Article pubs.acs.org/IECR
Hydrodeoxygenation of 4‑Propylguaiacol (2-Methoxy-4propylphenol) in a Microreactor: Performance and Kinetic Studies Narendra Joshi* and Adeniyi Lawal New Jersey Center for Microchemical Systems (NJCMCS), Department of Chemical Engineering and Materials Science, Stevens Institute 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 pyrolysis oil, was performed in a packed-bed microreactor. The effects of various processing conditions were investigated using presulfided NiMo/Al2O3 catalyst. The conversion of 4-propylguaiacol is highly influenced by temperature. Although many products were formed, 4-propylphenol predominated under the operating conditions selected with presulfided NiMo catalyst. External and internal mass-transfer resistances and heat-transfer resistance were investigated and found to be negligible. Reaction rate expressions were based on proposed reaction mechanisms using the Langmuir−Hinshelwood approach. Nonlinear regression was performed to obtain kinetic constants. The best-fitting rate equations were further validated by comparing experimental data obtained from an integral reactor with predictions obtained using the Runge−Kutta method based on the rate equations. A difference of less than 10% between the experimental data and the predicted data for the integral reactor was found. of laminar flow, mixing in microchannels occurs predominantly by interdiffusion of reactants.7 However, because of short transverse diffusional distance, rapid and effective mixing is attainable in a microreactor that can quickly bring reactants into contact with catalyst in a heterogeneous reaction.8 Fast mixing in two-phase flows can also be achieved by selecting appropriate inlet T-orientations to provide a short slug length.9 Most hydrodeoxygenation studies have focused on sulfided CoMo- and NiMo-based catalysts, which are industrial hydrotreating catalysts for the removal of sulfur, nitrogen, and oxygen from petrochemical feedstocks.10 Platinum,11 vanadium nitride,12 and ruthenium13 have also been used for hydrodeoxygenation. In this study, we used the sulfided form of NiO/MoO3 on Al2O3. Model compounds as representatives of pyrolysis oil are typically used in the hydrodeoxygenation process to better elucidate reaction mechanisms and kinetics. Zhao et al.14 conducted the gas-phase hydrodeoxygenation of guaiacol on a series of novel hydroprocessing catalysts such as Ni2P/SiO2, Fe2P/SiO2, MoP/SiO2, Co2P/SiO2, and WP/SiO2. They found that the activities of the catalysts for the HDO of guaiacol followed the order Ni2P > Co2P > Fe2P, WP, MoP and that the major products formed were phenol, benzene, and methoxybenzene, with no catechol formed at higher contact times whereras, at lower contact times, catechol was the major product for Co2P and WP. It was mentioned that no catechol was observed for the HDO of guaiacol with Ni2P even at low contact times. They also found that the commercial 5% Pd/ Al2O3 catalyst was more active than the metal phosphides at lower contact times and that the major product was catechol. They also used commercial CoMoS/Al2O3 catalyst for HDO,
1. INTRODUCTION The consumption of fossil fuels is increasing steadily and expected to grow by 53% in 2035 compared to 2008,1 as the world population comes close to 9 billion. The known reserves of fossil fuels are continuously decreasing, and excessive use of fossil fuels has increased the emissions of greenhouse gases, particularly carbon dioxide, which is considered to contribute strongly to global warming. These factors have led to increasing interest in the use of biomass-derived fuels that do not contribute any new carbon dioxide to the atmosphere. Nonfood lignocellulosic biomass feedstocks such as corn stover, straw, wood chips, switch grass, and other waste products can be used for the production of fuels. These biomass feedstocks consisting of cellulose, hemicellulose, and lignin are converted to pyrolysis oil (PO) through a thermochemical route known as fast pyrolysis. Pyrolysis oil is further processed by hydrodeoxygenation (HDO) to obtain transportation fuel. However, more than 300 oxygenated organic compounds in pyrolysis oil complicate the study of its reaction mechanisms and kinetics. Many cellulose-derived components of pyrolysis oil have been investigated, but the lignin-derived components have not received much attention, and there is limited understanding of the reaction networks and kinetics of HDO of these components.2 In this work, we have selected 4-propylguaiacol as a model compound to investigate the hydrodeoxygenation process in a microreactor. 4-Propylguaiacol represents some of the major lignin-derived components present in pyrolysis oil such as benzene, phenol, guaiacol, anisole, propyl anisole, propylphenol, and propylbenzene. The presence of phenolic compounds in pyrolysis oil is the cause of polymerization and coke formation during hydrodeoxygenation at temperatures above 300 °C. By conducting a study on the hydrodeoxygenation of 4-propylguaiacol, we hope to understand a mechanism and kinetics for the upgrading of pyrolysis oil. Better selectivity, high yield, improved product quality, and safe operation are attainable in microreactors because of very fast heat and mass transfer.3−6 Studies have shown that, because © 2013 American Chemical Society
Received: Revised: Accepted: Published: 4049
January 4, 2013 February 25, 2013 February 26, 2013 February 26, 2013 dx.doi.org/10.1021/ie400037y | Ind. Eng. Chem. Res. 2013, 52, 4049−4058
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conversion, selectivity, and space-time yield were investigated using presulfided Ni−Mo/Al2O3 catalyst.
but it deactivated quickly and showed little activity for the HDO of guaiacol. Another group2 also used guaiacol as a model of lignin-derived components in pyrolysis oil for the hydrodeoxygenation to elucidate the reaction network and to predict oxygen removal. The conversion of guaiacol took place with a high selectivity for aromatic carbon−oxygen bond cleavage relative to the accompanying methyl-group-transfer reactions. The catalytic conversion of guaiacol in the presence of hydrogen catalyzed by Pt/Al2O3 was found to involve three major classes of reactions, namely, hydrogenation, hydrodeoxygenation, and transalkylation, with selectivities to hydrodeoxygenation products comparable to the selectivities to the accompanying transalkylation products. Bio-oil upgrading at Pacific Northwest National Laboratory (PNNL) focused on heterogeneous catalytic hydroprocessing of model compounds using various commercial catalysts in a batch reactor.15 Catalysts such as CoMo, NiMo, NiW, Ni, Co, Pd, and CuCrO 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 at 400 °C. Of the catalysts tested, the sulfided form of CoMo was most active, producing a product containing 33.8% benzene and 3.6% cyclohexane at 400 °C. Another group16 synthesized and tested mono- and bimetallic (Pt−Sn alloy) monoliths for the hydrodeoxygenation of guaiacol and anisole. Both Pt−Sn/ Inconel and Pt−Sn/carbon nanofiber (CNF)/Inconel were able to fully deoxygenate guaiacol and anisole. Coating with CNFs increased the surface area of the monoliths more than 10 times, allowing for a higher metal uptake during the activephase incorporation compared to monoliths without coating. According to the researchers, the Pt−Sn/CNF/Inconel monolith is a promising catalyst for the upgrading of pyrolysis oil. Massoth et al. conducted hydrodeoxygenation of methylsubstituted phenols in a microreactor at 300 °C and 2.85 MPa hydrogen pressure using a sulfided CoMo/Al2O3 catalyst.17 Methyl-substituted benzene, cyclohexene, cyclohexane, and H2O were the primary products. Based on an analysis of the results, the authors suggested two independent reaction paths, one leading to aromatics and the other leading to partially or completely hydrogenated cyclohexanes. A kinetics study was conducted using Langmuir−Hinshelwood model to obtain adsorption and rate constants characterizing the two reaction paths. The same adsorption constant found for the two reaction paths suggested that a single catalytic site center is operative in both reactions. Similarly, Edelman et al. performed the vaporphase catalytic hydrodeoxygenation of benzofuran at 300 °C and 35 atm total pressure over a presulfided NiMo/Al2O3 catalyst in a microreactor.18 The results showed that benzofuran reacted through hydrogenation and hydrogenolysis to form 2,3-dihyrobenzofuran, o-ethylphenol, and phenol. The subsequent hydrodeoxygenation products of the phenols were ethylbenzene, toluene, benzene, and ethylcyclohexane. The authors suggested that the hydrogenation of benzofuran can be modeled as pseudo-first-order in the benzofuran concentration and that the hydrodeoxygenation reaction can be modeled as non-first-order kinetics, possibly −1 order in oxygenated compounds. The objective of the work presented here was to evaluate the hydrodeoxygenation of 4-propylguaiacol and to study reaction mechanisms and kinetics in a packed-bed microreactor. The effects of various processing conditions such as hydrogen partial pressure, reactor diameter, temperature, and residence time on
2. EXPERIMENTAL SECTION 2.1. Materials. 4-Propylguaiacol was purchased from SigmaAldrich. Hydrogen gas was purchased from Praxair. Nitrogen was used as a tracer to perform a material balance. The feed mixture 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 particles with diameters in the range of 75−150 μm. The average surface area of the sieved catalysts was 164 m2/g, and the average pore diameter was 106 Å. The surface area and pore diameter were obtained by using the multipoint Brunauer−Emmett−Teller (BET) technique on a Quantochrome Autosorb-1 instrument. The catalyst was reduced with 5.0 sccm (standard cubic centimeters per minute) of hydrogen at 593 K and 3.45 MPa for 2 h. The average surface area of the reduced catalysts was 209.0 m2/g, and the average pore diameter was 92.0 Å. The catalyst activity remained constant for 7 h of on-stream time. In each 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 (Porter model 201) were used to control the flow rates of hydrogen and nitrogen. A high-performance liquid chromatography pump (Laboratory Alliance Series III) was used to control the flow rate of 4-propylguaiacol. The ranges of flow rates used for 4-propylguaiacol, hydrogen, and nitrogen gases were 0.03− 0.18 mL/min, 30−120 sccm, and 10−60 sccm respectively. The liquid and gas phases were combined in a T-junction mixer (Upchurch) with a 508 × 10 −6 m through-hole. The Reynolds number for the combined flow was less than 100 for all experiments, indicating laminar flow. The fluids exiting from the T-junction exhibited a Taylor flow pattern with a liquid slug length in the range of 0.001−0.003 m, whereas gas bubble length varied from 0.001 to 0.005 m. The lengths of the liquid slugs and gas bubbles were measured by observing the Taylor flow pattern exited from the T-junction with the help of a ruler and magnifying eyepiece. A microreactor was prepared from a 0.0016-m 316 stainless steel tubing with a 762 × 10 −6 m internal diameter (i.d.) that was gravity-filled with catalyst. The total length of the packed-bed microreactor varied from 0.025 to 0.18 m. Hastelloy micrometer filter cloth (200 × 1150 mesh, Unique Wire Weaving Co., Hillside, NJ) was placed at the ends of the reactor to retain the catalyst. The reactor system was pressurized 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. The pressure drop along the reactor varied from 0.07 to 0.2 MPa depending on reactor length. A schematic flow diagram of the microreactor setup is shown in Figure 1. 2.3. Analysis. Analysis of the liquid HDO product was conducted by gas chromatography with mass spectrometry (GC/MS). The GC/MS analysis was performed using a Varian instrument (GC 3900, equipped with a Varian CP-1177 split/ splitless injector and a Varian CP-8410 autosampler), and for detection, an ion-trap mass spectrometer (Varian Saturn 2100T) was used. The capillary column used for gas chromatograph was a Factor Four VF-5 ms column (30 m in length with a 2.5 × 10−4 m diameter and a 2.5 × 10−7 m film thickness). 4050
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deoxygenation by removal of the hydroxyl group. Formation of phenol, 4-ethylphenol, and cresol indicates the dealkylation reaction. Slightly more formation of 4-propylbenzene compared to phenol suggests that dehydroxylation is favored compared to dealkylation. The reaction of 4-propylguaiacol was characterized in terms of conversion, yield, space-time consumption (STC), rate of disappearance of 4-propylguaiacol, and rate of formation of 4propylphenol, which are defined as follows conversion (%) =
mass of 4PG reacted × 100% mass of 4PG fed
(1)
yield (%) = mass of product formed × 100% theoretical mass of product that could be formed Figure 1. Experimental setup.
(2)
The HDO of 4-propylguaiacol involves complex reactions consisting of series and parallel reactions possibly forming 4propylphenol, 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 phenolic compounds2,14,16,19−21 and product analysis, a possible reaction network of hydrodeoxygenation of 4-propylguaiacol using presulfided NiO/MoO3/Al2O3 is shown in Figure 2. The
space‐time consumption (STC) =
mass of feed reacted mass of catalyst × time (3)
rate of disappearance of 4‐propylguiacol mass of 4PG reacted = mass of catalyst × time
(4)
rate of formation of 4‐propylphenol mass of 4PP formed = mass of catalyst × time
(5)
3. RESULTS AND DISCUSSION 3.1. Catalyst Activity. A presulfided NiMo/Al2O3 catalyst was investigated for its activity. The experiment was conducted for 7 h, and a sample was collected every hour. The results shown in Figure 3 indicate that the catalyst did not lose its activity for 7 h of on-stream time.
Figure 2. Reaction network for the hydrodeoxygenation of 4propylguaiacol. Figure 3. Catalyst activity. Reaction conditions: temperature, 623 K (350 °C); pressure, 2.07 MPa (300 psig); gas phase, hydrogen and nitrogen; liquid phase, 4-propylguaiacol; liquid flow rate, 8.33 × 10−10 m3/s (0.05 mL/min).
predominant products of the hydrodeoxygenation of 4propylguaiacol catalyzed by presulfied-NiMo/Al2O2 catalyst were 4-propylphenol, 4-propylbenzene, 4-ethylphenol, phenol, and cresol with trace amounts of benzene and toluene; the highest selectivity was toward 4-propylphenol followed by 4propylbenzene, 4-ethylphenol, phenol, and cresol. Guaiacol was not detected, but based on the products formed and literature reviews, it was assumed to be converted to cresol, catechol, phenol, and methxybenzene. Catechol and methoxybenzene were not quantified because of a lack of standards. Higher selectivity toward 4-propylphenol indicates that deoxygenation by removal of the methoxy group is more favorable than
3.2. Effects of Temperature. 4-Propylguaiacol was hydrodeoxygenated at various temperatures but at a constant total pressure of 2.07 MPa (300 psig). The residence time was kept constant by varying the reactor length (catalyst loading) to compensate for the change in gas velocity. Temperature was varied from 200 to 450 °C. Many products were detected for the HDO product samples. The products identified and 4051
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3.4. Effects of the Liquid Flow Rate of 4-Propylguaiacol. Experiments were conducted to study the effects of the liquid flow rate of 4-propylguaiacol on the conversion and yield by varying the liquid flow rate from 0.03 to 0.15 mL/min. Other operating conditions such as temperature, pressure, and gas flow rate were kept constant. In these experiments, the actual volumetric gas flow rate was at least 1 order of magnitude higher than the liquid flow rate; hence, the residence time was essentially constant. The results shown in Figure 6 indicate that,
quantified were 4-propylphenol, phenol, cresols, ethylphenol, and propylbenzene, with 4-propylphenol having the largest yield. Other products, which amounted to less than 10%, were not quantified because of a lack of standards. Based on the products formed, the selectivity toward the formation of 4propylphenol was dominant. The results shown in Figure 4 indicate that the conversion of 4-propylguaiacol and yield of 4propylphenol products increased as the temperature increased.
Figure 4. Effects of temperature on conversion and yield. Reaction conditions: pressure, 2.07 MPa (300 psig); gas phase, hydrogen and nitrogen; liquid phase, 4-propylguaiacol; catalyst, sulfided NiMo/ Al2O3; liquid flow rate, 8.33 × 10−10 m3/s (0.05 mL/min).
Figure 6. Effects of 4-propylguaiacol liquid flow rate on conversion and 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.
3.3. Effects of Hydrogen Partial Pressure. A set of experiments was carried out at 400 °C in the range of 1.65− 3.30 MPa (240−480 psig) to study the effects of the inlet hydrogen partial pressure on the conversion and yield. Reaction temperature and residence time were kept constant. The residence time was kept constant by varying the reactor length (catalyst loading). The H2 partial pressure was varied by changing the total pressure. The results in Figure 5 indicate that, initially, the conversion increased as the hydrogen partial pressure increased but, after the hydrogen partial pressure 2.20 MPa (320 psig), the conversion remain constant, indicating that adsorbed hydrogen on the catalyst surface had reached a maximum (saturated) value.
as the liquid flow rate of 4-propylguaiacol increased, the conversion decreased. During the experiments, a steady increase in liquid slug length was observed as the liquid flow rate was increased from 0.03 to 0.15 mL/min. As mass transfer from the gas to liquid slugs through the hemispherical caps of gas bubbles was a strong function of liquid slug length, the convective mass-transfer rate of hydrogen to the catalyst surface through 4-propylguaiacol decreased when the liquid velocity was increased,22 which was consistent with our findings. 3.5. Effects of Residence Time. Residence time was varied by increasing the reactor length (catalyst loading) while keeping the flow rate, temperature, and pressure constant to study the effects on conversion and yield. The result in Figure 7 shows that the conversion steadily increased as the residence time increased.
Figure 7. Effects of residence time on conversion and 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; liquid flow rate, 8.33 × 10−10 m3/s (0.05 mL/ min).
Figure 5. Effects of inlet hydrogen partial pressure on conversion and yield. Reaction conditions: temperature, 400 °C; gas phase, hydrogen and nitrogen; liquid phase, 4-propylguaiacol; catalyst, sulfided NiMo/ Al2O3; liquid flow rate, 0.05 mL/min. 4052
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3.6. Effect of Reactor Diameter. A set of experiments was conducted to study the effect of the reactor diameter on the space-time consumption (STC) of 4-propylguaiacol. In these experiments, reactor internal diameters of 0.0008, 0.0032, and 0.0064 m were used, as the temperature, pressure, residence time, and superficial velocity were kept constant. The results shown in Figure 8 indicate that the STC decreased as the
catalyst particle size. Two different particle size ranges of (38− 45) × 10−6 and (75−150) × 10−6 m were selected for a study of the effect of catalyst particle size on the space-time yield of 4propylguaiacol. The space-time yields of 4-propylguaiacol for the particle size ranges of (38−45) × 10−6 and (75−150) × 10−6 m were 0.0366 and 0.0372 kg of 4-propylguaiacol per kilogram of catalyst per second, respectively, which indicates that there were no diffusional mass-transfer limitations for the particle 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 reaction with respect to hydrogen and 4-propylguaiacol28,29 according to the equation30 ′ ⎞ d p ⎛ ρp r4PG ⎜⎜ ⎟ φexp (Thiele modulus) = 6 ⎝ DeC H2 ⎟⎠
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.
0.5
(6)
where −r′4‑propylguaiacol is the reaction rate; ρp is the catalyst particle density; and De is the effective diffusivity, which was estimated using the equation De = (DABφpσc)/τ, where DAB is the binary diffusivity of hydrogen in the liquid reactant. DAB was estimated 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 to the rate of reaction that would result if the entire interior surface were exposed to the external catalyst surface conditions (CAs, Ts). Therefore, it can be concluded that the reaction rate of 4-propylguaiacol was not limited by internal mass transfer within the catalyst particles. 3.7.3. Heat-Transfer Limitations. A qualitative analysis of radial heat-transfer limitations was conducted by calculating the Damkohler number (Da) for heat transfer and comparing it to the value of 0.4(RTw/Ea),32 as shown by the equation
reactor diameter increased. The higher STC for the microreactor with a diameter of 0.0008 m (
