Catalytic upgrading of beech wood pyrolysis oil over iron- and zinc-promoted hierarchical MFI zeolites

Fuel 264 (2020) 116813

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Full Length Article

Catalytic upgrading of beech wood pyrolysis oil over iron- and zincpromoted hierarchical MFI zeolites A. Palizdar, S.M. Sadrameli

T

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Process Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran

ARTICLE INFO

ABSTRACT

Keywords: Bio-oil Upgrading MFI Hierarchical Deoxygenation Iron Zinc Coke

The catalytic upgrading of beech wood pyrolysis oil has been carried out over iron- and zinc-promoted hierarchical Mordenite Framework Inverted (MFI) zeolites to produce a blendable stream with fluid catalytic cracking (FCC) feed. The iron and zinc were introduced to the zeolite structure in different loadings through the impregnation technique using metal nitrate solutions with various concentrations. The aim was to investigate the effect of zeolite acidity control along with mesoporosity of HZSM-5 zeolite on catalytic upgrading parameters. The results showed that the metal doping reduced textural properties through coverage of the catalyst surface by metal species. The existence of metals also altered acid site distribution of the hierarchical zeolite retaining crystallinity. The results of bio-oil upgrading experiments revealed a rather significant effect of extra framework metal species, particularly in 2 wt% loading, on the deoxygenation activity, coke formation potential, and selectivity towards desired products. Indeed, the improved accessibility of original and new weak and strong acid sites, as well as controlled acidity by incorporation of metals caused the enhanced conversion of large oxygenates to desired products. Zn-modified hierarchical HZSM-5 showed high selectivity towards desired oxygenates such as ketones via the promotion of carboxylic acids ketonization and reduced the formation of polycyclic aromatics. The analysis of coke also indicated that the coke yield and oxidation temperature decreased by 15% and 10%, respectively, over Zn-promoted catalyst compared with hierarchical sample showing desirable characteristics for cost-effective catalyst regeneration.

1. Introduction Bio-oil is a promising alternative source to substitute fossil fuels involving a wide range of highly oxygenated biomass-derived compounds like phenols, furans, acids, alcohols, aldehydes, esters, ketones, etc. [1–4]. The great oxygen content weakens the physicochemical and fuel properties of bio-oil such as heating value, acidity, miscibility, and chemical stability and makes it undesired for combustion [5–8]. Therefore, a catalytic upgrading step is crucial to improve the quality and form valuable compounds suited for fuels through deoxygenation and cracking reactions [9–13]. There are several strategies to accomplish the catalytic pathways with differences in upgrading targets, levels, techniques and main chemical reactions that may take place during the process [14]. When bio-oil is supposed to use directly in engines, a high-level upgrading would be needed that is feasible only in the presence of a high-pressure hydrogen stream. This hydroprocessing approach is cost-intensive with low safety. Another promising strategy in catalytic upgrading is to partially improve the quality of bio-oil up to the level that it can be blended with FCC feedstock. This approach can

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reduce the cost of upgrading and improve the product of FCC unit from an environmental viewpoint. In this approach, bio-oil is upgraded to a stream miscible with conventional feedstock of the FCC unit (vacuum gas oil) in existing refineries. It is believed that such an approach is a feasible route economically and technically due to the ability of FCC catalysts to remove oxygen from bio-oil and eliminate the need for external hydrogen source to obtain deep deoxygenation [15,16]. With this regard, the microporous MFI zeolites have exhibited a great performance in bio-oil upgrading due to its high acidity, high selectivity towards valuable hydrocarbons and appropriate hydrothermal stability [17]. However, the pore structure of these zeolites (0.52 nm × 0.56 nm) hinders large oxygenate molecules to access catalytic active sites within the micropores and also extends diffusion path length which causes to produce undesired products and coke precursors resulting in pore blockage [18,19]. A promising way to overcome this challenge is to introduce mesopores within the pore structure of MFI zeolites via post-treatment method which can improve the accessibility of the active sites, enhance the catalyst effectiveness and activity in catalytic deoxygenation, and increase the stability of the

Corresponding author. E-mail address: [email protected] (S.M. Sadrameli).

https://doi.org/10.1016/j.fuel.2019.116813 Received 18 August 2019; Received in revised form 13 November 2019; Accepted 2 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 264 (2020) 116813 [29]

[28]

[27]

[26]

[25]

[24]

[23]

[22]

[21]

[20]

catalyst against deactivation. Nevertheless, this porosity improvement has a negative effect on the density and strength of active acid sites and thus, a further improvement to tailor the zeolite acidity seems to be necessary. It is hypothesized that incorporation of various metals can promote the activity, selectivity, and stability of mesoporous or hierarchical HZSM-5 zeolite in the upgrading process through the synergetic effect of enhanced accessibility of active sites and tuned acidity to achieve the best upgradation result. To the best of our knowledge, a few efforts have been carried out on the efficient utilization of metal-modified hierarchical MFI zeolites with different metal types and loadings in catalytic upgrading of bio-oil. A summary of these studies is presented in Table 1. The results entirely show a rather high potential of modified mesoporous MFI zeolites to efficiently improve the bio-oil quality. However, the metals used as a promoter over hierarchical zeolite so far (like cerium, lanthanum, gallium, and platinum) were mostly expensive. Also, the effect of metal loading on the performance of mesoporous catalysts in bio-oil upgrading was studied through a narrow range of parameters such as aromatic yield. Therefore, a gap exists in the literature for a comprehensive study of the effect of metal modification in terms of different parameters. On the other hand, a few studies have been focused recently on the effect of various metal contents over mesoporous MFI zeolites to find the optimum loading. Some aspects of the synergetic effect of mesoporosity and metal doping on catalytic upgrading of bio-oil still remain unknown and further comprehensive studies are required. The present work discusses the effect of iron and zinc metals with various loading over hierarchical HZSM-5 along with mesoporosity on the upgrading parameters involving upgrading factor, degree of deoxygenation, higher heating value, effective hydrogen index, the relative content of various compounds, distribution of oxygenates and coke yield. The main reasons for the selection of these two metals are the good performance for bio-oil upgrading in previous studies over other supports [30–35] as well as relatively low cost [36,37]. In addition, the effect of doping these metals over the hierarchical HZSM-5 on the biooil upgrading has not been fully investigated, yet. For the first time, a more realistic parameter to investigate the performance of catalysts in the bio-oil upgrading process, upgrading factor (UF), is introduced and discussed in detail. The parameter includes different aspects of catalysts performance like degree of deoxygenation, effective hydrogen index and selectivity towards undesired products. It is finally evidenced that metal-modified hierarchical MFI zeolites (particularly Zn-promoted) with proper metal loading (2 wt%) are efficient catalysts for bio-oil catalytic upgrading due to higher selectivity towards desired products and lower coke yield and oxidation temperature.

550 2, 5, 8, 11 Ni

2.1. Materials Beech wood sawdust, as feedstock for the catalytic pyrolysis, was crushed to the desired particle size (< 2 mm) prior to use. The parent MFI zeolite (ZSM-5, CBV-5524G, Si/Al = 50) was purchased from Zeolyst International in ammonium form (NH4-ZSM-5), hence, it was calcined in air at 550 °C for 5 h and labeled as HZ. Hierarchical MFI zeolite was prepared using alkaline treatment (desilication) of the parent ZSM-5 with aqueous NaOH solution. Approximately 10 g of the parent zeolite was mixed with 100 ml of NaOH (98%, Neutron Pharmachemical Co., Iran) with a certain concentration (0.5 M) for 2 h at 70 °C by using a heater-stirrer device. The solution was then kept within an ice-water bath for 10 min to stop further desilication reaction. After quenching, the solution was centrifuged, washed with deionized water and dried at 100 °C for 3 hr. The dried Na-form sample was transformed into H-form by 0.2 M NH4NO3 solution (99%, Chem-Lab NV) at 80 °C for 24 h. After overnight drying at 100 °C, the resultant was calcined at 550 °C for 5 h and coded as MZ5. The hierarchical

10

9

8

7

6

5

4

3

2

2. Experimental

Corn cob

Cellulose

1

double-shot pyrolyzer

500

– 0.1, 0.5, 1 Rice straw

Ni, Cu, Zn, Ga Fe, Ga

Tandem microreactor Microreactor

500 5 Rape straw

La

Slow/In situ

500 10 Eucalyptus woodchips

Fast/In situ

450 1 Wood

Sn, Cu, Ni, Mg Mg, Zn

Fast/Ex situ

500 1, 5 Ga

Fast/In situ

– 0.5 Pt

Miscanthus sinensis var. purpurascens Palm kernel shell

Slow/In situ

0.5 Pt Rice husk

Flash pyrolyzer

500

Cerium-incorporated zeolite exhibited selectivities greatly shifted from aromatics to oxygenated chemicals. The addition of Pt made the Meso-MFI catalyst even more active in deoxygenation and in the production of aromatics. Both cracking and dehydrogenation reactions were promoted over Pt/meso-MFI, resulting in enhanced deoxygenation and aromatization. Ga-promoted zeolite indicated the best performance on aromatics formation and bio-oil deoxygenation. In all the cation-loaded hierarchical zeolites, the incorporation of metallic species at the ion exchange sites decreases the production of aromatics. The incorporation of MgO and ZnO resulted in a higher yield of the organic compounds and decreasing the formation of undesired polyromantic and coke. La/HZSM-5 zeolite had high catalytic activity and exhibited good potential and a beneficial nature for efficient preparation of high-valued bio-oil from rape straw. The loading of a suitable amount of Ni and Cu produced more aromatics due to the improved deoxygenation reactions leading to higher yields of olefins. Both Fe and Ga modification increased the the selectivity of monoaromatics at the expense of polyaromatics. Value-added bio-oil with high selectivities of aromatics was produced via nickelpromoted hierarchical ZSM-5 catalyst. 600 Fast/In situ 1.92 Ce

Effect of cerium-incorporated MFI zeolite on catalytic pyrolysis of biomass Effects of meso-MFI and Pt/meso-MFI on biomass catalytic pyrolysis Using Pt/meso-MFI to produce phenolics and aromatics in pyrolysis of miscanthus Use of a cascade system of various catalysts for in-situ catalytic pyrolysis of biomass The effect of metal loaded hierarchical MFI zeolites on bio-oil deoxygenation Using ZSM-5 zeolites modified with MgO and ZnO for catalytic pyrolysis of biomass Catalytic pyrolysis of rape straw over hierarchical La/HZSM-5 Effect of metal-modified hierarchical MFI on catalytic fast pyrolysis of rice straw Effect of metal-modified hierarchical MFI on catalytic fast pyrolysis of cellulose catalytic fast pyrolysis of biomass over Ni modified hierarchical ZSM-5 1

Glucose

Loading (wt %) Metal Feed Description Entry

Table 1 Summary of the literature on catalytic upgrading of bio-oil over metal-modified hierarchical MFI zeolites.

Processing mode

T (°C)

Major remarks

Ref.

A. Palizdar and S.M. Sadrameli

2

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A. Palizdar and S.M. Sadrameli

HZSM-5 sample was promoted with Fe and Zn through varying metal loading ratios (2, 4, 6 and 8 wt%) via dry impregnation method. The aqueous solutions were prepared with a (solution volume)/(total pore volume) ratio of 2 using Fe(NO3)3·9H2O (98%, BIOCHEM) and Zn (NO3)2·6H2O (99%, CDH Ltd.) as the metal sources. The solutions were gradually added to the mesoporous zeolite at room temperature during the impregnation. After each blending operation, the impregnated sample was dried at 100 °C for 12 h and consequently calcined at 550 °C for 5 h. The resultants were designated as MZFex and MZZnx where x represents the wt% of metal promoters in each zeolite sample.

under atmospheric pressure. After filling the reactor, the temperature was increased to the desired value for pyrolysis and catalytic reactions (500 °C) by a heating rate of 25 °C/min using an electrical furnace. Prior to each test 100 ml/min of the nitrogen carrier gas was used to remove the entrained oxygen in the reactor [38]. During the experiments, the N2 stream was also used to continuously sweep the pyrolysis vapor into the quenching section. The liquid bio-oil product was collected in a vessel inside an ice-cooled condenser. After completion of the reactions, the reactor was cooled down to ambient temperature and the liquid and solid (char + tar) products were extracted and weighted. Finally, the aqueous phase and organic liquid products were also separated and weighed. Characterization of the organic phase was performed utilizing a gas chromatograph (Agilent 6890 series, USA) equipped with a triple quadrupole mass spectrometer (Agilent 5973 Network, USA) and an electron impact ionization detector (EI). The GC injector was kept at 280 °C, and the injection split ratio and the injection volume was 30:1 and 2 μl, respectively. The condensable pyrolysis products were separated with an HP-1 MS column (30 m × 0.25 mm i.d. × 0.5 μm film thickness, Agilent Technologies, Inc., USA). The temperature ramp of the GC column oven began with a constant temperature step at 50 °C for 2 min and then increased by a rate of 10 °C/min up to a final temperature of 280 °C which was held constant for 15 min. Helium (99.9999% purity) was used as a carrier gas with a 1 ml/min volume flow. The MS ionization mode was electron impact at 70 eV and the mass scan range was from 40 to 500 m/z. The various pyrolysis products were identified by comparing all chromatogram spectra to the NIST mass spectral search program and the Wiley mass spectrum library. The amount of coke deposited on the catalysts was measured by Thermogravimetric Analysis (TGA) utilizing Q600 (TA, USA) in an air atmosphere at a heating rate of 20 °C/min in the range of 40 °C–800 °C. The results were analyzed through several parameters representing the catalytic performance of the catalysts in the bio-oil upgrading. These parameters were upgrading factor (UF), degree of deoxygenation (DOD%), the yield of pyrolysis products, relative peak area (RPA) of the different compounds detected in bio-oil and coke yield which were calculated according to the following equations:

2.2. Catalyst characterization All the characterization techniques were applied according to standard procedures. The crystallinity of the catalysts was identified using X-ray Diffraction (XRD). Powder XRD patterns were obtained by using the PW1730 diffractometer (Philips, Netherland) operated at 40 kV and 30 mA using Cu as an anode material (k = 1.54 Å). The scanning step size was 0.05°/min with 1 s per step in the 2Θ range of 5°–65°. To characterize the morphology of the zeolite surface and size of the crystals, Scanning Electron Microscopy (MIRA III, TESCAN, Czech Republic) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (SAMX, France) was utilized. The EDS analysis was used to detect the metal elements incorporated into the mesoporous zeolite framework. Nitrogen adsorption isotherms were measured by a BELSORP-mini II (BEL Japan Inc., Japan) at 77 K. Prior to analysis, the samples were degassed under vacuum at 200 °C for 6 hr. The total surface area of the catalysts was determined based on the BrunauerEmmett-Teller (BET) method and their total pore volume was calculated at p/p0 = 0.99. The Barrett-Joyner-Halenda (BJH) method was utilized for determining micropore and mesopore surface area and pore size distribution. The acidity of the samples was determined by the Temperature Programmed Desorption of Ammonia (NH3-TPD) technique using a NanoSORD-NS91 (Sensiran Co., Iran) analyzer. 2.3. Catalytic activity The catalytic pyrolysis of biomass was carried out in a fixed-bed tubular reactor as illustrated in Fig. 1. The reactor tube was made of quartz with an internal diameter of 2 cm. For each experiment, 5 g of biomass was loaded into the center of the reactor followed by 2 g of the catalyst which is held by a quartz wool bed. The process was carried out

Pyrolysis product yield (wt%) =

Mass of a product (g) × 100 Mass of biomass feed (g) (1)

Fig. 1. Schematic diagram of the biomass pyrolysis and catalytic upgrading system. 3

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A. Palizdar and S.M. Sadrameli

Fig. 2. X-ray diffraction patterns of the metal-modified MFI zeolites.

Relative peak area(%) The relative content of a compound = × 100 The relative content of overall compounds

Coke yield(%) =

Mass of deposited coke (g) × 100 Mass of biomass feed (g)

Degree of deoxygenation(%) =

Oi

Oj Oi

× 100

HZSM-5 as well as metal-modified samples represents a combination of types I and IV isotherms with a hysteresis loop at intermediate and a plateau at high relative pressures indicating that mesopores have been introduced to the zeolite microporous structure [26,27,29]. Table 2 summarizes the textural properties of the zeolite samples in terms of total pore volume (Vpore), BET surface area (SBET), micropore, and mesopore surfaces (Smicro and Smeso) and micropore and mesopore volumes (Vmicro and Vmeso). As expected, the mesopore surface and volume were notably raised by desilication at the expense of the micropore area. Moreover, metal doping decreased the BET surface, micropore and mesopore volume of the mesoporous zeolite possibly due to the blockage of the pores by metal species [24,25]. This suggests that the Fe and Zn species are partially incorporated in the micropores in addition to both mesopores and external surfaces of the catalyst particles. On the other hand, the mesopore volume decreased more slightly compared to the micropore. This may be attributed to the large volume of mesopores compared to the number of metal species [29]. The values of average pore size for metal-modified samples are in the range of 4.74 nm to 6.79 nm indicating that the hierarchical structure of mesoporous HZSM-5 retains after metal incorporation. The pore size distributions for the zeolite catalysts are displayed in Fig. 5. The treatment by using a 0.5 M solution creates mesopores with varying diameters and distributions. Similar results were reported by Chen et al. [27] and Dai et al. [29]. In addition, the intensity of mesopores with the diameters of 10 nm and 22 nm was sharply enhanced for the MZ5 sample. These sharp peaks become milder after metal impregnating, still being more severe than those of the parent zeolite. This may be attributed to the reduction in the mesopore and external surface, particularly, by metal loading. The density and strength of accessible acid sites of the parent, desilicated, and modified MFI zeolites were probed by NH3-TPD analysis. The resultant profiles are shown in Fig. 6. The distribution of acid sites in zeolite samples is also presented in Table 3. The parent HZ catalyst indicates two desorption peaks at around 210 °C and 430 °C which can be ascribed to the NH3 desorbed from weak and strong acid sites, respectively. Although it is difficult to distinguish between weak and strong acidity by NH3-TPD results, it has been indicated that a hightemperature peak of NH3 desorption can be mainly attributed to zeolite's strong Brønsted acid sites associated with framework Al atoms [45]. On the other hand, the interaction of Lewis acid sites with the NH3 may be indicated by the desorption peak at low temperatures. As

(2) (3) (4)

where Oi and Oj represent oxygen content of raw pyrolysis oil and upgraded bio-oil on a dry basis, respectively. 3. Results and discussion 3.1. Catalyst characterization To analyze the effect of metal doping on crystalline phases of the hierarchical zeolite, the related XRD patterns are depicted in Fig. 2. The peaks at 2Θ ranges of 7–10 and 23–25 confirm the MFI structure of the ZSM-5 zeolite samples [4,39–41]. The analogous patterns for all the metal-modified catalysts also reveal that the crystalline structure of ZSM-5 still preserves after metal impregnation [42,43]. The XRD results do not display any notable peak related to metal-containing crystalline phases showing that metal oxide aggregates (ZnO and Fe2O3) have not been detected by the analysis [34]. This output does not reject the presence of iron and zinc oxides in different forms on the external surface or in the micropores. These species may actually be present as an amorphous phase or highly-dispersed nanoparticles in the external zeolite surface [2,3,23,24]. The metal content of the impregnated zeolites was determined by EDS and the results together with other characteristics of the catalytic samples are summarized in Table 2. The SEM images of the parent, hierarchical, and metal-modified hierarchical zeolites are illustrated in Fig. 3. As can be seen, all the samples exhibit well-crystallized particles of the nanometer-scale which form agglomerates. It is also observed from images that desilication breaks up some of the particles into smaller fragments [44]. These fragments can also be seen by the images of metal-promoted zeolites. The nitrogen adsorption-desorption isotherms of the parent, mesoporous, and metal-modified HZSM-5 are depicted in Fig. 4. Contrarily to the parent zeolite which shows a type I isotherm, the hierarchical 4

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A. Palizdar and S.M. Sadrameli

Table 2 Textural properties and elemental compositions of the zeolite samples. Catalyst

Vpore (cm3/gr)

Vmicro (cm3/gr)

Vmeso (cm3/gr)

SBET (m2/gr)

Smicro (m2/gr)

Smeso (m2/gr)

Average pore size (nm)

Actual metal loading (wt%)

HZ MZ MZFe2 MZFe4 MZFe6 MZFe8 MZZn2 MZZn4 MZZn6 MZZn8

0.348 0.808 0.639 0.628 0.621 0.599 0.644 0.636 0.607 0.586

0.150 0.123 0.113 0.109 0.11 0.107 0.116 0.109 0.099 0.102

0.198 0.685 0.526 0.519 0.511 0.492 0.528 0.527 0.508 0.484

424.75 438.94 414.11 397.02 378.41 372.58 403.68 387.03 361.92 357.68

357.53 298.04 284.76 270.08 257.93 256.45 272.97 259.68 241.48 238.80

67.22 140.90 129.35 126.94 120.48 116.13 130.71 127.35 120.44 118.88

3.28 7.36 6.43 5.41 5.07 4.74 6.79 6.57 6.48 6.38

0.00 0.00 1.73 3.85 5.19 8.16 2.14 4.61 5.47 7.14

observed in Fig. 6, both low and high-temperature peaks for hierarchical sample (MZ5) were slightly shifted toward lower temperatures. It suggests that the strength of weak and strong acid sites will decrease after desilication. Likewise, the density of both weak and strong acid sites decreased in the MZ5 sample due to the preferential extraction of Si from the catalyst structure. Introducing the metals with different loadings to the hierarchical sample causes a series of variations in the density and strength of the catalyst acid sites. For all metal-modified samples, the high-temperature peak disappeared resulting in a reduction of strong acidity. This may be related to the fact that Brønsted acid sites are ion-exchanged with metal cations or covered by metal species making the strong active sites inaccessible for the reactant molecules [21,22,46,47]. A different trend is perceived for the density and strength of weak acid sites. When Fe and Zn were incorporated into the hierarchical support by 2 wt%, the density of weak acid sites, as well as their strength, is raised since the corresponding desorption peak is shifted to higher temperatures. As suggested by Fermoso et al. [25] who faced a similar observation in their study, the loading of metals causes the generation of additional Lewis acid sites, probably related to the metal oxide particles. It is expected that the higher density and strength of these new weak acid sites compared to the original ones in the zeolite structure significantly affect the bio-oil upgrading process. The density of those acid sites was, however, reduced in higher loadings while their strength was still enhanced. By increasing the metal loading, the size of metal clusters is also enlarged and consequently, the active Lewis acid sites may be covered by these clusters without generating a large number of new weak acid sites. In iron-promoted hierarchical zeolite, different Fe complexes may exist varying in electrical charge or local configuration. Depending upon the thermodynamic stability, interconversion of these complexes occurs in zeolite voids. Li et al. [48] have widely discussed in detail the state of iron in the Fe/ZSM-5 bifunctional catalyst. Based on their results, mononuclear oxygenated iron cations such as [FeO]+ show lower stability and tend to self-organize into binuclear cationic clusters. On the other hand, preliminary Fe4O6 clusters decompose into binuclear species via protonation over Brønsted acid sites during the impregnation procedure. These transformations are followed by dehydration to oxygen-bridged [Fe-O-Fe]2+ species which are stable particularly under reducing conditions. In the presence of oxygen, this complex is oxidized into [Fe-O2-Fe]2+ binuclear clusters that act as active Lewis acid sites in the deoxygenation reactions. Other possible Fe complexes are not stable enough under usual bio-oil upgrading conditions. Only at high temperatures, the formation of Fe3+ ions may occur under a reducing condition. Likewise, in zinc-promoted meso-HZSM-5, different zinc species can exist throughout the zeolite structure, mainly such as Zn2+ cations that substitutes two protons and interacts with two aluminum sites bridged by oxygen [O−-Zn2+-O−] or [Zn-O-Zn]2+ formed at nearly framework Al pairs, zinc hydroxyl ZnOH+ which located at Brønsted acid sites, and highly dispersed ZnO [49–51]. Niu et al. [52] have reported that when the Zn/ZSM-5 is prepared by impregnation, only 28% of zinc species

may exist as well-dispersed ZnO particles in the zeolite channel. The ZnOH+ as Lewis acid site of zinc species is generated at the cationic positions in which hydrogen atom pulls Al3+ making the hydrogen atom tend to be protonated, and then a combination of Zn and acidic hydroxyl can generate ZnOH+ [47,53,54]. The [Zn-O-Zn]2+ cations may also be created via two ZnOH+ species through an endothermic reaction (2Z-Zn-OH → Z-Zn-O-Zn-Z + H2O) [52]. The possible existing forms of Zn species on the hierarchical zeolite are illustrated in Fig. 7. To sum up, the incorporation of Fe and Zn metals in the hierarchical zeolite framework changes the charge balancing protons reducing the concentration of Brønsted acid sites and forming new Lewis sites as various oxycation complexes. From the density of Brønsted and Lewis sites via NH3-TPD results, it is clear that more Lewis sites are formed and a portion of Brønsted sites are consumed (particularly in 2 wt% loading) or covered (in higher loadings) [56]. 3.2. Catalytic activity in bio-oil upgrading 3.2.1. Thermal pyrolysis The chemical compounds of beech wood pyrolysis oil along with their relative peak area are summarized in Table 4. As detected by GC–MS analysis, the bio-oil product totally consists of oxygenated hydrocarbons with various functional groups such as phenolics, carboxylic acids, furans, aldehydes, and esters. Highly presence of oxygenate compounds causes a series of problems like undesired combustion, chemical instability, and further polymerization, high acidity and immiscibility with petroleum fuels when using the bio-oil as fuel. These challenges necessitate the deoxygenation of the bio-oil through catalytic routes [14]. Converting the bio-oil to a blendable feedstock for FCC units is a possible method that needs a partial deoxygenation step via catalytic pyrolysis without the external hydrogen supply required. Considering the similar nature of the catalytic cracking of heavy hydrocarbons and catalytic deoxygenation reactions, it is demonstrated that a blend of 10–20 wt% partially deoxygenated bio-oil and vacuum gasoil (VGO) can be a suitable feedstock for the existing FCC units with negligible changes required in current facilities. Also, FCC catalysts are able to crack heavy hydrocarbons to smaller ones and to remove oxygen from bio-oil through deoxygenation reactions. In addition, a high amount of water in the bio-oil leads to dilute hydrocarbon feedstock, improves the heat balance of the process and well fluidizing of catalytic bed. Therefore, the coprocessing of bio-oil in the FCC unit is the most promising and economical way to upgrade the bio-oil [15,57,58]. However, the properties of raw bio-oil presented in Table 5, reveal several drawbacks to use it as FCC feedstock. The high amount of small, medium and large oxygenates in the bio-oil (based on the number of oxygen atoms in molecular structure) suggests high polarity and consequently low immiscibility with VGO as the main feedstock of an FCC unit. Particularly, the high presence of large oxygenates mostly derived from lignin fragmentation also donates high coke forming potential to the bio-oil. Furthermore, polycyclic hydrocarbons produced during the catalytic pyrolysis play the role of coke precursors. Although the 5

Fuel 264 (2020) 116813

A. Palizdar and S.M. Sadrameli

Fig. 3. SEM images of the (a) HZ, (b) MZ5, (c) and (d) MZFe2, (e) and (f) MZZn2, (g) MZFe4, (h) MZFe6, (i) MZFe8, (j) MZZn4, (k) MZZn6, and (l) MZZn8 samples.

coprocessing method modifies the coke forming ability of bio-oil via hydrogen transfer from the VGO stream to the hydrogen-deficient biooil, it is still necessary to improve the bio-oil quality before coprocessing from a coke formation point of view. The acidity of the bio-oil mainly stems from carboxylic acids, which is known as another possible source of some problems in the coprocessing [59]. In this study, the upgrading experiments over the parent and modified catalysts were performed to eliminate these disadvantages as much as possible.

The ability of a hydrocarbon stream to form coke can be defined as effective hydrogen to carbon efficiency (H/Ceff) or effective hydrogen index (EHI) calculated by the following equation [57]:

EHI =

H

2O C

3N

(5)

where H, O, N and C are the mole fractions of hydrogen, oxygen, nitrogen, and carbon in the stream. The small value of this parameter for 6

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A. Palizdar and S.M. Sadrameli

catalyst obviously improved the quality of bio-oil with respect to density, higher heating value, degree of deoxygenation and effective hydrogen index. The decrease in oxygen content from 39.27 wt% for raw bio-oil to 18.64 wt% for upgraded one exhibits the promotion in the deoxygenation reactions. In the latter case, a DOD of 52.55% was achieved resulting in a decrease in the O/C ratio from 0.729 to 0.252. This oxygen removal also causes an improvement in HHV of the bio-oil up to 34.88 MJ/kg. The reduction in the value of density displays that the cracking reactions have notably occurred. Interestingly, the potential of the bio-oil to coke formation during further operations was significantly reduced owing to the vast enhancement in the EHI parameter from 0.366 to 0.806. In terms of the relative content of various compounds, the number of carboxylic acids was considerably diminished up to 70% showing rather large progress in the decarboxylation pathway. Deoxygenation of carboxylic acids is generally accomplished via two well-known paths: decarboxylation and ketonization. Decarboxylation of carboxylic acids releases CO2 and alkane per each mole of reactant, while in ketonization, the corresponding ketone along with CO2, H2O is produced. The decarboxylation path is promoted by strong acid sites while the ketonization is mostly catalyzed via weak acidity [19,63]. The Lewis acid sites that play a crucial role in ketonization of formic and acetic acids can improve the deoxygenation efficiency in the upgrading step [24,25]. Regarding the results of Table 5, decarboxylation was the main route to oxygen removal from the carboxylic acids over the parent HZSM-5. A similar trend was evidenced for the content of phenolics to 29.79 wt% contrarily to that of monocyclic and polycyclic aromatics (MAH and PAH) which increases up to 44.98 wt%. Indeed, the reduction in the relative content of phenolics and the increment in that of aromatics also may represent the deoxygenation of phenolics to form aromatics [64]. Generally, the hydrogen-deficient environment of the bio-oil upgrading reactor favors aromatics because they are the most stable hydrocarbons when hydrogen content is low. In addition, strong Brønsted acid sites are mainly responsible for aromatization through cracking and dehydration reactions [2,19,21–23,61,63,65–67]. The content of furans, ketones, and esters was reduced over HZ, while aldehydes completely disappeared. The removal of carbonyl, hydroxyl and methoxyl functionalities through decarbonylation are mostly promoted via Lewis acidity [2,68]. Light phenols are typically produced through secondary catalytic cracking or deoxygenation of lignin-derived oxygenates (e.g. demethoxylation of guaiacols and syringols) over active acid sites [69]. The extent of medium and large oxygenates was detracted by catalytic upgrading showing that the cracking reactions were considerably promoted. The large portion of medium oxygenates is related to acetic acid which and has contributed by 34.53 wt% in the raw bio-oil. Accordingly, the decrease in acetic acid content up to 13.08 wt% over HZ caused a great reduction in medium oxygenates. The majority of furanic compounds in the upgraded bio-oil are formed via catalytic cracking of levoglucosan as a dominant product in cellulose pyrolysis [16]. Furfural is an abundant compound in the raw biooil and is converted to furans and CO through decarbonylation reaction [25,49,70] and the resultant furans consequently produce aromatics during complex reaction network [71]. It can be concluded that weak and strong acidities of HZSM-5 have a high ability in catalytic cracking and aromatization routes due to the large density of strong acidity. Taking the results of Table 5 into account, bio-oil upgrading over hierarchical HZSM-5 improved the characteristics of the liquid products compared to the parent zeolite. In particular, the UF which represents the bio-oil upgrading performance exhibits a remarkable enhancement by 0.735 for MZ5 against 0.54 for HZ. Indeed, improved external and mesoporous surface area and accessibility of bulk oxygenates found in the raw bio-oil to active acid sites are the main reasons for the progress of cracking and deoxygenation reactions resulting in DOD increment. The mentioned accessibility was achieved at the expense of a decline in the zeolite crystallinity and the relative loss in active sites, particularly strong acidity. Similar results have been reported by Mohammed et al.

Fig. 4. N2 adsorption–desorption isotherms for the parent, hierarchical, and (a) Fe- and (b) Zn-promoted zeolites.

the raw bio-oil (0.366) discloses a high potential of the stream to form coke during upgradation. Therefore, it is necessary to enhance the EHI of the bio-oil by partial deoxygenation in the catalytic pyrolysis step. In order to have a more realistic sense toward the performance of the various catalysts in catalytic upgrading and also to have a criterion for comparing the results, Upgrading Factor (UF) was defined as below [60]:

UF =

DOD × EHI The relative content of undesired components

(6)

where, the undesired compounds consist of large oxygenates (O3 and O4), aromatic hydrocarbons and carboxylic acids. Indeed, the UF represents all the aims considered for the catalytic upgrading involving deoxygenation activity (DOD), stability (EHI and also coke precursors) and selectivity. This parameter provides the ability to the comparison of the results considering different aspects of catalytic upgrading simultaneously and the high value of the UF discloses a well-performed upgrading experiment. 3.2.2. Microporous and hierarchical MFI zeolites The effect of various catalysts and metal loadings on the pyrolysis product yields are illustrated in Fig. 8. As observed, catalytic upgrading the bio-oil generally reduced the yield of the organic phase and increased that of the aqueous phase and also gas product. Such the effect shows progress in deoxygenation reactions [11,61]. In particular, a slight increase in aqueous phase yield verified the promotion of dehydration path. The minimum bio-oil yield was achieved in the presence of HZ and MZ5 indicating high deoxygenation ability. Table 5 presents the elemental analysis and characteristics of the raw and upgraded bio-oil using different catalysts. The parent HZSM-5 7

Fuel 264 (2020) 116813

A. Palizdar and S.M. Sadrameli

Fig. 5. Pore size distributions for the parent, hierarchical, and (a) Fe- and (b) Zn-promoted zeolites.

[72,73] who showed that a certain increase in HZSM-5 mesoporosity could significantly enhance DOD and HHV of the Napier grass pyrolytic oil. The relative content of phenolics increased by incorporation of mesoporosity within the zeolite structure while the distribution of large oxygenates was notably reduced. This is in good agreement with the results presented by Dai et al. [29] and reveals that large phenolic oxygenates may crack into the medium or small ones, mostly phenolics, over strong acid sites of the external or mesoporous surfaces. As depicted in Fig. 6, although the number of strong acid sites in MZ5 is lower than HZ, a larger reduction in the number of carboxylic acids was achieved over this catalyst [23,69]. This may be related to the role of weak acidity to promote ketonization path. This output is in a reasonable consistency with the content of ketones which increases by mesoporosity. The results are supported by the study of Veses et al. [24] who disclosed the reverse trend in aromatics and ketones production. The relative content of MAH and PAH was reduced by the creation of mesopores that is desired particularly for the PAHs because of its potential in the coke formation. The detriment in promoting the

aromatization pathway over hierarchical zeolite can be interpreted as a lower strength of strong acidity than the microporous one on the contrary of improved accessibility to active sites [28,44,45]. Various oxygenates were converted to the smaller ones by introducing mesopore within the zeolite structure (large oxygenates to medium ones and medium oxygenates to small ones). The relative content of oxygenates was altered toward small ones and the percent of medium and large oxygenates was considerably reduced. This shows a proper performance of the hierarchical zeolite in catalytic upgrading of pyrolysis oil. It seems that higher conversion of large oxygenates over MZ5 compared to HZ is directly attributed to the better accessibility of these molecules to weak and strong active acid sites. Li et al. [26] have reported a similar effect in catalytic upgrading of pyrolysis vapor from rape straw over HZSM-5 with a hierarchical structure. The relative content of small oxygenates (O1) increased using mesoporous zeolite while that of medium and large (O2 and O3) decreased. Analogously with the present work, they concluded that not only the total content of oxygenates is reduced after the upgrading, but also the oxygenated compounds are more converted to O1 compounds. 8

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Fig. 6. NH3-TPD profiles of the parent, hierarchical, and (a) Fe- and (b) Zn-promoted zeolites.

the higher the bio-oil yield. This effect which demonstrates a less cracking strength of the metal-promoted zeolites may be due to their lower acidity. As seen in Table 5, the bio-oil properties were significantly affected by the incorporation of the metals into the hierarchical zeolite with different loadings. The main idea of metal doping on the hierarchical sample was to exploit tuned acidity distribution along with mesoporosity and tailored pore size, simultaneously. The majority of the biooil characteristics were reduced by increasing the metal loading. In the case of iron, the DOD was continuously decreased from 46.9% for MZFe2 to 21.46% for MZFe8. A similar trend was obtained for zincpromoted samples in which the parameter was altered from 49.3% for MZZn2 to 23.93% for MZZn8. Likewise, the value of other parameters involving density, higher heating value, and EHI exhibits the same results. The relative content of phenolic compounds and carboxylic acids was raised in the presence of metals showing consistency with the values for DOD. However, the key effect of metals on bio-oil properties is revealed in the relative content of ketones and aromatics. The number of ketones over the Fe-promoted samples was firstly increased to

Table 3 The distribution of acid sites in the parent, hierarchical, and metal-modified catalysts. Catalyst

Total acidity (mmol/gr)

Weak acidity (mmol/gr)

Strong acidity (mmol/gr)

Weak acidity (%)

Strong acidity (%)

S/W

HZ MZ5 MZFe2 MZFe4 MZFe6 MZFe8 MZZn2 MZZn4 MZZn6 MZZn8

0.506 0.455 0.505 0.399 0.396 0.372 0.540 0.434 0.406 0.456

0.274 0.246 0.284 0.217 0.224 0.215 0.325 0.256 0.240 0.268

0.232 0.209 0.221 0.182 0.172 0.157 0.215 0.178 0.166 0.188

54 54 56 54 57 58 60 59 59 59

46 46 44 46 43 42 40 41 41 41

0.85 0.85 0.78 0.84 0.77 0.73 0.66 0.70 0.69 0.70

3.2.3. Metal-modified hierarchical MFI zeolites As shown in Fig. 8, the bio-oil yield increased when Fe and Zn were incorporated into the hierarchical zeolite. The more the metal loading, 9

Fuel 264 (2020) 116813

A. Palizdar and S.M. Sadrameli

Fig. 7. The possible forms of Zn species on the extra framework position of the hierarchical zeolite [53,55].

reduction [26]. Therefore, the catalytic performance of the samples with higher loadings than 2 wt% is obviously decreased. Considering more appropriate results for MZFe2 and MZZn2 samples, the catalytic performance of these catalysts will be investigated in comparison with HZ and MZ5 zeolites to demonstrate their positive effects on upgraded bio-oil. As depicted in Figs. 9 and 10, the value of UF for mesoporous HZSM5 and Fe- and Zn-promoted zeolites is higher than HZ. This is mainly due to the better accessibility to the active sites within the zeolite pores. This can be related to a notable reduction of undesired compounds selectivity even though the value of DOD and EHI decreased for MZFe2 and MZZn2. The relative peak area (RPA) for various compounds is illustrated in Fig. 11. The key point is a higher RPA of ketones and furans against lower RPA of aromatics, particularly PAHs for metalpromoted HZSM-5. However, the conversion of carboxylic acids over these catalysts approximately equals to MZ5. Thus, it can be concluded that the incorporation of Fe and Zn into the hierarchical zeolite alters the deoxygenation reactions from cracking and aromatization pathways to the ketonization route. This is in agreement with the acid site distribution of the catalysts displayed in Fig. 12. As observed, both ironand zinc-modified samples have higher and stronger weak acid sites than the mesoporous zeolite. It is the main responsible for the promotion of ketonization reaction. The increment of weak acidity in the presence of metals is attributed to the new Lewis acid sites generated as cations or oxycation complexes in various forms. On the other hand, the NH3-TPD patterns also indicate that zinc species e.g. [Zn-O-Zn]2+ and ZnOH+ provide a stronger weak acidity rather than iron species such as [Fe-O2-Fe]2+. This difference makes a more suitable UF for the Znmodified catalyst than the other. Catalytic upgrading of bio-oil over MZZn2 resulted in the minimum RPA of PAHs, maybe because of the lower density of strong acidity, and the maximum RPA of ketones. These results are supported by previous works. Neumann et al. [20] applied a cerium-promoted hierarchical MFI zeolite to catalytic pyrolysis of biomass and the results showed a higher selectivity toward valuable oxygenates instead aromatics. In addition, the catalyst was able to catalyze the ketonization of carboxylic acids and consequently made the bio-oil more stable. Veses et al. [24] also examined several metal-modified hierarchical HZSM-5 on bio-oil upgrading and reported a large activity in deoxygenation and ketonization of carboxylic acids particularly over Mg-promoted sample. For all the catalysts, incorporation of the metals into the support reduced the selectivity of aromatics due to lower Brønsted acidity. Fermoso et al. [25] impregnated lamellar and pillared ZSM-5 zeolites by zinc and magnesium oxides and investigated their catalytic performance in bio-oil upgrading. The doping of metals lowered the density of original Lewis and Brønsted acid sites while a great amount of new Lewis sites was generated. The results revealed less activity of metal oxides doped samples in undesired further reactions and thus, low selectivity of PAHs. Ketonization of carboxylic acids was also promoted over these catalysts. Ketonization is a useful alternative route to reduce the oxygen content of bio-oil particularly when the upgrading process is operated without an external hydrogen source. In this reaction, two carboxylic acid molecules are joined together producing one molecule of corresponding ketone, and one molecule of water and carbon dioxide [74,75]:

Table 4 Chemical composition of beech wood raw pyrolysis oil obtained by GC–MS. Entry

RT (min)

Area (%)

1 2 3 4 5 6 7 8 9 10 11 12 13

1.48 1.61 1.71 1.86 2.6 3.75 4.84 5.33 5.95 6.52 7.15 7.7 8.32

0.72 4.48 1.77 1.37 34.53 3.73 5.36 1.26 1.38 3.34 1.85 2.53 1.85

14 15 16 17 18 19 20 21 22 23

9.2 9.39 11.05 12.13 12.35 13.22 14.52 14.65 15.53 17

3.44 3.07 1.85 1.63 1.32 6.4 4.11 1.61 2.62 2.31

24 25

17.58 17.86

4.47 0.74

26

18.23

2.26

Formula Acetone Acetic acid, methyl ester Formic acid 2,3-Butanedione Acetic acid Butanedial Furfural 2-Propanone, 1-(acetyloxy)2(5H)-Furanone 1,2-Cyclopentanedione 2-Furancarboxaldehyde, 5-methylOxazolidine, 2,2-diethyl-3-methyl2-Cyclopenten-1-one, 2-hydroxy-3methylPhenol, 4-methoxyPhenol, 2-methoxyPhenol, 2-methoxy-4-methyl1,2-Benzenediol, 3-methoxyPhenol, 4-ethyl-2-methoxyPhenol, 2,6-dimethoxyBenzoic acid, 4-hydroxy-3-methoxyPhenol, 2-methoxy-4-(1-propenyl)5-tert-Butylpyrogallol Benzaldehyde, 4-hydroxy-3,5dimethoxyPhenol, 2,6-dimethoxy-4-(2-propenyl)Ethanone, 1-(4-hydroxy-3,5dimethoxyphenyl)1-Butanone, 1-(2,6-dihydroxy-4methoxyphenyl)-

C3H6O C3H6O2 CH2O2 C4H6O2 C2H4O2 C4H6O2 C5H4O2 C5H8O3 C4H4O2 C5H6O2 C6H6O2 C8H17NO C6H8O2 C7H8O2 C7H8O2 C8H10O2 C7H8O3 C9H12O2 C8H10O3 C8H8O4 C10H12O2 C10H14O3 C9H10O4 C11H14O3 C10H12O4 C11H14O4

15.04% for MZFe4 and then decreased for higher loadings. Nevertheless, in the presence of zinc species, the content of ketones showed steadily a descending trend. The relative content of aromatics was also diminished by increasing the Fe and Zn loadings. This may be related to a reduction in strong acid sites when doping metals due to the coverage or ion-exchange by these metal species. Eventually, the value of UF for upgraded bio-oils using metal-modified zeolites was detracted with increasing the metal loadings. To explain the effect of higher metal loadings on bio-oil deoxygenation, it should be noted that insertion of metal species into the extra framework position of zeolite structure causes three distinct phenomena [25,26,29]: (1) some empty voids within the zeolite pores are occupied by metal species, thus average pore volume is consequently reduced; (2) some weak and strong acid sites are covered or ion-exchanged by metal species, therefore the accessibility of reactant molecules to these active sites will be limited; (3) some new active sites are generated in various form e.g. metal cations, oxycation complexes and metal oxides. A desired catalytic performance in bio-oil upgrading can be achieved by accurate control of the acid site distribution using proper metal loading. When Fe and Zn are loaded on the hierarchical HZSM-5 support by 2 wt%, new generated weak acid sites with higher strength have a considerable effect on bio-oil characteristics. This effect is at the expense of a decrease in zeolite pore volume and accessibility to the original acid sites. Further loadings will, however, reduce pore volume and accessibility to the original acid sites more significantly while new generated active sites are not able to compensate for this

R1COOH + R2COOH 10

catalyst

R1C = OR2 + CO2 + H2 O

(7)

Fuel 264 (2020) 116813

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Table 5 Elemental analysis and characteristics of the raw and upgraded bio-oil over different catalysts. Thermal

HZ

MZ5

MZFe2

MZFe4

MZFe6

MZFe8

MZZn2

MZZn4

MZZn6

MZZn8

Elements (wt%) C H O N

53.80 6.60 39.27 0.25

74.06 7.30 18.64 0.00

74.48 7.55 17.65 0.42

72.15 7.28 20.85 0.00

66.10 7.06 26.80 0369

63.16 6.97 29.54 0.22

62.00 6.85 30.84 0.40

72.16 7.36 19.90 0.46

68.37 7.14 23.95 0.75

63.23 6.88 29.32 0.68

63.46 6.77 29.88 0.00

Components (wt%) Phenolics Carboxylic acids Ketones Furans Esters Aldehydes Other oxy. ALH MAH PAH

35.83 36.30 8.54 11.12 4.48 3.73 0.00 0.00 0.00 0.00

29.79 13.08 4.41 5.37 1.85 0.00 0.52 0.00 23.55 21.43

34.37 10.59 8.56 4.45 1.58 0.00 0.46 1.66 22.68 15.65

32.32 12.05 11.86 9.84 1.86 0.00 0.00 0.00 20.12 11.95

42.48 15.49 15.04 6.36 2.38 0.00 0.00 1.45 8.74 8.06

43.66 17.08 14.26 6.27 5.24 0.00 0.00 0.96 5.83 6.7

46.91 20.16 13.64 5.48 2.83 0.00 0.00 1.56 1.42 8.00

28.74 10.93 16.31 9.15 2.19 0.00 0.00 1.79 21.02 9.87

37.24 14.51 11.75 8.71 1.92 0.00 0.00 2.95 15.27 7.65

45.43 18.89 9.67 8.85 2.65 0.00 0.00 2.67 2.78 9.06

44.82 18.38 8.52 12.61 2.3 0.00 0.00 0.00 5.82 7.55

Oxygenates (wt%) Small (O1) Medium (O2) Large (O3 + O4)

3.25 70.95 25.80

7.86 26.74 20.42

19.98 25.57 14.46

17.48 33.10 17.35

11.93 45.03 24.79

6.37 50.38 29.76

6.69 50.1 32.23

22.82 29.47 15.03

13.22 39.39 21.52

7.17 44.58 33.74

4.92 49.37 32.34

Others Density (gr/cm3) HHVa (MJ/kg) DOD (%) EHI H/C O/C UF

1.02 21.84 0.00 0.366 0.123 0.729 0.000

0.90 34.88 52.55 0.806 0.098 0.252 0.540

0.89 35.54 55.05 0.847 0.101 0.237 0.735

0.90 33.01 46.90 0.779 0.100 0.289 0.594

0.93 29.19 31.75 0.661 0.107 0.405 0.368

0.94 27.25 24.78 0.615 0.110 0.468 0.257

0.98 26.48 21.46 0.564 0.111 0.497 0.196

0.89 33.63 49.30 0.795 0.102 0.276 0.690

0.93 30.82 39.02 0.700 0.104 0.350 0.463

0.97 27.28 25.32 0.583 0.109 0.464 0.229

0.97 27.23 23.93 0.574 0.107 0.471 0.214

HHV

( ) = 5.22(C ) MJ kg

2

319(C )

1647(H ) + 38.6(CH ) + 133(N ) + 21028 [62].

0.9

45

EHI HHV (MJ/kg)

40

EHI

0.8

35 0.7 30 0.6

0.5

HHV (MJ/kg)

a

25

HZ

MZ5

MZFe2

MZZn2

20

Fig. 10. The effect of various catalysts on the upgrading parameters: EHI and HHV.

where R1 and R2 are alkyl substituents. For a monocarboxylic acid, such as acetic acid, the reaction decreases the oxygen content by 75% and increases the chain length without a loss in bio-oil yield revealing an excellent deoxygenation performance [76,77]. Many studies have been carried out on the ketonization reaction and effect of various catalysts (mostly metal oxides) on its promotion. Also, several mechanisms have been proposed to indicate the molecular pathway over different catalysts [74,76,78,79]. It is believed that the ketonization route mainly proceeded over extra framework metal species in various form especially metal oxides and probably oxycation complexes during bio-oil upgrading [65]. Kumar et al. [80] discussed numerous proposed mechanisms for biomass-derived carboxylic acid ketonization. The “ketene mechanism” is known as one of the main reaction pathways for the ketonization of carboxylic acids over mixed oxides involved the decarboxylative coupling of an adsorbed surface ketene with a carboxylate species. These surface ketene species may form through dehydration of adsorbed bidentate carboxylate species [80]. After the formation of these species, they can couple with an adsorbed

Fig. 8. The yield of various pyrolysis products in catalytic upgrading of bio-oil over the focused catalysts.

0.7

70

UF DOD (%)

60

UF

0.6 0.5

50

0.4 40

0.3 0.2

30

0.1

0

DOD (%)

0.8

HZ

MZ5

MZFe2

MZZn2

20

Fig. 9. The effect of various catalysts on the upgrading parameters: UF and DOD.

11

Fuel 264 (2020) 116813

A. Palizdar and S.M. Sadrameli

35

HZ

MZ5

MZFe2

MZZn2

Relative peak area (%)

30 25 20 15 10 5 0

Fig. 11. The effect of various catalysts on the relative peak area of different compounds in bio-oil upgrading.

carboxylate to produce a corresponding ketone. Such a mechanism for acetic acid over mixed oxides is presented in Fig. 13. The distribution of oxygenates shown in Fig. 14, indicates that the percent of small and medium oxygenates still remains large even after doping the Zn species. However, by Fe loading, the medium oxygenates will be enhanced at the expense of small ones. Such a result discloses a higher ability of zinc species to crack the large and medium oxygenates to smaller ones through decarbonylation, decarboxylation, dehydration, and ketonization pathways.

weight curve DTG. The lower the peak temperature the easier the oxidation of coke. The TG and DTG curves of the parent, hierarchical, and metal-modified hierarchical spent catalysts are depicted in Fig. 15. The coke analysis results are also summarized in Table 6. Two distinct trends can be seen for zeolites modified by iron and zinc with different loadings. The coke yield firstly increased up to 8.33 wt% for MZFe2 by raising the iron doping and then decreased. Contrarily, the coke yield firstly decreased to 5.41 wt% for MZZn2 by raising the zinc loading and then increased. In each case, a reverse trend can be seen for peak temperatures. On the other hand, the high coke yield of MZ5 can be attributed to the improved accessibility of strong acid sites. It is proven that the aromatic hydrocarbons especially polycyclic aromatics are the main precursors for coke formation through consequent oligomerization and polymerization. Strong acid sites have a high potential to promote such reactions [81]. As displayed in Fig. 16, the incorporation of Fe and Zn into the hierarchical HZSM-5 reduced the coke yield (22% and 15%, respectively) and also peak temperature (24% and 15%, respectively) compared with the MZ5. These reductions show a soft nature of coke produced resulting in milder oxidation. This also exhibits the high ability of the metal-modified catalysts to cost-effective regeneration.

3.2.4. Coke analysis As mentioned before, the hydrogen-deficient environment of bio-oil upgrading reactors is highly able to form coke particularly when acidic active sites are available. It is believed that the introduction of mesopores within the zeolite structure can control the ability of coke formation. It is due to the shorter diffusion path of the reactant and product molecules and lower residence time. Zhu et al. [44] demonstrated that the coke produced during bio-oil upgrading over hierarchical HZSM-5 is inherently two types with a difference in oxidation temperature during regeneration. This temperature can be found by thermogravimetric analysis (TGA) at corresponding peaks in the derivative

0.08

HZ

0.07

MZFe2

0.06

TCD Signal (A.U.)

MZ5

Weak acidity Strong acidity

MZZn2

0.05 0.04 0.03

0.02 0.01 0

100

200

300

400

500

600

Temperature (°C) Fig. 12. Acid site distribution of the parent, hierarchical, and metal-modified samples at 2 wt% loading. 12

Fuel 264 (2020) 116813

A. Palizdar and S.M. Sadrameli

Fig. 13. A proposed mechanism for ketonization of acetic acid to acetone: “Ketene mechanism” [80]. Table 6 The coke yield and peak temperature for the spent catalysts.

Fig. 14. The effect of various catalysts on the distribution of oxygenates in biooil upgrading.

Catalyst

Percent of weight change (%)

Coke yield (wt%)

Tpeak (°C)

HZ MZ5 MZFe2 MZFe4 MZFe6 MZFe8 MZZn2 MZZn4 MZZn6 MZZn8

13.74 16.91 13.62 17.44 15.43 12.24 14.87 12.42 12.65 13.81

6.41 7.98 6.19 8.33 7.28 5.53 6.76 5.41 5.68 6.40

622.23 541.60 410.47 356.51 366.15 400.28 488.71 517.67 512.72 493.09

MZ5 zeolite in bio-oil upgrading, a relatively high coke yield is a notable challenge. Modification of the hierarchical catalyst using iron and zinc metals could reduce high coke yield as well as oxidation temperature. This improvement was achieved through efficient control of the zeolite strong acidity.

Indeed, the main reason for the mentioned coke behavior is to control the acidity of support zeolite using metal doping and lower selectivity of the catalysts toward PAHs. The same coke behavior has been evidenced by Neumann et al. [20], Veses et al. [24], Fermoso et al. [25], and Li et al. [26]. To sum up, although the appropriate performance of 100

0

98

Weight (%)

-0.02

94 92

-0.03

90 -0.04

88 HZ

86

MZFe2

82

80

-0.05

MZ5

84

-0.06

MZZn2

30

100

170

240

310

380

450

520

590

660

730

800

-0.07

Temperature (°C) Fig. 15. The TG and DTG curves of the parent, hierarchical, and metal-promoted spent catalysts. 13

Derive. Weight (%/°C)

-0.01

96

Fuel 264 (2020) 116813

A. Palizdar and S.M. Sadrameli

Coke yield Tpeak

Coke yield (wt%)

8

7.5 7

500

6

450

5.5 5

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400

4.5 4

600 550

6.5

References

650

Tpeak (°C)

8.5

HZ

MZ5

MZFe2

MZZn2

350

Fig. 16. The coke yield and peak temperature for the spent catalysts.

4. Conclusions In this study, microporous, hierarchical, and iron- and zinc-modified hierarchical MFI zeolites were used to the catalytic upgrading of beech wood pyrolysis oil in order to produce a blendable stream with FCC feed. The introduction of mesopores within the zeolite structure was performed through desilication by alkaline NaOH solution. Also, metals were incorporated into the hierarchical zeolite by the impregnation method with various metal loadings. The aim was to study the effect of mesoporosity and acidity control on catalytic upgrading features. The zeolite samples were characterized using XRD, SEM, EDS, BET and NH3TPD analyses. The main conclusions of this study can be summarized as below:

• Metal doping decreased the textural properties of the hierarchical • • •

zeolite through coverage of the catalyst surface by metal species. It also altered acid site distribution of the hierarchical zeolite without a significant effect on the zeolite crystallinity. For hierarchical HZSM-5 zeolite, improved accessibility of various acid sites required to promote cracking and deoxygenation reactions caused to the enhanced conversion of large oxygenates to smaller ones resulting in better upgradation of the raw bio-oil. Zn-modified hierarchical HZSM-5 at the loading of 2 wt% showed high selectivity towards desired oxygenates such as ketones via the promotion of carboxylic acid ketonization and reduced the formation of polycyclic aromatics. The coke yield and oxidation temperature decreased by 15% and 10%, respectively, over Zn-promoted catalyst relative to hierarchical sample indicating desirable characteristics for cost-effective catalyst regeneration.

Author contribution Ali Palizdar and Seyed Mojtaba Sadrameli carried out the experiment on the application of catalysis on the pyrolysis of Beech wood for the upgrading of produced bio-oil. We wrote the manuscript with support from the research department of Tarbiat Modares University and Iran National Science Foundation. The first author is a Ph.D. student and the second author supervised the project. Declaration of Competing Interest 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. Acknowledgments The financial supports received from the research department of Tarbiat Modares University and Iranian National Science Foundation (INSF) during the research is gratefully acknowledged. 14

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