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Synthesis and transformation of thiols and organic sulfides on MCM-41 mesoporous molecular sieves
MESOPOROUS MOLECULAR SIEVES 1998 Studies in Surface Science and Catalysis, Vol. 117 L. Bonneviot, F. B61and, C. Danumah, S. Giasson and S. Kaliaguine (Editors) o 1998 Elsevier Science B.V. All rights reserved.
509
Synthesis and transformation of thiols and organic sulfides on MCM-41 mesoporous molecular sieves M. Ziolek, I. Nowalc, P. Decyk and J. Kujawa A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznat, Poland
Hydrosulfurisation of methanol and ethanol and the decomposition of ethanethiol and diethyl sulfide on MCM-41, AIMCM-41, NbMCM-41 and their hydrogen and Ni-impregnated forms were studied by means of GC analysis. The results were compared with those on the zeolites. The mesoporous materials were characterized by XRD, N 2 adsorption-desorption isotherms, and their acidity- by the isopropanol conversion. The catalytic studies of the CH3OH + HzS reaction have shown a higher selectivity in the synthesis of methanethiol on HNbMCM-41 than that on HA! - material which was related to various acidity of the samples. Nickel impregnated mesoporous sieves were active in the transformation of ethanethiol and diethyl sulfide and their activity and selectivity were influenced by the H2S (the reaction product) chemisorption.
1. INTRODUCTION Industrial synthesis of light thiols and organic sulfides is carried out in the hydrosulfurisation process, i.e., the catalytic reaction between alcohol and hydrogen sulfide. The Bronsted and Lewis acidic eentres of the catalysts as well as basic sites can be employed in this reaction [ 1,2]. However, the same kind of the active sites can be involved in the competitive reactions, namely: i) the decomposition of alcohol to olefins and oligomers, ii) the transformation of the products, i.e., thiols and organic sulfides. Therefore, the optimisation of the catalysts needs a wide study of the synthesis and the transformation of sulfur organic compounds. The aim of our study was to check whether mesoporous molecular sieves of MCM-41 type can be applied in the hydrosulfurisation of methanol and ethanol as well as to measure their activity in the decomposition of ethanethiol and diethyl sulfide. The latter is important also in the study of the removal of toxic sulfur compounds from the exhaust gases. The earlier studies indicated that low or medium strength acidic centres together with basic sites are involved in the formation of CH3SH from CH3OH and I-IzS [ 1,2]. Therefore, the study on the use of the mesoporous molecular sieves were concentrated on the Nb- containing MCM-41 materials, because their hydrogen forms are less acidic than AI- containing one [3]. NiY zeolite exhibited the highest activity (among others Y zeolites modified with various cations) in the ethanethiol dehydrosulfurisation [4]. Moreover, NiMCM-41 material was active in the hydrodesulfurisation of thiophene [5]. These observations provoked us to start our study from the transformation of ethanethiol and diethyl sulfide on mespoporous sieves modified with nickel.
510
2. EXPERIMENTAL Silica (Ventron GMBI-I), NaY (Katalistiks) with Si/AI = 2.56 and mesoporous molecular sieves of MCM-41 type were used as parent materials for preparation of the modified catalysts. Mesoporous silica (MCM-41), alumino-silica (AIMCM-41 with Si/AI = 16) and niobio-silica (NbMCM-41 (16), NbMCM-41 (32) and NbMCM-41 (64)- numbers in brackets indicate the Si/Nb ratio) were prepared according to the procedure described in our earlier paper [6] and based on that presented by Beck et al. [7]. Hydrogen forms of all these materials were obtained via ion exchange with 0.25 M solution of NH4CI followed by heating in situ at 673 K in a flow of pure, dried helium. Nickel nitrate was used for impregnation of NaY, SiO2, MCM-41 and AIMCM-41 samples. 5 % wt of Ni was incorporated to the samples at room temperature. The samples were dried at 393 K for 12 h and calcined at 673 K for 2 h. The characterization of the prepared materials has been accomplished using XRD, sorption measurements and a test reaction. X-ray powder diffraction was carried out on a TUR 42 diffractometer using CuK a radiation. Nitrogen adsorption-desorption isotherms were determined at 77 K with Micrometrics 2010 using a conventional volumetric technique. The isopropanol dehydration and dehydrogenation (a test reaction) was performed using a pulse microreactor with a helium flow rate of 24 x 10.4 m s h"~. The catalyst bed (0.05 g) was first activated for 2 h at 673 K in helium flow. 5 )~1 pulses of alcohol were introduced at 523 K. Isopropanol and the reaction products were analysed on a CHROM 5 gas chromatograph on line with the micro reactor. A pulse technique was also applied in the study of the ethanethiol and diethyl sulfide transformation. The microreactor was filled with 0.02 g of the dehydrated form of the catalyst. Before the reaction the samples were activated for 2 h at 673 K in a helium flow. The reactions were carried out at 623 K. Pulses of I I~l of thiol or sulfide were introduced. Products were analysed using an on line gas chromatograph equipped with a FID detector. The hydrosulfurisation of methanol and ethanol was carried out in the flow system at 623 K. Portions of 0.1 g of dehydrated catalysts were used. For each catalytic experiment, the materials were pelleted without binder, ground, sieved to 0.5 - 1.0 mm diameter range, and activated in situ for 2h under a pure and dried helium flow at 673 K. Mixtures containing hydrogen sulfide (3 vol. %), alcohol (1.5 vol. %) and helium as a carrier gas were passed through the catalyst bed into a gas chromatograph. The total pressure was 1 atm and the flow rate 24 x 10-4 m s h -l. The reagents and products were analysed on line using a gas chromatograph, model SKI with a FID and a sulfur FPD detectors. The catalytic activity is presented as percentage of methanol conversion.
3. RESULTS AND DISCUSSION 3.1. Characterization of materials Powder X-ray diffraction patterns of niobium containing mesoporous materials were shown in our earlier papers [6,8]. They confirmed the presence of the hexagonal MCM-41 phase showing the decrease in the (100) peak intensity from Si/Nb = 64 to Si/Nb = 16. That feature was also registered in the nitrogen adsorption / desorption isotherms presented in Figure 1.
511
-- Adsorption
8OO
C~ 800 700 600
5OO
500
~400
eo
400,
1r o E2oo
300 200
Nb-MCM-41 (1G)
lOO 0
9
0.0
v
~
w
0.2
-
9
0.4
J
v
0.6
-
--
0.8
Nb-MCM.41 (32)
100 1.0
o.o
0;2
0;4
o16
o.e
1.o
1000 84
va
,=,,
~ 0 0 "
Figure 1. Nitrogen adsorption isotherms for niobium containing MCM-41 materials with various Si/Nb ratio (16, 32, 64).
Ee00. O
o
,,oo
"o r
IE20O
Nb-MCM-41 (64)
...,..
> O
-
0.0
w
-
b
'
-
v
-
0'.2 0.4 0.6 0.8 1.0 Relative pressure, P/Po
The hysteresis loop caused by a capillary condensation in the secondary inter particle
mesopores was higher when the concentration of Nb was higher (compare the isotherms for Si/Nb = 16 and Si/Nb = 64). That indicates the less ordering of the materials consisting the
higher amount of niobium (Si/Nb = 16). The impregnation of mesoporous sieves with Ni(NO3)2x6l-lzO did not destroyed the hexagonal phase of MCM-41 materials which was evidenced by XRD patterns. The acidity of MCM-41, AIMCM-41, NbMCM-41 and their hydrogen forms was presented in a previous paper [3]. It was shown that the number and strength of the Bronsted acid sites were much higher on HAIMCM-41 than that on HNbMCM-41. Moreover, the latter exhibited a higher concentration of Lewis acid sites than HAl- related materials. In this paper, the isopropanol conversion and the selectivity of this reaction characterized the acidic properties of the materials. Table I shows the results. Some samples were also tested after sulfidation carried out in the flow of hydrogen sulfide and helium at 623 K for 1 h. Hydrogen sulfide is a reagent
512 or product in the reactions studied and its chemisorption can change the surface properties of the catalysts as was often observed [for example: 10-12]. Table 1 Isopropanol conversion at 523 K Catalyst
i-propanol conv., % pure sample ' after I-I~S ads. 100 HNaY HAIMCM-41 88 HNbMCM-41 (16) 37 Ni/NaY 95 70 Ni/AIMCM-41 29* 4* Ni/MCM-41 3.5 16' Ni/SiO 2 3 MCM-41 2.5 ._SiO~ 0.5 * - acetone in the reaction products The main reaction product, on all investigated samples, was propene formed either on the Bmnsted acid sites or on the pairs of Lewis acid and basic centres [9]. Moreover, diisopropyl ether, produced with the participation of medium or strong Lewis acid base pairs, was registered on HNbMCM-41, Ni/MCM-41 and Ni/SiOz samples. Acetone was observed only in few experiments marked by a star. The results presented in Table 1 indicate that Ni impregnated materials exhibited rather low acidity which significantly increased after the sulfidation with H2S ( especially for the Ni/AIMCM-41 material). The increase of the Bmnsted acidity of NiNaY after the sulfidation with H2S was observed by Koranyi at al. [ 13]. The same can be proposed for Ni supported on mesoporous materials and silica taking into account an increase of the isopropanol conversion after the H2S pretreatment. The fact that sulfided Ni/MCM-41 and Ni/SiO2 showed a selectivity of about 10 % towards acetone suggests that the sulfidation change also the basicity of these samples. The acidity of the materials studied, concluded on the basis of the isopropanol conversion, was much higher for Y zeolites than MCM-41 materials. Among the mesoporous materials, HAIMCM-41 exhibited the acidity comparable with that of the HNaY and Ni/NaY zeolites. 3.2. H y d r o s u l f u r i s a t i o n o f alcohols
Figure 2 exhibits the results of the reaction between methanol and hydrogen sulfide on hydrogen forms of niobium- and aluminum - containing MCM-41 materials. The highest methanol conversion on HAIMCM-41 was accompanied by the formation of various products: methanethiol, dimethyl sulfide, dimethyl ether and hydrocarbons (not plotted in the figure). The selectivity to sulfur organic products was about 50 %. That means that the reactions competitive to hydrosulfurisation (i.e. the transformation of methanol in ether and hydrocarbons) took place on this catalyst. Moreover, the yields of thiol and sulfide were not very different so, one cannot obtain a high selectivity to one of these products. The same behaviour was observed earlier on HNaY zeolites [2,14]. Contrary, HNbMCM-41 materials showed a lower activity than HAIMCM-41 sample and a very high selectivity towards
513 methanethiol (38 % yield for--40 % of the methanol conversion). The activity of HNbMCM41 materials and the yield of thiol decreased with the growth of Si/Nb ratio. In the same order the number of Lewis acid sites [3] and the selectivity to thiol increased. 90
m~mXm__m~m
80
~,
70.
40'
~R
0~0~0~0~0~
"0
660" o
C
0
0
40,
0 c r
30"
.~30-
~176176 ~--0~0~0-----0~. 0
1010
0
(~0
1:20
1~)
2~)
0
Time on stream, min
60
1",20
1~0
2,io
Time on stream, mln
24,
40-
20. 3O
;r
;~ 16
G)
>,, o') 12.
9~, 2oOr
t~l
"r" O
1-
8
o
A
"
6()
1:~)
180
Time on stream, min
O
. - i i - - HAl MCM-41 ---O--- HNb MCM-41 (16) - - O . - - HNb MCM-41 (32) - - & . - - HNb MCM-41 (64)
lO
.
2~10
6()
I:L~)
1~)
2d~0
Time on stream, min
Figure 2. Activity and selectivity of hydrogen forms of mesoporous molecular sieves in the reaction between methanol and hydrogen sulfide at 623 K. The participation of Lewis acid sites in the methanethiol formation was stressed many times in the literature [for example: 1, 2, 15-17]. It is important to note that NbMCM-41 materials showed very low activity in this reaction which can be correlated with the low number of the Lewis acid sites [3] (147 and 208 x 1017 per 1 g for Si/Nb = 16 and 32, respectively) to compare with HNbMCM-41 (493 and 522 x 1017 per I g for Si/Nb = 16 and 32, respectively).
514 The reaction between ethanol and hydrogen sulfide hardly occurred at 623 K. Table 2 shows that the conversion of ethanol on hydrogen forms of Y zeolite and niobium containing MCM-41 material was very high but the yield of sulfur organic compounds was very low. That behaviour can be due to the high dehydration activity of these samples. Nickel impregnated MCM-41 materials presented very low activity in the hydrosulfurisation of ethanol. Table 2 The activity and the yield of products in the reaction between ethanol and hydrogen sulfide at 623 K (the results at the stationary state) Catalyst HNaY HNbMCM-41 (16) Ni/MCM-41
Ethanol conv., % ' 100 97 5
Ethene 97 83
Yield~ % Etahnethiol Diethyl sulfide 0.3 0.1 8 -
0.3
3
Other HC* 2.6 6
-
1.7
* H C = hydrocarbons 3.3. Transformation of ethanethiol and diethyl sulfide Hydrogen forms of mesoporous molecular sieves which were active in the dehydration of ethanol (in the C2I-I5OH+ I-I2S reaction), did not exhibit a high activity in the transformation of ethanethiol (Table 3). It can be due to the low concentration of Bronsted acid sites in the mesoporous materials [3] (much lower than in HNaY) and the absence of extra lattice cations strong enough for coordination of sulfur organic compounds. An impregnation of these materials with Ni nitrate caused a great increase in the transformation of both ethanethiol and diethyl sulfide (Table 3). All Ni impregnated mesoporous materials and silica showed a higher activity in the transformation of ethanethiol than that for diethyl sulfide. The main reaction products were hydrocarbons and hydrogen sulfide. As the results collected in Table 3 show the formation of other sulfur products was rather casual. Table 3 Transformation of sulfur organic compounds at 623 K - the results for the first pulse Catalyst
C2HsSH con% %
NaY MCM-41
26 5
HAIMCM-41 HNbMCM--41 (16) Ni/NaY Ni/MCM-41 Ni/AIMCM-41 Ni/SiO~..
11 8 100 97 100 98
Yield, % (C2I-Is):S Yield 2 % (C~H,)2S C4H4S cony., % C,H~SH c4H4's 3 0.2
0.4 traces
2 traces 1.5 traces traces 4 9 1 5 2.5 3 . 0.5
24 7
5 4
13 8 94 25 48 33
5 2 traces 0.2 0.5 0.8
0.3 0. I
traces 0.3 6 2 1.6 0.3
However, it is important to note that all Ni impregnated materials were not stable in the transformation of ethanethiol. Their activity decreased with the number of pulses. As nickel play the role of active sites [4] the decrease of the activity should be due to the poisoning effect
515
of the active centres. The toxicity effect of hydrogen sulfide and thiophene was described in the literature [ 13] for the benzene hydrogenation on Ni/SiO 2. As the main reaction products in our studies were hydrocarbons and hydrogen sulfide, and the decrease of the activity was registered for the transformation of ethanethiol, we supposed that hydrogen sulfide could be responsible for the deactivation of Ni- containing catalysts. Therefore, the experiments with the hydrogen sulfide pretreated catalysts (in a helium flow at 623 K) were performed. The results were similar for all samples studied and are shown in Figure 3. The deactivation of Ni/NaY zeolite is not as fast as that of mesoporous materials. The chemisorption of hydrogen sulfide led to a significant decrease of the activity in spite of the increase in the catalyst acidity. This shows the dominant role of nickel in the active site. The hydrogen sulfide preadsorption causes the decrease in diethyl sulfide conversion too. In this reaction, the H2S chemisorption influences the selectivity. Figure 4 shows the example plotted for the Ni/AIMCM-41 mesoporous sieve. First, one should stress that the activity of this material in the conversion of diethyl sulfide was stable on both pure and sulfided sample. Moreover, the transformation of diethyl sulfide into ethanethiol increased with the number of pulses on the pure material contrary to that on the sulfided catalyst on which the selectivity to ethanethiol was high in the first pulse and decreased with the number of pulses. That behaviour can be correlated with the generation of the Bronsted acid sites after chemisorption of hydrogen sulfide [13]. The Bronsted acid centres are involved in the transformation of diethyl sulfide to ethanethiol. One cannot exclude the participation of the chemisorbed hydrogen sulfide as a reagent in the reaction with diethyl sulfide: (CH3) 2 S + H2S --~ 2 CH3SH.
~100
,,,-'-----X~X
X
~80
80 m
dm
7o7~
~90 C
o60
0
O
- . - ~ - N i I/q MCM.41- p u r 9 --O~-NIIAIMCM.41 after H2S ads.
r-
o80
"~.\^
o70
t~
050
J --@-NiO~I --X-- N ! I NaY
"U
~.40 n
r-
~"---A ...... -
m50 e-
~
~
50"* ~r 40om
A'''~
o.
30~
=30
Q
@ r-
m40
m
60=
. . . . . ~ ..... ~ ..... 4 - ..... ~ ..... @ .....
O". . . . . 9 . . . . . . 1
2
9 ...... 3
Number
9 ...... 4
O~
6
.....
5
. . . . . . . 9 ...... O-i-
glO
6
7
of pulses
Figure 3. Ethanethiol conversion at 623 K on Ni-impregnated materials (solid linepure sample; dashed line- after H2S ads.)
QO
~
........ .ff-":--e o-
1
2
3
Number
4
..A
10~
o,,'~
5
2o~
6
7
o~
of pulses
Figure 4. Activity and selectivity of Niimpregnated AIMCM-41 in the diethyl sulfide transformation at 623 K (solid lineconversion; dashed line- selectivity).
4. CONCLUSIONS Among the mesoporous sieves of MCM-41 type, the HAIMCM-41 sample showed the highest conversion of methanol in the reaction between CH3OH and I-IzS. However, only about half of alcohol reacted with H2S towards both sulfur products: thiol and sulfide. The less acidic samples, i.e., HNbMCM-41 mesoporous materials exhibited a lower methanol
516 conversion but a very high selectivity to methanethiol. The activity of HNb- containing MCM41 materials decreased with Si/Nb ratio showing the role of Nb content in this reaction. The hydrogen forms of the mesoporous sieves exhibited a low activity in the decomposition of ethanethiol and diethyl sulfide but an impregnation of the materials with nickel increased significantly the activity. The influence of I~S (the reaction product) on the reaction pathway was observed. The adsorption of I-hS decreased the catalytic activity of the Ni- samples in spite of the acidity growth. The presence of hydrogen sulfide on the catalyst surface caused an increase of the ethanethiol formation in the reaction of diethyl sulfide. The presented results did not indicate that the mesoporous molecular sieves of MCM.41 type exhibit a higher activity in the reactions studied than the traditional zeolites. However, the facts that they are active and in the case of HNbMCM-41 - very selective in the methanethiol formation, are important for the studies of the synthesis and transformation of larger sulfur organic molecules which can hardly occur in the channels of the zeolites. ACKNOWLEDGEMENTS This work was supported by the fund from A. Mickiewicz University, Faculty of Chemistry. The assistance of M. Kubiak, H. Nowicka and A. Rauhut in the experimental work is acknowledge.
REFERENCES
1. A.V. Mashkina, Russian Chemical Reviews, 64 (1995) 1131. 2. M. Ziolek, J. Czyzniewska, J. Kujawa, A. Travert, F. Mauge and J.C. Lavalley, Zeolites, in press. 3. M. Ziolek, I. Nowak and J.C. Lavalley, Catal. Lett., 45 (1997) 259. 4. M. Sugioka and K. Aomura, Prep. Am. Chem. Soc.,Div. Petrol. Chem., 25 (1980) 245. 5. J. Cui, Y.H. Yue, Y. Sun, W.Y. Don8 and Z. Gao, Stud. Surf. Sci. Catal., 105 (1997)687. 6. M. Ziolek and I. Nowak, Zeolites, 18 (1997) 356. 7. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Olson, E.W. Sheppard, S.B. McCullen, J.B. HJggins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 8. I. Nowak and M. Ziolek, Proc. 3rd Polish German Zeolite Colloqium, (M. Rozwadowski, Ed.) NCU Press, Torun 1998, p. 161. 9. A. Gervasini, J. Fenyvesi and A. Auroux, Catal. Lett., 43 (1997) 219. 10. M. Ziolek, J. Kujawa, O. Saur and J.C. Lavalley, J. Molec. Catal. A: Chemical, 97 (1995) 49. 11. M. Sugioka, React. Kinet. Catal. Lea., 41 (1990) 345. 12. A. Diaz, L.M. Gandia, J.A. Odriozola and M. Montes, J. Catal., 162 (1996) 349. 13. T.I. Koranyi, F. Moreau, V.V. gozanov and E.A. Rozanova, J. Mol. Struc., 410-411 (1997) 103. 14. M. Ziolek and I. Bresinska, Zeolites, 5 (1985) 245. 15. A.V. Mashkina and V.N. Yakovleva, Kinetika i Kataliz, 32 (1991) 636. 16. A.V. Mashkina, V.M. Mastikhin, V.Yu. Mashkin, A.V. Nosov and V.M. Kudenkov, Kinet. Katal., 34 (1993) 880. 17. M. Ziolek, J. Kujawa, O. Saur and J.C. Lavalley, J. Phys. Chem., 97 (1993) 9761.
509
Synthesis and transformation of thiols and organic sulfides on MCM-41 mesoporous molecular sieves M. Ziolek, I. Nowalc, P. Decyk and J. Kujawa A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznat, Poland
Hydrosulfurisation of methanol and ethanol and the decomposition of ethanethiol and diethyl sulfide on MCM-41, AIMCM-41, NbMCM-41 and their hydrogen and Ni-impregnated forms were studied by means of GC analysis. The results were compared with those on the zeolites. The mesoporous materials were characterized by XRD, N 2 adsorption-desorption isotherms, and their acidity- by the isopropanol conversion. The catalytic studies of the CH3OH + HzS reaction have shown a higher selectivity in the synthesis of methanethiol on HNbMCM-41 than that on HA! - material which was related to various acidity of the samples. Nickel impregnated mesoporous sieves were active in the transformation of ethanethiol and diethyl sulfide and their activity and selectivity were influenced by the H2S (the reaction product) chemisorption.
1. INTRODUCTION Industrial synthesis of light thiols and organic sulfides is carried out in the hydrosulfurisation process, i.e., the catalytic reaction between alcohol and hydrogen sulfide. The Bronsted and Lewis acidic eentres of the catalysts as well as basic sites can be employed in this reaction [ 1,2]. However, the same kind of the active sites can be involved in the competitive reactions, namely: i) the decomposition of alcohol to olefins and oligomers, ii) the transformation of the products, i.e., thiols and organic sulfides. Therefore, the optimisation of the catalysts needs a wide study of the synthesis and the transformation of sulfur organic compounds. The aim of our study was to check whether mesoporous molecular sieves of MCM-41 type can be applied in the hydrosulfurisation of methanol and ethanol as well as to measure their activity in the decomposition of ethanethiol and diethyl sulfide. The latter is important also in the study of the removal of toxic sulfur compounds from the exhaust gases. The earlier studies indicated that low or medium strength acidic centres together with basic sites are involved in the formation of CH3SH from CH3OH and I-IzS [ 1,2]. Therefore, the study on the use of the mesoporous molecular sieves were concentrated on the Nb- containing MCM-41 materials, because their hydrogen forms are less acidic than AI- containing one [3]. NiY zeolite exhibited the highest activity (among others Y zeolites modified with various cations) in the ethanethiol dehydrosulfurisation [4]. Moreover, NiMCM-41 material was active in the hydrodesulfurisation of thiophene [5]. These observations provoked us to start our study from the transformation of ethanethiol and diethyl sulfide on mespoporous sieves modified with nickel.
510
2. EXPERIMENTAL Silica (Ventron GMBI-I), NaY (Katalistiks) with Si/AI = 2.56 and mesoporous molecular sieves of MCM-41 type were used as parent materials for preparation of the modified catalysts. Mesoporous silica (MCM-41), alumino-silica (AIMCM-41 with Si/AI = 16) and niobio-silica (NbMCM-41 (16), NbMCM-41 (32) and NbMCM-41 (64)- numbers in brackets indicate the Si/Nb ratio) were prepared according to the procedure described in our earlier paper [6] and based on that presented by Beck et al. [7]. Hydrogen forms of all these materials were obtained via ion exchange with 0.25 M solution of NH4CI followed by heating in situ at 673 K in a flow of pure, dried helium. Nickel nitrate was used for impregnation of NaY, SiO2, MCM-41 and AIMCM-41 samples. 5 % wt of Ni was incorporated to the samples at room temperature. The samples were dried at 393 K for 12 h and calcined at 673 K for 2 h. The characterization of the prepared materials has been accomplished using XRD, sorption measurements and a test reaction. X-ray powder diffraction was carried out on a TUR 42 diffractometer using CuK a radiation. Nitrogen adsorption-desorption isotherms were determined at 77 K with Micrometrics 2010 using a conventional volumetric technique. The isopropanol dehydration and dehydrogenation (a test reaction) was performed using a pulse microreactor with a helium flow rate of 24 x 10.4 m s h"~. The catalyst bed (0.05 g) was first activated for 2 h at 673 K in helium flow. 5 )~1 pulses of alcohol were introduced at 523 K. Isopropanol and the reaction products were analysed on a CHROM 5 gas chromatograph on line with the micro reactor. A pulse technique was also applied in the study of the ethanethiol and diethyl sulfide transformation. The microreactor was filled with 0.02 g of the dehydrated form of the catalyst. Before the reaction the samples were activated for 2 h at 673 K in a helium flow. The reactions were carried out at 623 K. Pulses of I I~l of thiol or sulfide were introduced. Products were analysed using an on line gas chromatograph equipped with a FID detector. The hydrosulfurisation of methanol and ethanol was carried out in the flow system at 623 K. Portions of 0.1 g of dehydrated catalysts were used. For each catalytic experiment, the materials were pelleted without binder, ground, sieved to 0.5 - 1.0 mm diameter range, and activated in situ for 2h under a pure and dried helium flow at 673 K. Mixtures containing hydrogen sulfide (3 vol. %), alcohol (1.5 vol. %) and helium as a carrier gas were passed through the catalyst bed into a gas chromatograph. The total pressure was 1 atm and the flow rate 24 x 10-4 m s h -l. The reagents and products were analysed on line using a gas chromatograph, model SKI with a FID and a sulfur FPD detectors. The catalytic activity is presented as percentage of methanol conversion.
3. RESULTS AND DISCUSSION 3.1. Characterization of materials Powder X-ray diffraction patterns of niobium containing mesoporous materials were shown in our earlier papers [6,8]. They confirmed the presence of the hexagonal MCM-41 phase showing the decrease in the (100) peak intensity from Si/Nb = 64 to Si/Nb = 16. That feature was also registered in the nitrogen adsorption / desorption isotherms presented in Figure 1.
511
-- Adsorption
8OO
C~ 800 700 600
5OO
500
~400
eo
400,
1r o E2oo
300 200
Nb-MCM-41 (1G)
lOO 0
9
0.0
v
~
w
0.2
-
9
0.4
J
v
0.6
-
--
0.8
Nb-MCM.41 (32)
100 1.0
o.o
0;2
0;4
o16
o.e
1.o
1000 84
va
,=,,
~ 0 0 "
Figure 1. Nitrogen adsorption isotherms for niobium containing MCM-41 materials with various Si/Nb ratio (16, 32, 64).
Ee00. O
o
,,oo
"o r
IE20O
Nb-MCM-41 (64)
...,..
> O
-
0.0
w
-
b
'
-
v
-
0'.2 0.4 0.6 0.8 1.0 Relative pressure, P/Po
The hysteresis loop caused by a capillary condensation in the secondary inter particle
mesopores was higher when the concentration of Nb was higher (compare the isotherms for Si/Nb = 16 and Si/Nb = 64). That indicates the less ordering of the materials consisting the
higher amount of niobium (Si/Nb = 16). The impregnation of mesoporous sieves with Ni(NO3)2x6l-lzO did not destroyed the hexagonal phase of MCM-41 materials which was evidenced by XRD patterns. The acidity of MCM-41, AIMCM-41, NbMCM-41 and their hydrogen forms was presented in a previous paper [3]. It was shown that the number and strength of the Bronsted acid sites were much higher on HAIMCM-41 than that on HNbMCM-41. Moreover, the latter exhibited a higher concentration of Lewis acid sites than HAl- related materials. In this paper, the isopropanol conversion and the selectivity of this reaction characterized the acidic properties of the materials. Table I shows the results. Some samples were also tested after sulfidation carried out in the flow of hydrogen sulfide and helium at 623 K for 1 h. Hydrogen sulfide is a reagent
512 or product in the reactions studied and its chemisorption can change the surface properties of the catalysts as was often observed [for example: 10-12]. Table 1 Isopropanol conversion at 523 K Catalyst
i-propanol conv., % pure sample ' after I-I~S ads. 100 HNaY HAIMCM-41 88 HNbMCM-41 (16) 37 Ni/NaY 95 70 Ni/AIMCM-41 29* 4* Ni/MCM-41 3.5 16' Ni/SiO 2 3 MCM-41 2.5 ._SiO~ 0.5 * - acetone in the reaction products The main reaction product, on all investigated samples, was propene formed either on the Bmnsted acid sites or on the pairs of Lewis acid and basic centres [9]. Moreover, diisopropyl ether, produced with the participation of medium or strong Lewis acid base pairs, was registered on HNbMCM-41, Ni/MCM-41 and Ni/SiOz samples. Acetone was observed only in few experiments marked by a star. The results presented in Table 1 indicate that Ni impregnated materials exhibited rather low acidity which significantly increased after the sulfidation with H2S ( especially for the Ni/AIMCM-41 material). The increase of the Bmnsted acidity of NiNaY after the sulfidation with H2S was observed by Koranyi at al. [ 13]. The same can be proposed for Ni supported on mesoporous materials and silica taking into account an increase of the isopropanol conversion after the H2S pretreatment. The fact that sulfided Ni/MCM-41 and Ni/SiO2 showed a selectivity of about 10 % towards acetone suggests that the sulfidation change also the basicity of these samples. The acidity of the materials studied, concluded on the basis of the isopropanol conversion, was much higher for Y zeolites than MCM-41 materials. Among the mesoporous materials, HAIMCM-41 exhibited the acidity comparable with that of the HNaY and Ni/NaY zeolites. 3.2. H y d r o s u l f u r i s a t i o n o f alcohols
Figure 2 exhibits the results of the reaction between methanol and hydrogen sulfide on hydrogen forms of niobium- and aluminum - containing MCM-41 materials. The highest methanol conversion on HAIMCM-41 was accompanied by the formation of various products: methanethiol, dimethyl sulfide, dimethyl ether and hydrocarbons (not plotted in the figure). The selectivity to sulfur organic products was about 50 %. That means that the reactions competitive to hydrosulfurisation (i.e. the transformation of methanol in ether and hydrocarbons) took place on this catalyst. Moreover, the yields of thiol and sulfide were not very different so, one cannot obtain a high selectivity to one of these products. The same behaviour was observed earlier on HNaY zeolites [2,14]. Contrary, HNbMCM-41 materials showed a lower activity than HAIMCM-41 sample and a very high selectivity towards
513 methanethiol (38 % yield for--40 % of the methanol conversion). The activity of HNbMCM41 materials and the yield of thiol decreased with the growth of Si/Nb ratio. In the same order the number of Lewis acid sites [3] and the selectivity to thiol increased. 90
m~mXm__m~m
80
~,
70.
40'
~R
0~0~0~0~0~
"0
660" o
C
0
0
40,
0 c r
30"
.~30-
~176176 ~--0~0~0-----0~. 0
1010
0
(~0
1:20
1~)
2~)
0
Time on stream, min
60
1",20
1~0
2,io
Time on stream, mln
24,
40-
20. 3O
;r
;~ 16
G)
>,, o') 12.
9~, 2oOr
t~l
"r" O
1-
8
o
A
"
6()
1:~)
180
Time on stream, min
O
. - i i - - HAl MCM-41 ---O--- HNb MCM-41 (16) - - O . - - HNb MCM-41 (32) - - & . - - HNb MCM-41 (64)
lO
.
2~10
6()
I:L~)
1~)
2d~0
Time on stream, min
Figure 2. Activity and selectivity of hydrogen forms of mesoporous molecular sieves in the reaction between methanol and hydrogen sulfide at 623 K. The participation of Lewis acid sites in the methanethiol formation was stressed many times in the literature [for example: 1, 2, 15-17]. It is important to note that NbMCM-41 materials showed very low activity in this reaction which can be correlated with the low number of the Lewis acid sites [3] (147 and 208 x 1017 per 1 g for Si/Nb = 16 and 32, respectively) to compare with HNbMCM-41 (493 and 522 x 1017 per I g for Si/Nb = 16 and 32, respectively).
514 The reaction between ethanol and hydrogen sulfide hardly occurred at 623 K. Table 2 shows that the conversion of ethanol on hydrogen forms of Y zeolite and niobium containing MCM-41 material was very high but the yield of sulfur organic compounds was very low. That behaviour can be due to the high dehydration activity of these samples. Nickel impregnated MCM-41 materials presented very low activity in the hydrosulfurisation of ethanol. Table 2 The activity and the yield of products in the reaction between ethanol and hydrogen sulfide at 623 K (the results at the stationary state) Catalyst HNaY HNbMCM-41 (16) Ni/MCM-41
Ethanol conv., % ' 100 97 5
Ethene 97 83
Yield~ % Etahnethiol Diethyl sulfide 0.3 0.1 8 -
0.3
3
Other HC* 2.6 6
-
1.7
* H C = hydrocarbons 3.3. Transformation of ethanethiol and diethyl sulfide Hydrogen forms of mesoporous molecular sieves which were active in the dehydration of ethanol (in the C2I-I5OH+ I-I2S reaction), did not exhibit a high activity in the transformation of ethanethiol (Table 3). It can be due to the low concentration of Bronsted acid sites in the mesoporous materials [3] (much lower than in HNaY) and the absence of extra lattice cations strong enough for coordination of sulfur organic compounds. An impregnation of these materials with Ni nitrate caused a great increase in the transformation of both ethanethiol and diethyl sulfide (Table 3). All Ni impregnated mesoporous materials and silica showed a higher activity in the transformation of ethanethiol than that for diethyl sulfide. The main reaction products were hydrocarbons and hydrogen sulfide. As the results collected in Table 3 show the formation of other sulfur products was rather casual. Table 3 Transformation of sulfur organic compounds at 623 K - the results for the first pulse Catalyst
C2HsSH con% %
NaY MCM-41
26 5
HAIMCM-41 HNbMCM--41 (16) Ni/NaY Ni/MCM-41 Ni/AIMCM-41 Ni/SiO~..
11 8 100 97 100 98
Yield, % (C2I-Is):S Yield 2 % (C~H,)2S C4H4S cony., % C,H~SH c4H4's 3 0.2
0.4 traces
2 traces 1.5 traces traces 4 9 1 5 2.5 3 . 0.5
24 7
5 4
13 8 94 25 48 33
5 2 traces 0.2 0.5 0.8
0.3 0. I
traces 0.3 6 2 1.6 0.3
However, it is important to note that all Ni impregnated materials were not stable in the transformation of ethanethiol. Their activity decreased with the number of pulses. As nickel play the role of active sites [4] the decrease of the activity should be due to the poisoning effect
515
of the active centres. The toxicity effect of hydrogen sulfide and thiophene was described in the literature [ 13] for the benzene hydrogenation on Ni/SiO 2. As the main reaction products in our studies were hydrocarbons and hydrogen sulfide, and the decrease of the activity was registered for the transformation of ethanethiol, we supposed that hydrogen sulfide could be responsible for the deactivation of Ni- containing catalysts. Therefore, the experiments with the hydrogen sulfide pretreated catalysts (in a helium flow at 623 K) were performed. The results were similar for all samples studied and are shown in Figure 3. The deactivation of Ni/NaY zeolite is not as fast as that of mesoporous materials. The chemisorption of hydrogen sulfide led to a significant decrease of the activity in spite of the increase in the catalyst acidity. This shows the dominant role of nickel in the active site. The hydrogen sulfide preadsorption causes the decrease in diethyl sulfide conversion too. In this reaction, the H2S chemisorption influences the selectivity. Figure 4 shows the example plotted for the Ni/AIMCM-41 mesoporous sieve. First, one should stress that the activity of this material in the conversion of diethyl sulfide was stable on both pure and sulfided sample. Moreover, the transformation of diethyl sulfide into ethanethiol increased with the number of pulses on the pure material contrary to that on the sulfided catalyst on which the selectivity to ethanethiol was high in the first pulse and decreased with the number of pulses. That behaviour can be correlated with the generation of the Bronsted acid sites after chemisorption of hydrogen sulfide [13]. The Bronsted acid centres are involved in the transformation of diethyl sulfide to ethanethiol. One cannot exclude the participation of the chemisorbed hydrogen sulfide as a reagent in the reaction with diethyl sulfide: (CH3) 2 S + H2S --~ 2 CH3SH.
~100
,,,-'-----X~X
X
~80
80 m
dm
7o7~
~90 C
o60
0
O
- . - ~ - N i I/q MCM.41- p u r 9 --O~-NIIAIMCM.41 after H2S ads.
r-
o80
"~.\^
o70
t~
050
J --@-NiO~I --X-- N ! I NaY
"U
~.40 n
r-
~"---A ...... -
m50 e-
~
~
50"* ~r 40om
A'''~
o.
30~
=30
Q
@ r-
m40
m
60=
. . . . . ~ ..... ~ ..... 4 - ..... ~ ..... @ .....
O". . . . . 9 . . . . . . 1
2
9 ...... 3
Number
9 ...... 4
O~
6
.....
5
. . . . . . . 9 ...... O-i-
glO
6
7
of pulses
Figure 3. Ethanethiol conversion at 623 K on Ni-impregnated materials (solid linepure sample; dashed line- after H2S ads.)
QO
~
........ .ff-":--e o-
1
2
3
Number
4
..A
10~
o,,'~
5
2o~
6
7
o~
of pulses
Figure 4. Activity and selectivity of Niimpregnated AIMCM-41 in the diethyl sulfide transformation at 623 K (solid lineconversion; dashed line- selectivity).
4. CONCLUSIONS Among the mesoporous sieves of MCM-41 type, the HAIMCM-41 sample showed the highest conversion of methanol in the reaction between CH3OH and I-IzS. However, only about half of alcohol reacted with H2S towards both sulfur products: thiol and sulfide. The less acidic samples, i.e., HNbMCM-41 mesoporous materials exhibited a lower methanol
516 conversion but a very high selectivity to methanethiol. The activity of HNb- containing MCM41 materials decreased with Si/Nb ratio showing the role of Nb content in this reaction. The hydrogen forms of the mesoporous sieves exhibited a low activity in the decomposition of ethanethiol and diethyl sulfide but an impregnation of the materials with nickel increased significantly the activity. The influence of I~S (the reaction product) on the reaction pathway was observed. The adsorption of I-hS decreased the catalytic activity of the Ni- samples in spite of the acidity growth. The presence of hydrogen sulfide on the catalyst surface caused an increase of the ethanethiol formation in the reaction of diethyl sulfide. The presented results did not indicate that the mesoporous molecular sieves of MCM.41 type exhibit a higher activity in the reactions studied than the traditional zeolites. However, the facts that they are active and in the case of HNbMCM-41 - very selective in the methanethiol formation, are important for the studies of the synthesis and transformation of larger sulfur organic molecules which can hardly occur in the channels of the zeolites. ACKNOWLEDGEMENTS This work was supported by the fund from A. Mickiewicz University, Faculty of Chemistry. The assistance of M. Kubiak, H. Nowicka and A. Rauhut in the experimental work is acknowledge.
REFERENCES
1. A.V. Mashkina, Russian Chemical Reviews, 64 (1995) 1131. 2. M. Ziolek, J. Czyzniewska, J. Kujawa, A. Travert, F. Mauge and J.C. Lavalley, Zeolites, in press. 3. M. Ziolek, I. Nowak and J.C. Lavalley, Catal. Lett., 45 (1997) 259. 4. M. Sugioka and K. Aomura, Prep. Am. Chem. Soc.,Div. Petrol. Chem., 25 (1980) 245. 5. J. Cui, Y.H. Yue, Y. Sun, W.Y. Don8 and Z. Gao, Stud. Surf. Sci. Catal., 105 (1997)687. 6. M. Ziolek and I. Nowak, Zeolites, 18 (1997) 356. 7. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Olson, E.W. Sheppard, S.B. McCullen, J.B. HJggins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 8. I. Nowak and M. Ziolek, Proc. 3rd Polish German Zeolite Colloqium, (M. Rozwadowski, Ed.) NCU Press, Torun 1998, p. 161. 9. A. Gervasini, J. Fenyvesi and A. Auroux, Catal. Lett., 43 (1997) 219. 10. M. Ziolek, J. Kujawa, O. Saur and J.C. Lavalley, J. Molec. Catal. A: Chemical, 97 (1995) 49. 11. M. Sugioka, React. Kinet. Catal. Lea., 41 (1990) 345. 12. A. Diaz, L.M. Gandia, J.A. Odriozola and M. Montes, J. Catal., 162 (1996) 349. 13. T.I. Koranyi, F. Moreau, V.V. gozanov and E.A. Rozanova, J. Mol. Struc., 410-411 (1997) 103. 14. M. Ziolek and I. Bresinska, Zeolites, 5 (1985) 245. 15. A.V. Mashkina and V.N. Yakovleva, Kinetika i Kataliz, 32 (1991) 636. 16. A.V. Mashkina, V.M. Mastikhin, V.Yu. Mashkin, A.V. Nosov and V.M. Kudenkov, Kinet. Katal., 34 (1993) 880. 17. M. Ziolek, J. Kujawa, O. Saur and J.C. Lavalley, J. Phys. Chem., 97 (1993) 9761.