Hangyu Zhao1, Ronghua Yu1, Hao Qiao1, Cunli Liu1. 1. College of Biology and Pharmaceutical Engineering Nanjing University of Technology, Nanjing 211816, China.
Abstract
The preparation of glycine by the hydantoin method is currently a relatively advanced process. The raw materials of this process are nontoxic, the operation process is simple and safe, the side reactions are few, and the yield of glycine is high. The core reaction of the hydantoin method is the hydrolysis of hydantoin. The hydrolysis is divided into two steps: first, hydantoin is hydrolyzed into hydantoin acid, and hydantoin acid is further hydrolyzed into glycine. At a temperature of 423.15 K, a molar ratio of sodium hydroxide to hydantoin of 1:3, and a total reaction time of 6 h, the conversion rate of hydantoin reached 100% and the yield of glycine reached 91%. At the same time, by calculating the hydrolysis kinetic parameters, the reaction was determined to be a first-order series reaction, and a kinetic model was established, which laid the foundation for the development of a green glycine process and a new reactor.
The preparation of glycine by the hydantoin method is currently a relatively advanced process. The raw materials of this process are nontoxic, the operation process is simple and safe, the side reactions are few, and the yield of glycine is high. The core reaction of the hydantoin method is the hydrolysis of hydantoin. The hydrolysis is divided into two steps: first, hydantoin is hydrolyzed into hydantoin acid, and hydantoin acid is further hydrolyzed into glycine. At a temperature of 423.15 K, a molar ratio of sodium hydroxide to hydantoin of 1:3, and a total reaction time of 6 h, the conversion rate of hydantoin reached 100% and the yield of glycine reached 91%. At the same time, by calculating the hydrolysis kinetic parameters, the reaction was determined to be a first-order series reaction, and a kinetic model was established, which laid the foundation for the development of a green glycine process and a new reactor.
The main process of producing
glycine in China[1] uses ammonia, ammonia,
and chloroacetic acid as raw materials,[2] uses tropine as a catalyst, and simultaneously
adds a large amount of methanol or ethanol. Although this method effectively
solves the problem that ammonium chloride is difficult to remove,
it increases the cost,[3] the reaction time
is as long as 48 h, and the catalyst is difficult to recover, resulting
in serious environmental pollution. Today, under the policy of environmental
protection and high pressure, the chloroacetic acid method can no
longer meet the policy requirements, and this process has been eliminated
abroad.[4]The process of producing
glycine abroad mainly uses formaldehyde,
ammonia, hydrocyanic acid, and carbon dioxide as raw materials in
a tubular reactor,[5] but hydrocyanic acid[6] is a highly toxic substance, and the process
takes a long time, the post-treatment is complicated, the operation
requirements are harsh, and it is also unable to meet domestic environmental
requirements.The most effective herbicide currently used in
the world is glyphosate.[7] There is no new
herbicide to replace glyphosate.
Most glycine is used as the main raw material for glyphosate production;
thus, it is very important to ensure its production capacity. The
total annual production capacity of glyphosate in China can reach
560,000 tons. The herbicide consumption of glycine is as high as 440,000
tons/year, and 20% is used in food, medicine, and other industries.
However, under the conditions of environmental protection and high
pressure, the operating rate of manufacturers has decreased year-by-year,
and the supply of glycine is slightly insufficient.[8] At the same time, glycine is used in medicine, food, and
other fields.[9] Therefore, it is urgent
to explore a new production process of glycine.This subject
uses hydantoin, sodium hydroxide, and water as raw
materials to hydrolyze hydantoin in a high-pressure reactor. Generally,
hydantoin is produced by the reaction of hydroxyacetonitrile, ammonium
bicarbonate, and carbon dioxide at 85 °C for 4 h. Hydantoin undergoes
two steps of hydrolysis to produce glycine. The raw materials are
nontoxic, and the reaction proceeds in an aqueous solution. The experimental
operation is convenient and safe. The first step of the hydrolysis
has almost no byproducts, and the yield is stable and higher than
that of the conventional process. Therefore, the hydantoin method
is a relatively advanced method.In this paper, the two-step
hydrolysis process for the preparation
of glycine by hydantoin was studied. The effects of the stirring rate,
temperature, hydrolysis time, and alkali dosage on the reaction were
discussed, and a hydrolysis kinetic model was established.[10]
Experimental Steps
Experimental Instruments and Reagents
Instrument
Miniature reaction kettle
(Beijing Century Senlang Co., Ltd.), Agilent 1260 Infinity LC (Agilent
Instruments), pH detector MP511 (Samsung Precision Instruments), circulating
water vacuum pump SHZ-D(III) (Gongyi City Yuhua Instrument Factory),
and ultrasonic cleaner KQ2200E (Kunshan Ultrasonic Instrument Co.,
Ltd.) were used.
Reagent
The
water was obtained
from “Milli-Q purification unit”, and hydantoin (homemade),
sodium hydroxide (AR) (Xiyu Science Co., Ltd.), ammonium dihydrogen
phosphate (GR) (National Pharmaceutical Group Chemical Reagent Co.,
Ltd.), acetonitrile [high-performance liquid chromatography (HPLC)]
(Zhongguo Group Chemical Reagent Co., Ltd.), methanol (HPLC) (Zhongguo
Group Chemical Reagent Co., Ltd.), and phosphoric acid (AR) (Zhongguo
Group Chemical Reagent Co., Ltd.) were used.
Experimental Procedure
The hydrolysis reactor used in this
subject is a microreactor. Weigh 2.0 g of hydantoin and a certain
molar ratio of sodium hydroxide, dissolve in 27 mL of pure water (the
water was obtained from “Milli-Q purification unit”),
and pass the prepared solution into the reaction kettle to rapidly
heat up the hydrolysis. Hydrolyzate is detected by HPLC.[11]Specific steps for calculating kinetic
parameters: input the experimentally measured concentration data into
Excel cells, in A1–A7, enter the reactant concentrations measured
at various times at a certain temperature, and in B1–B7 also
input the measured values measured at various times at a certain temperature.
For reactant concentration, enter the concentration data measured
at all temperatures in this way. Enter the reaction time in F1–F7,
set the variable cell N1, enter the initial value n value 2, enter the formula 1/c(n – 1) in G1–G7, put n = 2 and the
concentration data at a certain temperature. Bring in, set the target
cell K1, enter the correl function, use macro programming to solve,
set the constraints: $ K $ 1 ≤ 1, and click
solve; you can find the best n value, so that the target cell value
is closest to 1, in which n is the reaction order.[12]
Detection
Conditions of HPLC
The
detection conditions are as follows :HPLC for Agilent 1260, using
Hedera-C18 column, room temperature, detection wavelength is 205 nm,
mobile phase: 1:9:0.5 (v/v/v) (acetonitrile, phosphate buffer solution,
triethylamine), phosphate buffer salt is formulated with ammonium
dihydrogen phosphate and water in a certain ratio. pH 3.0, flow rate:
1.2 mL/min, injection volume: 20 μL.[13] Preparation of the test solution: take 1 mL of the reaction solution
and prepare a 25 mL sample to be tested with a mobile phase.
Hydrolysis Equation
Hydantoin is
usually hydrolyzed in weak alkaline environment, and substituted hydantoin
at C-5 position can generate corresponding amino acids. Figure shows that hydantoin is hydrolyzed
to hydantoin acid in sodium hydroxide solution. Weigh 2 g of hydantoin
and a certain molar ratio of sodium hydroxide. At normal temperature,
first dissolve the sodium hydroxide in water to form a sodium hydroxide
solution, and then pour the hydantoin into the sodium hydroxide solution
to dissolve and configure the reaction solution. The reaction liquid
is passed into the reaction kettle, and hydrolysis is performed at
a certain temperature. The initial pressure is not set, and the pressure
is self-boosting. Generally, within the first hour, hydantoin is completely
hydrolyzed.
Figure 1
Hydrolysis of hydantoin to hydantoin acid.
Hydrolysis of hydantoin to hydantoin acid.Figure shows that
all of the hydantoin in the reactor are converted to hydantoin acid,
and hydantoin acid starts to convert to glycine, and the pressure
of the reactor rises; therefore, the reaction conditions do not need
to be changed at this time.
Figure 2
Hydrolysis of hydantoin acid to glycine.
Hydrolysis of hydantoin acid to glycine.
Results and Discussion
Effect of Stirring Rate on Hydrolyzation of
Hydantoin
Experimental Conditions
The molar
ratio of hydantoin to sodium hydroxide is 1:2. The hydrolysis temperature
is 383.15 K, the hydrolysis time is 30 min, and the stirring rate[14] is 100–600 rpm. It can be seen from Figure that when the stirring
rate is between 100 and 300 rpm, the conversion of hydantoin is obvious,
and when the stirring rate is between 300 and 400 rpm, the increase
is gentle, and after 400 rpm, the conversion rate of hydantoin is
no longer increased. This is because during the reaction, proper agitation
can make hydantoin dissolve more fully in the sodium hydroxide solution,
and at the same time, make the entire reaction solution heated evenly,
improving the mass and heat transfer efficiency. When the rotation
speed exceeds 400 r/min, the effect of heat and mass transfer is the
best, even if the speed is increased, the effect will not be improved.
The hydrolysis reaction is carried out in an aqueous solution, the
phase of the reactants is unchanged, and the hydantoin is completely
dissolved in the alkaline solution; therefore, the hydrolysis reaction
is considered to be a homogeneous reaction, and the entire reaction
system is controlled by kinetics.[15] Therefore,
the hydantoin hydrolysis experiment selected a stirring rate of 400
rpm.
Figure 3
Effect of the stirring speed on the conversion rate α of
hydantoin.
Effect of the stirring speed on the conversion rate α of
hydantoin.
Effect
of Hydrolysis Temperature and Hydrolysis
Duration on the Hydrolysis Process of Hydantoin
Experimental
Condition
The mole
ratio of hydantoin and sodium hydroxide is 1:2, the hydrolysis temperature
is 373.15–443.15 K, and the hydrolysis time is 7 h. Samples
are taken every 5 min in the first hour, and every other hour in the
second to sixth hours. Figure shows that when the hydrolysis time reaches 30 min, the conversion
rate of hydantoin is almost 100%. Figure shows that the yield of glycine reaches
the highest at 6 h at each temperature, and when the temperature is
423.15 K, the glycine yield is higher than other temperatures. However,
after the seventh hour, the yield of glycine decreases. This is because
under high temperature and high pressure, glycine will polymerize
to form glycine polymers such as diglycine, trihepatic peptide, and
so forth. At the same time, the long-term reaction is partially unreacted.
The side reaction of finished hydantoin acid and its derivatives will
inhibit the formation of glycine to a certain extent.[16] Therefore, it is more suitable to choose the temperature
of 423.15 K and the hydrolysis time of 6 h. The following is a graph
of glycine yield and hydantoin conversion rate.
Figure 4
Effect of molar ratio
of hydantoin to sodium hydroxide on hydrolysis
of hydantoin.
Figure 5
Effect of molar ratio of hydantoin to sodium
hydroxide on glycine
yield β.
Effect of molar ratio
of hydantoin to sodium hydroxide on hydrolysis
of hydantoin.Effect of molar ratio of hydantoin to sodium
hydroxide on glycine
yield β.
Effect
of Molar Ratio of Hydantoin to Sodium
Hydroxide on the Hydrolysis Process
The hydrolysis was carried
out at 423.15 K, the hydrolysis time was 6 h, and the molar ratio
of hydantoin to sodium hydroxide was 1:1, 1:2, 1:3, 1:4, and 1:5. Figure shows that hydantoin
is completely hydrolyzed only when the molar ratio is 1:1, and the
hydantoin is completely hydrolyzed when the amount of alkali is increased. Figure shows that when
the molar ratio is 1:3, the yield of glycine is much higher than that
of the other four groups, and when the alkali dosage is more than
1:3, the yield of glycine is significantly reduced. This is due to
the fact that glycine is an amino acid and contains amino and carboxyl
groups. The reaction with acid is the reaction between the amino group
and acid. The reaction with alkali is the reaction between the carboxyl
group and alkali. Both are acid–base neutralization reactions
to produce salt. The presence of excess sodium hydroxide will form
sodium glycine with glycine, which reduces the yield of glycine. At
the same time, hydantoin acid and hydantoin acid can be hydrolyzed
in a weakly alkaline environment. Too high an alkaline solution will
cause new side reactions, and these intermediates will play a competitive
role in the formation of glycine. The byproducts produced during the
degradation process at high temperatures are indeed detected by HPLC.
Therefore, the molar ratio of hydantoin to sodium hydroxide is 1:3,
which is a suitable condition.
Figure 6
Effect of molar ratio of hydantoin to
sodium hydroxide on hydrolysis
of hydantoin.
Figure 7
Effect of molar ratio of hydantoin to sodium
hydroxide on glycine
yield.
Effect of molar ratio of hydantoin to
sodium hydroxide on hydrolysis
of hydantoin.Effect of molar ratio of hydantoin to sodium
hydroxide on glycine
yield.
Determination
of Hydrolysis Condition
Based on the above experimental results,
it is determined that the
stirring rate is 400 rpm, reaction temperature is 423.15 K, hydrolysis
time is 6 h, and molar ratio of sodium hydroxide to hydantoin is 3:1,
which is a suitable condition for the hydrolysis of hydantoin to glycine.
HPLC Test Results
Test results of
standard:Chromatogram of glycine, hydantoin acid, and hydantoin
(Figures , 9, and 10) standards:
Figure 8
Glycine standard.
Figure 9
Hydantoin acid standard.
Figure 10
Hydantoin
standard.
Glycine standard.Hydantoin acid standard.Hydantoin
standard.Detection results of hydantoin
hydrolysate:Figure shows
the situation within the first hour of the hydrolysis of hydantoin.
This stage is mainly due to the hydrolysis of hydantoin to generate
hydantoic acid. Figure shows the situation within 2–6 h of the hydrolysis
of hydantoin. This stage is mainly due to the hydrolysis of hydantoin
to glycine. As shown in Figure , within the first hour, the hydantoin has been completely
hydrolyzed, while a little glycine is produced, and the next five
hours are the hydrolysis of hydantoin. As the temperature increases,
hydantoin decreases and glycine is formed. Impurity 1 and impurity
2 are formed during the reaction, and impurity 2 appears in 353.15–383.15
K. This may be because the hydantoin derivative cannot be hydrolyzed
when the temperature is not high, and impurity 2 is completely degraded
as the temperature increases, and it is found that impurity 1 is always
present in it. The content of hydrolysate is low at 353.15–443.15
K, and there is no decomposition in this temperature range, which
may be caused by the temperature that is not high enough, but considering
that too high temperature will aggravate the polymerization of glycine,
in order to suppress impurities. In the future, it may be necessary
to control the reaction time and consider using other types of bases.[17]
Figure 11
First hour of hydantoin hydrolysate.
Figure 12
Hydantoin
hydrolysate at 2–6 h.
First hour of hydantoin hydrolysate.Hydantoin
hydrolysate at 2–6 h.Table shows that
the peak times of the three standards are 6.813, 6.550, and 10.169
min, respectively. The average peak times of the three standards in Table are 6.835, 6.558,
and 10.003 min, which are basically consistent with the peak time
of the standard product. Therefore, the external standard method can
be used to calculate the content of each substance in the hydrolyzate. Table shows that with the
increase of temperature, the peak time of hydantoin and hydantoin
acid is delayed, while the peak time of glycine is advanced, which
may be related to the content of three substances. With the increase
of temperature, the content of glycine in hydrolysate increases, and
its elution ability increases, while the decrease of hydantoin and
hydantoin acid makes the elution ability decrease.
Table 1
Peak Time Table for Standard Chromatographic
Detection
standard
peak time/min
hydantoin
6.813
hydantoinacid
6.550
glycine
10.169
Table 2
Chromatographic
Peak Time Table of
Hydantoin Hydrolysate at Various Temperatures
hydantoin hydrolyzate temperature/°C
hydantoin peak time/min
hydantoin acid peak time/min
glycine peak time/min
80
6.811
6.536
10.119
90
6.827
6.538
10.103
100
6.803
6.532
10.381
110
6.798
6.534
10.350
120
6.796
6.528
10.257
130
6.785
6.523
10.194
140
6.897
6.602
9.576
150
6.868
6.598
9.527
160
6.888
6.595
9.756
170
6.878
6.596
9.767
mean
6.835
6.558
10.003
Establishment of Dynamic Model
Rate
Equation
In alkaline condition,
hydantoin is formed by ring opening, while hydantoin can be hydrolyzed
to glycine. Therefore, the reaction is considered as a series reaction.A—hydantoin,
B—hydantoin acid,
D—glycine, P—sodium hydroxide, k1—hydantoin hydrolysis rate constant, k2—hydantoin acid hydrolysis rate constant, cA—hydantoin concentration, cB—hydantoin acid concentration, cD—glycine concentration, and cP—sodium hydroxide concentration. The kinetic model
is establishedBecause the type and concentration
of the base are stable, k1cPα can
be considered as a constant, denoted as k3, and the above formula is
Determination of Reaction Order and Rate
Constant
The method of determining the reaction order usually
has the differential method, integral method, and half-life method.[18] However, only by these methods, the engineering
quantity error is very large, and the reaction order of this experiment
is solved by Excel macroplanning.[19] For
the first-order reaction, ln c–t is linear, while for the non-first-order reaction, 1/c(–t is linear.[20] Enter the reaction time
at different temperatures and the concentration of the corresponding
time obtained in the experiment in Excel software. When n is equal to 1, solve the linear relationship of ln c–t; when n is not equal
to 1, set different n (level) value, applying the
correl function to find the maximum r2 value of 1/c(–t at different temperatures.Table shows the actual
concentration measured in each time period during the hydrolysis of
hydantoin. These concentration data will be brought into the calculation
step in Section .
Table 3
Experimental Concentration of Hydantoin
time/s
373.15 K/mol·L–1
383.15 K/mol·L–1
393.15 K/mol·L–1
403.15 K/mol·L–1
413.15 K/mol·L–1
423.15 K/mol·L–1
433.1 K/mol·L–1
443.15 K/mol·L–1
600
0.329749
0.157115
0.053487
0.055487
0.048896
0.018899
0.017912
0.019429
900
0.117230
0.062910
0.015442
0.017442
0.009708
0.003795
0.004485
0.003982
1200
0.043781
0.020927
0.006382
0.006082
0.002208
0.000793
0.000882
0.000890
1500
0.013721
0.006536
0.002036
0.002036
0.000422
0.000166
0.000156
0.000166
1800
0.005585
0.002533
0.000673
0.000673
0.000093
0.000038
0.000037
0.000035
Table shows the
actual concentration measured in each time period during the hydrolysis
of hydantoin acid. These concentration data will be brought into the
calculation step in Section .
Table 4
Experimental Concentration of Hydantoin
Acid
time/h
373.15 K/mol·L–1
383.15 K/mol·L–1
393.15 K/mol·L–1
403.15 K/mol·L–1
413.15 K/mol·L–1
423.15 K/mol·L–1
433.15 K/mol·L–1
443.15 K/mol·L–1
1
0.413169
0.330824
0.270305
0.244336
0.193102
0.109209
0.008037
0.106563
2
0.400001
0.311372
0.243429
0.202233
0.131090
0.075904
0.006040
0.038540
3
0.383225
0.289074
0.220139
0.164727
0.096965
0.058330
0.003000
0.014861
4
0.371206
0.274501
0.197632
0.137663
0.070656
0.039308
0.001807
0.005552
5
0.358740
0.259353
0.177944
0.115719
0.050184
0.027557
0.001010
0.002312
6
0.345741
0.245985
0.157755
0.093778
0.036142
0.019089
0.000501
0.000810
Table shows the
calculation results of the first step of the hydrolysis of hydantoin, n = 2 is the preset number during the calculation of the
series, and 1/c(n – 1) is
the calculation result after bringing the preset number. According
to the calculation method in Section , when the initial value is 2, the calculated
five 1/c(n – 1) values at
each temperature are shown in the table. When Excel starts to solve,
the system will automatically introduce different preset values n in addition to 2, so that r2 is close to 1, then n value is the required sequence.
According to Table , the average reaction order of the first step of the hydrolysis
of hydantoin is 1.03.
Table 5
Result of the Series
Calculation of
the First Step of the Hydrolysis of Hydantoin
temperature (K)
373.15
383.15
393.15
403.15
413.15
423.15
433.15
443.15
when n = 2, the value of 1/c(n–1)
3033
6365
18,696
18,022
21,324
52,914
55,830
51,469
8530
15,896
64,757
57,332
103,009
263,525
222,969
251,141
22,841
47,785
156,699
164,429
452,960
1,260,398
1,133,414
1,123,394
72,879
153,002
491,070
491,070
2,370,736
6,030,588
6,417,608
6,030,588
179,039
394,742
1,484,825
1,484,825
10,697,475
26,624,197
27,352,434
28,935,338
r2
0.9994
0.9986
0.9981
0.9993
0.9991
0.9992
0.9987
0.9992
optimal value of n
1.000000
1.000000
1.0003107
1.094726
1.039055
1.065041
1.002150
1.029439
average of n
1.02884
Table shows the
calculation results of the second stage of the hydrolysis of hydantoin.
According to Table , the average reaction order of the first step of the hydrolysis
of hydantoin is 1.05.
Table 6
Calculated Results
of the Second Step
of the Hydrolysis of Hydantoin
temperature
(K)
373.15
383.15
393.15
403.15
413.15
423.15
433.15
443.15
when n = 2, the value of 1/c(n–1)
2420
3023
3700
4093
5179
9157
124,428
9384
2500
3212
4108
4945
7628
13,175
165,557
25,947
2609
3459
4543
6071
10,313
17,144
333,322
67,290
2694
3643
5060
7264
14,153
25,440
553,373
180,124
2788
3856
5620
8642
19,926
36,288
990,393
432,616
2892
4065
6339
10,663
27,669
52,386
1,996,008
1,234,568
r2
0.9986
0.9991
0.9996
0.9997
0.9944
0.9989
0.9994
0.9987
optimal value of n
1.007797
1.000000
1.008638
1.058607
1.000092
1.000000
1.123223
1.182364
average of n1
1.04759
The average series calculation results in Tables and 6 are 1.029 and
1.048, respectively. This indicates that the two-step hydrolysis reaction
of hydantoin is a first-order reaction, and the total reaction is
a first-order series reaction.[21]
Rate Constant of Hydantoin Hydrolysis
Figures and 14 show the linearity of ln c and t. It can be seen from the figure that the five groups of
data are in a linear relationship, and the relationship expression
is ln c = −kt + b. According to Table , the rate constant[22] increases with the
rise of temperature, and the rate constant in the first step of hydantoin
hydrolysis is larger than that in the second step; thus, the hydrolysis
reaction in the second step is considered as the rate control step
of the whole hydantoin hydrolysis process.[23] Compared with most serial reactions, the difference of this subject
is that the target product is not an intermediate, that is, the target
product is glycine rather than hydantoin acid. Therefore, in addition
to improving the first step reaction rate and conversion rate of hydrolysis,
it is necessary to improve the second step rate and conversion rate,
so as to improve the yield of glycine.
Figure 13
Hydrolysis rate constant
of hydantoin.
Figure 14
Hydrolysis rate constant of hydantoin
acid.
Table 7
Calculation Results
of Rate Constants k1 and k2
reaction temperature/K
rate constants k1
rate constants k2
373.15
3.43 × 10–3
1.0 × 10–5
383.15
3.51 × 10–3
2.0 × 10–5
393.15
3.59 × 10–3
3.0 × 10–5
403.15
3.66 × 10–3
5.0 × 10–5
413.15
5.19 × 10–3
9.0 × 10–5
423.15
5.22 × 10–3
2.0 × 10–4
433.15
5.25 × 10–3
3.0 × 10–4
443.15
5.28 × 10–3
4.0 × 10–4
Hydrolysis rate constant
of hydantoin.Hydrolysis rate constant of hydantoin
acid.R-squared is the
ratio of the sum of squared regressions
to the sum of squared deviations. SSR is the sum of squared residuals.[24] It represents the effect of random errors. The
smaller the sum of squared residuals of a set of data, the better
the fit is. Sum squared residual[25] calculation
formulaŶ is the fitted value
of Y.The fitting
results from Tables and 9 show that the values
of the five groups of SSR are all close to 0, and both the R2 value and the corrected R2 value are close to 1, indicating that the fitting result
is good.
Table 8
Fit of Rate Constant k1
fitting result parameter
(K)
373.1
383.15
393.15
403.15
413.15
423.15
433.15
443.15
intercept
0.9490
0.30928
–0.82043
–0.72028
0.05698
–0.89164
–0.78983
–0.75681
slope
–0.00300
–0.00351
–0.00359
–0.00366
–0.00519
–0.00519
–0.00525
–0.00528
SSR
0.01140
0.017631
0.02231
0.00204
0.00538
0.00418
0.03259
0.00619
R-squared
0.99892
0.99841
0.9981
0.99983
0.99978
0.99983
0.99869
0.99976
adjusted R2
0.99857
0.99788
0.99744
0.99978
0.99970
0.99977
0.99825
0.99967
Table 9
Fit of Rate Constant k2
fitting result parameter
(K)
373.1
383.15
393.15
403.15
413.15
423.15
433.15
443.15
intercept
–1.05164
–1.19745
–1.22200
–1.33776
–1.85464
–4.11761
–1.29869
–1.05164
slope
–0.00002
–0.00003
–0.00005
–0.00009
–0.00010
–0.00016
–0.00027
–0.00002
SSR
0.00021
0.00015
0.00041
0.00177
0.00448
0.05311
0.00643
0.00021
R-squared
0.99666
0.99927
0.99935
0.99908
0.99788
0.99056
0.99961
0.99666
adjusted R2
0.99583
0.99908
0.99919
0.99884
0.99735
0.98820
0.99951
0.99583
Model Significance Analysis
In
order to verify the applicability of the kinetic model, the t-sample test[26] and F-sample test[27] were performed using Origin
software to test the regression coefficient and overall significance
of the model. The t-sample test uses the t-distribution theory to infer the probability of the occurrence
of the difference. The F-test is a test under the
null hypothesis (H0), and the statistical
values follow the F-distribution to compare the difference
between the two averages significantly. In the analysis results of
Origin, if the probability of P > |t| and the probability of P > |F| are less than 0.05, there is a statistical difference, if they
are less than 0.01, there is a significant statistical difference,
and if they are less than, there is an extremely significant statistical
difference.The analysis results in Table are the analysis of the results of the
rate constant k1 curve fitting at different
temperatures. SEM is the standard error. Generally speaking, the value
of SEM represents the representativeness of the batch of samples to
the overall sample. The smaller the standard error, the stronger the
strap is. According to Table , we can see that 373.15–423.15 K. In the temperature
range of K, the standard errors are close to 0, and the standard errors
of only 433.15 and 443.15 K are slightly higher, which shows that
the samples used in k1 fitting are more
representative. At the same time, under each temperature model of
the result of the t-test sample, the value of P > |t|. Less than 0.05, and the variance
of P > |F| in the analysis is
less
than 0.05, indicating that the model is good. The results show that
within this temperature range, the k1 fitting
results are credible.[28]
Table 10
k1 Sample t-Test
and Sample F Test
statistical
parameter
models at different temperatures/K
SEM
t statistic
F
P > |t|
P > |F|
373.15
0.72882
–4.35169
2.59571
0.01214
0.03067
383.15
0.74442
–5.23689
0.00635
393.15
0.76267
–6.72716
0.00254
403.15
0.7758
–6.5848
0.00275
413.15
1.10123
–5.60448
0.00498
423.15
1.10047
–6.4666
0.00295
433.15
1.1143
–6.36196
0.00313
443.15
1.12035
–6.33166
0.00318
The analysis results in Table are the analysis of the results of the
rate constant k2 curve fitting at different
temperatures. It
can be seen from Table that the standard errors in the temperature range of 373.15–443.15
K are close to 1, which shows that the samples used in k2 fitting are highly representative. At the same time,
under each temperature model of the result of the t-test sample, the value of P > |t |. Less than 0.05, and the variance of P > |F| in the analysis is less than 0.05, indicating that the
model is good. The results show that in this temperature range, the k1 fitting result is credible.
Table 11
k2 Sample t-Test
and Sample F Test
statistical
parameter
models at differenttemperatures/K
SEM
t statistic
F
P > |t|
P > |F|
373.15
0.02728
–35.6655
28.76019
3.2 × 10–7
1.1 × 10–13
383.15
0.04550
–27.68664
1.2 × 10–6
393.15
0.08165
–19.24591
7.0 × 10–6
403.15
0.14499
–13.00898
4.8 × 10–5
413.15
0.25272
–9.87385
1.8 × 10–4
423.15
0.26553
–11.56252
8.5 × 10–5
433.15
0.43301
–14.07014
3.3 × 10–5
443.15
0.73823
–6.34086
0.00144
Activation Energy Calculation
According
to the Arrhenius equation[29]which is plotted as ln k–T–1.According to the fitting results
in Figures and 16, in the first step of hydantoin hydrolysis, ln k1 and T–1 show a linear relationship in the temperature range of 373.15–403.15
and 413.15–443.15 K, while in the second step of hydantoin
hydrolysis, ln k2 and T–1 show a linear relationship in the temperature
range of 373.15–413.15 and 423.15–443.15 K, respectively,
and R2 is close to 1; thus the fitting
results are good. The obtained slopes are γ1 = −327,
γ2 = −105, γ3 = −8550,
and γ4 = −9652, and the four direct slopes
are substituted into the Arrhenius derivation formula to obtain the
reaction activation energy.[30]
Figure 15
Relationship between ln k1 and T–1.
Figure 16
Relationship between ln k2 and T–1.
Relationship between ln k1 and T–1.Relationship between ln k2 and T–1.Kinetic modelAccording to the calculated activation energy,
it is found that
the activation energy of each step of the reaction of hydantoin changes,
and the boundary temperature is 413.15 K, which indicates that for
the reaction of hydantoin hydrolysis, a stepwise hydrolysis method
can be adopted. In the first step of hydrolysis, the activation energy
is in the temperature range of 373.15–403.15 K. C is 3.1 times the interval of 413.15–443.15 K, and the rate
constant increases with increasing temperature, taking into account
the range of 413.15–443.15 K. The rate constant does not increase
much; therefore, we choose 413.15 K as the first hydrolysis temperature
of hydantoin. Because the hydantoin is completely hydrolyzed within
1 h, it is preliminarily determined that the first hydrolysis period
is 1 h. In the second step of the hydrolysis of hydantoin, the activation
energy in the temperature range of 423.15–443.15 K is only
1.1 times the temperature range of 373.15–413.15 K, but the
rate constant is much larger than the temperature range of 373.15–413.15
K, taking into account the target product of this series of reactions
is glycine rather than hydantoin, so the reaction rate for increasing
the second step reaction is the main target. 443.15 K was selected
as the temperature of the second stage of hydantoin hydrolysis, and
the glycine yield reached the highest at the 6th hour; therefore,
the second hydrolysis period was initially determined to be 5 h.
Conclusions
Through experimental research,
the optimal reaction conditions
were determined as follows: stirring rate 400 rpm, temperature 423.15
K, molar ratio of hydantoin sodium hydroxide 1:3, and reaction time
of 6 h, so that the hydantoin conversion rate reached 100%. The yield
of glycine reached 91%. At the same time, the kinetic parameters of
hydantoin hydrolysis were calculated, proving that the total reaction
of hydantoin hydrolysis was a first-order series reaction, and the
rate constants of the two-step hydrolysis at 423.15 K were 5.22 ×
10–3 and 2.0 × 10–4, respectively,
and the kinetic model of hydrolysis was established, and the significance
and reliability of the kinetic model of hydrolysis of Hein were verified
by the origin statistical function. The kinetic model was regarded
as a reliable model.The calculation of the rate constant and
the activation energy
Sof the hydrolysis reaction provide a new way for the hydrolysis of
hydantoin. The original single constant temperature hydrolysis method
is changed to the segmented constant temperature hydrolysis. The segmented
temperature is 413.15 and 443.15 K, respectively, and the segmented
hydrolysis time is 1 and 5 h, respectively. This is the innovation
of this topic. At present, most of the intermediate products of the
series reaction are the final products needed; thus, most of the means
are to delay its degradation. The target product of hydantoin hydrolysis
is glycine rather than intermediate; therefore, accelerating the degradation
of intermediate hydantoin acid is also the focus of the study. The
research of this subject is to increase the rate of the two-step reaction,
provide basic data for the design and development of a new glycine
reactor, and lay a foundation for the industrialization of glycine
production by the hydantoin method.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16