Experimental Study on Pyrolysis of Rice Straw Catalyzed by CaO/Al2O3-Phosphate Mixture

CaO and phosphates showed synergistic effects in the regulation of pyrolysis products in the pyrolysis of when they were directly mixed with camphor wood. The alkyl phenol yield increased during the pyrolysis process of corn straw fermentation residue directly mixed with KH2PO4 supported by γ-Al2O3. Rice stalks from agricultural crops are often disposed as waste. However, the potassium phosphate impregnated raw straw pyrolysis with CaO and Al2O3 has not been reported. This paper studied the synergistic effects of CaO or Al2O3 and three potassium phosphates (i.e., KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O) in the rice straw pyrolysis through pyrolysis–gas chromatography-mass spectrometer (Py-GC/MS) experiments. The results showed that CaO/Al2O3 and potassium phosphates showed synergistic effects in the regulation of the types or contents of phenols, ketones, aldehydes, etc. and increased the contents of phenols, aldehydes, acids, and levoglucosan (LG) from most samples and increased those of ketones compared with those catalyzed by potassium phosphates alone. They were suitable for the production of ketone-rich and acid-low bio-oil, which is an important precursor for the preparation of power or jet fuel. The highest contents of ketones (HCK) reached 56.65% and 56.02% in the pyrolysis of K3PO4·3H2O impregnated rice straw with CaO or Al2O3, respectively. The lowest contents of acids and acetic acid (LCA) were nearly or equal to 0, respectively. HCK and LCA were respectively significantly higher and lower than the values reported in the literatures for biomass catalytic pyrolysis using CaO/Al2O3 and potassium phosphates alone or in combination. Dehydration reactions, etc. were further promoted under the co-catalysis of the two catalysts, and some phenols could be converted to benzene products, etc. For 50% K3PO4·3H2O impregnated sample, the yields of furans reduced sharply after CaO addition. For most impregnated samples except 50% K2HPO4·3H2O and 30% and 50% K3PO4·3H2O samples, the contents of total furans and furfural increased after Al2O3 addition. CaO/Al2O3 and potassium phosphates showed synergistic effects in the regulation of the types or contents of phenols, ketones, aldehydes, etc. They were suitable for the production of ketone-rich and acid-low bio-oil, which was an important precursor for the preparation of power or jet fuel. Dehydration reactions, etc. were further promoted under the co-catalysis of the two catalysts, and some phenols could be converted to benzene products, etc. The lowest contents of acids and acetic acid were nearly or equal to 0, respectively. CaO/Al2O3 and potassium phosphates showed synergistic effects in the regulation of the types or contents of phenols, ketones, aldehydes, etc. They were suitable for the production of ketone-rich and acid-low bio-oil, which was an important precursor for the preparation of power or jet fuel. Dehydration reactions, etc. were further promoted under the co-catalysis of the two catalysts, and some phenols could be converted to benzene products, etc. The lowest contents of acids and acetic acid were nearly or equal to 0, respectively.

decreased, the yield of organic components in the pyrolysis products decreased, and the water content increased. Wang et al. [29,30] proved that CaO could inhibit the generation of acid substances in the corn cob pyrolysis products. Veses et al. [31] studied the pyrolysis characteristics of pine with CaO and found that CaO decreased the oxygen content and increased the calori c value of bio-oil during the catalytic process. Acids, furans, phenols with high oxygen content decreased, while the contents of phenol, small molecular ketones and cyclic ketones with low oxygen content increased. Mahadevan et al. [32] studied the in uence of CaO, MgO and ZSM-5 in the biomass pyrolysis process. It was found that after CaO addition, acids decreased, the contents of furan and ketone products increased. MgO is a mild catalyst with only a slight change in the quality of the bio-oil. ZSM-5 has no signi cant effect on the acid content of the bio-oil.
Phosphate can provide lots of acidic sites or active sites and plays a great catalytic role in the process of pyrolysis. Zhang, et al. [33,34] used Py-GC/MS to study the catalytic pyrolysis of poplar wood by phosphates and found that K 3 PO 4 , K 2 HPO 4 and KH 2 PO 4 promoted the generation of phenolic products.
And the selectivity of phenolic products from high to low were K 3 PO 4 , K 2 HPO 4 and KH 2 PO 4 . Lu et al. [35] found that K 3 PO 4 inhibited the devolatilization of cellulose to form organic volatiles, while promoted the decomposition of lignin to generate phenolic compounds. Li Kai [36] found that loading K 3 PO 4 on the surface of rice husk was bene cial to reduce the reaction activation energy of rice husk pyrolysis and change the pyrolysis transformation path of rice husk. Wang et al. [37] found that the alkyl phenol yield in bio-oil signi cantly increased to about 50% during the catalytic process of corn straw with KH 2 PO 4 supported by activated alumina.
Both metallic oxides and potassium phosphates` are useful catalysts in the biomass pyrolysis. In our previous studies, CaO and phosphates showed synergistic effects in the pyrolysis of camphor wood. CaO and calcined K 3 PO 4 ·3H 2 O together reduced the relative contents of 3-methoxy-2,5,6-trimethylphenol, 3methoxy-1,2-benzenediol signi cantly, while enhanced those of isoeugenol, trans-isoeugenol, and furfural. CaO and calcined K 2 HPO 4 ·3H 2 O together increased the relative contents of phenols and cyclopentanones. It can be assumed that metallic oxides and potassium phosphates would show some bene cial synergistic effects in the biomass pyrolysis process. Besides, Rice stalks from agricultural crops are often disposed as waste. Catalytic pyrolysis, which convert rice stalk into high-valuable bio-oil or chemicals, is an important technical means for rice stalk utilization. A lot of researches have been done on the catalytic pyrolysis of rice stalk. In this paper, Py-GC/MS was utilized to study the fast pyrolysis of rice stalk with the co-catalysis of two metallic oxides (e.g., CaO, Al 2 O 3 ) and three types of potassium phosphates, (e.g., KH 2 PO 4 , K 2 HPO 4 ·3H 2 O and K 3 PO 4 ·3H 2 O).
Besides, potassium phosphate can be added to biomass by direct mixing and impregnation. In impregnation process, potassium phosphate can penetrate into the pores of biomass particles, disperse evenly, contact with biomass fully, and the catalytic cracking reaction is more intense. Lu et al. found that the products of biomass pyrolysis catalyzed by phosphate impregnation and phosphate direct mixing are quite different. In our previous work, CaO and calcined phosphate were mixed rst, and then mixed with biomass. In this paper, the biomass was impregnated with potassium phosphate and then mixed with metal oxide. The way of adding catalyst is different, and the catalytic process is different, and the pyrolysis products are also different.
In this paper, the distributions of pyrolysis products were analyzed and the synergistic effects of CaO or Al 2 O 3 and three potassium phosphates (e.g., KH 2 PO 4 , K 2 HPO 4 ·3H 2 O and K 3 PO 4 ·3H 2 O) in the rice stalk pyrolysis were investigated, so as to provide a basis for the production of valuable bio-oil in the pyrolysis of biomass with the joint catalysis of CaO/Al 2 O 3 and potassium phosphates.

Experimental materials and preparation
The biomass used in the research was rice stalk purchased from Jiaxing City, Zhejiang Province. First, the rice stalks were crushed by a shredder, and then were screened through standard sample screening. Rice stalks with particle size of 160-200 mesh (0.075-0.1mm) were selected for this experiment. Before each experiment, the biomass was dried in a drying oven at 105°C for 12h. The proximate and elemental analysis of the rice stalk are shown in Table 1. As illustrated in the introduction, potassium phosphates were added to biomass by impregnation method.
The impregnation method was as follows: rstly, according to the impregnation ratio, a certain amount of potassium phosphates was weighed and put into distilled water, then 2g of camphor wood was added and stirred by magnetic stirrers for 12h in order to ensure su cient contact with the solution. Finally, the mixed sample was placed in a drying oven for 12h at 120 °C and sealed for use.

Instruments and methods
The pyrolysis-gas chromatography-mass spectrometry test rig (Py-GC/MS) used in this study consisted of a Pyroprobe 5200 HP-R (CDS Analytical) and GCMS-QP2020 (Shimadzu, Japan). Samples was placed in the middle of the quartz tube, and quartz wool was placed at both sides of the quartz tube to avoid the escape of the particles. Helium (99.999%) was the carrier gas with a ow rate of 0.99 mL/min and a split ratio of 1:50. The GC oven program was maintained at 50°C for 2 min, then 10°C/min to 200°C followed by 15°C/min to 280°C and nally 5 min at 280°C. The ion source temperature of the mass spectrometer was 250°C, and the mass spectrometer was operated in EI mode at 70 eV with a mass scanning range (m/z) of 28-400 amu. The standard mass spectrometry library used was NIST2014. For each product, the changing of its relative content among the detected products can be determined by comparing its peak area% values.

Co-catalytic pyrolysis by CaO and potassium phosphates
As illustrated above, calcined CaO was mechanically mixed with rice stalk samples impregnated with KH 2 PO 4 , K 2 HPO 4 ·3H 2 O and K 3 PO 4 ·3H 2 O, respectively, in the mass ratios of 0:1, 0.5:1, 1:1 and 1.5:1, in which 0:1 indicated no addition of CaO. The pyrolysis products were divided into phenols, ketones, acids, aldehydes, furans, saccharides, and a few others. The contents were represented by the relative peak areas.
(1) Phenols Fig. 1(a) shows the relative yields of total phenols and Fig. 1(b)-(j) shows the relative yields of different phenols produced from the catalytic pyrolysis of rice stalk with CaO, KH 2 PO 4 , K 2 HPO 4 ·3H 2 O, and K 3 PO 4 ·3H 2 O. R is the mass ratio of CaO to impregnated biomass. The total content of phenols from Rs sample is 16.63%. For the Rs samples, the total yields of phenols slightly decrease and then increase with the increase of CaO addition ratios. The relative yields of 2-methoxy-4-vinylphenol, mequinol are high in the Rs sample pyrolysis. The relative yields of phenol, p-cresol and 3-methyl-phenol increase and those of 2-methyl-1,3-benzenediol decrease markedly after CaO addition. When R is greater than or equal to 1, 2methyl-1, 3-benzenediol could not be detected.
It can be seen that except with 10%KH 2 PO 4 addition (10%K1-RS), the total yields of phenols increase, indicating that potassium phosphates alone can promote the generation of phenols, which is in accordance with Zhang  3-phenylenediol decrease to 0. This is because potassium phosphate can further promote the deoxidation, demethylation, demethoxylation reactions, etc., and also promote the removal of alkyl side chains on benzene ring in some phenols [41].
On the whole, the total contents of phenols co-catalyzed by CaO and potassium phosphates decrease compared with those catalyzed by potassium phosphates alone. The total contents of phenols decrease obviously from 27.69% to 17.57% with the increase of R when co-catalyzed by CaO and 50% K 3 PO 4 ·3H 2 O. This is because CaO can further catalyzes dehydration reactions, etc. to convert phenols to form benzene products, etc. Some phenols from some impregnated samples are further promoted after CaO addition, such as 2-methoxy-4-vinylphenol from the pyrolysis of 10% K1-Rs, mequinol from the pyrolysis of 50% K2-Rs or 30% K3-Rs, p-cresol and 3-methyl-phenol from the pyrolysis of K2-Rs. Some phenols from some impregnated samples are inhibited after CaO addition on the whole, such as 2-methoxy-4-vinylphenol, 4ethyl-2-methoxy-phenol from the pyrolysis of K3-Rs samples, 4-ethyl-2-methoxy-phenol from the pyrolysis of 50% K1-Rs and 50% K2-Rs, 2,6-Dimethoxy-phenol and phenol from the pyrolysis of 50% K3-Rs, etc. The yields of 3-methyl-phenol, 3-methoxy-1,2-benzenediol, 2-methyl-1,3-benzenediol from the pyrolysis of 50% K3-Rs, 50% K2-Rs and most impregnated samples, respectively can even be reduced to 0 after CaO addition. The yields of some phenols from some impregnated samples reach the highest value at certain R, such as phenol from 30% K1-Rs, 10% K2-Rs, 30%K3-Rs samples at R=0.5, p-cresol from 30% K2-Rs and 50% K3-Rs samples at R=0.5 and 10% K2-Rs and 10% K3-Rs samples at R=1. CaO and potassium phosphates show some synergistic effects in the regulation of the type of phenols.
(2) Ketones Fig. 2(a) shows the relative yields of total ketones and Fig. 2 [42]. The yields of ketones show the highest value, with 34.47% in the products in the pure rice stalk pyrolysis, indicating that rice stalk is suitable for prepare ketone-rich bio-oil After CaO addition only, the total yield of ketones increased signi cantly, with maximum value of 45.05% at R=1. It can be seen from Figure 3(b)-(f) that the relative contents of most ketones except 1,2-cyclopentanedione increase after CaO addition only. On the whole, the total contents of ketones co-catalyzed by CaO and potassium phosphates further increase compared with those catalyzed by potassium phosphates alone. From R=0 to R=1.5, the yields of ketones increase from 44.62% to the highest value 56.65% from the pyrolysis of 10% K3-Rs. CaO and potassium phosphates show some synergistic effects in the regulation of the type of ketones and are suitable for the production of ketone-rich bio-oil.
(3) Acids Fig. 3(a) shows the relative yields of total acids and Fig. 3(b) shows the relative yields of acid produced from the catalytic pyrolysis of rice stalk with CaO, KH 2 PO 4 , K 2 HPO 4 ·3H 2 O, and K 3 PO 4 ·3H 2 O. It can be seen from Figure 3(a) that CaO alone signi cantly reduce the total content of acids from 8.44% to 3.45% at R=0 to R=1.5. This is because CaO can effectively react with acids or their precursors to generate calcium carboxylates, which can decompose into ketones such as acetone, as shown in Equations (1) and (2).
The total contents of acids from the pyrolysis of rice stalk co-catalyzed by CaO and potassium phosphates further decrease signi cantly compared with those catalyzed by potassium phosphates alone. Except for 10% and 50% K1-Rs, the contents of acetic acid can decrease to 0 after CaO addition. CaO and potassium phosphates together show strong synergistic effects in decreasing acids, which is in accordance to our previous ndings.
(4) Aldehydes Fig. 4(a) shows the relative yields of total aldehydes and Fig. 4 On the whole, the total contents of aldehydes co-catalyzed by CaO and potassium phosphates from most samples decrease compared with those catalyzed by potassium phosphates alone. Other samples have uctuated content of total aldehydes. The decrease is also because CaO can further catalyzes dehydration and deoxidation reactions. The yields of pentanal from the pyrolysis of 30%, 50% K1-Rs and 10%K2-Rs further increase after CaO addition on the whole.
Some aldehydes from some impregnated samples are inhibited after CaO addition on the whole, such as hydroxy-acetaldehyde from the pyrolysis of most samples, hydroxy-acetaldehyde from the pyrolysis of 10%, 30% K2-Rs and 10% K3-Rs, pentanal from the pyrolysis of 30% K3-Rs. Some aldehydes from some impregnated samples show uctuated trends with the increase of R. CaO and potassium phosphates show some synergistic effects in the regulation of the type of aldehydes.
(5) Furans   Fig. 5 shows the relative yields of total furans produced from the catalytic pyrolysis of rice stalk with CaO, KH 2 PO 4 , K 2 HPO 4 ·3H 2 O, and K 3 PO 4 ·3H 2 O. It can be seen that CaO alone increase the total content of furans. This is because CaO promotes the depolymerization, ring opening and dehydration of xylan and 4-O-methylglucuronic acid units, etc. 50% KH 2 PO 4 alone increase the yields of total furans to 9.52% from 5.31% in the pyrolysis products of pure rice stalk. For 50% K3-Rs sample, the yields of furans reduce sharply after CaO addition.
(6) Levoglucosan (LG) Levoglucosan (LG) is an important product in the biomass pyrolysis. Fig. 6 shows the relative yields of Levoglucosan (LG) produced from the catalytic pyrolysis of rice stalk with CaO, KH 2 PO 4 , K 2 HPO 4 ·3H 2 O, and K 3 PO 4 ·3H 2 O. CaO alone changes the yields of LG a little.
On the whole, most potassium phosphates alone reduce LG [43]. K 3 PO 4 ·3H 2 O shows the highest promotion effects. This is because alkaline environment induced by K 3 PO 4 ·3H 2 O is harmful for the production of ketone products [44]. 30%, 50% KH 2 PO 4 impregnation increase the yields of LG.
On the whole, the contents of LG co-catalyzed by CaO and potassium phosphates decrease compared with those catalyzed by potassium phosphates or CaO alone. For 50% K2-Rs and all the K3-Rs samples, they can decrease to 0 after CaO addition.
On the whole, the contents of most phenols co-catalyzed by Al 2 O 3 and potassium phosphates decrease compared with those catalyzed by potassium phosphates alone. For certain samples, three small phenols, i.e., phenol, p-cresol, 3-methyl-phenol can reach its highest value at certain R, such as phenol at R=1, 1.5, 1.5, 0.5 for 30%, 50% K1-Rs samples and 10%, 50%K2-Rs samples, respectively. For 50% impregnation samples, the yields of total phenols decrease maximumly from 27.69-8.17% at R=0 to 1.5. The yields of 2-methyl-1,3-benzenediol from all the samples decrease to 0 after Al 2 O 3 addition. The yields of 3-methoxy-1,2-benzenediol from 30%, 50% K2-Rs samples decrease to 0 after high CaO addition amounts. Al 2 O 3 and potassium phosphates show some synergistic effects in the regulation of the type of phenols.
(3) Ketones   Fig. 8(a) shows the relative yields of total phenols and Fig. 8 synergistic effects in the regulation of the type of ketones and are suitable for the production of ketonerich bio-oil.
(3) Acids Fig. 9(a) shows the relative yields of total acids and Fig. 8 (4) Aldehydes Fig. 10(a) shows the relative yields of total aldehydes and Fig. 8 Furans   Fig. 11(a) shows the relative yields of total furans and Fig. 11 (2) After CaO addition alone, the relative yields of phenol, p-cresol and 3-methyl-phenol increase and those of 2-methyl-1,3-benzenediol decrease markedly. The yields of ketones increase signi cantly, furans increase a little, and those of acids, aldehydes decrease.
(4) After co-catalyzed by CaO/Al 2 O 3 and potassium phosphates, the total contents of phenols, aldehydes, acids, LG from most samples decrease and those of ketones increase compared with those catalyzed by potassium phosphates alone. CaO/Al 2 O 3 and potassium phosphates show synergistic effects in the regulation of the type or content of phenols, ketones, aldehydes, etc. and are suitable for the production of ketone-rich bio-oil. Dehydration reactions, etc. are further promoted under the co-catalysis of the two catalysts, and some phenols can be converted to benzene products, etc. The contents of acetic acid can decrease to 0.