ZrO2 Alloy Catalyst for Benzene Selective Hydrogenation to Cyclohexene

Available online at www.sciencedirect.com

ScienceDirect Journal of Natural Gas Chemistry 15(2006)319-326

Journalot Natural Gar Chemistry

SCIENCE PRESS

www.eIsevier.codocate/jllocsteljngc

Article

Study on the Nanosized Amorphous Ru-Fe-B/ZrOa Alloy Catalyst for Benzene Selective Hydrogenation to Cyclohexene Shouchang Liu*,

Zhongyi Liu,

Shuhui Zhao,

Yongmei Wu,

Zheng Wang,

Peng Yuan

Department of Chemistry, Zhengzhou University, Zhengzhou 450052, Henan, China [Manuscript received May 29, 2006; revised July 31, 20061

Abstract: A novel nanosized amorphous Ru-Fe-B/ZrOz alloy catalyst for benzene selective hydrogenation to cyclohexene was investigated. The superior properties of this catalyst were attributed to the combination of the nanosize and the amorphous character as well as to its textural character. In addition, the concentration of zinc ions, the content of ZrOa in the slurry, and the pretreatment of the catalyst were found to be effective in improving the activity and the selectivity of the catalyst. Key words: Ru-Fe-B/ZrOz amorphous catalyst; benzene selective hydrogenation; cyclohexene

1. Introduction Selective hydrogenation of benzene to cyclohexene has received considerable attention owing to its environmentally benign process, atomic economy, and potentially wide industrial applications [l-71. However, it appears that such selective hydrogenation is very difficult because cyclohexene is chemically active due to the presence of the double bond, on which further hydrogenation to cyclohexane can occur easily. Therefore, considerable effort has been made t o improve the selectivity and increase the yield of cyclohexene, and these efforts have met with significant progress [8-131. Recently, amorphous alloy material as a good-quality, novel type of catalyst has received increasing attention because of its excellent activity and superior selectivity in many hydrogenation reactions [14,15], and it has also been used in the selective hydrogenation of benzene to cyclohexene [16-181. However, a large part of the work on the amorphous alloy catalyst has been carried out only in laboratories; to date, there are no published reports regarding its application in industries. As the amorphous alloy is thermodynamically metastable, the crystal-

lization deactivation process of the amorphous alloy could occur spontaneously during the reaction, especially at high temperatures, which limits the industrial application of amorphous alloy catalysts. In this study, a novel nanosized amorphous Ru-Fe-B/Zr02 alloy catalyst prepared by chemical reduction for benzene hydrogenation to cyclohexene was developed in a pilot study, which exhibited higher activity and better selectivity. In our previous study, reaction conditions such as suitable temperature, appropriate pressure, optimal ratio of water to benzene in the reaction system, and stirring rate that favored benzene selective hydrogenation over the amorphous alloy Ru-FeB/ZrOz catalyst were studied in detail [19]. It was found that a chemical environment around the catalysts is crucial to improve the selectivity t o cyclohexene. In this study, the catalysts were subjected to various characterizations using XRD, TEM, SAED, and N2 physisorption. In particular, the effects of a chemical environment on the performance of this RuFe-B/ZrO:! catalyst under conditions for a pilot study were investigated. The aim of this study is to investigate the prospects of industrial application for the catalysts.

* Corresponding author. Tel and Fax: +86-371-67763706; E-mail: Iiushouchang@zzu,edu.cn

320

Shouchang Liu et al./ Journal of Natural Gas Chemistry Vol. 15 No. 4 2006

2. E x p e r i m e n t a l

2.1. C a t a l y s t p r e p a r a t i o n The Ru-Fe-B/ZrOz catalyst sample was prepared as follows. An appropriate amount of zirconium dioxide was added to 50 ml RuC13 and FeSO4 solution (0.05 mol/L) with stirring for 30 min; 50 ml NaBH4 solution (0.5 niol/L) was then added drop by drop to the above solution (mass ratio Ru/Zr02=10-20%). The agitation was continued for 5 min. The black precipitate was kept in liquor solution for a while and then filtered and washed thoroughly with distilled water until neutrality; the nanosized amorphous Ru-FeB/ZrOz alloy catalyst was obtained. 2.2. C a t a l y t i c test

The selective hydrogenation of benzene was carried out in a 1-L autoclave. A total of 280 ml H20, 19.6 g ZnS04.7H20, and 4 g catalyst were introduced. The autoclave was sealed and then filled with Hz more than four times t o exclude air. Initially, the stirring rate was adjusted to 600 r/min and the H2 pressure was maintained at 3.0 MPa. When the temperature increased up to 413 K, 140 ml of benzene was charged into the reactor. After this, the stirring rate was adjusted to 1000 r/min and the pressure of H2 was elevated to 5.0 MPa and the reaction was considered to have begun. The reaction process was monitored by taking a small amount of reaction mixture a t intervals, followed by analysis in a gas chromatograph equipped with FID. The quantification of benzene, cyclohexene, and cyclohexane was carried out using calibration curves. Definition:

benzene converted by 1 g of catalyst per hour when the conversion of benzene is at 40 mol%, and it is a general industrial target for evaluating catalytic activity. S40 indicates cyclohexene selectivity when benzene conversion is 40 mol%. Benzene conversion and the cyclohexene selectivity were plotted as a function of time, respectively. S40 would be obtained from this plot. 7 4 0 is given by:

where, V is the volume of benzene (ml); ~ B is Z the density of benzene (0.88 g/ml); XBZis the conversion of benzene (40 mol%); t40 is reaction time (h) a t 40 mol% benzene conversion; Adcat.is the mass of active component Ru in the catalyst (g).

2.3. Pilot s t u d y of the Ru-Fe-B/ZrOz catalyst A total of 14 L of distilled water and 0.2 kg of Ru-Fe-B/ZrOz catalyst were introduced into a 50-L autoclave. Then the autoclave was sealed and air was flushed out using nitrogen gas several times. Next, hydrogen replaced the nitrogen and its pressure was maintained at 3.0 MPa. The initial stirring rate was 300 r/min, and the temperature was raised at the rate of 80 K/h. When the desired temperature, 403 K, was reached, 7 L of benzene was introduced. Subsequently, the pressure was adjusted to 4.5 MPa, the stirring rate was elevated to 600 r/min, and the reaction was considered to have begun. The reaction temperature was controlled at 4 1 3 f 2 K. The analysis of the product was carried out as mentioned above. 2.4. C h a r a c t e r i z a t i o n methods

XBZ =

mole of reacted benzene x 100% mole of initial benzene

SHE=

mole of cyclohexene formed x 100% mole of reacted benzene

YHE=

mole of cyclohexene formed x 100% mole of initial benzene

Where, XBEis benzene conversion, SHEis cyclohexene selectivity, YHE is cyclohexene yield. The catalyst activity and selectivity is given by 740 and 5’40, respectively. 740 indicates the mass of

The phases of the Ru-Fe-B/ZrOz catalyst were determined by X-ray diffraction (XRD) using Cu K, radiation; the tube voltage was 40 kV,and the tube current was 40 mA. The surface morphology of the active component on the support and the particle size were determined with the aid of a high-resolution transmission electron microscope (HRTEM, JEM-201 l),using an accelerating voltage of 100 kV. The amorphous character of the as-prepared catalysts was verified by selected area electron diffraction (SAED). The textural character of the as-prepared catalyst was determined by N2 physisorption a t 77 K on a Micronieritics TriStar 3000 apparatus.

Journal of Natural Gas Chemistry Vol. 15 N o . 4 2006

3. Results and discussion

Characterization of the Ru-Fe-B/ZrOZ catalyst 3.1.

3.1.1. XRD and TEM of the Ru-Fe-B/ZrOz

Figure 1 shows the changes in the XRD patterns of the Ru-Fe-B/ZrOZ catalyst during the heat pretreatment. As shown in Figure 1, when the sample was treated at temperatures below 673 K, no significant change in the XRD patterns was observed. Only the diffraction peaks of monoclinic zirconia were observed. No distinct peaks corresponding to Ru phase was seen in the patterns from 293 to 673 K. Therefore, it can be safely assumed that the Ru-Fe-B amorphous alloy is quiet stable below 673 K, which was attributed to the high dispersion of the Ru-FeB amorphous alloy particles on the ZrO2 matrix and the stabilizing effect of a small amount, of the additive of Fe. This is consistent with that reported in the literature [16]. However, various crystalline diffractional peaks corresponding to metallic Ru appeared on the XRD patterns of Ru-Fe-B/ZrOz sample when the temperature was above 673 K , indicating the crystallization of Ru-Fe-B amorphous alloy and the formation of crystallized Ru. The intensity of these Ru crystallized peaks increased gradually with the increase in treating temperature from 673 K to 873 K. Therefore, it could be concluded that the crystallization process of the Ru-Fe-B/ZrOa amorphous catalyst proceeded stepwise during which the crystallized Ru phase formed simultaneously.

32 1

Figures 2 and 3 show the TEM and SAED images of the Ru-Fe-B/ZrOz catalyst, respectively. The light gray circular or elliptic flakes shown in Figure 2 were naiiosized ZrOz crystallites, and the black particles were the active components comprising RUB and Fe amorphous alloys, with the particle diameter ranging from 3 to 6 nm. As shown in Figure 3, the Ru-FeB/ZrOs sample showed a number of diffraction circles and some small, white flecks on the diffraction circle that were identified as Zr02 crystallites, indicating the typical amorphous structure.

Figure 2. TEM image of the Ru-Fe-B/ZrOz catalyst

Figure 3. SAED pattern of the Ru-Fe-B/ZrOz catalyst

3.1.2. Texture character of the Ru-Fe-B/ZrOz catalyst .-0 m C

c

10

20

30

40

50

60

70

281(O )

Figure 1. XRD patterns of the Ru-Fe-B/ZrOa catalyst at different temperatures (1) 293 K , (2) 373 K , (3) 473 K, (4) 573 K , (5) 673 K , (6) 773 K , ( 7 ) 873 K

Figure 4 shows the curves of the N2 adsorptiondesorption isotherm of the Ru-Fe-B/ZrOz catalyst. It can be concluded from the hysteresis curves that the shape of the pores of this catalyst is similar to that of a capillary tube, with both sides open and the pore size distribution mainly ranging from 2 to 50 nm. The relative pressure ( p / p o ) at the separate region in the adsorption curves and desorption curves was more than 0.8, indicative of the catalyst with bigger pore diameter. Figure 5 shows the differential curve of the pore size distribution of the Ru-Fe-B/ZrOZ catalyst. It wa5

322

Shouchang Liu et al./ Journal of Natural Gas Chemistry Vol. 15 No. 4 2006

observed that the pore size distribution of the catalyst is between 2 nm and 100 nm and the most probable pore diameter is about 28 nm. The results of the measurement show that the BET surface area of RuFe-B/ZrOz catalyst is about 29 ni2.g-l , the specific pore volume is about 0.20 cm3.g-', and the average pore diameter ( 4 V I A by BET) is about 28 nm, which is in good agreement with the data obtained from the differential curve of pore size distribution.

100

I

d

B

f s: % $ C

so

0

0.2

0.4 0.6 Relative pressure @ l p , )

1 .o

0.8

90 85 -

. $

......

80

:

7s

-

70

-

i

mi

65

, , , , I , , ,

,

I

,

,

,

.

I

,

,

,

,

'

~

,

(

,

1

~

~

,

~

l

~

~

~

,

l

Figure 4. The NZadsorption and desorption isotherm of the Ru-Fe-B/ZrOz catalyst

C

I

-

I

J

I

\

x 0

I

I

10

100

Dlnm

Figure 5. The differential curve of pore distribution of the Ru-Fe-B/ZrOz catalyst

Activity and selectivity of the Ru-FeB/ZrOz catalyst 3.2.

Figure 6 shows the catalytic performance of the Ru-Fe-B/ZrOz in a stirring autoclave. It was observed that with the increase in benzene conversion, the cyclohexene yield was more than 50% (see Figure 6 (a)), and the selectivity to cyclohexene over the catalyst is beyond 85% in the initial stage (see Figure 6 (b)). From Figure 6, it can be observed that t40 is 20.4 min

and that ,940 is 81.6% a t the benzene conversion of 40% by interpolation method. The mass of active component Ru in the catalyst is 0.64 g. On the basis of these data and according to equation (1) mentioned above, 740 can be calculated as follows: 740=226 h-' , i.e. 226 g of benzene is converted by 1 g of catalyst per hour at benzene conversion of 40%. In comparison with the data reported thus far in the literature, tlit Ru-Fe-B/ZrOz catalyst has exhibited higher activity and better selectivity to cyclohexene. According to the various characterizations described above, the superior catalytic properties of the Ru-Fe-B/ZrOZ catalysts could be attributed to the following. First, the combination of the ultrafine size and the amorphous character, which could offer more active centers for benzene hydrogenation. Second, its textural character, which is helpful to the exterior diffusion of the intermediate cyclohexene, thereby avoiding its further hydrogenation to cyclohexane. Third, the promoting effect of the iron and boron species dispersed among

~

,

,

~

323

Journal of Natural Gas Chemistry VoJ. 1 5 No. 4 2006

the ruthenium particles is also an important factor because iron has low electronic affinity and boron is a n electron-deficient element, which could promote the water adsorption on the catalyst surface, thus greatly enhancing the hydrophilicity of the catalyst and suppressing further hydrogenation of cyclohexene to cyclohexane [20].

3.3. Operation conditions of the Ru-FeB/ZrOz catalyst The reaction for benzene selective hydrogenation to cyclohexene over the Ru-Fe-B/ZrOz nanosized catalyst comprises four phases: vapor (hydrogen), oil, aqueous, and solid catalyst. Besides, suitable amounts of zinc sulfate and zirconium dioxide are always added into the aqueous solution. Water can form a stagnant layer around the catalyst surface, competing with cyclohexene on surface adsorption because of the low solubility of benzene in water, thereby not favoring the hydrogenation of cyclohexene to cyclohexane. Catalyst hydrophilicity can be improved by adsorbing zinc sulfate on the catalyst surface, which is beneficial for enhancing the selectivity to cyclohexene. Zirconia as a dispersing agent can prolong the life of the catalyst and improve the selectivity to cyclohexene. Our previous studies have shown that for the selective hydrogenation of benzene over the catalysts, the suitable temperature was 408-413 K, the appropriate pressure was 4.5-5.0 MPa, the optimal volume ratio of water to benzene in the reaction system was 2:1, and the stirring rate should be high enough t o exclude the effect of diffusion. On the basis of the above-mentioned detailed studies, we further investigated in particular the influences of the composition of the reaction system, including the concentration of zinc ions in the aqueous solution, the content of ZrOz in the slurry, and the catalyst pretreatment, on the properties of the catalyst in pilot units so as to acquire some valuable information for industrial application for the catalysts. Then, the operation condition of the Ru-Fe-B/ZrOa catalyst is determined. 3.3.1. Influences of concentration of zinc ions

in the aqueous solution The influence of the concentration of zinc ions in the aqueous solution, in the absence of ZrOa, on the performance of the catalysts is shown in Table 1.

Table 1shows that benzene conversion was higher, whereas the selectivity to cyclohexene was lower in the absence of zinc sulfate in the slurry, whose pH value was 7.2. With the addition of zinc sulfate to the slurry, the pH value declined, benzene conversions decreased, and selectivities to cyclohexene showed a dramat,ic increase. However, the concentration of zinc up to 0.8 mol/L led to the highest benzene conversion and lower selectivity to cyclohexene, indicating that the yield of the byproduct cyclohexane increased. It was determined that the optimal concentration of Zn2+ in the slurry is 0.50-0.60 mol/L, with the pH values of 5.4-5.5. Under these conditions, both higher selectivity and yield of cyclohexene can be obtained at an acceptable benzene conversion rate. Table 1. Selectivity and yield over Ru-Fe-B/ZrOz catalyst for conversion of benzene to cyclohexene at different concentrations of the zinc ion 0

7.2

50.9

35.2

17.6

0.10

6.1

25.0

0.30

5.8

26.1

71.9 76.7

18.0 20.0

0.50

5.5

37.6

74.0

0.60

5.4

39.9

70.6

27.9 28.2

0.80

5.2

44.6

62.3

27.8

Reaction conditions: 280 ml H20, 140 ml C6&, 4 g Ru-FeB/ZrOz catalyst (0.64 g Ru), 140 p H , = 5 MPa, 1000 r/min, without pretreatment at reaction time of 5 min.

"c,

According to the above-mentioned results, the pilot study was carried out in a 50-L stirring autoclave with the amount of [Zn2+] being 0.50 and 0.60 mol/L, respectively, with other conditions remaining unchanged. The results of the reactions are shown in Figure 7. From Figure 7 (c), it can be seen that when the amount of [Zn2+]in the slurry was 0.5 mo1.L-' (curve Y H E ( ~a) maximum ), yield of cyclohexene of 35 mol% was obtained at the reaction time of 45 min, corresponding to benzene conversion of 66 mol% (Figure 7 (a)); when the amount of [Zn2+] in the slurry was 0.6 mo1.L-' (curve Y H E ( ~a) maximum ), yield of cyclohexene of 39 mol% was obtained at the reaction time of 19 min, corresponding to benzene conversion of 64 mol% (Figure 7 (a)). Therefore, it can be concluded that in the absence of ZrO2, bett,er results were obtained when the concentration of zinc ions in the slurry was 0.6 mo1.L-l compared with 0.5 mo1.L-l.

324

Shouchang Liu et al./ Journal of Natural Gas Chemistry Vol. 15 No. 4 2006

80 -

. s.

y:

60 40

-

20

-

0 0

0 0

10

20

30

40

50

60

I

,

10

l

,

l

,

l

30

20

40

,

l

.

50

.-

l

60

0

10

20

rlmin

rlmin

30

40

SO

60

rlmin

Figure 7. Benzene conversion (a), cyclohexene selectivity (b) and yield (c) over Ru-Fe-B/ZrOz catalyst at different zinc ion concentrations ( I ) [Zn2+]=0.5mol.L-l, (2) [Zn2+]=0.6 mo1.L-l

3.3.2. Influences of the content of ZrOz i n the slurry Apart from the presence of zinc sulfate, nanosized zirconia as a dispersing agent is added t o the slurry 100 -

90

80

-

80

60

-

40

-

in the presence of different concentrations of [Zn'']. Figure 8 shows the influence of content of zirconia in the slurry on the performance of the Ru-Fe-B/ZrOz catalyst in the presence of 0.5 mo1.L-l Zn2+.

70

s.

> 60 2

50

20 -

0

40

L

..

0

10

20

30

tlmin

40

50

60

0

20

40

60

X,, I mol%

80

100

0

10

20

30

40

50

60

70

tlmin

Figure 8. Effect of ZrO2 on the performance of Ru-Fe-B/ZrO2 catalyst at [Zn2+]=0.5 mo1.L-' (a) Benzene coversion, (b) Cyclohexene selectivity, (c) Cyclohexene yield; (1) without ZrO2, (2) Cat./ZrOz=l:l, (3) Cat./ZrO2=1:2, (4) Cat./ZrOz=1:2.5

From Figure 8 (a), it can be seen that with the increase in the amount of ZrOz in the slurry, there is a corresponding, simultaneous increase in benzene conversions because of the dispersing effect of zirconia on the catalyst. From Figure 8 (b), it can be observed that with the increase in the amount of ZrO2, the selectivity to cyclohexene varies in a complicated way and too high benzene conversion results in a decrease in selectivity at the mass ratio of catalyst to zirconia of 1:2.5 (curve 4), and among them, the highest selectivity to cyclohexene was observed at the mass ratio of catalyst to zirconia of 1:2 (curve 3). From Figure 8 (c), it can be seen that the highest yield of cyclo-

hexene can be achieved at the mass ratio of catalyst to zirconia of 1:2. From Figure 8 (a)-(c), it can be observed that the optimal amount of ZrOz in the slurry is equivalent to the mass ratio of catalyst to zirconia of 1:2 in the presence of 0.5 mo1.L-' Zn2+, a t which the highest yield of cyclohexene achieved is 46 mol%, with 65% selectivity t o cyclohexene a t 70 mol% benzene conversion and with the corresponding reaction time being approximately 22 min. In comparison with the data shown in Table 1 in the absence of zirconia, it is suggested that a suitable amount of zirconia in the slurry can not only enhance activity of the catalyst but also significantly improve the selectivity and

325

Journal of Natural Gas Chemistry Vol. 15 No. 4 2006

yield of cyclohexene. The hydrogenation reaction was carried out by fixing the mass ratio of catalyst to zirconia at 1:2 and by altering the concentration of zinc sulfate from 0.5 t o 0.6 mo1.L-l. The results show that the maximum yield of cyclohexene achieved was 43 mol% with 62% selectivity to cyclohexene at 70 mol% benzene conversion and the corresponding reaction time being approximately 17 min. In contrast to the two results discussed above, it, can be observed that the performance of the catalyst for benzene selective hydrogenation to cyclohexene is closely related to the chemical environment around the catalyst particles. When zirconia and zinc sulfa,te are used in combination in the slurry, the variations in benzene conversion and in the selectivity and yield of

cyclohexene are considerable different from the case where only zinc sulfate is present in the slurry. It should also be noted that the experiment was carried out in a pilot study, and few similar studies have been reported previously.

3.3.3. Influence of pretreatment to the Ru-FeB/ZrOz catalyst The pretreatment of the catalyst was also carried out in the pilot study. The operation is that under reaction conditions the catalyst runs for a period of time in tJheslurry in the absence of benzene. Figure 9 shows the comparison of performance of the runs with and without pretreatment for 12 h for the nanosized amorphous Ru-Fe-B/ZrO:! alloy catalysts. 50

0

10

20

30

40

50

60

I

0

10

20

30

40

50

60

tlmin

timin

Figure 9. Selectivity and yield for conversion of benzene to cyclohexene on Ru-Fe-B/ZrOn catalyst without pretreatment (1) and with pretreatment for 12 h (2)

From Figure 9 (a), it can be seen clearly that in the case of the pretreated catalyst, the benzene conversion showed a dramatic decrease, whereas the selectivities to cyclohexene showed a dramatic increase. From Figure 9 (b), it can be seen that for the pretreated catalyst the yields of cyclohexene increased gradually until the reaction time was about 70 min, and the maximum yield of cyclohexene was not achieved. This is considerably different from the case of the catalyst with no pretreatment. In the case of the pretreated catalyst, when the benzene conversion reached 40 mol%, a cyclohexene selectivity of more than 80% and a cyclohexene yield of more than 32 mol% were achieved in a reaction time of 40 min. Moreover, our pilot study also showed that the nanosized amorphous Ru-Fe-B/ZrOz alloy catalysts became more stable as a result of pretreatment. However, sedimentation and separation performance of the pretreated catalysts could evidently be improved,

thereby effectively avoiding the loss of catalysts during the continuous process of product separation. It is believed that the improvement of the catalytic properties of the nanosized amorphous Ru-Fe-B/ZrOz alloy catalysts may well be responsible for changing from hydrophobicity to hydrophilicity via the pretreatment of catalyst. On the basis of the results of the pilot studies, it is proposed that the Ru-Fe-B/ZrOz catalysts would have potential industrial application. 4. Conclusions Using various characterizations and studies on the performance of the novel nanosized amorphous Ru-FeB/ZrOz alloy catalyst prepared by chemical reduction with NaBH4 in a pilot study, the main conclusions are as follows. The Ru-Fe-B/ZrOz catalyst belongs t o an amor-

326

Shouchang Liu et a1./ Journal of Natural Gas Chemistry Vol. 15 No. 4 2006

phous alloy material of nanosize. The textural character shows that the shape of the pores of this catalyst is similar to a capillary tube, with both sides open and the pore size distribution mainly ranging from 2 to 50 nm, with the most probable pore diameter being approximately 28 nm. The high activity and excellent selectivity of this catalyst for benzene selective hydrogenation to cyclohexene is the result of its structural as well as textural characters. The hydrogenation reaction over the Ru-FeB/ZrOZ catalyst has to be carried out in a suitable chemical environment that aids in the formation of cyclohexene molecules. The slurry containing zinc sulfate and zirconia is necessary for enhancing the activity and selectivity of the catalysts to cyclohexene. The results of pilot studies show that the appropriate concentration of zinc sulfate in the aqueous solution is 0.50 mo1.L-’ and the suitable amount of zirconia in the slurry is equivalent to the mass ratio of catalyst t o zirconia of 1:2. It also shows that the performance of the catalysts can be considerably enhanced by pretreatment. These conclusions are considered to be very important information for the potential industrial application of the Ru-Fe-B/ZrOz catalyst. References [l] Van der Steen P J, Scholten J J F. Appl Catal, 1990, 58(1): 281 [2] Struyk 3, Scholten J J F. Appl Catal, 1990, 62(1): 151

[3] Struijk J, Scholten J J F. Appl Catal A , 1992, 82(2): 277 [4] Struijk J, D’Angremond M, Lucas-De Regt W J M, Scholten J ,J F. Appl Catal A , 1992, 83(2): 263 [5] Milone C, Neri G, Donato A, Musolino M G, Mercadante L. J Catal, 1996, 159(2): 253 [6] Dobert F, Gaube J. Chem Eny Scz, 1996, 51(1): 2873 [7] Hronec M, Cvengrosova*Z.Kralik M, Palma G, Corain B. J Mol Catal, 1996, 105(1-2): 25 [8] Nagahara H, Ono M, Konishi M, Fukuoka Y. Appl Surf Sci, 1997, 121/122: 448 [9] Hu S C, Chen Y W. Ind Eng Chem Res, 1997, 36(12): 5153 [lo] Ronchin L, Toniolo L. Catal Today, 1999, 48(1-4): 255 [ll] Ronchin L, Toniolo L. Catal Today, 2001, 66(2-4): 363 [12] Rorichin L, Toniolo L. Appl Catal A , 2001, 208(1,2):

77 [13] Spinace E V, Vaz J M. Catal Commun, 2003, 4(3): 91 [14] Hou Y J, Wang Y Q , He F, Han S, Mi Z T, Wu W, Min E Z. Material Lett, 2004, 58(7-8): 1267 [15] Hou Y J, Wang Y Q, He F, Mi W L, Li Z H, Mi Z T, Wu W, Min E Z. Appl Catal A , 2004, 259(1): 35 [16] Li H X, Wang W J , Li H, Deng J F. J Catal, 2000, 194(2): 211 [17] Li H X, Li H, Dai W L, Qiao M H. Appl Catal A , 2003, 238(1): 119 [18] Xie S H, Qiao M H, Li H X, Wang W J , Deng J F. Appl Catal A , 1999, 176(1): 129 [19] Liu S C, Luo G, Wang H R, Xie Y L, Yang B J, €Ian M L. Cuihua Xuebao (Chin J Catal), 2002, 23(4): 317 [20] Han M L, Liu S C, Yang X D, Wang K, Qiao Y Q, Zhang S F. Fenzi Cuihua ( J Mol Catal), 2004, 18(1): 47