Microwave-Assisted Catalytic Fast Pyrolysis of Biomass for Hydrocarbon Production with Physically Mixed MCM-41 and ZSM-5

To delve into the law of hydrocarbon production in microwave-assisted catalytic fast pyrolysis (MACFP) of corn straw, physical mixed Mesoporous Crystalline Material-41 (MCM-41) and Zeolite Socony Mobile-5 (ZSM-5) catalyst prototypes were exploited in this study. Besides, the effects exerted by temperature of reaction and MCM-41/ZSM-5 mass ratio were explored. As revealed from the results, carbon outputs of hydrocarbons rose initially as the temperature of MACFP rose and reached the maximal data at 550 °C; subsequently, it declined as reaction temperature rose. Moreover, the MCM-41/ZSM-5 mass ratio of 1:2 was second-to-none for hydrocarbon formation in the course of biomass MACFP. It was reported that adding MCM-41 can hinder coke formation on ZSM-5. Furthermore, MCM-41/ZSM-5 mixture exhibited more significant catalytic activity than ZSM-5/MCM-41 composite, demonstrating that hydrocarbon producing process can be stimulated by a simple physical MCM-41 and ZSM-5 catalysts mixture instead of synthesizing complex hierarchically-structured ZSM-5/MCM-41 composite.


Introduction
Over the past few years, energy consumption and environmental pollution have risen remarkably, so renewable energy should be developed to tackle energy and environmental issues [1]. In terms of various alternative energy resources, biomass energy exhibits the maximal potential for its merits of non-pollution, renewability and carbon neutral [2,3]. Accordingly, biomass exploitation should be advanced to extract useful energy. Indeed, biomass energy has achieved extensive applications recently following direct or indirect technical routes (e.g., landfill, combustion, and fermentation) [4,5]. However, all the mentioned treatment approaches can cause secondary pollution to risk human health, which are becoming hard to accept [6,7].
Catalytic fast pyrolysis (CFP) refers to an emerging and promising thermochemical conversion technique to yield liquid fuels (termed bio-oil) from biomass in the absence of oxygen [8][9][10], combining fast pyrolysis and zeolitic catalyst and allowing for biomass volume reduction and high-quality bio-oil production. It is noteworthy that the zeolitic catalyst is employed to expel oxygen and facilitate hydrocarbon formation during CFP [11], which improves bio-oil quality. Over the past few decades, numerous microporous and mesoporous catalysts have been screened and assessed for biomass CFP transformation [12,13], and it is identified that ZSM-5 catalyst optimally contributes to hydrocarbon production for its great deoxygenating capacity [14][15][16]. As a microporous zeolite, the inner pore structure of ZSM-5 is characterized by the straight 0.53 × 0.56 nm channels connected with zigzag 0.51 × 0.55 nm channels. In the course of biomass CFP, when the biomass primary pyrolytic vapors

Influence of Temperature of MACFP on Pyrolysis Product Yields
The product distribution from MACFP of corn stalk is shown in Figure 1 as a function of temperature. As observed, the carbon outputs of aromatics, C 2 -C 4 olefins, C 5 compounds and overall petrochemicals (aromatics + C 2 -C 4 olefins + C 5 compounds) rose initially with mounting temperature of MACFP and were peaked at 550 • C, and then dropped off when the reaction temperature further increased. Conversely, the carbon output of unidentified oxygenates initially declined with the increasing temperature, arrived at the minimum number of 11.0% at 550 • C, and then rose with the rise in the temperature. The carbon outputs of overall petrochemicals and total unidentified oxygenates were 5.7%-17.1% versus 11.0%-27.5%, respectively. Likewise, the maximal carbon output of overall petrochemicals (17.1%) and minimum carbon output of unidentified oxygenates (11.0%) were obviously obtained at a temperature of MACFP of 550 • C, suggesting that 550 • C is the optimal temperature for MACFP of corn stalk to maximize hydrocarbon output. Moreover, the carbon outputs of methane, CO 2 and CO obviously tend to augment with the upswing reaction temperature. The underlying cause was Catalysts 2020, 10, 685 3 of 11 that a higher temperature of MACFP can facilitate the transformation of more precursor compounds into the mentioned non-condensable light gases via secondary cracking reactions [26]. Moreover, it is found that the carbon output of solid residue (char + coke) decreased when the temperature of MACFP ranged from 450 to 650 • C. This could be explained by the fact of greater primary thermal decomposition of biomass and secondary thermal decomposition of the solid carbonaceous residue at increased temperature of MACFPs [27]. For the overall generated hydrocarbons, aromatics and C 2 -C 4 olefins account for the maximal proportion from 91.8% to 100%. The aromatic hydrocarbon compounds primarily involved benzene, toluene, xylene and naphthalene, whereas the C 2 -C 4 olefin compounds are comprised of ethylene, propylene, butene, and butadiene.
Catalysts 2020, 10, x FOR PEER REVIEW  3 of 11 reactions [26]. Moreover, it is found that the carbon output of solid residue (char + coke) decreased when the temperature of MACFP ranged from 450 to 650 °C. This could be explained by the fact of greater primary thermal decomposition of biomass and secondary thermal decomposition of the solid carbonaceous residue at increased temperature of MACFPs [27]. For the overall generated hydrocarbons, aromatics and C2-C4 olefins account for the maximal proportion from 91.8% to 100%. The aromatic hydrocarbon compounds primarily involved benzene, toluene, xylene and naphthalene, whereas the C2-C4 olefin compounds are comprised of ethylene, propylene, butene, and butadiene. After fast pyrolysis via microwave heating and cracking via MCM-41 catalyst, the biomass primary pyrolytic vapors were transformed into hydrocarbons by ZSM-5 catalyzed transformation. Typified by a unique three-dimensional pore system with 10-member ring, ZSM-5 catalyst exhibited inside straight 0.53 × 0.56 nm channels linked to zigzag 0.51 × 0.55 nm channels. In the interim of biomass CFP, ZSM-5 catalyst retained the performance to adsorb oxygenated chemicals from biomass primary pyrolytic vapors in its confined pore channels and intersection cavities, along with assorted deoxygenation (e.g., dehydration, dehydroxylation, decarbonylation, and decarboxylation reactions) and isomerization responses to expel oxygen atoms as H2O, CO2 and CO and produced hydrocarbons [18]. Furthermore, the pore diameter of ZSM-5 catalyst is obviously consistent with the dynamic diameters of benzene, toluene and xylene, so ZSM-5 catalyst exerts a prominent shape-selective catalytic influence on the formation of aromatic hydrocarbons during biomass CFP [15,28].
For the specific catalytic mechanism of ZSM-5, it is revealed that hydrocarbons were shaped via a universal intermediate termed as "hydrocarbon pool" in this zeolite framework. After the "hydrocarbon pool" was formed within ZSM-5 in the course of CFP, oxygenated chemicals in primary product vapored from biomass pyrolysis went into the inner pores of catalyst once they came into contact with ZSM-5, and were transformed into alkanes, olefins, and aromatic hydrocarbons by reacting with this "hydrocarbon pool" intermediate [29]. On closer inspection, a duplex aromatic-and olefin-based carbon pool cycle took charge of hydrocarbon formation in ZSM-5 during biomass CFP run [30,31]: C2 olefin (ethylene) and aromatic hydrocarbons were produced from the aromatic-based methylation catalytic cycle (aromatic carbon pool) while >C2 olefins were generated from another olefin-based one (olefin carbon pool). It is noteworthy that the formation of ethylene was different from that of other olefins. As observed, ethylene was involved in aromatic carbon pool that aromatic hydrocarbons could perform as active catalyzed species.  After fast pyrolysis via microwave heating and cracking via MCM-41 catalyst, the biomass primary pyrolytic vapors were transformed into hydrocarbons by ZSM-5 catalyzed transformation. Typified by a unique three-dimensional pore system with 10-member ring, ZSM-5 catalyst exhibited inside straight 0.53 × 0.56 nm channels linked to zigzag 0.51 × 0.55 nm channels. In the interim of biomass CFP, ZSM-5 catalyst retained the performance to adsorb oxygenated chemicals from biomass primary pyrolytic vapors in its confined pore channels and intersection cavities, along with assorted deoxygenation (e.g., dehydration, dehydroxylation, decarbonylation, and decarboxylation reactions) and isomerization responses to expel oxygen atoms as H 2 O, CO 2 and CO and produced hydrocarbons [18]. Furthermore, the pore diameter of ZSM-5 catalyst is obviously consistent with the dynamic diameters of benzene, toluene and xylene, so ZSM-5 catalyst exerts a prominent shape-selective catalytic influence on the formation of aromatic hydrocarbons during biomass CFP [15,28].
For the specific catalytic mechanism of ZSM-5, it is revealed that hydrocarbons were shaped via a universal intermediate termed as "hydrocarbon pool" in this zeolite framework. After the "hydrocarbon pool" was formed within ZSM-5 in the course of CFP, oxygenated chemicals in primary product vapored from biomass pyrolysis went into the inner pores of catalyst once they came into contact with ZSM-5, and were transformed into alkanes, olefins, and aromatic hydrocarbons by reacting with this "hydrocarbon pool" intermediate [29]. On closer inspection, a duplex aromatic-and olefin-based carbon pool cycle took charge of hydrocarbon formation in ZSM-5 during biomass CFP run [30,31]: C 2 olefin (ethylene) and aromatic hydrocarbons were produced from the aromatic-based methylation catalytic cycle (aromatic carbon pool) while >C 2 olefins were generated from another olefin-based one (olefin carbon pool). It is noteworthy that the formation of ethylene was different from that of other olefins. As observed, ethylene was involved in aromatic carbon pool that aromatic hydrocarbons could perform as active catalyzed species.
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 11 This investigation was implemented to examine the influence of joint exploitation of MCM-41 and ZSM-5 as catalysts in MACFP of corn stalk. In the present section, a constant temperature of MACFP of 550 °C was maintained during all runs, and the effect of MCM-41/ZSM-5 mass ratio on pyrolysis product yields was researched. In respective experimental process, the mass ratio of corn stalk to catalyst (MCM-41 + ZSM-5) remained at 1:2 (corn stalk: 1 g, catalyst mixture: 2 g), and MCM-41 and ZSM-5 catalysts were employed at various mass ratios (MCM-41 only, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, and ZSM-5 only). Figure 2 shows the product distributions from MACFP of corn stalk under different MCM-41/ZSM-5 mass ratios. It is observed that with a higher fraction of ZSM-5 catalyst in the catalyst mixture, C2-C4 olefins and overall petrochemicals rose from 1.8%, 0.8% and 2.6% (MCM-41 only) to the maximal values of 10.1%, 6.2% and 17.1% (MCM-41/ZSM-5 = 1:2), and then decline to 8.2%, 4.5% and 13.1% (ZSM-5 only). Moreover, when MCM-41/ZSM-5 mass ratio was 1:2, the carbon output of unidentified oxygenates reaches the minimum of approximately 11.0%. The mentioned results prove that MCM-41 and ZSM-5 as catalyst mixtures are capable of promoting hydrocarbon formation and facilitating oxygen expel in MACFP of biomass, consistent with existing studies [32]. For non-condensable gaseous products (methane, CO2 and CO), the carbon outputs of CO2 and CO rose with the augment of used amount of ZSM-5 catalyst, and then decreased when ZSM-5 amount continued to rise. Conversely, the carbon output of methane increased with the declining MCM-41/ZSM-5 mass ratio. The carbon output of total gaseous product (methane + CO2 +CO) rose with the addition of ZSM-5 catalyst and reached the maximal of 36.8% at the MCM-41 to ZSM-5 ratio of 1:2. The use of MCM-41 allowed for the cracking of large-molecular chemicals (e.g., polysaccharide or lignin derivatives) in primary pyrolytic vapors into small-molecular ones, followed by subsequent transformation of the mentioned small-molecular compounds into non-condensable gaseous products and targeted hydrocarbons by hydrocarbon pool in the narrow pores of microporous ZSM-5 ( Figure 3) [20,21]. As suggested from the mentioned experimental results, the MCM-41/ZSM-5 ratio of 1:2 is optimal for MACFP of biomass. For non-condensable gaseous products (methane, CO 2 and CO), the carbon outputs of CO 2 and CO rose with the augment of used amount of ZSM-5 catalyst, and then decreased when ZSM-5 amount continued to rise. Conversely, the carbon output of methane increased with the declining MCM-41/ZSM-5 mass ratio. The carbon output of total gaseous product (methane + CO 2 +CO) rose with the addition of ZSM-5 catalyst and reached the maximal of 36.8% at the MCM-41 to ZSM-5 ratio of 1:2. The use of MCM-41 allowed for the cracking of large-molecular chemicals (e.g., polysaccharide or lignin derivatives) in primary pyrolytic vapors into small-molecular ones, followed by subsequent transformation of the mentioned small-molecular compounds into non-condensable gaseous products and targeted hydrocarbons by hydrocarbon pool in the narrow pores of microporous ZSM-5 ( Figure 3) [20,21]. As suggested from the mentioned experimental results, the MCM-41/ZSM-5 ratio of 1:2 is optimal for MACFP of biomass.

The Formation of Coke on ZSM-5 during MACFP
It is mentioned that ZSM-5 ineluctably suffers from significant acid-catalyzed polymerization of a large proportion of large reactant molecules derived from biomass pyrolysis, thereby forming detrimental bulky molecules (termed "coke") on its external surface. Overall, the coke yield from biomass CFP is lower than 5.0 wt.%, whereas the attached coke will cause the blockage of pore and rapid inactivation of catalysts, which inhibits the anticipated shape-selective catalytic reactions [18,19]. In this study, coke, char, and consumed catalyst could not be efficiently differentiated and separated as all these were totally mixed in the microwave chamber. To explore coke formation on ZSM-5 during MACFP, additional experiments were performed with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) instrument.
MACFP of corn stalk was conducted in a CDS Pyroprobe 5200 pyrolyser. Before the experimental processes, we filled the open ended quartz tube successively with some quartz wool, 1.00 mg ZSM-5 catalyst samples, some quartz wool, some MCM-41 catalyst samples, some quartz wool, 0.50 mg corn stalk particles, some quartz wool, some MCM-41 catalyst samples, some quartz wool, 1.00 mg ZSM-5 catalyst samples, as well as some quartz wool. The schematic cross-section of the quartz tube is illustrated in Figure 4. In the experimental processes, the feedstock was pyrolyzed at 600 °C, which was held for 20 s. Moreover, the heating rate was set at 20 °C/ms. Furthermore, the high-purity helium (99.999%) was carrier gas at a constant flow of 1.0 mL/min. When respective experimental process was achieved, the used ZSM-5 catalyst was harvested and went through the drying process at 120 °C for 1 h with a drying oven. Afterwards, the samples dried were arranged in a muffle furnace to be combusted (650 °C, 2 h), for the determination of the coke yield (wt.%) on ZSM-5. Table 1 lists the experimental outcomes. It is suggested that, in

The Formation of Coke on ZSM-5 during MACFP
It is mentioned that ZSM-5 ineluctably suffers from significant acid-catalyzed polymerization of a large proportion of large reactant molecules derived from biomass pyrolysis, thereby forming detrimental bulky molecules (termed "coke") on its external surface. Overall, the coke yield from biomass CFP is lower than 5.0 wt.%, whereas the attached coke will cause the blockage of pore and rapid inactivation of catalysts, which inhibits the anticipated shape-selective catalytic reactions [18,19]. In this study, coke, char, and consumed catalyst could not be efficiently differentiated and separated as all these were totally mixed in the microwave chamber. To explore coke formation on ZSM-5 during MACFP, additional experiments were performed with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) instrument.
MACFP of corn stalk was conducted in a CDS Pyroprobe 5200 pyrolyser. Before the experimental processes, we filled the open ended quartz tube successively with some quartz wool, 1.00 mg ZSM-5 catalyst samples, some quartz wool, some MCM-41 catalyst samples, some quartz wool, 0.50 mg corn stalk particles, some quartz wool, some MCM-41 catalyst samples, some quartz wool, 1.00 mg ZSM-5 catalyst samples, as well as some quartz wool. The schematic cross-section of the quartz tube is illustrated in Figure 4. In the experimental processes, the feedstock was pyrolyzed at 600 • C, which was held for 20 s. Moreover, the heating rate was set at 20 • C/ms. Furthermore, the high-purity helium (99.999%) was carrier gas at a constant flow of 1.0 mL/min.

The Formation of Coke on ZSM-5 during MACFP
It is mentioned that ZSM-5 ineluctably suffers from significant acid-catalyzed polymerization of a large proportion of large reactant molecules derived from biomass pyrolysis, thereby forming detrimental bulky molecules (termed "coke") on its external surface. Overall, the coke yield from biomass CFP is lower than 5.0 wt.%, whereas the attached coke will cause the blockage of pore and rapid inactivation of catalysts, which inhibits the anticipated shape-selective catalytic reactions [18,19]. In this study, coke, char, and consumed catalyst could not be efficiently differentiated and separated as all these were totally mixed in the microwave chamber. To explore coke formation on ZSM-5 during MACFP, additional experiments were performed with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) instrument.
MACFP of corn stalk was conducted in a CDS Pyroprobe 5200 pyrolyser. Before the experimental processes, we filled the open ended quartz tube successively with some quartz wool, 1.00 mg ZSM-5 catalyst samples, some quartz wool, some MCM-41 catalyst samples, some quartz wool, 0.50 mg corn stalk particles, some quartz wool, some MCM-41 catalyst samples, some quartz wool, 1.00 mg ZSM-5 catalyst samples, as well as some quartz wool. The schematic cross-section of the quartz tube is illustrated in Figure 4. In the experimental processes, the feedstock was pyrolyzed at 600 °C, which was held for 20 s. Moreover, the heating rate was set at 20 °C/ms. Furthermore, the high-purity helium (99.999%) was carrier gas at a constant flow of 1.0 mL/min. When respective experimental process was achieved, the used ZSM-5 catalyst was harvested and went through the drying process at 120 °C for 1 h with a drying oven. Afterwards, the samples dried were arranged in a muffle furnace to be combusted (650 °C, 2 h), for the determination of the coke yield (wt.%) on ZSM-5. Table 1 lists the experimental outcomes. It is suggested that, in When respective experimental process was achieved, the used ZSM-5 catalyst was harvested and went through the drying process at 120 • C for 1 h with a drying oven. Afterwards, the samples dried were arranged in a muffle furnace to be combusted (650 • C, 2 h), for the determination of the coke yield Catalysts 2020, 10, 685 6 of 11 (wt.%) on ZSM-5. Table 1 lists the experimental outcomes. It is suggested that, in comparison with ZSM-5 catalyst alone, adding MCM-41 offers the merits of reduction in coke formation, demonstrating that the addition of MCM-41 and their combined application is an effective approach to hinder coke formation on ZSM-5 catalyst.

Comparison between MCM-41/ZSM-5 Mixture and Hierarchical ZSM-5/MCM-41 Composite
Another approach to combine the merits of MCM-41 (cracking) and ZSM-5 (deoxygenation) to retard coke generation and facilitate hydrocarbon formation refers to synthesizing an innovative hierarchically structured ZSM-5/MCM-41 composite zeolite catalyst through the growth a layer of MCM-41 structure onto the external surface of ZSM-5 zeolite seeds. Gratifying outcomes are expected to able be gained in biomass MACFP to such mix. While the primary biomass pyrolytic vapors were passing via the hierarchically-structured ZSM-5/MCM-41 composite catalyst, all heavy oxygenated compounds in vapors would be cracked into smaller ones in the outer MCM-41 structure, thereby being further altered into a range of hydrocarbons in the inter ZSM-5 core. In the continuous attempts of the synthesis of zeolite micromesoporous composites, the optimization goal of the biomass MACFP system is to combine the key advantages of ZSM-5 zeolite, namely its considerable deoxidation capacity, along with the facile cracking of bulky molecules based on the orderly-constructed mesopore-based MCM-41 configuration. In this study, the distinction between simple MCM-41/ZSM-5 mixture and hierarchical ZSM-5/MCM-41 composite was analyzed.
For the synthesis of hierarchical ZSM-5/MCM-41 composite zeolite, hydrothermal crystallization technique is the most common with ZSM-5 powders as the original seed source in both lab-and industrial-scale operation [33]: First, parent ZSM-5 powders were alkali-leached by NaOH solution (2.0 mol/L) with mechanical stirring, which formed desired aluminosilicate fragments in solution. Then, a certain amount of CTAB (hexadecyl trimethyl ammonium bromide) solution (10 wt.%) was added drop wise to this zeolite solution. In this place, CTAB was MCM-41 precursor. Further, an autoclave with the lining was employed for crystallization of the resultant solution (110 • C, 24 h). After cooling the autoclave to ambient temperature and regulating the solution pH to 8.5, a second crystallization was implemented (110 • C, 24 h). Subsequently, to obtain Na-ZSM-5/MCM-41 composite zeolite, filtration, washing, drying, and calcination steps were conducted. Lastly, Na-ZSM-5/MCM-41 was transformed into ZSM-5/MCM-41 composite catalyst via NH 4 Cl solution leaching, filtration, washing, drying and calcination treatment.
During MACFP of biomass, the mass ratio of corn stalk to total catalyst remained at 1:2 (corn stalk: 1 g, total catalyst: 2 g) and the temperature of MACFP was maintained at 550 • C. For MCM-41/ZSM-5 mixture, the MCM-41/ZSM-5 mass ratio was 1:2. The experimental results are concluded in Figure 5. Interestingly, it is clear that compared with ZSM-5/MCM-41 composite, the simple MCM-41/ZSM-5 mixture gives rise to higher carbon outputs of aromatics, olefins, overall petrochemical products and gaseous products, demonstrating that MCM-41/ZSM-5 mixture can achieve more significant catalytic effect. This differs from the result reported in previous researches [34,35]. There are two reasons to explain this fact.

Biomass Feed
Corn stalk taken from a farmland in Nanjing, Jiangsu, China was used as biomass feedstock in this study. The biomass feedstock was dehydrated at 105 °C for a day and sifted to 40-mesh in size before the experiment. As revealed from the proximate analyzing process, the corn stalk after air drying has 5.19 wt.% moisture, 86.14 wt.% volatile, 5.72 wt.% fixed carbon, and 2.95 wt.% ash. Moreover, 37.91 wt.% carbon, 0.32 wt.% nitrogen, 45.77 wt.% oxygen, and 6.86 wt.% hydrogen of dried corn stalk has been shown in elemental composition analysis.

Biomass Feed
Corn stalk taken from a farmland in Nanjing, Jiangsu, China was used as biomass feedstock in this study. The biomass feedstock was dehydrated at 105 • C for a day and sifted to 40-mesh in size before the experiment. As revealed from the proximate analyzing process, the corn stalk after air drying has 5.19 wt.% moisture, 86.14 wt.% volatile, 5.72 wt.% fixed carbon, and 2.95 wt.% ash. Moreover, 37.91 wt.% carbon, 0.32 wt.% nitrogen, 45.77 wt.% oxygen, and 6.86 wt.% hydrogen of dried corn stalk has been shown in elemental composition analysis.

Experimental Setup
As shown in Figure 6, MACFP of biomass was conducted in a system of microwave oven reactor (Jinhaifeng, Nanjing, China). The composition of the reactor system included: (1) semi-continuous feeder (corn stalk and MCM-41/ZSM-5 mixture); (2) quartz entry linker with the length of 140 mm and 24.6 mm diameter; (3) microwave oven (MAX, CEM Corporation, 750W and 2450 MHz); (4) two necks quartz reacting element which is especially manufactured with fused silica, with the diameter of 127 mm and the length of 128 mm; (5) quartz beaker contained with microwave absorbent (SiC); (6) K-type thermocouple; (7) quartz exit linker with the diameter of 24.6 mm and the length of 148 mm; (8) bio-oil collecting element; (9) beaker containing ice water; (10) vacuum pump connecting process. In the course of pyrolysis experimental process, MD-2000 digital display microwave detector was used for measuring microwave leakage to ensure safety. necks quartz reacting element which is especially manufactured with fused silica, with the diameter of 127 mm and the length of 128 mm; (5) quartz beaker contained with microwave absorbent (SiC); (6) K-type thermocouple; (7) quartz exit linker with the diameter of 24.6 mm and the length of 148 mm; (8) bio-oil collecting element; (9) beaker containing ice water; (10) vacuum pump connecting process. In the course of pyrolysis experimental process, MD-2000 digital display microwave detector was used for measuring microwave leakage to ensure safety.

Experimental Methods
In this study, SiC was microwave resorbent to up-regulate the internal heating rate in CFP for its unique capacity of absorbing the microwave. Before the process of experiment, quartz reactor was placed in a quartz cup. Subsequently, 1000 g SiC with 30 grit particle size were placed in the quartz beaker surrounding the quartz reactor. Before each run, mixed compound of corn stalk and catalyst with the total weight of 3 g was well synthesized in advance by physical mixing ways. The mass ratio of corn stalk and catalyst (MCM-41 + ZSM-5) remained at 1:2. The microwave oven operated for heating treatment when the outlet tubes were connected with the quartz inlet. When the temperature was the experimental setting value, the synthesized mixed sample can be semi-continually added to the quartz reactor via the feeding element for MACFP reacting processes. The pulse width modulation (PWM) control was used for keeping the microwave on and off per 15 s to maintain the required experimental temperature. To keep the process of MACFP always under the atmosphere of inert and vacuum condition, the system should be vacuumed with a vacuum pump at 250 mm Hg for no less than 10 min before microwave-based heat process. A collecting element could gather biooil in an ice water bath. Non-condensable gas was gathered by employing a 1 L Tedlar air bag to achieve the analyzing process. Respective experimental process operated for 45 min.
After the reacting processes, to conduct the combustion of char and coke, we introduced oxygen to the quartz reacting element via the material feeding element at the speed of 100 mL/min and 850 °C. In the course of the combustion, the produced CO was then transformed into CO2 at 250 °C in the presence of CuO/Al2O3 catalyst. When the exhaust gas passed the desiccator, CO2 could be completely gathered with commercial ascarite CO2 trap. Ascarite CO2 trap and CuO/Al2O3 catalyst were bought from Sigma-Aldrich Corporation. This study ensured the carbon output of char and coke by weighing the discrepancy of CO2 trap. Under identical conditions, each experiment was conducted at least twice, so the reproducibility could be confirmed, and experimental uncertainty could be avoided to some extent by employing and calculating the standard deviation values.

Experimental Methods
In this study, SiC was microwave resorbent to up-regulate the internal heating rate in CFP for its unique capacity of absorbing the microwave. Before the process of experiment, quartz reactor was placed in a quartz cup. Subsequently, 1000 g SiC with 30 grit particle size were placed in the quartz beaker surrounding the quartz reactor. Before each run, mixed compound of corn stalk and catalyst with the total weight of 3 g was well synthesized in advance by physical mixing ways. The mass ratio of corn stalk and catalyst (MCM-41 + ZSM-5) remained at 1:2. The microwave oven operated for heating treatment when the outlet tubes were connected with the quartz inlet. When the temperature was the experimental setting value, the synthesized mixed sample can be semi-continually added to the quartz reactor via the feeding element for MACFP reacting processes. The pulse width modulation (PWM) control was used for keeping the microwave on and off per 15 s to maintain the required experimental temperature. To keep the process of MACFP always under the atmosphere of inert and vacuum condition, the system should be vacuumed with a vacuum pump at 250 mm Hg for no less than 10 min before microwave-based heat process. A collecting element could gather bio-oil in an ice water bath. Non-condensable gas was gathered by employing a 1 L Tedlar air bag to achieve the analyzing process. Respective experimental process operated for 45 min.
After the reacting processes, to conduct the combustion of char and coke, we introduced oxygen to the quartz reacting element via the material feeding element at the speed of 100 mL/min and 850 • C. In the course of the combustion, the produced CO was then transformed into CO 2 at 250 • C in the presence of CuO/Al 2 O 3 catalyst. When the exhaust gas passed the desiccator, CO 2 could be completely gathered with commercial ascarite CO 2 trap. Ascarite CO 2 trap and CuO/Al 2 O 3 catalyst were bought from Sigma-Aldrich Corporation. This study ensured the carbon output of char and coke by weighing the discrepancy of CO 2 trap. Under identical conditions, each experiment was conducted at least twice, so the reproducibility could be confirmed, and experimental uncertainty could be avoided to some extent by employing and calculating the standard deviation values.

Gaseous Product Analysis
By employing GC/TCD (Varian CP4900, Agilent, Santa Clara, CA, USA) that is equipped with a Porapak Q column and a 5A molecular sieve column (Shimadzu, Kyoto, Japan), this study analyzed the gaseous products. Both the injector and the detector were kept at 110 • C. The Porapak Q column and the 5A molecular sieve respectively remained at 150 • C and 80 • C. The carbon output of non-condensable gas was calibrated by a standard gas mixture that included methane, carbon monoxide, carbon dioxide, ethylene, ethane, propene, propane, butene and butane. The calibrating gas was provided by Nanjing special gas limited company (China), purity > 99.999%.
A GC/MS (7890A/5975C, Agilent, Santa Clara, CA, USA) instrument was employed to measure the bio-oil chemical composition. High-purity helium was the carrier gas with a constant gas flow rate of 1.2 mL/min. The temperature of oven remained at 40 • C for 3 min at first and it increased to 290 • C finally at the heating rate of 5 • C/min, while being maintained at 290 • C for 5 min. The GC/MS injector temperature was set at 250 • C while the temperature of detector was 230 • C. Moreover, the process of GC isolation was executed by a capillary column called HP-5 MS (Agilent, Santa Clara, CA, USA). A split ratio of 1:10 was employed with the injection size of 1 µL. The chromatographic hydrocarbon peaks are identified and qualified based on the National Institute of Standards and Technology (NIST) mass spectral data library.

Conclusions
To facilitate hydrocarbon formation in the MACFP of biomass, we employ physical mixed MCM-41 and ZSM-5 catalysts in this study, and the impacts exerted by reaction temperature and mass rate of MCM-41 to ZSM-5 are explored. Experimental results show that the carbon outputs of hydrocarbons (aromatics and olefins) are promoted first with raised temperature of MACFP and are peaked values at 550 • C, and then decrease when the reaction temperature further increases. In addition, the MCM-41/ZSM-5 mass ratio of 1:2 can optimally form hydrocarbon during biomass MACFP, and it is revealed that the addition of MCM-41 is capable of inhibiting coke formation on ZSM-5. Furthermore, MCM-41/ZSM-5 mixture can achieve a more significant catalyzing outcome than ZSM-5/MCM-41 composite, demonstrating that hydrocarbon production can be facilitated by a simple MCM-41 and ZSM-5 catalysts mixed physically, instead of synthesizing a complex hierarchical ZSM-5/MCM-41 composite.
Author Contributions: Conceptualization, Z.X. and Z.Z.; experiment design, Z.X. and B.Z.; data analysis, Z.X.; writing-original draft preparation, Z.X.; writing-review and editing, Z.Z. and B.Z.; supervision, Z.Z. All authors have read and agreed to the published version of the manuscript.