Article pubs.acs.org/EF
Catalytic Hydrothermal Liquefaction of Soy Protein Concentrate Ligang Luo,†,‡ Liyi Dai,† and Phillip E. Savage*,‡,§ †
Shanghai Key Laboratory of Green Chemistry and Green Process, Department of Chemistry, East China Normal University, No. 500 Dongchuan Road, Shanghai 200241, People’s Republic of China ‡ Chemical Engineering Department, University of Michigan, 3074 H.H. Dow Building, 2300 Hayward St., Ann Arbor, Michigan 48109-2136, United States § Chemical Engineering Department, Pennsylvania State University, 160 Fenske Lab, University Park, Pennsylvania 16802, United States ABSTRACT: We report herein on the hydrothermal catalytic liquefaction of soy protein concentrate. Catalyst screening experiments have shown that metal catalysts (e.g., 5 wt % Pt, Pd, or Ru) supported on porous solids (e.g., carbon, Al2O3) produce crude bio-oils with less heteroatom content than does noncatalytic liquefaction of this material under otherwise identical conditions. The catalysts had little influence on the biocrude yield. Of the different catalytic materials tested, Ru/C had the greatest influence on the biocrude composition. The heating value of the biocrude produced from liquefaction at 350 °C for 120 min with a 20 wt % loading of Ru/C was 16% higher than the heating value from the noncatalytic run (37 MJ/kg vs 43 MJ/kg). Moreover, the heteroatom content of this biocrude from catalytic liquefaction was less than half that of its noncatalytic counterpart (16.6 wt % vs 7.8 wt %). The catalyst reduced sulfur levels in the biocrude to below detection limits and the nitrogen level to 45% of that in the noncatalytic product. Additional experiments with the Ru/C catalyst showed only modest improvements in biocrude quality with increasing catalyst loading or liquefaction temperature or time. The presence of the Ru/C catalyst shifted the distribution of products amenable to gas chromatography (GC) analysis from being primarily heterocyclic nitrogen-containing compounds and fatty acid amides to being primarily single-ring aromatic hydrocarbons and phenolics. Taken collectively, these results show that heterogeneous catalysts can be effective during hydrothermal liquefaction of material rich in protein.
1. INTRODUCTION Microalgae can serve as a renewable feedstock for liquid fuels. Algal biomass has been attracting attention because of the faster growth rates, shorter growing cycles,1,2 and higher photosynthetic efficiencies3 of microalgae, compared with terrestrial lignocellulosic biomass. Traditional algal biofuel is produced through the extraction of lipids from dried algal biomass by organic solvents, followed by transesterification or hydrotreatment.4 This route is expensive, requires energy for biomass drying, and requires solvents and reagents derived from nonrenewable, fossil-based resources. Hydrothermal liquefaction (HTL), on the other hand, obviates drying and the need for petroleum-derived solvents. HTL involves treating biomass in compressed liquid water at 200−350 °C to break down the biomolecules into smaller, liquid-fuel-range molecules.5,6 The biocrude produced by HTL has about twice the energy density of the original algal biomass, but it still retains much of the oxygen, nitrogen, and sulfur that were present in the biomass. Algae HTL biocrudes typically contain ∼10 wt % O, 5 wt % N, and 1 wt % S.5 These heteroatoms must be removed to produce a fully hydrocarbon renewable crude that could serve as a drop-in replacement for petroleum. Two approaches have been explored for heteroatom removal from the biocrude. One approach is to upgrade the biocrude, either thermally or catalytically, in a separate step after HTL (see, e.g., refs 7−9). The other approach is to use catalysts during the liquefaction process, with the goal of removing more heteroatoms than would be removed via HTL alone.10,11 The latter approach offers the promise of process simplification and intensification, © 2015 American Chemical Society
but only modest enhancements in heteroatom (especially N) removal have been reported for algae HTL. Microalgae are a complex feedstock consisting primarily of carbohydrates, proteins, and lipids, but also containing pigments, inorganic compounds, and nucleic acids. This complexity makes it difficult to elucidate all of the reactions occurring during HTL when one works with algal biomass. Therefore, experiments with model systems can provide information unavailable from experiments with actual algal biomass. Unlike lipids and carbohydrates, the protein fraction of microalgae contains all three major heteroatoms (N, S, and O). Therefore, examining the catalytic HTL of protein could provide new insights into catalytic HTL of algae, as well as other protein-rich biomass. To the best of our knowledge, there has been no prior work on catalytic HTL of protein alone. The reactions of proteins in hot compressed water has certainly received previous attention (see, e.g., ref 12), and there is even some very recent work wherein the products from the hydrothermal treatment of proteins were worked up using the same methods used to make biocrude from algal biomass.13,14 However, these previous studies did not employ heterogeneous catalysts. The purpose of the present study is to determine the effects of different heterogeneous catalysts on the biocrude produced from HTL of a model protein. We used soy protein, because it Received: February 9, 2015 Revised: March 16, 2015 Published: April 16, 2015 3208
DOI: 10.1021/acs.energyfuels.5b00321 Energy Fuels 2015, 29, 3208−3214
Article
Energy & Fuels
with helium and then, in some experiments, further charged with hydrogen (1−5 MPa at room temperature). The loaded reactors were placed in a preheated fluidized sandbath and continuously agitated using a wrist-action shaker. After the desired time had elapsed, the reactors were removed from the sand bath, cooled, and kept under ambient conditions for at least 12 h to allow the liquid−gas system to reach equilibrium prior to performing the gas-phase analysis. A sample of the gaseous components was introduced to a GC via a gas-sampling valve. The total mass of gas present was calculated as the difference between the mass of the cooled reactor after reaction and its mass after venting the gases. After analyzing the gas fraction, the reactors were opened and 9 mL of dichloromethane was used to recover all of the material, including the catalysts. The samples were centrifuged at 1500 rcf for 3 min to facilitate phase separation, and the organic and aqueous phases were transferred to separate vials using a glass pipet. The residual solids (containing catalyst and organics) were retained in the original test tube, and the organic, aqueous, and solid phases were dried under a vacuum of 20 kPa at 35 °C for 90 min. The mass of each dried product was recorded. The material in the organic phase that remains after solvent removal is the biocrude. The yield of each product fraction was calculated using the following equations:
is readily available and has an amino acid profile that is similar to that of protein in many species of microalgae.15 We recognize, of course, that protein derived from different plants (e.g., soybeans versus microalgae) have different features that could conceivably influence the outcome of HTL experiments. Nevertheless, we believe that this first report on catalytic HTL of a biomass protein fraction provides information and insight that advances the field of hydrothermal treatment of biomass. We report herein on the effects of several different commercially available catalytic materials (HZSM-5 (Si:Al = 80:1), 5 wt % Pt/C, 5 wt % Pd/C, 5 wt % Ru/C, 5 wt % Pd/γAl2O3, 5 wt % Pt/γ-Al2O3, CoMo/γ-Al2O3 (sulfided), Mo2C, and MoS2). For Ru/C, which was the most effective catalyst, we also determined the influence of temperature (250, 300, 350, 400 °C), reaction time (60−360 min), catalyst loading (10−50 wt %), and initial hydrogen pressure (1−5 MPa) on the yield and composition of the biocrude.
2. EXPERIMENTAL SECTION 2.1. Materials. Soy protein concentrate (76% protein) was obtained from a local health food store. Table 1 gives its elemental
⎛ mass of biocrude ⎞ yield of biocrude (wt %) = ⎜ ⎟ × 100 ⎝ mass of protein ⎠
Table 1. Elemental and Biochemical Composition of Soy Protein Concentrate
yield of solid residue (wt %) ⎛ mass of solid − mass of catalyst loaded ⎞ =⎜ ⎟ × 100 mass of protein ⎝ ⎠
Biochemical Composition (wt %)
Elemental Composition (wt %) C
H
Oa
N
S
ash
lipid
protein
othersb
48.0
6.76
26.6
13.6
0.5
4.56
4.54
75.8
15.1
⎛ mass of gas − mass of H 2 loaded ⎞ yield of gas (wt %) = ⎜ ⎟ × 100 mass of protein ⎝ ⎠
Calculated by difference: O (%) = 100 − C − H − N − S − ash. b Calculated by difference: others (%) = 100 − lipids − proteins − ash. a
Duplicate independent runs were conducted under nominally identical conditions to determine the uncertainties in the experimental results. Results reported herein represent the mean values for the two independent trials. 2.3. Analytical Chemistry. The gas-phase products were analyzed with an Agilent Technologies Model 6890N gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). The procedure for gas-phase products, except C3−C5 alkanes, was outlined previously.6,16 For the analysis of C3−C5 alkanes, a 6 ft × 1/8 in. OD stainless steel column, packed with 60 × 80 mesh Porapak Q was used to separate each component in the mixture. Argon (15 mL/min) served as the carrier gas for the analysis. The column was initially held at 35 °C for 2 min. The temperature was ramped to 225 °C at 20 °C min−1 and held isothermally for 8 min. Mole fractions of the gaseous products were determined from calibration curves obtained by analysis of gas standards with known compositions. Separation of the liquid-phase products was achieved by using an Agilent 6890 GC equipped with a 30 m × 0.23 mm × 0.32 μm HP-5 MS capillary column. The injection port temperature was 310 °C. The column was initially held at 40 °C for 4 min. The temperature was ramped to 300 °C at 4 °C min−1 and held isothermally for 10 min. The biocrude products were identified using a mass spectrometry (MS) detector (Agilent). The concentration of ammonia in the aqueous phase was measured with a nitrogen ammonia reagent set (HACH). We diluted the aqueous phase (1:200) with deionized water and added the diluted sample to the HACH reagents. The absorbance of the solution at 655 nm was obtained using a Thermo Scientific Model Genesys 20 spectrophotometer, and the ammonia concentration was determined by analysis of standards with known concentrations. Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), and elemental analyses were performed as previously described.9,17,18 The Dulong formula was used to estimate the higher heating value (HHV) of the biocrudes on the basis of their elemental compositions.6
and biochemical composition. Distilled and deionized water was used throughout the experiments. All solvents were from Sigma−Aldrich in >95 wt % purity and used as received. CoMo/γ-Al2O3 (sulfide with 3.4−4.5 wt % CoO, 11.5−14.5 wt % Mo2O3) was obtained from Alfa Aesar. All other catalysts were obtained from Sigma−Aldrich. The catalyst particle size was ∼25 μm. All other chemicals were purchased from Fisher Scientific in high purity and used as received. Helium, hydrogen, and argon were obtained from Cryogenic Gases. Gas standards for instrument calibration were purchased from Air Liquide Specialty Gases. The reactors were constructed from a 1/2 in. Swagelok port connector and cap and a 1/2 in. to 1/8 in. reducing union. Each reactor was fitted with a 9 in. length of 1/8 in. outer diameter (OD) stainless steel tubing, which was connected to a shut-off valve (High Pressure Equipment Company). The total volume of the reactor assembly is ∼4.3 mL. 2.2. Procedure. All experiments were conducted in 316 stainless steel batch reactors that allowed for the recovery and analysis of both the liquid- and gas-phase products in a single run. Prior to use in reactions, all reactors were loaded with 2 mL of deionized water and heated to 350 °C for 8 h to expose the reactor walls to a hydrothermal environment. The reactors were then gradually cooled to ambient temperature and thoroughly washed with acetone and air-dried. In a typical run, the amount of water loaded in the reactors was such that it would expand to fill 95% of the reactor volume at the reaction temperature. Sufficient soy protein concentrate was added to the reactor to constitute 15 wt % of the mass of concentrate and water in the reactor. Next, catalyst (0−50 wt % of the protein concentrate mass) was loaded into the reactor, and the reactor assembly was connected to the shut-off valve. The air inside the reactor was displaced with helium by repeatedly applying a vacuum (−0.093 MPa) and charging with He (1 MPa). Several helium flushing-vacuum cycles were conducted. The reactor was next pressurized to 1 MPa (gauge) 3209
DOI: 10.1021/acs.energyfuels.5b00321 Energy Fuels 2015, 29, 3208−3214
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Energy & Fuels
The yields of gases were in the range of ∼25−39 wt %. These yields are higher than those observed for HTL of microalgae (see, e.g., refs 5, 6, and 10), but they are consistent with other recent work in our laboratory for HTL of this protein concentrate.14 Table 2 provides the elemental compositions of the biocrudes produced with the different catalysts and the higher heating value (HHV) of each estimated from the elemental composition. The HHV of the biocrude produced without catalyst was 37 MJ/kg. The HHVs of the biocrudes acquired with the different catalysts are all higher, and those produced from HTL over metal catalysts reach 43 MJ/kg. These improved heating values are due to the biocrudes from catalytic HTL over metals having a higher carbon and hydrogen content and lower O, N, and S content. All of the materials tested, except HZSM-5, produced biocrudes with O, N, and S contents lower than the biocrude from uncatalyzed HTL. The catalysts also appeared to promote hydrogenation reactions, as the H/C ratio in the biocrudes from catalytic HTL exceeded that from uncatalyzed HTL (1.58). Of the different catalysts tested, HZSM-5 showed no catalytic activity for deoxygenation or denitrogenation. The Ru/ C catalyst provided the greatest extent of oxygen, nitrogen, and sulfur removal. The biocrude from HTL of protein concentrate over this catalyst was 92% C and H, just 5% O, 2.9% N, and had a sulfur content below the detection limits. These values for the oxygen and nitrogen content are about half of the corresponding values for the biocrude produced in the absence of a catalyst. The results in Table 2 show that catalytic HTL can produce biocrudes of measurably higher quality than can undergo HTL under the same conditions in the absence of catalysts. Interestingly, such significant reductions in the nitrogen content of the biocrude were not observed from catalytic HTL of algal biomass,10,11 the only protein-rich biomass for which the literature permits comparison with the present results. It seems that some component(s) present in the biomass or the accompanying growth media inhibit the activity of heterogeneous catalysts when employed during algae HTL. Table 3 shows the fractions of C and H atoms in the soy protein concentrate that partition into the biocrude and gas phases after HTL. HZSM-5 had no measurable effect on these metrics, but all of the metal-containing catalysts showed increased partitioning of carbon and hydrogen into the biocrude, relative to that observed for HTL in the absence of catalysts. Pt and Pd performed similarly, whether supported on
3. RESULTS AND DISCUSSION This section first provides the results from catalytic HTL of protein concentrate with the different catalysts. It then explores the influence of temperature, time, catalyst loading, and initial hydrogen pressure on the results from experiments with the best catalyst. 3.1. Comparison of Different Catalysts. Figure 1 shows the effects of the various catalysts on the yields of the solids,
Figure 1. Yields of product fractions from HTL of soy protein concentrate with different catalysts (350 °C, 3 MPa H2, 120 min, 20 wt % catalyst loading).
gases, and biocrude from the liquefaction of soy protein concentrate with high-pressure hydrogen (3 MPa loading at room temperature). The gas, solid, and biocrude products accounted for ∼70 wt % of the initial mass of protein concentrate, regardless of which catalyst was used. The remaining ∼30 wt % of the initial material must have been transformed to products that partitioned into the aqueous phase after HTL. The yields of biocrude ranged from 32 wt % (HZSM-5) to 40 wt % (Pt/C). Given the uncertainties of ±3− 4 wt % that typically accompany determination of biocrude yields from HTL using these mini-batch reactors, we cannot declare with certainty that any of these biocrude yields differ from the yield of 34 wt % obtained from uncatalyzed liquefaction.
Table 2. Elemental Composition of Biocrudes from Different Catalysts (350 °C, 3 MPa H2, 120 min, 20 wt % Catalyst Loading) Elemental Composition (wt %)
a
entry
catalyst
C
H
O
N
S
H/C
O/C
N/C
higher heating value, HHV (MJ/kg)
1 2 3 4 5 6 7 8 9 10
none HZSM-5 Pd/C Pt/C Pd/γ-Al2O3 Pt/γ-Al2O3 Mo2C MoS2 CoMo/γ-Al2O3 Ru/C
73.67 73.41 78.25 77.63 77.10 78.35 77.18 78.54 80.12 80.41
9.67 9.84 10.73 11.05 10.89 10.54 10.80 11.36 11.63 11.76
9.92 10.32 7.04 7.92 7.61 6.74 6.68 6.17 5.18 4.97
6.35 5.87 3.98 3.16 4.23 4.11 5.34 3.78 3.07 2.86
0.39 0.56 n.d.a 0.24 0.17 0.26 n.d.a 0.15 n.d.a n.d.a
1.58 1.61 1.64 1.70 1.69 1.61 1.68 1.73 1.74 1.75
0.10 0.11 0.067 0.076 0.062 0.065 0.049 0.057 0.045 0.046
0.074 0.069 0.043 0.034 0.046 0.044 0.052 0.041 0.034 0.030
37.0 37.1 40.5 40.6 40.3 40.4 40.3 41.7 42.8 43.1
n.d. = not detected. 3210
DOI: 10.1021/acs.energyfuels.5b00321 Energy Fuels 2015, 29, 3208−3214
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Energy & Fuels
small enough that it is comparable to the standard deviation of this measurement. Table 4 shows the yields of biocrude from HTL of the soy protein concentrate over Ru/C at different temperatures, times, and catalyst loadings. The HTL temperature had little influence on the biocrude yields obtained after 120 min, which were in the range of 36−40 wt %. This variability does not exceed the run-to-run variation typically observed in these experiments. Likewise, the effect of the reaction time for HTL at 350 °C had little effect on the biocrude yield. The data in Table 4 also illustrate the effect of the HTL temperature for a 2 h experiment and the effect of HTL reaction time at 350 °C on the composition of the biocrude. Increasing the time from 1 h to 2 h increases the carbon and hydrogen content of the biocrude and decreases the oxygen, nitrogen, and sulfur content. As a result, the HHV of the biocrude increases by ∼5%. Increasing the time beyond 2 h provides additional, but more modest, improvement in the heating value of the biocrude. Increasing the temperature from 250 °C to 400 °C led to a steady reduction in the oxygen and nitrogen content and a steady increase in the carbon content of the biocrude. The catalyst loading in the reactor is also a process variable that influences the biocrude composition. Table 4 shows that higher loadings generally produced a biocrude richer in carbon and hydrogen and poorer in oxygen, nitrogen, and sulfur. A catalyst loading of 50 wt % produced a biocrude in 2 h that had a composition very similar to that which required 6 h with a catalyst loading of 20 wt %. It seems that there may be the ability to use the HTL reaction time and the catalyst loading as variables for optimizing the process. In general, longer reaction times, higher temperatures, and higher catalyst loadings produced biocrudes with higher HHV, hydrogen content, and carbon content, and lower oxygen and nitrogen contents. These findings indicate that deoxygenation and denitrogenation or hydrodeoxygenation and hydrodenitrogenation proceeded during the catalytic HTL of soy protein concentrate over Ru/C. Use of this catalyst for HTL, under the conditions examined in the present study, produced biocrude that contained
