Bruno M Santos
email: bruno_m@petrobras.com.br

José L Zotin

Françoise Maugé

Laetitia Oliviero

Weitao Zhao

Mônica A P Da Silva

Strong and weakly acidic OH groups of HY zeolite into the different routes

Keywords: "operando IR", "cyclohexene", "isomerization", "hydrogen transfer", "zeolite", "HY", "acidity" Graphical

IR operando study of cyclohexene reaction on HY zeolite (Si/Al = 15) was performed to determine the role of the OH groups located in the different zeolitic cages and the silanol groups (SiOH). A parallel between the product yield and type and amount of OH groups poisoned by coking and pyridine pulses was established. The role of silanol and zeolitic OH groups during cyclohexene conversion and the coverage rate of surface groups of the dealuminated HY zeolite, were evaluated by FT-IR operando study.

At the beginning of the reaction, zeolite exhibited a very intense deactivation due to coke formation. Coke is initially deposited on strong acid sites, inhibiting the H-transfer activity.

Further, coke formation decreased. At this stage of the reaction, pyridine addition markedly decreased the isomerization activity. In parallel, it is noted that mainly SiOH, OH groups at hexagonal prisms, and perturbed OH groups in sodalite cages were mainly affected, indicating that these groups are responsible for the isomerization steps. These results showed that even weakly acidic sites are active sites for isomerization reaction.

Introduction

Zeolites are widely used in petroleum industry as acid catalysts, for which the Brønsted acid sites (BAS) are usually associated with cracking and isomerization reactions. In hydrocracking processes, catalysts contain metals in addition to zeolites, which include a hydrogenation/dehydrogenation activity, resulting in bifunctional catalysts [START_REF] Du | The chemistry of selective ring-opening catalysts[END_REF]. Due to stringent Strong and weakly acidic OH groups of HY zeolite into the different routes... 3 specifications of diesel, the incorporation of aromatic-rich streams to this fuel is limited because of their detrimental effect on density and cetane number. In this case, aromatic hydrogenation and ring-opening reactions of naphthenic compounds have been studied on bifunctional catalysts to improve such properties, which allows the processing of aromatic-rich streams to diesel fuel in refineries [START_REF] Mcvicker | Selective Ring Opening of Naphthenic Molecules[END_REF][START_REF] Santana | Evaluation of different reaction strategies for the improvement of cetane number in diesel fuels[END_REF][START_REF] Stanislaus | Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production[END_REF]. Effects of support acidity on hydroprocessing catalysts were previously investigated in our research group using NiMo catalysts supported on alumina, silica-alumina or alumina-HUSY zeolite [START_REF] Ferraz | Influence of support acidity of NiMoS catalysts in the activity for hydrogenation and hydrocracking of tetralin[END_REF]. The results indicated that zeolite addition increased not only isomerization and ring-opening reactions, but also hydrogenation reactions. In other work [START_REF] Ferraz | Influence of Support Acidity of NiMo Sulfide Catalysts for Hydrogenation and Hydrocracking of Tetralin and Its Reaction Intermediates[END_REF], it was proposed that the improvement of hydrogenation activity of zeolite-based catalysts could be associated with the formation of protonated species on zeolite acid sites, which would be easier to be hydrogenated on NiMoS sites. However, further studies of such reactions over zeolites are necessary in order to have a better understanding of this behavior.

Regarding reactions involving cyclic compounds, cyclohexene is a suitable model compound.

Because it is well known as an intermediate for hydrogenation (or hydrogen transfer) reactions [START_REF] Cheng | Conversion of cyclohexene over Y-zeolites: A model reaction for hydrogen transfer[END_REF] in addition to cracking, isomerization and coke formation. Conversion of cyclohexene is extensively studied in the literature [START_REF] Aboul-Gheit | Effect of combining palladium, iridium or rhenium with platinum supported on H-ZSM-5 zeolite on their cyclohexene hydroconversion activities[END_REF][START_REF] Romero-Rivera | Cyclohexene hydrogenation with molybdenum disulfide catalysts prepared by ex situ decomposition of ammonium thiomolybdate-cetyltrimethylammonium thiomolybdate mixtures[END_REF][START_REF] Vít | Synergetic effects of Pt and Ru added to Mo/AlO sulfide catalyst in simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene[END_REF][START_REF] Des Rochettes | Kinetic study of hydrogen transfer of olefins under catalytic cracking conditions[END_REF][START_REF] Dwyer | Bimolecular hydrogen transfer over zeolites and SAPOs having the faujasite structure[END_REF][START_REF] Li | Selective hydrodesulfurization of gasoline on Co/MoS 2±x catalyst: Effect of sulfur defects in MoS 2±x[END_REF][START_REF] Bukin | Performance of a zeolite-containing catalyst and catalysts based on noble metals in intermolecular hydrogen transfer between С6 hydrocarbons[END_REF][START_REF] Boudart | Catalytic hydrogenation of cyclohexene[END_REF][START_REF] Soto-Puente | Synthesis, characterization and cyclohexene hydrogenation activity of high surface area molybdenum disulfide catalysts[END_REF], and its reaction scheme is shown in Figure 1.

Disproportioning of cyclohexene may result in cyclohexane and benzene formation by the hydrogen transfer (H-transfer) process. Additionally, coking process may also allow H-transfer reactions, forming cyclohexane; benzene formation is too low or not observed in this case. On the other hand, isomerization reactions may occur, resulting in methylcyclopentenes formation, which can be further converted to methylcyclopentane (MCP) by H-transfer reactions. It is well known in the literature that methylcyclopentenes are intermediates for ring-opening of six-ring cycles, which can be further cracked into smaller molecules depending on zeolite acidity [START_REF] Du | The chemistry of selective ring-opening catalysts[END_REF]. Infrared spectroscopy is one of the most widely used techniques for solid catalyst characterization. In general, catalyst properties are investigated after in situ activation and using probe molecules. Under working conditions, coverage of different species changes the catalyst surface, and the characterization of such species is essential to understand their role in the studied process. Operando spectroscopy is a technique that allows the observation of catalysts in such conditions [START_REF] Alayoglu | Nanocatalysis II: In Situ Surface Probes of Nano-Catalysts and Correlative Structure-Reactivity Studies[END_REF], whose setup is available in the literature [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF][START_REF] Lesage | Studying the NOx-trap mechanism over a Pt-Rh/Ba/Al2O3catalyst by operando FT-IR spectroscopy[END_REF]. Regarding cyclohexene conversion and n-alkane cracking in zeolites, operando spectroscopy studies have been used by several authors [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF][START_REF] Jolly | Situ Ft-Ir Study of Acid Sites Responsible for Cyclohexene Conversion on Hy Zeolites[END_REF][START_REF] Jolly | The effect of 2,6dimethylpyridine poisoning on the activity in n-hexane cracking of dealuminated HY zeolites[END_REF][START_REF] Jolly | FT-IR characterization of carbenium ions, intermediates in hydrocarbon reactions on H-ZSM-5 zeolites[END_REF][START_REF] Jolly | FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions[END_REF][START_REF] Li | Monomolecular Cracking Rates of Light Alkanes over Zeolites Determined by IR Operando Spectroscopy[END_REF][START_REF] Kadam | Impact of Zeolite Structure on Entropic-Enthalpic Contributions to Alkane Monomolecular Cracking: An IR Operando Study[END_REF]. In such works, high frequency (HF) OH groups are associated with H-transfer reactions, and they show high sensitivity to coke. Besides, HF OH groups perturbed by extraframework species, with strong Brønsted acidity, are considered active sites for cracking reactions and coke formation.

On the other hand, isomerization reactions are described as not sensitive to coke and they occur even on weak acid sites. However, the role of low frequency (LF) OH groups and silanol (SiOH) on the reaction is not well known, as well as the coverage rate of each group during the reaction. The effects of nitrogen species in the feed were also investigated in the literature. Their inhibitor effects were associated with the neutralization of BAS [START_REF] Mendes | Quantification of the available acid sites in the hydrocracking of nitrogen-containing feedstocks over USY shaped NiMo-catalysts[END_REF]. Previous IR operando study has shown that the addition of 2,6-dimethylpyridine in the feed led to the adsorption of nitrogen species at the expense of the intermediate for alkene isomerization reaction [START_REF] Travert | IR study of hydrotreating catalysts in working conditions: comparison of the acidity present on the sulfided phase and on the alumina support[END_REF], resulting in catalyst deactivation. This work aims to evaluate, by FT-IR operando study, the coverage rate of each OH group present on HY zeolite and their role in coke formation, hydrogen transfer and isomerization steps during cyclohexene conversion.

Experimental

Catalytic tests were carried out using a stabilized HY zeolite (Zeolyst Inc., bulk Si/Al = 15, framework Si/Al = 25) with 547 m² g -1 of micropore area (measured by t-plot method) and specific surface area of 771 m 2 g -1 (measured by BET method).

Characterizations of OH groups and acidic sites were performed by FTIR spectroscopy. Each sample was pressed into a self-supported wafer (approximately 8 mg cm -2 ) that was pre-treated in situ in the IR cell by heating from room temperature (RT) to 623 K (1 K min -1 ), followed by an isotherm at 623 K for 1 hour under secondary vacuum (10 -4 Pa) and a decrease to 423 K. Spectra were recorded on a Nicolet iZ10 equipped with an MCT detector, with 64 scans and 4 cm -1 resolution. All spectra were normalized to a constant disc mass of 500 mg cm -2 .

Characterization of acid sites of pre-treated samples was performed by pyridine adsorption at 133 Pa and 423 K, followed by desorption at 12 Pa from 423 to 623 K at a rate of 1 K min -1 .

Regarding band overlapping, quantification of area of each band was obtained after spectral decomposition on OMNIC 9 peak fit. The bands were adjusted considering Voigt fit analysis.

Operando study was performed in a setup previously described, consisting of a reactor cell made by stainless steel tubing, equipped with two CaF 2 windows and sealed with a Kalrez Oring for each window [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF][START_REF] Lesage | Studying the NOx-trap mechanism over a Pt-Rh/Ba/Al2O3catalyst by operando FT-IR spectroscopy[END_REF][START_REF] Travert | IR study of hydrotreating catalysts in working conditions: comparison of the acidity present on the sulfided phase and on the alumina support[END_REF]. Samples were pressed into self-supported wafers, as described above. Prior to the tests, the zeolite was activated at 623 K for 2.5 h (heating rate of 3 K min -1 ) under a flow of 30 mL min -1 of N 2 . After that, the temperature was decreased to 523 K at a ramp of 3 K min -1 , and two different reaction tests were carried out. In the first one, defined as test 1, the reaction gas (cyclohexene and N 2 flow) was added to the cell just after the catalyst activation. In the second one, defined as test 2, the catalyst was poisoned with approximately Strong and weakly acidic OH groups of HY zeolite into the different routes... 6 4.2 μmol of pyridine (at 523 K) before adding reaction gas to the cell. Then, after approximately 1.2 h of time on stream (TOS), an additional dose of 2.1 μmol of pyridine was introduced into the cell. In both tests, the temperature was 523 K, the reaction gas flow was 100 mL min -1 and its molar composition was 1.3% of cyclohexene and 98.7% of N 2 . FTIR spectra of the catalyst surface were recorded every approximately 10 minutes on a Nicolet 6700 equipped by an MCT detector with 64 scans and 4 cm -1 resolution, while the composition of reactor effluent was determined by GC, utilizing a Bruker 450-GC equipment, equipped with a column BP1 50 m x 0.32 mm and a FID. Cyclohexene conversion was determined as defined in Equation [START_REF] Du | The chemistry of selective ring-opening catalysts[END_REF], where X cyclohexene is the conversion of cyclohexene, C cyclohexene is the cyclohexene molar composition in effluent gas, and C i is the molar composition of the other compounds in effluent gas.

(1) Product yields were calculated by Equation [START_REF] Mcvicker | Selective Ring Opening of Naphthenic Molecules[END_REF], where Y i is the yield of product i.

(

The ratio between isomerization (1-methyl-1-cyclopentene and methylcyclopentane) and hydrogen transfer activities (cyclohexane and methylcyclopentane) was calculated according to Equation (3).

(

) 3
where ISO and HYD are the isomerization and hydrogen transfer products, respectively, and C is the molar composition of the compound indicated.

Quantification of each types of zeolitic OH groups, silanol groups, Brønsted acid sites (BAS) and Lewis acid sites (LAS), defined as η (µmol), were performed by Equation (4), where A is the IR band area, s is the area of the self-supported wafer (cm 2 ), and ε is the molar absorption coefficient (µmol -1 cm). [START_REF] Stanislaus | Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production[END_REF] Strong and weakly acidic OH groups of HY zeolite into the different routes...

7

The coverage (ϴ) of silanol and zeolitic OH groups at given TOS was calculated for each type of OH groups according to Equation [START_REF] Ferraz | Influence of support acidity of NiMoS catalysts in the activity for hydrogenation and hydrocracking of tetralin[END_REF], where the quantity corresponds to the amount of covered silanol or zeolitic OH groups (in µmol -calculated by Equation ( 4)). These values are obtained from the difference between the spectra taken at given TOS and the activated sample.

(5)

Results and discussion

Surface sites of HY zeolite

The IR spectrum of OH groups of the activated HY zeolite and the band decomposition is shown in Figure 2. This zeolite exhibited a band corresponding to SiOH groups at 3739 cm -1 [START_REF] Gabrienko | Direct Measurement of Zeolite Brønsted Acidity by FTIR Spectroscopy: Solid-State 1H MAS NMR Approach for Reliable Determination of the Integrated Molar Absorption Coefficients[END_REF]. Additionally, two bands corresponding to zeolitic high frequency (HF) OH groups were found. The first one, at 3625 cm -1 , was assigned to non-perturbed OH groups inside the supercages [START_REF] Khabtou | Quantitative infrared study of the distinct acidic hydroxyl groups contained in modified Y zeolites[END_REF], and the second one, at 3606 cm -1 , was ascribed to perturbed OH groups inside the supercages. These latter OH groups are known as superacidic due to their interaction with the extraframework phase [START_REF] Malicki | A new model for acid sites in dealuminated Y zeolites[END_REF][START_REF] Vimont | Analysing and understanding the active site by IR spectroscopy[END_REF]. Regarding low frequency (LF) OH groups, three bands were distinguished. The first one, at 3558 cm -1 , was assigned to non-perturbed OH groups located inside the sodalite cages [START_REF] Vimont | Analysing and understanding the active site by IR spectroscopy[END_REF]; the second one, at 3541 cm -1 , corresponded to OH groups inside the sodalite cages perturbed by the extraframework phase [START_REF] Thibault-Starzyk | Infrared Spectroscopy[END_REF], and the last one, at 3516 cm -1 , was attributed to OH groups in hexagonal prisms of zeolite [START_REF] Long | Cyclopentadiene transformation over H-form zeolites : TPD and IR studies of the formation of a monomeric cyclopentenyl carbenium ion intermediate and its role in acid-catalyzed conversions[END_REF]. The concentration of the various groups was obtained considering a molar absorption coefficient of 7.5 µmol -1 cm for HF OH, 5.6 µmol -1 cm for LF OH (29) and 3.0 µmol -1 cm for SiOH [START_REF] Gallas | Comparative study of the surface hydroxyl groups of fumed and precipitated silicas. 4. Infrared study of dehydroxylation by thermal treatments[END_REF]. The results are summarized in Table 1. The Brønsted (BAS) and Lewis (LAS) acidity of the HY zeolite has been probed by pyridine adsorption at 423 K and subsequent desorption by heating from the same temperature to 623 K.

The obtained spectrum after pyridine desorption at 423, 523 and 623 K are shown in Figure 3 and exhibited bands at 1454 and 1621 cm -1 , corresponding to LAS, two bands at 1543 cm -1 and 1634 cm -1 associated with BAS, and another one related to both sites at 1490 cm -1 . Overall, BAS were quantified from the band area at 1543 cm -1 of the spectrum recorded at 423 K, considering a molar absorption coefficient of 1.8 µmol -1 cm (29). In contrast, overall, LAS were determined from the area of the band at 1454 cm -1 in the same spectrum, with a molar absorption coefficient of 1.5 µmol -1 cm [START_REF] Hadjiivanov | Identification and Characterization of Surface Hydroxyl Groups by Infrared Spectroscopy[END_REF]. The resulting values obtained were 200 μmol g -1 of BAS and 140 μmol g - 1 of LAS. Spectra recorded at 523 and 623 K exhibit the BAS and LAS with higher strength. At 523 K, zeolite presented 170 μmol g -1 of BAS and 105 μmol g -1 of LAS and, at 623 K, were found 99 μmol g -1 and 78 μmol g -1 of BAS and LAS, respectively, indicating that approximately half of BAS and LAS on the zeolite are very strong sites.

Operando IR study during cyclohexene reaction

The reaction of cyclohexene on HY zeolite was followed in operando by FTIR spectroscopy and GC, simultaneously. As for test 1, cyclohexene conversion (X), product yield (Y), and ISO/HYD ratio are shown in Figures 4a andb. The main products formed were 1-methyl-1cyclopentene, methylcyclopentane (MCP) and cyclohexane.

Cyclohexene conversion and coke formation

The results indicated a rapid decrease in catalyst activity (Figure 4a) in the first 0.2 h, followed by a smaller one at greater TOS. Meanwhile, coke deposition was evidenced. On the IR spectrum taken after 10 min in test 1 (Figure 5A), bands related to coke (~1591, 1379 and 1348 cm -1 ) were observed. According to Jolly et al. [START_REF] Jolly | Situ Ft-Ir Study of Acid Sites Responsible for Cyclohexene Conversion on Hy Zeolites[END_REF], the band at 1591 cm -1 is associated with graphitic coke, while the band at 1348 cm -1 refers to pre-graphitic carbons. Additionally, a band was found at 1496 cm -1 that can be associated with unsaturated carbenium ions [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF][START_REF] Paweewan | Coking and deactivation during n-hexane cracking in ultrastable zeolite Y[END_REF], an intermediate of cyclohexene reaction through H-transfer [START_REF] Jolly | The effect of 2,6dimethylpyridine poisoning on the activity in n-hexane cracking of dealuminated HY zeolites[END_REF]. The band at 1450 cm -1 was referred to as a combination of contributions from adsorbed intermediate species and coke [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF][START_REF] Jolly | The effect of 2,6dimethylpyridine poisoning on the activity in n-hexane cracking of dealuminated HY zeolites[END_REF]. The amount of deposited coke during the reaction was estimated according to the intensity of the 1591 cm -1 band, as reported in Figure 4a. Thus, fast coke deposition was evidenced in the first 0.2 h of operation and then coke formation became smaller, with a small change in slope at approximately 0.7 h.

Evolution of coke with TOS is inversely parallel to the cyclohexene conversion.

To understand the origin of the rapid initial deactivation, a second test (test 2) was carried out in which the zeolite was previously contacted with 4.2 μmol of pyridine in order to selectively poison the strong acid sites and observe the catalyst behavior in terms of sites coverage, activity and selectivity. Figure 5 (B andC) compares the surface species after initial pyridine addition in test 2, as well as after the subsequent 10 minutes of TOS in both tests. The spectrum taken before the reaction in test 2 (Figure 5B) presents the same pyridine bands as already described in Figure 3. The quantity of BAS and LAS initially poisoned in test 2 was then determined to be After 10 min of reaction in test 2, the bands associated with coke were observed in addition to those related to pyridine species, already present on the sample before reaction (Figure 5C).

However, bands of pyridine species at 1454 cm -1 and 1490 cm -1 were very close to the ones related to the adsorption of intermediate species of cyclohexene reaction. It can be noted that globally the intensity of the IR bands at 1348, 1379 and 1591 cm -1 was lower in test 2 than in test 1, showing that the initial poisoning by pyridine has decreased the coke formation during the first 10 min of reaction. Meanwhile, the initial cyclohexene conversion is lower in test 2 than in test 1 (Figures 4a andc)

In test 2, after 1.2 h of TOS, a second pyridine pulse was injected. IR spectra taken before (B) and after (A) the new pyridine introduction are shown in Figure 6. The bands typical of coke

Strong and weakly acidic OH groups of HY zeolite into the different routes...

13

were observed at 1591, 1379 and 1348 cm -1 , i.e., at very close wavenumbers as in Figure 5. One can note the absence of the band at 1496 cm -1 related to intermediate species of cyclohexene reaction through H-transfer. This inobservance agrees with the low amount of hydrogen transfer products formed after 1.2 h. The subtracted spectrum (Figure 6 (A-B)) showed a small decrease of the coke band at 1591 cm -1 that is very limited compared to the overall intensity of the band, as was also observed by Jolly et al. [START_REF] Jolly | FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions[END_REF]. This could be ascribed to a limited desorption of coke by pyridine adsorption.

Besides, the second pulse led to pyridine band appearance similar to those observed previously, although with a weaker intensity. Amounts of BAS and LAS poisoned by this second pyridine injection were calculated from (A-B) spectrum and corresponded to 16 and 17 µmol g -1 , respectively.

Selectivity changes with TOS

The selectivity of the reaction was followed for tests 1 and 2 (Figures 4a andc). At the beginning of test 1, the high formation of methylcyclopentane decreased very fast, while the formation of 1-methyl-1-cyclopentene increased. After that, 1-methyl-1-cyclopentene yield decreased very slowly, while methylcyclopentane yield became very small and coke formation moderately increased. The behaviors followed by cyclohexane and cracking products were similar to that of methylcyclopentane, but their yields are very small. Figure 4b points out that methylcyclopentane formation was greater than that of cyclohexane, as well as its deactivation.

As a result of these evolutions, ISO/HYD ratio increased continuously with TOS, indicating a higher deactivation of hydrogen transfer than isomerization activity with TOS.

Evolution of product yields and ISO/HYD ratio for test 2 are shown in Figures 4c andd.

Comparison with test 1 shows that the first dose of pyridine, introduced before test 2, initially resulted in a lower cyclohexene conversion but that the nature of the main products, as well as their evolution with TOS and the ISO/HYD ratio during the first hour of reaction were similar to test 1.

The second addition of pyridine, performed at approximately 1.2 h (indicated by a red line in Figures 6c andd), led to a supplementary decrease of cyclohexene conversion. This can be explained by competition between pyridine species and cyclohexene reaction intermediates for the acid sites. It should be noted that ISO/HYD ratio stabilized after the second dose of pyridine. Regarding product yields, 1-methyl-1-cyclopentene presented the highest deactivation, methylcyclopentane suffered a decrease in its yield, likely related to the 1-methyl-1cyclopentene formation decrease, and the effect on cyclohexane formation was very low.

Therefore, after the second pyridine poisoning, isomerization activity was more affected than hydrogen transfer.

Evolution of the coverage rate of hydroxyl groups with TOS

The IR spectra obtained in the operando study allows to follow not only the coke deposition and intermediate species formation on the catalyst surface but also interestingly the coverage of hydroxyl groups during the reaction. Hence, the coverage rate (fraction of occupied hydroxyl groups calculated from Equation 5) of SiOH and zeolitic OH groups during test 1 is shown in Figures 7a andb, respectively. Regarding HF OH groups (Figure 7a), the non-perturbed OH groups inside the supercages (3625 cm -1 ) were covered almost instantaneously and remained so during the test. In contrast, the perturbed OH groups inside the supercages (3606 cm -1 ) also presented a strong coverage at the beginning of the reaction and showed some small coverage modification with TOS. Indeed, a slight recovery is first noted, and further, a small coverage increase occurred throughout the reaction.

Regarding LF OH groups (Figure 7b), the coverage of the non-perturbed OH groups inside the sodalite cages (3558 cm -1 ) was almost instantaneous and complete and remained stable during the test. Otherwise, the perturbed OH groups inside the sodalite cages (3541 cm -1 ) also presented a strong initial coverage, although lower, that is followed by a progressive increase with TOS. Then, both OH groups at hexagonal prisms (3516 cm -1 ) and SiOH groups (3739 cm - 1 ) exhibited similar behavior with gradual coverage during TOS. Besides, Figure 7b The coverage of zeolitic OH groups and SiOH groups in test 2 is shown in two ways to distinguish the TOS effect from that of the pyridine addition effect. The concentrations of occupied groups after the first pyridine poisoning (before the reaction), as well as before and after the second pyridine dose, are listed in Table 2. Besides, the coverage during the reaction is displayed in Figure 7c andd.

After the first pyridine poisoning, the concentration of poisoned perturbed supercage OH groups was higher than that of non-perturbed ones (Table 2, Figure 7c). As for LF OH groups, the quantity of poisoned perturbed sodalite cage OH groups was almost similar to that of nonperturbed ones. In contrast, the quantity of OH groups located at hexagonal prisms poisoned by pyridine was smaller (Figure 7d). Silanol groups presented the lowest poisoning by pyridine since only approximately 10% of such groups were in interaction with pyridine (Figure 7d).

At this initial step, pyridine adsorbed on the strongest acid sites of the zeolite. So, the results regarding the poisoning of HF groups are consistent with those reported in previous works [START_REF] Malicki | A new model for acid sites in dealuminated Y zeolites[END_REF][START_REF] Vimont | Analysing and understanding the active site by IR spectroscopy[END_REF], where perturbed HF groups of HY zeolite are considered superacidic. The smaller difference observed between the poisoning of perturbed and non-perturbed groups in sodalite cages could be associated with the lower accessibility to pyridine of such groups (32). For test 2, the evolution of coverage rates between the first and second doses of pyridine (left of the red line in Figure 7c andd) revealed the impact of the interaction with reactants and products as well as coking. Thus, comparing the OH group coverages between tests 1 and 2 allowed determining the impact of pyridine poisoning. Hence, except HF OH groups, all OH groups presented a lower coverage after 1.2 h of reaction in test 2 than in test 1, which could be directly linked to the lower coke formation. Accordingly, the initial blockage of the stronger acidic sites by pyridine adsorption inhibits coke formation and leads to a lower OH group poisoning by coke in reaction conditions.

The second dose of pyridine (right of the red line in Figures 7c andd) increased the coverage with TOS for SiOH, OH groups at hexagonal prisms and perturbed OH groups in sodalite cages (this last one was fully covered after this second pulse). Interestingly, it did not impact the coverage of both supercage OH groups and non-perturbed sodalite cage OH groups that kept coverages of 80% and 95%, respectively.

Deactivation process

The fast deactivation process identified at the beginning (up to 0.2 h) of the tests 1 and 2 can be associated with the reduction of hydrogen transfer reactions since methylcyclopentane and cyclohexane were the products that suffered the highest decrease in their formation. Hence, hydrogen transfer reactions are more affected by fast coke formation than isomerization reactions, as was reported by Joly et al. [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF]. Coke formation has a dual impact. Indeed, it blocks the active sites for H-transfer reactions, but coke formation is also a source of hydrogen, allowing a hydrogenation activity in the absence of H 2 in the flow.

After this initial stage, a slow deactivation stage occurred. This was associated with the decrease of 1-methyl-1-cyclopentene formation and that could be ascribed to the increase of "slow" coke.

However, ISO/HYD ratio continuously increased. This can be explained by the higher deactivation of hydrogen transfer reactions, more dependent on the coking process than isomerization. Indeed, a decrease in coke formation made hydrogen availability more limited.

Thus, hydrogen transfer reactions are decreased, favoring 1-methyl-1-cyclopentene formation.

These observations are consistent with the fact that hydrogen transfer reactions are more sensitive to coke than isomerization reactions, as reported by Joly et al. [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF]. Finally, considering the H-transfer products and reaction scheme shown in Figure 1, the conversion of 1methyl-1-cyclopentene to methylcyclopentane was the preferential route at the expense of the direct formation of cyclohexane, indicating that cyclohexene is preferentially isomerized before H-transfer step.

One can also note a small coke amount increase at 0.7 h of TOS. One could note that a decrease of intermediate species band intensity, shown in Online Resource 1 (S2), was observed at the same TOS. This is in accordance with previously reported work [START_REF] Joly | Infrared in situ characterization of HY zeolite acid sites during cyclohexene transformation[END_REF], where a decrease of such species was associated with the stabilization of coke. Also, in parallel, a slope change in the coverage rate of SiOH and LF OH groups occurred, as shown in Figure 7b, indicating that the additional coke was formed on such sites.

Active sites for the various pathways of cyclohexene reaction Figures 7c andd showed that, in test 2, the first introduction of pyridine (before the reaction) led to a perturbation of all OH group types. Nevertheless, the interaction with supercage OH groups and non-perturbed sodalite OH groups is more marked. The interaction is maximum with the supercage perturbed OH groups (50%) that with the non-perturbed ones (37%) (Figure 7c). As a consequence of the smaller amount of available acid sites, the initial conversion was clearly lower in test 2 (36%) than in test 1 (58%). Comparison between tests 1 and 2 (Figures 4a andc) reveals that pyridine poisoning did not affect 1-methyl-1-cyclopentene yield (~5%), while methylcyclopentane yield sharply decreased (from 43% in test 1 to 24% in test 2). This allows concluding the hydrogen transfer required strongly acidic zeolitic OH groups. Meanwhile, the ISO/HYD ratio was similar for tests 1 and 2 at the beginning of the reaction. This suggests that initial addition of pyridine blocked the same sites as "fast" coke species.

After the second dose of pyridine (test 2), a supplementary decrease of cyclohexene conversion was observed (Figure 4c). In parallel, Figure 7d showed an increase in the poisoning of the weakly acidic sites as silanol, OH groups in hexagonal prisms and perturbed sodalite cage OH groups. At this stage, no supplementary poisoning of the more acidic species (HF OH species and non-perturbed sodalite cage) were detected. It can be mentioned that these species were already strongly poisoned either by the first pyridine pulse or by coke formed with TOS, although not completely poisoned (80-95% -Figures 7c andd). It was also noticeable that the second pyridine pulse markedly affected the ISO/HYD ratio (Figure 4d). Indeed, the ratio stopped its increase with TOS and became constant. Besides, the second pyridine pulse led to a decrease of 1-methyl-1-cyclopentene yield (isomerization product). The decrease of methylcyclopentane yield observed was likely a consequence of the low amount of 1-methyl-1cyclopentene formed. The other products whose yield was very low were almost unaffected.

Thus, one can conclude that SiOH, OH groups in hexagonal prisms and sodalite cages perturbed OH groups are involved in the isomerization route.

Strong and weakly acidic OH groups of HY zeolite into the different routes...

21

Regarding SiOH groups, test with pure silica (shown in Online Resource 1, S3) revealed that such groups are inactive for cyclohexene reactions. However, it is known that the presence of a small amount of Al into silica generates Brønsted acid sites and can give a substantial activity to silica [START_REF] West | The catalytic activity of silica gel[END_REF][START_REF] Crépeau | Nature, structure and strength of the acidic sites of amorphous silica alumina: An IR and NMR study[END_REF]. This could explain the activity in cyclohexene isomerization of the silanol groups of the zeolite.

On the other hand, the second pyridine addition did not influence hydrogen transfer reactions nor the coverage of supercage OH groups, confirming that such groups are responsible for hydrogen transfer reactions. More specifically, according to other authors [START_REF] Pine | Prediction of cracking catalyst behavior by a zeolite unit cell size model[END_REF], HF OH groups perturbed by extraframework Al species are responsible for such reactions because neighboring Al sites are required to facilitate hydrogen transfer between sorbed carbenium ions and alkenes.

Lewis acid sites related to the extraframework Al content in the zeolite [START_REF] Liu | Nature and Catalytic Role of Extraframework Aluminum in Faujasite Zeolite: A Theoretical Perspective[END_REF][START_REF] Maache | Acidity of zeolites Beta dealuminated by acid leaching: An FTIR study using different probe molecules (pyridine, carbon monoxide)[END_REF] were also poisoned by pyridine. However, no direct association between the poisoning of such sites and product distribution was found, as reported by others [START_REF] Jolly | FT-IR study of hydrocarbon conversion on dealuminated HY zeolites in working conditions[END_REF].

Coke formation

A lower coke amount was formed in test 2 than in test 1. It can be associated with the poisoning of the strongest acid sites of HY catalyst by the initial pyridine pulse in test 2. This agrees with previous work [START_REF] Jolly | The effect of 2,6dimethylpyridine poisoning on the activity in n-hexane cracking of dealuminated HY zeolites[END_REF], which showed that coking process depends on initial BAS concentration and is favored by strong acid sites. However, it should be noted that only limited changes in the evolution of coverage of OH groups in supercages and sodalite cages occurred between the two tests. Then, comparing the coverages at the end of the test without pyridine (test 1) and before the second pyridine poisoning (test 2), significantly lower coverage of SiOH and OH groups at hexagonal prisms is observed in the presence of pyridine. The evolution of coverage of OH groups in hexagonal prims occurred continuously in test 1, while in test 2, it increased more rapidly and achieved a limit value after 0.2h. These behaviors agree with the evolution of coke formation during both tests. Based on these results, coke deposition initially occurred on the strong acid sites, which promoted an intense deactivation at the beginning. After that, the slow and limited deactivation rate observed was attributable to coke deposition on OH groups with lower acidity as SiOH, non-perturbed sodalite cage OH groups and OH groups in hexagonal prisms.

One point to mention is that the coverage rate of supercage OH groups is never complete (Figure 7) whereas OH groups inside sodalite cages can be fully covered after 1.5 h of reaction.

As mentioned before, the coverage of these groups remained after cutting off cyclohexene feed, indicating that they were irreversibly poisoned by coke. Thus, one possible reason for the partial coverage of supercage OH groups could be related to the blockage of the accessibility of such groups by coke.

Conclusions

This IR operando study allows to highlight the role of the various acidic OH groups of HY zeolite for the different pathways of cyclohexene transformation reaction. Parallel between the type and amount of OH groups poisoned by coking or by different pyridine pulses and the reaction product yields were performed.

First fast and then slow deactivation occurred during cyclohexene reaction. The fast coke formation mostly poisoned the stronger acidic sites of HY, i.e., perturbed and non-perturbed supercage OH groups and non-perturbed sodalite cages ones. In parallel, a strong decrease in Htransfer activity is observed. Similarly, the introduction of pyridine before the reaction that led to preferential poisoning of the stronger acid sites also decreased the methylcyclopentane and cyclohexane yields. This showed that these strongly acidic OH groups are responsible for Htransfer activity.

During the slow deactivation stage, a decrease of the 1-methyl-1-cyclopentene yield occurred.

After one hour of reaction, the introduction of a pyridine pulse led to a marked decrease of the isomerization product and stabilized the ISO/HYD ratio at a constant value. Simultaneously, specific poisoning of the silanol and zeolitic hexagonal prism and perturbed sodalite cage OH groups was observed. This led to the conclusion that these weakly acidic sites are active sites for isomerization reaction.

These findings contributed to a complete understanding of the HY zeolite behavior during the cyclohexene reaction. They will support further studies on the evaluation of the role of metals, such as Ni and Mo, into zeolite, which are present in hydrocracking catalysts.

Figure 1 .

 1 Figure 1. Pathways of cyclohexene conversion. HT: Hydrogen Transfer. I: Isomerization.

Figure 2 .

 2 Figure 2. IR spectrum of HY zeolite in the OH region after activation at 623 K.

Figure 3 .

 3 Figure 3. IR spectrum of pyridine after desorption at 423 K, 523 K and 623 K over HY zeolite.

Figure 4 .

 4 Figure 4. Catalytic data observed in Test 1 (a and b) and Test 2 (c and d). Graphs a and c present

Figure 5 .

 5 Figure 5. IR spectra of HY before and during cyclohexene reaction. A: spectrum taken in test 1,

Figure 6 .

 6 Figure 6. IR spectra of HY during cyclohexene reaction (test 2). Spectrum taken before (B) and

  revealed a slightly more marked coverage at approximately 0.7 h for all groups. At the end of the test, only the LF OH groups inside sodalite cages (perturbed and non-perturbed) were fully covered (coverage near to 1). After 1.4 h of the test, the cyclohexene flow was cut off. The catalyst was maintained under inert gas flow in order to distinguish the coverage due to the adsorption of intermediate species from that due to coke. The results are shown in Online Resource 1 (S1). The coke species remained constant. In contrast, the intermediate species decreased but did not disappear, indicating that the sites remained covered due to a combination of coke and intermediate species with strong interaction. In parallel, after cyclohexene flow cutting off, only the OH groups in the hexagonal prisms and SiOH groups presented a decrease of coverage, which indicates the release of weakly intermediate adsorbed species. By contrast, the other groups remained covered by coke.

Figure 7 .

 7 Figure 7. Coverage rates of the HY zeolite OH group during Test 1 (a and b) and Test 2 (c and

Table 2 .

 2 Concentration of HY acidic sites poisoned by first and second pyridine addition in Test

		2.
		Unit	After 1 st	Before 2 nd	After 2 nd
			poisoning	poisoning	poisoning
	TOS	h	0	1.1	1.3
	Non-perturbed OH	µmol g -1	15	34	33
	supercages
	High frequency
	OH
	Perturbed OH	µmol g -1	27	43	42
	supercages
	Non-perturbed OH	µmol g -1	21	39	39
	sodalite cages
	Low frequency
	OH
	Perturbed OH sodalite	µmol g -1	25	78
	cages

Acknowledgments

The authors would like to thank Dr. Philippe Bazin for the support in the pyridine adsorption experiments and operando setup development, as well as Dr. Alexandre Vimont and Yoann Levaque. The authors are also grateful to PETROBRAS and LCS for the financial support and the authorization for using the facilities for carrying out the experiments.

Conflict of interest

The authors declare that they have no conflict of interest.


C E Santolalla-Vargas

Victor Santes

V A Suarez-Toriello

J A De

Los Reyes

B Pawelec

J L G Fierro

E Dominguez

L Oliviero

Françoise Maugé

J A De Los Reyes

E Dominguez Garcia

Effect of sulfidation pressure on the structure and activity of

  L'archive ouverte pluridisciplinaire

Introduction

The development of hydrotreating catalysts is of great importance for the refinery to comply the worldwide environmental legislations [START_REF] Hughes | Chapter 12 Legislation Relating to Highway Pollution[END_REF]. The current trend of crude oil supply provides heavy crudes containing increasingly high percentages of heteroatoms (S, N, O) that are, therefore, more difficult to process. Besides, the demand for production of clean fuels is increasing. In this sense, the environmental legislations are stricter with the sulfur (S) content in road fuels i.e. diesel and gasoline. In this sense, a decrease of S content of about < 0.05 wt% has been indicated [START_REF] Vasudevan | A Review of Deep Hydrodesulfurization Catalysis[END_REF]. Moreover, the S content of maritime fuel are now regulated (IMO 2020).

To satisfy the environmental requirements, the development of hydrotreatment catalysts more active is necessary. Currently, industrial catalysts are sulfided CoMo, NiMo and NiW on alumina-based support. The NiW catalysts have interesting properties in HDS specifically for hydrotreatment of heavy feeds [START_REF] Gachet | Optimization of the composition of Ni-W/A1203 hydrotreating catalysts using model molecules and real feedstock conversion studies[END_REF]. However, NiW catalysts generally exhibit low sulfidation degree due to strong metal-support interaction which limits their activity [START_REF] Scheffer | Temperature-programmed reduction of NiOWO3/Al2O3 Hydrodesulphurization catalysts[END_REF][START_REF] Yang | Hydrotreating of crude 2-ethylhexanol over Ni/Al2O3 catalysts: Influence of the Ni oxide dispersion on the active sites[END_REF].

To increase the activity of NiW catalyst, the preparation has been modified at different stages. For instance, modification of the conventional Al 2 O 3 suppport [START_REF] Palcheva | TiO2 nanotubes supported NiW hydrodesulphurization catalysts: Characterization and activity[END_REF][START_REF] Tavizón-Pozos | Deep Hydrodesulfurization of Dibenzothiophenes Over NiW Sulfide Catalysts Supported on Sol-Gel Titania-Alumina[END_REF], has been considered through the pre-impregnation of additive as F and Ga [START_REF] Sun | The effects of fluorine, phosphate and chelating agents on hydrotreating catalysts and catalysis[END_REF][START_REF] Díaz De León | Hydrodesulfurization of sulfur refractory compounds: Effect of gallium as an additive in NiWS/γ-Al2O3 catalysts[END_REF]. Among other modifications, the addition of chelating agent has been also explored [START_REF] Koizumi | Simultaneous promotion of hydrogenation and direct desulfurization routes in hydrodesulfurization of 4,6-dimethyldibenzothiophene over NiW catalyst by use of SiO2-Al2O3 support in combination with trans-1,2diaminocyclohexane-N,N,N′,N′-tetraacetic acid[END_REF][START_REF] Palcheva | NiMo catalysts supported on the Nb modified mesoporous SBA-15 and HMS: Effect of thioglycolic acid addition on HDS[END_REF]. In this respect, the co-impregnation of chelating agent such as nitrilotriacetic acid (NTA), citric acid, ethylenediaminetetraacetic acid (EDTA) and 1,2 cyclohexanediamine-N,N,N',N'tetraacetic acid (CyDTA) within the metal precursor impregnation solution has shown an increase of HDS activity for sulfur molecules such as 4,6 dimethyl dibenzothiophene [START_REF] Suárez-Toriello | Influence of the solution pH in impregnation with citric acid and activity of Ni/W/Al2O3 catalysts[END_REF][START_REF] Santolalla-Vargas | Effects of pH and chelating agent on the NiWS phase formation in NiW/γ-Al2O3 HDS catalysts[END_REF] and thiophene [START_REF] Kishan | Sulfidation and Thiophene Hydrodesulfurization Activity of Nickel Tungsten Sulfide Model Catalysts, Prepared without and with Chelating Agents[END_REF]. The activity increase was related to the formation of a Ni complex with the chelating agent in the impregnation solution. These complexes increase the reduction temperature for Ni, and favor the formation of NiWS species. These features can explain the increment of activity for HDS of 4,6 dimethyl dibenzothiophene of ca. three times in comparison with the counterpart prepared without chelating agent [START_REF] Suárez-Toriello | Influence of the solution pH in impregnation with citric acid and activity of Ni/W/Al2O3 catalysts[END_REF][START_REF] Santolalla-Vargas | Effects of pH and chelating agent on the NiWS phase formation in NiW/γ-Al2O3 HDS catalysts[END_REF]. As a general manner, Zuo et al. showed on a series of NiW/Al 2 O 3 with different Ni/W ratios, that increasing the WS 2 dispersion and the amount of W 4+ species are beneficial to provide more edge sites for the formation of the NiWS phase [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF]. On Mo-based systems, Chen et al. were able to show that changing the morphology, i.e. the S-/M-edge ratio, of the sulfide slab is also a way to improve the catalytic activity. In this sense, it was shown that metal amount, citric acid addition and sulfiding conditions (T, P) can modify the sulfide slab morphology [START_REF] Chen | IR spectroscopy evidence of MoS2 morphology change by citric acid addition on MoS2/Al2O3 catalysts -A step forward to differentiate the reactivity of M-edge and S-edge[END_REF][START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF]. Indeed, high sulfidation pressure increases the S-edge/M-edge ratio leading to more truncated MoS 2 slabs. As a result, the thiophene HDS activity significantly increases after high pressure sulfidation. These data points out that CUS sites on M-edges and on S-edges do not present the same activity. As a consequence, the sulfidation parameters such as temperature [START_REF] Hensen | Insight into the formation of the active phases in supported NiW hydrotreating catalysts[END_REF] and pressure modified the sulfidation degree [START_REF] Dugulan | Effect of pressure on the sulfidation behavior of NiW catalysts: A 182W Mössbauer spectroscopy study[END_REF], but has also an important role in the formation and structure of WS 2 phase [START_REF] Dugulan | Effect of pressure on the sulfidation behavior of NiW catalysts: A 182W Mössbauer spectroscopy study[END_REF][START_REF] Van Der Meer | Characterization and thiophene hydrodesulfurization activity of amorphous-silica-alumina-supported NiW catalysts[END_REF]. Dugulan et al. have shown that sulfidation of NiW/Al 2 O 3 under high pressure increases the crystallinity of WS 2 slabs as compared with the counterpart sulfided at low pressure [START_REF] Dugulan | Effect of pressure on the sulfidation behavior of NiW catalysts: A 182W Mössbauer spectroscopy study[END_REF]. In addition, van Meer et al reported for NiW/ASA that high sulfidation pressure increases the average slab length [START_REF] Van Der Meer | Characterization and thiophene hydrodesulfurization activity of amorphous-silica-alumina-supported NiW catalysts[END_REF]. These authors also showed a marked positive effect of sulfidation pressure on the thiophene HDS activity of NiW/ASA [START_REF] Van Der Meer | Characterization and thiophene hydrodesulfurization activity of amorphous-silica-alumina-supported NiW catalysts[END_REF].

Hence, previous works have shown that chelating agent addition and high-pressure sulfidation can lead to preparation of more active catalysts. However, in-depth understanding of the impact of these parameters on the structure of active sites is lacking.

In this work, NiW/Al 2 O 3 catalysts were prepared with or without CyDTA addition at different pH of impregnation, and sulfided at different pressures (1 bar or 40 bar). The thiophene HDS activity of these catalysts was studied after sulfidation at different conditions. Furthermore, these catalysts were characterized by temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy with CO adsorption (IR/CO) to account for the role of CyDTA addition, pH and sulfidation pressure on the structure of the active phase.

Experimental

Synthesis of the catalysts

The W/γ-Al 2 O 3 catalyst base (W nominal loading of 20 wt.%) was prepared by incipient wetness impregnation [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF] of a commercial γ-Al 2 O 3 (specific surface area=245m 2 /g, PIE=7.0, Vp=1. . The pH of the Ni-CyDTA solution was adjusted with the addition of ammonium hydroxide (25% vol.) to obtain pH values of 4.0 and 8.7. After impregnation, the catalysts aged for 24 h and then all solids were dried at 393 K in air. No calcination was performed before sulfidation. The W loading was 20 wt % and 4.4 wt % for Ni. The catalysts are called NiW(x), where x is the pH value of the Ni-CyDTA solution. NiW/Al 2 O 3 catalyst with a similar amount of Ni and W but without CyDTA was prepared and used as a reference. Also, W/Al 2 O 3 catalyst with a similar amount of W was prepared. Ni/CyDTA = 1, pH=8.7

Thiophene test

Before HDS activity measurements, the oxide catalysts were sulfided in situ in a fixed bed stainless steel reactor at a rate of 5 K/min up to 673 K under a flow of 10 % H 2 S/H 2 (30 mL/min) at 1 bar or 40 bar for 4 h. The thiophene HDS reaction was carried out in a continuous flow reactor at 623 K and atmospheric pressure with 50 mg of sulfided catalyst. Thiophene was introduced into the reactor by passing 70 mL.min -1 of H 2 flow through a thiophene saturator maintained at 291 K and mixed with a flow of 20 mL.min - 1 10 % H 2 S/H 2 . The partial pressure of thiophene, H 2 and H 2 S in the mixture are 0.08 bar , 0.912 bar and 0.021 bar, respectively. After 15 h reaction, the outlet gas was analyzed by a Varian 3900 chromatograph equipped with flame ionization (FID) detector. The concentrations of butane, 1-butene, trans-2-butene, cis-2-butene, tetrahydrothiophene (THT), as well as thiophene were recorded. The reaction rate was calculated as rHDS= (F/m)*X, were F/m is the molar flow rate of thiophene per gram of catalyst and X is the thiophene HDS conversion which was near to 10 %.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) spectra of the sulfided catalysts (10 % H 2 S in H 2 , 673 K, 1 or 40 bar) were measured at room temperature using a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and a Mg Kα (hν=1253.6 eV) X-ray source. The freshly sulfided catalyst (ex situ sulfidation) were deposited into small glass cylinders containing isooctane in order to avoid contact with air and mounted onto a manipulator which allowed transfer from the preparation chamber into the analysis chamber of the spectrometer. The details of the XPS measurements by this spectrometer are reported elsewhere [START_REF] Navarro | Deep hydrodesulfurization of DBT and diesel fuel on supported Pt and Ir catalysts[END_REF]. The S/(Ni+W) or S/W ratios were considered to determine the sulfidation degree for the NiW and W catalysts respectively.

Temperature programmed reduction

TPR experiments were carried out in a U-shape quartz reactor. After ex situ sulfidation (under flow of 10 % H 2 S/H 2 at 673 K, 1 bar or 40 bar) about 0.1 g catalyst (precisely weighted) was placed in the reactor and purged under nitrogen flow for 1 h at room temperature (RT). Then the gas flow was switched to hydrogen and the sample was heated at a rate 5 K/min up to 1323 K. Mass spectra of released gasses were analyzed with a quadrupole Thermo Prolab mass-spectrometer. The overall accuracy on the temperature scale is about ±5 K.

IR spectroscopy of adsorbed CO

Due to the high absorption of IR signal by the sulfided W phase, this technique is restricted to materials with loading lower than 10 wt% of W content. Thus in the present work, the NiW catalysts (4.4 wt% Ni, 20 wt% W) have to be diluted into Al 2 O 3 , an inert diluent, before transmission IR experiments. Consequently, the IR spectra of the diluted samples will show the bands of CO adsorbed on the NiW/Al 2 O 3 catalyst as well as on the alumina used as support and diluent.

The (Ni)W catalysts have been diluted into pure alumina (50 % catalyst -50 % Al 2 O 3 ). The mixture was very carefully crushed and pressed into a self-supporting wafer (ca. 17 mg, precisely weighted, for a disc of 2 cm 2 ) and put into an IR quartz cell. Before the adsorption experiments, the sample was sulfided in situ in the IR cell. The catalyst was first heated to 673 K in a gas mixture containing 10 % H 2 S/H 2 (30 mL/min) at atmospheric pressure and using heating rate of 5 K/min. The catalyst was kept at this temperature for 4 h. After sulfidation, the cell was evacuated at 673 K for 1 h and then cooled down under vacuum. CO adsorption was performed at low temperature (~100 K) to avoid any reaction between CO and the catalyst surface (as Ni(CO) 4 formation), and to favor interaction with all the surface sites. Small calibrated doses of CO were introduced in the IR cell up to an equilibrium pressure of 133.3 Pa. CO was subsequently evacuated at low temperature (~100K) up to a residual pressure of 10 -3 Pa, and finally at room temperature. All the spectra presented are normalized to a disc of 10 mg and 2 cm 2 of W-based sulfided catalyst diluted into alumina.

Results and discussion

Thiophene HDS

The effects of preparation conditions and sulfidation pressure on the thiophene HDS reaction rate are reported in Table 2 andTable 3. The thiophene activity was measured at low conversion and pseudo-steady state (time-on-stream of 15 h).

For the catalysts sulfided at atmospheric pressure (Table 2), the NiW catalyst prepared without CyDTA exhibited a 11-fold greater thiophene HDS activity than the W catalyst. This promotion factor is in good agreement with previous papers [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF][START_REF] Villasana | Pollutant reduction and catalytic upgrading of a Venezuelan extra-heavy crude oil with Al2O3-supported NiW catalysts: Effect of carburization, nitridation and sulfurization[END_REF]. For the NiW samples prepared with chelating agent, a significant increase of activity was observed as compared with the NiW sample prepared without. Interestingly, the activity for the NiW(4.0) sample was approximately 40 % higher than that for the NiW sample prepared without chelating agent. Changing the pH of the impregnating solution can improve the preparation but in a limited manner. Indeed, the catalyst prepared at low pH (4.0) displayed a slightly greater HDS activity than that prepared at pH = 8.7 (46 vs 43 mol/h/kg). 3 shows that the pressure of sulfidation markedly influences the HDS activity. Indeed, after sulfidation under 40 bar, the NiW catalysts (prepared with or without CydTA) present an HDS activity greater by about 50% compared to the corresponding catalysts sulfided under 1 bar. Note that after sulfidation under 40 bar, the catalyst prepared at high pH (8.7) displayed only a slightly greater HDS activity than that prepared at pH = 4.0 (66.9 vs 64.5 mol/h/kg) For all the studied catalysts, the products detected in the thiophene HDS test were: tetrahydrothiophene, 1-butene, cis-2-butene, trans-2-butene and n-butane (Table 2 and Table 3). W catalyst displayed similar concentration of tetrahydrothiophene and butenes (0.4 %). For all the Ni-promoted catalysts, the main products were trans-2-butenes, cis-2-butenes, and n-butane. The detection of tetrahydrothiophene on the W catalyst was an indication of its high hydrogenation ability as compared to the C-S bond cleavage one. In contrast, promotion by Ni improved the formation of butenes. The CyDTA incorporation hardly modified the selectivity of NiW catalysts. But high pressure sulfidation slightly enhances the ratio butene/butane.

Comparison of Table 2 and Table

X-ray photoelectron spectroscopy

In order to measure the effect of sulfidation pressure and condition of preparations on the sulfidation degree, the catalysts were characterized by XPS. The amount of different W species, and atomic ratios W/Al, Ni/Al as well S/M ratio are presented in Table 4.

Table 4 shows that the W/Al and Ni/Al stayed almost constant for all the NiW catalysts. By contrast, the amount of W 6+ and W 4+ species is modified by the preparation procedures and composition. Thus, after sulfidation under 1 bar H 2 S/H 2 , the NiW catalyst exhibits greater S/W ratio (1.97) as compared to unpromoted W (1.57) indicating that Ni addition improves the sulfide amount. Nevertheless, XPS detected amount of W 6+ on all the catalysts sulfided at atmospheric pressure indicating presence of oxide or oxysulfide species. Table 4 also points out that the addition of chelating agent increases the sulfidation degree of NiW phase of about 10 % -from 1.97 to 2.15. The beneficial effect of CyDTA does not change with the pH values of the impregnating solution when sulfidation is performed at 1 bar. After sulfidation under high pressure, the NiW catalysts exhibit greater amount of W sulfide phase as well as greater sulfidation degree than the catalysts treated at low sulfidation pressure. In this regard, the S/W ratio of the NiW(8.7) catalyst increases of about ~7% when sulfidation is performed at 40 bar compared to 1 bar.

Figure 1 presents the S/(Ni+W) ratios for the various NiW catalysts. These data confirm the clear beneficial effect of high pressure sulfidation, addition of chelating agent is beneficial for sulfidation degree. It shows that increasing the pH values of Ni-CyDTA is beneficial when sulfidation is performed under high pressure. 2 and Figure 3. Different zones of H 2 S released can be distinguished in these TPR patterns. The H 2 S release at high temperature (T>1023 K) corresponds to bulk WS 2 reduction. It has been used to normalize the curves and correct the temperature scale, according to ref [START_REF] Afanasiev | On the interpretation of temperature programmed reduction patterns of transition metals sulphides[END_REF]. Therefore, the area marked with a rectangle will not be discussed further. Note that the maximum intensity of this high-temperature peak does not depend on chemistry but only on the total amount of WS 2 of the studied sample [START_REF] Afanasiev | On the interpretation of temperature programmed reduction patterns of transition metals sulphides[END_REF]. Below 1000 K, three zones of H 2 S release can be distinguished.

The first peak corresponds to weakly bounded sulfur. On NiW and NiW(4.0) catalysts, this H 2 S release was greater and shifted towards greater temperature when sulfidation is performed under higher pressure. Indeed, on NiW sample the peak shifts from 451 K after sulfidation at 1 bar, to 481 K after sulfidation at 40 bar (Figure 2). For NiW(4.0) catalyst, the peak shifts from 497 K for low sulfidation pressure to 521 K for high sulfidation pressure (Figure 3) These two features can be related to a greater sulfur coverage of the WS 2 edges after sulfidation under high pressure as well as to better crystallized edges, i.e. more "rigid", with more stable S-M bonds. Such phenomenon has already been reported for MoS 2 catalysts sulfided at different temperatures [START_REF] Baubet | The influence of MoS2 slab 2D morphology and edge state on the properties of alumina-supported molybdenum sulfide catalysts[END_REF]. An increase of sulfidation pressure should lead to the same effect as an increase of temperature, since in both cases the chemical potential of sulfur is increased. The On both NiW catalysts, a small peak is observed at about 580 K that is not usually encountered in the TPR patterns of sulfided catalysts. To unravel its nature, the MS patterns of all the effluents have been examined during the TPR experiments on NiW catalyst. Figure 3 shows that the position of the H 2 S release peak ~578 K is close to the maximum of the second peak of water release at 593 K. The first broad water release peak (380-450 K) corresponds to the catalyst dehydration, the second water release peak (520-673 K) can be due to a reduction process. It indicates that some oxide and oxysulfide species that were present in the NiW catalysts prepared with and without chelating agent, were reduced in this temperature range. This is in agreement with the detection of oxy(sulfide) W species by XPS.

A third H 2 S peak is detected between 823 K and 973 K on the NiW catalysts. Litterature assigned this H 2 S release to the reduction of nickel -containing sulfide species. Zuo et al. [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF] suggested that this peak is directly related to the presence of NiWS species. However, this peak can also arise from a separate NiS x phase. First, it can be noted that on both NiW and NiW(4.0) catalysts, the intensity of this peak is slightly greater when the sulfidation is performed under 1 bar than under 40 bar. Second, it appears that this peak presents a greater intensity for NiW catalyst than for the one prepared with CyDTA. Considering only these results, it cannot be decided whether the H 2 S release comes from the NiSx or NiWS phases.

In addition, TPR patterns of NiW catalyst showed a maximum of this peak at 925 K and 915 K for sulfidation at 1 bar and 40 bar, while TPR patterns of NiW with chelating agent exhibited peaks at 850 K and 800 K for sulfidation at 1 bar and 40 bar. First, as a general manner, the high temperature of the peak 3 is related to the formation of sulfide species which are extremely difficult to reduce, indicating strong metal-support interactions [START_REF] Scheffer | Temperature-programmed reduction of NiOWO3/Al2O3 Hydrodesulphurization catalysts[END_REF][START_REF] Yang | Hydrotreating of crude 2-ethylhexanol over Ni/Al2O3 catalysts: Influence of the Ni oxide dispersion on the active sites[END_REF]. Second, it is shown that high pressure sulfidation decreases the strength of the metal-support interactions. This effect of high pressure sulfidation has already been reported for other sulfided systems [START_REF] Hattori | spectra of adsorbed CO on Pt/Al2O3 catalysts with super-low Pt content by a diffuse reflectance fourier transform I.R. spectrometer[END_REF]. Third, it appears that addition of CyDTA also decreases the strength of the metal-support interactions. In this regard, Scheffer et al. [START_REF] Scheffer | Sulfidability and HDS activity of Co-Mo/Al2O3 catalysts[END_REF] suggested that the higher activity of CoMoS phase is attributed to the weaker active phase-support interaction. In the same way Chen et al showed that addition of chelating agent as citric acid limits the metal support interaction and leads to the formation of more active M and CoMo catalysts [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF][START_REF] Chen | Role of Citric Acid in Preparing Highly Active CoMo/Al2O3 Catalyst: From Aqueous Impregnation Solution to Active Site Formation[END_REF]. Consequently, the catalyst NiW with chelating agent could forms NiWS species more active than NiW.

Note at last that the high-temperature reduction peak was accompanied with additional water release (Figure 3). That indicated that the sulfidation was not complete in both catalysts. This is in agreement with XPS data that pointeds out some hardly reducible W oxide remaining in the NiW and NiW(4.0) catalysts.

IR spectroscopy of adsorbed CO probe

Infrared spectroscopy of adsorbed CO provides interesting information for investigation of the surface species of sulfide catalysts like the NiW ones. In particular, IR signal of CO (IR/CO) allows discriminating unpromoted W phase from NiWS one [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF][START_REF] Zavala-Sanchez | Structure and Quantification of Edge Sites of WS2/Al2O3 Catalysts Coupling IR/CO Spectroscopy and DFT Calculations[END_REF][START_REF] Zavala-Sanchez | Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy[END_REF] Figure 5 to Figure 7 show the IR spectra of CO adsorbed on sulfided W, NiW and NiW(4.0), respectively. Bands at 2197, 2188, 2160, 2119, 2064 and 2050 (sh) cm -1 are observed on W catalyst (Figure 5). Their intensity increases with the amount of CO introduced. From previous studies [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF], these bands can be attributed to coordination of CO on Lewis acid sites of the alumina support (2197 and 2188 cm -1 ), CO in hydrogen bonding with the support hydroxyl groups (2160 cm -1 ) and to CO in interaction with the edges of the WS 2 crystallites (2119 , 2064 and 2050(sh) cm -1 ). Previous works already reported [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF]30,[START_REF] Duchet | Carbon monoxide and oxygen chemisorption and functionalities of sulphided Ni-W/Al2O3 hydrotreating catalysts[END_REF] these two main bands at 2119 and 2064 cm -1 characterized WS 2 edge sites. Note that their frequencies can be shifted according to the support nature [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF]30,[START_REF] Duchet | Carbon monoxide and oxygen chemisorption and functionalities of sulphided Ni-W/Al2O3 hydrotreating catalysts[END_REF]. Recent parallel between IR/CO spectroscopy and DFT calculations allow assigning these two bands to CO adsorbed on M-and S-edge sites of the WS 2 slab, respectively [START_REF] Zavala-Sanchez | Structure and Quantification of Edge Sites of WS2/Al2O3 Catalysts Coupling IR/CO Spectroscopy and DFT Calculations[END_REF] . It should be mentioned that, on Mo sulfided catalysts, IR/CO spectroscopy also allows to distinguish Mo sites located on M-edge from Mo sites located on S-edge [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF].

Figure 5: IR spectra of increasing doses of CO adsorbed on sulfided W catalyst ((from 5 to 129 µmol of CO per g of sulfided catalyst and further 133 Pa CO at equilibrium).

In the case of NiW catalysts, NiS x , WS 2 and NiWS [START_REF] Le | Solution synthesis of the unsupported Ni-W sulfide hydrotreating catalysts[END_REF] phases can coexist which makes the assignment of CO adsorption bands more complicated. The IR spectra of CO adsorbed on sulfided NiW catalyst are shown in Figure 6. From these IR spectra, it is observed that the addition of Ni to W brings three new bands at 2127, 2093 and 2079 cm -1 . These wavenumbers are very close to those reported by Duchet et al. [START_REF] Duchet | Carbon monoxide and oxygen chemisorption and functionalities of sulphided Ni-W/Al2O3 hydrotreating catalysts[END_REF], Zuo et al. [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF], Travert et al. [START_REF] Travert | CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study[END_REF] and more recently by Zavala-Sanchez [START_REF] Zavala-Sanchez | Structure and Quantification of Edge Sites of WS2/Al2O3 Catalysts Coupling IR/CO Spectroscopy and DFT Calculations[END_REF][START_REF] Zavala-Sanchez | Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy[END_REF]. These authors proposed that these three bands characterize interaction between Ni and W but with different environments. Zuo et al. [START_REF] Zuo | The formation of the active phases in sulfided NiW/Al2O3 catalysts and their evolution during post-reduction treatment[END_REF] proposed that 2127 and 2079 cm -1 characterized NiWS sites whereas 2094 cm -1 would characterize Ni particles perturbed by WSx phase. Zavala-Sanchez et al [START_REF] Zavala-Sanchez | Structure and Quantification of Edge Sites of WS2/Al2O3 Catalysts Coupling IR/CO Spectroscopy and DFT Calculations[END_REF][START_REF] Zavala-Sanchez | Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy[END_REF] + new ref refined these assignements. Hence, by considering the experimental work of Dominguez [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology[END_REF] on NiMo with accurate variation of the promotion degree of the catalysts as well as the DFT calculation of CO adsorbed on NiMo [START_REF] Travert | CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study[END_REF], it is proposed that 2094 and 2078 cm -1 characterize WS 2 edges partially promoted by Ni wheraeas the 2129 cm -1 band would characterize WS 2 edges completely promoted by Ni. It should be mentioned that the IR spectra of CO recorded on the sulfided NiW does not exhibit any clear band at 2119 cm -1 corresponding to unpromoted W sites on M-edge. It suggested that, on the NiW sample, the promoter Ni covers all the M-edge sites of WS 2 slabs. For the band at ~2060 cm -1 that characterizes unpromoted S-edge sites, it is difficult to draw any clear conclusion due to the broadness of the present bands. The IR spectra of CO adsorbed recorded on the sulfided NiW(4.0) (Figure 7) showed bands at wavenumbers close to those observed on NiW catalyst (Figure 6). Nevertheless, all the bands characterizing CO on the sulfided phase are shifted toward higher wavenumber (+3-5 cm -1 ) on catalyst prepared with CyDTA. Note that the shoulder at about 2084 cm -1 , that is detected from the first doses of CO, appears much more markedly on NiW(4.0) than on NiW catalyst (Figure 7). In addition, Figure 8 shows that the intensity ratio between the bands at ~ 2093 cm -1 and ~2127 cm -1 is stronger on NiW(0.4) than on NiW catalysts. However the most striking feature is the greater intensity (almost 3 times) of the CO uptake on NiW(4.0) than on NiW catalyst. This points out the formation of more NiWS sites by CyDTA addition (Figure 8).

Discussion

The NiW catalyst prepared with chelating agent displayed higher thiophene HDS activity than the reference NiW catalyst without CyDTA. After high pressure sulfidation, the various NiW catalysts present greater activity compared to their counterparts sulfided at atmospheric pressure. These modifications in the preparation stages of NiW catalysts also induce some changes in the selectivity since some improvements of the butene-over-butane ratio can be obtained after high pressure sulfidation. Note that the Ni-promotion of the W catalyst, not only strongly increases the HDS activity but also enhances butene-to-butane ratio. In order to understand the origin of these catalytic changes, the compositions and structures of the sulfide particles were studied.

Figure 9 compares the HDS activity and the sulfiding degree, as determined by XPS, measured on the catalysts after low and high sulfidation pressure. After sulfidation under 1 bar, the most active catalyst (NiW(4.0)) shows the highest S/(Ni+W) ratio (1.21). After high sulfidation pressure, the NiW(8.7) presents the highest sulfidation degree (2.34) as well as is the greatest HDS activity. Figure 9 points out the appearance of a linear correlation between the HDS activity and the S/(Ni+W) ratio for the NiW catalyst sulfided under 1 and 40 bar. Similar trends were previously reported for dibenzothiophene HDS on CoMo/Al 2 O 3 sulfided at 673 K and 4 MPa compared to the samples sulfided at at 673 K and atmospheric pressure [START_REF] Dugulan | High-pressure sulfidation of a calcined CoMo/Al2O3 hydrodesulfurization catalyst[END_REF]. As a first conclusion, sulfidation degree is indeed a key parameter to obtain high activity of NiW-based catalyst. However, the structure of the sulfide phase has also to be considered.

Figure 9: Relation between catalytic activity and sulfidation degree of the NiW catalysts

The activity of the HDS catalysts is generally related to the coordinatively unsaturated sites (CUS) sites formed on the edges of the promoted sulfide nanoslabs [START_REF] Byskov | DFT Calculations of Unpromoted and Promoted MoS2-Based Hydrodesulfurization Catalysts[END_REF][START_REF] Ramos | HRTEM and molecular modeling of the MoS2-Co9S8 interface: understanding the promotion effect in bulk HDS catalysts[END_REF]. In the TPR experiments, the H 2 S release between 800 and 925 K (zone 3) is particularly informative since it is related to the CUS created on Ni-based phases i.e. on NiWS phase and/or NiSx phase. Whatever the sulfidation pressure, NiW(4.0) showed lower reduction temperature (800 K and 850 K) than NiW (915 K and 925 K). Thus, TPR patterns indicated that the creation of CUS is easier for catalysts prepared with CyDTA and when sulfidation is performed under high pressure (Figure 2, Figure 3). This is related to the strength of the metal-support interaction. These results are in agreement with the conclusions about the sulfidation pressure effect obtained on (Co)Mo catalysts. Hence, a parallel appears between the creation of CUS and the HDS performances. Indeed, NiW prepared with chelating agent or sulfided under high pressure present greater activity. Similarly, Dugulan et al. [START_REF] Dugulan | High-pressure sulfidation of a calcined CoMo/Al2O3 hydrodesulfurization catalyst[END_REF] showed that the high sulfidation pressure enhanced the activity as well as the Co-promotion and the formation of the mixed CoMoS phase.

Finally, on W catalysts, CO adsorption on sulfided phase showed bands at 2119 cm -1 and 2064 cm -1 indicating that the WS 2 slabs present both M-and S-edges. On NiW catalyst, CO spectra displayed mostly the bands at 2127 and 2093 cm -1 which are ascribed to NiWS sites, completely or partially promoted by Ni, respectively [START_REF] Duchet | Carbon monoxide and oxygen chemisorption and functionalities of sulphided Ni-W/Al2O3 hydrotreating catalysts[END_REF]. On NiW(4.0), NiWS sites are also characterized by two bands 2130 and at 2097 cm -1 , i.e. at slightly greater wavenumbers that the counterpart without chelating agent. These (CO) frequencies are in agreement with the change in the strength of the metal-support interaction. However, the most striking features is the strong increment of the intensity of the NiWS bands on NiW(4.0) compared to NiW. This indicates the formation of a greater concentration of NiWS on NiW(4.0) catalyst. This is in agreement with the high catalytic activity in thiophene HDS of the catalyst prepared with CyDTA addition.

Hence, more active NiW catalysts are obtained by addition of CyDTA and sulfidation under high pressure. Furthermore, since refineries have to remove sulfur compounds selectively preventing the hydrogenation of olefins and maintaining the high octane number of gasoline [START_REF] Palcheva | 12-Tungstophosphoric heteropolyacid supported on modified SBA-15 as catalyst in HDS of thiophene[END_REF]. This study showed that the formation of NiWS species modified the HDS thiophene products as compared to W catalysts. The NiW and NiW(4.0) displayed an increment of the ratio butenes/butanes as compared to W (Table 2), and therefore, an increase of olefins production. In this sense, NiW(4.0) displayed lower hydrogenation selectivity that would be favorable for the quality of gasoline [START_REF] Brunet | On the hydrodesulfurization of FCC gasoline: a review[END_REF].

Conclusions

In this work, sulfided NiW/γ-Al 2 O 3 catalysts synthesized with Ni-CyDTA solutions at two pH values were studied after sulfidation at atmospheric or high pressure (40 bar). The addition of CyDTA clearly improved the thiophene HDS activity of the NiW catalyst by a factor of 1.4. However, the influence of the pH of impregnation was limited. All catalysts sulfided under 40 bar exhibited a significantly greater HDS activity (by factor 1.5) as compared with the ones sulfided at low pressure. However, the beneficial effect of CyDTA addition measured on catalysts sulfided at 1 bar or 40 bar was similar. The CO adsorption showed the formation of NiWS phase on the catalysts prepared without and with CyDTA, although in a strongly greater amount for the catalyst prepared with the chelating agent. A correlation between catalytic activity and sulfidation degree was evidenced as well as with the intensity of the ν(CO) band ascribed to NiWS sites. Finally, CyDTA and high pressure sulfidation improved the sulfidation degree of the catalysts, the amount of sulfide phase CUS sites and decrease the sulfide phase-support interactions. This leads to a strong enhancement of the hydrodesulfurization activity and slightly lowered the hydrogenation route.

Figure 1 : 3 . 3

 133 Figure 1: Effect of the sulfidation pressure on the sulfidation degree of the NiW catalysts

  1 st peak show that the CyDTA addition also leads to more rigid WS 2 edges.

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2: H 2 S release during TPR experiments for NiW catalyst sulfided under 1 bar (a) and 40 bar (b)

Figure 6 :

 6 Figure 6: IR spectra of CO adsorbed at 100 K on sulfided NiW with increasing doses (from 5 to 134 µmol of CO per g of sulfided catalyst and further 133 Pa CO at equilibrium).

Figure 7 :

 7 Figure 7: IR spectra of CO adsorbed on sulfided NiW(4.0) with increasing (from 5.7 to 134 µmol of CO per g of sulfided catalyst and further 133 Pa CO at equilibrium).

Figure 8 :

 8 Figure 8: Comparison of IR spectra of 133 Pa of CO adsorbed on sulfided W (a), NiW (b) and NiW(4.0) (c) catalysts.

  1 cm 3 /g) with aqueous solution of ammonium metatungstate (NH 4 ) 6 H 2 W 12 O 40 •xH 2 O (Sigma-Aldrich) at pH of 4.0. After sample drying at 393 K, the Ni was incorporated onto the W/Al 2 O 3 catalyst base using appropriate amount of Ni(NO 3 ) 2 •6H 2 O (98%, Sigma-Aldrich) to obtain Ni/(Ni+W)=0.4 atomic ratio. To study the effect of chelating agent and pH conditions, two NiW/Al 2 O 3 samples were prepared at different pH by addition of 1,2-cyclohexanediamine-tetraacetic acid (C 14 H 22 N 2 O 8 •H 2 O (99%); CyDTA) to an aqueous solutions of the nickel nitrate (CyDTA/Ni molar ratio of 1

Table 1 : Catalysts and synthesis conditions

 1

	Catalyst	Synthesis conditions
	W	Without CyDTA
	NiW	Without CyDTA
	NiW (4.0)	Ni/CyDTA = 1, pH=4.0
	NiW (8.7)

Table 2 :

 2 Reaction rate a and product yields in thiophene HDS over (Ni)W/Al 2 O 3 catalysts after sulfidation at atmospheric pressure

						Reaction
	Catalyst	Conv. (mol.%)	Butenes (mol.% )	Butane (mol. %)	Butenes/Butane Ratio	rate (RR) at steady (mol h-1 kg-state	Promotion effect RR NiW /RR W
						1)
	W	1.0	0.4	0.2	2.0	3.0	-
	NiW	9.0	6.9	2.5	2.76	34.0	11.3
	NiW (4.0) 12.0	8.6	3.1	2.77	46.0	15.3
	NiW (8.7) 11.0	8.1	2.9	2.79	43.0	14.3

Table 3 :

 3 Reaction rate and product yields in thiophene HDS over (Ni)W/Al 2 O 3 catalysts after sulfidation under high pressure (40 bar)

	Catalyst	Conv. (mol.%)	Butenes (mol.%)	Butane (mol. %)	Butenes/Butane Ratio	Reaction rate (RR) at steady state (mol h-1 kg-1)	Effect sulfidation pressure on of reaction rate RR 40bar /RR 1bar
	NiW	14.0	10.2	3.4	3.00	51.5	1.51
	NiW (4.0)	16.0	12.1	4.0	3.02	64.5	1.40
	NiW (8.7)	17.0	12.4	4.3	2.88	66.9	1.56

Table 4 :

 4 Amount of different W forms and atomic ratios for (Ni)W/Al 2 O 3 catalysts sulfided at atmospheric and high pressure, as determined by XPS

	Catalyst -	W 6+	W 4+	W/Al	Ni/Al	S/(Ni+W) S/W
	Sulfidation pressure	(%)	(%)
	W -1 bar	22.0	78.0	0.069	-	-	1.57
	NiW -1 bar	40.0	60.0	0.089	0.066	1.13	1.97
	NiW (4.0 ) -1 bar	33.0	67.0	0.082	0.062	1.21	2.13
	NiW(8.7) -1 bar	18.0	82.0	0.079	0.063	1.19	2.15

Acknowledgements

In memory of Professor José Luis García Fierro. Authors thank Dr. Pavel Afanasiev for the TPR analyses. J.A. de los Reyes and V.A. Suarez Toriello acknowledge CONACYT-SENER funding for financial support. The authors thank financial support from Instituto Politécnico Nacional (Proyecto SIP 20201116 and 20201629) and the Mexican National Council for Science and Technology (CONACyT; project CB-2017-2018 # A1-S-32418).