Study of the liquid activation of CoMo and NiMo catalysts

Silva A. a,b,1, Corre T. b, Lemos F.a

a Instituto Superior Técnico, Avenida Rovisco Pais 1, 1049-001 Lisboa, Portugal b IFP Energies nouvelles, Rond-point de l'échangeur de Solaize, BP 3, 69360 Solaize, France

ABSTRACT

In this work, CoMoP and NiMoP catalysts were prepared with similar chemical composition, differing between them just in terms of the nature of

the promoter and the final treatment given: drying, calcination and additivation. After the preparation, the liquid sulfidation of the catalysts with

SRGO+2%DMDS was followed with time and temperature, through the DMDS decomposition. Finally the catalysts were characterized by

elemental analysis and XPS.

It was seen that the boosted catalysts are always the ones which present better results when compared with the other catalysts, fact that may be

correlated with the presence of an additive that leads to the delay of the DMDS decomposition. In contrast, the calcined catalysts present after the

liquid sulfidation, the worst values in terms of global degree of sulfidation and degree of promotion when comparing with the other catalysts.

Furthermore, the dried catalysts present, for CoMoP and NiMoP, higher global degree of sulfidation and promotion than the calcined ones.

A classification in terms of global degree of sulfidation and degree of promotion was established for both series:

CoMoP: boosted=dried>>calcined

NiMoP: boosted≈dried>calcined

Finally, the effect of increasing the pressure and time in the sulfidation of the NiMoP catalysts was studied. No change in the sulfidation and

dispersion in the catalyst was registered.

Keywords: Diesel; liquid sulfidation; CoMoP; NiMoP; DMDS decomposition

1. Introduction

Due to the importance of the transportation sector, the global

demand of the middle distillates like gasoil is expected to increase in

the years to come. Associated to this increase, the reduction of its sulfur

content and the tight legislation due to this compound pollutant

characteristics, associated with the competition in the refinery market,

requires an improvement of the processes and catalysts of

hydrotreatment (HDT). The most commonly commercial catalysts

used in the HDT are the molybdenum catalysts promoted with cobalt

or nickel supported on a gamma alumina. It is possible to give different

final treatments to the oxide form of the catalysts: drying, calcination

or additivation. When the oxide catalyst is synthetized, its active form

must be obtained by sulfidation. In the industry, the sulfidation is

usually performed in-situ in the liquid phase, with the feedstock spiked

with a sulfiding agent (normally dimethyldisulfide also called DMDS).

Many studies report the influence of the sulfidation conditions in

the final activities of the catalysts as well as the effect of the addition

of the additives to the catalyst. However, a lack of information is found

when studying the influence of the final treatment of the catalyst during

the liquid sulfidation, and its relation with the HDT activities.

1 Corresponding author: ana.carolina.silva@tecnico.ulisboa.pt

Moreover, it is known that, after the liquid sulfidation, the ranking

of activities of the CoMo and NiMo catalysts can be described as:

boosted> dried≥ calcined

The liquid phase activation is the one which is used at the industrial

level. In this process, the catalyst is wetted at low temperature by the

spiked feedstock, in order to put in contact the sulfur compounds with

the oxide phases to be converted. Once the catalyst is completely

wetted, the temperature is raised progressively up to the reaction

temperature under a stream of H2, and the sulfidation takes place

during this heat treatment. However, the increase of temperature must

be very slow, in order to facilitate the decomposition of the spiking

agent in sulfur compounds such as H2S, and to limit the reduction of

the oxides by H2 (since reduced species are more difficult to sulfide)

(1).

A spiking agent is a sulfur-containing organic compound which

releases H2S at a much lower temperature than the sulfur compounds

present in the normal feedstock. Among all the spiking agents, the

dimethyldisulfide (DMDS) is the one more used industrially due its

low vapor pressure and low flammability, its substantial sulfur content

and low decomposition temperature, but also due its low cost when

compared with polysulfides. In addition, the low temperatures at which

the DMDS decomposes in H2S maintains a good concentration of

sulfur, allowing also, the protection of the catalysts from undergoing

reduction by the hydrogen present (2).

From the DMDS decomposition it is possible to distinguish

different steps (2) (3), as schematized in the Figure 1.

Figure 1-Mechanism of DMDS decomposition. Adapted from (2).

Texier et co-workers (4) studied the decomposition of DMDS

during the activation procedure, at 40 bar of total pressure in the

presence of a NiMo/Al2O3 catalyst. They saw that the decomposition

of that organosulfur compound takes place between 150°C and 350°C.

In a first step, below 230°C, the DMDS is completely decomposed in

methylmercaptan (CH3SH or MeSH). In a second step, at higher

temperatures (230-330°C) the CH3SH decomposes, by hydrogenolysis

reactions, into CH4 and H2S. In parallel (T=230-330°C), the CH3SH

also leads to the formation of dimethylsulfide (C2H6S or DMS) and

H2S that subsequently leads to H2S and CH4.

Figure 2- Variation in molar fractions of reactant (DMDS) and

products (MeSH, DMS, CH4, H2S) (1).

From the on-line analysis of this process of decomposition (Figure

2) it is possible to see that the amount of MeSH (CH3SH) reaches a

maximum, decreasing after, while the CH4 concentration increases

symmetrically (between 230-330°C). The DMS also reaches a

maximum value of concentration around the temperature of 290°C,

decreasing after for higher temperatures of activation (due to the

appearance of H2S and CH4). A gap between the appearance of the CH4

and H2S is also seen, due to the sulfidation process of the catalyst. It is

important to refer that in the liquid phase, the active decomposition of

sulfur containing compounds (that include the hydrogen sulfide

production) can be considered when the methane starts to appear in the

gas analysis. Hydrogen sulfide appears latter in relation with the CH4

since this compound participates in the sulfidation of the catalyst,

being only detected when the active metal is mainly saturated with H2S

(5). Finally, at 330°C the DMDS is completely decomposed in CH4

and H2S.

During the decomposition of the spiking agent, the sulfidation can

be divided in two parts: low temperature sulfidation (in which the

sulfur agent is mainly the methylmercaptan) and high temperature

sulfidation (in which the sulfur agent is the H2S) where the sulfiding

reaction competes with reduction. The sulfidation of the catalyst

results from the contact between the sulfur agents and the oxide

precursors, forming gradually in the surface a sulfide phase. This

sulfide phase initiate the decomposition of DMDS and methylmercaptan, leading to the appearance of CH4 and H2S in the gas

analysis.

2. Experimental part

2.1 Catalysts preparation

In this study CoMoP and NiMoP catalysts were prepared by

following the steps described in the Figure 3. All the catalysts were

prepared by incipient wetness impregnation using a -alumina support

with a trilobe extrude shape, with a length of 2-4 mm and no particular

treatment was performed to the support. In terms of chemical

composition all the catalysts present the same mass content in

molybdenum and the same promoter/molybdenum ratio. The catalysts

are doped with phosphorous and were prepared with the same P/Mo

molar ratio.

After the maturation, the total amount of CoMoP (or NiMoP)

catalysts was divided in three parts, in order to proceed to the final

treatment: drying, drying and calcination or drying and additivation).

Figure 3- Main steps in the preparation of the CoMoP and NiMoP

catalysts.

Preparation of the impregnation solution

The preparation of the impregnation solution was made by

dissolution of the oxide precursors in a water volume equal to 95% of

the total amount of water necessary in the solution (leaving some

fraction to assure that the washing is well done and to avoid the loss of

precursors). The Table 1 summarizes the main characteristics of the

products used to prepare both series of catalysts.

Table 1- Precursors used in the preparation of the CoMoP and

NiMoP catalysts, and their principal properties

Oxide Precursor Molar mass

(g/mol) Purity (%)

MoO MoO3 143,94 100

P2O5 H3PO4 98,00 85

CoO Co(OH)2 92,93 95

NiO Ni(OH)2 92,71 95

In a beaker with the distillated water was added the H3PO4,

followed by the MoO3 and finally by the promoter precursor (Co(OH)2

or Ni(OH)2). In order to proceed to the total dissolution of the

precursors, the solution was left in agitation with heating (around

T=90°C) until was translucent.

Impregnation

To perform the incipient wetness impregnation is used a rotating

beaker where the support is placed. The solution is added drop by drop

to the support. To improve the impregnation process the operator

promotes the support movement by using a spatula. This step lasts

around 10 minutes.

2

Maturation

The main goal of maturation is to give time to the solution to enter

properly in the pores of the support, in other words, to allow the

precursors to diffuse in the support, in order to obtain a well dispersed

metal phase. This step can have different durations, being in our case

of 12 hours, in an atmosphere saturated in water, at T=25°C.

Drying

The drying process is performed at T=90°C in an oven with air.

The aim of this step is to remove the solvent in excess from the support.

The dried oxide catalysts are obtained at this point of the preparation.

Calcination

The calcination step is not a mandatory one, since the catalyst can

be directly sulfided after the drying step. The calcination allows the

breaking of undesirable ions that may be present on the impregnated

and dried support as well as the formation of an oxide phase that differs

from the one obtained after drying.

This stage in the catalyst preparation was made with a flow of air

which varies with the mass of catalyst to be calcined, in a fixed bed.

The calcination is performed with an air flow equal to 1,5

L/h/gcatalyst, a temperature of 450ºC during 2 hours, being the heating

ramp of 5ºC/min.

Additivation

The additive is an organic compound added on the catalyst after

the drying step by incipient wetness impregnation. After the addition

of the appropriate amount of additive, the catalyst is dried in an oven

at a temperature close to 100ºC.

2.2 In-situ liquid sulfidation

The sulfidation procedure was performed in the unit T58. This

procedure allows evaluating the performances of the catalysts during

the liquid phase activation step, in a fixed bed up-flow reactor, under

hydrogen pressure. The liquid charge is composed by SRGO spiked

with DMDS.

The general scheme of the unit is presented in Figure 4.

Figure 4-Schematic description of the T58 unit

The unit can be divided in several sections, namely the charge, gas,

reaction, separation, pressure and measurement sections.

Online analysis system

The gas phase from is analyzed online by a gas chromatography

mass spectrum Balzers Pfeiffer, model TSU065D. This apparatus allow

us to follow in time the formation of different products, namely the

products that result from the DMDS decomposition (Figure 1):

CH3SH, C2H6S, CH4 and H2S.

Reactor charge and pressure test

The reactor, with an intern diameter of 20 mm and 435 mm of

length, is loaded with 20 cm3 of catalyst. In order to accomplish a good

distribution of catalyst and to get a homogeneous dispersion of the

heat, the catalytic bed comprises 3 different zones divided by two

grids:

•Zone 1: Glass beads with a diameter of 5 mm (to filtrate little particles

and to improve the liquid distribution) and silicon carbide (also known

as carbondurum, SiC) with a grain of 1,68 mm to ensure the good

thermal diffusion;

•Zone 2: 20 cm3 of catalyst;

•Zone 3: SiC with a grain of 1,68 mm plus glass beads with a diameter

of 5 mm.

After the charging and installation of the reactor, a pressure test

needs to be made, in order to verify that no leaks are remaining in the

unit. This test is made at least at 10% above the pressure of the

sulfidation test being valid if the pressure drop is lower than 0,5 bar/h.

Sulfidation process

The two series of catalysts, CoMoP and NiMoP, were sulfided in

the liquid phase in the unit T58. The liquid charge consisted in Straight

Run Gasoil (SRGO) with 2% in mass of DMDS, being the main

characteristics of the gasoil presented in Table 2:

Table 2- Important gasoil characteristics.

Sulfur (wt. ppm) 4189

Density (g/cm3) 0,8491

The DMDS was used as spiking agent due its chemical

characteristics: low vapor pressure, high sulfur content, low

decomposition temperature and low cost when compared with

polysulfides. The DMDS decomposition (Figure 1) leads to the

formation of H2S, compound that sulfides the catalyst.

The procedure of the liquid activation used in the unit T58 is

described as a temperature profile in the Figure 5.

Figure 5- Temperature profile of each test.

3

Catalyst wetting at 50 °C

Sulfidation

SRGO+2%DMDS

Plateau 8 h

Washing with Toluene

(3h) + drying (1h, Patm)

Each test comprises the catalyst wetting at 50°C during 1,5 hours

in order to assure a good contact between the solid and the liquid. This

step is followed by an increment of temperature at a rate of 8°C/h until

350°C, in which the temperature is maintained constant during 8 hours

in a plateau (sulfidation). Then, the catalyst is washed with an organic

solvent with low ebullition point (in this case, toluene), in order to

remove the charge that is still found in the catalyst bed. The washing

is carried out at 200°C during three hours. After that time, the toluene

injection is stopped and the drying process takes one hour at 200ºC and

atmospheric pressure. Finally, the reactor is unloaded in an inert

atmosphere in order to avoid the re-oxidation of the sulfide phase

(since X-Ray photoelectron spectroscopy, XPS, analyses are

performed to the sulfided catalysts).

The sulfidation conditions used in the unit were adapted from other

studies performed at IFPEN and are close to the industrial ones. The

Table 3 summarizes the operating conditions used in the sulfidation:

Table 3- Operational conditions of the sulfidation in the T58 unit.

Pressure (bar) 30 or 60

Temperature (°C) 50-350

Volume of catalyst (cm3) 20

LHSV (h-1) 2

H2/Hc (NL/L) 240

Fliquid (g/h) 34

Fgas (NL/h) 9,6

Heating ramp (°C/h) 8

The liquid and gas feeds (𝐹𝐿 and 𝐹𝐺 , respectively) were established

from the operational conditions by use of the calculations that follows:

𝐹𝐿 = 𝑉𝑐𝑎𝑡 × 𝐿𝐻𝑆𝑉 × 𝜌𝐿 𝐸𝑞 1

Being 𝑉𝑐𝑎𝑡 de volume of catalyst (cm3) used during the test, 𝐿𝑆𝐻𝑉

the liquid hourly space velocity (h 1) and 𝜌𝐿 the density of the liquid

feed (g/cm3).

𝐹𝐺 = (𝐻2/𝐻𝑐) × 𝐹𝐿 𝐸𝑞 2

Where (𝐻2/𝐻𝑐) is the volume ratio of hydrogen and liquid feed

(NL/L).

During the sulfidation process the DMDS decomposition is

followed by mass spectroscopy in line. This apparatus allow us to track

the decomposition products (see Figure 1) with the time (and indirectly

with the temperature), being possible to compare qualitatively the

sulfidation of different catalysts, at the same operational conditions.

Concerning the liquid phase analysis, samples were taken from time to

time during the increasing in temperature, in order to measure their

sulfur content by XRF (X-Ray fluorescence). These samples were

taken with 30 minutes (minimum) and 2h (maximum) of interval.

It is important to take into account that, once the unit is not

completely automatized, the switch from one step to another needs to

be made manually. For this reason, in order to complete a test

(comprising reactor loading, pressure test, wetting, sulfidation,

washing, drying and unloading) four days are necessary, thus meaning

it is only possible to make one test per week.

3. Results

3.1 In-situ liquid sulfidation study

CoMoP catalysts

In order to understand the difference between the CoMoP catalysts

dryed, calcined and boosted, concerning the activation procedure, each

DMDS decomposition product was analyzed separately. The signal

obtained from the GC mass spectrometer was normalized in order to

make the comparison easier and more accurate.

The evolution of the signal for the CH4 with time and temperature

is shown in the Figure 6.

Figure 6- Online analysis of CH4 for the three catalysts, with time and

temperature

The moment of appearance of the CH4 corresponds to the

beginning of the sulfidation, since the H2S appears simultaneously

with the methane, resulting both from the direct decomposition of the

CH3SH.

From the analysis of the Figure 6 it is possible to see that the dried

catalyst present the same trend as the calcined respecting to the

evolution of the signal of the methane (CH4). For both catalysts the

CH4 appears around 21 h (218ºC). However, for the boosted catalyst,

the CH4 curve appears latter, around 23 h (234ºC). It is clear that a gap

in the appearance of this product between the curves corresponding to

the calcined and boosted catalysts exists. This gap, of about 2 h (16ºC)

seems constant until the stabilization of the concentration of CH4.

In this way it is possible to conclude that a delay in the DMDS

decomposition is seen during the sulfidation of the boosted catalyst.

We can, in a first analysis, conclude that the presence of the additive

in the catalysts leads to a delay in the DMDS decomposition.

A similar analysis can be made for the H2S signal. Figure 7 shows

the evolution of the signal with time and temperature for H2S.

Figure 7- Online analysis of H2S for the three catalysts with time and

temperature.

The appearance of the H2S for the calcined catalyst takes place at

around 25 h (T=250ºC), about 4 hours after the appearance of the CH4.

In respect to the presence of H2S (Figure 7), its behavior is similar

to the CH4 compound with the difference that the catalyst dried follows

the same trend as the boosted catalyst. In this way we can already take

∆≈2 h≈16°C

∆≈2 h≈16°C

∆≈0

4

one conclusion regarding the speed of the sulfidation for the CoMoP

dried. Since for this catalyst the CH4 appears earlier than for the

boosted, and that the H2S appears at the same time (27 h, T= 266ºC),

we may suggest that the dried catalyst is the one that takes more

time to sulfide, from the three catalysts.

Concerning the sulfidation process of the calcined and boosted

catalysts, a careful analysis should be made. On one side, we can notice

that it takes around the same time, for both catalysts to be sulfided since

the ∆t between the appearance of CH4 and H2S for each catalyst is of

around 4 h. This is the analysis that can be made to the so called low

temperature sulfidation, and assuming that the rate of sulfidation is the

same between catalysts. On the other side, both curves reach the

plateau at the same time, which may suggest that the rate of

sulfidation at high temperatures is higher for the boosted catalyst:

in the beginning of the curves the same gap as for CH4 maintains (2h

corresponding to 16ºC), however this gap tends to decrease until

disappears completely before the beginning of the temperature plateau.

We may conclude that the boosted catalyst sulfides faster than the

other catalysts, assumption that can be confirmed if the degree of

sulfidation in the catalyst, after the activation step, is at least equal to

the calcined one.

It is also possible to follow, during the DMDS decomposition the

behavior of the signal of the intermediate products, namely, the CH3SH

and the C2H6S. Figure 8 presents the variation of the signal of the

CH3SH with the time and temperature.

Figure 8- Online analysis of CH3SH for the three catalysts, with time

and temperature.

For all the CoMoP catalysts, it is not very clear due to the noisy

signals obtained due to stability problems of the GC mass

spectrometer, but in trend, all the curves appear at the same time,

meaning that no special effect is seen in the presence of each kind of

catalyst. The CH3SH is present in the gas phase starting around 18 h

(T=195°C), value that is in accordance with the described in literature

in similar conditions. It is also possible to verify that the maximum of

the curves (that correspond to the maximum concentration) appears at

around 240°C-250°C, temperature at which this compound apparently

starts to decompose in other sub-products.

In respect to the C2H6S, its evolution with the time and temperature

in the presence of the three CoMoP catalysts is depicted in Figure 9.

Figure 9- Online analysis of C2H6S for the three catalysts, with time

and temperature.

A similar analysis to the previous one may be made for this

compound. The moment at which the C2H6S appears, for the three

catalysts, seems to be the same. However, a gap between the maximum

concentration of the compounds seems to exist, being the biggest

difference registered between the CoMoP calcined and boosted

(2h≈16ºC). Another conclusion can be taken: the relative

concentration of the C2H6S is always lower than the relative

concentration of the CH3SH, leading to the conclusion that the

most significant part of the H2S is obtain directly by the direct

decomposition of the CH3SH.

In this way, from the analysis of the Figure 8 and Figure 9, it is

possible to confirm that the order of appearance of the species, in the

presence of catalyst, is the one described in the literature. Furthermore

we can conclude that the DMDS decomposition is complete during the

sulfidation procedure since in the end, all the intermediates present

signals practically zero, since they decompose completely in order to

form CH4 and H2S.

In order to have more information about the sulfidation process for

each catalyst, liquid samples were taken during the sulfidation process.

The content in sulfur was then measured by XRF. This analysis

complements the gas-phase analysis, making possible to take more

accurate conclusions about the process.

The content in sulfur in the liquid samples for the three catalysts is

present in the Figure 10.

Figure 10- Sulfur analysis of the liquid effluent from the T58 unit, for

the three catalysts.

During the sulfidation, as expected, the content of sulfur in the

liquid decreases with the time, since the DMDS decomposition leads

to the formation of products in the gas phase. It is observed that at low

temperatures (T<260°C) the content in sulfur, when the boosted

catalyst is present, is always higher than for the others. This data

confirms the gas phase analysis, since a delay in the decomposition of

Gas oil initial sulfur content

5

DMDS by the boosted catalyst leads necessarily to a higher

concentration of sulfur in the liquid phase.

Additionally, by the end of the 8 hours plateau of temperature at

350ºC, the sulfur concentration in the liquid is the same for all the

catalysts around 0,2 %wt. This value confirms that the DMDS is

completely decomposed since it is lower than the initial content of

sulfur found in the SRGO. In the end of the activation test, the gasoil

is slightly desulfurized. However, in order to confirm these

conclusions, G.C. analysis should be made to the liquid effluent in

order to know the compounds present in the liquid effluent.

NiMoP catalysts

A similar study to the one presented before was performed for the

NiMoP/Al2O3 catalysts. The Figure 11 represents the evolution with

time and temperature of the products that result from the DMDS

decomposition for the NiMoP calcined.

Figure 11- Results from the mass spectrum, DMDS decomposition

with time, from the start of the heating ramp (8°C/h) until the end of

the 8 h plateau, for the NiMoP calcined.

Despite the fact that the DMDS decomposition show the same

behavior for all the catalysts, the trend of the curves is very different

from the one found for the CoMoP. The main differences that it is

possible to highlight are the fact that none of the intermediates

(CH3SH and C2H6S) exhibit a maximum in their concentration and

the H2S does not stabilize even after reaching the temperature

plateau.

Taking a closer look to each one of the decomposition products,

the atypical behavior of the curves (when compared with the ones

obtained for the CoMoP series) is confirmed.

Figure 12- Online analysis for the three catalysts (calcined in blue,

boosted in red and dried in green), with time and temperature. Left:

CH3SH right: DMS (C2H6S).

From Figure 12 (left) the first difference in relation with the

CoMoP (see Figure 8) is that when the curve reaches the maximum

seems to stabilize, instead of decreasing in benefit of the CH4 and H2S

appearance.

When following the C2H6S formation with time and temperature

(Figure 12, right), no large variation in the signal is seen and no clear

maximum exists, despite what is observed for the CoMoP (Figure 9).

In this way, from this data, it may be suggested that the

decomposition of the DMDS is not complete, since the results obtained

are not in accordance with the decomposition mechanism proposed in

the literature, for similar conditions.

Figure 13- Online analysis of the CH4 and CH3SH for the NiMo

calcined.

It is clear, from de comparison between the signals of the CH3SH

and CH4 with time and temperature that they present the same trend,

stabilizing in the beginning of the temperature plateau, around the

same time.

This information, may suggest an alteration in the mechanism of

DMDS decomposition (Figure 14). In accordance with the literature

(3), the DMS route represents a minor percentage of the CH4 and H2S

formed in the process. In our working conditions in the presence of the

NiMoP catalysts, the DMS route is almost negligible, leading to an

equilibrium between the formation of CH3SH and the methane and

H2S.

Figure 14- Modifications proposed in the mechanism of DMDS

decomposition, in the presence of NiMoP catalysts, in our working

conditions

In respect to the sulfidation of the catalysts the behavior of the

methane was studied (Figure 15), in order to know at which

temperature the H2S would start to be provided to the catalyst, on other

words, when the sulfidation process starts.

Figure 15- Online analysis of CH4 for the three catalysts, with time

and temperature.

6

End of the 8 h plateau

(T=350°C)

The effect of delay in the DMDS decomposition caused by the

presence of the additive in the boosted catalysts is once again

confirmed. In respect to the calcined catalyst, this seems to be the one

with the slowest decomposition of the spiking agent, since the CH4

appears before the boosted catalyst and stabilizes in the beginning of

the plateau after it. Furthermore, regarding to the temperature of

appearance, and like in the CoMoP series, the boosted catalyst present

the highest temperature of appearance (T=246°C, 24,5 h ), followed by

the calcined (T=211°C, 20 h) and finally by the dried (194°C, 18h).

In Figure 16 H2S curve is presented, for the three NiMoP catalysts

with the time and temperature.

Figure 16- Online analysis of H2S for the three catalysts, with time

and temperature.

From the study of this figure, we may suggest that the sulfidation

process of the NiMoP catalysts, in the end of the 8 hours plateau,

seems to be incomplete. This observation is made since no

stabilization of the curve is seen, suggesting that, if the CH4 is already

stabilized, the difference in H2S concentration in time is due to the fact

that sulfur changing is still occurring. This analysis should be made

carefully, since the HDS of the gasoil can also lead to an increase of

the H2S formed.

Furthermore, the appearance of H2S seems to occur at the same

time for the three catalysts, thus suggesting that the boosted catalyst

sulfides quickly, at least at lower temperatures. However, in a first

analysis, no conclusion can be taken for the sulfidation process since

the hydrogen disulfide does not reach a plateau (in the end of the test,

we cannot assume that the quantity of sulfur incorporated in the

catalyst is the same for the three catalysts, being necessary to perform

their characterization).

In Figure 17 is represented the sulfur content in the liquid effluent,

with time and temperature, for the three catalysts.

Figure 17- Sulfur analysis of the liquid effluent from the T58 unit, for

the three catalysts.

From the liquid analysis one of the conclusions that can be made

is the same that the one for the CoMoP catalysts: the content in sulfur

is always higher for the boosted catalysts at low temperatures

(T<260°C), even though for all the catalysts the trend is the same. In

addition, another interesting observation can be drawn: in respect to

the DMDS decomposition, in the presence of the NiMoP catalysts,

after the 8 hours of plateau, the content in sulfur is lower than the initial

one in the gasoil. In this way an important conclusion can be taken: the

DMDS is probably completely decomposed, being the effect proposed

in its mechanism confirmed. Furthermore, the final content of sulfur

found in the gasoil in the end of the test, for the NiMoP and CoMoP is

practically the same. This supports the assumption that after 45 hours

of sulfidation, the NiMoP catalysts are still changing sulfur with the

feed, being the constant increase of the H2S curve due to this fact and

not to the HDS of the gasoil.

Since in the present working conditions, it seems that the NiMoP

catalysts are not completely sulfided, a test in different conditions was

made. In this way a test at total pressure equal to 60 bar, remaining the

other parameters constants, was performed. The catalyst used was the

NiMoP calcined.

However it was observed that in the end of the plateau of 8 h at

350ºC, the H2S curve was not stabilized despite the stabilization of the

CH4 (indicating that the sulfidation process was still running). In this

way, the duration of the plateau at 350ºC was extended 24 h.

Figure 18- Online analysis of H2S for the three catalysts, with time

and temperature.

From the H2S curve (Figure 18), we confirm that at 60 bar of total

pressure, the catalysts do not seem sulfided in the end of the 8 h, since

no stabilization of the signal was detected. In the end of 32 h of plateau,

the H2S curve seems to start to stabilize, however this tendency is not

very clear.

In this way it is concluded that no pressure and time effect on the

sulfidation of the NiMoP catalysts seems to exist.

End of the 8 h plateau

(T=350°C)

7

3.2 Catalysts characterization

The spent catalysts were analyzed making use of the elemental

analysis CHNS and also the XPS surface technique.

The Table 4 present the content in carbon, hydrogen and nitrogen

present in the active catalysts, from the CHNS for the CoMoP and

NiMoP catalysts (dried, calcined and boosted) subject to the liquid

sulfidation in the T58.

Table 4- CHNS results for the CoMoP and NiMoP catalysts, after the

sulfidation in the unit T58.

(%wt) C ±0,4 H ±0,2 N ±0,1 GSDT

(%)

CoMoP

Dried 3,2 1,2 0,4 88

Calcined 2,4 1,0 0,3 76

Boosted 3,7 1,0 0,3 92

NiMoP

Dried 2,7 1,1 0,3 88

Calcined 2,6 0,9 0,3 88

Boosted 3,5 1,1 0,3 92

The content in nitrogen is equal between catalysts and relatively

low, as expected, since the activation was made with SRGO, gasoil

poor in this component.

Table 4 also reports the value of the global sulfidation degree

(GSDT). From the results obtained, two important remarks should be

made:

Firstly, it is clear that, for both catalyst series, the ones that present

a higher global sulfidation degree, thus are probably better

sulfided are the boosted ones, with a value of 92%. In a first

approach is expected that a catalyst with a better sulfidation lead

to better activities.

Secondly, these CoMoP and NiMoP boosted catalysts present the

highest carbon content, suggesting that they are more susceptible

that the other catalysts to coke (we may suggest that some part of

the additive still rests in the catalyst, leading to higher C content).

In addition, for CoMoP and NiMoP catalysts the same tendency

is observed: the calcined catalyst is the one with the lowest carbon

content followed by the dried and by the boosted.

More important, the CoMoP dried and boosted, as well as all the

NiMoP catalysts, achieve higher values of degree of sulfidation when

comparing with the CoMoP calcined that presents a value of around

76%. Moreover, despite the fact that, during the liquid sulfidation of

the NiMoP catalysts, the H2S did not stabilize (indicating that the

sulfidation was not complete), all the catalysts from this step present

satisfactory values of global degree of sulfidation.

Globally, the results of the content of each species in the catalyst

surface obtained by XPS are the ones presents in Table 5. The results

are expressed in mass percentage with a maximal relative error of +/-

10%. These values are normalized relatively to the initial theoretical

content of metal in the oxide form present in the catalyst. It is important

to notice that the normalization was made by dividing the value

obtained by a constant, in order to be possible to do the comparisons

between catalysts.

Since the XPS is a surface technique, higher contents of each

compound detected by this method means that a lower stacking degree

exists and consequently a better dispersion.

Table 5-Results from the XPS analysis for the CoMoP and NiMoP

catalysts (+/- 10% maximum of relative error)

Mo Co Ni

CoMoP

Dried 0,56 0,71 0

Calcined 0,67 0,72 0

Boosted 0,72 1,06 0

NiMoP

Dried 0,60 0 0,90

Calcined 0,65 0 0,94

Boosted 0,68 0 0,97

For the CoMoP catalysts it is seen that the dried catalyst is the

one that presents a worst distribution of the active phase when

activated in the presence of the SRGO spiked with DMDS, since is the

one with lower content in Mo and Co detected. It can be assumed that

the calcined and boosted catalysts present the same Mo degree of

dispersion (0,67 and 0,72 for the CoMoP calcined and boosted

respectively). In respect to the Co degree of dispersion, the boosted

catalyst presents a value of 1,06, higher than the ones found for the

calcined (0,72) and dried (0,71).

The same analysis can be made for all the NiMoP catalysts with

the exception that, concerning the dispersion of the promoter nickel,

there are no differences between catalysts, showing that the dried,

calcined and boosted catalysts present practically the same promoter

dispersion.

On the other side, the boosted catalyst has a remarkably good

distribution of the active phase: the contents seen with the XPS

methods are very close of the global content in the catalyst (values in

Table 5 are near the unit), meaning that a good distribution of the

species was achieved with success.

Table 6 summarizes the results obtained by XPS, being possible to

compare the global degree of sulfidation (GDS), Mo degree of

sulfidation (MoDS) and degree of promotion (DP), defined by the

equations 3, 4 and 5, respectively. From these values it will be possible

to take more conclusions respecting the liquid sulfidation procedure

for all the catalysts.

𝐺𝐷𝑆(%) =𝐶𝑡𝑜𝑡𝑎𝑙 𝑠𝑢𝑙𝑓𝑢𝑟

(𝐶𝑡𝑜𝑡𝑎𝑙 𝐶𝑜 𝑂𝑅 𝑁𝑖 × 𝑥

𝑦+ 2 × 𝐶𝑡𝑜𝑡𝑎𝑙 𝑀𝑜 )

× 100 𝐸𝑞 3

𝑀𝑜𝐷𝑆(%) =𝐶 𝑀𝑜𝑆2

𝐶𝑡𝑜𝑡𝑎𝑙 𝑀𝑜× 100

𝐸𝑞 4

𝐷𝑃(%) =𝐶 𝐶𝑜𝑀𝑜𝑆 𝑂𝑅 𝑁𝑖𝑀𝑜𝑆 𝑝ℎ𝑎𝑠𝑒

𝐶𝑡𝑜𝑡𝑎𝑙 𝐶𝑜 𝑂𝑅 𝑁𝑖 × 100

𝐸𝑞 5

Where C is the concentration expressed in atomic % and 𝑥 and 𝑦

correspond respectively to 8 and 9 for the CoMoP and to 1 and 1 for

the NiMoP catalysts.

8

Table 6-Global degree of sulfidation, degree of sulfidation of the molybdenum and degree of promotion obtained by XPS for the CoMoP and NiMoP

sulfided at 30 bar during 8 hours.

MoDS(%) GDS (%) DP (%) Co(Ni)/Mo slabs

CoMoP

Dried

Calcined

Boosted

73

57

78

84

76

84

43

25

46

0,30

0,19

0,34

NiMoP

Dried

Calcined

Boosted

79

72

82

76

72

79

56

47

60

0,43

0,38

0,43

When comparing these results we can see that the boosted catalyst

is the one with better results during the sulfidation in the liquid phase,

in the present working conditions, since is the one which presents, not

only better dispersion of the species, but also presents the higher

degrees of sulfidation (with MoDS=78, GDS=84 and DP=46 for the

CoMoP series and MoDS=82, GDS=79 and DP=47 for the NiMoP).

Despite the first analysis made during the liquid sulfidation of the

NiMoP catalysts (where the H2S curve suggested that the sulfidation

process was not complete), when examining the degree of sulfidation

for these catalysts, the values are satisfactory. However, we may

suggest that higher values of degree of sulfidation can be achieved for

the NiMoP catalysts.

The calcined catalyst present values too different from the ones

expected, pointing out a poor sulfidation when comparing with the

other catalysts, in both series, being this difference more accentuated

for the CoMoP. This result may suggest that the sulfidation conditions

are not the optimal for the calcined catalyst, since the GDS does not

reach the same values as for the boosted and dried catalysts.

A ranking in terms of global degree of sulfidation/degree of

promotion can be established, for both series:

CoMoP: boosted=dried>>calcined

NiMoP: boosted≈dried>calcined

It is important to refer that, even if the dried catalysts have

systematically a better degree of sulfidation and promotion, the results

also indicate to what seems to be a worst dispersion of the metals in

the surface, when compared with the calcined. In this way, it is not

possible to affirm that the dry catalyst is more active, since both

dispersion and degree of sulfidation contribute to the catalyst activity.

Furthermore, activity tests should be made in order to take more

conclusions.

The results of XPS for both NiMoP calcined sulfided at different

pressures and different times on the 350ºC temperature plateau, are

summarized in Table 7:

Table 7- Global degree of sulfidation, degree of sulfidation of the

molybdenum and degree of promotion obtained by XPS.

NiMoP

calcined MoDS(%) GDS (%) DP (%)

Ni/Mo

slabs

P=30 bar, 8 h

P=60 bar, 32 h

72

73

72

74

47

51

0,38

0,38

As expected by following the CH4 and H2S curves during the liquid

sulfidation with SRGO spiked with DMDS, no difference is seen in the

sulfided catalysts, being possible to conclude that no pressure and time

effect were seen in the activation of the catalyst.

4. Conclusions

The main objective of this work was to study the liquid activation

of CoMo and NiMo catalysts in order to understand the differences in

activity of boosted, calcined and dried catalysts.

Two series of catalysts (CoMoP and NiMoP) were prepared with

the same content in molybdenum, the same Co(Ni)/Mo and P/Mo

ratios, being then divided in three parts, each one with a different final

treatment: drying, calcination and additivation. After the preparation,

the liquid sulfidation of the catalysts with SRGO+2%DMDS was

followed with time and temperature, through the DMDS

decomposition. Finally the catalysts were characterized by elemental

analysis CHNS and XPS.

From this study, it was possible to conclude that the boosted

catalysts is always the one which presents better global degree of

sulfidation, better degree of promotion and also better dispersion.

Being these parameters all positives we can conclude that probably this

catalyst is the most active in HDT, confirming the ranking of activities.

Furthermore, it was concluded that the presence of additive leads to the

delay of the DMDS decomposition, fact that may be related with the

better results for the boosted catalyst.

In contrast, the CoMoP/NiMoP calcined catalyst present, after the

liquid sulfidation, the worst values in terms of global degree of

sulfidation and degree of promotion when comparing with the other

catalysts, showing low values in these parameters. This may suggest

that higher values (as the ones found for the boosted and dried

catalysts) may be achieved in future works, by changing the

operational conditions.

Moreover, the dried catalyst presents, for CoMoP and NiMoP,

higher degree of sulfidation and promotion than the calcined one.

However, their dispersion is lower, not being possible to take

conclusions concerning the activities of these catalysts in HDT,

without catalytic tests. In this way it is not possible to confirm the

raking of activities for the dried and calcined catalysts without further

studies.

9

Finally, the effect of increasing the pressure and time in the

sulfidation of the NiMoP catalysts was studied. No change in the

sulfidation and dispersion in the catalyst was registered, being possible

to conclude that no effect was seen.

5. References

1. Hervé Toulhoat, Ed. e Pascal Raybaud, Ed., Catalysis by transition

metal sulphides, I,II et III, Paris, France: TECHNIP, 2013.

2. Humblot, Francis e Srinivas, Vijay. Activation of hydroprocessing

catalysts: An in depth understanding of dymethyldisulfide (DMDS)

decomposition chemistry of hydroprocessing catalysts during

activation via sulfiding.

3. Texier, Samuel, et al., Activation of an alumina-supported

hydrotreating catalysts by organosulfides or H2S: Effect of the H2S

partial pressure used during the activation process, Applied catalysis

A: general, 293 (2005) 105-119

4. Texier, Samuel, et al., Activation of alumina-supported

hydrotreating catalysts by organosulfides: comparison with H2S and

effect of different solvents, Journal of catalysis, 223 (2004) 404-418.

5. Pashigreva, A, et al., Activity and sulfidation behaviour of the

CoMo/Al2O3 hydrotreating catalyst: The effect of drying conditions,

Catalysis Today, 149 (2010) 19-27.

10