Production of aromatic hydrocarbons by co-cracking of bio-oil and ethanol over Ga2O3/HZSM-5 catalysts

Xing Zhang,Jingfeng Wu,Junhao Chen,Liang Lu,Lingjun Zhu*,Shurong Wang*

State Key Laboratory of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

Keywords: Biomass Biofuel Co-cracking Catalyst Preparation method Si/Al ratio

ABSTRACT The catalytic cracking of bio-oil is important to produce aromatic hydrocarbons,which can partially replace gasoline or diesel to greatly reduce carbon emissions from transportation.To further promote the formation of aromatic hydrocarbons,this work studied the effects of the preparation method and the acid strength of Ga2O3/HZSM-5 on catalytic cracking of the bio-oil distilled fraction systematically.The preparation method of Ga2O3/HZSM-5 had an important effect on its catalytic activity:the Ga2O3/HZSM-5 prepared by physical mixing showed the low dispersion of active phases and poor pore structure,resulting in its insufficient activity and severe coke deposition;the Ga2O3/HZSM-5 prepared by precipitation exhibited the higher activity,while many polycyclic aromatic hydrocarbons unfavorable for the subsequent utilization were in the oil phase;the Ga2O3/HZSM-5 prepared by impregnation showed the highest activity and 35.5% (mass)selectivity of the oil phase,including 80.3% monocyclic aromatic hydrocarbons and 12.0% polycyclic aromatic hydrocarbons.The Br?nsted acidity of Ga2O3/HZSM-5 decreased with Si/Al ratio,leading to the decline in reactant conversion,oil phase selectivity and quality.Meanwhile,the polymerization between monocyclic aromatic hydrocarbons and oxygenates was promoted to produce many polycyclic aromatic hydrocarbons and even coke,causing catalyst deactivation.

1.Introduction

Bio-oil produced by fast pyrolysis and hydrothermal liquefaction of biomass is an attractive green alternative to unsustainable fossil fuels [1].However,some undesirable properties of crude bio-oil—such as high oxygen content(35% –55% ,mass),high viscosity (40–100 cP,50 °C),strong corrosiveness (pH=2–3),and low heating value(HHV=16–19 MJ?kg-1)—limit its utilization as transportation fuels [2].Further upgrading is necessary to improve the quality of bio-oil.Various bio-oil upgrading technologies,including hydrogenation,hydrodeoxygenation,cracking,esterification,emulsification,and steam reforming,have been developed[3].Catalytic cracking over zeolite catalysts can convert oxygenated biooil into aromatic hydrocarbons [4].

Crude bio-oil is a complicated mixture of numerous oxygenates,including acids,ketones,alcohols,aldehydes,esters,phenolics,sugars,etc.[1].The complex composition and the low (H/C)eff((H-2O)/C)ratio[5]of crude bio-oil make it impractical for direct cracking.Severe coke deposition (up to 19% (mass)) has been observed during crude bio-oil cracking [5].Previous studies on the cracking activities of different oxygenated families in bio-oil have revealed that alcohols,ketones,and aldehydes could be readily converted to hydrocarbons,while high-molecule-weight phenolic oligomers and sugars strongly tended to form coke,resulting in rapid catalyst deactivation [6,7].To improve cracking activity and stability,advanced bio-oil separation technology is required to extract the bio-oil components suitable for cracking.

Molecular distillation is an efficient separation technology for the separation of complex and thermally sensitive molecules.Compared with conventional distillation technologies,molecular distillation has the advantages of low operating temperature,high separation efficiency,and short condensation time [8].In recent years,molecular distillation has been developed to separate biooil [9,10]:acids,aldehydes,and ketones were typically concentrated in the distilled fraction,while phenolic oligomers and sugars remained in the residual fraction [9].A previous study [11] has shown that compared to crude bio-oil,the catalytic cracking performance of the distilled fraction was much improved.However,the deoxygenation of the distilled fraction could generate light olefin intermediates with a higher (H/C)effratio,accompanied by the formation of coke with a lower (H/C)effratio [12].Therefore,the ethanol with a high (H/C)effratio of 2—produced by sugar fermentation—can be used as the co-reactant in the cracking process to improve the cracking activity by increasing the overall (H/C)effratio [11].

The HZSM-5 zeolite is considered as an efficient catalyst for catalytic cracking because of its sufficient Br?nsted acid sites.Deoxygenation reactions,such as dehydration,decarbonylation,and decarboxylation,first occurred on these active sites to remove oxygen from bio-oil in the form of CO,CO2,and H2O.The subsequent aromatization reaction also occurred on Br?nsted acid sites,and the special pore structure (i.e.,shape selectivity) in HZSM-5 favored the formation of aromatic hydrocarbons[13,14].However,the massive release of C2H4,a key intermediate from the deoxygenation of bio-oil,showed the insufficient aromatization activity of HZSM-5.Therefore,metal oxides have been usually doped into the HZSM-5 zeolite to further promote the formation of aromatic hydrocarbons.By comparing the activities of different metal oxide-modified HZSM-5 catalysts for bio-oil cracking,our group[15] found that the incorporation of Ga2O3could promote the aromatization reaction and improve the cracking stability.The activity and stability of catalyst are highly dependent on its preparation method in most cases.Common methods for the preparation of solid catalysts include impregnation,precipitation,physical mixing,ion-exchange,etc.The preparation method can make an important effect on the physicochemical properties of catalysts,including texture,structure,dispersion of active phases,acidity,redox property,etc.[16].Furthermore,the acid strength of HZSM-5 zeolite,greatly depending on its Si/Al ratio,is also a key factor determining its deoxygenation and aromatization activities.Therefore,in this work,the effects of the preparation method and the acid strength of Ga2O3/HZSM-5 on catalytic cracking of the biooil distilled fraction were studied systematically.

2.Experimental

2.1.Catalyst preparation

Ga2O3/HZSM-5 catalysts were prepared by physical mixing,precipitation,and impregnation,while Ga2O3/Al2(SiO3)3was selected for comparison.According to our previous study [17],the loading of Ga2O3was set at 15% (mass)to promote the aromatization reaction.The HZSM-5 zeolites(Si/Al=25,70,170,and 350)were purchased from the Catalyst Plant of Nankai University;Al2(-SiO3)3and Ga(NO3)3were obtained from Sinopharm Chemical Reagent;Ga2O3was supplied by Aladdin Industrial Corporation.

Physical mixing:certain amounts of Ga2O3and HZSM-5 were physically mixed by crushing and tableting.

Precipitation:a certain amount of HZSM-5 was added to Ga(NO3)3aqueous solution under constant stirring at ambient temperature.Ammonia solution (NH3?H2O) was used to titrate the above suspension until pH was around 4.0.After aging for 2 h,the precipitate was washed,dried overnight at 110°C,and calcined in air at 550 °C (5 °C?min-1) for 6 h.

Impregnation:a certain amount of HZSM-5 was added to Ga(NO3)3aqueous solution at ambient temperature,followed by constant stirring for 12 h.After that,the mixture was dried overnight at 110 °C and calcined in air at 550 °C (5 °C?min-1) for 6 h.

The obtained samples are denoted as Ga2O3/HZSM-5-M/P/I (n),where M,P,and I represent physical mixing,precipitation,and impregnation,respectively,andnrepresents Si/Al ratio.The procedure used to prepare Ga2O3/Al2(SiO3)3was the same as that of Ga2O3/HZSM-5-I,except for the support.The abovementioned catalysts were pressed,milled,sieved to 40–60 mesh,and then activated at 550 °C (5 °C?min-1) for 6 h before the experiment.

2.2.Catalyst characterization

Powder X-ray diffraction(XRD)patterns were recorded from 5°to 90° on a PANalytical X’Pert PRO X-ray diffractometer equipped with Cu Kα radiation (λ=0.15406 nm) at 40 kV and 40 mA.

Acid strengths of Ga2O3/HZSM-5-I and Ga2O3/Al2(SiO3)3were determined by NH3-temperature programmed desorption (NH3-TPD) using a Micromeritics AutoChem Ⅱ2920 apparatus.0.03 g of sample was pretreated in flowing N2at 550 °C for 2 h and then cooled down to 100 °C.After the adsorption of NH3for 1 h,the sample was purged with flowing He for another 1 h to remove the physically adsorbed NH3.Finally,the sample was heated from 100°C to 600°C with a rate of 10°C?min-1,while the desorbed NH3species were detected on a mass spectrometer.

Acid sites of the Ga2O3/HZSM-5-I with different Si/Al ratios were determined by the infrared spectra of adsorbed pyridine(Py-IR) on a TENSOR27 Fourier transform infrared spectrometer(Bruker,German).The sample was pretreated at 450 °C for 1 h under vacuum,and the background spectrum was collected.After cooling to ambient temperature,the sample was equilibrated with pyridine vapor,followed by removing the weakly adsorbed pyridine.The sample was then evacuated at 200 °C for 1 h,and the IR spectrum was collected again.

N2physisorption measurement was carried out on a Micromeritics Tristar II 3020 instrument to determine the Brunauer-Emmett-Teller(BET) specific surface areas and the average pore volumes of the fresh and spent catalysts.

Thermogravimetry analysis(TGA)was performed on a TGA8000 Perkin-Elmer instrument to obtain the coke yield on a spent catalyst.The sample was heated up in flowing air (30 ml?min-1) from ambient temperature to 900°C with a heating rate of 20°C?min-1.

2.3.Catalytic run

Some typical compounds in the bio-oil distilled fraction were chosen as model compounds,including acetic acid,hydroxyacetone,guaiacol,and furfural;their detailed mass ratio was set at 50% (mass) acetic acid,30% (mass) hydroxyacetone,10% (mass)guaiacol,and 10% (mass) furfural,based on the composition of the bio-oil distilled fraction [18,19].Based on our previous study on bio-oil co-cracking,the mass ratio of the distilled fraction and ethanol was set at 2:3[11].Therefore,the composition of feedstock was 20% (mass)acetic acid,12% (mass)hydroxyacetone,4% (mass)guaiacol,4% (mass) furfural,and 60% (mass) ethanol.

The catalytic reaction was carried out on a stainless steel fixedbed reactor(Fig.1)with an inner diameter of 8 mm,in which 2 g of catalyst was supported on quartz wool.The feedstock was nebulized with N2and fed to the reactor with the aid of a highperformance liquid chromatography (HPLC) pump.The outlet gas was cooled and separated into liquid and non-condensable gas.The weight hourly space velocity (WHSV) of the reactant was 2 h-1.The reaction temperature and pressure were kept at 400 °C and 2 MPa,respectively,which had been proved to benefit the formation of aromatic hydrocarbons [11].The reaction pressure was maintained by flowing N2(40 ml?min-1).

The composition of liquid products was identified on a gas chromatograph–mass spectrometer(GC–MS,TraceDSQ II)with an Agilent DB-WAX capillary column,and the relative contents of various compounds were calculated by peak area normalization.The oven temperature was kept at 40 °C for 1 min,then increased to 240 °C at a rate of 8 °C?min-1and maintained at this temperature for 10 min.The concentrations of the unconverted reactants and liquid products were determined using a gas chromatograph (Agilent 7890A) with an INNOWAX capillary column and quantified by external standard method.The temperature program was consistent with that of GC–MS.

Fig.1.An apparatus flowchart of the reaction system.

The conversions of reactants(Xi,i=acetic acid/hydroxyacetone/guaiacol/furfural/ethanol) and the selectivities of products (Sj,j=oil phase/aqueous phase)are defined by Eqs.(1)and(2),where the symbol “m”represents the mass of corresponding substances.

3.Results and Discussion

3.1.Catalyst characterization

3.1.1.XRD

Fig.2.XRD patterns of the Ga2O3/HZSM-5 (25) prepared by different methods.

Fig.2 illustrates the XRD patterns of the Ga2O3/HZSM-5 (25)prepared by different methods.The diffraction peaks at 7°–9°and 22.5°–25.0° were attributed to a typical MFI zeolite framework,indicating the intact crystal structure of HZSM-5 in the Ga2O3/HZSM-5 (25) prepared by different methods.The strong diffraction peaks of Ga2O3/HZSM-5-M (25) at 30.5°,31.7°,33.5°,35.2°,37.4°,38.4°,45.8°,and 64.7° were ascribed to Ga2O3(PDF#43-1012),indicating the local agglomeration of Ga2O3.This is because the randomness of Ga2O3diffusion resulted in the low dispersion of active phases during physical mixing.Conversely,almost no Ga2O3peak was present in Ga2O3/HZSM-5-P (25) and Ga2O3/HZSM-5-I (25),indicating that Ga2O3was likely highly dispersed or amorphous.Recently,Zhanget al.[20] and Razzaqet al.[21] also reported the absence of metal oxide peak in metal oxide-modified HZSM-5 catalysts prepared by precipitation and impregnation,indicating the high dispersion of metal oxides.

3.1.2.Acidity

Fig.3(a) shows the NH3-TPD patterns of the Ga2O3/HZSM-5-I with different Si/Al ratios.Two typical desorption peaks appeared around 200 °C and 370 °C,which were attributed to weak and strong acid sites,respectively.Two acid sites were obvious at the Si/Al ratio of 25,while almost no acid sites were observed at the Si/Al ratios of 170 and 350.This showed that a higher Si/Al ratio led to a weaker intensity of desorption peaks,i.e.,a decrease in the total acidity,because acid sites were formed on the framework Al sites.Compared with Ga2O3/HZSM-5-I,Ga2O3/Al2(SiO3)3only showed a strong desorption peak at a low-temperature region,which was attributed to weak acid sites.This was consistent with the NH3-TPD pattern of Pt(Pd)Ni/Al2(SiO3)3[22].

The Br?nsted and Lewis acid sites of the Ga2O3/HZSM-5-I with different Si/Al ratios were determined by Py-IR,as shown in Fig.3(b).For the Ga2O3/HZSM-5-I,the bands around 1550 cm-1and 1450 cm-1were attributed to the pyridine adsorbed on Br?nsted and Lewis acid sites,respectively [23,24].The Br?nsted acid sites were generated from the protons adsorbed near oxygen bridges in the zeolite framework;the Lewis acid sites stemmed from the extra-framework Al or the coordinatively unsaturated Al sites created by the structural defects of the zeolite [25].The strengths of Br?nsted and Lewis acids were highest at the Si/Al ratio of 25 and then decreased with Si/Al ratio,which was consistent with a gradual decrease in the total acidity.Compared to Ga2O3/HZSM-5-I,only the Lewis acid site stemming from Al sites was detected in Ga2O3/Al2(SiO3)3.

3.1.3.BET and TG

The textural properties of the fresh and spent catalysts are listed in Table 1.Different from Si/Al ratio,catalyst preparation method had a significant effect on its specific surface area (SBET):the specific surface area of Ga2O3/HZSM-5-I (25) was the highest,i.e.,283.9 m2?g-1,followed by those of Ga2O3/HZSM-5-P (25)(264.9 m2?g-1)and Ga2O3/HZSM-5-M(25)(239.6 m2?g-1).The lowest specific surface area of Ga2O3/HZSM-5-M (25) might result from the low dispersion of active phases,which was inferred from the XRD pattern.The specific surface area and pore volume (VPore)of Ga2O3/Al2(SiO3)3were 59.5 m2?g-1and 0.178 cm3?g-1,respectively,while its pore size (Dpore) was up to 13.05 nm,suggesting its underdeveloped pore structure.This was also supported by a previous study on Pt(Pd)Ni/Al2(SiO3)3[22].At the low Si/Al ratio of 25,Ga2O3/HZSM-5 catalysts had large pore sizes independent of the preparation method.The specific surface areas and pore volumes of all catalysts decreased after the reaction,while the pore sizes increased,indicating that some pores collapsed.The specific surface area and average pore volume of the spent Ga2O3/HZSM-5-M (25) showed the most obvious decrease to 48.4 m2?g-1and 0.079 cm3?g-1,respectively;its high coke/catalyst value suggested a high yield of coke formed over the catalyst blocking its pore structure.Although the spent Ga2O3/Al2(SiO3)3showed the lowest coke yield of 0.09 g?g-1,its specific surface area was still the lowest,only 21.9 cm3?g-1,which was because of its undeveloped pore structure.For the Ga2O3/HZSM-5-I with different Si/Al ratios,the coke yield was the lowest,i.e.,0.15 g?g-1,at the Si/Al ratio of 70;the higher coke yield at the Si/Al ratio above 170 might be because of the insufficient acid strength and catalytic activity;the drop of Si/Al ratio to 25 resulted in a slight increase in the coke yield,which was due to the strong acidity of the catalyst.Cordero-Lanzacet al.[26] also found that condensation reactions of biooil could be promoted by acid sites.

Fig.3.(a) NH3-TPD and (b) Pyridine-IR patterns of Ga2O3/HZSM-5-I and Ga2O3/Al2(SiO3)3.

Table 1 The textural properties of the fresh and spent catalysts

3.1.4.SEM

Fig.4 shows the SEM images of Ga2O3/Al2(SiO3)3,Ga2O3/HZSM-5-M(25),Ga2O3/HZSM-5-P(25),and Ga2O3/HZSM-5-I(25).Ga2O3/HZSM-5-M (25) showed a brick-like structure,which was attributed to the presence of HZSM-5 (25),and there were almost no Ga species on the surface of HZSM-5(25).In contrast,Ga species could be successfully doped into supports (i.e.,HZSM-5 and Al2(SiO3)3)by impregnation and precipitation methods.Compared to Ga2O3/Al2(SiO3)3and Ga2O3/HZSM-5-P (25),a lot of pores were observed in Ga2O3/HZSM-5-I(25),which led to a higher specific surface area.

3.2.Co-cracking of bio-oil and ethanol

3.2.1.Effect of catalyst preparation method

Ga2O3/HZSM-5(25)catalysts were prepared by physical mixing,precipitation,and impregnation to study the effect of preparation method on catalytic cracking.As shown in Fig.5(a),all reactants except guaiacol were almost completely converted over all catalysts.Hydroxy ketones,aldehydes,and acids were found to exhibit the excellent activity in catalytic cracking,because their oxygencontaining functional groups were active in the deoxygenation reaction [27].The lower activity of guaiacol was due to its low(H/C)effratio and high bond strengths of Caryl–OCH3(422 kJ?mol-1)and Caryl–OH (468 kJ?mol-1) [28].The conversion of guaiacol over Ga2O3/HZSM-5-I (25) (95.3%) was superior to that over Ga2O3/HZSM-5-P (25) (90.9%) or Ga2O3/HZSM-5-M (25) (87.9%).The XRD and BET patterns showed the low dispersion of Ga2O3and poor pore structure in Ga2O3/HZSM-5-M (25),leading to its insufficient catalytic activity.

Ga2O3/HZSM-5-M (25) showed the lowest oil phase selectivity below 30% (mass) with the highest aqueous phase selectivity up to 38.3% (mass),as shown in Fig.6.The oil phase selectivity of Ga2O3/HZSM-5-I (25) (35.5% (mass)) or Ga2O3/HZSM-5-P (25)(35.2% (mass)) was obviously higher than that of Ga2O3/HZSM-5-M (25).Meanwhile,the C2H4selectivity of Ga2O3/HZSM-5-M (25)(8.9% (mass)) was significantly higher than those of Ga2O3/HZSM-5-I (25) and Ga2O3/HZSM-5-P (25) (2.4% (mass) and 4.6% (mass),respectively).The reaction pathways for catalytic cracking of the oxygenated families in bio-oil can be illustrated as shown in Fig.7.Previous studies [12,15] on the cracking mechanism of biooil indicated that oxygenated compounds underwent deoxygenation (e.g.,dehydration,decarboxylation,and decarbonylation) to produce light olefin intermediates,such as C2H4.These light olefins underwent three major reactions,which mainly included the following:i) aromatization to form aromatic hydrocarbons,ii)oligomerization and isomerization to form aliphatic hydrocarbons,iii) oligomerization to form coke [29].Bio-oil with a low (H/C)effwas converted to light olefins with a higher(H/C)effand coke with a lower (H/C)effby catalytic cracking,resulting in the deactivation of catalyst [5].Compared to bio-oil,ethanol exhibited higher deoxygenation activity to produce olefins because of its high (H/C)effof 2 [30].Meanwhile,the surplus hydrogen atoms were observed after the aromatization of these olefins.By co-cracking of bio-oil and ethanol,these hydrogen atoms could be used to help the deoxygenation of bio-oil to produce olefins [11].The strong Br?nsted acid strength and shape selectivity of HZSM-5 were favorable for the formation of aromatic hydrocarbons,while the insufficient aromatization activity resulted in the formation of a large number of light olefins [15].The introduction of Ga could promote the decarbonylation of reactants and the aromatization of light olefins without changing the initial reaction pathway,which increased the yield of aromatic hydrocarbons [31].The uneven active phases and poor pore structure of Ga2O3/HZSM-5-M (25) resulted in the insufficient ability of olefin aromatization.This explained why there was more C2H4and less oil phase over Ga2O3/HZSM-5-M (25).The oligomerization of light olefins was then promoted to cause severe coke deposition.Compared with the Ga2O3/HZSM-5-M (25),the activities of Ga2O3/HZSM-5-I (25)and Ga2O3/HZSM-5-P (25) were well maintained in the cracking process,which was closely related to their higher specific surface areas and dispersion of active phases.

Fig.4.SEM images of (a) Ga2O3/Al2(SiO3)3,(b) Ga2O3/HZSM-5-M (25),(c) Ga2O3/HZSM-5-P (25),and (d) Ga2O3/HZSM-5-I (25).

Fig.5.Reactant conversion versus (a) catalyst preparation method and (b) Si/Al ratio.

Fig.6.Liquid phase selectivity versus catalyst preparation method and Si/Al ratio.M—physical mixing;P—precipitation;I—impregnation.

The products of the oil phase were classified according to chemical families,as listed in Table 2.The oil phase over Ga2O3/HZSM-5-M (25) consisted of 76.2% aromatic hydrocarbons and 20.9% oxygenates,similar to that over Ga2O3/HZSM-5-I (350) with weak acidity.The insufficient activity of Ga2O3/HZSM-5-M (25) was attributed to the low dispersion of active phases and poor structure rather than low acid strength.Compared to the Ga2O3/HZSM-5-M(25),Ga2O3/HZSM-5-I (25) and Ga2O3/HZSM-5-P (25) showed an improved quality of the oil phase:the content of aromatic hydrocarbons was～90% ,with that of oxygenates less than 10% .However,for Ga2O3/HZSM-5-P (25),the content of polycyclic aromatic hydrocarbons as high as 36.3% was even higher than that of Ga2O3/HZSM-5-M (25).Notably,the presence of polycyclic aromatic hydrocarbons was not beneficial for the subsequent storage,transportation,and combustion.Monocyclic aromatic hydrocarbons were found to be formed on the acidic inner surface of HZSM-5,while they could further react with oxygenates to produce polycyclic aromatic hydrocarbons on the acidic outer surface [32].For Ga2O3/HZSM-5-P(25),it could be inferred that the acidic outer surface of HZSM-5 might be not well adjusted by Ga2O3,resulting in aromatic ring polymerization.Compared to the spent Ga2O3/HZSM-5-I (25),the spent Ga2O3/HZSM-5-P (25) showed a higher yield of coke,which also confirmed the abovementioned conclusion.

Table 2 Composition of the oil phase over the Ga2O3/HZSM-5 (25) prepared by different methods

Table 3 Composition of the oil phase over the Ga2O3/HZSM-5-I with different Si/Al ratios

3.2.2.Effect of Si/Al ratio

The Ga2O3/HZSM-5-I catalysts with Si/Al ratios of 25,70,170,and 350 were used to investigate the effect of acid strength on catalytic cracking,while Ga2O3/Al2(SiO3)3was selected for comparison.As shown in Fig.5(b),the conversions of ethanol and furfural were close to 100% over all catalysts,which was due to the high activities of alcohol hydroxyl and aldehyde groups[27,33].The conversions of hydroxyacetone and acetic acid over Ga2O3/HZSM-5-I (25) were also high,reaching 99.1% and 99.2% ,respectively;however,increasing Si/Al ratio led to a significant decrease in their conversions,where the conversion of acetic acid decreased more obviously;their conversions over Ga2O3/HZSM-5-I (350) were as low as 82.6% and 54.0% ,respectively.As mentioned above,deoxygenation,aromatization,and hydrogen transfer reactions mainly occurred on acid sites,especially Br?nsted acid sites [17].Both total and Br?nsted acidity decreased with Si/Al ratio,hindering the initial deoxygenation of oxygenates.The obvious decrease in the conversions of acetic acid and hydroxyacetone might be due to the lower activities of carboxyl and carbonyl groups than alcohol hydroxyl and aldehyde groups [27,33].This result indicated that decarboxylation and decarbonylation reactions were sensitive to acid strength.The conversion of guaiacol over Ga2O3/HZSM-5-I (25) was the lowest,i.e.,95.3% ;its conversion decreased in a certain degree with Si/Al ratio and reached 84.9% at the Si/Al ratio of 350.A previous study by Chantalet al.[34] on the cracking mechanism of phenolics over HZSM-5 reported that phenolics were first converted to the highly unsaturated ketene intermediates through protonation over Br?nsted acid sites,and the intermediates were converted to light olefins with the assistance of the surplus hydrogen atoms produced by ethanol cracking,followed by further aromatization to form aromatic hydrocarbons.The high bond strength of Caryl–OH rendered the direct deoxygenation of guaiacol and ring-opening of aromatic rings difficult during catalytic cracking.Much guaiacol usually underwent alkylation to form other monophenols,or condensation to form polymers and even coke.Phenolics had been found to tend to form aromatic coke on the catalyst micropores,resulting in rapid catalyst deactivation during the reaction[27].The conversion of acetic acid over Ga2O3/Al2(SiO3)3was only 74.4% ,while the conversions of other reactants exceeded 97% .Although the conversion of reactant over Ga2O3/Al2(SiO3)3was high,its activity needed tobe evaluated by the composition of the oil phase,which was due to the absence of its Br?nsted acid sites.

Fig.7.Proposed reaction pathway for catalytic cracking of the oxygenated families in bio-oil.

The oil phase selectivity over Ga2O3/HZSM-5-I (25) was the highest,i.e.,35.5% (mass),as shown in Fig.6.The oil phase selectivity gradually decreased with Si/Al ratio.The oil phase selectivities over Ga2O3/HZSM-5-I (70,170,350) catalysts were 32.8% (mass),28.9% (mass),and 25.3% (mass),respectively,indicating the benefit of strong acidity for the formation of gasoline phase.The liquid product comprised two separable layers,i.e.,the oil and aqueous phase products.As the acidity decreased,these two layers gradually became blurry.Some unconverted acetic acid,hydroxyacetone,and generated oxygenates entered the aqueous phase.Notably,Ga2O3/Al2(SiO3)3only showed a slightly yellow homogeneous liquid product with a selectivity of 89.7% (mass).As mentioned above,the aromatization reaction highly depended on Br?nsted acid sites,so it was impossible to generate aromatic hydrocarbons over Ga2O3/Al2(SiO3)3with only a few Lewis acid sites.The C2H4selectivity over Ga2O3/HZSM-5-I (25) was only 2.4% (mass) and increased to 3.6% (mass),9.7% (mass),and 9.6% (mass) over Ga2O3/HZSM-5-I (70,170,350) catalysts,respectively.This result showed that the Br?nsted acid strength and aromatization activity of catalyst decreased with Si/Al ratio,while the coke yield decreased firstly and then increased.Little C2H4with a selectivity of 2.5% (mass) was detected over Ga2O3/Al2(SiO3)3.This might be due to the pyrolysis of the distilled fraction promoted by some Lewis acid sites.Liet al.[35] had applied Lewis-acid catalyst Nb2O5in catalytic fast pyrolysis of lignin.However,the absence of Br?nsted acid sites rendered further aromatization to get the gasoline phase difficult.

The family compounds in the oil phase over the Ga2O3/HZSM-5-I with different Si/Al ratios are summarized and listed in Table 3.The major compounds in the oil phase were aromatic hydrocarbons,followed by oxygenates and few aliphatic hydrocarbons.With Si/Al ratio,the content of monocyclic aromatic hydrocarbons decreased,while that of polycyclic aromatic hydrocarbons increased,indicating the growing trend of aromatic ring polymerization.The oil phase over Ga2O3/HZSM-5-I (25) was composed of 92.3% aromatic hydrocarbons,2.6% aliphatic hydrocarbons,and 5.1% oxygenates.The content of monocyclic aromatic hydrocarbons reached 80.3% ,mainly including benzene,toluene,and xylene;polycyclic aromatic hydrocarbons with the content as low as 12.0% comprised naphthalene and methyl naphthalene;the typical compound among aliphatic hydrocarbons was 2-methyl butane;oxygenates mainly consisted of cresol,dimethylphenol,and ethylphenol produced by the incomplete deoxygenation and alkylation of guaiacol.The content of monocyclic aromatic hydrocarbons over Ga2O3/HZSM-5-I(350)was less than 43% ,while those of polycyclic aromatic hydrocarbons and oxygenates reached 35.1% and 19.7% ,respectively.This indicated the insufficient deoxygenation activity of the catalyst,which was in line with the low conversions of reactants.The content of oxygenates in the homogeneous phase over Ga2O3/Al2(SiO3)3was 99% ,including ethyl acetate,hydroxyl ketones,and various phenolic derivatives,indicating that Lewis acid sites catalyzed the esterification,isomerization,and alkylation reactions.

4.Conclusions

This work studied the effects of catalyst preparation method and Si/Al ratio on co-cracking of bio-oil distilled fraction and ethanol over Ga2O3/HZSM-5.Although physical mixing had a simple process,the dispersion of active phases was low.The oil phase selectivity was less than 30% (mass),mainly including 76.2% aromatic hydrocarbons and 20.9% oxygenates,accompanied by a high coke yield.The catalysts prepared by impregnation and precipitation showed the similar oil phase selectivity (～35% (mass)) and aromatic hydrocarbon content (～90%);however,the monocyclic aromatic hydrocarbon content of the former (80.3%) was significantly higher than that of the latter (51.8%).The acidic outer surface of HZSM-5 might be not well adjusted by Ga2O3during the precipitation process,resulting in aromatic ring polymerization.With Si/Al ratio,the Br?nsted acid strength of catalysts decreased,leading to a gradual decrease in catalytic activity.Ga2O3/HZSM-5-I(25) showed the highest activity:the conversions of all reactants reached above 95% ;the oil phase selectivity was as high as 35.5% (-mass),including 80.3% monocyclic aromatic hydrocarbons,12.0% polycyclic aromatic hydrocarbons,and 5.1% oxygenates.Therefore,Ga2O3/HZSM-5-I (25) is promising for aromatic hydrocarbon production via bio-oil catalytic cracking.This work confirms the feasibility of the production of aromatic hydrocarbons by co-cracking of bio-oil and ethanol.This upgrading strategy will be a beneficial reference for the future utilization of bio-oil.

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.

Acknowledgements

This work was supported by the Innovative Research Groups of the National Natural Science Foundation of China (51621005).