Biomass upgrading in staged autothermal reactors with multifunctional catalysts

Abstract

Biomass contains a significant weight percentage of oxygen which must be removed to produce biofuel. In addition, the distributed nature of biomass necessitates the development of small-scale biorefineries which will also require reactors capable of processing and distributing biofuel locally. For this we turn to small-scale autothermal reactors capable of thermochemical processing of biomass to biofuel. An autothermal staged reactor consists of an upstream catalyst responsible for catalytic partial oxidation (CPO) which generates hydrogen and process heat needed to drive reactions on a downstream zeolite stage.Reactions in an autothermal staged reactor are heat-integrated and small-scale, making these reactors potentially useful for a biorefinery. The challenge in developing autothermal staged reactors is in integrating multiple catalytic functions requiring different operating conditions such as temperature and space velocity. In this thesis, the autothermal staged reactor is developed by initially studying relatively simple systems such as butanol dehydration and isomerization in Chapter 2 and eventually extending the analysis to more complex and realistic systems such as cellulose upgrading in Chapter 5. Integration of CPO and zeolite functionalities is discussed in Chapter 2, where butanol, considered an advanced biofuel, is dehydrated and isomerized to butene isomers. Higher butene yields were obtained from butanol when the catalyst used did not contain Bronsted acid sites such as gamma-Al2O3 or when the zeolite used, such as HFER, contained pores that were too small to permit competing reactions such as oligomerization to occur. Up to a 95% butene yield was obtained with HFER at temperatures from 280-350 C.The staged autothermal reactor concept was extended in Chapter 3 from butanol to hexane, decane, and 2-decanone which were used to probe the extent of isomerization, cracking and deoxygenation as a function of carbon chain length and oxygen functionality. While the kinds of reactions butanol underwent were largely limited to dehydration, paraffins and alkanones can undergo isomerization and cracking over zeolites in a staged autothermal reactor. A maximum 36% aromatics yield was obtained from a 2-decanone feed at 400 C over zeolite USY in contrast with a less than 5% aromatics yield from decane under identical conditions.The staged autothermal reactor was applied in Chapter 5 to cellulose upgrading, a feedstock more similar to actual biomass than either butanol or paraffins. The staged autothermal reactor enables solids such as cellulose to be upgraded over heterogeneous catalysts such as zeolites by going through a pyrolysis vapor intermediate. Microcrystalline cellulose particles were fed to the reactor and pyrolyzed to volatile organic compounds that were subsequently upgraded over a second stage containing HZSM-5 at 500 C. A maximum 24% yield of aromatics and 20% yield of C2-4 olefins and paraffins were obtained from the pyrolysis vapor entering the second stage. Pyrolysis oil contains species such as ketones whose acidity and corrosiveness make chemical storage difficult. Bifunctional catalysts such as those containing both metal and acid functionality can be applied to upgrade ketones more effectively than monofunctional catalysts. Upgrading of butanone over Pt/gamma-Al2O3 mixed with HZSM-5 is discussed in Chapter 4. A 99% selective stream of butane was obtained from butanone at 67% conversion and 160 C with minimal C-C bond scission, an important characteristic in any biofuel process.Glycerol conversion to olefins is possible with a three-stage isothermal reactor over a combination of HZSM-5, HBEA, and Pt or Pd catalysts and is discussed in Chapter 6. Glycerol can be converted to propanal which can undergo aldol condensation to form olefins ranging in size from C2-5 along with small amounts of aromatics and minimal amounts of CO. A maximum yield of 70% was obtained when propanal was reacted over HBEA in the absence of acetaldehyde at 500 C. Catalyst lifetime was also extended from 10 min to at least 150 min for a pure propanal feed.In this thesis, the chemical functionality of autothermal reactors and multifunctional catalysts is expanded and developed for a wide scope of applications--from dehydration of simple alcohols to cellulose upgrading. The autothermal staged reactor represents an attempt at designing a small-scale and heat-integrated system for potential use in a biorefinery.