Materials Chemistry and Physics, 9 (1983) 457-466 457

COMPARATIVE STUDY OF ALUMINA SULFATION FROM H2S AND SO2 OXIDATION

H6lGne SAUSSEY, Alain VALLET and Jean-Claude LAVALLEY

Laboratoire de Spectrochimie, ERA 824

I.S.M.Ra UniversitL de Caen 14032 Caen Cedex (France)

Received 6 April 1983; accepted 25 April 1983

ABSTRACT

Alumina sulfation was studied under static conditions from HzS2zr SO2 oxida- tion, using infrared spectroscopy to determine the amounts of SOI, formed. On

heating without 02 traces, no sulfate appears. With a large excess of 02, some

HzS sulfation occurs even at room temperature. It does not depend on the alumina hydroxyl content and increases with the oxidation temperature, Tax. It reaches a limit value, _ 2.2 pmol m-', when To, b 723 K. SO2 sulfation is more difficult as it occurs at higher oxidation temperatures than the HpS reaction. tJater and hydroxyl groups promote it. It reaches the same limit value as the H2.S reaction when Tax >/ 723 K.

Two mechanisms are proposed to explain HzS sulfation. When To, < 473 K, there is direct sulfation, involving an H2S chemisorbed species. Radicals are certain- ly an intermediate species. Fo$_higher To, values, H2S is first transformed into

so2; this explains why the SOI, amounts then formed do not depend on the star- ting compound, H2S or SO2. A mechanism involving in the first step formation of hydrogen sulfite species accounts well for the results obtained on SO2 sulfation.

INTRODUCTION

Alumina is the most common catalyst used to remove HPS from sour gases, actor

ding to two processes:

2 H2S + SO2 -+ ; S, + 2 Hz0 (Claus reaction)

or H2S + 102 + 2

$ s, + Hz0

One of the principal problems is catalyst deactivation due to poisoning. Sul-

fate poisoning is reported to decrease the Claus activity drastically [I]. How-

ever sulfate formation has not yet been investigated systematically; to our

knowlegde only one paper has been devoted to such a fundamental study: Quet c

al. [Z] h s owed that neutral and bad sulfates exist for the Cldus reaction on alu-

mina and they suggested possible sites and mechanisms for their formation. They

concluded that the poisoning is due to sulfates formed when oxygen is present in

the gases. Oxygen is reported elsewhere not to be essential, sulfur itself being

the oxidising agent b].

0254-0584/83/$3.00 0 Elsevier Sequoia/Printed in The Netherlands

458

The lack of fundamental studies on the sulfation of oxides is certainly due

to difficulties in the determination of the SOI, 2-

content. In the present study,

it has been determined using infrared spectroscopy in a way which avoids the so-

lubilization of alumina [4]. After reporting the method, we present the results

we obtained on HpS or SO:! sulfation in varying the amount of oxygen, the oxida-

tion temperature, the amounts of HzS or SO2 and water. Some mechanisms are pro-

posed involving a different way of sulfate formation from HsS or SO:, oxidation.

These mechanisms take account of the results we have already obtained studying

HpS [5] and SO;! [6] adsorption on alumina.

EXPERIMENTAL METHODS

The aluminium oxide used was DECLJSSA C (a mixture of y and &alumina) with -1

a surface area of 100 m2 8 . Its sodium content was quite low (< 50 ppm).

The sulfation experiments were carried out in the IR cell described in a

previous study [4]. Its volume is about 300 ml. The powder was pressed (~a.

3x10' N me2) into the form of - 30 mg discs (diameter = 16 mm). The samples were

activated at the given temperature Tae by heating for 1 h under oxygen, followed

by an evacuation for i h (P < 5~10~~ N mm2), then by treatment under hydrogen

(Ih), then by another evacuation (2h, P < 1.3~10~~ N m-'>.

The oxidation experiments were carried out, under static conditions, on the

activated discs: after introducing the starting products (HsS or SOP, and even-

tually O2 or HsO) at room temperature , we heated them at the chosen oxidation

temperature, To,, for 14 h, then we evacuated at 723 K (Zh, P < 1.3x10 -3 N mm2>.

Infrared spectra were recorded at 315 K using a Perkin-Elmer 580 grating ins-

trument. The materials used were all chemically pure Reagent grade and were used

without further purification.

RESULTS

A typical IR spectrum of sulfates on alumina is reported Fig. I. It shows

two bands, at 1390 and 1100 em -1

, characteristic of SOW 2-

ions [4]. It is quite

similar to the spectrum of sulfated anatase [7]. Such bands have been considered -1

as being due to covalent sulfate species. From the 1390 cm band integrated in-

tensity, using calibration curves @], we deduced the SOI+ Z-

amount on the sample.

The same S0,,2- spectrum is obtained either from HeS or SO, oxidation [4].

The starting alumina sample activated at 723 K (further noted AlzOs-723)

shows mainly three v(OH) bands: 3790 (weak), 3730 and 3690 cm

fated sample spectrum (Fig. I) shows a very weak band at -' CJ' The persul-

3790 cm and a strong

one at 3692 cm -1

with a broad tail (shoulders near 3640, 3500 and 3320 cm-').

This broad band is certainly due to Briinsted H+ sites as further addition of

NH:, gives rise to NHI,~ species p].

459

60.

:ooo i . . . .

3200 ’ , , . . . s .

1600 1200 cm-1

Fig. I: IR spectrum of an evacuated Al 2 s-723 sample heated at 623 K for 14 h 0 with a mixture of 500 pm01 SzT of SOa and a large excess of Oz. ( -----__ spectrum obtained after background substraction).

ic so42- amount

/ pm01 g-1 200.

O- 373 473 5h 6h

*

Fig. 2: Variation, with the oxidation temperature To,, of the Sob Z-

amount formed from oxidation of SO0 nmol g&l of $02 (a) or HzS (b) with a large excess of 02 on A1203-723.

a: from SO2 oxidation (A) b: from H2S oxidation (CI)

dotted line : difference between b and 8

Before reporting the detailed results, it is important to mention first that,

without any traces of oxygen, heating at 723 K 500 nmol g -I

of HzS or SO2 on

Alz03-723 did not lead to the SOI* 2-

species (amount lower than 5 umol g-l). The

460

same result was obtained on A1203-283 or on A1203-723 by heating with an excess

of water. Therefore, under our experimental conditions, oxygen ions on alumina

do not transform HzS or SO;! into SOL, 2-

ions.

Sulfation from HzS

Effect of the oxidation temperature, ToL

Using a large excess of oxygen (P = 2x104 N m -2 in the cell) it appeared that

oxidation of 500 ~1rn01 g -1

of HzS into SO4 2-

ions occured on AlpOa-723, even at

room temperature (- 18 nmol g-l}_ kle report in Fig. 2 the variation of the SOI, 2-

amount formed under such conditions with To,: it increases with To, and tends

towards a limit (- 220 nmol g-') when Tax >, 723 IL The experiments were comple-

ted by analysis of the gas phase content by gas chromatography. We noted SO2

formation when To, > 473 K. Between 473 and 573 K, both gases, HeS and Son, were

present while, when To, > 573 K, only SOa was detected. Under such experimentaL

conditions, the whole H2S excess is therefore transformed into sulfur dioxide.

Influence of the amount of HsS or 02 introduced

To determine which of the two compounds limits the SOI+ 2-

ion formation, expe-

riments have been carried out with known quantities. of each gas:

In Fig. 3, we report the SO4 2- amount fOXUI& an A1203-723 by oxidation of

limited quantities of H2S in the presence of a large excess of oxygen (P =

2~10~ N m -2

in the cell). Different Tax values were used. When To, = 723 K, the

TQxm623K ----____c__

Fig. 3: Variation of the SO@ 2-

amount formed with the introduced quantities of H,S (broken Lines) or SOP (solid lines) at different oxidation temperatures ia the presence of a large excess of Oz.

461

H2S introduced is almost fully transformed into SOI, 2- -1

species up to 150 nmol g .

Then it tends to the limit value (220 umol g-l). When To, = 623 or 523 K, lower

so 2- -;

amounts were formed (Fig. 3) (the limit values were then 140 and 90 umol

g respectively). Again, when Tax = 623 K, it appears that the first quantities

of HzS introduced are almost completely transformed into sulfate ions.

In another experiment, on AlpOa-723, we found that oxidation at 723 K of

500 urn01 g -1

of HzS with a limited quantity of 02 (750 nmol g-l) led to only

75 urn01 g -1

of sulfate ions, instead of the 220 nmol g -I

expected.

We conclude that a large excess of oxygen is necessary to fulfil the forma-

tion of SO4 2-

ions under our experimental conditions. On the other hand, it

appears that the first doses of H2S introduced are almost fully transformed into

SO4 2-

ions. This suggests that, at least for low values of Tax, it is a part of

the irreversible species given by H2S adsorption which leads to sulfate ions. To

check this result, two experiments were carried out with 50 nmol g -1

of H2S on

A1~03-723. In the first one, this quantity was directly heated at To, = 373 K

with a large excess of oxygen. The second one was the same but the eventual H2S

excess introduced (reversible species) was evacuated at T = 373 K before intro-

ducing 02 and heating. The same amount of SOS 2-

ions (12 nmol g-l) was found,

showing that these ions indeed result from the oxidation of H2S irreversibly ad-

sorbed species.

Influence of water or of alumina hydroxyl content

In a series of experiments we have studied this influence, limiting the HeS

amount to 50 ymol g -1

but using a large excess of oxygen. We observe (Fig. 4~)

that the amount of SO4 2-

ions formed with respect to Tax does not depend on the

hydroxyl content of alumina: the points relative to A1209-723 and A1203-293 are

on the same curve. The same appears when some water is introduced in the 02. We

conclude that HzS sulfation does not seem to involve OH groups on alumina. How-

ever it is possible that water may be present in all experiments, even on A1203--

723, as it could be formed from a partial oxidation of HzS into sulfur.

Sulfation from SO:!

Influence of Tax

Using Al,O,-723 and a large excess of 02, it appears (Fig. 2) that SO, 2-

for-

mation is quite low (< 20 umol g-l) when To, < 523 K. Then, the SOI, 2-

amount

increases with To,, until To, = 723 K, for which the same limit value (220 nmol

g-l), as previously observed in the case of H2S, is reached. Note that curves

obtained from H2S and SO;! (Fig. 2) are almost indentical when To, > 623 K,

showing that the SO,, 2-

amount formed, under such conditions, does not depend

on the starting compound, either HaS or SOa. On the other hand, when To, < 523 K

(Fig. 2), it appears that the sulfation from H2S is easier than that from SOP.

462

Influence of the amount of SO2 or 0s introduced

Experiments with limited quantities of SO2 or 02 led to the following resulb:

On Ala03-723 and for To, = 723 K, we obtained, from oxidation of known quan-

tities of SO2 with large amounts of 02, SO+ 2-

quantities quite similar to those

found from H2S oxidation under the same conditions (Fig. 3). Again the amount

of SO2 introduced is almost fully transformed into SOI, 2-

up to 150 urn01 g -I. On

the other hand, when To, = 623 K, the amount of SOS 2-

species formed is much

lower (Fig. 3) confirming the previous result on the relative ease of sulfation

of both compounds.

On Al203-723 and for To, = 723 K, oxidation of 500 umol g

same amount of 02 gave rise to only 65 nmol g -1 of sob2- spe;;e;f so2 w1th the

We conclude, as in the case of H2S sulfation, that under our static condi-

tions and for To, = 723 K, it is the amount of oxygen which is the limiting

quantity in the formation of sulfate ions.

Influence of water or of alumina hydroxyl content

Using the same conditions (50 nmol g -1

of SOa, large excess of 02) as in the

case of H2S (Fig. 4) we confirm that the sulfation of SO2 on AlzOa-723 is only

effective when To, > 523 K (Fig. 4a). On the other hand, on A1203-293 (i.e.

highly hydroxylated), we observe SO4 2-

formation at a lower temperature (Tox >

373 K). The SO4 2-

amount (Fig. 4b) is always higher than on the well activated

sample. The same curve (Fig. 4b) is obtained on Al203-723 when SO2 is heated

under oxygen and water (Po2/PH20 = 30). It is concluded that hydroxyl groups or

water promote SOS 2-

formation from SO2 oxidation in the same way.

A

40.

SOd2- amount / pm01 g-l

20.

_c-

lox

0 /",

673 773 ^

-Fig. 4: Variation, with the oxjdation temperature Tax, of the SO+ L-

med by oxidation of 50 umol g of H2S (broken lines) or SO2 (solid a large excess of 02 : effect of water and alumina dehydroxylation. a= SO2 sulfation on AlaOs- b= SO2 sulfation on AlsO,- (A) or on A1203-723 by heating with 02 + H,O (v) C= H2S SUlfatiOn OII Al,O,-723 (0) or on Alc03-293 (D)

amount for- lines) with

a mixture of

463

DISCUSSION

This study clearly shows that oxygen is necessary to transform SO2 or H2S

into sulfates on alumina. Note that the alumina activations involved a final

pretreatment with hydrogen. This avoids the presence of molecular oxygen, in an

adsorbed sate, which has been found to be reactive towards oxidation of sulfur

compounds [G]. Without 02 traces, alumina does not show such oxidizing proper-

ties.

With an excess of oxygen, the SO+ 2-

amount formed reaches a limit value (2.2

nmol mw2). It corresponds to about 2 % by weight, or 1.3 SOL,*- -2

ions nm .

Attempts to obtain more sulfates, either by heating at higher temperatures

(823 K) or by heating a longer time (36 h) were unsuccessful. This limit would

therefore correspond to a complete surface coverage. It is in good agreement

with Quet et al.'s results, although the latter were obtained under dynamic con-

ditions, at lower T ox

: on AlaOs-543, in the presence of Claus gas, they found

4.5 % sulfates by weight, which corresponds to 1.0 SOI, 2- -2

nm [2].

Curves reported in Fig. 2 show that H2S oxidation is much easier than S02:

it occurs at lower oxidation temperatures. This could explain why the sulfate

level reported by Quet et al. [2] with the Claus gas is far above the one obtai-

ned when alumina is treated by a mixture of SO2 + 02 without H2S. Yoreover, with

the Claus gas, water is formed, promoting SO2 sulfation, as shown in the present

study (Fig. 4).

Considering curves reported in Figs. 2 and 4, we deduce the possibility of

two processes to transform HaS into sulfates. The first one, which occurs for

low values of To,, involves a direct transformation, without any passage via SO2 -

(the SOti2- quantity formed in that manner has been deduced by subtracting curves

2a from 2b). On the other hand, when To, > 473 K, SO2 was found in the gas phase,

suggesting a mechanism like:

H2S + SO2 -t SOL,~-

Both mechanisms are discussed below.

Direct sulfation from H2S

The present results show that this reaction involves H2S chemisorbed species.

It has been found that about 100 nmol g -1

of H2S are irreversibly adsorbed, at

room temperature, on AlaOs- b]. D' association occurs leading to -OH and -SH

species [5]. Th e same adsorption was found on NaX zeolithe [IO]. It is however

possible that part of the chemisorption leads to S 2-

species. They can be very

reactive towards oxygen, giving rise directly to SOr 2-

ions. In a recent paper

relative to the photocatalytic oxidation of sulfur on TiO2, it has been reported

that a large amount of SOI, 2- is produced from NaaS solutions on anatase Fl].

The authors suggested mechanisms with radicals. We discuss such a possibility

underneath.

464

Let US mention first that no ESR signal was reported when alumina was heated

in the presence of HzS Bz]. The same result was found when A1203 was exposed to

0~ b3]. However, Steijns et al. b4] did observe the formation of sulfur radi-

cals, in the presence of H2S and in the absence of oxygen, on pure alumina and

NaX zeolite. Moreover they reported a yellow color for the catalysts, indicating

the formation of sulfur. Dudzik and Ziolek discussed these results p5] and

suggested that the activation procedure used in p4] allowed some oxygen species

to be present on the surface. Both studies agree with the generation of surface

radicals such as 'S *. x

They react with molecular oxygen. Steijns et al. sugges-

ted the formation of a product, noted [SxO.z], an oxidized sulfur, which could be

+s-(s&-s,;-

and %#ould constitute the first step of the sulfur and water formation.

Dudzik and Ziolek proposed other reactions on zeolites, giving rise to a

superoxide ion OZ.- p5]:

'S-(S)x_2-s*+ o* -f .s-(s)x_*S+ + 02-

02 is very reactive and could lead in particular to HOi and HCTradicals. Such

radicals were proposed as reactive species for the direct sulfate production

from sulfur on anatase 011:

2s + 4HO*(ads) + HaS + 2H+ + SOh2-

In fact the radical mechanisms are certainly very complex and other ways

could be proposed, involving for instance HS- and+ radicals [15]. If such

species are formed on alumina, they could react with oxygen as the polysulfur

radicals do n4], leading to oxidized sulfur species, which may be adsorbed on

alumina as sulfate ions.

Sulfation from SO2

The present paper shows that HsS oxidation with an 02 excess on alumina leads

to some SO:!in the gas phase when To, > 473 K. This was explained according to

the following mechanism b4]:

l s-(S)x_1-SO9 + 's-(s)x_2-s' + so:! SO2 formation allows one to understand why the sulfation curves obtained from

HzS or SO2 oxidation tend to be similar at high To, values (Figs. 2, 4).

SO2 adsorption on alumina gives rise to several species, Infrared spectros-

copy shows the formation of sulfites u6] and coordinated species [6] on well

dehydrated samples. On hydroxylated alumina, hydrogen sulfite species were evi-

denced f6]. Moreover, when SO2 treated alumina is heated at 5C0 K, ES9 shows the

formation of SOz- species [17].

Results reported in Fig. 4 clearly show that the presence of hydroxyl groups

on A1203 (or water) favours sulfate formation. They are in good agreement with

465

Lunsford's results on MnY zeolites as it was found that the amount of SOI, 2-

formed per unit cell in the fully hydrated samples was more than an order of

magnitude greater than the amount formed in the corresponding dehydrated samples

cl*! * The following mechanism was proposed:

s”*w -+ "'(ads)

SOP(ads) 2 + .!. 02 + so3

SO+ Hz0 + HaSO4

or SO3 + 02- 2-

(lattice) -f 'OS (ads)

We suggest another mechanism involving hydrogen sulfite species. Such species

are formed on hydroxylated alumina f6] and in aqueous sulfur dioxide solutions

[19] - They have already been involved in mechanisms of oxidation of aqueous SO:

solutions with homogeneous manganese [Id and iron [Id catalysts [19].

The first step is the formation of hydrogen sulfite species from interaction

between SO:! and basic OH groups of alumina. This interaction is quite easy as it

has been found that about 160 nmol g -1

of SO2 are irreversibly adsorbed in that

manner on Alz03-293 @]:

SOz + OH- -t HS03-(adsj

The second step

llSos (ads) 1

+?02 +H+ + S0'2-(ads)

would be more difficult as it requires heating at To, > 450 K on A120s-293

(Fig. 4).

This reaction is not a catalytic one as the basic hydroxyl groups are not

regenerated. This could partly explain why a limit value of SOI, 2-

ions is

reached.

Experiments are in progress to find precisely the poisoning effect of sulfates

on the H2S transformation into sulfur and water with oxygen.

ACKNOWLEDGMENTS

The authors wish to thank MM. .I. Preud'homme and A. Janin for help and the

D.G.R.S.T. and IGOL-Normandie for financial support.

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