Al2O3Supported Ni2P catalysts

Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) 9 2006 Elsevier B.V. All rights reserved

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Comparison of Structural Properties of SiO2, A!203, and C/A!203 Supported NiEP catalysts Yong-Kui Lee and S. Ted Oyama Environmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, Virginia Tech, Blacksburg, Virginia, 2406 l, USA This paper describes the catalytic activity of nickel phosphide supported on silica, alumina, and carbon-coated alumina in the hydrodesulfurization of 4,6-dimethyldibenzothiophene. The catalysts are made by the reduction of phosphate precursors. On the silica support the phosphate is reduced easily to form nickel phosphide with high catalytic activity, but on the alumina support interactions between the phosphate and the alumina hinder the reduction. The addition of a carbon overlayer on alumina decreases the interactions and leads to the formation of an active phosphide phase. 1. INTRODUCTION Metal phosphides are a novel class of catalysts for deep hydrotreating which have received much attention due to their high activity in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of petroleum feedstocks [ 1-6]. Previous studies of supported Ni2P catalysts were carried out mostly with SiO2 supports [1,2,4,5]. It is the objective of this work to investigate "r as a support and to compare the structural properties of the Ni2P with SiO2-supported samples. Particular attention will be placed on understanding the effect of carbon coatings on the 7-A120~ support and the catalytic and structural behavior, as the v-A1203 support is reported to inhibit the formation of Ni2P due to the high interaction with phosphorus [7,8]. The present study also includes the use of X-ray absorption fine structure (XAFS) spectroscopy to study the structure of the dispersed phosphide phases. 2. EXPERIMENTAL The supports used in this study were SiO2 (Cabot, Cab-O-Sil) of high surface area (EH5, 350 m 2g~) and 7-A1203 (AkzoNobel, 230 m 2g-~)and were used as received. A carbon-coated 7-A120~ support (CA1203) was prepared by pyrolysis of ethylene at high temperature. Ethylene was decomposed onto 10 g of the 7-A1203 sample at 973 K at a flowrate of 200 pmol s-~ The supported Ni2P catalysts were prepared on these supports with excess phosphorus (Ni/P-I/2) and a loading of 1.16 mmol Ni/g support (12.2 wt% Ni2P/ C-A120.~). Previous studies [4,5] had shown that this composition and loading level gave high activity and stability in hydroprocessing reactions. A sample prepared with the carbon-coated y-A120~ was denoted as Ni2P/C-A120~. The synthesis of the catalyst involved two steps [4,5]. In the first step, a supported nickel phosphate precursor was prepared by incipient wetness impregnation of a solution of nickel nitrate and ammonium phosphate, followed by calcination at 673 K. In the second step, the supported metal phosphate was reduced to a phosphide by temperatureprogrammed reduction (TPR). In catalyst preparation, larger batches using up to 5.50 g of supported nickel phosphate were prepared in a similar manner by reduction to 853 K, 1230 K, and 1873 K for Ni2P/Si02, Ni2P/A1203, and Ni2P/C-A120~, respectively. Sulfided Ni-Mo/AI203 (CR 424) was used as reference. The prepared catalysts were characterized by x-ray diffraction (XRD), N2 adsorption and CO chemisorption. Also, X-ray absorption spectroscopy (XAS) at the Ni K edge (8.333 keV) of reference and catalyst samples was carried out in the energy range 8.233 to 9.283 keV at beamline XI 8B of the

358

National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). Hydrotreating was carried out at 3.1 MPa (450 psig) and 613 K (340 ~ in a three-phase upflow fixed-bed reactor (Figure 1). The feed liquid was prepared by combining different quantities of 1.0 wt% tetralin (Aldrich, 99%), 0.02 wt% N as quinoline (Aldrich, 99%), 0.05 wt% S as 4,6dimethyldibenzothiophene (4,6-DMDBT, Fisher, 95%), 0.3 wt% S as dimethyldisulfide (DMDS, Aldrich, 99%), n-octane (Aldrich, 99%), and balance n-tridecane (Alfa Aesar, 99%). Liquid product compositions were determined with a Hewlett-Packard 5890A chromatograph equipped with a 50 m dimethylsiloxane column (Chrompack, CPSil 5B) of 0.32 mm i.d. Reaction products were identified by matching retention times with commercially available standards. In this paper, the HDS conversion is defined as

HDS Conversion(%) = lOOx (1-

MCHT + DMBCH + 3,3DMBP

)

4,6DMDBT,,

Vent

Feed U(:luid

~

Filter

Check Valve On-Off Valve

.,~

Pressure Regulator Pressure Gauge

Eked Sample

ff

A

UCluidPumD

M~ Thermocouple

/

Drain

9

[~

Rotameter

Y

Reserver

Figure 1. Experimental set-up for hydrotreating. 3. R E S U L T A N D D I S C U S S I O N

Figure 2 shows the TPR profiles of the calcined phosphate precursors of the Ni2P/SiO2, Ni2P/AI203, and Ni2P/C-A1203 catalysts. .,

1_

400

500

600

700 800 900 Temperature / K

1000

1100

1200

Fig. 2. Temperature-programmed reduction profiles for Ni2P/SiO2, Ni@/A1203, and Ni2P/C-A1203. The Ni2P/SiO2 and Ni2P/C-A1203 catalysts have distinct reduction peaks at around 850 K and 873 K respectively, while the Ni2P/A1203 has two broad reduction peaks at around of 810 and 1150 K. This indicates that the nature of the oxidic precursor of the Ni2P/AI20~ is different from that of the Ni2P/SiO2 or Ni2P/C-AI203. As will be shown in the EXAFS analysis below, the reduction of the oxidic precursor for the Ni2P/A1203 goes through Ni at a lower temperature of 853 K, followed by Ni2P

359 at a higher temperature 1230 K. Figure 3 shows the Fourier transforms of the Ni K-edge EXAFS spectra for the freshly prepared samples before (A) and after reduction (B) and bulk reference samples (C,D). For the oxidic precursor (A) of the Ni2P/Si02 catalyst the Fourier transform gives three main peaks centered at 0.09, 0.16, and 0.27 nm, respectively. These peaks are located at almost the same positions as those of the bulk nickel phosphate reference (C). Unlike the case of the Ni2P/SiO2, the oxidic precursor (A) of the Ni2P/AI~O~ gives two main peaks, a smaller peak at 0.16 nm and a larger peak at 0.25 nm, being similar in appearance to those of the bulk nickel oxide reference (C). This indicates the formation of the NiO phase during the impregnation and calcination step. This is related to the high interaction between the A1203 support and phosphorus, which results in the formation of an AIPO4 phase during the calcination [7,8]. For the oxidic precursor of the Ni2P/C-AI~O3 the Fourier transform gives three main peaks of which locations are almost identical to those of the bulk nickel phosphate reference (C). This suggests that the carbon coating readily reduce the interaction between the support and phosphorus. For the bulk Ni2P the Fourier transform (D) gives two main peaks, a smaller peak at 0.171 nm, and a larger peak at 0.228 nm, corresponding to the Ni-P and Ni-Ni bonding, respectively. For the Ni2P/SiO2 (B) there are also two main peaks corresponding to the Ni~P phase. For the Ni~P/AI~O~ a lower reduction temperature of 853 K led to the formation of Ni metal, while a higher temperature of 1230 K gave rise to a Ni2P phase. For the Ni~P/C-A120~ (B) there are two main peaks, again, corresponding to a Ni2P phase. It is thus likely that the carbon coating on the alumina support lessened the interaction between A1203 and P and inhibited the formation of nickel phosphate after calcination and also lowered the reduction temperature. Structural models of the supported Ni2P catalysts are shown in Figure 4.

A) Oxidic precursors

-

q I B) Reduced

samples

9

iO

0

1

2

3

4

5 0 1 2 Distance / 0.1 nm

3

4

5

6

Fig. 3. EXAFS analysis results for A) oxidic precursors before reduction, B) fresh samples after reduction, and C), D) bulk reference samples. Table 1 compares the physical properties of the catalyst samples and the catalytic activities in the HDS of 4,6-DMDBT at 613 K and 3.1 MPa. The catalytic activity for the catalyst samples in the HDS of 4,6-DMDBT followed the order, Ni2P/A1203 < Ni2P/C-AI20~ (-.- Ni-MoS/AI20~) < Ni2P/SiO: under 0.35 % S, 0.02 % N, and 1 % tetralin. The order correlated well with the amount of CO uptake. These results thus suggest that the HDS activity of the Ni2P catalysts highly depend on the dispersion of the Ni2P phase.

360

SiO2 ~

AI203

4

)

y

~NiO

~ . Ni2P

AIPO4

r

Ni metal

~

Ni2P

Impregnation/ TPR/843 K o ~ o TPR/1230 K Calc./673 K fCarbon _ Ni,(PO4)y / Ni2P y o ysis of ~ ethylene/973 K

preg ato ~ Calc./673K ~

TPR/873 ~ ~ " ~

Fig. 4. Proposed structure model for Ni2P/Si02.Ni~P/AI_,O~, and Ni2P/C-AI203. Table 1. Physical properties and catalytic activities of the catalyst samples Samples Ni2P/SiO2

CO uptake / pmol g-~

BET area m2 g

4,6-DMDBT HDS conversion b/ %

102 240 59 81 Ni2P/C-A1203 69 92 Ni-Mo-S/A120.s 287 a 155 a Atomic oxygen uptake, b Based on equal site loadings of 230 ~mol in the reactor

Ni2P/A1203

99 52 71 75

4. CONCLUSION Nickel phosphide (Ni2P) catalysts supported on SiO_~, A1203, and carbon-coated AI_~O~, were successfully prepared. The Ni2P/Si02 showed high HDS activity for 4,6-DMDBT compared to a commercial Ni-Mo-S/A1203 catalyst at 613 K (340 ~ and 3.1 MPa based on equal sites (230 ~mol) loaded in the reactor. Unlike the case of SiO2 support, the AI20~ support gave rise to strong interactions with phosphorus and formed AIPO~ and NiO during the initial synthesis steps of impregnation followed by calcination. Reduction to obtain the Ni2P phase required high temperature (1230 K). The carbon coating on the AI~O~ support, however, led to a considerable decrease in the interaction between A1203 and phosphorus, with the reduction temperature being lowered to 873 K. The EXAFS analysis also indicates that the oxidic precursor of nickel phosphate is readily formed on the carbon-coated A1203 support. The Ni2P/C-A1203catalyst also showed comparable HDS activity for 4,6-DMDBT with the Ni-Mo-S/A1203 catalyst. The HDS activity of the supported Ni2P catalysts was closely related to the dispersion of the Ni2P on the supports. Therefore, it will be of great interest to increase the dispersion of Ni2P phase on the carbon-coated A120~ support. ACKNOWLEDGMENT Support for this work came from the U.S.Department of Energy, Office of Basic Energy Sciences, through Grant DE-FG02-963414669 and Brookhaven National Laboratory under grant 3972.

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