Thiophene synthesis by dehydrogenation of tetrahydrothiophene on chromium catalysts

M.Guisnet et al. (Editors), Heterogenwus Catalysis and Fine Chemicals Ill CD 1993 Elsevier Science Publishers B.V. All rights reserved.

369

Thiophene synthesis by dehydrogenation of tetrahydrothiophene on chromium catalysts A. Commarieua, E. Arretza, D. Duprezb and C. Guimonc a Groupement de Recherches de Lacq, ELF, BP 34,64170 ARTlX France b Laboratoire de Catalyse, URA 350,86022 POITIERS Cedex France

C

Laboratoire de Physico-Chimie Moleculaire, URA 474,64000 PAU France

Abstract Dehydrogenation of tetrahydrothiophene (THT) was carried out at 380°C, 1 atm on supported chromium catalysts (20 wt.-% Cr2O3). Different supports were investigated : A12O3, Si02, TiO2, C and the results were compared to those obtained with an unsupported chromium catalyst. The following rank of activity was obtained (mmole h-lg-1) : Cr/C, 33 > Cr/A1203, 12 > Cr/SiO2, 2-12 == Cr/TiO2, 7. The selectivities to thiophene, relatively constant in the 40-100% range of conversion,were higher for C, Si02 and Ti02-supported catalysts (87-92%) than for Cr/A1203 (79%). The catalysts were characterized by XRD, XPS, TPR and oxygen uptake. The activity depends on the crystallite size of Cr2S3 (very small on Cr/C), on the degree of sulfidation (residual 02- ions being poison of the reaction) and on the surface reduction of chromium (CrII species).

1. INTRODUCTION Thiophene is utilized in Organic Chemistry as an intermediate in the synthesis of pharmaceuticals and of phytoproducts. Moreover since the go's, the polymerization of thiophene has been extensively studied, pol ythiophene being a conductor polymer with potential application in Electrochemistry.

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Thiophene can be prepared (i) by sulfurization-cyclization of hydrocarbons (butane, butene or butadiene) or of alcohols (butanol, butanediol-1,4) (ii) by substitution of the oxygen by sulphur in the molecule of furan or (iii) by cyclization of linear sulfided compounds (diethylsulfide for instance). In the processes, the selectivity into thiophene is generally lower than 70% [l-41. An alternative way for thiophene synthesis is the dehydrogenation of tetrahydrothiophene (THT). This molecule is easily produced, with an excellent yield, by oxygedsulfur substitution in tetrahydrofuran. THT dehydrogenation was studied by different authors on sulfided Ni, Mo or Co-Mo catalysts. Recently Lacroix et al [5] performed an extensive study of the reaction on unsupported sulfides. In this paper we present a kinetic study of THT dehydrogenation on supported chromium catalysts which were, concurrently with ruthenium ones [ 5 ] , the most promising catalysts for this reaction. Two main reactions can occur : Dehydrogenation (DH) :

4 2 -H2 THT -> Dihydrothiophene -> Thiophene

Desulfurization (DS) :

- H2S THT > Butadiene

H2 ->

(1)

Butene, Butane (2)

The catalysts were also characterized by different techniques before and after their use in reaction.

2. EXPERIMENTAL

Six supports were used : GFS = RhBne Poulenc GFS alumina (200 m2g-1, impurities < 500 ppm). A = RhBne Poulenc A alumina (350 m2g-1, 0.4% Na), C = Picatal E 612 carbon (950 m2g-1,0.7% K), DBM = RhBne Poulenc DBM 250 silica (250 m2g-1, 0.2% Na), AERO = Degussa Aerosil silica (200 m2g-1, impurities < 500 ppm), P25 = Degussa P25 titania (50 m2g-1, anatase + rutile, impurities c 1000 ppm). The supports were peptized (AERO and P25), crushed and sieved to 0.1-0.2 mm. The catalysts were prepared by impregnation of these supports with aqueous solutions of chromium salts (generally, the nitrates). They are refered to as CrnX, where n is the percentage of chromium oxide (equivalent Cr2O3) and X the code of the support. They were calcined at 450°C for 4h and in situ sulfided in a stream of 10% H2S/H2 for 4h at 500°C and cooled to the ambient temperature in the sulfiding mixture. Without any contact with air, they were heated to the operating temperature under a 1.38 v o I . 4 H2S/N2 flow. The reaction was carried out at 380°C in a microreactor (THT, 5.2 torr; H2S, 10.4 torr; N2, 745 totr; weight hourly space

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velocity : 0.3 to 1.2 h-1). The products were analyzed by G.C. using a PORAPLOT Q capillary column (25 m, i.d. 0.32 mm, 60-200°C, 6°C min-1). XRD characterizations were carried out in a Siemens D 200 diffractometer. JCPDS sheets 06-0504 of Cr2O3 and 10-0340 of Cr2S3 were used as references. XPS experiments were carried out in a SSI-301 spectrometer (source : AlKa at 1486.6 eV, 120 W). The spectra were standardized by using hexatriacontane as a reference (Cls at 284.6 eV). The major peaks of the supports were Cls at 284.2 eV for C, Si2p at 103.7 eV for AERO and at 104.0 eV for DBM, and A12p at 74.4 eV for A. Pure compounds (Cr2O3, Cr2S3, CrC13 and CrC12) were also used for measuring binding energies of Cr2p3/2 in different valence states and in different chemical environments. Oxygen chemisorptions were carried out on the calcined catalysts reduced in H2 (temperature-programmed reduction from 25 to 500°C at 4°C min-1) in order to evaluate the surface reducibility of chromium ions. The same technique was applied to sulfided catalysts in accordance with a method already used with NiMo [6] and CoMo [7] catalysts.

3. RESULTS AND DISCUSSION 3.1 Activity and selectivity of the bare supports The reaction was first carried out on the sulfided supports (no Cr added). The results are shown in Fig.1. The two silicas (AERO and DBM) and the carbon which were found totally inactive are not represented on the figure.

U

u)

w

Conversion

DHT

k

2

el

I-

0 w

-I

Thiophene c4

w

u)

A1203

C

A1203 A

Ti02 P 25

Fig. 1 : Activity and selectivity of the sulfided oxides (no Cr added). Catalyst weight : 120 mg. DHT = dihydrothiophene,C4 = butenes+butadiene

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The two aluminas and the titanium oxide show a significant activity in THT conversion. Nevertheless the selectivity to thiophene is relatively poor particularly on aluminas (C4 formation). Active sites of the support (probably - SH groups) catalyze desulfurization at the expenses of dehydrogenation, thus confirming the high reactivity of THT (compared to thiophene) in desulfurization [8]. By-products (C1C3 hydrocarbons, alkylthiophenes and butylmercaptan) are produced in significant proportion (15-20%) on A1203 while Ti02 was the most active and selective support in dehydrogenation.

3.2 Activity and selectivity of the sulfided chromium catalysts Two series of experiments were carried out : at a constant space velocity (120 mg) and at a conversion close to 5096, obtained by changing the catalyst weight. The catalytic properties of an unsupported chromium oxide (29 m2g-1) were also determined. The results are given in Table 1. Table 1 Activity and selectivity of the sulfided chromium catalysts (1h-on-stream). Catalyst or precursor

Weight

Conversion %

0 2 0 3 (bulk) Cr20 A Cr20 GFS Cr20 DBM Cr20 AERO Cr20 c Cr20 P25

120 120 120 120 120 120 120

97.6 96.6 9x3 44.4 91.0 99.5 82.7

0 2 0 3 (bulk) Cr20 A Cr20 AERO Cr20 c

25 30 30 10

54.0 52.2

52.4 48.1

Activity (mrnol/g cat.h)

-__ __

Selectivities Thiophene DHT

C4 others

7.3

91.6 81.9 70.8 87.4 93.0 88.5 87.0

0.1 0 0 3.0 0.1 0 0.5

10.8

0.9 1.5 10.0 3.3 1.1 7.5 1.7

15.5 12.3 12.4 32.8

90.5 79.5 89.4 89.4

2.0 2.4 3.0 2.6

6.9 17.4 6.7 5.6

0.6 0.7 0.9 2.4

2.4

---

7.4 16.6 19.2 6.3 5.8

4

Chromium sulfide is the active component for the dehydrogenation : alumina which possesses desulfurization sites, confers a poor selectivity to the Cr/A1203 catalyst. We can note the very high activity of the Cr/C catalyst. The selectivity to dihydrothiophene (DHT) is always higher at a 50% conversion than at high conversion (80-100%). Results obtained at low conversion [9] confirm this tendency, the selectivity to DHT reaching 15-2096 at a 10-20% conversion, in accordance with the consecutive scheme (Eqn. 1) proposed for the dehydrogenation.

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3.3 Stability of the sulfided chromium catalyst The changes in relative activity with time-on-stream are represented on Fig.2. The initial conversions are those, close to 50%, given in Table 1.

TIME (h) Fig.2 Deactivation of the supported chromium catalysts The following order of the catalyst stability is obtained : Cr20 DBM > Cr20 A > Cr20 C > Cr20 AERO. At this stage, no correlation can be found between the rates of deactivation and the nature of support (the two silicas DBM and AERO lead to catalysts with quite different stabilities). This stability probably is a complex function (i) of the morphology of catalyst, (ii) of its state of sulfidation and (iii) of the formation of coke precursors (particularly butenes and butadiene) during the reaction 191.

374

3.4 Catalyst characteristics The results obtained with the different methods of characterization are given in Table 2. The sulfidation is not total on Si02-supported catalysts while approaching 100% on Cr20 C. The extent of bulk sulfidation rs is an important parameter for the reaction : increasing rs by a prolonged duration of sulfidation increases catalyst activity (Table 2a). Even though there remains some chromium oxide in Cr/SiOz catalysts, the surface appears to be essentially sulfided (see XPS data in Table 2d : surface Cr2S3 and surface Cr2O3 can be distinguished by their differences in binding energies). TPR profiles (not represented here) show two main peaks : a lowtemperature peak in the 320-380°C temperature range and a high-temperature peak at 480-490°C. Hydrogen uptakes during TPR and oxygen uptakes after TPR (Table 2b) can be interpreted as follows : in the oxided catalyst, Cr is present in the tn valence state (with n 2 3, obtained probably by a mixture of t3 and t6 species). During TPR, chromium is essentially reduced in Cr2O3 plus a small amount of CrII species (x), so that : WCr = n - 3 t x

(1)

After TPR, these CrII are re-oxidized into CrIII by oxygen : O/Cr = XI2

(2)

Except in Cr20 A (45% CrVI), chromium is essentially (CrlSiO2) or exclusively (Cr/C) present as Cr2O3 (Table 2c). There exists a correlation between the reducibility of Cr (measured by the percentage of CrII after TPR) and the catalytic activity. A similar correlation is obtained with the O/Cr values measured after sulfidation. Owing to differences in the temperature programme (4h at 500°C in H2S/H2 versus l h at 500°C in HYAr after the temperature rampes) oxygen uptakes are higher after sulfidation than after TPR. However, after a 4 h-reduction in H2, the ratio O/Cr for Cr20 C is very close (0.18) to the value found after sulfidation (0.21). It seems thus that, both on reduced and on sulfided catalysts, oxygen titrates coordinatively unsaturated sites (c.u.s.) of CrII which would be the active sites of dehydrogenation. XRD and XPS results show that chromium is highly dispersed on carbon even though a slight sintering occurs upon reduction and sulfidation. In the calcined catalyst, all the chromium is in the t3 state and the support favors the formation of CrII c.u.s., which makes this catalyst particularly active in THT dehydrogenation. It was shown by XPS that the difference between the binding energies of Cr3+ and Cr2+ amounts to 1.9 eV (Cr2p3/2 at 577.4 eV for CrC13 and at 575.5 eV for CrC12. Nevertheless, owing to the high sensitivity of sulfided catalysts to air, CrII species could not be detected by this technique despite the use of a glove box coupled with the introduction chamber of the spectrometer.

375

Table 2 Catalyst characteristics (a) Extent of sulfdation rs (96 Cr2O3 ->

Cr20 DBM Cr20 AERO Cr20 c

29% 50% 1s = 100% IS= IS=

Cr2S3) versus activity A (mmol h - I g - 1 )

A = 2.4 A = 12.4 A = 32.8

IS 1s

= 65% = 73%

A = 3.8 A = 16.8

(b)Reducibility of chromium species (oxided catalysts) HICr VPR) 0.165 0.094 1.37 0.18

Cr20 DBM Cr20 AERO Cr20 A Cr20 c

(c)x

OICr (after TPR) 0.007 0.032 0.009 0.09

n

% CrII

3.15 3.03 4.35 3.00

1.4% 6.4% 1.8% 18%

OICr (after sulf.) 0.005 0.040 0.071 0.21

m:

Cr203 180-200 %, cristallites on the two silicas Cr2S3 evident on the two silicas, particularly on 0 2 0 AERO. Cr203 remains visible after sulfidation. No clear diffractogram of 0 2 0 3 and Cr2S3 on Cr20 C (well-dispersed Cr) Oxided : Sulfided :

-

(d) XPS :surface state of chromium State Cr20 DBM

calcined reduced (H2) sulfided Cr20 AERO calcined reduced sulfided Cr20 c calcined reduced sulfided Cr203 unsupp.,calc. 20 Cr203+80 Si02 mech. mixt. 20 C12O3+8OC mech. mixl. Cr2S3 unsupp.,re-sulf.

Cr2p.712 (eV) B.E. width (at Hl2) 576.3 3.3 575.8 3.1 574.0 3.0 3.0 576.4 576.4 2.9 574.6 3.9 576.3 2.6 576.2 2.7 575.0 2.4 2.9 576.4

CdSi (Cr20 Si02) or CdC ((2120 C) 2 20 4 1 2 2 20 11 6 (S/Cr=2.6)

__

19.5 4

574.5

2.1

--

376

4. CONCLUSIONS Chromium sulfide is a selective catalyst of THT dehydrogenation minimizing the C-S bond cleavage. The activity seems to be correlated to the reducibility of chromium into CrIl species. Carbon is an excellent support promoting both the dispersion of the active phase and favoring the creation of coordinatively unsaturated chromium species (probably CrII species). However further investigations are required to improve its stability.

5. REFERENCES

U.S. Patent 3 939 179 to Pennwalt (1974). U.S. Patent 3 822 2x9 to Tar Distillers (1973). U.S. Patent 4 143 052 to SNEA(P) (1979). Ger. Offen. 1 224 749 to BASF (1964). M. Lacroix, H. Marrakchi, C. Calais, M. Breysse and C. Forquy, in M. Guisnet el al., Ed., 2nd Int. Symp. Heter. Catal. Fine Chem., Poitiers, 1990, Stud. Surf. Sci. Catal. Vo1.59, Elsevier, Amsterdam, 1991, p.277. S. Brunet, S. Karmal, D. Duprez and G. Perot, Catal. Lett., 1 (19x8) 255. S. Karmal, Thesis, Poitiers (l98X) W.R. Moser, G.A. Rossetti Jr, J.T. Gleaves and J.R. Ebner, J . Catal., 127 (1991) 190. A. Commarieu, Thesis, Poitiers (1990).