From peanut shell to 2, 5-dimethylfuran: the role of Zr-micro/mesoporous zeolites

doi:10.21203/rs.3.rs-3757524/v1

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From peanut shell to 2, 5-dimethylfuran: the role of Zr-micro/mesoporous zeolites

https://doi.org/10.21203/rs.3.rs-3757524/v1

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Catalytic pyrolysis of biomass is a promising process for the conversion of agricultural residues into renewable bio-oils, wherein zeolites play a key role in their upgrading. In this work, peanut shell pyrolysis was catalysed by micro/mesoporous ZSM-11 zeolites modified by the incorporation of zirconium. Mesopores were generated in the microporous zeolitic structure by post synthesis alkaline treatment using potassium hydroxide as basic medium. The effect of this treatment and its parameters were studied in detail. The obtained materials were extensively characterized. Results showed that micro/mesoporous ZSM-11 zeolites maintained their crystalline structure after the desilication process and presented excellent textural properties, such as high BET surface. Zirconium incorporation increased the amount of Lewis and Brønsted acid sites. Pyrolysis tests in presence of this material remarkably increased the selectivity to 2,5-dimethylfuran (DMF), a potential biofuel with high energy density, among other high value products for the chemical industry. Therefore, micro/mesoporous zeolite modified with Zr is presented as an excellent catalyst for pyrolysis of agricultural wastes.

5-dimethylfuran

micro/mesoporous ZSM-11

Zr-zeolites

pyrolysis

agricultural waste

It is well known the serious problem caused by biomass residues all around the world. Particularly, in the peanut industry the shells are not yet conveniently used. Peanut shells fast pyrolysis is proposed as an adequate treatment to obtain high value products for the chemical industry. Considering the importance of active catalyst with high thermal stability for the process, micro/mesoporous zeolites modified with zirconium are excellent candidates to improve the performance and quality of the liquid products obtained. These materials showed good performance toward obtaining DMF, a renewable liquid fuel of new generation.

ZSM-11 (Zeolite Socony Mobil 11) zeolites, also known as MEL zeolites (Mobil Eleven), are microporous crystalline aluminosilicates constructed by tetrahedral SiO₄ and AlO₄⁻ linked together through the oxygen vertices forming a three-dimensional network. Their structural advantages such as uniform and well-defined pores, number and strength of acid sites inside the micropores, large specific surface area, high thermal and hydrothermal stability, high ion exchange capacity (with the ability to introduce new strong acid sites) and shape selectivity make these materials ideal candidates for multiple purposes [1, 2]. Particularly, the use of zeolites as heterogeneous catalysts in oil reforming, coal conversion, petrochemical industry and organic synthesis to produce high value-added chemicals and fuels have been abundantly reported in the scientific literature [3–5]. Furthermore, biomass conversion via catalytic pyrolysis and bio-oil upgrading using these materials as catalysts are among the emerging processes that have attracted significant interest in recent years [6, 7].

Despite their numerous advantages and excellent properties, the presence of micropores in zeolites often cause limitations in reactions involving reactants or products with sizes beyond the micropores dimensions that cannot diffuse to and from the active sites inside the zeolite channels. This restriction reduces molecules mass transfer, decreasing the reaction rate and causing polymerization problems. Finally, catalytic active sites within the microporous channels get blocked. This affects the catalytic efficiency producing deactivation of the zeolites [2, 8].

An effective solution to overcome these diffusional problems is the introduction of secondary porosity, in most cases mesopores (2–50 nm), into the microporous structure. Thus, the so-called hierarchical zeolites are obtained. Mesopores presence represents a real advantage since mass transfer limitations are reduced. Therefore, bulky molecules can be transformed during the reaction through the auxiliary mesoporous channels [6].

Hierarchical zeolites preparation pathways could be summarized in two main techniques: direct synthesis, by adding mesoporous templates in the synthesis gel, and the post-synthetic treatments. The former involves the use of auxiliary mesotemplates such as cationic surfactants (CTAB) [9], cationic polymers [10] or natural macromolecular polymers (glucose, sucrose, starch, carboxymethyl cellulose) [11, 12], among others. The mesopore template is introduced into the zeolite framework during the synthesis process and further is thermally removed to generate additional porosity in the zeolitic structure.

In contrast, post-synthesis modifications involve the extraction or selective leaching of structural portions of the synthesized microporous zeolite by controlled destruction of the crystals. In this sense, the best-known post synthetic strategies are dealumination and alkaline treatments, known as desilication.

Dealumination causes the cleavage of Si-O-Al bonds and, further, the selective removal or extraction of aluminium atoms. This acid treatment leads to vacancies formation and structure collapse. The vacancies in the zeolite framework constitute an additional secondary porosity, usually in the mesopore range. Nevertheless, this treatment could be difficult to control. Then, poorly interconnected mesopores are randomly generated in the zeolitic structure, which limits large molecules transport [13].

In contrast to dealumination, alkaline treatments basically consist in the selective extraction of silicon atoms from the zeolite framework. Si is hydrolysed and dissolved in the presence of an alkali such as KOH, LiOH, Na₂CO₃, NH₄OH or NaOH. The intracrystalline mesopores induced by this method are interconnected allowing access from the external surface to the zeolite crystal, which is an advantage for reactions with limited access and impeded diffusion [14].

In the present paper, the preparation of hierarchical ZSM-11 zeolites by alkaline treatment using potassium hydroxide (KOH) solution as alkaline medium is reported. The effect of desilication on zeolite structure, crystallinity, textural properties and morphology was studied. Moreover, zirconium was incorporated in the hierarchical zeolites. This metal was selected since its moderate acidity and oxidizing capability present it as an ideal candidate for biomass derivatives reactions. Thus, modified materials were also characterized and evaluated as catalysts [15–17].

Finally, considering the importance of micro/mesoporous zeolites in different chemical processes, these zirconium–modified materials were evaluated in pyrolysis reactions for the valorization of peanut shell, an agricultural waste. According to the United States Department of Agriculture (USDA), peanut global production is around 50,535 (1000 MT) for 2023 [18]. From that 25% corresponds to the shell that represents the residue of peanut’s production process. Considering the huge volume of bio-waste regularly generated, its valorization is mandatory.

The catalytic pyrolysis proposed for peanut shell valorization consists in the thermal biomass decomposition in oxygen absence, followed by a catalytic bed where controlled reactions take place [19]. From this process three products lines are obtained: bio-oil, bio-char and gas. The liquid product is expected to be an important source of platform molecules for the chemical industry and fuels in the future. Particularly, 2,5-dimethylfuran (DMF) emerges as a high potential biofuel. According to Román-Leshkov et al [20], DMF possess high RON (119) and energy density (30 MJ/L) that make it a good candidate for energy purposes. Hence, selective catalysts are required that promote it obtaining from renewable sources. In this work Zr-zeolites effect in DMF yields, among other interesting products, from peanut shell pyrolysis is studied.

2.1 Synthesis of microporous ZSM-11 zeolite

The materials employed as reactants for the synthesis of traditional ZSM-11 zeolite were silicon dioxide (SiO₂, Aldrich) as silica source, sodium aluminate (NaAlO₂, Strem Chemical) as aluminum source, tetrabutylammonium hydroxide (TBAOH 40% in water, Fluka); as structure directing agent and distilled water. Microporous ZSM-11 zeolites were synthesized following the hydrothermal treatment reported by Chu [21]. A typical preparation could be described as follows: NaAlO₂, SiO₂ and TBAOH were dissolved in distilled water under continuous stirring at room temperature (25 ºC) until getting a homogenous gel. Then, reaction gel was introduced into a Teflon lined stainless steel autoclave for crystallization. Conventional heating at 140°C was employed under autogenous pressure for 14 days. After crystallization processes, the autoclave and the vessel reactors were cooled at room temperature. The obtained solid product was filtered, washed with distilled water until neutral pH, and dried at 110°C overnight. Finally, the solid was desorbed under N₂ flow (20 mL/min) at 500°C and calcined in static air in a muffle furnace at the same temperature for 8 h to remove the organic structure directing agent trapped in the pores. The obtained material is called ZSM-11 T.

The ammonium form of the zeolite (NH₄-ZSM-11 T) was obtained by ionic exchange with a 1 M ammonium chloride aqueous solution (NH₄Cl, Biopack) at 80°C during 40 h. Finally, the sample was filtered, washed with distilled water and dried at 110°C for 24 h. To obtain the protonic form of the final material (H-ZM-11 T), the NH₄-ZSM-11 T zeolite was desorbed under N₂ flow (20 ml/min) at 500°C and calcined in static oxidant atmosphere at the same temperature for 8 h.

2.2 Preparation of hierarchical ZSM-11 zeolite

The post-synthesis modifications of traditional matrix ZSM-11 was executed by alkaline treatment with KOH solution (Strem Chemical). The traditional zeolite was treated with 0.3 M KOH solution under magnetic stirring at 65 ºC during 30 min. For a better temperature control, a thermostated water bath was employed. The process was stopped using an ice bath and the obtained product was filtered, washed with distilled water until neutral pH and dried overnight at 110°C. The micro/mesoporous zeolite obtained was denoted as ZSM-11-KOH.

Finally, NH₄-ZSM-11-KOH was prepared following the same protocol previously described for the ZSM-11 material.

2.3 Zirconium incorporation on traditional and micro/mesoporous zeolites

The zeolites treated with NH₄-zeolites (microporous and hierarchical) were modified by the incorporation of Zr by wet impregnation. This method consisted in suspending the NH₄-form of the zeolites in an aqueous solution of zirconium chloride octahydrate (ZrCl₄.8H₂O, Sigma Aldrich), loading 2 wt% of the metal. Samples were then introduced in a rotator-evaporator at 80°C under vacuum until complete dryness. Finally, the zeolites were thermally treated under nitrogen flow at 500 ºC (8 h), followed by calcination in static air at the same temperature to obtain the Zr-zeolites. The prepared materials were named Zr-ZSM-11 T and Zr-ZSM-11-KOH.

The catalytic performances of the as-prepared zeolites were evaluated in fast pyrolysis of peanut shell to produce platform molecules of great interest in the fine chemical industry. The liquid product of the process (bio-oil) is a blend of organic compounds. It is expected to be an important fount of chemicals and fuels in the future, since biomass is the only renewable energy source that contains carbon.

Peanut shell was the feedstock selected for the evaluation of the catalytic performance of ZSM-11 zeolites. Complete characterization of this biomass can be found elsewhere [22].

Biomass pyrolysis reactions were done in a fixed bed glass reactor (23 mm i.d., 290 mm length) under 60 mL/min of N₂ for 10 min. For each experience 1 g of the shells was located in a sample carrier over the catalytic bed. The latter, being a mixture of 1 g of the selected zeolite and 7 g of milled quartz. The reactor was introduced in an electric furnace at 500°C with a thermocouple to monitor the temperature during the reaction. The studies were carried out for 10 min, under inert atmosphere with a N₂ flow of 60 mL/min regulated by a mass flow controller.

Condensable vapors were collected at the reactor output in a condenser ( < − 10°C). Three repeats of every experimental run were done. Average values are reported.

4.1 Zeolites

X-ray diffraction (XRD) patterns were obtained with a Philips X`pert PANanalytical diffractometer using CuKα radiation. Diffraction data were recorded between 2θ = 5–60º at an interval of 0.026º. A scanning speed of 5º/min was used.

Textural properties of the samples were determined by means of N₂ adsorption and desorption isotherms in a Micromeritics ASAP 2000 at − 196°C. The samples were outgassed for 2 h at 300°C before each measurement. The surface areas were calculated from the adsorption branch employing the Brunauer–Emmett–Teller (BET) method. The pore size distribution was calculated using the Barrett–Joyner–Halenda (BJH) model. Microporous volume (V_Micro) was determined from the t-plot method, while the total pore volume (V_total) was determined at P/P₀ of 0.974, assuming complete filling of the pores. Mesoporous volumes (V_Meso) were obtained from V_total and V_Micro differences.

Si/Al ratios were determined by inductively coupled plasma analysis (ICP-AES, PerkinElmer 2100DV) after microwave digestion of the solids. Zirconium content in the zeolites was determined by X-ray fluorescence spectroscopy (EDX-720, Shimadzu).

The coordination of Al atoms in the hierarchical ZSM-11 framework was determined by Solid-state magic angle spinning nuclear magnetic resonance (MAS NMR). ²⁷Al MAS NMR spectra were recorded on a 7T Bruker Avance II-300 spectrometer equipped with a magic angle spin probe at room temperature by a single pulse and the Al chemical shifts were referenced to Al(NO₃)₃ 9 H₂O.

Fourier-Transform Infrared (FTIR) spectroscopy was applied in order to determine acid sites type and concentration in the materials. Spectra were recorded with a Thermo Nicolet iS10 after outgassed and later pyridine absorption as probe molecule.

4.2. Pyrolysis products

Products yields were calculated employing equations (1), (2) and (3), where \({M}_{bio-oil}\), \({M}_{bio-char}\) and \({M}_{o}\) are the masses of bio-oil, bio-char and peanut shells, respectively.

\({Y}_{bio-oil} \left(wt\%\right)=\frac{{M}_{bio-oil}}{{M}_{o}}\bullet 100\) Eq. (1)

\({Y}_{bio-char} \left(wt\%\right)=\frac{{M}_{bio-char}}{{M}_{o}}\bullet 100\) Eq. (2)

\({Y}_{gas} \left(wt\%\right)=100-\left[{Y}_{bio-oil}+{Y}_{bio-char}-{Y}_{coke}\right]\) Eq. (3)

The compounds present in the bio-oil were identified by the GC-MS technique in a Perkin Elmer Claurus 600 Gas Chromatograph coupled with an ion trap mass spectrometer in full scale mode (m/z 40 to 550) and an Elite 5MS capillary column. He was used as carrier gas. The NIST MS library was employed to identify the chromatographic peaks. Peak area calibration was accomplished through specific response factors for each chemical group. Compounds selectivity was calculated by Eq. (4), where \({A}_{i}\) is the absolute peak area of compound \(i\) and \({R}_{i}\) is the response factor of compound \(i\).

\({S}_{i}\left(\%\right)=\frac{{A}_{i}\bullet {R}_{i}}{\sum _{i=1}^{n}{A}_{i}\bullet {R}_{i}}x 100\) Eq. (4)

5.1 Characterization of the microporous and hierarchical zeolites

In order to confirm the structure and crystallinity of the prepared materials, XRD was done. The XRD patterns of traditional and hierarchical ZSM-11 zeolites are compared in Fig. 1. As can be seen, both materials exhibit the typical diffraction peaks of microporous traditional ZSM-11 zeolite which are in the range between 2θ = 7–9° and 2θ = 23–24°, with well resolved peaks, without signals of other crystalline phases or amorphous phases (widening of the base of the peaks). These XRD results reveal that KOH is an active base for the extraction of silica in the zeolite framework without deteriorating the original matrix structure.

Figure 2 shows N₂ adsorption and desorption isotherms obtained at − 196 ºC. Pore size distributions were calculated from the BJH adsorption branch of the isotherms for both materials. The traditional ZSM-11 zeolite exhibits a typical type I isotherm, according to the IUPAC classification, without hysteresis loops, which is a characteristic of microporous materials. This isotherm type is concave respect to the P/P₀ axis. As could be seen, it rapidly increases N₂ adsorption amounts at low relative pressure (P/P₀ < 0.1) indicating the existence of a microporous structure. However, the hierarchical zeolite (ZSM-11-KOH) presents a combination of type I (in the low-pressure range) and type IV isotherms (in the intermediate and high-pressure ranges), characteristics of mesoporous materials. A pronounced hysteresis loop could be observed at P/P₀ > 0.44, confirming the creation of secondary porosity in this material [23].

From the inset in Fig. 2, it could be seen that hierarchical zeolite presents a wider pore size distribution than the microporous one (ZSM-11 T). Size pore is between 1.7–31 nm, with a maximum centered on 11 nm, confirming additional mesoporosity formation in its structure. According to literature, a very wide size pore distribution is characteristic of hierarchical zeolites obtained by desilication. This post-synthesis alkaline treatment causes an uncontrolled dissolution of the silicon present in the zeolite structure, generating cavities in the range of mesopores [14].

Textural properties of the materials determined by this technique are shown in Table 1. It is well known that the total surface area (S_BET) and the micro and mesopore volumes are good indicators of the quality of hierarchical zeolites. The sample treated with KOH solution presents a BET surface area higher than that of ZSM-11 T. As could be observed from the table, the significant increase in the external surface (S_EXT) after the alkaline treatment is attributed to some auxiliary mesopores generation in the zeolite framework. In addition, the total pore volume and mesopore volume of the hierarchical sample (ZSM-11-KOH) are higher compared with the traditional ZSM-11. However, this gain in mesoporosity is accompanied with a small loss in microporosity that could be explained by some micropores destruction caused by the alkaline treatment.

Table 1

Textural parameters of traditional and micro/mesoporous ZSM-11 zeolites.
Samples	S_BET (m²/g)	S_EXT (m²/g)	V_Micro (cm³/g)	V_Meso (cm³/g)	V_Total (cm³/g)
ZSM-11 T	283	85	0.129	0.066	0.195
ZSM-11-KOH	368	172	0.106	0.310	0.416

During desilication, the alkaline solution donates its hydroxyl group (OH⁻) to carry out the hydrolysis of the Si-O-Si bonds, which leads to the dissolution of the silica on the external surface of the crystals, causing the creation of larger pores in the starting matrix. In turn, zeolites undergo an ionic exchange with cations excess from the alkaline solution. The resulting product has positive ions (Na⁺, K⁺) that act as counterions to compensate the negative charge of the structural tetrahedral aluminum. In this sense, cations play an important role in the charge balance within the zeolite framework.

Silicon dissolution rate depends on the type of base used. Groen et al. [14] reported the formation of mesoporosity in ZSM-5 matrices by desilication. They found that dissolution kinetics increase in the order LiOH < NaOH < KOH, that is, potassium hydroxide has the highest dissolution rate of silicon species compared to other alkaline bases.

The tetrahedral coordinated Al species in the zeolite framework have a crucial role in the alkaline treatment because they determine the susceptibility towards Si extraction and act as an effective pore directing agent. The latter, due to their ability to regulate the formation of intracrystalline mesopores in the zeolites [24]. For this reason, it is important to analyse the Si/Al ratio in the microporous matrix since alkaline treatment selectively extracts Si over Al. Si/Al ratio of traditional ZSM-11 and hierarchical ZSM-11-KOH zeolite determined by ICP elemental analysis are 19.36 and 9.73, respectively. This significantly decrease in the Si/Al molar ratio in the micro/mesoporous zeolite is caused by the silicon atoms removal from the zeolite framework during the alkaline treatment, thus demonstrating the effectiveness of the desilication process using KOH as base.

According to Fathi et al [25], the percentage of aluminium content increases with the gradual removal of silicon species during the desilication process. Therefore, the low Si/Al molar ratio in the sample treated with KOH suggests a higher proportion of aluminium present in this material. In addition, the decrease in the molar ratio reveals that the Si extracted during the chemical treatment was dissolved in the alkaline solution and did not recrystallize [26].

As desilication process can also cause Al dissolution from the zeolitic framework, extra red aluminum could be generated on the surface of the material. These species could form an amorphous layer that may block the micropores. For this reason, a soft acid washing is often necessary in order to eliminate these species and reopen the generated micro and mesopores [2, 27]. To corroborate this hypothesis the material modified after alkaline treatment was analysed by ²⁷Al MAS NMR spectroscopy. The ²⁷Al MAS NMR spectra of microporous and hierarchical ZSM-11 zeolites are illustrated in Fig. 3.

As can be observed, both samples exhibit a single well intense peak centered at 54 ppm that corresponds to the tetrahedrally coordinated Al species in the zeolite framework. It should be noticed that the δ = 0 ppm signal corresponding to octahedral coordinated Al is not observed in any case. That means that not extra framework aluminium species could be detected by NMR, confirming the preservation of the structural Al species after KOH treatment. The high effectiveness of the desilication procedure is confirmed again.

Zeolites morphology was analysed by scanning electron microscopy (SEM). As shown in Fig. 4A, microporous ZSM-11 zeolite presents the typical hexagonal crystals of the MEL morphology indicating high crystallinity. Meanwhile, hierarchical zeolite (Fig. 4B) preserved the hexagonal crystals after alkaline treatment but with different size than those of the traditional ZSM-11 matrix. Furthermore, in the micrograph of the ZSM-11-KOH, the presence of an amorphous phase was not observed, which agrees with the results obtained by XRD (Fig. 1). Therefore, SEM images confirmed that desilicated ZSM-11 is exclusively formed by a micro/mesoporous phase.

5.1 Characterization of the zirconium modified materials

X-ray diffraction patterns of the materials obtained by incorporation of Zr in both traditional and post-treated ZSM-11 zeolites are shown in Fig. 5. As depicted, both materials clearly show the presence of the typical crystalline structure of ZSM-11 zeolites, with well-defined diffraction peaks of high structural order. These XRD results demonstrate that Zr incorporation did not affect the ordered zeolitic structure. In addition, the peaks corresponding to the tetragonal phase of ZrO₂ (2θ = 30.2°; 34.7º; 35.1º; 50° and 60 º) and the monoclinic phase (2θ = 28.2º and 31.6º ) are not found, which would suggest a good dispersion of this metal in the zeolite matrix [28].

The amount of zirconium incorporated in both materials was determined by X-ray fluorescence spectroscopy, since the metal is a thermal and chemically stable element, difficult to digest and detect by other techniques, such as ICP. The metal content effectively incorporated in Zr- ZSM-11 T and Zr-ZSM-11-KOH samples were 2.0 and 2.1 wt%, respectively.

Besides, Zr-zeolites acid sites characteristics (type and amount) were assessed by FT-IR spectroscopy. Pyridine (Py) was adsorbed as probe molecule at room temperature after water desorption treatment under vacuum conditions (10^− 4 Torr) and 400°C. Samples were further heated at 400°C for an hour for Py desorption. Brønsted and Lewis acid sites quantification (µmol site/g) for each solid was done from 1545 cm^− 1 and 1455 cm^− 1 absorption bands, respectively. These values were calculated using literature data of the integrated molar extinction coefficients [29]. Table 2 presents the number of Lewis and Brønsted acid sites for both samples.

Table 2

Quantification of the acid sites of the Zr modified catalysts and the parent H-ZSM-11 T.
Samples	Lewis(L)*	Brønsted (B)*	L + B*
H-ZSM-11 T Zr- ZSM-11 T	5.20 36.18	60.90 26.85	66.10 63.03
Zr-ZSM-11-KOH	63.15	79.99	143.14
*(µmol /g)

As can be seen, Zr incorporation in the zeolites modifies the number of the acid sites, favouring Lewis acid sites over Brønsted ones, in comparison with the parent H-ZSM-11. Zr ions interact with the OH bridges and behave like extra-framework Al, thus contributing positively to Lewis acid sites [15].

Particularly, in the case of the hierarchical zeolite, Zr incorporation generates a significant increase in the total acid sites concentration, even when compared with the microporous Zr-ZSM-11 zeolite. This higher amount of acid sites could be attributed to the low Si/Al ratio (Si/Al = 9.73), and their better accessibility in this material, as expected by its textural properties (Table 1). All this would guarantee a greater number of acid sites than the protonated microporous matrix.

Holm et al. [30] proposed that in desilicated zeolites, the silicon or aluminium species dissolved by the alkaline treatment are reinserted in the vacancies generated by the treatment and new Si-OH groups are created. In this way, internal defects are drastically reduced and an external surface of zeolite crystals (mesoporous surface) with isolated sites of silanols (Si-OH) is generated. This observation could explain the increase in Brønsted acid sites in the hierarchical Zr-ZSM-11 zeolites. Accordingly, these results evidence that the higher external surface area of the sample treated with KOH solution not only provided a better access and transport of large molecules to the active sites, but also offered new Brønsted acid sites.

5.3 Catalytic activity results

The porosity, crystallinity and acidity are crucial properties of zeolites in biomass catalytic pyrolysis. Acid sites catalyse most of the reactions that take place during pyrolysis process, such as dehydration, decarbonylation, decarboxylation, polymerization, alkylation, cyclization, aromatization, isomerization and dihydroxylation. Therefore, the use of suitable catalysts allows selective control of the biomass pyrolysis and favours those mechanisms that lead to the desired products [31]. On the other hand, the introduction of secondary mesoporosity after KOH leaching may improve the molecular diffusion of reactants and products into the active sites of zeolites during the pyrolysis process.

Figure 6 shows mass balance of PS pyrolysis reactions catalysed by Zr-zeolites. Thermal process (without catalyst) and the protonated matrix results (H-ZSM-11 T) are presented for comparison reasons.

When comparing the thermal reactions with the catalysed ones, no great variations in the yields of the different products are obtained. However, the hierarchical sample, Zr-ZSM-11-KOH, shows an increase in the bio-oil yield (51.10 wt %) with a detriment in the gas yield (16.80 wt%) with respect to microporous materials (Zr-ZSM-11 and H-ZSM-11 T). This could be explained by the larger amount of acid sites (Lewis and Brønsted) available for deoxygenation and decarboxylation/decarbonylation reactions that favour the production of bio-oil [32, 33].

In addition, when comparing bio-oil yields of Zr-zeolites, the higher one is obtained with the sample treated with KOH. This behaviour could be related to the easier diffusion thorough the mesoporous channels. Moreover, the small variations observed in gas yields when microporous materials are used, is a consequence of condensable products cracking, thus increasing gas production.

As commonly known, the liquid product obtained from biomass pyrolysis is rich in oxygenated compounds, that produce bio-oils with low combustion heat and calorific value, high acidity as well as corrosivity and viscosity which restrict their potential use as alternative fuel [34, 35]. Generally, oxygenates that compose bio-oil are defined as compounds containing oxygen such as phenols, furans, acids, esters, ethers, alcohols, aldehydes, ketones and nitrogenous compounds. The acids, aldehydes, ketones, esters, ethers and nitrogenous are considered undesirable compounds in the bio-oil. Acids can cause corrosion while aldehydes and ketones are associated with the instability during transport and storage. On the other hand, ethers and esters reduce the heating value of the bio-oil [36]. It should be mentioned that the liquid obtained from thermal pyrolysis of peanut shells (Fig. 7) contained 52 wt% of undesirable oxygenated compounds. In this sense, the development of a second stage of catalytic cracking using hierarchical zeolites in the pyrolysis process can convert oxygenates into chemical stable compounds and reduce the content of undesirable oxygenates improving the quality of the bio-oil.

The composition of undesirable oxygenates in the bio-oil obtained from different evaluated reactions is presented in Fig. 7. As can be observed, the incorporation of zirconium in both materials (micro and micro/mesoporous zeolites) significantly modified the undesirable oxygenated products distribution. The content of acids was reduced by the hierarchical Zr- ZSM-11- KOH sample, with respect to the other catalysts; while ketones, esters and nitrogen compounds completely disappeared, thus improving the quality of the bio-oil. The deoxygenation of acids is generally accomplished via decarboxylation (promoted by strong acid sites). The acids could be decomposed into smaller molecules, such as CO₂, resulting in a reduction of their yields in the bio-oil [37, 38]. On the other hand, ketones removal from the liquid could probably follows a decarbonylation pathway favoured by the additional porosity of Zr-ZSM-11 KOH sample [36].

Furthermore, the higher deoxygenation activity of the hierarchical zirconium zeolite is attributed to the mesopores and external surface area in combination with the greater number of total acids sites [39]. As proposed by Tan et al. [40], and confirmed by FTIR results (Table 2), Zr promotes acidity properties that boost catalytic activity. This acid sites enhancement improves cracking activity of the material and also favours selectivity towards hydrocarbons [41]. These results demonstrated that Zr incorporation in the hierarchical sample helps to decrease acid content and to eliminate most of undesirable oxygenated compounds of the bio-oil.

On the other hand, alcohols, phenols and furans are considered desirable oxygenated compounds. Alcohols are required for biofuels production, while phenols and furans are regarded as high value-added chemicals. Phenols are important intermediates in the agrochemical, pharmaceutical and polymer industries [39]. It is well known that these compounds can be used to produce phenolic resins, synthetic adhesives and fuel additives [42, 43]. Moreover, the use of furans as oxygenated fuels is promising, since several of these compounds have high calorific value, low water solubility and excellent volatility [44].

By modifying the hierarchical ZSM-11 zeolite with Zr, an increase in alcohols content was found. Selectivity towards these compounds was 11.46 wt% when using H-ZSM-11, but it nearly double when Zr-ZSM-11-KOH was employed (20.35 wt%). The higher selectivity towards alcohols is possibly due to the larger amount of total acid sites in combination with the micro and mesoporous structure of the zeolite. The obtained results indicate that this material improved alcoholic products concentration in bio-oil, which enhance the stability of bio-oil [45].

When analyzing phenolic compounds, it was observed that the hierarchical Zr-zeolite promoted their formation. Selectivity towards phenols was 7.41 wt% when Zr-ZSM-11 KOH was used, while it was 3.30 wt% when Zr-ZSM-11 T was employed. The enhanced production of phenols caused by the hierarchical Zr-zeolite could be attributed to its unique micro/mesoporous structure, which combines the advantages of microporous ZSM-11 matrix (stronger deoxygenation activity and shape selectivity) and larger pore size and number of acids sites. This observation is in accordance to that reported by Park et al. [46] who demonstrated that ZSM-5 zeolite with additional porosity in the range of mesopores was superior than microporous matrix in terms of phenolics production. This, as result of mass transfer and diffusional rates improvement.

Figure 8 shows the results of selectivity towards the furans family in the bio-oil produced by thermal and catalytic pyrolysis as a function of acids sites concentration. From the figure it is possible to observe that Zr-ZSM-11 KOH guaranteed the highest selectivity towards furans. This could be attributed to the substantial increase of total acid sites in this material. Furans are obtained by thermal decomposition and dehydration of cellulose, whose products then undergo a deoxygenation and decarboxylation process [47, 48]. The high Lewis acidity of this material combined with Brønsted acid sites, its high BET and external surface areas, and the micro/mesoporous structure promoted the dehydration of the saccharides and the dehydrogenation of cyclopentanones to increase the furan content [49].

As consequence of the global increment on energy demand, renewable energies have gained remarkable importance. Particularly, the energy obtained from biomass can replace the non-renewable energy from petroleum. One of the most potential new fuels that could be obtained from biomass valorisation processes is 2,5-dimethylfuran (DMF) [50]. It presents outstanding advantages and excellent physical and chemical properties such as high energy density (31.5 MJ/L), high octane number (RON = 119) and low volatility [51, 52]. When Zr-zeolites were used as catalysts, DMF was obtained (Fig. 8). According to Lim and Rashidi [53], this oxygenated molecule is formed from 5-hydroxymethylfurfural (HMF) in two stages. Initially, lignocellulosic biomass is transformed into sugars (e.g., glucose and fructose), followed by their catalytic conversion into HMF. This product is reported as the most common intermediate in DMF synthesis pathway by hydrogenation and hydrogenolysis reactions [54]. The higher activity and selectivity found for Zr-ZSM-11- KOH towards DMF could be related to the higher concentration of Lewis acid sites of this sample with respect to the other catalysts. This observation is consistent with that reported by Zhu et al [55] who indicated that the presence of Lewis acid sites of Zr^+ 4 corresponding to the vacant orbit of Zr, activated the oxygen in the CH₂OH groups of HMF favouring its hydrodeoxygenation. In addition, Zr^+ 4 species tend to activate the oxygen in the carbonyl group of HMF, followed by hydrogenolysis to generate DMF.

Micro/mesoporous ZSM-11 zeolite was successfully obtained via KOH desilication process. This material was further modified by Zr incorporation in order to be tested as effective catalyst in pyrolysis reactions. The as prepared catalysts were characterized by different physicochemical techniques. ZSM-11 structure and crystallinity were confirmed in all the materials. Besides, Zr-hierarchical zeolite possess the highest acid sites concentration of the evaluated materials. This characteristic, added to its greater pore volume and surface area, makes this material behave as an excellent catalyst for the biomass pyrolysis process. In this sense, Zr-ZSM-11- KOH showed the greatest yields to 2,5-dimethylfuran (DMF) when shells, waste of the industrial process of peanut, were pyrolyzed. DMF is a new sustainable fuel with remarkable properties that could be obtained from biomass.

Acknowledgments

We would like to thank Universidad Tecnológica Nacional and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

Funding

This work was supported by CONICET (PIP 11220200103249CO) and Universidad Tecnológica Nacional (MSTCCO0008681TC).

Data Availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Declarations of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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From peanut shell to 2, 5-dimethylfuran: the role of Zr-micro/mesoporous zeolites

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Abstract

Figures

Statement of Novelty

1. Introduction

2. Experimental

2.1 Synthesis of microporous ZSM-11 zeolite

2.2 Preparation of hierarchical ZSM-11 zeolite

2.3 Zirconium incorporation on traditional and micro/mesoporous zeolites

3. Biomass pyrolysis

4. Characterization

4.1 Zeolites

4.2. Pyrolysis products

5. Results and Discussion

5.1 Characterization of the microporous and hierarchical zeolites

5.1 Characterization of the zirconium modified materials

5.3 Catalytic activity results

Conclusions

Declarations

References

Supplementary Files

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