https://doi.org/10.1007/s12649-023-02153-0 ORIGINAL PAPER
Pyrolysis of Palm Fronds Waste into Bio‑Oil and Upgrading Process Via Esterification‑Hydrodeoxygenation Using Cu–Zn Metal Oxide Catalyst Loaded on Mordenite Zeolite
Ahmad Nasir Pulungan1 · Ronn Goei2 · Fauziyah Harahap3 · Lisnawaty Simatupang1 · Cicik Suriani3 · Saharman Gea4 · Muhammad Irvan Hasibuan1 · Junifa Layla Sihombing1 · Alfred Iing Yoong Tok2
Received: 17 December 2022 / Accepted: 22 April 2023
© The Author(s), under exclusive licence to Springer Nature B.V. 2023
Abstract
Bio-oil from the pyrolysis of palm fronds waste has potential to be used as an alternative fuel. However, due to its poor physicochemical properties, it requires further treatment to improve its overall quality. In this study, the conversion of palm fronds waste into bio-oil was carried out using the semi-fast pyrolysis at 500 °C and continued with esterification and hydrodeoxygenation (HDO). Metal oxides of Cu and Zn with a zeolite carrier mordenite (Mor) were used as bifunctional catalysts in the HDO reaction performed in a fixed-bed system reactor with temperature variations (250, 300, and 350 °C) for 2 h. Upgraded bio-oil (UBO) products at optimum conditions were analyzed for their physicochemical properties. In the pyrolysis process, the highest bio-oil conversion of 46.3% was obtained at a sample size of 60 mesh with the main compo- nent of furan compounds (46.31%). In the upgrading process through the esterification pretreatment, the optimum condition of HDO was determined to be at 300 °C, this can be seen from the high yield of liquid phase products produced on each catalyst (Mor; 89.85%, and CuO-ZnO/Mor; 88.25%). The physicochemical properties of upgraded bio-oil obtained under optimum conditions showed an increase in the quality of bio-oil with a decrease in water content (up to 23%), an increase in higher heating value (HHV) (up to 14.67% in CuO-ZnO/Mor). It is known that HDO with CuO-ZnO/Mor catalyst has a higher selectivity than Mor catalyst in converting aromatic hydrocarbon compounds such as methyl cyclohexane which is a potential component of a fuel.
Keywords Bio-oil · Esterification-Hydrodeoxygenation · Catalyst · CuO · ZnO · Mordenite
Introduction
The problem associated with the depletion of non-renewable fossil fuels has always been a hot issue that has prompted great interest in exploring alternative renewable energy sources. The potential of lignocellulosic biomass as an alter- native energy source is very promising due to its availability, low cost and renewability [1, 2]. These characteristics can be found in palm frond biomass which is one of the planta- tion wastes that has not been widely used [3]. The biomass can be converted through a pyrolysis process with thermal decomposition into a fuel source in the form of bio-oil [4].
In order to produce optimum amount of bio-oil in the pyroly- sis process, important factors must be considered, such as heating rate, temperature, and short feed residence time [5].
Semi-fast pyrolysis is one of several pyrolysis techniques to decompose biomass thermally, it is called semi-fast pyrol- ysis because the operating conditions used in the process
* Ahmad Nasir Pulungan [email protected]
* Alfred Iing Yoong Tok [email protected]
1 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Negeri Medan, Jl. Willem Iskandar Pasar V Medan Estate, Medan 20221, Indonesia
2 School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
3 Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Negeri Medan, Jl. Willem Iskandar Pasar V Medan Estate, Medan 20221, Indonesia
4 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Jl. Bioteknologi No. 1, Medan 20155, Indonesia
are almost close to the fast pyrolysis process, but in some literature, this process can be referred to as intermediate pyrolysis., Semi-fast pyrolysis (intermediate pyrolysis) is a process that promises a higher percentage of liquid product to be obtained from the biomass. High temperature heat- ing (300–500 °C) and fast heat transfer rate at the reaction interface are important factors for producing bio-oil. This requires a finely ground biomass feed to increase the surface area [6]. Reducing the size of the feed particles is expected to increase the yield of pyrolysis bio-oil. However, there is no specific optimum feed size specification for the pyroly- sis process. This can be seen from the results of previous studies which showed differences in optimum conditions.
For example, Pattiya et al. (2012) reported that a particle size of 0.25–0.425 mm produced the maximum bio-oil in pyrolysis of cassava stems in a fluidized bed reactor [7], while the results obtained by Kim et al. (2010) showed that a smaller feed size (0.425–0.85 mm) produced more oil products in palm kernel shells pyrolysis compared to a feed size of 0.85–1 mm [8]. The as-produced bio-oil cannot be used directly as a fuel due to its weak physicochemi- cal properties, such as high oxygen and water content, low pH, and high viscosity. In addition, the content of oxygen- ate compounds such as aldehydes, acids, ketones, phenols and alcohols found in bio-oil causes instability, low calorific value of bio-oil, and is naturally corrosive [1, 2]. The various weaknesses of bio-oil have prompted many researchers to develop various techniques to improve the physicochemical properties of bio-oil including emulsification [9], catalytic cracking [10], steam reforming [11], hydrogenation [12], hydrodeoxygenation [13], and esterification [14]. The esteri- fication and hydrodeoxygenation (HDO) methods are more desirable because the esterification process can reduce the amount of acidic compounds by converting them into more stable ester compounds thus reducing the acid number in bio-oil [14, 15]. The hydrodeoxygenation process is known to reduce the oxygen content contained in bio-oil through several reactions such as decarboxylation, hydrogenation, demethoxylation, demethylation, and deoxygenation. With the reduced oxygen content, the physicochemical properties of bio-oil such as calorific value and stability will increase [16]. Therefore, by combining the two upgrading methods (esterification-hydrodeoxygenation) it is possible to obtain bio-oil with superior physicochemical properties.
The use of catalysts in the HDO reaction has been widely reported to optimize the process. Currently, the most devel- oped type of catalyst is a bifunctional catalyst with an active catalyst and support material. Transition metals are widely used as catalysts due to their low price and high activity [17]. As one of the transition metals, zinc (Zn) exhibits excellent catalytic activity and stability for the conversion of oxygenated compounds in bio-oil into hydrocarbons, this is inseparable from its coke-resistant and acidity properties
[18]. Cheng et al. [19] showed that the use of ZnO metal could increase the yield of bio-oil with the highest hydro- carbon content of 68.95% in HDO bio-oil from pine sawdust waste. These results are in line with Phourzolfaghar et al.
[20] that the incorporation of Zn as the active site above SiO2 in phenol HDO causes an increase in the concentration of acid sites on the catalyst thereby increasing the selectiv- ity in cyclohexane formation with a conversion efficiency of up to 80%, in addition, the catalyst shows extraordinary resistance after regeneration multiple rounds of reuse. Mean- while, another transition metal that is widely used is Cu metal (copper). In catalysts with a 2-metal system, Cu acts as a promoter that can withstand reducibility, which causes reduction to occur at low temperatures [21]. This metal has a high selectivity in hydrogenation reaction, as reported by Li et al. [22] studied the effect of metal Cu deposition on Ni/
HZSM-5 catalyst on coke formation and catalyst deactiva- tion process in the HDO bio-oil process. This is because the bimetallic catalyst consisting of Ni and Cu can activate hydrogen at a lower reaction temperature, which favours increased hydrogenation and inhibits the polycondensation reaction at the same time. The same results were reported by Sitthisa et al. [23] that the Cu catalyst embedded in SiO2 was more selective in hydrogenating the C = O bond in the conversion of aldehyde compounds containing C = O and C = C bonds than the metal catalysts Pd and Ni in the reac- tion HDO fulfural.
In addition to the active metal, the support material is also a key factor determining the overall catalytic performance of bifunctional catalysts [24]. Support serves as an anchor to disperse metal nanoparticles and carries active sites for HDO reactions. Some of the widely used supports include silica [25], alumina [26], zeolites [27] and zirconia [28].
Compared to the other catalyst supports, zeolites possess a good thermal stability and provide suitable acidic anchor sites, so they have great potential in the HDO reaction catal- ysis process [29].
Mordenite is an aluminosilicate crystal with a fairly large pore size [30]. Mordenite zeolites exhibit chemical resist- ance and also have excellent mechanical and thermal stabil- ity due to the flexibility of their framework [31]. The high content of Brønsted acid sites, the large pore system and sur- face areas can provide a place to anchor metal nanoparticles to facilitate accessibility to the active site during the catalytic process. These properties make mordenite suitable for use as a material support in bifunctional catalyst systems [32]. Hu et al. [33] found that the influence of the Ru catalyst embed- ded in the mordenite zeolite showed good performance in the HDO reaction of the lignin model compound. Most of the oxygenated compounds disappeared and were converted to 74.36% cycloalkanes. In this study, CuO and ZnO metal oxides were used as a catalyst with mordenite zeolite as a support material. We investigate their activity and selectivity
in upgrading bio-oil obtained from palm frond waste with a two-stage process through esterification and HDO.
Materials and Methods
MaterialsThe materials used were palm fronds obtained from plan- tations in North Sumatra (Indonesia), synthetic mor- denite zeolite (HSZ-640HOA, Tosoh Corporation Japan), ZnSO4·7H2O (E. Merck), Cu(NO3)2·3H2O (E. Merck), etha- nol (E.Merck), H2SO4 (E.Merck), and double distilled water (DDW). Hydrogen gas, Oxygen gas, and Nitrogen gas were purchased from PT. Aneka Gas, Medan, Indonesia.
Catalysts Preparation
Wet impregnation method is used for the catalyst preparation following our previous work [27]. Mordenite was calcined at 500 °C for 2 h under N2 gas flow at a flow rate of ± 5 mL/sec.
Each metal precursors (1 wt% vs mordenite) were dissolved in DDW then the mixture of metal solution and mordenite was refluxed and stirred at 80 °C for 5 h. Afterwhich, the catalyst mixturewas dried, then calcination under O2 atmos- phere at a flow rate of ± 5 mL/sec at 500 °C for 2 h. Through this method, CuO/Mor, ZnO/Mor, and CuO-ZnO/Mor cata- lysts were obtained.
Catalysts Characterization
Crystallographic properties and mineral types on the catalyst were characterized using X-ray diffraction (XRD Shimadzu 6100). The degree of crystallinity is calculated using Eq. 1, and the determination of the crystal area on the XRD diffrac- togram is assisted by the origin pro 2018 software. Surface morphology was analyzed using Scanning Electron Micro- scope with Energy-Dispersive X-Ray Spectroscopy series model (SEM–EDS Type Zeiss EPOMH 10Zss). Nitrogen gas adsorption–desorption test was performed using Gas Sorption Analyzer (NOVA 1200e) Quantachrome instru- ment, Surface area calculated by Brunauer–Emmett–Teller (BET) method, while pore volume and pore size were ana- lyzed from desorption band using Barret-Joyner-Halenda (BJH) method.
Bio‑oil Preparation
The conversion of palm frond waste into bio-oil was carried out using the semi-fast pyrolysis method with a fixed-bed system reactor (Fig. 1). In each pyrolysis process, 50 g of mashed palm fronds were used with various feed sizes of 10 mesh, 60 mesh and 100 mesh. The pyrolysis process condi- tions were set at 500 °C with N2 gas flow for 1 h. The yield distribution of the resulting product is calculated by the fol- lowing equations:
(1) Degree of crystallinity(%) = Crystal area under2𝜃∶7−70oof bifunctional catalyst
Crystal area under2𝜃∶7−70oof Mor ×100%
Fig. 1 Schematic of semi fast pyrolysis process with fixed-bed reactor system
Upgrading Bio‑Oil Via Esterification and Hydrodeoxygenation
The esterification procedure followed the method reported by [34] in which bio-oil and ethanol were mixed in a ratio of 1:1 (w/w) into a flask. H2SO4 (96%) was added to the mixture (1% wt%). The mixture was stirred at 60 °C for 2 h.
The esterified bio-oil was subjected to a hydrodeoxygenation process using the prepared catalyst. The hydrodeoxygenation process was carried out by mixing 1 g of catalyst and 50 g of esterified bio-oil [16]. The HDO process was carried out in a fixed-bed system reactor (Fig. 2) with H2 gas flow for 2 h and at various temperatures (250, 300, and 350 °C). The liquid product with the optimum yield is further analyzed for its physicochemical properties.
Bio‑Oil Product Analysis
Bio-oil was analyzed for its physicochemical properties before and after the HDO-esterification process. The tests include elemental analysis (C,H,N,O) using CHN Analyzer LECO-CHN 628, water content analyzed using Metrohm 870 KF Titrano Plus, determination of acid numbers using the titrimetric method, HHV using the Sheng and Azvedo formulas, density analysis using a pycnometer, viscosity (2) Yliquid(%) = Liquid fraction collected
initial sample mass ×100 %
(3) Ycoke(%) = coke fraction collected
initial sample mass ×100 %
(4) Ygas(%) =100 % − (Yliquid+ Ycoke)
using Ostwald Viscometer, and Gas Chromatography Mass Spectrometry (GC–MS QP2010 Plus Shimadzu brand) to determine the constituent of the liquid products.
Results and Discussions
Crystallinity of the CatalystXRD analysis is used to identify the crystalline phase (crystallinity) of the catalyst material (Table 1). The dif- fractogram obtained was then compared to the pattern of peaks formed between mordenite zeolite and metal oxide impregnated zeolite as shown in Fig. 3.
Figure 3 shows the comparison of the diffractogram of the mordenite catalyst with and without the addition of metal catalyst. It can be observed that peaks located at 6.51; 9.68; 13.52; 19.70, 22.32; 25.69, and 27.62° are typical peaks for mordenite type zeolite (JCPDS 06-0239) [35, 36]. For the catalyst with the addition of Cu, the formation of CuO crystals was indicated by the appear- ance of a distinctive peak at 2θ = 35.80; 48.14; 53.19; and 57.88 ◦ which is correlated with the data JCPDS files of 05–0661 [37, 38]. While the ZnO crystals formed were characterized by the appearance of a distinctive peak at 2θ = 34.24; 36.61; and 47.62 ◦ in line with the data
Fig. 2 Schematic of the HDO process in a fixed-bed reactor
Table 1 Comparison of degrees
of crystallinity of catalysts Catalyst Degree of crystallinity ( %)
Mor 100
CuO/Mor 84.2
ZnO/Mor 80.4
CuO-ZnO/Mor 60.7
JCPDS 36-1451 [19, 39]. In addition, there was a shift in the main peak which was not too significant after metal deposition was carried out. This is because the thermal treatment includes oxidation and calcination, besides that the metal deposition process slightly affects the diffrac- tion pattern of the catalyst.
Based on the tabulated data presented in Table 1, metal oxide loading on the mordenite zeolite resulted in a decrease in the intensity of the resulting peak. As the intensity of the metal oxides-modified catalyst decreases, the degree of crystallinity of the zeolite also decreases.
These results indicate that the metal oxide treatment loaded into mordenite affects the internal and exter- nal surfaces of the mordenite structure, resulting in a decrease in the peak intensity of the catalyst [40]. This decrease is caused by infiltration of the deposited metal oxides are in the zeolite structure, partially covering the
sides of the zeolite crystal and resulting in a decrease in the Si/Al ratio (Table 2). These results are also in line with those obtained by Ji et al. [41] in modifying HZSM-5 with transition metals (Cu, Zn, and La).
Morphology of the Catalysts
The surface morphology of the catalysts was analyzed by SEM. The particle size distribution of the catalysts is shown in Fig. 4. Mordenite consists of thin cylindrical crystalline aggregates that form larger particles with smoother outer surface [35, 42, 43]. As for the particle size distribution data, zeolite mordenite has an average diameter of 361.3 nm.
However, after metal deposition there was an increase in the average particle diameter in almost all catalysts except CuO/Mor, and the morphology of the catalyst surface was becoming more homogeneous, especially in the CuO-ZnO/
Mor catalyst.
Catalyst Composition
The elemental components contained in each catalyst were analyzed by EDX and have been summarized in Table 2.
Mordenite zeolite experienced a decrease in Si content in all modified catalysts. The decrease in Si content indicates changes that occur during the impregnation, calcination, and oxidation processes. Calcination at high temperatures can cause part of the skeleton to collapse which results in the release of Si and Al from the zeolite framework [44]. Cu, Zn, and Cu–Zn metals impregnated with CuO/Mor, ZnO/
Mor and CuO-ZnO/Mor catalysts were detected at 1.33%, 1.18%, 0.88 and 1.17% (wt%). This shows that the metal has been successfully distributed on the mordenite.
Fig. 3 Comparison of XRD diffractograms of mordenite, CuO/Mor, ZnO/Mor, and CuO-ZnO/Mor catalysts
Table 2 Composition of Mor, CuO/Mor, ZnO/Mor, and CuO-ZnO/
Mor catalysts Composition
(Mass%) Mor CuO/Mor ZnO/Mor CuO-ZnO/Mor
Si 36.67 32.42 31.88 31.64
Al 3.15 3.18 2.93 3.12
O 55.01 58.72 58.79 55.59
Cu – 1.33 – 0.88
Zn – – 1.18 1.17
Impurities 5.16 4.35 5.22 7.6
Si/Al 11.64 10.19 10.88 10.14
Fig. 4 Comparison of surface morphology (SEM) and particle size distribution histograms of (a) Mordenite, (b) CuO/Mor, (c) ZnO/Mor, and (d) CuO-ZnO/Mor catalyst
Fig. 5 Elemental mapping of the catalyst (a) Mordenite, (b) CuO/Mor, (c) ZnO/Mor, (d) CuO-ZnO/Mor
Elemental mapping on the catalyst was analyzed using SEM-Maping (Fig. 5). Based on Fig. 5, each element is rep- resented by a certain color. The darker the color, the more these elements are distributed on the surface of the zeolite.
In Fig. 5 it can be seen that the CuO/Mor and ZnO/Mor catalysts are dominated by red, yellow, green and a little blue. This is in accordance with the catalyst composition data by EDX analysis where the largest composition is O (green) 58 wt%, Si (red) 31–32 wt%, Al (yellow) 2–3 wt%, and blue represents Cu and Zn metals which are not too dominant because only a small amount (1%). Meanwhile, for the bimetallic CuO-ZnO/Mor catalyst, there is a slight difference in symbols where the element O is represented in red, which predominates, and the metals Cu and Zn are represented in green and blue respectively, and it can be seen that in the bimetallic catalyst, the distribution of Cu and Zn metals is more even than the catalyst monometal. In addi- tion, the metals deposited on the zeolite can be loaded both inside and outside the zeolite framework depending on the size of the metal [16].
N2 Gas Sorption Analysis
Further analysis was carried out to determine the effect of the metal oxide-bearing process on the character of the cata- lyst including surface area, total volume and pore diameter.
The surface area was calculated by the BET method, while the pore volume and average pore diameter were analyzed from the desorption band using the BJH method. The results of measurements of surface area, pore volume and average pore diameter of each catalysts are summarized in Table 3.
Table 3 shows that the modification of zeolite by carrying metal oxides causes changes in the surface area, average pore diameter, and pore volume. The CuO-ZnO/Mor catalyst has the largest increase in surface area (438.6 m2/g) among other catalysts. Catalysts with the addition of single metal oxide exhibits decrease in the surface of the catalyst. Meanwhile, the average pore volume for each catalyst containing metal oxide decreased. This changes could be caused by the evapo- ration of impurity molecules such as water and gas which are chemically bound to the surface of the zeolite during the calcination process leaving behindempty spaces within the zeolite lattice. In the CuO-ZnO/Mor catalyst, the infiltration of metal resulted in the expansion of the space between the zeolite layers so that a larger pore system was formed. In addition, it is also possible that the metal oxides are scat- tered on the external surface of the catalyst which causes the surface area to increase. As the surface area increases, the absorption process will be enhanced [45]. Meanwhile, for CuO/Mor and ZnO/Mor catalysts, a decrease in surface area may occur because the pores of the carrier with a smaller radius will be clogged causing overall decrease of the sur- face area of the catalyst [40]. The pore diameter is almost the same as Mor, this shows that metal oxides partially cover the pores but the zeolite lattice structure does not change [41].
Table 3 Surface area, pore volume, and average pore diameter of each catalyst
Catalyst Surface
area (m2/g) Pore volume (cc/g) Average pore diameter (nm)
Mordenite 415.84 0.0560 3.75
CuO/Mor 185.78 0.0007 3.34
ZnO/Mor 385.88 0.0068 3.35
CuO-ZnO/Mor 438.60 0.0490 3.76
Fig. 6 Comparison of graphs of N2 adsorption–desorption isotherms of (a) Mordenite, and (b) CuO-ZnO/Mor catalysts
The graph of the adsorption–desorption isotherm for each catalyst is shown in Fig. 6. The graph shows the pres- ence of a hysteresis loop type H4 at a relative pressure of 0.43–0.99 so that this isotherm graph is classified as a type IV isotherm (IUPAC). The presence of a H4-type hysteresis loop and a large increase in the relative pressure indicates that the catalyst has meso-sized pores (2–50 nm) which has the characteristics of aggregation between crystals in the zeolite [27, 35].
Based on the analysis that has been carried out, the CuO- ZnO/Mor catalyst shows superior characteristics compared to the ZnO/Mor and CuO/Mor catalysts. Therefore, CuO- ZnO/Mor and Mor catalysts are used as catalysts in the esterification process of bio-oil hydrodeoxygenation or UBO (Upgraded Bio-Oil).
Raw Bio‑Oil Characterization
The sample used for bio-oil production is palm fronds that have been mashed. Kaur et al. [46] reported that bio-oil conversion can be influenced by several things including lignocellulose content in the sample, catalyst, temperature, process pressure, heating time, and the size of the feed par- ticle used. In this study, the particle size variation of the sample with variations of 10 mesh, 60 mesh and 100 mesh was prepared to understand the effect of particle size on bio-oil production. The distribution of the resulting product is shown in Fig. 7.
Based on Fig. 7, the most converted bio-oil products were at sample sizes of 10 mesh and 60 mesh with conversions reaching 41.70 and 41.81%, while at a sample size of 100 mesh the yield of bio-oil obtained was only 38.36%. From the product distribution data, it can be seen that the sample size affects the conversion of pyrolysis products, the smaller
the sample size, the more gas products formed and the less coke products produced. This observation is in the agree- ment with the research of Montoya et al. [47] who carried out the pyrolysis of bagasse with variations in sample size.
The larger yield of the smaller particle size sample could be caused by a more effective heat transfer between parti- cles. Small particle size sample would increase mass and heat transfer between particles in the reactor resulting in fast biomass decomposition and short residence time in the reactor. On the other hand, the large particle size sample would result in less heat transfer from the outer surface to the inside of the particles and supports coke formation [48].
Compounds Presence in the Bio‑Oil
A GC–MS analysis of bio-oil was carried out on various sample sizes (10 mesh, 60 mesh, and 100 mesh) to find out more about the compounds contained in bio-oil. There are 28 compound components presence in the bio-oil (Table 4) which can be grouped into several groups including acids, aldehydes, esters, ketones, phenols, and alcohols. The effect of sample size on the decomposition of the group of com- pounds that make up bio-oil is summarized in Fig. 8.
From the data in Fig. 8, the compounds in bio-oil are dominated by furan, aldehyde and ketone groups which are decomposition product of cellulose. It is known that the par- ticle size of the sample affects the components of the bio-oils where the aldehyde group increases with decreasing sample particle size (16.31 to 22.02%). The highest furan concen- tration was found in the sample size of 60 mesh (46.31%), while the decomposition of lignin derivatives, namely phe- nol, increased with decreasing sample particle size (6.95 to 14.83%). The results of the pyrolysis compound components were in line with the lignocellulosic components contained in the oil palm midrib samples, indicating that the cellulose component (31.5–56.03%) had the largest percentage com- pared to hemicellulose (19.2–46%) and lignin (14–20.48%) [49]. The furan compound group which consists of 2-Fur- ancarboxaldehyde, 2-Furancarboxaldehyde-5-methyl, and 2-Furanmethanol were only found in a sample size of 100 mesh. Furan group compounds such as furancarboxalde- hyde which is a fulfural compound dominate the compound components in raw bio-oil and are estimated to be formed at a temperature of 400–500 °C [50]. It is known that furan derivative compounds such as methyl furan are one of the potential compounds that can be used as an alternative to gasoline because of their very attractive combustion perfor- mance in engines [51].
The largest group of phenolic compounds was found in the sample size of 100 mesh, consisting of phenol (10.64%), phenol, 2-methoxy (1.23%), and phenol, 2,6-dimethoxy (2.96%). The phenol group of compounds is estimated to be formed around a temperature of 350–500 °C [50]. Lignin
Fig. 7 Comparison graph of bio-oil yield on sample particle sizes of 10 mesh, 60 mesh, and 100 mesh
Table 4 Composition of bio-oil compounds from pyrolysis of palm fronds with sample sizes of 10, 60 and 100 mesh
Compounds Structure Area (%)
10 mesh 60 mesh 100 mesh
Pentadecanoic acid 1.98 2.21 3.43
Hexadecanoic acid 10.83 10.80 14.14
2-Furancarboxaldehyde 38.59 43.27 19.57
2-Furancarboxaldehyde, 5-methyl 2.28 3.04 2.43
2-Furanmethanol – – 2.34
Tridecanedial 0.43 – –
Butanedial – 0.24 –
9-Octadecenal, (Z) 15.88 19.92 22.02
1,2-Ethanediol – 0.18 1.20
Hexadecanoic acid, methyl ester 0.70 – –
Hexadecanoic acid, 2-hydroxy-1-(hydroxymethyl)ethyl ester 0.89 – –
9-Octadecenoic acid (Z)-, methyl ester 1.09 0.36 –
Octadecanoic acid, 2-hydroxy-1,3-propanediyl ester 1.62 1.89 2.12
Anodendrine, iodide, methyl ester – 0.13 –
Ethanone, 1-(2-furanyl) 0.38 0.58 0.79
1-Hydroxy-2-Butanone – 0.42 1.34
3-Hexanone, 2-methyl – – 0.89
Table 4 (continued)
Compounds Structure Area (%)
10 mesh 60 mesh 100 mesh
2-Cyclopenten-1-one, 2-hydroxy-3-methyl 0.74 – 1.82
2-Cyclopenten-1-one, 2-methyl – – 0.78
Cyclopentadecanone, 2-hydroxy- 15.65 5.44 10.02
Cyclopentanone – 0.34 –
1,2-Cyclopentanedione – – 0.78
syringyl acetone 1.23 1.19 1.51
Phenol 4.76 7.52 10.64
Phenol, 2-methoxy 0.39 0.81 1.23
Phenol, 2,6-dimethoxy 1.27 1.67 2.96
Phenol, 3-[(trimethylsilyl)oxy]- 0.53 – –
N-Butyl-tert-butylamine 0.75 – –
which consists of phenylpropane structural units in a three- dimensional framework is the cause of the formation of phe- nolic compounds [52]. In general, the pyrolysis of lignin compounds mainly occurs through free radical reactions.
In the molecular structure of lignin, the oxygen bonds link- ing the phenyl-propane units and the side chains are eas- ily broken when heated. Active free radicals containing a benzene ring are formed, and these can easily react with other molecules or free radicals to produce macromolecules with a more stable structure, eventually forming charcoal.
Generally lignin compounds decompose at 280–500 °C. Fur- thermore, when compared to the cellulose, more charcoal is produced from the pyrolysis of lignin. The main reactions during the pyrolysis of lignin are the conversion of alkyl chains including the breaking of bonds between lignin units at a temperature of 150–420 °C to produce small molecular gases such as CO and CO2, conversion of aromatic rings and char formation reactions at 380–800 °C [53]. With increas- ing temperature, the functional groups react with phenyl- propane units such as phenolic hydroxyl groups, carboxyl groups, and benzyl groups to form phenolic compounds and aromatic hydrocarbons. Due to the low dissociation energy of methoxyl in the lignin structure, the demethoxylation reaction of guaiacol produces phenol and CH4 (Fig. 9). Gen- erally, the gases produced in the pyrolysis process are CO, CO2, H2, CH4, and C2H4. As for the range of 400–550 °C, hemicellulose decomposition produces the most gas among other components [52].
The most dominant aldehyde component found in the bio-oil produced was 9-Octadecenal (Z) compound with the highest content at a sample size of 100 mesh (22.02%).
Aldehyde compounds can be formed from the decomposi- tion of cellulose, hemicellulose compounds or from the tri- glyceride content in the sample [54]. Based on the reaction
mechanism presented in Fig. 9, the compound 9-Octadece- nal (Z) could be formed from a further reaction of a carbox- ylic acid compound in the form of octadecanoic acid which undergoes deoxygenation by releasing oxygen and allows the formation of CO and CO2 gases which are one of the contributors to the high gas content obtained in the pyrolysis process of the sample size of 100 mesh.
Some of the ketone compounds found in all variations in sample sizes were syringyl acetone, and Cyclopentade- canone-2-hydroxy. The largest components of the ketone group were obtained at sample sizes of 10 mesh (18%) and 100 mesh (17.93%). Ketone compounds can be produced from the decomposition of cellulose and hemicellulose [52].
Syringyl acetone or syringol is a ketone compound formed from the decomposition of lignin through a radical reac- tion and rearrangement. Meanwhile, Cyclopentadecanone- 2-hydroxy was also found in the study of Salema et al. [55]
with biomass derived from oil palm, namely oil palm shells carried out by microwave pyrolysis to produce Cyclopenta- decanone-2-hydroxy of 5.3%. The possibility of these com- pounds can be formed from the cyclization reaction and rear- rangement process of cellulose or hemicellulose compounds.
The formation of carboxylic acid and ester compounds such as hexadecanoic acid, and 9-Octadecenoic acid (Z)- methyl ester, is in agreement with previous work of by Aziz et al. [34]. Several carboxylic acid components and esters were also obtained, such as Dodecanoic acid, octadecanoic acid, 2,4-Hexadienedioic acid, 3,4-diethyl-dimethyl ester, Hexadecanoic acid-ethyl ester, and 9-Octadecenoic acid (Z)- ethyl ester. The fatty acid and ester compounds contained in the bio-oil may be derived from the decomposition of triglycerides contained in the palm fronds.
Overall, the pyrolysis of palm frond waste biomass was dominated by the decomposition of cellulose and hemicellu- lose derived compounds. Meanwhile, when viewed from the variation in sample size, it can be assumed that at a sample size of 100 mesh the lignin decomposition is more optimal, while the decomposition of cellulose and hemicellulose is optimal at a sample size of 60 mesh. The proposed reaction mechanism in the pyrolysis process can be seen in Fig. 9.
Physicochemical Properties of Raw Bio‑Oil
Analysis of physicochemical properties of bio-oil products includes elementary content, calorific value, moisture con- tent, viscosity, density, and analysis of acid number. The physicochemical properties are summarized in Table 5 The water content in bio-oil is quite high (73.15%), which indi- cates that the H2O compound formed is quite significant during the decomposition process. High water content was also found by Sakulkit et al. [56] in the pyrolysis of oil palm trunks with the highest content reaching 76.36%. In addi- tion, the oxygen content of bio-oil is also very high, this
Fig. 8 Composition of bio-oil prepared from sample of particle sizes 10 mesh, 60 mesh, and 100 mesh
is correlated with the amount of water content it has and also the content of oxygenated compounds that are pre- dominantly formed during the decomposition process. The abundant content of carboxylic and oxygenic acids makes the acid number of bio-oil very high (134 mg KOH/g oil).
Water content and high oxygen content (78.72%) in bio-oil results in the low HHV value of only 11.57 MJ/kg. The phys- icochemical properties of bio-oil are in the agreement with result reported by Abatyough et al. [57] and Sembiring et al.
[3]. Meanwhile, the presence of oxygenated compounds that
Fig. 9 The proposed reaction mechanism on the pyrolysis of cellulose, hemicellulose, lignin, and lipids of palm frond samples
have a high molecular weight would increase the viscosity and density of the bio-oil [58]. Sakulkit et al. [56] obtained a bio-oil density of around 1.005 g/cm3 on the pyrolysis of oil palm trunks. Meanwhile Qureshi et al. [59] also reported a higher density of bio-oil in the range of 1.040 g/cm3 with the highest viscosity reaching 23.70 cP. Based on the character- istics of raw bio-oil that has been obtained, raw bio-oil with a sample size of 60 mesh is used to continue the upgrading process through esterification-hydrodeoxygenation, this is based on a sample size of 60 mesh which has an optimum bio-oil yield, and also has several potential compounds that are larger than other sample sizes.
Bio‑Oil Upgrading Via Esterification‑HDO
The hydrodeoxygenation (HDO) process is carried out to reduce the oxygen content of oxygenate compounds in bio-oil into hydrocarbon compounds and their derivatives, resulting in a more stable bio-oil [60]. Before going through the HDO process, the bio-oil is pretreated through the esteri- fication process. This is done to convert the acid compound into a more stable ester form, improving the physicochemi- cal properties of bio-oil such as increasing HHV and lower- ing the water content of raw bio-oil [61]. Mordenite (Mor) and CuO-ZnO/Mor catalysts are used in the esterified HDO upgraded bio-oil (UBO) process. Temperature variations of 250, 300, and 350 °C were chosen to understand the effect of temperature on the hydrodeoxygenation process. The comparison results of HDO products catalyzed by Mor and CuO-ZnO/Mor are shown in Fig. 10.
From Fig. 10, it is observed that temperature has an influ- ence on the resulting liquid phase product with the most optimum results both at a temperature of 300 °C of 89.85%
and 88.25% on each catalysts. The liquid phase yield, which is not much different at 300 °C, is associated with the char- acteristics of the two catalysts, such as pore volume pore diameter, and Si/Al ratio which are not much different. In the bimetallic catalysts case, coke formation could occur because the Lewis acid sites on the metal can be quickly cov- ered by oxygenic compounds, while the Brønsted acid sites in zeolites donate protons to oxygenated compounds leading to the formation of coke formation precursor carbocations on the catalyst surface [16]. The results of the temperature variation show an increase in the liquid phase product when the temperature is 300 °C. When the temperature is raised to 350 °C there is a decrease in the product obtained (87.11%
and 78.10%), this is inversely proportional to the gas pro- duced when the temperature was 300 °C (6.8% and 8.68%) vs 350 °C (highest 10.38% and 19.19%). The higher gas
Table 5 Physicochemical properties of UBO with Mor and CuO- ZnO/Mor catalysts
a calculated based on the difference
b HHV is calculated based on the following formula: HHV (MJ/
kg) = − 1.3675 + (0.3137 C) + (0.7009 H) + (0.0318 O)
Properties Raw bio-oil Mor CuO-ZnO/Mor
Elemental analysis (wt.%)
C 11.46 16.73 20.78
H 9.76 8.28 10.46
N 0.04 0 0
Oa 78.72 74.97 68.75
Water content (%) 75.15 52.31 51.68
Density (g/cm3) 1.03 0.95 0.92
HHV (MJ/kg)b 11.57 12.07 14.67
Viscosity (cP) 1.13 1.14 1.16
Acid number (mg KOH/g oil) 124.56 50.49 39.27
H/C 10.15 5.95 6.01
O/C 5.15 3.36 2.48
DOD (%) - 34.75 51.84
Fig. 10 Distribution of upgraded bio-oil products catalyzed by (a) Mor and (b) CuO-ZnO/Mor
content indicates that increase in the pyrolysis temperature promotes gasification or hydrocracking reaction of bio-oil so that it converts organic compounds into gaseous products [62]. Ly et al. [63] reported the main gas products formed during the HDO bio-oil process from Saccharina japonica Alga includes CO, CO2, CH4 and several other hydrocar- bon gases. As for the resulting residue, solids are formed which may occur due to polymerization and condensation of bio-oil during the HDO process. Based on the available data, it could be concluded that 300 °C is the optimum HDO operating temperature with the highest conversion of liquid phase products on each catalyst. The liquid phase product produced at this optimum temperature is further analyzed to determine its physicochemical properties.
Catalyst Selectivity in HDO Process
The compounds contained in the bio-oil component after the HDO process were analyzed using GC–MS. These com- pounds are grouped into several groups including acids, esters, phenols, furans, alcohols, aldehydes, ketones, and cycloalkanes which are summarized in Fig. 11.
Based on Fig. 11, compounds in raw bio-oil from pyrol- ysis of palm fronds are dominated by cellulose deriva- tives which includes acids (12.81%), aldehydes (16.31%), furans (40.87%), ketones (18%). The formation of small amount of lignin derivatives such as phenol (6.95%) was also detected. The result suggests that the palm oil frond samples consist mainly of cellulose components with small amount of hemicellulose and lignin [64]. The compound
2-furancarboxaldehyde or furfural was the most concen- trated component (38.59%) found in raw bio-oil. In the HDO process, fulfural compounds undergo hydrogenation to pro- duce furfuryl alcohol compounds. The addition of a weak acid site or Lewis acid next to the metal site encourages the opening of the furfuryl alcohol ring followed by hydrogena- tion and rearrangement to form ketone compounds [65]. This can be seen by decreasing the amount of furan compounds and increasing amount of the ketone groups produced.
Meanwhile, the decrease in acid content in raw bio-oil and the increase in ester compounds produced suggest that the esterification process is able to convert organic acids into a more stable ester compounds [66], this is correlated with the data on physicochemical properties in Table 5. where there was a significant decrease in the acid number of upgraded bio oil (UBO).
The presence of phenolic compounds were not detected after the HDO process. The phenolic compounds that were presence in the raw bio-oils had been converted into hydro- carbon compounds in the form of cyclohexane, which could be proven by the increase in the amount of cycloalkane com- pounds, especially when CuO-ZnO/Mor catalyst was used (28.77%). Hydrodeoxygenation in phenol has various routes as the final product, one of which is through a direct deoxy- genation reaction by breaking the C-O bond to form benzene compounds [20, 67]. However, in this study, the more domi- nant route is the formation of methyl cyclohexane which is characterized by an increase in the percentage of alcohol area in HDO catalyzed by CuO-ZnO/Mor, where one of the compounds formed is methyl cyclohexanol which is an inter- mediate compound during the formation of methyl cyclohex- ane (Fig. 12). In addition to methyl cyclohexanol, methyl cyclohexanone is also formed which causes an increase in ketone group compounds after the HDO. The process for the formation of methyl cyclohexane from phenol is divided into four stages: (i) alkylation of phenol to cresol, (ii) hydrogena- tion of cresol to methyl cyclohexanone, (iii) hydrogenation of methyl cyclohexanone to methyl cyclohexanol, and (iv) dehydration (deoxygenation) of methyl cyclohexanol. to methyl cyclohexane [68, 69]. Meanwhile, the high content of unconverted ketones in the UBO catalyst Mor makes the oxygen content of the upgraded bio-oil still high with the degree of deoxygenation not too significant (Table 5).
The conversion of guaiacol derivatives such as phenol is affected by the number of active sites available [70]. In the study of Li et al. [71] showed that Cu loaded on ZSM-5 zeolite could increase the strength of its acidic site, which may have affected the active site available for the cracking reaction. This shows that the CuO component in the CuO- ZnO/Mor catalyst contributes to increasing the strength of the active site during the HDO process. In addition, with a large surface area and pores, the CuO-ZnO/Mor catalyst is able to facilitate more reaction pathways. Cu-based catalysts
Fig. 11 Composition of compounds in raw bio-oil, esterified bio-oil (UBO), and hydrodeoxygenation (HDO)-upgraded bio-oil (UBO) product catalyzed by Mor, and CuO-ZnO/Mor at an optimum tem- perature of 300 ℃
are known to be more selective in dehydration, decarboxyla- tion and decarbonylation reactions [40]. Meanwhile, Zn can selectively break C–O bonds in various lignin molecules under relatively mild reaction conditions [19, 20, 72]. This shows that the catalyst with both CuO and ZnO impregnated on Mordenite zeolite support (CuO-ZnO/Mor) has better selectivity than the catalyst without metal (Mor) bearing in the formation of aromatic hydrocarbon compounds. Methyl cyclohexane is one of the potential compounds produced among other cycloalkane compounds because it has a high octane number and thus is promising to be used as an alter- native fuel [70].
Acidic solid supports such as Mordenite zeolite in bifunctional catalyst systems (CuO-ZnO/Mor) are known to have a strong catalytic effect on dehydration reactions.
The bifunctional catalyst in the HDO process is indicated to have two functions, one side of the catalyst on the sur- face between the metal (CuO-ZnO) and the support (Mor) functions to activate the oxy compound by accepting a free electron pair from the oxy compound and the other side on the metal functions as a proton donor (H+) (Fig. 13). In the hydrodeoxygenation process, hydrogen gas is adsorbed on the metal surface of the catalyst, and then the sigma bonds between hydrogens are broken and form metal-H bonds. The carbon–oxygen double bond or carbon–carbon double (pi bond) in these compounds will capture electrophiles (E+) like H+. Then the Brønsted acid site provided by the metal
(CuO-ZnO) will provide a proton (H+) to break the bond on the OH group and form H2O [73].
Physicochemical Properties of Upgraded Bio‑oil Analysis of physicochemical properties of HDO UBO prod- ucts includes elementary content, calorific value, viscos- ity, acid number, and analysis of compound content using GC–MS. The physicochemical properties are summarized in Table 5.
Based on Table 5, upgraded bio-oil produced through the esterification and hydrodeoxygenation processes possess superior physicochemical properties. The esterification- hydrodeoxygenation process with the Mor and CuO-ZnO/
Mor catalysts succeeded in reducing the oxygen content in the raw bio-oil, thus increasing the HHV value and decreas- ing the overall acidity of the bio-oil. This shows that organic acid compounds contained in bio-oil have been reduced. It is also known that there is a decrease in density after HDO is performed. It is assumed that the catalyst can effectively convert the macromolecular compounds present in the bio- oil into monomers or light molecular compounds through hydrogenation, decarbonylation, dehydration, and cracking in the upgraded bio-oil [17, 24].
Meanwhile, the smaller O/C molar ratio of UBO indicates that a direct deoxygenation reaction pathway has occurred
Fig. 12 Possible reactions that occur in the hydrodeoxygena- tion process of upgraded bio-oil
by releasing water. This is supported by the reduced water content and the degree of deoxygenation (DOD) reaching 34.75% with Mor catalyst and 51.84% with CuO-ZnO/Mor catalyst. UBO catalyzed by CuO-ZnO/Mor has a higher H/C ratio than Mor catalyst. This correlates with the GC–MS data which suggests that mixed metal oxide catalysts are more selective in the deoxygenation and hydrogenation reactions. Water content could be decreased through dehy- dration reactions in the deoxygenation reaction pathway, zeolites with high acidity (related to dehydration reactions) also work to reduce water content. However, the structure of the zeolite framework tends to collapse at a high tem- perature which causes the dehydration reaction could not carried out properly. UBO with the highest HHV (14.67 MJ/
kg) was achieved with CuO-ZnO/Mor catalyst, which could be ascribed to its lower water content and higher aromatic hydrocarbon content compared to Mor catalyzed UBO. In addition, the reduced acidity in UBO can be caused by the esterification treatment which converts acidic compounds to form more stable ester compounds [14, 15].
Conclusions
In this study, the mixed metal oxide CuO-ZnO/Mor cata- lyst has better characteristics than the catalyst with one metal oxide (CuO/Mor and ZnO/Mor), including having
an even distribution of metal, the highest surface area reaching 438.6 m2/g, pore volume (0.049 cc/g) and mean pore diameter (3.14 nm). These properties are important in the HDO process, therefore they are used later in the HDO process with mordenite zeolite. A temperature of 300 °C was deemed as optimum pyrolysis temperature for the hydrodeoxygenation process. This can be seen from the high yield of liquid products produced on each catalyst (Mor: 89.85%, and CuO-ZnO/Mor: 88.25%). The physico- chemical properties of the upgraded bio-oil at optimum conditions showed an increase in the quality of bio-oil with a decrease in water content (up to 23%), an increase in HHV (the highest was 14.67% in CuO-ZnO/Mor), and an increase in the degree of deoxygenation (51.84% in CuO-ZnO/Mor, 34.75% on Mor). It is known that HDO with CuO-ZnO/Mor catalyst has a higher selectivity than Mor catalyst in converting aromatic hydrocarbon com- pounds such as methyl cyclohexane which is a potential compound as a fuel.
Author Contributions ANP: conseptualization, methodology, data curation, investigation, writing—original draft. RG: conseptualiza- tion, formal analysis, writing—original draft. FH: formal analysis, data curation, writing—review and editing. LS: formal analysis, writ- ing—review and editing. CS: data curation, writing—review and edit- ing. SG: supervision, data curation, writing—review and editing. MIH:
software, investigation, writing—review and editing. JLS: visualiza- tion, investigation, writing—original draft. AIYT: supervision, data curation, writing – review and editing.
Fig. 13 Schematic of the possible effect of metal oxides CuO and ZnO on the catalyst in the HDO reaction
Funding The authors would like to thank the Institute for Research and Community Service (LPPM) Universitas Negeri Medan for the financial support provided through the collaborative research scheme between institutions and universities with grant number No. 002/
UN33.8/PPKM/IKU/2022.
Data Availability The datasets generated during and/or analyzed dur- ing the current study are available from the corresponding author on reasonable request.
Declarations
Conflict of Interest The authors declared that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Ethical Approval Not applicable.
Consent to Participate Not applicable.
Consent fot publication The authors hereby confirm that all authors mutually agree for submitting their manuscript and that the manuscript is original work of the authors.
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