(243a) Size Selectivity in Catalytic Fast Pyrolysis

There are several routes being studied to convert solid biomass to a liquid fuel, which involve multiple steps thus greatly increasing the cost of biomass conversion.¹  Catalytic fast pyrolysis (CFP) offers a simple single-step process to produce aromatic fuel molecules directly from biomass.  Previously, model compounds of glucose, cellobiose, cellulose and xylitol were used as feedstocks with ZSM-5 catalyst to form aromatics from pyrolysis.  It was shown² that a single step process involving catalytic fast pyrolysis of biomass and model compounds can produce significant yields of gasoline range aromatics.  At high catalyst-to-feedstock ratios (~19:1 wt/wt), the fast catalytic pyrolysis of biomass model compounds over a ZSM-5 catalyst produces up to ~40 wt% yield of aromatics, of which the major aromatic component is naphthalene.  The distribution of aromatics produced from the pyrolysis of the different feedstocks; glucose, xylitol, cellulose and cellubiose; was the same, indicating that these compounds undergo the same mechanism of catalytic conversion.  The aromatics produced include benzene, toluene, trimethylbenzene, ethylbenzene, ethyltoluene, methylbenzene, indane, naphthalene and xylenes.  At lower catalyst to glucose ratios (1.5:1 wt/wt), oxygenates are produced, including hydroxyacetylaldehyde, acetic acid, furan, furfural, methyl furan, 4-methyl furfural, and furan-2-methanol.  This indicates that these oxygenates are intermediates to the formation of the aromatic products at higher catalyst-to-feed ratios.

Weisz et al.³ recognized that it was important to consider the size of the reacting biomass compounds with respect to the pore size of the zeolite catalyst.  These workers³ reported the catalytic pyrolysis of seed oils and latex using ZSM-5 catalyst.  They³ showed a high level of conversion to LPG and gasoline-range hydrocarbons. 

Several important questions arise, namely:

(1)   What are the sizes of the reacting molecules and the products compared to the zeolite catalyst pore? 

(2)   Given the size selectivity of ZSM-5, do the reactions take place within the pores or at the surface sites?  Is there an optimum pore size for aromatic yield in CFP.

We calculated the dimensions of molecules, in particular the kinetic diameter and critical diameter, as a screening method for size selectivity of zeolite catalysts.  Estimations of the molecular diameters were also determined from calculations performed with Gaussian '03⁴ using density functional theory. 

The pore size of zeolite catalysts are typically given as the crystallographic diameters based on atomic radii e.g. 5.5-5.6 Å for ZSM-5 ⁵. Cook and Conner⁶ have shown that using Norman radii for Si and O atoms, the crystallographic pore diameters are 0.7Å larger, consistent with the diffusion of molecules of larger diameter than the crystallographic diameter reported, e.g. cyclohexane in silicalite.

Figure 1. Schematic of zeolite pore diameter (d_N) compared to the kinetic diameter of glucose feedstock, oxygenate intermediates and hydrocarbon catalytic pyrolysis products.

Experimental data on the aromatic yields observed for zeolite catalysts over a range of pore sizes (3.7-7.5 Å) was obtained using a model 2000 pyroprobe analytical pyrolizer, (CDS Analytical Inc.) coupled to model 5890 gas chromatograph (GC) interfaced with a Hewlett Packard model 5972A mass spectrometer). Glucose was used as a model compound for the cellulosic component of biomass.  This was pyrolyzed at 600°C with a ramp rate of 1000°C/s 

The glucose molecule (in the cyclic form) is significantly larger (~9Å) than the pore size of ZSM-5, however, this rapidly decomposes with heating to 600°C, therefore the pyrolysis intermediates and products are of specific importance for the diffusion in the zeolite catalyst.

The kinetic diameter of the gases and oxygenate pyrolysis intermediates are smaller than the pore size of ZSM-5 (6.2Å) therefore these will ready diffusion into the zeolite catalysts to react.

Of the aromatic products, naphthalene is the aromatic that is produced in the highest yield.  It is known that this larger polyaromatic has very slow diffusion in ZSM-5 ⁷ and therefore, it might be speculated that naphthalene is not formed within the pores.  Indeed, naphthalene has a kinetic diameter (~6.2 Å⁸) very close to the pore diameter of  ZSM-5 (~6.2 Å with Norman radii adjustment⁶).  However, at the elevated reaction temperature (600˚C), the energetic barrier to diffusion is likely to be decreased.  Hence, it is possible naphthalene is formed within the pores rather than on the surface.  Suggested from this analysis, the larger aromatics formed (e.g. pyrene and 1,5-dimethylnaphthalene) may be formed on the surface.  ZSM-5 excludes the formation of larger coke molecules. However, it should be noted that from the methanol-to-gasoline process using ZSM-5 catalyst, molecules with kinetic diameter as large as 7Å are formed.

Figure 2 shows the carbon yield versus the maximum pore size of catalysts for catalytic fast pyrolysis of glucose, using the py-GCMS. The aromatic yield reaches a maxima with a pore size range of 5.7 to 6.6Å which is sufficient to allow the diffusion of oxygenated intermediate molecules such as methyl furfural (kinetic diameter of 5.9 Å) into the zeolite framework. For small pore zeolites such as SAPO-34 and ZK-5 which have pore size of ~4.5 Å, no aromatics were produced and coke was predominantly formed.  On the other hand, when the pore size is greater than the diameter of the aromatic products (> 6.3 Å) such as β- and Y-zeolite, aromatic formation was not favored and coke formation increased.  The pores are large enough for the formation of coke molecules.

Figure 2. Carbon yield as a function of max pore size for a set of catalysts in catalytic fast pyrolysis of glucose. ♦: aromatics □: oxygenates▲: carbon dioxide ●: carbon monoxide ■: coke.

In conclusion, a simple molecule size model based on the kinetic diameter of molecules was developed to demonstrate the size selectivity in catalytic fast pyrolysis.  A pore size of 6.3Å (ZSM-5) allows the diffusion of oxygenate intermediates and smaller aromatic products, while excludes the larger aromatic molecules. This technique can be used to determine the appropriate catalyst pore sizes for selective reactions of biofuel molecules.

Complementary experimental results from catalytic fast pyrolysis of glucose with a range of zeolite catalyst pore sizes showed that 5.7 to 6.6Å was the optimum pore size range for maximum aromatic yield.

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