(429g) 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
volumeincreasing 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.