J . Phys. Chem. 1991, 95, 8826-8831
8826
derived from twisted top layer HOPG structures are identified by a long-range superstrumre whose periodicity diverges with a decaying amplitude as the nontwisted domain is approached. The sccond class of i m result from a crystalline or graphite contaminated tip that enoountm a low work function surface defect. These images are fundamentally different in that they have cmtant l o ~ r m g~~, e CWLFtBRt amplitudes, and abrupt domain boundary termlnafion. Such images are indicative of extensive tip commination. It should be notal that all the images
of this type which have been encountered in our laboratory may be successfully interpreted based on this model.
Acknowledgment. This restarch was supported in part through the Air Force Office of Scientific Research and by the National Science Foundation under Grant CHE-89 12674. We are also grateful to Prof. D. M. Kalyon and Dr. J. Wang for the use of their Silicon Graphics Iris facility to implement our model. Registry No. Graphite, 7782-42-5.
J. hnyer,* I(. Karin, W.J. Smith, N. E.Thompson, Department of Chemistry, UMIST, P.O.Box 88, Manchester, M60 1 QD England Robin K . Harris, and David C . Apperley Department of Chemistry, University of Durham, Durham, DHl 3LE England (Received: December 12, 1990)
Synthetic cubic faujasites (Si/AI = 4) are synthesized hydrothermally and compared with (i) materials of similar framework composition produced by secondary synthesis from zeolite Y using silicon hexafluoride and (ii) zeolite ZSM-20. Solid-state NMR (%i and 27AI),XRD, FTIR, and surface analysis are used to characterize the materials, which are also evaluated as catalysts by using n-hexane conversion as a test reaction. %i NMR results suggest that the materials produced by primary synthesis have aluminum ordering differing from that observed in materials with similar framework composition (Si/AI) but produced by secondary synthesis. The difference evident in the %i NMR may account for the higher catalytic activity observed with the products of direct synthesis. Surface analysis using depth profiling shows that the siliceous frameworks produced by direct synthesis are more homogeneous than materials produced by secondary synthesis.
Introduction Zeolites having the faujasitic structure are the active components in fluid catalytic cracking (FCC) catalysts. Both the catalytic activity and selectivity of acidic zeolites can be related to the strength and density of the acid sites, which depend mainly upon the structure and composition of the framework, although nonframework species can also influence catalysis.' Until quite recently it was not possible to synthesize zeolites having the cubic faujasite structure with a framework siliconto-aluminum ratio greater than 3. Consequently, dealumination procedures using hydrothermal treatments2or secondary synthesis using appropriate silica sources such as Sic43 or SiF6F4have been used to modify the framework composition of faujasites. However, the direct synthesis of faujasitic zeolites with enhanced framework Si/AI has recently been achieved using organic templates. Both cubic and hexagonal forms of faujasite are r e p ~ r t e d and , ~ intergrowths of both structures (ZSM-20) are also describede6 Currently there is little information regarding the characterization and physical properties of these new materials, which are clearly of interest in studying the role of framework composition as it relates to the strength and density of acid sites and in providing more stable and active components for FCC processing. In the present study, cubic faujasitic zeolites having siliceous frameworks are synthesized, characterized by using several physical techniques, and evaluated as catalysts for the conversion of n-hexane. Comparisons are made with intergrowths of the cubic and hexagonal forms and with siliceous faujasites generated from zeolite Y by secondary synthesis.
Experimental Section The cubic faujasites (CUB-Y) with gel composition ( 10Si02, Al2O3, 1 .ONaF, 2.4Na20, 1.05template, 140H20] were syn*Towhom correspondenceshould be addressed. 0022-36S4/91/2095-8826$02.50/0
thesized by use of the 15-crown-5 ether following the procedure reported by Delprato et ala5The Si/AI ratio of CUB-Y material was altered by varying the composition of the gel. The secondary synthesis (CSY) materials were prepared from zeolite Y by treatment with SiF6'-? and ZSM-20 was synthesized by using tetramethyl orthosilicate.n The as-made samples of CUB-Y and ZSM-20 were calcined first in argon at 450 OC for 3 h, after heating to temperature at 2 OC/min, and then in dry air at 550 OC for 16 h. All the samples were ion-exchanged 6-7 times with ammonium sulfate (1.5 m ) . The XRD data were obtained with an XDS 2000 SCINTAG diffractometer (Cu K a radiation) over a range of 2-60° 28, at a scan rate of 0.01 deg/min. The unit cell dimensions were determined by using the TREOR method.'* Nitrogen sorption isotherms and surface areas were determined by a Micromeritics ASAP2400 porosimeter at Crosfield Chemicals Ltd. Samples were heated under vacuum at 270 "C overnight, and nitrogen was adsorbed at liquid nitrogen temperature. Total surface areas were determined by using the BET method. Scanning electron micrographs were taken with a Philips SEM Model 505 instrument. (1) Dwyer, J. Stud. Surf. Sci. Curd. 1989, 37.
(2) McDaniel. C. V.; Maher, P. K. U S . Patent No. 3,292,192, 1966; 3,449,010, 1969. (3) Beyer, H. K.; Belenzkaya, I. In Curulysfs by Zeolites; Imelik, B., et al., Eds.; Elsevier Scientific Publishing: Amsterdam, 1980; p 203. (4) Skaceels, G. W.; Breck, D. W. P m .6rh Inr. Zeolite Cod..,Reno, 1983; US. Patent 4,503, 023. (5) Delprato, F.; Guth, J.; Huve, L.; Delmotte, L. Zeolites 199fJ,fO, 546. ( 6 ) Newsam. J. M.: Tracev. M. M.: Vauahan. D. E. W.: Strochemaicr.K. J.; Mortier, W.'J. J . Chem. Soc., Chem. b m m u n . 1989i8. 493. (7) (a) Ernst,.S.; Kokotailo, 0 . T.; Weitkamp, J. Zeolites 1987, 7, 180. (b) Dwyer, J.; Millward, D.; OMalley, P. J.; Araya, A.; Corma, A. J. Chem. SOC.,Furaday Trons. 1990, 86 (6). 1001.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8821
Comparison of Siliceous Faujasitic Zeolites
TABLE III: Sorption of Nitrogen TABLE I: Unit Cell Compoeition of Silieeotg Faujdtes av pore size micropore UCS, Si/AI Si/AI Si/AI framework unit cell Si/AI surface m 2 g-l diameter, vol, cm3 zeolite A (XRD) (EDAX) (NMR) composition (NMR) zeolite (NMR) BET Langmuir A g-I NH4Y 24.69 2.33 2.5 2.5 (AQhASi02)137 NH4Y 2.5 860 894 15.2 0.32 CSY 24.58 3.28 3.4 3.4 (A102)43(Si02)149 846 CSY 3.4 817 CSY 24.50 4.37 15.5 0.30 4.7 4.4 (A102)dSiO~)I 5, CUB-Y 3.8 869 945 14.3 0.34 CUB-Y 24.57 3.40 3.5 3.8 (AIOz)o(Si02)I 52 3.6 894 930 CUB-Y 24.62 15.9 0.3 1 2.88 3.2 3.1 (AIOZ)~~(S~OZ)I~SZSM-20 ZSM-20 24.57 3.40 3.7 3.6 (A102)42(Si02)I b TABLE I V High-Frequency Hydroxyl Band (DOH) in Faujasitic Zeolites TABLE II: Frequencies (cm-') of the Main Absorptions in the Mid-IR Swctra of CUB-Y, ZSM-20. and CSY zeolite Si/AI VoH, cm-' zeolite Si/AI VOH, cm-' asymmetric symmetric CSY 4.4 3634 CUB-Y 3.1 3636 stretch stretch Si/AI CSY 5.3 3631 ZSM-20 3.6 3630 CUB-Y 3.8 3633 EX" INb D6R zeolite (NMR) EX' INb areal
ZSM-20 CUB-Y CSY NH4Y
3.6 3.8 3.4 2.5
1143 1162 1158 1130
1026 1033 1032 1018
796 801 798 787
728 727 725 717
583 583 582 577
stacking of faujasite sheets in ZSM-20 has been reportedS6 The mid-IR framework absorption bands of the materials under study are characteristic of faujasites and are shifted toward a higher frequency compared with the conventional NH4Y (Si/AI 'External. *Internal. = 2.51, indicating the higher framework Si/AI ratio. Results are given.in Table 11. The silicon-29 and aluminum-27 NMR spectra were recorded The scanning electron micrographs of CUB-Y and ZSM-20 (usually without cross-polarization) at 59.6 and 78.2 MHz, reare shown in Figure 2. The crystals of CUB-Y are of a regular spectively, with a Varian VXR-300 multinuclear spectrometer octahedral (cubic symmetry) shape (2 pm in size). The crystals a t the University of Durham Industrial Research Laboratories. of a M - 2 0 have an irregular hexagonal shape and range in size All spectra were obtained with magic-angle spinning (MAS) and up to 2-pm diameter. The SEM photographs indicate the absence at ambient temperature ( E 2 2 OC)* The 29si were recorded of amorphous material in the as-synthesized as well as in the with high-power proton decoupling using a 7-mm-0.d. Doty MAS calcined materials. probe with spinning rates between 4 and 5 kHz. The "Al spectra Figure 3 shows the nitrogen sorption isotherms for the CUB-Y, were recorded with a Varian MAS 7-mm-0.d. Probe with spinning c s y , and ZSM-20 samples, and sorption data are summarized rates of approximately 3 kHz. Chemical shifts are quoted relative in Table 111. AII isotherms are of type I, which is characteristic to tetramethylsilane for %i and for 27Alrelative to a 1 M solution of sorption in microporous solids. The absence of hysteresis of AICIJ, with the high-frWencY positive a"tion throughout. between the adsorption and desorption branches of the isotherm The IR spectroscopic study was Performed using a Mattson indicates the absence of an extensive mesopore system in these Cygnus 100 Fourier transform infrared Spectrometer. F ~ a n ~ ~ o r kmaterials. The average pore size and micropore volume of CUB-Y region absorption bands were obtained by using KBr disks, while are similar to those for ZSM-20 and CSY, the hydroxyl spectra were obtained by preparing self-supporting The SIMS depth profiles for CUB-Y, CSY, and NaY are zeolite wafers. The ammonium forms of the samples were acpresented in Figure 4. F~~csy, the s i + / A ~ +ratio ( ~ 1 ~ats ) tivated at 400 OC and IO" Torr for 3 h. the surface is higher than in the bulk, and this ratio decreases with The surface compositional depth Profiles were obtained with increasing depth into the zeolite crystallite. This contrasts with a VG S l M S spectrometer. An ion beam of IO-kV energy cUB-Y and conventional N a y , where a constant Si+/Al+ ratio and 20-pAcurrent was used to etch the surface. The depth profile (SIMS) is observed at all depths, between surface and bulk. Thus, was calculated by using the procedures reported elsewhere.* SIMS analysis suggests that the CUB-Y zeolite is homogeneous Catalytic activity of the zeolites was determined by using nin while the surface of CSY is slightly enriched with hexane conversion as a test reaction. The reaction was carried silicon, indicating that the dealumination process with SiF62- is out in a fixed-bed, stainless steel tube, intermittent flow reactor diffusion limited the zeolite pores, composition gradients at 400 OC* The +hexane feed (g9e7%) was furtherPurified by during depth profiling for zeolite Y dealuminated by SiF6z-and fractional distillation. Before the catalytic measurements, the EDTA have been reported previously.8,9 Extraction of A1 using catalysts were activated in a nitrogen stream at 450 OC overnight. EDTA can lead to considerable composition gradients, but CSY The reaction products were analyzed On line by use Of a materials show much shallower gradients which, indeed, are not 6000 G C (FID -t TCD). The WHSV/contact time Was varied observed readily where depth analysis is limited,9 by changing the flow rate of the feed, over a fixed volume of the The acidic nature of the faujasitic materials can be compared fresh catalyst (40-60 mesh size). by observing the hydroxyl spectra using FTIR. Figure 5 shows the hydroxyl stretching region of the CUB-Y and CSY zeolites, Results and Discussion and the wavenumbers (pOOH)for high-frequency bands are given The XRD patterns of faujasite-type zeolites, i*e*ZSM-20, in Table IV. All the materials show the characteristic highand are shown in Figure and the unit frequency (HF) and low-frequency (LF) absorption bands around parameters are given in 1. The CUB-Y shows 3640 and 3550 cm-1, respectively, associated with the faujasite all the characteristic peaks associated with Y-type ~eolites.'~ The structure. A shift in BOH for the HF band to lower wavenumber peaks are sharp and shifted toward values of 28 higher than those with increase in the s ~ ratio / ~ ofI faujasitic zeolite Y dealuminatd Observed with typical synthetic Y-type z d i t e * This shift toward and synthesized by various methods has been reported by many higher 28 values is related to the higher silicon-to-aluminum ratio, authors.lo ~ ~data are ~ showni in ~i~~~~ ~ 6. ~ l associated with the framework of the zeolite. A similar shift or Neither CUB-Y nor ZSM-20 (Figure 5 ) shows any significant decrease in unit cell size is observed in CSY zeolites. In addition adsorption at 3740 cm-~,indicating the absence of extensive defect to reflections characteristic of faujastic material, there are some or chain-terminating, silanols. additional reflections related to hexagonal symmetry in the powder The 29SiMAS NMR spectra of zeolites CSY, ZSM-20, and pattern for ZSM-20. In particular, there is an intense reflection CUB-Y are shown in ~i~~~~ 7. The relative of various at a value of 5' 28. The intergrowth of cubic and hexagonal csyq
'*
(8) Dwyer, J.; Fitch, F. R.;Machado, F.; Qin, G.; Smyth, erman, J. C. J . Chem. Soc., Chem. Commun. 1981,422.
S.M.; Vick-
(9)Chapple, A. P.; Ness, J. N.; Joyner, D. J. Zeolites 1989, 9. 250. (IO) Ward, J. W. Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monogr. 1976, 171, 118.
8828 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 Q
Dwyer et al.
A
10
20
40
30 20
J
0
Figure 2. Scanning electron micrograph of (bottom) CUB-Y (Si/Al)NMR = 3.8 and (top) ZSM-20 (Si/AI)NMR = 3.6.
20 20
0-
1600 LL
-
v,
%! 1202
0
>
I
t
1
la
20
-
80 40 -
29
Figure 1. (a) X-ray diffraction patterns of faujasitic zeolites: (A) CUB-Y, as synthesized, crown ether template, a. = 24.576 (7) A; (B) CSY, NH4-Y reacted with (NH4)fiiF6 to give Si/Al = 7.3, a, = 24.458 (9) A. (b) X-ray diffraction patterns of ZSM-20; a, = 24.573 (4) A (assigned in cubic symmetry). The lower figure is an expansion of the upper figure.
Figure 3. Nitrogen sorption isotherms of faujasitic zeolites: (a) CUB-Y (Si/AI = 3.8), (b) ZSM-20 (Si/AI = 3.6), (c) CSY (Si/AI = 3.3). Si/AI ratios determined by NMR.
Si(nAl) determined from 29SiMAS NMR, where n which can be 0, 1,2,3, or 4 denotes the number of aluminum atoms linked to a given silicon atom, are summarized in Table V. The spectrum of the CUB-Y and CSY zeolites includes, depending on Si/AI ratio, four peaks corresponding to Si(3AI), Si(2AI), Si( 1Al), and Si(0AI) configurations. Calculation of Si/Al from 29SiNMR intensities agrees quite well with.the chemical (EDAX) analysis (Table I), indicating that aluminum is sited solely in framework posi tions. The relative intensities of the Si(nA1) configurations demonstrate an interesting difference in the distribution of "T" atoms in the CUB-Y and ZSM-20 framework tetrahedra compared with the CSY. In CUB-Y and ZSM-20 the population of silicon in
Si(1Al) sites is higher and that in Si(0AI) sites is lower than is observed for high-silica Y materials dealuminated by chemical or physical methods (Figure 7). The 29SiMAS NMR spectra of dealuminated zeolites, however, can contain a contribution (originating from the silanol (Si-OH) group) which can be hidden under the Si( 1Al) signal. Any contribution of silanol groups and defect sites to the Si(1AI) signal can be identified by 29Si-'H cross-polarization and from IR spectra of the hydroxyl band at =3740 cm-I. The 29Si-'H cross-polarization results (Figure 8) for CURY zeolite demonstrate a negligible increase in the relative intensity of the Si( 1Al) peak, indicating only a limited number of defect sites as confirmed by the FTIR spectra for the hydroxyl region (Figure 5 ) . Consequently, the relatively high value for the
I
I
0.2
I
I
0.4
I
I
I
I
I
0.6 0.8 1.0 RELATIVE PRESSURE, (P/Po)
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8829
Comparison of Siliceous Faujasitic Zeolites
=
8
'* le16-
x:..--*o, 0.4040.2
-
=
8
e'E
*:;E$-
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A
A
A
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~
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A
A
A
A
A
A
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IJW-
-
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-
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CUB-Y (1990) SiCI' Minowski 1986) 0 Sic4 (De Canio 1986)
c
ZSM-20(1990)
5\m
6
-4:
-3FE -2
3
-l2
WAVENUMBER /c&
-100
-80 0
I \
0 4
-120
k i h m
Figure 7. %i MAS NMR spectra of (a) CUB-Y, (b) ZSM-20, and (c)
3800
3700
Moo
3300
3400
3500
WAVENUMBER /c m-' 1
4
I
I
I
l
I
I
I
I
I
CSY zeolites, with Si/AI = 3.8, 3.6, and 4.4, respectively. (a) and (b) were obtained following a 90° pulse with a 60-s delay between pulses. (c) was obtained following a 45O pulse with 120-s between pulses. A total of 920,932, and 484 transients were accumulated for (a), (b), and (c), respectively. Intensity comparisons should only be made internally for a given spectrum.
3630 ?543
0 3800 36W
3ooo WAVENUMBER /cm-'
3400 3200
Figure 5. Infrared spectra; hydroxyl region of faujasitic zeolites. CUB-Y synthesized by using crown ether template.
intensity of the Si( 1Al)peak in the T)si NMR spectrum of CUB-Y and ZSM-20 material is not due to the defect sites and appears to be due to a significant difference in the distribution of AI within the framework resulting in more Si( 1Al) and less Si(OA1) configurations. The difference in relative intensities of Si(nA1) coordinations in the 29SiMAS NMR spectrum of CUB-Y and of ZSM-20 as compared with CSY of similar composition suggests that the ordering of "T" atoms in CUB-Y and ZSM-20, synthesized by using organic templates, is different from that of the CSY generated by secondary synthesis or dealumination. Whereas it is possible to calculate 29SiN M R parameters for a faujasitic structure where the aluminum ordering is known, it is not generally possible to define aluminum distributions from experimental NMR spectra. Approaches to the specification of aluminum ordering in faujasitic frameworks include both the
I
-60
1
-80
I
-100
1
-120
-60
I
I
I
-80
-100
-120
6Sihm
Figure 8. %i MAS NMR spectra of CUB-Y zeolite (Si/AI = 3.8)
without (a) and with (b) 'H-% cross-polarization. Experimental conditions for (a) were as those for Figure 7a, and for (b) 55 780 transients were accumulated with a 1-s relaxation delay and 5-ms contact time. consideration of preferred unique structures"J2 and more detailed statistical m o d e l ~ . ~Although ~J~ it is simplistic to presume that "si NMR spectra for a sample of zeolite crystals can be simulated precisely by consideration of a single aluminum distribution, it is instructive to use specific distributions for comparative pur(11) Thomas, J. M.; Ramdas, S.;Klinowski, J.; Fyfe. C. A.; Hartman, J. S.J . Chem. SOC.,Faraday Trans. 1982.2 (78), 1025. (12) Dempey,E. J. Caral. 1974.33.497; 1975,39, 155; 1977,49, 1 IS.
(1 3) Peters, A. W. Book of Absrracrs, 183rd National Meeting of ACS, Washington, DC, 1982; PETER 15, p 482. (14) Beagley. B.; Dwyer, J.; Fitch, F. R.; Mann, R.; Walters, J. J. Phys. Chem. 1984,88, 1744.
8830 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
Dwyer et al.
TABLE V: Percentage Dhtrikdioe of SI(nA1) Codgumtiom Derived from %i MAS NMRO framework zeolite Si/AI Si(4AI) Si(3AI) Si(2AI) NHdY 2.5 2 12 42 (-85.1) (-89.4) (-94.7) CSY 3.4 6 29 (-88.0) (-93.1) CSY 4.4 20 (-94.7) CUB-Y 3.1 5 28 (-89.2) (-94.3) CUB-Y 3.8 25 (-94.8) ZSM-20 3.6 29 (-94.6) CUB-Yb 3.4 5 25 (-89.0) (-93.9)
"Chemical shifts in ppm. "Davis,
Si(0AI)
IO (-105.4) 23 (-105.1) 29 (- 105.2) 12 (-106.0) 18 (- 106.1) 19 (-107.1) 17 (-1 05.25)
57 (-100.5) 53 (-100.3) 53 (-99.6)
M. E. Private communication.
TABLE VI: Relative Population of Si(mAl) Units, per sod.lite Cage of Y zeolites, Derived Tbcoreticallv from Sekcted ordering ModclsO 2%i N M R Si(nAl) configuration structure Si(OAl):Si(lAI):Si(2AI):Si(3AI):Si(4AI) Si/AI D6R linkages a 25:705:00 5 4MiM2 b 26:53:21 :00 3.8 3MiMz/ 1MI2M2' C I2:5033:5:0 3 2MlM2/ 2M12M2
d I
100
"See Figure 9. Silicon NMR Parameter
Si( 1Al) 34 (-100.3) 38 (-100.0) 51 (-100.2) 55 (-100.0)
SVAI
,
i ,
I
0
b hL
-100
100
-100
UPPm
Figure 10. 27Al MAS N M R spectra of CUB-Y zeolite (Si/AI = 3.8) synthesized using a crown ether template: (a) as-made, (b) calcined and exchanged with NH,. Both spectra were obtained with 05s relaxation delays, 30° pulses, and 2000 transients.
/
0.16
2&53:Zl: 0:O
0
= 0.12
t
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Figure 9. Si, AI ordering schemes of faujasitic structures, having a center of symmetry.
poses." For example, in the present study two of the synthetic siliceous cubic faujasites have framework composition (determined by NMR) with Si/AI ratios of 3.8 and 3.1, which are very close to values for centrosymmetric structured2 with 40 and 48 AI per unit cell (3.8 and 3.0,respectively). In Figure 9 the faujasitic framework is represented by two &cages,11and the aluminum ordering shown leads to calculated 2%i NMR parameters that are reasonably close to the experimental values (Tables V and VI). The structure shown here does not represent the minimum-energy structure" although it must be expected that minimum-energy structures for crystallizing systems in contact with mother liquors may well differ considerably from those. calculated" for isolated structures. Interestingly, the structure suggested here for CUB-Y with AI/UC = 48 (Si/Al = 3.0) is similar to that proposed for Y zeolite dealuminated by hydrothermal treatment and has the same values for NMR parameter^.'^ However, the (IS) Engelhardt, G.; Lohse, U.; Patzelova. V.; Magi, M.;Lippmaa, E. Zeolites 1983, 3, 239.
0
02
04
06 00 FO CONTACT TIME (s?
Figure 11. n-Hexane cracking Over CSY (Si/AI = 4.4), C U B Y (Si/AI = 3.8), and ZSM-20 (Si/AI = 3.6). TABLE VII: n-HexaeCneLhn over Acidic zcditea at 400 O zeolite Si/AI k , , s-I kprs-' CSY 4.4 0.03 0.22 CUB-Y 3.8 0.08 0.33 ZSM-20 3.6 0.17 0.67
C
"k, refers to the initial slower rate constant at low conversion, and k2 refers to the subsequent more rapid rate constant. present study shows cleariy that Y zeolite dealuminated by using SiFt- gives aluminum ordering, as revealed by %i NMR,which differs from that obtained by direct synthesis. Comparison of the present with the previous study15is, however, possibly complicated by uncertainty in the relative intensity of the Si( 1AI) peak in this earlier worki5 since no supporting cross@arization or infrared studies were reported, so that it is not possible to eliminate the effect of silanol defects, which are common in hydrothermally
J. Phys. Chem. 1991, 95, 8831-8836 treated zeolites, on NMR intensities. The 27AIMAS NMR spectra of the zeolites under study contain one resonance at -60 ppm (Figure IO) due to tetrahedrally coordinated framework aluminum. There is no apparent signal due to extraframework octahedral aluminum species either in the synthesized or in the calcined materials, suggesting a high thermal stability for the CUB-Y material. The catalytic activity of the ZSM-20, CUB-Y, and CSY materials is compared by using n-hexane conversion as a model reaction. Figure 11 shows the apparent first-order plots, and rate constants are given in Table VII. At low contact times or very low conversion, the existence of an induction period in the n-hexane reaction over Y-type faujasite materials is evident.I6 The order of the activity, ZSM-20 > CUB-Y > CSY, is observed for materials having similar composition. This order of activity could arise from differences in crystal quality or from differences in the amounts of residual sodium ions. In all cases crystal quality as revealed by XRD is good, but microporosity is reduced for CSY as compared to the synthetic materials (Figure 3) and this could influence activity. Ion-exchange procedures are extensive but, although there is evidence for differences in sodium content, it is difficult to exclude the effect of traces of sodium ions on catalytic activity. However, the high activity in ZSM-20 and CUB-Y compared with that in CSY could be due to the different ordering of the T atoms, observed by 29SiNMR, resulting in differences (16) Dwyer, J.; Karim, K. Work to be submitted for publication. Karim, K. Ph.D. Thesis, UMIST, 1990. (17) heck, D. W. Zeolife Molecuhr Sieves;Wiley: New York, 1974. (18) Werner, P.E. Documentation for TREOR, Smithsonian Institution 1984.
8831
in active site distribution. n-Hexane cracking over CUB-Y material also demonstrates interesting effects on product selectivity and hydrogen-transfer functions, which will be discussed in more detail in a separate communication. Conclusion
Faujasitic zeolites synthesized at higher framework Si/AI ratio may have either cubic or hexagonal (or mixed) symmetry. They can be synthesized with excellent crystallinity and few defects and show higher surface areas and a more uniform composition than materials of similar framework composition made by secondary synthetic methods or by dealumination of zeolite Y. The materials made by primary synthesis appear to have an aluminum distribution different from that of the dealuminated materials in that they have a greater population of Si(lA1) units, and reduced population of Si(OA1) units, as revealed by 29SiMAS NMR. Structures are tentatively proposed to account for the 29SiNMR spectra in the siliceous cubic faujasites. Differences in the aluminum ordering between siliceous faujasites produced by primary synthesis and those produced by secondary synthesis are reflected in the catalytic properties of these zeolites in hydrocarbon transformations.
Acknowledgment. We thank Crosfield Catalysts (Wamngton), IC1 Chemicals (Wilton), and Shell Research (Amsterdam) for the financial support of K. Karim, N. P. Thompson, and W. J. Smith, respectively. We also thank SERC for provision of a grant (GR/F 22777). Re&@ No. 15-Crown-5,33100-27-5; nitrogen, 7727-37-9; n-hexane, 110-54-3.
Molecular Orientation In Mlxed ?r-Conjugated Polymer Monolayers Studied by Second Harmonic Generation T. Kurata,* A. Tsumura, H. Fuchigami, and H. Koezuka Materials and Electronic Devices Laboratory, Mitsubishi Electric Corporation, 1 - 1 Tsukaguchi-Honmachi 8-Chome, Amagasaki, Hyogo 661. Japan (Received: January 4, 1991; In Final Form: May 3, 1991) Stable mixed Langmuir-Blodgett (LB) films, composed of a soluble *-conjugated polymer, poly(3-hexylthiophene) (PHT), and an amphiphilic diacetylene derivative, pentacosa-IO,12-diynoicacid (DA), were prepared. Their structures have been investigated primarily by second harmonic generation (SHG). The mixed monolayers partly contain a double-layered structure consisting of PHT and DA. It has been concluded that the SHG originated from slightly twisted PHT in the layer, which is supported by the calculations of the molecular tilt angle of PHT repeating units. The molecular hyperpolarizability was evaluated at the same time. The UV polymerization of DA molecules inside the LB films and the successive heat treatment have enhanced the SHG intensities. The mechanism of the SHG enhancement will be also discussed.
1. Introduction Organic molecules have attracted much attention for the fabrication of future devices in optical and molecular electronic fields. Among them, *-conjugated polymers are one of the most promising candidates. Their electronic properties can be modified at will by chemical and electrochemical doping methods. The ?r electrons are loosely bound to the backbones, so they can quickly respond to the electric field of irradiated light. It is important to make the thin films of these materials in terms of device fabrications. The Langmuir-Blodgett (LB) technique has been widely studied because of the easy preparation of molecular oriented films. The molecular orientation of LB films has been investigated by various methods. The Fourier transform infrared (IT-IR) technique has been utilized to characterize the monolayers.'*2 The transmission spectra pick up the vibrations (1) Kimura,
F.; Umemura, J.; Takenaka, T. Lungmuir
1986, 2, 96.
0022-365419 1/2095-883 1$02.50/0
having the transition moments parallel to the film, while the vibration with the transition moments normal to the surface can be selectively observed in the reflection-absorption (RA) spectra. Comparison between the two spectra gives the information on molecular ~ r i e n t a t i o n . ~However, ,~ it cannot be often applied to mixed LB films consisting of several molecules when the vibrations due to each component overlap. Second harmonic generation (SHG) is a well-known phenomenon among the second nonlinear optical effects. SHG requires noncentrosymmetry of the samples and is very sensitive to the properties at the interface between the different kinds of materials. This technique has been increasingly utilized to detect the slight change, for example, at the surface of the electrode during (2) Dote, J.; Mowery, R. L. J . Phys. Chem. 1988, 92, 1571. ( 3 ) Chollet, P.-A.; Messier, J.; Rosilio, C. J . Chem. fhys. 1976, 64, 1042. (4) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J . fhys. Chem. 1990, 94, 62.
0 1991 American Chemical Society
