Estimation of the void structure and pore dimensions of molecular sieve zeolites using the hydroconversion of n-decane

Estimation of the void s ucture and pore dimensions of molecular sieve zeolites using the hydroconversion of n-decane. Johan 'A. Martens, Mia Tielen and Peter A. Jacobs Centrum voor Oppervlaktescheikunde en Collo~dale Scheikunde, K. U. Leuven, Kard. Mercierlaan 92, B-3030 Leuven (Heverlee), Belgium and Jens Weitkamp Engler-Bunte-Institute, University of Karlsruhe, D-7500 Karlsruhe, FRG (Received 12 December 1983)

INTRODUCTION AND SCOPE One of the ultimate goals of fundamental research in catalysis is to design tailor-made catalysts. Homogeneous catalytic systems are generally known for their high selectivity, while heterogeneous catalysts, although less selective, can be used in continuous processes with high throughput. The selective production of chemicals in large quantities will therefore often require the immobilization of selective homogeneous catalysts on solid adsorbents. One book and the proceedings of a series of conferences 2--4 dealing with the relationships between homogeneous and heterogeneous catalysis prove how successful this approach has been in the past. Classical adsorbents such as silica and alumina as well as ion exchange resins and polymers have received considerable attention in this respect. A very promising family of solids is constituted by the zeolite-molecular sieve group of materials. Their highly crystalline porous structure makes them unique among the high-surface-area solids. A priori, the following characteristics make them specific supports for transition-ion-based catalytic functions: (1) They are anionic in nature and may retain cations as well as cationic complexes by electrostatic forces. (2) The charge density and consequently the number of cationic catalytic species in a given structure is easily controllable; their concentration may vary from extreme dilution up to a concentration of 7 mmol per gram of zeolite. At the same time the properties of the intracrystalline void space may change from very hydrophillic to highly hydrophobic. (3) A second catalytic function as acidity or basicity is easily built in, depending upon the method of preparation and pretreatment. (4) A wide variety of pore dimensions and pore structures is available. Pore sizes ranging from 0.4 up to 1.3 nm make these materials real sieves for substrate molecules. Access to the intracrystalline void space of zeolites or molecular 0144-2449184/040088-10503.00

~) Butterworth & Co. (Publishers) Ltd. 98

ZEOLITES, 1984, Vo/ 4, Apri/

sieves is regulated by circular or ellipsoidal 8-, 10- or 12-membered rings of oxygen atoms, which may be deformed to a certain extent depending upon the structure, number, nature and charge of the exchangeable cations. It is even possible to block these rings partially by changing the nature and dimensions of these charge compensating cations. Currently encountered void structures have either:

- - monodimensional pores of tubular or undulated nature, consisting of 8-, 10- or 12-membered rings (MR); - - bidimensional intersecting pores, which give rise to the formation of intersections with larger volumes than the effective pore diameter; --tridimensional networks of cages which are interconnected via windows consisting again of 8-, 10-or 12-MR. (5) As a result of the molecular sieve character of the support, a fourth catalytic function is inherent to these materials, namely shape selectivity. Indeed, the structure and dimensions of the pores may impose steric restrictions on the formation of bulky transition states, a phenomenon defined as restricted transition state shape selectivity (TSS) 5 or the diffusion rate of reactants or products through the windows may be affected, which is known as reactant or product shape selectivity (RSS or PSS) 6. In zeolite structures with distinct sets of pores, characterized by different sizes and interconnected, a new type of shape selectivity that regulates the diffusion rate of molecules of different sizes is known as Molecular Traffic Control (MTC) 7. A quick overlook of the existing literature indicates that zeolite-molecular sieves are promising supports for catalytic functions in selective catalytic conversions. A catalyst derived from zeolite N a - X by ion-exchange with the Rh(NH3)sCI +~ complex is used in the homologation reaction of methanol. This

Review

catalyst has virtually the same activity as homogeneous ones and is very selective for the formation of acetic acid or methyl acetate 8. Metal clusters in A and Y zeolites produce a FischerTropsch product pattern which deviates from the generally encountered Flory-Schulz distribution law. Since a so-called 'cut-off effect on the product chain length exists at a certain carbon number, selectivity is improved and can be monitored to a certain extent by the zeolite structure. This effect has been related to the particle size of the metal clusters inside the zeolites, to the typical pore structure of the zeolite cage (the length of the largest hydrocarbon formed is equal to the size of the zeolite cage) and to the existence of residual acidity9. The unique properties of zeolites in terms of structure or geometry of cages and pores are also evident from their behaviour in the water-gas-shift reaction. Indeed, after appropriate activation of Ru (III) hexamine in faujasite-type zeolites l°, a cationic Ru (I)-complex seems to be formed which apparently has no homogeneous equivalent, which is very active and 100% selective in the shift conversion and the stabilization ofwhich requires the specific geometry of a faujasite supercage. Recently, several reports appeared on the selective hydrogenation of olefins on noble metal loaded shape selective zeolite supports of the ZSM-5 type I I. Forty zeolite minerals and approximately 150 synthetic materials have been reported to date, each of which can be diversified by changing the charge density and the number and nature of the charge compensating cations. All these structures can undoubtedly be determined using X-ray diffraction techniques on monocrystals. However, monocrystals of zeolites are rarely available and structure determination is a very time-consuming process which, generally speaking, cannot be done in catalytically meaningful conditions. New materials are claimed in the patent literature at a rate of at least three a month, and it seems that all the elements are available now which a catalytic chemist needs to design tailor-made catalyst formulations. However, a quick screening method is lacking in order to select a structure with the most suitable intracrystalline void space for a given catalytic application or to determine it for a newly synthetized material. Adsorption of a whole set of well-selected probe molecules with increasing kinetic diameter can also be used to characterize effective sizes of zeolite pores. This technique has been applied very often m the patent literature to characterize newly claimed molecular sieve zeolites. Its main drawbacks are: (1) the adsorption temperature has to be limited to the pre-catalytic temperature region; (2) it can only determine total pore volumes and effective pore sizes, which is insufficient to get insight into the void structure of the molecular sieve material. It has been the aim of this work to develop a test reaction suitable for the determination of zeolitic pore size and void structures at elevated temperature. This reaction is first tested using known zeolites with different pore structures and size and subsequently it

is applied to zeolitic materials, the crystal structure of which has not been determined yet. Requirements o f a g o o d test reaction for the determination o f the structure o f a zeolitic void space Since the aim is to characterize the intracrystalline void space of molecular sieve zeolites and to use the data for the design of catalytic formulations in other reactions, a test reaction is used. The ideal reaction has to fulfil the following requirements: (1) The experimental set-up should be simple and give maximum information. Therefore a detailed product analysis is required. The model reaction advanced here requires only that a continuous flow reactor is combined with a capillary GLC. (2) The test reaction should be performed in non-deactivating conditions. Indeed, the product selectivities should refer to steady-state conditions and not be the result of transient effects. Furthermore, it is useful to be able to measure the conversion dependence of the reaction selectivity on the same catalyst sample. The reaction proposed is of a bifunctional nature, and consequently no deactivation occurs during the determination of the selectivity at different conversions. (3) The disappearance of the reactant should not be diffusion controlled, since otherwise selectivities exhibited by the catalyst might not be obse'rved. Since supports with pore sizes ranging from 0.4 to 1.3 nm are investigated, a n-paraffin should be a suitable probe molecule. The reaction conditions can then be easily chosen so that a catalytic efficiency close to one can be reached. Under these conditions the distribution of the reaction products is entirely determined by TSS or PSS. (4) A single measurement should give as much information as possible on existing reaction pathways and selectivities exhibited by the support. The longer the hydrocarbon chain, the higher the number of possible conversion pathways will be. However, the latter will only be observed provided a detailed product analysis can still be made. Therefore, a compromise will be needed between the chain length of the feed molecule and the complexity of the analysis. In case of the bifunctional conversion of n-paraffins over zeolites, the reaction products can be analysed individually up to a carbon number of 10. (5) In order to be able to determine selectivity for supports with a wide range of pore sizes and structures, products with a wide variety of molecular size should be formed. This is the case for the bifunctional conversion ofn-decane since methylnonanes, ethyloctanes, propylheptane, dimethyloctanes as well as trimethylheptanes are within the expected isomeric products. The cracked products contain normal as well as branched paraffins ranging from carbon number 3 to 7.

ZEOLITES, 1984, Vol 4, April 99

Review

(6)

Since from product distributions pore size and structure should be derived, the detailed reaction mechanism should be relatively well understood. This is so for the reaction considered. (7) The information on the pore structure should be the result of several independent sets of data. For the n-decane hydroconversion the following independent criteria are used: - - relative distribution of the individual monobranched isomers at the level of the methylnonanes and ethyloctanes; - - relative distribution of the feed isomers in terms of the degree of branching: mono-, diand tribranched structures; - - d i s t r i b u t i o n of carbon numbers of the cracked products; - - yields of isomers in the cracked products. Bifunctional c o n v e r s i o n o f n - d e c a n e o n metalloaded acid-zeolites In order to obtain information on the zeolite pore size and structure from the products of the n-decane conversion, the active sites should be located inside the pores. The rate-determining events in the reaction mentioned are the rearrangements of carbenium ions and consequently the active sites are Bronsted acid sites. It is therefore necessary that residual acid sites at the external surface of the zeolite crystals should be pre-poisoned in order not to disturb initial selectivities by secondary isomerization. It is our experience that this can be done easily by poisoning with triphenylchlorosilane'according to the following stoichiometry:

Z-OH

+ ClSi(

)3

100oC )

Z-O-Si

(~)

)3 + HCI

in which Z stands for any zeolite structure. This silane molecule is unable to enter even the largest pores of any known zeolite. In order to exhibit bifunctional behaviour, the Weisz intimacy criterion concerning the average distance between metal and acid site should be obeyed 12. For diffusion coefficients of olefins determined in ZSM-5 zeolites (D = 10-3 - 10-8 cm2/s) I~, this critical distance is in the range 0.2 to 20 ~tm. In other words for zeolite crystals of the order of i-5 I.tm, the metal should be dispersed inside the pores. To conclude, in order to observe bifunctional behaviour in the zeolite catalysed hydroconversion of n-decane, all the acid sites and a substantial part of the noble metal should be inside the pores. Mechanistically, the overall reaction network is represented in Scheme I. The paraffin-olefin equilibrium is rapidly established on the metal phase, provided enough metal surface is available within the zeolite crystal. Feed isomerization and cracking occur on the Bronsted acid sites in a consecutive manner. Ci and C2 products are not formed via this mechanism, since it would involve the formation of primary carbenium ions, which is hardly conceivable in the reaction conditions (reaction temperature below 523 K.). IfCi and C2 are formed as major reaction products, this

100

ZEOLITES, 1984, Vol 4, 1984, April

n-decane

isodecanes

Pt

H+zeolite

Pt

n-decenes=-'-~n-decylcarbenium ion ~H+__

isodecylcarbenium Ion crack-~e; products ( paraf finic )

Scheme I n-decane hydroconversion according to a bifunctional mechanism

can only be the result of feed hydrogenolysis on the metal phase, and is indicative of a metal phase not homogeneously distributed throughout the zeolite crystal or present at its external surface. V o i d characteristics o f zeolites o f k n o w n structure u s e d as standard catalysts in the n - d e c a n e hydroconversion The void characteristics, cage, pore and window dimensions, sample notation and framework composition are shown in Table 1 for zeolites of known structure which will be used as standard materials. The Table contains representative structures with either mono-, di- or tridimensional void structure, limited by either 8-, 10- or 12-membered rings of oxygen atoms, and consisting of a cage and windowtype void structure or of tubular and lobate pores. Diagrammatic on-scale representations of intersections through the void space are given in Figure 1 for all these structures together with the kinetic free diameter of a few representative organic molecules. Several zeolites with crystallographically not-yetresolved structures are shown in Table 2, together with the information available on their porosity and catalytic activity. These structures, in their hydrogen form and loaded with 1% by weight of Pt, have been subjected to the n-decane hydroconversion test in order to gather information on their void structure. Overview o f criteria u s e d to d e t e r m i n e v o i d structure and d e r i v e d from the p r o d u c t s o f the n-decane h y d r o c o n v e r s i o n reaction The zeolites are converted into their hydrogenforms and loaded with 1% of Pt via an ion-exchange or impregnation technique. To ensure maximum dispersion of the metal, it is preferable to pre-calcine the catalyst prior to reduction. For small pore zeolites (the 8-MR structures), these techniques may turn out to give insufficient dispersion, so that it is necessary to add a Pt-complex during the synthesis of the material. The detailed product distribution is then determined every 10 K, from 400 up to 523 K, covering this way the whole n-decane conversion range. Only a period of 24 h is needed to collect the data. The total reaction pressure is atmospheric, the H2/hydrocarbon molar ratio is 100. It should be noted that the reaction is zero order in the hydrocarbon and has a negative first order in hydrogen37. For the present purpose, distributions of several classes of products are determined which constitute independent criteria in the estimation of the zeolite void structure. It should be noted that such other catalyst

Review

Table 1

Zeolite characteristics

Void characteristics Zeolite

Notation

Si/AI

Origin ref.

Structure

Ferrierite

FER

23.4

S, 16

BIDIM d

ZSM-5

MFI

37.0

S, 17

BIDIM

ZSM-11 Clinoptilolite

MEL CLI

60.0 5.2

S, 18 N, 19

BIDiM BIDIM

EAB

EAB

2.9

S, 20

Alpha

ALP

5.6

S, 21

Erionite

ERI

3.7

N, 22

Chabasite

CHA

3.0

N, 23

Mordenite

MOR

5.0

S, 24

Cage + wind. BIDIM Cage + wind. TRIDIM d Cage + wind. TRIDIM Cage + wind. TRIDIM MONODIM d

Offretite

OFF

2.5

S, 25

MONODIM

L

LTL

3.0

S, 26

Ultrastable Y

FAU*

5.0

S, 27

MONODIM + lobes TRIDIM

Pore

Dimensionsa(nm)

10-MR 8-MR 10-MR 10-MR 10-MR 10-MR 8-MR 8-MR 8-MR cage 8-MR cage 8-MR cage 8-MR cage 12-MR 8-MR 12-MR 8-MR 12-MR Lobe 12-MR cage

0.43 x 0.34 x 0.51 x 0.52 x 0.51 x 0.72 x 0.40 x 0.41 x 0.37 x 0.6 x 0.41 1.14c 0.36 x 0.63 x 0.36 x 0.65 x 0.67 x 0.29 x 0.64 0.36 x 0.71 0.75 0.74 1.2c

0.55 0.48 0.55 0.54 0.55 0.44 0.55 0.47 0.48 1.0b 0.52 1.50c 0.37¢ 1.10 0.70 0.57 0.52

a After Ref. 14; b Estimated; ¢ After Ref. 15; d MONODIM: monodimensional; BIDIM: bidimensional; TRIDIM: tridimensional

2 Existing structures Table

Structure Ref.

TRI

MONO

MONO

MONO

.2MR

F,u

-

~

o.E

BI

BI

BI

29

TRI

ALP

°"kinetic 0 diometer

30

ZSM-12

31

0

ERI

ZSM-34

32 33

ZSM-47 ZSM-48 T

34 33 35

BI

0

/-h EAB

V Bulone

PHI BI

TRI

CH,

28

,TL

v

TRI eMR +cocje

oF,

-

Isobutone Neopentone Tributylomine

0

0

on some

unknown

zeolite

Available information

MONO BETA

o

information

0

Figure 1 Diagrammatic on-scale representation of sections through the void space of zeolite structures and of the kinetic diameter of representative organic molecules. Sections touching each other in bidimensional systems are indicative of intersecting pores (BI = bidimensional; MONO = monodimensional; TRI = tridimensional)

36

Absorbs 19.3 w t % of cyclohexane; diphenylmethane formed out of benzene and trioxane Phenyldodecanes formed out of dodecene and benzene Large pore characteristics; absorbs 3.5 w t % neopentane; 8 wt% tertiary perfluorobutylamine Selective formation of p a r a 2- and 3octylphenols out of phenol and 1-octanol (slightly more selective than MOR); pores intermediate between faujasite and pentasil zeolites CP = 2

M e m b e r of the offretite-erionite family. Intergrowth of very small erionite domains. Higher n-hexane sorption capacity than any other known offretite or erionite Sorption of 2.7 wt% cyclohexane CP at 510°C = 5.3; 538°C = 3.4 Sorption of 0.9 wt% cyclohexane; member of the erionite-offretite family Able to isomerize xyrenes

" Cl = constraint index, proportional to the ratio of the rate constants of n-hexane and 3-methylpentane cracking (Ref. 41 )

ZEOLITES, 1984, Vol 4, April

101

Review

characteristics as number and strength distribution of the acid sites may be deduced from the same data 3s, however this is outside the scope of the present article.

50

1. Overall distribution of the feed isomers according to their degree of branching

4o

Normal long chain hydrocarbons are methylbranched via so-called protonated cyclopropane (PCP) intermediates. The latter have to be invoked, since otherwise primary carbenium ions are needed to explain isomerization at these low reaction temperatures. This is schematically represented for n-decane in Scheme H.. Consecutive methylbranching of ndecane via PCP structures may give rise to mono-, diand multibranched isomers which become progressively more bulky in nature. Since isomerization and hydrocracking are consecutive phenomena, the isomer selectivity against conversion goes through a maximum. The relative distribution of mono- against dibranched feed isomers obtained for several zeolites of known structure at this maximum isomerization conversion is shown in Figure 2. At this maximum isomerization conversion, equilibrium between the groups of mono- and dibranched isomers is reached for the open structures. As a result, deviations from it reflect the presence of steric constraints. This Figure

/~/~/-~/-"x//'"

'i" v

v

®

Scheme II

Methyl-branching of a 5-decylcarbocation assuming PCP-type intermediates

45 ~ P H ~

~,BETA ~.ZSM-12

35

~.

~;,,, M:34\

~o ~

~ ",~,~sM-47

I,r o~ °

1-~

~'4r2,~0

SM-48

,,

1o

I

5 0

I 55

I

I 65

I

I

I

75 % MONOBRANCHED

I 85

~. ~vj 95

Figure 3 Yield of dibranched feed isomers against the yield of monobranched onces at the m a x i m u m isomerization conversion of n-decane o n u n k n o w n zeolite structures

makes a clear distinction between the 10- and 12-MR zeolites. From the former, only minor amounts of dibranched isomers desorb. Among each group, the effective pore diameter of a particular zeolite determines its position on the line. The cage-zeolites accessible through 8-MR can be easily distinguished from the 12-MR zeolites, but overlapping with the group of 10-MR zeolites occurs, mainly for those with relatively small cages. According to this first criterion, the unknown zeolite structures considered (Figure 3) apparently belong to the following groups:

50

PHI, BETA, ZSM-12, ZSM-34: 12-MR structures 4~I'D

T, ZSM-48: cages with 8-MR

35-

ZSM-47: either a highly deformed 12-MR or a large cage with 8-MR

30~ tu O ul

25

ci

z ~ gl a

N 15 1¢

\

mt:L

\\

50

I

55

I

60

I

I

I

I

I

65 70 75 80 85 MONOBRANCHEO ISOMERS

\

, "q

EAB~

I

90

I

95

100

Figure 2 Relative distribution of mono- against dibranched isomers from n-decane on different known zeolite structures, obtained at the maximum isomerization conversion (-MR = -membered ring)

102

ZEOLITES, 1984, Vol 4, April

Since meaningful data can only be taken at high total isomerization conversions, this parameter is expected to be less sensitive to structural effects and susceptible to changes caused by secondary isomerization and consecutive hydrocracking, therefore has to be handled with caution. This is shown for the clinoptilolite mineral which has a high external surface (CLI*) (Figure 2). It forms a higher amount of dibranched isomers, through secondary isomerization on OHgroups at the external surface, than the sample (CLI) of the same mineral, the OH-groups on the outer surface of which were poisoned with triphenylchlot~osilane. The crystallineEAB sample, deammoniated at 573 K was still highly crystalline compared to the one heated at 673 K which is denoted as EAB* (Figure 2). The amorphous material in the latter sample ~s responsible for the second branching of the monobranched isomers.

Review

e

sion. It follows that: (a) ZSM-48 is definitely a I0-MR pore material. Its classification among the 8-MR + cage materials accoi'ding to the first criterion can only be the result of secondary PCP isomerization at the external surface; (b) Zeolite T is a 12-MR pore structure or a cage with 8-MR. The latter assignment is m agreement with its X-ray diffractogram, classifying it as an offretite-erionite intergrowth;

0

Scheme III Formation of 3-ethyloctane from methylnonanes via alkyl-shifts

20

e

~.FAU" ,"

%

_

%

UJ

~),.LTL , ~ H ~ RI

w

Od>z, ,"

®

Scheme IV Formation of 4-ethyloctane from methylnonanes via alkyl-shifts (a, b) and interconversion of 3- and 4-ethyloctane via ethyl-shift (c) ALP EAB

2. Relative contribution of the ethyl-isomers to the monobranched isomers at low isomerization conversion Equilibration of the methylbranched isomers can occur via classical methyl- or alkyl-shifts. The latter give rise to the appearance of ethyloctanes, which have a larger effective kinetic diameter than the corresponding methylnonanes. In Schemes I l l and IV the formation of the ethyloctane isomers is illustrated via alkyl-shifts. It looks straightforward, therefore, to assume that not only the diffusion of these molecules, but also their rate of formation will be hindered sterically in the smaller pores. The relative amount of the ethyloctanes in the monobranched feed isomers at 5% isomerization conversion is given in Figure 4, for known zeolite structures. At this low conversion there is much less chance that the data are perturbed by secondary isomerization and as a result this criterion is considered to be very sensitive to structural differences. Zeolites with 10-MR pores inhibit the formation of ethyloctanes in their void space. Even the straight 12 MR-pores only allow minor quantities of ethyloctanes to be formed; for 12 MR-pores with lobes and large cages, more and more ethyloctanes appear although even for the faujasite sample, thermodynamic equilibrium is far from being reached. This indicates that this criterion can be of use even for more open structures. Overlapping occurs between 12-MR pore structures and structures with cages and 8-MR windows. The ethyloctane yield in the monobranched isomers from n-decane is represented in Figure 5 for the unknown structures and at low isomerization conver-

90

MEt MFI 100

=E MC9 % Figure 4 Yield of ethyloctane (EC8) against methylnonane (MC9) isomers from n-decane on zeolites of known structure, obtained at 5% isomerization conversion; the shaded-area represents thermodynamic composition in the temperature range investigated 20

%. \ m 1(~ W

5

,

("

\~ETA

", Oe #O

,,. Ns.-,,, %

°...\

eo

I 90 .~ M C 9 / %

lOO

Figure 5 Total ethyloctane against methylnonane yield from n-decane at 5% isomerization conversion on zeolites with unknown structure

ZEOLITE•, 1984, Vol 4, April

103

Review

second criterion attributes to this zeolite a slightly larger pore diameter; - - zeolite T is clearly an O F F - E R I intergrowth and ZSM-47 has pore characteristics very close to

3E/4EC~ 1,2 1.1

E R I ;

1.0 "~ZSM -47

0.9

EA~

0.8 O.;

- - ZSM--48 has the pore characteristics of a 10-MR pore sieve, since no ethyloctanes are desorbed from this material. - - ZSM-34 shows a behaviour similar to M O R , with slightly smaller pore diameters.

4. Relative distribution of the individual monomethylbranched isomers at low isomerization conversion

0,( 0.~

THERM. EQUILIBRIUM

I

04

423

i

z

~

i

[

i

i

473 reaction temperature / K

i

i

]

523

i

Figure 6 Ratio of 3-ethyloctane against 4-ethyloctane from n-decane at different reaction temperatures over 1 Pt/H-zeolites

(c) Z S M - 1 2 , - 3 4 , PHI and BETA are 12-MR structures or have cages and 8-MR. Their extremely high yield of dibranched isomers at high isomerization conversion has to be caused by secondary effects; (d) ZSM-47 is located at the borderline o f 8 - M R + cage and 12-MR pore zeolites.

3. Ratio of 3-ethyl- to 4-ethyloctane The formation of 3-ethyloctane may occur via ethyl-shift on a 3-methyl-4-nonyl cation (SchemeIII, a) or a pentyl-shift on a 4-methyl-3-nonyl cation (Scheme III, b). However, it is straightforward to assume that the occurence of an ethyl-shift will be more probable than ofa pentyl-shift. 4-Ethyloctane cannot be formed via an ethyl-shift: it is obtained either via a propyl-shift on a 4-methyl-5-nonyl cation (Scheme IV, b) or via a butyl-shift on a 5-methyl-4-nonyl cation (Scheme IV, a). It can also be formed by secondary isomerization of 3-ethyloctane via an ethyl-shift

(Scheme IV, c). At low isomerization conversions, even for the Y-type zeolites, the ratio of 3-ethyloctane to 4ethyloctane (3E/4EC8) is not at equilibrium. Figure 6 shows the decrease of this ratio with increasing reaction temperature. It is seen that the position of the line representing this ratio against reaction temperature is very much structure dependent. The Figure allows one to draw the following conclusions: - -

- -

- -

- -

104

tlae faujasite structure is the most open material investigated; zeolite Beta seems to have structural characteristics similar to these of L T L (12 M R + lobe). According to the second criterion (Figures 4 and 5) this structure consists also of 12-MR pores and lobes with dimensions smaller than those in L T L are present; zeolite PHI shows a behaviour similar to that of M O R , with slightly enhanced effective pore sizes. The same conclusion is reached using the previous criterion; ZSM-12 consists definitely of 12-MR pores, with effective dimensions between M O R and OFF. The

ZEOLITES, 1984, Vol 4, April

In Y-type zeolites the relative distribution of methylnonanes among each other approaches thermodynamic equilibrium from medium conversion 39,40 At low conversion a kinetic hindrance in the rate of formation of 2-methylnonane is always observed4°: 2-methylnonane is always formed half as fast as 3- or 4-methylnonane. This was ascribed to the lower number of PCP structures which can be formed from n-decane giving the 2-methyl isomer compared to the 2 others. It is schematically illustrated in Scheme V. In Z S M - 5 zeolites, as a result of transition state shape selective effects and over the whole conversion range, the rate of 2-MC9 formation is enhanced compared to that of 4- and 5-MC938. Therefore, the relative distribution of the methylnonanes is expected to be very sensitive to pore size and structure, particularly for low isomerization conversions, at which disturbance of the initial picture by secondary isomerization is non-existent. Therefore, the yield of the isomer preferred in small structures (2-MC9) is plotted in Figure 7 against the yield of the more bulky isomer (5-MC9). The 12-MR zeolites can again be reasonably well distinguished from the other materials. The behaviour of the structures with cages accessible through 8-MR is overlapping to some extent with that of the 10-MR zeolites, although the structures of major catalytic interest (ERI, ALP, CHA) can be discriminated from 10-MR zeolites as well as from 12-MR structures. According to Figure 8, zeolites BETA and PHI behave as 12-MR materials. Z S M - 3 4 and -12 have effective diameters which are at the limit between

___

~_.~'J

•

\7

\f

[

\7

"

"

\

--I=

2MC9

" ~2,3.c9

Scheme V Explanation via PCP-methylbranching of the low rate of formation of 2-methylnonane compared to the 3- and 4-methylbranched isomers. Equal concentration of the n-decylPCP-structures and identical rates for their transformation are assumed

Review

5. Absoluteyield of isopentane in the hydrocrackedproducts at low hydrocracking conversions

~-'~IFI eFER

The molar distribution of hydrocracked products from n-decane is found to be symmetrical among C5 .in faujasite 42. The absolute yield ofisopentane formed from Ci0 paraffins is a parameter which is very sensitive to structural effects and insensitive to secondary reactions. Indeed, the more a n-C10 hydrocarbon is branched before cleavage, the higher the probability will be for central [$-scission, which wi41 result in the generation of an increased number of iC5 fragments. Therefore, in larger pores a higher degree of branching will be possible via PCP isomerization, before cracking via the more energetic [3-scission pathway 38'4s will occur. As a result, from larger pores higher absolute amounts of iC5 will desorb. The yield ofiC5 was preferred over that ofiC6 and iC7 isomers for the following reasons:

\',, 45

\

\ \ \

0 cu\ ¢~ 35 o~

- - s e c o n d a r y cracking of iC5 is negligible at low cracking conversions and therefore at low reaction temperatures since it would require that primary carbenium ions are involved; - - the iC5 and in general the C5 yield is not affected by secondary cracking of C7 and C6 fragments, yielding C.~ + C3 and C3 + C3 fragments, respectively.

CHAo\\

~

25-

\ =OFF \\\ ~'-~L

15

5

I 10

I 15

20

The absolute yield ofiC5 formed over the respective Pt loaded H-zeolites is shown in Figure 9. As expected the highest yields are formed over the 12-MR zeolites.

°~ 5 M C 9 Figure 7 Relative concentration of 2-MC9 against 5-MC9 in the monomethylbranched isomers from n-decane at 5% isomerization conversion obtained from known zeolite structures

12-MR and 8-MR + cage materials. ZSM-47 and T are found near the large pore 10-MR or 8-MR + cage structures. The position of ZSM-48 is rather dubious, most probably as a result of secondary isomerization. The same position has already been noticed for its relative yield of dibranched isomers (Figure 3). Mobil workers defined a constraint index (CI) 41 as the ratio of the cracking constants of hexane and 3-methylpentane and used it to discriminate between different classes of zeolite structures. In this way the Pentasil group of zeolites becomes easily distinguishable from any other family of porous crystalline structures. From the data of Figures 7 and 8, it results that the ratio of 2-MC9 to 5-MC9 in the isomers at low isomerization conversion should also constitute an index to discriminate between various families of porous crystalline materials. Since selectivity of the bifunctional methylbranching of n-decane is probably transition state controlled 38, this refined constraint index (CI') will not reflect differences in diffusional effects on the ingoing feed molecules, but rather, differences in size and geometry of the transition state or differences in the diffusion of the mono-branched olefins from the acid site to a metal cluster. It follows that CI' is a more subtle index since it allows one to distinguish between the endmembers of the Pentasil zeolites and consequently will be able to determine intergrowths in this family of zeolites.

\ \

45

\

~ \\ \

.

\

0

\ \

"~

\

ezse

48

o,

~E 35 ¢N

\

o~

\..ZSM-47 v

,.~eZSM_12 \

\\

2~

\

15

I 10

BETA

I 15

20

% 5MC9 Figure 8 Relative yield of 2-MC9 against 5-MC9 from unknown zeolite structures

ZEOLITES, 1984, Vol 4, April

105

Review

The latter zeolites can be isolated from all other structures according to this criterion. The 10-MR structures and 8-MR + cage zeolites overlap at least partially. According to this 5th criterion zeolites BETA, PHI, ZSM-12 and -34 behave as 12-MR structures, the effective pore sizes decreasing from BETA to ZSM-34. ZSM-48 belongs to the family of 10-MR structures while ZSM-47 and zeolite T can be found in the region common to 10-MR and 8-MR + cage materials.

IC 5 mol/1OOmol

60-

FAU

<.--BETA

5(;


-

6. General assignment of the unknown structures In Table 3 is collected the pertinent information on the void structure of zeolites with unknown crystal structure according to the five different criteria handled. Although some criteria alone do not give unique information on the pore structures, a combination of the information from the 5 criteria allows one to make an assignment of the pore structure in terms of structures with the following characteristics: 10-MR, 8-MR + cage and 12-MR zeolites. Moreover, it is possible to rank average diameters of pores and cages and to compare the porous structure with that of known zeolites. It is also noteworthy that the information of Table 2 on these structures is always in line with the assignment proposed here.

-


EAB

30-

20

I--ZSM-34

CHA

~-MEL

4~_T ZSM-47

ERI ALP --CLIN

lOI

CONCLUSIONS

_ZSM-48

--FER,MFI

Figure 9 Absolute yield of iC5 formed over different zeolite structures through hydrocracking of n-decane at 5% cracking conversion

In this work it is shown that a relatively simple catalytic test reaction can be used to get detailed insight into the void structure of unknown zeolites. In the n-decane isomerization and hydrocracking reaction over Pt loaded hydrogen-zeolites, five independent criteria can be handled which give consistent and supplementary evidence on the structure and dimensions of the void space of unknown zeolites. The latter

Table 3 Characterization of the void structure of unknown zeolite materials using the hydroconversion of n-decane

Zeolite

Criterion 1 % Dibr. isomers

2 2MC9/ 5MC9

3 % ethyloctanes

4 3EC8 4EC8

5 iC5

General assignment

ZSM-48

8MR + cage

8MR + cage or 10 MR

10 MR

10 MR

10 MR

10 MR zeolite

ZSM-47

12MR or 8MR + cage

8MR + cage or 10 MR

12MR pore or 8MR + cage

like ERI smaller than OFF

8MR + cage or IOMR

8MR + cages like ERI

ZSM-12

12MR

12MR or 8MR + cage

12MR pore or 8MR + cage

between MOR and OFF

12MR

12MR pores, between MOR and OFF

ZSM-34

12MR

12MR or 8MR + cage

12MR lobe or 8MR + cage

like MOR

12MR

12MR, comparable to MOR

PHI

12MR

12MR

12MR pore or 8MR + cage

between MOR and LTL

12MR

12MR + lobe, smaller than LTL

BETA

12MR

12MR

12MR lobe or 8MR + cage

between FAU* and LTL (largest lobe)

12MR

12MR + lobe, larger than PHI

8MR + cage

8MR + cage or 10 MR

12MR lobe or 8MR + cage

between ERI and OFF

IOMR or 8MR + cage

8MR + cage, OFF-ERI intergrowth

T

106

ZEOLITES, 1984, Vol 4, April

.Rev~w

can be characterized in terms of pores with 10membered rings of oxygen atoms, cages of different dimensions accessible through 8- or 12-membered rings or with straight or lobate pores circumscribed by 12-membered rings. ACKNOWLEDGEMENT j. A. M. and P. A. J. acknowledge from the Belgian Fund of Scientific Research (NFWO-FNRS), a fellowship (aspirant) and a research position (Onderzoeksleider), respectively. Funding from the same institution and from the Belgian Government (Diensten Wetenschapsbeleid, Geconcerteerde Actie Catalyse) is highly appreciated.

16 17 .18 19 20 21 22 23 24 25 26 27

REFERENCES 1 'Catalysis by Supported Complexes', (Eds. Y. I. Yermakov, B. N. Kuznetsov and V. A. Zakharov), Elsevier Scientific Publ. Co., Amsterdam, Oxford, New York, 1981 2 Proceedings First Int. Symp. on Relations Between Homogeneous and Heterogeneous Catalysis, in 'Catalysis, Heterogeneous and Homogeneous' (Eds. B° Delmon and G. James), Elsevier, 1975 3 Second Int. Symp. on Relations Between Homogeneous and Heterogeneous Catalysis, Lyon, J. Mol. Catal. 1977, 3, 1/3 4 Third Int. Symp. on Relations Between Homogeneous and Heterogeneous Catalysis, Groningen, J. Mol. Catal. 1981, 11, 2/3 5 Csicsery, S. M., ACS Monograph 171 led. J. A. Rabo), Am. Chem. Soc., Washington D.C. 1976, p. 680 6 Weisz, P. B., Proceed. 7th ICC. Tokyo, 1980, paper 1 7 Derouane, E. G. and Gabelica, Z. Jo CataL 1980, 65, 486 8 For a review see: Ben Taarit, L. and Che, M. 'Catalysis by Zeolites' (Eds. B. Imilik et aL), Studies in Surface Science and Catalysis, 5, Elsevier, 1980, p. 167 9 For a review see: Jacobs, P. A. and Van Wouwe, D. J. MoL CataL 1982, 17, 145 10 Nijs, H. H., Jacobs, P. A. and Uytterhoeven, J. B. J. Chem. Soc., Chem. Commun. 1979, 180 11 Dessau, R. M., US Pat. 4 347 394 12 Weisz, P. B. Adv. CataL 1962, 13, 137 13 Haag, W. O., Lago, R. M. and Weisz, P. B. Faraday Disc. Chem. Soc. 1981, 72, 317 14 Meier, W. M. and Olson, D. H. 'Atlas of Zeolite Structure Types', Publ. by the Structure Commission of the Int. Zeolite Association, Juris Druck + Verlag AG, Z~rich, Switzerland, 1978 15 Barrer, R. M. 'Zeolites and Clay Minerals as Sorbents and

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Molecular Sieves', Academic Press, London, New York, San Francisco, 1978 Synthetized after Euro Pat. 0012473, Ex. 8 Description of this material: Jacobs, P. A. and von Ballmoos, R. J. Phys. Chem. 1982, 86, 3050 Same synthesis method as published earlier: Jacobs, P. A., Martens, J. A., Weitkamp, J. and Beyer, Ho K. Faraday Disc. Chem. Soc. 1982, 72, 353 Description of this material: Detrekoy, E. J. and Jacobs, P. A. Proceed. third Int. Conf. on Molecular Sieves, Zilrich, lEd. J. B. Uytterhoeven), Leuven, University Press, 1973, p. 373 S. Cartlidge, PhD Thesis ETH ZOrich, 1983 Synthetized after US Pat. 4 191 663, (1980) assigned to Mobil Oil Co. Natural sample of erionite from Nevada Description of this material: Beyer, M. K., Jacobs, P. A., Uytterhoeven, J. B. and Till, F. J. Chem. Soc., Faraday Trans. I, 1977, 73, 1111 Zeolon sample from Norton Company Synthesized after US Pat. 1 188 043 Commercial Linde type SK-45 zeolite Same material as described earlier: Steijns, M., Froment, G., Jacobs, P. A., Uytterhoeven, J. B. and Weitkamp, J. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 654 Tobias, M. A., US Pat. 3 728 408, (1973), assigned to Mobil Oil Corp. Young, L. B., Eur. Pat. Appl. 0 030 084, (1980), assigned to Mobil Oil Corp. Grose, R. W. and Flaningen, E. M., US Pat. 4 124 686 (1978), assigned to Union Carbide Corp. Young, L. B., Eur. Pat. Appl. 0 029 333, (1981), assigned to Mobil Oil Corp. Haag, W. O. and Lago, R. M., US Pat. 4 326 994, (1982), assigned to Mobil Oil Corp. Rubin, M. K., Rosinski, E. J. and Planz, C. J., US Pat. 4 086 186, (1978), assigned to Mobil Oil Corp. Kokotailo, G. T. and Sawruk, S., US Pat. 4 187 283, (1980), assigned to Mobil Oil Corp. Givens, E. N., Plank, C. J. and Rosinski, E. J., US Pat. 4 079 095, (1978), assigned to Mobil Oil Corp. Olive, M. F., Dovey, G. and Keen, I. M., PS. 1 291 928, (1972), assigned to the British Petroleum Co. Ltd. Steijns, M. and Froment, G. F. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 660 Jacobs, P. A., Martens, J. A., Weitkamp, J. and Beyer, H. K. Faraday Disc. Chem. SOCo1981, 72, 353 Weitkamp, J. 'Hydrocracking and Hydrotreating' lEd. J. W. Ward and S. A. Quader) Am. Chem. Soc. Syrup. Ser. 1975, 20, 28 Weitkamp, J. Erd61; Kohle, Erdgas, Petrochem. 1978, 31, 13 Frilette, K. J., Haag, W. O. and Lago, R. M. J. CataL 1981, 67, 223 Steijns, M., Froment, G. F., Jacobs, P. A., Uytterhoeven, J. B. and Weitkamp, J. Erd61, Kohle, Erdgas, Petroleum 1978, 31,513 Weitkamp, J. and Jacobs, P. A. Prepr. Div. Petr. Chem., Am. Chem. SOCo1981, 26, 9

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