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Organic–inorganic hybrid catalysts for acid- and base-catalyzed reactions
Microporous and Mesoporous Materials 35–36 (2000) 143–153 www.elsevier.nl/locate/micromeso
Organic–inorganic hybrid catalysts for acid- and base-catalyzed reactions S. Jaenicke*, G.K. Chuah, X.H. Lin, X.C. Hu Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Singapore Received 25 March 1999; received in revised form 5 July 1999; accepted for publication 9 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science
Abstract Stable heterogeneous catalysts with adjustable base strength have been prepared by grafting organic amine bases into the the pores of the inorganic mesoporous material, MCM-41, and to a porous styrene–divinylbenzene resin. The activities of these catalysts are compared for the formation of the monoglyceride from lauric acid and glycidol, and the Knoevenagel condensation of heptaldehyde with benzaldehyde to form a-n-amylcinnamaldehyde ( jasminaldehyde). Also included in the comparison are catalysts prepared by incorporating K O, BaO and K O/La O into 2 2 2 3 MCM-41. The resin-based catalyst suffers from poor thermal and mechanical stability. The organic–inorganic hybrid material containing the strong hindered amine base, TBD (1,5,7-triazabicyclo[4,4,0]dec-5-ene), performs well for the monoglyceride reaction at 110°C, and the catalyst can be re-used for at least 11 cycles with little loss of activity. However, it deactivates if it is used in the coupling reaction with aldehydes. This is obviously caused by loss of the base at the higher reaction temperature of 170°C, and by poisoning of the strong basic sites with benzoic acid which is formed by oxidation or through the Cannizzaro disproportionation of benzaldehyde. The more weakly basic catalysts based on MCM-41 with K O/La O can be used at a higher reaction temperature to compensate for their 2 2 3 lower intrinsic activity, and their activity can be restored by calcination. © 2000 Elsevier Science B.V. All rights reserved. Keywords: MCM-41; Heterogenized homogeneous catalysts; Organic–inorganic hybrid catalysts
1. Introduction Heterogeneous catalysts have well-known advantages over homogeneous catalysts. Therefore, there have been frequently attempts to ‘heterogenize’ homogeneous catalysts in order to combine the advantages of a homogeneous catalyst, such as high selectivity, with those of a heterogeneous catalyst, such as robustness and * Corresponding author. Tel: +65+874-2918; fax: +65-779-1691. E-mail address: [email protected] (S. Jaenicke)
ease of separation [1]. Haag et al. [2,3] were the first to propose that transition metal complexes could be immobilized on polymer resins. It was hoped that this would lead to improved catalysts for hydrogenation and selective oxidation reactions, as well as for CMC bond formation. However, noble metal or transition metal complexes covalently attached to a carrier have not met with much practical success because the need for facile ligand exchange, required for the catalytic transformation, also favors removal of the metal from its binding site on the support ( leaching). More recent work [4] indicates that it should be
1387-1811/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 21 5 - 2
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possible to overcome this problem by judicious choice of the tethering ligand. Another important class of reactions are those catalyzed by acids or bases, and there is a strong incentive to find solid acids [5] or bases [6 ] that can replace their liquid counterparts in large-scale reactions. Ion-exchange resins have been available for many years. Acidic ion exchangers are generally based on a crosslinked polymer with acid functionalities in the form of MSO H or MCOOH groups. 3 Similarly, strongly basic ion exchangers contain basic groups such as the quarternary amine, –N(CH )+OH−. The disadvantage of the standard 33 materials, which are based on porous polymers such as polystyrene crosslinked with a few per cent of divinylbenzene (DVB), is their poor thermal stability and limited mechanical strength which restricts their use in packed beds under pressure. Microporous (gel-type) ion-exchange resins have to be used in a solvent-swollen form in order to make the internal active sites accessible. The more rigid porous structure of macroreticulated polymers overcomes this problem. Perfluorinated polymeric materials such as NafionA [7] have a higher acidity due to the electron-withdrawing effect of the CF group. They also have much better thermal 2 stability, but a relatively low loading with acid groups. Because of their polar nature, the material is hydrophilic and swells only in aqueous medium, whereas in organic solvents access into the pores is restricted and only acid groups at the surface of the particles are available for reaction. Attempts have therefore been made to use porous inorganic materials with their superior thermal and mechanical properties as the carrier. The preparation of organic-modified silica gels has been disclosed [8–10]. Hybrid catalysts with organic groups attached to the support by standard silica functionalization techniques have been proposed. It is apparent that materials with wide pores are required in order to accommodate the functional groups and to allow easy access of reactants to the active sites. Following this line of thought, Cauvel et al. [11] functionalized the mesoporous material, MCM-41, with 1-triethoxysilyl-3-aminopropane. They also developed a generalized synthesis method to introduce other organic bases, by initially functionalizing the MCM-41 carrier with 1-trimethoxysilyl-
3-chloropropane, and then reacting the chloride with a secondary amine. Subba Rao et al. [12,13] pointed out that this route will lead to immediate poisoning of the strongest basic sites by the HCl liberated in the synthesis. They proposed to use instead the commercially available 3-methoxysilylpropoxymethyloxirane with a glycidyl group as the binding site (Scheme 1). Besides functionalizing MCM-41 materials or porous silicas, the direct sol–gel synthesis starting from mixtures of tetraethylorthosilicate ( TEOS ) and a functionalized silane containing an organic rest and three hydrolyzable ester groups, with and without structure-directing agents, has also been evaluated [14,15]. This method has been used to introduce acidic functionalities in the form of tethered sulfonic acid or carboxylic acid groups into hybrid organic–inorganic materials. Jones et al. [16 ] synthesized a BEA zeolite in the presence of (phenylethyl )trimethoxysilane and thereby introduced phenylpropyl groups into the pore space. Subsequently, the phenyl rings were sulfonated by reaction with gas-phase SO . Van Rhijn 3 et al. [17] used a similar strategy to bind a thiol into the walls of MCM-41-like material. This functional group is then easily oxidized to the sulfonic acid with H O . Harmer et al. [18,19] 2 2 describe nanocomposites prepared by the sol–gel condensation of TEOS in the presence of a colloidal solution of NafionA ion-exchange material. The same authors also report the preparation of an active acidic catalyst by co-gelation of TEOS with a silylester-functionalized perfluorinated molecule that carries a sulfonyl group [20]. In the present study, we address the following problems: (1) how does the catalytic activity depend on the strength of the base introduced? and (2) have hybrid catalysts a potential in continuous processes? In order to assess the merits of the hybrid materials, we compare them with a commercial-resin-based immobilized guanidine base, and with a purely inorganic catalyst prepared by incorporating K O and KLaO in the pores of 2 2 MCM-41[21]. For these tests, reactions between relatively bulky molecules were selected: the formation of monoglycerides from the base-catalyzed reaction between lauric acid and glycidol under
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Scheme 1. Surface modification with 3-trimethylsilylpropoxymethyloxirane.
mild reaction conditions, and the condensation of benzaldehyde with heptanal to form a-n-amylcinnamaldehyde ( jasminaldehyde) (Scheme 2). The synthesis of jasminaldehyde, a compound used in the flavor industry, is not only a reaction of commercial interest, but is also an interesting test reaction to assess the selectivity of the catalyst. The aliphatic aldehyde, heptaldehyde, is more reactve than the aromatic aldehyde, benzaldehyde, and therefore the dimerization of heptaldehyde is a parallel reaction which severely reduces the yield to jasminaldehyde. In order to improve the selectivity and to suppress the dimerization reaction, one has to work with a high excess of benzaldehyde. However, this leads to additional cost in product separation. Alternatively, the concentration of heptaldehyde in the reaction mixture can be kept low by adding this compound slowly over a long time to the reaction mixture. This leads to a semi batchwise operation, which cannot easily be adapted to
a continuous process. Recently, Climent et al. [22] proposed a one-step synthesis of jasminaldehyde with an acidic catalyst. The authors showed that an acidic Al-MCM-41 can catalyze the one-pot synthesis of jasminaldehyde from benzaldehyde and heptaldehyde at low pH. The concentration of free heptaldehyde is kept low by first transforming the aldehyde into its hemiacetal by reaction with methanol. MCM-41 is an excellent catalyst for this reaction [23]. It is then possible to synthesize jasminaldehyde with good yield and selecivity at a low ratio of benzaldehyde:heptaldehyde of 1.5:1. We investigated here the possibility of synthesizing jasminaldehyde directly by the base-catalyzed condensation reaction. Most of the tests were done in a batch reactor, either with slow addition of the heptaldehyde or by addition of the pre-mixed reactants to the catalyst. The results of the latter studies are compared with tests in a fixed-bed continuous flow reactor.
Scheme 2.
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2. Experimental 2.1. Catalyst preparation 2.1.1. Organic–inorganic hybrid catalysts The all-silica mesoporous MCM-41 was synthesized from tetraethylorthosilicate as described earlier [24]. The quality of the material was confirmed by powder X-ray diffraction ( XRD) and sorption measurements. The dried material was functionalized with 3-triethoxysilyl-1-chloropropane (Fluka) or 3-glycidylpropyltrimethoxysilane (Fluka) in toluene following the procedure of Cauvel et al. [11] and Subba Rao et al. [12,13]. Different amine bases were then covalently bound to the organic spacer. The bases used — piperidine, pyrolidine, 2,4,6-triaminopyrimidine and 1,5,7-triazabicyclo[4,4,0]dec-5-ene ( TBD), are shown in Scheme 3. A typical synthesis is as follows: 3.0 g of vacuum-dried MCM-41 in 40 cm3 of dry toluene was refluxed for 24 h with 1.06 g (4.5 mmol ) of trimethoxysilylpropoxymethyloxirane. The solid was recovered by filtration and washed with dry toluene. The glycidylated MCM-41 was then stirred with 0.92 g (6.6 mmol ) of TBD, or 0.47 g (6.6 mmol ) of pyrrolidine in 40 cm3 of dry toluene at room temperature for 10 h. Excess TBD or pyrrolidine was removed by lengthy Soxhlet
extraction with CH Cl . The resulting hybrid cata2 2 lysts were dried at 80°C overnight and finally under vacuum for 3 h at 100°C. 2.1.2. K O/MCM-41 and BaO/MCM-41 2 A 1.0 g quantity of MCM-41 was stirred at 70°C for 2 h in 20 cm3 of a methanolic solution containing the appropriate amount of CH COOK to achieve a loading of 5 or 10 wt% 3 K O, respectively. The methanol was removed 2 quickly under vacuum in a rotary evaporator. Similarly, catalysts with BaO loadings of 5 and 10 wt% were prepared with an aqueous solution of Ba(NO ) . The material was subsequently 32 heated at the rate of 1°C min−1 in air, and the MCM-41 impregnated with CH COOK was 3 calcined at 500°C for 5 h, whereas MCM-41supported Ba(NO ) was calcined at 600°C. The 32 temperatures required for the formation of the oxide had been determined by thermogravimetric analysis ( TGA). 2.1.3. KLaO /MCM-41 2 Binary KLaO /MCM-41 materials, containing 2 equimolar amounts of both potassium and lanthanum, were prepared in two different ways; i.e., by wet impregnation and solid-state reaction [25]. Wet impregnation involved stirring about 1.0 g of MCM-41 in 20 cm3 of methanol solution contain-
Scheme 3. Structures of the amine bases used for grafting MCM-41 and their pK values. a
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ing equimolar amounts of CH COOK and 3 La(NO ) · 6H O for 3 h at 60°C, followed by 33 2 quick drying and subsequent calcination in air at 500°C for 5 h. For the solid-state reaction, the MCM-41 support was carefully ground by hand in an agate mortar with the appropriate amount of CH COOK and La(NO ) · 6H O. The mixture 3 33 2 was calcined at 500°C for 5 h in air with a heating rate of 1°C min−1. The samples are identified with W or S following the sample code. The oxide loading was 5%, 10% and 20% calculated as KLaO . 2 2.1.4. TBD/polymer A catalyst with TBD bound via a C -spacer to 1 polystyrene crosslinked with 2% DVB is commercially available (Fluka 90603). The material contains ca. 2.8 mmol base/g resin. 2.2. Sample analysis The hybrid materials were characterized by infrared (IR) spectroscopy ( KBr pellet technique). The absence of an absorption band in the 3600–3800 cm−1 range (not shown) indicated that almost all surface silanol groups reacted with the silylating agent. The amount of base bound to the surface was determined by potentiometric titration in aqueous suspension, and from elemental analysis and the weight loss in TGA [26 ]. The samples were also analyzed by XRD. 2.3. Catalyst testing Testing of the monoglyceride synthesis was done in a stirred reactor, whereas the formation of jasminaldehyde was investigated both in a batchwise operating slurry reactor and in a continuous flow system with a packed catalyst bed. 2.3.1. Monoglyceride synthesis The reactor was a 100 cm3 round-bottomed flask equipped with an internal magnetic stirrer, heated on a temperature-controlled oil bath. All reactions were done under a protective argon atmosphere. For the monoglyceride synthesis, 1 g of the catalyst was suspended in 25 cm3 toluene, and 2 g (10 mmol ) of lauric acid and 0.74 g
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(10 mmol ) of glycidol were added. The mixture was heated with stirring to 110°C. Samples were withdrawn periodically and analyzed by gas chromatograpy (Supelco PetrocolA; ramp from room temperature to 350°C; FID). The products, unreacted lauric acid and the mono, di- and triglyceride, were identified by their retention times after initial gas chromatography–mass spectrometry (GC–MS) analysis. Glycidol could not be quantified for a complete mass balance because it elutes together with the solvent. 2.3.2. Catalytic synthesis of a-namylcinnamaldehyde (jasminaldehyde) in a batch reactor For the synthesis of jasminaldehyde, 0.2 g of the carefully dried catalyst was suspended in 1.59 g (15 mmol ) of benzaldehyde under an atmosphere of argon. The reaction mixture was heated on an oil bath to different temperatures (120, 150 and 170°C ), and heptaldehyde (1.08 g, 10 mmol ) was added either immediately, or dropwise over a time of 1 h. Samples were withdrawn through a syringe filter and analyzed by gas chromatography (packed column GP 5% Sp1200/1.75% Bentone 34 on 100/120 Supelcoport; FID). Products were identified by comparison with authentic samples. 2.3.3. Tests in a continuously operating plug-flow reactor The TBD/MCM-41catalyst was also tested in a flow reactor. As reactor, a stainless steel narrowbore high-performance liquid chromatography ( HPLC ) column (inner diameter 2.1 mm, length 250 mm; obtained from Supelco) was used. In order to limit the pressure drop through the packed bed to an acceptable value, the catalyst had to be granulated. For this, the catalyst powder was stirred with water into a slurry, dried, crushed, and the fraction between 425 and 125 mm was sieved out and used for the experiment. About 0.7 g of the material was dry-packed into the HPLC column. The column was placed inside an electrically heated oven kept at 150°C, and a slow flow (0.5 cm3 h−1; LHSV=2 h−1) of the reagent mixture (benzaldehyde:heptaldehyde=1.5:1) was introduced with a syringe pump.
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3. Results and discussion Some typical diffraction patterns for the parent material and the organic-modified materials are shown in Fig. 1. The high quality and good longrange order of the parent material is apparent from the diffractogram. The grafted materials show only a slight reduction in peak intensities. Spectra for the materials incorporating BaO, K O or 2 KLaO are reproduced in Fig. 2. They show an 2 attenuation of the signals at low angle, which is characteristic for the MCM-41 structure. The attenuation is most severe for the K O-loaded 2 material. Here, the MCM-41 structure appears to collapse partially. This collapse of the pore system may be brought about by the presence of the alkali together with steam generated by combustion of the acetate precursor during calcination. The structural collapse is also reflected in the decrease of both surface area and pore volume of this catalyst. However, the BaO- and KLaO -containing 2 catalysts maintain the MCM-41 structure. In agreement with the observations of Kloestra et al.
Fig. 1. XRD of MCM-41 (top), and of the hybrid catalysts Pyrr/MCM-41(middle) and TBD/MCM-41(bottom). The range above 2h=4° has been magnified in the MCM-41 diffractogram to show the reflexes at higher angles.
Table 1 Structural data for some fresh and used catalysts Sample
V p (cm3 g−1)
d p˚ (A )
BET area (m2 g−1)
MCM-41 Fresh TBD/MCM-41 Once-used TBD/MCM-41 Twice-used TBD/MCM-41 Fresh 10% K O/MCM-41 2 Fresh 10% BaO/MCM-41 Fresh 10% KLaO /MCM-41W 2 Used 10% KLaO /MCM-41W 2 Re-calcined 10% KLaO / 2 MCM-41W
1.11 0.61 0.46 0.42 0.12 0.50 0.62 0.41 0.57
33.4 21.4 21.0 21.3 23.9 30.0 30.8 26.4 30.8
982 674 521 395 92 535 646 562 641
[21], no new diffraction lines at higher angles were seen. Thus, the BaO and KLaO particles depos2 ited in the pore space are too small to give rise to an X-ray signal. However, the nitrogen-adsorption isotherms indicate that deposition of the alkali metal oxide clusters reduces the pore space, resulting in a lower total pore volume and a smaller pore radius. Similarly, the organic-functionalized materials show some loss of surface area and a pronounced reduction in the pore volume. The calculated ‘pore diameter’ is reduced from a value ˚ in the parent MCM-41 to about 21 A ˚ in of 34 A the catalyst TBD/MCM-41. The textural properties of several of the catalysts both before and after catalytic testing are listed in Table 1. The final loading of MCM-41 with base groups was in the order of 0.8 to 1.0 meq g−1, rather independent of the base used ( Table 2). For the catalysts prepared with the propylchloride precursor, the amount of base calculated from elemental analysis and from the weight loss in TGA is substantially higher than that determined by potentiometric titration. A considerable amount of chloride was detected in these catalysts; the sum of titrated free base and of chloride adds up to the value of total organic base as determined from TGA. This indicates that a considerable fraction of the potential basic sites (30–40%) is indeed blocked by HCl. The potentiometric titration ( Fig. 3) indicates that the base strength in aqueous suspension is lower than that of the free organic bases; the measured pH decreased continuously during titration with acid, indicating a wide distri-
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Fig. 2. Powder XRD of the inorganic-modified MCM-41 catalysts BaO/MCM-41, KLaO /MCM-41 and K O/MCM-41, together 2 2 with the spectrum of the parent compound. There are no reflexes above 2h=10°, and this range is therefore not shown. Table 2 Base and chloride content of the hybrid MCM-41 catalysts Catalyst
Base (mmol g−1)a
Base (mmol g−1)b
Cl− (mmol g−1)c
Cl/MCM-41 Pip/MCM-41d Pyr/MCM-41d TBD/MCM-41d Pyr/MCM-41e TBD/MCM-41e
0.76 0.77 0.80 0.79 0.98 0.79
Not applicable 0.50 0.53 0.58 – –
Not determined 0.29 0.35 0.20 Not detected Not detected
a From TGA. b By potentiometric titration. c From wet chemical analysis. d Precursor: propylchloride. e Precursor: propoxymethyloxirane.
bution of base strength for the surface-bound species. However, the potentiometric measurements show that the order of base strength: TBD>piperidine#pyrrolidine>primary amine, is the same as that for the free bases. We investigated catalysts with different organic bases with respect to the monoglyceride synthesis. The commercial access to these compounds is based on the transesterification of fat with excess
glycerol, catalyzed by strong base [27]. We investigate an alternative path, using readily available free fatty acids and glycidol. The results have been published [26 ]. Some of the results are shown in Table 3. The tests reveal that the base strength is of prime importance for the rate of the reaction: the catalyst with TBD has the highest activity, and the one with propylamine the lowest. However, the strong base TBD catalyzes not only the forma-
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Fig. 3. Titration curves of (a) 1 mmol of free and (b) 1 g of MCM-bound piperidine. (c) [ H+] concentration (=10−pH) as function of added acid for the carrier-bound catalyst. The equivalence point at 0.48 meq g−1 is found by back-extrapolating the straight line.
tion of the desired product, but leads also to the subsequent esterification of the remaining OH groups, so that an appreciable amount of diglycerides is present in the equilibrium mixture. After the catalysts had been used once for 24 h of reaction, they were recovered and examined. Pore volume and surface area of the used catalysts were considerably smaller and, also, the intensity of the Table 3 Activity of organic–inorganic hybrid catalysts for the formation of monoglyceride Catalyst
Run
2h
6h
24 h
PM/MCM-41
1 2 1 2 3 4 1 2 1 2 1 2
31.1 32.0 17.1 16.2 14.9 14.1 40.4 35.5 42.5 36.0 68.2 58.3
46.7 48.9 30.8 36.2 38.0 40.0 70.6 72.4 73.1 79.1 59.8 84.1
62.4 79.3 32.7 80.9 84.2 75.8 78.5 82.1 81.2 86.2 43.8 95.0
NH /MCM-41 2
Pyr/MCM-41 Pip/MCM-41 TBD/MCM-41
XRD reflections was weaker. However, when the catalysts were used again they showed a similar activity and, in most cases, an improved selectivity. This is attributed to the fact that residual (acidic) surface OH groups, which can catalyze the unproductive oligomerization of glycidol, have been removed by glycidylation during the first reaction cycle. Experiments were undertaken to determine if leaching of the catalyst takes place. It seems that this is not a problem. The TBD-containing catalyst was re-used for 11 reaction cycles, each of 2 h duration, and showed very little loss of activity throughout. The reactivity of aldehydes for a condensation reaction is lower than that of the oxirane for esterification. Therefore, more drastic reaction conditions are required. We used the same ratio of benzaldehyde:heptaldehyde=1.5:1 as Climent et al. [22] for our base-catalyzed synthesis. Analysis of the reaction mixture identified the desired product, jasminaldehyde, together with unreacted benzaldehyde and heptaldehyde; side reactions resulted in the dimerization product of heptaldehyde, and some benzylalcohol and benzoic
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Table 4 Effect of reaction temperature on the conversion of heptaldehyde and the selectivity to a-n-amylcinnamaldehyde ( jasminaldehyde). Catalyst: TBD/polymer Reaction temperature
2h
4h
6h
8h
(°C )
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
120 150 170 Reflux; 180
32.4 67.7 94.2 97.9
56.1 77.7 88.1 90.8
46.9 80.7 98.4 99.3
60.4 79.1 86.4 91.6
61.1 85.5 99.3 99.3
62.7 79.9 89.4 90.6
68.1 93.3 99.4 99.4
61.2 80.4 89.5 88.5
Table 5 Activity of solid base catalysts for the synthesis of jasminaldehydea Catalyst
10% K O/MCM-41 2 10% KLaO /MCM-41W 2 10% KLaO /MCM-41S 2 10% BaO/MCM-41 TBD/MCM-41 Pyrrolidine/MCM-41 TBD/polymerd
2h
6h
Conversionb (%)
Selectivityc (%)
Conversionb (%)
Selectivityc (%)
12.6 61.9 24.7 7.20 40.6 23.8 94.5
58.0 53.4 55.9 30.8 61.1 42.8 46.2
37.4 98.0 79.4 15.4 79.4 58.8 95.1
41.8 66.0 64.2 38.4 69.7 48.05 49.0
a Reaction conditions: benzaldehyde:heptaldehyde=1.5:1; both reactants were mixed, added to the catalyst powder, and kept at a temperature of 160°C for the time indicated. b Based on heptaldehyde. c To jasminaldehyde. d Fluka 90603
acid. The latter are the products of the Cannizzaro reaction, which benzaldehyde undergoes in alkaline medium. The benzoic acid formed in this step is detrimental because it poisons the basic sites at the catalyst. It was found that a higher temperature favors the formation of jasminaldehyde, especially when the heptaldehyde was added slowly in order to keep its concentration in the reaction mixture low. A selectivity of about 90% can be obtained over the TBD-containing polymeric catalyst ( Table 4). When the reaction conditions were that of a true batch reactor, a much lower selectivity is apparent, as seen from the data in Table 5. TBD bound to the styrene–divinylbenzene resin has the highest activity, and 95% conversion (based on heptaldehyde) was obtained with this catalyst after 2 h at 160°C. However, the selectivity to the desired product is below 50%. Also, the catalyst suffered
severe deactivation at this temperature and cannot be successfully recycled. TBD immobilized on MCM-41 results in a catalyst that is considerably more stable thermally, and shows a better selectivity. This is probably an indication for a shapeselective reaction in the pore system. The pyrrolidine-based catalyst had less activity than the one containing TBD, despite its higher loading with base groups. This shows that the strength of the base is also of importance for this reaction. Catalysts prepared by impregnating the MCM pore system with BaO show some activity, but are less selective than the TBD-based catalysts. However, the best results with respect to product yield were obtained with the binary inorganic catalyst, KLaO /MCM-41. Some differences due 2 to the preparation method are apparent, with the catalyst prepared by wet impregnation performing
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Table 6 Products formed over TBD/MCM under continuous flow conditions in a packed beda Temperature (°C )
RT 150 150 160 160
Cumulative time (h)
0.5 1 2 3 4
Product analysis (wt%) Heptaldehyde
Benzaldehyde
Benzoic acid
Dimer
Jasminaldehyde
39.8 34.3 34.8 35.0 35.4
59.3 53.7 54.7 55.7 55.6
0.73 1.81 2.20 3.00 2.14
0.12 3.92 3.29 2.60 2.86
0.00 6.20 5.05 3.74 3.93
Conversion (%)
Selectivityb (%)
0.3 17.0 14.3 11.3 12.0
0.0 44.1 43.1 41.9 40.8
a Reaction conditions: 0.7 g TBD/MCM-41, benzaldehyde:heptaldehyde =1.5:1, reagent flow=0.5 cm3 h−1, LHSV=2 h−1. b To jasminaldehyde.
better than that from the solid-state synthesis. The presence of the rare earth metal not only improves the thermal stability of the material, but improves also the selectivity. Table 1 shows the results of structural analysis for some of the catalysts over several test cycles. The used catalysts had been exhaustively extracted with methylene chloride to remove all soluble organic deposits before being subjected to sorption measurements for pore-size analysis. It is apparent that the hybrid catalyst suffers from fouling and blocking of the pore system, most probably by incompletely removed products. The used inorganic catalyst shows a similar loss of surface area, but the catalyst can be re-calcined, and this process restores almost all of the original surface area. Testing of the TBD/MCM-41 catalyst in a packed bed was conducted in a micro-reactor. The results of the test are shown in Table 6. The conversion is lower than in the slurry reactor experiments, as a consequence of the shorter contact time (t#30 min), and seems to decrease somewhat with time-on-stream of the catalyst. A higher temperature does not improve the conversion. The selectivity to jasminaldehyde is lower than under batch processing conditions. The selectivity did not increase when the reaction was run at a higher temperature. The formation of byproducts, especially benzoic acid, was much more pronounced than in the batch reactor. It is possible that air in the catalyst bed, which had not been efficiently removed, is responsible for oxidation of the aldehydes, because we did not detect the corresponding quantity of benzylalcohol.
4. Conclusion Solid base catalysts show promise for certain applications in fine chemical synthesis. Hybrid catalysts, based on a stable MCM-41 mesoporous support, have better thermal stability than ionexchange resin type catalysts and can be used repeatedly at temperatures as high as 170°C if proper care is taken to remove oxygen and other oxidants. Indeed, TBD/MCM-41 delivers some of the advantages expected from a solid base: the free guanidine base with a pK #25 is of comparable a strength to KOH, and can be used in non-aqueous medium. However, the pK value of the grafted a base is much reduced compared with the free base, and even a strong organic base like TBD has only moderate basicity in the grafted form. Also, the loading with active groups at 1 meq g−1 catalyst is only about 50% of that obtainable with organic resins. The synthesis of monoglycerides, which poses some problems under technical conditions due to limited solubility of the reaction partners and foaming, can be smoothly catalyzed under mild conditions with a catalyst based on a largepore molecular sieve such as MCM-41. However, it is necessary to use reactive reactants such as glycidol so that the reaction can take place under mild conditions. High yield and high product selectivity to the desired monoglycerides are achieved. The mesoporous hybrid materials show fewer advantages in reactions with more inert compounds. Because of the relatively low basicity of the material, higher temperatures are required to bring about reaction in a reasonable time. In
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such cases, solid base catalysts using a combination of rare earth and alkali metal oxides on a mesoporous carrier seem to offer advantages, primarily with respect to catalyst regeneration. The presence of the rare earth metal in these catalysts is necessary to improve thermal stability, and has also a positive influence on the product selectivity. However, tests with the hybrid catalyst under continuous flow conditions are encouraging, and verify the potential of these solid bases.
Acknowledgement This work was RP972669.
funded
by NUS under
References [1] P. Panster, S. Wieland, B. Cornils, W.A. Herrmann ( Eds.), Applied Homogeneous Catalysis with Organometallic Compounds vol. 2, VCH, Weinheim, 1996, p. 605. [2] W.O. Haag, D.D. Whitehurst, BE 721 686, 1968 (assigned to Mobil Oil Corp.). [3] W.O. Haag, D.D. Whitehurst, in: Proc. 5th Int. Cong. Catal. (1972) 465. [4] B. Pugin, M. Mu¨ller, M. Guisnet et al. ( Eds.), Heterogeneous Catalysis and Fine Chemicals vol. III, Elsevier, Amsterdam, 1993, pp. 107–114. [5] A. Corma, Chem. Rev. 95 (1995) 559. [6 ] H. Hattori, Chem. Rev. 95 (1995) 537. [7] G.A. Olah, P.S. Iyer, G.K.S. Prakash, Synthesis (1986) 513.
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[8] P. Panster, W. Buder, P. Kleinschmit, DE 2 834 692, 1978 (assigned to Degussa AG). [9] P. Panster, P. Kleinschmit, DE 3 029 599, 1980 (assigned to Degussa AG). [10] P. Panster, P. Kleinschmit, DE 3 131 954, 1981 (assigned to Degussa AG). [11] A. Cauvel, G. Renard, D. Brunel, J. Org. Chem. 62 (1997) 749. [12] Y.V. Subba Rao, D.E. De Vos, P.A. Jacobs, Angew. Chem. 109 (1997) 2776. [13] Y.V. Subba Rao, D.E. De Vos, P.A. Jacobs, Angew. Chem. Int. Ed. Engl. 36 (1997) 2261. [14] D.J. Macquarrie, Chem. Commun. (1996) 1961. [15] C.E. Fowler, S.L. Burkett, S. Mann, Chem. Commun. (1997) 1769. [16 ] C.W. Jones, K. Tsuji, M.E. Davis, Nature 393 (May) (1998) 52. [17] W.M. Van Rhijn, D.E. De Vos, B.F. Sels, W.D. Bossaert, P.A. Jacobs, Chem. Commun. (1998) 317. [18] M.A. Harmer, W.E. Farneth, Q. Sun, J. Am. Chem. Soc. 118 (1996) 7708. [19] M.A. Harmer, Q. Sun, M.J. Michalczyk, Z. Yang, Chem. Commun. (1997) 1803. [20] M.A. Harmer, W.E. Farneth, Q. Sun, Adv. Mater. 10 (1998) 1255. [21] K.R. Kloetstra, M. van Laren, H. van Bekkum, J. Chem. Soc., Faraday Trans. 93 (1997) 1211. [22] M.J. Climent, A. Corma, R.. Guil-Lopez, S. Iborra, J. Primo, J. Catal. 175 (1998) 70. [23] Y. Tanaka, N. Sawamura, M. Iwamoto, Tetrahedron Lett. 39 (1998) 9457. [24] L.Y. Chen, Z. Ping, G.K. Chuah, S. Jaenicke, G. Simon, Microporous Mesoporous Mater. 27 (1999) 231. [25] F. Yagi, N. Kanuka, H. Tsuji, H. Kita, H. Hattori, in: H. Hattori, M. Misono, Y. Ono (Eds.), Acid–Base Catalysis vol. II, Elsevier-Kodansha, Tokyo, 1994, p. 349. [26 ] X.-H. Lin, G.K. Chuah, S. Jaenicke, J. Mol. Catal. A. (1999) in press. [27] N.O. v. Sonntag, J. Am. Oil Chem. Soc. 59 (1982) 795.
Organic–inorganic hybrid catalysts for acid- and base-catalyzed reactions S. Jaenicke*, G.K. Chuah, X.H. Lin, X.C. Hu Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Singapore Received 25 March 1999; received in revised form 5 July 1999; accepted for publication 9 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science
Abstract Stable heterogeneous catalysts with adjustable base strength have been prepared by grafting organic amine bases into the the pores of the inorganic mesoporous material, MCM-41, and to a porous styrene–divinylbenzene resin. The activities of these catalysts are compared for the formation of the monoglyceride from lauric acid and glycidol, and the Knoevenagel condensation of heptaldehyde with benzaldehyde to form a-n-amylcinnamaldehyde ( jasminaldehyde). Also included in the comparison are catalysts prepared by incorporating K O, BaO and K O/La O into 2 2 2 3 MCM-41. The resin-based catalyst suffers from poor thermal and mechanical stability. The organic–inorganic hybrid material containing the strong hindered amine base, TBD (1,5,7-triazabicyclo[4,4,0]dec-5-ene), performs well for the monoglyceride reaction at 110°C, and the catalyst can be re-used for at least 11 cycles with little loss of activity. However, it deactivates if it is used in the coupling reaction with aldehydes. This is obviously caused by loss of the base at the higher reaction temperature of 170°C, and by poisoning of the strong basic sites with benzoic acid which is formed by oxidation or through the Cannizzaro disproportionation of benzaldehyde. The more weakly basic catalysts based on MCM-41 with K O/La O can be used at a higher reaction temperature to compensate for their 2 2 3 lower intrinsic activity, and their activity can be restored by calcination. © 2000 Elsevier Science B.V. All rights reserved. Keywords: MCM-41; Heterogenized homogeneous catalysts; Organic–inorganic hybrid catalysts
1. Introduction Heterogeneous catalysts have well-known advantages over homogeneous catalysts. Therefore, there have been frequently attempts to ‘heterogenize’ homogeneous catalysts in order to combine the advantages of a homogeneous catalyst, such as high selectivity, with those of a heterogeneous catalyst, such as robustness and * Corresponding author. Tel: +65+874-2918; fax: +65-779-1691. E-mail address: [email protected] (S. Jaenicke)
ease of separation [1]. Haag et al. [2,3] were the first to propose that transition metal complexes could be immobilized on polymer resins. It was hoped that this would lead to improved catalysts for hydrogenation and selective oxidation reactions, as well as for CMC bond formation. However, noble metal or transition metal complexes covalently attached to a carrier have not met with much practical success because the need for facile ligand exchange, required for the catalytic transformation, also favors removal of the metal from its binding site on the support ( leaching). More recent work [4] indicates that it should be
1387-1811/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 21 5 - 2
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possible to overcome this problem by judicious choice of the tethering ligand. Another important class of reactions are those catalyzed by acids or bases, and there is a strong incentive to find solid acids [5] or bases [6 ] that can replace their liquid counterparts in large-scale reactions. Ion-exchange resins have been available for many years. Acidic ion exchangers are generally based on a crosslinked polymer with acid functionalities in the form of MSO H or MCOOH groups. 3 Similarly, strongly basic ion exchangers contain basic groups such as the quarternary amine, –N(CH )+OH−. The disadvantage of the standard 33 materials, which are based on porous polymers such as polystyrene crosslinked with a few per cent of divinylbenzene (DVB), is their poor thermal stability and limited mechanical strength which restricts their use in packed beds under pressure. Microporous (gel-type) ion-exchange resins have to be used in a solvent-swollen form in order to make the internal active sites accessible. The more rigid porous structure of macroreticulated polymers overcomes this problem. Perfluorinated polymeric materials such as NafionA [7] have a higher acidity due to the electron-withdrawing effect of the CF group. They also have much better thermal 2 stability, but a relatively low loading with acid groups. Because of their polar nature, the material is hydrophilic and swells only in aqueous medium, whereas in organic solvents access into the pores is restricted and only acid groups at the surface of the particles are available for reaction. Attempts have therefore been made to use porous inorganic materials with their superior thermal and mechanical properties as the carrier. The preparation of organic-modified silica gels has been disclosed [8–10]. Hybrid catalysts with organic groups attached to the support by standard silica functionalization techniques have been proposed. It is apparent that materials with wide pores are required in order to accommodate the functional groups and to allow easy access of reactants to the active sites. Following this line of thought, Cauvel et al. [11] functionalized the mesoporous material, MCM-41, with 1-triethoxysilyl-3-aminopropane. They also developed a generalized synthesis method to introduce other organic bases, by initially functionalizing the MCM-41 carrier with 1-trimethoxysilyl-
3-chloropropane, and then reacting the chloride with a secondary amine. Subba Rao et al. [12,13] pointed out that this route will lead to immediate poisoning of the strongest basic sites by the HCl liberated in the synthesis. They proposed to use instead the commercially available 3-methoxysilylpropoxymethyloxirane with a glycidyl group as the binding site (Scheme 1). Besides functionalizing MCM-41 materials or porous silicas, the direct sol–gel synthesis starting from mixtures of tetraethylorthosilicate ( TEOS ) and a functionalized silane containing an organic rest and three hydrolyzable ester groups, with and without structure-directing agents, has also been evaluated [14,15]. This method has been used to introduce acidic functionalities in the form of tethered sulfonic acid or carboxylic acid groups into hybrid organic–inorganic materials. Jones et al. [16 ] synthesized a BEA zeolite in the presence of (phenylethyl )trimethoxysilane and thereby introduced phenylpropyl groups into the pore space. Subsequently, the phenyl rings were sulfonated by reaction with gas-phase SO . Van Rhijn 3 et al. [17] used a similar strategy to bind a thiol into the walls of MCM-41-like material. This functional group is then easily oxidized to the sulfonic acid with H O . Harmer et al. [18,19] 2 2 describe nanocomposites prepared by the sol–gel condensation of TEOS in the presence of a colloidal solution of NafionA ion-exchange material. The same authors also report the preparation of an active acidic catalyst by co-gelation of TEOS with a silylester-functionalized perfluorinated molecule that carries a sulfonyl group [20]. In the present study, we address the following problems: (1) how does the catalytic activity depend on the strength of the base introduced? and (2) have hybrid catalysts a potential in continuous processes? In order to assess the merits of the hybrid materials, we compare them with a commercial-resin-based immobilized guanidine base, and with a purely inorganic catalyst prepared by incorporating K O and KLaO in the pores of 2 2 MCM-41[21]. For these tests, reactions between relatively bulky molecules were selected: the formation of monoglycerides from the base-catalyzed reaction between lauric acid and glycidol under
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Scheme 1. Surface modification with 3-trimethylsilylpropoxymethyloxirane.
mild reaction conditions, and the condensation of benzaldehyde with heptanal to form a-n-amylcinnamaldehyde ( jasminaldehyde) (Scheme 2). The synthesis of jasminaldehyde, a compound used in the flavor industry, is not only a reaction of commercial interest, but is also an interesting test reaction to assess the selectivity of the catalyst. The aliphatic aldehyde, heptaldehyde, is more reactve than the aromatic aldehyde, benzaldehyde, and therefore the dimerization of heptaldehyde is a parallel reaction which severely reduces the yield to jasminaldehyde. In order to improve the selectivity and to suppress the dimerization reaction, one has to work with a high excess of benzaldehyde. However, this leads to additional cost in product separation. Alternatively, the concentration of heptaldehyde in the reaction mixture can be kept low by adding this compound slowly over a long time to the reaction mixture. This leads to a semi batchwise operation, which cannot easily be adapted to
a continuous process. Recently, Climent et al. [22] proposed a one-step synthesis of jasminaldehyde with an acidic catalyst. The authors showed that an acidic Al-MCM-41 can catalyze the one-pot synthesis of jasminaldehyde from benzaldehyde and heptaldehyde at low pH. The concentration of free heptaldehyde is kept low by first transforming the aldehyde into its hemiacetal by reaction with methanol. MCM-41 is an excellent catalyst for this reaction [23]. It is then possible to synthesize jasminaldehyde with good yield and selecivity at a low ratio of benzaldehyde:heptaldehyde of 1.5:1. We investigated here the possibility of synthesizing jasminaldehyde directly by the base-catalyzed condensation reaction. Most of the tests were done in a batch reactor, either with slow addition of the heptaldehyde or by addition of the pre-mixed reactants to the catalyst. The results of the latter studies are compared with tests in a fixed-bed continuous flow reactor.
Scheme 2.
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2. Experimental 2.1. Catalyst preparation 2.1.1. Organic–inorganic hybrid catalysts The all-silica mesoporous MCM-41 was synthesized from tetraethylorthosilicate as described earlier [24]. The quality of the material was confirmed by powder X-ray diffraction ( XRD) and sorption measurements. The dried material was functionalized with 3-triethoxysilyl-1-chloropropane (Fluka) or 3-glycidylpropyltrimethoxysilane (Fluka) in toluene following the procedure of Cauvel et al. [11] and Subba Rao et al. [12,13]. Different amine bases were then covalently bound to the organic spacer. The bases used — piperidine, pyrolidine, 2,4,6-triaminopyrimidine and 1,5,7-triazabicyclo[4,4,0]dec-5-ene ( TBD), are shown in Scheme 3. A typical synthesis is as follows: 3.0 g of vacuum-dried MCM-41 in 40 cm3 of dry toluene was refluxed for 24 h with 1.06 g (4.5 mmol ) of trimethoxysilylpropoxymethyloxirane. The solid was recovered by filtration and washed with dry toluene. The glycidylated MCM-41 was then stirred with 0.92 g (6.6 mmol ) of TBD, or 0.47 g (6.6 mmol ) of pyrrolidine in 40 cm3 of dry toluene at room temperature for 10 h. Excess TBD or pyrrolidine was removed by lengthy Soxhlet
extraction with CH Cl . The resulting hybrid cata2 2 lysts were dried at 80°C overnight and finally under vacuum for 3 h at 100°C. 2.1.2. K O/MCM-41 and BaO/MCM-41 2 A 1.0 g quantity of MCM-41 was stirred at 70°C for 2 h in 20 cm3 of a methanolic solution containing the appropriate amount of CH COOK to achieve a loading of 5 or 10 wt% 3 K O, respectively. The methanol was removed 2 quickly under vacuum in a rotary evaporator. Similarly, catalysts with BaO loadings of 5 and 10 wt% were prepared with an aqueous solution of Ba(NO ) . The material was subsequently 32 heated at the rate of 1°C min−1 in air, and the MCM-41 impregnated with CH COOK was 3 calcined at 500°C for 5 h, whereas MCM-41supported Ba(NO ) was calcined at 600°C. The 32 temperatures required for the formation of the oxide had been determined by thermogravimetric analysis ( TGA). 2.1.3. KLaO /MCM-41 2 Binary KLaO /MCM-41 materials, containing 2 equimolar amounts of both potassium and lanthanum, were prepared in two different ways; i.e., by wet impregnation and solid-state reaction [25]. Wet impregnation involved stirring about 1.0 g of MCM-41 in 20 cm3 of methanol solution contain-
Scheme 3. Structures of the amine bases used for grafting MCM-41 and their pK values. a
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ing equimolar amounts of CH COOK and 3 La(NO ) · 6H O for 3 h at 60°C, followed by 33 2 quick drying and subsequent calcination in air at 500°C for 5 h. For the solid-state reaction, the MCM-41 support was carefully ground by hand in an agate mortar with the appropriate amount of CH COOK and La(NO ) · 6H O. The mixture 3 33 2 was calcined at 500°C for 5 h in air with a heating rate of 1°C min−1. The samples are identified with W or S following the sample code. The oxide loading was 5%, 10% and 20% calculated as KLaO . 2 2.1.4. TBD/polymer A catalyst with TBD bound via a C -spacer to 1 polystyrene crosslinked with 2% DVB is commercially available (Fluka 90603). The material contains ca. 2.8 mmol base/g resin. 2.2. Sample analysis The hybrid materials were characterized by infrared (IR) spectroscopy ( KBr pellet technique). The absence of an absorption band in the 3600–3800 cm−1 range (not shown) indicated that almost all surface silanol groups reacted with the silylating agent. The amount of base bound to the surface was determined by potentiometric titration in aqueous suspension, and from elemental analysis and the weight loss in TGA [26 ]. The samples were also analyzed by XRD. 2.3. Catalyst testing Testing of the monoglyceride synthesis was done in a stirred reactor, whereas the formation of jasminaldehyde was investigated both in a batchwise operating slurry reactor and in a continuous flow system with a packed catalyst bed. 2.3.1. Monoglyceride synthesis The reactor was a 100 cm3 round-bottomed flask equipped with an internal magnetic stirrer, heated on a temperature-controlled oil bath. All reactions were done under a protective argon atmosphere. For the monoglyceride synthesis, 1 g of the catalyst was suspended in 25 cm3 toluene, and 2 g (10 mmol ) of lauric acid and 0.74 g
147
(10 mmol ) of glycidol were added. The mixture was heated with stirring to 110°C. Samples were withdrawn periodically and analyzed by gas chromatograpy (Supelco PetrocolA; ramp from room temperature to 350°C; FID). The products, unreacted lauric acid and the mono, di- and triglyceride, were identified by their retention times after initial gas chromatography–mass spectrometry (GC–MS) analysis. Glycidol could not be quantified for a complete mass balance because it elutes together with the solvent. 2.3.2. Catalytic synthesis of a-namylcinnamaldehyde (jasminaldehyde) in a batch reactor For the synthesis of jasminaldehyde, 0.2 g of the carefully dried catalyst was suspended in 1.59 g (15 mmol ) of benzaldehyde under an atmosphere of argon. The reaction mixture was heated on an oil bath to different temperatures (120, 150 and 170°C ), and heptaldehyde (1.08 g, 10 mmol ) was added either immediately, or dropwise over a time of 1 h. Samples were withdrawn through a syringe filter and analyzed by gas chromatography (packed column GP 5% Sp1200/1.75% Bentone 34 on 100/120 Supelcoport; FID). Products were identified by comparison with authentic samples. 2.3.3. Tests in a continuously operating plug-flow reactor The TBD/MCM-41catalyst was also tested in a flow reactor. As reactor, a stainless steel narrowbore high-performance liquid chromatography ( HPLC ) column (inner diameter 2.1 mm, length 250 mm; obtained from Supelco) was used. In order to limit the pressure drop through the packed bed to an acceptable value, the catalyst had to be granulated. For this, the catalyst powder was stirred with water into a slurry, dried, crushed, and the fraction between 425 and 125 mm was sieved out and used for the experiment. About 0.7 g of the material was dry-packed into the HPLC column. The column was placed inside an electrically heated oven kept at 150°C, and a slow flow (0.5 cm3 h−1; LHSV=2 h−1) of the reagent mixture (benzaldehyde:heptaldehyde=1.5:1) was introduced with a syringe pump.
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3. Results and discussion Some typical diffraction patterns for the parent material and the organic-modified materials are shown in Fig. 1. The high quality and good longrange order of the parent material is apparent from the diffractogram. The grafted materials show only a slight reduction in peak intensities. Spectra for the materials incorporating BaO, K O or 2 KLaO are reproduced in Fig. 2. They show an 2 attenuation of the signals at low angle, which is characteristic for the MCM-41 structure. The attenuation is most severe for the K O-loaded 2 material. Here, the MCM-41 structure appears to collapse partially. This collapse of the pore system may be brought about by the presence of the alkali together with steam generated by combustion of the acetate precursor during calcination. The structural collapse is also reflected in the decrease of both surface area and pore volume of this catalyst. However, the BaO- and KLaO -containing 2 catalysts maintain the MCM-41 structure. In agreement with the observations of Kloestra et al.
Fig. 1. XRD of MCM-41 (top), and of the hybrid catalysts Pyrr/MCM-41(middle) and TBD/MCM-41(bottom). The range above 2h=4° has been magnified in the MCM-41 diffractogram to show the reflexes at higher angles.
Table 1 Structural data for some fresh and used catalysts Sample
V p (cm3 g−1)
d p˚ (A )
BET area (m2 g−1)
MCM-41 Fresh TBD/MCM-41 Once-used TBD/MCM-41 Twice-used TBD/MCM-41 Fresh 10% K O/MCM-41 2 Fresh 10% BaO/MCM-41 Fresh 10% KLaO /MCM-41W 2 Used 10% KLaO /MCM-41W 2 Re-calcined 10% KLaO / 2 MCM-41W
1.11 0.61 0.46 0.42 0.12 0.50 0.62 0.41 0.57
33.4 21.4 21.0 21.3 23.9 30.0 30.8 26.4 30.8
982 674 521 395 92 535 646 562 641
[21], no new diffraction lines at higher angles were seen. Thus, the BaO and KLaO particles depos2 ited in the pore space are too small to give rise to an X-ray signal. However, the nitrogen-adsorption isotherms indicate that deposition of the alkali metal oxide clusters reduces the pore space, resulting in a lower total pore volume and a smaller pore radius. Similarly, the organic-functionalized materials show some loss of surface area and a pronounced reduction in the pore volume. The calculated ‘pore diameter’ is reduced from a value ˚ in the parent MCM-41 to about 21 A ˚ in of 34 A the catalyst TBD/MCM-41. The textural properties of several of the catalysts both before and after catalytic testing are listed in Table 1. The final loading of MCM-41 with base groups was in the order of 0.8 to 1.0 meq g−1, rather independent of the base used ( Table 2). For the catalysts prepared with the propylchloride precursor, the amount of base calculated from elemental analysis and from the weight loss in TGA is substantially higher than that determined by potentiometric titration. A considerable amount of chloride was detected in these catalysts; the sum of titrated free base and of chloride adds up to the value of total organic base as determined from TGA. This indicates that a considerable fraction of the potential basic sites (30–40%) is indeed blocked by HCl. The potentiometric titration ( Fig. 3) indicates that the base strength in aqueous suspension is lower than that of the free organic bases; the measured pH decreased continuously during titration with acid, indicating a wide distri-
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Fig. 2. Powder XRD of the inorganic-modified MCM-41 catalysts BaO/MCM-41, KLaO /MCM-41 and K O/MCM-41, together 2 2 with the spectrum of the parent compound. There are no reflexes above 2h=10°, and this range is therefore not shown. Table 2 Base and chloride content of the hybrid MCM-41 catalysts Catalyst
Base (mmol g−1)a
Base (mmol g−1)b
Cl− (mmol g−1)c
Cl/MCM-41 Pip/MCM-41d Pyr/MCM-41d TBD/MCM-41d Pyr/MCM-41e TBD/MCM-41e
0.76 0.77 0.80 0.79 0.98 0.79
Not applicable 0.50 0.53 0.58 – –
Not determined 0.29 0.35 0.20 Not detected Not detected
a From TGA. b By potentiometric titration. c From wet chemical analysis. d Precursor: propylchloride. e Precursor: propoxymethyloxirane.
bution of base strength for the surface-bound species. However, the potentiometric measurements show that the order of base strength: TBD>piperidine#pyrrolidine>primary amine, is the same as that for the free bases. We investigated catalysts with different organic bases with respect to the monoglyceride synthesis. The commercial access to these compounds is based on the transesterification of fat with excess
glycerol, catalyzed by strong base [27]. We investigate an alternative path, using readily available free fatty acids and glycidol. The results have been published [26 ]. Some of the results are shown in Table 3. The tests reveal that the base strength is of prime importance for the rate of the reaction: the catalyst with TBD has the highest activity, and the one with propylamine the lowest. However, the strong base TBD catalyzes not only the forma-
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Fig. 3. Titration curves of (a) 1 mmol of free and (b) 1 g of MCM-bound piperidine. (c) [ H+] concentration (=10−pH) as function of added acid for the carrier-bound catalyst. The equivalence point at 0.48 meq g−1 is found by back-extrapolating the straight line.
tion of the desired product, but leads also to the subsequent esterification of the remaining OH groups, so that an appreciable amount of diglycerides is present in the equilibrium mixture. After the catalysts had been used once for 24 h of reaction, they were recovered and examined. Pore volume and surface area of the used catalysts were considerably smaller and, also, the intensity of the Table 3 Activity of organic–inorganic hybrid catalysts for the formation of monoglyceride Catalyst
Run
2h
6h
24 h
PM/MCM-41
1 2 1 2 3 4 1 2 1 2 1 2
31.1 32.0 17.1 16.2 14.9 14.1 40.4 35.5 42.5 36.0 68.2 58.3
46.7 48.9 30.8 36.2 38.0 40.0 70.6 72.4 73.1 79.1 59.8 84.1
62.4 79.3 32.7 80.9 84.2 75.8 78.5 82.1 81.2 86.2 43.8 95.0
NH /MCM-41 2
Pyr/MCM-41 Pip/MCM-41 TBD/MCM-41
XRD reflections was weaker. However, when the catalysts were used again they showed a similar activity and, in most cases, an improved selectivity. This is attributed to the fact that residual (acidic) surface OH groups, which can catalyze the unproductive oligomerization of glycidol, have been removed by glycidylation during the first reaction cycle. Experiments were undertaken to determine if leaching of the catalyst takes place. It seems that this is not a problem. The TBD-containing catalyst was re-used for 11 reaction cycles, each of 2 h duration, and showed very little loss of activity throughout. The reactivity of aldehydes for a condensation reaction is lower than that of the oxirane for esterification. Therefore, more drastic reaction conditions are required. We used the same ratio of benzaldehyde:heptaldehyde=1.5:1 as Climent et al. [22] for our base-catalyzed synthesis. Analysis of the reaction mixture identified the desired product, jasminaldehyde, together with unreacted benzaldehyde and heptaldehyde; side reactions resulted in the dimerization product of heptaldehyde, and some benzylalcohol and benzoic
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Table 4 Effect of reaction temperature on the conversion of heptaldehyde and the selectivity to a-n-amylcinnamaldehyde ( jasminaldehyde). Catalyst: TBD/polymer Reaction temperature
2h
4h
6h
8h
(°C )
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
120 150 170 Reflux; 180
32.4 67.7 94.2 97.9
56.1 77.7 88.1 90.8
46.9 80.7 98.4 99.3
60.4 79.1 86.4 91.6
61.1 85.5 99.3 99.3
62.7 79.9 89.4 90.6
68.1 93.3 99.4 99.4
61.2 80.4 89.5 88.5
Table 5 Activity of solid base catalysts for the synthesis of jasminaldehydea Catalyst
10% K O/MCM-41 2 10% KLaO /MCM-41W 2 10% KLaO /MCM-41S 2 10% BaO/MCM-41 TBD/MCM-41 Pyrrolidine/MCM-41 TBD/polymerd
2h
6h
Conversionb (%)
Selectivityc (%)
Conversionb (%)
Selectivityc (%)
12.6 61.9 24.7 7.20 40.6 23.8 94.5
58.0 53.4 55.9 30.8 61.1 42.8 46.2
37.4 98.0 79.4 15.4 79.4 58.8 95.1
41.8 66.0 64.2 38.4 69.7 48.05 49.0
a Reaction conditions: benzaldehyde:heptaldehyde=1.5:1; both reactants were mixed, added to the catalyst powder, and kept at a temperature of 160°C for the time indicated. b Based on heptaldehyde. c To jasminaldehyde. d Fluka 90603
acid. The latter are the products of the Cannizzaro reaction, which benzaldehyde undergoes in alkaline medium. The benzoic acid formed in this step is detrimental because it poisons the basic sites at the catalyst. It was found that a higher temperature favors the formation of jasminaldehyde, especially when the heptaldehyde was added slowly in order to keep its concentration in the reaction mixture low. A selectivity of about 90% can be obtained over the TBD-containing polymeric catalyst ( Table 4). When the reaction conditions were that of a true batch reactor, a much lower selectivity is apparent, as seen from the data in Table 5. TBD bound to the styrene–divinylbenzene resin has the highest activity, and 95% conversion (based on heptaldehyde) was obtained with this catalyst after 2 h at 160°C. However, the selectivity to the desired product is below 50%. Also, the catalyst suffered
severe deactivation at this temperature and cannot be successfully recycled. TBD immobilized on MCM-41 results in a catalyst that is considerably more stable thermally, and shows a better selectivity. This is probably an indication for a shapeselective reaction in the pore system. The pyrrolidine-based catalyst had less activity than the one containing TBD, despite its higher loading with base groups. This shows that the strength of the base is also of importance for this reaction. Catalysts prepared by impregnating the MCM pore system with BaO show some activity, but are less selective than the TBD-based catalysts. However, the best results with respect to product yield were obtained with the binary inorganic catalyst, KLaO /MCM-41. Some differences due 2 to the preparation method are apparent, with the catalyst prepared by wet impregnation performing
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Table 6 Products formed over TBD/MCM under continuous flow conditions in a packed beda Temperature (°C )
RT 150 150 160 160
Cumulative time (h)
0.5 1 2 3 4
Product analysis (wt%) Heptaldehyde
Benzaldehyde
Benzoic acid
Dimer
Jasminaldehyde
39.8 34.3 34.8 35.0 35.4
59.3 53.7 54.7 55.7 55.6
0.73 1.81 2.20 3.00 2.14
0.12 3.92 3.29 2.60 2.86
0.00 6.20 5.05 3.74 3.93
Conversion (%)
Selectivityb (%)
0.3 17.0 14.3 11.3 12.0
0.0 44.1 43.1 41.9 40.8
a Reaction conditions: 0.7 g TBD/MCM-41, benzaldehyde:heptaldehyde =1.5:1, reagent flow=0.5 cm3 h−1, LHSV=2 h−1. b To jasminaldehyde.
better than that from the solid-state synthesis. The presence of the rare earth metal not only improves the thermal stability of the material, but improves also the selectivity. Table 1 shows the results of structural analysis for some of the catalysts over several test cycles. The used catalysts had been exhaustively extracted with methylene chloride to remove all soluble organic deposits before being subjected to sorption measurements for pore-size analysis. It is apparent that the hybrid catalyst suffers from fouling and blocking of the pore system, most probably by incompletely removed products. The used inorganic catalyst shows a similar loss of surface area, but the catalyst can be re-calcined, and this process restores almost all of the original surface area. Testing of the TBD/MCM-41 catalyst in a packed bed was conducted in a micro-reactor. The results of the test are shown in Table 6. The conversion is lower than in the slurry reactor experiments, as a consequence of the shorter contact time (t#30 min), and seems to decrease somewhat with time-on-stream of the catalyst. A higher temperature does not improve the conversion. The selectivity to jasminaldehyde is lower than under batch processing conditions. The selectivity did not increase when the reaction was run at a higher temperature. The formation of byproducts, especially benzoic acid, was much more pronounced than in the batch reactor. It is possible that air in the catalyst bed, which had not been efficiently removed, is responsible for oxidation of the aldehydes, because we did not detect the corresponding quantity of benzylalcohol.
4. Conclusion Solid base catalysts show promise for certain applications in fine chemical synthesis. Hybrid catalysts, based on a stable MCM-41 mesoporous support, have better thermal stability than ionexchange resin type catalysts and can be used repeatedly at temperatures as high as 170°C if proper care is taken to remove oxygen and other oxidants. Indeed, TBD/MCM-41 delivers some of the advantages expected from a solid base: the free guanidine base with a pK #25 is of comparable a strength to KOH, and can be used in non-aqueous medium. However, the pK value of the grafted a base is much reduced compared with the free base, and even a strong organic base like TBD has only moderate basicity in the grafted form. Also, the loading with active groups at 1 meq g−1 catalyst is only about 50% of that obtainable with organic resins. The synthesis of monoglycerides, which poses some problems under technical conditions due to limited solubility of the reaction partners and foaming, can be smoothly catalyzed under mild conditions with a catalyst based on a largepore molecular sieve such as MCM-41. However, it is necessary to use reactive reactants such as glycidol so that the reaction can take place under mild conditions. High yield and high product selectivity to the desired monoglycerides are achieved. The mesoporous hybrid materials show fewer advantages in reactions with more inert compounds. Because of the relatively low basicity of the material, higher temperatures are required to bring about reaction in a reasonable time. In
S. Jaenicke et al. / Microporous and Mesoporous Materials 35–36 (2000) 143–153
such cases, solid base catalysts using a combination of rare earth and alkali metal oxides on a mesoporous carrier seem to offer advantages, primarily with respect to catalyst regeneration. The presence of the rare earth metal in these catalysts is necessary to improve thermal stability, and has also a positive influence on the product selectivity. However, tests with the hybrid catalyst under continuous flow conditions are encouraging, and verify the potential of these solid bases.
Acknowledgement This work was RP972669.
funded
by NUS under
References [1] P. Panster, S. Wieland, B. Cornils, W.A. Herrmann ( Eds.), Applied Homogeneous Catalysis with Organometallic Compounds vol. 2, VCH, Weinheim, 1996, p. 605. [2] W.O. Haag, D.D. Whitehurst, BE 721 686, 1968 (assigned to Mobil Oil Corp.). [3] W.O. Haag, D.D. Whitehurst, in: Proc. 5th Int. Cong. Catal. (1972) 465. [4] B. Pugin, M. Mu¨ller, M. Guisnet et al. ( Eds.), Heterogeneous Catalysis and Fine Chemicals vol. III, Elsevier, Amsterdam, 1993, pp. 107–114. [5] A. Corma, Chem. Rev. 95 (1995) 559. [6 ] H. Hattori, Chem. Rev. 95 (1995) 537. [7] G.A. Olah, P.S. Iyer, G.K.S. Prakash, Synthesis (1986) 513.
153
[8] P. Panster, W. Buder, P. Kleinschmit, DE 2 834 692, 1978 (assigned to Degussa AG). [9] P. Panster, P. Kleinschmit, DE 3 029 599, 1980 (assigned to Degussa AG). [10] P. Panster, P. Kleinschmit, DE 3 131 954, 1981 (assigned to Degussa AG). [11] A. Cauvel, G. Renard, D. Brunel, J. Org. Chem. 62 (1997) 749. [12] Y.V. Subba Rao, D.E. De Vos, P.A. Jacobs, Angew. Chem. 109 (1997) 2776. [13] Y.V. Subba Rao, D.E. De Vos, P.A. Jacobs, Angew. Chem. Int. Ed. Engl. 36 (1997) 2261. [14] D.J. Macquarrie, Chem. Commun. (1996) 1961. [15] C.E. Fowler, S.L. Burkett, S. Mann, Chem. Commun. (1997) 1769. [16 ] C.W. Jones, K. Tsuji, M.E. Davis, Nature 393 (May) (1998) 52. [17] W.M. Van Rhijn, D.E. De Vos, B.F. Sels, W.D. Bossaert, P.A. Jacobs, Chem. Commun. (1998) 317. [18] M.A. Harmer, W.E. Farneth, Q. Sun, J. Am. Chem. Soc. 118 (1996) 7708. [19] M.A. Harmer, Q. Sun, M.J. Michalczyk, Z. Yang, Chem. Commun. (1997) 1803. [20] M.A. Harmer, W.E. Farneth, Q. Sun, Adv. Mater. 10 (1998) 1255. [21] K.R. Kloetstra, M. van Laren, H. van Bekkum, J. Chem. Soc., Faraday Trans. 93 (1997) 1211. [22] M.J. Climent, A. Corma, R.. Guil-Lopez, S. Iborra, J. Primo, J. Catal. 175 (1998) 70. [23] Y. Tanaka, N. Sawamura, M. Iwamoto, Tetrahedron Lett. 39 (1998) 9457. [24] L.Y. Chen, Z. Ping, G.K. Chuah, S. Jaenicke, G. Simon, Microporous Mesoporous Mater. 27 (1999) 231. [25] F. Yagi, N. Kanuka, H. Tsuji, H. Kita, H. Hattori, in: H. Hattori, M. Misono, Y. Ono (Eds.), Acid–Base Catalysis vol. II, Elsevier-Kodansha, Tokyo, 1994, p. 349. [26 ] X.-H. Lin, G.K. Chuah, S. Jaenicke, J. Mol. Catal. A. (1999) in press. [27] N.O. v. Sonntag, J. Am. Oil Chem. Soc. 59 (1982) 795.