The synthesis procedure of alkyl benzene sulfonic acid is as shown in the References [21] - [24] , and the molecular structure of alkyl aryl benzene sulfonic acid as following in Scheme 1.

In typical procedure: 15 g starch and 15 g sulfonic acid were mixed together and transferred to the quartz furnace. The mixture was heated to 200˚C from room temperature with the heating rate of 1˚C/min and remained at 200˚C for 12 h under nitrogen atmosphere. After cooling to room temperature, obtained black solid was washed with deionized water until no acidity detected in the filtrate. The novel solid acid was obtained after drying at 120˚C overnight in an oven and grinding it into fine flour. P-toluene sulphonic acid carbon-based solid acid catalyst, tetradecyl benzene sulphonic acid carbon-based solid acid catalyst, hexadecyl benzene sulphonic acid carbon-based solid acid catalyst and octadecyle benzene sulphonic acid carbon-based solid acid catalyst are abbreviated in what follows as cat 1, cat 2, cat 3, cat 4 respectively.

Mono-fatty alcohol polyoxyethylene maleate esters were synthesized by 0.1 mol (9.806 g) of maleic anhydride (MAH) reacted to 0.1 mol (31.695 g, M = 317.695 g/mol) of fatty alcohol polyoxyethylene (AEO-3) and 0.1% (wt) p-toluene sulphonic acid served as a catalyst for 8 h with heating at 80˚C. Standard titration solution of NaOH was used to determine the system acid value, and when the variation of acid value was less than 1 mg/h, it was regarded as near the end point of esterification. The crude product was recrystallized from ethanol and water (V/V = 1:1) to three times. Compound 1a was obtained in 95.66%.

0.05 mol (20.515 g) of compound 1 reacted with 0.025 mol (2.253 g) of 1,4-butanediol and 1% (wt) of carbon- based solid acid served as catalyst under vacuum degree of −0.09 mpa at 150˚C reacting 10 h. and real-time detection of acid value. The esterification rate was calculated using the following formula [25] - [27] : (Er) = (1-

Scheme 1. The molecular structure of alkyl aryl benzene sulfonic acid.

AV_a/AV_b) × 100%, where Er is esterification rate, AV_a is Final acid value, AV_b is Initial acid value. The typical procedure (Scheme 2).

The FT-IR spectrum Figure 1 showed that the carbon-based solid acid contain resident functionalities including,

Cat 1: p-toluene sulphonic acid catalyst, Cat 2: tetradecyl benzene sulphonic acid catalyst

Cat 3: hexadecyl benzene sulphonic acid catalyst, Cat 4: octadecyle benzene sulphonic acid catalyst

Scheme 2. Synthesis route of the reaction.

Scheme 3. Hydrothermal method to synthesis of carbon-based solid acid.

1007 cm⁻¹, 1032 cm⁻¹ (S=O str). 1118 cm⁻¹ (SO₃H str), 1710 cm⁻¹ (C=O bend), 3450 cm⁻¹ (OH str) On the other hand, the bands due to ?OH stretching at 3420 cm⁻¹, and C=C bonds stretching at 1620 cm⁻¹ were observed for both samples independent of the sulfonation. It is shown that the carbon-based solid acid have the group of -SO₃H.

The XRD pattern (Figure 2) contains two broad and weak diffraction peaks C (002) diffraction peak (2θ = 10˚ - 35˚) attributable to amorphous carbon composed of aromatic carbon sheets oriented in a considerably ran

dom fashion. A definitive C (101) diffraction peak (2θ = 35˚ - 45˚) due to the a axis of the graphite structure can be seen. The X-ray diffractogram is similar to that of char because the sulfonic acid groups were attached to edges of isotropic carbon sheets and the solid acid exhibits broad diffraction peaks of amorphous carbon [31] - [33] .

The optimum reaction conditions from Table 1 was obtained as follows: m_starch:m_{cat 1} = 1:1, reaction temperature: 220˚C, reaction time: 10 h, All results were repeated for three times: Esterification rate were 91.85%, 90.63% and 90.71% respectively, average value was 91.06%.

The optimum reaction conditions from Table 2 was obtained as follows: reaction temperature: 180˚C; reaction

^*Average 1 is experiments 1, 4, 7; Average 2 is 2, 5, 8; Average 3 is 3, 6, 9.

^*Average 1 is experiments 1, 4, 7; Average 2 is 2, 5, 8; Average 3 is 3, 6, 9.

time: 10 h; m_starch:m_cat2 = 1:1, All results were repeated for three times: Esterification rate were 88.04%, 86.98% and 90.82%, average value was 88.61%.

The optimum reaction conditions from Table 3 was obtained as follows: reaction temperature: 220˚C, reaction time: 10 h, m_starch:m_cat3 = 1:3, All results were repeated for three times: Esterification rate were 91.08%, 89.42% and 89.92%, average value was 90.14%.

The optimum reaction conditions from Table 4 was obtained as follows: reaction temperature: 180˚C; reaction time: 12 h; m_starch:m_cat4 = 1:1, All results were repeated for three times: Esterification rate were 90.07%, 91.98% and 93.06%, average value was 91.70%.

^*Average 1 is experiments 1, 4, 7; Average 2 is 2, 5, 8; Average 3 is 3, 6, 9.

^*Average 1 is experiments 1, 4, 7; Average 2 is 2, 5, 8; Average 3 is 3, 6, 9.

The esterification at 0.05 mol (20.515 g) of compound 1a reacted to 0.025 mol (2.253g) of 1,4-butanediol and 1% (wt) of carbon-based solid acid was used as catalyst under vacuum degree of −0.09 mpa at 150˚C reacting 10 h. From Figure 3 we can see that alkyl benzene sulfonic acid carbon-based solid acid catalyst (cat2, cat3, cat4) has the same catalytic activity with p-toluene sulphonic acid carbon-based solid acid catalyst (cat1) in the reaction of esterification of Mono-fatty alcohol polyoxyethylene maleate esters with 1,4-butanediol.

The reusability of the alkyl benzene sulfonic acid carbon-based solid acid catalyst was investigated for the reaction of esterification. After the reaction had reached equilibrium, the novel carbon-based acid was simply recovered by filtration and recycled for further reaction. It was confirmed that the activity remained unchanged, even after the catalyst had been recycled five times (Figure 4).