Transformation of South African coal fly ash into ZSM-5 zeolite and its application as an MTO catalyst

Abstract

This study presents a way of using South African coal fly ash by extracting metals such as Al and Fe with concentrated sulphuric acid, and then using the solid residue as a feedstock for the synthesis of ZSM-5 zeolite. The percentage of aluminium and iron oxides decreased from 28.0 ± 0.2% and 5.0 ± 0.1% in coal fly ash to 24.6 ± 0.1% and 1.6 ± 0.01% in the acid treated coal fly ash respectively. The fly ash-based zeolite ZSM-5 sample synthesised from the solid residue after extraction of Al and Fe, contained 62% of ZSM-5 zeolite pure phase with a number of Brønsted acid site density of 0.61 mmol per g zeolite . By properly treating the as-prepared coal fly ash-based ZSM-5 zeolite, an active and selective methanol-to-olefins acid catalyst could be designed, leading to full methanol conversion during 15 h on stream. The optimised catalyst exhibited a cumulative methanol conversion capacity of 71 g(MeOH converted )/g(catalyst) and a light olefin productivity of 21 g(C 2¼ eC 4¼ )/g(catalyst).

Conversion of South African Coal Fly Ash into High-Purity ZSM-5 Zeolite without Additional Source of Silica or Alumina and Its Application as a Methanol-to-Olefins Catalyst

“…Several routes to prepare various zeolite types have been suggested based on the pre-treatment of the fly ash, alkalinity of the hydrothermal solution, molarity of alkaline agents, solution/solid ratio, templating agent, time, temperature and time of incubation [2][3][4][5][6][7][8][9]. ZSM-5 is synthesized from a hydrothermal solution containing Al and Si sources, a mineralizing agent (OH − or F − ) and an organic structure-directing agent (OSDA) [10][11][12][13], with tetrapropylammonium cation (TPA + ) being the most-used OSDA [9,[14][15][16]. The OSDAs that give pure ZSM-5 increase as follows: alcohols < amines < tetrapropylammonium halide.…”

“…It has been reported that ZSM-5 zeolite can be synthesized from fly ash. In most cases, a source of silica needs to be added [9,13,21,22], which does not promote the use of fly ash as feedstock in the synthesis of ZSM-5 at large scale. However, Xing and Hongyan reported the synthesis of ZSM-5 from an alkaline supernatant of a pulverized fuel ash without additional source of silicon or aluminum [23].…”

“…However, Xing and Hongyan reported the synthesis of ZSM-5 from an alkaline supernatant of a pulverized fuel ash without additional source of silicon or aluminum [23]. Moreover, the presence of other mineral phases in the final ZSM-5 product hinders its catalytic effectiveness [13].…”

“…ZSM-5 has also been used to convert alcohols into hydrocarbons [12,19,26]. In South Africa, the methanol conversion to hydrocarbons can be exploited at an industrial scale and adds more value to coal and coal fly ash, as methanol can be synthesized from coal [27], and ZSM-5 can be produced from coal fly ash [9,13]. It was reported that the post-synthesis treatment of ZSM-5 with oxalic acid solution improved its catalytic efficiency [13,28].…”

“…In South Africa, the methanol conversion to hydrocarbons can be exploited at an industrial scale and adds more value to coal and coal fly ash, as methanol can be synthesized from coal [27], and ZSM-5 can be produced from coal fly ash [9,13]. It was reported that the post-synthesis treatment of ZSM-5 with oxalic acid solution improved its catalytic efficiency [13,28]. Several studies have been conducted using fusion of fly ash with sodium hydroxide as a pre-treatment step in order to dissolve Si and Al prior to the synthesis of high-purity zeolites [3,29].…”