The Effects of Hierarchical Pore Structure ZSM-5 on the Catalytic Fast Pyrolysis of Biomass

Catalytic fast pyrolysis (CFP) is an effective process for conversion of biomass to liquid hydrocarbons and fuels. MFI type zeolites have been proven to be the best catalyst for CFP.¹ However, low bio-oil yields and coke formation continue to hamper industrialization of CFP. One solution for the aforementioned challenges may be the introduction of mesoporosity in the zeolite pore structure. This may reduce diffusion limitations of bulky oxygenates formed in the initial stages of pyrolysis. The objective of this study is to develop hierarchical ZSM-5 zeolites via two different top-down approaches, and investigate their effectiveness with regards to biomass CFP.

Commercially available ZSM-5 (CBV 8014) was modified using the top-down techniques of desilication and a surfactant assisted method.² Desilication (DS) was performed via alkaline treatment (0.1 M NaOH), followed by an acid wash, triple ion exchange, and calcination. Two materials were created with the surfactant assisted method. Both zeolites were treated with NaOH (0.1 M for SA_mild, 0.3 M for SA_strong) followed by acid wash, triple ion exchange, and calcination. Each zeolite was characterized using N₂ adsorption (BET method), X-ray Diffraction (XRD), diffuse reflectance FTIR, pyridine adsorption and ICP. Testing of the zeolites was performed using a benchtop pyrolysis gas chromatograph (PyGC-MS). Briefly, biomass (cellulose or miscanthus) was physically mixed in a 5:1 catalyst to biomass (C/B) ratio. The mixture was packed into a quartz microreactor, and pyrolyzed at 600 °C.

Figure 1. N₂ adsorption and desorption isotherms (left) and BJH adsorption pore size distribution (right) for ZSM-5 and hierarchical ZSM-5 zeolites

XRD confirms that all materials exhibit the MFI type crystalline structure, and no significant crystallinity was depleted from the top-down prepared materials with respect to the parent ZSM-5. ICP confirms the Si/Al ratio of all materials were between 29 and 41, and DRIFT-FTIR of the -OH stretching region showed that all zeolites contain significant Brönsted acidity. Additionally, the materials prepared with the SA method showed an increased peak at 3775 cm^-1, most likely a result of the formation of silanol nests. Pyridine adsorption tests indicate reduction of Brönsted acidity followed by an increase in Lewis acidity for the mesoporous zeolites.

Figure 1 shows the N₂ adsorption isotherms for each material and the BJH adsorption pore size distribution. The DS material has a much more broad pore diameter range, spanning between 40 and 150 Å. Each material exhibits a unique pore size distribution, with mesopore volume increasing in the following order: CBV8014 < SA_mild < DS  < SA_strong. All materials had very comparable micropore volumes, but CBV 8014 (0.14 cm³/g) was the highest, and SA_strong (0.10 cm³/g) the lowest.

Figure 2. Liquid product distribution from CFP of miscanthus

CFP of miscanthus was performed, and the liquid product distribution and yields were determined, and are shown in Figure 2. CFP with all catalysts produced significant yields to aromatic compounds. The conversions to benzene, toluene and xylene were relatively constant among all the other catalysts evaluated. This could be because the micropore volume was relatively constant across all catalysts tested. The maximum micropore diameter of ZSM-5 type catalysts is approximately 5.5 Å, which has been shown to provide the ideal shape selectivity with regards to the formation of these three compounds.¹ The formation of alkyl benzenes, indenes,  naphthalenes and higher order PAHs is significantly increased with the introduction of mesoporosity. CFP with the desilicated zeolite produced the most alkyl benzenes and indenes. Increasing the catalyst mesoporosity further shifts the product distribution to naphthalenes  and then higher order PAHs (SA_strong).

A further analysis of the product distribution shows that the increased yields of larger compounds is accompanied with a decrease in the formation of solid carbon (coke). This may be because bulky coke precursors, such as naphthalenes and PAHs are allowed to diffuse out of the pore structure due to the presence of mesopores. These coke precursors become trapped in the microporous catalyst, polymerize and are eventually deposited as coke.

Figure 3 Average carbon number of the bio-oil produced from CFP of cellulose compared to the mesopores volume of each catalyst

Figure 3 shows the average carbon number of the bio-oil produced from CFP of cellulose compared to the mesopore volume of each catalyst.  Average carbon number is defined as the fractional selectivity to each compound multiplied by the number of carbon atoms in the molecule. Average carbon number is indicative of average size of carbon atoms in each molecule of the constituent bio-oil. CFP of cellulose with CBV8014 resulted in a bio-oil with the lowest average carbon number of about 8.9. The average carbon number then significantly increased with mesopore volume, until a final carbon number of approximately 9.25 was achieved with the SA_strong material.

References

(1)        Jae, J.; Tompsett, G. A.; Foster, A. J.; Hammond, K. D.; Auerbach, S. M.; Lobo, R. F.; Huber, G. W. Investigation into the shape selectivity of zeolite catalysts for biomass conversion. J. Catal. 2011, 279, 257–268.

(2)        Li, K.; Valla, J.; Garcia-Martinez, J. Realizing the Commercial Potential of Hierarchical Zeolites: New Opportunities in Catalytic Cracking. ChemCatChem 2014, 6, 46–66.