## 3 Results and Discussion
### Proposed Catalytic Pyrolysis Mechanism for Tar Formation  
As discussed above, AD had an important influence on the pine pyrolysis rate and contributed little to the thermo-chemical equilibrium. Figure 7 shows the catalytic pyrolysis pathway. The three main components, cellulose, hemicellulose, and lignin in pine, melted with AD first. The functional groups, such as hydroxyl, carbonyl, and ether, combined with metallic oxide in AD to form C-O-M clusters by hydrogen bond or van der Waals forces. The number of radical fragments trapped in residue increased rapidly to 10.97 \(\times\) 10\({}^{19}\) spins/g at 300 \({}^{\circ}\)C, as shown in Figure 3. The decomposition of functional groups was suppressed so that the peak 1 area in the sub-curve of DTG decreased. The cleavage of ether linkage in cellulose, such as C1, was accelerated by AD, leading to an increment in the peak 2 area in the DTG sub-curves. Moreover, oxygenated aromatic carbons were activated by AD to break facilely at higher temperature. As the C-O-M clusters aided to form a macromolecular network and hence aggregate structures by a conjugation effect, a number of carbonyl and hydroxyl groups remained in the PADC residue. Figure 5: Variation of the aliphatic carbon structure area percentage assigned at chemical shift. Figure 6: Variation of the aromatic carbon structure area percentage assigned at chemical shift. Tar compositions from fixed bed pyrolysis at 500 and 700 \({}^{\circ}\)C were analyzed by GC-MS and are shown in Figure 8.  
Figure 8: Comparison of the tar composition from the pyrolysis of pine and a mixture at 500 \({}^{\circ}\)C and 700 \({}^{\circ}\)C. (**a**) 500 \({}^{\circ}\)C and (**b**) 700 \({}^{\circ}\)C.  
Figure 7: Proposed reaction pathway of catalytic pyrolysis on AD. The primary tar, mainly including furans, ketones, phenol, alkylphenols, and methoxyphenols, was formed by a free radical reaction. Furans and ketones were formed from cellulose and hemicellulose by the cleavage of C-O bonds. The catalytic pyrolysis accelerated the breakage of C-O bonds connecting C1 (shown in Figure 7) in cellulose to form ketones by scheme 1 in Figure 9[12]. Part of the glycosidic bonds were cleaved after activation of C1 in hemicellulose by AD. A substantial amount of active intermediate the radicals further decomposed into active monomers containing hydroxyl radicals and macromolecular radicals. C-C bonds in active monomers broke down to form smaller radicals, and then ketones. The ketones from catalytic pyrolysis were higher than those from non-catalytic pyrolysis at 500 \({}^{\circ}\)C, suggesting that the primary pyrolysis for ketone formation via scheme 1 was accelerated. This was in agreement with the larger area percentage of peak 2 in the DTG sub-curves of catalytic pyrolysis. With a pyrolysis temperature of 700 \({}^{\circ}\)C, the amount of ketones from pure pine pyrolysis changed insignificantly. However, the ketones from catalytic pyrolysis decreased from 10.80% to 4.86% as a result of secondary pyrolysis to generate gas on AD. Figure 10 shows the formation scheme of furans. It was first initiated by the thermal cleavage of the C-O bond in the hexatomic ring of a cellulose or hemicellulose monomer[12; 13]. Then, the dihydroxylation and transformation of the structure configuration were induced to form furan rings (5-hydroxy methyl fur furural, HMF). HMF decomposed to form furans. As discussed in Figure 5, AD enhanced the cleavage of the C-O bond in C1 of cellulose, forming selective furans and ketones. The percentage of furans was 22.23% from catalytic pyrolysis, higher than 17.45% of pure pine pyrolysis at 500 \({}^{\circ}\)C.  
Figure 10: Scheme 2 for the formation of furans from cellulose and hemicellulose catalyzed by AD. Figure 9: Scheme 1 for formation of ketones from cellulose and hemicellulose catalyzed by AD. Phenols were the major components in primary bio-tar, formed mainly via lignin pyrolysis, as illustrated by Scheme 3 in Figure 11[15; 16; 17].