Kinetic studies of model reactions to transform biomass into fuels

Dhairya D Mehta, Purdue University

Abstract

Second-generation biofuels utilizing lignocellulosic biomass are considered to be a promising alternative to fossil-based fuels. Lignocellulosic biomass is structurally diverse and therefore requires detailed understanding of the thermal depolymerization and catalytic hydrodeoxygenation reactions to optimize the overall process. This dissertation describes the experimental work using model compounds to elucidate the role of bimetallic catalyst and control the reaction operating parameters such as temperature and hydrogen pressure to maximize energy recovery in the liquid product from biomass resource. Water-gas shift (WGS) is a well-known reaction to produce hydrogen and finds application industrially in steam-reforming of methane and other fossil-based feedstocks. Traditionally, gas-phase WGS has been studied on noble metal based catalysts in experimental conditions relevant to reforming of methane. Recently, aqueous-phase reforming of oxygenated organic compounds has been considered to be one of the promising routes to produce hydrogen from sustainable biomass derivatives. One of the hypothesis in that study was that WGS played a vital role in production of hydrogen and thus it is important to quantify and qualify the contribution of WGS under aqueous reforming conditions. Platinum (Pt) and platinum-molybdenum (Pt-Mo) catalysts supported on multi-walled carbon nanotubes (MWCNT) were chosen since they were studied previously for aqueous-phase reforming of glycerol. Under these conditions, kinetics of WGS were significantly altered, especially CO order which is ∼0.9 compared to ∼0.1 for gas-phase WGS. Furthermore, Mo is shown to alter the kinetics of WGS reaction in a similar way both in gas and liquid-phase WGS. Fast-hydropyrolysis followed by in-line catalytic hydrodeoxygenation has been shown to have the potential to produce hydrocarbon fuels using hydrogen as co-feed. The objective of the research project was to identify and quantify the primary products of fast hydropyrolysis as well as establish the effect of temperature on overall product distribution. A torch igniter typically used in rocket engines was modified to design a reactor that is able to continuously feed biomass at a g min-1 scale and complete the entire process from fast hydropyrolysis to condensation of bio-oil in a matter of <70 ms at 3.6 MPa hydrogen pressure.>Levoglucosan, cellobiosan, glycolaldehyde and glucopyranosyl-β-aldehyde were the major products from cellulose pyrolysis at 500°C. As hydropyrolysis temperature was increased in the range of 500 to 700°C, the bio-oil product distribution shifted toward C2-C5 light oxygenates. The identification and quantification of products with molecular weight higher than the monomer (levoglucosan) at 70 ms residence time compared to their absence in reactors with residence time of 2-3 s indicate that levoglucosan is not the sole primary product of cellulose pyrolysis. Thus a portion of levoglucosan is a result of dimer (cellobiosan) and trimer (cellotriosan) degradation. Hydropyrolysis of cellulose resulted in a product distribution that retained most of the oxygen from parent cellulose. Catalytic hydrodeoxygenation (HDO) therefore assumes the vital part of selectively removing that oxygen as water and thus transforming the oxygenated organic compounds to fungible hydrocarbon fuels. Furfural and dihydroeugenol were the chosen model compounds to represent cellulose and lignin fraction of biomass. Discerning the role of catalyst descriptors and hydrogen pressure were the main objectives of the study. A bimetallic catalyst system comprising platinum as a hydrogenation function and an oxophilic promoter molybdenum supported on multi-walled carbon nanotubes (MWCNT) was used to achieve 100% hydrodeoxygenation. Furthermore, a series of Pt-Mo catalysts were tested to elucidate roles of Pt and Mo in the reaction network. Finally hydrogen partial pressure was shown to have a defining say in the dominant reaction scheme in the network and hence in the final product distribution as well. Scanning transmission electron microscopy combined with electron energy loss spectroscopic analysis on the 5%Pt-2.5%Mo/MWCNT revealed that 77% of nanoparticles were bimetallic Pt-Mo. X-ray absorption spectroscopy also confirmed the presence of Pt-Mo alloy and that Mo was partially reduced with an average oxidation state of 0 and +4. In case of furfural, four major routes observed were: Decarbonylation, reduction of aldehyde, furan-ring hydrogenation and furan-ring opening. Out of these decarbonylation was undesirable since it leads to loss of carbon in the form of carbon monoxide whereas reduction and furan-ring opening reaction are a result of C-O scission and hence desirable. Increasing the hydrogen pressure as well the relative Mo loading on the catalysts are reported to have significant role in increasing the C-O bond scission selectivity. Dihydroeugenol (DHE) is an aromatic ring with three substituents: alkyl, hydroxyl and methoxy. These substituents are characteristic of most of the compounds derived from a variety of lignin depolymerization processes such as pyrolysis/hydropyrolysis, organosolv extraction et cetera. Previously, the efficacy of Pt-Mo bimetallic catalyst system in complete hydrodeoxygenation of DHE to propylcyclohexane at 2.4 MPa hydrogen pressure was shown. In this work, hydrogen pressure was shown to have an impactful role in the distribution of aromatic to saturated hydrocarbon. At atmospheric pressure hydrogen, the yield of propylbenzene was 93.2%, and the pathway was deduced to be a result of direct deoxygenation of 4-propylphenol. Methoxy deoxygenation is proposed to occur through 4-propyl-1,2-benzenediol intermediate which subsequently deoxygenated to 4-propylphenol. Additionally, reaction site time yields and selectivity were compared to monometallic Pt and Mo catalyst in order to gain an understanding of their individual roles. Combination of results from catalyst characterization and kinetic studies over a series of Pt-Mo catalysts suggest that the overall site time yield was proportional to Pt loading, whereas the selectivity toward C-O scission products increased as the relative Mo to Pt ratio increased.

Degree

Ph.D.

Advisors

Agrawal, Purdue University.

Subject Area

Alternative Energy|Chemical engineering

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