Langmuir 1999, 15, 5781-5784
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Relation between Chemisorption and Catalytic Transformation of R2S Compounds on Faujasite-Type Zeolites† Maria Ziolek* and Piotr Decyk A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland Received August 28, 1998. In Final Form: February 25, 1999 The adsorption and transformation of sulfur organic compounds on faujasite-type zeolites are considered using ethanethiol and diethyl sulfide molecules as examples. Alkali-metal-exchanged and -protonated forms of the zeolites presenting various nature and various strength of the active centers were applied. The difference in thiol and sulfide chemisorption on MNaY zeolites (M ) Li, K, Rb, Cs) relies on the formation of the hydrogen bond between ethanethiol and the basic sites of zeolites together with the coordination bond which occurs for both thiol and sulfide. That implicates the high activity of MNaY zeolites in the decomposition of ethanethiol. Protonated forms of zeolites are highly active in the transformation of diethyl sulfide to ethene and ethanethiol thanks to the formation of hydrogen bonding species followed by the protonation of the sulfide molecule.
Introduction Sulfur organic compounds of R2S type (R ) H, CH3, C2H5, ...) are formed in biological or chemical reactions such as thermal or anaerobic decomposition of organic materials. They are also known as strong pollutants which are emitted to the atmosphere from various industries. The main industrial sources are petroleum, paper, viscose, and food industries.1 Hydrodesulfurization (HDS)2,3 i.e., the catalytic reaction between organic sulfur compounds and hydrogen is widely applied for the removal of toxic sulfur materials from petroleum products. Hydrogen sulfide and hydrocarbons are formed in this process. H2S is next transformed to sulfur in the Claus process. Zeolites are active catalysts in the decomposition of thiols and sulfides to H2S and hydrocarbons without the use of expensive hydrogen, in the dehydrosulfurization process (DHS).4-10 The competitive reaction for DHS is the transformation of thiol to sulfide and vice versa. The selectivity of the reaction depends on the nature of the active centers on the zeolite surface. The aim of this study is the identification of the nature of the active centers responsible for various chemisorbed species formed after adsorption of diethyl sulfide on NaX, MNaY and MHNaY (M ) Li, K, Rb, Cs) zeolites, the * To whom correspondence should be addressed. Fax: (+4861) 8658008. E-mail:
[email protected]. † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. (1) Martin, G., Laffort, P., Bersillon, K. M. Odors and Deodorisation in the Environment; VCH Publ.: New York, 1994; Chapter 8. (2) Sugioka, M.; Tochiyama, C.; Matsumoto, Y.; Sado, F. Stud. Surf. Sci. Catal. 1995, 94, 544. (3) Sugioka, M.; Tochiyama, C.; Sado, F.; Maesaki, N. Stud. Surf. Sci. Catal. 1996, 100, 551. (4) Sugioka, M.; Kamanaka, T.; Aomura, K. Bull. Jpn. Pet. Inst. 1976, 18, 14. (5) Sugioka, M.; Aomura, K. J. Chem. Soc. Jpn. (Nippon Kagaku Kaishi) 1973, 7, 1279. (6) Ziolek, M.; Decyk, P.; Derewinski, M.; Haber, J. Stud. Surf. Sci. Catal. 1989, 46, 305. (7) Ziolek, M.; Decyk, P. Stud. Surf. Sci. Catal. 1994, 84, 579. (8) Ziolek, M.; Decyk, P.; Czyzniewska, J.; Karge, H. G. Stud. Surf. Sci. Catal. 1997, 105 (B), 1625. (9) Ziolek, M.; Czyzniewska, J.; Lamotte, J.; Lavalley, J. C. React. Kinet. Catal. Lett. 1994, 53, 339. (10) Sarbak, Z. Appl. Catal. A 1996, 147, 47.
comparison with the adsorbed species of ethanethiol described earlier4,7,8 and the correlation of the type of the adsorbed species with the possible reaction pathway. Two reaction pathways for each compound will be taken into account:
2C2H5SH f C2H5SC2H5 + H2S
(1a)
C2H5SH f C2H4 + H2S
(2a)
C2H5SC2H5 f C2H5SH + C2H4
(1b)
C2H5SC2H5 f 2C2H4 + H2S
(2b)
The obtained results will be correlated with those published for hydrogen sulfide adsorption.11 Experimental Section The following catalysts were used as parent materials: NaY, Katalistiks, with Si/Al ) 2.56 and NaX, Linde Lot No. 2565330, with Si/Al ) 1.13. NaY was purified by an exchange with NaCl prior to the modification. The modified forms were prepared by an ion exchange with 0.25 M solution of NH4Cl and/or respective alkali metal chlorides. Hydrogen zeolites were produced by the calcination of ammonium forms at 673 K in a flow of pure, dried He. Alkali metal-ammonium forms (MNH4NaY) were prepared by ammonium cation exchange followed by alkali-metal cation exchange procedure. The obtained catalysts are listed in Table 1. The conversion of organic sulfur compounds was measured in a pulse microreactor filled with 0.02 g of the dehydrated form of the zeolite. Before the reaction, the catalysts were activated for 4 h at 673 K in a helium flow. The reactions were carried out at 623 K. Pulses of 1 µL of thiol or sulfide were syringed by a membrane into the reactor heated to the reaction temperature. Products were analyzed using an on-line gas chromatograph with a flame ionization detector and a 4 m column filled with Chromosorb W (60-80 mesh) and silicon oil DC with 5% addition of stearic acid as the active phase. Catalytic experiments were also conducted after poisoning acid centers with pyridine and base sites with sulfur dioxide. Pyridine was syringed by a membrane into the reactor kept at the reaction temperature and in the flow of helium. The first pulse of the sulfur compound was introduced into the poisoned catalyst after (11) Karge, H. G.; Rasko, J. J. Colloid Interface Sci. 1978, 64, 522.
10.1021/la9811261 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999
5782 Langmuir, Vol. 15, No. 18, 1999
Ziolek and Decyk
Table 1. Zeolite Compositiona catalyst
Na/Al
M/Al
NH4/Al
catalyst
Na/Al
M/Al
LiNaNH4Y NaNH4Y KNaNH4Y RbNaNH4Y CsNaNH4Y
0.31 0.81 0.19 0.22 0.29
0.26
0.35 0.19 0.19 0.20 0.17
LiNaY NaY KNaY RbNaY CsNaY
0.59 1.00 0.46 0.41 0.32
0.41
a
0.54 0.56 0.54
0.54 0.59 0.68
M ) Li, K, Rb, Cs.
a helium flow at 623 K for 15 min. The poisoning with sulfur dioxide was performed using a mixture containing helium and sulfur dioxide (2.5 vol %) which flowed through the catalyst bed at 623 K for 1 h. After this time, pure helium was passed through the reactor for 15 min. The samples were investigated by means of FTIR spectroscopy (Brucker FTIR IFS 113V spectrometer). Self-supporting disks of the samples (10-15 mg/cm2) were pressed and subsequently placed in a sample holder in the IR cell. Zeolites were evacuated at 673 K for 4 h at pressure about 10-4 mbar. The adsorption of ethanethiol or diethyl sulfide (p ) 20 Torr) was carried out at 423 K. The desorption was conducted at room temperature and 423, 523, and 623 K. Electronegativities of zeolites and the adsorbed compounds were calculated on the basis of Sanderson’s equation12 adapted to zeolites by Mortier.13
Results and Discussion Three kinds of the R2S chemisorbed species on zeolites have been found: (i) dissociative adsorbed molecules, (ii) R2S coordinated with the extra lattice cations and (iii) hydrogen-bonded species.8,11,14-17 The transformation of R2S compounds on zeolites depends on the nature of the adsorbed species and on the strength of C-S and S-H bonds in the adsorbed molecule. The dissociation of the S-H bond in the sulfur compound followed by the connection of proton with the zeolite lattice oxygen can be predicted on the basis of the electronegativity calculations. The electronegativities of the zeolites used in this work oscillate between 3.2 and 4.0 whereas that for hydrogen sulfide is 3.74 and those for ethanethiol and diethyl sulfide are below 3.0. According to Mortier’s description,13 during the formation of an adsorption complex, the equalization principle predicts an intermediate electronegativity for the entire compound. If the zeolite has a higher electronegativity than the adsorbed molecule, an equalization can be promoted by a proton transfer from the zeolite to the molecule. And, of course, if the molecule has a higher electronegativity than the zeolite, the proton transfer to the zeolite lattice occurs. In this equalization of the electronegativity, the difference in the values of the zeolite electronegativity and an electronegativity of an adsorbed molecule seems to limit the possibility and rate of proton transfer. When the electronegativity of the zeolites was lower than 3.51, i.e., much lower than the electronegativity of hydrogen sulfide,11,18 the dissociation of H2S adsorbed was registered by IR spectroscopy. On the basis of that consideration one should exclude the possibility of the S-H bond dissociation in ethanethiol adsorbed on the zeolites used in this paper because the electronegativity (12) Sanderson, R. T. Inorganic Chemistry, Reinhold: New York, 1967. (13) Mortier, W. J. J. Catal. 1978, 50, 138. (14) Sugioka, M.; Aomura, K. Bull. Jpn. Pet. Inst. 1975, 17, 51. (15) Kamanaka, T.; Sugioka, M.; Aomura, K. Bull. Jpn. Pet. Inst. 1977, 19, 41. (16) Garcia, C. L.; Lercher, J. A. J. Mol. Struct. 1993, 293, 235. (17) Sugioka, M.; Aomura, K. J. Chem. Soc. Jpn. (Nippon Kagaku Kaishi) 1973, 3, 471. (18) Ziolek, M.; Nowinska, K.; Leksowska, K. Zeolites 1992, 12, 710.
Table 2. ν(CH) Symmetric Vibration in Ethanethiol and Diethyl Sulfide Adsorbed on MNaYsFT IR Results wavenumber, cm-1 zeolite
ethanethiol ads
diethyl sulfide ads
LiNaY NaY KNaY RbNaY CsNaY
2980 2979 2970
2975 2974 2969 2966 2964
2966
of ethanethiol is lower than that for all the zeolites applied in this study. FTIR Adsorption Measurements. Ethanethiol can be considered like hydrogen sulfide in which one proton is replaced by C2H5 group. That, of course, changes the acidic properties of the molecule (ethanethiol is less acidic than hydrogen sulfide), which determines the chemisorption species formed. The question arises whether ethanethiol can dissociate on the same zeolites such as hydrogen sulfide, and which bond can be broken, C-S or S-H. The rupture of the S-H bond should lead to the formation of OH groups as a result of a proton attack on the zeolite oxygen. That can be, of course, visible in the IR spectra. Moreover, after the cleavage of the S-H bond the S-H vibration should not be observed in the IR spectra. The FTIR study of C2H5SH adsorption on NaX and NaY zeolites indicated that the cleavage of the S - H bond does not occur because a band at ∼2550 cm-1 (ν(S-H)) is present in both IR spectra (on NaX and NaY) and no OH groups are generated.4 The lower wavenumber of the S-H band compared with that for the normal value of ca. 2590 cm-1 for thiols, suggests a hydrogen bonding of ethanethiol to the zeolite surface. Moreover, a broad band at ∼3400 cm-1 was observed, showing the hydrogen bonding between C2H5SH molecules and the zeolite surface. The difference in the chemisorption of ethanethiol on the both zeolites is due to the thermal stability of chemisorbed species which is higher on the more basic NaX zeolite. The influence of the nature of the extraframework cations on the chemisorption of ethanethiol was visible when IR spectra were scanned after C2H5SH adsorption at 623 K on various alkali-metal-exchanged Y zeolites7s Table 2. The shift for the ν(CH) band at ∼2980 cm-1 (Table 2) depends on the nature of the alkali-metal cation, indicating the formation of the species coordinatively bonded to the extraframework cations. Such a coordinative bonding between sulfur from thiol and an alkali-metal cation of the zeolite leads to the weakness of the C-S bond which can be easily broken.4 Hydrogen forms of zeolites strongly interact with thiols such as with alcohols. Garcia and Lercher,15 on the basis of the spectroscopic measurements, proposed the following models for the thiol adsorbed structures on the hydrogen forms of the zeolites:
The thiol interaction with the acidic OH groups of the zeolites depends on both the strength of the zeolite acidity and the nature of thiol. Hydrogen bonding was observed
R2S Compounds on Faujasite-Type Zeolites
Langmuir, Vol. 15, No. 18, 1999 5783
Figure 1. FTIR spectra of diethyl sulfide adsorbed on MNaY zeolites at 423 K (the spectra were recorded at room temperature).
Figure 3. Conversion of ethanethiol and diethyl sulfide at 623 K on MNaY and MNaHY zeolites.
Figure 2. FTIR spectra of diethyl sulfide adsorbed on NaHY zeolite (recorded at room temperature): a, after NaHY evacuation at 673 K; b, after diethyl sulfide adsorption at 423 K and short evacuation.
when ethanethiol was adsorbed on NaHY zeolite.7,15 Broad bands in the region of 3500-3000 cm-1 indicated the presence of hydrogen bonds between the zeolite and the adsorbed molecule. The band at 2560 cm-1 was well visible, showing that S-H bond in the thiol molecule was not broken and its position suggests the hydrogen bonding to the surface. It is important to stress that some part of ethanethiol chemisorbed on NaHY zeolite was strongly held because the evacuation at 623 K for 1 h did not remove all adsorbed species. That can suggest either (i) the formation of complex b proposed above which, due to its six-ring structure, should be more stable or (ii) most likely, an attachment of the hydrogen-bonding molecule to the species strongly adsorbed on the Lewis acid sites. The role of alkali-metal cations in the chemisorption of diethyl sulfide is also evident from FT IR spectra presented in Figure 1. The observed shift of ν(C-H) vibration (Table 2) depending on the cation nature suggests the coordinatively bonded sulfide to the alkali-metal cation, like for ethanethiol. However, one should stress the difference in the chemisorption of the both compounds, namely C2H5SH is additionally hydrogen bonded to the lattice oxygen which is very important for further transformation of the molecule. The participation of Brønsted acid sites in the chemisorption of diethyl sulfide was confirmed by the results of the infrared spectroscopy study (Figure 2). The adsorption of diethyl sulfide on HNaY at 423 K caused the decrease of the intensity of the IR bands from acidic hydroxyls in the large cavities of the zeolite. The sulfide molecules did not interact with the Brønsted acid centers
in the small cavities (the intensity of the IR band at 3550 cm-1 was stable). In the C-H vibration region, the typical diethyl sulfide chemisorbed species were registered. The broad IR bands at ∼2700 cm-1 and 2358 cm-1 origin from the zeolite OH groups hydrogen bonded to sulfur from the diethyl sulfide molecules (so-called AB bands). They are due to the 2δ(OH) vibration in interaction with the perturbed ν(OH) one.19-21 Two bands AB appear usually in the IR spectra of complexes bonded with the hydrogen bond of medium strength. According to the literature21 neutral complex or pairs of ions can be formed on acidic hydroxyl groups:
B‚‚‚HO (solid) T BH+‚‚‚-O (solid) If acid strength of the protonated reagent molecule is lower than that of the solid, the ion pairs are formed, and if it is opposite, the neutral complex appears. On the basis of the IR study, it is difficult to postulate which form dominates after the adsorption of diethyl sulfide on NaHY. Catalytic Measurements. The activity of the zeolites used in the conversion of thiol and sulfide is presented in the diagramssFigure 3. Alkali-metal cation-exchanged Y zeolites are more active in the transformation of thiol than their hydrogen forms. The conversion of ethanethiol decreases with the decrease of the extraframework cations Lewis acidity. That indicates the great role of the nature of the alkali-metal cation in this process which confirms the results obtained in the adsorption studies. However, not only the Lewis acid sites involved in the coordinative adsorption of C2H5SH participate in the thiol transformation but also the Lewis basic centers. The experiments with the poisoning the Lewis acid sites with pyridine and the basic sites with sulfur dioxide showed that both decrease the activity of NaX zeolite (Figure 4). The effect of blocking the basic sites is less pronounced because sulfur dioxide is not so strongly held on the basic centers as (19) Pelmenschikov, A. G.; van Santen, R.A.; Janchen, J.; Meijer, E. J. Phys. Chem. 1993, 97, 11071. (20) Claydon, M. F.; Sheppard, N. J. Chem. Soc. Chem. Commun. 1969, 1431. (21) Kubelkova, L.; Korla, J.; Florian, J. J. Phys. Chem. 1995, 99, 10285.
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Ziolek and Decyk Table 3. Selectivity of Zeolites in Ethanethiol Decomposition at 623 K (the First Pulse) product distribution, %
Figure 4. Influence of the acidic and basic sites poisoning (by pyridine and sulfur dioxide, respectively) on the ethanethiol conversion at 623 K on NaX zeolite.
Figure 5. Influence of the acidic and basic sites poisoning (by pyridine and sulfur dioxide, respectively) on the diethyl sulfide conversion at 623 K on NaX zeolite.
pyridine on the Lewis acid sites, under the conditions used in this work. That is in agreement with the adsorption results indicating the formation of the hydrogen bond between proton from thiol and the zeolite lattice oxygen. Contrary to that, MNaY zeolites present very low activity in the transformation of diethyl sulfide (Figure 3) although sulfide is chemisorbed on the alkali-metal cations. However, (C2H5)2S is not chemisorbed on the pairs of active centers such as C2H5SH, and therefore the cleavage of C-S bond is not too easy. SO2 adsorption does not change the zeolite activity (Figure 5) in the sulfide conversion suggesting that the basic centers do not participate in this process. Hydrogen forms of the zeolites exhibit the high conversion of diethyl sulfide which increases with the increase of the zeolite Brønsted acidity. A very important question concerns the selectivity of both processes. Hydrogen sulfide and hydrocarbons are formed by desulfurization of ethanethiol and diethyl sulfide. Ethanethiol can also be transformed to sulfide and vice versa. Moreover, thiophene can be formed as a side reaction product. From the practical point of view such reaction pathways which lead to the formation of a new organic sulfur compound are not desirable. The product distribution resulting from the conversion of ethanethiol on alkali-metal-exchanged zeolites varies from that obtained on their protonated forms (Table 3). On MINaY a high selectivity to thiophene was observed whereas, MINaHY exhibited a high selectivity to diethyl sulfide. The selectivity to diethyl sulfide increases with the decrease of the acidic strength. It can be due to the lower activity of the less acidic samples in the transformation of diethyl sulfide formed in the first step of the
zeolite
ethene
C4-C6
thiophene
(C2H5)2S
aromatics
LiNaY NaY KNaY CsNaY NaX LiNaHY NaHY KNaHY RbNaHY CsNaHY
10 16 24 32 74 69 52 58 54 38
47 21 24 38 3 21 20 7 7 12
40 24 35 19 14 traces traces traces traces traces
1 16 5 8 3 8 23 31 33 46
2 23 12 3 6 2 4 3 5 4
Table 4. Selectivity of Zeolites in Diethyl Sulfide Decomposition at 623 K (the First Pulse) product distribution, % zeolite
ethene
C3-C6
C2H5SH
NaX LiNaHY NaHY KNaHY RbNaHY CsNaHY
92 32 29 52 52 44
8 12 15 7 8 14
56 56 41 40 42
reaction or can be due to the fact that diethyl sulfide is produced faster on the more basic samples. The other possibility for the observed diethyl sulfide selectivity trend could be the participation of both Brønsted acid and basic sites of the catalysts in the reaction mechanism (one reguired in the production of C2H5+ and the second in the formation of C2H5S- species). The strength of both kinds of centers should be similar for the interaction of both formed ions and the formation of sulfide. The increase of the selectivity to diethyl sulfide is accompanied by the decrease of the selectivity to ethene. During the decomposition of diethyl sulfide on MNaHY zeolites (Table 4) only traces of aromatics and thiophene were produced. However, ethanethiol was the significant product next to ethene. That confirms the reaction pathway proposed above on the basis of FTIR study. There is no clear-cut relation between the strength of acidity of the samples and the selectivity to the products listed in Table 4. Conclusions Hydrogen bonding of the molecule adsorbed on the zeolite surface and proton transfer are very important steps in the catalytic transformation of the organic sulfur compounds on faujasite type zeolites. Ethanethiol is chemisorbed on the pairs of Lewis acid and basic centers on MNaY zeolites. Such kind of chemisorption via coordination and hydrogen bonding increases the rate of the C-S bond cleavage and the transformation of thiol. Therefore, MNaY zeolites present high activity in the ethanethiol conversion. Diethyl sulfide is chemisorbed on alkali metal exchanged zeolites only via the coordination bonding and that is a reason of the low activity of these zeolites. The presence of the Brønsted acid sites in MHNaY zeolites involves the formation of hydrogen bonding between diethyl sulfide and the acidic hydroxyl groups of the zeolites. On LiHNaY and NaHY there is the optimum strength of the chemisorbed form which leads to the high conversion of diethyl sulfide to ethanethiol. Acknowledgment. This work was supported by the fund from A. Mickiewicz University, Faculty of Chemistry. The authors thank Dr. J. Czyzniewska for the preparation of the MHNaY zeolites. LA9811261
