3.1. Characterization of the Biomass Sample and Catalysts
Table 1 shows the proximal and final analysis outcomes, which are atomic H/C and O/C ratios, calorific value and content of the biomass sample. PS contains volatile matter (VM) of 83.84% and little amount of ash (inorganic content) of 0.91, thus making this more advantegous for thermal processes since a high yield of bio-oils and biogas are obtained [
26]. Raw material has low moisture contents (5–9%), average carbon (48.53%) and no sulfur content with a H/C ratio of 1.45. Biomass chemical composition has significant effects on pyrolytic properties. In this way, in this study, the chemical structure of the plum seed is revealed by FT-IR, 1H-NMR, as well as the final and proximal analysis.
Figure 1a shows the plum seed spectrum. The overlapping bands between 3600 and 3100 cm
−1 attributed to OH stretching vibrations in hydroxyl groups (mainly due to the moisture contents in the raw material), acidic or phenolic groups can be seen in the spectrum. Asymmetric and symmetric C–H bands which submit the presence of alkyl groups for aliphatic and olefinic structure are seen with two strong bands at 2924 and 2857cm
−1 respectively. The stretching vibration band between 1770 and 1650 cm
−1 is related to carbonyl groups. The C=C vibrations in aromatic structure are detected between 1650 and 1600cm
−1 that demonstrates the lignin presence. The bands between 1060 and 1100 cm
−1 are because of the C–O vibrations in olefinic and aromatic structures such as saturated ethers which denote the presence of hemicellulose, cellulose, and lignin [
27]. The
1H-NMR spectra of plum seed is submitted in
Figure 1b. It is clear in
Figure 1 that biomass sample contains 80% aliphatic and 20% aromatic compounds.
DTG, TGA, and DTA analyses of plum seed are submitted in
Figure 2a,b and
Table 2. Lignocellulosic biomass predominantly consists of natural biopolymers like cellulose, hemicelluloses, as well as lignin. There is general agreement that the major pyrolytic degradation of the lignocellulosic structure is related to the decomposition of hemicellulose (220-315 ºC); cellulose (315-400ºC) and lignin degradation (150-450ºC) [
28]. Pyrolysis reactions can be defined by DTG peaks at the lower temperature up to 200 ºC after the release of moisture at approximately 90– 105 ºC with a weight loss of 7.89% in the sample. The maximum loss of weight was detected at 228–400ºC temperature ranges. Hemicellulose begins to degrade at 228ºC and ends at 353 ºC, whereas the cellulose decomposition range is between the temperatures of 355–393ºC. Therefore, the losses of weight mentioned here may be associated with the decomposition of these two. Degradation of lignin takes between the temperatures of 230-540ºC, but, lignin continues to degrade up to higher temperatures [
29]. Since hemicellulose is a heterogeneous polysaccharide composed of different hexoses and pentoses with a lower polymerization degree, its decomposition temperature is lower than that of cellulose; thus, the intermolecular bond strength is lower than that in cellulose. Being a homogeneous polysaccharide consisting of only D-glucopyranose, has a uniform crystal structure. Hemicellulose and cellulose increase the formation of or volatile compounds, and lignin increases the formation of char [
30,
31]. Devolatilization begins at about 200 ºC and removing the volatiles is completed at approximately 530 ºC. The maximum heat losses and the pyrolytic reactions took place below 550 °C, the most active temperature range. After 540 ºC, no weight loss was seen. Therefore the optimum pyrolysis temperature convenient to maximize bio-oil was 550 ºC. CS lost 95.199 and 97.994% of its beginning value up to the final temperature of 900 °C in nitrogen and air environments respectively.
Analysis of XRD is an efficacious technology in detecting the crystalline structures of the zeolites. The structure of the catalysts examined with XRD patterns is given in
Figure 3. The featural peaks regarding the orthorhombic, hexagonal, monocilinic, cubic aluminosilicates structure of the zeolites were observed in the XRD patterns, in line with the literature. A significant diffraction peak was near 2θ=10-20º and 21–35°, akin to the zeolite crystalline [
32].
Table 3 shows the results from the XRF analysis of the catalysts are given in
Table 3. The Si/Al (w/w) ratio was of 5.30, 116.57 and 1.06 in the clinoptilolite, ZSM-5 and Purmol -CTX (as revealed by XRF) respectively. The application of zeolites in biomass advancement is related to the acidic properties of the zeolite as well as its structure and textural properties. The lower silica to alumina ratio was beneficial for cracking, in addition to converting bio-oil oxygenates into aromatics with the help of successive actions such as dehydration, cracking, decarbonylation, decarboxylation, oligomerization, alkylation, cyclization and aromatization as well as surface acidity and enhanced thermal stability. Lowering the Si/Al ratio increases the acidity of catalyst which at the same time alters the surface area and the particle size of zeolites [
26]. Clinoptilolite and PURMOL CTX were attributed to a higher proportion of K and Na, respectively.
3.2. Pyrolysis Yields
Pyrolysis temperature is the most important parameter impacting the chemical compositions of the char, liquid, and gas products as well as the yields. Bhoi et al. [
3] summarized that majority biomasses had an active pyrolytic temperature between 400–600 ºC obtained by thermogravimetric analysis due to their alike compositions in terms of lignin, cellulose, and hemicellulose. In this study, firstly, pyrolysis experiments were conducted at temperature values of 400, 450, 500, 550 and the rate of a sweeping gas flow was 100 cm
3 min
−1 under a steady heating rate of 100 °C min
−1. The sweeping gas deports the volatiles from the pyrolysis environment in course of the reaction. Due to the pyrolysis reactions the nitrogen flow affects the residence duration of the produced gas; thus, minimazing the secondary reactions including char formation, recondensation, and repolymerization [
33]. The temperature-dependent relationships of plum seed yields are given in
Figure 4. As shown in the figure, while the pyrolysis temperature increases, the gas yields also increase, however, the solid yields (char) decrease. For the most part, a temperature increase assisted the gasification of the formed tar; so, less liquid was acquired at elevated temperatures (550 °C). It can be assumed that at elevated temperatures, secondary reactions of the liquid fraction of volatiles and more decomposition of the char particles achieved in the reactor [
29]. Accordingly, the highest yield of char processed at 400 °C was 29%, and the lowest yield of gas processed at the equal temperature was 16.3%. The bio-oil water content declined from 29% to 19%, as the temperature was shifted from 400 to 600 °C. At a pyrolysis temperature of 550 °C, the yield of liquid attained the highest which was 29.43%. Prior studies stated that the optimum temperature of pyrolysis to maximize the oil yield was between 500 and 600 °C [
7,
19,
34,
35,
36,
37,
38]. Further temperature increases reaching to 600°C only boosted the products of gas. The highest yield of gas yield procured at 600◦C pyrolysis temperature was 29.06%. As the temperature increases, the rate of reaction also increases and the long-chained compounds are divided into smaller pieces resulting in an increase in the yield of gas. Similarly, Naqvi et al. found that at values higher than 450 - 600 °C, the yield of gas was enhanced due to the secondary oil cracking and the biochar decomposition.
During the second part, experiments of pyrolysis were performed at heating rates of 10, 50 and 100 °C min
−1; the rate of the sweeping gas flow was steady at 100 cm
3 min
−1 and the temperature of pyrolysis was 550 °C. Gained results are submitted in
Figure 5. As the heating rate increased, the yields of liquid also increased whereas the yields of char declined. According to the results, the yield of char was 28.70% at 10 °C min
-1 but at 100 °C min
-1 it declined to 25.24%. The pyrolysis reactions and their related order as well as the composition of products and overall yield are affected by the rate of the heating. Due to the low energy input per unit of time, slow heating rates result in no cracking of biomass contributing to higher coke and biochar formation [
39]. When compared, a higher rate of heating lowers the exposure duration of biomass; thus, limiting the primary and secondary cracking reactions to interfere with one another. Higher heating enables diminshing secondary reactions and promotes decomposing of the products formed earlier. Therefore, the yield of bio-oil enhances with higher rates of heating compared to less rates of heating. In studies made by Ateş et al. [
25,
40] pyrolysis of wheat straw carried ot at 500◦C in a fixed-bed reactor. They determined that the yield of bio-oil was as 19.1% in 7
◦C
/min heating rate, it reached up to 31.9% in 300
◦C
/min heating rate.
In order to procure the peak yields of oil, catalytic pyrolysis experiments were conducted in the optimum conditions in which the rate of heating was 100 °C min
−1, the temperature of pyrolysis was 550 °C and the rate of sweeping gas flow was 100 cm
3 min
−1 in the presence of synthetic zeolites ZSM-5, Purmol CTX and NZ. As can be seen from the results given in
Figure 6, the highest gas yield procured was 20.21% with NZ, whereas the highest liquid obtained was 33.20% with ZSM-5 which is an acidic catalyst, and acknowledged for generating nearly 40% of the bio-oil yield [
41]. Akhtar and Saidina Amin [
42] studied the impact of severity on zeolite catalysed biomass pyrolysis on the highest yield of the targeted bio-oil. Lappas et al. [
43] assessed the zeolite acid catalyst application in biomass pyrolysis for transportation fuel production. Putun et al. [
7] carried out the catalytic pyrolysis of cotton seed cake with natural zeolite (clinoptilolite) selected as the catalyst at pyrolysis temperature of 550ºC with the sweeping gas flow rate of 100 mLmin
-1. The highest yiled of liquid was 30.84% with catalyst the amount of 20wt.% of raw material. The liquid products obtained as a result of pyrolysis processes posses a high amount of oxygen which leads to an acidic, corrosive and unstable result with a comparatively low energy density when highly present in the bio-oil. Despite the fact that they are utilized in numerous applications for generating heat and power, they induce an efficiency decrease if employed in motors and turbines. These oils may be enhanced by catalytic cracking for lowering the content of oxygen with the aim of using them directly as conventional transport fuel. Soongprasit et al.[
44] studied fast pyrolysis of millettia (Pongamia) pinnata waste at 400–600 ºC under sweeping gas(He) flow rate of 5 mL/ min with 30% zeolite (USY) catalyst loading in a micro-batch pyrolyzer PY- 2020iD. Non catalytic bio-oil included 34.1–66.5% of oxygenated compounds. The catalytic pyrolysis enhanced the yield of hydrocarbon to 99%, at which the conversion of the oxygenated compounds into aromatic and aliphatic hydrocarbon using decarboxylation and dehydration took place.
Table 4 shows the elemental compositions of the non-catalytic and catalytic bio-oils. There were higher carbon content, less oxygen content and higher energy density in the catalytic bio-oil compared to non-catalytic bio-oil. The pyrolysis oil oxygen content was 25.03% and declined to 21.33%, 16.54%, and 17.30%, with NZ, ZSM-5, and Purmol CTX, respectively by catalytic treatment. As indicated, the zeolites action for removing the oxygen from the pyrolysis oil is obvious. Removing more oxygen causes the calorific value of the fuel to increase.
In terms of environment, a higher H/C ratio which is 1.58 for heavy fuel oil, 1.8 for diesel fuel, and 2 for gasoline enables lower greenhouse gas emissions along with enhanced fuel qualifications. Obviously, catalytic enhancement raises the H/C ratio of bio-oil. The H/C ratio of pyrolysis oil found to be 1.52 was raised using catalytic enhancement and the subsequent the ratios were 1.69, 1.75, and 1.77 with NZ, ZSM-5, and Purmol CTX, respectively. Comparison of H/C ratios with conventional fuels showed that the H/C ratios of the oils procured within the scope of this study are between light and heavy petroleum products. The catalysts advanced the bio-oils calorific value to 24.97-41.75 MJ/kg corresponding to other conventional fuels like LPG (45.75 MJ/kg), petroleum (43 MJ/kg), and kerosene (41 MJ/kg) [
24]. When Zhang et al. [169] employed ZSM-5 for ex-situ mode catalytic pyrolysis of corncobs with a fluidized bed reactor, the obtained bio-oil demonstrated a decrease for oxygenated compounds by 25% with a high calorific value of 34.6 MJ/kg, and that was akin to the values of heavy fuel oil and diesel [
45].
Table 5 shows the results obtained from adsorption chromatography. Pyrolysis oil of plum seeds contained 54% asphaltenes and 46% n-pentane solubles. The oil fraction was 19% for aliphatic, 36% for aromatic and 45% for polar. The fraction of maltenes (n-pentane solubles) was raised to nearly 71% applying catalytic pyrolysis. The reason for this increase can be explained by the cracking degree in the course of catalytic pyrolysis. The percentage of the maximum aromatics procured with ZSM-5 was 45.45%, and the percentage of the maximum aliphatics procured with NZ was 24.28% both of which may be due to the properties of zeolites. The increase in aromatics (e.g., toluene and benzene) and aliphatics (e.g. alkanes and iso-paraffins) are considered favorable for using products as value-added chemicals and fuels. Also, using catalyst leads to a decrease in polar fractions (generally oxygenated groups). The polar fraction includes primarily carboxylic acids, phenols, aldehydes, furans, and ketones. Thus, the polar fraction of non-catalytic pyrolysis oil declined from 57.34% to 363.34% subsequent to the catalytic application with ZSM-5.
1H NMR is reported to be a requisite and precise technique in order to identify hydrogen distributions in bio-oil. [
46,
47]. If the hydrogen atoms (major isotope
1H) are abundant in an organic compound, this feature makes it better suited for a
1H NMR spectroscopy analysis to detect bio-oil constituents. Thus, this technique offers a faster analysis with more precise results [
48].
Table 6 shows a summary of the hydrogen percentages of bio-oils procured from both catalytic and non-catalytic pyrolysis of plum seed. The
1H NMR spectra were splitted in three interest regions related to the chemical shifts of specific proton types. The 0.5-3.0 ppm chemical shift region is where the aliphatic resonances appear, 4.5-6.0ppm region is where the olefinics resonances appear and 6.0-9.0 ppm region is where the aromatic resonances appear. High hydrogen content in aliphatic CH
3–CH
2– and CH– group is typical for all studied oils. The non-catalytic pyrolysis oil aliphatic content (68%) rose to 72.60% with ZSM-5. These results are coherent with the column chromatography findings. The aromatic hydrogen intensity occurring mostly between the 6.4–7.5 ppm suggests that the aromatic species are mostly phenolic. Together with phenols, IR spectroscopy pointed out that ketones/aldehydes and carboxylic acids are also significant organics containing oxygen found in the polar fractions.
1H-NMR spectra of the bio-oils demonstrate that the aroma of the natural zeolite bio-oil of catalytic pyrolysis is greater than the non catalytic and other catalytic pyrolysis oils. Bio-oil procured with NZ catalytic pyrolysis gave the highest aromatic hydrocarbons percentage of 35.23 %. Bio-oils procured with natural zeolite catalytic pyrolysis contain more paraffin and aromatics in comparison to that of uncatalyzed and other catalysts products. Several authors have investigated bio-oil structure with NMR spectroscopy [
46,
47,
48,
49].Tessarolo et al. utilized
1H NMR for examining bio-oils made from sugarcane bagasse and pine wood. At various temperatures, the bio-oils were produced through non-catalyzed and ZSM-5-catalyzed pyrolysis. All bio-oil samples’
1H NMR chemical shift integration ranges are shown. In comparison to non-catalytic sugarcane bagasse bio-oil, the bio-oil from sugarcane bagasse pyrolyzed with ZSM-5 had a higher hydrogen content from aromatic and conjugated alkenes and a lower hydrogen content from oxygen-containing groups. The identical ZSM-5 catalyst effect was seen on pine wood bio-oils [
49].
Abnisa et al. utilized FTIR for deciding the chemical structure of purge natural product brunches, mesocarp fiber buildups, and palm shells [
50]. The FTIR spectra appear the comparable useful bunches in both the EFB and mesocarp strands as evident within the shapes and the power of their spectra. The infrared spectrum of palm oil shell demonstrates weaker IR absorbance in comparison with those of mesocarp fiber and EFB, which reflects the nearness of lower volatile substance compared to the mesocarp strands.
Figure 7 shows the FT-IR spectra of the oil. The presence of alcohols and phenols is indicated by the O–H stretching vibrations between 3200-3400 cm
-1; the presence of alkenes is indicated by the C–H stretching vibrations between 2800-3000 cm
-1 and C–H deformation vibrations between 1350-1475 cm
-1. The presence of aldehydes or ketones is indicated by the C=O stretching vibrations between 1650 and 1750 cm
-1. The presence of aldehydes or ketones are indicated by the C=O stretching vibrations between 1650 and 1750 cm
-1. As an indicative of aromatics and alkenes, the C=C stretching vibrations are represented by the absorbance peaks between 1575-1675 cm
-1. It is seen from the spectra that the functional groups of the oils in compliance with those of the oils and chromatographic functions procured from cotton-seed cake.
Gas chromatography/mass spectrometry (GC/MS) is considered a fast, advantageous and an effective tool to define bio-oil samples which are heterogeneous and complex [
51]. The bio-oil chemical composition was examined with GC/MS tool for the purpose of throughly understanding the way catalysts affected the biomass pyrolysis chemistry.
Figure 8 shows the bio-oil gas chromatograms achieved with and without catalyst. In addition to being utilized as fuel in boilers and diesel engines, pyrolysis bio-oils are also evaluated as a beneficial source for organic chemicals. The rates of different compounds such as hydrocarbons (aliphatic + aromatic), carbonyls, acid, phenolics and alcohols found in the non-catalytic and catalytic plum seed bio-oils can be seen in
Figure 9. According to the above-mentioned findings,
Figure 9 should be studied more carefully in terms of aliphatics percentage which is evident to be greater with catalysts. It can be concluded that utilizing the catalyst leads to a advantegous result since aliphatics are critical compounds regarding the alikeness with fuels [
52]. Being invaluable industrial products (such as resins, solvents, medicine raw materials, and pesticides) due to the high commercial value they have [
40], phenols are the second most crucial compound found in plum seed bio-oil as can be seen in
Figure 9. Phenols, alkyl phenols, and methoxy phenols are the primary phenolic compounds detected in bio-oils as oligomers and monomeric units extracted from lignin. The total percentage of phenolics compounds was 27.17% at non-catalitic bio-oils and increased to 29.28%, 30% and 35% with ZSM-5, Purmol, NZ catalysts respectively. Pattiya et al. examined cassava rhizome pyrolysis with 4 catalysts and concluded that the most efficient catalyst was ZSM-5 since it caused a noteworthy rise in phenols and aromatics [
53]. Using commercial catalysts in-bed and ex-bed mode, Samolada et al. found out that phenols hightened in both modes in comparison with non-catalytic application [
54]. It is acknowledged that biomass liquids exhibit an acidic structure. However, the presence of acids in pyrolysis oils is undesirable because of their corrosive effects. In this study, carboxylic acids were reduced in bio-oils utilizing catalysts. The total percentages of the carboxylic acids observed in the oils were 4.6%, 3.1%, 1.22% and 0.95% in non-catalytic experiments and using ZSM-5, PURMOL, NZ catalysts, respectively. Low percentage acidic compounds observed in bio-oils of catalytic pyrolysis of plum seed can be evaluated as a superiority regarding the final quality of the fuel. Since the carbonyls undergo condensation reaction resulting in the formation of higher molecular weight components and enhanced viscosity, their presence leads to the issue of instability. Yet, the amount of carbonyls in the bio-oil is decreased by the catalyst. Subsequent to the deoxygenation and cracking of the oil which occurs in the catalyst pores, the processes of cyclization, isomerization, and aromatization takes place. Based on the results of GC/MS, comparing the products procured from the pyrolysis regarding the oxygenated compounds, it is found that the compunds are reduced using a catalyst. In their study which focused on advancing fast pyrolysis bio-oils with various catalysts in a fixed-bed micro-reactor, Adjaye and Bakshi [
55,
56] observed that H–Y, silicaalumina, and silicate provided more aliphatic hydrocarbons than aromatic hydrocarbons whereas HZSM-5 and H-mordenite provided more aromatic hydrocarbons than aliphatic hydrocarbons.
Table 7 shows a detailed component analysis of the aliphatic sub-fractions of pentane soluble bio-oil employing GC/MS, which includes the compound name, the peak area, and the retention time without catalyst and with a catalyst conditions. Primarily, three groups were formed to divide the bio-oil carbon number which were respectively C
5-C
11 (gasoline fraction), C
12-C
18 (kerosene-diesel fraction), and C20-C38 (heavy oil fraction). For non-catalyst pyrolysis processes, these fractions were distributed as follows: 3.64wt% (C
5-C
11), 57.94 wt% (C
12-C
18) and 38.41 wt% (C
20-C
38). For catalytic pyrolysis, the carbon number distribution was of C
5-C
11, C
12-C
18, and C
20-C
38; in ZSM-5 pyrolysis bio-oil was 13.08, 66.32 and 20.59 wt% ; in NZ pyrolysis bio-oil was 12.57, 69.09 and 18.33 wt%, and in PURMOL CTX pyrolysis bio-oil was 18.58, 68.80 and 12.60 wt%, respectively. These results show that after the catalytic application, the long chains of alkanes and alkenes of the pyrolysis oil were transformed into hydrocarbons which were lower in weight. Branched hydrocarbons of 3.29% obtained without catalyst raised 8.11%, 10.60%, and 11.97% using ZSM-5 Purmol CTX and NZ catalysts, relatively.