Catalytic Activity of Mordenite from Natural Source in the Dimerization of α-methylstyrene

Studying the activity of any catalyst and selectivity toward special products is easy by selecting a module reaction, and choosing appropriatecondition. In our case the reaction of α- methylstyrene dimerization was a choice to study the activity of mordenite type zeolite crystallized from natural clay mineral (kaolin P-2), and also the selectivity toward dimers.The prepared catalyst in the form of hydrogen showed that the acceptable activity and selectivity to linear dimers were (72.5%), at 80 °C, and atmospheric pressure. Different products from the dimerization of α- methylstyrene over prepared catalyst was investigated, and the conversion rate reached to 73%wt.


1.
Introduction For decades chemicals manufacturing, petrochemicals, and oil industry are depended on the implementation of solid acid catalysts (heterogeneous), dominating commercial catalysis. [1] Mordenite (MOR) zeolite, establishes private position among the other kinds of zeolite due to its special properties, and thus MOR-zeolite has been applied as catalyst for significant reaction, such as isomerization, alkylation, cracking, etc… [2,3,4].
Generally powdered MOR-zeolite in their Na-form prepared using aqueous solution of Nasilicates and Na-aluminates [5]. MOR-zeolite exists naturally and it can be synthesized.In the development of the industrial process for the production of particular products, it is almost necessary to solve the problems of reducing its cost and simplifying technology for the synthesis, as well as taking into account the environmental consideration. In the production of zeolites, these mentioned problems are tried to solve, including the partial or complete replacement of synthetic sources of Al and Si, into natural aluminosilicates. One of the most suitable raw materials is kaolin [6,7].
Recently many synthesis procedures of powdered MOR-zeolite have been published.The synthesis is depending on raw materials as source of Al and Si from natural and synthetic sources [8][9][10][11][12].
Using natural clay minerals as an alternative source of raw materials (Si and Al) in the zeolite synthesis documented in [13].
Prior to use kaolin as source for Si and Al in the synthesis of MOR-zeolite, it requires applying high temperature (calcination at 600 C for 4h) to convert kaolin into amorphous metakaolin [14].  Dimers produced from alpha-MS (α-methylstyrene) dimerization reaction can be well manufactured over solid acid catalyst crystallized from zeolites. N. G. Grigor'eva et al. and V. P. Talzi et al. showed that the selectivity of linear dimers over Y-zeolite is 90-92% [15,16].
The products of alpha-MS dimerization resulting in the formation of linear, cyclic dimers, and trimers, are described in [17].
It is familiar that using clay minerals as a source material of crystallization of zeolite in the alpha-MS dimerization reaction differfrom other kinds of catalysts. Therefore we excited to study the activity of MOR-zeolite obtained from Prosyanovsk kaolin P-2 in the dimerization of alpha-MS as module reaction.

Chemicals
Commercial clay mineral Prosyanovsk kaolin abbreviated as P-2 used as source of Si and Al to prepare MOR type zeolite. The chemical composition of P-2 is given in table 1. Ufaorgsyntes Ufa, Russia was provided the alpha-MS (α-methylstyrene), and prior to use alpha-MS subjected to distillation.

Synthesis of MOR type zeolite
MOR type zeolite from kaolin P-2 (P-2 converted to metakaolin prior to synthesis MOR) and MOR from aqueous solution of Na-aluminate and Na-silicate were prepared in their Na-forms as described in [18].

Synthesis of Solid Acid Catalysts
The both samples; MOR-zeolite from kaolin P-2 and from aqueous solution of silicate and aluminate, converted to NH 4 -form of zeolite using ion exchange of Na + ion by NH 4 + ion in aqueous solution of NH 4 NO 3 . Following the exchange of Na + ion by NH 4 + ion, samples were thermally treated at 540 o C for four hours to synthesis of the solid acid catalysts 0.96 HNaMOR from kaolin P-2, and 0.94 HNaMOR from aqueous solution of aluminate and silicate.

Evaluation of Solid Acid Catalysts Activity
The activity of 0.96 HNaMOR, and 0.94 HNaMOR catalysts in the dimerization reaction of alpha-MS was performed. Before testing catalysts, both samples were thermally treated at 300-350 o C for three hours under inert atmosphere using argon.
Three-neck flask used as reactor provided with mechanical stirrer. Chlorobenzene used as solvent. 0.96 HNaMOR, and 0.94 HNaMOR catalysts activity was determined as a function of alpha-MS conversion(K), and the dimerization products selectivity.

Solid Acid Catalysts and Products Characterization
Different techniques were applied to characterize the zeolite samples and dimerization products. Zeolites and 0.96 HNaMOR, and 0.94 HNaMOR catalysts well characterized by x-ray diffraction technique, and classic temperature programmed desorption of NH 3 system (TPD-NH 3  adsorption capacity of the solid catalysts was measured by the vapours of C 7 H 16 , C 6 H 6 , and H 2 O at 20 o C and P/Ps = 0.7 -0.8 using the desiccator method which described in [19]. The analysis of products from alpha-MS dimerization reaction were performed by (Tsvet) chromatograph with a flame-ionization detector, a (2mx2mm) column packed with 5% SE-30-coated Chromaton HMDS, in the style of temperature programming from 50 to 280 ° C at a rate of (8 K/min), and detectors and evaporator temperatures of 250 and 300° C, respectively, using He as a carrier gas at a flow rate of 30 ml/min.
Collected data on catalytic activity in dimerization reaction and physicochemical properties of zeolite samples and catalyst from kaolin P-2 were compared with conventional MOR zeolite catalyst which crystalized from aqueous solution of Na-silicate and Na-aluminate.

Characterizations of the Catalysts
Acidic properties obtained by TPD-NH 3 and thermal stability distinguish MOR zeolite from other types of zeolite. The catalysts used in this work were 0.96 HNaMOR zeolite and 0.94 HNaMOR zeolite, and the acidity for both samples is high enough to serve as acid catalyst as shown in table 2. Solid acid catalyst 0.96 HNaMOR contains less (strong) acidic sites than 0.94 HNaMOR. It is clear from the data listed in table 2 of adsorption capacity by water, benzene, and nheptane vapours that the 0.96 HNaMOR catalyst has higher adsorption capacity of benzene, and nheptane vapours than the 0.94 HNaMOR catalyst. We suppose that, during crystallization process of mordenite type zeolite from kaolin two or more crystalline particles are joined together to form a structure in addition to surface of zeolite that use the internal space between these particles to uptake more molecules of heptane and benzene.
The condition of crystallization and chemical composition of prepared zeolite from the reaction mixture (3Na 2 O.Al 2 O 3 .12.SiO 2 .300H 2 O) are given in table 3.

Evaluation the Catalytic Activity of Catalysts
The results that we have in this study of the activity of zeolite samples and prepared catalysts in the dimerization reaction of alpha-MS are illustrated in table 4. This Table shows that, there are no βlinear dimers in the products of dimerization in the presence of Na-form zeolite from aqueous solution and 0.94 HNaMOR catalyst, but the selectivity of MOR zeolite from kaolin toward β-linear dimers is 0.6%. This allows us to suppose that, the low amount of TiO 2 , Fe 2 O 3 and CaO in raw kaolin used in crystallization is promoting the forming β-linear dimers. The amount of linear dimers increased over catalysts from kaolin P-2 and aqueous solution as a source of Al and Si by 69.6% and 16.4% respectively. Thus, under the condition of dimerization reaction that we used, linear dimers dominate the portion. For both samples used in this study products of trimer species were investigated.
It is evidence that, from the results of dimerization reaction the selectivity of linear dimers rely on the type of material used for the zeolite crystallization. In addition the selectivity towards linear dimers increased after ion exchange of Na + ion by NH 4+ ion followed by heat treatment to get H-form of zeolite.
As stated by the adsorption capacity measured by water, benzene, and n-heptane vapours, and XRD-data as shown in figure 1, all zeolite samples prepared and used in this study have the crystallization degree close to 100%.   The alpha-MS conversion over 0.96 HNaMOR catalyst is increased by 64.6 wt% after ion exchange process, and by 44.5 wt. % over 0.94 HNaMOR catalyst. Thus, the conversion percentage of prepared catalyst from kaolin P-2 is better than conventional catalyst crystallized form aqueous solution of silicates and aluminates.
Since the purpose of this study was to observe the activity of MOR catalyst crystallized from kaolin, it was fixed that the catalyst prepared from kaolin P-2 is active in the dimerization reaction of alpha-MS.
This high activity registered, could be due to the concept of almost acidic sites of 0.96 HNaMOR catalyst are reachable by alpha-MS molecules and the products of dimerization can desorbed out easily from porous and the channels of prepared catalyst.
It is important to point out that, without using of solvent in the dimerization reaction of alpha-MS led to fast deactivation of 0.96 HNaMOR and 0.94 HNaMOR catalysts as mentioned in [18]. Therefore, the conversion of alpha-MS in dimerization reaction is preferred in the presence of solvent, in our case chlorobenzene. Besides to the dimerization reaction of alpha-MS over tested samples in chlorobenzene solvent, formation of oxidation products was also examined. Table 4.

Note.
The results on large pores MOR and 0.94 HNaMOR were published in [18], and provided in this work as reference for the comparison with MOR from kaolin and prepared catalysts.

Conclusion
Mordenite type zeolite exhibit unique properties with respect to both activity and selectivity. The dimerization reaction of alpha-MS over zeolite catalysts allowed us to investigate the activity and selectivity of 0.96 HNaMOR catalyst crystallized from kaolin followed by ion exchange process. In this study 0.96 HNaMOR catalyst converts alpha-MS to the products of dimerization more effectively than 0.94 HNaMOR which associated with the SiO 2 /Al 2 O 3 molar ratio and with the different content of acid sites.
A comparison of the catalytic activity, acidic characteristics, and adsorption capacity of the tested zeolite catalysts showed that, synthesized a promising catalyst for dimerization of alpha-MS based on the crystallization of clay mineral (kaolin P-2) in sodium silicate solution with the reaction mixture (3Na 2 O.Al 2 O 3 .12.SiO 2 .300H 2 O) and SiO 2 /Al 2 O 3 molar ratio equal to 10.1.