The heterogeneous sulfunic acid that grafted outside of amino functionalized porous material (HY and MCM-41) have been characterized as follows:
3.1. FT-IR Analysis
FT-IR spectra of pure HY zeolite, HY-NH2, HY-N-SO3H and MCM-N-SO3H are presented in Fig.1.a-d. The FT-IR spectra of all samples indicate an intense band about ca.1022 cm-1 attributable to the asymmetric stretching of T–O–T (T= Si, Al) chain of porous materials. The symmetric stretching and bending frequency bands of T–O–T framework of porous materials appear at ca.791 and 463 cm-1, respectively . Fig.1a,c shows FT-IR spectrum of HY zeolite and MCM-41 and Fig.1.b,d shows spectrum of HY-N-SO3H, MCM-N-SO3H, the characteristic absorption correlated to S=O stretching appeared at 1255 cm-1, the stretching band and out of plane bending related to O-H group appeared at 3400-3500 and 797 cm-1, respectively which confirm the presence of -SO3H groups function on the porous materials .
3.2. XRD Analysis
The immobilized catalysts were also characterized by powder X-ray diffraction. The patterns for HY, HY-NH2 and HY-N-SO3H are shown in Fig.2.a-c. While the XRD analysis is not quantitative, the diffraction patterns after the binding of 3-APTES and also after the grafting of sulfunic acid are similar to that of the pure zeolite support. According to similar studies, in the present study no changes were detected in the crystalline structure of zeolite during the immobilization procedures, also the percentage of amorphous phase is small . Fig.3.a show the XRD pattern in the low angle ranges HY-N-SO3H, which there are not any reflectance, which confirmed the mesoporous phase, is not ordered in the HY-N-SO3H. In the case of MCM-41, Fig.3.b three (100), (110) and (200) lines related to hexagonal pore symmetry of MCM-41 can be observed which after functionalize with sulfunic acid the intensity of them decreased due to distortion that occurs on the channels of the mesoporous materials .
3.3. TG-DTA Analysis
To illustrate the thermal stability of HY-N-SO3H materials, TG-DTA analysis of these samples has been carried out under N2 flow. TG- DTA plots and DTG pattern for these materials are shown in Fig.4 TG curve of HY-N-SO3H revealed an initial weight loss of 7.509 % up to 200 ◦C due to the decomposition of physical and chemical adsorbed water in it. The second steps, show the complete loss weight of the organic species and hydroxyl groups (20.94 %), which observed in the temperature range of 250-450 ◦C. In the last step, the structure of zeolite is changed. On TGA curve of HY zeolite, there are two obvious peaks: a peak around 120 ◦C caused by the loss of zeolite-adsorbed water and a peak at 1100 ◦C, probably caused by self-dehydroxylation of –OH group on the surface of the zeolite[ref* 35new]. Moreover, the investigation of the DTA pattern show some small endothermic peak around 250-450 ◦C attributed to the dehydration of the hydroxyl groups inside the micro and meso phases [36,23] and a broad endothermic peaks related to amine and sulfunic acid decomposition. These results confirmed the immobilization of organic materials on the zeolite.
[ref* 35 new] Li, Z., Xie, K., & Slade, R. C. (2001). Studies of the interaction between CuCl and HY zeolite for preparing heterogeneous CuI catalyst. Applied Catalysis A: General, 209(1-2), 107-115.
3.4. FESEM, EDS, TEM, BET and Elemental Analysis
In Fig.5.a,b we have seen the FESEM micrographs of heterogenized HY and sulfunic acid (HY-N-SO3H) sample with a wide range of shapes and sizes of particles. The presence of well-defined zeolite crystals with some shadow indicated presence of organic group on the external surface. The particle sizes for HY are around 500 nm. The particle sizes for HY-N-SO3H are about 150-200 nm that the decrease of size for modified zeolite is due to the complex formation on the support . Also, in Fig.5.c the shows TEM some worm like changing in the surface of zeolite which may be due to disorder meso structure in the surface of HY-N-SO3H zeolite .
The presence of C, O, Al, Si, S and N atoms was observed in the EDX spectrum Fig.5.d. It is observed that the peaks corresponding to Si, Al and O were contributed by aluminosilicates present in the sample. Remarkably, the results Elemental Analysis show the ratio ~4 for Si/Al, which confirms the present of HY zeolite without any changes in its structure and the result of CHN confirm the C/N ratio about 3 for HY-N-SO3H.
Nitrogen absorption-desorption and BJH plots show in the Fig.6.a,b. Noteworthy, in the case of pure HY zeolite which based on IUPAC data Nitrogen absorption-desorption show the I type curve which confirmed the microporous structure. HY-N-SO3H with the specific surface areas and pore volume of the catalysts that were estimated by nitrogen adsorption at relative pressures (P/P0) in the range of 0.008-0.35 (microporous) and 0.35-0.9 with a hysteresis related to mesoporous show in Table 1 and Fig.6.b confirmed the preset micro-meso structure for catalyst .
In comparison of HY zeolite with HY-N-SO3H materials exhibited decrease in specific surface area and nano porous volumes after functionalization amine and sulfunic acid (Table 1). Since the zeolite framework, structure is not affected by functionalization as shown by the XRD pattern, the reduction of surface area and pore volume provides direct evidence for the presence of organic materials in the surface and cavities .
pH-analysis of HY-N-SO3H to an aqueous solution of NaCl (1M, 25 ml) with initial pH 5.93, 500 mg of HY-N-SO3H was added and mixture stirred for 3h after which the pH of solution decreased to 1.74. This is equal to a loading of 0.9 mmol SO3H obtained back-titration analysis of the catalyst.
- Catalytic studies
4.1. Preparation of coumarins via pechmann condensations
Here, we examined the catalytic activity of HY-N-SO3H and comprised with NaY-N-SO3H and MCM-N-SO3H as catalyst for the synthesis of coumarin compounds under pechmann condensation. At first, we selected Resorcinol as the primery substance of reaction mixed with ethyl acetoacetate in presence different amounts of synthesized catalyst (0.02-0.1 g) at 110 ˚C to investigated the effect of catalyst amount on percentage conversion (Table 2, entry 1-4). The result show that the 0.08 g of catalyst is the best amount of catalyst (entry 3) which use to prepared of coumarin. The results in Table 2 show that the yield of reaction is trace without using of catalyst for two derivative (entry 17). To consider the effect of solvents on the preparation of coumarin with functionalized HY zeolite with sulfunic acid, in the optimum conditions some usual solvents were used (entries 6-9). The result show that with use of these solvents comparing to the solvent free condition the yield of reaction decreased. It seems the interaction between these polar solvents and oxygen groups in the catalyst sites to inhibit the formation coumarin. The creating solvent free conditions for organic reaction are the advantageous in this work, which avoids the use of environmental hazardous and toxic solvents that are importance problems for green chemistry. Literature review show that there are not definite mechanism but based on other works a suggested mechanism shows in Scheme 2 to the formation of coumarin. In the first, acidic catalyst activated the hydroxyl of phenolic groups and protonated ethyl acetoacetate and phenolic groups and then resorcinol making nucleophilic attack to the carbonyl group. In the next steps, between reactants, transesterification was occurred and proton absorbed to catalyst again. Finally, EtOH and H2O removing and coumarin derivatives were synthesized.
4.2. Formylation of amines
At first, aniline has been selected as the model reaction and all reaction conditions have been studied and optimized which results summarized in Table 3. The result show that without using of catalyst, no creating any product (Table 3, entry 9). Therefore, it seems that the acidity of the catalyst is effective for this reaction. As obtained, best yields and reaction times was obtained using 10 mol % (0.02 g) of catalyst in the 9 min at solvent free condition (entry 3). Literature reviews show that solvent plays an important role in catalytic behavior because it can make different phases uniform, thus promoting mass transportation which could also change the reaction mechanism by affecting the intermediate species, the surface properties of catalysts and reaction pathways [39 41]. The effect of solvents were considered in the (entries 6, 7 and 8), results show that the time of reaction was reduced. As we known, one of the most importance problems for green chemistry are the creating solvent free conditions for organic reaction, which avoids the use of environmental hazardous and toxic solvents. Then, we have continued our work in solvent free condition.
We can see the proposed mechanism in Scheme 3 which based on this proposed mechanism acidity of the catalyst is effective for this reaction because help to protonated hydroxyl group and creating a positive carbon which suitable to attack of amine group and lead to formylation reactions.
Therefore, in this optimized condition, several aniline derivatives have been N-formylated to corresponding formamides in good yields and short reaction times (Table 4). In comparison of different substituents reveals that electron releasing groups such as Methyl (entry 2) accelerate the reaction and lead to shorter reaction times, on the other hands, electron with drawing groups such as Cl and NO2 (entries 3-7) whereas lower the rates and lead to longer reaction times. In addition, orthophenylendiamine has been formylated in both amine groups (entry 8).
To consider the effect of acidity and type of porous size in the catalyst, we compared the catalytic activity of HY-N-SO3H with pure NaY, HY, MCM-41, NaY-N-SO3H and MCM-N-SO3H (Table 2 entries 10-16 and Table 3 entries 10-14). The result show that the acidity of zeolite can be effective on the two set reaction (Table 2 and 3 entries 10,11) but the increasing the size of porous alone doesn’t any significant effect on the formylation of amine and synthesis of coumarine (Table 2 and 3 entries 12). In the other hand with functionalize sulfunic acid on the MCM-41 the acid site increasing which caused the increasing of yield (Table 2 entries 15,16 and Table 3 entry 14). Finally, in the case of HY-N-SO3H, the both acidity of zeolite and sulfunic acid can be useful for organic reactions. It seems functionalized of HY zeolite by sulfunic acid with a micro-meso structure have bigger porous size and higher stability respect to HY and MCM-41 which suitable for entering bulk molecule and increased the yield and lifetime and reusibilty of the catalyst [40 42 44]. 42,43 in table
- Recyclability and comprising with other works
To investigate the catalytic efficiency of the recycled catalyst, successive cycles of the model reaction were run under the optimal reaction conditions using recycled HY-N-SO3H from the previous run for synthesis of coumarines and formamides (Fig.7). A few change in the yield after each cycle can be due to micro-mesopore structure catalyst that let to catalyst release of products. In Fig.8, we investigated the FT-IR and XRD of catalyst after final run was comprised with first run the catalyst was easily recovered by simple filtration, adequately washed with ethanol, dried and was recycled in the model reaction for two set catalysts. The result show that there isn’t any change in catalyst and could be reused without any significant loss of activity up to six cycles for N-formylation and pechmann condensation.
The catalytic activity of HY-N-SO3H with micro-meso structure comparised with other reported heterogeneous catalysts for combination of resorcinol and ethyl acetoacetate to preparing coumarin shows in Table 5.
It is clearly showed from these comparison table that this catalyst display a higher activity and loading under the same reaction condition. Remarkably, the slight amount of catalyst, excellent yield, short reaction time, mild, ecofriendly and solvent free condition lead to excell other works.
In final, this protocol for formylation of aniline has been compared with some other reported methods in the Table 6. Although these methods are useful or efficient but most of them need to elevated temperature, toxic reagents, long reaction time and large amount of catalyst. Finally they also obtain pour efficiency and side hazardous products. As this table, this methodology improved yields, reaction times or reaction conditions. It seems that both sulfate groups as an acidic agent and HY zeolite catalyze this reaction together and intensify each other’s effects.
Then, the sulfunic acid immobilize on the HY zeolite create low cost heterogeneous active catalyst under mild condition for organic reactions and indicating cooperative effect of zeolite support. It seems ,micro-meso structure of synthesized catalyst was provide or was obtained bigger acidic porous size and acidity for entering organic molecule which increased the yield and lifetime of the catalyst.