- Email: [email protected]
Toluene disproportionation catalysis using euo type zeolites: Initial optimisation and development
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
851
TOLUENE DISPROPORTIONATION CATALYSIS USING EUO TYPE ZEOLITES: INITIAL OPTIMISATION AND D E V E L O P M E N T J.L. Casci a and A. Stewart b
a- Synetix, RT&E Department, PO Box 1, Billingham TS23 1LB, UK. b - ICI Technology, Science Support Group, PO Box 90, Wilton, TS90 6JE, UK. This paper is a combined preparation and catalysis study, which explores the relationship between catalyst preparation and performance, in particular how the conditions of manufacture impact on catalyst deactivation. The commercially important reaction of toluene disproportionation (TDP) was studied using catalysts based on zeolite EU-1. This report describes initial results from a screening study, where it was found that different samples of EU-1 decayed at markedly different rates; these were samples of different compositions synthesised at different temperatures for different periods of time. The decay characteristics are rationalised in terms of synthesis reaction over-run during which there is a marked variation in both the pH of the reaction mixture and the surface composition of the isolated material, as determined by x-ray photoelectron spectroscopy (XPS). This over-run phenomena, not previously reported for high-silica zeolites, is related to synthesis temperature (and composition) and is likely to be important for other zeolite catalysts. I. Introduction
The preparation and manufacture of (Zeolite- based) a catalyst is multi-stage process comprising zeolite synthesis, activation (typically calcination and ion exchange) and forming (e.g. extrusion after addition of a binder). All stages can have a profound influence on the catalyst performance. Transalkylation reactions are widely used in petrochemical industries where they are used to upgrade the economic value of, chemical feedstocks. An example is toluene disproportionation 1 (TDP) in which toluene is reacted to generate benzene and (ortho-, meta- and para-) xylene: 2C7H 8 ~ C6H6 + C8H10 schemel Thus a material, toluene, which has only "fuel value" can be upgraded to the more valuable chemical feedstocks benzene and para-xylene, key intermediates (for example) for nylon and polyethyleneterephthalate respectively. In addition this process has the advantage of generating a low ethylbenzene C 8 stream for further processing. The reaction, however, is not quite as simple as scheme 1 suggests with a series of side and/or consecutive reactions yielding light gas (by dealkylation) and heavier products (C 9, C 10 etc. by further reaction of xylenes). There are numerous reports of zeolite-based catalysts for TDP with most referring to either MOR or MFI materials 1, with a modified MFI also being reported 2, for Selective TDP in which a p-xylene rich product is generated rather than the thermodynamic distribution of xylene isomers. While MFI consists of intersecting 10-ring channels 3, the MOR framework is based on unidimensional 12-ring channels linked by 8-rings 3. Based on these features alone one might consider the EUO framework type to be of interest in
852
TDP since it is based on 10-ring channels with deep 12-ring side-pockets 3'4. The EUO framework was first discovered as EU-15, 6 by workers at Edinburgh University and ICI then later as TPZ-3 (Teijin) and ZSM-50 (Mobil) - see reference 3 for further details. This paper (which is the first in a series which will examine the development and optimisation of a TDP catalyst based on the EUO, and related, frameworks) will concentrate on the relationship between the synthesis conditions and catalyst decay. It will explore, it is believed for the first time, the effect of crystallisation over-run on catalytic performance and surface composition; these effects are likely to have profound implications for other materials and other reactions. 2. Experimental
Samples of EU-1 were synthesised using fumed silica (Cab-o-Sil, M5, BDH Ltd.), solid sodium aluminate (approximate composition 1.35 Na20 A120 3 5H20, BDH Ltd.) and sodium hydroxide pellets (BDH Ltd.). The organic template used was hexamethonium bromide: hexane-l,6-bis(trimethylammonium bromide)(Fluka). Demineralised water was used for all solution make-up. The reaction mixture was prepared by dividing the required water into three approximately equal portions. The sodium aluminate and sodium hydroxide were dissolved in the first portion, hexamethonium bromide was dissolved in the second and the third aliquot used to disperse the fumed silica. The solutions of hexamethonium bromide and sodium aluminate/sodium hydroxide were mixed then added, with stirring, to the silica dispersion. Stirring was continued until a smooth gel resulted (about 5 minutes) following which the mixture was transferred to the reactor. Syntheses were carried out in 1 or 2 litre stirred (4-blade, 45 ~ pitched-paddle impellers operating at 300rpm) stainless steel autoclaves. The reactors were electrically heated with temperature control to + 2~ Reaction times were taken from when the autoclave contents reached operating temperature. The autoclaves could be sampled. All samples and final products (in this study) were activated by calcination (static air: 450~ 48 hours; 550~ 24 hours) followed by ion-exchange (1M HC1, 2 x 4 hours, 60oc, 50cc per g zeolite). After exchanged materials were filtered, washed and dried before a final calcination (static air; 550oc, 24 hours). Materials were characterised by powder xrd (Philips APD 1700 diffractometer, Cu Ka radiation), chemical analysis (AAS + ICP) and XPS (Kratos XSAM 800 spectrometer using Mg Kct radiation (hv = 1253.6 eV). Catalytic evaluation was carried out on 0.5-1.0g fractions of catalyst (undiluted zeolite) in a 3mm I.D. stainless steel tube with quartz wool used to maintain the bed in the isothermal zone. TDP was carried out in the absence of H 2 and at atmospheric pressure, at temperatures between 300 and 500~ and at a WHSV of about 0.4 (see later for exact temperatures and space velocities). Products (and unreacted toluene) were analysed by GC. 3. Results and Discussion
Initial screening of a series of EU-1 samples in TDP revealed a spread in catalytic properties. Figure 1 presents some typical data for toluene conversion against time-on-line.
853
Only four sets of data are shown to avoid confusion and these samples have been chosen simply to exemplify the variability observed. Large differences in decay rate can be clearly seen. Detailed examination suggested that there are two types of behaviour: EU-1 types I and II. Three examples of type I materials (A, B and C in the figure) are shown as solid symbols and display the very rapid decay in activity. The type II material, displayed in open symbols, decays in "two stages": an initial rapid decay (c.f. type I) then a long period of slow decay. This paper will rationalise the variability between the type I materials; subsequent papers will describe the differences between types I and II (composition, crystal size etc.) and a detailed analysis of the decay kinetics. 9O r.~ ;~
o
8O
m
70
n
!0_2~0.0C_ m180-C---.A-!_~6--0__~IC
m
~
60
~
so
e.l 9 ....,
40
u A
A o
m
30
~
20
A
0
0
50
100
C rystallisation
Figure 1.
150
200
Tim
250
300
e (hours)
Toluene Conversion against Time-on-Line (hours) for a series of EU-1 samples Temperature = 330~ WHSV = 0.4
It must be stressed that the designations, Type I and II, used here are employed simply to distinguish between the different catalytic behaviours observed. Examination, by XRD, of all samples ( A - D) in Figure 1 showed that the materials were EUO types. Some minor differences are observed (e.g. in xrd peak width) but these were fully consistent with "simple" differences in crystallite size. All three type I materials (A, B and C in the figure) were activated under identical conditions, but there were some significant differences in the method of crystallisation see Table 1. A typical reaction mixture composition was: 60 SiO 2 x A1203 10 Na20 10 HexBr 2 3000H20 (where HexBr 2 is the template hexamethonium bromide referred to above) Table 1 Comparison of EU-I Type I samples in Figure 1 Type I materials in Fig 1 SiO 2/A120 3 Temperature/ o C 200 60/0.77 180 160 60/1
Time / hours 51.5 166 163
All three materials had similar sized crystals: ellipsoidal aggregates approximately 1- 5 microns in length which have been described previously 7 although sample C also contained some smaller crystals. The significance of these smaller crystals and the (slight)
854 differences in SiO2/A120 3 will be described in subsequent papers. External surface areas for samples A-C (typical of Type I materials) were about 1-10 m2gn. One of the most notable differences in Table 1 is the crystallisation temperature. It can be noted that the materials' decay in activity (Figure 1) follows this temperature i.e. sample A which is crystallised at the highest temperature decays more rapidly than B, than C. Thus the lower the crystallisation temperatures the more stable the resulting catalyst. All three materials had xrd patterns (see above), which were virtually identical, but it was known 7 that after maximum crystallinity was reached there were some changes in reaction mixture pH. An example of this and, it is believed the reason behind the variation in performance is shown in Figure 2.
120 100
-
9•
80 -
t~
60 -
I._
40 . 20 t
0T
ca.
12.1
--_
12
,
11.9
9
11.8
.
11.7
9
11.6
9
11.5
b
.
.
I
1 1.4
,
I
I
100
150
120
lOO o < ~r
80
~
40
~
20
=
0
r~
60
t
0
50
Time (hours) Figure 2. Plot of % Crystallinity (XRD), pH and surface composition Vs Time
200
855 Figure 2 is based on an EU-1 Synthesis of composition: 60 SiO 2 0.77 AI20310 Na2010 HexBr 2 3000H20 carried out at 180~ It can be seen that maximum crystallinity is reached at about 48 hours and thereafter remains constant. The pH, however, after reaching a maximum at a similar time to xrd thereafter shows a marked decline - it was this phenomenon which first suggested that the reaction was not "static" in this period and that further changes were taking place. There are a number of possibilities for the change (decline) in pH but it is believed the most likely is the (Hofmann) degradation of the template which consumes hydroxyl ions and liberates trimethylamine (and generates an olefinic species). Regardless of its origin, the decline in pH will have a marked influence on the silica that remains in solution. If it is assumed that at full crystallisation an equilibrium is established between the crystalline EU-1 and the silica remaining in solution, then a decrease in pH will perturb the equilibrium and generate a solution supersaturated in silica (most or all of the alumina will have been consumed). This supersaturation will be discharged by the deposition of silica: either as an amorphous phase or as a coherent, crystalline silica-rich "over layer" - in both cases effectively "coating" the EU-1. It is believed that it is this coating which is responsible for the more rapid deactivation of certain EU-1 samples (Type I) in TDP. At any given crystallisation temperature "over-running" the reaction will lead to this decline in pH and the resulting loss in performance. The effect is purely a time based phenomenon which is most pronounced at higher crystallisation temperatures, that is, EU-1 samples of similar surface composition and "integrity" can be made at all temperatures. However, the "time window" for this high integrity EU-1 is much reduced at higher temperatures: it is only a few hours at 200~ This is illustrated by Figure 3, which shows the variation in surface composition (SiOJA1203 ratio determined by XPS) as a function of time for series of reactions carried out at different temperatures.
90
~ x
80 70
O
m I
60 O
,A
I
50 40
j,#
B 0
D-2-0OCm180C ,A 160C
I
~
20
r~
10
A
A
A
0
50
1O0
150
200
250
300
Crystallisation Time (hours) Figure 3. Plot of surface SIO2/A1203 against crystallisation time for a series of EU-1 samples crystallised at different temperatures. (Reaction mixtures had similar compositions to those shown in Figure 2.)
856
Measurements were made on isolated samples taken after 100% crystallisation had been reached, that is, measurements were only carried out on samples of fully crystalline zeolite EU-1 (as determined by xrd analysis). It can be seen that both "initial" and "final" compositions (SIO2/A1203 ratios) are similar for all preparations (about 30 and 90 respectively) regardless of crystallisation temperature and that the composition increases steadily with t i m e - although the rate of increase is more marked at the higher temperatures. The minimum value of the surface SIO2/A1203 ratio (30) represents the maximum aluminium loading that can be obtained for EU-1 crystallised using the hexamethonium template. At this value it would be expected that catalysts would have maximum activity since acid site density is at its highest- this presupposes that all sites have similar intrinsic acidities. What is less obvious is why such materials also have the greatest stability. This topic will form the basis of subsequent papers.
Conclusion The "optimum" EU-1, type I materials (for TDP catalysis) are prepared by isolating the sample at maximum pH (as soon as full crystallinity is reached) and this is most readily accomplished at lower (say 160~ crystallisation temperatures. A similar surface composition (SIO2/A1203 ratio) can be obtained from reactions carried out between 160 and 200~
Acknowledgements The authors gratefully acknowledge the technical expertise of Mr N. Malone (XPS), Mr S. Huntley (zeolite synthesis and activation) and Mr P.J. Hogan, M.A. Martin and S. Johnson (TDP catalysis) and thank Synetix and ICI Technology for permission to publish. References 1 A. Azzouz,V. Hulea,B. Zaoui,M. Attou and E. Dumitriu,J. Soc. Alger. Chim., 3 (1993) 38. 2 For example, J.S Abichandani, J.S. Beck, R.H. Fischer, I.D. Johnson and D.L.Stern, U.S. Patent 5625103. [See also Chemical Week, August 10, 18 (1994).] 3 W . M . Meier, D.H. Olson and L. McCusker, "Atlas of Zeolite Structure Types" Fourth Revised Edition, Elsevier, 1996. 4 N.A. Briscoe, D.W. Johnson, M.D. Shannon, G.T. Kokotailo and L.B. McCusker, Zeolites, 8 (1988) 74. 5 J.L. Casci, Ph.D. Thesis, University of Edinburgh, 1982. 6 J.L. Casci, B.M. Lowe and T.V. Whittam, E.P. 42226. 7 J.L. Casci, B.M. Lowe and T.V. Whittam, Proc. VIth Int. Zeolite Conf., 894, Butterworths, 1984.
851
TOLUENE DISPROPORTIONATION CATALYSIS USING EUO TYPE ZEOLITES: INITIAL OPTIMISATION AND D E V E L O P M E N T J.L. Casci a and A. Stewart b
a- Synetix, RT&E Department, PO Box 1, Billingham TS23 1LB, UK. b - ICI Technology, Science Support Group, PO Box 90, Wilton, TS90 6JE, UK. This paper is a combined preparation and catalysis study, which explores the relationship between catalyst preparation and performance, in particular how the conditions of manufacture impact on catalyst deactivation. The commercially important reaction of toluene disproportionation (TDP) was studied using catalysts based on zeolite EU-1. This report describes initial results from a screening study, where it was found that different samples of EU-1 decayed at markedly different rates; these were samples of different compositions synthesised at different temperatures for different periods of time. The decay characteristics are rationalised in terms of synthesis reaction over-run during which there is a marked variation in both the pH of the reaction mixture and the surface composition of the isolated material, as determined by x-ray photoelectron spectroscopy (XPS). This over-run phenomena, not previously reported for high-silica zeolites, is related to synthesis temperature (and composition) and is likely to be important for other zeolite catalysts. I. Introduction
The preparation and manufacture of (Zeolite- based) a catalyst is multi-stage process comprising zeolite synthesis, activation (typically calcination and ion exchange) and forming (e.g. extrusion after addition of a binder). All stages can have a profound influence on the catalyst performance. Transalkylation reactions are widely used in petrochemical industries where they are used to upgrade the economic value of, chemical feedstocks. An example is toluene disproportionation 1 (TDP) in which toluene is reacted to generate benzene and (ortho-, meta- and para-) xylene: 2C7H 8 ~ C6H6 + C8H10 schemel Thus a material, toluene, which has only "fuel value" can be upgraded to the more valuable chemical feedstocks benzene and para-xylene, key intermediates (for example) for nylon and polyethyleneterephthalate respectively. In addition this process has the advantage of generating a low ethylbenzene C 8 stream for further processing. The reaction, however, is not quite as simple as scheme 1 suggests with a series of side and/or consecutive reactions yielding light gas (by dealkylation) and heavier products (C 9, C 10 etc. by further reaction of xylenes). There are numerous reports of zeolite-based catalysts for TDP with most referring to either MOR or MFI materials 1, with a modified MFI also being reported 2, for Selective TDP in which a p-xylene rich product is generated rather than the thermodynamic distribution of xylene isomers. While MFI consists of intersecting 10-ring channels 3, the MOR framework is based on unidimensional 12-ring channels linked by 8-rings 3. Based on these features alone one might consider the EUO framework type to be of interest in
852
TDP since it is based on 10-ring channels with deep 12-ring side-pockets 3'4. The EUO framework was first discovered as EU-15, 6 by workers at Edinburgh University and ICI then later as TPZ-3 (Teijin) and ZSM-50 (Mobil) - see reference 3 for further details. This paper (which is the first in a series which will examine the development and optimisation of a TDP catalyst based on the EUO, and related, frameworks) will concentrate on the relationship between the synthesis conditions and catalyst decay. It will explore, it is believed for the first time, the effect of crystallisation over-run on catalytic performance and surface composition; these effects are likely to have profound implications for other materials and other reactions. 2. Experimental
Samples of EU-1 were synthesised using fumed silica (Cab-o-Sil, M5, BDH Ltd.), solid sodium aluminate (approximate composition 1.35 Na20 A120 3 5H20, BDH Ltd.) and sodium hydroxide pellets (BDH Ltd.). The organic template used was hexamethonium bromide: hexane-l,6-bis(trimethylammonium bromide)(Fluka). Demineralised water was used for all solution make-up. The reaction mixture was prepared by dividing the required water into three approximately equal portions. The sodium aluminate and sodium hydroxide were dissolved in the first portion, hexamethonium bromide was dissolved in the second and the third aliquot used to disperse the fumed silica. The solutions of hexamethonium bromide and sodium aluminate/sodium hydroxide were mixed then added, with stirring, to the silica dispersion. Stirring was continued until a smooth gel resulted (about 5 minutes) following which the mixture was transferred to the reactor. Syntheses were carried out in 1 or 2 litre stirred (4-blade, 45 ~ pitched-paddle impellers operating at 300rpm) stainless steel autoclaves. The reactors were electrically heated with temperature control to + 2~ Reaction times were taken from when the autoclave contents reached operating temperature. The autoclaves could be sampled. All samples and final products (in this study) were activated by calcination (static air: 450~ 48 hours; 550~ 24 hours) followed by ion-exchange (1M HC1, 2 x 4 hours, 60oc, 50cc per g zeolite). After exchanged materials were filtered, washed and dried before a final calcination (static air; 550oc, 24 hours). Materials were characterised by powder xrd (Philips APD 1700 diffractometer, Cu Ka radiation), chemical analysis (AAS + ICP) and XPS (Kratos XSAM 800 spectrometer using Mg Kct radiation (hv = 1253.6 eV). Catalytic evaluation was carried out on 0.5-1.0g fractions of catalyst (undiluted zeolite) in a 3mm I.D. stainless steel tube with quartz wool used to maintain the bed in the isothermal zone. TDP was carried out in the absence of H 2 and at atmospheric pressure, at temperatures between 300 and 500~ and at a WHSV of about 0.4 (see later for exact temperatures and space velocities). Products (and unreacted toluene) were analysed by GC. 3. Results and Discussion
Initial screening of a series of EU-1 samples in TDP revealed a spread in catalytic properties. Figure 1 presents some typical data for toluene conversion against time-on-line.
853
Only four sets of data are shown to avoid confusion and these samples have been chosen simply to exemplify the variability observed. Large differences in decay rate can be clearly seen. Detailed examination suggested that there are two types of behaviour: EU-1 types I and II. Three examples of type I materials (A, B and C in the figure) are shown as solid symbols and display the very rapid decay in activity. The type II material, displayed in open symbols, decays in "two stages": an initial rapid decay (c.f. type I) then a long period of slow decay. This paper will rationalise the variability between the type I materials; subsequent papers will describe the differences between types I and II (composition, crystal size etc.) and a detailed analysis of the decay kinetics. 9O r.~ ;~
o
8O
m
70
n
!0_2~0.0C_ m180-C---.A-!_~6--0__~IC
m
~
60
~
so
e.l 9 ....,
40
u A
A o
m
30
~
20
A
0
0
50
100
C rystallisation
Figure 1.
150
200
Tim
250
300
e (hours)
Toluene Conversion against Time-on-Line (hours) for a series of EU-1 samples Temperature = 330~ WHSV = 0.4
It must be stressed that the designations, Type I and II, used here are employed simply to distinguish between the different catalytic behaviours observed. Examination, by XRD, of all samples ( A - D) in Figure 1 showed that the materials were EUO types. Some minor differences are observed (e.g. in xrd peak width) but these were fully consistent with "simple" differences in crystallite size. All three type I materials (A, B and C in the figure) were activated under identical conditions, but there were some significant differences in the method of crystallisation see Table 1. A typical reaction mixture composition was: 60 SiO 2 x A1203 10 Na20 10 HexBr 2 3000H20 (where HexBr 2 is the template hexamethonium bromide referred to above) Table 1 Comparison of EU-I Type I samples in Figure 1 Type I materials in Fig 1 SiO 2/A120 3 Temperature/ o C 200 60/0.77 180 160 60/1
Time / hours 51.5 166 163
All three materials had similar sized crystals: ellipsoidal aggregates approximately 1- 5 microns in length which have been described previously 7 although sample C also contained some smaller crystals. The significance of these smaller crystals and the (slight)
854 differences in SiO2/A120 3 will be described in subsequent papers. External surface areas for samples A-C (typical of Type I materials) were about 1-10 m2gn. One of the most notable differences in Table 1 is the crystallisation temperature. It can be noted that the materials' decay in activity (Figure 1) follows this temperature i.e. sample A which is crystallised at the highest temperature decays more rapidly than B, than C. Thus the lower the crystallisation temperatures the more stable the resulting catalyst. All three materials had xrd patterns (see above), which were virtually identical, but it was known 7 that after maximum crystallinity was reached there were some changes in reaction mixture pH. An example of this and, it is believed the reason behind the variation in performance is shown in Figure 2.
120 100
-
9•
80 -
t~
60 -
I._
40 . 20 t
0T
ca.
12.1
--_
12
,
11.9
9
11.8
.
11.7
9
11.6
9
11.5
b
.
.
I
1 1.4
,
I
I
100
150
120
lOO o < ~r
80
~
40
~
20
=
0
r~
60
t
0
50
Time (hours) Figure 2. Plot of % Crystallinity (XRD), pH and surface composition Vs Time
200
855 Figure 2 is based on an EU-1 Synthesis of composition: 60 SiO 2 0.77 AI20310 Na2010 HexBr 2 3000H20 carried out at 180~ It can be seen that maximum crystallinity is reached at about 48 hours and thereafter remains constant. The pH, however, after reaching a maximum at a similar time to xrd thereafter shows a marked decline - it was this phenomenon which first suggested that the reaction was not "static" in this period and that further changes were taking place. There are a number of possibilities for the change (decline) in pH but it is believed the most likely is the (Hofmann) degradation of the template which consumes hydroxyl ions and liberates trimethylamine (and generates an olefinic species). Regardless of its origin, the decline in pH will have a marked influence on the silica that remains in solution. If it is assumed that at full crystallisation an equilibrium is established between the crystalline EU-1 and the silica remaining in solution, then a decrease in pH will perturb the equilibrium and generate a solution supersaturated in silica (most or all of the alumina will have been consumed). This supersaturation will be discharged by the deposition of silica: either as an amorphous phase or as a coherent, crystalline silica-rich "over layer" - in both cases effectively "coating" the EU-1. It is believed that it is this coating which is responsible for the more rapid deactivation of certain EU-1 samples (Type I) in TDP. At any given crystallisation temperature "over-running" the reaction will lead to this decline in pH and the resulting loss in performance. The effect is purely a time based phenomenon which is most pronounced at higher crystallisation temperatures, that is, EU-1 samples of similar surface composition and "integrity" can be made at all temperatures. However, the "time window" for this high integrity EU-1 is much reduced at higher temperatures: it is only a few hours at 200~ This is illustrated by Figure 3, which shows the variation in surface composition (SiOJA1203 ratio determined by XPS) as a function of time for series of reactions carried out at different temperatures.
90
~ x
80 70
O
m I
60 O
,A
I
50 40
j,#
B 0
D-2-0OCm180C ,A 160C
I
~
20
r~
10
A
A
A
0
50
1O0
150
200
250
300
Crystallisation Time (hours) Figure 3. Plot of surface SIO2/A1203 against crystallisation time for a series of EU-1 samples crystallised at different temperatures. (Reaction mixtures had similar compositions to those shown in Figure 2.)
856
Measurements were made on isolated samples taken after 100% crystallisation had been reached, that is, measurements were only carried out on samples of fully crystalline zeolite EU-1 (as determined by xrd analysis). It can be seen that both "initial" and "final" compositions (SIO2/A1203 ratios) are similar for all preparations (about 30 and 90 respectively) regardless of crystallisation temperature and that the composition increases steadily with t i m e - although the rate of increase is more marked at the higher temperatures. The minimum value of the surface SIO2/A1203 ratio (30) represents the maximum aluminium loading that can be obtained for EU-1 crystallised using the hexamethonium template. At this value it would be expected that catalysts would have maximum activity since acid site density is at its highest- this presupposes that all sites have similar intrinsic acidities. What is less obvious is why such materials also have the greatest stability. This topic will form the basis of subsequent papers.
Conclusion The "optimum" EU-1, type I materials (for TDP catalysis) are prepared by isolating the sample at maximum pH (as soon as full crystallinity is reached) and this is most readily accomplished at lower (say 160~ crystallisation temperatures. A similar surface composition (SIO2/A1203 ratio) can be obtained from reactions carried out between 160 and 200~
Acknowledgements The authors gratefully acknowledge the technical expertise of Mr N. Malone (XPS), Mr S. Huntley (zeolite synthesis and activation) and Mr P.J. Hogan, M.A. Martin and S. Johnson (TDP catalysis) and thank Synetix and ICI Technology for permission to publish. References 1 A. Azzouz,V. Hulea,B. Zaoui,M. Attou and E. Dumitriu,J. Soc. Alger. Chim., 3 (1993) 38. 2 For example, J.S Abichandani, J.S. Beck, R.H. Fischer, I.D. Johnson and D.L.Stern, U.S. Patent 5625103. [See also Chemical Week, August 10, 18 (1994).] 3 W . M . Meier, D.H. Olson and L. McCusker, "Atlas of Zeolite Structure Types" Fourth Revised Edition, Elsevier, 1996. 4 N.A. Briscoe, D.W. Johnson, M.D. Shannon, G.T. Kokotailo and L.B. McCusker, Zeolites, 8 (1988) 74. 5 J.L. Casci, Ph.D. Thesis, University of Edinburgh, 1982. 6 J.L. Casci, B.M. Lowe and T.V. Whittam, E.P. 42226. 7 J.L. Casci, B.M. Lowe and T.V. Whittam, Proc. VIth Int. Zeolite Conf., 894, Butterworths, 1984.