Hydrodeoxygenation of 4‑Propylguaiacol (2-Methoxy-4-propylphenol) in a Microreactor: Performance and Kinetic StudiesNarendra Joshi* and Adeniyi Lawal

New Jersey Center for Microchemical Systems (NJCMCS), Department of Chemical Engineering and Materials Science, StevensInstitute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030, United States

ABSTRACT: Catalytic hydrodeoxygenation of 4-propylguaiacol, a model compound for lignin-derived components of pyrolysisoil, was performed in a packed-bed microreactor. The effects of various processing conditions were investigated using presulfidedNiMo/Al2O3 catalyst. The conversion of 4-propylguaiacol is highly influenced by temperature. Although many products wereformed, 4-propylphenol predominated under the operating conditions selected with presulfided NiMo catalyst. External andinternal mass-transfer resistances and heat-transfer resistance were investigated and found to be negligible. Reaction rateexpressions were based on proposed reaction mechanisms using the Langmuir−Hinshelwood approach. Nonlinear regressionwas performed to obtain kinetic constants. The best-fitting rate equations were further validated by comparing experimental dataobtained from an integral reactor with predictions obtained using the Runge−Kutta method based on the rate equations. Adifference of less than 10% between the experimental data and the predicted data for the integral reactor was found.

1. INTRODUCTIONThe consumption of fossil fuels is increasing steadily andexpected to grow by 53% in 2035 compared to 2008,1 as theworld population comes close to 9 billion. The known reservesof fossil fuels are continuously decreasing, and excessive use offossil fuels has increased the emissions of greenhouse gases,particularly carbon dioxide, which is considered to contributestrongly to global warming. These factors have led to increasinginterest in the use of biomass-derived fuels that do notcontribute any new carbon dioxide to the atmosphere. Nonfoodlignocellulosic biomass feedstocks such as corn stover, straw,wood chips, switch grass, and other waste products can be usedfor the production of fuels. These biomass feedstocks consistingof cellulose, hemicellulose, and lignin are converted to pyrolysisoil (PO) through a thermochemical route known as fastpyrolysis. Pyrolysis oil is further processed by hydrodeoxyge-nation (HDO) to obtain transportation fuel. However, morethan 300 oxygenated organic compounds in pyrolysis oilcomplicate the study of its reaction mechanisms and kinetics.Many cellulose-derived components of pyrolysis oil have beeninvestigated, but the lignin-derived components have notreceived much attention, and there is limited understandingof the reaction networks and kinetics of HDO of thesecomponents.2 In this work, we have selected 4-propylguaiacolas a model compound to investigate the hydrodeoxygenationprocess in a microreactor. 4-Propylguaiacol represents some ofthe major lignin-derived components present in pyrolysis oilsuch as benzene, phenol, guaiacol, anisole, propyl anisole,propylphenol, and propylbenzene. The presence of phenoliccompounds in pyrolysis oil is the cause of polymerization andcoke formation during hydrodeoxygenation at temperaturesabove 300 °C. By conducting a study on the hydro-deoxygenation of 4-propylguaiacol, we hope to understand amechanism and kinetics for the upgrading of pyrolysis oil.Better selectivity, high yield, improved product quality, and

safe operation are attainable in microreactors because of veryfast heat and mass transfer.3−6 Studies have shown that, because

of laminar flow, mixing in microchannels occurs predominantlyby interdiffusion of reactants.7 However, because of shorttransverse diffusional distance, rapid and effective mixing isattainable in a microreactor that can quickly bring reactants intocontact with catalyst in a heterogeneous reaction.8 Fast mixingin two-phase flows can also be achieved by selectingappropriate inlet T-orientations to provide a short slug length.9

Most hydrodeoxygenation studies have focused on sulfidedCoMo- and NiMo-based catalysts, which are industrialhydrotreating catalysts for the removal of sulfur, nitrogen,and oxygen from petrochemical feedstocks.10 Platinum,11

vanadium nitride,12 and ruthenium13 have also been used forhydrodeoxygenation. In this study, we used the sulfided form ofNiO/MoO3 on Al2O3.Model compounds as representatives of pyrolysis oil are

typically used in the hydrodeoxygenation process to betterelucidate reaction mechanisms and kinetics. Zhao et al.14

conducted the gas-phase hydrodeoxygenation of guaiacol on aseries of novel hydroprocessing catalysts such as Ni2P/SiO2,Fe2P/SiO2, MoP/SiO2, Co2P/SiO2, and WP/SiO2. They foundthat the activities of the catalysts for the HDO of guaiacolfollowed the order Ni2P > Co2P > Fe2P, WP, MoP and that themajor products formed were phenol, benzene, and methox-ybenzene, with no catechol formed at higher contact timeswhereras, at lower contact times, catechol was the majorproduct for Co2P and WP. It was mentioned that no catecholwas observed for the HDO of guaiacol with Ni2P even at lowcontact times. They also found that the commercial 5% Pd/Al2O3 catalyst was more active than the metal phosphides atlower contact times and that the major product was catechol.They also used commercial CoMoS/Al2O3 catalyst for HDO,

Received: January 4, 2013Revised: February 25, 2013Accepted: February 26, 2013Published: February 26, 2013

Article

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© 2013 American Chemical Society 4049 dx.doi.org/10.1021/ie400037y | Ind. Eng. Chem. Res. 2013, 52, 4049−4058

but it deactivated quickly and showed little activity for theHDO of guaiacol. Another group2 also used guaiacol as a modelof lignin-derived components in pyrolysis oil for the hydro-deoxygenation to elucidate the reaction network and to predictoxygen removal. The conversion of guaiacol took place with ahigh selectivity for aromatic carbon−oxygen bond cleavagerelative to the accompanying methyl-group-transfer reactions.The catalytic conversion of guaiacol in the presence ofhydrogen catalyzed by Pt/Al2O3 was found to involve threemajor classes of reactions, namely, hydrogenation, hydro-deoxygenation, and transalkylation, with selectivities to hydro-deoxygenation products comparable to the selectivities to theaccompanying transalkylation products. Bio-oil upgrading atPacific Northwest National Laboratory (PNNL) focused onheterogeneous catalytic hydroprocessing of model compoundsusing various commercial catalysts in a batch reactor.15

Catalysts such as CoMo, NiMo, NiW, Ni, Co, Pd, andCuCrO were used to hydrodeoxygenate phenol at 300 and 400°C. p-Cresol, ethylphenol, dimethylphenol, trimethylphenol,naphthol, and guaiacol were also tested with a CoMo catalyst at400 °C. Of the catalysts tested, the sulfided form of CoMo wasmost active, producing a product containing 33.8% benzeneand 3.6% cyclohexane at 400 °C. Another group16 synthesizedand tested mono- and bimetallic (Pt−Sn alloy) monoliths forthe hydrodeoxygenation of guaiacol and anisole. Both Pt−Sn/Inconel and Pt−Sn/carbon nanofiber (CNF)/Inconel wereable to fully deoxygenate guaiacol and anisole. Coating withCNFs increased the surface area of the monoliths more than 10times, allowing for a higher metal uptake during the active-phase incorporation compared to monoliths without coating.According to the researchers, the Pt−Sn/CNF/Inconelmonolith is a promising catalyst for the upgrading of pyrolysisoil.Massoth et al. conducted hydrodeoxygenation of methyl-

substituted phenols in a microreactor at 300 °C and 2.85 MPahydrogen pressure using a sulfided CoMo/Al2O3 catalyst.17

Methyl-substituted benzene, cyclohexene, cyclohexane, andH2O were the primary products. Based on an analysis of theresults, the authors suggested two independent reaction paths,one leading to aromatics and the other leading to partially orcompletely hydrogenated cyclohexanes. A kinetics study wasconducted using Langmuir−Hinshelwood model to obtainadsorption and rate constants characterizing the two reactionpaths. The same adsorption constant found for the two reactionpaths suggested that a single catalytic site center is operative inboth reactions. Similarly, Edelman et al. performed the vapor-phase catalytic hydrodeoxygenation of benzofuran at 300 °Cand 35 atm total pressure over a presulfided NiMo/Al2O3catalyst in a microreactor.18 The results showed thatbenzofuran reacted through hydrogenation and hydrogenolysisto form 2,3-dihyrobenzofuran, o-ethylphenol, and phenol. Thesubsequent hydrodeoxygenation products of the phenols wereethylbenzene, toluene, benzene, and ethylcyclohexane. Theauthors suggested that the hydrogenation of benzofuran can bemodeled as pseudo-first-order in the benzofuran concentrationand that the hydrodeoxygenation reaction can be modeled asnon-first-order kinetics, possibly −1 order in oxygenatedcompounds.The objective of the work presented here was to evaluate the

hydrodeoxygenation of 4-propylguaiacol and to study reactionmechanisms and kinetics in a packed-bed microreactor. Theeffects of various processing conditions such as hydrogen partialpressure, reactor diameter, temperature, and residence time on

conversion, selectivity, and space-time yield were investigatedusing presulfided Ni−Mo/Al2O3 catalyst.

2. EXPERIMENTAL SECTION2.1. Materials. 4-Propylguaiacol was purchased from Sigma-

Aldrich. Hydrogen gas was purchased from Praxair. Nitrogenwas used as a tracer to perform a material balance. The feedmixture was 1.1 M 4-propylguaiacol in research-grade hexane.Presulfided NiO/MoO3/Al2O3 catalyst (Ni, 1−5 wt %; Mo,10−20 wt % from the literature15) obtained from Albemarle(Houston, TX; sulfided and supplied by Eurecat USA,Pasadena, TX) was ground and sieved to obtain particleswith diameters in the range of 75−150 μm. The average surfacearea of the sieved catalysts was 164 m2/g, and the average porediameter was 106 Å. The surface area and pore diameter wereobtained by using the multipoint Brunauer−Emmett−Teller(BET) technique on a Quantochrome Autosorb-1 instrument.The catalyst was reduced with 5.0 sccm (standard cubiccentimeters per minute) of hydrogen at 593 K and 3.45 MPafor 2 h. The average surface area of the reduced catalysts was209.0 m2/g, and the average pore diameter was 92.0 Å. Thecatalyst activity remained constant for 7 h of on-stream time. Ineach experiment, a fresh catalyst was used for each data point,and the sample was collected within 2 h of the run.

2.2. Experimental Setup. Mass flow controllers (Portermodel 201) were used to control the flow rates of hydrogenand nitrogen. A high-performance liquid chromatographypump (Laboratory Alliance Series III) was used to controlthe flow rate of 4-propylguaiacol. The ranges of flow rates usedfor 4-propylguaiacol, hydrogen, and nitrogen gases were 0.03−0.18 mL/min, 30−120 sccm, and 10−60 sccm respectively. Theliquid and gas phases were combined in a T-junction mixer(Upchurch) with a 508 × 10 −6 m through-hole. The Reynoldsnumber for the combined flow was less than 100 for allexperiments, indicating laminar flow. The fluids exiting fromthe T-junction exhibited a Taylor flow pattern with a liquid sluglength in the range of 0.001−0.003 m, whereas gas bubblelength varied from 0.001 to 0.005 m. The lengths of the liquidslugs and gas bubbles were measured by observing the Taylorflow pattern exited from the T-junction with the help of a rulerand magnifying eyepiece. A microreactor was prepared from a0.0016-m 316 stainless steel tubing with a 762 × 10 −6 minternal diameter (i.d.) that was gravity-filled with catalyst. Thetotal length of the packed-bed microreactor varied from 0.025to 0.18 m. Hastelloy micrometer filter cloth (200 × 1150 mesh,Unique Wire Weaving Co., Hillside, NJ) was placed at the endsof the reactor to retain the catalyst. The reactor system waspressurized using a back-pressure regulator (GO Regulator Co.,Spartanburg, SC). The entrance and exit pressures of the fluids(liquid and gas combined) in the reactor were measured. Thepressure drop along the reactor varied from 0.07 to 0.2 MPadepending on reactor length. A schematic flow diagram of themicroreactor setup is shown in Figure 1.

2.3. Analysis. Analysis of the liquid HDO product wasconducted by gas chromatography with mass spectrometry(GC/MS). The GC/MS analysis was performed using a Varianinstrument (GC 3900, equipped with a Varian CP-1177 split/splitless injector and a Varian CP-8410 autosampler), and fordetection, an ion-trap mass spectrometer (Varian Saturn2100T) was used. The capillary column used for gaschromatograph was a Factor Four VF-5 ms column (30 m inlength with a 2.5 × 10−4 m diameter and a 2.5 × 10−7 m filmthickness).

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The HDO of 4-propylguaiacol involves complex reactionsconsisting of series and parallel reactions possibly forming 4-propylphenol, 4-guaiacol, catechol, cresols, ethylphenol,benzene, propylbenzene, cyclohexane, cyclohexene, toluene,xylene, methoxycyclohexane, propylcyclohexane, and anisole.Based on a review of various articles on the HDO of phenoliccompounds2,14,16,19−21 and product analysis, a possible reactionnetwork of hydrodeoxygenation of 4-propylguaiacol usingpresulfided NiO/MoO3/Al2O3 is shown in Figure 2. The

predominant products of the hydrodeoxygenation of 4-propylguaiacol catalyzed by presulfied-NiMo/Al2O2 catalystwere 4-propylphenol, 4-propylbenzene, 4-ethylphenol, phenol,and cresol with trace amounts of benzene and toluene; thehighest selectivity was toward 4-propylphenol followed by 4-propylbenzene, 4-ethylphenol, phenol, and cresol. Guaiacol wasnot detected, but based on the products formed and literaturereviews, it was assumed to be converted to cresol, catechol,phenol, and methxybenzene. Catechol and methoxybenzenewere not quantified because of a lack of standards. Higherselectivity toward 4-propylphenol indicates that deoxygenationby removal of the methoxy group is more favorable than

deoxygenation by removal of the hydroxyl group. Formation ofphenol, 4-ethylphenol, and cresol indicates the dealkylationreaction. Slightly more formation of 4-propylbenzene comparedto phenol suggests that dehydroxylation is favored compared todealkylation.The reaction of 4-propylguaiacol was characterized in terms

of conversion, yield, space-time consumption (STC), rate ofdisappearance of 4-propylguaiacol, and rate of formation of 4-propylphenol, which are defined as follows

= ×conversion (%)mass of 4PG reacted

mass of 4PG fed100%

(1)

=

×

yield (%)mass of product formed

theoretical mass of product that could be formed100%

(2)

‐ =×

space time consumption (STC)mass of feed reacted

mass of catalyst time(3)

‐

=×

rate of disappearance of 4 propylguiacolmass of 4PG reacted

mass of catalyst time (4)

‐

=×

rate of formation of 4 propylphenolmass of 4PP formed

mass of catalyst time (5)

3. RESULTS AND DISCUSSION3.1. Catalyst Activity. A presulfided NiMo/Al2O3 catalyst

was investigated for its activity. The experiment was conductedfor 7 h, and a sample was collected every hour. The resultsshown in Figure 3 indicate that the catalyst did not lose itsactivity for 7 h of on-stream time.

3.2. Effects of Temperature. 4-Propylguaiacol washydrodeoxygenated at various temperatures but at a constanttotal pressure of 2.07 MPa (300 psig). The residence time waskept constant by varying the reactor length (catalyst loading) tocompensate for the change in gas velocity. Temperature wasvaried from 200 to 450 °C. Many products were detected forthe HDO product samples. The products identified and

Figure 1. Experimental setup.

Figure 2. Reaction network for the hydrodeoxygenation of 4-propylguaiacol.

Figure 3. Catalyst activity. Reaction conditions: temperature, 623 K(350 °C); pressure, 2.07 MPa (300 psig); gas phase, hydrogen andnitrogen; liquid phase, 4-propylguaiacol; liquid flow rate, 8.33 × 10−10

m3/s (0.05 mL/min).

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quantified were 4-propylphenol, phenol, cresols, ethylphenol,and propylbenzene, with 4-propylphenol having the largestyield. Other products, which amounted to less than 10%, werenot quantified because of a lack of standards. Based on theproducts formed, the selectivity toward the formation of 4-propylphenol was dominant. The results shown in Figure 4indicate that the conversion of 4-propylguaiacol and yield of 4-propylphenol products increased as the temperature increased.

3.3. Effects of Hydrogen Partial Pressure. A set ofexperiments was carried out at 400 °C in the range of 1.65−3.30 MPa (240−480 psig) to study the effects of the inlethydrogen partial pressure on the conversion and yield. Reactiontemperature and residence time were kept constant. Theresidence time was kept constant by varying the reactor length(catalyst loading). The H2 partial pressure was varied bychanging the total pressure. The results in Figure 5 indicatethat, initially, the conversion increased as the hydrogen partialpressure increased but, after the hydrogen partial pressure 2.20MPa (320 psig), the conversion remain constant, indicatingthat adsorbed hydrogen on the catalyst surface had reached amaximum (saturated) value.

3.4. Effects of the Liquid Flow Rate of 4-Propylguaia-col. Experiments were conducted to study the effects of theliquid flow rate of 4-propylguaiacol on the conversion and yieldby varying the liquid flow rate from 0.03 to 0.15 mL/min.Other operating conditions such as temperature, pressure, andgas flow rate were kept constant. In these experiments, theactual volumetric gas flow rate was at least 1 order of magnitudehigher than the liquid flow rate; hence, the residence time wasessentially constant. The results shown in Figure 6 indicate that,

as the liquid flow rate of 4-propylguaiacol increased, theconversion decreased. During the experiments, a steadyincrease in liquid slug length was observed as the liquid flowrate was increased from 0.03 to 0.15 mL/min. As mass transferfrom the gas to liquid slugs through the hemispherical caps ofgas bubbles was a strong function of liquid slug length, theconvective mass-transfer rate of hydrogen to the catalyst surfacethrough 4-propylguaiacol decreased when the liquid velocitywas increased,22 which was consistent with our findings.

3.5. Effects of Residence Time. Residence time was variedby increasing the reactor length (catalyst loading) whilekeeping the flow rate, temperature, and pressure constant tostudy the effects on conversion and yield. The result in Figure 7shows that the conversion steadily increased as the residencetime increased.

Figure 4. Effects of temperature on conversion and yield. Reactionconditions: pressure, 2.07 MPa (300 psig); gas phase, hydrogen andnitrogen; liquid phase, 4-propylguaiacol; catalyst, sulfided NiMo/Al2O3; liquid flow rate, 8.33 × 10−10 m3/s (0.05 mL/min).

Figure 5. Effects of inlet hydrogen partial pressure on conversion andyield. Reaction conditions: temperature, 400 °C; gas phase, hydrogenand nitrogen; liquid phase, 4-propylguaiacol; catalyst, sulfided NiMo/Al2O3; liquid flow rate, 0.05 mL/min.

Figure 6. Effects of 4-propylguaiacol liquid flow rate on conversionand yield. Reaction conditions: pressure, 2.07 MPa (300 psig);temperature, 400 °C; gas phase, hydrogen and nitrogen; liquid phase,4-propylguaiacol; catalyst, sulfided NiMo/Al2O3.

Figure 7. Effects of residence time on conversion and yield. Reactionconditions: pressure, 2.07 MPa (300 psig); temperature, 400 °C; gasphase, hydrogen and nitrogen; liquid phase 4-propylguaiacol; catalyst,sulfided NiMo/Al2O3; liquid flow rate, 8.33 × 10−10 m3/s (0.05 mL/min).

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3.6. Effect of Reactor Diameter. A set of experiments wasconducted to study the effect of the reactor diameter on thespace-time consumption (STC) of 4-propylguaiacol. In theseexperiments, reactor internal diameters of 0.0008, 0.0032, and0.0064 m were used, as the temperature, pressure, residencetime, and superficial velocity were kept constant. The resultsshown in Figure 8 indicate that the STC decreased as the

reactor diameter increased. The higher STC for the micro-reactor with a diameter of 0.0008 m (<1 mm) can be attributedto the higher concentration of hydrogen on the catalyst surfacedue to intensified recirculation of liquid slugs.23−27 Based onthe results presented in Figure 8, it can be deduced that, as thereactor diameter increased, the overall performance of thereactor diminished. Hence, comparatively, the microreactor(i.d. < 1 mm) performs better than larger-diameter reactors interms of STC.3.7. Kinetic Study. 3.7.1. External Mass-Transfer Limi-

tations. The heterogeneous reaction of 4-propylguaiacol andhydrogen on the sulfided NiMo/Al2O3 catalyst surface involvestransfer of hydrogen into the liquid phase and diffusion throughthe liquid phase to the catalyst through a boundary layersurrounding the catalyst surface. As mass-transfer rates throughboth the gas−liquid and liquid−solid interfaces are affected byflow velocity, the space-time consumption was measured as afunction of total (gas and liquid) superficial velocity while theresidence time was kept constant by varying the catalyst bedvolume. Figure 9 shows that, for the range of velocities studied,

the superficial velocity had no effect on the STC. This indicatesthat, at the lowest velocity selected for this study, the boundarylayer was so thin that it no longer offered any significantresistance to diffusion; hence, hydrodeoxygenation under theseconditions is not limited by the external mass transfer.3.7.2. Internal Mass-Transfer Limitations. The effect of

internal mass-transfer limitations was studied by varying the

catalyst particle size. Two different particle size ranges of (38−45) × 10−6 and (75−150) × 10−6 m were selected for a studyof the effect of catalyst particle size on the space-time yield of 4-propylguaiacol. The space-time yields of 4-propylguaiacol forthe particle size ranges of (38−45) × 10−6 and (75−150) ×10−6 m were 0.0366 and 0.0372 kg of 4-propylguaiacol perkilogram of catalyst per second, respectively, which indicatesthat there were no diffusional mass-transfer limitations for theparticle size range of (75−150) × 10−6 m.Internal mass-transfer limitations can be estimated by

calculating the Thiele modulus for the particle size range of(75−150) × 10−6 m assuming a pseudo-first-order reactionwith respect to hydrogen and 4-propylguaiacol28,29 according tothe equation30

φρ

=′⎛

⎝⎜⎜

⎞⎠⎟⎟

d r

D C(Thiele modulus)

6expp p 4PG

e H

0.5

2 (6)

where −r4‑propylguaiacol′ is the reaction rate; ρp is the catalystparticle density; and De is the effective diffusivity, which wasestimated using the equation De = (DABφpσc)/τ, where DAB isthe binary diffusivity of hydrogen in the liquid reactant. DAB wasestimated to be 1.23 × 10−8 m2/s according to the Wilke−Chang equation,31 using typical values for the porosity (φp),constriction factor (σc), and tortuosity (τ) of 0.4, 0.8, and 3,respectively. The Thiele modulus was estimated to be 0.45,which corresponds to an internal effectiveness factor of unity,indicating that the actual overall rate of reaction was equal tothe rate of reaction that would result if the entire interiorsurface were exposed to the external catalyst surface conditions(CAs, Ts). Therefore, it can be concluded that the reaction rateof 4-propylguaiacol was not limited by internal mass transferwithin the catalyst particles.

3.7.3. Heat-Transfer Limitations. A qualitative analysis ofradial heat-transfer limitations was conducted by calculating theDamkohler number (Da) for heat transfer and comparing it tothe value of 0.4(RTw/Ea),

32 as shown by the equation

λ=

−Δ − − ϵ+

<DaH r R

T bRTE

( )(1 )(1 )

0.4obs o2

w

w

a (7)

If this relation holds, then the radial temperature difference inthe reactor will be less than 5%. In eq 7, the heat of reaction(ΔH) is −83.03 kJ/mol, the observed reaction rate (−robs) is139.0631 mol/m3·s at a temperature (Tw) of 673 K, the radiusof the tubular reactor (Ro) is 0.00038 m, the bed porosity (ϵ) isassumed to be 0.3, the effective thermal conductivity of porouscatalyst (λ) is on the order of 0.1 W/m·K,33 and the ratio ofdiluent to catalyst volume (b) is 0. The activation energy (Ea)for the reaction is 34 kJ/mol, and R is the gas constant.Calculation showed that the left-hand side in eq 7 was 2 ordersof magnitude smaller than the right-hand side. Therefore, radialheat-transfer effects in the microreactor were not a factor.

3.7.4. Differential Method for Rate Analysis. A set ofexperiments was conducted under different operating con-ditions to determine the kinetic rate expression for thehydrodeoxygenation of 4-propylguaiacol. The overall velocitywas maintained in such a way that it would ensure the absenceof external mass-transfer limitations, and the conversion ofreactants was confined to 10% or below so that the dependenceof the reaction rates on the initial reactant concentrations canbe assumed. The concentration of hydrogen in the reaction

Figure 8. Effect of reactor diameter on STC. Reaction conditions:pressure, 2.07 MPa (300 psig); temperature, 400 °C; gas phase,hydrogen and nitrogen; liquid phase, 4-propylguaiacol; catalyst,sulfided NiMo/Al2O3.

Figure 9. Effect of total fluid flow velocity on reaction rate. Reactionconditions: pressure, 2.07 MPa (300 psig); temperature, 400 °C; gasphase, hydrogen and nitrogen; liquid phase, 4-propylguaiacol.

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medium was obtained from the solubility data for hydrogen inhexane.34

The effect of the hydrogen concentration on the rate ofdisappearance of 4-propylguaiacol was investigated in a range of0−0.15 mol/L at 4-propylguaiacol concentration of 1.1 mol/Lin hexane at different temperatures. The results shown inFigure 10 indicate that the rate of disappearance of 4-

propylguaiacol increased with increasing hydrogen concen-tration. Figure 11 shows a log−log plot of rate versus hydrogen

concentration. The slopes of the lines represent the orders ofthe rate dependence on the hydrogen concentration, which isdifferent at different temperatures. This indicates that a simplepower-law model would not be appropriate for the rateexpression.The effect of the hydrogen concentration on the rate of

formation of 4-propylphenol was investigated in the range of0−0.15 mol/L at a 4-propylguaiacol concentration of 1.1 mol/Lin hexane at different temperatures. The results shown inFigure 12 indicate that the rate of formation of 4-propylphenolincreased with increasing hydrogen concentration at lower H2concentrations and then remained constant at higher H2concentrations.The effect of the 4-propylguaiacol concentration on the rate

of disappearance of 4-propylguaiacol was investigated in theconcentration range of 0.1−2.1 mol/L at a hydrogen pressureof 1.43 MPa (208 psig) at different temperatures. The resultsshown in Figure 13 indicate that the rate increased initially as

the 4-propylguaiacol concentration increased but thenremained constant at higher 4-propylguaiacol concentrations.

Similarly, the effect of the 4-propylguaiacol concentration onthe rate of formation of 4-propylphenol was investigated in theconcentration range of 0.1−2.1 mol/L at a hydrogen pressureof 1.43 MPa (208 psig) at different temperatures. The resultsshown in Figure 14 indicate that the rate of formation of 4-propylphenol increased linearly at lower 4-propylphenolconcentrations but then remained constant at higherconcentrations.

3.7.5. Kinetic Modeling. The reaction pathway of hydro-deoxygenation of 4-propylguaiacol on sulfided NiMo catalyst isshown in Figure 15 for the purpose of the kinetic study.Rate equations for the reaction in Figure 15 were developed

using the Langmuir−Hinshelwood model. After eliminatingexternal mass transfer, internal mass diffusion, and heat-transferlimitations, a catalytic reaction in the Langmuir−Hinshelwoodmodel involves a sequence of three steps:

(1) adsorption of the reactants on the catalyst surface,(2) reaction of the reactants on the catalyst surface, and(3) desorption of the products from the catalyst surface.

Figure 10. Effect of H2 concentration on the rate of disappearance of4-propylguaiacol.

Figure 11. Plot of the log−log form of the rate of disappearance of 4-propylguaiacol with respect to hydrogen concentration.

Figure 12. Effect of hydrogen concentration on the rate of formationof 4-propylphenol.

Figure 13. Effect of the 4-propylguaiacol concentration on the rate ofdisappearance of 4-propylguaiacol.

Figure 14. Effect of the 4-propylguaiacol concentration on the rate offormation of 4-propylphenol.

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These steps occur on the molybdenum surface denoted by“s” for the reaction in Figure 15.The mechanistic schemes for reactions assuming competitive

reaction with nondissociative adsorption of hydrogen, com-petitive reaction with dissociative adsorption of hydrogen,noncompetitive reaction with nondissociative adsorption ofhydrogen, and noncompetitive reaction with dissociativeadsorption of hydrogen are shown in Figures 16−19,respectively.

Different rate equations were derived for each of thesemechanistic schemes mentioned assuming each step as the rate-limiting step. These rate equations are listed in Tables 1−4.The rate data obtained from the experiments were best-fitted

to the tabulated rate equations by conducting a nonlinearregression analysis to estimate the kinetic constants. It wasfound that the rate data were best-fit by a rate equation derivedfor the surface reaction as the rate-controlling step assumingnoncompetitive adsorption of reactants and nondissociativeadsorption of hydrogen.Surface reaction as the rate-controlling step can be expressed

as

Figure 15. Reaction pathway for the hydrodeoxygenation of 4-propylguaiacol.

Figure 16. Mechanistic scheme for competitive reaction withnondissociative adsorption of hydrogen.

Figure 17. Mechanistic scheme for competitive reaction withdissociative adsorption of hydrogen.

Figure 18. Mechanistic scheme for noncompetitive reaction withnondissociative adsorption of hydrogen.

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− ′ =+ +

rk K C K C

K C K C

( )( )

(1 )(1 )4PGHeql

H 4PGeql

4PG

Heql

H 4PGeql

4PG

2 2

2 2 (8)

The second best-fitting rate equation was found to be the rateequation for surface reaction with the assumption of non-competitive adsorption of reactants and dissociative adsorptionof hydrogen; however, the optimization variables from

nonlinear regression were less than 95% for all threetemperatures. Therefore, this rate equation was not consideredfurther.The values of the kinetic constants obtained for eq 8 are

listed in Table 5.

The activation energy and heats of adsorption of 4-propylguaiacol and hydrogen for hydrodeoxygenation weredetermined from kinetic constants using the Arrheniusequations and are reported in Table 6.

The rate equation and its kinetic parameters were validatedby comparing the experimental data obtained under integralconditions with the theoretical predictions using the Runge−Kutta method. A set of integral experiments were carried out atdifferent amounts of catalysts but constant residence timeunder kinetically controlled conditions at different temper-

Figure 19. Mechanistic scheme for noncompetitive reaction withdissociative adsorption of hydrogen.

Table 1. Rate Equations Derived for Competitive Reactionwith Nondissociative Adsorption of Hydrogen

no. rate-limiting step rate equation

1 adsorption of 4-propylguaiacol − ′ =

+r

kCK C(1 )4PG

4PG

Heql

H2 2

2 adsorption of hydrogen− ′ =

+r

kC

K C(1 )4PGH

4PGeql

4PG

2

3 surface reaction− ′ =

+ +r

k K C K C

K C K C

( )( )

(1 )4PGHeql

H 4PGeql

4PG

4PGeql

4PG Heql

H2

2 2

2 2

4 desorption of product − ′ =r k4PG

Table 2. Rate Equations Derived for Competitive Reactionwith Dissociative Adsorption of Hydrogen

no. rate-limiting step rate equation

1 adsorption of 4-propylguaiacol − ′ =

+r

kC

K C(1 )4PG4PG

Heql

H1/2

2 2

2 adsorption of hydrogen− ′ =

+r

k C

K C

( )

(1 )4PGH

1/2

4PGeql

4PG

2

3 surface reaction− ′ =

+ +r

k K C K C

K C K C

( )( )

(1 )4PGHeql

H 4PGeql

4PG

4PGeql

4PG Heql

H2

2 2

2 2

4 desorption of product − ′ =r k4PG

Table 3. Rate Equations Derived for NoncompetitiveReaction with Nondissociative Adsorption of Hydrogen

no. rate-limiting step rate equation

1 adsorption of 4-propylguaiacol

− ′ =r kC4PG 4PG

2 adsorption of hydrogen − ′ =r kC4PG 4PG

3 surface reaction− ′ =

+ +r

k K C K C

K C K C

( )( )

(1 )(1 )4PGHeql

H 4PGeql

4PG

Heql

H 4PGeql

4PG

2 2

2 2

4 desorption of product − ′ =r k4PG

Table 4. Rate Equations Derived for NoncompetitiveReaction with Dissociative Adsorption of Hydrogen

no. rate-limiting step rate equation

1 adsorption of 4-propylguaiacol

− ′ =r kC4PG 4PG

2 adsorption ofhydrogen

− ′ =r k C( )4PG H1/2

2

3 surface reaction− ′ =

+ +r

k K C K C

K C K C

( )( )

(1 )[1 ( ) ]4PGHeql

H 4PGeql

4PG

4PGeql

4PG Heql

H1/2 2

2 2

2 2

4 desorption ofproduct

− ′ =r k4PG

Table 5. Kinetic Constants for the Rate Equations

T(°C) k(mol/g·h)

K4‑propylguaiacol(L/mol) KH2

(L/mol) R2

250 22.21 ± 0.77 0.02 ± 0.002 0.3 ± 0.004 0.97300 41.40 ± 4.21 0.1 ± 0.01 0.1 ± 0.01 0.96350 77.78 ± 0.01 0.6 ± (9.7 × 10−5) 0.02 ± (1.6 × 10−6) 0.96

Table 6. Pre-exponential Factors, Activation Energies, andHeats of Adsorption for the Reaction

intrinsic constant value

Pre-exponential Factorko (×10

−4 mol/g·h) 5.29 ± 0.66K4‑propylguaiacol,o (×10

−7 L/mol) 2.750 ± 0.14KH2,o (×10

8 L/mol) 1.6 ± 0.13

Activation Energy and Heat of AdsorptionEa (kJ/mol) 33.86 ± 2.70ΔH4‑propylguaiacol (kJ/mol) 91.85 ± 2.69ΔHH2

(kJ/mol) −72.69 ± 2.63

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dx.doi.org/10.1021/ie400037y | Ind. Eng. Chem. Res. 2013, 52, 4049−40584056

atures. The experimental data and predictions for theconversion of 4-propylguaiacol are shown in Figure 20. The

difference of less than 10% between the predictions and theexperimental data indicates that the rate parameters representthe data obtained under both differential and integralconditions.

4. CONCLUSIONSThe hydrodeoxygenation reaction of 4-propylguaiacol overreduced sulfided NiMo/Al2O3 catalyst in a microreactor hasbeen studied. The experimental results indicate that theconversion of 4-propylguaiacol is highly influenced by temper-ature. The conversion and product yields were found toincrease as temperature was increased from 200 to 450 °C.Although many products formed, 4-propylphenol predomi-nated under the operating conditions selected with presulfidedNiMo catalyst. The conversion and yields increased for diluted4-propylguaiacol in hexane, which could be due to the increasein solubility of hydrogen in hexane. The effect of hydrogenpartial pressure indicated that conversion initially increased aspressure increased from 1.65 to 3.30 MPa (from 200 to 400psig) and then remained constant for higher hydrogenpressures.As the liquid flow rate of 4-propylguaiacol increased, the

conversion and yield decreased because of the decrease in theconvective mass-transfer rate of hydrogen from gas bubbles tothe catalyst surface through the 4-propylguaiacol slug. Theeffect of residence time on conversion was studied by increasingthe reactor length, and it was found that the conversion steadilyincreased as the residence time increased. The conversion of 4-propylguaiacol decreased as the internal reactor diameterincreased because of the decreased concentration of hydrogenon the catalyst surface because of the diminishing rate ofrecirculation.A packed-bed microreactor was used to develop an intrinsic

kinetic rate equation for the hydrodeoxygenation of 4-propylguaiacol. Many studies have shown that mass- andheat-transfer limitations in microreactors are negligible. Theoverall flow velocity selected was 2.5 m/s or above, which wasfar beyond the external mass-transfer resistance (<0.26 m/s) fora packed-bed microreactor.35 Internal mass-transfer and heat-transfer limitations were also found to be negligible for theoperating conditions selected. Therefore, the hydrodeoxygena-tion of 4-propylguaiacol in a microreactor can be considered tobe kinetically controlled under the selected operatingconditions. For the hydrodeoxygenation of 4-propylguaiacol,the Langmuir−Hinshelwood model was found to be suitablefor rate expressions. Based on the experimental data, it wasfound that the surface reaction was the rate-controlling step.

Kinetic constants were obtained from nonlinear regression ofthe experimental data in a differential microreactor. The derivedkinetics was able to predict the integral reactor behavior in thekinetic-controlled regime.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: njoshi@stevens.edu.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

N.J. gratefully acknowledges support from the Robert C.Stanley Graduate Fellowship Program of Stevens Institute ofTechnology and GK-12 fellowship of the National ScienceFoundation under Grant DGE-0740462. In addition, we aregrateful to Dr. James Manganaro for his technical contributionto this work and to Steve Mayo (Albemarle) for providing thecatalysts.

■ NOMENCLATURE

Ao = pre-exponential factor for a reaction (mol/g·h)b = ratio of diluent to catalyst volumeC4PG = concentration of 4-propylguaiacol in the liquidmixture (mol/L)CH2

= H2 concentration in the liquid (kmol/m3)CH2

= concentration of hydrogen in the liquid mixture (mol/L)Da = Damkohler numberDe = effective diffusivity of hydrogen in the bulk liquid (m2/s)dp = catalyst particle diameter (m)Ea = activation energy (kJ/mol)k = intrinsic kinetic rate constant (mol/g·h)k = reaction rate constant (mol/g·h)K = equilibrium constant (L/mol)ko = pre-exponential factor (mol/g·h)Ko = pre-exponential factor (L/mol)K4‑propylguaiacol,o = pre-exponential factor for 4-propylguaiacol(L/mol)K4PGeql = equilibrium constant for 4-propylguaiacol (L/mol)

KH2

eql = equilibrium constant for hydrogen (L/mol)KH2,o = pre-exponential factor for hydrogen (L/mol)R = ideal gas constantRo = radius of the tubular reactor (m)r4‑propylguaiacol′ = reaction rate of 4-propylguaiacol (kmol/kg·s)−r4PG′ = reaction rate for HDO of 4-propylguaiacol (mol/g·h)−robs = observed reaction rate (mol/m3·s)Tw = reactor wall temperature (K)ΔH = heat of reaction (kJ/mol)ΔH4‑propylguaiacol = heat of adsorption of 4-propylguaiacol (kJ/mol)ΔHH2

= heat of adsorption of hydrogen (kJ/mol)ϵ = bed porosityλ = effective thermal conductivity (W/m·K)ρp = particle density (kg/m3)σc = constriction factorτ = tortuosityφexp = Thiele modulusφp = porosity

Figure 20. Experimental and predicted conversions of 4-propylguaia-col under integral reaction conditions.

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