Tianhan Zhu ,Hua Song ,*,Xueya Dai,Hualin Song

1 Provincial Key Laboratory of Oil&Gas Chemical Technology,College of Chemistry&Chemical Engineering,Northeast Petroleum University,Daqing 163318,China

2 Key Laboratory of Cancer Prevention and Treatment of Heilongjiang Province,Mudanjiang Medical University,Mudanjiang 157011,China

1.Introduction

With the development of global economy,the demand of energy has been increasing rapidly,which leads to the decline of fossil fuel sources and environmental issues.To reduce our reliance on fossil fuels,more and more attention was paid to biomass due to its potentialas feedstock for the production ofliquid fuels[1,2].As an efficient method of biomass conversion,pyrolysis has been industrially realized[3,4].Unfortunately,pyrolysis oil contains about 35 wt%-50 wt%of oxygen[5].The high oxygen content of pyrolysis oil leads to higher viscosity,poor stability,and lower caloric value[6].As a result of those negative impacts,the pyrolysis oil requires upgrading to form eligible liquid oil.To improve quality and stability of pyrolysis oil,the oxygen must be removed.Hydrodeoxygenation(HDO)process is thought to be an available solution to remove oxygen from pyrolysis oil,in which oxygen can be removed as water and carbon oxides.Conventional HDO catalysts,including supported metal sulfides[7](such as Co and Mo)and noble metal catalysts(such as Pd[8]and Pt[9]),exhibit relatively high HDO activity for oxygen removal from pyrolysis oil.However,for supported metal sulfides,an additional S-containing agent is needed to inhibit deactivation of sulfide catalyst caused by the loss of S,thereby,some byproducts of harmful to our environment would be generated inevitably[10,11].Noble metal catalysts show higher activity than metal sulfide catalysts,but their applications on scale are limited due to their high cost.In recent years,nickel phosphide is reported to be suitable catalyst for HDO process and numbers of studies on nickel phosphide have been reported.The results showed that nickel phosphide is a promising catalyst for HDO[12,13].

It is known that the choice of supports is very important for improving catalyst performance[14].The mesoporous silica SBA-15 has some advantages,its excellent structural properties,such as uniform pore size and high surface area,contribute to their applications as support[15-17].Yang et al.[18]investigated the influence of the Ni/P ratio and Ni loading on the performance of NixPy/SBA-15 catalysts for the HDO of methyl oleate.They found that the formation of different active phases(Ni2P,Ni12P5or Ni3P)depending on the P/Ni molar ratio and the Ni2P phase showed the highest activity for this process.Tan[19]proposed that SBA-15 has nearly no catalytic activity for the hydroxyalkylation of phenol with formaldehyde to bisphenol F,however,the mesoporous M(Al,Zr,Al-Zr)-SBA-15 catalysts showed high activity for the hydroxyalkylation reaction.Among these tested metals,the Al-doped SBA-15 catalyst was in favor of the formation of 4,4′-isomer,and the selectivity of bisphenol F reached 93%.Showing that the metal Al doped into SBA-15 plays an important role in selective hydroxyalkylation.As far as we know,little studies on HDO performance over Ni2P supported on Al incorporated SBA-15 support have been reported.Therefore,it is of interest to gain insight into the effect of Al on HDO performance over a Ni2P/SBA-15 catalyst.

In this work,the supported nickel phosphide on mesoporous SBA-15 and Al-SBA-15 catalysts were synthesized according to a previously described method[13]from ammonium hypophosphite and nickel chloride at a lower reduction temperature of 400°C.To obtain information of the effect of Al on the catalytic structure and nature of the active sites of Ni2P/SBA-15 catalyst,the prepared samples were characterized by several techniques,such as XRD,nitrogen adsorption,CO uptake,H2-TPR,XPS,NH3-TPD,and TEM.Finally,the HDO activities of BF over corresponding catalysts were compared.

2.Experimental

2.1.Materials

The mesoporous SBA-15 and Al-SBA-15 were obtained from NKU and were employed as support materials.The chemicals NiCl2·6H2O(Aladdin,98%)and NH4H2PO2(Macklin,97%),were used as received.The chemicals used as reactants were benzofuran(Maya Reagent,99%)and decahydronaphthalene(Ourchem,99.5%).

2.2.Preparation of catalysts

The supported Ni2P catalyst precursor was prepared by impregnating an ammonium hypophosphite(NH4H2PO2)and nickel chloride(NiCl2·6H2O)solution with the mesoporous SBA-15,following procedures previously described by our group[20,21].After the water was evaporated,the impregnated solid was dried at90°C overnight.The precursor was then pressed in disks,crushed and sieved to 16-20 mesh.For the reduction,precursor was placed in a fixed-bed reactor by heating from room temperature to 400 °C,at a rate of 3 °C·min-1in a flow of H2(200 ml·min-1),held for 2 h,then cooled to 100 °C in a continuous H2flow and held for 1 h under flowing air(20 ml·min-1).Additionally,the support SBA-15 doped with Al was prepared,and in the same way,the obtained solid catalysts with Ni theoretical loading of 10 wt%and an initial Ni/P molar ratio of 1/2 were designed as Ni2P/SBA-15 and Ni2P/Al-SBA-15,respectively.

2.3.Characterization methods

The X-ray diffraction(XRD)analysis was carried out on a D/max-2200PC-X-ray diffractometer using Cukα radiation under the setting conditions of 40 kV,30 mA,and scan range from 10°to 80°at a rate of 10(°)·min-1.

The typical physico-chemical properties of supports and catalysts were analyzed by BET method using Micromeritics adsorption equipment of NOVA2000e.All the samples were out gassed at 200°C until the vacuum pressure was 800 Pa.The adsorption isotherms for nitrogen were measured at-196°C.

The COadsorption capacity of the catalysts was measured by pulsing calibrated volumes of CO into a He carrier.CO uptake was calculated by measuring the decrease in the peak areas caused by adsorption in comparison with the area of a calibrated volume.

The X-ray photoelectron spectroscopy(XPS)spectra were acquired using an ESCALAB MKII spectrometer under vacuum.XPS measurements have been performed for Mg radiation(E=1253.6 eV)and equipped with a hemi-sphericalanalyzer operating at a fixed pass energy of 40 eV.The recorded photoelectron binding energies were referenced against the C 1s contamination line at 284.8 eV.

The reducibility of precursors was characterized by the H2temperature-programmed reduction(H2-TPR)using a quartz Utube reactor(inner diameter of 6 mm),in which 0.05 g of catalyst was loaded in the thermostatic zone.Reduction was conducted at a heating rate of 10 °C·min-1in a 10 vol%H2/Ar flow(30 ml·min-1).The TPR spectrum was determined using a thermal conductivity detector(TCD)to monitor hydrogen consumption.

The temperature-programmed desorption of ammonia(NH3-TPD)was adopted to measure the catalystacidity.100 mg sample was cleaned with heliumand adsorption of ammonia at40°C until the TCD signalwas stable.NH3-TPD was performed between 40 °C and 800 °C at a beating rate of 10 °C·min-1using a TCD detect the desorbed NH3.

The acid type of catalysts was analyzed by Fourier transform infrared(FT-IR)measurement of adsorbed pyridine using the TENSOR 27 spectrometer with a 4 cm-1resolution.Samples,in the form of selfsup ported wafers,were pretreated in vacuum for 1 h at 340°C and exposed to pyridine at 50 °C.After desorption at 150 and 340 °C,the spectra were recorded.

Transmission electron microscope(TEM)examinations were performed using the JEM-1010 instrument supplied by JEOL.The samples were dispersed in ethanol and placed on a carbon grid before TEM examinations.

2.4.Catalytic activities

The HDO of BF over prepared catalysts was carried out in a flowing high-pressure fixed-bed stainless steel catalytic reactor(8 mm in diameter,and 400 mm in length),using a feed consisting of a decal in solution of BF(2 wt%).The conditions of the HDO reaction were 300°C,3.0 MPa,WHSV=4 h-1and hydrogen/oil volume ratio of 500.The activities of each catalyst were measured at different time.The feed and reaction product were analyzed by FID gas chromatography with a GC-14C-60 column.

The BF conversion was calculated from the ratio of converted BF to initial BF.The selectivity to the different reaction products was calculated based on the moles of each chemical as follows:ethyl benzene(EB),ethylcyclohexane(ECH),methyl benzene(MB),benzene(B),methyl cyclohexane(MCH),2,3-dihydrobenzofuran(2,3-DHBF),2-ethylphenol(2-EtPh),and phenol(Ph).The conversion of BF(yBF)and selectivity(si)were defined as follows:

Turnover frequency(TOF)values of the samples containing nickel phosphide were calculated using Eq.(3):[22]

where F is the molar rate of BF fed into the reactor(μmol·s-1),W is the mass of catalyst(g),y is the conversion of BF(%),and M is the mole of sites loaded which is decided by the CO uptake.

3.Results and Discussion

3.1.XRD

Fig.1 shows XRD patterns for the Ni2P/SBA-15 and Ni2P/Al-SBA-15 catalysts.All the patterns show a broad peak at around 2θ =22°,this is due to the amorphous nature of mesoporous structure.For all catalysts,the peaks at 2θ =40.6°,44.5°,47.1°,54.1°,and 54.8°(PDF:03-0953)can be ascribed to Ni2P phase.There is no other peaks related to Niand P to be observed,indicating that Ni2P was the main active phase for each catalyst.As compared to the Ni2P/SBA-15,the peaks of Ni2P phase for the Ni2P/Al-SBA-15 were wider with low intensity,indicating that the Ni2P crystallites were small and highly dispersed on the Al-SBA-15 support.The average size of the Ni2P crystallites(column 5 of Table 1)estimated by the Scherrer equation was 13.1 nm for Ni2P/Al-SBA-15,smaller than that for Ni2P/SBA-15(18.4 nm).This indicates that the incorporation of Al into the SBA-15 support can promote the formation of smaller,highly dispersed Ni2P particles on the catalyst.

Fig.1.XRD patterns for Ni2P/SBA-15 and Ni2P/Al-SBA-15.

3.2.N2 adsorption-desorption

N2adsorption-desorption isotherms of supports and catalysts were displayed in Fig.2 and textual properties were listed in Table 1.As shown in Fig.2,all of the supports and related catalysts exhibited type IV isotherms,showing the presence of some mesopores.The surface area of SBA-15 was 619 m2·g-1,incorporation of Al into SBA-15 led to an increase in surface area(805 m2·g-1).The possible reason may be that the incorporation of Al resulted in blockage of large pores and thus more mesopores were generated.The surface areas and pore volumes of SBA-15 and Al-SBA-15 decreased with increasing pore diameter upon loading the nickel phosphide phase.This is possibly because a number of small pores of supports are blocked by the nickel phosphide.

3.3.CO uptakes

CO uptakes were measured by pulsing calibrated volumes of CO into a He carrier.Itisknown that COwasabsorbed at Nisites and the amount of CO absorbed on P sites can be ignored[23].The CO uptakes of the as pre pared samples are listed in column 6 of Table 1.It can be seen that the CO uptakes of Ni2P/Al-SBA-15 were 45 μmol·g-1,higher than that of Ni2P/SBA-15(38 μmol·g-1).This is possibly because the Al-SBA-15 support possesses the highest surface area,which is beneficial to dispersion of Ni2P particles.In addition,the incorporation of Al would inhibit the free lateral growth of the Ni2P crystallite.Such inhibition leads to smaller crystallites and consequently higher dispersion.In other words,the Al interaction contributes to the better dispersion.This will be discussed later in XPS analysis.

3.4.H2-TPR

H2-TPR pro files of supported catalyst precursors are shown in Fig.3.According to our previous study[20],the hydrogen consumption peaks at around 260°C are attributed to the reduction of Ni2+to Ni and the peak at around 320°C is ascribed to the reduction of phosphorus species.Berhault et al.[24]reported that the reduction of Ni2P from a precursor prepared by(NH4)2HPO4(or NH4H2PO4)and Ni(NO3)2does notoccur untilthe temperature reaches 550°C.The high reduction temperature is attributed to the highly thermodynamic stability of P--O bond.Compared to the results obtained by Barnaul et al.,the reduction temperature is decreased remarkably(at least 230°C),this is because the phosphorus species with low valence electron can be easily reduced at much lower temperatures.

3.5.X-ray photoelectron spectroscopy

The XPS technique of samples was performed to gain further insight into the states of metals and surface composition of samples.Fig.4 shows the XPS spectra for Ni 2p and P 2p lines of the catalysts.As shown in Fig.4(a),for all catalysts,the banding energy centered at 852.3-852.5 eV can be assigned to Niδ+(0 ＜ δ ＜ 2)in Ni2P and the banding energy centered at 856.2-856.6 eV,corresponding to the possible interaction of Ni2+ions with phosphate ions,as a consequence of a super ficial passivation,along with the broad shake-up peak at approximately 6.0 eV higher than that of the Ni2+species[25,26].In Fig.4(b),the peaks centered at 129.1-129.2 eV can be assigned to Pδ-(0＜δ＜1)species in the Ni2P phase[27]and the peak at 134.6-134.7 eV can be assigned to P5+due to the super ficial oxidation of the Ni2P particles[28,29].For Ni2P/Al-SBA-15 catalyst,it is interesting that a new band at 135.2 eV was observed,which can be assigned to P in AlPO4[30].This shows that the Al existed in support has strong interaction with P species[28,31].As shown in Table 2,for Ni2P/Al-SBA-15 sample,the binding energy of Ni2+and Pδ-shifts to a slightly lower value with the incorporation of Al,indicating that there is an interaction between the Ni2P particles and the Al-SBA-15 support.To be specific,the electrons in Al3+species can be easily transferred and injected into the Ni 3d species through band under reduction.This is because the Al3+is an electron donor and Ni2P is an excel lentelectric conductor.In addition,the increase in electron density will give rise to minor stabilization of the Ni 3d levels and a small Ni→P charge transfer[32].This specialeffectis beneficial to the reduction PO43-into Ni2P.Hence,the incorporation of Al into bulk Ni2P can promote the formation of active Ni2P phase and more exposed Ni sites on the surface.

3.6.Superficial atomic ratio

Spectral parameters of catalysts and atomic ratios obtained from XPS and EDS analysis were listed in Table 2.The initial Ni/P molar ratio was 1/2.However,the data from XPS analysis showed that all the samples exhibited lower Ni/P ratio than 1/2,which may be due to the enrichment of phosphorous on the surface of the catalysts.It is noticed that the Ni/P atomic ratio of Ni2P/Al-SBA-15 catalyst is slightly higher than that of Ni2P/SBA-15,indicating that the incorporation Al into SBA-15 contributed to suppress the enrichment of phosphorous,and therefore increase the amount of exposed Ni sites on the surface of catalyst,which conformed by the CO uptake results.The Ni/Si of Ni2P/Al-SBA-15 is higher than that of Ni2P/SBA-15,which also confirmed the results.Obviously,the Ni2P/Al-SBA-15 showed more Ni sites than Ni2P/SBA-15,both Ni/P and Ni/Si ratios were higher than that for Ni2P/SBA-15,which agreed with the results obtained by XPS analysis.

Table 1Properties of supports and catalysts

Fig.2.N2 adsorption capacity of samples(a)supports;(b)supported catalysts.

Fig.3.H2-TPR pro files of supported catalyst precursors.

3.7.Acidity of catalysts

NH3-TPD is used to test the acidity of the catalysts and results are shown in Fig.5.The Ni2P/SBA-15 catalyst showed two peaks centered atabout137 °C and 230 °C with a shoulder ata hig hertemperature,corresponding to the weak acid and medium strong acid sites,respectively[33].Compared with Ni2P/SBA-15,the peaks of the Ni2P/Al-SBA-15 catalyst shifted towards higher temperature,which centered at 160 and 293°C.It is known that the peak temperature represents the acid strength.Obviously,the Ni2P/Al-SBA-15 catalyst showed higher weak acid strength than those of Ni2P/SBA-15 catalyst,which showed that there exists an interaction among the Ni,P and Al[34].This is possibly because the incorporation of Al3+sites to the SBA-15 framework may activate the hydroxyl groups in the framework and generate acid sites.The pyridine FT-IR analysis is also performed at 150°C to investigate the acid types of catalysts and results are shown in Table 2.The results showed that Ni2P/Al-SBA-15 possessed more Lewis and Br?nsted acid sites than Ni2P/SBA-15,which agrees with the NH3-TPD analysis.

3.8.TEM

Fig.6 showed the morphologies and particle size distribution of Ni2P/SBA-15 and Ni2P/Al-SBA-15 catalysts.As can be seen from Fig.6(a),for Ni2P/SBA-15,it is obvious that the Ni2P particles were aggregated and formed large Ni2P particles with the average particle size of about 16.3 nm.For Ni2P/Al-SBA-15(Fig.6(b)),the distribution of Ni2P particles was much uniform with the average particle size of about 10.6 nm.Combined with the XRD and CO uptake results,it could be concluded thatthe incorporation ofAlcontributesto the highly dispersion of Ni2P particles and the formation of small Ni2P particles on the support.

Fig.4.XPS spectra in the Ni(2p3/2)and P(2p3/2)regions for samples.(a)Ni 2p3/2 core level spectra;(b)P 2p core level spectra.

Table 2 Spectral parameters of catalysts

3.9.Catalytic results and reaction pathways

Fig.7 and Table 1 depict the HDO conversion and product distribution as a function of the reaction time.The BF conversion over both the Ni2P/SBA-15 and Ni2P/Al-SBA-15 catalysts increased and then remained stable with increasing reaction time.However,the BF conversion over Ni2P/Al-SBA-15 was faster than that over Ni2P/SBA-15.The BF conversion over Ni2P/SBA-15 was only 85.0%at the steady reaction conditions.While the BF conversion over Ni2P/Al-SBA-15 reached 93.1%,which is an increase of 8.1%when compared with that found for Ni2P/SBA-15.Above results showed that the incorporation of Al could increase the BF HDO activity of Ni2P/SBA-15.As can be seen from Table 1,the HDO TOF(calculated at BF conversion is less than 50%)of Ni2P/Al-SBA-15(0.0016 s-1)is much higher than that of Ni2P/SBA-15(0.0011 s-1),indicating that the intrinsic activity of sites is increased 45.5%by the incorporation of Al.

It can be seen from Fig.7 that the deoxygenated product ECH was the main product over both the Ni2P/SBA-15 and Ni2P/Al-SBA-15 catalysts.As compared with the Ni2P/SBA-15(64.8%),the selectivity to ECH over Ni2P/Al-SBA-15(88.4%)increased significantly.The total deoxygenated product selectivity for Ni2P/Al-SBA-15 was 90.3%,while for Ni2P/SBA-15 was only 70.9%,showing that the incorporation of Al not only could increase the HDO conversion of Ni2P/SBA-15 but also could promote the desired HDO reaction over Ni2P/SBA-15.The detected deoxygenated product selectivities over Ni2P/SBA-15 catalyst after 7 h on stream decreased in the order:ethylcyclohexane(ECH)＞ethylbenzene(EB)＞methylcyclohexane(MCH)＞methylbenzene(MB)＞benzene(B).While the O-containing intermediate selectivities decreased in the order:2,3-dihydrobenzofuran(2,3-DHBF)＞2-ethylphenol(2-EtPh)＞phenol(Ph).According to our previous study[13]and on the basis of the product distribution,the possible reaction routes that may occur during HDO of BF under the experimental conditions over the Ni2P/SBA-15 catalyst are shown in Fig.8.The selectivities to MB(2.3%),B(1.0%)and Ph(2.6%)are very low,showing that the corresponding reactions are very slow in speed.

Fig.6.TEM images and particle size distribution of(a)Ni2P/SBA-15 and(b)Ni2P/Al-SBA-15.

Fig.7.Conversion and product selectivity of BF HDO over Ni2P/SBA-15 and Ni2P/Al-SBA-15 catalysts.(a)Ni2P/SBA-15;(b)Ni2P/Al-SBA-15.(Reaction conditions:T=300°C,p=3.0 MPa,WHSV=4.0 h-1,and hydrogen/oil volume ratio=500.

Fig.8.Reaction pathways for BF HDO over Ni2P/SBA-15 catalyst.

For the Ni2P/Al-SBA-15 catalyst,all the products obtained over Ni2P/SBA-15 can be detected.However,the selectivities to MB(0.3%),B(0.5%)and Ph(0.5%)are much lower than those obtained over Ni2P/SBA-15 catalyst,showing that the corresponding reactions are almost hardly performed.Therefore,the possible reaction routes that may occur during BF HDO under the experimental conditions over the Ni2P/Al-SBA-15 catalyst could be simplified to Fig.9.

Fig.10 shows the totaldeoxygenated productover the prepared catalysts.The totaldeoxygenated productyield over Ni2P/SBA-15 was only 70.9%at the steady reaction conditions.However,the total deoxygenated product yield over Ni2P/Al-SBA-15 reached 90.3%,which is an increase of 19.4%when compared with that found for Ni2P/SBA-15.The excellent HDO performance over Ni2P/Al-SBA-15 can be attributed to the smaller Ni2P crystallites and better dispersion of the active Ni2P phases,as well as the lower coverage of P when compared with Ni2P/SBA-15.Besides,the acid sites over Ni2P/Al-SBA-15,which is more and stronger that of Ni2P/SBA-15,are bene ficial for BF HDO.This is understandable,since Al induced additional acid sites for the adsorption of the BF,and when the activated hydrogen dissociated by Ni2P spilledover to the acid sites,it interacted and hydrogenated with the adsorbed BF.Silva-Rodrigo et al.[35]reported similar results by studying thiophene HDS over NiMo/TM41 and NiW/TM41 catalysts, finding that milder surface acidity seemed to favor the thiophene conversion.This also agrees with the result reported by Cairon et al.[36].

4.Conclusions

Fig.9.Reaction pathways for BF HDO over Ni2P/Al-SBA-15 catalyst.

Fig.10.The total deoxygenated product yield over Ni2P/SBA-15 and Ni2P/Al-SBA-15 catalysts.(Reaction conditions:T=300°C,p=3.0 MPa,WHSV=4.0 h-1,and hydrogen/oil volume ratio=500.

In the present work,Ni2P/SBA-15 and Al-doped Ni2P/Al-SBA-15 catalysts were synthesized by temperature programmed reduction at a relatively low reduction temperature of 400°C using ammonium hypophosphite and nickel chloride,and the effect of the incorporation of Al on the catalytic properties for BF HDO was investigated.The XRD results showed that,as compared to the Ni2P/SBA-15,the peaks of Ni2P phase for the Ni2P/Al-SBA-15 were wider with low intensity,indicating that the incorporation of Al is bene ficial to the formation of smaller size of Ni2P particles.The TEM images confirmed that the incorporation of Al can suppress aggregation of Ni2P particles and therefore can promote formation of much uniform and small Ni2P particles(TEM,10.6 nm).Moreover,the CO uptakes of Al-doped Ni2P/Al-SBA-15 was much higher than that of Ni2P/SBA-15 by 18.4%,showing that the Al contributes to the increase in the amount of exposed Ni atoms on the surface of catalyst.The XPS analysis showed that the enrichment of P on the surface could be suppressed to some extentby addition of Al and this is another reason for high CO uptake for Ni2P/Al-SBA-15.Meanwhile,NH3-TPD analysis confirmed that the incorporation of Al can modify the acidic nature of catalyst and enhance the acid strength(both weak acid and medium strong acid),which is caused by the interaction among the Ni,P and Al.The Al-doped Ni2P/Al-SBA-15 catalystexhibited higher BF conversion of 93.1%and the total deoxygenated product yield of 90.3%.As compared with the Ni2P/SBA-15(70.9%),the total deoxygenated product yield over Ni2P/Al-SBA-15 increased by 19.4%,showing that the incorporation of Al not only could increase the HDO conversion of Ni2P/SBA-15 but also could promote the desired HDO reaction over Ni2P/SBA-15.In summary,the results suggested that the Ni2P/Al-SBA-15 is a promising catalyst for deep HDO.

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