Highly-active nickel phosphide hydrotreating catalysts prepared in situ using nickel hypophosphite precursors

Abstract

An in situ method for the preparation of nickel phosphide (Ni 2 P) on silica, alumina, and amorphous silica-alumina (ASA) supports is described. The synthesis avoids the use of nickel and phosphorus salts by employing the reaction between nickel hydroxide (Ni(OH) 2) and hyphosphorus acid (H 3 PO 2), allowing the impregnation of nickel hypophosphite (Ni(H 2 PO 2) 2) onto the oxide supports in the absence of salt byproducts. Temperature-programmed reduction (TPR) in flowing hydrogen at 573-773 K yields phase pure Ni 2 P on the supports with small average particle sizes (3-4 nm) as measured using transmission electron microscopy. The conversion of Ni(H 2 PO 2) 2 to Ni 2 P and related reactions were probed using TPR with on-line mass spectral analysis of the gas effluent. Unsupported Ni(H 2 PO 2) 2 reacts in flowing hydrogen to produce PH 3 and H 2 O at 468 and 482 K, respectively; the reaction is shifted to increasingly higher temperatures for Ni(H 2 PO 2) 2 supported on SiO 2 , Al 2 O 3 and ASA. The hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) properties of the Ni 2 P catalysts were probed using a mixed feed containing carbazole and benzothiophene. While Ni 2 P/SiO 2 catalysts prepared by the different methods exhibited similar HDN and HDS activities, the in situ prepared Ni 2 P/Al 2 O 3 and Ni 2 P/ASA catalysts were substantially more active than their ex situ counterparts prepared from hypophosphite-and phosphate-based precursors.

Hydrodesulfurization Properties of Nickel Phosphide on Boron‐treated Alumina Supports

“…The TOFs of the optimized catalysts, 2.32 × 10 À 4 and 2.72 × 10 À 4 s À 1 for the Ni 2 P/0.8BÀ Al 2 O 3 -hypo and Ni 2 P/1.2BÀ Al 2 O 3 -phos catalysts, respectively, can be compared to silica-supported Ni 2 P prepared similarly and tested under the same reactor conditions. Drawing from data published previously, [12] 4,6-DMDBT HDS TOFs of 3.13 × 10 À 4 and 7.57 × 10 À 5 s À 1 can be calculated for boron-free Ni 2 P/SiO 2 -hypo and Ni 2 P/SiO 2 -phos catalysts at a reaction temperature of 573 K. This comparison reveals that B addition at optimal loadings to γ- ) than in hypophosphite (H 2 PO 2 À ), [2,11,[55][56] and the results of the current study on B-modified γ-Al 2 O 3 supports are consistent with these earlier findings. Thus, while weakened interactions of Ni and P with the alumina support in the Ni 2 P/xBÀ Al 2 O 3 catalysts enabled more complete reduction of the Ni 2 P, clear advantages remain for using hypophosphite-based precursors owing to the lower TPR temperatures used in the reduction of the precursors.…”

“…[38] For phosphorus, the strong interaction of impregnated P (in the form of phosphate) with γ-Al 2 O 3 was previously identified as an obstacle in preparing Ni 2 P/xBÀ Al 2 O 3 catalysts having high HDS and HDN activities, due to the need to use a large excess of P in the catalyst precursors and the tendency to form AlPO 4 at the support surface. [4,11] Infrared spectroscopy and CO 2 adsorption have been used to probe the interactions of phosphate (PO 4 3À ) with γ-Al 2 O 3 and the results of these studies provide insights for the current work. [52][53][54] Similar to borate, phosphate (in the form of H 3 PO 4 ) reacts with basic hydroxyl groups on alumina when impregnated onto γ-Al 2 O 3 ; it follows that the interaction of impregnated phosphate onto B-modified Al 2 O 3 supports will be weakened relative to B-free alumina.…”

“…Carbon monoxide chemisorption measurements were carried out using a Micromeritics AutoChem 2950HP instrument outfitted with a thermal conductivity detector, following procedures described previously. [11] Catalyst samples were degassed in 60 mL/min He for 30 min, heated to 673 K at 10 K/min in 60 mL/min H 2 and held for 1 h in continued H 2 flow. The sample was then purged in 60 mL/min He for 1 h at 673 K, cooled to room temperature in continued He flow.…”

“…[1][2] The most widely employed method for preparing metal phosphide catalysts, either in bulk or oxide-supported form, is temperatureprogrammed reduction (TPR) in flowing hydrogen of phosphate-based precursors prepared from solutions of metal and phosphate salts (e. g. Ni(NO 3 ) 2 + NH 4 H 2 PO 4 ) either evaporated to dryness or impregnated onto the support material. [2] In the case of Ni 2 P, the highest hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities have been achieved when the phosphide phase is supported on silica (SiO 2 ), [3][4][5][6][7][8][9][10][11][12] although high activities have been achieved on Al-containing oxide supports when hypophosphite (H 2 PO 2 À ) or phosphite (H 2 PO 3 À ) is used in place of phosphate (PO 4 3À ) as the P source. [11,13] Alternatively, phosphine (PH 3 ) can be used as a gas phase phosphiding reagent, [14][15][16] but the high toxicity of PH 3 makes it unattractive for the large scale synthesis of metal phosphide catalysts.…”

Effects of calcination and reduction temperature on the properties of Ni-P/SiO2 and Ni-P/Al2O3 and their hydrodenitrogenation performance

“…Nevertheless, nickel phosphide catalysts supported on SiO 2 and Al 2 O 3 exhibit very different catalytic activity in HDN and HDS. 9 Moreover, when Al 2 O 3 is used as a support, more phosphide is used to produce Ni 2 P because phosphide reacts with aluminum to form AlPO 4 .…”