Study on conversion of glucose into HMF based on coordination of MIL-101(Cr)/Cr(OH)3 @[A-15 x%] composite with DBD plasma

Glucose was highly economical as a reactant for the catalytic formation of 5-hydroxymethylfurfural (HMF). However, the regulation method of glucose conversion to HMF requires high temperatures. Dielectric barrier discharge (DBD) plasma could realize glucose conversion in low-temperature conditions. Glucose could be isomerized to fructose on Lewis acid, and remove H2O to form HMF on Brønsted acid fructose. A variety of by-products were generated in the process of isomerization and dehydration, and thus the regulation of the Brønsted-Lewis acid site in the catalyst was essential in the targeted formation of HMF. Using Lewis acidic MIL-101(Cr) compounded with Brønsted acidic Amberlyst-15, a bi-functional catalyst MIL-101(Cr)/Cr(OH)3@[A-15 x%] was obtained. By coordinating it with DBD plasma on glucose conversion, 80.6% of the glucose conversion rate and 5.3% of HMF yield were obtained.


Introduction
Glucose was highly economical as a reactant to produce 5-hydroxymethylfurfural(HMF) [1] .However, the selectivity of HMF was usually relatively inferior compared to fructose as the reactant.Dielectric barrier discharge (DBD) plasma had attracted widespread attention in biomass refining processes due to its unique chemical activity and high energy [2] .DBD plasma could be used as a glucose conversion technology.It could ionize and dissociate to form H + radicals, accelerating the glucose isomerization reaction [3] .DBD plasma coordination with catalysts could shorten the reaction time of the glucose pyrolysis reaction.The micro-regulation of the Brønsted-Lewis acidic site in the catalyst and its acidity played a vital role in the targeted formation of HMF [4] .The transformation of glucose to fructose determined the reaction rate.In the process of dehydration of fructose, a wide variety of by-products were generated.Glucose could be oxidized to lactic acid on weak Lewis catalysts [5] .When Lewis acid or Brønsted acid sites were excessive [6] .CrCl 3 could shorten the rotation time between glucose isomers and accelerate glucose isomerization into fructose [7] .However, as a homogeneous catalyst, CrCl 3 has the disadvantages of poor environmental protection and difficult recovery.MIL-101(Cr), as a Cr-based solid acid material, is a recyclable heterogeneous catalyst with a unique structure.However, due to the single Lewis acid site of MIL-101(Cr), the yield of HMF is not high.The composite Brønsted acid catalyst can effectively improve the yield of HMF.Amberlyst-15 was an ion exchange resin with strong Brønsted acidity provided by the -SO 3 H functional group.The pore structure of Amberlyst-15 was very close to the adsorbent.Amberlyst-15 could be used as a carrier for compounding with other catalysts, where -SO 3 H could ligate to the catalyst surface to enhance the catalyst performance.The -SO 3 H in Amberlyst-15 was used as a linking porphyrin functional group to prepare a novel metal-free catalyst [8] .Mahiro [9] used Amberlyst-15 and CrMgAl-LDH for the one-pot formation of furfural with 59% of HMF yield.

DBD plasma coordinate with MIL-101(Cr)/Cr(OH) 3 @[A-15 2%] for glucose conversion
DBD plasma reactor was shown in Figure 1.Firstly, we added the catalyst sample to the reactor, then pumped the glucose solution into the reactor, and after that, clamped the liquid inlet and outlet to keep the solution in the reactor.High-purity N 2 was supplied from the air inlet, and the gas-gathering condensate bottle collected the non-condensing products.After the pressure inside the reactor stabilized, turn on the plasma power supply.At regular intervals, collected products at the sampling port for analysis.Figure 2(b) showed the XRD results of prepared catalysts.The diffraction peaks at 19.7°, 25.3°, 28.2°, and 38.2° were highly coincidence with the Cr(OH) 3 peaks.While the peaks at 7.9° and 9.0° were corresponded to MIL-101 [10] .For compound catalyst samples, there were distinct Amberlyst-15 characteristic peaks between 15°-30° compared to MIL-101(Cr)/Cr(OH) 3 .

Py-IR and NH 3 -TPD results.
The soret band that appeared at 1556 was on account of the Brønsted acid site (BAS) binding to pyridine molecules (BAS), the soret band at 1452 was attributed to the strong Lewis acid site (LAS), and the soret band at 1462 was because of weak Lewis acid site [11]   .As shown in Figure 5  NH 3 -TPD results were shown in Table 1.Compared with the uncompounded catalyst, with the addition of Amberlyst-15, the BAS to LAS ratio(B/L) significantly increased, indicating that Brønsted acid was effectively introduced by the compound method.Also, the introduction of Amberlyst-15 led to a decrease in strong LAS to total LAS ratio(L^/L), while leading to an increase in strong BAS to total BAS ratio(B^/B).

Glucose pyrolysis performance of MIL-101(Cr)/Cr(OH) 3 @[A-15 x%]
Using an oil-bath reaction to test the catalytic performance of prepared catalysts, the results were shown in Figure 6.When the addition of Amberlyst-15 was 1 mol%, the HMF yield was 17.0%.As the addition of Amberlyst-15 increased to 2 mol%, HMF reached 31.2%.However, as the addition of Amberlyst-15 increased to 3 mol%, HMF yield decreased to 22.49%, thus, MIL-101(Cr)/Cr(OH) 3 -[Amb 2%] was chosen as the catalyst for the glucose conversion by DBD plasma.

Glucose conversion by DBD plasma
The results of the glucose conversion rate using DBD plasma technology was shown in Figure 7(a).When using DBD alone to convert glucose, the glucose conversion rate reached 77.6% after 2 h of discharge time.When DBD plasma coordinated with 0.01 wt% Cr-MOFs/Cr(OH) 3 @[A-15 x%] catalyst, the glucose conversion rate increased to 80.6%.HMF yield using DBD plasma was shown in Figure 7(b).When the discharge time of DBD alone reached 2 h, HMF yield was 0.57%; by coordinating with 0.01 wt% Cr-MOFs/Cr(OH) 3 @[A-15 x%], HMF yield reached 5.3% after 2 h.

Conclusion
DBD plasma served as a new method for glucose conversion to HMF.It was found that when the discharge voltage was 10.2 kV and the discharge time was 2 h, the glucose conversion rate was 77.6%, and the HMF yield was 0.57%.Prepared a new bi-functional composite MIL-101(Cr)/Cr(OH) 3 @[A-15 x%] as catalysts for glucose conversion.Changing the addition amount of Amberlyst-15 could regulate Brønsted-Lewis acid sites in MIL-101(Cr)/Cr(OH) 3 @[A-15 x%].As the additional amount of Amberlyst-15 increased, the Brønsted-Lewis acid ratio(B/L) increased too.MIL-101(Cr)/Cr(OH) 3 @[A-15 2%] showed the best catalytic performance on glucose conversion to HMF.When DBD plasma coordinated with MIL-101(Cr)/Cr(OH) 3 @[A-15 2%] for glucose conversion, the converting ratio of glucose was 80.6% and the yield of HMF was 5.3% at 2 h of reaction.This research provided a new maneuver on the catalytic conversion system of glucose to HMF and enriched the method and catalytic system of glucose conversion to HMF.
3.1.2SEM and EDS-mapping results.SEM images of the samples were demonstrated in Figure3.MIL-101(Cr)/Cr(OH) 3 @[A-15 2%] showed a rod-like structure.It could expose more acid sites on the edge of catalysts, and this low crystalline structure contained a large number of uncoordinated sites, which improved catalytic performance.

Figure 4
Figure4showed the results of EDS-mapping spectra of MIL-101(Cr)/Cr(OH) 3 -[Amb 2%].The results demonstrate that the preparation of MIL-101(Cr)/Cr(OH) 3 -[Amb 2%] was successful and simultaneously the elements were well distributed.It can be seen that the sample was rich in N. Higher N element content was favorable to anchor Cr element and provide more Lewis metal active sites, and S element was derived from -SO 3 H in Amberlyst-15, which can provide effective Brønsted acid sites.
(a), compared to MIL-101(Cr)/Cr(OH) 3 , the composite catalysts had weak LAS.As shown in Figure 5(b), with the additional amount of Amberlyt-15 increased, LAS decreased and BAS increased.

Table 1 .
BAS and LAS ratio of prepared catalysts.