Znd. Eng. Chem. Res. 1991, 30,1151-1157 Henderson, M. A.; Worley, S. D. An Infrared Study of the Hydrogenation of Carbon Dioxide on Supported Rhodium Catalysk An Inverse Spillover. J. Phys. Chem. 1986,89, 1417-1423. Hori, B.; Takezawa, N.; Kobayashi, H. Sampling Devices for the Transient Response Study of the Catalyst Reaction. Ind. Eng. Chem. Fundam. 1986,24, 397-398. Iiiuka, T.; Tanaka, Y. Dissociative Adsorption of C02 on Supported Rhodium Catalyst. J. Catal. 1981, 70,449-450. Iizuka, T.; Tanaka, Y.; Tanabe, K. Hydrogenation of CO and COz over Rhodium Catalyst Supported on Various Metal Oxides. J. Catal. 1982, 76, 1-8. Inui, T.; Funabiki, M.; Suehiro, M.; Sezume, T.; Iwana, T. Methanation of Carbon Dioxide and Carbon Monoxide over Supported Ni-Laz03-Ru Catalyst. Nippon Kagakukaishi 1978, No. 4, 517-524.
1151
Phys. Chem. 1983,87,437&4387. Ponec, V. Some Aspecta of the Mechanism of Methanation and Fischer-Tropsch Synthesis. Catal. Rev.-Sci. Eng. 1978, 18, 151-171.
Solymosi, F.; Erdohelyi, A. Hydrogenation of C02 and CO over Alumina-Supported Nobel Metals. J. Mol. Catal. 1980, 6, 471-474.
Solymosi, F.; Erdohelyi, A.; Kocsis, M. Surface Interaction between Hz and H2on Rh/A1203, Studied by Adsorption and Infrared Spectroscopic Measurement. J. Catal. 1980,65,428-436. Solymosi, F.; Erdohelyi, A.; Bansagi, T. Methanation of C02 on Supported Rhodium Catalyst. J. Catal. 1981,68,371-382. Underwood, R.P.; Bennett, C. 0. The CO/H2 Reaction over Nickel-Alumina Studied by the Transient Method. J. Catal. 1984,86, 245-253.
Kobayashi, H.; Kobayashi, M. Transient Response Method in Heterogeneous Catalysis. Catal. Reu. 1974,10, 139-176. McCarty, J. G.; Wise, H. Hydrogenation of Surface Carbon on Alumina Supported Nickel. J. Catal. 1979,57,406-416. Mills, G. A.; Steffgen, F. W. Catalytic Methanation. Catal. Rev. 1973,83, 159-210.
Ozdogan, S. Z.; Gochis, P. D.; Falconer, J. L. Carbon and Carbon Monoxide Hydrogenation on Nickel: Support Effects. J. Catal.
Vannice, M. A. The Catalytic Synthesis of Hydrocarbons from Eng. 1976,14, Carbon Monoxide and Hydrogen. Catal. Rev.-% 153-191.
Weatherbee, G. D.; Bartholomew, C. H. Hydrogenation of C02 on Group VI11 Metals. J. Catal. 1981, 68, 67-76. Zagli, E.; Falconer, J. L. Carbon Dioxide Adsorption and Methanation on Ruthenium. J. Catal. 1981,69, 1-8. Received for review September 26,1989 Reoised manuscript receioed August 27, 1990 Accepted January 6, 1991
1983,83,257-266.
Peebles, D. E.; Goodman, D. W.; White, J. M. Methanation of Carbon Dioxide on Ni(100) and the Effects of Surface Modifiers. J.
MATERIALS AND INTERFACES NMR and Intrinsic Viscosity Study of Two Different Phenol-Formaldehyde Resol Resins Moon G.Kim* Forest Products Laboratory, Mississippi State University, Mississippi State, Mississippi 39762
Larry W.Amos Weyerhaeuser Company, Tacoma, Washington 98477
Two different phenol-formaldehyde (PF) resol resins used as binders in wood products manufacture were subjected to acetylation and fractionation, and the fractions were studied by quantitative NMR, intrinsic viscosity, and number average molecular weight measurements. The NMR results described the structures of these resin fractions reasonably well as polymeric methylene (hydroxymethy1)phenols. However, the solution 13C NMR results, obtained with long delay times and nuclear Overhauser effect (NOE) suppression, showed significant deviations from the structure, apparently due to the limitations of the method used. While higher molecular weight fractions deviated more than low molecular weight fractions, resin A fractions deviated less than resin B fractions, reflecting the structural differences between the two resins. Mark-Houwink correlation results for resin A fractions indicated nonuniform structures, and for resin B fractions the correlation resulted in an “a” value of 0.21 in chloroform and 0.12 in benzene, indicating a compact structure typical of branched polymers. Intrinsic viscosities of resin A fractions expanded less than those of resin B fractions upon a solvent change from benzene to chloroform. Resin B fractions showed higher, more variable Huggins’ constants than those of resin A fractions, indicating their higher tendency of molecular associations in solution, which was interpreted to give rise to their higher deviations in solution l9C NMR results. Introduction Phenol-formaldehyde (PF) resol resins used as binders in the wood products industry are known to be composed of polymeric methylene (hydmxymethy1)phenols(Megson, 1958). But defining the polymer molecular structure and 0888-5885/91/2630-1151$02.50/0
properties with respect to the performance or synthetic procedures has been difficult due to the lack of adequate analytical procedures and in part due to the instability of resol resins. The reported molecular weight determinations by gel permeation chromatography or ultracentrifuge 0 1991 American Chemical Society
1152 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 Table I. Fractionation Data
Fractions 1-10 were obtained from Eh, ether-insoluble fraction. E,,:
(Gollob et al., 1985; Wooten et al., 1988) or by intrinsic viscosity and number average molecular weight measurements (Tobiason et al., 1973) all resulted in high or varying values, apparently because of the molecular association of the polymer molecules occurring when the medium was changed from a strongly alkaline aqueous to a less alkaline aqueous or to an organic solvent. The stability of resins has also been of concern in these analytical procedures. Extensive solution *Hand 13CNMR studies reported in the past resulted in qualitative descriptions of P F resol resin structures in terms of the numbers of methylene and hydroxymethyl groups substituted on ortho and para carbons of a phenolic ring. More recent solid-state 13Cand deuterium NMR studies (Fyfe et al., 1980; Maciel et al., 1984; Kelusky et al., 1986) showed qualitatively that, as a PF resol resin polymerizes further as in curing, the methylene bond content increases at the expense of hydroxymethyl groups and at the end the resin attains a typical glassy structure. In a study using a quantitative 13C NMR procedure in alkaline aqueous DMSO solution and a gel permeation chromatographic method with 0.1 N sodium hydroxide solution as carrier (Kim et al., 1990), a branched structure with a relatively low average molecular weight was proposed for a wood adhesive PF resol resin. Their NMR results still appeared to be affected by the molecular associations and by other factors, and since the resin studied had a wide molecular weight distribution, the polymer structures were less clear, especially with respect to those of high molecular weight components. Although sorely needed for the structure-property relationship, the quantitative determination of the extent of polymerization or structures for the high molecular weight components of PF resol resins by NMR or any other methods has been difficult for various reasons. The intrinsic viscosity of a linear high molecular weight polymer represents its hydrodynamic volume and is related to its molecular weight by the Mark-Houwink equation (Flory, 1953). This relationship has also been shown to be useful for low molecular weight polymers (Perico and Rossi, 1970). In this report two P F resol resins that were similarly high in the degree of polymerization but synthesized differently for different applications were acetylated for stabilization and subjected to fractionation to obtain high, narrow molecular weight fractions. Select fractions were examined by NMR, intrinsic viscosity, and number average molecular weight measurements to investigate the structure and molecular properties of the high molecular weight fractions and also to differentiate the two resins for their structure-property relationship. Experimental Section Resin A is a typical wet-process hardboard binder type P F resin designed to effect a complete resin precipitation upon acidification in a dilute aqueous solution. The resin was prepared as follows: To a reactor with a stirrer, cooling condenser, and cooling coil were charged 453.4 g of phenol, 721.0 g of 50% formaldehyde, and 526.0 g of water while the temperature was being adjusted at 30 "C. Added to this reaction mixture was 299.0 of 50% sodium hydroxide salution in drops over a 25-min period, while the temperature was allowed to rise steadily to 90 "C. In the following 10-min period the reaction mixture was al-
Ether-soluble fraction.
1
1
1
ABElutionVol (mL) 40 50
MWx1O1
1
80 70
I
1
l
l
l
l
l
l
l
l
80 90 100 110120130 140 150 180170
100 70 5040 30252015107543121 1 2515
'Reference (Kim, et al. 1-
Figure 1. Gel permeation chromatograms of resins A and B/sephacryl/O.Ol N NaOH.
lowed to reach reflux at 102 "C, which was maintained for 15 min. The reaction mixture was then cooled to 84 "C, which was kept constant until the resin viscosity reached 300 CP (1 h 45 min). The resin was cooled and stored frozen until the analysis. Resin B was a commercial wood particleboard binder P F resin (Borden 83PB), and its manufacturing procedure was expected to be quite different from that of resin A because of its different use properties. The molecular weight distribution results obtained on a Sephacryl gel permeation chromatography (GPC) column using 0.1 N sodium hydroxide solution, according to a reported procedure (Kim et al., 19901, also showed significant differences (Figure 1). Acetylation was performed according to a known procedure (Woodbrey et al., 1965) with minor modifications as follows: A 300-g amount of liquid resin was freeze-dried to a syrup and dissolved in 400 g of a 1:l pyridineDMSO mixture, and the solution was stirred in an ice bath. A 300-g amount of acetic anhydride was dropped in over a period of 1h, and the reaction was continued for 2 more h in an ice bath and overnight at room temperature. The reaction mixture was then evaporated on a rotary evaporator at 40 "C, and the residue was taken up in 500 mL of chloroform, washed with brine and water, and dried on anhydrous sodium sulfate. Evaporation of the solution resulted in 162 g of product from resin A and 112 g of product from resin B. In order to separate out the low molecular weight components that could unduly complicate the fractionation process, the acetylated products were dissolved in 500 mL of ethyl acetate, and 500 mL of anhydrous ether was slowly added to this solution while it was vigorously stirred. The ether-insoluble precipitates were collected and used for fractionation,and ether-soluble low molecular weight components were recovered by evaporating the solvent (Table I). Fractionation of Acetylated Resin A. When the precipitation procedure used for resin B (below) was tried for resin A, the fractionation work became very difficult to perform. The amounts of solvent that needed to be distilled off for a reasonable amount of precipitates were small and variable, resulting in erratic amounts of precipitates. The hexane additions were similarly very difficult to control. Therefore, a column procedure was used as follows. A 36-g portion of ether-insoluble acetylated resin A was dissolved in 400 mL of chloroform, and the solution was mixed with 1600 g of cleaned sand. The chloroform was evaporated, while the sand was mixed
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1153 continuously with a spatula to result in a uniform resincoated sand that was then packed in a 5.0-cm (diameter) glass column. After the column was washed with 400 mL of hexane, fraction AI, was collected by eluting with a mixed solvent of hexane and toluene; the toluene content in hexane was increased until the ratio of hexane to toluene reached to 4~96.About 1L of mixed solvent was used for fraction A,,,. Then, while the hexane content in toluene was gradually reduced to zero, the elution was carried out by collecting 200-mL aliquots. Each aliquot was evaporated right away on a rotary evaporator to obtain the polymer, which was combined, if necessary, with the next aliquot to secure a workable amount of polymer for each fraction. The elution volumes for each fraction were: 400 mL (A9),200 mL (A8), 400 mL (A,), 400 mL (A& 800 mL (A5), 1200 mL (A4),and 1200 mL (A3). Fraction A2 was then collected by eluting with 500 mL of 1.0% chloroform in toluene. The residual polymer was collected as fraction AI by washing with chloroform (Table I). Fractionation of Acetylated Resin B. A 40-g portion of ether-insoluble product of resin B was equilibrated at 23 "C overnight in a mixed solvent of 1500 mL of benzene and 3500 mL of toluene. The insoluble fraction was isolated as fraction B1. Fractions B2+ were successivelyobtained by partially distilling out the solvent and equilibrating the residual solution overnight at 23 "C. The solution volumes reached when each fraction was collected were 4100 mL (B2),3100 mL (B3),2300 mL (BJ, 1300 mL (B5),and 600 mL (Be). The residual solution was diluted to 1900 mL with toluene, and fractions B7+ were collected after precipitating with hexane, heating the mixture to 60 "C, and equilibrating overnight at 23 "C. The amounts of hexane added in succession to obtain each fraction were 100 mL (B,), 210 mL (Be),and 400 mL (B9). Fraction Blo was obtained by evaporating the whole solution (Table I). 'H NMR spectra were obtained on a Varian EM-360 on 6.3% concentrations in CDC13with TMS as the frequency reference. 13C NMR spectra were obtained on a Varian CFT-20 on 26.0% solutions in the same solvent. All I3C NMR solutions were low in viscosity except fractions A, and B1, which were in a fairly mobile range of about 100 cP. The decoupler was gated *on" during the acquisition (0.892 s) and "off" during the delay (12.0 s) to suppress the nuclear Overhauser effect (NOE). The pulse time was 18 p s (90"). Between 4000 and 5000 scans were accumulated, and peak areas were integrated by using expanded spectra. Solid-state I3C NMR spectra were obtained on a General Electric S-100 spectrometer using standard cross-polarization techniques with magic angle spinning, observing I3C at 25.2 MHz and decoupling 'H at 100.2 MHz. The carbon and proton spins were contacted for 2.0 ms with an effective power of 12 G, followed by signal acquisition for 25 ms as 512 data points and a 1.5-9 recovery delay. The sum of 4000 transients was treated with a 10-Hz exponential line broadening, zero-filled to 2048 data words and Fourier transformed. The spinning was at 3500 rps. Spinning side bands from the aromatic carbons were included in the aromatic carbon integrals. Several simple acetylated model compounds were prepared by acetylating the corresponding phenolic compounds. Compounds B, C, D, and F were isolated in impure form from the ether-soluble fraction of acetylated resin B by silica gel column chromatography using ether-hexane as eluent. Chemical shift assignments for these model compounds (Table 11) were used to identify the compounds. Intrinsic viscosity was measured by using a CannonUbbelohde four-bulb shear dilution viscometer designed
A
B C D
E F G H I
151.10 149.50 150.77 148.97 148.90 147.54 147.65 146.20 146.22
121.70 128.70 121.86 127.57 128.48 129.44 128.17 129.17 129.14
129.40 129.60 129.47 130.70 130.20 130.48 130.04 130.07 130.04
125.60 126.17 134.07 126.11 134.14 134.30 138.13 138.43 138.75
130.36
122.93
131.60 129.50
122.37 122.96
130.81
122.83
131.04
129.14
o-CH2*OAc,61.1-61.8 ppm; -CO*OAr, 169.2-168.6 ppm; p CH2*OAc, 65.2-65.5 ppm; -CH,, 20.2-20.6 ppm; -CO*OCH2, 170.5-170.7 ppm. (I
for minimizing the kinetic effect (Cannon Instrument Co., State College, PA; catalog no. CUSDC) at 25.0 f 0.1 "C. The duplicate measurements usually agreed within 0.2 s with an efflux time range of 45-120 Y for four 10-mL dilutions, which started with a 15.0-mL stock solution containing 0.25-0.50 g of polymer. Since no shear-rate dependence was observed in this range, the eight data points were averaged. From the solution densities measured with a pycnometer, four specific viscosities, qBp,were obtained from the four dilution efflux times, and Huggins' plots were constructed with respective volume concentrations from the equation qsp = [ q ] + k'[qI2c. Least-squares regression analysis resulted in the Huggins' constant (k9 and intrinsic viscosity ( [ q ] )with 1.2 values greater than 0.98 in most cases. The number average molecular weight was measured in acetone at 37 "C with a Hewlett-Packard Model 302B vapor pressure osmometer (vpo) using benzophenone as a calibration standard (Bersted, 1973). The duplicate, four dilution measurements were fitted to a linear regression equation to obtain the molecular weight. Most r2 values were greater than 0.90.
Results and Discussion Fractionation. The fractionation results (Table I) appeared to be satisfactory for resin B, as indicated by the comparative vpo molecular weight data (Table IV). The column fractionation used for resin A appeared to have occurred satisfactorily in the narrow molecular weight range that was expected from the GPC results (Figure 1). But the attempted Mark-Houwink correlation of resin A fractions (Figure 5) indicates that the higher solubilities of early fractions in comparison to those of later fractions were principally due to their structural features rather than due to their lower molecular weights. NMR Results. Solution 13CNMR spectral assignments of acetylated model compounds (Table 11) were made parallel to those of corresponding unacetylated compounds from earlier reports (Sojka et al., 1979; Kim et al., 1990). 'H NMR assignments with the attendant calculation
1154 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 Table 111. NMR Data of Acetylated P F Resin Fractions' CH,O/Ar CH2/Arb H" CNMR HNMR ~NMR A, 0.94 0.95 1.26 0.89 1.24 1.01 0.97 0.88 A; 1.01 0.84 1.05 A8 0.88 0.78 0.88 0.94 0.87 A, 0.64 1.06 1.27 0.77 A, 1.20 0.60 0.47 0.66 B1 B4 B8
BEL
B,
0.58 0.50 0.65 0.86
0.52 0.58 0.64 0.84
1.05 0.89 1.05 0.59
0.72 0.72 0.83 0.57
CH,~
O U D ratio.
I
[UBI
d
('/&)/ArmC
PP
Po
00
0 0 0 9 0 0.35 0.50 0.57 0.52 1.09
0.26 0.10 0.06 0.33 0.19 0.88 0.54 0.41 0.18
1.26 1.04 1.05 1.22 1.06 1.38 1.29 1.35 0.91 1.33
33 26 27 31 36 30 27 27 30 35
39 39 41 42 43 47 42 42 46 40
29 36 32 27 21 23 31 31 24 25
-0.07
a Abbreviations: AI, resin A fraction 1, etc.; EL,ether-insoluble fraction; E ,,, ether-soluble fraction; Ar, phenol ring; [U,],unsubstituted Cp,4,6 per phenol ring; d , term defined in eq 1; pp:po:oo, methylene group orientation. bMethylene peaks were broad. 'The ratio obtained of the total aromatic carbon divided by the phenolic carbonyl carbon intensity. from
methods were reported by Woodbrey et al. (1965). Analogous calculation methods can be used for 13CNMR results, and refinement is accessible by the added resolutions (Kim et al., 1990). Typical 'H and '3c NMR spectra (fraction B,) are shown in Figure 2a,b with the chemical shift assignments made on the basis of the model compounds. All resin fractions were well represented by the model compounds, and their structures were qualitatively in accord with the structure of polymeric methylene (hydroxymethy1)phenols. However, although the 13CNMR spectra were obtained under optimized conditions, the results (Table 111)were not straightforward to interpret because of the inconsistenciesobserved among integration values of various carbons. The number of hydroxymethyl or methylene groups per aromatic ring was found to be appreciably lower when they were calculated from the intensities of the respective groups than when phenolic carbonyl group values (ArOCO*-) were used as the basis of the phenolic ring. The abnormally higher intensities of aromatic carbons reported as the ratio of aromatic carbon intensity (X'/s) to phenolic carbonyl carbon intensity may be partly due. at least for resin B fractions, to the unreacted ortho ar.U para carbons, which can affect the 13Crelaxation and NOE processes under the experimental conditions used. In calculating the numbers of hydroxymethyl or methylene groups per phenolic ring, the phenolic carbonyl intensities were used, as this method resulted in more reasonable values. Nonetheless, the deviations were still significant for certain fractions. Since cyclic products are unlikely to be present in these fractions, the extent of deviation ( d ) was estimated by eq 1,where d = 3.0 - 2(CH2/Ar) - (CH20H/Ar) - [U,] (1) CH2/Ar is the number of methylene groups, CH20H/Ar the number of hydroxymethyl groups and [U,]the number of unreacted ortho and para carbons, respectively, per phenolic ring. The causes of the varying d values observed are not clear but appear to be related to molecular associations that occur even in dilute solutions (see below). Depressed signals occur in 13CNMR spectra of polymers when molecular motions are restricted, as in cross-linked polymers (Ford and Balakrishnan, 1983) or in severely branched polymers (Andreis and Koenig, 1989). These types of polymers show peak broadening in 'H NMR spectra. The methylene peaks in 'H NMR spectra of higher molecular weight fractions of resins A and B were broad (0.7 ppm), resulting in CH2/Ar ratios appreciably higher than unity. All resin fractions of this study were completely soluble in benzene or chloroform, and crosslinked structures are unlikely, but the extent of branching could be severe. In addition, resin A fractions are quite different from resin B fractions with deviation ( d ) values
1
0
180
E
s
180
7
140
8
120
5
100
3
4
80
80
2
40
20ppm
Figure 2. (a, top) Proton NMR spectrum of resin fraction B4 in chloroform. (b, bottom) Carbon-13NMR spectrum of resin fraction B4 in chloroform.
reflecting the certain structural differences. Solid-state I3C NMR spectra obtained for fraction Bz and B8 (Figure 3a,b) are very similar to the corresponding solution spectra and reflect the structural features of polymeric methylene (hydroxymethy1)phenols. The observed values for the CH2/Ar ratio, 1.10 (fraction B,) and 1.00 (fraction BJ, are very reasonable in view of the various factors involved that affect the quantitative aspects of this NMR method (Maciel et al., 1981). A more deteiled study is needed for probing the accuracy or the presence of cross-links, which is indicated at least for fraction Bz. The degree of polymerization, n,can be calculated from the CH2/Ar ratio or other group ratios (Woodbrey et al., 1965; Kim et al., 1990). However, the solution 13C NMR results for these values were not accurate or consistent
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1155 Table IV. Intrinsic Viscosity (IV, mL/g), Molecular Weight (M,,), Specific Volume (SpV, mL/g), and the Hug%ns' Confitant (k') for PF Resin Fractions in Benzene (B) and Chloroform (C) (25 O C ) " in benzene in chloroform fraction IV SPV k' IV SPV k' CIB M. 3.85 5.19 6.85 9.36 4.23 4.64 4.58 4.89
AE, Aa A4 A1
BE, B8 B4
Bl
0.793 0.775 0.776 0.769 0.793 0.788 0.779 0.800
0.79 1.07 0.74 0.73 2.60 1.32 3.02 4.36
6.97 6.00 8.76 12.86 5.76 5.24 6.44 7.47
0.762 0.752 0.769 0.776 0.803 0.769 0.801 0.733
1.88 1.96 1.06 1.01 1.46 0.98 1.23 0.99
1.81 1.16 1.28 1.37 1.36 1.13 1.41 1.53
4300 4610 4690 6070 1650 3060 3230 5600
"SpV, values obtained as the average between the first and last dilutions in IV measurements that were in the range of (10-46) X 10" g/g in benzene and (5-25) X g/g in chloroform; k', constant obtained in most cases with R2 values greater than 0.98 in least-squares regression plots; C/B, intrinsic viscosity in chloroform/in benzene; M,,vapor pressure osmometric molecular weight with most values obtained with R* values greater than 0.90 in least-squares plots. EL,ether-insoluble fraction.
3w
200
1W
0
.lo0 PPM
Figure 3. (a, top) Solid-state NMR spectrum of resin fraction B.I (b, bottom) Solid-state NMR spectrum of resin fraction Bs.
enough for these equations to be useful when compared with the vpo molecular weight data (Table IV). The greater than unity CH2/Ar ratios in '3c or 'H NMR results need to be further examined in the future with respect to the presence of cross-links by varying NMR experimental conditions. Maciel et al. (1984) suggested the formation of ether bonds through phenolic hydroxyl groups and the emergence of methine groups between phenolic rings in cured P F resol resins based on the observation of certain broad peaks in solid-state NMR spectra. Although observation of these bonds in high molecular weight fractions of this study would confirm their findings, none were shown in
the solution or solid-state '3c NMR spectra. An interesting observation is that the phenolic carbonyl peaks are resolved into three distinct peaks for fractions A4 and A8, which have low d and low [U,]values, as shown by peaks pl, p2,and p3 in Figure 4. On the basis of the observation of numerous other similar spectra, these three peaks were tentatively assignable, as shown, to the phenolic ring structures that are substituted by various numbers of methylene groups. Hydroxymethyl groups are also often separated into two peaks, cla and Clb, as shown, which correspond to the suggested phenolic ring structures. The p1:p2:p3peak area ratios were approximately 1:21 in these fractions, which, granted the tentative assignments, indicate that the polymer structures have these ratios for the linear, terminal, and branched phenolic ring segments. Further study is needed on this carbon as well as on substituted phenolic ring carbons to obtain the extent of branch structures. Resin B fractions did not show such resolutions for phenolic carbonyl peaks, probably because of the presence of unreacted ortho or para carbons. Intrinsic Viscosity and Molecular Weight. Intrinsic viscosity is sensitive to the presence of cross-links or branching (Longi et al., 1969). The intrinsic viscosity values obtained for resin A and B fractions (Table IV) increased as the vpo molecular weight increased, and their magnitudes are in a comparable range with those of PF novolac resins (Tobiason et al., 1972; Kamide and Miyakawa, 1978). Furthermore, the intrinsic viscosity values were greater in chloroform (C) than in benzene (B),i.e., the hydrodynamic volume expanded. The intrinsic viscosity ratio in these solvents (C/B), an indicator of molecular expansion, increased as the molecular weight increased. The magnitudes of the C/B ratio increases were also comparable to the values observed in novolac PF resin fractions (Tobiason et al., 1972),which are known to have no cross-linked components. Therefore, resin A and B fractions appear to be devoid of cross-links. Furthermore, resin B fractions showed more expansion than resin A fractions, which appears to indicate that resin B fractions have stronger molecular association interactions in benzene. Huggins' constant normally ranges from 0.3 to 0.4 for linear polymers (Flory, 1953). The Huggins' constants obtained (Table IV) increased as the molecular weight increased and were generally higher, and especially high for resin B fractions in benzene. The molecular association in resin B fractions must be increasing more rapidly as the concentration increases, a reflection of the polymer structures that favor the solute-solute interactions. The more variable 13C NMR intensity anomalies of resin B fractions appear to be related to this tendency of molecular associations that can restrict the molecular motions.
1156 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991
!
(Ih
I
OXi
OAC
CLC
I
I
I
I
1
I
I
173
170
167
60
50
~
1-
~
~~
40
I
30 PPm
Figure 4. Carbon-13 NMR spectra of carboxyl, hydroxymethyl, and methylene groups of resin A fractions.
low molecular weight fractions. These property differences observed between resin A and resin B fractions appear in line with the structural differences expected of the synthetic procedures. S
5t 1x10~
6 1
I
2x103
3x103
I
1
5x103
I
l
l
1
1
104
Mn
Figure 5. Mark-Houwink plot of resin A and B fractions.
The vpo molecular weight and intrinsic viscosity data of resin B fractions were plotted by using the MarkHouwink equation in eqs 2 and 3 (Figure 5). The “a” [q] = [q] =
1.196Mn0*213 resin B in CHC1,
(2)
resin B in C6H6
(3)
1.735M,0.120
1650 < M, < 5600 value of 0.21 obtained in chloroform indicates a compact branched structure, which evidently becomes more so in benzene with an “an value of 0.12. The data for Resin A fractions, however, resulted in poor correlations. In practical terms, resin A fractions showed reductions in intrinsic viscosity similar to those of resin B fractions, but their molecular weight values did not show a similar reduction. This incongruence appears to indicate the nonuniform structures of resin A fractions: the low molecular weight fractions are more compact than its higher molecular weight fractions, and if the extent of their solutesolvent interaction is assumed to be similar among the fractions, more branched structures are indicated for the
Conclusion The polymer structure of the high molecular weight fractions of PF resol resins can be described as polymeric methylene (hydroxymethy1)phenols from NMR results. However,. the functional group ratios were not able to describe the structures quantitatively, revealing the limitations of the method used. And the different NMR deviations observed between fractions of resin A and resin B apparently are due to the differences in polymer structure. Resin A fractions showed less intrinsic viscosity increases than resin B fractions in a solvent change from benzene to chloroform, and Mark-Houwink correlation results indicated a nonuniform molecular structure for resin A fractions in that the higher molecular weight fractions showed appreciably higher intrinsic viscosities than the low molecular weight fractions, even though they all remained in a narrow molecular weight range. This structure difference appears to indicate that the degree of branching is greater for lower molecular weight fractions than for higher molecular weight fractions of resin A. Resin B fractions were typical compact polymers in benzene and chloroform. The high, varying Huggins’ constants observed for resin B fractions indicated the higher tendency of molecular associations and appeared to give rise to the varying 13C NMR results. The differences observed in this study between resin A and resin B appear to be in line with the structures expected of the synthetic procedures and provided a useful methodology for future studies of PF resol resin properties with respect to the performance and synthetic procedures. Acknowledgment
We appreciate the many helpful discussions with Dr. Fred Tobiason.
1157
I n d . Eng. Chem. Res. 1991,30, 1157-1165
Registry No. A, 122-79-2; B, 1575-87-7; C, 2937-64-6; 38782-65-9; E, 2937-70-4; F, 2219-90-1; G, 132492-03-6; 132492-04-7; (PhOH)(HCOH) (copolymer), 9003-35-4.
D, H,
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PROCESS ENGINEERING AND DESIGN Reactor Simulation Studies of Methane Oxidative Coupling on a Na/NiTi03 Catalyst Jesus M.Santamaria,?Eduardo E. M i r o , t and Eduardo E. Wolf* Chemical Engineering Department, University of Notre Dame, Notre Dame, Indiana 46556
A reactor-reaction model for the oxidative coupling of methane on a 1.6% Na/NiTi03 catalyst has been developed. The reaction was assumed to take place both in the gas phase and on the catalytic surface. Kinetic rate constants experimentally obtained under differential conditions were used in a four-species kinetic model. Simulation solutions of the external field and particle equations for the temperature and concentrations were achieved by using the orthogonal collocation method combined with a Runge-Kutta procedure. The model predicted fairly well integral experimental results under various reaction conditions, and it was used to investigate the effect of several operating variables on the conversion and selectivity obtained in the methane oxidative coupling process. A simulated distributed oxygen feed system was found to improve hydrocarbon selectivity and yield at the reactor exit. Introduction In the past decade, the oxidative coupling of methane to ethane, ethylene, and higher hydrocarbons has attracted
* To whom correspondence should be addressed. 'Present address: Department of Chemical ~ ~ ~ University of Zaragoza, 5009 Zaragoza, Spain. $ Present addrese: INCAPE, Universidad Nacional del Litoral, Sgo.de1 Estero 2829 3000 Santa Fe, Argentina. 0888-5885/91/2630-ll57$02.50/0
the a t t e n t i o n of m a n y laboratories in the world, as evidenced b y the number of review papers on the subject (i.e., Bhasin, 1988;Scurrel, 1987; Lee and Oyama, 1988). In s p i t e of t h i s effort, m a n y aspects of the catalyzed process are not yet well understood, and the highest yield reported ini the literature ~ ~ up~to date ~ (about i 25%) ~ is ~Still too , low to m a k e the process commercially feasible.
Several experimental studies have been reported on the development of active and selective catalysts for the 0 1991 American Chemical Society
