J. Phys. Chem. 1992, 96, 2669-2675
2669
Adsorption and Surface Reactions of Thiophene on ZSMS Zeolites Cristina L. Garcia and Johannes A. Lercher* Institut fur Physikalische Chemie und Christian Doppler Laboratorium fur Heterogene Katalyse, Getreidemarkt 9, A - 1060 Vienna, Austria (Received: August 21, 1991)
The adsorption and surface chemistry of thiophene on Na-, K-, and H-ZSMS were studied by means of IR spectroscopy, mass spectrometry, and gravimetry. On hydroxyl groups, thiophene is reversibly adsorbed at rmm temperature via hydrogen bonding. On acid hydroxyl groups (strong Bronsted acid sites) ring opening and oligomerization were observed for longer exposures, indicating that these processes are relatively slow. The main primary products resulting from ring opening were concluded to be unsaturated thiol-like species. These species may undergo cracking while the fragments may rearrange to give various aromatic sulfur (and to a smaller extent sulfur free) compounds. The results indicate easy formation and breakng of sulfur carbon bonds on acidic zeolite catalysts.
experiments for the H-ZSMS sample. Introduction IR Spectroscopy, TPD, and Mass Spectrometry. The samples Thiophene adsorption on various oxides and zeolites surfaces were investigated by means of transmission-absorption IR has been previously de~cribed.I-~The appearance of time-despectroscopy (Bruker IFS 88,4 cm-I resolution). Self-supporting pendent IR bands characteristic of CH stretching and deformation disks of the samples (8-10 mg/cm2) were pressed and subsequently vibrations in aliphatic compounds and complex spectra at waveplaced in a sample holder at the center of a small furnace in the numbers lower than 2000 cm-I were noticed upon thiophene IR beam. The IR cell was evacuated to pressures below lod mbar. adsorption carried out on different materials, even at room temFor activation, the sample was heated in situ up to 873 K with perature. Some authors attributed the appearance of these bands a heating rate of 10 K-min-l. The sorption experiments were to the formation of nonidentified saturated species, through up to 1 mbar cleavage of the C-C and/or C-S bonds of the thiophene r i ~ ~ g . l > ~carried out in situ at room temperature, from partial pressures of thiophene (Merck > 99% purity). The pressure The possibility of thiophene polymerization at room temperature was kept constant during equilibration by means of differential on Si02was also invoked.8 pumping of the adsorption manifold. The bands of the lattice Moreover, the formation of cyclic saturated structures, attached vibrations between 2090 and 1740 cm-I were used to normalize through one, two, or even four points to the surface were proposed the intensities of all the bands.’O A curve-fit program (LAB. CALC., upon adsorption of thiophene on 7-A1203, cobalt molybdate, MoS, Galactic Ind. Corp.) which allows to calculate the best fit and 7-Alz03supported MoSz surfaces (with participation of OH (Gaussian, Lorentzian, or Gaussian-Lorentzian mixtures bands) surface groups, in the case of y-A1203).44 was used to deconvolve a set of overlapping bands. On HY zeolite, De Angelis et aL2 suggested that the cleavage After evacuation at room temperature, temperature-programof the aromatic ring resulted in SH groups linked to organic med desorption was carried out in situ with a temperature inresidues and/or HIS, and dienic species probably polymerized. crement of 10 Kemin-’ up to 873 K. The gas phase was analyzed On H-montmorillonite, the formation of trimers, containing two with a quadrupole mass spectrometer (Balzers QMG 420) directly thiophene and one thiophane ring, was reported.’ connected to the vacuum system. In this contribution, we attempt to clarify the role of the strong Thermogravimetry. For the thermogravimetric measurements Bronsted acidity of H-ZSM5 in determining the complex ada Cahn RG electrobalance was used. The system was evacuated sorption behavior of thiophene. For comparison, results of to pressures below mbar and the samples were pretreated thiophene adsorption on Si02and on cation-exchanged Na- and as described for the IR experiments. The concentrations of adK-ZSM5, under the same experimental conditions, are discussed. sorbed species were determined for equilibrium pressures from IR spectroscopy, temperature-programmed desorption-mass to 1 mbar. spectrometry (TPD-MS), and thermogravimetry were used for characterization of the adsorption system and the gas phase. Results Thiophene Adsorption on SOz. The difference between the Experimental Section spectrum of SiOz in contact with 1 mbar thiophene and the Materials. S O 2 , Aerosil 300, provided by Degussa Corp., spectrum of the activated Si02 sample is shown in Figure 1. In NH4-ZSM5 zeolite (Si/Al = 35.9, provided by Mobil, and the this mode of presentation, bands pointing upwards are higher and sodium and potassium forms of ZSM5 were used as adsorbents. those pointing downwards are lower in intensity, compared to the The hydrogen form of ZSMS was obtained by heating the spectrum of the activated sample. NH,+-exchanged form in vacuum (P< 10” mbar) for 1 h at 873 Because of the decrease in the intensity of the band at 3745 K. Na- and K-ZSMS were obtained by ion exchange of the cm-l we concluded that thiophene interacts with the SiOH groups. hydrogen form in 1 M nitrate solutions of the corresponding cation, The broad band which appeared at 3620 cm-’ was assigned to at 363 K. An acid site density of 2.42 acid sites per unit cell was the perturbed SiOH stretching vibration. The shift of 125 cm-I gravimetrically determined by pyridine adsorption-desorption to lower wavenumbers is in agreement with values reported for the interaction of r-electron donor molecules with S O H groups through a hydrogen-bonding type of interaction.” This value ( 1 ) Ulendeeva, A.; Lygin, V.; Lyapina, N. Kinet. Xaral. 1979, 20 (4). 978. agrees very well with previously reported data on thiophene ad(2) De Angelis, B.; Appierto, G. J . Colloid. Interface Sci. 1975, 53(1), 14. (3) Kolboe, S. Can. J . Chem. 1969, 47, 352. sorbed on Si02.1.8 (4) Nicholson, D. Anal. Chem. 1960, 32, 1365. Bands of adsorbed thiophene were detected at 31 16, 3080 cm-’ (5) Nicholson, D. Anal. Chem. 1962, 34, 370. (C-H stretching vibrations), 1410 cm-’ (fundamental ring (6) Sultanov, A.; Khakimov, U.; Talipov, G.; Shchekochinin, J. React. Kinel. C a r d Leu. 1975, 2(3), 243.
(7) Lorprayoon, V.;Condrate, R. Appl. Spectrosc. 1982, 36(6), 696. (8) Rochester, C. H.; Terrell, R. J. J . Chem. Soc., Faraday Trans. 1977,
73, 596. (9) Blyholder, G.; Bowen, D. J . Phys. Chem. 1962, 66, 1288.
(10) Jentys, A,; Lercher, J. Stud. Surf. Sci. Calal. 1989, 46, 585. ( 1 1 ) Knozinger, H. The Hydrogen Bond; Schuster, Zundel, Sandorfy, Eds.; North Holland: Amsterdam, 1976; Vol. 111, p 1275.
0022-3654/92/2096-2669$03.00/0 0 1992 American Chemical Society
Gar&a and Lercher
2670 The Journal of Physical Chemistry, Vol. 96,No. 6,1992
,3500
3000
2500 2000 1500 wavenumbers/cm- 1
b
Figure 1. Difference spectrum between the spectrum of Si02 in contact with 1 mbar thiophene and the spectrum of the Si02 activated sample (Aerosil 300).
s -0
',4--:a.
-8m
p = 0.001 m b a r l
0
0.01 0
i.1D 0
.05 -
i . --.
0-
-.01
0.02
C
.04
QO1
0.03
C
I
J,
0
-.02 3500
2500
1500
--.
1396an.lI 14lOcm-'
I'
I
3500
2500 wovenumbers/cm- 1
1500
3500
2500
1500
wovenumbers/cm- 1
Figure 2. Differences between the spectra of (a) thiophene in contact with Na-ZSMS and the spectrum of the Na-ZSMS activated sample and (b) thiophene in contact with K-ZSMSand the spectrum of the K-ZSMS activated sample for the equilibrium pressures indicated.
stretching vibration v5) and at 1398 cm-l (C-H out of plane bending mode). The positions of these bands correspond reasonably well to those reported for thiophene in the liquid and/or gas phase,12-14suggesting that the molecules are weakly hydrogen bonded. Thiophene adsorption on Si02was completely reversible at room temperature. Thiophene Adsorption on Na- and K-ZSM5. The differences between the spectra of Na- and IS-ZSM5 in contact with thiophene and the spectra of the activated Na- and IC-ZSM5, respectively, are shown in Figure 2, for equilibrium pressures from up to 1 mbar. Increasing the thiophene partial pressure up to lW3mbar caused the appearance of bands of adsorbed thiophene at 3109 and 3089 cm-' (C-H stretchingvibrations), at 1683, 1630, 1598, and 1450 cm-' (combination of ring and C-H in plane bending vibrations) and at 1396 cm-'. The band at 1396 an-' was assigned to the perturbed fundamental ring stretching vibration v5, which was concluded to be 14 cm-l shifted to lower wavenumbers as compared with thiophene adsorbed on SiOz. Thus, all bands (12) Rico, M.; Orza, J.; Morcillo, J. Spectrochim. Acta 1965, 21, 689. (13) Loisel, J.; Lmenzelli, V. Spectrochim. Acta 1967, 23A, 2903. (14) Lord, R.; Miller, F. J. Chem. Phys. 1942, IO, 328.
pf mbar
observed are attributed to modes of vibration of thiophene molecules strongly interacting with the Na+ and K+cations. It should be mentioned that an enhanced extinction coefficient of the bands which are a combination of the in-plane bending vibrations was observed upon interaction of thiophene with the cations, as compared with both gas- and liquid-phase spectra and also with the spectrum of thiophene adsorbed on Si02. After equilibration at 10-l mbar and, subsequently,at 1 mbar, the decrease in the intensity of the bands at 3745 and at 3725 cm-l showed that thiophene was adsorbed at SiOH groups. In parallel, new broad bands appeared at approximately 3620 and 3500 cm-l, which were assigned to perturbed OH vibrations. In analogy to the IR spectra of thiophene on Si02, we attributed the bands at 3122,3077 cm-' (C-H stretching vibrations), 1587 and 1410 cm-l (ring stretching vibrations), and 1396 cm-l (CH out-of-plane bending vibration) to thiophene molecules weakly hydrogen bonded to SiOH groups. Deconvolution and quantification of the bands between 3200 and 3000 cm-l and between 1450 and 1380 cm-l was effected for the spectra of thiophene in contact with both Na- and K-ZSM5. As an example, Figure 3a shows the results of the deconvolution for the spectrum recorded at 10-l mbar equilibrium pressure of thiophene in contact with Na-ZSM5. The normalized integral intensities of the bands as a function of the thiophene equilibrium pressure are additionally shown in Figure 3b,c (for the Na-ZSM5 sample). The intensity of the bands at 3109, 3088, and 1396 cm-' (assigned to species strongly interacting with cations) reached fairly constant values for loM2 mbar and higher equilibrium pressures on both samples. Thus, complete
The Journal of Physical Chemistry, Vol. 96, No. 6,1992 2671
Reactions of Thiophene on ZSMS Zeolites m.s.respo se a,tl.unlts
a
I
-.l{
3500
2500 2000 ravenumberdcm-
1500
3000
4W
500 TIKW
700
,
3000
,
,
,
2500
2000
1500
rovenumberdcm- 1
'1
4 1
300
,
3500
800
m.s.response arb.unlts C
520K
C -.4 J
-054
,
5500
5000
2500
2000
1500
,
3500
, 3000
,
,
,
2500
2000
1500
ravenumberdcm-
wovenumberdcm- 1
I
m/e=58 0
300
400
50OTIK 600
700
800
Figure 4. TPD of thiophene: (a) from the Na-ZSMS sample after outgassing at room temperature; (b) from the K-ZSMS sample after outgassing at room temperature; (c) from the Na-ZSMS sample after outgassing at 423 K; (d) from the K-ZSMS sample after outgassing at 393 K.
coverage of the cationic sites was achieved at mbar. The parallel increase of the bands at 1396 cm-' and at 1410 cm-I (for adsorption on SiOH attributed to C H out-of-plane bending and ring stretching vibrations, respectively), when the equilibrium pressure of thiophene was increased from 1 0-2 to lo-' mbar and from 10-l to 1 mbar, indicates adsorption on SiOH groups. A 1:1 stoichiometry for the thiophene-alkali-metal cation was determined gravimetrically at mbar equilibrium pressure. At the higher equilibrium pressures, clusters with up to three thiophene molecules per adsorption site were formed. After outgassing the sample at room temperature, TPD was carried out up to 873 K. In a similar experiment, the samples were outgassed at 393 K (K-ZSMS sample) and at 423 K (NaZSMS sample) to desorb weakly adsorbed molecules. In both cases, thiophene desorbed completely without reaction. Two maxima of desorption were determined, centered at 423 and 520 K for Na-ZSM5 and at 393 and 471 K for K-ZSM5 (see Figure 4), corresponding to weakly and strongly adsorbed molecules, respectively. Thiophene Adsorption on H-ZSMS. Adsorption af Room Temperature. Figure 5a-e shows the changes in the IR spectra of H-ZSMS induced by successive increments of the thiophene equilibrium pressure from 10" to 1 mbar. The spectra were monitored until adsorption/desorption equilibrium was achieved for each pressure step. After equilibration with lo4 mbar thiophene, the intensity of the band at 3610 cm-' decreased in intensity indicating the interaction of thiophene with the SiOHAl groups (BrBnsted acid sites). In parallel, a broad band appeared at 3278 cm-I, which was attributed to the stretching vibration of the SiOHAl groups perturbed by hydrogen bonding of thiophene molecules. The bands observed at 3114, 1410, and at 1400 cm-l were assigned to thiophene hydrogen bonded to the SiOHAl sites (CH stretching, fundamental ring stretching v5 vibrations and CH out-of-plane bending, respectively).I *-14
3500
Moo
2Mo
2000
wovenumberdcm- 1
1503
3500
3000
2500
2000
rorenumberr/cm-
'
1iO;
Figure 5. Changes in the IR spectra of H-ZSMS induced by successive increments of thiophene equilibrium pressure: (a) 104-10-5mbar; (b) 10-3-104mbar; (c) l@Ll@3 mbar; (d) 10-'-10-2mbar; (e) 1-10-' mbar; ( f ) difference between the spectrum collected after 70 min outgassing at room temperature after thiophene adsorption on H-ZSMS and the spectrum of the activated sample.
Bands observed at 3106, 1610,and 1394 cm-l were assigned to the perturbed vibrations of thiophene molecules strongly interacting with extralattice alumina. The intensity of these bands did not increase with subsequent increases of the equilibrium pressure. It should be noted that the band at 3745 cm-' lost some intensity after equilibration at lo4 mbar, indicating an initial interaction of the thiophene molecules with terminal SiOH groups. We would like to speculate that these OH groups are located close to the extralattice alumina. Similar conclusions were drawn by Lavalley et al. for pyridine adsorption on dealuminated H Y zeolites. Increasing the partial pressure up to mbar caused the intensity of the broad band at 3270 cm-' to increase (note also the shift to lower wavenumbers) in parallel with the decrease in the intensity of the band at 3610 cm-I. The intensity of the bands at 31 14,1410,and 1400 cm-' further increased and new bands appeared a t 3078 and 1360 cm-l, which were also attributed to the thiophene ring hydrogen bonded to the SiOHAl sites (CH stretching and ring stretching fundamental u4 vibrations, respectively).l2-I4 At 1W2mbar equilibrium pressure the intensity of all previously mentioned bands further increased in comparison to spectra after lower equilibrium pressures. The position of the perturbed OH stretching band shifted downwards to 3228 cm-I. The disappearance of the band at 3610 cm-'suggests that complete coverage of the SiOHAl (Brbnsted acid) sites was achieved at this pressure step. A 1 :1 stoichiometry was confirmed gravimetrically. ( 1 5 ) Lavalley, J.; Janin, A,; Maache, M.; Joly, J.; Raatz, F.; Szydlowski, N. Zeolites 1991, I ! , 391.
2672 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
GarGa and Lercher
1I
,
,
,
,
,
3100
3400
E .06e -
,
,
,
I
2800
,
I
wovenumbers
1
2200
2500
/ cm-'
C 0
; '-..02"
-.02\ MOO
w
. 5300 . . 3100 . . 2900 . . 2700 . . 2SOO . , 2soO , , 2100 wovenumbers/cm-
'
I
I
I
1660
,
I
I
I
1580
I
l
,
I
1500
wovenumbers
,
I
I
I
1420 /
cni
I
I
'
I
1310
Figure 7. Spectra of 0.1 mbar thiophene in contact with H-ZSMS collected during 4 h, with variable time resolution.
-
.
0
2
i
i
1640 1600 1560 1520 1480 1440 1400 1360 1520
wovcnumbersjcm-
T/I
