Abstract
LTA (4A) and NaX (13X) zeolite have been grown in the macrostructure of four ‘unmodified’ carbons (MAST, willow, pine and rattan) without blocking the meso/micropores of the substrate. A lack of modification reduces environmental/production costs, avoids weakening the substrate and prevents side reactions. The MAST/NaX composites can be rapidly regenerated in-situ by electrical heating using the resistivity of the carbon substrate. Calcination of the composites to remove the carbon produces a zeolite-only replica of the carbon substrate and (though fragile) the zeolite structures may be of use in their own right. Zeolite loading could be increased almost 200% (from 3.3% to 9.6% mass) by extending substrate time in the precursor solution, or further increased (to 28-44% MAST) by gravity deposition of the zeolite on the substrate. Extending the synthesis time may have produced a different zeolite inside the pores to that produced externally. SEM on a 5-week synthesised LTA sample suggests that the zeolite hydroxy sodalite (H-SOD) has formed inside the pores while LTA developed externally. Hence, growth rates were found to be different inside the carbon pores as crystal growth continued after it was arrested in the external solution. However, XRD and 29Si MAS-NMR analysis indicated that, in the first 14 h, NaX zeolite synthesis was slower inside the carbon pores than the external liquor. Acidity was expected to be important in the ability of a material to adsorb the basic molecule ammonia, so, to assess if this is indeed the case, LTA, carbon and composites were H+ exchanged using HCl before ammonia adsorption. Ammonia adsorption was not found here to be significantly increased by acid exchange. Indeed, Na+ LTA was found to adsorb NH3 just as extensively (6.6mmol/g) as H+LTA (6.8mmol/g) and adsorption in H+ LTA zeolite exchanged using the NH4+ LTA method (then heated to remove NH3) was decreased by 50%, and only 3.3mmol/g was adsorbed (on the 6.6mmol/g adsorbing LTA) after NH4+ exchange. The carbon substrate, however, performed much better when acid treated. Boehm titration results indicated that acidity ranges from 0.14mmol/g to 1.36mmol/g, even in non-acidified synthetic carbons, depended on the activation method performed (steam, CO2, etc.) and when acid treated the carbon NH3 adsorption increased 10 fold (from 0.3mmol/g to 3.3mmol/g). NaX was found to be the best zeolite for NH3 adsorption (5.1mmol/g for a 2 h outgas, 9.7mmol/g for a 4 h outgas and 18.6mmol/g for a 10 h+ outgas), if properly pre-conditioned. Such intense preconditioning was found to be less important when the zeolite was contained in the carbon pores and a composite of MAST/NaX (containing 22% NaX) adsorbed 3.7mmol/g NH3. Therefore, the NaX zeolite adsorbed 16.8mmol/g when supported on a carbon substrate after a 3 h outgas compared to 18.6mmol/g unsupported NaX after a 10 h outgas. Dynamic MS-RGA TPD of NH3 highlighted interesting water/ammonia/adsorbent kinetics and NH3 desorbs in a series of pulses from very humid zeolite/composites. No advantage to using the templating agent TMAH was found, during carbon/zeolite composite characterization. However, use of a templating agent as a precursor in the zeolite synthesis solution has been shown to have advantages when adsorbing certain molecules (such as HFC-134a) and disadvantages when adsorbing others (such as NH3). NaX(TMAH) in the macropores of MAST carbon was 35% more effective at adsorbing HFC-134a than unsupported (and without TMAH). [NaX (without TMAH) = 1195umol/g (Ea 140kJ/mol; Tmac348K), MAST/NaX (with TMAH) = 1632μmol/g(NaX) (Ea 100kJ/mol; Tmax358K)] (To verify this, unsupported NaX (with TMAH) was assessed and found to adsorb 1767μmol (HFC-134a)/g, no TPD or Ea has yet been performed on this sample). In the case of NH3, however, it is interesting to note that the MAST/NaX (with TMAH) composite, adsorbed NH3 ~40% as well as non templated (no TMAH precursor) NaX. /MAST composite at 7mmol/g(NaXTMAH) (Ea 50kJ/mol; Tmax368, 402K) and 17mmol/g(NaX) (Ea 62kJ/mol; Tmax 351, 480K) respectively.