高手话不多Washed colloidal suspensions of nanocrys-tals were dried overnight in polystyrene boats at 80°C,and then ground into a powder form for powder XRD,FT-IR,29Si MAS NMR spec-troscopy,TG analysis,ICP OES,and nitrogen adsorption–desorption measurements.Powder XRD(Siemens D-500di?ractometer using Cu K a radiation)was used to determine sample crystal-linity.Data was collected in step mode every0.02°for a1s duration.FT-IR(Bruker Equinox55) was used to analyze sample structures(0.5cmà1 resolution).FT-IR samples were prepared by mixing a small amount of washed zeolite Y pow-der with dried KBr and pressing into13mm di-ameter circular wafers using3metric tons of force for10s.
ICP OES analyses were used to determine the chemical formula of the zeolite Y nanocrystal.A Perkin Elmer3000DV inductively coupled plasma optical emission spectrophotometer was used to analyze samples for Al,Si and Na by dissolving washed zeolite Y powder samples in appropriate amounts of2wt%HNO3solution.In order to calculate the amount of TMA in the samples with ICP OES,the assumption was made that the ze-olite frameworkwas charge balanced.Therefore, every aluminum atom in the frameworkshould be balanced by a cation.The only cations in our system above trace concentrations were Na and TMA,so it was assumed that either a Na or TMA cation balanced each frameworkaluminum atom.
29Si Magic angle spinning solid-state NMR(29Si MAS NMR)spectra were acquired on a Bruker DSX400spectrometer(1H frequency400.13MHz, 29Si frequency79.495MHz)equipped with a4mm double resonance MAS probe.1H decoupling ?elds of83kHz were used along with a MAS of10 kHz.These analyses veri?ed the Si/Al ratio deter-mined by the ICP OES chemical analyses.The technique for performing the Si/Al ratio determi-nation is described thoroughly in the workof Thomas and Klinowski[33].
TG(Cahn D-200)analyses were carried out at 10°C/min in air and were used to observe water content and the decomposition of TMA in the zeolite Y framework.TMA content was deter-mined using TG analyses by calculating the weight lost from the sample between280and700°C.TG analyses were used to verify the TMA composition calculated from the ICP OES chemical analyses.
Suspended solids concentrations(grams sus-pended solids per gram synthesis sol)were deter-mined by?rst weighing the amount of synthesis sol being centrifuged as it was put in a clean poly-propylene centrifuge tube.Then the sample was centrifuged three times at48,500g for2h,and each time followed by decanting with DDI water. The washed material was then dried in a polysty-rene boat at80°C overnight and weighed.The suspended solid concentration in the synthesis sol could then be calculated.
Yield(g/g)is de?ned as the ratio of the weight of calcined zeolite to the dry weight of Al2O3and SO2in synthesis solution.The weight of zeolite was determined by subtracting the zeolite water and template content(from TG experiments)from the weight of zeolite crystals.
Particle number concentration in the synthesis solution was determined using the suspended sol-ids concentration(g suspended solids/g synthesis solution)and the mass median diameter in the synthesis solution determined using DLS.All par-ticles were assumed to be spherical.Suspended solid concentration was pided by the faujasite frameworkdensity1.9?10à21g/nm3and the av-erage particle volume in the solution to give the number of particles per gram of synthesis solution.
16 B.A.Holmberg et al./Microporous and Mesoporous Materials59(2003)13–28
Nitrogen adsorption–desorption measurements were carried out at)196°C on a Micromeritics ASAP2010instrument to determine the Bru-nauer–Emmett–Teller(BET)surface area and to estimate the mesopore size distribution using the Barrett–Joyner–Halenda(BJH)calculation proce-dure[34].Before each measurement,samples were evacuated overnight at100°C and<1l m Hg. Higher temperatures were not used so that oc-cluded template in the zeolite structure would not decompose or volatilize.The total pore volume, V total,was taken from the desorption branch of the isotherm at P i=P o?0:995assuming complete pore saturation.The volume of the micropores,V micro, was determined by the t-plot method.The volume of the mesopores,V meso,was estimated from the equation:V meso?V totalàV micro.
3.Results and discussion
3.1.Zeolite Y crystallization with additional TMABr
The molar ratios for TMAOHeQT,and TMABreRTwere varied independently in order to determine the e?ect of the ratios(TMA)2O/Al2O3 and TMABr/TMAOHeSTon crystal size.In the ?rst investigation,Q was?xed at2.39[22],and R ratios of0,0.6and1.2were evaluated.Detailed molar compositions are shown in Table1.These synthesis batches were referred to as T2.4,T3.0 and T3.6based on their respective(TMA)2/Al2O3 ratios.
The XRD patterns of batches T2.4,T3.0and T3.6were taken over time to investigate the e?ect of additional TMABr on crystallization,though only data on T2.4and T3.6are presented(Fig.2). Crystalline material emerges in Batch T2.4near42 h of crystallization at100°C,and its crystallinity increases with increasing crystallization time(Fig. 2A).The crystalline material of batch T3.6emer-ges between42and54h crystallization at100°C, and its crystallinity does not increase with crys-tallization times longer than54h(Fig.2B).XRD patterns of batch T3.6and T2.4possess identical peakpositions,indicating that the addition of TMABr in the synthesis has not a?ected the crystallinity of the zeolite Y nanocrystals.
TEM images of products from batches T2.4and T3.6after75h crystallization both contain well-formed nanocrystals(Fig.3)with the lattice spacing of14 A attributed to the(111)planes of the FAU structure[35].The TEM image at higher magni?cation(Fig.3C)clearly indicates that the nanocrystals of Batch T3.6are highly crystalline despite the addition of TMABr in the synthesis solution.
FT-IR spectra for washed samples from batches T2.4and T3.6are shown in Fig.4.Peakposition and height are nearly identical for the two samples. The peakat460cmà1is assigned to the structure insensitive internal TO4(T?Si or Al)tetrahedra bending peakof zeolite Y.The peakat565cmà1is attributed to the double ring external linkage peak assigned to zeolite Y in literature.The peaks at685 and775cmà1are assigned to external linkage symmetrical stretching and internal tetrahedra symmetrical stretching respectively.The peaks at 1010and1080cmà1are assigned to internal tet-rahedra asymmetrical stretching and external link-age asymmetrical stretching respectively.Overall, the FT-IR spectra match well with the character-istic zeolite Y absorption peaks laid out in past literature[36].
Table1
Synthesis conditions for batches with increasing TMABr con-tent and DLS and TEM sizing data after75-h synthesis dura-tion
Control25%more
(TMA)2O/
Al2O3
50%more
(TMA)2O/
Al2O3 Batch T2.4T3.0T3.6
Q,(TMA)2O{2OHà}/
Al2O3
2.39 2.39 2.39
R,(TMA)2O{2Brà}/
Al2O3
0.000.60 1.19 (TMA)2O/Al2O3 2.39 2.99 3.58
S,TMABr/TMAOH0.000.250.50
OHàcharged(mol/L) 1.08 1.08 1.08
Mass median
diameter at75h(nm)
564534 Average TEM size at
75h(nm)
51–32
Mass median diameter
at216h(nm)
120–37
B.A.Holmberg et al./Microporous and Mesoporous Materials59(2003)13–2817
3.2.E?ect of amount of additional TMABr on crystal size
Fig.5A shows the mass median diameter and suspended solids concentration in T2.4and T3.6 with respect to time.After the three-day aging period,both T2.4and T3.6had gel particle sizes of 17nm at a concentration of about0.0400g sus-pended solids/g sol.For the?rst42h,T2.4and T3.6mass median diameter and suspended solid concentrations behaved similarly.As the crystal-lization time increased,the concentration of sus-pended solids decreased sharply to less than0.0040 g suspended solids/g sol in both batches at42h crystallization.Beyond the?rst42h,T2.4and T3.6crystallized very di?erently.Near42h the particles in T2.4began to crystallize(Fig.2A). Crystals in T2.4after54h(39nm,0.0040g sus-pended solids/g sol),continued to grow with crystallization until sampling used up the
synthesis
solution.The ?nal observed particle size at 216h was 120nm with a concentration of 0.0115g sus-pended solids/g sol.Between 42and 54h the particles in T3.6began to crystallize,reaching full crystallinity by 54h (Fig.2B).After 54h,particles in T3.6continued to grow from 32nm (0.0069g suspended solids/g sol)to 37nm (0.0087g sus-pended solids/g sol)by the 90th hour of crystalli-zation (Fig.5A).After 90h,crystal size peaked at 40nm and stabilized at 37nm (0.0096g suspended solids/g sol)after 138h.T3.6suspended solids concentrations improved to 0.0112g suspended solids/g sol at 216h.At 54h,T3.6showed 75%higher suspended solid concentrations than T2.4.After durations longer than 66h,the crystalline suspended solid concentration in T2.4surpassed that of T3.6,but by no more than 10%.The DLS results are consistent with average particle size determined from TEM images.The average crystal sizes from TEM measurements of crystal samples taken after 75h crystallization are estimated to be 51and 32nm for batch T2.4and T3.6,respectively (Table 1,Fig.3).From DLS,the mass median diameters for T2.4and T3.6after 75h crystallization were determined to be 56and 34nm,respectively.Zeolite yield on an Al 2O 3and SiO 2basis fol-lowed the same trend as suspended solids con-centrations.At 54h,T3.6(32nm,0.076g/g)showed 73%higher zeolitic Al 2O 3and SiO 2yield than T2.4(39nm,0.044g/g).At 215h,T3.6(37nm,0.123g/g)gave nearly the same zeolitic yield as T2.4(120nm,0.126g/g).Fig.5B compares the particle number concen-tration per gram of synthesis sol over time for T2.4and T3.6.Again,for the ?rst 42h,T2.4and T3.6behaved similarly;particle number concentrations for both batches were comparable.Between 42and 54h,T3.6particle concentration in T3.6doubled.After 54h T3.6particle concentration held ap-proximately constant.Batch T2.4particle con-centration fell continually after 42h.The number concentration of crystals in T2.4at 54h was only 35%of the particle number concentration at 42h.We were unable to clearly identify the nucle-ation mechanism in T2.4with the data we col-lected.It has been proposed by Mintova et al.[22]
