Abstract
The dwindling of global petroleum deposits and worsening environmental issues have triggered researchers to find an alternative energy such as biodiesel. Biodiesel can be produced via transesterification of vegetable oil or animal fat with alcohol in the presence of a catalyst. A heterogeneous catalyst at an economical price has been studied widely for biodiesel production. It was noted that various types of natural waste shell are a potential calcium resource for generation of bio-based CaO, with comparable chemical characteristics, that greatly enhance the transesterification activity. However, CaO catalyzed transesterification is limited in its stability and studies have shown deterioration of catalytic reactivity when the catalyst is reused for several cycles. For this reason, different approaches are reviewed in the present study, which focuses on modification of waste-shell derived CaO based catalyst with the aim of better transesterification reactivity and high reusability of the catalyst for biodiesel production. The catalyst stability and leaching profile of the modified waste shell derived CaO is discussed. In addition, a critical discussion of the structure, composition of the waste shell, mechanism of CaO catalyzed reaction, recent progress in biodiesel reactor systems and challenges in the industrial sector are also included in this review.
Highlights
The drastic depletion of fossil fuels and continuous anthropogenic greenhouse gas emissions have prompted the search for alternative renewable and sustainable fuels with efficiency similar to the conventional fuels being used today [1,2]
Natural waste shells are rich in calcium that is potentially convertible into Ca-based catalyst for biodiesel synthesis via transesterification/esterification pathways
The calcium is present in the form of calcium carbonate (95–99%), and further treatment is required in order to expose the calcium active phases for further reaction
Summary
The drastic depletion of fossil fuels and continuous anthropogenic greenhouse gas emissions have prompted the search for alternative renewable and sustainable fuels with efficiency similar to the conventional fuels being used today [1,2]. It was discovered that the waste shells are beneficial for the production of a wide range of value-added chemicals and products, such as 1,2,3-triazoles [44], cyclopentanone [45], 7,8-dihydro-4H-chromen-5(6H)-one [46], dimethyl carbonate [47], steam gasification [48], wastewater treatment [49], soil improvement [50], Portland cement replacement [51] and artificial stone [52]. We aim to discuss the remarkable progress of waste shell-derived CaO for biodiesel production from 2011–2020, besides a comparative study of transesterification activity summarized for conventional CaO, waste shell-derived CaO and modified wasteshell-derived CaO catalysts. The review will highlight the reaction mechanism of both CaO and modified CaO derived from waste shells for biodiesel production. The review summarizes the existing technology of reactor systems for advance biodiesel production
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