Preparation of a Cellulase-imitated Solid Acid Catalyst With High Acid Density and Its Evaluation for Hydrolysis of Cellulose

: In this paper, tannic acid, a polyphenolic substance rich in plants, is modified 8 by the glutamic acid and cross-linked with formaldehyde to prepare a high acid density 9 tannin-glutamate acid resin-based imitation enzyme solid acid catalyst (T-Glu-R), which 10 is completely different from traditionally carbon-based solid acid synthesized by 11 concentrated sulfuric acid and carbonized matter. The solid acid catalyst was 12 characterized by Fourier transform infrared spectroscopy, scanning electron microscope, 13 thermogravimetry, and X-ray photoelectron spectroscopy. The catalytic activity and 14 cycle performance of T-Glu-R in the cellulose hydrolysis reaction were evaluated. The 15 results show that the acid density of T-Glu-R reached 7.28 mmol/g, which is much 16 higher than that of the highest acid density of carbon-based solid acid. Microcrystalline 17 cellulose was hydrolyzed in distilled water at 180 °C for 2 h, the yield of total reducing 18 sugars reached 72.15%. After four cycles of hydrolysis, the yield was only reduced by 19 4.32%, showing excellent cycle performance and stability. The study provides a new 20 strategy with the synthesis of solid acid catalyst for hydrolysis of cellulose converted 21 into platform compounds without concentrated sulfuric acid.

The use of traditional fossil energy has brought various environmental problems, 25 like air pollution and global warming. Fossil resources will not be able to meet the 26 future energy needs of humanity due to their non-renewability. As the only renewable 27 organic carbon source on Earth, biomass is regarded as the best substitute for fossil binding and catalytic groups in the structure. Catalytic groups, like the carboxyl and oxidizing the lactone, ether, quinine, and other functional groups on the surface of the 75 activated carbon to -COOH, but also dissolving the ash on the surface of the activated 76 carbon to allow the grafting of more -SO3H onto the activated carbon. The acid density 77 of the catalyst increased from 1.58 mmol/g to 2.23 mmol/g, and the glucose yield 78 increased to 62.2%. In summary, high-density acid centers are essential to cellulose 79 hydrolysis. 80 Most of the above solid acids use -SO3H as the catalytic group. However, the -SO3 -81 formed after the sulfonic acid group releases H + will be hydrolyzed and fall off in the 82 aqueous acidic medium, causing the loss of the sulfonic acid group (Min et al. 2013 obtain T-Glu. Subsequently, 4 g of T-Glu, 3 mL of formaldehyde, 2 mL of HCl, and 20 125 mL of distilled water were added to a 100-mL Teflon-lined stainless-steel autoclave and 126 reacted hydrothermally at 120 °C for 12 h to obtain a brown resin, which was washed to 127 the solution is neutral. The product was ground and passed through a 100-mesh sieve to 128 obtain a resin-based solid acid catalyst, which was named T-Glu-R. The preparation's 129 reaction equation is shown in Fig. 1.     The thermal behavior of the solid acid catalyst is shown in Fig. 4    The influence of different times on the acid density of the prepared catalyst is 268 shown in Fig. 6b. When the temperature was 120 °C and the molar ratio of TA to T-Glu 269 was 1:3, the total acid, phenolic hydroxyl, and carboxyl group densities were the highest   with cellulose, thereby shortening the distance with cellulose and playing a binding role.

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The carboxyl group can cut the glycosidic bond of the cellulose chain, which acts 305 similarly as the catalytic group of cellulase. Fig. 7 presents a schematic of the 306 hydrolysis of cellulose with the T-Glu-R solid acid catalyst. where C0 represents the initial cellulose concentration.

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The kinetic analysis of hydrolyzed cellulose was carried out at different 349 temperatures according to Formula (9). The kinetic fitting results are shown in Table 2 350 and Fig. 9. Figure 9 indicates that as the reaction temperature increased, both k1 and k2 351 showed an increasing trend, indicating that increasing the temperature promotes the  The recyclability of the catalyst plays an important role in practical applications.

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The hydrolysis performance of the catalyst after three recoveries is shown in Fig. 10.

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The TRS yield gradually decreases as the recycling iteration increases. After four  for the synthesis of solid acid, which has potential market application value.

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Conflicts of interest 404 The authors declare no conflicts of interest.