(653e) Hierarchical Pore Structure Zeolites for the Catalytic Upgrading of Tars

Hierarchical pore structure zeolites

for the catalytic upgrading of tars

Ameya Akkalkotkar, Shoucheng Du, George M. Bollas,
Julia Valla

Introduction

Thermochemical conversion
of biomass via gasification has attracted increasing scientific interest lately.
However, one of the most important technical barriers towards the successful
commercialization of biomass gasification is the efficient removal of
impurities present in the synthesis gas (syngas), like tars. Tars are a complex
mixture of light and heavy molecular weight Poly-Aromatic Hydrocarbons (PAH). PAHs
deactivate catalysts, cause blockages in transfer lines, damage downstream
units and lower the overall process efficiency. Moreover, due to their
carcinogenic nature, the disposal of tars is impossible.

Modifying the gasification
operation parameters can affect and potentially reduce the tars content in the
syngas, while newly developed entrained flow gasifiers produce almost no tars,
but require very flexible designs and are associated with high capital cost.
Downstream thermal cracking is another way to reduce tars. However, it involves
heating the tars to very high temperature (1000°C). Catalytic cracking of tars appears to be one of
the most promising options for tars reduction, because it has the potential to
convert tars into lighter hydrocarbons, which can later be reformed to valuable
syngas.

Zeolites are microporous materials
that have the potential to catalytically eliminate tars from syngas. Zeolites
exhibit a unique network of properties like acidity, microporosity and shape
selectivity. For this reason they are extensively used during the Fluid
Catalytic Cracking (FCC) Unit, where heavy hydrocarbons are cracked into
lighter valuable products, like gasoline and diesel. Notwithstanding the positive
effect of the presence of micropores with respect to shape selectivity, the
zeolite micropore structure may often induce a negative effect due to the low
diffusion rates of heavy molecules into the zeolite crystals, as well as
unwanted adsorption effects of the reactants and/or products that can lead to
the formation of coke.

The aim of our research is
twofold: a) to test the effectiveness of zeolites for tars cracking, and b) to
modify the zeolites in order to create hierarchical pore structure network that
will reduce the diffusion limitations of the heave PAHs in and out of the
zeolite pores, hence improving their cracking reactions.

Experimental Section

Model Compounds

Model compounds, representative of the tars mixture, have
been used during this study including: benzene, naphthalene, fluoranthrene and
coronene, as they are the most abundant compounds in the groups of light PHA
and heavy PHA. The structure of the above model compounds is shown in Figure
(1).

Figure
1. Tars model
compounds used in this study: (a) benzene, (b) naphthalene, (c) fluoranthrene
and (d) coronene

Experimental Set-up

In Figure (2) the
experimental set up for the tars cracking tests is shown. 5 gm of each model
compound is placed in a stainless steel vessel and heated up just below its
boiling point. Nitrogen is bubbled through the liquid compounds and picks up
the model compound. This mixture then passes over the catalyst bed (0.5 gm),
which is held in a quartz tube reactor (7mm ID). The reactor is placed in a
furnace for the catalytic cracking experiments. Two different temperatures have
been tested (625^oC and 700^oC). Stainless steel piping is
used to withstand the high system temperature. The piping is heat taped to
avoid tars condensation inside the system. The products are passed through a train
of impingers, which contain isopropanol as a solvent. The solvent dissolves the
heavy tars, which are then analyzed using a GC equipped with Simulated
Distillation and GC Mass Spectroscopy. The gas is analyzed using GC equipped
with Thermal Conductivity Detector (TCD).

Figure
2. 
Experimental set-up for the tars model compounds cracking reactions

Materials Preparation
and Characterization

In this study ZSM-5 and Y
zeolites obtained from Zeolyst International were used. Hierarchical mesoporous
zeolites ZSM-5 and Y zeolites were prepared by sequential alkaline-acidic
treatment [1]. During the
treatment, the ammoniated form of the zeolite is calcined at 550^oC
to obtain the protonated form. This zeolite is then subjected to treatment with
NaOH (30 ml/g of zeolite) at 65^oC for 30 min. The suspension is quenched,
filtered, washed till neutral pH and dried. The powder is then treated with HCl
(100 ml/g of zeolite) at 65^oC for 6 hrs. It is again filtered,
washed and dried. The product is then ion-exchanged 3 times with 0.1N NH₄NO₃
for 12 hrs each. The resultant suspension is again filtered, washed and dried
and then calcined at 550^oC. The materials were characterized using
X-ray Diffraction (XRD) and N₂ adsorption. The coke deposited on the
catalyst after the cracking experiments was analyzed using Thermogravimentric
Analysis (TGA).

Results and Discussion

N₂ adsorption
of the parent Y and ZSM-5 zeolites, as well as the various mesoporous USY and
mesoporous ZSM-5 are shown in Figure 3. There are two important observations:
1) the concentration of the alkaline media (NaOH) plays a key role on the
extent of the desilication of the zeolites and consequently on the introduction
of mesoporous and the destruction of microporous structure of the zeolites, and
2) the acid treatment after the alkaline treatment (desilication) is a
necessary step that helps to the unblocking of the microporous due to the
formation of extra framework aluminium [2]. Based on the N₂
adsorption results, it has been revealed that the optimum conditions for
maximum mesoporosity incorporation and microporosity retention is 0.1M
NaOH/0.02M HCl and 0.2M NaOH/0.02MHCl for the USY and the ZSM-5, respectively.

Figure
3.  N₂
adsorption/desorption curves for the parent and the mesoporous USY and ZSM-5 zeolites

Catalytic cracking
experiments have been conducted using both benzene and naphthalene. Benzene is
the smallest aromatic hydrocarbon, while naphthalene is the smallest PAH. In
Figure 4 the benzene conversion to gas, liquids and coke at two temperatures
(625^oC and 700^oC) using parent ZSM-5 (a), mesoporous
ZSM-5 (b), parent Y (c) and mesoporous Y (d) is presented.

Figure 4. Benzene conversion to gas, liquids
and coke using ZSM-5 (a), Mesoporous ZSM-5 (b), Y (c) and Mesoporous Y (d).

Firstly,
parent Y zeolite (Figure 4(b)) leads to slightly higher benzene conversion than
ZSM-5 (Figure 4(a). This was expected since the parent Y zeolite has higher
both microporous and mesoporous area. Secondly, for both parent ZSM-5 and Y
zeolites (Figure 4(a) and Figure 4(b)), increasing the reaction temperature
from 625^oC to 700^oC favors the overall benzene conversion.
With respect to the mesoporous zeolites, mesoporous ZSM-5 (Figure 4(c)) does
not favor the conversion of benzene, compared to the parent ZSM-5. The above
results lead us to the conclusion that the reaction of the benzene to the ZSM-5
is not diffusion limited. On the contrary, benzene conversion is favored by the
active sites in the ZSM-5, which are partially destroyed during the
introduction of mesoporosity. In the case of Y zeolites, the results are more
complicated. It appears that at low temperature (625^oC) mesoporous Y
favors the conversion of benzene compared to the parent Y. However, at high
temperature (700^oC) no difference in benzene conversion was observed
between the parent and the mesoporous Y. The above results as well as the
results of the catalytic cracking of naphthalene over Y, ZSM-5 and the
corresponding mesoporous zeolites will be discussed.

Conclusion

A laboratory experimental
set up has been designed and built for the evaluation of zeolites and
hierarchical pores structure zeolites on the catalytic cracking of PAHs derived
from biomass gasification. Hierarchical pore structure ZSM-5 and Y zeolites
have been prepared using the desilication method. Preliminary cracking tests using
benzene and naphthalene as model compounds at two different reaction
temperatures and using the parent and the corresponding hierarchical zeolites
have been conducted and the results will be presented and discussed.

Acknowledgement. The study is funded by the National Science
Foundation Award CBET-1236738.

References

[1]
Radwan A.M.; Kyotani T.; Tomita A., Appl. Catal. A., 2000, 192,
43.     

[2]
Groen J.C.; Moulijn J. A.; and J. P¨¦rez-Ram¨ªrez, Microporous Mesoporous Mater.,
2005, 87(2), 153.