**10. Catalytic Co-pyrolysis (CCP)**

Investigators recognized that pyrolysis produced improved product quality when a second feed was added to the process [22, 24]. The co-feed can include coal, plastics, tire, and sludge. N.Y. Chen et al. first reported in 1986 that the hydrogen to carbon effective ratio (H/Ceff) can predict the outcome of pyrolysis [177]. This ratio is calculated according to the following formula (Eq. (2)).

$$\frac{\mathbf{H} / \mathbf{C}\_{\mathrm{eff}} = \mathbf{H} - \mathbf{2} \left( \mathbf{O} \right)}{\mathbf{C}} \tag{2}$$

When this ratio is less than one, coke is expected to be made in high yields. For example, glucose has a ratio of zero, sorbitol has 0.333, glycerol has 0.667, xylitol has 0.400 and petroleum is greater than two [160]. As a feed, lignocellulose is highly deficient in hydrogen. It might be plausible to increase this ratio by

### *Catalytic Fast Pyrolysis of Lignocellulosic Biomass to Benzene, Toluene, and Xylenes DOI: http://dx.doi.org/10.5772/intechopen.99946*

co-feeding a material rich in hydrogen [19, 27]. By increasing this ratio, not only is coke reduced, but higher yields of aromatics and olefins result. Other means to increase this ratio is to pretreat the feed by hydrogenolysis [178]. An ex situ copyrolysis strategy to increase BTX yield was cleverly conducted by removing BTX from the product stream to leave behind polyaromatic hydrocarbons. This high molecular weight material was then hydrogenated to produce polycyclic aliphatic hydrocarbons. This hydrogenated fraction was then co-pyrolyzed with glycerol to produce higher yields of BTX [176].

The upgrading of furanics to aromatics was benefitted by the co-addition of olefins [42, 119, 163, 177, 179, 180]. While co-addition of ethylene and propylene was effective to produce more aromatics, investigators recognized that the process must now incorporate flammable gases. An alternative to olefins utilization was to co-add methanol to the pyrolysis. Under the upgrading conditions, methanol to in situ olefins can occur to produce the dienophiles required for furanic Diels Alder reactions. Therefore, 2-methylfuran (2-MF) and methanol co-addition resulted in boosting aromatics yield while coke was reduced [168]. When the MeOH/2-MF ratio was increased from zero to 3:1, 2-MF conversion was increased from 39.8% to as high as 96.5% at an optimal ratio of 2:1. Of course, not all furanics were equally converted. A comparison of 2,5-dimethylfuran (DMF), furan, and furfural revealed that electron-rich DMF quickly reacted while the electron-deficient furfural lagged behind: DMF > furan > furfural [94]. In addition to aromatics, CO2, CO, and other olefins were also made at 400–600 °C [38]. Unavoidably, formation of graphitic coke on catalyst surfaces resulted in its deactivation within 30 minutes. Methane was examined as a co-feed in the catalytic upgrading [164]. This co-feed was beneficial, an indication that methane could be activated by this catalyst to yield more aromatics, but coke formation also increased.

CCP was applied to a mixture of sawdust and methanol. This combination reduced coke and char and improved aromatics yields [77, 93, 181]. However, the researchers could not exclude the possibility that a background methanol to aromatics process was also occurring. Another team examined CCP of other co-feeds such as methanol, polyethylene terephthalate (PET), and calcium formate with pinewood [182]. Of these three, calcium formate was found to be most effective at calcium formate/pinewood of 3:1. In fact, co-feeding calcium formate did not produce a large increase in aromatics vs. control (10.65% vs. 9.23%), but this co-pyrolysis reduced the polyaromatic hydrocarbons yield (1.94% vs. 2.49%). The authors speculated that the calcium salt helped to maintain catalyst activity within the hydrocarbon pool of the zeolite.

A patent was granted to Anellotech for co-feeding oxygenates such as acetone into pyrolysis to enhanced pX yield [183]. The oxygenates utilized are low value byproducts from other industrial processes. Such oxygenates include acetone from the phenol process, furfural, hexanol, and hexanoic acid from other sources. Furfural was less effective, but the formation of xylenes was enhanced by hexanol and hexanoic acid co-additions. Other oxygenates include fermentation products [184]. Co-pyrolysis of citrus unshiu peel and alcohols, ketones, and furanics has been reported [119]. In this case, two different zeolites were compared: HZSM-5 (23) and HBEA (25). The key difference between both zeolites was that HBEA produced more coke.

Tail Gas Reactive or Recycle Pyrolysis (TGRP) is an interesting process that recycles non-condensable pyrolysis gases to the pyrolysis step [150, 166, 185, 186]. Recycling this gas to pyrolysis provides a reductive, low acidic atmosphere which benefits CFP by increasing the H/Ceff. The condensable gas fraction was separated, including unwanted carboxylic acids and furfural which are detrimental to vapor upgrading. Up to 10X more BTX was produced when TGRP was incorporated in comparison to CFP-only.

Other non-conventional processes could also improve CFP yields. One example is addition of ball bearings to the catalyst bed to increase residence time which provided higher hot surface areas [187]. This modification increased cracking reaction and produced 3X more BTX. The downside is formation of more polyaromatic hydrocarbons. An alternative to thermal heating, microwave energy can also be used. Microwave assisted pyrolysis (MOP) does not require agitation and fluidization [56, 188].
