Bonded organo-pellicular packings for chromatographic columns

Highly effective chromatographic column packings are prepared by reacting hydroxyl groups on a silica surface with SiCl.sub.4 and then reacting the chlorosilylated surface with a polyglycol or polymeric glycol ester in a slurry reaction. Residual chlorosilane groups on the reacted surface are neutralized by reaction with methanol or other lower alkanol. The resulting modified silica has a bonded, essentially monomolecular organic surface film which provides thermal stability, uniform efficiency, and rapid analysis when the material is used as a column packing in gas-liquid chromatography.

BACKGROUND OF THE INVENTION 
This invention relates to improved silica packings for use in 
chromatographic analysis and to a process for making them. It relates 
particularly to finely divided silica supports having a very thin 
polymeric organic film chemically bonded to their surface. 
The problem of optimizing chromatographic performance is one that has 
persisted throughout the history of chromatographic separations. Although 
significant advances have been made in the application of gas-liquid 
chromatography to analytical problems, most of these have resulted from 
improvements in apparatus rather than in the column packing itself. 
In order to avoid or minimize problems caused by active sites on the 
surface of a silica chromatographic column packing, coated packings have 
been prepared by heating an activated silica with an appropriate alcohol 
under conditions which allow continuous removal of water, thereby causing 
etherification of the alcohol with silanol groups on the silica surface, 
see Halasz et al., J. Chrom. Sci., 12, 161 (1974). However, the coating 
obtained is often not uniform and the method gives poor reproducibility. 
Stehl, U.S. Pat. No. 3,664,967 describes a method whereby a silica or 
alumina gel is reacted with an organohalosilane, the halosilane groups 
thus attached to the surface are reacted with an alcohol and the product 
is halogenated to provide a haloorganic coating bonded to the support 
surface. However, the improved chromatographic packings thus obtained are 
still not entirely satisfactory. 
SUMMARY OF THE INVENTION 
It has now been found that novel chromatographic packings of uniformly high 
quality are produced by a process which comprises (1) contacting an 
activated silica support having a significant proportion of hydroxyl 
groups bonded to silicon atoms at the support surface with silicon 
tetrachloride at about 50.degree. C.-300.degree. C. for a time sufficient 
to react essentially all of said hydroxyl groups, thereby producing a 
chlorosilylated surface, (2) reacting by contacting the chlorosilylated 
surface with an inert solvent solution of a polyol having an average 
molecular weight of at least about 3,000 at a temperature of about 
100.degree. C.-250.degree. C., (3) contacting the polyol-chlorosilylated 
surface reaction product with a lower alkanol in sufficient quantity to 
neutralize residual chlorosilyl groups, and (4) separating the neutralized 
product from the reaction mixture as an essentially pure and dry solid. 
The reaction product has a uniform, chemically bonded organic surface 
coating which is essentially monomolecular in thickness. This bonded 
coating provides substantially complete surface coverage and reduces 
surface activity to a minimum. 
DETAILED DESCRIPTION OF THE INVENTION 
The chromatographic packings of this invention offer the advantages of high 
thermal stability, increased selectivity, controllable functionality, 
reduced analysis time, low reactivity to sample components, and sharp 
separation of solute species at lower temperatures than those required for 
elution on conventional coated packings. 
These advantages are obtained by following the above-described process 
steps and they are maximized by following those steps in their preferred 
modes of operation. For example, the surface of a silica support is 
preferably specially activated to provide a larger number of hydroxyl 
groups bonded to the surface silicon atoms by treating a cleaned silica 
with vaporized concentrated hydrochloric acid at about 100.degree. 
C.-300.degree. C. for 0.5-5 hours. The vaporized aqueous HCl is most 
conveniently applied as a mixture with an inert gas such as nitrogen, 
argon, helium or the like. 
The hydroxy (or silanol) groups on the silica surface are then reacted with 
silicon tetrachloride in either a slurry reaction with the liquid reagent 
or, preferably, by a gas-solid reaction in which SiCl.sub.4 vapors are 
contacted with a bed of the silica particles. In either case, the 
SiCl.sub.4 reaction is carried out at about 50.degree. C.-300.degree. C., 
preferably at about 150.degree. C.-250.degree. C. for the gas phase 
reaction, at a somewhat lower temperature for the reaction with liquid 
silicon tetrachloride in order to avoid excessive reactor pressure. This 
chlorosilylation reaction is preferably carried to the extent of about 
0.002 to about 0.01 gram atoms of silicon-bound chlorine per square meter 
of silica surface. 
The reaction of the chlorosilylated product with the polyol or polyester 
polyol is carried out at about 100.degree. C.-250.degree. C. by contacting 
the liquid polyol reactant with the solid chlorosilylated silica in a 
slurry reaction, preferably in the presence of an inert solvent for the 
polyol. Suitable solvents are those boiling at or above 100.degree. C. and 
inert to both reactants under the reaction conditions. Aromatic 
hydrocarbons such as xylene, diethylbenzene, and durene are examples. 
Lower alkylene polyglycols of at least 3,000 average molecular weight are 
preferred polyol reactants. These include polyethylene glycol, 
polypropylene glycol, polybutylene glycol, block copolymers of two or more 
of these oxyalkylene units, and physical mixtures of any of these. The 
minimum molecular weight is a measure of the minimum length of molecule 
required to give effective surface coverage and consequent surface 
deactivation. For polyethylene glycol, the minimum molecular weight 
indicates a chain of 65-70 oxyethylene units in the average molecule. For 
polypropylene and polybutylene glycols, molecular weights of about 4,000 
and 5,000, respectively, correspond to molecules of similar length. 
Polyglycols having an average molecular weight of about 100,000 represent 
a practical maximum molecular size limit. 
Polyester polyols are another class of polyol reactant. Polymers made by 
esterifying an alkylene diol of 2-16 carbon atoms with a dicarboxylic acid 
of 3-10 carbon atoms are preferred examples. Alkylene diols include 
ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, 
triethylene glycol, dibutylene glycol, trimethylene glycol, 
1,4-butanediol, and 1,12-dodecanediol, also mixtures of these. Aliphatic 
dicarboxylic acids such as malonic acid, succinic acid, and sebacic acid 
are preferred although aromatic diacids such as terephthalic acid and 
isophthalic can also be used, alone or in mixture with acids defined 
above. The polyesterification reaction is normally carried out for 
convenience with the diacid chloride. Other reactive dihalides can be 
mixed in minor proportion with the diacid chloride reactant in the 
polyesterification reaction to vary the properties of the resulting 
polymer, for example, organic silicon dichlorides, disulfonyl dichlorides, 
and the like. A minimum molecular weight of about 3,000 is also 
appropriate for polyester polyol reactants. Preferably, the polyester 
polyol is prepared in situ, in the presence of the chlorosilylated silica 
so that the polyesterification reaction and the reaction of the polyol 
molecules with the chlorosilyl groups take place more or less 
simultaneously. 
When the reaction of the polyol or polyester polyol reactant with the 
chlorosilylated silica has essentially ceased, a small proportion of 
unreacted chlorosilyl groups remains on the silica surface. In order to 
eliminate these highly undesirable active groups, they are neutralized by 
adding a lower alkanol such as methanol, ethanol, or isopropyl alcohol to 
the reaction mixture and heating as before. Preferably, an intermediate 
neutralization with a lower molecular weight and consequently more 
reactive polyol is carried out, most preferably with a series of such 
polyols of progressively decreasing molecular weight. In this way, the 
silica surface is blanketed to the greatest extent possible with bonded 
molecules of maximum length. For example, chlorosilylated silica can be 
reacted with polyethylene glycol of 20,000 molecular weight and remaining 
silicon-bound chlorine atoms then neutralized by successive reactions with 
polyethylene glycols of 5,000 and 1,000 molecular weight, then with 
triethylene glycol, and finally with methanol to ensure the netralization 
of all possible residual chlorosilyl groups. 
Preparation of Silica Surface 
About 100 g portions of 30-100 mesh Chromosorb W, a flux-calcined celite 
diatomaceous silica specially processed for chromatographic use by 
Johns-Manville Corp., were extracted for 24-72 hours in a Soxhlet 
extraction apparatus with constant boiling hydrochloric acid. The 
extracted silica was put in a washing column and washed for 12-24 hours 
with deionized water at room temperature using a fluid bed back-flushing 
procedure to remove acid and fines. The washed silica was rinsed 
thoroughly with methanol and then dried by passing filtered air through 
the column for about two hours. The dried silica was stored in closed 
glass bottles until subjected to surface reaction. 
The silica surface was activated by packing about 30 g of the 
acid-extracted silica in a glass reactor tube heated by a clamshell 
electric furnace and passing about 45 ml/min nitrogen through the bed 
while its temperature was raised in 40.degree. C. steps to 200.degree. C. 
over a period of about 40 minutes, then the incoming nitrogen was switched 
through a conc. HCl bubbler so that the nitrogen passing through the bed 
was essentially saturated with HCl and water vapor. The HCl-saturated 
nitrogen stream was continued at 200.degree. C. at the same rate for three 
hours, then the bubbler was bypassed and the bed was flushed with pure 
nitrogen for one hour, also at 200.degree. C. 
Reaction with SiCl.sub.4 
At this point, a bubbler charged with SiCl.sub.4 was connected into the 
nitrogen supply line and the silica bed was contacted with SiCl.sub.4 
vapor in nitrogen for 90 minutes, the temperature and nitrogen flow rate 
remaining constant at the prior levels. The bed of chlorosilylated silica 
was then flushed with nitrogen as before for 15 minutes and allowed to 
cool to room temperature after removal of the furnace with continued flow 
of nitrogen. 
The following procedure was employed with modifications as noted for the 
reaction of the chlorosilylated silica with a polyglycol. A somewhat 
modified procedure was used for the corresponding reaction of a polyester 
polyol as described in those examples. 
Reaction Procedure 
A glass reactor flask equipped with reflux condenser and nitrogen inlet was 
charged with about 10 g of polyglycol reactant and 300 ml of o-xylene and 
the contents refluxed for about an hour with dry nitrogen flush to remove 
small amounts of water, then the contents were cooled to 110.degree. C. 
and about 30 g of the chlorosilylated silica were added under nitrogen. 
Nitrogen flow through the flask was reestablished and the reaction mixture 
was heated at reflux temperature for 2-24 hours. The reaction was then 
quenched by successive addition of polyethylene glycols as described in 
the examples while maintaining reflux temperature. The reactor was then 
cooled to 100.degree. C., the heat source was removed, and 50 ml of 
anhydrous methanol were added slowly to neutralize any residual 
chlorosilane groups. After the addition was completed and the reaction 
mixture had cooled to about 55.degree. C.-60.degree. C., the liquid in the 
reactor flask, consisting essentially of xylene and unreacted polyglycols, 
was decanted and the reacted silica was washed by decantation with three 
portions of methanol followed by three portions of chloroform. The washed 
silica was then carefully transferred to a washing column where it was 
thoroughly washed by gravity flow with successive 300 ml portions of 
methanol, chloroform, and methylene chloride. The washed silica was then 
dried by drawing filtered air through the column for about an hour. The 
finished bonded silica packing was stored in sealed glass bottles until 
used. The product was a free-flowing fine white powder.

EXAMPLE 1 
Chlorosilylated 100-120 mesh Chromosorb W was reacted with polyethylene 
glycol of 20,000 average molecular weight (E-20,000) by the procedure 
described above. The reaction was quenched by adding about 2 g of melted 
polyethylene glycol of 6,000 average molecular weight (E-6,000), refluxing 
the reaction mixture for about 20 minutes and repeating this procedure 
with successive 2 g portions of polyethylene glycols E-4,000, E-1,000, and 
E-400 and, finally, diethylene glycol. The reacted silica was then treated 
with methanol and washed and dried by the previously described procedure. 
For purpose of comparison, "bonded" or coated silica packings were prepared 
by coating 100-120 mesh Chromosorb W-HP with polyethylene glycol of 20,000 
molecular weight in quantities sufficient to produce loadings of 5 percent 
and 3 percent by weight using the conventional slurry method (described by 
Halasz et al., J. Chrom. Sci., 12, 161 (1974). These packings were 
preconditioned for 12 hours at 220.degree. C. and 60 ml/min helium flow. 
These packings were compared in a 2.1 mm .times.160 cm column maintained at 
75.degree. C. and using 300 .mu.g/ml n-dodecane in methylene chloride as 
the test solute and helium as the carrier gas. Column efficiency for each 
packing at optimum carrier flow was calculated from the plot of test 
results and is listed in Table 1. 
TABLE 1 
______________________________________ 
Optimum 
Column Plate Height, Carrier Flow 
Packing mm/Theoretical Plate 
ml/min 
______________________________________ 
bonded 0.34 55 
coated, 5% 
0.67 27 
coated, 3% 
0.62 32 
______________________________________ 
It is apparent that the bonded packing provided substantially greater 
efficiency in terms of plate height and also offers faster analysis times 
since the optimum carrier flow was about twice that for the coated 
packings prepared by a previously known method. 
EXAMPLE 2 
A bonded packing was prepared as in Example 1 using 80-100 mesh Chromosorb 
W and about 8 g of polyethylene glycol of 4,000 average molecular weight 
(E-4,000). Polyglycols used in the quench cycle of the process were 
polyethylene glycols of 1,000 and 600 molecular weight, tetraethylene 
glycol, and diethylene glycol respectively. This packing was compared with 
a conventionally prepared polyester-coated silica packing by the method of 
Example 1 in the analysis of impurities in 1,2-dibromo-3-chloropropane, a 
commercial soil fumigant. Both packings showed the presence of allyl 
chloride and 1,2,3-tribromopropane in the product but the bonded packing 
of this invention also showed the presence of 1,2,5,6-tetrabromohexane 
which was not previously observed using the conventional packing. 
Additionally, use of the bonded packing cut the analysis time in half. 
EXAMPLE 3 
Bonded packings were also prepared by the method of Examples 1 and 2 using 
polyethylene glycols with average molecular weights of about 6,000 and 
about 1,500. Chromatographic testing showed excellent results for the 
first of these, comparable with results obtained with the products of 
Examples 1 and 2. However, the bonded packing made with E-1,500 showed 
severe peak tailing, characteristic of high surface activity. Evidently, 
for packings having a bonded polyethylene glycol layer, the minimum 
average molecular size that provides adequate surface coverage is in the 
molecular weight range of about 3,000, corresponding to polymers having 
about 65-70 oxyethylene units in the polyglycol molecule. 
This conclusion was supported by the properties of a bonded polyglycol 
packing described in Example 4 where the polyglycol reactant was a 
polypropylene glycol of 4,000 average molecular weight, corresponding to 
about 65-70 oxypropylene units per molecule. The bonded packing was 
evidently at about the lower molecular size limit for the bonded molecules 
covering the silica surface, for it showed some peak tailing although 
successful chromatographic separations were obtained. 
EXAMPLE 4 
A bonded chromatographic column packing was prepared with polypropylene 
glycol of 4000 average molecular weight (P-4000) and subjected to the same 
evaluation as described in Example 1. When examined using n-tetradecane at 
100.degree. C. as in Example 1, theoretical plate heights of 0.88 mm were 
observed for the P-4000 product as compared to 0.45 mm for the bonded 
E-4000 packing. These hydrocarbons are eluted faster at the same 
temperatures from the bonded polypropylene glycol columns. As a result, 
the number of components that can be separated in a given length of time 
is virtually identical on both bonded columns and both are significantly 
better than a conventionally coated column packing as noted in Table I. 
In addition to the improved efficiency, the longer molecules represented by 
the polypropylene glycols afford this high efficiency at lower flow rates 
than the corresponding bonded polyethylene glycols, allowing observation 
and detection of more volatile components. 
EXAMPLE 5 
The procedure described above for the preparation of the bonded polyglycol 
packings was modified to make a corresponding silica having surface-bonded 
diethylene glycol succinate polymer. Equimolar quantities of diethylene 
glycol and succinyl chloride (0.0472 g mole each) in o-xylene solution 
were added from separate dropping funnels to a flask reactor containing 
about 30 g of chlorosilylated Chromosorb W-AW in refluxing o-xylene under 
a nitrogen atmosphere. The resulting reaction mixture was quenched by 
adding about 3 g of diethylene glycol and refluxing an additional half 
hour. The mixture was then cooled to about 100.degree. C. and 50 ml of 
methanol were added dropwise with gradual cooling to about 55.degree. C. 
Liquid was decanted off and the coated silica was washed and dried as 
before. 
The polyester-coated silica was packed into a column similar to that of 
Example 1 and the column was used to separate a mixture of closely related 
phenols and cresols (300 .mu.g in ether). The components of the mixture 
(phenol, o-cresol, o-chlorophenol, p-chlorophenol, 6-chloro-o-cresol, 
4-chloro-o-cresol, 2,4-dichlorophenol, and 4,6-dichloro-o-cresol) were all 
separated efficiently and sharply. 
EXAMPLE 6 
The procedure of Example 5 was followed in the reaction of chlorosilylated 
Chromosorb W-AW with 1,12-dodecanediol sebacate polymer, the polymer being 
formed in situ by reaction of the diol and the acid chloride as before. 
The reaction mixture was quenched by the addition of 2 g of molten 
dodecanediol with a one-hour reflux followed by addition of 2 g of 
diethylene glycol and another 20 minutes of reflux. 
The bonded packing thereby produced was found to be particularly useful for 
analyzing mixtures of nonpolar compounds. For example, it was highly 
effective for the isothermal chromatographic analysis of three alkanes 
(C.sub.14, C.sub.15, and C.sub.16) in hexane. It also provided efficient 
separation of polychlorinated dibenzo-p-dioxin isomers. 
EXAMPLE 7 
A bonded silica packing was made by the procedure described in Example 5 
except that the succinyl chloride was replaced by a mixture of 6.6 g 
(0.0424 mole) succinyl chloride and 1.2 g (0.0048 mole) 
.gamma.-cyanopropyl phenyl dichlorosilane. The bonded coating thereby 
obtained was a polymeric diethylene glycol succinate wherein a tenth of 
the succinyl groups were replaced by .gamma.-cyanopropyl phenyl silyl 
moieties. The reaction product was quenched by reacting with diethylene 
glycol and then with methanol as in Example 5. 
The highly polar nature of the bonded silica packing thereby obtained 
permitted efficient gas chromatographic separation of brominated 
pentaerythritols, the dibromo and tribromo compounds both eluting in sharp 
peaks with minimal tailing. This packing also provided improved separating 
power and considerably reduced analysis time as compared to a conventional 
coated silicone packing in the separation of components present in crude 
pentabromochlorocyclohexane. 
In all of the bonded polyester packing products described in Examples 5-7 
and in other such bonded polyester packings prepared similarly from other 
diol and dibasic acid reactants, the polyester moieties had a 
comparatively broad distribution of molecular weights in the approximate 
range of 1,000 to 20,000 based on examination of the nonbonded polyester 
byproduct of the reaction.