Model planar alumina catalyst preparation and aging under hydrotreating conditions

Applied Catalysis, 50 (1989) 65-77 Elsevier Science Publishers B.V., Amsterdam -

65 Printed in The Netherlands

Model Planar Alumina Catalyst Preparation and Aging Under Hydrotreating Conditions N. NOURBAKHSH

and T.T. TSOTSIS

University of Southern California, Department of Chemical Engineering, University Park, Los Angeles, CA 90089-1211 (U.S.A.) and LA. WEBSTER*“ Unocal Science and Technology Division, Unocal Corporation, P.O. Box 76, Brea, CA 92621 (U.S.A.) (Received 12 September 1988, revised manuscript received 28 December 1988) ABSTRACT Pore blockage and metal deposition on low surface area porous alumina films was monitored at several positions along a hydrotreater which processed heavy Arabian atmospheric residuum. Scanning Electron Microscopy and Energy Dispersive Analytical X-ray techniques were used to study the alumina film’s surface morphology and degree of contamination after the aging process. After a period of two months in the hydzotreater, pore blockage at the reactor entrance was observed to occur within pores of ca. 1000 A diameter. Pores of the same size located further down the bed show no evidence of blockage, however metallic and non-metallic contaminants are still present. The porous alumina films from the second bed of the hydrotreater show no trace of any contaminants. In fact, the porous surface looks remarkably clean as compared with the inlet surface. Porous anodic alumina films are demonstrated to be suitable probes for monitoring catalyst deactivation.

INTRODUCTION

Hydroprocessing catalysts are metal sulfides supported on high surface area aluminas. The aging behavior of catalysts used in coal liquefaction, and heavy oil processing, has been recently reviewed by Thakur and Thomas [ 11.During operation, a catalyst deactivates through carbon and metal deposition. In petroleum residua, the metal contaminants are vanadium, nickel and iron; for coal the major contaminants are iron and titanium. Deposition of carbon (coke) causes loss of catalytic activity by chemical modification of the surface and physical blocking of the pores. Coke deposition in heavy oil processing is believed to start on the external surface of the catalyst extrudate in a fixed bed reactor [ 2-41, and its concentration in the interior of the catalyst increases with process time. aCurrent address: Unocal Co., Corporate Division, 1201 West 5th Street, P.O. Box 7600, Los Angeles, CA 90051, U.S.A. 0166-9834/89/$03.50

0 1989 Elsevier Science Publishers B.V.

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Metal deposition causes further loss of catalytic activity by active site poisoning and eventually pore mouth plugging. In fixed bed reactors, metal deposition decreases from the inlet to the outlet of the bed and the majority of the metals are believed to be present as sulfides on the catalyst support. In order to determine the rate and mechanism of deactivation, it is instructive to characterize the spent catalyst for coke and metal contents, however the tortuous nature of the catalyst support and the presence of other ingredients on commercial catalysts complicate this task. Visual observation of how the pore structure of a hydroprocessing catalyst constricts during operation is lacking. With the objective of visualizing the aging process in a hydroprocessing catalyst at the single pore level, we have made model planar alumina supports and aged them under commercial heavy oil hydrotreating conditions. The model supports exhibit no tortuosity and are ideal for analysis by scanning electron microscopy (SEM), energy dispersive analytical X-ray (EDAX) and X-ray photoelectron spectroscopy (XPS ) . The anodization of aluminum in acidic solutions produces ideal porous structures. This phenomenon has been reviewed by Diggle et al. [ 51, O’Sullivan and Wood [ 61 and more recently, with an emphasis on their possible application to existing catalytic problems, by Cocke et al. [7]. The pores are vertical channels, surrounded by hexagonal cells, which are closed at the lower end by the cell wall material. The expected ideal porous structure is shown in Fig. 1. Various methods of obtaining films of porous alumina with uniform pore diameter from 60-1500 A have been reported [ 631. We have made such model alumina supports and aged them in a catalytic oil hydroprocessor. Catalyst aging behavior was studied in the two beds of a residuum oil hydrotreater to determine coke and metal deposition in the alumina’s macropores. PORE

CENTER

DISTANCE ORE DIAMETER

MICROCRYSTALLINE ALUMINA BRIDGING PORES MlCROCRVSTALLlN ALUMINA LINING PORE

PURE ALUMINA

Fig. 1. Idealized porous type anodic oxide pore structures exhibiting both pore lining and bridging by microcrystalline alumina.

67 Porous planar anodized alumina was chosen over conventional catalyst supports of high surface area because of the anodized alumina’s regular non-tortuous, unimodal porous structure. The porous film’s pore diameter was approximately 1000 A. The porous films were characterized by XPS and their morphology was imaged by SEM. We realize that 1000 A pores are certainly large for use in a commercial catalyst. However, we simply note that we elected to use such large pores because; - SEM-EDAX was analytically possible, - pores of the size lo3 A are in the size range of the macropores in a bimodal heavy oil upgrading catalyst. Conceptually the macropores feed the micropores. EXPERIMENTAL

Alumina preparation and characterization The porous model alumina catalyst supports consisted of thin layers of alumina (about 10 pm thick ) on an aluminum substrate. They were prepared by anodizing pure aluminum sheets, 99.45% (International Foils, Placentia, CA), in a spherical electrochemical cell (Princeton Applied Research, N.J.). The cathode was platinum (Fischer Scientific ). Electrolytes were prepared from analytical grade reagents (Baker) and distilled water. Aluminum samples were anodized at room temperature, potentiostatically, in 0.4 M H,PO, solutions for 60 min at varying voltages (lo-110 V) with good electrolyte mixing. The samples were subsequently gold coated for SEM. SEM was carried out with a Cambridge S4-10 microscope equipped with an EDAX energy spectrometer operating at 20 keV. XPS was performed on a film grown at 90 V to assess chemical composition. The XPS measurements were made at lop7 Pa with a Physical Electronics, Model 548, ESCA/ Auger electron spectrometer equipped with a Mg anode operating at 15 kV and 20 mA. A high resolution scan over 20 eV range was performed of the Al,, region. After the determination of film morphology by SEM and chemical composition by XPS, several films were prepared at 90 V under identical conditions for the hydroprocessing aging study. Aging of anodic aluminas in a hydrotreater The model supports were fixed to each reactor tube’s thermocouple well, at the approximate locations detailed in Fig. 2. The reactor was then operated to upgrade heavy Arabian atmospheric residuum, (11.8 API gravity, 31 ppm nickel, 102 ppm vanadium, 3.9 wt.-% sulfur, 0.3 wt.-% nitrogen and 13 wt.-%

68 HEAVY AAABIAN RESIO

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Fig. 2. Placement of anodically prepared alumina catalysts in the two-bed, bench-scale hydrotreater.

asphaltenes) under typical hydrotreating conditions, over a proprietary Unocal catalyst. The run lasted for approximately 60 days, during which time the temperature was gradually increased, to maintain catalyst activity. Post-run, the catalyst bed was hydrogen stripped and then dumped in sections to locate the alumina samples. The aged samples were subsequently prepared for SEM and EDAX analysis. RESULTS AND DISCUSSION

SEA4 study of the freshy prepared porous film Figs. 3 and 4 are SEM micrographs of the surface of the porous anodic alumina films prepared in 0.4 M H,PO, at varying voltages (10-110 V). Figs. 5a and b were plotted from computations made on the SEMs. Fig. 5a shows the dependence of the average pore diameter on the applied anodizing voltage. Pore diameter correlates linearly with applied anodization voltage with a gradient of 11.3 A/V. Examination of the SEM micrograph of the film grown at 10 V does not show the formation of any pores. Chu and Ruckenstein [ 81 have already reported the observation of pores as small as 60 A. They used transmission electron microscopy (TEM) which does not require

69

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Fig. 3. SEMs of porous alumina anodic films produced potentiostatically from 10 to 110 V (0.4 M H,PO, electrolyte, 25 aC) . Marker bar represents 1 pm.

the sample to be coated. For this reason, the region below 30 V in Fig. 5a is shaded to indicate the uncertainty of observing small pores with the SEM technique. Fig. 5b shows the effect of anodizing voltage on the pore density. For each data point on the graph pores in a 1 ym* area on the micrographs were counted. Pore density is inversely proportional to the voltage. Both of the above observations are in good agreement with the data of O’Sul-

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Fig. 4. SEMs of porous alumina anodic films produced potentiostatically from 10 to 110 V (0.4 M H,PO, electrolyte, 25°C). Marker bar represents 0.1 pm.

livan and Wood [6], who found a value of 12.9 A/V for the gradient of pore diameter vs. voltage graph. The film thickness is also known to be proportional to the anodizing time (O’Sullivan and Wood [ 61) , although no data in support

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Fig. 5. Dependence on the applied anodizing voltage of the average pore diameter (a); and of the pore density (b ) .

of this fact is presented here. Collectively these three measurements give one the predictive capability of tailor making model aluminas of a desired pore diameter and pore length. XPS analysis of the freshly prepared porous film A survey scan of the film formed at 90 V is shown in Fig. 6. A detailed scan of the Al,, peak was obtained. The Alappeak was symmetric. About 6 eV charging was observed for all measured peaks when C1, was set to 285 eV as an internal reference. This indicated that the oxide film is thick relative to the depth probed (30 A). The corrected binding energies for the Alzp (74.6 eV) and 01, (531.9 eV) give a good match for A1203. No evidence for the detection of zero valent aluminum was noted. The survey scan indicates the presence of phosphorus, which must originate from the phosphoric acid electrolyte. A relative surface composition calculated by XPS is Al 34%, 0 62%, P 3% and N 1%. SEM and EDAX analysis of the aged model support SEM and EDAX results of the study of the surface of the anodized aluminum samples, aged and recovered from the bench scale hydrotreater are shown in

72

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Fig. 7. SEM and EDAX results of Run 3IW139, heavy Arabian AR, sample position: Rl, inlet (sample 282-10). (a) Magnification20 000 x (marker bar = 1 pm), (b) magnification 50 000 x (marker bar = 1 pm), (c) EDAX spectrum.

Fig. 8. SEM and EDAX results of Run 3IW139, heavy Arabian AR, sample position: Rl, middle (sample 282-11). (a) Magnification 10 000 X (marker bar = 1 pm), (b) magnification 20 000 x (marker bar = 1 pm), (c) magnification 50 000 x (marker bar = 1 pm), (d) EDAX spectrum.

Figs. 7-10. Each of these figures gives the position of the sample in the reactor (see top left), low and high magnification SEM’s, and an EDAX spectrum over the area depicted in the low magnification SEM. Conceptually we observe the following: (i) Sample 282- 10 (Fig. 7 ) has many surface deposits. This is expected since the sample was located at the reactor train inlet and as such contacts oil having a high contaminant level. However, the porous structure of the alumina is still visible beneath the deposits. It is reasonable to assume that the overlayer in sample 282-10 is primarily carbonaceous. (EDAX does not detect carbon.) (ii) Sample 282-11 (Fig. 8), removed from the mid-section of the first reactor bed (Fig. 1)) shows a much cleaner surface. The 1000-A pores are clearly rriaihln l?‘nAK * IVIP_nIb.YY‘%IL

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major metal deposits. Each of these metal contaminants is in the crude feedstock to the reactor. In contrast to the bed inlet sample 282-10, the aluminum peak is strong. This suggests the existence of a thinner deposit overlayer, which is in agreement with SEM observations. (iii) Sample 282-14 (Fig. 9) removed from the outlet of the first reactor shows vanadium, manganese and iron are still present, though in much smaller quantities, than measured upstream in the reactor. Once again the SEM results indicate a clean surface. (iv) Sample 282-19 (Fig. 10) shows no evidence of any contaminants. The SEM’s show a surface free of any deposits. The axial concentration profiles for the deposited metals vanadium, manganese, iron and nickel each exhibit a maximum. The position of the maximum is located downstream from the reactor catalyst bed inlet. Representative profiles are depicted in Fig. 11. The existence of these “middle bed maxima” suggests that the demetallation reaction mechanism is simply not a single-step reaction of a metallorganic to

Fig. 10. SEM and EDAX results of Run 3IW139, heavy Arabian AR, sample position: R2, outlet (sample 282-19). (a) Magnification 10 000 X (marker bar = 1 pm), (b) magnification 50 000 x (marker bar = 1 pm), (c) magnification 100 000 X (marker bar = 0.1 pm), (d) EDAX spectrum.

OUTLET

REACTOR BED LOCATION

Fig. 11. Axial concentration profiles along the reactor catalyst bed for vanadium, manganese, iron and nickel.

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a deposit, which would be expected to follow first order kinetics. If this were true, we would expect monotonically decreasing concentration profiles from bed inlet to outlet. Rather, the observed profiles are consistent with a sequential HDM mechanism involving reactive intermediates. These mechanisms have already been proposed by Wei’s group [g-111,during experimentation with metalloporphyrins. CONCLUSIONS

We have demonstrated the new use of anodic alumina films in the study of residuum upgrading catalyst deactivation. Porous anodic alumina films can provide insight into the aging phenomena occurring with real catalysts. Anodic alumina catalysts have a non-tortuous porous structure, which simplifies direct observation by SEM. We believe it is possible to observe such phenomena as coking, pore mouth plugging and metal contaminant profiles within the model catalyst. Our major conclusions in using the anodic alumina films in residuum upgrading are: (i) The catalyst experiences the heaviest coking at the reactor inlet. We observed pore blockage in our anodic films of pores an order of magnitude larger than those used in commercial residuum upgrading catalysts. It is logical to expect that the effect of pore blockage would be more significant with the more commonly used smaller pores. /..\ _ (ii ) Downstream from t’ne beci inlet we noted a region with the highest nickel and vanadium deposits. The existence of this region at a location removed from the bed inlet is consistent with a sequential catalytic hydrodemetalation (HDM) mechanism as already discussed by Ware and Wei [ 9,101and Webster and Wei [ 111.We did not oLserve any pore blockage of our 1000-A pores in this region. The surface, according to SEM, was remarkably clean. (iii) Towards the bed outlet, and following the high metal deposition region above, the catalyst surface is clean. The surface has little metal or carbonaceous deposits as measured by SEM/EDAX. We would like to suggest that model anodized alumina films, once impregnated, could be universally employed as model catalysts in aging studies. Their structure and geometry make them ideal for analysis by modern UHV spectroscopic techniques. Microtomed sections of aged films subjected to Transmission Electron Microscopy (TEM) offer exciting possibilities for examining aged samples to obtain a visual picture of deactivation phenomena at a single pore level. The results of these studies will be presented in a future publication

[121. ACKNOWLEDGEMENTS

This work results from a collaborative research effort between Unocal Science and Technology Division and the Chemical Engineering Department at

77

University of Southern California. I.A. Webster thanks Unocal Corporation for the opportunity to participate in this effort. IAW also thanks the following Unocal personnel: D. Kelly, hydrotreater supervision, J. Jaco (SEM/EDAX analytical) and M. Hogelund (artwork). Acknowledgement is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, U.S. Department of TTnora, ~mrltha TlCf-’ T?nn,,lt., T?ooa.a,.oh Tmnn.,nt;r\n T&,-A fnw no&;.,1 o,wmw-.rt nf UIIV,I&y UllU Irllb ““V I ca,ul~y Ib-zU-z’cIIbII lllll”“cL~I”II I’ UIIU l.“I parucu oupp”‘b “I this work.

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