Preparation of bimetal Co–Ni supported on Mg–Al oxide for chemocatalytic upgrading of tailored fermentation products to energy intensive fuels

Jiajun Liu,Kui Wu,Zhengke Li,Wensong Li,Yuqing Ning,Weiyan Wang ,Yunquan Yang

School of Chemical Engineering,Xiangtan University,Xiangtan,Hunan,411105,China

Abstract The great challenge in the aldol condensation of tailored fermentation products(acetone–butanol–ethanol,ABE)into energy intensive fuels is to develop a suitable catalyst with high activity and low–cost.In this study,Co,Ni,and Co–Ni supported on Mg–Al oxide catalysts were prepared and their pore diameters were enlarged via adding active carbon as a hard template into Mg–Al hydrotalcite.During the aldol condensation reaction,the catalyst activity was enhanced after enlarging the pore diameter and Co–Ni bimetal supported catalyst presented the highest activity,which was resulted from that the electron transfer between Co and Ni in Co–Ni alloy enhanced the dehydrogenation activity and large pore lowered the mass transfer resistance.After optimizing the reaction conditions,acetone conversion and the total selectivity of C5–C11 desired products in the aldol condensation of ABE reached up to 76%and 90%,respectively.The stability study showed that the activity was decreased with the increase of reaction number because of the oxidation of metallic Co and Ni,but this could be solved via a simple hydrogen reduction method.

Keywords: Fermentation products;Aldol condensation;Active carbon;Energy intensive fuel;Co–Ni alloy

1.Introduction

In the past decades,the excessive depletion of traditional fossil fuel and the emergence of environmental pollution problems have accelerated the exploitation and development of clean and sustainable energy sources [1].As a new carbonaceous feedstock with the advantages of carbon neutrality,renewability and easy accessibility,the conversion of biomass into small molecular compounds or biofuels has attracted much attention and became one of the research hotspots in renewable energy field [2].Low–value sugars,carbohydrates or lignocellulosic biomass can be converted into acetone–butanol–ethanol (ABE) mixtures via an anaerobic fermentation process byClostridium acetobutylicum[3].However,these ABE mixtures have short carbon chain length,which is hard to satisfy the carbon number range of gasoline(C5–C13)[4].To increase the energy density,C–C coupling reactions such as aldol condensation,etherification,ketonization,acylation require to be proceeded [5–7].Based on the functional groups (nucleophilic α–carbons of the acetone and the electrophilic α–carbon of the alcohols) in ABE mixtures,aldol condensation reaction is a good approach to allow them converted into energy–intensive fuel components[8],where some oxygen is simultaneously removed without consuming extra hydrogen.

It had been reported that aldol condensation reaction of aldehydes or ketones with alcohols included two important steps:dehydrogenation of the alcohols into aldehyde over metallic sites and aldol condensation of aldehydes with ketones and dehydration on acid–base sites[9,10].Consequently,a suitable catalyst requires three prerequisites:metal,acidity and basicity.Park et al.[11] had found that α alkylation of ketones with primary alcohols on Pd supported catalyst was enhanced in the presence of alkali.Inspired by this result,Toste et al.[12]had prepared Pd/C–K3PO4catalyst for aldol condensation of ABE fermentation products into fuel precursors of gasoline,jet,and diesel.After optimizing the alkalis,only a small amount of acetone self-condensation products and no guerbet products were observed in aldol condensation reaction,indicating the important role of alkali.Xu et al.[13] had also reported this similar result.However,the produced water can dissolve the added alkali,which not only lowers the activity but also cause some environmental pollution problems.To overcome these weaknesses,Lee et al.[14]had prepared Pd/C–CaO heterogeneous catalyst for solvent–free α–alkylation of ABE mixtures,and found that the catalyst deactivation during the reaction was caused by the slight agglomeration of Pd/C and the formation of CaCO3.Therefore,how to construct an inexpensive and high active catalyst for the transformation of fermentation products into practicable fuel is a great challenge.

Mg–Al hydrotalcite (HT) is a good support for dispersing metal components and possesses acidic and basic sites [15–17],which had also applied into aldol condensation reaction[18].To lower the catalyst cost,Tan et al.[19] had used inexpensive first–row transition metals to substitute noble metals and found that Ni–based catalysts presented the highest activity for upgrading ABE mixture.However,due to the production of long carbon–chain compounds in aldol condensation reaction,the mass–transfer resistance of products in the catalyst is consequentially increased.In addition,enlarging pore size can promote the dispersion of metal component and prevent the metal particles from sintering[20,21].Therefore,Mg–Al oxide,derived from hydrotalcite after calcination at high temperature,was used as carriers to support Ni and Co in this study,and its pore size was enlarged by adding active carbon as a hard template.The effects of pore size,reduction temperature and synergy between Ni and Co on the activity of Co–Ni supported on Mg–Al oxide in the chemocatalytic upgrading of tailored fermentation products into energy intensive fuels were studied in detailed.

2.Experimental

2.1.Materials and chemicals

Mg(NO3)2·6H2O,Al(NO3)3·9H2O,Ni(NO3)2·6H2O,Co(NO3)2·6H2O,activated carbon (AC),NaOH and anhydrous Na2CO3were purchased from Maclin.N–butanol(99.5%) and ethanol (99.7%) were purchased from Sinopharm Chemical Reagent Co.,Ltd.Acetone (99.5%),n–dodecane and n–decane (internal standard) were purchased from Aladdin.

2.2.Catalyst preparation

The support precursor was prepared as following.Na2CO3solution (0.26 mol L-1,35 mL) and AC (0.77 g) were added into a 500 mL three–necked flask under stirring at 25°C for 20 min.Then,35 mL solution that contained Mg(NO3)2·6H2O(5.34 g) and Al(NO3)3·9H2O (2.87 g) was added into the above flask with a flow rate of 3 mL min-1under vigorous stirring condition.At the same time,NaOH solution(1 mol L-1,70 mL) was also added to control the pH of reaction liquid to 9.5–10.5.After co-precipitation reaction,the suspension was put into a sealed reactor and heated at 95°C for 10 h.The produced black precipitate was separated and dried at 105°C for 4 h.

Co–Ni bimetal supported on Mg–Al oxide catalysts were prepared as following.Co(NO3)2·6H2O (0.42 g) and Ni(NO3)2·6H2O (0.61 g) were dissolved in 10 mL of water.The as–prepared support precursor(2.80 g)was added into the above water solution and treated by ultrasonic technology for 60 min.After drying,the solid powder was calcined in air at 500°C for 8 h in a muffle furnace(10°C min-1)to ensure the transformation of hydrotalcite into oxide and complete removal of active carbon.Then,the precursors were reduced under hydrogen atmosphere at the designed temperature for 6 h with a ramping rate of 4°C min-1.The resultant catalyst was named as Co–Ni/Mg–Al–C–T,where T represented the reduction temperature.For comparison,Mg–Al hydrotalcite was also used to support Co and Ni.

2.3.Catalyst characterization

X–ray diffraction(XRD)measurement was carried out on a D/max2500 18 KW Rotating anode X–Ray Diffractometer with monochromatic Cu Kα radiation (λ=1.5418 ?) radiation at voltage and current of 40 kV and 250 mA.The 2θ was scanned over the range of 5–90°at a rate of 10°min-1.The specific surface area was measured by a Quantachrome’s NOVA–2100e Surface Area instrument by physisorption of nitrogen at 77 K.All samples were dehydrated at 300°C using vaccum degassing for 12 h before experiments.The morphologies of catalysts were determined by transmission electron microscopy (TEM) on JEOL JEM–2100 transmission electron microscope with a lattice resolution of 0.19 nm and an accelerating voltage of 200 kV.Elemental mapping was recorded by Oxford detector.The surface composition and surface electronic state were analyzed by X–ray Photoelectron Spectroscopy (XPS) using Kratos Axis Ultra DLD instrument at 160 eV pass energy.Al Kα radiation was used to excited photoelectrons.The binding energy value of each element was corrected using C1s=284.6 eV as a reference.The H2temperature-programmed reduction (H2–TPR),ammonia temperature-programmed desorption (NH3–TPD) and CO2temperature-programmed desorption(CO2–TPD)were carried out in Micrometrics Auto Chem II 2920 instrument equipped with a thermal conductivity detector.

2.4.Activity measurement

The catalytic activity tests were carried out in a 100 mL sealed batch reactor.Acetone (1.58 g),n–butanol (3.24 g),ethanol(0.55 g),catalyst(1.00 g)andn–dodecane(12.35 g)as solvent were added into the reactor.Air in the reactor was evacuated by pressurization–depressurization cycles with nitrogen.The reaction system was heated to the designed temperature,then pressurized with nitrogen and stirred at 900 r min-1.After reaction,liquid samples were withdrawn from the reactor and analysed by Agilent 6890/5973N GC–MS and 7890 gas chromatography using a flame ionization detector (FID) with a 30 m AT–5 capillary column.The experiments were repeated at least twice.The mean standard deviation for these experiments was within 3%,and the carbon balance in each experiment was higher than 90%.

3.Results and discussion

3.1.Characterization of Co–Ni supported on Mg–Al oxide

The crystal structures of Co,Ni and Co–Ni supported on Mg–Al oxide catalysts were characterized by XRD.Because the precursors were reduced at 750°C under hydrogen atmosphere,metallic Co or Ni would appear in the catalysts.As shown in Fig.1a,two peaks at 2θ=44.1°and 51.8°were observed,corresponding to the (1 1 1) and (2 0 0) reflections of metallic Co or Ni [22,23],respectively.Since hydrotalcite has transformed into oxide and active carbon has been thoroughly removed after calcination at 500°C,no diffraction peak could be assigned to hydrotalcite and carbon structure in Fig.1a [24,25].The Raman spectra also evidenced that no carbon was remained in the resultant catalyst (Fig.S1).In addition,some other peaks at 2θ=36.6°,43.0°and 62.8°were attributed to(3 1 1)reflections of Co or Ni oxide species,(2 0 0)and(2 2 0)reflections of MgO[26],respectively.These Co or Ni oxide species might be caused by the formation of Co(Ni)Al2O4spinel–like structure.

Fig.1b presents the effects of reduction temperature on crystal structure of Co–Ni bimetal supported on Mg–Al oxide catalyst.Due to almost the same atomic radius and crystalline structure,metallic Co and Ni preferred to form alloy,especially at such high reduction temperature [27,28],which was further confirmed by XPS and element mapping results.The diffraction peaks at 2θ=44.1°,51.8°and 75.7°were attributed to metallic Co/Ni or Co–Ni alloy.These peaks became sharp with the increase of reduction temperature,which was caused by the aggregation of metal particles at high temperature.Since the spinel–like structure was easy to form at high temperature,some other peaks at 2θ=31.1°,36.7,59.4°and 65.2°were observed in the XRD pattern of Co–Ni/Mg–Al–C–900,well matching with the characteristic(2 2 0),(3 1 1),(5 1 1) and (4 4 0) reflections of Co(Ni)Al2O4[29,30].

XPS characterization was employed to insight the chemical states of Co and Ni in the catalyst surface.Due to the formation of spinel–like structure,Co and Ni oxide species in these Mg–Al supported catalysts were hard to be reduced to metallic states,leading to the dominant peaks to metal oxides in the XP spectra in this study.As shown in Fig.2a,two small peaks at the binding energies of 778.3 eV and 794.5 eV in the XP spectra of Co/Mg–Al–C–750 were ascribed to the metallic Co0[31,32].After introducing Ni,the peaks to metallic Co0were increased and the corresponded binding energies were negatively shifted by 0.5 eV in comparison with that in Co/Mg–Al–C–750,suggesting the enhanced reduction of Co oxides in the presence of Ni,which was resulted from the spillover hydrogen on metallic Ni and further confirmed by H2–TPR.Ni/Mg–Al–C–750 presented two peaks at the binding energies of 853.1 eV and 871.7 eV (Fig.2b),assigning to metallic Ni0[31,33],but these peaks were weakened and positively shifted by 0.5 eV in Co–Ni/Mg–Al–C–750.These XPS data demonstrated that some electron was transferred from metallic Ni0to Co0,presenting a strong interaction between Ni0and Co0[34–36],which well evidenced the formation of Co–Ni alloy.

Fig.1.XRD patterns of (a) Co and Ni and (b) Co–Ni bimetal supported catalysts.

Fig.2.XP spectra of (a) Co 2p and (b) Ni 2p levels of Co,Ni and Co–Ni supported on Mg–Al oxide.

Fig.3.H2–TPR profiles of Co,Ni and Co–Ni supported on Mg–Al oxide.

H2–TPR was carried out to investigate the differences on the reduction of Co and Ni oxides.As shown in Fig.3,Co/Mg–Al–C precursor presented three peaks at 291°C,340°C and 902°C,corresponding to the reduction of Co3+to Co2+,Co2+to Co0and Co2+in spinel CoAl2O4to Co0[37,38],respectively.In comparison with the reduction temperatures of Co oxides to metallic Co in Co–base supported catalysts in previous investigations (310°C and 450°C) [39,40],it was obvious that Co oxides were easier to be reduced in this study,which might be caused by its high dispersion in Mg–Al hydrotalcite.For Ni/Mg–Al–C precursor,two reduction peaks at 378°C and 771°C were assigned to the reduction of Ni2+to Ni0and Ni2+in spinel NiAl2O4to Ni0[29,41],respectively.However,the reduction temperatures were changed to 312°C and 816°C for Co–Ni bimetal supported catalyst precursor,suggesting a strong interaction between Co and Ni [42].

The acidity and basicity of Co,Ni and Co–Ni supported catalysts were characterized by NH3–TPD and CO2–TPD,as shown in Fig.4.The NH3–TPD profiles presented a weak adsorption peaks at the temperature range of 300–450°C,especially for Ni contained catalysts.On the other hand,due to the high content of Mg oxide in the support,a broad peak was observed at the temperature range of 170–400°C in the CO2–TPD profiles.These suggested that acidic and basic sites coexisted in all resultant catalysts.The relative area of these peaks indicated that the acidic sites were decreased with the following order:Co supported catalysts >Co–Ni supported catalysts >Ni supported catalysts,which confirmed the interaction between Co and Ni again.In addition,the introduction of active carbon as a hard template could enhance the acid sites but lower the basic sites,which might be resulted from the change of surface composition.

The element composition and dispersion were analyzed by TEM and element mapping.As shown in Fig.5,the TEM image of Co–Ni/Mg–Al–C–750 presented some white particles with a size range of 5–10 nm,which was much smaller than that in Co/Mg–Al–C–750 and Ni/Mg–Al–C–750.The element mapping displayed that these particles were composed by metallic Co and Ni,where Co and Ni were uniformly mixed.The line scanning result also showed that these particles contained very little oxygen and the dominant components were metallic Co and Ni.The EDS results showed that Co and Ni contents in Co–Ni/Mg–Al–C catalyst were 5.02% and 7.49%,respectively,which were almost the same as the designed contents.

Fig.4.(a) NH3–TPD and (b) CO2–TPD profiles of Co,Ni and Co–Ni catalysts.

Fig.5.TEM image and element dispersion in Co–Ni/Mg–Al–C catalysts.

The effects of adding active carbon as a hard–template agent on the surface area,pore volume and average pore size of the resultant catalysts are summarized in Table 1.When Mg–Alhydrotalcite was used as a support,the corresponded catalyst presented mesoporous structure and the average pore size was about 17 nm.After adding active carbon as a hard template,the surface area of the resultant catalyst was almost unchanged,but both the pore volume and pore size were enlarged obviously.For example,the pore volume and pore size of Co/Mg–Al–C–750 and Ni/Mg–Al–C–750 were increased to 0.58 mL g-1and 21.7 nm,0.68 mL g-1and 19.7 nm,respectively.These indicated that the active carbon was a good hard–template agent for enlarging the pore volume and pore size,which was well consistent with previous investigations[21,43].In comparison with the monometal supported catalyst,Co–Ni bimetal supported on Mg–Al oxide had lower surface area.Moreover,with the increase of reduction temperature,the surface area was gradually decreased while the pore size was progressively increased,which was mainly caused by the collapse of some small pores at high temperature.

Table 1Surface area,pore volume and pore size of the samples.

3.2.Aldol condensation reaction on Co and Ni supported on Mg–Al oxide

To reveal the synergistic effect between Co and Ni,the catalytic activity of Co and Ni supported on Mg–Al oxide was firstly tested in the aldol condensation of acetone withn–butanol.This reaction includes dehydrogenation of alcohol to aldehyde and aldehyde ketone condensation,yielding α,β–unsaturated ketone,which is further hydrogenated to saturated ketone or alcohol.As shown in Fig.6,C7(2–heptanone,2–heptanol,3–hepten–2–one),C8(2–ethylhexanol),C11(6–undecanone and 6–undecanol)were the target products,which were easy to be deoxygenated into hydrocarbons as alternative fuel.Butylbutyrate produced via Tischenko pathway and some higher–molecular–weight(MW)products produced via multi–condensation were also detected [15,44],but these were undesired.After reaction at 200°C,acetone conversion and C7–C11products selectivity on Co/Mg–Al–750 were 42% and 85%,respectively,but butylbutyrate selectivity was high to 13%.Due to the higher dehydrogenation activity of Ni than Co,butanol was easier to be converted into butaldehyde on Ni/Mg–Al–750 catalyst,providing more aldehyde for condensation reaction and then promoting the ketone conversion,which caused to that acetone conversion and target products selectivity were increased to 58%and 88%,respectively.Moreover,C11products selectivity was increased to 31%,but the undesired product butyl–butyrate selectivity was decreased to 2%.These suggested that the high dehydrogenation activity of Ni could enhance the aldol condensation of acetone with butanol,yielding C7–C11compounds.

Fig.6.Aldol condensation of acetone with n–butanol on Co and Ni supported catalysts.Reaction conditions:4.05 g n–butanol,1.58 g acetone,1.0 g catalyst,14.35 g dodecane,200 °C,1.7 MPa N2 pressure and 20 h.

Enlarging the pore size of the catalyst had a great effect on acetone conversion and products distribution.As shown in Fig.6,in comparison with the aldol condensation of acetone withn–butanol on Co/Mg–Al–750,after reaction at 200°C,although the total selectivity of C7–C11products on Co/Mg–Al–C–750 was almost unchanged,acetone conversion was increased from 43% to 69% while butylbutyrate selectivity was decreased to 3%.Acetone conversion on Ni/Mg–Al–C–750 was almost the same as that on Co/Mg–Al–C–750,but C11products selectivity was increased to 28% and butylbutyrate selectivity was decreased to <1%.These indicated that enlarging pore diameter was beneficial to enhance the activity,which was mainly caused by the decreased mass transfer resistance of reactants and long carbon–chain products[45,46].However,although Ni/Mg–Al–C–750 exhibited good activity in aldol condensation of acetone withn–butanol,the high dehydrogenation activity of Ni and large pore were inevitable to enhance the deep condensation reaction,yielding higher MW products,which required to be further modified.

The effects of Co content on the conversion and product distribution in the aldol condensation of acetone withn–butanol are shown in Fig.7.With the increase of Co content,acetone conversion was increased firstly and then decreased while C7products selectivity was gradually increased.The decrease of the 6–undecanol selectivity and the increase of 2–heptanone selectivity indicated that the dehydrogenation activity of metallic Ni in the presence of Co was decreased.When the Co content was increased to 5%,Co–Ni/Mg–Al–C–750 exhibited the optimal catalytic property,giving 85%acetone conversion,94% selectivity of the target C7–C11products and only 1% selectivity of undesired butylbutyrate.These above results were mainly caused by the synergistic effect between metallic Co and Ni.Metallic Ni had high reactivity towardsn–butanol dehydrogenation while metallic Co had strong affinity to C=O species[47].The formation of Co–Ni alloy allowed the uniform dispersion of Co and Ni and resulted in an electron transfer between Co and Ni.The electron–defect of Ni enhanced the dehydrogenation activity[48],leading to the high conversion and C7–C11products selectivity.However,the addition of excess Co would cover some active Ni sites and then lower the dehydrogenation activity,leading to a decline of acetone conversion.Therefore,forming Co–Ni alloy was a good strategy to enhance the aldol condensation of acetone withn–butanol to produce C7–C11fuel precursors.

Fig.7.Aldol condensation of acetone with n–butanol on Co–Ni/Mg–Al–C–750 with different Co content at 200 °C for 20 h.Reaction conditions:4.05 g n–butanol,1.58 g acetone,1.00 g catalyst,14.35 g dodecane and 1.70 MPa N2.

Given the high activity,Co–Ni/Mg–Al–C–750 was applied into the aldol condensation of ABE mixtures (the molar ratio of acetone :n–butanol :ethanol=2.3:3.7:1).Fig.8a shows the effect of reduction temperature of catalyst on the activity.Undesired product butylbutyrate was not detected,suggesting that the introduction of ethanol inhibited the tischenko pathway.With the raise of reduction temperature,both acetone conversion and the total selectivity of C5–C11products were increased firstly and then decreased,which might be related to the increased metal content and metal particle size.As confirmed by H2–TPR,when the reduction temperature lowered to 600°C,the Co and Ni ions in spinel structure were not reduced to metal state,especially for Co.In contrast,the metallic Co–Ni particles preferred to aggregate together at high temperature,resulting in larger particles and less active sites for aldol condensation reaction.Therefore,750°C was the optimum reduction temperature for obtaining Co–Ni supported on Mg–Al oxide catalyst with the highest activity.Fig.8b and c show the effects of reaction parameters such as nitrogen pressure and reaction temperature on the conversion and products distribution.Since the dehydrogenation of alcohol is a volume–increased and endothermic reaction,acetone conversion was decreased at high pressure but both C11products selectivity and higher MW products selectivity were increased at high temperature.Under the optimized reaction conditions (1.7 MPa nitrogen pressure and 200°C),acetone conversion and C5–C11products selectivity reached to 76% and 90%,respectively.These suggested that Co–Ni/Mg–Al–C–750 was a potential catalyst for the aldol condensation of tailored fermentation products into C5–C11fuel precursors.

Fig.8.Aldol condensation of ABE mixtures over(a)Co–Ni bimetal catalysts reduced at different temperatures under 200 °C and 1.7 MPa N2 pressure,(b)Co–Ni/Mg–Al–C–750 at 200 °C under different nitrogen pressures and (c) Co–Ni/Mg–Al–C–750 at different reaction temperatures under 1.7 MPa N2 pressure.

Fig.9.Reaction routes in the aldol condensation of the ABE mixtures.Highlighted field:substrates (pink),mechanistic considerations (red,take acetone and ethanol as examples),target products (blue),by–products (box).

Based on the detected products and previous investigations[7,15,49,50],the probable reaction routes and mechanism for the aldol condensation of the ABE mixtures on Co–Ni bimetal supported on Mg–Al oxide were deduced and shown in Fig.9.Toste et al.[51]had investigated the kinetics of ABE condensation reaction and concluded that the dehydrogenation was the rate-determining step.In this study,acetone was dehydrogenated α–H on the basic sites of Mg–Al oxide to form oxygen radical anion species in the first step and then converted into enolate species.Since the alkoxide was easy to be formed over basic oxides,the abstraction of hydrogen from alcohols was thought to proceed through alkoxide that generated by breaking the O–H bond of alcohols on basic sites at the same time,yielding carbanion intermediate [52].Then,the produced enolate species acted as a nucleophile to attack the produced carbanion intermediate,forming a new C–C bond.The dehydrogenated α–H combined with oxygen radical anion species to form β–hydroxy ketone,but this was not stable due to the active α–H and immediately converted into enolate structure.The β–hydroxy was further adsorbed on acid sites and removed via an E1 mechanism,yielding α,β–unsaturated ketones with a stable conjugated double bond,which was further hydrogenated into saturated ketones and converted into alcohols by the hydrogen from metal sites.If the produced ketones contained α–H,it would further react with aldehydes derived from alcohols to produce long carbon–chain compounds such as 2–heptanone and 6–undecanone.During the aldol condensation reaction,some undesired higher MW products were also produced via self–condensation reaction or guerbet reaction.

Fig.10.The stability study of Co–Ni/Mg–Al–C–750 in aldol condensation of ABE mixtures at 200 °C for 20 h.Reaction conditions:4.05 g n–butanol,1.58 g acetone,1.00 g catalyst,14.35 g dodecane and 1.70 MPa N2.

The stability of Co–Ni/Mg–Al–C–750 in aldol condensation of ABE mixtures was also studied,as shown in Fig.10.With the increase of reaction number,both the conversion and product distribution were greatly changed.After 3 cycles,acetone conversion was decreased to 32%,and the yields of all products were decreased,except for 2–heptanone,which suggested a deactivation.The spent catalyst was collected and characterized by Raman,XRD and XPS.The Raman result(Fig.S1) showed that no deposit carbon was formed on catalyst surface,but the XRD (Fig.S2) and XPS (Fig.S3)results presented that the metallic Co and Ni were oxidized and their contents were markedly decreased.This suggested that the oxidation of metal Co and Ni was the main reason for catalyst deactivation.To further confirm this conclusion,the spent catalyst was regenerated via simple reduction in the presence of hydrogen at 750°C.As shown in Fig.10,acetone conversion was raised to 62%.Hence,this deactivated catalyst could be easily regenerated after a simple treatment.

4.Conclusions

Co–Ni supported on Mg–Al oxide catalysts,possessing metal,acidity and basicity,were prepared for chemocatalytic upgrading of tailored fermentation products to energy intensive fuels.Active carbon was added as a hard template to enlarge their pore size.During the aldol condensation of acetone withn–butanol,C7(2–heptanone,2–heptanol,3–hepten–2–one),C8(2–ethylhexanol),C11(6–undecanone and 6–undecanol) were the desired products.Due to the high dehydrogenation activity of Ni,undesired product butyl–butyrate selectivity on Ni/Mg–Al–750 was decreased to 2%.After enlarging the pore size,mass transfer resistances of reactants and products were decreased,leading to that C11products selectivity was increased to 28%while butyl–butyrate selectivity was decreased to <1%.Because of the formation of Co–Ni alloy and electron transfer from metallic Ni0to Co0,acetone conversion and target C7–C11products selectivity on Co–Ni bimetal supported on Mg–Al oxide were increased to 85% and 94%,respectively.This bimetal catalyst also exhibited high activity in the transformation of tailored fermentation products into C5–C11fuel precursors.Acetone conversion and the total selectivity of C5–C11desired products reached up to 76%and 90%,respectively.Although the catalyst showed a deactivation during the stability study,its activity could be easily recovered after a simple hydrogen reduction.

Conflict of interest

The authors declared that they have no conflicts of interest to our manuscript titled:“Preparation of bimetal Co–Ni supported on Mg–Al oxide for chemocatalytic upgrading of tailored fermentation products to energy intensive fuels”.We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No.21776236 and 21676225),Scientific Research Fund of Hunan Provincial Education Department (19A478),Natural Science Foundation of Hunan Province (2018JJ2384),Engineering Research Centre of Chemical Process Simulation and Optimization of Ministry of Education,and Students’ innovation and entrepreneurship training program of Hunan province.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2020.10.011.