Characterization of SAP041 synthesized conventionally and in the presence of fluoride ions

Puyam S. Singh, R. Bandyopadhyay and B. S. Rao* Catalysis Division, National Chemical Laboratory, Pune 41 1 008, India

SAPO-11 samples have been synthesized by the conventional route and from an aqueous solution containing F- anions. Charac- terization of these samples by X-ray diffraction (XRD), chemical analysis, X-ray photoelectron spectroscopy (XPS), thermal analysis, NMR, adsorption measurements and catalytic test reaction reveals that the samples are highly crystalline, but differ in some physico-chemical properties. These differences in the properties of SAPO- 1 1 obtained via different synthesis routes are correlated with different ordering of silicon substitution within the framework.

1. Introduction For silica-containing molecular sieves, the replacement of OH- by F- as a mineralizer makes it possible to obtain crys- talline microporous solids and lower pH values. The role of fluoride as a mobilizing agent in hydrothermal synthesis is known. Fluoride has been used in the crystallization of silicalite-1.' It has been reported2v3 that the solubility of silica by the formation of hexafluorosilicate species increases signifi- cantly in the presence of fluoride anions and it has been assumed that the hydrolysis of fluorosilicate anions yields polycondensable hydroxylated species whose condensation leads to the crystalline material. The details are discussed in the present paper.

The synthesis of crystalline microporous silicoalumino- phosphate was reported4 in 1984. For various structure types, the relative amounts of silicon in the framework can be varied such that gradual transitions from phosphate-based to silicate-based frameworks are possible. Characterization of the samples was performed with the aim of elucidating the mecha- nism and ordering of silicon substitution.

2. Experimental 2.1 Synthesis of SAPO-11

2.1.1 In the absence of HF. SAPO-11 was synthesized using the conventional method of preparation based on the reported l i t e r a t~ re .~ The gel composition is given in Table l(a). A general procedure is as follows. An aluminophosphate gel was prepared by mixing an appropriate amount of 85% orthophosphoric acid (Merck), aluminium isopropoxide (Fluka) and distilled water. Silica sol (28% SO,) was added to this mixture with constant stirring until the gel was homo- geneous. A templating agent, dipropylamine (99%, Aldrich) was added and the final gel stirred for ca. 10 min. The mixture was divided into three portions and each portion was sealed in a Teflon vessel inside the stainless-steel autoclave and heated at 200°C for 4, 7 and 24 h. The samples are designated as SAPO-11 (P4), SAPO-11 (P7) and SAPO-11 (P24). The crystallites are collected by decanting the sedimented pro- ducts. Yield of the fully crystalline product, SAPO-11 (P24) is ca. 95-98%. Yield (wt.%) is calculated as (experimental zeolite yield/theoretical zeolite yield) x 100, where the theoretical zeolite yield is the maximum yield, assuming full incorpor- ation of aluminium initially present in the gel into sili- coaluminophosphate. The samples were calcined slowly at 200°C for 24 h, then the calcination temperature was increased at 10°C h- ' and finally held at 550°C for 24 h

under evacuation. The calcined samples were stored in a desiccator.

2.1.2 In the presence of fluoride ions. The synthesis of SAPO-11 in the presence of fluoride ions [HF : SiO,(moles) = 1 : 0.51 was carried out by adding H F prior to the addition of the template following the procedure, discussed earlier. The gel composition is give in Table l(a). The final gel was divided into three portions and then each portion was sealed in a Teflon vessel inside a stainless-steel autoclave and heated at 200°C for 4, 7 and 24 h and desig- nated as SAPO-11 (F4/2), SAPO-11 (F7/2) and SAPO-11 (F24/2). Other samples with different H F to SiO, mole ratios were also prepared as listed in Table l(a) and designated as SAPO-11 (F24/1) and SAPO-11 (F24/3). Yield of the fully crystalline product, SAPO-11 (F24/2) is ca. 95-98%.

2.2 Instrumental

XRD patterns of the as-synthesized and calcined samples were recorded on a Rigaku (model, D/MAX-VC, Japan) diffracto- meter with Ni filtered Cu-Ka radiation (A = 1.5404 A). Scan- ning electron microscopy (SEM) micrographs at different magnifications were taken using a (JEOL JSM 5200) scanning electron microscope. The bulk chemical analysis of the samples was performed with wet gravimetric analysis in com- bination with X-ray fluoroscence spectroscopy on an X-ray fluoroscence spectrometer (Rigaku 3700) and inductively coupled plasma (ICP), Jobin Yuon JY 38 VHR. A low- temperature nitrogen sorption was also carried out using an Omnisorp, 100CX. The surface chemical analysis was per- formed by XPS on an ESCA-3-MK(II) (VG Scientific, UK) using Mg-Ka radiation under evacuation at ca. lop8 Torr with a 4 mm slit. Thermal analysis was performed on an auto- matic derivatograph (SE-TARAM TG DTA 92) at a rate of 10°C min-' using finely powdered a-alumina as a reference material. 27Al, 29Si, 'P magic-angle spinning (MAS) NMR spectra were obtained at room temperature on a Bruker MSL-300 spectrometer, operating in Fourier-transform mode, using a 'one cycle' type measurement. 27Al MAS NMR spectra were recorded on a Bruker MSL-300 spectrometer at different spinning speeds; spectral width, 125 kHz; pulse length, 1 ps; delay, 500 ms, spectrometer frequency, 78.2 MHz. ,'Si MAS NMR spectra were measured at a spinning speed of 3 kHz; spectral width, 20 kHz; pulse length, 2 ps; delay, 2 s; spectrometer frequency, 59.6 MHz. The adsorption measure- ments were performed using standard' all-glass gravimetric units connected to high-vacuum system. The adsorption iso- therms were measured up to 500 Torr in the temperature

J . Chem. SOC., Faraday Trans., 1996,92( 1 l), 201 7-2026 2017

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

. View Article Online / Journal Homepage / Table of Contents for this issue

Table 1 (a) Molar composition of the reaction mixture for the synthesis of ALPO-11 and its silicon analogues

synthesis batch composition (mole)

sample A1203 p 2 0 5 S i 0 2 HZO DPA H F

ALPO-11 SAPO- 1 1 SAPO-11 (F24/1) SAPO-11 (F24/2) SAPO-11 (F24/3)

0 0.5 0.5 0.5 0.5

40 40 40 40 40

0 0 0.5 1 .o 1.5

(b) Elemental composition of the samples

sample

Si/AI (Si,AI,P,)02 fractions

surface bulk bulk surface

SAPO-11 (P24) (siO. 199A10.441 p0.360)02 (Si0.082A10.490P0.428~02

SAPO-11 (F24/1) (si0.279A10.396p0.323)02 (Si0.092A10.482P0.426)02

SAPO-11 (F24/2) ~s i0 .463A10.334p0.Z03~02 (Si0.10A10.474P0.424)02

SAPO-11 (F24/3) (si0.701A10. 157p0.142)02 (siO. 1 42A10.48 1'0.3 77)OZ

0.45 0.17 0.70 0.19 1.39 0.2 1 4.46 0.30

range 333-483 K. The temperature accuracy throughout the sorption measurements was within k 1 K. The sample was degassed at 723 K for 10 h and the temperature was lowered to the isotherm temperature where it was allowed to stabilize and was maintained for at least 2 h before the commencement of the measurement. In order to check the reversibility of the sorption, desorption measurements were also carried out. After each isotherm, the sample was evacuated at 723 K at

Torr for several hours. XRD was carried out for each sample, before and after the sorption measurements, to check the structural stability. The catalytic experiments were per- formed in a fixed-bed downflow silica reactor. The catalysts were pressed, pelleted and sieved to a size of 10-20 mesh. The amount cf catalyst taken was ca. 2 g, the height of the catalyst bed being 3 cm. The catalysts were activated at 550°C in the presence of air before each run. The liquid reactants were fed by a syringe pump (SAGE Instruments, USA). The products of the reactions were collected downstream from the reactor in a cold trap and analysed by gas chromatography, Shimadzu, Xylene master column (FID detector) for the alkylation and isomerisation reaction products, and an HP5880 (RGA) with TCD for the gas analysis.

3. Results and Discussion 3.1

As-synthesized SAPO- 11 obtained from F --mediated aqueous solution contains fluorine. However, on calcination fluorine escapes together with the decomposition of the organic material and the resulting calcined sample is essentially fluo- rine free. The bulk and surface chemical compositions are listed in Table l(b). It can be seen that the surface silicon content of the crystalline materials increases with the amount of H F added during the hydrothermal synthesis. The surface silicon content increases from 0.70 (Si/AI) for sample SAPO- 1 1 (F24/1) to 4.46 (Si/Al) for sample SAPO-11 (F24/3), whereas the bulk silicon content increases from 0.19 (Si/Al) for sample SAPO-11 (F24/1) to 0.30 (Si/AI) for sample SAPO-11 (F24/3). The amount of silicon uptake increases for the sample synthe- sized with HF. More silicon would dissolve as H F is added to the reaction mixture. Crystallization occurred in a more supersaturated medium, owing to the greater concentration of the dissolved species. Several a ~ t h o r s ~ ~ , ~ ~ * ~ have studied the incorporation of silicon into SAPO-based materials with dif- ferent silicon mole fraction. It is suggested that silicon substi- tution is by an SM2 rection when the silicon mole fraction is less and a combined SM2 and SM3 (homo-

Chemical analysis, XPS, SEM and XRD

geneous + heterogeneous) reaction when the silicon mole frac- tion is more. (See next section for definition of SM1 etc.) The increase in the build up of siliceous regions with silicon content results in composition gradients in the crystal. The formation of siliceous regions by simultaneous SM3 (homogeneous) and SM2 reactions compete with the SM3 (heterogenous) reaction. By the SM3 (heterogeneous) reaction that is by replacing systematically A1 and P atom pairs in a certain section of the crystal starting from the external surface, an electroneutral framework comprising ALP0 layers and topotactic SiO, overlayers is The incorporation of silicon preferentially into the surface of the crystal in the presence of fluoride ions may be associated with an SM3 (heterogeneous) reaction.

Fig. 1A and B show SE micrographs, at different magnifi- cation of SAPO-11 (F24/2) samples. These have larger, elon- gated and cylindrical crystals with a well defined boundary, whereas SAPO-11 (P24), (Fig. 1C and D) has spherical aggre- gates. No intergrowths or amorphous phase could be seen or detected in any of the samples. The addition of H F to a gel mixture is known3a,3c to influence the size and shape of the crystals. The crystals are always of good quality and the size exceeds generally the values observed in fluorine-free synthe- sis. The usual pH of the reaction mixture of ALPO-based solids is slightly acidic. The addition of HF results in a decrease in the initial gel pH which may influence the disso- lution, in the synthesis gel, of the species necessary for nucle- ation. As observed in earlier studies by Guth et aL2" and Xu et ~ l . , ~ ' the presence of fluoride ions reduces the induction time and leads to a slower rate of crystal growth. It is assumed2b that the presence of fluoride favours the rapid production of fewer nuclei, after which crystal growth consumes prefer- entially, but slowly, the precursors. The stability of the fluoro- complexes formed by solubilizing amphoteric oxides and hydroxides must not be so high that further reaction involving them is inhibited.

The X-ray diffractograms for the samples crystallized in the absence or in the presence of HF are given in Fig. 2. Some changes in intensity could be seen for the sample made with H F and none of the samples contain any other crystalline phases. The change in crystallinity of SAPO-based solids in the presence of fluoride ions was already seen in the case of SAPO-34.2b In both the conventional and the fluoride synthe- sis, it is found that the samples collected after 4 h of hydro- thermal treatment show the lowest percentage of crystallinity, whereas the samples collected after 24 h of hydrothermal treatment show 100% crystallinity .

2018 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

A C

B D

Fig. 1 synthesized SAPO-11, made by the conventional method.

SE micrographs of SAPO-11. A, B as-synthesized SAPO-11, from silicoaluminophosphate gel containing fluoride anions; C, D as-

3.2 MAS NMR spectroscopy

The 27Al MAS-NMR spectra of SAPO-11, synthesized with or without HF, are quite similar and do not show any large apparent quadrupole effects as in SAPO-34 and SAPO-37.6 The aluminium environment in these samples appears tetra- hedral and similar to that in AEL type materiak6v7 The crystal structure of as-synthesized ALPO-11 (space group Zma2) calls for three distinct tetrahedral crystallographic sites for aluminium in the population ratio 2 : 2 : 1 and five distinct crystallographic sites in the population ratio of 1 : 1 : 1 : 1 : 1 for hydrated calcined ALPO-11 (space group Pn~2, ) .~"9~ ' However, the lineshapes of the signal from four-coordinated

5.00 10.00 20.00 30.00 40.00 B/degrees

Fig. 2 XRD patterns of as-synthesized SAPO-11. A, SAPO-11 (P24); B, SAPO-11 (F24/2).

A1 (ca. 30-40 ppm) and the octahedral signal (ca. - 20 ppm) in 27Al NMR spectra might be broadened by second-order quadrupole interactions. All the three and five distinct crystal- lographically different tetrahedral sites for aluminium are not distinguished in as-prepared and hydrated calcined SAPO- 1 1 ~arnples.~".~** The 27Al MAS-NMR spectrum of the hydrated calcined sample [Fig. 3(b) and (d) ] shows at least two signals from four-coordinated A1 and a signal from a six-coordinated species. The results of 27Al MAS NMR spectroscopy at a spinning speed of 3-7 kHz, for calcined SAPO-11, synthesized hydrothermally in the presence of HF, are very similar to that of SAPO-11, synthesized in the absence of HF. 27Al MAS NMR investigation does not provide any information con- cerning the silicon incorporation. A similar observation has already been made by Jahn et aL7 Neither 27Al nor 31P MAS NMR were suitable to detect the isomorphous substitution of P by Si. However, by means of 29Si MAS NMR, different silicon environments can be detected. The theoretically pos- sible isomorphous substitution mechanisms are substitution of Si for A1 (SMl), Si for P (SM2) and two Si atoms for one A1 and one P simultaneously (SM3). SM1 generates net positive framework charges. The simultaneous substitution of Al, P by two Si leads to a neutrally charged framework. The possibility of SM1 is unlikely as it involves the generation of Si-0-P linkage^.'*^*'^ The observed chemical shift 6(29Si) for SAPO-11 (P24) [Fig. 3(e)] is in the range of -92 to -113 ppm. For SAPO-based solids the 29Si MAS-NMR data show more ~ a r i a b i l i t y . ~ ~ , ' , ~ Some SAPO-11 samples show a single dominant Si resonance while some show the presence of other ill defined lines in the chemical shift range from -92 to - 112 ppm. Reliable structural features cannot be re~olved'.~ as such spectra indicate more variability for siting of Si in these novel frameworks than in those with zeolite structural analogues. 29Si MAS NMR of our as-synthesized sample SAPO-11 (P24) is composed of four resonance lines centred at -92, -96,

J . Chem. SOC., Faraday Trans., 1996, Vol. 92 2019

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

I , , , , I , , , , I , ) , , I , , , , , 6

200 100 0 -100- -200

l , , I I I I I I t I I I I I I I I l i l 1 8 I I I I , , , , I , , , , I , , , , * 200 100 0 -100 -200 200 100 0 -100 -200

6 6

- -60 -80 -100 -120 -140 -160

6

Fig. 3 *'A1 MAS NMR spectra of (a) as-synthesized SAPO-11 (P24) at different spinning speeds: A, 5.5 and B, 6.4 kHz. (b) Calcined SAPO-11 (P24) at A, 5.8 and B, 6.7 kHz. (c) As-synthesized SAPO-11 (F24/2) at A, 6.3 and B, 7.0 kHz. (d) Calcined SAPO-11 (F24/2) at A, 5.7 and B, 6.7 kHz. (e) 29Si MAS-NMR spectrum of as-synthesized SAPO- 1 1 (P24).

- 103 and - 113 ppm. Derivation of the relative contribution from the substitution mechanism is not possible from 29Si MAS NMR spectra and thus, it is not possible to quantify the amount of Si(nA1) environments. However, the presence of

signals at ca. -92 ppm indicative of Si(4A1) of SAPO-type environment and at ca. - 113 pm indicative of Si(4Si) siliceous type environment is not q~estionable.~".~

3.3 Thermal analysis

The results of the thermal analysis are presented in Fig. 4. The TG curves reveal that dehydration occurs at ca. 298-443 K and decomposition of the template at ca. 443-823 K. More water fills in the voids of the sample during the earlier stages of crystallization [40-80% crystallinity; SAPO-11 (P4), SAPO-11 (P7), SAPO-11 (F4/2), SAPO-11 (F7/2)], however, the water content decreases relatively for 100% crystalline samples [SAPO-11 (P24) and SAPO-11 (F24/2)] (Table 2). Further, SAPO-11 synthesized in an HF medium is more hydrophobic than SAPO-11 synthesized without fluoride anions, however, the template content remains nearly the same (7.2 cf: 7.5 f 0.1) in all 100% crystalline samples irre- spective of the method of synthesis. The differential thermal analysis (DTA) curves clearly reveal the energetically different template sites [Fig. 4(a) and (b)] . It shows that the calcination process in air proceeds in five stages as in the case of MeAPSO-11 (Me = V, Mg, Zn, Cd, Ni or Cr).1'v12 Stage I shows the endothermic desorption of water. Stages II-V reveal the oxidative decomposition of template molecules and, in all stages, exothermic peaks are observed. For ALPO-11, the process of the removal of template proceeds in Stage I1 [Fig. 4(c) and (43. A certain amount of template interacts with the acid centres of ALPO-11 and is decomposed at higher temperatures (573-823 K). The acid centres of aluminophos- phate molecular sieves originate from terminal hydroxy bonds (T-OH, T = A1 or P) and lattice-defect-induced Lewis acid sites.13 IR spectra of chemisorbed pyridine at 25 and 100°C on ALPO-11 show absorbance at ca. 1447-1450 and 1543- 1548 em- which c ~ r r e s p o n d ~ ~ " . ~ ~ to coordinately bonded pyridine and pyridinium. This indicates the presence of both Lewis and protonic sites on the aluminophosphate. Temperature-programmed desorption (TPD) of ammoniaI4 shows a peak maximum at a lower temperature, indicative of weak acid sites of ALPO-11. For SAPO-11 (P4), SAPO-11 (P7) and SAPO-11 (P24) samples, the main process of the decomposition of template proceeds in Stage I1 and Stage IV [Fig. qa)] . A certain amount of template is converted into coke during the calcination and interacts strongly with the cationic framework; this can be burned out in air during Stage V at higher temperature. For SAPO-11 (P4) and SAPO-11 (P7) samples, the removal of template in Stage I11 is more pronounced, whereas for SAPO- 11 (P24), the removal of template in Stage I11 is extremely weak, as reported pre- viously.' ' ,12 For this sample, SAPO-11 (P24), the removal of template in Stage IV is quite large as compared with the SAPO-11 (P4) and SAPO-11 (P7) samples. The diffusibility of template during calcination depends upon the stage of the

Table 2 Thermal analysis

template loss (wt.%)

sample water loss (wt.%) I1 I11 + IV + v total template loss (wt.%)

298-443 K 443-573 K 573-823 K ALPO- 1 1 1.250 6.724 0.370 7.100 SAPO-11 (P4) 4.365 3.332 4.326 7.658 SAPO-11 (P7) 4.399 3.219 4.227 7.446 SAPO- 1 1 (P24) 2.150 3.216 4.299 7.5 1

298-433 K 433-683 K 683-823 K SAPO-11 (F24/1) 0.135 6.290 0.710 7.000 SAPO-11 (F4/2) 5.188 5.42 1 1.503 6.924 SAPO-11 (F7/2) 2.3 17 5.684 1.279 6.963 SAPO-11 (F24/2) 0.153 6.74 0.76 7.500 SAPO- 1 1 (F24/3) 0.212 7.16 0.46 7.62

2020 J . Chem. SOC., Faraday Trans., 1996, V d . 92

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

W I

100 200 300 400 500 600 700 800 900

te m pe ratu re1"C

\

I

F 2412 F 2412 F 712

F 712

I 1 1 I I I I

100 200 300 400 500 600 700 800 900

tern peratu re1"C

\ I

\i v 150 250 350 450 550

te m peratu re1"C 150 250 350 450 550 650

tern peratu re/"C

Fig. 4 TG and DTA patterns of SAPO-11 with different percentage of crystallinity, (a) synthesized by the conventional method. (b) Synthesized in the presence of fluoride anions. (c) DTA pattern of SAPO-11, synthesized in the presence of different amounts of fluoride anions and ALPO-11. (d) DTG pattern of SAPO-11, synthesized in the presence of different amounts of fluoride anions and ALPO-11.

template, anchored inside the pore. After completion of crys- tallization, more template acts as a charge compensating cation, interacting with framework Brnrnsted acidity owing to greater incorporation of silicon into the ALPO-11 frame- work. The diffusibility of template during calcination for the SAPO- 1 1 samples, calcination for the SAPO- 1 1 samples, SAPO- 1 1 (F4/2), SAPO- 1 1 (F7/2), SAPO- 1 1 (F/2) and SAPO-11 (F24/2) [Fig. 4(b)] reveals that the state of occluded template (Stage 11), is anchored relatively more strongly than that of SAPO-11 (P4), SAPO-11 (P7) and SAPO-11 (P24) samples. The main process of removal of the template for SAPO-11 (F4/2), SAPO-11 (F7/2) and SAPO-11 (F24/2) samples is in Stage I1 (433-683 K). The exothermic peak maximum of template removal in Stage I1 is shifted from 592 K for SAPO-11 (F4/2) to 666 K for SAPO-11 (F24/2) [Fig. 4(b)]. A small amount of template acts as charge- compensating cations in all SAPO- 11 (F4/2), SAPO-11 (F7/2) and SAPO-11 (F24/2) samples and is decomposed in Stage IV. The template-converted coke burned off in Stage V is less than in other stages. The diffusibility of the occluded template of SAPO-11 (F4/2), SAPO-11 (F7/2) and SAPO-11 (F24/2) samples in Stage I1 is relatively slow and it is decomposed within the temperature range 433-683 K cf: 443-573 K for SAPO-11 (P4), SAPO-11 (P7) and SAPO-11 (P24) samples. This phenomenon is also seen more clearly in the case of SAPO-11 (F24/1) and SAPO-11 (F24/3) samples [Fig. 4(c) and (43.

3.4 Adsorption measurements

Equilibrium sorption capacities, obtained from sorption mea- surements at PIP, = 0.8 and 298 K, of different sorbates in SAPO-11 materials, are shown in Table 3(4. It is observed that the sorption capacities of hydrocarbons are not a maximum in fully crystalline SAPO-11. A small amount of fully crystalline, calcined sample (pure white in colour) was analysed for carbon and hydrogen and found to contain ca. 0.32 wt.% C and ca. 0.53 wt.% H whereas the sample collected in the early stage of crystallization contains a negligible amount of carbon and hydrogen. A certain amount of tem- plate, converted into coke during calcination, interacts strong- ly with the cationic framework, and can be burned off in air only at higher temperature, however, some coke still remains and decreases the void volume. For SAPO-11, synthesized with or without HF, the sorption capacities of hydrocarbons increase with the increase in the percentage of crystallinity of the sample. The samples with greater crystallinity are expected to bring about an increase in void volume of the lattice. The sorption capacity of water increases from 15.29 wt.% of SAPO-11 (P4) to 17.56 wt.% of SAPO-11 (P24). As the per- centage of crystallinity increases, there is an increase in silicon incorporation. The incorporation of silicon into the lattice, which is responsible for Brnrnsted acid sites in the sample, gen- erates more hydrophilicity. However, the sorption capacity of water for SAPO-11 samples synthesized in the presence of HF

J . Chem. SOC., Faraday Trans., 1996, Vol. 92 2021

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

Table 3 (a) Sorption properties of ALPO-11 and SAPO-11 at 298 K and PIPo = 0.8

saturation capacity (wt.%)

sample C6H14 C6H12 ~~

ALPO-11 14.58 7.20 5.10 SAPO-11 (P4) 15.29 6.00 4.25 SAPO-11 (P7) 16.86 7.26 5.14

SAPO-11 (F4/2) 12.31 4.76 4.10 SAPO-11 (F7/2) 12.84 7.14 5.21 SAPO-11 (F24/2) 12.57 6.39 4.82

SAPO-11 (P24) 17.56 6.83 4.93

(b) Saturation capacities of ALPO-11 and SAPO-11

saturation capacity/molecules uc - temperature surface area

sample /K Langmuir BET Dubinin experimental unit cell composition /m2 g-I

ALPO-11 333 363 393 423

SAPO- 1 1 (P24) 333 363 393 423

SAPO-11 (F24/2) 333 363 393 423

20.49 16.00 5.85 3.10

17.15 14.24 6.58 4.88

14.28 10.86 3.66 2.79

12.06 9.05 4.33 2.27

11.43 8.58 4.87 3.17

9.82 6.75 3.10 2.00

17.46 14.28 5.21 1.93

16.61 10.12 5.78 4.66

11.47 10.17 3.35 2.55

20.22 A1,0.4P 19.6O80 17.76

245

7.45 4.72

16.89 H3. 2 19.6 1 7.2O80 214 13.93 8.98 4.94

13.03 11.24 5.07 3.66

208

remains nearly constant, irrespective of silicon incorporation. This is probably due to an increase in silicon patches over the surface, which is hydrophobic and not responsible for Brsnsted acidity. The sorption capacities of hydrocarbons for SAPO-11 (F24/2) is slightly lower than SAPO-11 (F7/2) but higher than SAPO-11 (F4/2) and the trend is similar to that of SAPO-11 (P4), SAPO-11 (P7) and SAPO-11 (P24) samples. However, as compared with SAPO-11 (P24), the sorption capacities of hydrocarbons for SAPO-11 (F24/2) is lower. This decreasing void volume is due to more silicon deposition on the external and internal surface of voids of the microporous materials [Table l(b)]. Isotherms of ammonia sorption are presented in Fig. 5(a) and exhibit type I isotherms according to Kiselev’s classification.’ With increase in tem- perature, the shape of the isotherm deviates from the Lang- muir type and becomes similar to a Henry’s type. The uptake of ammonia increases from SAPO-11 (F24/2) to SAPO-11 (P24). The factors such as Sanderson’s electronegativity, pol- arizability (e/r) of the extraframework cationic species, specific surface areas and void volumes of the lattice are expected to influence the sorption capacities and energetics of a polar mol- ecule such as ammonia. Fig. 5(b) shows the amount of ammonia retained irreversibly during desorption by evac- uation (up to Torr). The phenomenon of irreversibility of ammonia adsorption has been reported earlier for MnY l 6 and cation-exchanged Y1 zeolites at 303 K. The irreversibility of ammonia sorption in zeolite is attributed16 to (a) formation of ammonia complexes with intrazeolite cations, particularly in the case of exchanged zeolites and (b) chemisorption. The irre- versibly sorbed ammonia was found to be completely degassed by evacuating at Torr for several hours. The number of ammonia molecules held [Fig. 5(b)] on account of the different degrees of chemisorption may be taken as a measure of the acidity. Physically sorbed ammonia molecules are desorbed on degassing at low pressure without increasing the temperature. The degree of chemisorbed ammonia retained depends on the interaction of ammonia molecules with the acid centres of the sample. The acidity is generated in the sample by various modes such as the terminal hydroxy

group, Lewis and Brransted acid sites. The generation of Brransted acid sites due to substitution of Si for P in the ALPO-11 lattice leads to more ammonia molecules being strongly chemisorbed on its Br~nsted acid sites. The substitut- ion of two silicon atoms for a pair of A1 and P does not gener- ate Brsnsted acid sites. For a given site, it has been shown18 that the substitution of a single Si in place of P is energetically more favourable than the substitution of 2Si for an A1 and P pair. However, the substitution of a 2Si for an A1 and P pair is possible’ 8*1 when large amounts of silicon are available. SAPO-11, synthesized in the presence of HF, produces large amounts of siliceous domains. These results may be due to the high availability of silicon through formation of complexes when F- replaces OH- or H 2 0 5 during the crystallization period.

For SAPO- 1 1 (F24/2), relatively fewer ammonia molecules are held in the lattice, although it contains a relatively larger amount of silicon. This is due to the fact that only some sili- cons are responsible for Brsnsted acid sites which can interact with ammonia molecules producing chemisorbed ammonia.

3.4.1 Application of sorption isotherm equations. Dubinin, BET and Langmuir equation analysis of sorption data in terms of different isotherm equation models always provides useful information in identifying the nature of sorption centres and the physical state of sorbed molecules. The applicability of the Polanyi potential theory modified by Dubinin and Radushkevich2’ for ammonia sorption in different SAPO- 1 1 is tested. Dubinin plots so obtained exhibit excellent linearity [Fig. 6(a)]. The saturation capacities show close agreement with those obtained experimentally [Table 3(b)] and thus indi- cated the satisfactory nature of the Dubinin-Radhuskevich equation based on the volume-filling approach. The applica- tion of the BET equation to the sorption of ammonia in SAPO-11 yielded excellent linear plots [Fig. 6(b)]. The equa- tion is based on the multilayer assumption by assigning a much higher value to the heat of sorption for the first layer than that for the successive layers. The Langmuir sorption model is a localized sorption with monolayer approach and

2022 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

20

15

10

5

0

7 h 12.5 - - W u c ._ 5 10.0

g 7.5

v

v)

3 -

- . 2 c .O 5.0 i??

2 2.5

c

c C W

0 0

I0 z

100 200 300 400 500 Pflorr

P I

0 100 200 300 400 500 Pf lo r r

325 350 375 400 425 TIK

Fig. 5 Isotherm of ammonia sorption in (a) SAPO-11 (P24) at 1, 333; 2, 363; 3, 393; 4, 423; 5 , 453 and 6, 483 K and (b) SAPO-11 (F24/2) at the same temperatures. (c) Amount of ammonia retained irreversively as a function of temperature in 1, SAPO-11 (P24) and 2, SAPO-11 (F24/2).

1 : 1 correspondence between the sorbate molecule and the energetically equivalent sorption centre. Monolayer capacities obtained from the reciprocal of the slopes of the linear plots [Fig. 6(c)] show fair agreement with those obtained from the Dubinin-Radhushkevich equation. The saturation capacities decrease with SAPO-11 (F24/2) at all temperatures as com- pared with SAPO-11 (P24) [Table 3(b)].

3.4.2 Chemical affinity, selectivity of the sorbed phase and isosteric heat of ammonia sorption. A decrease in chemical potential results when a gas is transferred reversibly and iso- thermally from a gas phase at a standard pressure Po into an infinite amount of sorbent-sorbate mixture over an equi- librium pressure p.*' Plots of Ap (chemical potential) us. the amount sorbed provide useful criteria for the comparison of

the sorption affinities of a probe molecule in the lattice of molecular sieves. Fig. 7(a) shows that a drop in - A p with the coverage is similar in SAPO-11 (P24) and SAPO-11 (F24/2). This indicates that the strength of sorption centres in different SAPO-11 samples is similar; however, at a fixed coverage, SAPO-11 (P24) shows higher chemical affinity than SAPO-11 (F24/2). The isosteric heat of sorption is derived by applying the Claussius-Clapeyron equation at constant sorbate loading.22 The values of the isosteric heats obtained from the slopes of the linear isosteres are plotted against the coverage [Fig. 7(b)]. This shows that in SAPO-11 (P24) the Q,, value decreases with the coverage whereas in SAPO-11 (F24/2), Qs, remains nearly constant. This shows that initially ammonia interacts strongly with the stronger acid centres and later with the weak terminal hydroxy groups of SAPO-11 (P24). However in SAPO-11 (F24/2), the strength of different acid centres is not clearly defined.

3.5 Catalytic test reactions

The acidic sites formed in ALPO-11 and SAPO-11 are differ- ent than those of zeolites.' 5,23 The activity/acidity aspects of these materials are demonstrated in three different reactions.

Alkylation of toluene. Brmsted acid sites of solid acid cata- lysts such as zeolite catalyse toluene methylation. SAPO-11 (F24/3) showed less activity than SAPO-11 (P24) and among SAPO-11, synthesized in an H F medium, SAPO-11 (F24/3) showed the least activity [Table qa)]. ALPO-11 does not show any activity. A small conversion observed in toluene methylation and m-xylene isomerization over ALPO-11 may not, necessarily, be due to structural acidity but to some inherent acidic nature of ALPO-11, as discussed above. The conversions are in agreement with the acidity of the catalysts. The high para-isomer content amongst the xylenes and the formation of only the 1,2,4-TMB (trimethyl benzene) isomer in the trimethyl benzenes suggested a shape-selective effect and is attributed to the restricted transition-shape selectivity owing to the unidimensional elliptical pore structure. Additionally, as SAPO-11 is slightly acidic, the fast isomerisation of the primary product, p-xylene, is restricted, thus contributing to a high yield of the para-isomer in the product. A similar effect of shape selectivity is observed for the isomerization of m-xylene over SAPO-11 samples [Table 4(b)].

Dehydration of isobutanol. This reaction can be catalysed by all types of acid sites. It proceeds initially via isobutene forma- tion and then secondary carbenium ion formation as shown in Fig. 8. Abstraction of one proton from the neighbouring CH, or CH, group produces the corresponding but-1-ene and but-2-ene. Table 4(c) shows the comparison of the catalysts. At all temperatures SAPO-11 is most active, the activity is related to the acidity of the materials. The ratio of but-2-ene to the total amount of butenes increases from 0.67 for SAPO-11 (P24) to 0.84 for SAPO-11 (F24/3) at a reaction temperature of 473 K. A similar trend is observed at higher reaction temperatures (523 and 573 K). Over stronger acid sites, the tendency to form secondary carbenium ions is enhanced, followed by transformation to but- 1-ene and but-2- ene. This reveals that the acid strength of the samples synthe- sized in the presence of fluoride ion is less than in those syn- thesized using the conventional method.

Stereoselectivity in the case of the but-2-ene isomeric mixture varies among SAPO-11 samples. Abstraction of any H from the neighbouring -CH2 group of the secondary carb- enium ion has the same probability to form the cis- or trans- isomer since the carbenium ion has a nearly planar structure of an SP2 hybridized carbenium centre. However, the restriction due to pore size slows down the movement of the bulkier isomer and allows consecutive reaction. (Z)-but-2-ene

J . Chem. SOC., Faraday Trans., 1996, Vol. 92 2023

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

Table 4 (a) Alkylation of toluene with methanol

product distribution (wt . %)

conversion non- total total p-xylene 1,2,4-TMB catalyst (wt.%) aromatics benzene p-xylene m-xylene o-xylene xylenes TMB /total xylene /total TMB

ALPO-11 0.99 - 0.02 0.47 0.3 1 0.15 0.93 0.04 0.50 0.52 SAPO- 1 1 (P24) 23.20 0.12 0.30 7.74 6.82 3.04 17.60 6.45 0.44 0.75 SAPO- 1 1 (F24/ 1) 1 3.20 0.08 0.14 5.50 3.82 1.54 10.86 2.28 0.51 0.97 SAPO- 1 1 (F24/2) 1 1.22 0.06 0.09 5.03 3.03 1.42 9.48 1.70 0.53 0.96 SAPO-11 (F24/3) 4.57 0.03 0.06 2.00 1.24 0.62 3.86 0.67 0.52 0.94

Temperature 723 K, WHSV 2.4 h-', TOS 1.5 h, mole ratio (toluene/MeOH) 2 : 1, pressure 1 atm. Thermodynamic equilibrium concentration of xylene isomersz5 at 700 K: para 23.5, meta 52.1, ortho 24.4.

(b) m-Xylene isomerization

product distribution (wt. %)

conversion p-x ylene p-xylene 1,2,4-TMB cat a1 y s t (wt.%) benzene toluene + o-xylene TMB /p-xylene + o-xylene /total TMB

ALPO- 1 1 1 .00 - 0.10 0.87 0.05 0.57 0.47 SAPO-11 (P24) 23.83 - 0.43 23.07 0.33 0.66 1 .00 SAPO-11 (F24/1) 6.98 0.03 0.34 6.55 0.05 0.70 1 .00 SAPO-11 (F24/2) 5.59 0.04 0.1 1 5.37 0.07 0.68 1 .oo SAPO-11 (F24/3) 7.39 0.03 0.18 7.06 0.12 0.70 1 .OO

Temperature 723 K, WHSV 2.4 h-', TOS 1.0 h, pressure 1 atm.

(c) Dehydration of isobutanol

product distribution (wt.%) conversion (Z)-but-2-ene but-1-ene but-2-ene

catalyst T / K (wt . Yo) c3 butanes butenes /(E)-but-2-ene /(E)-but-2-ene /totalbutenes

ALPO-11 SAPO-11 (P24) SAPO-11 (F24/1) SAPO- 1 1 (F24/2) SAPO- 1 1 (F24/3)

ALPO-11 SAPO- 1 1 (P24) SAPO-11 (F24/1) SAPO- 1 1 (F24/2) SAPO-11 (F24/3)

ALPO-11 SAPO- 1 1 (P24) SAPO-11 (F24/1) SAPO- 1 1 (F24/2) SAPO-11 (F24/3)

473 473 473 473 473

523 523 523 523 523

573 573 573 573 573

- 13.86 12.20 11.20 0.43

12.10 86.00 75.00 73.00 36.00

70.00 100.00 100.00 100.00 90.10

- - - - -

-

0.05 0.03 - -

0.02 0.40 0.26 0.11 0.07

-

0.13 0.14 0.12 0.08

-

0.50 0.37 0.38 0.24

1.57 0.56 0.42 0.43 0.55

-

13.73 12.06 11.08 7.64

12.10 85.45 74.60 72.62 35.76

68.41 99.40 99.32 99.46 89.48

-

0.56 0.52 0.5 1 0.43

0.54 0.59 0.54 0.54 0.55

0.60 0.62 0.61 0.60 0.59

-

0.09 0.06 0.06 0.06

0.05 0.08 0.07 0.06 0.06

0.06 0.12 0.10 0.09 0.08

-

0.67 0.78 0.83 0.84

0.79 0.50 0.62 0.68 0.74

0.75 0.43 0.48 0.50 0.56

WHSV 2.4 h-', TOS 2 h, pressure 1 atm.

(molecular diameter, 0.55 nm) is relatively bulkier than (E)- but-2-ene (molecular diameter, 0.48) and possesses a relatively high dipole moment24 (0.3 D). Therefore, formation of (E)-but- 2-ene is enhanced in smaller pore molecular sieves. Among the SAPO-11 samples, SAPO-11 (F24/3) shows enhanced forma- tion of (E)-but-2-ene. This is attributed to the decrease in void volume and the narrow opening due to the deposition of non- acidic silicon species on the external and internal surface of the SAPO-11. Deposition of non-acidic silicon species on the external and the internal surfaces decreases the surface acidity and slows down the movement of the cis-isomer out of the pore system, leading to more (E)-but-2-ene. However, the dis- tribution of isomer ratios depends on the conversion level. The cisltrans ratio increases over SAPO-11 catalysts at higher reaction temperatures (473-573 K).

4. Conclusions SAPO-11 was synthesized hydrothermally from a sili- coaluminophosphate gel containing fluoride anions and com-

pared with that obtained by the conventional method. The synthesis in the presence of HF leads to higher quality, larger crystals than those obtained by the conventional method of synthesis. Fluoride ions solubilize silicon during the crys- tallization period and give rise to more silicon in the SAPO-11 samples, however, the distribution of silicon is not uniform. The amount of silicon incorporated is correlated with the amount of HF added during the synthesis. Thus, SAPO-11 with different composition gradients in the crystals can be synthesized by choosing appropriate conditions and an appropriate method of synthesis. The results of thermal analysis show that the prepared material, with or without F-, differs with respect to both morphology and physico-chemical properties. This is likely to be due to variations in the amount of silicon as siliceous domains on the external as well as on the internal surface.

Equilibrium sorption capacities of hydrocarbons (hexane, cyclohexane) are nearly the same, however, the SAPO-11 syn- thesized in the presence of fluoride anions exhibits less water sorption capacity. Ammonia sorption isotherms in different

2024 J . Chem. SOC., Faraday Trans., 1996, V d . 92

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

3 r I

300 -

7 4" - 200 1 a'

-

lool -7 0 100 200 300

1 03 P/ p0

1 5 0 0

a' 1 ' O 0 1

- 501 o n

100 200 300 PITorr

2 50

200

E %. a' m 100 z

'0 150

1

50

0 100 200 300 460 1 O3PIP0

100

80

60 1 a'

40

0 100 200 300 PITO r r

Fig. 6 Dubinin plots for ammonia sorption in (a) SAPO-11 (P24) at 1, 333; 2, 363; 3, 393 and 4, 423 K and (b) SAPO-11 (F24/2) at the same temperatures. BET plots for ammonia sorption in (c) SAPO-11 (P24) at 1, 333; 2, 363; 3, 393 and 4,423 K and (d) SAPO-11 (F24/2) at the same temperatures. Langmuir plots for ammonia sorption in (e) SAPO-11 (P24) at 1, 333; 2, 363; 3, 393 and 4, 423 K and (f) SAPO-11 (F24/2) at the same temperatures.

.-. 4 5 - - E I 7 x - I

40 - 6 -

0 5 10 15 20 2.0 2.5 3.0 3.5 4.0 NH3 concentration/molecules (unit cell)-' NH3 concentration/molecules (unit cell)-'

Fig. 7 (a) Chemical affinity for ammonia sorption at 333 K in 1, SAPO-11 (P24) and 2, SAPO-11 (F24/2). (b) Isosteric heats (Qs,) for ammonia sorption in 1, SAPO-11 (P24) and 2, SAPO-11 (F24/2).

J . Chem. SOC., Faraday Trans., 1996, Vol. 92 2025

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online

X 3% + - H’ CH~--~H-CH,-CH~

+

L/ Fig. 8 Reaction scheme for isobutanol dehydration over SAPO-11

SAPO-11 up to 400 Torr in the low-temperature region are found to be type I isotherms (Langmuir type) according to Kiselev’s classification. At higher temperature, they deviate from the Langmuir type to the Henry’s type. The amount of irreversibly chemisorbed ammonia is found to be dependent on the intrinsic acidity associated with the microporous molecular sieves. The saturation capacities obtained from linear Dubinin plots and monolayer capacities obtained from linear Langmuir plots are in fair agreement with the experi- mental sorption capacities. SAPO-11, synthesized by the con- ventional method, exhibit higher chemical potential and Qst

value than SAPO-11 synthesized in presence of fluoride anions.

The acidic properties reported in the hydrocarbon reactions indicate the difference in acid sites of ALPO-11 and SAPO-11. High para-selectivity amongst the products owing to restricted transition-state shape selectivity are observed in ALPO-11 and SAPO-11. Comparison of isobutanol dehydration over ALPO-11 and SAPO-11 has demonstrated the acidity/activity aspects and influence of pore confinement on the isomeric dis- tribution of but-2-ene.

The authors thank Dr. P. Ratnasamy and Dr. V. P. Shiralkar for stimulating discussions and encouragement. They also thank the referees for their suggestions which added much value to the paper; P.S.S. and R.B. thank CSIR (New Delhi) for Senior Research Fellowships.

References 1 E. M. Flanigen and R. L. Patton, US Pat. No. 4073865, 1978. 2 (a) J. L. Guth, H. Kessler and R. Wey, in Proc. 7th Int. Zeolite

Conf., ed. Y . Murakami, A. Lijima and J. W. Ward, Elsevier, Amsterdam, 1986, p. 21; (b) Y. Xu, P. J. Maddox and J. W. Couves, J. Chem. SOC., Faraday Trans., 1990, 86, 425; (c) F. Hoffner-Marcuculli, Thesis, Mulhouse, 1992.

3 (a) Y. Xu, P. J. Maddox and J. M. Thomas, Polyhedron, 1989, 8, 819; (b) S. A. Axon and J. Klinowski, J. Chem. SOC., Faraday Trans., 1993, 89, 4245; (c) H. Kessler, J. Patarin and C. Schott- Darie, Stud. S u r - Sci. Catal., 1995,85, 75.

4 (a) B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, J. Am. Chem. SOC., 1984, 106, 6092; (b) X. Wang, X. Liu, T. Song, J. Hu and J. Qiu, Chem. Phys. Lett., 1989, 157, 87; (c) A. F. Ojo, J. Dwyer, J. Dewing, P. J. O’Malley and A. Nabhan, J. Chem. SOC., Faraday Trans., 1992,88, 105; (6) P. P. Man, M. Briend, M. J. Peltre, A. Lamy, P. Beaunier and D. Barthomeuf, Zeolites, 1991, 12, 342; (e) J. A. Martens, P. J. Grobet and P. A. Jacobs, J. Catal., 1990,126,299. (a) P. J. Barrie, M. E. Smith and J. Klinowski, Chem. Phys. Lett., 1991, 180, 6; (b) J. M. Bennet, J. W. Richardson Jr., J. J. Pluth and J. V. Smith, Zeolites, 1987,7, 160.

6 C. S. Blackwell and R. L. Patton, J. Phys. Chem., 1988,92,3965. 7 E. Jahn, D. Muller and K. Becker, Zeolites, 1990, 10, 151. 8 E. M. Flanigen, B. M. Lok, R. A. Patton and S. T. Wilson, Stud.

Surf. Sci. Catal., 1988,37, 13. 9 E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, in ref.

2(a), p. 103. 10 M. Martens, J. A. Martens, P. J. Grobet and P. A. Jacobs, Guide-

lines for Mastering the Properties of Molecular Sieves, NATO AS1 Series, Series B, Physics, 1989, vol. 1, p. 221.

11 P. S. Singh, R. Bandyopadhyay and B. S. Rao, J. Mol. Catal., 1995,104, 103.

12 J. Kornatowski, G. Finger, K. Jancke, J. Richter-Mendau, D. Schultz, W. Joswig and W. H. Baur, J. Chem. SOC., Faraday Trans., 1991,90,2141. (a) V. R. Choudhary and D. B. Akolekar, J. Catal., 1987, 103, 115; (b) S. G. Hegde, R. Ratnasamy, L. M. Kustov and V. B. Kazansky, Zeolites, 1988, 8, 137; (c) J. Das, S. P. Lohokare and D. K. Chakrabarty, Indian J. Chem. A, 1992,31,742.

14 P. S. Singh, R. Bandyopadhyay, S. G. Hegde and B. S. Rao, Appl. Catal., 1995, 136, 249.

15 A. V. Kiselev, Discuss. Faraday SOC., 1965,40,205. 16 B. Coughlan and J. J. McEntee, Proc R . Irish Acad., 1976, 76,

Section B (28, 29, 30), 473. 17 V. P. Shiralkar and S. B. Kulkarni, J. Colloid Interface Sci., 1985,

1 0 8 , l . 18 S. Prasad and R. Vetrivel, J. Mol. Catal., 1993,84,299. 19 B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R.

Cannan and E. M. Flanigen, US Pat. No. 4,440,871, 1984. 20 M. M. Dubinin and L. V. Radushkevich, Proc. Acad. Sci., USSR,

1974,55, 327. 21 R. M. Barrer and B. Coughlan, Molecular Sieves, Society of

Chemical Industry, London, 1968, pp. 141,233,241. 22 V. P. Shiralkar and S. B. Kulkarni, J . Colloid Interface Sci., 1984,

109, 115. 23 P. S. Singh, R. Bandyopadhyay, R. A. Shaikh and B. S. Rao, Stud.

Surf. Sci. Catal., 1995,97, 343. 24 M. Richter, H-L. Zubowa, E. Schreier, B. Parlitz and J. Richter

Mendau, Zeolites, 1994, 14,414. 25 W. J. Taylor, D. D. Wagman, M. G. Williams, K. S. Pitzer and

R. D. Rossini, J. Res. Nut. Bur., 1946,37,95.

5

13

Paper 5/05118E; Received 1st August, 1995

2026 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by 5

1323

9 on

26/

09/2

013

14:5

1:12

.

View Article Online