Catalytic Isomerization of n-butane and n-pentane over Sulfided AghMordenite Zeolite


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Abstract accepted for poster presentation in ZMPC-Totttori(2006)

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Catalytic Isomerization of n-butane and n-pentane over Sulfided AghMordenite Zeolite

  1. 1. Catalytic Isomerization of n-butane and n-pentane over Sulfided AgHMordenite ZeoliteLebong Andalaluna1,2,* and Masatoshi Sugioka1Department of Applied Chemistry, Muroran Institute of Technology, 27-1 Mizumoto-cho,Muroran-shi 050-8585, Japan 2DTIEML,BPPT.* Corresponding author, e-mail, fax +62-21-8297433Catalytic isomerization of n-butane and n-pentane proceeded predominantly via bimolecularpathway over sulfided AgHMordenite zeolite (SAHMOR), as well as in HMordenite zeolite(HMOR). It was revealed that SAHMOR showed particular high enhanced activity which wasremarkably higher than that of parent HMOR in n-butane isomerization. However, the catalyst onlyshowed comparable catalytic activity with that of HMOR in n-pentane isomerization. It is proposedthat the formation of C10+ intermediate in n-pentane isomerization was inhibited by zeolite channelnarrowing as the result of silver-sulfide species (AgSx) formation in the sulfiding of AgHMordenite.Introduction In the previous works, we have reported that sulfiding of metal ion-exchanged zeolites andmesoporous material with hydrogen sulfide resulted remarkable activity enhancement for someacid-catalyzed reactions [1,2]. Mordenite is applied commercially as alkanes isomerization catalystwhich is an important process for producing high quality of automotive fuel. In current study, thecatalytic activities of sulfided AgHMordenite zeolite (SAHMOR) in the isomerization of n-alkanes(n-butane and n-pentane) was examined. The catalytic performance of the catalyst was associatedwith surface active sites and pore character generated over SAHMOR in the sulfiding.Experimental HMordenite (HMOR) was prepared using 0.1 mol/l HCl aqueous solution and AgHMordenite(AHMOR) was prepared by ion exchanging HMOR using 0.1 mol/l aqueous solution of AgNO3.All samples were calcined in air at 500ºC for 4 hours. Isomerization of n-butane and n-pentane werecarried out on a conventional closed circulating reactor at 250ºC and 220ºC, respectively. AHMORwere sulfided with 40 Torr of hydrogen sulfide at 300ºC for one hour and evacuated for 0.5 hour atthe same temperature to obtain SAHMOR. Infrared spectroscopic study of pyridine adsorption anddeuterium exchange were also performed by Jasco FT-IR 230S using an in-situ cellResults and Discussions Skeletal isomerization of n-butane proceed via bimolecular reaction route over acidic zeolite,involving the formation of C8 carbenium ion [3]. Mordenite has a one-dimensional microporesystem with opening channel of 6.5 x 7.0 Å. The one-dimensional pore of Mordenite [4] andinclusion of metal particle inside the zeolite channel [5] was proposed inhibit sterically thebimolecular pathway of the reaction. Figure 1 shows the catalytic isomerization of n-butane over HMOR and SAHMOR at 250ºC. Itwas observed that SAHMOR showed remarkable high catalytic activity in the isomerization, whichwas much higher than that of its parent HMOR. SAHMOR showed twice of HMOR n-butaneconversion after 4 hours reaction. Furthermore, SAHMOR was revealed exhibiting significantcatalytic activity at 200ºC with much shorter induction period than HMOR (figure not shown). The isomerization of n-pentane over HMOR and SAHMOR at 220ºC is shown in Figure 2. Theisomerization products was mainly C2-C5 hydrocarbons with low selectivity to iso-pentane whichshowed that the isomerization predominantly occured through bimolecular mechanism via C10+intermediate formation [6]. It was observed that SAHMOR showed high catalytic activity in theisomerization. However, in contrast with result in n-butane isomerization, the activity was onlyslightly higher than that of HMOR. Moreover, an induction period was observed over SAHMORin the isomerization. The initial product was predominantly iso-pentane which indicated thatmonomolecular reaction might occured in the initial reaction over SAHMOR. We assumed thatC10+ intermediate formation of bimolecular route in n-pentane isomerization was suppresed overZMPC 2006 1
  2. 2. 25 25 C1 C1 Amount of product (%) Amount of product (%) 20 C3 20 C3 i-C4 i-C4 i-C5 i-C5 15 15 n-C5 n-C5 10 10 5 5 0 0 0 120 240 360 480 0 120 240 360 480 Reaction time (min) Reaction time (min)Figure 1: Catalytic isomerization of n-butane over HMOR and SAHMOR zeolites, respectively. 20 20 C3 C3 Amount of product (%) Amount of product (%) i-C4 16 i-C4 16 n-C4 n-C4 i-C5 12 i-C5 12 8 8 4 4 0 0 0 60 120 180 240 0 60 120 180 240 Reaction Time (min) Reaction Time (min)Figure 2: Catalytic isomerization of n-pentane over HMOR and SAHMOR zeolites, respectively.SAHMOR. This kind of inhibition might occured as the result of narrowing of mordenite zeolitepore system due to formation of silver-sulfide species inside one-dimensional channel of mordenitezeolite. Such as inhibition was apparent in the isomerization of n-pentane but not in the case ofn-butane isomerization, possibly due to bulkier size of C10+ intermediate of n-pentane isomerizationthan C8 carbenium ion intermediate of n-butane isomerization.Conclusion Sulfided AgHMordenite zeolite (SAHMOR) showed remarkable high activity for n-butaneisomerization which was much higher than that of HMOR. On the other hand, the catalytic activityof for n-pentane isomerization was affected by suppression of C10+ intermediate formation as theresult of formation of silver-sulfide species (AgSx-) inside the mordenite zeolite pore system.References[1] M. Sugioka and L. Andalaluna, Stud. Surf. Sci. Catal., 105 (1997) 1995.[2] M. Sugioka, L. Andalaluna and J.K.A. Dapaah, Stud. Surf. Sci. Catal., 129 (2000) 823.[3] R.A. Asuquo, G. Eder-Mirth, J.A. Lercher, J. Catal. 135 (1992) 115.[4] H. Liu, G.D. Lei, W.M.H. Sachtler, Appl. Catal. A, 137, 167(1996).[5] P. Canizares, A. de Lucas, F. Dorado, D. Perez, Appl. Catal. A, 190 (2000) 233.[6] N. Essayem, Y. Ben Taarit, C. Feche, P.Y. Gayraud, G. Sapaly, C. Naccache, J. Catal., 219 (2003) 97.ZMPC 2006 2