Tailoring the mesopore structure of HZSM-5 to control product distribution in the conversion of propanal Xinli Zhu, Lance L. Lobban, Richard G. Mallinson, Daniel E. Resasco * Center for Biomass Refining, School of Chemical, Biological, and Materials Engineering, The University of Oklahoma, Norman, OK 73019, USA article info Article history: Received 23 September 2009 Revised 14 December 2009 Accepted 2 February 2010 Available online 6 March 2010 Keywords: Propanal HZSM-5 Desilication Biomass conversion Bio-oil upgrading abstract Conversion of propanal to gasoline-range molecules was investigated over a series of HZSM-5 catalysts with controlled mesoporosity generated by desilication. Characterization of the structure of the solid by powder X-ray diffraction (XRD), scanning electronic microscopy (SEM), ammonia and isopropylamine temperature programmed desorption (TPD), and n-butane diffusivity measurements confirmed the development of various degrees of mesoporosity in the zeolites. This structural modification seems to have little influence on Brønsted acid density. The catalyst stability was improved upon desilication due to an increase in coke tolerance. The product distribution of the propanal conversion was found to vary with the severity of the desilication. Increasing the extent of desilication gradually reduced the aro- matization and cracking reactions, due to a reduction in the fraction of micropores and in the diffusion path length. Mildly desilicated samples were found to exhibit the best stability on stream and inhibited coke formation. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Conversion of lignocellulosic biomass into liquid hydrocarbon fuels provides a CO 2 neutral energy production route, which poten- tially can reduce the dependency on fossil fuels [1–3]. In the ther- mochemical route (e.g. fast pyrolysis), the molecular structure of biomass is broken down into smaller fragments that subsequently undergo further conversion in the vapor and liquid phases con- densing into a complex product termed bio-oil. Some of the con- stituents of this product are larger than the desirable fuel range, others are shorter, but all of them contain significant amounts of oxygen. The chemically unstable, highly viscous, corrosive, and low-heating value liquid product includes acids, aldehydes, ke- tones, phenolic compounds, sugars, and dehydrosugars [4,5]. Deoxygenation of the larger oxygenated molecules (guaiacols, vanillins, cresols, catechol, etc.) is being extensively investigated [6–10]; and hydrogenation, hydrogenolysis, and decarbonylation are potential reaction pathways to improve the quality of these heavy molecules. By contrast, the short oxygenates (e.g., alde- hydes, acids, ketones) need to be condensed into larger molecules to become useful fuel components. Under hydrotreating conditions for refining the complete bio-oil, short oxygenates are converted to light hydrocarbons of low value while consuming substantial hydrogen. Alternative strategies that avoid discarding short oxy- genates should be considered since they constitute a significant fraction of the product [4,5]. Due to the highly complex nature of the bio-oil, understanding the reaction pathways for each kind of compound conversion is highly desirable for catalyst and process screening. Therefore, the study of model compounds is the first step in simplifying the complexity of the problem [11–14]. While our next studies will include more complex mixtures, in this work, propionaldehyde (propanal) has been selected as a model com- pound to investigate the conversion of short aldehydes into gaso- line-range molecules. The study of propanal conversion is also relevant to the utiliza- tion of glycerol, a major by-product of bio-diesel production. Glyc- erol is readily converted into acrolein by dehydration [15,16]. Subsequent hydrogenation produces propanal [17]. Thus, conver- sion of propanal may also represent a potential approach for the conversion of bio-diesel by-products to gasoline-range fuels. Previous studies on the conversion of small oxygenates (meth- anol, ethanol, etc.) to hydrocarbons (alkene/alkane, aromatics) over zeolites have addressed propanal conversion briefly. It has been re- ported that propanal can yield aromatics in higher selectivity than acetone and much higher than other C 3 oxygenates (alcohol, acid, ester) [18–20]. However, it was also found that propanal causes a rapid catalyst deactivation [19]. Zeolites are widely used in hydrocarbon conversion due to their high density of strong acid sites and their well-defined micropo- rous channel structure that enable shape selective reactions inside the pore channels. However, transport of both reactants and prod- ucts in and out of the micropores may be limited by diffusion. Con- ventional mesoporous materials such as MCM-41 and SBA-15 have superior diffusion properties but lower thermal/hydrothermal 0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2010.02.004 * Corresponding author. Fax: +1 405 325 5813. E-mail addresses: mallinson@ou.edu (R.G. Mallinson), resasco@ou.edu (D.E. Resasco). Journal of Catalysis 271 (2010) 88–98 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat