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Evaluation of a commercial alumina-based bimodalcatalyst (ComCat) and AMAC-based catalysts was carriedout in a pilot plant equipped with a Robinson–Mahoneyreactor, where catalysts were loaded in a 316SSmeshed basketattached to the stirring shaft. Once the reactor was set up, pre-sulfiding of the catalyst was carried out by flowing 1 wt.%CS2in naphtha (150 cm3/min) andH2 (76 l/h) under a total pressureof 30 kg/cm2, and stirring rate of 300 rpm. The reactortemperature was gradually increased in 50 8C steps and heldfor 6 h until reaching 300 8C. After this, the reactor pressurewas increased to 200 kg/cm2and vacuum residue was fed at50 cm3/h instead of naphtha; then, the reactor temperature roseto 330 8Cfor 12 h and theH2 supply ratewas 76 l/h.Afterward,temperature was gradually increased in approximately 20 8Csteps and maintained for 12 h until reaching 415 8C, and after24 h, stationary operations began and samplings were takenevery 24 h. Reaction conditions were fixed as follows: reactiontemperature, 415 8C; pressure, 200 kg/cm2;LHSV,0.5h 1;H2/hydrocarbon, 6000 ft3/barrel; total time-on-run, 600–700 h.2.4. Product analysis.Total sediments in the liquid product were determinedfollowing the ASTM D4870-99 method. Ni and V in the liquidproduct were determined by the ASTM D5863 methodusing atomic absorption on a Perkin-Elmer PE5000 spectro-photometer. Yields to different hydrocarbon fractions weremeasured by simulated distillation in an Agilent 6890N gaschromatograph, following the ASTM D5307 method. Totalsulfur in the liquid product was quantified in an X-ray HoribaSLFA1800 apparatus, following the ASTM4294 method.Total nitrogen was measured in an Antek9000, following theASTM4629 method. Conradson carbon residue (CCR) wasdetermined following the ASTM D189 method.3. Results and discussionA general preparation of the AMAC supports and catalystsused in this study is shown in Fig. 1, where the most distinctiveTable 1Composition and properties of feedstockMaya residue + VGO cut backSulfur (wt.%) 5.68Nitrogen (wppm) 5136Nickel (wppm) 69.1Vanadium (wppm) 345.9Sediment (wt.%) 0.05n-C7 insoluble (wt.%) 15.65CCR (wt.%) 20.78Gravity, 20/4 8C 1.0327API 3Dist. IBP 425.55% 49910% 530.520% 57130% 609.540% 655.550% 719Yield (wt.%)Gasoil 19.76Residue 80.24 and noteworthy aspects are the addition of carbon black and thenear-pyrolysis conditions (6 vol.% O2/N2) in the heat-treat-ment. Numerous attempts to obtain bimodal alumina-carboncomposites were pursued by using different carbon sourcessuch as polyvinyl alcohol and furfuryl alcohol, as well asdifferent peptizing/agglomerating additives, such as HNO3,NH4OH, acetic acid, sucrose, starch, and methyl-cellulose, allin different amounts and combinations. Nevertheless, most ofthe resulting extrudates were not made up of macropores. Thebest results were obtained when combining acetic acid andthe indicated amounts of carbon black. Since an oxidizingatmosphere burns off carbon, all heat-treatments were carriedout in 6 vol.% O2/N2, while carbon-loss at this condition wasneglected. A little O2 aids in removing volatile components(e.g., water) without removing carbon. Accordingly, carbonblacks exhibit a high reactivity towards polymerization underweakly oxidizing conditions; the cross-linking of carbon stacksresults in non-graphitizable hard carbon black [11].Some relevant properties of AMAC supports and catalystsare given in Table 2. AMAC-117 shows no macroporosity, andit was chosen as a comparison for the other AMACs. Notice thatmacroporous AMACs are on average 38% less mechanicallyresistant than the one without macropores (AMAC-117). Asmentioned above, a compromise between macropore formationand mechanical strength is crucial. Macropores are between14 and 22% of the total pore volume, measured by Hg-porosimetry. After Ni and Mo impregnation, AMAC catalystswere obtained, and their physical properties are shown in thelower part of Table 2. A comparison of side crushing strengthvalues between catalysts and supports revealed that somecatalysts (AMAC-117 and -128) increased their mechanicalresistance between 50 and 25%, probably because of metalscontribution; the other AMACs remained practicallyunchanged. A relative number of macropores remained nearthe one belonging to the related support, or rather, between 20and 24% of the total pore volume, suggesting that metalsimpregnation was sufficiently uniform to allow meso andmacropores to remain unplugged. Pore-size distribution ofAMAC catalysts, which is essentially the same as that ofAMAC supports, is shown in Fig. 2. Bimodal characteristics ofAMAC catalysts are evident, with macropore average values ofAMAC supports being 1400, 2600, 3500, and 3670 A ˚ ,depending on the formulation conditions. For clarity, pore-size distribution of AMAC-117 was not included in Fig. 2, but itcontains only mesopores, as indicated in Table 2.An evaluation of AMAC catalysts was carried out selecting aunimodal AMAC-117 and two bimodals, AMAC-128 and -142,in order to examine possible contributions of macropores,and they were all compared with ComCat. Stability of pilotplant operation was good in terms of reaction temperature415 5 8C (see Fig. 3) and average LHSV 0.54 0.02 to0.60 0.04 (see Fig. 4), except for AMAC-128 where LHSV variations were more pronounced. After about 140 h ofoperation, all catalysts showed stable and similar conversionlevels, between 55 and 65%, except for AMAC-128 which wasabout 70%; after 300 h conversion increased at about 80%, andremained stable for the rest of operation. Overall conversionwas unaffected by macropores and carbon black in AMACs(see Fig. 5).The level of sulfur removal or hydrodesulfurization (HDS),however, is greatly improved by macropores, as can beobserved in Fig. 6. For instance, the sulfur removal deactivationrate drops 46% faster in unimodal AMAC-117 than that inbimodal AMAC-128, -142, and ComCat. Moreover, initialsulfur removal levels of AMAC-128 and -142 are 92 and 89%,respectively, probably ascribed to a combination of macroporesand carbon black effects, while ComCat shows 86% HDS.Accordingly, HDS reactions are more favored on NiMo/C incomparison with those on NiMo/Al2O3 catalysts [5,6,12].Hydrodenitrogenation (HDN) was considerably higher inAMAC catalysts in comparison with that in ComCat,particularly in unimodal AMAC-117, as shown in Fig. 7.Thisresult suggests that the removal of nitrogen is more favored inmesopores than in macropores. Moreover, between 500 and700 h of operation, AMAC-117 (no macropores) and AMAC-128 (smallest macroporous AMAC, macropore peak@1450 A ˚ )showed higher HDN levels than ComCat and AMAC-142,