Co-Pyrolysis Behaviors of the Cotton Straw / PP Mixtures and Catalysis Hydrodeoxygenation of Co-Pyrolysis Products over NiMo / Al 2 O 3 Catalyst

The doping of PP (polypropylene) with cotton straw improved the bio-oil yield, which showed there was a synergy in the co-pyrolysis of the cotton straw and PP at the range of 380–480 °C. In a fixed-bed reactor, model compounds and co-pyrolysis products were used for reactants of hydrodeoxygenation (HDO) over Ni-Mo/Al2O3. The deoxygenation rate of model compounds decreased over Ni-Mo/Al2O3 in the following order: alcohol > aldehyde > acetic acid > ethyl acetate. The upgraded oil mainly consisted of C11 alkane.


Introduction
The decreasing supplies of fossil fuels and chemical feedstocks have made researchers and industry exploit alternative renewable resources.Biomass energy is one of the candidates.
Biomass is a clean and renewable energy [1], and can be converted into bio-oil by the pyrolysis method [2][3][4][5].Bio-oil mainly consists of oxygenated organic compounds with bad properties, such as OPEN ACCESS corrosiveness, instability, and low calorific value [6,7], which hinder its direct application as transportation fuel.Thus, it is necessary to improve the quality of bio-oil.A promising way to increase the quality and yield of bio-oil is the co-pyrolysis of biomass/synthetic polymer mixtures [8][9][10][11][12].The co-pyrolysis of mixtures of biomass and synthetic polymers has received attention in recent years [9].Plastics with approximately 14 wt.%, such as PE (polyethylene) and PP (polypropylene), provide hydrogen to biomass during co-pyrolysis and improve bio-oil quality [9,[13][14][15][16].Besides the co-pyrolysis of mixtures of biomass and synthetic polymers, other methods such as decarboxylation [17][18][19] and aqueous-phase reforming [20] can do the same.It is difficult to remove all oxygen from bio-oil with the aforementioned methods, even when catalysts are used.
Hydroprocessing [21,22] is one of the promising routes to upgrade pyrolysis oils.Hydrodeoxygenation (HDO) [23][24][25][26][27] has great potential on an industrial scale.Thus, pyrolysis oil upgraded by HDO has been investigated from different aspects [28][29][30].Many catalysts [31][32][33] were investigated.Priecel [34] studied the role of Ni species in the deoxygenation of rapeseed oil.The CoMo-, NiMo-, and NiW-supported catalysts were studied for their excellent activity as bi-metal catalysts [35,36].Many model compounds were used as reactants to study the HDO process [37][38][39].Stephen [40] reported the catalytic hydrodeoxygenation of two lignin model compounds (anisole and guaiacol) in the temperature range of 260 to 325 °C.Limin [41] investigated the deoxygenation of long-chain fatty acid esters at mild conditions (200 °C, 3.0 MPa), which provided an energy-economic route to upgrade bio-oils with high oxygen content.However, HDO of the model compounds is different from that of real crude oil over Ni-Mo/Al2O3, so HDO of real crude oil is required over Ni-Mo/Al2O3.Now, it was controversial to the synergistic interaction in the co-pyrolysis of biomass and synthetic polymers.For hydrodeoxygenation over Ni-Mo/Al2O3, the model compound was often used as a reactant, and crude oil as feedstock was seldom reported.In the study, the co-pyrolysis behavior of a mixture (PP and cotton straw) was investigated under an inert atmosphere by a thermogravimetric analyzer.Then the crude bio-oil was used as feedstock for HDO over Ni-Mo/Al2O3.The upgraded oil was analyzed by GC-MS.

Preparation of Crude Oil and Thermal Degradation
The effect of PP content on crude oil yield is listed in Table 1.The crude oil yield increased with PP in the range of 0-80 wt.%.The crude oil yield of 43 wt.% was obtained with a mixture of 80 wt.% PP as feedstock, which was 2.0 and 1.3 times that of the cotton straw and PP sample, respectively.The oil yield of the mixture was larger than the weighted sum of PP and cotton straw, indicating there was a synergistic effect between cotton straw and PP, which was contrary to the result reported by Han et al. [42].First, ΔW is defined as an interactive effect parameter and is formulated by Equation (4). Figure 1 shows the variation of ΔW with temperature; ΔW was less than 1% below 180 °C because PP and cotton straw were not decomposed below 180 °C, and there was no synergistic effect.We saw that ΔW was positive at the range of 180-320 °C, which was attributed to the fact that PP was softened at about 180 °C and further heated to produce a plastic state that inhibited the evolution of volatile matter in the cotton straw.Then, ΔW was negative at the range of 380-480 °C.In this stage, cotton straw and PP began to decompose simultaneously in the temperature range, and cotton straw decomposed to form a radical, which can promote PP to degrade, and the weight loss rate of the cotton straw/PP mixture was greater than the weighted average of one obtained from the separate pyrolysis of the sample.The result indicated there was a synergistic effect in the co-pyrolysis of the cotton straw/PP mixture.Based on elemental analysis (see Table 2), the calorific value of oil from the pyrolysis of straw was the lowest, which was attributed to the high oxygen content of the product.

Characterization and Test of Ni-Mo/Al2O3 Catalyst
Figure 2 shows XRD patterns of catalysts with different NiMo loadings.The catalyst with low NiMo loadings, such as in Equations ( 3) and ( 4), shows no noticeable diffraction peaks.XRD patterns of catalysts with higher NiMo loadings present the evidence of MoO3 and NiMoO4 crystalline phases.The bands at 2θ = 23.3° and 25.7° correspond to crystalline orthorhombic α-MoO3.XRD patterns of catalysts were analogous to that of γ-Al2O3, which was attributed to the good dispersion of NiO and MoO3 on γ-Al2O3.The dispersity of NiO on γ-Al2O3 was better than that of MoO3 on γ-Al2O3.The weak peaks at 2θ = 26.6°and 28.8°, 39.1° correspond to the β-and α-NiMoO4 phase, respectively, and were almost not  Acetic acid was selected as a reactant not only because it was known to be rich in the oil phase and aqueous phase from co-pyrolysis products but also because the derived products (ethanol, acetaldehyde, and ethyl acetate) were easily quantified by standard analytical techniques.Figure 3 shows acetic acid as a function of MoO3 content.The conversion of acetic acid increased with MoO3 content, and then decreased.The phenomenon was attributed to the fact that the activity of MoO3 and NiO was different, and the doping of MoO3 favored the dispersion of NiO (see Figure 2).The NiO-2.6 wt.%/γ-Al2O3 (MoO3-5.4wt.%) catalyst held the highest activity, and it was used as a catalyst in subsequent experiments.The catalyst was tested at the range of 230~280 °C and 3 MPa of hydrogen pressure for 2 h.The conversion of acetic acid was neglected over the Al2O3 catalyst at 280 °C and 3 MPa for 2 h.The conversion of acetic acid increased with temperature over the Ni-Mo/γ-Al2O3 catalyst, and a conversion of 98% was obtained at 280 °C, which may be considered a complete conversion.Acetic acid was converted via two paths: (1) hydrogenation and (2) esterification.A main reaction profile is given in Scheme 1. Product distribution also is shown in Figure 4. Selectivity of ethyl acetate decreased with the increase of temperature, and that of ethanol and aldehyde were contrary to that of ethyl acetate.Methane, CO, and CO2 were detected in the outlet stream above 280 °C.The result suggests that decarboxylation of acetic acid occurs under this condition.Crude oil from the co-pyrolysis of the cotton straw/PP mixture contained diverse compounds with the same functional group and different chain lengths.Thus, it was important to study the effect of the chain length of the model compound with an identical functional group on HDO on the Ni-Mo/γ-Al2O3 catalyst.Five model compounds with an identical functional group but different chain lengths were investigated in order to know the effect of chain length on HDO.Results are shown in Figure 6.It was well known that the chain length of the organic compound affected HDO on the Ni-Mo/γ-Al2O3 catalyst.The rate of HDO decreased in the following order: acetic acid > propanoic acid > butyric acid > valeric acid > stearic acid, which depended on the size of R of R-COOH.R of carboxylic acid molecules was larger; steric hindrance was more outstanding, so the rate of HDO decreased with the increase of R. The following section was carried out with crude oil at the same conditions (270 °C and 4 MPa).Figure 7 shows the GC-MS profile of crude oil and the upgraded oil.The chromatogram of the upgraded oil showed all compositions separating out before 20 min, and many peaks corresponding to a series of n-alkanes starting at about C11 regularly appeared.The chromatogram of the crude oil showed a longer retention time than that of the upgraded oil.A summary of these results is listed in Table 3. GC-MS analysis showed that the crude oil mainly consisted of high-carbon alcohol, long-chain hydrocarbons (unsaturated and saturated), and fatty acids.Conversely, the upgraded oil mainly consisted of alkanes.Alkane (3-methyldecane) was the richest, but other alkanes from C12 to C20 also were present in the upgraded oil.A principal route to form C11 was the hydrodeoxygenation of carboxylic acid, alcohol, and aldehyde with C11, or decarbonylation and decarboxylation of the oxygenic compounds with more than C11.The composition of crude oil was different to that of the upgraded oil.Crude oil derived from the co-pyrolysis of the mixture mainly consisting of alkane, alkene, alcohol, and ester.Meanwhile, the upgraded oil mainly consisted of alkanes.Based on these results, the most of the oxygen can be removed after hydrotreating, suggesting HDO is feasible to upgrade crude oil.

Experimental Section
Cotton straw was from the south of China and dried at 100 °C for 5 h, and then ground into ca.0.23 mm.Polypropylene (PP) is waste plastic from Shanghai Yangli Mechanical and Electrical Technology LTD., Shanghai, China, and was dried, and then ground into ca.0.23 mm.Elemental analysis of cotton straw and PP is listed in Table 4. Supported Ni-Mo catalyst was prepared by wet co-impregnation of aqueous solutions of Ni(NO3)26H2O and (NH4)6Mo7O244H2O on the support (γ-Al2O3).The loading is 8 wt.%.The atomic ratio of Ni to Mo is 5:4.The catalyst was loaded in a stainless steel tubular reactor (1.5 cm i.d, and 50 cm in length).Before reaction, the catalyst was pretreated at 400 °C for 1 h in H2.Hydrogen and oil were fed to the reactor at a ratio of H2/oil = 400.The liquid products were collected in a trap.GC-MS analysis was carried out on a Trace DSQ GC-MS system with an AB-5MS capillary column (30 m × 0.25 mm i.d, 0.25 μm film thickness).Helium was used as carrier gas, with a flow rate of 1 mL•min −1 .The column temperature was programmed from 60 to 300 °C at a rate of 10 °C•min −1 after an initial two-minute isothermal period.Then it was kept at the final temperature for 10 min.The inlet temperature was set to 300 °C, and the split ratio was 1:50.The mass spectrometer was set to an ionizing voltage of 70 eV with a mass range from 35 to 650 amu.Identifying organic compounds was accomplished by comparing the mass spectra of the resolved components using electronic library search routines.Elemental analysis was carried out on Elementar (Frankfurt, German, sensitivity to 0.1 g).C, H, N, and S was analyzed at He atmosphere with O as a combustion improver, and gas flow was 50 mL/min.The atomic ratio of Ni to Mo was measured by ICP-AES (Perkin-Elmer 3300 DV, Fremont, CA, USA).N2 adsorption-desorption isotherms at −196 °C were recorded with a Micromeritics ASAP 2010 automatic sorption analyzer (Micromeritics, Norcross, GA, USA).The detailed data are listed in Table 5. X-ray powder diffraction patterns of catalysts were recorded on a Bruker D8 Advance diffractometer (Bruker, Germany), using CuKa (1.5406 Å) radiation in the range of 10°-60° with a scanning rate of 1°/min.The amount of cotton straw and PP used throughout all experiments was 10 g.Co-pyrolysis of mixtures was performed in the self-made fixed reactor (300 mm × 20 mm).N2 (flow rate 200 mL•min −1 ) was used as carrier gas, temperature was programmed from 40 to 600 °C at 100 °C•min −1 and kept for 20 min at 600 °C, then was cooled to room temperature.The above experiment was repeated at least three times.During the process, pyrolysis products were cooled down and collected, and gas was evacuated.Pyrolysis products were placed and delaminated into two layers.The top and bottom layers were oil phase and aqueous phase, respectively (Scheme 2).Scheme 2. Co-pyrolysis equipment: 1, carrier gas; 2, valve; 3, flowmeter; 4, temperature monitor; 5, temperature controller; 6, furnace; 7, fixed bed reactor; 8, round-bottomed flask; 9, condenser.
The pyrolysis of cotton straw/PP mixture was performed on thermogravimetric analyzer (TA Instrument SDT Q600, New Castle, DE, USA) with N2 as carrier gas (60 mL/min), and the temperature was programmed from 20 to 900 °C at 10 °C/min.
The heating value of the products is approximated using Dulong's equation [43].

Caloricity (MJ•kg
where C, H, and O are the mass percent of carbon, hydrogen, and oxygen, respectively.The difference of weight loss Δ is defined as a function of the synergistic effect during pyrolysis.The conversion of reactant, crude oil yield, and synergy is calculated with Equations ( 2)-(4), respectively. ( (3) (4) where η and μ are the conversions of the reactant and crude oil yield, respectively.W0, Wt, and Ws are the weight of the reactant, residue, and bio-oil from co-pyrolysis, respectively, and ΔW represents, to a certain degree, the synergistic effect during co-pyrolysis.Wb is the weight loss of mixture of cotton straw and PP.Wi is the weight loss of each material at the same conditions.Xi is the weight fraction of each material in the mixture.
The conversions of model compounds and the product selectivity are calculated by the formulas: (5) where m1 and m2 represent the content of the aimed product and all products obtained from the HDO reaction, respectively.

Conclusions
The doping of PP with biomass can improve oil yield and oil quality, which is attributed to the synergy between cotton straw and PP at the range of 380-480 °C.The substrate structure had the determining effect on the HDO reaction over Ni-Mo/Al2O3.The rate of deoxygenation depended on the chain length and functional group of the organic compounds.The upgraded crude oil mainly consisted of C11 alkane from the HDO of carboxylic acid, alcohol, and aldehyde with C11, or decarbonylation and decarboxylation of the oxygenic compounds with more than C11.

Figure 1 .
Figure 1.Graph shows the ΔW curve of straw and PP mixture.
indicated that β-and α-NiMoO4 dispersed uniformly on γ-Al2O3.The good dispersion of the active components on support improves the activity.

Table 1 .
Effect of PP on oil yield.

Table 2 .
Elemental analysis of oil.

Table 3 .
Compositions of oil.

Table 4 .
Elemental analysis of cotton straw and PP.