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Monolayer coating of capillary Chromatographic columns with small zeolite crystals
ELSEVIER
Short
Communication
Monolayer coating of capillary chromatographic columns with zeolite crystals
small
Marc P.F. Dehnas LaRoche Air Systems, Baton Rouge, LA Douglas M. Ruthven Dept. of Chemical Engz’neering, University of Maine, A wall-coated intracrystalline the preparation Keywords:
capillary diffusion of such
Diffusion;
zeolite;
Chono,
ME
column provides a relatively simple macroscopic method in small (micron-sized) zeolite crystals. We report here columns. capillary
for measuring a procedure for
column
INTRODUCTION
COATING
PROCEDURE
The zeolite crystallites from which industrial adsorbents and catalysts are produced are generally small (-1-5 pm) since such crystals are manufactured more easily than larger crystals, and for most practical purposes, the small size is actually an advantage since the mass transfer resistance is reduced. However, the measuremen t of diffusion in small zeolite crystals presents significant practical difficulties. We demonstrated recently the possibility of making such measurements by using a capillary chromatographic column, the internal surface of which was coated with an (incomplete) monolayer of crystals.’ Such columns are also widely used for analytic chromatography as they have the advantage of yielding sharp peaks, with good resolution and a small retention volume. In commercially available zeolite-coated capillary columns the loading of the zeolite crystals is very low (less than 10% of monolayer coverage). This is too low for application in diffusion measurements, and we therefore had to develop our own experimental technique to prepare columns with sufficiently high crystal coverage. We present here a brief summary of this technique as a guide for those who may wish to prepare their own zeolite-coated capillary columns either for diffusion measurements or for analytic chromatography.
The general procedure involves filling the capillary with a dilute suspension of the zeolite crystallites in a volatile liquid which is then evaporated under vacuum to leave a layer of crystals, held by van der Waals forces, on the capillay wall. In the selection of the solvent the first criterion 1s the boiling point. By using a solvent with a low boiling point the vacuum required i,c reduced so that disturbances within the column at the location of the meniscus are less pronounced. The second criterion is the heat of vaporization.’ The temperature difference due to the heat of vaporization at the meniscus creates a gradient of surface tension at the gas-liquid interface, as shown in Fig-w-~1. The effects of several parameters were investigated.
Choice of solvent Despite several previous studies of the deposition of 13X zeolite from aqueous suspensions,“.4 we selected methanol for two reasons: the lower boiling point and greater uniformity of the crystallite suspension. The methanol must, however, be of high grade since any nonvolatile impurities will be concentrated as the methanol is evaporated, leading to a change in physical properties and consequent lack of uniformity.
Solid concentration Address Chemical Received 8 October
reprint requests to Dr. Ruthven at the Department of Engineering, University of Maine, Orono, ME 04469. 18 May 1995; revised 6 September 1995; accepted 1995
Zeolites 16:313-315, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas,
New
York,
NY 10010
Initial loadings between 0.5 and 5% (w/w) were tested (with several different solvents). For methanol the most uniform coating was obtained at 1.5%.
SSDI
0144.2449/96/$'~5.00 0144-2449(95)00128-X
Monolayer
coating:
M. P. F. Delmas
and
D. M. Ruthven
meniscus larger surface tension I smaller surface tension
.. . . .. .. .\c+ \
.
. l
.
l
.
.
*
Effect
sobent - crystals suspension
.
l
icl
surface tension gradient Figure
1
Dynamic
Surface
tension
versus
gradient
at the
liquid-gas
interface.
static
As has been frequently mentioned in the literature, the dynamic coating method is faster and easier to set up than the static method. Soulages and Brieva,” who first reported the preparation of 13X-coated PLOT columns, obtained better results by the dynamic technique. However, we found that neither the regular dynamic method nor the “four injections” method recommended by Schneider et al.s yielded uniformly coated columns. Therefore, the static method described by Grob” was modified, as described below, and this led, eventually, to a satisfactory procedure.
Sealing In any kind of column coating (liquid or solid), having an air-free solvent/seal interface becomes important since a single bubble will induce a breakthrough, leading to the loss of the column.’ A very simple and effective plugging device was used in this work. A piece of silicone was drilled at a diameter smaller than the external capillary tubing diameter and greased. The column was then forced into the hole, thus providing a very simple and leak-free end seal.
Pressurization
uum can cause excessive evaporation early in the process, resulting in non-uniform coating. Slightly increasing the vacuum with time during the first 20 min led to good coating, and this was adopted in the final procedure.
before pumping
Repressurization of the coating solution in the case of bubble breakthrough was first considered as a repair technique,’ but it was then applied as a safety precaution, prior to pumping.’ Leaving the filled and sealed column under an inert gas pressure for 20 min helps dissolve any air bubbles trapped in the column during the sealing operation, thus efficiently preventing breakthrough. This step was carried out carefully and systematically during all coating procedures.
of tubing pretreatment
A brief study was carried out to determine the effect of pretreatment of the capillary tubing with solutions of hydrofluoric acid at different concentrations. This was intended to smooth the inner wall of the column by dissolving any glass imperfections that might have resulted from the drawing process. A series of coating procedures, including an initial rinse step with a hydrofluoric acid aqueous solution prior to filling the column with the zeolite suspension, was tried. Several hydrofluoric acid concentrations were used ranging ji om 0.25 to 20 mol/liter. No correlation with column quality was observed, and therefore this step was eventually abandoned.
Adaptation
of the free release technique
Xu and Vermeulen showed that the only possibility of increasing the flow rate of solvent vapor through the column was to increase P, the pressure at the coating One alternative is to increase the solution front.‘“‘” coating temperature. Because of the design constraints of the rotating device in use (see Figure 2), immersing the column in a water bath was not convenient. We therefore inserted the rotating glass tube in which the capillary column was sitting into another glass tube about twice as large in diameter. This external glass tube was then connected to an air heater blowing warm air in the annulus thus created. A major advantage of using warm air as opposed to a water bath is that there is a temperature gradient along the tube, as a result of heat losses to the room. This means that as evaporation takes place and as the sol\,ent meniscus moves toward the closed end of the column (see Figure 2)) the temperature surrounding the capillary increases. The higher temperature thus compensates for the increase in static pressure due to the length of empty tubing. This helps keep the higher coating speed relatively constant through the process and eliminates the requirement for a temperatureprogrammed bath. The final procedure was as follows: air
rotating glass tube silicone cork
Effect
Since the effective vacuum at the meniscus is primarily controlled by the flow resistance exerted by the empty tube on the solvent vapor, increasing the vacuum source power should not increase the coating speed.” However, because of the short lengths of our columns (typically about 1 .O m) a valve was installed on the vacuum line. Pressures ranging between 0.66 and 1.0 atm were tested. It was found that too strong an initial vac-
314
/
of vacuum power
Zeolites
16:313-315,
1996
capillar); column
//
vacuum pump
outside glass tube Figure
2
Improved
static
coating
experimental
setup.
Monolayer Table
1
Void
Solvent Methanol Methanol Pentane Methanol Methanol Methanol Methanol All columns olite crystals mm.
fraction
of coated
Suspension concentration (% wt) 5 3 3 3 1.5 1.5 1.5
capillary
columns
Length km)
Void fraction (%)
78 26 23 45 45 61 40
a5 90 93 68 75 63 70
were prepared with I-2-urn 13X or 3-4~urn deposited in a capillary of internal diameter
M.P.F.
Delmas
and D.M.
Ruthven
Details of some of the columns prepared by this technique are given in Table 1. The total zeolite loading was determined from equilibrium isotherm measurements with n-octane under standard test conditions. The void fraction was then estimated relative to a complete monolayer. The absence of any significant multilayer deposition was confirmed by examination under an optical microscope.
REFERENCES
5A ze0.53
A 1.5% (w/w) suspension of the zeolite crystals in grade methanol was prepared and maintained under a nitrogen pressure of 18 psig to dissolve any air bubbles. The first 0.5-l cm of the capillary in which the evaporation process tends to start under atmospheric conditions was cut off and removed. A very modest vacuum (P-O.95 atm) was then applied. The air heater was turned on, and after 5 min the pressure was decreased to 0.8 atm; after a further 5 min the motor was switched on to rotate the column. After a further 10 min the pressure was reduced to 0.66 atm and maintained at this level until the solvent was fully evaporated and the coating procedure was completed.
coating:
ANAIAR
7 a 9 10 11
Delmas, M.P.F., Cornu, C. and Ruthven, D.M. Zeolites 1995, 15,45 Grob, K. High Res. Chrom. Chrom. Comm. 1978, 1, 93 Mohnke, M. and Heybey, J. J. Chromatogr. 1989,471, 37 Soulages, N.L. and Brieva, A.M. J. Chromatogr. 1974, 101, 365 Schneider, W., Frohne, J.C. and Bruderreck, H. J. Chromafogr. 1978, 155,311 Grob, K. Making and Manipulating Capillary Columns for Gas Chromatography, Dr. Alfred Hiithig Verlag, Heidelberg, 1986, p. 163 Grob, K. and Grob, G. High Res. Chrom. Chrom. C’omm. 1982, 5, ii9 Grob, K. and Grob, G. High Res. Chrom. Chrom. C’omm. 1985, 8,856 Grob, K. High Res. Chrom. Chrom. Comm. 1978, 1, 93 Xu, B. and Vermeulen, N.P.E. High Res. Chrom. Chrom. Comm. 1985, 8, 181 Xu, B. and Vermeulen, N.P.E. Chromatographia 1984, 18, 520
Zeolites
16:313-315,
1996
315
Short
Communication
Monolayer coating of capillary chromatographic columns with zeolite crystals
small
Marc P.F. Dehnas LaRoche Air Systems, Baton Rouge, LA Douglas M. Ruthven Dept. of Chemical Engz’neering, University of Maine, A wall-coated intracrystalline the preparation Keywords:
capillary diffusion of such
Diffusion;
zeolite;
Chono,
ME
column provides a relatively simple macroscopic method in small (micron-sized) zeolite crystals. We report here columns. capillary
for measuring a procedure for
column
INTRODUCTION
COATING
PROCEDURE
The zeolite crystallites from which industrial adsorbents and catalysts are produced are generally small (-1-5 pm) since such crystals are manufactured more easily than larger crystals, and for most practical purposes, the small size is actually an advantage since the mass transfer resistance is reduced. However, the measuremen t of diffusion in small zeolite crystals presents significant practical difficulties. We demonstrated recently the possibility of making such measurements by using a capillary chromatographic column, the internal surface of which was coated with an (incomplete) monolayer of crystals.’ Such columns are also widely used for analytic chromatography as they have the advantage of yielding sharp peaks, with good resolution and a small retention volume. In commercially available zeolite-coated capillary columns the loading of the zeolite crystals is very low (less than 10% of monolayer coverage). This is too low for application in diffusion measurements, and we therefore had to develop our own experimental technique to prepare columns with sufficiently high crystal coverage. We present here a brief summary of this technique as a guide for those who may wish to prepare their own zeolite-coated capillary columns either for diffusion measurements or for analytic chromatography.
The general procedure involves filling the capillary with a dilute suspension of the zeolite crystallites in a volatile liquid which is then evaporated under vacuum to leave a layer of crystals, held by van der Waals forces, on the capillay wall. In the selection of the solvent the first criterion 1s the boiling point. By using a solvent with a low boiling point the vacuum required i,c reduced so that disturbances within the column at the location of the meniscus are less pronounced. The second criterion is the heat of vaporization.’ The temperature difference due to the heat of vaporization at the meniscus creates a gradient of surface tension at the gas-liquid interface, as shown in Fig-w-~1. The effects of several parameters were investigated.
Choice of solvent Despite several previous studies of the deposition of 13X zeolite from aqueous suspensions,“.4 we selected methanol for two reasons: the lower boiling point and greater uniformity of the crystallite suspension. The methanol must, however, be of high grade since any nonvolatile impurities will be concentrated as the methanol is evaporated, leading to a change in physical properties and consequent lack of uniformity.
Solid concentration Address Chemical Received 8 October
reprint requests to Dr. Ruthven at the Department of Engineering, University of Maine, Orono, ME 04469. 18 May 1995; revised 6 September 1995; accepted 1995
Zeolites 16:313-315, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas,
New
York,
NY 10010
Initial loadings between 0.5 and 5% (w/w) were tested (with several different solvents). For methanol the most uniform coating was obtained at 1.5%.
SSDI
0144.2449/96/$'~5.00 0144-2449(95)00128-X
Monolayer
coating:
M. P. F. Delmas
and
D. M. Ruthven
meniscus larger surface tension I smaller surface tension
.. . . .. .. .\c+ \
.
. l
.
l
.
.
*
Effect
sobent - crystals suspension
.
l
icl
surface tension gradient Figure
1
Dynamic
Surface
tension
versus
gradient
at the
liquid-gas
interface.
static
As has been frequently mentioned in the literature, the dynamic coating method is faster and easier to set up than the static method. Soulages and Brieva,” who first reported the preparation of 13X-coated PLOT columns, obtained better results by the dynamic technique. However, we found that neither the regular dynamic method nor the “four injections” method recommended by Schneider et al.s yielded uniformly coated columns. Therefore, the static method described by Grob” was modified, as described below, and this led, eventually, to a satisfactory procedure.
Sealing In any kind of column coating (liquid or solid), having an air-free solvent/seal interface becomes important since a single bubble will induce a breakthrough, leading to the loss of the column.’ A very simple and effective plugging device was used in this work. A piece of silicone was drilled at a diameter smaller than the external capillary tubing diameter and greased. The column was then forced into the hole, thus providing a very simple and leak-free end seal.
Pressurization
uum can cause excessive evaporation early in the process, resulting in non-uniform coating. Slightly increasing the vacuum with time during the first 20 min led to good coating, and this was adopted in the final procedure.
before pumping
Repressurization of the coating solution in the case of bubble breakthrough was first considered as a repair technique,’ but it was then applied as a safety precaution, prior to pumping.’ Leaving the filled and sealed column under an inert gas pressure for 20 min helps dissolve any air bubbles trapped in the column during the sealing operation, thus efficiently preventing breakthrough. This step was carried out carefully and systematically during all coating procedures.
of tubing pretreatment
A brief study was carried out to determine the effect of pretreatment of the capillary tubing with solutions of hydrofluoric acid at different concentrations. This was intended to smooth the inner wall of the column by dissolving any glass imperfections that might have resulted from the drawing process. A series of coating procedures, including an initial rinse step with a hydrofluoric acid aqueous solution prior to filling the column with the zeolite suspension, was tried. Several hydrofluoric acid concentrations were used ranging ji om 0.25 to 20 mol/liter. No correlation with column quality was observed, and therefore this step was eventually abandoned.
Adaptation
of the free release technique
Xu and Vermeulen showed that the only possibility of increasing the flow rate of solvent vapor through the column was to increase P, the pressure at the coating One alternative is to increase the solution front.‘“‘” coating temperature. Because of the design constraints of the rotating device in use (see Figure 2), immersing the column in a water bath was not convenient. We therefore inserted the rotating glass tube in which the capillary column was sitting into another glass tube about twice as large in diameter. This external glass tube was then connected to an air heater blowing warm air in the annulus thus created. A major advantage of using warm air as opposed to a water bath is that there is a temperature gradient along the tube, as a result of heat losses to the room. This means that as evaporation takes place and as the sol\,ent meniscus moves toward the closed end of the column (see Figure 2)) the temperature surrounding the capillary increases. The higher temperature thus compensates for the increase in static pressure due to the length of empty tubing. This helps keep the higher coating speed relatively constant through the process and eliminates the requirement for a temperatureprogrammed bath. The final procedure was as follows: air
rotating glass tube silicone cork
Effect
Since the effective vacuum at the meniscus is primarily controlled by the flow resistance exerted by the empty tube on the solvent vapor, increasing the vacuum source power should not increase the coating speed.” However, because of the short lengths of our columns (typically about 1 .O m) a valve was installed on the vacuum line. Pressures ranging between 0.66 and 1.0 atm were tested. It was found that too strong an initial vac-
314
/
of vacuum power
Zeolites
16:313-315,
1996
capillar); column
//
vacuum pump
outside glass tube Figure
2
Improved
static
coating
experimental
setup.
Monolayer Table
1
Void
Solvent Methanol Methanol Pentane Methanol Methanol Methanol Methanol All columns olite crystals mm.
fraction
of coated
Suspension concentration (% wt) 5 3 3 3 1.5 1.5 1.5
capillary
columns
Length km)
Void fraction (%)
78 26 23 45 45 61 40
a5 90 93 68 75 63 70
were prepared with I-2-urn 13X or 3-4~urn deposited in a capillary of internal diameter
M.P.F.
Delmas
and D.M.
Ruthven
Details of some of the columns prepared by this technique are given in Table 1. The total zeolite loading was determined from equilibrium isotherm measurements with n-octane under standard test conditions. The void fraction was then estimated relative to a complete monolayer. The absence of any significant multilayer deposition was confirmed by examination under an optical microscope.
REFERENCES
5A ze0.53
A 1.5% (w/w) suspension of the zeolite crystals in grade methanol was prepared and maintained under a nitrogen pressure of 18 psig to dissolve any air bubbles. The first 0.5-l cm of the capillary in which the evaporation process tends to start under atmospheric conditions was cut off and removed. A very modest vacuum (P-O.95 atm) was then applied. The air heater was turned on, and after 5 min the pressure was decreased to 0.8 atm; after a further 5 min the motor was switched on to rotate the column. After a further 10 min the pressure was reduced to 0.66 atm and maintained at this level until the solvent was fully evaporated and the coating procedure was completed.
coating:
ANAIAR
7 a 9 10 11
Delmas, M.P.F., Cornu, C. and Ruthven, D.M. Zeolites 1995, 15,45 Grob, K. High Res. Chrom. Chrom. Comm. 1978, 1, 93 Mohnke, M. and Heybey, J. J. Chromatogr. 1989,471, 37 Soulages, N.L. and Brieva, A.M. J. Chromatogr. 1974, 101, 365 Schneider, W., Frohne, J.C. and Bruderreck, H. J. Chromafogr. 1978, 155,311 Grob, K. Making and Manipulating Capillary Columns for Gas Chromatography, Dr. Alfred Hiithig Verlag, Heidelberg, 1986, p. 163 Grob, K. and Grob, G. High Res. Chrom. Chrom. C’omm. 1982, 5, ii9 Grob, K. and Grob, G. High Res. Chrom. Chrom. C’omm. 1985, 8,856 Grob, K. High Res. Chrom. Chrom. Comm. 1978, 1, 93 Xu, B. and Vermeulen, N.P.E. High Res. Chrom. Chrom. Comm. 1985, 8, 181 Xu, B. and Vermeulen, N.P.E. Chromatographia 1984, 18, 520
Zeolites
16:313-315,
1996
315