Revista Latinoamericana de Metalurgia y Materiales, Vol. 22, N° 1,2002,47 - 51 47

CHARACTERIZATION OF ULTRA FINE SILICA SUPPORTED IRONCATALYSTS PREPARED BY THEAMMONIA METHOD

A. Loaíza-Gil", P. Rodríguez', W. VelasquezI,D.GÓmezI, B. FontaF, M. Reyes', T. Suárez/

l. Laboratorio de Cinética y Catálisis, Departamento de Química, Facultad de CienciasUniversidad de Los Andes, Mérida 5101, Venezuela.

e-mail: loaizag@ciens.ula.ve2. Laboratorio de Organometálicos, Departamento de Química, Facultad de Ciencias

Universidad de Los Andes, Mérida, 5101 Venezuela.

Abstraet

New ultra fine particle size and nonporous Fe/Si02 catalysts ranging 20-160 nm were prepared with a modified high loadingammonia method. Scanning electron microscopy (SEM) associate to x-ray energy dispersion spectroscopy and nitrogenfisisorption studies of the fresh catalysts dried at 353 K shows a non porous structure constituted by superimposed sheet ofrectangular crystals of Phyllosilicate type. The chemical and structural composition of such solids changes with the thermalpretreatment under hydrogen flow. FT-IR studies performed at programmed reduction temperature suggests that the freshcatalyst is constituted by an iron complex like Fe(H20)t sandwiched between bi-dimensional sheets of Sips phyllosilicate.The amount of -OH groups bonded to the iron complex and those associates to the Si20S polymeric groups decrease progres-sively to disappears by slowly heating at 1 Klmin from 353 to 973 K under hydrogen flow. At the final reduction temperature, anew catalyst structure constituted by silica support finely dispersed iron crystallites arise suggesting a catalyst morphologyand properties governed by the temperature.

Keyboards: Catalyst, Iron, ammonia method, Fischer Tropsch.

Resumen

Nuevos catalizadores no porosos de Fe/Si02 con tamaño de partículas en el orden de 20-160 nm fueron preparados con unamodificación del método del amonio para altas cargas de metal. Los catalizadores frescos, secados a 353 K presentan unaestructura no porosa de cristales en forma de laminas rectangulares sobrepuestas unas a otras similares a los filosilicatos. Lacomposición química y estructural de estos sólidos cambia con el pre-tratamiento térmico en flujo de hidrógeno. Estudios de IR-FT realizados a temperatura de reducciónprogramada sugieren que el catalizador fresco está formado por iones complejos de Fedel tipo Fe(H20)6 +3 atrapados entre laminas bidimensionales de filo silicatos Sips .

La cantidad de grupos -OH enlazados a el complejo de hierro así como aquellos asociados a los grupos poliméricos Si20sdisminuye progresivamente hasta desaparecer cuando se calienta lentamente el catalizador en presencia de hidrógeno desde353 hasta 973 K. A la temperatura de reducción final, el catalizador presenta una nueva estructura constituida por pequeñoscristalitos de hierro finamente dispersados sobre la superficie de la sílice sugiriendo que las propiedades finales del catalizadory su morfología están gobernadas por la temperatura.

1. Introduction

It is well known that transition metal (e.g. Fe, Co, Ni),unsupported or not, can catalyze the carbon monoxidehydrogenation reactions to forming either paraffins, olefinsan~ aIcohols. The activity and selectivity of such catalystsis strongly influenced by its cheinistry and structuralcomposition [1].

Non porous and small metal particle size catalysts seemsto be a desired structural composition for the carbonmonoxide hydrogenation processes because its avoid thepresence of diffusion limitations of the reactants and are

less influenced by the deactivation phenomena [2,3 ]. On theother hand, the iron based catalysts are shipper than cobaltcatalysts and presents an acceptable selectivity for lightolefins, gasoline and gas oil .A preparation method of ironbased catalyst with such characteristic is mostly desirablefor the synthesis of oxygenates or liquids hydrocarbonsfrom the carbon monoxide hydrogenation.

The ammonia method [4] was used recently to preparesilica supported cobalt catalyst with metal particles ofnanometric size and an acceptable metal dispersion [5]. Themethod consists of contacting silica aerosil 200 with a solu-tion ofCo(N03)3.6Hp to which arnmonia solution was added.

48 A. Loaiza-Gil y col. /Revista Latinoamericana de Metalurgia y Materiales

Cobalt complexes of the solution like [Co(HP) 6-0 (NH3) )+2,with n-e 6 reacts with the silica surface [6] forming bi-dimen-sional compounds, probably of the Phyllosilicate type. Thebasic structural feature of such compounds [7] is a compos-ite sheet in which a layer of octahedral coordinated cationsis sandwiched between two identicallayers of linked Si04

tetrahedra of Si.O, composition. Additional hydroxyl ionstogether with the oxygen from the tetrahedral complete theoctahedral coordination of the sandwiched cations. Thespace requirements of Fe3 and Fe2 ion s are similar to thoseof Nit", CO+2and Co+3• Such considerations, open the pos-sibility to prepare silica supported iron catalysts with a simi-lar method. The chemistry involved in the catalyst prepara-tion is expected to be similar to that found for nickel andcobalt.

This work was aimed to study the feasibility of use thearnmonia method to prepare nonporous silica supported ironcatalyst with metal particle size in the nanometric range.The chemistry involved during the preparation and thestructural changes induced by the thermal pretreatmentunder hydrogen flow was investigated by scanning elec-tron microscopy (SEM) associate to x-ray energy disper-sion spectroscopy (EDS), nitrogen physisorption, carbonmonoxide chemisorption and Fourier transformed infrared(FT-IR) at programrned reduction temperature in the 353 to973 K range and a heating rate of 1 KJmin.

2. Materials and Methods

2.1 The catalyst

The silica supported iron catalyst were preparedsubstantially according to the method described by Barbierandcoworkers [5] except for the inert atmosphere that wasdeemed not necessary because the salt precursor type. Sincethe sample ca1cinations were performeé without prevrous ,centrifugation. The preparation of the catalysts was asfollow: 1.6 g ofFeCN0

3)3' 9 HP (100% purity, J.T.Baker) is

added to 16 mL of distilled water at room temperature andstirred by a magnetic rod. The iron hydroxide becomeprecipitated by the addition of some drops of ammoniasolution (25% NH3, Riedel de-Haén). A large excess ofarnmonia solution ( 16-20 mL) is added to dissolve the formerprecipitated. After one hour stirring, 2 g. of silica aerosi1200(specific surface area of 200 m2/g, Degussa) are added tothe solution. The system is dried in an oyen at 353 K for 48hours after others 2 hours stirring. Except when otherwiseindicated, the samples were reduced under hydrogen flow(10 rnL/min) by slowly increasing the temperature at aheating rate of 1KJmin.

The final temperature was of 973 K and the reductiontime was of 55 hours.

2.2 Apparatus and procedures

Micrographs of the reduced and unreduced Fe/Si02catalysts were obtained in a scanning electron microscopeHitachi S-2500. The samples were dispersed by ultrasoundin ethanol before its deposition in an aluminium disc pan.

Nitrogen physisorption measurements of the freshsamples were performed in a standard SorptometerMicromeritics ASAP 2010. The samples, previously dried at353 K were degassed during 4 hours at 625 K and 100 mmHgpressure and then exposed to a 5% nitrogen-helium mixtureat 77 K. The carbon monoxide chemisorption of the reducedsamples was carried out in the same sorptometer using thechemisorption mode. The experiments were performed at 308K and a pressure range of 100-400 rnrnHg using the carbonmonoxide isotherm of silica aerosil200 as reference.

The infrared spectra of the samples obtained for eachstep of the reduction temperature in the 353-973 K rangewere obtained on KBr in a Perkin-Elmer 1725X FT-IR.

3. Results and Discussion

The structural feature of the Fe/Si02 catalysts obtainedafter precursors contact time of 2 hours dried in air at 353 Kis shown in Figure 1. The micrograph reveals that the cata-lyst is constituted by superimposed bi-dimensional struc-tures (sheets). The deformed rectangles presents in Figure 1are similar to those observed in natural Phy llosilicate [7].

Figure 2 shows a single platy block of sheets with a welldefined rectangular outline. Plastic deformations can be seenin both figures 1 and 2.

...•.

Fig. 1. Superimposed sheets of iron phyllosilicates synthesizedwith the ammonia method. Fresh catalysts dried at 353 K.

Revista Latinoamericana de Metalurgia y Materiales, Vol. 22, N° 1, 2002.

Both micrographs shows around the two main crystalsthe presence of abundant smaller broken material s composedby superimposed sheets with no defined form probably re-sulting of the rupture of major size structures. The thermalpretreatment under hydrogen flow performed at 973 K causeda dramatic change in the catalyst morphology. Indeed, anagglomerate of mostly irregular particles of nanometric sizesare presents in the micrograph of Figure 3.

Fig.. 2. Single platy block (book) of iron Phyllosilicate with well-defined rectangular outline, obtained with the ammonia method.Fresh catalyst dried at 353 K.

...•.

Fig. 3. Structural feature of the silica supported iron catalystobtained from iron phyllosilicates at 923 K during the hydrogenpretreatment.

49

The nitrogen physisorption studies at 77 K performedboth on the silica aerosil support and on the fresh silicasupport iron catalyst are shown in Figure 4. Both isothermsare quite similar and characteristic of non porous solids. Thesmall hysteresis effect observed in Figure 4 can beattributable to a short time of pressure equilibration or themicro pores presence in the sample. The specific surfacearea of the silica supported iron catalyst decrease respect tothat of support probably due to the presence of synterizationeffects and metal-support interactions. Such effect increaseswith the metalloading as can be seen in Table 1.

))0 .------------,

250

AEfrol200

2)0I]Js::,1f2 f:.-- (j)'U 1501: -~'L... M

O :=IfI ü 100.¡¡¡ ~>.s:u, 50

1 (p,{. FeBiO::::O+------¡--~--_.__-___l

O 0,25 0,5 0,75 1Relative Pressure (PJPo)

. Fig. 4. Nitrogen physisorption isotherrns at 77 K of the supportsilica aerosil (upside) and 10% Fe/Si02 fresh catalyst dried at 353K.

Table 1. B.E.T. Surface area (mvg)

Silica Aerosil 200 5% 10% 20%FelSiO') Fe/Si O" FelSiO?-.

! 199 ± 2 161 + 1 ! 137 + 1 126 + 2

Figure 5 shows the chemisorption isotherms after the re-duction process under hydrogen flow at 973 K. The experi-ments were performed for both the 10% Fe/Si02 catalyst andthe silica aerosil 200 support taken as reference. The silicaaerosil200 presents no adsorption of carbon monoxide (cal-culated at P == 0.0 mmHg) while the iron catalyst does it. Themetal particle size as measured from the carbon monoxidechemisorbed (at zero pressure) is about 76 nm for the 10%Fe/Si02 catalyst and increase with the metalloading ( Table2). This result can not be compared with the micrograph ofFigure 3 because of the SEM spectroscopy resolution limita-tion. However, mostly of the agglomerate of particles shownin Figure 3 presents a size of around 100 nm.

50 A. Loaiza-Gil y col. /Revista Latinoamericana de Metalurgia y Materiales

- 16t» 14-00

! 12"t:I 10-dioS S~ 6~lIS I

di 4~ 2-.;g O"

O

10%1=81SiO.2 -

100 200 300, 400 500Press ure (mmHg)

Fig. 5. Carbon monoxide chemisorption on the silica aerosilsupport (low side) and 10% Fe/SiOl catalyst (upside) after areduction process under hydrogen flow at 973 K.

Table 2. Influence of the metalloading on the metal particle size

CO Uptake Particle size MetalCatalyst Dispersion(cm3/g) (nm) (%)

5% 1.23± 0.03 23 ± 1 5.2Fe/Si02

10% 0.66± 0.02 76 ± 3 1.6Fe/Si02

20% 0.62± 0.02 160 ± 3 0.8Fe/Si02

Figure 6 shows the Ff-IR spectra of the reduced ironcatalysts in the 353 to 973 K temperature range. All of spec-tra shows the bands characteristic of silica. The shoulder ofabout 3468 and 3156 cm" can be attributable to O-H stretch-ing vibration modes of hydrogen bonded to OH of poly-meric association and hydrogen bonded to OH intermolecu-lar or chelate compounds. The syrnmetrie stretching Si-Ovibration of silica can be observed at around 1100 cm-l. Atlower frequencies the bands at around 820 and 470 cm:'corresponds to asyrnmetric Si-O stretching and Si-O bend-ing modes of silica in that order. At medium frequeneies thebands at around 1640 and 406 cm' corresponds to H-O-Hbend of crystallization water and N03 + vibrations of the ni-trate ion [8]. An inereasing of the reduction temperature inthe 353-898 K range does not affeet the shoulder of about3468 cm' assigned to the O-H bonded to polymeric associa-tion like bi-dimensional Sips' At 973 K all ofbands between3500 and 3000 cm' disappears while the band assigned tothe bonded O-H intermolecular shoulder at 3156 cm' disap-pears after 598 K. As the same way the intensity of the bandsassigned to crystallization water and NO/ ion s decreasewith the temperature increase up to disappears at 973 K. Atthe last reduction temperature the Ff-IR speetrum corre-sponds to that of the siliea (Si02).

Wavelength (cm')Fig. 6. FT-IR spectra of Fe/Si02 catalysts after prograrnmedtemperature reduction under hydrogen flow at 353-973 Ktemperature range and a heating rate of 1Klmin.

4. Conclusions

The arnmonia method modified was used to prepared silicasupported iron catalyst. The struetural feature of tbe freshcatalyst consist in superimposed sheets of rectangular crys-tals. Some of those presents a well defined outline andplastic deformations characteristic of the naturalPhyllosilicate. Such solids are nonporous. Its surface areadecrease with the metalloading increasing. Iron crystallitesof around 20 to 160 nm supported on silica (SiOz) are result-ing from tbe thermal pretreatment at high temperature underhydrogen flow. Fl- IR studies at prograrnmed temperaturereduction indicates the progressive leaving of OH groupsbonded to a polymeric structure like bi-dimensional Si.O,and intermolecular OH groups probably belonging to theiron complex like Fe(Hp)6 +3 suggesting a fresh catalyst struc-ture constituted by Fe(Hp)6 +3 complex sandwiched betweensheets of Si.O, phyllosilicates. Such structure disappears bya progressive leaving of the OH groups and a new structureconstituted by small iron crystallites dispersed on the silicasupport arises by effect of the final reduction temperature.Similar structures were found on silica support cobalt cata-lysts synthesized with the same metbod [5,9].

5. References

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Revista Latinoamericana de Metalurgia y Materiales, Vol. 22, N° 1,_2002. 51

5. A. Barbier, A. Hanif, lA.Dalmon, G.A.Martin, AppliedCata.,168(1998)333.

6. o;Clause, M. Kermarec, L. Bonneviot, F.Villain, M. Che,lAmer. Chem. Soc.114(1992)4709

7. W.A.Deer, R.A.Howie, l Zussman, "An Introduction tothe rack forming mineral" Longman, G.L. Press, London(1966)

8. Wilson & Wilson's, Analytical Infrared Spectroscopy,Vo1.6,G. SvehlaEditor, Elsevier (1976).

9. Alfonso Loaiza-Gil, Memorias V Congresso NacionalCatálisis .Mcbo-Venezuela (2001) 1001

AcknowledgementsThis work was supported by Consejo Nacional de

InvestigacionesCientificas (CONICIT- Venezuela) underGrant N° S1-2000000809 and Consejo de desarrollo Científico,Humanistico y tecnológico (CDCHT) de la Universidad deLos Andes, Project W C-942-99-08-AA.