Catalysis Communications 5 (2004) 79–82

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Comparison of vanadium carbide and nitridecatalysts for hydrotreating

Patrick Rodr�ııguez a,b, Joaqu�ıın L. Brito a, Alberto Albornoz a, Mary Labad�ıı a,Carolina Pfaff b, Santiago Marrero b, Delf�ıın Moronta b, Paulino Betancourt b,*

a Laboratorio de Fisicoqu�ıımica de Superficies, Centro de Qu�ıımica, Instituto Venezolano de Investigaciones Cient�ııficas (IVIC) Apartado 21827,

Caracas 1020-A, Venezuelab Universidad Central de Venezuela, Escuela de Qu�ıımica Facultad de Ciencias, Centro de Cat�aalisis, Petr�ooleo y Petroqu�ıımica,

Los Chaguaramos, Caracas AP 47102, Venezuela

Received 15 July 2003; accepted 20 November 2003

Published online: 30 December 2003

Abstract

Vanadium nitride and carbide were synthesized by the temperature-programmed reaction of ammonium vanadate (NH4VO3)

with pure NH3 and 20% CH4 in H2, respectively. Based on the XRD results, the catalysts showed VN and V8C7/V4C3 with an

amount of V2O3 in the bulk after nitridation and carburization. The hydrogenation, hydrodesulfurization and hydrodenitrogen-

ation reactions were studied and compared to vanadium sulfide catalyst. These catalysts (VC, VN) are stable under typical hy-

droprocessing conditions.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Vanadium nitride; Vanadium carbide; Vanadium sulfide; Hydrotreating

1. Introduction

Hydrotreating of distillate fuels, is receiving consid-

erable attention because of the increasingly more strin-

gent environmental regulations on the composition of

transportation fuel [1–7]. Therefore, research on the

development of new catalysts, which more efficientlyremove heteroatoms (S, N, O) from the hydrocarbon

feeds, is growing in importance. Most work so far has

been carried out with sulfides like Ni–Mo–S and Co–

Mo–S. On the other hand, transition metal carbides and

nitrides have shown excellent potential for use in hyd-

rodenitrogenation (HDN) [8–13] and hydrodesulfuriza-

tion (HDS) [8–10,12,14–16] reactions. Since the

discovery by Boudart and coworkers [17] that early

* Corresponding author. Tel.: +58-212-6051-649; fax: +58-212-6051-

220.

E-mail address: pbetanco@strix.ciens.ucv.ve (P. Betancourt).

1566-7367/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2003.11.011

transition metal carbides and nitrides can be produced

with high surface areas, there has been substantial in-

terest in their use as catalysts. These materials possess

catalytic properties that resemble those of the Pt-group

metals [17], and are among the most active hydrotreat-

ing catalysts known [18]. Several studies using transition

metal carbides and nitrides in hydroprocessing haveappeared. However, most of the work has concentrated

on molybdenum nitride and carbide catalysts. It was

found that these catalysts, both supported and unsup-

ported, were more active than a commercial catalyst in

hydrotreating [18]. Bulk vanadium sulfides, have been

shown to be efficient catalysts HDS, HDN and HYD of

aromatic molecules [19–21], but comparatively much

less work has been reported on the hydrotreatingproperties of the carbides and nitrides of this metal. In

the present investigation we describe the surface, bulk

and catalytic properties of vanadium carbide, nitride

and sulfide.

Table 1

Structural properties

Catalysts Crystalline phases Surface area (m2 g�1)

Fresh Spent

VC V4C3 and/or

V8C7 +V2O3

14 12

VN VN+V2O3 53 57

VS V3S4 44 16

80 P. Rodr�ııguez et al. / Catalysis Communications 5 (2004) 79–82

2. Experimental

2.1. Materials

Ammonium vanadate (Merck, 99.95%) was used as

precursor for the preparation of catalysts. The gases

employed were H2 (AGA, 99.99%), N2 (BOC, 99%),

H2S (Matheson, cp), CH4 (Matheson, 99%) and NH3

(BOC, 99.998%). For the reactivity test, the chemicals

employed were: thiophene (Aldrich, 99.9%), pyridine

(Aldrich, 99.9%), toluene (Fluka, 99.5%), heptane (Al-drich, >99.5%), dimethyl disulfide (Aldrich, 99%). All

chemicals were used as received. H2 and N2 for reac-

tivity experiments were passed through water/oxygen-

removing purifier cartridges.

2.2. Synthesis and characterization

Vanadium carbide or nitride were synthesized viatemperature-programmed reduction of NH4VO3 with

either 20% CH4 in H2 or pure NH3. A typical synthesis

consisted of loading 2 g of the vanadate precursor in a

quartz boat placed in a tubular furnace. The synthesis

was carried out in two stages. In the first, the temperature

of the reactor was increased to 523 K at 10 K min�1. In

the second stage, the temperature was raised linearly at a

heating rate of 5Kmin�1 to the final temperature (1253Kfor VC or 1148 K for VN) and held at that temperature

for a given time (20 min for VC and 0 min for VN). Once

the reaction was completed, the gas flow was switched to

nitrogen (0.5% O2) in order to passivate the sample.

Synthesis of vanadium sulfide was carried out as re-

ported elsewhere [21].

Surface area determinations by nitrogen adsorption

using BET were carried out in a Micromeritics ASAP2010 instrument. As pre-treatment, 100 mg of catalyst

were placed in quartz tube and evacuated for 1 h at

150 �C. The experimental error in the surface area

measures is ca. 10%. X-ray diffraction (XRD) mea-

surements were made using a Siemens D5000 X-ray

diffractometer with monochromatic radiation Cu Ka(k ¼ 1:5418 �AA) in step scanning mode in the range

5� < 2h < 90�. Qualitative phase analysis was carriedout using the Siemens Diffrac AT software package.

EPR spectra have been measured at room temperature

on a Varian E 104 A spectrometer operating a X band.

Varian pitch was used as reference for the calibration of

g values. XPS (VG 220i-XL, Mg source) analysis was

performed to study the surface composition of the fresh

catalysts. The C 1s peak at 285 eV was taken as the

reference for the binding energy.

2.3. Activity

The activity measurements in HDS, HYD and HDN

reactions were accomplished in a fixed-bed reactor

working at atmospheric pressure at 350 �C, with 0.5 g

catalyst. The catalysts were sulphided in situ at 350 �C.The reaction products were identified by GC-MS and

the results of the identification were confirmed by in-

jection of standard compounds. Only steady-state ac-tivity results are reported. The absence of any diffusional

effects was experimentally verified by showing that

similar conversions, as a function of contact time, were

obtained for two different weights of catalysts.

3. Results and discussion

Table 1 reports specific surface areas (SSA) measured

before and after catalytic tests and the crystalline phases

detected by XRD. The SSA were effectively unchanged

by catalytic reaction and demonstrate that VC and VN

catalysts are stable under reaction conditions. The fresh

vanadium sulphide catalyst had a BET surface area of

44 m2 g�1, which after HDS reaction decreases to 16 m2

g�1, implying a morphological change. Fig. 1 shows theXRD patterns of the three solids, vanadium sulphide,

nitride and carbide. The results of XRD analysis are

summarized in Table 1. The XRD patterns of the solids

prepared by temperature-programmed reaction indicate

that the compounds are not pure carbide or nitride

phases, with oxide impurities in the bulk (V2O3). These

solids were identified by a pattern search in the JCPDF

files.The atomic composition of pure vanadium sulphide is

V0:75S. Its XRD pattern shows intense peaks

(2h : 15:60; 35:32 and 45.14) corresponding to V3S4phase.

The EPR spectra of the carbide and nitride catalysts

showed only the signals corresponding to vanadium

oxide (V2O3) with a g value of 1.92. For the VS solid a

strong and symmetrical signal corresponding to vana-dium sulphide ðg ¼ 1:955Þ was observed. The broad

signal is due to dipolar coupling arising from the strong

interaction of near-neighbour vanadium atoms. These

results are supported by our XRD studies.

XPS measurements were made of the V 3d, S 2p, C

1s, and O 1s signals for the different catalysts. Table 2

presents these results before and after reaction. Carbon

(C 1s 285.0 eV) was taken as reference in all the analysis.The high carbon and oxygen contents in the fresh

Fig. 1. XRD patterns of (a) VS, (b) VN and (c) VC. j VN; m V8C7; d V2O3.

Table 2

Atomic concentration ratios of fresh and spent catalysts obtained by

XPS

Catalysts Fresh Spent

C/V O/V N/V S/V C/V O/V N/V S/V

VC 1.26 1.32 – 0.0 1.57 1.36 – 0.10

VN – 0.91 0.73 0.0 1.90 0.82 0.34 0.03

VS 0.00 1.41 – 0.84 1.28 0.10 – 1.35

P. Rodr�ııguez et al. / Catalysis Communications 5 (2004) 79–82 81

samples are due to their exposure to the ambient before

analysis. The C/V ratios in the passivated carbides in-

dicate the presence of excess free carbon on the surface.

The oxygen content on the surface is also high due to the

incorporation of oxygen during the passivation process.

The N/V ratio of vanadium nitride shows a slight defi-

ciency of nitrogen with respect to the VN stoichiometry.

This could be due to the presence of some nitrogen va-

cancies filled by oxygen or carbon. The carbon andoxygen contamination is typical for compounds exposed

to the atmosphere [22]. X-ray photoelectron spectro-

scopic analysis of the catalysts after the HDS reaction

revealed an increase in carbon in all samples, indicating

that carbon was deposited onto the catalyst surface

during the reaction (Table 2). There are only small

amounts of sulphur on the spent catalysts surface, so the

VS VN VC0

10

20

30

40

50

60

70

80

90

100C

onve

rsio

n / %

Catalyst

HYD HDS HDN

Fig. 2. Comparison of toluene hydrogenation, thiophene HDS and

pyridine HDN conversions of VS, VN and VC.

82 P. Rodr�ııguez et al. / Catalysis Communications 5 (2004) 79–82

catalysts are tolerant to sulfidation. This small amount

of sulphur may be helpful for hydrotreating reactions, as

it can participate slightly in carbon-heteroatom scission.

An interesting finding was that the sulphur on the VCsurface was predominantly in the form of sulphate

(169.1 eV), on the other hand, in the case of VN, the

sulphide (162.2 eV) phase was observed.

A comparison of the HDS, HYD and HDN con-

versions of VC and VN compared to VS is presented in

Fig. 2. The three studied catalysts exhibited similar re-

sults toward toluene hydrogenation. This result is sur-

prising, due to the structural differences of each solid.However, is well-known that vanadium sulphide (V3S4)

shows a high activity in aromatic compounds hydroge-

nation [21]. In the thiophene HDS reaction an impor-

tant conversion is observed for the VC and VN

catalysts. The conversion is three times superior to that

observed for vanadium sulphide.

In pyridine HDN, pentane was the major product

followed by pyperidine. For the HDN reaction vana-dium carbide showed to be the best catalyst, while the

VN catalyst presented a very low conversion. In this

regard, we remark that the kinetically important step in

HDN and HDS of many multiring heterocyclic com-

pounds is the hydrogenation of the heterocyclic ring.

Since the HDN reaction involves a dual-site consecutive

mechanism (hydrogenation and C–N bond breaking),

one possibility for explaining the higher selectivity forHDN would be that vanadium carbide could enhance

the hydrogenolysis of the pyridine by providing the

Brønsted acid sites where such reactions take place.

4. Conclusions

Well characterized catalysts of vanadium carbide/ni-tride were tested in hydrogenation, HDN and HDS of

model compounds, and compared with the corre-

sponding sulfide. The materials (VC, VN) exhibit similar

hydrogenation performance and are comparable with

vanadium sulfide. Vanadium carbide showed to be the

best catalyst for HDN, while the VN catalyst presented

a very low conversion. These catalysts are stable under

typical hydroprocessing conditions although a partialsulfidation of their surface during HDS cannot be

avoided.

Acknowledgements

This work was financially supported by FONACIT

(Project G-2000-1537).

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