Hydrodeoxygenation of Guaiacol as Model Compound for Pyrolysis Oil on Transition Metal Phosphide Hydroprocessing Catalysts

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Applied Catalysis A: General 391 (2011) 305310

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Applied Catalysis A: General

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

Hydrodeoxygenation of guaiacol as model compound for pyrolysis oil on

transition metal phosphide hydroprocessing catalysts

H.Y. Zhao a, D. Li a,b, P. Bui a, S.T. Oyama a,c,

a Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, United Statesb State Key Lab of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR Chinac Department of Chemical Systems Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

a r t i c l e i n f o

Article history:

Received 26 March 2010

Received in revised form 14 July 2010

Accepted 21 July 2010

Available online 30 July 2010

Keywords:

Hydrodeoxygenation

Guaiacol

Biooil

Transition metal phosphides

Ni2P

CoMoS

a b s t r a c t

Thegas phase hydrodeoxygenation (HDO) of guaiacol, as a model compound for pyrolysisoil, wastested

on a series of novel hydroprocessing catalysts transition metal phosphides which included Ni 2P/SiO2Fe2P/SiO2, MoP/SiO2, Co2P/SiO2 and WP/SiO2. The turnover frequency based on active sites titrated by

the chemisorption of CO followed the order: Ni2P >Co2P >F e2P, WP, MoP. The major products from

hydrodeoxygenation of guaiacol for the most active phosphides were benzene and phenol, with a smal

amount of methoxybenzene formed. Kinetic studies revealed the formation of reaction intermediate

such as catechol and cresol at short contact times. A commercial catalyst 5% Pd/Al2O3 was more activ

than themetal phosphides atlowercontact time butproducedonly catechol.A commercial CoMoS/Al2O

deactivated quickly andshowed littleactivityfor the HDO of guaiacol at theseconditions. Thus, transitio

metal phosphides are promising materials for catalytic HDO of biofuels.

2010 Elsevier B.V. All rights reserved

1. Introduction

Pyrolysis oil from thermal cracking of biomass is attracting

attention as an alternative liquid fuel because of the depletion

of petroleum deposits and the increasing environmental concern

with the burning of nonrenewable resources [1]. However, oxygen

removal is required toupgrade pyrolysisoil because itshighoxygen

content (2040%) leads to undesirable properties of the oil such as

low energy density, and thermal and chemical instability[2].The

subjects of hydrodeoxygenation[3]and pyrolysis oil treatment[4]

have been reviewed. The average composition of pyrolysis oils is

5065 wt% organic components, that include organic acids, alde-

hydes, ketones, furans, phenolic compounds, guaiacols, syringols

andsugarbasedcompounds,1530 wt%water and20 wt%colloidal

ligninfraction[5,6]. The phenolicscontent,a majorpartof thelignin

fraction may reach 30% of the organic component [68]. Guaiacol is

one of the most abundant of the lignin-derived products in biooil,

present at levels of approximately 0.180.51 wt% in switchgrass

and alfalfa derived pyrolysis oils[9].

Generally, there are two methods for removal of oxygen. In

the direct deoxygenation method, which is generally conducted at

atmospheric pressure,CO bonds are broken without theassistance

Corresponding author.

E-mail address: [email protected](S.T. Oyama).

of a reducing gas such as hydrogen [10], and in the hydrogena

tion route aromatic rings are hydrogenated before removal o

oxygen[11].The former has been reported on tungsten (IV) com

pounds[12] and acidic zeolites such as HZSM-5 [13]. The latte

process is carried out at high pressure and temperature, and i

related to hydrotreating of petroleum feedstocks for removal o

sulfur and nitrogen. Thus, this hydrodeoxygenation (HDO) pro

cess can potentially allow use of the existing petroleum refining

infrastructure for processing and transportation[14]. Conventiona

sulfide catalysts for petroleum hydroprocessing[15]and preciou

metal catalysts[16]have been studied for their reactivity in gua

iacol and model oxygenate compound HDO. Oxygenated group

in pyrolysis oil such as ketones, aldehydes, and organic acid

require lower temperatures for elimination of the reactive func

tionalities but guaiacol type molecules and other phenolic specie

require higher temperature [17]. Phenyloxygen bonds are cleaved

at 500650K using hydropressing catalysts under hydrogen pres

sure, in which theoxygen is ultimately removed as water.However

typical hydrodesulfurization catalysts, such as NiMoS/Al2O3 and

CoMoS/Al2O3 were found to quickly deactivate by coke deposi

tion in model HDO reactions because of the acidity of the reactan

[18]. Sulfides on neutral supports including carbon, silica and

alumina modified by K for HDO reactions have been reported

[1720].Yet the effect of modification for alumina supported cat

alysts has not been optimized. It was found that guaiacol coking

reactions were negligible with molybdenum sulfide supported on

0926-860X/$ see front matter 2010 Elsevier B.V. All rights reserved.

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306 H.Y. Zhao et al. / Applied Catalysis A: General391 (2011) 305310

activatedcarbon, and the catalyst showedgood stabilityand poten-

tial for catalytic hydrotreating system [21]. Noble metal Pt as an

active component was added to a conventional CoMoS catalyst and

showed no significant improvement[17].According to this study,

monometallic and bimetallic noble metal catalysts supported on

zirconia have lower coke formation than a CoMoS/Al2O3 catalyst

and in particular Rh-containing catalysts demonstrate potential in

biofuel upgrading. Given the propensity of biofuels to thermally

decompose to form coke and coke-like matter, catalysts with sup-

ports that are less active forcoke formation or more hydrogenating

catalysts permitting a rapid transformation of dioxygenated reac-

tants (guaiacol, catechol) into less coke-forming products (phenol)

would be highly desirable.

Hydrodeoxygenation of actual biooils has been studied with

two stages because of the thermal instability of the oil [22]. In

a first stage, a stabilization process was carried out at low tem-

perature to eliminate reactive compositions like ketones. In a

second stage, deoxygenation of the phenolic-type molecules was

carried at higher temperatures. Using diluted model oxygenated

compound solutions for hydrodeoxygenation study will give more

precise chemical information and avoid thermal polymerization

reactions [23]. In the present study, guaiacol (methoxyphenol)

is chosen as model compound for hydrodeoxygenation because

guaiacol and substituted guaiacols constitute a relatively high con-

centration of the lignin-derived fraction (up to 0.5 wt%) and these

have a high tendency to coke. Guaiacols contains two different

oxygenated functions (phenolic and methoxy groups), so are chal-

lenging molecules to completely deoxygenate. Transition metal

phosphides supportedon neutral silicaare a promisingclass of new

hydroprocessing catalysts [24,25],and it was of interest to investi-

gatethem for guaiacol catalytichydrodeoxygenationin comparison

to commercial catalysts such as CoMoS/Al2O3and 5% Pd/Al2O3.

2. Experimental

2.1. Materials

The 5% Pd/Al2O3 commercial catalyst was provided by BASF

Catalysts, In. and the CoMo/Al2O3hydrotreating catalyst was pro-

vided by Haldor Topse. Transition metal phosphides Ni2P/SiO2,

Fe2P/SiO2, MoP/SiO2, Co2P/SiO2and WP/SiO2 were synthesized as

will bedescribed below,usinga fumedsilicaEH-5supportprovided

by Cabot Corp. The chemicals used in the synthesis of the cata-

lysts were Ni(NO3)26H2O (Alfa Aesar, 99%), Fe(NO3)39H2O (Alfa

Aesar, 99%), (NH4)6Mo7O244H2O (Alfa Aesar,99%), Co(NO3)26H2O

(Alfa Aesar, 99%), (NH4)6W12O39xH2O (Aldrich, 99%), (NH4)2HPO4(Aldrich, 99%). The chemicals used for the reactivity tests were

guaiacol (Alfa Aesar, 98%). The gases employed were H2 (Airco,

Grade 5, 99.99%), He (Airco, Grade 5, 99.99%), CO (Linde Research

Grade, 99.97%), 0.5% O2/He (Airco, UHP Grade, 99.99%), O2 (Airco,

UHP Grade, 99.99%), N2

(Airco, Grade 5, 99.99%). Chemical stan-

dards forGC andmass spectrometry werebenzene,phenol, toluene,

methoxybenzene (Alfa Aesar, 98%), cyclohexane (Alfa Aesar, 99%).

2.2. Metal phosphides synthesis

Ni2P/SiO2[2628],Fe2P/SiO2[27],MoP/SiO2 [29,30],Co2P/SiO2[27] and WP/SiO2 [31] were prepared by temperature-programmed

reduction (TPR), following procedures reported previously

[32,33]. Briefly, the synthesis of the catalysts involved two

stages. First, solutions of the corresponding metal phos-

phate precursors were prepared by dissolving appropriate

amounts of Ni(NO3)26H2O, Fe(NO3)39H2O, (NH4)6Mo7O244H2O,

Co(NO3)26H2O, (NH4)6W12O39xH2O, with ammonium phosphate

in distilled water, and these solutions were used to impregnate

silica by the incipient wetness method. The obtained samples were

dried and calcined at 500 C for 6h, then ground with a mortar

and pestle, pelletized with a press (Carver, Model C), and sieved to

particles of 6501180m diameter (16/20mesh). Second, thesolid

phosphates were reduced to phosphides at 2 Cmin1 in flowing

H2 [1000 cm3 (NTP)min1 g1]. Reduction temperatures were

568 C for Ni2P/SiO2, 680C for Fe2P/SiO2, 680

C for Co2P/SiO2,

494 C for MoP/SiO2, and 527C for WP/SiO2. The samples were

kept at the reduction temperature for2 h, followed by cooling to RT

under He flow [100 cm3 (NTP)min1], and then were passivated

at RT in a 0.5% O2/He for 4 h. The Ni, Fe, Mo, Co, W molar loading

were all 1.6 mmolg1 (mmol per g of support), corresponding to

a weight loading of Ni2P of 8.6% with an initial Ni/P ratio of 1/2,

Fe2P of 8.3% with an initial Fe/P ratio of 1/2, MoP 12.8% with initial

Mo/P ratio of 1, Co2P of 8.6% with initial Co/P ratio 1, WP 19.9%

with initial W/P ratio 1.

2.3. Characterization

Irreversible CO uptake measurements were used to titrate the

surface metal atoms and to provide an estimate of the active sites

on the catalysts for the noble metals andthe transition metal phos-

phides. Usually, 0.3g of a passivated sample was loaded into a

quartz reactor. Noble metal catalysts were reduced in H 2at 325C

for 2 h while passivated transition metal phosphides were reduced

at 450 Cfor2 hw it hH2at 300 ml (NTP) min1. After cooling in He,

pulses of CO in a He carrier at 43 mols1 [65cm3 (NTP)min1]

were injected at RT through a sampling valve, and the mass 28

(CO) signal was monitored with a mass spectrometer. CO uptake

was calculated by measuring the decrease in the peak areas caused

by adsorption in comparison with the area of a calibrated vol-

ume (19.5mol). Low temperature O2chemisorption was used for

CoMo/Al2O3 applying the same technique. Prior to the measure-

ment the sample was sulfided in a flow of 10% H2S/H2at 400C.

Surface areas of the samples were obtained using the BET

method based on adsorption isotherms at liquid nitrogen temper-

ature, and using a value of 0.162 nm2 for the cross-sectional area of

a N2molecule. The measurements were performed in a volumetricadsorptionunit (Micromeritics ASAP 2000). X-raydiffraction (XRD)

patterns of the samples were obtained with PANalytical Xpert Pro

powder diffractometer operated at 45 kV and 40 mA, using Cu K

monochromatized radiation (= 0.154178nm).

2.4. Reactivity studies

Hydrodeoxygenation activity was measured in a packed bed

reactor at atmospheric pressure.Guaiacolwas introducedby means

of a saturator at 25 C with a concentration of 0.024 mol%. The

dimensionsof thereactorswere1.5 cmi.d.25.5 cmlong,and were

loaded with 30molactive sites. To start a reaction, catalysts were

placed in the catalytic reactor and pretreated at the same condi-

tions as used forchemisorption. After pretreatment, a flowmixtureof hydrogen and nitrogen ata ratio of4 to1 saturated with guaiacol

was introduced at 150 cm3 (NTP)min1. The hydrogen to guaiacol

molar ratio was 33. The catalysts were stabilized for 6 h after the

feed wasintroduced.Thensamples were taken every23 h untilthe

conversion of guaiacol reached steady-state. Reactivity testing was

performedas a function of temperature, starting at the highest tem-

perature of 300 C and was varied downwards and upwards with

the initial temperature repeated at the end. Generally it took about

100 h of on-stream time to collect the rate data at several different

temperatures. The reaction products were analyzed using an online

gas chromatograph (HewlettPackard, 5890A) equipped with a

0.32 mmi.d.50m fusedsilica capillarycolumnand a flame ioniza-

tion detector. The reactants and products were identified by their

retention time in comparison with commercially available stan-



H.Y. Zhao et al. / Applied Catalysis A: General391 (2011) 305310 30

dards and confirmed by gas chromatographymass spectrometry

(GCMS) (HewlettPackard, 58905972A). Response factors were

determinedexperimentally using pure compounds. Low molecular

weight products (methane and methanol) could not be separated

with the present column, and were not analyzed. However, since

they arise as by products of guaiacol reaction to phenol, their omis-

sion does not affect the calculation of guaiacol conversion.

Conversion[%]=

N(guaiacol)in N(guaiacol)out

N(guaiacol)in

100

The conversions of guaiacol and product distributions were cal-

culated based onthe analyzedgas phase.The conversion of guaiacol

was calculated from the initial and final amounts of guaiacol.

Turnover frequency [s1]

=

Reactant flowrate [mol/s] conversion

Quantityof sites [mol/g] Catalystweight [g]

The turnover frequency was calculated to compare the intrinsic

activity of different catalysts.

3. Results and discussion

3.1. CO chemisorption and BET areas

Table 1 reportsuptakes of CO at room temperature forthe metal

phosphide catalysts and the noble metal and uptakes of O2at dry-

ice acetone temperature forCoMoS/Al2O3. Table1 alsoprovidesBET

characterization results. Earlier studies have shown that uptakes

of the SiO2and Al2O3were negligible[3436].The CO chemisorp-

tion uptakes of the different samples varied in a wide range from

42 to 200mol/g. The dispersion (D) of metal sites was estimated

from the CO uptakes and the known loading of the samples (in all

cases 1.16 mmol g1 of total metal). The order of dispersion was

Table 1

Characterization results for catalysts including CO chemisorption, dispersion, par

ticle size, BET surface area.

CO uptake

(mol/g)

Dispersion (%) Partical size

(nm)

BET surface

area (m2 /g)

Ni2P/SiO2 134 12 8 309

Fe2P/SiO2 52 4 20 233

MoP/SiO2 214 19 5 207

Co2P/SiO2 42 4 25 307

WP/SiO2 70 6 15 147Pd/Al2O3 120 25 4 82

CoMo/Al2O3 100a 12 7 224

a O2 uptake for CoMo/Al2O3 .

Co2P/SiO2 < Fe2P/SiO2 < WP/SiO2 < Ni2P/SiO2 < MoP/SiO2 assuming

that each active site adsorbs one CO molecule. The particle size

(d) was then calculated using equationd0.9/D. It was found tha

nickel and molybdenum formed much smaller particles and were

better dispersedthan the othertransition metal phosphides.Excep

for WP/SiO2, all other transition metal phosphides show fairly high

BET surface area.

3.2. X-ray diffraction

X-ray diffraction (XRD) was used to ascertain the phase and

phase composition of the synthesized transition metal phos

phides by comparingwith the standardpowder diffraction pattern

(Fig. 1). The diffraction pattern for silica supported nickel phos

phide shows three major peaks at 40.5, 44.8 and 47.5, which

line up well with the standard pattern for Ni2P. The peaks are

significantly broadened indicating that small Ni2P crystals wer

formed. The three major peaks for cobalt phosphide line up wel

with the standard Co2P pattern suggesting that the major phase is

Co2P in our synthesis, which is consistent with result reported by

Bussell that Co2P is obtained by using a Co/P ratio of 1 [37].Th

Fig. 1. X-ray diffraction pattern for transition metal phosphides.




Fig. 2. Turnover frequency of guaiacol on transition metal phosphides.

Fe2P/SiO2XRD pattern shows three strong peaks which are in line

with Fe2P. Noticeably, the Fe2P/SiO2XRD pattern is similar to that

ofNi2P/SiO2 butitspeaksare much narrowerthan that ofNi2P/SiO2,

indicating that the Fe2P particle size is larger than that of the Ni 2P,

which is consistent with the CO chemisorption results (Table 1).

The WP/SiO2 XRD pattern shows strong narrow peaks lining upwell with the standard WP pattern. In the case of MoP/SiO2, no

peaks were detected for its XRD pattern, which indicated that MoP

is well dispersed on the silica. Combined with the CO chemisorp-

tion results, it suggests that the MoP crystallites are smaller than

5 nm and are not detectable by XRD.

3.3. Reactivity

3.3.1. HDO activity of transition metal phosphides

The hydroprocessing of biooils required very long contact

times to remove the oxygen content [38]. The reactivity tests

here were carried out at high contact time 20.2 min. A gas phase

feed containing 0.024 mol% guaiacol carried out with a mixture of

hydrogen and nitrogen ata ratio of 4 to 1 was usedto test the HDOactivity of the transition metal phosphides and commercial cata-

lysts. All the catalysts showed expected responses to temperature

with higher conversions at higher temperatures and reasonable

stability over the time course of the reactions. The catalytic activity

of transition metal phosphides are compared in Fig. 2in terms of

turnover frequency (TOF) based on sites titrated by the adsorption

of CO. These measurements were carried out starting with the

highest temperature of 300 C and then lowering and raising the

temperature back to 300 C to determine whether catalyst deacti-

vation was occurring. AsFig. 2shows, at the high temperature of

300 C, the TOF of the catalysts for the HDO of guaiacol follows the

order: Ni2P/SiO2 > Co2P/SiO2 > Fe2P/SiO2 > WP/SiO2 > MoP/SiO2.

For Ni2P/SiO2 in the temperature range 200300C, the con-

version of guaiacol varied from 31% to 93%, while for Co2

P/SiO2the conversion ranged from 21% to 82%. The smooth variation

of the conversion for Ni2P/SiO2 and Co2P/SiO2 with temperature

indicates that these catalysts were not deactivating. However with

Fe2P/SiO2, as the reaction temperature was lowered from 300 to

Table 2

Activation energy for guaiacol reaction on transition metal phosphide catalysts.

Conversion (%) Temperature (C)

200 225 250 275 300 Ea (kJ/mol)

Ni2P/SiO2 31 52 65 85 93 40

Co2P/SiO2 21 39 61 77 82 52

WP/SiO2 9 12 22 59 23

Fe2P/SiO2 15 70 a

MoP/SiO2 10 14 19 50 63

a Not calculated because of deactivation.

275 C the conversion decreased abruptly from 70% to 15%, and

the temperature was not lowered further. After 50 h at 300 C the

conversion of guaiacol decreased from 70% to 10%, so the Fe2P/SiO2catalyst obviously underwent deactivation, probably by coking,

which is commonly observed [39]. For WP/SiO2in the temperature

range 200300 C, the conversion of guaiacol varied from 9% to

59%, while for MoP/SiO2it varied from 10% to 50%. Here there was

some deactivation, especially for MoP/SiO2. In summary, Ni2P/SiO2and Co2P/SiO2are much more active than Fe2P/SiO2, WP/SiO2and

MoP/SiO2, and are stable in the HDO of guaiacol.

The apparent activation energy for guaiacol on the different

catalysts were estimated based on the reactivity at different tem-peratures and are reported in Table 2. To minimize the effect of

deactivation, the lower conversion data at lower temperature were

used to calculate the apparent activation energy (Table 2). The

activation energies obtained for the overall conversion of guaiacol

(2363 kJ/mol) are lower than expected for carbon-oxygen bond

rupture (>240kJ/mol)[40]. These lowactivation energies mayindi-

cate that the oxygen removal reactions occur by hydrogenation

of double bonds in the aromatic ring followed by elimination of

water [4144]. Although guaiacolhas a high cokingtendency, in our

continuous flowexperiments, theactivityof thecatalysts wasmea-

sured aftera stabilization period. Thus in ourcasethe lowactivation

energyreflects the high activity of the transition metal phosphides.

Products distribution and total conversion for HDO of guaiacol

are reported in Table 3. With the catalyst Ni2P/SiO2, products werecomposed of 30% phenol, 60% benzene and 10% methoxybenzene.

The major product benzene resulting from the complete HDO of

guaiacol is thedesirable product. ForCo2P/SiO2, products consisted

of 32% phenol, 52% benzene, 15% C3C5 and 1% methoxybenzene.

For Fe2P/SiO2, the products were mainly phenol up to 94% and 6%

methoxybenzene. With WP/SiO2, the product for HDO of guaiacol

was essentially 100% phenol. Similarly to Ni2P/SiO2and Co2P/SiO2,

the major products with MoP/SiO2were 28% phenol, 53% benzene,

15% C3C5 and 4% toluene. Thus, the major products for HDO of

guaiacol on transition metal phosphides were benzene, and very

small amounts of toluene. Partial HDO products were phenol and

small amounts of methoxybenzene.

3.3.2. Effect of contact time

The reaction of guaiacol HDO has been suggested to proceed

by first the hydrogenolysis of the methyloxygen bond of the

methoxy group to form catechol, followed by the elimination of

the hydroxyl groups to produce phenol and water and then hydro-

Table 3

Products distribution for HDO of guaiacol with a concentration of 0.024 mol% at 300 C, atmosphere pressure, at contact time 20.2min and space velocity 1.4 h1.

Total conversion (%) Product distribution (%)

Phenol Benzene Methoxybenzene Toluene C3C5

Ni2P/SiO2 80 30 60 10 0 0

Co2P/SiO2 70 32 52 1 0 15

Fe2P/SiO2 64 94 0 6 0 0

WP/SiO2 60 100 0 0 0 0

MoP/SiO2 54 28 53 0 4 15



H.Y. Zhao et al. / Applied Catalysis A: General391 (2011) 305310 30

Table 4

Effect of contact time on conversion of guaiacol and products distribution at 300 C.

Catalyst Contact time (min) Space velocity (h1) Conversion (%) Product distribution (%)

Phenol Benzene Methoxybenzene Catechol Cresol C3C5

Ni2P/SiO2 20.2 1.4 80 30 60 10 0 0 0

0.339 59 19 28 4 38 0 30 0

Co2P/SiO2 20.2 1.4 70 32 52 1 0 0 15

0.339 59 35 1 0 0 99 0 0

WP/SiO2 20.2 1.4 60 100 0 0 0 0 0

0.339 59 12 12 0 0 88 0 0

carbons[4144].This reaction sequence suggests that catechol is

the primary reaction product, which is transformed to phenol. Our

results did not show catechol as a product for HDO of guaiacol in

all cases. Thus reactivity tests with lower contact time were carried

out to identify the reaction intermediates and the possible reac-

tion pathway (Table 4).The contact time defined in the following

equation, was decreased from 20.2 to 0.339 min

Contact time [min]

=

Quantity of sites [mol/g] Catalyst weight [g]

Reactant flowrate [mol/min]

Three catalysts Ni2P/SiO2, Co2P/SiO2, WP/SiO2 were chosen for

the lower contact time reactivity tests because they had the high-

est activity and showed minimal deactivation. As the contact time

decreased from 20.2 to 0.339 min, the conversion of HDO guaiacol

decreased significantly for all the catalysts, as expected. However,

the decrease of the contact time from 20.2 to 0.339min was not

accompanied by a linear decrease of the conversion. Instead, the

conversion of guaiacol at the contact time of 0.339 min was higher

than expected, and this could be because the reactor operated in

integral manner (at high conversion) and there may have been

product inhibition by water.

In the case of Ni2P/SiO2 a new product, cresol, was observed

and more methoxybenzene was formed while less benzene

was produced. Selectivity towards phenol was 28%, which wasalmost the same as at higher contact time, but no catechol was

observed. This result might be due to a direct elimination of the

methoxy group by hydrogenolysis of the aromatic carbonoxygen

bond[17].

In the case of Co2P/SiO2, the products were 99% catechol and

only 1% phenol at lower contact time compared with 94% phe

nol and6% methoxybenzene at higher contact time. With WP/SiO2at lower contact time 88% catechol was observed instead of 100%

phenol. Thus, the intermediate catechol was observed for HDO o

guaiacol at lower contact time. The results indicate that the firs

bond to be broken is the bond between the oxygen and the methy

carbon, which is depicted in thereactionscheme illustrated in Fig.3

which shows the possible reaction pathway on transition meta

phosphides. The scheme was adapted from results of this study awell as work from others. Cyclohexanol was proposed as an inter

mediate in the HDO of phenol[45],while cresol was reported in

several studies[11,40].

3.3.3. HDO activity comparison

The commercial catalysts Pd/Al2O3 and CoMo/Al2O3 were also

tested for the HDO of guaiacol at the lower contact time 0.339 min

at 300 C to compare with the transition metal phosphides. Th

results are summarized inTable 5.The catalyst Pd/Al2O3 is mor

active in terms of guaiacol conversion at 70% than the most active

transition metal phosphide Co2P/SiO2 with 35% conversion. How

ever, catechol was the only product for conversion of guaiacol with

Pd/Al2O3. Catechol easily leads to coking, and this may explain the

lower performance of the Pd catalyst. With CoMoS/Al2O3the conversion rateof guaiacol decreasedquicklyand almost no conversion

was observed for guaiacol. The deactivation may have two origin

Fig. 3. Reaction network for HDO of guaiacol.




Table 5

Activity comparison between transition metal phosphides and commercial catalysts at contact time 0.339min and space velocity 59 h1 at 300 C.

Co nve rs ion ( %) Pro duct dis tributio n (% )

Phenol Benzene Methoxybenzene Catechol Cresol

Ni2P/SiO2 19 28 4 38 0 30

Co2P/SiO2 35 1 0 0 99 0

WP/SiO2 12 12 0 0 88 0

5% Pd/Al2O3 70 0 0 0 100 0

CoMo/Al2O3 1 0 0 100 0 0

[23],coking or poisoning. Thus, either coke or other heavy prod-

ucts block the active sites, or the primary product catechol adsorbs

strongly on them [46]. The main superiority of Ni2P/SiO2 at low

contact time over Pd/Al2O3 comes from the phosphides ability to

form phenol in a higher proportion. Then, this catalyst is able to

form products less susceptible to coke formation. Thus, Ni 2P/SiO2is betterthanPd/Al2O3 and CoMoS/Al2O3 underthe conditionsused

in this study.

4. Conclusions

A group of transition metal phosphides were evaluated for the

hydrodeoxygenation of guaiacol. The activity for HDO of guaiacolfollows the order: Ni2P > Co2P > Fe2P, WP, MoP. The major prod-

ucts for HDO of guaiacol are phenol, benzene, methoxybenzene,

with no catechol formed at higher contact time. At lower contact

time catechol is the major products for Co2P and WP. No catechol

was observed for HDO of guaiacol with Ni2P even at low contact

time. The commercial 5% Pd/Al2O3catalyst is more active than the

metal phosphides at lower contact time, but the major product is

catechol which is undesired. The commercial hydrotreating cat-

alyst CoMoS/Al2O3 deactivated quickly and showed little activity

for the HDO of guaiacol at these conditions. These results indicate

that transition metal phosphides are promising catalysts for the

treatment of bio-derived feedstocks.

Acknowledgments

This work wassupported bythe US Department ofEnergy, Office

of Basic Energy Sciences, through Grant DE-FG02-963414669,

the National Renewable Energy Laboratory through Grant DE-

FG3608GO18214, and the Japan Ministry of Agriculture, Forestry,

and Fisheries (Norinsuisansho).

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Documents

Hydrodeoxygenation of Guaiacol as Model Compound for Pyrolysis Oil on Transition Metal Phosphide Hydroprocessing Catalysts