Propylene Epoxidation using Titanium-containing Zeolite Catalysts

Korean Chem. Eng. Res., Vol. 44, No. 2, April, 2006, pp. 121-128 Š l oj mš { mk i m qçm kçmq Ço Çg p 402-701 p}e n 253 (2005 10o 13p r, 2006 1o 11p }ˆ) Propylene Epoxidation using Titanium-containing Zeolite Catalysts Han-Ju Ban, Kyu-Yong Lee, Joong-Ki Lee, Sung-Taik Chung and Wha-Seung Ahn Department of Chemical Engineering, Inha university, 253, Yonghyun-dong, Nam-gu, Inchon 402-751, Korea (Received 13 October 2005; accepted 11 January 2006) k Žˆ o rm p TS-1 Ti-MCM-22 pn l r l e pp mp, pl, p p p k,, p m, k, n l r s m. TS-1p n p t ps (45 o C, 7 atm, 0.5 g catalyst, 2.5 wtí H 2, ˆm n, 1,000 rpm )l 95Í p p H 2 r p 94Í p p (propylene oxide, PO) ˆ lp pl. k Š p (acetonitrile) n l p e p Ti-MCM-22 99Íp H 2 r p 100Íl r ˆ p, k l n pp m. h Abstract Propylene epoxidation by H 2 (30Í aqueous) as oxidant was studied in a semi-batch reactor using TS-1 catalyst: Effects of reaction temperature, time, pressure, solvent, catalyst and H 2 concentration on H 2 conversion (limiting reagent) and product distribution were investigated. Potential inhibition by propylene oxide on the epoxidation rate was also examined. Ti-MCM-22 with MWW zeolytic structure was found to exhibit better performance than TS-1 with MFI structure, provide that a proper choice of solvent(acetonitrile) is made. Key words: Propylene, Propylene Oxide, Epoxidation, TS-1, Ti-MCM-22 1. p lrp k n t ~p n ˆp ~p polyol rsl n vp. p v p p hydroperoxide epichlorohydrinp pn l e pp rs v o p r pn, p p rr p l, ~ rl ep vt l m. pm l r n p l e p r PO rp (Scheme 1 s), MFI(mobil five) s Žˆ o rm p titanium silicalite-1(ts-1) m p s l n l e p l [1, 2]. Clerici [3]p TS-1 p k s p m p l e pl p n p }p m. ˆm/ n pn l 40 Cl p p o 90 min p k 90Íp lp, 97Íp PO ˆ To whom correspondence should be addressed. E-mail: whasahn@inha.ac.kr ll. Propylene glycol(pg) pp monomethyl ether(pm) tn pl. Thiele Roland[4] p p tp o k m p ~ mp, t p Na 2 SO 4 k p NaH 2 PO 4 p lp PO ˆ p m. p p TS-1 l defect q p e p rp qn l p, p rp pr m Scheme 1. Propylene epoxidation reaction path and corresponding products using TS-1 catalyst and H 2. 121

122 tëp nëpt Ër ˆËk d p nkp t eˆ p m. Chen [5]p TS-1, Ti /Si xerogel, Ti-MCM-41 ˆ k eˆ ZSM-5l TiCl 4 } p peˆ(ti- ZSM-5) Žˆ o 4 v rs l p l e pl rn m. m kl pp v mp, p r pp p TOF m. mv TS-1 Ti-ZSM-5 p p ˆ lp, ~ op Žˆ q m s t p (hyrophobicity)p pl tn p p l. Ti-ZSM-5 qs k l p r p l PO diol ether t l. pm p v l p v v PO ˆ p r m, p n p ˆm n l pp v. p 550 Cl } p o mv, p Žˆ p p k n p m. Li [6]p e k n TS-1 vv~ m, k q~p rp p l POp pp sq l pp ˆ p PG p PMp o p., p vv~ n l TS-1p k spray coatingeˆ e rs l PO l n m, p p p v e l vv p PO ˆ p pp ˆ l. rqp wp diffusion POl PG/PMp p l p p r pp lp, POp pp l pp plp rp v r POp ˆ l l m., PO rl pl r p o p vrrp n el TS-1l ~ l m p ~ in-situ rs l pl n q p p l l [7-10]. p p t op l, anthrahydroquinone(ahq)p l Œp l m AHQ p l pn TS-1 p l e e PO l. m p ~ p peˆ p n v TS-1p po n,. k pl p n v n Žˆ o rm p p l. t q t n p q~ Ti-MCM-22 p, MCM-22 rm p q~[11-13] 12-membe- red ring(mr) 10-membered ring(mr)p 3 o s(mww s) rm p p. Ti-MCM-22 l fumed silica, tetrabutyl orthotitanate (TBOT), boric acid, deionized water s kr hexametyleneimine (HMI) piperidine(pi)p n m [14-16]. v Sim Ti gelp v MWW sp Žˆ o e q~ rs llp svv sr boric acid Žˆ n p rk l. Ti-MCM-22 Si/B=0.75 Si/ Ti=100-10p l PIm HMIp v s kr l lp, o template r o 530 o C l s p sheet pl r p op MWW s lpp k pl. Ti-MCM-22 cyclohexene 1-hexenep l e pl TBHP rl lp n p ˆ lp, rs p l n p p o44 o2 2006 4k Tatsumi Žl l n[14-16]p r p. l l e k k p pn l, k n l n eˆ p r pn l p rs k pl l m. p rp rm p MFI sp titanium silicalite-1(ts-1) t n l, PO rs pl tn r p m p s m., Ti-MCM-22 l l s mp, }pp k pl p p rn l TS-1 m p r m. 2. 2-1. { oo 2-1-1. TS-1p Žˆ op titanium ethoxide(teot, 98Í Aldrich), e o p tetraethylorthosilicate(teos, Aldrich) n m. v o l TEOT TEOSl }} r eˆ 2e k m, l 1M nkp tetrapr-opylammonium hydroxide(tpaoh, Aldrich)p ~ 30 min r e. s p s p TPA + /Si =0.9, Si /Ti =32.0, H 2 O/Si =80pl. 80 Cl o 6e k k mp v eˆ eˆ p, } l p l 175 Cl 3p o eˆ, l v } p 550 Cl e o m. 2-1-2. Ti-MCM-22p Ti-MCM-22 r B-MCM-22, 6 M v p pn l s tl r l Žˆ p } p q l lt p m. s r piperidine(pi, 99Í, Aldrich), e op fumed silica(cab-o-sil M5, Riedel-dehaen), Žˆ op tetrabutylorthotitante(tbot, 97Í, Aldrich), o p boric acid(99.5í, Aldrich)p n m. r Scheme 2 l r m. 2-2. { m o vp X r p X-ray diffractometer(rigaku-miniflex, Target: CuKα, Filter:Ni) n l m. vp pq k p o l SEM(Hitachi S-4200, S-4300)p n m, Žˆ p p o SEM/EDS(Kevex/Hitachi S-4200) n m. Žˆ p ˆ p o l UV-Vis p mp Varian Cary-3E double beam spectrometer l ˆ eˆ Si t v l 200~600 nm ol r m. FT-IR spectrum(bomem MB104)p e KBr l k eˆ wafer l 500~4,500 cm 1 ol Œ r mp, p rp Micromeritics ASAP 2000 automatic analyzer q pn l k~ v m l v e r m. rp Brunauer-Emmett-Teller(BET) p r m. 2-3. { m Titanium Silicalite-1(TS-1) [Si/Ti=40]p t pl n m. 125 ml n p batch p pn l 36.6 gp n k l 2.5 wtíp (30 wtí) l 40 gp nkp sr

Scheme 2. Synthesis procedure for Ti-MCM-22. Žˆ o rm p pn l e p 123 p m. p flame ioniz-ation detectorm packed column (50 m 0.32 mm) n SIMADZU GC-14A gas chromatography m. t p propylene oxide(po)p propylene glycol(pg) 2-propanol methyl ether(pm) l. 3. y 3-1. { m oo Š l l l 2 v Žˆ o, TS-1m Ti- MCM-22p XRD Ž p Fig. 1l e m. TS-1p n r r p MFI sl X r Ž p 2 θ k om 24.5 29.2 o l ˆ r s lt. Ti-MCM-22 p n rp XRD l om 3 6 p s o ˆ q ˆ p., 7~10 o o 12- membered-ring s ˆ, p l 20~30 pl o ˆ p 10-membered-ringp l k r p. } l p s tp s kr r l om ol XRD Ž p r~rp m., 3 6 sq r p p r tl dehydroxylation l p p, v p lp OH H 2 O lr p s p l s v p k t., 7 p q o p p s l sq 12-memberedring pocketp supercage p p. Ti-MCM-22 p s tl qs p Ž ~ op Žˆ p r o l 2 Mp v p }, XRD l pl lp rm p MWW s o v l. v p p SEMp kp, Fig. 2 l m. TS-1p n pq 0.2 µmp cubic p ˆ l, SEM/EDS e p Si/Ti k 40 pl. Žˆ p o Ti-MCM-22p n, } v k e m } e p SEM vp rq p v p r k l ƒ p 1,000 rpmp eˆ pp v m. k p l p l ˆm r, p e l 1-way valve l p tp back pressure gauge pn l pr k p o veˆ ˆl pp v m. p n p m sr rl p l l pm m, p mechanical stirrer pn m. k e p p 1/16" SUS p pr e p } l GC p s p m. p r p ˆ p ferroin indicator pn CeSO 4 (0.1 M)rr p pn l r m. Simulator(Aspen + 12.1) p e s l p p p p p plp, p r p p Fig. 1. XRD patterns of; (a) TS-1(calcined), (b) Ti-MCM-22(calcined), (c) Ti-MCM-22(as-syn.). Korean Chem. Eng. Res., Vol. 44, No. 2, April, 2006

124 tëp nëpt Ër ˆËk d Fig. 2. SEM images of; (a) TS-1(calcined), (b) Ti-MCM-22(calcined without acid treatment), (b) Ti-MCM-22(calcined with acid treatment). Fig. 4. FT-IR spectra of; (a) TS-1, (b) Ti-MCM-22. Fig. 3. UV-Vis spectra of; (a) TS-1(calcined), (b) Ti-MCM-22(as-syn.), (c) Ti-MCM-22(acid treated), (d) Ti-MCM-22(calcined). l p p s p pv, q p s r qkv v, p p s ov p p m. } v kp e p r p p p l k pp p v v kk. Žˆ o p UV-Visible Fig. 3l e m. Žˆ p rm p s tp l n Žˆ p 220 nmp ~ s, 260 nmp Ž ~ s 320 nmp k ˆr p d v sq pp, p t pp rp 220 nmp ~ s ˆ sq Žˆ sp k r p [17]. TS-1p n UV 220 nml t lp 270 nml p Ž ~ Ž ˆ p lp, 330 nmp k ˆr p p pv kk. rl p sq 220 nm p qžq yp p p p TS-1 e p Žˆ ˆ r~rp k Ž. p Ti-MCM-22p n v Žˆ ˆ el sq ppp k pp, } v p nl 270 nmp Žˆ d r r~rp Žˆ p p p p p. v, } l 4 ~ np Žˆ p mr r l op 270 nm }p sq m. o44 o2 2006 4k e p FT-IR Fig. 4l ˆ l. e l Si-O-Ti l 1 960 cm lp s l Žˆ p p m v lpp p pl., BET p p r l TS-1p n 500 m 2 /gpl, Ti-MCM-22p n 180 m /gp p TS-1p 2 p rp m. 3-2. m l e pp TS-1p t r l r p ˆ l pm, k, n l p lr l s q m. r e p t s p o l p k, p l r pp PO ˆ l e lp 90Í p p lp p p s p } q m. p m m k p p q l 45 o Cm 7 kp ˆ mp, n l e pl rp k v ˆmp tp n m. r p kl r pp s mp, Fig. 5l e m. 0.2 gl 0.6 gp p kp 0.1 gj eˆ 1e k pp r pp 0.2~0.5 gp ol 43l 92Í v p rp mp, 0.6 g p n r pp ll. PO ˆ p n p k p mp 88Íl 79Í v p rp m. 0.5 gp t p r m. r rp r pp v PO ˆ p mp, p nkp 30 wtí p o p m p POp pp v l PG p., PG n p ˆ m p l l rp PMp r l. p / p o rl m p pl vp r l m. p kp 0.5 g p reˆ 300, 500, 1,000 rpmp v eˆ pp m, Fig. 6l e m. 300 rpm l p r p(68í)p llrp 500 rpm p p

Žˆ o rm p pn l e p 125 Fig. 5. Effect of catalyst loading on C 3 epoxidation; (Ë) Conversion of H 2, (Ë) Selectivity to PO, (Ë) Selectivity to PM, (Ë) Selectivity to PG. Reaction conditions: 45 o C, 7 atm, 2.5 wt H 2, methanol solvent, 1,000 rpm stirring. Fig. 7. Effect of H 2 concentration on C 3 epoxidation; (Ë) Conversion of H 2, (Ë) Selectivity to PO, (Ë) Selectivity to PM, (Ë) Selectivity to PG. Reaction conditions: 45 o C, 7 atm, 0.5 g catalyst, methanol solvent, 1,000 rpm stirring. Fig. 6. Effect of stirring speed on C 3 epoxidation; (Ë) Conversion of H 2, (Ë) Selectivity to PO, (Ë) Selectivity to PM, (Ë) Selectivity to PG. Reaction conditions: 45 o C, 7 atm, 0.5 g catalyst, 2.5 wt H 2, methanol solvent. 92Í r pl lpp p m. p p pl o Ž l 1,000 rpm p tp m. nk(30 wtí)p r~ nkp 2.5, 5, 7.5 wtí 10 wtí p eˆ pp mp, Fig. 7l r m. r pp 92Íl 85Í p v l mp, PO ˆ le rp 82l 62Í m. p pp ve ˆp p plp, m / rp p 2.5 wtí t p s p ˆ m. p e l l p p t s p ˆ m. 3-3. m j m i r s l pm pl m p 45 C tp o 15 o C r p ol s m. m pl m p v ~w m p v l p Fig. 8. Effect of temperature on C 3 epoxidation; (a) (Ë) H 2 Conversion at 30 o C, (Ë) H 2 Conversion at 45 o C, (Ë) H 2 Conversion at 60 o C, (Ë) PO Yield at 30 o C, (Ë) PO Yield at 45 o C, (Ë) PO Yield at 60 o C, (b) (Ë) Selectivity to PO at 30 o C, (Ë) Selectivity to PO at 45 o C, (Ë) Selectivity to PO at 60 o C, (Ë) Selectivity to PM at 30 o C, (Ë) Selectivity to PM at 45 o C, (Ë) Selectivity to PM at 60 o C. Reaction conditions: 7 atm, 0.5 g catalyst, 2.5 wt H 2, methanol solvent, 1,000 rpm stirring. p v m w m v l p n n pp, p p npp pl qn m l. Fig. 8p p l m Korean Chem. Eng. Res., Vol. 44, No. 2, April, 2006

126 tëp nëpt Ër ˆËk d p p r pp mp, m l p n (k p Fig. 11(d) s) pl l p v p k p, m v l r p d (85l 94Í) PO ˆ (94l 70Í) l. p e tlv p s l p p l l p, m l p v p r pl tn m p v p p. POp pp tp p m o, p v 45 C s tl rp o Ž l. 3-4. h m i k p pl m p 4~10 k ol mp, Fig. 9l r m. k p v n l n p kp v, l e p l ps p, p l k v l r pp v. p e p lv e p / v p p k l m p v kp p. PO ˆ k v l 87l 80Í r mp, Fig. 10. Solvent effect on C 3 epoxidation; (a) (Ë ) Conversion PO in methanol, ( Ë ) Conversion PO in ethanol, ( Ë ) Conversion PO in acetonitrile, (Ë) Yield in methanol, (Ë) Yield in ethanol, (Ë ) Yield in acetonitrile, (b) (Ë ) Selectivity to PO in methanol, (Ë) Selectivity to PO in ethanol, (Ë) Selectivity to PO in acetonitrile, (Ë) Selectivity to PM in methanol, (Ë) Selectivity to PM in ethanol, (Ë) Selectivity to PM in acetonitrile. Reaction conditions: 45 o C, 7 atm, 0.5 g catalyst, 2.5 wt H 2, 1,000 rpm stirring. PO p k l lp 1 e p tp 76~78Í ˆ l m l m p ll. Fig. 9. Effect of pressure on C 3 epoxidation; (a) (Ë) H 2 Conversion at 4 atm, (Ë) H 2 Conversion at 7 atm, (Ë) H 2 Conversion at 10 atm, (Ë) PO Yield at 4 atm, (Ë) PO Yield at 7atm, (Ë) PO Yield at 10 atm, (b) (Ë) Selectivity to PO at 4atm, (Ë) Selectivity to PO at 7 atm, (Ë) Selectivity to PO at 10 atm, (Ë) Selectivity to PM at 4 atm, (Ë) Selectivity to PM at 7 atm, (Ë) Selectivity to PM at 10 atm. Reaction conditions: 45 o C, 0.5 g catalyst, 2.5 wt H 2, methanol solvent, 1,000 rpm stirring. o44 o2 2006 4k 3-5. k m i n k pl (1) p l n p, (2) rl p t ~ p p (3) n q~p pp p p p q eˆ p. l l ˆm, lˆm k Š p p n r l pp mp Fig. 10l ˆ l. p rp TS-1p n pl ˆmp q sp n k r pp, p m l ˆm(92Í) > lˆm(82í) > k Š p (26Í)p r pp ll. PO ˆ ˆm(82Í) > k Š p (79Í) > lˆm(67í) m, PO p ˆmp rp n m. n l p n Aspen+12.1p pn l l (Fig. 11 s) p n k Š p > lˆm > ˆmp ˆ l. pl n l p p p n p n m v rl p p t ~ p p p pl m p tn p

Žˆ o rm p pn l e p 127 l pp mp, Fig. 12l ˆ l. p r ~ k n p kp sr l pr ov m. p p s l p l pr p PO 1e p 92l 69Í r pp p pl., PO p l p e p v l l pp v l p PM PGp kp v m. p e p p PO l rp p l r t p r pp p PO ˆ ov ppp k pl. 3-7. TS-1 Ti-MCM-22m { srp MWW sp Ti-MCM-22p pp l TS-1 p ˆm k Š p 2 v n pn l t s l mp Fig. 13l e m. l e Ti-MCM-22 k Š p n l TS-1 n p ˆ l r p 99Íl p p v k 100Íl r PO ˆ p lp plp, p e l p PO ˆ p ov m. p Fig. 11. Vapor pressure of C 3 at 45 o C, 7 atm; (a) in Methanol, (b) in Ethanol, (c) in Acetonitrile, (d) solubility in Varius Solvent at different temperature.., e p l p p s l pp v rp np n p m p lp p. 3-6. m i p p PO l e pl m p s o l m p p l tp Fig. 12. Effect of C 3 O inhibition on C 3 epoxidation; (a) (Ë ) Conversion without PO added, ( Ë ) Conversion with PO added, (Ë ) Yield without PO added, (Ë ) Yield with PO added, (b) (Ë ) Selectivity to PO without PO added, (Ë ) Selectivity to PO with PO added, (Ë) Selectivity to PM without PO added, (Ë) Selectivity to PM with PO added. Reaction conditions: 45 o C, 7 atm, 0.5 g catalyst, 2.5 wt H 2, methanol solvent, 1,000 rpm stirring. Fig. 13. Comparison of catalytic performance by TS-1 and Ti-MCM- 22; (a) (Ë) conversion of TS-1(methanol), (Ë) conversion of Ti-MCM-22(acetonitrile), (Ë ) conversion of Ti-MCM-22(methanol), (b) (Ë) Selectivity to PO: TS-1(methanol), (Ë) Selectivity to PO: Ti-MCM-22(acetonitrile), (Ë) Selectivity to PO: Ti-MCM- 22(methanol), (Ë) Byproduct : TS-1(methanol), (Ë) Byproduct: Ti-MCM-22(acetonitrile), (Ë) Byproduct: Ti-MCM-22(methanol). Reaction conditions: 45 o C, 7 atm, 0.5 g catalyst, 2.5 wt H 2, 1,000 rpm stirring. Korean Chem. Eng. Res., Vol. 44, No. 2, April, 2006

128 tëp nëpt Ër ˆËk d p e l pp l. ˆ m n l TS-1 r p(15í) PO ˆ (67Í) ov l. kp UV l re m p p Ti d p sql Ti-MCM-22l n p p mnrp, n p p l k Š p (polarity 6.3) l p p ˆm(polarity 6.6)l m rp Žˆ sp TS-1l m p rp e. 1-Hexene l e pl p p Žˆ s l Ti-MCM-22 TS-1 n p p ˆ l. q Ti-MCM-22 p l n p l v rp l v p. 4. (30 wtí) r pn l l e pp l TS-1p l m. r rp, m, k p v l r pp v lp, p e p lvl PO ˆ p m. p nkp 30 wtí p o p m p POp p v l propylene glycol(pg)p p., PG n p ˆm p l l rp propylene monomethyl ether(pm)p r l. ˆmp q n n mp, pp p s l semi-batch p pn l v lp n pl m p v kk. p p pl lrr qn p p pl. Ti-MCM-22 k Š p n l TS-1 n p ˆ lp, p e l lp p v k rp qrp r pn k pl p pn l v Ž. p p 2003 v q qvo l (KRF- 2003-41-D0081)l p p lv n t p p, q p vol. y 1. Bu, J., Yun, S. H. and Rhee, H. K., Epoxidation of n-hexene and Cyclohexene over Titanium-Containing Catalysts, Korean J. Chem. Eng., 17(1), 76-80(2000). 2. Ko, Y. S. and Ahn, W. S., Characterization and Catalytic Properties of Titanium Silicalite-1 Catalyst, Korean J. Chem. Eng., 15(2), 182-191(1998). 3. Clerici, M. G., Bellussi, G. and Romano, U., Synthesis of Propylene Oxide from Propylene and Hydrogen Peroxide Catalyzed by Titanium Silicalite, J. Catal., 129, 159-167(1991). 4. Thiele, G. R. and Roland, E., Propylene Epoxidation with Hydrogen Peroxide and Titanium Silicalite Catalyst: Activity, Deactivation and Regeneration of the Catalyst, J. Mol. Catal. A: Chemical, 117, 351-356(1997). 5. Chen, L. Y., Chuah, G. K. and Jaenicke, S., Propylene Epoxidation with Hydrogen Peroxide Catalyzed by Molecular Sieves Containing Framework Titanium, J. Mol. Catal. A: Chemical, 132, 281-292(1998). 6. Li, G., Wang, X. S., Yan, H. S., Liu, Y. H. and Liu, X. W., Epoxidation of Propylene Using Supported Titanium Silicalite Catalysts, Appl. Catal. A: General, 236, 1-7(2002). 7. Laufer, W., Meiers, R. and Holderich, W., Propylene Epoxidation with Hydrogen Peroxide over Palladium Containing Titanium Silic, J. Mol. Catal. A: Chemical, 141, 215-221(1999). 8. Laufer, W. and Hoelderich, W. F., Direct Oxidation of Propylene and Other Olefins on Precious Metal Containing Ti-catalysts, Appl. Catal. A: General, 213, 163-171(2001). 9. Jenzer, G., Mallat, T., Maciejewski, M., Eigenmann, F. and Baiker, A., Continuous Epoxidation of Propylene with Oxygen and Hydrogen on a Pd-Pt/TS-1 Catalyst, Appl. Catal. A: General, 208, 125-133(2001). 10. Wang, C., Y., Wang, B. G., Meng, X. K. and Mi, Z. T., Study on Process Integration of the Production of Propylene Oxide and Hydrogen Peroxide, Catal. Today, 74, 15-21(2002). 11. Corma, A., Corell, C. and Perez-Pariente, J., Synthesis and Characterization of the MCM-22 Zeolite, Zeolites, 15, 2-8(1995). 12. Guray, I., Warzywoda, J., Bac, N. and JrG Sacco, A., Synthesis of Zeolite MCM-22 Under Rotating and Static Conditions, Micropor. Mesopor. Mater., 31, 241-251(1999). 13. He, Y. J., Nivarthy, G. S., Eder, F., Seshan, K. and A. Lercher, J., Synthesis, Characterization and Catalytic Activity of the Pillared Molecular Sieve MCM-36, Micropor. Mesopor. Mater., 25, 207-224(1998). 14. Wu, P., Tatsumi, T., Komatsu, T. and Yashima, T., A Novel Titanosilicate with MWW Structure. I. Hydrothermal Synthesis, Elimination of Extraframework Titanium, and Characterizations, J. Phys. Chem. B., 105, 2897-2905(2001). 15. Wu, P., Tatsumi, T., Komatsu, T. and Yashima, T., A Novel Titanosilicate with MWW Structure: II. Catalytic Properties in the Selective Oxidation of Alkenes, J. Catal., 202, 245-255 (2001). 16. Wu, P. and Tatsumi, T., Preparation of B-free Ti-MWW Through Reversible Structural Conversion, CHEM. COMMUN., 1026-1027 (2002). 17. Yoon, B. S. and Ahn, W. S., Synthesis, Characterization and Catalytic Properties of Titanium-containing Zeolite Beta Catalysts, HWAHAK KONGHAK, 40(1), 1-8(2002). o44 o2 2006 4k