Effect of alkali metal salts on the product distributionFigure 2 shows the product distribution from cellulose and bamboo catalytic pyrolysis with different alkali metal salts. From Fig. 2a. Cellulose pyrolysis with SiO2 generated higher yield of bio-oil (60 wt%) with some biochar and gas products. KCl showed no obvious effect on the product distribution of cellulose pyrolysis. K2SO4 promoted the generation of biochar and gas products a little, while inhibited the formation of bio-oil. Greatly different from KCl and K2SO4, K2CO3 significantly increased biochar yield, and largely decreased bio-oil yield to only 41 wt%, while gas yield increased 2 times to 29 wt%. To summarize, potassium salts facilitated the secondary fragmentation of volatiles, thereby increasing the formation of gas products and biochar, while reducing the yield of liquid products. This effect was particularly noticeable for K2CO3, with its reduction in liquid products likely due to its higher alkalinity compared to K2SO4. The catalytic effect of potassium salts on cellulose pyrolysis followed the order: K2CO3 > K2SO4 > KCl. Compared with potassium salts, NaCl inhibited the formation of bio-oil, and Na2SO4 showed a stronger inhibiting effect on bio-oil. Like K2CO3, Na2CO3 also greatly promoted the gas releasing (25 wt%), while decreased bio-oil yield (44 wt%). The catalytic effect of sodium salts on cellulose pyrolysis also followed the order: Na2CO3 > Na2SO4 > NaCl.Fig. 2: Pyrolysis product distribution.Effect of alkali metal salts in in-situ catalytic pyrolysis. a Cellulose pyrolysis. b Bamboo pyrolysis.Compared with cellulose, from Fig. 2b. Bamboo catalytic pyrolysis with SiO2 addition generated more biochar and gas products, while less bio-oil. Similar with cellulose, KCl addition had no obvious effect on the product distribution of bamboo pyrolysis. K2SO4 inhibited the bio-oil generation a little. While K2CO3 greatly promoted the formation of biochar (26 wt%) and gas products and inhibited the formation of bio-oil (39 wt%). The catalytic effect of potassium salts on bamboo pyrolysis also followed the order: K2CO3 > K2SO4 > KCl. Sodium salts also showed a similar order: Na2CO3 > Na2SO4 > NaCl. Besides, the biochar yield from bamboo pyrolysis was higher than that from cellulose, which may be due to it being easy for the secondary refinement of hemicellulose and lignin pyrolysis volatiles22,23,24. Metal salts may also enhance the interaction between biomass components, which results in the increase of biochar yield25,26.Effect of alkali metal salt on gas productsThe effect of potassium and sodium salts on the gas compositions of cellulose and bamboo pyrolysis is shown in Fig. 3. From Fig. 3a. Gas product from cellulose pyrolysis with SiO2 was mainly CO (2.1 mmol/g) and CO2 (1.4 mmol/g) with some CH4 (0.3 mmol/g) and H2 (0.1 mmol/g). KCl increased CO2 yield while decreasing the yield of other compositions slightly. K2SO4 not only promoted the generation of CO2 but also increased the yield of CH4 and H2. Greatly different from KCl and K2SO4, K2CO3 greatly increased the yield of all gas compositions that CO yield increased to 2.9 mmol/g, and the yield of CO2, CH4, and H2 increased 3 times (4.5 mmol/g), 2 times (0.4 mmol/g), and 9 times (1.2 mmol/g), respectively. It indicated that the catalytic effect of K2CO3 was significantly stronger than that of K2SO4 and KCl, which could not only accelerate the decarboxylation, demethylation, and dehydrogenation reactions but also promote the decarbonylation reactions of volatiles. It is related to its strong alkali characteristics of K2CO327,28,29. Compared with potassium salts catalysts, sodium salts showed some different catalytic properties. NaCl increased the yield of CO2 and CH4. Na2SO4 also promoted the release of CO2, CH4 and H2. Na2CO3 also greatly increased the yield of CO (2.4 mmol/g), CO2 (3 times to 4.0 mmol/g), CH4 (2 times to 0.5 mmol/g), and H2 (5 times to 0.6 mmol/g). It could be observed that the catalytic effect of potassium salts was obviously stronger than that of sodium salts. It may be ascribed to that the metal strength of K is larger than that of Na30.Fig. 3: Gas compositions.Effect of alkali metal salts on the gas products in in-situ catalytic pyrolysis. a Cellulose pyrolysis. b Bamboo pyrolysis.The case of bamboo pyrolysis was different from cellulose. From Fig. 3b. Bamboo pyrolysis with SiO2 generated more CO2 (2.1 mmol/g), CH4 (0.6 mmol/g), and H2 (0.2 mmol/g), while less CO (2 mmol/g), which was due to the decomposition of hemicellulose and lignin in bamboo. KCl slightly decreased the yield of all gas compositions from bamboo pyrolysis. In contrast, K2SO4 significantly increased the yield of all gas compositions. Although K2CO3 greatly promoted the formation of CO2 (2 times to 4.2 mmol/g), CH4 (0.8 mmol/g), and H2 (8 times to 1.4 mmol/g), it inhibited the generation of CO (only 1.7 mmol/g). Like potassium salts, NaCl showed little effect on gas compositions, and Na2SO4 greatly promoted the gas compositions formation, while K2CO3 only increased the yield of CO2 (1.5 times to 3.6 mmol/g), CH4 (0.7 mmol/g), and H2 (5 times to 0.9 mmol/g), and decreased CO yield to only 1.5 mmol/g. The catalytic effect of potassium salts and sodium salts on bamboo pyrolysis also showed the order: CO32− > SO42− > Cl−, and the catalytic effect of K+ and Na+ showed the order: K+ > Na+. The results were still in line with the alkalinity strength relationship. It could be pointed out that CH4 and H2 from bamboo catalytic pyrolysis (Fig. 3b) were significantly higher than that from cellulose (Fig. 3a), which was due to that metal salts promoted the cracking of -OCH3 of lignin and released more CH4 and H2. While the yield of CO2 and CO showed opposite tendency with the addition of K2CO3 and Na2CO3, which may be attributed to cellulose, with regular and ordered polysaccharide structure, the primary pyrolysis product of which was rich in pyran compounds, and K2CO3 and Na2CO3 could promote the reforming of pyran rings through releasing CO2 and CO23.Effect of alkali metal salts on bio-oil productsThe main organic compositions of bio-oil from cellulose and bamboo catalytic pyrolysis are classified into pyrans (including anhydrosugar and pyrone), phenols, furans, cyclopentanones and aliphatics (short-chain aliphatic compounds), and the content of these compositions is shown in Fig. 4. From Fig. 4a. Cellulose pyrolysis with SiO2 addition mainly generated pyrans (85%, mainly including LG, with a little of furans, cyclopentanones and aliphatics. KCl and K2SO4 showed similar catalytic effect, which largely decreased the content of pyrans, while increased that of furans (mainly including furfural (FF) and 5-hydroxymethylfurfural (5-HMF)), cyclopentanones and aliphatics accordingly, but pyrans were still the main component, accounting for 71%. The decrease in pyrans may be attributed to their lower stability of them, which easily decomposed into small molecular products with the effect of KCl and K2SO4. Greatly different from KCl and K2SO4, with the addition of K2CO3, the bio-oil components showed subversive change. There was nearly no pyrans generation, while the proportion of cyclopentanones and aliphatics increased significantly, and became the main components, which accounted for 53% (mainly including 2-cyclopenten-1-one, 2-methyl-2-cyclopenten-1-one, 3-methyl-2-cyclopenten-1-one) and 28% (mainly including 1-hydroxy-2-propanone), respectively. Interestingly, a few phenolic compounds also occurred with the addition of alkali metal carbonate, indicating that rearrangement reaction to form aryl compounds was promoted31. NaCl and Na2SO4 also largely decreased the content of pyrans (which decreased to 60% and 77%, respectively), while increased the content of other components. But the catalytic effect of NaCl was significantly stronger than that of Na2SO4. Na2CO3 showed the strongest catalytic effect, which also made cyclopentanones become the dominant component (58%) with 24% aliphatics. The order of catalytic effect was CO32− > Cl− > SO42−.Fig. 4: Composition of liquid products.Effect of alkali metal salts on the bio-oil in in-situ catalytic pyrolysis. a Cellulose pyrolysis. b Bamboo pyrolysis.The decrease of pyrans was mainly due to the decrease of LG, which was consistent with the increase of FF and 5-HMF. It indicated that Cl- promoted the ring-opened reactions of LG to form FF and 5-HMF. K2CO3 and Na2CO3 promoted the ring-opening reactions of cellulose, and the further cracking and condensation reactions of the intermediates27,32,33. Some intermediates underwent Grob cracking, dehydration reactions, and keto-alcohol isomerization reactions to form short-chain small molecule compounds and gas products23. The case was consistent with the sharp increase of gas products from Fig. 3a. and Supplementary Table 1.Different from cellulose, for bamboo pyrolysis liquid products in Fig. 4b. The major components were phenols (47%, mainly including 4-vinyl phenol, 4-ethyl phenol, 2,6-dimethoxy phenol, phenol, and p-cresol) and aliphatics (31%, mainly including acetic acid) with SiO2 addition. It could be observed that the content of LG was strikingly decreased, which was due to the presence of lignin and hemicellulose exerting a significant inhibitory effect on the pathway of cellulose pyrolysis. Lignin was the main source of phenols through a series of reactions like hydroxyl groups removal, cleavage of weak ether bonds and alkyl side chain removal34. While the hemicellulose will release acetic acid through the fragmentation of acetyl substituents35,36. KCl and K2SO4 showed no obvious effect on the bio-oil components, which slightly increased the content of phenols. However, K2CO3 greatly promoted the generation of phenols, the content of which increased to 81%, while inhibiting the formation of other components mainly aliphatics. Like potassium salts, there was no obvious change in bio-oil components after the addition of NaCl and Na2SO4, and Na2CO3 also sharply increased the content of phenols to 81%. It has been reported that potassium additives can additionally promote decarboxylation or decarbonylation reactions, along with the removal of unsaturated alkyl branch chains37. Moreover, as shown in Supplementary Table 1 and Table 2, acids and aldehydes were not detected with the addition of alkali metal carbonate, and proportion of ketones was lowered. For one thing, these could partially be attributed to competition among different biomass components, the remarkable catalytic effect of alkali metal carbonate on ketonization reactions was inhibited. For another, due to the stronger alkaline nature, they were more prone to react with -COOH functional groups and release CO2, which was consistent with the significant increase in CO2 in the pyrolysis gas.Figure 5 shows the composition of the phenols after the addition of K2CO3 and Na2CO3. After the addition of K2CO3 and Na2CO3, the content of phenol changed slightly. While K2CO3 and Na2CO3 promoted the formation of alky-phenols, and the content increased more than 2 times (reaching 47.9% and 47.7%, respectively). The content of methoxy-phenols remained at 20%. This was mainly due to the that demethoxylation reaction was promoted and alkylation happened during depolymerization and cracking of lignin38. Compared with OH radical the oxygen atom in the methoxy group has a higher electron density39, thus it was more preferentially for the methoxy group accounting to be adsorbed on K2CO3 and Na2CO3, resulting in the secondary decomposition.Fig. 5: Distribution of phenols.Respective Effect of K2CO3 and Na2CO3 on distribution of phenols in bamboo bio-oil was compared with control group (SiO2).The effect of K2CO3 and Na2CO3 presented above was mainly based on cellulose in-situ catalytic pyrolysis, during the pyrolysis process both solid-solid and gas-solid contact between raw material and alkali metal salt were included. Separation of pyrolysis and catalytic processes was necessary for studying the effect of catalysis mode. Given this, the experiments of K2CO3 and Na2CO3 as catalysts for cellulose ex-situ catalytic pyrolysis were set. The composition properties of bio-oil from cellulose ex-situ catalytic pyrolysis with K2CO3 and Na2CO3 catalysts are shown in Supplementary Fig. 1. The main products in bio-oil from cellulose ex-situ catalytic pyrolysis with SiO2 were FF and LG, and the content were 4.4% and 57.6%, respectively. After the addition of K2CO3, FF content increased over 2 times to 10.8%, while LG content decreased to 44.7%. Like K2CO3, the addition of Na2CO3 increased FF content (7.4%) and decreased LG content (47.9%). Moreover, K2CO3 and Na2CO3 also greatly increased the yields H2, CO2, CH4, and CO. For cellulose in-situ catalytic pyrolysis with K2CO3 and Na2CO3 catalysts, there were no FF and LG generation. It indicated that in-situ catalytic pyrolysis had stronger catalytic effect than ex-situ catalytic pyrolysis, as FF and LG were catalytic decomposed further. FF can be reduced to 2-furanmethanol under the action of H radical40. Furthermore, the demethoxylation and decarbonylation reactions could facilitate the supply of H radicals during in-situ pyrolysis41. Thus, the catalytic effect from solid-solid contact between raw material and alkali metal salts likely had quite an important role in assisting gas-solid contact pyrolysis.Effect of alkali metal salts on the chemical structure of biocharTo comprehensively elucidate the effect of potassium salts and sodium salts on biomass catalytic pyrolysis, the morphology structure of biochar is shown in Fig. 6. The peak at 2θ = 23°represents the peak (002) indicating the carbon layers are stacked in parallel42, and the peak (100) located at around 2θ = 44° is used as an indicator of an orderly stacking of aromatic layers43. Both of which are used as indicators of the degree of graphitization.Fig. 6: XRD pattern of biochar.a Cellulose pyrolysis. b Bamboo pyrolysis.For cellulose catalytic pyrolysis in Fig. 6a. KCl and K2SO4 showed similar effects on the biochar structure, with a slight decrease in intensity of the peak (002) in contrast to that of SiO2 control group. While K2CO3 made a rather broader peak (002) of biochar, which indicated a highly disordered structure. Besides, a slight left shift of the peak location was observed. These indicated that the crystallinity of carbon in the stacking direction was significantly reduced. For sodium salt catalysts, NaCl featured the highest peak (002) intensity, and the curve trend of Na2SO4 was similar with that of Na2CO3. Compared with K2CO3, the amorphous degree of carbon skeleton decreased a lot in Na2CO3 group. The peak (100) in all cellulose catalytic pyrolysis groups presented low intensity.Similar with cellulose pyrolysis, from Fig. 6b. The obvious asymmetry of peak (002) and the slight peak (100) occurred in biochar from bamboo catalytic pyrolysis. K2SO4 showed the highest broad of peak (002) in potassium salts, followed by KCl and K2CO3. Among these the peak (002) of K2CO3 turned left a lot, indicating the large amount of potassium impregnation into the biochar matrix during pyrolysis, wrecking the regular biochar structure6. In comparison, nearly all the sodium salts exhibited a higher intensity of the peak (002) than of potassium salts. Besides, the intensity of peak (100) in bamboo catalytic pyrolysis was little higher than that of cellulose catalytic pyrolysis. However, the degree of graphitization was still low with amorphous carbon being the main component.Figure 7 shows the FTIR spectrum of biochar from cellulose and bamboo catalytic pyrolysis. From Fig. 7a. Compared with cellulose catalytic pyrolysis with SiO2, the addition of KCl had little effect on the property of biochar. For the addition of K2SO4, the bending vibration peak of the aromatic ring C-H in the infrared spectrum of biochar was strengthened, and the maximum peak position shifted from 875 to 810 cm−1, indicating that the structure of the biochar matrix changed from complex position and poly-substituted aryl structure to simple one. The addition of K2CO3 strikingly increased the absorbance of C = C (1580 cm−1), promoting the development of aromatic structure. While absorbance of aromatic C-H stretching vibration and out of plane deformation were significantly weakened, indicating that the substitution of aromatic hydrogen atoms was strengthened and fused rings were formed44,45, This could be attributed to the promoting effect on cleavage of hydrogen bond by K2CO3, which was corresponding to the increase of H2 production (9 times that of SiO2 control group) in the gas products (Fig. 3a). Besides, the content of aryl C-O-C groups was observed to be a little higher than that of other groups. NaCl and Na2SO4 were similar with that of KCl and K2SO4, respectively. Na2CO3 showed relatively inferior effect on the aromatic skeletal vibration than that of K2CO3.Fig. 7: FTIR spectrum of biochar.a Cellulose pyrolysis. b Bamboo pyrolysis.For bamboo catalytic pyrolysis in Fig. 7b. The SiO2 control group showed that the peak intensity approximately from 1000 to 1500 cm−1 increased, representing more oxygen functional groups on the surface of biochar. KCl slightly undermined the absorbance of aromatic skeletal vibration. The addition of K2SO4 showed similar effect. At the same time, the C-O-C absorption peak of aryl ethers became more and more obvious. As to K2CO3, aromatic C-H structure was weakened. C-O group of aromatic rings became more obvious and the absorbance of aromatic skeletal vibration was also facilitated, along with more H2 production (Fig. 3b). As to sodium salts, in comparison, NaCl showed similar effects with KCl. Na2SO4 increased the aromatic skeletal vibration a lot compared with K2SO4. While Na2CO3 showed slightly inferior absorbance of functional groups than that of K2CO3.
