3.1 Co-catalytic pyrolysis by CaO and potassium phosphates
As illustrated above, calcined CaO was mechanically mixed with rice stalk samples impregnated with KH2PO4, K2HPO4·3H2O and K3PO4·3H2O, 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, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. 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, 2-methyl-1, 3-benzenediol could not be detected.
It can be seen that except with 10%KH2PO4 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 et al.’s studies [34, 38, 39]. With 50% K3PO4·3H2O impregnation only, the total yields of phenols content reached the highest value of 27.69%. Under most impregnation ratios, the catalytic effect for the rising value of the yields of phenols are in the descending order of K3PO4·3H2O, K2HPO4·3H2O, KH2PO4, and the values increase with the increase of impregnation amount [40]. The contents of 4-ethyl-2-methoxy phenol, phenol and p-cresol all increase after potassium phosphate impregnation only. After 50% KH2PO4 impregnation only, the relative yield of 2-methoxy-4-vinylphenol increases from 3.14% to 4.19%, with the highest value in phenols, and that of 4-ethyl-2-methoxy phenol increases from 0.72% to 2.55%. After 50% K2HPO4·3H2O impregnation only, the yield of 2, 6-dimethoxy phenol increases from 1.21% to 1.77% and that of phenol increases from 1.05% to 2.17%. After 50% K3PO4·3H2O impregnation only, the yield of mequinol increases from 2.18% to 3.94%, and that of 3-methyl-phenol increases from 0.61% to 1.89%. Some phenols are inhibited by potassium phosphates. After 30%, 50% K3PO4·3H2O impregnation only, the yields of 3-methoxy-1, 2-phenylenediol and 2-methyl-1, 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% K3PO4·3H2O. 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, 4-ethyl-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(b)-(g) shows the relative yields of different ketones produced from the catalytic pyrolysis of rice stalk with CaO, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. A lot of ketones are derived from the ring-opening reaction of cellulose, and acids are partially converted into ketones under catalytic pyrolysis [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 significantly, 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. The yields of 1-hydroxy-2-acetone show the highest value in the ketones, with 10.86% from pure rice stalk pyrolysis and maximum content 13.05% at R=1.
K3PO4·3H2O and K2HPO4·3H2O alone promote the formation of ketones, while KH2PO4 inhibits the formation of ketones. This is because K2HPO4·3H2O and K3PO4·3H2O are alkaline, while KH2PO4 is acidic. Alkaline environment is conducive to the production of ketone products. After KH2PO4 impregnation alone, the yields of 1-hydroxy-2-acetone and 2-hydroxy-3-methyl-2-cyclopentene-1-one decrease, the yield of 1-acetyloxy-2-propanone decrease and the yields of acetone and 1,2-cyclopentanedione don’t change much.
High impregnation amount of K2HPO4·3H2O reduces the contents of 1-hydroxy-2-acetone, 1, 2-cyclopentaradione. K2HPO4·3H2O increases the content of 2-hydroxy-3-methyl-2-cyclopentene-1-one, but has little effect on 1-acetyloxy-2-propanone. After 50% K3PO4·3H2O impregnation only, the yields of 2-hydroxy-3-methyl-2-cyclopentene-1-one increase from 1.48% to 2.35% and those of 1, 2-cyclopentanedione, 1-acetyloxy-2-propanone decrease to 0. After 10% K3PO4·3H2O impregnation only, the yields of 1-hydroxy-2-acetone increase from 10.86% to 13.3%, while those of 1-hydroxy-2-acetone decrease with high K3PO4·3H2O impregnation amount.
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, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen from Figure 3(a) that CaO alone significantly 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).
CaO+2RCOOH→(RCOO)2Ca+H2O (1)
(RCOO)2Ca→CaCO3+RCOR (2)
Acids mainly contain acetic acid. As shown in Fig. 4(b), CaO alone significantly reduce the total content of acid products from 4.67–2.33% at R=0 to R=1.5.
K3PO4·3H2O alone significantly reduce acids. After 50% K3PO4·3H2O impregnation, the yields of acids decrease to 2.39% and those of acetic acid decrease to 0. Except for 10% and 30% KH2PO4 addition alone, the contents of acetic acid decrease after potassium phosphates addition alone.
The total contents of acids from the pyrolysis of rice stalk co-catalyzed by CaO and potassium phosphates further decrease significantly 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 findings.
(4) Aldehydes
Fig. 4(a) shows the relative yields of total aldehydes and Fig. 4(b)-(d) shows the relative yields of different aldehydes produced from the catalytic pyrolysis of rice stalk with CaO, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen from Figure that CaO alone reduce the total content of aldehydes from 24.67% to 16.71% and those of hydroxy-acetaldehyde from 16.41% to 9.93% at R=0 to R=1.5. The contents of hydroxyl-acetaldehyde, succinaldehyde and pentanal all decrease after CaO addition alone.
Potassium phosphates alone reduce aldehydes, the higher the impregnation amount, the more the reduction of aldehyde content. K3PO4·3H2O shows the highest reduction effects. 50% K3PO4.3H2O alone reduce the yield of total aldehydes to 1.36%. Hydroxy-acetaldehyde and succinaldehyde decrease to 0 after 30%, 50% K3PO4·3H2O and 50% K2HPO4·3H2O impregnation alone. Pentanal decrease to 0 after 50% K3PO4·3H2O impregnation alone.
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 fluctuated 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 fluctuated 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, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. 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% KH2PO4 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, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. CaO alone changes the yields of LG a little.
On the whole, most potassium phosphates alone reduce LG [43]. K3PO4·3H2O shows the highest promotion effects. This is because alkaline environment induced by K3PO4·3H2O is harmful for the production of ketone products [44]. 30%, 50% KH2PO4 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.
3.2 Co-catalytic pyrolysis by Al2O3 and potassium phosphates
(1) Phenols
Fig. 7(a) shows the relative yields of total phenols and Fig. 7(b)-(j) shows the relative yields of different phenols produced from the catalytic pyrolysis of rice stalk with Al2O3, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen that the relative contents of total phenols, 4-ethyl-2-methoxy phenol, 3-methoxy-1,2-benzenediol and 2-methyl-1, 3-benzenediol decrease after Al2O3 addition alone.
On the whole, the contents of most phenols co-catalyzed by Al2O3 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 Al2O3 addition. The yields of 3-methoxy-1,2-benzenediol from 30%, 50% K2-Rs samples decrease to 0 after high CaO addition amounts. Al2O3 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(b)-(f) shows the relative yields of different phenols produced from the catalytic pyrolysis of rice stalk with Al2O3, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen that the relative contents of total ketones increase a lot after Al2O3 addition alone.
The contents of ketones co-catalyzed by Al2O3 and potassium phosphates increase a lot further compared with those catalyzed by potassium phosphates alone. The yields of ketones reach the maximum value 56.02% at 50% K3PO4.3H2O impregnation and R=0.5. On the whole, the yields of 1-hydroxy-2-propanone from 10% K1-Rs sample, 30%, 50%K2 samples and all K3-Rs samples, 1,2-cyclopentanedione from 30%, 50% K2-Rs samples and 10%, 30% K3-Rs samples, 1-acetyloxy-2-propanone from 50% K2-Rs sample decrease after Al2O3 addition. Al2O3 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. 9(a) shows the relative yields of total acids and Fig. 8(b) shows the relative yields of acetic acid produced from the catalytic pyrolysis of rice stalk with Al2O3, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen that the relative contents of total acids decrease after Al2O3 addition alone. The yields of acetic acid decrease from 4.67% to 2.55% at R=0 to 1.5.
The contents of acids co-catalyzed by Al2O3 and potassium phosphates decrease compared with those catalyzed by potassium phosphates alone. For 50% K2-Rs and 30% K3-Rs samples, the yields of acetic acid can decrease to 0 after Al2O3 addition.
(4) Aldehydes
Fig. 10(a) shows the relative yields of total aldehydes and Fig. 8(b)-(d) shows the relative yields of different aldehydes produced from the catalytic pyrolysis of rice stalk with Al2O3, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen that the relative contents of total aldehydes increase after Al2O3 addition alone.
On the whole, the contents of total and most aldehydes from most samples co-catalyzed by Al2O3 and potassium phosphates decrease compared with those catalyzed by potassium phosphates alone. The yields of hydroxy-acetaldehyde from 30%, 50% K1-Rs samples and 30%K2-Rs samples can decrease to 0 after Al2O3 addition.
(5) Furans
Fig. 11(a) shows the relative yields of total furans and Fig. 11(b)-(c) shows the relative yields of different furans produced from the catalytic pyrolysis of rice stalk with Al2O3, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. It can be seen that the relative contents of total furans decrease after Al2O3 addition alone. The yields of furfural increase a lot from 2.55% to 4.88% at R=0 to 1.5.
On the whole, the contents of total furans and furfural from most samples except 50% K2-Rs sample and 30%, 50%K3-Rs samples co-catalyzed by Al2O3 and potassium phosphates increase compared with those catalyzed by potassium phosphates alone. The yields of furfural from 50% K2-Rs sample and 30%, 50%K3-Rs samples can decrease to 0 after Al2O3 addition. The yields of 2,3-dihydro-benzofuran from all samples decrease after Al2O3 addition. Al2O3 and potassium phosphates show some synergistic effects in the decrease of furans and the regulation of the type of furans.
(6) Levoglucosan (LG)
Fig. 12 shows the relative yields of LG produced from the catalytic pyrolysis of rice stalk with Al2O3, KH2PO4, K2HPO4·3H2O, and K3PO4·3H2O. The relative contents of LG decrease after high amount of Al2O3 addition [45–47].
On the whole, the contents of LG co-catalyzed by Al2O3 and potassium phosphates decrease compared with those catalyzed by potassium phosphates alone. The contents of LG from most samples except for 10%, 30% K1-Rs samples can decrease to 0 after Al2O3 addition.