Investigation of a Complex Reaction Pathway Network of Isobutane/2-Butene Alkylation by CGC–FID and CGC-MS-DS

The mechanism of reaction in isobutane/2-butene alkylation systems is extremely complicated, accompanied by numerous side reactions. Therefore, a comprehensive understanding of the reaction pathways in this system is essential for an in-depth discussion of the reaction mechanism and for improving the selectivity of the major products (clean fuel blend components). The alkylation of isobutane/2-butene was studied using a self-made intermittent reaction device with a metering, cooling, reaction, vacuum and analysis system. The alkylates were qualitatively and quantitatively analyzed using a capillary gas chromatography-mass spectrometry-data system (CGC-MS-DS) and capillary gas chromatography with flame ionization detection (CCGC-FID), respectively, and the precision and recovery of the quantitative analytical methods were verified. The results showed that the relative standard deviation (RSD) of the standard sample was below 0.78%, and the recoveries were from 98.53% to 102.85%. Under the specified reaction conditions, 79 volatile substances were identified from the alkylates, and the selectivity of C8 and trimethylpentanes (TMPs) reached 63.63% and 53.81%, respectively. The changes of the main chemical components in the alkylation reaction with time were tracked and analyzed, based on which reaction pathways were determined, and a complex reaction network containing the main products’ and the by-products’ generation pathway was constructed.


Introduction
With the increasing stricter emission standards, the upgrading of gasoline is developing in a higher quality and environmentally friendly direction. The alkylation of isobutane/butene in the C4-fraction is a vital process in the petroleum refining industry for the production of alkyl compounds, in which the alkylated oil produced has the outstanding advantages of a high researched octane, with good anti-explosive properties, no olefins or aromatics, a low vapor pressure, a complete combustion, cleanliness, etc. It is the unique blending component that can simultaneously satisfy the requirements of a high octane value and a clean combustion [1][2][3][4][5][6][7][8]. Since 1990, U.S. refiners have been obliged to change their gasoline composition strategies to meet the mandatory specifications of the Clean Air Act (CAA). Since 1995, the US has been using methyl tertiary butyl ether (MTBE) in large quantities for gasoline additives. However, due to the MTBE having a severe environmental contamination problem, policies banning MTBE as a gasoline additive have been introduced in many countries. Since then, the alkylates have become the optimum mixture component in the gasoline pool of a traditional refinery. With the rapidly increasing demand for alkylated oil, the world's alkylated oil production exceeded 115.6 million tons/year [9][10][11][12].
Currently, liquid acids, ionic liquids and solid acids are used as catalysts for isobutanes and butenes alkylation reactions. However, alkylation technologies using ionic liquids and Meanwhile, the intermediate species and products distribution were investigated by the CGC-MS-DS and CGC-FID. According to the classical carbonium ion mechanism, as well as the results of the CGC-MS-DS identification of the alkylation products of isobutane and 2-butene and the alkylation product distribution with time, a detailed reaction pathway network was developed, which includes the reaction pathway of each by-product.

Optimization of the CGC-FID Analysis Conditions
Since almost all alkyl compounds are hydrocarbons, the inlet temperature was set up at 250 °C to rapidly vaporize the various components of the sample. The temperature of the detector was set up at 250 °C to prevent the generation of condensate in the detector, due to the temperature difference. The alkylates are a mixture of hydrocarbons with different carbon numbers, in which there are a large number of isomers with similar boiling points and molecular weights and wide boiling ranges, so the CGC separation was carried out by a programmed temperature rise. Following several experimental tests on the injection volume, the programmed ramp-up rate and the carrier gas flow rate, the CGC-FID analysis conditions were identified below: both the injector and flame ionization detector were set to 250 °C, and the injection was in separation mode (1:80) with an injection volume of 1 μL. The pressures of the carrier gas, air, and hydrogen were set to 0.12 MPa, 0.1 MPa, and 0.1 MPa, respectively. This analytical condition provided a good separation of the alkylate fractions, and the chromatogram of the alkylates is shown in Figure 1.

Relative Correction Factors
Three standard solutions of different concentrations were prepared separately using electronic-analytical balances with a precision as low as 0.0001 g. The standard solutions were measured five times in parallel under the specified chromatographic analysis conditions, and 2,2,4-trimethylpentane was selected as the reference material (S). The relative correction factors of each component were calculated by Equation (4), as shown in Table  1.
As seen in Table 1, the relative correction factors of the benchmark 2,2,4-trimethylpentane and the components were in the range of 0.99 to 1.07, and all of the

Relative Correction Factors
Three standard solutions of different concentrations were prepared separately using electronic-analytical balances with a precision as low as 0.0001 g. The standard solutions were measured five times in parallel under the specified chromatographic analysis conditions, and 2,2,4-trimethylpentane was selected as the reference material (S). The relative correction factors of each component were calculated by Equation (4), as shown in Table 1.
As seen in Table 1, the relative correction factors of the benchmark 2,2,4-trimethylpentane and the components were in the range of 0.99 to 1.07, and all of the components had relative correction factors close to 1. For the alkylates, most of the components are tautomers or homologs, and their structures are similar, so their relative correction factors are also close to each other. Firstly, three standard samples with different concentrations were prepared and quantified by the peak area normalization method and the corrected peak area normalization method. Next, the average value was taken for five parallel measurements, and then the relative deviations (RDs) were calculated according to Equation (1), and the results are shown in Table 2.
where Z i is the concentration of component i measured by different analytical methods, v i is the concentration of the initial standard samples of component i. As seen in Table 2, the maximum RD between the quantitative results of the peak area normalization method and the actual content is 3.11%, and the maximum RD between the quantitative results of the corrected peak area normalization method and the actual content of the sample is 2.52%. The difference between the quantitative results of the two ways was 0.59%. The deviation of the data measured by the two methods is small, so it is more convenient to choose the peak area normalization method for the quantification.

Precision
A standard sample was prepared, measured five times in parallel and the average value was calculated. Then, the measurement results' relative standard deviation (RSD) was calculated by Equation (2), and the results are shown in Table 3.
where S i is the standard deviation of component i, x ij is the mass fraction of component i, x i is the mean of the mass fraction of component i. Table 3, the RSD of the samples was less than 0.78%, indicating that the selected chromatographic conditions were reasonable and the excellent precision of the area normalization method's quantitative results met the determination requirements.

Recovery
A standard sample was prepared and measured five times in parallel to take the average value, and then a specific content of the measured substance was added and measured  Table 4. As shown in Table 4, the recoveries of the standard samples were 98.53% to 102.85%, and the accuracy of the selected analytical and quantitative methods was high.
where Y i is the measured value after the spiking of component i, y i is the measured value of the initial standard sample of component i, a i is the spiked value of component i.

Chemical Composition of the C 4 Alkylation Products
The alkylates were characterized using the CGC-MS-DS. The standard samples (listed in the materials) were first analyzed to determine their spectrum and relative retention time, and then the alkylate was divided into two equal parts. One was added to the standard samples, and the other was not added with any substance. Both samples were analyzed using identical instrument parameters. Their spectra were both searched using the NIST14, NIST14s, NIST20-1, NIST20-2 and NIST20s databases in the CGC-MS-DS program. Then, the substances without standard samples were directly searched and characterized by the databases, and other products in the alkylate were characterized by the databases, further confirmed by adding standards. A total of 86 substances were confirmed to be isolated from the isobutane/2-butene alkylation reaction products, and the identification of 79 compounds was identified. The results revealed that the retention times of the most important products of the isobutane/2-butene alkylation reaction, TMPs, ranged from 16.734 min to 22.407 min. The results of the isobutane/2-butene alkylation product components are shown in Table 5.

. Changes in the Composition of the Reaction Process
The isobutane/2-butene alkylation reaction was followed by the CGC-MS-DS and CGC-FID under the conditions of the molar ratio of isobutane to 2-butene of 10:1 (I/O = 10:1), the sulfuric acid/hydrocarbon volume ratio of 1:1 (A/H = 1:1), the reaction temperature of 7 • C, the reaction pressure of 0.5 MPa, the stirring speed of 1300 rpm and the changes of the reaction conversion and selectivity of each component with time were investigated. The results are shown in Figure 2.
As noted in Figure 2d, the conversion of 2-butene reached 97.12% at 2 min, which indicates that the alkylation of C 4 was a fast reaction, and most of 2-butene was already consumed rapidly at 2 min, and the conversion of 2-butene increased slightly after 2 min; it reached 98.08% at 5 min and finally stabilized, but the conversion did not reach 100%, perhaps because 2-butene had a certain saturation vapor pressure at the reaction temperature and failed to participate in the reaction completely. From Figure 2a-c, it could be seen that the selectivity of the C 8 component increased sharply from 0.5 to 5 min, the selectivity of the C 9 + components decreased significantly, and the selectivity of the C 5 -C 7 components increased slowly; after 5 min, the components stabilized. The alkylation reaction was completed after 2 min, but between 2 and 5 min, the selectivity of the TMPs continued to increase, the selectivity of the DMHs increased slightly, and the selectivity of the C 9 + high carbon fraction continued to decrease.
As noted in Figure 2d, the conversion of 2-butene reached 97.12% at 2 min, which indicates that the alkylation of C4 was a fast reaction, and most of 2-butene was already consumed rapidly at 2 min, and the conversion of 2-butene increased slightly after 2 min; it reached 98.08% at 5 min and finally stabilized, but the conversion did not reach 100%, perhaps because 2-butene had a certain saturation vapor pressure at the reaction temperature and failed to participate in the reaction completely. From Figure 2a-c, it could be seen that the selectivity of the C8 component increased sharply from 0.5 to 5 min, the selectivity of the C9 + components decreased significantly, and the selectivity of the C5-C7 components increased slowly; after 5 min, the components stabilized. The alkylation reaction was completed after 2 min, but between 2 and 5 min, the selectivity of the TMPs continued to increase, the selectivity of the DMHs increased slightly, and the selectivity of the C9 + high carbon fraction continued to decrease.

Reaction Pathway Network Construction
According to the CGC-MS-DS and CGC-FID tracing analysis results of the alkylation reaction products, it was known that the reaction generated C8 (TMPs) as the main product, while a large number of low carbon molecules, as well as high carbon molecule byproducts, were also generated, in which the high carbon molecules were generated because of the polymerization of the low carbon molecules. In the qualitative analysis of the CGC-MS-DS in Table 5, it was found that there were many isomers in the same carbon

Reaction Pathway Network Construction
According to the CGC-MS-DS and CGC-FID tracing analysis results of the alkylation reaction products, it was known that the reaction generated C 8 (TMPs) as the main product, while a large number of low carbon molecules, as well as high carbon molecule by-products, were also generated, in which the high carbon molecules were generated because of the polymerization of the low carbon molecules. In the qualitative analysis of the CGC-MS-DS in Table 5, it was found that there were many isomers in the same carbon number molecule, indicating that there was also an isomerization reaction in the alkylation reaction. It could be seen from Figure 2 that the alkylation reaction essentially ended at 2 min, and within 5 min, the selectivity of C 8 increased sharply, the selectivity of C 9 + decreased sharply, and the selectivity of C 5 -C 7 increased slightly. Indicating that at this stage, high carbon molecules underwent a scission reaction to form C 8 and the low carbon molecules C 5 -C 7 , among which TMPs are the main cleavage products. The selectivity of C 9 decreased most rapidly, indicating that it was one of the significant reactants in the fragmentation reaction. Thus, the alkylation of isobutene/2-butene was a complex reaction in which the primary reaction was an addition reaction to produce C 8 , accompanied by polymerization, fragmentation, isomerization and other side reactions. The reactions at each node in the alkylation reaction pathway network were as follows: (1) Isomerization reaction: Under an acidic environment, the reaction material 2-butene (2-C 4 = ) undergoes isomerization through a hydrogen transfer or methyl transfer to form 1-butene (1-C 4 = ) and isobutene (i-C 4 = ) and reaches equilibrium, and the thermodynamic equilibrium between butenes favors isobutene at the reaction temperature, so the selectivity of isobutene is the highest [30,31]. Similarly, other high-carbon carbocations undergo isomerization reactions through a similar hydrogen transfer or methyl transfer, which is why the alkylation products contain multiple isomers in the same carbon number molecule.
(2) Main reaction: It is well known that the isobutane alkylation reaction follows the classic carbonium ion mechanism by Schmerling et al. [18,32]. According to the classic carbonium ion mechanism, the unsaturated double bond in butene seizes H + in the acid catalyst to form C 4 + , and C 4 + further undergoes isomerization to generate the more stable tert-butyl carbocation (i-C 4 + ). i-C 4 + underwent addition reactions with 2-C 4 = (or i-C 4 = ) and 1-butene to generate TMPs + and DMHs + , respectively. Finally, the TMPs + seizure the H-of i-butane (i-C 4 ) to generate TMPs + and the DMHs + seizure the Hof reactants i-C 4 to generate DMHs [14,33].
(3) Polymerization reaction: Olefins underwent dimerization or multimerization reactions at high temperatures and under acidic conditions. The strong exotherm of the alkylation reaction led to high local temperatures and initiated the polymerization of olefins. The dimerization reaction between olefins produced C 8 + , and then C 8 + and short chain carbonium ions will continue to polymerize with C 4 -fractions to produce high carbon number molecules [34,35].
(4) Fragmentation reaction: The multimerization reaction between olefins generated high-carbon molecules, which obtained protons to form carbonium ions. Long-chain carbonium ions were unstable in an acidic environment and were easily broken into shortchain hydrocarbon molecules at the β position of the charged carbon atoms. The resulting short-chain hydrocarbons underwent further reactions under alkylation conditions [36]. Albright [37] believed that C 12 + and C 16 + were the most important intermediates, C 12 + and C 16 + underwent a cleavage reaction to generate short-chain carbanions and shortchain alkenes, and the short-chain carbanions further abstracted Hto generate short-chain alkanes, resulting in the generation of C 5 , C 6 , C 7 and other alkanes.
According to the classic carbonium ion mechanism, the results of the tracing analysis of the isobutane/2-butene alkylation products, and the discussion of the above reaction pathway network nodes, multiple reactions occur simultaneously in the alkylation system of isobutane and 2-butene with a large number of isoparaffins and corresponding carbocations. During the experiment, it was found that the change in the selectivity of the C 8 component was negatively correlated with the selectivity of C 9 + components before 5 min, with the largest change in the selectivity of the C 9 components. However, as shown by the conversion of 2-butene, the alkylation reaction was finished at 2 min. Therefore, the increase in the selectivity of the C 8 components between 2 to 5 min may be related to the fracture reaction of the C 9 components. In this work, an isobutane/2-butene alkylation reaction pathway network with main and side reactions was constructed on the basis of the results obtained from the tracing analysis, as shown in Figure 3.

Experimental Equipment
An independently designed set of C 4 alkylation batch reaction devices, which consists of a metering system, a cooling system, a reaction system, a vacuum system and an analysis system. The feeding and metering of the raw materials are controlled by the plunger-type metering pump into the pressurized reaction kettle, but the control of the feed by the plunger-type metering pump may cause an inaccurate metering [38]. To solve the problem of inaccurate measurement, a mass flow meter (D07-7B, Seven Star, Beijing, China) was used to accurately measure the raw material gas. Then the feed gas passed through a constant temperature circulating water tank into a cooling coil, where it condensed and finally passed into a stainless steel PTFE-lined autoclave (Dalian Jingyi Autoclave Co., Ltd., Dalian, China) to perform the reaction. The unit is also equipped with a vacuum system to remove the air in the system so that the feed gas can be better condensed in the cooling coil and provide a stable environment for the alkylation. To investigate the variation of the reaction product concentration distribution with time, sampling is required to follow the reaction process. Therefore, the analytical system in this paper uses the CGC-MS-DS (TQ8040, Shimadzu, Kyoto, Japan) and CGC-FID (GC9790, FULI, Hangzhou, China) to perform the qualitative and quantitative analyses for the alkylation products, respectively. The self-made reaction device is shown in Figure 4.

CGC-FID Analysis Conditions
Quantitative analysis was detected by FULI GC-9790 (Zhejiang Fuli Analytical Instruments Co., Hangzhou, China)with a FID detector and a HP-PONA capillary column (50 m × 0.2 mm × 0.5 µm). The temperature of both the injector and the flame ionization detector was set to 250 • C. The injections were made in a split mode (1:80) with an injection volume of 1 µL. The pressures of the carrier gas, air and hydrogen were set to 0.12 MPa, 0.1 MPa and 0.1 MPa, respectively. The temperature program was as follows: The column temperature was stabilized at an initial value of 60 • C (held for 1 min), ramped up to 80 • C (held for 2 min at a rate of 5 • C/min) and finally ramped up to 200 • C (held for 10 min).

Determination of the Relative Correction Factors
According to the benchmark of a particular component, it was necessary to configure the standard solutions of the different concentrations. Next, we performed the parallel determinations under the specified chromatographic conditions. Then, based on the obtained data, we calculated the relative correction factors of other components in the solution relative to the reference substance.
The calculation formula is shown as follows: where f is is the relative correction factor of component i, f i is the correction factor of component i, f s is the correction factor of a reference substance, m i is the mass of component i, m s is the mass of the reference substance, A i is the peak area of component i, A s is the peak area of the reference substance.

Determining the Mass% of the SAMPLE Components
Based on the peak area Ai and the relative correction factor f is of component i concerning the reference material calculated in Equation (4), the mass fraction of each component i can be calculated from Equation (5).
The calculation formula is shown as follows: where A i f is is the corrected peak area of component i, ∑A i f is is the sum of the corrected peak areas, and ω i is the mass% of component i.

Qualitative Analysis CGC-MS-DS Analysis Conditions
The instrument used for the qualitative analysis was a Shimadzu GC-MS-TQ8040, and the column installed was a HP-PONA (50 m × 0.2 mm × 0.5 µm) from Agilent, USA. The sample was injected in the split-flow model (1:100) with an injection volume of 0.2 µL. The column temperature was stabilized at an initial value of 60 • C (held for 2 min), ramped up to 80 • C (held for 2 min at a rate of 5 • C/min) and finally ramped up to 200 • C (held for 10 min). A high purity helium was used as the carrier gas with a pressure of 0.12 MPa, a total flow rate of 50 mL/min, and a purge flow rate of 3.0 mL/min. The total run time was 58 min. The inlet temperature was 250 • C. The mass spectra were obtained in a EI (electron ionization) mode at 70 eV. The ion source temperature was 250 • C, and the interface temperature was 280 • C. Full scan chromatograms with selected mass-to-charge ratios in the range of 20-300 m/z were used for the acquisition.

C 4 Alkylation Reaction
The isobutane/2-butene alkylation was carried out in a 0.1 L stainless steel PTFElined autoclave. An internal water-cooled coil is used to control the reaction temperature. A schematic of the experimental set-up is depicted in Figure 4. A certain amount of concentrated sulfuric acid catalyst was poured into the autoclave, and then closed and sealed the autoclave. The air was extracted out of the autoclave with a vacuum pump to an absolute pressure of approximately 0.005 MPa. Then, N 2 was introduced into the autoclave to bring the pressure to 0.5 MPa and held for 10 min to ensure that the autoclave did not leak. Next, the autoclave was purged three times by N 2 at 0.5 MPa to eliminate any remnant air. The cryostat was adjusted to keep the cooling coil at a low temperature, and the material gas entered the cooling coil to condense. When the autoclave contents were cooled to the desired temperature, N 2 was started to be charged to press the condensed material in the cooling coil into the autoclave while the alkylation reaction was carried out at a preset stirring rate. N 2 was charged through a manually controlled valve to ensure a constant pressure in the autoclave throughout the reaction. Online sampling was performed at the desired time points, and the samples were processed before the gas chromatography analysis. Once the reaction was finished, we removed the product after collecting the gas from the autoclave.

Conclusions
The method for the CGC-FID analysis of alkylates was established, and the relative correction factors of several vital components in the alkylated gasoline were in the range of 0.9907 to 1.0692, and the maximum error of the quantitative results was obtained by the peak area normalization method and the corrected peak area normalization method was 0.59%. In addition, the precision and recovery of the area normalization method were examined. The results showed that the relative standard deviations of the precision were less than 0.78%, and the recoveries ranged from 98.53% to 102.85%, which indicated that the selected gas chromatographic conditions were reasonable and the quantitative analysis results by the area normalization method and the precision met the requirements of the assay.
The products of the alkylation reaction of isobutane/2-butene were characterized by the CGC-MS-DS coupling technique and a total of 79 compounds were identified and followed up. In the early stage of the reaction, 2-butene was isomerized in an acidic environment to form isobutene and 1-butene. In the first 2 min, isobutane was alkylated with the isomerized butene to form the C 8 component (TMPs) and other by-products (C 5 -C 7 , C 9 + ); after 2 min, the alkylation reaction was completed, and the rearrangement between the components of the reaction products was carried out between 2 and 5 min, with the long chain The long-chain alkane component undergoes a breakage reaction and the short-chain alkane undergoes an isomerization reaction; after 5 min, the rearrangement reaction process is completed. A detailed network of the alkylation of isobutane/2-butene containing each of the by-products was established, and the primary reaction is the alkylation of isobutene and 2-butene, which is a fast reaction accompanied by side reactions such as isomerization, polymerization, fragmentation, etc.
This study provides a precise and sensitive method of qualitative identification and quantitative analysis for the C 4 alkylation, demonstrates a plausible complex reaction pathway network of alkylation to reveal the alkylation reaction mechanism, and provides a basis for the establishment of the detailed kinetic models, which in turn leads to the optimization reaction conditions and the design of the C 4 alkylation reactors.