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Catalytic activities of heterogeneous catalysts obtained by copolymerization of metal-containing 2-(acetoacetoxy)ethyl methacrylate

  • Giuseppe Romanazzi EMAIL logo , Piero Mastrorilli , Mario Latronico , Matilda Mali , Angelo Nacci and Maria Michela DelľAnna
Published/Copyright: June 1, 2018
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Open Chemistry
From the journal Open Chemistry Volume 16 Issue 1

Abstract

Among the synthetic strategies commonly used for supporting a metal complex onto an organic polymer in order to obtain an heterogenous catalyst, a valid choice is to synthesize a metal containing monomer (MCM), which can subsequently be subjected to polymerization with suitable comonomers and crosslinkers, achieving a supported transition metal catalyst as a metal-containing polymer (MCP). In this context, during the last two decades, we explored the use of 2-(acetoacetoxy)ethyl methacrylate (HAAEMA) as a ligand to prepare several MCMs for the relevant MCPs. In this review we summarize and discuss our developments in the studies of the catalytic activity of these “hybrid” catalysts. These catalysts have demonstrated high efficiency and/or excellent selectivity in several kinds of chemical reactions and very often they could be recovered and reused in multiple cycles maintaining their activity and selectivity without suffering from appreciable metal leaching.

Keywords: supported transition metal catalysts; heterogeneous catalysts; metal-containing monomers; metal-containing polymers; reductions; oxidations; C-C bond forming reactions

1 Introduction

In the last three decades, a large number of researches have been dealt with the possibility of supporting a metal complex onto an insoluble matrix to obtain a heterogeneous catalyst. The matrixes on which metal complexes can be immobilized are generally inert inorganic materials (typically oxides) or organic polymers but the latter, by choosing a suitable combination of comonomers and crosslinkers, in principle allows to control the flexibility of the catalyst as well as to fine-tune its physical properties (polarity, swellability, morphology, etc.) [1,2,3,4]. The only disadvantages of this approach are sometimes the low thermal capacity and mechanical strength of the obtained heterogeneous catalysts [5]. The most commonly used strategies for synthesizing a supported metal complex on organic polymer or resin are essentially those represented in Scheme 1.

Scheme 1
Scheme 1

Route a is the “classic” synthesis of a macromolecular ligand followed by the linking of a transition metal as salt or complex. [2,6] Although the route b is less frequently used, it is more interesting both from a synthetic and a catalytic point of view. The route b consists of the preparation of a metal containing monomers (MCM), i.e. a catalytically active transition metal complex with a ligand having a functionality, which can subsequently be subjected to polymerization with suitable comonomers and crosslinkers for achieving a supported transition metal catalyst as a metal-containing polymer (MCP). Moreover, the use of MCMs offers the advantage of a possible comparison of the activity between the homogeneous and the heterogeneous phase as well as a more uniform distribution of the transition metal in the support with respect to route a, and thus a more controllable dispersion of the metal in the polymer matrix. The use of MCMs for the obtainment of heterogeneous catalysts has been reviewed by some of us in the 2004 and more recently in 2013. [7,8] In this framework, during the last two decades, we explored the use of 2-(acetoacetoxy) ethyl methacrylate (HAAEMA) as a ligand to prepare several MCMs for the obtainment of relevant MCPs. HAAEMA (Scheme 2) is a clear or light-yellow liquid which finds use as versatile functional acrylic monomer [9,10,11,12,13,14] for making copolymers to be used in various applications such as, by way of example, dental resins, [15,16] coatings for glass and metal surfaces, [17] wound sealants, [18] waterborne coatings, [19,20] thermal nanoimprint lithography, [21] and nanoparticles [22,23,24,25]. On the other hand, exploiting the fact that the reactivity of the β-ketoester functionality in HAAEMA towards transition metal salts or complexes is very close to that of acetylacetone, we were able to prepare several transition metal complexes containing the ligand AAEMA- (Scheme 2).

Scheme 2
Scheme 2

The spectroscopic features of all AAEMA– complexes demonstrate that the β-ketoester moiety is the sole functionality involved in the coordination while the methacrylic tail is indeed left free for other reactions, such as radical polymerizations. Thanks to these characteristics, it was possible to synthesise and employ in catalysis several polymer-supported transition metal catalysts obtained from copolymerization of transition metal-containing AAEMA– with appropriate with appropriate comonomers and crosslinkers (Scheme 3). All resulting MCPs are non-hygroscopic powders which are insoluble in all solvents but they swelled well in water, acetone, halogenated solvents, dioxane, THF, DMF, and shrink when treated with diethyl ether, ethyl acetate or petroleum ethers. These swellabilities are very valuable in looking for new catalytic systems that could be recoverable and recyclable. In this review, we summarize and discuss our developments in the studies of catalytic activity of these “hybrid” catalysts. We will focus on their advantages in terms of recyclability without forgetting to point out some of their drawbacks. The discussion on the various MCPs has been subdivided into paragraphs each of which dealing with a single transition metal.

Scheme 3
Scheme 3

2 Catalytic activity of Fe-cat

Fe-cat was obtained as orange solid by copolymerizing the complex Fe(AAEMA)3 with DMAA as comonomer, and MBAA as crosslinker in DMF at 50°C for 24 h (Scheme 3) [26]. The first application of Fe-cat was in the catalytic aerobic epoxidation of olefins (Scheme 4) in presence of isovaleraldehyde as coreductant at room temperature in 1,2-dichloroethane (DCE). Under these reaction conditions, also known as “Mukaiyama’s conditions”, [27] Fe-cat proved to be highly active and selective for the epoxidation of unsaturated substrates such as norbornene, cholesteryl acetate, and cyclohexene. The catalytic system was recyclable without loss of activity except for the epoxidation of open chain alkenes such as 1-octene and 1-dodecene where Fe-cat suffered from a severe metal leaching.

Scheme 4
Scheme 4

Under “Mukaiyama’s conditions”, Fe-cat was also used as heterogeneous catalyst for the aerobic oxidation of α-hydroxyketones to α -diketones (Scheme 5) [28].

Scheme 5
Scheme 5

With a very low amount of catalyst, substrates such as benzoin, 4,4’-dimethyl-benzoin, and anisoin were smoothly oxidized to corresponding α-diketones in moderate to good yield, and Fe-cat was easily separable from the reaction mixture and recyclable at least once without suffering from metal leaching.

Finally, Fe-cat also catalysed a carbon–carbon bondforming reaction such as the cross-coupling between acyl chlorides and Grignard reagents, [29] affording several aliphatic or aromatic ketones in moderate to excellent yield (Scheme 6).

Scheme 6
Scheme 6

At least in one instance, it was possible to recycle Fe-cat, which showed a comparable activity and selectivity with respect to the first cycle.

3 Catalytic activity of Ni-cat

Ni-cat was obtained as pale green powder by copolymerizing the complex Ni(AAEMA)2 with DMAA as comonomer, and MBAA as crosslinker in DMF at 120°C for 24 h (Scheme 3) [26].

The first application of Ni-cat in catalysis was the aerobic epoxidation of olefins under “Mukaiyama’s conditions” and, like Fe-cat, Ni-cat proved to be active, selective, and recyclable for the epoxidation of norbornene, cholesteryl acetate, and cyclohexene. In the epoxidation of linear alkenes also this catalyst suffered from a severe metal leaching (Scheme 7).

Scheme 7
Scheme 7

The Ni-cat was also used as heterogeneous catalysts for Michael addition reactions (Scheme 8) [30].

Scheme 8
Scheme 8

When the Michael donor was methyl acetoacetate (X=CH3; Y=OCH3), the best yield (92%) of the relevant adduct was achieved within 19 h. The recycle test gave 76% yield after 24 h. This case represents the best compromise in terms of activity and metal leaching (20% of initial amount) of Ni-cat. For other donors, the adduct yields were lower and metal leaching for Ni-cat was even more significant (up to 87% of initial amount).

More recently, we explored the use of Ni-cat for accelerating the transfer hydrogenation of nitroarenes in aqueous media using NaBH4 as the reducing agent. Although preliminary catalytic tests using nitrobenzene as the representative substrate were encouraging (isolate yield up to 93%), attempts to recycle Ni-cat failed. In the second run, the yield into aniline dropped to 27% due to severe metal leaching. Nevertheless, we found that by submitting Ni-cat to thermal annealing at 300°C under nitrogen a polymer supported nickel nanomaterial (Ni-cat#) was formed which proved to be a very active and selective catalyst for the hydrogenation of several functionalized nitroarenes to corresponding aromatic amines in aqueous medium at room temperature in the presence of NaBH4 (Scheme 9) [31].

Scheme 9
Scheme 9

Remarkably, Ni-cat# was found highly selective in the hydrogenation of halonitrobenzene to haloaniline, avoiding the formation of hydro-dehalogenated side-products, and displayed excellent recyclability for five cycles without suffering from metal leaching.

4 Catalytic activity of Co-cat

Co-cat was obtained as pink solid by copolymerizing the complex Co(AAEMA)2 with DMAA as comonomer, and MBAA as crosslinker in DMF at 50°C for 24 h (Scheme 3) [26]. In the aerobic epoxidation of olefins under “Mukaiyama’s conditions”, like Fe-cat and Ni-cat, Co-cat was active, selective, and recyclable for the epoxidation of norbornene, cholesteryl acetate, and cyclohexene but again a severe metal leaching was observed in the epoxidation of linear alkenes (Scheme 10).

Scheme 10
Scheme 10

Under “Mukaiyama’s conditions”, Co-cat acted as heterogeneous catalyst also for the aerobic oxidation of α-hydroxyketones to α-diketones, displaying a higher activity than Fe-cat (Scheme 11) [28].

Scheme 11
Scheme 11

Benzoin, 4,4’-dimethyl-benzoin, and anisoin were smoothly oxidized to the corresponding α-diketones in good to excellent yields and Co-cat was recyclable at least once without appreciable metal leaching.

Moreover, Co-cat compared to Fe-cat and Ni-cat demonstrated to be the most versatile catalyst in the oxidation reactions. In fact, Co-cat was also an active and recyclable heterogeneous catalyst for the selective oxidation of disubstituted sulfides to sulfoxides and/or sulfones under “Mukaiyama’s conditions” (Scheme 12 and Table 1) [32].

Scheme 12
Scheme 12

Table 1

Oxidation of disubstituted sulphides catalysed by Co-Cat under Mukaiyama’s conditions.

EntrySubstrateOxidantTime (h)Yield in sulfoxide (%)Yield in sulfone (%)
1(CH3)2SAir880–
2(CH3)2SAir20–>99
3(n–Bu)2SAir218310
4(n–Bu)2SAir37892
5(t–Bu)2SO23–>99
6(PhCH2)2SO28–>99
7p–Tol–S–CH3O22–>99

The selective formation of the intermediate sulfoxide was achieved by lowering the partial pressure of the oxidant, using dried air in place of pure dioxygen, and matching with the time reactions.

Having optimized the reaction conditions for the obtainment of sulfoxides, the highly regioselective oxidation of a bis-sulfide such as 2-(2-p-tolylsulfanyl-ethylsulfanyl)-benzothiazole into the corresponding sulfoxide 2- [2- (toluene- 4-sulfinyl) - ethylsulfanyl] -benzothiazole in 93% yield was successfully obtained (Scheme 13) [33].

Scheme 13
Scheme 13

Co-cat also acted as an active and reusable catalyst for the oxidation of benzylic and secondary alcohols at 40°C under “Mukaiyama’s conditions” (Scheme 14) [34].

Scheme 14
Scheme 14

The corresponding carbonyl or ketone compounds were achieved in good to excellent yields and Co-cat was recycled at least four times without negligible metal leaching.

Pursuing our studies on the aerobic catalytic oxidation of organic substrates, we also found that Co-cat, under very mild conditions, behaved as a recyclable heterogeneous catalyst for the oxidation of alkyl and aryl phosphines to the corresponding oxides [35].

The Co-cat was an active heterogeneous catalyst for Michael addition reactions (Scheme 15) [30].

Scheme 15
Scheme 15

As in the case of Ni-cat, when the Michael donor was methyl acetoacetate (X=CH3; Y=OCH3), the best yield (97%) of the relevant adduct was obtained within 19 h while a 93% yield was obtained in the recycle test after 19h. Although Co-cat was a more active catalyst towards Michael reaction with respect to Ni-cat, its metal leaching was much higher. For example, in the case of methyl acetoacetate (X=CH3; Y=OCH3) it was 65% of initial amount (for Ni-cat, it was 20% of initial amount, see above).

5 Catalytic activity of Cu-cat

The supported copper complex Cu-cat, was obtained as green solid from copolymerization of Cu(AAEMA)2 with DMAA as comonomer, and MBAA as crosslinker in acetone/DMF (1/1) at 60°C for 5 h (Scheme 3). Cu-cat as Fe-cat and Co-cat was tested as metal catalyst in several oxidations under Mukaima’s condition, but the obtained results were not encouraging with respect to them. On the other hand, Cu-cat was an active catalyst (Scheme 16) in the oxidative coupling of 2-naphthol for obtaining 1,1’-bi-2-naphthol. However, the yield was moderate, and the catalyst was not recyclable [36].

Scheme 16
Scheme 16

6 Catalytic activity of Rh-cat

The synthesis of Rh-cat (Scheme 3) was achieved submitting the complex Rh(cod)AAEMA to copolymerization with DMAA and MBAA at 110°C in DMF in the presence of AIBN as radical initiator, obtaining an greenish-black powder. [37] Rh-cat catalysed the hydrogenation of several unsaturated substrates (Table 2) in methanol or CH2Cl2 with yields ranging from 64 to >99% under very mild conditions.

Table 2

Hydrogenation of unsaturated substrates catalyzed by Rh-cat (0.625 % mol).

EntrySubstratesolventT (°C)P (bar)Time (h)ProductYield (%)
11-hepteneCH3OH2114n-heptane>99
2cyclohexeneCH3OH2112cyclohexane>99
3(–)-carvoneCH3OH2116.5carvotanacetone77
4citralCH2Cl221154citronellal64
5cinnamaldehydeCH2Cl2212083-phenylpropanal68
6valeronitrileCH3OH21206dipentylamine85
7nitrobenzeneCH3OH50118aniline>99

Only cinnamaldehyde (entry 5), and valeronitrile (entry 6) required higher H2 pressure (20 bar) for their reduction. Five recycles of Rh-cat were made in the case of 1-heptene hydrogenation, without activity loss.

Encouraged by the good results achieved in nitrobenzene reduction (Table 2, entry 7), we employed Rh-cat in the reduction of p-halo-nitrobenzenes into p-halo-anilines (Scheme 17) [38].

Scheme 17
Scheme 17

Although the average turnover frequencies (TOF’s) of catalysis were very low (from 0.95 up to 7.6 h–1), the selectivities were very high, thus minimizing concurrent hydrodehalogenation. In the recycle tests Rh-cat was even more active (average TOF’s up to 12.0 h–1) but 6-8% of by-products such as p-halonitrosobenzene and 4,4’-dihaloazobenzene was also formed.

A chiral version (Rh-cat*) of Rh-cat was synthesised by exchanging the cod ligand in (cod)Rh(AAEMA) with the (S,S)-DIOP and copolymerizing the resulting complex [(+)-diopRh(AAEMA)] with DMAA and MBAA in presence of AIBN in toluene/DMF at 100°C (Scheme 18) [39].

Scheme 18
Scheme 18

Rh-cat* accelerated the hydrogenation of methyl-(Z)- α-N-acetamidocinnamate to N-acetyl-(S)-phenylalanine methyl ester with enantiomeric ratio (er) up to 76.5/23.5. However, Rh-cat* showed a lower enantioselectivity with respect to the homogeneous counterpart [(S,S)- DIOPRh(AAEMA)], which reached er=83.5/16.5. Thanks to CP/MAS 31P NMR studies [40,41,42], it was found that the lowering of enantioselectivity is due to the partial oxidation of the (S,S)-DIOP ligand that occurs during the copolymerization of [(S,S)-DIOPRh(AAEMA)]. In the recycle test reactions, Rh-cat* unfortunately halves its activity, lowering its enantioselectivity (er up to 65.0/35.0).

Finally, Rh-cat was also found to facilitate carboncarbon bond forming reactions. In fact, employing it as heterogeneous catalyst for the polymerization of phenylacetylene or p-tolylacetylene, polymers with excellent yields and very high stereoregularities were obtained [43]. In this framework, it was ascertained that Rh-cat could affect, in a reproducible manner, the size distribution of the nanospheres of the obtained polyphenylacetylenes [44].

7 Catalytic activity of Ru-cat

By copolymerizing the complex Ru(PPh3)2(AAEMA)2 with DMAA as comonomer and MBAA in DMF and in presence of AIBN at 60°C, the insoluble resin Ru-cat was obtained (Scheme 3) [45]. The catalytic figure of merit of Ru-cat were indeed modest: employing Ru-cat in the hydrogenation of alkenes in methanol at 65°C under 20 bar of H2, a low TOF (16 h–1) was achieved.

8 Catalytic activity of Pd-cat

Pd-cat is by far the most versatile and widely used catalyst among the family catalysts based on metal-containing AAEMA because it was able to accelerate several kinds of chemical reaction such as reduction, oxidation and carbon-carbon bond forming reactions as well as esterification and transesterification reactions. Its synthesis requires the use of comonomer not containing nitrogen [37] and was achieved in very reproducible way by reacting Pd(AAEMA)2, ethyl methacrylate (EMA) and ethylene glycol dimethacrylate (EGDMA), this latter added in portions, in THF under the light of a table lamp in the presence of AIBN [46].

The first catalytic application of Pd-cat in a reduction reaction was the hydrogenation of several unsaturated substrates such as olefins, alkynes, unsaturated aldehydes, citral, carvone, and nitro compounds under H2 atmosphere at RT (Table 3) [47].

Table 3

Hydrogenation of unsaturated substrates catalyzed by Pd-cat (0.625% mol) at RT.

EntrySubstratesolventP (bar)Time (h)ProductYield (%)
11-hepteneCH3OH12n-heptane>99
2cyclohexeneCH3OH12cyclohexane>99
31-octyneCH3OH15n-octane77
4phenylacethyleneCH3OH17ethylbenzene99
5diphenylacetyleneCH2Cl215.51,2-diphenylethane>99
62-cyclohexen-1-oneCH3OH12cyclohexanone91
72-cyclopenten-1-oneCH3OH13cyclopentanone>99
8cinnamaldehydeCH2Cl21333-phenylpropanal83
9citralCH3OH110.5citronellal70
10(–)-carvoneCH3OH17carvotanmenthone70
11benzaldehydeCH3OH101benzyl alcohol95
12benzaldehydeCH2Cl2104toluene>99
13nitrobenzeneCH3OH17aniline>99
142-nitrotolueneCH3OH17o-toluidine>99
152,4-dinitrotolueneCH3OH1492,4-diaminotoluene>99

In most cases, the yields exceeded 90% within very short time reactions. Only benzaldehyde (entry 11 and 12) needed higher H2 pressure to be first transformed into benzyl alcohol (t = 1 h) and then into toluene (4 h).

The refractoriness of benzaldehyde to be reduced prompted us to employ Pd-cat as a novel heterogenous catalyst in the one-pot direct amination reaction [48] using molecular hydrogen as reducing agent (Scheme 19) [49].

Scheme 19
Scheme 19

The catalytic system was found to be applicable, with slight differences, to both aliphatic and aromatic aldehydes and to a wide range of nitroarenes. Excellent yields of secondary amines were obtained under very mild conditions and the catalyst was recyclable at least 8 times without appreciable loss of activity and selectivity. TEM (transmission electron microscopy) analyses of Pd-cat demonstrated that the active species are insoluble palladium nanoparticles having a size distribution centred at 5 nm.

In looking for an eco-friendly and economic catalytic system for the reduction of nitroarenes to arylamines, we also found that Pd-cat could catalyse such reduction in the presence of NaBH4 and water as a mild reducing agent and a green solvent, respectively (Scheme 20) [50].

Scheme 20
Scheme 20

From moderate to excellent yields of aryl amines were obtained and the catalyst was recyclable at least 12 times without loss of activity and selectivity. In fact, TEM observations showed that the catalytic active species are very small Pd nanocrystallites (mean size diameter ca. 3 nm) formed in the presence of NaBH4.

The activity of Pd-cat in reduction reactions was also tested in the reduction of quinolines to 1,2,3,4-tetrahydroquinolines, which are important intermediates for the synthesis of biological active molecules, drugs, agrochemicals, dyes, and alkaloids [51]. However, the selective catalytic hydrogenation of polynuclear heteroaromatic nitrogen compounds such as quinolines is a difficult task because quinolines usually poison the traditional noble metal-based hydrogenation catalysts. [52,53,54] In spite of these assumptions, Pd-cat was found an active and selective catalyst for reduction of quinolines and quinoxalines (Scheme 21) [55].

Scheme 21
Scheme 21

In aqueous medium, high yields and excellent selectivity were achieved under not severe conditions (80°C and 10 bar of H2). The catalytic system maintained its activity and selectivity for nine cycles with negligible metal leaching. TEM analyses on catalyst pointed out that the active species were supported Pd nanoparticles, having a mean size of 4 nm, which did not aggregate with the recycles.

Searching for an eco-friendly and more economic catalytic system for the reduction of quinolines to 1,2,3,4-tetrahydroquinolines under milder conditions, we found that Pd-cat catalysed the reduction of quinolines in the presence of NaBH4 as a mild reducing agent and neat water as green solvent (Scheme 22) [56].

Scheme 22
Scheme 22

Although a slightly higher Pd-cat loading (2.0 mol %) was necessary, the catalytic system exhibited excellent activity and selectivity, which were maintained for at least seven reaction runs without metal leaching. By means of TEM studies, it was ascertained that the active species were supported Pd nanoparticles, having a smaller mean size (3 nm) with respect to the protocol under H2 pressure.

The first application of Pd-cat in accelerating a carbon-carbon bond forming reaction was the Heck crosscoupling reaction [57,58,59] between aryl iodides or bromides and different olefins, in the presence of potassium or sodium acetate as base (Scheme 23), in which Pd-cat also acted as phosphane free catalyst [60].

Scheme 23
Scheme 23

A very low Pd-cat loading was necessary to achieve trans disubstituted-olefins in moderate to excellent yields and the catalyst was recycled up to six times with slight loss of its activity. However, when the reaction mixture was hot filtered at 20% conversion of aryl halide, the activity of the mother liquor was almost comparable to that of fresh Pd-cat. On the other hand, if the filtration was carried out at 100% conversion of aryl halide, the activity of mother liquors was experimentally negligible. Thus, Pd-cat behaved as a reservoir of catalytically active palladium soluble species, which deactivated when the aryl halide was consumed [61].

This behaviour was found to be not operating in the asymmetric allylic alkylation reaction [62,63,64] promoted by Pd-cat in the presence of catalytic amount of chiral ligands, such as (R,S)-PPFA, (R,S)-JOSIPHOS, and (R,R)-NORPHOS (Scheme 24) [65].

Scheme 24
Scheme 24

The reaction between rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate in the presence of N,O- bis(trimethylsilyl) acetamide (BSA) and a catalytic amount of KOAc provided good yields and good enantioselectivity (er up to 96.5:3.5 in the presence of (R,S)-JOSIPHOS). However, only when chiral ferrocenyl ligands (R,S)-PPFA and (R,S)-JOSIPHOS) were employed, Pd-cat could be used in two subsequent runs with a severe metal leaching (for (R,S)-JOSIPHOS: 71% yield and er= 92.5/7.5 in the recycle but metal leaching up to 39% of initial amount). Furthermore, in the case of (R,R)-NORPHOS the metal leaching after the first cycle is already almost quantitative.

Pd-cat also displayed high efficiency in the Stille cross-coupling [66,67] of aryl iodides or bromides with various trimethyltin or tributyltin derivatives (Scheme 25) [68].

Scheme 25
Scheme 25

Yields from 63 to 99% were achieved in first cycle. In the case of aryl iodides, the catalyst was recycled up to six runs with appreciable lowering of catalytic activity due to the accumulation of tin derivatives onto the support, which hampered the migration of the substrates to the catalytically active sites.

Pd-cat was also successfully used as catalyst in the Suzuki-Miyaura cross-coupling reaction [69,70,71,72] of aryl bromides (or activated aryl chlorides) under air in neat water at 100°C, in the presence of K2CO3 as base (Scheme 26) [73].

Scheme 26
Scheme 26

From good to excellent yields in biaryls were achieved and the catalyst was recycled at least five times without suffering from a significant palladium leaching. By means of hot filtration tests of the mother liquors, we established that Pd-cat acted as a reservoir of catalytically active species (palladium nanoparticles) that leached out in a very little amount for each cycle in solution. The use of a transfer phase agent, such as tetrabutylammonium bromide (TBAB) accelerated the cross-coupling reaction because it facilitated the formation of smaller size (more active) palladium nanoparticles.

The good activity and reusability of Pd-Cat in water media for the reduction of nitroarenes or quinolines with H2 or NaBH4 as reducing agent, and for the Suzuki-Miyaura cross-coupling reaction pushed us to perform a detailed and accurate studies on palladium nanoparticles formed during their catalytic runs. By combining information obtained through high-resolution TEM, X-ray energy dispersive spectroscopy, and micro-IR spectroscopy we were able to study the morphological features of the Pd nanoparticles formed in Pd-cat, before and after its use in catalysis, and also the effect on their evolution in function of several parameters, such as different reducing agents (aryl boronic acid, dihydrogen, NaBH4), the presence of a phase transfer (TBAB) and the temperature [74]. Micro-IR spectroscopy showed that the support was chemically stable over the catalyst recycles in all the reactions tested, while TEM analyses pointed out that the macroporosity of the resin remained constant after each run and in all cases the pristine Pd(II) polymer supported complex was reduced in situ to Pd(0) forming nanoparticles (the active species) under reaction conditions. The organic support was always able to stabilize Pd nanoparticles during recycling without negligible agglomeration. High temperatures (80–100°C) favoured the formation of Pd nanoparticles of 9 nm average size, while at room temperature the Pd nanoparticles average size was smaller (2–6 nm).

The presence of a phase transfer agent (TBAB) led to formation of Pd nanoparticles with 4 nm average size, which leached into solution because of the well-known capacity of TBAB to stabilize the nanoparticles favouring their solubilization [75,76,77].

Concerning the use of different reducing agents on Pd nanoparticles, in the Suzuki-Miyaura coupling, the effect of the reductant such as aryl boronic acid was masked by the temperature reaction (100°C) and/or by the presence of TBAB (when used). On the contrary, for the reduction of nitroarenes or quinolines in water, the choice of the reductant (H2 or NaBH4) was fundamental to determinate the diameter size of Pd nanoparticles. Under H2 atmosphere, Pd nanoparticles with average diameter size from 6 to 10 nm were obtained, while in the presence of NaBH4 they ranged from 2 to 4 nm.

By summarizing the investigation results, in Suzuki-Miyaura reaction Pd-cat operateed as a reservoir of soluble Pd nanoparticles, which were released into the aqueous environment and they were effectively recaptured by the polymer matrix after substrate consumption. In fact, TEM images of Pd-cat recovered after any recycles showed the majority of Pd nanoparticles onto the surface of the polymer. This “release and catch” mechanism of active species, is commonly accepted by the scientific community for some heterogeneously catalysed C–C bond forming reaction [78,79,80]. On the other hand, in the hydrogenation reactions under H2 atmosphere or in the presence of NaBH4, Pd-cat operated by means of a truly heterogeneous catalytic mechanism because TEM images of Pd-cat recovered after any recycles showed a uniform distribution of Pd nanoparticles in the polymer matrix.

Pd-cat acted as heterogeneous catalyst for the oxidation of alcohols in water by air. Primary and secondary aromatic alcohols were oxidized to their corresponding carbonyl compounds in excellent yields with a very low Pd loading (0.5 mol%) in relatively short reaction times (Scheme 27) [81].

Scheme 27
Scheme 27

The catalyst was easily recovered and reused up to six cycles. Hot filtration tests indicated that Pd-cat really operated through a heterogeneous pathway.

Finally, Pd-cat was found to catalyse, under pressure of 2 bar of H2 and in the presence of catalytic amount of bromobenzene in excess of alcohols, the esterification (or transesterification) of aliphatic and aromatic carboxylic acids into their corresponding esters in good to excellent yields (Scheme 28) [82].

Scheme 28
Scheme 28

Bromobenzene was necessary to generate in-situ a little amount of HBr, which provided a mild acidic reaction environment. Under these reaction conditions the true active species were, again, stabilized Pd nanoparticles and Pd-cat was reusable for eight consecutive cycles with a negligible Pd leaching into solution. The catalytic system was also explored in preliminary studies for the preparation of partially hydrogenated fatty acid methyl esters, starting from a mixture composed by highly polyunsaturated esters and free carboxylic acids, taken as a model acidic feedstock for mimic a biodiesel upgrading [83,84].

9 Conclusions

Polymerization of transition metal-containing the polymerizable ligand AAEMA turned out to be a convenient alternative tool for the preparation of supported transition metal catalysts as MCPs. These MCPs employed as catalysts in several kinds of chemical reactions showed high efficiency and/or excellent selectivity and very often they could be recovered and reused in multiple cycles without significant loss of their activity and selectivity. Soon, other catalytic applications will be explored for these catalysts and new noble or non-noble transition metal-containing AAEMA will be synthesized and then polymerized with suitable comonomers and crosslinkers to obtain novel catalysts. Much efforts will be made to systematically ascertain when the solid matrix not only possess the role of heterogenizing the catalyst but also when it affects the chemical efficiency and the selectivity of a chemical process.

Acknowledgements

G.R., P.M., M.L. and M.M.D. acknowledge Politecnico di Bari for Fondi di Ricerca di Ateneo (FRA2016).

  1. Conflict of interest: Authors state no conflict of interest.

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Abbreviations

AAEMA–

deprotonated form of 2-(acetoacetoxy) ethyl methacrylate

AIBN

azobisisobutyronitrile (IUPAC name 2,2′-azobis (2-methylpropionitrile))

BSA

N,O-bis(trimethylsilyl)acetamide

cod

1,5-cyclooctadiene

CP/MAS NMR

Cross Polarization/ Magic Angle Spinning Nuclear Magnetic Resonance; (+)-diop - (S,S)-4,5-bis(diphenylphosphinomethyl)-2,2′-dimethyl-l,3-dioxolane

DCE

1,2-dichloroethane

DEG

diethylene glycol

DMAA

N,N-dimethylacrylamide

DMF

N,N-dimethylformamide

EGDMA

ethylene glycol dimethacrylate

EMA

ethyl methacrylate

er

enantiomeric ratio

MBAA

N,N’-methylenebisacrylamide

HAAEMA

2-(acetoacetoxy)ethyl methacrylate (IUPAC name: 2-(2-methylprop-2-enoyloxy)ethyl 3-oxobutanoate); (R,S)-JOSIPHOS – (R)-1-[(SP)-2-(diphenylphosphino) ferrocenyl]ethyldicyclohexylphosphine

MCM

metal-containing monomer

[(R,S)-PPFA

(R)-N,N-dimethyl-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethylamine

TBAB

tetra-n-butyl ammonium bromide

TEM

transmission electron microscopy

THF

tetrahydrofuran

TOF

turnover frequency

Received: 2018-03-07
Accepted: 2018-03-18
Published Online: 2018-06-01

© 2018 Giuseppe Romanazzi et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

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  27. Bio-accumulation of Polycyclic Aromatic Hydrocarbons in the Grey Mangrove (Avicennia marina) along Arabian Gulf, Saudi Coast
  28. Dynamic Change of Secondary Metabolites and spectrum-effect relationship of Malus halliana Koehne flowers during blooming
  29. Lipids constituents from Gardenia aqualla Stapf & Hutch
  30. Effect of using microwaves for catalysts preparation on the catalytic acetalization of glycerol with furfural to obtain fuel additives
  31. Effect of Humic Acid on the Degradation of Methylene Blue by Peroxymonosulfate
  32. Serum containing drugs of Gua Lou Xie Bai decoction (GLXB-D) can inhibit TGF-β1-Induced Epithelial to Mesenchymal Transition (EMT) in A549 Cells
  33. Antiulcer Activity of Different Extracts of Anvillea garcinii and Isolation of Two New Secondary Metabolites
  34. Analysis of Metabolites in Cabernet Sauvignon and Shiraz Dry Red Wines from Shanxi by 1H NMR Spectroscopy Combined with Pattern Recognition Analysis
  35. Can water temperature impact litter decomposition under pollution of copper and zinc mixture
  36. Released from ZrO2/SiO2 coating resveratrol inhibits senescence and oxidative stress of human adipose-derived stem cells (ASC)
  37. Validated thin-layer chromatographic method for alternative and simultaneous determination of two anti-gout agents in their fixed dose combinations
  38. Fast removal of pollutants from vehicle emissions during cold-start stage
  39. Review Article
  40. Catalytic activities of heterogeneous catalysts obtained by copolymerization of metal-containing 2-(acetoacetoxy)ethyl methacrylate
  41. Antibiotic Residue in the Aquatic Environment: Status in Africa
  42. Regular Articles
  43. Mercury fractionation in gypsum using temperature desorption and mass spectrometric detection
  44. Phytosynthetic Ag doped ZnO nanoparticles: Semiconducting green remediators
  45. Epithelial–Mesenchymal Transition Induced by SMAD4 Activation in Invasive Growth Hormone-Secreting Adenomas
  46. Physicochemical properties of stabilized sewage sludge admixtures by modified steel slag
  47. In Vitro Cytotoxic and Antiproliferative Activity of Cydonia oblonga flower petals, leaf and fruit pellet ethanolic extracts. Docking simulation of the active flavonoids on anti-apoptotic protein Bcl-2
  48. Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction
  49. A new selective, and sensitive method for the determination of lixivaptan, a vasopressin 2 (V2)-receptor antagonist, in mouse plasma and its application in a pharmacokinetic study
  50. Anti-EGFL7 antibodies inhibit rat prolactinoma MMQ cells proliferation and PRL secretion
  51. Density functional theory calculations, vibration spectral analysis and molecular docking of the antimicrobial agent 6-(1,3-benzodioxol-5-ylmethyl)-5-ethyl-2-{[2-(morpholin-4-yl)ethyl] sulfanyl}pyrimidin-4(3H)-one
  52. Effect of Nano Zeolite on the Transformation of Cadmium Speciation and Its Uptake by Tobacco in Cadmium-contaminated Soil
  53. Effects and Mechanisms of Jinniu Capsule on Methamphetamine-Induced Conditioned Place Preference in Rats
  54. Calculating the Degree-based Topological Indices of Dendrimers
  55. Efficient optimization and mineralization of UV absorbers: A comparative investigation with Fenton and UV/H2O2
  56. Metabolites of Tryptophane and Phenylalanine as Markers of Small Bowel Ischemia-Reperfusion Injury
  57. Adsorption and determination of polycyclic aromatic hydrocarbons in water through the aggregation of graphene oxide
  58. The role of NR2C2 in the prolactinomas
  59. Chromium removal from industrial wastewater using Phyllostachys pubescens biomass loaded Cu-S nanospheres
  60. Hydrotalcite Anchored Ruthenium Catalyst for CO2 Hydrogenation Reaction
  61. Preparation of Calcium Fluoride using Phosphogypsum by Orthogonal Experiment
  62. The mechanism of antibacterial activity of corylifolinin against three clinical bacteria from Psoralen corylifolia L
  63. 2-formyl-3,6-bis(hydroxymethyl)phenyl benzoate in Electrochemical Dry Cell
  64. Electro-photocatalytic degradation of amoxicillin using calcium titanate
  65. Effect of Malus halliana Koehne Polysaccharides on Functional Constipation
  66. Structural Properties and Nonlinear Optical Responses of Halogenated Compounds: A DFT Investigation on Molecular Modelling
  67. DMFDMA catalyzed synthesis of 2-((Dimethylamino)methylene)-3,4-dihydro-9-arylacridin-1(2H)-ones and their derivatives: in-vitro antifungal, antibacterial and antioxidant evaluations
  68. Production of Methanol as a Fuel Energy from CO2 Present in Polluted Seawater - A Photocatalytic Outlook
  69. Study of different extraction methods on finger print and fatty acid of raw beef fat using fourier transform infrared and gas chromatography-mass spectrometry
  70. Determination of trace fluoroquinolones in water solutions and in medicinal preparations by conventional and synchronous fluorescence spectrometry
  71. Extraction and determination of flavonoids in Carthamus tinctorius
  72. Therapeutic Application of Zinc and Vanadium Complexes against Diabetes Mellitus a Coronary Disease: A review
  73. Study of calcined eggshell as potential catalyst for biodiesel formation using used cooking oil
  74. Manganese oxalates - structure-based Insights
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  76. Long-Term Dissolution of Glass Fibers in Water Described by Dissolving Cylinder Zero-Order Kinetic Model: Mass Loss and Radius Reduction
  77. Topological study of the para-line graphs of certain pentacene via topological indices
  78. A brief insight into the prediction of water vapor transmissibility in highly impermeable hybrid nanocomposites based on bromobutyl/epichlorohydrin rubber blends
  79. Comparative sulfite assay by voltammetry using Pt electrodes, photometry and titrimetry: Application to cider, vinegar and sugar analysis
  80. MicroRNA delivery mediated by PEGylated polyethylenimine for prostate cancer therapy
  81. Reversible Fluorescent Turn-on Sensors for Fe3+ based on a Receptor Composed of Tri-oxygen Atoms of Amide Groups in Water
  82. Sonocatalytic degradation of methyl orange in aqueous solution using Fe-doped TiO2 nanoparticles under mechanical agitation
  83. Hydrotalcite Anchored Ruthenium Catalyst for CO2 Hydrogenation Reaction
  84. Production and Analysis of Recycled Ammonium Perrhenate from CMSX-4 superalloys
  85. Topical Issue on Agriculture
  86. New phosphorus biofertilizers from renewable raw materials in the aspect of cadmium and lead contents in soil and plants
  87. Survey of content of cadmium, calcium, chromium, copper, iron, lead, magnesium, manganese, mercury, sodium and zinc in chamomile and green tea leaves by electrothermal or flame atomizer atomic absorption spectrometry
  88. Biogas digestate – benefits and risks for soil fertility and crop quality – an evaluation of grain maize response
  89. A numerical analysis of heat transfer in a cross-current heat exchanger with controlled and newly designed air flows
  90. Freshwater green macroalgae as a biosorbent of Cr(III) ions
  91. The main influencing factors of soil mechanical characteristics of the gravity erosion environment in the dry-hot valley of Jinsha river
  92. Free amino acids in Viola tricolor in relation to different habitat conditions
  93. The influence of filler amount on selected properties of new experimental resin dental composite
  94. Effect of poultry wastewater irrigation on nitrogen, phosphorus and carbon contents in farmland soil
  95. Response of spring wheat to NPK and S fertilization. The content and uptake of macronutrients and the value of ionic ratios
  96. The Effect of Macroalgal Extracts and Near Infrared Radiation on Germination of Soybean Seedlings: Preliminary Research Results
  97. Content of Zn, Cd and Pb in purple moor-grass in soils heavily contaminated with heavy metals around a zinc and lead ore tailing landfill
  98. Topical Issue on Research for Natural Bioactive Products
  99. Synthesis of (±)-3,4-dimethoxybenzyl-4-methyloctanoate as a novel internal standard for capsinoid determination by HPLC-ESI-MS/MS(QTOF)
  100. Repellent activity of monoterpenoid esters with neurotransmitter amino acids against yellow fever mosquito, Aedes aegypti
  101. Effect of Flammulina velutipes (golden needle mushroom, eno-kitake) polysaccharides on constipation
  102. Bioassay-directed fractionation of a blood coagulation factor Xa inhibitor, betulinic acid from Lycopus lucidus
  103. Antifungal and repellent activities of the essential oils from three aromatic herbs from western Himalaya
  104. Chemical composition and microbiological evaluation of essential oil from Hyssopus officinalis L. with white and pink flowers
  105. Bioassay-guided isolation and identification of Aedes aegypti larvicidal and biting deterrent compounds from Veratrum lobelianum
  106. α-Terpineol, a natural monoterpene: A review of its biological properties
  107. Utility of essential oils for development of host-based lures for Xyleborus glabratus (Coleoptera: Curculionidae: Scolytinae), vector of laurel wilt
  108. Phenolic composition and antioxidant potential of different organs of Kazakh Crataegus almaatensis Pojark: A comparison with the European Crataegus oxyacantha L. flowers
  109. Isolation of eudesmane type sesquiterpene ketone from Prangos heyniae H.Duman & M.F.Watson essential oil and mosquitocidal activity of the essential oils
  110. Comparative analysis of the polyphenols profiles and the antioxidant and cytotoxicity properties of various blue honeysuckle varieties
  111. Special Issue on ICCESEN 2017
  112. Modelling world energy security data from multinomial distribution by generalized linear model under different cumulative link functions
  113. Pine Cone and Boron Compounds Effect as Reinforcement on Mechanical and Flammability Properties of Polyester Composites
  114. Artificial Neural Network Modelling for Prediction of SNR Effected by Probe Properties on Ultrasonic Inspection of Austenitic Stainless Steel Weldments
  115. Calculation and 3D analyses of ERR in the band crack front contained in a rectangular plate made of multilayered material
  116. Improvement of fuel properties of biodiesel with bioadditive ethyl levulinate
  117. Properties of AlSi9Cu3 metal matrix micro and nano composites produced via stir casting
  118. Investigation of Antibacterial Properties of Ag Doped TiO2 Nanofibers Prepared by Electrospinning Process
  119. Modeling of Total Phenolic contents in Various Tea samples by Experimental Design Methods
  120. Nickel doping effect on the structural and optical properties of indium sulfide thin films by SILAR
  121. The effect mechanism of Ginnalin A as a homeopathic agent on various cancer cell lines
  122. Excitation functions of proton induced reactions of some radioisotopes used in medicine
  123. Oxide ionic conductivity and microstructures of Pr and Sm co-doped CeO2-based systems
  124. Rapid Synthesis of Metallic Reinforced in Situ Intermetallic Composites in Ti-Al-Nb System via Resistive Sintering
  125. Oxidation Behavior of NiCr/YSZ Thermal Barrier Coatings (TBCs)
  126. Clustering Analysis of Normal Strength Concretes Produced with Different Aggregate Types
  127. Magnetic Nano-Sized Solid Acid Catalyst Bearing Sulfonic Acid Groups for Biodiesel Synthesis
  128. The biological activities of Arabis alpina L. subsp. brevifolia (DC.) Cullen against food pathogens
  129. Humidity properties of Schiff base polymers
  130. Free Vibration Analysis of Fiber Metal Laminated Straight Beam
  131. Comparative study of in vitro antioxidant, acetylcholinesterase and butyrylcholinesterase activity of alfalfa (Medicago sativa L.) collected during different growth stages
  132. Isothermal Oxidation Behavior of Gadolinium Zirconate (Gd2Zr2O7) Thermal Barrier Coatings (TBCs) produced by Electron Beam Physical Vapor Deposition (EB-PVD) technique
  133. Optimization of Adsorption Parameters for Ultra-Fine Calcite Using a Box-Behnken Experimental Design
  134. The Microstructural Investigation of Vermiculite-Infiltrated Electron Beam Physical Vapor Deposition Thermal Barrier Coatings
  135. Modelling Porosity Permeability of Ceramic Tiles using Fuzzy Taguchi Method
  136. Experimental and theoretical study of a novel naphthoquinone Schiff base
  137. Physicochemical properties of heat treated sille stone for ceramic industry
  138. Sand Dune Characterization for Preparing Metallurgical Grade Silicon
  139. Catalytic Applications of Large Pore Sulfonic Acid-Functionalized SBA-15 Mesoporous Silica for Esterification
  140. One-photon Absorption Characterizations, Dipole Polarizabilities and Second Hyperpolarizabilities of Chlorophyll a and Crocin
  141. The Optical and Crystallite Characterization of Bilayer TiO2 Films Coated on Different ITO layers
  142. Topical Issue on Bond Activation
  143. Metal-mediated reactions towards the synthesis of a novel deaminolysed bisurea, dicarbamolyamine
  144. The structure of ortho-(trifluoromethyl)phenol in comparison to its homologues – A combined experimental and theoretical study
  145. Heterogeneous catalysis with encapsulated haem and other synthetic porphyrins: Harnessing the power of porphyrins for oxidation reactions
  146. Recent Advances on Mechanistic Studies on C–H Activation Catalyzed by Base Metals
  147. Reactions of the organoplatinum complex [Pt(cod) (neoSi)Cl] (neoSi = trimethylsilylmethyl) with the non-coordinating anions SbF6– and BPh4–
  148. Erratum
  149. Investigation on Two Compounds of O, O’-dithiophosphate Derivatives as Corrosion Inhibitors for Q235 Steel in Hydrochloric Acid Solution
https://doi.org/10.1515/chem-2018-0055
Keywords for this article
supported transition metal catalysts; heterogeneous catalysts; metal-containing monomers; metal-containing polymers; reductions; oxidations; C-C bond forming reactions
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  89. A numerical analysis of heat transfer in a cross-current heat exchanger with controlled and newly designed air flows
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  94. Effect of poultry wastewater irrigation on nitrogen, phosphorus and carbon contents in farmland soil
  95. Response of spring wheat to NPK and S fertilization. The content and uptake of macronutrients and the value of ionic ratios
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  102. Bioassay-directed fractionation of a blood coagulation factor Xa inhibitor, betulinic acid from Lycopus lucidus
  103. Antifungal and repellent activities of the essential oils from three aromatic herbs from western Himalaya
  104. Chemical composition and microbiological evaluation of essential oil from Hyssopus officinalis L. with white and pink flowers
  105. Bioassay-guided isolation and identification of Aedes aegypti larvicidal and biting deterrent compounds from Veratrum lobelianum
  106. α-Terpineol, a natural monoterpene: A review of its biological properties
  107. Utility of essential oils for development of host-based lures for Xyleborus glabratus (Coleoptera: Curculionidae: Scolytinae), vector of laurel wilt
  108. Phenolic composition and antioxidant potential of different organs of Kazakh Crataegus almaatensis Pojark: A comparison with the European Crataegus oxyacantha L. flowers
  109. Isolation of eudesmane type sesquiterpene ketone from Prangos heyniae H.Duman & M.F.Watson essential oil and mosquitocidal activity of the essential oils
  110. Comparative analysis of the polyphenols profiles and the antioxidant and cytotoxicity properties of various blue honeysuckle varieties
  111. Special Issue on ICCESEN 2017
  112. Modelling world energy security data from multinomial distribution by generalized linear model under different cumulative link functions
  113. Pine Cone and Boron Compounds Effect as Reinforcement on Mechanical and Flammability Properties of Polyester Composites
  114. Artificial Neural Network Modelling for Prediction of SNR Effected by Probe Properties on Ultrasonic Inspection of Austenitic Stainless Steel Weldments
  115. Calculation and 3D analyses of ERR in the band crack front contained in a rectangular plate made of multilayered material
  116. Improvement of fuel properties of biodiesel with bioadditive ethyl levulinate
  117. Properties of AlSi9Cu3 metal matrix micro and nano composites produced via stir casting
  118. Investigation of Antibacterial Properties of Ag Doped TiO2 Nanofibers Prepared by Electrospinning Process
  119. Modeling of Total Phenolic contents in Various Tea samples by Experimental Design Methods
  120. Nickel doping effect on the structural and optical properties of indium sulfide thin films by SILAR
  121. The effect mechanism of Ginnalin A as a homeopathic agent on various cancer cell lines
  122. Excitation functions of proton induced reactions of some radioisotopes used in medicine
  123. Oxide ionic conductivity and microstructures of Pr and Sm co-doped CeO2-based systems
  124. Rapid Synthesis of Metallic Reinforced in Situ Intermetallic Composites in Ti-Al-Nb System via Resistive Sintering
  125. Oxidation Behavior of NiCr/YSZ Thermal Barrier Coatings (TBCs)
  126. Clustering Analysis of Normal Strength Concretes Produced with Different Aggregate Types
  127. Magnetic Nano-Sized Solid Acid Catalyst Bearing Sulfonic Acid Groups for Biodiesel Synthesis
  128. The biological activities of Arabis alpina L. subsp. brevifolia (DC.) Cullen against food pathogens
  129. Humidity properties of Schiff base polymers
  130. Free Vibration Analysis of Fiber Metal Laminated Straight Beam
  131. Comparative study of in vitro antioxidant, acetylcholinesterase and butyrylcholinesterase activity of alfalfa (Medicago sativa L.) collected during different growth stages
  132. Isothermal Oxidation Behavior of Gadolinium Zirconate (Gd2Zr2O7) Thermal Barrier Coatings (TBCs) produced by Electron Beam Physical Vapor Deposition (EB-PVD) technique
  133. Optimization of Adsorption Parameters for Ultra-Fine Calcite Using a Box-Behnken Experimental Design
  134. The Microstructural Investigation of Vermiculite-Infiltrated Electron Beam Physical Vapor Deposition Thermal Barrier Coatings
  135. Modelling Porosity Permeability of Ceramic Tiles using Fuzzy Taguchi Method
  136. Experimental and theoretical study of a novel naphthoquinone Schiff base
  137. Physicochemical properties of heat treated sille stone for ceramic industry
  138. Sand Dune Characterization for Preparing Metallurgical Grade Silicon
  139. Catalytic Applications of Large Pore Sulfonic Acid-Functionalized SBA-15 Mesoporous Silica for Esterification
  140. One-photon Absorption Characterizations, Dipole Polarizabilities and Second Hyperpolarizabilities of Chlorophyll a and Crocin
  141. The Optical and Crystallite Characterization of Bilayer TiO2 Films Coated on Different ITO layers
  142. Topical Issue on Bond Activation
  143. Metal-mediated reactions towards the synthesis of a novel deaminolysed bisurea, dicarbamolyamine
  144. The structure of ortho-(trifluoromethyl)phenol in comparison to its homologues – A combined experimental and theoretical study
  145. Heterogeneous catalysis with encapsulated haem and other synthetic porphyrins: Harnessing the power of porphyrins for oxidation reactions
  146. Recent Advances on Mechanistic Studies on C–H Activation Catalyzed by Base Metals
  147. Reactions of the organoplatinum complex [Pt(cod) (neoSi)Cl] (neoSi = trimethylsilylmethyl) with the non-coordinating anions SbF6– and BPh4–
  148. Erratum
  149. Investigation on Two Compounds of O, O’-dithiophosphate Derivatives as Corrosion Inhibitors for Q235 Steel in Hydrochloric Acid Solution
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