Materials Research Bulletin 70 (2015) 914–919

Characterizations and synergistic catalytic activity of bimetallicAl-Fe-TUD-1

Vinju Vasudevan Srinivasana, Anand Ramanathanb,*, Rajamanickam Maheswaria,**,Gaffar Imrana, Rajamanickam Rajalakshmia, Anbazhagan Nilamadanthaia

aDepartment of Chemistry, Anna University, Chennai 600025, IndiabCenter for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA

A R T I C L E I N F O

Article history:Received 26 April 2015Received in revised form 18 June 2015Accepted 24 June 2015Available online 29 June 2015

Keywords:Amorphous materialsSol–gel chemistryNuclear magnetic resonance (NMR)X-ray diffractionTransmission electron microscopy (TEM)Infrared spectroscopyCatalytic properties

A B S T R A C T

Amorphous 3D mesoporous silicate TUD-1 was incorporated with monometallic Al3+, Fe3+ and bimetallicAl3+–Fe3+ by direct one pot synthesis. The incorporation of these metal ions created both Lewis andBrønsted acid sites. A synergy between Al and Fe sites in TUD-1 was noticed and is found responsible forobserved higher activity in the synthesis of 1,4-dihydropyridine and b-amino carbonyl compoundsthrough Hantzsch and Mannich reactions respectively.

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1. Introduction

Solid acids such as zeolites are well known catalysts forpetroleum refining due to their well-defined number and nature ofacid sites [1,2]. However, for acid-catalyzed organic transforma-tions involving bulkier substrates, incorporation of heteroatomssuch as Al was successfully carried out in MCM-type (pore size2–3 nm) mesoporous molecular sieves [3,4]. In general, theincorporation of Al in these mesoporous silicates generates bothLewis and Brønsted acidity that can be tuned based on the loadingof Al. Further, for the synthesis of larger pore silicates (>5 nm, e.g.SBA-type materials), the incorporation of Al was achieved byadjusting the pH of the synthesis gel [5] under acidic conditions.On the other hand, amorphous disordered mesoporous materialwith relatively narrow pore size distribution, TUD-1, is preparedunder basic conditions. It was shown that with the non-surfactant—aggregated route for the mesopore formation ofTUD-1, a higher loading of Al (Si/Al = 4) can be achieved and itsBrønsted site is exploited as an ionic carrier [6]. We have shownthat Al-TUD-1 can be utilized as a Friedel–Crafts alkylation catalyst

* Corresponding author. Fax: +1 785 864 6051.** Corresponding author.

E-mail addresses: anand@ku.edu (A. Ramanathan),maheswarianand@annauniv.edu (R. Maheswari).

http://dx.doi.org/10.1016/j.materresbull.2015.06.0390025-5408/ã 2015 Elsevier Ltd. All rights reserved.

both in liquid and gas phase [7]. Independently, Al-TUD-1 wasalso shown to be an effective catalyst for degradation of highdensity polyethylene (HDPE) [8] and for transformation ofbio-mass based substrates such as conversion of furfuryl alcoholinto ethyl levulinate and conversion of saccharides into furanicaldehydes [9,10].

The acidity of Al-TUD-1 can further be tuned by incorporatinganother metal ion such as Zr4+ and a synergy between Brønstedand Lewis acid sites were demonstrated in Al–Zr-TUD-1 for Prinscyclization of citronellal [11]. Similarly, the incorporation of Fe toAl-MCM-41 leads to a higher catalytic activity in the t-butylation ofphenol compared to Al-MCM-41 [12]. The activity increase wasattributed to strengthening of the Brønsted acid sites (generatedby Al3+) by incorporation of Fe in tetrahedral co-ordinationin close proximity to the Brønsted acid site. Though bimetallicAl-Fe-SBA-15 was prepared by direct synthesis, leaching of Fespecies that are not close to framework Al ions was observed[13]. Moreover, similar catalyst (Fe–Al-SBA-15) is shown to beactive in benzene hydroxylation [14].

Encouraged by these results, we intent to synthesis and exploreincorporation of both Al and Fe into amorphous mesoporousTUD-1 material and its activity was compared with monometallicAl-TUD-1 and Fe-TUD-1. The synthesis of 1,4-dihydropyridines andb-amino carbonyl compounds through Hantzsch and Mannichreactions respectively studied by us earlier [15,16] was chosen asprobe reactions.

V.V. Srinivasan et al. / Materials Research Bulletin 70 (2015) 914–919 915

2. Experimental

2.1. Synthesis of bimetallic Al-Fe-TUD-1

Monometallic Al-TUD-1 and Fe-TUD-1 with (Si/M = 40, M = Al orFe) were synthesized according to previously reported literature[7,17]. Al-Fe-TUD-1 materials with Si/(Al + Fe) molar ratio of40 with Al/Fe ratio of 1 were synthesized by using triethanolamine(TEA) as complexing agent and tetraethyl ammonium hydroxide(TEAOH) as base in a one pot surfactant-free procedure. In a typicalsynthesis, aluminum isopropoxide (0.18 g, 98%, Aldrich) and iron(III) nitrate nanohydrate (0.36 g, 98%, Aldrich) were dissolved in a1:1 mixture of isopropanol (2 mL) and absolute ethanol (2 mL) andwere added to tetraethyl orthosilicate (15.0 g, 98%, Aldrich). Afterstirring for a few minutes, a mixture of TEA (10.5 g) and distilledwater (6 mL) was added, followed by addition of TEAOH (10.5 g,35 wt% in H2O, Aldrich) under vigorous stirring. The clear gelobtained after these steps was then aged at room temperature for24 h and dried at 98 �C for 24 h. Hydrothermal treatment wascarried out in a Teflon-lined autoclave at 180 �C for 8 h and finallythe organic moieties were removed by calcination in the presenceof air at 600 �C with a temperature ramp of 1 �C min�1.

2.2. Characterization

The powder X-ray diffraction patterns were recorded on aRigaku instrument with Cu-Ka (l = 1.54 Å) in the 2u range of10–65�. Nitrogen adsorption and desorption isotherms weremeasured at 77 K using a Quantachrome porosimeter (Quanto-sorbSI). Chemical compositions were analyzed by ICP-OES using aPerkinElmer OES Optima 5300 DV spectrometer. Fourier-transforminfrared spectroscopy (FT-IR) was carried out on a Nicolet 5700FT-IR instrument measured at 4 cm�1 resolutions. Samples weremixed and ground with KBr followed by pressing into pellets for IRmeasurement in the range of 4000–400 cm�1. Diffuse reflectanceUV–vis spectra were recorded by a Shimadzu UV-2401 spectro-photometer with BaSO4 as reference. Temperature programmeddesorption (TPD) of ammonia measurements were performedusing the Micromeritics TPR/TPD 2900 equipped with a thermalconductivity detector (TCD). For pyridine FTIR 100 mg of thecatalyst was dried at 200 �C for 2 h to remove the adsorbed species.Then the sample was wetted with pyridine and equilibrated for12 h. The sample was then heated at 110 �C in the atmosphericpressure to remove the physisorbed pyridine and FTIR analysis wascarried out in the absorption mode.

Fig. 1. Powder XRD patterns of Al-TUD-1, Al-Fe-TUD-1

2.3. Catalytic activity

For Hantzsch reaction [15], benzaldehyde (1 mmol), ethylacetoacetate (2 mmol), ammonium acetate (1.2 mmol), ethanol(4 mL) and 100 mg of catalyst (preheated at 200 �C for 3 h) weretaken in a 25 mL two neck round bottom flask fitted with refluxcondenser. The flask was then immersed in an oil bath and thereaction was carried out under reflux conditions (typically 80 �C for3 h). After completion of reaction (monitored by TLC using hexane:ethyl acetate 7:3), the reaction mixture was cooled down to roomtemperature and poured into crushed ice with stirring. The crudeproduct was filtered and washed with distilled water followed bytreatment with brine solution and mixed with ethyl acetate toseparate the compound and dried over anhydrous Na2SO4. Thecrude mixture was dissolved in hot ethanol to separate the solidcatalyst and the crude product was further purified by recrystalli-zation from ethanol. The isolated pure compound was confirmedby 1H NMR, 13C NMR and FT-IR (not shown) with the reportedliterature.

For Mannich reaction [16], a mixture of aniline (1 mmol),benzaldehyde (1 mmol), acetophenone (1 mmol) and catalyst(0.1 g) in ethanol (5 mL) were added to a 15 mL glass vial and thereaction mixture was stirred at room temperature for 10 h.The progress of the reaction was monitored by thin-layerchromatography (TLC). After reaction, the catalyst was filteredand the filtrate was washed with aqueous NaCl followed bydistilled water and dried over sodium sulphate (Na2SO4).After evaporation of the solvent, the crude yellow productwas further purified by recrystallization from ethanol. Theisolated pure compound was characterized and confirmed by1H NMR, 13C NMR and FT-IR (not shown) with the reportedliterature.

3. Results and discussion

The low angle powder XRD patterns of Al-TUD-1, Fe-TUD-1 andAl-Fe-TUD-1 are presented in Fig.1a. A growing peak starting about2� (2u) toward the low angle 0.5� (2u) was noticed in all thesesamples indicating mesostructured and amorphous nature ofthese materials [16–18]. A broad peak ranging from 2u = 15� to 30�

in the high angle XRD patterns of these samples (Fig.1b) confirmedthe amorphous nature of the silica. Further no crystallineoxide (Al2O3 or Fe2O3) species were evidenced in the highangle XRD suggesting homogeneous dispersion of Al and Fespecies in these samples.

and Fe-TUD-1 in (a) low angle and (b) high angle.

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The N2 physisorption isotherms of Al-TUD-1, Fe-TUD-1 andAl-Fe-TUD-1 (Fig. 2a) exhibit a sharp inflection at a relativepressure (P/P0) approximately from 0.45 to 0.85 indicatingcapillary condensation within mesopores, a characteristic oftypical mesoporous materials [16–18]. With Al-TUD-1, thestructure presented a hysteresis close to H1 type and withincreasing Fe content and in Fe-TUD-1, H2 type hysteresis wasnoticed. The specific surface area (BET), total pore volume andaverage pore diameter calculated from N2 adsorption isothermsusing the BJH model are summarized in Table 1. A relatively broadpore size distribution from 3 nm to 20 nm was noticed for all these

Fig. 2. (a) N2 sorption and (b) pore size distribut

Fig. 3. (a) FTIR spectra and (b) diffuse reflectance UV–v

Table 1Properties of TUD-1 catalysts.

Catalyst (Si/M)a Al (wt%)b Fe (wt%)b Si/Mc ratio (M = Al + Fe)

Al-TUD-1 (40) 1.3 – 33

Fe-TUD-1 (40) – 2.5 35

Al-Fe-TUD-1 (40) 0.6 (0.6)g 1.4 (1.1)g 35

a Molar ratio and wt% of Al and Fe in the synthesis gel.b The wt% of Al and Fe in calcined samples.c Actual molar ratio in sample determined by ICP-OES.d SBET = BET specific surface area.e Vtp = total pore volume at 0.99 P/P0.f dP,BJH = average pore diameter calculated from N2 adsorption isotherms using the Bg The wt% of Al and Fe after fourth cycle of Hantzsch reaction by ICP-OES.

samples (Fig. 2b). Nevertheless, the average pore diameter wasestimated to be 4.5–6.5 nm and the total pore volume increasedwith Al content and was observed in the range of 0.72–0.91 cm3/g.

FTIR spectra of KBr diluted Al-TUD-1, Fe-TUD-1 and Al-Fe-TUD-1 samples in the framework region are shown in Fig. 3a. The strongbands observed at around 1090 cm�1 and 806 cm�1 are character-istic of the Si��O��Si asymmetric and symmetric stretchingvibrations respectively. The absorption band at 1640 cm�1

corresponds to O��H bending vibration arising from absorbedwater in silica network. In addition, in all these samples a weakband at 966 cm�1 due to the presence of Si��O��M (M = Al or Fe)

ions of Al-TUD-1, Al-Fe-TUD-1 and Fe-TUD-1.

is spectra of Al-TUD-1, Al-Fe-TUD-1 and Fe-TUD-1.

SBETd (m2/g) Vtp

e (cc/g) dP,BJHf (nm) Total acidity (mmol NH3/g)

558 0.91 6.5 0.30634 0.72 4.4 0.15521 0.85 6.5 0.21

JH model.

Fig. 4. (a) NH3-TPD and (b) pyridine adsorbed FTIR spectra of Al-TUD-1, Al-Fe-TUD-1 and Fe-TUD-1.

Fig. 6. 27Al-MAS-NMR of (a) A-TUD-1 and (b) Al-Fe-TUD-1.

V.V. Srinivasan et al. / Materials Research Bulletin 70 (2015) 914–919 917

bending vibration, suggesting framework incorporation of Al or Feions in TUD-1 silicate network [19].

The diffuse reflectance spectra of Al-TUD-1, Fe-TUD-1 and Al-Fe-TUD-1 samples are depicted in Fig. 3b. No significant absorptionband was noticed for Al-TUD-1 but a sharp and intense peakcentered around 245–250 nm was observed for Fe-TUD-1 andAl-Fe-TUD-1 samples due to the presence of Fe3+ ions intetrahedral coordination [20,21]. However, a broad absorptionband ranges between 450 nm and 600 nm in Fe-TUD-1 indicatesthe formation of extra-framework small oligomeric Fe2O3

clusters on the surface of the material [20,21].The acidity of Al-TUD-1, Fe-TUD-1 and Al-Fe-TUD-1 samples

was characterized from ammonia-TPD studies and the results areshown in Fig. 4a. All the samples displayed a broad desorption ofammonia between 100 �C and 350 �C due to the presence of weakand medium type acid strength. From the amount of ammoniadesorbed, the total acidity of samples followed the orderAl-TUD-1 > Al-Fe-TUD-1 > Fe-TUD-1 (see Table 1). Further,Al-Fe-TUD-1 showed additional ammonia desorption between380 �C and 500 �C evidencing formation of strong acid sitesprobably from the interaction of Al and Fe species [12,22].

The FTIR spectra of pyridine adsorbed on Al-TUD-1,Fe-TUD-1 and Al-Fe-TUD-1 samples are shown in Fig. 4b. Apredominant intense band at 1448 cm�1 ascribed to strongLewis acid sites which is found to increase in the order of

Fig. 5. TEM images of Al-Fe-TUD-1 at a mag

Al-TUD-1 > Al-Fe-TUD-1 > Fe-TUD-1 similar to the observation inammonia-TPD studies. The Brønsted acid sites usually observed at1545 cm�1 are clearly absent in Fe-TUD-1 whereas it is foundweak in Al-Fe-TUD-1. On the other hand, increases in this band(1545 cm�1) intensity along with 1490 cm�1 band (due to bothLewis and Brønsted acid sites) were observed for Al-TUD-1.These observation clearly suggests that bimetallic Al-Fe-TUD-1sample presents both the Lewis and Brønsted acid sitesin moderate concentration due to stabilization and highdispersion of isolated Fe3+ sites which are shown to be activefor various acid catalyzed reactions [12,22].

nification of (a) 100 nm and (b) 10 nm.

R

O

O

O

O

O

O

O

NH4+

OO-

NH

RO

O O

O+

Ethanol, Refluxed at 80 ºC

Ethyl acetoacetate

Amm onium acetate

Aldehyde

Al-F e-TUD-1

Scheme 1. Hantzsch reaction over Al-Fe-TUD-1 catalyst.

O R2

O R1 +

NH2R3

Al-Fe-TUD-1NHO

R2R1

R3

Scheme 2. Mannich reaction over Al-Fe-TUD-1 catalyst.

Fig. 7. Reusability of Al-Fe-TUD-1 for Hantzsch and Mannich reaction.

918 V.V. Srinivasan et al. / Materials Research Bulletin 70 (2015) 914–919

The three-dimensional sponge-like mesoporous structure ofAl-Fe-TUD-1 is further evidenced in the TEM image (see Fig. 5a).Moreover, no diffraction fringes of Fe2O3 or Al2O3 were observedeven at higher magnification (see Fig. 5b), suggesting againhomogeneous distribution of Al3+ and Fe3+ species in Al-Fe-TUD-1.These results are in correlation with the observations from XRD,UV–vis and FTIR studies.

27Al-MAS-NMR measurement of Al-TUD-1 and Al-Fe-TUD-1(Fig. 6) displayed a major resonance peak at 53 ppm suggestingthat majority of aluminum is framework incorporated in tetrahe-dral coordination [7,23]. Further, a sharp resonance at 0.5 ppm inAl-TUD-1 indicates the presence of a small amount of extraframework Al species in the octahedral coordination. Al-Fe-TUD-1 also exhibited a peak at 12 ppm that can be assigned to sixcoordinated octahedral AlO6 or distorted tetrahedral Al species.The spinning sideband at 106 ppm arises due to the presence ofFe3+ in the Al-Fe-TUD-1 [24].

All the three catalysts are subjected to two different acidcatalyzed reaction (see Scheme 1) namely Hantzsch synthesis of1,4-dihydropyridines (DHP) [15] and Mannich reaction (synthesisof b-amino carbonyl compounds, Scheme 2) [16] and the resultsare presented in Table 2. Both these reactions did not proceed withsiliceous TUD-1 due to the absence of acid sites (typical acidity

Table 2Activity of TUD-1 catalysts for Hantzsch and Mannich reaction.

Entry Catalyst Time (h)a Hantzsch re

1 Al-TUD-1 (40) 3 52

2 Fe-TUD-1 (40) 3 71

3 Al-Fe-TUD-1 (40) 3 86

4 Zr-TUD-1 (40) 3 73

5 Zr-SBA-16 (50) 3 69

6 AlCl3 3 24

7 FeCl3 3 38

8 Hb 6 56

9 Montmorillonite K-10 2 60

10 Bentonite 8 48

a Completion of the time monitored by TLC.b Isolated yield.c Reaction conditions: catalyst (100 mg), benzaldehyde (1 mmol), ethyl acetoacetae (d Reaction conditions: catalyst (100 mg), benzaldehyde (1 mmol), aniline (1 mmol) a

�0.03 mmol NH3/g). Similarly, no desired product was obtainedwith homogeneous AlCl3 or FeCl3 (yield <10%) for Mannichreaction, whereas considerable yield of DHP (24% and 38%respectively) was obtained (Table 2, entries 6 and 7).

In sharp contrast, desired products with significantly higheryields were obtained over both the heterogeneous Al-TUD-1 andFe-TUD-1 catalyst. It is interesting to note that despite higher totalacidity of Al-TUD-1 significant enhanced activity was observedover Fe-TUD-1 catalyst (Table 2, entries 1 and 2). This couldpossibly be attributed to strong interaction of bases such asammonium acetate and aniline with strong Brønsted acid sites ofAl-TUD-1 thus an observed drop in the final yield of 1,4-DHP andb-Amino carbonyl compound respectively. Moreover, thesecatalysts performed excellent compared to their homogeneoussalts AlCl3 and FeCl3 (Table 2, entries 6 and 7). On the other hand,Al-Fe-TUD-1 having intermediate Lewis and Brønsted acid sitescompared to both Al-TUD-1 and Fe-TUD-1 gave significantly higheryields of 1,4-DHP and b-Amino carbonyl compound (Table 2, entry3). Zr-SBA-16 and Zr-TUD-1 having predominant Lewis acid sites[15,25] gave slightly higher yields in Hantzsch and Mannichreaction (Table 2, entries 4 and 5) compared to Fe-TUD-1(40),however, showed lower activity compared to bimetallic Al-Fe-TUD-1(40). Further, commercial catalyst such Hb, montmorilloniteK-10 and bentonite were also compared for this reaction and theresults are given in Table 2 (entries 8–10). Significant lower yield inthe strong Brønsted acid catalyst such as Hb clearly indicates that

action yield (%)b,c Time (h)a Mannich reaction yield (%)b,d

10 5810 6510 7410 708 72

10 Trace10 <1010 406 48

12 61

2 mmol) and ammonium acetate (1 mmol) at T = 80 �C.nd acetophenone (1 mmol) at room temperature.

V.V. Srinivasan et al. / Materials Research Bulletin 70 (2015) 914–919 919

the Brønsted acid sites lead to formation of intermediate and otherproducts both in Hantzsch and Mannich reaction. Hence, weconclude that the moderate Lewis and Brønsted acid sites arisingfrom incorporation of both Al and Fe sites in TUD-1 promotes theformation of 1,4-DHP and b-Amino carbonyl compound comparedto its monometallic counterparts as observed by other authors in adifferent type of reactions [12,22].

Further, to investigate the heterogeneity of the Al-Fe-TUD-1 catalyst, after reaction, the catalyst was washed with water andacetone and activated at 200 �C for 2 h, then the reactivatedcatalyst was employed in the reaction. Though the catalyst wasreusable up to four cycles slight lowering in the yields wasobserved (Fig. 7) which might be attributed to slow leaching of Fe3+

ions (about 27% of Fe in the solid was found leached, Table 2).

4. Conclusions

We have successfully synthesized bimetallic Al and Feincorporated TUD-1 material and is characterized by presence ofboth Lewis and Brønsted acid sites. Diffuse reflectance UV–visspectrum confirms the presence of isolated Fe3+ in the frameworkand further supported by FTIR studies. Al-Fe-TUD-1 is shown to bean efficient catalyst for the synthesis of 1,4-dihydropyridine andb-amino carbonyl compounds through Hantzsch and Mannichreaction due to the synergy between Al and Fe sites in TUD-1.Further, the catalyst can be easily recovered and reused withoutsignificant leaching of active metal ions.

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