**8. Catalyst deactivation**

In almost every publication concerning CFP catalyst or process development, investigators have identified catalyst deactivation to be the single greatest challenge to this process. The catalyst lifetime can be as short as minutes. There are three types of deactivations and of which, two are irreversible deactivations. The most common form of deactivation results from coke buildup on acid sites within the zeolite pores and renders the catalyst inactive [116]. Coking is most rapid when the catalyst/biomass ratio is less than one and particularly at low SAR. The second type of deactivation results from metal oxides deposited in zeolite which can result

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

in irreversible deactivation if these salts are not removed [117]. Thirdly, zeolite dealumination can occur and result in irreversible deactivation.

Coke deactivation most rapidly occurs among highly acidic zeolites of low silica/ alumina ratio (SAR) [118]. Ironically, low zeolite SAR are most active for aromatics [82, 119]. Individual lignocellulosic components caused deactivation differently [120]. Depolymerized lignin mainly deactivates by coking on zeolite surfaces because lignin monomers (mostly phenolics) cannot diffuse into the microchannels of HZSM-5 [121]. Cellulose pyrolysis products are responsible for coking within the interior of the zeolite and this leads to occlusion [121]. A study into deactivation mechanism was conducted from in situ CFP pine wood [121]. Fresh catalyst produced mainly aromatics and olefins with no detectable levels of oxygenates. As the campaign progresses, the catalyst/biomass ratio began to decrease, and oxygenates began to breakthrough, including phenolics. When the catalyst/biomass ratio reached 1:3, complete deactivation will have occurred.

Not all oxygenates deactivate HZSM-5 at the same rate [122]. One team examined the effects of upgrading ethanol-only, ethanol+acetic acid, ethanol+ethyl acetate, and ethanol+acetaldehyde. Acetic acid in the vapor strongly adsorbed onto zeolite surfaces and promoted formation of polyaromatic hydrocarbon precursors to coke. Acetaldehyde deactivated 10X more than ethanol. While ethyl acetate did not directly adsorb onto catalyst surfaces, the reaction conditions rapidly hydrolyzed ethyl acetate to acetic acid which then immediately deactivated the catalyst.

An extruded HZSM-5 (a.k.a. "technical catalyst") containing an alumina binder was also examined in its deactivation in ex situ CFP of cellulose and pinewood [44]. CFP of cellulose generates smaller oxygenates, which diffused into the zeolite to produce a "catalytic coke", made via a ring-growth mechanism involving H-transfer at temperatures above 200 °C [123]. Initial formation of monoaromatics led to formation of polyaromatic hydrocarbons, coke precursors. It is not surprising to learn that the coking mechanism of oxygenates is similar in pathways to coke formation in petroleum refining [118]. Lignin forms "thermal coke" from condensation of lignin monomers, and easier to remove because they accumulate on the surfaces of the zeolite [118]. High temperature combustion readily regenerates coke-deactivated HZSM-5.

The presence of water liberated from the pyrolysis can accumulate in the catalyst [118, 124]. Later, at high regeneration temperature, steaming can result in dealumination of the zeolite via leaching. In addition to removal of strong acid-site aluminum, there can be a loss of surface area even after one regeneration cycle [44, 51]. As a further complication, during the regeneration, hot spots can appear and irreversibly damage the catalyst. Successive regeneration increases the risk of further dealumination and reduction in micropore volumes, particularly at high regeneration temperature of 650–670 °C [118]. Therefore, these investigators recommended limiting the O2 concentration to 15% and include 5% steam to preserve catalyst lifetime [125]. Use of this method was found to preserve catalyst stability for up to 30 regeneration cycles. The formation of aromatics and olefins was stable at about 31.3% [125].

Alkaline and alkaline earth metal ions (AAEMs) are part of the plant's nutrient to enable the plant to grow [15, 126]. Lignocellulose is comprised of four different types of materials: lignin, cellulose, hemicellulose, and inorganics. AAEMs and other metal salts are represented by the inorganics fraction. A measurement of ash content provides a direct correlation of mineral concentration in the feed. Mechanistically, alkali metal salts also ion-exchanged with the proton of Brønsted acid sites and this reduces zeolite activity [118]. This was demonstrated from an onpurpose potassium ion exchange with HZSM-5 (up to 2.85 wt%) which resulted in drastic reduction of catalyst acidity, leading to lower yields of carbons [127]. Within the pyrolysis process itself, AAEMs can catalyze unwanted secondary reactions,

including cracking, decarboxylation, and dehydration, resulting in a loss of liquid yields [34]. Carbon oxides and water were increased by 10% due to these inorganic components, changing the product distribution [118, 128]. Among the metal ions that are highly detrimental to the process are K > Na > Ca, and the concentration of potassium in the feed is inversely proportional to aromatics [129]. Furthermore, Xiao et al. studied the effect of potassium salts on CFP of a model feed, 2-methylfuran [130]. The experiments were conducted by deposition of potassium salts into HZSM-5. Neutral KCl was less harmful than basic K2CO3 which produced lower yields of aromatic hydrocarbons and gases. The negative effects were due to loss of acidity and erosion of physical structure of HZSM-5, particularly when the K2CO3 concentration was as high as 1.0 wt%.

Magnesium is also present, but it is not as harmful as the other metal ions. Other inorganic ions that need to be monitored include Cl, S, P, and N [131]. Particularly, CFP of guayule, a perennial woody shrub, can produce upgraded products containing 80 to 700 ppm sulfur which can be detrimental to any sulfur sensitive downstream processes [132].

Because of the nature of the process, it is in situ CFP that is most sensitive to AAMEs in the feed. Even at levels as low as 0.1 wt% in the feed, AAEMs can greatly impact the pyrolysis process. Aggravating this situation is the variation of feedstock which also varies the inorganic component concentrations. Switchgrass as a feed results in high ash content [118, 131]. Versus pinewood (0.49 wt%), Switchgrass can contain around 2.6 wt% ash comprised of metal oxides. Crop selection is crucial to preserve performance of CFP [15, 133]. Feed pretreatment can be an option to minimize the deleterious effects of AAEMs.
