Small-Sized Mg–Al LDH Nanosheets Supported on Silica Aerogel with Large Pore Channels

14 Pages·2017·2.54 MB·English

nanomaterials
Article
Small-Sized Mg–Al LDH Nanosheets Supported on
Silica Aerogel with Large Pore Channels: Textural
Properties and Basic Catalytic Performance
after Activation
LijunWang* ID,YusenWang,XiaoxiaWang,XiaolanFeng,XiaoYeandJieFu
SchoolofChemistryandChemicalEngineering,ShaoxingUniversity,Shaoxing312000,China;
[email protected](Y.W.);[email protected](X.W.);[email protected](X.F.);
[email protected](X.Y.);[email protected](J.F.)
* Correspondence:[email protected];Tel.:[phone]
Received:15January2018;Accepted:13February2018;Published:16February2018
Abstract: Layered double hydroxides (LDHs) have been widely used as an important subset of
solidbasecatalysts. However,developinglow-cost,small-sizedLDHnanoparticleswithenhanced
surfacecatalyticsitesremainsachallenge. Inthiswork,silicaaerogel(SA)-supported,small-sized
Mg–Al LDH nanosheets were successfully prepared by one-pot coprecipitation of Mg and Al
ions in an alkaline suspension of crushed silica aerogel. The supported LDH nanosheets were
uniformlydispersedintheSAsubstratewiththesmallestaverageradialdiameterof19.2nmandthe
thinnestaveragethicknessof3.2nm,bothdimensionsbeingsignificantlylessthanthoseofthevast
majorityofLDHnanoparticlesreported. TheSA/LDHcompositesalsoshowedlargeporevolume
(upto1.3cm3·g)andporediameter(>9nm),andthereforeallowefficientaccessofreactantstothe
edgecatalyticsitesofLDHnanosheets. Inabase-catalyzedHenryreactionofbenzaldehydewith
nitromethane,theSA/LDHcatalystsshowedhighreactantconversionsandfavorablestabilityin
6successivecyclesofreactions. ThelowcostoftheSAcarrierandLDHprecursors,easypreparation
method,andexcellentcatalyticpropertiesmaketheseSA/LDHcompositesacompetitiveexampleof
solid-basecatalysts.
Keywords: layereddoublehydroxides;nanocatalysts;mesoporoussilica;support;solidbase
1. Introduction
Layereddoublehydroxides(LDHs)areaclassoftwo-dimensionalanionicclayscomposedof
positively-chargedlayersofmixedbivalentandtrivalentmetalhydroxideswithcharge-balancing
anions and hydrogen-bonded water molecules in the interlayer [1–4]. Mg–Al LDHs, represented
by the general formula [Mg2+ Al3+ (OH) ]x+(An–) ·mH O, have been widely used as an
1–x x 2 x/n 2
important subset of solid base catalysts [5–10]. In particular, calcined Mg–Al LDHs composed
of uniformly-dispersed magnesium–aluminum mixed oxides with variable proportions have
shown very attractive properties as catalysts for a broad spectrum of base-catalyzed organic
reactions,includingtransesterification[11,12],isomerizationofglucose[13–15],Henryreactions[16],
Cannizzaro reactions [17], epoxidation of olefins [18], alkylation of phenol [19], and aldol
condensation[20–22].
For flake-like LDH, the activated catalytic sites are mainly located at their edges [23–26].
Generally, LDH flakes with a smaller size (especially the radial size) possess higher surface area,
corresponding with more edge catalytic sites [27,28]. However, these smaller LDH nanoparticles
are readily agglomerated by ab-face stacking in solution due to their high charge density
and hydrophilicity [29–31]. Furthermore, small LDH nanoparticles are hard to separate after
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preparation and reaction in liquid systems [28]. A good way to solve these problems is to
support the LDH nanoparticles on high-surface-area carriers. A variety of materials such as
SBA-15[28,32],carbonnanofibers[27],multi-walledcarbonnanotubes[33],grapheneoxide[34,35],
mesoporous AlOOH [36], zeolite [37], hollow carbon nanospheres [38,39], silica nanofiber [1],
γ-Fe O [40],SiO nanospheres[30,41–46],biochar[47],Fe O [48],cellulose[49],Cunanowires[50],
2 3 2 3 4
and magnesium ferrite nanoparticles [51] have been employed as carriers to prepare supported
LDHs. The core/shell structured silica-sphere-supported LDHs have been especially studied by
severalgroups[30,41–46]veryrecentlyduetotheireasypreparationandexcellentpropertiessuchas
tunablesize,crystallinity,andmorphologyofLDH.However,thereisstillalackofextensiveresearch
ofmesoporoussilica(mSiO )-supportedLDHsystemswheremSiO canprovidemoresurfacearea,
2 2
farmorethanthatofsilicananosphere,forthedepositingofLDHnanoparticles.Thismaybeattributed
totworeasons: (1)InthemSiO -supportedHTstructure,thesupportedLDHnanoparticleslocatedat
2
theinnersurfaceofmSiO wouldblocktheporechannels,limitingtheaccessibilityofactivesitesof
2
theconfinedLDH;(2)ThehighercostandmorecomplexpreparationofmSiO comparedwiththe
2
SiO sphereusuallysynthesizedbyasimpleStöbermethod. Therefore,developingnovel,low-cost,
2
facilemSiO -supportedLDHswithsmallsizeandeasilyaccessibleedgesitesareexpected.
2
Silicaaerogels(SAs)haveattractedmuchattentioninscienceandtechnologyduetotheirhigh
surfaceareaandlargeporevolume[52,53]. Moreover,SAsareimportantlow-costmodernindustrial
materials for heat-insulation and heat-resistance applications. In this work, low-cost commercial
SAs with a large average pore diameter of 34.4 nm and large pore volume of 5.92 cm3·g were
selected as carriers to prepare supported Mg–Al LDH nanoparticles by a one-pot coprecipitation
method. The large pore diameter and pore volume facilitate the nucleation and growth of LDH
nanoparticles at the inner surface of SA and the minimization of pore blocking by the inner LDH
particles.Asexpected,theobtainedSA/LDHcompositesshowedlargeaverageporediameters(>9nm),
largeporevolume(upto1.3cm3·g), andhighsurfacearea(upto587.4m2·g)andcouldtherefore
provideeasyaccessofreactantstotheedgecatalyticsitesoftheincorporatedLDHnanosheets. Inthis
context,themorphology,texturalproperties,andbasesitestrengthofaseriesofSA/LDHcomposites
synthesizedatdifferentMg/AlratiosandhydrothermaltemperatureswerecharacterizedbyTEM,
XRD,EDS,BET,andCO -TPD.Abase-catalyticHenryreactionofbenzaldehydewithnitromethane
2
wasemployedasamodelreactiontoevaluatethecatalyticpropertiesofthesupportedLDHsusing
theunsupportedLDHnanosheetsandbareSAascontrolsamples.
2. MaterialsandMethods
2.1. Materials
Magnesium nitrate, aluminium nitrate, sodium hydrogen carbonate, nitromethane, ethanol,
dichloromethane,toluene,N,N-Dimethylformamide,tetrahydrofuran,andsodiumhydroxidewereall
ofanalyticalgradeandpurchasedfromAladdinReagentCo.,Ltd.(Shanghai,China).SAwasprovided
byNanoTechCo.,Ltd. (Shaoxing,China). Benzaldehyde(>98.0%,GC)ando-xylene(>98.0%,GC)
werepurchasedfromTCI(Tokyo,Japan).
2.2. Experimental
TheSApowderwascalcinedat823Kfor2htoremovesurfaceorganicgroups. ThetreatedSA
powder(500mg)wassuspendedin500mLofdeionizedwaterbysonicationfor30min. 10mLof
mixedsolutionA(mmol·L−1Mg(NO ) ·6H Oandnmol·L−1Al(NO ) ·9H O)wasaddedtotheSA
3 2 2 3 3 2
suspensionandtheobtainedmixturestirredfor30min. ThepHofthesolutionwasadjustedto8.8with
the mixed solution B (NaOH/NaHCO (0.5 M/0.5 M)). 140 mL of mixed solution A and various
3
amountsofsolutionBwereaddedalternatelytomaintainthepHat8.8,followedbyhydrothermal
treatmentat80◦C,105◦C,or150◦Cfor24h.TheMg:Alratiowasvariedsuchthatm+n=0.05mol·L−1
and m:n = 3:1, 5:2, and 2:1. The obtained products were concentrated by centrifugation, decanted
Nanomaterials2018,8,113 3of14
andwashedthreetimeswithdeionizedwater,andvacuum-dried. Thefinalsamplesweredenoted
asSA/LDH-Mg Al-y,wherexandyrepresenttheMg/Almoleratiointheprecursorsolutionand
x
thehydrothermaltemperature,respectively. ForthecontrolsampleofunsupportedLDHnanosheets
(LDH-Mg Al-80),thesyntheticmethodwasthesameasfortheSA/LDHcompositeexceptforusing
2
apHof9.5insteadof8.8soastoobtainLDHwithanMg/Alratioof~2:1. Allmaterialswerecalcined
at723Kfor2handcooledunderaflowingstreamofnitrogenbeforecatalysttesting.
2.3. Characterization
TransmissionelectronmicrographsweretakenusingaJEOLJEM-2100TransmissionElectron
Microscope (TEM) (JEOL Ltd., Tokyo, Japan) operating at an accelerating voltage of 200 kV.
ScanningelectronmicrographsandenergydispersivespectrawereobtainedusingaJEOLJSM-6360LV
scanningelectronmicroscope(SEM)equippedwithanX-actenergy-dispersiveX-ray(EDX)analyzer
(OxfordINCA,OxfordInstruments,Abingdon,Oxfordshire,UK).N adsorption–desorptionisotherms
2
wereobtainedusingaMicromeriticsASAPTriStarII3020poreanalyzer(MicromeriticsInstrument
Corp., Norcross, GA, USA) at 77 K under continuous adsorption conditions. The samples
were outgassed at 150 ◦C for 8 h before measurements. The specific surface areas were
calculated by the Brunauer–Emmett–Teller (BET) method, and pore size distributions were
measuredusingBarrett–Joyner–Halenda(BJH)analysisfromthedesorptionbranchesofthenitrogen
isotherms. X-ray diffraction (XRD) (Rigaku Corp., Tokyo, Japan) patterns were collected using
a Rigaku D/Max-2200 PC X-ray diffractometer with a Cu target (40 kV, 40 mA). CO and NH
2 3
temperature-programmed desorption (CO -TPD and NH -TPD) profiles of the (supported) LDH
2 3
sampleswereobtainedusingaQuantachromeChemBETPulsarequippedwithathermalconductivity
detector(TCD)(QuantachromeInstruments,BoyntonBeach,FL,USA).A100mgsamplewascalcined
for2hat773Kunderflowinghelium,andthencooledto323KtoadsorbNH orCO . Thephysical
3 2
adsorbedCO waspurgedbyflowingheliumat323K,andNH -andCO -TPDwassubsequently
2 3 2
carriedoutattherateof10Kmin−1upto1073K.TheMg/AlandMg/Al/Simoleratiosoftheprepared
seriesofSA/LDHcompositeswereassessedbysemiquantitativeEDSanalysis. Mgcontentsinfresh
andusedSA/LDH-Mg Al-80andtheMg/AlmoleratioofunsupportedLDHweredeterminedusing
2
inductivelycoupledplasmaatomicemissionspectroscopy(LeemanProdigyXPICP-AESspectrometer)
(TeledyneLeemanLabs,Hudson,NH,USA).
2.4. CatalyticEvaluation
The Henry reaction of benzaldehyde with nitromethane was performed in liquid phase
under atmospheric pressure to evaluate the catalytic properties of the SA/LDH composite series.
Benzaldehyde(1mmol),o-xylene(0.1g,internalstandard),nitromethane(5mL),catalyst(50mg),
and a magnetic stir bar were placed into a 50 mL Schott bottle. The bottle was then sealed and
put into a magnetic stirring bath (equipped with digital heating). The reaction started when the
temperature reached 80 ◦C. After stirring at 400 rpm for 6 h, the reaction was stopped by cooling
the bottle to room temperature. The solid catalyst was separated from the reaction product by
centrifugation. The reaction product was analyzed by a Shimadzu GC2010 gas chromatograph
(GC) (Shimadzu Corp., Kyoto, Japan) equipped with a flame ignition detector (FID), an AOC-20I
autosampler,andacapillarycolumn(Rtx-5,30m×0.25mm×0.25µm,crossbond5%diphenyl/95%
dimethylpolysiloxane). For6successivecyclesofreactions,theusedcatalystwaswashedwithethanol
orwatertopartlyremovethecarbonaceousproductsadsorbedandthenvacuum-driedforreusein
thefirstfivecyclesandcalcinedat823Ktothoroughlyremovethecarbonaceousproductsforreuse
inthesixth(last)cycle. ToevaluatetheeffectofsolventsonthecatalyticpropertiesoftheSA/LDH
catalyst,benzaldehyde(1mmol),o-xylene(0.1g,internalstandard),nitromethane(5mmol),solvent
(4.73mL),andcatalyst(50mg)wereadopted.
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3. ResultsandDiscussion
3. Results and Discussion
ThesynthesisofSA-supportedLDHnanosheetsisdescribedinScheme1. Briefly,asmallamount
The synthesis of SA-supported LDH nanosheets is described in Scheme 1. Briefly, a small
ofmixedsolutionA(Mg(NO ) /Al(NO ) )wasfirstaddedtotheSAsuspension,inwhichprocess
amount of mixed solution A 3(M2 g(NO3)23/A3l(NO3)3) was first added to the SA suspension, in which
theMg2+ andAl3+ wereadsorbedontothenegativelychargedinnerandoutersurfacesoftheSA.
process the Mg2+ and Al3+ were adsorbed onto the negatively charged inner and outer surfaces of
ThensolutionB(NaOH/NaHCO )wasaddedtoprecipitatetheMg2+andAl3+adsorbedinsituand
the SA. Then solution B (NaOH/N3 aHCO3) was added to precipitate the Mg2+ and Al3+ adsorbed in
formLDHprecursor(Pre-LDH)uniformlyonthesurfaceoftheSA.Thereafter,alargeamountofmixed
situ and form LDH precursor (Pre-LDH) uniformly on the surface of the SA. Thereafter, a large
solutionAandacalculatedamountofsolutionBwereaddedalternatelytomaintainthesolutionpH
amount of mixed solution A and a calculated amount of solution B were added alternately to
atacertainvalue. Finally,theSA/LDHcompositeformedthroughhydrothermaltreatmentofthe
maintain the solution pH at a certain value. Finally, the SA/LDH composite formed through
obtainedSA-supportedPre-LDH.
hydrothermal treatment of the obtained SA-supported Pre-LDH.
Small amount of
Mg2+ and Al3+
Mg2+
Al3+
SA SA/Mg2+-Al3+ SA/LDH with smaller
size
Mg2(SiO4)
Hydrothermal
Alternate addition
treatment
Large amount of High temperature
Mg2+ and Al3+;
NaOH/NaHCO
3
SA/Pre-LDH SA/Pre-LDH SA/LDH

SScchheemmee 11.. AAs cshcehmemataictiicl luilslutrsattriaotnioonf tohfe tfhaeb rficaabtriiocnatoiofns iloicfa saileircoag eale(rSoAge)-l s(uSpAp)-o rstuedpplaoyreterded ldayoeurbelde
hdyodurbolex ihdyed(rLoDxHid)en (aLnDoHsh) eneatns.osheets.
The morphology of initial template SA and the prepared SA/LDH composite were
ThemorphologyofinitialtemplateSAandthepreparedSA/LDHcompositewerecharacterized
characterized by TEM. Typically, SA shows disordered pore structure (Figure 1a) and low TEM
byTEM.Typically,SAshowsdisorderedporestructure(Figure1a)andlowTEMcontrastduetoits
contrast due to its low density and amorphous structure. In the current SA/LDH composite series
lowdensityandamorphousstructure. InthecurrentSA/LDHcompositeseries(Figure1b–f),alarge
(Figure 1b–f), a large amount of LDH nanosheets was observed located at the inner and outer
amountofLDHnanosheetswasobservedlocatedattheinnerandoutersurfacesoftheSAsubstrate.
TsuhrefaScAe/s LoDf Hthsee rSieAs ssyunbtshterasitzee. dTahtea SlAow/LtDemHp seerraiteusr esyonft8h0e◦sCizeadll saht oaw leodws mteamllpLeDrHaturraed ioafl d8i0a m°Cet earlsl
showed small LDH radial diameters and the average radial diameters were ca 19.2 nm, 31.4 nm,
and the average radial diameters were ca 19.2 nm, 31.4 nm, and 35.8 nm for SA/LDH-Mg Al-80,
2
SanAd/ L3D5.H8 -Mnmg Afolr- 8S0A, /aLnDdHS-MA/gL2ADlH-8-0M, gSAA/Ll-D80H,-rMesgp5Aeclt2i-v8e0l, ya(nTda bSleA1/LDanHd-MFigg3uArle-8S01, ).resHpeocwtievveelyr,
5 2 3
(Table 1 and Figure S1). However, the radial diameters of LDH increased to ~53.0 and 57.4 nm for
the radial diameters of LDH increased to ~53.0 and 57.4 nm for the SA/LDH-Mg Al-105 and
2
SthAe/ SLAD/HLD-MHg-MAgl-21A5l0-1,0w5h aicnhdw SeAre/LpDreHp-aMregd2Aatl-h1i5g0h, ewrtheimchp ewreartue rpesreopfa1r0e5d ◦aCt ahnigdh1e5r0 t◦eCm,preersapteucrteivse olyf.
2
105 °C and 150 °C, respectively. The same trend was found in the thickness change of the SA/LDH
The same trend was found in the thickness change of the SA/LDH series (Table 1 and Figure S2).
series (Table 1 and Figure S2). The thicknesses were 3.2 nm, 3.2 nm, and 3.6 nm for
The thicknesses were 3.2 nm, 3.2 nm, and 3.6 nm for SA/LDH-Mg Al-80, SA/LDH-Mg Al -80,
2 5 2
aSnAd/LSDAH/-LMDgH2A-Ml-g80A, Sl-A80/L,DreHsp-eMctgi5vAell2y-,8a0n, danindc rSeAa/sLeDdHto-M4.5g3aAnld-840.,3 rnemsp,efcotrivSeAly/, LaDnHd -iMncgreAasle-1d0 t5oa 4n.d5
3 2
SanAd/ L4.D3 Hn-mM, gfoAr Sl-A15/L0,DrHes-pMegct2iAvle-l1y0.5I tasnhdo SuAld/LbDeHno-Mtegd2Athla-1t5t0h,e rSesAp/eLctDivHelyco. mIt pshoosiutelds ybne tnhoesteizde dthaatt
2
the SA/LDH composite synthesized at higher pH values such as 9.5 showed significant destruction
higherpHvaluessuchas9.5showedsignificantdestructionofSAstructureduetothestrongalkali
of SA structure due to the strong alkali corrosion in the preparation. Therefore, the impact of pH
corrosioninthepreparation. Therefore,theimpactofpHvaluesonthestructureofthecompositeis
values on the structure of the composite is not discussed in this work. EDS mappings of the
notdiscussedinthiswork. EDSmappingsofthepreparedSA/LDHcomposites(FigureS3)showed
prepared SA/LDH composites (Figure S3) showed an almost complete overlap of the Si, Mg, Al, O
analmostcompleteoverlapoftheSi,Mg,Al,Oelementalmappings,furtherdemonstratingthatthe
elemental mappings, further demonstrating that the supported LDH nanosheets were uniformly
supported LDH nanosheets were uniformly dispersed on the SA substrate. However, the Mg/Al
dispersed on the SA substrate. However, the Mg/Al ratios in the obtained composites were not
ratiosintheobtainedcompositeswerenotconsistentwiththoseinprecursorsolutions(Table1)as
consistent with those in precursor solutions (Table 1) as only a fraction of the Mg(II) ions
only a fraction of the Mg(II) ions precipitated at the synthetic pH of 8.8 due to the high solubility
pprroecdiupcittacteodn satta tnhte( sKynt=he5t.i1c p×H1 o0f− 812.8) doufeM tog (tOheH h)igwh hsoilleuAbill(iItyII )parolmduocstt ccoonmstpalnett e(lKyspp =r e5c.1ip ×i t1a0t−e1d2) ionf
sp 2
tMheg(fOorHm)2 owfhAille( OAHl()III)w ailtmhoastl ocwomKplevteallyu peroefci1p.3ita×ted10 i−n3 t3hue nfdoremr t ohfe Asal(mOeHc)3o wnditihti ao nlosw. IKnspa vdadluiteio onf,
3 sp
t1h.3e ×M 1g0/−3A3 ulnmdoelre trhaet isoasmoef ScAon/dLiDtioHn-sM. Ign Aadl-d1i0t5ioann, dthSeA M/Lg/DAHl -mMogle Aral-t1io5s0 owf eSrAe/bLoDthHh-Miggh2eArlt-h1a0n5 tahnadt
2 2
SA/LDH-Mg2Al-150 were both higher than that of SA/LDH-Mg2Al-80 which is attributed to the
Nanomaterials2018,8,113 5of14
Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 15
of SA/LDH-Mg Al-80 which is attributed to the formation of magnesium silicate under the high
formation of m2agnesium silicate under the high synthetic temperatures as proven by the
synthetictemperaturesasprovenbythesubsequentXRDanalysis.
subsequent XRD analysis.
TheXRDpatternofthefabricatedSA/LDHcomposites(Figure2)allshowedtypicalbutweak
The XRD pattern of the fabricated SA/LDH composites (Figure 2) all showed typical but weak
LDHdiffractionpeaks(StandardPDFcard: #89-0460),whichcanbeindexedtothe(003),(006),(012),
LDH diffraction peaks (Standard PDF card: #89-0460), which can be indexed to the (003), (006), (012),
and(110)planes. FortheSA/LDHpreparedatahighsynthetictemperatureof105◦Cand150◦C,
and (110) planes. For the SA/LDH prepared at a high synthetic temperature of 105 °C and 150 °C,
thestrongestreflectionpeakofmagnesiumsilicateindexedtothe(101)plane(StandardPDFcard:
the strongest reflection peak of magnesium silicate indexed to the (101) plane (Standard PDF card:
#87-2037)appeared,whichisprobablybecauseSiO 2− derivedfromthepartialdissolutionofSAinan
#87-2037) appeared, which is probably because Si3O32− derived from the partial dissolution of SA in
alkalinepreparationsolutionofSA/LDHtendstoreactwithunprecipitatedMg(II)ionsintheLDH
an alkaline preparation solution of SA/LDH tends to react with unprecipitated Mg(II) ions in the
precursorathighsynthetictemperatures.
LDH precursor at high synthetic temperatures.
Figure3andFigureS4showtheN adsorption–desorptionisothermsoftheSA/LDHcomposites,
Figures 3 and S4 show the N2 adso2rption–desorption isotherms of the SA/LDH composites, SA
SAandtheunsupportedLDH,respectively. AllSA/LDHcompositesdisplayedatypeIVisotherm
and the unsupported LDH, respectively. All SA/LDH composites displayed a type IV isotherm with
with H hysteresis and a sharp increase in volume adsorbed at P/P ≈ 0.6 to 0.7, which indicates
H1 hyst1eresis and a sharp increase in volume adsorbed at P/P0 ≈ 0.06 to 0.7, which indicates that
thatSA/LDHcompositepartiallyretainedthemesoporousstructureoftheSA.TheBETsurfacearea
SA/LDH composite partially retained the mesoporous structure of the SA. The BET surface area
(S ), pore volume (V ), and pore diameter (D ) of the relative samples determined via nitrogen
(SBBEETT), pore volume (Vpp), and pore diameter (Dpp) of the relative samples determined via nitrogen
adsorption–desorption measurements are listed in Table 1. All SA/LDH samples showed a high
adsorption–desorption measurements are listed in Table 1. All SA/LDH samples showed a high
surfaceareaofmorethan400m2·g,alargeporevolumeofgreaterthan0.90cm3·g,andporediameters
surface area of more than 400 m2·g, a large pore volume of greater than 0.90 cm3·g, and pore
larger than 9 nm. SA/LDH-Mg Al-80 possesses an obviously higher surface area than the other
diameters larger than 9 nm. SA/L2DH-Mg2Al-80 possesses an obviously higher surface area than the
SA/LDHsamples,andthesurfaceareaofthesamplesdecreasedwithincreasingsynthetictemperature.
other SA/LDH samples, and the surface area of the samples decreased with increasing synthetic
Asexpected,thesurfacearea,porevolume,andporediameteroftheSA/LDHseriesalldecreasedin
temperature. As expected, the surface area, pore volume, and pore diameter of the SA/LDH series
comparisontothesubstrateSAduetothepartialblockageofSAporechannelsbyLDHnanosheets
all decreased in comparison to the substrate SA due to the partial blockage of SA pore channels by
andtheinherentlyhigherdensityoftheLDHthanthatoftheporousSAinthecomposite. Onthebasis
LDH nanosheets and the inherently higher density of the LDH than that of the porous SA in the
oftheresultsinTable1,itisconcludedthattheSA/LDHcompositeswithsmallerLDHsizeshavethe
composite. On the basis of the results in Table 1, it is concluded that the SA/LDH composites with
highersurfacearea,whichisattributedtothesmallerLDHnanosheetsminimizingtheblockingofSA
smaller LDH sizes have the higher surface area, which is attributed to the smaller LDH nanosheets
porechannels.
minimizing the blocking of SA pore channels.

Figure 1. TEM images of SA (a); SA/LDH-Mg2Al-80 (b); SA/LDH-Mg5Al2-80 (c); SA/LDH-Mg3Al-80
Figure1.TEMimagesofSA(a);SA/LDH-Mg Al-80(b);SA/LDH-Mg Al -80(c);SA/LDH-Mg Al-80(d);
2 5 2 3
(d); SA/LDH-Mg2Al-105 (e); and SA/LDH-Mg2Al-150 (f).
SA/LDH-Mg Al-105(e);andSA/LDH-Mg Al-150(f).
2 2
Nanomaterials2018,8,113 6of14
NNaannoommaatteerriiaallss 22001188,, 88,, xx FFOORR PPEEEERR RREEVVIIEEWW 66 ooff 1155
Table1.TexturalpropertiesoftheSA/LDHcompositeandthecontrolsamples.
TTaabbllee 11.. TTeexxttuurraall pprrooppeerrttiieess ooff tthhee SSAA//LLDDHH ccoommppoossiittee aanndd tthhee ccoonnttrrooll ssaammpplleess..
SSSaaammmpppllleeesss (((mSmmSSBB2B22EEE···gTgTTg ))) (((cccmmmVVV333ppp·· ·ggg))) ((n(nDDnDmmmppp ))) DDD((nn(LLLnDmDmDHmH)H) a a ) a TTT((HHHnn(nLmLLmDDmDH)H)H ) bb b MMMRRRagagagtt//t/iAiAiAoooll l MMMRgRRgga/A/a/atAAittloiil/looS//S S iii
SSSAAA///LLLDDDHHH--MM-Mggg22AA2All--l88-080 0 555888777...444 111...333000 111111..99.9 11919.9.22. 2 333..22.2 111...222444 000...555888::0:00..4.44666::1:11
SSASAA//LL/DDLDHHH--MM-Mgg22gAA2lAl--11l-001550 5 444666000...555 000...999000 999..33.3 55353.3.00. 0 444..55.5 111...444666 000...666999::0:00..4.44777::1:11
SSASAA//LL/DDLDHHH--MM-Mgg22gAA2lAl--11l-551005 0 444222999...555 000...999666 999..00.0 55757.7.44. 4 444..33.3 111...555111 000...777777::0:00..5.55111::1:11
SSSAAA//LL/DLDDHHH--MM-Mgg5g5AA5All22--l828-008 0 444555666...000 000...999888 999..99.9 33131.1.44. 4 333..22.2 222...111000 000...888333::0:00..3.33999::1:11
SA/LDH-Mg Al-80 496.1 1.12 9.2 35.8 3.6 2.27 0.78:0.34:1
SSAA//LLDDHH--MMgg33AA3ll--8800 449966..11 11..1122 99..22 3355..88 33..66 22..2277 00..7788::00..3344::11
SA 803.8 5.92 34.4 — — — —
SSAA 880033..88 55..9922 3344..44 —— —— —— ——
LDH-Mg Al-80 120.6 0.69 21.2 87.1 <20 1.94 —
2
LLDDHH--MMgg22AAll--8800 112200..66 00..6699 2211..22 8877..11 <<2200 11..9944 ——
aTheradialdiameterofLDH;bThethicknessofLDH.
aa TThhee rraaddiiaall ddiiaammeetteerr ooff LLDDHH;; bb TThhee tthhiicckknneessss ooff LLDDHH..

FFFiiiggguuurrreee 222... X XXRRRDDDp ppaatatttettreenrrnnssso fooSff ASSAA/L//LLDDDHHH-M--MMggg22A2AAll-l-8-88000( a((aa);));;S ASSAA///LLLDDDHHH---MMMggg222AAAlll---111000555 (((bbb)));;; S SSAAA///LLLDDDHHH---MMMggg222AAAlll---111555000 (((ccc)));;;
SSSAAA///LLLDDDHHH---MMMggg555AAAlll222--8-88000 ((d(dd));); ;aaannnddd SSSAAA//L/LDLDDHHH--MM-Mgg3g3AA3All--88l-008 0((ee())e.. ).

FFFiiiggguuurrreee 3.33.. NNN22 aaadddsssooorrrppptttiiiooonnn–––dddeeesssooorrrppptttiiiooonnn isiisosoothtthheeerrmrmmsss ofooffS ASS/AAL//DLLDDHHH-M--MMgggA22AAl-ll8--88000,,, SASSAA//L/LLDDDHHH---MMMggg22AAAlll---111000555,,,
2 2 2
SSSAAA///LLLDDDHHH---MMMggg22AAAlll---111555000,,, SSSAAA///LLLDDDHHH--MM-Mggg55AAAll22-l-88-008,,0 aa,nnaddn dSSAASA//LL/DDLHHD--HMM-ggM33AAgll--A880l0-..8 0.
2 5 2 3
TToo eevvaalluuaattee tthhee bbaassiicciittyy ooff tthhee SSAA//LLDDHH sseerriieess,, CCOO22--TTPPDD mmeeaassuurreemmeennttss wweerree ccoonndduucctteedd.. IInn
To evaluate the basicity of the SA/LDH series, CO -TPD measurements were conducted.
ggeenneerraall,, aaccttiivvaatteedd LLDDHHss ppoosssseessss wweeaakkllyy bbaassiicc ssiitteess,, mmooddeerraa2tteellyy bbaassiicc ssiitteess,, aanndd ssttrroonnggllyy bbaassiicc ssiitteess,,
Ingeneral,activatedLDHspossessweaklybasicsites,moderatelybasicsites,andstronglybasicsites,
aassccrriibbeedd ttoo tthhee ssuurrffaaccee hhyyddrrooxxyyll ggrroouupp,, mmeettaall––ooxxyyggeenn ppaaiirrss ((ee..gg..,, MMgg22++––OO22)),, aanndd llooww--ccoooorrddiinnaattiioonn
ascribedtothesurfacehydroxylgroup,metal–oxygenpairs(e.g.,Mg2+–O ),andlow-coordination
ooxxyyggeenn aanniioonnss,, rreessppeeccttiivveellyy [[5544]].. FFoorr CCOO22--TTPPDD,, tthhee bbaassiicc ssiitteess iinn tthhee hhiigghh2 ddeessoorrppttiioonn tteemmppeerraattuurree
rraannggee aarree uussuuaallllyy ddeessiiggnnaatteedd aass ssttrroonngg bbaassiicc ssiitteess,, aanndd vviiccee vveerrssaa [[5555]].. AAss sshhoowwnn iinn FFiigguurree 44 aanndd
TTaabbllee 22,, tthhee SSAA//LLDDHH sseerriieess sshhoowweedd aa bbrrooaadd CCOO22 ddeessoorrppttiioonn bbaanndd ffrroomm 223300 ttoo 779900 °°CC wwiitthh tthhee
Nanomaterials2018,8,113 7of14
oNxaynogmeanteraianlsi o20n1s8,, r8e, xs pFOecRt iPvEeElRy R[E5V4I]E.WF o rCO2-TPD,thebasicsitesinthehighdesorptiontempera7t oufr 1e5
rangeareusuallydesignatedasstrongbasicsites,andviceversa[55].AsshowninFigure4andTable2,
thhieghSAes/t LpDeaHk sienr itehses hhoigwhe dteambpreoraadtuCrOe radnegseo ropft i6o0n5b.7a ntod 6fr4o9m.3 °2C30 atnod7 9a0 b◦rCoawdi tshhtohueldheigr hpeesatkp einak thine
2
tlhoewh tigemhpteemraptuerraet urarnegrea nogfe 4o1f76.60 5t.o7 4to586.459 °.C3,◦ Cdeamnodnastbrarotiandg sthhoaut ltdheer spuerafkacien bthaesilco wstrteenmgtphe roaft uthree
rcaonmgepoosfi4te1s7 .w6taos 4w58id.5el◦yC ,ddisetmribountsetrda tainngd tthhaet tShAe/sLuDrfHac ecobmaspicossittreesn gptohssoefstsheedc ommapinolsyi tmesowdaesrawtei daenlyd
dstisrtornibgu bteadsea nsditetsh eoSriAg/inLaDtiHngc ofrmompo stihtees Op2o−s saensisoends manaidn lsyumrfoacdee rMateg–aOnd2− sptraoirnsg obfa sLeDsHite. sTohreig hiniagthinesgt
fdroesmortphteioOn 2−peaanki opnossaitniodns uorf faScAe/LMDgH–O-M2−g2pAali-r8s0,o fSLAD/LHD.HT-hMegh5iAghl2e-8s0t,d aensodr pStAio/nLDpHea-kMpgo3Asilt-i8o0n aolfl
SsAhi/ftLeDd Ht-oM ghigAhle-8r0 ,tSemA/pLerDaHtu-rMesg cAolm-p80a,readn dwSAith/ LSDAH/L-MDgH-AMl-g820Aall-l1s0h5i,f teSdAt/oLDhiHgh-Mergt2eAml-p1e5r0a,t uarneds
2 5 2 3
cLoDmHp-aMregd2Awli-t8h0,S Ain/dLicDatHin-Mg gthaAt l-t1h0e5 ,SSAA/L/DLDHH s-aMmgplAesl -s1y5n0,thaensdizLeDd Hat- MlogwA tle-m80p,einradtiucraet inhgavthe amttohree
2 2 2
SsAtr/onLgDlHy sbaamsipc lseistessy ntthhaens iztheodsaet lsoywnttheemsipzeerda tautr ehhigahv etemmopreersatrtounrgesly abnads icthseit eusntshuapnpthoortseeds yLnDthHes wizeitdh
alatrhgieg hsiztee manpde rsaotumree spaanrtdiclteh eagugnrseugpatpioonrt e(FdigLuDreH Sw5)i. thIn laardgdeitsioizne, aalnl dsasmompleesp sahrotiwcleeda ag grreelgataitvieolny
(wFiegaukr epSe5a)k. Iinn athded itteimonp,earlaltsuarme pralensgseh 1o2w1e.5d tao r1e6la0t.i6v °eCly, cwoerareksppeoankdiinngt hteo twemeapkelrya tbuarseicr asnitgees 1d2e1r.i5vetod
1f6ro0.m6 ◦tChe, cOorHre- sgproonudpisn ogft tohwe eLaDkHly. bTahseic tsoittaels pdeearkiv aerdefar oomf CtOhe2-OTPHD- gsriogunpalsso ifs tlhineeLaDrlHy .pTrohpeotorttiaolnpaela tko
athreea aomfCouOnt- ToPf DCOsig2 naadlssoirsbleinde aarnldy pcaronp tohrutiso nbael utosethde aasm ao suenmtio-fqCuaOntiatdatsiovreb epdaraanmdectaenr ftohru sasbseesussiendg
2 2
the number of basic sites of the catalysts. The basic site numbers of the samples are ranked as
asasemi-quantitativeparameterforassessingthenumberofbasicsitesofthecatalysts. Thebasicsite
nfoulmlobwesr s(Tofabthlee 2s)a:m SpAl/eLsDaHre-rMangk5Aedl2-a8s0f o> lSloAw/LsD(THab-Mleg22)A: Sl-A80/ L>D SHA-/MLDgHA-Ml -g830A>l-8S0A >/ LLDDHH--MMgg2AAll--8800 >>
5 2 2
SSAA//LLDDHH--MMgg2AAll--18005> >L DSAH/-LMDgHA-Ml-g802A>l-S1A50/.L TDhHe -hMigghAerl -b10a5si>c SsiAte/ LnDumHb-Merg oAf Sl-A15/L0.DTHhe-Mhigg5hAelr2-b8a0s iicn
3 2 2 2
sciotemnpuamribsoern otfoS tAha/tL oDfH S-AM/LgDAHl-M-80g2iAnlc-o8m0, pwahriiscohn htaost haa stmofalSlAer/ LLDDHH -sMizge aAnl-d8 0a, hwighhicehr hsausrfaacsme aarlleear,
5 2 2
may be due to the higher Mg content of the former than the latter. This discrepancy could also
LDHsizeandahighersurfacearea,maybeduetothehigherMgcontentoftheformerthanthelatter.
explain the higher observed basic site density of the unsupported LDH in comparison to those of
ThisdiscrepancycouldalsoexplainthehigherobservedbasicsitedensityoftheunsupportedLDHin
ctohme pSAar/iLsoDnHt-oMthgo2Asel-o10f5th aenSdA S/AL/DLHD-HM-MggA2Al-1l-01550a nwditShA g/rLeDatHer- MsugrfAacle-1 a5r0ewa iatnhdg rsemataelrlesru LrfDacHe asrizeea.
2 2
From these results, it is concluded that the basicity of LDH in this system is enhanced by multiple
andsmallerLDHsize. Fromtheseresults,itisconcludedthatthebasicityofLDHinthissystemis
factors including small LDH size, high surface area, and high Mg content.
enhancedbymultiplefactorsincludingsmallLDHsize,highsurfacearea,andhighMgcontent.

FFigiguurree4 4..C COO2--TTPPDD pprroofifilleess ooff SSAA//LLDDHH sseerriieess aanndd tthhee uunnssuuppppoorrtteedd LLDDHH nnaannoosshheeeettss..
2
NanomNaatneormiaalst e2r0ia1l8s,2 80,1 x8 ,F8O,R11 P3EER REVIEW 8 of 185 of14
Table 2. The semi-quantitative results of CO2-TPD measurements. .
Table2.Thesemi-quantitativeresultsofCO -TPDmeasurements.
2
CO2-TPD Peak Position (°C) Total Peak
Samples
Samples I CO2-TPDIPIeakPosition(◦C)III TotalPAeraeka (a.u.) a
SA/LDH-Mg2Al-80 160.6 I 455.6I I II6I49.3 Area(a.u.8)6a188.7
SA/LDH-Mg2ASl-A1/0L5 DH-Mg A1l3-810.3 160.6 430.465 5.6 6496.312.0 86188.768865.8
2
SA/LDH-Mg2ASlA-1/5L0D H-Mg A1l-4120.59 131.3 417.463 0.6 6126.005.7 68865.843227.1
2
SA/LDH-Mg5ASlA2-/8L0D H-Mg2A1l-4105.02 142.9 431.481 7.6 6056.743.1 43227.192283.9
SA/LDH-Mg3ASAl-/80L DH-Mg5A1l24-38.01 140.2 458.453 1.8 6436.128.9 92283.974727.8
SA/LDH-Mg Al-80 143.1 458.5 628.9 74727.8
LDH-Mg2Al-80 3 121.5 376.7 616.5 69046.4
LDH-Mg Al-80 121.5 376.7 616.5 69046.4
2
a Total peaaTko taalrepae aiks alirneeaaisrllyin pearrolpyoprrtoipoonratilo tnoa tlhtoe tahmeaomuonut notf oCfOCO2 a2dadsosorbrbeedd..
The base-catalyzed Henry reaction of conversion of benzaldehyde to
trans-1-phTehneyl-2-bnaistreo-c-eatthaalynzoeld and Htreannrsy-1-nitrreoa-c2t-ipohnenyloefthylecnoen v(Seprseicoines 1, o2f, andb e3n zinal dEeqhuyadtieon to
(1), trreasnpse-c1t-ipvheelyn)y lw-2a-sn ittarok-eenth aasn oal manoddterla nres-a1c-tnioitnro t-o2 -pevhaelnuyalteet htyhlee nbeas(Sicp eccaiteasly1t,i2c ,parnodp3eritnieEs qoufa ttihoen (1),
SA/LreDsHpe cctoimveplyo)siwteass. tTahkee nsoalsveanmt opdlaeylsr eaa cstigionniftiocaenvta rloulaet einth ceatbaalsyiscisc arteaalcyttiiocnpsr, ospuecrhti eass odfisthsoeluSAtio/nL DH
of recaocmtapnotssi taens.dT hperosdoulvcetsn,t apdlajuysstainsgig tnhiefi cianntterraocletioinn coatfa lryesaicstarenatsc tiaonnds ,psurochduacstsd iwssiotlhu ttihoen osufrrefaacceta nts
catalayntdic psriotedsu ocft sc,aatdajluyssttisn gantdh eaicnttiveraaticntigo nreoafctraenactst aanntsd ainndteprmroedduiacttessw. iTthhet hceatsaulyrftaicc epcraotpaelyrttiiecss iotfe sof
SA/LcDatHal-yMstgs2aAnld-8a0c twiviathti ntgher esamctaalnletssta nLdDiHnt esrimzee dinia tdeisf.feTrheentc astoallvyetinctsp rwopeerert igeisvoefnS Ain/ LTDabHle- M3.g 2IAn l-80
genewraitlh, tthhee rsemlaatilvleeslyt LhDigHh csoizneveinrsdioinffse rwenetres oolbvteanintsedw ienr ethgei vseonlvienntTsa wblieth3 .reIlnatigveen wereaal,kt hpeolraerliatyti,v ely
suchh aigsh ncitornovmeertshioannse,w deicrheloobrotamineethdainne,t haendso tlovleunetnsew, withhirlee ltahtiev seowlveeanktsp woliathri tsyt,rosnugch paoslanriittyro, msuecthh ane,
as etdhiacnholol,r oTmHeFt, hDaMneF,, aanndd wtoaltueern, eg,avwe hloilwe tchoenvseorlsvieonntss. Twhiitsh trsetnrodn wgaps osilmariiltayr, tsou tchhata fsouenthda inno tl,heT HF,
reacDtioMnF o,fa nnditrwobaetnerz,agldaevheyldoew wciothn vneirtsrioomnse.thTanheis [5tr6e]n adnwd acsousilmd iblaer attotritbhuattefdo uans dthien rtehaesornea tchtiaotn of
protnici tprooblaern szoallvdeenhtys dceomwpitehtendi twroimthe tthhea nneitr[o5m6]eathnadnceo (uplKdab =e 1a0tt.2ri1b)u ftoerd thaes stuhrefarecea sboansitch saittepsr ooft itchpe olar
catalsyosltv, ernetdsuccoinmgp tehteed nuwmitbhetrh oef nciatrtaolmyteicth saintees (apvKaaila=bl1e0 f.2o1r) thfoer rtehaectsiuonrf.a Wcehbeans incitsriotemseothfathnee wcaatas lyst,
usedr eadsu bciontgh tah esonluvmenbte arnodfc aa traelyatcitcasnitt,e sthaev ahiilgahbleestf ocrotnhveerresaicotnio onf. W96h.8e%n nwitarso mobettahianneedw oawsiunsge dtoa sthbeo tha
highseorl vceonntcaenndtraatrieoanc toafn tn,itthroemhiegthheasntec oannvde rtshiuons oa fh9i6g.8h%er wcoanstoabctta ipnreodbaobwiliintgy toof thneitrhoigmheetrhcaonnec ewnittrha tion
benzoaflndiethroymdee.t hInan aedadnidtiothnu, sthaeh aigdhsoerrpctoinotna cotfp trhoeb aybeillliotwyo pfrnoidtruocmt ettrhaanns-e1w-niitthrob-e2n-pzhaledneyhlyetdhey.lIennaed odnit ion,
the ctahteaalydssto wrpatsi ofnouonfdth feory ethlleo wreapcrtoiodnusc tintr aanlls s-1o-lnvietnrots-2 e-xpcheepnty floerth eythleanneool n(Ftihgeurcea tSa6ly).s tBwasaesdf ooun nthdefsoer the
resurletsa ctainodn stihnea llloswol veenntvsireoxncempetnfot rheathzaanrdol, (FniigtruormeSet6h).aBnea sewdaosn stheleesceterdes ualsts athned tshoelvloewnt enfovri rothnem ent
subsheaqzuaerndt, sntiutrdoimese. thanewasselectedasthesolventforthesubsequentstudies.
O OH
H Base cat. NO2 + NO2 (1)
+CH3NO2
1 2 3
Table 3. Henry reaction of benzaldehyde with nitromethane over SA/LDH-Mg2Al-80 in different
Table 3. Henry reaction of benzaldehyde with nitromethane over SA/LDH-Mg Al-80 in
solvents a. 2
differentsolventsa.
Entry Solvent Conversion of 1 (%) Selectivity to 2 (%) Selectivity to 3 (%)
1 EntryNitromethaSnoelv ent Co9n6.v8e rsionof1(%) Sel2e.c7t ivityto2(%) Sele9c7t.3iv ityto3(%)
2 Ethanol 28.4 11.7 88.3
1 Nitromethane 96.8 2.7 97.3
3 Dichloromethane 49.0 6.0 94.0
2 Ethanol 28.4 11.7 88.3
4 3 TolDueicnhel oromethane 41.6 49.0 5.0 6.0 95.0 9 4.0
5 4 DMF Toluene 5.9 41.6 15.8 5.0 84.2 9 5.0
6 5 Water DMF 16.7 5.9 32.3 15.8 67.7 8 4.2
7 6 THF Water 27.7 16.7 21.8 32.3 78.2 6 7.7
7 THF 27.7 21.8 78.2
a Reaction conditions: SA/LDH-Mg2Al-80, 50 mg; benzaldehyde, 1 mmol; o-xylene, 0.1 g;
nitroamReetahcatinone, c5o nmdimtioonl;s :sSoAlv/eLnDt,H 4-M.73g 2mAlL-8; 0t,im50em, g6; hb;e nrzeaalcdteiohnyd tee,m1pmemraotlu;or-ex, y8le0n °eC,0; .1SAg;/nLiDtroHm-Methga2nAel,-580m mol;
was saocltvivenatt,e4d.7 b3ym cLa;ltciimnea,ti6ohn; raeta 7ct2i3o nKt efmorp 2er hat ubreef,o8r0e ◦uCs;eS. A/LDH-Mg2Al-80wasactivatedbycalcinationat723K
for2hbeforeuse.
The catalytic properties of SA/LDH series catalysts with various textures are given in Table 4.
SA/LDH-Mg2Al-80, SA/LDH-Mg5Al2-80, and SA/LDH-Mg3Al-80 synthesized at a low temperature
all showed high reaction conversions of 96.8%, 98.2%, and 92.7%, respectively, which are
significantly higher than those of SA/LDH-Mg2Al-105 (59.3%) and SA/LDH-Mg2Al-150 (51.9%)
Nanomaterials2018,8,113 9of14
ThecatalyticpropertiesofSA/LDHseriescatalystswithvarioustexturesaregiveninTable4.
SA/LDH-Mg Al-80,SA/LDH-Mg Al -80,andSA/LDH-Mg Al-80synthesizedatalowtemperature
2 5 2 3
allshowedhighreactionconversionsof96.8%,98.2%,and92.7%,respectively,whicharesignificantly
higherthanthoseofSA/LDH-Mg Al-105(59.3%)andSA/LDH-Mg Al-150(51.9%)synthesizedat
2 2
hightemperatures(Table4), demonstrating thatthe reactantconversions are positively correlated
withthebasicityofthecatalysts. ItshouldbenotedthattheSA/LDH-Mg Al-80hasaselectivityof
2
97.3%toproduct3,whichissignificantlyhigherthanthoseoftheSA/LDH-Mg Al -80(78.6%)and
5 2
SA/LDH-Mg Al-80(87.2%),whichcouldbeattributedtothehigherAlcontentofSA/LDH-Mg Al-80,
3 2
endowing it with higher acidic site density and acidic strength than SA/LDH-Mg Al -80 and
5 2
SA/LDH-Mg Al-80 as shown by NH -TPD (Figure S7 and Table S1). This characteristic would
3 3
promotethedehydrationoftrans-1-phenyl-2-nitro-ethanoltogivetrans-1-nitro-2-phenylethylene[57].
ThisreactiondidnotproceedwhenusingbareSAasacatalyst,asexpected,andtheunsupported
LDH-Mg Al-80 showed a reactant conversion of 75.9%, lower than SA/LDH-Mg Al-80, which is
2 2
attributedtothehigherbasicstrengthofSA/LDH-Mg Al-80thanLDH-Mg Al-80. Summarizingthe
2 2
resultsabove,theSA/LDHsampleswithhighbasicityallshowedhighreactantconversions(>90%)
in the model Henry reaction, indicating the favorable accessibility of the surface basic sites of
SA/LDHcomposite.
To examine the stability of the SA/LDH catalyst against deactivation, 6 successive cycles
of reactions of benzaldehyde with nitromethane over SA/LDH-Mg Al-80 were conducted using
2
nitromethaneasasolvent. Aftereachcycle,theusedcatalystwaswashedwithethanolorwaterto
removetheproductsadsorbed. Forthelastcycle, thecarbonaceousproductsontheusedcatalyst
werethoroughlyremovedbycalcinationbeforethereaction. Inthefirstfivecycles,theconversions
decreased continuously from 96.4% to 75% for the catalyst reused by ethanol washing treatment,
whilethosedeclinedsharplyto24.8%forthecatalystreusedbywaterwashingtreatment(Figure5).
ThecontentsoftheactivecomponentMgare21.9%and21.2%inthefreshcatalystandusedcatalyst
aftersixcycles(ethanolwashing),respectively,determinedbyICP-MS,demonstratinganegligible
leachingofMg2+ions. Therefore,itcouldbeconcludedthatcatalystdeactivationwasmainlyderived
from thecoveringof carbonaceousproductsonthe surfaceactivesites ofthecatalyst andethanol
washingcouldeffectivelyremovethecarbonaceousproductsadsorbedonthecatalystandprevent
thedeactivationofcatalysts,whichisconsistentwiththeaboveobservationofaweakadsorptionof
theyellowproductonthecatalystwhenusingethanolasasolution. Inthesixthcycle,aftercalcining
the used catalysts, the conversions restored from 75% to 87.7% and from 24.8% to 84.3% for the
ethanol-washingsampleandthewater-washingsample,respectively. Thehighselectivitiestoproduct
trans-1-nitro-2-phenylethylenewerekeptinallcyclesinspiteoftheobviouschangesoftheconversions.
Table4.HenryreactionofbenzaldehydewithnitromethaneoverSA/LDHseriescatalystsa.
Entry Catalystsb Conversionof1(%) Selectivityto2(%) Selectivityto3(%)
1 SA/LDH-Mg Al-80 96.8 2.7 97.3
2
2 SA/LDH-Mg Al-105 59.3 30.9 69.1
2
3 SA/LDH-Mg Al-150 51.9 28.1 71.9
2
4 SA/LDH-Mg Al -80 98.2 21.4 78.6
5 2
5 SA/LDH-Mg Al-80 92.7 12.8 87.2
3
6 SA Trace — Trace
7 LDH-Mg Al-80 75.9 13.6 86.4
2
8 None Trace — Trace
aReactionconditions:catalyst,50mg;benzaldehyde,1mmol;o-xylene,0.1g;nitromethane,5mL;time,6h;reaction
temperature,80◦C;bAllthecatalystswereactivatedbycalcinationat723Kfor2h.
Nanomaterials2018,8,113 10of14
Nanomaterials 2018, 8, x FOR PEER REVIEW 10 of 15

a %)100 b %)100
y ( y (
vit 80 vit 80
cti cti
e Conversion of 1 e
Sel 60 Selectivity to 2 Sel 60
nd Selectivity to 3 nd
n a 40 n a 40 Reuse afer washing with water Calcination
o o
si Reuse afer washing with ethanol Calcination si
er 20 er 20
v v
n n
o o
C 0 C 0
1 2 3 4 5 6 1 2 3 4 5 6
Cycle Number Cycle Number

FFiigguurree 55.. CCaattaallyyttiicc ccaappaacciittiieess ooff SSAA//LLDDHH-M-Mgg2A2Al-l8-08 0inin 66 ssuucccceessssivivee ccyyccleless ooff rreeaaccttiioonnss: :((aa)) TThhee uusseedd
ccaattaallyysstt wwaass rerefrferesshheedd bbyy etehtahnaonlo wl wasahsihnign gini ntheth feirsfitr s5t c5ycclyesc laensda ncdalccianlactiinoant iionn thine tshixeths icxythclec;y (cble) ;
T(bh)e Tuhseedu sceadtaclaytsatl ywsatsw raesfrreesfhreesdh ebdy bwyawteart ewrawshaisnhgin ign itnhteh feirfisrt s5t 5cyccylcelse sanandd ccaalclcininaatitoionn inin tthhee ssiixxtthh
ccyyccllee.. RReeaaccttiioonn ccoonndditiitoionns:s:c actaatlaylsyts,t5, 05m0 gm;gb;e nbzeanlzdaelhdyedhey,d1e,m 1m molm;oo-lx; yole-xnyel,e0n.1e,g 0;.n1i tgro; mniettrhoamneet,h5amneL, ;
5t immLe,; 6timh;er,e 6a cht;i orneatcetmiopne treamtupreer,a8t0u◦reC, .80 °C.
Various solid base catalysts have been employed for catalyzing the Henry reaction of
Various solid base catalysts have been employed for catalyzing the Henry reaction of
benzaldehyde with nitromethane, and different reaction conditions were adopted based on the
benzaldehyde with nitromethane, and different reaction conditions were adopted based on the
inherent characteristics of the special catalysts. Table 5 gives a rough catalytic property comparison
inherentcharacteristicsofthespecialcatalysts. Table5givesaroughcatalyticpropertycomparison
of the SA/LDH catalysts with other solid base catalysts regardless of the specific reaction conditions.
oftheSA/LDHcatalystswithothersolidbasecatalystsregardlessofthespecificreactionconditions.
The SA/LDH catalysts showed the highest conversion of 98.2%, which is higher than those of most
The SA/LDH catalysts showed the highest conversion of 98.2%, which is higher than those of
of the other inorganic catalysts and comparable to those of the catalysts containing organic base
most of the other inorganic catalysts and comparable to those of the catalysts containing organic
groups with high cost and low thermal stability. Furthermore, compared to other inorganic
base groups with high cost and low thermal stability. Furthermore, compared to other inorganic
catalysts, the optimized SA/LDH catalyst gave a higher selectivity of 97.3% to the main product
catalysts, the optimized SA/LDH catalyst gave a higher selectivity of 97.3% to the main product
trans-1-nitro-2-phenylethylene, which is an important chemical intermediate for slimicides and
trans-1-nitro-2-phenylethylene, which is an important chemical intermediate for slimicides and
dyes [57].
dyes[57].
TTaabbllee 55.. CCoommppaarriissoonn ooff ccaattaallyyttiicc ccaappaacciittiieess ooff SSAA//LLDDHH ccaattaalylysststs wwitithh ootthheerr ssoolliidd bbaassee ccaattaallyyssttss iinn
HHeennrryy rreeaaccttiioonn ooff bbeennzzaallddeehhyyddee wwiitthh nniittrroommeetthhaannee..
Conversion of Yield of 2 Yield of 3
EEnntrtyr y CCaattaallyyssttss Conv1e r(%sio) nof1 Y(i%el)d ao f2 Yie(%ld) oa f3 TTeemmpp.. ((◦°CC)) TTimimee (h(h) ) RReeff..
(%) (%)a (%)a
1 SA/LDH-Mg2Al-80 96.8 2.6 94.2 80 6 —
2 1 SSAA//LLDDHH-M-Mgg5A2Al2-l8-800 9986..28 212.0.6 7974..22 8800 66 ——
3 2 SA/LDHCa-MO g5Al2-80 6908..12 —21 .0 7—7.2 18002 126 [5—8]
4 3 MCgaOO 630.0.1 2.—1 —— 16052 61 2 [[5598]]
56 45 AMAMllg22gOOOO3 3 ——3—.0 3537127 .bb1 b ———— ——6—5 18126 2 [[[[66560090]]]]
7 6 ModifiedM MggO-Al LDH —— 9551 b b —— —— 0.58 [[6600]]
7 ModifiedMg-AlLDH — 95b — — 0.5 [60]
8 Tb-MOF-NH2 87.0 — — 90 24 [61]
8 TMATOb-HM iOntFe-rNcaHla2ted 87.0 — — 90 24 [61]
9 TMAOHintercalated 55.0 41.8 3.74 R.T. 6 [62]
9 layered silicate 55.0 41.8 3.74 R.T. 6 [62]
layeredsilicate
101 0 ssiilliiccaa--aalluummiinnaa--NNHH22 —— —— 9999..00 110000 66 [[6633]]
111 1 SSiiOO22--NNHH22 —— —— 3377..00 110000 66 [[6633]]
121 2 SSii--ZZrr--TTii//PPAAII--HHFFss--NNHH22 8800 101.04. 4 6699..66 5500 44 [[6644]]
131 3 MMgg--NNHHCC —— 747 4 —— 6600 1122 [[6655]]
DDiaiammininoo mmooddifiifieedd- -
141 4 9966 0.04.84 8 9955..5522 7700 22 [[6666]]
LLDDHH//ssiliilcicaatete ccoommppoossitiete
a YaieYlide l=d C=oCnovnevresrsioionn ×× SSeelleeccttiivviittyy;; bb IIssoolalatetdedy iyeiledl.d.
4. Conclusions
4. Conclusions
In this work, small-sized Mg–Al LDH nanosheets supported on silica aerogel with a large
Inthiswork,small-sizedMg–AlLDHnanosheetssupportedonsilicaaerogelwithalargeaverage
average pore diameter and large pore volume were successfully prepared by a one-pot
porediameterandlargeporevolumeweresuccessfullypreparedbyaone-potcoprecipitationmethod.
coprecipitation method. The impact of synthesis parameters on the textural properties of the
Theimpactofsynthesisparametersonthetexturalpropertiesoftheobtainedcompositewasevaluated.
obtained composite was evaluated. Low Mg/Al precursor ratios and low hydrothermal treatment
temperatures result in small LDH sizes and high surface area of the SA /LDH composite. The