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Gas Phase Synthesis of Mtbe Over Acid Zeolites
Ouai, L a al. (Editors),New Frontkrs h Catafysir Proceedings of the 10th Intcmtional Congrew on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elscvicr science Publishers B.V.All rights reserved
GAS PHASE SYNTHESIS OF MTBE OVER ACID ZEOLITES A. Nikolopoulo.9~ T. P. Paluckab) P. V.Shertukdea, R. Oukacia) J. G. Goodwin,Jr.0 and
G. Marcelinab
*Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA bAltamira Instruments, Inc., 2090 William Pitt Way, Pittsburgh, PA 15238, USA
1. INTRODUCTION
Methyl tert-butyl ether (MTBE) is made commercially by the acid-catalyzed equilibrium reaction of methanol and iso-butylene using an ion-exchange resin catalyst, viz.:
CH,OH
+ (CH,)$=CH,
-
(CH,),COCH,
This reaction is typically carried out in the liquid phase at relatively low temperatures (T < 100°C). This low temperature limit is imposed partly by the increasingly unfavorable thermodynamics and partly by the instability of the resin at elevated temperatures. Although the reaction proceeds via an acid catalyzed pathway, little is known about the requirements of the catalyst to form the MTBE product selectively. .This work examines the effect of acid properties of various catalysts, namely zeolites, on the selective gas phase MTBE synthesis. Zeolites are ideal materials for this study since they have high acidity and can be readily prepared with a range of acid characteristics. Previous studies have addressed the use of zeolites for MTBE synthesis [l-31. However, a systematic investigation of the cffect of acidity on the reaction has not been reported. In this work, a number of HY zeolites with different acid properties were studied in an attempt to investigate their catalytic performance and correlate it with their acid properties. 2. EXPERIMENTAL
Six acid catalysts were studied. Four of these were HY zeolites with different acid properties. Zeolites LZ210-12, Y62, and Y82 were commercially obtained, while zeolite S(LZ12)8 was obtained by mild steam dealumination of LZ210-12. A silica-alumina catalyst (Si-Al-0) with lower acidity and Amberlyst-15 resin were also evaluated for comparison. The zeolites were initially in the ammonium form and were dearninated prior to reaction by heating to 400°C at a rate of 2"C/min under He flow and holding for 12 hours.
2602 The Si/Al ratios of the zeolites were determined by X R D and 2'Si and 27AI MASNMR. Temperature programmed desorption (TPD) of pyridine was carried out on all catalysts except Amberlyst in order to estimate acid strength. Adsorption was conducted at 100'C and desorption was conducted from 1 0 0 ° C to 7 0 0 ° C at a rate of 1O0C/niin. Acid strengths were also determined by n-pentane cracking reaction at 400'C [4]. The MTBE synthesis reaction was carried out using a small fixed bed reactor with on-line GC-FID analysis. Experiments were performed at atmospheric pressure, 100 C, i-butylene/methanol ratio of 1.8, and a space velocity of about 16 h-'. 3. RESULTS AND DISCUSSION
The structural and acid properties of the catalysts studied are summarized in Table 1. For the zeolites, acid site densities were calculated based on each structural Al resulting in a Bronsted site. In the case of the Si-AI-0, i t was not possible to accurately determine the acid site density, but a working range could be determined by taking the lower limit as the number of pyridine molecules adsorbed per gram of catalyst and the upper limit as the aluminum site density. I t should be noted that these catalysts have ;I different number of Brijnsted sites and acid strength of these sites, thus allowing comparison of the catalytic behavior t o both acid strength m c l site density. Table 1 Summary of structural and acid properties of catalysts Catalyst
Y 62 Y82 LZ2 10-12 S(LZ12)8 Si-AI-0 Am her ly s t - 15
Source Comniercia I (Linde) Com nie rcial (Lintle) Coin me r ci ;i I (Linde) Steani Dealum. LZ2 10-12 Co mme rc i a1 (Am .Cyannmicl ) Co m nie rci al (Rohm 24 Hans)
Si/AI Ratio 2.5"
Brijnstetl Site Density ( 102"/g) 27;1
Relative Acid StrengthC 1.0;'
Pyridi ne TI'D (pmol/g) 2200
5.1"
16"
-5.8"
1850
0.Oi'
l5;l
4.0"
I700
8.3"
10;'
8.8;'
1200
1.5
20-30''
< < 1'1
1650
> > 1'1
-
-
3oc
a. From reference [4]; b. From reference [ 5 ] ; c. See text; d. Estimate. e. Defined as ratio of TOF's over TOF for Y62 for n-pentane cracking at 400°C.
2603
Steady-state reaction data for all the catalysts studied are presented in Table 2. In general the zeolites were found to be superior to the Amberlyst-'15 resin in terms of activity and MTBE selectivity. The poorer resin selectivity is primarily due to formation of the C8 i-butylene dimer and dimethyl ether (DME). This may be due to the very strong acid character of the resin which favors many secondary reactions. The lower steady-state conversion value may simply be due to fast poisoning of the strong acid sites. An important characteristic of the zeolite which may also be important is shape selectivity. The resin structure is quite open having average an pore radius of 160A [6] allowing all reactions to occur within the pores. In contrast, the small zeolite pores can inhibit i-butylene diffusion and thus dimer formation, thus enhancing MTBE selectivity. Table 2 MTBE Synthesis over Acid Catalysts at 100°C. Comparison of Ion Exchange Resin and Zeolites at Steady-State Y82 LZ210-12 S(LZ12)8 Si-AI-0 Amberlyst YG2 Cat.Weight (g) 0.10 0.05 0.05 0.05 0.10 0.05 i-C,HIo/CH3OH 1.0 1.8 1.8 1.8 1.7 1.8 WHSV (hi') 15 17 16 16 14 18 CH30H conv.(%) 8.1 12.4 7.9 12.6 10.1 5.6 MTBE select.(%) 54.5 99.0 99.0 98.6 99.9 99.5 ANALYSIS MOL% Hydrocarbons: 0.0 0.0 0.0 0.0 0.6 C1-C7 1.5 2.5 2.1 0.9 39.4 3.7 1.4 C8 Oxygenates: DME 0.0 0.0 0.0 17.1 0.0 0.0 1.4 0.1 0.4 t-butanol 0.3 0.9 0.9 MTBE 41.3 95.4 97.7 96.1 97.2 98.7
The effect of acid strength on the initial and steady-state turnover frequency (rate of MTBE production per acid site) for all zeolites is shown in Figure 1. A linear correlation is observed between initial TOF and acid strength in the range of values studied. In contrast, no clear correlation is observed when the steady-state results are used. Interestingly, the change in activity between initial and steady-state, which is an indication of catalyst deactivation, is also an increasing function of acid strength. These results suggest that acid strength exerts a major influence on the catalytic behavior of zeolites for MTBE synthesis with increased acid strength is favoring the formation of MTBE. However, high acid strength also facilitates deactivation. Consequently, there seems to be an optimum set of values for acid strength and acid density. High acid strength results in a higher MTBE production rate but can lead to significant deactivation. Low site density tends to reduce overall production. Among the catalysts tested, zeolite LZ210-12 appeared to possess the optimum acidity characteristics.
2604 30 h
v
x 25 X
aJ
5 20
3
LLm
P
5 l5 0 Q1
5 10
E
/
A
Figure 1. Relationship between TOF for MTBE synthesis and acid strength. 4. CONCLUSIONS A kinetic study of gas phase MTBE synthesis reaction over acid zeolites showed that HY zeolites are in general superior to Amberlyst-15 resin in terms of activity and MTBE selectivity. This is due both to the acid and shape selective characteristics of the zeolites.
5, ACKNOWLEDGMENTS
We thank Ann McGowan for assistance in the TPD experiments. Funding for this research from the U.S. Department of Energy, under contract DE-AC22-90PC90047, is gratefully acknowledged. 6. REFERENCES 1. P. Chu and G.H. Kiihl, Ind. Eng. Chem. Res., 26 (1987) 365. 2. S.I. Pien and W.J. Hatcher, Chem. Eng. Comm., 93 (1990) 257. 3. R. Le Van Mao, R. Carli, H. Ahlafi, and V. Ragaini, Catal. Lett., 6 (1990) 321. 4. P. Shertukde, Ph.D. Dissertation, University of Pittsburgh (1991). 5 . F. Ancillotti, M. Massi Mauri, and E. Pescarollo, J. Catal., 46 (1977) 49. 6. G. Marcelin, D.C. Cronauer, R.F. Vogel, M.E. Prudich, and J. Solash, Ind. Eng. Chem. Process Des. Dev., 25 (1986) 747.
GAS PHASE SYNTHESIS OF MTBE OVER ACID ZEOLITES A. Nikolopoulo.9~ T. P. Paluckab) P. V.Shertukdea, R. Oukacia) J. G. Goodwin,Jr.0 and
G. Marcelinab
*Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA bAltamira Instruments, Inc., 2090 William Pitt Way, Pittsburgh, PA 15238, USA
1. INTRODUCTION
Methyl tert-butyl ether (MTBE) is made commercially by the acid-catalyzed equilibrium reaction of methanol and iso-butylene using an ion-exchange resin catalyst, viz.:
CH,OH
+ (CH,)$=CH,
-
(CH,),COCH,
This reaction is typically carried out in the liquid phase at relatively low temperatures (T < 100°C). This low temperature limit is imposed partly by the increasingly unfavorable thermodynamics and partly by the instability of the resin at elevated temperatures. Although the reaction proceeds via an acid catalyzed pathway, little is known about the requirements of the catalyst to form the MTBE product selectively. .This work examines the effect of acid properties of various catalysts, namely zeolites, on the selective gas phase MTBE synthesis. Zeolites are ideal materials for this study since they have high acidity and can be readily prepared with a range of acid characteristics. Previous studies have addressed the use of zeolites for MTBE synthesis [l-31. However, a systematic investigation of the cffect of acidity on the reaction has not been reported. In this work, a number of HY zeolites with different acid properties were studied in an attempt to investigate their catalytic performance and correlate it with their acid properties. 2. EXPERIMENTAL
Six acid catalysts were studied. Four of these were HY zeolites with different acid properties. Zeolites LZ210-12, Y62, and Y82 were commercially obtained, while zeolite S(LZ12)8 was obtained by mild steam dealumination of LZ210-12. A silica-alumina catalyst (Si-Al-0) with lower acidity and Amberlyst-15 resin were also evaluated for comparison. The zeolites were initially in the ammonium form and were dearninated prior to reaction by heating to 400°C at a rate of 2"C/min under He flow and holding for 12 hours.
2602 The Si/Al ratios of the zeolites were determined by X R D and 2'Si and 27AI MASNMR. Temperature programmed desorption (TPD) of pyridine was carried out on all catalysts except Amberlyst in order to estimate acid strength. Adsorption was conducted at 100'C and desorption was conducted from 1 0 0 ° C to 7 0 0 ° C at a rate of 1O0C/niin. Acid strengths were also determined by n-pentane cracking reaction at 400'C [4]. The MTBE synthesis reaction was carried out using a small fixed bed reactor with on-line GC-FID analysis. Experiments were performed at atmospheric pressure, 100 C, i-butylene/methanol ratio of 1.8, and a space velocity of about 16 h-'. 3. RESULTS AND DISCUSSION
The structural and acid properties of the catalysts studied are summarized in Table 1. For the zeolites, acid site densities were calculated based on each structural Al resulting in a Bronsted site. In the case of the Si-AI-0, i t was not possible to accurately determine the acid site density, but a working range could be determined by taking the lower limit as the number of pyridine molecules adsorbed per gram of catalyst and the upper limit as the aluminum site density. I t should be noted that these catalysts have ;I different number of Brijnsted sites and acid strength of these sites, thus allowing comparison of the catalytic behavior t o both acid strength m c l site density. Table 1 Summary of structural and acid properties of catalysts Catalyst
Y 62 Y82 LZ2 10-12 S(LZ12)8 Si-AI-0 Am her ly s t - 15
Source Comniercia I (Linde) Com nie rcial (Lintle) Coin me r ci ;i I (Linde) Steani Dealum. LZ2 10-12 Co mme rc i a1 (Am .Cyannmicl ) Co m nie rci al (Rohm 24 Hans)
Si/AI Ratio 2.5"
Brijnstetl Site Density ( 102"/g) 27;1
Relative Acid StrengthC 1.0;'
Pyridi ne TI'D (pmol/g) 2200
5.1"
16"
-5.8"
1850
0.Oi'
l5;l
4.0"
I700
8.3"
10;'
8.8;'
1200
1.5
20-30''
< < 1'1
1650
> > 1'1
-
-
3oc
a. From reference [4]; b. From reference [ 5 ] ; c. See text; d. Estimate. e. Defined as ratio of TOF's over TOF for Y62 for n-pentane cracking at 400°C.
2603
Steady-state reaction data for all the catalysts studied are presented in Table 2. In general the zeolites were found to be superior to the Amberlyst-'15 resin in terms of activity and MTBE selectivity. The poorer resin selectivity is primarily due to formation of the C8 i-butylene dimer and dimethyl ether (DME). This may be due to the very strong acid character of the resin which favors many secondary reactions. The lower steady-state conversion value may simply be due to fast poisoning of the strong acid sites. An important characteristic of the zeolite which may also be important is shape selectivity. The resin structure is quite open having average an pore radius of 160A [6] allowing all reactions to occur within the pores. In contrast, the small zeolite pores can inhibit i-butylene diffusion and thus dimer formation, thus enhancing MTBE selectivity. Table 2 MTBE Synthesis over Acid Catalysts at 100°C. Comparison of Ion Exchange Resin and Zeolites at Steady-State Y82 LZ210-12 S(LZ12)8 Si-AI-0 Amberlyst YG2 Cat.Weight (g) 0.10 0.05 0.05 0.05 0.10 0.05 i-C,HIo/CH3OH 1.0 1.8 1.8 1.8 1.7 1.8 WHSV (hi') 15 17 16 16 14 18 CH30H conv.(%) 8.1 12.4 7.9 12.6 10.1 5.6 MTBE select.(%) 54.5 99.0 99.0 98.6 99.9 99.5 ANALYSIS MOL% Hydrocarbons: 0.0 0.0 0.0 0.0 0.6 C1-C7 1.5 2.5 2.1 0.9 39.4 3.7 1.4 C8 Oxygenates: DME 0.0 0.0 0.0 17.1 0.0 0.0 1.4 0.1 0.4 t-butanol 0.3 0.9 0.9 MTBE 41.3 95.4 97.7 96.1 97.2 98.7
The effect of acid strength on the initial and steady-state turnover frequency (rate of MTBE production per acid site) for all zeolites is shown in Figure 1. A linear correlation is observed between initial TOF and acid strength in the range of values studied. In contrast, no clear correlation is observed when the steady-state results are used. Interestingly, the change in activity between initial and steady-state, which is an indication of catalyst deactivation, is also an increasing function of acid strength. These results suggest that acid strength exerts a major influence on the catalytic behavior of zeolites for MTBE synthesis with increased acid strength is favoring the formation of MTBE. However, high acid strength also facilitates deactivation. Consequently, there seems to be an optimum set of values for acid strength and acid density. High acid strength results in a higher MTBE production rate but can lead to significant deactivation. Low site density tends to reduce overall production. Among the catalysts tested, zeolite LZ210-12 appeared to possess the optimum acidity characteristics.
2604 30 h
v
x 25 X
aJ
5 20
3
LLm
P
5 l5 0 Q1
5 10
E
/
A
Figure 1. Relationship between TOF for MTBE synthesis and acid strength. 4. CONCLUSIONS A kinetic study of gas phase MTBE synthesis reaction over acid zeolites showed that HY zeolites are in general superior to Amberlyst-15 resin in terms of activity and MTBE selectivity. This is due both to the acid and shape selective characteristics of the zeolites.
5, ACKNOWLEDGMENTS
We thank Ann McGowan for assistance in the TPD experiments. Funding for this research from the U.S. Department of Energy, under contract DE-AC22-90PC90047, is gratefully acknowledged. 6. REFERENCES 1. P. Chu and G.H. Kiihl, Ind. Eng. Chem. Res., 26 (1987) 365. 2. S.I. Pien and W.J. Hatcher, Chem. Eng. Comm., 93 (1990) 257. 3. R. Le Van Mao, R. Carli, H. Ahlafi, and V. Ragaini, Catal. Lett., 6 (1990) 321. 4. P. Shertukde, Ph.D. Dissertation, University of Pittsburgh (1991). 5 . F. Ancillotti, M. Massi Mauri, and E. Pescarollo, J. Catal., 46 (1977) 49. 6. G. Marcelin, D.C. Cronauer, R.F. Vogel, M.E. Prudich, and J. Solash, Ind. Eng. Chem. Process Des. Dev., 25 (1986) 747.