**1. Introduction**

Volatile organic compounds (VOCs) are organic compounds with an appreciable vapour pressure at ambient temperature. They include naturally occurring and synthetic compounds and range in effect from harmless to toxic. Some VOCs have been shown to have malodorous, mutagenic or carcinogenic properties [1–3] and some have been implicated in causing air pollution, particularly in developing countries [4], and are partly responsible for the generation of photochemical ozone and smog precursors. They are thus considered as harmful pollutants [2,3]. Some industrial manufacturing processes, as well as the use of manufactured materials, can increase the emission of VOCs into the local environment [5,6]. As a consequence, the development of e ffective technologies to mitigate the emission of VOCs has received increasing attention [1]. Some reports have shown promising removal and recovery methods of VOCs from air and water through adsorption processes [7–9]. Solid adsorbents have been shown to be superior compared to other techniques of decontamination of air or water, owing to their relative low cost, wide range of applications, simplicity of design, easy operation, low harmful secondary products and the feasible regeneration of these solid adsorbents [10]. Traditional solids adsorbents such as zeolites and activated carbon (AC) can be used for sorption purposes but

they have shortcomings such as low surface area and the requirement of high temperature for their synthesis and regeneration [11,12]. Recent reports have shown that metal-organic frameworks (MOFs) have higher adsorption capacities and lower energy costs for regeneration [4,10,12].

Porous MOFs are crystalline frameworks with a wide range of possible configurations arising from the coordination of metal centres or clusters and organic linkers. MOFs can be designed to have high surface areas [10,13–15], easy functionalization, and tunable porosities, making them preferable to zeolites and activated carbon for many applications. Additionally, the coexistence of inorganic (hydrophilic) and organic (hydrophobic) moieties in MOFs structure may o ffer control of their interaction with gues<sup>t</sup> molecules [4]. Thus, MOFs are of interest for a wide range of applications such as gas sorption [4,7,16], storage [17], separation [18–21], and sensing [22–25]. The choice of organic linker is key to MOF properties. The most commonly used linkers are those that can coordinate to metal ions via oxygen or nitrogen donors. Prior studies in our laboratories [19] and elsewhere [26] have shown that combining carboxylate and pyridyl or triazole aromatic rings allows dynamic rotation between the aromatic rings which in turn generates flexible MOFs. This is a key feature for their selective sorption capacity [19,26]. The recognition of chemical information by an adsorbent MOF may be characterised by colour change known as chromism [27,28], or reversible change in structure size known as a breathing phenomenon [19,23]. The latter has been observed in both single ligand MOFs such as [Zn(34pba)2]n as well as in a mixed ligand MOF [Cd(34pba)(44pba)]n; where the channels react to stimuli caused by the temperature and size of the entering molecules such as alkyl alcohols, N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) [19,26]. However, it can be di fficult to characterise the sorbed product due to a loss of crystallinity after removal or inclusion of guests [27,29–31]. Furthermore, the selective sorption capacity for VOCs such as chlorinated solvents and amines are rarely investigated. In this paper we report the synthesis of three-dimensional isomorphous and isostructural MOFs from cobalt(II) and zinc(II) with two related ligands, 3-(4-pyridylbenzoate) (34pba) and 4-(4-pyridylbenzoate) (44pba). These non-interpenetrated frameworks retain the framework structure and crystallinity on activation under vacuum. Their sorption capacity for amines and chlorinated solvents was investigated, as was their relative selectivity for sorption of chlorinated VOCs.
