Effect of Pore Size of Silica Gels on the Separation of Hydrocarbons

K k

L n

N

P O

1

= Permeability constant = Partition coefficient

= Ratio of the liquid phase to the gas phase capacity

= Column length = Number of theoretical

plates

theoretical d a t e s = Number of effective

= Column outlet pres-

= Column inlet pressure = Column radius = Column surface = Retention time of an

inert gas = Retention time = Linear gas velocity a t

= Average linear gas ve-

sure

the column outlet

locity

VL = Liquid phase volume V O = Gas phase volume V N = Net retention volume Vads = Adsorption retention

v, = Specific retention vol-

W = Weight of liquid phase v = Carrier gas viscosity

volume

ume

LITERATURE CITED

(1) Deety, D. H., Hareanape, J. N., Whvman, B. H., ANAL. CHEM. 32. 302-( 1960).

30, 1387 (1958).

p. 31, Reinhold, New York, 1960.

(2) Eggertsen, F. T., Nelsen, F. M., Ibid.,

(3) Emmett, P. H., “Catalysis,” Vol. I ,

( 4 ) HalAsz, I., Horvath. C.. ANAL. CHEM. 34, 499 (1963).

71 (1963). (5) Halhz, I., Horvath, C., ’Vatwe 197,

(6) Keulemans, A. I. M., “Gas Chro-

matography,” p. 124, Reinhold, New York. 1957.

( 7 ) Leibnitz. W.. Mohnke. M.. (Ihem. 7 -

Tech.’(Bedin) 14, 753 (1962).- (8) Mohnke, M., Saffert, W., “Gas Chro-

matography 1962,” M. Van Swaay, ed., p. 216, Butterworths, London, 1963.

(9) Peterson, D., Hirsh, J., J . Lzpad Res. 1 , 32 (,1959).

(10) Petitjean, D., Leftault, C., Pitts- burgh Conf. Anal. Chem. Appld. Spectry., Pittsburgh, Pa., March 1962.

(11) Purnell, J. H., “Gas Chromatog- raphy,” p. 58, Wiley, Sew York, 1963.

(12) Ibid. , p. 113. (13) Ibzd., p. 126. (14) Ibzd., p. 158. (15) Schwartz, R., Brasseaux, D., Shoe-

make, G., AKAL. CHEM. 35, 496 (1963).

RECEIVED for review January 7 , 1964. Accepted May 12, 1964. Presented at 2nd International Symposium on ,4d- vances in Gas Chromatography, Uni- versity of Houston, Houston, Texas, March 23-26, 1964.

Effect of Pore Size of Silica Gels on the Separation of Hydrocarbons A. V. KISELEV, YU. S. NIKITIN, R. S. PETROVA, K. D. SHCHERBAKOVA, and YA. I. YASHIN

laboratory o f Adsorption and Gas Chromatography, Chemistry Department, M. V. lomonosov State University o f Moscow, Moscow, U.S.S.R.

b The effect of the geometrical structure of silica gels on the gas chromatography of hydrocarbons has been investigated. Different boiling ranges of the substances are characterized by an optimum porosity of the adsorbent. Heats o f adsorption of n- alkanes CI-Clo on silica gels with pore sizes from 32 to 4 100 A. have been determined from chromatograms. These heats are slightly lower than those determined in a calorimeter. For large-porous silica gels, the limiting dependence of the heat of adsorption of the numbet, n, of carbon atoms in a molecule of n-alkane has been obtained. The narrowing of pores increases the heats of adsorption, particularly for high n’s. The difference between the heats o f adsorption of saturated and unsaturated hydrocarbons, which characterizes the “specific” interactions of n-bonds with protonized surface hydroxy! groups are practically independent of the pore size. Absolute (per unit surface area) values of retention volumes for normal hydrocarbons have been determined. For large porous silica gels they are practically independent of the specific surface. The region of velocities of the carrier gas, corre- sponding to the minimum height of the equivalent theoretical plate, widens with an increase in the average diameter of the adsorbent pores and shifts towards higher velocities, thus

making it possible to use large-porous silica gels (as well as surface-porous glasses) for rapid analyses. One such silica gel has been used for the separation o f n-alkanes to CLQ.

N THE ANALYTICAL practice of gas I adsorption chromatography silica gel has found wide application (10, 14 ) . In recent years use has been made also of porous glasses which possess some advantages over silica gels because of the possibility of obtaining a more uniform porosity (17 , 68) and solid grains with surface porosity only (68). The separating power of adsorbents is determined by the selectivity and rate of mass transfer. The best separation, all other conditions being equal, will take place if the adsorption isotherms are linear up to working concentrations, the equilibrium constants differ sufficiently, and the mass transfer coefficient is high enough. The selectivity of adsorbents-Le., the differences between the equilibrium constants-are determined by the geometrical structure and the chemical nature of the surface. I t is well known that the main disadvantage of the gas-solid method as compared with the gas-liquid method is the nonlinearity of the isotherm of adsorption (owing to the inhomogeneity of the surface) which leads to asymmetrical peaks. In the case of graphitized thermal carbon blacks, however, the

effect of this disadvantage has been practically eliminated (17, 21, 22) . Of all the proposed ways for reducing the nonlinearity of the isotherms of adsorption on silica gels (see review in (28)) the most promising one is the preparation of large-pore and uniform- pore silica gels by geometrical and chemical modification of their skeleton and surface (17, 21) .

The adsorpt’ion kinetics are det’ermined by the time of penetration of the adsorbed molecules into the pores-i.e., by diffusion in the gas. In this respect’ the gas adsorption t,echnique has advantages over the gas-liquid one, as the kinetics of absorption in a liquid, in addition to adsorption on its surface, are determined by the low rate of diffusion inside the liquid film (15) , and t,he transition resistance a t the gas- liquid interface (It?), as well as by the adsorption on the surface of the support.

In this connection, of great interest is investigation of the effect of the geometrical structure and chemistry of the surface of silica gel and porous glasses upon adsorption properties and separating power. In t,his work, results for silica gels are presented. Some data for bulk- and surface-porous glasses have been published previously (27 , 28).

Chemical (8), deuterium-exchange (SO), adsorption (2 , 2O), and spectro- scopic (9) investigations have shown that the surface of silica gels in a maximally- hydroxylated state contains highly

1526 ANALYTICAL CHEMISTRY

protonized hydrogen ( 4 ) of surface hydroxyl groups. The latter may either be hydrogen-bonded or free (19). Vacuum heating removes first (up to 400” C.) the hydrogen-bonded and then gradually (up to 1000” C.) free hydroxyl groups. The concentration of these groups is independent of dispersity of silica (SO) and is determined by the conditions of hydroxylat’ion and the t’emperature of the subsequent evacua- tion of silica (8, SO). In addition to surface hydroxyl groups, silica gels con- t,ain also intraglobular hydroxyl groups whose quantity is dependent on the globule size-i.e.> on the dispersity and history of the silica sample (JO) .

The high concentrat,ion of free hydroxyl groups with partly protonized hydrogen on the hydroxylated surface of silica causes “specific” adsorption of molecules which cont’ain the part’s with peripheral concentration of electron density (19). These include both polar molecules containing, for example, atoms of oxygen or nitrogen with lone electron pairs (water, alcohols, et’hers, ketones, etc., ammonia, amines, pyr- idine, etc.) and nonpolar molecules with R-bonds [aromatic (9) and unsaturat,ed hydrocarbons, nitrogen]. The contribution of the energy of “specific” interact,ion of such molecules with protonized hydrogen of the surface hydroxyl groups to the total energy of adsorption during the dehydroxylation of silica decreases. Therefore the differential heats of adsorption of such substances fall off drastically during the dehydroxylation of the silica surface ( 9 , 19). These “specific” interactions have a common nature, for example, they act, in the case of “cationated” surfaces of zeolites ( I $ ) , although the contribution of classical and quantum- mechanical interactions (connected with the electron delocalization) is different for the different systems. An ordinary hydrogen bond is only a particular case of such “specific” interactions (19 ) .

Conversely, the heat of adsorption of substances whose molecules have sym- metrical electron shells or o-bonds (noble gases, saturated and cyclic hydrocarbons, carbon tetrachloride, et,c.) is practically independent of dehydroxylation of the silica surface (2 , 19) . These substances are adsorbed “non- specifically”-Le., only a t the expense of universal dispersion forces of inter- action with the adsorbent (the energy of classical induction forces is low). The features of specific and nonspecific int,eractions are distinctly manifested in infrared spectra of hydroxyl groups of silica ( 9 ) and in infrared and electron spectra of adsorbed molecules (9 , 1 9 ) .

The narrowing of silica gel pores cause:: an increase mainly in the contribution of the energy of additive dispersion interactions. Therefore, this narrowing leads to an int,ensive rise in the

Figure 1 . Isotherms of adsorption on large-porous silica gels Nos. 7, 8, and 14 Black dots indicate desorption. Specific surface area values are indicated a t curves. Benzene vapors a t 25’ C.

heat of adsorption of large-sized organic molecules and produces a weaker effect on the adsorption of small polar or quadrupole molecules such as water, ammonia, methanol, and nitrogen (18, 19).

Accordingly, the retention time and band broadening in gas adsorption chromatography (apart from the rate of the carrier gas and the packing of the adsorbent in the column) is determined by the geometrical structure of the adsor-

bent (grain size, specific surface, size of pore openings, and pore shape), thermal motion of the molecules- -Le., column temperature and molecular weights-- and, finally, by the rates of adsorption and desorpt’ion. The latter are determined by the temperature, geometry, and chemical nature of the adsorbent surface as well as by the electron density distribution and geometrical structure of the adsorbate molecules. Since for the purpose of rapid analysis it is essential to achieve the separation within a short time and with a minimum broadening of com- ponent peaks, there must be an optimum porosity of the particular adsorbent’ for each region of the boiling temperatures of the substances of similiar nature under analysis.

Investigations into the effect of the chemistry of the silica gel and porous glass surface upon gas chromatographic separation of hydrocarbons have been published (17, 22, 25, 28). The effect of the geometrical structure of silica gel upon gas chromatographic separation of light hydrocarbons was investigated (97). An increase in the specific surface and a decrease in the pore size of silica gel resuhed in a more complete separation of these substances.

The present paper considers the cffect of the geometrical structure of a number of silica gels with a hydroxylated surface upon the adsorption and gas chromatography of some hydrocarbons.

EXPERIMENTAL

Silica Gel Samples. Some characteristics of the fine-, medium- and large-porous silica gels with a hydroxylated surface are presented in Table I. Samples 1-6 contained some alumina and were used unwashed; bample 7 was washed silica gel; samples 8-20 were obtained from sample 7 by the hydrothermal treatmerit in the auto- clave ( I ) .

Static measurements of the adsorption of benzene vapor were made in a vacuum apparatus with the McBain- Baer micro balance. Chromatograms

Table 1. Some Structural Characteristics of Silica Gels (The samples are placed in the order of decreasing specific surface)

Average Specific pore Specific pore

Sample surface areaa diameter Sample surface areaa diameter No. s , m.2/gram d , A. N O . s, n 2 / g r a m d, A.

Average

715 650 520 525 485 375 300 117

32 46 70 22 82

104 125 300

11 12 13 14 15 16 17 18

37 34 31 23 20 15 13 9

1050 1100 1200 1800 1700 2500 2800 4100

9 50 f10 19 9 4100 10 4 5 900 20 6 . 6 4100

a Determined from isot,herms of adsorpt>ion of nitrogen, krypton, benzene, and methanol vapors by the BET method ( 7 ) .

VOL. 36, NO. 8, JULY 1964 1527

't

Figure 2. Initial portions of isotherms on large-porous silica gels Nos. 7 and 8 and nonporous silica (aerosil) with hydroxylated surface

Block dots indicote desorption. indicated on Figure. absolute values (per unit surface)

Pore sizes ore Benzene vapor a t 25' C.,

were obtained with a katharometer (carrier gas, nitrogen) and flame- ionization detector (carrier gases, nitrogen and hydrogen). The experimental conditions are indicated in the captions to the figures. Grain sizes were 0.25- 0.50 mm. unless otherwise indicated.

RESULTS AND DISCUSSION

To investigate the structure of silica gel pores, complete isot,herms of adsorption of benzene vapor on some of the silica gels a t 25" C. were determined. Figure 1 shows an example of a decrease in the specific surface of a silica gel and widening of its pores upon the hydrothermal treatment in the auto- clave.

The nature of the isotherms of adsorption of benzene in the mono- molecular region gives an idea of the nature of the silica gel surface. Figure 2 exhibits the initial portions of the isotherms of absolute values (per unit surface area) of the adsorption on hydroxylated samples 7 and 8 which coincide with the isotherm of absolute values of adsorption on the hydroxylated surface of a nonporous silica aerosil (solid line). This points to an identical nature of their surface and agrees with calorimetric measurements of differential heats of adsorption (2).

To study the effect of porosity upon gas chromatographic separation, chromatograms of hydrocarbons on all samples of silica gel were obtained. Figure 3 depicts chromatograms of mixtures of methane, ethane, ethylene, propane, propylene, and butane obtained on columns of equal length under identical conditions a t 80" C. on four fine-porous silica gels (samples 1 , 2, 3, and 6 inTable I) , and Figure 4 shows chromatograms of normal hydrocarbons CB-Cla a t 120" C. and aromatic ones (benzene, toluene, ethyl benzene, iso- propyl benzene) a t 140' C., obtained

on large-pore silica gels with a small and hydroxylat'ed surface (samples 8, 9, 12, and 20 in Table I.).

Although the selectivity of columns of equal length increases with increasing diameter of the pores of the silica gels under investigation (Figure 3), the value of the separation criterion Kl = ( t Z - t l ) /

(At2 + Atl ) , where t? and tl are the retent'ion times and Atz and Atl are widths of the bands (at half-height) of components 1 and 2, remains practically constant. For analyzing t,his mixture, however, it is more advant,ageous t,o use silica gels with a smaller specific surface area-e.g., sample 6-since in this case, with the criterion Kl remaining unchanged, time of analysis is considerably reduced and the peaks are less diffuse.

To investigate the effect of pore narrowing upon gas chromatographic separation, chromatograms on silica gels with identical surfaces but with different, average pore diameters were obtained (samples 3 and 4 in Table I, which had d = 70 A . and d = 22 .\., respectively, with the same s = 520 meterz/gram). Figure 5 shows chromatograms obtained in columns containing equal masses and, consequently, equal int'ernal surfaces of these samples under identical condit'ions. &is the average pore diameter increases from 22 to 70 ;1. in accordance with the decrease in the heat of adsorption (18) there is a drastic reduction in the retention time (approximately by a factor of 4). The values of K1 for a wide-pore sample, however, are somewhat higher due to the lower broadening of bands in the case of a large-pore sample (evidently owing t'o the higher rate of diffusion in wide pores). In this connection, of great interest is the use of grains of adsorbents having surface porosity only, thus making it possible to increase considerably t,he rate of diffusion and mass transfer with the surface and hence to reduce the time of analysis without' affecting the separating power (28).

To determine whether adsorption equilibrium is achieved--i.e., whether all t,he available internal surface of the silica gel participates in adsorption a t the employed velocities of the carrier gas-measurements were made, under identical conditions, of the retention volumes of methane, ethane, ethylene, propane, and propylene on silica gel with spherical grains of various sizes differing by a factor of 5 to 10 (sample 5 in Table I, s = 485 meters*/gram, d = 82 A .

The values of the retention volume per gram of silica gel, V R m , for various grain sizes (from 0.25 to 2.0 mm.) a t three temperatures, under the conditions of these experiments are practically independent of the grain size of this silica gel. This shows that in such cases nearly the whole available surface takes part in adsorption. Xlthough the

values of VR, a t peak maxima are not changed as the grain sizes change, columns with larger silica gel grains produce more diffused and asymmetrical peaks due to the differences introduced into the band broadening by the diffusion in the gas.

With the same grain size each porosity should be characterized by different optimum velocities of the carrier gas. Figure 6 demonstrates the effect of changes in the velocities of the carrier gas upon H , the height equivalent, to a theoretical plate, of the columns filled with silica gel of different structures with equal grain sizes. h s the pore size of silica gel increases, the region of optimum gas velocities widens and shifts toward higher values of these velocities. Therefore for the purpose of analysis it is better to use silica gels of large pore size (of course, retaining the separating power of the column) because they permit wider changes in the velocities of the carrier gas within which the optimum separating power of the

2

104d

- 4 0

2 I

Figure 3. Chromatograms of low boiling hydrocarbons obtained at 80" C. on gels Nos. 1, 2, 3, and 6 with different pore sizes (indicated near chromatograms)

Experimental conditions: column 150 cm. X 0.45 cm., rote of carrier gas (hydrogen) 50 ml./ min., sample size 0.02 ml. Gels were tlrst dried a t 1 5 O 0 - 2 O O 0 C. to constant weight


mm 8 4 o

1 a) 70A

i\ min - 7-70

I'

300A

min Q 8 4 o

Figure 4. Chromatograms of hydrocarbons obtained on different large-pore silica gels: Nos. 8, 9, 12, and 20 with different pore sizes (indicated near the chromatograms)

Experimental conditions: column 100 cm. X 0.4 cm., rate of carrier gos (hydrogen) 6 0 ml./min., sample size 0.01 ml., temperature 1 3 0 ' C.

2 r

Y S ~ P P ~ ~ ~ P ~ ~ ~ ~ ~ O I a e r ' o

Figure 5. Chromatograms of hydrocarbons obtained on two silica gels, Nos. 3 and 4, with rather similar surface areas but different average pore diameters Experimental conditions: temperature 50' C., somple sizes 0.02 ml.; rote of carrier gos (hydrogen) 50 ml./min.; column, (a) 100 cm. X 0.45 cm., ( b ) 58 cm. X 0.45 cm., odsorbent weight in both coses 8 grams

column is practically retained. Similarly, for surface-porous glass a considerably wider region of opt,imal velocities of the carrier gas is observed thitn for bulk-porous glass (88) in view of the high rate of diffusion and mass transfer.

To investigate the effect of the silica gel structure both upon the selectivit,y of gas chromatographic separation and its adsorption properties a t low coverages, heats of adsorption of hydrocarbons were determined chromato- graphically for all the samples given in Table I.

\\'hen comparing the heats of adsorption by silica gels obtained by gas chromatography with those determined from isostrres and measured in a calorimeter, the following should be kept in mind. First, gas chromatographic data, particularly those obtained with ionization detectors, refer to coverages so low that they are usually inaccessible by static investigations. In contrast to graphitized carbon black with a very homogeneous surface ( f ? ) 21, 22) , the c~xtrapolation of differential heats of adsorption, deter-

mined calorimetrically, to IOK coverages for a heterogeneoub surface of silica gels is difficult. Second, in contrast to evacuated adsorbents with a free surface, in gas c>hromatography experiments the most active portion of the surface (especially of hjdrophilic silica gels) could be a t least partly occupied by water molecules and other adsorbed molecules which screen off the most active portions of the surface. Third, gas chromatographic investigations are usually carried out a t higher temperatures, and this should also cause some reduction in the heats of adsorption on nonhomogeneous surfaces as determined from chromatograms. Xnd, finally, fourth, in gas chromatography experiments with porous adsorbents it is possible to achieve somewhat incomplete thermodynamic equilibrium. particularly on the most active portions of the surface in fine pores. l o study these l)roblems, it is necessary to make parallel investigations, not only of heats of adsorption at various coverages (by gas chromatography at low coverages, and by the static method a t higher coverages), but also detailed investiga-

1 . . . . . . . . . . J 2 6 10 14 18

u, "/= Figure 6. Dependence of HEW (for ethane) on linear velocity of carrier gas for silica gels of various porosities at 50" C.

t'ions of adsorption isotherms and the depcndences of heats of adsorption on the surface coveragc in sufficiently wide (overlapping), surface coveragr rangcs, using both methods.

In this connection chromatograms of propane were determined on silicma gi.1 5 with dts: 82 &\. a t four teml~cratures and with various amounts of h!dro- carbons up to the niasiniun~ possible ones (Figure 7). The picture ohtained is characteristic of chronlatograms during adsorption on a heterogrnous surface- Le., bands with a sharp front and extended tail. It, should be noted that in accordance with the some\vhat in- clined fronts of t,hese band.: thcir tail.< for different pulse sizes do not c o i n d e . This is because, as a wholc, the proccis of gas chromatography in the case of porous adsorbents is not strictly an equilibrium one. Figure 7 shows, however, that in the investigattd cas(' the divergencies are not too great, The location of the peak ohtainrd ivith an ionization detector---i.e., for vcry lo\v coverages-at the end of the tail (but not further) of the band olitained ivith a katharomrtri.-i.e., for high roverages--

VOL. 36, NO. 8, JULY 1 9 6 4 1529

b i i 3 4 5 i i a 9 o m i n

Figure 7. Chromatogram of propane on silica gel No. 5 at 28" C. Experimental conditions: column 100 cm. X 0.45 cm., adsorbent grain sizes 0.25-0.5 mm., velocity of carrier gar (hydrogen) about 35 ml./ min.; chromatograms carresponding to sample sizes 0.5, 2, 5, 8 , and 10 ml. have been measured with katharometer, and for 0.02 ml. with flame-ionization detector

indicates that the lower limit of integra- tion of the chromatogram may be determined with sufficient accuracy. Tntegra- tion (11) has produced the adsorption isotherms presented in Figure 8a, from which isosteric heats of adsorption have been computed as a function of surface coverage (Figure 8b). Un- fortunately, no direct measurements of the isotherms and heats of adsorption of propane on the same silica gel have been made by static methods as yet. Therefore, for comparison Figure 8b shows the values of heats obtained by extrapolation to these surface coverages of calorimetric data for heats of adsorption of n-alkanes on large-pore silica gels (13). The intervals of the surface coverage in gas chromatographic and static determinations do not over- lap, but nevertheless it may be inferred t'hat' in the case of silica gels (in contrast to t'he nonporous graphitized carbon blacks) the chromatograms yield lower heats of adsorption values. Among t'he above mentioned possible causes of divergence between chromatographic and st'atic determinations, of great,est importance in this case are probably the incomplete equilibrium and thc retention by the silica gel in the

column of some amount of water adsorbed on t,he most, act,ive portions of the heterogeneous >urface of t,he silica gel. From this 1)oint of view the gas chromatographic dependence of &. on B may refer to somrn-hat higher values of B , since part of the surface is occupied by water. Further detailed invcstiga- tions in this direction are essential.

In spite of the lower heats of adsorption on the silica gel surfaces determined by gas chromatography, they are very useful, for they can he determined rapidly and, as we shall see lat,er, they give a true qualitative picture of the effect of the geometry and chemistry of the adsorbent surface and the effect of the geometrical and electronic structure of t'he adsorbate molecules upon adsorpt'ion properties. Therefore in our later work heats of adsorption were det'ermined by the simplest method- i.e., from the dependence of the log- arithms of the ratios of retention volumes to the absolute temperature of the column, log V R j T , on the inverse values of this temperature (26). The velocities of the carrier gas corresponded to the minimum heights of theoretical plates (See Figure 6)--i.e., under near- equilibrium conditions. The surface coverages range from lo-' to lo-' for various silica gels.

Such low coverages in the case of geometrically heterogeneous adsorbents, in particular fine-porous ones, relate t.0 the region of a rapid drop of differential heat of adsorption with an increase in surface coverage and, accordingly, to the most convex (toward the adsorption axis) portion of the adsorption isotherm (18). However, gas chromatography experiments, unlike vacuum static measurements of differential heats of adsorption, are made a t higher temperatures This reduces the effect of heterogeneity and causes the adsorption isotherm to approach the Henry law (3). Some of the resulting heats of adsorption are listed in Table 11.

In accordance with the data of direct calorimetric det,erminations [see review ( I @ ] , gas chromat'ographic measurements indicate that a decrease in the pore size increases the heats of adsorption of hydrocarbons which is attributed

4

D. mm Ha

Figure 8. Adsorption of propane

A. Isotherms on gel No. 5 calculated from chromatograms of Figure 7

6. Dependence of heats of adsorption on surface coverage of gel. Curve I , isosteric heats calculated from isotherms of Figure 8 A ; Curve 2, heats obtained by extrapolation of calorimetric data (see Figure 96) to n = 3

t'o the increase in the potent,ial energy of the nonspecific dispersion forces upon the narrowing of adsorbent pores. This increase of the heat of adsorption rises with the growth of the adsorbate molecule.

Static measurements showed that for xide-pore and nonporous silica gels isotherms of absolute (per unit surface area) adsorption values coincide (2 , 20). The effect of pore narrowing with large pores practically does not change the values of the heats of adsorption. The values of Q. for all the samples of wide- pore silica gels coincide within errors of determination for each of the hydrocarbons investigated.

Figure 9a gives dependences of the heats of adsorption of saturated hydrocarbons CI-Cio on the number of atoms in a molecule for the silica gels with various porosities. As the pores widen, the limiting dependence of Q. on the number of carbon at'oms in a molecule of n-alkanes is achieved which is independent of the pore size and the surface area.

In Figure 9b this dependence of Q.

Table II. Differential Heats of Adsorption, Q,, of Hydrocarbons C1-C4 on Fine- and Medium-Pore Silica Gels (Obtained from gas chromatographic data at surface coverage 0 = T = 20"-90" C.)

Specific Average surfare pore Q C ~ H ~ Q c ~ H ~

S:tuiple area diameter Differential heats of adsorption, Qa, kcal./mole minus minus X o , 8, m2/gr:trn d , A. CH4 C2Hs CiH4 CzHa CaHs C4Hx &rzHs &rm

1 715 32 3 . 8 5 5 . 9 5 6 . 9 7 . 9 9 . 0 9 7 0 .95 2 650 46 3 . 7 5 5 . 6 6 . 5 7 5 8 . 7 9 4 0 9 3 520 i o 3 . 6 5 . 4 6 . 4 7 . 2 8 . 4 9.15 1 . o 6 375 104 3 . 4 5 . 2 5 6 . 2 6 . 9 8 . 1 8 . 6 0 . 9 5 8 117 300 3 . 5 4 . 7 . . . 5 . 8 . . . . . . . . .

, . . . . . 15 20 I TOO 3 . 2 4 . 0 . . 5 . 2 , . .

1 1 1 2 1 2 1 2

~~


Q,

5 -

heat of adForption value5 arr hinalli~r than those determined 11y htatic. methods.

The fairly true reflection of ad.-oq)tioii properties by gas chromatographic. data makes it possible to espre-qs thebe data in the form of physico-chemical constants characterizing thc nature of the adsorbate-adsorbent physico-chemical chara for the adsorption investigations arc the isotherms of abbolute (per unit surface area) values of adsorption itsclf and the dependences of the differciitial heats of adsorption on thesc absolute values of the adsorption ( 2 ) .

For nonporous and sufficiently la pore adsorbent. of a himilar naturrt t h f w characteristiw are indc1)cndent of tli.-- persity (specific burface area) and thfre- fore represent actual phybico-chemical constants. These physico-cheniical characteristics of the nature of ad-

tern. were ohtained for hernial carbon tilacks (12)

and hydroxylated nonporoub anti sufficiently large pore silicai ( 2 ) .

In accordance with this, thc gas chromatographic charact nat,ure of an adsorption absolute (also per unit surface area) value of the retention volume of the particular adsorbate-i.e., the value (21

/

&- ;:E LY 1700

0 2800 0 14w

10 -

.d

@ ifii 1 . 2 . . . . . . . . 0 4100

0 6 Q nc Figure 9. Heats of adsorption of n-alkanes

a.

b.

Calculated from linear dependence of log V R / T on 1 / f as function of number of carbon atoms n, in adsorbate molecule for silica gels with different porosities Obtained in calorimeter a t 20' C. as function of n at different coverages of surface of large- pore gel. Dotted line = heats calculated from lineor dependence of log VR/T on 1 / T a t 8 = 0.1

on n obtained 11) ga- chromatography for 0 = 0 1 is compared u i th the cor- responding dependences determined from calorimetric data foi n-alkanes C5-C, a t beveral burface coveiages (13). I t can be seen from the figure that as 0 decrease. and n increases, the calorimetric data inciease, deflecting upnard from linear dependence foi 6 = 0 5 It should be borne in mind that an increase in n rewlts in a piogre..ive displacement of the range of ga. chromatographic measurements ton ard higher temperatures, and this also can increaie the lag of ga- chromatographic

heats of adsorption from those det,ermined in the calorimeter. h comparison of gas chromatographic and static data for some hydrocarbons is given in Figure 10 as well.

I t is also evident from Table I1 that gas chromatography provides a correct sequence of the heats of adporption of alkanes, alkenes, as well as aromatic hydrocarbons (24) on the hydroxylated surface of silica-i.e., the growth of these values with transition from n- alkanes to alkenes and aromatic hydrocarbons with the same nuinber of carbon atoms (29)---although the changes of the

i O,A calorimetric, 80. 2 0 chromato raphic, , ns-ho-

5-. u 7 1 0,2 0.4 0.6 0,2 0,4 46 0.9 $4 0,s

0- Figure 10. Dependence of heats of adsorption of n- pentane, n-octane, and benzene on surface coverage of large-porous silica gel

I , heats obtained in calorimeter a t 20' C.; 2, heats calculoted from linear dependences of log Vn/T on 1 / T i black paints denote derorptlon

Indeed, it is shonn (21 ) that the xalue. of FIRs (at Ion 6) for variou- hTdro- carbons, acetone, ether, and cai bon tetrachloride on graphitized thermal carbon blacks: n ith a T ery honiogeneou. surface coincide for each adsorbate independent11 of di.prr.ity (of the value of 5).

In the c a v of ad-orbent. irith a heterogeneous .urfate thc value\ of

5m;g - Figure 1 1 . Dependence of retention volume per gram of silica gels, VRnl , of n-hexane at 100" C. on specific surface area, s, of large-pore silica gels

Surface coverage for different silica gels = from 0.01 to 0.1 of the complete monolayer

VOL. 36 , NO. 8, JULY 1 9 6 4 1531

ioo"

Y

Figure 12. Dependence of absolute values (per unit surface area) of retention volumes ( V R s ) of n-alkanes on number of carbon atoms in adsorbate molecule

Large-pore silica gels ot 100' C. and surface coverage from 10-3 to 0.1 of complete monolayer

ITR8 should depend on 0 and on the pore size even at low 0's (see Figure 7 ) . HoLvever, in accordance ivith the limiting dq)endence of Q, on n (P ' igui~ 9 a ) , thew shou d exist al>o mnstnnt niaxi- riiuni values of L7R8 for \vitle-pore silica gels.

From Figure 1 1 it follows that the per-1-gram values of the retention volume, I*Rmr of n-hexane, are indeed proportional to the s1)rcific surface area, s, for hydroxylated silica gels.

The values of T*Rm were calculated by the formula

where t R = the corrected retention time, m = the mass of the adsorbent, T = the absolute temperature of the column, T , and p , = the absolute temperature and presiure a t which the flow rate w ha< been measured, and

Table Ill. Approximate Values of Ab- solute Retention Volumes, V R s , (per unit surface area) of Some Hydrocarbons on Large-Pore Silica Gels at Various

Surface Coverages at 100°C. V R ~ , ml./meter2

Adsorbate e = lo-' 0 = lo-? 0 = n-C,H,, 0 030 0 034 n-CsHja 0 065 0 078 0 095 n-C.rHia 0 130 0 150 0 200 n-CeHls 0 2 i 0 32 0 40 n-C8H20 0 5.7 0 62

CaH, 0 32 0 47 0 69 ni-CjOH22 1 10 1 24

260'

, >-LA-, . , 1 . L E 2 4 6 I 0 12 f* I# I) P JJ I* i)l I W U JV,

mm

Figure 13. Chromatogram of purified paraffin on column 100 cm. X 0.5 cm. with very large-pore silica gel No. 15 at 260" C. Rate of carrier gas (helium) is 54 rnl./min., sample size 50 PI. of paraffin in solvent of n-alkanes C G - C ~ ~

p , and po = the pressures a t the en- trance and at the exit of the column (21).

Figure 12 gives the dependence of the values of V R s for n-alkanes on n for a large number of silica gels with a hydroxylated rurface. For dEcient ly wide-pore silica gels the values of V R s are practically indepcndent of s and therefore represent phy$ico-chemical constants for a given teml)erature and surface coverage depending only on the nature of the adsorbate-adsorbent system but not from the value of s. The narrowing of pores increases the value of V R a ; this increase is the greater, the higher is n in accordance with the adsorption data (18).

Table 111 lists some values of I r R s for the adsorption of different hydrocarbons on wide-pore silica gels with a hydrox- vlated surface. These values may be ;sed both for the identification of these hydrocarbons during the gai; chromatographic analysis of their rnixtures as well as for the determination of the specific surface area of nonporous and large pore silicas with the hydroxylated surfaces from gas chromatographic experiments by using Equation 1.

From the values presented in Table I1 it follows that the difference between the heats of adsorption of unsaturated and saturated hydrocarbons with the same number of carbon atoms in a molecule is independent of the pore size for silica gels with a hydroxylated surface, since it characterizes mainly the contribution of "specific" interactions of ?r-bonds of unsaturated hydrocarbons with protonized hjdinxyl groups of the hurface (6, 19). This difference, however, should decrease with increasing temperature! as for zeolites (25 ) .

The difference between the heats of adsor1)tioii of two (*onsecutive terms of a homologous series of hydrocarbons increases ivith an increase in thr s l~ r i f i c surface area and decreases with an increase in the average pore diameter. The selectivity of silica gels (the difference between specific values of reten-

tion times) increases in the >ame direction. I h t in this case the width of the peak increases in the same degree, so that the separating power of the column is practically retained (as we have seen, the h', criterion remains unchanged).

The broadening of the peaks prevents the use of high selectivity of silica gels with a large specific surface area and a smaller pore diameter. The retention times of the components being analyzed, as was mentioned above, are determined, on the one hand, by the geometrical structure and chemical nature of the silica gel surface and, on the other, by the molecular weight, the geometrical structure, and electron density distribution of a molecule of the sub- stance under investigation, as well as by the temperature of the experiment. Based on this and also on the results ohtained in the present work! it may be inferred that for each range of boiling temperatures of the substances of a similar nature being analyzed there is an optimum porosity of the adsorbent used which may lead to a comparatively rapid separation with a minimum broading of the peaks. So for the separation of low-boiling gases i t is de- sirahle to use silica gels with an average pore diameter not exceeding 20 LL, for the separation of hydrocarbons with boiling temperatures not exceeding 100" C. it is expedient to use silica gels with an average pore diameter from 50 to

and for the separation and of higher-boiling substances,

larger [)ore silica gels. Es1)ccially wide-pore silica gels may

be used for chromatographic analysis of many liquids and solids a t high temperatures. Figure 13 exemplifies a chromatogram of purified paraffin on silica gel (sample 15 in Table I) with s rr 20 metersz,'gram and d M 1700 .\. at, 260" C. In view of the high thermal stability of the geometrical structure (porosity) of large-pore adsorbents they may be used for chromatographic analysis even up to 600"-800" C. (in a partly dehydroxylated state).


LITERATURE CITED

111) Akshinskaya, ?i. I-., Kiselev, A. I-., Sikitin, Yu. S., J . Phys. Chem. (Mos- c . 0 ~ 1 ) 37, 927 (!963),,

( 2 ) Ihilikin, I. lu., hiselev, A. I-.) Zbid.,

( 3 ) Harrer, 11. AI., Iiees, L. 1‘. C., Trans.

(4) Rasila, M. R., J . Phmn. Phys. 35, 1151

15) I k z u s . A. ( i . . Ihevine. P..

11. 228.

I.‘arar/a!q SOC. 57, W!) (1961).

( 196 1 ).

C’hararter of Adsorption,” Ciarendon . . Press, Oxford, 1953.

( 7 ) Brunauer, S., I’:nirnett, P. H., Teller,

f S ) Fripiiit., J., I.ytterlioeven, J., J . Phys. f ’hrn i . 66, 800 (1962).

( 0 ) ( idkin, G. *4., Kiselev, A. I-., Lygin, 1.. I., Trans. Faradug Soc. 60, 461 (1964). 01 (ireene. S. A , . Pust. H.. . ~ N A L . CHEM.

J . . lm . Cheni. Soc. 60, 309 (1938).

I i

29, 105,~ i i r ~ j . 1 ) Huher, J. F. I<., J<eulernans, A. I. ‘ ,C; t ts Chromatographviy 1962,” RI. van Swaay, ed., p. 26, Butterworths, London, lW2. 2 ) Isirikyan, A. A , , Kiselev, A . V., J . Ph!/s. Phem. 65, 601 (1961).

(13) Isirikyan, 4. A, , Kiselev, A. V., Frolov, B. A . , J . P h y s . C h e m ( N o s c o w ) 33, 389 (1959).

(14) Janak, J., Collection Czech. Chem. Conmirn. 18, 798 (1953).

(15) Keulenians, A. I. &I., “(;as Chro- niatography,” C. C;. I-erver, ed., Xew York, London, 1958.

(16) Khan, R I . A. , “Gas Chromatography 1962,” XI. van Swaay, ed., p. 190, Butterworths, London, 1962.

(17) Kiselev, A4. I-., Ibid., p. XXXII- . (18) Kiselev, A. “Proc. 2nd Int.

Congress on Surface Activity,” Shull- man, ed., 1-01. 2, p. 179, Butterworths, London, lYt57.

(19) Iiiselev, A. V., Rev. Gen. Caoutchouc. 41, 377 (1964).

(20) Kiselev, A. J‘., “The Structure and Properties of Porous A\Iaterials,” 1). H. Everett, F. Stone, eds., p. 195, Butter- wvorths, London, 1958.

(21) Kiselev, I.., Paskonova, E. A, , Petrova, It. S., Shcherhakova, K. I)., J . Phrls. Phem. (illoscow) 38, 161 (1964).

( 2 2 ) Kiselev, A. I-., Shcherbakova, K. I)., “Abhandlungen der I)eutsc>hen Aka- deniie der LVissenschaften zu Berlin, Gas Chromatographie 1961 ,“ S-207-281~ Akadernie-I’erlag, Berlin, 1962.

(23) Kiselev, A. 1-.) Yashin, Ya. I., J .

Phys. Chem. (.WOSCOW) 37, 2614 ( l ! ) C i 3 ) . (24) Kiselev, A. I*.) Yashin, Ya. I . ,

Petrol. Chem. ( M O S C O U ) ~ , (l964), in press. (25) Petrova, 11. H., Khrapova, 1’.

Shcherbakova, ,K. I)., “C:as Chroma- tography 1902, M. van Swaay, ed., p. 18, Butterworths, London, 1962.

(26) Ross, S., Haelens, J. K., Olivier, J. P., J . Phys. Phem. 6 6 , 696 (1962).

(27) l?yakhirev, I ) . A . , Chernyaev, I. P., Bruk, A. I., J . Phys. (‘hem. (,li‘oscowj 34, 1096 (1960).

(28) Yashin, Ya. I., Zhdanov, S . I’., Kiselev, A. V., “(;as Chrornatogr:iphie 19633,” I‘ortrage des 4 Symposirinis nber Cras-Chroinatographie in Leuna, her- ausgegeben von H. P. Angele und H . C ; . Struppe.

(29) Zhdanov, S. P., Kiselev, A. I*., Yashin, Ya. I., Petrol. Chem. ( M o s c o w ) 2,417 (1‘363).

(:30) Ilavydov, 1.. Ya., Zhuravlev, L. T., Kiselev, A. I-., Trans. Faraday Soc. (1964), in press.

RECEIVED for review January 7 , IO(i4. Accepted April 1, 1964. Presented at 2nd International Symposium on Advanves in Gas Chromatography, University of Hous- ton, Houston, Texas, hlarch 2i3-26, 1064.

Gas Chromatographic Separation of Hydrocarbons (C, to C,) by Carbon Number Using Packed Capillary Columns WERNER SCHNEIDER and HARTMUT BRUDERRECK

Scholven-Chemie A. G., Gelsenkirchen-her, West Germany

I STVAN HALASZ

lnsfitut fur Physikalische Chemie der Universitut, Frankfurt am Main, West Germany

b Capillary columns were packed with graphited carbon black and impregnated with about 0.4 wt. 70 of squalane. The order of the retention volumes of hydrocarbons in the C1 to CS range corresponds, with a few exceptions only, to the number of carbon atoms in the molecules. Re- tentions relative to n-pentane on nonimpregnated and impregnated graphited carbon black are tabulated.

s 1956 Eggertson, Knight, and I Groennings ( 4 ) separated paraffins and naphthenes in the range Cs to Ci on I’elletes, a furnace black coated with l.5Cro squalane. They found that “a cwbon number analysis is ~)ossible, at least through C 7 . . . and probably higher.” Simmons and Snyder ( I S ) pointed out in 1958 some anomalies in the relation between retention time and boiling i~oint of saturated C; hydro- c-arhons on the same stationary pha3e.

I*ncslwcted separation.% were ob- t a i n d with gral~hited carbon black. ~ i ~ a i ) h i t c ~ . as the stationary iihase in

thin layer capillary columns (8, fI). A noteworthy result is the quick separation of t.he xylenes. Contrary to our experience with any other stationary phase, m-xylene under these conditions had a ret’ention volume smaller than that of p-xylene. Graphited carbon black was also used to produce coatings on glass beads, so called porous layer glass bed, (I’LGI3) (6). Also in this type of column, the graphite showed re- markable separating Iroperties. 13e- cause of its nonpolarity, it produces relative wtention times of nonpolar compounds rsceeding those of polar comlIorinds of the same atmospheric boiling 1)oint. While nonpolar liquid ,stationarj. / ) h a w sellarate nonpolar compontmts i~nighly by their atmospheric. hoiling Imint, gra1)hite shows a rlistinc.tly anornalow behaviour.

The authnw (6 , 1 1 ) explain this fact by assuming that the compounds are oriented on thtl nonpolar graphite with their longitudinal asis. This would mean that highly branched, higher boiling hydrocarbons have relative retention times shorter than those en-

countered with straight chain, lower boiling compounds.

This hypothesis is based on delibera- tions of Breshchenko ( f ) , who investigated the adsorptive properties of graphited carbon black during paraffin removal from high boiling hydrocarbon mixtures in the liquid phase. According t o his findings, paraffins (straight chain or low branched, irrespect’ive of their molecular weights) are adsorbed easier than highly branched paraffins or even aromatics. nrwhchenko explains the preferred adsorption of long chain hydrocarbons wit’h reference to t,he nonuniform strength distribution of the intermolecular forces, showing a maximum in the direction perpendicular to the axis of the molecule and a minimum parallel to the axis.

At any temperature, t8he heat, of adsorption of a given organic com- pound on graphite is lower than that 011 the conventional act ive solids used in gas chromatographic analysis. but higher than the heat of solution in the conventional liquid stationary phases. This latter fact explains the coni-

VOL. 36, NO. 8, JULY 1964 * 1533

Documents

Effect of Pore Size of Silica Gels on the Separation of Hydrocarbons