**1. Introduction**

With the depleting supplies of fossil fuel and increasing environmental problems, the catalytic conversion of biomass to fuel and chemicals has been gaining grea<sup>t</sup> attention [1–3]. Compared with other biomass, lignocellulose, which is derived from agricultural waste and forest residues, is much cheaper and more abundant. Therefore, the synthesis of fuel and chemicals with lignocellulose-derived platform molecules has been a research hotspot [4–8].

Cyclopentanone is a promising lignocellulose-derived platform molecule in the conversion of biomass to fuels and chemicals. It can be produced via aqueous-phase selective hydrogenation of furfural derived from hemicellulose [9,10]. Cyclopentanone can undergo a self-aldol condensation pathway, and as-obtained dimer can be used as either high-density fuel [11,12] or perfume precursors [13]. Generally, this reaction was catalyzed by solid base or acid catalysts, for example: commercial CaO, hydrotalcites [11], MOF-encapsulating phosphotungstic acid [14] or MgO-based catalysts [15] (as shown in Scheme 1). However, all of these catalysts need to be prepared by multiple steps or purchased additionally. From the point of view of green chemistry and economic cost, it is still expected to develop bio-based and cost-effective catalysts with high activity for cyclopentanone condensation.

**Scheme 1.** Reaction pathway of cyclopentanone aldol-condensation.

Waste seashells are one of the major food residues in China, especially in the southeastern coastal areas. The annual production of seafood in China was 35 million tons in 2017. Among these, the production of mollusks was over 12.7 million tons, which accounts for over 36% [16]. Currently, the shells of these mollusks are directly discarded. Thus, using these waste seashells as raw materials for catalyst preparation can not only minimize food residues, but also synthetize cost-e ffective catalysts. The major constituent of seashells (e.g., clams, conches) is CaCO3, which can be transformed to CaO by direct calcination at appropriate temperatures. In previous work, these calcined waste-shell derived catalysts were mainly employed to produce biodiesel from transesterification of soybean oil [17], palm olein oil [18], etc. In addition, they were also used as pretreatment materials for kraft lignin pyrolysis [19]. However, there is no report about using calcined waste shells as aldol condensation catalysts of bio-derived ketones.

The aim of this paper is to demonstrate the usage of waste seashells as a bio-based and cost-e ffective catalyst for cyclopentanone condensation with a high activity. Multiple characterizations were also investigated to illuminate the reason for activity di fferences.
