|Acid/base catalytic conversion of polysaccharide-rich food waste
|Tsang, Daniel (CEE)
|Hong Kong Polytechnic University -- Dissertations
Organic wastes -- Recycling
|Department of Civil and Environmental Engineering
|xv, 202 pages : color illustrations
|Food waste has been a monumental issue in many metropolitan cities. Approximately 1.3 billion tonnes food waste is generated worldwide annually. In Hong Kong, over 3,600 tonnes food waste is produced daily and ends up in landfills, which accounts for one-third of the municipal solid waste. The pressing situation demands new technologies and outlets for food waste recycling. Rice, noodles, and bread are common food from our diet which are rich in starch, a polysaccharide consisting of glucose. Vegetable is rich in cellulose, another polysaccharide which is also consisting of glucose. The carbohydrate in polysaccharide-rich food waste will be a potential source for value-added chemical production to substitute petrochemicals. This study has demonstrated vegetable food waste can be converted to levulinic acid (LA, 17 C mol% yield) in aqueous environment with the aid of microwave-assisted heating and Amberlyst 36 at 150 °C in 5 min. Addition of dimethyl sulfoxide (DMSO) in the reaction media resulted in 17 C mol% of hydroxymethylfurfural (HMF) at 150 °C in 5 min. Dilute sulphuric acid has been compared with Amberlyst 36, and the swelling structure of cellulose in the presence of dilute acid was considered to enhance cellulose dissolution and beneficial to LA production. The established reaction system is therefore validated for vegetable waste conversion. Bread waste has been selectively converted to glucose (> 70 C mol%) at moderate temperatures (120 - 135 °C) by Bronsted acid catalyst. Further conversion to HMF or LA was hindered by glucose isomerization to fructose, which was the rate-limiting step in the reaction. Elevating the reaction temperature resulted in loss of products due to humin formation in acid catalysis. Therefore, glucose isomerization has been investigated via base catalysis to enhance overall conversion and carbon efficiency. Bronsted base solid catalyst derived from melamine and spent coffee grounds has presented the capacity of glucose isomerization, where 12% glucose and high selectivity of fructose (84%) was converted in 20 min at 120 °C. Co-solvent of acetone in the system promoted fructose yield to 19% due to the increase of overall basicity in the media. Investigation on catalyst nature and chemical environment in glucose isomerization via base catalysis revealed that in addition to the basicity in the solution, intramolecular hydrogen bonding in the catalyst, or the intermolecular hydrogen bonding between catalyst-solvent-substrate system also has great impact on glucose activity. The type of amines, availability of the basic sites, and the stereochemistry of amines may also affect the catalytic performance by displacement of equilibrium between glucose isomerization to fructose and the competing side reactions.
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