Catalytic Pyrolysis of Char and Coke Precursors: An Investigation of the Formation Mechanisms of Char and Coke

Catalytic pyrolysis of char and coke precursors: an investigation of the formation mechanisms of char and coke

Shoucheng Du, David P. Gamliel, Julia A. Valla, George M. Bollas

Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT

Abstract

Fast catalytic pyrolysis of lignocellulosic biomass, in the presence of proper pore structure zeolite catalysts, such as ZSM-5, has proven to provide enhanced bio-oil selectivity to aromatic hydrocarbons. [1-3] However, not all carbon content in biomass is converted to bio-oil, due to the production of carbon oxides (CO and CO₂), olefins and solid residue (char and coke). The production of solid carbonaceous residue not only results in the loss of carbon from the product bio-oil, but also causes catalyst deactivation, which largely affects the product distribution and bio-oil selectivity. Control of coke and char formation during pyrolysis could be possible through innovative catalyst and process designs, in which the fundamental understanding of the formation mechanisms of char and coke should be viewed as a prerequisite. The latter requires insights into coke and char structural characteristics and formation chemistry. Brewer et al. [4] proposed a structure of char formed by slow and fast thermal pyrolysis of switch grass using ¹³C-NMR analysis. They showed that aromatic clusters of 7-8 rings terminated by carbonyl and hydroxyl groups are representative of the composition of thermal char. Valle et al. [5] studied catalytic upgrading of bio-oil with methanol over ZSM-5. They showed two origins of coke, thermal and catalytic, by performing temperature programmed oxidation (TPO). They also observed that catalytic coke is deposited mainly inside the zeolite crystal channels; whereas thermal coke is deposited mainly outside the zeolite crystals. Cheng and Huber [6] studied catalytic pyrolysis of furan over ZSM-5. They found the soluble coke mostly consists of aromatic rings and carbonyl groups. They also identified the molecular weight distribution of soluble coke by using gel permeation chromatography (GPC) and concluded that the maximum molecular weight of soluble coke is beyond the limitation of a GPC column (M_w >10⁴).

Figure 1 Some reaction pathways for coke and char formation. (a) RA 每 toluene self-alkylation via RA.1 (alkylation), RA.2 (dehydrogenative coupling), RA.3 (iso- merization), and RA.4 (hydrogen transfer and repetition of RA.1每RA.5); (b) RB 每 coke formation via RB.1 (alkylation on the nucleus with carbenium ions), RB.2 (side alkylation and isomerization), RB.3 (cyclization), and RB.4 (repetition of RB.2, RB.3); (c) RC 每 char formation from furfural via RC.1 (Diels每Alder with propylene), and aldol condensations (RC.2每RC.7)

Research efforts have focused on understanding the reaction mechanisms of biomass catalytic pyrolysis, and those of coke and char formation. Figure 1 illustrates some pathways (RA, RB and RC) for the formation of char and coke as identified previously [7]. For instance, when toluene is chosen as the precursor, coke forms under self-alkylation reactions [8] and/or side alkylation with propylene [9]. Starting from furfural, formation of char may undergo Diels-Alder reactions to form tolualdehyde, followed by a series of Aldol condensation steps.

Figure 2 Major (a) and secondary (b) product distributions from catalytic pyrolysis of toluene at 600 蚓, 1-5 catalyst to toluene ratios

An effective way to study reaction mechanisms is the employment of model compounds [6,10每13]. In this study, toluene and tolualdehyde are used as model compounds for the identification of reaction pathways, responsible for coke and char formation, and the role of the catalyst. Figure 2 shows the major product distribution (a) and more detailed liquid product selectivity (b) from the catalytic pyrolysis of toluene at 600 蚓 and varying catalyst to feed ratio. Specifically, as the catalyst to toluene ratio increases, the coke yield increases, whereas the yields to benzene, xylene and other aromatics, such as 1-methyl-9H-fluorene and 9, 10-dihydro-1-methyl-phenanthrene, decrease. This suggests that the aforementioned liquid products act as intermediates for the formation of coke during catalytic pyrolysis, which is consistent with the previously proposed mechanisms (RA). This also shows that the production of aromatics is competitive to coke and char formation. Moreover, the yield of 1-methyl-pyrene decreases with increasing coke yield, which implies that 1-methyl-pyrene is the largest coke precursor detectable by GC-MS. Furthermore, elemental analysis of the coke/char from catalytic pyrolysis of tolualdehyde with acetone under different catalyst to feed ratios show that as the catalyst to feed ratio increases, catalytic pyrolysis of tolualdehyde with acetone produces coke/char of higher H/C ratios. This translates to less fused aromatics with more side chains [4,7], or generally less condensed aromatic structures in the coke/char.

As indicated, liquid product analysis reflects the reaction intermediates in coke/char formation and the elemental analysis implies the structure of final coke/char products. Combining these analyses with other techniques, such as Raman spectroscopy and solid-state NMR, more detailed information on the reaction pathways towards coke and char is revealed. This allows for a comprehensive identification of the most significant pathways to coke and char in biomass catalytic pyrolysis. It also confirms that aromatics selectivity in biomass catalytic pyrolysis is directly competitive to coke and char formation.

Acknowledgement

The study is funded by the National Science Foundation Award CBET-1236738.

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