Products distribution during in situ and ex situ catalytic fast pyrolysis of Chinese herb residues

Catalytic fast pyrolysis (CFP) for biomass treatment is a research hotspot but there is little information about the difference between the in situ and ex situ methods. In present work, the Ni–Fe/CaO–Al2O3 catalysts with different Ni/Fe ratios have been synthesized by coprecipitation method, and the product distribution about the Chinese herb residue (CHR) catalytic fast pyrolysis by in situ and ex situ methods in a quartz tube reactor system has been investigated. The results show that the CFP pyrolysis would upgrade the quality of bio-oil but decrease the yields, no matter in situ or ex situ CFP process. During the in situ CFP process, heteroatoms may be absorbed by the catalyst support and cannot be transferred to the bio-oil, but the results of ex situ CFP are the opposite. In addition, the ex situ CFP reaction significantly increases the content of aromatic hydrocarbons. As to the gas products’ distribution, the effect of Fe in catalysts to promote CH4 formation is reflected in in situ CFP process, while the promotion effect of H2 generation for Ni added in catalyst is mainly reflected in ex situ CFP process. However, due to the high reaction temperature (800 °C), the adsorption of CO2 by CaO support is not particularly significant. The possible mechanism of CHR in CFP process has also been summarized for understanding the process of in situ and ex situ CFP, and this study may provide a choice or reference for CHR treatment.


Introduction
In recent years, the energy utilization of waste biomass had received great attention in order to alleviate the future fossil fuel overuse, global warming, and environmental pollution (Hu & Gholizadeh 2019). According to a recent report by Xing et al., the biomass had a significant potential for bioenergy utilization in China (Xing et al. 2021). But the shortcomings of biomass were also obvious, such as high moisture and volatile matter content, low bulk and energy density, and high ash content (Chen et al. 2018), which restrain the direct utilization, storage, and transportation of raw biomass. Therefore, it was important for biomass to convert bio-oil, biochar, and inflammable gas by thermochemical ways (Liao et al. 2020). The catalytic fast pyrolysis (CFP) was a preferred way to convert biomass into bio-oil with high heating values, which had been evaluated comprehensively by Xu et al.  and it was benefit to obtain high quality and yield bio-oil. Cai et al. (2021) summarized the raw materials and reactors for biomass fast pyrolysis; they thought that the key for energy conversion of biomass was to overcome the seasonal supply and low catalytic pyrolysis efficiency. Thus, a stable supply of biomass feedstock was conducive to the continuity of catalytic fast pyrolysis as well. Chinese herb residues (CHRs) were a solid biomass waste of which the production is more than 15 million tons every year; it was suitable for energy utilization and overcoming feedstock supply (Zhao et al. 2019). However, catalysts and catalysis methods had also a significant impact on the distribution and quality of the products of CFP. A review summarized by Kim et al. (2019) demonstrated that bifunctional catalysts were more suitable for the biomass pyrolysis in terms of remission catalyst deactivation and coke deposition. More recently, Jin et al. (2022) found catalytic pyrolysis significantly decreased the oxygen ratio of bio-oil and increased the higher heating value, and basic catalysts were expected to neutralize the acid in bio-oil.
Certainly, for pyrolysis catalysts, catalytic centers and the structures of the support were all important about their performance. The Ni-Fe-based catalyst had been widely reported due to their excellent mutual promotion catalytic performance in tar cracking and gas upgrading (He et al. 2020;Liang et al. 2020;Wang et al. 2011). Yue et al. (2021) synthesized a biochar-supported Ni catalyst to obtain methane-rich gas and found it conducive to the tar coverted into gas production. Lu et al. (2021) had investigated the deactivation of Fe catalyst and they found it to have been wrapped and covered by coke during the catalytic pyrolysis, which was the main reason for restraining the activity of Fe catalyst. Wang et al. (2020) found Fe-Ni bimetallic catalyst had a stable catalytic reaction performance, and it combined the advantages of two metal elements during the catalytic pyrolysis. Lin et al. (2021) had synthesized a char-supported Ni-Fe catalyst with the hierarchical structure; they found it had high reaction acitivity for cracking tar, and the special structure would enhace the ability of deactivation and sintering of the catalyst. Li et al. (2022) invesitaged the deactivation mechanism of Ni-Fe catalyst and they found the activity of catalyst decreased due to the cumulative biomass/catalyst ratio, but it was able to maintain the aromatic selectively of bio-oil, so it was necessary to control the ratio of biomass to catalysts. In addtion, the Ni-Fe catalyst had also application in another thermochemical conversion field, especially in fast pyrolysis of waste plastics (Cai et al. 2020;Chen et al. 2020;Yao et al. 2021). As to supports, Al 2 O 3 was a suitable carrier for pyrolysis catalysts because of its pysical and chemical stability (Arregi et al. 2018). Accordng to the report by Fernandez et al. (2021), Al 2 O 3 supports had severe catalystic acidity to increase the bio-oil cracking, which was conducive to the gas formation and enhance the CO content. Furthermore, in order to reduce the CO 2 compounds in gas, the CaO had also been used as one of the pyrolysis catalyst supports. Calcium oxide was a low-cost, highly effective, and non-toxic support material which was reviewed by Li et al. (2021), regardless if in pyrolysis or in gasification of biomass. Nam et al. (2020) found that the presence of CaO in biomass gasifiaction process would promote the water-gas shift reaction and enhance the H 2 concentration of products due to the CO 2 absorbed, and this phenomenon had also be reported by Liu et al. (2019) about chemical looping gasification of biomass. Besides in situ CO 2 capture, Hu et al. (2020) demostrated that CaO plays as the catalyst and forms a new phase by reacting with Fe 2 O 3 , which was also conducive to the yield of H 2 . Gupta et al. (2021) invesitaged the multi-route conversion of CaO catalyst and demostrated the effective and stability structure of CaO during the catalytic fast pyrolysis of biomass to upgrade the pyroltic products and obtained value-added chemical and fuel precursors. So the dual-support catalysts of Al 2 O 3 and CaO with Ni-Fe were worth exploring.
Furthermore, the in situ and ex situ catalytic methods were crucial for catalytic pyrolysis products. However, very few had simultaneously compared the difference of in situ and ex situ fast catalytic pyrolysis. The motivation of the present work was to investigate the production distribution of in situ and ex situ catalytic fast pyrolysis of CHR with the Ni-Fe loaded Al 2 O 3 /CaO, which may provide a new way for the energy utilization of CHR.

Materials
Chinese herb residue taken from Jiuzhitang Co., Ltd. (Changsha, China), and its utimate and proximate analysis are reported by our previous work . The ultimate and proximate analysis of CHR is shown in Table 1. Before performing the pyrolysis experiment, the raw CHR had been dehydrated and milled throngh a 100-mesh sieve. Some analytical reagents, including Ni ( and Na 2 CO 3 , were purchased from the Chemical Reagent Co., Ltd., China Pharmaceutical Group, which were used to synthesize catalysts.

Catalyst synthesis and characterization
The Ni-Fe/CaO-Al 2 O 3 catalysts with the different ratio of Ni/ Fe were synthesized by coprecipitation method. The mass ratio of reagents was performed by the ratio of the total of Ni-Fe and the total of CaO-Al 2 O 3 of 1:4. Particularly, the molar ratio of Ni and Fe was kept in 0:1, 1:3, 1:1, 3:1, and 1:0, and these catalysts were denoted as NiFe-1, NiFe-2, NiFe-3, NiFe-4, and NiFe-5, respectively. During the coprecipition process, reagents were mixed according to mass ratio and dissolved in deinoized water. This aqueous solution was heated to 70 °C and was constantly stirred. Later, precipitate solution was added which made up NaOH and Na 2 CO 3 of 1:1 and the remaining was stirred for 60 min. After 10 h of aging, the crystals precipitaed were separated from the solution and dried at 105 °C for 12 h and then calcined at 800 °C for 4 h in air atmosphere. Finally, the solid samples were reduced in 10 mL/min H 2 and 90 mL/N 2 atmosphere to obtian catalyst. In order to investigate the surface microstructure of the catalyst, the surface crystal structure of the catalyst was scanned in the range of 10° to 80° with XRD-6000 diffrotometer (Shimadzu Corporation, Japan), and the specific surface area and total pore volume were measured with KuBO-X1000 machine (Beijing Builder Electronic, China).

Pyrolysis conditions and product analysis
The in situ and ex situ catalytic fast pyrolysis experiments are performed in a quartz tube reactor system shown in Fig. 1, and the size of the device was the same as that in our previous report . During the in situ CFP process, catalyst and CHR were mixed in 1:1 ratio in quartz boat and the total amount is 10 g, and were then put into the tube of furnace. In a nitrogen flow of 100 mL/ min, samples were heated from room temperature to 800 °C with 10 °C/min. The ex situ CFP process was the same as in situ CFP process; only the CHR and catalyst were placed in quartz boat and catalytic device, respectively. After the tube furnace temperature was stabilized at 800℃, the quartz boat containing the sample was pushed into the constant temperature zone of the furnace by putter, and kept at constant temperature for 1 h. The products of bio-oil and gas were collected by condensation unit and gas collecting bag, respectively. Obviously, the char remains in the quartz boat. The mass balance calculation method of pyrolysis products, the extraction and analyzed method of bio-oil, and the gas analysis were consistent with our previous report ).

The characterization of catalysts
According to the XRD spectra in Fig. 2, the crystal structure of Al 2 O 3 , Fe 2 O 3 , CaO, Ca(OH) 2 , and NiO was detected in catalyst precursors. After hydrogen reduction, a new phase of metal Ni appeared in catalysts. A small amount of moisture was adsorbed and reacted by CaO support in the catalysts, which might be the only reason for detection of Ca(OH) 2 . The diffraction peak intensity of NiO/Ni and Fe 2 O 3 was consistent with the relative load of Ni and Fe element. The Fe 2 O 3 in catalysts had not been reduced because Fe atoms had stronger O affinity in catalyst which made it difficult to be reduced to Fe metal. In addition, the BET surface area and pore volume of catalysts are shown in Table 2, and it was clear that bimetal load samples like NiFe-3 and NiFe-4 have a smaller specific surface area and pore volume than single load like NiFe-1 and NiFe-5, but NiFe-2 has the largest specific surface area and pore volume.

The product distribution of in situ and ex situ pyrolysis
The distribution of gas, bio-oil, and char of in situ and ex situ CFP of CHR is shown in Fig. 3a and b, respectively. Generally, the char and bio-oil yields in in situ CFP pyrolysis are lower than those in ex situ, while the gas yields are relativey higher. This phenomenon is consistent with the different reaction mechanism of in situ and ex situ CFP. Catalysts interact directly with the solid biomass when they blend with CHR, which would favor the thermal decomposition of the feedstock and result in less residual char. However, in ex situ CFP process, catalysts are only for catalytic reforming of the volatiles after biomass   decomposition, and this process will result in the reformation of volatiles and the formation of bio-oil compunds, so ex situ CFP enhances the yield of bio-oil. As to the different ratios of Ni and Fe in catalysts, more Fe contents in catalysts seem to enable more char and bio-oil, while the gas generation should be promoted with the increase of Ni content in catalysts. This is due to the fact that Ni contributes to the secondary cracking of the bio-oil, which is more pronounced in ex situ CFP process (Lu et al. 2020). Nevertheless, the effect of different catalysts on product distribution is not very obvious, especilly in NiFe-2, NiFe-3, or NiFe-4, which may be due to some synergy between Ni, Fe, and the catalyst support. Compared to the CK group, both in situ and ex situ CFP seem to reduce the yield of bio-oil and increase the yield of gas, which may be due to the fact that the catalyst promotes the fragmentation of macromolecular structures in the feedstock and thermal decomposition volatile matter.

Gas composition of in situ and ex situ CFP
As can be seen in Fig. 4, there are relatively large differences between the gas composition of in situ and ex situ CFP process, but the regular patterns of Fe and Ni effects on gas composition are unified in two pyrolysis methods. Fe added in catalysts would promote the CH 4 formation and Ni added would enhance the H 2 yield Qu et al. 2021). The catatlytic effect of H 2 yield in in situ CFP is weaker than that in ex situ because the former catalysts act directly on biomass feedstock while the latter catalysts act on volatile matter, and the catalytic pyrolysis targeting volatiles is more likely to promote the cleavage of C-H to form H 2 ). CO 2 contents were less than those in our previous studies ) that was because a part of the CO 2 has been absorbed by CaO in catalyst support. And the CO content is increasing gradually with the Ni increasing in in situ CFP process, but it is not obvious the regularity in ex situ CFP pyrolysis; this may be due to the fact that Ni in the catalyst promotes the decarbonylation reaction mainly in the reaction with biomass feedstocks rather than volatile matter (Van de Velden et al. 2010). In addtion, the deposition coke in the surface of catalyst supports may react with CO 2 and convert it to CO at high temperature, which may be another resaon that the CO content is generally higher than CO 2 . This phenomenon is consistent with the results of Gao et al. (2021) that the presence of CaO would absorb CO 2 and increase the H 2 content in gas products. In the CK group without catalyst, the content of CH 4 seems to be higher than that in the CFP experimental group. But as the matter of fact, during the pyrolysis reaction, there are many conversion paths between CH 4 and H 2 , and CO and other gases, and the process is quite complicated (Yun et al. 2020), so it is difficult to follow or discuss in this part.

Effect of catalytic methods on bio-oil compounds
Bio-oil is the most important product for biomass CFP. It is also an important energy carrier and precursor of chemical products. Besides yield, the composition of bio-oil also plays a crucial role in its utilization value. The basic classification of in situ and ex situ catalyzed bio-oils are shown in Fig. 5a and b, respectively. Whether it is in situ or ex situ CFP process, the main components of bio-oil are aromatics which originate from the decomposition of aromatics  (Safdari et al. 2019). In composition, the content of heterocyclics and aliphatics is much lower, which was consistent with our previous studies because of the physical and chemical properties of CHR (Auersvald et al. 2020;Huang et al. 2022). The results also show that the catalytic method has little effect on the distribution of the three types of components in bio-oil. In contrast, the increase of Ni in the catalyst during the ex situ CFP process is more conducive to the selective generation of aromatics.
The detail functional compound contents of bio-oil are shown in Fig. 6. Apparently, aromatic hydrocarbons (Ar-H) is the most content in bio-oil, and the secondly is phenol.
This phenomenon also demostrated that the aromatization is a major reaction in the CFP process to form bio-oil . However, there are quite different effects about the Ar-H content in bio-oil between in situ and ex situ CFP process. In in situ CFP, higher Ni/Fe ratio in the catalyst tends to generate more aromatics, and the highgest is in NiFe-4, but it is not conducive to the formation of phenols. In Fig. 6a, the content of phenols decreases gradually with the increases of Ni/Fe ratio. This may be due to the fact that in the process of in situ CFP process, the total amount of aromatic substances generated by the interaction of raw materials and catalysts is constant, and they obey the distribution principle in the process of forming aromatic hydrocarbons and phenols. Moreover, after the in situ catalytic reaction is completed, the decomposed volatiles cannot form new aromatic hydrocarbons because they no longer have the effect of catalyst, so this phenomenon appears in the in situ CFP process. Compared with in situ CFP, there was less phenol apperance but more Ar-H content in ex situ CFP, which was also determined by two different methods of catalysis. In addtion, heteroatoms in in situ CFP bio-oil are lower than those in ex situ CFP; this may be due to CaO and Al 2 O 3 as catalyst supports in in situ CFP react with heterostoms, leaving them in the solid product, and resulting in relatively low content of heteroatoms in bio-oil. Meanwhile, in ex situ experiments, it is clearly showed that heteroatoms increase gradually with the increase of Ni/Fe ratios. Firstly, in ex situ CFP process, there are no catalyst and catalytic carrier during the thermal decomposition process of CHR, which results that the heteroatoms in CHR easily enter the second reaction stage with the release of volatile matter. Later, this volatile matter reacts on the catalyst surface to form bio-oil, and the heteroatoms in the volatile matter are transferred to the bio-oil. Secondly, as the Ni/Fe ratios increase, more hydrogen in the volatiles matter is converted into hydrogen by the nickel in the catalyst, which provieds more sites for the binding of heteroatoms to carbon atoms, and it may also be an important reason for the high content of heteroatoms in bio-oil obtained by ex situ CFP. As to another compound in bio-oil, such as alcohol, acid, and ester, they are present in extremely low amounts and with little regularity in their distribution, especially in ex situ experiments. In general, single iron-containing catalyst (NiFe-1) is beneficial for the formation of more functional species in bio-oil, either in situ or ex situ CFP. Furthermore, in situ CFP tended to form more alcohol and ketone ), but ex situ CFP tended to form more ethers.
The relative content of different carbon numbers in bio-oil compounds is shown in Table 3. In general, the main composition of CFP bio-oil is light compounds (C4-C11), and in in situ CFP process, more Fe added in catalyst is conducive to much light compounds. This phenomenon indicates that  heavy oil (C20 +) compounds may be more easily decomposed into aliphatic compounds under the action of catalysts with high Fe content and low Ni content. During the in situ catalytic CFP process with high Ni added, the content of diesel compunds (C12-C20) is also increased to a certain extent. The increase in the content of diesel compounds (C12-C20) can be clearly observed from Table 3, because of the significant secondary catalytic effect on volatiles in the ex situ CFP process.

Mechanism of CHR thermal decomposition during the in situ and ex situ CFP process
Based on the distribution of gas and bio-oil, the possible mechanism of CHR in CFP process is shown in Fig. 7. During the in situ CFP process (Fig. 7a), the catalyst is in direct contact with the CHR, and the catalyst support will also react directly with CHR . This process makes a small amount of heteroatoms in the CHR absorbed and bound by alumina oxide or calcium oxide in the catalyst, which reduces the probability of heteroatoms in the CHR entering the volatiles. Meanwhile, some CO 2 might be reacted by CaO in the catalyst support to form CaCO 3 when it is produced, which results in a lower CO 2 content in pyrolytic gas of in situ CFP than ex situ CFP. In addition, compared with ex situ CFP, the addition of Ni in the in situ catalyzed reaction catalyst does not seem to have a particulary significant effect on the H 2 yield, which may be due to the fact that Ni contributes to the generation of H 2 mainly in the secondary reaction of volatiles (Shen et al. 2014), while the contact time between the catalyst and the volatiles is very short and uneven during the in situ CFP process. And the direct contact between the catalyst and the CHR feedstock that affects the generation of H 2 promotion is very limited. However, the ex situ CFP process is shown in Fig. 7b; heteroatoms in CHR are released from feedstock after thermal decomposition and transferred to volatiles. The heteroatoms have been bound in the volatiles by covalent bonds, and the catalyst support does not seem to adsorb the heteroatoms in the volatiles during the ex situ CFP process, which leads to the majority of the heteroatoms generated during the ex situ CFP process entering into bio-oil. Nevertheless, the CFP process may be more conducive in the generation of aromatics in bio-oil.

Conclusion
The difference of in situ and ex situ CFP process about product distribution has been invesitaged in the present work, and the possible mechanism of CFP process has also been surmised. The results show that in situ and ex Fig. 7 Mechanism of CFP process of CHR: a in situ; b ex situ situ CFP of CHR have little effect on the types of pyrolytic products but have different effects on their content and changing trends. The quailty of bio-oil is improved for in situ CFP pyrolysis, while relatively more bio-oil can be obtained for ex situ CFP reaction. For the role of catalyst, the promotion effect of Fe on the generation of CH 4 is more significant in in situ CFP, while the promotion effect of Ni on the generation of H 2 is mainly reflected in ex situ CFP. The content of heteroatoms in the bio-oil of in situ CFP is significantly lower than that of ex situ CFP. Compared with the CK group, the adsorption of CO 2 on the CaO supports is mainly reflected in the in situ CFP process, but the amount of adsorption is small. Overall, both in situ and ex situ CFP can achieve the upgrading of the pyrolytic products of CHR, but the product distribution characteristics are also quite different for the different mechanisms of reactions. The research in the present work may provide a reference for the catalytic fast pyrolysis treatment and utilization of Chinese herb residues or other waste biomass.