WO1995011753A1

WO1995011753A1 - Hydrotreating catalyst and process

Info

Publication number: WO1995011753A1
Application number: PCT/EP1994/003543
Authority: WO
Inventors: Robert Charles Ryan
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1993-10-26
Filing date: 1994-10-26
Publication date: 1995-05-04
Anticipated expiration: 1996-04-26
Also published as: AU8105194A; EP0725680A1; NZ275799A; CA2174947A1; JPH09504228A

Abstract

Hydrotreating catalyst having improved hydrodesulphurization and hydrodenitrification activity comprising an alumina supported Group VIB and/or Group VIII metal-containing hydrogenating catalyst which has been impregnated with a liquid form of a silicon compound having general formula (I), wherein U, V, W, X, Y, and Z can indivdually be -R, -OR, -Cl, -Br, -SiH3, -COOR, -SiHnClm, R being either hydrogen, or an alkyl, cycloalkyl, aromatic, alkyl aromatic, alkylcycloalkyl radical having from 1 to 30 carbon atoms, 'n' and 'm' being whole numbers in the range of from 1 to 3 and 'a' being a whole number in the range of from 0 to 80, in an amount sufficient to deposit from 2.5 to 8.0 percent by weight of the total catalyst of Si, and subsequently has been calcined at a temperature ranging from 300 °C to 600 °C in an oxidizing atmosphere. Hydrotreating process using the above catalyst and process for improving hydrodesulphurization and hydrodenitrification activity of conventional alumina-supported Group VIB and/or Group VIII metal-containing hydrotreating catalysts.

Description

HYDROTREATING CATALYST AND PROCESS

The present invention relates to a hydrotreating catalyst, to a method for improving the hydrodesulphurization and hydro- denitrification activity of a hydrofining catalyst, and to a hydrotreating process. The sulphur-containing compounds and nitrogen-containing compounds found in petroleum fractions can cause a variety of adverse effects. For example, sulphur-containing compounds in fuels are known to adversely effect air qualit As a result, sulphur- containing compounds must be reduced in ^ .roleu fractions to a level within the air quality guidelines as set out by the various governmental instances. Nitrogen compounds can adversely affect the storage stability and octane value of naphthas and may poison also downstream catalysts. In addition, by removing nitrogen compounds, air quality is improved to some extent, since it lowers the potential for NO_χ formation during subsequent fuel combustion. Crude and other heavy petroleum fractions are typically subjected to hydrodesulphurization and hydrodenitrification in a hydrotreater to significantly reduce the sulphur and nitrogen compounds prior to being subjected to further processing. There has now been found a catalyst having improved hydrodesulphurization activity as well as hydrodenitrification activity. The more active catalyst can be operated at a lower temperature to obtain the same degree of sulphur and nitrogen conversion as a less active catalyst. A lower operating temperature will also prolong catalyst life and decrease operating expenses.

In the prior art several examples of modifications to catalysts are disclosed using silicon compounds as modifying agents. U.S. patent no. 4,038,337, for instance, discloses the treatment of alumina with silicon compounds to provide catalysts which are more active and selective for olefin isomerization. U.S. patents Nos. 4,013,589 and 4,013,590 and Reissue number 30,668 all disclose methods for improving the thermal and mechanical properties of alumina by treating it with silicon compounds. U.S. patent No. 3,089,845 discloses that the properties of naphtha catalysts are improved by treatment with a silicon compound such as tetraethyl orthosilicate. U.K. patent number 2,121,430 discloses also the treatment of isomerization catalysts by the treatment with silicon compounds such as ethyl orthosilicate.

The instant invention relates to a hydrotreating catalyst which comprises a Group VTB metal component and/or a Group VIII metal component on an alumina support, which catalyst has been impregnated with a liquid form of a silicon compound having the general formula

X U

I I

Y Si (OSi ) _aW

I I Z V

wherein U, V, W, X, Y, and Z can individually be -R, -OR, -Cl, -Br, -SiH , -COOR, -SiH_nCl_m, R being either hydrogen, or an alkyl, cycloalkyl, aromatic, alkyl aromatic, alkylcycloalkyl radical having from 1 to 30 carbon atoms, "n" and "m" being whole numbers in the range of from 1 to 3 and "a" being whole number in the range of from 0 to 80 in an amount sufficient to deposit from 2.5 to 8.0 percent by weight of the total catalyst of Si, and subsequently calcined at a temperature ranging from 300 °C to 600 °C in an oxidizing atmosphere. The instant catalysts have an enhanced hydrodesulphurization and hydrodenitrification activity.

The present invention also relates to a process for improving the hydrodesulphurization and hydrodenitrification activity of a hydrotreating catalyst which comprises a Group VTB and/or Group VIII metal component supported on an alumina support, which process comprises impregnating this catalyst with a liquid form of the above defined silicon compound and subsequently calcining the impregnated catalyst at a temperature ranging from 300 °C to 600 °C in an oxidizing atmosphere. The catalysts that are to be treated with the silicon- containing organo-compounds according to the method of the instant invention comprise Group VIB and/or Group VIII metals supported on an alumina support. Preferably, they comprise a Group VIB hydrogenating metal component selecte:, from nickel, cobalt and mixtures thereof and a Group VIII non-noble metal component selected from molybdenum, tungsten and mixtures supported on alumina. More preferably the catalysts comprise nickel and molybdenum supported on alumina or cobalt and molybdenum supported on alumina. The catalysts may optionally be promoted with phosphorous. The Group VIB and/or Group VIII metals present in the catalyst may be present in elemental form, as oxides, as sulphides or as a combination of two or more of these forms.

The metal-containing catalysts that are to be treated with silicon are catalysts that are known in the hydrocarbon hydroprocessing art. These catalysts are made in a conventional fashion as described in the prior art. For example, porous alumina pellets can be impregnated with solution(s) containing the Group VIB and/or Group VTII metal compounds and optionally phosphorous compounds, the pellets subsequently dried and calcined at elevated temperatures. Alternately, one or more of the components can be incorporated into an alumina powder by mulling, the mulled powder formed into pellets and calcined at elevated temperature. Combinations of impregnation and mulling can be utilized. Other suitable methods can be found in the prior art. Examples of catalyst preparative techniques can be found in U.S. patents

Nos. 2,510,189; 4,530,911; 4,520,128. The catalysts are typically formed into various sizes and shapes. They may be suitably shaped into particles, chunks, pieces, pellets, rings, spheres, wagon wheels, and polylobes, such as bilobes, trilobes and tetralobes. The metals-containing catalysts are impregnated with a liquid form of a silicon compound having the general formula

X U

I I Y Si—(OSi—)_a

I I Z V wherein U, V, W, X, Y, and Z can individually be -R, -OR, -Cl, -Br, -SiH3, -COOR, -SiH_nCl_m, R being either hydrogen, or an alkyl, cycloalkyl, aromatic, alkyl aromatic, alkylcycloalkyl radical having from 1 to 30 carbon atoms, "n" and "m" being whole numbers in the range of from 1 to 3 and "a" being a whole number in the range of from 0 to 80, preferably in the range of from 5 to 60, in an amount sufficient to deposit from 2.5 percent to 8.0 percent, preferably from 3.0 percent to 6.0 percent by weight of the total catalyst of Si, and subsequently calcining said impregnated catalyst at a temperature ranging from 300 °C to 600 °C in an oxidizing atmosphere. In a particularly preferred embodiment, the cobalt/molybdenum catalysts are impregnated with a liquid form of the aforementioned silicon compound wherein U, V, W, X, Y, and Z can individually be -R or -OR, R being either hydrogen, or an alkyl, cycloalkyl, alkylcycloalkyl radical having from 1 to 30 carbon atoms.

The silicon-containing compounds are used in the liquid form to impregnate the catalysts. The silicon compounds may be used neat when they are in liquid form and their viscosity is such that they can readily be impregnated. Viscosities of less than 100 centiStokes (cSt), measured at 40 °C are suitable, and viscosities of less than 75 cSt are preferred. The viscosity of the silicon compound is directly related to the value of "a" in the above formula. Listed below are the viscosities for silicon compounds of the above formula in which U, V, , X, Y, and Z are individually methyl and the corresponding value for "a" is as noted:

a Viscosity ( cSt )

0 0. 65

1 1. 0

2 1. 5

3 2. 0

5 3. 0

8 5 0

11 7 . 0

15 10. 0

25 20. 0

49 50. 0

79 100. 0

In order to facilitate impregnation, the silicon compounds may be dissolved in suitable organic solvents, such as lower alkanes, alcohols, ketones, aromatics and the like. Also, aqueous emulsions of the organo silicon compounds can be used. The silicone oils are particularly useful, either neat or diluted with an appropriate organic solvent or in aqueous emulsion form. These silicone oils are readily available commercially from various manufacturers, such Dow Corning, Aldrich Chemical Co. (e.g., Aldrich 14,615-3; Aldrich 17,563-3), Petrarch Systems (e.g., Silicone Antifoam aqueous emulsion with 11.8% Si; Petrarch PS039.5) and Union Carbide (e.g., L45 (350) ) . Preferred terminal groups for the silicone oils are trimethylsilyl groups. Silicone oils with t_* rminal hydroxy groups impregnate with difficulty, probably because of an interaction of the hydroxy group with the alumina support. In general terms the alumina supported metals-containing catalysts are impregnated with a liquid orga silicon compound and subsequently calcined in an oxidizing atmosphere in order to decompose the organo silicon compound to a silicon oxide. The oxidizing atmosphere is one that contains oxygen, and preferably is air. The air may be mixed with nitrogen during the initial stages of the calcination in order to prevent overheating of the catalyst as the silicone material oxidized. To obtain the benefits of the invention it is important that the silicon impregnation be carried out after the metals have been incorporated onto the carrier, rather than before. Thus, the silicon impregnation is applied to a finished catalyst comprising Group VTB and/or Group VIII metals supported on an alumina support.

As indicated, the preferred catalysts to be treated with the silicon compound described above comprise either cobalt and molybdenum or nickel and molybdenum supported on a porous alumina support, preferably comprising gamma alumina. It contains from 1 to 5, preferably from 2 to 4 percent by weight of cobalt or nickel (measured as the metal) and from 8 to 20, preferably from 12 to 16 percent by weight of molybdenum (measured as the metal) and, if present at all,' from 1 to 5, preferably from 2 to 4, percent by weight of phosphorous (calculated as the element) , all per total weight of the catalyst. The catalyst suitably has a surface area, as measured by the B.E.T. method (Brunauer et al, J. Am. Chem. Soc, 6_0, 309-16 (1938)) of greater than 120 m²/g and a water pore volume between 0.2 to 0.8, preferably between 0.4 to 0.7 ml/g.

The catalysts of the instant invention are normally presulphided prior to use. Typically, the catalysts are presulphided by heating in H2S/H2 atmosphere at elevated temperatures. For example, a suitable presulphiding regimen comprises heating the catalysts in a hydrogen sulphide/hydrogen atmosphere (5%v H2S/95%v H2) for about two hours at about 204 °C (400 °F) increasing the temperature to 316 °C (600 °F) and holding for 1 hour and finally increasing the temperature to 371 °C (700 °F) and holding for 2 hours. Other methods are also suitable for presulphiding and generally comprise heating the catalysts to elevated temperatures (e.g., 200 °C-500 °C) in the presence of hydrogen and a sulphur-containing material.

The instant invention also relates to a process for reducing the sulphur and nitrogen content of a hydrocarbon feedstock, i.e. a process for hydrogenating sulphur-containing and nitrogen-containing hydrocarbons present in a hydrocarbon feedstock, by contacting the feedstock with hydrogen in the presence of a catalyst as described hereinbefore at hydrotreating conditions, i.e., at conditions of temperature and pressure and amounts of added hydrogen such that significant quantities of sulphur-containing hydrocarbons and nitrogen-containing hydrocarbons are reacted with hydrogen to produce gaseous sulphur compounds and gaseous nitrogen compounds which are removed from the feedstock. The feedstock to be utilized is any crude or petroleum fraction containing in excess of 100 parts per million by weight (ppm) of sulphur in the form of sulphur-containing hydrocarbons and in excess of 20 parts per million by weight (ppm) of nitrogen in the form of nitrogen-containing hydrocarbons. Examples of suitable petroleum fractions include catalytically cracked light gas and heavy cracked oils, straight run heavy gas oils, light flash distillates, light cycle oils, vacuum gas oils, coker gas oil, synthetic gas oil and mixtures thereof. Hydrotreating conditions usually comprise temperatures ranging from 250 °C to 450 °C. The total pressure will typically range from 15 bar (200 psig) to 173 bar (2500 psig) . The hydrogen partial pressure will typically range from 8 bar (100 psig) to 153 bar (2200 psig) . The hydrogen feed rate will typically range from 36 to 1780 m³/m³ (from 200 to 10,000 standard cubic feet per barrel ("SCF/BBL") ) . The feedstock rate will typically have a liquid hourly space velocity ("LHSV") ranging from 0.1 to 15 (1/1.hr) .

The ranges and limitations provided in the instant specification and claims are those which are believed to particularly point out and distinctly claim the instant invention. It is, however, understood that other ranges and limitations that perform substantially the same function in substantially the same way to obtain the same or substantially the same result are intended to be within the scope of the instant invention as defined by the instant specification and claims. The invention will be described by the following examples which are provided for illustrative purposes and are not to be construed as limiting the invention. Example 1

A commercially available hydrotreating catalyst comprising cobalt and molybdenum on a gamma alumina support was used as a base catalyst. The base catalyst was dried at 480 °C (896 °F) for 0.5 hours and 195 grams were weighed into a plastic container and 21.0 grams of silicone fluid obtained from Aldrich (14,615-3) was used to impregnate the catalyst. After impregnation, the catalyst was then heated from 25 °C (77 °F) to 482 °C (900 °F) over a period of

30 minutes and then the catalyst was held at 482 °C (900 °F) for 1.5. hours. The catalyst was then cooled to room temperature in a desiccator. The catalyst contained about 3.7%wt of silicon, measured as the metal. This catalyst is denoted SPC-1 (Silicon- Promoted Catalyst #1) in the following examples. Example 2

A commercially available hydrotreating catalyst comprising cobalt and molybdenum supported en a gamma alumina support was used as a base catalyst. The base catalyst was dried at 399 °C (750 °F) for 2.0 hours and 100 grams were weighed into a plastic container and 10.94 grams of silicone fluid obtained from Dow Corning (Dow Corning 200 (5cSt) ) was used to impregnate the catalyst. After impregnation, the catalyst was then heated from 25 °C (77 °F) to 482 °C (900 °F) over a period of 30 minutes and then the catalyst was held at 482 °C (900 °F) for 1.5 hours. The catalyst was then cooled to room temperature in a desiccator. The catalyst contained about 3.6%wt of silicon, measured as the metal. This catalyst is denoted SPC-2 (Silicon-Promoted Catalyst #2) in the following examples. Example 3 A commercially available hydrotreating catalyst comprising cobalt and molybdenum supported on a gamma alumina support was used as a base catalyst. The base catalyst was dried at 399 °C (750 °F) for 1 hour and 100 grams were weighed into a plastic container and 4.98 grams of silicone fluid obtained from Dow Corning (Dow Corning 200 (5cSt) ) was used to impregnate the catalyst. After impregnation, the catalyst was then heated from 25 °C (77 °F) to

482 °C (900 °F) over a period of 30 minutes and then the catalyst was held at 482 °C (900 °F) for 2 hours. The catalyst was then cooled to room temperature in a desiccator. The catalyst contained about

1.8%wt of silicon, measured as the metal. This catalyst is denoted

SPC-3.

Example 4/Comparative Example 1

Three silicon-promoted catalysts (SPC-1, SPC-2 and SPC-3) were tested for their hydrodesulphurization activity and hydrodenitrific¬ ation activity on a diesel feedstock. Two of the catalysts, SPC-1 and SPC-2, illustrate specific embodiments of the present invention while SPC-3 illustrates a silicon-promoted catalyst which contains a less than optimum amount of silicon. For comparison, a base catalyst (no silicon added, catalyst denoted as CC-1 (comparative catalyst 1) was also tested. The properties of all catalysts are shown in Table 1 below.

TABLE 1: CATALYST PROPERTIES

CATALYST SPC-1 SPC-2 SPC-3 CC-1 Metals, %wt

Co 2.9 2.9 3.0 3.1

Mo 11.4 11.4 11.9 12.4

Si 3.7 3.6 1.8 -

CBD, glee 0.809 0.808 0.775 0.745

The catalyst testing was performed in pilot scaled microreactors using whole pellets. The catalysts were loaded and diluted with 60-80 mesh SiC to minimize feed channeling and to allow for uniform, isothermal operation of the reactor.

In this technique, each of 5 four ounce bottles are loaded with 18.2 grams of 60-80 mesh silicon carbide and 20% of the catalyst charge of 50cc is added to each bottle and loaded into the reactor. To determine the loading weight, the catalyst bulk density (CBD) was multiplied by 100.

The catalysts were sulfided with 5% hydrogen sulfide in hydrogen at a gas flow rate of 4.0 SCF/hr. Gas flow was established at room temperature and then the reactor temperature was increased to 204 °C for 2 hours, increased to 316 °C and held for 1 hour, and finally increased to 371°C and held for 2 hours. The following test conditions were used (LHSV = Liquid hourly space velocity) : LHSV : 1.0 hr^-1

Pressure 41 bar (600 psig) H2 Circulation 178 m³/m³ (1000 SCF/bbl) Temperature 363 °C (685 °F) For measuring hydrodesulphurization (HDS) activity and hydrodenitrification (HDN) activity, the catalysts were broken in at 363 °C (685 °F) for about 100 hours. Activities of the catalysts are shown in Table 2.

Properties of the feedstock utilized to illustrate the instant invention are also detailed in Table 2.

TABLE 2

CATALYST# SPC-1 SPC-2 SPC-3 CC-1 % Si 3.7%Si 3.6%Si 1.8%Si

TARGET

Temperature, °F 685 683 682.5 685.5 685

LHSV, hr-1 1 0.97 0.86 0.93 1.01

PRODUCT PROPERTIES FEEDSTOCK DENSITY 0.9032 0.8809 0.8808 0.8826

COMPOSITION HYDROGEN (%wt) 11.56 12.1855 12.17 12.1565 12.241

CARBON (%wt) 87.003 87.795 87.804 87.8985 87.615

OXYGEN (%wt) 0.17 0.04 0.02 0.04 0.04

SULPHUR (%wt) 1.216 0.03 0.0267 0.0393 0.041

NITROGEN (ppm) 916 268 194 248 406

Basic nitrogen (ppm) 319 18 10 24 101

HYDROGEN CONSUMPTION, SCF/bbl 369 362 344 415

TEMPERATURE REQUIRED FOR 400PPM S 671 670 689 686

OLEFINS Bromine number 10.9 0.7 0.93 0.54 1.05 (gm,Br/lOOgm)

FIA Analysis 0 0 0 0 0

AROMATICS FIA Analysis 59.7 47 50.3 46.4 50.2

COLOUR 2 2.5 3 4

CLOUD POINT 19 19 20 17

REFRACTIVE INDEX 1.5058 1.4915 1.4916 N/A 1.4907

CETANE NUMBER 34.9 35.9 N/A N/A 38.2

PRODUCT RECOVERY (%) 98.07 99.6 98.1 99.32 Example 5

As a base catalyst was used a commercially available hydrotreating catalyst comprising nickel, molybdenum and phosphorous supported on a gamma alumina support. The base catalyst was dried at 400°C for 1 hour and 201 grams were weighed into a 400 milliliter round bottom flask and 26.5 grams of silicone fluid obtained from Dow Corning (Dow Corning 200 (50cSt) ) was used to impregnate the catalyst. After impregnation, the catalyst was then heated from 121 °C (250°F) to 538°C (1000°F) over a period of 30 minutes and then the catalyst was held at 538°C (1000°F) for 2 hours. The catalyst was then cooled to room temperature in a desiccator. The catalyst contained about 3%wt of silicon, measured as the metal. This catalyst is denoted SPC-4.

SPC-4 was subsequently tested on an Arabian Heavy Flashed Distillate feedstock. For comparison, the base catalyst (no silicon) was also tested (CC-2) . The properties of the catalysts are shown in Table 3 below.

TABLE 3: CATALYST PROPERTIES

Properties of the feedstock utilized to illustrate the instant invention are detailed in Table 4 below. TABLE 4 : PROPERTIES OF ARABIAN HFD FEEDSTOCK

Physical Properties Density, 60°F, 70°C 0.9346, 0.8968 Viscosity (Cs ) , 60°C, 100°C 46.5, 11.7 Molecular t . 442

Elemental Content

Hydrogen 11.9380wt.%

Carbon 85.2970wt.%

Oxygen 0.1180wt.%

Nitrogen 0.1080wt.%

Sulphur 2.7300wt.%

Asphaltenes (wt.%)

0.07 ^c7 0.07

Carbon Residue (wt.%)

Ramsbottom 0.50 microcarbon 1.06

UV Aromatics Content (wt.%) MWR

Mono 4.7

Di 3.2

Tri 4.3

Tetra 4.4

Total 16.6

TABLE 4 (CONT'D): PROPERTIES OF ARABIAN HFD FEEDSTOCK

Boiling Point Distribution TBP- GLC

IBP 674 ^βE ¹ (357 °C)

10wt.% 764 (407)

30wt.% 837 (447)

50wt.% 892 (478)

70wt.% 945 (507)

86w .% 1000 (538)

The catalyst testing was performed in pilot scaled micro- eactors using whole pellets. The catalysts were loaded and sulphided in the same way as described in Example 4.

The following test conditions were used (WHSV = weight hourly spce velocity) :

WHSV 1.0 kg feed/1-catalyst-hr

Pressure 1725 psig (120 bar)

H2 Circulation 5000 SCF/bbl (1000 Nl/kg feed) For measuring hydrodenitrification (HDN) activity, the catalysts were broken in at 357oC for about 200 hours and then the temperature was increased to the temperature at which 7.5 ppm of nitrogen were found in the product. Catalysts with higher hydrodenitrification activities require lower temperatures.

It was found that for SPC-4 the temperature, at which 7.5 ppm of nitrogen were found in the product, was 383 °C (721 °F) , whereas for CC-2 this temperature was found to be 389 ^βC (733 °F) . From this it can be concluded that SPC-4 has a higher HDN activity than CC-2.

Claims

C A I M S

1. Hydrotreating catalyst comprising a Group VIB metal component and/or a Group VIII metal component supported on an alumina support, which catalyst has been impregnated with a liquid form of a silicon compound having the general formula

X U I I Y Si (OSi )_aW

I I Z V wherein U, V, W, X, Y, and Z can individually be -R, -OR, -Cl, -Br,

-SiH3, -COOR, -SiH_nCl_m, R being either hydrogen, or an alkyl, cycloalkyl, aromatic, alkyl aromatic, alkylcycloalkyl radical having from 1 to 30 carbon atoms, "n" and "m" being whole numbers in the range of from 1 to 3 and "a" being a whole numbers in the range of from 0 to 80, in an amount sufficient to deposit from 2.5 to 8.0 percent by weight of the total catalyst of Si, and subsequently has been calcined at a temperature ranging from 300 °C to 600 °C in an oxidizing atmosphere.

2. Catalyst according to claim 1, comprising a molybdenum component as the Group VIB metal component and/or a nickel or cobalt component as the Group VIII metal component.

3. Catalyst according to claim 2, comprising either molybdenum and nickel components or molybdenum and cobalt components.

4. Catalyst according to any one of the preceding claims, wherein U, V, W, X, Y and Z are individually hydrogen, methyl or ethyl.

5. Catalyst according to any one of the preceding claims, wherein "a" ranges from 5 to 60.

6. Catalyst according to any one of the preceding claims, wherein the silicon compound is in the form of a liquid having a viscosity of less than lOOcSt (measured at 40 °C) .

7. Catalyst according to any one of the preceding claims, wherein

8. Process for hydrogenating sulphur-containing and nitrogen- containing hydrocarbons in a hydrocarbon feedstock, which process comprises contacting at hydrofining conditions said feedstock with hydrogen in the presence of a catalyst according to any one of claims 1 to 7.

9. A method for improving the hydrodesulphurization activity and the hydrodenitrification activity of a hydrotreating catalyst which comprises a Group VIB metal component and/or a Group VIII metal component supported on an alumina support, said method comprising impregnating said hydrotreating catalyst with a liquid form of a silicon compound having the general formula

X U I I Y—Si (OSi—)_aW

I I Z V wherein U, V, W, X, Y, and Z can individually be -R, -OR, -Cl, -Br,

-SiH3, -COOR, -SiH_nCl_m, R being either hydrogen, or an alkyl, cycloalkyl, aromatic, alkyl aromatic, alkylcycloalkyl radical having from 1 to 30 carbon atoms, "n" and "m" being whole numbers in the range of from 1 to 3 and "a" being a whole number in the range of from 0 to 80, preferably 5 to 60, in an amount sufficient to deposit from 2.5 to 8.0, preferably 3.0 to 6.0 percent by weight of the total catalyst of Si, and subsequently calcining said impregnated catalyst at a temperature ranging from 300 °C to 600 °C in an oxidizing atmosphere.

10. The method of claim 9, wherein U, V, W, X, Y and Z are individually hydrogen, methyl or ethyl.

11. The method of claim 9 or 10, wherein the silicon compound is in the form of a liquid having a viscosity of less than lOOcSt

(measured at 40 °C) .

12. The method of any one of claims 9 to 11, wherein the hydrotreating catalyst comprises either cobalt and molybdenum or nickel and molybdenum.