Effects of Catalyst Properties on Biomass Conversion By Catalytic Fast Pyrolysis and Hydropyrolysis

Effects of Catalyst Properties on Biomass Conversion by Catalytic Fast Pyrolysis and Hydropyrolysis

David P. Gamliel and Julia A. Valla

Department of Chemical & Biomolecular Engineering, University of Connecticut

191 Auditorium Road, Unit 3222, Storrs, CT 06269-3222, USA,

Phone: +1-860-486 4602, e-mail: ioulia.valla@.uconn.edu

Biomass is a clean and renewable carbon source, capable of providing sustainable carbon for conversion to fuels and platform chemicals. Catalytic fast pyrolysis (CFP) of biomass is the thermochemical conversion of biomass to liquid bio-oil, gas and char under inert atmosphere at intermediate temperatures and high heating rate. Recently, catalytic pyrolysis of biomass under a hydrogen atmosphere at elevated pressure, termed hydropyrolysis, has been studied as an effective method for production of deoxygenated high-value bio-oil with plentiful aromatic and aliphatic hydrocarbons [1]. The objective of this work is to determine how catalyst properties correlate to CFP and hydropyrolysis product yields, and propose the ideal catalyst for the production of value-added products from biomass resources. 

A wide variety of catalysts were prepared and characterized for this study, such that the effects of transition metal choice, metal loading, support type and support acidity could be determined. Each catalyst was completely characterized using a variety of techniques to find the surface area, morphology, pore structure, metal oxidation state and crystal structure. Pd, Ru and Ni were chosen as candidate metals due to their high activity for deoxygenation and hydrogenation [2]. These metals were deposited via dry impregnation on a variety of alumino-silicate supports including alumina, silica and ZSM-5 zeolite. Furthermore, the Si/Al ratio of the ZSM-5 support was varied in order to determine the effects of acidity on the product distribution.

CFP was performed in a pyrolysis gas chromatograph (PyGC) unit, at 600 °C, under inert conditions and at atmospheric pressure. Increasing ZSM-5 catalyst acidity was found to increase yields of mono-aromatic hydrocarbons (MAHs), naphthalenes and CO, and decrease char formation. However, once the zeolite Si/Al ratio was lower than 25, coke formation reactions became dominant. It was found that significant increase of catalyst acid site density resulted in enhanced formation of coke and char precursors, such as poly-aromatic hydrocarbons (PAHs). The results are in agreement with the theory that the primary mechanism of acid catalyzed deoxygenation of pyrolysis vapors is decarbonylation and aromatization [3]. Impregnation of 3% of each transition metal slightly favored permanent gas, primarily in the form of CO, and decreased char yield. CFP with non-zeolitic support alone was found to have high yields to oxygenates and char.

Catalytic hydropyrolysis of biomass was performed in the same unit in pure H¬2 atmosphere, at 600 °C and 450 psig pressure. When hydropyrolysis was performed in the presence of ZSM-5 zeolite, a similar product distribution to CFP was observed. When Ni was incorporated on the catalyst, high amounts of CH4 were produced at the expense of solids, and alkanes were present in the liquid product, as shown in Figure 1. As the zeolite acidity was increased naphthalenes, PAHs and carbon oxide yields increased significantly at the expense of CH4 and liquid alkanes. This demonstrates the competition between acid-catalyzed decarbonylation, decarboxylation and aromatization and metal-catalyzed methanation and hydrogenation. As with CFP, hydropyrolysis with alumina or silica produced few liquids and high char yields. When Ni was incorporated onto silica, alkanes yield totaled about 6 wt.% carbon, and CH4 yield totaled over 50 wt.% carbon. Alternatively, when Ni was incorporated on the alumina support, lower CH4 yields and almost no alkanes were observed, along with higher solid yields, proving that significant Lewis acidity and large pore structure are detrimental for value-added product formation.

Catalyst design is essential should the thermochemical conversion of biomass reach large-scale implementation. Proper catalyst acidity aids in the formation of MAH compounds, while highly acidic catalysts result in high coke production. Incorporation of metals in the hydropyrolysis environment aids in the creation of alkanes and CH4, at the expense of char. In particular, metal-catalyzed methanation and hydrogenation reactions were found to compete with acid-catalyzed deoxygenation when both were present. 

 Acknowledgement

The study was funded by the National Science Foundation Award CBET-1236738 and the University of Connecticut GK12 program.

References

[1]           V.K. Venkatakrishnan, W.N. Delgass, F.H. Ribeiro, R. Agrawal, Green Chem. 17 (2015) 178–183.

[2]           T. Prasomsri, M. Shetty, K. Murugappan, Y. Román-Leshkov, Energy Environ. Sci. 7 (2014) 2660.

[3]           D.P. Gamliel, S. Du, G.M. Bollas, J.A. Valla, Bioresour. Technol. 191 (2015) 187–196.

 Figure 1. Bio-oil product distribution (left) and permanent gas product yields (right) for hydropyrolysis with ZSM-5 zeolites of various acidity with and without Ni incorporated.