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Effective Recovery of Vanadium from Oil Refinery Waste into Vanadium-Based Metal–Organic Frameworks Guowu Zhan, Wei Cheng Ng, Wenlin Yvonne Lin, Shin Nuo Koh, and Chi-Hwa Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04989 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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Environmental Science & Technology

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Effective Recovery of Vanadium from Oil Refinery

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Waste into Vanadium-Based Metal–Organic

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Frameworks

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Guowu Zhan,† Wei Cheng Ng,‡ Wenlin Yvonne Lin,† Shin Nuo Koh,§ and Chi-Hwa Wang*†

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Department of Chemical and Biomolecular Engineering, National University of Singapore, 4

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Engineering Drive 4, 117585, Singapore ‡

NUS Environmental Research Institute, National University of Singapore, 1 Create Way, Create

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Tower #15-02, 138602, Singapore §

Sembcorp Industries Ltd., 30 Hill Street #05-04, 179360, Singapore

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Submitted to

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*Corresponding author, Tel.: +65-65165079, Fax: +65-67791936, E-mail address:

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[email protected]

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ACS Paragon Plus Environment

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Graphical TOC Entry

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ABSTRACT. Carbon black waste, an oil refinery waste, contains a high concentration of

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vanadium (V) leftover from the processing of crude oil. For the sake of environmental

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sustainability, it is therefore of interest to recover the vanadium as useful products instead of

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disposing of it. In this work, V was recovered in the form of vanadium-based metal–organic

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frameworks (V-MOFs) via a novel pathway by using the leaching solution of carbon black waste

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instead of commercially available vanadium chemicals. Two different types of V-MOFs with

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high levels of crystallinity and phase purity were fabricated in very high yields (>98%) based on

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a coordination modulation method. The V-MOFs exhibited well-defined and controlled shapes

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such as nanofibers (length: >10 µm) and nanorods (length: ~270 nm). Furthermore, the V-MOFs

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showed high catalytic activities for the oxidation of benzyl alcohol to benzaldehyde, indicating

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the strong potential of the waste-derived V-MOFs in catalysis applications. Overall, our work

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offers a green synthesis pathway for the preparation of V-MOFs by using heavy metals of

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industrial waste as the metal source.

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KEYWORDS. Metal–organic frameworks, vanadium recovery, waste, catalysts, oil refinery.

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■ Introduction

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With the rapid growth of world population, the amount of resources consumed is increasing,

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and inevitably so is the amount of industrial waste generated in order to meet the world’s

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demand. These industrial wastes may contain components that still pose good market value but

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not recovered due to technological or economic limitations.1 An economically feasible solution

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for the recovery of metals from industrial wastes should therefore be developed as it is

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considered an important aspect to move towards a sustainable society.2-4 For instance, vanadium


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is present in crude oil, coal, oil shale and tar sands, and therefore is an unavoidable waste in the

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oil refinery industry and oil-fired electrical power stations.5, 6According to the 2016 BP

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Statistical Review of World Energy,7 Singapore is capable of processing 1.514 million barrels of

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crude oil per day (1.6% share of the world’s total). In Singapore, the oil refinery industry

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generates about 30 tons of carbon black waste (a type of petroleum coke) daily as an unavoidable

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by-product from the cracking of crude oil. The carbon black waste was found to contain a high

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concentration of V (>14000 ppm), which could be extracted by chemical leaching.8, 9 In 2013,

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the global vanadium production from natural resources containing V (e.g., vanadium ore) was

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roughly 151,000 tons, up 11% year on year. The expanding demand for vanadium has

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encouraged researchers to recover the vanadium from the industrial wastes. Accordingly, the

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carbon black waste can be a potential source of V metal, and we are interested in recovering and

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converting the waste metal ions into vanadium based metal–organic frameworks (abbreviated as

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V-MOFs).

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Metal–organic frameworks (MOFs) or supramolecular coordination polymers with one-, two-,

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or three-dimensional crystalline networks are a new class of functional self-assembly materials in

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which the metal ions/clusters are bridged through diverse organic ligands.10 Generally, the V-

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MOFs contain trivalent or tetravalent V as metal nodes.11, 12 MOFs have been reported to be

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useful in a wide variety of applications such as CO2 adsorption,13-15 heterogeneous catalysts,16-18

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cathode materials for lithium ion batteries,19 magnetic sensors,20 and membrane separation.21

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The conventional approach of fabricating MOFs generally requires the use of expensive metal

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ion precursors.22 In addition, the use of costly and non-reusable organic solvents further impedes

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an economically feasible production of MOFs for industrial applications.23 For this

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consideration, the proposed pathway of vanadium recovery into V-MOFs would be conducted in


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aqueous solutions by directly using the leachate of carbon black waste as the solvent. This

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strategy, besides being environmentally friendly, would also provide a feasible way to reduce the

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cost of production by eliminating the need for organic solvents.

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The coordination of multicarboxylic acids to metal ions is the most representative assembly

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manner in building MOFs, in which the M−O−M serves as primary structural motif to form

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extended networks.24-26 To implement this idea, in this work, the aqueous leaching solution of

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carbon black waste was used to prepare V-MOFs, with multicarboxylic acids as the building

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blocks (Figure 1a). Depending on the organic ligands employed, two different V-MOFs with

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different crystal morphologies and orientations were synthesized, i.e., V-BDC product (one-

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dimensional nanofiber) when using 1,4-benzenedicarboxylate (BDC2−) linker, and V-NDC

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product (nanorod) when using 1,4-naphthalenedicarboxylate (NDC2−) linker. Additionally, we

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demonstrated that the as-obtained V-MOFs are efficient heterogeneous catalysts for selective

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benzyl alcohol oxidation in liquid-phase (see Figure 1b), which is an important oxidation

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reaction from the viewpoint of establishing a green process for benzaldehyde production.

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■ MATERIALS AND METHODS

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Materials. Carbon black waste was collected from an oil refinery on Jurong Island in Singapore.

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The

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Benzenedicarboxylic acid (H2BDC, Aldrich, 98%), 1,4-Naphthalenedicarboxylic acid (H2NDC,

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Alfa Aesar, 98%), vanadium chloride (Aldrich, 97%), cetyltrimethylammonium bromide

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(CTAB, Fluka, 96%), sodium dithionite (Aldrich, 85%), hydrochloric acid (VWR Chemicals,

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32%), sodium hydroxide (Merck, 99%), benzyl alcohol (Alfa Aesar, 99%), tertbutyl

following

chemicals

were

used

as

received

without

further

purification:1,4-


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hydroperoxide (TBHP, Aldrich, 70% in H2O), n-dodecane (Alfa Aesar, 99%), ethyl acetate

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(Merck, 99.5%), and toluene (J.T.Baker, 99.5%). Deionized water was used in all experiments.

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Carbon black waste leaching solution. The moisture content in the as-received carbon black

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waste was about 45.7 wt% (see the TGA curve in Figure S1). So, the sample was first dried in an

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electric oven at 150oC for 12 h. Then, 2.5 g of the dry solid was dispersed in 50 mL of NaOH (1

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M) by sonication (150 W) for 1 h. Afterward, the leaching solution was collected by a membrane

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filter (Nylon, pore size: 0.45 µm) under vacuum filtration condition.

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Synthesis of V-MOFs. For the synthesis of V-BDC, 4 mL of the leaching solution was mixed

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with 4 mL of HCl solution (1 M). Then, 1 mL of sodium dithionite aqueous solution (0.5 M) was

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added. The color of the solution changed immediately upon the addition of the reducing agent.

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After stirring for 2 min, 80 mg of H2BDC linkers and 0.1 g of CTAB were added. The mixture

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was thoroughly stirred at room temperature for 10 min before hydrothermal treatment in a

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Teflon-lined steel autoclave (capacity: 25 mL) at 200oC for 15 h. The solid was collected by

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centrifugation and washing for two times with water/ethanol to remove sulfurous impurity. As

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for the synthesis of V-NDC, 80 mg of H2NDC was used following the same approach. Note that

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V-MOFs prepared without adding CTAB were labeled as V-BDC-i and V-NDC-i, respectively (i

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refers to the irregular shape of the products).

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Catalytic performance evaluations. Firstly, V-MOFs were activated in static air at 280oC for

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10 h before the tests. Afterwards, the benzyl alcohol oxidation reaction was carried out in a

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magnetically stirred flask (capacity: 50 mL). Typically, the initial reaction mixture was

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composed of 1 mmol of benzyl alcohol, 1 mmol of n-dodecane (as an internal standard), 2.5

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mmol of TBHP, 15 mL of toluene, and 30 mg of V-MOF catalyst. At a specific time, 0.2 mL of


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the sample was extracted with 1 mL of ethyl acetate. Catalyst solids were separated from the

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reaction mixture by syringe filters (PTFE, pore size: 0.45µm). The extracts were subsequently

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analyzed by a gas chromatography (GC).

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Characterization Techniques. Morphologies of V-MOF samples were characterized by

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scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM,

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JEM-2010). The crystallographic information was identified by selected-area electron diffraction

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(SAED, JEM-2100F) and X-ray diffraction (XRD, Bruker D8 Advance) equipped with a Cu Kα

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radiation source. The elemental mapping was carried out by energy-dispersive X-ray

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spectroscopy (EDX, Oxford Instruments, Model 7426). Brunauer–Emmett–Teller (BET) specific

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surface areas of the samples, were calculated using N2 physisorption isotherms at 77 K

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(Quantachrome NOVA-3000 system). The chemical compositions of the samples were analyzed

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by X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical). Thermogravimetric

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analysis (TGA) studies were carried out on a thermobalance (TGA-2050, TA Instruments) with

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flowing air atmosphere (flow rate: 50 mL/min) at a temperature ramping rate of 10°C/min. Metal

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concentrations in leaching solutions were measured by inductively coupled plasma-atomic

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emission spectrometry (ICP-OES, Optima 7300DV, Perkin Elmer).

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resonance (NMR) solution spectra were recorded on a Bruker AV500 (500MHz) spectrometer.

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■ RESULTS AND DISCUSSION

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Synthesis of V-MOFs. The carbon black waste contains a significant amount of heavy metals,

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such as vanadium (~14500 ppm), nickel (~3200 ppm), and iron (~1500 ppm). Chemical leaching

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with sodium hydroxide is an effective method to recover the vanadium ions from the carbon

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black waste. The concentration of V in the obtained leaching solution was determined as 705

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V nuclear magnetic


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ppm, and only trace amounts of Ni and Fe metal ions were detected (see Table S1). In this work,

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5 wt% of carbon black waste solid was used in the leaching process. Therefore, the leaching

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efficiency of vanadium was about 97.2%. It is worth noting that the leachate obtained may be

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further recycled through a cycling leaching process to increase the concentration of V in the final

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leaching solution (see Figure 1). Since the vanadium ions in the carbon black waste leaching

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solution were mainly in +5 oxidation state as monomeric anion VO43− (refer to 51V NMR data in

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Figure S2), VV was first reduced to VIII species by using sodium dithionite as a reducing agent.

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This approach was similar to a previous study on the reductive transformation of V2O5 powders

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to vanadium-based MOFs.27 Sodium dithionite is a widely used reductant in industry. It was

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chosen due to its high driving force for reducing VV to VIII ions.28 After only 1 second of

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reaction, the vanadate ions could be totally reduced as visualized from the color change of the

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leaching solution from yellow to gray. It should be noted that some sulfite and sulfate impurities

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may be present in the original MOF product,29 but these impurities can be easily removed by

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water washing treatment, as checked by thermogravimetric analysis (TGA) shown in Figure S3.

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Based on Figure 2a, one can see that the amount of sodium dithionite added is crucial to the

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transformative yield of vanadium. As a control, without the addition of sodium dithionite, no

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MOFs was formed because VV cannot be used to build MOFs. It was found that 0.6 mL of

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sodium dithionite (0.5 M) was the optimal value to achieve ~99% yield of V-BDC. Similarly, the

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yield for V-NDC was around 99.4% under the same circumstances. As expected, there was no

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solid product formed in the absence of organic linker even though the reducing agent was added.

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This observation further confirms that the coordination reactions between leaching solution and

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organic linkers produce MOFs in good yield. In addition, the production of V-MOFs was easily

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scalable using the same set of synthesis parameters (see the result in Figure S4). However, it was


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found that ascorbic acid was not an effective reducing agent for the transformation, as a

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significant amount (99%) of vanadium ions remained in the solution.

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Morphology control of V-MOFs. The morphology and size of the prepared V-BDC and V-

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NDC products were observed by SEM and TEM, as displayed in Figure 3. Interestingly, the

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derived V-BDC and V-NDC showed distinctly different shapes of nanofiber and nanorod,

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respectively. For example, the length of V-BDC nanofiber was typically larger than tens of

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micrometers with an average diameter of 40 nm. On the other hand, V-NDC exhibited short

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nanorod shape with an average length of 270 nm and diameter of 64 nm. EDX elemental maps

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further confirmed the chemical compositions of the prepared V-MOFs and also showed that V,

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C, and O were evenly distributed across the entire structures (Figure 3d,h). Albeit no difference

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in vanadium recovery rate, V-MOFs samples synthesized without CTAB (viz., V-BDC-i, and V-

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NDC-i) had an irregular shape with a broad size distribution, as presented in Figure S5. It is well

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known that CTAB, a capping agent, has frequently been used to control the morphology of MOF

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crystals by adjusting growth rates of different facets via the electrostatic interactions between

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CTA+ and the deprotonated carboxylic groups.30 In addition, it was also found that CTAB would

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facilitate the dissolution of organic linkers in aqueous solution, probably due to their van der

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Waals interactions. In control experiments, we also used VCl3 as precursors for preparing V-

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MOFs. Indeed, the as-synthesized V-BDC and V-NDC displayed similar shapes of nanofibers

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and nanorods, respectively (Figure S6). However, in the case of fabrications from VCl3, the

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vanadium yields for V-BDC-i, V-BDC, V-NDC-i, and V-NDC were only 49.7%, 29.9%, 44.8%,

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and 35.1%, respectively (Figure 2b). The enhanced yields by using leaching solutions in the

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recovery process were probably due to two factors: (i) the lower pH value of the synthesis


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solution offered a more suitable environment for recovery, and (ii) the reduction process

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promoted the coordination between VIII ions and the organic ligands.

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Crystal structures of V-MOFs. According to XRD pattern in Figure 2c, the designed V-BDC

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is [VIII(OH)(BDC)](H2BDC)x, which is known as MIL-47as and the asterisk marks the Bragg

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peaks belonging to the included guest H2BDC ligands. Upon activation in static air at 280oC for

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10 h, MIL-47as can be converted to VIV(O)(BDC) (or known as MIL-47, one of the most studied

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V-MOF materials) with high levels of crystallinity by removing the free organic linkers, along

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with the oxidation of VIII ions to VIV ions and the conversion of VIII–OH bonds to vanadyl

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(VIV=O) bonds. Interestingly, N2 physisorption measurements at 77 K revealed a drastic increase

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in the BET surface area and pore volume as the filled linkers were removed upon activation

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(SBET: 6.91 vs 408.3 m2 g-1; pore volume: 0.02 vs 0.21 cm3 g-1 referring to Figure S7). Besides

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this, the orthorhombic crystal system of V-BDC was also confirmed from the SAED pattern

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(Figure 4a), in which the series of spots can be attributed to (022), (101), and (123)

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crystallographic planes with the respective d-spacing of 0.55 nm, 0.61 nm, and 0.35 nm.12 The

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angle between the (022) and (101) spots is 71°, consistent with the theoretical value in the crystal

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structure. The SAED pattern also indicates that the nanofibers observed are oriented almost

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along the [11-1] zone axis with a single crystalline nature, and the reflections exhibit the

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most intense diffraction spots. As also verified from XRD pattern (Figure 2d), the prepared V-

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NDC is isostructural to [Al(OH)(NDC)].31 The presence of several peaks in the range (5 to 40o)

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is due to the substitution of (AlIIIOH)2+ sites by (VIIIOH)2+ sites.27 Different from V-BDC, there

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was no significant change in XRD patterns of V-NDC before and after activation treatments

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(Figure S8), indicating that crystalline structure of V-NDC was guest-free and the larger H2NDC

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molecules were unlikely possible to be captured within the pores. The BET surface area of V-


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NDC after activation treatment was around 300 m2 g-1, comparable to the value of Al-NDC.32

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Likewise, the tetragonal crystal system of V-NDC was confirmed from the SAED pattern,31 in

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which the reflections of (200), (101), and (301) are clearly visible (Figure 4b). The surface angle

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between the (200) and (101) facets is 73°, matching well with the crystal structure. Similar to

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most other MOFs,33 the V-BDC and V-NDC frameworks would partially degrade under high

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energy electron-beam irradiation, as the mentioned SAED diffraction spots were no longer

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evident after 30 seconds of illumination (Figure S9). In both V-MOFs, small changes in MOF

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crystal structures were found when CTAB surfactant was added to modulate the product shapes.

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TGA and XPS Characterizations. The thermal stability of the as-prepared V-MOFs was

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studied by TGA. Results shown in Figure 2e demonstrated that V-MOF structures were quite

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stable up to 300°C in air atmosphere. There was no noticeable difference between the different

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MOF crystals, and it was observed that the frameworks decomposed in the temperature range of

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300–400 °C, which led to the formation of V2O5 as residue. The molar ratios of V metal ions to

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organic linkers (either BDC2− or NDC2−) were close to 1: 1 in all the four MOFs, which matched

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well with the formulas. Moreover, the reduction of vanadate in leaching solution by sodium

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dithionite was further confirmed from XPS characterization of the V-MOFs. As revealed in

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Figure 2f, all the V2p3/2 levels exhibited a binding energy of 515.8±0.2 eV, indicating the

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presence of VIII rather than VV ions in the frameworks. The difference in binding energies (∆)

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between the O1s and V2p3/2 levels was around 15 eV, which matched well with the reported

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value (14.71 eV).20, 34 The XPS V2p3/2 levels of V-MOFs after activation treatments (280oC for

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10 h) are shown in Figure S10, indicating that vanadium ions in the frameworks were oxidized to

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VIV having a binding energy of 516.6 eV.35 The XPS peak deconvolution analysis indicates that

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the ratios of VIV/(VIV+VIII) were 88.9% and 90.0% in V-BDC and V-NDC samples, respectively.


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Catalytic performances of V-MOFs. Benzaldehyde is an important intermediate to many

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organic compounds, and it can be produced from the catalytic oxidation of benzyl alcohol in

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liquid-phase.36, 37 Herein, the catalytic performance of our prepared V-MOFs (V-BDC, V-BDC-i,

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V-NDC, and V-NDC-i) was evaluated for this reaction. Firstly, a control study of the oxidative

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reaction in the absence of V-MOFs showed only minor background reaction (

Effective Recovery of Vanadium from Oil Refinery Waste into

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