Biomass conversion: Novel analytical methods and techniques

Aleksei Vjunov, Donald M. Camaioni, Jian Z. Hu, John L. Fulton, Donghai Mei, Johannes A. Lercher

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

Abstract

Biomass derived fuels are becoming increasingly attractive in the light of growing demand for oil as well as political instability and economic tension. The wide abundance of biomass material, in particular lignin, makes it a promising feedstock for diesel-range fuel production. The conversion of lignin to liquid energy carriers requires novel catalytic approaches in order to obtain a uniform pool of hydrocarbons.[1] While lignin, a highly branched phenol-based polymer, contains much less oxygen than cellulose, the high oxygen content in the lignin-derived molecules still requires substantial amounts of hydrogen for complete reduction to hydrocarbons.[2] The hydrogenation of ligninderived bio-oils on metals, such as Pd or Ni, leads to a range of C6-C9 cycloalkanols. The alcohols must be dehydrated to olefins and then further alkylated and hydrogenated to obtain diesel-range alkanes. While a direct, one pot, conversion of lignin would be most advantageous, the dehydration step of the reaction cascade still poses significant complications and is rate-limiting for the reaction sequence. In the past hydrodeoxygenation (HDO) has been successfully performed on dual functional metal/acid catalysts even in liquid water, which is the solvent of choice due to its ubiquitous presence.[3] Also, it has been shown that phenol hydroalkylation on solid acids, such as HBEA, can take place in aqueous environment thereby providing a way to increase the bio-oil hydrodeoxygenation product carbon numbers.[4] This can be tentatively attributed to the unique environment of the zeolitic pores (often referred to as confinement and nest effects[5]) leading to better stabilization of the sorbed and transition states.[6] A fundamental study of the active centers, reaction intermediates and reaction dynamics/kinetics is critically needed to facilitate the development of catalysts for the practical conversion of biomass to energy-carrying alkanes. Therefore, we introduce the application of a combination of nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) spectroscopies that enable us to monitor the state of the reacting molecules and determine the catalyst active site structure. We have recently reported an in-situ study of a cyclohexanol reaction (a common cycloalkanol from phenol HDO) on HBEA in liquid water at elevated temperatures using high-resolution magic angle spinning (MAS) NMR spectroscopy.[7] In particular, this method not only enabled us to discriminate between different mechanisms of water elimination, but also to follow the distribution and catalyst adsorption of products during the reaction. It was shown that the initial rates of functional group migration in the reactants/products during cyclohexanol dehydration on HBEA in liquid water are better explained by the E1 mechanism. This new insight can lead to improved reaction control and process design for cycloalkanol dehydration in the future. The developed technique is also suitable for study of other industrially relevant reactions, e.g. alkylation and isomerization reactions. In a separate effort the structure of the catalysts, HBEA150 and HBEA25, was analyzed in unprecedented detail. As a note in passing, Al3+ substitutes Si in the tetrahedral (T) positions of the zeolite framework forming the catalytically active sites. HBEA has nine different T-sites. Previously, it has been difficult to determine experimentally the distribution of Al3+ amongst the different T-sites in the zeolite.[8] We report a quantitative analysis of the Al K-edge EXAFS that allowed us to determine the Al3+ lattice distribution. The experimental EXAFS spectra were interpreted by invoking a linear combination fit of the molecular dynamics (MD) EXAFS derived for each T-site from the DFT modeled structure. It has been determined that most of the Al3+ populating T-sites that are part of one or more 4- member framework rings. In contrast, dramatically different Al3+ distributions are found for the T-sites occupying only 5- and 6-member rings. Using the method described above, we studied the integrity of HBEA framework under hot liquid water conditions that are typical for HDO. Commonly there are two mechanisms describing zeolite lattice degradation: acid catalyzed dealumination, removal of Al3+ from the T-sites and base-catalyzed hydrolysis of siloxane bonds. It was determined that dealumination does not occur under high-temperature water conditions up to the point where the zeolite undergoes complete lattice degradation. It was also found that the Al3+ site retains its coordination and local structure, remaining identical to that of the initial HBEA T-site. For the same samples, however, we observed severe loss of crystallinity. These results suggest that the desilication mechanism is primarily responsible for the increasing number of framework defect sites, which form as a function of time. We believe future work in the field of catalyst design in particular in respect to framework Al3+ distribution will benefit greatly from this capability. In summary, a combination of two novel approaches for both reaction and catalyst studies is presented and their application is demonstrated. The new understanding of the dehydration reaction step as part of the overall HDO cascade is an important advance toward future industrial implementation of the bio-oil conversion to diesel-range fuels. The quantitative analysis of Al3+ T-site distribution and the zeolite framework stability in liquid water are highly relevant to refining. In particular, the optimization of zeolite performance and increase of catalyst life cycle will greatly reduce both environmental impact and refinery operation costs to the industry.

Original languageEnglish
Title of host publication17th Topical on Refinery Processing 2014 - Topical Conference at the 2014 AIChE Spring Meeting and 10th Global Congress on Process Safety
PublisherAIChE
Pages165-167
Number of pages3
ISBN (Electronic)9781634390644
StatePublished - 2014
Event17th Topical on Refinery Processing 2014 - Topical Conference at the 2014 AIChE Spring Meeting and 10th Global Congress on Process Safety - New Orleans, United States
Duration: 30 Mar 20143 Apr 2014

Publication series

Name17th Topical on Refinery Processing 2014 - Topical Conference at the 2014 AIChE Spring Meeting and 10th Global Congress on Process Safety

Conference

Conference17th Topical on Refinery Processing 2014 - Topical Conference at the 2014 AIChE Spring Meeting and 10th Global Congress on Process Safety
Country/TerritoryUnited States
CityNew Orleans
Period30/03/143/04/14

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