Catalytic performance of Metal-Organic-Frameworks vs. extra-large pore zeolite UTL in condensation reactions

Catalytic behavior of isomorphously substituted B-, Al-, Ga-, and Fe-containing extra-large pore UTL zeolites was investigated in Knoevenagel condensation involving aldehydes, Pechmann condensation of 1-naphthol with ethylacetoacetate, and Prins reaction of β-pinene with formaldehyde and compared with large-pore aluminosilicate zeolite beta and representative Metal-Organic-Frameworks Cu3(BTC)2 and Fe(BTC). The yield of the target product over the investigated catalysts in Knoevenagel condensation increases in the following sequence: (Al)beta < (Al)UTL < (Ga)UTL < (Fe)UTL < Fe(BTC) < (B)UTL < Cu3(BTC)2 being mainly related to the improving selectivity with decreasing strength of active sites of the individual catalysts. The catalytic performance of Fe(BTC), containing the highest concentration of Lewis acid sites of the appropriate strength is superior over large-pore zeolite (Al)beta and B-, Al-, Ga-, Fe-substituted extra-large pore zeolites UTL in Prins reaction of β-pinene with formaldehyde and Pechmann condensation of 1-naphthol with ethylacetoacetate.


INTRODUCTION
Condensation reactions, in which a carbonyl group undergoes nucleophilic attack by the enol form or enolate carbanions, are powerful tools to form C-C bonds affording for easy preparation of useful organic compounds (Li, 2005). Among them, Knoevenagel, Pechmann, and Prins reactions, being quite sensitive to the nature of active sites of a catalyst, are particularly interesting to be studied over solid acids, containing active sites of different type and strength.
impregnated on Montmorillonite (Yadav and Jasra, 2006), and Sn-SBA-15 (Selvaraj and Choe, 2010). However, doping of metals into mesoporous silica does not produce single site catalysts and due to the amorphous nature, metal-doped mesoporous materials are not stable enough due to a leaching of the active phase.
Due to extra-high porosity (Chae et al., 2004), the regular arrangement of a large number of active sites and mostly Lewis acidity metal-organic frameworks (MOFs) attract significant attention as perspective heterogeneous catalysts, which are able to compete with the traditionally used zeolites (Li et al., 1999;Eddaoudi et al., 2002;Chae et al., 2004;Corma et al., 2010;Dhakshinamoorthy et al., 2013). MOFs exhibit a high catalytic activity in Knoevenagel condensation reactions (Opanasenko et al., 2013a) and perform even better in transformations of bulky substrates than zeolites due to steric limitations. At the same time, other condensation reactions have been considerably less explored over MOFs (Opanasenko et al., 2013b).
Extra-large pore zeolites, possessing micropores larger than 0.85 nm, represent another group of porous materials with a great potential for application in fine chemistry catalysis (Jiang et al., 2010). Zeolite UTL (Corma et al., 2004;Paillaud et al., 2004;Shvets et al., 2008), containing two-dimensional system of intersecting large (12-rings) and extra-large channels (14-rings) with dimensions 0.85 × 0.55 nm and 0.95 × 0.71 nm, belongs to the thermally stable extra-large pore zeolites. Moreover, the catalytic activity of (Al)UTL was shown to be higher with respect to UTD-1 zeolite, having one-dimensional 14-ring channel system, in dealkylation of tri-isopropylbenzene and di-isopropylbenzene (Corma et al., 2004). Recently, higher activity and selectivity of Al-substituted UTL zeolites in acylation of p-xylene with benzoyl chloride in comparison to large-pore alumosilicate zeolite beta was shown (Shamzhy et al., 2013). However, despite extra-large pore zeolite UTL looks like promising solid catalyst for the liquid phase condensation reactions (i.e., Knoevenagel, Pechmann), as far as we know, there are no reports addressing this issue.
Our contribution is aimed at the investigation of catalytic properties of some zeolites and metal-organic-frameworks, in particular in comparison of B-, Al-, Ga-, Fe-substituted extralarge-pore zeolites UTL with large-pore alumosilicate zeolite (Al)beta and MOFs [Cu 3 (BTC) 2 , Fe(BTC)] in Knoevenagel, Pechmann, and Prins reactions. The selection of (Al)beta, Cu 3 (BTC) 2 and Fe(BTC) as the reference materials was made based on their wide availability and ample use as solid catalysts.
All reactants and solvents were obtained from Sigma Aldrich and used as received without any further treatment.

CHARACTERIZATION
The crystallinity of samples under study was determined by X-ray powder diffraction on a Bruker AXS D8 Advance diffractometer with a Vantec-1 detector in the Bragg-Brentano geometry using CuKα radiation. To limit the effect of preferential orientation of individual crystals a gentle grinding of the samples was performed before measurements.
Adsorption isotherms of nitrogen at −196 • C were determined using an ASAP 2020 (Micromeritics) static volumetric apparatus. In order to attain sufficient accuracy in the accumulation of the adsorption data, the ASAP 2020 was equipped with pressure transducers covering the 133 Pa, 1.33 kPa and 133 kPa ranges. Before adsorption experiments the samples were outgassed under turbomolecular pump vacuum at temperature of 150 • C for MOFs and 250 • C for zeolites. This temperature was maintained for 8 h.
The concentrations of Brønsted and Lewis acid sites in zeolites were determined by pyridine adsorption at 150 • C followed by FTIR spectroscopy (Nicolet 6700) using self-supporting wafer technique. Generally, a thin sample wafer of zeolite was activated prior to the experiment in a high vacuum (10 −4 Torr) at 450 • C overnight. Adsorption of pyridine proceeded at room temperature for 30 min at a partial pressure of 5 Torr and was followed by 20 min evacuation at the temperature 150 • C. For the quantitative determination of concentration of relevant acid sites the molar adsorption coefficients (Emeis, 1993)

CATALYSIS
The condensation reactions were performed in a liquid phase under atmospheric pressure in a multi-experiment work station StarFish (Radley's Discovery Technologies UK). Prior to use, 200 mg of the catalyst was activated at 150 • C (for MOFs) or 450 • C (for zeolites) for 90 min with a temperature rate 10 • C/min in a stream of air.

Pechmann condensation
Typically, 8.5 mmol of 1-naphthol, 0.5 g of n-dodecane (internal standard), 10 ml of nitrobenzene and 200 mg of catalyst were added to the 3-necked vessel, equipped with condenser and thermometer, stirred, and heated. 100 mmol of ethyl acetoacetate was added into the reaction vessel through a syringe when the temperature of 130 • C was reached.

Knoevenagel condensation
Typically, 6.0 mmol of aldehyde, 0.4 g of mesitylene (internal standard), 10 ml of p-xylene and 200 mg of catalyst were added to the 3-necked vessel, equipped with condenser and thermometer, stirred, and heated. 9.0 mmol of ethyl acetoacetate was added into the reaction vessel through a syringe when the temperature of 130 • C was reached.

Prins condensation
Typically, 8.0 mmol of PF, 0.4 g of mesitylene (internal standard), 10 ml of acetonitrile and 200 mg of catalyst were added to the 3necked vessel, equipped with a condenser and a thermometer, stirred, and heated. 4.0 mmol of β-pinene was added into the reaction vessel through a syringe when the temperature of 80 • C was reached.
Aliquots of the reaction mixture were sampled at the interval time of 0, 20, 60, 120, 180, 240, 300, 360 min in order to determine the equilibrium of the reaction. Zero point of conversion corresponds to the concentration of phenol, aldehyde, or β-pinene in starting solution in the presence of catalyst (to neglect the contribution of adsorption).
To evaluate a potential influence of leaching of active species from the heterogeneous catalysts, a part of the reaction mixture was filtered at the reaction temperature and the obtained liquid phase was further investigated in condensation reaction under the same reaction conditions.

Reaction product analysis
The reaction products were analyzed by gas chromatography (GC) using an Agilent 6850 with FID detector equipped with a non-polar HP1 column (diameter 0.25 mm, thickness 0.2 μm and length 30 m). Reaction products were indentified using GC-MS analysis (ThermoFinnigan, FOCUS DSQ II Single Quadrupole GC/MS).

CHARACTERISTICS OF THE CATALYSTS
The X-ray diffraction patterns of all the catalysts match well with those reported in the literature ( Figure A1) (Chui et al., 1999;Shvets et al., 2008;Dhakshinamoorthy et al., 2012).
The known frameworks of the catalysts under investigation are depicted on Figure 1. In Cu 3 (BTC) 2 framework, the Cu 2clusters are coordinated via carboxylate groups of benzene-1,3,5tricarboxylate to form a paddlewheel unit in a three-dimensional porous cubic network ( Figure 1A). Zeolite beta consists of an intergrowth of two distinct structures termed polymorphs A ( Figure 1B) and B. The polymorphs grow as two-dimensional sheets and the sheets randomly alternate. Both polymorphs have a three dimensional network of 12-ring pores (0.64 × 0.76 and 0.56 × 0.56 nm). The intergrowth of the polymorphs does not significantly affect the pores in two of the dimensions, but in the direction of the faulting, the pore becomes tortuous, but not blocked. Zeolite UTL (Figure 1C) belongs to the extra-large pore zeolites having 2D pore system of intersecting 14-(0.71 × 0.95 nm) and 12-ring channels (0.85 × 0.55 nm).  Crystals of UTL zeolites possess rectangular shape (Figure 2) and have the close size, except (Al)UTL ( Table 1). Crystals of zeolite (Al)beta are characterized by size about 0.5 μm. The crystals of Cu 3 (BTC) 2 are rectangular prisms with the length of the edges of about 7 μm, while the size of the crystals of Fe(BTC) is about 3 μm.
Nitrogen adsorption isotherms of the catalysts are depicted in Figure 3. All catalysts exhibit type I isotherm being characteristic for microporous solids. The presence of a hysteresis loop at p > 0.8 on the isotherm of (Al)UTL is probably connected with an interparticle adsorption. Textural properties of all catalysts are summarized in Table 1.
Acidic properties of the catalysts (i.e., type and concentrations of acid sites) were analyzed using adsorption of pyridine followed by FTIR. For zeolites, absorption bands around 1546 cm −1 (interaction of pyridine with Brønsted acid sites) and 1453-1455 cm −1 (interaction of pyridine with Lewis acid sites) were chosen as characteristic. To estimate the acidity of the Cu 3 (BTC) 2 band at 1069 cm −1 assigned to C-C out-of-plane vibrations of coordinatively bonded pyridine was chosen (Pérez-Mayoral et al., 2012). Characteristic bands in region 1400-1600 cm −1 were not used for the determination of the amount of acid sites in MOFs because of an overlap with the bands corresponding to MOF's framework. Table 2 lists the concentrations of Brønsted and Lewis acid sites for different temperatures of pyridine desorption.
It can be seen, that the concentration of acid sites increases in the following sequence: (B) < (Fe) < (Ga) < (Al)UTL << (Al)beta. The concentration of coordinatively bonded pyridine was determined to be equal to 2.30 mmol g −1 for Cu 3 (BTC) 2 activated at 200 • C (Pérez-Mayoral et al., 2012).

KNOEVENAGEL CONDENSATION
The catalytic performance of MOFs and isomorphously substituted extra-large pore zeolites UTL was studied in Knoevenagel condensation of cyclohexane carbaldehyde (ChCA) and benzaldehyde (BA) with ethylacetoacetate (EAA) and compared with large-pore alumosilicate zeolite (Al)beta.
The conversion of aldehydes increased over UTL zeolites in the following sequence: (B) < (Fe) < (Ga) < (Al)UTL (Figures 4A,  5A) matching well with the increasing concentration of accessible acid sites and particularly their acid strength. At the same time, while Fe-, Ga-and Al-substituted UTL zeolites appeared to be low-selective catalyst (Figure 4B) in condensation of ChCA with EAA (11.9 and 8% selectivity to product I, respectively), the selectivity 46% of ether I was achieved when using B-containing UTL zeolite as the catalyst. This is most probably related to the acceleration of the rate of secondary reactions, involving the primary product I, with increasing the concentration of acid sites in the range (B) < (Fe) ≈ (Ga) < (Al)UTL.
The initial conversions of ChCA in condensation reaction with EAA over (Al)beta and (Al)UTL were quite similar (23% at 60 min TOS, Figure 4A). With increasing TOS the slope of the curve "conversion-TOS" became flatter for (Al)beta indicating SCHEME 4 | Condensation of ChCA with EAA. SCHEME 5 | Condensation of BA with EAA.

FIGURE 5 | TOS dependence of BA conversion in condensation reaction with EAA over (B)UTL (--), (Ga)UTL (-•-), (Fe)UTL (--), (Al)UTL (-X-), (Al)beta (--), Fe(BTC) (--), Cu 3 (BTC) 2 (--) (A); the yield ( ) and selectivity ( ) (at 15% of BA conversion) to the product III in condensation of BA with EAA (B).
a lowering of the reaction rate. It may be caused by diffusion restrictions for substrates/products in the porous system of zeolite beta in comparison with extra-large pore zeolite UTL despite the crystal size of (Al)beta is much more lower in comparison with (Al)UTL (0.5 × 0.5 × 0.5 vs. 4.0 × 0.5 × 0.1 μm, Table 1). Moreover, the selectivity of (Al)beta possessing the highest concentration of strong acid sites, reached expectedly the lowest value (2%) among the investigated catalysts ( Figure 4B). Surprisingly, the lowest conversion of ChCA was found for Cu 3 (BTC) 2 (Figure 4A), possessing comparably high concentration of mild Lewis acid sites ( Table 2). This may be due to the relative weakness of the respective tetracoordinated Cu 2+ sites within the MOF for the activation of ChCA. However, Cu 3 (BTC) 2 demonstrated higher selectivity to the product I in comparison with UTL zeolites (52%, Figure 4B), which may be also caused by low strength of acid sites within Cu 3 (BTC) 2 .
When compared with ChCA, BA appeared to be easier activated substrate-the conversions of BA over all investigated catalysts were comparable or exceeded the respective values for ChCA (Figures 4A, 5A).
At the same time the increasing selectivity of the catalysts in the case of BA condensation with EAA may be caused by the electronwithdrawing effect of phenyl-group. It results in decreasing reactivity of the primary product III (Scheme 5) in the secondary reactions of Michael addition in comparison with the product I being transformed to product II (Scheme 4) over all investigated catalyst. Even though, Al-substituted zeolites beta and UTL were characterized by the lowest selectivity (20%, Figure 5B) due to a rapid hydrolysis of the target ether III on strong Brønsted acid sites followed by dicarboxylation with the formation of sideproduct IV (Scheme 5). At the same time, the product of BA oxidation-benzoic acid was found as the side product when using (B)UTL, Cu 3 (BTC) 2 , Fe(BTC), which appeared to be the most selective among the investigated ones.
The yields of the target products in Knoevenagel condensation of both ChCA and BA with EAA over the catalysts under investigation increase in the following sequence: (Al)beta < (Al)UTL < (Ga)UTL < (Fe)UTL < Fe(BTC) < (B)UTL < Cu 3 (BTC) 2 , which is caused by the enhancement of the catalyst's selectivity with the decreasing of the strength of active sites. The most active Cu 3 (BTC) 2 catalyst was proved to be structurally stable upon liquid phase Knoevenagel condensation with EAA at 130 • C when using p-xylene as the solvent (Figure 6A). In addition, a negligible increase in the conversion of initial aldehydes after removing of Cu 3 (BTC) 2 from the "hot" reaction mixture at 120 min TOS confirms the absence of homogeneous reaction contribution, which might be caused by the leaching of active sites to the liquid phase ( Figure 6B).
Despite Knoevenagel reaction can be catalyzed by both solid acids (activation of carbonyl component) and bases (activation of methylene component) (Tietze and Beifuss, 1991), it was mainly tested over basic catalysts. In ref. (Lau et al., 2008), Knoevenagel condensation of BA and EAA was performed in a microreactor using Cs-exchanged NaA membrane and NaX crystals. It was shown that the using catalyst film instead of powder results in suppression of undesirable side-reactions (increasing the selectivity from 55 to 78%) due to the rapid elaboration of Knoevenagel adduct from the porous system of the catalyst. Analogously, poor selectivity of (Al)beta in comparison with (Al)UTL may be caused by diffusional restrictions for the removal of the product from the porous system of the first catalyst. The influence of the strength of basic sites on the activity of Li-, Na-, K-, Rb-, Cs-impregnated simple oxide matrices (SiO 2 , Al 2 O 3 , Nb 2 O 5 ) was established (Calvino-Casilda et al., 2009). The best performance of K/Al 2 O 3 in Knoevenagel condensation of BA and EAA (90% of BA conversion and 89.8% of selectivity at 140 • C in 300 min of TOS) was connected with the mild strength of basic sites, found for this system (Calvino-Casilda et al., 2009). Analogously, in the range of Nb-MCM-41 catalysts, impregnated by alkali metals, Rb/Nb-MCM-41 demonstrated the best performance (80% of benzaldehyde conversion in 4 h of TOS at 140 • C) (Calvino-Casilda et al., 2010).

PECHMANN CONDENSATION
Pechmann condensation of 1-naphthol proceeds according to the Scheme 2 with the formation of 4-methyl-2H-benzo[h] chromen-2-one ( Figure A2) as the target product and some unidentified polycondensation products.
Superiority of MOFs over large-pore zeolite (Al)beta and extra-large pore zeolite UTL in Pechmann condensation of 1naphthol was observed. While the conversion of the initial substrate at 120 min TOS did not exceed 15% for zeolites, it totaled 33 and 44% for Cu 3 (BTC) 2 and Fe(BTC), respectively (Figure 7). The selectivity to the target product was about 98% for both MOFs. A lower initial reaction rate over Cu 3 (BTC) 2 in comparison with Fe(BTC) may be caused by a lower accessibility of the active sites within Cu 3 (BTC) 2 catalyst, possessing smaller pore windows in comparison to Fe(BTC) ( Table 1).
Higher conversion of 1-naphthol in Pechmann condensation over MOFs in comparison with zeolites may be connected with significantly higher concentration of acid sites of sufficient strength to activate the substrate. In addition, the facilitation of transport of both reagents and product within pore system of MOFs and extra-large pore UTL zeolites, most likely, prevents side-transformations resulting in 100% selectivity to the target product of 1-naphthol condensation with EAA. Only 80% selectivity was observed when (Al)beta was used.
XRD patterns of Cu 3 (BTC) 2 , elaborated from the reaction mixture after 24 h TOS, provide an evidence of the preservation of the framework of the MOF during the Pechmann condensation of 1-naphthol. At the same time no significant differences in the position of diffraction lines for the fresh and utilized Fe(BTC) were observed (Figure A3A).
Leaching test of the investigated MOFs removing them from the reaction mixture by "hot" centrifugation was not accompanied by the further increase in the conversion of 1-naphthol ( Figure A3B), which strongly denies the leaching of the active sites for Cu 3 (BTC) 2 and Fe(BTC). It should be noted that catalytic activity of and FeBTC is comparable with periodic mesoporous silica chloride (the yield of target product 75% at 130 • C and 180 min of TOS), (Karimi and Behzadnia, 2011) and SBA-15-Ph-Pr-SO 3 H (the yield of target product 65% at 130 • C and 240 min of TOS) (Kalita et al., 2010), which were shown to be one of the most promising catalysts for Pechmann condensation of 1-naphthol.

PRINS REACTION
In the Prins reaction of β-pinene with formaldehyde besides the formation of target product nopol (VI) we observed also the formation of the side products of β-pinene isomerizationlimonene (VII) or camphene (VIII) (Scheme 6).
Fe(BTC) showed the highest performance (47% of β-pinene conversion at 120 min of TOS, Figure 8) in Prins reaction of β-pinene with formaldehyde among the investigated catalysts and providing 100% selectivity to the target productnopol.
Both isomorphously substituted UTL and (Al)beta zeolites appeared to be less efficient catalysts in reaction of β-pinene with formaldehyde in comparison with Fe(BTC). It is obviously caused by the much lower concentration of active sites in zeolites ( Table 2). The conversion of β-pinene increases for www.frontiersin.org August 2013 | Volume 1 | Article 11 | 7 extra-large-pore UTL zeolites in the sequence: (B) < (Fe) < (Ga) ≈ (Al)UTL and does not exceed 15% in 120 min of TOS (Figure 8). The conversion of β-pinene over large-pore zeolite (Al)beta was higher than over UTL-based catalysts, which might be due to the higher concentration of active sites accessible for β-pinene (Table 2). In contrast to Fe(BTC), we observed the formation of side product VI over zeolite beta (40% selectivity to nopol). This result shows that the presence of relatively strong Brønsted acid sites leads to the reduction of the catalyst performance in reaction of β-pinene with formaldehyde. Surprisingly, only isomerization of initial substrate with the formation of camphene took place over Cu 3 (BTC) 2 , which similarly to Fe(BTC) is characterized by the mild exclusively Lewis acidity and close size of micropores. The reasons of camphene formation are not entirely clear on the moment.
To check the possibility of leaching of Fe 3+ ions from the framework to the solution, the most active Fe(BTC) catalyst was surveyed for leaching, which evidences a negligible increase in the conversion after removal by centrifugation of the solid catalyst from reaction mixture at 120 min of TOS, indicating that no leaching of the active species takes place ( Figure A4).
Thus, Fe(BTC), containing the largest amount of acid sites of appropriate strength to activate initial β-pinene, overtops largepore zeolite (Al)beta and B-, Al-, Ga-, Fe-substituted extra-large pore zeolites UTL in Prins reaction of β-pinene with PF.
When compared our results with the previously obtained, montmorillonite impregnated with ZnCl 2 prepared by wet impregnation technique was shown to be active in Prins reaction of β-pinene with PF (75% of β-pinene conversion and 97% selectivity after 24 h TOS). However, leaching of Zn 2+ took place, contributing into the overall conversion value (Yadav and Jasra, 2006). In (Selvaraj and Choe, 2010) Sn-doped SBA-15 was shown to be highly active catalyst for nopol synthesis via Prins reaction of β-pinene and PF. The activity of the catalyst increased with the increasing of the concentration of active tetracoordinated Sn 4+ species. The best results (β-pinene conversion of 99.3% and a nopol selectivity of 95.4% at 360 min of TOS) were achieved over Sn-SBA-15 (Si/Sn = 5), which was characterized by the highest amount of Lewis acid centers.

CONCLUSIONS
Catalytic performance of extra-large pore germanosilicate UTL zeolites prepared by incorporation of different heteroelements (e.g., B, Al, Ga, Fe) into the framework in Knoevenagel, Pechmann, Prins reactions was investigated and compared with that of large-pore aluminosilicate zeolite beta and Cu-, Fecontaining metal-organic-frameworks.
The performance of UTL zeolites in Knoevenagel condensation is mainly determinated by the strength of acid sites. While the conversion of initial ChCA and BA matches with the total concentration of acid sites and increases in the following sequence (B) < (Fe) < (Ga) < (Al)UTL, the selectivity of the catalysts depends on the strength of active centers and has the highest value for B-UTL, containing the weakest acid centers.
The yields of the target products in Knoevenagel condensation of both ChCA and BA with EAA over the catalysts under investigation increase in the following sequence: (Al)beta < (Al)UTL < (Ga)UTL < (Fe)UTL < Fe(BTC) < (B)UTL < Cu 3 (BTC) 2 , which is caused by the enhancement of the catalyst selectivity with the decreasing of the strength of active sites.
Fe(BTC) surpasses not only large-pore zeolite (Al)beta, but also isomorphously substituted extra-large pore zeolites UTL in Pechmann condensation of 1-naphthol with EAA and Prins reaction of β-pinene with PF, which seems to be caused by the much more higher concentration of accessible active sites within MOF and their appropriate strength.