**7. Metal doped HZSM-5**

The literature is replete with studies of metals doped HZSM-5 to investigate their effects of CFP catalyst. Zeolites metalated even at low concentrations beneficially resulted in accelerated deoxygenation by decarbonylation, decarboxylation, dehydration, and olefin aromatization of pyrolysis products. In some cases, there were reduced coke formation by decreasing selectivities to polycyclic aromatic hydrocarbons. **Table 1** summarizes the metals dopant and their references. Among these metals, gallium, iron, zinc, molybdenum, nickel, and copper increased for deoxygenations, dehydrocyclization, and aromatization.

Investigators also examined combinations of metals to determine whether there were accumulated benefits in CFP. For example, Ga and Ni are perhaps the most investigated metal additives, and they have been combined for CFP [63]. Ga-Ni reduced acidity, and reduced formation of polyaromatic hydrocarbons which could explain the reduced coking. Another combination is Cu-Ni which was examined in hydroprocessing (CFP under H2 atmosphere) [108]. Ni-Fe produced an additive effect. Individually, Fe increased yields of monoaromatics, while Ni increased yields of naphthalenes [82]. When combined, this catalyst generated more monoaromatics and naphthalenes. The doping of these metals onto HZSM-5 increased the concentration of strong acid sites, but also resulted in more coke formation.

Molybdenum is already a highly active metal when doped in HZSM-5 in zeolites [111]. When Mg was combined with Mo in HZSM-5, higher selectivities to monoaromatic hydrocarbons and decreased polyaromatics selectivities were achieved. Phosphorous modified HZSM-5 and doped with Zn decreased external acidity of catalyst surface while the internal acid sites were left unchanged. This modification resulted in improvements in this CFP catalyst [107]. HF modification of HZSM-5 and later doped by Ni produced a catalyst that increased yields of BTX [110]. The acid modification created mesopores and reduced Brønsted acidity via dealumination.

Certain combinations of metals and nonmetals behaved as "noble-metal-like" catalysts [106]. Therefore Ni-Mo2N was prepared and confirmed that Ni further extended performance of Mo2N [112].


**Table 1.** *Metals and additives to HZSM-5 for CFP.*
