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Catalytic Fast Pyrolysis of Lignin Isolated by Hybrid Organosolv—Steam Explosion Pretreatment of Hardwood and Softwood Biomass for the Production of Phenolics and Aromatics

Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Chemical Process and Energy Resources Institute, Centre for Research and Technology-Hellas (CPERI/CERTH), 57001 Thessaloniki, Greece
Biochemical Process Engineering, Division of Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, 971-87 Luleå, Sweden
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(11), 935;
Submission received: 11 October 2019 / Revised: 31 October 2019 / Accepted: 2 November 2019 / Published: 8 November 2019
(This article belongs to the Special Issue Recent Advances in Catalytic Sustainable Processes in Biorefineries)


Lignin, one of the three main structural biopolymers of lignocellulosic biomass, is the most abundant natural source of aromatics with a great valorization potential towards the production of fuels, chemicals, and polymers. Although kraft lignin and lignosulphonates, as byproducts of the pulp/paper industry, are available in vast amounts, other types of lignins, such as the organosolv or the hydrolysis lignin, are becoming increasingly important, as they are side-streams of new biorefinery processes aiming at the (bio)catalytic valorization of biomass sugars. Within this context, in this work, we studied the thermal (non-catalytic) and catalytic fast pyrolysis of softwood (spruce) and hardwood (birch) lignins, isolated by a hybrid organosolv–steam explosion biomass pretreatment method in order to investigate the effect of lignin origin/composition on product yields and lignin bio-oil composition. The catalysts studied were conventional microporous ZSM-5 (Zeolite Socony Mobil–5) zeolites and hierarchical ZSM-5 zeolites with intracrystal mesopores (i.e., 9 and 45 nm) or nano-sized ZSM-5 with a high external surface. All ZSM-5 zeolites were active in converting the initially produced via thermal pyrolysis alkoxy-phenols (i.e., of guaiacyl and syringyl/guaiacyl type for spruce and birch lignin, respectively) towards BTX (benzene, toluene, xylene) aromatics, alkyl-phenols and polycyclic aromatic hydrocarbons (PAHs, mainly naphthalenes), with the mesoporous ZSM-5 exhibiting higher dealkoxylation reactivity and being significantly more selective towards mono-aromatics compared to the conventional ZSM-5, for both spruce and birch lignin.

1. Introduction

Lignocellulosic biomass is nowadays considered to be an important renewable source for the production of fuels, energy, chemicals, polymers, and other products, with the (bio)catalytic processes playing a major role [1,2,3,4]. Lignocellulose comprises of two polysaccharides, cellulose (30–50 wt.%) and hemicellulose (15–30 wt.%), and an amorphous phenolic polymer, lignin (10–30 wt.%) [3,5,6,7]. Lignin is formed via dehydrogenative polymerization of three phenylpropane units (monolignols), i.e., sinapyl, coniferyl, and p-coumaryl alcohols, which are linked via ether and C–C bonds and correspond to the syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) building blocks in the structure of lignin, respectively [8,9,10,11,12,13,14]. The most abundant types of bonds in lignin structures are ether type linkages, like β-O-4 (40%–50%), and others, such as α-O-4, 5-5, β-5, β-1, dibenzodioxocin, spirodienone, and β-β linkages [8,10,15]. The composition and structure of lignin depend on the type and nature of plant biomass. Softwoods, such as spruce, pine, etc., contain 20 to 30 wt.% lignin, hardwoods, such as birch, beech, poplar, etc. 15 to 25 wt.%; and grasses, straw, and stover 10 to 20 wt.%. Likewise, softwood-derived lignins contain mainly coniferyl alcohol units, i.e., guaiacyl units. Hardwood lignins comprise of both guaiacyl and syringyl (sinapyl alcohol) (S) units, the latter having two methoxy groups, while lignin from grass also contains p-hydroxyphenyl (p-coumaryl alcohol) units in addition to G and S units [9,10,16].
Lignin is typically produced in vast amounts as a low value byproduct in the traditional pulp and paper industry (i.e., kraft and sulphite lignin) and is mainly utilized to generate heat and power through combustion. In more recent years, lignin has also been utilized in various sectors, such as an asphalt emulsifier, dispersant for clays, dyestuffs, insecticides, adhesive resins, soil upgrading agent, rubber reinforcing agent, adsorbent, biochar and carbon fibers, grinding agent for cement, viscosity reducer of heavy oil, surface active agent for oil extraction, lubricant additive, vanillin production, etc. [14,17,18,19,20,21,22]. In addition to the above classical technical lignins, the emerging biorefinery concept in recent years has generated the need for more efficient, sustainable, and environment-friendly biomass fractionation processes that will facilitate downstream conversion of carbohydrates and lignin to value-added products. For example, in the production of second-generation bioethanol, i.e., by utilizing lignocellulosic, mainly residual/waste, biomass, the initial pretreatment step is very important and aims to maximize the enzymatic hydrolysis of cellulose to glucose, which is then fermented to ethanol [23]. Within this context, two types of pretreatment have attracted increased interest due to their efficiency combined with green process characteristics, i.e., the hydrothermal (liquid hot water) [24,25,26] or steam explosion pretreatment [27,28,29] utilizing neat water/steam and the organosolv process [30,31,32] that uses mixtures of water with friendly solvents (i.e., ethanol) under relatively mild hydrothermal conditions (i.e., up to ca. 200 °C). Moreover, extraction of surface lignin with acetone and ethanol from hydrothermally pretreated biomass has recently been proposed as a method to overcome inhibitory effects imposed by lignin during enzymatic hydrolysis of cellulose [26]. With regard to the organosolv pretreatment, it was shown that the use of small amounts of acids (i.e., H2SO4 or H3PO4) may further improve the efficiency of the process [30,33]. More recently, a hybrid organosolv–steam explosion process was proposed for the pretreatment and efficient fractionation of biomass, leading to enhanced enzymatic hydrolysis and isolation of high-quality and purity lignin [34,35]. For example, by pretreating birch and spruce biomass with this method, solids with high cellulose (77.9 wt.% and 72 wt.%, respectively) and low lignin (delignification up to 86.2 wt.% and 79.4 wt.%, respectively) content were obtained. These solids exhibited high saccharification activity (yields up to 61% w/w for spruce and complete saccharification for birch), thus rendering them as ideal feedstocks for downstream (bio)catalytic valorization [34,35].
With regard to lignin depolymerization and further upgrading to phenolic and aromatic compounds, two of the most studied methods are catalytic hydrogenolysis and fast pyrolysis. The former usually takes place at low/moderate reaction temperatures (ca. 150–350 °C) under high H2 pressure (ca. 20–90 bar) and/or by using solvents that can act as hydrogen donors, in a neutral, basic, or acidic environment [36]. The most common catalysts comprise of noble and transition metals (i.e., Ru, Pd, Pt, Ni, Cu, etc.) supported on carbons, carbides, zeolites, and various oxides that are aimed at the scission of C–O (e.g., β–O–4 bonds) and C–C bonds towards the production of smaller phenolic fragments or phenolic, aromatic, and alkane monomers [8,37,38,39,40,41,42]. Fast pyrolysis is a thermochemical process that takes place at moderate/high temperatures of ca. 400 to 700 °C in the absence of oxygen/air. The process characteristics, i.e., fast heating and cooling rates, moderate temperatures, and short vapor contact times, favor the formation of a liquid product called bio-oil. In the case of lignin, the bio-oil mainly contains alkoxy-phenols and oxygenated aromatics, such as syringol, guaiacol, and their alkylated analogues, vanillin, syringaldehyde, 1,2,3-trimethoxy-benzene, and others. The composition of bio-oil is dependent on the origin of the lignin, i.e., the type of lignocellulosic biomass. Furthermore, it was recently shown that the composition of lignin, i.e., the type of building units, is depicted in the composition of the bio-oil, i.e., the bio-oil produced from a kraft softwood (spruce) lignin that was enriched in guaiacyl units contained only guaiacol-type (with one methoxy group) compounds [43]. Some gases, such as carbon dioxide, carbon monoxide, and methane, as well as solids (char), are also produced in lignin fast pyrolysis; the latter being in higher yields (ca. 40–50 wt.%) compared to the char produced in biomass fast pyrolysis (ca. 15–25 wt.%) [10,44,45,46,47]. Due to its phenolic nature and the relatively high homogeneity compared to the parent biomass fast pyrolysis oil, which in addition to phenolic compounds also contains furans, acids, esters, ketones, aldehydes, ethers, alcohols, and sugars due to cellulose/hemicellulose pyrolysis, lignin-derived bio-oil could be utilized in the production of phenol-based resins and polyurethanes, replacing petroleum phenol [48,49]. Furthermore, it can be subjected to hydrodeoxygenation (HDO) for the production of aromatics and cycloalkanes [50,51,52,53,54,55].
An alternative method for the deoxygenation of lignin bio-oil, avoiding the use of hydrogen, is the catalytic fast pyrolysis (CFP) of lignin, in analogy to the CFP of lignocellulosic biomass, where the initially produced biomass thermal pyrolysis vapors are further converted (in situ or ex situ) on the catalyst active sites [56,57,58,59,60]. Although different types of catalysts, i.e., acidic, basic, metal oxides, bifunctional metal-acidic, etc., have been studied in biomass CFP, leading to varying degrees of deoxygenation and bio-oil composition, the use of zeolites, mainly ZSM-5, induces deep deoxygenation via enhanced dehydration, decarbonylation, and decarboxylation reactions. Further transformation of the intermediate products via cracking, (de)alkylation, isomerization, aromatization, condensation, and oligomerization reactions on the zeolitic catalysts results in the production of a highly deoxygenated bio-oil that consists mainly of monocyclic aromatic hydrocarbons (i.e., benzene, toluene, xylenes, etc.), naphthalenes, and alkyl-phenols. The penalty is the increased production of water (aqueous fraction of bio-oil) and gases, as well as catalytic coke, at the expense of the organic fraction of bio-oil [57,60,61,62,63,64]. Similar effects have been observed in the case of lignin CFP, where the use of zeolitic catalysts, including ZSM-5, beta, mordenite, ferrierite, and USY (Ultra Stable Y), has mainly been investigated [43,63,65,66,67,68,69,70]. ZSM-5 has been shown to be very reactive towards aromatic compounds, due to its unique microporous structure and strong Brønsted acidity; however, USY zeolite, which possesses larger micropores than those of ZSM-5 zeolite, has also been shown to be quite efficient in the production of hydrocarbons, also exhibiting a relatively low formation of tar [68]. More recently, is has also been shown that nano-sized Al-MCM-41 mesoporous catalysts may exhibit a similar deoxygenation activity and yield of aromatics compared to strongly acidic microporous ZSM-5 zeolite in fast pyrolysis of alkali softwood lignin [71].
A more recent type of zeolitic catalysts, the so-called hierarchical zeolites, which exhibit both the improved diffusion properties of mesoporous materials and the strong acidity and stability of zeolites [72,73,74], has also been investigated in biomass CFP [75,76,77,78,79,80,81]. However, few studies have been reported so far on lignin CFP. In the work of Lee et al., it was shown that the production of mono-aromatics and polycyclic aromatic hydrocarbons (PAHs) with a mesoporous Y zeolite (prepared from USY) was substantially improved compared to a conventional Al-MCM-41, which mostly produced phenolics [82]. Li et al. compared a typical microporous ZSM-5 zeolite with an alkaline-treated mesoporous ZSM-5 and showed that with the latter catalyst, both aromatics and phenols were increased while the yield of char/coke was decreased [83]. Kim et al. compared a mesoporous MFI zeolite with an Al-SBA-15 mesoporous aluminosilicate and reported an enhanced formation of alkyl-phenols and mono-aromatics due to the strong Brønsted acidity of the mesoporous zeolite [84]. More recently, in our previous work, the catalytic fast pyrolysis of a kraft (softwood) lignin with different ZSM-5-based catalysts was shown, including a mesoporous ZSM-5 and a nano-sized ZSM-5 [43]. It was shown that the classical microporous ZSM-5 zeolites were slightly more selective towards mono-aromatics while the nano-sized and especially the mesoporous ZSM-5 exhibited, in addition to aromatics, a high selectivity towards alkyl-phenols. Furthermore, the nano-sized ZSM-5 induced lower yields of the organic bio-oil fraction and higher production of water, coke, and gases in comparison to the micro- and mesoporous ZSM-5 zeolites.
Within this context, in this work, we studied the thermal (non-catalytic) and catalytic fast pyrolysis of softwood (spruce) and hardwood (birch) lignins, isolated by the recently proposed organosolv—steam explosion pretreatment method [34,35], in order to further investigate the effect of lignin origin/composition on the product yields and lignin oil composition. Furthermore, the fast pyrolysis of two model compounds, i.e., guaiacol and syringol, was also investigated in order to elucidate possible reaction pathways in lignin pyrolysis. The catalysts studied were two microporous ZSM-5 zeolites with an Si/Al ratio equal to 11.5 and 40, meso-ZSM-5 zeolites with intra-crystal mesoporosity and different mesopore sizes (i.e., 9 and 45 nm) produced by alkaline treatment of commercial ZSM-5, as well as a nano-ZSM-5 zeolite produced by hydrothermal synthesis. Two pyrolysis set-ups were used, a pyrolyzer/gas chromatography-mass spectrometry (Py/GC-MS) system and a bench scale fast pyrolysis unit with a fixed bed reactor, while the obtained catalytic data were correlated and discussed in light of the catalysts’ textural and acidic characteristics, as well as the feed lignin properties.

2. Results and Discussion

2.1. Lignin Characteristics

The elemental analysis of the spruce- and birch-derived lignin samples, isolated by the hybrid organosolv—steam explosion method, and used as feedstock in the fast pyrolysis experiments is shown in Table 1. The content of C, H, and O was typical for this type of lignin while S and N contents were also very low [85,86,87,88]. The low content of S, as well as of inorganic ash, is a beneficial characteristic of organosolv-type lignins with regard to their valorization, compared, for example, to kraft lignin and lignosuphonates [43,67,89]. The spruce and birch lignins used in this study, which were isolated by the hybrid organosolv—steam explosion method, contained a very low amount of ash (<0.1 wt.%), as well as minimal carbohydrate impurities, i.e., hemicellulose (<2 wt.%) and cellulose (<1 wt.%) [34,35]. Their molecular weights were also in the range of previously reported values for similar organosolv-type lignins, with a relatively narrow molecular mass distribution as evidenced by the low polydispersity index (PDI) values [22,85,87,90,91].
The thermal decomposition profile of both lignins is depicted in the thermogravimetic analysis (TGA) and differential thermogravimetric analysis (DTG) curves shown in Figure 1. The initial weight loss of ~2% up to ca. 120 °C is due to the evaporation of humidity while the steep weight loss of ~51% for spruce lignin (DTG maximum 398 °C) and ~60% for birch lignin (DTG maximum 354 °C), in the range of ca. 180 to 600 °C, is attributed to the decomposition of lignin. The above differences in the weight loss and DTG maxima indicate the relatively higher stability of spruce-derived lignin. A further progressive limited weight loss was observed at higher temperatures, which can be attributed to enhanced condensation reactions and/or gasification of the formed carbonaceous material. The higher residual char at 950 °C produced by spruce lignin (~32%) compared to birch lignin (~25%) can also be related to the higher thermal stability of the former, as well as to enchanted repolymerization/condensation reactions that occur under the slow pyrolysis conditions of the TGA experiment.
The spruce and birch lignins, isolated by the hybrid organosolv—steam explosion method, were also characterized by 2D HSQC (Heteronuclear Single Quantum Coherence) NMR and the respective spectra that correspond to the inter-unit linkages and the types of aromatic units are shown in Figure 2. The HSQC cross peaks were attributed to specific aromatic units and linkages using previously reported data [92,93,94,95,96,97,98]. The assigned peaks are listed in Table 2 and the specific types of aromatic units (S, S’, G, G’, H) and inter-unit linkages (A, B, C, I, J, FA) are shown in Figure 3. The HSQC spectrum referring to the aromatic units of spruce lignin (Figure 2a; δCH 100–155/6.3–7.7 ppm) clearly shows that this lignin consists only of guaiacyl (G) units, as was expected due to the nature of spruce being a softwood. As a specific case of the guaiacyl units, a ferulate structure (FA structure) seems to be present to some extent (4.8 units per 100 Ar) since the C2–H2 and C6–H6 correlation peaks of the ferulate end-groups appear in the spectrum at δCH 111.2/7.35 ppm and 123.6/7.19 ppm, respectively. In this part of the spectrum, there are also two peaks at δCH 153.7/7.65 ppm and 125.9/7.0 ppm that correspond to the α and β atom of an aldehyde end-group (J structure, 4.5 units per 100 Ar). On the other hand, the HSQC spectrum of birch lignin (Figure 2c; δCH 100–155/5.9–7.9 ppm) shows that this lignin consists mainly of syringyl (S) lignin units. A lesser amount of guaiacyl (G) units and a small quantity of p-hydroxyphenyl structures (H units) is also present. Integrating the respective cross-peaks, the molar ratio of the S, G, and H units for the birch lignin was found to be S/G/H = 77.0/22.3/0.7%. Apart from the main lignin units, in this part of the birch lignin spectrum, there is evidence of some amounts of an aldehyde end-group (J structure, 2.9 groups per 100 Ar) with its Cα-Hα correlation peak at δCH 153.7/7.65 ppm and a ferulate structure (FA structure, 0.7 groups per 100 Ar) with its C2-H2 correlation peak at δCH 111.2/7.35 ppm. Small amounts of xylan were observed at δCH 72.5/3.07 ppm (X2 atom of xylan structure, 3.3 groups per 100 Ar) and δCH 74.75/3.55 ppm (X4 atom of xylan structure, 1.6 groups per 100 Ar) [92,97]. The cross-peak at δCH 131.4/6.47 ppm can be ascribed to the Cαα correlation peak of a not common structure that contains a conjugated double bond [94].
The part of the HSQC spectrum referring to inter-unit linkages for spruce lignin (Figure 2b; δCH 52–90/2.8–5.8 ppm) is dominated by the strong signals of both the phenylcoumaran structures (β–5′, type C linkage, 28.5 bonds per 100 Ar) and β-Aryl ethers (β–Ο–4′, type A linkage, 28.7 bonds per 100 Ar) while pinoresinols (β–β’, type Β linkage, 8.3 bonds per 100 Ar) exist in small amounts. The relative abundance of each type of linkage was found as A/B/C = 28.7/8.3/28.5% by integration of the respective peaks. Apart from these main types of linkages, in this part of the spectrum, the presence of an alcohol end-group (I structure, 4.9 groups per 100 Ar) is evidenced by its Cγ–Ηγ correlation peak that appears at 61.4/4.10 ppm. In the respective part of the birch lignin HSQC spectrum (Figure 2d; δCH 50–90/2.9–6.0 ppm), the main type of inter-unit linkages is that of β-aryl ethers (β–Ο–4′, type A linkage, 49.3 units per 100 Ar) while there is also a substantial quantity of pinoresinols (β-β’, type B linkage, 16.4 units per 100 Ar) and a small amount of phenylcoumaran structures (β–5′, type C linkage, 6.7 bonds per 100 Ar). The relative abundance of each type of linkage was found to be A/B/C = 49.3/16.4/6.7% by integration of the respective peaks. In this part of the spectrum, there is also a small peak at (δCH 61.4/4.1 ppm) that corresponds to the γ atom of an alcohol end-group (I structure, 2.2 groups per 100 Ar).

2.2. Characterization Data of ZSM-5 Zeolites

The XRD patterns of all the ZSM-5 zeolites tested, i.e., conventional microporous, mesoporous, and nano-ZSM-5, revealed their high crystallinity and exhibited diffraction peaks that are characteristic of the MFI structure (Figure 4), as was also previously shown for similar ZSM-5 zeolite catalysts [43]. The intensity of the peaks of the mesoporous and nano-sized ZSM-5 zeolites was slightly lower, which is consistent with the related reduction in the intrinsic density and scattering power of the crystals [73]. Incomplete crystallization and limited long-range ordering also cannot be excluded for the nano-sized zeolite. The N2 isotherms and the BJH pore size distribution curves of the catalysts can be seen in Figure 5. The microporous ZSM-5 zeolites, i.e., ZSM-5(40) and ZSM-5(11.5), exhibit I(a) adsorption isotherms based on the updated IUPAC (International Union of Pure and Applied Chemistry) classification [99], being typical for microporous zeolites (with micropores width of <~1 nm) without significant crystal and textural imperfections that may induce secondary meso/macroporosity [43]. The typical well-formed crystals with a parallelepiped shape can be seen in the TEM images of these zeolites (Figure S1, Supplementary Material). The BET total surface area of both ZSM-5(11.5) and ZSM-5(40) zeolites is around 430 m2/g and the micropore area is ~330 to 350 m2/g (Table 3). The N2 adsorption isotherm of the nano-ZSM-5 is also of type I(a), exhibiting, in addition, a steep increase of adsorbed nitrogen at high P/Po (≥0.9), which is representative of the high macropore and/or external surface area owing to inter-nanoparticle voids. This morphology can also be revealed by the SEM and TEM images of the nano-sized ZSM-5, which show the presence of primary nanocrystals of <20 nm in size (Figure 6a), forming polycrystalline particles of up to ca. 0.5 μm (Figure 6c), which in turn are aggregated into bigger particles of 5 to 10 μm (Figure 6d).
The N2 adsorption isotherm of the mesoporous ZSM-5 (9 nm) zeolite that was prepared by mild alkaline treatment and subsequent washing with dilute aqueous acid solution exhibits a combined type I(a) and ΙΙ, as well as a steep increase of sorbed N2 at P/Po >0.95, indicating the presence of typical zeolitic micropores, intracrystal mesopores with a broad size distribution, and increased macropore and/or external area, as was also previously shown for similar desilicated mesoporous ZSM-5 zeolite variants [43]. Indeed, in the BJH curves of Figure 5, a relatively broad pore size distribution from 2.5 to 30 nm, with a maximum at about 9 nm, can be seen for ZSM-5 (9 nm). The formation of intracrystal meso/macropores is also verified by the evenly distributed light contrast spots that can be identified in the TEM image of ZSM-5 (9 nm) (Figure 6b). Furthermore, the clearly observed zeolitic lattice fringes across the whole particle verify the high degree of crystallinity, in accordance with the XRD results. A relatively broad intra-crystal mesopore size distribution is typical for meso-ZSM-5 zeolites prepared by mild alkaline treatment of parent crystalline ZSM-5, in comparison to meso-ZSM-5 zeolites synthesized by the simultaneous use of the classical tetrapropylammonium cation (template of the MFI microporous structure) and of various mesoporous structure-directing agents [73,74,78,100]. The BET surface area of ZSM-5 (9 nm) has been significantly increased, i.e., by 28% from 437 to 560 m2/g, compared to that of the parent microporous ZSM-5 (40) zeolite, due to the substantial increase of the meso/macroporous and external area, with the simultaneous decrease of the microporous area (Table 3). This is usually observed in mesoporous ZSM-5 zeolites [73,78] and can be attributed to partial disordering of the microporous zeolitic structure especially close to the formed meso/macropores, as well as to the formation of amorphous silica-alumina impurities that block the micropores’ entrance. The second mesoporous ZSM-5 sample of this study, i.e., ZSM-5 (45 nm), exhibits similar textural and porosity characteristics to ZSM-5 (9 nm), with the difference that the former zeolite contains larger mesopores in the range of 15 to 90 nm (with average width of 45 nm) (Table 3).
ZSM-5 is a strongly acidic zeolite, with many applications in the cracking/pyrolysis of petroleum or biomass-derived feedstocks, containing mainly Brønsted acid sites as well as Lewis sites when extra-framework amorphous phases are present [43,101,102,103,104]. As expected, the amount of Brønsted acid sites of the parent commercial ZSM-5 zeolites increases as the Si/Al decreases, as can be seen in Table 3. Regarding the relative strength, as can be seen in Figure 7, ZSM-5 (40) possesses stronger Brønsted acid sites compared to those of ZSM-5 (11.5). The amount of Brønsted sites and the Brønsted to Lewis (B/L) sites ratio of the meso-ZSM-5 (9 nm) remain high and similar to those of the parent ZSM-5 (40). This is indicative that the mild alkaline treatment and subsequent mild acid “washing” (for removing Na+ cations and formed extra-framework aluminum phases) do not significantly alter the acidic properties of ZSM-5 zeolite, as was also previously shown for similar parent and desilicated mesoporous ZSM-5 zeolite catalysts [43]. Similarly, the acidity of ZSM-5 (45 nm) has not been altered substantially (Table 3). On the other hand, the nano-sized ZSM-5 contains about half the amount of Brønsted sites, despite having a similar Al content with ZSM-5 (40) (Table 3), possibly due to the relatively lower degree of crystallinity and inadequate organization of the zeolitic framework.

2.3. Non-Catalytic and Catalytic Fast Pyrolysis of Spruce and Birch Lignins (Py/GC-MS System)

Fast pyrolysis experiments with the use of silica sand as the inert heat carrier were performed on a Py/GC-MS system at 400 and 600 °C for the spruce- and birch-derived lignins. Representative chromatograms are shown in Figure 8 while full lists of the identified compounds are given in Tables S1 and S2 for the two lignins, respectively (Supplementary Material). The distribution of the compounds among the various groups, i.e., AR, PH, OxyPH, AC, etc. (see the experimental section), is also shown in Figure 9. The spruce lignin pyrolysis vapors contained mainly alkoxy-phenolic compounds with a single alkoxy group, i.e., of the guaiacol (G) type, such as guaiacol, creosol, trans-isoeugenol, 2-methoxy-4-vinylphenol, and 4-ethyl-2-methoxy phenol, as well as guaiacol-type compounds substituted with functional groups, such as vanillin, 3-(4-hydroxy-3-methoxyphenyl)-2-propenal (coniferyl aldehyde), and 1-(4-Hydroxy-3-methoxyphenyl)-2-propanone (guaiacylacetone). A similar product distribution profile was previously observed for the fast (non-catalytic) pyrolysis of spruce kraft lignin [43], this being additional proof that the predominant G units determined by the 2D HSQC NMR measurements in the structure of softwood spruce lignin isolated either by the organosolv–steam explosion pretreatment (this study) or the kraft process are also the main components of the produced fast pyrolysis oil. Very low concentrations of other types of compounds, such as alkyl-phenols (not oxygenated), furans, acids, esters, alcohols, ethers, ketones, and oxy-aromatics, were also produced in addition to the alkoxy-phenols, as can be seen in Figure 9a and Table S1. Dimers and higher molecular weight fragments were also identified in the Py/GC-MS spectra (Figure 8a).
Due to the nature of birch (hardwood), the fast pyrolysis vapors of the isolated lignin comprised mainly of syringol (S)-type compounds with two alkoxy-groups, i.e., syringol (2,6-dimethoxy phenol) and 2,6-dimethoxy-4-(2-propenyl)-phenol; syringol-type compounds substituted with functional groups, such as 3,5-dimethoxy-acetophenone and 4-hydroxy-3,5-dimethoxy-benzaldehyde; as well as oxygenated aromatics, such as 1,2,4-trimethoxybenzene (Figure 8b and Table S2). In addition to the S-type compounds, G-type compounds were also identified, i.e., creosol, guaiacol, 4-methoxy-3-(methoxymethyl)-phenol, etc. with an S/G ratio of 70–72/30–28 at both 400 and 600 °C. As in the case of the spruce lignin, it is clear that the distribution of S and G units in birch lignin identified by 2D HSQS NMR (S/G = 77/22) was also retained in the non-catalytic fast pyrolysis oil. In addition to alkoxy-phenols, alkyl-phenols (not oxygenated), acids, ketones, oxy-aromatics, and nitrogen-containing compounds were also produced, as can be seen in Figure 9b and Table S2.
The catalytic fast pyrolysis (CFP) study on the Py/GC-MS system was performed at 600 °C with the two lignins and the ZSM-5 zeolite catalysts described in the previous section (Table 3). The temperature of 600 °C was selected for the catalytic tests in order to enhance the initial thermal decomposition of lignin towards smaller oligomers and alkoxy-phenol monomers that can further react on the catalyst. The relative abundance of the different groups of compounds can be seen in Figure 9 and a detailed list of the products is given in Tables S1 and S2 for the spruce and birch lignin, respectively. As a general trend, the use of the conventional microporous ZSM-5 zeolite in the pyrolysis of both lignins induced a substantial conversion of the initially formed (by thermal pyrolysis) alkoxy-phenols towards BTX mono-aromatics (mainly toluene, 1,3-dimethyl-benzene, 1,2,3-trimethyl-benzene, benzene, o-xylene, etc.), PAHs (mainy naphthalenes, such as 1- or 2-methyl-naphthalene), and alkyl-phenols (such as phenol, 2- or 3-methyl-phenol, 2,5-dimethyl-phenol, etc.) in accordance with our previous work on spruce kraft lignin pyrolysis [43] and other related studies [65,67,69,71]. The deoxygenation activity and increased production of aromatics with ZSM-5 zeolite has been attributed to its strong Brønsted acidity and unique pore system comprising of tubular micropores of moderate size (~5.5 Å diameter) and slightly wider spherical intersections (~10 Å diameter), which promote decarbonylation, decarboxylation, dehydration, C–O (dealkoxylation), and C–C bond scission reactions, as well as the formation of aromatics via deoxygenation of the initially formed phenolics or via C2=/C3= aromatization; these small alkenes can be produced by thermal or catalytic cracking of side alkyl-chains or via dehydration of small intermediate alcohols [43,64,66,68,69]. The relative strength of the Brønsted acid sites of ZSM-5 zeolite is also important as the ZSM-5(40), which has less but stronger acid sites compared to ZSM-5 (11.5) (Table 3, Figure 7, and the related discussion above), exhibited slightly higher reactivity for the conversion of alkoxy-phenols and the production of mono-aromatics, phenols, and PAHS, for both lignins (Figure 9).
When comparing the influence of the type of lignin, i.e., spruce (softwood) vs. birch (hardwood), it can be seen that both microporous ZSM-5 zeolites, i.e., ZSM-5 with an Si/Al ratio of 11.5 and 40, are slightly more active in the conversion of guaiacyl-type compounds originating from spruce lignin compared to the syringyl compounds derived from birch lignin (Figure 9). This leads to a higher increase of alkyl- (non-oxygenated) phenols and PAHs with spruce lignin while the relative concentration of mono-aromatics is similar for both lignins. Furthermore, from the data in Tables S1 and S2, it can also be seen that there is no significant differentiation with regard to the selectivity towards individual mono-aromatic compounds. For both lignins, toluene and 1,3-dimethyl-benzene (m-xylene) were the most abundant aromatics, followed by benzene, 1,2,3-trimethyl-benzene, and mesitylene. Similarly, 1- or 2-methyl-naphthalene and naphthalene were the most abundant PAHs for both lignins while 2- or 3-methyl-phenol and 4-methyl-1,2-benzenediol were the alkyl-phenols with the higher concentration, especially with spruce lignin. Although there are few studies available in the literature that have investigated the fast pyrolysis of different types of lignins, i.e., from different raw biomass (in some cases, mixtures of different wood biomass) or method of isolation [66,105,106,107], the present work examines specific hardwood (birch) and softwood (spruce) lignins that have been isolated by the same method (organosolv–steam explosion), thus solely elucidating the effect of the botanical origin and excluding the changes induced by the different biomass fractionation methods or the mixing of different biomass sources.
The effect of intracrystal mesoporosity and high external surface (due to nanosized crystals) of ZSM-5 on the composition of the fast pyrolysis oil for the two lignins can be revealed by the data shown in Figure 9, as well as in Tables S1 and S2 (Supplementary Materials). It is clear that the dealkoxylation activity of all three hierarchical ZSM-5 zeolites, i.e., meso-ZSM-5 (45 nm), meso-ZSM-5 (9 nm), and nano-ZSM-5, was substantially higher than that of their corresponding microporous ZSM-5 (11.5) and ZSM-5 (40) zeolites, especially with the two former for which alkoxy-phenols (in the non-catalytic pyrolysis vapors) were almost completely eliminated for both lignins (Figure 9). It should be noted that the two meso-ZSM-5 zeolites have similar acidic characteristics with their microporous counterparts (Table 3, Figure 7), thus attributing their enhanced reactivity mainly to the presence of the intracrystal mesopores and the improved diffusion properties they offer as the average critical diameter of the related alkoxy-phenolics (at least the monomeric compounds produced initially via thermal pyrolysis of lignin) ranges from ca. 7.5 to 10 Å [108]. On the other hand, the nano-ZSM-5 contains fewer Brønsted acid sites, almost half of those of ZSM-5 (40) or meso-ZSM-5 (9.5 nm), as discussed in the previous section, but still exhibits relatively higher reactivity compared to that of microporous ZSM-5 (40) due to its high external surface.
A similar superior behavior of hierarchical ZSM-5 zeolites was also observed in our previous work on the catalytic fast pyrolysis of kraft spruce lignin [43]. However, a substantial difference was observed with those data, i.e., the spruce lignin of the present study (isolated by the organosolv–steam explosion method) offered a much higher concentration of mono-aromatics with the mesoporous zeolites than with the microporous ZSM-5 (Figure 9), compared to the kraft spruce lignin, which, on the other hand, induced enhanced formation of alkyl- (non-oxygenated) phenols, almost similar to that of aromatics [43]. With the organosolv–steam explosion spruce lignin of the present work, only the nano-ZSM-5 was capable of providing increased selectivity of both mono-aromatics and alkyl-phenols. These directly comparative data reveal the effect of the lignin isolation method on the CFP product selectivity, i.e., aromatics vs. phenols, which may arise from the different structural and compositional characteristics of the lignins. It is also interesting to note that the nitrogen compounds were almost completely eliminated, especially with the hierarchical ZSM-5 zeolites, compared to the non-catalytic pyrolysis vapors for both organosolv lignins (Figure 9 and Tables S1 and S2) while sulfur compounds were absent even in the non-catalytic pyrolysis vapors as both lignins contained only traces of sulfur (Table 1) in contrast to kraft spruce lignin [43].
With regard to the effect of hierarchical zeolites on the individual aromatic and phenolic compounds, as compared to the CFP with the conventional microporous ZSM-5, an increased concentration of the most abundant mono-aromatics was observed, i.e., toluene, 1,3-dimethyl-benzene, 1,2,3-trimethyl-benzene, mesitylene, o-xylene, etc., for both lignins (Tables S1 and S2). In addition, p-xylene was identified in an increased concentration with the meso-ZSM-5 zeolites. Alkyl-phenols, such as 2- or 3-methyl-phenol and dimethyl phenols, were further increased mainly by the nano-ZSM-5 zeolite while PAHs were slightly reduced or increased depending on the lignin origin, as discussed below, with the most representative being the same as those from the conventional ZSM-5s, i.e., naphthalene, 1- or 2-methyl-naphthalene, as well as dimethyl naphthalenes.
When comparing the influence of the type of lignin on the effect of the hierarchical porosity of ZSM-5 zeolite (Figure 9, and Tables S1 and S2), similar trends can be identified in the changes of mono-aromatics and alkyl-phenols for both lignins. In the case of PAHs, a slight variation can be observed, with the two meso-ZSM-5 zeolites inducing less PAHs compared to the microporous ZSM-5 with spruce lignin (in accordance also with the results in [43]) and slightly more with birch lignin. With regard to the individual compounds, no significant influence of the lignin origin can be identified with the exception of slightly increased dimethlyl naphthalenes (PAHs) with the birch-derived lignin (Table S1 vs. Table S2).

2.4. Non-Catalytic and Catalytic Fast Pyrolysis of Model Compounds (Py/GC-MS System)

There are several studies available in the literature that have focused on the fast pyrolysis of model lignin compounds, including dimers/oligomers containing β- and α-ether, β-aryl, and other representative bonds in the structure of lignin, as well as in situ spectroscopic investigations that aim to identify the formation of reactive intermediates [109,110,111,112,113]. The reported results and related discussion provide valuable insight but are highly dependent on the type of model compound and experimental pyrolysis/analysis system used, thus suggesting possible reaction mechanisms that cannot be widely adopted. On the other hand, there are many studies that have proposed mechanistic schemes based mainly on the composition of feed (lignin, model molecules) and the obtained final products (aromatics, phenolics, coke, etc.) by utilizing existing knowledge of the acid catalytic function of various zeolites [43,64,66,68,69]. In this work, we studied the fast pyrolysis of two of the most abundant compounds in the fast pyrolysis vapors of spruce and birch lignins (Figure 8 and Figure 9, and Tables S1 and S2), i.e., guaiacol and syringol, which are the most typical model monomer compounds of softwood and hardwood lignins, which also have critical diameters suitable for the pores of conventional and hierarchical ZSM-5 zeolites, as discussed above. Two representative catalysts were selected for this study, i.e., the conventional ZSM-5 (40) and the hierarchical meso-ZSM-5 (9 nm). The distribution of groups of compounds produced in the non-catalytic (silica sand) and catalytic pyrolysis of guaiacol and syringol is presented in Figure 10a,b, respectively. The first important information arises from the relatively higher stability of syringol at 600 °C in the absence of a catalyst (94% of the total GC-MS peak area) compared to guaiacol (52% of the total GC-MS peak area), which was mainly converted to 2-hydroxy-benzaldehyde (19%), alkyl-phenols, such as 2-methyl- and 2-ethyl-phenol (5% each), and catechol (5%). The formation of these three types of compounds has been previously proposed to occur via homolytic cleavage of the O–CH3 bond or via O–CH3 and benzyl ring rearrangement pathways involving radical intermediates [109,114]. On the other hand, the limited conversion of syringol induced the formation of mainly 2-hydroxy-3-methoxy-benzaldehyde, tetra-alkyl phenols, and 2-methoxy-6-methylphenol.
The catalytic pyrolysis of the model compounds followed the same trend with that of the non-catalytic tests. There is a significant difference in the remaining (not converted) guaiacol (39% of the total GC-MS peak area with ZSM-5 (40) and 19% with meso-ZSM-5 (9 nm)) and syringol (83% with ZSM-5 (40) and 49% with meso-ZSM-5(9 nm)). The substantial higher conversion of guaiacol with ZSM-5 (40) led to the formation of catechol (13%), 2-hydroxy-benzaldehyde (7.5%), 2-methyl-phenol (4.4%), 2,5-dimethyl-phenol (3.5%), phenol (2.8%), and toluene (1.5%) and lower percentages of methoxy-methyl-phenols, xylenes, naphthalenes, etc. (Figure 10a). With the mesoporous meso-ZSM-5 (9 nm) zeolite, the conversion of guaiacol was higher, leading to more catechol (20%), 2-methyl-phenol (11%), 2,5-dimethyl-phenol (11%), phenol (5.5%), and toluene (1.9%), as well as more of the rest of the alkyl-phenols, mono-aromatics, and naphthalenes (Figure 10a). It is clear, however, that with both catalysts, the preferred group of products is alkyl-phenols (up to 53%) instead of mono-aromatics (up to 6%). The conversion of syringol with the conventional microporous ZSM-5 (40) was limited, mostly towards mono-aromatics (8%), such as 1-methylene-1H-Indene, dimethyl- and trimethyl-benzenes, and toluene, and very few alkyl-phenols and PAHs (Figure 10b). The meso-ZSM-5 (9 nm) was again more reactive, mainly towards mono-aromatics (16%, as those with ZSM-5 (40), plus o-xylene, indenes, and benzene), alkyl-phenols (17%, mainly di- and trimethyl phenols), and PAHs (10%, mainly naphthalene and methyl- or dimethyl naphthalenes) (Figure 10b).
In summary, the main outcomes of the model compounds pyrolysis study were the higher conversion of guaiacol compared to syringol under both non-catalytic and catalytic pyrolysis conditions, the clear benefit of the intracrystal mesoporosity of the ZSM-5 zeolite as was also discussed above for the spruce and birch lignin pyrolysis, and the difference in product selectivity depending on the type of model reactant, i.e., guaiacol exhibiting high selectivity towards alkyl-phenols while syringol led to a more balanced formation of aromatics and phenols. The enhanced reactivity of guaiacol compared to syringol, as well as the higher selectivity towards alkyl-phenols with guaiacol as the reactant, are in accordance with the higher conversion of alkoxy-phenols from spruce lignin and the higher abundance of alkyl-phenols by the use of conventional and mesoporous ZSM-5 zeolites, compared to birch lignin, as discussed above (Figure 9, Tables S1 and S2). On the other hand, both mesoporous ZSM-5 zeolites induced almost complete conversion of the alkoxy-phenols in the thermal pyrolysis vapors of both spruce and lignin (Figure 9) while pure guaiacol and syringol were only partly converted under the same pyrolysis conditions (Figure 10). Furthermore, the clear beneficial effect of meso-ZSM-5 (9 nm) compared to ZSM-5 (40) towards alkyl-phenols from guaiacol (