Benzene hydrogenation on platinum and iridium catalysts. Variation of the toxicity of sulfur with the nature of the support

Applied Catalysis A: General, 101 (1993) 143-149 Elsevier Science Publishers B.V., Amsterdam

143

APCAT A2541

Benzene hydrogenation on platinum and iridium catalysts. Variation of the toxicity of sulfur with the nature of the support P. Ma&cot, J.R. Mahoungou and J. Barbier Universiti de Poitiers, URA CNRS 350, Laboratoire de Catalyse en Chimie Organique, 40, Avenue du Recteur Pineau, 86022 Poitiers Ce’dex (France) (Received 24 February 1993, revised manuscript received 3 April 1993)

Abstract Benzene hydrogenation was studied at 323 K on fresh and sulfurixed catalysts. The activity of small platinum and iridium particles was sensitive to the electronic properties of the metal, which, in turn, depended on the acidity of the support. The toxicity of sulfur, which varies with both the nature of the metal and the support, can be explained by the combination of geometric (negative) and electronic (negative or positive) effects. Key words: benzene hydrogenation; iridium; platinum; sulphide; toxicity

INTRODUCTION

Naphtha reforming catalysts are usually sulfided in order to improve their selectivity and stability [l-3]. Sulfurization can be carried out by treatment with various sulfur compounds, but in all cases, catalysts are finally subjected to a hydrogen flow at high temperature. Under such conditions, part of the adsorbed sulfur is eliminated, but some residual sulfur remains irreversibly adsorbed on the metallic phase of the catalyst [ 4-71. The aim of this work is to study the effect of this irreversibly adsorbed sulfur on the activity of platinum and iridium deposited on various supports for benzene hydrogenation. In order to improve the discussion in terms of ligand effects, the competitive hydrogenation of benzene and toluene was also investigated [ 8,9]. Initially, hydrogenation of benzene was regarded as a structure-insensitive reaction on metals [lo-141 but some divergences appeared particularly on nickel catalysts [E-18] or other metals deposited on various supports [ 19Correspondence to: Dr. P. Madcot, Universiti de Poitiers, URA CNRS 350, Laboratoire de Catalyse en Chimie Organique, 40, Avenue du Recteur Pineau, 86022 Poitiers Cddex, France. Tel. (+33)49453910, fax. (+33)49453499.

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0 1993 Elsevier Science Publishers B.V. AlI rights reserved.

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231. Therefore, the first part of the paper is devoted to the study of the effect of the nature of the carrier on the activity of supported platinum and iridium catalysts for the model reaction of benzene hydrogenation. This reaction will then further be used to define the modifications induced by sulfur adsorption. EXPERIMENTAL

The materials were a y-alumina (GFS C, Rhbne-Poulenc, 210 m2/g), a silica-alumina 26-74 (26 wt.-% Si02, 74 wt.-% Al,O,; Ketjen; 340 m2/g) and a silica (Shell, 266 m”/g). Supported metallic catalysts were prepared by impregnation of the supports with aqueous solutions of chloroplatinic or chloroiridic acids. After impregnation, the catalysts were dried overnight at 383 K, then calcined in an air stream at 723 K (platinum) or 543 K (iridium), and finally reduced in flowing hydrogen for 8 h at 773 K. The chloride content of the different catalysts was reduced by steam treatment under hydrogen at 673 K for 5 h after the initial reduction at 773 K. In this way, catalysts with comparable metallic dispersions and chloride contents were obtained on various carriers. Metallic accessibilities were determined by hydrogen chemisorption and hydrogen/oxygen titrations using a conventional volumetric technique. For hydrogen chemisorption, the double-isotherm method was used [ 241. The values of metal surface areas for platinum and iridium catalysts were calculated by assuming H/Pt = O/Pt= H/Ir= O/Ir = 1 [ 25-271. It was ascertained that residual chloride does not influence the chemisorption properties of metallic catalysts supported on silica by performing hydrogen chemisorption at 298 K with or without a “temperature jump” at 573 K [ 281. Benzene hydrogenation and competitive hydrogenation reactions of benzene and toluene were carried out at 323 K in a conventional flow reactor at atmospheric pressure. Injection of benzene and toluene were done using calibrated motor-driven syringes. The activity tests were carried out under the following conditions: Hydrogenation of benzene, P,=O.O5 atm (2 cm3/h); Pn1026 = 0.95 atm. Competitive hydrogenation, Pbenzene = 0.05 atm; Pn1026 = 0.90 atm; PtilUene varying between 0 and 0.05 atm. Helium was added in order to maintain the other pressures constant. The KTiB values (ratio of the adsorption coefficients of toluene and benzene) were determined as described previously [ 8,9]. The hydrogen sulfide adsorption was carried out in a continuous flow reactor at normal pressure following the procedure described in a previous work [ 71. On the support, sulfur is completely eliminated by hydrogen treatment at 773 K. As regards the surface acidities of the various supports, they were determined by thermodesorption of ammonia desorbed is stages between 373 and 773K [29].

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P. Markcot et al./Appl. Catal. A 101 (1993) 143-149 RESULTS AND DISCUSSION

Activity of fresh catalysts The various catalysts were tested in benzene hydrogenation at 323 K. The data listed in Table 1 show that the turnover frequency (TOF, h- ’ ) depends on the nature of the support whatever the metallic phase. Nevertheless, the effect of the carrier varies also with the nature of the metal: for example, iridium is more active when it is deposited on silica whereas the same support leads to the lowest activity for platinum. This evolution of the activity may be explained by the nature of the support and particularly by the increase of the acidity following the sequence silica < alumina < silica-alumina (Table 2) and therefore by the higher electrondeficient character of the small metallic particles deposited on the most acidic support (Table 1) . Indeed, in previous work [ 8,9] large KTIB ratios have been associated with the electron-deficient character of the metal (under such conditions the adsorption of the molecule with higher electronic density (toluene) is favoured). Moreover, on a same support and in accordance with ref. 30, the KTIB ratios are higher on iridium than on platinum. However, the electronic structure of iridium deposited on silica is akin to that of platinum deposited TABLE 1 Characterization of the fresh catalysts Catalyst

D (%)

Cl (WI

KT/B

A0

0.6% 0.6% 0.6% 0.6% 0.6% 0.6%

55 57 55 61 66 53

0.3 0.3 0.3 0.3 0.3 0.3

7.4 5.0 4.8 16.6 6.2 6.2

520 550 210 250 140 860

Pt/Si02-A&O3 Pt/A1203 Pt/SiO, Ir/Si02-A1,03 Ir/Al,O, Ir/SiO,

K,,e=ratio of the adsorption coefficients of toluene and benzene; A,=turnover number of the fresh catalysts for the reaction of benzene hydrogenation at 323 K (h-’ ) . TABLE 2 Determination of the acidity of the supports by ammonia thermodesorption support

Acidity (meq. H+/gX102)

Si02-Al,O, AI,O, SiO,

64 44 34

P. Ma&cot et al./Appl. Cat&. A 101(1993) 143-149

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Ao h-1

Ir/SiOz

600 -

600 PI/Al

2 O3 PtISiOz,

400

-AI2

O3

-

Ir/SiO,

-Al2

O3

0 200

Pt/SiOz

-

0

: 0

5

I

1

10

15

2o

K

TIE

Fig. 1. Turnover number (A,,) for benzene hydrogenation at 323 K as a function of the ratio of the adsorption coefficients KT,B. (KT,e were obtained from the competitive hydrogenation of toluene and benzene).

on silica-alumina. In other words, the electronic properties of platinum and iridium, evaluated by their ability to adsorb in competition toluene and benzene, are comparable when platinum is deposited on an acidic support and iridium deposited on a non-acidic one. As a result, Fig. 1, which represents the activity (A,) for benzene hydrogenation at 323 K versus the KTIB ratio of the metallic phase, shows a curve with a maximum for Ir/Si02 (the best electronic properties for the reaction under consideration), while platinum on a nonacidic support (silica) and iridium on acidic supports (alumina, silica-alumina) are less active.

Effectof sulfur The different catalysts were sulfurized at 773 K under the hydrogen sulfide/ hydrogen mixture and then treated at the same temperature under pure hydrogen flow. After such treatment some “irreversible sulfur” remains adsorbed on the metallic phase [4-71 which leads to the sulfur coverages reported in Table 3. In the same table are also recorded the activities, for benzene hydrogenation, of the fresh catalysts (A,), of the sulfurized ones (A,) and the A,/ A, ratios. The data point out that sulfurization induces a more or less large

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TABLE 3 Effect of sulfur on the turnover number of supported platinum and iridium catalysts Catalyst

0,

A0

A*

&IA,

0.6% Pt/Si02-A1203 0.6% Pt/Al,OB 0.6% Pt/SiOz 0.6% Ir/Sr02-Al2O3 0.6% Ir/Al,O, 0.6% Ir/SiO,

0.19 0.35 0.36 0.39 0.64 0.44

520 550 210 250 140 880

290 210 110 120 50 20

1.8 2.6 1.9 2.1 2.8 44.0

0,= Sulfur coverage (irreversible sulfur under hydrogen at 773 K); A,,= turnover number of the fresh catalysts for the reaction of benzene hydrogenation at 323 K (h-l); A,= turnover number of the sulfurized catalysts for the reaction of benzene hydrogenation at 323 K (h-r). (The accessibility of the fresh catalysts was used to calculate A, and A,). TABLE 4 Effect of sulfur on the Kr,n of supported platinum catalysts Catalyst

0.6% Pt/Si02-A&O3 0.6% Pt/A1203 0.6% Pt/SiOz

D (%)

55 57 55

K T/B Fresh

Sulfurized

7.4 5.0 4.8

7.8 6.4 5.8

decrease of the activity as shown by the evolution of the AJA, ratio. (The accessibility of the fresh catalyst was used to calculate A,, and A,). A simple geometric effect cannot by itself explain the results since the decrease of the activity is not directly proportional to the sulfur coverage, particularly in the case of the Ir/AlzO, and Ir/SiOp catalysts. On the other hand, it is known that sulfur, by its electron acceptor properties induces an electron deficient character on the unpoisoned metallic phase [ 8,311. Indeed, Table 4 shows that the sulfurization increases the KTIB ratios on the supported platinum catalysts. (These ratios were not determined on iridium catalysts for which the remaining activities are too low in the presence of sulfur). The increase of the electron deficient character of the metallic phase induced by sulfurization is reported in Fig. 2 which points out that sulfur, as compared to the unpoisoned metallic species, can result in higher activity (Pt/SiO, and Pt/A1203), lower activity (Ir/SiOp and Pt/Si02-A1203) or comparable activity ( Ir/AlzO, and Ir/SiOz-A1z03). However, the effect of the sulfurization on the catalyst activity depends also on the sulfur coverage of the metallic phase (0,). Thus, the high toxicity of sulfur on the Ir/SiOz catalyst can be explained by

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P. Mar&at et al./Appl. Catal. A 101(1993) 143-149

Ao ,. h-1.

Ir/Si02

Pt/SiOz

200

I

p-a/)

’

0

5

10

15

20 ’

T/B

Fig. 2. Effect of sulfur on the activity of the unpoisoned metallic species : ( /” ) increase of the activity, ( \ ) decrease of the activity, (+ ) comparable activity. --+: Increase of the KTjB ratio induced by sulfur on platinum samples. - - + : Expected increase of the KTiB ratio induced by sulfur on iridium samples.

the combination of two highly negative effects (geometric and electronic ). For Ir/A120, and Ir/Si02-AlsO catalysts, it is mainly the geometric effect that is responsible for the toxicity of sulfur since the catalytic properties of the fresh and sulfurized samples are quite similar (Fig. 2 ) . On platinum/silica and platinum/alumina catalysts, the sulfurization would induce a positive electronic effect on the metallic phase (Fig. 2), but, as the sulfur coverage is high, the geometric effect prevails and therefore the activity for benzene hydrogenation decreases. Nevertheless, such interpretation allowed Oudar et al. [ 321 to explain the increase of activity at low sulfur coverage for ‘Hz-2H2 exchange. In conclusion, it appears that the activity of small platinum or iridium particles for benzene hydrogenation at 323 K is sensitive to the electronic properties of the metal, which, in turn, depend on the acidity of the support. Therefore, sulfur, which itself induces a modification of the electronic properties of the metal by its electron acceptor character, will display different toxicities on the various catalysts. Therefore, the combination of two highly negative effects (geometric and electronic) can explain the high toxicity of sulfur on Ir/Si02, while opposite effects, a negative one by geometrical hindrance of the surface and a positive one by electronic transfer, can explain a low toxicity of sulfur. From a practical point of view, sulfur is often used as a agent promoting selec-

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