Delignification mechanism study of acid hydrotropes

doi:10.11949/0438-1157.20211779

Abstract

Abstract:

p-Toluenesulfonic acid (TSA) and maleic acid (MA) are important acid hydrotropes for lignocellulose fractionation in recent years. The delignification mechanism of TSA and MA fractionation of eucalyptus wood chips was studied. The results were as follows: (1) Both acid hydrotropes have high delignification efficiency. The delignification rate of TSA was 67.94% and that of MA was 65.14%; (2) The delignification rate of TSA was higher than that of MA at the same hydrotrope concentration; (3) TSA hydrotropic lignin has higher β-aryl ether linkage content than that of MA lignin under mild condition, the β-aryl ether linkage content of acid hydrotrope dissolved lignin was reduced with the increasing fractionation severity; (4) More than 90% cellulose was well retained after fractionation, but the hemicellulose degradation rate increased with the reaction severity; (5) The contact angle decreased with the increasing hydrotrope concentration, so the wetting ability of lignin increased, and delignification rate increased also, the size of lignin aggregate in acid hydrotropes decreased. In conclusion, delignification of hydrotropes is a combined effect of wetting dissolution, aryl-ether linkages cleavage and hemicellulose degradation etc. The research can provide a reference for the process optimization and mechanism of delignification under mild conditions.

Key words: acid hydrotrope, delignification, reaction kinetics, biomass, aggregation, contact angle

摘要：

系统探讨了酸性助水溶剂对甲苯磺酸(p-toluenesulfonic acid，TSA)和马来酸(maleic acid，MA)分离桉木各组分的工艺过程，并对其中木质素脱除机理进行了研究。通过分析对比两种优化后的工艺发现：(1)两种酸性助水溶剂都可以高效脱除木质素，TSA木质素脱除率为67.94%，MA为65.14%；(2)在相同质量分数下，TSA的木质素脱除率比MA更高；(3)在温和条件下，TSA木质素的β-芳醚键含量比MA的高，随着反应条件的加剧，TSA和MA处理后木质素产品中β-芳醚键含量逐渐减少；(4)两种酸性助水溶剂处理后，纤维素保留率都较高，可保持90%以上；但是半纤维素的降解程度随着反应条件的加剧而增加；(5)酸性助水溶剂质量分数越高，在疏水表面的接触角越小，对木质素的助溶作用越明显，脱除木质素效率越高，溶液中木质素聚集体的粒径越小。综上所述，酸性助水溶剂对木质素的脱除基于润湿溶解、木质素芳醚键断裂、半纤维素降解等的综合作用。相关研究可为后续实现温和条件脱木质素工艺优化及机理提供参考。

关键词: 酸性助水溶剂, 脱木质素, 反应动力学, 生物质, 聚集(作用), 接触角

CLC Number:

TQ 352.62

Min WANG, Jinlan CHENG, Xin LI, Jingjing LU, Chongxin YIN, Hongqi DAI. Delignification mechanism study of acid hydrotropes[J]. CIESC Journal, 2022, 73(5): 2206-2221.

王敏, 程金兰, 李鑫, 陆晶晶, 尹崇鑫, 戴红旗. 酸性助水溶剂脱除木质素机理分析[J]. 化工学报, 2022, 73(5): 2206-2221.

Figures/Tables 20

Table 1 TSA experiment factors design

水平	质量分数/%	反应温度/℃	反应时间/min
1	25	70	30
2	25	80	60
3	25	90	90
4	40	70	60
5	40	80	90
6	40	90	30
7	55	70	90
8	55	80	30
9	55	90	60

Table 2 MA experiment factors design

水平	质量分数/%	反应温度/℃	反应时间/min
1	40	100	60
2	40	120	120
3	40	140	180
4	55	100	120
5	55	120	180
6	55	140	60
7	70	100	180
8	70	120	60
9	70	140	120

Table 3 Chemical composition of eucalyptus chip material

化学成分	含量/%
灰分	0.3±0.01
苯醇抽提物	0.57±0.03
酸溶木质素	4.09±0.02
酸不溶木质素	22.89±0.1
葡聚糖	36.54±1.1
木聚糖	14.55±0.8

Table 4 Chemical composition of fractions

样品编号	固体组分							液体组分
样品编号	灰分/%	得率/%	酸溶木质素/%	酸不溶木质素/%	木质素脱除率/%	葡聚糖/%	木聚糖/%	木糖/（g/L)
P25T70t30	0.34±0.02	93.59±0.20	4.72（96.08）±0.02	22.15（90.57）±0.27	9.43±1.22	41.42（99.17）±1.00	13.47（86.62）±0.33	1.70±0.05
P40T70t60	0.25±0.00	72.56±0.18	4.47（83.09）±0.00	19.05（68.90）±0.59	31.30±0.20	43.78（96.50）±0.97	12.57（71.34）±0.99	2.68±0.04
P25T80t60	0.24±0.04	80.02±0.20	3.56（78.35）±0.02	18.94（65.21）±0.30	33.79±0.39	42.42（98.86）±0.30	10.56（58.07）±0.78	4.67±0.02
P55T80t30	0.17±0.00	75.49±0.04	3.08（54.94）±0.05	16.29（53.20）±0.68	47.80±1.03	46.59（99.54）±0.78	8.52（42.94）±0.88	5.45±0.03
P55T70t90	0.09±0.00	73.36±0.15	2.55（45.89）±0.01	15.14（50.57）±0.79	47.43±1.08	56.49（102.89）±0.39	6.78（37.05）±0.90	6.78±0.02
P40T90t30	0.06±0.00	70.97±0.13	2.08（29.76）±0.03	13.54（46.95）±0.20	55.05±1.09	58.39（100.90）±0.65	6.88（35.95）±0.20	10.07±0.03
P40T80t90	0.08±0.1	69.38±0.07	1.56（18.92）±0.00	13.48（42.80）±0.01	57.37±0.98	60.30（105.48）±0.76	5.15（25.60）±0.44	13.92±0.06
P25T90t90	0.09±0.03	63.73±0.07	1.01（8.58）±0.04	12.95（37.29）±0.30	62.91±0.79	59.61（102.34）±0.89	5.25（24.81）±0.77	14.78±0.08
P55T90t60	0.06±0.02	58.50±0.20	0.53（7.59）±0.01	12.54（32.06）±1.49	67.94±0.90	72.14（98.73）±0.45	4.08（16.42）±0.02	15.08±0.10
M40T100t60	0.31±0.01	91.63±0.19	4.89（96.28）±0.10	22.17（88.76）±0.67	11.24±0.76	46.77（94.49）±1.29	12.33（77.65）±1.08	1.89±0.01
M55T100t120	0.28±0.04	74.50±0.03	4.38（83.56）±0.03	20.58（75.99）±0.40	24.01±1.02	56.59（95.59）±0.45	9.52（58.73）±0.99	7.50±0.02
M70T120t60	0.23±0.00	72.24±0.16	3.68（59.27）±0.02	19.05（56.33）±0.30	43.67±1.07	59.00（93.72）±0.64	8.51（45.07）±0.97	12.03±0.01
M40T120t120	0.18±0.05	71.01±0.16	3.04（51.57）±0.04	18.16（53.46）±0.99	46.54±1.11	49.10（95.43）±0.82	9.24（37.56）±0.76	13.92±0.04
M55T140t60	0.19±0.00	67.98±0.18	2.74（49.04）±0.03	17.80（49.90）±0.39	50.10±0.20	62.52（95.61）±0.84	7.65（35.74）±0.56	15.20±0.02
M70T100t180	0.02±0.01	65.60±0.09	2.03（47.22）±0.01	16.06（47.80）±1.01	52.20±0.49	56.66（93.97）±0.39	6.54（32.25）±0.88	15.98±0.07
M55T120t180	0.05±0.01	63.81±0.19	1.78（35.07）±0.04	15.68（45.04）±0.20	54.96±0.59	66.73（94.71）±0.59	6.57（28.38）±0.99	16.06±0.03
M40T140t180	0.03±0.01	62.11±0.11	1.34（28.42）±0.02	13.21（36.33）±0.30	63.67±0.50	59.66（91.28）±0.58	6.44（26.17）±0.30	16.60±0.80
M70T140t120	0.06±0.03	60.58±0.13	0.67（13.68）±0.01	12.75（34.86）±0.22	65.14±1.02	66.72（90.13）±0.67	4.10（17.63）±0.20	17.06±0.13

Table 4 Chemical composition of fractions

样品编号	固体组分							液体组分
样品编号	灰分/%	得率/%	酸溶木质素/%	酸不溶木质素/%	木质素脱除率/%	葡聚糖/%	木聚糖/%	木糖/（g/L)
P25T70t30	0.34±0.02	93.59±0.20	4.72（96.08）±0.02	22.15（90.57）±0.27	9.43±1.22	41.42（99.17）±1.00	13.47（86.62）±0.33	1.70±0.05
P40T70t60	0.25±0.00	72.56±0.18	4.47（83.09）±0.00	19.05（68.90）±0.59	31.30±0.20	43.78（96.50）±0.97	12.57（71.34）±0.99	2.68±0.04
P25T80t60	0.24±0.04	80.02±0.20	3.56（78.35）±0.02	18.94（65.21）±0.30	33.79±0.39	42.42（98.86）±0.30	10.56（58.07）±0.78	4.67±0.02
P55T80t30	0.17±0.00	75.49±0.04	3.08（54.94）±0.05	16.29（53.20）±0.68	47.80±1.03	46.59（99.54）±0.78	8.52（42.94）±0.88	5.45±0.03
P55T70t90	0.09±0.00	73.36±0.15	2.55（45.89）±0.01	15.14（50.57）±0.79	47.43±1.08	56.49（102.89）±0.39	6.78（37.05）±0.90	6.78±0.02
P40T90t30	0.06±0.00	70.97±0.13	2.08（29.76）±0.03	13.54（46.95）±0.20	55.05±1.09	58.39（100.90）±0.65	6.88（35.95）±0.20	10.07±0.03
P40T80t90	0.08±0.1	69.38±0.07	1.56（18.92）±0.00	13.48（42.80）±0.01	57.37±0.98	60.30（105.48）±0.76	5.15（25.60）±0.44	13.92±0.06
P25T90t90	0.09±0.03	63.73±0.07	1.01（8.58）±0.04	12.95（37.29）±0.30	62.91±0.79	59.61（102.34）±0.89	5.25（24.81）±0.77	14.78±0.08
P55T90t60	0.06±0.02	58.50±0.20	0.53（7.59）±0.01	12.54（32.06）±1.49	67.94±0.90	72.14（98.73）±0.45	4.08（16.42）±0.02	15.08±0.10
M40T100t60	0.31±0.01	91.63±0.19	4.89（96.28）±0.10	22.17（88.76）±0.67	11.24±0.76	46.77（94.49）±1.29	12.33（77.65）±1.08	1.89±0.01
M55T100t120	0.28±0.04	74.50±0.03	4.38（83.56）±0.03	20.58（75.99）±0.40	24.01±1.02	56.59（95.59）±0.45	9.52（58.73）±0.99	7.50±0.02
M70T120t60	0.23±0.00	72.24±0.16	3.68（59.27）±0.02	19.05（56.33）±0.30	43.67±1.07	59.00（93.72）±0.64	8.51（45.07）±0.97	12.03±0.01
M40T120t120	0.18±0.05	71.01±0.16	3.04（51.57）±0.04	18.16（53.46）±0.99	46.54±1.11	49.10（95.43）±0.82	9.24（37.56）±0.76	13.92±0.04
M55T140t60	0.19±0.00	67.98±0.18	2.74（49.04）±0.03	17.80（49.90）±0.39	50.10±0.20	62.52（95.61）±0.84	7.65（35.74）±0.56	15.20±0.02
M70T100t180	0.02±0.01	65.60±0.09	2.03（47.22）±0.01	16.06（47.80）±1.01	52.20±0.49	56.66（93.97）±0.39	6.54（32.25）±0.88	15.98±0.07
M55T120t180	0.05±0.01	63.81±0.19	1.78（35.07）±0.04	15.68（45.04）±0.20	54.96±0.59	66.73（94.71）±0.59	6.57（28.38）±0.99	16.06±0.03
M40T140t180	0.03±0.01	62.11±0.11	1.34（28.42）±0.02	13.21（36.33）±0.30	63.67±0.50	59.66（91.28）±0.58	6.44（26.17）±0.30	16.60±0.80
M70T140t120	0.06±0.03	60.58±0.13	0.67（13.68）±0.01	12.75（34.86）±0.22	65.14±1.02	66.72（90.13）±0.67	4.10（17.63）±0.20	17.06±0.13

Table 5 TSA orthogonal experiment and range analysis

水平	因素			木质素脱除率/%
水平	A质量分数/%	B反应温度/℃	C反应时间/min	木质素脱除率/%
1	25	70	30	9.43
2	25	80	60	34.79
3	25	90	90	62.71
4	40	70	60	31.10
5	40	80	90	57.20
6	40	90	30	53.05
7	55	70	90	49.43
8	55	80	30	46.80
9	55	90	60	67.94
K₁	107	0.90	190
K₂	141	139	134
K₃	164	184	169
k₁	36	30	36
k₂	47	46	45
k₃	55	61	56
极差R	19	31	20
最优方案	A3	B3	C3
极差R排序		BCA

Table 6 MA orthogonal experiment and range analysis

水平	因素			木质素脱除率/%
水平	A质量分数/%	B反应温度/℃	C反应时间/min	木质素脱除率/%
1	40	100	60	11.24
2	40	120	120	43.67
3	40	140	180	63.67
4	55	100	120	24.01
5	55	120	180	54.96
6	55	140	60	50.10
7	70	100	180	52.20
8	70	120	60	46.54
9	70	140	120	65.14
K₁	119	96	108
K₂	140	147	142
K₃	164	179	173
k₁	40	32	36
k₂	47	49	47
k₃	55	60	58
极差R	15	11	22
最优方案	A3	B3	C3
极差R排序		CAB

Table 7 Fitting parameters for Eqs. （4）—（7）

参数	单位	CDF		CHF
参数	单位	TSA	MA	TSA	MA
α，α′	—	35.46	11.75	26.79	9.01
β，β′	—	0.14	9.9E-05	0.10	0.10
E，E′	J/mol	95800	44187	78958	49852
f，f′	—	0.00036	0.015	0.0072	0.08
θ，θ′	—	0.64	0.73	0.88	0.37
θ_Re，θ $R e'$	—	0.33	0.35	0.14	0.00042

Table 7 Fitting parameters for Eqs. （4）—（7）

参数	单位	CDF		CHF
参数	单位	TSA	MA	TSA	MA
α，α′	—	35.46	11.75	26.79	9.01
β，β′	—	0.14	9.9E-05	0.10	0.10
E，E′	J/mol	95800	44187	78958	49852
f，f′	—	0.00036	0.015	0.0072	0.08
θ，θ′	—	0.64	0.73	0.88	0.37
θ_Re，θ $R e'$	—	0.33	0.35	0.14	0.00042

Fig.1 CDF and CHF curves of TSA [(a), (c)] and MA [(b), (d)]

Table 8 CDF and CHF values of TSA and MA fractionation

处理条件	CDF/（min·mol/L）	CHF/（min·mol/L）
P25T70t30	363.2	21.0
P40T70t60	1419.4	78.6
P25T80t60	1881.2	91.8
P55T80t30	3121.8	139.7
P55T70t90	3615.9	180.6
P40T90t30	4517.6	191.3
P40T80t90	5514.3	258.2
P25T90t90	6935.4	289.1
P55T90t60	15344.9	586.3
P55T90t90	23017.3	879.4
M40T100t60	21.4	0.3
M55T100t120	57.5	1.1
M70T120t60	82.5	1.6
M40T120t120	88.2	2.1
M55T140t60	114.2	2.5
M70T100t180	119.8	2.8
M55T120t180	178.0	3.7
M40T140t180	254.7	4.9
M70T140t120	317.5	8.9
M70T140t180	476.2	13.4

Fig.2 Microscope images of the solid fraction of TSA(a) and MA(b)（100倍）

Table 9 Fiber quality analysis of TSA and MA solid fraction

处理条件	数均长度/mm	重均长度/mm	宽度/μm	扭结指数/%	卷曲指数/%	细小纤维含量/%	长宽比
P25T70t30	0.596	0.578	21.5	15.0	5.0	30.2	26.9
P40T70t60	0.509	0.540	20.6	14.9	4.9	31.9	26.2
P25T80t60	0.479	0.532	19.2	14.2	4.8	32.9	27.7
P55T80t30	0.403	0.468	19.0	14.0	5.2	33.8	24.6
P55T70t90	0.379	0.457	19.0	14.3	5.8	35.8	24.1
P40T90t30	0.309	0.456	18.7	14.6	6.0	37.0	24.4
P40T80t90	0.380	0.489	19.3	14.9	6.4	45.9	25.3
P25T90t90	0.346	0.410	19.2	14.2	6.2	50.3	21.4
P55T90t60	0.301	0.335	19.2	12.7	5.7	56.1	17.4
M40T100t60	0.568	0.599	21.8	12.2	4.9	33.1	27.5
M55T100t120	0.485	0.556	19.2	11.8	5.3	39.5	28.6
M70T120t60	0.437	0.536	19.4	10.3	5.4	39.9	27.6
M40T120t120	0.425	0.474	19.1	9.3	5.4	46.1	24.8
M55T140t60	0.402	0.456	18.3	9.0	5.5	48.3	24.9
M70T100t180	0.304	0.448	18.6	8.9	5.7	50.9	24.1
M55T120t180	0.317	0.467	19.2	9.2	5.8	55.3	24.3
M40T140t180	0.302	0.407	19.0	8.1	5.1	58.6	21.4
M70T140t120	0.299	0.324	18.9	7.2	4.4	60.0	17.1

Fig.3 FTIR spectrum of the solid fraction of TSA (a) and MA (b)

Fig.4 FTIR spectrum of lignin of TSA (a) and MA (b)

Fig.5 The aromatic regions (δC/δH 90—150/6.0—8.0) and aliphatic region (δC/δH 50—90/2.0—6.0) of lignin in 2D-HSQC NMR spectra

Table 10 Assignments of the lignin 13C-1H correlation peaks in the 2D-HSQC spectra of lignin

信号	δC/δH	信号归属
S_2/6	103.9/6.84	C_2/6-H_2/6 in syringyl units (S)
S $2 / 6'$	106.2/7.35	C_2/6-H_2/6 in oxidized S units (S′)
S_con	106.5/6.48	condensed C_2/6-H_2/6in syringyl units
G₂	110.9/7.05	C₂-H₂ in guaiacyl units (G)
G $2'$	110.9/7.34	C₂-H₂ in oxidized(C_α-O)guaiacyl units (G′)
G₅	114.5/6.71	C₅-H₅ in guaiacyl units (G)
G₆	118.9/6.77	C₆-H₆in guaiacyl units (G)
A_γ	59.9/3.63	C_γ-H_γ in γ-hydroxylated β-O-4′ substructures (A)
B_γ	62.6/3.72	C_γ-H_γ in β-β resinol (B)
C_γ	62.4/3.43	C_γ-H_γ in phenylcoumaran (C)
A $γ'$	64.8/4.31	C_γ-H_γ in γ-acylated β-O-4(A′ )
(A,A′)_ɑ	71.5/5.00	C_α-H_α in β-O-4′ substructures (A)
B_ɑ	86.9/5.55	C_α-H_α in phenylcoumaran substructures(B)
C_ɑ	86.8/5.47	C_α-H_α in β-β′ resinol substructures (C)
A_β(G)	83.8/4.48	C_β-H_β in β-O-4′ substructures (A) linked to a G-unit
A_β(S)	86.0/4.25	C_β-H_β in β-O-4′ substructures (A) linked to a S-unit
B_β	53.5/3.46	C_β-H_βin phenylcoumaran substructures(B)
C_β	53.1/3.06	C_β-H_β in β-β′resinol substructures (C)
methoxyl	55.6/3.58	methoxyl
HK_γ	67.1/4.19	C_γ-H_γ in Hibbert’s ketone (HK)
E_γ(MA)	128.0/6.2, 132.9/6.4, 68.3/4.37	C_γ-esterified by maleic acid

Table 10 Assignments of the lignin 13C-1H correlation peaks in the 2D-HSQC spectra of lignin

信号	δC/δH	信号归属
S_2/6	103.9/6.84	C_2/6-H_2/6 in syringyl units (S)
S $2 / 6'$	106.2/7.35	C_2/6-H_2/6 in oxidized S units (S′)
S_con	106.5/6.48	condensed C_2/6-H_2/6in syringyl units
G₂	110.9/7.05	C₂-H₂ in guaiacyl units (G)
G $2'$	110.9/7.34	C₂-H₂ in oxidized(C_α-O)guaiacyl units (G′)
G₅	114.5/6.71	C₅-H₅ in guaiacyl units (G)
G₆	118.9/6.77	C₆-H₆in guaiacyl units (G)
A_γ	59.9/3.63	C_γ-H_γ in γ-hydroxylated β-O-4′ substructures (A)
B_γ	62.6/3.72	C_γ-H_γ in β-β resinol (B)
C_γ	62.4/3.43	C_γ-H_γ in phenylcoumaran (C)
A $γ'$	64.8/4.31	C_γ-H_γ in γ-acylated β-O-4(A′ )
(A,A′)_ɑ	71.5/5.00	C_α-H_α in β-O-4′ substructures (A)
B_ɑ	86.9/5.55	C_α-H_α in phenylcoumaran substructures(B)
C_ɑ	86.8/5.47	C_α-H_α in β-β′ resinol substructures (C)
A_β(G)	83.8/4.48	C_β-H_β in β-O-4′ substructures (A) linked to a G-unit
A_β(S)	86.0/4.25	C_β-H_β in β-O-4′ substructures (A) linked to a S-unit
B_β	53.5/3.46	C_β-H_βin phenylcoumaran substructures(B)
C_β	53.1/3.06	C_β-H_β in β-β′resinol substructures (C)
methoxyl	55.6/3.58	methoxyl
HK_γ	67.1/4.19	C_γ-H_γ in Hibbert’s ketone (HK)
E_γ(MA)	128.0/6.2, 132.9/6.4, 68.3/4.37	C_γ-esterified by maleic acid

Table 11 Semi-quantitative analysis of lignin units and ratio and linkages based on 2D-HSQC spectra

样品编号	CDF/(min·mol/L)	S_2/6	S′_2/6	S_con	S	G	S/G	β-O-4/%	β-5/%	β-β/%
MWL	0	55.56	5.98	0	61.54	42.79	1.44	52.31	2.04	6.67
P40T70t60	1419.4	72.44	2.87	10.55	85.86	14.14	6.07	50.72	0.92	5.23
P40T80t90	5514.3	71.65	2.23	22.18	96.06	3.94	24.40	31.23	0.79	4.20
P55T90t60	15344.9	24.98	2.13	71.74	98.85	1.15	86.21	0	0	1.84
M55T100t120	57.5	58.56	4.10	31.15	93.81	6.19	15.15	31.33	1.00	3.87
M55T120t180	178.0	23.14	2.77	71.42	97.33	2.67	36.49	1.53	0.40	1.03
M70T140t120	317.5	16.13	1.30	81.62	99.05	0.95	104.67	0	0	0.39

Fig.6 The effect of acid hydrotrope concentration on the average particle size of lignin aggregates

Table 12 The contact angle of acid hydrotropes

质量分数/%	TSA溶液接触角/（°）	MA溶液接触角/（°）
10	107.94±1.90	103.36±0.40
20	94.70±1.70	103.10±1.80
30	89.98±0.70	102.71±0.02
40	84.14±0.02	98.27±0.04
50	83.84±0.04	96.14±0.01

Fig.7 The contact angle images of acid hydrotropes

Fig.8 Schematic diagram of delignification of acid hydrotropes

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