Mixed-Metal MOF-Derived Carbon Sponges for Oil Absorption
Authors: González Cesar M. Oliva , de Monserrat Navarro Tellez Ana , Kharisov Boris I. , Hernández Juan Marcos Guillén , Quezada Thelma E. Serrano , González Lucy T. , de la Fuente Idalia Gómez 
Published: February 2021

Aims:

In this work, we propose the implementation of three carbon sponges, generated from the
carbonization of melamine-formaldehyde sponges coated with different HKUST-type metal-organic frameworks (MOFs)
in different thermal conditions.

Background:

Nowadays, numerous investigations are focused on the development of new technologies for the rapid
separation of water/oil mixtures. Several these processes use hydrophobic materials of different nature for efficient oil
capture. Despite these efforts, the water/oil separation still remains a great challenge. The main oil absorbers that are
commercially available tend to be expensive and have complex synthesis; however, they usually have an acceptable
cost/benefit ratio. Despite this, the passage of time has brought us new generations of materials, which seek to solve the
problems in a more efficient way, as in the case of metal-organic frameworks (MOFs), which stand out for the great ease
with which their morphological and surface aspects can be controlled. MOFs are extensively investigated in the fields of
adsorption and catalysis; the MOF coated sponges do not meet the selectivity and stability standards to be applied in oil
spills in water. However, this completely changes when subjected to the pyrolysis process, giving the material an increase
in its surface area, hydrophobic and magnetic properties in addition to making the material suitable for this application.

Objective:

Creation of a low-cost 3D template and the study of morphological properties of MOFs, for the formation of
carbon-based materials by a fast, simple and low-cost method, promoting the use of new generations of materials to more
effectively solve persistent environments.

Methods:

The employed MOF precursors are trimesic acid (BTC), nickel and cobalt salts. The monometallic HKUST
type MOFs were synthesized using a simple method of controlled precipitation, which starts from two precursor solutions.
The first one consisted of a ligand solution, dissolving the BTC in deionized water. In the case of mixed-metal MOFs, they
were synthesized using the same procedure described for monometallic MOFs, but in this case a mixture of metal salts
with a 1:1 molar ratio was performed. The methodology for the production of the sponges decorated with MOF was
carried out in two steps. In the first stage, the sponges were subjected to a wash to remove dust and impurities, being
rinsed with acetone in an ultrasonic bath for 30 min. The sponges were subsequently immersed in deionized water and subjected to an ultrasonic bath for 10 min. Finally, the sponges were dried at 60°C for 3 h. The second step is the addition
of the HKUST-type MOFs to the sponges was carried out by means of the immersion method, preparing a dispersion of
the corresponding MOFs in ethanol.

Results:

It was revealed that the carbon sponges can selectively absorb the oil in the water/oil mixture, possessing
magnetic and enhanced hydrophobic and super hydrophobic properties. All the pyrolyzed carbon sponges, obtained at 500
and 700°C, were not the most optimal since they had absorption capacities of around 25 g/g and only supported up to 4
absorption cycles. On the other hand, the carbon sponges, obtained at 300°C, had absorption capacities greater than 40
g/g, in addition to being able to be reused up to 12 times without showing significant changes in their absorption capacity
and having acceptable hydrophobic characteristics for the removal of oil dispersed in water. Among the three sponges
obtained at 300°C, we highlight the sponges coated with BTC-Co which have the highest absorption capacity (54 g/g)
among all fabricated sponges.

Conclusions:

The sponges obtained in the present work are a promising alternative to the materials that are traditionally
used, since they have great advantages such as their simple production method, low-cost starting materials and good
absorption capacities. This work sheds light on the production of carbon materials from 3D templates decorated with
MOFs, through a one-step carbonization process and we demonstrate that these materials have characteristics that make
them applicable in the removal of oil dispersed in water, giving us a practical, economic and friendly alternative to the
environment.