1738
Ind. Eng. Chem. Res. 1987, 26, 1738-1743
Supercritical Gas Extraction of Wood with Methanol in a Tubular Reactor Michel G. Poirier, Aziz Ahmed, Jean-L. Grandmaison, and Serge C. F. Kaliaguine* Department of Chemical Engineering, Lacal I 'niuersity, Ste-Foy, Quebec, Canada G1K 7P4
A tubular type reactor has been designed and built for the supercritical extraction of Populus tremuloides in methanol. A Box-Behnken design, employing 15 experiments, is used to investigate the effects of 3 independent variables (temperature T , pressure P, and solvent flow rate D)on wood conversion and oil yield. Two corresponding regression equations have been calculated for values
of T ranging from 250 to 350 "C, P from 3.4 to 17.2 MPa, and D from 0.5 t o 2.5 L j h . The temperature explained 90.2% of the total sum of squares for the wood conversion, whereas for oil yield temperature and pressure contribute 46.3% and 49.2%, respectively. Gel permeation chromatography has been performed on oils. It is shown t h a t for a given extraction temperature, an increase in pressure corresponds simultaneously to higher oil yield and higher molecular weight. A t constant pressure, increasing the temperature results in higher oil yields and lower molecular weights. The nature of oils as a function of the temperature of extraction is also discussed. Supercritical gas extraction (SGE) is one of the various processes currently under study for the direct liquefaction of biomass. Wood liquefaction usually yields very complex mixtures which have found little utility yet because, due to their large oxygen content, their heating value is usually low and they may show acidic corrosive properties. The potential of such processes lies in the separation of various fine chemicals from these mixtures. It is therefore of primary importance to develop analytical techniques relating the operation conditions of a liquefaction process and the chemical nature of the liquids produced. In SGE of Populus tremuloides with methanol, by use of a continuous stirred tank reactor (CSTR) (Labrecque et al., 1984), it was found that temperature and pressure are the two main parameters governing the process of wood conversion and oil yield. The solvent flow rate has also some effect, as it affects the residence time of the solvent and of dissolved oil in the reactor. A decrease in solvent flow rate favors recombination of some fragments in the oil onto the char, corresponding to a decrease in oil yield and an increase in char. The present paper reports new results for the SGE of Populus tremuloides in methanol obtained by using a tubular reactor in which recombination should be avoided due to smaller residence time of the dissolved oil. In this study, the emphasis is on the statistical analysis of the wood conversion and oil yield data and on the GPC characterization of the oil produced. Such an approach should allow correlation of chemical information. namely, the molecular weight distribution, with the operating conditions of the extraction process.
Experimental Setup and Procedures Figure 1 shows a diagram of the experimental setup. Thirty grams of wood chips are loaded in a basket (13) placed in a 150-cm' tubular reactor (12) (Autoclave Engineers). The tubular reactor is first filled with cold solvent and hermetically closed. The cold solvent is then continuously fed by using a plunger pump (2) (Milroyal), the reactor is isolated from the circulating solvent by means of three pneumatic valves (ll),and the solvent flows through a preheater autoclave (7) (Aminco) directly to the condenser (14) during the warm-up period of the autoclave. When the solvent reaches the experimental temperature, the pneumatic valves direct the hot solvent through the reactor. T w o expansion valves (16) allow an accurate control of the pressure in the reactor, and the liquid is 0888-5885I87 12626- 1738$01.50/0
collected by condensation at 0 "C. The gaseous fraction passes through a second condenser, and it is stored in a floating piston gasometer (19). The reactor is heated by an external heating element (6b) of 2.6 kW and the preheater autoclave by an external heating element (6a) of 3.9 kW. The pyrolytic oil is separated from the solvent by vacuum evaporation to dryness at 75 "C in a rotating vacuum evaporator (Buchi). GPC analysis of oils was performed by using an ALC/ GPC-201 liquid chromatograph (Waters Associates) equipped with a Model R-401 refractometer; the columns (30 cm x 7.8 mm id.) employed were 10-nm pstyragel, lo4, lo3, 500, and 100 A, and the mobile phase was redistilled T H F with a flow rate of 1.0 mL/min. The calibration curve was obtained by using the following standards: polystyrene 17 500, 9000, 4000, 2000, and 800); poly(ethylene glycol) 500 and 400); and eugenol, vanillin, and guaiacol ( M , 164, 152, and 124, respectively). The solvent collected during vacuum evaporation was analyzed for formic and acetic acids as their methyl esters by using a Perkin-Elmer Sigma 115 gas chromatograph equipped with a Porapak-Q column (6 ft x 'la in. 0.d.) and a FID detector. The solid residues from the experiments were recovered following the procedure described by Labrecque et al. (1984), and detailed results concerning their chemical characterization and their contents in carbohydrates, lignin, and recondensed material were reported elsewhere (Ahmed et al., 1986). The gas fraction was not further analyzed. Statistical Analysis. An optimum statistical design, a Box-Behnken design (Box and Behnken, 1960), was used to investigate the effects of three independent variables (temperature T , pressure P , and flow rate D) upon the conversion of wood into gas, pyrolytic oil, and char. This procedure allowed the determination of a regression equation describing the results of supercritical extraction of P. tremuloides in methanol. As shown in Figure 2, the limit values in the Box-Behnken design were 250 and 350 "C for T , 3.4 and 17.2 MPa for P , and 0.5 and 2.5 L/h for D. This statistical design is an incomplete 3 x 3 factorial which leads with only 15 experiments to the determination of the first-order ( T ,P, and D), the second-order (TT,PP, and D D ) and the double-interaction ( T P , T D , and P D ) effects for the three independent variables, yielding 9 parameters. The calculations were performed by using the
(a,
(aw
C 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1739 Table I. Experimental Conditions a n d Results for t h e SGE Extraction of Populus tremuloides i n Methanol (% Are Meant for w t % on Dry Wood Basis) expt 22 25 17 15
T, "C
P, MPa
D, l / h
OY,%
c, %
wc, %
FA, %
AA, 70
250 250 250 250
3.4 10.3 10.3 17.2
1.5 0.5 2.5 1.5
11.7 21.4 28.0 25.8
74.8 70.7 69.7 68.4
25.2 29.3 30.3 31.6
0 0 1.14 0.96
3.81 1.74 5.03 4.25
1050 1060 1820 3350
16 20 9 21 12 23 18
300 300 300 300 300 300 300
3.4 3.4 10.3 10.3 10.3 17.2 17.2
0.5 2.5 1.5 1.5 1.5 0.5 2.5
19.5 18.0 42.1 39.9 42.0 44.0 49.8
40.5 31.0 16.4 15.7 15.9 22.4 15.0
59.5 69.0 83.6 84.3 84.1 77.6 85.0
1.56 0.92
3.71 1.86
2.42
4.53
2.42 4.40
3.62 5.63
840 590 650 750 810 720 1540
27 14 11 8 24
330 350 350 350 350
17.2 3.4 10.3 10.3 17.2
1.2 1.5 0.5 2.5 1.5
55.8 30.2 46.0 57.5 54.5
8.4
91.6
26.9 18.2 10.2 6.1
73.1 81.8 89.8 93.9
4.81 5.35
5.25 2.73
6.70
6.46
M
W
470 420 690 830
.-j ' L
4 Figure 1. Schematic description of the experimental setup: feed tank (1);plunger pump (2); surge tank (3); pressure gauge (4); thermocouples (5); preheater autoclave heating element (6a); reactor heating element (6b); preheater autoclave (7); magnetic stirrer (8); heating controller (9); valves (10); pneumatic valves (11); tubular reactor (12); basket (13); condenser (14); filter (15); expansion valves (16); collecting flask (17); mercury contact (18); floating piston gasometer (19).
Statistical Analysis System software (SAS User's Guide, 1979) on an IBM 4381 (C.T.I., Universit6 Laval).
Experimental Conditions and Results All results reported here were obtained by utilizing wood from the stem of a 20 year old P. tremuloides. This wood was cut into small blocks (5 x 5 X 2 mm) and dried for 24 h at 95 "C in an oven. Table I reports experimental conditions and results for the whole set of experiments. A combination of the nine temperatures, pressures, and flow rates was used. The limit of 350 "C was imposed by the thermal cracking of methanol, which was observed at temperatures of 370-380 "C (Labrecque et al., 1984). The oil yield OY, defined as the weight percent of pyrolytic oil recovered with respect to the mass of original dry wood, ranges from 11.7% up to 57.5%. The lower value (11.7%) was obtained at lower temperature (250 OC) and a t lower pressure (3.4 MPa), whereas the higher value (54.5%) was obtained, for the same flow rate, at the higher temperature (350 "C) and higher pressure (17.2 MPa). The wood conversion WC was defined as wc = 100 - c (1) where C is percentage char expressed as the weight percent of solid recovered in the reactor over the mass of the or-
2 50
T
,*c
350
Figure 2. Box-Behnken experimental design.
iginal dry wood. The conversion was 25.2-31.6% a t 250 "C and reached 59.5-85.0% at 300 "C and 73.1-93.970 at 350 "C. The data corresponding to experiments 9, 21, and 12 performed in identical conditions at the central point of the Box-Behnken experimental design allow appreciation of the reproducibility of these results. For these three experiments, oil yields range between 39.9% and 42.1% and wood conversions from 83.6% to 84.3%. This reproducibility seems therefore reasonable. The figures reported for the contents in formic acid (FA) and acetic acid (AA) are expressed in weight percent of each of these species with respect to original dry wood calculated from the results for formic and acetic esters in evaporated methanol. For a given flow rate, the production of formic acid increases rapidly with temperature; at 1.5 L/h, for example, the value was below 1.14% at 250 "C but reached 4.81-6.70% at 350 "C. On the other hand, the supplementary production of acetic acid was small at temperatures higher than 250 "C, with values of 3.81-4.1570 at 250 "C and 5.25-6.4670 at 350 "C. Finally, the sum of OY, C, FA, and AA represented the identified part of the products with respect to the initial material. The missing fraction was associated to the volatile and light compounds formed during the experiment, presumably water, methanol, etc., and not isolated or identified; water is among these volatile compounds, as
1740 Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987
the water content of the oils is typically below 0.5% as determined by the Karl Fischer analysis. This whole fraction is designated as nonquantified volatile fraction
(NQV). A regression was performed on the OY and WC experimental values of the 15 experiments, and the Fisher F-test was used to determine which of the 9 parameters have a significant effect on the dependent variables. The regression on OY displays four significant effects at the 10% confidence level of the F-test: the linear effect of temperature (T)contributing 46.3% to the total sum of squares; the linear and quadratic effects of pressure (P and PP) contributing 40.5 and 8.7%, respectively; and the solvent flow rate (D) contributing only 2.3%. The regression equation is OY = 39.557 + 12.663T* + 11.838P* + 2.8000* - 7.870P*P* (2) where T* = T - 300 50 ~
p * = P - 10.3 6.9 ~
D* = D - 1.5 1.0
1
~
The maximum deviation of the experimental results of OY to the regression calculated limit is 5.1%. The regression on WC shows that all nine effects, except for the PD interaction, are significant a t the 10% confidence level of the F-test. This large number of significant parameters is associated with the small value obtained for the variance of the residual error (2.88 for the 5 df of the error compared to a variance of 957.17 for the 9 df of the model: df = degrees of freedom). In such circumstances, the formal choice of significant variables may include parameters with rather small contributions to the sum of squares. In the present case, the more important are however the temperature terms: the linear and quadratic effects of temperature (T and TT) explaining 71.6% and 18.6% of the sum of squares. The regression equation is WC = 84.000 + 27.775T* + 7.663P* + 3.238D* - 21.513T*T* - 6.538P*P* - 4.688D*D* 3.600T*P* + 1.750T*D* (3)
+
The maximum deviation of the experimental results of WC to the regression calculated value is 2.4%. Equations 2 and 3 may be used for calculations of all values of OY and WC inside the experimental domain. Parts A and B of Figure 3 show respectively the calculated values for oil yield (OYj and char (Cj expressed as weight percent of original dry wood, as functions of temperature for the three experimental pressures. Figure 4 shows examples of GPC chromatograms of SGE oils for experiments 22, 6, 17, and 15 (experiment 6 was in the same experimental conditions as experiment 25; see Table I). These results indicate dramatic changes in molecular weight distribution as the extraction pressure is increased. From GPC chromatograms, one can calculate the mass of oil constituents within a given molecular weight range per unit mass of original dry wood. Such data are reported in cumulative form in Figure 5 which shows the evolution of the molecular weight distribution with both extraction pressure and temperature. From the GPC chromatogram, one can also calculate the weight-averaged molecular weight (MW) of each oil. The values of MW are reported in Table I and presented in Figure 6 as a function of oil yield. A regression analysis was also performed for the.Mwdata. At the 10% confidence level of the F-test only, three parameters were found to have significant effects: the effect of temperature ( T ) , pressure (PI,and interaction (TP)contributed respectively
!
250
I
1
350
300 TEMPERATURE T
*
, O C
t
60
-
c I
8. 5c u
4 0 1
a
1
3012o IC
r -
,
1
250
300 TEMPERATURE T I " C
350
*
Figure 3. Regression analysis of experimental results for (A, top) oil yield and (B, bottom) char yield.
38.3%, 19.7%, and 12.2% to the sum of squares. The regression equation is
M w = 1037.3 - 608.7T* + 436.2P* - 485.OT*P* (4) Figure 6 shows clearly the interrelation between ATW,OY, T, and P. For a given extraction temperature, an increase in pressure corresponds to higher oil yields and higher average molecular weights. At constant pressure, increasing the temperature results in higher oil yields and lower molecular weights.
Discussion As the temperature of extraction increases, the overall wood liquefaction process is more effective as indicated by the linear increase in oil yield (OY) with temperature (Figure 3A); this is also reflected by the fact that wood conversion (WC) increases with temperature specially in the range 250-300 O C . Moreover, whatever the temperature, an increase in pressure from 3.4 to 10.3 MPa corre-
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1741 b
MOLECULAR WEIGHT ( M w l 10000 5000 1
1000
500 300
1
4000
-
3
'2
+ 3000I
(3 W
3 a
I
U
\
-I I) 0
y 2000 0 5 W (3
U
a
A,
W
> a 1000
-
\/
Figure 6. Correlation of oil average molecular weight and oil yield.
,
ELUTION VOLUME , m$
Figure 4. Gel permeation chromatograms (GPC) of oils produced during four experiments a t 250 "C.
1m
6o
2 5 I/h
A
I
txpt
ne. a
114 '117 !
t
oil
110 , I
' 110
t
I Figure 7. Fast pyrolysis/molecular beam sampling/mass spectrometry (FP/MBS/MS). Analysis of oil from experiment 6 (250 "C, 10.3 MPa; m / z values in daltons).
250
300
350
TEMPERATURE ( T I , O C
Figure 5. Distributions of oil yields in molecular weight fractions: M , (a) 5000.
sponds to a very significant increase in oil yield (OY), whereas increasing the pressure from 10.3 to 17.2 MPa yields only a minor additional increase in oil yield. As will be discussed below, our various results concerning the chemical composition of the oil, as well as some analytical results obtained for the chars (Ahmed e t al., 1986), confirm our previous proposition that pressure increases the oil yield by increasing the dissolving power of the solvent (Kaliaguine et al., 1983). Oils Produced at 250 "C. It was reported earlier from GC/MS analysis of SGE oils obtained a t 250 "C that only a fraction of the lignin is broken down to fragments present in the oil along with pyrolysis products of hemicellulose (Grandmaison e t al., 1983). Evans e t al. (1983) have performed fast pyrolysis/molecular beam sampling/mass spectrometry on oil samples from our experiment 6 (250 "C, 10.3 MPa). Their results, reported in Figure 7, show
the presence of lignin fragments with peaks a t m / z 124, 138, 154, 168,180, 182, 194, 208, and 210 corresponding to various guaiacol and syringol derivatives, as well as peaks a t m / z 272, 302, 332, and 418 corresponding to guaiacol and syringol dimeric compounds (see Table I1 for identification). Although these results indicate mainly the presence of lignin in the oil produced a t 250 "C, we believe that products of hemicellulose decomposition are also present in this oil. Indeed hemicellulose is known to pyrolyze at 200-260 "C (Soltes and Elder, 1981; Hansen and April, 1982). Moreover, we have found that most of the acetic acid is produced below 250 "C (Labrecque e t al., 1984). This acid is known to derive mostly from pentose, especially xylose from hemicellulose (Brown, 1958). I t seems thus that although the analysis of the solid residues from our experiments has shown that both hemicellulose and cellulose are thermally degraded a t 250 "C (Ahmed et al., 1986), mostly hemicellulose-derived products are present in the oil in addition to guaiacol and syringol derivatives coming from lignin. I t is interesting to note from GPC analysis (Figure 4) and from the molecular weight distribution of the oils (Figure 5) that the increase in oil yield with pressure at
1742 Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 Table 11. Partial List of Products Identified in Oils, with an Assignment of Their Original Polymer ( m / z Values in daltons) mlz identification origin" H 86 butanedione H 96 furfural C furfuryl alcohol 98 5-methylfurfural C 110 C 112 5-methylfurfuryl alcohol 3-hydroxy-2-penteno-1,5-lactone H 114 or dianhydropentose 124 L guaiacol C 5-hydroxymethylfurfural 126 132 H 1,6-anhydropentose L 4-methylguaiacol 138 syringol L 154 levoglucosan C 162 4-methyls yringol L 168 coniferyl alcohol L 180 L syringaldehyde 182 L 194 4-propen yls yringol L 208 sinapaldehyde L sinapyl alcohol . 210 L 272 guaiacyl-like dimer (Cl6HI6O4) L guaiacyl-syringyl-like dimer (CI7Hl8O5) 302 L 332 syringyl-like dimer (C18H2006) L 418 sinapyl alcohol dimer " H = hemicellulose; C = cellulose; L = lignin.
250 "C is mostly associated with the extraction of higher molecular weight fragments as pressure is increased. It is believed that these high molecular weight fragments are lignin and hemicellulose derivatives retained in the matrix of the solid residues when pressure is not high enough for these compounds to dissolve. The increase in solubilizing power of a solvent when pressure is raised to and above its critical pressure is the basis for supercritical gas extraction. The increase in the molecular weight of the compounds extracted as pressure is increased may therefore be associated with the higher solubility of these compounds at high pressure. This suggests that molecular products of thermal degradation are generated in the solid matrix and submitted to the competitive actions of mass transfer following solubilization and recondensation reactions producing char, Increasing the pressure of extraction results in an increase of the solubility of the fragment of all sizes but in a more pronounced increase in the solubility of the higher molecular weight fragments. This results therefore in simultaneous increases in oil yield and oil average molecular weight and decrease in char yield. A similar discussion of the effect of solvent flow rate can be made. Increasing this flow rate results in a decrease in the instantaneous oil concentration in the solvent. This should increase the rate of extraction and have an effect similar to the one of an increase in pressure. This is confirmed by the statistical analysis of oil yield and wood conversion data which show that the parameters P* and D* are both significant and appear in eq 2 and 3 with coefficients of the same sign. D* however was not found to have a significant effect on A& (eq 4) presumably due to the narrow experimental range for the solvent flow rate. Oils Produced at 300 and 350 "C. As seen from Figures 6 and 7, the increase in oil yield when temperature is raised from 250 to 300 and 350 "C is associated with a decrease in average molecular weight of the oil component. From Figure 5 it can be seen that this decrease is mostly associated with an increase in the yield of the fractions with low molecular weights (fractions a + b), as the mass of the fractions with molecular weights higher than 1000 (fractions d + e) is only showing a minor decrease. This small decrease is mostly detectable a t a pressure of 17.2
194
oil
180 154
, expt
no. 8
, 210
Figure 8. Fast pyrolysis/molecular beam sampling/mass spectrometry (FP/MBS/MS). Analysis of oil from experiment 8 (350 "C, 10.3 MPa; m / z values in daltons).
MPa, and it is obviously associated with the thermal degradation of soluble polymers. The decrease in oil average molecular weight is however mostly determined by the dramatic increase a t temperatures of 300 and 350 "C of the mass of the oil fraction with molecular weights below 1000 (fractions a + b + c, on Figure 5). This fraction is associated with an additional production of lignin derivatives (Grandmaison et al., 1983) as well as with the degradation products of cellulose. Indeed, formic acid, which is considered a degradation product of hexoses (Goldstein, 1980), mainly appears at 300 and 350 "C (see Table I and Labrecque et al. (1984)). The appearance of compounds originating from cellulose in the oil can be further demonstrated by the result obtained by fast pyrolysis/ molecular beam sampling/mass spectrometry using our oil 8 (350 "C, 10.3 MPa). These results are reported on Figure 8 which, when compared to Figure 7, shows the appearance of peaks a t m / z 98,110, 112,126, and 162. As seen from Table 11, these compounds may be obtained by pyrolysis of hexoses. Obviously, as pointed out by Evans et al. (1983), the efficiency of detection of lignin derivatives by this technique is much higher than the one for carbohydrate derivatives, so that the spectra in Figures 7. and 8 cannot be used for a quantitative assessment of lignin products in the oils.
Conclusions With regards to oil yield, this study has led to the following conclusions: Oil yield increases linearily with extraction temperature over the range 250-350 "C. Increasing solvent pressure increases oil yield by raising the solubilizing power of the solvent. This effect is mostly significant within the range 3.4-10.3 MPa and becomes less important in the range 10.3-17.2 MPa. However, the main result of this study is the demonstration that the nature of SGE oil can be very much affected by the conditions of extraction and particularly by pressure. At 250 "C raising the pressure from 3.4 to 17.2 MPa yields a spectacular increase in the molecular size of the lignin and hemicellulose derivatives extracted from the reacting wood with molecular weights reaching 5-10000. This corresponds to an increase in oil yield from 10-12% to 25-30%. Simultaneously small lignin derivatives are present in the oil. At this temperature cellulose is probably already transformed, as indicated by char analysis (Ahmed et al., 1986), but its products are not released to the liquid phase. Raising the temperature to 300 and 350 "C improves the oil yield by subsequent conversion of the lignin and cellulose derivatives still present in the solid residue. This increase in oil yield is mostly associated with the extraction of smaller fragments, with molecular weights typically below 500.
I n d . Eng. Chem. Res. 1987,26, 1743-1746
In the whole range of temperature studied, the average molecular weight, Mw, of the oil is far below the M, of original cellulose, hemicellulose, and lignin in wood.
Acknowledgment This work was supported by the Natural Sciences and Engineering Council of Canada (NSERC) and by the FCAR fund. J.L.G. thanks the Ministere de la Science et de la Technologie (Qugbec) for support through its FSES fund. Registry No. CH,OH, 67-56-1.
Literature Cited Ahmed, A.; Grandmaison, J. L.; Kaliaguine, S. J . Wood Chem. Technol. 1986, 6(2), 219-248. Box, G. E. P.; Behnken, D. W. Technometrics 1960, 2, 455-475. Brown, F. L. “Theories of the Combustion of Wood and its Control” Report 2136, 1958; Forest Service, US.Department of Agriculture, Forest Products Laboratory, Washington, D.C.
1743
Evans, R. J.; Soltys, M. N.; Milne, T. A. “Fundamental Pyrolysis Studies”, Report SERI/PR-234-2286, 1983; Solar Energy Research Institute, Washington, D.C. Goldstein, I. S. Conversion of Carbohydrates to Basic Chemicals and Polymers by Chemical Approaches; Corn Refiners: Washington, D.C., 1980. Grandmaison, J. L.; Kaliaguine, S.; Ahmed, A. “Characterization of Pyrolytic Oils Produced by Dense Gas Extraction”, NRCC 23130, 1983; Sherbrooke, Qudbec, Canada. Hansen, S. M.; April, G. C. Znd. Eng. Chem. Prod. Res. Deu. 1982, 21(4), 621-626. Kaliaguine, S.; Daniel, H.; Grandmaison, J. L. “Production of Pyrolytic Oils from Aspen Wood by Dense Gas Extraction”, NRCC 23130, 1983; Sherbrooke, QuBbec, Canada. Labrecque, R.; Kaliaguine, S.; Grandmaison, J. L. Ind. Eng. Chem. Prod. Res. Deu. 1984, 23(1), 177-182. SAS User’s Guide; SAS Institute: Cary, NC, 1979. Soltes, E. J.; Elder, T. J. “Pyrolysis”, in Organic Chemicals from Biomass; Goldstein, I. S., Ed.; CRC: Boca Raton, FL, 1981.
Received for review April 29, 1986 Revised manuscript received January 5, 1987 Accepted April 20, 1987
Alkylation of o -Xylene with Styrene by Superacid Catalysts Rajeev A. Rajadhyaksha* and Dilip D. Chaudhari Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
Alkylation of hydrocarbons by styrene t o give 1-phenyl-1-arylethane is a n industrially useful class of reactions. In the present work, alkylation of o-xylene by styrene has been investigated on a variety of superacid catalysts, which include Nafion-H, triflic acid, and recently described “sulfate-treated’’ inorganic oxide catalysts. T h e reaction was also investigated on poly(styrenesu1fonic acid) cationexchange resin catalyst (CER) for comparison. Amongst the inorganic oxide catalysts, zirconia exhibited the highest activity. The activity of zirconia was comparable to that of Nafion-H and CER, in spite of t h e much lower concentration of acid sites. Selectively para-substituted product was obtained, and no formation of styrene dimer could be detected. T h e results demonstrate potential of zirconia catalyst for industrial use, particularly in view of its lower cost and high thermal stability. The alkylation of aromatic hydrocarbons with styrene to give 1-phenyl-1-arylethane is generally carried out using homogeneous Bransted or Lewis acid catalysts (Friedman and Patinkin, 1964). However, it suffers from the drawbacks such as formation of styrene oligomers and separation of soluble catalysts from the reaction mixture. Hence, use of heterogeneous acid catalysts for this system has clear advantages. In the present work, alkylation of o-xylene with styrene, which is an important reaction belonging to this group, has been investigated on a variety of heterogeneous superacid catalysts. The product phenylxylylethane (PXE) is used extensively as heat-transfer and hydraulic fluid, transformer oil, and electrically insulating oil (Grigor’ev et al., 1981). It has been employed as an ingredient of corrosion protective coatings of chlorinated rubber and chlorine-containing synthetic resins (Torii, 1973). It has also found applications in high energy fuel for jets, turbojets, rockets, missile engines, and high stability lubricants (Eldon, 1966). This reaction has been studied using various acid catalysts which include sulfuric acid (Malan, 1972), hydrofluoric acid (Mitsubishi, 1980), silica-alumina (Matsuzaka et al., 1978), prefluorinated resin sulfonic acid Nafion-H (Gotoh et ai., 1981), triflic acid cation-exchange resin Amberlyst-15 and boron trifluoride etherate (Hasegawa and Higashimura, 1980). Most of this information, however, is in the form of patents, and the only research paper
published on the reaction is by Hasegawa and Higashimura. Hasegawa and Higashimura obtained nearly quantitative and specific monoalkylation by employing acid catalysts like boron trifluoride etherate, Amberlyst-15, and superacid catalysts such as Nafion-H and triflic acid. The selectivity was found to depend greatly on the catalysts, especially in the reactions of less activated aromatics such as benzene, toluene, and p-xylene. The extent of the formation of styrene dimers as a byproduct was found to be less on solid acid catalysts. However, these workers have carried out the reactions at extremely low concentrations of styrene (styrene was 1.18% (w/w) of o-xylene in the initial reaction mixture), which is not suitable for industrial use. In the present work, the reaction was explored using recently introduced solid superacid catalysts based on inorganic oxides such as “sulfate-treated’’ zirconia, titania, and iron oxides. These superacid catalysts were shown to be highly active for various acid-catalyzed reactions such as isomerization, cracking, esterification, etc., under mild conditions (Arata and Wino, 1980, 1981). In addition to the advantages of ease of separation and reusability which are also offered by polymer resin catalysts, these catalysts have high thermal stability and low cost. The reaction was also carried out using Nafion-H, triflic acid, and CER for comparison. The reactions were carried out using a higher concentration of styrene.
08s8-5885/87/2626-1743$01.50~0 0 1987 American Chemical Society
