Conversion of n-Octanol and Bio-Butanol over Supported Nickel and Zeolite Catalysts for the Production of Transportation Fuel and Building Block Chemicals

Palla, V C S and Shee, Debaprasad (2017) Conversion of n-Octanol and Bio-Butanol over Supported Nickel and Zeolite Catalysts for the Production of Transportation Fuel and Building Block Chemicals. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

The transportation fuels and commodities derived from organic chemicals play an extremely vital role in the daily life of our modern civilization. At present more than 80% of energy and around 90% of organic chemicals in the world are derived from finite fossil fuels. Specifically, the transportation fuels are one of the leading energy-consuming sectors and solely contributing ~28% of global energy consumption. Moreover, the earth environment is increasingly intoxicated due to the large-scale release of harmful and greenhouse gases due to consumption of fossil fuels. Availability of carbonneutral renewable resources for the production of transportation fuels and organic chemicals is, therefore, highly essential to reduce dependency on finite fossil fuels. The biomass, an abundant renewable resource, has the enormous potential for sustainable production of an array of bio-fuels and organic chemicals in an integrated biorefinery. The fast pyrolysis is one of the promising technologies for the thermochemical conversion of lignocellulosic biomass directly into liquid product commonly known as bio-oil. The bio-oil is, however, inappropriate as feedstock for the production organic chemicals due to the presence of wide variety of compounds and functionality with low concentration. The bio-oil is also unsuitable for direct usage as transportation fuels because of its high water and oxygen contents, immiscibility with petroleum fuels, low heating value, poor storage stability, and high corrosiveness. The removal of oxygen from bio-oils is thus indispensable for the production of liquid transportation fuels. The hydrodeoxygenation (HDO) is a promising technology for removal of oxygen from bio-oils. Understanding mechanism of HDO of whole bio-oils is, however, highly challenging because of the presence of diverse functionalities. vii The present work is, therefore, commenced to explore the fundamental understanding of HDO of n-octanol as a model aliphatic alcoholic functionality of bio-oil. The biofuels such as bio-methanol, bio-ethanol, bio-butanol, and biodiesel are considered as the promising alternative to fossil fuel derived transportation fuels. These oxygenated bio-fuels are, however, suffering from drawbacks of incompatibility with existing engines and poor fuel mileage compared to fossil fuels derived transportation fuels. Application of these bio-fuels is, therefore, limited to blending with petroleum-derived transportation fuel to a certain extent for use in an unmodified internal combustion engine. On the other hand, the organic chemicals are primarily produced from a set of building block chemicals (syngas, olefins, and aromatics). The biomassderived platform chemicals provide an attractive alternative for the production of organic chemicals in a sustainable manner. However, the production and downstream transformation of these oxygenated platform chemicals involve completely new chemistry and hence unsuitable with existing petrochemical industry infrastructures. Therefore, the production of hydrocarbon transportation fuels and building block chemicals from biomass is highly desirable to circumvent the development of capital-intensive new infrastructures. The bio-butanol derived from sugar/starchy and cellulosic biomass is an excellent alternative for the production of hydrocarbon transportation fuels and building block chemicals. A few attempts in the primitive level have been made to produce hydrocarbon transportation fuel and building block chemicals from n-butanol. A comprehensive investigation for the production of hydrocarbon transportation fuels and building blocks chemicals from n-butanol is, therefore, considered in the present work. The followings are the objectives of present work. viii (i) HDO of n-octanol over several supported (γ-Al2O3, SiO2, and HZSM-5) nickel catalysts with different degree of acidity to (a) delineate a holistic reaction mechanism and (b) understand the effect of various process conditions on conversion of n-octanol and selectivity to products. (ii) Selective conversion of n-butanol over HZSM-5 at atmospheric pressure for clear identification of temperature and weight hourly space velocity (WHSV) window to maximize the yield of gasoline range transportation fuel, butylenes, and aromatics in single step. (iii) Selective conversion of n-butanol to gasoline range hydrocarbons at high pressure over HZSM-5 to obtain (a) the optimum process conditions (pressure, temperature, and WHSV) for maximum yield of aromatics-free gasoline range hydrocarbons and (b) characterization of the physicochemical properties of the liquid product to understand its suitability as a transportation fuel in an unmodified combustion engine. (iv) Selective conversion of n-butanol to aromatics at high pressure over various solid acid catalysts (HZSM-5, H-Beta, and Al2O3) to (a) understand the role of acidity of the catalyst on the yield of aromatics and (b) obtain the optimum of process conditions for maximum yield of aromatics. In the present work, γ-Al2O3, SiO2, and HZSM-5 supported nickel catalysts (0 to 20 wt%) were prepared by incipient wetness impregnation method. The supports and prepared catalysts were characterized by BET, powder X-ray diffraction (XRD), H2 pulse chemisorption, temperature programmed reduction, temperature programmed desorption of NH3 (NH3-TPD), UV-vis spectroscopy, Fourier transformed infrared spectroscopy (FTIR), and pyridine IR spectroscopy. The spent catalysts were characterized by BET, XRD, FTIR, pyridine IR, NH3-TPD, and thermo gravimetric analyzer. The HDO of n-octanol and conversion of n-butanol to hydrocarbons were performed in an isothermally operated high-pressure fixed-bed reactor. The ix HDO of n-octanol was performed using nitrogen and/or hydrogen as carrier gas. The conversion of n-butanol to hydrocarbons was performed using nitrogen as carrier gas. The products were identified by a gas chromatograph equipped with mass spectroscopy (GC-MS). The liquid products were quantified using an offline gas chromatograph equipped with flame ionization detector (FID). The gaseous products were quantified using two online gas chromatographs equipped with FID and thermal conductivity detector. For HDO of n-octanol, n-heptane, n-octane, isomers of heptene (heptenes) and octene (octenes), octanal, di-n-octyl ether (DOE), tetradecane, and hexadecane were identified in the liquid samples. A small amount of H2, CO, CO2, and CH4 were detected in gaseous products. During the conversion of n-butanol, H2, aliphatic hydrocarbons (C1-C12), aromatics (C6-C12), and oxygenated compounds (butanal and di-n-butyl ether (DBE)) were mainly observed as the products. The paraffins and olefins were observed as the principal aliphatic hydrocarbons with only trivial quantity of cyclic paraffins (naphthene). On the other hand, benzene, toluene, ethylbenzene and xylene (BTEX) were observed as the leading aromatics. The HDO of n-Octanol follows two different routes on two different sites of the catalysts: acid and metal. In the first route, n-octanol undergoes dehydration over acid sites to octenes either directly or through intermediate etherification reaction with the formation of DOE. The octenes subsequently undergo either hydrogenation to n-octane or oligomerization followed by hydrogenation to hexadecane. Following the second route, n-octanol undergoes dehydrogenation reaction over active metal sites of the catalysts leading to the formation of n-octanal. The n-octanal consequently converted to either n-heptane by decarbonylation reaction or isomers of heptenes by dehydroformylation reaction over metal sites of the catalysts. The heptenes are then either hydrogenated to n-heptane or oligomerized followed by x hydrogenated to tetradecane. Formation of CO2 and CH4 may be due to water gas shift reaction and methanation of CO/CO2, respectively. The product distribution observed for HDO of n-heptanol and n-hexanol was also fully in line with proposed mechanism. Characterization of supported nickel (or nickel oxide) catalysts revealed the existence of dispersed as well as bulk nickel (or nickel oxide) depending on the extent of nickel loading and nature of supports. The acidity of γ-Al2O3 supported nickel catalysts were decreased with increasing nickel loading on γ-Al2O3. The C7 hydrocarbons (heptane and heptenes) were observed as primary products for γ-Al2O3 and SiO2 supported nickel catalysts. However, C8 hydrocarbons were primarily formed over acidic catalysts such as pure HZSM-5 and HZSM-5 supported nickel catalysts. The n-octanol conversion was increased with increasing nickel loading on γ-Al2O3, and temperature and decreasing pressure and WHSV. The selectivity to products was strongly influenced by temperature, nickel loading on γ-Al2O3, pressure, and types of carrier gases (nitrogen and hydrogen). The selectivity to C7 hydrocarbons was favored over catalysts with increased nickel loading on γ-Al2O3 at elevated temperature and lower pressure. The conversion of n-butanol to hydrocarbons mainly proceeds through two parallel routes: (i) dehydration and (ii) dehydrogenation. Following the first route, the n-butanol undergoes dehydration to 1-butylenes over acidic sites of the catalyst either directly or through the formation of an intermediate DBE. 1-Butylene further isomerizes over acidic sites of the catalyst forming four different isomers: i-butylene, trans-2-butylene, cis-2-butylene, and 1- butylene. In the second route, the n-butanol transforms to butanal via dehydrogenation reaction over extra framework alumina of HZSM-5. The butanal subsequently converted to propylene and propane over the metallic center of catalyst via dehydroformylation and decarbonylation reaction, xi respectively. The dehydrogenation route was negligible over the HZSM-5 catalyst. BUY isomers and propylene further transformed to C5-C12 olefins via oligomerization reaction over acidic sites of the zeolite. A fraction of the C2-C12 olefins undergo hydrogenation reaction forming C2-C12 paraffins. The C6-C12 olefins further undergo cyclization reaction leading to formation of C6-C12 cyclic paraffins (naphthene). The cyclic paraffins are, then, readily transformed to aromatics by dehydrogenation reaction. The aromatics formed during the reaction subsequently undergo (a) transalkylation, (b) disproportionation, and (c) skeletal isomerization under the reaction conditions over acidic sites of the catalyst leading to distributed aromatics in the product. The hydrocarbons formed from n-butanol were broadly lumped into five different categories based on their probable applications for simplicity of discussion: (i) oxygenates (DBE and butanal), (ii) light hydrocarbons (C1-C2), (iii) LPG range hydrocarbons (C3-C4), (iv) gasoline range hydrocarbons (C5-C12), and (v) aromatics. During the atmospheric pressure study, the n-butanol selectively converted to BUY at about 473 K with low WHSV over HZSM-5 under atmospheric pressure. The optimal operating condition for selective conversion of nbutanol to aromatics-free gasoline range hydrocarbons (with ~55% selectivity) was found to be 523 K with low WHSV (0.75 h-1 ). The gasoline range hydrocarbons were mainly composed of C5-C12 hydrocarbons centered on C8. The most suitable operating condition for the production of BTEXrich aromatics (~75% of the aromatic fraction) was 673 K with low WHSV (≤ 2.99 h-1 ). The percentage of benzene, toluene, and combined ethylbenzene and xylenes in BTEX fraction was 19.0, 47.0, and 34.0%, respectively. The distribution of p-xylene, o-xylene, m-xylene, and ethyl benzene in C8 aromatics was 62.0%, 21.0%, 12.0%, and 5.0%, respectively at 673 K and 0.75 h-1 . The reaction predominantly proceeds through dehydration route at xii low reaction temperatures below 523 K. The dehydrogenation route, however, becomes prominent beyond 523 K. The selective conversion of n-butanol to gasoline-range hydrocarbons was performed over the HZSM-5 catalyst at high pressure. The selectivity to gasoline-range hydrocarbons (C5-C12) increased with increasing pressure up to 20 bars and temperature up to 543 K and with decreasing WHSV. The selectivity to gasoline-range hydrocarbons remained practically constant beyond 20 bars. The selectivity to C3-C4 hydrocarbons decreased with increasing pressure up to 20 bars and temperature and decreasing WHSV. The optimum reaction conditions for maximum yield of aromatics-free gasoline range hydrocarbons were 20 bars, 543 K, and WHSV of 0.75 h-1 . The maximum selectivity to gasoline-range hydrocarbons was found to be 80.0% with about 11.0% and 9.0% selectivity to C3-C4 paraffin and olefins, respectively. The selectivity to C5-C12 hydrocarbons decreased slightly with increasing time-on-stream (TOS) with a concurrent increase of selectivity to C3-C4 hydrocarbons. The physicochemical properties and distillation characteristics of the virgin liquid product and hydrogenated liquid product were compared with petro-gasoline. The boiling temperature of the hydrogenated liquid product deviated significantly from petro-gasoline. The selective conversion of n-butanol to the aromatics was carried out at high pressure over various solid acid catalysts (HZSM-5, H-Beta, and Al2O3). The yield of aromatics and other hydrocarbons depends on the acidity of zeolites. The HZSM-5 showed consistent catalytic activity up to 28 h of TOS. The maximum selectivity to aromatics was found to be 49.2% (29.4% BTEX) under 20 bars pressure at 623 K and 0.75 h-1 WHSV. 20 bars and 623 K with low WHSV is thus considered as the optimum reaction conditions for maximum selectivity to aromatics.

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