CATALYTIC HYDROGENATION OF FU COMPOUNDS S.
H. W O J C I K
H O O K E R E L E C T R O C H E M I C A L C O M P A N Y , N I A G A R A F A L L S , N. Y .
'
A brief history of high pressure catalytic hydrogenation is discussed which includes equipment used and types of catalyst with regard to performance w-ithin certain pressure and temperature ranges, economy, and selectivity. Furan and furfural are versatile compounds, and a partial review of the literature lists a large number of interesting derivatives which may be prepared by the introduction of hydrogen into their molecules. Results obtained on the hydrogenation of furfural to furfuryl alcohol and to tetrahydrofurfuryl alcohol in the pilot plant and on a commerdial basis are included.
reductions; it is a rugged catalyst, quite inexpensive and rather active toward a variety of functional groups. I n the copper series, copper-chromium oxide is an exceptionally good catalyst for the ieduction of carbonyl groups. The hydrogenation of cyclic compounds, such as furfural to tetrahydrofurfuryl alcohol, benzene to cyclohexane, or pyridine t o piperidine, is not successfully accomplished with copper catalysts. Xolybdenum and tungsten sulfide catalysts require fairly high temperatures and pressures, conditions favoring hydrogenolysis as well as straight hydrogenation. The tendency in the study of catalysts a t present is toward increased selectivity-for example, the conversion of an acetylenic linkage to ethylene or the reduction of a carbonyl structure, ~ i t h o u effecting t a change in an ethylene bond. Pressure and temperature requirements are governed primarily by the type of functional group to be altered and the molecular configuration surrounding the functional group. A review of the literature pertaining to the catalytic hydrogenation of furan compounds shows that the investigations reported to date represent only a minor portion of the studies which can be carried out on these interesting and potentially useful materials. I t is the object of this presentation to indicate the numerous products which can be made from furan, furfural, and their derivatives by treatment with hydrogen under proper conditions. K O attempt will be made to cover all of the compounds investigated or to comment in detail on results reported.
A
LTHOUGH catalytic hydrogenation of organic compounds dates back to the experiments of Sabatier carried out in 1897 and to the use of superatmospheric pressures for such reductions by Ipatieff in 1904, the application of high pressure catalytic hydrogenation to industrial operations is still in the process of gradual acceptance and is viewed with some degree of awe. Since furfuryl and tetrahydrofurfuryl alcohols were among the guinea pigs in the industrial development of this new technique, it may be in order to make a few general remarks about the equipment, catalysts, and conditions employed before discussing the hydrogenation of furan compounds. High pressure catalytic hydrogenations may be defined, in the light of present-day practice, as a reaction involving the addition of hydrogen to an organic molecule in the presence of a suitable catalyst a t elevated temperatures (50-500 O C.) and superatmospheric pressures (50-10,000 pounds per square inch). The reaction can be carried out either ih the liquid phase, with or without a solvent present, or in the gas phase. In recent years a number of interesting articles and a few good books have been published on catalytic hydrogenations (1, 4, 6, 9, 10, 13, 15, $4, $6). Calingaert and Edgar's (6)article is especially informative. It not only describes in detail a practical pilot plant batch hydrogenation process for the conversion of furfural to furfuryl alcohol but also covers the preparation of a copper-chromium oxide catalyst. High pressure equipment design and construction have been perfected to a point where, with a minimum amount of precaution and maintenance, the units can be operated safely and efficiently over a period of years. The present tendency is toward the use of higher pressures to increase reaction rates and the adaptations of continuous systems whenever warranted by volume. Catalysts for hydrogenation appear to fall into four groups based on performance within certain pressure and temperature ranges, economy, and selectivity: ( a ) platinum-palladium series, ( b ) nickel series, (c) copper-mixed oxide series, and ( d ) molybdenum-tungsten sulfide series. Platinum-palladium catalysts are useful a t low temperatures and pressures. They are seldom used alone on a commercial scale because they invariably prove to be too expensive. Small amounts of platinum or palladium are sometimes employed in conjunction with other catalysts as promoters. Nickel catalysts prepared by various processes are especially useful, Raney nickel, made by leaching a nickel-aluminum alloy with caustic, is finding extensive application in pressure
FURAN-ALKYL FURANS
The conversion of furan and its alkyl derivatives to the corresponding tetrahydrofurans by hydrogenation presents no serious problem. Such compounds as furan, methylfuran, or,a,-dimethyl furan, ethylfuran, and the alkene furans are rapidly and smoothly converted to the saturated derivatives in good yields under rather mild reection conditions (Table I, see page 212). Under more drastic conditions, especially increased temperature and the presence of water, the furan ring undergoes hydrogenation and hydrogenolysis yielding diols, ketones, and secondary and primary alcohols. Burnette (3)showed that methylfuran hydrogenated over partially activated Raney nickel at atmospheric pressure and 200" C. is converted to methyltetrahydrofuran and appreciable quantities of 2-pentanone and 2pentanol.
OH CHBCH2CHBhH--C&
...
By treating methylfuran with hydrogen a t 250" C. in the presence of copper-chromium oxide, Adkins (1) obtained methyl tetrahydrofuran, 1-pentanol, and 2-pentanol. 210
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1948
15% CHaCHzCHzCHzCHzOH
30%
OH
t:
CHaCHzCHz H-CHa
33 %
Schniepp, Geller, and Von Korff (88) reported that the hydrogenation of methylfuran in an aqueous medium containing small amounts of acidic material results in the formation of 1,4-pentanediol (5040% yield). I t was postulated that the reduction proceeds in accordance with the mechanism proposed by Topchiev (R6).,
FURFURAL
FURFUYL ALCOHOLPRODUCTION. Furfural occupies a unique position among organic compounds capable of yielding a variety of products by catalytic hydrogenation. Gilman and his associates (6, 7) attributed superaromatic properties to furan compounds based on nitration and Friedel-Crafts reactions involving 2-fury1 phenyl ketone and other furan derivatives. These observations might lead one to suspect that the hydrogenation of furfural may follow the behavior of benzaldehyde. I t is exceedingly difficult to obtain good yields of benzyl alcohol by the reduction of the aldehyde. Hydrogenolysis predominates and results in the formation of toluene and water or methyl cyclohexane and water. Furfural, on the other hand, can be smoothly and rapidly converted to furfuryl alcohol in virtually quantitative yields.
211
TETRAHYDROFURFURYL ALCOHOLPRODUCTION. By mbrely changing the catalyst, replacing copper-chromium oxide with Raney nickel or using a mixture of the two catalysts, furfural or furfuryl alcohol can be transformed to tetrahydrofurfuryl alcohol.
IrJlH '0
Raney Ni Cu-CrOZ 1000-1500 lb./sq. in. 170-180" C.
or Raney Xi
1
Cu-Cro
+
Yields for this conversion by either of the methods indicated range from 70 to 80%. Literature and patent disclosures claim 95% yields, especially when the reaction is carried out at a low temperature, 100-125 'C., pver an extended period of time. To increase the reaction rate, plant operation is maintained a t 170180' C., whereby the hydrogenations are completed in 45-90 minutes. All three methods shown in the scheme have been used a t one time or another for the commercial production of tetrahydrofurfuryl alcohol. Using a mixture of Raney nickel and copper-chromium oxide and carrying out the conversion in one step from furfural is the most economical process for the production of tetrahydrofurfuryl alcohol. PREPARATION OF METHYLFURAN. Methylfuran was obtained in excellent yields according to Burnette (3) and Schniepp, Geller, and Von Korff (8.3) by treating furfural with hydrogen in the vapor phase at 200-250 ' C. over copper-chromite catalyst dispersed on charcoal. PREPARATION OF POLYHYDROXYL ALKYLS. According to Leuck, Porkorny, and Peters (167, furfural can be made to yield 1,4-pentanediol and 1,2,5-pentanetriol by hydrogenation in an acidulated aqueous medium. HzC-CHI
I
I
H29 CH-CHzOH H b AH 0'
Hz, HzO (H+) Raney Ni 1000 lb./sq. in. 160" C.
HzC-CHZ Hz
Ac: I
I
H-CH3
+ HzO
.
HO OH Adkins ( I ) hydrogenated furfuryl alcohol and obtained methylfuran, 1-pentanol, 1,5-pentanediol, and 1,a-pentanediol.
Hz Cu-CrO 1000-1500 lb./sq. in. 175' C.
U-cHZoH
Cu-CrO HZ 2000 lb./sq. in. 250" C.
Q-cH3 '0
36%
CH,CHZCHZCHZCH,OH 36% Commercially the reduction of furfural to furfuryl alcohol is carried out in 110-gallon autoclaves a t 1000-1500 pounds per aquare inch and 175" C. in the presence of 1-2% of copperchromium oxide catalyst. Technical furfural is suitable for the conversion. The copper-chromium oxide catalyst has little or no effect on the furan ring a t 175' C.; thus yields amounting to 96-99% of theory for furfuryl alcohol are consistently obtained. This hydrogenatik can be conducted in the presence of other catalysts, but in most cases it is difficult to stop the reaction a t t h e furfuryl alcohol stage.
CHzCHzCHzCHzCHz
AH
15%
AH
CH~CH-CHZCH~CHS
14%
AH AH He also showed that, a t 175" C. over a considerably longer reaction period, furfuryl alcohol yields a 70% mixture of 1,2-pentanediol and l,&pentanediol. Adkins believes the products result from the hydrogenolysis of the unsaturated alcohol, since
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INDUSTRIAL AND ENGINEERING CHEMISTRY
212
__
TABLE I. HYDROGESATION Starting Material
Reference
01" FURAN A N D ALKYL
Catalysta
Temp., O
C.
Pressure, Atm.
DERIVATIVES Yield,
To
Vol. 40, No. 2 0
,iJ-.CII-CII('-IIil 0
Product
HB+ .&i 21500 Ib. /sq. in. 300-315' C.
CIT,ICIIn:jCH3 307,
R.P.08-99" C.
a
I t has been suggcetcd t,liat this reaction might have possibilities as a commercial preparation of pure hept,ant:. Before continuing with the reduction of other furan derivatives, it is interesting t o show t,he variety of compounds which can readily be mado from furfural by direct hydrogenation (Figure 1). Apparently the first point of att'ack on furfurnl occurs at the carbonyl structure which is easily transformed t o an alcohol. Once furfuryl alcohol is formed, the t,ypes of products made and the yield of each obtained are governed by the conditions, the type of catalyst used, and the presence or , absence of n-ater during the reduction. Furfural can be converted in good yields t o furfuryl alcohol, tetrahyrlrofurfuryl alcohol, methglfuran, methyltetrahydrofuran, 3 ,2-pent8anediol, 1 , 4 pent anediol, 1,5-peutanedioI, 1,2,5pentanetriol, a mist,ure of 1-pentanol and 2-pentanol, and finally pentane.
S i - R , Raney nickel: Xi-K, nickel on kieselguhr; Pd-A, palladium on asbestos.
TABLE 11. HYDROGEN-~TIOK OF FUROIC ACID I~ERIVATIVXS Reference
Starting Material
Temp., Catalysta C.
(14)
PtO
Pressure, Atm.
Yield,
%
Product 0
50
2
11
40
\OrG-OH (20)
Ni-R
110
100
89
0 (1)
C&C!-OC,H, 0
r)
Ni-K
200
100
0;
0 IT)-/,
\o
I"7-f 1 7?
\o (18
Ni-R
175
100
io
70
C)Cb-OCH:Ni-R, Raney nickel: Xi-K, nickel on kieselguhr: P t O , platinum oxide. Q
C-OH
,o
c-oc2wi
-C-OCHn-l
7
FUROlC ACID A N D I T S DERIVATIVES
\o
Furoic acid and its metallic salts, esters, and amides readily react Kith hydrogen to form the saturated compounds, Kaufman and Bdams ( 1 4 ) prepared tetrahydrofuroic acid by hydrogenating an et,hanolsolution of furoic acid with a platinum oxide catalyst. Paul and Hilly ( d o ) demonstrated that sodium furoate undergoes rapid reduction in aqueous solutions at 110' and 750 pounds per square inch with Raney nickel, to result in the forma-
tetrahydrofurfuryl alcohol is more stable toward hydrogenolysis, and results in the formation of only 1,S-pentanediol at a sufficiently high temperature. PREPARATION OF HYDROCAREOX. Orlov and Radchenko (18) were successful in converting furfural to a mixture of hydrocarbons by treatment with hydrogen at 330TABLE 111. HYDROGENATION OF FCROIC ACID DERIVATIVES 350' C. and 2200 pounds per square Presinch in the presence of molybdenum ReferTemp., sure, Yield, sulfide catalyst,. Starting AIaterial ence Catalyst C. Atm. % Product
HZ + hIoSs 2200 lb./sq. in.
330-350" C.
Pentane Paraffins Napthalene Aromatics (1) Orlov (17) investigated the scope of this reaction and, on the basis of other experiments, claims it to be general for furan derivatives. In the case of furylacrolein pure heptane was isolated in 30% yields.
,-.
n
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1948
213
in 50% yields by reacting furfural dissolved in ammoniacal methanol solution Products with hydrogen a t 150' C. and 3500-5000 pounds per square inch in the presence ~~CH~CH~CHzcHzCH20H \n of Raney nickel. Adkins and Winans ( 2 ) and FVinans (28) prepared mixed secondary CHaOH amines by hydrogenating the reaction products of furfural OH O H and primary amines, as, for \o - H- Hexample, butyl amine. They have also shown that hyd r o f u r a m i d e (1) can be made to yield tetrahydrofurfuryl and ditetrahydrofurfuryl amines by reduction in ethanol solution a t 200" C. with nickelon kieselguhr.
OF SOME FURFURAIFALDEKYDE CONDENSATION PRODUCTS TABLE IV. HYDROGENATION
Starting Material
Reference
Catalyst'
(11)
Ni-R
175
150-200
90
(1)
Ni-K
150
100-200
93
Temp., Pressure, Yield, OC. Atm. %
0 CH=CH-CH=CHC-H
a
/I
Ni-R, Raney nickel; Ni-K, nickel on kieselguhr.
. tion of the tetrahydrofuroate. The saturated acid can also be prepared in excellent yields from the calcium salt. From ethyl furoate the saturated ester is obtained in 97% yields. Furfuryl furoate has been successfully hydrogenated to tetrahydrofuryltetrahydrofuroate. In this case it has not been established whether one of the furanoid nuclei is more susceptible to hydrogenation than the other (Table 11). Because of the rather energetic conditions required for the conversion of amides to amines with copper-chromium oxide, the hydrogenation of furamide and N-substituted furamides does not produce furfuryl amines. Instead, the tetrahydro derivatives are obtained in substantial yields. Adkins (1) reported the results shown in Table I11 for the reduction of tetrahydrofuramide and N-furoyl piperidine (Table 111).
1 74
c) ~
PREPARATION OF AMINES
The preceding section showed that tetrahidrofuramide, on treatment with hydrogen under proper conditions, yields the primary and secondary amines. Also furoyl piperidine was successfully converted to N-tetrahydrofurfurylpiperidine. Robinson and Snyder (II)reported the preparation of furfuryl amine
Treating P-(Zfuryl)-vinyl cyanide (1) with hydrogen in the presence of Raney nickel a t 50' C. resulted in the formation of 3-(2-furyl)-propyl amine (59%) and di[3-(Zfuryl)-propyl]amine
(21%).
HZC-CH~
HzC--CH?
~--CHO
P.
4
HzC---CHz -Hz&
OH I
HIC-CHZ
AHzCHa bH
HzA OH I
~HzCHS
AH
HzC-CHz 4
H3h
~Hz-CHZOH I
Figure 1.
Furfural hydrogenation
INDUSTRIAL AND ENGINEERING CHEMISTRY
214
TABLET.'.
Vol. 40, No. 2
HYDROGEXATIOS OF TYPICAL FURFURAL-KETONE CONDEXSATION PRODUCTS Refer- Cataence lysta
Starting AIaterinl
Pressure, Atm.
Yield,
100-125
89
Temp., O C.
c/o
Products OH
160
a
\o
CHzCHz1CHcH8
Xi-R, Raney nickel: Xi-K, nickel on kieselguhr..
TABLE VI. HYDROCENUIOU O F FURFURAL-ESTER. ETHYL FUROaTE-I
