Applied Catalysis A: General 342 (2008) 119–125

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Applied Catalysis A: General

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Stability, reutilization, and scalability of activated hydrotalcites in aldolcondensation

Sonia Abello a, D. Vijaya-Shankar a, Javier Perez-Ramırez a,b,*a Institute of Chemical Research of Catalonia (ICIQ), Avinguda Paısos Catalans 16, 43007 Tarragona, Spainb Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluıs Companys 23, 08010 Barcelona, Spain

A R T I C L E I N F O

Article history:

Received 21 December 2007

Received in revised form 11 February 2008

Accepted 4 March 2008

Available online 13 March 2008

Keywords:

C–C bond formation

Solid bases

Aldol condensation

Hydrotalcite

Deactivation

Reutilization

Regeneration

CO2 poisoning

Scale-up

A B S T R A C T

A number of studies have shown that solid bases, among others activated hydrotalcites, are highly

efficient catalysts for C–C bond formation reactions. A widely studied case is the aldol condensation of

citral and acetone, where rehydrated Mg-Al hydrotalcite shows higher yields to pseudoionone compared

to NaOH solutions. Despite this fact, the fine chemical industry still operates with the traditional liquid

inorganic bases. This manuscript addresses technical aspects that can explain the limited implementa-

tion of activated hydrotalcites in aldol condensations. For this purpose, the catalyst stability in air, its

reusability and regenerability after reaction, and the process scalability were investigated. The

conciliation of activity data of the fresh hydrotalcite in batch laboratory (ml) and bench (l) reactors with

on line ATR analysis is excellent, revealing that the citral–acetone reaction over hydrotalcite can be

upscaled. However, the poisoning of the active Brønsted basic centers in the rehydrated hydrotalcite by

CO2 is very fast (50% activity loss after 1 h exposure to ambient). Besides, the catalyst after one run is

inactive due to the presence of strongly adsorbed products and requires time-consuming (and not fully

complete) regeneration. Basic centers of Lewis nature in calcined hydrotalcites (basically MgO) are more

stable, but their activity is very low compared to the rehydrated counterpart. Both the technical

disadvantages of current solid bases and the lack of stringent environmental regulations motivate the

conservatism of industry to use alkaline solutions.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Inorganic bases in solution are traditionally applied in industryas homogeneous catalysts in reactions for the synthesis of finechemicals, including isomerizations, C–C bond formation, addi-tions, cyclizations, and oxidations [1–3]. High operating costs andserious environmental issues associated with base neutralization,product separation and purification, corrosion, and waste genera-tion motivated substantial efforts toward the development ofprocesses mediated by heterogeneous catalysts. Research in thisfield during the last decade led to the identification of various solidbases with remarkable activity and selectivity in many of the abovereactions [1]. However, these materials have not been widelyexploited in large scale, i.e. manufacturers still rely on thetraditional alkaline hydroxide and ethoxide solutions.

A well-known example is the aldol condensation of citral andacetone to pseudoionone (PS) (Scheme 1), an important precursor

* Corresponding author. Fax: +34 977 920 224.

E-mail address: jperez@iciq.es (J. Perez-Ramırez).

0926-860X/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.03.010

for vitamin A, fragrances, and pharmaceutical products. Indust-rially, this reaction is catalyzed by NaOH with typical PS yields of60–80% [4,5]. Different families of solid bases have been foundactive in this reaction, including alkali-exchanged zeolites (Cs-beta) [6], alkaline-earth metal oxides (MgO, CaO) [7,8], alumino-phosphate oxynitrides [9], ion-exchange resins [10,11], andactivated Mg-Al hydrotalcites [12–17]. In particular, pseudoiononeyields of 100% have been demonstrated over the latter group ofmaterials by optimal treatment of the as-synthesized clay [16] andproper selection of reaction conditions [18].

On the basis of the excellent aldolization performance ofactivated Mg-Al hydrotalcites in terms of activity and selectivityand taking into account the drawbacks of homogeneous catalysts,one can question why these solid bases have not broadly reachedthe industrial scene. Of course, the low cost of alkaline hydroxideor ethoxide solutions and the lack of stringent environmentalpolicies are strong reasons to stick to liquid bases. However, the100% PS yield of rehydrated Mg-Al hydrotalcite reported in lab-tests coupled to the expected reduction in operating costs andwaste generation is a good motivation to establish the long-term

Scheme 1. Base-catalyzed aldol condensation of acetone and cis-citral to pseudoionone.

S. Abello et al. / Applied Catalysis A: General 342 (2008) 119–125120

effectiveness of the overall process over the heterogeneous system.Technical aspects that may prevent application of activatedhydrotalcites can be related to questions like: is the catalystpreparation reproducible in large scale? How stable is the catalystupon exposure to air? Can the reaction be effectively upscaled? Isthe catalyst reusable? Can it be easily and completely regenerated?To the best of our knowledge, none of these items have beenquantitavely addressed in any of the numerous publicationsdealing with base catalysts and can be important to devise futureresearch strategies in this field.

Herein we have studied technical aspects that can result criticalfor industrial implementation of solid bases, taking the citral–acetone condensation over rehydrated hydrotalcites as case study.Aspects related to stability, reusability, regenerability, andscalability were systematically investigated by means of catalytictests in lab and bench-scale reactors as well as characterization ofthe samples.

2. Experimental

2.1. Catalyst preparation

Mg-Al hydrotalcite (Mg6Al2(OH)16CO3�4H2O) was prepared bycoprecipitation at pH 10 using the in-line dispersion-precipitation(ILDP) method (Fig. 1, left) detailed elsewhere [19]. The resultingslurry was aged in a mechanically stirred vessel at 298 K for 12 h,followed by filtration, washing, and drying at 353 K for 12 h. Thesolid was calcined in static air at 723 K for 15 h at 10 K min�1,followed by liquid-phase reconstruction [16]. For this purpose, thecalcined sample was immersed in decarbonated water at roomtemperature for 1 h, leading to meixnerite (Mg6Al2(OH)18�4H2O).Extreme care was taken to avoid contact of the rehydratedhydrotalcite with air using inert gas atmosphere in all operationsduring preparation, processing, storage, and use of the solid. Alongthe manuscript, the as-synthesized, calcined, and rehydratedhydrotalcites are denoted as HT-as, HT-ca, and HT-rh, respectively.

Fig. 1. Platforms used for preparation of hydrotalcite by the ILDP m

2.2. Catalyst characterization

The chemical composition of the as-synthesized hydrotalcitewas determined by AAS (Hitachi Z-8200) and ICP-OES (PerkinEl-mer Plasma 400). Powder X-ray diffraction patterns were acquiredin transmission on a D8 Brucker-Nonius Advance Series 2u/2upowder diffraction system using Cu Ka radiation. Data werecollected in the 2u range of 5–708 with an angular step of 0.0168and a counting time of 10.4 s per step. Thermogravimetric analysis(TGA) was carried out in a Mettler Toledo TGA/SDTA851emicrobalance equipped with a 34-positions sample robot and70 ml a-alumina crucibles. Analyses were performed in dry airflow of 50 ml STP min�1, ramping the temperature from 298 to1173 K at 5 K min�1. Transmission electron microscopy (TEM) wascarried out in a JEOL JEM-1011 microscope operated at 80 kV andequipped with a SIS Megaview III CCD camera. A few droplets of thesample suspended in ethanol were placed on a carbon-coatedcopper grid followed by evaporation at ambient conditions. N2

adsorption–desorption isotherms at 77 K were measured on aQuantachrome Autosorb 1-MP analyzer. Before analysis, thesamples were degassed in vacuum at 393 K for 16 h. Tempera-ture-programmed desorption of CO2 was measured in a ThermoTPDRO 1100 unit. Before TPD experiments, the sample (50 mg)was pretreated at 353 K with 3.5 vol.% CO2 in He flow(20 ml STP min�1) for 1 h, followed by removal of weakly adsorbedCO2 at 373 K, and ramping in He (20 ml STP min�1) from 373 to1173 K at 10 K min�1. A SuperCRCTM microcalorimeter fromOmnical was used to determine the heat evolved upon contactingcitral (Aldrich, 95%) and acetone (Aldrich, 99%) with the catalysts.Ca. 50 mg of solid was introduced in a glass vial sealed with aseptum and located inside a Teflon reactor under Ar atmosphere.The reactor was heated at 313 K and the reactant was dosed bymeans of a syringe. The catalyst mass was 16% of the mass of citralor acetone injected. An empty vial loaded with the same reactantamount was used as the reference. Both vials were magneticallystirred and the heat flow was recorded at intervals of 3 s during

ethod [19], parallel catalyst testing [18], and reactor scale-up.

Fig. 2. XRD patterns and schematic structures resulting from activation of Mg-Al

hydrotalcite (HT-as) by calcination (HT-ca) and rehydration (HT-rh). Phases: (&)

hydrotalcite and (&) periclase. The reflections marked by asterisk belong to the

Pt90-Rh10 heater strip.

S. Abello et al. / Applied Catalysis A: General 342 (2008) 119–125 121

100 min. The enthalpy associated with the process was determinedby integration of the heat flow versus time profiles.

2.3. Lab tests

The aldol condensation of citral and acetone was studied in anautomated MultimaxTM liquid-phase parallel lab-reactor systemfrom Mettler Toledo. The set-up and operating procedures weredescribed in detail elsewhere [18]. Briefly, 16-glass vessels (17 mmi.d. and total volume 50 ml) were filled with catalyst, carefullysealed to prevent any contact with CO2 in air, and mounted in thereactor block (Fig. 1, middle). Citral and acetone were dosed to thereactors and samples were collected by a syringe attached to an xyz

robot at 8 different times in the reaction period of 85 min. Based onour previous study [18], tests were carried out at the followingconditions: molar acetone-to-citral ratio = 3, temperature = 333 K,amount of catalyst with respect to citral = 16%, particle sizefraction = 75–300 mm, and stirring speed = 500 rpm. Fresh, used,decanted, and regenerated catalysts were evaluated. The proce-dures to obtain these samples are described in specific sections ofresults and discussion.

The liquid samples were analyzed off-line by a gas chromato-graph (Agilent 6890N/G1530N) equipped with a flame ionizationdetector and HP-5 capillary column. Tetradecane was used asinternal standard at a molar citral-to-tetradecane ratio = 5.3. Thecitral conversion and pseudoionone selectivity were calculated as[mole citral reacted/mole citral initial] and [mole pseudoiononeformed/mole

PPj], respectively, where

PPj is the sum of all

reaction products. The PS yield was computed as the product ofcitral conversion and pseudoionone selectivity.

2.4. Scale-up tests

Scale-up tests with the fresh rehydrated hydrotalcite werecarried out in a LabMaxTM Automatic Laboratory Reactor systemfrom Mettler Toledo. The glass double-jacketed reactor (12 cm i.d.and total volume 2 l, Fig. 1, right) was connected to a refluxcondenser and purged by nitrogen, followed by dosing of thereactants. The mixture was stirred at 150 rpm by means of avertical glass anchor impeller with a diameter of 9.3 cm. Thereactor temperature was controlled by a thermostat unit using aPt-100 sensor located in the reaction mixture. After the set pointtemperature was reached and stabilized, the reactor was filledwith catalyst particles. Tests were carried out at the sameconditions described for lab-scale experiments in Section 2.3. Liquidsamples (0.1 ml) were collected for off-line GC analysis by a syringeduring a reaction time of 200 min. Stirring was discontinued for 10 sin order to avoid capture of catalyst particles during sampling. Thereaction was also monitored on line by real-time ATR-FTIRspectroscopy using a React-IRTM 4000 spectrometer equipped withan ASI Applied Systems 16 mm-probe with a diamond crystal(DiCompTM) as the sensing element. The probe was introduced in thereactor and data collection was initiated after loading the catalyst.Spectra were collected in the range 650–4000 cm�1 by co-additionof 32 scans at a nominal resolution of 4 cm�1. The spectrum of theempty reactor in nitrogen was taken as the background. TheConcIRTTM Opus I software was used for spectral quantification.

3. Results and discussion

3.1. Synthesis and activation of hydrotalcite

Hydrotalcite-like compounds are typically prepared by batchcoprecipitation of the metal salts at increasing, decreasing, orconstant pH [20]. Owing to the discontinuous operation of the

synthesis method, the residence time of the precipitate particlesand concentration of reactants change throughout the precipita-tion process. This prevents a constant product quality, which isoften essential for the large-scale implementation of the resultingmaterials [21,22]. The continuous in-line dispersion-precipitation(ILDP) method [19,23] was used to synthesize hydrotalcites withuniform quality in the kg scale, as one of the main objectives of thiswork is to scale up the aldol condensation reaction. The ILDP routeencompasses the precipitation of Mg-Al hydrotalcite at constantpH (10) and fixed residence time (36 s) in a system integrating amicro-reactor (volume of 6 ml) with a high-shear stirrer and in-line pH control (Fig. 1, left). In addition to significant improvementin product uniformity, productivities in this highly intensifiedmethod largely exceed those typically attained by conventionalprecipitation [19].

Fig. 2 shows the XRD patterns of the as-synthesized material(HT-as, molar Mg/Al ratio of 2.9) and the products of activation bycalcination (HT-ca) and rehydration (HT-rh). The parent sampleshows hydrotalcite as the only crystalline phase (powderdiffraction file 89-460 from ICDD). The surface area of this sampledetermined by the BET method [24] was 26 m2 g�1. Thermaltreatment at 723 K for 15 h originates a poorly crystalline mixedoxide with rock-salt periclase structure (MgO, powder diffractionfile 45-0946 from ICDD) and a BET surface area of 250 m2 g�1.Reconstruction of the mixed oxide by rehydration in decarbonatedwater at 303 K leads to the pristine hydrotalcite structure in themeixnerite form, i.e. with OH� as the compensating interlayeranion instead of CO3

2�. The BET surface area of the reconstructedhydrotalcite amounted to 225 m2 g�1. The basicity of the sampleswas determined by temperature-programmed desorption of CO2.As expected, the HT-as sample led to negligible CO2 uptake, whileHT-ca and HT-rh presented a total CO2 uptake of 686 mmol g�1 and605 mmol g�1, respectively. Basic sites of different nature arepresent in these samples. HT-ca contains low and medium-strength Lewis O2� basic sites [25], while the hydroxyl groups inthe interlayer of HT-rh possess Brønsted basic character [16,26].

3.2. Comparison of bases

Despite the vast literature reporting catalytic data in aldolcondensation, no single study has systematically confronted thevarious catalyst families in Scheme 1. Reaction conditions (time,temperature, pressure, catalyst amount, acetone-to-citral ratio)

Fig. 3. Lab-performance of selected bases in the cross-condensation of citral and

acetone. Conditions: T = 333 K, A/C = 3, Wcat/Wcitral = 16%, t = 1 h.

Fig. 4. Pseudoionone yield vs. time over fresh HT-rh in lab (ml) and bench (l)

reactors. Symbols: off-line GC analysis; Solid line: on line ATR-FTIR analysis.

Conditions: T = 333 K, A/C = 3, Wcat/Wcitral = 16%.

Fig. 5. Waterfall ATR-FTIR plot of the citral–acetone condensation vs. time over

fresh HT-rh. Conditions: T = 333 K, A/C = 3, Wcat/Wcitral = 16%. The band at the

position of the asterisk was taken for quantification of the PS yield in Fig. 4.

S. Abello et al. / Applied Catalysis A: General 342 (2008) 119–125122

and reactor operation (batch vs. continuous, gas vs. liquid phase)often differ considerably to enable a proper comparison of catalyticperformance reported by different authors [7,12,14,16,27,28].Even more striking is the lack of studies including NaOH solutionsas a reference catalyst to benchmark the performance of theparticular solid base under investigation. For this reason, previousto conducting scalability and reusability studies over rehydratedMg-Al hydrotalcite, we have tested representative bases under thesame experimental conditions in a parallel-batch reactor system(Fig. 1, middle).

The citral conversion and pseudoionone selectivity over thesamples is shown in Fig. 3. The rehydrated hydrotalcite (HT-rh) isunequivocally the most efficient catalyst, leading to a 100% PS yieldafter a reaction time of 1 h. The NaOH solution leads to citralconversion and pseudoionone selectivity of 80% in the same periodof time. As reported elsewhere [16], as-synthesized and calcinedMg-Al hydrotalcites are selective to the cross-aldolization product(90%), although the citral conversion was ca. 10% after 1 h. Similarperformance was measured over MgO (not shown). In spite of thesimilar total density of basic sites in the calcined and rehydratedsamples determined by CO2-TPD, a completely different order ofcatalytic activity was attained. This can be explained attending tothe higher efficiency in the reaction of interlayer Brønsted OH�

sites in the rehydrated hydrotalcite HT-rh compared to the LewisO2� sites in the calcined hydrotalcite (and generally MgO) [12]. Theactivity of calcined hydrotalcites (or MgO) can be promoted byaddition of alkaline metals (Li, Na, K) [7,29,30]. However,significant leaching of the doping agents to the reaction mediumupon consecutive runs makes this approach highly unattractive[30].

3.3. Reaction scale up and on line analysis

A 2-l vessel (Fig. 1, right) was used to scale up the citral–acetonecondensation. This bench-scale reactor is two orders of magnitudelarger than those used for catalyst screening (Fig. 1, middle) andwas equipped with an ATR probe to quantitatively monitor thereaction in real time. This spectroscopic technique is increasinglyapplied in kinetic studies of liquid-phase reactions over hetero-geneous catalysts [31–34]. The experimental conditions used inlaboratory tests were transferred to the bench-scale reactor.

Fig. 4 compares the pseudoionone yield over the fresh HT-rhcatalyst in both lab and bench-scale reactor systems. The concilia-tion of catalytic data in the two platforms is excellent, enabling toconclude that the reaction over activated hydrotalcite can be

successfully upscaled. Representative infrared spectra showing theevolution of the reaction in the 1500–1850 cm�1 region are shown inFig. 5. The different components were assigned by recording thespectra of the pure liquid components, which enables to determinethe overlapped bands. The fast decrease of the bands assigned tocitral at 1673 and 1632 cm�1, attributed to the C O stretching in thea,b-unsaturated aldehyde and the C C stretching conjugated withC O [35], respectively, is coupled to the increased absorbance of thepseudoionone bands at 1666, 1632, and 1585 cm�1. The 1666 cm�1

band is related to the C O stretching in the a,b-unsaturated ketone,and the 1632 cm�1 band is assigned to the C C stretching in transform with C O [35]. The band at 1585 cm�1 has been controversiallyassigned to different strongly adsorbed species or enolate anions[36–38]. The PS yield was quantified from the absorbance of theband at 1585 cm�1 (asterisk in Fig. 5), which did not overlap withany other component. The resulting profile, which is depicted by thesolid line in Fig. 4, demonstrates the notable agreement between online ATR-FTIR and off-line GC analyses. Accordingly, this spectro-scopic technique can be elegantly applied to acquire real timeinfrared spectra in liquid-phase applications under operandoconditions, thus avoiding the laborious batch-wise sampling.

3.3. Sensitivity to ambient

So far, inert atmosphere was used in all the steps associatedwith the rehydration treatment and further use of the activatedhydrotalcite. Due to the presence of Brønsted basic sites in thismaterial and the acidic properties of CO2 present in air, it is highly

Fig. 6. Initial rate of pseudoionone formation over HT-rh after different exposure

times to ambient. Conditions: T = 333 K, A/C = 3, Wcat/Wcitral = 16%.

Fig. 7. Citral conversion and pseudoionone selectivity over Mg-Al hydrotalcites.

Sample codes: HT-rh (fresh rehydrated), HT-u1 and HT-u2 (used), HT-d (decanted),

and HT-rg (regenerated). Conditions: T = 333 K, A/C = 3, Wcat/Wcitral = 16%.

S. Abello et al. / Applied Catalysis A: General 342 (2008) 119–125 123

relevant to assess the stability of the basic sites upon exposure ofthe freshly rehydrated to ambient. Although it is commonknowledge that carbon dioxide poisons basic sites, the deactiva-tion kinetics of rehydrated hydrotalcites by CO2 and its implicationin aldol condensation has not been explicitly quantified in theliterature. Fig. 6 shows the initial rate of pseudoionone formationover the activated HT-rh previously exposed to ambient fordifferent times. A 15 min exposure to air reduced the reaction rateby 50%, while a 22 h exposure rendered a virtually inactivecatalyst. The citral conversion after 1 h of reaction dropped from100% in the fresh rehydrated hydrotalcite to 80% and <2% in thesample exposed during 15 min and 22 h in air, respectively. DiCosimo et al. [25] demonstrated by means of infrared studies atroom temperature that the interaction of CO2 with hydroxylgroups leads to the formation of bicarbonates in the interlayerspace (OH� + CO2! HCO3

�). Accordingly, the deactivation of therehydrated Mg-Al hydrotalcite in air is likely caused by thereaction of CO2 with active interlayer hydroxyl groups leading tobicarbonate species as compensating anions. It is worth addingthat the selectivity to pseudoione was 100% independently of theair exposure period and the degree of citral conversion. Thisstrongly suggests that the CO2 gradually reduces the number ofactive OH� without altering the quality of the remaining centersfor the cross-aldolization reaction. The extremely fast deactivationof rehydrated hydrotalcite by CO2 in the ambient atmosphere is aconsequence of the highly active nature of interlayer hydroxylgroups and the few number of utilized centers in these layeredmaterials, basically only those located at the edges and/or defectsof the platelets [16,17,39]. This aspect severely limits the practicalimplementation of these materials, as extreme precautions asregards inert atmosphere during handling, storage, reactor loading,and use are required.

3.4. Reutilization and regenerability

The aldol condensation performance of the fresh HT-rh vs. time isshown in Fig. 7. The reaction is very fast, reaching 85% citralconversion at 25 min and total conversion after ca. 1 h. Thepseudoionone selectivity is kept at 100% along the reaction period,in concordance with our previous observations [18]. Reusabilitytests were conducted over a used catalyst from a typical experimentafter cooling down the reactor to room temperature, recovery of thesolid by filtration under Ar atmosphere, and drying (samples HT-u1and HT-u2). The citral conversion over the catalyst after one run

(HT-u1) is extremely low (5% at t = 85 min) and the catalyst after twoconsecutive runs (HT-u2) showed no activity. More important froma fundamental perspective is the decreased PS selectivity, from 100%in the fresh HT-rh catalyst to 75% in the used HT-u1 catalyst. In thelatter case, diacetone alcohol (DAA), the acetone self-condensationproduct, was also obtained. The deactivation of these samples maybe due to adsorption of reaction products (polyunsaturatedmolecules such as pseudoionone and derived compounds) and/orproducts derived from oligomerization during the reaction and thecooling down of the reactor at the end of the catalytic run. Thesedeposits likely modify the nature of the few active OH� centers leftand/or its accessibility by reactants in subsequent reactions due tosteric hindrance, leading to lower pseudoionone selectivity.

The adsorption of reactants over the fresh and used sampleswas studied by microcalorimetry. Fig. 8 shows the heat flowprofiles upon interaction of citral and acetone with the fresh (HT-rh) and used (HT-u1) catalysts at 313 K. The enthalpy of citralinteraction with the fresh catalyst (17.8 kJ mol�1) was ca. 40 timeshigher than that of acetone (0.5 kJ mol�1). These results, aspreviously concluded by several authors, indicate a strongeradsorption of citral with the catalyst as compared to acetone[18,40]. The adsorption of both citral and acetone over the usedcatalyst is hindered, hardly producing an apparent signal. Theenthalpies of citral and acetone interaction with the usedhydrotalcite were as low as 0.9 and 0.2 kJ mol�1, respectively.

Fig. 8. Microcalorimetic profiles upon contacting citral and acetone with the fresh

(gray line, HT-rh) and used (black line, HT-u1) hydrotalcites. Conditions: T = 313 K,

Wcat/Wcitral or Wcat/Wacetone = 16%.

Fig. 9. Thermogravimetric profiles of the fresh, used, and regenerated hydrotalcites.

Fig. 10. TEM of the fresh, used, and regenerated hydrotalcites. The scale bar applies

to all the samples.

S. Abello et al. / Applied Catalysis A: General 342 (2008) 119–125124

This drastic reduction evidences the inactivity of the used catalystowing to the impossibility of the reactants to adsorb on the sample.

The presence of adsorbed species in the used catalysts wasdemonstrated and quantified by thermal analysis, after filtering thereaction mixture and drying the solid under inert atmosphere andvacuum conditions. The total weight loss of the rehydrated samplewas 37% (HT-rh, Fig. 9), with the two characteristic steps related todehydration and dehydroxylation of the layered structure, respec-tively [41]. The used sample (HT-u1) also displayed the two-stepbehavior, although important differences were observed above623 K, thus reaching a total weight loss of 50%. The deposits duringthe aldol condensation alter the morphology and porosity of thefresh rehydrated Mg-Al hydrotalcite. The typical platelet morphol-ogy of these layered materials [20] is hardly visualized in the usedsample (Fig. 10). Besides, the BET surface area and total pore volumeof HT-rh (225 m2 g�1 and 0.39 cm3 g�1, respectively) dropped to48 m2 g�1 and 0.22 ml g�1 in HT-u1.

Taking into consideration the reusability procedure of HT-u1 andHT-u2, we can put forward that the residues that deactivate thecatalyst are mainly formed during the cooling down of the reactorafter the run. In order to confirm this, an additional experiment wasconducted by decanting the reaction mixture at the end of thereaction (minimizing any significant drop of temperature) andadding a new batch of reactants, previously preheated at thereaction temperature (HT-d, Fig. 7). This led to a minor decrease of

the citral conversion with respect to HT-rh, although the PSselectivity decreased from 100 to 75%. This suggests that a certainamount of adsorbed species formed during the reaction significantlyaffects the quality and/or accessibility of the basic sites, inducing theself-condensation of acetone. In line with this, a previous work [42]suggested that the deactivation of Mg-Al mixed oxide in the acetoneoligomerization is due to the obstruction of active sites by residuesformed during the reaction. In any case, the above results discard thereutilization of the same hydrotalcite load in multiple runs, andtherefore, regeneration after each use is required.

The regenerated hydrotalcite (HT-rg) was obtained by calcina-tion at 723 K for 15 h (i.e. the same thermal treatment as to activateHT-as) in order to remove carbonaceous deposits, followed byrehydration in decarbonated water under inert atmosphere. Thecitral conversion was largely recovered (Fig. 7), although it wasslightly lower compared to the fresh sample (65% vs. 82% att = 25 min). Importantly, the PS selectivity returns to 100% as in thefresh catalyst. The total weight loss of the regenerated hydrotalcite(HT-rg) approaches that of the fresh sample (Fig. 9), although it issomewhat higher in the former specimen (40% vs. 37%). The first

S. Abello et al. / Applied Catalysis A: General 342 (2008) 119–125 125

weight loss was similar over both samples, and the higher weightloss in the second step is assigned to strongly adsorbed carbonac-eous deposits from the reaction, which were not decomposed bycalcination of HT-u1 at 723 K. Although the typical platelet-likemorphology was largely recovered (Fig. 10), the somewhat lowersurface area of HT-rg (SBET = 185 m2 g�1) compared to HT-rh(SBET = 225 m2 g�1) also substantiates that the applied calcinationtemperature was not sufficient to completely eliminate thecarbonaceous deposits. Therefore, the regeneration of the hydro-talcite was not complete, explaining the slightly lower citralconversion on HT-rg with respect to HT-rh. It can be put forwardthat the remaining carbonaceous deposits reduce the number ofactive sites in the sample, rendering lower catalytic activity, but donot affect the quality of the regenerated centers, as the PS selectivityremains at 100%.

4. Conclusions

This manuscript evaluates technical reasons explaining whyhydrotalcite-type bases have not widely reached the industrialscene as alternative to liquid inorganic bases. For this purpose, theintensively researched aldol condensation of citral and acetoneover activated hydrotalcite was taken as case study. The latterfamily of solid materials is very popular and often causes optimismamong the scientific community due to remarkable catalyticproperties in lab scale. In fact, fresh rehydrated Mg-Al hydrotalciteshows excellent yields to pseudoionone compared to NaOHsolutions and upscaling catalyst synthesis and reactor have beensuccessfully accomplished. However, technical barriers makingactivated hydrotalcites less attractive in the fine chemical industryderive from the extremely fast poisoning of the active OH� centersin contact with air, unfeasible reusability, and time-consumingregeneration steps. This can be likely extrapolated to otheraldolizations and C–C bond formation reactions where OH�

centers are required. These technical adversities of solid bases,the low cost of alkaline solutions, and the lack of environmentalpolicies associated with their use, motivate the conservatism ofchemical industry to use the traditional process. In our opinion,moving forward in the topic of base catalysis requires radicallynew fundamental directions. Calcined hydrotalcites with Lewisbasicity are more stable but much lower catalytic activitycompared to the rehydrated counterpart is often attained.Scientists active in this area should endeavor new strategies todesign solid bases, and this likely involves finding the optimalcompromise between activity, selectivity, and stability.

Acknowledgments

This research was funded by the Spanish MEC (project CTQ2006-01562/PPQ and Consolider-Ingenio 2010, grant CSD2006-

003, and fellowship PTQ05-01-00980) and the ICIQ Foundation. Dr.G. Colet and Dr. C. Jimenez are acknowledged for assistance in thescaling up tests.

References

[1] Y. Ono, J. Catal. 216 (2003) 406.[2] H. Hattori, Appl. Catal. A 222 (2001) 247.[3] K. Tanabe, W.F. Holderich, Appl. Catal. A 181 (1999) 399.[4] P.S. Gradeff, US Patent 3,840,601, 1974.[5] P.W.D. Mitchell, US Patent 4,874,900, 1989.[6] M.J. Climent, A. Corma, S. Iborra, J. Primo, J. Catal. 151 (1995) 60.[7] V.K. Dıez, C.R. Apesteguıa, J.I. Di Cosimo, J. Catal. 240 (2006) 235.[8] C. Noda, G.P. Alt, R.M. Werneck, C.A. Henriques, J.L.F. Monteiro, Braz. J. Chem. Eng.

15 (1998) 120.[9] M.J. Climent, A. Corma, H. Garcıa, R. Guil-Lopez, S. Iborra, V. Fornes, J. Catal. 193

(2001) 385.[10] H. Naka, Y. Kaneda, T. Kurata, J. Oleo Sci. 50 (2001) 813.[11] V. Subramoni, V.K. Rajeevan, B.R. Prasad, Ion Exchange Technol. (1984) 367.[12] J.C.A.A. Roelofs, A.J. van Dillen, K.P. de Jong, Catal. Today 60 (2000) 297.[13] A. Corma, M.J. Climent, A. Velty, S. Iborra, M. Susarte, ES 2159259A1, 2001.[14] M.J. Climent, A. Corma, S. Iborra, A. Velty, Catal. Lett. 79 (2002) 157.[15] C. Noda Perez, C.A. Perez, C.A. Henriques, J.L.F. Monteiro, Appl. Catal. A 272 (2004)

229.[16] S. Abello, F. Medina, D. Tichit, J. Perez-Ramırez, J.C. Groen, J.E. Sueiras, P. Salagre, Y.

Cesteros, Chem. Eur. J. 11 (2005) 728.[17] F. Winter, X. Xia, B.P.C. Hereijgers, J.H. Bitter, A.J. van Dillen, M. Muhler, K.P. de

Jong, J. Phys. Chem. B 110 (2006) 9211.[18] S. Abello, S. Dhir, G. Colet, J. Perez-Ramırez, Appl. Catal. A 325 (2007) 121.[19] S. Abello, J. Perez-Ramırez, Adv. Mater. 18 (2006) 2436.[20] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173.[21] P. Courty, C. Marcilly, in: G. Poncelet, P. Grange, P.A. Jacobs (Eds.), Preparation of

Catalysts III, Stud. Surf. Sci. Catal., Elsevier Science, Amsterdam, 1983, p. 485.[22] S. Schimpf, M. Lucas, P. Claus, in: W.F. Maier, R. Potyrailo (Eds.), Combinatorial and

High-Throughput Discovery and Optimization of Catalysts and Materials, Taylor& Francis CRC Press, United States, 2006, p. 149.

[23] M. Santiago, M.S. Yalfani, J. Perez-Ramırez, J. Mater. Chem. 16 (2006) 2886.[24] S. Brunauer, P.H. Emmet, E. Teller, J. Am. Chem. Soc. 60 (1938) 309.[25] J.I. Di Cosimo, V.K. Dıez, M. Xu, E. Iglesia, C.R. Apesteguıa, J. Catal. 178 (1998) 499.[26] K.K. Rao, M. Gravelle, J. Sanchez Valente, F. Figueras, J. Catal. 173 (1998) 115.[27] C. Noda Perez, C.A. Henriques, O.A.C. Antunes, J.L.F. Monteiro, J. Mol. Catal. A:

Chem. 233 (2005) 83.[28] P. Kustrowski, D. Sulkowska, L. Chmielarz, R. Dziembaj, Appl. Catal. A 302 (2006)

317.[29] J.I. Di Cosimo, C.R. Apesteguıa, V.K. Dıez, Appl. Catal. A 137 (1996) 149.[30] S. Abello, F. Medina, D. Tichit, J. Perez-Ramırez, X. Rodrıguez, J.E. Sueiras, P.

Salagre, Y. Cesteros, Appl. Catal. A 281 (2005) 191.[31] G. Hamminga, G. Mul, J.A. Moulijn, Chem. Eng. Sci. 59 (2004) 5479.[32] I. Dolamic, T. Burgi, J. Phys. Chem. B 110 (2006) 14898.[33] W.Z. Xu, X. Li, P.A. Charpentier, Polymer 48 (2007) 1219.[34] J.C. Groen, G.M. Hamminga, J.A. Moulijn, J. Perez-Ramırez, Phys. Chem. Chem.

Phys. 9 (2007) 4822.[35] G. Socrates, Infrared Characteristic Group Frequencies, John Wiley & Sons, New

York, 1980, p. 64.[36] B.E. Hanson, L.F. Wieserman, G.W. Wagner, R.A. Kaufmanlb, Langmuir 3 (1987)

549.[37] J. Weitkamp, M. Hunger, U. Rymsa, Microporous Mesoporous Mater. 48 (2001)

255.[38] M.I. Zaki, M.A. Hasan, L. Pasupulety, Langmuir 17 (2001) 768.[39] S. Abello, F. Medina, D. Tichit, J. Perez-Ramırez, Y. Cesteros, P. Salagre, J.E. Sueiras,

Chem. Commun. (2005) 1453.[40] M.J. Climent, A. Corma, S. Iborra, A. Velty, Green Chem. 4 (2002) 474.[41] W. Yang, Y. Kim, P.K.T. Liu, M. Sahimi, T. Tsotsis, Chem. Eng. Sci. 57 (2002) 2945.[42] V.K. Dıez, C.R. Apesteguıa, J.I. Di Cosimo, Lat. Am. Appl. Res. 33 (2003) 79.