A novel method for synthesis of a Ni/Al2O3 catalyst with a ...

Fig. 4 TEM images of calcined (a) Ni-H1, (b) Ni-NS, and (c) Ni-N1.Fig. 5 XRD patterns of (a) mesoporous alumina prepared using stearicacid, and (b) mesoporous Ni/Al 2 O 3 prepared using nickel stearate aftercalcination at 450 uC.and the phase for active alumina was retained up to 600 uC. Asshown in Fig. 5, mesoporous alumina prepared using stearicacid as a chemical template shows an active alumina phase. Onthe other hand, mesoporous Ni/Al 2 O 3 is not a pure aluminamaterial but co-exists with metal oxide. In addition, the activealumina phase overlaps with that of nickel oxide or a nickelaluminate-like material.Magnesium ion in the magnesium stearate was exchangedwith nickel ion, and nickel stearate was then used as a chemicaltemplate and metal source on the inner pore surface. Metalchloride or metal hydroxide was used as an additional metalsource on the outer pore surface. At low pH, the nickel loadingcontent was decreased with increasing HCl content (i.e. 8 wt%for Ni-H1 and 2 wt% for Ni-H3). At high pH, the nickelcontent was about 7–8 wt%. Thus, whether HCl or NH 4 OH isused to make metal salts, the metal contents of the preparedcatalysts are similar (i.e. 8 wt% for Ni-H1, 7 wt% for Ni-N1,and 8 wt% for Ni-N2) because the initial metal content is thesame. However, calcined Ni/Al 2 O 3 has a different color, i.e.Ni-H1 is dark-green, like an alumina incorporated with astoichiometric nickel oxide, Ni(II), and Ni-N1 is dark gray likealumina with nonstoichiometric nickel oxide, Ni(III). 11 Such adifference in color implies that calcined nickel catalysts showdifferent metal-to-support interactions. This was confirmedthrough the TPR and 27 Al MAS NMR analysis.As shown in Fig. 6, the three catalysts exhibit quite differentpeaks for H 2 uptake. The nickel species produced from nickel–support interactions vary depending on the chemical andphysical properties of the support, 11 and the preparationconditions. 12 At temperatures below 500 uC, the reduction peakshows an easy-to-reduce Ni species with weak metal-to-supportinteractions, while above 800 uC, the reduction peak shows ahard-to-reduce Ni species (i.e., nickel aluminate-like species).Therefore, Ni-N1 materials are more easy to reduce thanNi-H1, while Ni-NS shows a low H 2 uptake compared to theothers due to low metal loading contents (v2 wt%). This resultclearly indicates that the nature of the Ni species on the supportvaries with the preparation conditions. The presence of hardto-reduceNi species above 800 uC was confirmed with 27 AlMAS NMR.The coordination of nickel-to-aluminium atoms on activealumina was investigated by 27 Al MAS NMR. As-madealumina has mainly 6-coordinated Al atoms, in the form oflayered aluminium hydroxide. After calcination, 4, 5, and6-coordinated Al atoms are present (Fig. 7). Tetrahedral(Al IV ) and octahedral (Al VI ) coordinated Al are produced asthe result of dehydration and dehydroxylation of aluminiumhydroxide, and pentahedral (Al V ) coordinated Al is producedby migration toward tetrahedral and/or octahedral sites. 13 Thestructure of both c- and g-alumina was a defective spinel(%Al 2 ,Al 6 O 12 or Al 3 ,%Al 5 O 12 ; % vacancy), 14 and hence theirvacancy in these spinel structures allowed the migration ofaluminium atoms during the phase transformation. In the caseof inverse bimetallic spinels such as NiAl 2 O 4 the Al 31 ionsFig. 6 TPR curves of (a) Ni-NS, (b) Ni-H1, and (c) Ni-N1 beforecalcination.Fig. 7 27 Al MAS NMR patterns of (a) Ni-H1 and (b) Ni-N1.2356 J. Mater. Chem., 2003, 13, 2353–2358

Fig. 8 TEM images of (a) Ni-H1 and (b) Ni-N1 materials afterreduction.would also be expected to be in tetrahedral coordinationsites, while nickel oxide supported on active alumina allowsNi 21 ions to diffuse into the surface lattice vacancies of thealumina spinel structure. 15 The surface lattice vacancies of thetetrahedral coordinated Al were occupied with the Ni 21 ionsthat diffused during dehydroxylation and dehydration, and theelectron field of Al IV was then deshielded compared to that ofAl IV with vacancies. Therefore, the position of the chemicalshift (d) of tetrahedral Al atoms in metal supported alumina(d(Al IV ) ~ 71.788) shifted to the left (deshielding) as opposedto that in pure active alumina (d(Al IV ) ~ 69.289, not shownhere). This migration of metal ions into the alumina matrix is acommon phenomenon in metal supported catalysts and may belimited to the first few layers of the support, eventuallyproducing a hard-to-reduce metal oxide (i.e. high metal-tosupportinteraction). For pentahedral Al, unsaturated Al isproduced due to the oxygen vacancies that are formed asa result of the dehydroxylation of precursors. This Al Vcoordinated Al serves as an electron acceptor, and acts asLewis acid sites. 15,16Dispersion of nickel particlesAfter reduction at 450 uC, the nickel particle size was measuredby TEM analysis (Fig. 8). The nickel particles supported on thereduced Ni-H1 material were larger than the reduced Ni-N1.Their metal loading contents were similar. However, theyshowed different metal particle sizes. This may be due to thedifferent starting agents used as chemical templates. As shownin Fig. 9, the HCl-treated solution used for the synthesis ofNi-Hx exhibited a characteristic peak of carboxylic acid(–COOH, 1700 cm 21 ) and a symmetric carboxylate (–COO 2 ,1335–1440 cm 21 ). The NH 4 OH-treated precipitate for thesynthesis of Ni-Nx, however, shows only carboxylate (asym.and sym. –COO 2 ) chararacteristics. This indicates that bothstearic acid and a stearic acid salt were formed in the HCltreatedsolution, but a stearic acid salt (i.e., metal stearate) waspresent in the NH 4 OH treated precipitate.When an NH 4 OH-treated precipitate is used as a chemicaltemplate, nickel stearate may be present in the inner pores as achemical template, where it then acts as a source of highlydispersed nickel. When an HCl-treated solution is used, stearicacid acts as a chemical template, and metal salts (i.e. metalchloride) on the outerside or near-entrance of the pores act asanother metal source. After reduction, this metal source on theouterside or near-entrance of the pores will aggregate, thusproducing larger nickel particles. Therefore, Ni-Nx representsmore highly dispersed nickel particles than Ni-Hx.Hydrogenation of linoleic acidA catalyst prepared by the traditional method (i.e. metalimpregnation onto mesoporous alumina 7,17 ) has featuressimilar to the Ni-Hx catalyst prepared by the one-potmethod as described in this work. These materials showedaggregated and large nickel particles on the pores and/orsurface of the structure, while the Ni-Nx catalyst showed smallFig. 9 FT-IR patterns of (a) HCl-treated solution for Ni-Hx and (b)NH 4 OH-treated precipitate for Ni-Nx used as chemical template.and highly dispersed nickel particles (Fig. 8). The Ni-H1 andNi-N2 catalysts have the same metal content (8 wt%), and thus,were tested in a hydrogenation reaction of linoleic acid (C18:2,n D ~ 1.469) as a potential application. Linoleic acid contains 2unsaturated (9C and 12C) bonds. In this reaction the acidwould be saturated with hydrogen and changed into oleic acid(C18:1, n D ~ 1.459) or stearic acid (C18:0), and thus the degreeof saturation would be increased. Approximately 0.1 g ofreduced catalyst was added to 50 ml of linoleic acid, which wasplaced in the reactor. The temperature was increased to 160 uCunder nitrogen, and hydrogen gas was then added. Theprogress of the reaction was easily analyzed by measuring theiodine value or by means of a refractometer (N-3000E, n D1.435–1.520, Atago). 18 At the beginning of the hydrogenation,almost all of the double bonds are present in the cis form. Atthe end, almost of the C cis 18:2, however, was converted toC trans 18:2 and C trans 18:1, and a small portion of C cis 18:1 orC trans 18:1 were saturated, forming C18:0. 19 Therefore, theselective hydrogenation of linoleic acid to oleic acid wasachieved.As shown in Fig. 10, Ni-N2 is a more reactive catalyst thanNi-H1 due to the high dispersion of metal particles, the highsurface area, and many active sites for hydrogenation. Thisreaction showed that the Ni-N2 with highly dispersed metal is amore suitable and efficient catalyst for the hydrogenation offats than one-pot synthesized mesoporous alumina (e.g. Ni-H1)or metal impregnated mesoporous alumina, which containlarge metal particles.In conclusion, an easy and fast synthesis method for thepreparation of Ni/Al 2 O 3 with a mesoporous structure usingnickel stearate as a chemical template and a metal source isdescribed. The prepared Ni/Al 2 O 3 supports could be used ascatalysts only after a reduction step. Ni-Hx materials preparedusing a HCl-treated solution show a well-developed frameworkporosity and a regular pore distribution, while Ni-Nx preparedFig. 10 Hydrogenation of linoleic acid using (a) Ni-H1 and (b) Ni-N2catalysts.J. Mater. Chem., 2003, 13, 2353–2358 2357

using a NH 4 OH-treated precipitate show a developed frameworkand textural porosity and higher surface area thanNi-Hx. The nickel content in Ni/Al 2 O 3 was found to be 0.25times the magnesium content, and the nickel species on thesupport varies with the conditions used in the preparation. 27 AlMAS NMR and TPR analysis confirm that Ni ions diffuse intothe surface lattice vacancies of the alumina spinel structure; as aresult, hard-to-reduce Ni species are produced, and this Ni-Nxhas more highly dispersed nickel particles than Ni-Hx, after thereduction step. In addition, some feature of the catalysts, suchas metal dispersion and surface area, affected the catalyticactivity.AcknowledgementsThis work was supported by the National Research Laboratory(NRL) of the Korean Science and Engineering Foundation(KOSEF).References1 F. Vaudry, S. Khodabandeh and M. E. Davis, Chem. Mater.,1996, 8, 1451.2 M. Yada, M. Machida and T. Kijima, Chem. Commun., 1996, 769.3 S. Valange, J.-L. Cuth, F. Kolenda, S. Lacombe and Z. Gabelica,Microporous Mesoporous Mater., 2000, 35–36, 597.4 W. Zhang and T. J. Pinnavaia, Chem. Commun., 1998, 1185.5 S. Cabrera, J. E. Haskouri, J. Alamo, A. Beltran, S. Mendioroz,M. D. Marcos and P. Amoros, Adv. Mater., 1999, 11(5), 379.6 Y. Kim, B. Lee and J. Yi, Korean J. Chem. Eng., 2002, 19(5), 908.7 Y. Kim, C. Kim, J. W. Choi, P. Kim and J. Yi, Stud. Surf. Sci.Catal., 2003, 146, 209.8 Q. Huo, D. I. Margolese, U. Ciesla, P. Feng, T. E. Gier, P. Sieger,R. Leon, P. M. Petroff, F. Schuth and G. D. Stucky, Nature, 1994,368, 317.9 P. T. Tanev and T. J. Pinnavaia, Chem. Mater., 1996, 8, 2068.10 W. Deng, P. Bodart, M. Pruski and B. H. Shanks, MicroporousMesoporous Mater., 2002, 52, 169.11 C. Li and Y.-W. Chen, Thermochim. Acta, 1995, 256, 457.12 S. Wang and G. Q. Lu, Appl. Catal. A, 1998, 169, 271.13 X. Krokidis, P. Raybaud, A.-E. gobichon, B. Rebours, R. Euzenand H. Toulhoat, J. Phy. Chem. B, 2001, 105, 5121–5130.14 K. Sohlberg, S. T. Pantelides and S. J. Pennycook, J. Am. Chem.Soc., 2001, 123, 26–29.15 S. Chokkaram, R. Srinivasan, D. R. Milburn and B. H. Davis,J. Mol. Catal. A, 1997, 121, 157.16 L. J. Alvarez, A. L. Bumenfeld and J. J. Fripiat, J. Chem. Phys.,1998, 108, 1724–1729.17 Y. Kim, C. Kim, P. Kim, J. C. Park and J. Yi, Adsorption Scienceand Technology, ed. C. Lee, World Scientific, Singapore, 2003,p. 605.18 D. Javonovic, R. Radovic, L. Mares, M. Stankovic andB. Markovic, Catal. Today, 1998, 43, 21.19 D. Javonovic, Z. Cupic, M. Stankovic, L. Rozic and B. Markovic,J. Mol. Catal. A: Chem., 2000, 159, 353.2358 J. Mater. Chem., 2003, 13, 2353–2358