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The world currently relies on coal, oil and gas, not only for fuel but also as raw materials for the production of chemicals. With reserves of these fossil fuels running out, and a growing awareness of the CO2 pollution their use causes, it is becoming increasingly important to develop sustainable carbon sources. One option is to use biomass, dry plant matter, and a team of researchers have demonstrated a new method for converting biomass into butene gas, which in turn can be processed into the chemicals used in the production of polymers and resins. Their work uses Gamma-valerolactone (GVL), a chemical processed from biomass raw material, and the catalyst Zn/ZSM-15, and demonstrates that it is possible to use a renewable source material to produce benzene, toluene and xylene. They used high-resolution X-ray powder diffraction (SRXD) on Ill to examine the structure of their samples, which yielded important information about the reaction mechanism. This was the first time that SXRD had been used to investigate the structures of adsorbed structures of the Gamma-valerolactone GVL and immobilized Zn-species used in the research. The results are a step towards affordable, sustainable chemical prodction.
![Figure 1: (a) Product distribution over various ZSM-5 based catalysts (5 g) by feeding 40 wt%
<br/>aqueous GVL solution to the preheated catalyst bed with weight hourly space velocity (WHSV)
<br/>1.0 h-1 at 10 bar after 12 h (the time to collect the liquid products for analysis) at complete
<br/>GVL conversions. The mole % yield is based on the moles of the GVL reactant to the products.
<br/>(b) Long term catalytic performance of Zn/ZSM-5 with WHSV 1.0 h-1 at 450 oC. [Reused from
<br/>doi:10.1002/anie.201704347(2017)]](/dam/jcr:98bbdabe-4648-42d5-b171-0a30ba3e886a/p56fig1.jpg)
The depleting supplies of non-renewable oil, coal and gas and their associated CO2 emissions are of significant concerns1. Biomass has long been regarded as a potential alternative to mitigate these problems. Lactones, such as gamma-valerolactone (GVL) can be favourably produced from lignocellulosic biomass and be applied to the production of BTX (benzene, toluene, xylene)2. However, the commercialisation of a biomass process is commonly limited by the process efficiency, tolerance to impurities and the stability of catalyst, etc. In the GVL conversion, the key challenges are to reduce the catalysis into a single simple step without the need for water separation and keeping high productivity in BTX3.
After screening a number of modified ZSM-5 samples, aluminosilicatezeolites, we have identified the Zn incorporation into H-ZSM-5 which gives the best catalytic activity (Fig. 1a). The long-term testing shows that water molecule is essential for keeping the high yield of BTX over the Zn/ZSM-5 (Fig. 1b). We are interested to elucidate the structure of Zn/ZSM-5 and its interaction with GVL/water for the production of BTX in this catalysis.
However, the modified zeolite powder although is in microcrystalline form, it is difficult to get single crystal for its structure determination. In addition, the common lab-source X-rays techniques for powder diffraction are not sensitive and accurate to determine the Zn/ZSM-5 structure. Recently, the significant advancement of modern X-ray diffraction facilities using the high brightness synchrotron has provided chemists with a powerful tool in studying powder crystalline materials. The environment of Zn and the adsorption geometry of GVL in the H-ZSM-5 were therefore elucidated by SXRD in Beamline I11 in combined with Rietveld refinement in order to rationalize the catalytic mechanism of this reaction.
![Figure 2: The refined structures of Zn/ZSM-5 catalyst derived from SXRD and refinements.
<br/>(a) Fresh Zn/ZSM-5; (b) GVL adsorbed on fresh Zn/ZSM-5; (c) A close-up view of Zn(2)-OH
<br/>interacting with the carbonyl of GVL; BAS (Brønsted acidic sites) in O6 assists the regeneration
<br/>of depleted Zn-OH by water molecule activation (d) A published crystal structure showing
<br/>a complex intermediate of Zn-OH of T199A-CA II enzyme with adsorbed CO2 for comparison
<br/>(PDB: 1CAM) [Crystal Database from National Center for Biotechnology Information]4. In all
<br/>the refined structures, the stick/ball model is used O = red, Si = grey, Al = green, Zn = blue, C
<br/>= black. In (c) and (d), only the activated complexes with carbonyl adsorbates are colored. A
<br/>mirror plane symmetry has been disregarded for clarity, view down the x-axis (the sinusoidal
<br/>channel). Site 199 assists the regeneration of depleted Zn-OH by water molecule activation.
<br/>[Reused from doi:10.1002/anie.201704347(2017)]](/dam/jcr:cea2ec29-f12d-4397-b724-892587b02b68/p56fig2.jpg)
The best-fit synchrotron X-ray powder diffraction (SXRD) result (Fig. 2a) shows the space group of the Zn/ZSM-5 is Pnma. Two isolated Zn species have been identified within the zeolite cavity. One hexa-aqua Zn2+ (named Zn-1) with Zn-O(H)2 distance of 2.12(3) Å is found located at the intersection region of the sinusoidal and straight channels. The site occupancy factor (SOF) of the Zn-1 is 0.104(2). Taking the symmetry of the Pnma space group into account, there are 0.104 × 4 = 0.42 Zn-1 per unit cell of Zn/ZSM-5. Interestingly, another tetra-coordinated Zn2+ (named Zn-2) attached to three wall-oxygens (next to Al(T6)) of ZSM-5 and one oxygen of dissociated H2O is also identified. The SOF of Zn-2 is 0.071(3) corresponding to 0.071 × 8 = 0.57 Zn-2 per unit cell.
![Figure 3: The proposed mechanism for aromatics production via butene formation whereby the initial step of cooperative ring opening hydrolytic decarboxylation of gamma-valerolactone (GVL)
<br/>with water molecule activation over Zn/ZSM-5 is similar to that of Zn-containing enzymatic catalysis. [Reused from SI of doi:10.1002/anie.201704347(2017)]](/dam/jcr:2dc18c3b-1144-4a19-a42c-31918455726c/p57fig3.jpg)
In conclusion, using SXRD combined with Rietveld refinement, we demonstrate for the first time the presence of Zn-OH and neighbour Brønsted acid site in Zn/ZSM-5. They provide the synergetic active sites to convert the GVL and water to BTX via the initial ring opening and decarboxylation of the GVL molecule (Fig. 3). The catalytic mechanism is also shown to be comparable to that of the reported CA II enzyme containing similar Zn-OH framework. Thus, the immobilized Zn2+ for catalytic hydrolysis may provide inspirations to the chemical industry on how to harness biomass to produce useful products.
Funding acknowledgement: The authors wish to thank EPSRC, UK and Diamond Light Source Ltd and SRIFT-SINOPEC for the financial support of this collaborative work and are grateful to the Office of China Postdoctoral Council to grant a fellowship to LY to work at Oxford.
Corresponding author: Prof Edman Tsang, University of Oxford, [email protected]
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