Znd. Eng. Chem. Res. 1993,32, 11-19
11
Reactions of Aqueous Glucose Solutions over Solid-Acid Y -Zeolite Catalyst at 110-160 "C Khavinet Lourvanij and Gregory L. Rorrer* Department of Chemical Engineering, Oregon State University, Corvallis, Oregon 97331
Reactions of glucose with solid-acid Y-zeolite catalyst were studied to see if this heterogeneous system could produce oxygenated hydrocarbons by shape-selective, acid-catalyzed processes a t fairly low temperatures. Experimentally, aqueous solutions of glucose (12 w t %) were reacted with HY-zeolite powder in a well-mixed batch reactor a t temperatures ranging from 110 to 160 "C and catalyst concentrations ranging from 2 t o 20 g/150 mL. Unreacted glucose and oxygenated hydrocarbon products were measured by HPLC as a function of reaction time (0-24 h) and process conditions. Glucose conversions of 100% were obtained a t 160 "C after an 8-h reaction time. The apparent activation energy based on glucose conversion was 23.25 f 0.40 kcal/mol. Several acid-catalyzed reactions were identified, including isomerization of glucose to fructose, partial dehydration of glucose to 5-(hydroxymethy1)furfural (HMF), rehydration and cleavage of HMF to formic acid and 4-oxopentanoic acid, and carbonization. Polymers of HMF and seven minor additional products in the lower molecular weight organic acids/aldehydes/ketones elution range were also isolated by HPLC. High yields of organic acids relative to HMF and lowered selectivity of HMF in the bulk phase relative to the homogeneous acid-catalyzed reaction suggests the possibility of molecular sieving reactions within the Y-zeolite in addition to reactions on the outer surface of the Y-zeolite particle.
Introduction The catalytic conversion of glucose to oxygenated hydrocarbons can occur by oxidation, hydrogenation, and dehydration processes (Kieboom and van Bekkum, 1986). In particular, the partial dehydration of glucose, fructose, or other hexose sugars to furan derivatives (5-(hydroxymethy1)furfural) and organic acids (formic acid, 4-oxopentanoic acid) is promoted by aqueous mineral acids (McKibbins et al., 1962; Kuster and Van der Baan, 1977; Kuster, 1977; Kuster and Temmink, 1977; Baugh and McCarty, 1988)or acidic ion exchange resins (Schraufnagel and Rase, 1975) in aqueous or polar solutions at temperatures ranging from 100 to 200 "C. It is possible that glucose can be catalytically converted to useful chemicals by molecular-sievingzeolite catalysts. Although largely unstudied, the reactions of glucose with zeolite catalysts deserve consideration because they can act as molecular sieves to improve the yield and selectivity of acid-catalyzed reactions (Chen and Garwood, 1986). Since the dehydration of glucose by hydronium-ion catalysts is well-known, it is appropriate to explore how solid-acid zeolite catalysts promote the dehydration of glucose to oxygenated hydrocarbon products, particularly furan derivatives and organic acids. Previous studies on the reaction of glucose or other carbohydrates with solid-acid zeolite catalysts focused on hydrocarbon production at high temperatures. Chen et al. (Chen and Koenig, 1985; Chen et al., 1986) explored the dehydration of glucose to gasoline-rangehydrocarbons over a H-ZSM5 zeolite catalyst in a water/methanol mixture at temperatures ranging from 300 to 650 "C. The overall yield of aliphatic and aromatic hydrocarbons was 20% based on carbon retention. The remaining carbon went to form carbon monoxide, carbon dioxide, and coke. Similar results were later obtained by Haniff and Dao (1988) and Dao et al. (1988). Hydrogenolysis of glucose by zeolite catalysts is also possible, as reported by Jacobs and Herve (19891, who reacted an aqueous suspension of starch with Na-Y zeolite powder containing 3% ruthenium in a well-mixed batch reactor at 453 K pressurized with 55 atm of hydrogen. Starch hydrolyzed to glucose, and glucose was subsequently hydrogenated to yield a polyhydroxy alcohol mixture containing 96% D-ghcitol, 1% D-mannitol, and 1% xylitol. 0888-5885/93/2632-0011$04.00/0
Previous studies with medium-pore zeolite catalysts (e.g., ZSM-5) did not attempt to exploit the molecular-sieving capability of zeolite catalysts for the selective dehydration of glucose to oxygenated hydrocarbons. Furthermore, the high temperature regime (>300 "C) promoted complete dehydration of glucose to hydrocarbons and coke. The goal of this present work is to determine if large-pore, solid-acid Y-zeolite catalysts can promote the shape-selective,partial dehydration of glucose to oxygenated hydrocarbons at low temperatures where nonselective carbonization processes are potentially minimized. Shape selectivity is provided by the Y-zeolite pore matrix, which consists of 0.75-nm pores connected to 1.3-nm cages arranged in cubic symmetry. In order for the glucose molecule to undergo shape-selective rearrangement, the glucose molecule must diffuse into the HY-zeolite pore matrix and dehydrate to stable molecules. It is well established that Bronsted acids in polar solvents catalyze the cleavage of the pyranose ring of glucose to a linear l,Zenediol, which is unstable (Feather and Harris, 1973). The unstable 1,2-enediol then dehydrates to the stable molecule 5-(hydroxymethyl)furfural(HMF), which can be catalytically rehydrated by hydronium ions to formic acid and 4-oxopentanoic acid in equimolar amounts. In a proposed parallel mechanism within solid-acid Y-zeolite catalysts, the cyclic 0.9-nm glucose molecule is cleaved to a linear 1,2-enediol by Bronsted acid sites imbedded on the outer surface of the HY-zeolite particle. Although the glucose molecule is too large to diffuse into the 0.75-nmdiameter pores directly, the linear enediol could thread through the 0.75-nm pores within the Y-zeolite matrix and eventually enter a 1.3-nm cage. Inside the cage, the unstable enediol could react on Bronsted acid sites to form stable molecules such as HMF. Bronsted acid sites within the cage could promote the subsequent rehydration and cleavage of HMF to formic and 4-oxopentanoic acids. Since the bulky 0.82-nm HMF molecule is trapped within the 1.3-nm cage, only smaller linear molecules of formic and 4-oxopentanoic acid could thread out through the 0.75-nm pores and diffuse out into the liquid phase. Thus linear organic acids would be selectively produced over cyclic furan compounds such as HMF. The reactions of aqueous glucose solution with solid-acid Y-zeolite powder will be studied in a well mixed batch 0 1993 American Chemical Society
12 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
flow
Nz
> control unit thermocouple
Figure 1. Stirred autoclave reactor and instrumentation.
reactor by measuring the conversion of glucose and the yield of furan and organic acid products as a function of reaction time and process conditions. Specific process conditions of interest are mixing speed, temperature, and catalyst loading. The temperature range chosen for study will be between 110 and 160 OC, which is high enough to promote the partial dehydration of glucose to furan derivatives and organic acids, but low enough to minimize complete dehydration to hydrocarbons and carbonaceous residues. However, coke formation and catalyst deactivation will also be considered. All of this data will be used to identify acid-catalyzed reactions of glucose on HYzeolite catalysts. Also, the hypothesis of a shape-selective mechanism for partial dehydration of glucose to organic acids within Y-zeolite catalysts will be considered. Experimental Section Materials. Crystalline D-glucose (MW 180.16 g/mol, mp 146-150 "C) was obtained from Sigma Chemical Co. Ultrastable Y-zeolite powder in hydrogen form was obtained from the PQ Corp. under the product label VALFOR CP 300-35. This solid-acid HY-zeolite is a Faujasite aluminosilicate possessing an internal surface area of 700 m2/g, unit cell size of 2.435 nm, Si02/A103molar ratio of 6.5, and NazO composition of 0.18 wt %. The Y-zeolite pore matrix consists of 0.75-nm-diameter pores connected to 1.3-nm-diametercages arranged in cubic symmetzy. The particle size of the powder is mostly less than 38 pm (71 wt % ) as determined by sieve shaking. Batch Reactor. A 300-mL stirred autoclave reactor manufactured by Parr Instruments was used for all reaction studies. This batch reactor is schematically illustrated in Figure 1. The reactor is constructed of 316 stainless steel and is rated at 2000 psig maximum pressure and 350 "C maximum temperature. The reactor head plate has six ports to accommodate a thermocouple well with type-J thermocouple, cooling water heat exchanger loop, gas inlet/outlet porta with shut-off valve, liquid inlet port with shut-off valve, pressure relief valve, and liquid sampling line. The liquid sampling line (4.5 mm) has a l0-wm stainless steel mesh filter at the inlet and a shut-off valve at the outlet. The reactor contents are mixed by an overhead mixer with four-blade pitched turbine impeller. Mixing speed is controlled electronically and displayed on a digital tachometer. Temperature in the reactor is maintained by an external heating jacket and an internal cooling water loop. A PITI controller regulates power input to the heating jacket and also controls the solenoid valve that regulates water flow through the cooling loop. This arrangement provides precise control of reactor temperature to h0.5 OC. Reaction Procedure. The desired amount of crystalline D-glucose and distilled/deionized water were weighed separately to a precision of 0.01 g. The total mass of the
glucose and the water was fixed at 170 g. The glucose crystals were dissolved in the water. The initial concentration of glucose used in this study was 12 w t % (20 g of glucose/l50 mL of water). The glucose solution and a preweighed amount of Y-zeolite powder were charged to the 300-mL reactor vessel. The head plate was mounted onto the vessel and sealed. The mixing speed was set to the desired value, typically 300 rpm. The sealed reactor was purged with nitrogen gas for 10 min, and then the headspace over the liquid was pressurized at 30 psig nitrogen to facilitate removal of liquid samples from the reactor. The reactor was then heated up to the desired set point temperature. After the reaction temperature was established, the gas pressure over the liquid did not increase over the course of the reaction, implying that production of gaseous products was not significant. The liquid phase was sampled periodically at l-h intervals for the first 10 h, at 2-h intervals from 10 to 16 h, and finally at 24 h. About 0.5 mL of the reactor contents was removed during sampling. The 10-pm frit at the inlet of the sample line kept the catalyst powder in the reactor. Each liquid sample was immediately placed in an ice bath, weighed to a precision of 0.1 mg, and then diluted in a 6-mL aqueous solution containing 18 mg of butyric acid (internal standard for HPLC organic acids analysis) and 75 mg of myo-inositol (internal standard for HPLC monosaccharides analysis). The diluted sample was mixed thoroughly and then filtered through a 0.45-pm filter. The filtrate was stored frozen in the dark until analysis by HPLC. HPLC. The liquid samples from the reactor were analyzed by high performance liquid chromatography (HPLC). The HPLC system consisted of a Waters 501 dualpiston precision isocratic pump, Eldex column oven, Rheodyne 725 zero dead volume injector with 20-pL sample loop, Waters 484 variable wavelength UV/vis detector, Altex 156 differential refractive index detector, and SRI Instruments PeakSimple I1 chromatography computer data station. Sugars and oxygenated hydrocarbons were analyzed separately. The sugars in the reaction sample were separated on a BIO-RAD HPX-87P monosaccharide analysis column (300 X 19 mm) at 85 "C using He-degassed HPLC water at 0.6 mL/min as the eluting solvent. Column effluent sugars were detected by differential refractive index with respect to the solvent. Retention times for glucose, fructose, and myo-inositol (the internal standard) were 12.7, 16.8, and 21.0 min, respectively. Sugars were detected by refractive index because they have a very attenuated UV absorption at wavelengths greater than 190 nm. Organic acids (formic acid, 4-oxopentanoic acid), furan derivatives (HMF), and other oxygenated hydrocarbons in the reaction sample were separated on a BIO-RAD HPX-87H organic acids analysis column at 65 "C using Hedegassed 0.005 M H#04 solution as the eluting solvent. Column effluent products were detected by UV absorption at either 210 or 270 nm. Retention times (tR), UV wavelength absorption maxima (Am,), and extinction coefficients (c) for formic acid, 4-oxopentanoic acid, butyric acid (the internal standard), and HMF are provided in Table I. Butyric acid was chosen as an internal standard because its peaks was well separated from the product peaks of interest and because its retention time did not overlap with any of the other peaks in the product profile. However, since the ,A, for butyric acid is 208 nm, all quantitative analyses using the internal standard were carried out at 210 nm. The large difference in extinction coefficients between the compounds complicated quantitative analysis.
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 13 Table I. HPLC Analysis with the BIO-RADHPX-87H Column retention response factor, time, ,,A Rf,at 210 nm component , t R (mid (nm) t ((rV.s)/rg) 8.34 X lo' 13.5 205 44.67 formic acid 2.63 X 10' 15.3 270 25.12 4-oxopentanoicacid 4.47 X lo' 21.0 210 63.09 butyric acid (internal standard) 5-(hydroxymethyl)-229.3 284 18197 1.55 X lofi furfural
IS0
~~
A t 210 nm, the ratio of response factors for equimolar amounts of 4-oxopentanoicacid and butyric acid was 0.78, whereas the ratio of response factors for equimolar amounts of HMF and butyric acid was 49.5 (a factor of 63X higher), indicating that 4-oxopentanoic acid was much harder to detect than HMF. For both sugar and organic acids chromatograms, component peaks and internal standard peaks were integrated, and the concentration of each identified component, expressed as grams of component per gram of solution, was quantified by the internal standard method of data analysis. Solid Residue Analysis. A t the end of the reaction, typically 24 h, the reactor contents were vacuum filtered. The filtrate was clear yellow to dark brown in color, depending on the severity of the reaction conditions. The insoluble cake, which consisted of a brown residue dispersed on the catalyst powder, was dried under vacuum at room temperature until the total weight of the residue did not change. A 1-g portion of the dried residue was removed, washed repeatedly with tepid water, and then redried under vacuum until the weight did not change. From this gravimetric data, the amount of carbonaceous residue per unit mass of fresh catalyst Waf)determined. The residue and catalyst were then dried at 500 "C in a muffle furnace for 12 h under flowing nitrogen to boil off volatile components in a nonoxidizing environment. The amount of coke remaining on the catalyst was determined by weighing. Catalyst Acidity. The hydrogen form of the Y-zeolite catalyst has Bronsted acid sites (hydrogen atoms) exchanged for sodium atoms in the aluminosilicate crystalline matrix. The concentration of acid sites, expressed as either the number of sites per gram of powder or millimoles of H+ equivalents per gram of powder, was determined by a nonaqueous titration technique (Kladnig, 1976) using 0.125 N n-butylamine in dry benzene as titrant and 0.1 wt ?% 4-phenylazodiphenylamine (pK, +1.5) in dry benzene as the Hammett indicator. This indicator was picked because its molecular dimensions are small enough to enable diffusion of the indicator molecule through the Yzeolite pore matrix and because the end point coloration is readily observable (Kladnig, 1979). Before titration, the catalyst sample was heated in a muffle furnace under flowing nitrogen at 500 "C for 12 h to drive off all traces of water. The dried catalyst was weighed and loaded to three dry screw-cap vials. After 3 mL of dry benzene (stored over 4-A molecular sieves) and two drops of the indicator solution were added to each vial, the vial contents were mixed for 24 h under continuous shaking to let the indicator molecules completely diffuse into the Y-zeolite pore matrix. The color of the solution turned purple following addition of the indicator solution. The titrant was added to each sealed vial in 0.03-mL increments with a ~OOO-HLmicropipet every 24 h under continuous shaking until the color of the solution and the catalyst powder changed completely from purple to yellow. The acidity was determined from the amount of titrant used.
3
1
2. fructor* 8 formk reid 1. 4 ~ o x o p ~ n t r n o 1r c l d 10. butyrlc rcld (Inlrmrl alrndrrd)
2
1 1 . HMf
1
IS0
1
11
I I
I!
J .20
0
IO
20
30
40
Time (mln)
Figure 2. HPLC profile of water soluble reaction producta on a BIO-RADHPX-87H organic acids analysis column. See Experimental Section for analytical conditions. Sample: 130 "C; 12 w t % initial glucose concentration; 10 g of catalyst/l50 mL;12-h reaction time. The void peak 1 is representative of polymerized HMF; seven additional unknown compounds (peaks 3-5,7, 9,12, 13) were also found.
Results Reactions of aqueous glucose solution with HY-zeolite powder were carried out in a well-mixed, 300-mL batch reactor. Consumption of glucose and production of selected oxygenated hydrocarbon products were measured by HPLC as a function of reaction time at a fixed set of process conditions. Process conditions included mixing speed, temperature (110-160 "C), and catalyst concentration. Coking and catalyst acidity were also measured as a function of process conditions. HPLC Analysis of Oxygenated Hydrocarbons. Before the effects of process conditions on the aqueous glucose/Y-zeolite reaction system are considered, the oxygenated hydrocarbon products present in the water soluble reaction products must be identified. The water soluble reaction products were amber to dark brown in color, depending on reaction conditions. As described earlier, oxygenated hydrocarbons in the water soluble reaction products were separated by HPLC using a BIORAD HPX-87H organic acids analysis column with UV detection. A sample HPLC profile at 210 and 270 nm is shown in Figure 2. The primary oxygenated hydrocarbon products derived from the acid-catalyzed dehydration of glucose, including HMF, formic acid, and 4-oxopentanoic acid were identified in the reaction mixture. 4-Oxopentanoic acid was difficult to accurately quantitate because of a very low UV detection response, as described in the Experimental Section. Both formic and 4-OXOpentanoic acid peaks were not resolved to the base line which further complicated quantification. At the analysis conditions specified, seven minor unknown products were also isolated in the elution range where low molecular weight oxygenated hydrocarbons such as aldehydes, ketones, and furan derivatives are typically found (Pecina et al., 1984). Possible candidates for unknown peaks are methylglyoxal and dihydroxyacetone, which Bobleter and Bonn (1983) identified as secondary products of glucose hydrothermolysis in dilute acid following HPLC profiling of the water soluble products on a BIO-RAD HPX-87H column under similar analysis conditions. Varying the wavelength from 210 to 270 nm improved the response of two unknown products, slightly improved the response to 4-oxopentanoic acid, but greatly depressed the response for butyric acid (the internal standard). The trailing void peak from retention times of 6-9 min is rep-
14 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 12
Wt%
1 ,
glucose
I
1 0 g zeoliteI150 mL 1 3 0 OC
C
.-
?
o.s
>
C
0
ii
0.6
1
o.2
teei
0
I
1200 rpm 300 rpm
1 .o
0.01 1
0.0 b
0
5
10
15
20
25
Time (h)
Figure 3. Glucose conversion vs reaction time a t mixing speeds of 300 and 1200 rpm. 1 2 w t X glucose 1 0 g zeo11Wt 50
mL
I 1BOOC
C
.-0 I L
al E
8 m
0 0
a 0
5
10
15
20
0.0022 0.0024 0.0026 0.0028 1IT ( K
)
Figure 5. Arrhenius plot based on apparent rate constant (kapp)for glucose conversion a t temperatures ranging from 110 to 160 ' C . Slope -E/R = 11700 f 203 (Is, n = 4, r2 = 0.9994).
At 130 "C, 80% conversion was achieved within 24 h, whereas, at 160 "C, 100% glucose conversion was achieved within 8 h. In control experiments, 12% glucose solution with no catalyst was reacted at 130 and 160 "C within the batch reactor at 300 rpm. After 8 h, 5% of the glucose degraded at 130 "C and 30% of the glucose degraded at 160 "C. After 24 h, 10% of the glucose degraded at 130 "C and 35% of the glucose degraded 160 "C. These values are upper limits because in the real reaction system consumption of glucose will be promoted primarily by the Y-zeolite catalyst. Glucose conversion (X,)vs time data were fitted to the linear form of the pseudo-first-order equation, which is given by
25
Time (h)
Figure 4. Glucose conversion vs reaction time at temperatures ranging from 110 to 160 O C . The solid lines represent the fit of the data to a pseudo-first-order rate expression.
resentative of higher molecular weight materials, possibly polymers of HMF. D-Fructose, the product of acid-catalyzed isomerization of glucose, was also identified by HPLC with the organic acids analysis column and UV detection, but was quantified using the BIO-RAD HPX-87P monosaccharide analysis column and refractive index detection. Mass-Transfer Considerations and Effect of Mixing Speed. External mass-transfer resistances created by insufficient mixing can limit the overall glucose conversion rate if the difference between the glucose concentration in the bulk solution and the glucose concentration at the particle outer surface is large. For the batch reaction of glucose solution in a Y-zeolite catalyst slurry, external mas-transfer resistances were minimized by increasing the mixing speed until the glucose conversion vs time profile did not change. In Figure 3, a mixing speed of 300 rpm was sufficient to minimize external mass-transfer resistances to the glucose conversion rate at 130 "C. Effect of Temperature. Glucose conversion vs reaction time profiles at temperatures ranging from 110 to 160 "C are shown in Figure 4. At each temperature, the mixing speed was fixed at 300 rpm so that external resistances to mass transfer were minimized. The catalyst loading was fixed at 10 g of catalyst/l50 mL of water, and the initial glucose concentration was fEed at 20 g of glucose/ 150 mL of solvent (12% glucose by weight) so that the glucose to catalyst ratio was always fixed at 2:l by weight. Both the rate of glucose conversion and the maximum glucose conversion increased with increasing temperature.
The apparent rate constant kappwas estimated from the least-squares slope of In [1/(1 - XA)]vs t data at a fixed temperature. The solid lines in Figure 4 are the predicted XAvs time profiles given the fitted values for Itapp. An Arrhenius plot of In Itapp vs 1/T is shown in Figure 5. An apparent activation energy of 23.25 f 0.40 kcal/mol (1s) was determined from the least-squares slope of the line. Measured product yields of fructose, HMF, formic acid and 4-oxopentanoic acid are plotted as a function of time at temperatures ranging from 110 to 160 "C in Figure 6A-D, respectively. A t i60 "C, the concentration of all measured water-soluble reaction products increases rapidly and then decreases, indicating formation to other products. A t higher temperatures and longer reaction times, measured product recoveries were lower. Coke deposition and Y-zeolite catalyst acidity after a 24-h reaction time are plotted as a function of reaction temperature in Figure 7. The extent of coking increased dramatically at 130 "C and then appeared to level off. Effect of Catalyst Concentration. Glucose conversion vs time profiles at catalyst concentrations ranging from 2 g of catalyst/l50 mL of solvent to 20 g of catalyst/l50 mL of solvent are presented in Figure 8. At each catalyst concentration, the reaction temperature was fixed at 130 "C, and the initial glucose concentration was fixed at 20 g of catalyst/l50 mL of solvent (12 wt %). The mixing speed was also fixed at 300 rpm to minimize external mass-transfer resistances. When the catalyst concentration increased, the rate of glucose conversion also increased. The solid lines in Figure 8 represent the fit of X Avs time data to the pseudo-first-order rate equation, where k,,, is the fitted parameter at the given catalyst concentration.
50
0 0
C
r
7
X
X
o,
401
40 0
0
-a -m 30 a'u' 20
0
a
0
30
E" 2 w
20
a
0
c 0
.-0
-m
e
E lo
s 10
Q
5
0
10
15
20
25
E o
10
5
0
Time (h)
15
20
25
20
25
Time (h) 0
50
I
I
B
0 0
501
D
X 0
400
3
ll0OC
0
120% 130°C i6OOC
v
cn 0
-ca
0
0
30-
-D 4 0 -
0
.
30-
0 m
g
20
E E
gio x
?
P
0
5
10
15
20
25
-m- O-
0
5
10
15
Time (h) Time (h) Figure 6. Product yield vs reaction time profiles a t temperatures ranging from 110 to 160 "C: (A) fructose; (B)HMF; (C) formic acid; (D) 4-oxopentanoic acid. 0
3
0.5
residua belora caicinalion r e d d u e allel caicinalicn acidity after clicinellon
1 2 wt% glucose, 130 0 i0.5
I
1
1 .o
c
20 g zeolite11 50 mL
0.8 0.6 0.4
0.2 00
0
5
10
15
20
25
Temperature (OC) Figure 7. Coke formation and Y-zeolite acidity after 24-h reaction time at temperatures of 110-160 "C. Calcination conditions: 500 "C; 12 h; N2 atmosphere.
Time (h) Figure 8. Glucose conversion vs reaction time a t 130 "C and Yzeolite catalyst concentrations ranging from 2 to 20 g of catalyst/l50 mL of solvent. The solid lines represent the fit of the data to a pseudo-first-order rate expression.
A plot of kappvs catalyst concentration is shown in Figure 9. To account for the effect of catalyst concentration on the glucose conversion rate, the apparent rate constant can be rewritten as
plot of kappvs (&.Acat. has slope a and intercept log k (Figure 9). For the data given in Figure 9, the reaction order a was estimated at 0.45 f 0.01 (ls), using the fresh catalyst activity (Acat,)of 0.52 f 0.02 (1s) mmol of H+/g of catalyst. However, this relatively low value for the reaction order may be due to a reduction in catalyst acidity as a result of coking on the catalyst. Measured product yields of fructose, HMF, formic acid, and 4-oxopentanoic acid are plotted as a function of time at increasing catalyst concentrations in Figure 10A-D, respectively. Product yields and production rates also increase with increasing catalyst concentration, but the
kapp = k[Ccat.Acet,I"
where k is an intrinsic rate constant for glucose conversion, C,, is the catalyst concentration in the reactor (grams of catalyst per liter of solvent), A,,, is the catalyst activity is the (moles of acid sites per gram of catalyst), and reaction order for the catalyst concentration. A log-log
16 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 I
I
'
5i
41
4 5
10
2
3
1
4 5
100
C C a t . A c a t(rnrnole H+/L)
Figure 9. Reaction order plot for catalyst concentration. Catalyst concentration is expressed as millimoles H+ equivalents per liter of ranging from 6.9 to 68.7 mmol of H+/L, reaction volume (CCnLACnL C, ranging from 2 to 20 g of Y-zeolite/lBO mL). The acidity of the fresh catalyst (0.52 mmol of H+/g) was used for all calculations. Slope = 0.45 0.01 (18, n = 4, r2 = 0.9956).
dependence is fairly weak, especially at high catalyst concentrations. Coke deposition and Y-zeolite catalyst acidity after a 24-h reaction time are plotted as a function of catalyst concentration in Figure 11. The extent of coking increased with decreasing catalyst concentration because the glucose to catalyst ratio increased.
Discussion The reaction of aqueous glucose solution with solid-acid Y-zeolite powder was studied in a well-mixed batch reactor
at temperatures ranging from 110 to 160 "C, catalyst loadings ranging from 2 to 20 g of catalyst/l50 mL, and mixing speeds sufficient to minimize external mass-transfer resistances (1300 rpm). Conversion of glucose and production of fructose, HMF, formic acid, and 4-oxopentanoic acid were measured as a function of reaction time up to 24 h. Maximum glucose conversions were greater than 80% at 130 "C and achieved 100% at 160 "C. Increasing the catalyst concentration also increased the glucose conversion rate, and control experiments conducted over the same range of process conditions but with no catalyst added showed minor decomposition of glucose to carbonaceous residue but not to the aforementioned products. Thus, although thermal degradation of glucose did occur, the conversion of glucose was promoted primarily by the solid-acid Y-zeolite catalyst at fairly low temperatures of 110-160 The apparent activation energy for glucose conversion was estimated at 23.25 f 0.40 kcal/mol (Is), which is lower than activation energies reported for the aqueous mineral acid-catalyzed dehydration of glucose. For example, McKibbins et al. (1962) estimated an apparent activation energy of 32.7 kcal/mol for dehydration of glucose in 0.025-0.8 N sulfuric acid solution, and more recently Baugh and McCarty (1988) reported an apparent activation energy of 28.9 kcal/mol for dehydration of glucose in 0.001-0.1 N sulfuric acid at 170-230 "C. The relative high value of 23.25 kcal/mol for the apparent activation energy suggests that dehydration of glucose with Y-zeolite powder was rate limiting rather than diffusion limiting. Analysis of the water soluble reaction products by HPLC revealed that the solid-acid Y -zeolite catalyst promoted
"c.
0
0
30
v-
X
m W 0
0
-aD 20
-m
z .
U .0
m 10
.-0
E0
c
E
. m
5
0
10
15
20
25
0 0
5
Time (h)
B
25
20
25
X
0
F
3
K
v
m
-aD
20
30
~~
2 U
15
Time (h)
"I
0 0
10
0
20-
m
68.7 mmol H+/L 34.3 mmol H+/L 13.7 mmol H+/L 6.9 mmol H+/L
10
u
-a Q 20
. E
E a
-u
:
c
10
E
a X
P
9
0
5
10
15
20
25
: o -0
5
10
15
Time (h) Time (h) Figure 10. Product yield vs reaction time profiles at 130 OC and varying Y-zeolite catalyst concentrations. Catalyst concentration is expressed as millimoles of H+ equivalents per liter of reaction volume (C,,,A,,, ranging from 6.9 to 68.7 mmol of H+/L, C, ranging from 2 to 20 g of Y-zeolite/lBO mL). (A) Fructose; (B)HMF; (C) formic acid; (D)4-oxopentanoic acid.
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 17
* 0.5 I
i
rmiidue belole cetcinillon residue 11111 cilclnillon icldlly alter Cilclnillon
REACTION PROCESSES I
0.5
(1) Dehydration
- 0.4 -
0.3
m
E
- 0.2
U
-
E 0
v)
A
3
1:I
0
. e f,
25
OH F=F-$--~~CHzOH HOHHOHH
,
H
H
~
:
~
o
&
HMF
1,2 ENEDlOL
3H20
HO OH
H
D-FRUCTOSE (2) Parllal Rehydration CH 3- C-CH & y C -
CCat (g Y-zeolite/l 5 0 rnL)
Figure 11. Coke formation and Y-zeolite acidity after 24-h reaction time a t 130 OC and catalyst concentrations ranging 2 to 20 g of catalyst/l50 mL of solvent. Calcination conditions: 500 "C; 12 h; N2 atmosphere.
H'
D-GLUCOSE
4 3
m
H
hE
H+
H x o c H p
.
a
4-OXOPENTANOIC ACID
+ H -C-
HMF
t
OH
b
a
2H20
OH
FORMIC ACID
(3) Isomerization
the partial dehydration of glucose to HMF and the subsequent rehydration of HMF to formic acid and 4-OXOpentanoic acid, presumably by reaction chemistry similar to that for the hydronium-ion-catalyzed partial dehydration of glucose. Although quantification of formic acid and Coxopentanoic acid was difficult, molar formic acid yields were consistently higher than molar 4-oxopentanoic acids yields (30% for formic acid v8 15-2090 for Coxopentanoic acid). Since the reaction stoichiometry of HMF rehydration requires equimolar production of formic acid and 4-oxopentanoic acid, the difference might be due to reactions of 4-oxopentanoic acid within the Y-zeolite. Within the Y-zeolite, reactions of 4-oxopentanoic acid might be preferred over formic acid, because 4-OXOpentanoic acid molecule is larger than formic acid and thus diffuses slower through the Y-zeolite, allowing a longer residence time for secondary reactions. The maxima in both formic acid and 4-oxopentanoic acid yield vs time profiles at 160 "C are further evidence for secondary reactions of these compounds. Acid sites on the outer surface of the Y-zeolite catalyst also promoted the isomerization of glucose to fructose; the maxima in the fructose yield vs time curves suggest that fructose subsequently underwent partial dehydration to furan and organic acid products by a mechanism similar to that for acid-catalyzed partial dehydration of glucose. Several minor unknown products, possibly low molecular weight oxygenated hydrocarbons related to ketones, aldehydes, or furan derivatives where also isolated by HPLC. These compounds may be secondary dehydration products such as methylglyoxal and dihydroxyacetone (Bobleter and Bonn, 1983) or derivatives of the primary dehydration products. In future work, these compounds may be identified by gas-liquid chromatography/mass spectrometry at conditions suggested by Shaw et al. (1967) for analysis of minor compounds produced by the hydronium-ion-catalyzed dehydration of fructose. Polymers of HMF were also tentatively identified. Polymerization of HMF is promoted by the acid catalyst, and HMF polymers are known precursors to coke formation (Kuster, 1977; Kuster and Temmink, 1977; Kuster and Van der Baan, 1977). Over the range of reaction conditions studied, coke deposition on the catalyst increased with increasing temperature and decreasing catalyst concentration. The increase in coking was accompanied by a decrease in catalyst acidity, but it is not known if coke deposited on the outer surface of the particle or within the pores of the Y-zeolite.
H
CH .OH
H+
*
L-
li bH
D- GLUC 0S E
I A !
D-FRUCTOSE
(4) Carbonlzailon CH20H
H+
CxHy CARBON DEPOSIT
HMF
Figure 12. Reaction processes catalyzed by HY-zeolite catalysts.
The light brown color of the catalyst after reaction suggests that at least some of the coke deposited on the outer surface. In summary, acid-catalyzed reaction processes which described glucose conversion and product formation were identified as the isomerization of glucose to fructose, the dehydration of glucose to HMF, rehydration of HMF to formic acid and 4-oxopentanoic acid, and carbonization of g l u m or oxygenated hydrocarbons (Figure 12). In Figure 13, the individual reaction processes are assembled into a reaction scheme involving the Y-zeolite catalyst. A closer look at the aqueous glucose/Y-zeolite reaction system shows three reaction zones: the bulk fluid, the particle outer surface, and the intracrystalline pore-cage network within the Y-zeolite. Did the solid-acid Y-zeolite catalyst promote the shape-selective dehydration of glucose within the Y-zeolite matrix? Molar yields of each organic acid always exceeded yields for HMF in the bulk phase by at least of factor of 2, even though HMF is the first reaction product. However, the appearance of HMF in the bulk phase suggests that some of the glucose dehydrated to HMF on the outer surface of the particle, because a purely shape-selective dehydration mechanism would produce and trap HMF only within the cages of the Y-zeolite. Also, attraction of polar water molecules to the aluminosilicate pore matrix could further encourage surface reaction by limiting access of solute molecules to intraparticle catalytic sites. To help distinguish between surface/ bulk-phase reactions and possible intraparticle reactions, selectivities of HMF for the heterogeneous dehydration of glucose over Y-zeolite were compared to selectivities of HMF for the homogeneous dehydration of glucose by aqueous mineral acids at the same temperature and equivalent acid concentration (Figure 14). Glucose conversion and HMF yield data for
18 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 Aqueous Phase
Catalyst Phase
4-Oxopentanoic acid
I
/ cy-c-cyH,-c-oH
8
8
4-Oxopentanoic acid
+ Y-c-
8
OH
Formic acid +
i
I
I
Fructose Figure 13. Proposed reaction scheme of glucose with HY-zeolite catalyst illustrating both surface and molecular sieving reactions. D-
1 2 w l % glucose, 1 3 0
OC
34 3 mmol H'IL
-
"
0)
.
L
SHUF. aqueous H 2 S 0 , SHUF. zeolite
I
: I 8
0.3
Time (h) Figure 14. Comparison of HMF yield selectivity data for the homogeneous mineral acid-catalyzed glucose dehydration vs glucose dehydration catalyzed by HY-zeolite powder.
the homogeneous process were obtained by reacting 20 g of glucose with 150 mL of 17.2 mM H2S04solution (34.3 mmol of H+/L, an acidity equivalent 10 g of zeolite/l50 mL) at 130 "C using procedures described in the Experimental Section. Yield selectivities of HMF for the HYzeolite catalyst were lower than yield selectivities of HMF for the homogeneous process. Since HMF is the fmt stable product of a two step reaction process for organic acids production, lowered values for the yield selectivity of HMF relative to the homogeneous process imply that some of the HMF was formed within Y-zeolite cages. However, since HMF was present in the bulk phase, it is likely that glucose dehydration also occurred on the outer particle surface. Thus, when the glucose molecule is cleaved to the linear 1,2-enediol by a Bronsted acid site on the outer surface of the particle, the 1,2-enediol either dehydrates to HMF or diffuses into the Y-zeolite matrix for further reaction. Although the 1,2-enediol intermediate can diffuse into the pore matrix of the Y-zeolite crystal, glucose may not be able to directly diffuse into the pore matrix because the size of the cyclic glucose molecule (9-8, diameter) is too large to diffuse directly through the 7.5-A pore openings of the Y-zeolite crystal. However, on the basis of chromatographic experiments for sorption of glucose on Ca-Y
zeolites, Ho et al. (1987) stated that intracrystalline diffusional resistance is negligible and the sorption kinetics are controlled entirely by the combined effects of macropore diffusion (in the zeolite crystal binder) and external resistances. Since they did not directly measure the intracrystalline diffusion coefficient, the question of whether or not glucose can directly diffuse into the Y-zeolite pore matrix is still an open question worthy of future research. The above discussion suggests that shape-selective reactions of glucose within the pore matrix might be promoted by molecular-sievingacid catalysts which have pore sizes slightly larger than the glucose molecule (Le., >10 A), such as the 12-A zeolite synthesized by Davis et al. (1988) or pillared clays with 10-15-A galley heights (Pinnavia et al., 1985; Rightor et al., 1991). Study of the intracrystalline diffusion and reaction of glucose within large-pore (>lo-& molecular sieving catalysts is planned in future work, along with measurements of intracrystalline diffusivities and development of a heterogeneous kinetic model. Since glucose is a hydrogen-deficient compound, combined hydrogenolysis and dehydration of glucose to lower molecular weight alcohols in water/methanol solutions over bifunctional large-pore molecular seiving catalysts is also worthy of future consideration.
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research through Grant No. 24866-G5. Literature Cited Baugh, K. D.; McCarty, P. L. Thermochemical Pretreatment of Lignocellulose to Enhance Methane Fermentation: I. Monosaccharide and Furfurals Hydrothermal Decomposition and Product Formation Rates. Biotechnol. Bioeng. 1988, 31, 50-61. Bobleter, 0.; Bonn, G. The Hydrothermolysis of Cellobiose and Its Reaction Product D-Glucose. Carbohydr. Res. 1983,124,185-193. Chen, N. Y.; Koenig, L. R. Process for Converting Carbohydrates to Hydrocarbons. US.Patent 4,503,278, 1985. Chen, N. Y.; Garwood, W. E. Industrial Application of Shape-Selective Catalysis. Catal. Rev.-Sci. Eng. 1986, 28, 185-264. Chen, N. Y.; Deganan, T . F.; Koenig, L. R. Liquid Fuel from Carbohydrates. CHEMTECH 1986, 16, 506-511. Dao, L. H.; Haniff, M.; Houle, A.; Lamothe, D. Reactions of Model Compounds of Biomass-Pyrolysis Oils over ZSM-5 Catalysts. In Pyrolysis Oils from Biomass; Soltes, E. J., Milne, T. A., Eds.; ACS Symposium Series 376; American Chemical Society: Washington, D.C., 1988; pp 328-341.
Ind. E n g . C h e m . Res. 1993, 32, 19-26 Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. VPI-5: The First Molecular Sieve with Pores Larger than 10 Angstroms. Zeolites 1988, 8, 362-366. Feather, M. S.; Harris, J. F. Dehydration Reactions of Carbohydrates. Ado. Carbohydr. Chem. Riochem. 1973, 28, 161-224. Haniff, M. I.; Dao, L. H. Deoxygenation of Carbohydrates and Their Isopropylidene Derivatives over ZSM-5 Zeolite Catalysts. Appl. Catal. 1988, 39, 33-47. Ho, C. C.; Ching, C. B.; Ruthven, D. M. Comparative Study of Zeolite and Resin Adsorbents for the Separation of Glucose-Fructose Mixtures. Znd. Eng. Chem. Res. 1987,26, 1407-1412. Jacobs, P.; Herve, H. Single-Step Catalytic Process for the Direct Conversion of Polysaccharides to Polyhydric Alcohols by Simultaneous Hydrolysis and Hydrogenation. Eur. Patent E P 329923, 1989. Kieboom, A. P. G.; van Bekkum, H. Chemical Conversion of Starch-Based Glucose Syrups. In Starch Conversion Technology; van Beynum, G. M. A,, Roels, J. A., Eds.; Dekker: New York, 1986. Kladnig, W. Surface Acidity of Cation Exchange Y-zeolite. J . Phys. Chem. 1976,80, 262-269. Kladnig, W. Use of Hammett Indicators for Acidity Measurements in Zeolites. J . Phys. Chem. 1979, 83, 765-766. Kuster, B. F. M. The Dehydration of D-Fructose (formation of 5hydroxymethyl-2-furaldehydeand levulinic acid): Part 111. The Influence of Water Concentration on the Dehydration of DFructose. Carbohydr. Res. 1977,54, 177-183. Kuster, B. F. M.; Temmink, H. M. G. The Dehydration of DFructose (formation of 5-hydroxymethyl-2-furaldehydeand levu-
19
linic acid): Part IV. The Influence of pH and Weak-Acid Anions on the Dehydration of D-Fructose. Carbohydr. Res. 1977, 54, 185- 191. Kuster, B. F. M.; Van der Baan, H. S. The Dehydration of DFructose (formation of 5-hydroxymethyl-2-furaldehyde and levulinic acid): Part 11. The Influence of Initial and Catalyst Concentrations on the Dehydration of D-Fructose. Carbohydr. Res. 1977, 54, 165-176. McKibbins, S. W.; Harris, J. F.; Saeman, J. F.; Neill, W. K. Kinetics of Acid Catalyzed Conversion of Glucose to 5-Hydromethyl-2Furaldehyde and Levulinic Acid. For. Prod. J. 1962, 12, 17-23. Pecina, R.; Bonn, G.; Burtscher, E.; Bobleter, 0. High-Performance Liquid Chromatographic Elution Behavior of Alcohols, Aldehydes, Ketones, Organic acids, and Carbohydrates on a Strong CationExchange Stationary Phase. J . Chrornatogr. 1984,287,245-258. Pinnavaia, T. J.; Tzou, M A . ; Landau, S. D. New Chromia Pillared Clay Catalysts. J. A m . Chem. SOC.1985, 107, 4783-4785. Rightor, E. G.; Tzou, M.-S.; Pinnavaia, T. J. Iron Oxide Pillared Clay with Large Gallery Height: Synthesis and Properties. J . Catal. 1991, 130, 29-40. Schraufnagel, R. A.; Rase, H. F. Levulinic Acid from Sucrose Using Acidic Ion-Exchange Resins. Ind. Eng. Chern. Prod. Res. Dev. 1975,14, 40-44. Shaw, P. E.; Tatum, J. H.; Berry, R. E. Acid-Catalyzed Degradation of D-Fructose. Carbohydr. Res. 1967, 5, 266-273.
Received f o r review July 13, 1992 Revised manuscript received September 18, 1992 Accepted October 5, 1992
Gas-Liquid Mass Transfer and Holdup in Vessels Stirred with Multiple Rushton Turbines: Water and Water-Glycerol Solutions M. Nocentini, D. Fajner, G. Pasquali, and F. Magelli* Dipartimento di Ingegneria Chimica e d i Processo, University of Bologna, viale Risorgimento 2, I-40136 Bologna, Italy
Gas-liquid mass-transfer coefficient, holdup, and power consumption were measured in vessels stirred with multiple Rushton turbines. Water and glycerol-water solutions were used, with viscosity up to 70 mPa.s. For measuring kLa,the dynamic oxygen electrode technique was adopted. The following For ah-water correlation is applicable for the water-glycerol solutions: k,a = C(P,/ V)a(Us)B(p/~,m)6. systems the parameter C , while being the same as that for single impellers, is different from that for water-glycerol solutions. At equal P / V and Us,the mass-transfer coefficient for solutions of low glycerol concentration is higher than tkat for water, whereas it decreases at an increase in glycerol concentration. This behavior can be attributed to the noncoalescing characteristics of these solutions that are superimposed to the influence of viscosity. Cross-checking of kLa and holdup data confirms the interpretation given. The suitability of several equations available in the literature in interpreting the present data is also discussed. 1. Introduction Gas-liquid mass transfer in stirred vessels is a process of considerable importance in the chemical and biochemical industries. Excellent review papers were published on the matter, the most relevant aspects covered being the measuring techniques for mass-transfer coefficients and the way to correlate them to the operating parameters and system properties (e.g., van’t Riet, 1979; Sobotka et al., 1982; Linek et al., 1987; Zlokarnik, 1978; Judat, 1982; Nienow and Ulbrecht, 1985; Shah, 1991). Attention has mainly been focused on the behavior of vessels stirred with single impellers of several styles. However, even for these, general correlations that prove completely reliable and always useful for design are not available (Tatterson, 1991). The reason for this seems to be the specific role that the
* To whom correspondence should be addressed. 0888-5885/93/2632-0019$04.00/0
various system properties play. In spite of their practical interest (for instance, in the fermentation industry), much fewer efforts have been devoted to the study of multiple-impeller vessels. The few exceptions regard equipment stirred with either standard radial turbines (Taguchi and Kimura, 1970; Roustan et al., 1978; Ramanarayanan and Sharma, 1982; Jurecic et al., 19@ Hickman and Nienow, 1986; Ho et al., 1987; Spanihel et al., 1987;Machon et al., 1988, Cooke et al., 1988; Oldshue et al., 1988), mixed-flow impellers (Hickman and Nienow, 1986), or proprietary devices (Kipke, 1978; Hickman and Nienow, 1986; Cooke et al., 1988; Oldshue et al., 1988). Gas holdup, which is an index of the gas-liquid masstransfer ability of the system, has also been the subject of many investigations. In this case, too, relatively few data and correlations are available for multiple-impeller equipment (Lodi et al., 1982; Abrardi et al., 1988; Nocentini et al., 1988a; Smith, 1991). 0 1993 American Chemical Society
