Effect of the acid-base properties of Mg-Al mixed oxides on the catalysts deactivation during aldol condensation reactions
V.K. Díez, C.R. Apesteguía and J.I. Di Cosimo*
Instituto de Investigaciones en Catálisis y Petroquímica
-INCAPE- (UNL-CONICET).
Santiago del Estero 2654, (3000) Santa Fe, Argentina.
e-mail: dicosimo@fiqus.unl.edu.ar
* To whom correspondence should be addressed
Abstract— The effect of chemical composition of Mg-Al mixed oxides on both the acid-base properties and the deactivation process during the gas phase self-condensation of acetone was studied. The activity and selectivity for acetone oligomerization depended on the catalyst acid-base properties. Mg-rich catalysts selectively yielded mesityl oxides whereas Al-rich MgyAlOx oxides produced mainly isophorone. The initial deactivation rate, increased linearly with the density of surface basic sites, thereby suggesting that although MgyAlOx oxides promote the self-condensation of acetone by both acid- and base-catalyzed mechanisms, the deactivation rate would be closely related to the surface basic properties. The MgyAlOx activity declines in the acetone oligomerization reaction due to a blockage of both base and acid active sites by a carbonaceous residue formed by secondary reactions. The amount and the nature of the carbon deposits were characterized by temperature-programmed oxidation technique. MgyAlOx and Al2O3 formed more and heavier coke than pure MgO but the latter deactivates faster. The deactivation rate and coke composition are defined by the nature of the active site involved in the coke-forming reactions at different catalyst compositions rather than by the carbon amount or polymerization degree.
Keywords— Catalyst Deactivation. Acetone Oligomerization. Acid-Base Catalysis.
I. INTRODUCTION
Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition (Di Cosimo et al., 2000). These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Brönsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations (Reichle, 1985), alkylations (Velu and Swamy, 1996) and alcohol eliminations (Di Cosimo et al., 2000). In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a ,b -unsaturated ketones such as mesityl oxides and isophorone (Di Cosimo et al., 1998a). Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. In this work, we studied the deactivation of Mg-Al mixed oxides in the gas phase oligomerization of acetone. We prepared and characterized calcined Mg-Al hydrotalcites with Mg/Al atomic ratios of 1-9. The effect of composition on both the surface and catalytic properties as well as the catalyst deactivation were investigated by combining several characterization methods with catalytic data.
II. METHODS
A. Synthesis Procedures
Mg/Al hydrotalcite with Mg/Al atomic ratios (y) of 1, 3, 5 and 9 were prepared by coprecipitation of Mg(NO3)2.6 H2O and Al(NO3)3.9 H2O at 333 K and pH = 10. After drying overnight at 348 K, precursors were decomposed in N2 at 673 K in order to obtain the MgyAlOx mixed oxides. More experimental details are given elsewhere (Di Cosimo et al., 2000). Reference samples of Al2O3 and MgO were prepared following the same procedure.
B. Methods for the Sample Characterization
The crystalline structure of the samples was determined by powder X-ray diffraction (XRD) methods. Acid site densities were measured by temperature-programmed desorption (TPD) of NH3 preadsorbed at room temperature. The structure of CO2 chemisorbed on hydrotalcite-derived samples was determined by infrared spectroscopy (IR). CO2 adsorption site densities were obtained from TPD of CO2 preadsorbed at 298 K. Total surface areas (Sg) were measured by N2 physisorption at 77 K.
C. Acetone Self-Condensation Procedures
Self-condensation of acetone was carried out at 473 K and 100 kPa in a flow
system with a differential fixed-bed reactor. Acetone was vaporized in H2 (H2/acetone
= 12) before entering the reaction zone. The standard contact time (W/F0Acetone)
was 49 g catalyst h/mole acetone. Main reaction products were mesityl oxides
(MO's), isophorone (IP) and mesitylene (MES). Traces of phorone and light
hydrocarbons were also identified.
Coke formed on the catalysts was characterized after reaction, ex-situ, in
a temperature-programmed oxidation (TPO) unit. The TPO experiments were carried
out in a microreactor loaded with 50 mg of catalyst and using a 3 % O2/N2 carrier
gas. Sample temperature was increased linearly from room temperature to 973
K at 10 K/min. The reactor exit gases were fed into a methanator operating
at 673 K to convert COx in methane and then analyzed by flame ionization
detector.
III. RESULTS AND DISCUSSION
A. Catalyst Characterization
The hydroxycarbonate precursors showed diffraction patterns consistent with
the layered double hydroxide structure proposed for hydrotalcite-like compounds,
i.e., brucite-like sheets formed by OH groups and cations (Mg2+ and
Al3+) in octahedral coordination. Water molecules and charge-compensating
CO32- anions are located in the interlayers.The hydrotalcite-like
precursor composition is with r =
Al/(Al+Mg); the stoichiometric hydrotalcite compound, Mg6Al2(OH)16CO3.4H2O,
is obtained for r = 0.25. Upon thermal decomposition at 673 K, the
hydroxycarbonate precursors lose water and CO2. This gaseous evolution
develops a significant porous structure and gives rise to relatively high
surface area mixed oxides as shown in Table 1. In the compositional range
of this study, the analysis by XRD of the MgyAlOx oxides
showed only one crystalline phase, MgO periclase (ASTM 4-0829). No traces
of hydrotalcite phases were detected after the thermal decomposition at 673
K.
Table 1 presents the acid and base site densities measured for all the samples
by TPD of preadsorbed NH3 and CO2, respectively. The
base site density (nb) of the MgyAlOx oxides
was always between the values measured for MgO and Al2O3.
Basic sites of different chemical nature were observed by FTIR of CO2 and
deconvolution of the CO2 TPD traces; the strength of the basic sites
follows the order: isolated low-coordination O2- ions > O2- in
metal-oxygen pairs > OH groups (Di Cosimo et al., 1998b). The density
of metal–oxygen pairs measured by CO2 TPD decreased with increasing
the Al content at expenses of the surface hydroxylation.
Table 1. Chemical Composition, Crystalline Structure, Surface Area and
Acid-Base Properties of MgyAlOx, MgO and Al2O3.
The acid site density (na), on the other hand, increases with increasing Al content. By deconvolution of the NH3 TPD traces it was found that the MgyAlOx oxides contain both Brönsted OH groups (low temperature peak) and Lewis acid sites provided by metal cations (high temperature peak) (Prinetto et al., 2000; Berteau et al., 1991). Since NH3 adsorption was performed at room temperature, the measured Brönsted acidity may be attributed on all the samples to the reversible H-bonded NH3 adsorption on surface hydroxyls. Lewis acidity of MgyAlOx oxides corresponds to the irreversible coordinated NH3 adsorption on surface defects with accessible Al3+ cations whereas on alumina, the Lewis acidity was more likely assigned to Al3+ cations in the abundant surface Al3+-O2- pairs. On MgO, on the other hand, the high temperature peak may result from the irreversible coordinated NH3 adsorption on Mg2+ cations combined with heterolytic dissociative adsorption leading to NH2- - H+ species on Mg2+- O2- pairs (Garrone and Stone, 1985).
B. Sample Composition, Surface Acid-Base Properties and Catalytic Activity
Acetone aldol condensation proceeds on either acidic or basic catalysts. On basic catalysts, the reaction products are mainly a ,b -unsaturated ketones (Di Cosimo et al., 1996) whereas on acidic materials formation of aromatics and olefins is favored (Chang and Silvestri, 1977). In our catalytic tests, the main reaction products were mesityl oxides (MO's) and isophorone (IP). Over all the samples the reaction rate diminished as a function of time-on-stream as shown in Figure 1 for the Mg9AlOx sample which lost about 60 % of its initial activity after 10 h-run. Initial reaction rates (r0j) and product selectivities (S0j) were calculated by extrapolating the reaction rates vs. time curves to zero.
Figure 1: Time dependence of the acetone conversion and product formation
rates on Mg9AlOx (T = 473 K, PAcetone = 7.7
kPa)
In Figure 2 we have represented both the r0Acetone values and the total measured site density (nT = nb + na) as a function of catalyst composition. Qualitatively, the variation of r0Acetone with increasing Al content is similar to that followed by nT thereby suggesting that acetone conversion depends on both acid and base sites. Pure MgO was the most active catalyst whereas Al2O3 showed the lowest activity. This is because Al-O pairs are much less active than Mg-O pairs for promoting the proton abstraction and carbanion stabilization steps involved in aldol condensation reactions. We have shown (Di Cosimo et al., 2000) that the aldolization rate is controlled on basic catalysts by the number of metal cation-oxygen anion surface pairs. Mg-rich MgyAlOx oxides are less active than MgO because they exhibit a lower base site density and also poor acidic properties. In contrast, Al-rich MgyAlOx oxides are more active than Al2O3 due to a proper combination of acid and base sites.
Figure 2: Initial acetone conversion rate and total number of active
sites as a function of composition of MgyAlOx oxides
(T = 473 K, PAcetone = 7.7 kPa, W/F0Acetone =
49 g h/mol)
In Figure 3 the initial product selectivities measured at isoconversion are represented as a function of catalyst composition. It is observed that the initial selectivities toward IP and MES increase with increasing the Al content whereas high selectivity toward MO's was obtained on Mg-rich catalysts. The presence of MES in low concentrations on Al-rich samples is indicative of an acid-catalyzed reaction pathway (Chang and Silvestri, 1977). Brönsted OH groups present on the surface of Al-rich catalysts, probably account for MES formation in accord with previous work (Di Cosimo et al., 1998b) by X-ray photoelectron spectroscopy in which we demonstrated that in Al-rich MgyAlOx mixed oxides, the binding energy of O 1s signal approached the values of surface OH groups thereby confirming a higher surface hydroxylation degree compared to Mg-rich catalysts.
Figure 3: Initial product selectivity as a function of composition on
MgyAlOx catalysts (T = 473 K, PAcetone = 7.7
kPa, X0 Acetone = 12-14 %)
Fig. 4 shows the main reaction scheme for the aldol condensation sequence leading to IP, where the dotted lines represent the reaction pathways to MES and IP formation that take place exclusively on acidic catalysts. The dimer, MO is the detected primary product formed by the initial self-condensation of acetone after fast dehydration of the unstable diacetone alcohol whereas IP is the final product arising from the consecutive aldol condensation between MO and acetone.
Figure 4: Acid- and base-catalyzed pathways for acetone conversion by aldol-like reactions on MgyAlOx catalysts.
C. Coke Formation and Characterization
In a previous work (Di Cosimo and Apesteguía, 1998), we found that MgO-based catalysts are deactivated during acetone oligomerization because of the formation of carbon deposits that block pores and surface active sites. Here, the coke formed on the samples after the 10-h catalytic runs was characterized by TPO technique. The TPO curves are shown in Figure 5 and quantitative results are summarized in Table 2. The amount of carbon measured on the samples was between 12 and 22 wt. % and, as a general trend, MgyAlOx and Al2O3 formed more coke than pure MgO.
Table 2. Coke Characterization
Figure 5: Coke burning experiments after acetone aldol condensation
at 473 K on MgyAlOx catalysts. r = Al/(Al + Mg)
The TPO curves of Fig. 5 show the existence of two different carbon deposits;
the main one gives a broad oxidation peak centered at 650-700 K (low-temperature
peak); the other one arises as a shoulder at temperatures above 720 K (high-temperature
peak). The low-temperature peak maximum shifts to higher temperatures with
increasing Al content in the samples (Table 2). In order to obtain more insight
on the nature of the carbon deposits, stripping experiments in pure N2 were
carried out. In these experiments, coked samples were first treated in N2 from
room temperature up to 973 K at 10 K/min before performing standard TPO runs.
The TPO curves obtained after the N2 treatment are identified here
as TPO(N) and are displayed in dotted lines in Fig. 5. The amount of carbon
oxidized in TPO(N) experiments was between 4 and 8 wt % (Table 2). The shape
and temperature maximum of the TPO(N) curves were similar to those of the
high-temperature peaks in the corresponding TPO curves. These results show
that the nitrogen treatment removes almost completely the carbon deposits
that account for low-temperature peaks in TPO traces of Fig. 5. Both kind
of carbon deposits seem to be related to the catalyst acid-base properties
since both peak maxima gradually shifted to higher temperatures at increasing
Al contents as presented in Table 2.
In previous work (Di Cosimo and Apesteguía, 1998) we postulated that
the coke precursor species on MgO-based catalysts are highly unsaturated linear
compounds, such as 2,6-dimethylhepta-2,5-dien-4-one (phorone), 4,6-dimethylhepta-3,5-dien-2-one
and 4,4'-dimethylhepta-2,6-dione which are trimeric compounds formed from MO
and acetone by secondary aldol condensation reactions as shown in Figure 4.
These linear trimers (C9 ketones) are non-detectable in the gas
phase during reaction because they remain bound to the surface due to the strong
interaction between the unsaturated chain and the basic centers. All of them
are able to give IP by consecutive internal aldol or Michael rearrangements
(Reichle, 1980). Selective formation of one trimer in preference to the others
depends on the catalyst acid-base properties. Phorone is the preferred intermediate
on basic catalysts whereas formation of MES on acidic catalysts (Fig. 3) takes
place only from 4,6-dimethylhepta-3,5-dien-2-one (Reichle, 1980). To obtain
more information on the heavy compounds present in trace amounts in the gaseous
products, we performed a GC-Mass Spectroscopy analysis of the reactor effluent
collected in a condenser during acetone aldolization on Mg1AlOx catalyst.
Besides acetone, MO's, IP and MES, we detected the trimer 4,6-dimethylhepta-3,5-dien-2-one
(m/z = 138), tetramers like isoxylitones (m/z = 178) and also tetramethyltetralones
(m/z = 202) and other heavy polymers of m/z = 218 and 240.
Results can be explained by considering that the catalyst surface acid-base
properties determine the preferential formation of a given trimeric intermediate
which in turn defines both the formation of the product released to the gas
phase (MO, IP or MES) and the nature of the carbon deposit. This interpretation
is depicted in Figure 6. Coke formed from the 4,6-dimethylhepta-3,5-dien-2-one
intermediate on acidic catalysts (dotted lines in Fig. 6) will be fundamentally
aromatic in composition since MES can further react to xylenes and other C9+ aromatic
hydrocarbons (Chang and Silvestri, 1977; Salvapati et al., 1989). On
the other hand, coke formed on more basic materials from phorone, will be mostly
heavy a ,b -unsaturated
ketones such as isoxylitones, tetralones and other C12+ oxygenates
(Salvapati et al., 1989; Lippert et al., 1991). Then, the gradual
shift from one intermediate to another causes a concomitant change in coke
composition and it could explain the higher temperatures needed to burn the
carbon deposits at increasing Al content in the samples, as observed in Fig.
5.
Figure 6: Coke-forming reactions during acetone oligomerization on MgyAlOx
D. Relationship between catalyst deactivation and acid-base properties
The relative catalytic activity is defined as a = rt /r0 where rt and r0 are the reaction rates at time t and zero time respectively. Fig. 7 shows the activity decay of MgO as a function of time at different contact times. To quantify the effect of contact time on catalyst activity, the experimental points of Fig. 7 were fitted with first order deactivation functions and from the slopes of the curves, we calculated the initial deactivation rate as d0 = [- da/dt ]t=0. A plot of the resulting d0 values as a function of contact time is presented in Fig. 8; it is observed that the initial deactivation rate decreases rapidly with increasing contact time. This result shows that the activity decline of MgO is not related to the formation of any terminal product in the consecutive reaction pathways. In Fig. 8 we also plotted the initial selectivities toward MO and the final product (IP) as a function of contact time. The fact that d0 parallels the MO selectivity dependence confirms that IP is not responsible for MgO deactivation. We can also discard water as responsible for deactivation of MgO since IP synthesis involves the concomitant formation of two water molecules; water partial pressure on the catalyst surface should therefore increase with increasing contact time, which is the opposite trend observed for d0 in Fig. 8. Then, the effect of water as a surface poison able to convert the Lewis acid sites (Mg2+) of MgO into inactive Mg-OH surface species can be neglected because although generation of these species in situ is possible, the labile nature of the surface bond probably makes them unstable under reaction conditions. In fact, we have shown previously by IR of CO2 the weak character of bicarbonate species formed on surface hydroxyls of MgO (Di Cosimo et al., 1998b).
Figure 7: Acetone condensation activity as a function of time at different
contact times (W/F0 Acetone) over MgO (T = 573 K, PAcetone =
7.7 kPa)
Figure 8: Initial catalyst deactivation and product selectivities as
a function of contact time over MgO (T = 573 K, PAcetone = 7.7 kPa)
Since MO itself is not a strong coking agent (Reichle, 1980), it is reasonable
to assume that on MgO coke may be formed from the unsaturated trimers as postulated
in Fig. 6. Furthermore, in Fig 9 we present a linear correlation between the d0 values
and the carbon amount measured by TPO after the catalytic runs of Fig. 7,
indicating that the carbon deposit formed on the very active Mg-O pair sites
causes the deactivation of the MgO catalyst.
Plots of the relative catalytic activity as a function of time on stream at
473 K (Fig. 10) showed that the activity decay on Mg-Al mixed oxides is faster
than on Al2O3 but slower than on MgO. Pure MgO presented
a rapid deactivation and its residual activity after the 10-h run was very
low. From the slope of the plots of Fig. 10, the initial deactivation rate
was once more calculated. In Fig. 11, the d0 values obtained
for all the samples are represented in open symbols as a function of the carbon
content measured after the catalytic runs (Table 2). Clearly, it does not exist
any correlation between d0 and the amount of coke for MgyAlOx catalysts,
in contrast to what was obtained on pure MgO (Fig. 9). Initial deactivation
is lower on Al2O3 (d0 = 0.14 h-1)
than on MgO (d0 = 0.53 h-1), in spite that alumina
forms more coke during reaction and that the coke is more difficult to oxidize
as compared to MgO (Table 2 and Fig. 5).
Figure 9: Initial catalyst deactivation as a function of the carbon
amount deposited on MgO after 10-h catalytic runs at different contact times
(T = 573 K, PAcetone = 7.7 kPa)
Figure 10: Acetone condensation activity as a function of time over MgyAlOx,
MgO and Al2O3 catalysts (T = 473 K, PAcetone = 7.7 kPa,
W/F0 Acetone = 49 g h/mol) ▲ r
= 0.11; ● r = 0.18; ■ r
= 0.24; ♦ r = 0.47
Figure 11: Initial catalyst deactivation of MgyAlOx oxides,
MgO and Al2O3 as a function of the base site density
and carbon amount deposited after 10-h catalytic runs (T = 473 K, PAcetone =
7.7 kPa, W/F0 Acetone = 49 g h/mol)
These results show that neither the coke amount nor its polymerization degree
account for the catalyst deactivation order observed in Fig. 10. A better
explanation is obtained by considering the nature of the surface sites that
are responsible for the formation of coke precursors on pure Al2O3 or
MgO. Alumina contains Brönsted (OH groups) and Lewis (metal cations)
acidic sites and probably coke forms preferentially on Brönsted sites
rather than on Al-O acid-base pair sites. Contrarily to the known catalytic
properties of Brönsted OH sites in liquid-phase heterogeneously catalyzed
acetone aldolization at low temperatures, these sites are not active for the
gas-phase reaction to unsaturated ketones and the reaction would proceed,
therefore, at an appreciable rate on Al-O pairs even in the presence of a
high carbon content deposited on the Brönsted sites.
Contrarily to what has been discussed above about disregarding the poisoning
effect of water on MgO, we have no evidence on the role of water in poisoning
Lewis acid sites of alumina. In fact, surface transformations caused by water
leading to Al-H and Al-OH species with the consequent diminution of active
surface Al-O pairs should be also taken into account in addition to coke formation
when dealing with deactivation of alumina or Al-rich mixed oxides.
On MgO, the fast deactivation is due to the fact that aldolization and coke
formation reactions compete for the same active site, the Mg-O pairs, thereby
showing a linear correlation between d0 and the carbon amount
(Fig. 9). The aldol reaction stops almost completely even when the carbon deposition
is less intense than on Al-containing samples. On MgyAlOx oxides,
coke will mainly form on Mg-O pairs (Mg-rich samples) or Brönsted acid
sites (Al-rich samples) following the Al content in the sample as shown in Fig. 6.
In Fig. 11, the d0 values obtained for MgO, Al2O3 and
MgyAlOx oxides were plotted as a function of the
density of basic sites, nb (Table 1). It is observed that the initial
deactivation correlates linearly with the density of basic sites. No correlation
was found between the NH3 site density (na, Table 1)
and d0. These results show that although MgyAlOx oxides
promote the self-condensation of acetone by both acid- and base-catalyzed pathways
(Fig. 2), the initial deactivation rate is essentially related to the surface
basic properties. This dependence of d0 with nb for
MgyAlOx oxides probably reflects the fact that
the acetone oligomerization rate decline is significantly higher when coke
poisons very active Mg-O pair sites than when eliminates moderately active
Al-O pair site because of the more basic features of the former.
IV. CONCLUSIONS
The activity and selectivity for acetone oligomerization on MgyAlOx oxides
depend on the catalyst acid-base properties, which in turn are determined
by the aluminum content in the sample. Mg-rich catalysts selectively yield
mesityl oxides whereas Al-rich MgyAlOx oxides
produce mainly isophorone. MgyAlOx samples are
more active than alumina. Intermediate Mg/Al compositions show unique catalytic
properties because fulfill the reaction requirements for the density and strength
of basic and acid sites.
The MgyAlOx activity declines in the acetone oligomerization
reaction due to a blockage of both basic and acidic active sites by a carbonaceous
residue formed by secondary aldol condensation or hydrocarbon-forming reactions,
respectively. The key intermediate species for coke formation are highly unsaturated
linear trimers formed by aldol condensation of mesityl oxide with acetone that
remain strongly bound to the catalyst surface. The catalyst surface acid-base
properties determine the preferential formation of a given trimeric intermediate,
which in turn defines the chemical nature of the carbon deposit. Aromatic hydrocarbons
are the main component of coke formed on acidic Al-rich MgyAlOx samples
whereas heavy a ,b -unsaturated
ketones preferentially form on basic Mg-rich catalysts.
Alumina forms more and heavier coke than MgO but the latter deactivates faster.
This is explained by the fact that on MgO primary aldol condensation reactions
and formation of coke precursors species take place on the same sites, i.e.,
the Mg-O pairs in which oxygen presents moderate basic properties. In contrast,
main reaction to ketones takes place on the Al-O pairs of alumina whereas CHx coke
precursors form on Brönsted OH groups that are inactive for aldol condensation
reactions to unsaturated ketones.
Deactivation of MgyAlOx oxides is caused by carbon
deposition but neither the carbon amount nor its polymerization degree are
key factors in terms of the deactivation rate. The nature of surface active
sites for coke formation, which depend on the catalyst chemical composition,
seems to define the activity decay rate.
Deactivation of MgyAlOx oxides during acetone
oligomerization reactions is essentially related to the surface basic properties.
This is because the activity decline is significantly higher when coke poisons
very active basic Mg-O pairs than when eliminates moderately active acidic
Al-O pairs.
Acknowledgements
Support of this work by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina and by the Universidad Nacional del Litoral, grant CAI+D 94-0858-007-049, Santa Fe, Argentina, is gratefully acknowledged.
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Received: September 16, 2001.
Accepted for publication: July 26, 2002.
Recommended by Guest Editors J. Cerdá, S. Díaz and A. Bandoni.