JOURNAL OF CATALYSIS 171, 130–147 (1997) ARTICLE NO. CA971777
Isobutanol and Methanol Synthesis on Copper Catalysts Supported on Modiﬁed Magnesium Oxide
Mingting Xu, Marcelo J. L. Gines, Anne-Mette Hilmen, Brandy L. Stephens, and Enrique Iglesia1 Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94710
Received January 27, 1997; revised June 9, 1997; accepted June 10, 1997
for 2-methyl-1-propanol (isobutanol) because of its steric Alcohols are selectively produced from CO/H2 on K–CuMgCeOx hindrance and the lack of the two -hydrogens required catalysts, but synthesis rates are strongly inhibited by CO2 formed for aldol condensation reactions. Therefore, isobutanol be- during reaction. Reaction pathways involve methanol synthesis comes a preferred end product of alcohol chain growth re- on Cu, chain growth to C2+ alcohols, and metal–base bifunc- actions on basic oxides. Isobutanol synthesis appears to be tional coupling of alcohols to form isobutanol. Ethanol reactions limited by the rate of ethanol formation from synthesis gas – on K Cu0.5Mg5CeOx show that Cu catalyzes both alcohol dehydro- (4, 6), which limits the concentration of the aldehydic inter- genation and aldol condensation reactions. CeO increases Cu dis- 2 mediates required for chain growth. High reactor pressures persion and MgO surface area and K decreases Cu dispersion, but increases the density of basic sites. Reactions of acetaldehyde and favor the synthesis of higher alcohols and bimolecular con- 13C-labeled methanol lead to 1-13C-propionaldehyde, a precursor densation reactions during CO hydrogenation, but lead to to isobutanol. The density and strength of basic sites were mea- unfavorable thermodynamics for alcohol dehydrogenation 12 13 sured using a CO2/ CO2 isotopic jump method that probes the steps that may be required in order to form aldehyde inter- number and chemical properties of basic sites available at typical mediates required in aldol coupling pathways. isobutanol synthesis temperatures. K or CeO2 addition to CuMgOx Our study addresses the reaction pathways and catalytic increases the density and strength of basic sites and the rates of site requirements for the synthesis of branched alcohols base-catalyzed ethanol condensation reactions leading to acetone on (metal–base) bifunctional catalysts at low temperatures. and n-butyraldehyde. The presence of CO in the He carrier during These bifunctional catalysts consist of Cu metal crystallites temperature-programmed surface reactions of ethanol preadsorbed supported on high-surface-area basic oxides based on mod- on Cu Mg CeO decreases the rate of base-catalyzed condensation 0.5 5 x iﬁed MgO. Speciﬁcally, we describe the synthesis proce- reactions of preadsorbed ethanol, possibly due to the poisoning of dures, catalyst structural characterization data, and surface basic and Cu sites by the CO2 formed from CO via water–gas shift reactions. c 1997 Academic Press and catalytic properties for a class of materials based on K-modiﬁed CuMgCeOx (6–8). These K–CuMgCeOx cata- lysts are among the best reported catalysts for alcohol synthesis at low temperatures and pressures (6–8). MgO- INTRODUCTION based solids also catalyze condensation reactions of al- cohols and aldehydes with high activity and selectivity Isobutanol is a potential fuel additive and a precursor (9, 10). to isobutene and methyl-tert-butyl ether. Currently avail- CuMgCeOx samples were prepared by controlled pH able catalysts for the synthesis of isobutanol from CO/H2 coprecipitation methods designed to maximize the num- mixtures require high temperatures (750–800 K) and pres- ber of Cu surface atoms and basic sites (8). Site titration sures (10–30 MPa) and produce isobutanol with relatively and isotopic switch methods were used to determine the low productivity and selectivity under milder reaction con- density and chemical properties of exposed Cu and ba- ditions (1–5). Typical isobutanol yields reported in the lit- sic sites. Isobutanol and linear alcohol synthesis pathways erature are shown in Table 1. were studied using the adsorption and surface reactions Isobutanol synthesis requires the initial formation of of alcohols and aldehydes in steady-state catalytic reac- methanol and higher linear alcohols and their subsequent tors and in a temperature-programmed surface reaction chain growth processes leading to 2-methyl-branched alco- (TPSR) apparatus. The role of individual elements in multi- hols with high selectivity (3). Chain growth rates are low component catalysts was explored by varying the composi- tion of K–CuMgCeOx catalysts and determining the effect 1 To whom correspondence should be addressed. E-mail: [email protected] of each component on surface properties and on reaction cchem.berkeley.edu. Fax: (510) 642-4778. steps.
130 0021-9517/97 $25.00 Copyright c 1997 by Academic Press All rights of reproduction in any form reserved. ISOBUTANOL AND METHANOL SYNTHESIS 131
TABLE 1 low-Cu catalysts in order to get the same amount of CuO charge in the reactor. After the temperature was lowered to Representative Isobutanol Synthesis Catalysts (6, 7) 3 300K,a5%H2/He mixture (100 cm /min) was introduced GHSV Isobutanol into the reactor and the temperature was then increased 1 Catalyst T (K) P (MPa) CO/H2 (h ) yield (g/kgcat/h) to 623 K at a rate of 0.17 K/s. The evolution of H2O and the consumption of H2 during reduction were followed by Cu–Zn–Cr–Cs 583 9.1 1.0/0.45 5330 51 Cu–Mg–Ce–K 593 5.1 1.0/1.0 1832 7.2 on-line mass spectrometry (H200M Gas Analyzer, Leybold Pd–Zr–Zn–Mn–Li 593 5.1 1.0/1.0 1832 1.1 Inﬁcon Inc.). Cu–Zn–Mn–K 643 10.0 0.5/1.0 10000 175a Zn–Cr–K 693 32.5 1.0/2.2 15000 125a Pd–Zr–Zn–Mn–Li 700 25.0 1.0/1.0 20000 740a Site Density and Reactivity
a g/litercat/h. Cu surface areas were measured by titrating Cu sur- face atoms in prereduced samples with N2O (Matheson, ultra high purity) at 363 K. N2O was introduced as pulses EXPERIMENTAL METHODS (1.60 mol N2O/pulse) into a He stream. The number of chemisorbed O atoms was obtained from the amount of Catalyst Synthesis N2O consumed and the N2 produced. A saturation cover- age of 0.5 : 1 O : Cu was used to estimate Cu surface area CuMgCeO samples were prepared by coprecipitation s x and dispersion (13, 14). Cu dispersion is deﬁned as the ratio of mixed nitrate solutions with an aqueous solution of of surface Cu (Cu ) to the total number of Cu atoms in the KOH (2 M) and K CO (1 M) at 338 K and a constant pH s 2 3 catalyst. of 9 in a well-stirred thermostated container. The solids In temperature-programmed CO desorption (TPD) formed were ﬁltered, washed thoroughly with 300–500 cm3 2 measurements, samples (50 mg) were ﬁrst pretreated in of deionized water at 333 K, and dried at 353 K overnight. ﬂowing He ( 100 cm3/min) at 723 K for 0.3 h and then Dried samples were treated in ﬂowing air at 723 K for 4 h reduced in 5% H /He at 623 K for 0.5 h. After exposure in order to form the corresponding mixed metal oxides. 2 to CO (Cambridge Isotope Laboratories, Inc.) for 0.15 h Residual potassium in all dry samples was less than 0.1 wt.% 2 at room temperature, the samples were ﬂushed with He to before alkali impregnation. The controlled pH coprecipita- remove gas phase and weakly adsorbed CO . The temper- tion technique produces uniform precipitates, which lead, 2 ature was then increased up to 723 K at a rate of 0.5 K/s, after thermal treatment, to mixed metal oxides with high and the desorption proﬁle of CO was monitored by mass surface area and intimate contact between components (8). 2 spectrometry. CuZnAlO samples were prepared using the same proce- x The density of basic sites was determined using a 13CO / dures as for CuMgCeO , except that the pH was held at 7.0 2 x 12CO isotopic exchange method. This method provides a and the dried powders were treated in ﬂowing air at 623 K 2 direct measure of the number of basic sites kinetically avail- for 4 h (11, 12). K and Cs were introduced by incipient able for CO adsorption at typical reaction temperatures. wetness of the oxidized samples using K CO (0.25 M) and 2 2 3 This technique also provides the reactivity distribution of CH COOCs (0.25 M) aqueous solutions (K CO : Fisher 3 2 3 basic sites and a direct measure of surface nonuniformity. In Scientiﬁc, A.C.S. certiﬁed; CH COOCs: Strem Chemicals, 3 this method, a prereduced catalyst was exposed to a 0.1% 99.9%). 13 CO2/0.1% Ar/He (Cambridge Isotope Laboratories, Inc.) stream at 573 K. After 13CO reached a constant concen- Structural Characterization 2 tration in the efﬂuent stream (0.5 h), the ﬂow was switched 12 Powder X-ray diffraction (XRD) patterns were collected to 0.1% CO2/He (Cambridge Isotope Laboratories, Inc.). 13 using a D5000 Siemens Diffractometer and monochromatic The decay in the concentration of CO2 as it was replaced 12 Cu-K radiation. Total surface areas were determined us- on the surface by CO2 was followed by mass spectromet- 13 ing the single point BET method by measuring N2 phy- ric analysis of CO2 and Ar concentrations as a function of sisorption at 77 K using a continuous ﬂow Quantasorb Sur- the time elapsed after the isotopic switch. The presence of face Area Analyzer (Quantachrome Corp.). Bulk catalyst Ar permitted corrections for gas holdup and hydrodynamic compositions were measured by atomic absorption spec- delays within the apparatus. troscopy (AAS). Temperature programmed surface reaction studies were The reduction behavior of Cu catalysts was studied using carried out by exposing a prereduced catalyst (33.0 mg) temperature-programmed reduction (TPR). This method to an ethanol/He stream at room temperature for 0.2 h. involves treating a Cu sample (15–100 mg) in He at 723 K Ethanol (C2H5OH: Fisher Scientiﬁc, A.C.S. certiﬁed) was for 0.3 h in order to remove adsorbed water and carbonates. introduced by passing He through a saturator containing Different amounts of samples were used for the high- and ethanol at 273 K. The catalyst sample was then purged with 132 XU ET AL.
He (100 cm3/min) at room temperature in order to remove purity). The reactant mixture was circulated continuously gas phase and weakly adsorbed species and the reactor tem- through the catalyst sample and reaction products were perature was increased to 723 K at 0.5 K/s. Desorption prod- sampled by syringe extraction after various contact times. ucts were continuously monitored by mass spectrometric Reactant and product concentrations were measured by analysis of the efﬂuent stream. The TPSR apparatus and the mass spectrometric and ﬂame ionization detection, after detailed experimental procedures will be described else- separation with a 5% phenyl-methyl-silicone capillary col- where (15). H2 and He were puriﬁed by passing through umn. The effect of CO2 (Matheson: research grade) during a molecular sieve trap (Matheson, Model 452 : 4A) in or- ethanol reactions on 1 wt.% K–CuyMg5CeOx was explored der to remove H2O and through an oxygen scavenging trap by adding different amounts of CO2 (0–20 kPa) to the 13 (Matheson, Model 64-1008A). ethanol reactants. Cross-coupling reactions of CH3OH (13C: 99%, 18O: 12%; Isotec Inc.) with acetaldehyde (Fisher Catalytic Evaluation and Mechanistic Studies Scientiﬁc, Reagent Grade) were carried out using the following reactant mixture: 12C H O/13CH OH/CH /He = High-pressure isobutanol synthesis studies were per- 2 4 3 4 4.0/8.0/2.7/86.6 kPa. 13CH OH was puriﬁed by several formed in a catalytic microreactor consisting of a single-pass 3 freeze–pump–thaw cycles before catalytic experiments. tubular reactor (1.27 0.071 cm, 304 stainless-steel tube, length 33.0 cm) held within a three-zone heated furnace in order to ensure uniform axial temperatures ( 1 K). The RESULTS AND DISCUSSION catalyst (2 g) was reduced in situ in pure H2 (Matheson, 99.99%) at 583 K and atmospheric pressure for 12 h be- Structural and Textural Properties of fore catalytic experiments were carried out at 4.5 MPa us- K–CuMgCeOx Catalysts = ing synthesis gas (H2/CO/Ar 0.45/0.45/0.10 at.; Matheson: The X-ray diffraction pattern of a dried precipitate sam- 99.99% CO, 99.99% Ar). Ar was used as an internal stan- ple shows the presence of Mg(OH)2 and Ce(OH)4/CeO2 dard in order to ensure accurate material balances. Syn- phases (Fig. 1a). The poor sample crystallinity and the co- thesis gas was puriﬁed using activated carbon (Sorb-Tech incidence of diffraction lines for Ce oxide and hydroxide RL-13) in order to remove metal carbonyls. H2 was pu- phases make it difﬁcult to determine conclusively the form riﬁed by passing through a catalytic puriﬁer (Matheson, of Ce in dried samples. Precipitates did not show any diffrac- Model 64-1008). Tracesof water were removed from H2 and tion lines corresponding to layered hydrotalcite-type struc- H2/CO mixtures using molecular sieves (Matheson, Model tures. 452 : 4A). Separate MgO and CeO2 phases were detected by X-ray Reactants and products were analyzed on-line using a diffraction after air treatment at 723 K (Fig. 1b). Diffrac- gas chromatograph (Hewlett–Packard, Model 5890 II Plus) tion lines occurred at the positions expected for the equipped with a 10-port sampling valve and two sample pure monometallic oxides and provided no evidence for loops. One sample loop was injected into a 5% phenyl- MgCeOx mixed oxides. CuO crystallites were not detected, methyl-silicone capillary column (HP-5, 50 m, 0.32 mm di- suggesting that the Cu component is well dispersed, either ameter, 1.05 m ﬁlm thickness) and the other into a packed column (Porapak Q, 1.8 m length, 0.32 cm diameter). A ther- mal conductivity detector (TCD) was used to measure the concentration of Ar, N2,CO,CO2, and H2O in the efﬂu- ent from the packed column. A ﬂame ionization detector (FID) was used to measure the concentrations of all or- ganic compounds eluting from the capillary column. Con- densable products were collected and used to conﬁrm the identity of species in reaction products by mass spectro- scopic analysis after capillary chromatography (Hewlett– Packard, Model 5890 II Plus GC; Hewlett–Packard, Model 5972 Mass Selective Detector). Alcohol and aldehyde coupling reactions were carried out in a gradientless recirculating reactor unit (RRU). The catalyst (18.0 mg) was ﬁrst reduced in H2 (10% H2/He) at 623 K for 0.5 h. The temperature was lowered to 573 K and ethanol was introduced along with a small amount of methane, used as an unreactive internal standard (reaction = mixture: C2H5OH/CH4/He 4.0/2.7/94.6 kPa; C2H5OH: FIG. 1. Powder X-ray diffraction patterns of Cu/Mg/Ce = 0.5/5.0/1.0 Fisher Scientiﬁc, A.C.S. certiﬁed; CH4: Matheson, ultra high (atomic ratio) samples (a) uncalcined and (b) treated in air at 723 K. ISOBUTANOL AND METHANOL SYNTHESIS 133
TABLE 2 Composition, Surface Area, and Basic Site Density of Mixed Metal Oxides
BET Exchangeable CO2 desorbed during CO2 exchange a c K content surface area Cu CO2 at 573 K TPD below 573 K rate constant Sample (wt.%) (m2/g) dispersionb (10 6 mol/m2) (10 6 mol/m2)(k, 103 s 1)
MgO 0.01 191 — 0.38 0.50 18 Cu0.1MgOx 0.2 118 0.06 — — — Mg5CeOx 0.01 188 — 0.95 0.84 5.0 Cu0.5Mg5CeOx 0.1 167 0.23 1.2 0.62 4.8 Cu0.5Mg5CeOx 1.0 147 0.14 2.3 0.64 2.4 Cu0.5Mg5CeOx 3.5 62 0.062 5.2 0.65 2.6 Cu7.5Mg5CeOx 1.2 92 0.047 3.3 0.91 3.0 Cu/ZnO/Al2O3 0 62 0.05 1.1 0.72 13
a Bulk composition measured by atomic absorption. b Dispersion calculated from the ratio of surface Cu (determined by N2O decomposition at 363 K (13, 14)) to the total number of Cu atoms in the catalyst. c Obtained at the peak maximum of basic site distribution curve.
as small crystallites or as a solid solution within the CeO2 was rereduced at 473 K instead of 623 K after the titration lattice. Mixed Cu/Ce oxide solid solution can form during measurements. Surface oxygen atoms on Cu surface can be coprecipitation and remain after high temperature oxida- removed at 473 K in H2, but reduction of CeO2 is likely to tive treatments (16, 17). occur only at higher reduction temperatures. Thus, reduc- The total surface area of Cu0.5Mg5CeOx samples de- tion at 473 K should remove oxygen from Cu surface but creased with increasing K concentration (Table 2). not from CeOx. The Cu surface area of the sample reduced Lunsford and co-workers (18, 19) have reported that the at 473 K was 19.3 m2/g, a value identical to that obtained on addition of Li also decreases the surface area of MgO. the fresh sample after reduction at 623 K (19.0 m2/g). This The copper dispersion measured by N2O decomposition shows that CeOx does not contribute to the measured cop- on Cu0.5Mg5CeOx is 0.23 (Table 2); this value is among the per dispersion and that measured dispersion values indeed highest reported in the literature for Cu-based methanol reﬂect the presence of very small Cu metal crystallites of di- and higher alcohol synthesis catalysts. This copper disper- ameter about 5 nm. An additional exposure of this titrated sion value corresponds to a crystallite size of about 5 nm as- sample to 5% H2 at 423 K resulted in a Cu surface area of 2 suming hemispherical crystals. A Cu0.5Mg5Ox sample with 17.9 m /g, suggesting that oxygen atoms chemisorbed on Cu similar Cu content as Cu0.5Mg5CeOx has a copper disper- can be almost completely removed by H2 at temperatures sion of only 0.06. Thus, the presence of Ce oxide species as low as 423 K. favors the formation of small Cu metal crystallites on MgO Copper dispersion decreased with increasing K loading supports. (Table 2) because of a decrease in the surface area of the The decomposition of N2O on reduced ZnO sites has Mg5CeOx support and also because K species may block been found to corrupt Cu surface area measurements surface Cu atoms or inhibit their reduction to Cu metal, on Cu/ZnO catalysts (20). It is possible that decompo- resulting in a decrease in the number of exposed surface Cu sition of N2O on reduced CeOx species in prereduced atoms. CeOx addition to Cu0.1MgOx increased both surface Cu0.5Mg5CeOx catalysts also corrupts N2O titration data area and copper dispersion (Table 2), suggesting that CeOx and leads to observed high copper dispersion values. N2O acts as structural promoter. did not decompose on Mg5CeOx samples prereduced at The temperature-programmed reduction proﬁles of 623 K, suggesting that any reduced Ce species present do MgO-based Cu samples are shown in Fig. 2. The onset and not decompose N2O at 363 K. CeO2 may only reduce, how- peak maximum temperatures for H2 consumption and H2O ever, when Cu metal is available in order to dissociate H2. formation appeared at the same temperature for each sam- In effect, the presence of Cu may promote the reduction of ple. The high-temperature tail in the H2O peak is caused by CeO2. a strong interaction between H2O and MgO. This tail was A reduction–oxidation cycle was carried out in order not observed for the H2 peak, but the signal-to-noise ratio to rule out any contribution from reduced Ce species of the H2 peak was lower than that of H2O because of the to N2O titration measurements of Cu surface area. A high H2 background pressure in the mass spectrometer. Cu0.5Mg5CeOx sample was reduced at 623 K and N2O titra- The presence of CeOx in Cu0.5Mg5CeOx decreases the tion data were obtained. The measured Cu surface area reduction temperature of CuO (from 508 to 436 K). CeOx was found to be 19.0 m2/g (0.23 Cu dispersion). This sample addition also increases Cu dispersion and decreases Cu 134 XU ET AL.
Ea, was 84 kJ/mol for the reduction of pure CuO whereas Ea decreased to 77 kJ/mol for the reduction of the CuO–ZnO catalysts (22), in agreement with the promoting effect of ZnO on the reducibility of CuO. Similar processes are likely to occur during CuO reduction on CuMgCeOx samples. A better contact between CeOx and Cu is expected with in- creasing CeOx/Cu ratio, leading to CuO reduction at lower temperatures. In contrast with CeOx , K addition to Cu-containing sam- ples inhibits CuO reduction, as shown by the shift of the re- duction peak to higher temperatures (Fig. 3). The effect of K is stronger on low-Cu (Cu0.5Mg5CeOx; 1T = 79 K) than on high-Cu (Cu7.5Mg5CeOx; 1T = 57 K) catalysts. Also, the effect of K did not depend on the presence of CeOx; the re- duction temperatures increased to a similar extent (1T = 70 K) on K–Cu0.5Mg5Ox and K–Cu0.5Mg5CeOx. K appears 2 + FIG. 2. Temperature-programmed reduction proﬁles obtained in 5% to increase the stability of Cu ions and to make them H2/He of MgO-based Cu catalysts: (a) Cu0.5Mg5CeOx; (b) Cu7.5Mg5CeOx; more difﬁcult to reduce by H2. A similar effect was re- 3 (c) Cu0.1Mg5Ox. [Heating rate: 0.17 K/s; 15–100 mg of sample, 100 cm /min ported on Cs-promoted Cu/ZnO/Cr2O3 (25). In this study, 5% H2/He mixture; pretreatment temperature, 723 K.] the presence of Cs retards CuO reduction by about 50 K. Campos-Martin et al. (25) suggest that the inhibited reduc- tion of CuO is associated to closer interaction between the CuO and alkali phases retards H activation. particle size apparently because of strong interactions be- 2 The observed inhibition of copper reduction by K can tween Cu and CeO . The large Cu particles in Cu Mg x 7.5 5 be explained by inhibited activation of H , as proposed CeO , however, can also be reduced at temperature lower 2 x by Klier and co-workers (25), or by the strengthening of than that on Cu MgO . The reduction proﬁles (Fig. 2) 0.1 x Cu–O bonds upon K addition. As reported in the literature suggest that the promoting effect of CeO on copper ox- x (26, 27), the bond energy of surface oxygen for CuO is about ide reduction increases with increasing Ce/Cu ratio. CeO x 42 kJ/mol and it increases to 63 kJ/mol upon the addition has been reported previously to promote metal oxide re- of 10–25 at.% MgO. Addition of MgO weakens the Cu–Cu duction for Pd/CeO /Al O catalysts (21). The presence 2 2 3 bonds and strengthens Cu–O bonds. In a similar way, the in- of CeO shifts the reduction peak temperature of PdO x corporation of K O into CuO during catalyst synthesis may from 437 to 376 K. Moreover, the reduction behavior of a 2 Pd/CeO2/Al2O3 sample prepared by the coprecipitation of Pd and cerium nitrates differs from that of a Pd/CeO2/Al2O3 prepared by successive impregnation of Al2O3 with CeO2 and Pd. Some reduction occurs at room temperature on the sample prepared by the coprecipitation methods as a conse- quence of a higher degree of PdO–CeOx contact area (22). Lamonier et al. (16) have found that Cu2+ insertion into CeOx occurs during the synthesis of CuCeOx samples by coprecipitation methods. Four different species, present as CuO monomers, dimers, clusters, and small particles, were detected in these CuO/CeO2 samples (16). ZnO has a similar effect as CeOx on copper reduction. Garcia-Fierro et al. (22, 23) reported that the fraction of copper oxide strongly interacting with ZnO increases with increasing Zn/Cu ratio and that such copper oxide species reduced a low temperature. A kinetic model of reduction kinetics of CuO/ZnO suggests that the promoting effect of ZnO on copper reduction is caused by the dissociative FIG. 3. Temperature-programmed reduction proﬁles obtained in 5% adsorption of H2 on ZnO surfaces or on Cu metal clus- H /He of MgO-based Cu catalysts: (a) 1.0 wt.% K–Cu Mg CeO ; (b) ters closely associated with ZnO (24). The spillover of the 2 0.5 5 x + 1.2 wt.% K–Cu7.5Mg5CeOx; (c) 1.1 wt.% K–Cu0.1Mg5Ox. [Heating rate: 2 3 hydrogen atoms formed enhances Cu reduction. In fact, 0.17 K/s; 15–100 mg of sample, 100 cm /min 5% H2/He mixture; pretreat- kinetic analysis showed that the apparent activation energy, ment temperature, 723 K.] ISOBUTANOL AND METHANOL SYNTHESIS 135
FIG. 4. Isotopic switch method for probing adsorption–desorption processes dynamics on nonuniform surface of K–Cu0.5Mg5CeOx. increase the bond strength of Cu–O and therefore retard of Fig. 4a. The exchange capacity is determined from the 13 CuO reduction. area of the CO2 desorption curve immediately after a 13 12 switch from CO2 to CO2. When corrected for gas holdup, physical hydrodynamic delays, and CO mass spectromet- Density and Strength of Basic Sites 2 ric response factors, this area corresponds to the number of The density and strength of basic sites on modiﬁed MgO basic sites that are kinetically accessible for exchange reac- samples were measured from the exchange capacity and tions at 573 K. Weakly interacting sites are mostly unoccu- 13 12 rates obtained using isotopic CO2/ CO2 switch experi- pied by CO2; strongly interacting sites do not exchange in ments at typical reaction temperatures (573 K). Site den- the time scale of the isotopic relaxation experiment. These 13 sities obtained from the area under the CO2 relaxation strongly interacting and weakly interacting sites are also curves are shown in Table 2 for several catalytic materi- unlikely to contribute to catalytic reactions at 573 K. Site als. As a comparison, basic site densities determined by densities obtained from these measurements are shown in TPD of preadsorbed CO2 are also listed in Table 2. CO2 Table 2. TPD measurements involve the adsorption of CO2 on pre- The local slope in the semi-logarithmic plots of Fig. 4a re- reduced samples at room temperature, the removal of gas ﬂects the dynamics of the ﬁrst-order CO2 exchange reaction phase and weakly adsorbed CO2 by ﬂowing He, and a linear and thus the exchange rate constant on available basic sites. temperature increase with on-line monitoring of the CO2 The curved semi-logarithmic plots show that Cu0.5Mg5CeOx evolution rate by mass spectrometry. CO2 TPD basic site surfaces contain sites with a distribution of exchange rate densities were obtained by integration of the evolution rate constants, because uniform surfaces with only one type of curves up to a desorption temperature of 573 K. adsorption site would lead to straight lines in Fig. 4a. Ex- Although temperature-programmed desorption of CO2 change rate constants depend on the thermodynamics of is frequently used for measuring basic site densities and binding interactions between CO2 and basic sites through strength, the area under TPD curves gives the total number linear free energy relations commonly used to estimate acti- of basic sites and not the number of basic sites kinetically vation energies for chemical reactions (29). Large exchange available for adsorption and catalytic reaction at reaction rate constants and the concomitant short relaxation times temperature. Desorption peak temperatures provides only (e.g. on 0.1 wt.% K) reﬂect short CO2 surface lifetimes and qualitative information about the basic strength. In con- weaker binding of CO2 molecules on available basic sites. 13 12 trast, CO2/ CO2 exchange methods can provide not only The distribution of exchange rate constants was obtained a direct measure of the number of these basic sites that re- for each catalyst sample from the relaxation dynamics us- versible chemisorb CO2 at 573 K, but also a distribution of ing inverse Laplace transform deconvolution methods (30). reactivity for such basic sites (28). These kinetic distributions are shown for 0.1 and 1.0 wt.% 13 12 The relaxation responses obtained from CO2/ CO2 iso- K–Cu0.5Mg5CeOx catalysts in Figs. 4b and 4c. The distri- topic switch experiments on Cu0.5Mg5CeOx catalysts with bution curves were normalized to give an area of unity. 0.1 and 1.0 wt.% K are shown in the semi-logarithmic plots The y-axis represents the distribution function f(k), where 136 XU ET AL. f(k) dk gives the fraction of the total number of accessi- TABLE 3 ble basic sites with exchange rate constants between k and Activities and Selectivities for Isobutanol Synthesis k + dk. on K-Promoted Cu0.5Mg5CeOx The density of sites that reversibly bind CO2 at typi- cal catalytic turnover time obtained by isotopic exchange Catalyst: Ref. (7) This study This study methods on MgO at 573 K is 0.38 10 6 mol/m2; this is Composition: 0.9 wt.% K 1.0 wt.% K 2.9 wt.% Cs lower than the value of 18.3 10 6 mol/m2 reported for Cu0.5Mg5CeOx Cu0.5Mg5CeOx Cu/ZnO/Al2O3 Space velocity the surface density of oxygen atoms in MgO (31). This 3 13 12 [cm /g-cat.h]: 1832 1500 6000 is not unexpected since the CO2/ CO2 isotopic switch CO conversion [%]: 15.5a 14.7b 15.4c method only detects those basic sites that participate in ex- CO reaction rate change reactions near 573 K. The density of available basic [mol CO conv./g-cat.h]: 6.3 10 3 4.4 10 3 17.0 10 3 d sites in Cu0.5Mg5CeOx increases with increasing K loading Carbon selectivities [%] (Table 2). Potassium also leads to slower exchange pro- Methanol 57.7 62.7 67.6 cesses, suggesting that stronger basic sites form when alkali Ethanol 1.9 1.7 4.4 is present on modiﬁed MgO surfaces (Figs. 4b and 4c). An 1-Propanol 2.6 3.0 6.9 increase in K loading from 1.0 to 3.5 wt.% increases basic Isobutanol 10.4 6.2 6.9 site density from 2.3 to 5.2 10 6 mol/m2, but has no de- Other higher alcohols 2.2 9.2 9.5 and oxygenates tectable effect on the exchange rate constant and site dis- Dimethylether 1.2 6.8 0.9 tribution (Table 2). The presence of small amounts of CeOx Hydrocarbons 24.3 9.9 2.5 in MgO (Mg/Ce = 5 at.) increases both the density and the CO2 31.0 23.4 22.6 strength of basic sites (Table 2). a Reaction conditions: 593 K, 5.1 MPa, H2/CO = 1. In contrast with the CO2 TPD method, in which both tem- b Reaction conditions: 583 K, 4.5 MPa, H2/CO = 1. 13 12 c perature and surface coverage vary with time, CO2/ CO2 Reaction conditions: 580 K, 4.5 MPa, H2/CO = 1. d exchange methods probe the density and reactivity of basic Hydrocarbon and alcohol selectivities normalized on a CO2-free basis sites under conditions of chemical equilibrium and constant (except CO2). temperature. Exchange methods also avoid contributions from the decomposition of catalytically irrelevant and un- drocarbons, and higher alcohols or by the readsorption of reactive carbonates and from the desorption of strongly the water formed along with these products. adsorbed CO . This method probes the number of sites 2 The higher CO selectivity reported by Apesteguia et al. that can be made available for base-catalyzed reactions at 2 (7) may reﬂect the slightly higher reaction temperature temperatures typical of isobutanol synthesis. As we discuss in their studies and their higher selectivity to hydrocar- below, competitive adsorption between CO and aldehy- 2 bons. Also, higher alcohol-to-hydrocarbon ratios were ob- dic species on such sites renders basic sites unavailable for tained on our samples than on those studied by Apesteguia alcohol synthesis reactions on Cu0.5Mg5CeOx catalysts. et al. (7). Our Cu0.5Mg5CeOx catalyst, however, shows much higher selectivity to dimethylether. Both hydrocar- Isobutanol Synthesis from CO/H Mixtures 2 bons and dimethylether are formed on acid sites but the Cs–Cu/ZnO/Al2O3 and K–Cu0.5Mg5CeOx catalysts show formation of hydrocarbons requires subsequent reactions the highest isobutanol synthesis rates and alcohol-to- of dimethylether. These results suggest that our samples hydrocarbon product ratios among reported catalysts for contain fewer residual sites than catalysts of similar nominal the synthesis of isobutanol at low temperatures (<623 K) composition reported by Apesteguia et al. (7), apparently and pressures (6, 7, 32). Alcohol synthesis rates on K- as a result of more effective alkali dispersion. Potassium modiﬁed Cu0.5Mg5CeOx catalysts depend strongly on potas- appears to titrate residual acid sites formed during copre- sium concentration. Intermediate K concentrations (1.0– cipitation or CuO reduction processes. Potassium also in- 1.5 wt.%) lead to the highest isobutanol synthesis rates and creases the density and strength of basic sites required for selectivities (7). chain growth reactions of alcohols (Table 2, Fig. 4). Reaction products obtained on 1.0 wt.% K–Cu0.5Mg5 Alkali dispersion appears to be sensitive to the details CeOx (583 K and 4.5 MPa) and 2.9 wt.% Cs–Cu/ZnO/Al2O3 of the impregnation procedure. For example, the addition (580 K and 4.5 MPa) are shown in Table 3 at similar CO of potassium before oxidative treatment of dry precipitates conversion values. The data reported by Apesteguia et al. (calcination) leads to catalysts with much higher selectivity (7) on similar K–Cu0.5Mg5CeOx samples are also shown to hydrocarbons than on samples impregnated with K after in Table 3. Methanol, isobutanol, and CO2 were the most oxidation. Higher potassium contents (e.g., 3.5 wt.%) are abundant reaction products. CO2 is formed in CO oxidation undesirable because the more complete titration of residual or water–gas shift reactions, using adsorbed oxygen atoms acid sites is accompanied by sintering of the metal oxide formed directly during the synthesis of dimethylether, hy- support and in the number of accessible Cu atoms measured ISOBUTANOL AND METHANOL SYNTHESIS 137
FIG. 5. CO conversion and product selectivities vs space velocity on (a) Cs–Cu/ZnO/Al2O3 and (b) K–Cu0.5Mg5CeOx. [583 K, 4.5 MPa, CO/H2 = 1. Product selectivities are on CO2-free basis.]
by N2O decomposition methods. These surface Cu atoms Methanol synthesis rates decrease with decreasing space are required for methanol synthesis and alcohol conversion velocity on both catalysts, because the methanol synthesis to aldehydes during CO hydrogenation to higher alcohols reaction (Table 2). CO + 2H2 = CH3OH The effects of space velocity (bed residence time) on al- cohol synthesis rates and selectivities are shown in Fig. 5 approaches equilibrium under our reaction conditions; for 2.9 wt.% Cs–Cu/ZnO/Al2O3 and 1.0 wt.% K–Cu0.5 these conditions were chosen in order to maximize yields Mg5CeOx. CO conversion should increase linearly with in- of higher alcohols. Higher alcohol yields are obtained at creasing bed residence time at the low CO conversion levels higher temperatures and long bed residence times. When in this study. The observed decrease in the slope for the CO methanol yields become limited by equilibrium, methanol conversion curve with increasing bed residence time (Fig. 5) synthesis rates become proportional to space velocity, be- reﬂects the gradual approach to equilibrium by the primary cause CO conversion to methanol remains constant with methanol synthesis step and possibly also the inhibition of increasing bed residence time. methanol synthesis reactions by CO2 and H2O formed as On typical methanol synthesis catalysts (i.e., Cu/ZnO/ side products of hydrocarbon and higher alcohol synthe- Al2O3), the presence of CO2 in modest concentrations sis. Isobutanol and higher alcohols form in secondary chain (1–2% mol) actually increases methanol synthesis rates growth reactions, such as methanol carbonylation or aldol (33). High CO2 concentrations (>10% mol), however, lead coupling reactions, which are favored by longer bed resi- to higher steady-state oxygen surface coverages and inhibit dence times. C2–C3 alcohols are intermediate products that methanol synthesis on these catalysts (33). Cs-modiﬁed undergo chain growth; therefore, their selectivity reaches a methanol synthesis catalysts reach maximum rates at very maximum value at intermediate values of residence time. low CO2 concentrations (<2%) (34). Steady-state oxygen Isobutanol is a kinetic end product, because it lacks coverages during methanol synthesis on Cu surfaces de- the two -hydrogens required for facile chain growth via pend on the relative rates of oxidation and reduction of aldol condensation pathways. Therefore, isobutanol se- surface Cu atoms. At high CO2 concentrations the rate of lectivity increases with increasing residence time. This oxidation increases relative to reduction, leading to higher increase, however, is less marked on K–Cu0.5Mg5CeOx steady-state oxygen coverages and to a (reversible) de- than on Cs–Cu/ZnO/Al2O3, because base-catalyzed chain crease in the number of surface Cu atoms available for growth reactions are inhibited by CO2 more strongly on methanol synthesis. These surface metal atoms on Cu crys- K–Cu0.5Mg5CeOx than on Cs–Cu/ZnO/Al2O3. These ef- tallites catalyze methanol synthesis reactions of CO/CO2/ fects are discussed in detail below. CO2 appears to inhibit H2 mixtures (33, 35). both methanol synthesis on Cu sites and aldol coupling re- In order to examine the details of CO2 inhibition effects actions on basic sites during isobutanol synthesis reactions on methanol and isobutanol synthesis, CO2 was systemati- on K-promoted Cu0.5Mg5CeOx catalysts. The weaker basic cally added to the synthesis gas feed. The results are shown sites on Cs–Cu/ZnO/Al2O3 (Table 2) appear to be less sen- in Fig. 6 as the reciprocal of methanol and isobutanol syn- sitive to CO2 poisoning; as a result isobutanol selectivities thesis rates versus CO2 concentrations. Typical Langmuir– reach higher values than on K–Cu0.5Mg5CeOx with increas- Hinshelwood rate expressions would lead to straight lines ing bed residence time. when data are presented in this form. Methanol synthesis 138 XU ET AL.
FIG. 6. Reciprocal methanol (a) and isobutanol (b) productivity vs average CO2 partial pressure for 1 wt.% K–Cu0.5Mg5CeOx, 2.9 wt.% Cs–Cu/ 3 ZnO/Al2O3, and 0.9 wt.% K–Cu7.5Mg5CeOx. [583 K, 4.5 MPa, H2/CO = 1, 3000 cm /g/h.] rates for catalysts with high Cu contents decreased slightly isobutanol from ethanol. Also, as we discuss later, aldol as the CO2 concentration was increased. Methanol syn- coupling reactions of alcohols require both Cu and basic thesis rates on 1.0 wt.% K–Cu0.5Mg5CeOx were more sites; therefore any blocking of surface Cu atoms by oxy- strongly inhibited by CO2. These results reﬂect the opera- gen can lead to a decrease in isobutanol synthesis rates. tion of these catalysts near methanol synthesis equilibrium. CO2 inhibition of isobutanol synthesis reactions may also Catalysts containing a larger number of Cu surface sites reﬂect the reversible titration of basic sites on oxide surfaces (e.g., 1.0 wt.% K–Cu7.5Mg5CeOx) can maintain equilibrium by acidic molecules, such as CO2. The inhibiting effect of methanol concentrations even after a signiﬁcant fraction of CO2 is greater on 1 wt.% CuMg7.5CeOx than on 2.9 wt.% such sites are covered with oxygen adatoms during steady- Cs–Cu/ZnO/Al2O3, possibly because of the stronger basic- state catalysis; therefore no effect of CO2 on the methanol ity of the former (detected by the dynamics of CO2 ex- productivity is observed until larger amounts of CO2 are change). On low Cu catalysts, alcohol chain growth appear added. Catalysts containing fewer Cu sites (e.g., 1.0 wt.% to be limited by the availability of minority Cu sites and K–Cu0.5Mg5CeOx) become unable to maintain methanol thus CO2 inhibition of Cu such sites decrease chain growth synthesis equilibrium conversions at lower CO2 concentra- rates. Higher Cu contents lead to coupling rates controlled tions than catalysts with higher Cu site densities. by reactions on basic sites, which are blocked by CO2 most The effect of CO2 on the rate of higher alcohol synthesis effectively on the more strongly basic MgO-based materi- has not been as thoroughly studied as on methanol synthesis als. Basic sites on 0.9 wt.%CuMg7.5CeOx appears to be more rates. Tronconi et al. (36) observed lower yields of higher sensitive to CO2 than on 2.9 wt.% Cs–Cu/ZnO/Al2O3. This alcohols on K–ZnCrOx at 673 K when CO2 (3–6%) was is consistent with the faster dynamics of CO2 exchange on added to the H2/CO feed. These authors proposed that wa- the latter catalysts. ter, produced from CO2 in the reverse water–gas–shift re- action, titrated active sites. Elliot (37) observed that higher Alcohol Coupling Reaction Pathways alcohol synthesis over Cu/ZnO was enhanced by the addi- Alcohol and aldehyde coupling reactions were studied in tion of small amounts of CO2 (6%) at 557–561 K. Calver- order to probe reaction pathways involved in branched and ley and Smith (38) observed that the higher alcohol yields linear higher alcohols synthesis from methanol, ethanol, reach a maximum at intermediate CO2 concentrations and propanol intermediates formed initially during CO hy- (0–9% CO2) on 0–0.5 wt.% K–Cu/ZnO/Cr2O3 at 558 K. drogenation reactions at high pressures. These studies also They also observed that the effect of CO2 becomes stronger address the effects of CO2 on alcohol dehydrogenation re- as alkali concentration increases. actions and on chain growth reactions and the roles of Cu, Figure 6b shows the reciprocal isobutanol synthesis rate K, and Mg–Ce oxides in the synthesis of branched alco- as a function of the average CO2 partial pressure within hols. These experiments were carried out on Mg5CeOx, the catalyst bed. The catalyst with low Cu content was in- Cu0.5Mg5CeOx,K–Cu0.5Mg5CeOx, and K–Cu7.5Mg5CeOx hibited more strongly by the presence of CO2 than cat- under atmospheric pressures in a gradientless batch reac- alysts with higher Cu contents. To some extent, this may tor. Two types of reactants were used: pure ethanol and 13 reﬂect the reversible inhibition of Cu metal sites by CO2, CH3OH–acetaldehyde mixtures. which leads to lower methanol yields on catalysts with low Acetaldehyde was the initial product of reactions of pure Cu content. Although methanol is not involved in the ini- ethanol on Cu0.5Mg5CeOx (Fig. 7). Dehydrogenation reac- tial formation of ethanol (39) on Cu/MgCeOx catalysts, it tions occur much faster than chain growth reactions; the acts as a C1 precursor in the synthesis of 1-propanol and latter lead to the formation of acetone, n-butyraldehyde, ISOBUTANOL AND METHANOL SYNTHESIS 139
Acetaldehyde reaches a maximum concentration at inter- mediate contact times during ethanol reactions and then de- creases gradually, suggesting that acetaldehyde concentra- tions approach equilibrium. Acetone and n-butyraldehyde were the predominant condensation products. The nonzero initial slopes on acetone and n-butyraldehyde curves are inconsistent with products formed through consecutive re- actions (Fig. 7). Therefore, condensation products are ei- ther formed directly by ethanol self-condensation or via acetaldehyde condensation reactions with zero-order kinet- ics. Methyl ethyl ketone (by acetaldehyde self-condensation with oxygen retention reversal (40, 41)), 2-pentanone (by acetaldehyde–acetone condensation), and ethyl acetate (by ethanol–acetaldehyde reactions) were detected in much smaller concentrations among reaction products. FIG. 7. Site yields as a function of contact time on 1.0 wt.% Reaction pathways involved in the formation of all de- K–Cu0.5Mg5CeOx in ethanol reactions. [573 K, 101.3 kPa total pressure, tected products are shown in Fig. 8. Acetaldehyde is formed 4.0 kPa ethanol, balance He.] via ethanol dehydrogenation (step I). Aldol species (butan- 1-al-3-ol) formed from direct condensation reactions of ethanol and from self-condensation of acetaldehyde pro- and other oxygenates. Site yields (y-axis, Fig. 7) are deﬁned duce n-butyraldehyde (steps II–IV) and methyl ethyl ke- as the number of moles of ethanol converted to products tone (steps II–V) (40). The aldol species converts to the keto per g-atom Cu (total) in the catalyst charge. The total num- form via H transfer (step VI); methyl ethyl ketone is also ber of Cu atoms in the sample is a convenient normalizing formed by dehydration–dehydrogenation reactions of the parameter. The local slope of the curve in Fig. 7 gives the keto form (step VII). Acetone is formed via two pathways: rate of formation of each product. When divided by the by reaction of aldol intermediates with surface oxygen fol- fractional Cu dispersion, this becomes a site-time yield for lowed by decarboxylation (steps VIII–IX) and by reverse each product, at least for reactions with rate-determining aldol condensation of the keto form of reaction intermedi- elementary steps requiring Cu surface atoms. ates (step X). Formaldehyde formed in the latter reaction
FIG. 8. Reaction scheme for ethanol reactions on K–CuMg5CeOx catalysts. 140 XU ET AL.
Effects of Cu- and K-Loading on Ethanol Consumption and Product Formation on Mg5CeOx
Rates of formation Ethanol dehydrogenation Acetone Butyraldehyde Cu K Areal rate Turnover rate Areal rate Turnover rate Areal rate Turnover rate a b c d c d (wt.%) (wt.%) r1 r1 r2 r2 r3 r3
0 0 4.0 10 9 — 6.2 10 11 6.5 10 5 NDe NDe 7 0.1 3.6 10 7 0.24 3.0 10 9 2.2 10 3 4.5 10 10 3.4 10 4 7 1.0 2.4 10 7 0.23 4.3 10 9 1.8 10 3 7.6 10 10 3.2 10 4 49 1.2 9.4 10 7 0.24 4.4 10 8 1.0 10 2 1.2 10 9 2.8 10 4
a 2 r1 is the rate of ethanol consumption as mol/m total s. b Turnover rates per Cu surface atom as s 1. c 2 r2 and r3 are the rates of product formation as mol/m MgCeOx s. d 13 12 Turnover rates per accessible site from CO2/ CO2 as mol/mol CO2 exchangeable s. e Not detected.