Synthesis and acidity study of Na-ZSM-5 zeolite

Бюллетень науки и практики / Bulletin of Science and Practice

UDC 544.47                                         

Different zeolite and zeotype frameworks are of special interest due to their superior sorption and catalytic properties [1-3]. ZSM-5 zeolite has general formula |Na x (H 2 O) 16 | [AlxSi 96-x O 192 ]-(x<27), it’s structure consist from straight channels connected to other parallel channels via the sinusoidal channels[2]. ZSM-5 zeolite synthesis using structure directing agents is known since 1970 [1, 4], however due to high price of structure directing agents these methods are purely applicable for industrial applications. Therefor a lot of scientific teams were working on development of easy and reliable methods for template free synthesis of ZSM-5 zeolite [5-10]. In this article a template free method of ZSM-5 synthesis was developed using local reagents as reactive precursors.

Materials and methods

For NaZSM-5 zeolite synthesis chemical grade sodium hydroxide, sodium aluminate, silica gel and Na-ZSM-5 HKC with purity not less than 99%were purchased from local supplier and used as is. Distillate water was purified using DE-25 aqua distillation system. Prior to synthesis silica gel was crashed in laboratory milling machine to obtained 10-100 μm particles fraction. For synthesis of first sample (Table) sodium hydroxide, sodium aluminate, silica gel, one gram of Na-ZSM-5 sample, and distillate water were placed in Parr instrument top bench 4566 autoclave reactor equipped with impeller mixer. Mixer rotation rate was set to 600 rpm and temperature was set to 1800C. Hydrothermal synthesis was provided for 24 hours, then autoclave was cooled to ambient temperature. In all other zeolite synthesis (Table 1) sodium hydroxide and silica gel and 150 ml of water was placed in autoclave at 600 rpm and 700C for one hour to form gel. Then sodium aluminate solution in 100 ml of water and one gram of Na-ZSM-5 HKC was added to gel and temperature was set to 1800C for 24 hours. Reaction mixture was placed in IEC HN-SII centrifuge and zeolite was separated from reaction solution. Zeolite was washed with distillate water three times and dried in laboratory drier at 1400C. Synthesized samples have theoretical Si/Al rate from 7.5 to 2000 (Table).

Table

REACTION CONDITIONS FOR AND NA-ZSM-5 SYNTHESIS

Ammonia chemosorption experiments were made in order to evaluate acidic properties of synthesized samples using AutoChem HP chemosorption analyzer. For ammonia desorption experiments synthesized samples were placed in quartz cuvette and placed in analyzer module. Where sample was heated in argon atmosphere up to 800℃ cooled down to 1500C flashed with mixture of 10 v.% ammonia in helium for one hour followed by flashing with pure helium for one hour. Afterwards sample was heated to 8000C with temperature gradient of 100C /min and ammonia desorption curve was recorded (Figure 1). Quantity of acid cites were calculated according to quantity of chemosorbed ammonia using preliminary made calibration curve.

Yield of synthesized zeolite was made by dividing of dried solid weight on theoretical weight of Na-ZSM-5 sample.

Results and discussions

After Na-ZSM-5 hydrothermal synthesis crystallite solid residue of zeolite was formed (Figure 1). Synthesis of Na-ZSM-5 sample from solid precursors (Sample 1, Table) compared to synthesis from preliminary made precursors solutions (Sample 2, Table) showed lower for 35% yield, beside quantity of surface acid cites decreased for 15-16%. Therefor synthesis of Na-ZSM-5 from liquid solutions of precursors is superior compared to synthesis from solids with solutions formation directly in reaction autoclave.

Increasing of Si/Al ratio from 7.5 to 100 results in solid yield decrease from 97% to 41% and increase of surface acidity from 0.066 to 0.265 mmol/g (Table) that can be explained by optimal ratio of used precursors. Further increasing of Si/Al ratio up to 2000 lead to solid yield stabilization, however drastic decrease of surface acidity down to 0.002 mmol/g can be noticed, that can be explained by alumina ions lack in reaction solution.

Repeated synthesis of Na-ZSM-5 samples 8-1 and 8-2 showed high dispersion in solid yield and surface acidity that can be explained by different rates of zeolite species nucleation.

Figure 1. Samples of dried Na-ZSM-5

Time of hydrothermal synthesis was 24 hours under 180 ℃, one g of Na-ZSM-5 was added to reaction mixture before synthesis.

Ammonia desorption curves of Na-ZSM-5 sample 1, table 1 synthesized using solid precursor showed formation of broad peak from 2000C to 5000C that can be subscribed to weak and strong Bronsted acid sites (Figure 2). Sample 2, table 1 synthesized from precursor solutions showed presence of two partly imposed peaks, first in 200–350 C region and second one in 350–500 C. The increase of Si/Al ratio from 7.5 up to 100 results in appropriate increase of chemosorbed ammonia peak, broad peak shape remains practically the same. Further increase of Si/Al ratio up to 2000 results formation of small peak in 150–200 C that can be subscribed to physiosorbed ammonia in zeolite micropores.

Conclusions

Na-ZSM-5 synthesis using preliminary produced solutions of precursors was found to be more superior compared to direct solid precursors dissolution in reaction autoclave. Increasing of

Si/Al ratio from 7.5 up to 100 results in appropriate increase of surface acidity from 0.066 mmol (NH 3 )/g(ZSM-5) up to 0.265 mmol(NH 3 )/g(ZSM-5). Further increase of Si/Al ratio up to 2000 results in appropriate decrease of surface acidity down to 0.002-0.005 mmol (NH 3 )/g(ZSM-5). Reliable and easy method Na-ZSM-5 synthesis was designed providing zeolite yield up to 97%.

Figure 2. Ammonia chemosorption curves for synthesized Na-ZSM-5 samples

Acknowledgments

The reported study was jointly funded by Russian Foundation for Basic Research (RFBR), Sirius University of Science and Technology, JSC Russian Railways and Educational Fund “Talent and success”, project number 20-38-51001.