# Quantification of competitive adsorptions between sulfur and olefinic model compounds representative of a FCC gasoline over a CoMoS/Al2O3 catalyst. Theoretical and experimental approaches

Alan Silva Dos Santos, Etienne Girard, Philibert Leflaive, Sylvette Brunet

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Alan Silva Dos Santos, Etienne Girard, Philibert Leflaive, Sylvette Brunet. Quantification of competitive adsorptions between sulfur and olefinic model compounds representative of a FCC gasoline over a CoMoS/Al2O3 catalyst. Theoretical and experimental approaches. Applied Catalysis A: General, 2024, 687, pp.119962. 10.1016/j.apcata.2024.119962. hal-04702874

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# Quantification of competitive adsorptions between sulfur and olefinic model compounds representative of a FCC gasoline over a CoMoS/Al₂O₃ catalyst. Theoretical and experimental approaches

Alan Silva dos Santos¹, Etienne Girard², Philibert Leflaive² and Sylvette Brunet¹*

¹ Université de Poitiers, Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), UMR 7285 CNRS-, 4 rue Michel Brunet, TSA 71106-86073 Poitiers Cedex9, France

² IFP Energies nouvelles, Rond-point de l'échangeur de Solaize, BP 3, 69360 Solaize, France

* Corresponding author: email: [sylvette.brunet@univ-poitiers.fr](mailto:sylvette.brunet@univ-poitiers.fr) (Sylvette Brunet)

Tel: (+33) 5 49 45 36 27

Fax: (+33) 5 49 45 38 97
# 1 Introduction

While the overall composition of a FCC gasoline has been known since a long time, the exact composition within each family is more difficult to estimate. This is a very important parameter as the content can vary depending on the origin of the raw material. Indeed, gasoline pool comes from FCC process for 40-80% [1]. The challenge since few decades is too respect the environmental regulations corresponding to a level of sulfur lower 10 ppm since 2009 in European Union and reinforced in the forthcoming China V and US Tier 3 regulations for the final product more recently [2-4]. Although the use of fossil fuels is changing rapidly with the advent of electric motors, their use is still essential, and hydrodesulfurization (HDS) processes still need to be optimized with respect for the environment to achieve the regulations. The content of sulfur compounds could vary greatly according to the origin of the crude oil [5]. FCC gasoline pool which contributes to over than 90% of all sulfur content present in the gasoline pool [1,6] is a complex blend of more than 400 compounds, mainly including thiophenic compounds, paraffins, olefins, naphtenes and aromatics. The composition of a FCC gasoline varies depending on the feedstock and could be composed basically of paraffins and isoparafins (~30 wt%), olefins (~30 wt%), aromatic compounds (~30 wt%) and impurities such as sulfur compounds, derivate alkyl and nitrogen compounds. As reported in the literature, sulfur compounds are present as mercaptans, thiophenes, benzothiophenes and their alkyl derivatives [7-9]. Furthermore, Magyar et al. [10] have shown that the amount of olefins present in a feedstock consist in majority by pentenes (such as 2-methylbut-2-ene and pent-2-ene) and hexenes (such as hex-2-ene, 2-methylpent-1-ene). It is remarkable that in different feedstocks the amount of internal olefins present in the gasoline is higher than terminal olefins, which represents about 80% of C6 olefins present in the FCC gasoline [11]. The main point of a successful hydrotreatment of a FCC gasoline cut is to achieve a high hydrodesulfurization (HDS) activity to remove the sulfur compounds from fuel and a low hydrogenation (HYD) activity, to avoid the hydrogenation of olefins and, consequently, decrease the octane loss in
the final product. The amount of sulfur compounds in the gasoline varies also according the
feedstock. However it is well known that benzothiophene, C1-thiophenes and C2-thiophenes
corresponds to more than 60% of the sulfur compounds present in a FCC gasoline [12].
Optimising the process requires i) a better understanding of the composition and the behaviour
of all these molecule families when they are mixed together, i.e. their impact on the
transformation of each other ii) a better knowledge of the properties of the catalyst. Articles in
the literature reported kinetic studies of the transformation of olefins alone or in mixture with
sulfur compounds representative of the various families or directly FCC Naphta [13-16].
Daudin et al [13,17] elaborated a microkinetic model considering a Langmuir Hinshelwood
mechanism rate, heterogeneous dissociation of H<sub>2</sub>S and H<sub>2</sub>, the addition of the first proton as
the determining step and involving only one type of site. Regarding the hydrogenation
transformation of 2 model molecules i.e 2,3-dimethylbu-2-ene (hydrogenation : HYD) as model
olefin and 2-methylthiophene (Hydrodesulfurization : HDS) as sulfur compound over bulk
metallic sulfide catalysts, an optimum of selectivity (HYD/HDS) was related to a maximum for
the NiMoS bulk catalyst corresponding to a metal sulfide energy (E(MS)) of 128 kJ/mol. In the
same way, C.M. Celis-Cornejo [16], under the same kinetic hypothesis than [13], observed
competitive adsorptions over a CoMoS catalyst surface between a sulfur compound (2-
methylthiophene) and an olefin (trimethylpentene) while the selectivity depended on their
concentrations. Badawi et al [14] showed by a kinetic study that the rate in hydrogenation of
3,3-dimethylbut-1-ene, of cyclopentane and 1-methylcyclopentene decreased with the increase
of the substitution of the double bond over a CoMoS based catalyst. More recently, Dos Santos
et al [18] showed that benzothiophene sulfur model molecule was more reactive than 3-
methylthiophene over a CoMo/Al<sub>2</sub>O<sub>3</sub> catalyst. When these two molecules were studied in
mixture, a mutual inhibiting effect was observed which was more pronounced for
benzothiophene than 3-methylthiophene. These results were explained considering the
adsorption constants of these two molecules where the adsorption constant of benzothiophene
is higher than the adsorption constant of 3-methylthiophene. These values were determined
from Langmuir-Hinshelwood a kinetic model.

This paper focuses on the comparison of the reactivity of various model olefins representative of C₆ olefins present in FCC gasoline. The C₆ olefins (predominant in gasolines) studied are: hex-1-ene, 4-methylpent-1-ene (4MP1N), 3,3-dimethylbut-1-ene (33DMB1N) and 2,3-dimethylbut-2-ene (23DMB2N). These were chosen in order to highlight the effect of the structure (position of the double bond and branching). First, their reactivity was measured alone to establish a scale of reactivity (in terms of isomerization and hydrogenation reactions) according to their structure. Their reactivity was then measured in the presence of sulfur model molecules (3-methy thiophene and benzothiophene) in order to compare their behavior under conditions close to HDS of a FCC gasoline. All of these results have been quantified and validated through a unique kinetic model considering a Langmuir Hinshelwood formalism developed from [17] and by [18] for the transformation of 3-methylthiophene and benzothiophene in the same operating conditions.

## 2 Experimental part

### 2.1 Catalyst and chemicals

As reported previously [18], the alumina supported catalyst was provided by IFPEN with the following composition: CoO (3%wt), MoO₃ (10%wt). The catalyst was crushed and sieved to size range between 250 and 315 µm and then sulfided in situ under H₂S/H₂ flow (10 mol% H₂S) for 10 h at 400°C at atmospheric pressure. 3-methylthiophene (98% purity), benzothiophene (95% purity), n-heptane (>99% purity), 2,3-dimethylbut-2-ene (98% purity), hex-1-ene (97% purity) were purchased from Sigma-Aldrich unlike 4-methylpent-1-ene (97% purity) and 3,3-dimethylbut-1-ene (95% purity) from Alfa Aesar. They were used without further purification. Hydrogen sulfide (1 vol% in mixture with H₂) and hydrogen were purchased from Air Liquide.
## 2.2 Reaction conditions

Catalytic activity measurements were carried out in a fixed bed reactor at 250°C under a total pressure of 2 MPa with a H₂/feed ratio of 360 NL/L [18,19-21]. The model feed diluted in n-heptane was injected in the reactor by a HPLC Gilson pump (307 series, pump's head volume: 5 cm³).

The feed was composed by 10 wt% of olefin (hex-1-ene, 4MP1N, 33DMB1N or 23DMB2N) and 1000ppm S of sulfur compound (3MT, BT or H₂S) diluted in n-heptane. H₂S was introduced with a flowmeter. N-heptane was not converted under these experimental conditions. No catalyst deactivation was observed for all experiments.

## 2.3 Products analysis

The reaction products for the olefins and sulfur molecules were injected on-line by means of an automatic sampling valve into a Varian gas chromatograph. The reaction products were analyzed on-line by means of a Varian gas-chromatograph equipped with an automatic sampling valve, a 50 m PONA (HP) capillary column (inside diameter: 0.2 mm; film thickness: 0.5 µm), a flame ionization detector. A cryogenic system was used to start the analysis at 0°C, to ensure a good separation of the olefin transformation products. The conditions of analysis were the following: a temperature programme from 0°C (15 min) to 20°C (1.3°C/min) then from 20°C to 50°C (2°C/min) and then from 50°C to 250°C (7°C/min). The identification of the products was made possible by GC-MS coupling. Finally, the temperature of the FID detector and injector was set at 250°C.

Activity for the transformation of olefins and sulfur compounds was calculated after stabilization under conditions where a linear relationship between conversion and residence time was obtained (conversion around 30%). The contact time is defined as the ratio between the total amount of feed and the mass of catalyst in the oxide form. To obtain a large range of conversions, the contact time was modified by changing the amount of catalyst and/or the feed flow rate. The activity is defined as the number of mole of reactants (olefins and sulfur
compounds) transformed (conversion) per hour and per gram of catalyst calculated for conversions lower than 30% in a differential regime.

More specifically, isomerization (ISO) and hydrogenation (HYD) activities are defined as the number of mole of products resulting of isomerization (ISO) and hydrogenation (HYD) reactions from olefins formed per hour and per gram of catalyst.

### 2.3.1 Kinetic modeling

Experimental results were modeled through a theoretical approach using the Langmuir-Hinshelwood model and the Arrhenius equation. Kinetic and adsorption parameters were first set from single component experiments using ReactOp software® and were then used to fit binary experimental results without further adjustment. The model used was previously described for the transformation of the sulfur compounds (3MT and BT) alone and in mixture [18] considering:

1. Competition reactions between the adsorbed molecules;

2. ISO and HYD reaction rate of 23DMB2N, 33DMB1N, 4MP1N and hex-1-ene and first-order relative to the olefinic compound and hydrogen;

3. Heterolytic dissociation for adsorption of H₂ and H₂S on catalytic sites;

4. Addition of the first hydrogen as the determining step for the transformation of olefins;

5. A single type of catalytic site for HYD and ISO reactions.

The rate constants of isomerization (ISO) and hydrogenation (HYD) of the olefins are determined as followed equation 1 and equation 2 respectively.

$$r(T, P_{H_2S}, P_{H_2}) = k_{ISO} \cdot e^{\frac{Ea_{ISO}}{RT}} \frac{K_R P_R \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}}{\left(1 + K_R P_R + \frac{P_{H_2S}}{K_S P_{H_2}} + \frac{K_{H_2} P_{H_2} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}}{\sqrt{K_{H_2} S P_{H_2 S}}} + \sqrt{K_{H_2} S P_{H_2 S}} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}\right)^2}$$
Equation 1

$$r(T, P_{H_2S}, P_{H_2}) = k_{HYD} e^{-\frac{E_{HYD}}{RT}} \frac{K_R P_R \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}}{\left( 1 + K_R P_R + \frac{P_{H_2S}}{K_S P_{H_2}} + \frac{K_{H_2} P_{H_2} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}}{\sqrt{K_{H_2S} P_{H_2S}} + \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}} \right)^2} \quad \text{Equation 2}$$

with $K_R$ and $P_R$ are defined as adsorption constant and the partial pressure of olefin (reactant).

The adsorption constants ($K_{H2S}$) of H₂S and ($K_{H2}$) of H₂ have been fixed and are the same as those determined previously for the modeling of sulfur compounds alone and as a mixture [18].

To model the mutual competitive effects between sulfur compounds and olefins, a term of inhibition for sulfur compounds (RS) was added to the denominator of the global rate equation (Equation 3), corresponding to the product of partial pressure for the sulfur compound (RS) and adsorption constant ($K_{RS}$) calculated by modeling these compounds alone ($K_{RS} \cdot P_{RS}$).

The modeling was performed also considering 10% by weight of the olefins in mixture with 1000 ppmS (as H₂S, 3MT or BT) of sulfur compounds and the same parameters obtained from the modeling of olefins alone and sulfurs alone [18].

$$r(T, P_{H_2S}, P_{H_2}) = k_{ISO(HYD)} e^{-\frac{E_{ISO(HYD)}}{RT}} \frac{K_R P_R \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}}{\left( 1 + K_R P_R + K_{RS} P_{RS} + \frac{P_{H_2S}}{K_S P_{H_2}} + \frac{K_{H_2} P_{H_2} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}}{\sqrt{K_{H_2S} P_{H_2S}} + \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_S P_{H_2}}}} \right)^2}$$

With RS = 3MT or BT
Finally, the impact of olefins (R) in the rate of the transformation of the sulfur compounds (RS = 3MT or BT) was considered with the equation as followed :

$$r(T, P_{H_2S}, P_{H_2}) = k_{HDS} \cdot e^{\frac{Ea_{HDS}}{RT}} \frac{K_{RS} P_{RS} \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_{S}P_{H_2}}}}{\left(1 + K_R P_R + K_{RS} P_{RS} + \frac{P_{H_2S}}{K_{S}P_{H_2}} + \frac{K_{H_2} P_{H_2} \sqrt{\frac{P_{H_2S}}{K_{S}P_{H_2}}}}{\sqrt{K_{H_2S} P_{H_2S}}} + \sqrt{K_{H_2S} P_{H_2S}} \sqrt{\frac{P_{H_2S}}{K_{S}P_{H_2}}}\right)^2}$$

# 3 Results

## 3.1 Transformation of single olefins compounds

The transformation of hex-1-ene, 4-methylpent-1-ene (4MP1N), 3,3-dimethylbut-1-ene (33DMB1N) and 2,3-dimethylbut-2-ene (23DMB2N) was therefore studied over a CoMoS / Al₂O₃ catalyst in the presence of 1000 ppmS of H₂S to avoid catalyst deactivation and are representative of the amount of sulfur under operating conditions of HDS (T = 250°C, P = 20 bar).

There is a linear increase of the conversion for contact time lower than 2s for hex-1-ene and 4MP1N and 5s for 33DMB1N and 23DMB2N (Figure 1). However important differences are noted. For example, for a contact time of 2 s, the conversion of hex-1-ene is 85%, for 4MP1N 65%, for 33DMB1N 15% and finally for 23DMB2N 5%. The catalytic activity of the CoMoS/Al₂O₃ catalyst (calculated at conversion lower than 30%) is higher for the transformation of hex-1-ene than 4-methylpent-1-ene, 3,3-dimethylbut-1-ene and 2,3-dimethylbut-2-ene (Table 1). As reported in Figure 2a and Figure 2b, two main reactions can be considered: isomerization (ISO) (double bond and structural) and hydrogenation (HYD) reactions respectively for the various substrates, the isomerization being the main reaction whatever the conversion of hex-1-ene followed by 4MP1N and 33DMB1N. Conversely, the hydrogenation reaction was mainly observed with 23DMB2N, 33DMB1N, 4MP1N and hex-1-ene respectively. These two main reactions of isomerization (skeletal and/or position of the
double bond) and hydrogenation are observed for the transformation of all olefins and involve
the properties in terms of Brønsted acidity of the catalyst, which are involved in the formation
of intermediate carbocations.

Regarding the distribution of products for the different olefins, the transformation of hex-1-ene
leads mainly to the formation of cis and trans-hex-3-ene and cis and trans-hex-2-ene as primary
products from direct isomerization of the double bond of the reagent (Figure 3a). n-hexane is
formed by the direct hydrogenation of the different alkenes and appears as a secondary product
(Figure 3a). In addition, it should be noted that olefins are still the main products. From these
results, the reaction scheme (Scheme 1) established is in agreement with that proposed by Dos
Santos et. al. [22] (T = 190°C, P = 20 bar, CoMo/Al₂O₃) and Tu et. al. [23] (T = 195°C, P = 16
bar, CoMo/Al₂O₃) under operating conditions close to the operating conditions used in this
work.

Only few information is present in the literature for the transformation of 4MP1N under the
same operating conditions as reported in this work and in the presence of sulfide catalysts. For
the conversion of 4MP1N, cis and trans-4-methylpent-2-ene (4MP2N) (product of
isomerization reaction of the double bond) are in the majority up to practically total conversion
and appear as primary reaction products (Figure 3b). 2-methylpent-2-ene (2MP2N) and 2-
methylpent-1-ene (2MP1N) (formed from 4MP2N) are in small amounts up to conversions of
about 60%. Then, their yield increases strongly for higher conversions. 2-methylpentane (2MP)
(hydrogenation product) appears as a primary product which seems to show that its formation
comes mainly from 4MP1N (Scheme 2).

The transformation of 33DMB1N (Figure 3c) leads to the formation of 2,2-dimethylbutane
(22DMB) (hydrogenation product). The isomerization products (2,3-dimethylbut-2-ene and
2,3-dimethylbut-1-ene) appear as by-products. 2,3-dimethylbutane (23DMB) is observed in
small quantities, resulting from the hydrogenation of 23DMB2N and 23DMB1N (Scheme 3).
This reaction scheme is in agreement with those reported in the literature under different
operating conditions by Badawi et al. [24] (T = 150°C, P = 1 atm, CoMo/Al₂O₃), Irvine et al. [25] (T = 150°C, P = 1 atm, catalyst SA2165) and Karmal et. al. [26] (T = 200°C, P = 1 atm, bulk CoMoS). Skeletal isomerization of 33DMB1N occurs by opening a protonated cyclopropane to form 23DMB2N, as also proposed by Irvine et al. [25].

Finally, in accordance with Mey et al [19], the transformation of 23DMB2N mainly leads to 23DMB1N as an isomerization product where the thermodynamic equilibrium between the two isomers is so rapid that it was not possible to quantify the rate of this step (Figure 3d). Therefore the reactant was considered as the sum of the two isomers. 23DMB is the main hydrogenation product.

A scale of reactivity of these C6-olefins was demonstrated from experimental data considering the overall reactivity and according to the isomerizing and hydrogenating pathways.

Thus, hex-1-ene is more reactive overall and for the isomerization step than 4MP1N and 33DMB1N. 23DMB2N was not considered because the isomerization step could not be quantified (rapid isomerization between 23DMB2N and 23DMB1N at thermodynamic equilibrium). The hydrogenation of olefins follows the order: 33DMB1N > 4MP1N > hex-1-ene > 23DMB2N. This can be explained in one hand by the position of the double bond and on the other hand by the stability of the carbocations involved. Indeed, the least reactive molecule (23DMB2N) has an internal double bond which is more stable than a terminal double bond and therefore more difficult to hydrogenate (Scheme 4). Among the branched molecules (4MP1N and 33DMB1N), the difference in reactivity of these olefins can also be explained by the stability of the intermediate carbocation. Indeed, the formation of a tertiary carbocation (33DMB1N) is more stable than the formation of a secondary carbocation (4MP1N), which means that 4MP1N is more reactive than 33DMB1N. Finally, among the olefins studied, the only linear molecule with a terminal double bond (hex-1-ene) is the least stable and therefore the most reactive. These results are in agreement with the literature, which confirms that olefins with a terminal double bond are more reactive than olefins with an internal double bond [11,19].
## 3.2 Transformation of the model feed

The impact of the nature of the sulfur compounds ($H_2S$, 3MT or BT) on the olefin conversion (ie, hex-1-ene, 4-methylpent-1-ene (4MP1N), 3,3-dimethylbut-1-ene (33DMB1N), and 2,3-dimethylbut-2-ene (23DMB2N)) is reported respectively Figure 4a, Figure 4b, Figure 4c, Figure 4d. Differences can be observed depending on the structure of the olefins and the sulfur compounds. Regarding the transformation of hex-1-ene, its conversion is higher in the presence of 3MT than BT and $H_2S$ (Figure 4a) mainly at low conversions corresponding to a higher inhibiting effect of $H_2S$ and BT than 3MT. For the transformation of 4MP1N (Figure 4b), a higher inhibiting effect of BT is demonstrated with respect to the same transformation in the presence of $H_2S$ or 3MT. The difference of impact between $H_2S$ and 3MT is very low. This confirms, therefore, that BT is more strongly adsorbed on the catalytic site than 3MT and $H_2S$ as previously reported by Dos Santos and al [18]. In the same way, no difference between $H_2S$ and 3MT was observed for a wide range of conversions and contact times (between 0 and 10 s) for the transformation of 33DMB1N. In the presence of BT, a significant inhibition of 33DMB1N conversion was also observed (Figure 4c). For the transformation of 23DMB2N only the total hydrogenation product (23DMB) was considered due to the very fast thermodynamic equilibrium between 23DMB2N and 23DMB1N. No differences were noted of the impact of BT, 3MT or $H_2S$ in the transformation of 23DMB2N. Taking into account of the low reactivity of the olefin in comparison with the reactivity of 3MT and BT, higher contact time were applied to obtain a conversion of 23DMB2N. Under these conditions, the sulfur compounds (BT and 3MT) are completely converted and therefore the effect observed is due exclusively to the presence of 1000 ppmS of $H_2S$ produced by the total transformation of 3MT or BT. Indeed, a total conversion of the sulfur compounds is obtained for a contact time of 2s (Figure 4a [18]) corresponding to a very low conversion of 23DMB2N (around 10%, Figure 4d).
Regarding isomerization and hydrogenation, the main two reactions involved in the transformation of olefins, the isomerization reaction (ISO) is less impacted by 3MT than by H₂S and BT, especially for the transformation of hex-1-ene and 4MP1N (Figure 5a, Figure 5b respectively). For the transformation of 33DMB1N (Figure 5c) a very small difference is observed between 3MT and H₂S. The isomerization step of 23DMB2N is not presented because, as already indicated above, it corresponds to the thermodynamic equilibrium between 23DMB2N and 23DMB1N.

The hydrogenation reaction is also more impacted by the presence of BT than H₂S and 3MT regardless the olefin structure (Figure 6a, Figure 6b, Figure 6c respectively for hex-1-ene, 4MP1N and 33DMB1N), except for the 23DMB2N (Figure 4d). In this case, the formation of 23DMB, the unique product of hydrogenation and of reaction, corresponds also to the conversion of 23DMB2N, for contact time implied, the sulfur compounds are completely converted thus corresponding to H₂S effect.

Taking into account of the challenge of achieving 10 ppmS in FCC gasolines with the minimum of hydrogenation of olefins, the evolution of the hydrogenation yield involved in the transformation of olefins are reported as function of hydrodesulfurization yield (HDS) of sulfur compounds (BT and 3MT). There is, indeed, an increase in the hydrogenation yield of olefins with the HDS level (Figure 7). However, important differences between them are highlighted. Indeed, the increase is more important with hex-1-ene (Figure 7a) and 4MP1N (Figure 7b) than with 33DMB1N (Figure 7c) and especially 23DMB2N (Figure 7d). Moreover, in all cases, the presence of BT impacts more the hydrogenation of the olefins than 3MT and H₂S. These phenomena could be explained by mutual competitive adsorption on the catalytic surface where BT is the more strongly adsorbed than 3MT and H₂S as reported previously by Dos Santos et al [18].

In addition, there is also a negative impact of olefins on sulfur compounds. The inhibiting effect of olefins (hex-1-ene, 4MP1N, 33DMB1N and 23DMB2N) on the transformation of 3MT and
BT is reported Figure 8a and Figure 8b, respectively. 33DMB1N is the most inhibiting molecule for the transformation of 3MT and BT followed by 4MP1N and hex-1-ene and 23DMB2N. This shows, therefore, a greater catalytic surface adsorption for 33DMB1N than for the other olefins. Therefore, a higher inhibiting effect is demonstrated for the transformation of 3MT than that of BT. This could be explained by the different values of the adsorption constant (Kads) previously determined by Dos Santos *et al.* [18], BT (Kads = 12 bar⁻¹) being more strongly adsorbed than 3MT (Kads = 7, 1 bar⁻¹). Indeed, for a same contact time of 2 s, for example, the conversion of 3MT alone is 78% and in mixture with 33DMB1N of 22%. On the other hand, the conversion of BT alone is 84% and in mixture with 33DMB1N of 40%. In addition, under the same conditions, a reduction of 50% is achieved for the conversion of 3MT mixed with 23DMB2N, while for the conversion of BT the decrease is only 10%. Calculations of catalytic activity (at the same conversion around 30%) for sulfur compounds (Table 2) show a minimum reduction of 45% for the transformation of 3MT in the presence of 23DMB2N and 77% in the presence of 33DMB1N, regarding BT transformation, the decrease in BT reactivity being 32 and 56%, respectively.

## 4 Kinetic modeling

From the hypotheses proposed by considering the Langmuir-Hinshelwood formalism with the heterolytic dissociation of H₂ and H₂S, a kinetic model for each olefin in the presence of 1000 ppmS of H₂S taking into account the reaction temperature of 250°C and a total pressure of 20 bar. The adsorption constants of H₂S and H₂ were fixed and are the same as those determined previously in the same operating conditions and used for the modeling of sulfur compounds alone and in mixture [18].

This study was carried out to determine the kinetic parameters of the reactions and the adsorption constants of olefins alone, in order to better understand the reactivity of these compounds as well as the mutual competitive adsorption on the catalytic sites.
For kinetic modeling of olefin transformation, the sum of all isomerization products was considered as a single product. Thus two different types of reactions were considered:

1/ Olefin isomerization reactions by lumping the isomerization products;

2/ Olefin hydrogenation.

Thus, from the simplified reaction scheme, the reactions considered for the kinetic modeling of 4MP1N for the isomerization (R1) and hydrogenation (R2) steps are reported below.

$$ \text{4MP1N} \xrightarrow{\text{+H}^+} \left[ \text{4MP2N} + \text{2MP2N} + \text{2MP1N} \right] \quad \text{R1} $$

$$ \left[ \text{4MP1N} + \text{4MP2N} + \text{2MP2N} + \text{2MP1N} \right] + \text{H}_2 \rightarrow \text{2MP} \quad \text{R2} $$

For the transformation of 33DMB1N, three reactions were considered:

1/ The isomerization reaction of 33DMB1N into 23DMB2N and 23DMB1N (R3);

2/ The direct hydrogenation of 33DMB1N into the main product of hydrogenation (22DMB).

The direct hydrogenation of 33DMB1N into 22DMB (R4);

3/ The hydrogenation of 23DMB2N and 23DMB1N into 23DMB (R5). This hydrogenation was considered from the sum of 23DMB2N and 23DMB1N taking into account the thermodynamic equilibrium [19].

$$ \text{33DMB1N} \xrightarrow{\text{+H}^+} \left[ \text{23DMB2N} + \text{23DMB1N} \right] \quad \text{R3} $$
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

For the transformation of hex-1-ene, two main reactions were considered

1/ The isomerisation of hex-1-ene into cis and trans-hex-2-ene and cis and trans-hex-3-ene (R6);

2/ The hydrogenation of all isomers leading to the formation of n-hexane (R7).

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

With the same and unique model, a good fit of all the experimental results was obtained considering the transformation of the olefins alone i) for the conversion (Figure 1) ii) the molar yield in isomerization (Figure 2a) and hydrogenation (Figure 2b) products. This model also explained the mutual inhibiting effect between sulfur and olefin compounds in mixture. To model the mutual competition effects between sulfur compounds and olefins, a term of inhibition for sulfur compounds (RS) was added to the denominator of the global rate equation (Equation 3, experimental part), corresponding to the product of partial pressure for the sulfur compound (RS) and the adsorption constant ($K_{RS}$) calculated by modeling these compounds alone ($K_{RS} \cdot P_{RS}$). The model was established considering 10% by weight of the olefins in
mixture with 1000 ppmS (as H₂S, 3MT or BT) of sulfur compounds and the same parameters obtained from the modeling of olefins alone and sulfurs alone [18]. Good fits were observed regarding the impact of the sulfur compounds in the conversion of the olefins (Figure 4), in the isomerization (Figure 5) and hydrogenation (Figure 6) reactions in one hand and the impact of the olefins in the transformation of sulfur compounds (Figure 8) in the other hand.

Kinetic and adsorption parameters calculated are reported Table 3. The kinetic parameters confirm the order of reactivity of olefins observed experimentally where the rate constant of the isomerization reaction involved in the transformation of hex-1-ene (k=2.9E+08 h⁻¹) is higher than that of 4MP1N (k=1.8E+08 h⁻¹) and 33DMB1N (k=7.0E+07 h⁻¹). Regarding the hydrogenation reactions, 4MP1N (k=1.2E+08 h⁻¹) is more reactive than hex-1-ene (k=4.4 E+07 h⁻¹) and 23DMB2N (k=3.8E+07 h⁻¹). The comparison with the hydrogenation of 33DMB1N is more difficult because two kinetic constants for hydrogenation reaction were calculated from the model due to that two hydrogenation steps are involved. This scale of reactivity could be partly explained regarding the adsorption constant determined from the model: 33DMB1N is the most adsorbed compound on the catalyst surface (Kads = 0.052bar⁻¹) followed by 4MP1N (Kads = 0.012bar⁻¹), hex-1-ene (Kads = 0.0096bar⁻¹) and finally 23DMB2N (Kads = 0.007 bar⁻¹).

In the same way, the good fit between modeling and experimental data for the mutual inhibiting effect of olefins and sulfur compounds validates the unique kinetic model chosen as well as the hypothesis for the transformation of the olefinic and sulfur compounds taken alone (kinetic and adsorption constants). Indeed, the differences observed can be explained satisfactorily from the adsorption constants (Kads) of the olefins and sulfur compounds and taking into account the differences in concentration (C) between olefins (10% by weight) and sulfur compounds (1000 ppmS) in the feed which is represented by the product of Kads x C (Table 3). First of all, the adsorption constants of the sulfur compounds are greater than those of the olefins, but taking into account the concentration of sulfur compounds represented by the Kads x C product, it is
noted that the differences are greatly reduced. This would reinforce why sulfur compounds are
also impacted by the presence of olefins. 33DMB1N has the largest impact followed by hex-1-
ene, 4MP1N and finally 23DMB2N. This corresponds to the decrease of the Kads x C product.
Finally, the robustness of the model chosen already validated by modeling the mixture between
2MT and BT [18] and between olefins and 3MT or BT was demonstrated. Similar conclusions
were reported by Dos Santos *et al.* [22] for the transformation of 3MT in the presence of hex-
1-ene under close operating conditions (T = 180 ° C, P = 20 bar, CoMoS / Al₂O₃). Indeed, in
this study, the HDS rate constant of 3MT decreases with the amount of hex-1-ene added. In
addition, Valencia *et al.* [27] noted inhibition by various types of olefins of 2MT conversion
over a CoMoS/Al₂O₃ catalyst at 270 ° C and 50 bar ie under operating conditions close of those
used in this work.

5 Conclusion

A reactivity scale of olefins depending on their structure, involving two main reactions (isomerization and hydrogenation), was established. Thus, hex-1-ene is the most reactive with respect to the isomerization reaction, then 4-methylpent-1-ene and finally 3,3-dimethylbut-1-ene. For the hydrogenation reaction, the most hydrogenating molecule is 3,3-dimethylbut-1-ene then 4-methylpent-1-ene, hex-1-ene and finally 2,3-dimethylbut-2-ene. Indeed, these differences are mainly due to the stability of intermediate carbocations. This order of reactivity is not modified by the presence of sulfur compounds (3MT and BT) but their apparent reactivity decreases like that of sulfur compounds. This can be explained from a unique and robustness kinetic model (i.e. for sulfur compounds alone, olefins alone and/or in mixture) by mutual competitive adsorption on the surface of the catalyst. All these phenomena have been well quantified from the adsorption constants of each compound and taking into account of their concentration. BT is the most inhibiting sulfur compound because it has the highest adsorption constant, while it is 33DMB1N for olefins. Finally, with this methodological approach
combining experiments and kinetic modeling, this work gives further insights on HDS/HYD selectivity under FCC gasoline hydrodesulfurization conditions and it is possible to select catalysts optimizing the HDS of FCC gasoline.

## Acknowledgements

A. Silva dos Santos thanks CNPq - Brasil for a PhD grant and IFPEN Lyon for the fruitful discussions.

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<table><thead><tr><th></th><th>A<sub>total</sub> (mmol/g.h)</th><th>A<sub>ISO</sub> (mmol/g.h)</th><th>A<sub>HYD</sub> (mmol/g.h)</th></tr></thead><tbody><tr><td>Hex-1-ene</td><td>595.7</td><td>549.2</td><td>46.5</td></tr><tr><td>4MP1N</td><td>309.1</td><td>188</td><td>121.1</td></tr><tr><td>33DMB1N</td><td>40.8</td><td>23.3</td><td>17.5</td></tr><tr><td>23DMB2N</td><td>-</td><td>-</td><td>23.9</td></tr></tbody></table>

Table 1: Transformation of various olefins. Total activity (A<sub>total</sub>), Isomerization activity (A<sub>ISO</sub>) and Hydrogenation activity (A<sub>HYD</sub>). (T = 250°C, P = 20 bar, CoMo/Al<sub>2</sub>O<sub>3</sub>, H<sub>2</sub>/feed = 360 NL/L, iso-conversion :30%).

4MP1N : 4-methyl-pent-1-ene, 3MP1N : 3-methyl-pent-1-ene, 23DMB2N : 2,3-dimethyl-but-2-ene
<table><thead><tr><th></th><th>Olefin</th><th>A<sub>3MT</sub> (mmol/g.h)</th><th>A<sub>BT</sub> (mmol/g.h)</th></tr></thead><tbody><tr><td>Single</td><td>-</td><td>5.2</td><td>6.6</td></tr><tr><td rowspan="4">In mixture</td><td>4MP1N</td><td>2.6</td><td>4.5</td></tr><tr><td>23DMB2N</td><td>2.9</td><td>4.7</td></tr><tr><td>33DMB1N</td><td>1.2</td><td>2.9</td></tr><tr><td>Hex-1-ène</td><td>2.1</td><td>4.9</td></tr></tbody></table>

**Table 2 : Transformation of sulfur compounds. Impact of the presence of olefins (T = 250°C, P = 20 bar, CoMo/Al<sub>2</sub>O<sub>3</sub>, H<sub>2</sub>/feed = 360 NL/L, iso-conversion (≈ 30%)).**

4MP1N : 4-methyl-pent-1-ene, 3MP1N : 3-methyl-pent-1-ene, 23DMB2N : 2,3-dimethyl-but-2-ene, BT : Benzothiophene, 3MT : 3-methylthiophene
<table><caption>Table 3: Kinetic and adsorption constants of olefins and sulfur compounds. (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L).</caption><thead><tr><th>Olefin</th><th>K<sub>ads</sub> (bar<sup>-1</sup>)</th><th>K<sub>ads</sub> x C</th><th>Reaction</th><th>k (h<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="2">4MP1N</td><td rowspan="2">0.012</td><td rowspan="2">12</td><td>ISO</td><td>1.8E+08</td></tr><tr><td>HYD</td><td>1.2E+08</td></tr><tr><td rowspan="3">33DMB1N</td><td rowspan="3">0.052</td><td rowspan="3">52</td><td>ISO</td><td>7.0E+07</td></tr><tr><td>HYD<sup>1</sup></td><td>9.8E+08</td></tr><tr><td>HYD<sup>2</sup></td><td>3.8E+07</td></tr><tr><td>23DMB2N</td><td>0.007</td><td>7.2</td><td>HYD</td><td>3.8E+07</td></tr><tr><td rowspan="2">Hex-1-ene</td><td rowspan="2">0.0096</td><td rowspan="2">9.6</td><td>ISO</td><td>2.9E+08</td></tr><tr><td>HYD</td><td>4.4E+07</td></tr><tr><td rowspan="2">3MT</td><td rowspan="2">10.2</td><td rowspan="2">10.2</td><td>HDS</td><td>5.5E+12</td></tr><tr><td>HYD</td><td>5.8E+12</td></tr><tr><td rowspan="3">BT</td><td rowspan="3">12.1</td><td rowspan="3">12.1</td><td>HDS<sup>3</sup></td><td>1.9E+13</td></tr><tr><td>HDS<sup>4</sup></td><td>3.4E+09</td></tr><tr><td>HYD</td><td>1.2E+13</td></tr></tbody></table>

4MP1N: 4-methyl-pent-1-ene, 3MP1N: 3-methyl-pent-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene, BT: Benzothiophene, 3MT: 3-methylthiophene

*K<sub>ads</sub> x C: multiplication of the adsorption constant and the concentration (10 wt% for olefins)

¹HYD of 3,3-dimethylbut-1-ene (33DMB1N) into 2,2-dimethylbutane (22DMB),

²HYD of 2,3-dimethylbut-2-ene (23DMB2N+23DMB1N) into 2,2-dimethylbutane (23DMB),

³HDS of Dihydrobenzothiophene (DHBT) into Ethylbenzene (EB),

⁴ direct HDS of Benzothiophene (BT) into Ethylbenzene (EB)
<figure><img src="image_6.png" /><figcaption>Scheme 1 : Transformation of hex-1-ene</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Scheme 2 : Transformation of 4-methyl-pent-1-ene (4MP1N)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Scheme 3 : Transformation of 3,3-dimethylbut-1-ene (33DMB1N)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Scheme 4: Transformation of 2,3-dimethylbut-2-ene (23DMB2N) (23DMB1N: 2,3-dimethylbut-1-ene, 23DMB: 2,3-dimethylbutane)</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 1: Transformation of olefins versus contact time. (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L). Comparison of simulation results (dotted line) and experimental data (points).</figcaption></figure>

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene.
<figure><img src="image_11.png" /><figcaption>Figure 2: Transformation of olefins. a) Isomerization products b) hydrogenation products versus conversion. (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L).</figcaption></figure>

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene, hex-1: hex-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene.

c/t hex-3: cis/trans hexe-3-ene, c/t hex-2: cis/trans hexe-2-ene, nC₆: n-hexane, c/t 4MP2N: cis/trans 4-methylpent-2-ene, 2MP: 2-methylpentane, 2MP2N: 2-methylpent-2-ene, 2MP1N: 2-methylpent-1-ene, 22DMB: 2,2-dimethylbutane, 23DMB1N: 2,3-dimethyl-but-1-ene, 23DMB: 2,3-dimethylbutane,
<figure><img src="image_12.png" /><figcaption>Figure 3 : Transformation of olefins : molar yield of products. a) hex-1-ene, b) 4MP1N, c) 33DMB1N et d) 23DMB2N versus conversion. (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L).</figcaption></figure>

4MP1N : 4-methyl-pent-1-ene, 33DMB1N : 3,3-dimethylbut-1-ene, 23DMB2N : 2,3-dimethyl-but-2-ene.

c/t hex-3 : cis/trans hexe-3-ene, c/t hex-2 : cis/trans hexe-2-ene, nC₆ : n-hexane, c/t 4MP2N : cis/trans 4-methylpent-2-ene, 2MP : 2-methylpentane, 2MP2N : 2-methylpent-2-ene, 2MP1N : 2-methylpent-1-ene, 22DMB : 2,2-dimethylbutane, 23DMB1N : 2,3-dimethyl-but-1-ene, 23DMB : 2,3-dimethylbutane,
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>Figure 4: Transformation of olefins: Effect of the presence of H₂S, 3MT (3-Methylthiophene) and BT (Benzothiophene) versus contact time. a) Hex-1-ene, b) 4MP1N, c) 33DMB1N, d) 23DMB2N. Comparison of simulation results (dotted line) and experimental data (points). (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L)</figcaption></figure>

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene. Comparison of simulation results (dotted line) and experimental data (points).
<figure><img src="image_17.png" /><figcaption>Figure 5: Transformation of olefins. Isomerization (ISOM) yield versus contact time. Effect of the presence of H₂S, 3MT (3-Methylthiophene) and BT (Benzothiophene). Comparison of simulation results (dotted line) and experimental data (points).</figcaption></figure>

a) Hex-1-ene, b) 4MP1N, c) 33DMB1N ($T = 250^{\circ}C$, $P = 20$ bar, $CoMo/Al_2O_3$, $H_2/feed = 360$ NL/L)

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene. Comparison of simulation results (dotted line) and experimental data (points).
<figure><img src="image_18.png" /><figcaption>Figure 6: Transformation of olefins. Hydrogenation (HYD) yield versus contact time. Effect of the presence of H₂S, 3MT (3-Methylthiophene) and BT (Benzothiophene). Comparison of simulation results (dotted line) and experimental data (points). a) Hex-1-ene, b) 4MP1N, c) 33DMB1N (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L)</figcaption></figure>

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene. Comparison of simulation results (dotted line) and experimental data (points).
<figure><img src="image_19.png" /><figcaption>Figure 7: Transformation of olefins. Hydrogenation (HYD) yield versus HDS yield. Effect of the presence of 3MT (3-Methylthiophene) and BT (Benzothiophene) a) Hex-1-ene, b) 4MP1N, c) 33DMB1N, d) 23DMB2N (T = 250°C, P = 20 bar, CoMo/Al₂O₃, H₂/feed = 360 NL/L)</figcaption></figure>

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene.
<figure><img src="image_20.png" /><figcaption>Figure 8. Transformation of sulfur compounds in the presence of olefins. Comparison of simulation results (dotted line) and experimental data (points). a) 3MT et b) BT. (T = 250°C, P = 20bar, CoMo/Al₂O₃, H₂/charge = 360 NL/L).</figcaption></figure>

4MP1N: 4-methyl-pent-1-ene, 33DMB1N: 3,3-dimethylbut-1-ene, 23DMB2N: 2,3-dimethyl-but-2-ene.