A series of NiMo-Al2O3 catalysts containing boron and phosphorus Part I. Synthesis

Applied Catalysis A:General 260(2004)

137–151

A series of NiMo/Al 2O 3catalysts containing boron and phosphorus

Part I.Synthesis and characterization

D.Ferdous a ,A.K.Dalai a ,?,J.Adjaye b

a

Catalysis and Chemical Reactor Engineering Laboratories,Department of Chemical Engineering,University of Saskatchewan,

Saskatoon,Canada SK S7N 5C9

b Syncrude Canada Ltd.,Edmonton Research Center,Canada

Received 13March 2003;received in revised form 9October 2003;accepted 9October 2003

Abstract

In this work,a series of nickel–molybdenum,nickel–molybdenum–boron and one nickel–molybdenum–phosphorus catalysts were prepared by incipient wetness impregnation method.Calcination temperature,nickel,molybdenum and boron concentrations in ?-Al 2O 3were varied from 450to 600?C,1.8to 3.1wt.%,10.6to 13.2wt.%and 0.5to 1.7wt.%,respectively.Nickel–molybdenum–phosphorus catalyst was prepared using 2.7wt.%phosphorus.All catalysts were thoroughly characterized.The catalyst containing 10.6wt.%Mo and 2.4wt.%Ni in ?-Al 2O 3gave maximum BET area of 211m 2/g.In these catalysts,the molybdenum oxides were present predominantly as polymolybdate and tetrahedral form.Addition of boron to NiMo/Al 2O 3caused an increase in weak acid centers,whereas phosphorus caused the formation of acid centers with intermediate strength.Also,addition of boron caused the formation of crystalline B 2O 3and MoO 3on the catalyst surface,evident from both XRD and SEM analyses.Whereas addition of phosphorus caused the formation of irregular particle size and agglomeration on the catalyst surface,evident from SEM-EDS analysis.New Lewis and Br?nsted acid sites on the catalyst surface were observed from FTIR analysis because of the addition of boron and phosphorus to NiMo/Al 2O 3.?2003Elsevier B.V .All rights reserved.

Keywords:NiMo/Al 2O 3catalysts;Boron;Phosphorus;TPR;TPD;XRD;NMR;FTIR;HDN;HDS

1.Introduction

Oil sand bitumen and the bitumen derived gas oil contain high levels of nitrogen and sulfur compounds.Thus,the products such as heavy gas oil obtained from the processing of bitumen also contain very high levels of nitrogen.Cat-alytic hydrodenitrogenation (HDN)is the only process used commercially for reducing the level of nitrogen content in those types of feed stocks.Because of high nitrogen content hydrodenitrogenation (HDN)of this heavy gas oil is more dif?cult and less effective with conventional NiMo/Al 2O 3catalyst.

In the past some attempts have been made to modify NiMo/Al 2O 3catalyst using boron to improve hydrodenitri-genation (HDN)of model nitrogen compounds.Borate ion incorporated on alumina forms a system exhibiting acidic

?

Corresponding author.Tel.:+1-306-966-4771;fax:+1-306-966-4777.

E-mail address:dalai@https://www.sodocs.net/doc/118002534.html,ask.ca (A.K.Dalai).

properties.Peil et al.[1]and Wang and Chen [2]showed that incorporation of borate ions increased alumina acid-ity.Moreover,borate ions brought about an increase in the activity of CoMo/Al 2O 3and NiMo/Al 2O 3catalysts in the reaction of HDS [3,4].Lulic [5]investigated the perfor-mance of NiMo/Al 2O 3–B 2O 3catalyst on the HDN activity and concluded that the catalysts containing boron are more active than commercial HDN catalyst.DeCanio and Weiss-man [6]studied the characterization and activity of boron (0.3–1.8wt.%)modi?ed commercial NiMo/Al 2O 3catalyst.At higher boron loadings of over 1.8wt.%boron,poor cat-alytic performance was observed due to the presence of bulk borate phase.Lewandowski and Sarbak [7]studied the effect of boron addition on texture and structure of NiMo/Al 2O 3catalysts.They found that addition of boron did not change the pore volume signi?cantly;however,it increased the pore radius to 20,25,30and 40?in comparison with the unmod-i?ed catalyst.Lewandowski and Sarbak [8]studied the HDS and HDN activities of boron modi?ed NiMo/Al 2O 3catalyst using quinoline,carbazole and coal liquid.The support with

0926-860X/$–see front matter ?2003Elsevier B.V .All rights reserved.doi:10.1016/j.apcata.2003.10.010

138 D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

borate ions increased the acidity of Ni–Mo catalyst;in partic-ular,the amount of acid centers of intermediate strength.The HDN activity of the catalyst containing9.1and11.7wt.% boron was considerably higher than that of NiMo catalyst. They concluded that the HDN of carbazole indeed depends on the number of acid centers present on the catalyst surface. On the other hand,in the case of quinoline,the acid centers also play a certain role;however reaction may proceed on centers of weak strength.They also concluded that a support modi?cation with borate ions led to a decrease in deactiva-tion of the Ni–Mo catalyst in the HDN of coal liquid.

The presence of small amounts of phosphorus in sul?ded NiMo/Al2O3catalyst has been shown to have a positive effect on both their HDN and HDS activities.In addition, these catalysts are less susceptible to coking[9].Various explanations are proposed for this improvement.It has been reported that phosphorus enhances the solubility of the molybdate by the formation of phosphomolybdate com-plexes,which make the catalyst preparation easier and may lead to a different dispersion of the Mo and Co/Ni.Also, addition of phosphorus causes the formation of easily re-ducible and sul?ded forms,such as multilayered molybdate, crystalline Al2(MoO4)3and MoO3on the catalyst surface [9–11].On the other hand,phosphorus causes interactions of metal atoms with alumina support surface in case of sul?ded cobalt and nickel[12].However,Iwamoto and Grimblot[13] reported that phosphorus modi?es the textural characteristic and acidity of the catalysts.From work on the unpromoted MoP/Al2O3catalyst,they have shown that the thiophene HDS activity was not promoted by phosphorus loading, while large amount of phosphorus decreased the catalytic performances,probably by formation of bulk MoO3in ox-ide precursor.Eijsbouts et al.[14]found bene?cial effects of phosphorus in studies of quinoline HDN.They have sug-gested that,phosphorus may play a direct role in C–N bond cleavage.On the other hand,Lewis et al.[11]and Eijsbouts et al.[14]did not?nd any positive effect of phosphorus dur-ing hydrodesulfurization of thiophene.This discrepancy was explained by assuming that the HDN and HDS reactions take place on different catalytic sites and phosphorus has differ-ent in?uence on them.Jian et al.[15]found no effects in the HDN of piperidine and pyridine over NiMo/Al2O3catalyst. But in the presence of phosphorus the HDN activity de-creased as the phosphorus content in the catalyst increased. They have concluded that phosphorus has a negative effect on the C–N bond cleavage in piperidine,as well as on the subsequent hydrogenation of the C5alkene intermediates. In the past,most of the research has been performed by modifying the support with boron.However,little informa-tion is available on the modi?cation of NiMo/Al2O3itself with boron.Although a number of researchers and com-mercial operations use P modi?ed NiMo/Al2O3catalysts,in the open literature,most studies have been conducted using HDN of model compounds and for short run https://www.sodocs.net/doc/118002534.html,r-mation on performance of B and P on the hydroprocessing of actual feed is scarce.The correlation between catalyst properties with the HDN activity is also missing especially

when the feed is gas oils derived from the oil sands.Conse-

quently,in the present work,a series of NiMo/Al2O3cata-

lyst was prepared using different concentrations of B and P,

and thoroughly characterized(Part I).The impact of B and

P on the catalytic properties as well as on HDN and HDS

performances of heavy gas oil derived from oil sands bitu-

men were also investigated(the results of which constitutes

Part II of this work).

2.Experimental

2.1.Catalyst preparation

A series of NiMo/Al2O3was prepared using incipient

wetness coimpregnation method using extruded?-Al2O3

(～1.4mm of diameter)obtained from Sud Chemical In-

dia Ltd.,New Delhi.In this method an ammoniacal so-

lution(28wt.%concentrated NH3)was prepared at a

pH of9.5using required amount of ammonium hep-

tamolybdate[(NH4)6Mo7O24·4H2O]and nickel nitrate [Ni(NO3)2·6H2O].The solution was heated at70?C until it produced a clear solution,and was impregnated with

support(dried at120?C for overnight).The impregnated

support was dried for12h at120?C.The catalyst was then

calcined at different temperatures(450–600?C)for4h.

Catalysts were prepared at different Ni(1.8–3.1wt.%)and

Mo(10.6–13.2wt.%)concentrations.

NiMoB/Al2O3catalysts were prepared by impregnating

appropriate solution of boric acid on to a NiMo/Al2O3cat-

alyst.The pH of the solution was maintained at～7.The

resulting modi?ed catalysts were then dried at120?C for

12h.Finally the catalysts were calcined at500?C for4h.

NiMoP/Al2O3catalyst was prepared using incipient wet-

ness coimpregnation method.In this method,a solution

containing required amount of ammonium heptamolybdate

[(NH4)6Mo7O24·4H2O],nickel nitrate[Ni(NO3)2·6H2O] and phosphoric acid(H3PO4)in water was impregnated onto an Al2O3support.The pH of the solution was main-tained～3.5using28wt.%concentrated NH3solution. Impregnation of the support was followed by drying for 12h at120?C and calcined at500?C for4h.

2.2.Catalyst characterization

Catalyst characterization was carried out to identify the properties of the catalysts that are responsible for the catalyst activity.Different characterization techniques used in this study are described below.

2.2.1.Elemental analysis

The analysis of the elemental composition of Ni,Mo,B and P in the calcined catalyst was performed using inductiv-ity coupled plasma-mass spectrometer(ICP-MS).The pro-cedure is described by Ferdous[16].

D.Ferdous et al./Applied Catalysis A:General260(2004)137–151139

2.2.2.BET surface area,pore volume and pore size measurement

BET surface area,pore volume and pore size measure-ments of the calcined catalysts(～0.1g)were performed us-ing a Micromeritics adsorption equipment(Model ASAP 2000,manufactured by Micromeritics Instruments Inc.,Nor-cross,GA,USA)at78K using liquid N2.Before analysis, catalyst sample was evacuated at200?C for4h in a vacuum of5×10?4atm to remove all adsorbed moisture from the catalyst surface and pores.

2.2.

3.Temperature programmed reduction

The temperature programmed reduction(TPR)analysis of the catalyst(～0.03g)was performed using Quantachrome equipment(Model ChemBET3000,Manufactured by Quan-tachrome Corporation,FL,USA).The sample U tube was placed in an electric furnace and heated from22to1050?C at20?C/min at1atm in a reducing gas of3mol%H2in N2(obtained from Praxiar,Mississauga,Ont.,Canada)with a?ow rate of30ml/min.H2consumption during the TPR experiments was measured with a thermal conductivity de-tector(TCD).The TPR plots were logged using an on-line data acquisition system.

2.2.4.Temperature programmed desorption

The temperature programmed desorption(TPD)of ammo-nia was performed using Quantachrome equipment(Model ChemBET3000,Manufactured by Quantachrome Corpo-ration,FL,USA)for the measurement of the acidic prop-erties of catalysts.For TPD analysis,0.1g catalyst sample was placed in an adsorption vessel(U-shaped),heated with a?ow of0.1%NH3in N2at the?ow rate of20ml/min and at heating rate of10?C/min up to400?C and kept at this temperature for1h.Then it was cooled to room tempera-ture using this gas mixture at the same?ow rate.Finally,the TPD was run from room temperature to1050?C at heating rate of10?C/min with He?ow at20ml/min.The amount of NH3desorbed was measured using a TCD.

2.2.5.X-ray diffraction

Powder X-ray diffraction(XRD)studies were performed on the calcined sample in order to identify the crystalline species present.XRD analysis was performed using Rigaku diffractometer(Rigaku,Tokyo,Japan)using Cu K?radi-ation?ltered by a graphic monochromator at a setting of 40kV and130mA.The powdered catalyst samples were smeared on glass slide with methanol and dried at room tem-perature.The X-ray diffraction analysis was carried out in the scanning angle(2θ)range of3–60?at scanning speed of 5?/min.

2.2.6.Scanning electron microscopy–energy dispersion spectroscopy

Scanning electron microscopy(SEM)analysis was per-formed on NiMo/Al2O3catalysts using Hitachi S-2700with Princeton Gamma-Tech Prism EDS Detector at Syncrude Canada Ltd.For doing this analysis,a few pellets of the cat-alyst samples were attached onto an aluminum stub using epoxy as the adhesive.Before doing the analysis the sam-ples were coated with carbon by vacuum evaporation to re-duce charging and improve the secondary electron signals for imaging.Analysis was done for the surface and also for the radial direction of the catalyst samples at the magni?ca-tion of12000×.In addition,energy dispersion spectroscopy (EDS)was also used to obtain the elemental analysis for selected particles.

2.2.7.Nuclear magnetic resonance

NMR analysis of all calcined catalysts was performed using an Advance DRX NMR spectrometer,manufactured by Bruker Analytik GmbH.The study looks at the different hydroxyl structures present on the catalyst surface.For this analysis1H(proton)nucleus(360.13MHz)was observed. Approximately0.5g of sample was packed in the rotor.The rotor was placed in the probe and spinning started to a speed of5kHz.The sample sits at an angle of54.7?,which is referred to as magic angel spinning(MAS).The data was collected for2.2min and then transformed to produce the spectra.

2.2.8.Fourier transform infrared spectroscopy

Fourier transform infrared(FTIR)analysis of all catalysts was performed using an FTIR spectrometer attached to a photo acoustic spectroscopy(PAS)cell.Before doing analy-sis all samples were powdered and dried in an oven at400?C for3h,cooled in the desiccator until it reached about50?C whereupon it was transferred into a sample cup stored in the desiccator.The loaded cup was inserted into the PAS mea-surement chamber and the atmosphere control valve rotated to the open purge position to allow helium gas to?ow over the sample.Then the rear lever was rotated to the closed purge position and the sample allowed to cool to room tem-perature,in the sealed cell,under helium.All spectra were recorded after512scans at a resolution of8cm?1in order to achieve desired signal to noise ratio.

3.Results and discussion

3.1.Catalyst characterization

3.1.1.Elemental analysis

Table1shows the elemental analysis of the different cat-alyst formulations.It is observed that for all catalysts the measured concentrations are comparatively lower than those of targeted ones particularly in case of Mo.For example, for the catalyst with the targeted Ni and Mo concentrations of4and14wt.%,the actual concentrations were3.1and 11.1wt.%,respectively.Part of this discrepancy is due to the inherent nature of these compounds to form consider-able metal vacancies,depending on the preparation or heat treatment conditions[17].Also,the signi?cant difference in

140 D.Ferdous et al./Applied Catalysis A:General 260(2004)137–151

Table 1

Summary of ICP results of different NiMo/Al 2O 3,NiMoB/Al 2O 3,and NiMoP/Al 2O 3catalysts calcined for 4h Catalyst

Calcination temperature (?C)Targeted (wt.%)Measured (wt.%)Ni Mo B P Ni Mo B P A 45031400 2.712.200B 50031400 2.913.200C 60031400 3.013.200D 50021400 1.812.500B 50031400 2.913.200E 50041400 3.111.100F 50031200 2.410.600B 50031400 2.913.200G 5003140.60 2.611.70.530H 500314 1.20 2.611.8 1.10I 500314 1.30 2.89.1 1.20J 500314 1.40 2.511.8 1.30K 500314 2.00 2.210.2 1.70L

500

3

14

3

3.1

12.5

2.7

expected and targeted Mo concentrations could be due to the hygroscopic nature of the Mo precursor (ammonium hepta-molybdate)thus preventing complete impregnation of Mo from the solution to the support.

3.1.2.BET surface area,pore volume,and pore size measurement

The BET surface area,pore volume and pore size of dif-ferent catalysts are given in Table 2.From this table,it is observed that the surface area,pore volume,and pore diam-eter of the support decreased with the incorporation of Ni,Mo,B and P in ?-Al 2O 3,due to:(a)the pore blocking of alumina with impregnated material [7],and (b)the partial solubility of alumina in acid and alkaline media [18].It is also seen that the increase in calcination temperature caused

Table 2

BET surface area,pore volume and pore diameter of different NiMo/Al 2O 3,NiMoB/Al 2O 3and NiMoP/Al 2O 3catalysts calcined for 4h Ni Mo (wt.%)B P Calcination temperature (?C)BET (m 2/g)Pore volume (cm 3/g)Pore diameter (?)Support ––––2350.6571122.712.2––4501830.452992.913.2––5001930.4871013.013.2––6002010.479951.812.5––5001860.44952.913.2––5001930.4871013.111.1––5001970.469952.410.6––5002110.482912.913.2––5001930.4871012.611.70.5–5001950.5221072.611.8 1.1–5001940.444922.89.1 1.2–5001920.459962.511.8 1.3–5001850.4741032.210.2 1.7–5001640.4221033.1

12.5

–

2.7

500

194

0.43

96

an increase in surface area of the catalyst.For example,it increased from 183to 201m 2/g with calcination tempera-ture from 450to 600?C (see Table 2for catalysts A–C).On the other hand Brito and Laine [19]reported no signif-icant change in surface area but slight increase in the pore volume with the increase in calcination temperature from 400to 500?C.This is probably due to migration of Ni and Mo metal from pore to surface,with the increase in calci-nation temperature.The surface area increased from 186to 197wt.%with the increase in Ni concentration from 1.8to 3.1wt.%(Table 2)(catalysts B,D and E).These results are in agreement with those results reported by Brito and Laine [19].However,an increase in Mo concentration from 10.6to 13.2wt.%(catalysts B and F)caused a decrease in surface area from 211to 193m 2/g.

It is seen from Table 2that the surface area (192–195m 2/g)did not change signi?cantly with the addition of 0.5–1.2wt.%B to the NiMo/Al 2O 3catalyst (catalysts G–I)but it de-creased signi?cantly with the increase in B to 1.7wt.%(catalysts J and K).This decrease is due to the blockage of pores and the formation of bulk and crystalline B 2O 3at higher boron loading.Lewandowski and Sarbak [7]also re-ported a decrease in surface area of NiMoB/Al 2O 3catalyst from 163to 123m 2/g with an increase in boron concentra-tion from 2.8to 11.7wt.%.The surface area of the catalyst did not change with the addition of P to this catalyst.In contrast,Jian and Prins [20]reported decrease in surface area from 212to 174m 2/g with the addition of 2wt.%P to NiMo/Al 2O 3.The pore volume and pore diameter of all catalysts varied between 0.42and 0.52cm 3/g,and 91and 107?,respectively.

3.1.3.Temperature programmed reduction

Temperature programmed reduction (TPR)analysis was performed to determine the nature of different oxide species present on the calcined catalyst surface as well as to study

D.Ferdous et al./Applied Catalysis A:General 260(2004)137–151141

the reducibility of the catalyst.TPR has been shown to be a sensitive technique for studying reducibility [21]and has been applied successfully for the characterization of CoMo/Al 2O 3[22],NiO/Al 2O 3[23],MoO 3/Al 2O 3[23,24],and NiMo/Al 2O 3[23]

.Fig.1.TPR pro?les of different NiMo/Al 2O 3catalysts.Effects of:(a)calcination temperature,(b)Ni concentration,(c)Mo concentration,(d)B concentration,and (e)P concentration.

The TPR pro?les of different NiMo/Al 2O 3catalysts and ?-Al 2O 3support are given in Fig.1.Fig.1a for NiMo/Al 2O 3shows that the lower temperature peak,which is referred to as Mo I ,shifts from 520to 490?C,and the higher temperature peak,which is referred to Mo II ,shifts from 915to 900?C,

142 D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

with the increase in calcination temperature from450to

600?C.The peak at520?C is probably the result of the par-

tial reduction of well-dispersed polymolybdate-like species,

i.e.Mo6+→Mo4+.The peak at775?C(see Fig.1a)is because of the reduction of Ni[19].Its intensity decreased

signi?cantly with the increase in calcination temperature

from450to600?C.Brito and Laine[19]reported that at

the calcination temperature<600?C,all Ni reduced under

the Ni i band.The reducibility of Ni depends upon the cal-

cination temperature of the catalyst.It was reported that in

NiMo/Al2O3catalyst,most of the Ni reduced simultane-

ously with Mo under the low temperature peak[25].But at

higher calcination temperatures>600?C,essentially all the

nickel present reduces with Mo II[19].In contrast,Park et al.

[23]reported that the peak at775?C is probably because

of the further reduction of polymolybdate,which exerts a

stronger interaction with the support.The peak at higher

temperature(915?C)represents complete reduction of poly-

molybdate plus the reduction of the tetrahedrally coordinated

molybdate group i.e.Mo6+→Mo4+and Mo4+→Mo0+ [23].Fig.1a also shows that increase in calcination temper-ature caused a decrease in peak intensity at520?C,proba-bly because of Mo loss at the higher calcination temperature [26].However,hydrogen consumption for these catalysts did not change signi?cantly with the increase in calcination temperature from450to600?C,which remained at～0.2l/g of catalyst for all calcination temperatures.

The lower temperature peak shifts from520to490?C and

peak intensity decreased with increase in Ni concentration

from1.8to3.1wt.%(catalysts B,D and E)but the higher

temperature peak did not shift(Fig.1b).The hydrogen con-

sumption for these catalysts decreased from0.3to0.2l/g

of catalyst with the increase in Ni concentrations from1.8

to3.1wt.%.This may be due to the interaction between

Ni and Mo is suggested by Dufresne et al.[27],producing

Ni–Mo–O phase,which is the precursor of the Ni–Mo–S,

known as the active site for HDN reaction.A possible role

of Ni in enhancing Mo reducibility could be to facilitate

hydrogen activation(i.e.decomposition of H2to atomic hy-

drogen and migration of polymolybdates e.g.by spill over)

[19].Higher Mo concentration did not change the reduction

temperature of the lower temperature peak but decreased the

reduction temperature for higher temperature peak slightly

from920to915?C(Fig.1c).No signi?cant change in total

hydrogen consumption(～0.2l/g of catalyst)was observed

for these catalysts.

The effects of B concentration on TPR pro?le are given in

Fig.1d,which shows that the addition of B to NiMo/Al2O3

catalyst(catalysts G–K)decreases the reducibility of the

catalyst.The reduction temperature of polymolybdate in-

creased from520to605?C with increase in boron concen-

tration from0to1.7wt.%.Increase in boron concentration

also changed the molybdenum oxide structure and caused

the formation of less-reducible molybdenum borate.This is

con?rmed by DeCanio and Weissman[6]using FTIR tech-

niques by NO adsorption that addition of B up to0.3wt.%to NiMo/Al2O3catalyst changes molybdenum oxide struc-ture,rendering it more susceptible to reduction.However, by adding B from0.3to1.8wt.%,decreased the intensity of the Ni–Mo band which indicates that some molybdenum converts to a less easily reducible form such as molybde-num borate.This study con?rms the decrease in reducibil-ity of B(0.6–1.7wt.%)containing catalysts.The hydrogen consumption for these catalysts increased from0.2to0.5l/g of catalyst with the increase in B concentration from0to 1.7wt.%.This indicates that addition of B to NiMo/Al2O3 catalyst caused the migration of higher amounts of Ni and Mo oxides to the catalyst surface.

The reduction temperature of polymolybdate and tetramolybdate increased from520to550?C and920 to1000?C with the addition of2.7wt.%phosphorus to NiMo/Al2O3catalyst(see Fig.1e).The reducibility of the catalyst decreased probably because of the strong interac-tion of Ni with aluminum phase,and because of the forma-tion of AlPO4at the support surface[12].The hydrogen consumption increased from0.2to0.4l/g of catalyst with the incorporation of2.7wt.%P to NiMo/Al2O3catalyst.

3.1.

4.Temperature programmed desorption Temperature programmed desorption(TPD)of adsorbed ammonia was used to measure the acidic properties of the catalyst.NH3can be absorbed on the catalyst surface phys-ically and chemically.The physically absorbed NH3is des-orbed at lower temperature whereas chemically absorbed NH3is desorbed at higher temperature.From the literature, it is observed that there are three ranges of temperature for the desorption of ammonia,namely,20–200,200–350and 350–550?C,which correspond to weak,intermediate,and strong acid centers,respectively[8].On the other hand,the desorption peak intensity and area measures the number of acid sites,i.e.if the peak intensity and area are high,the number of acid sites is more and vice versa[28].

The pro?les in Fig.2show the acid center for support (Al2O3)as well as for different NiMo/Al2O3catalysts cal-cined at different temperatures and those containing dif-ferent concentrations of Ni,Mo,B,and P.From this?g-ure,two peaks from100to400?C and500to1000?C for the desorption of NH3are observed,corresponding to weak–intermediate and strong acid centers.The?gure shows that the intermediate acid strength increased and the strength of strong acid centers decreased by impregnation of sup-port with molybdenum,nickel,boron,and phosphorus ions, which are evident from the peak intensity and area.

The effect of calcination temperature on TPD pro?le shows that increase in calcination temperature from450to 600?C caused an increase in intermediate acid centers but a decrease in strong acid centers(Fig.2a for catalysts A–C). Fig.2b shows that the increase in Ni concentration caused a decrease in number of acid sites as well as in intermediate acid strength.However,the number of strong acid sites, and those with intermediate and higher strength of support increased with increase in Ni concentration.The increase

D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

143

Fig.2.TPD pro?le of different NiMo/Al2O3catalysts.Effects of:(a)calcination temperature,(b)Ni concentration,(c)Mo concentration,(d)B concentration,and(e)P concentration.

in Mo concentration from10.6to13.2wt.%caused an in-crease in number of intermediate and strong acid centers (see Fig.2c).

Increase in boron loading from0.5to1.3wt.%(cata-lysts G–J)caused the formation of intermediate acid centers,whereas1.7wt.%boron(catalyst K)concentration caused the formation of weak acid centers as observed from their desorption peak temperature(Fig.2d).However,addition of phosphorus to NiMo/Al2O3catalyst(catalyst L)caused the formation of intermediate acid centers(Fig.2e).

144 D.Ferdous et al./Applied Catalysis A:General 260(2004)137–151

3.1.5.Scanning electron microscopy–energy dispersion spectroscopy

Scanning electron micrographs are used to study the sur-face morphology of the fresh catalysts whereas EDS is used to quantify the elemental composition of a single particle present on the catalyst surface.

With reference to SEM micrographs from the surfaces of samples calcined in the range of 450–600?C (catalysts A–C),it is seen that an increase in calcination temperature up to 500?C caused an increase in particle size from ～0.2to ～0.4?m (Fig.3b and c ),after which the particle size decreased to ～0.25?m (Fig.3d ).At 500?C,sintering due to Al 2O 3transformation by forming Al 2(MoO 4)3might play a role in enhancing the particle size.Decrease in particle size at >500?C is probably because of sublimation of MoO

3

Fig.3.Effects of calcination temperatures on surface morphology of different NiMo/Al 2O 3catalysts:(a)?-Al 2O 3,(b)450?C,(c)500?C,and (d)600?C.

as reported by Kasztelan et al.[29].It is seen that at the calcination temperature of 500?C (Fig.3c ),the particle sizes are more uniform,which may lead to ef?cient reduction of metals during hydrotreating process.This indicates that the optimum calcination temperature may be 500?C.

Increase in Ni concentration in NiMo/Al 2O 3catalyst (cat-alysts B,D and E)did not show any signi?cant change on the catalyst surface [16].

SEM analysis of NiMoB/Al 2O 3showed that the increase in B concentration in the catalyst (catalysts G–I)caused agglomeration of particles (Fig.4a–c ).The radial direction analysis (end view)showed that increase in B concentration caused the increase in needle-like crystals [16].Addition of phosphorus to NiMo/Al 2O 3catalyst (catalyst L)showed irregular particle sizes and agglomeration (Fig.5b ).EDS

D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

145

Fig.4.Effects of B concentration of surface morphology of different NiMo/Al2O3catalysts calcined at500?C for4h:(a)0wt.%B,(b)0.5wt.%B,and (c)1.1wt.%B.

analysis indicated the presence of P in these agglomerated particles.

The EDS results(Table3)are based on elemental analysis of selected catalyst particles.It shows that NiMoP/Al2O3 agrees in concentration values,however,for most of the analysis,the concentrations were completely different from measured values.From this table,2wt.%Ni was detected for NiMo/Al2O3catalyst calcined at450and500?C,whereas at higher calcination temperature(600?C),Ni concentration was～15wt.%,probably because of the migration and/or ag-glomeration of Ni on the surface at600?C.The Mo concen-tration obtained from EDS analysis was signi?cantly higher than that obtained from bulk elemental analysis,indicating that Mo may have migrated as well as agglomerated more on the surface at a given calcination temperature.Boron is a light component and is not detectable from EDS analysis. The concentrations of Ni,Mo,and P for NiMoP/Al2O3cat-alyst from EDS analysis were3,14and3wt.%,which are exactly same as the targeted ones.As the EDS analysis is based on a selected particles,it is expected that EDS analy-sis gives different elemental composition than that in bulk. In summary,the EDS analysis,which is based on elemental

146 D.Ferdous et al./Applied Catalysis A:General260(2004)

137–151

Fig.5.Effects of P concentration on surface morphology of different NiMo/Al2O3catalysts calcined at500?C for4h:(a)0wt.%P and(b) 2.7wt.%P.

analysis of selected particles on the catalyst surface has limitations in quantifying the catalyst surface composition but gives quantitative data on localized spots on the surface, which may be different from bulk composition.

3.1.6.X-ray diffraction

X-ray diffraction(XRD)study was used to detect different crystalline phases present in the catalyst.X-ray diffraction of various calcined NiMo/Al2O3catalysts revealed no peaks other than that of alumina(?gure not shown),indicating high dispersion of Ni and Mo on the catalyst surface.Increase in calcination temperature from450to600?C(catalysts A–C) did not change the crystallinity of the catalyst.These results are in agreement with those reported by Brito and Laine[19].Table3

Summary of EDS analysis of different NiMo/Al2O3,NiMoB/Al2O3,and NiMoP/Al2O3catalysts calcined at500?C for4h

Calcination

temperature

(?C)

Measured by EDS

(wt.%)

Measured by ICP

(wt.%)

Ni Mo B P Ni Mo B P 450 2.712.20021500 500 2.913.20021700 600 3.013.200152500 500 1.812.50042500 500 2.913.20021700 500 3.111.100112700 500 2.410.60073000 500 2.913.20083400 500 2.611.70.5–417n/d a0 500 2.611.8 1.1–332n/d0 500 2.89.1 1.2–320n/d0 500 2.511.8 1.3–424n/d0 500 2.210.2 1.7–224n/d0 500 3.112.5– 2.731403

a Not detectable.

Addition of0.5wt.%B to NiMo catalyst caused a de-crease in the intensity of the alumina diffraction line,due to the formation of highly dispersed microcrystalline alu-minum borate phase on the catalyst surface[30].With the further increase in boron(1.1–1.7wt.%)(catalysts H–K),a crystalline borate phase as well as MoO3phase became more prominent[31–33],when the B concentration was increased to1.7wt.%.Addition of P to NiMo/Al2O3catalyst(cata-lyst L)did not show any signi?cant effect on crystallinity of this catalyst,which con?rmed the study by Decanio et al.

[9].

3.1.7.Nuclear magnetic resonance

Because of the paramagnetic nature of Co and Ni ions, solid state NMR has seldom been used for studying hy-drotreating catalyst.To our knowledge,no1H NMR analy-sis for NiMo/Al2O3,NiMoB/Al2O3and NiMoP/Al2O3has been done before.In our case,analysis was performed us-ing dry as well as for air exposed samples.No signi?cant difference in results was observed.

The1H NMR spectra of different NiMo/Al2O3catalysts containing different Ni,Mo,B and P concentrations cal-cined at different calcination temperatures(450–600?C) are given in Fig.6.The corresponding NMR peak areas for different catalysts are given in Table4.Fig.6also show the1H NMR spectra for support(?-Al2O3)for compari-son.?-Al2O3possesses a defective spinal lattice structure, which is terminated by a surface hydroxyl group.During the preparation of catalyst,the metal species interact with the alumina hydroxyl groups to produce dispersed metal oxide phases[34].Also,new hydroxyl groups can also form on the catalyst surface,which is responsible for the acidity of these catalysts.From NMR analysis,only peak

D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

147

Fig.6.NMR pro?les of different NiMo/Al2O3catalysts.Effects of:(a)calcination temperature,(b)Ni concentration,(c)Mo concentration,(d)B concentration,and(e)P concentration.

at～4.62ppm was observed for all catalysts.This peak can be attributed to the Al–OH group present in alumina[35]. Table4shows that there is no signi?cant change in the NMR peak area with the incorporation of Ni,Mo,B,and P to the support.3.1.8.Fourier transform infrared spectroscopy

Fourier transform infrared(FTIR)spectroscopy in combination with photoacoustic spectroscopy(PAS),is particularly useful to characterize heterogeneous catalysts. Application of this technique to catalysts provides useful

148 D.Ferdous et al./Applied Catalysis A:General 260(2004)137–151

Table 4

NMR peak area of different NiMo/Al 2O 3,NiMoB/Al 2O 3and NiMoP/Al 2O 3catalysts calcined for 4h Ni Mo (wt.%)B P Calcination

temperature (?C)NMR peak area (a.u.)Support –––– 1.92.712.2––450 2.12.913.2––500 1.93.013.2––600 2.01.812.5––500 2.02.913.2––500 1.93.111.1––500 2.12.410.6––500 2.12.913.2––500 1.92.611.70.5–500 2.12.611.8 1.1–500 2.22.89.1 1.2–500 2.12.511.8 1.3–500 2.22.210.2 1.7–500 2.23.1

12.5

–

2.7

500

2.2

Fig.7.FTIR spectra of different NiMo/Al 2O 3catalysts.Effects of:(a)calcination temperature,(b)Ni concentration,(c)Mo concentration,and (d)B concentration.

microstructural information that can be correlated with catalytic activity.Each experiment was performed for 2–3times.The results were quite reproducible.To study the effects of absorbed water on the catalyst surface properties,FTIR analysis of the calcined (400?C for 3h)and uncalcined alumina support was performed.Uncalcined sample showed strong band in the range of 3000–3580cm ?1due to the pres-ence of absorbed water.So,before performing FTIR analysis all catalysts samples were dried in the oven at 400?C for 3h.FTIR spectra for different NiMo/Al 2O 3catalysts as well as support are given in Figs.7and 8.The region 3000–4000cm ?1in FTIR spectra is termed the hydroxyl stretching region,whereas the region 900–1800cm ?1is termed as frequency multiplied range representing the characteristics vibration of Al–O and B–O stretching.The positions of the hydroxyl bands have been shown to be de-pendent on the acidity of the hydroxyl group;the basic hy-droxyl groups gives rise to the IR bands between 3762and 3727cm ?1,while the most acidic hydroxyl groups produce bands at 3677cm ?1[9].FTIR spectra of alumina at 3778

D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

149

Fig.8.Effects of P concentration on FTIR Spectra of NiMO/Al2O3catalysts calcined at500?C for4h:(a)0–5000cm?1,(b)1200–1500cm?1,and(c) 2800–4000cm?1.

and3684cm?1can be attributed to the presence of the most basic hydroxyl group,and the most acidic hydroxyl group, respectively,whereas the band at3580and1670cm?1can be attributed to the presence of hydrogen-bonded water [6]and the adsorbed water on the catalyst surface[36], respectively.The band at1100cm?1,though has not been previously reported in the literature[37,38],represents Al–O stretching vibration of alumina[6].Fig.8a shows that the incorporation of Ni and Mo on the support caused a somewhat increase in intensity of the bands at1670and 1100cm?1.

Fig.7b shows that the increase in Ni concentration from 1.8to3.1wt.%(catalysts A–C)caused a decrease in the intensity of the band at1670cm?1and increase in Al–O stretching vibration band at1100cm?1.However,there was no change in basic and acidic hydroxyl groups with the increase in Ni content in the catalyst.

Fig.7c shows that with the increase in Mo concentration from10.6to13.2wt.%(catalysts B and F)in NiMo/Al2O3, the band intensity at1670cm?1,the stretching of Al–O vi-bration(at1100cm?1)and the basic hydroxyl group(band at3778cm?1)did not change with increase in Mo concen-tration from10.6to13.2wt.%.

Fig.7d shows that the addition of B to the NiMo/Al2O3 increased the intensity of the bands at1100and1408cm?1. Addition of B resulted in depletion of the IR band at 3778cm?1,indicating that boric acid reacts with basic alumina[6].Acid hydroxyl group intensity remained un-changed,whereas intensity of the band at3580cm?1 increased with the increase in B concentration from0.5 to1.7wt.%(catalysts G–K).A new band at1550cm?1 appeared with the addition of B,due to the formation of Br?nsted acid sites,which was con?rmed by Decanio and Weissman[6]from FTIR analysis of NiMo/Al2O3 using pyridine adsorption.This band intensity increased with the increase in B concentration from0.5to1.7wt.%, which proves that increase in B concentration causes an increase in acidity of the catalyst.The band intensity at 1100cm?1increased signi?cantly with the addition of B.

A new band at1480cm?1assigned to the presence of B–O stretching vibration,appeared with the addition of

B to NiMo/Al2O3,whose intensity increased with the increase

150 D.Ferdous et al./Applied Catalysis A:General260(2004)137–151

in B concentration.The band can also be assigned to Lewis acid sites,as indicated by Decanio and Weissman[6]. Fig.8a shows no signi?cant effect of P on FTIR spec-tra.However,when the spectra are enlarged for the range of 1200–1550and2800–4000cm?1(Fig.8b and c),some dif-ferences appear.From Fig.8b,it is seen that the addition of P to NiMo/Al2O3catalyst(catalyst L)caused the formation of bands at1378,1425,and1471cm?1,probably due to the formation of new Lewis acid sites on the catalyst surface. The sharp band at3666cm?1(see Fig.8c)could be due to the–OH associated with phosphate[9].This–OH group on the catalyst surface is responsible for the formation of Br?n-sted acid sites.The bands at3594and3386cm?1could be due to the formation of new–OH groups on the catalyst surface in the presence of phosphorus.

3.2.Implication of the characterization results

on HDN activity

There was no signi?cant difference in surface area with the addition of small concentrations of B and P to NiMo/Al2O3catalyst.However,increase in B concentra-tion above1.3wt.%caused a signi?cant decrease in surface area.From the literature,it is observed that higher HDN activity is related to decrease in surface area.Lewandowski and Sarbak[7]reported a decrease in surface area and an in-crease in HDN activity with an increase in B concentration from2.8to6.3wt.%.Similarly,Jian et al.[15]also reported a decrease in surface area and increase in HDN activity with the addition of2wt.%P to NiMo/Al2O3.However,it is dif?cult to correlate the HDN activity with surface area, since HDN activity is dependent not only on the physical surface area but also on the other surface physicochemical properties of the catalyst.

Addition of B and P caused a decrease in reducibility of the catalyst.The reducibility of NiMo oxide catalysts favored by their conversions to sul?ded state is an impor-tant factor for the HDN activity.Several attempts have been made to correlate the reducibility of oxidic NiMo catalyst to hydroprocessing activity of their sul?ded state[25].It may be noted that in Mo supported catalysts and under normal sul?ding conditions,O–S exchange is a more important re-action than reduction with hydrogen[26].Thus,it is not generally true that phases more easily reducible are more sul?dable and/or active after sul?dation[19].

It is indicated in the literature that high activity for HDN is associated with an increase in the number of acid centers of weak and intermediate strength[8].From TPD analy-sis it was observed that addition of B from0.5to1.2wt.% to NiMo/Al2O3catalyst led to the formation of strong acid centers and the further addition of B caused the formation of weak acid centers.So,higher HDN activity can be antic-ipated from these catalysts.

It is known that two types of reactions occur during HDN reaction:(1)hydrogenation and(2)hydrogenolysis.Two cat-alytic sites have been proposed in the literature to account for these two types of reaction.These two catalytic sites can be viewed as an equilibrium between Lewis(L)and Br?n-sted(B)acid sites[39,40]

:

Kwart et al.[39]reported that the B-sites are responsible for hydrogenation reactions whereas L-sites for hydrogenol-ysis.So,B-sites would act as the electron-withdrawing sites for hydrogenation reactions and L-sites would act as the electron-donating sites for hydrogenolysis of C sp2–X bonds [41].Consequently,high HDN activity of NiMoB/Al2O3 and NiMoP/Al2O3is anticipated because of the formation of new Lewis and Br?nsted sites on these catalysts and the acidity of the catalyst would increase with the increase in B concentration from0.5to1.7wt.%,as is evident from FTIR analysis.

It is known that addition of P to NiMo/Al2O3has a ge-ometric effect,such as an increase in the number of stacks [42].When H3PO4is used during catalyst preparation, AlPO4is formed by partial solubilization of Al cations [12],which increases the amount of molybdenum fractions in easily sul?dable forms,such as multilayered molybdate or MoO3[10].Large nitrogen containing compounds,such as heavy gas oil molecules,can better coordinate to the active sites at the top of the MoS2stacks than to sites close to the Al2O3surface[12].So,it can be anticipated that the HDN activity of B and P containing NiMo/Al2O3catalysts will give higher HDN activity of heavy gas oil than those without B and P.The reactivity test of these catalysts will be presented in Part II of this paper.

4.Conclusions

A number of NiMo/Al2O3catalysts promoted with

B and P were prepared and extensively characterized.The maxi-mum BET area of211m2/g was obtained for the catalyst containing10.6wt.%Mo and2.4wt.%Ni calcined at500?

C for4h.In these catalysts,polymolybdenum and tetramolyb-denum oxides were predominantly present.Due to the ad-dition of Ni and Mo to support,the acidity on the catalyst surface decreased to some extent.However,the addition of B from0.5to1.2wt.%to NiMo/Al2O3catalyst led to the formation of strong acid centers of intermediate strength and the further addition of B caused the formation of weak acid centers of intermediate strength.SEM-EDS analyses indi-cated the formation of crystalline B2O3and MoO3due to addition of B,whereas addition of P caused the formation irregular particle size and agglomeration on the catalyst sur-face.NMR analysis showed the presence of Al–OH groups

D.Ferdous et al./Applied Catalysis A:General260(2004)137–151151

on the catalyst surface.No characteristic peak for B–OH was observed for NiMoB/Al2O3catalyst.New Lewis and Br?nsted acid sites were observed from FTIR analysis of NiMoB/Al2O3and NiMoP/Al2O3,which complemented the results obtained from TPD analysis. Acknowledgements

The?nancial supports to A.K.Dalai from(1)NSERC Collaborative Research and Development Grant,(2)Syn-crude Canada Ltd.and NSERC IPS2scholarship to D.Fer-dous are acknowledged.The authors are grateful to Drs.L.S. Kotlyar and Z.Li at National Research Council,Ottawa, Canada for FTIR and to Mr.P.Abel at Syncrude Research Center,Edmonton,Canada for SEM-EDS analysis. References

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从实践的角度探讨在日语教学中多媒体课件的应用

从实践的角度探讨在日语教学中多媒体课件的应用 在今天中国的许多大学，为适应现代化，信息化的要求，建立了设备完善的适应多媒体教学的教室。许多学科的研究者及现场教员也积极致力于多媒体软件的开发和利用。在大学日语专业的教学工作中，教科书、磁带、粉笔为主流的传统教学方式差不多悄然向先进的教学手段而进展。 一、多媒体课件和精品课程的进展现状 然而，目前在专业日语教学中能够利用的教学软件并不多见。比如在中国大学日语的专业、第二外語用教科书常见的有《新编日语》（上海外语教育出版社）、《中日交流标准日本語》（初级、中级）（人民教育出版社）、《新编基础日语（初級、高級）》（上海译文出版社）、《大学日本语》（四川大学出版社）、《初级日语》《中级日语》（北京大学出版社）、《新世纪大学日语》（外语教学与研究出版社）、《综合日语》（北京大学出版社）、《新编日语教程》（华东理工大学出版社）《新编初级（中级）日本语》（吉林教育出版社）、《新大学日本语》（大连理工大学出版社）、《新大学日语》（高等教育出版社）、《现代日本语》（上海外语教育出版社）、《基础日语》（复旦大学出版社）等等。配套教材以录音磁带、教学参考、习题集为主。只有《中日交流標準日本語（初級上）》、《初級日语》、《新编日语教程》等少数教科书配备了多媒体DVD视听教材。 然而这些试听教材，有的内容为日语普及读物，并不适合专业外语课堂教学。比如《新版中日交流标准日本语（初级上）》，有的尽管DVD视听教材中有丰富的动画画面和语音练习。然而，课堂操作则花费时刻长，不利于教师重点指导，更加适合学生的课余练习。比如北京大学的《初级日语》等。在这种情形下，许多大学的日语专业致力于教材的自主开发。 其中，有些大学的还推出精品课程，取得了专门大成绩。比如天津外国语学院的《新编日语》多媒体精品课程为2007年被评为“国家级精品课”。目前已被南开大学外国语学院、成都理工大学日语系等全国40余所大学推广使用。

新视野大学英语全部课文原文

Unit1 Americans believe no one stands still. If you are not moving ahead, you are falling behind. This attitude results in a nation of people committed to researching, experimenting and exploring. Time is one of the two elements that Americans save carefully, the other being labor. "We are slaves to nothing but the clock,” it has been said. Time is treated as if it were something almost real. We budget it, save it, waste it, steal it, kill it, cut it, account for it; we also charge for it. It is a precious resource. Many people have a rather acute sense of the shortness of each lifetime. Once the sands have run out of a person’s hourglass, they cannot be replaced. We want every minute to count. A foreigner’s first impression of the U.S. is li kely to be that everyone is in a rush -- often under pressure. City people always appear to be hurrying to get where they are going, restlessly seeking attention in a store, or elbowing others as they try to complete their shopping. Racing through daytime meals is part of the pace

aseriesof…作主语谓语动词用单数还是用复数

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aseriesof一系列

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会遭人厌恶。 在欧美，对不受欢迎的客人不说“请脱下外套”，所以电视剧中的侦探哥隆波总是穿着外套。访问日本家庭时，要在门厅外脱掉外套后进屋。穿到屋里会不受欢迎的。 这些习惯只要了解就不会出问题，如果因为不知道而遭厌恶、憎恨，实在心里难受。 过去，我曾用色彩图画和简短的文字画了一本《关键时刻的礼仪》（新潮文库）。如今越发希望用各国语言翻译这本书。以便能对在日本的外国人有所帮助。同时希望有朝一日以漫画的形式画一本“世界各国的习惯与礼仪”。 练习答案 ５、 （１）止める並んでいる見ているなる着色した （２）拾った入っていた行ったしまった始まっていた

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新视野大学英语第三版第二册读写课文翻译 Unit 1 Text A 一堂难忘的英语课 1 如果我是唯一一个还在纠正小孩英语的家长，那么我儿子也许是对的。对他而言，我是一个乏味的怪物：一个他不得不听其教诲的父亲，一个还沉湎于语法规则的人，对此我儿子似乎颇为反感。 2 我觉得我是在最近偶遇我以前的一位学生时，才开始对这个问题认真起来的。这个学生刚从欧洲旅游回来。我满怀着诚挚期待问她：“欧洲之行如何？” 3 她点了三四下头，绞尽脑汁，苦苦寻找恰当的词语，然后惊呼：“真是，哇！” 4 没了。所有希腊文明和罗马建筑的辉煌居然囊括于一个浓缩的、不完整的语句之中！我的学生以“哇！”来表示她的惊叹，我只能以摇头表达比之更强烈的忧虑。 5 关于正确使用英语能力下降的问题，有许多不同的故事。学生的确本应该能够区分诸如their/there/they're之间的不同，或区别complimentary 跟complementary之间显而易见的差异。由于这些知识缺陷，他们承受着大部分不该承受的批评和指责，因为舆论认为他们应该学得更好。 6 学生并不笨，他们只是被周围所看到和听到的语言误导了。举例来说，杂货店的指示牌会把他们引向stationary（静止处），虽然便笺本、相册、和笔记本等真正的stationery（文具用品）并没有被钉在那儿。朋友和亲人常宣称They've just ate。实际上，他们应该说They've just eaten。因此，批评学生不合乎情理。 7 对这种缺乏语言功底而引起的负面指责应归咎于我们的学校。学校应对英语熟练程度制定出更高的标准。可相反，学校只教零星的语法，高级词汇更是少之又少。还有就是，学校的年轻教师显然缺乏这些重要的语言结构方面的知识，因为他们过去也没接触过。学校有责任教会年轻人进行有效的语言沟通，可他们并没把语言的基本框架——准确的语法和恰当的词汇——充分地传授给学生。

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常见同义词替换 A A range of=an array of= a list of= a collection of=a series of= a set of一系列 Accelerate=speed up加速 Accomplishment=achievement 成就，完成Accomplish=achieve （V）完成，达成，成就 Attend=participate in 参加 Achieve=obtain=gain=access to sth 得到、获得 Aim.n=objective=goal 目的 Aim at=focus on=concentrate on=emphasize on 集中、强调 Attempt to do=try to do=intend to do尝试作，想要作Approximately=roughly=about=some=around 大约 As a result=consequently=as a consequence=hence因此、从而 Associate with=connect with=link with=relate to=involve in与……相关联 Attach to=connect to=link to与……相连接、附着在……上At will=at random＝by chance=by accident=accidentally 随意地

Be similar to=closely resembled =have something in common 相似 Be used to=be devoted to专门用于 Be equivalent to=be equal to 等同于 Be taken aback=be surprised 吃惊 C Calculate=measure 衡量 Catch up with=keep pace with跟上步伐 Chief=major=dominant=principle主要的 Chronic=long term 慢性病的 Concern n. =focus关心的问题 Concept =conception=notion=idea=understanding概念、理解 Confront=encounter=in the face of面临 Consist of=comprise由什么组成 Contribute to=result in=lead to=link to=responsible for 导致 Cooperate with=work with 与……协作 Convert to=turn into 转为、转变为