Production of C 4 Hydrocarbons from Modified Fischer−Tropsch Synthesis over Co−Ni−ZrO 2 /Sulfated-ZrO 2 Hybrid Catalysts

Production of C4 Hydrocarbons from ModifiedFischer-Tropsch Synthesis over Co-Ni-ZrO2/

Sulfated-ZrO2 Hybrid Catalysts

Raphael O. Idem*

Industrial/Petroleum Systems Engineering, University of Regina, 3737 Wascana Parkway,Regina, Saskatchewan, Canada S4S 0A2

Sai P. R. Katikaneni

Advanced Technology Group, Fuel Cell Energy Inc., 3 Great Pasture Road,Danbury, Connecticut 06813

Ramakrishnan Sethuraman and Narendra N. Bakhshi

Catalysis and Chemical Reaction Engineering Laboratory, Department of ChemicalEngineering, 110 Science Place, University of Saskatchewan,

Saskatoon, Saskatchewan, Canada S7N 5C9

Received March 15, 2000. Revised Manuscript Received June 8, 2000

Fischer-Tropsch synthesis was carried out at atmospheric pressure in a fixed-bed microreactorat temperatures and weight hourly space velocities (WHSV) ranging from 513 to 533 K and 5 to25 h-1, respectively, over hybrid catalysts (physical mixtures) containing Co-Ni-ZrO2 andsulfated-ZrO2 catalysts. The sulfated-ZrO2/Co-Ni-ZrO2 catalyst weight ratios (SZ/CN) rangedfrom 0 to 3, whereas sulfate concentrations in sulfated-ZrO2 catalyst (sulfate loading) rangedfrom 5 to 15 wt %. Fischer-Tropsch synthesis over Co-Ni-ZrO2 catalyst alone produced amaximum C4 hydrocarbon selectivity of 14.6 wt % at a temperature of 523 K and WHSV of 15h-1. There was an impressive increase in C4 hydrocarbons selectivity to a maximum of 32.4 wt% when catalyst HB5,1 (SZ/CN of 1 and sulfate loading of 5 wt %) was used. This catalyst alsogave an extremely high selectivity for isobutane (maximum of 10.6 wt % of total hydrocarbonproducts) as compared to 0.1 wt % obtained with Co-Ni-ZrO2 catalyst. A time-on-stream studyon catalyst HB5,1 showed a decrease in activity of this catalyst with reaction time. In contrast,the use of hybrid catalyst HB5,0.5 (SZ/CN of 0.5 and sulfur loading of 5) where the overall sulfurcontent was low resulted in almost no deactivation. However, the activity obtained in the case ofcatalyst HB5,0.5 was lower than that obtained for catalyst HB5,1 but was much higher thanthat for Co-Ni-ZrO2 catalyst. On the other hand, for hybrid catalysts HB5,2 and HB15,1,whichhad high overall concentrations of sulfur, there was no activity at all. The results show thatinteractions brought about by close proximity of Fischer-Tropsch catalyst active sites and acidsites produce favorable effects when the overall sulfur content in the hybrid catalyst is low.

Introduction

Isobutane, isobutylene, and n-butane are highly de-sirable hydrocarbons. Isobutylene is used in the produc-tion of methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) which are used as oxygenateadditives in producing reformulated gasoline. This typeof gasoline is regarded as the fuel most likely to meetthe stringent requirements of the U.S.A. Clean Air Actof 1990.1,2 On the other hand, n-butane is widely usedfor the production of acetic acid, maleic anhydride, andbutanediol which are important feedstocks for themanufacture of resins and other fine chemicals. All the

C4 hydrocarbons (i.e., i-butane, n-butane, i-butylene, 1-and 2-butenes) are used for alkylation processes toproduce alkylate gasoline which has a high octanerating.

Traditionally, C4 hydrocarbons are obtained frompetroleum sources such as natural gas and steamcracking of naphtha and gas oil.3 However, consideringthe large need for C4 hydrocarbons, it is worthwhile toinvestigate alternative sources of these hydrocarbonsother than the conventional petroleum sources. As iswell-known, a variety of hydrocarbons can be producedfrom synthesis gas using the Fischer-Tropsch (FT)chemistry.4-15 Synthesis gas (syngas) is generally pro-

* Author to whom all correspondence should be addressed. Fax:(306) 585-4855. E-mail: [email protected].

(1) Parkinson, G. Chem. Eng. 1992, April, 35.(2) Unzelman, G. H. Oil Gas J. 1990, April, 91.

(3) Gary, J. H.; Handwerk, G. E. Petroleum Refining Technology andEconomics; Marcel Dekker: New York, 1994.

(4) Anderson, R. B. The Fischer-Tropsch Synthesis; AcademicPress: Orlando, 1984.

1072 Energy & Fuels 2000, 14, 1072-1082

10.1021/ef000052+ CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 07/22/2000

duced from steam reforming of methane or naphtha. Inaddition to these feedstocks, the potential also exists ofproducing syngas from renewable biomass materialssuch as crop residues and biomass derived chars. Thus,by utilization of biomass or waste biomass-type materi-als for syngas production, Fischer-Tropsch processesmay have an environmental impact as well.

Generally, catalysts for Fischer-Tropsch synthesisconsist of metals such as cobalt, nickel, iron, andruthenium impregnated on supports ranging from metaloxides to zeolites.16,17 As is well-known,18 FT synthesislacks specificity for a particular hydrocarbon type orchain length. However, literature16-28 indicates thatspecificity may be improved by using a second catalystin conjunction with the FT catalyst.

In our earlier work,29 we employed such an approachto improve C4 hydrocarbon specificity (or selectivity) byusing a follow bed reactor setup. The Fischer-Tropschsynthesis catalyst used in the study (placed in the firstcatalyst bed) was Co-Ni-ZrO2 whereas the modifiercatalysts (placed in the second catalyst bed) was sulfated-ZrO2 solid acid catalyst. The setup was such that theproduct from the first catalyst bed flowed over thesecond catalyst bed. It was observed that this setupresulted in a fairly high selectivity for C4 hydrocarbons.However, in addition to the follow bed reactor scheme,it appears30 there is the possibility of improving C4hydrocarbon selectivity from modified FT process throughthe use of a mixed (FT + solid acid) catalysts bed. Themajor reason is that both the FT and acid sites areavailable side by side. Thus, the scheme provides thepossibility of very close contact of products of the FTreactions to immediately react over the acid sites. It istherefore possible to effect reactions of hydrocarbons

produced from FT synthesis thus leading to improve-ment in C4 hydrocarbon selectivity.

In this work, we studied improvement in selectivityfor C4 hydrocarbons using a physical mixture consistingof Co-Ni-ZrO2 catalyst and sulfated-ZrO2 solid acidcatalyst (hybrid catalyst) for modified FT synthesis atatmospheric pressure at temperatures and space veloci-ties in the range 513-533 K and 5-25 h-1, respectively,as a function of sulfated-ZrO2/Co-Ni-ZrO2 catalystweight ratio (SZ/CN) and sulfate loading. All thecatalysts were characterized thoroughly in order toobtain an understanding of the relationship betweencatalyst characteristics and catalyst performance.

Experimental Section

Preparation of Catalysts. Co-Ni-ZrO2 Catalyst. TheCo-Ni-ZrO2 catalyst was prepared by first preparing a driedNi-Zr coprecipitate and then incorporating Co in the copre-cipitate. Ni-Zr coprecipitate was prepared by coprecipitationtechniques involving dropwise addition of a 400 mL aqueoussolution containing zirconium dinitrate and nickel nitrate (92g of zirconium dinitrate oxide (ZrO(NO3)2.xH2O, obtained fromAlpha Products, Denver, CO) and 12.5 g of nickel nitrate (Ni-(NO3)2‚6H2O, obtained from BDH Chemicals, Poole, England)in 1000 mL of distilled water) to a continuously stirred 300mL of 2.78 mol/L aqueous sodium hydroxide solution. Anairtight container was used for coprecipitation to preventdiffusion of atmospheric carbon dioxide into the system duringcoprecipitation. We found that if carbon dioxide were present,it would react with sodium hydroxide to form a carbonatewhich we also found to lead to production of a poor qualitycatalyst. Thus, prior to and during coprecipitation, the con-tainer was purged with nitrogen gas to ensure the system wascarbon dioxide free. After the precipitation step, the Ni-Zrcoprecipitate was filtered and washed several times withdistilled water until the filtrate was neutral, then twice withacetone, and finally dried in an oven at 333 K for 24 h.

The dried coprecipitate was ground and sieved into particlesizes ranging from 15 to 74 µm before impregnating cobalt byadding 15 mL of 1.14 mol/L aqueous cobalt nitrate (Co(NO3)2‚6H2O, obtained from BDH, Poole, England) solution to all ofthe dried and sized Ni-Zr coprecipitate obtained from theprevious coprecipitation step. The impregnated catalyst wasdried at 333 K for 24 h and then calcined at 773 K for 16 h.The Co-Ni-ZrO2 catalyst was designated as CN.

Earlier characterization29 showed that this catalyst had acomposition of 5.6 and 3.8 wt % Co and Ni, respectively. Also,the BET surface area was 188 m2/g (see Table 1), while thepore volume and average pore size were 0.35 mL/g and 7 nm,

(5) Pichler, H.; Schulz, H.; Kuhne, D. Brennst-Chem. 1968, 49, 344.(6) King, D. L.; Cusumano, J. A.; Garten, R. L. Catal. Rev.sSci. Eng.

1981, 23, 233.(7) Dry, M. E. In CatalysissScience and Technology; Anderson, J.

R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 1, Chapter4, pp 159-255.

(8) Anderson, R. B. In Catalysis; Emmett, P. H., Ed.; Reinhold: NewYork, 1956; Vol. 4.

(9) Shaw, Y. T.; Perotta, A. J. Ind. Eng. Chem. Prod. Res. Div. 1976,15, 123.

(10) Vannice, M. A. Catal. Res.sSci. Eng. 1976, 14, 153.(11) Denny, P. J.; Whan, D. A. In Catalysis; The Chemical Society:

London, 1978; Vol. 2.(12) Bartholomew, C. H.; Pannel, R. B. J. Catal. 1980, 65, 390.(13) Holm, V. C. F.; Bacles, G. C. U.S. Patent 3,032,599, 1962.(14) Hini, M.; Kobayashi, S.; Arata, K. AIChE J. 1979, 101, 6439.(15) Rostrup-Nielson, J. R. Catal. Today 1994, 21, 305-324.(16) Bruce, L. A.; Hope, J. G.; Mathews, J. F. Appl. Catal. 1983, 8,

349.(17) Adesina, A. A. Appl. Catal. A: General 1996, 138, 345-367.(18) Bruce, L. A.; Mathews, J. F. Appl. Catal. 1982, 4, 353-370.(19) Chang, C. D.; Lang, W. H.; Silvestri, A. J. J. Catal. 1979, 56,

268.(20) Snell, R. Catal. Rev.sSci. Eng. 1987, 29, 361.(21) Bruce, L. A.; Hope, G. J.; Mathews, J. F. Appl. Catal. 1984, 9,

351.(22) Varma, R. L.; Bakhshi, N. N.; Mathews, J. F.; Ng, S. H. Can.

J. Chem. Eng. 1985, 63, 612.(23) Haag, W. O.; Huang, T. J. U.S. Patent 4,279,830, 1981.(24) Guo, C.; Liao, S.; Qian, Z.; Tanabe, K. Appl. Catal. A: General

1994, 107, 239-248.(25) Guo, C.; Yao, S.; Cao, J.; Qian, Z. Appl. Catal. A: General 1994,

107, 229-238.(26) Comelli, R. A.; Canavese, S. A.; Vaudagna, S. R.; Figoli, N. S.

J. Catal. 1996, 135, 287-299.(27) Song, X.; Sayari, A. Appl. Catal. A: General 1994, 110, 121-

136.(28) Song, X.; Sayari, A. Catal. Rev.sSci. Eng. 1996, 38, 329.(29) Sethuraman, R; Katikaneni, S. P. R.; Idem, R. O.; Bakhshi, N.

N. Fuel. Proc. Technol., submitted.(30) Song, X; Sayari, A. Energy Fuels 1996, 10, 561-565.

Table 1. BET Surface Areas of Sulfated-ZrO2,Co-Ni-ZrO2, and Hybrid Catalystsa

catalystidentity

sulfated-ZrO2/Co-Ni-ZrO2

wt ratio

sulfate loading onsulfated-ZrO2

catalyst, wt %BET surfacearea, m2/g

SZR5 N/A 5 64SZR7 N/A 7 63SZR10 N/A 10 55SZR12 N/A 12 58SZR15 N/A 15 89CN N/A N/A 188HB5,1 1 5 119HB5,0.5 0.5 5 135HB5,2 2 5 105HB7,1 1 7 116HB10,1 1 10 120HB12,1 1 12 122HB15,1 1 15 136

a CN ) unmodified Co-Ni-ZrO2 catalyst; N/A ) not applicable.

C4 Hydrocarbons from Modified Fischer-Tropsch Synthesis Energy & Fuels, Vol. 14, No. 5, 2000 1073

respectively. Furthermore, hydrogen and carbon monoxidechemisorption tests, and X-ray line broadening measurementsfor Co-Ni-ZrO2 catalyst reduced with hydrogen at 673 K29

showed that the average crystallites size of Ni species was verysmall (<3 nm) whereas that for Co was 1.36 nm. Thus, thedispersion of Ni on ZrO2 was high while that of Co was low(25.1%) thus giving a moderate overall joint dispersion of Niand Co on ZrO2.

Sulfated-ZrO2. Altogether, six sulfated-ZrO2 catalysts ofsulfate concentrations in the range 0-15 wt % were preparedaccording to the procedure described by Song and Sayari.27

This procedure involved preparation of the ZrO2 supportfollowed by incorporation of sulfate ions in the support. Thisincorporation was followed by drying at 383 K for 24 h, andthen by calcination of the dried catalysts at 923 K for 4 h.

The sulfate concentrations in the calcined sulfated-ZrO2

catalysts were: 0, 5, 7, 10, 12, and 15 wt %. Sulfated-ZrO2

catalysts with the above sulfate concentrations were desig-nated as SZR0, SZR5, SZR7, SZR10, SZR12, and SZR15,respectively. The BET surface areas of these catalysts weredetermined from our earlier studies.29 Those for catalystsSZR5, SZR7, SZR10, and SZR12 were more or less the sameat ≈55-64 m2/g whereas in the case of catalyst SZR15 therewas a sudden increase in BET surface area to 89 m2/g (seeTable 1).

Hybrid Catalysts. The hybrid catalysts were obtained bythoroughly mixing each of the sulfated-ZrO2 catalysts withCo-Ni-ZrO2 catalyst. The particle size of these catalystsranged between 15 and 74 µm. The sulfated-ZrO2/Co-Ni-ZrO2

catalyst weight ratios of the hybrid catalysts (SZ/CN) were 0,0.5, 1, 2, and 3. These hybrid catalysts were designated asHBc,r where c represents the sulfate concentration in thesulfated-ZrO2 catalyst (sulfate loading) and r represents theSZ/CN catalyst weight ratio.

Characterization of Catalysts. The hybrid catalysts(HBc,r) were thoroughly characterized in order to determineboth their physical and chemical characteristics.

Physical Property Characterization. Physical properties suchas BET surface area, pore volume, pore size, and pore sizedistribution were measured using a Micromeritics adsorptionequipment (model ASAP 2000) with N2 (99.995% purity;obtained from Praxair, Calgary, Canada) as the analysis gas.Prior to BET measurements, each catalyst was evacuated at573 K at a vacuum of 0.54 kPa for 10 h.

Chemical Property Characterization. The chemical propertycharacterization techniques were employed principally forevaluating the acidity, reducibility, and metal-support inter-actions in the hybrid catalysts as a function of both sulfateconcentration and SZ/CN weight ratio.

Acidity. The nature of acid sites and the acid site distribu-tion in the hybrid catalyst were determined using the FTIRtechnique. IR measurements were made on powdered catalystsamples using a Biorad Infrared Spectrometer (model FTS 40,Digilab Division). Only the pyridine region (1400-1759 cm-1)of the IR spectra was explored since the region can providethe necessary information regarding the nature and distribu-tion of acid sites.36 Analysis in this region requires the use ofpyridine chemisorbed catalyst samples and these were ob-tained by passing pyridine vapor over the catalysts at 373 Kfor 45 min.29,33 After pyridine adsorption, each sample was

allowed to cool to room temperature and subsequently usedfor IR analysis.

Reducibility and Metal-Support Interactions. Reducibilityas well as interactions between either Co or Ni or both andZrO2 in the hybrid catalysts were determined using thetemperature-programmed reduction (TPR) technique. TPRwas performed on the hybrid catalysts as well as on CoO andNiO (both obtained, respectively, by decomposition of cobaltnitrate and nickel nitrate by calcination at 573 K for 4 h) atatmospheric pressure in a conventional flow system on thebasis of the principle outlined by Jenkins et al.37 The equip-ment and procedure used were similar to the ones describedearlier by Idem and Bakhshi.35,38

Experimental Setup and Run Procedure. Fischer-Tropsch synthesis experiments were carried out in a fixed-bed stainless steel (316 SS) microreactor of 4 mm i.d. and 230mm overall length at atmospheric pressure at temperaturesand CO weight hourly space velocities (WHSV) ranging from513 to 533 K and from 5 to 25 h-1, respectively. The reactorwas placed in a furnace whose temperature was controlled bya Thermo Electric temperature controller (model SWT-100018,supplied by Thermo Electric, NJ) using a thermocoupleinserted into a heating block placed concentrically between thefurnace and reactor. A separate thermocouple (located in thecatalyst bed) was used to monitor the temperature of thecatalyst bed. This was displayed on a multi-point temperatureindicator. The feed was a gas mixture containing hydrogen,carbon monoxide, and argon in the ratio 2:2:1. Argon servedboth as a diluent and as an internal standard for productanalysis. The experimental system was equipped with apreheater whose temperature was monitored using a K-typethermocouple connected to the multi-point temperature indica-tor. Also, the product from the reactor passed directly to thegas chromatograph (GC) for product analysis and then vented.

The tubing connecting the reactor to the GC was heatedusing electrical heating tapes to ca. 493 K so as to maintainthe product stream in the vapor phase and to prevent productfractionation during GC analysis. The flow of the feed gas wasmonitored using a Matheson mass flow meter (model 8141,Matheson, East Rutherford, NJ) which was calibrated usingthe feed gas itself.

Typical Experimental Procedure. Runs were performed overunmodified Co-Ni-ZrO2 catalyst (i.e., HBc,0) as well as overthe hybrid catalysts.In each case, a typical experimental runwas carried out as follows: The reactor was loaded with 0.1 gof the Co-Ni-ZrO2 catalyst mixed with the amount ofsulfated-ZrO2 catalyst (of desired sulfate concentration) re-quired to obtain the desired catalyst weight ratio. The catalystwas reduced in hydrogen at 673 K for 16 h. After reduction,nitrogen gas was allowed to flow through the reactor systemfor 30 min during which time the catalyst bed was allowed tocool to a temperature slightly lower than the desired temper-ature. The feed gas was then introduced into the reactorsystem.

Fischer-Tropsch synthesis is a net exothermic reaction andconsequently, there was a net increase in reactor temperatureupon introduction of the feed. Thus, cooling the catalyst bedto a temperature lower than the desired temperature was donein order to employ the heat of reaction in heating andstabilizing the reactor at the desired temperature. For ex-ample, to make a test run at 523 K, the reactor was initiallycooled to ca. 518 K and then allowed to stabilize at 523 K.This stabilization procedure took about 5 min and the reactorremained isothermal after then. Product sampling and analy-ses were performed at intervals of 1 h after the introductionof feed into the reactor. However, the first sample for each set

(31) Das, D.; Chakrabarty, D. K. Energy Fuel 1998, 12, 109-114.(32) Parvulescu, V.; Coman, S.; Grange, P.; Parvulescu, V. I. Appl.

Catal. 1999, 176, 27-43.(33) Sethuraman, R. M.Sc. Thesis, University of Saskatchewan,

Saskatoon, Canada, 1996.(34) Davis, B. H.; Keogh, R. A.; Srinivasan, R. Catal. Today 1994,

20, 219.(35) Idem, R. O.; Bakhshi, N. N. Ind. Eng. Chem. Res. 1994, 33,

2047-2055.(36) Vedrine, J. C.; Auroux, A.; Bolis, V.; Dejaifve, P.; Naccache, C.;

Wierzchowski, P.; Derouane, E. G.; Nagy, J. R.; Gilson, J. P.; Jan, H.C.; van Hooff, J. P. V. D.; Wolthuizen, J. J. Catal. 1979, 59, 248-262.

(37) Jenkins, J. W.; McNicol, B. D.; Robertson, S. D. Chem. Tech.1977, 7, 316-320.

(38) Idem, R. O.; Bakhshi, N. N. Can. J. Chem. Eng. 1996, 74, 288-300.

1074 Energy & Fuels, Vol. 14, No. 5, 2000 Idem et al.

of reaction conditions was taken after the reactor was stabi-lized at the desired temperature (5 min after introduction offeed).

Product Analysis. The product stream leaving the reactorwas analyzed online using a Hewlett Parkard gas chromato-graph (model 5880) equipped with both a flame ionizationdetector (FID) and thermal conductivity detector (TCD) andtwo sets of columns. These were (i) SP2100 column in serieswith a Chromosorb 102 column attached to the FID, and (ii) aCarbosieve SII column attached to the TCD. The productcomponents in the C1-C16 hydrocarbon range were analyzedusing the SP2100 column in series with a Chromosorb 102column. The Chromosorb column was used specifically toseparate various isomers as well as paraffinic and olefiniccomponents in the C1-C4 hydrocarbon range. On the otherhand, carbon monoxide, argon, carbon dioxide, and methane,which served as a tie component, were analyzed using aCarbosieve SII column.

Results and Discussion

Characteristics of Hybrid Catalysts. PhysicalProperties. The BET surface areas of calcined sulfated-ZrO2/Co-Ni-ZrO2 hybrid catalysts are given in Table1 as a function of sulfate loading and SZ/CN weightratio. The table shows that BET surface area of thehybrid catalysts decreased with an increase in SZ/CNwt ratio. These results show that BET surface areas ofthe hybrid catalysts are essentially a weighted averageof the BET surface area of Co-Ni-ZrO2 catalyst (188m2/g) and that of the applicable sulfated-ZrO2 catalyst(in the range 55-89 m2/g).29 The table also shows that,in general, the BET surface area of the hybrid catalystsincreased with an increase in the sulfate loading.

Chemical Properties. Acidity Characteristics. Thesulfated-ZrO2 component in the hybrid catalysts wasemployed to enhance the acid-catalyzed oligomerization,alkylation, and isomerization characteristics of theoverall reaction. As is well-known, the extents of thesereactions depend on both the type and distribution ofthe acid sites on the catalyst. The source of acid sitesin the hybrid catalysts is sulfated-ZrO2 catalyst whichis reported28-34 to contain both Brønsted and Lewis acidsites. Thus, it is desirable to first know how these

various acid sites are distributed in unmodified sulfated-ZrO2 catalysts as a function of sulfate concentration.Figure 1 shows the FTIR spectra of unmodified pyridineadsorbed sulfated-ZrO2 catalysts. The figure shows thatthe catalysts exhibited five bands at frequencies of ca.1640, 1610, 1545, 1490, and 1445 cm-1. According toParvulescu et al.,32 Rahman et al.,39 and Borade andClearfield,40 the band at the frequency of 1640 and 1545cm-1 represents Brønsted acid sites whereas the one at1490 cm-1 represents the presence of a mixture of bothBrønsted and Lewis acid sites. On the other hand, theabsorption bands at 1445 and 1610 cm-1 represent thepresence of Lewis acid sites. Figure 1 shows that as thesulfate loading increases, the intensity of all the absorp-tion bands also increases thus indicating an increasein the amounts of Brønsted, Lewis, and total acid siteswith sulfate concentration. Also, Figure 1 indicates thatan increase in sulfate concentration in the catalystresults in a slight shift of the absorption bands towardhigher wavenumbers. This implies a slight increase inthe strengths of the various acid sites with an increasein sulfate loading. These results are in perfect agree-ment with those of Parvulescu et al.32

Information regarding the type, strength, and distri-bution of both Brønsted and Lewis acid sites in thehybrid catalysts were also determined using FTIRspectroscopy of pyridine adsorbed samples (frequencyin the range 1425-1775 cm-1).36 Typical IR spectra ofpyridine adsorbed samples showing a comparison be-tween hybrid catalysts (such as HB5,1 and HB15,1) andthe sulfated-ZrO2 catalysts of corresponding sulfateconcentrations (i.e., SZR5 and SZR15) are presented inFigure 2. As in unmodified sulfated-ZrO2 catalysts(Figure 1), Figure 2 shows that all the hybrid catalystsexhibited five bands at frequencies of ca. 1640, 1610,1545, 1490, and 1445 cm-1. Their assignments are thesame as those given previously for sulfated-ZrO2 cata-lyst. Figure 2 shows that as the sulfate loading in thecatalyst increased, the intensity of all the absorption

(39) Rahman, A.; Lemay, G.; Adnot, A.; Kaliaguine, S. J. Catal.1988, 112, 453-463.

(40) Borade, R. B.; Clearfield, A. J. Mol. Catal. 1994, 88, 249-266.

Figure 1. FTIR spectra of sulfated-ZrO2 catalysts of various sulfate loadings. 1, SZR5; 2, SZR7; 3, SZR10; 4, SZR12; 5, SZR15.


bands also increased. This result indicates an increasein the amounts of total, Brønsted and Lewis acid siteswith sulfate loading, as in the case of unmodifiedsulfated-ZrO2 catalysts. Also, Figure 2 shows that anincrease in sulfate loading in the catalyst results in aslight shift of the absorption bands toward higherwavenumbers. This implies a slight increase in thestrengths of the various acid sites with an increase insulfate loading. Again, this is similar to what wasobtained for unmodified sulfated-ZrO2 catalysts.

On the other hand, it is seen in Figure 2 that thereare other changes in the distribution of the Brønstedand Lewis acid sites in the hybrid catalysts as comparedto that in sulfated-ZrO2 catalysts of correspondingsulfate loading. For example, it is seen that the intensi-ties of the absorption bands representing Lewis acidsites (i.e., 1445 and 1610 cm-1) were higher in thehybrid catalysts than in the pure sulfated-ZrO2 catalystsindicating a larger fraction of Lewis acid sites in thehybrid catalysts than in unmodified sulfated-ZrO2 cata-lysts. In contrast, the intensities of the absorption bandsrepresenting Brønsted acid sites (i.e., 1545 and 1640cm-1) were lower in the hybrid catalysts than in thepure sulfated-ZrO2 catalysts indicating a smaller frac-tion of Brønsted acid sites in the hybrid catalysts thanin unmodified sulfated-ZrO2 catalysts. Furthermore,Figure 2 shows that the wavenumbers for correspondingacid sites were more or less the same for unmodifiedsulfated-ZrO2 catalysts as for the hybrid catalysts. Theabove results show that there are no changes in thestrengths of the various types of acid sites whensulfated-ZrO2 catalysts are physically mixed Co-Ni-ZrO2 catalyst. Thus, in terms of acidity, it can beconcluded that the only consequence of physically mix-ing a Co-Ni-ZrO2 catalyst with a sulfated-ZrO2 cata-lyst is that of increasing the fraction of Lewis acid siteswhile decreasing the fraction of Brønsted acid sites.

Reducibility and Metal-ZrO2 Interactions. Figure 3shows the TPR profile of Co-Ni-ZrO2 catalyst (curve3) as well as those for CoO (curve 1) and NiO (curve 2).Curve 3 of Figure 3 shows that there are two reductionmaxima at 643 and 778 K for Co-Ni-ZrO2 catalyst. Thepeak at 643 K represents the reduction of the oxide ofCo while that at 778 K represents the reduction of theoxide of Ni. Figure 3 also indicates that the pure oxidesof Ni (NiO) and Co (CoO) reduce at 618 and 543 K,

respectively. Thus, there is a difference between theTPR peak temperatures of the oxides of Co and Ni, andthose of the corresponding metal oxides in Co-Ni-ZrO2catalyst. The shift to higher temperatures of the TPRpeak temperatures for Co and Ni oxide species in thecase of Co-Ni-ZrO2 catalyst suggests the existence ofinteractions between ZrO2 and Co oxide as well asbetween ZrO2 and Ni oxide. However, these shifts mayalso be due to formation of types of Ni and Co oxides onCo-Ni-ZrO2 catalyst which are different from NiO andCoO, respectively.

Figure 3 also shows typical TPR profiles for the hybridcatalysts. For example, curves 4 and 5 of the figureshows TPR profiles for HB5,1 and HB15,1 hybridcatalysts (SZ/CN weight ratio of unity and respectivelycontaining 5 and 15 wt % sulfate loading). It was shownearlier that the TPR peak temperatures for Co and Nioxide species in Co-Ni-ZrO2 catalyst (Curve 3 of Figure3), were 643 and 778 K, respectively. Curves 4 and 5show that TPR peaks for these species were obtainedat temperatures quite different from those of Co-Ni-ZrO2 catalyst. Also, curves 4 and 5 show enhancementsin the sizes of the higher temperature peaks for thehybrid catalysts. This enhancement was observed toincrease with sulfate concentration in the sulfated-ZrO2component in the hybrid catalyst. Thus, the enhance-

Figure 2. A comparison of FTIR spectra of hybrid catalystswith those of sulfated-ZrO2 catalysts of corresponding sulfateloadings. 1, SZR5; 2, HB5,1; 3, SZR15; 4, HB15,1.

Figure 3. TPR spectra of CoO, NiO, Co-Ni-ZrO2 catalyst,HB5,1 hybrid catalyst, and HB15,1 hybrid catalyst. 1, CoO;2, NiO; 3, Co-Ni-ZrO2 catalyst; 4, HB5,1 hybrid catalyst; 5,HB15,1 hybrid catalyst.


ment in the size of the high-temperature peak can beattributed to sulfate decomposition that has been foundto occur within this temperature range.29 Therefore, theimplication is that the high-temperature peak repre-sents both the reduction of the oxide of Ni in the Co-Ni-ZrO2 catalyst component as well as sulfate decom-position in the sulfated-ZrO2 catalyst component.

Curve 5 of Figure 3 shows that the peak temperatureof the peak ascribed to the combined processes of Nioxide species reduction and sulfate decomposition shiftedto ca. 853 K for catalyst HB15,1 (hybrid catalyst withthe higher sulfate loading) whereas that for HB5,1(hybrid catalyst with the lower sulfate loading) was at778 K. Thus, a comparison of the TPR spectra of thesehybrid catalysts (curves 4 and 5) shows that thestrengths of both the Ni-ZrO2 interaction and sulfate-ZrO2 bond increase with sulfate loading. On the otherhand a comparison of the TPR spectra Co-Ni-ZrO2catalysts and hybrid catalysts HB5,1 and HB15,1(curves 3, 4, and 5 of Figure 3) shows that the TPR peaktemperatures for reduction of the oxides of Co and Niwere lower for HB5,1 hybrid catalyst and higher forHB15,1 hybrid catalyst than for Co-Ni-ZrO2 catalyst.It is important to note that the types of oxides of Coand Ni in Co-Ni-ZrO2 catalyst are, respectively, thesame as those in hybrid catalysts HB5,1 and HB15,1,as these oxides all come from the same Co-Ni-ZrO2catalyst we prepared. This scenario is quite unlike thatin the case of curves 1, 2, and 3 where the types of oxidesof Co and Ni in Co-Ni-ZrO2 catalyst may have been,respectively, different from CoO and NiO. Thus, theobserved shifts in TPR peak temperatures for HB5,1and HB15,1 as compared to those for Co-Ni-ZrO2catalyst are certainly due to changes in the strengthsof both the metal-ZrO2 interactions and sulfate-ZrO2bond and not due to any possible differences in the typesof oxides of Ni and Co. These results show that thepresence of sulfur has an influence on the strengths ofinteraction between the metals and ZrO2 as well asbetween sulfate ions and ZrO2 support. They also showthat a low sulfate concentration results in a decreasein the strengths of Co-ZrO2 and Ni-ZrO2 interactionsas well as sulfate-ZrO2 bond, whereas a high sulfateconcentration produces the opposite effect.

Activation is an important step in the preparation ofany catalyst and, to a large extent, it determines thelevel of performance of the catalyst. Usually, Fischer-Tropsch synthesis catalysts are activated by reductionin a hydrogen atmosphere prior to reaction runs. In thecase of the Co-Ni-ZrO2 catalyst used in the presentwork, this type of activation is intended to result in thepresence of metallic Co and Ni species in the catalystwhich are active for Fischer-Tropsch synthesis.16-18

Thus, it is important to determine whether physicallymixing the CN and sulfated-ZrO2 catalyst (to producethe hybrid catalyst) will have any effect on its reduc-ibility with hydrogen. This was done using the TPRprofiles of the various catalysts (Figure 3).

It is seen that for HB5,1 hybrid catalyst, a completereduction of the Co oxide will occur prior to bothreduction of Ni oxide and sulfate decomposition sincethe Co oxide reduction peak appears at a lower tem-perature and is also completely resolved from the Nioxide peak. On the other hand, complete reduction of

the Co oxide will not occur prior to the start of reductionof Ni oxide and sulfate decomposition for HB15,1 hybridcatalyst since there is no complete resolution of the twopeaks. Also, while Co and Ni oxides reduction peaks forHB5,1 moved to lower peak temperatures, those forHB15,1 moved to higher reduction peak temperaturesboth as compared to Co-Ni-ZrO2 catalyst. Theseresults suggest that both Co and Ni oxides in HB5,1hybrid catalyst were easier to reduce than those in Co-Ni-ZrO2 catalyst, whereas those in HB15,1 hybridcatalyst were more difficult to reduce. These changesin reducibility characteristics imply that, at the reduc-tion temperature of 673 K used in this study for catalystactivation, there is complete reduction of the compoundsof Co and some reduction of those of Ni in catalystHB5,1 (i.e., hybrid catalyst with 5 wt % sulfate loading)resulting in the availability in the catalyst system of alarge amount of active metallic Co species as well assome amount of active metallic Ni species. On the otherhand, there is only a small extent of reduction of thecompounds of Co and potentially no reduction of Nicompounds in catalyst HB15,1 (i.e., hybrid catalyst with15 wt % sulfate loading), thus resulting in the presencein the catalyst of only a small amount of active metallicCo species and a negligible amount of active metallicNi species.

Fischer-Tropsch Synthesis over Sulfated-ZrO2/Co-Ni-ZrO2 Hybrid Catalyst. The performance ofthe catalysts were evaluated in terms of CO conversion(eq 1) as well as the yield (as in eq 2) and selectivity (asin eq 3) for C4 hydrocarbons. However, to obtain anunderstanding of the possible mechanism responsiblefor changes in the selectivity for C4 hydrocarbons,changes in yields and selectivities for carbon dioxide andother hydrocarbons due to changes in catalyst charac-teristics were also evaluated.

where COin ) CO in feed stream (g/min), COout ) COin product stream (g/min)

where Pp ) product of interest (g/min) and COin )carbon monoxide in feed (g/min)

where PI ) hydrocarbon product of interest (g/min) andPTH ) total hydrocarbon product (g/min)

Tables 2 and 3 show CO conversions as well as yieldsfor various products for runs carried out at the indicatedtemperatures and space velocities over the hybridcatalysts as functions of sulfate loading and SZ/CNweight ratio, respectively.

CO Conversion. It is seen in Table 2 that maximumCO conversion was obtained for runs with unmodifiedCo-Ni-ZrO2 catalyst. In the case of hybrid catalysts,CO conversion decreased with an increase in the sulfateloading for a constant SZ/CN weight ratio. CO conver-sion became zero for a sulfate loading g15 wt %. Also,Table 2 shows that for a fixed sulfate loading, CO

(i) CO conversion (X), (wt %) )100 × (COin - COout)/COin (1)

(ii) yield of product (wt %) )100 × (Pp/COin) (2)

(iii) hydrocarbon product selectivity (wt %) )100 × (PI/PTH) (3)


conversion decreased with an increase in the SZ/CNweight ratio and became zero for an increase in the SZ/CN ratio g2. These results can be explained on the basisof the reducibility results obtained and discussed earlierusing TPR results of Figure 3 for hybrid catalysts.

It was shown in these results that one of the effectsof a high sulfate loading (either due to a high SZ/CNweight ratio or a high sulfate loading) was that ofincreasing the strength of interaction between the twocatalyst constituents of the hybrid catalysts (i.e., sulfated-ZrO2 and Co-Ni-ZrO2). Also, it was shown that in-creased interaction results in a decrease in reducibilityof the hybrid catalyst. The implication is that at thetemperature used for catalyst reduction (673 K), theamounts of the compounds of Co and Ni, respectively,reduced to active metallic Co and Ni species decreaseas either SZ/CN weight ratio or sulfate loading in-creases. This decrease in the amounts of metallic Co andNi species is responsible for the observed decrease inCO conversion with an increase in either the SZ/CNweight ratio or the sulfate loading.

Yields of Various Products. Tables 2 and 3 show thatmaximum total hydrocarbon yields were obtained withunmodified Co-Ni-ZrO2 catalyst. Also, it is seen inTable 2 that, in general, the yield of total hydrocarbonproducts decreased with both an increase in sulfateloading and SZ/CN weight ratio. Total hydrocarbon yieldwas zero for catalyst weight ratios g2 and sulfateloading g15 wt %. Furthermore, Table 2 shows thatthere was a maximum in the relationships between theyield of total C4 hydrocarbons and both SZ/CN weightratio and sulfate loading. For example, maximum yieldsof total C4 hydrocarbons of 1.5 and 1.3 wt % wereobtained, respectively, for the hybrid catalyst with 5 and12 wt % sulfate loadings at a SZ/CN weight ratio ofunity. Also, the results show that the overall maximumtotal C4 hydrocarbon yield of 1.5 wt % was obtained fora sulfate loading of 5 wt % at a SZ/CN weight ratio ofunity.

A major reason for the decrease in total hydrocarbonyields with an increase in sulfate loading is the decreasein CO conversion as the sulfate loading increases. Onthe other hand, the exhibition of a maximum in the yieldof total C4 hydrocarbons with either SZ/CN weight ratioor sulfate loading shows the net result of opposing

effects of decreased reducibility and increased acidityin hybrid catalysts as compared to Co-Ni-ZrO2 catalystor sulfated-ZrO2 catalyst, respectively. Changes in theyield of C4 hydrocarbon occur due to changes in theextents of acid-catalyzed hydrocarbon reactions suchas alkylation, oligomerization, isomerization, andcracking,29-32 as shown in eqs 4-12.

where n ) 2, ..., 5, x ) 1, ..., 5,

where n ) 2, ..., 5, x ) 1, ..., 5 and Z represents an acidsite.

It is well-known that the extents of these reactions aredetermined by the amounts and strengths of the acidsites present on the catalyst.27-32,41 Also, it was shownearlier from FTIR results that the fractions of thevarious acid sites increased as either SZ/CN weight ratioor sulfate loading or both increased. Increased acidityresults in an increase in the extents of these hydrocar-bon reactions (eqs 4-11) and should result in a mono-tonic increase in the yield of total C4 hydrocarbons asreported in the literature.27,28 Instead, a maximum isobserved in the relationship between the yield of totalC4 hydrocarbons and either SZ/CN weight ratio (Table3) or sulfate loading in the hybrid catalyst (Table 2).The major reason for the observed maximum is that anincrease in the value of any of these two independentvariables also results in a decrease in reducibility of thehybrid catalyst. As was discussed earlier, decreasedreducibility results in a decrease in CO conversion andconsequently, in the amount of total hydrocarbonsproduced by Fischer-Tropsch reactions in the firstplace. These hydrocarbons obtained by CO conversionare necessary for subsequent acid-catalyzed hydrocar-bon reactions (eqs 4-12) for producing additional C4hydrocarbons.27,28,41 Thus, these results show that thehigh extent of hydrocarbon reactions to produce C4hydrocarbons (due to increased acidity) is not able tocompensate for the small amount of total hydrocarbons

(41) Garwood, W. E. Am. Chem. Soc. Symp. Ser. 1983, 218, 383.

Table 2. CO Conversion and Yields of Various Productsas a Function of Sulfate Loadinga

catalyst

Co-Ni-ZrO2 HB5,1 HB7,1 HB10,1 HB12,1 HB15,1

sulfateloading,wt %

0 5 7 10 12 15

COconversion,wt %

14.6 12.3 12.1 11.9 11.1 0

CO2 yield,wt %

9.3 3.9 4.3 3.3 3.6 0

Yield of Hydrocarbons, wt %total 6.8 4.6 4.4 3.9 4.2 0C1 1.3 0.6 0.6 0.5 0.5 0C2 0.4 0.2 0.2 0.1 0.2 0C3 1.2 0.8 0.7 0.6 0.7 0C4 1.0 1.5 1.3 1.0 1.3 0C5

+ 2.9 1.5 1.7 1.7 1.6 0

a Reactions conducted at 1 atm, at 523 K,and a space velocity(WHSV) of 15 h-1.

2Z + CH4 f Z-H + Z-CH3 (methane activation)(4)

Z-CH3 + C3H6 f Z-C4H9 (propylene alkylation)(5)

Z-C4H9 + Z-H f 2Z + C4H10 (hydrogenation) (6)

2Z + C2H6 f Z-H + Z-C2H5 (ethane activation)(7)

Z-C2H5 + C2H4 f Z-C4H9 (ethylene alkylation)(8)

Z-C4H9 + Z-H f 2Z + C4H10 (hydrogenation) (9)

Z-CnH2n + xCnH2n f Z-Cn(x+1)H2n(x+1)

(oligomerization) (10)

Z-Cn(x+1)H2n(x+1) f Z-CnH2n + xCnH2n (cracking)(11)

n-C4H10 f iso-C4H10 (isomerization) (12)


produced by low CO conversion (due to decreasedreducibility).

Hydrocarbon Product Selectivity. Effect of SulfateLoading. The selectivities for various hydrocarbonproducts are shown in Table 4 as a function of sulfateloading for a SZ/CN weight ratio of unity. The tableshows a decrease in the selectivities for C1, C2, C3, andC4 hydrocarbons as sulfate loading increased from 5 to10 wt %. Further increase in sulfate loading to 12 wt %resulted in an increase in the selectivities of thesehydrocarbons. Beyond 12 wt % sulfate loading (i.e., at15 wt % sulfate loading), the selectivities for C1-C4hydrocarbons were all zero essentially because nohydrocarbons were formed since there was no COconversion. For example, CO conversion was zero forhybrid catalyst HB15,1 with 15 wt % sulfate loading.As was discussed previously, this result for hybridcatalyst HB15,1 is attributed to low reducibility of thecatalyst. In contrast to the results for selectivities of C1-C4 hydrocarbons, there was an increase in the selectivityfor C5

+ hydrocarbons as sulfate loading increased from5 to 10 wt %. Further increase in sulfate loading (forexample to 12 wt % sulfate) resulted in a decrease inC5

+ hydrocarbon selectivity. At a sulfate loading of 15wt %, C5

+ hydrocarbon selectivity was zero. Again, thiswas attributed to zero conversion obtained as a resultof low reducibility of hybrid catalyst with this highsulfate loading.

As was discussed earlier, the extents of acid-catalyzedhydrocarbon reactions (eqs 4-12) increase as theamounts and strengths of the acid sites present on the

catalyst increase. Also, it was shown from FTIR resultsthat the amounts of the various acid sites increased asthe sulfate loading increased. In this set of reactions,eqs 5 and 8 indicate the alkylation of both ethylene andpropylene with methane and ethane, respectively toproduce C4 hydrocarbons. It is important to note thatit is also possible to obtain C5

+ hydrocarbons fromalkylation if, for example, ethane and propylene orpropane and propylene or C4 hydrocarbons are involvedin the alkylation process. These latter types of alkylationprocesses are responsible for the boost in selectivity forC5

+ hydrocarbons. As such, alkylation reactions shouldresult in a decrease in the selectivities for C1-C4hydrocarbons for an increase in sulfate loading. Thisdecrease can be seen in Table 4 which shows a decreasein selectivities for C1-C4 hydrocarbons up to 10 wt %sulfate. This result implies that there is an increase inthe extent of alkylation with increased acidity (i.e., withan increase in sulfate loading). Equation 10 representsan oligomerization reaction. For the reaction systemunder study, this involves mainly ethylene and propy-lene but may also involve C4 olefins. Consequently,oligomerization reaction can also result in the formationof C5

+ hydrocarbons at the expense of C2-C4 olefins, asin the case of alkylation reactions. Table 4 shows adecrease in the selectivity for C2-C4 olefins up to asulfate loading of 10 wt % implying an increase in theextent of oligomerization with increased acidity (i.e.,increase in sulfate loading).

The above results show that both alkylation andoligomerization result in the formation of C5

+ hydro-carbons. Thus, these reactions have to be accompaniedby cracking (eq 11) in order to produce the desired C4hydrocarbons.27,28 As was shown earlier, the extents ofalkylation and oligomerization reactions increased asthe amount and strengths of acid sites increased (i.e.,as the sulfate loading increased). Also, it is well-knownthat the extent of cracking increases as the amount andstrengths of acid sites increase (i.e., as the sulfateloading increases). However, it has been shown41 thatcracking increases with an increase in acid site strengthto a greater extent than both alkylation and oligomer-ization. Thus, beyond a certain acid site strength in thecatalyst, an increase in acid site strength results in moredepletion of C5

+ hydrocarbons by cracking than thereare produced by alkylation and oligomerization.

Thus, the increase in selectivity for C5+ hydrocarbons

and the decrease in selectivities for C1-C4 hydrocarbonsfor an increase in sulfate loading (up to 10 wt % sulfate)

Table 3. CO Conversion and Yields of Various Products as a Function of Sulfated-ZrO2/Co-Ni-ZrO2 Weight Ratioa

catalyst

Co-Ni-ZrO2 HB5,0.5 HB5,1 HB5,2 Co-Ni-ZrO2 HB12,0.5 HB12,1 HB12,2

sulfated-ZrO2/Co-Ni-ZrO2 wt ratio

0 0.5 1 2 0 0.5 1 2

CO conversion, wt % 14.6 12.3 12.3 0 14.6 12.6 11.1 0CO2 yield, wt % 9.3 4.7 3.9 0 9.3 3.9 3.6 0

Yield of Hydrocarbons, wt %total 6.8 4.9 4.6 0 6.8 3.4 4.2 0C1 1.3 0.5 0.6 0 1.3 0.4 0.5 0C2 0.4 0.2 0.2 0 0.4 0.1 0.2 0C3 1.2 0.6 0.8 0 1.2 0.4 0.7 0C4 1.0 1.1 1.5 0 1.0 0.7 1.3 0C5

+ 2.9 2.5 1.5 0 2.9 1.7 1.6 0a Reactions conducted at 1 atm, at 523 K, and a space velocity (WHSV) of 15 h-1.

Table 4. Hydrocarbon Product Selectivity as a Functionof Sulfate Loadinga

catalyst

Co-Ni-ZrO2 HB5,1 HB7,1 HB10,1 HB12,1 HB15,1

sulfateloading,wt %

0 5 7 10 12 15

C1 19.5 12.9 12.8 12.1 12.2 0C2 6.0 4.2 4.1 3.7 3.8 0C3 17.4 17.6 15.4 14.3 15.7 0C4 14.6 32.4 28.8 26.1 30.4 0C5

+ 42.5 32.9 38.9 43.8 37.9 0C2-C4 olefins 25.4 28.8 26.1 24.8 26.1 0isobutane 0.1 10.6 6.9 4.3 9.6 0n-butane 6.3 12.4 12.3 11.8 11.4 0isobutylene 5.5 6.2 6.2 6.3 6.1 0other butenes 2.7 3.2 3.4 3.7 3.3 0

a Reactions conducted at 1 atm, at 523 K, and a space velocity(WHSV) of 15 h-1 with catalysts of sulfated-ZrO2/Co-Ni-ZrO2 wtratio of 1.


implies that oligomerization and alkylation occur to agreater extent than cracking. On the other hand, thereverse situation for selectivities of C1-C4 and C5

+

hydrocarbons beyond 10 wt % sulfate loading (i.e., at12 wt % sulfate loading) implies that cracking becomesmore dominant than both alkylation and oligomeriza-tion as the strengths of acid sites increase. These resultscorroborate our earlier characterization results (Figures1 and 2) for unmodified sulfated-ZrO2 solid acid cata-lysts and hybrid catalysts, respectively, which showedthat both the amount and strength of the acid sitesincreased with sulfate loading.

Effect of Sulfated-ZrO2/Ni-Co-ZrO2 Catalyst (SZ/CN) Weight Ratio. Table 5 shows the selectivities forvarious hydrocarbons as a function of SZ/CN weightratio for sulfate loadings of 5 and 12 wt %. The tableshows that the selectivities for C1-C4 hydrocarbonsincreased as the SZ/CN weight ratio was increased from0.5 to 1. In contrast, there was a decrease in C5

+

hydrocarbon selectivity as the SZ/CN weight ratioincreased from 0.5 to 1. A further increase in SZ/CNweight ratio beyond 1 (for example, to 2) resulted in zeroselectivity for all hydrocarbons (i.e., C1-C5

+).These results can be explained on the basis of the

acidity and reducibility characteristics of the catalysts.As was shown earlier, an increase in the SZ/CN weightratio results in an increase in Lewis acid sites, adecrease in Brønsted acid sites, and a decrease in thereducibility of the hybrid catalysts. The literature30,42,43

indicates that the presence of Lewis acid sites promotesalkylation, oligomerization, cracking, and isomerizationreactions, whereas our present work shows that de-creased reducibility inhibits the primary Fischer-Tropsch reaction. An increase in C1-C4 hydrocarbonsand a decrease in C5

+ hydrocarbons with an increasein SZ/CN weight ratio up to 1 (Table 5) implies thatcracking as opposed to alkylation and oligomerizationbecame the dominant reaction as the SZ/CN weightratio increased within the range 0-1 (i.e., as the Lewisacid sites increased). Also, it is seen in Table 5 that thereis an increase in the selectivity of iso-C4 hydrocarbonswith the SZ/CN weight ratio, showing that isomerizationincreases with Lewis acidity. However, beyond a SZ/CN weight ratio of unity, the detrimental effect of low

reducibility becomes very dominant, leading to zero COconversion and consequently to an apparent hydrocar-bon selectivity of zero.

Comparison of Optimum Performance of Vari-ous Reaction Schemes. C4 hydrocarbon selectivitywas used as gauge of performance for various reactionschemes. These were: the optimum hybrid catalyst(HB5,1,), the optimum follow bed configuration (Co-Ni-ZrO2 catalyst bed followed by SZR15 catalyst bed29),and unmodified Co-Ni-ZrO2 catalyst. Table 6 showsthe selectivity for C4 hydrocarbons obtained with theseschemes. The table shows that a maximum C4 hydro-carbons selectivity of 32.4 wt % was obtained for HB5,1.In the case of FT reactions using the optimum followbed configuration and unmodified Co-Ni-ZrO2 catalyst,the maximum C4 hydrocarbons selectivities were 24 and14 wt %, respectively. Thus, the table shows that underidentical operating temperatures and space velocities,hybrid catalyst HB5,1 provided the best scenario formaximum C4 hydrocarbon selectivity as compared tounmodified Co-Ni-ZrO2 catalyst as well as to SZR15and Co-Ni-ZrO2 catalysts in a follow bed scheme.

It was interesting to observe that dramatic changesin the compositions of individual C4 hydrocarbons (i.e.,n-butane, isobutane, isobutylene, and other butenes)occurred if different reaction schemes (i.e., optimumhybrid catalyst in a single bed, optimum follow bedreaction scheme, and unmodified Co-Ni-ZrO2 catalyst

(42) Pinna, F.; Signoretto, M.; Strukul, G.; Morterra, C. Catal. Lett.1994, 75, 1185.

(43) Clearfield, A.; Serrette, G. P. D.; Khazi-Syed, A. H. Catal. Today1994, 20, 295.

Table 5. Hydrocarbon Product Selectivity as a Function of Sulfated-ZrO2/Co-Ni-ZrO2 Weight Ratioa

catalyst

Co-Ni-ZrO2 HB5,0.5 HB5,1 HB5,2 Co-Ni-ZrO2 HB12,0.5 HB12,1 HB12,2

sulfate loading, wt % 0 5 5 5 0 12 12 12sulfated-ZrO2/

Co-Ni-ZrO2 wt ratio0 0.5 1 2 0 0.5 1 2

C1 19.5 9.9 12.9 0 19.5 12.2 12.2 0C2 6.0 3.1 4.2 0 6.0 3.9 3.8 0C3 17.4 12.0 17.6 0 17.4 13.1 15.7 0C4 14.6 22.9 32.4 0 14.6 21.4 30.4 0C5

+ 42.5 52.1 32.9 0 42.5 49.4 37.9 0C2-C4 olefins 25.4 21.2 28.8 0 25.4 23.5 26.1 0isobutane 0.1 3.7 10.6 0 0.1 1.2 9.6 0n-butane 6.3 11.0 12.4 0 6.3 11.2 11.4 0isobutylene 5.5 5.2 6.2 0 5.5 5.7 6.1 0other butenes 2.7 3.0 3.2 0 2.7 3.3 3.3 0

a Reactions conducted at 1 atm, at 523 K, and a space velocity (WHSV) of 15 h-1.

Table 6. C4 Hydrocarbon Selectivity for VariousOptimum Reaction Schemesa

catalyst Co-Ni-ZrO2 HB5,1 Co-Ni-ZrO2 +SZR15

sulfated-ZrO2/Co-Ni-ZrO2wt ratio

0 1 2

sulfate loading,wt %

0 5 15

reactionscheme

unmodifiedFischer-Tropschin a single bed

hybridcatalyst in asingle bed

follow bed

C4 hydrocarbonselectivity,wt %

14.6 32.4 24

isobutaneselectivity,wt % oftotal C4

0.1 32.7 19.1

isobutyleneselectivity,wt % oftotal C4

37.7 19.1 29.6

a Reactions conducted at 1 atm, at 523 K, and a space velocity(WHSV) of 15 h-1.


in a single bed) were used for reaction. For example,the composition of isobutane and isobutylene obtainedfor the run with unmodified CO-Ni-ZrO2 catalyst were0.7 and 38.4 wt % of the total C4 hydrocarbons,respectively. These selectivities changed, respectively,to 19.1 and 29.6 wt % of the total C4 hydrocarbons whenthe optimum follow bed configuration was used and to32.7 and 19.1 wt % of the total C4 hydrocarbons whenthe optimum hybrid catalyst was used. The presence oftotal iso-C4 hydrocarbon product composition of 51.5 wt% for the optimum hybrid catalyst when compared to48.7 wt % for the optimum dual bed configuration andto 38.4 wt % for Co-Ni-ZrO2 catalyst implies thatisomerization occurred to a larger extent in the hybridcatalyst than in the other two cases. However, thepresence of a 32.7 wt % isobutane composition for theoptimum hybrid catalysts as compared to 19.1 and 0.7wt % for dual bed configuration and unmodified, re-spectively, shows that hydrogenation (eq 9) was a moredominant reaction in the case of the hybrid catalystscheme than in the other two schemes.

As indicated in Tables 2-6, the optimum catalyst forthe hybrid scheme was HB5,1 (i.e., catalyst with asulfate loading of 5 wt % and SZ/CN weight ratio of 1).The results for this catalyst shows that there arebenefits if FT active sites (on Co-Ni-ZrO2 catalyst) lieside by side with acid sites (on sulfated-ZrO2 catalyst).However, when the results for HB5,1 catalyst arecompared with those of hybrid catalyst HB15,1 (i.e., asulfate loading of 15 wt % and SZ/CN weight ratio of 1)where there was practically no Fischer-Tropsch activity(and as such zero CO conversion), it is seen that closeproximity between these two types of sites also has adetrimental effect. This detrimental effect of decliningactivity comes about due to a decrease in reducibilityof the hybrid catalyst and an increased tendency forsulfur poisoning of Co and Ni with an increase in sulfurconcentration in the hybrid catalyst. The implicationtherefore is that maximum benefit (in terms of obtaininga high yield or selectivity of C4 hydrocarbons) is obtainedwhen the sulfur concentration in the hybrid catalyst islow either by keeping the sulfate loading e10 or bykeeping SZ/CN weight ratio <1.

Deactivation Behavior. As was mentioned previ-ously, all the reaction data discussed so far wereobtained after a 5 min time on stream (TOS). On theother hand, it is highly desirable to maintain thecatalyst activity for a long period of time if it is to beused for industrial operations. It is known27-32 that theuse of sulfated-ZrO2 catalyst for acid-catalyzed reactionsresults in rapid deactivation of the catalyst which isattributed to loss in acidity, pore plugging, and coking.In the present study, we have attempted an evaluationof the contribution of various acidity influencing factorssuch as reaction scheme, sulfate loading, and SZ/CNweight ratio on catalyst deactivation characteristicsduring Fischer-Tropsch synthesis using catalysts HB5,1,HB5,0.5, HB5,2, HB15,1, and HB0,0 (i.e., unmodifiedCo-Ni-ZrO2 catalyst) as well as the optimum followbed scheme (containing SZR15 and Co-Ni-ZrO2 cata-lyst in a 2:1 wt ratio29). Catalyst stability as a functionof TOS was evaluated in terms of selectivity for produc-tion of total C4 hydrocarbons.

Figure 4 shows the selectivities for total C4 hydro-

carbons obtained for TOS studies for all the catalystsand reaction schemes used. The figure shows that therewas no decrease in the selectivities for total C4 hydro-carbons for unmodified FT catalyst as a function of TOS,indicating that there was no deactivation of the un-modified FT catalyst. On the other hand, Figure 4 showsa decrease in selectivity for total C4 hydrocarbons as afunction of TOS for both HB5,1 hybrid catalyst schemeas well as the optimum follow bed scheme, indicatingthat there was deactivation for these two catalysts. Thefigure shows that in both cases there was a rapid dropin selectivity for total C4 hydrocarbons up to 120 minTOS. After 120 min, the drop in selectivity for total C4hydrocarbons was not so rapid. Also, the figure showsthat the rate of decrease in selectivities for total C4hydrocarbons for HB5,1 hybrid catalyst was essentiallythe same as that for the optimum follow bed reactionscheme for TOS up to 240 min. This result indicatesthe rates of deactivation for the two cases were more orless the same. However, the activity (in terms ofselectivity for total C4 hydrocarbons) for HB5,1 hybridcatalyst was much higher than that for the follow bedscheme throughout the 240 min period. In the case ofthe follow bed scheme, our previous studies29 showedthat loss in activity (i.e., decrease in the selectivity fortotal C4 hydrocarbons) was due to loss of solid acid sites.This in turn was attributed to both pore blocking bycarbon deposition on the surface of sulfated-ZrO2 cata-lysts as well as carbon deposition on the acid sitesthemselves. An additional cause for acid site loss wasthe reduction of sulfate as a result of exposure to ahydrogen atmosphere during catalyst activation (i.e.,reduction with hydrogen) and during reaction. Thesecauses are also applicable in the case of hybrid catalystsbut are certainly modified by changes in reducibility andacidity characteristics of the hybrid catalysts due toclose proximity of Co-Ni-ZrO2 and sulfated-ZrO2 cata-lysts.

On the other hand, in the case of HB5,0.5 hybridcatalyst, even though its activity was lower than thatfor HB5,1 and initially slightly lower than that for the

Figure 4. Variation of C4 hydrocarbon selectivity with time-on-stream for various catalysts and reaction schemes. Num-bers in the legend represent SZ/CN weight ratio; 2.0 (mixedbed) represents data (zero for all TOS) obtained for both HB5,2and HB15,1 hybrid catalysts.


follow bed scheme, there was only a slight decrease inC4 selectivity up to a TOS of 60 min. After this time,the catalyst maintained a stable activity which washigher than that for the follow bed scheme. This impliesthat deactivation is negligible for this hybrid catalyst.In contrast, in the case of hybrid catalysts HB5,2 andHB15,1, there was no Fischer-Tropsch activity at all,and so, no hydrocarbons were formed. As was discussedearlier, this suppression of Fischer-Tropsch activity isattributed to the increased interaction between FT sitesand solid acid sites as either the sulfate loading or thecatalyst weight ratio increased. The results for HB5,0.5,HB5,2, and HB15,1 hybrid catalysts again shows that,in terms of deactivation characteristics, beneficial effectsof close proximity between FT site and acid sites areobtained by selection of an appropriate sulfate loadingor sulfated-ZrO2/Co-Ni-ZrO2 weight ratio such as tokeep the sulfur content in the catalyst low.

In summary, an increase in sulfate loading in sulfated-ZrO2 solid acid catalyst and hybrid catalyst results inan increase in acid site strength and a decrease inreducibility of the hybrid catalyst. Also, there are a hugebenefits that accrue due to close proximity between CNsites and acid sites. These include improvements in C4

hydrocarbon selectivity and deactivation characteristics.These improvements were obtained with hybrid cata-lysts HB5,1 and HB5,0.5., respectively. The maximumC4 hydrocarbons selectivity for Fischer-Tropsch syn-thesis over Co-Ni-ZrO2 catalyst at a temperature of523 K and WHSV of 15 h-1 was 14.6 wt %. Under asimilar temperature and space velocity, a large increaseto a maximum of 32.4 wt % was observed for reactionsover HB5,1 hybrid catalyst (SZ/CN weight ratio of 1 anda 5 wt % sulfate loading). An extremely high selectivityfor isobutane (maximum of 10.6 wt % of total productsand 32.7 wt % of total C4 hydrocarbons) was obtainedwith hybrid catalyst HB5,1 as compared to 0.1 and 0.7wt %, respectively, obtained with Co-Ni-ZrO2 catalyst.Furthermore, the interaction between the CN sites andsolid acid sites in hybrid catalyst HB5,0.1 preventedsulfate reduction during CN reactions, thereby main-taining the acidity level of the catalyst. Deactivation wasalmost completely absent and activity of the HB5,0.5hybrid catalyst was stable up to a maximum TOS of 4h used in the studies.

EF000052+


Documents

Production of C 4 Hydrocarbons from Modified Fischer−Tropsch Synthesis over Co−Ni−ZrO 2 /Sulfated-ZrO 2 Hybrid Catalysts