The Effect of Calcination Temperature on the Properties and

Chapter 13

Downloaded by MONASH UNIV on June 14, 2013 | http://pubs.acs.org Publication Date (Web): June 11, 2013 | doi: 10.1021/bk-2013-1132.ch013

The Effect of Calcination Temperature on the Properties and Hydrodeoxygenation Activity of Ni2P Catalysts Prepared Using Citric Acid Victoria M. L. Whiffen and Kevin J. Smith* Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada *E-mail: [email protected]

The effect of calcination temperature on the preparation of unsupported high surface area Ni2P catalysts, synthesized by adding citric acid (CA) to an aqueous solution of nickel nitrate and diammonium hydrogen phosphate, is reported. The addition of CA led to increased surface area, decreased particle size, and increased CO uptake of the reduced Ni2P. However, increases in the Ni2P-CA calcination temperature from 773 to 823 and to 973 K led to a deterioration in the catalyst properties. All Ni2P catalysts deactivated following the hydrodeoxygenation (HDO) of 4-methylphenol (4-MP) at 623 K and 4.4 MPa. The deactivation was due to coking and was modeled by an exponential decay law. All Ni2P catalysts had similar deactivation parameters, indicating the loss in activity was due to C deposition on similar sites. The Ni2P-CA catalysts, with crystallite size in the range of 34–50 nm, had comparable initial TOFs, indicating that the HDO of 4-MP was structure insensitive over Ni2P catalysts of this size.

Introduction Pyrolysis oil derived from the fast pyrolysis of wood-waste is gaining attention as an alternative, renewable energy source. Compared with fossil fuels, pyrolysis oils generate far less green house gases, NOx, and SOx emissions (1). However, crude pyrolysis oils contain approximately 25 wt % moisture and 40–50 wt % O © 2013 American Chemical Society In Novel Materials for Catalysis and Fuels Processing; Bravo-Suárez, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


(2–4). The oxygen results in detrimental properties of the fuel such as instability, phase separation, high viscosity, low heating value, high acidity, and immiscibility with hydrocarbons (4, 5). The presence of oxygen is the primary reason for the observed differences in the properties of pyrolysis oil compared to fossil fuels. Therefore, the oxygen in pyrolysis oils must be removed to be comparable and competitive with fossil fuels. Several studies have shown that metal phosphides are active for the hydrodeoxygenation (HDO) of pyrolysis oil model compounds (6–8), with more recent studies paying particular attention to Ni2P (9–13). Previous work by Whiffen et al. (8) has shown that an “optimum” calcination temperature exists for the preparation of unsupported high surface area MoP in the presence of citric acid (CA). A catalyst calcination temperature of 823 K maximized CO uptake, HDO conversion of 4-methylphenol, and hydrogenation (HYD) selectivity over this catalyst. In a similar way an “optimal” calcination temperature might also exist for the preparation of Ni2P-CA. In the present study, unsupported Ni2P prepared using CA calcined at various temperatures, has been investigated and compared for the HDO of 4-methylphenol, a refractory model compound present in pyrolysis oils. The effect of calcination temperature on the reduced catalyst properties was investigated at temperatures of 773, 823, and 973 K. The chosen calcination temperatures were based on previous work by Whiffen et al. (8).

Experimental Catalyst Preparation Unsupported high surface area Ni2P catalysts were synthesized using the traditional Ni phosphate temperature programmed reduction method and a P:Ni molar ratio of 1:1 (16). Aqueous solutions of nickel nitrate (99.9% Ni(NO3)2.6H2O Fisher Scientific) and diammonium hydrogen phosphate (99% (NH4)2HPO4 Sigma-Aldrich) were prepared in de-ionized water (8, 9, 14, 15). The CA (99.8% Fisher Scientific) was added to the salt solution to give a 2:1 CA:Ni molar ratio (8, 9, 15). Subsequently the precursor solutions were aged for 24 h in a covered beaker held at 363 K in a water-bath and dried in an oven at 397 K for 24 h. The dried samples were calcined by heating at 5 K min−1 to 773, 823, or 973 K in stagnant air and held for 5 h at the final temperature. Ni2P prepared in the absence of CA was calcined to 773 K only. Approximately 0.7 g of the calcined catalyst precursors were ground to a powder (dP < 53 μm) and converted to Ni2P by temperature-programmed reduction (TPR) in UHP H2 at a flow rate of 160 cm3(STP) min−1 and a heating rate of 5 K min−1 to 573 K, followed by a heating rate of 1 K min−1 to 923 K. The final temperature was held for 2.5 h. The samples were then cooled to room temperature in a He flow and passivated in a flow of 1 mol % O2/He for 3 h prior to removal from the quartz U-tube reactor for characterization purposes. Other catalysts used for activity measurements were transferred directly from the quartz U-tube used for reduction, under a He flow (25 cm3(STP) min−1) into ~15 cm3 of decalin. They were then transferred to the reactor for activity measurements, without exposure to air (8). A P:Ni ratio of 2:1 for Ni2P prepared with CA has shown better physical properties and 288 In Novel Materials for Catalysis and Fuels Processing; Bravo-Suárez, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

higher activity for the HDS of 4,6-dimethyldibenzothiophene compared to Ni2P prepared with a P:Ni ratio of 1:1 (16). However, in the present study a P:Ni ratio of 1:1 was used to reduce the production of PH3 and to reduce P sublimation during preparation. The reduced catalysts are identified as Ni2P-CA-ttt K, where ttt is the precursor calcination temperature (K). The reduced catalysts prepared in the absence of CA are designated as Ni2P-noCA.


Catalyst Characterization Elemental C analysis was performed on the reduced and passivated Ni2P catalysts and on the used Ni2P samples (extracted following the 5 h HDO reaction) using a Perkin-Elmer 2400 Series II CHNS/O analyzer operated in the CHN mode. P analysis of selected samples was carried out using a colorimetric method, with direct comparison to a standard (17). Ni concentrations of select samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP–AES). Powder X-ray diffraction (XRD) patterns of the reduced and passivated Ni2P catalysts were collected using a Bruker D8 Focus (LynxEye detector) with a Co Kα X-ray source of wavelength 1.79 Å. Brunauer–Emmett–Teller (BET) surface areas of the reduced and passivated Ni2P catalysts were determined from N2 adsorption isotherms measured at 77 K using a Micromeritics Flowsorb II 2300. Samples were degassed in 30 mol % N2/He at 15 cm3(STP) min−1 for 16 h (7, 8). The CO uptake of the reduced Ni2P catalysts was measured by pulsed chemisorption using a Micromeritics AutoChem II 2920 unit. The reduced Ni2P samples were prepared from their calcined precursors by in situ reduction of approximately 0.1 g of sample in 9.5 mol % H2/Ar (50 cm3(STP) min−1) while heating at 5 K min−1 to 573 K followed by a ramp of 1 K min−1 to 923 K with the final temperature held for 2.5 h (replicating the standard reduction procedure of the calcined Ni2P precursors). The sample was then cooled in 50 cm3(STP) min−1 He to room temperature prior to injecting pulses of CO (7–9). Transmission electron microscopy (TEM) images of the reduced and passivated Ni2P catalysts were obtained using a 120 kV Hitachi H7600 with a tungsten filament and a FEI Tecnai TEM operated at 200 kV with a LaB6 filament. Log-normal particle size distributions were obtained by editing the images in Pixcavator 4.0 Image Analysis software and measuring the particle diameters and widths. Greater than 50 particles were measured for Ni2P-CA catalysts whereas the Ni2P-noCA particle size was based on >10 particles. Catalyst Activity The HDO reactions were carried out in a 300 cm3 stirred-batch reactor operated in slurry mode with 0.36 g of reduced Ni2P catalyst at 623 K and 4.4 MPa H2 with 2.96 wt % of 4-methylphenol (4-MP) (99%, Sigma-Aldrich), used as a model reactant, in 100 cm3 decalin (98%, Sigma-Aldrich). The concentration-time profiles were determined by withdrawing liquid samples 289 In Novel Materials for Catalysis and Fuels Processing; Bravo-Suárez, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


from the reactor periodically and analyzing them using a 14-A Shimadzu gas chromatograph (GC) equipped with a flame ionization detector (FID) and an AT-5 25 m × 0.53 mm capillary column. The product distribution was confirmed by GC mass spectrometry (MS) analysis using a Shimadzu QP-2010S GC–MS and a Restek RTX5 30 m × 0.25 mm capillary column (7–9). The reactant and product concentration data measured over time was used to determine the kinetic parameters of the HDO reaction over the Ni2P catalysts. Because the catalysts were observed to deactivate, the kinetics of the decomposition of 4-methylphenol was assumed to follow the exponential decay law given in eq 1:

where t is the HDO reaction time (s), Ccat is the concentration of the catalyst in the reactor at ambient conditions (gNi2P cm−3), Ca is the 4-methylphenol concentration at time t, k is the reaction rate constant (cm3 min−1 gNi2P−1), and kd is the deactivation rate constant (cm3 min−1 gNi2P−1).

Results and Discussion Catalyst Characterization The properties of the reduced and passivated Ni2P catalysts are summarized in Table 1. The Ni2P prepared in the absence of CA (Ni2P-noCA) was free of C. However, the catalysts prepared with CA contained a C:Ni-ratio of 0.6:1 for all calcination temperatures. Previous work by Whiffen et al. (8) reported that the calcination temperature used for the preparation of MoP-CA catalysts significantly affected their C content. The C:Mo ratio was high for MoP-CA-773 K at 2:1, whereas MoP-CA-973 K had a C:Mo of 0.5:1 (8). This implies that C is more easily removed from the Ni2P-CA catalysts at calcination temperatures below 973 K compared with that of the MoP-CA catalysts. The Ni:P ratio of the Ni2P-CA catalysts increased from 1:1 for the calcined Ni2P-CA and noCA precursors to 2.3:1 for the reduced and passivated Ni2P-CA catalysts and was 2.1:1 for the reduced and passivated Ni2P-noCA catalyst. The increase in metal content was due to P losses during reduction of the Ni2P calcined precursor, which led to PH3 generation. Table 1 also shows that the addition of CA to the catalyst precursors significantly increased the surface area of the reduced Ni2P catalysts. Compared to Ni2P-noCA calcined at 773 K, the surface area of the Ni2P-CA-773 K increased by a factor of ~10 due to the formation of a metal citrate (8). As the calcination temperature of Ni2P-CA catalysts increased from 773 to 973 K, the surface area decreased from 101 to 51 m2 gcat−1. The decrease was caused by sintering of the Ni2P-CA catalysts at higher calcination temperatures that led to agglomeration of the Ni2P crystallites. Ni2P prepared in the absence of CA had a crystallite size of 57 nm estimated by Scherrer’s equation using the (210) plane of Ni2P, whereas its particle size from TEM imaging was determined to be 259 nm. This again indicates significant agglomeration of the metal crystallites in the Ni2P-noCA 290 In Novel Materials for Catalysis and Fuels Processing; Bravo-Suárez, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

catalyst. Ni2P-CA-773 K had a crystallite size of 34 nm compared to 42 nm for the Ni2P-CA-823 K catalyst and 50 nm for the Ni2P-CA-973 K catalyst. Ni2P-noCA had a CO uptake of < 1 μmol gNi2P−1 compared to that of 20 μmol gNi2P−1 for the Ni2P-CA-773 K catalyst. The CO uptake of Ni2P-CA-773 K was also greater than both the Ni2P-CA-823 K and the Ni2P-CA-973 K catalysts that had CO uptakes of 9 and 10 μmol gNi2P−1, respectively. These results further indicate that nearly complete C removal and particle sintering occurred at calcination temperatures above 773 K, leading to inferior Ni2P-CA properties.


Table 1. Physical and chemical properties of Ni2P prepared with and without CA Ni2P

Ni:P Ratio

C:Ni Ratio

SBET (m2 gcat−1)

dXRD (hkl) (nm)

dTEM (STDev) (nm)

CO Uptake (μmol gNi2P−1)

CA-773 K

2.3

0.6

101

34 (210)

36 (11)

20

CA-823 K

2.3

0.6

75

42 (210)

54 (26)

10

CA-973 K

2.3

0.6

51

50 (210)

54 (15)

9

noCA

2.1

0

6

57 (210)

259 (44)

The Effect of Calcination Temperature on the Properties and

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