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Alkylation of deactivated aliphatic compounds on zeolites.

Introduction

Zeolites are crystalline aluminosilicates with ordered micropores (dmicro < 2 nm) which are applied to adsorption, catalytic and ion exchange properties. However, the sole presence of micropores in these materials often confines intracrystalline diffusion, resulting in low utilization of the zeolite active volume in catalyzed reactions. The pore width of mesopores (2 nm <dmeso < 50 nm) are bigger than micropores, thus they induce new properties and reveal unexpected applications. The major drawbacks in most industrial reactions are the low thermal, hydrothermal stability and weak acidity due to their amorphous pore wall. Therefore, micro/mesoporous materials which combine microporosity of zeolite units with mesoporous of amorphous phases have been received much attention due to their potential applications in recent years [1-3]. Shape-selective catalyses by zeolites occur by differentiating reactants, products, and/or reaction intermediates according to their shape and size in sterically restricted environment of the zeolite. Only molecules of which dimensions are less than the pore size of the zeolite can enter their channel, and react at internal catalytic sites. Bulky molecules are excluded from the channels, resulting in the formation of the slim isomers, and only molecules which can diffuse through the channels will appear in the products. On the other hand, if the spaces of zeolite channels are large enough to accommodate the reactant and/or the products, or if the reactants and the products are smaller than the spaces inside the channels, the reaction is controlled kinetically due to the reactivity of each position at lower temperatures, and to the thermodynamic stability of the products at higher temperatures [4].

Beers [5] reported on the acylation of anisole with octanoic acid using zeolite H-BEA. The catalytic activity appeared to depend on the mesoporosity and the Si/Al ratio of the zeolite. Derouane [6,7] studied adsorption, reaction, and deactivation of zeolite H-BEA in the acetylation of anisole with acetic acid as the acylating agent. Deactivation of the catalyst occurred due to both product inhibition by p-methoxyacetophenone and dealumination by acetic acid formed during the reaction. These authors concluded that competitive adsorption effects could play a major role when applying zeolites to fine chemical synthesis. In certain cases, very strong adsorption of the reaction products on the active sites can even lead to catalyst deactivation [8]. Such competitive adsorption effects can be expected in Friedel-Crafts-type alkylation reactions with zeolites, because molecules of completely different physiochemical nature are involved (i.e., aromatic substrate and alkylating agents such as alcohols, esters, anhydrides, etc.). Therefore, the aim of this work is to demonstrate our idea for modification of ZSM-5 zeolite, and to carry out two typical catalytic reactions, toluene methylation with methanol, isopropylation and 2-propanol.

Experimental:

Preparation of modified ZSM-5 zeolite

The ZSM-5 zeolite samples with Si/Al ratios of 60-170 were synthesized using tetrapropylammonium bromide (TPABr) as a template. N-cetylpyridinium bromide (>0.98 mass fraction), a cationic surfactant from Merck with a concentration of 20 mmol [dm.sup.-3] was used for modifying the external surface of the zeolites. Due to the relatively low solubility of CPB in water, a water-ethanol mixture of 8:2 (w/w) was used as solvent. Briefly each batch of zeolite (1.0 g) was added to a vial (150 ml) and 100 ml of CPB solution (20 mmol [dm.sup.-3]; higher than cmc) was poured into the vial. Then, the sample was placed on a shaker at room temperature for 48 h. The amount of the surfactant adsorbed on the zeolite surface was determined by difference of initial concentration versus the concentration of surfactant in the filtrate solution [9, 10].

Preparation of unmodified [H.sub.3]P[O.sub.4]/ZSM-5 zeolite:

A series of ZSM-5 zeolites (not treated with surfactant) with varying [H.sub.3]P[O.sub.4] content were prepared by impregnating calculated amounts of [H.sub.3]P[O.sub.4] dissolved in deionized water on unmodified ZSM-5 zeolites. The impregnated catalysts dried at 383 K for 12 h and then calcined in air at 873 K for 4 h. All the impregnated catalysts pressed into wafers, crushed and sieved to 20-40 mesh before use.

Results and Discussion

Toluene alkylation with methanol:

Alkylation of toluene with methanol produces primarily a mixture of xylenes the distribution of which in the product largely depends on the characteristics of the catalyst. Side reactions might also take place, beside alkylation, namely, disproportionation.

Effect of phosphorous content on the product distribution:

The activity of the unloaded zeolite is lower than the activities of all [H.sub.3]P[O.sub.4] loaded catalysts between 623 and 773 K. Hence, [H.sub.3]P[O.sub.4] incorporation in the zeolite has a significant role for activating the catalysts during the methylation reaction. Table 1 shows the changes in the total toluene conversion, and the yield of ortho, Meta and para-xylenes on the unmodified catalysts as a function of phosphorous content. Data of this table shows that the unloaded zeolite is significantly less active for xylenes production. Interestingly no trimethylbenzenes (TMBs) were detected in our products. It should be mentioned that a small amounts of ethylbenzene was also detected.

Fig. 1 (left), shows the deactivation of the modified catalyst in a period of 8 h. In compare with the deactivation of the unmodified catalyst, it can be seen that more deactivation occurred on the modified catalyst. Fig. 2 (left), shows the deactivation of the unmodified catalyst. It can be seen from Figs. 1 and 2 that the 2.1 wt.% P/ZSM-5-surf-170 shows high selectivity toward para-xylene after 8 h time-on stream (over 91% after 8 h), and also there is no significant decrease in paraxylene selectivity. It should be pointed out that the catalysts can be regenerated in a conventional manner to remove the coke deposits and restore its activity.

Effect of toluene to methanol molar ratio on the catalytic activity:

The effect of the amount of toluene on the methylation over modified 2.1 wt.% P-ZSM-5-surf170 is presented in Fig. 3. The conversion of toluene increased with decreasing the amount of toluene in the feed, from toluene: methanol molar ratio of 8:1 to 1:2, and benzene (from disproportionation of toluene and/or toluene dealkylation) decreased from 7.8% to 0.0. When the toluene: methanol molar ratio of 1:2 was employed, the apparent conversion of toluene reached to its maximum value of 43%. This conversion might be marginal; however the conversion of toluene under an ample supply of methanol would decrease, because of the stronger tendencies of the surface of the catalyst for adsorption of methanol.

Effect of the Si/Al ratio in the modified and unmodified catalysts on toluene conversion and pxylene selectivity:

A total of four ZSM-5 zeolites (Si/Al = 60, 80, 120, 170) are used, respectively, to support 2.1 wt.% P for the toluene methylation at 698 K. Tables 2 and 3 show the toluene conversion and xylenes selectivity's obtained for the unmodified and modified catalysts, respectively. For the unmodified catalyst, the toluene conversion and p-xylene selectivity increases by increasing the Si/Al ratio, as can be seen in the Table 4. Modified catalyst shows the same trend in toluene conversion but there is a big increase in p-xylene selectivity from 60.3% which reaches to maximum value of 100% in compare with unmodified catalyst. These results are in support of our previous study [40] and we decided to use the 2.1 wt.% P-ZSM-5-surf-170 for our detailed studies.

Isopropylation of toluene with 2-propanol:

Cymene (methylisopropylbenzene) production is commercially carried out by alkylation of toluene with propene. The alkylation produces a mixture of cymene isomers (i.e. ortho, meta and para). The most preferred isomer distribution requires low orthocymene content, since ortho-cymene is difficult to oxidize and inhibits the oxidation of the other isomers. In the present study the alkylation of toluene with isopropanol to produce cymenes over ZSM-5 zeolite has been carried out on two series of H3PO4 loaded ZSM-5 zeolite catalysts, same as the catalysts which were prepared and used for toluene methylation and the results are presented in the following sections.

Effect of reaction temperature on toluene conversion and p-cymene selectivity:

In order to achieve better toluene conversion and maximum p-cymene selectivity, we carried out toluene isopropylation in the temperature range of 483-583 K in steps of 25 K on the modified catalyst. The product distribution is presented in Table 4. All the reactions were carried out at optimized feed ratio toluene:2-propanol (1:4) and WHSV (0.8 [h.sup.-1]). This clearly depicts that the toluene conversion increases up to 533 K. Maximum p-cymene selectivity reached to 94% in 533 K and it did not go up to 100% like toluene methylation reaction over this catalyst.

Idea behind the ZSM-5 modification:

Sorption of cationic surfactants from solution onto solid surfaces has undergone extensive study in the past [11]. Sorption on solids with high surface charge density includes some clay minerals, mica, and zeolites. Several mechanisms, including ion exchange, ion pairing, acid-base interaction, polarization of P electrons, dispersion force, and hydrophobic bonding, were attributed to the sorption of cationic surfactants onto solid surfaces. Among them, ion exchange and hydrophobic bonding were the important ones [12]. The small units probably could penetrate into the pores and the long units has to deposit at the entrance of the pores and if we were lucky enough, these phosphate units do not land on the lipophilic sites loaded with the surfactant molecules. Therefore, if this happens, we should end up with a mass that has some phosphate units inside the pores of the zeolite and, if the phosphate units are being deposited around the mouths of the pores, they might block the entrances or in the best case these phosphate units may have decorated the entrances with a new architecture. The new question in this stage would be. The substitution of the Bronsted acidic hydroxyl groups by the [H.sub.2]P[O.sub.4] groups implies to different consequences on the strength and density of the Bronsted acid sites: (1) strong Bronsted acid sites are converted into weak Bronsted acid sites, and it is clear that the resulting terminal hydroxyl groups have a lower acid strength than the bridged hydroxyl groups. It should be mentioned that the weak Bronsted acid sites are possibly AlOH groups on extra-lattice regenerated by dealumination during calcination steps or SiOH groups of hydroxyl nests formed by the same process. (2) The number of (weak) acid sites increased. (3) This substitution should not change the polar character of the surface. Interesting data were obtained by studying the FTIR spectra of the untreated ZSM-5-170, H3PO4/ZSM-5surf-170 (2.1 wt.% P), and the unmodified H3PO4/ZSM-5-170 samples treated with different amounts of H3PO4 (0.7-3.5 wt.% P) which were measured with a diffuse reflectance type apparatus. We refer you to the IR measurements data presented in our previous article [40]. We have demonstrated that in the unmodified catalysts, [H.sub.3]P[O.sub.4] becomes distributed on the outer surface as well as inside the pores, but in the case of the modified ZSM-5, [H.sub.3]P[O.sub.4] distributed on the mouths of the pores with a new architecture, and probably inside the pores. It was summarized in our previous investigation [40] on Fries rearrangement that the external modification of ZSM-5 zeolite with surfactant and increasing the hydrophobicity of its outer surface has important effects on the shape selectivity of this industrially important zeolite. As we concluded in that work, in the unmodified catalysts, [H.sub.3]P[O.sub.4] becomes distributed on outer surface as well as inside the pores, but in the case of modified [H.sub.3]P[O.sub.4]/ZSM-5 catalysts, only the mouths of the pores and the internal pores of the zeolite should function as host sites for [H.sub.3]P[O.sub.4]. Thus, we would expect to observe different acid sites distribution, and different pore mouth's diameter for the modified and unmodified zeolites. The IR data supported this hypothesis because by excluding the phosphoric acid from reaching to the hidden external Bronsted acid sites, we observed the corresponding absorption band (3610 [cm.sup.-1]) in the modified samples in contrast with the unmodified samples. Hence, we should expect for the modified zeolites that there should be strong Bronsted acid sites on the external surface, but these modified zeolites have lost the strong Bronsted acid sites in the channels and at the entrance of the pores. Also, for the modified zeolites we expect to have more deposited aluminum phosphate because of dealumination, and more polymeric phosphate species in the channels.

Toluene methylation:

The toluene conversion on the H3PO4 loaded catalysts is found to follow the order of: 2.1 wt.% PZSM-5 > 3.5 wt.% PZSM-5 > 4.9 wt.% P-ZSM-5 > 0.7 wt.% P-ZSM-5 > ZSM-5.By accident the optimized phosphoric acid content is equal to that of our results in the Fries rearrangement [13]. Perhaps, by increasing the [H.sub.3]P[O.sub.4] loading from 2.1 to 4.9 wt.% P, the activity of the catalyst decreased, because of the covering the most active sites by blocking the pore mouth with higher concentrations of phosphoric acid. However, why the alkylation of toluene with alcohol should not takes place on the strong Bronsted acid sites? Probably, the alcohol molecules cluster around the strong Bronsted acid sites through hydrogen bonding and there will be an inappropriate environment for toluene as a hydrophobic molecule. Therefore, one should not observe the occurrence of the alkylation reaction on the external surface of the modified ZSM-5. On the other hand, impregnation of phosphoric acid on the zeolite restricted the pore diameter (channel) and favors the formation of less voluminous components. Less voluminous reaction products will diffuse faster to the outside of the pore, so that pxylene diffuses 1000 times faster than o- and m-xylene, and thus pxylene in the product might surpass the thermodynamic equilibrium values significantly [51]. Catalytic performance of modified and unmodified [H.sub.3]P[O.sub.4]/ ZSM-5 catalysts with time-on-stream is compared and the results are shown in Figs. 1 and 2. It can be seen from Figs. 1 and 2 that the 2.1 wt.% P/ZSM-5-surf-170 shows high selectivity toward pxylene even after 8 h time-on-stream (over 91% after 8 h). The relative selectivity's for xylenes were not affected by lowering the molar ratio of toluene: methanol from 1:2 to 1:4 and 1:8. In the toluene methylation with methanol over acidic zeolite catalysts, methanol is strongly adsorbed in competitive adsorption with toluene [14]. The relative selectivities for xylenes were practically constant under kinetic control with an ortho, meta, and para-xylene selectivity of 2:1:1 [15]. On the whole, the selectivity profiles illustrated in Fig. 3 are reasonable. The conversion of toluene increased with increasing amount of methanol. However, the maximum conversion of toluene was not more than 43%, even when a 8-fold excess amount of methanol was employed, which also demonstrates that the adsorption of toluene was weaker than that of methanol. It is generally accepted that the alkylation of toluene with methanol proceeds via the formation of methoxonium ion, which requires Bronsted acid sites [16]. Thus, the surface concentration of methoxonium ion, and therefore the catalytic activity for ring alkylation would depend on the density and strength of the Bronsted acid sites. One of the remarkable features of the selectivities shown in Table 4 is that demethylation of toluene increased significantly with decreasing the amount of methanol. The results of toluene WHSV are summarized in Table 6. It was found that the conversion decreases from 49% to 36% over 2.1 wt.% P-ZSM-5-170, when the WHSV increased from 0.5 to 2.32 h_1, the perceived low toluene conversion at higher space velocities could be accounted for in terms of the shorter contact time (faster diffusion) [17].

Conclusion:

We have demonstrated a unique process for modification of ZSM-5zeolite, which can enhance the selectivity of para-xylene, probably by deposition of phosphoric acid on the entrances of the pores and, inside of the pores of the zeolite. The 2.1 wt.% PZSM-5-surf-170 and 0.7 wt.% P-ZSM-5-surf-170 are suitable catalysts for alkylation of toluene with methanol and 2-propanol, respectively. The appropriate acidic sites, needed for catalyzing alkylation of toluene with methanol and 2-propanol, are supplied by loading 2.1 and 0.7 wt.% P on ZSM5 with optimum Si/Al ratio of 170. The external modification of catalyst with surfactant before [H.sub.3]P[O.sub.4] loading, led to passivation of the unselective Bronsted acid sites and to narrowing of the pores of the zeolite. Both effects resulted in enhanced shape selectivity of para-xylene in the toluene alkylation reaction. Therefore, this novel modification technique is a promising way to improve the selectivity of p-xylene and pcymene of products in toluene methylation and isopropylation industrially important reactions.

Acknowledgements

Thanks are due to the Research Council of Firozkouh branch, Islamic Azad University for supporting of this work.

References

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(1) F. Sheikholeslami Farahani, (2) 0. Moradi and F. Najafi

(1) Department of Chemistry, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran.

(2) Department of Chemistry, ShahreQods Branch, Islamic azad University, ShahreQods, Iran

(3) Department of Chemistry, Roudehen Branch, Islamic azad University, Roudehen, Iran

Corresponding Authors

F. Sheikholeslami Farahani, Department of Chemistry, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran.

E-mail: Sheikholeslami@iaufb.ac.ir

Table 1: Effect of the [H.sub.3]P[O.sub.4] loading on the catalytic
activity of the unmodified zeolite

P(wt.%)   p-Xylene   o-Xylene   m-Xylene   Ethylbenzene   Benzene
          (%)        (%)        (%)        (%)            (%)

0         46         27         18         <0.5           9
0.7       42         35         16         ND             7
2.1       30         48         21         <0.5           <0.5
3.5       32         49         18         ND             ND
4.9       37         39         18         ND             6

P(wt.%)   Toluene
          conversion (%)

0         14
0.7       21
2.1       46
3.5       40
4.9       35

Table 2: Effect of Si/Al ratio on the catalytic activity of
unmodified zeolites

Si/Al   p-Xylene   o-Xylene   m-Xylene   Ethylbenzene   Benzene
        (%)        (%)        (%)        (%)            (%)

60      21         59         18         ND             2
80      24         58         16         <0.5           1
120     27         52         19         <0.5           1
170     30         48         21         <0.5           1

Si/Al   Toluene
        conversion (%)

60      39
80      40
120     44
170     46

Table 3: Effect of Si/Al ratio on the catalytic activity of modified
zeolites

Si/Al   p-Xylene   o-Xylene   m-Xylene   Ethylbenzene   Benzene
        (%)        (%)        (%)        (%)            (%)

60      60         23         16         <0.5           1
80      69         17         13         ND             ND
120     96         2          1          <0.5           ND
170     100        ND         ND         ND             ND

Si/Al   Toluene
        conversion (%)

60      35
80      38
120     39
170     43

Table 4: Effect of the reaction temperature on the catalytic activity
of the modified zeolite

Temperature/K   p-Xylene   o-Xylene   m-Xylene   Ethylbenzene
                (%)        (%)        (%)        (%)

483             82         3          13         1
508             87         4          6          1
533             94         1          2          1
558             91         1          1          2
583             90         1          1          3

Temperature/K   Benzene   Toluene
                (%)       conversion (%)

483             1         48
508             2         62
533             2         75
558             4         59
583             5         52
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Title Annotation:Original Article
Author:Farahani, F. Sheikholeslami; Moradi, O.; Najafi, F.
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:7IRAN
Date:Apr 1, 2013
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