Utilization of zeolite catalysts in biomass exploitation: a minireview

This minireview discusses the use of zeolites as catalysts in all stages of lignocellulose biomass (LCB) valorization process. A summary of some synthesis and characterization methods for zeolitic materials is provided. Utilization of various active sites of zeolites is explored with the focus on Brønsted and Lewis acid sites and the importance of hierarchical structures in the processes involving bulky biobased molecules. Zeolites modified by top-down methods, specifically desilication with various basic reagents, are good candidates for applications in LCB valorization. Some novelty methods such as a dry milling for incorporating metal nanoparticles into zeolite structure are mentioned. An overview of studies utilizing zeolites in processes such as catalytic pyrolysis of LCB, acid-catalyzed hydrolysis and reactions of platform molecules (ethanol, furans, glycerol, bio-hydrocarbons, lactic acid, succinic acid, levulinic acid, sorbitol, xylitol) is discussed. Special attention is dedicated to reactions of biomass-derived alcohols and value-added chemicals produced from them, e.g., esters. The most successful industrial zeolite application—fluid catalytic cracking—is also summarized for comparison. Challenges of zeolite stability in reactions carried out in liquid water conditions, as well as possibilities of catalyst stabilization, e.g., with extra-framework aluminum, or hydrophobization of the surface, are explored.


Introduction
Biomass as a natural resource proposes a very promising source of organic carbon for synthesis of biofuels, chemicals, and other complex/composite materials. Its main advantage is the creation of a closed CO 2 loop with its periodic generation, making it a sufficient and carbon neutral alternative for fossil resources [1]. In turn, the main challenges of utilizing biomass lie in its varied chemical composition, containing species with oxygen and particularly nitrogen, which gives a high potentiality for a direct preparation of more complex molecules in comparison with crude oil, or natural gas as sources. Disadvantages of biomass are high water content, elements that cause catalyst poisoning and environmental issues tied to its growth, collection, transport, and processing [2]. Hence, for syntheses of fuels and chemicals from biomass to become economically and ecologically viable, catalytic systems performing at acceptable conversion and selectivity levels must be developed.
Many reactions necessary for biomass valorization, such as hydrolysis, pyrolysis, isomerization, alkylation, and other reactions of platform molecules, require an acid catalyst [3]. Homogeneous catalysts e.g., mineral acids are more efficient than heterogeneous ones due to superior mass transfer. The main disadvantages of their use are difficult product separation and catalyst regeneration, corrosivity, and toxicity in cases of volatile compounds, such as HF [4,5]. Therefore, replacing homogeneous catalysts with greener heterogeneous catalysts is a trend towards more environmentally friendly biomass conversion.
Zeolites are established as catalysts for some oil refining processes, such as fluid catalytic cracking (FCC), for a long time (approximately from 1964). Initially, Mg-stabilized Y zeolite (structure code FAU) comprised the main active component of the FCC catalyst. Later renditions utilized rare earth-stabilized Y zeolite, as well as USY, produced by steam dealumination [6,7]. Zeolite ZSM-5 (structure code MFI) has also been used in FCC catalyst, mainly to increase the yield of propylene [8].
The most important properties of zeolites are their intrinsic Brønsted and Lewis acidity, and relatively "homogeneous microporous structure", with pores from about 0.4 nm up to tens of nanometers in case of special zeolites [9]. Besides acid catalytic processes their applications in bifunctional catalysis as a functional (acid) catalyst support is important. Hence, there is a huge potential in their use in not only acid-base reactions but also redox reactions in biomass valorization. In fact, current research suggests that zeolitebased catalysts can be utilized in reactions such as catalytic pyrolysis, hydrolysis of biomass, and transformation of platform molecules into value-added chemicals (Fig. 1).
Acidity in zeolites promotes important cracking reactions of bulky biomass components. As there is also a need for C-O bond breaking in biomass processing (not only C-C bonds breaking as in cracking of fossil derived compounds) an adjustment in number and strength of acid sites is required. In general, zeolites with higher Si/ Al ratio have a better catalytic performance in biomass cracking [10,11].
Brønsted acid sites lead to production of aromatics, when combined with medium pore structure, and smaller oxygenates and coke in case of small-pore zeolites [12]. Extra-framework aluminum species (EFAl) as well as metal substitutions result in Lewis acidity in zeolites [13][14][15].
It is necessary to consider that in both cases of Brønsted and Lewis acid sites, protic solvents such as water lead to diminution of catalytic activity by preventing access of substrate to the sites [16,17].
Zeolite porosity is an important feature that results in their shape-selective properties, as well as "a molecular traffic control" in zeolites with two or more types of pores. This effect eliminates the effects of counterdiffusion [18]. Therefore, tuning the size of pores during the synthesis and modification is one of the challenges of using zeolite catalysts in biomass valorization. When working with biomass, the substrate usually consists of large molecules that can only interact with active sites on the surface of zeolites. In turn, smaller molecules that can diffuse into pores can form larger oligomers that later turn to coke and deactivate the catalyst. This effect can be countered by introducing mesoporous structures into zeolites (Fig. 2). Hierarchical structures have been shown to improve zeolite performance in reactions such as cellulose and hemicellulose hydrolysis, lignocellulose pyrolysis, methanol-to-gasoline (MTG) process and some condensation reactions [19][20][21][22]. The role of zeolites and mesoporous materials in the conversion of biomass to fuel was described by Perego and Bosetti [23]. A longer lifetime shorter deactivation) of zeolites with a mesoporous structure was emphasized. Zeolites usually exhibit good stability in gas-phase reactions up to 700 °C [24,25]. A shift towards liquidphase reactions often applied in biomass valorization affects the stability of catalysts used in these processes. Liquid solvents, mostly water, can cause irreversible deactivation of a zeolite. Processes such as hydrolysis, leaching of some elements or metal particle sintering can occur, especially at higher temperatures. Aside from solvents, some reactants or products can be corrosive towards heterogeneous catalysts as well [26,27]. A good review concerning effect of temperature and reaction environment on deactivation of zeolite-based catalysts, including metal-modified zeolites, in LCB treatment processes was recently published by Clatworthy et al. [28] who define the so-called "Comfort Zone". This is below 100 °C for liquid water reaction systems with metal-modified zeolite catalysts. Clatworthy et al. [28] have also given the list of processes outside of their comfort zone with focusing on deactivation represented mainly by metal leaching, fouling, site restructuring, amorphization and coke formation.
There are several possible strategies for improving the stability of zeolites in liquid-phase reactions. As EFAl have a stabilizing effect on a zeolite, introducing them to the structure can improve zeolite stability in aqueous conditions. This can be achieved by means of hydrothermal stabilization or basic realumination [29].
Another possible approach to increase the stability of zeolite type catalysts is a surface modification with the aim of higher hydrophobicity. Dealumination results in a more hydrophobic zeolite, however, it results in fewer acid catalytic sites. Modification of the zeolite surface with hydrophobic functional groups proves efficient whilst not affecting pore volume. The main drawback of this method is a significant rise in production cost by reagents such as organosilanes [30,31].
Other factors affecting zeolite stability worth consideration are catalyst fouling and poisoning. Biomass and its fractionation products contain heavy components that do not crack or evaporate at operational temperatures, hence causing coke formation. Smaller but highly reactive molecules can also form these heavy molecules in situ [32].
Although there are several ways to mitigate the coke formation, it is impossible to completely avoid it. Zeolite regeneration is therefore necessary. Coke burning is a viable option for zeolites because of their good thermal stability.
Acid sites in zeolites can be poisoned by basic compounds of nitrogen, as well as Ca + or K + ions. This neutralization may be mitigated by pre-processing of the feed or use of ion exchange [32].
Biomass valorization consists of multiple steps, starting with pyrolysis, hydrolysis, or other type of fractionation. Products of these processes are separated into platform molecules which are then transformed to the end-products. This review focuses on the potential use of zeolite catalysts in various stages of these processes. However, a short survey about physical chemical properties of zeolites and their synthesis is involved with aim to offer a deeper elucidation of reactions with bulkier molecules.

Synthesis and characterization of zeolites
Conventional synthetic zeolites are obtained by crystallization from a gel containing a source of alumina, silica, and/or possibly other metals, water, and usually basic agents. The crystallization is carried out under hydrothermal conditions at temperatures between 60 and 200 °C and autogenous pressures [33]. Only a limited amount of structure types can be obtained from such synthesis gels. New structure types were discovered thanks to the addition of a structure directing agent (SDA), which is usually an organic compound capable of modifying the gel chemistry, as a template or a pore filler [34]. Crystallization of zeolites can be accelerated by seeding-a method in which a source of nuclei is added to the synthesis gel. This method can to an extent eliminate the need for an organic template [35]. Zeolites can be prepared in a pure silica form, resulting in materials with almost no acidity and ion-exchange capacity. These are usually applied as catalyst support [36]. Metallosilicates with zeolite structure can also be prepared by a replacement of Al 3+ by a source of Sb 3+ , B 3+ , Ga 3+ , Fe 3+ , and others [37].
The uniform micropore shapes and sizes of synthetic zeolites is their main advantage in shape-selective catalysis. When utilizing zeolites as catalysts for bulkier substrates, it is however a hindrance that prohibits the access of molecules to the active sites inside the pores. There are several strategies to enhance the accessibility of synthetic zeolites for processes such as biomass valorization [38].
Typical classical zeolites have pore sizes ranging from about 0.4 nm up to a few tens of nanometers [9]. Small pore sizes imply a high diffusion resistance (Fig. 3), particularly for pores with diameter lower than 1 nm. Generally, the Thiele modulus and the effectiveness factor [40] are introduced for evaluation of heterogeneous catalytic processes. The Thiele modulus (ϕ) is a dimensionless number composed of the square root of the characteristic reaction divided by the characteristic diffusion rate in the pores. The effectiveness factor (η) has been defined as the ratio of the "real reaction rate of the catalyst particle" to the "imaginary reaction rate when the whole particle is assumed to have the surface reactant concentration", i.e., no diffusional resistance affects the process. For materials with small pores, the effectiveness factor is much smaller than 1. Considering η < 0.25, 1st order reaction and isothermal conditions, the effectiveness factor is [41] as follows: where D eff = effective diffusional coefficient, L = characteristic length that a particle travels before impact, e.g., radius of the particle; k r = reaction constant. Equation (1) allows an easy estimate of parameter values: a low value of diffusivity (D eff ) and a larger particle-higher value of L decrease the rate of reactant conversion. A low value of the reaction rate constant (k r ) allows to reach a higher value of the effectiveness factor. It is also noteworthy that increase in temperature increases the reaction rate constant much more in comparison to the increase of diffusivity. Therefore, diffusion hinderance appears more significantly at higher temperature. An example of this phenomenon is the catalytic cracking of naphtha over ZSM-5 zeolite using Macro-ZSM-5 (2 μm) and Nano-ZSM-5 (100 nm) [42]. An increase in the reaction rate in experiments with smaller particles, as well as lower deactivation by coking in comparison with larger particles, were observed. In order to obtain maximum utilization of catalytic sites (in case of zeolites: mainly acidic ones) and maintain good handling and mechanical properties of the catalyst, materials with meso-microstructure are produced. In accordance with the IUPAC definition [43], pores can be divided into the following categories, depending on their diameter: micropores (< 2 nm), mesopores (2-50 nm), macropores (> 50 nm). Micropores lower than 0.7 nm are denoted as ultramicropores. Macro-and mesopores allow a faster transport of reaction components (Fig. 3) and micropores possess a higher concentration of catalytic sites.
Synthesis strategies for zeolite crystals smaller than 100 nm utilize clear solutions or gels. Synthesis conditions must be controlled, so that nucleation is preferred over crystal growth. Small particles can aggregate during the synthesis, which can be avoided by organic SDAs and reduction of the alkali cations. However, template-free syntheses are preferable, as the subsequent calcination of the nano-zeolite can lead to particle aggregation [24].
Another approach is to synthesize zeolites with hierarchical structures. Some publications call these materials "mesoporous zeolites," but as there are other pore dimensions than mesopores present in these materials, this label can be misleading [33]. There are several types of hierarchy in zeolites (Fig. 4). A pore system of larger pores can be subdivided into smaller-size pores (type I) or there can be an interconnected pore system, in which small pores branch off from a continuous large pore (type II) [44]. A peculiar type of zeolites is layered ones [45] which were successfully applied for cracking alkylation and other reactions.
Methods for the synthesis of hierarchical zeolites can be generally divided into two main groups-the top-down and the bottom-up methods [7,40,44,46].
Top-down methods are used when zeolites are used to create a hierarchical system by post-treatment procedures. Bottom-up methods are those, during which the hierarchical system is created during the synthesis [7].
Top-down methods include dealumination, desilication, and dissolution with subsequent recrystallization.
Dealumination is a technique mainly used for the production of USY for FCC. Dealumination can be carried out by steam, acid, or heat treatment. Steam dealumination results in Al released from the zeolite framework that remains on the surface and inside the pores of the zeolite as EFAl. These can be removed via a mild acid treatment (or "acid wash") [47].
Desilication is a method that causes hydrolysis of Si-O-Si bonds by an alkali solution, e.g., diluted NaOH. This treatment selectively removes Si from the framework and results in mesopore formation. The zeolite largely retains its microporous character, while interconnected mesopores are formed [48].
Zeolite recrystallization comprises of two steps. In the first one, a part of the zeolite is dissolved, usually by an alkaline solution, which is followed by a reassembly of dissolved and dispersed species. By this approach, several types of hierarchical structures can be prepared. The resulting zeolite can consist of the same zeolitic phase as the parent material and is coated by a thin film of mesoporous material. A composite material consisting of both a zeolitic and a mesoporous phase can also be prepared. This mesoporous phase can be amorphous or crystalline. A third type is a material that has been completely recrystallized and only contains a mesoporous phase, usually MCM-41 [49].
Bottom-up methods utilize soft templates, hard templates, and there are also template-free syntheses.
A hierarchical zeolite synthesis can be based on an organic SDA and a mesoporogen. This is a dual-template method, in which SDA serves as a template for micropores and mesoporogen is responsible for mesopore creation. Mesoporogens like surfactants, polymers, or organosilanes are considered soft templates. Both SDA and mesoporogens can be removed through calcination after zeolite synthesis. A two-in-one template can also be utilized. This agent usually consists of a hydrophobic alkyl chain and a hydrophilic group, such as a quaternary ammonium ion [50].
Hard templates are solid materials with rigid structure that act as a template, around which the zeolite is formed. After synthesis, the template is usually removed by calcination, revealing a hierarchical meso-or microporous structure. Carbon materials, polymers, or biological materials can be used as hard templates. Calcination of these supports often requires high temperatures, which can result in loss of crystallinity [38].
Syntheses that do not rely on any templates use an addition of seeds. These are small proto-zeolitic particles from early stages of zeolite crystallization. By using them, an improved crystallinity and mesoporosity can be achieved. However, the synthesis mechanism is not yet fully understood [50].
Zeolitic catalytic materials are characterized by the following methods [40]: • Chemical composition, including surface analysis: inductively coupled plasma-optical emission spectrometry, energy dispersive X-ray analysis, X-ray fluorescence analysis, X-ray photoelectron spectroscopy, thermogravimetric analysis. • Zeolitic character, crystal structure, phase identification: powder X-ray diffraction, small-angle X-ray scattering, small-angle neutron scattering, selected area electron diffraction, high-resolution transmission electron microscopy. • Active sites: temperature programmed desorption, Fourier transform infrared spectroscopy, magic angle spinning nuclear magnetic resonance. • Microporosity: gas physisorption. • Textural properties: gas physisorption, Hg-porosimetry, small-angle X-ray scattering, scanning electron microscopy (SEM), transmission electron microscopy, highresolution transmission electron microscopy, electron tomography, focused ion beam combined with SEM, 3D electron tomography. • Pore interconnectivity: 3D electron tomography, positronium annihilation lifetime spectroscopy, Hg-porosimetry, scanning techniques, thermoporometry, focused ion beam, pulsed field gradient nuclear magnetic resonance, in situ small-angle X-ray scattering and small-angle neutron scattering, Xe nuclear magnetic resonance. • Mass transport, characteristic diffusion path length and diffusivities: tapered element oscillating microbalance, zero length column, pulsed field gradient nuclear magnetic resonance, quasi elastic neutron spectroscopy, micro imaging with infrared microscopy and interference microscopy. • Chemical stability: steaming coupled with magic angle spinning nuclear magnetic resonance, in situ powder X-ray diffraction, gas physisorption. • Mechanical stability: pressing coupled with magic angle spinning nuclear magnetic resonance, gas physisorption. • Catalytic activity and selectivity: a test reaction coupled with gas chromatography, gas chromatography-mass spectrometry, thermogravimetric analysis, Fourier trans-1 3 form infrared spectroscopy, nuclear magnetic resonance, ultraviolet-visible spectroscopy. • Lifetime and coke formation: a test reaction coupled with gas chromatography, gas chromatography-mass spectrometry, thermogravimetric analysis, nuclear magnetic resonance, gas physisorption, Xe nuclear magnetic resonance.
For characterization of metal containing zeolitic catalysts, the above set of methods is completed by the following: • Temperature programmed reduction. • Temperature programmed oxidation. • Chemisorption of CO or H 2 (specific surface of metal crystallites). • X-ray photoelectron spectroscopy (oxidation states of metal species).
Apart from experimental techniques, nowadays, molecular modelling plays important role for characterization of zeolitic materials, e.g., evaluation of adsorption and diffusion of C 1 to C 4 alkanes in dual-porosity ZSM-5 zeolites [51]. Calculated values of diffusional coefficients and adsorption constants were in very good agreement with experimental data.
In the listed references (see Refs. ) a lot of experimental data and illustration pictures about characterization of conventional and hierarchical zeolites can be found. In this paper we are giving an example by Fig. 6.
The following examples illustrate the effects of various synthesis techniques and related physicochemical properties of zeolites on their catalytic activity.
Michalcik et al. [53] carried out a series of experiments to determine the effect of zeolite structure type on the pyrolysis of various types of LCB. Of the tested catalysts, H-FER and H-MOR performed most similarly to the experiment without a catalyst, with regards to yields of aromatics, solids, non-condensable gases, and other condensables. This can be ascribed to relatively small pore sizes (0.39-0.51 and 0.42-0.58 nm, respectively). Beta and ZSM-5 zeolites achieved higher yields of 9-15 aromatics. H-ZSM-5 also increased the likelihood of the formation of substituted benzenes and naphthalenes, especially when pyrolyzing cellulose.
Considering that most of the LCB-derived oxygenates have a y-dimension larger than 0.7 nm, it could be deduced that none of these compounds could enter the pores of ZSM-5 and only a fraction of them could enter the pores of Beta and Y zeolites, hence most of these compounds would be converted on their surface. Yu et al. [54], however, achieved good results in their experiments with pyrolysis of lignin. The best performance of ZSM-5 zeolite in conversion of lignin to aromatics can be explained by the following two concepts: (I) Because the external surface accounted for 6.5-15.4% of the total surface area of the tested zeolites, the surface activity is not neglectable. Several researchers have suggested that the external surface acid site can contribute to crack and dehydrate large oxygenates into small ones that can then enter the pores of zeolite. (II) Thermal distortion of zeolite pore structure at high temperatures can considerably enlarge the "effective pore size" of the zeolites. Therefore, molecules that are larger than the determined pore size of zeolites can enter the pores at high temperatures needed for pyrolysis.
A comparison of pyrolytic performance of a conventional and a hierarchical ZSM-5 zeolite prepared by both top-down and bottom-up method was carried out by Qiao et al. [55,56]. The experiments showed a significant improvement in aromatics yield up to about 40% in both studies.
H-USY zeolite was chosen for a study of hydrolysis of cellulose and hemicellulose by Zhou et al. [57] due to its large-pore structure. Conventional H-USY was compared to a hierarchical H-USY-meso prepared by oxalic acid leaching. Although the acidity of H-USY-meso decreases compared to the parent zeolite due to dealumination, the increase in surface area, total pore volume, external surface area and mesopore volume suffices to result in a more active catalyst. The conversion of α-cellulose increases from 12.7% with H-USY to 24.9% with H-USY-meso.
Salakhum et al. [58] attempted to synthesize a hierarchical ZSM-5 zeolite from nano-aluminosilicate extracted from sand and scrap glass. Nano-aluminosilicate was dissolved in an alkali solution and crystallized after an addition of an organic mesoporogenic SDA-tetra(n-butyl)phosphonium hydroxide. XRD analysis confirmed the presence of ZSM-5 phase with crystallinity over 90%. After four days of crystallization, a material was created with similar textural properties to a conventional ZSM-5 zeolite with even larger S BET and micropore volume. ZSM-5 zeolite has micropore dimensions of 0.53 × 0.56 nm and 0.51 × 0.55 nm, hence it tends not to be active in fructose transformation to HMF. The mesoporous ZSM-5 prepared from scrap glass provided high accessibility of bulky molecules to active sites. It achieved a HMF yield of 77.05% while with conventional ZSM-5, the yield was only 60.37%. Selectivity of HMF production was similar in both cases-about 85%.

Catalytic pyrolysis of biomass
Pyrolysis, or thermal decomposition without the presence of oxygen, is one of the most effective ways of transforming biomass into renewable organic compounds. Particularly, fast and flash pyrolysis convert biomass into mostly liquid bio-oil, which is an attractive source of chemicals with high energy content. Bio-oils produced by non-catalytic transformation of biomass are usually low-quality, with a high content of oxygenates such as aldehydes, ketones, acids, ester, furans, etc. They are corrosive, reactive, and viscous [59]. Therefore, removing the majority of all heteroatoms from bio-oils using catalysts is a necessary step for a more efficient conversion of biomass.
Zeolites have proven effectiveness in catalytic upgrading of petrochemicals due to their strong Brønsted and Lewis acidity, as well as a high specific surface and porous structure. It has also been confirmed that zeolites are active in deoxygenation of biomass. Some of the structure types, such as ZSM-5, have good shape selectivity towards aromatic compounds in pyrolysis of LCB [53]. Oleaginous biomass can undergo catalytic pyrolysis resulting in an 85 to 90% yield of liquid products in the presence of a zeolite catalyst. This liquid condensate consists of mainly paraffins, olefins and fatty acids, which largely contribute to the corrosive properties of the liquid [60].
The effect of various zeolite structure types on catalytic fast pyrolysis of LCB is summarized in Table 1.
Although ZSM-5 has been mentioned as an excellent catalyst for high aromatics content in products, it does not perform as well in terms of deoxygenation. In this regard, large-pore zeolites, such as Y or Beta, provide better results [54].
Despite the fact that it has been reported that acid zeolites without metal modification did not promote deoxygenation and hydrogenation of some aromatic oxygenates at low temperatures (< 400 °C) [64][65][66], especially Brønsted acid sites at higher temperatures are active in isomerization, transalkylation, cracking and dehydration, resulting in the transformation of phenolics into benzene, toluene, and xylene. Higher Brønsted acidity in zeolites promotes secondary reactions, which result in polyaromatics or coke formation, hence causing the deactivation of the catalyst [67,68].
While Brønsted acid sites are catalytically active in aromatization and cracking reactions, Lewis acid sites promote ketonization of acids with aldehydes or dehydration [69]. Other research, however, suggests that Lewis acid sites do not exhibit an apparent activity in bio-oil upgrading [70].
Since purely acid zeolites are susceptible to deactivation by coke, metal modification is employed to not only improve coking resistance due to zeolite acidity altering, but also to introduce another type of active site [71]. Hydrodeoxygenation of phenolics can be promoted by transition metals (Ni, Co), and especially noble metals (Pt, Pd, Ru). Modification by alkali and alkaline earth metals introduces new Lewis acid sites and decreases the amount of Brønsted acid sites, hence preventing excessive cracking of the bio-oil and subsequent coke formation and deactivation of the catalyst [72]. The particle size and distribution crucially affect the catalytic activity of a bifunctional catalyst. On the one hand, metal modification can cause significant decrease in acid site density due to pore blockage. On the other hand, pore dimensions of the zeolite support influence metal particle size, distribution, and loading [73]. The synergistic effect between zeolite support and metal can be observed in the study of Shafaghat et al. [74]. The conversion of phenolics was in this case carried out via transformation of bio-oil to alcohols over a metal particle and subsequent alcohol dehydration over an acid site, hence achieving a complete O atom elimination.
Further performance improvement can be achieved by loading two different metals on a zeolite support. The study of Kumar et al. [75] developed a bimetallic CuNi/zeolite. The results showed that oxygenated compounds in bio-oil feed first deoxygenated by Cu sites and the remaining were further deoxygenated by Ni sites. The degree of deoxygenation with this catalyst was higher than with monometallic catalysts.
Pore size modification of zeolites by introducing mesopore structures can improve catalytic performance by enabling an easier access to acid centers inside the pores, as well as metal particles, in case of metal-modified zeolites [55,56,76]. Selected results of experiments with various hierarchical zeolites are summarized in Table 2. Zeolites modified by top-down methods, specifically desilication with various basic reagents, give the best performance if the treatment is mild, thus making the desilication more controllable, resulting in a higher number and strength of acid sites. Such zeolites result in higher yields of aromatics and decreased amount of coke in pyrolysis. The bottom-up methods of synthesizing zeolites involve using mesopore-directing agents such as 3-(phenylamino)propyltrimethoxysilane (PAPTS) or (3-aminopropyl)trimethoxysilane (APTS). The best aromatics yield is produced by catalysts with high crystallinity and a balance of micropores and mesopores in zeolite structure. The study of Gamliel et al. [77] has confirmed the findings that mild desilication produces the most active catalyst in conversion of biomass to aromatics. It also concludes that a comparison of zeolites synthesized by top-down and bottom-up methods is not straightforward, as there are differences in number of acid sites as well as their location.
When utilizing hierarchical zeolites which otherwise perform poorer as deoxygenation catalysts, a possible solution is co-pyrolysis of biomass with agents that generate hydrogen in situ or hydrogen-rich feedstocks, such as alcohols, plastics, tires, etc. Co-feeding of different waste plastics (HDPE, LDPE, PP, PET, PS) has been observed in multiple studies to decrease the proportion of oxygenated compounds and increase the C 5 -C 12 hydrocarbon content [78][79][80][81]. The addition of plastic materials also prolongs catalyst lifetime. Decomposition of plastics produces olefins which either react with furans or undergo dehydrogenation. Both pathways inhibit polymerization of oxygenates, which is the main cause of coke formation [82].
Methanol-to-hydrocarbons reaction is carried out within a temperature range of 350-550 °C, which is the same as some types of biomass pyrolysis. It is, therefore, possible to observe synergistic effects of these two reactions in copyrolysis of biomass and methanol. These include not only increased aromatics yield but also a decreased water content, which would otherwise cause catalyst destabilization [83]. Co-pyrolysis with other alcohols, such as methanol-ethanol mixture or fusel alcohol, has also been reported to produce similar results [84,85].

Acid-catalyzed hydrolysis
The main challenge in transitioning from mineral acids to heterogeneous catalysts in biomass hydrolysis are issues with mass transport. Bulky macromolecules can access acid sites only on the surface of microporous catalysts. There are several strategies to overcome this issue, including pre-processing the feedstock, creating mesoporous structures within the catalyst, or utilizing supercritical conditions. LCB can be pre-treated by physical, chemical, or biological methods. Physical methods include mechanical, thermal, ultrasound, microwave-assisted methods, etc. [86]. As reported by Millett et al., physical pre-treatment is an effective means to reduce particle size and crystallinity. With sufficient milling, the carbohydrates of the tested varieties of LCB were totally accessible to enzymes and hydrolysis was almost complete [87].
Biomass can be also pre-treated with acid or alkali solutions, organic reagents, or ionic liquids. Acid and alkali solutions attack mainly linkages between cellulose/hemicellulose and lignin and increase "porosity of a treated material", thus enabling a better penetration of reagents in the next technological steps. Organic solvents interact with lignin, increasing the accessibility of cellulose and hemicellulose. Ionic liquids react with cellulose due to its high polarity and hydrogen bonds, disrupting the three-dimensional network of cellulose [88]. According to a screening of 21 ionic liquids, 1-ethyl-3-methylimidazolium acetate was the most efficient one in cellulose and lignocellulose dissolution. A complete dissolution of 1% (w/w) Avicel was achieved within 20 min of treatment with this ionic liquid. Moreover, it can dissolve multiple types of wood partially or completely [89]. Lignin can be extracted from a biomass using deep eutectic solvents (DESs) in a form that is further utilizable as a source of phenolic compounds [90]. Jablonský et al. [91] examined delignification of kraft wood pulp by mixtures of alanine and lactic acid or choline chloride and lactic acid in ratio of 1:9. The achieved delignification efficiency was 37.8% and 43.3%, respectively. The mechanical properties of kraft wood pulp were similarly influenced by both DESs. In comparison to traditional oxygen delignification, application of DESs is an attractive alternative. This research team also tested six different DESs in fractionation of wheat straw [92]. The results have indicated that along lignin, a small amount of other components were extracted as well, therefore the selectivity of lignin isolation is low. The highest amount of lignin (57.9%) was extracted with a 1:1 mixture of choline chloride and oxalic acid dihydrate. The best selectivity (the least amount of holocellulose compared to extracted lignin) was achieved with a 1:10 mixture of choline chloride and lactic acid.
Biological pre-treatment methods utilize bacteria and fungi to degrade biomass with their enzymes. Some whiterot fungi have been reported to effectively delignify various LCB [93]. Other bacteria, on the other hand, produce cellulases [94]. The main drawback of these methods is their dependence on physicochemical conditions, such as temperature or pH [95].
Some of the possible modifications of heterogeneously catalyzed LCB hydrolysis to improve its efficiency are summarized in Table 3.
As previously stated, zeolites in general are not stable in hot liquid water conditions. Moreover, introducing hierarchical structure is often accompanied by a loss of framework aluminum and, therefore, loss of acidity. Both issues can be partially improved by modifying the zeolite with -SO 3 H functional groups. Several studies have confirmed that mesoporous SO 3 H-modified zeolites were more stable than their unmodified counterparts [96][97][98]. In the study of Hoang et al., sonication was used to assist the mixing and mass transfer inside the reaction. The two different solvent systems had an effect of 5-(hydroxymethyl)furfural (5-HMF). The dimethyl sulfoxide (DMSO)/H 2 O system achieved a higher yield due to its higher polarity and the deactivation of acid sites by THF [98].
Metal-modified zeolites can exhibit not only their inherent acidity, but also the ability to catalyze hydrolytic hydrogenation. Incorporation of metals usually weakens the acid sites, hence inhibiting further dehydration of monosaccharides to 5-HMF. In some cases, utilization of renewable insitu generated hydrogen can be useful. Luque [99] proposed a novel methodology for lignin deconstruction with formic acid and microwave assisted approach using metal nanoparticles supported on aluminosilicates The methodology involved the novel preparation of supported nanoparticle systems on aluminosilicates using a dry milling methodology. Noble (Pd, Pt, Ru, and Rh) and transition metals (Ni and Cu) were located on the external surface. Hence good accessibility and consequent catalytic activity was achieved for acidolytic bond cleavage of lignin.
The main disadvantage of metal-modified zeolites is the loss of surface area and pore volume caused by pore blocking [100]. Aspromonte et al. [101] additionally carried out hydrolysis of cellulose in water by supercritical conditions (400 °C, 25 MPa and 0.03 s) followed by an addition of Ag-modified zeolites. Supercritical conditions solubilized more than 99.9% of cellulose into soluble oligomers that further underwent catalytic hydrolysis into monosaccharides. The highest yield of glucose (77.0%) was achieved with mesoporous Ag-MOR. Using ionic liquids for LCB hydrolysis can improve zeolite stability, as well as promote cellulose dissolution. Such systems eliminate the need for biomass pre-treatment. According to Cai et al. [102], hydrolysis of cellulose in water yielded trace amounts of products. In contrast, hydrolysis in [BMIm]Cl under identical conditions yielded 15.6% cellobiose, 21.5% glucose, and 3.3% 5-HMF.
Acid-catalyzed hydrolysis of LCB in ionic liquids could be further accelerated by microwave irradiation [103].

Reactions of platform molecules
Platform molecules are a group of biobased chemicals that could be considered basic building blocks for the production of biofuels and chemicals. Their updated list according to Bozell and Petersen [105] is summarized in Table 4. These molecules have been chosen according to several criteria, such as capacity to replace existing petrochemicals, scalability, and existing technologies for their production from renewable carbon.
Zeolites exhibit catalytic activity in many reactions for production and subsequent utilization of platform molecules. The following chapters discuss the catalytic use of zeolites in various processes including platform molecules.

Furans (furfural, 5-HMF, furfurylalcohol)
Hydrolysis of hemicellulosic and cellulosic fractions of biomass produces various C 5 and C 6 sugars. These sugars are a feedstock to produce furanics, by means of acidcatalytic dehydration. In the case of furfural, this process is economically superior to synthesis from fossil fuels via catalytic oxidation of 1,3-dienes.
Furan compounds are starting materials for production of many value-added chemicals (Fig. 5), as well as very good fuel additives. The energy density of some furanic compounds is comparable to gasoline or diesel fuel [106,107]. Moreau et al. [108,109] were the first to examine the possibility of utilizing zeolites (ZSM-5, Beta, Y, mordenite) in dehydration of fructose to 5-HMF. Reactions were carried out in a biphasic water-methyl isobutyl ketone system. According to their findings, large-pore zeolites (dealuminited Y and Beta) produced high amounts of humins, hence the selectivity towards 5-HMF was low. Mordenite, thanks to a rapid diffusion of product from pores, produced 5-HMF with selectivity of ~ 90%. A study of H-mordenites with varying Si/Al ratios showed that lower ratios, resulting in highly acidic zeolites, promoted subsequent reactions, (polymerization, formation of levulinic and formic acid) hence decreased selectivity to 5-HMF. The optimal Si/Al ratio for fructose dehydration was 10-11.
Salakhum et al. [58] synthesized hierarchical ZSM-5 zeolites from scrap glass and sand by conventional hydrothermal synthesis. Water content in the synthesis gel and crystallization time were studied variables were. Higher water content led to a larger specific surface area, as illustrated by SEM images (Fig. 6). Longer crystallization times resulted in a higher porosity, crystallinity, and Si/Al ratio. Catalytic activity of these zeolites was tested in fructose dehydration into 5-HMF. Zeolite synthesized with 14-mol water content and crystallization time of 4 days resulted in 88.3% selectivity to 5-HMF at 87.3% fructose conversion. These results are superior to conventional ZSM-5, which achieved 85.8% selectivity to 5-HMF at 70.4% fructose conversion.
5-HMF can be synthesized in one step from cellulose, as demonstrated by Nandiwale et al. [110] Hierarchical ZSM-5 zeolite prepared by desilication enabled to achieve 46% 5-HMF yield at the 67% cellulose conversion.
Furfural can be synthesized from hexoses and pentoses. Industrial production is based on xylan-rich LCB (oat bran, wood chips, corncobs) by hydrolysis with mineral acids, commonly H 2 SO 4 , at temperatures of 150-240 °C. These conditions result in furfural yields between 50 and 90%. Heterogeneous catalysts could improve this process, as it would eliminate the use of corrosive reactants and production of high amounts of waste [111].
Kim et al. [16] examined synthesis of furfural from D-xylose in three solvent systems (DMSO, water, water -toluene) in the presence of zeolites. As water deactivates acid sites in hydrophilic zeolites by solvation, the best results were achieved with water -toluene system. No clear trend with various structure types of zeolites in different solvent systems was observed. Experiments with varying Si/Al ratios showed a decline in both D-xylose conversion and furfural yield with a rise in Si/Al ratio, which corresponds to a decrease in number and strength of acid sites.
Other research suggests that yields of furfural can be further improved by γ-butyrolactone-water solvent systems [112], as well as utilizing metallic salts [113]. Zhang et al. [113] further examined the effect of the γ-valerolactonewater solvent system in combination with Beta zeolite modified by ion exchange with ZrCl 4 , FeCl 3 , and SnCl 4 . In comparison to DMSO-water, which favored the formation of 5-HMF, the γ-valerolactone-water system produced higher furfural yields. Sn-Beta zeolite achieved a furfural yield of 66% and glucose conversion of 69.3%.
Furfuryl alcohol (FAL) is the most important derivate of furfural. Roughly 65% of world furfural production is converted into FAL. This chemical is further used mainly in the production of furanic resins. The conventional FAL production is based on furfural hydrogenation in the presence of copper chromite, i.e., Adkins catalyst [114].
Conventional catalysts are deactivated easily by sintering or leaching of the metal particles. Cao et al. [115] developed a more durable catalyst by encapsulating Cu nanoparticles into TS-1 zeolite in situ. Complexes of Cu 2+ ions and polyethylenepolyamine were introduced into the zeolite during the synthesis, as depicted on Fig. 7. For comparison, a catalyst prepared by conventional impregnation and both variants modified by Na + ion exchange were also prepared. The best results in furfural hydrogenation to FAL were achieved with Na + nanoparticle zeolite. Selectivity of FAL formation was 98.1% at furfural conversion of 93.0%. Apart from Cu nanoparticles, Na + ion exchange also contributes to the performance by reducing the number of strong Brønsted acid Paulino et al. [116] developed a direct route to a FAL production from xylose in the presence of a zeolite catalyst. This reaction consists of xylose dehydration into furfural and its subsequent hydrogenation with hydrogen formed in situ with the aid of an alcohol. The time-course curves of reactions with Beta, USY, and ZSM-5 zeolites are depicted in Fig. 8. The best results were achieved with Beta zeolite.
The shape of selectivity curves suggests that xylose first converts to xylulose, which then dehydrates into FAL. The second step is catalyzed by a strong Brønsted acid site. Experiments with varying water content in the system showed that water inhibits xylose conversion by deactivating Brønsted acid sites.

Levulinic acid
Levulinic acid (LevA) is another important platform molecule. It is a water-soluble organic acid with a carboxylic and a carbonyl group. LevA is a platform molecule for the production of succinic acid, acrylic acid, γ-valerolactone and others (Fig. 9). Conventional LevA production is based on hydrolysis of cellulose to monosaccharides, their transformation to 5-HMF and its subsequent rehydration to LevA and formic acid as a by-product. All these reactions are catalyzed by mineral acids, which cannot be considered green catalysts for reasons explained in previous chapters [117].  Jow et al. [119] were the first to examine D-fructose transformation to LevA using Y zeolite. The product distribution in time is depicted on Fig. 10. Compared to hydrolysis catalyzed with heterogeneous acids, 5-HMF yield is substantially lower. This can be ascribed to the shape selectivity of the used zeolite. As evident from the time-course of the reaction, the presumed reaction mechanism starts with a hexose ring opening catalyzed by a Lewis acid site located on the outer surface of the zeolite. Resulting linear sugar can diffuse into zeolite pores, where it dehydrates into 5-HMF. The molecule is too big to diffuse out of the pores, therefore it further dehydrates into LevA.n-Butyl levulinate is an important intermediate to produce solvents, plasticizers, and fragrances. Zeolites can be utilized as acid catalysts in esterification of LevA and n-butanol. Maheria et al. [118] tested various structure types in this reaction. Zeolite ZSM-5 performed poorly due to diffusion limitations of big molecules into the pores and formation of bulky intermediates inside the cavities. Consequently, a deactivation of the catalysts occurs. Mordenite had the highest amount of acid sites, to which the reactants adhered strongly and caused pore blockage. Zeolite Y had the lowest amount of strong acid sites; therefore, its catalytic activity was low. Zeolite Beta achieved the best performance because of an appropriate combination of pore size, specific surface, and moderate acidity.
As platform molecules are generally low-weight the goal of transformation of platform molecules into fuels can be achieved by condensation reactions. Thanks to the carbonyl group, LevA can undergo aldol condensation. Cueto et al. [120] studied aldol condensation of LevA and furfural catalyzed by zeolites. The examination of the reaction mechanism showed two possible products, resulting from an attack of hydrogen located on β-or δ-carbon. Preferred product was a result of the β-carbon attack. Zeolites ZSM-5 (Si/Al = 23), ZSM-5 (Si/Al = 80), MOR (Si/Al = 20), and BEA (Si/Al = 25) were tested. A correlation between results and structure types was not observed. The highest yield of both products (50%) was achieved with the ZSM-5 (Si/Al = 23) catalyst.γ-Valerolactone is a feedstock for synthesis of valeric acid, valeric esters, α-methylene-γ-valerolactone and other chemicals. These can be utilized as fuel additives. Conversion of LevA to valeric esters takes place in the following three steps: (i) hydrogenation of LevA to γ-valerolactone catalyzed by Pt/ TiO 2 ; (ii) acid-catalyzed ring-opening of γ-valerolactone to valeric acid; and (iii) esterification of valeric acid with alcohols (Fig. 11). For this process, Muñoz-Olasagasti et al. [121] developed a series of Pd-based catalysts using various supports (amorphous SiO 2 -Al 2 O 3 , ZSM-5, and Beta zeolite). The catalysts were prepared by incipient wetness impregnation with an aqueous solution of (NH 4 ) 2 PdCl 4 . A catalyst based on amorphous silica provided too weak acid sites for step (ii), and thus its activity was low. In contrast, acid sites of Beta zeolite were too strong and promoted subsequent reactions, therefore producing valeric esters with low selectivity. The best catalytic performance was achieved with the Pd/ZSM-5 catalyst-92% yield of desired products after 8 h of reaction. This activity can be ascribed to moderate acidity, which, apart from step (ii) of the mechanism) also affects Pd particle dispersion.

Lactic acid
Lactic acid (LA) or 2-hydroxypropionic acid, finds a wide range of use in the food industry and pharmaceuticals. It is an emerging building block of biodegradable plastics and solvents. It is also a starting material to produce acrylic acid, propylene glycol, acetaldehyde, and others (Fig. 12). The large-scale production of LA is based on batch-wise fermentation of glucose solution under anaerobic conditions. The fermentation requires continuous addition of calcium hydroxide over a period of 2-4 days to balance the pH on values from the range 5-8 depending on the process (types of enzymes) to ensure optimal performance of the bacteria. This process also includes complex purification methods. Other, potentially more economically viable methods for LA production include isomerization of C 3 sugars or conversion of cellulose [122,123].
Lewis acid zeolite catalysts made by an incorporation of Ti, Zr, and Sn have been proven active in dihydroxyacetone and glyceraldehyde isomerization to LA [125]. Catalysts based on transition metals such as tin have multiple drawbacks like complex and lengthy syntheses and scarce availability of metal precursors for the syntheses.
On this premise, West et al. [122] examined the catalytic activity of commercially available zeolites (H-USY, H-Beta, H-ZSM-5, H-MOR, H-montmorillonite) in this reaction. Importance of acidity is underlined by the dehydration to pyruvaldehyde, which is the first step of isomerization, as corroborated by the observed reaction time-course. Selectivity to LA/methyl lactate was similar in water and methanol used as solvents. H-USY with Si/Al ratio of 6 provided the best results. The next structure types in rate of performance were H-Beta and H-ZSM-5 zeolites. All other zeolites had lower Si/Al ratios than the best H-USY.
Dapsens et al. [126] prepared a series of hierarchical ZSM-5 zeolites by desilication for dihydroxyacetone isomerization into LA. The most effective catalysts were synthesized by treatment with NaOH solution. This method promoted realumination, leading to generation of Lewis acid sites, which did not occur with steaming, impregnation with aluminum ions, or other realumination methods. A concentration of 0.6 M proved to be the best method. LA selectivity and dihydroxyacetone conversion achieved with this catalyst were both over 90%.
Cellulose can also be a feedstock for LA production. It requires a three-step reaction of cellulose: (i) hydrolysis, (ii) glucose isomerization to fructose, and (iii) fructose transformation to LA via retro-aldol reaction. As these reactions require both Brønsted and Lewis acid sites, Ye et al. [123] developed bifunctional yttrium modified Beta zeolites for this reaction. Beta zeolite dealuminated with HNO 3 was ground with yttrium nitrate. 100% conversion of cellulose was achieved with all the catalysts with different yttrium loading, as well as parent H-Beta and dealuminated H-Beta.
Product distributions of these reactions are summarized in Fig. 13. Main by-products were glucose, fructose, and 5-HMF. Catalysts with 10% and 15% loadings provided the best LA yields-about 50%. Activity of the modified zeolites was also evaluated on several types of LCB-bamboo, pine, and rice husk. Based on the C 5 and C 6 monosaccharide units in the raw biomass, the LA yields were 75.9%, 60.8%, and 44.0%, respectively, using the 10% yttrium catalyst. Acetaldehyde is one of the possible products of LA conversion into value-added chemicals. It can be further transformed into acetic acid, acetate esters, pentaerythritol, and pyridine bases. Current production relies on ethylene oxidation in the presence of PdCl 2 -CuCl 2 catalyst, the socalled Wacker process. Interesting modification/heterogenization of the Wacker process has been introduced by Imbao et al. [127], who anchored Pd-Cu species into the Y (SiO 2 /Al 2 O 3 = 5.1) zeolite by the ion exchange method. Yields equal to more than 90% were achieved with the best catalysts in a fixed-bed plug flow reactor at 378 K with a gas feed consisting of ethylene, water, and oxygen (1:7:10 molar ratio) diluted in helium at 378 K. Sad et al. [124] synthesized acetaldehyde in gas phase using LA as a starting material in the presence of three zeolites-H-MCM-22, H-ZSM-5, and Na-ZSM-5. The most promoted reaction pathway in all the three cases was LA decarbonylation into acetaldehyde. Other by-products were acrylic acid, propanoic acid, 2,3-pentanedione, and lactic acid oligomers. H-MCM-22 contained the highest acid site density; hence it promoted LA condensation and polymerization. Acetaldehyde yield was the lowest in this case. H-ZSM-5 contained less strong Brønsted acid sites, therefore it performed with a higher acetaldehyde yield. The amount of coke formed from LA oligomers was still significant. Na-ZSM-5 produced the pest results, as its acid sites were mostly of weak or intermediate strength. Acetaldehyde yield in this case was 64.0% at 100% conversion of LA. LA can be an alternative feedstock to produce acrylic acid, which is the primary building block of all acrylate polymers and plastics. Its current conventional production is based on propylene oxidation. The main disadvantage of the alternative route are the competing reactions of acetaldehyde formation, decarboxylation, and hydrogenation to propanoic acid. Sun et al. [128] demonstrated that Na-Y zeolite modified with various potassium salts are selective and durable in this reaction. As expected, acetaldehyde was the main by-product. The best results were achieved with the Na-Y zeolite modified by potassium iodide-67.9% selectivity for acrylic acid at 97.6% LA conversion. Zeolites modified by potassium halogenides in general produced better results than other salts used (nitrate, sulfate, phosphate, etc.).

Succinic acid
Succinic acid (SA) or 1,2-ethanedicarboxylic acid is an important C 4 platform molecule with applications in food, agriculture, and pharmaceutical industry. It can potentially replace maleic anhydride in the production of several bulk chemicals such as γ-butyrolactone, tetrahydrofuran, 1,4-butanediol, succinimide, succinic esters, and others. Petrochemical production of SA is based on paraffin oxidation or maleic acid and maleic anhydride hydrogenation. Alternatively, bio-based routes for SA production include mainly fermentation of C 5 and C 6 sugars, as well as their catalytic dehydration and subsequent furan ring opening.
Conversion of SA to various products requires a high purity of SA crystals (99.0%). During bacterial fermentation, by-products are generated, mainly acetic acid and formic acid. As purification of bio-based SA from the fermentation medium represents more than 60% of the total cost of the process, it is necessary to develop a more efficient process for bio-SA to be economically competitive [129].
Separation of SA from fermentation broth and fermentation byproducts has been attempted by Efe et al. [130] in sorption columns packed with zeolite pellets (HiSiv 3000, Na + form). The results show that SA could be efficiently separated from its salts, as well as from acetic acid, which has lower affinity to the zeolite. The sorbent could be regenerated easily by desorption with water at 80 °C. Fermentation components caused fouling which could be solved by calcination.
Fergani et al. [131,132] synthesized SA by catalytic wet oxidation of glucose in the presence of Nb-zeolites. Several structure types (Beta, Y, ZSM-5) were impregnated with Nb ethoxide after dealumination. Reactions with glucose were carried out in liquid water with oxygen as a sole oxidation reagent. A two-step mechanism has been suggested for this reaction. In the first step, EFAl species dehydrate glucose to LevA, followed by its oxidation on nano-Nb 2 O 5 particles within zeolite channels (Fig. 14).
As the reaction is initiated by a reaction with EFAl, it is easily explained that zeolites with lower initial Si/Al ratio perform with better selectivity to SA, since a higher amount of EFAl is generated when these zeolites undergo dealumination. Catalytic wet oxidation of glucose produced SA with better selectivity than reactions with 5-HMF. In these reactions, two other major products were formed-LevA and maleic acid.
Palai et al. [133] developed a method of SA synthesis based on Baeyer-Villiger oxidation of furfural with H 2 O 2 in the presence of Lewis acids. Zeolites with tetravalent cations (Ti 4+ , Sn 4+ ) in the framework instead of aluminum exhibit only Lewis acidity. The zeolite TS-1 has been reported to be active in furfural oxidation to maleic acid [134]. On the other hand, Sn-Beta zeolite catalyzed furfural oxidation to SA. In the study, Sn-Beta catalyst was prepared by dealumination and subsequent grinding with Sn(II)acetate. This catalyst produced SA with a selectivity of 53%; maleic acid, malic acid and 2(5H)-furanone as the main by-products.
Important fine chemicals and polymers are produced by esterification of SA. Acid catalyzed esterification can be carried out in the presence of H-form zeolites.
Le et al. [135] synthesized diphenyl succinate via a direct esterification of SA with phenol over various structure types of zeolites. Zeolite H-Beta demonstrated the best catalytic activity, with SA selectivity of over 99% and phenol conversion of 51%. This could be ascribed to an interconnecting 3D channel structure, which out of the tested zeolites only H-Beta possessed. The study of Si/Al ratio effect showed that with an increase of the ratio, and parallel decrease in acidity, up to 150, SA yield sharply increased up to about 80%, followed by a rapid decline to about 45% at Si/Al ratio of 440.
Parmar et al. [136] tested the catalytic activity of H-ZSM-5 zeolite in SA esterification with ethanol. Kinetic measurements showed that the first step, i.e., the creation of monoester, was much more rapid than the subsequent esterification of the second acid site, as evident by Fig. 15. An increase in molar ratio of ethanol to SA led to a higher yield of diester after 9 h of reaction.

Alcohols
Production of several types of alcohols from biomass has been already well established. For short-chain alcohols, such as methanol, ethanol, or butanol, the mass industrial production by fermentation is already well established [137][138][139]. Some alcohols can be synthesized by catalytic transformation of synthesis gas, for which biomass can be a feedstock [140][141][142]. The main by-product of oleaginous biomass transformation into biodiesel is glycerol. Its worldwide production reached 36 billion liters in 2018, [143] hence its utilization would greatly improve the economic feasibility of the biodiesel industry. Industrial production of cyclic carbon chain alcohols from biomass has not yet been established.
There is, however, research on their production from platform molecules, such as catalytic synthesis of cyclopentanol from furanic compounds [144,145]. These examples prove that biomass is a feasible feedstock for production of alcohols, which can be then utilized in a variety of processes.
An interaction of alcohol with a zeolite can lead to the following three possible outcomes: (i) ether by condensation, (ii) olefin by dehydration, or (iii) carbonyl compound through complex reactions. A reaction of an alcohol over a Brønsted acid site causes dehydration, either intermolecular, producing an ether, or intramolecular, resulting in an olefin. An olefin can further isomerize, oligomerize or form coke, therefore deactivating the zeolite. Carbonyl compounds are formed by an interaction with a basic site capable of dehydrogenation. Zeolites containing alkali metal cations are active in this type of reaction [146].
Rownaghi et al. [147] synthesized a symmetrical dimethyl ether by dehydration of methanol in the presence of ZSM-5 zeolite with various particle sizes. Crystal sizes of prepared zeolites were 0.12 μm, (Nano-ZSM-5), 0.30 μm (Meso-ZSM-5), and 1.20 μm (Con-ZSM-5) (Fig. 16). The highest methanol conversion was achieved with Nano-ZSM-5. Compared to conventional ZSM-5, Meso-ZSM-5 also resulted in a higher conversion. Reaction temperature 270 °C led to a subsequent reaction of C 1 to C 10 carbohydrates from dimethyl ether. The decrease in selectivity to dimethyl ether was the highest with the conventional ZSM-5. At 320 °C, all catalysts produced these carbohydrates exclusively.
Zeolites have proved to be active in synthesis of unsymmetric ethers as well. Several possible strategies can be utilized in the production of these ethers.
Soták et al. [148] synthesized cyclopentyl methyl ether by intermolecular dehydration of cyclopentanol and methanol using various structure types of zeolites. The main challenge of this reaction was catalyst optimization towards the highest selectivity to the unsymmetric ether, as the competing reactions led to two possible symmetric ethers, as well as cyclopentanol dehydration to cyclopentene. Strong acidity, low Si/Al ratios as well as high reaction temperatures promoted cyclopentene formation. The optimal combination of pore structure and acidity of zeolite ZSM-5 (Si/Al = 25) produced the highest selectivity to the unsymmetric ether (> 85%) at cyclopentanol conversion of 96%.
Alkyl tetrahydrofurfuryl ethers have shown promising for bio-diesel production. Cao et al. [149] synthesized this type of compound in two steps by etherification of furfuryl alcohol with methanol or ethanol and subsequent hydrogenation of the ether. Two catalysts were used: H-ZSM-5 with varying Si/Al rations for the first step and Raney Ni catalyst for the second step. The first step provided a bigger challenge for reasons similar to the research of Soták et al. The best results were achieved with a Si/Al ratio of 25 (58.9% selectivity of methyl ether production at 94.2% furfuryl alcohol conversion). Hydrogenation with Raney Ni catalyst was almost quantitative with both methyl and ethyl ether.
Yang et al. [150] also examined the production of furanic ethers. tert-Butoxy methyl furfural was synthesized from 5-HMF and tert-butanol in the presence of various zeolites, as well as H 2 SO 4 and p-toluenesulfonic acid for comparison. Zeolite Y (Si/Al = 6) achieved the best performance, thanks to a network of three-dimensional channels and supercages allowing a large intermediate formation. The ether formation selectivity was 94% at 59% 5-HMF conversion. H 2 SO 4 and p-toluenesulfonic acid promoted mainly 5-HMF dimerization.
Challenges of unsymmetric ether synthesis from two alcohols can be avoided by addition of an alcohol to an olefin. Ruppert et al. [151] studied the effect of various heterogeneous catalysts on addition of bio-glycerol and glycols on 1-octene. Reactions with glycerol resulted in mono-and diethers. Triethers did not form due to steric constraints. From the tested structure types (H-Beta, H-USY, H-ZSM-5), H-Beta with Si/Al ratio of 12.5 achieved the best performance, with 95% cumulative selectivity mono-and diether at 19% glycerol conversion. This catalyst possessed an appropriate pore structure, as well as hydrophilic properties not demonstrated by zeolites with higher Si/Al ratio. This catalyst was also active with glycols and other olefins-1-hexene, 1-dodecene, and 1-hexadecene.
Etherification is the first step in the methanol to olefins process. This reaction consists of three following steps: Ramasamy and Wang [152] investigated the alcohols to hydrocarbons process catalysed by H-ZMS-5 with Si/Al ratio of 30. Schemes of these reactions for methanol, ethanol and n-butanol are summarized in Fig. 17. After 2 h time on stream, the majority of products with all alcohols were aromatics. After 24 h, the products further transformed into olefins. The catalyst lifetime was longer for n-propanol and n-butanol due to low activation energy for the oligomerization of the respective dehydration products. n-Propanol and n-butanol primarily generate olefin-rich compounds, which can be hydrogenated to produce high-value paraffin hydrocarbons.
Tynjälä et al. [153] researched formation of olefins from methanol, ethanol, and C 3 alcohols. Catalysts used in this reaction were H-ZSM-5, P1ZSM-5, and P2ZSM-5. P1ZSM-5 and P2ZSM-5 were prepared by impregnation and chemical vapor deposition of trimethyl phosphite. This modification, which targeted Brønsted acid sites, has resulted in weaker acid sites and improved stereoselective properties. Methanol conversion catalyzed by these modified zeolites stopped at the first step due to acid sites being too weak for ether conversion to hydrocarbons. After longer reaction times, the ether converted to methane, water, and carbon monoxide. Ethanol converted to diethyl ether and subsequently to ethylene. The second step (production of olefins) requires higher reaction temperatures. The parent zeolite catalyzed formation of higher hydrocarbons from ethylene, while modified zeolites did not further promote the reaction. Propyl alcohols readily converted to linear hydrocarbons without any unconverted ethers at the end of the reaction.
Biofuel industry also involves the production of fatty acid methyl esters (FAME) which can be, among other uses, utilized as a biodiesel. Their production relies on transesterification of vegetable oils and animal fats. This process requires a basic catalyst, which is conventionally homogeneous-a solution of alkali metal hydroxides. Leclercq et al. [154] developed a cesium modified NaX zeolite catalyst for transesterification of rapeseed oil with methanol. Catalysts prepared by ion exchange with cesium chloride, cesium content of about 30% exhibited the best performance, with oil conversion of 70% and selectivity to methyl ester of 96%. Catalysts with higher cesium contents performed poorer due to steric constraints and aggregates formation.
Suppes et al. [155] prepared a series of NaX and ETS-10 (titanium-containing zeolite) catalysts modified by potassium, cesium, as well as NaX occluded with sedum azide and acetate. All these catalysts were tested on transesterification of soybean oil with methanol. They also examined if metal shavings of the walls of reactor affects transesterification. As the metal shavings demonstrated catalytic activity, nickel in particular, tests with prepared zeolites were carried out in a glass vial. From the modified versions of NaX zeolite, catalysts with occluded sodium acetate and azide species performed the best (yield of methyl esters > 90% at 100% triglyceride conversion) across all reaction temperatures, due to an increase in number and strength of basic sites. NaX zeolites ion-exchanged with potassium and cesium performed similarly to parent zeolite-15-20% methyl ester yield at 35-40% triglyceride conversion. ETS-10 zeolite performed overall better than all modified NaX catalysts, with 90-95% yield of methyl esters at total triglycerides conversion.

Exploitation of glycerol
Glycerol is an abundant product of biomass-based oils transesterification. There are several ways to utilize this chemical in production of value-added chemicals and many of these processes are possible to catalyze by zeolites (Fig. 18).
Acid-catalyzed dehydration of glycerol produces acrolein. This compound finds use in polymer and detergent production. A variety of heterogeneous catalysts have been reported for this reaction in gas phase, including zeolites. One of the main challenges of utilizing zeolites in this process is their deactivation by coke formation. This phenomenon can be avoided by introducing mesoporous structure into the zeolite. Fernandes et al. [157] examined the difference in activity of conventionally prepared silicoaluminophosphate SAPO-40 and its hierarchical counterpart synthesized with the aid of an SDA. The mesoporosity considerably reduced the constraints to reactant and product diffusion, enabling a better access to the acid sites and reducing coke formation. Compared to conventional SAPO-40 in the same reaction conditions, the hierarchical catalyst was more active and significantly less deactivated by coke.
Glycerol can also be used as a feedstock for pyrolysis. While non-catalytic pyrolysis results in a range of gaseous products (H 2 , CO, CH 4 , C 2 H 4 , etc.), catalytic pyrolysis can produce low molecular weight aromatics (bio-BTX). In this process, glycerol first dehydrates into oxygenates (acrolein, acetaldehyde), which then form aromatics after a series of acid-catalyzed reactions. He et al. [158] investigated ZSM-5(23)/bentonite catalyst formulations in a bench scale unit. BTX yields from several consecutive runs in between thermal regenerations of the catalyst show that the catalyst deactivated within the first 5 h (decrease in yield from ~ 8% to 7-5%). Deactivation occurred due to coke formation. After one regeneration, only about 95% of initial activity was regained. This could happen due to collapse of the bentonite structure or reduction of catalyst acidity.

Summary and outlook
This mini review summarizes some of the potential use of zeolites as catalysts throughout the downstream processes of biomass valorization. Based on the already established processes of crude oil refining in which zeolites pose as catalysts, the logical extension of their use is for other, renewable carbon(-oxygen) feedstocks. The most attractive properties of zeolites are their intrinsic acidity, both Brønsted and Lewis, of varying strength, their porous structure and large surface area. Zeolites can undergo ion exchange, therefore are excellent supports for various metal particles that can then provide additional Lewis acid centers, as well as redox functionality to the catalyst. Replacing homogeneous acid catalysts in biomass transformation processes such as furfural production could diminish their ecological impact, as use of homogeneous acids causes difficult product separation, catalyst re-generation, and are corrosive and toxic in some cases. The main challenges of utilizing zeolites in biomass exploitation are high molecular weight compounds in the substrate that are unable to diffuse into pores of pristine zeolites and their poor stability in hot liquid water conditions and with corrosive reactants/products. Introducing hierarchical porosity is a strategy that can combat both these issues to an extent. There is a need to further research these processes, mainly to achieve their economic feasibility. Apart from optimization of reaction conditions for the best possible selectivity of the desired products, developing methods for synthesis of durable zeolites with stable catalytic activity, good recyclability and good regenerability could greatly improve process costs. Multifunctional zeolite catalysts with macro-microporous hierarchical structure seem to be the best candidates for applications. For example, zeolites modified by top-down methods, specifically desilication with various basic reagents, give the best performance if the treatment is mild, thus making the desilication more controllable, resulting in a higher number and strength of acid sites. This can be inspiration for further research and development.

Data availability Not applicable.
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