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Upgrading of Biomass via Catalytic Fast Pyrolysis (CFP)

Charles A. Mullen

USDA-Agricultural Research Service, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, PA, USA

1.1 Introduction

Defined as heating of an organic material in a nonoxidative environment, pyrolysis has been recognized for decades as the most efficient process for converting lignocellulosic biomass into a dense liquid, commonly called pyrolysis oil or bio-oil [1-3]. The most commonly used conditions for conversion of biomass to liquid have been high heating rates to temperatures of around 500?°C, at atmospheric pressure, the so-called fast pyrolysis process [1-3]. The fast pyrolysis process offers many advantages that make it attractive for conversion of biomass to bio-fuel intermediates and production of renewable chemicals. These advantages include high liquid yields (>60% in some cases) and production of a potentially valuable coproduct in bio-char. This solid, consisting of fixed carbon and minerals, has been shown to be a good soil amender and a potential route to sequester carbon [4-6]. With the potential utilization of the combustible off gases, and if needed some of the bio-char, pyrolysis can be powered by its own energy, making it a nearly self-sufficient process requiring few other inputs [3].

Bio-oil contains hundreds of oxygenated compounds derived from the cellulose, hemicellulose, and lignin that comprise the biomass. In recent years, much has been made of bio-oil as a potential intermediate to the production of advanced hydrocarbon transportation fuels or as a feedstock from which to isolate renewable chemicals. However, commercial or even precommercial success for utilization of these bio-oils has been limited to lower value applications such as use as boiler-type fuels for heat and power [3,7] or utilization as an asphalt-like material [8]. The technical reason for these limitations is that the composition of the bio-oil, comprising high concentrations of reactive oxygenated functional groups, plus the presence of catalytic microsolids, makes the mixture thermally unstable [9-11]. Therefore, processing technologies requiring even moderate heating of the bio-oil mixture, such as distillation, result in production of intractable materials [12]. While catalytic hydrodeoxygenation (HDO) has been the post-production upgrading choice for refining of bio-oil to hydrocarbons to be used as fuels, the unstable nature of the bio-oil also makes this HDO process difficult. The most effective post-production deoxygenation processes developed require multiple catalytic, high pressure hydrotreating steps, at significant cost, making the production of a low margin fuel product of questionable economic viability [13-15].

Because of these limitations, researchers have sought to develop processes that alter the chemical pathways during pyrolysis to produce a more stable bio-oil product with more favorable compositions for various end-use applications, including HDO. Utilization of heterogeneous catalysts during the pyrolysis process, termed catalytic fast pyrolysis (CFP), has received the most attention. Because of the interest in advanced hydrocarbon bio-fuels, the most common goal of catalytic pyrolysis has been to produce a partially deoxygenated, thermally stable pyrolysis oil that is more amenable to final HDO-type upgrading to fuel-range hydrocarbons. However, alternative processes have aimed to converge chemical pathways toward production of various individual compounds or groups of compounds for petrochemical or fine chemical uses. In this chapter we will discuss CFP processes both aimed at general deoxygenation and those aimed at targeted classes of molecules. For the purposes of this chapter, we will consider catalytic pyrolysis processes that fall within the following definitions: (i) heterogeneous catalytic processing of biomass pyrolysis vapors either in situ (pyrolysis and catalysis occur in the same reactor zone) or ex situ (pyrolysis and vapor phase catalysis are decoupled) and (ii) reactions taking place in inert or reactive (but non-oxidative) atmospheres at near atmospheric pressure. Therefore, catalytic hydrothermal or solvent liquefaction [16] and other technologies such as pressurized hydropyrolysis [17] are outside the scope of this chapter.

As mentioned earlier, CFP encompasses processes where solid biomass contacts the catalyst and both pyrolysis and vapor upgrading occur in the same reactor, the so-called in situ methods, and processes where the pyrolysis and catalytic vapor upgrading occur in separate reactors, called ex situ or vapor upgrading methods. Simplified schematics of the two processes are presented in Figure 1.1. The main advantage of the in situ method is its simplicity - the "one-pot" reaction system saves capital cost as it does not require a second reactor. However, there are several advantages that decoupling the two steps allows for [18]. While it is well known that temperatures in the 500?°C range produce the maximum yield of condensable range species from pyrolysis of most biomass, the ideal range for various catalytic processes may be significantly different, depending on the catalyst and the desired end products. Furthermore, because the in situ process requires contact of solid biomass with solid catalyst, reactors for the unit operation tend to be limited to only fluidized beds. Decoupling the process allows the catalysis step to occur in either a fixed or fluidized bed and also allows for other various reactor types to accomplish the pyrolysis. Another very important factor is that the ex situ process allows for removal of bio-char and its associated metal content prior to introduction of the catalyst. This has important implications for catalyst lifetimes as inorganic materials contained in the biomass, particularly Group 1 and Group 2 metals, can poison catalysts, leading to more frequent catalyst replacement or replenishment and a higher catalyst demand. This is an especially important consideration for zeolite-catalyzed processes because they are highly susceptible to this type of deactivation, as described later.

Figure 1.1 Simplified process schematics comparing (a) in situ and (b) ex situ catalytic pyrolysis.

1.1.1 Catalytic Pyrolysis Over Zeolites

Zeolites and similar materials have been by far the most common catalysts employed in CFP processes. Zeolites are crystalline substances with a structure characterized by a framework of linked tetrahedra, each consisting of four oxygen atoms surrounding a cation [19-21]. Zeolites occur naturally, but the advent of synthetic zeolites in the 1950s, free of the defects and impurities found in nature, is when their use in chemical catalysis took off [20]. Industrial scale catalytic use of zeolites started in 1962, with the use of zeolites X and Y for fluid catalytic cracking (FCC) of heavy petroleum fractions, a process that is still today one of the highest volume chemical processes used. Other petrochemical uses include hydrocracking, isomerization, disproportionation, and alkylation of aromatics. The framework of zeolites is usually silica and alumina based. This framework contains open cavities in the form of channels and cages [19]. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable [19]. In silica-alumina-based materials, the need for the extra-framework cation is to balance the charge imbalance created by the four coordinate Al(III) sites, as opposed to the neutral Si(IV) sites (Figure 1.2) [19-21]. Often, in the active catalysts, some or all of the extra-framework cations are Brønsted acid (H+) active sites, which along with the Lewis acidity of other sites are responsible for initiating the catalytic reactions. The channels allow the passage of guest species to these active sites [19-21]. In biomass CFP these guest species are molecules derived from the breakdown of the biopolymers caused by the initial pyrolysis reactions. The pore size and shape, Brønsted acid strength and site density, and the presence of other types of active sites are important factors in the activity and selectivity of the catalysts.

1.1.1.1 Catalytic Pyrolysis Over HZSM-5

Several different types of zeolites have been tested as catalysts for the deoxygenation of pyrolysis vapors including Zeolite Socony Mobil-5 (ZSM-5), Y, beta, mordenite, and ferrierite [22-26]. Among these types of zeolites, the ZSM-5 (also called MFI for mordenite framework inverted)-based materials have shown the most promise and received the most attention and will be the focus of this section. ZSM-5-type zeolites have been shown to selectively convert molecules from a wide variety of sources to aromatic hydrocarbons. Among these processes are methanol to olefins and gasoline and cracking of waste hydrocarbon plastics [27,28]. Aromatic...