity reduction is in the order alumina > carbon-covered alumina > carbon. This correlates well with the inherent acidities of these supports' which would contribute to cracking reactions. Upon the addition of a Ni-Mo catalyst, the significant decrease is in density whereas only a slight improvement in viscosity reduction is observed. The improvement in density (decrease) is due to increased hydrogenation and a correlation exists between density and hydrogen uptake. The addition of quinoline or thiophene to the gas oil does not change the significant drop in density and viscosity observed under thermal conditions. Using quinoline-spiked gas oil, similar trends exist like those observed for the blank supports and Ni-Mo catalysts for the unspiked feed. As expected, both the density and viscosity decrease with reaction time. What is interesting to note is that, for the Ni-Mo catalysts, an improvement in density is seen for the carbon supported catalyst, no change for the carbon-covered alumina catalyst, and a detrimental effect (an increase) for the alumina supported catalyst is noticed when quinoline-spiked feed is compared with the unspiked feed. If changes in density can be linked to hydrogenation, this suggests that the metal hydrogenation sites are less susceptible to deactivation by basic nitrogen compounds in the feed when supported on carbon. This is not the case when comparing A2 to C2 for the thiopheneand vanadium-spiked feeds. Although the density is reduced significantly compared to only thermal treatment, A2 gives a greater reduction in density than C2. Furthermore, the change in viscosity does not correlate with these density changes. I t is unusual to have density and viscosity opposing each other and leads to the conclusion that the mechanism by which thiophene and vanadium deactivate catalysts is different than for quinoline. The propensity for coke to deposit on these catalysts will be discussed in a later section (see the section on Characterization of Spent Catalysts after Spiked Gas Oil Hydroprocessing) Sulfur and Nitrogen Removal. The removal of heteroatoms, particularly sulfur and nitrogen, from petroleum fractions is necessary before subsequent refining reactions can be undertaken and to satisfy environmental concerns. In our previous study! our results indicated that the ability of the catalyst to facilitate sulfur removal is directly related to both the catalyst's ability to hydrogenate the feed and its ability to facilitate cracking reactions. In this study, we examine sulfur and quinoline removal from gas oil in more detail. Firstly, sulfur removal in the absence of a catalyst (thermal only) does occur to some extent (53% removal in Table 11). When the gas oil is spiked with 4.11 X mol of quinolinelg of feed, less sulfur is eliminated by thermal mechanisms in the absence of any additive (28% reduction in Table 111),suggesting that nitrogen and sulfur compounds compete with each other for hydrogen. In fact, Satterfield et a1.16 report thiophene HDS to be inhibited by pyridine at all the temperatures and pressures they studied (200-400 "C, 1.14-7 MPa). The addition of blank supports does not significantly change the % sulfur removed from the quinoline-spiked feed and actually decreases the % sulfur removed from unspiked feed, con-
.
(16)Satterfield, C. N.; Modell, M.; Wilkens, J. A. Ind. Eng. Chem. Process Des. Deu. 1980, 19, 154.
Energy & Fuels, Vol. 6, No. 3,1992 306
Hydroprocessing Study of Ni-Mo Catalysts H ~ ~ ~ O ~ ~ C C 3Chovn S S C with ~
BO 7
C2
1 4.50
Support -A-
- 3.00
Thermal hydroprocessing
*
Alumina C-covered aIumina
-
0 0.00 0.0 0.5 1.0 1.5 2.0 2.6 3.0 3.5
Unprocessed gas 011
Reaction time (hour.)
Figure 3. A comparison of quinoline HDN and sulfur removal with Ni-Mo catalysts.
Figure 2. Thiophene-spiked gas oil hydroprocessing NMR studies.
firming the need for active sites, i.e., supported metal sulfides, to facilitate sulfur removal. Upon hydroproceasing over metal sulfides, a significant decrease in wt % sulfur is noted for all three supports. Clearly then, a catalytic mechanism is needed for efficient HDS. The N M R spectra can also be used to give an indication of thiophene removal. All spectra are approximately the same volume percent and are all plotted relative to the CDC13signal which has been maximized. Thus, these spectra can only be used in a semiquantitative manner with regards to the thiophene removal. Figure 2 shows the aromatic region of the 13C NMR spectrum for thiophene spiked gas oil. The two sharp peaks are due to thiophene. It shows that thiophene removal does occur to some extent by thermal reactions but that, for complete elimination of thiophene, a catalyst is required. Even in the presence of quinoline, the % sulfur removed for the A2 hydroprocessed quinoline spiked gas oil is comparable to the unspiked gas oil (89.0 w 89.4% removal, respectively). However, an improvement in % sulfur reduction is noticed for C2, the carbon supported catalyst. Also, C2 has the highest quinoline HDN conversion. Figure 3 compares quinoline HDN and sulfur removal for Ni-Mo supported on alumina, carbon-covered alumina, and carbon. All three support systems show a similar activity vs time profile for quinoline HDN and HDS reaching what appears to be steady-state activity after 3 h. It appears that, for high nitrogen content feeds, sulfur removal is comparable for alumina and carbon supported catalysts but better HDN is achieved using a carbon supported catalyst. This is consistent with the idea that HDN is much more catalyst dependent than HDS which has a significant thermal component. If the carbon supported catalysts are less susceptible to deactivation by coking in high nitrogen content feedstocks, this would explain the results observed. If this is the case, it is perplexing as to why the carbon-covered alumina catalyst shows the poorest activity. If the metal sites are the active sites for the HDN reaction, a mechanism by which these sites are deactivated to a greater extent for carbon-covered alumina catalysts may explain this result. If the metals are preferentially deposited onto the alumina surface for the carbon-covered alumina catalysts, subsequent coking of these sites would give rise to a catalyst surface which is highly carbonaceous in nature. Even though the total
amount of carbon deposited may be less than for the alumina supported catalysts (see the section on Characterization of Spent Catalysts after Gas Oil Hydroprocessing), the carbon-covered alumina catalyst surface would be highly covered with carbon when considering the original 11.28 wt % carbon. This high surface coverage with carbon may seriously limit the access of quinoline and sulfur compounds to active sites in the smaller pores which may now be blocked to a great degree. This is plausible since a coverage of 11.28 wt % carbon on y-alumina corresponds to quite a high surface coverage of -~-40%.~ In support of this explanation, it has been reported that coke deactivation is strictly the result of site blockage rather than destruction of active sites through carbide formation and ESCA results show more coke to deposit on the surface than in the interior of ~atalysts.'~The presence of additional sulfur (5 wt % thiophene) or vanadium in the feed does not appear to dramatically influence the removal of sulfur when A2 and C2 are employed as catalysts. Table V tabulates the gas chromatography analyses for the quinoline-spiked gas oil hydroprocessing experiments and the quinoline HDN results. Under thermal conditions or with just the blank supports, there is a small amount of quinoline HDN occurring, although 165% of the quinoline has reacted. The addition of a Ni-Mo catalyst significantly increases the quinoline HDN for all three support systems. This is also shown in Figure 4, a and b. The unprocessed gas oil shows nine peaks due to quinoline (peaks at 128.99 and 128.93 ppm appear as one peak) which decrease in intensity correlating with the GC analysis after hydroprocessing thermally or with just the blank supports. All quinoline peaks disappear when hydroprocessed with a Ni-Mo catalyst, even after only 1 / 2 h of hydroprocessing. Analysis of the product distribution shows that quinoline is hydrogenated primarily to 1tetrahydroquinoline (1THQ) which is then converted to o-propylaniline (OPA).The upper reaction pathway in Figure 1 appears to be the favored one under the conditions of our experiment. Furthermore, the hydrogenation of quinoline occurs quite rapidly as evidenced by the total disappearance of quinoline, even after 1 / 2 h of hydroprocessing, and thus it is the C-N bond hydrogenolysis that is the rate-determining step. Characterization of Spent Catalysts after Spiked Gas Oil Hydroprocessing. The catalysts recovered after gas oil hydroprocessing were characterized to give an insight into the deactivation properties of the catalysts. The results are presented in Table VI. The amount of de(17) Simpson, H. D. Aspects of coke deactivation in hydroprocessing catalysts. Abstract No. D26 presented at the 12th North American Meeting of the Catalysis Society, Lexington, KY, May 1991.
Boorman and Chong
306 Energy & Fuels, Vol. 6, No. 3, 1992
I I
Hydmprocc~ocd3 hours wifh A I
Table VI. Composition and Surface Areas of Spent Catalysts after Spiked Gas Oil Hydroprocessing surface area catalyst w t % carbon" m2/g % decrease quinline spiked Al, 3 h 4.93 146 12.6 A2,3 h 4.05 94 38.6 A2,2 h 4.61 94 38.6 A2,l h 5.71 104 32.0 A2,1/2 h 6.05 99 35.3 CA1,3 h 4.25 115 14.8 CA2,3 h 1.00 90 13.5 CA2,2 h 1.26 90 13.5 CA2,l h 1.39 86 17.3 CA2, 112 h 2.01 C1,3 h 92 93.2 125 84.2 C2,3 h C2,2 h 84 89.4 C2,l h 96 87.9 C2, 112 h 95 88.0 sulfur spikedb A2 3.78 103 32.7 c2 96 87.9 vanadium spikedb A2 3.76 109 28.8 c2 123 84.5
I
Thmnsl hydmpmccuing
Unnoiked. unomccsscd ens oil
a Based on the support only, i.e., A2 and CA2 adjusted for metal sulfides present and carbon-covered alumina catalysts adjusted for the original 11.28 w t % carbon present. bThese experiments were of 3-h duration.
Hydropraessed 3 hours with A2
and released and another type which is unreactive and undergoes condensation and polymerization reactions to form unreactive coke on the catalyst surface thereby deactivating the catalyst. The idea that two types of coke can be present on the catalyst surface has previously been suggested by Ternan et alS4This would provide an explanation for our results where, over time, the amount of unreactive coke increases on the catalyst surface reaching an equilibrium value and deactivating the metal centers which suppresses the adsorption and hydrogenation of "reactive" carbon species. Also, it is clear that a carbonaceous support material reduces the propensity for coke depositing on the catalyst surface. However, the reduction in wt % carbon for CA1 vs A1 is not as dramatic as for CA2 vs A2. This suggests that the metal phase itself may be suppressing coke deposition by promoting hydrogenation reactions when it is associated with carbon. This is not surprising since a carbon surface lacks surface acidity (even when Ni-Mo are deposited) and therefore has a very low affiity for the basic quinoline molecules unlike an alumina surface, especially when loaded with metals. This is also shown when looking at the A2 catalyst for all three spiked feeds. The wt % carbon deposited is less as is the percentage decrease in surface area for the thiophene- and vanadium-spiked feeds. This supports the conclusion that basic nitrogen compounds have an affinity for acidic catalyst surfaces and undergo coking reactions thereby deactivating metal centers and reducing the surface area. The surface areas of the spent catalysts decrease to a greater extent for the alumina supported catalysts than the carbon-covered alumina catalysts. This is most likely a function of the amount of carbon deposited. Also, there is no significant change in surface area over time. What is interesting is when we compare A1 to A2. The blank alumina support shows a much smaller percentage decrease in surface area compared to when metals are loaded even though the wt % carbon deposited is not significantly different. This suggests that coking by quinoline is selective when metals are present but random in the absence of metals. It is reported that an alumina support loaded
t Hydroprocessed 3 hours with C2
Hydroprocessed 1/2 hour with C2
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,
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Figure 4. (a, top) Quinoline-spiked gas oil hydroprocessingNMR studies. (b, bottom) Quinoline-spikedgas oil hydroprocessing NMR studies.
posited carbon on the spent catalysts and reduction in surface area give an indication of the catalyst's susceptibility to deactivation by coking. Looking at the wt % carbon deposited on alumina and carbon-covered alumina catalysts, an interesting phenomenon is noticed. There appears to be more coke deposited on these catalysts initially (after 1/2 h) and the amount of coke deposited actually decreases with time. The spent catalysta are thoroughly washed and dried prior to analysis. Thus, it is unlikely that there would be any residual gas oil remaining on the catalyst surface. Instead, it is more likely that two types of carbon are depositing on the catalyst surface initially-one type of "reactive" carbon species which can be hydrogenated by the metal centers
,
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1
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,
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,
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Hydroprocessing Study of Ni-Mo Catalysts
Energy & Fuels, Vol. 6, No. 3, 1992 307
with metal sulfides such as MoSz develops Bronsted and Lewis acidity at high temperatures (4000C).'8 It appears then that quinoline selectively seeks out these acidic centers which are located within the pore structure of alumina and subsequent coking plugs these pores and reduces the surface area In the absence of Ni-Mo, quinoline randomly cokes the alumina surface which does not degrade the pore structure to as great an extent and consequently results in a smaller percentage decrease in surface area. This is supported by our previous resultas using unspiked gas oil which showed a much smaller difference in percentage decrease in surface area between A1 and A2 suggesting that it is the additional quinoline in the feed that is responsible for the dramatic percentage decrease in surface area. This may also be the case when carbon-covered alumina is used as the support. Although the addition of metals (CA2) does not significantly change the percentage decrease in surface area of the spent catalysts, there is less coke deposited on the catalyst surface and this probably accounts for the apparently small percentage decrease in surface area. The surface area decrease of the carbon supported catalysts is much more drastic (>84% reduction in surface area). Although the amount of coke deposited on carbon supported catalysts may be no more and probably lea than for alumina supported catalysts, this result questions the utility of much of the surface area of carbon which is in very small pores and consequently eliminated, either by physical collapse of the pores under the rigour of our experiment or by coking reactions. This drastic reduction in surface area does not appear to have a severe detrimental effect in the hydroprocessing of gas oil and, in fact, carbon appears to be the superior support when the feed is concentrated with basic nitrogen compounds.
thermal hydrogenation of the gas oil. 2. Quinoline, thiophene, and vanadium all suppress the hydrogen uptake for gas oil hydroprocessing with Ni-Mo catalysts supported on alumina and carbon-covered alumina but Ni-Mo supported on carbon is insensitive to these compounds. Based on the % aromatic reduction, the deactivating effect on a Ni-Mo/ y-alumina catalyst is in the order vanadium > thiophene > quinoline. 3. Quinoline HDN and thiophene HDS do occur thermally to a small extent but a catalyst (supported metal sulfides) is required for efficient heteroatom removal. Under the conditions used for our experiments, quinoline is rapidly hydrogenated to 1-tetrahydroquinoline and subsequent C-N bond hydrogenolysis is the rate-determining step. For the quinoline-spiked gas oil feed, the carbon supported catalyst is comparable to the alumina supported catalyst for sulfur removal but is superior for quinoline HDN. 4. Basic nitrogen compounds, such as quinoline, have an affmity for acidic catalyst surfaces and undergo coking reactions thereby deactivating metal centers and reducing the surface area. It was found for the alumina supported Ni-Mo catalyst that quinoline selectivelycokes the catalyst surface, seeking out the metal centers within the pore structure and plugging these pores. The use of carboncovered alumina as a support reduces the amount of coke deposited and the concomitant reduction in surface area. The carbon supported catalysts show a drastic reduction in surface area after hydroprocessing which questions the utility of the surface area in the micropores of the support. However, this drastic reduction in surface area does not have a severe detrimental effect on gas oil hydroprocessing and, in fact, carbon appears to be the superior support when a high nitrogen content feed is used.
Conclusions 1. Spiking the gas oil feed with quinoline or a vanadium compound does not inhibit thermal hydrogenation from occurring. However, the addition of thiophene retards
Acknowledgment. This work was funded by the Alberta Oil Sands Technology and Research Authority (AOSTRA). We also thank Dr. R. A. Kydd and J. M. Lewis for helpful discussions during the preparation of this paper. R&tW NO.I, 91-22-5; II, 110-02-1;V, 7440-62-2;C,7440-444 A1203, 1344-28-1; Ni,7440-02-0;Mo,7439-98-7.
(18)Topsoe, N. Y.; Topsoe, H.; Maasoth, F. E.J.Catal. 1989,119,252.
