Ind. Eng. Chem. Rod. Res. Dev. 1081, 20, 109-114
109
a reversible second-order reaction model, with an apparent activation energy of 14.5 kcal/g-mol, in the temperature range of 350-450 OC. For the falling rate period, the activity of the catalyst can be described by the exponential decay function, B = &e-&, where m is approximately equal to 3 x and t is time on stream expressed in minutes. Acknowledgment Shashidhar Bhavikatti thanks the Indian Institute of Technology, Bombay, for the research fellowship. Literature Cited
\ 1
60
ieo
.
1
loo
I
240
I
aoo
Timr -on - s t r e o m (m;n>
I
-
I
*PO
Figure 7. Test for exponential decay of catalyst activity,
adequately, with the value of index m as 3 X wherein time on stream ( t )is expressed in minutes. Conclusions The kinetics of toluene disproportionation over 0.2 w t % nickel-loaded H-M(23) can adequately be described by
AI, M.: EcMgoya, E.: Ozakl, A. Bun. Jpn. Pet. Inst. lW5, 7, 48. Aneke, L. E.; Qewben, L. A,: Eibm, J.; Trkn, R. J . Cael. 1979, 59, 37. Beechor, R.; VoOmh, A., Jr.; Eberl~,P. E., Jr. Ind. €m. - chem.Frcd. k . Dev. 1968, 7 , 203. Iwamua, T.: Otani, S.: Sato, M. Budla Jpn. Pet. Inst. 1971, 13, 118. Izuml, Y.: SMba, T. Kogyo Kagaku ZassM 19W, 66, 1817. Levensplel, 0. "Chemical Reaotkn Engineering”, WRey Eastern m a t o Ltd.: New Dsehl. 1974. MI&, J.-N.; .&, N. Y.; welsz, P. B. J. Catel. iew, 6 , 218. Ogawa, D.: byedrl, S.; Matsumura, K.; Iwamua, T.: Sato, M.: Otanl, S. K w o &@ku .?asshi 1969, 72, 2185: C h m . Abstr. lW, 72,785405. Ruthven, D. M. J. Catal. 1972, 25, 259. Thbk, E. W. Ind. €ng. C!”. 1999, 31, 918. Vvendenskli, A. A., “Thermodlnamlcheskb raschetl neftekhknlchesklkh protesessoe”,Gtxtoptekhnkdat: Leningrad, 1980. Wang, K. M.; Lunsford, J. M. J . &tal. 1972, 24, 282. wheeler, A. A&. &&I. 1951, 3, 249. Yaahhkne, T.: Moskhi, H.: bra, N. BUR. Jpn. Pet. Inst. 1970, 12, 108. Yoshida, F.; Ramaswamy, D.: Hougen, 0. A. AIChEJ. 1962, 8, 5.
Received for reuiew February 8, 1980 Accepted August 8, 1980
C, to C4 Oxygenated Compounds by Promoted Pyrolysis of Cellulose Chung Chlh Hsu and A. Norman Hlxson’ Department of Chembl €nglneerlng, UnlversHy of Pennsylvania, Philade@hla, Pennsylvenla 19 104
A low-temperature process for cellulose pyrolysis to produce short-chain oxygenated hydrocarbons was explored. Experiments were conducted In a fixed-bed reactor using a helium gas stream at atmospherk pressure. With pure cellulose, the liquid product was mainly an aqueous solution of carbohydrate derivatives such as levoglucosan with small amounts of furans. Using a solid &OH-cellulose mlxture, the organic products were CH30H, C&l,OH, and 2-methyl-2-propen-1-01. An aqueous NaOH pretreatment of the cellulose produced hydroxyacetone as the p r a dominant organic product. Results with alkali pretreated cellulose at 280 O C gave 36% gas, 26% solid residue, and 38% liquids (of which 6% was organic). The gas was 64 mol % CO, 35% COO.
Introduction The shortage in the supply of organic chemical feedstocks from petroleum and natural gas has brought an increased interest in the potential of cellulose-containing residue as a substitute resource. The primary attraction is that it is naturally renewable and occurs in large amounts as a waste byproduct. The processes reported have involved either a biochemical approach or a thermal treatment using a variety of atmospheres and physical conditions. The work reported in this paper falls in the latter category. Pyrolysis of cellulose under an inert atmosphere occurs between 250 and 360 “C producing a permanent gas mixture of CO and COz, volatile compounds including water, levoglucosan, furfural, and other furans as major constituents, and a solid char residue. 0798-4321 I81 I1 220-01 09$01 .0010
High temperature gasification (Bailie and Ishida, 1972; Burton et aL, 1974)gave a gas product containing H2,C02, CH4,CO, and C2Hb High concentrations of hydrogen were obtained by concurrent steam reforming (Halligan et al., 1975). Pyrolytic experimentation at high pressures used hydrogen, carbon monoxide, or compounds such as tetralii to provide a reducing atmosphere with the objective of producing low molecular weight oxygenated compounds or oil (Boomer et al., 1935;Gurkan, 1945;Appell et al., 1975;Gupta et al., 1976). Nickel, zinc, and cobalt catalysts were used in these studies. A great deal of study has been made of the effect of additives as flame retardants in the pyrolysis of cellulose. For example, sodium carbonate (Madorsky et al., 1956), potassium bicarbonate (Wodley, 19711,and Lewis acids 0 1981 American Chemical Society
110
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
-
,001
2
1
,
,
,
7.W
140
280
320 360
TEMPERATURE ( O C )
u u
11
Figure 1. Fixed bed pyrolysis system: 1, valve; 2, flowmeter; 3, pressure gauge; 4, preheating column; 5, furnace; 6, tubular reactor; 7, thermocouple; 8, heating tape;9, water-cooled condenser; 10, liquid collector; 11, dry ice-cooled traps; 12, drying tube; 13, gas sampling tubes; 14,water column; 15,temperature controller; 16,power source; 17, recorder.
(Shafiiadeh et al., 1973) have been used. Most drastically reduced the production of volatile levoglucosan and enhanced char formation. Shafizadeh et al. (1973) used ground sodium hydroxide mixed with cellulose. Their interest was predominantly in the products formed in the reaction residue rather than in the volatile products. It was decided to explore the low-temperature pyrolysis of cellulose at atmospheric pressure to produce useful organic liquid products in the C1 to C4 molecular weight range. It was considered likely that a catalyst or additive would be necessary to promote the reaction. Experimental Section Materials Used. The cellulose employed in these experiments was pure crystalline a-cellulose (Avicel PH-101, FMC Corporation). The particle size distribution was -100 to +325 mesh. All samples were dried to 2.0% moisture. To produce a solid NaOH-cellulose mixture, pellets of NaOH (reagent grade) were predried at 130 "C under a nitrogen atmosphere for 12 h, ground to a powder, dried for a further 10 h, and mixed with cellulose. Another method of introducing sodium hydroxide was to pretreat cellulose by suspension in an aqueous solution of known NaOH concentration. The cellulose was filtered and dried at 40 "C under a nitrogen atmosphere. The elemental sodium content was assayed using an atomic absorption spectrophotometer. Temperature Range Measurements. Two instruments were used to determine the temperature range of the pyrolysis of pure cellulose and the effect of NaOH mixtures. A differential scanning calorimeter (DSC) measured the energy absorbed or released as the temperature of the sample was increased at a constant rate. A thermogravimetric analyzer (TGA) recorded the loss of sample weight as the temperature was increased. This combination provided the basis for selecting the most promising temperatures for the fixed-bed reactor experiments. Fixed-Bed Pyrolysis System. The reaction system is shown in Figure 1. The fixed-bed reactor was a type 316 stainless steel tube, 60 cm long, 1.91 cm o.d., and 1.34 cm i.d. A U-shaped stainless steel tube, 150 cm long, was connected to the lower part of the reactor for heating the helium carrier gas. Quartz wool pads on wire gauze supports were placed above and below the cellulose sample to maintain its position. Reaction-bed temperatures were measured by two chromel-alumel thermocouples; the lower one just touched the bottom of the packed bed, and the upper one, which penetrated the bed to a depth of 1.27 cm, was used for
Figure 2. DSC thermograms of NaOH pretreated cellulose at different Na/CeHloOb mole ratios: -, 0; 4-,0.31;- - -,0.50; 0 , 0.66; 0, 1.13; heating rate = 5 OC/min.
temperature control. A test with packed N203 showed that the variation between temperatures at the top and bottom of the bed was f 2 "C. Thus, it approached isothermal conditions. The control was an off-on type on the heat source. There was no method of removing heat. A strongly exothermic reaction would cause the temperature to rise abruptly above the control point for a short period of time. The volatile products from the reaction zone were continuously removed by a helium gas stream to reduce further decomposition. The average residence time in the reaction zone varied roughly from 0.05 to 0.5 s depending on the amount of gas produced. The liquid product was recovered by a condensation system consisting of a water cooled condenser and four successive dry ice-acetone cooled vapor traps. The uncondensed gaseous stream passed through a drying tube into a mercury-displacement gas sampling device. The stream was then purged into the air through a water column which maintained a back-pressure on the entire system. Procedure. The system was continuously purged with helium until no trace of air was detected by GC analysis. The helium flow was set at 80 mL/min and the reactor heated. It took about 20 min to reach the set temperature. Each pyrolysis reaction lasted 120 min. Most of the volatile products were condensed in the water-cooled condenser; only trace amounts were found in the dry iceacetone cooling traps. Product Analyses. The organic chemical compounds in the liquid product were divided into two groups. The low molecular weight products from Cs on down were identified using a GC-mass spectrometer equipped with a W-AW 80/100,10% Chromosorb column. Isobutyl alcohol was the internal standard. To analyze the carbohydrate derivatives, the liquid product was trimethylsilylated (Sweeley et al., 1963), and separated in a GC column-3% OV-1 100/120 Chromosorb $. Inositol was used as an internal standard. The water fraction of the liquid product was determined by a Porapak Q 50/80 GC column. The gas product was analyzed in a Porapak R 50/80 column and a molecular sieve 5A, 60/80 column connected in series. The Porapak column was operated at 100 "C, the molecular sieve column at room temperature. Carbon dioxide and carbon monoxide were analyzed with helium as the carrier while hydrogen was analyzed using nitrogen. The total moles of each gaseous constituent were calculated using a Newton-Cotes integration of gaseous product concentrations determined at known time intervals. The char residue was washed with distilled water and dried at 100 "C under nitrogen. The carbon, hydrogen, oxygen, and sodium content was quantitatively measured. Results Temperature Scanning. Figure 2 shows the DSC thermograms for pure cellulose and for four samples pre-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 111 Table I. Product Distribution of Cellulose Pyrolysis at 290,310, and 320 "C wt fraction of cellulose
0
'~~~~~
0
product liquid
290°C 0.18 -
310°C
320°C
0.17 -
0.30 __
water organic
0.16 0.02 0.38 -
0.14 0.03 0.44 ___
0.24 0.06 0.31 -
0.32 0.06 0.43 -
0.37 0.07 0.42
0.22 0.09 0.39 -
1.03
1.00
gas
100 150 2 0 0 250 300 350
TEMPERATURE ("C)
Figure 3. TGA thermograms of NaOH pretreated cellulose at different Na/CsHloOsmole ratios: -, 0; -0-, 0.31; - - -,0.50; 0,0.66; 0,1.13; heating rate = 10 OC/min.
3 solid residue overall
~
0.99
Table 11. Composition of Principal Organic Constituents Obtained from Cellulose Pyrolysis at 290,310, and 320 "C
=g :::Il - --
,
,
I
200
240
280
400
180
v
1
320
TEMPERATURE (OC)
Figure 4. DSC thermograms. Cellulose and solid NaOH mixtures at different mole ratios: NaOH/C&loOs -, 0; -0-, 1.3;- - -, 2.5; 0, 5.1; heating rate = 5 OC/min.
treated with aqueous NaOH containing 0.31,0.50,0.66, and 1.13 mole ratios of sodium to glucose unit (C6HI0O5).The trace for pure cellulose is the unbroken line. Above 250 "C it drops rapidly below the base line indicating a strongly endothermic reaction. Each addition of NaOH reduced the energy requirement, but only the largest amount, 1.13 mole ratio, showed a small exothermic reaction near the end. However, the overall reaction for all samples was endothermic, but less than with pure cellulose. The TGA thermogram for this same series of samples is shown in Figure 3. Pure cellulose starts to decompose a t 250 "C and does so precipitously between 300 and 310 "C. The pretreated NaOH samples reduced the onset of decomposition almost 100 "C and caused the reaction to occur over a much broader range. The extent of the difference is seen to be a function of the sodium content. Three samples of cellulose mixed with dry, solid NaOH in mole ratios of 1.3,2.5,and 5.1 (NaOH/glucose unit) were tested on the DSC. These results are shown in Figure 4 in comparison with pure cellulose. The reaction switched to being strongly exothermic and started vigorously at 190 "C. There appeared to be a reaction peak at 230 "C; at temperatures above 310 "C the trace fluctuated rapidly. There was certainly evidence of a drastic alteration of the reaction mechanism. Fixed-Bed Pyrolysis. The fixed-bed reactor was used to determine the nature and amounts of products produced by pyrolysis of cellulose using the two different methods of NaOH addition. For comparison pure cellulose was pyrolyzed at 290,310, and 320 "C. These results are shown in Table I. In all cases 5 g of cellulose was used and the gas, liquid, and residues shown are in weight fractions of the original 5 g. At 310 "C the fractions were: liquid, 0.17, gas, 0.44, and residue, 0.42, each determined indepsndently. The total of 1.03 is the material balance. The liquid product was 82.4 w t % water and 17.6 w t % organic compounds. Table I1 shows the analysis of the liquid organic constituents. The predominant compounds were carbohydrate, presumably levoglucosan, and these increased with temperature. There was no noticeable trend in the shorter
wt % of liquid product
product total organic portion carbohydrate derivatives furfural 5-methylfurfural 2-acetylfuran methanol ethanol
0
ti
10
290°C 310°C 320°C 11.11 _ _ -17.64 _ _19.25 7.08 14.50 16.39 1.48 1.37 1.39 0.27 0.26 0.25 0.19 0.19 0.18 0.04 0.63 0.09 0.05 0.16 0.06
15
20
25
RETENTION TIME (MINUTE)
Figure 5. GC chromatogram of liquid product obtained from pure cellulose pyrolysis at 320 O C : 1, formaldehyde; 3, methanol; 4, ethanol; 6, methyl ethyl ketone; 9, 2-pentanol; 10, 1-butanol; 11, 2methyl-2-propen-1-ol;12, 2-buten-1-01; 13, cyclopentanol; 14, hydroxyacetone; 15, cyclohexanol; 16, furan; 17, furfural; 18,2-acetylfuran; 19, 5-methylfurfural; 20, furfuryl alcohol.
chain products. Figure 5 is a GC chromatogram of the organic liquids at 320 "C. Mixtures of cellulose and dry crushed NaOH were reacted at 240 "C. The NaOH/glucose unit ratios employed were 2.02, 4.05, and 8.1. The charge contained 5 g of cellulose. The temperature performance in the reactor for all three samples was quite different from previous experiments. When the reaction started the temperature rose rapidly to 330 OC accompanied by a large evolution of H2,CO, and vapors which condensed. Owing to the lack of a cooling system, it could not be controlled a t 240 "C. It took about 8 min to return to the control point where it remained for the rest of the experiment. This confirmed the exothermic nature of the reaction as shown by the DSC. Table III gives the product weight fractions of the pure cellulose charged in these experiments compared with those for pure cellulose at 310 "C. The NaOH content of the residues made it deliquescent and an accurate weight could not be measured directly. In these three cases it was calculated from an overall material balance of liquid and gaseous products and the original cellulose weight.
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I d . Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
Table 111. Product Distribution of Cellulose-NaOH Pyrolysis wt. fraction of cellulose cellulose, 310 "C
cellulose-NaOH mixture, 240 "C
mole ratio, NaOH/glucose unit product liquid water organic gas
0
2.02
0.37 0.07
COZ
co
HZ solid residue water soluble water insoluble total
4.05
8.10 0
0.17_ 0.12 0.20_ -0.28 _ _ _ _ 0.14 0.11 0.18 0.26 0.03 0.01 0.02 0.02 0.44 0.18 0.26 0.27 - - - 0.06 0.09 ---b 0.03 0.42_ 0.70" _ _ _ c 0.15 0.42 0.55 1.03 1.00
Obtained by difference. 0.01.
0.05 0.19 0.02 0.54a - -
0.06 0.16 0.05 0.45"
0.24 0.30 1.00
0.24 0.21 1.00
Not detected.
Less than
mole ratio, NaOH/glucose unit product total organic constituents carbohydrates derivatives volatile organic constituent methanol ethanol 2-methyl-2-propen1-01 2-buten-1 -01 cyclopen tanol hy droxyacetone furfural 5-methylfurfural %acetylfuran Less than 0.01%.
4.05
8.10
310°C 240°C 240°C 240°C 17.6
8.33
14.5
---'
3.1
8.33
9.83
6.76
0.63 0.16
1.28 2.67 0.28
2.13 1.29 0.64
1.29 0.39 1.15
0.04 0.01 0.29 0.12 1.20
0.11 0.11 1.10
0.48 1.05 0.22
0.37
--b ---b
---b
---b
0.03
a
a a a 1.37 0.26 0.19
...b
15
20
25
30
the residue increased. The gaseous decrease was predominantly in C02,undoubtedly absorbed by the NaOH in the residue. On the other hand, the increase in weight of CO and the appearance of significant amounts of H2, are indicators of an altered reaction mechanism. Water extracts of the residues, being highly alkaline, were neutralized first with C02 and finally with aqueous HC1 at a pH of 4.1, a dark brown precipitate was obtained which had an atomic ratio of C6H7.2201.97. It was soluble in acetone, ethyl alcohol, and dimethyl sulfoxide, but not in nonpolar solvents, indicating the presence of somewhat polar hydrogen atoms. The composition of the principal liquid organic constituents is shown in Table IV and in Figure 6. The presence of the dry NaOH caused a shift from carbohydrates and furfural to low molecular weight alcohols: methanol, ethanol, and 2-methyl-2-propen-1-01. For the next series of experiments five sodium hydroxide pretreated samples were pyrolyzed at 280 OC. The mole ratios of Na/glucose unit were 0.07,0.31,0.50, 1.13, and 1.86. Unlike the performance with the dry NaOH mixtures, there was no sudden increase in temperature and the reaction was easily controlled. It is significant to note that the 1.86 mole ratio used here is appreciably higher than the 1.3 ratio of the dry NaOH mixture that showed a strong exotherm on the DSC. The product weight fractions based on the weight of pure cellulose in the pretreated sample are shown in Table V. For comparison, typical results with pure cellulose and a dry NaOH-cellulose mixture are listed. The NaOH pretreatment increased the aqueous liquid product and considerably reduced the solid residue. The composition of the gaseous product also changed. Although the total
wt % of liquid product
2.02
IO
Figure 6. CC Chromatogram of liquid product obtained from the pyrolysis of cellulose and solid NaOH mixture at 8.1 NaOH/CJll0O5 mole ratio and 240 "C: 3, methanol; 4, ethanol; 6, methyl ethyl ketone; 7, methyl isopropyl ketone; 9,%-pentanol;10,l-butanol; 11, 2-methyl-2-propen-1-01;12,2-buten-l-01;13, cyclopentanol; 14, hydroxyacetone; 18, 2-acetylfuran; 20, furfuryl alcohol.
Table IV. Composition of Principal Organic Constituents at Different NaOH/Glucose Unit Mole Ratio
0
5
RETENTION TIME (MINUTE)
Not detected.
The data show that the presence of NaOH caused the organic liquid and the gaseous fraction to decrease while
Table V. Fraction of Cellulose Converted into Liquid Product, Gas Product, and Solid Residue (Pyrolysis of Sodium Pretreated Cellulose) wt fraction of cellulose
product liquid water organic gas
:o z H2
solid residue total a
Obtained by difference.
cellulose, 310 "C
NaOH pretreated cellulose, 280 "C mole ratio, Nalglucose unit
NaOH-cellulose mixture, 2.02 mole ratio, 240 "C
0.07
0.31
0.50
1.13
1.86
0.12 0.11 0.01 0.18 -
0.19 0.18 0.01 0.49
0.34
0.41 -
0.47 -
0.40 __
0.32 0.02 0.33 -
0.39 0.02 0.38 ___
0.44 0.03 0.43 __
0.34 0.15
0.16 0.17
0.16 0.22
0.20 0.23
0.38 0.02 0.41 __ 0.20 0.21
0.17 __ 0.14 0.03 0.44 0.37 0.07
...b
0.42 1.03 Not detected.
0.06 0.10 0.02 O.7Oa 1.00
__.
_._ b 0.29 0.97
.__
---b
___
0.29
0.28
0.18
0.19
0.96
1.07
1.08
1.00
b
-_b
---b
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 113 I
I
RETENTION TIME ( M I N U T E )
Figure 7. GC chromatogram of liquid product obtained from the pyrolysis of NaOH pretreated cellulose at 1.13 Na/C$Ilo06 mole ratio and 280 O C : 2, acetone; 3, methanol; 4, ethanol; 5,2-propanol; 8,2-butanol;9,a-pentanol;10,l-butanol; 11,2-methyl-2-propen-l-ol; 13, cyclopentanol;14, hydroxyacetone; 15, cyclohexanol; 16, furan; 18, 2-acetylfuran; 19,5-methylfurfur& 20, furfuryl alcohol. Table VI. Weight Percent of Organic Componenta and Ita Major Constituents in the Liquid Product Obtained from the Pyrolysis of Pretreated Cellulose wt % NaOH pretreated cellulose, 280 "C mole ratio Nalglucose unit product
0.07 0.31 0.50 1.13 1.86
total organic components hy droxyacetone 2-propanol ethanol furan 2-acetyl furan
furfural furfuryl alcohol a
Less than 0.01%.
7.30 3.78 0.18 0.06 0.53 0.10 0.55 0.24
5.43 2.99 0.41 0.09 0.37 0.19 0.45 0.16
5.96 4.40 0.26 0.07 0.45 0.18 0.21 0.17
7.12 5.59 0.02 a 0.46 0.07
5.74 4.45 0.11
--b
--b
a
0.32 0.02
0.44 0.51
Not detected.
amount of gas did not vary much from that with pure cellulose, the COz/CO ratio was drastically altered with CO being the major constituent. Apparently, the COZ was not absorbed by the NaOH as in the case of the dry mixture nor was H2 detected. The totals of the independently measured weight fractions for the pretreated samples is evidence of the accuracy of the technique. The liquid product was primarily an aqueous solution of C1 to C4oxygenated compounds. No carbohydrates were found. As shown in Table VI and Figure 7, the primary difference in all of these experiments was that hydroxyacetone was the major constituent. The composition of the solid residue ratioed to a c6 unit varied from C6H5.0202.43to C6H3.&.12Na,,.~ The higher the ratio of Na/glucose unit, the more sodium was permanently retained in the residue. Discussion The product distribution of cellulose pyrolysis strongly depends on the sodium hydroxide additives and the way they are added. The formation of volatile organic compounds such as levoglucosan and the furan derivatives by pyrolysis of pure cellulose involves the splitting out of an individual glucose unit from the polymer chain which then rearranges into levoglucosan, furan, and furan derivatives. On the other hand, the formation of water can result from the dehydration within a unit without separation from the chain. The fact that the carbohydrate fraction (levoglucosan) increased at increased temperature for pure cellulose indicated that this favored breaking the 3.j C1 C4 bond between glucose units. In the presence of solid NaOH, the water content of the volatile products increased and there were no carbohydrates in the smaller organic portion. The organic portion consisted primarily of a series of oxygenated compounds such as 2-methyl-2-propen-1-01, methanol, and ethanol
-
RETENTION TIME (MINUTE)
Figure 8. GC chromatogram of liquid product obtained from pyrolysis of periodate pretreated celluloee at 280 O C : 1, formaldehyde; 3, methanol; 4, ethanol; 11,2-methyl-2-propen-l-ol; 12,2-buten-l-ol; 13, cyclopentanol; 14, hydroxyacetone; 15, cyclohexanol; 16, furan; 17, furfural, 18,2-acetylfuran; 19,B-methylfurfural,20, furfuryl alcohol.
providing evidence of the extensive ring opening caused by the caustic. The preaence of H2 in the gaseous product from the solid NaOH-cellulose experiments could result from the reaction of cellulose with fused NaOH. This reaction at 200 "C (Othmer et al., 1942)was formerly used as a commercial process for producing oxalic acid from sawdust. Hydrogen was a byproduct. No hydrogen was detected in any results other than those using solid NaOH. Apparently, the water-gas reaction did not take place. The presence of hydroxyacetone as the predominant organic product from the pyrolysis of the NaOH-pretreated cellulose indicates a different reaction path. In the formation of alkali cellulose it has been shown (Sugihara, 1973)that the hydroxyl group on the c6 position is more reactive than that on the Cz and C3 positions. It is also known (Morrison and Boyd, 1973) that the presence of alkali in the aging of alkali cellulose caused the scission of the /3 C1 C4 glucosidic bonds. Assuming that this scission occurs at a chain end, the path to hydroxyacetone involves an opening of the ring structure of the glucose residue. The bond that would appear most susceptible is between the C2and C3positions. The presence of the NaO group on a c6 position may also exert an attractive force on the Cz, C3 hydroxyl groups of an adjacent chain. To obtain evidence for this reaction pathway, pure cellulose was oxidized in an aqueous solution of sodium periodate, a procedure which opens the ring structure at the Cz, C3 positions forming aldehyde groups on each carbon (Mark, 1965). The amount of oxidation was controlled at 50% of the glucose units. The sample was washed, dried, and pyrolyzed at 280 "C. As shown in Figure 8, the organic constituents contained hydroxyacetone, as well as furfural and furan as the major constituents. The gaseous product contained about 50 mol 90 C and 50 mol % COz. The higher hydroxyacetone concentration and CO content are very similar to the results with the NaOH pretreated cellulose. Evaluation of Results The tubular reactor proved to be a convenient piece of equipment for comparisons between various methods of pyrolyzing cellulose. In particular, coupled with the analytical techniques developed, it was possible to obtain excellent material balances between the amount of material charged and the gas, liquid, and solid products produced. It was also possible to obtain reproducible results from experiments run under duplicate conditions. It did not, however, fulfill satisfactorily one of the parameters desired-a short time of contact at temperature. The sweeping flow of helium certainly aided in removing the volatile products quickly, but the nonvolatile material
-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
remained throughout the entire run. Evidence of the effect is that experiments in the tube with pure cellulose gave residues of about 40% whereas the TGA showed residues of 10% or less. In view of this, it is interesting to note that the residues with NaOH pretreated cellulose were as low as 20%, approaching closely the TGA values. The weight fraction of cellulose that was produced as an organic liquid product varied from 0.01 to 0.06 in all of the experiments. The effect of additives was shown in the composition. With pure cellulose over 16 w t % was pure carbohydrate, probably levoglucosan, with the remainder chiefly furfural and 5-methyl furfural. The dry NaOH-cellulose combination produced predominantly alcohols, methanol, ethanol, and 2-methyl-2-propen-1-01. The alkali pretreated material gave a product predominantly hydroxyacetone, 3-5 wt %. It is obvious that a substantial difference was produced. The amount of gas produced remained practically constant at about 0.40 weight fraction of cellulose except in the case of the dry NaOH mixtures which absorbed most of the COP It is noteworthy that with pure cellulose, the gas analyzed 77 mol % C02 and 23 mol % CO whereas with NaOH pretreatment it ran consistently 37% COzand 63% CO. This is strong evidence of the difference in mechanism of pyrolysis and presents the possibility of obtaining a useful gaseous product. The only other gas found was in the dry NaOH experiments where H2 was given off. In this case, on a weight fraction basis, the analysis was 0.06 C02,0.16 CO, and 0.05 H2which converts to 78% H2, 18% CO, and 4 % C02on a mole basis. Most of the C 0 2 was absorbed by the excess NaOH. The residue results are probably the least informative. The char formation results from some type of polymerization of free radical-containing compounds as the result of cracking. As shown by the analyses, the ratios of H and 0 to C dropped from that of the original glucose unit. The lower weight of residue in the NaOH pretreated experiments seems to be reflected in the larger amount of H20 that appeared in the liquid products. The results show that it is possible to use NaOH in the pyrolysis of cellulose and produce oxygenated hydro-
carbons in the Cl to C4range under conditions of moderate temperature and atmospheric pressure. However, the yield of these products (2%) is entirely too small for a viable process. This study has indicated a new line of attack toward developing a process. By a combination of experiments with a DSC,a TGA, and a batch, tubular reactor it has provided evidence as to the mechanism by which cellulose decomposes, and the influence of NaOH on this mechanism. For example, pretreatment with aqueous NaOH results in the production of hydroxyacetone. The same product was obtained by oxidation with periodate which breaks the glucose unit ring between the C2and Cs carbon atoms. Apparently, NaOH accomplishes this same type of ring opening under pyrolysis conditions. There are certainly other methods of accomplishing these effects which should be explored. Literature Cited Appell, H. R.; Fu, Y. C.; I#g, E. Q.; Steffgem, F. W.; M l k , R. D. U . S . Bw.
h#na Rep. Invest. 1975, 8013. BaM, R. C.; Ishlda, M. AIChESymp. Ser. 1972, 68, 73. Boomer. E. H.; Argue, 0. H.; Edwarde. J. Can. J . Res. 1935, 138, 337. Burton, R. S.; Belle, R. C.; Alpert, S. AIChESynlp. Ser. 1974, 70, 116. Gupta, D. V.; Kranlch, W. L.; W e b , A. H. Ind. €ng. Chem. Rocess Des. Dev. 1976, 15, 256. Gukan, H. H. Ph.D. Dbsertetkn, COkmMa University, 1945. HaMgan, J. E.; Herzog, K. L.; Parker, H. W. Ind. Eng. Chem. process Des. Dev. 1975, 14v64. Madofsky, S. L.; Hart, V. E.; Straw, S. J. Res. b t l . Bur. Stend. 1956, 56, 343. Mark, H. F., Ed. "Encyclopedia of Polymer Science and Technobgy", Interscience: New York, 1965 Vd. 3, p 171. Monlson, R. T.; Boyd, R. N. "Organic Chemistry", 3rd 4.;Allyn and Bacon: Boston, 1973; p 1128. Othmer, D. F.; &mer, C. H.; Jacobs, J. J., Jr. I d . €ng. 0".1942, 34, 262. Shaflzadeh, F.; McIntyre, C. R.; Lundstrom, H.; Fu, Y. L. Roc. Montene Aced. Scl. 1473, 33, 65. suglhara. J. M. A&. Ca&h@. Chem. 1973, 8, 1. Sweeley, C. C.; Bentky, R.; Makite, M.; Wells, W. W. J . Am. Chem. Soc. 1963, 85, 2497. Wodley, F. A. J. Appl. Potym. Scl. 1971, 15, 835.
Received for review April 25, 1980 Accepted September 23,1980 Paper presented at AIChE 72nd Annual Meeting, San Francisco, Calif., Nov 1979.
