Perspectives in Molecular Sieve Science - ACS Publications

Chapter 1

N M R Spectroscopy and Zeolite Chemistry

Downloaded by UNIV OF PRINCE EDWARD ISLAND on January 23, 2015 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch001

J. B.Nagy and E . G. Derouane Laboratory of Catalysis, Center for Advanced Materials Research, Facultés Universitaires Notre Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium

The characterization of catalysts before, during, and after their catalytic evaluation is essentiel. Nuclear magnetic resonance (NMR) is a well-suited technique to observe subtle changes in zeolite catalysts as their composition is made of several NMR-sensitive nuclei such as Al, Si, and O as well as others, i.e., L i , Na, etc... corresponding to exchange cations. In addition, the method can also be used to study the behavior of adsorbed molecules and of carbonaceous deposits during or after testing. In this review paper, we focus our attention on the characterization of the as-synthesized zeolites, the changes occurring during their pretreatment and/or subsequent modifications (calcination, steaming, dealumination, isomorphous substitution in their framework), and the conversion of adsorbed reactants. Si-NMR enables the determination of the Si/Al ratio of zeolitic frameworks and of the number of crystallographically distinct sites. Indeed the chemical shift of Si is influenced by both the geometry of the sites (Si-O-T angles, T = Si,Al,B, ...) and their chemical environment (substitution of Si by other atoms (Al,B, ...) in the second coordination sphere). The distribution of the Al atoms can be either random or specific (partly or completely). The recognition of this fact may impact on our picture of the zeolite action as catalysts. Finally, the observation of SiOH defect groups in precursor zeolites, and of their modification during calcination, steaming, or dealumination, etc... does largely contribute to our understanding of the chemistry of these materials. Al-NMR characterizes aluminium species of different coordination (tetra- or octahedral), and their interaction with the surroundings. Quantification of 27Al-n.m.r. measurements can be achieved in well-defined conditions. The nature of the species resulting from dealumination can be inferred by studying the chemical shift and linewidth variations of the observed resonances. Li- and Na-NMR probe the exchange cations compensating the framework ([Si-O-Al]) or defect (SiO ) negative charges. The distribution of alkali cations in the as-synthesized catalysts throws some light on their capability to accompany the aluminium atom in the structure. In addition, the method can also be used to investigate the chemical behavior of precursor gels. 27

29

17

7

23

29

27

7

23

-

-

0097-6156/88/0368-0002$08.75/0 © 1988 American Chemical Society

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

1. B.NAGY AND DEROUANE

NMR Spectroscopy and Zeolite Chemistry

3

Interactions between the molecules occluded during synthesis and the zeolitic framework can be examined by C-NMR The behavior of reactants can also be monitored before (mobilities), during (reactivity) and after (reaction products and coking) the catalytic tests. Some typical examples of such in-situ N M R studies will be discussed.


13

Zeolites are microporous crystalline silicoaluminate materials (1). They have been extensively studied during the last 30 years, leading to the synthesis of novel structural species (1,2) and to a great number of applications (ion-exchange, molecular sieve, catalytic reactions,...) (1.2). Microporous materials with optimal properties (composition, acidity, thermal stability, pore size,...) desired for various applications can be obtained either by modifying classical zeolites (to reach higher Si/Al ratios using various dealumination procedures or substituting Si and/or A l by other elements) or by synthesizing novel zeolitic structures (borosilicates, borosilicoaluminates, gallosilicates, phosphoaluminates, ...) (2-4). In this review paper we focus our attention on the characterization of the as-synthesized zeolites, the changes occurring during their pretreatment and subsequent modifications (calcination, steaming, dealumination, isomorphous substitution in their framework), and the conversion of adsorbed reactants. Several review papers appeared recently, which deal with the application of N M R to the characterization of zeolites (5-7) or with the study of the adsorbent-adsorbate interaction (7-10). Most of them, however, emphasize the importance of the N M R measurements (5.6.8.9) and only two reviews treat the problems from essentially a catalytic point of view (7.10). This short review paper will be devoted to our own results obtained in the Laboratory of Catalysis and reference will only be done to illustrative publications in the literature. EXPERIMENTAL N M R spectra were recorded at room temperature using a Bruker CXP-200 high power spectrometer. The relevant N M R parameters are described in Table I. Table I. N M R parameters for the various nuclei Nucleus on 1 3

2 3 7

c

Na Li

v(MHz)

Reference

39.7 50.3 52.1 52.9 77.7

tpulse^)

TMS TMS A 1 ( N 0 ) aq NaC10 aq L i C l aq 3

4

3

64 tetrahedra characterized by the different interatomic distances and angles of the Si-O-T linkages (T = Si or Al) (13.14). 2 9

In silicates the total range of S i - N M R chemical shifts is appreaciable (from -60 to -120 ppm) and is subdivided into characteristic ranges for monosilicates (Q())> disilicates and chain end groups (Q\), middle groups in chains and cycles (Q2), chain branching sites (Q3) and the three-dimensional cross-linked framework (Q4) (12) (Figure 1). Substitution of silicon by aluminium leads to an additional paramagnetic shift. In addition, the sharpness of the resonance lines reflects the degree of crystallinity and depends on the regularity of the Si,Al distribution. From the relative intensities of the different Si(nAl) N M R lines, quantitative determination of S i / A l ratios is possible, provided the A l avoidance rule of Loewenstein is obeyed (15): t n

_Si_ Al

=

0

I Si(nAl)

}

NMR

4 I 0.25 n=0

n

I A

Si(nAl)

This formula is easy to understand, if one realizes that each Si-O-Al linkage accounts for 0.25 A l atoms. For zeolites characterized only by one type of crystallographic site, the computation of the Si/Al ratio is quite obvious. This is illustrated in Figure 2, where the S i - N M R spectra of zeolite Y and ZSM-20 are compared. The spectrum of zeolite Y includes five lines corresponding to Si(4Al), Si(3Al), Si(2Al), Si(l Al) and Si(OAl) configurations while that of ZSM-20 consists of four lines of Si(3Al), Si(2Al), Si(lAl) and Si(OAl) configurations. These latter resonance lines are systematically located at lower chemical shifts relative to the lines of zeolite Y (£), due to the higher Si content of the ZSM-20 zeolite. The Si/Al ratio computed from the relative N M R line intensities is equal to 4.3 and agrees quite well with the 2 9



B.NAGY AND DEROUANE

Figure 1.


Si-NMR chemical shifts of silic(oalumin)ates.


PERSPECTIVES IN MOLECULAR SIEVE SCIENCE


6




7

chemical E D X analysis (4.2) (1£). Indeed, zeolite ZSM-20 is a silica-rich zeolite, possessing a faujasite-like character but having a hexagonal symmetry unit cell (16.17). The high silica content could result in higher (hydro)thermal stability and acid strength, and better resistance to deactivation relative to the Y-type zeolites. The computation of the Si/Al ratio becomes more tedious, when several crystallographic sites are present, resulting in overlapping of various Si(nAl) j configurations (T^: ith crystallographic site) (i&). Several methods have recently been proposed based on one of the following assumptions regarding aluminium site location : specific location of A l on certain crystallographic sites (19.20) (see below), a fully random distribution of A l atoms (20.21) or a random distribution with some preferential location of A l on specific sites (22.23).


X

The Generalized Random with some Constraint model (GRC) is based on the following hypotheses (22): i) On each type of Tj site the Si, A l distribution is random, but the A l per Tj site is different for each i type crystallographic site ii) The Loewenstein's rule is obeyed. This model is applied to the study of A l distribution in offretite and omega zeolites, which both possess two different crystallographic sites. The probability p for a S i j i atom to have an A l atom as a Tj neighbour is described by : Pi = PtAlxi : S i ]

p = p[Al

X 2

: Si ]

p =p[Al

p =p[Al

x l

: Si ]

T 1

3

x 2

: Si ] T 1

2

4

T 2

x 2

(Note that in ref. 22 only two probabilities p^ and p were taken into account.) 2

Including Loewenstein's rule into the calculation, these probabilities have the following simple expressions (22): Pi

=

Al

x l

/Si

p = Al^/Si^

x l

2

p = (a/b) ( A l ^ / S i x x )

p = (b/a) ( A l ^ / S i ^ )

3

4

where a = S i i + A l j and b = S i o + A 1 are the numbers of crystallographic sites 1 and 2 per unit cell, respectively. The relative amounts S i j and A l j can be expressed as a function of two variables, i.e. R = Si/Al (global ratio) and x = Al-H/Al-nOa): X

X

X

X 2

X

Si

x l

= [a(R+l) (x+1) - x(a+b)]/(R+l) (x+1)

Si

X 2

= [b(R+l) (x+1) - (a+b)/(R+l) (x+1)

A1

X 1

Al

X 2

X

=x(a+b)/(R+l)(x+l) = (a+b)/(R+l)(x+l)

Finally the relative intensity of the N M R lines are computed: x

( Si(nAl))Ti = [ S i / ( S i T i

x l

+ S i ^ ) ] x Si(nAl) x I xi

t o t

where I is the total line intensity and Si(nAl) j the fraction of Si(nAl) j atoms, determined by combinational analysis : t o t

X


X


8

Si(nAl)

3

T1

SiCnAl)^

(n ) ( 1 - ) "

2

n P

l

4

n

1 1

3

n

2

2 4

) (1-P2)

n

3

n

4-11

n

n

(

2

2

3

P2 " P4 l

r/4) - 2) - 2 ) ] ( l - p ) - (l-n)V2 ~ "n 4-n (

1

(l-p ) + (4*-n) (1-Pi) " P i " p

£ ) d - P 2 ) - P^(l-P4) + ( +


n

P l

2

P4

These expressions are formally similar to those reported in ref. 22, they differ however in the meaning of the probability p^. Finally the spectra are simulated by varying R, x, and w, where w is the linewidth at half maximum. The experimental and theoretical spectra for steam dealuminated omega and offretite samples are illustrated in Figure 3. The correspondence between the experimental and theoretical spectra are excellent. One has also to emphasize, that the computation yields R = Si/Al, where the A l atoms are only those in tetrahedral sites of the structure noted as (Si/Al)jy in Figure 3. The Si/Al ratio determined by chemical analysis corresponds, on the other hand, to global composition including both tetrehedral framework and tetra-and/or octahedral non-framework aluminium atoms. The presence of extraframework A l can be noted in both cases because the Si/Al values are systematically lower than the (Si/Al)jy values. It was mentioned above that the ^ S i - N M R chemical shifts are also sensitive to the different interatomic distances and angles of the Si-O-T linkages (13.14). Indeed, the spectrum of the monoclinic form of highly siliceous ZSM-5 (or silicalite 1) shows as many as twenty one different resonance lines (Figure 4) (2£). It is now well known that the structure becomes orthorhombic at higher temperatures (2£) and under the influence of adsorbed molecules (22). The SiOR (R = H , metal or organic cation) defect groups are also readily detected by S i - N M R . In zeolites of quite different structures the corresponding resonance line is at ca -103 ppm (13.19.28). due probably to the similar average Si-O-T angles. Cross-polarization enhances the line characterized by a large contribution of SiOH groups (7.28) (Figure 5). The amount of SiOR defect groups increases with decreasing A l content of the ZSM-5 samples (29.30) (Figure 6) and as many as 32 SiOR per unit cell can be detected in highly siliceous samples. As four SiOR groups could correspond to each missing tetrahedral site (see below), this means that some eight T sites would be non occupied in the structure. These results confirm the model previously proposed for the formation of ZSM-5 zeolite, where double five-membered rings were supposed to condense to lead to a structure containing a high number of defect groups (2Q). 2 9

2

The ^ A 1 - N M R spectra are essentially used to characterize tetrahedral framework and octahedral extraframework A l species (6.31). (Figure 7). However, quite recently, Lippmaa et al. succeded in determining the precise chemical shift of A l using special N M R techniques, such as 2D M A S N M R or measuring the first or second spinning side-bands of the least shifted first satellite (±3/2, ±1/2) transition (32). They found a linear correlation between the chemical shift and the mean Al-O-Si bond angles in framework silicoaluminates. This method will thus be quite complementary to the S i - N M R to characterize the zeolite structures. It can also be 2 9



B.NAGY AND DEROUANE


Figure 3. Experimental and theoretical Si-NMR spectra of steam dealuminated offretite and omega. (Reproduced with permission from ref. 23. Copyright 1988 Elsevier.)

I

i

i

i

i

[

-108 «4

i

-113 p.p.m.

i

i

i

i

i

-118

Figure 4. Si-NMR spectrum of highly siliceous ZSM-5 zeolite. (Reproduced with permission from ref. 25. Copyright 1987 Macmillan Magazines.)



T

1

1

1

r


Si(OAI)

I

-70

1

1

-90

1

i

-110 8(p.p.m.)

i

-130

Figure 5. Si-NMR spectra of mordenite dealuminated by acid leaching and subsequent steaming. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)


B.NAGY AND DEROUANE

1

1

1


r

~

40h

• Na* O LI* OK* A Rb+

A


\ 30

\

• \

NP q

1

\ 20F

•

V

\

A

\ A ^

O \

10

•

O

JL 2 Al/u.c. -

Figure 6. Variation of the SiOR/u.c. (R = H, M, and TPA) as a function of Al/u.c. in the ZSM-5 zeolites. (Reproduced with permission from ref. 41. Copyright 1987 Butterworth.)




Figure 7. Al-NMR spectra of H-Mordenite recorded at two different flip angles. (Reproduced with permission from ref. 34. Copyright 1988 Elsevier.)




13


used to position A l atoms in the different crystallographic sites, provided the bond angles are accurately determined. For quantitative measurements, special care should be taken for the flip angle used. Indeed, it was shown previously, that in the case of quadrupolar nuclei the magnetization corresponding to different transitions rotate at different rates around the rf field (22). This results in a large variation of the line intensity. Figure 7 shows the different line intensities for the tetra- and octahedral A l atoms obtained at different flip angles (34). It has to be emphasized, that for adequate quantitative measurements short pulse anlges (0 < %I6) should be used (33.34). Figure 8 illustrates the variation of normalized line intensity as a function of flip angle. It should be noted, that for the two separate components, the intensities are normalized with respect to the total A l atoms in the sample. It is clear that the correct relative intensities can only be computed from small flip angles, id ca. 20° in this case. The method for quantitative analysis is the direct comparison of the initial slopes. From the initial slopes, the ratio of the octahedral to the tetrahedral A l species can be computed as (24) * slope (octahedral species) =27% slope (tetrahedral species) -NMR has also been applied quite recently to the characterization of zeolites (35.36). Figure 9 shows the wide line N M R spectra of Linde A (having only Si-O-Al bonds), SiC>2 (for Si-O-Si bonds), the sum of these spectra and the spectrum of Linde Y . The quadrupolar coupling constants as well as the chemical shifts of Si atoms in either Si-O-Al or Si-O-Si bonds are sufficiently different and the spectrum of zeolite Y can be decomposed into two contributions. This decomposition also leads to an easy determination of the Si/Al ratio. Indeed : 2[Si-0-Si] + [Si-O-Al] Si/Al = [Si-O-Al] where the concentrations of the Si-O-Si and Si-O-Al bonds are directly determined from the spectra^ The 0 - N M R brings thus complementary informations obtained by S i - and A 1 - N M R . The basic difficulty resides in the low sensitivity of natural compounds and hence 0-enriched materials should be used to achieve spectra of high signal to noise ratios. 1 7

2 9

27

17

7

2

L i - and ^ N a - N M R techniques are useful to study the hydration state of these alkali cations as well as their interactions with the negative centers of the framework. Relatively few studies have been reported either on the behaviour of L i (37-39) or that of Na+ ions (29.37.40). Figure 10 illustrates the L i - N M R spectra of the precursor and calcined ZSM-5 zeolite samples. Two lines can be distinguished in the spectrum of the precursor : a broad line (AH = 4 kHz) and a narrow line (AH = 0.5 kHz) both centered at ca 0 ppm (41). The former is assigned to partially hydrated or anhydrous L i cations, while the latter is characteristic of fully hydrated cations. Indeed, thermal gravimetry (TG) data indicate that the available water is not sufficient to hydrate fully the total amount of L i cations (Table II). For a full hydration of L r cation ca four water molecules are required (42). A relatively slow exchange may occur between the two species. The relative amount of Lihydr decreases with decreasing amount of A l / u . c . Indeed, it is accompanied oy a decrease of the amount of those L i ions which are counterions to [Al-O-Si]" framework negative charges. In this state these ions are then fully hydrated. Similar +

7

+

+

+



100

•

AKN0 )3 aqueous solution 3


o - H-Mordenite

•-

H-Mordenlte (tetrahedral Al)

•-

H-Mordenite (octahedral Al)

30 60 Pulse angle (degrees)

Figure 8. Normalized Al-NMR line intensity as a function of flip angles. (The intensities for tetra- and octahedral Al atoms are normalized to the total amount of Al in mordenite.) (Reproduced with permission from ref. 34. Copyright 1988 Elsevier.)

STATIC, 67.8 MHz

«*-ppm— Figure 9. O-NMR spectra of zeolites A, Y, and Si0 . (Reproduced from ref. 36. Copyright 1986 American Chemical Society.) 2


1.

B.NAGY AND DEROUANE


[

2kHz,

/

'

[

'


(Li, TPA) ZSM-5

)

(Li) ZSM-5

J

15

\

I—LI (H0)n 2

_JL

7

Figure 10. Li-NMR spectra of the precursor and calcined ZSM-5 samples (Reproduced with permission from ref. 41. Copyright 1987 Butterworth.)

Table II. a

Li O Al 0 2

2

a 3

7

Li-NMR and chemical analysis of (TPA,Li)-ZSM-5 zeolites

7

Li-NMR A H (kHz) AH°(kHz) hydr

Si/Al

d

Al/u.c.

d

Li/u.c.

c

SiOLi/u.c.

f

+

Li ..."[Al-0-Si]

f

&

HQ 2

Li

/u.c.

-xlOO

/Li

total

ôtal

0.25 1

2 1 1 1

1 1 1 0.6 0.3 0

4.0 4.2 4.5 4.1 4.2 4.3

0.62 0.50 0.42 0.54 0.54 0.54

28 30 15 15 19

39 39 41 69 85 390

2.4 2.4 2.3 1.4 1.1 0.25

0.4 1.3 1.7 1.2 2.6 1.2

0 0.6 0.9 1.4 3.1 -

0.6 0.9 0.9 0 0 -

2.3 0.9 0.6 2.0 1.1 -

a) Relative amounts in initial gels : xTPABr-y(TPA)2O-zLiÔ-wAl2O3-60SiO2-700H2O; x + 2y = 8 (constant TPA content) and y + z = 2 (constant alkalinity) b) Broad NMR line c) Narrow NMR line d) PIGE (proton induced y-ray emission) measurements e) Atomic absorption values f) Values computed from combined chemical analysis, Si-NMR and thermal analysis of TPA (41) g) TG (thermal gravimetry) data combined with chemical analysis 29

SOURCE: Reproduced with permission from ref. 41. Copyright 1987 Butterworth.


16

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE +

conclusions have been arrived at, when considering the state of N a ions in the ZSM-5 zeolite samples (43.44). It is also clear from Table II, that the non- or less-hydrated L i species are the SiOLi defect groups, where the less delocalized negative charge of the O atom interacts preferentially with less solvated cations (41.45V


+

Both linewidth and the chemical shift of ^ N a depend on the relative amount of A l in the ZSM-5 samples (29.401 A chemical shift close to 0 is obtained for highly siliceous samples, while the high Al-samples exhibit a spectrum characterized by a 8 = -15 - -20ppm (29.40.45). The linewidth is also greatly influenced by the relative hydration of the cation. A systematic work is still lacking including both the relative importance of SiONa and (Si-0-Al)~Na groups in the zeolitic samples together with the degree of hydration of the N a cations. +

+

High resolution M A S - N M R is a promising method to investigate the nature of carbon-containing molecules occluded into the zeolitic channels or cavities during the synthesis. The ^ C - N M R data show clearly that tetrapropylammonium ions are occluded intact in the ZSM-5 zeolite channels (10.46.47). This is also true for tetrabutylammonium and -phosphonium cations which direct the ZSM-11 structure (10.46). Moreover, two types of methyl groups are distinguished, the difference in chemical shift being due to different chemical environments. In ZSM-5 zeolite every channel intersection is occupied by a tetrapropylammonium ion. Oppositely, tetrabutyl-ammonium and -phosphonium cations occupy preferentially the large cavities in ZSM-11 zeolite, the small cavities being only partially occupied. The alkyl chains of all these organic cations occluded in both ZSM-5 and ZSM-11 zeolites extend in the channel system in order to fill completely the available channel length. 3

Note that ^ C - N M R has also been used to detect the presence of small ZSM-5 particles, which must contain less than 3 or 4 unit cells in thickness, corresponding to a size which is beyond the X R D detection limit (48.49). A change in the interaction occurs between the tetrapropylammonium ions and the ZSM-5 framework during calcination and the doublet splitting of the methyl groups disappears (50.51). The new species formed is described as a relaxed tetrapropylammonium ion occluded in a partially healed ZSM-5 structure (51). It was shown quite recently that in the synthesis of zeolite ZSM-48 in presence of an alkylamine and tetramethylammonium ( T M A ) ions, only the n-alkylamine plays a structure-directing role, while T M A ions are the main species incorporated during the growth process, until complete filling of the channels is achieved (£2). For example, 0.2 molecule of n-octylamine (2.6 A) and 1.9 T M A ions (13.1 A) per unit cell are occluded intact in final crystalline samples. +

+

In contrast to zeolite ZSM-48, the formation of ZSM-39 seems to be exclusively directed by the T M A ions. Indeed, a slight modification in the gel composition (no alkylamine present, increase in the A l content) and in the synthesis procedure, leads to preferential formation of zeolite ZSM-39. Its crystallization is accompanied by the dequaternation of T M A ions, producing trimethylammonium ions in equilibrium with trimethylamine. These latter two organic compounds are essentially occluded in the large cavities of ZSM-39 (52) (Figure 11). Note that the species adsorbed on external surface can be distinguished from those occluded in internal cavities by considering the much broader N M R lines of the former. +

+

In presence of hexamethenium (HM++) ( ( C r ^ + N - C r ^ - C I ^ - C H ^ ions, either zeolite ZSM-48 or ZSM-50 can be formed, depending on the initial




17

+ +

aluminium concentration. A decomposition of H M ions in the synthesis conditions only occurs if the precursor hydrogel contains ammonium ions (Figure 12). This decomposition leads to the formation of hexyltrimethylammonium ions, which are also occluded into the zeolite crystals. It is concluded, mat H M ions act as structure-directing agents and the total filling of the void volume is achieved in both cases (52). + +


Dealumination and realumination of zeolites In order to increase their thermal stability and the strength of their acid sites, high A l content zeolites are submitted to various dealumination treatments (7.19.53-55).Some procedures will be illustrated by the dealumination of zeolite mordenite, where the S i - N M R results are shown to lead to the determination of A l distribution in the samples, as well as to the proposition of a dealumination mechanism (19). 2 9

27

The A 1 - N M R spectra of the parent Na-mordenite (commercial Na-Zeolon (Norton)) and of its analogues dealuminated by acid leaching at various temperatures and for various times are presented in Figure 13. The Na-Z spectrum is characterized by a unique resonance line at 8 = 50 ppm vs A1(H20)£ , corresponding to tetrahedralry coordinated A l of the framework (Alj). An additional line at 8 = 0 ppm appears in the spectra of acid-treated samples and it belongs to octahedral A l species extracted from the lattice (AIQ). The narrowness of this line (0.8 kHz) suggests that the octahedral A l ions are fully hydrated and occupy probably cationic positions. From these spectra, the relative amounts of tetrahedral and octahedral A l atoms can be determined. (Note that all the spectra were taken at small flip angles providing quantitative detection of all A l atoms (34)). These data together with the Si/Al ratios obtained by chemical analyses (Table HI) allow one to compute the real composition of the zeolitic framework, if it is assumed that all A l y atoms belong to the lattice (Table IV). 3 +

+ 3

Table m . sample Na-Z H-Z-l H-Z-2 H-Z-3 H-Z-4 H-Z-5

a

Dealumination of mordenite by acid leaching

exptl conditions

b

4 M H N 0 , 293 K , 8 h 4 M H N O 3 , 323 K , 24 h 4 M H N O 3 , 363 K , 24 h 6 M H N O 3 , 363 K , 24 h 14 M H N O 3 , 363 K , 24 h 3

AA 5.5 6.1 5.8 20.5 20.6 31.2

Si/Al* EDX

27

A1-NMR 5.4 6.3 7.1 21.6 27.6 30.7

5.6 6.6 7.1 19.2 22.3 28.0 2 7

av 5.5 6.3 6.7 20.4 23.5 30.0

A A atomic absorption; E D X energy-dispersive X-ray analysis; A l N M R results obtained from a calibration curve " Commercial Na-Zeolon. SOURCE: Reproduced from ref. 19. Copyright 1986 American Chemical Society.


PERSPECTIVES IN MOLECULAR SIEVE SCIENCE TMAm +


recursor

(Calcined

J

I

70

I

I

i

I

at530°c)

I

-ppm-

13

Figure 11. C - N M R spectra of zeolite ZSM-39 prepared without nalkylamine: precursors and partially calcined sample at 530 °C to remove externally adsorbed species. (Reproduced with permission from ref. 52. Copyright 1988 Elsevier.)

ZSM-48

a: H M ^ b: hexylTMAm+ c: TMAm+

ZSM-50

ppm13

Figure 12. Characteristic C - N M R spectra of a ZSM-48 sample prepared in the presence of (NH ) 0 and low Al content and of a ZSM-50 sample obtained without (NH ) 0 and with a high Al content in the gel. (Reproduced with permission from ref. 52. Copyright 1988 Elsevier.) 2

4

2



B.NAGY AND DEROUANE


Figure 13. High-power solid-state Al-NMR spectra of Na-Zeolon and its dealuminated forms. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)


20


Table IV. Determination of the Si/Al ratios of the zeolite framework from the relative amounts of tetrahedral (T) and octahedral (O) A l and the overall chemical composition of the Na-and acid-treated mordenite samples

samples

2 7

chem. compn. Si/Al


a

Na-Z H-Z-l H-Z-2 H-Z-3 H-Z-4 H-2-5

A1 NMR A l % A1 %

latticecompn Si/A1 Alj/uc

100 81 75 89 93 93

5.5 7.7 8.9 22.9 25.3 32.2

x

5.5 6.3 6.7 20.4 23.5 30.0

D

Q

0 19 25 11 7 7

c

7.4 5.5 4.8 2.0 1.8 1.4

a From Table m J Si/Al) = [(Alp + A1 )/A1 ] (Si/Al) global A lj/uc = number oftetrahedral A l atoms per unit cell; A l / u c = 48/[l + ( S i / A l ) ] l a t t i c e

X

T

c

T

lattice

SOURCE: Reproduced from ref. 19. Copyright 1986 American Chemical Society. 1 y

Figure 14 shows the high-resolution M A S S i - N M R spectra of Na-mordenite and its dealuminated counterparts. The spectrum of Na-Z consists of three resonance lines at -99, -105 and -110 ppm, corresponding to Si(2Al), Si(lAl) and Si(OAl) configurations respectively. In addition, silanol groups at defect lattice sites contribute to the intensity of the N M R line at -105 ppm. As dealumination proceeds, the lines at -99 and -105 ppm decrease, while the relative intensity of the -110 ppm line increases (Table V).

z y

Table V.

Variation of relative H R M A S Si-NMR line intensities of mordenite at various degrees of dealumination by acid leaching

samples

Si/Al

Na-Z H-Z-l H-Z-2 H-Z-3 H-Z-4 H-Z-5

5.5 7.7 8.9 22.9 25.3 32.2

a

relative line intensities" (%) at 8 = -99 8 = -105 8 = -110 13 12 10 5 4 2

45 45 43 23 21 16

42 43 47 72 75 82

Si/Al

c

5.6 5.8 6.3 12 14 20

a

Lattice composition (Table IV). " Relative to the total N M R intensity. Recalculated by using the relation Si/Al = It/( X 0.25 n Isi(nAlp S SiOH defect groups do not contribute to the -105 ppm line intensity. SOURCE: Reproduced from ref. 19. Copyright 1986 American Chemical Society.

c

a s s u m m


m

a

t



21

This shows unambiguously the extraction of A l from the lattice leading to a decrease of the number of Si(lAl) and Si(2Al) configurations and to an increase of Si(OAl) configurations. However, the Si/Al ratios calculated from the general formula (see above) are not in good agreement with those obtained by combined ' A l - N M R and chemical analyses (Table V). The origin of this discrepancy can be explained by assuming a substantial contribution of SiOH groups to the -105 ppm line. Direct evidence for these SiOH groups are obtained by cross-polarization S i - H measurements for samples shown in Figure 5. The acid-leaching was followed by subsequent steaming. Consequently, the line at -105 ppm contains the contribution of both Si(lAl) configuration and defect SiOH groups : 2


2 9

J

-105

= I

J

and as *Si(lAl)

c a n b

e

+ I

Si(lAl)

1

SiOH

J

-99 = Si(2Al>

computed from *total

Si/A1

)lattice 2 x 0.25 x I 2 A l ) +

1 x

0

2

5

x

J

Si(

a n d

J

l

Si(lAl)

x

Si(0Al) = -110 + S i O H

The relative intensities of the different configurations so obtained are listed in Table VI. The number of SiOH groups at defect lattice sites per extracted A l atom is shown as a function of the number of extracted A l atoms per unit cell (Figure 15). In the beginning of dealumination up to four SiOH defect groups per extracted A l atom are generated in the structure (5©. Table V I : Evolution of the different Si configurations and tne number of SiOH defect groups generated per extracted A l during mordenite dealumination by acid leaching

samples

lattice compn Si/Al Al/uc

Na-Z H-Z-l H-Z-2 H-Z-3 H-Z-4 H-Z-5

5.5 7.7 8.9 22.9 25.9 32.2

a

7.4 5.5 4.8 2.0 1.8 1.4

Si configurations (% of total Si) SiOH Si(2Al) Si(lAl) Si(OAl) 0 17 18 15 14 8

42 61 66 87 89 90

45 28 24 8 7 8

13 11 10 5 4 2

b

SiOH/Al^ 3.7 3.0 1.3 1.2 0.6

b

Number of SiOH generated per A l extracted, S i ( O A l ) = Si(OAl) + SiOH SOURCE: Reproduced from ref. 19. Copyright 1986 American Chemical Society. t o t a l


2 1



Si(1 Al)

-80

-100

-120

5 (ppm/TMS) 29

Figure 14. High-resolution MAS Si-NMR spectra of Na-Zeolon and its dealuminated forms. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)

I 0

I 1

I 2

I 3 Al

I 4

e x t r a c t e d / U.C.

I 5

I 6

I 7

•

Figure 15. Variation of the total number of SiOH groups and the amount generated per extracted Al atom with the amount of Al extracted per unit cell. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)




Si I 0 I H

III Si I 0 Downloaded by UNIV OF PRINCE EDWARD ISLAND on January 23, 2015 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch001

23

= Si-0-A|H-0-Si= 1 0

—rr *=Si-0-H ?°/2H 0 4H

H—0—Si=

2

H

1

i

Si III

0 | Si +AI(H 0)J 2

+

The decrease of this number as dealumination proceeds is the result of a partial healing of the structure as it was also emphasized by other research groups (57.581 Figure 5 also shows that in addition to SiOH defect groups some Si(OH)9 groups are also present in the highly dealuminated sample. (Although on the basis of chemical shifts some other species like A10Si(OH)2 or ( A l O ^ S i O H could also be present, we regard them as minor components in these highly dealuminatied samples.) In addition, the S i - N M R spectrum without cross-polarization reflects the presence of different crystallographic sites, the attribution of the N M R lines was made on the correlation between chemical shifts and mean Si-O-Si angles (59). 2 9

When the dealumination is carried out using SiCl4, the extracted A l is deposited in the channels as extra-framework aluminous species. In addition, the SiCl4 treatment only partly removes A l from the lattice. This is caused by both reduced diffusivity of SiCl4 in the channels of mordenite, and by the presence of residual N a cations. The reduced diffusivity of SiCl4 is responsible for the partial dealumination, while the residual N a cations can react with the aluminium chlorides to form NaAlCl4 which is precipitated in the channels. Finally, these chlorides are hydrolyzed during washing to produce A1(H90)5 species. Note that relative to the acid leaching process the dealumination by & C I 4 generates only a few defect SiOH groups (IS). +

+

3+

As it is emphasized above, it is possible to determine the relative ^ S i - N M R line intensities from different types of A l distributions in the lattice. Figure 16 shows two possible distributions for which the theoretical and experimental Si(nAl) configurations are compared. It is obvious, that the fully random distribution among all crystallographic T j , Tj, T 3 and T 4 sites cannot account for the experimental results. The same conclusion holds for a random distribution in the 6-ring sheets, i.e., on sites T j and T 2 (12). The random distribution on sites T 3 + T 4 , i.e. in the 4-membered rings, or on T j only or on T 2 only do not reproduce the experimental values of the Si(nAl) configurations either (19). In order to explain adequately the experimental results, it is of paramount importance to include the mechanism of


24



dealumination in the correct computation of the A l distribution. Figure 16 b shows, that if the A l atoms are only located on sites T 3 and T 4 and the configurations with only one A l atom in the 4-rings are excluded, a very good agreement is found between the calculated and experimental values of the Si(nAl) configurations. (Some discrepancy can be noted for the slightly dealuminated and the initial samples, which suggests that additional informations should be included for the as-synthesized samples. See also ref. 23). We therefore propose an adequate picture for the mechanism of mordenite dealumination by acid leaching. In the initial sample, nearly all 4-rings contain two A l atoms. As dealumination proceeds, 4-rings with no A l atoms appear in the structure and the remaining 4-rings thus contain two A l atoms. In addition, SiOH defect groups appear, the number of which is close to four per extracted A l atom. The model implies, that both A l atoms are quasi simultaneously extracted from the 4-ring. During subsequent dealumination, the structure is reorganized (even at 353 K) by recombining the SiOH defect groups. Indeed, the number of SiOH per extracted A l atom decreases from 4 to 0.6 during dealumination. Note that both X-ray diffraction studies (60) and theoretical calculations (61) suggest the preferential siting of A l in the 4-rings of the structure. Very recently, pentacoordinated A l at 8 = 30 ppm was shown to exist in steamed crystalline or amorphous siHc(oalurnin)ate samples (Figure 17) (62). It was suggested that the presence of pentacoordinated A l may reduce the extent of coke formation during hydrocarbon cracking reactions .However, this interpretation is questioned and it is suggested that this line should belong to tetrahedral extraframework A l (63.64). Using 2DNMR, two different tetrahedral A l species can be distinguished in dehydrated Y zeolite samples. They give rise to separate signals in the F l display where the characteristic lineshapes depend on the ratio of the quadrupole interaction constant and the rf field strength co f. The framework tetrahedral A l is characterized by a lower quadrupole constant, while the strong quadrupolar interaction for the non-framework A l nucleus indicates that its tetrahedral symmetry is heavily distorted (£4). r

Incorporation of A l in the zeolite as aterracoordinatedspecies by treatment of zeolites with A I C I 3 is strongly supported by A 1 - N M R spectroscopy (65-67). The proposed mechanism for the alumination of ZSM-5 involves the reactive hydroxyl nests (see above) both on the external surface and in the internal channels (67). Indeed, the FTIR results also show clearly that the intensity of SiOH groups (at 3740 c m " ) decreases concomitantly to an increase in the Al(OH)Si acidic groups (at 3610 cm" ) (fi&£Z). 27

1

1

1 3

Application of C - N M R to reactions on zeolites l ^ C - N M R spectroscopy proves to be a powerful tool to characterize adsorbed species on various zeolites (7-10). The nature of the adsorbed species can be inferred from the usual chemical parameters, i.e. chemical shifts, linewidths and relaxation times. These latter allow one to study the mobility of the molecules on the surface. As an analytical tool, C - N M R spectroscopy can also be used to determine the concentration of reactants and products as a function of time and hence kinetic constants can easily be determined. 1 3

The reactivity of 2-isopropanol adsorbed on K- and Cs-ZSM-5 zeolites is examined in a batch-reactor (68). Propylene is formed by dehydration at 200°C (Figure 18). The subsequent polymerization of propene to form paraffinic compounds is quite significant on K-ZSM-5, while Cs-ZSM-5 yields only ca 10 %


1.

B.NAGY AND DEROUANE


1001

1

1

10

1

1


1

i

20

1

|

1

I

30

1

1

10

0

i

20

Si / Al

1

1

r

30

•

Figure 16. Comparison between the variation of the experimental Si(nAl) configurations and those computed from two Al distributions in the lattice: a) random among all sites and b) Al only on sites T and with two or no Al atoms in the 4-rings. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.) 3

T

I

I

200

i

i

i

i

i

0

100

5 (ppm)

i

i

-100

— •

Figure 17. Al-NMR spectrum of hydrothermally dealuminated faujasite. (Reproduced with permission from ref. 62. Copyright 1987 The Royal Society of Chemistry.)


25

40


(K) Z S M - 5


1

1

1

0 min

/

(Cs) Z S M - 5

I

>

\

/\ C»C

J 20 min

J

K

75 min

A/

-

105 min

i 150

i

20 min

30 min

i

i

.

0 70 min

6 (ppm) 13

Figure 18. Evolution of high-resolution low-power C - N M R spectra of 2C-isopropanol adsorbed on K - and Cs-ZSM-5 zeolites. Reaction temperature: 200 °C, measuring temperature: 25 °C. (Reproduced with permission from ref. 68. Copyright 1985 Elsevier.) 13





27

of paraffins after 90 % conversion of isopropanol. A certain deactivation of the catalysts occurs, due to probable pore blocking by polymers and/or coke formation (68). On K-ZSM-5, olefins different from propylene are also detected (Figure 18). It is important to note that no diisopropylether has been detected on any of the catalysts. After 90-100 % conversion, the polymerized products are cracked at 370°C. The strongly adsorbed species entrapped in the channels as a result of pore blocking are analyzed by high-resolution M A S solid state ^ C - N M R (Figure 19). The Cs-ZMS-5 shows the presence of propene, cis- and trans-2-butenes, and cis-2-pentene, together with some aromatic compounds (35 mol %) and butane (65 %). The K-ZSM-5 only yields cis- and trans-2-butenes as olefins and toluene and ethylbenzene as aromatics (15 %), while the distribution of paraffinic compounds (85 %) is larger, including butane, isobutane, pentane and isopentane. These results emphasize the shape selective role of the alkali cations occluded into ZSM-5 channels. Molecular traffic control (62), and other network tortuosity effects (22) have been postulated to affect the selectivity of zeolite-catalyzed organic conversion. To investigate and further discuss these hypotheses, the conversion of ^C-labeled ethylene (*CH2=*CH2) is studied on the H-form of three intermediate pore size (ca 0.55 nm) pentasil-type catalysts : H-ZSM-5, H-ZSM-11 and H-ZSM-48. As the A l content (Si/Al = 30,28 and 21 respectively) and the particle size (ca 0.5 }j.m) of these catalysts are comparable and assuming that non-homogeneous A l distribution (clustering, zoning, etc.) effects are negligible or small, these catalysts thus differ only by their channel network tortuosities and intersection dimensions. Figure 20 shows that the oligomerization product of ethylene at ambient temperature is characterized by three distinct N M R lines, corresponding to terminal methyl (8 = 13.2 ppm), penultimate methylene (8 = 23.5 ppm) and inner methylene groups (8 = 31.9 ppm), respectively (71). H-ZSM-5 and H-ZSM-11 behave analogously, as expected, both having interconnected channels of comparable size and length : the oligomer chains are composed of 36 and 38 carbons respectively. (Note that the methyl and methylene line intensities are semi-quantitatively comparable using cross polarization with 5.0 ms contact time and a recycle time of 4.0 s.) On the other hand, H-ZSM-48 yields shorter oligomers (chain length of 22 carbons) and shows the presence of a rather high amount of unreacted ethylene (Figure 20). These ethylene molecules correspond to species trapped between occluded oligomers, which have no access to active sites because of the one-dimensional nature of the ZSM-48 network. These results provide clear evidence for an augmentation in molecular traffic when the dimensionality of the zeolite channel network increases, and for the need to distinguish between aging resulting from either pore or site blockage (22). CONCLUSIONS High resolution M A S multinuclear N M R is a very valuable tool to characterize zeolitic catalysts in the as-synthesized form, to detect changes that occur during their pretreatment and subsequent modifications and to follow the conversion of adsorbed reactants. The HR ^^Si-NMR is able to resolve crystallographically different sites (e.g. highly siliceous ZSM-5) and helps the characterization of new type zeolites (ZSM-20) by comparing spectra with zeolites of known structures (Y zeolite). The A l distribution can be inferred from the ^ S i - N M R spectra provided adequate models are elaborated. Finally, a large amount of SiOH defect groups are identified in highly siliceous ZSM-5 samples.The quantitative determination of either tetra-or octahedral A l in the zeolitic samples requires the use of small flip angles for the A l 2

2 7




Figure 19. High-resolution MAS solid-state C-NMR spectra of the cracked products of the dehydration-polymerization products of 2- C-isopropanol. (Reproduced with permission from ref. 68. Copyright 1985 Elsevier.) 13



B.NAGY AND DEROUANE

i — i — i

150

i—»


»

100 —

«

i

i

i

i

50

i

«

«

«

•

«

0

8 (ppm)

13

Figure 20. CP C-MAS-NMR spectra of oligomerized ethylene on H-ZSM-5, H-ZSM-11, and H-ZSM-48 zeolites. (Reproduced with permission from ref. 71. Copyright 1986 Elsevier.)


30


nuclei. The Ô-NMR, almost unexploited at present, gives encouraging results for the analysis of Si-O-Si and Si-O-Al bonds. The ^ L i - and ^ N a - N M R spectra are quite sensitive on the degree of hydration and the nature of the counterions (either SiO" or (Al-O-Si)" groups). The C - N M R analysis yields valuable information on the molecules incorporated into the zeolitic cages or channels during the synthesis. These molecules may remain intact (e.g. tetrapropylammonium in ZSM-5 or hexamethonium in ZSM-50) or are partially decomposed (e.g. tetramethylammonium in ZSM-39 and hexamethonium in ZSM-48 in presence of ammonium ions). Downloaded by UNIV OF PRINCE EDWARD ISLAND on January 23, 2015 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch001

13

The combined used of ^ S i - and ^ A l - N M R leads to the determination of the silicon-aluminium ordering in mordenite. The A l atoms preferentially occupy tetrahedral positions in the four-membered rings of the structure. In addition, a mechanism of dealumination can be inferred, consisting in removing the Al atoms two by two from the four-membered rings. Four SiOH groups are generated per extracted Al atom in the beginning of dealumination and this number gradually decreases to two, suggesting mat a substantial reorganization must necessarily occur for substantial dealumination. The presence of pentacoordinated Al atoms is suggested by ^ A l - N M R in steamed crystalline or amorphous silic(oalumin)ate samples. The conversion of reactants is illustrated by the dehydration-polymerization of 2-isopropanol adsorbed on K- and Cs-ZSM-5 zeolites as followed by C-NMR. On the other hand, the polymerization of ethylene shows a clear-cut difference between three-dimensional channel systems (ZSM-5 and ZSM-11) able to promote the molecular traffic of reactants and on one-dimensional channel systems (ZSM-48) where some unreacted ethylene is still detected after polymerization. 13

Acknowledgments The authors thank all the collaborators whose names appear in the publications. They are also indebted to Mr. G. Daelen for his skillful help in taking the NMR spectra, Mr. F. Vallette for the preparation of drawings and Mrs.S. Lefebvre-Schwarz for the typing of the manuscript. Literature cited 1 Breck, D.W. Zeolites Molecular Sieves, Structure, Chemistry and Use, Wiley, New York, 1974 2 Barrer, R.M. Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982 3 Lok, B.M.; Cannan, T.R.; Messina, C.A. Zeolites, 1983, 3, 282 4 Wilson, S.T.; Lok, B.M.; Messina, C.A.; Cannan, T.R.; Flanigen, E.M., J. Am. Chem. Soc., 1982, 104, 1146 5 Klinowski, J. Prog. NMR Spectrosc. 1984, 16, 237 6 Thomas J.M.; Klinowski, J. Adv. Catal. 1985, 33, 199 7 B.Nagy, J.; Engelhardt, G.; Michel, D. Adv. Colloid Interf. Sci. 1985, 23, 67 8 Pfeifer, H. in "NMR Basic Principles and Progress", Diehl, P.; Fluck, E.; Kosfeld, K., Eds. Springer, Berlin, 1972, p. 53 9 Duncan, T.M.; Dybowski, C. Surf. Sci. Rep. 1981, 1, 157 10 Derouane, E.G.; B.Nagy, J. in "Catalytic Materials : Relationship Between Structure and Reactivity"; Whyte, T.E. Jr.; Dalla Betta R.A.; Derouane E.G.; Baker R.T.K. Eds, ACS Symposium Series 248, Washington, 1984, p. 101 11 Lippmaa, E.T.; Alla, M.A.; Pekk, T.J.; Engelhardt, J. J. Am. Chem. Soc. 1978, 100, 1929





12 Lippmaa, E., Mägi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A.R. J. Am. Chem. Soc., 1980,102,4889 13 Lippma, E. Mägi, M.; Samoson, A.; Tarmak, M.; Engelhardt, G. J. Am. Chem. Soc., 1981, 103, 4992 14 Fyfe, C.A.; Gobby, G.C.; Klinowski, J.; Thomas, J.M.; Ramdas, S. Nature 1982, 296, 530 15 Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak, M.; Mägi, M. Z. Anorg. Allg. Chemie, 1981, 482, 49 16 Derouane, E.G.; Dewaele, N.; Gabelica, Z.; B.Nagy, J. Appl. Catal. 1986, 28, 285 17 Ernst, S.; Kokotailo, G.T.; Weitkamp, J. Zeolites, 1987, 7, 180 18 Fyfe, C.A.; Gobbi, G.C.; Murphy, W.J.; Ozubko, R.S.; Slack, D.A. Chem. Lett. 1983, 1547 19 Bodart, P.; B.Nagy, J.; Debras, G.; Gabelica, Z.; Jacobs, P.A. J. Phys. Chem. 1986,90,5183 20 Bodart, P.; B.Nagy, J.; Gabelica, Z.; Derouane, E.G. Proc. Int. Conf. on Occurrence, Properties and Utilization of NaturalZeolites,Budapest, 1985, in press 21 Raeder, J.H. Zeolites, 1984, 4, 311 22 Lillerud, K.P.,Zeolites,1987,7,14 23 Raatz, F.; Roussel, J.C.; Cantiani, R.; Ferre, G.; B.Nagy, J. in Proc. Int. Symp. "Innovation in Zeolite MaterialsSciences",Nieuwpoort, Belgium, Sept. 13-17, 1987 24 Klinowski, J.; Anderson, M.W., J. Chem.Soc.,Faraday Trans. 1, 1986, 82, 569 25 Fyfe, C.A.; O'Brien, J.H.; Strobl, H.Nature,1987, 326, 281 26 Hay, D.G.; Jaeger, H.; West, G.W. J. Phys. Chem. 1985,89,1070 27 Fyfe, C.A.; Kokotailo, G.T.; Kennedy, G.J.; Lyerla, J.R.; Fleming, W.W. J. Chem. Soc. Chem. Commun. 1985, 740 28 B.Nagy, J.; Gabelica, Z.; Derouane, E.G.; Jacobs, P.A. Chem. Lett. 1982, 1105 29 Debras, G.; Gourgue, A.; B.Nagy, J.; De Clippeleir, G., Zeolites, 1986, 6, 161 30 van Santen, R.A.; Keijsper, J.; Ooms, G.; Kortbeek, A.G.T.G., Stud. Surf. Sci. Catal., 1986,28,169 31 B.Nagy, J.; Gabelica, Z.; Debras, G.; Derouane, E.G.; Gilson, J.-P.; Jacobs, P.A. Zeolites, 1984, 4, 133 32 Lippmaa, E.; Samoson, A.; Mägi, M., J. Am. Chem. Soc. 1986,108,1730 33 Schmidt, V.H. Proc. Ampère Int. Summer SchoolII,Basko polje, 1972, p. 75 34 Fernandez, C.; Lefebvre, F.; B.Nagy, J.; Derouane, E.G. in Proc. Int. Symp."Innovation in Zeolite MaterialsScience",Nieuwpoort, Belgium, Sept. 13-17, 1987 35 Timken, H.K.C.; Turner, G.L.; Gilson, J.-P.; Welsh, L.B.; Oldfield, E. J. Am. Chem. Soc., 1986,108,7231 36 Timken, H.K.C.; Janes, N.; Turner, G.L.; Lambert, S.L.; Welsh, L.B.; Oldfield, E. J. Am. Chem. Soc1986,108,7236 37 B.Nagy, J.; Bodart, P.; Gabelica, Z.; Derouane, E.G.; Nastro, A. Stud. Surf. Sci. Catal. 1986, 28, 231 38 Bodart, P. Ph. D.Thesis,FacultésUniversitaires de Namur, 1985 39 Melchior, M.T.; Vaughan, D.E.W.; Jacobson, A.J.; Pictorski, C.F. in "Proc. 6th Int. Zeolite Conference", Reno, 1983, D. Olson, A. Bisio, eds. Butterworths, Guildford, 1984, p. 684 40 Scholle, K.F.M.G.J. Ph. D.Thesis,Nijmegen, 1985 41 B.Nagy, J.; Bodart, P.; Collette, H., El Hage-Al Asswad, J.; Gabelica, Z.; Aiello, R.; Nastro, A.; Pellegrino, C. Zeolites, in press


31


32

Sci.


42 Burgess, J. Metal Ions in Solution, Elis Horwood, New York, 1978, p. 141 43 Tianyo, S.; Ruren, X.; Liyun, Lo; Zhaohui, Y. Stud. Surf. Sci. Catal., 1986, 28,201 44 Nastro, Α.; Aiello, R.; Crea, F.; Pellegrino, C. Zeolites in press 45 B.Nagy, J.; Bodart, P.; Collette, H.; Gabelica, Ζ.; Nastro, Α.; Aiello, R. in preparation. 46 Boxhoorn, G.; van Santen, R.A.; van Erp, W.A.; Hays, G.R.; Huis, R.; Clague, D.; J. Chem. Soc. Chem. Commun. 1982, 264 47 B.Nagy, J.; Gabelica, Z.; Derouane, E.G. Zeolites 1983,3,43 48 Gabelica, Z.; B.Nagy, J.; Debras, G. J. Catal. 1983,84,256 49 Gabelica, Z.; B.Nagy, J.; Debras, G.; Derouane, E.G. Acta Chim. Hung. 1985,119,275 50 Boxhoorn, G.; van Santen, R.A.; van Erp, W.A.; Hays, G.R.; Alma, N.C.M.; Huis, R.; Clague, A.D.H. in "Proc. 6th Int. Zeolite Conf., Reno 1983", D. Olson and A. Bisio, eds., Butterworths, Guildford, 1984, p. 694 51 El Hage-Al Asswad, J.; Dewaele, N.; B.Nagy, J.; Hubert, R.A.; Gabelica, Z.; Derouane, E.G.; Crea, F.; Aiello, R.; Nastro, A. Zeolites, in press 52 Dewaele, N.; Gabelica, Z.; Bodart, P.; B.Nagy, J.; Giordano, G.; Derouane, E.G. in Proc. Int. Symp. "Innovation in Zeolite Materials Science", Nieuwpoort, Belgium, Sept. 13-17, 1987 53 Scherzer, J. A.C.S. Symp. Ser., "Catalytic Materials : Relationship Between Structure and Reactivity" (Whyte, J.E., Jr. et al., eds). 1984,248,157 54 Kaliaguine, S.; B.Nagy, J.; Gabelica, Z.; in "Keynotes in Energy-Related Catalysis", Stud. Surf. Sci. Catal., in press 55 Debras, G.; Gougue, Α.; B.Nagy, J.; De Clippeleir, G.Zeolites,1986, 6, 241. 56 Kerr, G.T.J.Catal.1969, 15, 200 57 Beyer, H.K.; Belenykaja, I.M.; Mishin, I.W.; Boberly, Y. Stud. Surf. Sci. Catal. 1984,18,133 58 Fejes, P.; Hannes, I.; Kiricsi, I.; Pfeifer, H.; Freude, D.; Oehme, W. Zeolites 1985, 5, 45 59 Thomas, J.M.; Klinowski, J.; Ramdas, S.; Hunter, B.K.; Tennakoon, D.T.B. Chem. Phys. Lett. 1983,102,158 60 Schlenker, J.L.; Pluth, J.J.; Smith, J.V. Mater. Res. Bull. 1968,13,169 61 Derouane, E.G.; Fripiat, J.G. in Proc. 6th Int. Zeolite Conf., Reno, 1983; Olson, D., Bisio, Α.; eds.; Butterworths, Guildford, U.K., 1984, p. 717 62 Gilson, J.-P.; Edwards, G.C.; Peters, A.W.; Rajagopalan, K.; Wormsbecher, R.F.; Roberie, T.G.; Shatlock, M.P.; J. Chem. Soc., Chem. Commun. 1987, 91 63 Freude, D. in Proc. Int. Symp. "Innovation in Zeolite Materials Science", Nieuwpoort, Belgium, Sept. 13-17, 1987, in press. 64 Samoson, Α.; Lippmaa, E.; Engelhardt, G.; Lohse, G.; Jerschkewitz, H.G., Chem. Phys. Lett., 1987, 134, 589. 65 Jacobs, P.A.; Tielen, M.; B.Nagy, J.; Debras, G.; Derouane, E.G.; Gabelica, Z. in Proc. 6th Int. Zeolite Conf., Reno, 1983; Olson, D., Bisio, Α., eds.; Butterworths, Guildford, U.K., 1984, p. 783 66 Dessau, R.M.; Kerr, G.T., G.T. Zeolites, 1984,4,315 67 Chang, C.D.; Chu, C.T.-W. Miale, J.N.; Bridger, R.F.; Calvert, R.B.; J. Am. Chem. Soc.1984,106,8143 68 B.Nagy, J.; Lange, J.-P.; Gourgue, Α.; Bodart, P.; Gabelica, Z. Stud. Surf. Catal. 1985, 20, 127 69 Derouane, E.G.; Gabelica, Z.; J. Catal. 1980,65,486 70 Mirodatos, C.; Barthomeuf, D., J. Catal. 1985,93,246 71 Derouane, E.G.; Lefebvre, C.; B.Nagy, J. J. Molec. Catal. 1986,38,387 72 Beekman, J.W.; Froment, G. Ind. Eng. Chem. Fund. 1979,18,245 RECEIVED February 3, 1988 In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Perspectives in Molecular Sieve Science - ACS Publications

Recommend Documents