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Structure Sensitivity in Zeolite Catalysts
R.K. Grasselli and A.W. Sleight (Editors), Structure-Actmty and Sdectrulty Relationships in Heterogeneous Catalysis 1991 Elsevier Science Publishers B.V., Amsterdam
179
STRUCTURE SENSITIVITY IN ZEOLITE CATALYSTS
F. G. DWYER Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, NJ 08066 ABSTRACT Almost from its beginning, zeolite catalysis has had a strong dependency upon the catalyst structure. Shape selective catalysis using zeolites has controlled selectivity by constraining the reactants that can be admitted to the catalytic sites, the products that can emit from the zeolite pores and the products that can be formed within the zeolite cavity. Although advances continue to be made in this area, they are mainly subtle refinements of the original concepts. A major challenge is and has been to combine the shape selective constraints with homogeneous and/or enzymatic catalysis. The major obstacle has been the limited size of zeolite pores and cavities to accommodate the host homogeneous or enzymatic agents. With the recent discovery of the 18 membered AIP04, VPI-5/MCM-9, hope for the synthesis of zeolites to accommodate such systems has been renewed. INTRODUCTION
When we speak of structure sensitivity in zeolite catalysis, we are really referring to what is more commonly referred to as shape selectivity. This term, originally coined by Weisz and Frilette in 1960 ( l ) , embodies several structure sensitive phenomena. Size exclusion, in which reactants are prohibited in reaching the catalytic surface because of their size or products are barred from returning from the catalytic surface to the bulk, was the first phenomenon studied. Subsequently spatiospecific selectivity, where reactions are inhibited by the inability of reaction intermediates to form in the confines of the zeolite cavity, and diffusive selectivity were observed and applications developed. This unique ability to control the selectivity and essentially direct chemical reactions by purely physical constraints has always led to speculation of how we can combine these effects with catalytic surfaces or sites not found in zeolites. Although multifunctional catalysts have been prepared by incorporating metals into zeolites, the metal functions have complemented the zeolite catalysis rather than been enhanced by the zeolite’s shape selective constraints. For the sake of this discussion catalytic cracking has not been included. It is generally accepted that in catalytic cracking the large, high molecular weight hydrocarbons in the gas oil feed are first cracked either thermally or catalytically on the amorphous matrix component of the catalyst and it is the cracked products that can now diffuse into the zeolite portion of the catalyst reacting further. It is speculative whether the selectivity would be further enhanced, more gasoline/less coke, if the zeolite pores were large enough to admit the large hydrocarbon
180
feed molecules. Nevertheless if we were to extend our definition of shape selectivity, even catalytic cracking with zeolite catalysts would be included. ZEOLITE FUNDAMENTALS
For the purpose of this discussion, I will define zeolites as porous crystalline materials formed by the connection of numerous elements, usually tetrahedrally coordinated, through oxygen bridges. They usually have ion exchange capacity and can be readily converted into a variety of catalytic forms. The pore systems are regular and well-defined and can afford access to the interior of the crystal in a unidirectional mode, such as a tube, or by dual pore systems, intersecting and nonintersecting, or even a three-dimensional access. To demonstrate some of these characteristics, Figure 1 depicts the structure of faujasite, a large pore zeolite with threedimensional access. In Figure 2, the two-dimensional pore system and the 010 projection of
ZSM-5 are shown. Zeolites of interest in catalysis are usually classified according to the number of tetrahedral units forming the pore opening: 12 - large, 10 - intermediate, and 8 -
small. Examples of zeolites classified in this manner are shown in Figure 3. TYPES OF SHAPE SELECTIVITY
As has been stated before, shape selectivity in zeolite catalysis can be divided into three
general categories: size exclusion, spatiospecific and diffusive. We will discuss each type separately citing examples and, where applicable, processes built on these concepts.
Size Excluslon Early examples of size exclusive selectivity were the selective dehydration of n-butanol in the presence of i-butanol over the zeolite CaA (2) and the selective cracking of n-hexane in the presence of 3-methylpentane (3),Tables 1 and 2. Commercial applications of this
characteristic were demonstrated in the post-catalytic reforming processes, Selectoforming and M-forming, in which the low octane n-paraffins in the reformate were selectively cracked to lighter hydrocarbon fragments resulting in higher octane number product. Selectoforming employed erionite as the zeolitic catalytic component which had an 8-membered ring pore structure and M-forming utilized ZSM-5 having a 10-membered ring pore structure. The selective cracking phenomenon is illustrated in Tables 3 and 4 showing the compositional
changes brought about by the process with the corresponding octane number increase. By far the broadest application of size exclusion is in catalytic dewaxing processes. In these processes the low temperature rheology is changed to a more fluid product by cracking normal and slightly branched paraffins in the feed to lighter products. Mobil Distillate Dewaxing, MDDW, is the process for improving the low temperature fluidity of diesel fuel, jet fuel and
heating oils while Mobil Lube Dewaxing, MLDW, is the process for lube stocks. Both processes
181
Figure 1
Faujasite
Figure 2
-
ZSM 5 ZSM-5 (010) Projection
182
Figure 3
Pore Openings of Zeolite Structures Phase
Tetrahedral Units
Faujasite, X,Y Mazzite, ZSM-4 Mordenite Offretite Cancrinite Heulandite Stilbite Ferrierite ZSMd Zeolite A Erionite
Dimensions
12 12 12 12 12 10 10 10 10 8 8
(A)*
7.4 7.4 6.7 x 7.0 6.4 6.2 4.4 x 7.2 4.1 x 6.2 4.3 x 5.5 5.4 x 5.6 4.1 3.6 x 5.2
* Based on X-Ray Structure Determination Using an Oxygen Radius of
1.35i
Table 1
Dehydration of Primary Butyl Alcohols 1 Atm. Pressure
Temperature "C
130
230
260
(2 18
<2 60
46 9
85 64
Over Linde CaA Molecular Sieve lsobutyl Alcohol, Wt% n-Butyl Alcohol, Wt% sec-Butyl Alcohol, Wt%
N
O
Over Faujasite-Type CaX Molecular Sieve lsobutyl Alcohol, Wt% n-Butyl Alcohol, Wt% sec-Butyl Alcohol, Wt%
82
183
Table 2
Molecular-ShapeSelective Cracking Time on Stream (Min) 10 to 33 10 to 20 10 to 20
Temperature
Conversion
370 320 to 540 510 to 540
47 to 30 0 to 0.7 0.4 to 1.9
H Erionite
n-Hexane 2-Methylpentane 2-Methylpentane
26 26 26
320 430 540
52.1 1.o 4.7
H Chabazite H Chabazite
n-Hexane 2-Methylpentane
30
260 540
10.0
Catalyst H Gmelinite H Gmelinite H Gmelinite
H Erionite H Erionite
Hydrocarbon Charge n-Hexane 2-Methylpentane Methycyclopentane
10
Table 3
W,
("/.I
1.5
Selectoforming a C5-82 C light Naphtha Composition
+ Ethane ~~
Methane
-
Feed
Product at 395°C 425°C 1.5
5.0
21.9
24.7
lsobutane
0.4
1.3
n-Butane
6.0
7.7
Propane
lsopentane
22.2
22.1
21.7
n-Pentane
23.4
11.1
2.8
54.4
37.0
31.5
67.4
79.6
84.0
6+
C5+ Octane, R + O
at 28 Atm., 1.6 LHSV
184 employ a ZSM-5 containing catalyst tailored to the specific process. Figure 4 shows the relative effect of hydrocarbon components of petroleum stocks on the pour point, the property we use to measure low temperature fluidity. Another interesting aspect of dewaxing is the effect the shape of the pore opening can have on the effectiveness of the process. ZSM-23 is a zeolite which like ZSM-5 has a 10-memberedpore opening but with a tear-shaped opening. Figure 5 shows the projections of the pore openings for ZSM-5 and ZSM-23 with their corresponding effective and crystallographic dimensions. When dewaxing a distillate stock to the same pore point, ZSM-23 performs less conversion than ZSM-5 and gives a product with a higher viscosity index, a measure of the temperature/viscosity response (Table 5). This result is interpreted as due to the further restriction of the pores of ZSM-23 reducing the amount of branched paraffins that are cracked in the dewaxing process. Finally, this size exclusive property of ZSM-5 is also exhibited when used as an octane enhancing additive catalyst in catalytic cracking. In this operation, low octane n-paraffins and olefins are cracked to light gases while some olefin isomerization also occurs. Table 6 compares operation with and without ZSM-5 for both FCC and TCC. SpatlosDeclflclty When both the reactant molecule and the product molecule are small enough to diffuse through the zeolite pore channels, but the reaction intermediates are larger than either the reactants or products and are spatially constrained either by their size or orientation, we refer to this as spatiospecific selectivity. Spatioselectivity or transition state selectivity is independent of crystal size and activity, but depends on the pore diameter and zeolite structure. This type of selectivity was first proposed by Csicsery (4) in 1971. Spatioselectivity plays a major role in the selective cracking of paraffins in medium pore zeolites. For example, n-hexane and 3-methylpentane are readily sorbed by ZSM-5, yet the singly-branched molecule cracks at a significantly slower rate than the straight chain molecule. 3-methylpentane, being a bulkier molecule than n-hexane, apparently requires more space than n-hexane to form the reaction intermediate as shown in Figure 6. A very significant commercial application of spatioselectivity is in the isomerization of
xylenes using ZSMB containing catalysts. Xylenes can undergo isomerization via intramolecular 1,2-methyl shift as well as a disproportionation via diphenylmethanes, Figure 7. The intracrystalline cavity of ZSM-5 cannot readily accommodate the disproportionation reaction intermediate, hence shifting the selectivity dramatically towards isomerization. Figure 8 shows this selectivity effect in comparing the relative amounts of disproportionation and isomerization for ZSM-5 vs faujasite, a large pore zeolite with a much larger intracrystalline cavity.
185
Table 4
CompositionalChange at Different M-Forming Severities C6-80°C Mid-Continent Naphtha, 28 Atm. Feed
Product
Product
84.5
89.6
92.7
c11
18.5 23.4 0.6 0.2 0 0
16.9 22.5 1.1 2.9 2.0 0.3
-1.6 -0.9 +0.5 t2.7 t2.0 +0.3
16.3 21.7 1.5 3.7 3.3 0.4
+ +
Total
42.7
45.8
+3.0
46.8
+4.1
30.9 12.9 0.2 0
26.2 9.7 0 0
-4.7 -3.2 -0.2 0
23.4 0.7 0.3 0
-7.5 -4.2 to.l 0
44.0
35.9
-8.1
32.4
-11.6
1.4
1.6 8.6 8.1
t0.2 +0.6 +4.2
1.1
C,+,R+O Aromatics B
T
X C9 c 10
-2.2 -1.7 +0.9 3.5 + 3.3 0.4
Paraffins
Naphthenes Pentanes Butanes and Lighter
8.0
3.9
-0.3
+ 1.0 + 6.8
9.0 10.7
Figure 4
Conversion of Alkane and Alkylbenzene Molecular Classes During Dewaxing Over ZSM-5 1
.a Fraction Converted
:enes
.6 .4
.2
2+1-Me-Alkanes
0
I
I
I
I
1
1
1
1
-30
-20
-10
0
10
20
30
Pour Point, "C
186
Figure 5
Projections of ZSM-5 and 23 Structures Crystallograehic Pore Size (A) 5.1 x 5.5
-
ZSM 5
4.5 x 5.6 ZSM - 23
Table 5
Oewaxing Over ZSM-5 and ZSM-23 ZSM-23
Product Pour Point, "C
ZSM-5 15 -12
Viscosity Index
101.0
108.7
Catalyst Conversion, Wt%
11 -12
Table 6
CommercialTest of ZSM-5 TCC Base Catalyst Time on Stream, Days Conversion, Vol% C5+ Gasoline, Vol% Butenes, Vol% Propene, Vol% Light Fuel Oil, Vol% Coke, Wt% Research Octane Number R + 0 Motor Octane Number M + 0 Potential Alkylate, Vol% Total Gasoline, Vol%
FCC Catalyst Containlng ZSM-5
0
72
106
42.3 3.8 3.7 29.9
40.9 4.6 4.7 29.1
41.0 4.2 4.2 26.4
86.0 77.4 12.7 55.0
90.2 79.2 15.7 56.6
91.2 79.5 14.2 55.2
-_
__
I
Base Catalyst
Catalyst Containing ZSM-5
0 732-53.9 7.9 6.2 26.5 6.7 87.3 78.1 24.9 78.8
37 7 ? ’ ) 51.6 9.0 7.0 26.5 6.7 89.0 78.7 28.3 79.9
187
Figure 6
Mechanism of ParaffinCracking C
-
n hexane
C
/C1 0
C
c is+ 4 ’
6+,H
H' 0O8 \ -
Al
Si
-
6-
-
Cross Section
C
9,
6 x 7 i
C' 6-
- 0 0 0O6 \ 1
Al
Si
Figure 7
Xylene lsomerization C
C
Disproportionation
jyC+ b,--,o C
i
Si
3 methylpentane
C
4.9 x 6
+"& C
Si
188
Figure 8
Disproportionation vs. lsomerization
/I
m-Xylene Feed, 573 K (300°C) Comparison of ZSM-5 with Faujasite* 20
/
% Disproportionation
10
L
Faujasite
0.2
i
0.1
ZSMB 0
0
10
20
0
% lsomerization
"Lanewalda & Bolten, J. Org. Chem. 2,3107(1969)
Figure 9
1 10 -2
10
D cm2/Sec
10 a 10 -8 10 -10
10 -12 10 -14
I,, Configurational
1
10
100
Angstroms
Diffusivity and Size of Aperture (Pore); the Classical Regions of Regular and Knudsen and the New Regime of Configurational Diffusion
1
Ir.
10
189
Table 7
Hydrogenationof a Mixture of Trans- and Cis- Butene-2 Temp.,
initial Composition
- 120 78.7 21.3
“C
Trans.
Cis.
Final Composition
Conversion , Wt%
Trans.
Cis.
- -
n-Butane
Trans.
37.1
17.0
--
Cis.
45.9
52.9
20.8
-
kTrans
kcis 3.3
103
78.7
21.3
57.3
19.8
22.9
27.2
7.1
4.3
98
78.7
21.3
69.4
20.9
9.7
11.8
1.8
7.0
Figure 10
Kinetic Model for p-Xylene Preference with ZSM-5 Catalyst meta
ortho
para
Rel. Intrinsic k’s
1
1
Rel. Apparent k s
0.08
1
rn 104
Dp’Do
r----m A
I
Dm , I
m
-
P o1
Do
I
DP I
-0 -P
190
Table 8
Toluene Disproportionation Temperature, "C
WHSV
Feedstock
Large Crystal ZSM-5 550 30 Toluene
Mg ZSMQ 550 3.5 Toluene
13.2
10.9
35 46 19
88 10 2
Thermodynamic Equilibrium ~~
Conversion, Wt% Toluene % Xylenes
Para Meta Ortho
23 51 26
Diffusive Selectlvltv Diffusion in zeolites does not conform to the principles we usually apply in diffusive processes. The diffusive process in zeolites has been described by Weisz ( 5 ) as configurational diffusion. In this diffusion regime, even a subtle change in the dimensions of a molecule can result in a large change in its diffusivity as shown in Figure 9. This type of diffusion and its effect on catalytic selectivity was shown (Chen and Weisz) (6) in the hydrogenation of transand cis-butene where, although the two molecules differ in size by only 0.26, the much higher diffusion rate of trans-butene results in a much higher reaction rate (Table 7). Commercial processes utilizing this structure sensitive property include several aromatics processing applications such as xylene isomerization, selective toluene disproportionation (STDP), paraethyl toluene synthesis and p-diethylbenzene synthesis. In xylene isornerizationthe slight difference in the size of the para-isomer but large difference in diffusivity leads to a paraselective product as shown in Figure 10.
.
In the case of toluene disproportionation, an equilibrium mixture of xylenes is obtained
using a standard ZSMQ catalyst containing ZSMB crystals (0.5 Nm. Increasing the crystal size to >3 urn or chemically modifying the crystal with Mg to reduce pore size results in a dramatic
shift to para-selectivity, Table 8.
Chemical modification has also been shown to be effective in the alkylation of toluene
with ethylene to make p-ethyltoluene, Table 9, and ethylbenzene alkylation with ethylene to make p-diethylbenzene, Table 10. It should be pointed out that these structure sensitive phenomena in zeolites are not
mutually exclusive and in many reactions the phenomena occur simultaneously.
191
Table 9
Toluene-EthyleneAlkylation Unmodlfied HZSM-5
350
Temperature, "C WHSV Toluene C2H4
Toluene/C2H4 (Mole)
Selectivlty l o Products, % Benzene Ethylbenzene Xylenes p-Ethynoluene m-Ethyltoluene 0-Ethyltoluene Normalized Ethyltoluene Para Meta Ortho
Equilibrium
33.1 49.9 16.3
350
6.9 0.5 4.511
6.9 0.5 4.311
20.7 94.4
15.3 71.2
0.8 1.4 1.4
0.5
Conversion, Wi% Toluene C2H4
5-
0.8 0.8
23.3 53.3 8.3
90.6
27.4 62.8 9.8
97.0 3.0 0
2.8 0
Table 10
EthylbenzeneDisproportionation Large Crystal HZSM-5 Temperature, "C
Mg-P-ZSM-5
500
525
3.5
30.2
89.6
22.5
60.8 10.4 3.9 1.1 1.8 4.1
62.4 1.3
WHSV
EB
Conversion, % EB Selectivity to Products, Wt% Benzene ToI uene Xylenes, EB Ethyltoluene Diethylbenzene @her Aromatics Light Gas c5-c9
Gas
Total
-
1.9 16.0
0
0.6
15.4
2.7 11.6
-
I
100.0
100.0
33.3
99.3 0.7
Diethylbenzene Para Meta Ortho
65.0 1.l
0
192 FUTURE OPPORTUNITIES
The application of shape selective catalysis of zeolites is far from being exhausted. Extension to other petroleum based and petroleum related feedstocks has shown potential with existing zeolitic materials. Applications of shape selective catalysis in fine chemical manufacture seem to be relatively unexplored. Combining homogeneous catalysis with the shape selectivity of zeolites has found no success because of size incompatibility. With the discovery of larger pore zeolitic like materials such as VPI-5, we might be able to overcome the barrier. LITERATURE CITED 1 2 3 4 5
P. B. Weisz and V. J. Frilette, J. Phys. Chem., 64 (1960) 382. P. B. Weisz, V. J. Frilette, A. W. Maatman and E. B. Mower, J. Cat., 1 (1962) 307 J. N. Miale, N. Y . Chen and P. 9.Weisz, J. Cat., 6 (1966) 278. S. M. Csicsery, J. Cat., 23 (1971) 124. P.B. Weisz, Chemtech, 3 (1973) 498.
179
STRUCTURE SENSITIVITY IN ZEOLITE CATALYSTS
F. G. DWYER Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, NJ 08066 ABSTRACT Almost from its beginning, zeolite catalysis has had a strong dependency upon the catalyst structure. Shape selective catalysis using zeolites has controlled selectivity by constraining the reactants that can be admitted to the catalytic sites, the products that can emit from the zeolite pores and the products that can be formed within the zeolite cavity. Although advances continue to be made in this area, they are mainly subtle refinements of the original concepts. A major challenge is and has been to combine the shape selective constraints with homogeneous and/or enzymatic catalysis. The major obstacle has been the limited size of zeolite pores and cavities to accommodate the host homogeneous or enzymatic agents. With the recent discovery of the 18 membered AIP04, VPI-5/MCM-9, hope for the synthesis of zeolites to accommodate such systems has been renewed. INTRODUCTION
When we speak of structure sensitivity in zeolite catalysis, we are really referring to what is more commonly referred to as shape selectivity. This term, originally coined by Weisz and Frilette in 1960 ( l ) , embodies several structure sensitive phenomena. Size exclusion, in which reactants are prohibited in reaching the catalytic surface because of their size or products are barred from returning from the catalytic surface to the bulk, was the first phenomenon studied. Subsequently spatiospecific selectivity, where reactions are inhibited by the inability of reaction intermediates to form in the confines of the zeolite cavity, and diffusive selectivity were observed and applications developed. This unique ability to control the selectivity and essentially direct chemical reactions by purely physical constraints has always led to speculation of how we can combine these effects with catalytic surfaces or sites not found in zeolites. Although multifunctional catalysts have been prepared by incorporating metals into zeolites, the metal functions have complemented the zeolite catalysis rather than been enhanced by the zeolite’s shape selective constraints. For the sake of this discussion catalytic cracking has not been included. It is generally accepted that in catalytic cracking the large, high molecular weight hydrocarbons in the gas oil feed are first cracked either thermally or catalytically on the amorphous matrix component of the catalyst and it is the cracked products that can now diffuse into the zeolite portion of the catalyst reacting further. It is speculative whether the selectivity would be further enhanced, more gasoline/less coke, if the zeolite pores were large enough to admit the large hydrocarbon
180
feed molecules. Nevertheless if we were to extend our definition of shape selectivity, even catalytic cracking with zeolite catalysts would be included. ZEOLITE FUNDAMENTALS
For the purpose of this discussion, I will define zeolites as porous crystalline materials formed by the connection of numerous elements, usually tetrahedrally coordinated, through oxygen bridges. They usually have ion exchange capacity and can be readily converted into a variety of catalytic forms. The pore systems are regular and well-defined and can afford access to the interior of the crystal in a unidirectional mode, such as a tube, or by dual pore systems, intersecting and nonintersecting, or even a three-dimensional access. To demonstrate some of these characteristics, Figure 1 depicts the structure of faujasite, a large pore zeolite with threedimensional access. In Figure 2, the two-dimensional pore system and the 010 projection of
ZSM-5 are shown. Zeolites of interest in catalysis are usually classified according to the number of tetrahedral units forming the pore opening: 12 - large, 10 - intermediate, and 8 -
small. Examples of zeolites classified in this manner are shown in Figure 3. TYPES OF SHAPE SELECTIVITY
As has been stated before, shape selectivity in zeolite catalysis can be divided into three
general categories: size exclusion, spatiospecific and diffusive. We will discuss each type separately citing examples and, where applicable, processes built on these concepts.
Size Excluslon Early examples of size exclusive selectivity were the selective dehydration of n-butanol in the presence of i-butanol over the zeolite CaA (2) and the selective cracking of n-hexane in the presence of 3-methylpentane (3),Tables 1 and 2. Commercial applications of this
characteristic were demonstrated in the post-catalytic reforming processes, Selectoforming and M-forming, in which the low octane n-paraffins in the reformate were selectively cracked to lighter hydrocarbon fragments resulting in higher octane number product. Selectoforming employed erionite as the zeolitic catalytic component which had an 8-membered ring pore structure and M-forming utilized ZSM-5 having a 10-membered ring pore structure. The selective cracking phenomenon is illustrated in Tables 3 and 4 showing the compositional
changes brought about by the process with the corresponding octane number increase. By far the broadest application of size exclusion is in catalytic dewaxing processes. In these processes the low temperature rheology is changed to a more fluid product by cracking normal and slightly branched paraffins in the feed to lighter products. Mobil Distillate Dewaxing, MDDW, is the process for improving the low temperature fluidity of diesel fuel, jet fuel and
heating oils while Mobil Lube Dewaxing, MLDW, is the process for lube stocks. Both processes
181
Figure 1
Faujasite
Figure 2
-
ZSM 5 ZSM-5 (010) Projection
182
Figure 3
Pore Openings of Zeolite Structures Phase
Tetrahedral Units
Faujasite, X,Y Mazzite, ZSM-4 Mordenite Offretite Cancrinite Heulandite Stilbite Ferrierite ZSMd Zeolite A Erionite
Dimensions
12 12 12 12 12 10 10 10 10 8 8
(A)*
7.4 7.4 6.7 x 7.0 6.4 6.2 4.4 x 7.2 4.1 x 6.2 4.3 x 5.5 5.4 x 5.6 4.1 3.6 x 5.2
* Based on X-Ray Structure Determination Using an Oxygen Radius of
1.35i
Table 1
Dehydration of Primary Butyl Alcohols 1 Atm. Pressure
Temperature "C
130
230
260
(2 18
<2 60
46 9
85 64
Over Linde CaA Molecular Sieve lsobutyl Alcohol, Wt% n-Butyl Alcohol, Wt% sec-Butyl Alcohol, Wt%
N
O
Over Faujasite-Type CaX Molecular Sieve lsobutyl Alcohol, Wt% n-Butyl Alcohol, Wt% sec-Butyl Alcohol, Wt%
82
183
Table 2
Molecular-ShapeSelective Cracking Time on Stream (Min) 10 to 33 10 to 20 10 to 20
Temperature
Conversion
370 320 to 540 510 to 540
47 to 30 0 to 0.7 0.4 to 1.9
H Erionite
n-Hexane 2-Methylpentane 2-Methylpentane
26 26 26
320 430 540
52.1 1.o 4.7
H Chabazite H Chabazite
n-Hexane 2-Methylpentane
30
260 540
10.0
Catalyst H Gmelinite H Gmelinite H Gmelinite
H Erionite H Erionite
Hydrocarbon Charge n-Hexane 2-Methylpentane Methycyclopentane
10
Table 3
W,
("/.I
1.5
Selectoforming a C5-82 C light Naphtha Composition
+ Ethane ~~
Methane
-
Feed
Product at 395°C 425°C 1.5
5.0
21.9
24.7
lsobutane
0.4
1.3
n-Butane
6.0
7.7
Propane
lsopentane
22.2
22.1
21.7
n-Pentane
23.4
11.1
2.8
54.4
37.0
31.5
67.4
79.6
84.0
6+
C5+ Octane, R + O
at 28 Atm., 1.6 LHSV
184 employ a ZSM-5 containing catalyst tailored to the specific process. Figure 4 shows the relative effect of hydrocarbon components of petroleum stocks on the pour point, the property we use to measure low temperature fluidity. Another interesting aspect of dewaxing is the effect the shape of the pore opening can have on the effectiveness of the process. ZSM-23 is a zeolite which like ZSM-5 has a 10-memberedpore opening but with a tear-shaped opening. Figure 5 shows the projections of the pore openings for ZSM-5 and ZSM-23 with their corresponding effective and crystallographic dimensions. When dewaxing a distillate stock to the same pore point, ZSM-23 performs less conversion than ZSM-5 and gives a product with a higher viscosity index, a measure of the temperature/viscosity response (Table 5). This result is interpreted as due to the further restriction of the pores of ZSM-23 reducing the amount of branched paraffins that are cracked in the dewaxing process. Finally, this size exclusive property of ZSM-5 is also exhibited when used as an octane enhancing additive catalyst in catalytic cracking. In this operation, low octane n-paraffins and olefins are cracked to light gases while some olefin isomerization also occurs. Table 6 compares operation with and without ZSM-5 for both FCC and TCC. SpatlosDeclflclty When both the reactant molecule and the product molecule are small enough to diffuse through the zeolite pore channels, but the reaction intermediates are larger than either the reactants or products and are spatially constrained either by their size or orientation, we refer to this as spatiospecific selectivity. Spatioselectivity or transition state selectivity is independent of crystal size and activity, but depends on the pore diameter and zeolite structure. This type of selectivity was first proposed by Csicsery (4) in 1971. Spatioselectivity plays a major role in the selective cracking of paraffins in medium pore zeolites. For example, n-hexane and 3-methylpentane are readily sorbed by ZSM-5, yet the singly-branched molecule cracks at a significantly slower rate than the straight chain molecule. 3-methylpentane, being a bulkier molecule than n-hexane, apparently requires more space than n-hexane to form the reaction intermediate as shown in Figure 6. A very significant commercial application of spatioselectivity is in the isomerization of
xylenes using ZSMB containing catalysts. Xylenes can undergo isomerization via intramolecular 1,2-methyl shift as well as a disproportionation via diphenylmethanes, Figure 7. The intracrystalline cavity of ZSM-5 cannot readily accommodate the disproportionation reaction intermediate, hence shifting the selectivity dramatically towards isomerization. Figure 8 shows this selectivity effect in comparing the relative amounts of disproportionation and isomerization for ZSM-5 vs faujasite, a large pore zeolite with a much larger intracrystalline cavity.
185
Table 4
CompositionalChange at Different M-Forming Severities C6-80°C Mid-Continent Naphtha, 28 Atm. Feed
Product
Product
84.5
89.6
92.7
c11
18.5 23.4 0.6 0.2 0 0
16.9 22.5 1.1 2.9 2.0 0.3
-1.6 -0.9 +0.5 t2.7 t2.0 +0.3
16.3 21.7 1.5 3.7 3.3 0.4
+ +
Total
42.7
45.8
+3.0
46.8
+4.1
30.9 12.9 0.2 0
26.2 9.7 0 0
-4.7 -3.2 -0.2 0
23.4 0.7 0.3 0
-7.5 -4.2 to.l 0
44.0
35.9
-8.1
32.4
-11.6
1.4
1.6 8.6 8.1
t0.2 +0.6 +4.2
1.1
C,+,R+O Aromatics B
T
X C9 c 10
-2.2 -1.7 +0.9 3.5 + 3.3 0.4
Paraffins
Naphthenes Pentanes Butanes and Lighter
8.0
3.9
-0.3
+ 1.0 + 6.8
9.0 10.7
Figure 4
Conversion of Alkane and Alkylbenzene Molecular Classes During Dewaxing Over ZSM-5 1
.a Fraction Converted
:enes
.6 .4
.2
2+1-Me-Alkanes
0
I
I
I
I
1
1
1
1
-30
-20
-10
0
10
20
30
Pour Point, "C
186
Figure 5
Projections of ZSM-5 and 23 Structures Crystallograehic Pore Size (A) 5.1 x 5.5
-
ZSM 5
4.5 x 5.6 ZSM - 23
Table 5
Oewaxing Over ZSM-5 and ZSM-23 ZSM-23
Product Pour Point, "C
ZSM-5 15 -12
Viscosity Index
101.0
108.7
Catalyst Conversion, Wt%
11 -12
Table 6
CommercialTest of ZSM-5 TCC Base Catalyst Time on Stream, Days Conversion, Vol% C5+ Gasoline, Vol% Butenes, Vol% Propene, Vol% Light Fuel Oil, Vol% Coke, Wt% Research Octane Number R + 0 Motor Octane Number M + 0 Potential Alkylate, Vol% Total Gasoline, Vol%
FCC Catalyst Containlng ZSM-5
0
72
106
42.3 3.8 3.7 29.9
40.9 4.6 4.7 29.1
41.0 4.2 4.2 26.4
86.0 77.4 12.7 55.0
90.2 79.2 15.7 56.6
91.2 79.5 14.2 55.2
-_
__
I
Base Catalyst
Catalyst Containing ZSM-5
0 732-53.9 7.9 6.2 26.5 6.7 87.3 78.1 24.9 78.8
37 7 ? ’ ) 51.6 9.0 7.0 26.5 6.7 89.0 78.7 28.3 79.9
187
Figure 6
Mechanism of ParaffinCracking C
-
n hexane
C
/C1 0
C
c is+ 4 ’
6+,H
H' 0O8 \ -
Al
Si
-
6-
-
Cross Section
C
9,
6 x 7 i
C' 6-
- 0 0 0O6 \ 1
Al
Si
Figure 7
Xylene lsomerization C
C
Disproportionation
jyC+ b,--,o C
i
Si
3 methylpentane
C
4.9 x 6
+"& C
Si
188
Figure 8
Disproportionation vs. lsomerization
/I
m-Xylene Feed, 573 K (300°C) Comparison of ZSM-5 with Faujasite* 20
/
% Disproportionation
10
L
Faujasite
0.2
i
0.1
ZSMB 0
0
10
20
0
% lsomerization
"Lanewalda & Bolten, J. Org. Chem. 2,3107(1969)
Figure 9
1 10 -2
10
D cm2/Sec
10 a 10 -8 10 -10
10 -12 10 -14
I,, Configurational
1
10
100
Angstroms
Diffusivity and Size of Aperture (Pore); the Classical Regions of Regular and Knudsen and the New Regime of Configurational Diffusion
1
Ir.
10
189
Table 7
Hydrogenationof a Mixture of Trans- and Cis- Butene-2 Temp.,
initial Composition
- 120 78.7 21.3
“C
Trans.
Cis.
Final Composition
Conversion , Wt%
Trans.
Cis.
- -
n-Butane
Trans.
37.1
17.0
--
Cis.
45.9
52.9
20.8
-
kTrans
kcis 3.3
103
78.7
21.3
57.3
19.8
22.9
27.2
7.1
4.3
98
78.7
21.3
69.4
20.9
9.7
11.8
1.8
7.0
Figure 10
Kinetic Model for p-Xylene Preference with ZSM-5 Catalyst meta
ortho
para
Rel. Intrinsic k’s
1
1
Rel. Apparent k s
0.08
1
rn 104
Dp’Do
r----m A
I
Dm , I
m
-
P o1
Do
I
DP I
-0 -P
190
Table 8
Toluene Disproportionation Temperature, "C
WHSV
Feedstock
Large Crystal ZSM-5 550 30 Toluene
Mg ZSMQ 550 3.5 Toluene
13.2
10.9
35 46 19
88 10 2
Thermodynamic Equilibrium ~~
Conversion, Wt% Toluene % Xylenes
Para Meta Ortho
23 51 26
Diffusive Selectlvltv Diffusion in zeolites does not conform to the principles we usually apply in diffusive processes. The diffusive process in zeolites has been described by Weisz ( 5 ) as configurational diffusion. In this diffusion regime, even a subtle change in the dimensions of a molecule can result in a large change in its diffusivity as shown in Figure 9. This type of diffusion and its effect on catalytic selectivity was shown (Chen and Weisz) (6) in the hydrogenation of transand cis-butene where, although the two molecules differ in size by only 0.26, the much higher diffusion rate of trans-butene results in a much higher reaction rate (Table 7). Commercial processes utilizing this structure sensitive property include several aromatics processing applications such as xylene isomerization, selective toluene disproportionation (STDP), paraethyl toluene synthesis and p-diethylbenzene synthesis. In xylene isornerizationthe slight difference in the size of the para-isomer but large difference in diffusivity leads to a paraselective product as shown in Figure 10.
.
In the case of toluene disproportionation, an equilibrium mixture of xylenes is obtained
using a standard ZSMQ catalyst containing ZSMB crystals (0.5 Nm. Increasing the crystal size to >3 urn or chemically modifying the crystal with Mg to reduce pore size results in a dramatic
shift to para-selectivity, Table 8.
Chemical modification has also been shown to be effective in the alkylation of toluene
with ethylene to make p-ethyltoluene, Table 9, and ethylbenzene alkylation with ethylene to make p-diethylbenzene, Table 10. It should be pointed out that these structure sensitive phenomena in zeolites are not
mutually exclusive and in many reactions the phenomena occur simultaneously.
191
Table 9
Toluene-EthyleneAlkylation Unmodlfied HZSM-5
350
Temperature, "C WHSV Toluene C2H4
Toluene/C2H4 (Mole)
Selectivlty l o Products, % Benzene Ethylbenzene Xylenes p-Ethynoluene m-Ethyltoluene 0-Ethyltoluene Normalized Ethyltoluene Para Meta Ortho
Equilibrium
33.1 49.9 16.3
350
6.9 0.5 4.511
6.9 0.5 4.311
20.7 94.4
15.3 71.2
0.8 1.4 1.4
0.5
Conversion, Wi% Toluene C2H4
5-
0.8 0.8
23.3 53.3 8.3
90.6
27.4 62.8 9.8
97.0 3.0 0
2.8 0
Table 10
EthylbenzeneDisproportionation Large Crystal HZSM-5 Temperature, "C
Mg-P-ZSM-5
500
525
3.5
30.2
89.6
22.5
60.8 10.4 3.9 1.1 1.8 4.1
62.4 1.3
WHSV
EB
Conversion, % EB Selectivity to Products, Wt% Benzene ToI uene Xylenes, EB Ethyltoluene Diethylbenzene @her Aromatics Light Gas c5-c9
Gas
Total
-
1.9 16.0
0
0.6
15.4
2.7 11.6
-
I
100.0
100.0
33.3
99.3 0.7
Diethylbenzene Para Meta Ortho
65.0 1.l
0
192 FUTURE OPPORTUNITIES
The application of shape selective catalysis of zeolites is far from being exhausted. Extension to other petroleum based and petroleum related feedstocks has shown potential with existing zeolitic materials. Applications of shape selective catalysis in fine chemical manufacture seem to be relatively unexplored. Combining homogeneous catalysis with the shape selectivity of zeolites has found no success because of size incompatibility. With the discovery of larger pore zeolitic like materials such as VPI-5, we might be able to overcome the barrier. LITERATURE CITED 1 2 3 4 5
P. B. Weisz and V. J. Frilette, J. Phys. Chem., 64 (1960) 382. P. B. Weisz, V. J. Frilette, A. W. Maatman and E. B. Mower, J. Cat., 1 (1962) 307 J. N. Miale, N. Y . Chen and P. 9.Weisz, J. Cat., 6 (1966) 278. S. M. Csicsery, J. Cat., 23 (1971) 124. P.B. Weisz, Chemtech, 3 (1973) 498.