Catalytic pyrolysis of Napier grass with nickel-copper core-shell bi-functional catalyst

Highlights

• The Ni-Cu/mCMs catalysts gave a high efficiency for catalytic pyrolysis of Napier grass.

• The Ni-Cu/mCMs catalyst remarkably increased alkyl-phenols up to four-fold.

• The oxygen removal was about 24 % more than in the non-catalytic bio-oil.

Abstract

Nickel-copper (Ni-Cu) core-shell catalysts supported on phosphorus-modified carbon microspheres (mCMs) were prepared at three Ni/Cu mass ratios, and applied to the catalytic pyrolysis of Napier grass (Pennisetum purpureum) under atmospheric pressure. The catalytic pyrolysis was carried out inside a custom-built one-shot two-stage fixed-bed reactor. The catalyst structure was confirmed by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Remarkably, the Ni-Cu/mCM catalysts increased the produced alkyl-phenols by up to fourfold, and these products are an excellent gasoline blend stock due to their high blending octane number. At the same time, the mass of undesired compounds in the bio-oil including alcohols, ketones, and furan were markedly decreased compared to the case without a catalyst. The oxygen content of the bio-oil was reduced by up to 24 % compared to the non-catalytic case. The hydrogen/carbon and oxygen/carbon molar ratios were also improved after all catalytic treatments. Finally, a reaction mechanism was proposed, including the pathways of oxygen removal through dehydration, decarbonylation, and dealkoxylation. The presence of NiO in the Ni-Cu/mCM catalyst promoted the alkylation reaction to yield more alkyl-phenols.

Introduction

Nowadays, the demand for energy especially in transportation fuels is increasing rapidly, due to the growth of world population and economic development [1]. This demand is expected to keep increasing and double before 2050 [2]. At the same time, the reliance on fossil fuels has led to serious environmental concerns. There is a strong desire for alternative fuels that can replace conventional ones, be compatible with the existing infrastructure, and reduce the environmental impact including the carbon dioxide (CO₂) emission [1]. Recently, the upgrading of lignocellulosic biomass has attracted much research interest, since this method could generate fuels from renewable resources (mainly waste plant materials such as forestry wastes, energy crops, and agricultural residues) without competing with food production [3]. Napier grass (Pennisetum purpureum) is one of the best potential energy crops for efficient and economic bioenergy systems due to its valuable properties, such as a high biomass yield, compatibility with conventional farming practices, the ability to outcompete weeds, very little or no need for supplementary nutrients, and producing up to four harvests a year [4].

An important method to produce liquid fuel from biomass [5] is pyrolysis, in which the biomass is heated at a fast rate to 500–600 °C in the absence of oxygen and with a short vapor residence time. Compared to conventional diesel derived from petroleum, bio-oils generated by pyrolysis can come from a variety of biomass feedstocks, are easier to handle and transport, and have lower nitrogen and sulfur contents [6]. However, the high content of oxygen compounds in the pyrolysis bio-oil results in undesirable properties including a high acidity, a low heating value, corrosiveness, high viscosity, and immiscibility with hydrocarbon fuel. Recently, fast hydropyrolysis in a hydrogen (H₂) atmosphere has been developed to overcome these drawbacks of conventional fast pyrolysis. The produced bio-oil displayed improved stability, lower oxygen/carbon (O/C) molar ratio, and a higher content of saturated hydrocarbons [7,8]. However, deoxygenation of the bio-oil must be performed before using it as transportation fuel or blending feedstock.

Hydrodeoxygenation (HDO) is one of the common processes to selectively remove oxygen from bio-oil to produce renewable liquid fuels (including gasoline and diesel), by saturating the CO bonds in the presence of H₂ and a suitable catalyst [9]. The product of HDO has even better quality than conventional petroleum-based transportation fuel [10,11], making this process stand out from other deoxygenation processes. However, the existing HDO reaction of bio-oil or other model compounds is usually performed in a batch reactor operated under a very high H₂ pressure, which increases the costs of equipment and operation [1].

Bi-functional metal/acid catalysts are known to have outstanding efficiency in the HDO of phenolic compounds in bio-oil. The transformation of phenolic compounds over the metal and acid sites of these bi-functional catalysts lies in the different types of pathways. In general, metal sites promote hydrogenation reactions while acid sites promote hydrogenolysis/dehydration reactions [[12], [13], [14]]. Therefore, the bi-functional catalysts are very promising for the HDO of bio-oil that contains a variety of compounds [15]. Compared to noble metals and other transition metals, nickel (Ni) is a good candidate for HDO catalysts due to its high hydrogenation activity, alloying efficiency, and relatively low cost [16]. Meanwhile, copper (Cu) was found to be a good promoter when used with Ni, by preventing the methanation reaction of oxy-organics at a higher temperature, hindering the activity for CC bond breaking, lowering the reduction temperature of Ni, and preventing excessive carbon deposition on the Ni [17]. As a result, the Ni-Cu catalyst showed high catalytic activity in the HDO of some model compounds, such as guaiacol, phenol, and anisole [[18], [19], [20]].

Carbon spheres are a new interesting carbon material that has attracted a high level of interest in many fields, such as adsorbents, electrodes, and H₂ storage. Compared to activated carbon, they have high wear resistance, high mechanical strength, good fluidity, high bulk density, as well as allow controllable surface modification [21]. Furthermore, carbon spheres could be produced from waste biomasses or waste materials that contain carbohydrate precursors, such as glucose, fructose, starch, and xylose [22]. However, fresh carbon microspheres (CMs) are not suitable for use as a catalyst support due to their very poor porosity and low surface area. Meanwhile, there are very few reports on the modification of CMs for use as a catalyst support.

This research aimed to develop new and highly efficient bi-functional Ni-Cu catalysts supported on phosphorous modified CMs (mCMs) for the catalytic pyrolysis of Napier grass under atmospheric pressure in a custom one-shot two-stage fixed-bed reactor.