5842
Ind. Eng. Chem. Res. 2002, 41, 5842-5847
Structural Catalytic Packing for Reaction-Distillation Columns Arjomand Mehrabani,*,† Mohammad Mehdi Akbarnejad,‡ and Hossein Hosseini† Chemical Engineering Department, Isfahan University of Technology, Isfahan, Iran 84156, and Research Institute of Petroleum Industry (RIPI), National Iranian Oil Company, Tehran, Iran
This study was undertaken in order to implement the active cation-exchange resin catalyst on the surface of a well-shaped structured packing metallic system for application of a reactiondistillation column. Sol-gel technology using silane class compounds has been employed in order to achieve proper stability of the coating and to maintain suitable strength properties. Structural packings produced by this method have unique physical characteristics and could be implemented in reaction-distillation systems. Introduction The catalytic distillation method has gained remarkable attention in chemical industries because of its simplicity and high efficiency. The methods are based on the combination of two simultaneous processes of chemical reaction and distillation in one tower. The major advantages of the catalytic distillation include less operational equipment, energy savings, and increased chemical system efficiency due to thermodynamic advantages of in situ removal of products by distillation. The catalytic distillation technology is based on the knowledge of producing catalytic packings and fixing them within the reaction-distillation column. Clearly, effective contact between the liquid and gas phases in this system has a great influence on the system operation. Effective contact not only increases the efficiency of the reactions but also improves the separation of the products. The first industrial application of the reactiondistillation system is the production of methyl tert-butyl ether (MTBE) in the presence of an acidic cationexchanged resin catalyst. The ion-exchange resins are in the form of beads, with diameters varying from 0.5 to 1.2 mm. These beads cannot be used directly as packings in catalytic distillation columns. There are several reasons for this. The most important one is the increase of the column pressure drop. The conventional method for implementation of these beads in a reaction-distillation tower is to sew the beads into a wire net or alternatively into a glass fiber fabric called “bales”. This method is used widely in this technology and was introduced to the market originally by CDTECH Co., Houston, TX. This technique has disadvantages, including (i) mass-transfer deficiency in the liquid-solid interface and (ii) unutilizable empty space between the catalyst beads in the fabric and the existence of spaces among the bales.1-7 The structured metallic packing with a washcoat metallic support is developed by Switzerland Co. of Sulzar in Winterthur, Switzerland.8 In this regard the work on structured packings at the University of Delft, Delft, The Nether* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (98)+(311)+(3915609). Fax: (98)+(311)+(3912677). † Isfahan University of Technology. ‡ National Iranian Oil Co. E-mail:
[email protected].
lands, should be mentioned.9 The development of the structured catalyst and reactors is summarized by Cybulski and Moulijn.10 The idea of coating ceramic or metallic systems by a catalyst to utilize the advantages of catalytic packings in the form of structural packings has found little attention in the literature. The research group of Spes11 attempted to prepare packing material based on sintered polymer bodies, and this work is considered to be the first work in this era. In this work, the controlling sintering process temperature is reported to be difficult, and the catalytic activity of the resin is reduced as a result of covering the pores of the ion-exchange resin by the inert polymer. The mechanical stability of this packing also was very low. Rehfinger12 made catalytic packings of raschig rings by polymerization of a monomer mixture diluted with a pore-forming agent for the production of a macroporous ion-exchange catalyst. The samples prepared by Rehfinger were comparable to commercial resins with respect to activity; only mechanical stability was law. Kunz and Hoffmann13 introduced the ion-exchange resin into a pore volume of ceramic as the support. They added ceramic rings during polymerization of the ion-exchange resin to a mixture of monomer and a pore-forming agent. In the next stage, the coated rings were sulfonated to obtain catalytic properties. In this method swelling forces can crack the ceramic support at high polymer load. An appropriate load of the polymer led to a stable and active catalyst for reactive distillation processes. Also the existing bond between the ceramic and catalytic resin became weak and in the presence of different solvents were broken. There are few publications related to techniques for coating the random distillation packing materials with catalyst resins for reaction-distillation systems. The published literature deals mainly with random ceramic packing such as raschig ring or spherical-shaped materials. The first aim of this study is to extend the state of the art technology to metallic surfaces in order to be able to apply the active catalyst on the surface of well-shaped structured packings. It is obvious that such systems possess many advantages including proper and effective gas-liquid-solid interactions. The second aim of this study is to deal with the stability problem of coated materials and to study the different parameters which affect the swelling problem and indignity of finished products.
10.1021/ie010638z CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5843
Sol-Gel Technology For coating of packings by specific catalysts, intermediate components named adhesion promoters must be used. In this research, vinyltriethoxysilane is used as the adhesion promoter under the sol-gel process. The initial objective in the application of sol-gel technology was production of components from substrate and organic coatings having high adhesiveness. The bond of the components must be stable under various conditions. There are various techniques for preparing surfaces in order to gain maximum adhesiveness. Some of them are the removal of surface contaminations, change of the surface profile, and chemical modifications. For improving coating technology, implementation of adhesion promoters (coupling agent) is an effective tool. The coupling agent could form an initial bonding to the substrate and/or organic coating. Not only can adhesion promoters improve adhesiveness, but also they could adjust the bulk properties such as the viscosity of the coating. The main problem in using adhesion promoters is the shortage of quantitative information about their bond strengths. Published information regarding this matter is only limited to the empirical knowledge of some workers.14,15 Probably the first implementation of adhesion promoters was in the mid-1950s.16 Silane components were the first types of these materials. In the mid-1970s, the undesired effect of water on the strength of the coating bonds was studied.16 Adhesion of the organic coating to substrates has various applications such as protection of substrates. Organic coatings must adhere to substrates completely. Its bond must be sustained in all solutions. Production of sol-gel by implementation of silane components (as adhesion promoters) made a great change in surface-coating industries. Formation of solgel on various material metals was considered in 1982.16 Coating of silane over holes and openings of windows and glasses for the reduction of heat losses has been one of its applications. Control of the surface and interface of materials during manufacturing was the overall aim in the production of materials in the form of sol-gel. These substances have unique physical properties and could be produced by mineral and organic components, having unique physical properties.14,15 There are problems in previous studies of this subject especially about the strength and stability of catalytic coatings upon exposure to various solutions. In this research, there is an attempt to coat structural packings such as aluminum by ion-exchange catalysts by implementing the sol-gel process of silane components. From one side, there is a chemical bond between silane components and the ion-exchange catalyst, and from the other side, a chemical bond between silane components and metallic packing exists. The most important problem regarding this matter is swelling of ion-exchange catalysts in various solutions. This problem has been reduced greatly by the application of sol-gel technology. Also, because of this application, characteristics of the structural catalytic packing such as surface area and porosity are controlled in the production process. Structural packings produced by this method have unique physical characteristics. By implementation of this method, nearly all existing problems in the production of catalytic packing can be resolved.
Table 1. Results of Tetraethoxysiliane Hydrolysis by a Solvent of Dioxane at 25 °C sample
HCl concn (mol/L)
S0 (mol/L)
M0 (mol/L)
R ) M0/S0
MTS-00 MTS-01 MTS-02 MTS-03
0.1575 0.0702 0.004 distilled water
0.19 0.2 0.2 0.22
1.43 1.59 1.20 1.35
7.50 7.95 6.00 6.10
Catalytic Packing Tests Catalytic ion-exchange resin cannot be coated over various packings easily because no chemical bonding can be generated between the catalyst and packing. Therefore, intermediate materials of silane components have been implemented. The general structure of the used silane components is R-Si(OM)3, where R is an organic functional group such as vinyl, amine, etc., and M is a group of alkoxy or halide in order to gain property of hydrolysis. 1. Sol-Gel Process of Silane Components. Hydrolysis of silane components starts in the presence of acidic or hydroxide catalyst easily. Because water and silane components are insoluble, a mutual solvent for the homogeneous reaction is used. Hydrolysis of tetraethoxysilane and dehydration of the reaction products can be shown as follows:
nSi(OC2H5)4 + 4nH2O f nSi(OH)4 + 4nC2H5OH nSi(OH)4 f (SiO2)n + 2nH2O In the above reactions, the total amount of removed water that is equivalent to the difference of reacted water in the first reaction and produced water in the second one can be shown as
HT )
M0 - M S0
where parameters are HT ) total amount of removed water [mol/mol of Si(OC2H5)4], M0 ) moles of water at time t0 per liter (mol/L), M ) moles of water at time t per liter (mol/L), and S0 ) initial concentration of tetraethoxysilane [mol/mol of Si(OC2H5)4]. The amount of produced alcohol cannot be measured based on direct chemical methods of hydroxide group measurement. Because all reaction products have the same groups, separation of alcohols is done by vacuum distillation implementing a vacuum rotary. By measurement of the density, the concentration of ethanol in the mixture can be calculated based on following equation:
HH )
WA - (MVB × 18.02)/1000 1000 Ct VB S0 × 46.07
where HH ) amount of produced alcohol [mol/mol of Si(OC2H5)4], Ct ) ethanol concentration (by weight; g/cm3), VB ) volume of the sample before distillation (L), and WA ) weight of the distillate (g). Dioxane as the solvent and hydrochloric acid as the catalyst are used in the experiments. Tests were carried out on batch reactors. The volume of the reactants is diluted to 1 L by addition of the solvent. Table 1 represents the conditions of the most important experiments.
5844
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002
In all samples, silica gels were produced in a certain time depending on the initial conditions of the reactions. As this gel is produced, it is exposed to a variety of sequential processes as follows. (i) The samples are cast separately as generated. (ii) Ammonium hydroxide is added to samples to set their pH to 12. (iii) They are stored at a temperature of 25 °C for 1 day. (iv) The gels are immersed in the liquid completely for continuation of condensation polymerization. This reaction increases the strength of the gels in order for them to be stable during the drying process. This stage is called the aging operation. (v) The gels were stored in a 120 °C furnace for a period of 6 days. This process removes liquids that exist in the pores. This stage is called the drying operation. (vi) The samples are exposed to a furnace having a temperature of 600 °C. This operation removes silane bonds (Si-OH) in the network of gel pores that causes porous gels having an effective area. This stage is called the chemical stabilization operation. Within this stage, Si-O components are produced that can be used for the coating of various packing surfaces. These components could create chemical bonds between gel components and various materials. In one experiment, the gel-sol process is carried out in the presence of sodium chloride (0.05 M). The pore diameter of the produced gel is multiplied. This gel is more suitable for the coating of catalysts. 2. Preparation of the Surface of Structural Packings for Coating with Silica Gel. Structural packing is mainly metallic. The preparation of the surface of every metal for coating has its own specific procedures that are standard. Aluminum surfaces have been used for experiments. They have systematic and small holes. The procedure for the preparation of the aluminum surface was according to ASTM-D2651. In this stage, contamination and cover layers such as thin films of oxides are removed from the surface of the metal. This operation increases the strength of the surface bonds between aluminum and its coating. There is 16 h of time after this stage for the formation of very adhesive oxide layers for the bonding of silane components to the aluminum metal. 3. Sol-Gel Production of Silica over Aluminum Bases. At this stage aluminum has been prepared for bonding to silane components. All described stages for the production of silica sol-gel are carried out completely, but in the presence of aluminum packings. In this way the aluminum surfaces are coated with the produced silica gel. Probably, there are chemical reactions among aluminum and Si-O components. In this stage, for the production of silica sol-gel, vinyltriethoxysilane is used. The vinyl groups of this substance have the necessary ability of making a chemical bond with the catalyst. At this stage the aluminum surface has the capability of being coated with the catalysts (sulfonated polystyrenedivinylbenzene). The thickness of the silica gel films is measured by an electronic microscope. Infrared spectroscopy is used for structural analysis of the silica films over aluminum. For measurement of the porosity of the silica films during stages of the sol-gel process, a Quantasorb instrument based on the Brunauer-Emmett-Teller
(BET) method is used. These results are represented in the Results section completely. 4. Preparation of the Catalytic Packing of Aluminum with an Ion-Exchange Resin. Vinyl groups of vinyltriethoxysilane are not involved in hydrolysis and condensation polymerization based on the results taken from infrared spectroscopy. Thus, vinyl groups in one of the following mechanisms could make a chemical bond between polystyrene and divinylbenzene: (i) chemical bond theory; (ii) deformable layer theory; (iii) restrained layer theory; (iv) reversible hydrolytic bond theory. The method of adding the catalytic resin to aluminumsilica gel is described here. Initially, a mixture of 95 g of styrene and 5 g of divinylbenzene (with a purity of 65% from Merck Co.) is poured in a batch reactor. Then, 1 g of benzoyl peroxide as the initiator and a mixture consisting of 5 g of normal hexadecane and 5 g of normal octadecane as the pore-forming agent are added to the system. The resulting mixture is agitated for a period of 10 min. Silica gel coated aluminum packing is added to the monomer mixture in order to be coated completely with it. The temperature is increased to 90 °C to start precipitation polymerization. After 3 h the aluminum packing is removed from the reactor and cooled. For removing pore-forming components of hexadecane and octadecane from the packing, a chloroform solvent is used. The thickness of the film formed on aluminum directly depends on the residence time of aluminum in the monomer mixture. Probably, the chemical bond between silica gel and ion-exchange resin based on the chemical bond theory is due to the vinyl groups of the silica gel as shown in the following reaction. polymerization at 90 °C
styrene + vinyl group 98 poly(vinylstyrene) The produced sample is inserted in concentrated sulfuric acid at 100 °C for several hours for the sulfonation process. By this process, the resin is converted to sulfonated polystyrene-divinylbenzene. The produced samples are washed for removal of acids, and then they are dried at a temperature of 100 °C. One would expect that coated aluminum would preserve the metallic surface from attack of sulfuric acid during the sulfonation step of catalyst preparation. However, this method, as described in this paper, was intended to represent a general idea in order to take advantage of metallic-sol-gel-polymer structured packing materials for catalytic distillation systems. In this regard, the metallic system can be chosen in order to maximize the integrity of the metal support during the sulfonation step and also under the conditions of reactive systems. 5. Specifications of Catalytic, Physical, and Chemical Characteristics of the Catalytic Packing. Catalytic properties of the produced packings are the most important factor because these packings as the catalyst are used in the MTBE production process. The most important parameters that are considered regarding this matter are the pore volume, surface area, and acidic capacity. The pore volume and surface area are measured by a Quantasorb instrument implementing the BET method. The acidic capacity is determined by the titration method. The results are represented in the Results section.
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5845 Table 2. Results of Tetraethoxysilane Hydrolysis in Hydrochloric Acid
sample MTS-00 MTS-01 MTS-02 MTS-03
M (mol/L) 1.43 1.01 0.98 1.59 1.19 1.15 1.2 0.85 1.35 1.28
HT [mol/mol of Si(OC2H5)4]
HH [mol/mol of Si(OC2H5)4]
0.0
0.0
2.2 0.0
4.1 0.0
92
2.3 0.0 1.6 0.0 0.3
3.5 0.0 3.5
70 0.0 108
0.32
R (%)
time (min) 0 14 130 0 17 77 0 10200 0 11760
The unnotch izod of packings is measured based on the standards of ASTM-D256. At the end, chemical properties of the produced packings for evaluation of the bond strength between aluminum T silica gel and silica gel T ion-exchange resin are measured in the presence of 25 °C water for a period of 3 days. By implementation of an electronic microscope, the swelling percentage based on the difference of the film thickness before and after exposure of the samples to the water is calculated. These data are also represented in the Results section. Results Results of this research are categorized into three groups of the sol-gel process, structural analysis of the produced film, and characteristics of the catalytic packings. 1. Results of the Sol-Gel Tetraethoxysilane Process. In two experiments of MTS-00 and MTS-01, rates of reactions are very high because of the high concentration of acid used. Thus, the final amount of the produced alcohol (HH) cannot be determined. However, in the two experiments of MTS-02 and MTS-03, reactions are slow because of the low concentration of acid used. Therefore, rates of reactions are very low and the amount of HH can be determined easily. The concentrations of alcohol and water based on methods defined in the experimental section are measured. These are represented in Table 2. Parameter R is the converting degree of the product dehydration process.
R ) 2(1 - HT/HH) The results of experiments MTS-02 and MTS-03 show that the hydrolysis reaction sets the reaction rate. However, the dehydration process also is involved in setting of the reaction rate in cases of high HCl concentration. In concentrated HCl solutions, variation of the water concentration is very fast and the gel is formed very quickly. However, in diluted HCl solutions, variation of the water concentration is slower and therefore formation of the gel requires more time. Pore diameters of the gel produced by this method are about 5 nm. However, as told in the experimental section, usage of 0.05 M sodium chloride increases the pore diameters to 35 nm. This is better for coating of the catalyst over substrates, because catalyst pore diameters are less than this amount. Increasing the pore diameter in this case was due to the formation of a double electric layer around of the ions. Variation of the density for sample MTS-00 in different temperatures during different stages of sol-
Figure 1. Variation of the gel density with temperature for sample MTS-00.
gel production is measured. These data are represented in Figure 1. As shown by increasing the temperature, the density of the produced gel is increased to a constant value of 1.5 g/cm3. 2. Structural Analysis of the Silane Film over Aluminum. The silane film formed over aluminum packing is analyzed by an electronic microscope. Their thicknesses are about 0.05 nm. They are in the forms of discrete agglomerates. Probably, this happens during drying of the gels. Then the structure of the film of vinyltriethoxysilane formed on the aluminum surface is studied by infrared spectroscopy. On the basis of the analysis of the spectroscopy test, the following information can be extracted. (i) There is a relatively strong absorbing band near the frequency of 1090 cm-1. This absorbing band is related to the polymeric component of Si-O-Si. Thus, the produced film is polysiloxane. (ii) There is an absorbing band near the frequency of 3080 cm-1. This absorbing band is due to a vinyl group (CHdCH2). In fact, vinyl groups in the polymerization process do not change. They are available for reaction with styrene. This can be explained by the chemical bond theory mechanism. (iii) There is a relatively weak absorbing band near the frequency of 3390 cm-1. This frequency is for a hydroxide group (Si-OH). One can conclude that most of the hydroxide group is probably converted to groups of Si-O-. These groups have the ability of reaction with aluminum or some of the other metals (covalence bond). Also, the absorbing band at the frequency of 1108 cm-1 is seen. This shows up as the group Si-O-. The structure shown in Figure 2 is proposed for the coating of vinyltriethoxysilane. The pore volume of formed siloxane in sample MTS00 during various stages of sol-gel formation is measured. This information is shown in Figure 3. As represented, the pore volume of these films is much more than the pore volume of ion-exchange resins. This indicates that polysiloxane is a suitable substrate for the catalyst coating. 3. Catalytic, Physical, and Chemical Characteristics of the Catalytic Packings. The experimental methods described are carried out for measuring properties of the produced catalytic packings. The results of experiments are represented in Table 3. The swelling
5846
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002
Discussion and Suggestions
Figure 2. Proposed structure of vinyltriethoxysilane coatings over aluminum.
Figure 3. Variation of the pore volume for produced gel of sample MTS-00 with time. Table 3. Catalytic, Physical, and Chemical Characteristics of Catalytic Packings
experiment type
units
acidic capacity (based on the resin weight) pore volume surface area unnotch izod swelling
mequiv of H+/g cm3/g m2/g lb‚ft/in. %
packing with a coupling agent
packing without a coupling agent
3.20
2.70
0.11 35.60 2.20 72
0.23 215.80 2.60 8
item is the estimated swelling of the catalyst for a period of 3 days in 25 °C water. The acidic capacity is the most important catalytic property of resins. The value of this property is relatively good. This value is less for catalytic packings with a coupling agent in comparison with catalytic packings without a coupling agent. The reason for this difference is the existence of chemical reactions among the resin and polysiloxane. Catalytic packings made of a coupling agent have relatively suitable values for catalytic, physical, and chemical properties. Therefore, these can be used as catalytic packings for a reactive distillation tower. The experiments are performed on different sizes and shapes of ceramic and aluminum. This includes ceramic bales, rasching rings, berl saddles (1/8, 1/4, and 3/8 in.), and sheets of aluminum and structured aluminum packings.
The novel method presented in this research for production of structural catalytic packings is based on precipitation polymerization over mineral materials. This is suitable for the preparation of metal of catalytic packings and ion-exchange resins for the catalytic distillation process. The advantage of this method is the coating of the resins over substrates having various shapes such as saddle, raschig ring, spherical, and structural forms. The thickness of the coated resin film over packings can be controlled in various stages of the polymerization. By increasing the amount of resin on packings, the acidic capacity of the packings may be increased, which improves catalytic characteristics. There are two issues, which are as follows: (1) There is mass-transfer limitation in the sulfonation process for a high amount of resins that cover packings. (2) A high amount of resin coating does not guarantee an increase of the catalytic reaction rate. The only problem in systems with catalytic packings is swelling of the resin in liquids such as water and alcohols. This issue removes the catalyst from the packing surface. To prevent this problem, this project offers two effective proposals: (1) Increasing the amount of divinylbenzene during the production of the catalyst. (2) Implementation of adhesion promoters during the packing preparation. Although the suggested increase in the cross-linker concentration during catalyst preparation can solve the mechanical problem, a higher cross-linking will also influence the catalyst activity and selectivity. Thus, an optimum condition must be obtained based on experimental work in the future. A high amount of the pore volume, surface area, and bonding factors of packings are due to polysiloxane materials that have high volume porosities and surface areas. Also, because of the existence of these materials, the unnotch-izod property of these packings is increased. The swelling percentage of the resin in aqueous solutions is highly reduced because of the existence of chemical bonds between polysiloxane with substrates and the ion-exchange resin. On the basis of the analysis of infrared spectroscopy of polysiloxane coating films on aluminum, the polysiloxane are in linear form. Also, because of the presence of free bonds of Si-O, there is the capability of reactions with substrates. From the other side, because of the existence of vinyl groups, there is the capability of reactions with ion-exchange groups. Therefore, polysiloxane acts as an intermediate material for making chemical bonds between the ion-exchange catalyst and metallic bases. In regards to recently issued information about the production of ion-exchange resins, there is a proposal for improving the catalytic properties of the packings. Based on this, during the production of the resin, methyl methacrylate should be used. It increases the pore volume, glass transition temperature, and structural stability of the pores. This improves the properties of the produced packings. This process requires research in order to produce catalytic packings having optimized characteristics.
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5847
Nomenclature HT ) total amount of removed water, mol/mol of Si(OC2H5)4 M0 ) moles of water at time t0 per liter, mol/L M ) moles of water at time t per liter, mol/L S0 ) initial concentration of tetraethoxysilane, mol/mol of Si(OC2H5)4 HH ) amount of produced alcohol, mol/mol of Si(OC2H5)4 Ct ) ethanol concentration, g/cm3 VB ) volume of the sample before distillation, L WA ) weight of the distillate, g R ) conversion degree of the dehydration process for the product, %
Literature Cited (1) Akbarnejad, M. M.; Safekordi, A. A.; Zarrinpashneh, S. A study on the Capacity of Reactive Distillation Bole Packings, Experimental Measurements, Evaluation of the Existing Models and Preparation of a New Model. Ind. Eng. Chem. Res. 2000, 39, 3051. (2) Akbarnejad, M. M.; Sadeghi, M. B.; Shokrieh, A.; Yousefi, H.; Forsat, Kh.; Zarrinpashne, S. Catalytic Distillation Process for Production of MTBE. First National Congress of Iranian Chemical Engineering, Tarbiat-Modarres University, Tehran, Iran, 1984; p 31. (3) Kunin, R. Ion Exchange Resins; Wiley: New York, 1950. (4) Osborn, G. H. Synthetic Ion Exchangers; Macmillan Co.: New York, 1959. (5) Technical Bulletin; Rohm and Haas Co.: Philadelphia, PA, 1980.
(6) Kunin, R.; Meitzer, E.; Botnick, M. Macroreticular IonExchange Resin. J. Am. Chem. Soc. 1962, 84, 305. (7) Gates, B. S.; Rodriguez, W. General and Specific Acid Catalysis in Sulfonic Acid Resin. J. Catal. 1973, 31, 27. (8) Stankiewicz, A. I.; Moulijn, J. A. Process Intensification: Transforming Chemical Engineering. Chem. Eng. Prog. 2000, 1, 22. (9) http://www.dct.tudelft.nl/race/Research1/frame1.html. (10) Cybulski, A.; Moulijn, J. A. Structured Catalyst and Reactors; Marcel Dekker: New York, 1998. (11) Spes, H. Deutsches Patent 1285170, 1966. (12) Rehfinger, A. Reaction-technical investigation to the liquidphase synthesis of MTBE at a strong acidic macro-porous ionexchanger resin catalyst. Ph.D. Dissertation, Clausthal Technical University, Clausthal-Zeller feld, Germany, 1988. (13) Kunz, U.; Hoffmann, U. Preparation of Catalytic Polymer/ Ceramic Ionexchange Packings for Reactive Distillation Columns. Sixth International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, Sept 1994. (14) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: Boston, 1990. (15) Skeist, I. Handbook of Adhesives; van Nostrand: New York, 1990. (16) Wilson, A. D.; Nicholson, J. W.; Prosser, H. J. Surface coatingss1; Elsevier Applied Science: London, 1987.
Received for review July 27, 2001 Revised manuscript received August 23, 2002 Accepted August 29, 2002 IE010638Z
